Hypothesis

A distinct population of senescent enteric glial cells, defined by p16^INK4a expression, secretes a localized SASP that travels along vagal afferents to induce region‑specific microglial activation and transcriptomic aging in the brain.

Rationale

• Spatial transcriptomics shows p16^+ senescent cells have tissue‑restricted secretomes while p21^+ cells release a conserved SASP (1).
• The vagus nerve transmits gut‑derived pathology to the brain; vagotomy blocks AD‑like pathology in mice (2).
• Aging remodels the enteric nervous system, shifting lineage composition and causing a 52% loss of neurons accompanied by M1‑skewed macrophages (3,4).
• Enteric glia, derived from neural crest, persist throughout the gut and can become senescent, yet their contribution to the gut‑brain axis has not been mapped.

Novel Mechanistic Insight

We propose that senescent enteric glia, unlike senescent epithelial or immune cells, retain the ability to release metabolites and extracellular vesicles that preferentially engage Toll‑like receptor 4 on vagal terminals. This triggers a NF‑κB‑dependent calcium wave that propagates to the dorsal motor nucleus of the vagus, then to downstream brain nuclei such as the hippocampus and prefrontal cortex. Because glial SASP includes matrix metalloproteinases and specific chemokines (e.g., CCL2) that differ from epithelial SASP, the signal imposes a region‑specific aging signature on microglia that mirrors the spatial pattern seen in spatial aging clocks (5).

Testable Predictions

1. In aged mice, p16^+ enteric glial density will correlate with increased p‑NF‑κB signaling in the ipsilateral vagal ganglion and with elevated microglial Iba1^+ activation in hippocampus and cortex.
2. Selective ablation of p16^+ enteric glia (using a p16‑3MR suicide system) will reduce vagal‑dependent phospho‑tau accumulation in the brain without affecting peripheral inflammation.
3. Spatial transcriptomics of vagal ganglia after gut‑specific LPS challenge will reveal a ligand‑receptor pair between glial‑derived CCL2 and neuronal CCR2 that predicts downstream microglial aging clusters in the brain.
4. Vagotomy will uncouple the glial‑SASP signal, abolishing the brain‑region‑specific aging transcriptomic changes despite persistent gut senescence.

Experimental Design (outline)

• Use p16‑3MR mice; administer ganciclovir to clear p16^+ cells in the gut only (via intrarectal delivery) and compare to systemic clearance.
• Measure vagal ganglion phosphorylated NF‑κB, retrograde tracer uptake, and brain region‑specific microglial transcriptomes (single‑cell spatial RNA‑seq).
• Perform blockade of CCR2 with antagonists to test the ligand‑receptor hypothesis.
• Include young controls, aged wild‑type, and aged vagotomized groups.
• Statistical analysis: linear regression linking glial senescence load to brain region‑specific aging scores; significance set at p<0.05.

If the predictions hold, the hypothesis establishes senescent enteric glia as a spatially encoded source of inflammaging that travels via the vagus, offering a precise target for intercepting brain aging.