The brain's most intimate boundary — the blood-brain barrier — has long been known to falter in Alzheimer's disease, but the mechanism behind that failure has remained elusive. Now, researchers at Columbia University have traced a significant portion of the damage to fibronectin, a protein the body deploys to heal wounds, which in the presence of the APOE4 gene variant accumulates around brain blood vessels and quietly dismantles the seal meant to protect the mind. The discovery, published in Nature Aging, does not yet offer a cure, but it illuminates a new corridor in the architecture of Alzh