Hypothesis

Autophagic substrate hierarchy is tuned by a phosphorylation‑dependent affinity switch in selective autophagy receptors, and age‑related kinase/phosphatase imbalance rewires this switch, causing misprioritization that drives selective pathway failure.

Mechanistic rationale

Selective autophagy receptors—p62/SQSTM1, NBR1, OPTN, NDP52—contain ubiquitin‑binding domains (UBDs) and LC3‑interacting regions (LIRs) whose affinity for cargo and autophagosomal machinery is modulated by post‑translational phosphorylation. Stress‑activated kinases such as TBK1, ULK1, and CK2 phosphorylate serine/threonine residues within these domains, increasing ubiquitin affinity for damaged mitochondria (via OPTN/NDP52) or ER fragments (via SEC62/OPTN) while decreasing affinity for bulk cytosolic proteins[[1][5]]. This creates a rapid, signal‑dependent triage: under mitochondrial damage, phosphorylated OPTN/NDP52 outcompete p62 for LC3, directing autophagosomes to mitophagy; ER stress triggers SEC62 phosphorylation, favoring ER‑phagy[[1]].

In aging, chronic low‑grade inflammation and oxidative stress shift the kinase/phosphatase balance toward phosphatases (e.g., PPM1B, PP2A) and reduce TBK1/ULK1 activity[[2][4]]. The resulting hypophosphorylation of receptors flips the affinity hierarchy: p62, which remains constitutively active, gains relative advantage, promoting non‑selective bulk autophagy and sequestering autophagosomes away from damaged mitochondria and ER[[3][6]]. Consequently, mitophagy and ER‑phagy decline while lipofuscin and ubiquitin‑positive aggregates accumulate[[4][7]].

Testable predictions

1. Phospho‑state profiling – Quantitative phosphoproteomics of isolated autophagic receptors from young vs. aged mouse brain will show decreased phosphorylation at TBK1/ULK1 sites on OPTN, NDP52, and SEC62, and increased phosphorylation (or unchanged) on p62[[2][4]].

2. Rescue by phosphomimetic mutants – Neuronal expression of phosphomimetic OPTN/S473D or SEC62/S355D in aged primary cultures will restore preferential mitophagy/ER‑phagy (measured by mt‑Keima and ER‑phagy reporters) and reduce lipid‑droplet formation[[3][5]].

3. Kinase inhibition phenocopies aging – Acute TBK1 inhibition in young neurons will shift receptor competition toward p62‑dependent bulk autophagy, increase mitochondrial ROS, and trigger inflammasome activation, mirroring aged phenotypes[[6][7]].

4. Phosphatase overexpression accelerates decline – Overexpressing PPM1B in young mice will reproduce the aged autophagy hierarchy defect, leading to accelerated accumulation of lipofuscin and neuroinflammatory markers[[2][4]].

Experimental approach

• Use SILAC‑based phosphoproteomics on FACS‑sorted neurons from 3‑month and 24‑month mice.

• Deploy CRISPR knock‑in of phospho‑dead (S→A) and phosphomimetic (S→D/E) alleles for OPTN, NDP52, SEC62.

• Monitor flux with mCherry‑GFP‑LC3, mt‑Keima, and SEC62‑GFP‑ER‑phagy reporters; quantify by flow cytometry and confocal microscopy.

• Measure downstream readouts: mitochondrial membrane potential (TMRE), ER stress (CHOP), NLRP3 inflammasome activation (caspase‑1 p10), and cytokine release (ELISA).

If the phosphorylation‑switch model holds, manipulating receptor phosphorylation state will selectively reroute autophagic cargo without globally altering autophagosome formation, confirming that the hierarchy—not the bulk process—determines cellular fate in aging.