The undifferentiated state of muscle stem (satellite) cells (MuSCs) is maintained by the canonical Notch pathway. Although three bHLH transcriptional factors, Hey1, HeyL, and Hes1, are considered to be potential effectors of the Notch pathway exerting anti-myogenic effects, neither HeyL nor Hes1 inhibits myogenic differentiation of myogenic cell lines. Furthermore, whether these factors work redundantly or cooperatively is unknown. Here, we showed cell-autonomous functions of Hey1 and HeyL in MuSCs using conditional and genetic null mice. Analysis of cultured MuSCs revealed anti-myogenic activity of both HeyL and Hes1. We found that HeyL forms heterodimeric complexes with Hes1 in living cells. Moreover, our ChIP-Seq experiments demonstrated that, compared with HeyL alone, HeyL-Hes1 heterodimer bound with high affinity to specific sites in the chromatin including the cis-element of Hey1. Finally, the analyses of myogenin promoter activity showed that HeyL and Hes1 acted synergistically to suppress myogenic differentiation. Collectively, those results suggest that HeyL and Hey1 function redundantly in MuSCs, and that HeyL requires Hes1 for effective DNA binding and biological activity.

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