Hypothesis: Microbiome‑Driven Urolithin A/Phenylacetic Acid Ratio Determines Mitophagy Efficiency and Inflammaging

Core idea Urolithin A (UA) stimulates mitophagy via PINK1/Parkin activation and AMPK‑ULK1 signaling, while phenylacetic acid (PAA) promotes endothelial senescence and mitochondrial H₂O₂ production. We propose that the UA : PAA plasma ratio, shaped by individual gut microbiome composition, predicts the net effect on mitochondrial quality control and systemic inflammation. A high ratio yields enhanced mitophagy, reduced ROS, and improved CD8⁺ T‑cell naïvity; a low ratio blunts UA benefits and accelerates inflammaging.

Novel mechanistic insight UA restores ER‑mitochondria‑lysosome calcium signaling, a process that PAA‑induced oxidative stress disrupts by oxidizing ER calcium‑release channels (IP₃R) and lysosomal TRPML1. When UA predominates, calcium fluxes rebound, facilitating autophagosome‑lysosome fusion and mitophagy completion. Conversely, excess PAA sustains ER stress, blocks calcium‑dependent ULK1 recruitment, and shifts autophagy toward incomplete flux, thereby converting UA’s mitophagy signal into a futile cycle.

Testable predictions

1. In a cohort of middle‑aged adults, baseline plasma UA : PAA ratio will correlate positively with mitophagy markers (LC3‑II/I, PINK1 accumulation) in isolated PBMCs and negatively with senescence‑associated secretory phenotype (SASP) cytokines (IL‑6, IL‑8, TNF‑α).
2. Four‑week supplementation with 1000 mg/day UA will increase the UA : PAA ratio only in participants whose baseline microbiota harbor high‑abundance Ellagitannin‑utilizing taxa (e.g., Gordonibacter, Eggherthella) and low‑abundance phenylalanine‑degrading taxa (e.g., Clostridium spp., Morganella morganii).
3. Simultaneous administration of a prebiotic fiber that selectively suppresses phenylalanine‑degrading bacteria (e.g., resistant starch type 2) will raise the UA : PAA ratio in non‑responders to UA alone, rescuing mitophagy and lowering SASP.
4. Pharmacological chelation of ER calcium (using EG‑TA‑AM) will abolish the lifespan‑extending effect of UA in C. elegans even when the UA : PAA ratio is high, confirming calcium signaling as the downstream effector.

Experimental design

• Human trial: 120 participants aged 45‑65, stratified by baseline UA producer status (determined via fecal ellagitannin conversion). Randomize to (a) UA 1000 mg/day, (b) UA + resistant starch (30 g/day), (c) resistant starch alone, (d) placebo for 8 weeks. Measure plasma UA and PAA (LC‑MS/MS), shotgun metagenomics, PBMC mitophagy flow cytometry (mt‑Keima), SASP plasma panel, and CD8⁺ naïve/memory phenotype.
• Validation in model organisms: Treat C. elegans with UA ± PAA, with or without EG‑TA‑AM, and assess motility, lifespan, and mitochondrial ROS. Use ip‑3r and trpml1 mutants to test pathway specificity.

Falsifiability If UA supplementation improves mitophagy and reduces inflammaging independent of the UA : PAA ratio and microbiota composition, or if manipulating the ratio fails to alter downstream calcium signaling or mitophagy flux, the hypothesis would be refuted. Conversely, confirmation of the predicted correlations and rescue effects would support the mechanistic link between gut‑derived metabolite balance, ER‑lysosome calcium signaling, and systemic aging.