Catenin-dependent and -independent functions of vascular endothelial cadherin

Reversal of pulmonary veno-occlusive disease phenotypes by inhibition of the integrated stress response

Pulmonary veno-occlusive disease (PVOD) is a rare form of pulmonary hypertension arising from EIF2AK4 gene mutations or mitomycin C (MMC) administration. The lack of effective PVOD therapies is compounded by a limited understanding of the mechanisms driving vascular remodeling in PVOD. Here we show that administration of MMC in rats mediates activation of protein kinase R (PKR) and the integrated stress response (ISR), which leads to the release of the endothelial adhesion molecule vascular endothelial (VE) cadherin (VE-Cad) in complex with RAD51 to the circulation, disruption of endothelial barrier and vascular remodeling. Pharmacological inhibition of PKR or ISR attenuates VE-Cad depletion, elevation of vascular permeability and vascular remodeling instigated by MMC, suggesting potential clinical intervention for PVOD. Finally, the severity of PVOD phenotypes was increased by a heterozygous BMPR2 mutation that truncates the carboxyl tail of the receptor BMPR2, underscoring the role of deregulated bone morphogenetic protein signaling in the development of PVOD.

Reversal of pulmonary veno-occlusive disease phenotypes by inhibition of the integrated stress response

Pulmonary veno-occlusive disease (PVOD) is a rare form of pulmonary hypertension arising from EIF2AK4 gene mutations or mitomycin C (MMC) administration. The lack of effective PVOD therapies is compounded by a limited understanding of the mechanisms driving the vascular remodeling in PVOD. We show that the administration of MMC in rats mediates the activation of protein kinase R (PKR) and the integrated stress response (ISR), which lead to the release of the endothelial adhesion molecule VE-Cadherin in the complex with Rad51 to the circulation, disruption of endothelial barrier, and vascular remodeling. Pharmacological inhibition of PKR or ISR attenuates the depletion of VE-Cadherin, elevation of vascular permeability, and vascular remodeling instigated by MMC, suggesting potential clinical intervention for PVOD. Finally, the severity of PVOD phenotypes was increased by a heterozygous BMPR2 mutation that truncates the carboxyl tail of BMPR2, underscoring the role of deregulated BMP signal in the development of PVOD.

Long-term effect of sodium selenite on the integrity and permeability of on-chip microvasculature

Development of the robust and functionally stable three-dimensional (3D) microvasculature remains challenging. One often-overlooked factor is the presence of potential anti-angiogenic agents in culture media. Sodium selenite, an antioxidant commonly used in serum-free media, demonstrates strong anti-angiogenic properties and has been proposed as an anticancer drug. However, its long-term effects on in vitro microvascular systems at the concentrations used in culture media have not been studied. In this study, we used a five-channel microfluidic device to investigate the concentration and temporal effects of sodium selenite on the morphology and functionality of on-chip preformed microvasculature. We found that high concentrations (∼3.0 μM) had adverse effects on microvasculature perfusion, permeability, and overall integrity within the first few days. Moreover, even at low concentrations (∼3.0 nM), a long-term culture effect was observed, resulting in an increase in vascular permeability without any noticeable changes in morphology. A further analysis suggested that vessel leakage may be due to vascular endothelial growth factor dysregulation, disruption of intracellular junctions, or both. This study provides important insight into the adverse effects caused by the routinely present sodium selenite on 3D microvasculature in long-term studies for its application in disease modeling and drug screening.

Maternal high fat diet-induced obesity affects trophoblast differentiation and placental function in mice†

Evidence suggests that maternal obesity (MO) can aggravate placental function causing severe pathologies during the perinatal window. However, molecular changes and mechanisms of placental dysfunction remain largely unknown. This work aimed to decipher structural and molecular alterations of the placental transfer zone associated with MO. To this end, mice were fed a high fat diet (HFD) to induce obesity before mating, and pregnant dams were sacrificed at E15.5 to receive placentas for molecular, histological, and ultrastructural analysis and to assess unidirectional materno-fetal transfer capacity. Laser-capture microdissection was used to collect specifically placental cells of the labyrinth zone for proteomics profiling. Using BeWo cells, fatty acid-mediated mechanisms of adherens junction stability, cell layer permeability, and lipid accumulation were deciphered. Proteomics profiling revealed downregulation of cell adhesion markers in the labyrinth zone of obese dams, and disturbed syncytial fusion and detachment of the basement membrane (BM) within this zone was observed, next to an increase in materno-fetal transfer in vivo across the placenta. We found that fetuses of obese dams develop a growth restriction and in those placentas, labyrinth zone volume-fraction was significantly reduced. Linoleic acid was shown to mediate beta-catenin level and increase cell layer permeability in vitro. Thus, MO causes fetal growth restriction, molecular and structural changes in the transfer zone leading to impaired trophoblast differentiation, BM disruption, and placental dysfunction despite increased materno-fetal transfer capacity. These adverse effects are probably mediated by fatty acids found in HFD demonstrating the need for obesity treatment to mitigate placental dysfunction and prevent offspring pathologies.

Endothelial Permeability Assays In Vitro

The endothelium is a thin layer of squamous cells that acts as a semipermeable barrier regulating vascular permeability to let molecules pass through it thereby maintaining tissue fluid homeostasis. Physiological increase in endothelial or vascular permeability is transient, transpired by post-tissue injury during the initial phases of healing, whereas pathological permeability is persistent commonly witnessed in conditions such as atherosclerosis, chronic inflammation, tumor growth, and diabetic retinopathy. The in vivo or in situ use of animal models in the assessment of permeability not only raises inevitable ethical concerns but also confers difficulty to apply to high-throughput screening. Therefore, there is an ever-increasing dependency on in vitro studies to assess drug permeability, and various research programs have suffered to develop appropriate in vitro assays for measurement and prediction. In vitro models that both mimic in vivo microvascular endothelium and can be utilized to record changes in endothelial permeability are vital in delineating the mechanisms involved in the prevention and treatment of disorders related to vascular permeability. The Transwell^® and the electric cell-substrate impedance sensing (ECIS) assays are extensively used to assess the trans-endothelial permeability of solutes such as albumin, dextrans, and sucrose across endothelial monolayers and based on electrical resistance, etc. These models have several advantages such as the ease to perform and avoid the complexities of using a live animal.

VE-PTP stabilizes VE-cadherin junctions and the endothelial barrier via a phosphatase-independent mechanism

Juettner et al. describe a novel phosphatase-activity–independent mechanism by which the phosphatase VE-PTP restricts endothelial permeability. VE-PTP functions as a scaffold that binds and inhibits the RhoGEF GEF-H1, limiting RhoA-dependent tension across VE-cadherin junctions and decreasing VE-cadherin internalization to stabilize adherens junctions and reduce endothelial permeability.

Vascular endothelial (VE) protein tyrosine phosphatase (PTP) is an endothelial-specific phosphatase that stabilizes VE-cadherin junctions. Although studies have focused on the role of VE-PTP in dephosphorylating VE-cadherin in the activated endothelium, little is known of VE-PTP’s role in the quiescent endothelial monolayer. Here, we used the photoconvertible fluorescent protein VE-cadherin-Dendra2 to monitor VE-cadherin dynamics at adherens junctions (AJs) in confluent endothelial monolayers. We discovered that VE-PTP stabilizes VE-cadherin junctions by reducing the rate of VE-cadherin internalization independently of its phosphatase activity. VE-PTP serves as an adaptor protein that through binding and inhibiting the RhoGEF GEF-H1 modulates RhoA activity and tension across VE-cadherin junctions. Overexpression of the VE-PTP cytosolic domain mutant interacting with GEF-H1 in VE-PTP–depleted endothelial cells reduced GEF-H1 activity and restored VE-cadherin dynamics at AJs. Thus, VE-PTP stabilizes VE-cadherin junctions and restricts endothelial permeability by inhibiting GEF-H1, thereby limiting RhoA signaling at AJs and reducing the VE-cadherin internalization rate.

