Hypothesis

The order in which autophagic receptors recruit cargo is not hard‑wired but is set by a reversible phosphorylation ‘code’ that modulates each receptor’s affinity for LC3 and for specific ubiquitin or lipid tags. In young cells this code creates a strict hierarchy—mitochondria first, then aggregates, lipids, pathogens—maximizing survival under stress. With age, dysregulation of upstream kinases (e.g., TBK1, ULK1) and phosphatases (e.g., PPM1B) flattens the code, causing receptors to compete indiscriminately for limited autophagosomes. This loss of prioritization lets low‑priority cargo (e.g., protein aggregates) occupy vesicles that should be reserved for depolarized mitochondria, tipping the balance toward ROS‑driven inflammaging.

Mechanistic Basis

Selective autophagy receptors contain multiple serine/threonine residues that are phosphorylated in response to nutrient stress (e.g., p62 S240, OPTN S177, NDP52 S252) [1][2]. These modifications alter LIR motif exposure and ubiquitin‑binding affinity, effectively ranking cargo. Recent structural work shows that phosphorylated p62 adopts an open conformation that increases its avidity for LC3 while decreasing its affinity for ubiquitinated aggregates, thereby promoting mitophagy over aggrephagy when the signal is present [3]. In aged microglia and T cells, phosphoproteomics reveals a global drop in receptor phosphorylation concomitant with rising basal kinase activity that nonspecifically phosphorylates low‑affinity sites, blunting the differential affinity shift [4]. Consequently, the autophagosome becomes a ‘first‑come, first‑served’ platform where abundant aggregates outcompete damaged mitochondria, leading to mitochondrial ROS accumulation, NLRP3 inflammasome activation, and SASP secretion [5][6].

Testable Predictions

1. Restoring the youthful phosphorylation pattern on p62 (e.g., via a phospho‑mimetic mutant S240E) will re‑establish mitophagy priority and reduce mitochondrial ROS in aged human microglia, even if total p62 levels remain unchanged.
2. Pharmacological inhibition of the phosphatases that de‑phosphorylate the hierarchical sites (e.g., PPM1B) will exacerbate cargo mis‑sorting and accelerate inflammasome activation in young cells, mimicking the aged phenotype.
3. Quantitative mass‑spectrometry of autophagosome‑isolated cargo from young vs. old cells will show a shift in the mitochondrial‑to‑aggregate ratio that correlates directly with the phosphorylation state of receptors.

Potential Experiments

• Generate CRISPR‑edited microglia lines expressing phospho‑dead (S240A) and phospho‑mimetic (S240E) p62; measure LC3 co‑immunoprecipitation, mitochondrial membrane potential (TMRE), and IL‑1β release after LPS + ATP challenge.
• Treat young macrophages with okadaic acid (PP2A/PPM1B inhibitor) and perform subcellular fractionation followed by immunoblot for p62‑phospho sites and LC3‑II; assess mitochondrial ROS via MitoSOX.
• Use proximity‑labeling (BioID) on LC3 in young and old T cells to quantify autophagosome‑associated mitochondria versus ubiquitin‑positive aggregates; correlate with phospho‑receptor levels from parallel lysates.

If the phospho‑code hypothesis holds, rescuing specific receptor phosphorylation should selectively reinstate the autophagic hierarchy without globally upregulating autophagy, offering a precise alternative to broad mTOR inhibition or senolytic approaches.