Alteration of endothelial cell monolayer integrity triggers resynthesis of vascular endothelium cadherin

Human Immunodeficiency Virus (HIV) and Human Cytomegalovirus (HCMV) Coinfection of Infant Tonsil Epithelium May Synergistically Promote both HIV-1 and HCMV Spread and Infection

Mother-to-child transmission (MTCT) of human immunodeficiency virus type 1 (HIV-1) and human cytomegalovirus (HCMV) may occur during pregnancy, labor, or breastfeeding. These viruses from amniotic fluid, cervicovaginal secretions, and breast milk may simultaneously interact with oropharyngeal and tonsil epithelia; however, the molecular mechanism of HIV-1 and HCMV cotransmission through the oral mucosa and its role in MTCT are poorly understood. To study the molecular mechanism of HIV-1 and HCMV MTCT via oral epithelium, we established polarized infant tonsil epithelial cells and polarized-oriented ex vivo tonsil tissue explants. Using these models, we showed that cell-free HIV-1 and its proteins gp120 and tat induce the disruption of tonsil epithelial tight junctions and increase paracellular permeability, which facilitates HCMV spread within the tonsil mucosa. Inhibition of HIV-1 gp120-induced upregulation of mitogen-activated protein kinase (MAPK) and NF-κB signaling in tonsil epithelial cells, reduces HCMV infection, indicating that HIV-1-activated MAPK and NF-κB signaling may play a critical role in HCMV infection of tonsil epithelium. HCMV infection of tonsil epithelial cells also leads to the disruption of tight junctions and increases paracellular permeability, facilitating HIV-1 paracellular spread into tonsil mucosa. HCMV-promoted paracellular spread of HIV-1 increases its accessibility to tonsil CD4 T lymphocytes, macrophages, and dendritic cells. HIV-1-enhanced HCMV paracellular spread and infection of epithelial cells subsequently leads to the spread of HCMV to tonsil macrophages and dendritic cells. Our findings revealed that HIV-1- and HCMV-induced disruption of infant tonsil epithelial tight junctions promotes MTCT of these viruses through tonsil mucosal epithelium, and therapeutic intervention for both HIV-1 and HCMV infection may substantially reduce their MTCT. IMPORTANCE Most HIV-1 and HCMV MTCT occurs in infancy, and the cotransmission of these viruses may occur via infant oropharyngeal and tonsil epithelia, which are the first biological barriers for viral pathogens. We have shown that HIV-1 and HCMV disrupt epithelial junctions, reducing the barrier functions of epithelia and thus allowing paracellular penetration of both viruses via mucosal epithelia. Subsequently, HCMV infects epithelial cells, macrophages, and dendritic cells, and HIV-1 infects CD4⁺ lymphocytes, macrophages, and dendritic cells. Infection of these cells in HCMV- and HIV-1-coinfected tonsil tissues is much higher than that by HCMV or HIV-1 infection alone, promoting their MTCT at its initial stages via infant oropharyngeal and tonsil epithelia.

Mechanisms Ensuring Endothelial Junction Integrity Beyond VE-Cadherin

Endothelial junctions provide blood and lymph vessel integrity and are essential for the formation of a vascular system. They control the extravasation of solutes, leukocytes and metastatic cells from blood vessels and the uptake of fluid and leukocytes into the lymphatic vascular system. A multitude of adhesion molecules mediate and control the integrity and permeability of endothelial junctions. VE-cadherin is arguably the most important adhesion molecule for the formation of vascular structures, and the stability of their junctions. Interestingly, despite this prominence, its elimination from junctions in the adult organism has different consequences in the vasculature of different organs, both for blood and lymph vessels. In addition, even in tissues where the lack of VE-cadherin leads to strong plasma leaks from venules, the physical integrity of endothelial junctions is preserved. Obviously, other adhesion molecules can compensate for a loss of VE-cadherin and this review will discuss which other adhesive mechanisms contribute to the stability and regulation of endothelial junctions and cooperate with VE-cadherin in intact vessels. In addition to adhesion molecules, endothelial receptors will be discussed, which stimulate signaling processes that provide junction stability by modulating the actomyosin system, which reinforces tension of circumferential actin and dampens pulling forces of radial stress fibers. Finally, we will highlight most recent reports about the formation and control of the specialized button-like junctions of initial lymphatics, which represent the entry sites for fluid and cells into the lymphatic vascular system.

Bioprinting Approaches to Engineering Vascularized 3D Cardiac Tissues

PURPOSE OF REVIEW

3D bioprinting technologies hold significant promise for the generation of engineered cardiac tissue and translational applications in medicine. To generate a clinically relevant sized tissue, the provisioning of a perfusable vascular network that provides nutrients to cells in the tissue is a major challenge. This review summarizes the recent vascularization strategies for engineering 3D cardiac tissues.