Vascular Endothelial (VE)-Cadherin, Endothelial Adherens Junctions, and Vascular Disease

Endothelial cell-cell adherens junctions (AJs) supervise fundamental vascular functions, such as the control of permeability and transmigration of circulating leukocytes, and the maintenance of existing vessels and formation of new ones. These processes are often dysregulated in pathologies. However, the evidence that links dysfunction of endothelial AJs to human pathologies is mostly correlative. In this review, we present an update of the molecular organization of AJ complexes in endothelial cells (ECs) that is mainly based on observations from experimental models. Furthermore, we report in detail on a human pathology, cerebral cavernous malformation (CCM), which is initiated by loss-of-function mutations in the genes that encode the three cytoplasmic components of AJs (CCM1, CCM2, and CCM3). At present, these represent a unique example of mutations in components of endothelial AJs that cause human disease. We describe also how studies into the defects of AJs in CCM are shedding light on the crucial regulatory mechanisms and signaling activities of these endothelial structures. Although these observations are specific for CCM, they support the concept that dysfunction of endothelial AJs can directly contribute to human pathologies.

RGD cadherins and α2β1 integrin in cancer metastasis: A dangerous liaison

We propose a new cadherin family classification comprising epithelial cadherins (cadherin 17 [CDH17], cadherin 16, VE-cadherin, cadherin 6 and cadherin 20) containing RGD motifs within their sequences. Expression of some RGD cadherins is associated with aggressive forms of cancer during the late stages of metastasis, and CDH17 and VE-cadherin have emerged as critical actors in cancer metastasis. After binding to α2β1 integrin, these cadherins promote integrin β1 activation, and thereby cell adhesion, invasion and proliferation, in liver and lung metastasis. Activation of α2β1 integrin provokes an affinity increase for type IV collagen, a major component of the basement membrane and a critical partner for cell anchoring in liver and other metastatic organs. Activation of α2β1 integrin by RGD motifs breaks an old paradigm of integrin classification and supports an important role of this integrin in cancer metastasis. Recently, synthetic peptides containing the RGD motif of CDH17 elicited highly specific and selective antibodies that block the ability of CDH17 RGD to activate α2β1 integrin. These monoclonal antibodies inhibit metastatic colonization in orthotopic mouse models of liver and lung metastasis for colorectal cancer and melanoma, respectively. Hopefully, blocking the cadherin RGD ligand capacity will give us control over the integrin activity in solid tumors metastasis, paving the way for development of new agents of cancer treatment.

VE-cadherin RGD motifs promote metastasis and constitute a potential therapeutic target in melanoma and breast cancers

We have investigated the role of vascular-endothelial (VE)-cadherin in melanoma and breast cancer metastasis. We found that VE-cadherin is expressed in highly aggressive melanoma and breast cancer cell lines. Remarkably, inactivation of VE-cadherin triggered a significant loss of malignant traits (proliferation, adhesion, invasion and transendothelial migration) in melanoma and breast cancer cells. These effects, except transendothelial migration, were induced by the VE-cadherin RGD motifs. Co-immunoprecipitation experiments demonstrated an interaction between VE-cadherin and α2β1 integrin, with the RGD motifs found to directly affect β1 integrin activation. VE-cadherin-mediated integrin signaling occurred through specific activation of SRC, ERK and JNK, including AKT in melanoma. Knocking down VE-cadherin suppressed lung colonization capacity of melanoma or breast cancer cells inoculated in mice, while pre-incubation with VE-cadherin RGD peptides promoted lung metastasis for both cancer types. Finally, an in silico study revealed the association of high VE-cadherin expression with poor survival in a subset of melanoma patients and breast cancer patients showing low CD34 expression. These findings support a general role for VE-cadherin and other RGD cadherins as critical regulators of lung and liver metastasis in multiple solid tumours. These results pave the way for cadherin-specific RGD targeted therapies to control disseminated metastasis in multiple cancers.

Small Rho GTPase-mediated actin dynamics at endothelial adherens junctions

VE-cadherin-based cell-cell junctions form the major restrictive barrier of the endothelium to plasma proteins and blood cells. The function of VE-cadherin and the actin cytoskeleton are intimately linked. Vascular permeability factors and adherent leukocytes signal through small Rho GTPases to tightly regulate actin cytoskeletal rearrangements in order to open and re-assemble endothelial cell-cell junctions in a rapid and controlled manner. The Rho GTPases are activated by guanine nucleotide exchange factors (GEFs), conferring specificity and context-dependent control of cell-cell junctions. Although the molecular mechanisms that couple cadherins to actin filaments are beginning to be elucidated, specific stimulus-dependent regulation of the actin cytoskeleton at VE-cadherin-based junctions remains unexplained. Accumulating evidence has suggested that depending on the vascular permeability factor and on the subcellular localization of GEFs, cell-cell junction dynamics and organization are differentially regulated by one specific Rho GTPase. In this Commentary, we focus on new insights how the junctional actin cytoskeleton is specifically and locally regulated by Rho GTPases and GEFs in the endothelium.

Leukocytes Crossing the Endothelium: A Matter of Communication

Leukocytes cross the endothelial vessel wall in a process called transendothelial migration (TEM). The purpose of leukocyte TEM is to clear the causing agents of inflammation in underlying tissues, for example, bacteria and viruses. During TEM, endothelial cells initiate signals that attract and guide leukocytes to sites of tissue damage. Leukocytes react by attaching to these sites and signal their readiness to move back to endothelial cells. Endothelial cells in turn respond by facilitating the passage of leukocytes while retaining overall integrity. In this review, we present recent findings in the field and we have endeavored to synthesize a coherent picture of the intricate interplay between endothelial cells and leukocytes during TEM.

The actin-binding protein EPS8 binds VE-cadherin and modulates YAP localization and signaling

Vascular endothelial (VE)-cadherin transfers intracellular signals contributing to vascular hemostasis. Signaling through VE-cadherin requires association and activity of different intracellular partners. Yes-associated protein (YAP)/TAZ transcriptional cofactors are important regulators of cell growth and organ size. We show that EPS8, a signaling adapter regulating actin dynamics, is a novel partner of VE-cadherin and is able to modulate YAP activity. By biochemical and imaging approaches, we demonstrate that EPS8 associates with the VE-cadherin complex of remodeling junctions promoting YAP translocation to the nucleus and transcriptional activation. Conversely, in stabilized junctions, 14-3-3-YAP associates with the VE-cadherin complex, whereas Eps8 is excluded. Junctional association of YAP inhibits nuclear translocation and inactivates its transcriptional activity both in vitro and in vivo in Eps8-null mice. The absence of Eps8 also increases vascular permeability in vivo, but did not induce other major vascular defects. Collectively, we identified novel components of the adherens junction complex, and we introduce a novel molecular mechanism through which the VE-cadherin complex controls YAP transcriptional activity.

