Line Vildbrad Kristensen:
Studies on molecular mechanisms underlying spinocerebellar ataxia type 3

Date: 07-04-2016    Supervisor: Rasmus Hartmann-Petersen

The polyglutamine (polyQ) disorders comprise nine diseases characterized by an expanded polyQ tract within the respective proteins. These disorders are rare but include the well-known Huntington’s disease, and several spinocerebellar ataxias (SCAs). The diseases usually strike midlife and progress over 10-30 years being ultimately fatal. The expanded polyQ tract confers toxicity to the disease protein leading to aggregation, neuronal dysfunction, and loss in various brain areas. Besides the polyQ tract, the polyQ proteins are unrelated, resulting in distinct clinical features and neuropathology. Even though a range of mechanisms contributing to polyQ diseases have been uncovered, there is still no treatment available. One of the more common polyQ diseases is SCA3, which is caused by a polyQ expansion in the ataxin-3 protein that normally functions as a deubiquitinating enzyme involved in protein quality control. In SCA3 patients polyQ expanded ataxin-3 forms intranuclear inclusions in various brain areas, but why the polyQ expansion of ataxin-3 leads to neuronal dysfunction is still not well understood. This thesis describes molecular biological investigations of ataxin-3 biology, aimed at furthering our understanding of SCA3 disease mechanisms.

In manuscript I, we investigated if post-translational modifications of ataxin-3 were changed by the polyQ expansion. The ubiquitin chain topology and ubiquitination pattern of ataxin-3 were unaltered by the polyQ expansion. In contrast, phosphorylation was enhanced at several known and novel positions in expanded ataxin-3.

In manuscript II, novel interaction partners of ataxin-3 were identified by proteomics. Several proteins related to the ubiquitin-proteasome system were identified along with a large number of mitochondrial proteins. These proteins represent new candidates for ataxin-3 interactors, and in particular indicate that ataxin-3 could influence mitochondrial biology.

In manuscript III, the effect of polyQ expanded ataxin-3 was investigated in a SCA3 mouse model. This study showed that ataxin-3 led to alterations in mitochondrial markers, proteasome activity, and autophagy-related proteins in the synapse. This further indicates a role for ataxin-3 in mitochondrial biology and indicates that synapses are highly vulnerable to ataxin-3 expression.

Collectively the studies suggest that the polyQ expansion in ataxin-3 may alter some interactions and post-translational modifications. Moreover, ataxin-3 may play a larger role in mitochondrial biology than previously recognized. Further, more focus should be given to the implications of ataxin-3 function in the synapse, as this location appeared particularly vulnerable to expression of polyQ expanded ataxin-3.