RECENT FINDINGS

Considerable steps towards the generation of macroscopic sizes for engineered cardiac tissue with efficient vascular networks have been made within the past few years. Achieving a compact tissue with enough cardiomyocytes to provide functionality remains a challenging task. Achieving perfusion in engineered constructs with media that contain oxygen and nutrients at a clinically relevant tissue sizes remains the next frontier in tissue engineering. The provisioning of a functional vasculature is necessary for maintaining a high cell viability and functionality in engineered cardiac tissues. Several recent studies have shown the ability to generate tissues up to a centimeter scale with a perfusable vascular network. Future challenges include improving cell density and tissue size. This requires the close collaboration of a multidisciplinary teams of investigators to overcome complex challenges in order to achieve success.

Soluble vascular endothelial-cadherin and auto-antibodies to human vascular endothelial-cadherin in human diseases: Two new biomarkers of endothelial dysfunction

Vascular endothelial-cadherin is the most important transmembrane component of endothelial adherens junctions, exclusively expressed by endothelial cells in all types of vessels. Targeting either the extracellular domain or the cytoplasmic tail deleteriously affects the junctional strength and leads to vascular permeability. Recently, cytokine-induced phosphorylation of the vascular endothelial-cadherin cytoplasmic domain was reported to trigger cleavage of its extracellular domain, producing the soluble form of the protein – soluble vascular endothelial-cadherin. Hence, the presence of soluble vascular endothelial-cadherin or auto-antibodies to human vascular endothelial-cadherin in human serum could signalize the presence of vascular abnormalities. This systematic review covers many human studies reporting increased levels of soluble vascular endothelial-cadherin, as well as auto-antibodies to human vascular endothelial-cadherin, which could be promising biomarkers of endothelial dysfunction in a large panel of diseases.

Tyrosine Kinase Receptor B Protects Against Coronary Artery Disease and Promotes Adult Vasculature Integrity by Regulating Ets1-Mediated VE-Cadherin Expression

Objective—

Tyrosine kinase receptor B (TrkB) is a high-affinity receptor for brain-derived neurotrophic factor. In addition to its nervous system functions, TrkB is also expressed in the cardiovascular system. However, the association of TrkB and coronary artery disease (CAD) remains unknown. We investigated the role of TrkB in the development of CAD and its mechanism.

Approach and Results—

We performed a case–control study in 2 independent cohort of Chinese subjects and found –69C>G polymorphisms of TrkB gene significantly associated with CAD. TrkB –69C homozygotes, which corresponded to decreased TrkB expression by luciferase reporter assay, showed increased risk for CAD. Immunofluorescence analysis revealed that TrkB was expressed in the aortic endothelium in atherosclerotic lesions in humans and ApoE –/– mice. TrkB knockdown in the aortic endothelium resulted in vascular leakage in ApoE –/– mice. Mechanistic studies showed that TrkB regulated vascular endothelial cadherin (VE-cadherin) expression through induction and activation of Ets1 transcriptional factor. Importantly, TrkB activation attenuated proatherosclerotic factors induced-endothelial hyperpermeability in human vascular endothelial cells.

Conclusions—

Our data demonstrate that TrkB protects endothelial integrity during atherogenesis by promoting Ets1-mediated VE-cadherin expression and plays a previously unknown protective role in the development of CAD.

Preeclampsia does not alter vascular growth and expression of CD31 and vascular endothelial cadherin in human placentas

Preeclampsia is characterized by maternal endothelial dysfunction (e.g., increased maternal vascular permeability caused by the disassembly of endothelial junction proteins). However, it is unclear if preeclampsia is associated with impaired vascular growth and expression of endothelial junction proteins in human placentas. Herein, we examined vascular growth in placentas from women with normal term (NT) and preeclamptic (PE) pregnancies using two endothelial junction proteins as endothelial markers: CD31 and vascular endothelial-cadherin (VE-Cad). We also compared protein and mRNA expression of CD31 and VE-Cad between NT and PE placentas, and determined the alternatively spliced expression of CD31 using PCR. We found that CD31 and VE-Cad were immunolocalized predominantly in villous endothelial cells. However, capillary number density (total capillary number per unit villous area) and capillary area density (total capillary lumen area per unit villous area) as well as CD31 and VE-Cad protein and mRNA levels were similar between NT and PE placentas. PCR in combination with sequence analysis revealed a single, full-length CD31, suggesting that there are no alternatively spliced isoform of CD31 expressed in placentas. These data indicate that preeclampsia does not significantly affect vascular growth or the expression of endothelial junction proteins in human placentas.

