Hypothesis

Chronic activation of JNK/AP-1 in intestinal epithelial cells after transient barrier injury initiates a self‑amplifying circuit that reprograms CNS microglia into a senescent state, which then reinforces epithelial JNK signaling via vagal and humoral pathways, thereby converting acute stress resolution into persistent inflammaging.

Mechanistic Model

1. Acute gut injury → mitochondrial ROS → JNK phosphorylation → AP‑1–driven transcription of pro‑inflammatory cytokines (IL-1β, IL-6) and danger‑associated molecular patterns (HMGB1, S100A8/9) [1].
2. Circulating mediators cross a compromised blood‑brain barrier or signal via afferent vagal fibers, activating JNK/AP-1 in microglia [2].
3. Microglial JNK/AP-1 shifts cells toward a senescence‑associated secretory phenotype (SASP) that includes TNF‑α, IL-1β, glutamate, and increased release of corticotropin‑releasing hormone (CRH) [6].
4. Brain‑derived signals feed back to the gut: (a) vagal efferent activity, normally anti‑inflammatory via α7nAChR, becomes desensitized under chronic CRH exposure, reducing cholinergic restraint; (b) circulating CRH and substance P increase epithelial permeability and reactivate JNK/AP-1 [3].
5. The loop sustains epithelial hyperplasia, stem‑cell exhaustion, and microglial senescence, locking both tissues into a chronic inflammatory state that drives systemic inflammaging [4], [5].

Testable Predictions

• Prediction 1: In aged mice, intestinal epithelial pJNK levels will positively correlate with microglial pJNK and SASP markers (e.g., p16^INK4a^, Ccl2) across individuals; disrupting the correlation (e.g., by epithelial‑specific JNK1 KO) will uncouple the two tissues.
• Prediction 2: Pharmacological inhibition of microglial JNK (using a brain‑penetrant JNK inhibitor) after induced colitis will reduce gut epithelial pJNK and barrier permeability, even when the initial injury is unchanged.
• Prediction 3: Chemogenetic activation of vagal efferents will exacerbate epithelial JNK activation and cytokine release in wild‑type mice but not in mice with epithelial‑specific JNK deletion.
• Prediction 4: Microbiome “age” estimated by epigenetic clocks will predict the strength of the brain‑gut JNK loop; transplanting aged microbiota into young germ‑free recipients will accelerate the onset of correlated epithelial and microglial JNK activation.

Potential Pitfalls & Controls

• Off‑target effects of JNK inhibitors require use of genetic models (epithelial‑specific Jnk1^fl/fl^ Vil‑Cre^+, microglia‑specific Jnk1^fl/fl^ Cx3cr1‑Cre^+).
• Vagal manipulations must confirm specificity via electrophysiology and exclude sympathetic confounders (e.g., propranolol co‑administration).
• Microbiota transfer experiments should control for diet and circadian influences by pair‑feeding and housing under identical light‑dark cycles.

If any of these predictions fail—particularly if blocking microglial JNK does not attenuate gut epithelial JNK activation or barrier dysfunction—the hypothesis that a JNK/AP-1–mediated brain‑gut feedforward loop underlies the transition from acute stress to chronic inflammaging would be falsified.