Phosphorylation of eIF2α Is a Translational Control Mechanism Regulating Muscle Stem Cell Quiescence and Self-Renewal.

Regeneration of adult tissues depends on somatic stem cells that remain quiescent yet are primed to enter a differentiation program. The molecular pathways that prevent activation of these cells are not well understood. Using mouse skeletal muscle stem cells as a model, we show that a general repression of translation, mediated by the phosphorylation of translation initiation factor eIF2α at serine 51 (P-eIF2α), is required to maintain the quiescent state. Skeletal muscle stem cells unable to phosphorylate eIF2α exit quiescence, activate the myogenic program, and differentiate, but do not self-renew. P-eIF2α ensures in part the robust translational silencing of accumulating mRNAs that is needed to prevent the activation of muscle stem cells. Additionally, P-eIF2α-dependent translation of mRNAs regulated by upstream open reading frames (uORFs) contributes to the molecular signature of stemness. Pharmacological inhibition of eIF2α dephosphorylation enhances skeletal muscle stem cell self-renewal and regenerative capacity.

Next-generation sequencing analysis of miRNA expression in control and FSHD myogenesis.

Emerging evidence has demonstrated that miRNA sequences can regulate skeletal myogenesis by controlling the process of myoblast proliferation and differentiation. However, at present a deep analysis of miRNA expression in control and FSHD myoblasts during differentiation has not yet been derived. To close this gap, we used a next-generation sequencing (NGS) approach applied to in vitro myogenesis. Furthermore, to minimize sample genetic heterogeneity and muscle-type specific patterns of gene expression, miRNA profiling from NGS data was filtered with FC ≥ 4 (log(2)FC ≥ 2) and p-value<0.05, and its validation was derived by qRT-PCR on myoblasts from seven muscle districts. In particular, control myogenesis showed the modulation of 38 miRNAs, the majority of which (34 out 38) were up-regulated, including myomiRs (miR-1, -133a, -133b and -206). Approximately one third of the modulated miRNAs were not previously reported to be involved in muscle differentiation, and interestingly some of these (i.e. miR-874, -1290, -95 and -146a) were previously shown to regulate cell proliferation and differentiation. FSHD myogenesis evidenced a reduced number of modulated miRNAs than healthy muscle cells. The two processes shared nine miRNAs, including myomiRs, although with FC values lower in FSHD than in control cells. In addition, FSHD cells showed the modulation of six miRNAs (miR-1268, -1268b, -1908, 4258, -4508- and -4516) not evidenced in control cells and that therefore could be considered FSHD-specific, likewise three novel miRNAs that seem to be specifically expressed in FSHD myotubes. These data further clarify the impact of miRNA regulation during control myogenesis and strongly suggest that a complex dysregulation of miRNA expression characterizes FSHD, impairing two important features of myogenesis: cell cycle and muscle development. The derived miRNA profiling could represent a novel molecular signature for FSHD that includes diagnostic biomarkers and possibly therapeutic targets.