Mechanotransduction at the basis of endothelial barrier function

Destabilization of cell-cell contacts involved in the maintenance of endothelial barrier function can lead to increased endothelial permeability. This increase in endothelial permeability results in an anarchical movement of fluid, solutes and cells outside the vasculature and into the surrounding tissues, thereby contributing to various diseases such as stroke or pulmonary edema. Thus, a better understanding of the molecular mechanisms regulating endothelial cell junction integrity is required for developing new therapies for these diseases. In this review, we describe the mechanotransduction mechanism at the basis of adherens junction strengthening at endothelial cell-cell contacts. More particularly, we report on the emerging role of α-catenin and EPLIN that act as a mechanotransmitter of myosin-IIgenerated traction forces. The interplay between α-catenin, EPLIN and the myosin-II machinery initiates the junctional recruitment of vinculin and α-actinin leading to a drastic remodeling of the actin cytoskeleton and to cortical actin ring reshaping. The pathways initiated by tyrosine phosphorylation of VE-cadherin at the basis of endothelial cell-cell junction remodeling is also reported, as it may be interrelated to α-catenin/ EPLIN-mediated mechanotransduction mechanisms. We also describe the junctional mechanosensory complex composed of PECAM-1, VE-cadherin and VEGFR2 that is able to transmit signaling pathway under the onset of shear stress. This mechanosensing mechanism, involved in the earliest events promoting atherogenesis, is required for endothelial cell alignment along flow direction.

Elastase and cathepsin G from primed leukocytes cleave vascular endothelial cadherin in hemodialysis patients

Aims. To test the hypothesis that primed PMNLs in blood of chronic kidney disease patients release the active form of elastase and cathepsin G causing degradation of vital proteins and promote tissue damage. Methods. RT-PCR, immunocytochemical staining, immunoblotting, and FACS analyses were used to study these enzymes in hemodialysis patients (HD) versus healthy normal controls (NC). Using PMNLs and endothelial cells cocultivation system we measure the effect of HD PMNLs on the endothelial VE-cadherin, an essential protein for maintaining endothelial integrity. Results. Levels of elastase and cathepsin G were reduced in PMNLs of HD patients, while mRNA enzymes levels were not different. Elevated levels of the active form of these enzymes were found in blood of HD patients compared to NC.HD plasma had higher levels of soluble VE-cadherin present in three molecular forms: whole 140 kDa molecule and two fragments of 100 and 40 kDa. Cocultivation studies showed that primed PMNLs cleave the endothelial cadherin, resulting in a 100 kDa fragment. Conclusions. Elastase and cathepsin G are elevated in the plasma of HD patients, originating from primed PMNLs. In these patients, chronic elevation of these enzymes contributes to cleavage of VE-cadherin and possible disruption of endothelial integrity.

HIV-associated disruption of tight and adherens junctions of oral epithelial cells facilitates HSV-1 infection and spread

Herpes simplex virus (HSV) types 1 and 2 are the most common opportunistic infections in HIV/AIDS. In these immunocompromised individuals, HSV-1 reactivates and replicates in oral epithelium, leading to oral disorders such as ulcers, gingivitis, and necrotic lesions. Although the increased risk of HSV infection may be mediated in part by HIV-induced immune dysfunction, direct or indirect interactions of HIV and HSV at the molecular level may also play a role. In this report we show that prolonged interaction of the HIV proteins tat and gp120 and cell-free HIV virions with polarized oral epithelial cells leads to disruption of tight and adherens junctions of epithelial cells through the mitogen-activated protein kinase signaling pathway. HIV-induced disruption of oral epithelial junctions facilitates HSV-1 paracellular spread between the epithelial cells. Furthermore, HIV-associated disruption of adherens junctions exposes sequestered nectin-1, an adhesion protein and critical receptor for HSV envelope glycoprotein D (gD). Exposure of nectin-1 facilitates binding of HSV-1 gD, which substantially increases HSV-1 infection of epithelial cells with disrupted junctions over that of cells with intact junctions. Exposed nectin-1 from disrupted adherens junctions also increases the cell-to-cell spread of HSV-1 from infected to uninfected oral epithelial cells. Antibodies to nectin-1 and HSV-1 gD substantially reduce HSV-1 infection and cell-to-cell spread, indicating that HIV-promoted HSV infection and spread are mediated by the interaction of HSV gD with HIV-exposed nectin-1. Our data suggest that HIV-associated disruption of oral epithelial junctions may potentiate HSV-1 infection and its paracellular and cell-to-cell spread within the oral mucosal epithelium. This could be one of the possible mechanisms of rapid development of HSV-associated oral lesions in HIV-infected individuals.

Auto-antibodies to vascular endothelial cadherin in humans: association with autoimmune diseases

