Hierarchical Triage in Selective Autophagy: A Testable Model of Metabolite‑Reprogramming

Core hypothesis

Urolithin A reconfigures the selective autophagy hierarchy by preferentially activating the mitophagy receptor BNIP3 through AMPK‑dependent phosphorylation, thereby suppressing ER‑phagy downstream of TNF‑α signaling and preserving mitochondrial homeostasis in aged skeletal muscle.

Mechanistic rationale

1. Signal integration at the receptor level – Both BNIP3 (mitophagy) and OPTN (ER‑phagy) compete for limited LC3/GABARAP pools. Their affinity is modulated by phosphorylation: AMPK phosphorylates BNIP3 at Ser17 and Ser24, increasing its LC3‑interacting region (LIR) affinity, while TBK1 phosphorylates OPTN at Ser473/Ser478, enhancing ubiquitin binding but reducing LC3 access under low‑energy conditions.
2. Urolithin A as a metabolic cue – UA activates AMPK and inhibits mTORC1, shifting the cellular energy sensor state. It's known that this predicts increased AMPK‑mediated BNIP3 phosphorylation and decreased TBK1 activity toward OPTN (via reduced upstream IKK signaling), tilting the receptor competition toward mitophagy.
3. Temporal hierarchy disruption – In inflammatory stress, TNF‑α normally drives rapid TBK1 activation, promoting OPTN‑mediated ER‑phagy before BNIP3 can engage mitochondria. Don't overlook that if UA‑induced AMPK activation precedes or outweighs TBK1 signaling, the order reverses: mitochondria are cleared first, limiting ROS‑driven ER stress and preventing the secondary ER‑phagy wave that otherwise depletes ER reserves and fuels inflammaging.

Predictions and falsifiable tests

• Prediction 1: In C2C12 myotubes treated with TNF‑α, UA will increase phospho‑BNIP3 (Ser17/Ser24) and decrease phospho‑OPTN (Ser473/Ser478) within 30 min, detectable by Western blot.
• Prediction 2: Knock‑in of phospho‑deficient BNIP3 (S17A/S24A) will abolish UA‑induced mitophagy flux (measured by mt‑Keima) and restore ER‑phagy dominance (ER‑Tracker‑Green clearance) despite UA presence.
• Prediction 3: Pharmacological inhibition of TBK1 (using MRT67307) will phenocopy UA’s effect on receptor phosphorylation, whereas TBK1 overexpression will override UA’s bias toward mitophagy.
• Prediction 4: Aged mice (24 mo) fed UA‑supplemented diet will show higher BNIP3 phosphorylation, lower OPTN phosphorylation, improved mitochondrial respiration (Seahorse), and reduced ER‑stress markers (CHOP, BiP) in gastrocnemius versus controls; the effect will be lost in muscle‑specific AMPKα1/α2 double knockouts.

Experimental outline

1. Cellular assays – Treat C2C12 myotubes with TNF‑α (10 ng/mL) ± UA (10 µM) for 0‑2 h. Harvest lysates for phospho‑specific antibodies. Perform mt‑Keima and ER‑phagy (GFP‑SEC61b) flux assays.
2. Genetic manipulation – CRISPR‑knock‑in BNIP3 S17A/S24A and OPTN S473A/S478A lines; assess flux as above.
3. In vivo validation – Feed young (6 mo) and aged (24 mo) C57BL/6 mice control or UA (500 ppm) diet for 4 mo. Isolate muscle, quantify receptor phosphorylation, mitochondrial function, ER stress, and grip strength.
4. Rescue experiments – Treat aged UA‑fed mice with TBK1 activator (e.g., glutathionylation inducer) to test whether forcing ER‑phagy reverses UA benefits.

If UA fails to alter receptor phosphorylation hierarchy or if phospho‑deficient BNIP3 does not block its mitophagy effect, the hypothesis is falsified. Conversely, consistent shifts in receptor modification correlating with functional outcomes would support the model that metabolite‑driven reprogramming of selective autophagy hierarchy determines cellular fate during aging.