A local VE-cadherin and Trio-based signaling complex stabilizes endothelial junctions through Rac1

Endothelial cell-cell junctions maintain a restrictive barrier that is tightly regulated to allow dynamic responses to permeability-inducing angiogenic factors, as well as to inflammatory agents and adherent leukocytes. The ability of these stimuli to transiently remodel adherens junctions depends on Rho-GTPase-controlled cytoskeletal rearrangements. How the activity of Rho-GTPases is spatio-temporally controlled at endothelial adherens junctions by guanine-nucleotide exchange factors (GEFs) is incompletely understood. Here, we identify a crucial role for the Rho-GEF Trio in stabilizing junctions based around vascular endothelial (VE)-cadherin (also known as CDH5). Trio interacts with VE-cadherin and locally activates Rac1 at adherens junctions during the formation of nascent contacts, as assessed using a novel FRET-based Rac1 biosensor and biochemical assays. The Rac-GEF domain of Trio is responsible for the remodeling of junctional actin from radial into cortical actin bundles, a crucial step for junction stabilization. This promotes the formation of linear adherens junctions and increases endothelial monolayer resistance. Collectively, our data show the importance of spatio-temporal regulation of the actin cytoskeleton through Trio and Rac1 at VE-cadherin-based cell-cell junctions in the maintenance of the endothelial barrier.

Junctional complex and focal adhesion rearrangement mediates pulmonary endothelial barrier enhancement by FTY720 S-phosphonate

RATIONALE

Modulation of pulmonary vascular barrier function is an important clinical goal given the devastating effects of vascular leak in acute lung injury (ALI). We previously demonstrated that FTY720 S-phosphonate (Tys), an analog of sphingosine 1-phosphate (S1P) and FTY720, has more potent pulmonary barrier protective effects than these agents in vitro and in mouse models of ALI. Tys preserves expression of the barrier-promoting S1P1 receptor (S1PR1), whereas S1P and FTY720 induce its ubiquitination and degradation. Here we further characterize the novel barrier promoting effects of Tys in cultured human pulmonary endothelial cells (EC).

METHODS/RESULTS

In human lung EC, Tys significantly increased peripheral redistribution of adherens junction proteins VE-cadherin and β-catenin and tight junction protein ZO-1. Inhibition of VE-cadherin with blocking antibody significantly attenuated Tys-induced transendothelial resistance (TER) elevation, while ZO-1 siRNA partially inhibited this elevation. Tys significantly increased focal adhesion formation and phosphorylation of focal adhesion kinase (FAK). Pharmacologic inhibition of FAK significantly attenuated Tys-induced TER elevation. Tys significantly increased phosphorylation and peripheral redistribution of the actin-binding protein, cortactin, while cortactin siRNA partially attenuated Tys-induced TER elevation. Although Tys significantly increased phosphorylation of Akt and GSK3β, neither PI3 kinase nor GSK3β inhibition altered Tys-induced TER elevation. Tys significantly increased Rac1 activity, while inhibition of Rac1 activity significantly attenuated Tys-induced VE-cadherin redistribution and TER elevation.

CONCLUSION

Junctional complex, focal adhesion rearrangement and Rac1 activation play critical roles in Tys-mediated barrier protection in pulmonary EC. These results provide mechanistic insights into the effects of this potential ALI therapy.

VE-cadherin-independent cancer cell incorporation into the vascular endothelium precedes transmigration

Metastasis is accountable for 90% of cancer deaths. During metastasis, tumor cells break away from the primary tumor, enter the blood and the lymph vessels, and use them as highways to travel to distant sites in the body to form secondary tumors. Cancer cell migration through the endothelium and into the basement membrane represents a critical step in the metastatic cascade, yet it is not well understood. This process is well characterized for immune cells that routinely transmigrate through the endothelium to sites of infection, inflammation, or injury. Previous studies with leukocytes have demonstrated that this step depends heavily on the activation status of the endothelium and subendothelial substrate stiffness. Here, we used a previously established in vitro model of the endothelium and live cell imaging, in order to observe cancer cell transmigration and compare this process to leukocytes. Interestingly, cancer cell transmigration includes an additional step, which we term ‘incorporation’, into the endothelial cell (EC) monolayer. During this phase, cancer cells physically displace ECs, leading to the dislocation of EC VE-cadherin away from EC junctions bordering cancer cells, and spread into the monolayer. In some cases, ECs completely detach from the matrix. Furthermore, cancer cell incorporation occurs independently of the activation status and the subendothelial substrate stiffness for breast cancer and melanoma cells, a notable difference from the process by which leukocytes transmigrate. Meanwhile, pancreatic cancer cell incorporation was dependent on the activation status of the endothelium and changed on very stiff subendothelial substrates. Collectively, our results provide mechanistic insights into tumor cell extravasation and demonstrate that incorporation is one of the earliest steps.

VE-cadherin at a glance

Although being a monolayer the vascular endothelium controls fundamental vessel functions such as permeability, leukocyte extravasation and angiogenesis. The endothelial selective transmembrane constituent of adherens junctions, Vascular Endothelial- (VE-) cadherin plays a crucial role in the regulation of such activities. The signaling pathways controlled by VE-cadherin as well as the ones that regulate VE-cadherin activity start to be elucidated. This delineates a complex network of molecular and functional interactions that can be altered in pathologies.

Reactive oxygen species in inflammation and tissue injury

Abstract Reactive oxygen species (ROS) are key signaling molecules that play an important role in the progression of inflammatory disorders. An enhanced ROS generation by polymorphonuclear neutrophils (PMNs) at the site of inflammation causes endothelial dysfunction and tissue injury. The vascular endothelium plays an important role in passage of macromolecules and inflammatory cells from the blood to tissue. Under the inflammatory conditions, oxidative stress produced by PMNs leads to the opening of inter-endothelial junctions and promotes the migration of inflammatory cells across the endothelial barrier. The migrated inflammatory cells not only help in the clearance of pathogens and foreign particles but also lead to tissue injury. The current review compiles the past and current research in the area of inflammation with particular emphasis on oxidative stress-mediated signaling mechanisms that are involved in inflammation and tissue injury.

Mechanical control of the endothelial barrier

The integrity of the endothelial barrier is controlled by the combined action of chemical and mechanical signaling systems. Permeability-regulating factors signal through small GTPases to regulate the architecture of the cytoskeleton and this has a strong impact on the morphology and stability of VE-cadherin-based cell-cell junctions. The details of how structural and mechanical properties of the actin cytoskeleton influence cell-cell adhesion and how this impacts the dynamic regulation of the endothelial barrier, are beginning to be elucidated. In this review, we discuss the physical and regulatory interactions between the VE-cadherin complex and the actomysoin cytoskeleton, as they are the main determinants of cell-cell adhesion and the mechanical architecture of the cytoskeleton. We discuss, based on recent in vitro data, how a balance between Linear Adherens Junctions, paralleled by cortical actin bundles and Focal Adherens Junctions, connected to radial action bundles, determines endothelial barrier function. We discuss how small GTPases control this balance by regulating the spatial organization and mechanics of actomyosin. We propose a hypothetical model of how biochemical and mechanical signals cooperate locally, at the actomyosin-adhesion interface to open and re-seal the barrier in a rapid and controlled manner.