To identify patients with autoimmune diseases who are at high risk of developing vascular cell dysfunction, early biomarkers must be identified. This study was designed to detect and characterize circulating autoantibodies to VE-cadherin (AAVEs) in patients with early-stage autoimmune diseases. An enzyme-linked immunosorbent assay (ELISA) was developed to capture autoantibodies, using a recombinant human VE-cadherin fragment covering the extracellular domains as a target antigen. AAVEs specificity for the target antigen was confirmed by western blotting. Basal AAVEs levels were determined for healthy donors (n=75). Sera from patients (n=100) with various autoimmune diseases, including rheumatoid arthritis (n=23), systemic lupus erythematosus (SLE, n=31), systemic sclerosis (n=30), and Behçet's disease (BD, n=16) were also tested. Levels of AAVEs were significantly higher in rheumatoid arthritis (P<0.0001), SLE (P<0.05), and BD (P<0.05) populations than in healthy subjects. Purified immunoglobulin G (IgG) from a BD patient with exceptionally high AAVEs levels recognized the EC1-4 fragment in western blots. Further characterization of the epitopes recognized by AAVEs showed that BD patients had antibodies specific for the EC3 and EC4 domains, whereas SLE patients preferentially recognized the EC1 fragment. This suggests that distinct epitopes of human VE-cadherin might be recognized in different immune diseases. Purified IgG from BD patients was found to induce endothelial cell retraction, redistribution of VE-cadherin, and cause the formation of numerous intercellular gaps. Altogether, these data demonstrate a potential pathogenic effect of AAVEs isolated from patients with dysimmune disease. This is the first description of AAVEs in humans. Because regions EC1 and EC3-4 have been shown to be involved in homophilic VE-cadherin interactions, AAVEs produced in the course of dysimmune diseases might be specific biomarkers for endothelial injury, which is part of the early pathogenicity of these diseases.

HIV-associated disruption of mucosal epithelium facilitates paracellular penetration by human papillomavirus

The incidence of human papillomavirus (HPV)-associated epithelial lesions is substantially higher in human immunodeficiency virus (HIV)-infected individuals than in HIV-uninfected individuals. The molecular mechanisms underlying the increased risk of HPV infection in HIV-infected individuals are poorly understood. We found that HIV proteins tat and gp120 were expressed within the oral and anal mucosal epithelial microenvironment of HIV-infected individuals. Expression of HIV proteins in the mucosal epithelium was correlated with the disruption of epithelial tight junctions (TJ). Treatment of polarized oral, cervical and anal epithelial cells, and oral tissue explants with tat and gp120 led to disruption of epithelial TJ and increased HPV pseudovirion (PsV) paracellular penetration in to the epithelium. PsV entry was observed in the basal/parabasal cells, the cells in which the HPV life cycle is initiated. Our data suggest that HIV-associated TJ disruption of mucosal epithelia may potentiate HPV infection and subsequent development of HPV-associated neoplasia.

Epstein-Barr virus transcytosis through polarized oral epithelial cells

Although Epstein-Barr virus (EBV) is an orally transmitted virus, viral transmission through the oropharyngeal mucosal epithelium is not well understood. In this study, we investigated how EBV traverses polarized human oral epithelial cells without causing productive infection. We found that EBV may be transcytosed through oral epithelial cells bidirectionally, from both the apical to the basolateral membranes and the basolateral to the apical membranes. Apical to basolateral EBV transcytosis was substantially reduced by amiloride, an inhibitor of macropinocytosis. Electron microscopy showed that virions were surrounded by apical surface protrusions and that virus was present in subapical vesicles. Inactivation of signaling molecules critical for macropinocytosis, including phosphatidylinositol 3-kinases, myosin light-chain kinase, Ras-related C3 botulinum toxin substrate 1, p21-activated kinase 1, ADP-ribosylation factor 6, and cell division control protein 42 homolog, led to significant reduction in EBV apical to basolateral transcytosis. In contrast, basolateral to apical EBV transcytosis was substantially reduced by nystatin, an inhibitor of caveolin-mediated virus entry. Caveolae were detected in the basolateral membranes of polarized human oral epithelial cells, and virions were detected in caveosome-like endosomes. Methyl β-cyclodextrin, an inhibitor of caveola formation, reduced EBV basolateral entry. EBV virions transcytosed in either direction were able to infect B lymphocytes. Together, these data show that EBV transmigrates across oral epithelial cells by (i) apical to basolateral transcytosis, potentially contributing to initial EBV penetration that leads to systemic infection, and (ii) basolateral to apical transcytosis, which may enable EBV secretion into saliva in EBV-infected individuals.

Endothelialization approaches for viable engineered tissues

One of the main limitation in obtaining thick, 3-dimensional viable engineered constructs is the inability to provide a sufficient and functional blood vessel system essential for the in vitro survival and the in vivo integration of the construct. Different strategies have been proposed to simulate the ingrowth of new blood vessels into engineered tissue, such as the use of growth factors, fabrication scaffold technologies, in vivo prevascularization and cell-based strategies, and it has been demonstrated that endothelial cells play a central role in the neovascularization process and in the control of blood vessel function. In particular, different "environmental" settings (origin, presence of supporting cells, biomaterial surface, presence of hemodynamic forces) strongly influence endothelial cell function, angiogenic potential and the in vivo formation of durable vessels. This review provides an overview of the different techniques developed so far for the vascularization of tissue-engineered constructs (with their advantages and pitfalls), focusing the attention on the recent development in the cell-based vascularization strategy and the in vivo applications.

