VE-cadherin regulates endothelial actin activating Rac and increasing membrane association of Tiam

Cell-cell adhesion in lung endothelium

Homotypic cell-cell adhesion is essential for tissue and organ development, remodeling, regeneration, and physiological function. Whereas a significant number of homotypic cell-cell adhesion molecules have been identified, much more is known about those concentrated in epithelia than in endothelia. Among the endothelial cell-cell adhesion molecules, very little is known that is specific to endothelium in the pulmonary and bronchial circulations. This review focuses primarily on homotypic cell-cell adhesion molecules that are or are likely to be important in lung endothelium.

VEGF controls endothelial-cell permeability by promoting the beta-arrestin-dependent endocytosis of VE-cadherin

How vascular endothelial growth factor (VEGF) induces vascular permeability, its first described function, remains poorly understood. Here, we provide evidence of a novel signalling pathway by which VEGF stimulation promotes the rapid endocytosis of a key endothelial cell adhesion molecule, VE-cadherin, thereby disrupting the endothelial barrier function. This process is initiated by the activation of the small GTPase Rac by VEGFR-2 through the Src-dependent phosphorylation of Vav2, a guanine nucleotide-exchange factor. Rac activation, in turn, promotes the p21-activated kinase (PAK)-mediated phosphorylation of a highly conserved motif within the intracellular tail of VE-cadherin. Surprisingly, this results in the recruitment of beta-arrestin2 to serine-phosphorylated VE-cadherin, thereby promoting its internalization into clathrin-coated vesicles and the consequent disassembly of intercellular junctions. Ultimately, this novel biochemical route by which VEGF promotes endothelial permeability through the beta-arrestin2-dependent endocytosis of VE-cadherin may help identify new therapeutic targets for the treatment of many human diseases that are characterized by vascular leakage.

Vascular endothelial cadherin controls VEGFR-2 internalization and signaling from intracellular compartments

Receptor endocytosis is a fundamental step in controlling the magnitude, duration, and nature of cell signaling events. Confluent endothelial cells are contact inhibited in their growth and respond poorly to the proliferative signals of vascular endothelial growth factor (VEGF). In a previous study, we found that the association of vascular endothelial cadherin (VEC) with VEGF receptor (VEGFR) type 2 contributes to density-dependent growth inhibition (Lampugnani, G.M., A. Zanetti, M. Corada, T. Takahashi, G. Balconi, F. Breviario, F. Orsenigo, A. Cattelino, R. Kemler, T.O. Daniel, and E. Dejana. 2003. J. Cell Biol. 161:793–804). In the present study, we describe the mechanism through which VEC reduces VEGFR-2 signaling. We found that VEGF induces the clathrin-dependent internalization of VEGFR-2. When VEC is absent or not engaged at junctions, VEGFR-2 is internalized more rapidly and remains in endosomal compartments for a longer time. Internalization does not terminate its signaling; instead, the internalized receptor is phosphorylated, codistributes with active phospholipase C–γ, and activates p44/42 mitogen-activated protein kinase phosphorylation and cell proliferation. Inhibition of VEGFR-2 internalization reestablishes the contact inhibition of cell growth, whereas silencing the junction-associated density-enhanced phosphatase-1/CD148 phosphatase restores VEGFR-2 internalization and signaling. Thus, VEC limits cell proliferation by retaining VEGFR-2 at the membrane and preventing its internalization into signaling compartments.

Regulation of cGMP-dependent protein kinase expression by Rho and Kruppel-like transcription factor-4

Type I cGMP-dependent protein kinase (PKG I) plays a major role in vascular homeostasis by mediating smooth muscle relaxation in response to nitric oxide, but little is known about the regulation of PKG I expression in smooth muscle cells. We found opposing effects of RhoA and Rac1 on cellular PKG I expression: (i) cell density-dependent changes in PKG I expression varied directly with Rac1 activity and inversely with RhoA activity; (ii) RhoA activation by calpeptin suppressed PKG I, whereas RhoA down-regulation by small interfering RNA increased PKG I expression; and (iii) PKG I promoter activity was suppressed in cells expressing active RhoA or Rho-kinase but was enhanced in cells expressing active Rac1 or a dominant negative RhoA. Sp1 consensus sequences in the PKG I promoter were required for Rho regulation and bound nuclear proteins in a cell density-dependent manner, including the Krüppel-like factor 4 (KLF4). KLF4 was identified as a major trans-acting factor at two proximal Sp1 sites; active RhoA suppressed KLF4 DNA binding and trans-activation potential on the PKG I promoter. Experiments with actin-binding agents suggested that RhoA could regulate KLF4 via its ability to induce actin polymerization. Regulation of PKG I expression by RhoA may explain decreased PKG I levels in vascular smooth muscle cells found in some models of hypertension and vascular injury.

The multiple languages of endothelial cell-to-cell communication

Intercellular adhesion plays a key role during development and maintenance of tissue homeostasis. Within the vascular system, cell-cell adhesion is particularly important for the correct formation, networking, and remodeling of vessels. Although in vascular endothelial cells adhesive junctions account for the integrity of the vessel wall, they are not to be considered as static molecular structures that function as intercellular glue. This becomes evident during the remodeling of the endothelium in various physiological and pathological processes, requiring highly dynamic vascular adhesion complexes. Moreover, it has recently become evident that, besides their structural functions, adhesion molecules involved in endothelial cell-cell interaction play an important role in inducing and integrating intracellular signals that, in turn, impact on several aspects of vascular cell physiology. In this review, we describe these recent findings focusing on junctional proteins at adherens and tight junctions. The role of this adhesion molecule-mediated signaling is discussed in the context of developmental and pathological angiogenesis.

