Polyglutamine-Expanded Ataxin-3 Accelerates CFTR Degradation Through K63-Linked Ubiquitination to Exacerbate Microglial Inflammation.

The deubiquitinase Ataxin-3 causes spinocerebellar ataxia type 3 (SCA3) upon polyglutamine (polyQ) expansion. While expressed in the nervous system, the function of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel therein remains unclear, as does its potential regulation by Ataxin-3. This study reveals that Ataxin-3 interacts with and promotes CFTR degradation in human microglia by its K63-linked polyubiquitination, thereby shortening CFTR's half-life. Paradoxically, K63-linked polyubiquitin chains also promote the degradation of Ataxin-3 itself, suggesting a complex feedback mechanism. The pathogenic Ataxin-3Q80 mutant exerts a stronger effect than the wild-type protein. Consequently, this Ataxin-3-CFTR axis drives microglial polarization toward a pro-inflammatory phenotype and amplifies neuroinflammation. We thus identify a novel "Ataxin-3-K63 ubiquitin chain-CFTR" pathway that controls microglial activation, offering new mechanistic insight and therapeutic targets for SCA3. Abbreviations: MJDM: achado-Joseph disease; SCA3: spinocerebellar ataxia type 3; PolyQ: polyglutamine; CNS: central nervous system; CFTR: cystic fibrosis transmembrane conductance regulator; CF: cystic fibrosis; UIMs: ubiquitin-interacting motifs; MEM: Minimum Essential Medium; FBS: fetal bovine serum; P/S: penicillin/streptomycin; siRNA: small interfering RNA; BSA: bovine serum albumin; Co-IP: Co-immunoprecipitation; LPS: lipopolysaccharide; WT-CFTR: wild-type CFTR; CHX: Cycloheximide; 3-MA: 3-Methyladenine; IF: Immunofluorescence; IB: immunoblot; Ub: ubiquitin.