Vascular endothelial cadherin modulates renal interstitial fibrosis

BACKGROUND/AIMS

Renal interstitial fibrosis is a final common pathway of all chronic, progressive kidney diseases. Peritubular capillary rarefaction is strongly correlated with fibrosis. The adherens junction protein vascular endothelial cadherin (VE-cadherin) is thought to play a critical role in vascular integrity. We hypothesized that VE-cadherin modulates the renal microcirculation during fibrogenesis and ultimately affects renal fibrosis.

METHODS

Unilateral ureteral obstruction (UUO) was used as a renal fibrosis model in VE-cadherin heterozygote (VE+/-) and wild-type (WT) mice, and the kidneys were harvested at days 3, 7, and 14. Peritubular capillary changes and fibrogenesis were investigated.

RESULTS

VE+/- mice had lower levels of VE-cadherin protein than WT mice at 3 and 7, but not 14 days after UUO. Vascular permeability was significantly greater in VE+/- mice 7 days after UUO, while peritubular capillary density was not significantly different in VE+/- and WT mice. Interstitial myofibroblast numbers and collagen I and III mRNA levels were significantly higher in VE+/- mice, consistent with a stronger early fibrogenic response. Expression of the pericyte marker neuron-glial antigen 2 was upregulated after UUO, but was not greater in VE+/- mice compared to the WT mice.

CONCLUSION

Our data suggest that VE-cadherin controls vascular permeability and limits fibrogenesis after UUO.

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.

Placenta growth factor-1 exerts time-dependent stabilization of adherens junctions following VEGF-induced vascular permeability

Increased vascular permeability is an early event characteristic of tissue ischemia and angiogenesis. Although VEGF family members are potent promoters of endothelial permeability the role of placental growth factor (PlGF) is hotly debated. Here we investigated PlGF isoforms 1 and 2 and present in vitro and in vivo evidence that PlGF-1, but not PlGF-2, can inhibit VEGF-induced permeability but only during a critical window post-VEGF exposure. PlGF-1 promotes VE-cadherin expression via the trans-activating Sp1 and Sp3 interaction with the VE-cadherin promoter and subsequently stabilizes transendothelial junctions, but only after activation of endothelial cells by VEGF. PlGF-1 regulates vascular permeability associated with the rapid localization of VE-cadherin to the plasma membrane and dephosphorylation of tyrosine residues that precedes changes observed in claudin 5 tyrosine phosphorylation and membrane localization. The critical window during which PlGF-1 exerts its effect on VEGF-induced permeability highlights the importance of the translational significance of this work in that PLGF-1 likely serves as an endogenous anti-permeability factor whose effectiveness is limited to a precise time point following vascular injury. Clinical approaches that would pattern nature's approach would thus limit treatments to precise intervals following injury and bring attention to use of agents only during therapeutic windows.

The motor protein myosin-X transports VE-cadherin along filopodia to allow the formation of early endothelial cell-cell contacts

Vascular endothelium (VE), the monolayer of endothelial cells that lines the vascular tree, undergoes damage at the basis of some vascular diseases. Its integrity is maintained by VE-cadherin, an adhesive receptor localized at cell-cell junctions. Here, we show that VE-cadherin is also located at the tip and along filopodia in sparse or subconfluent endothelial cells. We observed that VE-cadherin navigates along intrafilopodial actin filaments. We found that the actin motor protein myosin-X is colocalized and moves synchronously with filopodial VE-cadherin. Immunoprecipitation and pulldown assays confirmed that myosin-X is directly associated with the VE-cadherin complex. Furthermore, expression of a dominant-negative mutant of myosin-X revealed that myosin-X is required for VE-cadherin export to cell edges and filopodia. These features indicate that myosin-X establishes a link between the actin cytoskeleton and VE-cadherin, thereby allowing VE-cadherin transportation along intrafilopodial actin cables. In conclusion, we propose that VE-cadherin trafficking along filopodia using myosin-X motor protein is a prerequisite for cell-cell junction formation. This mechanism may have functional consequences for endothelium repair in pathological settings.

Structure of artificial and natural VE-cadherin-based adherens junctions

In vascular endothelium, adherens junctions between endothelial cells are composed of VE-cadherin (vascular endothelial cadherin), an adhesive receptor that is crucial for the proper assembly of vascular structures and the maintenance of vascular integrity. As a classical cadherin, VE-cadherin links endothelial cells together by homophilic interactions mediated by its extracellular part and associates intracellularly with the actin cytoskeleton via catenins. Although, from structural crystallographic data, a dimeric structure arranged in a trans orientation has emerged as a potential mechanism of cell-cell adhesion, the cadherin organization within adherens junctions remains controversial. Concerning VE-cadherin, its extracellular part possesses the capacity to self-associate in solution as hexamers consisting of three antiparallel cadherin dimers. VE-cadherin-based adherens junctions were reconstituted in vitro by assembly of a VE-cadherin EC (extracellular repeat) 1-EC4 hexamer at the surfaces of liposomes. The artificial adherens junctions revealed by cryoelectron microscopy appear as a two-dimensional self-assembly of hexameric structures. This cadherin organization is reminiscent of that found in native desmosomal junctions. Further structural studies performed on native VE-cadherin junctions would provide a better understanding of the cadherin organization within adherens junctions. Homophilic interactions between cadherins are strengthened intracellularly by connection to the actin cytoskeleton. Recently, we have discovered that annexin 2, an actin-binding protein connects the VE-cadherin-catenin complex to the actin cytoskeleton. This novel link is labile and promotes the endothelial cell switch from a quiescent to an angiogenic state.