Rac1 and RhoA as regulators of endothelial phenotype and barrier function in hypoxia-induced neonatal pulmonary hypertension

Hypoxia is a common cause of persistent pulmonary hypertension in the newborn (PPHN), a condition associated with endothelial dysfunction and abnormal pulmonary vascular remodeling. The GTPase RhoA has been implicated in the pathogenesis of PPHN, but its contribution to endothelial remodeling and function is not known. We studied pulmonary artery endothelial cells (PAECs) taken from piglets with chronic hypoxia-induced pulmonary hypertension and from healthy animals and analyzed the roles of Rho GTPases in the regulation of the endothelial phenotype and function under basal normoxic conditions, acute hypoxia, and reoxygenation. The activities of RhoA, Rac1, and Cdc42 were correlated with changes in the endothelial cytoskeleton, adherens junctions, permeability, ROS production, VEGF levels, and activities of transcription factors hypoxia-inducible factor (HIF)-1alpha and NF-kappaB. Adenoviral gene transfer was used to express dominant-negative GTPases, kinase-dead p21-activated kinase (PAK)-1, and constitutively activated Rac1 in cells. PAECs from pulmonary hypertensive piglets had a stable abnormal phenotype with a sustained reduction in Rac1 activity and an increase in RhoA activity, which correlated with an increase in actin stress fiber formation, increased permeability, and a decrease in VEGF and ROS production. Cells from pulmonary hypertensive animals were still able to respond to acute hypoxia. They also showed high activities of HIF-1alpha and NF-kappaB, likely to result from changes in the activities of Rho GTPases. Activation of Rac1 and its effector PAK-1 as well as inhibition of RhoA restored the abnormal phenotype and permeability of hypertensive PAECs to normal.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Mechanotransduction in endothelial cell migration

The migration of endothelial cells (ECs) plays an important role in vascular remodeling and regeneration. EC migration can be regulated by different mechanisms such as chemotaxis, haptotaxis, and mechanotaxis. This review will focus on fluid shear stress-induced mechanotransduction during EC migration. EC migration and mechanotransduction can be modulated by cytoskeleton, cell surface receptors such as integrins and proteoglycans, the chemical and physical properties of extracellular matrix (ECM) and cell-cell adhesions. The shear stress applied on the luminal surface of ECs can be sensed by cell membrane and associated receptor and transmitted throughout the cell to cell-ECM adhesions and cell-cell adhesions. As a result, shear stress induces directional migration of ECs by promoting lamellipodial protrusion and the formation of focal adhesions (FAs) at the front in the flow direction and the disassembly of FAs at the rear. Persistent EC migration in the flow direction can be driven by polarized activation of signaling molecules and the positive feedback loops constituted by Rho GTPases, cytoskeleton, and FAs at the leading edge. Furthermore, shear stress-induced EC migration can overcome the haptotaxis of ECs. Given the hemodynamic environment of the vascular system, mechanotransduction during EC migration has a significant impact on vascular development, angiogenesis, and vascular wound healing.

The challenges of abundance: epithelial junctions and small GTPase signalling

Small GTPases of the Ras superfamily play critical roles in epithelial biogenesis. Many key morphogenetic functions occur when small GTPases act at epithelial junctions, where they mediate an increasingly complex interplay between cell-cell adhesion molecules and fundamental cellular processes, such as cytoskeletal activity, polarity and trafficking. Important recent advances in this field include the role of additional members of the Ras superfamily in cell-cell contact stability and the capacity for polarity determinants to regulate small GTPase signalling. Interestingly, small GTPases may participate in the cross-talk between different adhesive receptors: in tissues classical cadherins can selectively regulate other junctions through cell signalling rather than through a global influence on cell-cell cohesion.

Regulation of epithelial tubule formation by Rho family GTPases

Previous work has established that the integrin signal transduction pathway plays an important role in the regulation of epithelial tubule formation. Furthermore, it has been demonstrated that Rho-kinase, an effector of the Rho signaling pathway, is an important downstream modulator of collagen-mediated renal and mammary epithelial tubule morphogenesis. In the present study, MDCK cells that expressed mutant dominant-negative, constitutively active Rho family GTPases were used to provide further insight into Rho-GTPase signaling and the regulation of epithelial tubule formation. Using collagen gel overlays on MDCK cells as a model system, we observed phosphorylated myosin light chain (pMLC) at the leading edge of migrating lamellipodia. This epithelial remodeling led to the formation of multicellular branching epithelial tubular structures with extensive tight junctions. However, in cells expressing dominant-negative RhoN19, MLC phosphorylation, epithelial remodeling, and tubule formation were inhibited. Instead, only small apical lumens with a solitary tight junctional ring were observed, providing further evidence that Rho signaling through Rho-kinase is important in the regulation of epithelial tubule formation. Because the present model for the Rho signaling pathway proposes that Rac plays a prominent but reciprocal role in cell regulation, experiments were conducted using cells that expressed constitutively active RacV12. When incubated with collagen gels, RacV12-expressing cells formed small apical lumens with simple tight junctions, suggesting that Rac1 signaling also has a prominent role in the regulation of epithelial morphogenesis. Complementary collagen gel overlay experiments with wild-type MDCK cells demonstrated that endogenous Rac1 activation levels decreased over a time course consistent with lamellipodia and tubule formation. Under these conditions, Rac1 was initially localized to the basolateral membrane. However, after epithelial remodeling, activated Rac1 was observed primarily in lamellipodia. These studies support a model in which Rac1 and RhoA are important modulators of epithelial tubule formation.

Epac1 regulates integrity of endothelial cell junctions through VE-cadherin

We have previously shown that Rap1 as well as its guanine nucleotide exchange factor Epac1 increases cell-cell junction formation. Here, we show that activation of Epac1 with the exchange protein directly activated by cAMP (Epac)-specific cAMP analog 8CPT-2'O-Me-cAMP (007) resulted in a tightening of the junctions and a decrease in the permeability of the endothelial cell monolayer. In addition, 007 treatment resulted in the breakdown of actin stress fibers and the formation of cortical actin. These effects were completely inhibited by siRNA against Epac1. In VE-cadherin knock-out cells Epac1 did not affect cell permeability, whereas in cells re-expressing VE-cadherin this effect was restored. Finally, the effect of Epac activation on the actin cytoskeleton was independent of junction formation. From these results we conclude that in human umbilical vein endothelial cells Epac1 controls VE-cadherin-mediated cell junction formation and induces reorganization of the actin cytoskeleton.

