Hypothesis

Chronic elevation of gut‑derived kynurenine (Kyn) activates vagal afferents, driving a NTS→LC→EC signaling cascade that locally amplifies GSK3β‑mediated tau phosphorylation, making the entorhinal cortex (EC) the primary site of tau initiation in aging.

Mechanistic Rationale

1. Gut source of Kyn – Indoleamine 2,3‑dioxygenase (IDO1) in intestinal epithelial cells and immune cells converts tryptophan to Kyn under inflammatory conditions; circulating Kyn crosses the intestinal lumen and reaches the lamina propria where it can stimulate vagal afferent terminals[6].
2. Vagal afferent preference – Approximately 80% of vagus fibers are afferent, with terminations concentrated in the nucleus tractus solitarius (NTS)[2]. Kyn binds to the aryl hydrocarbon receptor (AhR) on vagal sensory neurons, increasing their firing rate and glutamate release onto NTS neurons.
3. NTS‑LC‑EC axis – NTS projections excitatory to the locus coeruleus (LC) via glutamatergic synapses; LC noradrenergic neurons then send dense, modality‑specific projections to the lateral entorhinal cortex, a region that shows the earliest tau pathology[3][4]. Noradrenaline potentiates astrocytic GABA release, which paradoxically enhances neuronal GSK3β activity through reduced inhibitory tone on PP2A phosphatase, fostering tau hyperphosphorylation.
4. Local amplification – Elevated Kyn also reaches the EC via systemic circulation and directly activates neuronal AhR, synergizing with noradrenergic signaling to sustain GSK3β activation and inhibit PP2A, creating a feed‑forward loop that seeds tau aggregation preferentially in EC.

Testable Predictions

• Prediction 1: Chemogenetic inhibition of vagal afferent terminals (e.g., using hM4Di DREADDs under the Advillin promoter) will reduce colonic Kyn‑induced EC tau phosphorylation without affecting tau levels in hippocampal CA1.
• Prediction 2: Selective antagonism of AhR in the NTS (via intra‑NTS injection of CH‑223191) will blunt LC firing and prevent EC‑specific tauopathy despite high systemic Kyn.
• Prediction 3: Administering an IDO1 inhibitor (e.g., epacadostat) peripherally will lower portal Kyn, decrease vagal afferent firing, and attenuate EC tau accumulation in aged mice.
• Prediction 4: Vagotomy (subdiaphragmatic) will abolish the EC‑specific tau phenotype induced by chronic gut inflammation, while sparing tau pathology induced by direct hippocampal injection of tau fibrils.

Experimental Design (mouse model)

1. Groups (n=10 per group): WT aged mice receiving (a) vehicle, (b) DSS‑induced colitis to raise gut Kyn, (c) colitis + vagal afferent hM4Di + CNO, (d) colitis + NTS AhR antagonist, (e) colitis + IDO1 inhibitor, (f) subdiaphragmatic vagotomy + colitis.
2. Readouts (after 8 weeks):
   • Vagal afferent firing measured ex vivo via extracellular recordings from nodose ganglia.
   • LC noradrenaline levels (HPLC).
   • EC phosphorylated tau (AT8 immunostaining) vs. hippocampal tau (quantified by confocal microscopy).
   • Spatial memory performance (Morris water maze) linked to grid‑cell integrity (in vivo calcium imaging of EC layer II stellate cells).
3. Statistical analysis: Two‑way ANOVA with factors treatment and region, followed by Tukey post‑hoc.

Falsifiability

If vagal afferent inhibition, NTS AhR blockade, or peripheral IDO1 suppression fails to reduce EC‑specific tau phosphorylation relative to colitis controls, the hypothesis that gut‑derived Kyn propagates tau via the vagal‑NTS‑LC‑EC axis is falsified. Conversely, a robust, region‑specific rescue would support the model and suggest that longevity strategies targeting intestinal tryptophan metabolism or vagal afferent signaling could delay EC‑initiated tauopathy.