PROJECT SUMMARY During homeostasis and proper acute immune responses, interleukin-18 (IL-18) acts as a facilitator of higher cerebral functions and as an alarmin molecule to signal incoming danger to cells. Our studies led us to discover endogenous counter-regulatory components in the IL-18 pathway that are impaired in Alzheimer's disease (AD), which results in excessive and prolonged signaling. More importantly, we found compelling evidence that this process plays a major role in the dysregulation of tau and accumulation of its pathological species. Investigating the underlying mechanisms by which the IL-18 pathway is affected by and drives tau pathology in AD could, therefore, yield novel means to mitigate this insidious disease. Here we propose three aims to investigate this important issue. In aim 1, we hypothesize that β-amyloid (Aβ) disrupts the endocytic protein Tollip (toll interacting protein), which is the leading regulator driving the homologous desensitization of the IL-18 receptor (IL-18R). We will genetically up- or downregulate Tollip in neuronal cell cultures and in an AD mouse model. The outcome of this aim may show that the buildup of pathological forms of tau induced by chronic IL-18 signaling in neurons is, at least in part, caused by the upregulation of IL-18R due to the impairment of its trafficking and degradation mediated by Tollip. For the second aim, we will test the hypothesis that the deficiency in the production of IL- 18's endogenous decoy receptor, IL-18 binding protein (IL-18BP), also plays a role in the chronic activation of this pathway in AD. We will use an adeno associated virus to upregulate IL-18BP levels in AD mice and determine its therapeutic efficacy on AD-like pathology. Finally, our last aim utilizes biochemical and proteomic methods to map the neuronal intracellular networks affected in response to IL-18. We will apply pharmacological and genetic tools to link potential candidates altered by IL-18, such as protein kinases and phosphatases, to the hyperphosphorylation of tau and subsequent synaptic loss and cognitive decline. Establishing these intracellular cascades could allow us to identify novel strategies to inhibit the pathological effects of IL-18, while preserving its relevant physiological functions. Overall, our studies are significant for two reasons: first, they will provide greater insights into the IL-18 signaling cascade and second, they will uncover the critical steps specifically involved in the IL-18 pathway in AD, and may result in the identification of new therapeutic candidates to potentially translate our discoveries into the clinic.