A mechanosensory complex that mediates the endothelial cell response to fluid shear stress

Shear stress is a fundamental determinant of vascular homeostasis, regulating vascular remodelling, cardiac development and atherogenesis, but the mechanisms of transduction are poorly understood. Previous work showed that the conversion of integrins to a high-affinity state mediates a subset of shear responses, including cell alignment and gene expression. Here we investigate the pathway upstream of integrin activation. PECAM-1 (which directly transmits mechanical force), vascular endothelial cell cadherin (which functions as an adaptor) and VEGFR2 (which activates phosphatidylinositol-3-OH kinase) comprise a mechanosensory complex. Together, these receptors are sufficient to confer responsiveness to flow in heterologous cells. In support of the relevance of this pathway in vivo, PECAM-1-knockout mice do not activate NF-kappaB and downstream inflammatory genes in regions of disturbed flow. Therefore, this mechanosensing pathway is required for the earliest-known events in atherogenesis.

Tyrosine Phosphorylation of VE-cadherin Prevents Binding of p120- and β-Catenin and Maintains the Cellular Mesenchymal State

In several pathological conditions, epithelial cells demonstrate a breakdown of barrier function and acquire an invasive phenotype. Endothelial cells in particular are maintained in a mesenchymal state during the cell invasion phase of angiogenesis. We show here that tyrosine phosphorylation of the adherens junction protein VE-cadherin at two critical tyrosines, Tyr-658 and Tyr-731, via tyrosine kinase activation or phosphatase inactivation was sufficient to prevent the binding of p120- and beta-catenin, respectively, to the cytoplasmic tail of VE-cadherin. In fact, phosphorylation at either site led to the inhibition of cell barrier function. Cells expressing wild-type VE-cadherin showed decreased cell migration compared with cells lacking VE-cadherin, whereas expression of VE-cadherin with a simple phosphomimetic tyrosine-to-glutamic acid mutation of Y658E or Y731E was sufficient to restore the migratory response. These findings demonstrate that a single phosphorylation event within the VE-cadherin cytoplasmic tail is sufficient to maintain cells in a mesenchymal state.

Endothelial intercellular adhesion molecule (ICAM)-2 regulates angiogenesis

Endothelial junctions maintain endothelial integrity and vascular homeostasis. They modulate cell trafficking into tissues, mediate cell-cell contact and regulate endothelial survival and apoptosis. Junctional adhesion molecules such as vascular endothelial (VE)-cadherin and CD31/platelet endothelial cell adhesion molecule (PECAM) mediate contact between adjacent endothelial cells and regulate leukocyte transmigration and angiogenesis. The leukocyte adhesion molecule intercellular adhesion molecule 2 (ICAM-2) is expressed at the endothelial junctions. In this study we demonstrate that endothelial ICAM-2 also mediates angiogenesis. Using ICAM-2-deficient mice and ICAM-2-deficient endothelial cells, we show that the lack of ICAM-2 expression results in impaired angiogenesis both in vitro and in vivo. We show that ICAM-2 supports homophilic interaction, and that this may be involved in tube formation. ICAM-2-deficient cells show defective in vitro migration, as well as increased apoptosis in response to serum deprivation, anti-Fas antibody, or staurosporine. ICAM-2 signaling in human umbilical vein endothelial cells (HUVECs) was found to activate the small guanosine triphosphatase (GTPase) Rac, which is required for endothelial tube formation and migration. These data indicate that ICAM-2 may regulate angiogenesis via several mechanisms including survival, cell migration, and Rac activation. Our findings identify a novel pathway regulating angiogenesis through ICAM-2 and a novel mechanism for Rac activation during angiogenesis.

RhoA and Rac mediate endothelial cell polarization and detachment induced by T-cadherin

T-cadherin (T-cad) is an atypical GPI-anchored member of the cadherin superfamily. Ligation of T-cad receptors on endothelial cells prevents cell spreading, promotes elongation and polarization, decreases adhesion to the matrix, and facilitates migration. This study investigates involvement of Rho GTPases in T-cad signaling. Human umbilical vein endothelial cells were infected with adenoviral vectors expressing dominant-negative and/or constitutively active mutants of RhoA (N19RhoA/RhoA63), ROCK (RB/PH(TT)/CAT), and Rac1 (N17RAC). Mutant-infected and empty vector-infected cells were compared with respect to their ability to detach and polarize when plated on substratum containing recombinant T-cad protein used as a ligand mimicking homophilic T-cad interactions. ROCK involvement was also studied using specific inhibitor Y-27632. Adhesion assays, analysis of cell phenotype, and actin cytoskeleton organization using TRITC-labeled phalloidin demonstrated that T-cad-induced cell polarization includes two complementary components: RhoA/ROCK pathway is necessary for cell contraction, stress fiber assembly, and inhibition of spreading, whereas Rac is required for formation of actin-rich lamellipodia at the leading edges of polarized cells. Individual repression of either pathway only partially prevented cell polarization and detachment, while simultaneous repression of RhoA and Rac pathways fully eliminated responses to homophilic T-cad ligation. In conclusion, these data suggest that T-cad induces cell deadhesion and polarization via RhoA-ROCK- and Rac-dependent mechanisms.

VE-cadherin links tRNA synthetase cytokine to anti-angiogenic function

A natural fragment of an enzyme that catalyzes the first step of protein synthesis-human tryptophanyl-tRNA synthetase (T2-TrpRS) has potent anti-angiogenic activity. A cellular receptor through which T2-TrpRS exerts its anti-angiogenic activity has not previously been identified. Here T2-TrpRS was shown to bind at intercellular junctions of endothelial cells (ECs). Using genetic knock-outs, binding was established to depend on VE-cadherin, a calcium-dependent adhesion molecule, which is selectively expressed in ECs, concentrated at adherens junctions, and is essential for normal vascular development. In contrast, T2-TrpRS binding to EC junctions was not dependent on platelet endothelial cell adhesion molecule type-1, another adhesion molecule found at EC junctions. Pull-down assays confirmed direct complex formation between T2-TrpRS and VE-cadherin. Binding of T2-TrpRS inhibited VEGF-induced ERK activation and EC migration. Thus, a VE-cadherin-dependent pathway is proposed to link T2-TrpRS to inhibition of new blood vessel formation.

