Irisin fusion protein and uses thereof

Abstract

Provided are an Irisin fusion protein, a polynucleotide encoding the fusion protein, a vector comprising the polynucleotide, a cell and applications thereof. Specifically, provided is a fusion protein comprising an Irisin polypeptide and an immunoglobulin Fc domain, wherein the Irisin polypeptide is covalently linked to the immunoglobulin Fc domain via a linker comprising an enzyme recognition sequence. Also provided are a polynucleotide encoding the fusion protein, a vector comprising the polynucleotide, a cell and a composition thereof, and use of the fusion protein, the polynucleotide, the vector, the cell and the composition in medicine and / or in the preparation of a medicament for treating or preventing metabolic diseases, complications or other related diseases associated with disorders of glucose metabolism and / or lipid metabolism.

Description

Irisin fusion protein and its use Technical Field The present invention relates to the field of biomedicine, and in particular, to an Irisin fusion protein, a polynucleotide encoding the fusion protein, a vector comprising the polynucleotide, a cell and applications thereof. Background Art Irisin is a myogenic factor discovered in 2012. Exercise induces the activation of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1α) in muscles and the expression of fibronectin type III domain containing 5 (FNDC5); the amino acid sequence of FNDC5 contains a signal peptide, two fibronectin domains and a hydrophobic domain. When the glutamic acid at position 112 is hydrolyzed by enzymes, the product is irisin[1]. Irisin has the function of regulating energy metabolism. According to research reports, Irisin promotes the expression of uncoupling protein 1 (UCP1), a key molecule responsible for heat production in adipocytes, and promotes the browning of white fat [1]. After high-fat-fed mice were transfected with FNDC5 lentivirus, the levels of serum triglycerides, cholesterol, and free fatty acids were significantly reduced. At the same time, the expression level of lipid droplet coating protein in fat and the diameter of adipocytes were also significantly reduced [2]. Irisin promotes the uptake of glucose and fatty acids by primary human skeletal muscle cells [3]. After obese and diabetic mice were given Irisin, the membrane translocation of glucose transporter 4 (GLUT4) in skeletal muscle cells was promoted, thereby enhancing the uptake of glucose [4,5]. In primary hepatocytes of obese mice, Irisin reduced cholesterol content by inhibiting sterol regulatory element binding protein 2 (SREBP2) [6]. In summary, Irisin has therapeutic effects on metabolic diseases, but Irisin is rapidly metabolized in vivo, with a half-life of less than 1 hour in mice[7], which affects the sustained effect of the drug. Therefore, there is an urgent need to develop Irisin drugs with significantly extended half-life in vivo, without affecting the in vivo activity of Irisin and conducive to drug development. Summary of the invention The present invention discloses an Irisin fusion protein with good in vivo metabolic characteristics and significantly prolonged half-life, and the fusion protein can be obtained by a variety of ways or methods. The present invention covalently links the Irisin polypeptide to the immunoglobulin (e.g., IgG) Fc domain through a linking portion including an enzyme recognition sequence to form an Irisin fusion protein, which is specifically cleaved by an enzyme in a specific tissue in the body (e.g., liver) to release the Irisin polypeptide, thereby playing a preventive or therapeutic role in the disease (as shown in Figure 2). One of the advantages of the present invention is that while maintaining the activity of the Irisin polypeptide, the half-life of the Irisin polypeptide is significantly prolonged, and it has good preventive and therapeutic effects in human diseases and animal disease models. One aspect of the present invention provides a fusion protein comprising an Irisin polypeptide and an immunoglobulin Fc domain, wherein the Irisin polypeptide and the immunoglobulin Fc domain are covalently linked via a linker comprising an enzyme recognition sequence. In certain embodiments, in the Irisin fusion protein of the present invention, the Irisin polypeptide is derived from or derived from human Irisin protein or mouse Irisin protein. In certain embodiments, in the Irisin fusion protein of the present invention, the Irisin polypeptide is derived from or derived from human Irisin protein. In certain embodiments, in the Irisin fusion protein of the present invention, the Irisin polypeptide is human Irisin protein. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity with SEQ ID NO: 1 or SEQ ID NO: 2, but still retains the function or activity of Irisin (e.g., stimulating β cells to secrete insulin, promoting insulin synthesis, and increasing the number of pancreatic β cells in a stimulant-dependent manner such as glucose, fatty acids, etc.). In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 1 or SEQ ID NO: 2. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2. In certain embodiments, the immunoglobulin Fc domain comprises or is an IgG2-Fc domain. In certain embodiments, the IgG2-Fc domain is an Fc domain from human IgG2. In certain embodiments, the IgG2-Fc domain comprises one, two or three amino acid substitutions selected from the group consisting of C222S, A330S and P331S. In certain embodiments, in the Irisin fusion protein of the present invention, the IgG2-Fc domain has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity compared to the amino acid sequence shown in SEQ ID NO: 3, and comprises one, two or three amino acid substitutions selected from the group consisting of C222S, A330S and P331S. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity with SEQ ID NO: 1 or SEQ ID NO: 2, and the immunoglobulin Fc domain comprises or is an IgG2-Fc domain, and comprises one or more amino acid substitutions selected from the group consisting of C222S, A330S and P331S. Without being bound by any theory, it is believed that the activity or function of the Irisin fusion protein or Irisin polypeptide of the present invention can be optimized by modification through genetic engineering. In certain embodiments, in the Irisin fusion protein of the present invention, the IgG2-Fc domain has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity compared to the amino acid sequence shown in SEQ ID NO: 3 or SEQ ID NO: 4, and comprises A330S and P331S substitutions. In certain embodiments, in the Irisin fusion protein of the present invention, the IgG2-Fc domain has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity compared to the amino acid sequence shown in SEQ ID NO: 3 or SEQ ID NO: 4, and comprises C222S substitution, A330S substitution and P331S substitution. In certain embodiments, in the Irisin fusion protein of the present invention, the IgG2-Fc domain has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity compared to the amino acid sequence shown in SEQ ID NO: 3 or SEQ ID NO: 4. In certain embodiments, the amino acid sequence of the IgG2-Fc domain is shown in SEQ ID NO: 3. In certain embodiments, the amino acid sequence of the IgG2-Fc domain is shown in SEQ ID NO: 4. In some embodiments, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, and the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 3. In some embodiments, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, and the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 4. In some embodiments, the Irisin polypeptide is located at the N-terminus or C-terminus of the immunoglobulin Fc domain. In some embodiments, the Irisin polypeptide is located at the C-terminus of the immunoglobulin Fc domain. In certain embodiments, in the Irisin fusion protein of the present invention, the enzyme recognition sequence is an enterokinase (EK) recognition sequence or a trypsin recognition sequence. In certain embodiments, the enzyme recognition sequence is an EK recognition sequence. In certain embodiments, the EK recognition sequence includes DDDDK (SEQ ID NO: 5). In certain embodiments, the EK recognition sequence is DDDDK (SEQ ID NO: 5). In certain embodiments, in the Irisin fusion protein of the present invention, the linking portion including the enzyme recognition sequence further includes a linker. In certain embodiments, the linker further includes: a cleavable linker, a non-cleavable linker, a flexible linker, a rigid linker, a helical linker, and a non-helical linker. In some embodiments, the linker further comprises a connecting peptide. In some embodiments, the connecting peptide comprises a linker containing glycine and serine. In some embodiments, the linker containing glycine and serine comprises one, two, three, four or more repeats as shown in SEQ ID NO:6 (GGGS), SEQ ID NO:7 (GGGGS), SEQ ID NO:8 (GGGGGS) or SEQ ID NO:9 (GGGGGGGS). In some embodiments, in the Irisin fusion protein of the present invention, the connecting peptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:18, SEQ ID NO:19 and SEQ ID NO:20. In some embodiments, the connecting peptide comprises an amino acid sequence as shown in SEQ ID NO:10. In certain embodiments, the amino acid sequence of the connecting peptide is as shown in SEQ ID NO:10. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 3, and the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 4, and the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 3, the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5, and the amino acid sequence of the connecting peptide is shown in SEQ ID NO: 10. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 4, the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5, and the amino acid sequence of the connecting peptide is shown in SEQ ID NO: 10. In certain embodiments, the Irisin fusion protein of the present invention comprises or has an amino acid sequence selected from any one of SEQ ID NOs: 23 to 32, or an amino acid sequence having at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, or at least about 99% sequence identity with the above sequence. In certain embodiments, the Irisin fusion protein of the present invention comprises or has an amino acid sequence selected from any one of SEQ ID NOs: 23 to 32, or an amino acid sequence having at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, or at least about 99% sequence identity with the above sequence, and the immunoglobulin Fc domain comprises one, two, or three amino acid substitutions selected from the group consisting of C222S, A330S, and P331S. In certain embodiments, the Irisin fusion protein of the present invention comprises or has an amino acid sequence selected from any one of SEQ ID NOs: 23 to 32, or an amino acid sequence having at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity thereto, and the immunoglobulin Fc domain comprises A330S and P331S substitutions. In certain embodiments, the Irisin fusion protein of the present invention comprises or has an amino acid sequence selected from any one of SEQ ID NOs: 23 to 32, or an amino acid sequence having at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity thereto, and the immunoglobulin Fc domain comprises C222S, A330S and P331S substitutions. In certain embodiments, the Irisin fusion protein of the present invention comprises or has an amino acid sequence as shown in SEQ ID NO: 24 or an amino acid sequence having at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity compared to the amino acid sequence as shown in SEQ ID NO: 24. In certain embodiments, the Irisin fusion protein of the present invention comprises an amino acid sequence as shown in SEQ ID NO: 24. In certain embodiments, the Irisin fusion protein of the present invention has an amino acid sequence as shown in SEQ ID NO: 24. In certain embodiments, the Irisin fusion protein of the present invention consists of an amino acid sequence as shown in SEQ ID NO: 24. In some embodiments, the Irisin fusion protein of the present invention further comprises a signal peptide. In some embodiments, the signal peptide is a human CD33 signal peptide. In some embodiments, the amino acid sequence of the signal peptide is shown in SEQ ID NO: 21. In certain embodiments, the half-life of the Irisin fusion protein of the invention in a subject is at least 1 day, at least 2 days, at least 3 days, at least 4 days, at least 5 days, at least 6 days, at least 7 days, at least 8 days, at least 9 days, at least 10 days, at least 11 days, at least 12 days, at least 13 days, or at least 14 days. Another aspect of the present invention provides a fusion protein dimer comprising two identical peptide chains connected by a disulfide bond, wherein each peptide chain comprises the Irisin fusion protein of the present invention. Another aspect of the present invention provides a nucleic acid molecule comprising a polynucleotide sequence encoding the Irisin fusion protein of the present invention. In certain embodiments, the nucleic acid molecule comprises a polynucleotide sequence encoding an Irisin fusion protein of the present invention (e.g., an Irisin fusion protein as shown in any of the amino acid sequences of SEQ ID NOs: 23 to 32). In certain embodiments, the nucleic acid molecule comprises a polynucleotide sequence as shown in SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38 or SEQ ID NO: 39, or a polynucleotide sequence having at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, at least about 96%, at least about 97%, at least about 98% or at least about 99% sequence identity with a polynucleotide sequence as shown in SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38 or SEQ ID NO: 39. In another embodiment, the nucleic acid molecule comprises or has a polynucleotide sequence as shown in SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38 or SEQ ID NO: 39. In some embodiments, the nucleic acid molecule comprises or has a polynucleotide sequence as shown in SEQ ID NO: 36. In some embodiments, the nucleotide sequence of the nucleic acid molecule is as shown in SEQ ID NO: 36. In yet another aspect of the present invention, a vector comprising the nucleic acid molecule is provided. In yet another aspect of the present invention, a recombinant cell is provided, which comprises the nucleic acid molecule or vector of the present invention. In another aspect of the present invention, a pharmaceutical composition is provided, which comprises the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector or cell, and optionally a pharmaceutically acceptable carrier. Another aspect of the present invention provides a method for constructing a recombinant cell, comprising introducing a nucleic acid molecule encoding the Irisin fusion protein of the present invention into a vector to construct an expression vector; and introducing the expression vector into a recombinant or natural cell to obtain a recombinant cell, preferably, the cell is a CHO-S cell. Another aspect of the present invention provides a method for producing a fusion protein, comprising the step of obtaining the fusion protein using the recombinant cell of the present invention or the cell prepared by the construction method of the present invention. Another aspect of the present invention provides an Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or composition for preparing a drug. In certain embodiments, in the application of the present invention, the drug is an Irisin receptor agonist, a drug that activates an intracellular signaling pathway (e.g., an autophagy-lysosome pathway, a SIRT3 / AMPK signaling pathway, etc.). Another aspect of the present invention provides use of the Irisin fusion protein, nucleic acid molecule, vector or pharmaceutical composition in the preparation of a drug for treating or preventing a disease. Another aspect of the present invention provides a method for treating or preventing a disease in a subject in need thereof, the method comprising administering to the subject a therapeutically effective amount of the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or composition. Another aspect of the present invention provides an Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or composition for treating or preventing a disease. In certain embodiments, the disease is a metabolic disease related to disorders of glucose metabolism or lipid metabolism or a complication of a metabolic disease. In some embodiments, the metabolic disease related to the dysregulated carbohydrate metabolism or lipid metabolism is selected from the group consisting of diabetes (e.g., type 2 diabetes), nonalcoholic steatohepatitis (NASH), nonalcoholic fatty liver disease (NAFLD), obesity, insulin resistance, impaired glucose tolerance, hyperinsulinemia, hypoinsulinemia, high fatty liver, hyperuricemia, fatty liver, hepatic steatosis, liver fibrosis, cirrhosis, and metabolic syndrome. In some embodiments, the metabolic disease related to the dysregulated carbohydrate metabolism or lipid metabolism is nonalcoholic fatty liver disease (NAFLD). In some embodiments, the nonalcoholic fatty liver disease includes nonalcoholic steatohepatitis (NASH). In certain embodiments, the complications of the metabolic disease include cardiovascular complications, renal complications or hepatic complications caused by the metabolic disease. The Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or pharmaceutical composition of the present invention can also be administered in combination with an additional therapeutic agent. In certain embodiments, the present invention provides a drug combination comprising the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or pharmaceutical composition of the present invention, and an additional therapeutic agent. In certain embodiments, in the method of the present invention, the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or pharmaceutical composition can also be administered in combination with a drug for treating diabetes. In certain embodiments, the drug for treating diabetes can be a drug currently on the market, such as insulin, metformin, GLP-1 analogs, sulfonylureas mainly including glimepiride, glibenclamide, gliclazide, gliquidone, etc., α-glucosidase inhibitors such as acarbose, etc., and also include other drugs for treating diabetes that have been on the market and are being developed. In certain embodiments, the diabetes drug is metformin or insulin. In certain embodiments, the Irisin fusion protein or its composition is administered by parenteral, intravenous, subcutaneous or intramuscular routes. In certain embodiments, the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or pharmaceutical composition is administered before, after or simultaneously with the drug for treating diabetes to the subject. The Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or pharmaceutical composition of the present invention can also be co-administered with an additional therapeutic agent for treating or preventing a disease, for example, for treating or preventing metabolic diseases or complications of metabolic diseases associated with dyslipidemia or dyslipidemia. In certain embodiments, an Irisin fusion protein having an amino acid sequence as shown in SEQ ID NO: 24 or a pharmaceutical composition comprising the same is co-administered with a drug for treating diabetes for treating or preventing metabolic diseases or complications of metabolic diseases associated with dyslipidemia or dyslipidemia. In certain embodiments, in the method of the present invention, the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell or pharmaceutical composition can also be co-administered with γ-aminobutyric acid (GABA). In certain embodiments, the Irisin fusion protein having the amino acid sequence shown in SEQ ID NO:24 or a pharmaceutical composition comprising it is co-administered with γ-aminobutyric acid for the treatment or prevention of metabolic diseases or complications of metabolic diseases associated with dysglycometabolism or lipid metabolism. In certain embodiments, the Irisin fusion protein having the amino acid sequence shown in SEQ ID NO:24 or a pharmaceutical composition comprising it is co-administered with γ-aminobutyric acid for the treatment or prevention of non-alcoholic fatty liver disease (NAFLD). In certain embodiments, the Irisin fusion protein having the amino acid sequence shown in SEQ ID NO:24 or a pharmaceutical composition comprising it is co-administered with γ-aminobutyric acid for the treatment or prevention of non-alcoholic fatty liver disease (NASH). In certain embodiments, the dosage of the Irisin fusion protein or the composition thereof contains about 0.2 mg to 20 mg of the Irisin fusion protein. In certain embodiments, the dosage preferably contains about 0.25 mg, about 0.3 mg, about 0.35 mg, about 0.4 mg, about 0.45 mg, about 0.5 mg, about 0.55 mg, about 0.6 mg, about 0.65 mg, about 0.7 mg, about 0.75 mg, about 0.8 mg, about 0.85 mg, about 0.9 mg, about 0.95 mg, about 1 mg, about 1.5 mg, about 2 mg, about 2.5 mg, about 3 The invention relates to an Irisin fusion protein of the present invention, wherein the present invention comprises: a) about 1.5 mg, about 2.5 mg, about 3.5 mg, about 4 mg, about 4.5 mg, about 5 mg, about 5.5 mg, about 6 mg, about 6.5 mg, about 7 mg, about 7.5 mg, about 8 mg, about 8.5 mg, about 9 mg, about 9.5 mg, about 10 mg, about 11 mg, about 12 mg, about 13 mg, about 14 mg, about 15 mg, about 16 mg, about 17 mg, about 18 mg, about 19 mg or about 20 mg of Irisin fusion protein. In certain embodiments, the Irisin fusion protein or the composition thereof is administered once every 3 days, once a week, or once every two weeks. In certain embodiments, the Irisin fusion protein or its composition is administered once a week and then discontinued, followed by administration once a week. In another preferred embodiment, the discontinuation period is 2 weeks. Another aspect of the present invention provides a method for restoring the autophagy-lysosome pathway in a subject, comprising administering to the subject the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. Another aspect of the present invention provides a method for promoting autophagosome degradation and / or increasing autophagic flux in a subject, comprising administering to the subject the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. Another aspect of the present invention provides a method for enhancing lysosomal function in a subject, comprising administering to the subject the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. Another aspect of the present invention provides a method for regulating the SIRT3 / AMPK signaling pathway in a subject, comprising administering to the subject the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. Another aspect of the present invention provides a method for upregulating SIRT3 activity, promoting AMPK phosphorylation, or inhibiting mTOR phosphorylation in a subject, comprising administering to the subject the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. Another aspect of the present invention provides a method for improving mitochondrial dynamics imbalance in a subject, comprising administering to the subject the Irisin fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. Other features and advantages of the present invention are described in detail below in the examples. The detailed description and specific examples are given only by way of illustration in the preferred embodiments of the present invention, and various changes and modifications within the spirit and scope of the present invention will be apparent to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of the vector pcDNA3.1 / IgG2-Fc / Irisin. Figure 2 is a schematic diagram showing the design concept of the preferred embodiment of the present invention. Figure 3 is an SDS-PAGE staining diagram of the IgG2-Fc / Irisin fusion protein. Wherein: R represents the reduced form (plus DTT, single-chain molecule), and NR represents the non-reduced form (without DTT, double-chain molecule). Figure 4 shows the intervention protocol of the Irisin fusion protein in high-fat diet-induced obese and fatty liver model mice. Wherein: (A) prevention group; (B) treatment group. Figure 5 shows that the IgG2-Fc / Irisin fusion protein reduces the weight gain of mice induced by a high-fat diet. Figure 6 shows that IgG2-Fc / Irisin fusion protein improves impaired glucose tolerance and insulin resistance in mice induced by a high-fat diet. (A) IPGTT blood glucose-time curves of each group of mice at week 12 of dietary intervention. (B) Statistical results of the area under the IPGTT curve (AUC). (C) IPITT blood glucose-time curves of each group of mice at week 12 of dietary intervention. (D) Statistical results of the area under the IPITT curve (AUC). (n=5 mice per group), compared with the NCD group, *P<0.05, **P<0.01; compared with the HFD group, #P<0.05, ##P<0.01. Figure 7 shows that IgG2-Fc / Irisin fusion protein reduces transaminase and blood lipid levels in mice fed a high-fat diet (n=5 mice in each group), compared with the NCD group, *P<0.05, **P<0.01; compared with the HFD group, #P<0.05, ##P<0.01. Figure 8 shows that IgG2-Fc / Irisin fusion protein reduces PA-induced lipid accumulation in HepG2 cells. Oil red O staining of cells in each treatment group and quantitative statistics of the cumulative optical density value / area relative level of red lipid droplet particles in each group. The image magnification is 200X, and the data are expressed as mean ± standard deviation, n = 4, compared with the Control group, *P < 0.05, **P < 0.01; compared with the PA group, #P < 0.05, ##P < 0.01. Figure 9 shows that IgG2-Fc / Irisin fusion protein improves PA-induced imbalance of mitochondrial dynamics in HepG2 cells (A) Western blot detection (quantitative data) of p-DRP1 expression levels and quantitative statistics in each treatment group. (B) RT-qPCR detection of mitochondrial fission-related gene expression levels in each group. (C) Representative images of TOMM20 immunofluorescence in cells of each treatment group and quantitative statistics of mitochondrial shape coefficient and aspect ratio. (D) Western blot detection (quantitative data) of MFN2 expression levels and quantitative statistics in each treatment group. (E) Western blot detection (quantitative data) of OPA1 expression levels in cells of each treatment group. (F) RT-qPCR detection of mitochondrial fusion-related gene expression levels in each group. The magnification of the staining images was 400X, and the data were expressed as mean ± standard deviation, n = 3-4. Compared with the Control group, *P < 0.05, **P < 0.01; compared with the PA group, #P < 0.05, ##P < 0.01. Figure 10 shows that IgG2-Fc / Irisin fusion protein improves PA-induced mitochondrial dysfunction in HepG2 cells. (A) Relative level statistics of MitoSOX staining and mean fluorescence intensity (quantitative data) of cells in each treatment group. (B) Representative images of JC-1 fluorescence staining and relative level statistics of red / green fluorescence intensity ratio of cells in each treatment group. Data are expressed as mean ± standard deviation, n = 3, compared with the Control group, *P < 0.05, **P < 0.01; compared with the PA group, #P < 0.05, ##P < 0.01. Figure 11 shows that IgG2-Fc / Irisin fusion protein increases the content of fatty acids entering mitochondria for oxidative decomposition. BODIPY FL C16 fluorescent dye (green) and TOMM20 immunofluorescence (red) staining colocalized in each treatment group and the Pearson correlation coefficient was calculated. Data are expressed as mean ± standard deviation, n = 3, compared with the Control group, *P < 0.05, **P < 0.01; compared with the PA group, #P < 0.05, ##P < 0.01. Figure 12 shows that IgG2-Fc / Irisin fusion protein promotes liver β-oxidation and inhibits lipid synthesis. (A) RT-qPCR detection of the expression levels of genes related to fatty acid β-oxidation in cells of each treatment group. (B) RT-qPCR detection of the expression levels of genes related to lipid synthesis in cells of each treatment group. Data are expressed as mean ± standard deviation, n = 3, compared with the Control group, *P < 0.05, **P < 0.01; compared with the PA group, #P < 0.05, ##P < 0.01. Figure 13 shows that IgG2-Fc / Irisin fusion protein activates PKA, upregulates SIRT3 and inhibits activation of mTOR signaling pathway. Western blot detection (quantitative data) of the expression levels and quantitative statistics of p-mTOR, p-PKA and SIRT3 proteins in cells of each treatment group. Figure 14 shows that IgG2-Fc / Irisin fusion protein up-regulates SIRT3 to reduce lipid accumulation in HepG2 cells induced by PA. Representative images of Oil Red O staining in each treatment group and quantitative statistics of the relative level of cumulative optical density / area of ​​red lipid droplet particles in cells of each group. Data are expressed as mean ± standard deviation, n = 5, compared with the Control group, *P < 0.05, **P < 0.01; compared with the PA group, #P < 0.05, ##P < 0.01; compared with the PA+Irisin group, $P < 0.05, $$P < 0.01. DETAILED DESCRIPTION The following is a detailed description to help those skilled in the art practice the present invention. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those of ordinary skill in the art to which the present invention belongs. The terms used in the present invention are only used to describe specific embodiments and are not intended to limit the present invention. All publications, patent applications, patents, drawings and other references mentioned herein are incorporated by reference in their entirety. I. Definitions or Terms Unless otherwise stated, the defined terms and terms used herein should be understood to refer to dictionary definitions, definitions in incorporated documents, and / or commonly understood meanings of the defined terms. All references, patents, and patent applications referred to herein are incorporated by reference into the subject matter of each cited document, in some cases in their entirety. All features disclosed in this specification may be combined in any manner. Each feature disclosed in this specification may be replaced by an alternative feature for the same, equivalent or similar purpose. Therefore, unless otherwise expressly stated, each feature disclosed is merely an example of a series of equivalent or similar features. As used herein, the terms "peptide", "polypeptide", "protein" and "protein" refer to an amino acid chain formed by the connection of two or more natural or non-natural amino acid residues, regardless of whether there is post-translational modification (e.g., glycosylation or phosphorylation). The polypeptide in the present invention may include, for example, 3 to 3500 natural or non-natural amino acid residues. The protein may be a single peptide chain or a multi-subunit protein (e.g., it may be composed of 2 or more polypeptides). The "peptide", "polypeptide", "protein" and "protein" described herein can be used interchangeably and may contain natural amino acids, non-natural amino acids, or analogs and mimetics of amino acids. The peptides, polypeptides, proteins or proteins described in this application can be obtained by any method known in the art, such as, but not limited to, by natural separation, recombinant expression, chemical synthesis, etc. As used herein, the term "amino acid" refers to a 2 ) and carboxyl (-COOH) functional groups and side chains unique to each amino acid. The amino acid names are also represented in this application by standard single-letter or three-letter codes, which are summarized as follows: "FNDC5" refers to Fibronectin type III domain containing 5, which is a transmembrane glycoprotein that can be cleaved and secreted by Irisin to regulate glucose and lipid metabolism and cardiovascular homeostasis. The FNDC5 protein described in the present application may refer to FNDC5 protein derived from any vertebrate source, including mammals, such as primates (e.g., humans and monkeys) and rodents (e.g., mice and rats). An exemplary sequence of human FNDC5 protein is shown in UniProtKB accession number Q8NAU1 (its amino acid sequence is shown in SEQ ID NO: 1). An exemplary sequence of mouse FNDC5 protein is shown in UniProtKB accession number Q8K4Z2 (its amino acid sequence is shown in SEQ ID NO: 22). "Irisin polypeptide", "Irisin protein" or "irisin" is a FNDC5 fragment formed by enzymatic hydrolysis of FNDC5 protein. In the present application, "Irisin polypeptide" includes any variant, conformation, isoform or species homolog of Irisin protein naturally expressed by cells or expressed by recombinant cells transfected with Irisin gene. For example, "Irisin polypeptide" in the present application includes: 1) natural unprocessed Irisin protein, full-length Irisin protein or naturally occurring Irisin variant (e.g., splice variant or allelic variant); 2) any form of Irisin protein processed in cells; or 3) full-length, fragment (e.g., truncated form or extracellular / transmembrane domain) or modified form (e.g., mutant form, glycosylation / pegylation or immunofluorescence fusion form) of Irisin protein produced by recombinant methods. In certain embodiments, the Irisin polypeptide described in the present application is a polypeptide produced by removing the signal peptide of FNDC5 protein and enzymatic hydrolysis of glutamic acid at position 143. In certain embodiments, the Irisin polypeptide described in the present application comprises the amino acid sequence shown in SEQ ID NO: 2. In certain embodiments, the Irisin polypeptide described in the present application consists of the amino acid sequence shown in SEQ ID NO: 2. In the present application, "fusion protein containing Irisin" (also referred to as "Irisin fusion protein" in the present application) refers to a single-chain protein formed by operably linking an Irisin polypeptide to at least one other heterologous peptide. The Irisin fusion protein described in the present application can be produced by a recombinant method, a chemical method or any other method. For example, in the present application, the fusion protein formed by the Irisin polypeptide and the Fc domain of immunoglobulin IgG is called "Irisin / IgG-Fc" or "IgG-Fc / Irisin" fusion protein, wherein "Irisin / IgG-Fc" means that the Irisin polypeptide is at the N-terminus of the fusion protein and IgG-Fc is at the C-terminus of the fusion protein; "IgG-Fc / Irisin" means that IgG-Fc is at the N-terminus of the fusion protein and the Irisin polypeptide is at the C-terminus of the fusion protein. By analogy, in the present application, the fusion protein formed by the Irisin polypeptide and the Fc domain of immunoglobulin IgG2 is called "Irisin / IgG2-Fc" or "IgG2-Fc / Irisin" fusion protein, wherein "Irisin / IgG2-Fc" means that the Irisin polypeptide is at the N-terminus of the fusion protein, and IgG2-Fc is at the C-terminus of the fusion protein; "IgG2-Fc / Irisin" means that IgG2-Fc is at the N-terminus of the fusion protein, and the Irisin polypeptide is at the C-terminus of the fusion protein. As used herein, the term "polynucleotide" or "oligonucleotide" refers to two or more covalently linked nucleotides. Unless the context clearly states otherwise, the term generally includes, but is not limited to, deoxyribonucleotides (DNA) and ribonucleotides (RNA), which can be single-stranded (ss) or double-stranded (ds). For example, the polynucleotide molecules or polynucleotides of the present invention can be composed of single-stranded and double-stranded DNA, DNA, single-stranded and double-stranded RNA, and RNA as a mixture of single-stranded and double-stranded regions. A mixture of single-stranded and double-stranded regions, a hybrid molecule comprising DNA and RNA, can be single-stranded or more typically double-stranded or a mixture of single-stranded and double-stranded regions. In addition, the polynucleotide molecule can be composed of a triple-stranded region comprising RNA or DNA or both RNA and DNA. As used herein, the term "oligonucleotide" generally refers to a polynucleotide having a length of no more than 200 base pairs, and can be single-stranded or double-stranded. The sequence provided herein can be a DNA sequence or an RNA sequence, but it should be understood that the sequence provided includes DNA and RNA, as well as complementary RNA and DNA sequences, unless the context clearly states otherwise. For example, the sequence 5'-GAATCC-3' is understood to include 5'-GAAUCC-3', 5'-GGATTC-3', and 5'-GGAUUC-3'. As used herein, the term "sequence identity" or "sequence identity" refers to the percentage of sequence identity between two amino acid sequences or two nucleotide sequences. To determine the percentage identity of two amino acid sequences or two nucleotide sequences, the sequences are aligned for optimal comparison purposes (for example, gaps can be introduced in the sequence of a first amino acid or nucleotide sequence to optimally align the amino acid or nucleotide sequence with a second amino acid or nucleotide sequence). The amino acid residues or nucleotides at corresponding amino acid positions or nucleotide positions are then compared. When a position in the first sequence is occupied by the same amino acid residue or nucleotide as the corresponding position in the second sequence, the molecules are identical at that position. The percentage of identity between the two sequences is a function of the number of identical positions shared by the sequences (i.e., percentage of identity = number of identical overlapping positions / total number of positions × 100%). In one embodiment, the two sequences are the same length. A mathematical algorithm can also be used to determine the percentage of identity between two sequences. A preferred, non-limiting example of a mathematical algorithm for comparing two sequences is the Karlin-Altschul algorithm [8], later modified to the Karlin-Altschul algorithm [9]. This algorithm is incorporated into the NBLAST and XBLAST programs.

