Hypothesis: Oral urolithin A (UA) or its ellagitannin precursors act as a gut‑derived mitophagy signal that sets the brain’s mitochondrial homeostasis baseline, thereby driving longevity benefits independent of central nervous system‑directed interventions.

Mechanistic rationale

• UA is produced by specific gut bacteria from dietary polyphenols (e.g., punicalagins) and reaches systemic circulation via the portal vein.
• Recent work shows UA induces mitophagy in muscle and macrophages through the SIRT1‑PINK1‑Parkin axis, but its transport across the blood‑brain barrier (BBB) and direct neuronal effects remain untested.
• We propose that UA crosses the BBB via monocarboxylate transporters (MCT1) expressed on brain endothelial cells, where it activates SIRT1 in pericytes and astrocytes, leading to up‑regulation of PINK1‑mediated mitophagy in microglia and neurons.
• Enhanced mitophagy reduces mitochondrial ROS, lowers endothelial senescence, and tightens BBB integrity by increasing claudin‑5 and occludin expression (see [1] for SCFA‑BBB parallels).
• A secondary gut‑mediated effect occurs when UA‑producing taxa (e.g., Gordonibacter) shift the metabolite profile: they decrease phenylalanine‑catabolizing bacteria, lowering circulating phenylacetic acid (PAA) that drives endothelial senescence ([2]).
• The combined increase in neuronal mitophagy and decrease in circulating PAA restores neurovascular unit function, which in turn normalizes vagal afferent firing to the nucleus tractus solitarius, resetting autonomic tone toward parasympathetic dominance.
• This vagal feedback further suppresses pro‑inflammatory cytokine release from the gut, creating a positive feedback loop that sustains low‑grade inflammation resolution.

Testable predictions

1. In aged mice, chronic oral UA (50 mg kg⁻¹ day⁻¹) will increase brain SIRT1 activity and PINK1‑dependent mitophagy markers (LC3‑II/I ratio, phospho‑Ubiquitin) in isolated neurons and microglia compared with vehicle.
2. The same treatment will reduce circulating PAA levels and elevate fecal butyrate concentrations, reflecting a shift away from phenylalanine‑catabolizing taxa.
3. BBB integrity, measured by Evans blue extravasation and claudin‑5 immunostaining, will improve in UA‑treated mice.
4. Subdiaphragmatic vagotomy will abolish the UA‑induced improvements in brain mitophagy, BBB integrity, and spatial memory performance (Morris water maze), confirming the dependence on vagal afferent signaling.
5. Germ‑free mice colonized with a defined UA‑producing consortium (e.g., Gordonibacter pamelaeae + Ellagibacter isourolithinifaciens) will recapitulate the neuroprotective effects of UA supplementation, whereas colonization with a PAA‑elevating consortium will not.

Falsification criteria

• If UA fails to increase neuronal mitophagy markers or BBB tight junction expression despite achieving plasma concentrations >1 µM, the hypothesis is unsupported.
• If vagotomy does not diminish the cognitive or neurovascular benefits of UA, the proposed afferent pathway is not required.
• If microbiota shifts do not correlate with reduced PAA or increased SCFA, the gut‑mediated metabolite arm is unlikely.

By positioning UA as a gut‑derived upstream signal that directly tunes brain mitochondrial quality control and indirectly shapes the circulating metabolite milieu, this hypothesis inverts the typical CNS‑outflow view of the gut‑brain axis and suggests a bottom‑up longevity stack centered on polyphenol‑microbiome interactions.