Actrii-ALK4 antagonists and methods of treating heart failure

Abstract

In some aspects, the disclosure relates to ActRII-ALK4 antagonists and methods of using ActRII-ALK4 antagonists to treat, prevent, or reduce the progression rate and / or severity of heart failure (HF), particularly treating, preventing or reducing the progression rate and / or severity of one or more HF-associated comorbidities. The disclosure also provides methods of using an ActRII-ALK4 antagonist to treat, prevent, or reduce the progression rate and / or severity of heart failure associated with a variety of conditions including, but not limited to, heart failure associated with aging.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority from U.S. Provisional Application No. 63 / 159,059, filed March 10, 2021. The specification of the foregoing application is incorporated herein by reference in its entirety. BACKGROUND The prevalence of heart failure (HF) depends on the definition applied, but it affects approximately 1-2% of the adult population in developed countries, rising to >10% among people 70 years of age. Among people 65 years of age presenting to primary care with breathlessness on exertion, one in six will have unrecognized HF (mainly HFpEF). The lifetime risk of HF at age 55 years is 33% for men and 28% for women. The proportion of patients with HFpEF ranges from 22 to 73%, depending on the definition applied, the clinical setting (primary care, hospital clinic, and hospital admission), age and sex of the studied population, previous myocardial infarction and the year of publication. With 50% of all heart failure diagnoses and 90% of all heart failure deaths occurring in adults over the age of 70, heart failure is undeniably tied to aging. The Framingham Heart Study found a prevalence of HF in men of 8 per 1000 at age 50 to 59 years, increasing to 66 per 1000 at ages 80 to 89 years; and similar values (8 and 79 per 1000) were noted in women. The prevalence in African-American populations is reported to be 25 percent higher than in white populations. While aging in and of itself is not a cause of heart failure, age does lower the threshold for manifestation of the disease. With the success of treatment options for ischemic and valvular diseases, there is an increasing number of older individuals with some degree of cardiac damage, which are increasingly imperiled by the diminished cardiac reserve associated with normal aging. Commonly, heart failure in aging patients falls under the umbrella of heart failure with preserved ejection fraction (HFpEF). Currently, there is no approved therapy specifically for HFpEF. Therefore, there is a high, unmet need for effective therapies for treating heart failure associated with aging. Accordingly, it is an object of the present disclosure to provide methods for treating, preventing, or reducing the progression rate and / or severity of heart failure, particularly treating, preventing or reducing the progression rate and / or severity of one or more heart failure-associated comorbidities. SUMMARY As demonstrated herein, an ActRII-ALK4 antagonist is effective in treating heart failure. In particular, an ActRIIB-ALK4 heterodimer protein demonstrated cardio-protective effects in a murine model of physiological cardiac aging using aged C57BL6 mice, displaying characteristics of heart failure associated with preserved ejection fraction (HFpEF). For example, data presented herein shows that treatment with an ActRIIB-ALK4 heterodimer has positive effects on various complications associated with this heart failure model including, but not limited to, LV contractility, hypertrophy, LV wall thickness, heart weight, systolic function, and serum biomarkers of cardiac injury (e.g., cTnl serum levels). While not wishing to be bound to any particular mechanism, it is expected that the effects of the ActRIIB-ALK4 heterodimer on heart failure is caused primarily by antagonizing ligandsignaling as mediated by one or more ligands that bind to the ActRIIB-ALK4 heterodimer protein including, but not limited to, activin A, activin B, GDF8, GDF11, BMP6, and / or BMP 10 (referred to herein as “ActRII-ALK4 ligands” or “ActRII-ALK4 ligand”). Regardless of the mechanism, it is apparent from the data presented herein that ActRIIB-ALK4 heterodimers have significant positive effects in ameliorating various complications associated with heart failure and further suggests that other ActRII-ALK4 antagonists may also be useful in treating heart failure associated with aging. As disclosed herein, the term “ActRII-ALK4 antagonist” refers a variety of agents that may be used to inhibit signaling by one or more ActRII-ALK4 ligands including, for example, antagonists that inhibit one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, and / or BMP10); antagonists that inhibit one or more ActRII-ALK4 ligand associated receptors (e.g., ActRIIA, ActRIIB, ALK4, and ALK7); and antagonists that inhibit one or more downstream signaling components (e.g., Smad proteins such as Smads 2 and 3). ActRII-ALK4 antagonists to be used in accordance with the methods and uses of the disclosure include a variety of forms, for example, ActRII-ALK4 ligand traps (e.g., soluble ActRIIA polypeptides or ActRIIB polypeptides including variants as well as heteromultimers and homomultimers thereof), ActRII-ALK4 antibody antagonists (e.g., antibodies that inhibit one or more of activin A, activin B, GDF8, GDF11, BMP6, BMP 10, ActRIIB, ActRIIA, ALK4 and / or ALK7), small molecule antagonists (e.g., small molecules that inhibit one or more of activin A, activin B, GDF8, GDF11, BMP6, BMP 10, ActRIIB, ActRIIA, ALK4 and / or ALK7) and nucleotide antagonists (e.g., nucleotide sequences that inhibit one or more of activin A, activin B, GDF8, GDF11, BMP6, BMP10, ActRIIB, ActRIIA, ALK4 and / or ALK7). In certain aspects, the disclosure provides ActRII-ALK4 antagonists comprising soluble ActRIIB, ActRIIA, ALK4, ALK7, or follistatin polypeptides to antagonize the signaling of ActRII-ALK4 ligands generally, in any process associated with heart failure associated with aging. ActRII-ALK4 antagonists of the disclosure may antagonize one or more ligands of ActRII-ALK4, such as activin A, activin B, GDF8, GDF11, BMP6, or BMP 10, and may therefore be useful in treating, preventing, or reducing the progression rate and / or severity of heart failure associated with aging, or one or more comorbidities of heart failure (e.g. anemia, angina, arterial hypertension, arthritis, atrial fibrillation, cachexia, cancer, cognitive dysfunction, coronary artery disease (CAD), diabetes, erectile dysfunction, gout, hypercholesterolemia, hyperkalemia, hyperkalemia, hyperlipidemia, hypertension, iron deficiency, kidney dysfunction, metabolic syndrome, obesity, physical deconditioning, potassium disorders, pulmonary disease (e.g., asthma, COPD), sarcopenia, sleep apnea, sleep disturbance, and valvular heart disease (e.g., aortic stenosis, aortic regurgitation, mitral regurgitation, tricuspid regurgitation)). In certain aspects, an ActRII-ALK4 antagonist to be used in accordance with the methods and uses disclosed herein (e.g., treating, preventing, or reducing the progression rate and / or severity of heart failure associated with aging, or one or more complications of heart failure) is an ActRII-ALK4 ligand trap polypeptide antagonist including variants thereof as well as heterodimers and heteromultimers thereof, an ActRII-ALK4 antibody antagonist, an ActRII-ALK4 polynucleotide antagonist, and / or an ActRII-ALK4 small molecule antagonist. ActRII-ALK4 ligand trap polypeptides include TGF-P superfamily-related proteins, including variants thereof, that are capable of binding to one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP 10). Therefore, an ActRII-ALK4 ligand trap generally includes polypeptides that are capable of antagonizing one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP10). As used herein, the term “ActRII” refers to the family of type II activin receptors. This family includes activin receptor type IIA (ActRIIA) and activin receptor type IIB (ActRIIB). In some embodiments, an ActRII-ALK4 antagonist comprises an ActRII-ALK4 ligand trap. In some embodiments, an ActRII-ALK4 ligand trap comprises an ActRIIB polypeptide, including variants thereof, as well as homomultimers (e.g., ActRIIB homodimers) and heteromultimers (e.g., ActRIIB-ALK4 or ActRIIB-ALK7 heterodimers). In some embodiments, an ActRII-ALK4 ligand trap comprises an ActRIIA polypeptide, including variants thereof, as well as homomultimers (e.g., ActRIIA homodimers) and heteromultimers (e.g., ActRIIA-ALK4 or ActRIIA-ALK7 heterodimers). In other embodiments, an ActRII-ALK ligand trap comprises a soluble ligand trap protein including, but not limited to, or a follistatin polypeptide as well as variants thereof. In some embodiments, an ActRII-ALK4 antagonist comprises an ActRII-ALK4 antibody antagonist. In some embodiments, an ActRII-ALK4 antagonist comprises an ActRII-ALK4 small molecule antagonist. In some embodiments, an ActRII-ALK4 antagonist comprises an ActRII-ALK4 polynucleotide antagonist. In part, the disclosure provides methods of treating heart failure associated with aging, comprising administering to a patient in need thereof an effective amount of an ActRII-ALK4 antagonist. The disclosure also provides methods of treating, preventing, or reducing the progression rate and / or severity of one or more comorbidities of heart failure associated with aging, comprising administering to a patient in need thereof an effective amount of an ActRII-ALK4 antagonist. In some embodiments, the patient is at least 40 years old. In some embodiments, the patient is between about 40 and about 100 years old. In some embodiments of the present disclosure, the heart failure is heart failure with preserved ejection fraction (HFpEF). In some embodiments, a patient has a left ventricular ejection fraction (LVEF) of >50%. In some embodiments, the patient has normal systolic function. In some embodiments of the present disclosure, the patient has dyspnea. In some embodiments, methods of the present disclosure decrease dyspnea. In some embodiments of the present disclosure, the patient has cardiovascular structural remodeling selected from the group consisting of an increase in vascular intimal thickness, an increase in vascular stiffness, an increase in left ventricular (LV) hypertrophy, and an increase in left atrial enlargement. In some embodiments, methods of the present disclosure improve cardiovascular structural remodeling in the patient selected from the group consisting of an increase in vascular intimal thickness, an increase in vascular stiffness, an increase in LV hypertrophy, and an increase in left atrial enlargement. In some embodiments of the present disclosure, the patient has LV hypertrophy. In some embodiments, methods of the present disclosure decrease LV hypertrophy in the patient. In some embodiments, the method decreases left ventricular hypertrophy in the patient, wherein the patient’s LV hypertrophy is decreased by at least 1% (e.g., 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or at least 50%). In some embodiments, methods of the present disclosure decrease cardiac filling pressure in the patient. In some embodiments, the method improves early diastolic cardiac filling in the patient. In some embodiments of the present disclosure, the patient has left atrial enlargement. In some embodiments, methods of the present disclosure decrease atrial enlargement in the patient. In some embodiments, the method decreases left atrial enlargement in the patient, wherein the patient’s left atrial enlargement is decreased by at least 1% (e.g., 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or at least 50%). In some embodiments, methods of the present disclosure decrease vascular intimal thickness in the patient. In some embodiments, methods of the present disclosure decrease vascular stiffness in the patient. In some embodiments of the present disclosure, the patient has a change in ventricular structure in the heart, selected from the group consisting of LV hypertrophy, an increase in cardiomyocyte size, a loss of cardiomyocytes, and a decrease in LV end-diastolic volume. In some embodiments, methods of the present disclosure improve changes in ventricular structure in the patient’s heart, selected from the group consisting of LV hypertrophy, an increase in cardiomyocyte size, a loss of cardiomyocytes, and a decrease in LV end-diastolic volume. In some embodiments, the method decreases cardiomyocyte size in the patient. In some embodiments, the method prevents the loss of cardiomyocytes from worsening in the patient. In some embodiments, the method increases LV end-diastolic volume in the patient. In some embodiments of the present disclosure, the patient has a change in atrial structure in the heart selected from the group consisting of left atrial hypertrophy, arrhythmia, atrial dilation, aortic root dilation, and atrial fibrillation. In some embodiments, methods of the present disclosure improve changes in atrial structure in the patient’s heart selected from the group consisting of left atrial hypertrophy, arrhythmia, atrial dilation, aortic root dilation, and atrial fibrillation. In some embodiments of the present disclosure, the patient has a functional change in the heart selected from the group consisting of change in diastolic heart function, change in systolic heart function, and change in electrical heart function. In some embodiments, methods of the present disclosure improve a functional change in the patient’s heart selected from the group consisting of change in diastolic heart function, change in systolic heart function, and change in electrical heart function. In some embodiments, the patient has a change in diastolic function. In some embodiments, the patient has diastolic dysfunction. In some embodiments, methods of the present disclosure improve diastolic dysfunction in the patient. In some embodiments, the patient has decreased ventricular relaxation and increased filling pressures. In some embodiments, the method increases ventricular relaxation and decreases filling pressures in the patient. In some embodiments, diastolic dysfunction in the patient is measured by a ratio of early diastolic transmitral flow to early diastolic mitral annular tissue velocity (E / e'). In some embodiments, the patient’s E / e' ratio is increased in comparison to healthy people of similar age and sex. In some embodiments, the patient’s E / e' ratio is less than 8. In some embodiments, the patient’s E / e' ratio is between 8 and 15. In some embodiments, the patient’s E / e' ratio is greater than 15. In some embodiments, methods of the present disclosure decrease a patient’s E / e' ratio, wherein the patient’s E / e’ ratio is decreased by at least 5% (e.g., by at least 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100%). In some embodiments, a patient’s E / e' ratio is decreased by at least 1 (e.g., by at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 35, 40, 45, or 50). In some embodiments, the method decreases a patient’s E / e’ ratio to below 8. In some embodiments, the patient has a diastolic dysfunction grade of normal. In some embodiments, the normal grade of diastolic dysfunction of the patient comprises a ratio of early diastolic transmitral flow velocity to late diastolic transmitral flow velocity (E / A) of between 1 and 2, an E / e' of < 8, a normal left atrium volume index (LAVI), and a deceleration time (DT) of <1 60 ms relative to a healthy person of similar age and sex. In some embodiments, the patient has a diastolic dysfunction grade of 1. In some embodiments, Grade 1 diastolic dysfunction of the patient comprises an E / A ratio of < 1 due to impaired relaxation, an E / e' of < 8, a normal or increased LAVI, and an increased deceleration time relative to a healthy person of similar age and sex. In some embodiments, the patient has a diastolic dysfunction grade of 2. In some embodiments, Grade 2 diastolic dysfunction of the patient comprises an E / A between 1 and 2, an E / e' of between 8 and 15, an increased LAVI, and a decreased deceleration time relative to a healthy person of similar age and sex. In some embodiments, the patient has a diastolic dysfunction grade of 3. In some embodiments, Grade 3 diastolic dysfunction of the patient comprises an E / A > 2, an E / e' of greater than 15, an increased LAVI, and a very short E deceleration time ( < 140 ms) due to severely reduced LV compliance and high LV filling pressure relative to a healthy person of similar age and sex. In some embodiments, methods of the present disclosure improve the patient’s diastolic dysfunction grade. In some embodiments, the method improves the patient’s diastolic dysfunction grade from Grade 3 to Grade 2. In some embodiments, the method improves the patient’s diastolic dysfunction grade from Grade 3 to Grade 1. In some embodiments, the method improves the patient’s diastolic dysfunction grade from Grade 3 to normal. In some embodiments, the method improves the patient’s diastolic dysfunction grade from Grade 2 to Grade 1. In some embodiments, the method improves the patient’s diastolic dysfunction grade from Grade 2 to normal. In some embodiments, the method improves the patient’s diastolic dysfunction grade from Grade 1 to normal. In some embodiments, methods of the present disclosure increase the patient’s LV diastolic function (e.g., at least 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100%). In some embodiments of the present disclosure, the patient has an ejection fraction of at least 50% (e.g., 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100%). In some embodiments of the present disclosure, the patient is assessed for electric functional changes using electrocardiography. In some embodiments, the patient’s changes in electrocardiogram measurements are selected from the group consisting of an increase in P-wave duration, P-R interval and Q-T interval, and T-wave voltage and a leftward shift of the QRS axis. In some embodiments, methods of the present disclosure improve a patient’s electrocardiogram measurements selected from the group consisting of a decrease in P-wave duration, a decrease in P-R interval, a decrease in Q-T interval, an increase in T-wave voltage, and a shift of the QRS axis to a normal position. In some embodiments of the present disclosure, the patient is assessed for diastolic dysfunction using stress diastolic testing. In some embodiments, the diastolic stress test is performed on a bicycle fixed to a catheterization table. In some embodiments, the diastolic stress test is performed using echocardiography. In some embodiments, the patient has an abnormal diastolic stress test with parameters selected from the group consisting of a septal e' velocity < 7 cm / s or lateral e' velocity < 10 cm / s at rest, an average E / e' > 14 or septal E / e' ratio >15 with exercise a peak tricuspid regurgitation (TR) velocity > 2.8 m / s with exercise, and an left atrium volume index (LAVI) of > 34 mL / m2. In some embodiments, methods of the present disclosure increase the patient’s septal e' velocity to > 7 cm / s or lateral e' velocity to > 10 cm / s at rest, decreases average E / e' to below 14 or septal E / e' ratio to below 15 with exercise, decreases peak tricuspid regurgitation (TR) velocity to < 2.8 m / s with exercise, and decreases left atrium volume index (LAVI) to < 34 mL / m2. In some embodiments, methods of the present disclosure decrease a patient’s H2FPEF score (e.g., by 1, 2, 3, 4, 5, 6, 7, 8, 9 points). In some embodiments, the patient is assessed for heart failure using right heart catheterization. In some embodiments, the patient has a pulmonary capillary wedge pressure (PCWP) of > 15 mmHg at rest and / or a PCWP of >25 mmHg during exercise. In some embodiments, methods of the present disclosure decrease the patient’s PCWP at rest to at least below 15 mm Hg, and / or decreases PCWP during exercise to at least below 25 mm Hg. In some embodiments of the present disclosure, the patient has a European Heart Failure Association (EHFA) score of > 5 points. In some embodiments, an EHFA score of > 5 points indicates HFpEF. In some embodiments, the patient has an EHFA score of between 2 and 4 points. In some embodiments, an EHFA score of between 2 and 4 points indicates that the patient has HFpEF. In some embodiments, the patient has an EHFA score of 1 point or less. In some embodiments, an EHFA score of 1 or less indicates that the patient does not have HFpEF. In some embodiments of the present disclosure, the patient has one or more major functional EHFA criteria for HFpEF. In some embodiments, the major functional criterion is selected from the group consisting of a septal e’ velocity < 7 cm / s, a lateral e’ velocity < 10 cm / s at rest, an average E / e' > 14 or septal E / e' ratio > 15 with exercise and a TR velocity > 2.8 m / s with exercise. In some embodiments, methods of the present disclosure improve one or more major functional criterion selected from the group consisting of increasing septal e’ velocity to > 7 cm / s, increasing lateral e’ velocity to > 10 cm / s at rest, decreasing E / e' to < 14 or septal E / e' ratio to < 15 with exercise and decreasing TR velocity to < 2.8 m / s with exercise. In some embodiments of the present disclosure, the patient has one or more major morphological EHFA criteria for HFpEF. In some embodiments, the major morphological criterion is selected from the group consisting of a LAVI > 34 mL / m2 and an LVMI >149 g / m2 for men and >122 g / m2 for women and RWT > 0.42. In some embodiments, methods of the present disclosure improve one or more major morphological criterion selected from the group consisting of decreasing LAVI to < 34 mL / m2 and decreasing LVMI to < 149 g / m2 for men and <122 g / m2 for women, and decreasing RWT to < 0.42. In some embodiments of the present disclosure, the patient has one or more major biomarker EHFA criteria for HFpEF. In some embodiments, the major biomarker criterion is sinus rhythm, with NT-proBNP > 220 pg / mL and / or BNP > 80 pg / mL. In some embodiments, the major biomarker criterion is atrial fibrillation, with NT-proBNP > 660 pg / mL and / or BNP > 240 pg / mL. In some embodiments, the method improves sinus rhythm, comprising decreasing NT-proBNP to < 220 pg / mL and / or decreasing BNP to < 80 pg / mL. In some embodiments, methods of the present disclosure improve atrial fibrillation, comprising decreasing NT-proBNP to < 660 pg / mL and / or decreasing BNP to < 240 pg / mL. In some embodiments of the present disclosure, the patient has one or more minor EHFA criteria for HFpEF. In some embodiments, the patient has one or more minor functional EHFA criteria for HFpEF. In some embodiments, the minor functional criterion is selected from the group consisting of an average E / e' 9-14 and a GLS < 16%. In some embodiments, methods of the present disclosure improve minor functional criteria, comprising decreasing E / e' to 8 or below and increasing GLS to > 16%. In some embodiments of the present disclosure, the patient has one or more minor morphological EHFA criteria for HFpEF. In some embodiments, the minor morphological criterion is selected from the group consisting of a LAVI 29-34 mL / m2, an LVMI >115 g / m2 for men, an LVMI of 95 g / m2 for women, a RWT > 0.42, and an LV wall thickness >12 mm. In some embodiments, methods of the present disclosure improve one or more minor morphological criterion selected from the group consisting of decreasing LAVI to <34 mL / m2, decreasing LVMI to < 115 g / m2 for men, decreasing LVMI to below 95 g / m2 for women, decreasing RWT to < 0.42, and decreasing LV wall thickness to < 12 mm. In some embodiments of the present disclosure, the patient has one or more minor biomarker EHFA criteria for HFpEF. In some embodiments, the minor biomarker criterion is sinus rhythm, with 5-NT-proBNP 125-220 pg / mL and / or BNP 35-80 pg / mL. In some embodiments, the minor biomarker criterion is atrial fibrillation, with NT-proBNP 365-660 pg / mL and / or BNP 105-240 pg / mL. In some embodiments, methods of the present disclosure improve sinus rhythm, comprising decreasing 5-NT-proBNP to < 220 pg / mL and / or decreasing BNP to < 80 pg / mL. In some embodiments, methods of the present disclosure improve atrial fibrillation, comprising decreasing NT-proBNP to < 660 pg / mL and / or decreasing BNP to < 240 pg / mL. In some embodiments, methods of the present disclosure decrease the patient’s EHFA score (e.g., by 1, 2, 3, 4, 5, 6, 7, or 8 points). In some embodiments of the present disclosure, a patient has New York Heart Association (NYHA) Class I HF. In some embodiments, a patient has NYHA Class II HF., or. In some embodiments, a patient has NYHA Class III HF. In some embodiments, a patient has NYHA Class IV HF. In some embodiments, methods of the present disclosure reduce a patient’s NYHA Class. In some embodiments, the method reduces a patient’s NYHA Class from Class IV to Class III. In some embodiments, the method reduces a patient’s NYHA Class from Class IV to Class II. In some embodiments, the method reduces a patient’s NYHA Class from Class IV to Class I. In some embodiments, the method reduces a patient’s NYHA Class from Class III to Class II. In some embodiments, the method reduces a patient’s NYHA Class from Class III to Class I. In some embodiments, the method reduces a patient’s NYHA Class from Class II to Class I. In some embodiments of the present disclosure, a patient has American College of Cardiology Foundation / American Heart Association (ACCF / AHA) stage A heart failure. In some embodiments, a patient has ACCF / AHA Stage B heart failure. In some embodiments, a patient has ACCF / AHA Stage C heart failure. In some embodiments, a patient has ACCF / AHA Stage D heart failure. In some embodiments, methods of the present disclosure reduce a patient’s ACCF / AHA stage. In some embodiments, the method reduces a patient’s ACCF / AHA stage from Stage D to Stage C. In some embodiments, the method reduces a patient’s ACCF / AHA stage from Stage D to Stage B. In some embodiments, the method reduces a patient’s ACCF / AHA stage from Stage D to Stage A. In some embodiments, the method reduces a patient’s ACCF / AHA stage from Stage C to Stage B. In some embodiments, the method reduces a patient’s ACCF / AHA stage from Stage C to Stage A. In some embodiments, the method reduces a patient’s ACCF / AHA stage or from Stage B to Stage A. In some embodiments of the present disclosure, a patient has Killip Classification of HF complicating AMI Class I heart failure. In some embodiments, a patient has Killip Classification of HF complicating AMI Class II heart failure. In some embodiments, a patient has Killip Classification of HF complicating AMI Class III heart failure. In some embodiments, a patient has or Killip Classification of HF complicating AMI Class IV heart failure. In some embodiments, methods of the present disclosure reduce a patient’s Killip Classification of HF complicating AMI class. In some embodiments, the method reduces a patient’s Killip Class from Class IV to Class III. In some embodiments, the method reduces a patient’s Killip Class from Class IV to Class II. In some embodiments, the method reduces a patient’s Killip Class from Class IV to Class I. In some embodiments, the method reduces a patient’s Killip Class from Class III to Class II. In some embodiments, the method reduces a patient’s Killip Class from Class III to Class I. In some embodiments, the method reduces a patient’s Killip Class or from Class II to Class I. In some embodiments of the present disclosure, a patient has one or more major Framingham criteria for diagnosis of HF. In some embodiments, a patient has one or more conditions selected from the group consisting of paroxysmal nocturnal dyspnea or orthopnea, jugular vein distension, rales, radiographic cardiomegaly, acute pulmonary edema, S3 gallop, increased venous pressure greater than 16 cm of water, circulation time greater than or equal to 25 seconds, hepatojugular reflex, and weight loss greater than or equal to 4.5 kg in 5 days in response to treatment. In some embodiments of the present disclosure, a patient has one or more minor Framingham criteria for diagnosis of HF. In some embodiments, a patient has one or more conditions selected from the group consisting of bilateral ankle edema, nocturnal cough, dyspnea on ordinary exertion, hepatomegaly, pleural effusion, decrease in vital capacity by 1 / 3 from maximum recorded, and tachycardia (heart rate greater than 120 / min). In some embodiments of the present disclosure, a patient has at least two Framingham major criteria. In some embodiments, a patient has at least one major Framingham criteria and at least two minor Framingham criteria. In some embodiments, methods of the present disclosure reduce the number of Framingham criteria for heart failure that a patient has. In some embodiments, the method decreases the number of major Framingham criteria for heart failure that a patient has. In some embodiments, the method decreases the number of minor Framingham criteria for heart failure that a patient has. In some embodiments of the present disclosure, a patient has one or more conditions selected from the group consisting of typical symptoms, less typical symptoms, specific signs, and less specific signs of HF. In some embodiments, a patient has one or more symptoms selected from the group consisting of breathlessness, orthopnea, paroxysmal nocturnal dyspnea, reduced exercise tolerance, fatigue, tiredness, increased time to recover after exercise, and ankle swelling. In some embodiments, a patient has one or more less typical symptoms selected from the group consisting of nocturnal cough, wheezing, bloated feeling, loss of appetite, confusion (especially in the elderly), depression, palpitations, dizziness, syncope, and bendopnea. In some embodiments of the present disclosure, a patient has one or more signs of HF. In some embodiments, a patient has one or more signs of HF selected from the group consisting of elevated jugular venous pressure, hepatojugular reflux, third heart sound (gallop rhythm), and laterally displaced apical impulse. In some embodiments, a patient has one or more less specific signs of HF. In some embodiments, a patient has one or more less specific signs of HF. In some embodiments, a patient has one or more less specific signs of HF selected from the group consisting of weight gain (>2 kg / week), weight loss (in advanced HF), tissue wasting (cachexia), cardiac murmur, peripheral edema (ankle, sacral, scrotal), pulmonary crepitations, reduced air entry and dullness to percussion at lung bases (pleural effusion), tachycardia, irregular pulse, tachypnoea, Cheyne Stokes respiration, hepatomegaly, ascites, cold extremities, oliguria, and narrow pulse pressure. In some embodiments, methods of the present disclosure reduce the number of signs and / or symptoms of heart failure that a patient has. In some embodiments, the method decreases the number of signs of heart failure that a patient has. In some embodiments, the method decreases the number of symptoms of heart failure that a patient has. In some embodiments of the present disclosure, a patient has elevated brain natriuretic peptide (BNP) levels as compared to a healthy patient. In some embodiments, a patient has a BNP level of at least 35 pg / mL (e.g., 35, 40, 50, 60, 70, 80, 90, 100, 150, 200, 300, 400, 500, 1000, 3000, 5000, 10,000, 15,000, or 20,000 pg / mL). In some embodiments, methods of the present disclosure decrease BNP levels in a