[0010] BLAST nucleotide searches can be performed using the NBLAST nucleotide program parameter set, e.g., score = 100, wordlength = 12, to obtain nucleotide sequences homologous to a particular nucleic acid molecule. BLAST protein searches can be performed using the XBLAST program parameter set, e.g., score = 50, wordlength = 3, to obtain amino acid sequences homologous to protein molecules described herein. To obtain gapped alignments for comparison purposes, Gapped BLAST can be used.

[0011] Alternatively, PSI-BLAST can be used to perform an iterated search to detect distant relationships (Id.) between molecules. When using BLAST, Gapped BLAST, and PSI-Blast programs, the default parameters of the respective programs (e.g., XBLAST and NBLAST) can be used (see, e.g., the NCBI website). Another preferred, non-limiting example of a mathematical algorithm for sequence comparison is the algorithm proposed by Myers and Miller

[0012] , this algorithm is incorporated into the ALIGN program (version 2.0), which is part of the GCG sequence alignment software package. When using the ALIGN program to compare amino acid sequences, a PAM120 weight residue table, a gap length penalty of 12, and a gap penalty of 4 can be used. The percent identity between two sequences can be determined using techniques similar to those described above, with or without allowing gaps. When calculating percent identity, only exact matches are usually calculated. In the present invention, "conservative substitution" of amino acids refers to a substitution in which one amino acid residue is replaced by another amino acid residue without eliminating the desired properties of the protein. Suitable conservative substitutions of amino acids can be performed by replacing amino acids with similar hydrophobicity, polarity and R-chain lengths with each other. Examples of conservative substitutions include replacing another non-polar residue (e.g., alanine, isoleucine, valine, leucine or methionine) with a non-polar (hydrophobic) residue, replacing another polar residue (e.g., between arginine and lysine) with a polar (hydrophilic) residue, between glutamine and asparagine, between glycine and serine, replacing another basic residue (e.g., lysine, arginine or histidine) with a basic residue, or replacing another acidic residue (e.g., aspartic acid or glutamic acid) with an acidic residue. The phrase "conservative substitution" also includes the use of chemically derived residues or non-natural amino acids to replace non-derivatized residues, provided that the polypeptide exhibits the necessary activity. In the present invention, the term "fusion protein" refers to a protein comprising two or more polypeptides forming different functional domains. For example, in certain embodiments, the Irisin fusion protein described herein comprises an Irisin polypeptide and an immunoglobulin Fc domain; in certain embodiments, the Irisin fusion protein described herein comprises an Irisin polypeptide, an enzyme recognition sequence, and an immunoglobulin Fc domain; in certain embodiments, the Irisin fusion protein described herein comprises an Irisin polypeptide, an enzyme recognition sequence, a linker, and an immunoglobulin Fc domain. In the present invention, the term "linker" refers to any chemical moiety that can covalently link one part to another part. For example, a "linker" can be an artificial amino acid sequence having 1, 2, 3, 4 or 5 amino acid residues, or a length between 5 and 15, 20, 30, 50 or more amino acid residues, connected by peptide bonds, and used to connect one or more polypeptides. The linker may or may not have a secondary structure. Linker sequences are known in the art, for example, see Holliger et al., Proc. Natl. Acad. Sci. USA 90: 6444-6448 (1993); Poljak et al., Structure 2: 1121-1123 (1994). In the present invention, the term "CH2" refers to constant heavy chain 2, which is one of the domains of the immunoglobulin heavy chain. Similarly, the term "CH3" refers to constant heavy chain 3, which is another domain of the immunoglobulin heavy chain. In the present invention, the term "hinge" when used in the context of immunoglobulins (eg, IgG) refers to the flexible region between the antigen binding fragment (Fab) and the crystallizable fragment (Fc). The term "vector" as used herein refers to a vehicle into which a genetic element can be operatively inserted and the genetic element can be expressed, such as producing a protein, RNA or DNA encoded by the genetic element, or replicating the genetic element. The vector can be used to transform, transduce or transfect a host cell so that the genetic element it carries is expressed in the host cell. For example, vectors include: plasmids, phagemids, cosmids, artificial chromosomes such as yeast artificial chromosomes (YAC), bacterial artificial chromosomes (BAC) or P1-derived artificial chromosomes (PAC), phages such as lambda phage or M13 phage, and animal viruses, etc. The vector may contain a variety of elements that control expression, including promoter sequences, transcription start sequences, enhancer sequences, selection elements and reporter genes. In addition, the vector may also contain a replication initiation site. The vector may also include components that assist it in entering the cell, including but not limited to, viral particles, liposomes or protein shells. The vector may be an expression vector or a cloning vector. In the present invention, the term "pharmaceutical grade" means that the chemical purity or proportion of drugs, biomacromolecules or reagents meets the requirements of drug production. In the present invention, the term "treatment" refers to administering an effective amount of a compound or composition or formulation to a subject, which may consist of a single administration, or alternatively include a series of procedures. As is well known in the art, "treatment" is a method for obtaining beneficial or desired results, including clinical results. Beneficial or desired clinical results may include, but are not limited to, alleviating or improving one or more symptoms or symptoms, reducing the extent of the disease, stabilizing (i.e., not worsening) the disease state, preventing the spread of the disease, reversing the disease, improving or alleviating the disease, and the alleviation of the disease state (whether partial or complete, or temporary). In the present invention, the term "subject" is also referred to as a patient, and includes all animals including mammals as used herein, and preferably refers to humans. "Subject" may also be livestock animals, such as cattle, pigs, sheep, poultry and horses; or rodents, such as rats and mice; or primates, such as apes, monkeys, chimpanzees, gorillas, orangutans and baboons; or domestic animals, such as dogs and cats. In the present invention, the term "pharmaceutically acceptable carrier" refers to any carrier, agent, or excipient that is biologically or otherwise acceptable. Unless the carrier, agent, or excipient is incompatible with the active ingredient, its use in a therapeutic formulation is acceptable. The use of such pharmaceutically acceptable carriers is well known in the art. For example, various ingredients that may be included in a pharmaceutical formulation are described in reference 13.

