Hypothesis

Aging-related decline in colonic motility stems not only from reduced SCFA‑driven TPH1 expression in enterochromaffin (EC) cells but also from altered bile acid sensing that converges on the same serotonergic output pathway. Restoring both microbial SCFA signaling and bile acid receptor activity will rescue EC‑cell serotonin release and improve propulsive motility more effectively than either intervention alone.

Mechanistic Basis

• EC cells express the G‑protein‑coupled receptor TGR5, which is activated by secondary bile acids such as deoxycholic acid (DCA) and lithocholic acid (LCA) (source).
• TGR5 activation raises intracellular cAMP, potentiating Ca²⁺‑dependent vesicle exocytosis and thus amplifying serotonin release downstream of TPH1 (source).
• Age‑associated microbiome shifts diminish butyrate‑producing taxa and concurrently increase bile‑acid 7‑α‑dehydroxylating bacteria, elevating luminal DCA/LCA while butyrate falls (source).
• We propose that EC cells act as coincidence detectors: optimal serotonin output requires both sufficient SCFA‑mediated TPH1 transcription (providing substrate) and bile‑acid‑driven TGR5 signaling (providing secretory potency). Loss of either input reduces serotonin release; loss of both produces the severe motility deficits observed in aged colons.

Testable Predictions

1. In vitro: Human EC‑cell lines treated with physiological butyrate (1 mM) plus physiological DCA (50 µM) will show >3‑fold increase in 5‑HT release compared with either agent alone, measured by ELISA (source).
2. Ex vivo: Colonic strips from aged mice will exhibit rescued propulsive contractions when perfused with a cocktail of sodium butyrate (2 mM) and a TGR5 agonist (e.g., INT‑777, 1 µM) versus single treatments.
3. In vivo: Aged mice receiving a combined prebiotic (high‑fiber diet to boost SCFA) and a bile‑acid modulator (e.g., cholestyramine to lower toxic secondary bile acids) will restore colonic TPH1 and TGR5 expression, normalize tissue 5‑HT levels, and accelerate transit time measured by carmine red assay.
4. Falsifiability: If combined SCFA and TGR5 modulation fails to improve motility beyond single treatments, or if EC‑specific TGR5 knockout abolishes the butyrate‑induced serotonin increase, the coincidence‑detector model is refuted.

Experimental Approach

• Use qPCR and Western blot to quantify TPH1 and TGR5 in isolated EC cells from young (3 mo) and aged (24 mo) mice under diet interventions.
• Measure luminal SCFA and bile‑acid concentrations by GC‑MS and LC‑MS/MS to correlate metabolite shifts with EC‑cell signaling.
• Employ optogenetic serotonin sensors in vivo to visualize real‑time 5‑HT dynamics during colonic perfusion.

Significance

This hypothesis reframes EC‑cell aging as a reversible signaling hub where microbiome‑derived metabolites and host‑modified bile acids intersect. Demonstrating that coordinated restoration of both arms rescues motility would provide a mechanistic basis for combined prebiotic‑bile‑acid therapies targeting age‑related gastrointestinal dysfunction.