ALS-associated mutation SOD1^G93A leads to abnormal mitochondrial dynamics in osteocytes

While the death of motor neuron is a pathological hallmark of amyotrophic lateral sclerosis (ALS), defects in other cell types or organs may also actively contribute to ALS disease progression. ALS patients experience progressive skeletal muscle wasting that may not only exacerbate neuronal degeneration, but likely has a significant impact on bone function. In our previous published study, we have discovered severe bone loss in an ALS mouse model with overexpression of ALS-associated mutation SOD1^G93A (G93A). Here we further provide a mechanistic understanding of the bone loss in ALS animal and cellular models. Combining mitochondrial fluorescent indicators and confocal live cell imaging, we discovered abnormalities in mitochondrial network and dynamics in primary osteocytes derived from the same ALS mouse model G93A. Those mitochondrial defects occur in ALS mice after the onset of neuromuscular symptoms, indicating that mitochondria in bone cells respond to muscle atrophy during ALS disease progression. To examine whether ALS mutation has a direct contribution to mitochondrial dysfunction independent of muscle atrophy, we evaluated mitochondrial morphology and motility in cultured osteocytes (MLO-Y4) with overexpression of mitochondrial targeted SOD1^G93A. Compared with osteocytes overexpressing the wild type SOD1 as a control, the SOD1^G93A osteocytes showed similar defects in mitochondrial network and dynamic as that of the primary osteocytes derived from the ALS mouse model. In addition, we further discovered that overexpression of SOD1^G93A enhanced the expression level of dynamin-related protein 1 (Drp1), a key protein promoting mitochondrial fission activity, and reduced the expression level of optic atrophy protein 1 (OPA1), a key protein related to mitochondrial fusion. A specific mitochondrial fission inhibitor (Mdivi-1) partially reversed the effect of SOD1^G93A on mitochondrial network and dynamics, indicating that SOD1^G93A likely promotes mitochondrial fission, but suppresses the fusion activity. Our data provide the first evidence that mitochondria show abnormality in osteocytes derived from an ALS mouse model. The accumulation of mutant SOD1^G93A protein inside mitochondria directly causes dysfunction in mitochondrial dynamics in cultured MLO-Y4 osteocytes. In addition, the ALS mutation SOD1^G93A-mediated dysfunction in mitochondrial dynamics is associated with an enhanced apoptosis in osteocytes, which could be a potential mechanism underlying the bone loss during ALS progression.