Cytoskeletal dynamics and lung fluid balance

This article examines the role of the endothelial cytoskeleton in the lung's ability to restrict fluid and protein to vascular space at normal vascular pressures and thereby to protect lung alveoli from lethal flooding. The barrier properties of microvascular endothelium are dependent on endothelial cell contact with other vessel-wall lining cells and with the underlying extracellular matrix (ECM). Focal adhesion complexes are essential for attachment of endothelium to ECM. In quiescent endothelial cells, the thick cortical actin rim helps determine cell shape and stabilize endothelial adherens junctions and focal adhesions through protein bridges to actin cytoskeleton. Permeability-increasing agonists signal activation of "small GTPases" of the Rho family to reorganize the actin cytoskeleton, leading to endothelial cell shape change, disassembly of cortical actin rim, and redistribution of actin into cytoplasmic stress fibers. In association with calcium- and Src-regulated myosin light chain kinase (MLCK), stress fibers become actinomyosin-mediated contractile units. Permeability-increasing agonists stimulate calcium entry and induce tyrosine phosphorylation of VE-cadherin (vascular endothelial cadherin) and β-catenins to weaken or pull apart endothelial adherens junctions. Some permeability agonists cause latent activation of the small GTPases, Cdc42 and Rac1, which facilitate endothelial barrier recovery and eliminate interendothelial gaps. Under the influence of Cdc42 and Rac1, filopodia and lamellipodia are generated by rearrangements of actin cytoskeleton. These motile evaginations extend endothelial cell borders across interendothelial gaps, and may initiate reannealing of endothelial junctions. Endogenous barrier protective substances, such as sphingosine-1-phosphate, play an important role in maintaining a restrictive endothelial barrier and counteracting the effects of permeability-increasing agonists.

In utero exposure to the anti-androgen flutamide influences connexin 43 and β-catenin expression in porcine fetal gonads

Recent reports have indicated a role of cell-to-cell interactions during gonadal development and functions. Numerous reports indicate that fetal hormonal disruption induces abnormalities in the developing reproductive system and, therefore, may interfere with reproductive functions later in adult life. Hence, this study investigated the effect of androgen deficiency during late prenatal periods on the gap junction-associated connexin 43 (Cx43) and the adherens junction-associated β-catenin expression in the fetal porcine gonads. Thus, pregnant gilts were injected with anti-androgen flutamide (for 7 d, 50 mg/kg BW per day) or corn oil (control groups) starting at 83 (GD90) or 101 (GD108) gestational day. On GD90 and GD108 the fetuses were excised and fetal gonads were obtained. To assess Cx43 and β-catenin expression real-time PCR and immunohistochemistry were performed. In fetal testes, Cx43 was localized between Leydig cells, whereas β-catenin was observed mainly within the seminiferous tubules. In fetal ovaries, Cx43 was detected between interstitial cells and between granulosa cells of forming follicles, whereas β-catenin was found within egg nests, in oocytes' membrane, and in granulosa cells of forming follicles. Immunohistochemistry showed decreased Cx43 and β-catenin expression in fetal gonads from flutamide-treated pigs compared with respective controls. However, the ovaries from animals treated with flutamide on GD108 showed increased Cx43 expression. The changes of Cx43 and β-catenin expression after prenatal flutamide treatment were confirmed at the mRNA level. These findings suggest that androgen deficiency during late gestation may lead to disturbed intercellular interactions in fetal porcine testes affecting testicular functions, as well as impaired follicular formation in fetal ovaries. Our results further signify the role of androgens in the regulation of cell-to-cell interactions within fetal porcine gonads.

p120-catenin binding masks an endocytic signal conserved in classical cadherins

p120 regulates adhesive junction dynamics through binding to a dual-function motif in classical cadherins that alternately serves as a p120-binding interface and an endocytic signal.

p120-catenin (p120) binds to the cytoplasmic tails of classical cadherins and inhibits cadherin endocytosis. Although p120 regulation of cadherin internalization is thought to be important for adhesive junction dynamics, the mechanism by which p120 modulates cadherin endocytosis is unknown. In this paper, we identify a dual-function motif in classical cadherins consisting of three highly conserved acidic residues that alternately serve as a p120-binding interface and an endocytic signal. Mutation of this motif resulted in a cadherin variant that was both p120 uncoupled and resistant to endocytosis. In endothelial cells, in which dynamic changes in adhesion are important components of angiogenesis and inflammation, a vascular endothelial cadherin (VE-cadherin) mutant defective in endocytosis assembled normally into cell–cell junctions but potently suppressed cell migration in response to vascular endothelial growth factor. These results reveal the mechanistic basis by which p120 stabilizes cadherins and demonstrate that VE-cadherin endocytosis is crucial for endothelial cell migration in response to an angiogenic growth factor.

Claudin-5 controls intercellular barriers of human dermal microvascular but not human umbilical vein endothelial cells

OBJECTIVE

To assess the role claudin-5, an endothelial cell (EC) tight junction protein, plays in establishing basal permeability levels in humans by comparing claudin-5 expression levels in situ and analyzing junctional organization and function in 2 widely used models of cultured ECs, namely human dermal microvascular (HDM)ECs and human umbilical vein (HUV)ECs.

METHODS AND RESULTS

By immunofluorescence microscopy, ECs more highly express claudin-5 (but equivalently express vascular endothelial-cadherin) in human dermal capillaries versus postcapillary venules and in umbilical and coronary arteries versus veins, correlating with known segmental differences in tight junction frequencies and permeability barriers. Postconfluent cultured HDMECs express more claudin-5 (but equivalent vascular endothelial-cadherin) and show higher transendothelial electric resistance and lower macromolecular flux than similarly cultured HUVECs. HDMEC junctions are more complex by transmission electron microscopy and show more continuous claudin-5 immunofluorescence than HUVEC junctions. Calcium chelation or dominant negative vascular endothelial-cadherin overexpression decreases transendothelial electric resistance and disrupts junctions in HUVECs, but not in HDMECs. Claudin-5 overexpression in HUVECs fails to increase transendothelial electric resistance or claudin-5 continuity, whereas claudin-5 knockdown in HDMECs, but not in HUVECs, reduces transendothelial electric resistance and increases antibody accessibility to junctional proteins.

CONCLUSIONS

Claudin-5 expression and junctional organization control HDMEC and arteriolar-capillary paracellular barriers, whereas HUVEC and venular junctions use vascular endothelial-cadherin.

The tyrosine phosphatase SHP2 regulates recovery of endothelial adherens junctions through control of β-catenin phosphorylation

Reconstitution of the endothelial barrier involves SHP2-mediated dephosphorylation of VE-cadherin–associated β-catenin, leading to reassembly of adherens junctions and thereby closing the gaps between endothelial cells.

Impaired endothelial barrier function results in a persistent increase in endothelial permeability and vascular leakage. Repair of a dysfunctional endothelial barrier requires controlled restoration of adherens junctions, comprising vascular endothelial (VE)-cadherin and associated β-, γ-, α-, and p120-catenins. Little is known about the mechanisms by which recovery of VE-cadherin–mediated cell–cell junctions is regulated. Using the inflammatory mediator thrombin, we demonstrate an important role for the Src homology 2-domain containing tyrosine phosphatase (SHP2) in mediating recovery of the VE-cadherin–controlled endothelial barrier. Using SHP2 substrate-trapping mutants and an in vitro phosphatase activity assay, we validate β-catenin as a bona fide SHP2 substrate. SHP2 silencing and SHP2 inhibition both result in delayed recovery of endothelial barrier function after thrombin stimulation. Moreover, on thrombin challenge, we find prolonged elevation in tyrosine phosphorylation levels of VE-cadherin–associated β-catenin in SHP2-depleted cells. No disassembly of the VE-cadherin complex is observed throughout the thrombin response. Using fluorescence recovery after photobleaching, we show that loss of SHP2 reduces the mobility of VE-cadherin at recovered cell–cell junctions. In conclusion, our data show that the SHP2 phosphatase plays an important role in the recovery of disrupted endothelial cell–cell junctions by dephosphorylating VE-cadherin–associated β-catenin and promoting the mobility of VE-cadherin at the plasma membrane.

