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 mechanisms by which tauopathies occur remains poorly understood. We propose that MAPT mutations drive tau aggregation and neuronal dysfunction through altered proteostasis. In preliminary studies, we have shown that induced pluripotent stem cell derived-neurons expressing MAPT R406W 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 R406W 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 manipulate tau proteoforms using genetic or molecular methods to define the mechanisms by which tau proteoforms disrupt proteostasis in tauopathies. We hypothesize that tau proteoforms are sufficient to destabilize proteostasis 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 cause impaired tau phenotypes and proteostasis characteristic of tauopathy. We will also generate a systematic genetic interaction map to elucidate connections between MAPT mutations, proteostasis, 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.