Localization signals within ataxin-3 influence the formation of intranuclear aggregates in spinocerebellar ataxia type 3

Spinocerebellar Ataxia Type 3 (SCA3) or Machado-Joseph disease (MJD), is an autosomal dominantly inherited neurodegenerative disorder caused by the expansion of a CAG stretch in the MJD1 gene resulting in an expanded polyglutamine repeat in the encoded ataxin-3 protein. SCA3 therefore belongs to the group of the so called polyglutamine diseases.

Ataxin-3 is mainly localized in the cytoplasm. However, one hallmark of SCA3 is the formation of ataxin-3 containing intranuclear aggregates. We recently demonstrated in vivo using transgenic mouse models the importance of intracellular localization of expanded ataxin-3 for the pathogenic process: The nuclear localization of ataxin-3 is required for the manifestation of symptoms in SCA3 whereas cytoplasmically localized ataxin-3 is not able to induce a phenotype even with a very high number of polyglutamine repeats.

Up to now it is not known why and how ataxin-3 leaves the cytoplasm and translocates into the nucleus. The identification of nuclear localization (NLS) or nuclear export signals (NES) within ataxin-3 would make an important contribution to the answer of this question. Recently, using computerized comparison with known sequence profiles, ataxin-3 was proposed to contain both one NLS and one NES. We screened ataxin-3 in silico for additional signals and performed intracellular localization studies of putative localization signals in tissue culture. Using different assays, we could identify two NES and one NLS within ataxin-3. Mutating crucial amino acids inactivated the functionality of these signals. In addition, we could demonstrate that the inactivation of these signals influence the formation of intranuclear inclusion bodies: Inactivating the NES increased the number of aggregates whereas the inactivation of the NLS reduced their number. This data demonstrates the importance of the identified signals for the pathogenesis in SCA3, help to understand the intracellular localization of ataxin-3 and might lead to novel possibilities for therapeutically intervention.