A decline in intestinal autophagy initiates a retrograde signal through vagal afferents that suppresses central TFEB activity, leading to reduced brain autophagy and accelerated neurological aging.

Mechanistic Rationale

1. Gut autophagy loss → metabolite shift – Impaired ATG5/ATG7 function in enterocytes and enteric glia diminishes clearance of damaged mitochondria, increasing luminal succinate and decreasing butyrate production (3).
2. Vagal afferent activation – Elevated succinate activates succinate receptor 1 (SUCNR1) on vagal nodose neurons, increasing afferent firing (1). Concurrently, enteric glia release ATP via pannexin‑1 channels, stimulating P2X3 receptors on the same afferents.
3. Signal transduction to the brainstem – Vagal afferents release glutamate and PACAP in the nucleus tractus solitarius (NTS), which drives local norepinephrine release onto the dorsal motor vagal nucleus and subsequently to the paraventricular hypothalamus via the corticotropin‑releasing hormone (CRH) pathway.
4. Central TFEB inhibition – Norepinephrine activates β‑adrenergic receptors on hypothalamic neurons, raising cAMP and PKA activity. PKA phosphorylates TFEB at Ser142, promoting its cytoplasmic retention. Simultaneously, increased neuronal calcium activates calcineurin‑dependent deacetylation of HDAC6, which deacetylates TFEB lysine residues, further reducing its transcriptional activity (2).
5. Outcome – Reduced nuclear TFEB diminishes transcription of ATG5, ATG7, ATG12 and lysosomal genes, causing a brain‑wide autophagy deficit that mirrors gut impairment.

Testable Predictions

• Prediction 1: In aged mice, genetic restoration of ATG5 specifically in intestinal epithelial cells (using Villin‑Cre‑ATG5^fl/fl) will increase vagal afferent firing (measured by ex vivo vagal nerve electrophysiology) and elevate nuclear TFEB in the hypothalamus and cortex.
• Prediction 2: Pharmacological blockade of SUCNR1 (with NF‑56EJ40) or vagotomy will prevent the rescue of brain TFEB nuclear localization and autophagy flux despite intestinal ATG5 restoration.
• Prediction 3: Administering a β‑adrenergic antagonist (propranolol) intracerebroventricularly will block the downstream effects of vagal activation on TFEB phosphorylation, confirming the cAMP/PKA step.
• Prediction 4: HDAC6 inhibition (tubacin) in the brain will rescue TFEB nuclear localization and autophagy markers even when intestinal autophagy remains deficient, placing HDAC6 downstream of the vagal signal.

Falsifiability

If intestinal ATG5 restoration fails to alter vagal firing, or if vagal blockade does not impede the brain TFEB response, the hypothesis is falsified. Similarly, if β‑adrenergic or HDAC6 interventions do not affect brain TFEB status despite intact vagal signaling, the proposed mechanism is invalid.

Experimental Outline

1. Models – 24‑month‑old C57BL/6 mice; Villin‑Cre‑ATG5^fl/fl for gut‑specific rescue; SUCNR1^−/−; vagotomized cohorts.
2. Readouts –
   • Vagal afferent firing (in vivo electrophysiology).
   • Nuclear vs cytoplasmic TFEB (immunofluorescence, subcellular fractionation).
   • Brain autophagy flux (LC3‑II turnover with bafilomycin A1, p62 levels).
   • Behavioral assays (Morris water maze, grip strength) to link molecular changes to functional aging.
3. Interventions – Gut‑specific ATG5 rescue, SUCNR1 antagonist, vagotomy, propranolol, tubacin.
4. Statistical Plan – Two‑way ANOVA with factors (gut rescue × vagal blockade) and post‑hoc Tukey; n≥8 per group for adequate power.

This hypothesis shifts the longevity research focus from top‑down CNS modulation to a bottom‑up gut‑to‑brain autophagy axis, offering a clear, mechanistic route to test whether intestinal health sets the brain’s aging baseline.