NADPH oxidase mediates vascular endothelial cadherin phosphorylation and endothelial dysfunction

Vascular endothelial activation is an early step during leukocyte/endothelial adhesion and transendothelial leukocyte migration in inflammatory states. Leukocyte transmigration occurs through intercellular gaps between endothelial cells. Vascular endothelial cadherin (VE-cadherin) is a predominant component of endothelial adherens junctions that regulates intercellular gap formation. We found that tumor necrosis factor (TNF) caused tyrosine phosphorylation of VE-cadherin, separation of lateral cell-cell junctions, and intercellular gap formation in human umbilical vein endothelial cell (HUVEC) monolayers. These events appear to be regulated by intracellular oxidant production through endothelial NAD(P)H (nicotinamide adenine dinucleotide phosphate) oxidase because antioxidants and expression of a transdominant inhibitor of the NADPH oxidase, p67(V204A), effectively blocked the effects of TNF on all 3 parameters of junctional integrity. Antioxidants and p67(V204A) also decreased TNF-induced JNK activation. Dominant-negative JNK abrogated VE-cadherin phosphorylation and junctional separation, suggesting a downstream role for JNK. Finally, adenoviral delivery of the kinase dead PAK1(K298A) decreased TNF-induced JNK activation, VE-cadherin phosphorylation, and lateral junctional separation, consistent with the proposed involvement of PAK1 upstream of the NADPH oxidase. Thus, PAK-1 acts in concert with oxidase during TNF-induced oxidant production and loss of endothelial cell junctional integrity.

VE-cadherin expression and clustering maintain low levels of survivin in endothelial cells

Survivin is strongly expressed in embryonic organs and in tumor cells but is low or absent in differentiated normal tissues. Resting endothelium expresses low levels of survivin but can up-regulate its synthesis on activation to proliferate. The mechanisms responsible for survivin down-regulation in resting conditions are still unknown. We report here that confluence and vascular endothelial-cadherin (VE-cadherin) expression induce contact inhibition of cell growth and survivin down-regulation in the endothelium. Using beta-catenin null and positive isogenic endothelial cell lines we found that the effect requires beta-catenin expression and its association to VE-cadherin cytoplasmic tail. Furthermore, in allantois organ cultures, survivin expression is up-regulated in areas of growing vessels where VE-cadherin is partially dismantled from junctions or in VE-cadherin -/- specimens. Overall, these data indicate that VE-cadherin and beta-catenin may negatively regulate survivin synthesis in endothelial cells. Consistently, in epidermal and pancreatic cell lines or ovarian tumors, epithelial-cadherin (E-cadherin) and survivin expression is inversely related, suggesting a non-cell-specific role of cadherins in reducing survivin synthesis.

Beta-catenin is required for endothelial-mesenchymal transformation during heart cushion development in the mouse

During heart development endocardial cells within the atrio-ventricular (AV) region undergo TGFβ-dependent epithelial-mesenchymal transformation (EMT) and invade the underlying cardiac jelly. This process gives rise to the endocardial cushions from which AV valves and part of the septum originate. In this paper we show that in mouse embryos and in AV explants TGFβ induction of endocardial EMT is strongly inhibited in mice deficient for endothelial β-catenin, leading to a lack of heart cushion formation. Using a Wnt-signaling reporter mouse strain, we demonstrated in vivo and ex vivo that EMT in heart cushion is accompanied by activation of β-catenin/TCF/Lef transcriptional activity. In cultured endothelial cells, TGFβ2 induces α-smooth muscle actin (αSMA) expression. This process was strongly reduced in β-catenin null cells, although TGFβ2 induced smad phosphorylation was unchanged. These data demonstrate an involvement of β-catenin/TCF/Lef transcriptional activity in heart cushion formation, and suggest an interaction between TGFβ and Wnt-signaling pathways in the induction of endothelial-mesenchymal transformation.

p21-activated kinase regulates endothelial permeability through modulation of contractility

Endothelial cells lining the vasculature have close cell-cell associations that maintain separation of the blood fluid compartment from surrounding tissues. Permeability is regulated by a variety of growth factors and cytokines and plays a role in numerous physiological and pathological processes. We examined a potential role for the p21-activated kinase (PAK) in the regulation of vascular permeability. In both bovine aortic and human umbilical vein endothelial cells, PAK is phosphorylated on Ser141 during the activation downstream of Rac, and the phosphorylated subfraction translocates to endothelial cell-cell junctions in response to serum, VEGF, bFGF, TNFalpha, histamine, and thrombin. Blocking PAK activation or translocation prevents the increase in permeability across the cell monolayer in response to these factors. Permeability correlates with myosin phosphorylation, formation of actin stress fibers, and the appearance of paracellular pores. Inhibition of myosin phosphorylation blocks the increase in permeability. These data suggest that PAK is a central regulator of endothelial permeability induced by multiple growth factors and cytokines via an effect on cell contractility. PAK may therefore be a suitable drug target for the treatment of pathological conditions where vascular leak is a contributing factor, such as ischemia and inflammation.

Endothelial cell-to-cell junctions: molecular organization and role in vascular homeostasis

Intercellular junctions mediate adhesion and communication between adjoining endothelial and epithelial cells. In the endothelium, junctional complexes comprise tight junctions, adherens junctions, and gap junctions. The expression and organization of these complexes depend on the type of vessels and the permeability requirements of perfused organs. Gap junctions are communication structures, which allow the passage of small molecular weight solutes between neighboring cells. Tight junctions serve the major functional purpose of providing a "barrier" and a "fence" within the membrane, by regulating paracellular permeability and maintaining cell polarity. Adherens junctions play an important role in contact inhibition of endothelial cell growth, paracellular permeability to circulating leukocytes and solutes. In addition, they are required for a correct organization of new vessels in angiogenesis. Extensive research in the past decade has identified several molecular components of the tight and adherens junctions, including integral membrane and intracellular proteins. These proteins interact both among themselves and with other molecules. Here, we review the individual molecules of junctions and their complex network of interactions. We also emphasize how the molecular architectures and interactions may represent a mechanistic basis for the function and regulation of junctions, focusing on junction assembly and permeability regulation. Finally, we analyze in vivo studies and highlight information that specifically relates to the role of junctions in vascular endothelial cells.

