A Multiple Hit Model for the Diverse Pathogenic Effects Specific to Huntingtin Aggregate Stage in Cells

Huntington's disease is a fatal neurodegenerative disorder characterized by the aggregation of polyglutamine-expanded huntingtin into oligomers and fibrils. How protein aggregation leads to cellular dysfunction is not well understood. To address this question, we combined in-cell single molecule fluorescence spectroscopy with quantitative proteomics to define how huntingtin engages in aberrant protein interactions during its progressive aggregation. We find that huntingtin interacts preferentially with key components of distinct cellular processes as aggregation proceeds from soluble oligomers to end-stage inclusions. The aberrant interactions of soluble oligomers are highly enriched on RNA-binding proteins and with proteins functioning in ribosome biogenesis, translation, transcription, and vesicle transport. A significant characteristic of these interactors is the presence of extended low-complexity sequence regions. Compared to the soluble aggregates, the interactome of insoluble inclusions is significantly less complex and is enriched in protein quality control components. Our results suggest a 'multiple hit' model for polyglutamine interactions in pathogenesis, with detrimental effects on cell function occurring in an aggregation stage-dependent manner.