Hypothesis

In aged endothelial cells, limited autophagic flux creates a competitive hierarchy where p62‑SQSTM1–mediated mitophagy outcompetes OPTN‑selective autophagy for eNOS‑containing caveolae, preserving mitochondrial mass at the cost of eNOS mislocalization and reduced NO output.

Mechanistic Basis

Shear stress normally activates ULK1‑dependent phagophore formation, allowing parallel clearance of damaged mitochondria via p62 and signaling platforms via OPTN. With age, lysosomal capacity declines (3), raising the effective Km for autophagic cargo. p62 binds ubiquitin‑tagged mitochondria with higher affinity than OPTN binds caveolar eNOS complexes (1), so under flux limitation p62 sequesters the majority of available autophagosomes. This selective advantage accelerates mitochondrial turnover while leaving eNOS‑rich microdomains undegraded, leading to their accumulation in cholesterol‑rich lipid rafts where eNOS becomes uncoupled (4). Uncoupled eNOS generates superoxide instead of NO, further inhibiting autophagic flux through oxidative modification of ATG proteins (5). The resulting feed‑forward loop sustains arterial stiffness.

Novel Insight

The hypothesis adds a receptor‑specific competition layer to the existing view of autophagy as a bulk quality‑control system. It predicts that manipulating the relative expression or activity of p62 versus OPTN will shift the substrate hierarchy without altering overall flux, thereby decoupling mitochondrial health from eNOS function.

Testable Predictions

1. In endothelial cells from old mice, p62‑positive autophagosomes will be enriched for mitochondrial markers, whereas OPTN‑positive autophagosomes will be depleted for caveolin‑1/eNOS compared with young cells.
2. Acute OPTN overexpression in aged endothelial cells will rescue eNOS localization and NO production without changing mitochondrial mass or mitophagy rates.
3. Pharmacological inhibition of p62‑ubiquitin interaction (e.g., using a small‑molecule blocker) will increase OPTN‑dependent eNOS caveolae turnover and improve NO bioavailability, even when lysosomal capacity remains low.
4. Heterochronic parabiosis will normalize the p62/OPTN activity ratio in aged endothelium, correlating with restored NO‑mediated vasodilation.

Experimental Approach

• Model: Primary human umbilical vein endothelial cells (HUVECs) subjected to replicative senescence or aged mouse aorta explants.
• Readouts: Tandem fluorescent‑tagged LC3 (mRFP‑GFP‑LC3) flux assay; immunofluorescence for p62, OPTN, mitochondrial COXIV, caveolin‑1, eNOS; DAF‑FM NO assay; MitoSOX ROS; pulse‑wave velocity in vivo.
• Manipulations: siRNA/shRNA knockdown or CRISPR‑activation of p62 or OPTN; overexpression vectors; peptide disruptors of p62‑Ub binding; lysosomal inhibitor chloroquine to modulate capacity.
• Analysis: Quantify colocalization coefficients (Pearson’s r) between autophagosome markers and cargo; compare NO output and mitochondrial respiration (Seahorse) across conditions; statistical testing with ANOVA and post‑hoc correction. }