Dark GPCR signaling underlying the Microbiome-Gut-Brain Axis for Alzheimer's Disease and Related Dementia

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PROJECT SUMMARY Cumulative evidence indicates the Microbiome-Gut-Brain axis plays a crucial role in Alzheimer’s disease (AD) and supports the potential of microbiome-targeted therapies as treatments for AD and AD-related dementia (ADRD). However, the precise mechanism of the Microbiome-Gut-Brain axis and the identity of actionable gut microbial biomarkers underlying AD/ADRD pathogenesis, disease progression, and modification remind understudied. Recent advances in chemogenomic technologies have demonstrated that G-protein-coupled receptors (GPCRs, the largest druggable target family in the human genome, as defined by the NIH-funded Illuminating the Druggable Genome Program) mediate much of the Microbiome-Gut-Brain axis, especially for gut microbiota-derived metabolites such as medium-chain fatty acids (MCFAs). Our preliminary experiments reveal strong significant associations between gut-microbiota MCFA metabolites (e.g., 5-phenylvaleric acid) and dark GPCR signaling (e.g., GPR84) in AD using multi-omics approaches and an AD patient-induced pluripotent stem cells (iPSC) model. Furthermore, we identified targeting gut microbial metabolite pathways improve cognitive behaviors in germ-free mice. We posit that combining AD patient-induced iPSC, cerebral organoids, and germ-free mouse models, along with multimodal analyses of plasma and hippocampus gut microbial metabolomics data, will enable improved mechanistic understanding of precise protective mechanisms of the Microbiome-Gut-Brain axis in AD/ADRD. Our central unifying hypothesis is that identifying likely molecular drivers (e.g., gut microbial metabolites) and druggable GPCR signaling networks underlying the Microbiome- Gut-Brain axis will elicit potential prevention and treatment strategies for AD. Aim 1 will test dark GPR84 (a putative microglial gene) and its signaling activation underlying the Microbiome-Gut-Brain axis of MCFAs via fecal microbiota transplantation (FMT) in germ-free mice by assessing AD-related cognitive and pathological phenotypes and mechanisms. We will evaluate differential gut microbial communities, untargeted and targeted gut metabolomics analyses of plasma and hippocampus in GPR84-/-, 5xFAD, and cross (5xFAD;GPR84-/-) germ-free mice during pre-FMT and post-FMT. Aim 2 will screen, test and validate dark GPCRs and signaling network perturbation by gut microbiota-derived MCFA metabolites using AD patient-derived iPSC lines in conjunction with cerebral organoid models. Specifically, we will evaluate physical binding of the gut microbial metabolite-GPCR interactome using complementary Calcium flux, cAMP glosensor, and β-arrestin Tango assays. Aim 3 will conduct supervised analyses of gut microbial metabolite biomarker discovery for, and prediction modeling of, clinically relevant AD pathological features using patient plasma targeted and untargeted gut microbial metabolomics. In summary, our multidisciplinary approach comprising germ-free mice, AD patient- derived iPSC, and cerebral organoid models, along with human plasma gut microbial metabolomics, will identify potential microbiome-targeted prevention and treatment approaches to be directly tested in people with AD.