Metabolic control of muscle and muscle stem cell function

Skeletal muscle composed of myofibers and a small amount of muscle stem cells (MuSCs). It plays important roles in energy metabolism. Some of the key metabolites, such as NAD+ and acetyl-CoA were recently found to regulate muscle and MuSCs function. Most of these functions are related to the cellular mitochondria. In my PhD thesis projects, we aimed at exploring the link between NAD+ levels, mitochondrial function, muscle structural homeostasis and MuSCs senescence. We also investigated the impact of acetyl-CoA on MuSCs activation and proliferation, through the control of protein acetylation. These projects mainly focus on three aspects: Rejuvenation of adult stem cells by NAD+ repletion. Aging is accompanied by SCs senescence. However, the initial signaling events mediating SCs senescence are still not well defined. We demonstrate the importance of mitochondrial activity as a pivotal modulator of MuSCs senescence. MuSCs from aged mice have reduced NAD+ levels, mitochondrial content and capacity for oxidative respiration. Importantly, the induction of the mitochondrial unfolded protein response (UPRmt), subsequent to increasing cellular NAD+ with the precursor nicotinamide riboside (NR), prevents MuSCs senescence and improves muscle function and regeneration in aged mice. NR also prevents MuSCs dysfunction in the mdx mouse, a known mouse model of accelerated MuSCs senescence. Extending these observations to other SC pools and on the organism as a whole, we demonstrate that NR delays neural and melanocyte SCs senescence, while also increasing mouse lifespan. Control of MuSCs function by acetyltransferase KAT2A. MuSCs are essential for skeletal muscle homeostasis and repair. The complete mechanism controlling MuSCs maintenance and differentiation, however, remains elusive. We found that the acetyltransferase KAT2A expression is essential for myogenic differentiation of MuSCs. KAT2A loss-of-function in cells blocks MuSCs differentiation, and in mice impairs muscle regeneration after damage. The regulation of KAT2A in MuSCs function might rely on its ability to enzymatically acetylate other proteins, and in particular the transcription factor PAX7. NAD+ repletion improves muscle function in muscular dystrophy. Duchenne muscular dystrophy (DMD) is caused by dystrophin gene mutations that lead to progressive muscle degeneration. Our bioinformatics analysis in mice and DMD patients suggests a role for NAD+ in protecting the muscle from metabolic and structural degeneration. Validating these findings, we reveal that the mdx mouse is characterized by reductions in muscle NAD+ levels, concurrent to increased PARP activity and reduced expression of nicotinamide phosphoribosyltransferase (NAMPT), the key enzymes for NAD+ consumption and biosynthesis. Replenishing the NAD+ pool by dietary NR supplementation benefits muscle function in mdx and mdx/Utr-/- mice, an effect replicated in C. elegans models of DMD. The beneficial effects of NAD+ repletion in muscular dystrophy are pleiotropic and rely on the improvement in mitochondrial function, structural protein expression and on reductions in general PARylation, inflammation and fibrosis. In summary, our work demonstrates the critical role of NAD+ and Acetyl-coA in maintaining muscle and MuSCs function. Repletion of NAD+ levels, by NR administration, and/or boosting the function of acetyltransferase KAT2A might be attractive strategies to maintain muscle homeostasis and MuSCs function in aging and muscular dystrophy.