Hypothesis

Aging induces a senescence‑associated secretory phenotype (SASP) in intestinal immune and stromal cells that suppresses Piezo2 expression in a specific mechanosensitive enterochromaffin (EC) subpopulation via HDAC‑mediated deacetylation of its promoter, leading to reduced calcium‑dependent 5‑HT release, slowed motility, and constipation‑like phenotypes; restoring a youthful microbiome or clearing senescent cells rescues Piezo2 activity and EC function.

Mechanistic Rationale

ECs produce ~90% of body 5‑HT and rely on Piezo2 for mechanotransduction that triggers calcium influx and vesicle release Enterochromaffin cells produce approximately 90% of the body's serotonin A specific mechanosensitive EC subpopulation uses Piezo2 channels to trigger calcium‑dependent 5‑HT release. With age, the gut microbiome shifts toward pro‑inflammatory taxa, increasing luminal LPS and secondary bile acids that activate TLR4 on lamina propria macrophages and fibroblasts Microbiota directly influences EC 5‑HT synthesis through metabolites including short‑chain fatty acids and indole derivatives. These senescent cells secrete IL‑6, TNF‑α, and TGF‑β components of the SASP, which have been shown to upregulate class IIa HDACs (HDAC4/5) in epithelial cells SASP components drive epithelial HDAC activation in aging tissues. HDAC4/5 deacetylate histone H3 at the Piezo2 promoter, reducing chromatin accessibility and transcription, a mechanism observed in other mechanosensors during inflammaging HDAC‑mediated silencing of mechanotransducers in chronic inflammation. Lower Piezo2 diminishes EC calcium spikes in response to bacterial metabolites like isovalerate, decreasing 5‑HT output despite unchanged TPH1 levels Human organoid models demonstrate EC responses to bacterial metabolites like isovalerate. The resulting 5‑HT deficit reduces activation of 5‑HT3/4 receptors on enteric neurons and smooth muscle, slowing peristalsis.

Predictions

1. Aged mice will show ↓ Piezo2 mRNA and protein specifically in the Piezo2⁺ EC subset, without changes in total EC number or TPH1 activity.
2. Colonic lysates from aged mice will exhibit ↑ HDAC4/5 occupancy at the Piezo2 promoter (ChIP‑qPCR).
3. Fecal microbiota transplantation (YMT) from young donors will normalize microbiome composition, lower SASP cytokines, reduce HDAC4/5 binding, and restore Piezo2 levels and 5‑HT release.
4. Pharmacological HDAC inhibition (e.g., tubastatin A) or senolytic treatment (dasatinib + quercetin) will mimic YMT effects on Piezo2 and motility.
5. EC‑specific overexpression of Piezo2 will rescue motility in aged mice even when the microbiome remains aged.

Experimental Design

• Mouse cohorts: Young (3 mo) vs. aged (24 mo) C57BL/6J; subsets receive YMT, senolytic cocktail, HDAC inhibitor, or vehicle.
• Readouts: (a) FACS‑sorted Piezo2⁺ ECs → qPCR/Western for Piezo2, TPH1; (b) Calcium imaging in EC organoids stimulated with isovalerate; (c) 5‑HT ELISA from basal supernatants; (d) ChIP‑qPCR for HDAC4/5 at Piezo2 promoter; (e) Colonic transit time via carmine red assay; (f) 16S rRNA sequencing to confirm microbiome shifts.
• Controls: Isotype ChIP, scrambled siRNA, germ‑free mice colonized with defined aged or young consortia.

Potential Outcomes and Interpretation

If aged ECs show reduced Piezo2 correlating with increased HDAC4/5 promoter binding, and YMT/senolitic/HDACi reverses these molecular changes while improving calcium flux, 5‑HT release, and transit time, the hypothesis is supported. Failure to rescue Piezo2 despite normalized microbiome would refute the mechanistic link, suggesting alternative age‑dependent pathways (e.g., direct oxidative damage to Piezo2). Conversely, if HDAC inhibition restores Piezo2 without altering microbiota, it affirms that inflammaging‑driven epigenetic silencing is sufficient to drive EC dysfunction. This work would define a testable axis connecting microbiome‑induced senescence, epigenetic regulation of mechanotransduction, and age‑related gastrointestinal motility decline.