From Dysbiosis to Blood-Brain Barrier Disruption: The Metabolite-Mediated Gut-Brain Axis in Alzheimer's Disease.

Alzheimer's disease (AD) is not merely a central nervous system disorder; rather, it is a systemic condition profoundly influenced by the peripheral internal environment. Recent research has revealed that imbalances in the gut microbiota (GM) and metabolite disturbances contribute to AD onset and progression. Clinical and animal studies have indicated that AD patients commonly exhibit reduced GM diversity, decreased populations of short-chain fatty acid (SCFA)-producing and indole-producing bacteria, disrupted bile acid (BA) profiles, and elevated levels of trimethylamine N-oxide (TMAO) and kynurenine pathway (KP) activity. These alterations not only reflect gut dysbiosis, but also impair blood-brain barrier (BBB) integrity and amplify neuroinflammation by modulating tight junction proteins and inflammatory signaling through their effects on receptors and transporters such as G protein-coupled receptors(GPR41/43), aryl hydrocarbon receptor(AhR), Farnesoid X receptor(FXR)/ Takeda G protein-coupled receptor 5(TGR5), L-type amino acid transporter 1(LAT1), and Major Facilitator Superfamily Domain containing 2A(MFSD2A). From an integrative perspective, these changes -including short-Chain Fatty Acids (SCFAs) deficiency, elevated TMAO and toxic BA levels, overactivation of the KP, and Lipopolysaccharides (LPS) leakage-often act synergistically, collectively forming key pathological nodes in the "metabolic network-BBB-AD" axis. GM-targeted strategies such as dietary interventions, probiotics and fecal microbiota transplantation (FMT) have demonstrated potential in improving metabolite profiles and BBB homeostasis. Future research should utilize induced pluripotent stem cell-derived organoids and multi-omics integration approaches to elucidate the spatiotemporal dynamics of metabolites within the gut-brain axis (GBA), thereby laying the foundation for precise microbiome-based interventions in AD.