Project Summary: Dementia due to Alzheimer’s disease (AD) affects 1 in 8 Americans over the age of 65, and is currently not well treated. While therapeutic development has largely focused on clearing brain amyloid via antibody approaches, brain metabolism is also known to be substantially altered in the disease. Altering the metabolic state—for example, via ketogenic diet—can improve cognition through incompletely understood mechanisms. Previous studies indicate that acute supplementation with the metabolite β-hydroxybutyrate (BHB), one of the ketone bodies produced as a result of ketogenesis, improves cognitive function both in people with AD dementia and in mouse models of AD. However, the factors—apart from diet—that impact BHB levels, as well as the specific mechanisms by which BHB may exert positive impacts on the brain are unknown. Our research team has generated several important leads that better inform the factors that impact BHB levels, as well as discovering that BHB impacts AD pathology through inhibition of the inflammasome in microglia. While previously underappreciated in studies of ketogenic diet, gut microbiome has a significant impact on BHB levels. Using gnotobiotic mice, we provide preliminary evidence brain levels of BHB can be altered by precise manipulation of the gut microbiota. We have also found that modifying the abundance of BHB through long-term direct administration in the drinking water results in remarkably diminished plaque burden and microgliosis in 5XFAD mice. Further, in our studies of tissue from individuals in the Wisconsin ADRC with AD dementia who came to autopsy, we found that brain levels of BHB levels were lower compared to individuals without AD dementia at death. In the proposed study, we will follow up these findings to determine how BHB modulates disease progression and address knowledge gaps that would facilitate therapeutic use of this metabolite. Answering these questions has immediate translational implications and is expected to lead to novel strategies to prevent or slow the course of AD. Here, we hypothesize that BHB protects against AD-associated pathology by inhibiting Nlrp3 inflammasome activation through activation of Hcar2 in microglia. We will determine the features of the inflammasome that mediate the effects of BHB on AD pathology in the 5XFAD mouse models of amyloid β plaque deposition, determine the extent to which gut microbiome impacts BHB levels via butyrate producing bacteria, and finally, using human metagenomic and biomarker data we will determine the extent to which gut microbiome composition and BHB are associated with AD pathology using fluid biomarkers. The work proposed here will provide a deeper understanding of the interplay between the innate immune system, gut microbes, and metabolism in AD, generating the needed data that will support the development of novel strategies to prevent or slow the course of AD.