Loss of C9orf72 impacts the peripheral neuromuscular system via immune dysregulation and accelerates the progression of amyotrophic lateral sclerosis in SOD-1 mutant mice.

BACKGROUND: Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder where neuromuscular health is central to disease progression. The degeneration of motor neurons (MNs) leads to muscle weakness and paralysis, underscoring the critical importance of neuromuscular junctions (NMJs) and axonal integrity. Among the genetic contributors to ALS, mutations in the C9orf72 gene are the most common, accounting for both ALS and frontotemporal dementia (FTD). While the role of C9orf72 has been studied across various cells and compartments, its function in the peripheral nervous system (PNS), a compartment crucial for maintaining neuromuscular connectivity in ALS,remains largely unexplored. MAIN BODY: Our study investigates the role of C9orf72 loss-of-function in ALS, focusing on its neuromuscular effects. C9orf72 expression is localized in MNs, microglia, oligodendrocytes, and Schwann cells (SCs) in the sciatic nerve (SN), but not in astrocytes. The absence of C9orf72 in mice is associated with hypomyelination and axonal sorting defect in the SN, but not with MNs loss in lumbar spinal cord. Additionally, we identified immune dysregulation, with elevated CD8+ T cells transcript and increased major histocompatibility complex I (MHCI) expression in SCs, in association with enhanced NMJ denervation in C9orf72-deficient ALS mice, suggesting a potential contribution of immune dysregulation to disease progression. These changes contributed to NMJ denervation characterized by increase in the expression of acetylcholine receptor gamma (AChRγ). The combination of C9orf72-deficiency with the ALS-linked SOD1G93A mutation resulted in a more severe phenotype and accelerated disease progression, despite no additional spinal MN loss. CONCLUSION: Our findings underscore the critical role of C9orf72 in maintaining neuromuscular health through its influence on myelination, immune response regulation, and NMJ integrity. Loss of C9orf72 function exacerbates ALS progression by promoting SC dysfunction and immune dysregulation. This highlights the significance of preserving normal C9orf72 function in ALS therapies through antisense oligonucleotides strategies. Furthermore, targeting immune responses and myelination pathways may offer novel avenues for ALS treatment strategies.