Biophysical and mechanistic impacts of small molecule and polyanionic aggregation modulators on tau4RD

Abstract

Tau is an intrinsically disordered protein (IDP) that pathologically aggregates in Alzheimer's disease and over twenty other neurodegenerative diseases known as tauopathies. Unlike structured proteins that exhibit one (or several) folded conformations, IDPs populate dynamic and interchanging conformational ensembles. This makes IDPs difficult to study structurally and challenging as drug targets. During aggregation, tau's ensemble is perturbed so that the disordered monomers assemble into ordered cross-beta-sheet amyloid structures. Recent advances in cryo-EM have revealed that many of the amyloid fibrils generated in different tauopathies display disease-specific morphologies. These morphologies are reproducible through prion-like seeding both in vivo and in vitro. While these observations imply that tau has a tunable self-assembly landscape, the structural and kinetic mechanisms that control tau's amyloid morphology remain unclear. Further, there could be great therapeutic potential in targeting species early in tau's aggregation pathway; aggregation intermediates (i.e., mid-stage oligomers) are considered toxic in neurodegenerative disease. Here, we describe the discovery and characterization of tryptanthrin and its synthetic analogs as extremely potent tau inhibitors that target the earliest stages of aggregation. We follow this with a pulsed hydrogen-deuterium exchange mass spectrometry (HDX-MS) investigation into the origins of tau's amyloid heterogeneity and find evidence that distinct amyloid morphologies are encoded at the start of aggregation. Together, these findings address critical open questions regarding drug development for IDPs and the timeline of amyloid structural divergence.