HuR-mediated regulation of mTOR mRNA stability promotes the commitment of satellite cells towards myogenesis.

The RNA-binding protein HuR has been shown to promote the differentiation of cultured muscle cells into muscle fibers. HuR mediates this process by differentially regulating, at different stages of this process, mRNA targets encoding pro-myogenic factors. Despite these advancements, the role of HuR, in vivo, at various stages of the myogenic process and its impact on muscle formation and function remain elusive. Towards this end, we used the Myf5-Cre loxP system to knock out HuR at a stage where muscle precursor cells (satellite cells, SCs) commit to myogenesis. Using these mice, we found that the muscle-specific depletion of HuR impairs, physiologically, its formation during embryogenesis and in response to injury. These mice exhibited smaller skeletal muscles and reduced exercise endurance. We demonstrate, using these mice, that this effect is due, in part, to the HuR-mediated regulation of mTOR (mechanistic target of rapamycin) mRNA expression. Using primary and cultured muscle cells, we show that HuR associates with this message, regulating its stability. In doing so, HuR facilitates the commitment of satellite cells toward myogenesis, thus preventing their transdifferentiation toward adipogenesis. These findings thus identify HuR as a master regulator of SCs' commitment to myogenesis and uncover a potential target for manipulating muscle myogenic capacity in both normal and pathological conditions.