Satellite cell expansion is mediated by P-eIF2α-dependent Tacc3 translation

Translational control of gene expression is an important regulator of adult stem cell quiescence, activation and self-renewal. In skeletal muscle, quiescent satellite cells maintain low levels of protein synthesis, mediated in part through the phosphorylation of eIF2 alpha (P-eIF2 alpha). Pharmacological inhibition of the eIF2 alpha phosphatase with the small molecule sa1003 maintains P-eIF2 alpha and permits the expansion of satellite cells ex vivo. Paradoxically, P-eIF2 alpha also increases the translation of specific mRNAs, which is mediated by P-eIF2 alpha-dependent read-through of inhibitory upstream open reading frames (uORFs). Here, we ask whether P-eIF2 alpha-dependent mRNA translation enables expansion of satellite cells. Using transcriptomic and proteomic analyses, we show a number of genes associated with the assembly of the spindle pole to be upregulated at the level of protein, without corresponding change in mRNA levels, in satellite cells expanded in the presence of sa1003. We show that uORFs in the 5' UTR of mRNA for the mitotic spindle stability gene Tacc3 direct P-eIF2 alpha-dependent translation. Satellite cells deficient for TACC3 exhibit defects in expansion, self-renewal and regeneration of skeletal muscle.