Hypothesis: Rapamycin re-programs endothelial lysosomal biogenesis to sustain vasoprotective autophagy without invoking systemic famine signaling\n\nCore proposition\nRapamycin extends vascular healthspan by directly activating a TFEB‑driven lysosomal autophagy program in endothelial cells, a mechanism that is decoupled from organism‑wide nutrient‑stress pathways. Consequently, the longevity benefits persist after drug withdrawal because the endothelial lysosomal network remains primed, whereas systemic famine‑mimicry signals (e.g., lowered IGF‑1, elevated FGF21) normalize quickly.\n\nTestable predictions\n1. Endothelial‑specific TFEB knockdown will abolish rapamycin‑mediated improvements in arterial stiffness and NO bioavailability, despite intact systemic mTORC1 inhibition.\n2. Transient rapamycin treatment will increase endothelial lysosomal mass and cathepsin activity, measurable by lysosomal-associated membrane protein 2 (LAMP2) fluorescence, and these changes will endure for at least 8 weeks post‑treatment.\n3. Systemic markers of famine signaling (plasma IGF‑1, FGF21, ketone bodies) will return to baseline within 48 h of rapamycin cessation, yet endothelial lysosomal signatures will remain elevated.\n4. Pharmacological blockade of lysosomal acidification (e.g., with bafilomycin A1) during the rapamycin window will prevent the lasting vascular benefits, confirming that lysosomal function—not merely mTORC1 suppression—is essential.\n\nExperimental approach\n- Use inducible, endothelial‑specific Cre‑TfeB floxed mice and wild‑type controls.\n- administer rapamycin (14 ppm diet) for 3 months starting at 12 months of age, then withdraw.\n- Measure pulse wave velocity (PWV), endothelial NO synthase (eNOS) phosphorylation, and aortic collagen content at baseline, post‑treatment, and at 2‑, 4‑, 8‑week intervals after withdrawal.\n- Quantify endothelial lysosomal mass via LAMP2‑GFP flow cytometry and cathepsin B activity assays.\n- Monitor plasma IGF‑1, FGF21, β‑hydroxybutyrate as systemic famine readouts.\n- In a parallel arm, treat a subset with bafilomycin A1 (low dose) concurrent with rapamycin to test lysosomal dependence.\n\nInterpretation\nIf endothelial TFEB loss eliminates the enduring vascular improvements while systemic famine markers normalize, the data would support the hypothesis that rapamycin’s vascular longevity effect stems from a cell‑autonomous lysosomal rejuvenation program rather than from impersonating a harder life. Conversely, if benefits persist despite endothelial TFEB loss, the famine‑mimicry model would gain support, indicating that downstream systemic adaptations are sufficient.\n\nBroader impact\nThis mechanistic refinement would shift the focus from whole‑body metabolic deceit to targeted organelle‑level rejuvenation, informing intermittent dosing strategies that maximize endothelial lysosomal priming while minimizing off‑target immunosuppression.