Establishment of cell-cell junctions depends on the oligomeric states of VE-cadherin

Specifically expressed at intercellular adherens junctions of endothelial cells, VE-cadherin is a receptor that exhibits particular self-association properties. Indeed, in vitro studies demonstrated that the extracellular part of VE-cadherin elaborates Ca(++)-dependent hexameric structures. We hypothesized that this assembly could be at the basis of a new cadherin-mediated cell-cell adhesion mechanism. To verify this assumption, we first demonstrated that VE-cadherin can elaborate hexamers at the cell surface of confluent endothelial cells. Second, mutations were introduced within the extracellular part of VE-cadherin to destabilize the hexamer. Following an in vitro screening, three mutants were selected, among which, one is able to elaborate only dimers. The selected mutations were expressed as C-terminal green fluorescent protein fusions in CHO cells. Despite their capacity to elaborate nascent cell-cell contacts, the mutants seem to be rapidly degraded and/or internalized. Altogether, our results suggest that the formation of VE-cadherin hexamers protects this receptor and might allow the elaboration of mature endothelial cell-cell junctions.

The thrombogenicity of human umbilical vein endothelial cell seeded collagen modules

Modular tissue-engineered constructs are assembled from sub-mm sized cylindrical collagen gel modules which are covered with a surface layer of human umbilical vein endothelial cells (HUVEC). The resulting construct is permeated by a network of interconnected endothelial cell lined channels to facilitate blood perfusion and nutrient delivery. This design strategy relies critically on the endothelial cells' layer behaving in a non-thrombogenic manner on the module surface and the objective here was to characterize this thrombogenicity. HUVEC prolonged clotting times in whole blood-module mixtures, and enabled slightly heparinized whole blood perfusion of an assembled modular construct in vitro with no increase in platelet loss compared to background levels. Flow cytometry and scanning electron microscopy indicated that HUVEC seeded modules reduced platelet activation and deposition but not leukocyte activation, compared to collagen only modules. Plasma recalcification times on non-stimulated HUVEC were longer compared to stimulated HUVEC but not different than that on collagen only module films and were not prolonged by incubation with a tissue factor blocking antibody. Together these data suggest that a functional non-thrombogenic layer of EC was generated on the module surface and that this layer should be sufficient to maintain continuous blood flow through an engineered modular tissue. In/ex vivo studies are warranted to confirm this conclusion.

gamma-Secretase: a multifaceted regulator of angiogenesis

Physiological angiogenesis is essential for development, homeostasis and tissue repair but pathological neovascularization is a major feature of tumours, rheumatoid arthritis and ocular complications. Studies over the last decade have identified γ-secretase, a presenilin-dependent protease, as a key regulator of angiogenesis through: (i) regulated intramembrane proteolysis and transmembrane cleavage of receptors (e.g. VEGFR-1, Notch, ErbB-4, IGFI-R) followed by translocation of the intracellular domain to the nucleus, (ii) translocation of full length membrane-bound receptors to the nucleus (VEGFR-1), (iii) phosphorylation of membrane bound proteins (VEGFR-1 and ErbB-4), (iv) modulation of adherens junctions (cadherin) and regulation of permeability and (v) cleavage of amyloid precursor protein to amyloid-β which is able to regulate the angiogenic process. The γ-secretase-induced translocation of receptors to the nucleus provides an alternative intracellular signalling pathway, which acts as a potent regulator of transcription. γ-secretase is a complex composed of four different integral proteins (presenilin, nicastrin, Aph-1 and Pen-2), which determine the stability, substrate binding, substrate specificity and proteolytic activity of γ-secretase. This seeming complexity allows numerous possibilities for the development of targeted γ-secretase agonists/antagonists, which can specifically regulate the angiogenic process. This review will consider the structure and function of γ-secretase, the growing evidence for its role in angiogenesis and the substrates involved, γ-secretase as a therapeutic target and future challenges in this area.

VE-cadherin: the major endothelial adhesion molecule controlling cellular junctions and blood vessel formation

Vascular endothelial (VE)-cadherin is a strictly endothelial specific adhesion molecule located at junctions between endothelial cells. In analogy of the role of E-cadherin as major determinant for epithelial cell contact integrity, VE-cadherin is of vital importance for the maintenance and control of endothelial cell contacts. Mechanisms that regulate VE-cadherin-mediated adhesion are important for the control of vascular permeability and leukocyte extravasation. In addition to its adhesive functions, VE-cadherin regulates various cellular processes such as cell proliferation and apoptosis and modulates vascular endothelial growth factor receptor functions. Consequently, VE-cadherin is essential during embryonic angiogenesis. This review will focus on recent new developments in understanding the role of VE-cadherin in controlling endothelial cell contacts and influencing endothelial cell behavior by various outside-in signaling processes.

