Unravelling the function of ataxin-3 as a deubiquitinating enzyme: is this function altered in Machado-Joseph disease?

Machado-Joseph disease (MJD) is a genetic form of neurodegeneration that is typically diagnosed around the fourth decade of life and results in progressive loss of movement, balance and coordination. Despite the single cause of MJD being established almost three decades ago, it remains a fatal disease with no effective treatments. In MJD, mutation of the ATXN3 gene results in polyglutamine expansion of the ataxin-3 protein, enhancing its propensity to misfold, aggregate, and form the neuronal intranuclear inclusions which are characteristic of MJD. Ataxin-3 is known to function as a deubiquitinating enzyme within the ubiquitin-proteasome system, and an increasing body of evidence suggests that its deubiquitinating function is altered by polyglutamine expansion in MJD. Whether polyglutamine expansion results in a gain or loss of ataxin-3's deubiquitinating function, as well as the downstream consequences of this on targets of ataxin-3, is yet to be completely unravelled.

This thesis identified that wild-type and polyglutamine expanded human ataxin-3 alter the abundance of K63- and K48-ubiquitinated proteins. A list of proteins with differential co-interaction with K48-linked ubiquitin due to the expression of polyglutamine-expanded human ataxin-3 was subsequently generated by LFQ proteomics, and three proteins of interest – matrin-3 (matr3), polyadenylate-binding protein 1 (PABPC1), and ELAV-like protein 1 (ELAVL1) – were subsequently explored. This thesis identified that matr3 is K48-ubiquitinated, has an altered abundance in ATXN3-expressing N2A cells and in a MJD mouse model, and co-localises with ataxin-3. However, polyglutamine expansion of ataxin-3 does not impact the subcellular localisation of matr3, the abundance of matr3 co-interacting with K48-linked ubiquitin, or the degree of co-localisation between matr3 and ataxin-3. The abundance of PABPC1 and ELAVL1, as well as the abundance of PABPC1 co-interacting with K48-linked ubiquitin chains were also found to be unaltered by polyglutamine expansion in human ataxin-3. Overall, this thesis contributes to our understanding of how polyglutamine expansion in ataxin-3 impacts its function as a deubiquitinating enzyme, and the consequences of this on the abundance, subcellular localisation, and ubiquitination of downstream proteins, with the hope of identifying novel treatment avenues for MJD.