PROJECT SUMMARY Alzheimer's Disease (AD) is the 6th leading cause of death in the United States and more than 5,8 million Americans suffer from this disease, This year the estimated cost of AD and other dementias is $305 billion, and could rise to $1,1 trillion by 2050, The cause of AD remains elusive and is likely multifactoriaL The greatest risk factors are known to be age, genetics, and inheritance, Genome-wide association studies have shown that variants of TREM2 are associated with two to four times the risk of developing AD, TREM2 is an innate immune receptor expressed in microglia that has been shown to interact with phospholipids, apoptotic cells, lipoproteins, and oligomeric forms of amyloid-13 (Al3), This protein has been shown to be required for microglial proliferation, survival, and phagocytosis of Al3, TREM2 is a transmembrane protein with no intracellular signaling domains; signal transduction requires binding to DAP12, Upon microglial activation and clustering, TREM2 is cleaved from the membrane generating soluble TREM2 (sTREM2), Since both membrane-bound TREM2 and sTREM2 coexist during different stages of AD, it has been difficult to differentiate the extent of which each form independently contributes to disease pathology, It was recently shown that after intrathecal administration, sTREM2 binds to microglia, transmitting intracellular signals, promoting survival, improving cognitive function, and mitigating Al3 pathology; it has been postulated that sTREM2 acts through a putative yet unknown receptor, In this proposal we will utilize a CRISPR/Cas9 whole genome screen to identify the receptor that binds sTREM2 on microglia and promotes signal transduction, Identified candidate genes will be validated in human microglial cells and knockout mouse models will be generated to further explore the importance of each in the development of AD and its pathophysiology, These models will serve as tools that will permit us to identify how sTREM2 modulates microglia independently from membrane-bound TREM2, Such discovery will serve as the basis for novel therapeutics aimed at modulation of the sTREM2 receptor, sTREM2 has also been shown to bind Al3 plaques, We will exploit this observation and develop sTREM2-fraction crystallizable(sTREM2-Fc) fusion proteins that bind Al3 plaques and promote activation of microglia and phagocytosis, The sTREM2-Fc fusion proteins will be validated using slices of brains from 5XFAD mice that accumulate A plaques typical of AD, We will also perform intrathecal injections, which will allow us to evaluate the efficacy of sTREM2-Fc in eliminating plaques and activating microglia, Taken together, these are novel approaches that may help to develop therapies that enhance clearance and prevention of amyloid-13 plaque formation for the treatment of AD.