The roles of G proteins in the activation of TRPC4 and TRPC5 transient receptor potential channels

TRPC4 and TRPC5 channels are important regulators of electrical excitability in both gastrointestinal myocytes and neurons. Much is known regarding the assembly and function of these channels including TRPC1 as a homotetramer or a heteromultimer and the roles that their interacting proteins play in controlling these events. Further, they are one of the best-studied targets of G protein-coupled receptors and growth factors in general and Gαq protein coupled receptor or epidermal growth factor in particular. However, our understanding of the roles of Gαi/o proteins on TRPC4/5 channels is still rudimentary. We discuss potential roles for Gαi/o proteins in channel activation in addition to their known role in cellular signaling.

Rickettsiae induce microvascular hyperpermeability via phosphorylation of VE-cadherins: evidence from atomic force microscopy and biochemical studies

The most prominent pathophysiological effect of spotted fever group (SFG) rickettsial infection of microvascular endothelial cells (ECs) is an enhanced vascular permeability, promoting vasogenic cerebral edema and non-cardiogenic pulmonary edema, which are responsible for most of the morbidity and mortality in severe cases. To date, the cellular and molecular mechanisms by which SFG Rickettsia increase EC permeability are largely unknown. In the present study we used atomic force microscopy (AFM) to study the interactive forces between vascular endothelial (VE)-cadherin and human cerebral microvascular EC infected with R. montanensis, which is genetically similar to R. rickettsii and R. conorii, and displays a similar ability to invade cells, but is non-pathogenic and can be experimentally manipulated under Biosafety Level 2 (BSL2) conditions. We found that infected ECs show a significant decrease in VE-cadherin-EC interactions. In addition, we applied immunofluorescent staining, immunoprecipitation phosphorylation assay, and an in vitro endothelial permeability assay to study the biochemical mechanisms that may participate in the enhanced vascular permeability as an underlying pathologic alteration of SFG rickettsial infection. A major finding is that infection of R. montanensis significantly activated tyrosine phosphorylation of VE-cadherin beginning at 48 hr and reaching a peak at 72 hr p.i. In vitro permeability assay showed an enhanced microvascular permeability at 72 hr p.i. On the other hand, AFM experiments showed a dramatic reduction in VE-cadherin-EC interactive forces at 48 hr p.i. We conclude that upon infection by SFG rickettsiae, phosphorylation of VE-cadherin directly attenuates homophilic protein-protein interactions at the endothelial adherens junctions, and may lead to endothelial paracellular barrier dysfunction causing microvascular hyperpermeability. These new approaches should prove useful in characterizing the antigenically related SFG rickettsiae R. conorii and R. rickettsii in a BSL3 environment. Future studies may lead to the development of new therapeutic strategies to inhibit the VE-cadherin-associated microvascular hyperpermeability in SFG rickettsioses.

Biphasic effect of danazol on human vascular endothelial cell permeability and f-actin cytoskeleton dynamics

Breakdown of endothelial barrier function is a hallmark event across a variety of pathologies such as inflammation, cancer, and diabetes. It has also been appreciated that steroid hormones impart direct biological activity on endothelial cells at many levels. The purpose of this investigation was to explore the effect of the androgen-like steroid, danazol, on endothelial cell barrier function in vitro. Primary human endothelial cells exposed to 0.01-50 μM danazol were evaluated for changes in permeability. We found that danazol altered endothelial permeability in a biphasic manner in which nanomolar concentrations enhance barrier function while micromolar concentrations are detrimental. Monitoring of trans-endothelial electrical resistance demonstrated that these barrier enhancing effects were rapid (within 5 min) and lasted for over 24h. Analysis of intracellular f-actin organization showed that barrier enhancement also correlated with the formation of a submembranous cortical actin ring. Conversely, at higher danazol concentrations, contractile cell phenotypes were observed, represented by stress fiber formation. Competitive binding studies performed using steroid hormone receptor antagonists proved that this activity is the result of androgen and estrogen receptor ligation. These findings suggest that low dose danazol may provide a therapeutic window for diseases involving vascular leakage.

iNOS activation regulates β-catenin association with its partners in endothelial cells

BACKGROUND

Signals that disrupt β-catenin association to cadherins may influence the translocation of β-catenin to the nucleus to regulate transcription. Post-translational modification of proteins is a signalling event that may lead to changes in structural conformation, association or function of the target proteins. NO and its derivatives induce nitration of proteins during inflammation. It has been described that animals treated with NO donors showed increased permeability due to modulation of VE-cadherin/catenin complex. We, therefore, aim to evaluate the effect of iNOS activation on the expression, nuclear localisation and function of β-catenin in endothelial cells.

METHODOLOGY/PRINCIPAL FINDINGS

Expression, nuclear localisation, post-translational modifications and function of β-catenin was analysed by cell fractionation, immunoprecipitation, immunoblots, QRT-PCR and permeability assays in murine endothelial cells (H5V). Influence of macrophage activation on expression of VE-cadherin/p120-catenin/β-catenin complex in co-cultured H5V cells was also assessed. Activation of macrophages to produce NO provoked a decrease in VE-cadherin/p120-catenin/β-catenin expression in H5V cells. Phosphorylation of β-catenin, p120-catenin and VE-cadherin, and reduction in the barrier properties of the cell monolayer was associated with iNOS induction. Moreover, high NO levels provoked nitration of β-catenin, and induced its translocation to the nucleus. In the nucleus of NOS activated cells, nitration levels of β-catenin influenced its association with TCF4 and p65 proteins. High levels of NO altered β-catenin mediated gene expression of NFκB and Wnt target genes without affecting cell viability.

CONCLUSIONS

NOS activity modulates β-catenin post-translational modifications, function and its association with different partners to promote endothelial cell survival. Therapeutic manipulation of iNOS levels may remove a critical cytoprotective mechanism of importance in tumour angiogenesis.

Neisseria meningitidis induces platelet inhibition and increases vascular endothelial permeability via nitric oxide regulated pathways

Despite antibiotic therapy, infections with Neisseria meningitidis still demonstrate a high rate of morbidity and mortality even in developed countries. The fulminant septicaemic course, named Waterhouse-Friderichsen syndrome, with massive haemorrhage into the adrenal glands and widespread petechial bleeding suggest pathophysiological inhibition of platelet function. Our data show that N. meningitidis produces the important physiological platelet inhibitor and cardiovascular signalling molecule nitric oxide (NO), also known as endothelium-derived relaxing factor (EDRF). N. meningitidis -derived NO inhibited ADP-induced platelet aggregation through the activation of soluble guanylyl cyclase (sGC) followed by an increase in platelet cyclic nucleotide levels and subsequent activation of platelet cGMP- and cAMP- dependent protein kinases (PKG and PKA). Furthermore, direct measurement of horseradish peroxidase (HRP) passage through a vascular endothelial cell monolayer revealed that N. meningitidis significantly increased endothelial monolayer permeability. Immunfluorescence analysis demonstrated NO dependent disturbances in the structure of endothelial adherens junctions after co-incubation with N. meningitidis . In contrast to platelet inhibition, the NO effects on HBMEC were not mediated by cyclic nucleotides. Our study provides evidence that NO plays an essential role in the pathophysiology of septicaemic meningococcal infection.

m-Calpain induction in vascular endothelial cells on human and mouse atheromas and its roles in VE-cadherin disorganization and atherosclerosis

BACKGROUND

Although dysfunction of VE-cadherin-mediated adherence junctions in vascular endothelial cells (ECs) is thought to be one of the initial steps of atherosclerosis, little is known regarding how VE-cadherin is disrupted during atherogenic development. This study focused on the role of calpain, an intracellular cysteine protease, in the proteolytic disorganization of VE-cadherin and subsequent progression of atherosclerosis.

