NOTCH inhibition promotes myoblast fusion by releasing HEYL repression on TMEM8C regulatory regions in foetal skeletal muscles

Differentiation and fusion are two intricate processes involved in skeletal muscle development. The close association of differentiation and fusion makes it difficult to address the process of fusion independently of differentiation. Using the fusion marker myomaker, named TMEM8C in chicken, we found that both TMEM8C transcripts and the differentiated and fusion-competent MYOG+ cells are preferentially regionalized in the central regions of limb foetal muscles in chicken embryos. Because the NOTCH signalling pathway is a potent inhibitor of muscle differentiation during developmental myogenesis, NOTCH function in myoblast fusion was not addressed so far. We analysed the consequences of NOTCH inhibition for myoblast fusion and TMEM8C expression during foetal myogenesis using in vitro and in vivo chicken systems. NOTCH inhibition following chicken embryo immobilisation or in myoblast cultures increased TMEM8C expression and myoblast fusion. Moreover, we showed that NOTCH inhibition induced the un-binding of the HEYL transcriptional repressor from the TMEM8C regulatory regions in limb muscles and myoblast cultures. These results identify a molecular mechanism underlying the fusion-promoting effect of NOTCH-inhibition during foetal myogenesis.