Early loss of bone mass in the SOD1(G93A) ALS mouse model is associated with loss of sensitivity to osteogenic factors and enhanced bone senescence
Amyotrophic lateral sclerosis (ALS) is conceptualized as a progressive motor neuron degeneration, but emerging evidence suggests systemic manifestations beyond the neuromuscular system. Altered bone structure and metabolism has been shown to be part of the ALS clinical presentation; it remains unclear whether the bone phenotype is secondary to muscle denervation and reduced loading, or it is due to an autonomous process. We investigated skeletal involvement in the SOD1(G93A) mouse model at presymptomatic (P45) and symptomatic (P110) stage through biomechanical and transcriptomic approaches. Biomechanical three-point bending tests revealed significant reductions in femoral rigidity and maximum bending force in SOD1 mutants at P45, indicating early structural deficits. Micro-CT analysis demonstrated reduced trabecular bone mineral density and thickness at P45, with progressive trabecular loss and cortical thinning by P110. Histological examination revealed marked osteoblast loss at P45 with minimal changes in osteoclast activity, suggesting impaired bone formation as the primary early mechanism. Transcriptomic analysis of bone tissue and cultured osteoblasts from P45 mice identified dysregulation of bone differentiation pathways, including downregulation of osteoblast differentiation genes and upregulation of negative regulators of ossification. Notably, unfolded protein response was upregulated in SOD1 osteoblasts and E2F targets and G2M checkpoint genes were significantly downregulated. Immunohistochemistry confirmed increased p16Ink4a level in SOD1 osteoblasts, indicating cellular senescence as a key pathological mechanism. These findings suggest that bone deterioration in ALS reflects intrinsic defects in bone that precede motor symptoms. Understanding these mechanisms may lead to targeted interventions to preserve skeletal integrity in ALS patients and potentially reveal novel insights into ALS pathogenesis beyond the neuromuscular system.