PROJECT SUMMARY Alzheimer’s disease (AD) is a chronic neurodegenerative disease characterized by extracellular β-amyloid (Aβ), microtubule-associated protein tau intracellular neurofibrillary tangles (NFTs), cellular loss, and cognitive decline. NFTs correlate significantly to the onset of clinical symptoms, yet current disease-modifying therapies do not target tau aggregation mechanisms. Pathogenic misfolded tau propagates across neuronal cells in AD and causes a ‘gain-of-function’ misfolding of normal tau proteins in a templated manner. Evidence from our group and others has identified specific RNA-binding proteins (RBPs) that co-aggregate with pathological tau in AD brains. A majority of these RBPs are insoluble, mislocalized to the cytoplasm, and display loss of splicing function in AD. Importantly, a specific class of RBPs that correlate with increased AD pathology and cognitive decline in AD contain disordered low complexity (LC) structural domains that are rich with serine and arginine residues (SR-rich). It is currently unknown if SR-rich RBPs alter tau aggregation in disease. I propose that this class of RBPs has distinct interactions with tau in AD through their SR-rich LC domains. My central hypothesis is that RBPs co-aggregating with tau in AD will seed and enhance tau aggregation via the SR-rich LC domain. I will use a combination of biochemical, cellular, and in vitro assays to examine how SR-rich LC domains alter tau aggregation. The long-term goal of this project is to identify points of therapeutic intervention by modulating tau aggregation in AD and enhancing our mechanistic understanding of the aggregation process. Aim 1 will determine if RBPs mediate tau aggregation via SR-rich LC domains through co-localization with tau fluorescent reporter cells and thioflavin-T fibrillization experiments. In Aim 2, I will identify co-aggregating RBPs through a novel tau proximity labeling system in mammalian cells. This proteomic screen provides the ability to resolve additional co-aggregating RBPs through mass spectrometry techniques and subsequent bioinformatic analyses. These two parallel aims will allow me to determine mechanisms that underlie tau seeding and aggregation in disease. By understanding how the SR-rich LC domain of RBPs implicated in AD promote tau aggregation, I will contribute to the understanding of AD pathological proteins and provide impactful insight toward therapeutic targets to reduce tau burden. The training provided through successful completion of the proposal aims will significantly prepare me for a career as an independent researcher in neurodegeneration.