PROJECT SUMMARY Alzheimer’s disease (AD) is a progressive neurodegenerative disorder associated with aging and is the most common cause of dementia. In recent years, genetic studies have revealed a number of risk loci in or near genes expressed in microglia, suggesting a key role for microglia in AD pathogenesis. Inositol polyphosphate-5- phosphatase D (INPP5D) is one of the microglia-specific targets converged on by these genetic studies. INPP5D is primarily expressed by cells of the hematopoietic lineage, including microglia, and encodes a 5’ phosphatase protein known as SHIP1. INPP5D/SHIP1 is a negative regulator of immune cell surface receptors, including TREM2/TYROBP, by catalyzing the conversion of PI(3,4,5)P3 to PI(3,4)P2 to prevent receptor activation and downstream signaling, counteracting the PI3K/mTOR pathway. SHIP1 limits microglia functions associated with survival, proliferation and phagocytosis. INPP5D could be an important target for microglia-focused AD therapies, although it is still unknown how decreasing or increasing INPP5D expression affects pathology or behavioral outcomes in vivo. Inhibiting INPP5D expression promotes microglial proliferation, and phagocytosis, each of which is considered to be an important potential cellular phenotype for preventing or reducing AD pathology and improving disease outcomes. The aim of this proposal is to increase our understanding of the function of INPP5D/SHIP1 in the microglial response in diseased brain. We will use Inpp5dflox mice crossed with an inducible CX3CR1-CreER, in which tamoxifen-induced Cre expression results in persistent recombination in long-lived microglia, but not short-lived myeloid cells. For each aim, we will induce microglia-specific knockdown in models representing the pathological hallmarks of AD, Aβ deposition (APPKM670/671NL/PSEN1Δexon9) in Aim 1 and tauopathy (MAPTP301S) in Aim 2. We hypothesize that Inpp5d/SHIP1 deficiency outcomes are driven by TYROBP overexpression/upregulation, i.e. ameliorated pathogenic forms of amyloid pathology and cognitive deficits in amyloid mouse model but exacerbated tau pathology and cognitive deficits in tauopathy model. Importantly, pharmacological targeting both to activate and inhibit SHIP1 are being explored. It is necessary to understand the role of Inpp5d/SHIP1 in diseased brain before further drug testing. It is our hope that with the understanding gained in this proposal, we will be able to appropriately target Inpp5d/SHIP1 for disease and begin the next steps to test these compounds for their efficacy in brain with the intent to translate to the clinic.

PROJECT NARRATIVE The overall goal of this project is to evaluate how microglia-specific knockdown of Inpp5d/SHIP1 influences AD pathogenesis by utilizing Inpp5dflox mice crossed with an inducible CX3CR1-CreER, in which tamoxifen- induced Cre expression results in persistent recombination in long-lived microglia, but not short-lived myeloid cells. We will induce microglia-specific knockdown in models representing the pathological hallmarks of AD, β- amyloid deposition (APPKM670/671NL/PSEN1Δexon9) and tauopathy (MAPTP301S) so that we can evaluate how Inpp5d/SHIP1 knockdown impacts the unique microglia response to each hallmark, particularly in terms of cognition, neuroinflammation, and spatial transcriptomics/single-cell transcriptomic analysis. This proposal aims to provide valuable insight on the role of Inpp5d/SHIP1 in brain, how Inpp5d/SHIP1 drive heterogeneity among microglia populations and provide evidence for further evaluation of INPP5D/SHIP1 as a potential target for AD therapies.