Abstract Microglia, the immune cells of the brain, are key cellular players in the pathogenesis and progression of Alzheimer’s disease (AD). As the immune cells of the brain, microglia are the cell type predominantly involved in phagocytosis of protein aggregates such as amyloid b (Ab), which aggregates to form the characteristic plaques present in AD. A decrement of microglia phagocytic capability, combined with a pro-inflammatory phenotype, may lead to a brain environment permissive for Ab plaque formation. Recent genome-wide association studies have identified mutations in the immune-cell specific gene CD33, expressed almost exclusively on microglia in the brain, as a risk factor for development of AD. CD33 is a member of the Siglec family of receptors, and has an immunosuppressive effect upon detection sialic acids. Reduced expression of CD33 is protective against formation of Ab plaques by stimulating phagocytosis and altering pro-inflammatory phenotype in microglia. Thus, CD33 represents an important therapeutic target for treatment and prevention of AD. However, an appropriate timepoint for anti-CD33 treatments has not been established due to technical challenges with manipulating CD33 in the brain of transgenic animal models of AD. Here, we propose to use a brain- and microglia-validated method for delivery of CRISPR to the brain to knock down expression of CD33 at various times during the course of disease progression in a transgenic animal model of AD. We predict that downregulation of CD33 early and late in disease will differentially alter microglia phagocytosis of Ab, change plaque load, abrogate neurotoxicity, and improve memory deficits. Further, we aim to understand the heterogeneity of microglia CD33 expression in human AD patients by examining existing single-cell transcriptomics datasets. We predict that microglia with reduced CD33 will display a phagocytic and neuroprotective gene signature. We predict that this gene signature will also be evident in CD33-high and CD33- low microglia from transgenic AD mice. Finally, we will test sialic acid binding as a potential mechanism by which CD33 regulates phagocytosis of Ab. Overall, these studies will enhance our current understanding of how CD33 modifies microglia function in vivo and will elucidate the benefits of early (preventative) vs. late (interventional) CD33 knockdown which will inform potential future treatments of CD33-targeting therapeutics.

Project Narrative Mutations in the gene for CD33, which is expressed on microglia cells, are a risk factor for Alzheimer’s disease. However, it is not clear how CD33 alters microglia function or how it contributes to Alzheimer’s disease initiation or progression. In this proposal we will investigate the function of CD33 on microglia using cellular and animal models of Alzheimer’s disease and closely examine existing human microglia datasets.