Hypothesis

Aging is accompanied by an increase in ileal apical sodium‑dependent bile‑acid transporter (ASBT) expression, contrary to the assumed decline. This upregulation enhances hepatic bile‑acid pool size and shifts the composition toward more hydrophobic primary bile acids (e.g., cholic acid). The altered pool activates intestinal FXR less effectively, reducing FGF19 secretion and disinhibiting hepatic CYP7A1, thereby boosting de novo bile‑acid synthesis. Excess bile acids spill into the colon where microbiota convert them to cytotoxic secondary bile acids such as deoxycholic acid (DCA). DCA activates TGR5 on enterochromaffin cells, provoking serotonin release that stimulates vagal afferents and drives sustained microglial activation via the nucleus tractus solitarius–mediated neuroimmune axis. Consequently, a self‑reinforcing gut‑brain loop amplifies systemic inflammaging, contributing to age‑related cognitive decline and neurodegeneration.

Testable Predictions

1. In aged mice, ileal ASBT mRNA and protein levels will be higher than in young adults.
2. Pharmacologic or genetic inhibition of ileal ASBT in aged animals will: a. Lower serum concentrations of hydrophobic primary bile acids. b. Decrease colonic DCA levels. c. Reduce vagal afferent firing (measured by electrophysiology) and downstream microglial Iba1 expression in the hippocampus. d. Improve performance in spatial memory tasks (Morris water maze).
3. Conversely, forced overexpression of ASBT in young mice will recapitulate the aged bile‑acid profile, elevate colonic DCA, increase vagal‑mediated microglial activation, and impair cognition.
4. Administration of a TGR5 antagonist or a 5‑HT3 receptor blocker will blunt the vagal‑microglial axis despite high ASBT, rescuing inflammatory markers.

Mechanistic Rationale

The enterohepatic circulation normally reclaims ~95% of bile acids via ileal ASBT, maintaining a stable pool size (2). Age‑related alterations in this reabsorption set point would directly affect the feedback loops governing hepatic synthesis through FXR/FGF19 (3). Increased ASBT would paradoxically raise hepatic extraction, yet because the transporter works near saturation, excess bile acids escape ileal uptake, enriching the luminal pool. Hydrophobic bile acids are poorer FXR agonists, weakening the FGF19‑mediated brake on CYP7A1, thus augmenting synthesis—a concept supported by bile‑acid signaling to the CNS via FXR and TGR5 on vagal afferents (1). The resulting surge of colonic DCA, a known TGR5 agonist, triggers serotonin release from enterochromaffin cells, activating vagal pathways that convey pro‑inflammatory signals to the brain, culminating in microglial priming and inflammaging. This mechanistic chain links a transport protein alteration to microbiota‑derived metabolites, vagal signaling, and neuroimmune outcomes, offering a concrete, falsifiable framework for gut‑brain contributions to aging.

Experimental Approach

• Use ileal‑specific Villin‑CreERT2;ASBT^fl/fl mice for knockout and Villin‑CreERT2;Rosa26^LSL‑ASBT for overexpression, induced at 3 months (young) or 18 months (aged).
• Quantify bile‑acid pools by LC‑MS serum, feces, and colon content.
• Measure colonic DCA, serum FGF19, hepatic CYP7A1 expression.
• Record vagal afferent activity with ex vivo vagus nerve electrophysiology.
• Assess hippocampal microglial morphology (Iba1 immunostaining) and cytokine profile (IL‑1β, TNF‑α).
• Conduct behavioral testing (novel object recognition, Morris water maze).
• Apply pharmacological blockers (TGR5 antagonist SB‑756050; 5‑HT3 antagonist ondansetron) to test pathway necessity.

If predictions hold, the data will demonstrate that age‑dependent ASBT up‑regulation initiates a bile‑acid‑driven gut‑brain inflammatory circuit, positioning microbial bile‑acid metabolism as a controllable driver of inflammaging rather than a passive bystander.