Canonical Wnt Signaling Suppresses Brain Endothelial Cell Transcytosis to Maintain Blood-Brain Barrier Integrity.

Canonical Wnt signaling is essential for blood-brain barrier (BBB) development and maintenance. However, the subcellular mechanisms underlying this critical regulation have remained elusive. In this study, we use a physiological paradigm examining an early phase of acutely attenuated canonical Wnt signaling in adult brain endothelial cells (ECs) to investigate how the pathway regulates BBB integrity. Following canonical Wnt signaling attenuation via EC-specific knockout of β-catenin, we find that there is increased transcytosis in brain ECs, including a striking diversity of morphologically distinct vesicles, indicating multiple pathways are involved. In addition, we find that although the molecular composition of tight junctions (TJs) is altered following canonical Wnt signaling attenuation, such that Claudin-5 and ZO-1 expression is downregulated, TJs remain impermeable to molecules as small as 1.9 kDa. These findings reveal previously underappreciated role of Wnt signaling in regulating brain EC transcytosis and help illuminate subcellular mechanisms of BBB maintenance in adulthood, which is crucial for improving delivery of therapeutics to the brain.