Hypothesis

We hypothesize that intestinal autophagy generates a diffusible signal that travels to the brain through vagal afferents, where it alters the BCL-2/Beclin-1 complex in neurons to promote autophagy and thereby extend lifespan. This signal is a microbiota‑derived metabolite whose production depends on intestinal autophagy, and its action requires intact vagal signaling and neuronal AMPK activity.

Mechanistic Rationale

1. Intestinal autophagy controls metabolite release – Autophagy in gut epithelial cells shapes the composition of the luminal microbiome and the secretion of specific metabolites (e.g., indole‑3‑propionic acid, serotonin, or short‑chain fatty acids) that can cross the epithelial barrier 1. When intestinal autophagy is impaired, the profile of these signaling molecules shifts toward pro‑inflammatory ligands.
2. Metabolite activates vagal afferents – Certain metabolites stimulate receptors on vagal sensory neurons (e.g., 5‑HT receptors for serotonin, FFAR2 for acetate) leading to action potential propagation to the nucleus tractus solitarius 2. Vagal firing triggers downstream neuromodulatory pathways that converge on the hypothalamus and cortex.
3. Vagal signaling engages neuronal AMPK – Activation of vagal afferents increases norepinephrine release in the brainstem, which activates AMPK in downstream neurons via β‑adrenergic signaling. AMPK phosphorylates Beclin‑1 at Ser93/Ser96, reducing its affinity for BCL‑2 and liberating Beclin‑1 to initiate autophagosome formation 3.
4. BCL-2/Beclin-1 modulation determines neuronal autophagy – In aging, NF‑κB‑driven BCL‑2 overexpression sequesters Beclin-1, suppressing autophagy and fostering inflammasome activation [4,5]. By decreasing BCL‑2 binding, the gut‑derived signal restores neuronal autophagy, mitigating proteostatic stress and inflammasome‑mediated inflammation.
5. Feedback to the gut – Enhanced neuronal autophagy improves autonomic output (e.g., vagal efferent tone), which can further support intestinal barrier function and autophagy, creating a positive gut‑brain loop.

Testable Predictions

• Prediction 1: Intestine‑specific overexpression of bec-1 (or atg-18) in C. elegans will increase lifespan only when neuronal bec-1 is functional; neuronal bec-1 knockdown will abolish the lifespan extension.
• Prediction 2: The lifespan benefit of intestinal bec-1 overexpression will be lost after subdiaphragmatic vagotomy or chemogenetic inhibition of vagal afferents (e.g., using halorhodopsin in C. elegans homologs).
• Prediction 3: Supplementation with the candidate metabolite (e.g., indole‑3-propionic acid) will mimic the lifespan extension of intestinal autophagy enhancement, and this effect will require neuronal AMPK activation (blocked by Compound C).
• Prediction 4: Neuronal Beclin‑1 will show reduced co‑immunoprecipitation with BCL‑2 and increased phosphorylation at AMPK sites in worms with intact intestinal autophagy; this shift will be reversed in vagal‑deficient animals.
• Prediction 5: Metabolomic profiling of long‑lived worms with intestine‑specific autophagy upregulation will reveal elevated levels of the putative signal compared with controls.

Experimental Approach (Outline)

• Use tissue‑specific CRISPR or RNAi to manipulate bec-1 in intestine versus neurons.
• Measure lifespan, motility, and autophagy reporters (LGGP‑1::GFP, mCherry::LGGP‑1) in each condition.
• Perform vagal ablation via laser surgery or optogenetic silencing; assess whether autophagy reporters and lifespan changes persist.
• Administer purified metabolites and test dependence on neuronal AMPK via pharmacological inhibition or aak-2 mutation.
• Conduct co‑immunoprecipitation and phospho‑specific Western blotting for Beclin‑1 in isolated neurons.
• Conduct LC‑MS metabolomics on intestinal extracts to correlate metabolite levels with longevity phenotypes.

Falsifiability

If intestinal autophagy enhancement extends lifespan independently of neuronal autophagy, vagal integrity, or AMPK activity, the hypothesis is falsified. Likewise, if vagal stimulation fails to alter neuronal BCL-2/Beclin-1 binding or autophagic flux, the proposed gut‑to‑brain signaling route is insufficient.

Implications

Confirming this model would invert the prevailing top‑down view of the gut‑brain axis in aging, positioning intestinal autophagy as a primary regulator of neuronal proteostasis. Interventions targeting the gut microbiome, intestinal autophagy, or specific metabolite production could then be leveraged to modulate brain aging without direct neuronal manipulation.