Enteric TF Oscillator Hypothesis

Core idea We propose that aging flips a bistable transcription‑factor (TF) circuit in the myenteric nNOS+/calbindin+ intrinsic primary afferent neurons (IPANs) of the colon, converting a homeostatic gut‑brain signal into a self‑reinforcing inflammaging loop.

Mechanistic basis In young tissue, Sox10 and Phox2b form a cooperative TF module that maintains IPAN excitability, promotes nNOS‑derived nitric oxide, and supports barrier‑friendly microbial metabolites. Age‑associated epigenetic drift—marked by increased H3K27me3 at Sox10/Phox2b promoters and decreased DNA methylation at Nr4a2 enhancers—shifts the balance toward Nr4a2 dominance. Nr4a2 drives a senescence‑associated secretory phenotype (SASP) in IPANs, releasing IL‑1α, CCL2, and ATP that activate muscularis macrophages to produce IL‑6 and CSF‑1 (see [5]).

From circuit to signal The altered IPAN output changes vagal afferent firing patterns: reduced tonic nNOS signaling lowers afferent threshold, while sporadic ATP bursts evoke aberrant spikes that are interpreted by the brainstem as danger signals (see [2]). This skews microglial activation toward a pro‑inflammatory state, elevating cortical IL‑1β and TNF‑α (see [1]).

Feedback amplification Corticotropin‑releasing hormone released from the hypothalamus via the HPA axis increases gut permeability and suppresses antimicrobial peptide release, further favoring dysbiotic LPS producers. LPS reinforces muscularis macrophage CSF‑1 secretion, which feeds back to sustain Nr4a2 activity in IPANs via ERK‑mediated phosphorylation, closing a positive‑feedback loop (see [3]).

Testable predictions

1. Single‑cell multi‑omics (scRNA‑seq + scATAC‑seq) of colonic IPANs from 3‑month vs 24‑month mice will show: a) increased Nr4a2 transcript and chromatin accessibility; b) decreased Sox10/Phox2b activity; c) enrichment of SASP‑related gene signatures.
2. Colon‑specific Nr4a2 knock‑down (using AAV‑Cre in Nr4a2^fl/fl mice crossed with Advillin‑CreER) will reduce circulating IL‑6 by >30 % and improve performance in the Morris water maze in aged mice.
3. Optogenetic restoration of tonic nNOS‑dependent firing in IPANs (via Chrimson) will normalize vagal afferent burst frequency and lower microglial Iba1 intensity in the hippocampus.
4. Pharmacological blockade of CSF‑1R in aged animals will break the IPAN‑macrophage loop, decreasing colonic Nr4a2 expression without affecting neuronal numbers.

Falsifiability If aged mice show no shift in Nr4a2/Sox10/Phox2b balance, or if manipulating Nr4a2 fails to alter vagal firing, cytokine levels, or cognition, the hypothesis is refuted.

Implications This framework moves beyond metabolite‑centric views of the gut‑brain axis, positioning transcription‑factor bistability in a defined neuronal subset as the mechanistic hub that translates microbial aging into systemic inflammaging.