Abstract
Regenerative coordination and remodeling of the intramuscular motoneuron network and neuromuscular connections are critical for restoring skeletal muscle function and physiological properties. The regulatory mechanisms of such coordination remain unclear, although both attractive and repulsive axon guidance molecules may be involved in the signaling pathway. Here we show that expression of a neural secreted chemorepellent semaphorin 3A (Sema3A) is remarkably upregulated in satellite cells of resident myogenic stem cells that are positioned beneath the basal lamina of mature muscle fibers, when treated with hepatocyte growth factor (HGF), established as an essential cue in muscle fiber growth and regeneration. When satellite cells were treated with HGF in primary cultures of cells or muscle fibers, Sema3A message and protein were upregulated as revealed by reverse transcription-polymerase chain reaction and immunochemical studies. Other growth factors had no inductive effect except for a slight effect of epidermal growth factor treatment. Sema3A upregulation was HGF dose dependent with a maximum (about 7- to 8-fold units relative to the control) at 10-25 ng/ml and occurred exclusively at the early-differentiation stage, as characterized by the level of myogenin expression and proliferation (bromodeoxyuridine incorporation) of the cells. Neutralizing antibody to the HGF-specific receptor, c-met, did not abolish the HGF response, indicating that c-met may not mediate the Sema3A expression signaling. Finally, in vivo Sema3A was upregulated in the differentiation phase of satellite cells isolated from muscle regenerating following crush injury. Overall, the data highlight a heretofore unexplored and active role for satellite cells as a key source of Sema3A expression triggered by HGF, hence suggesting that regenerative activity toward motor innervation may importantly reside in satellite cells and could be a crucial contributor during postnatal myogenesis.