Dynamic Phosphorylation of the Myocyte Enhancer Factor 2Cα1 Splice Variant Promotes Skeletal Muscle Regeneration and Hypertrophy

Abstract

The transcription factor MEF2C (Myocyte Enhancer Factor 2C) plays an established role in the early steps of myogenic differentiation. However, the involvement of MEF2C in adult myogenesis and in muscle regeneration has not yet been systematically investigated. Alternative splicing of mammalian MEF2C transcripts gives rise to two mutually exclusive protein variants: MEF2Ca2 which exerts a positive control of myogenic differentiation, and MEF2Ca1, in which the a1 domain acts as trans-repressor of the MEF2C pro-differentiation activity itself. However, MEF2Ca1 variants are persistently expressed in differentiating cultured myocytes, suggesting a role in adult myogenesis. We found that overexpression of both MEF2Ca1/a2 proteins in a mouse model of muscle injury promotes muscle regeneration and hypertrophy, with each isoform promoting different stages of myogenesis. Besides the ability of MEF2Ca2 to increase differentiation, we found that overexpressed MEF2Ca1 enhances both proliferation and differentiation of primary myoblasts, and activates the AKT/mTOR/S6K anabolic signaling pathway in newly formed myofibers. The multiple activities of MEF2Ca1 are modulated by phosphorylation of Ser98 and Ser110, two amino acid residues located in the a1 domain of MEF2Ca1. These specific phosphorylations allow the interaction of MEF2Ca1 with the peptidylprolyl isomerase PIN1, a regulator of MEF2C functions. Overall, in this study we established a novel regulatory mechanism in which the expression and the phosphorylation of MEF2Ca1 are critically required to sustain the adult myogenesis. The described molecular mechanism will represent a new potential target for the development of therapeutical strategies to treat musclewasting diseases. STEM CELLS 2017;35:725-738 SIGNIFICANCE STATEMENTA deep understanding of the mechanisms that control adult muscle mass is crucial for developing strategies to counteract muscle wasting-associated disorders. Our work demonstrate that MEF2C transcription factor promotes a balanced muscle growth by acting at two cellular levels: by stimulating muscle differentiation of stem cell-derived muscle precursors and by activating the protein synthesis pathway in differentiated myofibers. We show that the timely coordination of these activities is ensured by a molecular mechanism involving the cross-talk of alternative splicing and protein phosphorylation. Our findings supply evidence that modulating MEF2C function might be a valuable therapeutic strategy for muscle wasting therapies.

MEF2 as upstream regulator of the transcriptome signature in human skeletal muscle during unloading

“…The general significance of MEF2C for skeletal muscle integrity is demonstrated by the fact that skeletal muscle-specific deletion of MEF2C in mice results in disorganized myofibers and perinatal lethality (49). Also, in rodents, recent investigations have shown a significant role of MEF2C in overall growth, metabolism, and muscle hypertrophy and regeneration (5,12,36). Indeed, the MEF2 family, and in particular MEF2C, appears to control skeletal muscle regeneration through activation of satellite cells (36).…”

The co-existence of transcriptional activator and transcriptional repressor MEF2 complexes influences tumor aggressiveness

“…Dominant epigenetic regulations and/or the existence of multiple MEF2 transcriptional partners contribute to orchestrate the context-dependent MEF2 transcriptional landscape [3,4,8]. The four family members and their splicing variants can provide further layers of complexity to the MEF2 transcriptome [10–13]. Furthermore, MEF2 can be converted into transcriptional repressors after the binding to Cabin1, G9a or class IIa HDACs [14–16].…”

Deletion of metal transporter Zip14 (Slc39a14) produces skeletal muscle wasting, endotoxemia, Mef2c activation and induction of miR-675 and Hspb7

“…The up-regulation of Mef2c in muscle from the KO mice at basal steady state is also very relevant to the muscle wasting of the Zip14 KO phenotype. Mef2c is a major transcriptional regulator of genes responsible for skeletal muscle growth and differentiation 16,21 . Hspb7, a gene that codes of a protein that functions in muscle atrophy and autophagy is regulated by Mef2 transcription factors 18,19 .…”

“…Dysregulated pathways that influence phosphorylation could contribute to the biochemical mechanism of the muscle wasting phenotype found with Zip14 ablation. p38 mitogen-activated protein kinase (p38), when activated by LPS leading to increased phosphorylation 20 , has been shown to phosphorylate transcription factors including Mef2 21 . While p38 protein levels were similar and p-p38 levels were increased this likely suggests Zip14 influences phosphorylation of this inflammation-related transcription factor (Fig.…”