TY - JOUR T1 - Fibril branching dominates self-assembly of mutant huntingtin exon-1 aggregates <em>in vitro</em> JF - bioRxiv DO - 10.1101/195297 SP - 195297 AU - Anne S. Wagner AU - Antonio Z. Politi AU - Anne Steinhof AU - Kenny Bravo-Rodriguez AU - Alexander Buntru AU - Nadine U. Strempel AU - Lydia Brusendorf AU - Christian Hänig AU - Annett Boeddrich AU - Stephanie Plassmann AU - Juan M. Ramirez-Anguita AU - Katharina Baum AU - Elsa Sanchez-Garcia AU - Jana Wolf AU - Erich E. Wanker Y1 - 2017/01/01 UR - http://biorxiv.org/content/early/2017/10/19/195297.abstract N2 - Huntingtin (HTT) fragments with extended polyglutamine (polyQ) 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 selfassembly of an aggregation-prone N-terminal HTT exon-1 fragment with 49 glutamines (Ex1Q49). We demonstrate 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 monomers, and (2) a fast secondary fibril-dependent nucleation process, which involves 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 primary nucleation and delays Ex1Q49 fibril assembly, comprehensive mathematical and computational modelling studies, and seeding experiments with small, preformed fibrillar Ex1Q49 aggregates. All results indicate that in vitro, HTT exon-1 fibrillar aggregates are formed by a branching mechanism. ER -