Post-transcriptional regulatory mechanisms involved in physiological and pathological skeletal muscle myogenesis

Many adult stem cells exist in a quiescent state until they are activated in response to regenerate tissue (Simons and Clevers, 2011). Adult stem cells require tight regulation of translation, with increased or decreased rates of translation impairing stem cell function (Signer et al., 2015). Our results provide new insight into mechanisms that regulate translation to hold satellite cells (SCs) in a quiescent state. Specifically, the translation initiation factor eIF2a is phosphorylated in the quiescent satellite cell and rapidly dephosphorylated when they are activated to enter the myogenic program. We therefore propose a role for P-eIF2a in maintain somatic stem cell properties (self-renew and quiescence), mediated by a general repression of translation such that specific mRNAs are silenced or selectively translated. The general repression of translation caused by P-eIF2a is expected to create competition for available translation initiation complexes, making microRNA and RNA binding protein mediated silencing platforms more robust (Crist et al., 2012). Moreover, emerging evidence has demonstrated that microRNA sequences can regulate skeletal myogenesis not only at the quiescent state of satellite cells, but also by controlling the process of myoblast proliferation and differentiation. In particular, microRNA-1, -206 and -133a/b were defined as myomiRNAs to emphasize their crucial role in myogenesis (Zhao et al., 2005; Chen et al., 2006). Not surprisingly, miRNAs dysregulation has been found to be involved in muscle dysfunctions, as FSHD (Cheli et al., 2012; Eisenberg et al., 2007; Dmitriev et al., 2013). To date, miRNA studies reported for FSHD were essentially based on the analysis of a restricted number of known miRNA sequences, thus not allowing the derivation of the full miRNA-based dysregulation network. To close this gap, here we report miRNAs expression analysis, derived by next-generation sequencing (NGS), in primary muscle cells from healthy and FSHD subjects during differentiation. During normal in vitro myoblast differentiation, we reported the modulation of 38 microRNAs, including myomiRNAs. Indeed, the in vitro differentiation of myoblasts derived by FSHD patients report the modulation of only 14 microRNAs. Interestingly, myomiRNAs are found to be regulated also during FSHD myogenesis, but the fold expression result lower compared to the control myogenesis. These results further support the involvement of microRNAs in the differentiation defect occurring in patient muscle cells (Barro et al. 2010).