Title : Abnormal mitochondrial transport and morphology as

Spinal muscular atrophy (SMA), characterized by specific degeneration of spinal motor neurons, is caused by mutations in the survival motor neuron 1 (SMN1) gene and subsequent decreased levels of functional SMN. How the deficiency of SMN, a ubiquitously expressed protein, leads to spinal motor neuron-specific degeneration in SMA patients remains unknown. In this study, we examined the role of SMN on mitochondrial axonal transport and morphology in human motor neurons by generating SMA type 1 patient-specific induced pluripotent stem cells (iPSCs) and then differentiating these cells into spinal motor neurons. The initial specification of spinal motor neurons was not affected, but these SMA spinal motor neurons specifically degenerated following long-term culture. Moreover, at an early stage in SMA spinal motor neurons, but not in SMA forebrain neurons, mitochondrial number, area, and transport were significantly reduced in axons. Knocking down of SMN expression led to similar mitochondrial defects in spinal motor neurons derived from human embryonic stem cells (hESCs), confirming that SMN deficiency results in impaired mitochondrial dynamics. Finally, the application of N-acetylcysteine (NAC) mitigated the impaired mitochondrial transport and morphology, and then rescued motor neuron degeneration in SMA long-term cultures. Furthermore, NAC ameliorated the reduced mitochondrial membrane potential in SMA spinal motor neurons, suggesting that NAC may rescue apoptosis and motor neuron degeneration by improving mitochondrial health. Together, our data demonstrate that SMN deficiency results in abnormal mitochondrial transport and morphology and subsequent reduced mitochondrial health, which are implicated in the specific degeneration of spinal motor neurons in SMA. D is ea se M o de ls & M ec ha ni sm s • D M M • A dv an ce a rt ic le