Project Summary: Extensive research has established that progressive degeneration of the neuromuscular junction (NMJ) contributes to sarcopenia and motor deficits in old age. Hence, preserving the integrity of the NMJ is likely to be critical in maintaining muscle mass and motor function during aging. For these reasons, significant efforts continue to be devoted to identifying mechanisms that prevent age-related decline of NMJs. To date, there is a wealth of information about the roles of skeletal muscles and motor neurons in NMJ aging. In stark contrast, the role of perisynaptic Schwann cells (PSCs) in NMJ aging remains unknown. PSCs are synaptic glia that exclusively associate with NMJs and are essential for its maturation, stability, function and repair. Highlighting their importance, targeted ablation of PSCs results in axonal degeneration and postsynaptic loss. While several studies have provided clues that PSCs may impact the course of NMJ degeneration with aging, a comprehensive examination of progressive age-related changes in PSCs, as they relate to NMJ deterioration, is a significant knowledge gap. The overarching objective of this proposal is to uncover the cellular and molecular underpinnings of PSC aging to determine their contribution to age-related NMJ and muscle degeneration. Aim 1 will examine the progressive nature of PSC aging and its relationship to NMJ degeneration and muscle atrophy. Accordingly, the timing of age-related morphological changes in PSCs, NMJs and muscle fibers will be determined by light and electron microscopy. The biophysical properties of PSCs will be tracked by calcium imaging. A novel transgenic mouse line along with RNA-Seq and ATAC-Seq will be used to identify molecular pathways intrinsic to PSCs dysregulated during aging. In initial molecular studies, the NGR1-III and MEGF10 were identified as promising regulators of PSC aging. Motor axon-derived NRG1-III is perhaps the best described molecular mediator of PSC physiology. However, downstream effectors of the NRG1-III pathway in PSCs have not been identified and we do not understand how it is impacted by aging. Aim 2 will assess the role of NRG1- III signaling in PSCs aging through gain- and loss-of-function experiments. Preliminary data demonstrate that NRG1-III signaling is heightened in aged PSCs, indicating that curtailing this signaling pathway may protect PSCs during aging. Additional data suggests that NRG1-III signaling affects aging of PSCs by inhibiting MEGF10 expression. MEGF10 is a well-known modulator of cellular spatial organization and synaptic remodeling in the central nervous system (CNS); however, its function in PSCs has not been explored. Aim 3 will examine the role of MEGF10 in specifying the organization and repair functions of aging PSCs using a MEGF10fl/fl mouse line. These studies will be the first to define the physiological, cellular and molecular changes that precipitate aging of PSCs. This proposal will also be the first to ...