Hypothesis

Intestinal epithelial PXR signaling, triggered by microbiota‑derived indole‑3‑propionic acid (IPA), is the primary upstream driver of vagal afferent activity that maintains hippocampal BDNF expression and neurogenesis; loss of gut‑specific PXR reduces vagal cholinergic tone, decreasing hippocampal plasticity and accelerating age‑related cognitive decline, independent of neuronal PXR function.

Mechanistic Rationale

1. IPA‑PXR tightens the barrier and boosts enteroendocrine cholinergic output – In human colonic organoids, IPA activates PXR to upregulate Occludin (3.2‑fold) and ZO‑1 (4.1‑fold) 1. PXR activation also increases expression of the choline transporter CTL1 and vesicular acetylcholine transporter VAChT in enterochromaffin cells, raising basal acetylcholine (ACh) release into the lamina propria.
2. Enteroendocrine ACh stimulates vagal afferents via nicotinic α7 receptors – Elevated luminal ACh activates α7‑nAChR on vagal nodose ganglia, increasing afferent firing rate to the nucleus tractus solitarius and subsequently to the dorsal vagal complex, which projects to the hippocampus via the locus coeruleus‑noradrenergic pathway.
3. Vagal afferent tone sustains hippocampal BDNF and neurogenesis – Chronic vagal stimulation raises hippocampal BDNF transcription through CREB phosphorylation, promoting synaptic plasticity and adult neurogenesis. Reduced vagal input lowers BDNF, impairing memory consolidation.
4. Directionality of causality – If gut PXR loss precedes neuronal changes, systemic IPA bioavailability drops first, weakening vagal cholinergic signaling and initiating hippocampal decline. Neuronal PXR activation would then be a secondary, compensatory response rather than the primary driver.

Experimental Design

Model: Aged (18‑month) C57BL/6J mice with inducible, cell‑type‑specific PXR knockouts: (i) intestinal epithelial PXR KO (Villin‑CreERT2;Nr1i2^fl/fl), (ii) forebrain neuronal PXR KO (CamKII‑CreERT2;Nr1i2^fl/fl), and (iii) global PXR KO as control. Interventions: (a) IPA supplementation (10 mg/kg/day via gavage) or (b) vagal afferent blockade (capsaicin‑treated to desensitize sensory fibers). Readouts (collected at baseline, 4 wks, 8 wks):

• Serum IPA levels (LC‑MS/MS)
• Intestinal permeability (FITC‑dextran flux)
• Vagal afferent firing (ex vivo vagal nerve electrophysiology)
• Hippocampal BDNF ELISA and BrdU/NeuN neurogenesis quantification
• Cognitive performance (Morris water maze, novel object recognition)
• Microglial activation (Iba1 immunostaining)

Expected Outcomes & Falsifiability

• If gut‑first hypothesis is correct: Intestinal epithelial PXR KO will reduce serum IPA, increase permeability, lower vagal firing, decrease hippocampal BDNF/neurogenesis, and impair cognition despite intact neuronal PXR. Neuronal PXR KO will show milder or no cognitive deficits unless vagal signaling is also compromised. IPA supplementation will rescue vagal tone and hippocampal outcomes only in intestinal KO mice.
• Falsification: If neuronal PXR KO alone reproduces the full cognitive phenotype with normal vagal firing and hippocampal BDNF, or if intestinal PXR KO fails to alter vagal activity or hippocampal markers despite barrier loss, the gut‑to‑brain causal direction is refuted.

This framework provides a clear, testable mechanism by which gut‑derived IPA‑PXR signaling sets the brain’s homeostatic baseline via vagal cholinergic tone, shifting the focus of longevity interventions from top‑down neuroprotection to bottom‑up intestinal barrier maintenance.