Project Summary/Abstract Alzheimer's Disease (AD) is a severe neurodegenerative disorder that poses a major burden on human society. The pathology of AD is characterized by the accumulation of beta-amyloid plaques and hyperphosphorylated tau protein as well as neuroinflammation involving microglia. There is no effective treatment for AD currently, and new therapeutic strategies based on the mechanistic understanding of AD pathology are critically needed. Cannabidiol (CBD) is a non-psychoactive phytocannabinoid that has broad anti-inflammatory effects. Several recent studies have reported the ability of CBD to reduce neuroinflammatory responses and improve behavioral functions in AD mouse models. However, the in vivo actions and mechanisms of CBD on microglia and AD pathology remain unknown. The overall objective of this proposal is to identify the mechanistic effects of CBD on AD-related cellular pathology in order to optimize its therapeutic potentials. Several lines of evidence suggest that microglia are a strong candidate to mediate the in vivo actions of CBD and contribute to its therapeutic potentials in AD models. First, the accumulation of reactive microglia around amyloid plaques has long been recognized as a hallmark of AD pathology. Recent human genetic studies have further identified multiple AD risk genes involved in microglia function, implicating a casual role of microglia in AD pathogenesis. Second, single-cell RNA-sequencing studies have revealed significant activation diversity in AD microglia population, including disease associated microglia (DAM). Experimental manipulations that reprogram reactive microglia to restore their homeostatic functions have shown protective effects against AD pathology, suggesting the therapeutic potentials of microglia. Third, CBD is known to interact biochemically with multiple membrane receptors and intracellular signaling molecules that are expressed in microglia, including cannabinoid receptors. Fourth, prior in vitro cellular studies and our preliminary chronic in vivo imaging data show that CBD modulates microglia activity and morphology. How CBD will interact with microglia and impact amyloid plaques in AD transgenic models remains to be investigated. Our central hypothesis is that CBD treatment will alter the cellular activity and molecular profile of microglia and reduce AD pathology in the brain. To test this hypothesis, we specifically aim to 1) determine CBD's impacts on microglia activity and beta amyloid pathology in AD transgenic mouse models by chronic in vivo imaging, and 2) evaluate CBD's effects on the molecular phenotypes of microglia in AD models by single-cell RNA sequencing. Identifying the effects of CBD on microglia and AD-related pathology in vivo will not only provide novel insights into the cellular and molecular mechanisms underlying CBD's action in the brain, but also suggest new routes to develop therapeutic strategies for AD.