Leukocyte extravasation is an essential step during the immune response and requires the destabilization of endothelial junctions. We have shown previously that this process depends in vivo on the dephosphorylation of VE-cadherin-Y731. Here, we reveal the underlying mechanism. Leukocyte-induced stimulation of PECAM-1 triggers dissociation of the phosphatase SHP2 which then directly targets VE-cadherin-Y731. The binding site of PECAM-1 for SHP2 is needed for VE-cadherin dephosphorylation and subsequent endocytosis. Importantly, the contribution of PECAM-1 to leukocyte diapedesis in vitro and in vivo was strictly dependent on the presence of Y731 of VE-cadherin. In addition to SHP2, dephosphorylation of Y731 required Ca 2+ -signaling, non-muscle myosin II activation, and endothelial cell tension. Since we found that βcatenin/plakoglobin mask VE-cadherin-Y731 and leukocyte docking to endothelial cells exert force on the VE-cadherin-catenin complex, we propose that leukocytes destabilize junctions by PECAM-1-SHP2-triggered dephosphorylation of VE-cadherin-Y731 which becomes accessible by actomyosin-mediated mechanical force exerted on the VE-cadherin-catenin complex.

Cadherin-mediated cell adhesion requires anchoring via the β-catenin-α-catenin complex to the actin cytoskeleton, yet, α-catenin binds F-actin only weakly. A covalent fusion of VE-cadherin to α-catenin enhances actin anchorage in endothelial cells and strongly stabilizes endothelial junctions in vivo, blocking inflammatory responses. Here, we have analyzed the underlying mechanism. We found that VE-cadherin-α-catenin constitutively recruits the actin adaptor vinculin. However, removal of the vinculin binding region of α-catenin did not impair the ability of VE-cadherin-α-catenin to enhance junction integrity. Searching for an alternative explanation for the junction stabilizing mechanism, we found that an antibody-defined epitope, normally buried in a short α1-helix of the actin binding domain (ABD) of α-catenin, is openly displayed in junctional VE-cadherin-α-catenin chimera. This epitope, we found to become exposed in normal α-catenin upon triggering thrombin-induced tension across the VE-cadherin complex. These results suggest, that the VE-cadherin-α-catenin chimera stabilizes endothelial junctions due to conformational changes in the ABD of α-catenin, which support constitutive strong binding to actin.