Tau filaments are the pathological hallmark of >20 neurodegenerative diseases including Alzheimer's disease. Six tau isoforms exist that can be grouped into 4-repeat (4R) tau and 3-repeat (3R) tau based on the presence or absence of the second of four microtubule binding repeats. Recent evidence suggests that tau filaments can transfer between cells and spread through the brain. Here we demonstrate in vitro that seeded filament growth, a prerequisite for tau spreading, is crucially dependent on the isoform composition of individual seeds. Seeds of 3R tau and 3R/4R tau recruit both types of isoforms. Seeds of 4R tau recruit 4R tau, but not 3R tau, establishing an asymmetric barrier. Conformational templating of 4R tau onto 3R tau seeds eliminates this barrier, giving rise to a new type of tau filament. These findings provide fundamental mechanistic insights into the seeding, propagation, and diversification of tau filaments.

Tau fibrils are the main proteinacious components of neurofibrillary lesions in Alzheimer disease. Although RNA molecules are sequestered into these lesions, their relationship to Tau fibrils is only poorly understood. Such understanding, however, is important, as short fibrils can transfer between neurons and nonproteinacious factors including RNA could play a defining role in modulating the latter process. Here, we used sedimentation assays combined with electron paramagnetic resonance (EPR), fluorescence, and absorbance spectroscopy to determine the effects of RNA on Tau fibril structure and growth. We observe that, in the presence of RNA, three-repeat (3R) and four-repeat (4R) Tau form fibrils with parallel, in-register arrangement of β-strands and exhibit an asymmetric seeding barrier in which 4R Tau grows onto 3R Tau seeds but not vice versa. These structural features are similar to those previously observed for heparin-induced fibrils, indicating that basic conformational properties are conserved, despite their being molecular differences of the nucleating agents. Furthermore, RNA sustains template-assisted growth and binds to the fibril surface and can be exchanged by heparin. These findings suggest that, in addition to mediating fibrillization, cofactors decorating the surface of Tau fibrils may modulate biological interactions and thereby influence the spreading of Tau pathology in the human brain.

Background: K18/K19 Tau protein isoforms can aggregate into different amyloid-β-like fibrils.Results: Different K18/K19 oligomers can be templates for fibril growth via cross-seeding between K18 and K19.Conclusion: K18 and K19 octamers create different cross-seeding barriers promoting K18 growth on K19 seeds but preventing K19 growth on K18.Significance: Conformational selection of compatible states during cross-seeding of amyloid species is general in amyloid-related diseases.

The propagation of Tau pathology in Alzheimer’s disease (AD) is thought to proceed through templated conversion of Tau protein into fibrils and cell-to-cell transfer of elongation-competent seeds. To investigate the efficiency of Tau conversion, we adapted the protein misfolding cyclic amplification assay used for the conversion of prions. Utilizing heparin as a cofactor and employing repetitive cycles of shearing and growth, synthetic Tau fibrils and Tau fibrils in AD brain extract are progressively amplified. Concurrently, self-nucleation is suppressed. The results highlight breakage-induced replication of Tau fibrils as a potential facilitator of disease spread.

Seeded conversion of tau monomers into fibrils is a central step in the progression of tau pathology in Alzheimer’s disease and other neurodegenerative disorders. Self-assembly is mediated by the microtubule binding repeats in tau of which either three or four are present, depending on the protein isoform. Here we used double electron-electron resonance spectroscopy to investigate the conformational ensemble of four-repeat tau fibrils. We observe that single point mutations at key positions in the protein (ΔK280, P301S, P312I, D314I) markedly change the distribution of fibril conformers after template-assisted growth, whereas other mutations in the protein (I308M, S320F, G323I, G326I, Q336R) do not. These findings provide unprecedented insights into the seed selection of tau disease mutants and establish conformational compatibility as an important driving force in tau fibril propagation.