Neurodegeneration in Huntington's disease (HD) is accompanied by the aggregation of fragments of the mutant huntingtin protein, a biomarker of disease progression. A particular pathogenic role has been attributed to the aggregation-prone huntingtin exon 1 (HttEx1) fragment, whose polyglutamine (polyQ) segment is expanded. Unlike amyloid fibrils from Parkinson's and Alzheimer's diseases, the atomic-level structure of HttEx1 fibrils has remained unknown, limiting diagnostic and treatment efforts. We present and analyze the structure of fibrils formed by polyQ peptides and polyQ-expanded HttEx1. Atomic-resolution perspectives are enabled by an integrative analysis and unrestrained all-atom molecular dynamics (MD) simulations incorporating experimental data from electron microscopy (EM), solid-state NMR, and other techniques. Visualizing the HttEx1 subdomains in atomic detail helps explaining the biological properties of these protein aggregates, as well as paves the way for targeting them for detection and degradation.

Huntington's disease (HD) is a neurodegenerative disorder caused by the expansion of the polyglutamine (polyQ) segment in the exon 1 of the huntingtin (HttEx1) protein. This polyQ expansion leads to protein misfolding and the formation of β-sheet-rich fibrillar aggregates. Several studies have shown that these protein deposits can cause cytotoxicity, suggesting the development of small molecule aggregation inhibitors as potential modulators of HD pathogenesis. This requires a molecular understanding of the impacts of such modulators on the interplay of aggregation, polymorphism and toxic gain-of-function. Here, we study how a polyphenol modulates the HttEx1 aggregation mechanism at sub-stoichiometric ratios. Moreover, we examine how the disrupted aggregation process impacts the protein's misfolded structure and neurotoxic properties. We combine measurements of aggregation kinetics, electron microscopy, solid-state NMR, cytometry, and cytotoxicity assays. A notable delay of protein aggregation was observed even at sub-stoichiometric ratios of curcumin relative to the HttEx1 protein. Mechanistically, extension of the lag phase indicates an impact on the primary nucleation process that underpins the complex HttEx1 aggregation pathway, with an apparent role for β-hairpin formation. Remarkably, the deposits formed (more slowly) in presence of inhibitor displayed reduced toxicity in cultured neuronal cells, seemingly derived from their modulated structures. Thus, curcumin has a multifaceted effect based on delaying the fibril formation, while also changing the toxic properties of formed fibrils. Our findings highlight the ability of small molecule inhibitors to modulate the protein misfolding landscape, with potential implications for treatment strategies in HD and other protein deposition disorders.