The protective influence of indole‑3‑propionic acid (IPA) on intestinal mesenchymal cells depends on pregnane X receptor (PXR) activation, yet no study has examined whether PXR expression itself changes with age. We hypothesize that advancing age triggers an epigenetic downregulation of PXR in intestinal fibroblasts and pericytes, rendering these cells unresponsive to IPA despite normal ligand levels. This receptor loss converts a protective signaling circuit into a senescent, pro‑fibrotic state, thereby uncoupling microbiota‑derived metabolites from barrier maintenance.

Mechanistically, age‑associated decline in NAD+ reduces SIRT1 deacetylase activity, leading to hyperacetylation of histone H3 lysine 27 at the PXR promoter. Increased H3K27ac recruits polycomb repressive complex 2, which deposits H3K27me3 and compacts chromatin, silencing Nr1i2 transcription. Consequently, IPA‑bound PXR fails to suppress NF‑κB‑driven IL1β and IL5 secretion, and fibroblast‑derived collagen synthesis proceeds unchecked, reproducing the fibrosis seen in germ‑free or antibiotic‑treated young mice.

The hypothesis is testable and falsifiable. First, quantify Nr1i2 mRNA and PXR protein in isolated intestinal mesenchymal cells from young (3 mo), middle‑aged (12 mo), and old (24 mo) conventionally raised mice; a progressive reduction would support the claim. Second, assess promoter acetylation and methylation states via ChIP‑qPCR for H3K27ac and H3K27me3 at the Nr1i2 locus across the same ages. Third, rescue experiments: treat old mice with the NAD+ precursor nicotinamide riboside or a SIRT1 activator and determine whether PXR expression, IPA‑mediated anti‑inflammatory signaling, and collagen deposition revert to youthful levels. Fourth, use germ‑free animals colonized with an IPA‑producing strain to isolate ligand availability from receptor competence; if barrier function remains impaired despite adequate IPA, the defect lies downstream of ligand binding.

Falsifiability is built in: if Nr1i2 expression, promoter chromatin state, or functional responsiveness to IPA shows no significant age‑related change, or if NAD+ supplementation fails to restore PXR activity and barrier integrity, the hypothesis would be refuted. This work would shift the focus from ligand scarcity to receptor competence as a central determinant of age‑associated intestinal vulnerability, integrating NAD+ metabolism, epigenetic regulation, and xenobiotic sensing into a unified mechanism of barrier decline.