patient by at least 5% (e.g., 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or at least 80%). In some embodiments, methods of the present disclosure decrease BNP levels in a patient by at least 5 pg / mL (e.g., 5, 10, 50, 100, 200, 500, 1000, or 5000 pg / mL). In some embodiments, methods of the present disclosure decrease BNP levels to normal levels (i.e., <100 pg / ml). In some embodiments of the present disclosure, a patient has elevated N-terminal proBNP (NT-proBNP) levels as compared to a healthy patient. In some embodiments, a patient has an NT-proBNP level of at least 10 pg / mL (e.g., 10, 25, 50, 100, 150, 200, 300, 400, 500, 1000, 3000, 5000, 10,000, 15,000, or 20,000 pg / mL). In some embodiments, methods of the present disclosure decrease NT-proBNP levels in a patient by at least 5% (e.g., 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or at least 80%). In some embodiments, methods of the present disclosure decrease NT-proBNP levels in a patient by at least 10 pg / mL (e.g., 10, 25, 50, 100, 200, 500, 1000, 5000, 10,000, 15,000, 20,000, or 25,000 pg / mL). In some embodiments, methods of the present disclosure decrease NT-proBNP levels to normal levels (i.e., <100 pg / ml). In some embodiments of the present disclosure, a patient has elevated troponin levels as compared to a healthy patient. In some embodiments, methods of the present disclosure decrease troponin levels in a patient by at least 1% (e.g., 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or at least 80%). In some embodiments, methods of the present disclosure reduce a patient’s hospitalization rate by at least 1% (e.g., 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%.). In some embodiments, methods of the present disclosure reduce a patient’s rate of worsening of heart failure by at least 1% (e.g., 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%.). In some embodiments, methods of the present disclosure reduce the need to for the patient to stay at the hospital. In some embodiments, methods of the present disclosure reduce the number of total patient hospital visits. In some embodiments, methods of the present disclosure increase the time to initial hospitalization of the patient. In some embodiments, methods of the present disclosure increase the length of life of the patient. In some embodiments, methods of the present disclosure increase the time between hospital visits. In some embodiments, methods of the present disclosure decrease the number of recurrent hospital visits. In some embodiments, methods of the present disclosure increase a patient’s cardiac output by at least 5% (e.g, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or at least 80%). In some embodiments, methods of the present disclosure increase exercise capacity of a patient. In some embodiments, a patient has a 6-minute walk distance from 150 to 400 meters. In some embodiments, methods of the present disclosure increase a patient’s 6-minute walk distance. In some embodiments, methods of the present disclosure increase a patient’s 6-minute walk distance by at least 10 meters (e.g., at least 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200, 250, 300, or more than 400 meters). In some embodiments, methods of the present disclosure reduce a patient’s Borg dyspnea index (BDI). In some embodiments, methods of the present disclosure reduce a patient’s BDI by at least 0.5 index points (e.g., at least 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or 10 index points). In some embodiments of the disclosure, a patient is assessed for heart failure using echocardiography. In some embodiments, a patient is assessed for heart failure using cardiac magnetic resonance imaging (CMR). In some embodiments, a patient is assessed for heart failure using CMR with late gadolinium enhancement (LGE). In some embodiments, a patient is assessed for one or more of conditions selected from the group consisting of LV structure and systolic function (e.g., measured by M-mode in a parasternal short axis view at the papillary muscle level), including, but not limited to LV wall thickness (LVWT), LV mass (LVM), LV end diastolic diameter (LVEDD), LV end systolic diameter (LVESD), fractional shortening (FS) (calculated using the equation FS = 100% x [(EDD -ESD) / EDD]), LV end diastolic volume (LVEDV), LV end systolic volume (LVESV), ejection fraction (calculated using the equation EF = 100% x [(EDV - ESV) / EDV]), Hypertrophy index (calculated as the ratio of LVM to LVESV), and relative wall thickness (calculated as the ratio of LVWT to LVESD). In some embodiments, heart failure in a patient is assessed using cardiac imaging selected from the group consisting of multigated acquisition (MUGA), Chest X-Ray, single-photon emission computed tomography (SPECT) and radionucleotide ventriculography, positron emission tomography (PET), coronary angiography, and cardiac computing tomography (CT). In some embodiments, methods of the present disclosure further comprise administering to a patient an additional supportive therapy or active agent. In some embodiments, the additional supportive therapy or active agent is selected from the group consisting of: angiotensin-converting enzyme (ACE) inhibitors, beta blockers, angiotensin II receptor blockers (ARB), mineralocorticoid / aldosterone receptor antagonists (MRAs), glucocorticoids, statins, Sodium-glucose co-transporter 2 (SGLT2) inhibitors, an implantable cardioverter defibrillator (ICD), angiotensin receptor neprilysin inhibitors (ARNI), and diuretics. In some embodiments, the additional active agent and / or supportive therapy is selected from the group consisting of: benazepril, captopril, enalapril, lisinopril, perindopril, ramipril, trandolapril, zofenopril, acebutolol, atenolol, betaxolol, bisoprolol, carteolol, carvedilol, labetalol, metoprolol, nadolol, nebivolol, penbutolol, pindolol, propranolol, sotalol, timolol; losartan, irbesartan, olmesartan, candesartan, valsartan, fimasartan, azilsartan, salprisartan, telmisartan, progesterone, eplerenone and spironolactone, beclomethasone, betamethasone, budesonide, cortisone, deflazacort, dexamethasone, hydrocortisone, methylprednisolone, prednisolone, methylprednisone, prednisone, triamcinolone, finerenone, atorvastatin, fluvastatin, lovastatin, pravastatin, pitavastatin, simvastatin, rosuvastatin, canagliflozin, dapagliflozin, empagliflozin, ertugliflozin, valsartan and sacubitril (a neprilysin inhibitor), furosemide, bumetanide, torasemide, bendroflumethiazide, hydrochlorothiazide, metolazone, indapamidec, spironolactone / eplerenone, amiloride triamterene, hydralazine and isosorbide dinitrate, digoxin, digitalis, N-3 polyunsaturated fatty acids (PUFA), and If-channel inhibitor. In some embodiments of the present disclosure, a patient has a comorbidity selected from the group consisting of anemia, angina, arterial hypertension, arthritis, atrial fibrillation, cachexia, cancer, cognitive dysfunction, coronary artery disease (CAD), diabetes, erectile dysfunction, gout, hypercholesterolemia, hyperkalemia, hyperkalemia, hyperlipidemia, hypertension, iron deficiency, kidney dysfunction, metabolic syndrome, obesity, physical deconditioning, potassium disorders, pulmonary disease (e.g., asthma, COPD), sarcopenia, sleep apnea, sleep disturbance, and valvular heart disease (e.g., aortic stenosis, aortic regurgitation, mitral regurgitation, tricuspid regurgitation). In some embodiments, one or more comorbidities to consider in HF are selected from the group consisting of anemia, atrial fibrillation, coronary artery disease (CAD), and sleep apnea. .In some embodiments of the present disclosure, an ActRII-ALK4 antagonist comprises an ActRIIA polypeptide. In some embodiments, an ActRII-ALK4 antagonist is a heteromultimer. In some embodiments of the present disclosure, an ActRIIA polypeptide comprises an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence that begins at any one of amino acids 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30 of SEQ ID NO: 366 and ends at any one of amino acids 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, or 135 of SEQ ID NO: 366. In some embodiments of the present disclosure, an ActRIIA polypeptide comprises an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence of SEQ ID NO: 367. In some embodiments of the present disclosure, an ActRIIA polypeptide comprises an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence of SEQ ID NO: 368. In some embodiments of the present disclosure, an ActRIIA polypeptide is a fusion polypeptide comprising an ActRIIA polypeptide domain and one or more heterologous domains. In some embodiments, the fusion polypeptide is an ActRIIA-Fc fusion polypeptide. In some embodiments, the fusion polypeptide further comprises a linker domain positioned between the ActRIIA polypeptide domain and i) the one or more heterologous domains or ii) Fc domain. In some embodiments, a linker domain is selected from: TGGG (SEQ ID NO: 265), TGGGG (SEQ ID NO: 263), SGGGG (SEQ ID NO: 264), GGGGS (SEQ ID NO: 267), GGG (SEQ ID NO: 261), GGGG (SEQ ID NO: 262), and SGGG (SEQ ID NO: 266). In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 380. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 378. In some embodiments of the present disclosure, an ActRII-ALK4 antagonist is a homodimer polypeptide. In some embodiments, an ActRII-ALK4 antagonist is a heteromultimer polypeptide. In some embodiments, a heteromultimer polypeptide comprises an ActRIIA polypeptide and an ALK4 polypeptide. In some embodiments, the heteromultimer polypeptide comprises an ActRIIA polypeptide and an ALK7 polypeptide. In some embodiments of the present disclosure, an ALK4 polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence selected from the group consisting of SEQ ID NOs:84, 85, 86, 87, 88, 89, 92, 93, 247, 249, 421, and 422. In some embodiments of the present disclosure, an ALK7 polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence selected from the group consisting of SEQ IDNOs: 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 133, and 134. In some embodiments of the present disclosure, an ALK4 polypeptide is a fusion polypeptide comprising an ALK4 polypeptide domain and one or more heterologous domains. In some embodiments, an ALK7 polypeptide is a fusion polypeptide comprising an ALK7 polypeptide domain and one or more heterologous domains. In some embodiments, the fusion polypeptide is an ALK4-Fc fusion polypeptide. In some embodiments, the fusion polypeptide is an ALK7-Fc fusion polypeptide. In some embodiments, the ALK4-Fc fusion polypeptide further comprises a linker domain positioned between the ALK4 polypeptide domain and i) the one or more heterologous domains or ii) Fc domain. In some embodiments, the ALK7-Fc fusion polypeptide further comprises a linker domain positioned between the ALK7 polypeptide domain and i) the one or more heterologous domains or ii) Fc domain. In some embodiments, the linker domain is selected from: TGGG (SEQ ID NO: 265), TGGGG (SEQ ID NO: 263), SGGGG (SEQ ID NO: 264), GGGGS (SEQ ID NO: 267), GGG (SEQ ID NO: 261), GGGG (SEQ ID NO: 262), and SGGG (SEQ ID NO: 266). In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 13, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 13; b) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 14, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 14; c.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 15, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 15; d.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 16, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 16; and e.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 17, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 17. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 13, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 13; b.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 14, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 14; c.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 15, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 15; d.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 16, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 16; and e.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 17, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 17. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 18, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 19; and b.) the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 19, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 18. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% 19 identical to the amino acid sequence of SEQ ID NO: 18, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 19; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 19, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 18. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 20, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 21; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 21, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 20. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 20, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 21; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 21, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 20. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 22, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 22. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 22, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 22. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 24, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 25; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 25, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 24. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 24, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 25; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 25, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 24. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 26, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 27; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 27, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 26. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 26, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 27; and b.) The ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 27, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 26. In some embodiments of the present disclosure, an ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 28, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 29. In some embodiments of the present disclosure, an ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 28, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 29. In some embodiments of the present disclosure, an ActRIIA-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, glutamic acid at amino acid position 138, a tryptophan at amino acid position 144, and a aspartic acid at amino acid position 217, and wherein the ALK4-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at position 146 an arginine at amino acid position 162, an arginine at amino acid position 179, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ActRIIA-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, glutamic acid at amino acid position 138, a tryptophan at amino acid position 144, and a aspartic acid at amino acid position 217, and wherein the ALK7-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at position 146 an arginine at amino acid position 162, an arginine at amino acid position 179, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 28, and the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 29. In some embodiments of the present disclosure, an ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 28, and the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 29. In some embodiments of the present disclosure, an ALK4-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, glutamic acid at amino acid position 138, a tryptophan at amino acid position 144, and a aspartic acid at amino acid position 217, and wherein the ActRIIA-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at position 146 an arginine at amino acid position 162, an arginine at amino acid position 179, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ALK7-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, glutamic acid at amino acid position 138, a tryptophan at amino acid position 144, and a aspartic acid at amino acid position 217, and wherein the ActRIIA-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at position 146 an arginine at amino acid position 162, an arginine at amino acid position 179, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 30, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23. In some embodiments of the present disclosure, an ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 30, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23. In some embodiments of the present disclosure, an ActRIIA-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, a tryptophan at amino acid position 144, and a arginine at amino acid position 435, and wherein the ALK4-Fc fusion polypeptide Fc domain comprises cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at amino acid position 146, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ActRIIA-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, a tryptophan at amino acid position 144, and a arginine at amino acid position 435, and wherein the ALK7-Fc fusion polypeptide Fc domain comprises cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at amino acid position 146, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 30, and the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23. In some embodiments of the present disclosure, an ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 30, and the ActRIIA-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23. In some embodiments of the present disclosure, an ALK4-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, a tryptophan at amino acid position 144, and a arginine at amino acid position 435, and wherein the ActRIIA-Fc fusion polypeptide Fc domain comprises cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at amino acid position 146, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ALK7-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, a tryptophan at amino acid position 144, and a arginine at amino acid position 435, and wherein the ActRIIA-Fc fusion polypeptide Fc domain comprises cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at amino acid position 146, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ActRII-ALK4 antagonist comprises an ActRIIB polypeptide. In some embodiments of the present disclosure, an ActRII-ALK4 antagonist is a heteromultimer. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence that begins at any one of amino acids 20-29 (e.g., amino acid residues 20, 21, 22, 23, 24, 25, 26, 27, 28, or 29) of SEQ ID NO: 2 and ends at any one of amino acids 109-134 (e.g., amino acid residues 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, or 134) of SEQ ID NO: 2. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 29-109 of SEQ ID NO: 2. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 25-131 of SEQ ID NO: 2. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 20-134 of SEQ ID NO: 2. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 53. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 388. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 389. In some embodiments of the present disclosure, an ActRIIB polypeptide is a fusion polypeptide comprising an ActRIIB polypeptide domain and one or more heterologous domains. In some embodiments, the fusion polypeptide is an ActRIIB-Fc fusion polypeptide. In some embodiments, the fusion polypeptide further comprises a linker domain positioned between the ActRIIB polypeptide domain and the one or more heterologous domains or Fc domain. In some embodiments, the linker domain is selected from: TGGG (SEQ ID NO: 265), TGGGG (SEQ ID NO: 263), SGGGG (SEQ ID NO: 264), GGGGS (SEQ ID NO: 267), GGG (SEQ ID NO: 261), GGGG (SEQ ID NO: 262), and SGGG (SEQ ID NO: 266). In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 5. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 12. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises one or more amino acid substitution with respect to the amino acid sequence of SEQ ID NO: 2 selected from the group consisting of: A24N, S26T, N35E, E37A, E37D, L38N, R40A, R40K, S44T, L46V, L46I, L46F, L46A, E50K, E50P, E50L, E52A, E52D, E52G, E52H, E52K, E52N, E52P, E52R, E52S, E52T, E52Y, Q53R, Q53K, Q53N, Q53H, D54A, K55A, K55D, K55E, K55R, R56A, L57E, L57I, L57R, L57T, L57V, Y60D, Y60F, Y60K, Y60P, R64A, R64H, R64K, R64N, N65A, S67N, S67T, G68R, K74A, K74E, K74F, K74I, K74R, K74Y, W78A, W78Y, L79A, L79D, L79E, L79F, L79H, L79K, L79P, L79R, L79S, L79T, L79W, D80A, D80F, D80G, D80I, D80K, D80M, D80N, D80R, F82A, F82D, F82E, F82I, F82K, F82L, F82S, F82T, F82W, F82Y, N83A, N83R, T93D, T93E, T93G, T93H, T93K, T93P, T93R, T93S, T93Y, E94K, Q98D, Q98E, Q98K, Q98R, V99E, V99G, V99K, E105N, F1081, F108L, F108V, F108Y, El 1 ID, El 11H, El 1 IK, 11 IN, El 1 IQ, El 11R, RI 12H, RI 12K, RI 12N, RI 12S, RI 12T, Al 19P, Al 19V, G120N, E123N, P129N, P129S, P130A, P130R, and A132N. In some embodiments, an ActRIIB polypeptide comprises one or more amino acid substitution with respect to the amino acid sequence of SEQ ID NO: 2 selected from the group consisting of: L38N, E50L, E52D, E52N, E52Y, L57E, L57I, L57R, L57T, L57V, Y60D, G68R, K74E, W78Y, L79E, L79F, L79H, L79R, L79S, L79T, L79W, F82D, F82E, F82I, F82K, F82L, F82S, F82T, F82Y, N83R, E94K, and V99G. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an L substitution at the position corresponding to E50 of SEQ ID NO: 2. In some embodiments, an ActRIIB polypeptide comprises an N substitution at the position corresponding to L38 of SEQ ID NO: 2. In some embodiments, an ActRIIB polypeptide comprises a G substitution at the position corresponding to V99 of SEQ ID NO: 2. In some embodiments, an ActRIIB polypeptide comprises a R substitution at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, an ActRIIB polypeptide comprises an T substitution at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, an ActRIIB polypeptide comprises an H substitution at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 276. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 278. In some embodiments, the polypeptide comprises an I substitution at the position corresponding to F82 of SEQ ID NO: 2 and an R substitution at the position corresponding to N83. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 279. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 332. In some embodiments, the polypeptide comprises a K substitution at the position corresponding to F82 of SEQ ID NO: 2 and an R substitution at the position corresponding to N83. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 333. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 335. In some embodiments, the polypeptide comprises a T substitution at the position corresponding to F82 of SEQ ID NO: 2 and an R substitution at the position corresponding to N83. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 336. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 338. In some embodiments, the polypeptide comprises a T substitution at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 339. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 341. In some embodiments, the polypeptide comprises an H substitution at the position corresponding to L79 of SEQ ID NO: 2 and an I substitution at the position corresponding to F82. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 342. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 344. In some embodiments, the polypeptide comprises an H substitution at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 345. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 347. In some embodiments, the polypeptide comprises an H substitution at the position corresponding to L79 of SEQ ID NO: 2 and a K substitution at the position corresponding to F82. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 348. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 350. In some embodiments, the polypeptide comprises an L substitution at the position corresponding to E50 of SEQ ID NO: 2. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 351. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 353. In some embodiments, the polypeptide comprises an N substitution at the position corresponding to L38 of SEQ ID NO: 2 and an R substitution at the position corresponding to L79. In some embodiments of the present disclosure, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 354. In some embodiments, the polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 356. In some embodiments, the polypeptide comprises an G substitution at the position corresponding to V99 of SEQ ID NO: 2. In some embodiments of the present disclosure, an ActRIIB polypeptide is a homodimer polypeptide. In some embodiments, an ActRIIB polypeptide is a heterodimer polypeptide. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence that begins at any one of amino acids 20-29 (e.g., amino acid residues 20, 21, 22, 23, 24, 25, 26, 27, 28, or 29) of SEQ ID NO: 2 and ends at any one of amino acids 109-134 (e.g., amino acid residues 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, or 134) ofSEQ ID NO: 2 and one or more amino acid substitutions at a position of SEQ ID NO: 2 selected from the group consisting of: L38N, E50L, E52N, L57E, L57I, L57R, L57T, L57V, Y60D, G68R, K74E, W78Y, L79F, L79S, L79T, L79W, F82D, F82E, F82L, F82S, F82T, F82Y, N83R, E94K, and V99G. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence that begins at any one of amino acids 20-29 (e.g., amino acid residues 20, 21, 22, 23, 24, 25, 26, 27, 28, or 29) of SEQ ID NO: 2 and ends at any one of amino acids 109-134 (e.g., amino acid residues 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, or 134) ofSEQ ID NO: 2 and one or more amino acid substitutions at a position of SEQ ID NO: 2 selected from the group consisting of: L38N, E50L, E52D, E52N, E52Y, L57E, L57I, L57R, L57T, L57V, Y60D, G68R, K74E, W78Y, L79E, L79F, L79H, L79R, L79S, L79T, L79W, F82D, F82E, F82I, F82K, F82L, F82S, F82T, F82Y, N83R, E94K, and V99G. In some embodiments of the present disclosure, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 29-109 of SEQ ID NO: 2. In some embodiments, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 25-131 of SEQ ID NO: 2. In some embodiments, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 20-134 of SEQ ID NO: 2. In some embodiments, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 53. In some embodiments, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 388. In some embodiments, an ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 389. In some embodiments, an ActRIIB polypeptide comprises one or more amino acid substitution with respect to the amino acid sequence of SEQ ID NO: 2 selected from the group consisting of: L38N, E50L, E52D, E52N, E52Y, L57E, L57I, L57R, L57T, L57V, Y60D, G68R, K74E, W78Y, L79E, L79F, L79H, L79R, L79S, L79T, L79W, F82D, F82E, F82I, F82K, F82L, F82S, F82T, F82Y, N83R, E94K, and V99G. In some embodiments of the present disclosure, a heteromultimer polypeptide comprises an ActRIIA polypeptide and an ALK4 polypeptide. In some embodiments, a heteromultimer polypeptide comprises an ActRIIA polypeptide and an ALK7 polypeptide. In some embodiments, an ALK4 polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence selected from the group consisting of SEQ ID NOs: 84, 85, 86, 87, 88, 89, 92, 93, 247, 249, 421, and 422. In some embodiments, an ALK7 polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence selected from the group consisting of SEQ ID NOs: 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 133, and 134. In some embodiments of the present disclosure, an ActRIIB polypeptide is a fusion polypeptide comprising an ActRIIB polypeptide domain and one or more heterologous domains. In some embodiments, an ALK4 polypeptide is a fusion polypeptide comprising an ALK4 polypeptide domain and one or more heterologous domains. In some embodiments, an ALK7 polypeptide is a fusion polypeptide comprising an ALK7 polypeptide domain and one or more heterologous domains. In some embodiments, an ActRIIB polypeptide is an ActRIIB-Fc fusion polypeptide. In some embodiments, an ALK4 polypeptide is an ALK4-Fc fusion polypeptide. In some embodiments, an ALK7 polypeptide is an ALK7-Fc fusion polypeptide. In some embodiments, an ActRIIB-Fc fusion polypeptide further comprises a linker domain positioned between the ActRIIB polypeptide domain and the one or more heterologous domains or Fc domain. In some embodiments, the ALK4-Fc fusion polypeptide further comprises a linker domain positioned between the ALK4 polypeptide domain and the one or more heterologous domains or Fc domain. In some embodiments, the ALK7-Fc fusion polypeptide further comprises a linker domain positioned between the ALK7 polypeptide domain and the one or more heterologous domains or Fc domain. In some embodiments, the linker domain is selected from: TGGG (SEQ ID NO: 265), TGGGG (SEQ ID NO: 263), SGGGG (SEQ ID NO: 264), GGGGS (SEQ ID NO: 267), GGG (SEQ ID NO: 261), GGGG (SEQ ID NO: 262), and SGGG (SEQ ID NO: 266). In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 13, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 13; b.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 14, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 14; c.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 15, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 15; d.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 16, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 16; and e.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 17, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 17. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 13, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 33 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 13; b.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 14, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 14; c.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 15, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 15; d.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 16, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 16; and e.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 17, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 17. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 18, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 19; and b) the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 19, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 18. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 18, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 19; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 19, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 18. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 20, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 21; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 21, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 20. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 20, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 21; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 21, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 20. