Post-translational regulation of TGF-beta superfamily ligands in skeletal muscle and during Xenopus embryogenesis

Post-translational modification of TGF-β superfamily ligands is instrumental in providing precise spatial and temporal control over signaling events. Without this regulatory mechanism, TGF-β signals would be indiscriminately activated throughout development and within inappropriate tissues. Here, I present data on the post-translational regulation of TGF-β superfamily ligand production and activity in skeletal muscle and during Xenopus embryogenesis. Myostatin is a negative regulator of skeletal muscle mass. I find that pro-myostatin is secreted from skeletal muscle fibers prior to maturation cleavage. This pool of extracellular pro-myostatin has not been observed previously and represents a novel point at which myostatin signals could be regulated in skeletal muscle. I have also observed extracellular pro-myostatin in cultured cells and determined that the proprotein convertase furin can cleave pro-myostatin extracellularly. In addition, I have identified a novel interaction between pro-myostatin and a protein that is required for activity of the canonical TGF-β ligands, latent TGF-β binding protein-3 (LTBP-3). In cultured cells, LTBP-3 retains pro-myostatin in the extracellular matrix. When co-expressed, LTBP-3 reduces myostatin-induced signals, and ectopic expression of LTBP-3 in skeletal muscle increases muscle fiber area, consistent with reduced myostatin activity. These observations delineate novel post-translational mechanisms to control myostatin signals. Post-translational control of TGF-β superfamily ligands also plays an important role in mesoderm induction and axis specification during Xenopus embryogenesis . I have demonstrated that TGF-β superfamily ligand maturation is limited in Xenopus embryos. While all of the components that are necessary to traffick and proteolytically cleave TGF-β superfamily ligands are present in an early embryo, ligands are sequestered in the endoplasmic reticulum for four or more developmental stages. Ligand translation and dimerization occur very soon after ectopic expression of ligands, however, movement through the secretory pathway and proteolytic cleavage is temporally limited. These observations suggest that TGF-β signals are limited in the early embryo by a slow progression through the secretory pathway.