Yes-associated protein (Yap) expression and Ser127 phosphorylation during satellite cell activation, differentiation and self-renewal.

Yap is a transcriptional co-factor known to regulate the proliferation, differentiation and/or survival of embryonic, intestinal, neuronal, epidermal stem cells as well as hepatic oval cells. We have recently reported that Yap is expressed in skeletal muscle and that Yap is a novel regulator of C2C12 myogenesis (Watt et al., 2010). The aim of the present study was to test whether Yap is expressed in satellite cells and whether inhibitory Yap Ser127 phosphorylation changes during satellite cell activation, proliferation, differentiation and self-renewal. Mouse muscle fibres were isolated with their satellite cells retained in their niche using an established protocol (Collins & Zammit, 2009). Fibres were then cultured in suspension, fixed at 0, 24, 48 or 72 h post isolation and immunostained either for total Yap or phospho-Yap Ser127 or for markers of quiescent and self-renewing satellite cells (Pax7), activated satellite cells (MyoD) or differentiating satellite cells (myogenin. Yap staining was low in quiescent Pax7+ satellite cells from freshly isolated fibres. However, after 24 and 48 hours in culture Yap protein expression increased with strong nuclear staining in activated, MyoD+ satellite cells. After 72 hours in culture Yap protein levels remain high in clusters that contain both differentiating (myogenin+) and self-renewing (Pax7+) satellite cell progeny. Phospho-Yap Ser127, however, appeared higher in differenting, myogenin+ than self-renewing, Pax7+ satellite cells. Our data suggest that both Yap expression and phospho-Yap Ser127 change during satellite cell activation, differentiation and self-renewal and are consistent with the hypothesis that Yap is a regulator of satellite cell fate.