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 22, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 22. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 22, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 22. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 24, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 25; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 25, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 24. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 24, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 25; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 25, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 24. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 26, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 27; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 27, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 26. In some embodiments of the present disclosure, a heteromultimer comprises an Fc domain selected from: a.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 26, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 27; and b.) The ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 27, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 26. In some embodiments of the present disclosure, an ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 28, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 29. In some embodiments of the present disclosure, an ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 28, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 29. In some embodiments of the present disclosure, an ActRIIB-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, glutamic acid at amino acid position 138, a tryptophan at amino acid position 144, and a aspartic acid at amino acid position 217, and wherein the ALK4-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at position 146 an arginine at amino acid position 162, an arginine at amino acid position 179, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ActRIIB-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, glutamic acid at amino acid position 138, a tryptophan at amino acid position 144, and a aspartic acid at amino acid position 217, and wherein the ALK7-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at position 146 an arginine at amino acid position 162, an arginine at amino acid position 179, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 28, and the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 29. In some embodiments of the present disclosure, an ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 28, and the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 29. In some embodiments of the present disclosure, an ALK4-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, glutamic acid at amino acid position 138, a tryptophan at amino acid position 144, and a aspartic acid at amino acid position 217, and wherein the ActRIIB-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at position 146 an arginine at amino acid position 162, an arginine at amino acid position 179, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ALK7-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, glutamic acid at amino acid position 138, a tryptophan at amino acid position 144, and a aspartic acid at amino acid position 217, and wherein the ActRIIB-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at position 146 an arginine at amino acid position 162, an arginine at amino acid position 179, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 30, and the ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23. In some embodiments of the present disclosure, an ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 30, and the ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23. In some embodiments of the present disclosure, an ActRIIB-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, a tryptophan at amino acid position 144, and a arginine at amino acid position 435, and wherein the ALK4-Fc fusion polypeptide Fc domain comprises cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at amino acid position 146, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ActRIIB-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, a tryptophan at amino acid position 144, and a arginine at amino acid position 435, and wherein the ALK7-Fc fusion polypeptide Fc domain comprises cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at amino acid position 146, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ALK4-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 30, and the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23. In some embodiments of the present disclosure, an ALK7-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 30, and the ActRIIB-Fc fusion polypeptide comprises an Fc domain that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 23. In some embodiments of the present disclosure, an ALK4-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, a tryptophan at amino acid position 144, and a arginine at amino acid position 435, and wherein the ActRIIB-Fc fusion polypeptide Fc domain comprises cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at amino acid position 146, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ALK7-Fc fusion polypeptide Fc domain comprises a cysteine at amino acid position 132, a tryptophan at amino acid position 144, and a arginine at amino acid position 435, and wherein the ActRIIB-Fc fusion polypeptide Fc domain comprises cysteine at amino acid position 127, a serine at amino acid position 144, an alanine at amino acid position 146, and a valine at amino acid position 185. In some embodiments of the present disclosure, an ActRII-ALK4 antagonist is a follistatin polypeptide. In some embodiments, the follistatin polypeptide an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence selected from the group consisting of SEQ ID NOs: 390, 391, 392, 393, and 394. In some embodiments of the present disclosure, an ActRII-ALK4 antagonist inhibits one or more ligands selected from the group consisting of activin A, activin B, GDF8, GDF11, BMP6, BMP 10, ALK4, ActRIIA, and ActRIIB. In some embodiments of the present disclosure, an ActRII-ALK4 antagonist is an antibody or combination of antibodies. In some embodiments, the antibody or combination of antibodies binds to one or more ligands selected from the group consisting of activin A, activin B, GDF8, GDF11, BMP6, BMP 10, ALK4, ActRIIA, and ActRIIB. In some embodiments, the antibody is a multispecific antibody. In some embodiments, the antibody is a bi-specific antibody. In some embodiments, the antibody is selected from the group consisting of garetosmab, trevogumab, stamulumab, domagrozumab, landogrozumab, and bimagrumab. In some embodiments of the present disclosure, an ActRII-ALK4 antagonist is a small molecule or combination of small molecules. In some embodiments, the small molecule or combination of small molecules inhibits one or more ligands selected from the group consisting of activin A, activin B, GDF8, GDF11, BMP6, BMP10, ALK4, ActRIIA, and ActRIIB. In some embodiments of the present disclosure, an ActRII-ALK4 antagonist is a polynucleotide or combination of polynucleotides. In some embodiments, the polynucleotide or combination of polynucleotides inhibits one or more ligands selected from the group consisting of activin A, activin B, GDF8, GDF11, BMP6, BMP10, ALK4, ActRIIA, and ActRIIB. BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows an alignment of extracellular domains of human ActRIIB (SEQ ID NO: 1) and human ActRIIA (SEQ ID NO: 367) with the residues that are deduced herein, based on composite analysis of multiple ActRIIB and ActRIIA crystal structures, to directly contact ligand indicated with boxes. Figure 2 shows the amino acid sequence of human ActRIIB precursor polypeptide (SEQ ID NO: 2); NCBI Reference Sequence NP_001097.2). The signal peptide is underlined, the extracellular domain is in bold (also referred to as SEQ ID NO: 1), and the potential N-linked glycosylation sites are boxed. SEQ ID NO: 2 is used as the wild-type reference sequence for human ActRIIB in this disclosure, and the numbering for the variants described herein are based on the numbering in SEQ ID NO: 2 Figure 3 shows the amino acid sequence of a human ActRIIB extracellular domain polypeptide (SEQ ID NO: 1). Figure 4 shows a nucleic acid sequence encoding human ActRIIB precursor polypeptide. SEQ ID NO: 4 consists of nucleotides 434-1972 of NCBI Reference Sequence NM_001106.4. Figure 5 shows a nucleic acid sequence (SEQ ID NO: 3) encoding a human ActRIIB(20-134) extracellular domain polypeptide. Figure 6 shows a multiple sequence alignment of various vertebrate ActRIIB precursor polypeptides without their intracellular domains (SEQ ID NOs: 358-363), human ActRIIA precursor polypeptide without its intracellular domain (SEQ ID NO: 364), and a consensus ActRII precursor polypeptide (SEQ ID NO: 365). Upper case letters in the consensus sequence indicate positions that are conserved. Lower case letters in the consensus sequence indicate an amino acid residue that is the predominant form, but not universal at that position. Figure 7 shows multiple sequence alignment of Fc domains from human IgG isotypes using Clustal 2.1. Hinge regions are indicated by dotted underline. Double underline indicates examples of positions engineered in IgGl (SEQ ID NO: 13) Fc to promote asymmetric chain pairing and the corresponding positions with respect to other isotypes IgG4 (SEQ ID NO: 17), IgG2 (SEQ ID NO: 14), and IgG3 (SEQ ID NO: 15). Figure 8A and Figure 8B show schematic examples of heteromeric polypeptide complexes comprising a variant ActRIIB polypeptide (indicated as “X”) and either an ALK4 polypeptide (indicated as “Y”) or an ALK7 polypeptide (indicated as “Y”). In the illustrated embodiments, the variant ActRIIB polypeptide is part of a fusion polypeptide that comprises a first member of an interaction pair (“Ci”), and either an ALK4 polypeptide or an ALK7 polypeptide is part of a fusion polypeptide that comprises a second member of an interaction pair (“C2”). Suitable interaction pairs include, for example, heavy chain and / or light chain immunoglobulin interaction pairs, truncations, and variants thereof such as those described herein [e.g., Spiess et al (2015) Molecular Immunology 67(2A): 95-106]. In each fusion polypeptide, a linker may be positioned between the variant ActRIIB polypeptide, ALK4 polypeptide, or ALK7 polypeptide and the corresponding member of the interaction pair. The first and second members of the interaction pair may be unguided, meaning that the members of the pair may associate with each other or self-associate without substantial preference, and they may have the same or different amino acid sequences. See Figure 8A. Alternatively, the interaction pair may be a guided (asymmetric) pair, meaning that the members of the pair associate preferentially with each other rather than self-associate. See Figure 8B. Figure 9 shows a multiple sequence alignment of various vertebrate ALK4 proteins and human ALK4 (SEQ ID NOs: 414-420). Figure 10 shows a multiple sequence alignment of various vertebrate ActRIIA proteins and human ActRIIA (SEQ ID NOs: 367, 371-377). Figures 11A and 11B show two schematic examples of heteromeric protein complexes comprising type I receptor and type II receptor polypeptides. Figure 11A depicts a heterodimeric protein complex comprising one type I receptor fusion polypeptide and one type II receptor fusion polypeptide, which can be assembled covalently or noncovalently via a multimerization domain contained within each polypeptide chain. Two assembled multimerization domains constitute an interaction pair, which can be either guided or unguided. Figure 1 IB depicts a heterotetrameric protein complex comprising two heterodimeric complexes as depicted in Figure 11A. Complexes of higher order can be envisioned. Figures 12 show a schematic example of a heteromeric protein complex comprising a type I receptor polypeptide (indicated as “I”) (e.g. a polypeptide that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99% or 100% identical to an extracellular domain of an ALK4 protein from humans or other species such as those described herein) and a type II receptor polypeptide (indicated as “II”) (e.g. a polypeptide that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99% or 100% identical to an extracellular domain of an ActRIIB protein from humans or other species as such as those described herein). In the illustrated embodiments, the type I receptor polypeptide is part of a fusion polypeptide that comprises a first member of an interaction pair (“Ci”), and the type II receptor polypeptide is part of a fusion polypeptide that comprises a second member of an interaction pair (“C2”). In each fusion polypeptide, a linker may be positioned between the type I or type II receptor polypeptide and the corresponding member of the interaction pair. The first and second members of the interaction pair may be a guided (asymmetric) pair, meaning that the members of the pair associate preferentially with each other rather than self-associate, or the interaction pair may be unguided, meaning that the members of the pair may associate with each other or self-associate without substantial preference and may have the same or different amino acid sequences. Traditional Fc fusion proteins and antibodies are examples of unguided interaction pairs, whereas a variety of engineered Fc domains have been designed as guided (asymmetric) interaction pairs [e.g., Spiess et al (2015) Molecular Immunology 67(2A): 95-106]. Figures 13A-13D show schematic examples of heteromeric protein complexes comprising an ALK4 polypeptide (e.g. a polypeptide that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99% or 100% identical to an extracellular domain of an ALK4 protein from humans or other species such as those described herein) and an ActRIIB polypeptide (e.g. a polypeptide that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99% or 100% identical to an extracellular domain of an ActRIIB protein from humans or other species such as those described herein). In the illustrated embodiments, the ALK4 polypeptide is part of a fusion polypeptide that comprises a first member of an interaction pair (“Ci”), and the ActRIIB polypeptide is part of a fusion polypeptide that comprises a second member of an interaction pair (“C2”). Suitable interaction pairs included, for example, heavy chain and / or light chain immunoglobulin interaction pairs, truncations, and variants thereof such as those described herein [e.g., Spiess et al (2015) Molecular Immunology 67(2A): 95-106]. In each fusion polypeptide, a linker may be positioned between the ALK4 or ActRIIB polypeptide and the corresponding member of the interaction pair. The first and second members of the interaction pair may be unguided, meaning that the members of the pair may associate with each other or self-associate without substantial preference, and they may have the same or different amino acid sequences. See Figure 13 A. Alternatively, the interaction pair may be a guided (asymmetric) pair, meaning that the members of the pair associate preferentially with each other rather than self-associate. See Figure 13B. Complexes of higher order can be envisioned. See Figure 13C and 13D. Figure 14 shows the purification of ActRIIA-hFc expressed in CHO cells. The protein purifies as a single, well-defined peak as visualized by sizing column (top panel) and Coomassie stained SDS-PAGE (bottom panel) (left lane: molecular weight standards; right lane: ActRIIA-hFc). Figure 15 shows the binding of ActRIIA-hFc to activin (top panel) and GDF-11 (bottom panel), as measured by Biacore™ assay. Figure 16A and Figure 16B show values for ligand binding kinetics of homodimeric Fc-fusion polypeptides comprising variant or unmodified ActRIIB domains, as determined by surface plasmon resonance at 37°C. Amino acid numbering is based on SEQ ID NO: 2. ND# indicates that the value is not detectable over concentration range tested. Transient* indicates that the value is indeterminate due to transient nature of interaction. Control sample is ActRIIB-GlFc (SEQ ID NO: 5). Figure 17 shows values for ligand binding kinetics of homodimeric Fc-fusion polypeptides comprising variant or unmodified ActRIIB domains, as determined by surface plasmon resonance at 37°C. Amino acid numbering is based on SEQ ID NO: 2. ND# indicates that the value is not detectable over concentration range tested. Transient binding* indicates that the value is indeterminate due to transient nature of interaction. Control sample is ActRIIB-GlFc (SEQ ID NO: 5). Figure 18 shows values for ligand binding kinetics of homodimeric Fc-fusion polypeptides comprising variant or unmodified ActRIIB domains, as determined by surface plasmon resonance at 25°C. ND# indicates that the value is not detectable over concentration range tested. Amino acid numbering is based on SEQ ID NO: 2. Figure 19 shows comparative ligand binding data for an ALK4-Fc:ActRIIB-Fc heterodimeric protein complex compared to ActRIIB-Fc homodimer and ALK4-Fc homodimer. For each protein complex, ligands are ranked by koff, a kinetic constant that correlates well with ligand signaling inhibition, and listed in descending order of binding affinity (ligands bound most tightly are listed at the top). At left, yellow, red, green, and blue lines indicate magnitude of the off-rate constant. Solid black lines indicate ligands whose binding to heterodimer is enhanced or unchanged compared with homodimer, whereas dashed red lines indicate substantially reduced binding compared with homodimer. As shown, the ActRIIB-Fc:ALK4-Fc heterodimer displays enhanced binding to activin B compared with either homodimer, retains strong binding to activin A, GDF8, and GDF11 as observed with ActRIIB-Fc homodimer, and exhibits substantially reduced binding to BMP9, BMP10, and GDF3. Like ActRIIB-Fc homodimer, the heterodimer retains intermediate-level binding to BMP6. Figure 20 shows comparative ActRIIB-Fc:ALK4-Fc heterodimer / ActRIIB-Fc: ActRIIB-Fc homodimer IC50 data as determined by an A-204 Reporter Gene Assay as described herein. ActRIIB-Fc:ALK4-Fc heterodimer inhibits activin A, activin B, GDF8, and GDF11 signaling pathways similarly to the ActRIIB-Fc:ActRIIB-Fc homodimer. However, ActRIIB-Fc:ALK4-Fc heterodimer inhibition of BMP9 and BMP 10 signaling pathways is significantly reduced compared to the ActRIIB-Fc: ActRIIB-Fc homodimer. These data demonstrate that ActRIIB:ALK4 heterodimers are more selective antagonists of activin A, activin B, GDF8, and GDF11 compared to corresponding ActRIIB:ActRIIB homodimers. Figure 21 shows comparative ligand binding data for an ActRIIB-Fc:ALK7-Fc heterodimeric protein complex compared to ActRIIB-Fc homodimer and ALK7-Fc homodimer. For each protein complex, ligands are ranked by koff, a kinetic constant that correlates well with ligand signaling inhibition, and listed in descending order of binding affinity (ligands bound most tightly are listed at the top). At left, yellow, red, green, and blue lines indicate magnitude of the off-rate constant. Solid black lines indicate ligands whose binding to heterodimer is enhanced or unchanged compared with homodimer, whereas dashed red lines indicate substantially reduced binding compared with homodimer. As shown, four of the five ligands with strong binding to ActRIIB-Fc homodimer (activin A, BMP10, GDF8, and GDF11) exhibit reduced binding to the ActRIIB-Fc:ALK7-Fc heterodimer, the exception being activin B which retains tight binding to the heterodimer. Similarly, three of four ligands with intermediate binding to ActRIIB-Fc homodimer (GDF3, BMP6, and particularly BMP9) exhibit reduced binding to the ActRIIB-Fc:ALK7-Fc heterodimer, whereas binding to activin AC is increased to become the second strongest ligand interaction with the heterodimer overall. Finally, activin C and BMP5 unexpectedly bind the ActRIIB-Fc:ALK7 heterodimer with intermediate strength despite no binding (activin C) or weak binding (BMP5) to ActRIIB-Fc homodimer. No ligands tested bind to ALK7-Fc homodimer. Figure 22 shows a multiple sequence alignment of ALK7 extracellular domains derived from various vertebrate species (SEQ ID NOs: 425-430). Figure 23 ActRIIB-Fc:ALK4-Fc rescued cardiac diastolic dysfunction during LV remodeling in the aged heart. Thirteen male mice at 24-months of age (“Old”) and 10 mice at 4-months of age (“Young”) were studied. Groups of “Old” and “Young” mice received phosphate-buffered saline (PBS) twice per week subcutaneously for 8 weeks (“YoungVehicle” or “Old-Vehicle”). Another group of “Old” mice received ActRIIB-Fc :ALK4-Fc (10 mg / kg) twice per week subcutaneously for 8 weeks (“Old-ActRIIB-Fc:ALK4-Fc”). The volume of vehicle and volume of ActRIIB-Fc: ALK4-Fc administered was the same. E / e', a measurement of diastolic dysfunction, was increased in “Old-Vehicle” (n=7) mice compared “Young-Vehicle” mice (n=10, p<0.01). ActRIIB-Fc:ALK4-Fc treatment in “Old-ActRIIB-Fc:ALK4-Fc” mice significantly reduced E / e' (n=6, p<0.05). DETAILED DESCRIPTION 1. Overview In certain aspects, the disclosure relates to methods of using TGF-P superfamily ligand antagonists, in particular ActRII-ALK4 antagonists, to treat heart failure. For example, ActRII-ALK4 antagonists as described herein may be used to treat, prevent, or reduce the progression rate and / or severity of heart failure associated with aging, or one or more complications of heart failure associated with aging. Heart Failure (HF) is a clinical syndrome characterized by symptoms that include breathlessness, ankle swelling and fatigue, that may be accompanied by signs that include elevated jugular venous pressure, pulmonary crackles and peripheral edema caused by a structural and / or functional cardiac abnormality. HF typically results in a reduced cardiac output and / or elevated intracardiac pressure at rest or during stress. Before clinical symptoms become apparent, patients may present with asymptomatic structural or functional cardiac abnormalities (e.g., systolic or diastolic left ventricular (LV) dysfunction), which are precursors of HF. Recognition of these precursors is important because they are related to poor outcomes, and starting treatment at the precursor stage may reduce mortality in patients with asymptomatic systolic LV dysfunction. Demonstration of an underlying cardiac cause is central to the diagnosis of HF. This usually includes a myocardial abnormality causing systolic and / or diastolic ventricular dysfunction. However, abnormalities of the valves, pericardium, endocardium, heart rhythm and conduction can also cause HF (and more than one abnormality is often present). Identification of the underlying cardiac problem is crucial for therapeutic reasons, as the precise pathology determines the specific treatment used (e.g., valve repair or replacement for valvular disease, specific pharmacological therapy for HF with reduced EF, reduction of heart rate in tachycardiomyopathy, etc.). TGF-P superfamily ligand signals are mediated by heteromeric complexes of type I and type II serine / threonine kinase receptors, which phosphorylate and activate downstream Smad proteins upon ligand stimulation (Massague, 2000, Nat. Rev. Mol. Cell Biol. 1:169178). These type I and type II receptors are all transmembrane polypeptides, composed of a ligand-binding extracellular domain with cysteine-rich region, a transmembrane domain, and a cytoplasmic domain with predicted serine / threonine specificity. Type I receptors are essential for signaling, and type II receptors are required for binding ligands. Type I and type II activin receptors form a stable complex after ligand binding, resulting in phosphorylation of type I receptors by type II receptors. Two related type II receptors, ActRIIA and ActRIIB, have been identified as the type II receptors for activins (Mathews and Vale, 1991, Cell 65:973-982; Attisanoet al., 1992, Cell 68: 97-108). Besides activins, ActRIIA and ActRIIB can biochemically interact with several other TGF-P family proteins, including BMP7, Nodal, GDF8, and GDF11 (Yamashita et al., 1995, J. Cell Biol. 130:217-226; Lee and McPherron, 2001, Proc. Natl. Acad. Sci. 98:9306-9311; Yeo and Whitman, 2001, Mol. Cell 7: 949-957; Oh et al., 2002, Genes Dev. 16:2749-54). Applicants have found that soluble ActRIIA-Fc fusion polypeptides and ActRIIB-Fc fusion polypeptides have substantially different effects in vivo, with ActRIIA-Fc having primary effects on bone and ActRIIB-Fc having primary effects on skeletal muscle. Ligands of the TGF-beta superfamily share the same dimeric structure in which the central 3-1 / 2 turn helix of one monomer packs against the concave surface formed by the beta-strands of the other monomer. The majority of TGF-beta family members are further stabilized by an intermolecular disulfide bond. This disulfide bonds traverses through a ring formed by two other disulfide bonds generating what has been termed a ‘cysteine knot’ motif [Lin et al. (2006) Reproduction 132: 179-190; and Hinck et al. (2012) FEBS Letters 586: 1860-1870], Activins are members of the TGF-beta superfamily and were initially discovered as regulators of secretion of follicle-stimulating hormone, but subsequently various reproductive and non-reproductive roles have been characterized. There are three principal activin forms (A, B, and AB) that are homo / heterodimers of two closely related P subunits (PaPa, PbPb, and PaPb, respectively). The human genome also encodes an activin C and an activin E, which are primarily expressed in the liver, and heterodimeric forms containing Pc or Pe are also known. In the TGF-beta superfamily, activins are unique and multifunctional factors that can stimulate hormone production in ovarian and placental cells, support neuronal cell survival, influence cell-cycle progress positively or negatively depending on cell type, and induce mesodermal differentiation at least in amphibian embryos [DePaolo et al. (1991) Proc Soc Ep Biol Med. 198:500-512; Dyson et al. (1997) Curr Biol. 7:81-84; and Woodruff (1998) Biochem Pharmacol. 55:953-963]. In several tissues, activin signaling is antagonized by its related heterodimer, inhibin. For example, in the regulation of follicle-stimulating hormone (FSH) secretion from the pituitary, activin promotes FSH synthesis and secretion, while inhibin reduces FSH synthesis and secretion. Other proteins that may regulate activin bioactivity and / or bind to activin include follistatin (FS) and a2-macroglobulin. As described herein, agents that bind to “activin A” are agents that specifically bind to the Pa subunit, whether in the context of an isolated Pa subunit or as a dimeric complex (e.g., a PaPa homodimer or a PaPb heterodimer). In the case of a heterodimer complex (e.g., a PaPb heterodimer), agents that bind to “activin A” are specific for epitopes present within the Pa subunit, but do not bind to epitopes present within the non-PA subunit of the complex (e.g., the pB subunit of the complex). Similarly, agents disclosed herein that antagonize (inhibit) “activin A” are agents that inhibit one or more activities as mediated by a Pa subunit, whether in the context of an isolated Pa subunit or as a dimeric complex (e.g., a PaPa homodimer or a PaPb heterodimer). In the case of PaPb heterodimers, agents that inhibit “activin A” are agents that specifically inhibit one or more activities of the Pa subunit, but do not inhibit the activity of the non-PA subunit of the complex (e.g., the Pb subunit of the complex). This principle applies also to agents that bind to and / or inhibit “activin B”, “activin C”, and “activin E”. Agents disclosed herein that antagonize “activin AB” are agents that inhibit one or more activities as mediated by the Pa subunit and one or more activities as mediated by the Pb subunit. The BMPs and GDFs together form a family of cysteine-knot cytokines sharing the characteristic fold of the TGF-beta superfamily [Rider et al. (2010) Biochem J., 429(1): 1-12]. This family includes, for example, BMP2, BMP4, BMP6, BMP7, BMP2a, BMP3, BMP3b (also known as GDF10), BMP4, BMP5, BMP6, BMP7, BMP8, BMP8a, BMP8b, BMP9 (also known as GDF2), BMP10, BMP11 (also known as GDF11), BMP12 (also known as GDF7), BMP13 (also known as GDF6), BMP14 (also known as GDF5), BMP15, GDF1, GDF3 (also known as VGR2), GDF8 (also known as myostatin), GDF9, GDF15, and decapentaplegic. Besides the ability to induce bone formation, which gave the BMPs their name, the BMP / GDFs display morphogenetic activities in the development of a wide range of tissues. BMP / GDF homo- and hetero-dimers interact with combinations of type I and type II receptor dimers to produce multiple possible signaling complexes, leading to the activation of one of two competing sets of SMAD transcription factors. BMP / GDFs have highly specific and localized functions. These are regulated in a number of ways, including the developmental restriction of BMP / GDF expression and through the secretion of several specific BMP antagonist proteins that bind with high affinity to the cytokines. Curiously, a number of these antagonists resemble TGF-beta superfamily ligands. Growth and differentiation factor-8 (GDF8) is also known as myostatin. GDF8 is a negative regulator of skeletal muscle mass. GDF8 is highly expressed in the developing and adult skeletal muscle. The GDF8 null mutation in transgenic mice is characterized by a marked hypertrophy and hyperplasia of the skeletal muscle (McPherron et al., Nature, 1997, 387:83-90). Similar increases in skeletal muscle mass are evident in naturally occurring mutations of GDF8 in cattle (Ashmore et al., 1974, Growth, 38:501-507; Swatland and Kieffer, J. Anim. Sci., 1994, 38:752-757; McPherron and Lee, Proc. Natl. Acad. Sci. USA, 1997, 94:12457-12461; andKambadur et al., Genome Res., 1997, 7:910-915) and, strikingly, in humans (Schuelke et al., N Engl J Med 2004;350:2682-8). Studies have also shown that muscle wasting associated with HIV-infection in humans is accompanied by increases in GDF8 polypeptide expression (Gonzalez-Cadavid et al., Proc Natl Acad Sci USA, 1998, 95:14938-43). In addition, GDF8 can modulate the production of muscle-specific enzymes (e.g., creatine kinase) and modulate myoblast cell proliferation (WO 00 / 43781). The GDF8 propeptide can noncovalently bind to the mature GDF8 domain dimer, inactivating its biological activity (Miyazono et al. (1988) J. Biol. Chern., 263: 6407-6415; Wakefield et al. (1988) J. Biol. Chern., 263; 7646-7654; and Brown et al. (1990) Growth Factors, 3: 35-43). Other polypeptides which bind to GDF8 or structurally related polypeptides and inhibit their biological activity include follistatin, and potentially, follistatin-related polypeptides (Gamer et al. (1999) Dev. Biol., 208: 222-232). Growth and differentiation factor-11 (GDF11), also known as BMP11, is a secreted protein (McPherron et al., 1999, Nat. Genet. 22: 260-264). GDF11 is expressed in the tail bud, limb bud, maxillary and mandibular arches, and dorsal root ganglia during mouse development (Nakashima et al., 1999, Meeh. Dev. 80: 185-189). GDF11 plays a unique role in patterning both mesodermal and neural tissues (Gamer et al., 1999, Dev Biol., 208:22232). GDF11 was shown to be a negative regulator of chondrogenesis and myogenesis in developing chick limb (Gamer et al., 2001, Dev Biol. 229:407-20). The expression of GDF11 in muscle also suggests its role in regulating muscle growth in a similar way to GDF8. In addition, the expression of GDF11 in brain suggests that GDF11 may also possess activities that relate to the function of the nervous system. Interestingly, GDF11 was found to inhibit neurogenesis in the olfactory epithelium (Wu et al., 2003, Neuron. 