Drivers of Pathological Tau Aggregation

PROJECT SUMMARY The bottleneck for tauopathy therapy development is the lack of validated tauopathy models, mouse, cell or in vitro. This is reflected in the current reality that tauopathy-specific fibril structures solved by cryo-EM from post mortem patient brain tissue have never been replicated outside a patient, i.e. not in a mouse, cell or in vitro. While the patient- derived tauopathy fibrils offer critical goal posts, they are not in and of themselves viable therapeutic targets. For example, the development of Positron Emission Tomography (PET) ligands to diagnose and track Alzheimer’s disease (AD) or corticobasal degeneration (CBD) disease progression relies on screening small molecule binding to CBD- or AD-phenotypic fibrils—the very construct that nobody knows how to build yet. There are many more factors to consider for replicating the pathological pathway of tau aggregation, but replicating disease phenotypic tau fibrils is a minimal and necessary requirement, and so far an unattained tool for therapy development. The major knowledge gap that this proposal aims to close is the mechanism and tools to replicate tauopathy specific fibrils in vitro (Aim 1), and the key cellular and molecular factors that initiate misfolding of tau in cell to disease phenotypic shapes and facilitate aggregation (Aim 2). If we can successfully replicate any one tauopathy-phenotypic tau fold, or even a part of a folded tau structure, such as a mini-hairpin fold of CBD or AD with seeding competency, it will have an immediate impact on ongoing therapy developments, such as on the development of tauopathy-specific PET ligands, antibodies and small molecule drugs. This team will employ an innovative set of structural biology tools encompassing pulsed double electron-electron resonance (DEER), TEM and cryo-EM, as well as computational tools to focus on capturing the full folding and aggregation pathway of the tau protein ensemble from its intrinsically disordered to partially folded and fully converged fibril states. This team will concurrently use innovative cell biological tools with a strong premise of the knowledge of a dedicated tau receptor and transporter that can enhance tau seeding by endosomal escape and the knowledge that enhanced hydrophobicity of the local environment of tau is a potent factor to initiate misfolding, aggregation and propagation. While discovering the defining property of a competent seed and achieving shape propagation with seeds developed in this proposal will be a breakthrough, independent of this success, we will have developed experimental and computational tools to evaluate whether seeded shape propagation has occurred, or whether all, part, or none of the shape propagates. To have the tools to evaluate the mechanism of shape propagation will be a game changer. The lack of progress in closing the above-described knowledge gap is not due to a lack of investment by top notch laboratories around the world, but due to shortcomings of existing concepts and tools. Han and Kosik, together with Shea, will rely on the convergence of their respective fields and investing concerted effort using innovative tools to address long-standing questions in tauopathy research.