Regional rescue of spinocerebellar ataxia type 1 phenotypes by 14-3-3ε haploinsufficiency in mice underscores complex pathogenicity in neurodegeneration

Spinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disease caused by the expansion of a CAG repeat encoding a polyglutamine tract in Ataxin-1 (ATXN1). Both WT and mutant ATXN1 interact with 14-3-3 proteins, and 14-3-3 overexpression stabilizes ATXN1 levels in cells and increases ATXN1 toxicity in flies. To determine whether reducing 14-3-3 levels might mitigate SCA1 pathogenesis, we bred Scal(154Q/+) mice to mice lacking one allele of 14-3-3 epsilon. 14-3-3 epsilon haploinsufficiency rescued cerebellar pathology and motor phenotypes but, surprisingly, not weight loss, respiratory dysfunction, or premature lethality. Biochemical studies revealed that reducing 14-3-3 epsilon levels exerted different effects in two brain regions especially vulnerable in SCA1: Although diminishing levels of both WT and mutant ATXN1 in the cerebellum, 14-3-3 epsilon haploinsufficiency did not alter ATXN1 levels in the brainstem. Furthermore, 14-3-3 epsilon haploinsufficiency decreased the incorporation of expanded ATXN1 into its large toxic complexes in the cerebellum but not in the brainstem, and the distribution of ATXN1's small and large native complexes differed significantly between the two regions. These data suggest that distinct pathogenic mechanisms operate in different vulnerable brain regions, adding another level of complexity to SCA1 pathogenesis.