Grb2/Sos1 signaling regulates the number of reserve cells in C2C12 cell culture.

Skeletal muscle regeneration depends on satellite cells that maintain tissue homeostasis through self-renewal and the production of myoblasts that differentiate into mature myofibers. Dysregulation of these processes can lead to muscle degeneration, highlighting the need to elucidate their molecular mechanisms. In this study, we investigated the role of the Grb2/Sos1 signaling pathway in regulating satellite cell self-renewal and differentiation using C2C12 cells. Knockdown of either Grb2 or Sos1 significantly reduced the formation of Bcl-2-positive reserve cells and increased the proportion of differentiated myotubes. Conversely, forced expression of Grb2 increased the number of reserve cells, whereas the Grb2 P49L mutant, which disrupts its interaction with Sos1, decreased reserve cell formation and resulted in thinner myotubes. Although forced expression of Sos1 alone did not significantly increase reserve cell numbers, the chimeric protein cSos-SH2, which combines elements of Grb2 and Sos1, produced a pronounced increase of reserve cells. These results demonstrate that a precise balance between Grb2 and Sos1, along with their coordinated subcellular localization, is critical for controlling reserve cell populations. Activated by growth factor receptor tyrosine kinases and extracellular matrix/integrin interactions, the Grb2/Sos1 signaling pathway is critical for maintaining the muscle satellite cell pool, thereby playing an essential role in muscle regeneration.