Synchronous enhancer decay in gut and brain stem cells couples microbiome aging to systemic inflammaging

Hypothesis

Aging‑driven loss of 3D chromatin architecture at shared neuroendocrine enhancers occurs concurrently in intestinal stem cells (ISCs) and neural stem cells (NSCs), leading to coordinated dysregulation of gut‑derived serotonin/neuropeptide output and brain‑derived vagal tone. This parallel enhancer decay converts age‑related microbiome shifts into a chronic inflammaging signal that accelerates systemic decline.

Mechanistic Rationale

• Age‑related reduction of CTCF/cohesin binding (observed in muscle SCs [5]) weakens enhancer‑promoter looping at loci such as Tph1 (serotonin synthesis) and Sst (somatostatin) that are active in both enteroendocrine cells derived from ISCs and serotonergic neurons derived from NSCs.
• Concurrent Polycomb weakening (see Drosophila ISC data [3]) and promoter DNA hypermethylation of TFs like Egr1, Irf1, FosB [4] further destabilizes these enhancers, causing leaky enteroendocrine signaling and aberrant neuropeptide release.
• The gut microbiota itself ages, producing altered metabolite profiles (e.g., decreased SCFAs, increased LPS) that preferentially activate NF‑κB in cells whose neuroendocrine enhancers are already decoupled, creating a feed‑forward loop of inflammation.
• Because the same enhancer sets are regulated in both compartments, their synchronous erosion amplifies the signal beyond what either tissue could produce alone.

Predictions & Experimental Design

1. Multi‑omics time course – Perform Hi‑C, ATAC‑seq, and RNA‑seq on purified ISCs and NSCs from young (3 mo), middle (12 mo), and old (24 mo) mice. Identify enhancer‑gene contacts that are lost in both cell types at the same genomic loci (e.g., Tph1, Sst, Chat).
2. Microbiota transplant – Transfer feces from old mice into young germ‑free recipients that have been engineered to express a degron‑tagged RAD21 (cohesin subunit) in ISCs and NSCs. Predict that preserving cohesin will blunt enhancer loss despite an aged microbiome.
3. Functional read‑outs – Measure serum IL‑6, TNF‑α, fecal LPS, colonic serotonin, and vagal nerve activity. Predict that mice with intact cohesin show lower inflammaging markers and improved motor/cognitive performance.
4. Rescue experiment – Use CRISPR‑dCas9‑VP64 to forcibly loop the Tph1 enhancer to its promoter in aged ISCs/NSCs. Expect restoration of serotonin signaling and reduction of systemic inflammation even without microbiome alteration.

Potential Outcomes & Falsifiability

• If enhancer contacts decay synchronously and preserving them reduces inflammaging, the hypothesis is supported.
• If enhancer loss occurs independently in gut and brain, or preserving chromatin architecture fails to attenuate systemic inflammation despite correcting microbiome signals, the hypothesis is falsified.
• If microbiota‑derived metabolites directly drive inflammation irrespective of enhancer state, the mechanism would need revision.

This framework turns the 'bidirectional mess' of the gut‑brain axis into a testable chromatin‑centric model of aging.