The invention relates to ALK7 soluble receptors and their uses as antagonists of the function of certain ligands such as GDF-8 (Myostatin) and GDF-11. The ALK7 soluble receptor of the invention is useful as antagonists of GDF-8 and GDF-11 in the treatment of neuronal diseases or conditions such as stroke, spinal cord injury, and all peripheral nerve diseases. The ALK7 soluble receptor of the invention is also useful as GH (growth hormone) equivalent, and for increasing muscle mass.