RNA-Seq profiling of C2C12 cells with exogenous expression of Mef2Da1 and Mef2Da2
Although splicing occurs in most multi-exon genes, the generation of distinct isoforms through the alternate use of mutually exclusive exons is less prevalent. As exon-switching events have the potential to give rise to isoforms with different cellular functions, we have explored the role of the muscle-specific (Mef2Da2) and ubiquitously expressed (Mef2Da1) isoforms of the transcription factor Mef2D in myogenesis. Here we show that both isoforms of Mef2D bind a largely overlapping subset of genomic loci, yet only the muscle-specific Mef2Da2 isoform can activate the late myogenic gene expression program. This differential ability to activate transcription is modulated by PKA signaling where Mef2Da1 is efficiently phosphorylated by the kinase to enhance its association with repressive HDAC-deacetylase complexes. In contrast, alternate exon usage in Mef2Da2 renders the protein resistant to PKA phosphorylation, allowing it to interact with transcriptionally permissive Ash2L-trithorax complex. Our findings support a model wherein alternative exon usage allows Mef2D to transition from a repressor to activator in a myogenic environment rich in PKA activity. Thus we have identified a novel paradigm in which a ubiquitously expressed transcription factor has evolved to undergo tissue-specific alternative exon usage to permit the proper temporal activation of a gene expression program during differentiation. Overall design: RNA-Seq profiling of C2C12 cells with exogenous expression of Mef2Da1 and Mef2Da2