RhoA effectors ROCK and SLK/LOK oppositely regulate VE-cadherin mobility at endothelial junctions
Cellular contractile forces regulate VE-cadherin dynamics, adhesion, and endothelial junction integrity. RhoA controls these forces through two parallel effector kinases: ROCK, which activates non-muscle myosin II motors, and SLK/LOK, which promote actin-membrane attachment through phosphorylation of ezrin, radixin, moesin proteins (ERM). How these two RhoA effector pathways differentially affect VE-cadherin dynamics at junctions is not known. Using single-molecule tracking of Halo-tagged VE-cadherin, we obtained a detailed characterization of VE-cadherin diffusive behavior inside and outside junctional regions in cultured human umbilical vein endothelial cells (HUVEC). VE-cadherin was substantially less mobile and more confined within junctions than in non-junctional membrane. Inhibiting ROCK with Y-27632 increased VE-cadherin mobility both inside and outside junctions, consistent with RhoA/ROCK-driven contractility restraining cadherin diffusion globally. Unexpectedly, inhibiting SLK/LOK produced the opposite effect, selectively reducing VE-cadherin mobility within junctions with no comparable change outside them. This effect was accompanied by an increase in reticular junctions, suggesting that SLK/LOK inhibition alters VE-cadherin mobility indirectly, through a change in junctional architecture, rather than through a direct effect of actin-membrane coupling on VE-cadherin itself. These findings identify SLK/LOK as a regulator of VE-cadherin dynamics acting in parallel to ROCK downstream of RhoA and point to junctional architecture as a previously unappreciated determinant of VE-cadherin mobility at endothelial cell-cell contacts.