Hypothesis

Aging diminishes the flux of microbiota‑derived acetate into neuronal nuclei through reduced ACSS2 translocation, causing localized histone hypoacetylation at genes governing synaptic plasticity and stress resilience. Restoring this gut‑to‑brain acetate supply—either by reshaping the microbiome or by delivering acetate precursors that bypass intestinal uptake—replenishes nuclear acetyl‑CoA, reactivates histone acetylation, and improves cognitive function independently of anti‑inflammatory pathways.

Mechanistic Model

1. Microbial source – SCFA‑producing taxa (e.g., Faecalibacterium, Roseburia) generate acetate that enters the portal circulation.
2. Cellular uptake – Acetate crosses the blood‑brain barrier via monocarboxylate transporters (MCT1/2) and is captured by neurons.
3. Nuclear routing – Neuronal ACSS2, activated by rising intracellular acetate, translocates to the nucleus where it acetylates histone H3K9/K27 at promoters of neuroprotective genes (e.g., Bdnf, Fbxo7, Tfeb). This step is potentiated by calcium‑calmodulin‑dependent kinase II (CaMKII) signaling that is triggered by vagal afferent firing in response to gut luminal SCFA levels.
4. Epigenetic outcome – Increased histone acetylation loosens chromatin, facilitating transcription of autophagy‑lysosomal genes and antioxidant enzymes, thereby mitigating age‑related epigenetic drift.
5. Age‑related breakpoints – (a) Microbiome shifts lower acetate output; (b) neuronal ACSS2 expression and nuclear import decline with age; (c) vagal tone weakens, reducing the activity‑dependent cue for ACSS2 nuclear entry.

Testable Predictions

• Prediction 1: In 24‑month‑old mice, nuclear ACSS2 levels and H3K9ac at the Bdnf promoter will be ~40 % lower than in 3‑month‑old controls; fecal acetate concentrations will correlate positively with these nuclear markers (r > 0.5, p < 0.01).
• Prediction 2: Oral administration of a high‑acetate prebiotic (e.g., inulin‑acetate conjugate) for 8 weeks will raise plasma acetate by ~30 %, increase neuronal nuclear ACSS2 and H3K9ac, and improve performance in the Morris water maze (escape latency ↓ 25 % vs. vehicle). This cognitive rescue will be absent in mice with subdiaphragmatic vagotomy.
• Prediction 3: Neuron‑specific ACSS2 knock‑out (Camk2a‑Cre; Acss2^fl/fl) will abolish the prebiotic‑induced histone acetylation and memory benefits despite elevated systemic acetate, confirming the necessity of neuronal ACSS2.
• Prediction 4: Centenarian‑derived fecal microbiota transplanted into aged mice will elevate colonic acetate production, boost neuronal nuclear ACSS2, and delay epigenetic age markers (e.g., Horvath clock) in the hippocampus by ~15 %.

Potential Caveats

• Acetate can also serve as a fuel for oligodendrocytes and astrocytes; cell‑type‑specific manipulations may be needed to isolate neuronal effects.
• Chronic high acetate might feedback‑inhibit ACSS2 via product accumulation; dosing regimens should avoid supraphysiological concentrations.
• Vagal afferents convey multiple gut signals (e.g., serotonin, peptide YY); isolating the acetate‑specific contribution may require pharmacological blockade of acetate receptors (FFAR2) in the nodose ganglion.

This hypothesis flips the conventional top‑down view of brain‑gut communication, positioning the intestine as a metabolic epigenostat that sets the brain’s chromatin landscape. Validating it would redirect longevity strategies toward microbiome‑engineered acetate supplementation as a direct means to oppose brain aging.