37:197-207). In part, the examples of the disclosure demonstrate that an ActRIIB:ALK4 heterodimer is effective to improve diastolic dysfunction as measured by E / e'. Ejection fraction was not reduced in aged mice compared to young mice, while BNP levels increased, indicative of HFpEF. The data further suggest that, in addition to ActRIIB:ALK4 heteromultimers, other ActRII-ALK4 antagonists may be useful in treating heart failure associated with aging. In certain aspects, an ActRII-ALK4 antagonist to be used in accordance with the methods and uses disclosed herein (e.g., treating, preventing, or reducing the progression rate and / or severity of heart failure associated with aging, or one or more complications of heart failure) is an ActRII-ALK4 ligand trap polypeptide antagonist including variants thereof as well as heterodimers and heteromultimers thereof, an ActRII-ALK4 antibody antagonist, an ActRII-ALK4 polynucleotide antagonist, and / or an ActRII-ALK4 small molecule antagonist. ActRII-ALK4 ligand trap polypeptides include TGF-P superfamily-related proteins, including variants thereof, that are capable of binding to one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, and / or BMP 10). Therefore, an ActRII-ALK4 ligand trap generally includes polypeptides that are capable of antagonizing one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, and / or BMP10). In some embodiments, an ActRII-ALK4 antagonist comprises an ActRII-ALK4 ligand trap. In 51 some embodiments, an ActRII-ALK4 ligand trap comprises an ActRIIB polypeptide, including variants thereof, as well as homomultimers (e.g., ActRIIB homodimers) and heteromultimers (e.g., ActRIIB-ALK4 or ActRIIB-ALK7 heterodimers). In some embodiments, an ActRII-ALK4 ligand trap comprises an ActRIIA polypeptide, including variants thereof, as well as homomultimers (e.g., ActRIIA homodimers) and heteromultimers (e.g., ActRIIA-ALK4 or ActRIIA-ALK7 heterodimers). In other embodiments, an ActRII-ALK ligand trap comprises a soluble ligand trap protein including, but not limited to, or a follistatin polypeptide as well as variants thereof. In some embodiments, an ActRII-ALK4 antagonist comprises an ActRII-ALK4 antibody antagonist (antibodies that inhibit one or more of activin A, activin B, GDF8, GDF11, BMP6, BMP10, ActRIIB, ActRIIA, ALK4 and / or ALK7). In some embodiments, an ActRII-ALK4 antagonist comprises an ActRII-ALK4 small molecule antagonist (e.g., small molecules that inhibit one or more of activin A, activin B, GDF8, GDF11, BMP6, BMP 10, ActRIIB, ActRIIA, ALK4 and / or ALK7). In some embodiments, an ActRII-ALK4 antagonist comprises an ActRII-ALK4 polynucleotide antagonist (e.g., nucleotide sequences that inhibit one or more of activin A, activin B, GDF8, GDF11, BMP6, BMP 10, ActRIIB, ActRIIA, ALK4 and / or ALK7). The terms used in this specification generally have their ordinary meanings in the art, within the context of this disclosure and in the specific context where each term is used. Certain terms are discussed below or elsewhere in the specification to provide additional guidance to the practitioner in describing the compositions and methods of the disclosure and how to make and use them. The scope or meaning of any use of a term will be apparent from the specific context in which it is used. The term “sequence similarity,” in all its grammatical forms, refers to the degree of identity or correspondence between nucleic acid or amino acid sequences that may or may not share a common evolutionary origin. "Percent (%) sequence identity" with respect to a reference polypeptide (or nucleotide) sequence is defined as the percentage of amino acid residues (or nucleic acids) in a candidate sequence that are identical to the amino acid residues (or nucleic acids) in the reference polypeptide (nucleotide) sequence, after aligning the sequences and introducing gaps, if necessary, to achieve the maximum percent sequence identity, and not considering any conservative substitutions as part of the sequence identity. Alignment for purposes of determining percent amino acid sequence identity can be achieved in various ways that are within the skill in the art, for instance, using publicly available computer software such as 52 BLAST, BLAST-2, ALIGN or Megalign (DNASTAR) software. Those skilled in the art can determine appropriate parameters for aligning sequences, including any algorithms needed to achieve maximal alignment over the full length of the sequences being compared. For purposes herein, however, % amino acid (nucleic acid) sequence identity values are generated using the sequence comparison computer program ALIGN-2. The ALIGN-2 sequence comparison computer program was authored by Genentech, Inc., and the source code has been filed with user documentation in the U.S. Copyright Office, Washington D.C., 20559, where it is registered under U.S. Copyright Registration No. TXU510087. The ALIGN-2 program is publicly available from Genentech, Inc., South San Francisco, Calif., or may be compiled from the source code. The ALIGN-2 program should be compiled for use on a UNIX operating system, including digital UNIX V4.0D. All sequence comparison parameters are set by the ALIGN-2 program and do not vary. “Agonize”, in all its grammatical forms, refers to the process of activating a protein and / or gene (e.g., by activating or amplifying that protein’s gene expression or by inducing an inactive protein to enter an active state) or increasing a protein’s and / or gene’s activity. “Antagonize”, in all its grammatical forms, refers to the process of inhibiting a protein and / or gene (e.g., by inhibiting or decreasing that protein’s gene expression or by inducing an active protein to enter an inactive state) or decreasing a protein’s and / or gene’s activity. The terms "about" and "approximately" as used in connection with a numerical value throughout the specification and the claims denotes an interval of accuracy, familiar and acceptable to a person skilled in the art. In general, such interval of accuracy is ± 10%. Alternatively, and particularly in biological systems, the terms "about" and "approximately" may mean values that are within an order of magnitude, preferably < 5-fold and more preferably < 2-fold of a given value. Numeric ranges disclosed herein are inclusive of the numbers defining the ranges. The terms "a" and "an" include plural referents unless the context in which the term is used clearly dictates otherwise. The terms "a" (or "an"), as well as the terms "one or more," and "at least one" can be used interchangeably herein. Furthermore, "and / or" where used herein is to be taken as specific disclosure of each of the two or more specified features or components with or without the other. Thus, the term “and / or" as used in a phrase such as "A and / or B" herein is intended to include "A and B," "A or B," "A" (alone), and "B" (alone). Likewise, the term "and / or" as used in a phrase such as "A, B, and / or C" is intended to encompass each of the following aspects: A, B, and C; A, B, or C; A or C; A or B; B or C; A and C; A and B; B and C; A (alone); B (alone); and C (alone). Throughout this specification, the word “comprise” or variations such as “comprises” or “comprising” will be understood to imply the inclusion of a stated integer or groups of integers but not the exclusion of any other integer or group of integers. 2. ActRII-ALK4 Ligand Trap Antagonists and Variants Thereof In certain aspects, an ActRII-ALK4 antagonist to be used in accordance with the methods and uses disclosed herein (e.g., treating, preventing, or reducing the progression rate and / or severity of heart failure associated with aging, or one or more complications of heart failure) is an ActRII-ALK4 ligand trap polypeptide including variants thereof as well as heterodimers and heteromultimers thereof. ActRII-ALK4 ligand trap polypeptides include TGF-P superfamily-related proteins, including variants thereof, that are capable of binding to one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, and BMP 10). Therefore, ActRII-ALK4 ligand trap generally include polypeptides that are capable of antagonizing one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, and BMP 10). For example, in some embodiments, an ActRII-ALK4 ligand trap comprises an ActRII polypeptide, including variants thereof, as well as homo- and hetero-multimers thereof (e.g., homodimer and heterodimers, respectively). As used herein, the term “ActRII” refers to the family of type II activin receptors. This family includes activin receptor type IIA (ActRIIA) and activin receptor type IIB (ActRIIB). In some embodiments, an ActRII-ALK4 ligand trap comprises an ActRIIB polypeptide, including variants thereof, as well as homomultimers (e.g., ActRIIB homodimers) and heteromultimers (e.g., ActRIIB-ALK4 or ActRIIB-ALK7 heterodimers). In some embodiments, an ActRII-ALK4 ligand trap comprises an ActRIIA polypeptide, including variants thereof, as well as homomultimers (e.g., ActRIIA homodimers) and heteromultimers (e.g., ActRIIA-ALK4 or ActRIIA-ALK7 heterodimers). In other embodiments, an ActRII-ALK ligand trap comprises a soluble ligand trap protein including, but not limited to, or a follistatin polypeptide as well as variants thereof. A) ActRIIB Polypeptides In certain aspects, the disclosure relates to ActRII-ALK4 antagonists comprising an ActRIIB polypeptide, which includes fragments, functional variants, and modified forms thereof as well as uses thereof (e.g., of treating, preventing, or reducing the progression rate and / or severity of heart failure (HF) or one or more complications of HF). As used herein, the term “ActRIIB” refers to a family of activin receptor type IIB (ActRIIB) proteins from any species and variant polypeptides derived from such ActRIIB proteins by mutagenesis or other modifications (including, e.g., mutants, fragments, fusions, and peptidomimetic forms) that retain a useful activity. Examples of such variant ActRIIB polypeptides are provided throughout the present disclosure as well as in International Patent Application Publication Nos. WO 2006 / 012627, WO 2008 / 097541, WO 2010 / 151426, WO 2011 / 020045, WO 2018 / 009624, and WO 2018 / 067874 which are incorporated herein by reference in their entirety. Reference to ActRIIB herein is understood to be a reference to any one of the currently identified forms. Members of the ActRIIB family are generally all transmembrane polypeptides, composed of a ligand-binding extracellular domain with cysteine-rich region, a transmembrane domain, and a cytoplasmic domain with predicted serine / threonine kinase specificity. The amino acid sequence of human ActRIIB precursor polypeptide is shown in Figure 2 (SEQ ID NO: 2) and below. Preferably, ActRIIB polypeptides to be used in accordance with the methods of the disclosure are soluble. The term “soluble ActRIIB polypeptide,” as used herein, includes any naturally occurring extracellular domain of an ActRIIB polypeptide as well as any variants thereof (including mutants, fragments and peptidomimetic forms) that retain a useful activity. For example, the extracellular domain of an ActRIIB polypeptide binds to a ligand and is generally soluble. Examples of soluble ActRIIB polypeptides include an ActRIIB extracellular domain (SEQ ID NO: 1) shown in Figure 3 as well as SEQ ID NO: 53. This truncated ActRIIB extracellular domain (SEQ ID NO: 53) is denoted ActRIIB(25-131) based on numbering in SEQ ID NO: 2. Other examples of soluble ActRIIB polypeptides comprise a signal sequence in addition to the extracellular domain of an ActRIIB polypeptide (see Example 4). The signal sequence can be a native signal sequence of an ActRIIB, or a signal sequence from another polypeptide, such as a tissue plasminogen activator (TPA) signal sequence or a honey bee melittin signal sequence. In some embodiments, ActRIIB polypeptides inhibit activity (e.g., Smad signaling) of one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP 10). In some embodiments, ActRIIB polypeptides bind to one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP 10). Various examples of methods and assays for determining the ability of an ActRIIB polypeptide to bind to and / or inhibit activity of one or more ActRII-ALK4 ligands are disclosed herein or otherwise well known in the art, which can be readily used to determine if an ActRIIB polypeptide has the desired binding and / or antagonistic activities. Numbering of amino acids for all ActRIIB-related polypeptides described herein is based on the numbering of the human ActRIIB precursor protein sequence provided below (SEQ ID NO: 2), unless specifically designated otherwise. The human ActRIIB precursor protein sequence is as follows: 1 MTATTWATA^^           RGEAETRECI YYNANWELER TgQSGLERCE 51 GEQDKRLHCY ASWRNSSGTI ELVKKGCWLD DFNCYDRQEC VATEENPQVY 101 FCCCEGNFCN ERFTHLPEAG GPEVTYEPPP TAPTLLTVLA YSLLPIGGLS 151 LIVLLAFWMY RHRKPPYGHV DIHEDPGPPP PSPLVGLKPL QLLEIKARGR 201 FGCVWKAQLM NDFVAVKIFP LQDKQSWQSE REIFSTPGMK HENLLQFIAA 251 EKRGSNLEVE LWLITAFHDK GSLTDYLKGN IITWNELCHV AETMSRGLSY 301 LHEDVPWCRG EGHKPSIAHR DFKSKNVLLK SDLTAVLADF GLAVRFEPGK 351 PPGDTHGQVG TRRYMAPEVL EGAINFQRDA FLRIDMYAMG LVLWELVSRC 401 KAADGPVDEY MLPFEEEIGQ HPSLEELQEV VVHKKMRPTI KDHWLKHPGL 451 AQLCVTIEEC WDHDAEARLS AGCVEERVSL IRRSVNGTTS DCLVSLVTSV 501 TNVDLPPKES SI (SEQ ID NO: 2, Figure 2) The signal peptide is indicated with a single underline; the extracellular domain is indicated in bold font; and the potential, endogenous N-linked glycosylation sites are indicated with a double underline. A processed (mature) extracellular ActRIIB polypeptide sequence is as follows: GRGEAETRECIYYNANWELERTNQSGLERCEGEQDKRLHCYASWRNSSGTIELVKK GCWLDDFNCYDROECVATEENPQVYFCCCEGNFCNERFTHLPEAGGPEVTYEPPPT APT (SEQ ID NO: 1, Figure 3). In some embodiments, the protein may be produced with an “SGR..sequence at the N-terminus. The C-terminal “tail” of the extracellular domain is indicated by a single underline. The sequence with the “tail” deleted (a Al5 sequence) is as follows: GRGEAETRECIYYNANWELERTNQSGLERCEGEQDKRLHCYASWRNSSGTIELVKK GCWLDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTHLPEA (SEQ ID NO: 386). A form of ActRIIB with an alanine at position 64 of SEQ ID NO: 2 (A64) is also reported in the literature. See, e.g., Hilden et al. (1994) Blood, 83(8): 2163-2170. Applicants have ascertained that an ActRIIB-Fc fusion protein comprising an extracellular domain of ActRIIB with the A64 substitution has a relatively low affinity for activin and GDF11. By contrast, the same ActRIIB-Fc fusion protein with an arginine at position 64 (R64) has an affinity for activin and GDF11 in the low nanomolar to high picomolar range. Therefore, sequences with an R64 are used as the “wild-type” reference sequence for human ActRIIB in this disclosure. The form of ActRIIB precursor protein sequence with an alanine at position 64 is as follows: 1 MTAPWVALAL LWGSLCAGSG RGEAETRECI YYNANWELER TNQSGLERCE 51 GEQDKRLHCY ASWANSSGTI ELVKKGCWLD DFNCYDRQEC VATEENPQVY 101 FCCCEGNFCN ERFTHLPEAG GPEVTYEPPP TAPTLLTVLA YSLLPIGGLS 151 LIVLLAFWMY RHRKPPYGHV DIHEDPGPPP PSPLVGLKPL QLLEIKARGR 201 FGCVWKAQLM NDFVAVKIFP LQDKQSWQSE REIFSTPGMK HENLLQFIAA 251 EKRGSNLEVE LWLITAFHDK GSLTDYLKGN IITWNELCHV AETMSRGLSY 301 LHEDVPWCRG EGHKPSIAHR DFKSKNVLLK SDLTAVLADF GLAVRFEPGK 351 PPGDTHGQVG TRRYMAPEVL EGAINFQRDA FLRIDMYAMG LVLWELVSRC 401 KAADGPVDEY MLPFEEEIGQ HPSLEELQEV VVHKKMRPTI KDHWLKHPGL 451 AQLCVTIEEC WDHDAEARLS AGCVEERVSL IRRSVNGTTS DCLVSLVTSV 501 TNVDLPPKES SI (SEQ ID NO: 387) The signal peptide is indicated by single underline and the extracellular domain is indicated by bold font. A processed (mature) extracellular ActRIIB polypeptide sequence of the alternative A64 form is as follows: GRGEAETRECIYYNANWELERTNQ SGLERCEGEQDKRLHC YAS WANS SGTIELVKK GCWLDDFNCYDROECVATEENPOVYFCCCEGNFCNERFTHLPEAGGPEVTYEPPPT APT (SEO ID NO: 388) In some embodiments, the protein may be produced with an “SGR..sequence at the N-terminus. The C-terminal “tail” of the extracellular domain is indicated by single underline. The polypeptide sequence of the alternative A64 form with the “tail” deleted (a Al5 sequence) is as follows: GRGEAETRECIYYNANWELERTNQSGLERCEGEQDKRLHCYASWANSSGTIELVKK GCWLDDFNCYDRQECVATEENPQVYFCCCEGNFCNERFTHLPEA (SEQ ID NO: 389) A nucleic acid sequence encoding the human ActRIIB precursor protein is shown below (SEQ ID NO: 4), representing nucleotides 25-1560 of GenBank Reference Sequence NM_001106.3, which encodes amino acids 1-513 of the ActRIIB precursor. The nucleotide sequence as shown encodes a polypeptide with an arginine at position 64 and may be modified to encode a polypeptide with an alanine instead. The signal sequence is underlined. 1 ATGACGGCGC CCTGGGTGGC CCTCGCCCTC CTCTGGGGAT CGCTGTGCGC 51 CGGCTCTGGG CGTGGGGAGG CTGAGACACG GGAGTGCATC TACTACAACG 101 CCAACTGGGA GCTGGAGCGC ACCAACCAGA GCGGCCTGGA GCGCTGCGAA 151 GGCGAGCAGG ACAAGCGGCT GCACTGCTAC GCCTCCTGGC GCAACAGCTC 201 TGGCACCATC GAGCTCGTGA AGAAGGGCTG CTGGCTAGAT GACTTCAACT 251 GCTACGATAG GCAGGAGTGT GTGGCCACTG AGGAGAACCC CCAGGTGTAC 301 TTCTGCTGCT GTGAAGGCAA CTTCTGCAAC GAACGCTTCA CTCATTTGCC 351 AGAGGCTGGG GGCCCGGAAG TCACGTACGA GCCACCCCCG ACAGCCCCCA 401 CCCTGCTCAC GGTGCTGGCC TACTCACTGC TGCCCATCGG GGGCCTTTCC 451 CTCATCGTCC TGCTGGCCTT TTGGATGTAC CGGCATCGCA AGCCCCCCTA 501 CGGTCATGTG GACATCCATG AGGACCCTGG GCCTCCACCA CCATCCCCTC 551 TGGTGGGCCT GAAGCCACTG CAGCTGCTGG AGATCAAGGC TCGGGGGCGC 601 TTTGGCTGTG TCTGGAAGGC CCAGCTCATG AATGACTTTG TAGCTGTCAA 651 GATCTTCCCA CTCCAGGACA AGCAGTCGTG GCAGAGTGAA CGGGAGATCT 701 TCAGCACACC TGGCATGAAG CACGAGAACC TGCTACAGTT CATTGCTGCC 751 GAGAAGCGAG GCTCCAACCT CGAAGTAGAG CTGTGGCTCA TCACGGCCTT 801 CCATGACAAG GGCTCCCTCA CGGATTACCT CAAGGGGAAC ATCATCACAT 851 GGAACGAACT GTGTCATGTA GCAGAGACGA TGTCACGAGG CCTCTCATAC 901 CTGCATGAGG ATGTGCCCTG GTGCCGTGGC GAGGGCCACA AGCCGTCTAT 951 TGCCCACAGG GACTTTAAAA GTAAGAATGT ATTGCTGAAG AGCGACCTCA 1001 CAGCCGTGCT GGCTGACTTT GGCTTGGCTG TTCGATTTGA GCCAGGGAAA 1051 CCTCCAGGGG ACACCCACGG ACAGGTAGGC ACGAGACGGT ACATGGCTCC 1101 TGAGGTGCTC GAGGGAGCCA TCAACTTCCA GAGAGATGCC TTCCTGCGCA 1151 TTGACATGTA TGCCATGGGG TTGGTGCTGT GGGAGCTTGT GTCTCGCTGC 1201 AAGGCTGCAG ACGGACCCGT GGATGAGTAC ATGCTGCCCT TTGAGGAAGA 1251 GATTGGCCAG CACCCTTCGT TGGAGGAGCT GCAGGAGGTG GTGGTGCACA 1301 AGAAGATGAG GCCCACCATT AAAGATCACT GGTTGAAACA CCCGGGCCTG 1351 GCCCAGCTTT GTGTGACCAT CGAGGAGTGC TGGGACCATG ATGCAGAGGC 1401 TCGCTTGTCC GCGGGCTGTG TGGAGGAGCG GGTGTCCCTG ATTCGGAGGT 1451 CGGTCAACGG CACTACCTCG GACTGTCTCG TTTCCCTGGT GACCTCTGTC 15 01 ACCAATGTGG ACCTGCCCCC TAAAGAGTCA AGCATC (SEQ ID NO: 4, Figure 4) A nucleic acid sequence encoding a processed extracellular human ActRIIB polypeptide is as follows (SEQ ID NO: 3). The nucleotide sequence as shown encodes a polypeptide with an arginine at position 64, and may be modified to encode a polypeptide with an alanine instead (See Figure 5, SEQ ID NO: 3). 1 GGGCGTGGGG AGGCTGAGAC ACGGGAGTGC ATCTACTACA ACGCCAACTG 51 GGAGCTGGAG CGCACCAACC AGAGCGGCCT GGAGCGCTGC GAAGGCGAGC 101 AGGACAAGCG GCTGCACTGC TACGCCTCCT GGCGCAACAG CTCTGGCACC 151 ATCGAGCTCG TGAAGAAGGG CTGCTGGCTA GATGACTTCA ACTGCTACGA 201 TAGGCAGGAG TGTGTGGCCA CTGAGGAGAA CCCCCAGGTG TACTTCTGCT 251 GCTGTGAAGG CAACTTCTGC AACGAACGCT TCACTCATTT GCCAGAGGCT 301 GGGGGCCCGG AAGTCACGTA CGAGCCACCC CCGACAGCCC CCACC (SEQ ID NO: 3) B)    Variant ActRIIB Polypeptides In certain specific embodiments, the present disclosure contemplates making mutations in the extracellular domain (also referred to as ligand-binding domain) of an ActRIIB polypeptide such that the variant (or mutant) ActRIIB polypeptide has altered ligand-binding activities (e.g., binding affinity or binding selectivity). In certain cases, such variant ActRIIB polypeptides have altered (elevated or reduced) binding affinity for a specific ligand. In other cases, the variant ActRIIB polypeptides have altered binding selectivity for their ligands. For example, the disclosure provides a number of variant ActRIIB polypeptides that have reduced binding affinity to BMP9, compared to a nonmodified ActRIIB polypeptide, but retain binding affinity for one or more of activin A, activin B, GDF8, GDF11, and BMP10. Optionally, the variant ActRIIB polypeptides have similar or the same biological activities of their corresponding wild-type ActRIIB polypeptides. For example, a variant ActRIIB polypeptide of the disclosure may bind to and inhibit function of an ActRIIB ligand (e.g., activin A, activin B, GDF8, GDF11 or BMP 10). In some embodiments, a variant ActRIIB polypeptide of the disclosure treats, prevents, or reduces the progression rate and / or severity of heart failure or one or more complications of heart failure. Examples of ActRIIB polypeptides include human ActRIIB precursor polypeptide (SEQ ID NO: 2 and SEQ ID NO:387), and soluble human ActRIIB polypeptides (e.g, SEQ ID NOs: 1, 5, 6, 12, 276, 278, 279, 332, 333, 335, 336, 338, 339, 341, 342, 344, 345, 347, 348, 350, 351, 353, 354, 356, 357, 385, 386, 387, 388, 389, 396, 398, 402, 403, 406, 408, and 409). In some embodiments, the variant ActRIIB polypeptide is a member of a homomultimer (e.g., homodimer). In some embodiments, the variant ActRIIB polypeptide is a member of a heteromultimer (e.g., a heterodimer). In some embodiments, any of the variant ActRIIB polypeptides may be combined (e.g., heteromultimerized with and / or fused to) with any of polypeptides disclosed herein. ActRIIB is well-conserved across nearly all vertebrates, with large stretches of the extracellular domain conserved completely. See, e.g., Figure 6. Many of the ligands that bind to ActRIIB are also highly conserved. Accordingly, comparisons of ActRIIB sequences from various vertebrate organisms provide insights into residues that may be altered. Therefore, an active, human ActRIIB variant may include one or more amino acids at corresponding positions from the sequence of another vertebrate ActRIIB, or may include a residue that is similar to that in the human or other vertebrate sequence. The disclosure identifies functionally active portions and variants of ActRIIB. Applicant has previously ascertained that an Fc fusion polypeptide having the sequence disclosed by Hilden et al. (Blood. 1994 Apr 15;83(8):2163-70), which has an alanine at the position corresponding to amino acid 64 of SEQ ID NO: 2 (A64), has a relatively low affinity for activin and GDF11. By contrast, the same Fc fusion polypeptide with an arginine at position 64 (R64) has an affinity for activin and GDF-11 in the low nanomolar to high picomolar range. Therefore, a sequence with an R64 (SEQ ID NO: 2) is used as the wild-type reference sequence for human ActRIIB in this disclosure, and the numbering for the variants described herein are based on the numbering in SEQ ID NO: 2. Additionally, one of skill in the art can make any of the ActRIIB variants described herein in the A64 background. A processed extracellular ActRIIB polypeptide sequence is shown in SEQ ID NO: 1 (see, e.g., Figure 3). In some embodiments, a processed ActRIIB polypeptide may be produced with an “SGR..sequence at the N-terminus. In some embodiments, a processed ActRIIB polypeptide may be produced with a “GRG...” sequence at the N-terminus. For example, it is expected that some constructs, if expressed with a TPA leader, will lack the N-terminal serine. Accordingly, mature ActRIIB sequences described herein may begin with either an N-terminal serine or an N-terminal glycine (lacking the N-terminal serine). Attisano et al. (Cell. 1992 Jan 10;68(l):97-108) showed that a deletion of the proline knot at the C-terminus of the extracellular domain of ActRIIB reduced the affinity of the receptor for activin. Data disclosed in WO2008097541 show that an ActRIIB-Fc fusion polypeptide containing amino acids 20-119 of SEQ ID NO: 2, “ActRIIB(20-l 19)-Fc” has reduced binding to GDF11 and activin relative to an ActRIIB(20-134)-Fc, which includes the proline knot region and the complete juxtamembrane domain. However, an ActRIIB(20-129)-Fc polypeptide retains similar but somewhat reduced activity relative to the wild type, even though the proline knot region is disrupted. Thus, ActRIIB extracellular domains that stop at amino acid 134, 133, 132, 131, 130 and 129 are all expected to be active, but constructs stopping at 134 or 133 may be most active. Similarly, mutations at any of residues 129-134 are not expected to alter ligand binding affinity by large margins. In support of this, mutations of Pl29 and Pl 30 do not substantially decrease ligand binding. Therefore, an ActRIIB-Fc fusion polypeptide may end as early as amino acid 109 (the final cysteine), however, forms ending at or between 109 and 119 are expected to have reduced ligand binding. Amino acid 119 is poorly conserved and so is readily altered or truncated. Forms ending at 128 or later retain ligand binding activity. Forms ending at or between 119 and 127 will have an intermediate binding ability. Any of these forms may be desirable to use, depending on the clinical or experimental setting. At the N-terminus of ActRIIB, it is expected that a polypeptide beginning at amino acid 29 or before will retain ligand binding activity. Amino acid 29 represents the initial cysteine. An alanine-to-asparagine mutation at position 24 introduces anN-linked glycosylation sequence without substantially affecting ligand binding. This confirms that mutations in the region between the signal cleavage peptide and the cysteine cross-linked region, corresponding to amino acids 20-29, are well tolerated. In particular, constructs beginning at position 20, 21, 22, 23 and 24 will retain activity, and constructs beginning at positions 25, 26, 27, 28 and 29 are also expected to retain activity. Data shown in WO2008097541 demonstrate that, surprisingly, a construct beginning at 22, 23, 24 or 25 will have the most activity. Taken together, an active portion of ActRIIB comprises amino acids 29-109 of SEQ ID NO: 2, and constructs may, for example, begin at a residue corresponding to amino acids 20-29 and end at a position corresponding to amino acids 109-134. Other examples include constructs that begin at a position from 20-29 or 21-29 and end at a position from 119-134, 119-133 or 129-134, 129-133. Other examples include constructs that begin at a position from 20-24 (or 21-24, or 22-25) and end at a position from 109-134 (or 109-133), 119-134 (or 119-133) or 129-134 (or 129-133). Variants within these ranges are also contemplated, particularly those having at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to the corresponding portion of SEQ ID NO: 1. The variations described herein may be combined in various ways. In some embodiments, ActRIIB variants comprise no more than 1, 2, 5, 6, 7, 8, 9, 10 or 15 conservative amino acid changes in the ligand-binding pocket, optionally zero, one or more non-conservative alterations at positions 40, 53, 55, 74, 79 and / or 82 in the ligand-binding pocket. Sites outside the binding pocket, at which variability may be particularly well tolerated, include the amino and carboxy termini of the extracellular domain (as noted above), and positions 42-46 and 65-73 (with respect to SEQ ID NO: 2). An asparagine-to-alanine alteration at position 65 (N65A) does not appear to decrease ligand binding in the R64 background [U.S. Patent No. 7,842,663]. This change probably eliminates glycosylation at N65 in the A64 background, thus demonstrating that a significant change in this region is likely to be tolerated. While an R64A change is poorly tolerated, R64K is well-tolerated, and thus another basic residue, such as H may be tolerated at position 64 [U.S. Patent No. 7,842,663]. Additionally, the results of the mutagenesis program described in the art indicate that there are amino acid positions in ActRIIB that are often beneficial to conserve. With respect to SEQ ID NO: 2, these include position 80 (acidic or hydrophobic amino acid), position 78 (hydrophobic, and particularly tryptophan), position 37 (acidic, and particularly aspartic or glutamic acid), position 56 (basic amino acid), position 60 (hydrophobic amino acid, particularly phenylalanine or tyrosine). Thus, the disclosure provides a framework of amino acids that may be conserved in ActRIIB polypeptides. Other positions that may be desirable to conserve are as follows: position 52 (acidic amino acid), position 55 (basic amino acid), position 81 (acidic), 98 (polar or charged, particularly E, D, R or K), all with respect to SEQ ID NO: 2. It has been previously demonstrated that the addition of a further N-linked glycosylation site (N-X-S / T) into the ActRIIB extracellular domain is well-tolerated (see, e.g., U.S. Patent No. 7,842,663). Therefore, N-X-S / T sequences may be generally introduced at positions outside the ligand binding pocket defined in Figure 1 in ActRIIB polypeptide of the present disclosure. Particularly suitable sites for the introduction of non-endogenous N-X-S / T sequences include amino acids 20-29, 20-24, 22-25, 109-134, 120-134 or 129-134 (with respect to SEQ ID NO: 2). N-X-S / T sequences may also be introduced into the linker between the ActRIIB sequence and an Fc domain or other fusion component as well as optionally into the fusion component itself. Such a site may be introduced with minimal effort by introducing an N in the correct position with respect to a pre-existing S or T, or by introducing an S or T at a position corresponding to a pre-existing N. Thus, desirable alterations that would create anN-linked glycosylation site are: A24N, R64N, S67N (possibly combined with anN65A alteration), E105N, R112N, G120N, E123N, P129N, A132N, RI 12S and RI 12T (with respect to SEQ ID NO: 2). Any S that is predicted to be glycosylated may be altered to a T without creating an immunogenic site, because of the protection afforded by the glycosylation. Likewise, any T that is predicted to be glycosylated may be altered to an S. Thus, the alterations S67T and S44T (with respect to SEQ ID NO: 2) are contemplated. Likewise, in an A24N variant, an S26T alteration may be used. Accordingly, an ActRIIB polypeptide of the present disclosure may be a variant having one or more additional, non-endogenous N-linked glycosylation consensus sequences as described above. In certain embodiments, a variant ActRIIB polypeptide has an amino acid sequence that is at least 75% identical to an amino acid sequence selected from SEQ ID NOs: 1, 2, and 53. In certain cases, the variant ActRIIB polypeptide has an amino acid sequence at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence selected from SEQ ID NOs: 1, 2, and 53. In certain cases, the variant ActRIIB polypeptide has an amino acid sequence at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to SEQ ID NO: 1. In certain cases, the variant ActRIIB polypeptide has an amino acid sequence at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to SEQ ID NO: 2. In certain cases, the variant ActRIIB polypeptide has an amino acid sequence at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to SEQ ID NO: 53. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of any one of SEQ ID NOs: 1, 2, 5, 6, 12, 31, 33, 34, 36, 37, 39, 40, 42, 43, 45, 46, 48, 49, 50, 51, 52, 53, 276, 278, 279, 332, 333, 335, 336, 338, 339, 341, 342, 344, 345, 347, 348, 350, 351, 353, 354, 356, 357, 385, 386, 387, 388, 389, 396, 398, 402, 403, 406, 408, and 409. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 1. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 1 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 2. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 2 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 5. