Wnt4 acts as a differentiation factor on C2C12 myoblasts and satellite cells

The molecular signals that regulate satellite cell function remain largely obscure. However, it was recently demonstrated that Wnts participate in the temporal control of satellite cell expansion versus differentiation during adult muscle regeneration (Brack et al, 2008). Thus, differentiation of myoblasts in vitro and in vivo is correlated with an upregulation of canonical Wnt signaling. Furthermore, ectopic Wnt induces premature muscle differentiation whereas inhibition of Wnt signaling interferes with muscle differentiation. In this context, the fact that myostatin -a member of the TGF-β superfamily that specifically regulates muscle mass- was shown to implicate Wnt4 signaling in postnatal skeletal muscle hypertrophy is of the upmost importance (Steelman et al, 2006). This was corroborated by Takata et al, (2007) showing the involvement of Wnt4 signaling during myogenic proliferation and dfferentiation of skeletal muscle. In this context, we first established by SQ-PCR that a limited number of Wnts was expressed during proliferation and differentiation of C2C12 myoblasts and satellite cells (SC). Amongst the 19 Wnts examined, we noticed that only the expression of Wnt4 was lacking during proliferation and was highly induced during differentiation of both cell types. The aim of this study was thus to characterize the role of Wnt4 protein in muscle homeostasis. The effects of Wnt4 on myogenic differentiation were examined by modulating the expression level of this factor. We showed that over-expression of Wnt4 in proliferative state by transient or stable transfection was responsible for spontaneous differentiation (C2C12) or an important myotube hypertrophy (SC). Conversely, inhibition of Wnt4 expression by siRNA silencing on SC leaded to an atrophy of differentiated myotubes (myotube areas correspond to 40% of siRNA luciferase controls) and a decrease in fusion index (24%). Treatment of C2C12 myoblasts by Wnt4 siRNA induced a strong inhibition of differentiation associated with a decrease in the expression of Myf5.