​BACKGROUND: Polycomb group (PcG) genes code for chromatin multiprotein complexes that are responsible for maintaining gene silencing of transcriptional programs during differentiation and in adult tissues. Despite the large amount of information on PcG function during development and cell identity homeostasis, little is known regarding the dynamics of PcG complexes and their role during terminal differentiation.
RESULTS: We show that two distinct polycomb repressive complex (PRC)2 complexes contribute toskeletal muscle cell differentiation: the PRC2-Ezh2 complex, which is bound to the myogenin (MyoG) promoter and muscle creatine kinase (mCK) enhancer in proliferating myoblasts, and the PRC2-Ezh1complex, which replaces PRC2-Ezh2 on MyoG promoter in post-mitotic myotubes. Interestingly, the opposing dynamics of PRC2-Ezh2 and PRC2-Ezh1 at these muscle regulatory regions is differentially regulated at the chromatin level by Msk1 dependent methyl/phospho switch mechanism involving phosphorylation of serine 28 of the H3 histone (H3S28ph). While Msk1/H3S28ph is critical for the displacement of the PRC2-Ezh2 complex, this pathway does not influence the binding ofPRC2-Ezh1 on the chromatin. Importantly, depletion of Ezh1 impairs muscle differentiation and thechromatin recruitment of MyoD to the MyoG promoter in differentiating myotubes. We propose thatPRC2-Ezh1 is necessary for controlling the proper timing of MyoG transcriptional activation and thus, in contrast to PRC2-Ezh2, is required for myogenic differentiation.
CONCLUSIONS: Our data reveal another important layer of epigenetic control orchestrating skeletalmuscle cell terminal differentiation, and introduce a novel function of the PRC2-Ezh1 complex in promoter setting.​