Hypothesis

In aging cells, the selective autophagy hierarchy becomes temporally disordered: essential organelles are prematurely degraded while damaged substrates persist, because the switching between cargo receptors (e.g., p62/SQSTM1 → OPTN/NDP52 → NBR1) is mistimed rather than globally suppressed. This mistripping drives age‑related decline by creating a mismatch between cellular needs and autophagic output.

Mechanistic Basis

• Normal triage: Under nutrient stress, cells first ubiquitinate lipid droplets and peroxisomes, recruiting p62/NBR1 for lipophagy; sustained stress then tags mitochondria, recruiting OPTN/NDP52 for mitophagy; finally, protein aggregates are cleared via aggrephagy using shared autophagy machinery. The order is set by kinase‑dependent phosphorylation of cargo receptors that modulates their affinity for ubiquitin and LC3 (e.g., AMPK‑mediated p62 phosphorylation promotes lipophagy, while mTORC1 inhibition enables OPTN activation for mitophagy).
• Aging defect: Chronic low‑grade mTORC1 activity and reduced AMPK signaling in aged tissues blunt the phosphorylation cascade, causing receptors to retain affinity for early‑stage substrates even when later damage accumulates. Consequently, p62/NBR1 continue to drive lipophagy while OPTN/NDP52 remain under‑engaged, leading to persistent mitochondrial damage despite adequate lipid turnover.
• Digital twin implication: The ratio of phosphorylated to total cargo receptors (p‑p62/p62, p‑OPTN/OPTN) serves as a dynamic readout of the autophagic triage state. Incorporating these ratios into a digital twin allows prediction of which autophagy modulator (rapamycin, spermidine, or an AMPK activator) will correctly re‑sequence substrate consumption for a given individual's damage profile.

Testable Predictions

1. In young mouse liver under fasting, p62 peaks early (0‑2 h) followed by OPTN/NDP52 accumulation (4‑6 h); in aged mice, p62 remains high while OPTN/NDP52 fails to rise.
2. Pharmacological AMPK activation (AICAR) in aged mice restores the normal temporal shift of receptor activation and reduces mitochondrial ROS without altering overall autophagic flux (LC3‑II turnover).
3. Digital twins calibrated with baseline p‑p62/p62 and p‑OPTN/OPTN ratios will accurately predict individual responsiveness to rapamycin versus spermidine: high p‑p62/p62 predicts better response to spermidine (enhanced lipophagy), whereas high p‑OPTN/OPTN predicts rapamycin sensitivity (mitophagy boost).
4. Causally, expressing a phospho‑deficient p62 mutant (S349A) in young hepatocytes mimics the aged triage defect, causing early lipid clearance but delayed mitophagy and increased apoptosis under stress.

Experimental Design

• Model: Primary hepatocytes from 3‑month vs 24‑month C57BL/6 mice; treat with EBSS to induce autophagy.
• Readouts: Time‑course immunoblotting for total and phosphorylated p62, OPTN, NDP52; subcellular fractionation to assess lipid droplet (BODIPY) and mitochondrial (MitoTracker) mass; Seahorse OXPHOS measurements; LC3‑II flux with bafilomycin A1.
• Interventions: AICAR (AMPK activator), rapamycin (mTORC1 inhibitor), spermidine (epigenetic autophagy inducer).
• Digital twin: Build a personalized ODE model linking receptor phosphorylation states to substrate degradation rates; fit to young data, then test predictive accuracy on aged data with and without interventions.
• Outcome: Successful rescue of the temporal receptor order and correlated functional improvement will falsify the null hypothesis that aging merely reduces overall autophagy capacity.

Implications

If validated, this hypothesis reframes age‑related autophagy decline as a signaling‑timing error rather than a cargo‑capacity loss. It offers a biomarker‑driven strategy to match autophagy modulators to an individual's dominant damage type, moving digital twins from generic age predictors to precision gerotherapeutic advisors.