Hypothesis

Autophagy functions as a competitive triage system where cargo receptors vie for limited LC3‑II binding sites. With aging, the expression and post‑translational modification of key receptors (p62/SQSTM1, OPTN, NDP52, TAX1BP1, FAM134B, ATG39/40) become skewed, altering the zero‑sum game and causing chronic misallocation of autophagic capacity. We propose that this receptor‑level dysregulation—not a global loss of autophagy flux—drives age‑associated organelle selectivity, preferentially degrading mitochondria while sparing ER or protein aggregates, thereby amplifying ROS‑mediated damage and inflammaging.

Mechanistic basis

1. Receptor saturation kinetics shift – In young cells, stress‑specific induction balances receptor levels (e.g., starvation ↑ ATG39/40, proteotoxic stress ↑ p62). Aging blunts stress‑responsive transcription factors (FOXO, NRF2) and elevates basal p62 phosphorylation, increasing its affinity for LC3 and oligomerization propensity. This gives p62 a persistent competitive edge, sequestering LC3‑II even when aggregate load is low.

2. Spatial exclusion amplified – Receptors such as FAM134B (ER‑sheet) and RTN3L (ER‑tubule) normally partition ER subdomains. Age‑related ER stress upregulates FAM134B, expanding its condensate footprint and sterically hindering RTN3L‑mediated tubule turnover. The result is a biased ER‑phagy that favors sheet removal, disrupting ER‑mitochondria calcium coupling.

3. Mitophagy starvation – Because p62 and OPTN also bind ubiquitinated mitochondria, their chronic overexpression outcompetes TAX1BP1 for LC3 sites, reducing Parkin‑dependent mitophagy efficiency. Experimental data show that enhancing p62 oligomerization activates pro‑survival autophagy but predicts reciprocal suppression of mitophagy when shared LC3‑II sites are saturated.

Testable predictions

• Prediction 1: In aged mouse liver, quantitative immunoblotting will reveal a higher p62/TAX1BP1 ratio and increased p62‑LC3 co‑immunoprecipitation compared with young controls, while total LC3‑II levels remain unchanged.
• Prediction 2: Fluorescence‑based organelle‑specific autophagy reporters (mt‑Keima for mitochondria, GFP‑Atg8‑ER for ER) will show elevated mitophagic flux in young cells under starvation but a selective decline in aged cells, whereas ER‑phagy flux remains stable or increases.
• Prediction 3: Genetic normalization of p62 expression (heterozygous knockout) or pharmacological inhibition of its oligomerization in aged mice will restore TAX1BP1‑mediated mitophagy, decrease mitochondrial ROS, and extend median lifespan without altering bulk autophagy flux (measured by long‑lived protein turnover).

Falsification

If aged tissues display proportional reductions in all receptor‑LC3 interactions and a uniform drop in organelle‑specific flux, the hypothesis of selective receptor competition would be falsified. Likewise, if manipulating p62 levels fails to rescue mitophagy or lifespan despite restoring receptor balance, the causal link between receptor hierarchy and aging would be refuted.

Broader implication

This reframes autophagy decline as a qualitative rewiring of selectivity rather than a quantitative loss, offering a precise intervention point: re‑balancing receptor competition to restore organelle‑specific turnover.