Hypothesis

Gut-derived indole-3-propionic acid (IPA) prolongs mammalian lifespan primarily by activating vagal afferent pathways that suppress neuroinflammation, not by modulating central brain outputs that then influence the gut. If vagal afferent signaling is blocked, IPA supplementation will fail to extend lifespan or reduce brain aging markers.

Mechanistic Rationale

IPA is a tryptophan metabolite produced by certain gut bacteria and has shown antioxidant and neuroprotective properties [https://www.alzheimers.gov/news/unique-gut-microbiome-patterns-linked-healthy-aging-increased-longevity]. We propose that IPA binds the aryl hydrocarbon receptor (AhR) expressed on enteroendocrine L-cells in the colon, triggering release of serotonin (5‑HT). Serotonin activates 5‑HT3 receptors on vagal afferent fibers, increasing their firing rate and signaling to the nucleus tractus solitarius (NTS). This activates the cholinergic anti‑inflammatory pathway, reducing microglial activation and peripheral cytokine production, thereby slowing brain aging. This cascade places the gut upstream of the brain, aligning with the observation that gut‑to‑brain interventions (prebiotics, FMT, IPA) yield large, reproducible mammalian lifespan effects [https://www.nad.com/news/prebiotic-increases-lifespan-delays-brain-aging-improving-gut-health][https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2024.1362239/full], whereas brain‑to‑gut manipulations remain largely unverified in aging mammals [https://pmc.ncbi.nlm.nih.gov/articles/PMC7585115/][https://www.aging-us.com/article/103996].

Experimental Design

1. Animal model: 20‑month‑old C57BL/6 mice, both sexes, n=30 per group.
2. Groups:
   • Control vehicle
   • IPA supplementation (10 mg/kg/day, oral)
   • IPA + subdiaphragmatic vagotomy (surgical)
   • IPA + chemogenetic inhibition of vagal afferents (AAV‑hM4Di expressed in nodose ganglion, CNO administration)
   • Sham surgery + vehicle
   • Sham surgery + CNO (control for chemogenetics)
3. Interventions: Begin treatments at 20 months, continue for 6 months.
4. Outcomes:
   • Survival (primary endpoint)
   • Brain histology: Iba1+ microglial density, NeuN+ neuronal counts in hippocampus and cortex
   • Plasma cytokines (IL‑6, TNF‑α, IL‑10)
   • Fecal IPA levels (LC‑MS)
   • Vagal afferent activity (ex vivo electrophysiology of nodose ganglion)
5. Statistical analysis: Kaplan‑Meier with log‑rank test for survival; two‑way ANOVA for histological and cytokine data; significance set at p<0.05.

Predictions and Falsifiability

• Prediction 1: IPA alone will increase median lifespan by ~25‑35% relative to vehicle, mirroring the prebiotic NAT effect [https://www.nad.com/news/prebiotic-increases-lifespan-delays-brain-aging-improving-gut-health].
• Prediction 2: Vagotomy or chemogenetic silencing of vagal afferents will abolish the lifespan extension and neuroprotective benefits of IPA, bringing survival and brain inflammation metrics back to control levels.
• Prediction 3: Sham‑operated mice receiving IPA will show elevated serum IPA, reduced microglial activation, and lower proinflammatory cytokines compared with controls. Falsification occurs if IPA extends lifespan equally regardless of vagal afferent interruption, or if vagal manipulation alone alters lifespan without IPA, indicating a dominant brain‑to‑gut direction.

Potential Implications

Confirming this hypothesis would reorient longevity research toward gut‑centric interventions that harness specific metabolite‑vagus nerve axes. It would also provide a mechanistic bridge between microbial tryptophan metabolism and the cholinergic anti‑inflammatory pathway, offering a clear target for drugs or engineered probiotics designed to amplify bottom‑up signaling in aging populations.