PROJECT SUMMARY Innate immune antiviral pathways are upregulated in Alzheimer’s disease (AD) and are thought to drive AD pathogenesis. The specific mediators of maladaptive antiviral responses in AD are still not understood. Innate immune system utilizes a series of nucleic acid sensors to detect viral genetic material to activate interferon expression. The central DNA sensor in the cell is cyclic GMP-AMP synthase (cGAS). cGAS signaling has emerged as an important player in many neurodegenerative diseases like Parkinson’s disease and amyotrophic lateral sclerosis. The role of cGAS signaling in tau-mediated neurodegeneration remains unstudied. We discover that cGAS signaling is hyperactivated in the brains of mice expressing mutant tau (P301S mice) and human AD patients. Using behavioral assays, electrophysiological readings and single nuclei RNA sequencing, our preliminary results indicate that cGAS activity modifies both the cellular responses to and the functional deficits caused by tauopathy. We observed that partial or complete loss of Cgas rescued tauopathy-associated spatial learning and memory deficits. cGAS is highly enriched in immune cells including microglia. We find that tau induces a cGAS-dependent interferon signature by triggering mitochondrial DNA stress in microglia and show that Cgas deletion alters microglial disease transformation, characterized by reduced expression of interferon genes in disease microglia. It is not known how tau can alter mitochondrial and nuclear dynamics to activate cGAS and how cGAS activation enhances neurotoxic effects of tau. This proposal will investigate the mechanisms and consequences of cGAS and nucleic acid sensing pathways in tauopathy. I aim to dissect the molecular mechanism of cGAS activation in response to tau by focusing on tau dependent cellular processes that induce mitochondrial stress and microglial senescence (Aim 1) and investigate how cGAS mediates maladaptive microglial responses and neuronal damage in tauopathy (Aim 2). Finally, since antiviral responses could be species-specific, there is strong rationale to extend mouse studies of innate nucleic acid sensing to human models. Human and mouse cGAS share only 60% amino acid similarity and have different activation requirements. I will establish human pluripotent stem cell (hPSC)-based platforms to study broader nucleic acid sensing pathways and perform unbiased CRISPR-screens to identify novel regulators of human nucleic acid sensing in tau toxicity (Aim 3). The experiments outlined in this proposal will lead to a better understanding of the role of nucleic acid sensors in tauopathy and AD which might prove translatable to humans.