Transcriptional Regulation of Muscle Contractility and Metabolism by a MicroRNA-Mediated Feed Forward Loop

In response to physiological stimuli, skeletal muscle alters its myofiber composition to significantly affect muscle performance and metabolism. This process requires concerted regulation of myofiber specific isoforms of sarcomeric and calcium regulatory proteins that couple action potentials to the generation of contractile force, and a concordant alteration of myofiber metabolism. This stress responsive phenotypic shift requires that extrinsic and intrinsic signals coordinately affect gene regulatory mechanisms to ensure a proper adaptive response. Messenger RNA transcripts coding for three myosin heavy chain contractile proteins, Myh6, Myh7, and Myh7b, encode microRNAs miR-208a, miR-208b, and miR-499 respectively, within their introns. Here, I demonstrate through gain and loss of function studies in vivo that this family of microRNAs, termed MyomiRs, functions in skeletal muscle to promote the conversion of glycolytic myofibers expressing fast isoforms of contractile proteins, to slow oxidative myofibers that confer improved muscular performance. The MyomiRs influence myofiber phenotype by negatively regulating the transcription factor Sox6, in addition to several other functionally related transcriptional repressors. Subsequent studies identified Sox6 as a fast myofiber enriched repressor of slow muscle gene expression. Mice lacking Sox6 specifically in skeletal muscle have an increased number of slow myofibers, elevated mitochondrial activity, and exhibit down regulation of the fast myofiber gene program, resulting in enhanced muscular endurance. This effect on skeletal muscle is mediated by the direct binding of Sox6 to conserved cis-regulatory elements upstream of slow myofiber enriched genes, leading to their transcriptional repression. Collectively, these results identify myosin heavy chain encoded microRNAs, and their target Sox6, as robust regulators of muscle contractile and metabolic phenotype, and elucidate a double negative feed-forward regulatory loop by which functionally related fiber type specific gene isoforms are collectively controlled in response to physiological stressors.