The myocyte enhancer factor-2 (MEF2) family mediates complex gene regulation in skeletal and cardiac myocytes
Desjardins, Cody Alan
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Regulation of striated muscle differentiation and development are complex processes coordinated by an array of transcription factors. MEF2 is a crucial transcription factor required for muscle differentiation, but the roles of the individual MEF2 family members, MEF2A-D, have not been extensively evaluated. Acute ablation of Mef2 expression in skeletal myoblasts revealed a required role for MEF2A activity in myoblast differentiation that was not shared with the other MEF2 factors. We hypothesized that a transcriptomic level analysis of Mef2-deficient skeletal myoblasts would reveal distinct regulatory roles for each MEF2 isoform. Comparative microarray analysis supported our hypothesis and we observed distinct gene programs preferentially-sensitive to individual MEF2 isoforms. While there was no variance in the consensus binding site associated with regulation by individual MEF2 isoforms, we did observe uniquely enriched binding sites for candidate co-regulatory proteins that mediate these complex regulatory patterns. Based on our observations in skeletal myoblasts, we performed a series of acute Mef2 knockdowns in neonatal cardiomyocytes and uncovered a requirement for MEF2A and -D, but not MEF2C in cardiomyocyte survival. Comparative microarray analysis confirmed that, similar to skeletal myoblasts, the MEF2 family regulated distinct but overlapping gene programs in cardiomyocytes. Additionally, this analysis uncovered a previously uncharacterized antagonistic regulation of a subset of cell cycle and sarcomere genes. Interestingly, Mef2a and -d knockdowns caused an upregulation of cell cycle markers and downregulation of sarcomere genes, with the opposite regulatory pattern in Mef2c knockdown. Further investigation of the proximal promoter region of these genes revealed enriched binding sites for transcription factors associated with key signaling pathways in the developing embryo, Hedgehog and Notch. Overexpression of constitutively active components of these signaling pathways revealed that Notch function requires the presence of MEF2A and -D, while Hedgehog does not appear to interact with these two isoforms. We have shown through our studies that MEF2, a core muscle transcription factor, takes part in complex regulatory interactions that are critical for the appropriate development of striated muscle tissues.