Exploring proteolytic strategies to improve huntingtin clearance in cell models for Huntington's disease

Huntington’s disease (HD) is a neurodegenerative disorder, caused by an expanded polyglutamine (polyQ) tract close to the N-terminus of the mutant huntingtin protein (mHTT). The primary goal of this thesis was to explore proteolytic strategies to improve mHTT clearance, as a therapeutic strategy to delay the onset of the disease. To study how proteasome complexes are altered in HD we used murine models of HD. In addition, cellular models for HD were used to modify (proteasomal) degradation to determine the effect on mHTT turnover. This thesis contributes to a better understanding of how changes in the proteostasis machinery affect the degradation of polyQ fragments and ultimately mHTT levels. While we found that PA28αβ and IDE upregulation improves the degradation of polyQ fragments, the opposite effect (decreased turnover) was observed on polyQ containing mHTT fragments. This suggests that not the polyQ stretch itself, but rather the (misfolded) protein context hinders the degradation of mHTT. Moreover, as observed with PA28αβ and IDE, an increase in mHTT aggregation was detected, despite no direct effect on the turnover of soluble mHTT levels was seen. This indicates that protein aggregation is not only driven by the total levels of an aggregation-prone protein, but is also (directly or indirectly) influenced by other factors, such as chaperone function. Finally, the regulation of the different pathways is tightly connected, and a defect in one system, leads to the upregulation of a compensatory pathway. Which was observed after RPN10 silencing.