Hypothesis

Aging reduces ileal ASBT (SLC10A2) expression and function, which, combined with age‑associated taurine depletion, creates a dual‑hit that diminishes reabsorption of taurine‑conjugated bile acids. This leads to excess loss of taurine‑conjugated bile acids in feces, a shift toward unconjugated bile acids, reduced ileal FXR activation, heightened hepatic CYP7A1 activity, and increased signaling through the membrane bile acid receptor TGR5 on immune cells. The resulting milieu promotes mitochondrial oxidative stress (due to lower taurine antioxidant capacity) and chronic low‑grade inflammation (inflammaging), independent of hepatic synthetic capacity.

Rationale

• Aging impairs intestinal epithelial stem cell function and barrier integrity, lowering enterocyte markers [Aging impairs ileal enterocyte function...].
• In vivo perfusion studies stress expressing transport per surface area to account for mucosal changes [Perfusion studies recommendation].
• Circulating taurine declines with age across mammals [Taurine declines with aging], and taurine is required for conjugation of primary bile acids in rodents.
• Dynamic biomarker trajectories predict mortality better than static levels [Dynamic biomarkers predict mortality], supporting functional assays over static serum measures.
• Intestinal amino acid transporter decline regulates longevity in Drosophila [Transporter longevity in Drosophila], suggesting gut‑specific transporter dysfunction can drive aging phenotypes.

Combining these points, we propose that the ileal ASBT‑taurine axis is a gut‑centric regulator of systemic aging.

Novel Mechanistic Insight

1. Loss of taurine‑conjugated bile acids reduces FXR‑dependent FGF19 secretion, disinhibiting hepatic CYP7A1 and expanding the bile acid pool with more hydrophobic, unconjugated species.
2. Unconjugated bile acids are potent agonists of TGR5 on macrophages and dendritic cells, triggering cAMP‑PKA‑CREB pathways that increase NLRP3 inflammasome activation and IL‑1β release.
3. Taurine deficiency diminishes mitochondrial membrane stabilization and scavenging of hypochlorous acid, heightening ROS production in enterocytes and systemic circulation.
4. The combined FXR hypo‑activity/TGR5 hyper‑activity creates a feedback loop where inflammation further damages ileal epithelium, worsening ASBT expression—a vicious cycle.

Testable Predictions

1. Expression/Function: Aged (24‑month) mice will show ↓ ASBT mRNA and protein in isolated ileal enterocytes and ↓ [^3H]‑taurocholate uptake per cm² of mucosa compared with young (3‑month) mice.
2. Bile Acid Pool: Fecal concentrations of taurocholate will be ↑, while serum unconjugated bile acids (e.g., deoxycholic acid) will be ↑; the ratio of conjugated/unconjugated bile acids will shift toward unconjugated.
3. Signaling: Ileal FXR target genes (Fgf15, Shp) will be ↓; hepatic Cyp7a1 will be ↑; colonic TGR5‑dependent cAMP production in immune cells will be ↑.
4. Physiological Outcomes: Aged mice will exhibit ↑ serum mitochondrial ROS markers (e.g., 8‑iso‑PGF2α), ↑ circulating IL‑1β/TNF‑α, and histologic signs of colonic low‑grade inflammation.
5. Rescue: Viral‑mediated overexpression of ASBT in ileum or dietary taurine supplementation (2 % w/w) in aged mice will normalize bile acid conjugation, reduce unconjugated bile acids, lower inflammatory cytokines, and improve mitochondrial function.

Experimental Design

• Animals: Young (3 mo) and aged (24 mo) C57BL/6J mice; n=8 per group.
• Measurements: qPCR/Western blot for ASBT; ex vivo ileal perfusion with [^3H]‑taurocholate (calculate nmol·cm⁻²·min⁻¹); fecal and serum bile acid profiling by LC‑MS; FXR/TGR5 pathway read‑outs (qPCR, cAMP ELISA); mitochondrial ROS (MitoSOX flow cytometry); cytokine ELISA.
• Intervention Sub‑cohorts: Aged mice receiving AAV8‑Vill‑ASBT or taurine‑supplemented diet for 8 weeks; same measurements post‑intervention.
• Statistical Analysis: Two‑way ANOVA (age × treatment) with post‑hoc Tukey; p<0.05 considered significant.

Potential Pitfalls & Alternatives

• Compensatory transporters: Upregulation of OSTα/OSTβ could mask ASBT loss; measure basolateral efflux.
• Microbiota influence: Altered bile acid biotransformation by gut microbes may confound measurements; include germ‑free or antibiotic‑treated controls.
• Species differences: Human taurine conjugation is minimal; validate relevance using human ileal biopsies or organoids from young vs. older donors.

If aged ileum shows no ASBT decline, or if manipulating ASBT/taurine fails to alter bile acid composition, inflammation, or mitochondrial markers, the hypothesis would be falsified, prompting re‑evaluation of gut‑centric contributions to aging.

Conclusion

By linking ASBT‑mediated bile acid reabsorption with taurine availability, this hypothesis provides a mechanistic, gut‑focused explanation for how intestinal transporter decline can drive systemic aging phenotypes. It is directly testable with functional perfusion assays, genetic or nutritional rescue, and multimodal phenotyping.