Age‑Related Bile Acid‑Driven Occludin‑S408 Phosphorylation Drives Colon‑Segment‑Specific Barrier Leak and Inflammaging

Human aging leaves tight junction mRNA levels largely unchanged, yet functional barrier loss emerges through post‑translational modifications that differ along the intestinal tract. We hypothesize that age‑related accumulation of secondary bile acids, particularly deoxycholic acid (DCA), in the distal gut activates the membrane receptor TGR5 on colonic epithelial cells, triggering a PKCζ‑dependent phosphorylation of occludin at serine‑408 (p‑occludin‑S408). This modification serves as a sorting signal for clathrin‑mediated endocytosis, removing occludin from the apical junctional complex without altering total protein abundance. Because bile acid concentrations rise progressively from the proximal to the descending colon with age, the descending epithelium experiences a higher rate of occludin internalization, producing a focal increase in paracellular permeability that permits bacterial antigens to lodge in the lamina propria. There, antigen presentation drives Th1‑skewed cytokine release (IFN‑γ, TNF‑α, IL‑6) which, while capable of further weakening junctions via MLCK, is secondary to the primary TGR5‑PKCζ occludin‑S408 axis. Consequently, the descending colon becomes a hotspot for inflammaging‑initiating barrier breach, whereas the ascending colon, exposed to lower DCA levels, preserves junctional stability despite identical gene expression patterns[1]. The hypothesis generates three testable predictions: (1) In paired biopsies from older versus young donors, the ratio of p‑occludin‑S408 to total occludin will be significantly higher in descending colon samples and will correlate positively with fecal DCA concentrations; (2) Pharmacological blockade of TGR5 (e.g., with antagonistic small molecule SB‑022212) or inhibition of PKCζ (using pseudo‑substrate inhibitor) in ex‑vivo human colonic organoids will prevent DCA‑induced occludin S408 phosphorylation and retard its internalization, while overexpression of a phospho‑deficient occludin‑S408A mutant will rescue barrier function under the same conditions; (3) Longitudinal follow‑up of a cohort of middle‑aged adults will show that an upward trend in descending‑colon p‑occludin‑S408 precedes measurable rises in systemic inflammation markers (serum IL‑6, TNF‑α, CRP) by at least six months, establishing a temporal, causal relationship. Demonstrating this region‑specific, post‑translational mechanism would explain the discordance between human and rodent aging data, highlight bile acid signaling as a druggable node, and provide a biomarker‑guided strategy to interrupt the inflammaging feed‑forward loop before mucosal immune activation becomes entrenched[4][5]