Hypothesis

Core idea: Age‑related dysbiosis increases microbial tryptophanase activity, flooding the host with indole derivatives that chronically activate the aryl hydrocarbon receptor (AhR) in intestinal epithelium and microglia. This persistent AhR signaling compromises barrier integrity and primes neuroimmune cells, creating a self‑reinforcing loop that accelerates inflammaging and cognitive decline.

Mechanistic rationale

1. Microbial source – Certain Bacteroides and Clostridia strains express tryptophanase that converts dietary tryptophan to indole, indole‑lactate, and indole‑acrylic acid (5). Metagenomic surveys show these taxa expand with age while SCFA producers decline.

2. AhR as a signaling hub – Indole ligands bind AhR, regulating transcription of tight‑junction proteins (claudin‑1, occludin) and mucin genes. In youth, transient AhR activation promotes barrier repair; chronic activation, however, drives epithelial hyperplasia and mucus depletion (4). In the brain, AhR activation in microglia shifts them toward a pro‑inflammatory phenotype, amplifying cytokine release (3).

3. Feedback to the microbiome – Barrier leakage lets microbial metabolites reach systemic circulation, where they further stimulate hepatic AhR, altering bile‑acid composition and favoring growth of tryptophanase‑positive microbes—a positive feedback loop.

4. Behavioral read‑out – Elevated indole‑AhR signaling correlates with reduced vagal tone and memory deficits in aged mice (2).

Testable predictions

• Prediction 1: Aged mice will show higher fecal concentrations of indole‑lactate and indole‑acrylic acid than young controls, and these levels will positively correlate with plasma CML and hippocampal IL‑1β.

• Prediction 2: Pharmacological inhibition of microbial tryptophanase (e.g., with indole‑3‑propionic acid supplementation) or genetic knockout of tryptophanase in commensals will lower indole load, restore AhR‑dependent barrier gene expression, reduce microglial activation, and improve performance on the Morris water maze.

• Prediction 3: Administration of an AhR antagonist (such as CH‑223191) specifically to the gut (via oral gavage) will rescue vagal signaling (measured by heart‑rate variability) and reverse age‑related memory decline without altering overall microbiota composition.

• Prediction 4: Fecal microbiota transplantation from young donors into aged, tryptophanase‑deficient mice will not confer cognitive benefits unless the transplanted community retains low tryptophanase activity, indicating that metabolite flux, not just taxonomy, drives the effect.

Falsifiability

If any of the above interventions fail to modify indole levels, AhR signaling markers, barrier permeability, or cognitive outcomes despite correct dosing, the hypothesis would be refuted. Conversely, consistent rescue across multiple independent approaches would support the model.

Broader implication

Targeting microbial tryptophan metabolism offers a precise lever to modulate the gut‑brain axis without needing to correct the entire dysbiotic community—a potentially simpler, more translatable strategy for mitigating inflammaging.