Interactions between cells and the extracellular matrix coordinate signaling pathways that control various aspects of cellular behavior. Integrins sense the physical properties of the extracellular matrix and organize the cytoskeleton accordingly. In turn, this modulates signaling pathways that are triggered by various other transmembrane receptors and augments the cellular response to growth factors. Over the past years, it has become clear that there is extensive crosstalk between integrins, Src-family kinases and Rho-family GTPases at the heart of such adhesion signaling. In this Commentary, we discuss recent advances in our understanding of the dynamic regulation of the molecular connections between these three protein families. We also discuss how this signaling network can regulate a range of cellular processes that are important for normal tissue function and disease, including cell adhesion, spreading, migration and mechanotransduction.

A specialized subset of VE-cadherin adhesions senses cytoskeletal force and recruits Vinculin to control the stability of endothelial cell–cell junctions during their force-dependent remodeling.

SummaryCells integrate biochemical and mechanical information to function within multicellular tissue. Within developing and remodeling tissues, mechanical forces contain instructive information that governs important cellular processes that include stem cell maintenance, differentiation and growth. Although the principles of signal transduction (protein phosphorylation, allosteric regulation of enzymatic activity and binding sites) are the same for biochemical and mechanical-induced signaling, the first step of mechanosensing, in which protein complexes under tension transduce changes in physical force into cellular signaling, is very different, and the molecular mechanisms are only beginning to be elucidated. In this Commentary, we focus on mechanotransduction at cell-cell junctions, aiming to comprehend the molecular mechanisms involved. We describe how different junction structures are associated with the actomyosin cytoskeleton and how this relates to the magnitude and direction of forces at cell-cell junctions. We discuss which cell-cell adhesion receptors have been shown to take part in mechanotransduction. Then we outline the force-induced molecular events that might occur within a key mechanosensitive system at cell-cell junctions; the cadherin-F-actin interface, at which a-catenin and vinculin form a central module. Mechanotransduction at cell-cell junctions emerges as an important signaling mechanism, and we present examples of its potential relevance for tissue development and disease.

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During wound healing, angiogenesis, and tumor invasion, cells often change their expression profiles of fibronectin-binding integrins. Here, we show that β1 integrins promote random migration, whereas β3 integrins promote persistent migration in the same epithelial cell background. Adhesion to fibronectin by αvβ3 supports extensive actin cytoskeletal reorganization through the actin-severing protein cofilin, resulting in a single broad lamellipod with static cell–matrix adhesions at the leading edge. Adhesion by α5β1 instead leads to the phosphorylation/inactivation of cofilin, and these cells fail to polarize their cytoskeleton but extend thin protrusions containing highly dynamic cell–matrix adhesions in multiple directions. The activity of the small GTPase RhoA is particularly high in cells adhering by α5β1, and inhibition of Rho signaling causes a switch from a β1- to a β3-associated mode of migration, whereas increased Rho activity has the opposite effect. Thus, alterations in integrin expression profiles allow cells to modulate several critical aspects of the motile machinery through Rho GTPases.

Background-Tissue edema and endothelial barrier dysfunction as observed in sepsis and acute lung injury carry high morbidity and mortality, but currently lack specific therapy. In a recent case report, we described fast resolution of pulmonary edema on treatment with the tyrosine kinase inhibitor imatinib through an unknown mechanism. Here, we explored the effect of imatinib on endothelial barrier dysfunction and edema formation. Methods and Results-We evaluated the effect of imatinib on endothelial barrier function in vitro and in vivo. In human macro-and microvascular endothelial monolayers, imatinib attenuated endothelial barrier dysfunction induced by thrombin and histamine. Small interfering RNA knock-downs of the imatinib-sensitive kinases revealed that imatinib attenuates endothelial barrier dysfunction via inhibition of Abl-related gene kinase (Arg/Abl2), a previously unknown mediator of endothelial barrier dysfunction. Indeed, Arg was activated by endothelial stimulation with thrombin, histamine, and vascular endothelial growth factor. Imatinib limited Arg-mediated endothelial barrier dysfunction by enhancing Rac1 activity and enforcing adhesion of endothelial cells to the extracellular matrix. Using mouse models of vascular leakage as proof-of-concept, we found that pretreatment with imatinib protected against vascular endothelial growth factor-induced vascular leakage in the skin, and effectively prevented edema formation in the lungs. In a murine model of sepsis, imatinib treatment (6 hours and 18 hours after induction of sepsis) attenuated vascular leakage in the kidneys and the lungs (24 hours after induction of sepsis). Conclusions-Thus, imatinib prevents endothelial barrier dysfunction and edema formation via inhibition of Arg. These findings identify imatinib as a promising approach to permeability edema and indicate Arg as novel target for edema treatment. (Circulation. 2012;126:2728-2738.)

During immune surveillance and inflammation, leukocytes exit the vasculature through transient openings in the endothelium without causing plasma leakage. However, the exact mechanisms behind this intriguing phenomenon are still unknown. Here we report that maintenance of endothelial barrier integrity during leukocyte diapedesis requires local endothelial RhoA cycling. Endothelial RhoA depletion in vitro or Rho inhibition in vivo provokes neutrophil-induced vascular leakage that manifests during the physical movement of neutrophils through the endothelial layer. Local RhoA activation initiates the formation of contractile F-actin structures that surround emigrating neutrophils. These structures that surround neutrophil-induced endothelial pores prevent plasma leakage through actomyosin-based pore confinement. Mechanistically, we found that the initiation of RhoA activity involves ICAM-1 and the Rho GEFs Ect2 and LARG. In addition, regulation of actomyosin-based endothelial pore confinement involves ROCK2b, but not ROCK1. Thus, endothelial cells assemble RhoA-controlled contractile F-actin structures around endothelial pores that prevent vascular leakage during leukocyte extravasation.

The vasculature is one of the most dynamic tissues that encounter numerous mechanical cues derived from pulsatile blood flow, blood pressure, activity of smooth muscle cells in the vessel wall, and transmigration of immune cells. The inner layer of blood and lymphatic vessels is covered by the endothelium, a monolayer of cells which separates blood from tissue, an important function that it fulfills even under the dynamic circumstances of the vascular microenvironment. In addition, remodeling of the endothelial barrier during angiogenesis and trafficking of immune cells is achieved by specific modulation of cell–cell adhesion structures between the endothelial cells. In recent years, there have been many new discoveries in the field of cellular mechanotransduction which controls the formation and destabilization of the vascular barrier. Force-induced adaptation at endothelial cell–cell adhesion structures is a crucial node in these processes that challenge the vascular barrier. One of the key examples of a force-induced molecular event is the recruitment of vinculin to the VE-cadherin complex upon pulling forces at cell–cell junctions. Here, we highlight recent advances in the current understanding of mechanotransduction responses at, and derived from, endothelial cell–cell junctions. We further discuss their importance for vascular barrier function and remodeling in development, inflammation, and vascular disease.