Rho, tyrosine kinase, Ca(2+), and junctions in endothelial hyperpermeability.

Increased microvascular permeability is a central hallmark of inflammation and the basis of edematous tissue injury in many acute and chronic pathological conditions, including ischemia-reperfusion injury, sepsis, and acute respiratory distress syndrome.1–4 Increased microvascular leakage may occur in any region of the vasculature, but it is most familiar in the microvasculature, especially the postcapillary venule. Ultrastructurally, anatomists recognize at least 3 types of endothelial cells: continuous, fenestrated, and discontinuous, which can all exhibit altered barrier in response to environmental and chemical factors. Early studies by Majno and Palade5 and later studies by Simionescu et al6 suggested that inflammatory mediators, such as histamine, thrombin, and serotonin, increase solute permeability in microvessels by enlarging interjunctional spaces and allowing the extravasation of fluid, protein, and leukocytes into the tissues. It is widely held that many inflammatory mediators (eg, thrombin, histamine, and bradykinin) use a common mechanism to control junctional exchange through a Ca/calmodulin (CaM) and myosin light chain kinase (MLCK)–regulated actomyosin contraction, which generates cytoskeletal tension. In theory, this tension promotes separation of junctional clefts and allows equilibration of solutes into the interstitium.7–10 Many recent studies have demonstrated that this contractile response is regulated to a large extent through the activity of several members of the small GTPase family that includes Rho, Rac, and Cdc42.7,8,11,12 The study by van Nieuw Amerongen et al13 in this issue of Circulation Research is the most recent of several studies that describe features of Rho-dependent endothelial permeability produced by mediators, especially thrombin. While Rho is thought to play a critical role in altered permeability, van Nieuw Amerongen et al13 propose 4 main targets in barrier disruption: RhoA, calcium, tyrosine kinase, and cell junctions. The present study shows that thrombin increases endothelial permeability by activating RhoA and Rho kinase to increase MLC phosphorylation. This simultaneously promotes the organization of actin stress fibers in the cell necessary for producing tension and the loss of cell-cell apposition and initiates actomyosin contraction. These effects were significantly reduced by inhibition of Rho kinase with Y-27632, which blocked stress fiber organization, contraction, and permeability. These events were additionally reduced by chelation of intracellular calcium with BAPTA. Importantly, the authors demonstrate that Rac, an important regulator of cell endothelial shape and motility,14,15 does not appear to be activated in this model.13