Muscle Stem Cell Response to Perturbations of the Neuromuscular Junction Are Attenuated With Aging

During aging and neuromuscular diseases, there is a progressive loss of skeletal muscle volume and function, which is often associated with denervation and a loss of muscle stem cells (MuSCs). A relationship between MuSCs and innervation has not been established however. Herein, we administered neuromuscular trauma to a MuSC lineage tracing model and observed a subset of MuSCs specifically engraft in a position proximal to the neuromuscular junction (NMJ). In aging and in a model of neuromuscular degeneration (Sod1-/-), this localized engraftment behavior was reduced. Genetic rescue of motor neurons in Sod1-/- mice reestablished integrity of the NMJ and partially restored MuSC ability to engraft into NMJ proximal positions. Using single cell RNA-sequencing of MuSCs, we demonstrate that a subset of MuSCs are molecularly distinguishable from MuSCs responding to myofiber injury. These data reveal unique features of MuSCs that respond to synaptic perturbations caused by aging and other stressors. Overall design: C57BL/6J wild-type female mice were obtained from Charles River Breeding Laboratories, the National Institute on Aging, or from a breeding colony at the University of Michigan (UM). All mice were fed normal chow ad libitum and housed on a 12:12 hour light-dark cycle under UM veterinary staff supervision. All procedures were approved by the University Committee on the Use and Care of Animals at UM and were in accordance with the U.S. National Institute of Health (NIH). Young female mice (4-6 months) and aged female mice (20-24 months) were randomly assigned to one of five groups: uninjured, day 3, and day 7 injured (n=4 per group). To induce skeletal muscle injury, mice were first anesthetized with 2% isoflurane and administered a 1.2% barium chloride (BaCl2) solution injected intramuscularly into several points of the tibialis anterior and gastrocnemius muscles for a total of 80 µL per hindlimb.