METHODS AND RESULTS

Increased expression of m-calpain was observed in aortic ECs in atherosclerotic lesions in humans and low-density lipoprotein receptor-deficient (ldlr(-/-)) mice. Furthermore, proteolytic disorganization of VE-cadherin was shown in aortic ECs in ldlr(-/-) and apolipoprotein E-deficient (apoE(-/-)) mice. Long-term administration of calpain inhibitors into these mice attenuated atherosclerotic lesion development and proinflammatory responses, as well as VE-cadherin disorganization, without normalization of plasma lipid profiles. Furthermore, in vivo transfection of m-calpain siRNA to ldlr(-/-) mice prevented disorganization of VE-cadherin and proatherogenic hyperpermeability in aortic ECs. Treatment of cultured ECs with oxidized LDL, lysophosphatidylcholine, or LDL pretreated with secreted phospholipase A(2) led to the induction of m-calpain but not of μ-calpain, thereby eliciting selective m-calpain overactivation. These data suggest that lysophosphatidylcholine-induced m-calpain directly cleaves a juxtamembrane region of VE-cadherin, resulting in dissociation of β-catenin from the VE-cadherin complex, disorganization of adherence junctions, and hyperpermeability in ECs.

CONCLUSIONS

Subtype-selective induction of m-calpain in aortic ECs during atherosclerotic progression is associated with proteolytic disorganization of VE-cadherin and proatherogenic hyperpermeability in cells. Thus, a strategy to selectively inhibit m-calpain may be useful for the therapeutic treatment of patients with atherosclerosis.

Do viral infections mimic bacterial sepsis? The role of microvascular permeability: A review of mechanisms and methods

A dysregulated immune response and functional immunosuppression have been considered the major mechanisms of the bacterial sepsis syndrome. More recently, the loss of endothelial barrier function and resultant microvascular leak have been found to be a key determinant of the pathogenesis of bacterial sepsis. Whether a similar paradigm applies to systemic viral syndromes is not known. Answering this question has far-reaching implications for the development of future anti-viral therapeutic strategies. In this review, we provide an overview of the structure and function of the endothelium and how its barrier integrity is compromised in bacterial sepsis. The various in vitro and in vivo methodologies available to investigate vascular leak are reviewed. Emphasis is placed on the advantages and limitations of cell culture techniques, which represent the most commonly used methods. Within this context, we appraise recent studies of three viruses - hantavirus, human herpes virus 8 and dengue virus - that suggest microvascular leak may play a role in the pathogenesis of these viral infections. We conclude with a discussion of how endothelial barrier breakdown may occur in other viral infections such as H5N1 avian influenza virus.

Arterial erectile dysfunction: reliability of new markers of endothelial dysfunction

OBJECTIVE

Blood endothelial progenitor cells (EPC) and microparticles (EMP) have been proposed as markers of endothelial dysfunction. Aim of this study was to evaluate a new immunophenotype of EPC and EMP in patients with arterial erectile dysfunction (AED) compared to psychogenic erectile dysfunction (PED).

MATERIALS AND METHODS

One hundred patients (63.2±2.6 yr) with AED were enrolled in this study. Their EPC and EMP concentrations were compared to those of 40 patients with PED (64.2±2.7 yr). EPC (CD45(neg)/CD34(pos)/ CD144(pos)) and EMP (CD45(neg)/CD144(pos)/AnnexinV(pos)) blood concentrations were evaluated by flow cytometry.

RESULTS

Patients with AED had significantly higher blood pressure, triglycerides, homeostasis model assessment index of insulin resistance, and cavernous artery acceleration time and intima-media thickness than PED; whereas international index of erectile function 5 score, HDL-cholesterol, and cavernous artery peak systolic velocity was lower than PED. Both EPC and EMP were significantly higher in patients with AED compared to patients with PED.

CONCLUSIONS

Patients with AED showed worse metabolic parameters, cavernous artery parameters, and higher EPC and EMP compared to patients with PED. This suggests that AED is an expression of endothelial dysfunction and that EPC and EMP may be considered predictors of endothelial dysfunction in patients with AED.

Msx genes define a population of mural cell precursors required for head blood vessel maturation

Vessels are primarily formed from an inner endothelial layer that is secondarily covered by mural cells, namely vascular smooth muscle cells (VSMCs) in arteries and veins and pericytes in capillaries and veinules. We previously showed that, in the mouse embryo, Msx1(lacZ) and Msx2(lacZ) are expressed in mural cells and in a few endothelial cells. To unravel the role of Msx genes in vascular development, we have inactivated the two Msx genes specifically in mural cells by combining the Msx1(lacZ), Msx2(lox) and Sm22α-Cre alleles. Optical projection tomography demonstrated abnormal branching of the cephalic vessels in E11.5 mutant embryos. The carotid and vertebral arteries showed an increase in caliber that was related to reduced vascular smooth muscle coverage. Taking advantage of a newly constructed Msx1(CreERT2) allele, we demonstrated by lineage tracing that the primary defect lies in a population of VSMC precursors. The abnormal phenotype that ensues is a consequence of impaired BMP signaling in the VSMC precursors that leads to downregulation of the metalloprotease 2 (Mmp2) and Mmp9 genes, which are essential for cell migration and integration into the mural layer. Improper coverage by VSMCs secondarily leads to incomplete maturation of the endothelial layer. Our results demonstrate that both Msx1 and Msx2 are required for the recruitment of a population of neural crest-derived VSMCs.

Stabilizing the VE-cadherin-catenin complex blocks leukocyte extravasation and vascular permeability

To determine whether leukocytes need to open endothelial cell contacts during extravasation, we decided to generate mice with strongly stabilized endothelial junctions. To this end, we replaced VE-cadherin genetically by a VE-cadherin-α-catenin fusion construct. Such mice were completely resistant to the induction of vascular leaks by VEGF or histamine. Neutrophil or lymphocyte recruitment into inflamed cremaster, lung and skin were strongly inhibited in these mice, documenting the importance of the junctional route in vivo. Surprisingly, lymphocyte homing into lymph nodes was not inhibited. VE-cadherin-α-catenin associated more intensely with the actin cytoskeleton as demonstrated by its membrane mobility and detergent extractability. Our results establish the junctional route as the main pathway for extravasating leukocytes in several, although not in all tissues. Furthermore, in these tissues, plasticity of the VE-cadherin-catenin complex is central for the leukocyte diapedesis mechanism.

Structure and binding mechanism of vascular endothelial cadherin: a divergent classical cadherin

Vascular endothelial cadherin (VE-cadherin), a divergent member of the type II classical cadherin family of cell adhesion proteins, mediates homophilic adhesion in the vascular endothelium. Previous investigations with a bacterially produced protein suggested that VE-cadherin forms cell surface trimers that bind between apposed cells to form hexamers. Here we report studies of mammalian-produced VE-cadherin ectodomains suggesting that, like other classical cadherins, VE-cadherin forms adhesive trans dimers between monomers located on opposing cell surfaces. Trimerization of the bacterially produced protein appears to be an artifact that arises from a lack of glycosylation. We also present the 2.1-Å-resolution crystal structure of the VE-cadherin EC1-2 adhesive region, which reveals homodimerization via the strand-swap mechanism common to classical cadherins. In common with type II cadherins, strand-swap binding involves two tryptophan anchor residues, but the adhesive interface resembles type I cadherins in that VE-cadherin does not form a large nonswapped hydrophobic surface. Thus, VE-cadherin is an outlier among classical cadherins, with characteristics of both type I and type II subfamilies.

