Yeast as a gateway to conquering protein misfolding diseases.

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Abstract. Certain proteins misfold to form self-seeding prion-like aggregates associated with disease. We focus on one such protein, TDP-43, because it is the major protein associated neuronal aggregates in several neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), frontotemporal dementia and LATE. LATE is a recently described prevalent TDP-43 proteinopathy that causes dementia that is often misdiagnosed as Alzheimer’s disease (AD). In addition, TDP-43 is found in aggregates associated with AD and Parkinson’s. Since TDP-43 forms aggregates and is toxic (inhibits growth) in yeast, a powerful approach to find therapeutic targets has been to identify yeast genes that modify TDP-43 toxicity. The relevance of the yeast model to human disease is clear because several yeast genes that modify toxicity of human misfolding disease proteins, including TDP-43, are homologs of new or known human disease risk factors. We will continue to study the genesis and toxicity of TDP-43 aggregates in yeast building on our expertise with yeast self-seeding prion proteins. We expect to learn how TDP-43 causes toxicity in yeast and in what ways this relates to TDP-43 toxicity in flies, primary cortical neurons and mice. One of our goals is to investigate the range of condensates, oligomers and aggregates formed by TDP-43 and their associated toxicities. Determining which species of TDP-43 is most toxic is an important step towards understanding of toxicity mechanisms. It is also largely unknown what cellular functions are targeted by toxic TDP-43 species and the affiliated mechanisms. We will identify and study cellular targets of toxicity focusing on TDP-43 gain of function toxicity. We will also explore new models of therapeutic approaches by investigating if overexpression of TDP-43 binding proteins can inhibit the formation of toxic TDP- 43 species, if titration of important proteins by TDP-43 toxic species contributes to toxicity, and if mutations in TDP-43 can protect WT TDP-43 expressed in the same cell from forming toxic aggregates. Another gap we seek to address is why TDP-43 is associated with different diseases. Importantly, as we showed for yeast prions, TDP-43 and other disease proteins can form distinct aggregate variants (strains), unrelated to mutation, that are associated with distinct characteristics. Thus, different variants of TDP-43 could affect neuronal types differently causing e.g. ALS vs. LATE. TDP-43 variants established in yeast would be important tools to identify disease specific variants and facilitate development of variant specific treatments. We will also investigate the idea that entry into liquid-like granules is an upstream trigger for toxic species formation to learn if liquid-like granules are therapeutic targets. We will quantify the relationship between entry of prion proteins into liquid condensates and stochastic formation of prions in yeast. We will also explore the new area of disease associated metabolite amyloid-like aggregates and the hypothesis that they nucleate prion-like/disease protein misfolding much as we showed cross-seeding between yeast prions. We hope this work will lead to new treatment approaches for protein misfolding diseases.