[0013] . In the present invention, the term "therapeutically effective amount" refers to any dose that causes a desired effect in a subject, and the desired effect refers to relieving symptoms, slowing disease progression, preventing disease onset, etc. This amount can be effectively administered multiple times and / or achieve the desired effect within a period of time. As used herein, the term can refer to an amount that causes a decrease in blood sugar in a subject. In the present invention, "combined administration" and the like refer to, for example, two or more substances (e.g., two or more compounds, two or more compositions, etc.) that are administered to a subject and that both have biological activity at the same time. The exact circumstances of administration will depend on the pharmacokinetics of the two or more substances in the presence of each other. The two or more substances administered in combination can be administered to a subject sequentially or simultaneously. In understanding the scope of the present invention, the term "comprise" and its derivatives as used herein are open terms that specify the features, elements, components, groups, integers and / or steps, but do not exclude the presence of other unspecified features, elements, components, groups, integers and / or steps. The above also applies to words with similar meanings, such as the terms "comprises", "having" and their derivatives. As used herein, the terms "consisting of," "consisting of," and their derivatives are closed terms that specify the presence of stated features, elements, components, groups, integers, and / or steps, and also exclude the presence of other unstated features, elements, components, groups, integers, and / or steps. Numerical ranges recited herein by endpoints include all numbers and fractions subsumed within that range (eg, 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.90, 4, and 5). It should also be understood that all numbers and fractions thereof are assumed to be modified by the term "about." In addition, as used herein, terms of degree such as "substantially", "approximately" and "roughly" represent a reasonable amount of deviation of the modified term so that the end result is not significantly changed. These terms of degree should be interpreted as including a deviation of at least ±5% of the modifier if such deviation would not negate the meaning of its modifier. More specifically, the term "about" refers to ±0.1-25%, ±1-20%, ±1-15% or ±1-10%, for example, up to 10%, up to 5% of the reference number. As used in this specification and the appended claims, the singular forms "a", "an" and "the" include plural references unless the content clearly dictates otherwise. Thus, for example, a composition containing "a compound" includes a mixture of two or more compounds. It should also be noted that the term "or" is generally used in its sense including "and / or" unless the content clearly dictates otherwise. In addition, the definitions and embodiments described in the specific parts are intended to be applicable to other embodiments described herein, and those skilled in the art will understand. For example, in the following paragraphs, different aspects of the present invention are defined in more detail. Each aspect defined in this way can be combined with any other aspect or aspects, unless it is clearly stated that it cannot be combined. In particular, any feature indicated as preferred or advantageous can be combined with any other one or more features indicated as preferred or advantageous. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, specific methods and materials are described in this embodiment. II. Proteins and Fusion Proteins In one aspect, the present invention provides a fusion protein comprising an Irisin polypeptide and an immunoglobulin Fc domain, wherein the Irisin polypeptide and the immunoglobulin Fc domain are covalently linked via a linker comprising an enzyme recognition sequence. The Irisin fusion protein provided by the present invention significantly prolongs the half-life of Irisin in vivo without affecting the activity of Irisin, and achieves unexpected technical effects. The following describes each part of the Irisin fusion protein provided by the present invention. a) Irisin peptide In certain embodiments, in the Irisin fusion protein described in the present application, the Irisin polypeptide is derived from or derived from human Irisin protein or mouse Irisin protein. In certain embodiments, in the Irisin fusion protein described in the present application, the Irisin polypeptide is derived from or derived from human Irisin protein, for example, it can be human Irisin protein or its variant. Irisin "variant" refers to an Irisin polypeptide that has an amino acid mutation, deletion, insertion or modification compared to the amino acid sequence of the wild-type Irisin protein, but still retains the function or activity of the Irisin polypeptide (for example, stimulating β cells to secrete insulin, promoting insulin synthesis, and increasing the number of pancreatic β cells in a manner dependent on stimulants such as glucose and fatty acids). In certain embodiments, the amino acid sequence of the human wild-type FNDC5 protein is as shown in SEQ ID NO:1. In certain embodiments, the amino acid sequence of the murine wild-type FNDC5 protein is as shown in SEQ ID NO:22. In certain embodiments, the amino acid sequence of the human wild-type FNDC5 protein is as shown in SEQ ID NO:1, wherein the polypeptide formed by amino acids 1-31 of SEQ ID NO:1 is a signal peptide. The polypeptide produced when the glutamic acid at position 143 of the FNDC5 amino acid sequence is enzymatically cleaved (i.e., amino acids 32-143 of SEQ ID NO:1) is the Irisin polypeptide. In certain embodiments, the amino acid sequence formed by amino acids 32-143 of SEQ ID NO:1 is as shown in SEQ ID NO:2. In certain embodiments, the Irisin polypeptide comprises the amino acid sequence as shown in SEQ ID NO:1. In certain embodiments, the amino acid sequence of the Irisin polypeptide is as shown in SEQ ID NO:1. In certain embodiments, the amino acid sequence of the Irisin polypeptide has at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity with SEQ ID NO:1. In certain embodiments, the amino acid sequence of the Irisin polypeptide has at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity with SEQ ID NO:1, but still retains the function or activity of Irisin (e.g., stimulates insulin secretion by β cells, promotes insulin synthesis, and increases the number of pancreatic islet β cells in a stimulant such as glucose, fatty acid-dependent manner). In certain embodiments, the Irisin polypeptide comprises an amino acid sequence as shown in SEQ ID NO: 2. In certain embodiments, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2. In certain embodiments, the amino acid sequence of the Irisin polypeptide has at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99%) sequence identity to SEQ ID NO: 2. In certain embodiments, the amino acid sequence of the Irisin polypeptide has at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity with SEQ ID NO: 2, but still retains the function or activity of Irisin (e.g., stimulating β cells to secrete insulin, promoting insulin synthesis, and increasing the number of pancreatic β cells in a manner dependent on stimuli such as glucose, fatty acids, etc.). Under the premise of not affecting the activity, the Irisin polypeptide or its variant of the present application may also contain non-natural amino acids. Non-natural amino acids include, for example, β-fluoroalanine, 1-methylhistidine, γ-methyleneglutamic acid, α-methylleucine, 4,5-dehydrolysine, hydroxyproline, 3-fluorophenylalanine, 3-aminotyrosine and 4-methyltryptophan. The Irisin polypeptide or variant thereof described in the present application may also be modified using methods known in the art, such as, but not limited to, PEGylation, glycosylation, amino-terminal modification, fatty acylation, carboxyl-terminal modification, phosphorylation, and methylation. It will be appreciated by those skilled in the art that after the Irisin polypeptide or variant thereof described in the present application is modified using methods known in the art, it still retains functions substantially similar to those of the Irisin polypeptide. In certain embodiments, the Irisin polypeptide variants described in the present application have conservative amino acid substitutions at one or more sites based on the amino acid sequence as shown in SEQ ID NO: 2, but still retain the function or activity of the Irisin polypeptide (for example, stimulating β cells to secrete insulin, promoting insulin synthesis, and increasing the number of pancreatic β cells in a manner dependent on stimulants such as glucose, fatty acids, etc.). In certain embodiments, the Irisin polypeptide variants provided in the present application have at least 90% sequence identity (for example, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity) with the amino acid sequence as shown in SEQ ID NO: 2, and still retain the function or activity of the Irisin polypeptide (for example, stimulating β cells to secrete insulin, promoting insulin synthesis, and increasing the number of pancreatic β cells in a manner dependent on stimulants such as glucose, fatty acids, etc.). b) Immunoglobulin Fc domain In certain embodiments, the immunoglobulin Fc domain described in the present application is an Fc domain derived from IgG. In certain embodiments, the immunoglobulin Fc domain described in the present application is an Fc domain derived from IgG1, IgG2, IgG3 or IgG4. In certain embodiments, the immunoglobulin Fc domain described in the present application is an Fc domain derived from IgG2. In certain embodiments, the immunoglobulin Fc domain described in the present application is an Fc domain derived from human IgG. In certain embodiments, the immunoglobulin Fc domain described in the present application is an Fc domain derived from human IgG1, human IgG2, human IgG3 or human IgG4. In certain embodiments, the immunoglobulin Fc domain described in the present application is an Fc domain derived from human IgG2. In the present application, "IgG-Fc" and "IgG / Fc" can be used interchangeably, both referring to the Fc domain of immunoglobulin IgG; "IgG2-Fc" and "IgG2 / Fc" can be used interchangeably, both referring to the Fc domain of immunoglobulin IgG2. In certain embodiments, the immunoglobulin Fc domain described in the present application comprises an IgG2-Fc domain. In another embodiment, the immunoglobulin Fc domain described in the present application is an IgG2-Fc domain. In certain embodiments, the IgG2-Fc domain described in the present application is derived from the constant region of human IgG2. In certain embodiments, the IgG2-Fc domain described in the present application comprises a hinge region. In certain embodiments, the IgG2-Fc domain described in the present application is an Fc domain from human IgG2. In certain embodiments, the IgG2-Fc domain described in the present application is the hinge-CH2-CH3 region of human IgG2. In certain embodiments, the IgG2-Fc domain comprises one, two or three amino acid substitutions selected from the group consisting of C222S, A330S and P331S. In certain embodiments, the IgG2-Fc domain comprises a C222S substitution. The C222S substitution of IgG2-Fc can increase the flexibility of the N-terminal hinge region by removing the disulfide bond between the two monomers of the homodimer. The increased flexibility of the N-terminal hinge region can reduce the binding affinity of the Fcγ receptor, thereby reducing antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). In another embodiment, the IgG2-Fc domain comprises an A330S substitution. The A330S substitution of IgG2-Fc reduces affinity for Fcγ receptors and C1q complement proteins. In another embodiment, the IgG2-Fc domain comprises a P331S substitution. The P331S substitution of IgG2-Fc reduces affinity for Fcγ receptors and C1q complement proteins. In certain embodiments, the IgG2-Fc domain comprises C222S, A330S, and P331S substitutions. The amino acid residue positions of the IgG2-Fc domain described in the present application include, for example, C222, A330 and P331, Corresponding to the position of human IgG2 as shown in Genbank Accession No. QRG33935.1. In certain embodiments, the IgG2-Fc domain can be 227 amino acids as shown in SEQ ID NO: 3. Those skilled in the art will easily recognize the residue positions of amino acids, such as the residue positions of C222, A330 and P331 in the short or long Fc fragments shown in the reference sequence SEQ ID NO: 3 or Genbank Accession No. QRG33935.1 as shown in Table 4 below. Table 4. Replacement sites In certain embodiments, the IgG2-Fc domain described herein comprises the amino acid sequence shown in SEQ ID NO: 3. In certain embodiments, the IgG2-Fc domain described herein consists of the amino acid sequence shown in SEQ ID NO: 3. In another embodiment, the IgG2-Fc domain described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in SEQ ID NO: 3. In certain embodiments, the amino acid sequence of the IgG2-Fc domain described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in SEQ ID NO: 3, but still retains the function of the Fc domain shown in SEQ ID NO: 3 (e.g., being able to extend the half-life of the Irisin polypeptide). In certain embodiments, the IgG2-Fc domain described herein has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, or at least about 95% sequence identity compared to the amino acid sequence shown in SEQ ID NO: 3, and comprises one or more amino acid substitutions selected from the group consisting of C222S, A330S, and P331S. The fusion protein has an improved half-life compared to an Irisin polypeptide without an IgG / Fc domain fusion. In certain embodiments, the IgG2-Fc domain has at least 90% (e.g., at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity compared to the amino acid sequence shown in SEQ ID NO: 3, and comprises A330S and P331S substitutions. Preferably, the IgG2-Fc domain has at least 90% (e.g., at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity compared to the amino acid sequence shown in SEQ ID NO: 3, and comprises C222S, A330S and P331S substitutions. In certain embodiments, the IgG2-Fc domain described herein comprises the amino acid sequence shown in SEQ ID NO: 4. In certain embodiments, the IgG2-Fc domain described herein consists of the amino acid sequence shown in SEQ ID NO: 4. In another embodiment, the IgG2-Fc domain described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in SEQ ID NO: 4. In certain embodiments, the amino acid sequence of the IgG2-Fc domain described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in SEQ ID NO:4, but still retains the function of the Fc domain shown in SEQ ID NO:3 (e.g., being able to extend the half-life of the Irisin polypeptide). In certain embodiments, in the Irisin fusion protein described in the present application, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, and the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 3. In certain embodiments, in the Irisin fusion protein described in the present application, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, and the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 4. In certain embodiments, in the Irisin fusion protein described in the present application, the Irisin polypeptide is located at the N-terminus of the immunoglobulin Fc domain. In another embodiment, in the Irisin fusion protein described in the present application, the Irisin polypeptide is located at the C-terminus of the immunoglobulin Fc domain. c) Enzyme recognition sequence In the present application, "enzyme recognition sequence" refers to an amino acid sequence that directly or further connects the Irisin polypeptide described in the present application and the immunoglobulin Fc domain through a linker, and can be specifically recognized and cleaved by a protease. In certain embodiments, the "enzyme recognition sequence" described in the present application can only be specifically recognized and cleaved by enzymes expressed in a few tissues (e.g., liver). In certain embodiments, the "enzyme recognition sequence" described in the present application can be specifically recognized and cleaved by enzymes regulated by diet. Without being limited by any theory, the "enzyme recognition sequence" can be specifically recognized and cleaved by enzymes expressed in a few tissues (e.g., liver). The enzyme recognition sequence that specifically recognizes and cleaves an enzyme expressed by the liver) is preferred in the present application because such an enzyme can release the Irisin polypeptide in the Irisin fusion protein in specific tissues in the body, thereby prolonging the in vivo half-life of the Irisin polypeptide as much as possible while retaining the function or activity of the Irisin polypeptide to the greatest extent. In some embodiments, the enzyme recognition sequence is an enterokinase (EK) recognition sequence or a trypsin recognition sequence. In some embodiments, the enzyme recognition sequence is an enterokinase (EK) recognition sequence. In some embodiments, the enterokinase (EK) recognition sequence includes DDDDK (SEQ ID NO: 5). In some embodiments, the enterokinase (EK) recognition sequence is DDDDK (SEQ ID NO: 5). d) Linker In certain embodiments, the linking portion comprising an enzyme recognition sequence further comprises a linker. In certain embodiments, the linker further comprises: a cleavable linker, a non-cleavable linker, a flexible linker, a rigid linker, a helical linker, or a non-helical linker. In the present invention, the term "cleavable linker" refers to a linker that is sensitive to factors such as proteases, pH or chemicals in vivo and is easily cleaved in the presence of the above factors. In the present invention, the term "non-cleavable linker" refers to a linker that is stable to in vivo proteases, pH or chemical factors and is not easily cleaved. In the present invention, the term "flexible linker" refers to a linker that can increase spatial extensibility when connecting different protein components so that the spatial folding and conformation of the protein components are not affected by each other as much as possible. In the present invention, the term "rigid linker" refers to a linker that can maintain a fixed distance between protein components when connecting different protein components. In the present invention, the term "helical linker" refers to a linker in which a rigid unit can form a helix (eg, α-helix) within itself or with the same adjacent sequence, thereby allowing the formed fusion protein to have a relatively stable three-dimensional conformation. In the present invention, the term "non-helical linker" refers to a linker that is unable to form a helical structure. In certain embodiments, the linker further comprises a connecting peptide. The Irisin polypeptide and immunoglobulin Fc domain of the fusion protein disclosed herein, such as IgG2-Fc domain, can be further connected by a connecting peptide. As used herein, the term "connecting peptide" refers to any part that connects different functional domains of a polypeptide together. The connecting peptide can have any suitable length and structure. In certain embodiments, the connecting peptide comprises a linker comprising glycine and serine. Preferably, the linker comprising glycine and serine comprises SEQ ID NO: 6 (GGGS), SEQ ID NO: 7 (GGGGS), One, two, three, four or more repeats of SEQ ID NO:8 (GGGGGS) or SEQ ID NO:9 (GGGGGGGS). In certain embodiments, the connecting peptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:18, SEQ ID NO:19, and SEQ ID NO:20. In certain embodiments, the connecting peptide has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, or at least about 95% sequence identity to the amino acid sequence shown in SEQ ID NO: 10. In certain embodiments, the connecting peptide comprises the amino acid sequence shown in SEQ ID NO: 10. In certain embodiments, the amino acid sequence of the connecting peptide is shown in SEQ ID NO: 10. In certain embodiments, in the Irisin fusion protein described in the present application, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 3, and the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5. In certain embodiments, in the Irisin fusion protein described in the present application, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 4, and the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5. In certain embodiments, in the Irisin fusion protein described in the present application, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 3, the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5, and the amino acid sequence of the connecting peptide is shown in SEQ ID NO: 10. In certain embodiments, in the Irisin fusion protein described in the present application, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 4, the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5, and the amino acid sequence of the connecting peptide is shown in SEQ ID NO: 10. e) Fusion protein The present application provides an Irisin fusion protein, which comprises an Irisin polypeptide and an immunoglobulin Fc domain, wherein the Irisin polypeptide and the immunoglobulin Fc domain are covalently linked via a linker comprising an enzyme recognition sequence. In certain embodiments, the Irisin fusion protein described in the present application comprises or consists of the following structure from the N-terminus to the C-terminus: P1-L1-ER-L2-P2, Wherein, P1 represents the immunoglobulin Fc domain, L1 represents the bond or the first linker, ER represents the enzyme recognition sequence, L2 represents the bond or the second linker, and P2 represents the Irisin polypeptide. The "L1-ER-L2" in the above structure is the "connection portion including the enzyme recognition sequence" described in the present application. In certain embodiments, the "connection portion including the enzyme recognition sequence" described in the present application consists only of the enzyme recognition sequence (e.g., enterokinase recognition sequence). In certain embodiments, P1 represents an IgG2-Fc domain, L1 represents a first linker, ER represents an enzyme recognition sequence, L2 represents a bond, and P2 represents an Irisin polypeptide. In certain embodiments, P1 represents an IgG2-Fc domain, L1 represents a first linker, ER represents an enterokinase recognition sequence, L2 represents a bond, and P2 represents an Irisin polypeptide. In certain embodiments, P1 represents the IgG2-Fc domain as shown in SEQ ID NO:3, L1 represents the first linker, ER represents the enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents the Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, P1 represents the IgG2-Fc domain as shown in SEQ ID NO:3, L1 represents the first linker as shown in SEQ ID NO:10, ER represents the enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents the Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, P1 represents the IgG2-Fc domain as shown in SEQ ID NO:4, L1 represents the first linker, ER represents the enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents the Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, P1 represents the IgG2-Fc domain as shown in SEQ ID NO:4, L1 represents the first linker as shown in SEQ ID NO:10, ER represents the enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents the Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, P1 represents the IgG2-Fc domain as shown in SEQ ID NO:4, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents an Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, the Irisin fusion protein described in the present application comprises or consists of the following structure from the N-terminus to the C-terminus: P1-L1-ER-L2-P2, Among them, P1 represents Irisin polypeptide, L1 represents a bond or a first linker, ER represents an enzyme recognition sequence, L2 represents a bond or a second linker, and P2 represents an immunoglobulin Fc domain. In certain embodiments, P1 represents an Irisin polypeptide, L1 represents a bond, ER represents an enzyme recognition sequence, L2 represents a second linker, and P2 represents an IgG2-Fc domain. In certain embodiments, P1 represents an Irisin polypeptide, L1 represents a bond, ER represents an enterokinase recognition sequence, L2 represents a second linker, and P2 represents an IgG2-Fc domain. In certain embodiments, P1 represents an Irisin polypeptide as shown in SEQ ID NO:2, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a second linker, and P2 represents an IgG2-Fc domain as shown in SEQ ID NO:3. In certain embodiments, P1 represents an Irisin polypeptide as shown in SEQ ID NO:2, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a second linker as shown in SEQ ID NO:10, and P2 represents an IgG2-Fc domain as shown in SEQ ID NO:3. In certain embodiments, P1 represents an Irisin polypeptide as shown in SEQ ID NO:2, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a second linker, and P2 represents an IgG2-Fc domain as shown in SEQ ID NO:4. In certain embodiments, P1 represents an Irisin polypeptide as shown in SEQ ID NO:2, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a second linker as shown in SEQ ID NO:10, and P2 represents an IgG2-Fc domain as shown in SEQ ID NO:4. In certain embodiments, the Irisin fusion protein described in the present application comprises an amino acid sequence as shown in any one of SEQ ID NOs: 23 to 32. In certain embodiments, the Irisin fusion protein described in the present application consists of an amino acid sequence as shown in any one of SEQ ID NOs: 23 to 32. In certain embodiments, the Irisin fusion protein described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in any one of SEQ ID NOs: 23 to 30. In certain embodiments, the Irisin fusion protein described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in any one of SEQ ID NOs: 23 to 30, and wherein the IgG2-Fc domain comprises one, two or three amino acid substitutions selected from the group consisting of C222S, A330S and P331S, for example, comprising A330S and P331S substitutions, or comprising C222S, A330S and P331S substitutions. In certain embodiments, the Irisin fusion protein described in the present application has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in any one of SEQ ID NOs: 23 to 30, and the Irisin polypeptide formed after enterokinase cleavage still retains the function or activity of the Irisin protein (for example, stimulating β cells to secrete insulin, promoting insulin synthesis, and increasing the number of pancreatic β cells in a manner dependent on stimulants such as glucose and fatty acids). In certain embodiments, the Irisin fusion protein described in the present application comprises the amino acid sequence shown in SEQ ID NO: 24. In certain embodiments, the Irisin fusion protein described in the present application consists of the amino acid sequence shown in SEQ ID NO: 24. In certain embodiments, the Irisin fusion protein