Dissecting the Epigenetic Signaling Underlying Early Myogenic Differentiation

Abstract

While skeletal myogenesis is tightly coordinated by myogenic regulatory factors, epigenetic determinants have emerged as vital mechanisms of myogenic regulation as well. The epigenetic network provides the blueprint by which extra-cellular signals, such as bexarotene, a clinically approved agonist of retinoid X receptor (RXR), impact lineage-specific gene expression. We have previously established that bexarotene promotes the specification and differentiation of the skeletal muscle lineage however, the role of RXR signaling and how it effects global gene expression to promote myogenic differentiation is unclear. Here, we utilized integral RNA-seq and ChIP-seq analyses to understand the mechanistic activation of myogenic regulatory elements and the functional mode of myogenic transcriptional regulators. Our results describe an enrichment of loci-specific histone acetylation at enhancers associated to the histone acetyltransferase, p300, particularly when it is recruited by muscle master regulator, MyoD and define a role for p300 in the establishment and regulation of myogenic loci. Similarly, dissection of rexinoid-responsive gene expression revealed that rexinoid-enhanced myoblast differentiation is reconciled largely via a direct regulation of MyoD and residue-specific histone acetylation to promote a coordination of exit from the cell cycle to an activation of muscle-related genes including the differentiation marker, myogenin. Accordingly, we also uncovered a significant association of myogenin with rexinoid-responsive gene expression and its distribution to poised enhancers where RXR signaling augments residue-specific histone acetylation, particularly H4K8ac and H3K9ac, at enhancers co-occupied by p300 and myogenin. Importantly, we also utilized global transcriptional profiling of rexinoid-enhanced differentiation to identify the novel myogenic target, Tmem182. We show for the first time that Tmem182 expression is MyoD dependent and that the transmembrane protein is important for myoblast differentiation. In addition, we report increased enrichment of p300 as well as rexinoid-responsive histone acetylation at the Tmem182 promoter such that it may represent global trends in which myogenin cooperates with p300, in the installation of an epigenetic signature associated to rexinoid responsive gene expression. Thus, we provide novel molecular insights into the mechanisms of transcriptional activation underlying myogenic programs in the context of RXR signaling. Our study presents a valuable foundation for pharmacological correction of epigenetic regulators to treat muscle-related diseases and for the identification of additional myogenic targets and molecular interactions for therapeutic development.