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 5 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 6. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 6 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 12. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 12 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 31. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 31 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 64 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 33. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 33 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 34. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 34 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 36. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 36 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 37. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 37 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 39. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 39 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 40. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 40 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 42. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 42 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 43. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 43 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 45. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 45 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 46. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 46 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 48. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 48 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 49. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 49 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 50. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 50 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 51. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 51 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 52. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 52 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 53. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 53 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 276. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 276 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 278. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 278 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 279. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 279 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 332. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 332 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 333. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 333 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 335. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 335 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 336. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 336 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 338. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 338 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 339. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 339 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 341. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 341 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 342. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 342 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 344. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 344 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 345. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 345 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 347. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 347 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 348. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 348 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 69 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 350. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 350 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 351. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 351 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 353. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 353 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 354. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 354 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 356. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 356 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 357. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 357 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 385. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 385 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 386. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 386 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 387. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 387 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 388. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 388 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 389. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 389 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 396. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 396 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 398. An ActRIIB-Fc fusion protein comprising SEQ ID 71 NO: 398 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 402. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 402 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 403. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 403 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 406. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 406 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 408. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 408 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, variant ActRIIB polypeptides or variant ActRIIB-Fc fusion polypeptides of the disclosure comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 409. An ActRIIB-Fc fusion protein comprising SEQ ID NO: 409 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to variant ActRIIB polypeptides comprising an amino acid sequence that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence that begins at any one of amino acids 20-29 (e.g., amino acid residues 20, 21, 22, 23, 24, 25, 26, 27, 28, or 29) of SEQ ID NO: 2 and ends at any one of amino acids 109-134 (e.g., amino acid residues 109, 72 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, or 134) ofSEQ ID NO: 2, and wherein the polypeptide comprises one or more amino acid substitutions at a position of SEQ ID NO: 2 selected from the group consisting of: K55, F82, L79, A24, K74, R64, P129, P130, E37, R40, D54, R56, W78, D80, and F82 as well as heteromultimer complexes comprising one or more such variant ActRIIB polypeptides. In certain aspects, the disclosure relates to variant ActRIIB polypeptides comprising an amino acid sequence that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence that begins at any one of amino acids 20-29 (e.g., amino acid residues 20, 21, 22, 23, 24, 25, 26, 27, 28, or 29) of SEQ ID NO: 2 and ends at any one of amino acids 109-134 (e.g., amino acid residues 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, or 134) ofSEQ ID NO: 2, and wherein the polypeptide comprises one or more amino acid substitutions at a position of SEQ ID NO: 2, but wherein the amino acid at position corresponding to 79 of SEQ ID NO:2 is leucine as well as heteromultimer complexes comprising one or more such variant ActRIIB polypeptides. In some embodiments, the variant ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 29-109 ofSEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 25-131 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 20-134 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 53. In some embodiments, the variant ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 12. In some embodiments, the variant ActRIIB polypeptide comprises an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 5. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to A24 of SEQ ID NO: 2. For example, in some embodiments, the substitution is A24N. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to S26 of SEQ ID NO: 2. For example, in some embodiments, the substitution is S26T. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to N35 of SEQ ID NO: 2. For example, in some embodiments, the substitution is N35E. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to E37 of SEQ ID NO: 2. For example, in some embodiments, the substitution is E37A. In some embodiments, the substitution is E37D. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to L38 of SEQ ID NO: 2. For example, in some embodiments, the substitution is L38N. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to R40 of SEQ ID NO: 2. For example, in some embodiments, the substitution is R40A. In some embodiments, the substitution is R40K. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to S44 of SEQ ID NO: 2. For example, in some embodiments, the substitution is S44T. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to L46 of SEQ ID NO: 2. For example, in some embodiments, the substitution is L46A. For example, in some embodiments, the substitution is L46I. For example, in some embodiments, the substitution is L46F. For example, in some embodiments, the substitution is L46V. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to E50 of SEQ ID NO: 2. For example, in some embodiments, the substitution is E50K. In some embodiments, the substitution is E50L. In some embodiments, the substitution is E50P. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to E52 of SEQ ID NO: 2. For example, in some embodiments, the substitution is E52A. In some embodiments, the substitution is E52D. In some embodiments, the substitution is E52G. In some embodiments, the substitution is E52H. In some embodiments, the substitution is E52K. In some embodiments, the substitution is E52N. In some embodiments, the substitution is E52P. In some embodiments, the substitution is E52R. In some embodiments, the substitution is E52S. In some embodiments, the substitution is E52T. In some embodiments, the substitution is E52Y. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to Q53 of SEQ ID NO: 2. For example, in some embodiments, the substitution is Q53R. For example, in some embodiments, the substitution is Q53K. For example, in some embodiments, the substitution is Q53N. For example, in some embodiments, the substitution is Q53H. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to D54 of SEQ ID NO: 2. For example, in some embodiments, the substitution is D54A. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to K55 of SEQ ID NO: 2. For example, in some embodiments, the substitution is K55A. In some embodiments, the substitution is K55E. In some embodiments, the substitution is K55D. In some embodiments, the substitution is K55R. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to R56 of SEQ ID NO: 2. For example, in some embodiments, the substitution is R56A. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to L57 of SEQ ID NO: 2. For example, in some embodiments, the substitution is L57R. In some embodiments, the substitution is L57E. In some embodiments, the substitution is L57I. In some embodiments, the substitution is L57T. In some embodiments, the substitution is L57V. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to Y60 of SEQ ID NO: 2. For example, in some embodiments, the substitution is Y60F. In some embodiments, the substitution is Y60D. In some embodiments, the substitution is Y60K. In some embodiments, the substitution is Y60P. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to R64 of SEQ ID NO: 2. For example, in some embodiments, the substitution is R64K. In some embodiments, the substitution is R64N. In some embodiments, the substitution is R64A. In some embodiments, the substitution is R64H. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to N65 of SEQ ID NO: 2. For example, in some embodiments, the substitution is N65A. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to S67 of SEQ ID NO: 2. For example, in some embodiments, the substitution is S67N. In some embodiments, the substitution is S67T. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to G68 of SEQ ID NO: 2. For example, in some embodiments, the substitution is G68R. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to K74 of SEQ ID NO: 2. For example, in some embodiments, the substitution is K74A. In some embodiments, the substitution is K74E. In some embodiments, the substitution is K74F. In some embodiments, the substitution is K74I. In some embodiments, the substitution is K74Y. In some embodiments, the substitution is K74R. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to W78 of SEQ ID NO: 2. For example, in some embodiments, the substitution is W78A. In some embodiments, the substitution is W78Y. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to L79 of SEQ ID NO: 2. For example, in some embodiments, the substitution is L79D. In some embodiments, the substitution does not comprise an acidic amino acid at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the substitution is not at position L79 of SEQ ID NO: 2. In some embodiments, position L79 of SEQ ID NO: 2 is not substituted. In some embodiments, the substitution does not comprise an aspartic acid (D) at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the substitution is L79A. In some embodiments, the substitution is L79E. In some embodiments, the substitution is L79F. In some embodiments, the substitution is L79H. In some embodiments, the substitution is L79K. In some embodiments, the substitution is L79P. In some embodiments, the substitution is L79R. In some embodiments, the substitution is L79S. In some embodiments, the substitution is L79T. In some embodiments, the substitution is L79W. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to D80 of SEQ ID NO: 2. For example, in some embodiments, the substitution is D80A. In some embodiments, the substitution is D80F. In some embodiments, the substitution is D80K. In some embodiments, the substitution is D80G. In some embodiments, the substitution is D80M. In some embodiments, the substitution is D80I. In some embodiments, the substitution is D80N. In some embodiments, the substitution is D80R. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to F82 of SEQ ID NO: 2. For example, in some embodiments, the substitution is F82I. In some embodiments, the substitution is F82K. In some embodiments, the substitution is F82A. In some embodiments, the substitution is F82W. In some embodiments, the substitution is F82D. In some embodiments, the substitution is F82Y. In some embodiments, the substitution is F82E. In some embodiments, the substitution is F82L. In some embodiments, the substitution is F82T. In some embodiments, the substitution is F82S. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to N83 of SEQ ID NO: 2. For example, in some embodiments, the substitution is N83A. In some embodiments, the substitution is N83R. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to T93 of SEQ ID NO: 2. For example, in some embodiments, the substitution is T93D. In some embodiments, the substitution is T93E. In some embodiments, the substitution is T93H. In some embodiments, the substitution is T93G. In some embodiments, the substitution is T93K. In some embodiments, the substitution is T93P. In some embodiments, the substitution is T93R. In some embodiments, the substitution is T93S. In some embodiments, the substitution is T93Y. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to E94 of SEQ ID NO: 2. For example, in some embodiments, the substitution is E94K. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to Q98 of SEQ ID NO: 2. For example, in some embodiments, the substitution is Q98D. In some embodiments, the substitution is Q98E. In some embodiments, the substitution is Q98K. In some embodiments, the substitution is Q98R. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to V99 of SEQ ID NO: 2. For example, in some embodiments, the substitution is V99E. In some embodiments, the substitution is V99G. In some embodiments, the substitution is V99K. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to E105 of SEQ ID NO: 2. For example, in some embodiments, the substitution is E105N. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to El06 of SEQ ID NO: 2. For example, in some embodiments, the substitution is E106N. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to Fl08 of SEQ ID NO: 2. For example, in some embodiments, the substitution is Fl081. In some embodiments, the substitution is F108L. In some embodiments, the substitution is Fl08V. In some embodiments, the substitution is F108Y. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to El 11 of SEQ ID NO: 2. For example, in some embodiments, the substitution is El 1 IK. In some embodiments, the substitution is El 1 ID. In some embodiments, the substitution is El 11R. In some embodiments, the substitution is El 11H. In some embodiments, the substitution is El 1 IQ. In some embodiments, the substitution is El 1 IN. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to RI 12 of SEQ ID NO: 2. For example, in some embodiments, the substitution is RI 12H. In some embodiments, the substitution is RI 12K. In some embodiments, the substitution is RI 12N. In some embodiments, the substitution is RI 12S. In some embodiments, the substitution is RI 12T. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to Al 19 of SEQ ID NO: 2. For example, in some embodiments, the substitution is Al 19P. In some embodiments, the substitution is Al 19V. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to G120 of SEQ ID NO: 2. For example, in some embodiments, the substitution is G120N. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to E123 of SEQ ID NO: 2. For example, in some embodiments, the substitution is E123N. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to Pl29 of SEQ ID NO: 2. For example, in some embodiments, the substitution is P129S. In some embodiments, the substitution is P129N. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to Pl30 of SEQ ID NO: 2. For example, in some embodiments, the substitution is P130A. In some embodiments, the substitution is P130R. In some embodiments, the polypeptide comprises an amino acid substitution at the amino acid position corresponding to A132 of SEQ ID NO: 2. For example, in some embodiments, the substitution is A132N. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises a substitution at a position of SEQ ID NO: 2 selected from the group consisting of: A24, E37, R40, D54, K55, R56, R64, K74, W78, L79, D80, F82, P129, and P130. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position A24 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position E37 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position R40 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position D54 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position K55 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position R56 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position R64 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position K74 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position W78 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position L79 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position D80 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position F82 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position Pl29 with respect to SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a substitution at position Pl30 with respect to SEQ ID NO: 2. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 31. In some embodiments, the variant ActRIIB polypeptide comprises an alanine at the position corresponding to K55 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 31 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 33. In some embodiments, the variant ActRIIB polypeptide comprises an alanine at the position corresponding to K55 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 33 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 34. In some embodiments, the variant ActRIIB polypeptide comprises a glutamic acid at the position corresponding to K55 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 34 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 36. In some embodiments, the variant ActRIIB polypeptide comprises a glutamic acid at the position corresponding to K55 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 36 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 37. In some embodiments, the variant ActRIIB polypeptide comprises an isoleucine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 37 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 39. In some embodiments, the variant ActRIIB polypeptide comprises an isoleucine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 39 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 40. In some embodiments, the variant ActRIIB polypeptide comprises a lysine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 40 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 42. In some embodiments, the variant ActRIIB polypeptide comprises a lysine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 42 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 43. In some embodiments, the variant ActRIIB polypeptide comprises a glutamic acid at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 43 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 45. In some embodiments, the variant ActRIIB polypeptide comprises a glutamic acid at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 45 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 336. In some embodiments, the variant ActRIIB polypeptide comprises a threonine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 336 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 338. In some embodiments, the variant ActRIIB polypeptide comprises a threonine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 338 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 342. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 342 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 344. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 344 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 348. In some embodiments, the variant ActRIIB polypeptide comprises a leucine at the position corresponding to E50 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 348 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 350. In some embodiments, the variant ActRIIB polypeptide comprises a leucine at the position 81 corresponding to E50 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 350 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 354. In some embodiments, the variant ActRIIB polypeptide comprises a glycine at the position corresponding to V99 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 354 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 356. In some embodiments, the variant ActRIIB polypeptide comprises a glycine at the position corresponding to V99 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 356 may optionally be provided with the lysine removed from the C-terminus. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises at least 2, 3, 4, 5, 6, 7, 8, 9, or 10 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 2 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 3 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 4 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 5 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 6 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 7 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 8 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 9 of any of the amino acid substitutions disclosed herein. In some embodiments, any of the variant ActRIIB polypeptides disclosed herein comprises 10 of any of the amino acid substitutions disclosed herein. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising two or more amino acid substitutions as compared to the reference amino acid sequence of SEQ ID NO: 2. For example, in some embodiments, the variant ActRIIB polypeptide comprises an A24N substitution and a K74A substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L79P substitution and a K74A substitution. In some embodiments, the variant ActRIIB polypeptide comprises a P129S substitution and a P130A substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L38N substitution and a L79R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a F82I substitution and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a F82K substitution and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a F82T substitution and aN83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L79H substitution and a F82K substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L79H substitution and a F82I substitution. In some embodiments, the variant ActRIIB polypeptide comprises a F82D substitution and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a F82E substitution and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L79F substitution and a F82D substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L79F substitution and a F82T substitution. In some embodiments, the variant ActRIIB polypeptide comprises a E52D substitution and a F82D substitution. In some embodiments, the variant ActRIIB polypeptide comprises an E52D substitution and a F82T substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57R substitution and a F82D substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57R substitution and a F82T substitution. In some embodiments, the variant ActRIIB polypeptide comprises a F82I substitution and an E94K substitution. In some embodiments, the variant ActRIIB polypeptide comprises a F82S substitution and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57R substitution and a F82S substitution. In some embodiments, the variant ActRIIB polypeptide comprises a K74A substitution and a L79P substitution. In some embodiments, the variant ActRIIB polypeptide comprises a K55A substitution and a F82I substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L79K substitution and a F82K substitution. In some embodiments, the variant ActRIIB polypeptide comprises a F82W substitution and a N83A substitution. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 276. In 83 some embodiments, the variant ActRIIB polypeptide comprises an isoleucine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an isoleucine at the position corresponding to F82 of SEQ ID NO: 2 and an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 276 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 278. In some embodiments, the variant ActRIIB polypeptide comprises an isoleucine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an isoleucine at the position corresponding to F82 of SEQ ID NO: 2 and an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 278 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 279. In some embodiments, the variant ActRIIB polypeptide comprises an lysine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a lysine at the position corresponding to F82 of SEQ ID NO: 2 and an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 279 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 332. In some embodiments, the variant ActRIIB polypeptide comprises an lysine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a lysine at the position 84 corresponding to F82 of SEQ ID NO: 2 and an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 332 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 333. In some embodiments, the variant ActRIIB polypeptide comprises a threonine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a threonine at the position corresponding to F82 of SEQ ID NO: 2 and an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 333 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 335. In some embodiments, the variant ActRIIB polypeptide comprises a threonine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a threonine at the position corresponding to F82 of SEQ ID NO: 2 and an arginine at the position corresponding to N83 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 335 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 339. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an isoleucine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2 and an isoleucine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 339 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 341. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an isoleucine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2 and an isoleucine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 341 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 345. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a lysine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2, and a lysine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 345 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 347. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a lysine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a histidine at the position corresponding to L79 of SEQ ID NO: 2, and a lysine at the position corresponding to F82 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 347 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 351. In some embodiments, the variant ActRIIB polypeptide comprises an asparagine at the position 86 corresponding to L38 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an arginine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an asparagine at the position corresponding to L38 of SEQ ID NO: 2, and an arginine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 351 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising an amino acid sequence that is at least 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 353. In some embodiments, the variant ActRIIB polypeptide comprises an asparagine at the position corresponding to L38 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an arginine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises an asparagine at the position corresponding to L38 of SEQ ID NO: 2, and an arginine at the position corresponding to L79 of SEQ ID NO: 2. In some embodiments, the amino acid sequence of SEQ ID NO: 353 may optionally be provided with the lysine removed from the C-terminus. