In certain aspects, the disclosure provides soluble heteromeric polypeptide complexes comprising an
extracellular domain of a type I
serine /
threonine kinase receptor of the TGF-beta family and an
extracellular domain of a type II
serine /
threonine kinase receptor of the TGF-beta family. In some embodiments, the disclosure provides soluble polypeptide complexes comprising an
extracellular domain of a type II
receptor selected from: ActRIIA, ActRIIB, TGFBRII, BMPRII, and MISRII. In some embodiments, the disclosure provides soluble polypeptide complexes comprising an extracellular domain of a
type I receptor selected from: ALK1, ALK2, ALK3, ALK4, ALK5, ALK6, and ALK7. Optionally the soluble complex is a heterodimer. In certain aspects, such soluble polypeptide complexes may be used to regulate (promote or inhibit) growth of tissues or cells including, for example,
muscle, bone,
cartilage, fat, neural tissue, tumors, cancerous cells, and / or cells of hematopoietic lineages, including red blood cells. In certain aspects, such soluble polypeptide complexes can be used to improve
muscle formation,
bone formation, hematopoiesis, metabolic parameters, and disorders associated with these tissues, cellular networks, and endocrine systems.