Abstract One of the greatest mysteries in ageing biology is to understand why many tissues and organs in our body, including skeletal muscle, decline in function as we get older? Recent studies suggest that the inability of muscle stem cells (MuSCs) to turn on the repair program after trauma is a major factor leading to the loss of muscle mass and strength observed in the elderly. However, given that the signals driving MuSCs into the regenerative state remains a mystery even in young adults, we are left with virtually no therapeutic options for boosting the repair potential of aging MuSCs in the clinic. To address this unmet need, my laboratory recently discovered a new “Super-Healing” adult stem cell activation program, driven by the transcription factor, FBJ osteosarcoma oncogene (FOS), that speeds up adult stem cell activation and enhances muscle repair. Intriguingly, a key downstream target gene of FOS in adult MuSCs is the NAD-consuming, cell surface enzyme, ADP-Ribosyl-Transferase 1 (Art1), which attaches an understudied post-translational modification (PTM), mono-ADP-Ribosylation (MARylation), to protein substrates. Excitingly, our preliminary data suggests that the FOS/ART1-MARylation pathway is disrupted in aged MuSCs, and thus, representing one of the earliest molecular alterations that diminish the regenerative potential of aged skeletal muscle. Thus, in this proposal, we will test the hypothesis that the FOS/AP-1 tissue regenerative program (including the ART1 pathway) is mis-regulated in aged skeletal muscle, triggering a cascade of molecular events that dampen stem cell activation potential and lead to the progressive deterioration of skeletal muscle with increasing age; and most importantly, that reversal of this molecular dysfunction will correct the stem cell activation and regenerative deficits seen in aged skeletal muscle. In Aim1, we will determine the expression patterns and functional significance of FOS/AP-1 gene targets in adult and aged MuSCs. In Aim2, we will determine the expression dynamics, functional requirements, and MARylated protein substrates of the newly discovered NAD/ART1-MARylation stem cell activation pathway in adult and aged MuSCs. Collectively, this work will highlight a new FOS/NAD/ART1-MARylation stem cell activation pathway that has been largely ignored in aging biology until now and whose further study will open new therapeutic avenues for improving muscle health in the elderly population.