Self-assembly of Mutant Huntingtin Exon-1 Fragments into Large Complex Fibrillar Structures Involves Nucleated Branching

• Published in: Journal of molecular biology 2018-06, Vol.430 (12), p.1725-1744
• Main Authors: Wagner, Anne S., Politi, Antonio Z., Ast, Anne, Bravo-Rodriguez, Kenny, Baum, Katharina, Buntru, Alexander, Strempel, Nadine U., Brusendorf, Lydia, Hänig, Christian, Boeddrich, Annett, Plassmann, Stephanie, Klockmeier, Konrad, Ramirez-Anguita, Juan M., Sanchez-Garcia, Elsa, Wolf, Jana, Wanker, Erich E.
• Format: Article
• Language: English
• Subjects: aggregation mechanism; Amyloid - chemistry; amyloidogenesis; Cell-Free System; Exons; Humans; Huntingtin fibrillogenesis; Huntingtin Protein - chemistry; Huntingtin Protein - genetics; Microscopy, Atomic Force; Microscopy, Electron; Models, Molecular; Mutation; nucleated fibril branching; nucleation; Peptides - chemistry; Protein Aggregates; Protein Structure, Secondary
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/j.jmb.2018.03.017

Huntingtin (HTT) fragments with extended polyglutamine tracts self-assemble into amyloid-like fibrillar aggregates. Elucidating the fibril formation mechanism is critical for understanding Huntington's disease pathology and for developing novel therapeutic strategies. Here, we performed systematic experimental and theoretical studies to examine the self-assembly of an aggregation-prone N-terminal HTT exon-1 fragment with 49 glutamines (Ex1Q49). Using high-resolution imaging techniques such as electron microscopy and atomic force microscopy, we show that Ex1Q49 fragments in cell-free assays spontaneously convert into large, highly complex bundles of amyloid fibrils with multiple ends and fibril branching points. Furthermore, we present experimental evidence that two nucleation mechanisms control spontaneous Ex1Q49 fibrillogenesis: (1) a relatively slow primary fibril-independent nucleation process, which involves the spontaneous formation of aggregation-competent fibrillary structures, and (2) a fast secondary fibril-dependent nucleation process, which involves nucleated branching and promotes the rapid assembly of highly complex fibril bundles with multiple ends. The proposed aggregation mechanism is supported by studies with the small molecule O4, which perturbs early events in the aggregation cascade and delays Ex1Q49 fibril assembly, comprehensive mathematical and computational modeling studies, and seeding experiments with small, preformed fibrillar Ex1Q49 aggregates that promote the assembly of amyloid fibrils. Together, our results suggest that nucleated branching in vitro plays a critical role in the formation of complex fibrillar HTT exon-1 aggregates with multiple ends. [Display omitted] •Spontaneous fibrillogenesis of Ex1Q49 is dominated by both primary and secondary nucleation events.•Ex1Q49 molecules rapidly self-assemble into complex amyloid fibrils with multiple ends and fibril branching points.•The small molecule O4 targets Ex1Q49 molecules and extends the lag phase in the fibril formation pathway in a concentration-dependent manner.•Molecular dynamics simulations suggest that O4 decreases the β-sheet content of HTTex1 molecules with expanded polyQ tracts.•A key role of fibril branching in the Ex1Q49 aggregation cascade is supported by mathematical modeling studies and comprehensive seeding experiments.