Mechanistic Investigation into the Regulation of Amyloid Motifs in Tau Aggregation and Disease

Amyloid formation of tau protein is a unifying theme in a multitude of neurodegenerative diseases, collectively called tauopathies. Missense mutations in the tau gene (MAPT) correlate with aggregation propensity and cause dominantly inherited tauopathies, but the molecular mechanism of how they promote tau assembly into amyloids is poorly understood. Many disease-associated mutations localize within tau's repeat domain proximal to amyloidogenic sequences, such as 306VQIVYK311. We use computational modeling, recombinant protein and synthetic peptide systems, cross-linking mass spectrometry, and cell models to investigate the biophysical mechanisms behind aggregation of the P301L/S and S320F mutants. We conclude that the aggregation prone 306VQIVYK311 motif forms metastable compact structures with its upstream sequence that modulates aggregation propensity. We report that disease-associated mutations such as P301L/S at the inter-repeat interface, isomerization of a critical proline, or alternative splicing are all sufficient to destabilize this local structure and trigger aggregation. In the study of S320F, we again demonstrate the role of local protective structures in the regulation of tau aggregation driven by amyloid motif and uncover that the S320F mutation allosterically exposes 306VQIVYK311 by retaining one of the protecting motifs in a stabilized local hydrophobic cluster. We show that rational design of this nonpolar cluster centered on position 320 based on tauopathy fibril structures maintains a spontaneous aggregation phenotype revealing new principles that govern tau aggregation. We uncover a nuanced balance of local protective structures that sequester amyloid motifs and how introduction of a hydrophobic mutation redistributes these interactions to drive spontaneous aggregation. Tau presents as an aggregation-resistant monomer and only in the presence of an inducer such as heparin, RNA or other polyanion will tau aggregate in vitro. Our studies use disease-causing mutations as a bridge to explain the basis of early conformational changes that may underlie genetic and sporadic tau pathogenesis. Our findings provide molecular insights into regulation of tau assembly, and we anticipate deeper knowledge of this process will begin control of tau aggregation into discrete structural polymorphs.