The role of the guanine nucleotide exchange factor Tiam1 in cellular migration, invasion, adhesion and tumor progression

While advances in molecular genetics have provided new insights into molecular alterations that lead to the development of many tumors, including breast carcinoma, the genetic and epigenetic alterations that result in metastatic spread of the disease, from which afflicted patients ultimately succumb, are much more poorly understood. Important biologic processes in the development of metastasis include increased migration and invasion of tumor cells. While the regulation of these processes is complex, they are controlled in part by small G proteins of the Rho family, including Rho, Rac, and Cdc42, that are involved in cytoskeletal organization. These proteins, active when bound to GTP, are, in turn, regulated by guanine nucleotide exchange factors (GNEFs) and guanine nucleotide activating proteins. The GNEF Tiam1 catalyzes nucleotide exchange for Rac in vivo, and Rac, Cdc42 and Rho in vitro. Tiam1 was identified first in 1994 by in vitro selection for invasiveness in T-lymphoma cells. Accordingly, Tiam1 has been shown to increase invasion in T-lymphoma cells, as well as to increase cellular migration in fibroblasts, and to promote motility in some neuronal cells. In contrast, Tiam1 has been demonstrated to increase cellular adhesion in some epithelial cell populations. Thus, Tiam1 has multiple roles in regulating cellular functions, likely dependent on the cell type, the substratum, transformation status of the cells, and the activation state of small G proteins in a given cell. Increasing evidence has focused on Tiam1's regulation, as well as Tiam1's role in cancer progression and metastasis. Recent results from other laboratories and ours have demonstrated that increased Tiam1 expression correlates with grade of breast cancer in humans and metastatic potential of human breast carcinoma cell lines in nude mice. This review will discuss Tiam1's cellular functions and methods of regulation, and will highlight Tiam1's contribution to cancer progression and metastasis.

VE-cadherin: adhesion at arm's length

VE-cadherin was first identified in the early 1990s and quickly emerged as an important endothelial cell adhesion molecule. The past decade of research has revealed key roles for VE-cadherin in vascular permeability and in the morphogenic events associated with vascular remodeling. The details of how VE-cadherin functions in adhesion became apparent with structure-function analysis of the cadherin extracellular domain and with the identification of the catenins, a series of cytoplasmic proteins that bind to the cadherin tail and mediate interactions between cadherins and the cytoskeleton. Whereas early work focused on the armadillo family proteins beta-catenin and plakoglobin, more recent investigations have identified p120-catenin (p120(ctn)) and a related group of armadillo family members as key binding partners for the cadherin tail. Furthermore, a series of new studies indicate a key role for p120(ctn) in regulating cadherin membrane trafficking in mammalian cells. These recent studies place p120(ctn) at the hub of a cadherin-catenin regulatory mechanism that controls cadherin plasma membrane levels in cells of both epithelial and endothelial origin.

R-cadherin influences cell motility via Rho family GTPases

Classical cadherins are the transmembrane proteins of the adherens junction and mediate cell-cell adhesion via homotypic interactions in the extracellular space. In addition, they mediate connections to the cytoskeleton by means of their association with catenins. Decreased cadherin-mediated adhesion has been implicated as an important component of tumorigenesis. Cadherin switching is central to the epithelial to mesenchymal transitions that drive normal developmental processes. Cadherin switching has also been implicated in tumorigenesis, particularly in metastasis. Recently, cadherins have been shown to be engaged in cellular activities other than adhesion, including motility, invasion, and signaling. In this study, we show that inappropriate expression of R-cadherin in tumor cells results in decreased expression of endogenous cadherins (cadherin switching) and sustained signaling through Rho GTPases. In addition, we show that R-cadherin induces cell motility when expressed in epithelial cells and that this increased motility is dependent upon Rho GTPase activity.

Vascular endothelial-cadherin regulates cytoskeletal tension, cell spreading, and focal adhesions by stimulating RhoA

Changes in vascular endothelial (VE)-cadherin-mediated cell-cell adhesion and integrin-mediated cell-matrix adhesion coordinate to affect the physical and mechanical rearrangements of the endothelium, although the mechanisms for such cross talk remain undefined. Herein, we describe the regulation of focal adhesion formation and cytoskeletal tension by intercellular VE-cadherin engagement, and the molecular mechanism by which this occurs. Increasing the density of endothelial cells to increase cell-cell contact decreased focal adhesions by decreasing cell spreading. This contact inhibition of cell spreading was blocked by disrupting VE-cadherin engagement with an adenovirus encoding dominant negative VE-cadherin. When changes in cell spreading were prevented by culturing cells on a micropatterned substrate, VE-cadherin-mediated cell-cell contact paradoxically increased focal adhesion formation. We show that VE-cadherin engagement mediates each of these effects by inducing both a transient and sustained activation of RhoA. Both the increase and decrease in cell-matrix adhesion were blocked by disrupting intracellular tension and signaling through the Rho-ROCK pathway. In all, these findings demonstrate that VE-cadherin signals through RhoA and the actin cytoskeleton to cross talk with cell-matrix adhesion and thereby define a novel pathway by which cell-cell contact alters the global mechanical and functional state of cells.

