The role of Sonic Hedgehog signalling in satellite cell-mediated myogenesis

Adult skeletal muscle regeneration depends on the existence of tissue-specifc stem cells known as satellite cells. Satellite cells are found in a quiescent state in homeostatic conditions but become activated, re-enter the cell cycle, proliferate and differentiate or self-renew in response to muscle injury, exercise or disease. These events are tightly regulated by intrinsic and extrinsic cues, including well-characterised embryonic signalling cascades. The Sonic Hedgehog (Shh) signalling pathway has multiple roles in tissue patterning, cell fate determination, cell survival and proliferation in the embryo. Previous studies have shown that during embryonic myogenesis, Shh signalling controls the specification, migration and proliferation of muscle progenitor cells, as well as muscle patterning by the regulation of genes encoding basement membrane proteins. As the myogenic program carried out by satellite cells recapitulates, to a certain extent, embryonic myogenesis, I hypothesised that Shh signalling controls satellite cell activity in a manner reminiscent to its effect on muscle progenitor cells in the embryo. In this study, through a combination of ex vivo and in vivo approaches, I showed that, although quiescent satellite cells are refractory to Shh signals, activated satellite cells respond to Shh signalling. Shh response persists during the expansion phase and declines as satellite cells enter differentiation. Through the use of pharmacological agonists and antagonists of Shh signalling, as well as of an inducible conditional knockout mouse line of the Smoothened receptor in satellite cells, I demonstrated that Shh signalling contributes to satellite cell proliferation ex vivo and in vivo and to muscle regeneration following injury. Analysis of cell cycle dynamics showed that Shh signalling promotes the entry of satellite cells into the cell cycle and their progression through G1/S phase. Thus, the present study demonstrates that Shh signalling is required for adult skeletal muscle regeneration and provides novel insights into the role of Shh signalling in the control of satellite cell progression through the cell cycle and through myogenesis.