Muscular Rapsyn safeguards neuromuscular junction integrity during development and adult maintenance

Background Rapsyn is a pivotal scaffolding protein required for clustering nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction (NMJ). Mutations in Rapsyn are a well-established cause of congenital myasthenic syndromes (CMS), leading to early-onset muscle weakness. However, although Rapsyn is expressed exclusively in skeletal muscle, the specific contribution of muscle-derived Rapsyn to NMJ development and stability has never been directly tested. Previous studies using in vivo Rapsyn silencing suggested a role for Rapsyn in adult NMJ maintenance, but the specific contribution of muscle-derived Rapsyn and the threshold level requirement to maintain adult NMJ integrity remain incompletely understood. Methods Using muscle-specific and inducible Rapsyn knockout mouse models, we examined the role of muscle-derived Rapsyn in NMJ formation, structural maintenance, and neuromuscular function across developmental and adult stages. Results Homozygous muscular Rapsyn ablation caused neonatal lethality and catastrophic NMJ disorganization. In contrast, grade reduction of Rapsyn expression in adult mice progressively induced fragmented nAChR clusters and activity-dependent neuromuscular dysfunction despite relatively preserved gross NMJ morphology. These findings indicate that reduced Rapsyn expression destabilizes postsynaptic receptor domains and produces features resembling postsynaptic dysfunction observed in myasthenic disorders. Conclusions This study demonstrates that sustained muscular Rapsyn expression is required for lifelong NMJ integrity and identifies a dose-dependent requirement for maintaining postsynaptic stability. Our findings complement previous studies demonstrating the protective effects of Rapsyn augmentation in experimental autoimmune MG by providing genetic evidence that sustained muscular Rapsyn expression is required for lifelong NMJ stability and postsynaptic resilience.