described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity to the amino acid sequence shown in SEQ ID NO:24. In certain embodiments, the Irisin fusion protein described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in SEQ ID NO: 24, and wherein the IgG2-Fc domain comprises one, two or three amino acid substitutions selected from the group consisting of C222S, A330S and P331S, for example, comprising A330S and P331S substitutions, or comprising C222S, A330S and P331S substitutions. In certain embodiments, the Irisin fusion protein described in the present application has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in SEQ ID NO: 24, and the Irisin polypeptide formed after enterokinase cleavage still retains the function or activity of the Irisin protein (for example, stimulating β cells to secrete insulin, promoting insulin synthesis, and increasing the number of pancreatic β cells in a manner dependent on stimulants such as glucose and fatty acids). As shown herein, the fusion of Irisin polypeptide with IgG2-Fc increases the half-life of Irisin fusion protein. As demonstrated in the examples of the present application, the Irisin fusion protein disclosed herein has a longer half-life after fusion modification, for example, the Irisin fusion protein has a half-life of about 50-70 hours. The Irisin fusion protein, nucleic acid, vector and recombinant cell described herein are suitable for preparing drugs and compositions thereof, and for corresponding drug treatment purposes. In certain embodiments, the half-life of the Irisin fusion protein described herein in a subject is at least 1 day, at least 2 days, at least 3 days, at least 4 days, at least 5 days, at least 6 days, at least 7 days, at least 8 days, at least 9 days, at least 10 days, at least 11 days, at least 12 days, at least 13 days or at least 14 days. In certain embodiments, the Irisin fusion protein described herein is of pharmaceutical grade. Another aspect of the present invention provides a fusion protein dimer comprising two identical peptide chains connected by a disulfide bond, wherein each peptide chain comprises the Irisin fusion protein as described in the present application. f) Signal peptide In certain embodiments, the Irisin fusion protein further comprises a signal peptide. As used herein, the term "signal peptide" refers to a polypeptide that causes the fusion protein to be secreted into the extracellular medium. Such polypeptides may also be referred to as "leader peptides", "polypeptide precursors", "propeptides", etc. The use of signal peptides to direct the secretion of proteins is known in the art (e.g., U.S. Pat. No. 8,658,174, the contents of which are incorporated herein by reference in their entirety). Examples of signal peptides include, but are not limited to, human CD33 signal peptides, human growth hormone-releasing hormone (GHRH) signal peptides, human α-1-microglobulin / bikunin precursor (AMBP) signal peptides, Gaussia luciferase signal peptides, mouse immunoglobulin heavy chain signal peptides, and mouse immunoglobulin κ light chain signal peptides. The signal peptide is cleaved during the secretion process. In some embodiments, the signal peptide is exogenous. In some embodiments, the signal peptide is a human CD33 signal peptide. In some embodiments, the signal peptide has at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, or at least about 95% sequence identity with the amino acid sequence shown in SEQ ID NO: 21 (MPLLLLLPLLWAGALA), and allows secretion of the fusion protein. In some embodiments, the signal peptide comprises an amino acid sequence as shown in SEQ ID NO: 21 or an amino acid sequence having at least 90% sequence identity with SEQ ID NO: 21. In some embodiments, the signal peptide consists of an amino acid sequence as shown in SEQ ID NO: 21. In some embodiments, the signal peptide is located at the N-terminus of the Irisin fusion protein. In some embodiments, the Irisin fusion protein described in the present application comprises or consists of the following structure from the N-terminus to the C-terminus: SP-L3-P1-L1-ER-L2-P2, Among them, SP represents absence or signal peptide, L3 represents bond or third linker, P1 represents immunoglobulin Fc domain, L1 represents bond or first linker, ER represents enzyme recognition sequence, L2 represents bond or second linker, and P2 represents Irisin polypeptide. In certain embodiments, SP represents a signal peptide, L3 represents a bond, P1 represents an immunoglobulin Fc domain, L1 represents a bond or a first linker, ER represents an enzyme recognition sequence, L2 represents a bond or a second linker, and P2 represents an Irisin polypeptide. In certain embodiments, SP represents a signal peptide, L3 represents a bond, P1 represents an IgG2-Fc domain, L1 represents a first linker, ER represents an enzyme recognition sequence, L2 represents a bond, and P2 represents an Irisin polypeptide. In certain embodiments, SP represents a signal peptide, L3 represents a bond, P1 represents an IgG2-Fc domain, L1 represents a first linker, ER represents an enterokinase recognition sequence, L2 represents a bond, and P2 represents an Irisin polypeptide. In certain embodiments, SP represents a signal peptide as shown in SEQ ID NO:21, L3 represents a bond, P1 represents an IgG2-Fc domain as shown in SEQ ID NO:3, L1 represents a first linker, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents an Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, SP represents a signal peptide as shown in SEQ ID NO:21, L3 represents a bond, P1 represents an IgG2-Fc domain as shown in SEQ ID NO:3, L1 represents a first linker as shown in SEQ ID NO:10, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents an Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, SP represents a signal peptide as shown in SEQ ID NO:21, L3 represents a bond, P1 represents an IgG2-Fc domain as shown in SEQ ID NO:4, L1 represents a first linker, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents an Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, SP represents a signal peptide as shown in SEQ ID NO:21, L3 represents a bond, P1 represents an IgG2-Fc domain as shown in SEQ ID NO:4, L1 represents a first linker as shown in SEQ ID NO:10, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a bond, and P2 represents an Irisin polypeptide as shown in SEQ ID NO:2. In certain embodiments, the Irisin fusion protein described in the present application comprises or consists of the following structure from the N-terminus to the C-terminus: SP-L3-P1-L1-ER-L2-P2, Among them, SP represents absence or signal peptide, L3 represents bond or third linker, P1 represents Irisin polypeptide, L1 represents bond or first linker, ER represents enzyme recognition sequence, L2 represents bond or second linker, and P2 represents immunoglobulin Fc domain. In certain embodiments, SP represents a signal peptide, L3 represents a bond, P1 represents an Irisin polypeptide, L1 represents a bond, ER represents an enzyme recognition sequence, L2 represents a second linker, and P2 represents an immunoglobulin Fc domain. In certain embodiments, SP represents a signal peptide, L3 represents a bond, P1 represents an Irisin polypeptide, L1 represents a bond, ER represents an enzyme recognition sequence, L2 represents a second linker, and P2 represents an IgG2-Fc domain. In certain embodiments, SP represents a signal peptide, L3 represents a bond, P1 represents an Irisin polypeptide, L1 represents a bond, ER represents an enterokinase recognition sequence, L2 represents a second linker, and P2 represents an IgG2-Fc domain. In certain embodiments, SP represents a signal peptide as shown in SEQ ID NO:21, L3 represents a bond, P1 represents an Irisin polypeptide as shown in SEQ ID NO:2, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a second linker, and P2 represents an IgG2-Fc domain as shown in SEQ ID NO:3. In certain embodiments, SP represents a signal peptide as shown in SEQ ID NO:21, L3 represents a bond, P1 represents an Irisin polypeptide as shown in SEQ ID NO:2, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a second linker as shown in SEQ ID NO:10, and P2 represents an IgG2-Fc domain as shown in SEQ ID NO:3. In certain embodiments, SP represents a signal peptide as shown in SEQ ID NO:21, L3 represents a bond, P1 represents an Irisin polypeptide as shown in SEQ ID NO:2, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a second linker, and P2 represents an IgG2-Fc domain as shown in SEQ ID NO:4. In certain embodiments, SP represents a signal peptide as shown in SEQ ID NO:21, L3 represents a bond, P1 represents an Irisin polypeptide as shown in SEQ ID NO:2, L1 represents a bond, ER represents an enterokinase recognition sequence as shown in SEQ ID NO:5, L2 represents a second linker as shown in SEQ ID NO:10, and P2 represents an IgG2-Fc domain as shown in SEQ ID NO:4. In certain embodiments, the Irisin fusion protein described in the present application comprises an amino acid sequence as shown in SEQ ID NO: 31 or SEQ ID NO: 32. In certain embodiments, the Irisin fusion protein described in the present application consists of an amino acid sequence as shown in SEQ ID NO: 31 or SEQ ID NO: 32. In certain embodiments, the Irisin fusion protein described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity to the amino acid sequence shown in SEQ ID NO:31 or SEQ ID NO:32. In certain embodiments, the Irisin fusion protein described herein has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in SEQ ID NO:31 or SEQ ID NO:32, and wherein the IgG2-Fc domain comprises one or more amino acid substitutions selected from the group consisting of C222S, A330S and P331S, for example, comprising A330S and P331S substitutions, or comprising C222S, A330S and P331S substitutions. In certain embodiments, the Irisin fusion protein described in the present application has at least 70%, at least 75%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% sequence identity with the amino acid sequence shown in SEQ ID NO: 31 or SEQ ID NO: 32, and the Irisin polypeptide formed after cleavage by enterokinase still retains the function or activity of the Irisin protein (e.g., stimulating insulin secretion by β cells, promoting insulin synthesis, and increasing the number of pancreatic islet β cells in a stimulant such as glucose, fatty acid-dependent manner). III. Nucleic Acids In another aspect, the present invention also provides a polynucleotide comprising a polynucleotide encoding the polypeptide or peptide described in the present invention (e.g., Irisin polypeptide, Fc fragment and fusion protein). In another aspect, the present invention also provides a nucleic acid molecule comprising a polynucleotide encoding the polypeptide or peptide described herein (e.g., Irisin polypeptide, Fc fragment and fusion protein). As used herein, the terms "nucleic acid" or "nucleotide" refer to deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) and polymers thereof in single-stranded or double-stranded form. Unless otherwise specified, a particular nucleotide sequence also implicitly encompasses its conservatively modified variants (e.g., degenerate codon substitutions), alleles, orthologs, SNPs, and complementary sequences, as well as the explicitly recited sequences. Specifically, degenerate codon substitutions can be achieved by generating sequences in which the third position of one or more selected (or all) codons is replaced with a mixture of bases and / or deoxyinosine residues (see Batzer et al., Nucleic Acid Res. 19:5081 (1991); Ohtsuka et al., J. Biol. Chem. 260:2605-2608 (1985); and Rossolini et al., Mol. Cell. Probes 8:91-98 (1994)). In certain embodiments, the polynucleotide described in the present application is codon-optimized, e.g., optimized for humans. The nucleic acid molecule can be used in the methods described herein. Standard protein chemistry techniques can be used to synthesize polypeptides and fusion proteins. In addition, automatic peptide synthesizers are commercially available (e.g., Advanced ChemTech Model 1396; Milligen / Biosearch 9600). Alternatively, the peptides, polypeptides, or fragments or variants thereof described herein can be recombinantly produced using various expression systems well known in the art. In certain embodiments, the polynucleotides described herein comprise a polynucleotide sequence as shown in SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38 or SEQ ID NO: 39, or have at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, at least 100%, at least 101%, at least 102%, at least 103%, at least 104%, at least 105%, at least 106%, at least 107%, at least 108%, at least 109%, at least 110%, at least 111%, at least 112%, at least 113%, at least 114%, at least 1 At least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity. In certain embodiments, the polynucleotide described herein comprises a polynucleotide sequence as shown in SEQ ID NO:36, or comprises a polynucleotide sequence having at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity to the polynucleotide sequence as shown in SEQ ID NO:36. The polynucleotide described in the present application comprises a polynucleotide sequence as shown in SEQ ID NO:37 or a polynucleotide sequence having at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity to the polynucleotide sequence as shown in SEQ ID NO:37. The polynucleotide described in the present application comprises a polynucleotide sequence as shown in SEQ ID NO:38 or a polynucleotide sequence having at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity to the polynucleotide sequence as shown in SEQ ID NO:38. The polynucleotide described in the present application comprises a polynucleotide sequence as shown in SEQ ID NO:39 or a polynucleotide sequence having at least 70% (e.g., at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99%) sequence identity to the polynucleotide sequence as shown in SEQ ID NO:39. IV. Vectors and Cells In yet another aspect, the present invention provides a vector comprising the polynucleotide described in the present application. Any vector suitable for the intended use can be used. For example, some vectors can be introduced into expression systems, such as mammalian, insect or bacterial expression systems, for expression and purification of expressed proteins. Some vectors can be used to produce viruses. These different vectors are well known in the art. In some embodiments, the vector is used together with an in vitro expression system to produce a fusion protein. The expression and purification of the fusion protein can be performed by any suitable method known in the art. Possible expression vectors include, but are not limited to, plasmids or modified viruses (e.g., replication-defective retroviruses, including lentiviral vectors, adenoviruses, and adeno-associated viruses, etc.). In certain embodiments, the expression vector that can be used with the fusion protein of the present invention is pcDNA3.1, which can be obtained commercially. The vector may contain appropriate regulatory sequences and components. Suitable regulatory sequences may be selected from a variety of sources, including bacteria, Fungi, viruses, mammals or insect genes. Examples of such regulatory sequences include: transcriptional promoters and enhancers or RNA polymerase binding sequences, ribosome binding sequences, including translation initiation signals. In addition, other sequences, such as replication origins, additional DNA restriction sites, enhancers and sequences that confer transcriptional induction ability, can be incorporated into the expression vector, depending on the cells to be transfected / infected / transduced and the vector used. In some embodiments, the regulatory sequence guides or increases expression in neural tissue and / or cells. In some embodiments, the vector is a viral vector. The recombinant expression vector can also contain a marker gene that helps select host cells transformed, infected or transfected with the vector for expressing the Irisin fusion protein described herein. The recombinant expression vector can also contain other expression cassettes encoding, for example, other fusion moieties that can help detect, including, for example, tags and markers described herein. In some embodiments, the vector comprises one or more. For example, in some embodiments, the vector comprising the polynucleotide encoding the Irisin fusion protein is a pcDNA3.1 / Irisin / IgG2-Fc expression vector. In some embodiments, the vector comprising the polynucleotide encoding the Irisin fusion protein is a pcDNA3.1 / IgG2-Fc / Irisin expression vector. A variety of methods for transducing cells can be used, including viral vectors, "naked" DNA, DNA in lipid or other nanoparticles, adjuvant-assisted DNA, gene guns, etc. For example, retroviral vectors such as lentiviral vectors can also be used to transduce cells in vivo. Other vector systems that can be used to practice the present invention include vectors based on adenovirus and adeno-associated viruses. Another aspect of the present invention provides a recombinant cell, wherein the cell comprises a polynucleotide encoding the Irisin fusion protein, or comprises the vector described in the present application. Stably expressing recombinant cells can be prepared, for example, by transforming, transfecting or transducing recombinant cells with a vector comprising a polynucleotide, preferably any of the polynucleotides described herein. In some embodiments, the cell described in the present application is a eukaryotic cell. In some embodiments, the eukaryotic cell is a mammalian cell. In some embodiments, the mammalian cell is a cell derived from a human. For example, the mammalian cell is a human embryonic kidney cell 293 (HEK293 cell), for example, a HEK293T cell, a HEK293S cell or a HEK293F cell. In some embodiments, the mammalian cell is a Chinese hamster ovary (CHO) cell, for example, a CHO-K1 cell, a CHO-S cell or a CHO-DG44 cell. In certain embodiments, the recombinant cells of the present invention are obtained by suspension acclimation of Chinese hamster ovary cells. V. Cell Construction Method and Fusion Protein Production Method In another aspect of the present invention, a method for constructing a recombinant cell is provided, comprising: introducing a polynucleotide encoding the Irisin fusion protein of the present invention into a vector to construct an expression vector; and introducing the expression vector into a recombinant or natural cell to obtain a recombinant cell. In certain embodiments, the cell is a CHO cell, for example, a CHO-K1 cell, a CHO-S cell, or a CHO-DG44 cell. In certain embodiments, the method for constructing the recombinant cell comprises the following steps: a) inserting the polynucleotide sequence shown in SEQ ID NO:36, SEQ ID NO:37, SEQ ID NO:38 or SEQ ID NO:39 into the insertion site of pcDNA3.1 vector to generate pcDNA3.1 / Irisin / IgG2-Fc or pcDNA3.1 / IgG2-Fc / Irisin expression vector; b) Introducing the pcDNA3.1 / Irisin / IgG2-Fc or pcDNA3.1 / IgG2-Fc / Irisin expression vector into CHO-S cells to obtain recombinant cells. In another embodiment, the method for constructing the recombinant cell comprises the following steps: a) inserting the polynucleotide sequence shown in SEQ ID NO:36, SEQ ID NO:37, SEQ ID NO:38 or SEQ ID NO:39 into the NcoI and HindIII sites of the pcDNA3.1 vector to generate a pcDNA3.1 / IgG2-Fc / Irisin plasmid vector or a pcDNA3.1 / Irisin / IgG2-Fc plasmid vector; b) The pcDNA3.1 / Irisin / IgG2-Fc or pcDNA3.1 / IgG2-Fc / Irisin expression vector is introduced into CHO-S cells to obtain recombinant cells. In another aspect of the present invention, a method for producing the Irisin fusion protein is provided, comprising the step of obtaining the fusion protein using the recombinant cells described herein or the cells prepared by the construction method described herein. As described herein, Irisin fusion proteins can be synthesized. As shown herein, the fusion proteins can also be prepared using recombinant cells, including, for example, recombinant Chinese hamster ovary cells that recombinantly express Irisin fusion proteins. Therefore, the present invention also provides a method for preparing Irisin fusion proteins, the method comprising culturing recombinant cells expressing Irisin fusion proteins, wherein the culturing comprises one or more process steps or materials described in the examples. In some embodiments, the recombinant cells expressing Irisin fusion protein are prepared using HEK293 cells (e.g., HEK293T cells, HEK293S cells, HEK293F cells) and / or CHO cells (e.g., CHO-K1 cells, CHO-S cells, CHO-DG44 cells), for example, under conditions suitable for in vivo use, the recombinant cells can be used to produce recombinant polypeptides and / or fusion proteins. In certain embodiments, in the Irisin fusion protein of the present invention, the Irisin polypeptide is derived from or is derived from human Irisin protein, the enzyme recognition sequence is an EK enzyme recognition sequence, and the immunoglobulin Fc domain is an Fc domain from human IgG2. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5, and the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 3. In certain embodiments, in the Irisin fusion protein of the present invention, the Irisin polypeptide is derived from or is derived from human Irisin protein, the enzyme recognition sequence is an EK enzyme recognition sequence, and the immunoglobulin Fc domain is an Fc domain from human IgG2. In certain embodiments, in the Irisin fusion protein of the present invention, the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO: 2, the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO: 5, and the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO: 4. VI. Pharmaceutical Compositions Another aspect of the present invention also provides a pharmaceutical composition comprising the Irisin fusion protein, fusion protein dimer, polynucleotide, vector or recombinant cell described in the present application. In certain embodiments, the composition may further comprise a suitable diluent or carrier. In another preferred embodiment, the carrier is a pharmaceutically acceptable carrier. In certain embodiments, the composition is a pharmaceutical composition. In certain embodiments, the composition is a pharmaceutical composition comprising an Irisin fusion protein and a pharmaceutically acceptable carrier. In certain embodiments, the composition further comprises buffered saline. In certain embodiments, the composition further comprises a carbohydrate. In certain embodiments, the composition further comprises a surfactant. The pharmaceutical composition of the present invention can be prepared, packaged and / or sold in batches as a unit dose and / or multiple unit doses. The composition can be prepared in a variety of forms. In certain embodiments, the composition comprises an Irisin fusion protein, formulated into a preparation, and the dosage is about 0.25 mg to about 20 mg, for example, about 0.25 mg, about 0.3 mg, about 0.35 mg, about 0.4 mg, about 0.45 mg, about 0.5 mg, about 0.55 mg, about 0.6 mg, about 0.65 mg, about 0.7 mg, about 0.75 mg, about 0.8 mg, about 0.85 mg, about 0.9 mg, about 0.95 mg, about 1 mg, About 1.5 mg, about 2 mg, about 2.5 mg, about 3 mg, about 3.5 mg, about 4 mg, about 4.5 mg, about 5 mg, about 5.5 mg, about 6 mg, about 7 mg, about 7.5 mg, about 8 mg, about 8.5 mg, about 9 mg, about 9.5 mg, about 10 mg, about 11 mg, about 12 mg, about 13 mg, about 14 mg, about 15 mg, about 16 mg, about 17 mg, about 18 mg, about 19 mg or about 20 mg of Irisin fusion protein. The Irisin fusion protein, polynucleotide, vector, recombinant cell or composition can be used to prepare a medicament and / or for administration, for example, parenterally, intravenously, subcutaneously or intramuscularly. In certain embodiments, the Irisin fusion protein or composition thereof can be administered parenterally or formulated into a preparation for parenteral administration. In certain embodiments, the Irisin fusion protein or the composition thereof can be administered subcutaneously or formulated into a preparation for subcutaneous administration. In certain embodiments, the Irisin fusion protein or the composition thereof can be administered intravenously or formulated into a preparation for intravenous administration. In certain embodiments, the Irisin fusion protein or the composition thereof can be administered intramuscularly or formulated into a preparation for intramuscular administration. Suitable diluents for Irisin fusion proteins and / or cells include, but are not limited to, saline solutions, pH buffered solutions, and the diluents described herein, as well as glycerol solutions or other solutions suitable for freezing polypeptides and / or cells. Suitable diluents for nucleic acids and / or vectors include, but are not limited to, saline solutions, pH buffered solutions, and the diluents described herein, as well as water and the like. In another preferred embodiment, the diluent is sterile. VII. Methods and uses for treating and preventing diseases As shown herein, the present invention covalently links the Irisin polypeptide with the Fc domain of an immunoglobulin (e.g., IgG) through an enzyme recognition sequence to form an Irisin fusion protein, which is specifically cleaved by an enzyme in a specific tissue in vivo (e.g., the liver) to release the Irisin polypeptide, thereby exerting a disease prevention or treatment effect. The Irisin fusion protein, nucleic acid, vector, and recombinant cell described herein are suitable for preparing drugs and their compositions and for corresponding pharmaceutical treatment uses. In another aspect of the present invention, the fusion protein, fusion protein dimer, polynucleotide, vector, cell, or pharmaceutical composition disclosed herein can be used in a subject to treat or prevent the onset of a disease or disorder or to slow its progression. In another aspect of the present invention, there is provided a use of the fusion protein, fusion protein dimer, polynucleotide, vector, cell, or pharmaceutical composition for treating, preventing, or slowing the progression of a disease or disorder. In another aspect of the present invention, there is provided a fusion protein, fusion protein dimer, polynucleotide, vector, cell, or pharmaceutical composition for preparing a drug for treating, preventing, or slowing the progression of a disease or disorder. In another aspect of the present invention, there is provided a method for treating, preventing, or slowing the progression of a disease or disorder by administering a therapeutically effective amount of the fusion protein, fusion protein dimer, polynucleotide, vector, cell, or pharmaceutical composition to a subject in need thereof. In another aspect of the present invention, there is provided a use of the fusion protein, fusion protein dimer, polynucleotide, vector, cell, or pharmaceutical composition described herein in the preparation of a drug for treating or preventing a disease. In another aspect of the present invention, there is provided a use of the fusion protein, fusion protein