Aggregation Mechanisms of PolyQ-containing Proteins: Structure and Cytotoxicity of the Metastable Intermediate Species

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Insoluble fibrillar aggregates are the major histophological feature of polyglutamine (polyQ)-repeat diseases however their role in the disease pathology is still controversial. Following three hypotheses could explain the reported in the literature poor correlation between toxicity and aggregate formation in vivo: 1) a sequential assembly pathway with multiple metastable species with different toxicity, 2) multiple competing aggregation pathways with various transient species with different stability and toxicity, and 3) various intra- (e.g., polyQ-flanking sequences) and intermolecular factors (e.g., aging, stress response) could alter the aggregation pathway(s). To test these hypotheses, we propose new in vivo experiments allowing to directly map the structure (e.g., sequences involved in the aggregate core, flexible regions) and the character (on- or off- pathway) of the aggregate species emerging in the aggregation pathway of the exon 1 encoded fragment of huntingtin (Htt) with expanded polyglutamine stretch. In addition, using a broad spectrum of in vivo and ex vivo approaches we aim to determine the structure of the metastable aggregates and the molecular interactions governing aggregate morphology, their propagation in the cellular context and their toxic properties. The link between the structure and cytotoxicity will be the mechanistic foundation for our further major goal: to develop strategies to disfavor the formation of the toxic species. Thereby, the main focus will be to explore a naturally evolved mechanism, the osmocompensatory response, and how different types of osmolytes can re-shape the aggregation pathway of polyQ proteins and promote the formation of non-amyloidogenic, non-toxic off-pathway species.