Tiam1 and Rac1 Are Required for Platelet-activating Factor-induced Endothelial Junctional Disassembly and Increase in Vascular Permeability

Abstract

It is known that platelet-activating factor (PAF) induces severe endothelial barrier leakiness, but the signaling mechanisms remain unclear. Here, using a wide range of biochemical and morphological approaches applied in both mouse models and cultured endothelial cells, we addressed the mechanisms of PAF-induced disruption of interendothelial junctions (IEJs) and of increased endothelial permeability. The formation of interendothelial gaps filled with filopodia and lamellipodia is the cellular event responsible for the disruption of endothelial barrier. We observed that PAF ligation of its receptor induced the activation of the Rho GTPase Rac1. Following PAF exposure, both Rac1 and its guanine nucleotide exchange factor Tiam1 were found associated with a membrane fraction from which they co-immunoprecipitated with PAF receptor. In the same time frame with Tiam1-Rac1 translocation, the junctional proteins ZO-1 and VE-cadherin were relocated from the IEJs, and formation of numerous interendothelial gaps was recorded. Notably, the response was independent of myosin light chain phosphorylation and thus distinct from other mediators, such as histamine and thrombin. The changes in actin status are driven by the PAF-induced localized actin polymerization as a consequence of Rac1 translocation and activation. Tiam1 was required for the activation of Rac1, actin polymerization, relocation of junctional associated proteins, and disruption of IEJs. Thus, PAF-induced IEJ disruption and increased endothelial permeability requires the activation of a Tiam1-Rac1 signaling module, suggesting a novel therapeutic target against increased vascular permeability associated with inflammatory diseases.

Small Rho GTPase-mediated actin dynamics at endothelial adherens junctions

“…68 Nevertheless, Rac1 activation also causes loss of cellcell contacts upon stimulation of endothelial cells with VEGF, PAF and TNF-a, as well as clustering of vascular cell adhesion molecule (VCAM)-1. 25,[69][70][71][72][73] These data seem to be conflicting; however, they in fact underlie the importance of understanding the particular GEFGTPase-effector complexes that are locally activated under resting and inflammatory conditions. We have summarized this in Figure 2.…”

Role of G protein-coupled receptors in inflammation

“…PAF profoundly affects microvascular permeability, allowing extravasation of plasma contents such as albumin [51,52] . Like BK, PAF acts on endothelial cells directly, increasing gap formation between endothelial cells through the actions of eNOS [53] , tyrosine phosphorylation of VE-cadherin [54] , and the Rho family small GTPases [55] .…”

Blood cells and endothelial barrier function

“…133 PAF, which is known to increase vascular permeability when engaged with its receptor on endothelial cells, disrupts the interendothelial junctions via Rac1-dependent relocation of junctional proteins (e.g., VE-cadherin, ZO-1) and actin polymerization. 134 Diseases associated with endothelial barrier dysfunction An injured or dysfunctional endothelial barrier has the potential to significantly impact tissue function and viability. Discontinuities or breaks in the endothelial lining can impair blood flow regulation by interfering with vasodilatory responses that are dependent on endothelial cell-cell communication (e.g., ascending vasodilation).…”