PROJECT SUMMARY Tauopathies are a set of clinically-significant neurodegenerative disorders characterized by accumulation of fibrillar aggregates composed of tau protein within the brain. Alzheimer's disease is the most prevalent tauopathy as it accounts for the majority cases of dementia worldwide. With the prevalence of dementia expected to at least double in the next few decades, there is a desperate need to discover novel fundamental information about the aberrant tau aggregation at the root of these diseases. The recent advent of cryoelectron microscopy allowed for an unprecedented level of structural insight into the tauopathies, revealing extensive structural polymorphism among the filaments at the root of these tauopathies. This discovery has many vital implications. First, it directly suggests that distinct pathologic cellular conditions shape the tau aggregates in disease. Second, all distinct polymorphs discovered thus far retain at least one of the two key hexapeptide nucleation motifs previously shown to be vital for aggregation, implying some degree of commonality in the aggregation process that could be exploited therapeutically. Third, the presence of unique structures in each disease directly suggests the possibility of developing structure-specific chemical probes. My preliminary data demonstrates the power of structural biology in elucidating key insights into all three of these areas, and the project proposed in this fellowship application aims to expand on these results to deliver fundamental insights into tau aggregation, polymorphism, and ligand binding. Aim 1 will characterize the structural impact of multiple pathological cellular condition mimics on the structure of tau filaments. This structural analysis specifically aims to uncover novel information about the potential etiology at the root of tauopathies. Aim 2 will derive detailed mechanisms for how two in vitro inducers initiate and shape the aggregation process. This approach will uncover novel insight into how tau aggregation is triggered and identify potential therapeutic targets for disruption of aggregation. Aim 3 will characterize the driving forces behind ligand binding to specific sites on tau filaments. Such fundamental mechanistic information will unveil specifics for high-affinity ligand design and aid in the targeted development of disease-specific imaging agents. Overall, this proposal is innovative because it will deliver novel insight into multiple key aspects of tauopathies utilizing a structure-driven approach. This research is significant as it will specifically uncover information on the etiology and pathogenesis of aberrant tau aggregation and polymorphism, thus providing novel avenues for much-needed therapeutic intervention. Additionally, this research is significant because the mechanistic information derived herein will aid in the future development of disease-specific positron emission tomography tracers, in turn assisting with many clinical ...