FoxM1 regulates re-annealing of endothelial adherens junctions through transcriptional control of beta-catenin expression

Repair of the injured vascular intima requires a series of coordinated events that mediate both endothelial regeneration and reannealing of adherens junctions (AJs) to form a restrictive endothelial barrier. The forkhead transcription factor FoxM1 is essential for endothelial proliferation after vascular injury. However, little is known about mechanisms by which FoxM1 regulates endothelial barrier reannealing. Here, using a mouse model with endothelial cell (EC)-restricted disruption of FoxM1 (FoxM1 CKO) and primary cultures of ECs with small interfering RNA (siRNA)-mediated knockdown of FoxM1, we demonstrate a novel requisite role of FoxM1 in mediating endothelial AJ barrier repair through the transcriptional control of β-catenin. In the FoxM1 CKO lung vasculature, we observed persistent microvessel leakage characterized by impaired reannealing of endothelial AJs after endothelial injury. We also showed that FoxM1 directly regulated β-catenin transcription and that reexpression of β-catenin rescued the defective AJ barrier–reannealing phenotype of FoxM1-deficient ECs. Knockdown of β-catenin mimicked the phenotype of defective barrier recovery seen in FoxM1-deficient ECs. These data demonstrate that FoxM1 is required for reannealing of endothelial AJs in order to form a restrictive endothelial barrier through transcriptional control of β-catenin expression. Therefore, means of activating FoxM1-mediated endothelial repair represent a new therapeutic strategy for the treatment of inflammatory vascular diseases associated with persistent vascular barrier leakiness such as acute lung injury.

The soluble fragment of VE-cadherin inhibits angiogenesis by reducing endothelial cell proliferation and tube capillary formation

Vascular endothelial-specific cadherin (VE-cadherin) is an endothelial cell-specific adhesion molecule, localized at cell-cell contact sites. It is involved in physiological and pathological angiogenesis. In this study, we showed that in vitro a soluble N-terminal fragment of VE-cadherin (EC1-3) corresponding to cadherin 1-3 ectodomains inhibited vascular endothelial growth factor-stimulated endothelial cell proliferation and capillary tube structure formation in the matrigel model. In vivo, EC1-3 was tested in a murine colon cancer model. EC1-3-expressing colon cancer C51 cells were subcutaneously grafted into nude mice, and tumor growth and angiogenesis were evaluated. At day 33, the mean volume of the tumors developed was reduced (510±104 versus 990±120 mm(3) for control). Similarly, injection of EC1-3 virus-producing cells into established C51 tumors resulted in an inhibition by 33% of tumor growth. Immunohistological staining of vessels on tumor sections showed a significantly reduced intratumoral angiogenesis. Furthermore, EC1-3 did not induce vessel injury in the lung, liver, spleen, heart and brain in the mice. These results suggest that the soluble N-terminal fragment of VE-cadherin EC1-3 could exert an antitumoral effect by targeting tumor angiogenesis, which included blocking endothelial cell proliferation and capillary tube formation with no obvious toxicity on normal organs.

Mechanical tugging force regulates the size of cell-cell junctions

Actomyosin contractility affects cellular organization within tissues in part through the generation of mechanical forces at sites of cell-matrix and cell-cell contact. While increased mechanical loading at cell-matrix adhesions results in focal adhesion growth, whether forces drive changes in the size of cell-cell adhesions remains an open question. To investigate the responsiveness of adherens junctions (AJ) to force, we adapted a system of microfabricated force sensors to quantitatively report cell-cell tugging force and AJ size. We observed that AJ size was modulated by endothelial cell-cell tugging forces: AJs and tugging force grew or decayed with myosin activation or inhibition, respectively. Myosin-dependent regulation of AJs operated in concert with a Rac1, and this coordinated regulation was illustrated by showing that the effects of vascular permeability agents (S1P, thrombin) on junctional stability were reversed by changing the extent to which these agents coupled to the Rac and myosin-dependent pathways. Furthermore, direct application of mechanical tugging force, rather than myosin activity per se, was sufficient to trigger AJ growth. These findings demonstrate that the dynamic coordination of mechanical forces and cell-cell adhesive interactions likely is critical to the maintenance of multicellular integrity and highlight the need for new approaches to study tugging forces.

Cell-cell contact formation governs Ca2+ signaling by TRPC4 in the vascular endothelium: evidence for a regulatory TRPC4-beta-catenin interaction

TRPC4 is well recognized as a prominent cation channel in the vascular endothelium, but its contribution to agonist-induced endothelial Ca²⁺ entry is still a matter of controversy. Here we report that the cellular targeting and Ca²⁺ signaling function of TRPC4 is determined by the state of cell-cell adhesions during endothelial phenotype transitions. TRPC4 surface expression in human microvascular endothelial cells (HMEC-1) increased with the formation of cell-cell contacts. Epidermal growth factor recruited TRPC4 into the plasma membrane of proliferating cells but initiated retrieval of TRPC4 from the plasma membrane in quiescent, barrier-forming cells. Epidermal growth factor-induced Ca²⁺ entry was strongly promoted by the formation of cell-cell contacts, and both siRNA and dominant negative knockdown experiments revealed that TRPC4 mediates stimulated Ca²⁺ entry exclusively in proliferating clusters that form immature cell-cell contacts. TRPC4 co-precipitated with the junctional proteins β-catenin and VE-cadherin. Analysis of cellular localization of fluorescent fusion proteins provided further evidence for recruitment of TRPC4 into junctional complexes. Analysis of TRPC4 function in the HEK293 expression system identified β-catenin as a signaling molecule that enables cell-cell contact-dependent promotion of TRPC4 function. Our results place TRPC4 as a Ca²⁺ entry channel that is regulated by cell-cell contact formation and interaction with β-catenin. TRPC4 is suggested to serve stimulated Ca²⁺ entry in a specific endothelial state during the transition from a proliferating to a quiescent phenotype. Thus, TRPC4 may adopt divergent, as yet unappreciated functions in endothelial Ca²⁺ homeostasis and emerges as a potential key player in endothelial phenotype switching and tuning of cellular growth factor signaling.

A noncoding antisense RNA in tie-1 locus regulates tie-1 function in vivo

Recently, messenger RNAs in eukaryotes have shown to associate with antisense (AS) transcript partners that are often referred to as long noncoding RNAs (lncRNAs) whose function is largely unknown. Here, we have identified a natural AS transcript for tyrosine kinase containing immunoglobulin and epidermal growth factor homology domain-1 (tie-1), tie-1AS lncRNA in zebrafish, mouse, and humans. In embryonic zebrafish, tie-1AS lncRNA transcript is expressed temporally and spatially in vivo with its native target, the tie-1 coding transcript and in additional locations (ear and brain). The tie-1AS lncRNA selectively binds tie-1 mRNA in vivo and regulates tie-1 transcript levels, resulting in specific defects in endothelial cell contact junctions in vivo and in vitro. The ratio of tie-1 versus tie-1AS lncRNA is altered in human vascular anomaly samples. These results directly implicate noncoding RNA-mediated transcriptional regulation of gene expression as a fundamental control mechanism for physiologic processes, such as vascular development.