EphA2 receptor tyrosine kinase regulates endothelial cell migration and vascular assembly through phosphoinositide 3-kinase-mediated Rac1 GTPase activation

Angiogenesis is critical for vascular remodeling during development and contributes to the pathogenesis of diseases such as cancer. Targeted disruption of several EphB class receptor tyrosine kinases results in vascular remodeling defects during embryogenesis. The role of EphA class receptors in vascular remodeling, however, is not well-characterized. We recently demonstrated that global inhibition of EphA receptors disrupts endothelial migration induced by ephrin, VEGF or tumor-derived signals, though the specific target remained undefined. Here, we report that EphA2 regulates endothelial cell assembly and migration through phosphoinositide (PI) 3-kinase-mediated activation of Rac1 GTPase in two model systems: primary bovine and murine pulmonary microvascular endothelial cells. EphA2-deficient endothelial cells fail to undergo vascular assembly and migration in response to ephrin-A1 in vitro. Ephrin-A1 stimulation induces PI3-kinase-dependent activation of Rac1 in wild-type endothelial cells, whereas EphA2-deficient cells fail to activate Rac1 upon stimulation. Expression of dominant negative PI3-kinase or Rac1 inhibits ephrin-A1-induced endothelial cell migration. Consistent with in vitro data, EphA2-deficient mice show a diminished angiogenic response to ephrin-A1 in vivo. Moreover, EphA2-deficient endothelial cells fail to assemble in vivo when transplanted into recipient mice. These data suggest that EphA2 is an essential regulator of post-natal angiogenesis.

Cdc42 Regulates the Restoration of Endothelial Barrier Function

The mechanisms involved in the restoration of endothelial cell junctions subsequent to barrier disruption remain unclear. It is known that formation of adherens junctions (AJs) affects cytoskeletal actin arrangement and that Rho GTPases regulate the state of actin polymerization. In the present study, we examined the role of the Rho GTPases, Rho, Rac, and Cdc42 in the reannealing of AJs. We studied the response to thrombin, which increases endothelial permeability through disassembly of AJs, followed by recovery of barrier function through junctional reannealing within 2 hours. Cdc42 was activated late, at ≈1 hour after thrombin exposure, concurrent with its translocation from the cytoplasm to the membrane. Activation and translocation of Cdc42 preceded the reformation of AJs. Expression of the dnCdc42 mutant (N17Cdc42) significantly delayed the reformation of the VE-cadherin-containing AJs and restoration of endothelial barrier function. We also studied the lung microcirculation to address the in vivo relevance of Cdc42 signaling in barrier restoration. N17Cdc42 expression in the mouse lung endothelium markedly attenuated the endothelial barrier recovery after the permeability increase induced by activation of the thrombin receptor protease-activated receptor-1. These findings demonstrate the critical function of Cdc42 in restoring AJ-dependent, endothelial cell homotypic adhesion and barrier function. The delayed activation of Cdc42 represents a negative-feedback mechanism that signals AJ reassembly after the increase in endothelial permeability induced by inflammatory mediators such as thrombin.

Distinct sites in E-cadherin regulate different steps in Drosophila tracheal tube fusion

We have investigated how E-cadherin controls the elaboration of adherens junction associated cytoskeletal structures crucial for assembling tubular networks. During Drosophila development, tracheal branches are joined at branch tips through lumens that traverse doughnut-shaped fusion cells. Fusion cells form E-cadherin contacts associated with a track that contains F-actin, microtubules, and Shot, a plakin that binds F-actin and microtubules. Live imaging reveals that fusion occurs as the fusion cell apical surfaces meet after invaginating along the track. Initial track assembly requires E-cadherin binding to beta-catenin. Surprisingly, E-cadherin also controls track maturation via a juxtamembrane site in the cytoplasmic domain. Fusion cells expressing an E-cadherin mutant in this site form incomplete tracks that contain F-actin and Shot, but lack microtubules. These results indicate that E-cadherin controls track initiation and maturation using distinct, evolutionarily conserved signals to F-actin and microtubules, and employs Shot to promote adherens junction-associated cytoskeletal assembly.

Requirement of Rac activity for maintenance of capillary endothelial barrier properties

Our previous experiments indicated that GTPases, other than RhoA, are important for the maintenance of endothelial barrier integrity in both intact microvessels of rats and mice and cultured mouse myocardial endothelial (MyEnd) cell monolayers ( J Physiol 539: 295–308, 2002). In the present study, we inhibited the endothelial GTPase Rac by Clostridium sordellii lethal toxin (LT) and investigated the relation between the degree of inhibition of Rac by glucosylation and increased endothelial barrier permeability. In rat venular microvessels, LT (200 ng/ml) increased hydraulic conductivity from a control value of 2.5 ± 0.6 to 100.8 ± 18.7 × 10–7cm·s–1·cmH2O–1after 80 min. In cultured MyEnd cells exposed to LT (200 ng/ml), up to 60% of cellular Rac was glucosylated after 90 min, resulting in depolymerization of F-actin and interruptions of junctional distribution of vascular endothelial cadherin (VE-cadherin) and β-catenin as well as the formation of intercellular gaps. To understand the mechanism by which inhibition of Rac caused disassembly of adherens junctions, we used laser tweezers to quantify VE-cadherin-mediated adhesion. LT and cytochalasin D, an actin depolymerizing agent, both reduced adhesion of VE-cadherin-coated microbeads to the endothelial cell surface, whereas the inhibitor of Rho kinase Y-27632 did not. Stabilization of actin filaments by jasplakinolide completely blocked the effect of cytochalasin D but not of LT on bead adhesion. We conclude that Rac regulates endothelial barrier properties in vivo and in vitro by 1) modulation of actin filament polymerization and 2) acting directly on the tether between VE-cadherin and the cytoskeleton.

Cadherin-mediated cellular signaling

Recent cadherin studies focusing on cellular signaling have shown that several pathways are activated by cadherin-mediated cell-cell contact. Cadherin-mediated contacts activate Rho family GTPases, regulate the availability of beta-catenin to participate in Wnt signaling, and function in receptor tyrosine kinase signaling. Although different classical cadherins bind to the same cytosolic proteins via their cytoplasmic tails, one message that is clear from the recent literature is that downstream signals emanating from cadherin-mediated contacts are both cadherin-specific and cell-context-specific.

The conditional inactivation of the beta-catenin gene in endothelial cells causes a defective vascular pattern and increased vascular fragility

Using the Cre/loxP system we conditionally inactivated beta-catenin in endothelial cells. We found that early phases of vasculogenesis and angiogenesis were not affected in mutant embryos; however, vascular patterning in the head, vitelline, umbilical vessels, and the placenta was altered. In addition, in many regions, the vascular lumen was irregular with the formation of lacunae at bifurcations, vessels were frequently hemorrhagic, and fluid extravasation in the pericardial cavity was observed. Cultured beta-catenin -/- endothelial cells showed a different organization of intercellular junctions with a decrease in alpha-catenin in favor of desmoplakin and marked changes in actin cytoskeleton. These changes paralleled a decrease in cell-cell adhesion strength and an increase in paracellular permeability. We conclude that in vivo, the absence of beta-catenin significantly reduces the capacity of endothelial cells to maintain intercellular contacts. This may become more marked when the vessels are exposed to high or turbulent flow, such as at bifurcations or in the beating heart, leading to fluid leakage or hemorrhages.

