Gut microbiota rescues vascular cognitive impairment by improving glymphatic function via the ILA-AHR-AQP4 pathway

Microbial tryptophan metabolites regulate cognitive function, but their role in vascular cognitive impairment (VCI) induced by chronic cerebral hypoperfusion (CCH) remains poorly understood. In the CCH cohort, we observed gut microbiota dysbiosis and reduced levels of microbial tryptophan metabolites, including indole-3-lactic acid (ILA), in cognitively impaired CCH patients, with these metabolites serving as protective factors. In mice with bilateral carotid artery stenosis, gut microbiota dysbiosis and ILA reduction preceded cognitive decline and were accompanied by astrocytic aryl hydrocarbon receptor (AHR) inactivation, aquaporin-4 (AQP4) depolarization, and glymphatic dysfunction. Fecal microbiota transplantation (FMT) reconstituted the gut microbiota and salvaged cognition. The cognitive improvement following FMT depended on the restoration of the glymphatic system. Tryptophan supplementation increased cerebral ILA concentrations, activated astrocytic AHR, and polarized AQP4, thereby rescuing glymphatic function and enabling cognitive recovery. Notably, ILA supplementation alone was sufficient to restore AQP4 polarization and cognitive function. Mechanistically, we identified dioxin-responsive elements within the promoters of the AQP4 polarization-associated genes alpha-syntrophin (Snta1) and dystroglycan 1 (Dag1). ILA supplementation activated AHR in astrocytes and upregulated the expression of these genes, whereas AHR-specific antagonist pretreatment inhibited the upregulation of Snta1 and Dag1 induced by ILA. These findings demonstrate an ILA-AHR-AQP4 axis linking microbial tryptophan metabolism to the glymphatic system, which is important for cognitive restoration and indicates a novel therapeutic insight for VCI associated with CCH. Given the exploratory nature of the clinical cohorts, the findings warrant validation in larger independent cohorts. (Trial registration: ClinicalTrials. gov. ID number: NCT04688138).