A Validated Transcriptomic NMJ Remodeling Score Reveals Synaptic Dysfunction Independent of Muscle Atrophy after Immobilization in a Microgravity Analog.

Muscle weakness after immobilization often exceeds that explained by loss of muscle mass alone, suggesting a role for neuromuscular synaptic changes. To quantify these adaptations, we developed a composite transcriptomic Neuromuscular Junction (NMJ) Remodeling Score and evaluated its behavior relative to classical atrophy pathways during short-term unloading. We analyzed vastus lateralis RNA sequencing data from adults undergoing 10 days of unilateral lower-limb suspension followed by a 21-day recovery, generating NMJ and atrophy scores for 15 and 10 genes, respectively. Transcriptome-wide testing across more than twenty thousand genes identified a broad pattern of metabolic suppression. The NMJ score showed a large effect increase during unloading and partial normalization with recovery, while the atrophy score rose more strongly and reversed during recovery. The two scores demonstrated weak correlation, consistent with distinct biological processes. Individual NMJ-related genes displayed coordinated regulation, including marked upregulation of several acetylcholine receptor subunits and modest downregulation of muscle signaling kinase (MuSK), reflecting a denervation-like transcriptional pattern. Directional replication in a 60-day bed rest cohort supported generalizability across disuse conditions. Together, these findings indicate that limb unloading elicits measurable transcriptomic remodeling at the NMJ that is only partially aligned with atrophy signaling, providing a framework for investigating neural contributions to immobilization-induced weakness.