Role and Regulation of FRA-2 During Skeletal Muscle Development

Regulation of skeletal muscle development and regeneration is critical to all metazoans and also of clinical relevance as muscle wasting is manifested in a variety of disorders. The events contributing to development and regeneration of skeletal muscle are primarily controlled by members of the myogenic regulatory factors (MRF) and myocyte enhancer factor 2 (MEF2) transcription factor families. Secondary factors also exert effects on myogenesis such as the activator protein 1 (AP-1) transcription factor which has a complex role in the differentiation process. The AP-1 subunit Fra-2 has a role in skeletal muscle development and regeneration but it is less defined. Here, the role of Fra-2 in skeletal myogenesis was investigated thereby extending the study of AP-1 in muscle. It was determined that Fra-2 is regulated by the ERK 1/2 MAPK pathway via phosphorylation at S320 which is important for Fra-2 protein stability. Gain of function studies exploiting stability of Fra-2 achieved by phosphomimetic mutations impacted differentiation negatively. Conversely, loss of function using siRNAs resulted in precocious differentiation suggesting an overall inhibitory role for Fra-2 in myogenic cells. Intriguingly, it was observed that AP-1 is differentially expressed in a differentiated culture of C2C12 myogenic cells in that Fra-2 expression is restricted to monomucleated reserve cells and not in the differentiated myotubes. Furthermore, it was determined that Fra-2 is expressed in Pax7 positive satellite cells in a single muscle fibre culture model and that it binds to the promoter of the mustn1 gene which, in turn, is also a novel satellite cell marker. In conclusion, Fra-2 protein stability is regulated by phosphorylation of ERK 1/2 in myogenic cells and its expression in quiescent reserve cells and in satellite cells suggests a possible role for Fra-2 in maintaining the undifferentiated state in myogenic progenitor cells.