PROJECT SUMMARY: Neurodegenerative diseases pose significant challenges to public health and our understanding of brain function. Central to these conditions is the pathological accumulation of protein aggregates, notably alpha-synuclein, beta- amyloid, and Tau. Specifically, Tau aggregates are expelled into the extracellular space as neurons degenerate, where they act as damage-associated molecular patterns (DAMPs) that are sensed by microglia. Microglial activation leads to inflammatory responses, which contribute to impaired neuronal function and cognitive decline in diseases like Alzheimer’s Disease (AD). Although various pattern recognition receptors (PRRs) have been reported to sense Tau, emerging evidence suggests that cyclic cGAMP synthase (cGAS), a DNA PRR, also plays a crucial role in promoting inflammatory responses. Recent reports suggest that Tau activates the cGAS pathway, and this would require an initial interaction with an adaptor, Polyglutamate Binding Protein 1 (PQBP1). In three separate studies, our group has shown a similar requirement for cGAS activation by HIV-1 infection. PQBP1 decorates HIV-1 capsids and recruits cGAS to the site of HIV-1 DNA synthesis. PQBP1 recruitment to the capsid is mediated by charge complementation, and its dimerization is required for cGAS activation. We hypothesize that PQBP1 binding to Tau is necessary for cGAS activation, which contributes to chronic neuroinflammation in AD and other tauopathies. Building on our experience and reagents from our HIV-1 studies on PQBP1 and cGAS, we propose to fill major gaps in knowledge regarding the structural requirements of PQBP1-Tau complex formation and its contribution to cGAS activation. In Aim 1, we will determine the regions of PQBP1 that are required for the recognition of Tau monomers and fibrils. Further, we will elucidate whether PQBP1 interacts with exogenous PQBP1 as well as PQBP1 expressed in microglia. In Aim 2, we will demonstrate that the cGAS pathway is active i