Hypothesis

Age-related decline in apical sodium-dependent bile acid transporter (ASBT) function in the terminal ileum reduces intracellular bile acid levels, leading to diminished activation of the enterocyte nuclear receptor constitutive androstane receptor (CAR). This loss of CAR signaling relieves its repression of the tight‑junction protein claudin‑2, increasing paracellular permeability to bacterial endotoxins. The resulting endotoxemia fuels hepatic inflammation, insulin resistance, and systemic metabolic aging.

Mechanistic Rationale

• ASBT as gatekeeper of ileal BA flux – Under normal conditions, ~95% of secreted bile acids are reabsorbed via ASBT, maintaining high luminal‑to‑cellular BA gradients that activate CAR and FXR in enterocytes[3].
• CAR regulates barrier integrity – CAR activation induces expression of barrier‑protective genes (e.g., Mdr1, Shp) and suppresses claudin‑2 transcription; CAR‑null mice show heightened claudin‑2–mediated leakiness and increased LPS translocation[1].
• Age‑related ASBT decline – Although direct measurements are missing, indirect evidence links aging to reduced ileal BA uptake, altered BA pools, and barrier dysfunction[1][2].
• Feed‑forward loop – Increased luminal BA loss triggers hepatic CYP7A1 up‑regulation, raising systemic BA concentrations that can further impair CAR signaling via oxidative stress, creating a vicious cycle.

Testable Predictions

1. Molecular – Aged mice will show decreased ASBT protein/mRNA, reduced CAR target gene expression (Cyp2b10, Shp), and elevated claudin‑2 mRNA/protein in ileal epithelium compared with young counterparts.
2. Functional – Ileal BA uptake efficiency (measured ex vivo using radiolabeled taurocholate) will be inversely correlated with intestinal permeability (FITC‑dextran assay) and serum LPS‑binding protein levels.
3. Causal – Pharmacologic activation of CAR (e.g., with TCPOBOP) or ileal‑specific ASBT overexpression in aged mice will normalize claudin‑2 levels, decrease permeability, lower endotoxemia, and improve glucose tolerance.
4. Rescue – Conversely, ileal‑specific CAR knockdown in young mice will mimic the aged phenotype (↑claudin‑2, ↑permeability, metabolic dysregulation) despite normal ASBT levels.

Experimental Approach

• Animal models: Young (3‑mo) and aged (24‑mo) C57BL/6 mice; ileal‑specific ASBT‑overexpressing (AAV8‑ASBT) and CAR‑knockout (Vil‑Cre;Car^fl/fl) lines.
• Readouts: qPCR/Western for ASBT, CAR, Cyp2b10, Shp, claudin‑2; BA uptake using [^3H]‑taurocholate in ileal segments; permeability via oral FITC‑dextran (4 kDa) and plasma fluorescence; serum LPS, LPS‑binding protein, TNF‑α, insulin tolerance test.
• Interventions: TCPOBOP (CAR agonist) administered intraperitoneally for 2 weeks; AAV‑mediated ASBT delivery.
• Analysis: Correlational and regression analyses; two‑way ANOVA with post‑hoc Tukey.

Potential Implications

If validated, this hypothesis repositions ileal ASBT not merely as a passive conduit but as a signaling hub whose activity modulates CAR‑dependent barrier integrity. It suggests that preserving ileal BA transport—or directly stimulating CAR—could mitigate age‑associated leaky gut, endotoxemia, and downstream metabolic diseases, offering a novel therapeutic avenue distinct from hepatic‑centric BA modulation.

References [1] https://pubmed.ncbi.nlm.nih.gov/38680685/ [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC12716074/ [3] https://en.wikipedia.org/wiki/Enterohepatic_circulation [4] https://eriba.umcg.nl/groups/gut-liver-axis-in-healthy-ageing/ [5] https://science.nasa.gov/biological-physical/investigations/understanding-the-brain-liver-gut-axis-during-spaceflight-and-aging/