dimer, polynucleotide, vector, cell, or pharmaceutical composition described herein for treating or preventing a disease. In another aspect of the present invention, there is provided a method for treating or preventing a disease, the method comprising: administering to a subject a therapeutically effective amount of the fusion protein, fusion protein dimer, polynucleotide, vector, cell, or pharmaceutical composition described herein. In certain embodiments, the drug comprising the fusion protein is administered in an amount of about 0.2 mg to about 20 mg (per person per time) of the fusion protein. In certain embodiments, the amount of the fusion protein is about 1 mg to about 10 mg. In certain embodiments, the amount of the fusion protein is about 1 mg to about 5 mg. In certain embodiments, the amount of the fusion protein is about 0.25 mg, about 0.3 mg, about 0.35 mg, about 0.4 mg, about 0.45 mg, about 0.5 mg, about 0.55 mg, about 0.6 mg, about 0.65 mg, about 0.7 mg, about 0.75 mg, about 0.8 mg, about 0.85 mg, about 0.9 mg, about 0.95 mg, about 1 mg, about 1.5 mg, about 2 mg, about 2.5 mg g, about 3 mg, about 3.5 mg, about 4 mg, about 4.5 mg, about 5 mg, about 5.5 mg, about 6 mg, about 6.5 mg, about 7 mg, about 7.5 mg, about 8 mg, about 8.5 mg, about 9 mg, about 9.5 mg, about 10 mg, about 11 mg, about 12 mg, about 13 mg, about 14 mg, about 15 mg, about 16 mg, about 17 mg, about 18 mg, about 19 mg or about 20 mg. Different dosage forms may be used, suitable dosage forms may include but are not limited to solutions, suspensions, pills, tablets. In certain embodiments, a treatment regimen may include multiple administrations. In certain embodiments, the Irisin fusion protein or the composition thereof is administered once every 3 days, once a week, or once every two weeks. In certain embodiments, the Irisin fusion protein, fusion protein dimer, polynucleotide, vector, cell or composition thereof is administered once a week followed by discontinuation of administration, followed by administration once a week. In another preferred embodiment, the discontinuation period is 2 weeks. In certain embodiments, the Irisin fusion protein, fusion protein dimer, polynucleotide, vector, cell or composition thereof can be administered for two weeks after a single administration, and then administered four times a week in succession. In another preferred embodiment, the treatment further comprises administering an initial dose of 1 mg one week before the start of the dosing regimen. In certain embodiments, the Irisin fusion protein, fusion protein dimer, polynucleotide, vector, cell or a composition thereof is administered parenterally, intravenously, subcutaneously or intramuscularly. In certain embodiments, the disease includes a metabolic disease associated with glucose metabolism or lipid metabolism disorder, a complication of a metabolic disease, or other related diseases. In some embodiments, the disease is a metabolic disease related to glucose metabolism or lipid metabolism disorder. In some embodiments, the metabolic disease related to glucose metabolism or lipid metabolism disorder is selected from the group consisting of diabetes (e.g., type 2 diabetes), nonalcoholic steatohepatitis (NASH), nonalcoholic fatty liver disease (NAFLD), obesity, insulin resistance, impaired glucose tolerance, hyperinsulinemia, hypoinsulinemia, high fatty liver, hyperuricemia, fatty liver, hepatic steatosis, liver fibrosis, cirrhosis and metabolic syndrome. In certain embodiments, the metabolic disease associated with the dysglycometabolism or lipid metabolism disorder is or includes diabetes. In certain embodiments, the metabolic disease associated with the dysglycometabolism or lipid metabolism disorder is type 2 diabetes. In another embodiment, the metabolic disease associated with the dysglycometabolism or lipid metabolism disorder is or includes obesity. In another embodiment, the metabolic disease associated with the dysglycometabolism or lipid metabolism disorder is or includes non-alcoholic fatty liver disease (NAFLD). In another embodiment, the metabolic disease associated with the dysglycometabolism or lipid metabolism disorder is non-alcoholic steatohepatitis (NASH). In certain embodiments, the subject is newly diagnosed or has been previously diagnosed with diabetes (e.g., type 2 diabetes). Diabetes can be diagnosed in a variety of ways, such as fasting blood glucose (FPG). According to the American Diabetes Association, diabetes is diagnosed when fasting blood glucose is greater than or equal to 126 mg / dl. In certain embodiments, the subject has an increased likelihood of developing diabetes (e.g., type 2 diabetes). For example, the subject may be susceptible to diabetes because the subject is obese or the subject has a genetic susceptibility, such as when the subject has a family history of diabetes. In certain embodiments, the subject is an obese patient. Obesity can be defined by reference to body mass index (BMI). For example, the World Health Organization (WHO) defines obesity as having a BMI equal to or greater than 30. In another embodiment, the subject has at least about 20 kg / m 2 In another embodiment, the subject may have a blood sugar level higher than the average level of peers of comparable weight, but not enough to be diagnosed as diabetes. In another embodiment, the subject may also be an individual with a family history of diabetes. In certain embodiments, the subject is a patient who has been newly diagnosed or previously diagnosed with NAFLD or NASH. In another embodiment, the subject has an increased likelihood of developing NAFLD or NASH. For example, the subject may have a genetic susceptibility to NAFLD or NASH. In certain embodiments, the complications of the metabolic disease include cardiovascular complications (e.g., coronary heart disease, sudden cardiac death, heart failure, etc.), renal complications (e.g., acute kidney injury, diabetic nephropathy) or liver complications caused by the metabolic disease. The fusion protein, fusion protein dimer, polynucleotide, vector, cell, or composition of the present invention can be combined with any other known drugs or therapies for treating diseases. In certain embodiments, the fusion protein, fusion protein dimer, polynucleotide, vector, cell, or composition disclosed herein can be used in combination with a drug for treating diabetes, which can be a drug currently on the market, such as insulin, metformin, sulfonylureas mainly including glimepiride, glibenclamide, gliclazide, gliquidone, etc., α-glucosidase inhibitors such as acarbose, etc., and also include other drugs for treating diabetes that are already on the market and under development. In certain embodiments, the diabetes drug is metformin or insulin. In certain embodiments, the diabetes drug is metformin or insulin. VIII. Others In another aspect, the present invention also provides a method for restoring the autophagy-lysosome pathway in a subject, comprising administering to the subject the fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. In another aspect, the present invention also provides a method for promoting autophagosome degradation and / or increasing autophagic flux in a subject, comprising administering to the subject the fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. In another aspect, the present invention also provides a method for enhancing lysosomal function in a subject, comprising administering to the subject the fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. In another aspect, the present invention also provides a method for regulating the SIRT3 / AMPK signaling pathway in a subject, comprising administering to the subject the fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. In another aspect, the present invention also provides a method for upregulating SIRT3 activity, promoting AMPK phosphorylation or inhibiting mTOR phosphorylation in a subject, comprising administering to the subject the fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition described in the present invention. In another aspect, the present invention also provides a method for improving mitochondrial dynamics imbalance in a subject, comprising administering to the subject the fusion protein, fusion protein dimer, nucleic acid molecule, vector, cell, or pharmaceutical composition of the present invention. The SEQ ID NOs and their corresponding amino acid sequences or nucleotide sequences mentioned in this application are shown in Tables 5 and 6. Table 5. Amino acid sequence Table 6. Nucleotide sequences Example Unless otherwise specified, the experimental methods used in the following examples are all conventional methods. Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources. Example 1. Preparation of Irisin fusion protein As shown in Figure 1, the inventors constructed a secretory expression vector pcDNA3.1 / IgG2-Fc / Irisin encoding an IgG2-Fc / Irisin fusion protein, which contains human IgG2-Fc (containing the hinge, CH2 and CH3 regions of the human IgG2 heavy chain, i.e., Hinge-CH2-CH3, and having C222S, A330S, P331S site mutations, and its amino acid sequence is shown in SEQ ID NO:4), an enterokinase recognition sequence (its amino acid sequence is shown in SEQ ID NO:5) and a human Irisin polypeptide (its amino acid sequence is shown in SEQ ID NO:2). A cDNA fragment encoding an IgG2-Fc / Irisin fusion protein (as shown in SEQ ID NO:36) was chemically synthesized and inserted into the NcoI and HindIII sites of the pcDNA3.1 vector to generate a pcDNA3.1 / IgG2-Fc / Irisin plasmid vector. The pcDNA3.1 / IgG2-Fc / Irisin stable expression vector was transformed into E. coli DH5α competent cells. The plasmid was extracted from the strain containing pcDNA3.1 / IgG2-Fc / Irisin and digested with PvuI. After electrophoresis, there was a target band with the correct molecular weight. The linearized plasmid was quantified after ethanol precipitation and used for stable transfection. To establish CHO-S cells stably expressing IgG2-Fc / Irisin fusion protein, 1x10 cells were transfected with 20 μg of linearized pcDNA3.1 / IgG2-Fc / Irisin by electroporation (electroporation). 7 CHO-S cells. 24 hours after transfection, the cells were cultured in CD-CHO medium containing resistance MSX (methionine sulfoximine, 100 μM / L), and fresh medium was replaced every 3 days until the recombinant plasmid was stably integrated into the genome. Single cells were inoculated in 96-well plates using the limiting dilution method, and the culture was gradually expanded to form a stable monoclonal cell line and then stored in liquid nitrogen. The selected cell line was cultured in a shake flask, and the IgG2-Fc / Irisin fusion protein was purified and separated using a Protein A chromatography column. The purified IgG2-Fc / Irisin fusion protein was detected and identified using SDS-PAGE protein electrophoresis, Irisin and Fc antibody Western Blot method. As shown in Figure 3, the cell culture supernatant was purified to obtain an IgG2-Fc / Irisin fusion protein (whose amino acid sequence is shown in SEQ ID NO: 24), and 10 μg was taken after quantification by the BCA method, and treated without or with DTT (reducing disulfide bonds in the molecule), respectively, and stained with Coomassie Brilliant Blue after SDS-PAGE electrophoresis, R represents the reduced form (with DTT, single-chain molecule), and NR represents the non-reduced form (without DTT, double-chain molecule), and the gel staining results show that a higher purity IgG2-Fc / Irisin fusion protein is finally obtained. Example 2. IgG2-Fc / Irisin fusion protein has a longer in vivo half-life The IgG2-Fc / Irisin fusion protein prepared in Example 1 was intraperitoneally injected into C57BL / 6J mice, and blood was collected at different time points before and after the injection to detect the content of the IgG2-Fc / Irisin fusion protein in the serum, and the metabolic half-life of the IgG2-Fc / Irisin fusion protein in vivo was calculated. 2.1. Experimental animals and blood collection C57BL / 6J mice (purchased from Shanghai Slake Experimental Animal Co., Ltd., 8 mice aged 6-8 weeks) were randomly divided into two groups: low-dose IgG2-Fc / Irisin fusion protein (0.5μg / g) and high-dose IgG2-Fc / Irisin fusion protein (1.5μg / g). IgG2-Fc / Irisin fusion protein was injected intraperitoneally once, and blood was collected from the retro-orbital venous plexus at 8 time points, including 2 hours before injection and 24 hours, 48 ​​hours, 72 hours, 96 hours, 120 hours, 144 hours, and 168 hours after injection. 2.2. Determination of Irisin fusion protein concentration in serum The whole blood was allowed to stand at room temperature for 30 minutes, centrifuged at 3000 rpm for 15 minutes at 4°C, and the supernatant was aliquoted and stored at -80°C for testing. The concentration of IgG2-Fc / Irisin fusion protein in serum was determined by enzyme-linked immunosorbent assay (ELISA), and the serum to be tested was diluted with the sample diluent in the kit. 2.3. Preparation of standard products The preparation steps of the standard are as follows: 1) Prepare the washing buffer: Mix the washing stock solution with deionized ultrapure water (ddH 2 O) dilution; 2) Prepare antibody working solution: dilute the biotinylated antibody with antibody diluent at a volume ratio of 1:100; 3) preparing a working solution of horseradish peroxidase-labeled avidin: diluting horseradish peroxidase-labeled avidin with avidin diluent at a volume ratio of 1:100; 4) Add 100 μl of each concentration of standard or sample to be tested to each well of a 96-well plate (set duplicate wells for each sample), cover with a plate sticker, and incubate at 37°C for 2 hours; 5) discard the liquid; 6) Add 100 μl of antibody working solution, cover with plate sticker, and incubate at 37°C for 1 hour; 7) Discard the liquid, add 200 μl of washing solution, let it stand for 2 min, then discard the liquid, and repeat 3 times; 8) Add 100 μl of avidin working solution, cover the plate with stickers, and incubate at 37°C for 1 hour; 9) Discard the liquid, add 200 μl of washing solution, let it stand for 2 min, then discard the liquid, and repeat 5 times; 10) Add 90 μl of substrate solution and color develop at 37°C in the dark for 20 min; 11) When the first 3-4 wells of the standard show an obvious blue gradient, and the color development in the last 3-4 wells is not obvious, add 50 μl of stop solution; 12) Measure the absorbance (OD value) at a wavelength of 450 nm using an enzyme-labeled instrument within 5 minutes, draw a standard curve and a quantitative formula based on the gradient standard concentration and OD value, and calculate the IgG2-Fc / Irisin fusion protein concentration in the sample serum according to the formula. Dilute and prepare the standards to the concentrations shown in Table 1. Table 1. Dilution and preparation of standard concentrations 2.4. Experimental results and half-life calculation A line graph was drawn based on the above results to show the relationship between the administration time and the concentration of IgG2-Fc / Irisin fusion protein, and the elimination rate constant and half-life t were calculated according to the formula k = (lnC0-lnC) / t. 1 / 2 . The results showed that the plasma concentration of IgG2-Fc / Irisin in the two groups of mice was about 13.6ng / ml under physiological conditions. The blood concentration reached its peak 24h after the injection of IgG2-Fc / Irisin fusion protein, and the peak blood concentration was about 134.5ng / ml and 350.6ng / ml, respectively. Then the blood concentration gradually decreased with time. The plasma concentration of Irisin measured at each time point in the two groups of mice was converted to its natural logarithm, i.e., lnC, and a scatter plot of lnC versus time (t) was drawn. A simple linear fit was performed to obtain the half-life of IgG2-Fc / Irisin fusion protein of about 51-76h. The half-life of IgG2-Fc / Irisin fusion protein in mice was significantly increased compared with that of Irisin in its natural state. The above experimental results suggest that the frequency of IgG2-Fc / Irisin fusion protein administration in subsequent animal experiments is twice a week. Example 3. Establishment of high-fat diet obesity and fatty liver model mice 3.1. Experimental animals and high-fat feeding Experimental animals: SPF grade C57BL / 6J male mice (5 weeks old) High-fat diet: purchased from Research Diets, USA (Cat. No.: D12492), the ingredients are shown in Table 2. Ordinary feed: purchased from Shanghai Proton Biotechnology Co., Ltd., the ingredients are shown in Table 3. Table 2. High fat diet composition Table 3. Common feed ingredients 3.2 Animal Experimental Protocol and Experimental Methods Experimental grouping, feeding and intervention of high-fat fed animals: After one week of adaptive feeding, the animals were randomly divided into three groups, namely, a normal chow diet control group (NCD), a high fat diet group (HFD), and a high fat diet plus IgG2-Fc / Irisin fusion protein (prepared in Example 1) intervention group (HFD+Irisin, IgG2-Fc / Irisin 1.5 μg / g). Animal sampling and metabolic, biochemical, and liver pathology tests include: weekly testing of body weight and food intake, glucose tolerance test, insulin tolerance test, serum biochemistry, and liver pathology. The experimental methods are as follows: 3.2.1 Intermediate glucose tolerance test (IPGTT) 1) Prepare 20% glucose injection: dissolve 2g glucose powder in 10ml ddHO 2 O, filtered with 0.22μm filter membrane, ready for use; 2) After fasting for 12 hours, the tail tip of the mice was cut off, about 1-2 mm, and blood was collected to test fasting blood glucose. Glucose injection was injected intraperitoneally at 2 g / kg (glucose / body weight) and timing was immediately started. Blood was collected from the tail at 15 min, 30 min, 60 min, 90 min and 120 min after injection, and the blood glucose levels of the mice were measured and recorded. 3.2.2 Insulin tolerance test (IPITT) 1) Prepare insulin solution (0.75UI / 10ml) for injection: dilute 400UI / 10ml insulin stock solution with pre-cooled saline at a dilution volume ratio of 1:533, place on ice after preparation, and use immediately after preparation; 2) After fasting for 6 hours, the tail tip of the mice was cut off by about 1-2 mm, and blood was collected for blood glucose testing. Insulin solution was injected intraperitoneally at a dose of 0.75 UI / kg body weight and immediately timed with a timer. Blood was collected from the tail at 15 minutes, 30 minutes, 60 minutes, 90 minutes and 120 minutes after injection, and the blood glucose value was measured and recorded. 3.2.3 HE staining of liver tissue 1) The tissue block was completely immersed in paraformaldehyde (concentration 4%) and fixed overnight to 24 hours; 2) Dehydration, paraffin embedding and sectioning; 3) Baking, dewaxing and hydration: xylene (I) → xylene (II) → xylene (III) → anhydrous ethanol → 90% ethanol for 5 min → 80% ethanol → 70% ethanol, soaking for 10 minutes in each step, take out and drain, then place in ddH 2 O, wash slowly on a shaker for 3 times, 5 min each time; 4) HE staining: stain with hematoxylin for 10 minutes. After staining, wash off the floating color and immerse the slices in eosin stain for about 1 to 5 minutes. Rinse the slices gently with running water. 5) Dehydration and transparency: Immerse the sections completely in a histochemical tank and replace them in the following order: 70% ethanol → 80% ethanol → 90% ethanol → 100% ethanol → xylene (I) → xylene (II), soaking for 5 minutes in each step; 6) Sealing: After the slices become transparent, quickly wipe off the xylene around the slices, add 10-15μl of neutral gum, seal the slices with a coverslip, and let them dry at room temperature. 3.2.4 Oil Red O staining of liver tissue 1) Fixation: The tissue block was completely immersed in paraformaldehyde (concentration 4%) and fixed overnight to 24 hours; 2) Embed the samples in liquid nitrogen using OCT embedding medium, slice them at -20 degrees Celsius, and store them in a -20 degree refrigerator for later use; 3) Oil Red O staining: The frozen sections were left at room temperature for 15-20 minutes. After the moisture disappeared and the sections were dry, they were placed in Oil Red O staining solution for 8 minutes. After the sections were dyed, they were gently rinsed with PBS buffer and 75% ethanol was added for differentiation. 4) Hematoxylin counterstaining and sealing: Hematoxylin staining for about 90 seconds, gently wash the sections with PBS, and seal the sections with glycerol gelatin after they are slightly dried. 3.2.5 Liver immunohistochemical staining 1) The steps of baking, dewaxing and hydrating paraffin sections are the same as HE staining; 2) Antigen retrieval; 3) 3% hydrogen peroxide treatment to remove endogenous enzymes; 4) Blocking: add PBS containing 10% goat serum and place in a wet box for blocking at room temperature for 30 minutes; 5) Primary antibody incubation: add primary antibody containing 1% goat serum and incubate overnight at 4°C; 6) Secondary antibody incubation: add secondary antibody containing 1% goat serum and incubate at room temperature for 1 hour; 7) DAB color development: Add freshly prepared DAB color development solution to the tissue section and color for about 3-5 minutes, then rinse gently with tap water to stop color development; 8) The steps of hematoxylin counterstaining, dehydration, transparency and sealing are the same as HE staining. 3.2.6 Immunofluorescence staining of liver tissue 1) The steps of baking, dewaxing and hydrating paraffin sections are the same as HE staining; 2) Antigen retrieval, blocking, primary antibody incubation and secondary antibody incubation are the same as immunohistochemical staining; 3) DAPI staining: add 0.1 μg / ml DAPI staining solution and incubate at room temperature for 5-10 minutes; 4) Sealing: Add anti-fluorescence quenching agent, seal the slide with a coverslip, and seal the edges of the coverslip with transparent nail polish. After fixing, the sections can be placed in a cool, dark place to dry at room temperature overnight, and then stored at 4°C in a dark place for a long time. 3.2.7 Detection of triglyceride (TG) content in liver tissue Weigh the mouse liver tissue, add anhydrous ethanol, and prepare the tissue homogenate in a tissue grinder. Set up three wells in a 96-well plate, add the standard, blank control and sample respectively, add the working solution, incubate at 37℃ for 10 minutes, and measure the absorbance (OD) value at 510nm. Calculate the TG content of liver tissue according to the following formula. TG content (mg / g) = (sample OD value - blank OD value) / (standard OD value - blank OD value) × standard concentration (mmol / L) × 639 g / mol. 3.2.8 Detection of total cholesterol (TC) content in liver tissue Weigh the mouse liver tissue, add anhydrous ethanol, and prepare the tissue homogenate in a tissue grinder. Set up three wells in a 96-well plate, add the standard, blank control and sample respectively, add the working solution, incubate at 37℃ for 10 minutes, and measure the absorbance (OD) value at 510nm. Calculate the TC content of liver tissue according to the following formula. TC content (mg / g) = (sample OD value - blank OD value) / (standard OD value - blank OD value) × standard concentration (mmol / L) × 386.65 g / mol. 3.2.9 Serum lipid and transaminase levels After blood collection, the whole blood was kept at room temperature for 30 min, centrifuged at 3000 rpm for 15 min at 4°C, and the supernatant was stored at -80°C for later use. The levels of triglycerides, total cholesterol, low-density lipoprotein, high-density lipoprotein, alanine aminotransferase, and aspartate aminotransferase in serum were detected using an automatic biochemical analyzer. 3.2.10 Tissue protein extraction 1) Prepare protein lysis buffer: add 50× protease inhibitor solution and 10× phosphatase inhibitor solution to 1× RIPA lysis buffer and mix thoroughly. Prepare and use immediately. 