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising three or more amino acid substitutions as compared to the reference amino acid sequence of SEQ ID NO: 2. In some embodiments, the variant ActRIIB polypeptide comprises a G68R substitution, a F82S substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a G68R substitution, a W78Y substitution, and a F82Y substitution. In some embodiments, the variant ActRIIB polypeptide comprises a E52D substitution, a F82D substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises an E52Y substitution, a F82D substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises an E52D substitution, a F82E substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises an E52D substitution, a F82T substitution, and aN83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises an E52N substitution, a F82I substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises an E52N substitution, a F82Y substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises an E50L substitution, a F82D substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57I substitution, a F82D substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57V substitution, a F82D substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57R substitution, a F82D substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57E substitution, a F82E substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57R substitution, a F82E substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57I substitution, a F82E substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57R substitution, a F82L substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57T substitution, a F82Y substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a L57V substitution, a F82Y substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide may comprise at least two of the amino acid substitutions described in any of the variant ActRIIB polypeptides above. In certain aspects, the disclosure relates to a variant ActRIIB polypeptide comprising four or more amino acid substitutions as compared to the reference amino acid sequence of SEQ ID NO: 2. For example, in some embodiments, the variant ActRIIB polypeptide comprises a G68R substitution, a L79E substitution, a F82Y substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a G68R substitution, a L79E substitution, a F82T substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises a G68R substitution, a L79T substitution, a F82T substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide comprises an E52N substitution, a G68R substitution, a F82Y substitution, and a N83R substitution. In some embodiments, the variant ActRIIB polypeptide may comprise at least two of the amino acid substitutions described in any of the variant ActRIIB polypeptides above. In some embodiments, the variant ActRIIB polypeptide may comprise at least three of the amino acid substitutions described in any of the variant ActRIIB polypeptides above. C) ActRIIA Polypeptides In certain embodiments, the disclosure relates to ActRII-ALK4 antagonists that comprise an ActRIIA polypeptide, which includes fragments, functional variants, and modified forms thereof as well as uses thereof (e.g., of treating, preventing, or reducing the progression rate and / or severity of heart failure (HF) or one or more complications of HF). As used herein, the term “ActRIIA” refers to a family of activin receptor type IIA (ActRIIA) proteins from any species and variant polypeptides derived from such ActRIIA proteins by mutagenesis or other modification (including, e.g., mutants, fragments, fusions, and peptidomimetic forms) that retain a useful activity. Examples of such variant ActRIIA polypeptides are provided throughout the present disclosure as well as in International Patent Application Publication Nos. WO 2006 / 012627 and WO 2007 / 062188, which are incorporated herein by reference in their entirety. Reference to ActRIIA herein is understood to be a reference to any one of the currently identified forms. Members of the ActRIIA family are generally transmembrane proteins, composed of a ligand-binding extracellular domain comprising a cysteine-rich region, a transmembrane domain, and a cytoplasmic domain with predicted serine / threonine kinase activity. Preferably, ActRIIA polypeptides to be used in accordance with the methods of the disclosure are soluble (e.g., an extracellular domain of ActRIIA). In some embodiments, ActRIIA polypeptides inhibit activity (e.g., Smad signaling) of one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP 10). In some embodiments, ActRIIA polypeptides bind to one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP10). Various examples of methods and assays for determining the ability of an ActRIIA polypeptide to bind to and / or inhibit activity of one or more ActRII-ALK4 ligands are disclosed herein or otherwise well known in the art, which can be readily used to determine if an ActRIIA polypeptide has the desired binding and / or antagonistic activities. Numbering of amino acids for all ActRIIA-related polypeptides described herein is based on the numbering of the human ActRIIA precursor protein sequence provided below (SEQ ID NO: 366), unless specifically designated otherwise. The canonical human ActRIIA precursor protein sequence is as follows: 1 MGAAAKLAVA VFLISCSSGA ILGRSETQEC LFFNANWEKD RTgQTGVEPC 51 YGDKDKRRHC FATWKglSGS IEIVKQGCWL DDINCYDRTD CVEKKDSPEV 101 YFCCCEGNMC NEKFSYFPEM EVTQPTSNPV TPKPPYYNIL LYSLVPLMLI 151 AGIVICAFWV YRHHKMAYPP VLVPTQDPGP PPPSPLLGLK PLQLLEVKAR 201 GRFGCVWKAQ LLNEYVAVKI FPIQDKQSWQ NEYEVYSLPG MKHENILQFI 251 GAEKRGTSVD VDLWLITAFH EKGSLSDFLK ANWSWNELC HIAETMARGL 301 AYLHEDIPGL KDGHKPAISH RDIKSKNVLL KNNLTACIAD FGLALKFEAG 351 KSAGDTHGQV GTRRYMAPEV LEGAINFQRD AFLRIDMYAM GLVLWELASR 401 CTAADGPVDE YMLPFEEEIG QHPSLEDMQE VWHKKKRPV LRDYWQKHAG 451 MAMLCETIEE CWDHDAEARL SAGCVGERIT QMQRLTNIIT TEDIVTVVTM 501 VTNVDFPPKE SSL (SEQ ID NO: 366) The signal peptide is indicated by a single underline; the extracellular domain is indicated in bold font; and the potential, endogenous N-linked glycosylation sites are indicated by a double underline. A processed (mature) extracellular human ActRIIA polypeptide sequence is as follows: ILGRSETQECLFFNANWEKDRTNQTGVEPCYGDKDKRRHCFATWKNISGSIEIVKQG CWLDDINCYDRTDCVEKKDSPEVYFCCCEGNMCNEKFSYFPEMEVTQPTSNPVTPK PP (SEQ ID NO: 367) The C-terminal “tail” of the extracellular domain is indicated by single underline. The sequence with the “tail” deleted (a Al5 sequence) is as follows: ILGRSETQECLFFNANWEKDRTNQTGVEPCYGDKDKRRHCFATWKNISGSIEIVKQG CWLDDINCYDRTDCVEKKDSPEVYFCCCEGNMCNEKFSYFPEM (SEQ ID NO: 368) A nucleic acid sequence encoding human ActRIIA precursor protein is shown below (SEQ ID NO: 369), corresponding to nucleotides 159-1700 of GenBank Reference Sequence NM_001616.4. The signal sequence is underlined. 1 ATGGGAGCTG CTGCAAAGTT GGCGTTTGCC GTCTTTCTTA TCTCCTGTTC 51 TTCAGGTGCT ATACTTGGTA GATCAGAAAC TCAGGAGTGT CTTTTCTTTA 101 ATGCTAATTG GGAAAAAGAC AGAACCAATC AAACTGGTGT TGAACCGTGT 151 TATGGTGACA AAGATAAACG GCGGCATTGT TTTGCTACCT GGAAGAATAT 201 TTCTGGTTCC ATTGAAATAG TGAAACAAGG TTGTTGGCTG GATGATATCA 251 ACTGCTATGA CAGGACTGAT TGTGTAGAAA AAAAAGACAG CCCTGAAGTA 301 TATTTTTGTT GCTGTGAGGG CAATATGTGT AATGAAAAGT TTTCTTATTT 351 TCCGGAGATG GAAGTCACAC AGCCCACTTC AAATCCAGTT ACACCTAAGC 401 CACCCTATTA CAACATCCTG CTCTATTCCT TGGTGCCACT TATGTTAATT 451 GCGGGGATTG TCATTTGTGC ATTTTGGGTG TACAGGCATC ACAAGATGGC 501 CTACCCTCCT GTACTTGTTC CAACTCAAGA CCCAGGACCA CCCCCACCTT 551 CTCCATTACT AGGTTTGAAA CCACTGCAGT TATTAGAAGT GAAAGCAAGG 601 GGAAGATTTG GTTGTGTCTG GAAAGCCCAG TTGCTTAACG AATATGTGGC 651 TGTCAAAATA TTTCCAATAC AGGACAAACA GTCATGGCAA AATGAATACG 701 AAGTCTACAG TTTGCCTGGA ATGAAGCATG AGAACATATT ACAGTTCATT 751 GGTGCAGAAA AACGAGGCAC CAGTGTTGAT GTGGATCTTT GGCTGATCAC 801 AGCATTTCAT GAAAAGGGTT CACTATCAGA CTTTCTTAAG GCTAATGTGG 851 TCTCTTGGAA TGAACTGTGT CATATTGCAG AAACCATGGC TAGAGGATTG 901 GCATATTTAC ATGAGGATAT ACCTGGCCTA AAAGATGGCC ACAAACCTGC 951 CATATCTCAC AGGGACATCA AAAGTAAAAA TGTGCTGTTG AAAAACAACC 1001 TGACAGCTTG CATTGCTGAC TTTGGGTTGG CCTTAAAATT TGAGGCTGGC 1051 AAGTCTGCAG GCGATACCCA TGGACAGGTT GGTACCCGGA GGTACATGGC 1101 TCCAGAGGTA TTAGAGGGTG CTATAAACTT CCAAAGGGAT GCATTTTTGA 1151 GGATAGATAT GTATGCCATG GGATTAGTCC TATGGGAACT GGCTTCTCGC 1201 TGTACTGCTG CAGATGGACC TGTAGATGAA TACATGTTGC CATTTGAGGA 1251 GGAAATTGGC CAGCATCCAT CTCTTGAAGA CATGCAGGAA GTTGTTGTGC 1301 ATAAAAAAAA GAGGCCTGTT TTAAGAGATT ATTGGCAGAA ACATGCTGGA 1351 ATGGCAATGC TCTGTGAAAC CATTGAAGAA TGTTGGGATC ACGACGCAGA 1401 AGCCAGGTTA TCAGCTGGAT GTGTAGGTGA AAGAATTACC CAGATGCAGA 1451 GACTAACAAA TATTATTACC ACAGAGGACA TTGTAACAGT GGTCACAATG 1501 GTGACAAATG TTGACTTTCC TCCCAAAGAA TCTAGTCTA(SEQ ID NO:369) A nucleic acid sequence encoding processed soluble (extracellular) human ActRIIA polypeptide is as follows: 1 ATACTTGGTA GATCAGAAAC TCAGGAGTGT CTTTTCTTTA ATGCTAATTG 51 GGAAAAAGAC AGAACCAATC AAACTGGTGT TGAACCGTGT TATGGTGACA 101 AAGATAAACG GCGGCATTGT TTTGCTACCT GGAAGAATAT TTCTGGTTCC 151 ATTGAAATAG TGAAACAAGG TTGTTGGCTG GATGATATCA ACTGCTATGA 201 CAGGACTGAT TGTGTAGAAA AAAAAGACAG CCCTGAAGTA TATTTTTGTT 251 GCTGTGAGGG CAATATGTGT AATGAAAAGT TTTCTTATTT TCCGGAGATG 301 GAAGTCACAC AGCCCACTTC AAATCCAGTT ACACCTAAGC CACCC (SEQ ID NO:370) ActRIIA is well-conserved among vertebrates, with large stretches of the extracellular domain completely conserved. For example, Figure 10 depicts a multi-sequence alignment of a human ActRIIA extracellular domain (SEQ ID NO: 367) compared to various ActRIIA orthologs (SEQ ID NOs: 371-377). Many of the ligands that bind to ActRIIA are also highly conserved. Accordingly, from these alignments, it is possible to predict key amino acid positions within the ligand-binding domain that are important for normal ActRIIA-ligand binding activities as well as to predict amino acid positions that are likely to be tolerant to substitution without significantly altering normal ActRIIA-ligand binding activities. Therefore, an active, human ActRIIA variant polypeptide useful in accordance with the presently disclosed methods may include one or more amino acids at corresponding positions from the sequence of another vertebrate ActRIIA, or may include a residue that is similar to that in the human or other vertebrate sequences. Without meaning to be limiting, the following examples illustrate this approach to defining an active ActRIIA variant. As illustrated in Figure 10, F13 in the human extracellular domain is Y in Ovis aries (SEQ ID NO: 371), Gallus gallus (SEQ ID NO: 374), Bos Taurus (SEQ ID NO: 375), Tyto alba (SEQ ID NO: 376), and Myotis davidii (SEQ ID NO: 377) ActRIIA, indicating that aromatic residues are tolerated at this position, including F, W, and Y. Q24 in the human extracellular domain is R in Bos Taurus ActRIIA, indicating that charged residues will be tolerated at this position, including D, R, K, H, and E. S95 in the human extracellular domain is F in Gallus gallus and Tyto alba ActRIIA, indicating that this site may be tolerant of a wide variety of changes, including polar residues, such as E, D, K, R, H, S, T, P, G, Y, and probably hydrophobic residue such as L, I, or F. E52 in the human extracellular domain is D in Ovis aries ActRIIA, indicating that acidic residues are tolerated at this position, including D and E. P29 in the human extracellular domain is relatively poorly conserved, appearing as S in Ovis aries ActRIIA and L in Myotis davidii ActRIIA, thus essentially any amino acid should be tolerated at this position. Moreover, as discussed above, ActRII proteins have been characterized in the art in terms of structural / functional characteristics, particularly with respect to ligand binding [Attisano et al. (1992) Cell 68( 1 ):97-108; Greenwald et al. (1999) Nature Structural Biology 6(1): 18-22; Allendorph et al. (2006) Proc Natl Acad Sci USA103(20: 7643-7648; Thompson et al. (2003) The EMBO Journal 22(7): 1555-1566; as well as U.S. Patent Nos: 7,709,605, 7,612,041, and 7,842,663]. In addition to the teachings herein, these references provide ample guidance for how to generate ActRII variants that retain one or more desired activities (e.g., ligand-binding activity). For example, a defining structural motif known as a three-finger toxin fold is important for ligand binding by type I and type II receptors and is formed by conserved cysteine residues located at varying positions within the extracellular domain of each monomeric receptor [Greenwald et al. (1999) Nat Struct Biol 6:18-22; and Hinck (2012) FEBS Lett 586:1860-1870]. Accordingly, the core ligand-binding domains of human ActRIIA, as demarcated by the outermost of these conserved cysteines, corresponds to positions 30-110 of SEQ ID NO: 366 (ActRIIA precursor). Therefore, the structurally less-ordered amino acids flanking these cysteine-demarcated core sequences can be truncated by about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, or 29 residues at the N-terminus and by about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 residues at the C-terminus without necessarily altering ligand binding. Exemplary ActRIIA extracellular domains truncations include SEQ ID NOs: 367 and 368. Accordingly, a general formula for an active portion (e.g., ligand binding) of ActRIIA is a polypeptide that comprises, consists essentially of, or consists of amino acids 30-110 of SEQ ID NO: 366. Therefore ActRIIA polypeptides may, for example, comprise, consists essentially of, or consists of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to a portion of ActRIIA beginning at a residue corresponding to any one of amino acids 21-30 (e.g., beginning at any one of amino acids 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30) of SEQ ID NO: 366 and ending at a position corresponding to any one amino acids 110135 (e.g., ending at any one of amino acids 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, or 135) of SEQ ID NO: 366. Other examples include constructs that begin at a position selected from 21-30 (e.g., beginning at any one of amino acids 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30), 22-30 (e.g., beginning at any one of amino acids 22, 23, 24, 25, 26, 27, 28, 29, or 30), 23-30 (e.g., beginning at any one of amino acids 23, 24, 25, 26, 27, 28, 29, or 30), 24-30 (e.g., beginning at any one of amino acids 24, 25, 26, 27, 28, 29, or 30) of SEQ ID NO: 366, and end at a position selected from 111-135 (e.g., ending at any one of amino acids 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134 or 135), 112-135 (e.g., ending at any one of amino acids 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134 or 135), 113-135 (e.g., ending at any one of amino acids 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134 or 135), 120-135 (e.g., ending at any one of amino acids 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134 or 135),130-135 (e.g., ending at any one of amino acids 130, 131, 132, 133, 134 or 135), 111-134 (e.g., ending at any one of amino acids 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, or 134), 111-133 (e.g., ending at any one of amino acids 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, or 133), 111-132 (e.g., ending at any one of amino acids 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, or 132), or 111-131 (e.g., ending at anyone of amino acids 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, or 131) ofSEQ ID NO: 366. Variants within these ranges are also contemplated, particularly those comprising, consisting essentially of, or consisting of an amino acid sequence that has at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity to the corresponding portion of SEQ ID NO: 366. Thus, in some embodiments, an ActRIIA polypeptide may comprise, consists essentially of, or consist of a polypeptide that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 30-110 ofSEQ ID NO: 366. Optionally, ActRIIA polypeptides comprise a polypeptide that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 30-110 of SEQ ID NO: 366, and comprising no more than 1, 2, 5, 10 or 15 conservative amino acid changes in the ligand-binding pocket. In some embodiments, ActRIIA polypeptide of the disclosure comprise, consist essentially of, or consist of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to a portion of ActRIIA beginning at a residue corresponding to amino acids 21-30 (e.g., beginning at any one of amino acids 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30) of SEQ ID NO: 366 and ending at a position corresponding to any one amino acids 110-135 (e.g., ending at any one of amino acids 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134 or 135) ofSEQ ID NO: 366. In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical amino acids 30-110 of SEQ ID NO: 366. In certain embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical amino acids 21-135 ofSEQ ID NO: 366. In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 94 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 366. In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 367. In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 368. In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 380. In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 381. In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 384. In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 364 In some embodiments, ActRIIA polypeptides comprise, consist, or consist essentially of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 378. D) ALK4 Polypeptides In certain aspects, the disclosure relates to ActRII-ALK4 antagonists comprising an ALK4 polypeptide, which includes fragments, functional variants, and modified forms thereof as well as uses thereof (e.g., of treating, preventing, or reducing the progression rate and / or severity of heart failure (HF) or one or more complications of HF). As used herein, the term “ALK4” refers to a family of activin receptor-like kinase-4 (ALK4) proteins from any species and variant polypeptides derived from such ALK4 proteins by mutagenesis or other 95 modifications (including, e.g., mutants, fragments, fusions, and peptidomimetic forms) that retain a useful activity. Examples of such variant ALK4 polypeptides are provided throughout the present disclosure as well as in International Patent Application Publication Nos. WO / 2016 / 164089, WO / 2016 / 164497, and WO / 2018 / 067879, which are incorporated herein by reference in their entirety. Reference to ALK4 herein is understood to be a reference to any one of the currently identified forms. Members of the ALK4 family are generally transmembrane proteins, composed of a ligand-binding extracellular domain with a cysteine-rich region, a transmembrane domain, and a cytoplasmic domain with predicted serine / threonine kinase activity. Preferably, ALK4 polypeptides to be used in accordance with the methods of the disclosure are soluble. The term “soluble ALK4 polypeptide,” as used herein, includes any naturally occurring extracellular domain of an ALK4 polypeptide as well as any variants thereof (including mutants, fragments and peptidomimetic forms) that retain a useful activity. For example, the extracellular domain of an ALK4 polypeptide binds to a ligand and is generally soluble. Examples of soluble ALK4 polypeptides include an ALK4 extracellular domain (SEQ ID NO: 86) shown below, Other examples of soluble ALK4 polypeptides comprise a signal sequence in addition to the extracellular domain of an ALK4 polypeptide. The signal sequence can be a native signal sequence of an ALK4 polypeptide, or a signal sequence from another polypeptide, such as a tissue plasminogen activator (TPA) signal sequence or a honey bee melittin signal sequence. In some embodiments, ALK4 polypeptides inhibit (e.g., Smad signaling) activity of one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP10). In some embodiments, ALK4 polypeptides bind to one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP 10). Various examples of methods and assays for determining the ability of an ALK4 polypeptide to bind to and / or inhibit activity of one or more ActRII-ALK4 ligands are disclosed herein or otherwise well known in the art, which can be readily used to determine if an ActRIIB polypeptide has the desired binding and / or antagonistic activities. Numbering of amino acids for all ALK4-related polypeptides described herein is based on the numbering of the human ALK4 precursor protein sequence provided below (SEQ ID NO: 84), unless specifically designated otherwise. A human ALK4 precursor polypeptide sequence (NCBI Ref Seq NP_004293) is as follows: 1 MAESAGASSF FPLWLLLAG SGGSGPRGVQ ALLCACTSCL QANYTCETDG ACMVSIFNLD 61 GMEHHVRTCI PKVELVPAGK PFYCLSSEDL RNTHCCYTDY CNRIDLRVPS GHLKEPEHPS 121 MWGPVELVGI IAGPVFLLFL IIIIVFLVIN YHQRVYHNRQ RLDMEDPSCE MCLSKDKTLQ 181 DLVYDLSTSG SGSGLPLFVQ RTVARTIVLQ EIIGKGRFGE VWRGRWRGGD VAVKIFSSRE 241 ERSWFREAEI YQTVMLRHEN ILGFIAADNK DNGTWTQLWL VSDYHEHGSL FDYLNRYTVT 301 IEGMIKLALS AASGLAHLHM EIVGTQGKPG IAHRDLKSKN ILVKKNGMCA IADLGLAVRH 361 DAVTDTIDIA PNQRVGTKRY MAPEVLDETI NMKHFDSFKC ADIYALGLVY WEIARRCNSG 421 GVHEEYQLPY YDLVPSDPSI EEMRKWCDQ KLRPNIPNWW QSYEALRVMG KMMRECWYAN 481 GAARLTALRI KKTLSQLSVQ EDVKI (SEQ ID NO: 84) The signal peptide is indicated by a single underline and the extracellular domain is indicated in bold font. A processed extracellular human ALK4 polypeptide sequence is as follows: SGPRGVQALLCACTSCLQANYTCETDGACMVSIFNLDGMEHHVRTCIPKVELVPAG KPFYCLSSEDLRNTHCCYTDYCNRIDLRVPSGHLKEPEHPSMWGPVE (SEQ ID NO: 86) A nucleic acid sequence encoding an ALK4 precursor polypeptide is shown in SEQ ID NO: 221), corresponding to nucleotides 78-1592 of GenBank Reference Sequence NM_004302.4. The signal sequence is underlined and the extracellular domain is indicated in bold font. ATGGCGGAGTCGGCCGGAGCCTCCTCCTTCTTCCCCCTTGTTGTCCTCCTGCTCGC CGGCAGCGGCGGGTCCGGGCCCCGGGGGGTCCAGGCTCTGCTGTGTGCGTG CACCAGCTGCCTCCAGGCCAACTACACGTGTGAGACAGATGGGGCCTGCAT GGTTTCCATTTTCAATCTGGATGGGATGGAGCACCATGTGCGCACCTGCATC CCCAAAGTGGAGCTGGTCCCTGCCGGGAAGCCCTTCTACTGCCTGAGCTCG GAGGACCTGCGCAACACCCACTGCTGCTACACTGACTACTGCAACAGGATC GACTTGAGGGTGCCCAGTGGTCACCTCAAGGAGCCTGAGCACCCGTCCATG TGGGGCCCGGTGGAGCTGGTAGGCATCATCGCCGGCCCGGTGTTCCTCCTGTTC CTCATCATCATCATTGTTTTCCTTGTCATTAACTATCATCAGCGTGTCTATCACAA CCGCCAGAGACTGGACATGGAAGATCCCTCATGTGAGATGTGTCTCTCCAAAGA CAAGACGCTCCAGGATCTTGTCTACGATCTCTCCACCTCAGGGTCTGGCTCAGGG TTACCCCTCTTTGTCCAGCGCACAGTGGCCCGAACCATCGTTTTACAAGAGATTA TTGGCAAGGGTCGGTTTGGGGAAGTATGGCGGGGCCGCTGGAGGGGTGGTGATG TGGCTGTGAAAATATTCTCTTCTCGTGAAGAACGGTCTTGGTTCAGGGAAGCAGA GATATACCAGACGGTCATGCTGCGCCATGAAAACATCCTTGGATTTATTGCTGCT GACAATAAAGATAATGGCACCTGGACACAGCTGTGGCTTGTTTCTGACTATCATG AGCACGGGTCCCTGTTTGATTATCTGAACCGGTACACAGTGACAATTGAGGGGAT GATTAAGCTGGCCTTGTCTGCTGCTAGTGGGCTGGCACACCTGCACATGGAGATC GTGGGCACCCAAGGGAAGCCTGGAATTGCTCATCGAGACTTAAAGTCAAAGAAC ATTCTGGTGAAGAAAAATGGCATGTGTGCCATAGCAGACCTGGGCCTGGCTGTC CGTCATGATGCAGTCACTGACACCATTGACATTGCCCCGAATCAGAGGGTGGGG ACCAAACGATACATGGCCCCTGAAGTACTTGATGAAACCATTAATATGAAACAC TTTGACTCCTTTAAATGTGCTGATATTTATGCCCTCGGGCTTGTATATTGGGAGAT TGCTCGAAGATGCAATTCTGGAGGAGTCCATGAAGAATATCAGCTGCCATATTAC GACTTAGTGCCCTCTGACCCTTCCATTGAGGAAATGCGAAAGGTTGTATGTGATC AGAAGCTGCGTCCCAACATCCCCAACTGGTGGCAGAGTTATGAGGCACTGCGGG TGATGGGGAAGATGATGCGAGAGTGTTGGTATGCCAACGGCGCAGCCCGCCTGA CGGCCCTGCGCATCAAGAAGACCCTCTCCCAGCTCAGCGTGCAGGAAGACGTGA AGATC (SEQ ID NO: 221) A nucleic acid sequence encoding an extracellular ALK4 polypeptide is shown in SEQ ID NO: 222. TCCGGGCCCCGGGGGGTCCAGGCTCTGCTGTGTGCGTGCACCAGCTGCCTCCAGG CCAACTACACGTGTGAGACAGATGGGGCCTGCATGGTTTCCATTTTCAATCTGGA TGGGATGGAGCACCATGTGCGCACCTGCATCCCCAAAGTGGAGCTGGTCCCTGC CGGGAAGCCCTTCTACTGCCTGAGCTCGGAGGACCTGCGCAACACCCACTGCTGC TACACTGACTACTGCAACAGGATCGACTTGAGGGTGCCCAGTGGTCACCTCAAG GAGCCTGAGCACCCGTCCATGTGGGGCCCGGTGGAG (SEQ ID NO: 222) An alternative isoform of human ALK4 precursor protein sequence, isoform B (NCBI Ref SeqNP_064732.3), is as follows: 1 MVSIFNLDGM EHHVRTCIPK VELVPAGKPF YCLSSEDLRN THCCYTDYCN RIDLRVPSGH 61 LKEPEHPSMW GPVELVGIIA GPVFLLFLII IIVFLVINYH QRVYHNRQRL DMEDPSCEMC 121 LSKDKTLQDL VYDLSTSGSG SGLPLFVQRT VARTIVLQEI IGKGRFGEVW RGRWRGGDVA 181 VKIFSSREER SWFREAEIYQ TVMLRHENIL GFIAADNKDN GTWTQLWLVS DYHEHGSLFD 241 YLNRYTVTIE GMIKLALSAA SGLAHLHMEI VGTQGKPGIA HRDLKSKNIL VKKNGMCAIA 301 DLGLAVRHDA VTDTIDIAPN QRVGTKRYMA PEVLDETINM KHFDSFKCAD IYALGLVYWE 361 IARRCNSGGV HEEYQLPYYD LVPSDPSIEE MRKVVCDQKL RPNIPNWWQS YEALRVMGKM 421 MRECWYANGA ARLTALRIKK TLSQLSVQED VKI (SEQ ID NO: 421) The extracellular domain is indicated in bold font. A processed extracellular ALK4 polypeptide sequence corresponding to isoform B above is as follows: 1 MVSIFNLDGM EHHVRTCIPK VELVPAGKPF YCLSSEDLRN THCCYTDYCN RIDLRVPSGH 61 LKEPEHPSMW GPVE (SEQ ID NO: 422) A nucleic acid sequence encoding the ALK4 precursor protein (isoform B) is shown below (SEQ ID NO: 423), corresponding to nucleotides 186-1547 of GenBank Reference Sequence NM_020327.3. The nucleotides encoding the extracellular domain are indicated in bold font. 1 ATGGTTTCCA TTTTCAATCT GGATGGGATG GAGCACCATG TGCGCACCTG 51 CATCCCCAAA GTGGAGCTGG TCCCTGCCGG GAAGCCCTTC TACTGCCTGA 101 GCTCGGAGGA CCTGCGCAAC ACCCACTGCT GCTACACTGA CTACTGCAAC 151 AGGATCGACT TGAGGGTGCC CAGTGGTCAC CTCAAGGAGC CTGAGCACCC 201 GTCCATGTGG GGCCCGGTGG AGCTGGTAGG CATCATCGCC GGCCCGGTGT 251 TCCTCCTGTT CCTCATCATC ATCATTGTTT TCCTTGTCAT TAACTATCAT 301 CAGCGTGTCT ATCACAACCG CCAGAGACTG GACATGGAAG ATCCCTCATG 351 TGAGATGTGT CTCTCCAAAG ACAAGACGCT CCAGGATCTT GTCTACGATC 401 TCTCCACCTC AGGGTCTGGC TCAGGGTTAC CCCTCTTTGT CCAGCGCACA 451 GTGGCCCGAA CCATCGTTTT ACAAGAGATT ATTGGCAAGG GTCGGTTTGG 501 GGAAGTATGG CGGGGCCGCT GGAGGGGTGG TGATGTGGCT GTGAAAATAT 551 TCTCTTCTCG TGAAGAACGG TCTTGGTTCA GGGAAGCAGA GATATACCAG 601 ACGGTCATGC TGCGCCATGA AAACATCCTT GGATTTATTG CTGCTGACAA 651 TAAAGATAAT GGCACCTGGA CACAGCTGTG GCTTGTTTCT GACTATCATG 701 AGCACGGGTC CCTGTTTGAT TATCTGAACC GGTACACAGT GACAATTGAG 751 GGGATGATTA AGCTGGCCTT GTCTGCTGCT AGTGGGCTGG CACACCTGCA 801 CATGGAGATC GTGGGCACCC AAGGGAAGCC TGGAATTGCT CATCGAGACT 851 TAAAGTCAAA GAACATTCTG GTGAAGAAAA ATGGCATGTG TGCCATAGCA 901 GACCTGGGCC TGGCTGTCCG TCATGATGCA GTCACTGACA CCATTGACAT 951 TGCCCCGAAT CAGAGGGTGG GGACCAAACG ATACATGGCC CCTGAAGTAC 1001 TTGATGAAAC CATTAATATG AAACACTTTG ACTCCTTTAA ATGTGCTGAT 1051 ATTTATGCCC TCGGGCTTGT ATATTGGGAG ATTGCTCGAA GATGCAATTC 1101 TGGAGGAGTC CATGAAGAAT ATCAGCTGCC ATATTACGAC TTAGTGCCCT 1151 CTGACCCTTC CATTGAGGAA ATGCGAAAGG TTGTATGTGA TCAGAAGCTG 1201 CGTCCCAACA TCCCCAACTG GTGGCAGAGT TATGAGGCAC TGCGGGTGAT 1251 GGGGAAGATG ATGCGAGAGT GTTGGTATGC CAACGGCGCA GCCCGCCTGA 1301 CGGCCCTGCG CATCAAGAAG ACCCTCTCCC AGCTCAGCGT GCAGGAAGAC 1351 GTGAAGATCT AA (SEQ ID NO: 423) A nucleic acid sequence encoding the extracellular ALK4 polypeptide (isoform B) is as follows: 1 ATGGTTTCCA TTTTCAATCT GGATGGGATG GAGCACCATG TGCGCACCTG 51 CATCCCCAAA GTGGAGCTGG TCCCTGCCGG GAAGCCCTTC TACTGCCTGA 101 GCTCGGAGGA CCTGCGCAAC ACCCACTGCT GCTACACTGA CTACTGCAAC 151 AGGATCGACT TGAGGGTGCC CAGTGGTCAC CTCAAGGAGC CTGAGCACCC 2 01 GTCCATGTGG GGCCCGGTGG AGCTGGTAGG (SEQ ID NO: 424) An alternative isoform of human ALK4 precursor polypeptide sequence, isoform C (NCBI Ref SeqNP_064733.3), is as follows: 1 MAESAGASSF FPLWLLLAG SGGSGPRGVQ ALLCACTSCL QANYTCETDG ACMVSIFNLD 61 GMEHHVRTCI PKVELVPAGK PFYCLSSEDL RNTHCCYTDY CNRIDLRVPS GHLKEPEHPS 121 MWGPVELVGI IAGPVFLLFL IIIIVFLVIN YHQRVYHNRQ RLDMEDPSCE MCLSKDKTLQ 181 DLVYDLSTSG SGSGLPLFVQ RTVARTIVLQ EIIGKGRFGE VWRGRWRGGD VAVKIFSSRE 241 ERSWFREAEI YQTVMLRHEN ILGFIAADNK ADCSFLTLPW EVVMVSAAPK LRSLRLQYKG 301 GRGRARFLFP LNNGTWTQLW LVSDYHEHGS LFDYLNRYTV TIEGMIKLAL SAASGLAHLH 361 MEIVGTQGKP GIAHRDLKSK NILVKKNGMC AIADLGLAVR HDAVTDTIDI APNQRVGTKR 421 YMAPEVLDET INMKHFDSFK CADIYALGLV YWEIARRCNS GGVHEEYQLP YYDLVPSDPS 481 IEEMRKWCD QKLRPNIPNW WQSYEALRVM GKMMRECWYA NGAARLTALR IKKTLSQLSV 541 QEDVKI (SEQ ID NO: 85) The signal peptide is indicated by a single underline and the extracellular domain is indicated in bold font. A processed extracellular ALK4 polypeptide sequence (isoform C) is as follows: SGPRGVQALLCACTSCLQANYTCETDGACMVSIFNLDGMEHHVRTCIPKVELVPAG KPFYCLSSEDLRNTHCCYTDYCNRIDLRVPSGHLKEPEHPSMWGPVE (SEQ ID NO: 87) A nucleic acid sequence encoding an ALK4 precursor polypeptide (isoform C) is shown in SEQ ID NO: 223, corresponding to nucleotides 78-1715 of GenBank Reference Sequence NM_020328.3. ATGGCGGAGTCGGCCGGAGCCTCCTCCTTCTTCCCCCTTGTTGTCCTCCTGCTCGC CGGCAGCGGCGGGTCCGGGCCCCGGGGGGTCCAGGCTCTGCTGTGTGCGTG CACCAGCTGCCTCCAGGCCAACTACACGTGTGAGACAGATGGGGCCTGCAT GGTTTCCATTTTCAATCTGGATGGGATGGAGCACCATGTGCGCACCTGCATC CCCAAAGTGGAGCTGGTCCCTGCCGGGAAGCCCTTCTACTGCCTGAGCTCG GAGGACCTGCGCAACACCCACTGCTGCTACACTGACTACTGCAACAGGATC GACTTGAGGGTGCCCAGTGGTCACCTCAAGGAGCCTGAGCACCCGTCCATG TGGGGCCCGGTGGAGCTGGTAGGCATCATCGCCGGCCCGGTGTTCCTCCTGTTC CTCATCATCATCATTGTTTTCCTTGTCATTAACTATCATCAGCGTGTCTATCACAA CCGCCAGAGACTGGACATGGAAGATCCCTCATGTGAGATGTGTCTCTCCAAAGA CAAGACGCTCCAGGATCTTGTCTACGATCTCTCCACCTCAGGGTCTGGCTCAGGG TTACCCCTCTTTGTCCAGCGCACAGTGGCCCGAACCATCGTTTTACAAGAGATTA TTGGCAAGGGTCGGTTTGGGGAAGTATGGCGGGGCCGCTGGAGGGGTGGTGATG TGGCTGTGAAAATATTCTCTTCTCGTGAAGAACGGTCTTGGTTCAGGGAAGCAGA GATATACCAGACGGTCATGCTGCGCCATGAAAACATCCTTGGATTTATTGCTGCT GACAATAAAGCAGACTGCTCATTCCTCACATTGCCATGGGAAGTTGTAATGGTCT CTGCTGCCCCCAAGCTGAGGAGCCTTAGACTCCAATACAAGGGAGGAAGGGGAA GAGCAAGATTTTTATTCCCACTGAATAATGGCACCTGGACACAGCTGTGGCTTGT TTCTGACTATCATGAGCACGGGTCCCTGTTTGATTATCTGAACCGGTACACAGTG ACAATTGAGGGGATGATTAAGCTGGCCTTGTCTGCTGCTAGTGGGCTGGCACACC TGCACATGGAGATCGTGGGCACCCAAGGGAAGCCTGGAATTGCTCATCGAGACT TAAAGTCAAAGAACATTCTGGTGAAGAAAAATGGCATGTGTGCCATAGCAGACC TGGGCCTGGCTGTCCGTCATGATGCAGTCACTGACACCATTGACATTGCCCCGAA TCAGAGGGTGGGGACCAAACGATACATGGCCCCTGAAGTACTTGATGAAACCAT TAATATGAAACACTTTGACTCCTTTAAATGTGCTGATATTTATGCCCTCGGGCTTG TATATTGGGAGATTGCTCGAAGATGCAATTCTGGAGGAGTCCATGAAGAATATC AGCTGCCATATTACGACTTAGTGCCCTCTGACCCTTCCATTGAGGAAATGCGAAA GGTTGTATGTGATCAGAAGCTGCGTCCCAACATCCCCAACTGGTGGCAGAGTTAT GAGGCACTGCGGGTGATGGGGAAGATGATGCGAGAGTGTTGGTATGCCAACGGC GCAGCCCGCCTGACGGCCCTGCGCATCAAGAAGACCCTCTCCCAGCTCAGCGTG CAGGAAGACGTGAAGATC (SEQ ID NO: 223) A nucleic acid sequence encoding the extracellular ALK4 polypeptide (isoform C) is shown in SEQ ID NO: 224. TCCGGGCCCCGGGGGGTCCAGGCTCTGCTGTGTGCGTGCACCAGCTGCCTCCAGG CCAACTACACGTGTGAGACAGATGGGGCCTGCATGGTTTCCATTTTCAATCTGGA TGGGATGGAGCACCATGTGCGCACCTGCATCCCCAAAGTGGAGCTGGTCCCTGC CGGGAAGCCCTTCTACTGCCTGAGCTCGGAGGACCTGCGCAACACCCACTGCTGC TACACTGACTACTGCAACAGGATCGACTTGAGGGTGCCCAGTGGTCACCTCAAG GAGCCTGAGCACCCGTCCATGTGGGGCCCGGTGGAG (SEQ ID NO: 224) ALK4 is well-conserved among vertebrates, with large stretches of the extracellular domain completely conserved. For example, Figure 9 depicts a multi-sequence alignment of a human ALK4 extracellular domain compared to various ALK4 orthologs. Many of the ligands that bind to ALK4 are also highly conserved. Accordingly, from these alignments, it is possible to predict key amino acid positions within the ligand-binding domain that are important for normal ALK4-ligand binding activities as well as to predict amino acid positions that are likely to be tolerant to substitution without significantly altering normal ALK4-ligand binding activities. Therefore, an active, human ALK4 variant polypeptide useful in accordance with the presently disclosed methods may include one or more amino acids at corresponding positions from the sequence of another vertebrate ALK4, or may include a residue that is similar to that in the human or other vertebrate sequences. Without meaning to be limiting, the following examples illustrate this approach to defining an active ALK4 variant. As illustrated in Figure 9, V6 in the human ALK4 extracellular domain (SEQ ID NO: 414) is isoleucine in Mus muculus ALK4 (SEQ ID NO: 418), and so the position may be altered, and optionally may be altered to another hydrophobic residue such as L, I, or F, or a non-polar residue such as A, as is observed in Gallus gallus ALK4 (SEQ ID NO: 417). E40 in the human extracellular domain is K in Gallus gallus ALK4, indicating that this site may be tolerant of a wide variety of changes, including polar residues, such as E, D, K, R, H, S, T, P, G, Y, and probably a non-polar residue such as A. S15 in the human extracellular domain is D in Gallus gallus ALK4, indicating that a wide structural variation is tolerated at this position, with polar residues favored, such as S, T, R, E, K, H, G, P, G and Y. E40 in the human extracellular domain is K in Gallus gallus ALK4, indicating that charged residues will be tolerated at this position, including D, R, K, H, as well as Q and N. R80 in the human extracellular domain is K in Condylura cristata ALK4 (SEQ ID NO: 415), indicating that basic residues are tolerated at this position, including R, K, and H. Y77 in the human extracellular domain is F in Sus scrofa ALK4 (SEQ ID NO: 419), indicating that aromatic residues