The gut‑brain axis is often viewed as a conduit for microbial metabolites to shape neural function, yet aging research neglects the reverse flow: brain‑derived systemic signals that could directly remodel gut stem cell epigenetics. We hypothesize that chronic activation of the hypothalamic‑pituitary‑adrenal (HPA) axis in aged brains elevates circulating glucocorticoids, which bind glucocorticoid receptors (GR) in intestinal stem cells (ISCs) and recruit histone deacetylase (HDAC) complexes to Wnt‑responsive enhancers. This recruitment removes acetyl groups from histone H3K27ac, leading to chromatin compaction, reduced accessibility at TCF/LEF binding sites, and a downstream decline in Wnt‑driven transcription that underlies ISC loss of regenerative capacity.

Key predictions follow from this model. First, aged mice will show increased GR occupancy and HDAC2 binding at Wnt enhancer regions in purified Lgr5+ ISCs, detectable by sequential ChIP‑ATAC (ChIP‑seq for GR/HDAC2 followed by ATAC‑seq on the same sorted cells). Second, pharmacologic blockade of GR signaling (e.g., with mifepristone) or genetic deletion of GR specifically in ISCs should prevent age‑related loss of ATAC‑seq signal at Wnt loci and preserve ISC proliferation, even without altering gut microbiota composition. Third, restoring youthful brain glucocorticoid rhythm—via timed low‑dose corticosterone replacement or optogenetic suppression of HPA axis activity in aged animals—will reopen those enhancers, rescuing both chromatin accessibility and functional readouts such as organoid formation speed and mucosal repair after injury.

These experiments are feasible with existing tools: single‑cell ATAC‑seq on FACS‑sorted ISCs from young and old mice, CUT&RUN for GR and HDAC2, and lineage‑tracing reporters (e.g., Axin2‑LacZ) to quantify Wnt activity. Falsifiable outcomes include: (a) no change in GR/HDAC2 binding at Wnt enhancers despite aging, (b) GR inhibition failing to improve ATAC‑seq signal or ISC function, or (c) normalization of glucocorticoid rhythms not reversing chromatin closure. If any of these occur, the hypothesis would be weakened, prompting exploration of alternative brain‑derived factors (e.g., IGF‑1, serotonin) or non‑endocrine routes (vagal afferents). Conversely, confirmation would reorient longevity strategies toward brain‑centric interventions—such as HPA‑axis modulators, GR antagonists, or central vagal stimulation—as primary drivers of gut epithelial rejuvenation, effectively flipping the gut‑brain axis from a bottom‑up to a top‑down therapeutic paradigm.