TGF-β1-induced endothelial transcytosis drives blood-brain barrier leakage during aging

Age-related breakdown of the blood-brain barrier (BBB) is associated with cerebrovascular and neurodegenerative diseases, yet its underlying mechanisms remain unclear. Here, we find that BBB leakage begins in midlife and is driven primarily by increased endothelial caveolar transcytosis rather than tight junction disruption. AAV-mediated knockdown of caveolin-1 or restoration of Mfsd2a expression reduces endothelial vesicle formation and BBB leakage in aged mice. Transforming growth factor (TGF)-β1, originating from both brain and systemic circulation, increases with aging and directly suppresses Mfsd2a transcription via the Tgfbr2-Smad2/4 signaling in brain microvascular endothelial cells, thereby promoting caveolar transcytosis and BBB leakage. Endothelial-specific Tgfbr2 knockout or pharmacological inhibition of TGF-β signaling reduces endothelial caveolar transcytosis and BBB leakage and attenuates neurological dysfunction in aged mice. These findings identify TGF-β1-induced endothelial transcytosis as a central mechanism underlying age-related BBB breakdown and offer potential therapeutic targets for preserving brain health in age-related neurovascular disorders.