Mitochondrial Oxidative Metabolism is a critical determinant of Muscle Satellite Cell fate & function

Muscle stem cells (MuSCs) give skeletal muscle the remarkable ability to completely regenerate following injury and play an important role in tissue maintenance over time under normal conditions. Decreases in MuSC number and functionality have been implicated in aging-associated muscle wasting and several forms of muscular dystrophy. In response to injury, quiescent MuSCs are activated, proliferate to refill the muscle niche, and differentiate into terminally differentiated, contractile myofibers. Across this process, differentiating MuSCs undergo extensive metabolic reprogramming that entails a switch to oxidative phosphorylation and substantial mitochondrial biogenesis, an effect that has long been assumed to be necessary because of the massive ATP demand of muscle contraction. Accordingly, mitochondrial impairments block myogenic differentiation, though the molecular mechanism(s) underlying this blockade are not understood. Mitochondria are hubs of metabolic activity in cells, and among many catabolic and biosynthetic pathways they harbor an evolutionarily conserved pathway that synthesizes fatty acids in the mitochondrial matrix (mtFAS). mtFAS pathway activity is crucial for TCA cycle activity, through the generation of the enzymatic co-factor lipoic acid, as well as for electron transport chain (ETC) complex assembly and function. We found that errors in the mtFAS pathway block the differentiation of C2C12 myoblasts in vitro and impede muscle regeneration following barium chloride injection-induced injury in vivo. Our preliminary data suggest that this block is early in differentiation, and therefore challenge the model that ATP for contraction is the crucial output of mitochondrial metabolism during differentiation. The experiments in this proposal will definitively determine the critical metabolic requirements of myogenic differentiation and uncover the molecular mechanisms by which these metabolic pathways impinge on differentiation signaling. In Aim 1, we will use mtFAS knockout as a window through which to examine the effect of mitochondrial impairment on myogenic regulatory factor (MRF) signaling, focusing on promoter binding, recruitment of co-activators, and epigenetic modifications at MRF-target promoters. In Aim 2, we will use targeted metabolic strategies to rescue individual aspect(s) of mitochondrial function downstream of mtFAS to test their requirement for differentiation, and employ an unbiased screening method to identify ways to rescue differentiation in mtFAS-deficient cells. Understanding the metabolic regulation of myogenesis will be crucial to developing the next generation of therapies for diseases of MuSC loss or dysfunction.