N-cadherin Signaling via PI3Kβ-Akt3 Stabilizes Occludin Tight Junctions in the Blood-Brain Barrier

Abstract

Tight junctions (TJs) seal brain endothelial cells (BECs) to form a restrictive blood-brain barrier (BBB) to selectively control the influx of circulating substances into the central nervous system. Critical to the integrity of BBB are pericytes, cells that physically interact with BECs through Neural (N)-cadherin adhesion. Using brain samples from donors of different ages, we observed a significant loss of N-cadherin junctions in middle-aged patients. This change in N-cadherin was associated with a significant disruption of occludin TJs, suggesting a potential important crosstalk between N-cadherin and TJs. Therefore, to better understand the role of N-cadherin function in the BBB, we generated an inducible, EC-specific knockout of N-cadherin in mice. We demonstrated that mutant mice lacking N-cadherin exhibited greater BBB permeability as well as impairments in TJ ultrastructure. Analyzing different TJ proteins in the microvasculature of the brain cortex, we also found significant disruption of occludin but not claudin TJs in these mutant mice. Furthermore, analysis of occludin kinetics in BEC monolayers lacking N-cadherin adhesion revealed significantly greater internalization rates of occludin from TJs, suggesting that N-cadherin signaling stabilizes occludin TJs. We showed that N-cadherin induces the activation of phosphoinositide 3-kinase (PI3K), leading to Akt phosphorylation. Depletion of PI3K p110b or Akt3, but not other p110 or Akt isoforms, increased occludin internalization from TJs in BECs with intact N-cadherin adhesion. Our data indicate that N-cadherin adhesion activates PI3K p110b-Akt3 signaling in BECs that is essential for the stabilization of occludin TJs to strengthen the BBB.