Novel RNA-binding activity of MYF5 enhances Ccnd1/Cyclin D1 mRNA translation during myogenesis

Skeletal muscle contains long multinucleated and contractile structures known as muscle fibers, which arise from the fusion of myoblasts into nucleated myotubes during myogenesis. The myogenic regulatory factor (MRF) MYF5 is the earliest to be expressed during myogenesis and functions as a transcription factor in muscle progenitor cells (satellite cells) and myocytes. In mouse C2C12 myocytes, MYF5 is implicated in the initial steps of myoblast differentiation into myotubes. Ribonucleoprotein immunoprecipitation (RIP) analysis showed that MYF5 bound a subset of myoblast mRNAs; prominent among them was Ccnd1 mRNA, which encodes the key cell cycle regulator CCND1 (Cyclin D1). Biotin-RNA pulldown, UV-crosslinking, and gel shift experiments indicated that MYF5 was capable of binding the 3' untranslated region (UTR) and the coding region (CR) of Ccnd1 mRNA. MYF5 silencing in proliferating growing myoblasts revealed that and MYF5 promoted CCND1 translation, and it also modestly increased transcription of Ccnd1 mRNA. Importantly, silencing MYF5 reduced myoblast growth as well as differentiation of myoblasts into myotubes, while overexpressing MYF5 in C2C12 cells upregulated CCND1 expression. We propose that MYF5 enhances early myogenesis in part by coordinately elevating Ccnd1 transcription and Ccnd1 mRNA translation. Four replicates were utilized from either Control (IgG) or MYF5-immunoprecipitated RNA samples from C2C12 cells growing in either growth medium (GM) or differentiation medium (DM) for a total of sixteen samples.