VE-cadherin simultaneously stimulates and inhibits cell proliferation by altering cytoskeletal structure and tension

Engagement of vascular endothelial (VE)-cadherin leads to the cessation of proliferation commonly known as 'contact inhibition'. We show that VE-cadherin inhibits growth by mediating changes in cell adhesion to the extracellular matrix. Increasing cell-cell contact decreased cell spreading and proliferation, which was reversed by blocking engagement of VE-cadherin. Using a new system to prevent the cadherin-induced changes in cell spreading, we revealed that VE-cadherin paradoxically increased proliferation. Treating cells with inhibitors of PKC and MEK abrogated the stimulatory signal at concentrations that disrupted the formation of actin fibers across the cell-cell contact. Directly disrupting actin fibers, blocking actin-myosin-generated tension, or inhibiting signaling through Rho specifically inhibited the cadherin-induced proliferative signal. By progressively altering the degree to which cell-cell contact inhibited cell spreading, we show that cell-cell contact ultimately increased or decreased the overall proliferation rate of the population by differentially shifting the balance between the two opposing proliferative cues. The existence of opposing growth signals induced by VE-cadherin that are both mediated through crosstalk with cytoskeletal structure highlights the complex interplay of mechanical and chemical signals with which cells navigate in their physical microenvironment.

Regulation of Tiam1-Rac signalling

The GTPases of the Rho family are molecular switches that play an important role in a wide range of cellular processes and are increasingly implicated in tumourigenesis. Unlike what was found for the Ras oncogenes in tumours, hardly any activating mutations have been found in the genes encoding Rho proteins. In the past, we have identified Tiam1 (T-lymphoma invasion and metastasis) as a specific activator for the Rho-like GTPase Rac. In vivo, Tiam1 deficiency protects against Ras-induced skin carcinogenesis, underscoring the consequences of deregulated signalling for the onset and progression of tumours. Thus, an important level of regulation of signalling via the Rho-like GTPases comes from the specific control of their activators. In this paper, we review what is known on the specific regulation of Tiam1 signalling towards Rac.

Minimal mutation of the cytoplasmic tail inhibits the ability of E-cadherin to activate Rac but not phosphatidylinositol 3-kinase: direct evidence of a role for cadherin-activated Rac signaling in adhesion and contact formation

Classic cadherins are adhesion-activated cell signaling receptors. In particular, homophilic cadherin ligation can directly activate Rho family GTPases and phosphatidylinositol 3-kinase (PI3-kinase), signaling molecules with the capacity to support the morphogenetic effects of these adhesion molecules during development and disease. However, the molecular basis for cadherin signaling has not been elucidated, nor is its precise contribution to cadherin function yet understood. One attractive hypothesis is that cadherin-activated signaling participates in stabilizing adhesive contacts (Yap, A. S., and Kovacs, E. M. (2003) J. Cell Biol. 160, 11-16). We now report that minimal mutation of the cadherin cytoplasmic tail to uncouple binding of p120-ctn ablated the ability of E-cadherin to activate Rac. This was accompanied by profound defects in the capacity of cells to establish stable adhesive contacts, defects that were rescued by sustained Rac signaling. These data provide direct evidence for a role of cadherin-activated Rac signaling in contact formation and adhesive stabilization. In contrast, cadherin-activated PI3-kinase signaling was not affected by loss of p120-ctn binding. The molecular requirements for E-cadherin to activate Rac signaling thus appear distinct from those that stimulate PI3-kinase, and we postulate that p120-ctn may play a central role in the E-cadherin-Rac signaling pathway.

Contact inhibition of VEGF-induced proliferation requires vascular endothelial cadherin, beta-catenin, and the phosphatase DEP-1/CD148

Confluent endothelial cells respond poorly to the proliferative signals of VEGF. Comparing isogenic endothelial cells differing for vascular endothelial cadherin (VE-cadherin) expression only, we found that the presence of this protein attenuates VEGF-induced VEGF receptor (VEGFR) 2 phosphorylation in tyrosine, p44/p42 MAP kinase phosphorylation, and cell proliferation. VE-cadherin truncated in beta-catenin but not p120 binding domain is unable to associate with VEGFR-2 and to induce its inactivation. beta-Catenin-null endothelial cells are not contact inhibited by VE-cadherin and are still responsive to VEGF, indicating that this protein is required to restrain growth factor signaling. A dominant-negative mutant of high cell density-enhanced PTP 1 (DEP-1)//CD148 as well as reduction of its expression by RNA interference partially restore VEGFR-2 phosphorylation and MAP kinase activation. Overall the data indicate that VE-cadherin-beta-catenin complex participates in contact inhibition of VEGF signaling. Upon stimulation with VEGF, VEGFR-2 associates with the complex and concentrates at cell-cell contacts, where it may be inactivated by junctional phosphatases such as DEP-1. In sparse cells or in VE-cadherin-null cells, this phenomenon cannot occur and the receptor is fully activated by the growth factor.

