Background

Endothelial autophagy functions as a ordered degradation system where mitophagy precedes bulk autophagy to preserve nitric oxide (NO) signaling [1][2]. Mitophagy limits mitochondrial ROS, protecting eNOS coupling, while general autophagy sustains glycolysis‑dependent ATP release that activates purinergic receptors and phosphorylates eNOS‑Ser1177 [3]. When this hierarchy collapses, mitophagy deficiency elevates oxidative stress, uncouples eNOS, and reduces NO bioavailability, correlating with arterial stiffening [4][5].

Hypothesis

We propose that the primary defect in aging endothelium is not a global loss of autophagic flux but a specific disruption of the mitophagy‑first hierarchy, causing a compensatory shift toward glycolytic ATP production that maladaptively amplifies purinergic signaling and drives eNOS dysfunction through chronic Ca2+ overload and oxidative stress.

Mechanistic Model

1. Hierarchy breakdown – Age‑related decline in PINK1/Parkin or FUNDC1 activity reduces mitophagy priority, allowing damaged mitochondria to accumulate.
2. ROS surge – Leaky mitochondria elevate superoxide, oxidizing tetrahydrobiopterin and uncoupling eNOS, shifting its output from NO to superoxide.
3. Compensatory glycolysis – To maintain ATP for ion homeostasis, endothelial cells upregulate glycolysis, increasing cytosolic ATP.
4. Purinergic overstimulation – Excess ATP release activates P2X/P2Y receptors, causing sustained intracellular Ca2+ elevations.
5. Ca2+‑dependent NOS uncoupling – Persistent Ca2+ activates calpains that cleave eNOS and promotes NADPH oxidase (NOX2) assembly, further amplifying ROS.
6. Feedback loop – ROS‑induced damage to glycolytic enzymes (e.g., GAPDH) reduces ATP yield, prompting more glycolytic flux and perpetuating the cycle.

Testable Predictions

• Prediction 1: In aged endothelial cells, mitophagy flux (measured by mt‑Keima) will be significantly lower than bulk autophagy flux (LC3‑II turnover) despite comparable expression of core autophagy genes (ATG5, BECN1).
• Prediction 2: Pharmacological rescue of mitophagy (e.g., urolithin A or FUNDC1 agonists) will restore the mitophagy‑first order, reduce mitochondrial ROS, and normalize eNOS coupling without altering total autophagic flux.
• Prediction 3: Inhibiting glycolysis (2‑DG) in mitophagy‑deficient cells will decrease ATP release, attenuate purinergic receptor activation, lower cytosolic Ca2+, and improve NO production despite persistent mitochondrial damage.
• Prediction 4: In vivo, endothelial‑specific overexpression of a constitutively active FUNDC1 will prevent age‑related arterial stiffening (pulse wave velocity) even when bulk autophagy is genetically attenuated (endothelial‑specific Atg7 knockdown).

Experimental Approach

• Use primary human umbilical vein endothelial cells (HUVECs) cultured under oscillatory shear to mimic atherosclerotic prone regions.
• Quantify mitophagy vs. bulk autophagy flux using mt‑Keima and tandem fluorescent LC3 reporters, respectively.
• Manipulate hierarchy: (a) knock down PINK1 or FUNDC1 with siRNA; (b) overexpress a mitophagy‑targeting construct (FUNDC1‑CAAX); (c) treat with urolithin A or 2‑DG.
• Measure mitochondrial ROS (MitoSOX), eNOS dimer/monomer ratio (low‑temperature SDS‑PAGE), NO production (DAF‑FM), ATP release (luciferase assay), purinergic receptor phosphorylation (ERK1/2), and cytosolic Ca2+ (Fluo‑4 AM).
• Validate findings in aged mice (24 mo) with endothelial‑specific FUNDC1 overexpression vs. controls; assess PWV, endothelial NO‑dependent relaxation (wire myography), and histological markers of oxidative stress (4‑HNE).

If the hierarchy‑first model holds, restoring mitophagy priority—rather than globally boosting autophagy—should rescue endothelial function and delay vascular stiffening, offering a precise therapeutic target for age‑related cardiovascular disease.