Hypothesis

Core claim: In aged brain endothelium, the shift from receptor‑mediated to caveolar transcytosis reduces crocetin BBB permeability, lowering intracellular crocetin concentrations that normally suppress HIF‑1α via PI3K/AKT inhibition. The resulting HIF‑1α stabilization drives VEGF‑dependent angiogenic remodeling that further impairs Aβ efflux, creating a feed‑forward loop of neurovascular dysfunction.

Mechanistic rationale

• Crocetin’s small size and hydrophilicity favor paracellular or receptor‑mediated transport; age‑dependent loss of specific transporters (e.g., LRPs) and increased caveolae formation diminish its Papp [2][3].
• In young endothelium, crocetin (≥10 µM) inhibits PI3K/AKT/ERK signaling, decreasing HIF‑1α translation and promoting its proteasomal degradation (inferred from HepG2/HUVEC data) [4][5].
• When crocetin influx falls below a threshold in aged cells, AKT remains phosphorylated, HIF‑1α accumulates, and VEGF‑A transcription rises, tightening junctional complexes and shifting transcytosis toward non‑specific caveolar routes, thereby further limiting crocetin entry—a positive feedback.

Testable predictions

1. BBB permeability: Primary bovine brain endothelial cells (BBECs) from young (3 mo) and aged (24 mo) rats will show a ≥2‑fold lower Papp for crocetin in aged monolayers; co‑application of the P‑gp inhibitor tariquidar will rescue Papp to young levels.
2. HIF‑1α signaling: Aged BBECs treated with crocetin (10 µM) will exhibit unchanged p‑AKT/AKT ratios and HIF‑1α protein, whereas young cells will display ≥60 % reduction; adding a PI3K inhibitor (LY294002) will mimic crocetin’s effect in aged cells.
3. Aβ clearance: Transwell assays with fluorescent Aβ1‑40 will reveal reduced basolateral efflux in aged BBECs; crocetin supplementation (10 µM) will restore efflux in young but not aged cells unless combined with caveolae disruptor (methyl‑β‑cyclodextrin).
4. In vivo validation: Aged APP/PS1 mice receiving oral crocetin (22.5 mg/kg) plus tariquidar will show increased brain crocetin levels (LC‑MS), decreased endothelial HIF‑1α immunostaining, and improved Aβ clearance versus crocetin alone.

Falsifiability

If crocetin’s Papp is not significantly lower in aged endothelium, or if HIF‑1α levels remain unchanged despite altered crocetin delivery, the hypothesis is refuted. Likewise, rescuing crocetin transport without affecting HIF‑1α or Aβ clearance would disprove the causal link.

References (inline)

[1] https://pubmed.ncbi.nlm.nih.gov/41628871/ [2] https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2020.579052/full [3] https://doi.org/10.1038/s41586-020-2453-z [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC8862147/ [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC10648070/ [6] https://doi.org/10.1038/s42003-022-03037-0