Cadherin engagement inhibits RhoA via p190RhoGAP

Cadherins are transmembrane receptors that mediate cell-cell adhesion in epithelial cells. A number of changes occur during cadherin-mediated junction formation, one of which is a rearrangement of the actin cytoskeleton. Key regulators of actin cytoskeletal dynamics in cells are the Rho family of GTPases. We have demonstrated in previous studies that cadherin signaling suppresses RhoA activity and activates Rac1. The signaling events downstream of cadherins that modulate the activity of Rho family proteins remain unknown. Here we have identified a pathway by which RhoA becomes inactivated by cadherins. To determine whether cadherins regulate RhoA through activation of a GTPase-activating protein (GAP) for RhoA, we used constitutively active RhoA to isolate activated GAPs. Using this assay, we have identified the RhoA-specific GAP, p190RhoGAP, downstream from engaged cadherins. We found that cadherin engagement induced tyrosine phosphorylation of p190RhoGAP and increased its binding to p120RasGAP. The increased precipitation of p190RhoGAP with 63LRhoA was blocked by addition of PP2 suggesting that Src family kinases are required downstream from cadherin signaling. The inhibition of RhoA activity by cadherins was antagonized by expression of a dominant negative p190RhoGAP. Taken together, these data demonstrate that p190RhoGAP activity is critical for RhoA inactivation by cadherins.

Sticky business: orchestrating cellular signals at adherens junctions

Cohesive sheets of epithelial cells are a fundamental feature of multicellular organisms and are largely a product of the varied functions of adherens junctions. These junctions and their cytoskeletal associations contribute heavily to the distinct shapes, polarity, spatially oriented mitotic spindle planes, and cellular movements of developing tissues. Deciphering the underlying mechanisms that govern these conserved cellular rearrangements is a prerequisite to understanding vertebrate morphogenesis.

Direct cadherin-activated cell signaling: a view from the plasma membrane

Classical cadherin adhesion molecules are key determinants of cell recognition and tissue morphogenesis, with diverse effects on cell behavior. Recent developments indicate that classical cadherins are adhesion-activated signaling receptors. In particular, early-immediate Rac signaling is emerging as a mechanism to coordinate cadherin-actin integration at the plasma membrane.

Rho GTPases and the regulation of endothelial permeability

Endothelial permeability depends on the integrity of intercellular junctions as well as actomyosin-based cell contractility. Rho GTPases have been implicated in signalling by many vasoactive substances including thrombin, tumour necrosis factor alpha (TNF-alpha), bradykinin, histamine, lysophosphatidic acid (LPA), vascular endothelial growth factor (VEGF), and hepatocyte growth factor (HGF). Two Rho family GTPases, Rho and Rac, have emerged as key regulators acting antagonistically to regulate endothelial barrier function: Rho increases actomyosin contractility, which facilitates breakdown of intercellular junctions, whereas Rac stabilizes endothelial junctions and counteracts the effects of Rho. In this review, we present evidence for the opposing effects of these two regulatory proteins and discuss links between them and other key signalling molecules such as cyclic AMP (cAMP), cyclic GMP (cGMP), phosphatidylinositide 3-kinases (PI3Ks), mitogen-activated protein kinases (MAPKs), and protein kinases C (PKCs). We also discuss strategies for targeting Rho GTPase signalling in therapies for diseases involving altered endothelial permeability.

Rac activation upon cell-cell contact formation is dependent on signaling from the epidermal growth factor receptor

Cadherins are transmembrane receptors that mediate cell-cell adhesion. They play an essential role in embryonic development and maintenance of tissue architecture. The Rho family small GTPases regulate actin cytoskeletal dynamics in different cell types. The function of two family members, Rho and Rac, is required for the stability of cadherins at cell-cell contacts. Consistent with the published data we have found that Rac is activated upon induction of intercellular adhesion in epithelial cells. This activation is dependent on functional cadherins (Nakagawa, M., Fukata, M., Yamaga, M., Itoh, N., and Kaibuchi, K. (2001) J. Cell Sci. 114, 1829-1838; Noren, N. K., Niessen, C. M., Gumbiner, B. M., and Burridge, K. (2001) J. Biol. Chem. 276, 3305-3308). Here we show for the first time that clustering of cadherins using antibody-coated beads is sufficient to promote Rac activation. In the presence of Latrunculin B, Rac can be partially activated by antibody-clustered cadherins. These results suggest that actin polymerization is not required for initial Rac activation. Contrary to what has been described before, phosphatidylinositol 3-kinases are not involved in Rac activation following cell-cell adhesion in keratinocytes. Interestingly, inhibition of epidermal growth factor receptor signaling efficiently blocks the increased Rac-GTP levels observed after contact formation. We conclude that cadherin-dependent adhesion can activate Rac via epidermal growth factor receptor signaling.

Cell-cell adhesion and signalling

Signalling pathways activated by Rho small GTPases have recently been identified that coordinate junction assembly, stability and function, as well as interactions of adhesive complexes with the underlying cortical cytoskeleton. Particularly exciting is the interplay between adherens junctions, activation of Rho proteins and the dynamics of microtubule, actin and intermediate filaments. This interplay has important implications for functional regulation of cell-cell adhesion, and points to a more integrated view of signalling processes.

A monoclonal antibody to vascular endothelial-cadherin inhibits tumor angiogenesis without side effects on endothelial permeability

Vascular endothelial cadherin (VE-cadherin) is an endothelial-specific, trans-membrane protein that promotes homophilic cell adhesion. Inhibition of VE-cadherin by the blocking monoclonal antibody (mAb) BV13 inhibited angiogenesis and tumor growth in vivo. However, this effect was accompanied by a marked increase in lung and heart permeability. In the present paper, we characterize a different VE-cadherin mAb (BV14) that is able to inhibit angiogenesis without affecting vascular permeability. In vitro studies show that BV14, in contrast to BV13, did not increase paracellular permeability of endothelial monolayers and did not disrupt VE-cadherin clusters at junctions. However, both antibodies could inhibit formation of vascularlike structures in collagen gels and increase migration of endothelial cells into wounded areas. In vivo, BV14 and BV13 were equally active in inhibiting angiogenesis in the mouse cornea and in reducing the growth of hemangioma and C6 glioma. In contrast to BV13, BV14 did not change vascular permeability in all the organs tested and at any dose used. BV14 and BV13 bind to VE-cadherin extracellular repeats EC4 and EC1, respectively. We propose that, in resting vessels, where junctions are stable and well-structured, antibody binding to EC1 but not EC4 disrupts their organization and increases permeability. In contrast, in growing vessels, where endothelial cells are migrating and junctions are weaker, antibody binding to EC4 may be sufficient to disrupt cell-to-cell adhesion and inhibit assembly of new vascular structures.