Abstract
SMAD2 is a transcription factor, the activity of which is regulated by members of the transforming growth factor beta (TGF beta) superfamily. Although activation of SMAD2 and SMAD3 downstream of TGF beta or myostatin signaling is known to inhibit myogenesis, we found that SMAD2 in the absence of TGF beta signaling promotes terminal myogenic differentiation. We found that, during myogenic differentiation, SMAD2 expression is induced. Knockout of SMAD2 expression in primary myoblasts did not affect the efficiency of myogenic differentiation but produced smaller myotubes with reduced expression of the terminal differentiation marker myogenin. Conversely, overexpression of SMAD2 stimulated myogenin expression, and enhanced both differentiation and fusion, and these effects were independent of classical activation by the TGF beta receptor complex. Loss of Smad2 in muscle satellite cells in vivo resulted in decreased muscle fiber caliber and impaired regeneration after acute injury. Taken together, we demonstrate that SMAD2 is an important positive regulator of myogenic differentiation, in part through the regulation of Myog.