are tolerated at this position, including F, W, and Y. P93 in the human extracellular domain is relatively poorly conserved, appearing as S in Erinaceus europaeus ALK4 (SEQ ID NO: 416) and N in Gallus gallus ALK4, thus essentially any amino acid should be tolerated at this position. Moreover, ALK4 proteins have been characterized in the art in terms of structural and functional characteristics, particularly with respect to ligand binding [e.g., Harrison et al. (2003) J Biol Chern 278(23):21129-21135; Romano et al. (2012) J Mol Model 18(8):3617-3625; and Calvanese et al. (2009) 15(3):175-183]. In addition to the teachings herein, these references provide ample guidance for how to generate ALK4 variants that retain one or more normal activities (e.g., ligand-binding activity). For example, a defining structural motif known as a three-finger toxin fold is important for ligand binding by type I and type II receptors and is formed by conserved cysteine residues located at varying positions within the extracellular domain of each monomeric receptor [Greenwald et al. (1999) Nat Struct Biol 6:18-22; and Hinck (2012) FEBS Lett 586:1860-1870]. Accordingly, the core ligand-binding domains of human ALK4, as demarcated by the outermost of these conserved cysteines, corresponds to positions 34-101 of SEQ ID NO: 84 (ALK4 precursor). The structurally less-ordered amino acids flanking these cysteine-demarcated core sequences can be truncated by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 residues at the N-terminus and / or by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25 residues at the C-terminus without necessarily altering ligand binding. Exemplary ALK4 extracellular domains for N-terminal and / or C-terminal truncation include SEQ ID NOs: 86, 87, and 422. In certain embodiments, the disclosure relates to heteromultimers that comprise at least one ALK4 polypeptide, which includes fragments, functional variants, and modified forms thereof. Preferably, ALK4 polypeptides for use as disclosed herein (e.g., heteromultimers comprising an ALK4 polypeptide and uses thereof) are soluble (e.g., an extracellular domain of ALK4). In other embodiments, ALK4 polypeptides for use as disclosed herein bind to and / or inhibit (antagonize) activity (e.g., induction of Smad signaling) of one or more TGF-beta superfamily ligands. In some embodiments, heteromultimers of the disclosure comprise at least one ALK4 polypeptide that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 84, 85, 86, 87, 88, 89, 92, 93, 421,and 422. In some embodiments, heteromultimers of the disclosure consist or consist essentially of at least one ALK4 polypeptide that is at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of SEQ ID NO: 84, 85, 86, 87, 88, 89, 92, 93, 422. In certain aspects, the disclosure relates to a heteromultimer that comprises an ALK4-Fc fusion polypeptide. In some embodiments, the ALK4-Fc fusion polypeptide comprises an ALK4 domain comprising an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to an amino acid sequence that begins at any one of amino acids 23-34 (e.g., amino acid residues 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34) SEQ ID NO: 84, 85, or 421 and ends at any one of amino acids 101-126 (e.g., amino acid residues 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, and 126) of SEQ ID NO: 84, 85, or 421. In some embodiments, the ALK4-Fc fusion polypeptide comprises an ALK4 domain comprising an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 34-101 of SEQ ID NOs: 84, 85, or 421. In some embodiments, the ALK4-Fc fusion polypeptide comprises an ALK4 domain comprising an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to amino acids 23-126 of SEQ ID Nos: 84, 85, or 421. In some embodiments, the ALK4-Fc fusion polypeptide comprises an ALK4 domain comprising an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to the amino acid sequence of any one of SEQ ID Nos: 84, 85, 86, 87, 88, 89, 92, 93, 247, 249, 421, 422. E) ALK7 Polypeptides In certain aspects, the disclosure relates to ActRII-ALK4 antagonists comprising an ALK7 polypeptide, which includes fragments, functional variants, and modified forms thereof as well as uses thereof (e.g., of treating, preventing, or reducing the progression rate and / or severity of heart failure (HF) or one or more complications of HF). As used herein, the term “ALK7” refers to a family of activin receptor-like kinase-7 (ALK7) proteins from any species and variant polypeptides derived from such ALK7 proteins by mutagenesis or other modifications (including, e.g., mutants, fragments, fusions, and peptidomimetic forms) that retain a useful activity. Examples of such variant ALK7 polypeptides are provided throughout the present disclosure as well as in International Patent Application Publication Nos. WO / 2016 / 164089 and WO / 2016 / 164503, which are incorporated herein by reference in their entirety. Reference to ALK7 herein is understood to be a reference to any one of the currently identified forms. Members of the ALK7 family are generally all transmembrane polypeptides, composed of a ligand-binding extracellular domain with cysteine-rich region, a transmembrane domain, and a cytoplasmic domain with predicted serine / threonine kinase specificity. The amino acid sequence of a human ALK7 precursor polypeptide is shown in (SEQ ID NO: 120) below. Preferably, ALK7 polypeptides to be used in accordance with the methods of the disclosure are soluble. The term “soluble ALK7 polypeptide,” as used herein, includes any naturally occurring extracellular domain of an ALK7 polypeptide as well as any variants thereof (including mutants, fragments and peptidomimetic forms) that retain a useful activity. For example, the extracellular domain of an ALK7 polypeptide binds to a ligand and is generally soluble. Examples of soluble ALK7 polypeptides include an ALK7 extracellular domain (SEQ ID NO: 123) below. Other examples of soluble ALK7 polypeptides comprise a signal sequence in addition to the extracellular domain of an ALK7 polypeptide. The signal sequence can be a native signal sequence of an ALK7, or a signal sequence from another polypeptide, such as a tissue plasminogen activator (TPA) signal sequence or a honey bee melittin signal sequence. In some embodiments, ALK7 polypeptides inhibit activity (e.g., Smad signaling) of one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP 10). In some embodiments, ALK7 polypeptides bind to one or more ActRII-ALK4 ligands (e.g., activin A, activin B, GDF8, GDF11, BMP6, BMP10). Various examples of methods and assays for determining the ability of an ALK7 polypeptide to bind to and / or inhibit activity of one or more ActRII-ALK4 ligands are disclosed herein or otherwise well known in the art, which can be readily used to determine if an ALK7 polypeptide has the desired binding and / or antagonistic activities. Numbering of amino acids for all ALK7-related polypeptides described herein is based on the numbering of the human ALK7 precursor protein sequence provided below (SEQ ID NO: 120), unless specifically designated otherwise. Four naturally occurring isoforms of human ALK7 have been described. The sequence of human ALK7 isoform 1 precursor polypeptide (NCBI Ref SeqNP_660302.2) is as follows: 1 MTRALCSALR QALLLLAAAA ELSPGLKCVC LLCDSSNFTC QTEGACWASV MLTNGKEQVI 61 KSCVSLPELN AQVFCHSSNN VTKTECCFTD FCNNITLHLP TASPNAPKLG PMELAIIITV 121 PVCLLSIAAM LTVWACQGRQ CSYRKKKRPN VEEPLSECNL VNAGKTLKDL IYDVTASGSG 181 SGLPLLVQRT IARTIVLQEI VGKGRFGEVW HGRWCGEDVA VKIFSSRDER SWFREAEIYQ 241 TVMLRHENIL GFIAADNKDN GTWTQLWLVS EYHEQGSLYD YLNRNIVTVA GMIKLALSIA 301 SGLAHLHMEI VGTQGKPAIA HRDIKSKNIL VKKCETCAIA DLGLAVKHDS ILNTIDIPQN 361 PKVGTKRYMA PEMLDDTMNV NIFESFKRAD IYSVGLVYWE IARRCSVGGI VEEYQLPYYD 421 MVPSDPSIEE MRKWCDQKF RPSIPNQWQS CEALRVMGRI MRECWYANGA ARLTALRIKK 481 TISQLCVKED CKA (SEQ ID NO: 120) The signal peptide is indicated by a single underline and the extracellular domain is indicated in bold font. A processed extracellular ALK7 isoform 1 polypeptide sequence is as follows: ELSPGLKCVCLLCDSSNFTCQTEGACWASVMLTNGKEQVIKSCVSLPELNAQVFCHS SNNVTKTECCFTDFCNNITLHLPTASPNAPKLGPME (SEQ ID NO: 123) A nucleic acid sequence encoding human ALK7 isoform 1 precursor polypeptide is shown below in SEQ ID NO: 233, corresponding to nucleotides 244-1722 of GenBank Reference Sequence NM_145259.2. ATGACCCGGGCGCTCTGCTCAGCGCTCCGCCAGGCTCTCCTGCTGCTCGCAGCGG CCGCCGAGCTCTCGCCAGGACTGAAGTGTGTATGTCTTTTGTGTGATTCTTC AAACTTTACCTGCCAAACAGAAGGAGCATGTTGGGCATCAGTCATGCTAACC AATGGAAAAGAGCAGGTGATCAAATCCTGTGTCTCCCTTCCAGAACTGAATG CTCAAGTCTTCTGTCATAGTTCCAACAATGTTACCAAAACCGAATGCTGCTT CACAGATTTTTGCAACAACATAACACTGCACCTTCCAACAGCATCACCAAAT GCCCCAAAACTTGGACCCATGGAGCTGGCCATCATTATTACTGTGCCTGTTTGC CTCCTGTCCATAGCTGCGATGCTGACAGTATGGGCATGCCAGGGTCGACAGTGCT CCTACAGGAAGAAAAAGAGACCAAATGTGGAGGAACCACTCTCTGAGTGCAATC TGGTAAATGCTGGAAAAACTCTGAAAGATCTGATTTATGATGTGACCGCCTCTGG ATCTGGCTCTGGTCTACCTCTGTTGGTTCAAAGGACAATTGCAAGGACGATTGTG CTTCAGGAAATAGTAGGAAAAGGTAGATTTGGTGAGGTGTGGCATGGAAGATGG TGTGGGGAAGATGTGGCTGTGAAAATATTCTCCTCCAGAGATGAAAGATCTTGGT TTCGTGAGGCAGAAATTTACCAGACGGTCATGCTGCGACATGAAAACATCCTTG GTTTCATTGCTGCTGACAACAAAGATAATGGAACTTGGACTCAACTTTGGCTGGT ATCTGAATATCATGAACAGGGCTCCTTATATGACTATTTGAATAGAAATATAGTG ACCGTGGCTGGAATGATCAAGCTGGCGCTCTCAATTGCTAGTGGTCTGGCACACC TTCATATGGAGATTGTTGGTACACAAGGTAAACCTGCTATTGCTCATCGAGACAT AAAATCAAAGAATATCTTAGTGAAAAAGTGTGAAACTTGTGCCATAGCGGACTT AGGGTTGGCTGTGAAGCATGATTCAATACTGAACACTATCGACATACCTCAGAAT CCTAAAGTGGGAACCAAGAGGTATATGGCTCCTGAAATGCTTGATGATACAATG AATGTGAATATCTTTGAGTCCTTCAAACGAGCTGACATCTATTCTGTTGGTCTGGT TTACTGGGAAATAGCCCGGAGGTGTTCAGTCGGAGGAATTGTTGAGGAGTACCA ATTGCCTTATTATGACATGGTGCCTTCAGATCCCTCGATAGAGGAAATGAGAAAG GTTGTTTGTGACCAGAAGTTTCGACCAAGTATCCCAAACCAGTGGCAAAGTTGTG AAGCACTCCGAGTCATGGGGAGAATAATGCGTGAGTGTTGGTATGCCAACGGAG CGGCCCGCCTAACTGCTCTTCGTATTAAGAAGACTATATCTCAACTTTGTGTCAA AGAAGACTGCAAAGCC (SEQ ID NO: 233) A nucleic acid sequence encoding the processed extracellular ALK7 polypeptide (isoform 1) is show in SEQ ID NO: 234. GAGCTCTCGCCAGGACTGAAGTGTGTATGTCTTTTGTGTGATTCTTCAAACTTTAC CTGCCAAACAGAAGGAGCATGTTGGGCATCAGTCATGCTAACCAATGGAAAAGA GCAGGTGATCAAATCCTGTGTCTCCCTTCCAGAACTGAATGCTCAAGTCTTCTGT CATAGTTCCAACAATGTTACCAAAACCGAATGCTGCTTCACAGATTTTTGCAACA ACATAACACTGCACCTTCCAACAGCATCACCAAATGCCCCAAAACTTGGACCCAT GGAG (SEQ ID NO: 234) An amino acid sequence of an alternative isoform of human ALK7, isoform 2 (NCBI Ref SeqNP_001104501.1), is shown in its processed form as follows (SEQ ID NO: 124), where the extracellular domain is indicated in bold font. 1 MLTNGKEQVI KSCVSLPELN AQVFCHSSNN VTKTECCFTD FCNNITLHLP TASPNAPKLG 61 PMELAIIITV PVCLLSIAAM LTVWACQGRQ CSYRKKKRPN VEEPLSECNL VNAGKTLKDL 121 IYDVTASGSG SGLPLLVQRT IARTIVLQEI VGKGRFGEVW HGRWCGEDVA VKIFSSRDER 181 SWFREAEIYQ TVMLRHENIL GFIAADNKDN GTWTQLWLVS EYHEQGSLYD YLNRNIVTVA 241 GMIKLALSIA SGLAHLHMEI VGTQGKPAIA HRDIKSKNIL VKKCETCAIA DLGLAVKHDS 301 ILNTIDIPQN PKVGTKRYMA PEMLDDTMNV NIFESFKRAD IYSVGLVYWE IARRCSVGGI 361 VEEYQLPYYD MVPSDPSIEE MRKVVCDQKF RPSIPNQWQS CEALRVMGRI MRECWYANGA 421 ARLTALRIKK TISQLCVKED CKA (SEQ ID NO: 124) An amino acid sequence of the extracellular ALK7 polypeptide (isoform 2) is as follows: MLTNGKEQVIKSCVSLPELNAQVFCHSSNNVTKTECCFTDFCNNITLHLPTASPNAPK LGPME (SEQ ID NO: 125). A nucleic acid sequence encoding the processed ALK7 polypeptide (isoform 2) is shown below in SEQ ID NO: 235, corresponding to nucleotides 279-1607 of NCBI Reference Sequence NM_001111031.1. ATGCTAACCAATGGAAAAGAGCAGGTGATCAAATCCTGTGTCTCCCTTCCAG AACTGAATGCTCAAGTCTTCTGTCATAGTTCCAACAATGTTACCAAAACCGA ATGCTGCTTCACAGATTTTTGCAACAACATAACACTGCACCTTCCAACAGCA TCACCAAATGCCCCAAAACTTGGACCCATGGAGCTGGCCATCATTATTACTGT GCCTGTTTGCCTCCTGTCCATAGCTGCGATGCTGACAGTATGGGCATGCCAGGGT CGACAGTGCTCCTACAGGAAGAAAAAGAGACCAAATGTGGAGGAACCACTCTCT GAGTGCAATCTGGTAAATGCTGGAAAAACTCTGAAAGATCTGATTTATGATGTGA CCGCCTCTGGATCTGGCTCTGGTCTACCTCTGTTGGTTCAAAGGACAATTGCAAG GACGATTGTGCTTCAGGAAATAGTAGGAAAAGGTAGATTTGGTGAGGTGTGGCA TGGAAGATGGTGTGGGGAAGATGTGGCTGTGAAAATATTCTCCTCCAGAGATGA AAGATCTTGGTTTCGTGAGGCAGAAATTTACCAGACGGTCATGCTGCGACATGA AAACATCCTTGGTTTCATTGCTGCTGACAACAAAGATAATGGAACTTGGACTCAA CTTTGGCTGGTATCTGAATATCATGAACAGGGCTCCTTATATGACTATTTGAATA GAAATATAGTGACCGTGGCTGGAATGATCAAGCTGGCGCTCTCAATTGCTAGTG GTCTGGCACACCTTCATATGGAGATTGTTGGTACACAAGGTAAACCTGCTATTGC TCATCGAGACATAAAATCAAAGAATATCTTAGTGAAAAAGTGTGAAACTTGTGC CATAGCGGACTTAGGGTTGGCTGTGAAGCATGATTCAATACTGAACACTATCGAC ATACCTCAGAATCCTAAAGTGGGAACCAAGAGGTATATGGCTCCTGAAATGCTT GATGATACAATGAATGTGAATATCTTTGAGTCCTTCAAACGAGCTGACATCTATT CTGTTGGTCTGGTTTACTGGGAAATAGCCCGGAGGTGTTCAGTCGGAGGAATTGT TGAGGAGTACCAATTGCCTTATTATGACATGGTGCCTTCAGATCCCTCGATAGAG GAAATGAGAAAGGTTGTTTGTGACCAGAAGTTTCGACCAAGTATCCCAAACCAG TGGCAAAGTTGTGAAGCACTCCGAGTCATGGGGAGAATAATGCGTGAGTGTTGG TATGCCAACGGAGCGGCCCGCCTAACTGCTCTTCGTATTAAGAAGACTATATCTC AACTTTGTGTCAAAGAAGACTGCAAAGCC (SEQ ID NO: 235) A nucleic acid sequence encoding an extracellular ALK7 polypeptide (isoform 2) is shown in SEQ ID NO: 236. ATGCTAACCAATGGAAAAGAGCAGGTGATCAAATCCTGTGTCTCCCTTCCAGAA CTGAATGCTCAAGTCTTCTGTCATAGTTCCAACAATGTTACCAAAACCGAATGCT GCTTCACAGATTTTTGCAACAACATAACACTGCACCTTCCAACAGCATCACCAAA TGCCCCAAAACTTGGACCCATGGAG (SEQ ID NO: 236) An amino acid sequence of an alternative human ALK7 precursor polypeptide, isoform 3 (NCBI Ref Seq NP_001104502.1), is shown as follows (SEQ ID NO: 121), where the signal peptide is indicated by a single underline. 1 MTRALCSALR QALLLLAAAA ELSPGLKCVC LLCDSSNFTC QTEGACWASV MLTNGKEQVI 61 KSCVSLPELN AQVFCHSSNN VTKTECCFTD FCNNITLHLP TGLPLLVQRT IARTIVLQEI 121 VGKGRFGEVW HGRWCGEDVA VKIFSSRDER SWFREAEIYQ TVMLRHENIL GFIAADNKDN 181 GTWTQLWLVS EYHEQGSLYD YLNRNIVTVA GMIKLALSIA SGLAHLHMEI VGTQGKPAIA 241 HRDIKSKNIL VKKCETCAIA DLGLAVKHDS ILNTIDIPQN PKVGTKRYMA PEMLDDTMNV 301 NIFESFKRAD IYSVGLVYWE IARRCSVGGI VEEYQLPYYD MVPSDPSIEE MRKVVCDQKF 361 RPSIPNQWQS CEALRVMGRI MRECWYANGA ARLTALRIKK TISQLCVKED CKA (SEQ ID NO: 121) The amino acid sequence of a processed ALK7 polypeptide (isoform 3) is as follows (SEQ ID NO: 126). This isoform lacks a transmembrane domain and is therefore proposed to be soluble in its entirety (Roberts et al., 2003, Biol Reprod 68:1719-1726). N-terminal variants of SEQ ID NO: 126 are predicted as described below. 1 ELSPGLKCVC LLCDSSNFTC QTEGACWASV MLTNGKEQVI KSCVSLPELN AQVFCHSSNN 61 VTKTECCFTD FCNNITLHLP TGLPLLVQRT IARTIVLQEI VGKGRFGEVW HGRWCGEDVA 121 VKIFSSRDER SWFREAEIYQ TVMLRHENIL GFIAADNKDN GTWTQLWLVS EYHEQGSLYD 181 YLNRNIVTVA GMIKLALSIA SGLAHLHMEI VGTQGKPAIA HRDIKSKNIL VKKCETCAIA 241 DLGLAVKHDS ILNTIDIPQN PKVGTKRYMA PEMLDDTMNV NIFESFKRAD IYSVGLVYWE 301 IARRCSVGGI VEEYQLPYYD MVPSDPSIEE MRKVVCDQKF RPSIPNQWQS CEALRVMGRI 361 MRECWYANGA ARLTALRIKK TISQLCVKED CKA (SEQ ID NO: 126) A nucleic acid sequence encoding an unprocessed ALK7 polypeptide precursor polypeptide (isoform 3) is shown in SEQ ID NO: 237, corresponding to nucleotides 244-1482 of NCBI Reference Sequence NM_001111032.1. ATGACCCGGGCGCTCTGCTCAGCGCTCCGCCAGGCTCTCCTGCTGCTCGCAGCGG CCGCCGAGCTCTCGCCAGGACTGAAGTGTGTATGTCTTTTGTGTGATTCTTCAAA CTTTACCTGCCAAACAGAAGGAGCATGTTGGGCATCAGTCATGCTAACCAATGG AAAAGAGCAGGTGATCAAATCCTGTGTCTCCCTTCCAGAACTGAATGCTCAAGTC TTCTGTCATAGTTCCAACAATGTTACCAAAACCGAATGCTGCTTCACAGATTTTT GCAACAACATAACACTGCACCTTCCAACAGGTCTACCTCTGTTGGTTCAAAGGAC AATTGCAAGGACGATTGTGCTTCAGGAAATAGTAGGAAAAGGTAGATTTGGTGA GGTGTGGCATGGAAGATGGTGTGGGGAAGATGTGGCTGTGAAAATATTCTCCTC CAGAGATGAAAGATCTTGGTTTCGTGAGGCAGAAATTTACCAGACGGTCATGCT GCGACATGAAAACATCCTTGGTTTCATTGCTGCTGACAACAAAGATAATGGAACT TGGACTCAACTTTGGCTGGTATCTGAATATCATGAACAGGGCTCCTTATATGACT ATTTGAATAGAAATATAGTGACCGTGGCTGGAATGATCAAGCTGGCGCTCTCAAT TGCTAGTGGTCTGGCACACCTTCATATGGAGATTGTTGGTACACAAGGTAAACCT GCTATTGCTCATCGAGACATAAAATCAAAGAATATCTTAGTGAAAAAGTGTGAA ACTTGTGCCATAGCGGACTTAGGGTTGGCTGTGAAGCATGATTCAATACTGAACA CTATCGACATACCTCAGAATCCTAAAGTGGGAACCAAGAGGTATATGGCTCCTG AAATGCTTGATGATACAATGAATGTGAATATCTTTGAGTCCTTCAAACGAGCTGA CATCTATTCTGTTGGTCTGGTTTACTGGGAAATAGCCCGGAGGTGTTCAGTCGGA GGAATTGTTGAGGAGTACCAATTGCCTTATTATGACATGGTGCCTTCAGATCCCT CGATAGAGGAAATGAGAAAGGTTGTTTGTGACCAGAAGTTTCGACCAAGTATCC CAAACCAGTGGCAAAGTTGTGAAGCACTCCGAGTCATGGGGAGAATAATGCGTG AGTGTTGGTATGCCAACGGAGCGGCCCGCCTAACTGCTCTTCGTATTAAGAAGAC TATATCTCAACTTTGTGTCAAAGAAGACTGCAAAGCC (SEQ ID NO: 237) A nucleic acid sequence encoding a processed ALK7 polypeptide (isoform 3) is shown in SEQ ID NO: 238. GAGCTCTCGCCAGGACTGAAGTGTGTATGTCTTTTGTGTGATTCTTCAAACTTTAC CTGCCAAACAGAAGGAGCATGTTGGGCATCAGTCATGCTAACCAATGGAAAAGA GCAGGTGATCAAATCCTGTGTCTCCCTTCCAGAACTGAATGCTCAAGTCTTCTGT CATAGTTCCAACAATGTTACCAAAACCGAATGCTGCTTCACAGATTTTTGCAACA ACATAACACTGCACCTTCCAACAGGTCTACCTCTGTTGGTTCAAAGGACAATTGC AAGGACGATTGTGCTTCAGGAAATAGTAGGAAAAGGTAGATTTGGTGAGGTGTG GCATGGAAGATGGTGTGGGGAAGATGTGGCTGTGAAAATATTCTCCTCCAGAGA TGAAAGATCTTGGTTTCGTGAGGCAGAAATTTACCAGACGGTCATGCTGCGACAT GAAAACATCCTTGGTTTCATTGCTGCTGACAACAAAGATAATGGAACTTGGACTC AACTTTGGCTGGTATCTGAATATCATGAACAGGGCTCCTTATATGACTATTTGAA TAGAAATATAGTGACCGTGGCTGGAATGATCAAGCTGGCGCTCTCAATTGCTAGT GGTCTGGCACACCTTCATATGGAGATTGTTGGTACACAAGGTAAACCTGCTATTG CTCATCGAGACATAAAATCAAAGAATATCTTAGTGAAAAAGTGTGAAACTTGTG CCATAGCGGACTTAGGGTTGGCTGTGAAGCATGATTCAATACTGAACACTATCGA CATACCTCAGAATCCTAAAGTGGGAACCAAGAGGTATATGGCTCCTGAAATGCT TGATGATACAATGAATGTGAATATCTTTGAGTCCTTCAAACGAGCTGACATCTAT TCTGTTGGTCTGGTTTACTGGGAAATAGCCCGGAGGTGTTCAGTCGGAGGAATTG TTGAGGAGTACCAATTGCCTTATTATGACATGGTGCCTTCAGATCCCTCGATAGA GGAAATGAGAAAGGTTGTTTGTGACCAGAAGTTTCGACCAAGTATCCCAAACCA GTGGCAAAGTTGTGAAGCACTCCGAGTCATGGGGAGAATAATGCGTGAGTGTTG GTATGCCAACGGAGCGGCCCGCCTAACTGCTCTTCGTATTAAGAAGACTATATCT CAACTTTGTGTCAAAGAAGACTGCAAAGCC (SEQ ID NO: 23 8) An amino acid sequence of an alternative human ALK7 precursor polypeptide, isoform 4 (NCBI Ref Seq NP_001104503.1), is shown as follows (SEQ ID NO: 122), where the signal peptide is indicated by a single underline. 1 MTRALCSALR QALLLLAAAA ELSPGLKCVC LLCDSSNFTC QTEGACWASV MLTNGKEQVI 61 KSCVSLPELN AQVFCHSSNN VTKTECCFTD FCNNITLHLP TDNGTWTQLW LVSEYHEQGS 121 LYDYLNRNIV TVAGMIKLAL SIASGLAHLH MEIVGTQGKP AIAHRDIKSK NILVKKCETC 181 AIADLGLAVK HDSILNTIDI PQNPKVGTKR YMAPEMLDDT MNVNIFESFK RADIYSVGLV 241 YWEIARRCSV GGIVEEYQLP YYDMVPSDPS IEEMRKWCD QKFRPSIPNQ WQSCEALRVM 301 GRIMRECWYA NGAARLTALR IKKTISQLCV KEDCKA (SEQ ID NO: 122) An amino acid sequence of a processed ALK7 polypeptide (isoform 4) is as follows (SEQ ID NO: 127). Like ALK7 isoform 3, isoform 4 lacks a transmembrane domain and is therefore proposed to be soluble in its entirety (Roberts et al., 2003, Biol Reprod 68:17191726). N-terminal variants of SEQ ID NO: 127 are predicted as described below. 1 ELSPGLKCVC LLCDSSNFTC QTEGACWASV MLTNGKEQVI KSCVSLPELN AQVFCHSSNN 61 VTKTECCFTD FCNNITLHLP TDNGTWTQLW LVSEYHEQGS LYDYLNRNIV TVAGMIKLAL 121 SIASGLAHLH MEIVGTQGKP AIAHRDIKSK NILVKKCETC AIADLGLAVK HDSILNTIDI 181 PQNPKVGTKR YMAPEMLDDT MNVNIFESFK RADIYSVGLV YWEIARRCSV GGIVEEYQLP 240 YYDMVPSDPS IEEMRKWCD QKFRPSIPNQ WQSCEALRVM GRIMRECWYA NGAARLTALR 301 IKKTISQLCV KEDCKA (SEQ ID NO: 127) A nucleic acid sequence encoding the unprocessed ALK7 polypeptide precursor polypeptide (isoform 4) is shown in SEQ ID NO: 239, corresponding to nucleotides 244-1244 of NCBI Reference Sequence NM_001111033.1. ATGACCCGGGCGCTCTGCTCAGCGCTCCGCCAGGCTCTCCTGCTGCTCGCAGCGG CCGCCGAGCTCTCGCCAGGACTGAAGTGTGTATGTCTTTTGTGTGATTCTTCAAA CTTTACCTGCCAAACAGAAGGAGCATGTTGGGCATCAGTCATGCTAACCAATGG AAAAGAGCAGGTGATCAAATCCTGTGTCTCCCTTCCAGAACTGAATGCTCAAGTC TTCTGTCATAGTTCCAACAATGTTACCAAAACCGAATGCTGCTTCACAGATTTTT GCAACAACATAACACTGCACCTTCCAACAGATAATGGAACTTGGACTCAACTTTG GCTGGTATCTGAATATCATGAACAGGGCTCCTTATATGACTATTTGAATAGAAAT ATAGTGACCGTGGCTGGAATGATCAAGCTGGCGCTCTCAATTGCTAGTGGTCTGG CACACCTTCATATGGAGATTGTTGGTACACAAGGTAAACCTGCTATTGCTCATCG AGACATAAAATCAAAGAATATCTTAGTGAAAAAGTGTGAAACTTGTGCCATAGC GGACTTAGGGTTGGCTGTGAAGCATGATTCAATACTGAACACTATCGACATACCT CAGAATCCTAAAGTGGGAACCAAGAGGTATATGGCTCCTGAAATGCTTGATGAT ACAATGAATGTGAATATCTTTGAGTCCTTCAAACGAGCTGACATCTATTCTGTTG GTCTGGTTTACTGGGAAATAGCCCGGAGGTGTTCAGTCGGAGGAATTGTTGAGG AGTACCAATTGCCTTATTATGACATGGTGCCTTCAGATCCCTCGATAGAGGAAAT GAGAAAGGTTGTTTGTGACCAGAAGTTTCGACCAAGTATCCCAAACCAGTGGCA AAGTTGTGAAGCACTCCGAGTCATGGGGAGAATAATGCGTGAGTGTTGGTATGC CAACGGAGCGGCCCGCCTAACTGCTCTTCGTATTAAGAAGACTATATCTCAACTT TGTGTCAAAGAAGACTGCAAAGCCTAA (SEQ ID NO: 239) A nucleic acid sequence encoding the processed ALK7 polypeptide (isoform 4) is shown in SEQ ID NO: 240. GAGCTCTCGCCAGGACTGAAGTGTGTATGTCTTTTGTGTGATTCTTCAAACTTTAC CTGCCAAACAGAAGGAGCATGTTGGGCATCAGTCATGCTAACCAATGGAAAAGA GCAGGTGATCAAATCCTGTGTCTCCCTTCCAGAACTGAATGCTCAAGTCTTCTGT CATAGTTCCAACAATGTTACCAAAACCGAATGCTGCTTCACAGATTTTTGCAACA ACATAACACTGCACCTTCCAACAGATAATGGAACTTGGACTCAACTTTGGCTGGT ATCTGAATATCATGAACAGGGCTCCTTATATGACTATTTGAATAGAAATATAGTG ACCGTGGCTGGAATGATCAAGCTGGCGCTCTCAATTGCTAGTGGTCTGGCACACC TTCATATGGAGATTGTTGGTACACAAGGTAAACCTGCTATTGCTCATCGAGACAT AAAATCAAAGAATATCTTAGTGAAAAAGTGTGAAACTTGTGCCATAGCGGACTT AGGGTTGGCTGTGAAGCATGATTCAATACTGAACACTATCGACATACCTCAGAAT CCTAAAGTGGGAACCAAGAGGTATATGGCTCCTGAAATGCTTGATGATACAATG AATGTGAATATCTTTGAGTCCTTCAAACGAGCTGACATCTATTCTGTTGGTCTGGT TTACTGGGAAATAGCCCGGAGGTGTTCAGTCGGAGGAATTGTTGAGGAGTACCA ATTGCCTTATTATGACATGGTGCCTTCAGATCCCTCGATAGAGGAAATGAGAAAG GTTGTTTGTGACCAGAAGTTTCGACCAAGTATCCCAAACCAGTGGCAAAGTTGTG AAGCACTCCGAGTCATGGGGAGAATAATGCGTGAGTGTTGGTATGCCAACGGAG CGGCCCGCCTAACTGCTCTTCGTATTAAGAAGACTATATCTCAACTTTGTGTCAA AGAAGACTGCAAAGCCTAA (SEQ ID NO: 240) Based on the signal sequence of full-length ALK7 (isoform 1) in the rat (see NCBI Reference Sequence NP_620790.1) and on the high degree of sequence identity between human and rat ALK7, it is predicted that a processed form of human ALK7 isoform 1 is as follows (SEQ ID NO: 128). 1 LKCVCLLCDS SNFTCQTEGA CWASVMLTNG KEQVIKSCVS LPELNAQVFC HSSNNVTKTE 61 CCFTDFCNNI TLHLPTASPN APKLGPME (SEQ ID NO: 128) Active variants of processed ALK7 isoform 1 are predicted in which SEQ ID NO: 123 is truncated by 1, 2, 3, 4, 5, 6, or 7 amino acids at the N-terminus and SEQ ID NO: 128 is truncated by 1 or 2 amino acids at the N-terminus. Consistent with SEQ ID NO: 128, it is further expected that leucine is the N-terminal amino acid in the processed forms of human ALK7 isoform 3 (SEQ ID NO: 126) and human ALK7 isoform 4 (SEQ ID NO: 127). In certain embodiments, the disclosure relates to heteromultimers that comprise at least one ALK7 polypeptide, which includes fragments, functional variants, and modified forms thereof. Preferably, ALK7 polypeptides for use in accordance with inventions of the disclosure (e.g., heteromultimers comprising an ALK7 polypeptide and uses thereof) are soluble (e.g., an extracellular domain of ALK7). In other embodiments, ALK7 polypeptides for use in accordance with the disclosure bind to one or more ActRII-ALK4 ligand. Therefore, in some embodiments, ALK7 polypeptides for use in accordance with the disclosure inhibit (antagonize) activity (e.g., induction of Smad signaling) of one or more ActRII-ALK4 ligands. In some embodiments, heteromultimers of the disclosure comprise at least one ALK7 polypeptide that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, or 99% identical to the amino acid sequence of SEQ ID NO:120, 123, 124, 125, 121, 126, 122, 127, 128, 129, 255, 133, and 134. In some embodiments, heteromultimers of the disclosure consist or consist essentially of at least one ALK7 polypeptide that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 97%, 98%, or 99% identical to the amino acid sequence of SEQ ID NO: 120, 123, 124, 125, 121, 126, 122, 127, 128, 129, 255, 133, and 134. ALK7 is well-conserved among vertebrates, with large stretches of the extracellular domain completely conserved. For example, Figure 22 depicts a multi-sequence alignment of a human ALK7 extracellular domain compared to various ALK7 orthologs. Accordingly, from these alignments, it is possible to predict key amino acid positions within the ligandbinding domain that are important for normal ALK7-ligand binding activities as well as to predict amino acid positions that are likely to be tolerant to substitution without significantly altering normal ALK7-ligand binding activities. Therefore, an active, human ALK7 variant polypeptide useful in accordance with the presently disclosed methods may include one or more amino acids at corresponding positions from the sequence of another vertebrate ALK7, or may include a residue that is similar to that in the human or other vertebrate sequences. Without meaning to be limiting, the following examples illustrate this approach to defining an active ALK7 variant. V61 in the human ALK7 extracellular domain (SEQ ID NO: 425) is isoleucine in Callithrix jacchus ALK7 (SEQ ID NO: 428), and so the position may be altered, and optionally may be altered to another hydrophobic residue such as L, I, or F, or a nonpolar residue such as A. L32 in the human extracellular domain is R in Tarsius syrichta (SEQ ID NO: 429) ALK7, indicating that this site may be tolerant of a wide variety of changes, including polar residues, such as E, D, K, R, H, S, T, P, G, Y, and probably a non-polar residue such as A. K37 in the human extracellular domain is R in Pan troglodytes ALK7 (SEQ ID NO: 426), indicating that basic residues are tolerated at this position, including R, K, and H. P4 in the human extracellular domain is relatively poorly conserved, appearing as A in Pan troglodytes ALK7 thus indicating that a wide variety of amino acid should be tolerated at this position. Moreover, ALK7 proteins have been characterized in the art in terms of structural and functional characteristics [e.g., Romano et al (2012) Journal of Molecular Modeling 18(8): 3617-3625]. For example, a defining structural motif known as a three-finger toxin fold is important for ligand binding by type I and type II receptors and is formed by conserved cysteine residues located at varying positions within the extracellular domain of each monomeric receptor [Greenwald et al. (1999) Nat Struct Biol 6:18-22; and Hinck (2012) FEBS Lett 586:1860-1870]. Accordingly, the core ligand-binding domains of human ALK7, as demarcated by the outermost of these conserved cysteines, corresponds to positions 28-92 of SEQ ID NO: 120. The structurally less-ordered amino acids flanking these cysteine-demarcated core sequences can be truncated by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, or 27 residues at the N-terminus and by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or 21 residues at the C-terminus without necessarily altering ligand binding. Exemplary ALK7 extracellular domains for N-terminal and / or C-terminal truncation include SEQ ID NOs: 123, 125, 126, and 127. Accordingly, a general formula for an active portion (e.g., a ligand-binding portion) of ALK7 comprises amino acids 28-92 of SEQ ID NO: 120. Therefore ALK7 polypeptides may, for example, comprise, consists essentially of, or consists of an amino acid sequence that is at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identical to a portion of ALK7 beginning at a residue corresponding to any one of amino acids 20-28 (e.g., beginning at any one of amino acids 20, 21, 22, 23, 24, 25, 26, 27, or 28) of SEQ ID NO: 120 and ending at a position corresponding to any one amino acids 92-113 (e.g., ending at any one of amino acids 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, or 113) of SEQ ID NO: 120. Other examples include constructs that begin at a position from 21-28 (e.g., any one of positions 21, 22, 23, 24, 25, 26, 27, or 28), 24-28 (e.g., any one of positions 24, 25, 26, 27, or 28), or 25-28 (e.g., any one of positions 25, 26, 27, or 28) of SEQ ID NO: 120 and end at a position from 93-112 (e.g., any one of positions 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, or 112), 93-110 (e.g., any one of positions 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, or 110), 93-100 (e.g., any one of positions 93, 94, 95, 96, 97, 98, 99, or 100), or 93-95 (e.g., any one of positions 93, 94, or 95) of SEQ ID NO: 120. Variants within these ranges are also contemplated, particularly those having at least 70%, 75%, 80%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity to the corresponding portion of SEQ ID NO: 120. The variations described herein may be combined in various ways. In some embodiments, ALK7 variants comprise no more than 1, 2, 5, 6, 7, 8, 9, 10 or 15 conservative amino acid changes in the ligand-binding pocket. Sites outside the binding pocket, at which variability may be particularly well tolerated, include the amino and carboxy termini of the extracellular domain (as noted above). F) Follistatin Polypeptides In other aspects, an ActRII-ALK4 antagonist is a follistatin polypeptide. As described herein, follistatin polypeptides may be used to treat, prevent, or reduce the progression rate and / or severity of heart failure associated with aging, particularly treating, preventing or reducing the progression rate and / or severity of one or more heart failure-associated complications. The term "follistatin polypeptide" includes polypeptides comprising any naturally occurring polypeptide of follistatin as well as any variants thereof (including mutants, fragments, fusions, and peptidomimetic forms) that retain a useful activity, and further includes any functional monomer or multimer of follistatin. In certain embodiments, follistatin polypeptides of the disclosure bind to and / or inhibit activin activity, particularly activin A. Variants of follistatin polypeptides that retain activin binding properties can be identified based on previous studies involving follistatin and activin interactions. For example, WO2008 / 030367 discloses specific follistatin domains ("FSDs") that are shown to be important for activin binding. As shown below in SEQ ID NOs: 392-394, the follistatin N-terminal domain ("FSND" SEQ ID NO: 392), FSD2 (SEQ ID NO: 394), and to a lesser extent FSD1 (SEQ ID NO: 393) represent exemplary domains within follistatin that are important for activin binding. In addition, methods for making and testing libraries of polypeptides are described above in the context of ActRII polypeptides, and such methods also pertain to making and testing variants of follistatin. Follistatin polypeptides include polypeptides derived from the sequence of any known follistatin having a sequence at least about 80% identical to the sequence of a follistatin polypeptide, and optionally at least 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or greater identity. Examples of follistatin polypeptides include the mature follistatin polypeptide or shorter isoforms or other variants of the human follistatin precursor polypeptide (SEQ ID NO: 390) as described, for example, in WO2005 / 025601. The human follistatin precursor polypeptide isoform FST344 is as follows: 1 MVRARHQPGG LCLLLLLLCQ FMEDRSAQAG NCWLRQAKNG RCQVLYKTEL 51 SKEECCSTGR LSTSWTEEDV NDNTLFKWMI FNGGAPNCIP CKETCENVDC 101 GPGKKCRMNK KNKPRCVCAP DCSNITWKGP VCGLDGKTYR NECALLKARC 151 KEQPELEVQY QGRCKKTCRD VFCPGSSTCV VDQTNNAYCV TCNRICPEPA 201 SSEQYLCGND GVTYSSACHL RKATCLLGRS IGLAYEGKCI KAKSCEDIQC 251 TGGKKCLWDF KVGRGRCSLC DELCPDSKSD EPVCASDNAT YASECAMKEA 301 ACSSGVLLEV KHSGSCNSIS EDTEEEEEDE DQDYSFPISS ILEW (SEQ ID NO: 390; NCBI Reference No. NP 037541.1) The signal peptide is underlined; also underlined above are the last 27 residues which represent the C-terminal extension distinguishing this follistatin isoform from the shorter follistatin isoform FST317 shown below. The human follistatin precursor polypeptide isoform FST317 is as follows: 1 MVRARHQPGG LCLLLLLLCQ FMEDRSAQAG NCWLRQAKNG RCQVLYKTEL 51 SKEECCSTGR LSTSWTEEDV NDNTLFKWMI FNGGAPNCIP CKETCENVDC 101 GPGKKCRMNK KNKPRCVCAP DCSNITWKGP VCGLDGKTYR NECALLKARC 151 KEQPELEVQY QGRCKKTCRD VFCPGSSTCV VDQTNNAYCV TCNRICPEPA 201 SSEQYLCGND GVTYSSACHL RKATCLLGRS IGLAYEGKCI KAKSCEDIQC 251 TGGKKCLWDF KVGRGRCSLC DELCPDSKSD EPVCASDNAT YASECAMKEA 301 ACSSGVLLEV KHSGSCN (SEQ ID NO: 391; NCBI Reference No. NP 006341.1) The signal peptide is underlined. The follistatin N-terminal domain (FSND) sequence is as follows: GNCWLRQAKNGRCQVLYKTELSKEECCSTGRLSTSWTEEDVNDN TLFKWMIFNGGAPNCIPCK (SEQ ID NO: 392; FSND) The FSD...

