Developing and Validating a Novel Tau Toxicity Model in the Budding Yeast

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PROJECT SUMMARY Many forms of human dementia are caused by aberrantly folded proteins and protein aggregates. Overexpressing these disease proteins, such as α-synuclein and TBD-43, in the budding yeast Saccharomyces cerevisiae, also causes cytotoxicity, allowing unbiased powerful genetic screens for modifiers that enhance or repress toxicity. Hence, this single-celled eukaryotic model has been a powerful genetic tool for discovering molecular pathways regulating human dementia caused by protein misfolding and aggregation, such as the Parkinson’s disease and frontotemporal dementia. Neurofibrillary tangles (NFTs) composed of hyperphosphorylated and aggregated Tau proteins are a major pathological hallmark of Alzheimer’s disease (AD), as well as other neurodegenerative disorders collectively termed Tauopathies. Despite the strong clinical association, how wildtype human Tau proteins become hyperphosphorylated and cytotoxic remains poorly understood. Distinguished from other protein aggregates that cause dementia, overexpressing human Tau is well tolerated in the budding yeast with no apparent growth phenotype, even though human tau proteins are also hyperphosphorylated and aggregated in yeast cells. Hence, no yeast genetic screen for Tau toxicity modifiers has been performed thus far. Yet, tremendous potential remains if Tau-induced cytotoxicity can be reproduced in the simple yeast model, permitting unbiased screens to uncover the molecular pathways involved in tau toxicity and related diseases. In a preliminary screen, we have identified several ORF deletion mutations that show toxicity when human wildtype Tau (2N4R) is expressed. Interestingly, two of these mutant genes, THP1 and SAC3, encoding for subunits of the evolutionarily conserved TREX2 complex, are involved in activating stress response gene expression and mRNA export. Hence, we hypothesize that timely stress response at the transcription level is necessary for yeast cells to antagonize the toxicity induced by misfolded Tau. It is also possible that Tau proteins in thp1∆ and sac3∆ mutants adopt a cytotoxic conformation/state reminiscent of the situation of Tau pathology in neurons. Indeed, our analysis of the NIAGADS database uncovered that AD-associated genetic variants were found in four of the five subunits of the human TREX2 complex, making it one of the top 15 protein complexes enriched for AD variants. For this small pilot project, we will 1) investigate the cause of Tau toxicity in strains mutated for TREX2 subunits using a combination of genetics, genomics and biochemical approaches; 2) develop a set of tools that can help establish a novel Tau toxicity model in the budding yeast, allowing future unbiased screens for genetic modifiers of Tau toxicity.