2) Weigh the mouse liver tissue, add protein lysis buffer, prepare tissue homogenate in a tissue grinder, centrifuge at 14000g for 30 min at 4°C, discard the precipitate, repeat the centrifugation at 14000g for 30 min at 4°C, keep the supernatant; add appropriate amount of 5× loading buffer, mix well, and store at -20°C. Example 4. IgG2-Fc / Irisin fusion protein prevents hepatic steatosis caused by high-fat diet In vivo study on the prevention of liver steatosis by IgG2-Fc / Irisin fusion protein: Mice in the high-fat-fed HFD+IgG2-Fc / Irisin fusion protein intervention group (HFD+Irisin) were intraperitoneally injected with the IgG2-Fc / Irisin fusion protein (1.5 μg / g) prepared in Example 1 every week for 12 weeks. The results are as follows: IgG2-Fc / Irisin fusion protein reduces high-fat diet-induced weight gain As shown in Figure 4A, 5-week-old male C57BL / 6J mice were randomly divided into three groups, namely, a normal diet control group (corresponding to the "NCD" group in Figure 4A), a high-fat diet group (corresponding to the "HFD" group in Figure 4A), and a high-fat diet + IgG2-Fc / Irisin fusion protein prevention and intervention group (corresponding to the "HFD+Irisin" group in Figure 4A). All groups were fed for 12 weeks. The mice in the HFD+Irisin group were intraperitoneally injected with 1.5 μg / g of IgG2-Fc / Irisin fusion protein twice a week while being fed with high fat until the end of feeding. The food intake and body weight changes of mice in each group were recorded and counted weekly. The body weight and body weight gain of mice in the high-fat diet (HFD) group were significantly higher; there was no significant difference in food intake between the HFD+Irisin group and the HFD group, and the body weight gain of the HFD+Irisin group was lower from the 4th week, which was statistically significant. The above results show that IgG2-Fc / Irisin fusion protein intervention does not change food intake. Figure 5 shows that IgG2-Fc / Irisin fusion protein can reduce high-fat diet-induced weight gain. IgG2-Fc / Irisin fusion protein improves high-fat diet-induced impaired glucose tolerance and insulin resistance IPGTT and IPITT experiments evaluated the effects of IgG2-Fc / Irisin fusion protein on glucose metabolism and insulin sensitivity. The results of IPGTT showed that the HFD+Irisin group improved impaired glucose tolerance; the results of IPITT showed that the intervention of IgG2-Fc / Irisin fusion protein significantly improved insulin sensitivity. The above results suggest that IgG2-Fc / Irisin fusion protein improves impaired glucose tolerance and insulin resistance caused by a high-fat diet. IgG2-Fc / Irisin fusion protein reduces elevated blood lipid and transaminase levels induced by high-fat diet The levels of liver transaminases and blood lipids in serum were measured to evaluate the protective effect of IgG2-Fc / Irisin fusion protein on the liver. The results showed that the levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in the serum of the HFD+Irisin group were significantly lower than those of the HFD group. At the same time, the levels of serum triglycerides (TG), total cholesterol (TC) and low-density lipoprotein (LDL) in the HFD+Irisin group were also significantly lower than those in the HFD group, and the levels of high-density lipoprotein (HDL) did not change significantly in the mice of each group. The above results suggest that IgG2-Fc / Irisin fusion protein can reduce the blood lipid levels of mice fed a high-fat diet and improve systemic lipid metabolism disorders; IgG2-Fc / Irisin fusion protein can reduce hepatocyte damage in mice fed a high-fat diet and play a liver protective role. IgG2-Fc / Irisin fusion protein reduces high-fat diet-induced body fat accumulation and liver lipid accumulation The liver, para-epididymal fat, and inguinal subcutaneous fat tissue were photographed, weighed, and compared to evaluate the body fat accumulation and liver fat deposition status. IgG2-Fc / Irisin fusion protein intervention can significantly reduce the weight gain of adipose tissue caused by HFD; the liver index (Liver index) results showed that the liver index of the HFD+Irisin group was significantly lower than that of the HFD group. These results suggest that IgG2-Fc / Irisin fusion protein can effectively alleviate the body fat accumulation and liver weight gain of mice fed a high-fat diet, and prevent liver enlargement and damage. Photos of liver tissue showed that the livers of the HFD+Irisin group were similar in size and morphology to those of the normal diet group (NCD group), with a dark red color, sharp edges, and no greasy feeling. The results of liver tissue TG and TC content determination showed that the TG and TC content in liver tissue under the intervention of IgG2-Fc / Irisin fusion protein was significantly decreased compared with the HFD group. HE staining was used to observe and compare the pathological changes of liver tissue. The results showed that the hepatocyte morphology and hepatic cord arrangement of mice in the HFD+Irisin group were close to normal, and the intracellular lipid droplet vacuoles and ballooning were alleviated. The NAFLD activity score (NAS) was significantly reduced, indicating that IgG2-Fc / Irisin fusion protein can play a protective role in the liver. Oil red O staining was used to observe and compare lipid deposition in liver tissue. The experimental results showed that IgG2-Fc / Irisin fusion protein intervention significantly reduced the red lipid droplets accumulated in hepatocytes, significantly alleviated lipid deposition, improved fatty degeneration, and significantly reduced the lipid content in the liver of mice fed a high-fat diet. IgG2-Fc / Irisin fusion protein reduces the levels of hepatic inflammatory factors in mice fed a high-fat diet Western blot experiments were used to detect the protein expression levels of inflammatory factors TNF-α and IL-6 in the liver. The results showed that IgG2-Fc / Irisin fusion protein intervention reduced the expression of inflammatory factors TNF-α and IL-6 proteins induced by HFD. Example 5. IgG2-Fc / Irisin fusion protein for the treatment of hepatic steatosis in mice fed a high-fat diet In vivo study of IgG2-Fc / Irisin fusion protein in the treatment of liver steatosis and NAFLD: As shown in FIG4B , after completing 12 weeks of high-fat feeding, HFD mice received 4 weeks of IgG2-Fc / Irisin fusion protein (prepared in Example 1) injection intervention (1.5 μg / g) for a total of 16 weeks. The results are as follows: 5.1. IgG2-Fc / Irisin fusion protein improves high-fat diet-induced impaired glucose tolerance and insulin resistance IPGTT and IPITT experiments evaluated the effects of IgG2-Fc / Irisin fusion protein on glucose metabolism and insulin sensitivity. The results of IPGTT showed (Figure 6A, 6B) that IgG2-Fc / Irisin fusion protein improved impaired glucose tolerance; the results of IPITT showed (Figure 6C, 6D) that IgG2-Fc / Irisin fusion protein significantly improved insulin sensitivity after intervention. The above results suggest that IgG2-Fc / Irisin fusion protein improves impaired glucose tolerance and insulin resistance caused by a high-fat diet. IgG2-Fc / Irisin fusion protein reduces serum transaminase and lipid levels induced by high-fat diet The results in Figure 7 show that IgG2-Fc / Irisin fusion protein intervention for 4 weeks reduced the serum ALT and AST levels induced by a high-fat diet, indicating that IgG2-Fc / Irisin fusion protein can inhibit liver damage induced by a high-fat diet; IgG2-Fc / Irisin fusion protein intervention reduced serum TG, TC and LDL levels. The results suggest that IgG2-Fc / Irisin fusion protein can reduce the blood lipid levels of mice fed a high-fat diet and play a role in protecting the liver. IgG2-Fc / Irisin fusion protein reduces high-fat diet-induced fat accumulation and hepatic lipid accumulation The inventors found that intervention with IgG2-Fc / Irisin fusion protein significantly reduced the weight of liver, para-epididymal fat, and inguinal subcutaneous adipose tissue (Table 7). The inventors compared the liver index to assess the degree of liver fat accumulation, and the results showed that the liver index decreased significantly after intervention with IgG2-Fc / Irisin fusion protein. Photos of liver tissue showed that the liver in the NCD group was bright dark red, soft in texture, and sharp in edges; the liver in the HFD group was significantly larger in volume, dark yellow in color, blunt in edges, and greasy in cross section; after intervention with IgG2-Fc / Irisin fusion protein (i.e., HFD+Irisin group), the color, volume, and texture of liver tissue were between the NCD group and the HFD group. The results of liver lipid content showed that intervention with IgG2-Fc / Irisin fusion protein significantly reduced the increase in liver fat content induced by a high-fat diet. Table 7. Weights of liver, para-epididymal and inguinal subcutaneous adipose tissue in each group of mice HE results showed that after the intervention of IgG2-Fc / Irisin fusion protein, the morphology of hepatocytes and the arrangement of hepatic cords returned to normal, and pathological changes such as the content of vacuolar lipid droplets in the cytoplasm, ballooning and inflammatory cell infiltration were significantly alleviated; the NAS score was significantly reduced. The results of Oil Red O staining (Table 8) showed that IgG2-Fc / Irisin fusion protein intervention could significantly reduce fatty degeneration in hepatocytes induced by a high-fat diet, and could significantly reduce the lipid content in the liver of mice induced by a high-fat diet. Table 8. Relative percentage of the accumulated area of ​​red lipid droplets in liver tissue of each group IgG2-Fc / Irisin fusion protein alleviates the elevated levels of hepatic inflammatory factors induced by a high-fat diet Western blot was used to detect the protein expression levels of inflammatory factors TNF-α and IL-6 in the liver. The results in Table 9 (quantitative data) showed that after 4 weeks of IgG2-Fc / Irisin fusion protein intervention (i.e., HFD+Irisin group), the expression level of TNF-α in the liver was significantly decreased, and the expression of IL-6 showed a downward trend, indicating that IgG2-Fc / Irisin fusion protein inhibited the liver inflammatory response induced by a high-fat diet. Table 9. Irisin reduces the levels of liver inflammatory factors in mice fed a high-fat diet Example 6. Construction of a cellular fatty degeneration model by treating HepG2 cells with PA 6.1 Cell Experiment Protocol and Experimental Methods 6.1.1 Preparation of Palmitic Acid (PA) Solution 1) Weigh 40 mg of NaOH and dissolve it in 10 ml of ddH2 O to prepare a working solution with a concentration of 0.1 mM and place it at room temperature for later use; 2) Weigh 1 g of bovine serum albumin (BSA) powder and dissolve it in 19 ml of ddHO. 2 O, and heated in a 55°C water bath for 30 min to help BSA dissolve fully; 3) Weigh 33.4 mg of palmitic acid (PA) powder and add it to 1 ml of NaOH (0.1 mM) solution, and heat it in a metal bath at 90°C for 30 min; 4) After the BSA and PA solutions are completely dissolved, quickly add 1 ml of NaOH solution containing PA to the BSA solution, mix well and continue heating in a 55°C water bath for 30 minutes. If the PA solution is clear without precipitation, it indicates that the 6 mM stock solution of PA has been successfully prepared; 5) Add 1 ml of NaOH solution without PA to another tube of BSA solution to obtain a control solution; 6) Filter and package in a clean bench, store at -20℃ for long term and 4℃ for short term. Heat in a 55℃ water bath before each use, and use after the solution is fully dissolved. 6.1.2 HepG2 cell recovery 1) Take out a cell cryopreservation tube from the liquid nitrogen tank, put it into a disposable film glove and quickly put it into a 37℃ water bath, gently shake it to completely thaw it within 1-2 minutes; 2) Wipe the cryotube with 75% alcohol and place it in a clean bench. Open the lid, aspirate the cell suspension with a pipette, add it to a 15 ml centrifuge tube, add more than 2 times the DMEM high-glucose medium solution, and mix by pipetting. 3) Centrifuge at 1000 rpm for 5 min; 4) Discard the supernatant, add 10 ml of fresh DMEM high-glucose medium, and gently pipette to form a cell suspension; 5) Transfer the cell suspension to a 6 cm culture dish, gently shake the dish back and forth to evenly distribute the cells, and culture it in a 37°C incubator. On the second day, observe the cell adhesion and growth and replace with fresh culture medium. 6.1.3 HepG2 cell passaging 1) Observe that the cells can be passaged when the cell growth density reaches 80-90% confluence; 2) Heat the DMEM high-glucose medium, PBS buffer, and trypsin solution in a 37°C water bath in advance; 3) Aspirate the culture medium in the original culture dish, rinse the cells once with 2-5 ml of sterile PBS solution to remove residual serum, then add 1 ml of trypsin digestion solution (0.25%) and place the culture dish in a 37°C incubator for about 1 min 20 s; 4) After digestion is complete, take the culture dish out of the incubator, remove the trypsin digestion solution, and add 2 ml of PBS solution; wash the cells once, and finally add 2-5 ml of fresh culture medium. Use a pipette to blow the cells and collect the cell suspension. Control the force when blowing to avoid a large number of bubbles; 5) Inoculate the cell suspension into three other clean and sterile culture dishes, add fresh culture medium, and place them at 37°C CO 2 The cells were cultured in an incubator and their adhesion and growth were observed on the second day. 6.1.4 Cell Oil Red O Staining 1) Remove the cell culture medium from the 12-well plate and gently rinse with PBS solution for 3 times; 2) Add 4% paraformaldehyde solution (about 500 μl per well) and fix at room temperature for 10-15 minutes; 3) Wash the cells 3 times by adding PBS solution, 3 min each time; 4) Add 500 μl of 60% isopropanol solution to each well, let stand at room temperature for 10 min, then aspirate and dry; 5) Add 1 ml of Oil Red O dye to each well and let stand at room temperature for 30 minutes; 6) Aspirate the Oil Red O dye solution and wash the plate with PBS solution 5 times, 3 minutes each time. After washing, dry the plate; 7) Add 500 μl of 60% isopropanol solution to each well for about 5 seconds until the intercellular matrix is ​​clear, then quickly aspirate and rinse again with PBS solution for 3 times, 3 minutes each time, and let dry; 8) Counterstain with hematoxylin for 1-2 min, then wash the cells three times with PBS solution to remove excess floating color and residues; 9) Seal the slides with glycerol and observe and photograph them under a microscope as soon as possible. 6.1.5 Cell immunofluorescence staining 1) Cell seeding and drug addition: Sterile slides were pre-placed in a 24-well plate, and then 500 μl of cell suspension was added to each well to evenly distribute the cells. When the cell growth confluence reached 70-80%, the original cell culture medium was removed, and fresh culture medium and corresponding drug treatment were added; 2) Fixation: Discard the original culture medium, add 1 ml PBS solution to each well to wash the cells twice, then add 4% paraformaldehyde solution to fix the cells at room temperature for 10-15 minutes, 400 μl per well; 3) Blocking: remove the fixative and add PBS to wash the cells twice, then add 10% sheep serum solution diluted with 1× PBS to block at room temperature for 1 hour; 4) Primary antibody incubation: Gently pick up the slides in the 24-well plate with tweezers and transfer them to a glass slide. Quickly add 70 μl of primary antibody (MFN2, rabbit, 1:50 or p-DRP1, rabbit, 1:800 or TOMM20, rabbit, 1:500) diluted in 5% BSA to the slides and incubate the cells. For the negative control group, only 5% BSA was added and incubated overnight at 4°C. 5) Secondary antibody incubation: Recover the primary antibody, add PBS buffer and slowly wash the slide 3 times, 5 minutes each time. Incubate the cells with FITC-labeled goat anti-rabbit fluorescent secondary antibody containing 2% goat serum, add about 70 μl to each slide, and incubate at room temperature in the dark for 1 hour; 6) DAPI nuclear staining: Wash slowly with PBS buffer for 3 times, 5 minutes each time, then add DAPI staining solution to the slide and incubate at room temperature in the dark for 5-10 minutes; 7) Sealing: Wash the cells 3 times with PBS buffer, 5 minutes each time, pick up the slide gently with tweezers, and absorb the water remaining on the edge of the slide with absorbent paper. Add 10μl of anti-fluorescence quencher on the slide, turn the slide upside down slowly and pay attention to prevent bubbles from forming, store at 4℃ away from light, observe under a microscope and take pictures as soon as possible. 6.1.6 Cell protein extraction 1) Take the cell plate out of the 37°C incubator, remove the original culture medium, and add 1 ml of PBS buffer to each well to wash the cells once; 2) Add freshly prepared RIPA protein lysis buffer containing protease inhibitors and phosphatase inhibitors to the well plate (6-well plate: 150 μl / well; 12-well plate: 120 μl / well), and place it in an ice box and shake vigorously on a shaker for 30 minutes; 3) Pipette the protein lysate from each well into the corresponding numbered 2 ml EP tube, centrifuge at 14000g for 30 min at 4°C, and pay attention to balancing; 4) Use a 10 μl pipette to remove the white gelatinous precipitate (i.e., genomic DNA and other complexes) at the bottom of the tube, perform BCA protein concentration determination, and use protein lysis buffer to adjust the concentration of each sample to be consistent; 5) Add 5× loading buffer and mix the protein solution by pipetting. Heat the sample in a 100°C metal bath for 10 min and store at -20°C. 6.1.7 Trizol method for extracting cellular RNA 1) Precool the isopropanol and 75% ethanol solutions to -20°C in advance; 2) After the cell treatment, the original culture medium was removed, 1× PBS solution was added to wash the cells three times and the residual liquid was removed; 3) Add Trizol reagent (12-well plate: 1 ml; 24-well plate: 500 μl) to fully cover the cell surface and let stand at room temperature for 15 min; 4) Repeatedly pipette to fully lyse the cells, and pipette the lysate from each well into the corresponding 2 ml EP tube with a number. Then add chloroform to each tube (12-well plate: 200 μl; 24-well plate: 100 μl), vortex vigorously, mix thoroughly, make it appear uniform pink, and let it stand at room temperature for 15 minutes; 5) Centrifuge at 12000g for 15 min at 4°C; 6) Carefully pipette the upper aqueous phase from the tube with a 100 μl pipette and transfer it to a new 2 ml EP tube free of RNase, then add an equal volume of isopropanol solution to the supernatant, mix by inverting 10 times, and let stand at room temperature for 15 min; 7) Centrifuge at 12000g for 10 min at 4°C; 8) Observe the bottom of the tube for white RNA precipitation and discard the supernatant, add 1 ml of 75% ethanol, and mix thoroughly by pipetting. If necessary, vortex the tube to mix thoroughly. 9) Centrifuge at 7500 g for 5 min at 4 °C; 10) Discard the supernatant and repeat steps 8-9 to wash the precipitate again; 11) Discard the supernatant, turn the EP tube upside down on clean absorbent paper, and let it stand for 10 minutes to allow the residual ethanol to evaporate and dry; 12) Add 20-30 μl DEPC water to fully dissolve the RNA. Keep the RNA sample on ice to prevent degradation. Use NanoDrop ultra-micro spectrophotometer to detect RNA purity and concentration. 6.1.8 Mitotracker Red live cell mitochondrial staining 1) Preparation of storage solution: Take a tube of 50 μg Mitotracker Red powder and centrifuge it at low speed for a few seconds to allow the trace amount of powder to settle to the bottom of the tube, then add 470 μl DMSO to fully dissolve it to obtain a storage solution with a concentration of 200 μM, and store it in aliquots at -20°C away from light. 2) Preparation of working solution: Dilute the Mitotracker Red stock solution to the working solution concentration of 200 nM at 1:1000 with an appropriate buffer such as Hank's solution or serum-free DMEM high-glucose medium. Pre-incubate the staining working solution at 37°C before use. 3) HepG2 cell mitochondrial staining: When the cell fusion density in the culture plate reaches the required abundance, remove the original culture medium and add 37°C preheated Gently rinse the cells 1-2 times with PBS solution and discard; Add Mitotracker Red working solution and incubate at 37°C for 30 min; Remove the Mitotracker Red working solution and add PBS solution to wash the cells 1-2 times; Add 1× Hoechst 33342 dye diluted with Hank's solution and incubate at 37°C for 2-3 minutes; Remove the 1× Hoechst 33342 stain and wash the cells again 1-2 times with PBS solution; Add serum-free DMEM high-glucose medium preheated at 37°C, wrap the culture plate with tin foil, observe the mitochondrial morphology under a microscope as soon as possible and take photos to record. 6.1.9 BODIPY FL C16 fluorescent probe staining Principle: BODIPY FL C16 is palmitic acid labeled with boron dipyrrole fluoride fluorescence (green fluorescence). This fluorescent dye can be used to observe the uptake of fatty acids by cells and indicate the activity of fatty acid oxidation metabolism. 1) Preparation of storage solution: Take a tube of 1 mg BODIPY FL C16 (molecular weight: 474 g / mol) powder and centrifuge it at low speed for a few seconds to allow the trace powder to settle to the bottom of the tube, then add 830 μl DMSO to fully dissolve it to obtain a storage solution with a concentration of 2.5 mM, and store it in aliquots at -20°C away from light; 2) Preparation of working solution: Dilute the BODIPY FLC16 stock solution at a ratio of 1:25,000 with serum-free DMEM high-glucose medium to a working solution concentration of 100 nM. 3) HepG2 cell staining: When the cell fusion density in the 24-well plate reaches the required abundance, remove the original culture medium, add 37°C preheated PBS solution to gently rinse the cells 1-2 times and discard; Add 500 μl of preheated BODIPY FL C16 working solution to each well and incubate in a 37°C CO2 incubator for 30 min; Aspirate the staining solution, gently rinse the cells 1-2 times with PBS solution and discard; After staining, the cells were fixed with 4% paraformaldehyde and the subsequent immunofluorescence staining experiment was performed with the same steps as described above. 6.1.10 MitoSOX live cell mitochondrial superoxide detection Principle: This fluorescent probe can penetrate living cell membranes and selectively enter mitochondria. The probe can be oxidized by superoxide to emit red fluorescence. 1) Preparation of stock solution: Take a tube of 50 μg of MitoSOX (molecular weight: 759.71 g / mol) powder and centrifuge it at low speed for 1 min to sediment the trace powder to the bottom of the tube. Then add 13 μl of DMSO to dissolve it completely to obtain a stock solution with a concentration of 5 mM. Aliquot and store it at -20 °C in the dark, avoiding excessive contact with air. 2) Preparation of working solution: Dilute the MitoSOX stock solution with Hank’s solution at a ratio of 1:1000 to the working solution concentration, i.e., 5 μM. 3) Staining of HepG2 cells: Aspirate the original culture medium in the 24-well culture plate, add pre-warmed Hank’s solution at 37 °C, gently wash the cells 1 - 2 times and discard; Add 500 μl of MitoSOX working solution to each well and incubate in the dark at 37 °C for 10 min; Aspirate the MitoSOX fluorescent probe working solution, add Hank’s solution to wash the cells 1 - 2 times; Add 1× Hoechst 33342 staining solution diluted with Hank’s solution and incubate at 37 °C for 2 - 3 min; Remove the 1× Hoechst 33342 staining solution and wash the cells again with Hank’s solution 1 - 2 times; Add pre-warmed serum-free high-glucose DMEM medium at 37 °C, wrap the culture plate with tin foil and perform microscopic examination as soon as possible. 6.1.11 Detection of JC-1 mitochondrial membrane potential Principle: When the mitochondrial membrane potential is high, the JC-1 fluorescent probe aggregates in the mitochondrial matrix to form polymers (J-aggregates), emitting red fluorescence; when the mitochondrial membrane potential is low, the JC-1 fluorescent probe cannot aggregate in the mitochondrial matrix, and at this time it is a monomer (J-monomer), emitting green fluorescence. The relative ratio of red / green fluorescence is commonly used to measure the degree of mitochondrial depolarization. 1) Preparation of working solution: Take an appropriate amount of JC-1 staining stock solution (200×), dilute it according to the ratio of adding 5 μl of JC-1 staining stock solution (200×) to every 1 ml of JC-1 staining buffer, and mix well by repeated pipetting. 2) Staining of HepG2 cells: Aspirate the original culture medium in the 24-well culture plate and wash the cells once with pre-warmed PBS; Add 500 μl of fresh high-glucose DMEM medium and 500 μl of JC-1 staining working solution to each well, mix well, and incubate in the dark at 37 °C for 30 min; During the incubation, prepare an appropriate amount of JC-1 staining buffer according to the ratio of adding 4 ml of ddH2O to every 1 ml of JC-1 staining buffer (5×) and place it in an ice bath; After the incubation at 37°C, the supernatant was removed by aspiration, and the cells were washed twice with JC-1 staining buffer and discarded; Add 1 ml of DMEM high-glucose medium to each well, wrap the culture plate with tin foil and observe under a microscope as soon as possible. 6.1.12 siRNA cell transfection 1) Sirt3 siRNA information: purchased from Shanghai Tuoran Biotechnology Co., Ltd. Primer name: sequence (5'→3') Sirt3(human)siRNA-158: rG / / rG / / rA / / rA / / rG / / rA / / rA / / rG / / rG / / rU / / rC / / rC / / rA / / rU / / rA / / rU / / rC / / rU / / rU / / TT Sirt3(human)siRNA-1411: rG / / rC / / rU / / rU / / rU / / rC / / rU / / rG / / rU / / rG / / rC / / rC / / rU / / rA / / rG / / rU / / rU / / rG / / rA / / TT Sirt3(human)siRNA-1597: rC / / rA / / rU / / rG / / rA / / rA / / rA / / rU / / rA / / rC / / rA / / rU / / rU / / rU / / rA / / rG / / rU / / rC / / rU / / TT 2) Preparation of storage solution: Before use, centrifuge briefly at low speed and gently open the tube cap. Add 125 μl RNase-free H2O to prepare 20 μM storage solution. Aliquot and store at -20°C to avoid repeated freezing and thawing. 3) Transfection experiment grouping: Blank control group: no transfection reagent and siRNA; Negative control siRNA group: negative control siRNA and transfection reagent Lipofectamine 3000 were added; Transfection reagent group: only the transfection reagent Lipofectamine 3000 was added without siRNA; Experimental siRNA group: Sirt3 siRNA and transfection reagent Lipofectamine 3000 were added; Positive control group: GAPDH siRNA and transfection reagent Lipofectamine 3000 were added. 4) HepG2 cell transfection steps (taking 24-well plate as an example): √24h before transfection, resuspend 0.5-2×105 cells in 500μl DMEM high-glucose medium without double antibody and inoculate into the well plate to make the cell confluency reach 30-50%; √ Use 50 μl Opti-MEM medium to dilute Sirt3 siRNA or Scrambled siRNA, pipette gently 3-5 times to mix thoroughly, and let stand at room temperature for 5 minutes; √ Slowly invert to mix Lipofectamine 3000 transfection reagent, dilute 1μl Lipofectamine 3000 with 50μl Opti-MEM medium, and gently pipette 3-5 times to mix thoroughly; √Mix the transfection reagent and siRNA diluent, pipette gently 3-5 times to mix well, and let stand at room temperature for 20 minutes; √ Add the transfection complex to the cells in the 24-well plate in sequence, 100 μl / well, and then add 400 μl of DMEM high-glucose medium without double antibody and serum to each well. Gently shake the culture plate back and forth and place it in a 37°C incubator; √Observe the cell growth status under a microscope 6 hours after transfection, discard the supernatant, and add fresh complete medium without double antibodies; √ Subsequent cell treatment and other operations can be started 24 hours after transfection. 6.1.13 Overexpression plasmid cell transfection 1) Sirt3 overexpression plasmid information Purchased from Shanghai Jikai Gene Medical Technology Co., Ltd. (vector name: GV658; element sequence: CMV enhancer-MCS-polyA-EF1A-zsGreen-sv40-puromycin) 2) Transfection experiment grouping Blank control group: no transfection reagent and plasmid; Negative control group: empty plasmid and transfection reagent Lipofectamine 3000 were added; Transfection reagent group: only the transfection reagent Lipofectamine 3000 was added without plasmid; Experimental overexpression plasmid group: Sirt3 overexpression plasmid and transfection reagent Lipofectamine 3000 were added. 3) HepG2 cell transfection steps (taking 24-well plate as an example): √24h before transfection, HepG2 cells were evenly seeded into 24-well plates to make the cell confluence about 70-80%; √50μl Opti-MEM medium was used to dilute the Sirt3 overexpression plasmid or the empty control plasmid containing GFP, and the mixture was mixed by pipetting 3-5 times gently, and then allowed to stand at room temperature for 5 minutes; √ Slowly invert to mix Lipofectamine 3000 transfection reagent, dilute 1μl Lipofectamine 3000 with 50μl Opti-MEM medium, and gently pipette 3-5 times to mix thoroughly; √Mix the transfection reagent and plasmid diluent, pipette gently 3-5 times to mix well, and let stand at room temperature for 20 minutes; √ Add the transfection complex to the cells in the 24-well plate in sequence, 100 μl / well, and then add 400 μl of DMEM high-glucose medium without double antibody and serum to each well. Gently shake the culture plate back and forth and place it in a 37°C incubator; √Observe the cell growth status under the microscope 6 hours after transfection, discard the supernatant, and add fresh complete medium without double antibody √ Subsequent cell treatment and other operations can be started 24 hours after transfection. 