Abstract
Background
Spinal muscular atrophy (SMA), caused by mutations in survival motor neuron 1 (SMN1), presents with severe muscle atrophy and prevalent osteoporosis. Transcriptomic profiling of patient muscle biopsies revealed enrichment of extracellular vesicle genes, yet the contribution of SMA-EVs to SMA-associated bone loss and their link to SMN deficiency remain undefined.Methods
Clinical CT/MRI images of SMA and control subjects were acquired to quantify osteoporosis and muscle atrophy. SMA model mice (Smn1hSMN2/hSMN2ROSA26hSMN2/+) were phenotyped at 6 weeks by micro-CT and histology. EVs were isolated from muscles, validated (western blot, transmission electron microscope, nano-flow cytometry, BCA protein assay), and compared between genotypes. DiL-labelled EV biodistribution was tracked in vivo; uptake by BMSCs/BMMs was confirmed by confocal microscopy. Cytotoxicity was assessed by live/dead staining. Dose-response experiments evaluated the osteogenic and anti-osteoclastic activity of SMA-EVs. Comparison of the effects of SMA-EVs and CON-EVs were performed with adequate doses in vitro and in vivo, followed by EV replenishment in SMA mice. Osteogenic and osteoclastogenic gene expression was quantified by qPCR; ALP activity by ELISA. Bone and cell parameters were assessed by HE staining, TRAP staining, COL-1 immunofluorescence staining, and micro-CT. RNA-seq data were validated by Western blot. Lentiviral shRNA and over-expression plasmids were used to generate muscle cells with stable SNAP23 knock-down or up-regulation, and AAV-mediated muscle-specific Snap23 over-expression was employed in mice to define the role of muscular SNAP23 in EV secretion and its impact on bone mass. Mice carrying extra SMN2 transgenic copies were analyzed to delineate the SMN-SNAP23 relationship.Results
SMA patients and mice exhibited a significantly diminished capacity of skeletal muscle to secrete EVs, which were readily internalized by BMSCs and BMMs, dose-dependently promote osteogenic differentiation and suppress osteoclast formation. Adequate-dose SMA-EVs matched CON-EVs efficacy, and SMA-EVs supplementation effectively rescued the osteoporotic phenotype in SMA. Transcriptomics indicated impaired SNARE complex-mediated vesicle secretion pathway. We further demonstrated that deficiency of SMN protein drives downregulation of its downstream key SNARE component, SNAP23, thereby impairing the efficiency of SMA-EV secretion.Conclusion
Our work elucidates a novel disease-specific mechanism for SMA osteoporosis-dysfunction of the SMN-SNAP23-EVs axis-and highlights the therapeutic potential of replenishing SMA-EVs or targeting this axis, offering a promising strategy to improve skeletal health in SMA.