Hypothesis

Urolithin A (UA) and the short‑chain fatty acid butyrate act together to amplify mitophagy in aged intestinal epithelial cells, but only when both metabolites reach the colonic lumen at physiologically relevant concentrations. We propose that simultaneous activation of AMPK‑SIRT1 signaling by UA and inhibition of mTORC1 by butyrate creates a cooperative switch that drives DRP‑1‑dependent mitochondrial fission and subsequent lysosomal engulfment, independent of PINK1/Parkin. This synergy restores ER‑mitochondria‑lysosome communication, reduces mitoROS, and lowers senescence‑associated secretory phenotype (SASP) in the gut epithelium, thereby attenuating systemic inflammaging.

Mechanistic Rationale

• UA‑driven calcium signaling promotes DRP‑1‑mediated fission and engages AMPK, ULK1/2, and SIRT1, biasing the response toward selective mitophagy over bulk autophagy[Urolithin A induces mitophagy via calcium‑DRP1 pathway].
• Butyrate is a known HDAC inhibitor that raises cellular acetyl‑CoA, activates AMPK, and suppresses mTORC1 signaling, thereby favoring autophagosome formation[Butyrate activates AMPK and inhibits mTORC1].
• Convergent AMPK activation phosphorylates ULK1/2 and stabilizes PINK1, while SIRT1 deacetylates downstream autophagy proteins, enhancing LC3B lipidation and p62 clearance[AMPK‑PINK1‑Parkin intersection].
• Simultaneous mTORC1 inhibition removes the brake on autophagy initiation, allowing the UA‑primed fission machinery to efficiently recruit lysosomal components[mTORC1 inhibition promotes autophagy].
• In vascular models, UA improves mitoROS and LC3B II/I ratios independently of PINK1/Parkin, suggesting that alternative pathways can sustain mitophagy when fission is strongly driven[UA vascular effects]; butyrate’s mTORC1 suppression may compensate for any PINK1/Parkin insufficiency.

Experimental Design

1. In vitro – Culture aged human colonic epithelial cells (or a suitable model such as Caco‑2 with induced senescence). Treat with: (a) UA alone, (b) butyrate alone, (c) UA + butyrate, (d) vehicle. Include conditions with AMPK inhibitor (Compound C) or SIRT1 inhibitor (EX‑527) to test dependence. Measure:
   • Mitochondrial mass (MitoTracker Green) vs. membrane potential (TMRM) to calculate mitophagy flux.
   • DRP‑1 phosphorylation (Ser616) and lysosomal colocalization (LAMP1).
   • p62/SQSTM1 turnover and LC3B II/I ratio via western blot.
   • SASP cytokines (IL‑6, IL‑8) in supernatant.
2. Ex vivo – Use mouse colonic crypts from aged (≥20 mo) mice treated intra‑luminally with UA, butyrate, or combination via rectal gavage. Assess mitophagy markers in isolated epithelial cells.
3. In vivo – Feed aged mice a diet supplemented with ellagitannins (to generate UA) and a butyrate‑producing prebiotic (e.g., resistant starch) versus controls. Include a group receiving a colon‑targeted UA‑butyrate nanoparticle to ensure luminal co‑delivery. Endpoints:
   • Fecal microbiota composition (16S rRNA) to confirm butyrate‑producer enrichment.
   • Colonic epithelial mitophagy (immunofluorescence for mito‑Keima).
   • Systemic inflammaging markers (serum TNF‑α, IL‑6).
   • Grip strength and treadmill endurance as functional readouts.

Expected Outcomes and Falsifiability

If the hypothesis is correct, the combination treatment will produce a >2‑fold increase in mitophagy flux compared with either metabolite alone, an effect abolished by AMPK or SIRT1 inhibition. Butyrate’s mTORC1 suppression will be essential, as rapamycin (mTORC1 inhibitor) should phenocopy the butyrate effect, while mTORC1 activation (via MHY1485) will block synergy. Failure to observe enhanced mitophagy or SASP reduction under co‑treatment, or demonstration that the effect persists despite AMPK/SIRT1 blockade, would falsify the proposed mechanism.

Broader Impact

Validating this synergy would provide a rational, microbiome‑based strategy to harness dietary polyphenols and fermentable fibers for targeted mitochondrial rejuvenation in the gut, a key origin of systemic aging signals. It also underscores the necessity of delivering postbiotics in formats that preserve their natural luminal ratios, moving beyond single‑compound supplementation toward precision postbiotic formulations.