MLL1 promotes myogenesis by epigenetically regulating Myf5

Objectives: Mixed lineage leukaemia protein-1 (MLL1) mediates histone 3 lysine 4 (H3K4) trimethylation (me3) and plays vital roles during early embryonic development and hematopoiesis. In our previous study, we found its expression was positively correlated with embryonic myogenic ability in pigs, indicating its potential roles in mammalian muscle development. The present work aimed to explore the roles and regulation mechanisms of MLL1 in myogenesis.

Materials and methods: The expression of MLL1 in C2C12 cells was experimentally manipulated using small interfering RNAs (siRNA). 5-ethynyl-2'-deoxyuridine (EdU) assay, cell cycle assay, immunofluorescence, qRT-PCR and Western blot were performed to assess myoblast proliferation and differentiation. Chromatin immunoprecipitation assay was conducted to detect H3K4me3 enrichment on myogenic factor 5 (Myf5) promoter. A cardiotoxin (CTX)-mediated muscle regeneration model was used to investigate the effects of MLL1 on myogenesis in vivo.

Results: MLL1 was highly expressed in proliferating C2C12 cells, and expression decreased after differentiation. Knocking down MLL1 suppressed myoblast proliferation and impaired myoblast differentiation. Furthermore, knockdown of MLL1 resulted in the arrest of cell cycle in G1 phase, with decreased expressions of Myf5 and Cyclin D1. Mechanically, MLL1 transcriptionally regulated Myf5 by mediating H3K4me3 on its promoter. In vivo data implied that MLL1 was required for Pax7-positive satellite cell proliferation and muscle repair.

Conclusion: MLL1 facilitates proliferation of myoblasts and Pax7-positive satellite cells by epigenetically regulating Myf5 via mediating H3K4me3 on its promoter.