Sympathetic brain-to-gut signaling drives intestinal stem cell chromatin erosion during aging

Hypothesis: Chronic activation of hypothalamic sympathetic outflow in aging increases circulating norepinephrine, which binds β‑adrenergic receptors on intestinal stem cells (ISCs) and triggers a PKA‑HDAC signaling cascade that deacetylates histones at enhancers of stemness genes, causing progressive chromatin accessibility loss and functional decline.

Mechanistic rationale:

• Aged mice exhibit heightened hypothalamic sympathetic tone and elevated plasma norepinephrine[2].
• β‑adrenergic receptor activation raises cAMP, activating PKA, which phosphorylates class IIa HDACs (HDAC4/5), promoting their nuclear translocation and histone deacetylase activity.[3]
• Histone deacetylation reduces nucleosome openness at enhancer regions bound by pro‑quiescence transcription factors (e.g., FOXO1, HNF4α), a pattern mirrored in aged neural stem cells where accessibility loss correlates with transcriptional dysfunction[7].
• Loss of enhancer accessibility diminishes expression of ISC maintenance genes (Lgr5, Ascl2), impairing regenerative capacity and promoting barrier dysfunction.

Experimental plan:

1. Model: Use aged (20‑month) C57BL/6 mice with chemogenetic inhibition (hM4Di DREADDs) of hypothalamic sympathetic neurons (via Dbh‑Cre) to reduce norepinephrine release.
2. Readouts:
   • Plasma norepinephrine ELISA.
   • ATAC‑seq on purified intestinal crypt stem cells (Lgr5‑EGFP+).
   • RNA‑seq for stemness and differentiation markers.
   • Functional assays: dextran sulfate sodium‑induced colitis recovery, EdU incorporation, crypt proliferation.
   • Lifespan and healthspan monitoring (frailty index, gait speed).
3. Controls: Vehicle‑treated aged mice, young (3‑month) mice, and aged mice treated with a β‑blocker (propranolol) to pharmacologically mimic sympathetic inhibition.

Predicted outcomes:

• Sympathetic inhibition will normalize plasma norepinephrine levels to young‑adult ranges.
• ATAC‑seq will show a significant regain of accessibility at ISC‑specific enhancers (fold‑change >1.5, FDR<0.05) compared with untreated aged controls.
• Corresponding upregulation of Lgr5, Ascl2, and downstream proliferative genes.
• Improved crypt regeneration, faster colitis recovery, and extended median lifespan (>15% increase).
• β‑blocker treatment should phenocopy genetic inhibition, confirming receptor specificity.

Falsifiability:

• If sympathetic inhibition fails to restore chromatin accessibility or ISC function despite verified norepinephrine reduction, the hypothesis is refuted.
• Conversely, if accessibility is restored without functional improvement, downstream mechanisms beyond chromatin state would be implicated, requiring model revision.

This brain‑to‑gut, neuroepigenetic framework flips the prevailing gut‑centric view of aging and offers a testable, targetable node for longevity interventions.