Characterization of SMAD3 function in skeletal muscle growth
Date of Issue2012
School of Biological Sciences
Smad3 is a key intracellular signaling mediator for Myostatin and TGF-β1, two TGF-β superfamily members and are key regulators of skeletal muscle growth and homeostasis. Despite extensive studies about the functions and signaling of Myostatin/TGF-β1, little is known about the regulatory role of Smad3 in muscle growth. Thus the main emphasis of this thesis is to investigate the role of Smad3 in post-natal myogenesis. Firstly, this thesis demonstrated that there is pronounced skeletal muscle atrophy in Smad3-null mice and this is due to increased Myostatin expression. Enhanced Myostatin signaling promoted sarcomeric protein degradation through the ubiquitin proteasome system in Smad3-null skeletal muscle. Increased Myostatin level also resulted in impaired SC proliferation, and reduced self-renewal resulting in reduced SC number in Smad3-null mice. Therefore, treatment of Smad3-null myotubes with Myostatin antagonist significantly increased the expression of sarcomeric proteins. Consistent with this result, double knockout (DKO) mouse (Smad3-/-/Myostatin-/-) had improved muscle weights and in vitro myogenic capacity when compared to that of Smad3-null mice. Therefore, up-regulated Myostatin expression is partially responsible for the muscle atrophy in Smad3-null mice. To further validate the impaired SC function found in Smad3-null mice, muscle regeneration induced by notexin injection was analyzed. Smad3-null mice showed impaired M. tibialis anterior (TA) muscle regeneration, with a reduced inflammatory response and less activated myoblasts and nascent myofibers formation. Moreover, Smad3-null regenerated muscle had remarkable reduced oxidative enzyme activity, which might be due to impaired mitochondrial biogenesis since the expression of TFAM, the master regulator of mitochondrial biogenesis, was down-regulated. Furthermore, Smad3 deficiency led to reduced scar tissue formation in the regenerated muscle. Most importantly, similar to the uninjured muscle, Smad3-null muscle had consistently up-regulated Myostatin expression during the course of muscle regeneration. Therefore, the inhibitory role of Myostatin on muscle growth may be one major molecular mechanism underlying the impaired muscle regeneration of Smad3-null mice. To advance our understanding of molecular mechanisms through which Smad3 regulates muscle growth and metabolism, gene expression analysis of Smad3-null muscle was performed. In total, 25 up-regulated and 31 down-regulated genes (P<0.05, fold change ≥ 1.5 or ≤ -1.5) were identified in Smad3-null muscle when compared to Wild-type control muscles. Altered gene expression changes may help to explain the muscle phenotype seen in Smad3-null mice. Up-regulated expression of genes encoding two inflammatory cytokines (S100a8 and S100a9), may help to explain the systemic inflammation observed in Smad3-null mice. Furthermore, down-regulated expression of 5 genes encoding ECM components in Smad3-null muscle suggests an indispensable role of Smad3 in muscle fibrosis. In addition, down-regulated expression of Pkm2, and up-regulated expression of Dlat suggests a reduced glycolysis and pyruvate oxidation in Smad3-null muscle. In conclusion, results presented in this thesis highlight the essential role of Smad3 in post-natal myogenesis. Using in vitro and in vivo myogenesis models, I demonstrated that Smad3 deficiency led to defective post-natal myogenesis, atrophied musculature and impaired muscle regeneration. Up-regulated Myostatin levels partially contribute to the post-natal myogenic defects in Smad3-null mice. Moreover, this thesis also sheds light on the regulatory role of Smad3 in skeletal muscle metabolism, as well as ECM synthesis.
DRNTU::Science::Biological sciences::Molecular biology