Role of nuclear transport receptor KPNB1 in the pathophysiology of spinocerebellar ataxia type 3

Abstract

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is the most common form of autosomal dominant hereditary ataxias and characterized by a pathological expansion of the polyglutamine (polyQ) tract within the ataxin-3 protein. Ataxin- 3 is mainly a cytoplasmic protein, although polyQ-expanded ataxin-3 accumulates in the nucleus of affected neurons and forms intranuclear aggregates, leading to neurotoxicity and cell death. Unravelling the underlying mechanisms in the nuclear localization of polyQ- expanded ataxin-3 and its involvement in neurotoxicity can provide insight into the pathogenesis of this disease and the development of novel therapeutic strategies. In this respect, investigation of the nucleocytoplasmic transport machinery and its implication for the pathogenesis of SCA3 and other polyQ diseases has gathered attention. Our lab has previously indicated the critical importance of karyopherin α-3 (KPNA3), a nuclear transport receptor, in the nuclear transport of ataxin-3 and its implication in the pathogenesis of SCA3. Since KPNA3 functions as an adaptor protein for karyopherin β-1 (KPNB1) in the nuclear transport of protein cargos, we aimed to explore the role of KPNB1 in the pathogenesis of SCA3 as well. Here, we report on SCA3 cell model-based analysis of the nuclear transport receptor KPNB1 and its implications for the pathogenesis of SCA3. We figured out that KPNB1 interacts directly with both wild-type and polyQ-expanded ataxin-3. However, modulating KPNB1 levels did not change the subcellular distribution of ataxin-3. Interestingly, KPNB1 overexpression reduced protein levels and aggregation of ataxin-3 and promoted its cleavage, whereas its knockdown and pharmacological inhibition led to an increase in soluble and insoluble levels of ataxin-3. Our data revealed that modulation of ataxin-3 was apparently based on protein fragmentation, independent of the classical SCA3-associated proteolytic pathways. Label-free quantitative proteomics and knockdown experiments indicated mitochondrial protease CLPP as a potential mediator of the ataxin-3-degrading effect induced by KPNB1 overexpression. We confirmed a reduction of KPNB1 protein levels in SCA3 by analyzing two SCA3 transgenic mouse models and induced pluripotent stem cells (iPSCs) derived from SCA3 patients. Our findings suggested a yet undescribed regulatory function of KPNB1 in modulating ataxin-3, thereby highlighting a new potential target of therapeutic value for SCA3.