Aging, Oxidative Stress, and Schwann Cell Regulation: Implications for Neuromuscular Health

Abstract

This dissertation investigates the complex interplay between muscle-resident Schwann cells and neuromuscular junction (NMJ) maintenance and remodeling with a focus on the implications for aging-associated muscle atrophy and motor unit adaptation. The primary objective is to elucidate the cellular and molecular mechanisms underlying skeletal muscle and glial function in denervating conditions. The research employs a comprehensive approach, combining advanced in vivo and in vitro techniques, to uncover key elements that sustain NMJ integrity and overall neuromuscular health. We first examined the role of senescent cell accumulation in the decline of muscle function in aging mice. A genetic strategy to ablate p16INK4A-expressing cells in aged mice revealed that their removal at 20 months of age leads to modest preservation above untreated controls in muscle mass (+11%) and force (+11.5%) by 26 months. This finding indicates a tangible benefit of senescent cell elimination in mitigating age-associated muscle decline and suggests that changes in proliferative cell dynamics may mediate muscle health during aging. We next shifted to focus on the transcriptional responses of Schwann cells under oxidative stress and their role in modulating AChR density in muscle cells. Findings from this segment demonstrate a notable upregulation in key repair markers like Ngfr, Gdnf, and Agrn in Schwann cells in culture exposed to a ROS-inducing agent. Conditioned media from these cells, when applied to myotubes in vitro, not only enhanced AChR clustering but also synergistically potentiated the effects of Agrin-induced AChR clustering, indicating a direct Schwann cell-to-muscle signaling mechanism supporting post-synaptic structures. Further investigations explore Schwann cell behavior under conditions of denervation and reinnervation in vivo using Sod1-/- mice, a recognized model of progressive NMJ degeneration. This part of the study identifies a key window for NMJ regeneration and offers a detailed single cell RNA-Sequencing analysis of muscle-resident Schwann cells. This analysis identifies multiple subpopulations of Schwann cells, including the novel identification of a synapse-promoting terminal Schwann cell (tSC) cluster that emerges during denervation. Particularly noteworthy is the discovery of enhanced secreted phosphoprotein 1 (SPP1) signaling emanating from myelin Schwann cells in denervated muscles. Validation studies showed that neutralizing intramuscular SPP1 signaling in wildtype mice with nerve injuries impaired or delayed muscle fiber reinnervation, accompanied by a reduced number of tSCs. These experiments reveal intricate gene expression profiles associated with the SPP1 pathway and Schwann cell repair mechanisms, providing a deeper understanding of muscle reinnervation processes. Overall, this dissertation synthesizes its findings to significantly advance the fields of Schwann cell biology and neuromuscular remodeling. The insights gained offer a profound step forward in our understanding of neuromuscular health. By identifying key gaps in current knowledge and suggesting future research directions, it lays the groundwork for novel therapeutic approaches targeting Schwann cells and NMJ integrity in aging and neuromuscular disorders.