Claims

1. A method of treating heart failure associated with aging, comprising administering to a patient in need thereof an effective amount of an ActRII-ALK4 antagonist.

2. The method of claim 1, wherein the heart failure is heart failure associated with preserved ejection fraction (HFpEF).

3. The method of claim 1 or 2, wherein the patient has left ventricular (LV) hypertrophy.

4. The method of any one of claims 1-3, wherein the method decreases LV hypertrophy in the patient.

5. The method of any one of claims 1-4, wherein the method increases ventricular relaxation and decreases filling pressures in the patient.

6. The method of any one of claims 1-5, wherein the patient’s a ratio of early diastolic transmitral flow to early diastolic mitral annular tissue velocity (E / e' ratio) is increased in comparison to healthy people of similar age and sex.

7. The method of any one of claims 1-6, wherein the patient has a diastolic dysfunction.

8. The method of any one of claims 1-6, wherein the method improves the patient’sdiastolic dysfunction.

9. The method of any one of claims 1-8, wherein the patient has elevated brain natriuretic peptide (BNP) levels as compared to a healthy patient.

10. The method of claim 9, wherein the method decreases BNP levels in the patient.

11. The method of any one of claims 1-10, wherein the ActRII-ALK4 antagonistcomprises an ActRIIA polypeptide.

12. The method of claim 11, wherein the ActRIIA polypeptide comprises an amino acid sequence that is at least 70% identical to an amino acid sequence that begins at any one of amino acids 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30 of SEQ ID NO: 366 and ends at any one of amino acids 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, or 135 of SEQ ID NO: 366.

13. The method of claim 11, wherein the ActRIIA polypeptide comprises an amino acid sequence that is at least 70% identical to an amino acid sequence of SEQ ID NO: 367.

14. The method of claim 11, wherein the ActRIIA polypeptide comprises an amino acid sequence that is at least 70% identical to an amino acid sequence of SEQ ID NO: 368.

15. The method of any one of claims 1-14, wherein the ActRIIA polypeptide is a fusion polypeptide comprising an ActRIIA polypeptide domain and one or more heterologous domains.

16. The method of claim 15, wherein the fusion polypeptide is an ActRIIA-Fc fusion polypeptide.

17. The method of any one of claims 15 or 16, wherein the fusion polypeptide further comprises a linker domain positioned between the ActRIIA polypeptide domain and i) the one or more heterologous domains or ii) Fc domain.

18. The method of claim 17, wherein the linker domain is selected from: TGGG (SEQ ID NO: 265), TGGGG (SEQ ID NO: 263), SGGGG (SEQ ID NO: 264), GGGGS (SEQ ID NO: 267), GGG (SEQ ID NO: 261), GGGG (SEQ ID NO: 262), and SGGG (SEQ ID NO: 266).

19. The method of any one of claims 16-18, wherein the polypeptide comprises an amino acid sequence that is at least 90% identical to the amino acid sequence of SEQ ID NO: 380.

20. The method of any one of claims 16-19, wherein the polypeptide comprises the amino acid sequence of SEQ ID NO: 380.

21. The method of any one of claims 16-18, wherein the polypeptide comprises an amino acid sequence that is at least 90% identical to the amino acid sequence of SEQ ID NO: 378.

22. The method of any one of claims 16-18 and 21, wherein the polypeptide comprises the amino acid sequence of SEQ ID NO: 378.

23. The method of any one of claims 1-10, wherein the ActRII-ALK4 antagonist is a heteromultimer polypeptide.

24. The method of claim 23, wherein the heteromultimer polypeptide comprises an ActRIIB polypeptide and an ALK4 polypeptide.

25. The method of claim 23, wherein the heteromultimer polypeptide comprises an ActRIIB polypeptide and an ALK7 polypeptide.

26. The method of claim 24, wherein the ALK4 polypeptide comprises an amino acid sequence that is at least 75% identical to an amino acid sequence selected from the group consisting of SEQ ID NOs: 84, 85, 86, 87, 88, 89, 92, 93, 247, 249, 421, and 422.

27. The method of claim 25, wherein the ALK7 polypeptide comprises an amino acid sequence that is at least 75% identical to an amino acid sequence selected from the group consisting of SEQ ID NOs: 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 133, and 134.

28. The method of any one of claims 24 or 26, wherein the ALK4 polypeptide is a fusion polypeptide comprising an ALK4 polypeptide domain and one or more heterologous domains.

29. The method of any one of claims 25 or 27, wherein the ALK7 polypeptide is a fusion polypeptide comprising an ALK7 polypeptide domain and one or more heterologous domains.

30. The method of claim 28, wherein the fusion polypeptide is an ALK4-Fc fusion polypeptide.

31. The method of claim 29, wherein the fusion polypeptide is an ALK7-Fc fusionpolypeptide.

32. The method of claim 30, wherein the ALK4-Fc fusion polypeptide further comprises a linker domain positioned between the ALK4 polypeptide domain and i) the one or more heterologous domains or ii) Fc domain.

33. The method of claim 31, wherein the ALK7-Fc fusion polypeptide further comprises a linker domain positioned between the ALK7 polypeptide domain and the i) one or more heterologous domains or ii) Fc domain.

34. The method of claim 32 or 33, wherein the linker domain is selected from: TGGG (SEQ ID NO: 265), TGGGG (SEQ ID NO: 263), SGGGG (SEQ ID NO: 264), GGGGS (SEQ ID NO: 267), GGG (SEQ ID NO: 261), GGGG (SEQ ID NO: 262), and SGGG (SEQ ID NO: 266).

35. The method of any one of claims 1-10, wherein the ActRII-ALK4 antagonist comprises an ActRIIB polypeptide.

36. The method of any one of claims 23-34, wherein the ActRII-ALK4 heteromultimer comprises an ActRIIB polypeptide.

37. The method of claim 35 or 36, wherein the ActRIIB polypeptide comprises an amino acid sequence that is at least 75% identical to an amino acid sequence that begins at any one of amino acid residues 20, 21, 22, 23, 24, 25, 26, 27, 28, or 29 of SEQ ID NO: 2 and ends at anyone of amino acid residues 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, or 134 of SEQ ID NO: 2.

38. The method of claim 35 or 36, wherein the ActRIIB polypeptide comprises an amino acid sequence that is at least 75% identical to amino acids 29-109 of SEQ ID NO: 2.

39. The method of claim 35 or 36, wherein the ActRIIB polypeptide comprises an amino acid sequence that is at least 75% identical to amino acids 25-131 of SEQ ID NO: 2.

40. The method of claim 35 or 36, wherein the ActRIIB polypeptide comprises an amino acid sequence that is at least 75% identical to amino acids 20-134 of SEQ ID NO: 2.

41. The method of claim 35 or 36, wherein the ActRIIB polypeptide comprises an amino acid sequence that is at least 75% identical to the amino acid sequence of SEQ ID NO: 53.

42. The method of claim 35 or 36, wherein the ActRIIB polypeptide comprises an amino acid sequence that is at least 75% identical to the amino acid sequence of SEQ ID NO: 388.

43. The method of claim 35 or 36, wherein the ActRIIB polypeptide comprises an amino acid sequence that is at least 75% identical to the amino acid sequence of SEQ ID NO: 389.

44. The method of any one of claims 35-43, wherein the ActRIIB polypeptide is a fusion polypeptide comprising an ActRIIB polypeptide domain and one or more heterologous domains.

45. The method of claim 44, wherein the fusion polypeptide is an ActRIIB-Fc fusion polypeptide.

46. The method of any one of claims 44 or 45, wherein the fusion polypeptide further comprises a linker domain positioned between the ActRIIB polypeptide domain and the one or more heterologous domains or Fc domain.

47. The method of claim 46, wherein the linker domain is selected from: TGGG (SEQ ID NO: 265), TGGGG (SEQ ID NO: 263), SGGGG (SEQ ID NO: 264), GGGGS (SEQ ID NO: 267), GGG (SEQ ID NO: 261), GGGG (SEQ ID NO: 262), and SGGG (SEQ ID NO: 266).

48. The method of any one of claims 45-47, wherein the fusion polypeptide comprises an amino acid sequence that is at least 90% identical to the amino acid sequence of SEQ ID NO: 5.

49. The method of any one of claims 45-47, wherein the fusion polypeptide comprises an amino acid sequence that is at least 90% identical to the amino acid sequence of SEQ ID NO: 12.

50. The method of any one of claims 35 and 37-49, wherein the ActRIIB polypeptide comprises one or more amino acid substitutions with respect to the amino acid sequence of SEQ ID NO: 2 selected from the group consisting of: A24N, S26T, N35E, E37A, E37D, L38N, R40A, R40K, S44T, L46V, L46I, L46F, L46A, E50K, E50P, E50L, E52A, E52D, E52G, E52H, E52K, E52N, E52P, E52R, E52S, E52T, E52Y, Q53R, Q53K, Q53N, Q53H, D54A, K55A, K55D, K55E, K55R, R56A, L57E, L57I, L57R, L57T, L57V, Y60D, Y60F, Y60K, Y60P, R64A, R64H, R64K, R64N, N65A, S67N, S67T, G68R, K74A, K74E, K74F, K74I, K74R, K74Y, W78A, W78Y, L79A, L79D, L79E, L79F, L79H, L79K, L79P, L79R, L79S, L79T, L79W, D80A, D80F, D80G, D80I, D80K, D80M, D80N, D80R, F82A, F82D, F82E, F82I, F82K, F82L, F82S, F82T, F82W, F82Y, N83A, N83R, T93D, T93E, T93G, T93H, T93K, T93P, T93R, T93S, T93Y, E94K, Q98D, Q98E, Q98K, Q98R, V99E, V99G, V99K, E105N, F1081, F108L, F108V, F108Y, El 1 ID, El 11H, El 1 IK, 11 IN, El 1 IQ, El 11R, RI 12H, RI 12K, RI 12N, RI 12S, RI 12T, Al 19P, Al 19V, G120N, E123N, P129N, P129S, P130A, P130R, and A132N.

51. The method of any one of claims 35 and 37-49, wherein the ActRIIB polypeptidecomprises one or more amino acid substitutions with respect to the amino acid sequence of SEQ ID NO: 2 selected from the group consisting of: L38N, E50L, E52D, E52N, E52Y, L57E, L57I, L57R, L57T, L57V, Y60D, G68R, K74E, W78Y, L79E, L79F, L79H, L79R, L79S, L79T, L79W, F82D, F82E, F82I, F82K, F82L, F82S, F82T, F82Y, N83R, E94K, and V99G.

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