Molecular Regulation of Muscle Stem Cell Fate during Skeletal Muscle Regeneration and Aging

Muscle stem cells (MuSCs) are the primary source of myogenic progenitors during muscle repair and are essential for the long-term regenerative capacity of skeletal muscle. Following myofiber injury, MuSCs transition from a quiescent to an activated state, and undergo expansion and differentiation at the site of injury to contribute to the tissue repair process. The proper transition between these different states not only determines the rate of myogenesis but also the long-term maintenance of the stem cell pool and the regenerative capacity of skeletal muscle. Thus, expanding our current knowledge of the molecular regulation of MuSC state transitions will be key to discover new approaches to promote muscle health. In the presented dissertation, I have taken a multi-disciplinary approach combining molecular profiling of MuSCs, as well as nutritional and genetic interventions in murine and human pre-clinical models, to study MuSC dynamics and identify actionable mechanisms to combat age-related loss of skeletal muscle regenerative capacity. Using a high-throughput screening approach on human myogenic progenitors (hMPs), nicotinamide (NAM) and pyridoxine (PN) were identified as potent drivers of MuSC amplification and commitment. Oral supplementation of NAM/PN enhances MuSC function in vivo by increasing the number of progenitors that participate to skeletal muscle regeneration, overall resulting in accelerated fiber repair in both young and aged mice. Results from a cohort of older people show that serum levels of NAM and bioactive PN inter-independently associate with muscle mass and walking speed, suggesting that endogenous inadequacy of these two metabolites could contribute to physical decline and loss of functional capacity during aging through MuSC dysfunction and altered myogenic turnover. Overall, we demonstrate that NAM/PN supplementation represents an effective therapeutic strategy to nutritionally stimulate endogenous repair by boosting MuSCs and mitigate skeletal muscle decline during aging. In a second project, we compare the molecular and metabolic requirements of deeply quiescent and primed MuSCs. Using a novel mouse model with two fluorescent reporters under the control of Pax7 and Myf5, we show that differential expression of Myf5 delineates two subpopulations of MuSCs with distinct activation and commitment dynamics. To assess the energy requirements of the two subpopulations, we optimized the SCENITH methodology in MuSCs to profile energy metabolism at the single-cell level by flow cytometry. Our functional metabolic profiling reveals that MuSCs with lower Myf5 expression have drastically lower metabolic requirements compared to Myf5-High cells, suggesting a potential role for metabolism in maintaining a deep quiescence state. Lastly, in a third project, we discovered a new population of quiescent MuSCs that co-expresses Pax7 and Myogenin at the protein level. We demonstrate that this population is primed for cell cycle entry and