The Mechanism Of Cerebellar Ataxia Caused By Purkinje Cell-specific Deletion Of Snx14 In Cerebellum

Autosomal-recessive cerebellar ataxias(ARCAs)are an inherited neurodegenerative disease which occurs in both children and young adults.Autosomal recessive spinocerebellar ataxia 20(SCAR20)is a newly identified form of early-onset ARCAs.Clinical manifestations of SCAR20 include cerebellar atrophy,motor ataxia and severe mental retardation.The whole-exon sequencing of the patients revealed that SCAR20 was caused by deleterious mutations of the sorting nexin 14(SNX14)gene.However,the underlying molecular mechanism is still unclear and no cure is available.We generated a mouse model with Snxl4 conditional deletion in the whole brain.Through behavioral and morphological analysis,we found that the mice recapitulate pathological manifestations in the patients,that is,the mice showed severe motor ataxia,and the massive loss of cerebellar Purkinje cells.Given that Purkinje cells are essential for motor coordination,we speculate that Purkinje cells may play a key role in the pathogenesis of SCAR20.Therefore,we generated a Purkinje cell-specific Snx14 conditional knockout mouse model(Snx14 cKO).In behavioral tests,we found that Snx14 cKO mice exhibit progressive ataxia.Morphologically,Snx14 cKO mice displayed age-dependent degeneration of cerebellar Purkinje cells,and microglial activation which was positively correlated with the loss of Purkinje cells.In addition,we observed an increased production of proinflammatory cytokines,suggesting that Purkinje cell degeneration induces microglial activation and neuroinflammation.Moreover,we observed axonal swelling in Snx14-deficient Purkinje cells,and the electron microscopy indicated the accumulation of organelles(especially mitochondrion)in the swollen axons.To determine whether defective axonal transport induces the accumulation of mitochondria in axons,we examined axonal transport of mitochondria in primary neurons using live-cell imaging,and idnetified a defect in axonal transport of mitochondria,especially anterograde transport in the Snx14deficient neurons.Furthermore,we found the impaired cell respiration and reduced ATP production in Snx14-deficient neurons.In summary,using a Snx14 conditional knockout mouse model,we found that SNX14 deficiency causes cell autonomous death of Purkinje cells,which is an important cause of SCAR20.SNX14 deficiency disrupted axonal transport of mitochondria and impaired mitochondrial function,ultimately leading to the death of Purkinje cells.Our identification of the role of SNX14 in mitochondrial dysfunction and Purkinje cell death establishes a new molecular mechanism of SCAR20 pathogenesis.