Endothelial barrier stabilization by a cyclic tandem peptide targeting VE-cadherin transinteraction in vitro and in vivo

Inflammatory stimuli result in vascular leakage with potentially life threatening consequences. As a key barrier component, loss of vascular endothelial (VE-) cadherin-mediated adhesion often precedes endothelial breakdown. This study aimed to stabilize VE-cadherin transinteraction and endothelial barrier function using peptides targeting the VE-cadherin adhesive interface. After modelling the transinteracting VE-cadherin structure, an inhibiting single peptide (SP) against a VE-cadherin binding pocket was selected, which specifically blocked VE-cadherin transinteraction as analyzed by single molecule atomic force microscopy (AFM). The tandem peptide (TP) consisting of two SP sequences in tandem was designed to strengthen VE-cadherin adhesion by simultaneously binding and cross-bridging two interacting cadherin molecules. Indeed, in AFM experiments TP specifically rendered VE-cadherin transinteraction resistant against an inhibitory monoclonal antibody. Moreover, TP reduced VE-cadherin lateral mobility and enhanced binding of VE-cadherin-coated microbeads to cultured endothelial cells, but acted independently of the actin cytoskeleton. TP also stabilized endothelial barrier properties against the Ca2+ ionophore A23187 and the inhibitory antibody. Finally, TP abolished endothelial permeability increase induced by tumour necrosis factor-α in microperfused venules in vivo. Stabilization of VE-cadherin adhesion by cross-bridging peptides may therefore be a novel therapeutic approach for the treatment of vascular hyperpermeability.

The role of adherens junctions and VE-cadherin in the control of vascular permeability

Endothelial cells control the passage of plasma constituents and circulating cells from blood to the underlying tissues. This specialized function is lost or impaired in several pathological conditions - including inflammation, sepsis, ischemia and diabetes - which leads to severe, and sometimes fatal, organ dysfunction. Endothelial permeability is regulated in part by the dynamic opening and closure of cell-cell adherens junctions (AJs). In endothelial cells, AJs are largely composed of vascular endothelial cadherin (VE-cadherin), an endothelium-specific member of the cadherin family of adhesion proteins that binds, via its cytoplasmic domain, to several protein partners, including p120, beta-catenin and plakoglobin. Endogenous pathways that increase vascular permeability affect the function and organization of VE-cadherin and other proteins at AJs in diverse ways. For instance, several factors, including vascular endothelial growth factor (VEGF), induce the tyrosine phosphorylation of VE-cadherin, which accompanies an increase in vascular permeability and leukocyte diapedesis; in addition, the internalization and cleavage of VE-cadherin can cause AJs to be dismantled. From the knowledge of how AJ organization can be modulated, it is possible to formulate several pharmacological strategies to control the barrier function of the endothelium. We discuss the possible use of inhibitors of SRC and other kinases, of agents that increase cAMP levels, and of inhibitors of lytic enzymes as pharmacological tools for decreasing endothelial permeability.

Endothelial and virgultar cell formations in the mammalian lymph node sinus: endothelial differentiation morphotypes characterized by a special kind of junction (complexus adhaerens)

The lymph node sinus are channel structures of unquestionable importance in immunology and pathology, specifically in the filtering of the lymph, the transport and processing of antigens, the adhesion and migration of immune cells, and the spread of metastatic cancer cells. Our knowledge of the cell and molecular biology of the sinus-forming cells is still limited, and the origin and biological nature of these cells have long been a matter of debate. Here, we review the relevant literature and present our own experimental results, in particular concerning molecular markers of intercellular junctions and cell differentiation. We show that both the monolayer cells lining the sinus walls and the intraluminal virgultar cell meshwork are indeed different morphotypes of the same basic endothelial cell character, as demonstrated by the presence of a distinct spectrum of general and lymphatic endothelial markers, and we therefore refer to these cells as sinus endothelial/virgultar cells (SEVCs). These cells are connected by unique adhering junctions, termed complexus adhaerentes, characterized by the transmembrane glycoprotein VE-cadherin, combined with the desmosomal plaque protein desmoplakin, several adherens junction plaque proteins including alpha- and beta-catenin and p120 catenin, and components of the tight junction ensemble, specifically claudin-5 and JAM-A, and the plaque protein ZO-1. We show that complexus adhaerentes are involved in the tight three-dimensional integration of the virgultar network of SEVC processes along extracellular guidance structures composed of paracrystalline collagen bundle "stays". Overall, the SEVC system might be considered as a local and specific modification of the general lymphatic vasculature system. Finally, physiological and pathological alterations of the SEVC system will be presented, and the possible value of the molecular markers described in histological diagnoses of autochthonous lymph node tumors will be discussed.

Cell adhesion dynamics at endothelial junctions: VE-cadherin as a major player

The regulation of endothelial cell contacts is of central importance for the barrier function of the blood vessel wall and for the control of leukocyte extravasation. In addition, the plasticity of endothelial cell contacts is regulated during angiogenesis by growth factors, such as vascular endothelial growth factor and angiopoietin-1. Despite the participation of several adhesion molecules and receptors in the control of endothelial cell contacts, most of the currently known mechanisms involve vascular endothelial cadherin (VE-cadherin), an essential adhesion molecule for the stability of endothelial junctions. Here, we focus on recent results showing how leukocytes and angiogenic factors regulate endothelial junctions.

The actin cytoskeleton in endothelial cell phenotypes

Endothelium forms a semi-permeable barrier that separates blood from the underlying tissue. Barrier function is largely determined by cell-cell and cell-matrix adhesions that define the limits of cell borders. Yet, such cell-cell and cell-matrix tethering is critically reliant upon the nature of adherence within the cell itself. Indeed, the actin cytoskeleton fulfills this essential function, to provide a strong, dynamic intracellular scaffold that organizes integral membrane proteins with the cell's interior, and responds to environmental cues to orchestrate appropriate cell shape. The actin cytoskeleton is comprised of three distinct, but inter-related structures, including actin cross-linking of spectrin within the membrane skeleton, the cortical actin rim, and actomyosin-based stress fibers. This review addresses each of these actin-based structures, and discusses cellular signals that control the disposition of actin in different endothelial cell phenotypes.

Suppression of retinal neovascularization with an antagonist to vascular endothelial cadherin

OBJECTIVES

To examine the role of vascular endothelial cadherin (VE-cadherin) in cellular processes underlying angiogenesis and the effects of VE-cadherin inhibition on retinal angiogenesis.

METHODS

Retinal neovascularization was induced in newborn mice by exposure to 75% oxygen (postnatal days 7-12) followed by room air and quantitated from histological sections. Mice received daily intraperitoneal injections of either a VE-cadherin antagonist or a control peptide from postnatal days 12 to 17. In vitro cell migration, proliferation, and tubule formation were examined in the presence of the VE-cadherin antagonist. The effect of antagonist treatment on the integrity of established cell junctions was examined by fluorescein isothiocyanate-dextran monolayer permeability and VE-cadherin immunocytochemistry.

RESULTS

Treatment with the VE-cadherin antagonist significantly reduced retinal angiogenesis. Inhibition of VE-cadherin function suppressed tubule formation in endothelial cells. The antagonist treatment also decreased cell migration and proliferation. The antagonist treatment did not affect the integrity of existing cell junctions. Immunostaining for VE-cadherin and rates of monolayer permeability were comparable to those in untreated controls.

CONCLUSION

Our study points to a pivotal role played by VE-cadherin in the angiogenic process.

CLINICAL RELEVANCE

Inhibition of VE-cadherin might be an effective strategy for pharmacological inhibition in proliferative retinopathies.