Hypothesis

With advancing age, the competitive balance among autophagy cargo receptors shifts: p62/SQSTM1 and NBR1, which preferentially ubiquitinate ER‑associated substrates, lose affinity for ubiquitinated cargo faster than the mitochondrial receptors OPTN/NDP52/TAX1BP1. This inversion creates a selective deficit in ER‑phagy while mitophagy remains relatively intact, leading to accumulation of stressed ER membranes, chronic activation of the IRE1α‑XBP1 and PERK‑ATF4 pathways, and secondary NLRP3 inflammasome activation. The resulting inflammaging drives tissue dysfunction independently of bulk autophagy flux.

Mechanistic rationale

1. Receptor‑specific ubiquitin sensing – Structural studies show that p62’s UBA domain has a lower binding affinity for K63‑linked ubiquitin chains under oxidative conditions, whereas OPTN’s UBZ domain retains higher affinity (see [1] for cargo receptor competition). Age‑related increases in cytosolic ROS preferentially modify p62 cysteines, reducing its ubiquitin capture efficiency.
2. Stress‑biased receptor expression – Transcriptomic data from aged human fibroblasts reveal a ~40 % decline in NBR1 mRNA and a compensatory rise in TAX1BP1 ([6]). NBR1 is a key ER‑phagy receptor; its loss skews the hierarchy toward mitochondrial substrates.
3. ER‑phagy deficit amplifies ER stress – Impaired clearance of peroxidized ER fragments sustains IRE1α oligomerization, promoting TRAF2‑dependent NF‑κB activation and NLRP3 priming. Concurrently, PERK‑eIF2α signaling reduces global translation, favoring selective translation of pro‑inflammatory cytokines (e.g., IL‑1β).
4. Feedback to mitophagy – Persistent ER stress elevates cytosolic Ca²⁺ via IP₃R leak, activating CaMKKβ‑AMPK, which can paradoxically stimulate mitophagy. Thus, mitophagy flux may appear preserved or even elevated despite overall autophagic decline, masking the underlying ER‑phagy failure.

Testable predictions

• Prediction 1: In aged mouse liver, immunoisolation of LC3⁺ autophagosomes will show a ≥50 % reduction in ER‑resident markers (e.g., calnexin, reticulon‑4) relative to young controls, while mitochondrial markers (TOM20, COXIV) remain unchanged. This can be quantified by mass spectrometry of autophagosome cargo ([4] provides a workflow for subtype‑specific isolation).
• Prediction 2: Pharmacological stabilization of p62/NBR1 ubiquitin binding (using a small‑molecule UBA agonist) will restore ER‑phagy flux, decrease p62‑positive puncta, and lower NLRP3 inflammasome activity (measured by caspase‑1 cleavage and IL‑1β release) in aged fibroblasts, without altering bulk LC3‑II turnover.
• Prediction 3: Genetic overexpression of NBR1 in aged Drosophila will selectively extend lifespan by ~15 % and reduce gut epithelial ER stress markers (XBP1s splicing), whereas overexpression of TAX1BP1 will not affect lifespan but will improve climbing activity via enhanced mitophagy.

Falsification

If aged tissues exhibit proportional loss of both ER‑ and mitochondrial autophagosome cargo, or if restoring ER‑phagy fails to attenuate inflammasome activation, the hypothesis would be refuted. Similarly, if NBR1 overexpression does not rescue ER‑phagy flux despite increased expression, the postulated receptor‑specific affinity loss would be questioned.

This hypothesis reframes autophagy decline not as a global slowdown but as a stratified sabotage of the organelle‑degradation hierarchy, offering a precise point of intervention for age‑related inflammatory disease.