Mechanistic Insight into Tau Fibril Cross-Seeding Barriers: Structural Order and Disorder

Filamentous deposition of microtubule-associated protein tau is a hallmark for a number of neurodegenerative diseases collectively termed tauopathies. While tau fibrils are directly linked to the etiology and pathogenesis of these diseases, fibril morphology and their phenotypic presentation can be quite disparate. Alternative splicing of tau results in two main isoform groups, four-repeat (4R) tau and three-repeat (3R), identified by their number of microtubule binding repeats. Some diseases show deposition of both 4R and 3R isoforms, while others show preferential deposition of only one type. The conformational templated growth scheme and trans-synaptic spreading of fibrils is influenced by fibril conformation. Therefore, conformationally distinct fibrils possessing different growth properties is one mechanistic explanation for disparate pathology; yet, molecular level insight on the core structure of these fibrils is lacking. Elucidating how distinct conformers arise, and how their structural features influence growth properties is critical to understanding fibril propagation. The growth properties and the structural impact that physiologically relevant events including truncation, mutations, and oxidation have on 4R fibrils were investigated. The ability of full-length 4R fibrils to recruit both 4R and 3R tau is influenced by residues on its C-terminal end, as a 3R growth barrier was observed for fibrils formed from C-terminally truncated variants. A 3R growth barrier was also observed for fibrils with specific disease associated mutations. Structural investigations highlighted a region of structural disorder in these fibrils that exhibit 3R growth barriers, in contrast to full-length non-mutated fibrils. Fibrils formed from an oxidized version of 4R tau, with native cysteines intramolecularly disulfide bonded, are structurally distinct and exhibit different recruitment properties from their reduced counterparts. These findings provide mechanistic insight on how truncations, mutations, and oxidation can dictate fibril conformation and thus influence propagation. Techniques for early diagnostic and therapeutic intervention of tauopathies are limited by their sensitivity of detection. Suppressing tau monomer self-nucleation is one factor critical to amplification assays used to detect low fibril concentrations. Increased suppression of self-nucleation was achieved using mutant tau and modifying assay parameters. These improvements bring this assay one step closer to detecting ultralow fibril concentrations