Contribution of annexin 2 to the architecture of mature endothelial adherens junctions

The vascular endothelial cadherin (VE-cad)-based complex is involved in the maintenance of vascular endothelium integrity. Using immunoprecipitation experiments, we have demonstrated that, in confluent human umbilical vein endothelial cells, the VE-cad-based complex interacts with annexin 2 and that annexin 2 translocates from the cytoplasm to the cell-cell contact sites as cell confluence is established. Annexin 2, located in cholesterol rafts, binds to both the actin cytoskeleton and the VE-cad-based complex so the complex is docked to cholesterol rafts. These multiple connections prevent the lateral diffusion of the VE-cad-based complex, thus strengthening adherens junctions in the ultimate steps of maturation. Moreover, we observed that the down-regulation of annexin 2 by small interfering RNA induces a delocalization of VE-cad from adherens junctions and consequently a destabilization of these junctions. Furthermore, our data indicate that the decoupling of the annexin 2/p11 complex from the VE-cad-based junction, triggered by vascular endothelial growth factor treatment, facilitates the switch from a quiescent to an immature state.

HIF-2alpha specifically activates the VE-cadherin promoter independently of hypoxia and in synergy with Ets-1 through two essential ETS-binding sites

The mechanisms that are responsible for the restricted pattern of expression of the VE-cadherin gene in endothelial cells are not clearly understood. Regulation of expression is under the control of an approximately 140 bp proximal promoter that provides basal, non-endothelial specific expression. A larger region contained within the 2.5 kb genomic DNA sequence located ahead of the transcription start is involved in the specific expression of the gene in endothelial cells. We show here that the VE-cadherin promoter contains several putative hypoxia response elements (HRE) which are able to bind endothelial nuclear factors under normoxia. The VE-cadherin gene is not responsive to hypoxia but hypoxia-inducible factor (HIF)-2alpha specifically activates the promoter while HIF-1alpha does not. The HRE, that are involved in this activity have been identified. Further, we show that HIF-2alpha cooperates with the Ets-1 transcription factor for activation of the VE-cadherin promoter and that this synergy is dependent on the binding of Ets-1 to DNA. This cooperative action of HIF-2alpha with Ets-1 most probably participates to the transcriptional regulation of expression of the gene in endothelial cells. This mechanism may also be involved in the expression of the VE-cadherin gene by tumor cells in the process of vascular mimicry.

The influence of biomaterials on endothelial cell thrombogenicity

Driven by tissue engineering and regenerative medicine, endothelial cells are being used in combination with biomaterials in a number of applications for the purpose of improving blood compatibility and host integration. Endothelialized vascular grafts are beginning to be used clinically with some success in some centers, while endothelial seeding is being explored as a means of creating a vasculature within engineered tissues. The underlying assumption of this strategy is that when cultured on artificial biomaterials, a confluent layer of endothelial cells maintain their non-thrombogenic phenotype. In this review the existing knowledge base of endothelial cell thrombogenicity cultured on a number of different biomaterials is summarized. The importance of selecting appropriate endpoint measures that are most reflective of overall surface thrombogenicity is the focus of this review. Endothelial cells inhibit thrombosis through three interconnected regulatory systems (1) the coagulation cascade, (2) the cellular components of the blood such as leukocytes and platelets and (3) the complement cascade, and also through effects on fibrinolysis and vascular tone, the latter which influences blood flow. Thus, in order to demonstrate the thrombogenic benefit of seeding a biomaterial with EC, the conditions under which EC surfaces are more likely to exhibit lower thrombogenicity than unseeded biomaterial surfaces need to be consistent with the experimental context. The endpoints selected should be appropriate for the dominant thrombotic process that occurs under the given experimental conditions.

Desquamation of human coronary artery endothelium by human mast cell proteases: implications for plaque erosion

OBJECTIVE

Endothelial erosion has emerged as an important contributor to the pathogenesis of atherosclerosis and its complications, but the molecular mechanisms have remained unclear. As activated mast cells capable of secreting neutral proteases are present in the intima of eroded coronary plaques, we investigated their potential roles in endothelial erosion.

