Hypothesis

In aging endothelium, autophagy does not merely fail to clear von Willebrand factor (VWF) and Factor VIII (FVIII); it actively rations these hemostatic proteins to preserve vascular integrity under chronic inflammatory and energetic stress. This selective retention is mediated by altered ER‑phagy specificity, where VWF and FVIII are spared from degradation because their luminal domains contain motifs that recruit chaperone complexes inhibiting FAM134‑mediated autophagosome formation. Consequently, interventions that globally induce autophagy (e.g., rapamycin, spermidine) may improve cell survival without reducing VWF/FVIII levels, explaining why they often fail to lower thrombotic risk in older adults.

Mechanistic Rationale

1. Energy‑dependent ER‑phagy switch – Aging endothelial cells exhibit mitochondrial dysfunction and lowered ATP, activating AMPK. AMPK phosphorylates FAM134B at Ser‑278, reducing its affinity for generic ER substrates while increasing binding to cytosolic HSP70‑HSP90 complexes that recognize exposed KFERQ‑like motifs on VWF and FVIII. This phosphorylation skews ER‑phagy toward bulk ER turnover but spares chaperone‑bound VWF/FVIII.
2. Inflammatory feedback loop – TNF‑α and IL‑6 stimulate NF‑κB‑driven VWF transcription and simultaneously inhibit lysosomal acidification via V‑ATPase downregulation, further decreasing the degradative capacity for spared VWF/FVIII. The retained VWF/FVIII then promote platelet adhesion and release of stored inflammatory mediators, amplifying the loop.
3. Adaptive versus maladaptive outcome – Acute sparing of VWF/FVIII supports clot formation during injury or infection, preserving hemostasis. Persistent sparing, however, elevates plasma VWF/FVIII beyond homeostatic set points, shifting the balance toward pathologic thrombosis.

Testable Predictions

• Prediction 1: Pharmacological activation of AMPK (e.g., AICAR) in cultured human endothelial cells will increase FAM134B phosphorylation, decrease colocalization of VWF/FVIII with LC3‑II, and raise secreted VWF/FVIII levels without altering total ER mass.
• Prediction 2: Endothelial‑specific expression of a phospho‑deficient FAM134B mutant (S278A) in aged mice will restore ER‑phagy of VWF/FVIII, reduce plasma VWF/FVIII by ~30 %, and decrease thrombus formation in a FeCl₃‑induced injury model.
• Prediction 3: Inhibiting lysosomal acidification with bafilomycin A1 will abolish the difference between wild‑type and S278A mice, confirming that the observed effect depends on degradative capacity rather than altered synthesis.

Experimental Approach

• Use primary human umbilical vein endothelial cells (HUVECs) treated with low‑dose TNF‑α to mimic chronic inflammation. Measure AMPK activity, FAM134B phosphorylation (Western blot with phospho‑specific antibody), and VWF/FVIII secretion (ELISA) under rapamycin or vehicle.
• Generate Cdh5‑CreERT2;FAM134B^S278A knock‑in mice. Induce mutation at 18 months, then assess plasma VWF/FVIII, tail‑bleeding time, and carotid artery thrombosis after oxidative injury.
• Include rescue experiments with lysosomal acidification enhancers (e.g., ML‑S1) to test whether restoring degradation normalizes VWF/FVIII levels regardless of FAM134B status.

Falsifiability

If phospho‑deficient FAM134B does not lower VWF/FITC levels or thrombosis risk in aged mice, or if AMPK activation reduces VWF/FVIII secretion contrary to the prediction, the hypothesis that autophagy selectively rations VWF/FVIII via FAM134B phosphorylation would be refuted. Conversely, confirmation would support a paradigm shift from viewing autophagy decline as passive clearance failure to recognizing it as an active, stress‑adaptive resource‑allocation decision that becomes maladaptive when prolonged.