Restoration of Shal/K4 proteostasis and motor function in a Drosophila model of spinocerebellar ataxia type 19/22.

Loss-of-function mutations in the human KCND3 gene encoding K4.3 K channels are linked to the autosomal dominant neurodegenerative disease spinocerebellar ataxia type 19/22 (SCA19/22). Previous biophysical and biochemical analyses in vitro support the notion that the autosomal dominant inheritance pattern of SCA19/22 is associated with the dominant-negative effects of disease-causing K4.3 mutants on proteostasis of their wild-type (WT) counterpart. Herein we aimed to explore whether the disease-causing mutants might perturb protein expression of endogenous K4.3 channel in human cells, as well as contributing to in vivo pathomechanisms underlying motor impairments and neurodegeneration in an animal model of SCA19/22. Substantial reduction in human K4.3 protein level was validated in skin fibroblasts derived from heterozygous SCA19/22 patients. Genetic knockdown of endogenous Shal, the fly ortholog of human K4.3, in Drosophila led to locomotor impairment, ommatidia degeneration, and reduced brain cortex thickness, all of which was effectively ameliorated by transgenic expression of human K4.3, but not K1.1 K channel. Transgenic expression of SCA19/22-causing human K4.3 mutants resulted in notable disruption of endogenous Shal proteostasis, locomotor function, and ommatidia morphology in Drosophila. Enhanced expression of the Drosophila molecular chaperones HSC70 and HSP83 in our fly model of SCA19/22 corrected Shal protein deficit, locomotor dysfunction, and neurodegeneration. Overexpression of Hsp90β also upregulated endogenous human K4.3 protein level in patient-derived skin fibroblasts. Our findings highlight Drosophila as a suitable animal model for studying K4.3 channelopathy in vivo, and accentuate a critical role of defective K4.3 proteostasis in the pathogenesis of motor dysfunction and neurodegeneration in SCA19/22.