Functional dissection of fibronectin type 3 domains of SORL1 in Alzheimer's disease associated microglia Alzheimer's disease (AD) is a multifactorial, complex disease and a leading cause of dementia among aged people. There are no effective pharmacotherapeutic options for prevention and treatment of AD due to lack of complete understanding of disease mechanisms. Interestingly, Genome wide association studies have established that the majority of AD associated loci are found in or near genes that are highly/uniquely expressed in microglia (resident macrophage cells of brain) suggesting the critical role of these cells in disease progression. SORL1 is genetically implicated in late and early onset forms of AD. Studies on postmortem brain tissues have shown reduced levels of SORL1 in AD patient brains. Although, among brain cells microglia show highest expression of SORL1, yet little is known how this protein regulates microglia phenotypes during health and disease. Our preliminary data suggest, that a specific isoform of SORL1 is enriched in microglia and it physically interact through its FN3 domain with WAVE regulator complex (WRC). WRC is emerging as critical regulator of cell migration and phagocytic response hence can serve as therapeutic target for tuning up phenotypes of AD related microglia. In this application, we propose to: (1) determine the molecular mechanism by which SORL1 regulate microglia functions e.g. phagocytosis, chemotaxis, and cytokine expression; (2) Measure SORL- FN3 abundance in AD and non-AD brain microglia from cohorts of deeply characterized individuals (ROSMAP) to establish the relationship among FN3 abundance vs. AD related traits; (3) Identify therapeutic targets within SORL1-WRC using our newly developed CRISPR-mediated saturating mutagenesis approach. Overall, by leveraging innovative CRISPR-Cas9 genome editing technologies (1 & 3) and novel SRM (selected reaction monitoring) based quantitative proteomics and genetics (2) this proposal will accelerate the transition of genetic discoveries to molecular mechanism that can open new avenues for novel therapeutics for AD.