Hypothesis

Aging reduces luminal short‑chain fatty acid (SCFA) concentrations, leading to diminished HDAC inhibition and increased histone deacetylation at the Piezo2 promoter in enterochromaffin (EC) cells. This epigenetic silencing lowers Piezo2 channel expression and/or alters its membrane micro‑environment, impairing mechanotransduction‑evoked serotonin release. Consequently, EC‑derived 5‑HT signaling to the epithelium and enteric nerves weakens, slowing intestinal motility and allowing further accumulation of pathogenic microbes, which exacerbates SCFA loss—a self‑reinforcing dysbiosis‑motility loop.

Testable Predictions

1. Piezo2 expression correlates with fecal SCFA levels – In young vs. aged mice (or human biopsies), EC‑cell‑specific Piezo2 mRNA and protein will positively associate with colonic butyrate concentrations.
2. Butyrate supplementation restores Piezo2 function – Treating aged mice with oral sodium butyrate (or a butyrate‑producing probiotic) will increase Piezo2 acetylation‑dependent transcription, rescue mechanosensitive 5‑HT release from isolated EC cells, and normalize colonic transit time.
3. Epigenetic inhibition mimics aging – Pharmacological HDAC activation (e.g., using sulforaphane) in young mice will reduce Piezo2 expression and serotonin release, reproducing the aged phenotype even with normal microbiota.
4. Engineered 5‑HT probiotics act downstream – Administration of an engineered 5‑HT‑secreting probiotic will improve motility in aged animals only if residual Piezo2‑dependent EC signaling remains; in Piezo2‑EC‑specific knockouts the probiotic will fail to accelerate transit, indicating that microbial 5‑HT cannot fully compensate for lost mechanosensitive release.

Experimental Approach

• Isolate EC cells from duodenum and colon of 3‑month (young) and 24‑month (old) mice; quantify Piezo2 by qPCR, Western blot, and immunostaining; measure butyrate levels in luminal content via GC‑MS.
• Perform patch‑clamp or calcium‑imaging assays on EC cells exposed to controlled stretch, comparing 5‑HT release (ELISA) before and after butyrate (1 mM) or HDAC inhibitor (trichostatin A) treatment.
• In vivo, feed aged mice a butyrate‑enriched diet (5 % w/w) for 4 weeks; assess colonic transit using carmine red assay, motility via manometry, and EC‑cell serotonin content.
• Use EC‑cell‑specific Piezo2 floxed mice crossed with Villin‑CreERT2 to delete Piezo2 in adulthood; test whether butyrate supplementation can still improve motility.
• Finally, colonize germ‑aged mice with either wild‑type or engineered 5‑HT‑producing E. coli Nissle to determine if microbial serotonin can bypass the mechanotransduction defect.

Potential Outcomes and Falsification

• If butyrate fails to increase Piezo2 expression or EC 5‑HT release in aged tissue, the epigenetic link is unsupported.
• If HDAC activation in young mice does not reduce Piezo2 function, the causal direction is questionable.
• If engineered 5‑HT probiotics rescue motility irrespective of EC‑cell Piezo2 status, then microbial serotonin can act independently of mechanosensitive EC signaling, challenging the feed‑forward model.

By integrating mechanotransduction, microbial metabolism, and epigenetic regulation, this hypothesis provides a concrete, falsifiable framework for why the serotonin‑motility axis deteriorates with age and how targeting SCFA‑Piezo2 signaling could break the dysmotility‑dysbiosis cycle.