Project Summary / Abstract Sarcopenia, the age-related decline in skeletal muscle mass and force, is characterized by muscle wasting and diminished contractile strength leading to physical frailty, decreased mobility, increased disability, and a loss of independence. The neuromuscular junction (NMJ) is the synapse responsible for transmitting action potentials from motor neurons to the motor endplate of muscle fibers. NMJs exhibit structural and functional disruptions including a complete loss of innervation during aging, but the mechanisms for this detrimental phenotype remain unknown. This study aims to investigate the role in NMJ degeneration of changes in subsynaptic myonuclei (SSM). Skeletal muscle fibers are large multinucleated cells, with different individual nuclei responsible for supporting localized domains. SSM are a population of nuclei positioned near the NMJ that play a crucial role in NMJ development and maintenance. Previous studies have produced conflicting results regarding associations between aging, SSM number, and NMJ degeneration emphasizing the need for further investigations to clarify the role of changes in SSM as potential cause or consequence of a loss of innervation. Our own data indicates no change in SSM number with age, despite high numbers of disrupted and denervated endplates in muscles of old animals. We also found no changes in SSM number following acute denervation, but denervated muscles showed a significant increase in LMNA gene expression as well as in the levels of the protein product Lamin A/C. Lamin A/C is the primary intermediate filament protein of the nuclear lamina and a key component in maintaining myonuclear structural integrity. The nuclear lamina plays a critical role in regulating gene expression by interacting with chromatin and nuclear membrane proteins. The expression of Lamin A/C has been reported to decrease with age, and its muscle-specific deletion accelerates NMJ degradation, resembling age-related NMJ decline. Our overall hypothesis is that Lamin A/C is required for mediating the process of reinnervation following a denervation event. We will address this hypothesis using a combination of powerful mouse models, surgical interventions, assessments of neuromuscular functional properties, and an array of molecular techniques combined with high-resolution confocal imaging. Specifically, we will [1] define the impact of denervation on the integrity of subsynaptic myonuclei in young and aged mice, and [2] determine the impact of skeletal muscle-specific LMNA deletion on reinnervation following nerve injury. We expect to see a denervation- induced upregulation of Lamin A/C that contributes to the maintenance SSM shape and SSM number leading to improved reinnervation of endplates. Therefore, we also predict that lack of Lamin A/C will result in impaired reinnervation after nerve injury leading to poor neuromuscular function. By elucidating key regulators of reinnervation, we aim to identify the...