Heterogeneous intracellular localization and expression of ataxin-3

Spinocerebellar ataxia type 3 or Machado-Joseph disease (SCA3/MJD) is an au tosomal dominant neurodegenerative disorder caused by an unstable and expan ded CAG trinucleotide repeat that leads to the expansion of a polyglutamine tract in a protein of unknown function, ataxin-3. We have generated and ch aracterized a panel of monoclonal and polyclonal antibodies raised against ataxin-3 and used them to analyze its expression and localization. In Hela cells, multiple isoforms are expressed besides the major 55-kDa form. While the majority of ataxin-3 is cytosolic, both immunocytofluorescence and sub cellular fractionation studies indicate the presence of ataxin-3, in partic ular, of some of the minor isoforms, in the nuclear and mitochodrial compar tments. We also show that ataxin-3 can be phosphorylated, in the brain, onl y one ataxin-3 isoform containing the polyglutamine stretch was detected, a nd normal and mutated proteins were found equally expressed in all patient brain regions analyzed. In most neurons, ataxin-3 had a cytoplasmic, dendri tic, and axonal localization. Some neurons presented an additional nuclear localization. Ataxin-3 is widely expressed throughout the brain, with a var iable intensity specific for subpopulations of neurons. Its expression is, however, not restricted to regions that show intranuclear inclusions and ne urodegeneration in SCA3/MJD. (C) 1998 Academic Press.