6.2. PA induces lipid accumulation in HepG2 cells HepG2 cells were treated with PA to establish a cellular fatty degeneration model. Oil red O staining results showed that compared with the control group, as the PA concentration increased, the content of red lipid droplets in the cells increased, indicating that the model was successfully established. PA induces increased SREBP-1c activation in HepG2 cells Western blot experimental results showed that PA promoted the activation of SREBP-1c in a concentration-dependent manner, that is, the expression level of mature SREBP-1c (mSREBP-1c) increased. 6.4. PA induces imbalance of mitochondrial dynamics in HepG2 cells HepG2 cells were treated with different concentrations of PA for 24 h, and the expression levels of mitochondrial fission protein p-DRP1, mitochondrial fusion proteins MFN2 and OPA1 were detected by Western blot and immunofluorescence assay, respectively. Mitochondrial morphological changes were evaluated using Mitotracker Red mitochondrial red fluorescent probe, and mitochondria were labeled with Translocase of outer membrane 20 (TOMM20). Immunofluorescence assay was used to detect PA-induced mitochondrial morphological changes and quantitative statistical analysis was performed. 6.4.1PA increases mitochondrial fission in HepG2 cells Immunofluorescence results showed that the fluorescence intensity of p-DRP1 (S616) protein gradually increased with the increase of PA concentration, showing concentration dependence and statistical significance; similarly, the results of Western blot experiments were consistent with the above results, indicating that PA induced an increase in p-DRP1 protein expression, suggesting that PA promotes mitochondrial fission in cells. Mitotracker Red staining was used to initially observe the mitochondrial morphology. It was found that after 24 hours of PA treatment of HepG2 cells, the mitochondrial morphology changed from a continuous network in the control group to discontinuous fragments and dots, and the mitochondrial fluorescence density in the PA-treated group was weaker than that in the control group, especially in the PA 300 μM group, indicating that PA induced increased mitochondrial fission. To further visualize and quantify the changes in mitochondrial length in each group, the results of TOMM20 immunofluorescence staining showed that the mitochondrial morphology of HepG2 cells in the control group was long rod-shaped or interconnected into a network. As the concentration of PA increased, the mitochondrial morphology was significantly fragmented, producing a large number of dot-shaped mitochondrial fragments. The ImageJ Mitochondria Analyzer plug-in was used to analyze and calculate the mitochondrial form factor (FF) and aspect ratio (AR). The FF calculation formula = (perimeter^2) / (4π*area); AR calculation formula = major axis / minor axis. The larger the FF and AR values, the closer the mitochondrial morphology is to a long rod, and the smaller the values, the more round the mitochondrial morphology is, and the more mitochondrial fissions there are. In addition, FF can also reflect the mitochondrial branching to a certain extent. The statistical results showed that compared with the control group, the FF and AR values ​​of the PA treatment group were negatively correlated with the PA concentration, indicating that PA induced an increase in mitochondrial fission in HepG2 cells. 6.4.2 PA reduces mitochondrial fusion in HepG2 cells Immunofluorescence results showed that the fluorescence intensity of MFN2 gradually decreased in a dose-dependent manner with the increase of PA concentration, and Western blot results also showed that PA reduced the expression level of MFN2 protein. In addition, Western blot was used to detect the expression of OPA1, a key protein responsible for mitochondrial inner membrane fusion, in each group of cells. The results showed that PA also reduced the expression level of OPA1 in a concentration-dependent manner. The fluorescence intensity of OPA1 in the PA concentration treatment group of 300 and 500 μM was statistically significant compared with that in the control group. Example 7. IgG2-Fc / Irisin fusion protein improves PA-induced HepG2 cell steatosis model IgG2-Fc / Irisin fusion protein alleviates PA-induced lipid accumulation in HepG2 cells In order to verify whether the IgG2-Fc / Irisin fusion protein prepared in Example 1 can play a direct protective role on hepatocytes, HepG2 cells were co-incubated with different concentrations of IgG2-Fc / Irisin fusion protein recombinant protein (25, 50, 100 nM) and 300 μM PA for 24 h, and the lipid deposition in each group of cells was detected by oil red staining. The results in Figure 8 show that IgG2-Fc / Irisin fusion protein can reduce PA-mediated lipid accumulation, and the red lipid droplet content in the IgG2-Fc / Irisin fusion protein concentration of 100 nM group is significantly lower than that in the PA group alone, which is statistically significant. IgG2-Fc / Irisin fusion protein improves PA-induced imbalance of mitochondrial dynamics in HepG2 cells In the in vitro experiments, immunofluorescence, Western blot and RT-qPCR experiments were used to clarify the regulatory effect of IgG2-Fc / Irisin fusion protein on mitochondrial dynamics at the protein and mRNA levels. In addition, HepG2 cells were co-incubated with different concentrations of IgG2-Fc / Irisin fusion protein (25, 50, 100 nM, prepared in Example 1) and 300 μM PA for 24 hours, and mitochondria were labeled with TOMM20 for immunofluorescence staining, and the mitochondrial morphology of each group of cells was observed and the shape coefficient and aspect ratio values ​​were calculated. 7.2.1 IgG2-Fc / Irisin fusion protein inhibits mitochondrial fission The results of immunofluorescence showed that IgG2-Fc / Irisin fusion protein (prepared in Example 1) inhibited PA-induced DRP1 activation, i.e., reduced the expression level of p-DRP1 (S616); the Western blot results in Figure 9A further verified that IgG2-Fc / Irisin fusion protein inhibited PA-induced p-DRP1 expression increase in a dose-dependent manner; the RT-qPCR experimental results in Figure 9B showed that PA significantly upregulated the expression of mitochondrial fission-related genes Drp1 and Mff, and IgG2-Fc / Irisin fusion protein intervention could reverse this effect. Figure 9C shows that TOMM20 immunofluorescence staining showed that IgG2-Fc / Irisin fusion protein could inhibit excessive mitochondrial fission induced by PA in a dose-dependent manner, restoring the mitochondrial morphology in cells from short dots to long rods. Compared with the PA group alone, IgG2-Fc / Irisin fusion protein intervention inhibited mitochondrial fission. These results show that IgG2-Fc / Irisin fusion protein inhibits mitochondrial fission. 7.2.2 IgG2-Fc / Irisin fusion protein promotes mitochondrial fusion The results of immunofluorescence and Western blot (Figures 9D and 9E) showed that the intervention of IgG2-Fc / Irisin fusion protein (prepared in Example 1) increased the expression levels of MFN2 and OPA1 proteins in a concentration-dependent manner. The RT-qPCR results of Figure 9F also found that the downregulation of the levels of mitochondrial fusion-related genes Mfn2, Mfn1 and Opa1 induced by PA could be corrected by IgG2-Fc / Irisin fusion protein, which was statistically significant. These results show that IgG2-Fc / Irisin fusion protein promotes mitochondrial fusion. IgG2-Fc / Irisin fusion protein reverses PA-induced mitochondrial dysfunction in HepG2 cells At the cellular level, HepG2 cells were co-incubated with PA and different concentrations of IgG2-Fc / Irisin fusion protein (prepared in Example 1) for 24 hours, and then the mitochondrial ROS levels and membrane potential changes of each treatment group were determined using MitoSOX and JC-1 fluorescent probes. The results of Figure 10 show that PA induced a significant increase in the ROS content in mitochondria, and 25nM IgG2-Fc / Irisin fusion protein can significantly reduce the ROS content and is concentration-dependent. Using JC-1 to detect mitochondrial membrane potential, a decrease in the red / green mean fluorescence intensity ratio represents a depolarization of the mitochondrial membrane potential. The results showed that IgG2-Fc / Irisin fusion protein can significantly inhibit the decrease in mitochondrial membrane potential induced by PA. IgG2-Fc / Irisin fusion protein increases the amount of fatty acids entering mitochondria After incubating HepG2 cells with 300 μM PA and different concentrations of IgG2-Fc / Irisin fusion protein (prepared in Example 1) for 24 h, the original culture medium was discarded and the cells were rinsed once with PBS, and then 500 μl of DMEM complete medium diluted with 100 nM BODIPY FL C16 (i.e., green fluorescent labeled palmitic acid) was added to each well to locate the long-chain fatty acids entering the cells. After incubation at 37° C. for 30 min, the cells were fixed and subsequently TOMM20 immunofluorescence staining was performed to mark mitochondria, and the Pearson correlation coefficient was calculated for co-localization analysis to determine whether IgG2-Fc / Irisin fusion protein affects the content of fatty acids entering mitochondria. The results of Figure 11 show that compared with the control group, the Pearson correlation coefficient of green fluorescent labeled palmitic acid and red fluorescent labeled mitochondria in the PA treatment group was significantly reduced, indicating that the content of fatty acids entering the mitochondria was low. The IgG2-Fc / Irisin fusion protein can increase the content of palmitic acid entering the mitochondria in a dose-dependent manner, and the Pearson correlation coefficient of the IgG2-Fc / Irisin fusion protein at 100nM is statistically significant compared with the PA group alone. This result suggests that the IgG2-Fc / Irisin fusion protein may improve the lipid accumulation of hepatocytes by increasing the content of fatty acids entering the mitochondria and promoting β-oxidation. Example 8. IgG2-Fc / Irisin fusion protein improves liver metabolic imbalance induced by high-fat diet 8.1. IgG2-Fc / Irisin fusion protein promotes liver β-oxidation and inhibits lipid synthesis The expression of CPT1α, NRF2 and SREBP-1c in the liver of each group of mice in Example 3 was detected by Western blot. The results in Table 10 (after protein quantification) show that the expression levels of CPT1α and NRF2 in the liver of mice in the high-fat diet group were significantly inhibited, and the expression of SREBP-1c was significantly increased compared with the normal diet control group. However, after 12 weeks of preventive intervention (i.e., HFD+Irisin group) or 4 weeks of therapeutic intervention with IgG2-Fc / Irisin fusion protein, the expression of CPT1α and NRF2 was significantly upregulated, and SREBP-1c activation was inhibited. Secondly, at the in vitro level, RT-qPCR was used to determine the expression of genes related to fatty acid β-oxidation and lipid synthesis. The results of Figures 12A and 12B show that IgG2-Fc / Irisin fusion protein can significantly increase the expression of genes related to β-oxidation and downregulate the level of genes related to lipid synthesis. The above results show that IgG2-Fc / Irisin fusion protein can promote fatty acid β-oxidation and inhibit lipid synthesis, and improve liver lipid metabolism disorders. Table 10. Expression levels of CPT-1α, NRF2 and SREBP-1c proteins in liver tissues of mice in each group in the IgG2-Fc / Irisin fusion protein intervention study 8.2. IgG2-Fc / Irisin fusion protein improves mitochondrial dynamics imbalance in hepatocytes induced by high-fat diet 8.2.1 IgG2-Fc / Irisin fusion protein inhibits mitochondrial fission In Examples 4 and 5, it was verified that the IgG2-Fc / Irisin fusion protein prepared in Example 1 reduced the lipid accumulation and inflammatory response in the liver of mice fed a high-fat diet. In order to verify that the IgG2-Fc / Irisin fusion protein regulates and corrects the imbalance of mitochondrial dynamics in the liver of mice induced by a high-fat diet, immunofluorescence staining, immunohistochemistry staining and Western blot experiments were used to detect the expression levels of mitochondrial fusion and fission-related proteins in the liver of each group of mice. Table 11 Western blot detection (quantitative data) results show that a high-fat diet increases the activation of DRP1, suggesting that a high-fat diet induces mitochondrial fission. However, the IgG2-Fc / Irisin fusion protein can significantly reduce the expression of p-DRP1 and inhibit mitochondrial fission, whether as a preventive or therapeutic intervention. 8.2.2 IgG2-Fc / Irisin fusion protein promotes mitochondrial fusion Consistent with the cell level, the results of Western blot detection (quantitative data) in Table 11 showed that IgG2-Fc / Irisin fusion protein could upregulate the expression of mitochondrial fusion-related proteins MFN2 and OPA1 in the liver of mice fed a high-fat diet in both prevention and treatment. The above studies suggest that IgG2-Fc / Irisin fusion protein can regulate and improve the imbalance of liver mitochondrial dynamics induced by a high-fat diet, inhibit excessive mitochondrial fission and promote mitochondrial fusion, and restore mitochondrial dynamic homeostasis. Table 11. Quantitative statistics of p-DRP1, MFN2 and OPA1 expression levels in liver tissues of mice in each group in the prevention and treatment intervention study of IgG2-Fc / Irisin fusion protein Example 9. Upstream signaling pathways of IgG2-Fc / Irisin fusion protein regulating mitochondrial dynamics 9.1. IgG2-Fc / Irisin fusion protein activates PKA, upregulates SIRT3 and inhibits activation of the mTOR signaling pathway HepG2 cells were co-incubated with 300 μM PA and different concentrations of IgG2-Fc / Irisin fusion protein (prepared in Example 1) for 24 h, and the changes in the above signaling pathways were detected by Western blot. The results showed that PA significantly inhibited PKA activation and SIRT3 expression, while promoting mTOR activation, while IgG2-Fc / Irisin fusion protein could reverse the above effects in a dose-dependent manner. The results of cell experiments were consistent with those of animal experiments. IgG2-Fc / Irisin fusion protein could significantly activate PKA, upregulate SIRT3 and inhibit mTOR activation, suggesting that IgG2-Fc / Irisin fusion protein could improve the imbalance of hepatocyte kinetics induced by palmitic acid or high-fat diet by regulating the above signaling pathways. 9.2. IgG2-Fc / Irisin fusion protein activates PKA to reverse PA-induced mitochondrial dynamics imbalance On the basis of co-incubating HepG2 cells with 300μM PA and 100nM IgG2-Fc / Irisin fusion protein (prepared in Example 1) for 24h, PKA agonist Forskolin and PKA inhibitor PKI (Protein kinase inhibitor) were added to further verify whether IgG2-Fc / Irisin fusion protein can regulate mitochondrial dynamics by acting on the PKA signaling pathway. 30min before PA and IgG2-Fc / Irisin fusion protein intervention cells, fresh culture medium containing 10μM Forskolin was added to incubate cells for 30min, then the original culture medium was discarded, and PA and IgG2-Fc / Irisin fusion protein intervention was given; 5μM PKI was added together with PA and IgG2-Fc / Irisin fusion protein for 24h intervention. The Western blot results in Figure 13 and Table 12 show that IgG2-Fc / Irisin fusion protein can increase the expression of p-PKA and SIRT3, inhibit the level of p-mTOR, reverse the PA-induced decrease of mitochondrial fusion proteins MFN2 and OPA1 and the increase in the expression of mitochondrial fission protein p-DRP1, and the above effects can be significantly inhibited by PKI. At the same time, Forskolin can work together with IgG2-Fc / Irisin fusion protein to synergistically activate PKA, upregulate the expression of SIRT3, MFN2 and OPA1, and inhibit the activation of mTOR and DRP1. The above findings suggest that IgG2-Fc / Irisin fusion protein reverses PA-induced mitochondrial dynamics imbalance by activating the PKA signaling pathway, and the expression of SIRT3 and mTOR activation are regulated by PKA. Table 12. Western blot detection of p-PKA, SIRT3 and p-mTOR protein expression levels and quantitative statistics in liver tissues of mice in each group in the Irisin prevention and intervention study 9.3. IgG2-Fc / Irisin fusion protein activates PKA to alleviate PA-induced lipid accumulation in HepG2 cells In order to further clarify whether PKA can affect hepatocyte lipid metabolism by regulating mitochondrial dynamics, the cells in the above-mentioned treatment groups were stained with Oil Red O to determine the lipid content. The results showed that 100nM IgG2-Fc / Irisin fusion protein (prepared in Example 1) intervention for 24h can significantly reduce PA-induced hepatocyte lipid accumulation, while PKI can increase intracellular lipid content, offsetting the fat-reducing effect of IgG2-Fc / Irisin fusion protein. On the other hand, Forskolin and IgG2-Fc / Irisin fusion protein work together to significantly reduce the content of intracellular brown-red lipid particles. It is verified that IgG2-Fc / Irisin fusion protein corrects mitochondrial dynamics imbalance by activating PKA, and ultimately improves hepatocyte lipid metabolism disorders. IgG2-Fc / Irisin fusion protein upregulates SIRT3 to reverse PA-induced mitochondrial dynamics imbalance In order to explore the role of SIRT3 signaling pathway in IgG2-Fc / Irisin fusion protein-mediated mitochondrial dynamics regulation under PA stimulation, HepG2 cells were transfected with small interfering RNA (siRNA) and overexpression plasmids for 24 hours to knock down and overexpress SIRT3, and then PA (300 μM) and IgG2-Fc / Irisin fusion protein (100 nM, prepared in Example 1) were intervened for 24 hours and cells were collected to extract protein for detection. First, a siRNA transfection concentration gradient (10, 50 nM) was set, and Sirt3 siRNA, Scrambled siRNA and positive control siRNA (verified siRNA for GAPDH knockdown) were transfected into HepG2 cells, and the siRNA with the highest knockdown efficiency was confirmed and used for subsequent experiments. Western blot results showed that compared with the PA+IgG2-Fc / Irisin fusion protein+Scrambled siRNA group, the expressions of p-mTOR and p-DRP1 in the PA+IgG2-Fc / Irisin fusion protein+Sirt3 siRNA group were significantly increased, and the levels of MFN2 and OPA1 were significantly decreased. However, the expression level of p-PKA did not change significantly in the two groups. Similarly, the effective dose of Sirt3 overexpression plasmid (Sirt3-OE) was first confirmed to be 0.5 μg. The inventors found that compared with the PA+IgG2-Fc / Irisin fusion protein+Vector group, the PA+IgG2-Fc / Irisin fusion protein+Sirt3-OE group had a greater decrease in p-mTOR and p-DRP1 expression, a more significant increase in MFN2 and OPA1 levels, and no significant change in p-PKA protein between the two groups. The above results indicate that the specific molecular mechanism of IgG2-Fc / Irisin fusion protein in regulating mitochondrial dynamics is to activate PKA and upregulate its downstream SIRT3 expression, thereby inhibiting mTOR activation. IgG2-Fc / Irisin fusion protein reduces PA-induced lipid accumulation in HepG2 cells and upregulates SIRT3 levels In order to verify the effect of SIRT3-mediated IgG2-Fc / Irisin fusion protein on regulating mitochondrial dynamics on lipid metabolism in hepatocytes, HepG2 cells were transfected with Sirt3 siRNA and overexpression plasmid (Sirt3-OE) for 24 hours, followed by PA and IgG2-Fc / Irisin fusion protein intervention. The quantitative results of Oil Red O staining in Figure 14 showed that knockdown of SIRT3 significantly inhibited the effect of IgG2-Fc / Irisin fusion protein on improving PA-induced lipid accumulation in hepatocytes, while SIRT3 overexpression and IgG2-Fc / Irisin fusion protein synergistically significantly reduced the intracellular lipid content. Example 10. IgG2-Fc / Irisin fusion protein alleviates liver steatosis by activating the autophagy SIRT3 pathway 10.1 IgG2-Fc / Irisin fusion protein reduces hepatic lipid accumulation associated with increased autophagy Three methods were used to estimate autophagic flux. First, Western blot showed that the protein levels of autophagosome marker LC3 and autophagy receptor protein P62 were increased in the HFD and PA groups, but the levels were reduced after treatment with IgG2-Fc / Irisin fusion protein in vivo and in vitro. Immunofluorescence analysis showed similar results. Compared with PA and CQ, LC3II levels were increased in HepG2 cells treated with IgG2-Fc / Irisin fusion protein (prepared in Example 1) and autophagosome degradation inhibitor CQ (Chloroquine), indicating that IgG2-Fc / Irisin fusion protein increased autophagic flux. These results indicate that IgG2-Fc / Irisin fusion protein promotes autophagosome degradation and enhances autophagic flux. 10.2IgG2-Fc / Irisin fusion protein does not affect the formation of liver phagocytic vacuoles The phosphorylation level of ULK1 was measured, which is essential for regulating the formation of phagocytic cells. The results showed that the IgG2-Fc / Irisin fusion protein (prepared in Example 1) had no significant effect on the expression of p-ULK1 in the liver of PA-treated HepG2 cells or high-fat diet-induced mice, indicating that the IgG2-Fc / Irisin fusion protein may not affect the formation of hepatic phagocytic vacuoles. 10.3 IgG2-Fc / Irisin fusion protein enhances hepatocyte autophagy and is associated with improved lysosomal function Lysosomes are important organelles for autophagy-induced lipid droplet degradation. Therefore, the effect of IgG2-Fc / Irisin fusion protein on lysosomal function was evaluated. Cathepsin B is a lysosomal protease that generally plays a role in protein degradation. First, Western blotting showed that cathepsin B protein levels were reduced in the HFD and PA-treated groups and significantly increased after treatment with IgG2-Fc / Irisin fusion protein (prepared in Example 1). Immunofluorescence analysis of cathepsin B confirmed the Western blotting results. Secondly, the acidity of lysosomes in HepG2 cells was measured using LysoTracker, a fluorescent dye that can selectively mark acidic organelles such as lysosomes. The results showed that PA reduced the fluorescence intensity of LysoTracker red staining, and treatment with different concentrations of IgG2-Fc / Irisin fusion protein gradually increased the fluorescence intensity. This finding suggests that IgG2-Fc / Irisin fusion protein alleviates PA-induced lysosomal acidic damage. Taken together, these findings suggest that IgG2-Fc / irisin fusion protein enhances autophagic degradation by enhancing lysosomal function. 10.4IgG2-Fc / Irisin fusion protein promotes autophagy in part by restoring SIRT3 / AMPK signaling pathway In order to study the possible pathway changes of IgG2-Fc / Irisin fusion protein in enhancing autophagy and pave the way for subsequent mechanism exploration, the inventors used western blot to detect the protein expression levels of SIRT3, AMPK phosphorylation, and mTOR phosphorylation in the NCD group, HFD group, and HFD+Irisin treatment group. The results showed that IgG2-Fc / Irisin fusion protein could significantly upregulate the expression levels of SIRT3 and p-AMPK that were reduced by HFD. IgG2-Fc / Irisin fusion protein could also downregulate the expression level of p-mTOR that was increased by HFD. The translocation of transcription factor TFEB was detected by immunofluorescence, and the results showed that IgG2-Fc / Irisin fusion protein reversed the nuclear translocation of TFEB that was inhibited by HFD. The above results suggest that IgG2-Fc / Irisin fusion protein can promote the nuclear translocation of TFEB by upregulating SIRT3, promoting AMPK phosphorylation, and inhibiting the activity of mTOR, thereby increasing autophagy activity. The above examples are listed as currently considered to present the contents of the preferred examples of the present application, but it should be understood that the present application is not limited to the disclosed examples. On the contrary, the present application is intended to cover various modifications and equivalent examples included in the spirit and scope of the appended claims. Although specific terms are used herein, these terms are for descriptive rather than limiting purposes. All publications, patents and patent applications are incorporated herein by reference in their entirety. Specifically, the sequences associated with each accession number provided herein, including, for example, accession numbers and / or biomarker sequences (such as proteins and / or nucleotides) provided in the table or elsewhere, are incorporated by reference in their entirety. The scope of the claims should not be limited to the preferred examples and embodiments, but should be understood as the broadest interpretation consistent with the specification. References 1. Bostrom, P., et al., A PGC1-alpha-dependent myokine that drives brown-fat-like development of white fat and thermogenesis. Nature, 2012.481(7382): p.463-8. 2.Xiong,X.Q.,et al.,FNDC5 overexpression and irisin ameliorate glucose / lipid metabolic derangements and enhance lipolysis in obesity.Biochim Biophys Acta,2015.1852(9):p.1867-75. 3.Huh,J.Y.,et al.,Exercise-induced irisin secretion is independent of age or fitness level and increased irisin may directly modulate muscle metabolism through AMPK activation.J Clin Endocrinol Metab,2014.99(11):p.E2154-61. 4.Yang,Z.,et al.,Decreased irisin secretion contributes to muscle insulin resistance in high-fat diet mice.Int J Clin Exp Pathol,2015.8(6):p.6490-7. 5.Yano,N.,et al.,Irisin counteracts high glucose and fatty acid-induced cytotoxicity by preserving the AMPK-insulin receptor signaling axis in C2C12 myoblasts.Am J Physiol Endocrinol Metab,2020.318(5):p.E791-E805. 6.Tang,H.,et al.,Irisin Inhibits Hepatic Cholesterol Synthesis via AMPK-SREBP2 Signaling.EBioMedicine,2016.6:p.139-148. 7.Kim,H.,et al.,Irisin Mediates Effects on Bone and Fat via alphaV Integrin Receptors. Cell,2018.175(7):p.1756-1768 e17. 8.Karlin,S.and S.F.Altschul,Methods for assessing the statistical significance of molecular sequence features by using general scoring schemes.Proc Natl Acad Sci U S A,1990.87(6):p.2264-8. 9.Karlin,S.and S.F.Altschul,Applications and statistics for multiple high-scoring segments in molecular sequences.Proc Natl Acad Sci U S A,1993.90(12):p.5873-7. 10.Altschul,S.F.,et al.,Basic local alignment search tool.J Mol Biol,1990.215(3):p.403-10. 11.Altschul,S.F.,et al.,Gapped BLAST and PSI-BLAST:a new generation of protein database search programs.Nucleic Acids Res,1997.25(17):p.3389-402. 12.Myers,E.W.and W.Miller,Optimal alignments in linear space.Comput Appl Biosci,1988.4(1):p.11-7. 13.Allen,L.V.,Jr.,Remington:The Science and Practice of Pharmacy:from the past into the future.Int J Pharm Compd,2012.16(5):p.358-62. 14.Bodanszky,M.,Principles of peptide synthesis.2nd rev.ed.Springer laboratory.1993,Berlin;New York:Springer-Verlag.xii,329 p.

