ABSTRACT Tauopathies may occur by familial mechanisms in which mutations in the MAPT gene are dominantly inherited causing frontotemporal lobar degeneration (FTLD-tau) or by sporadic mechanisms in which MAPT haplotypes are associated with increased disease risk (e.g. progressive supranuclear palsy and corticobasal degeneration). MAPT mutations and risk haplotypes have been proposed to drive disease pathogenesis through proteoforms that contain 3-microtubue binding domain repeats (3R tau), 4R tau, or both. However, the contribution of specific tau forms (proteoforms) to tau toxicity and the mechanisms by which tauopathies occur remains poorly understood. We hypothesize that MAPT mutations drive tau aggregation and neuronal dysfunction by altering tau metabolism and overall proteostasis at one of several potential nodes of regulation. In preliminary studies, we have shown that induced pluripotent stem cell derived-neurons expressing MAPT mutations exhibit changes in tau turnover compared to isogenic, control neurons, and we observed differences in the turnover of specific tau proteoforms in mutant neurons. Neurons expressing MAPT mutations exhibit enlarged lysosomal structures and secondary elevation of lysosomal enzymes, markers of lysosomes that are unable to properly degrade their contents. Correction of the mutant allele was sufficient to restore these lysosomal defects. This suggests that altered tau kinetics may be due to defects in the endolysosomal pathway. Thus, a unifying feature by which MAPT mutations drive tauopathy is through disrupted proteostasis. The objective of this study is to extend our preliminary findings to define the nodes and mechanisms by which tau proteoforms disrupt proteostasis in tauopathies. We hypothesize that specific tau proteoforms are sufficient to destabilize proteostasis, alter tau half-life and secretion, and to result in the accumulation of tau in vulnerable brain regions. To test this hypothesis, we will determine the extent to which MAPT mutations and genetic modifiers disrupt tau kinetics and how impaired proteostasis impacts tau secretion. We will also generate a systematic genetic interaction map to elucidate connections between MAPT mutations, alterations in the tau metabolism pathway, and associated therapeutic targets. Together, this study will reveal novel mechanisms underlying tauopathy that are driven by specific tau proteoforms and whether therapeutics designed to block specific tau proteoforms impact pathologic events. Project 2 will work synergistically with the Administration Core (Core A), Macromolecular Structure (Core B), Genomics and Transcriptomics Cores (Core C) and Project 1 to address the overall hypothesis that proper tau metabolism requires the precise, coordinated action of molecular chaperones, co-chaperones and lysosomal proteases. Tau Metabolism and Variant database (TMVdb) and Tau Polygenic Risk Score (TPRS) generated from this project will be an invaluable resource for the broader scientific community.