Claims

1. A fusion protein comprising an Irisin polypeptide and an immunoglobulin Fc domain, wherein: The Irisin polypeptide is covalently linked to the immunoglobulin Fc domain via a linker comprising an enzyme recognition sequence.

2. The fusion protein according to claim 1, wherein The Irisin polypeptide is derived from or originates from human Irisin protein or mouse Irisin protein.

3. The fusion protein according to claim 1 or 2, wherein The amino acid sequence of the Irisin polypeptide has at least 70% sequence identity with the amino acid sequence shown in SEQ ID NO: 1 or SEQ ID NO: 2, but still retains the function or activity of Irisin.

4. The fusion protein according to claim 3, wherein the amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO:

2.

5. A fusion protein according to any one of the preceding claims, wherein The immunoglobulin Fc domain comprises or is an IgG2-Fc domain.

6. The fusion protein according to claim 5, wherein The IgG2-Fc domain is an Fc domain from human IgG2.

7. The fusion protein according to claim 5 or 6, wherein The IgG2-Fc domain comprises one, two or three amino acid substitutions selected from the group consisting of C222S, A330S and P331S.

8. A fusion protein according to any one of the preceding claims, wherein The IgG2-Fc domain has at least 70% sequence identity with the amino acid sequence shown in SEQ ID NO: 3 or SEQ ID NO: 4, and comprises one or more amino acid substitutions selected from the group consisting of C222S, A330S and P331S.

9. A fusion protein according to any one of the preceding claims, wherein The amino acid sequence of the Irisin polypeptide has at least 70% sequence identity with the amino acid sequence shown in SEQ ID NO: 1 or SEQ ID NO: 2, the immunoglobulin Fc domain comprises or is an IgG2-Fc domain, and comprises one or more amino acid substitutions selected from the group consisting of C222S, A330S and P331S.

10. The fusion protein according to claim 8 or 9, wherein The IgG2-Fc domain has at least 90% sequence identity with the amino acid sequence shown in SEQ ID NO: 3 or SEQ ID NO: 4, and comprises A330S and P331S substitutions.

11. The fusion protein according to claim 10, wherein The IgG2-Fc domain also contains a C222S substitution.

12. A fusion protein according to any one of the preceding claims, wherein The amino acid sequence of the IgG2-Fc domain is shown in SEQ ID NO: 3 or SEQ ID NO:

4.

13. A fusion protein according to any one of the preceding claims, wherein The amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO:2, and the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO:

4.

14. A fusion protein according to any one of the preceding claims, wherein The Irisin polypeptide is located at the N-terminus or C-terminus of the immunoglobulin Fc domain.

15. A fusion protein according to any one of the preceding claims, wherein The enzyme recognition sequence is an enterokinase (EK) recognition sequence or a trypsin recognition sequence.

16. The fusion protein according to claim 15, wherein The enterokinase (EK) recognition sequence includes DDDDK (SEQ ID NO: 5).

17. A fusion protein according to any one of the preceding claims, wherein The linking moiety further comprises a linker.

18. The fusion protein according to claim 17, wherein The linker further includes: a cleavable linker, a non-cleavable linker, a flexible linker, a rigid linker, a helical linker or a non-helical linker.

19. The fusion protein according to claim 18, wherein The linker further includes a linker peptide.

20. The fusion protein according to claim 19, wherein The connecting peptide includes a linker containing glycine and serine.

21. The fusion protein according to claim 20, wherein The glycine- and serine-containing linker includes one, two, three, four or more repeats as shown in SEQ ID NO:6 (GGGS), SEQ ID NO:7 (GGGGS), SEQ ID NO:8 (GGGGGS) or SEQ ID NO:9 (GGGGGGGS).

22. The fusion protein according to any one of claims 19 to 21, wherein The connecting peptide includes an amino acid sequence selected from the group consisting of SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:18, SEQ ID NO:19 and SEQ ID NO:

20.

23. The fusion protein according to claim 22, wherein The connecting peptide comprises the amino acid sequence shown in SEQ ID NO:

10.

24. A fusion protein according to any one of the preceding claims, wherein The amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO:2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO:4, and the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO:

5.

25. The fusion protein according to any one of claims 19 to 24, wherein The amino acid sequence of the Irisin polypeptide is shown in SEQ ID NO:2, the amino acid sequence of the immunoglobulin Fc domain is shown in SEQ ID NO:4, the amino acid sequence of the enzyme recognition sequence is shown in SEQ ID NO:5, and the amino acid sequence of the connecting peptide is shown in SEQ ID NO:

10.

26. A fusion protein according to any one of the preceding claims, wherein The fusion protein comprises or has any one of the amino acid sequences selected from SEQ ID NOs: 23 to 32 or an amino acid sequence having at least 70% sequence identity with the above sequence.

27. The fusion protein according to claim 26, wherein The fusion protein comprises or has the amino acid sequence shown in SEQ ID NO: 24 or has at least 70% sequence identity with the amino acid sequence shown in SEQ ID NO:

24.

28. The fusion protein according to claim 27, wherein The fusion protein has the amino acid sequence shown in SEQ ID NO:

24.

29. The fusion protein according to any one of the preceding claims, further comprising a signal peptide.

30. The fusion protein according to claim 29, wherein The signal peptide is human CD33 signal peptide.

31. A fusion protein according to any of the preceding claims, wherein the fusion protein has a half-life in a subject of at least 1 day, at least 2 days, at least 3 days, at least 4 days, at least 5 days, at least 6 days, at least 7 days, at least 8 days, at least 9 days, at least 10 days, at least 11 days, at least 12 days, at least 13 days, or at least 14 days.

32. A fusion protein dimer comprising two identical peptide chains linked by a disulfide bond, wherein: Each peptide chain comprises the fusion protein according to any one of claims 1-31.

33. A nucleic acid molecule comprising a polynucleotide sequence encoding the fusion protein of any one of claims 1-31.

34. The nucleic acid molecule of claim 33, comprising a polynucleotide sequence as shown in SEQ ID NO:36, SEQ ID NO:37, SEQ ID NO:38 or SEQ ID NO:39 or a polynucleotide sequence having at least 70% sequence identity with a polynucleotide sequence as shown in SEQ ID NO:36, SEQ ID NO:37, SEQ ID NO:38 or SEQ ID NO:

39.

35. The nucleic acid molecule according to claim 34, comprising the polynucleotide sequence as shown in SEQ ID NO:36 or a polynucleotide sequence having at least 70% sequence identity with the polynucleotide sequence as shown in SEQ ID NO:

36.

36. A vector comprising the nucleic acid molecule of any one of claims 33-35.

37. A recombinant cell comprising the nucleic acid molecule of any one of claims 33-35, or comprising the vector of claim 36.

38. The cell of claim 37, wherein the cell is a prokaryotic cell or a eukaryotic cell.

39. The cell of claim 38, wherein the eukaryotic cell is a mammalian cell.

40. The cell of claim 39, wherein the mammalian cell is a human cell or a Chinese Hamster Ovary (CHO) cell.

41. The cell according to claim 39, wherein the mammalian cell is a human embryonic kidney cell 293 (HEK293 cell), a CHO-K1 cell, a CHO-S cell or a CHO-DG44 cell.

42. A pharmaceutical composition comprising the fusion protein of any one of claims 1-31, or the fusion protein dimer of claim 32, or the nucleic acid molecule of any one of claims 33-35, or the vector of claim 36, or the cell of any one of claims 37-41, and an optional pharmaceutically acceptable carrier.

43. A method for constructing a recombinant cell, comprising: a) introducing a nucleic acid molecule encoding the fusion protein according to any one of claims 1 to 31 into a vector to construct an expression vector; b) introducing the expression vector into a recombinant or natural cell to obtain a recombinant cell, preferably, the cell is a CHO-S cell.

44. The method for constructing a recombinant cell according to claim 43, comprising the following steps: a) inserting the polynucleotide sequence shown in SEQ ID NO:36, SEQ ID NO:37, SEQ ID NO:38 or SEQ ID NO:39 into the insertion site of pcDNA3.1 vector to generate pcDNA3.1 / Irisin / IgG2-Fc or pcDNA3.1 / IgG2-Fc / Irisin expression vector; b) Introducing the pcDNA3.1 / Irisin / IgG2-Fc or pcDNA3.1 / IgG2-Fc / Irisin expression vector into CHO-S cells to obtain recombinant cells.

45. A method for producing a fusion protein, comprising the step of obtaining the fusion protein using the recombinant cell according to any one of claims 37 to 41 or the cell prepared by the construction method according to claim 43 or 44.

46. ​​Use of the fusion protein according to any one of claims 1-31, or the fusion protein dimer according to claim 32, or the nucleic acid molecule according to any one of claims 33-35, or the vector according to claim 36, or the cell according to any one of claims 37-41, or the pharmaceutical composition according to claim 42 in the preparation of a medicament for treating or preventing a disease.

47. The use according to claim 46, wherein The disease is a metabolic disease related to glucose metabolism or lipid metabolism disorder or a complication of a metabolic disease.

48. The use according to claim 47, wherein The metabolic disease associated with disordered carbohydrate metabolism or lipid metabolism is selected from the group consisting of diabetes (e.g., type 2 diabetes), non-alcoholic steatohepatitis (NASH), non-alcoholic fatty liver disease (NAFLD), obesity, insulin resistance, impaired glucose tolerance, hyperinsulinemia, hypoinsulinemia, hyperfatty liver, hyperuricemia, fatty liver, hepatic steatosis, liver fibrosis, cirrhosis, and metabolic syndrome.

49. The use according to claim 48, wherein The metabolic disease associated with disordered carbohydrate metabolism or lipid metabolism is non-alcoholic fatty liver disease (NAFLD).

50. The use according to claim 49, wherein The non-alcoholic fatty liver disease includes non-alcoholic steatohepatitis (NASH).

51. The use according to claim 47, wherein The complications of the metabolic disease include cardiovascular complications, renal complications or liver complications caused by the metabolic disease.

52. A method for restoring the autophagy-lysosomal pathway in a subject, comprising administering to the subject a fusion protein according to any one of claims 1-31, or a fusion protein dimer according to claim 32, or a nucleic acid molecule according to any one of claims 33-35, or a vector according to claim 36, or a cell according to any one of claims 37-41, or a pharmaceutical composition according to claim 42.

53. A method for promoting autophagosome degradation and / or increasing autophagic flow in a subject, comprising administering to the subject a fusion protein according to any one of claims 1-31, or a fusion protein dimer according to claim 32, or a nucleic acid molecule according to any one of claims 33-35, or a vector according to claim 36, or a cell according to any one of claims 37-41, or a pharmaceutical composition according to claim 42.

54. A method for enhancing lysosomal function in a subject, comprising administering to the subject a fusion protein according to any one of claims 1-31, or a fusion protein dimer according to claim 32, or a nucleic acid molecule according to any one of claims 33-35, or a vector according to claim 36, or a cell according to any one of claims 37-41, or a pharmaceutical composition according to claim 42.

55. A method for regulating the SIRT3 / AMPK signaling pathway in a subject, comprising administering to the subject the fusion protein according to any one of claims 1-31, or the fusion protein dimer according to claim 32, or the nucleic acid molecule according to any one of claims 33-35, or the vector according to claim 36, or the cell according to any one of claims 37-41, or the pharmaceutical composition according to claim 42.

56. A method for upregulating SIRT3 activity in a subject, promoting AMPK phosphorylation or inhibiting mTOR phosphorylation, comprising administering to the subject the fusion protein according to any one of claims 1-31, or the fusion protein dimer according to claim 32, or the nucleic acid molecule according to any one of claims 33-35, or the vector according to claim 36, or the cell according to any one of claims 37-41, or the pharmaceutical composition according to claim 42.

57. A method for improving mitochondrial dynamics imbalance in a subject, comprising administering to the subject the fusion protein of any one of claims 1-31, or the fusion protein dimer of claim 32, or the nucleic acid molecule of any one of claims 33-35, or the vector of claim 36, or the cell of any one of claims 37-41, or the pharmaceutical composition of claim 42.

Images