METHODS AND RESULTS

Studies involving double immunostaining of mast cells (tryptase(pos) cells) and platelets (CD42b) in human coronary artery specimens indicated that the number of subendothelial mast cells correlated with the number of parietal microthrombi (P=0.001, rs 0.27). The number of parietal microthrombi was significantly higher (P<0.001) in areas of plaques than in areas of healthy intima. Of the microthrombi 86% were in the lesional coronary segments, of all subendothelial mast cells 15% were located under parietal microthrombi, and of all parietal microthrombi 49% were located over subendothelial mast cells. Double immunostaining revealed the mast cell to neutrophil ratio in the human coronary artery intima to be 5 : 1, and that mast cells are a major local source of cathepsin G. Scanning electron and light microscopy indicated that treatment of fresh human coronary arteries intraluminally with recombinant human (rh)-tryptase and rh-chymase induced endothelial damage characterized by retraction of endothelial cells, disruption of endothelial cell to cell adhesions and desquamation of endothelial cells. VE-cadherin and fibronectin, which are necessary for cell-cell interactions and endothelial cell adhesion, were degraded by tryptase and chymase and also by cathepsin G.

CONCLUSIONS

Activated subendothelial mast cells may contribute to endothelial erosion by releasing proteases capable of degrading VE-cadherin and fibronectin.

Hydrostatic pressure influences morphology and expression of VE-cadherin of vascular endothelial cells

Bovine aortic endothelial cells (BAECs) were exposed to hydrostatic pressures of 50, 100, and 150 mmHg and changes in morphology and expression of vascular endothelial (VE)-cadherin were studied. After exposure to hydrostatic pressure, BAECs exhibited elongated and tortuous shape without predominant orientation, together with the development of centrally located, thick stress fibers. Pressured BAECs also exhibited a multilayered structure unlike those under control conditions and showed a significant increase in proliferation compared with control cells. Western blot analysis demonstrated that protein level of VE-cadherin were significantly lower under pressure conditions than under control conditions. Inhibition of VE-cadherin expression, using an antibody to VE-cadherin, induced the formation of numerous randomly distributed intercellular gaps, elongated and tortuous shapes, and multilayering. These responses were similar to those of pressured BAECs. The exposure of BAECs to hydrostatic pressure may therefore downregulate the expression of VE-cadherin, resulting in loss of contact inhibition followed by increased proliferation and formation of a multilayered structure.

Maternal antibodies enhance or prevent cytomegalovirus infection in the placenta by neonatal Fc receptor-mediated transcytosis

How human cytomegalovirus (CMV) reaches the fetus across the placenta is unknown. The major viral cause of congenital disease, CMV infects the uterine-placental interface with varied outcomes depending on the strength of maternal humoral immunity and gestational age. Covering the surface of villi that float in blood, syncytiotrophoblasts express the neonatal Fc receptor (FcRn) that transports IgG for passive immunity. Immunohistochemical analysis of early-gestation biopsy specimens showed an unusual pattern of CMV replication proteins in underlying villus cytotrophoblasts, whereas syncytiotrophoblasts were spared. Found in placentas with low to moderate CMV-neutralizing antibody titers, this pattern suggested virion transcytosis across the surface. In contrast, syncytiotrophoblasts from placentas with high neutralizing titers contained viral DNA and caveolin-1-positive vesicles in which IgG and CMV glycoprotein B co-localized. In villus explants, IgG-virion transcytosis and macrophage uptake were blocked with trypsin-treatment and soluble protein A. Quantitative analysis in polarized epithelial cells showed that FcRn-mediated transcytosis was blocked by the Fc fragment of IgG, but not F(ab')(2). Our results suggest that CMV virions could disseminate to the placenta by co-opting the receptor-mediated transport pathway for IgG. These findings could explain the efficacy of hyperimmune IgG for treatment of primary CMV infection during gestation and support vaccination.

The human VE-cadherin promoter is subjected to organ-specific regulation and is activated in tumour angiogenesis

Vascular endothelial (VE)-cadherin is exclusively expressed at interendothelial junctions of normal and tumour vessels. In this report, we characterized the transcriptional activity of the human VE-cadherin promoter. Transient transfection assays revealed that sequences at positions --1135/-744 and -166/-5 base pairs are critical for promoter activity in endothelial cells. We show that specific sequences in the proximal region interact with Ets and Sp1 family members. Transgenic mice were created and the human VE-cadherin promoter was able to confer correct temporal and spatial expression on the LacZ gene in embryos. In adults, the transgene was specifically and strongly expressed in the lung, heart, ovary, spleen and kidney glomeruli, whereas expression was weak or absent in the vasculature of other organs, including the brain, thymus, liver and skeletal muscle. Neovessels in tumour grafts and Matrigel implants harboured strong stainings, indicating that promoter activity is enhanced in angiogenic situations. Furthermore, Matrigel and transfection assays showed that VE-cadherin promoter is subjected to bFGF induction. Transgene expression was also noticed in extravascular sites of the central nervous system, suggesting that silencer elements may be located elsewhere in the gene. These results are a first step towards addressing the organ- and tumour-specific regulation of the VE-cadherin gene.

Vascular endothelial-cadherin tyrosine phosphorylation in angiogenic and quiescent adult tissues

Vascular endothelial-cadherin (VE-cadherin) plays a key role in angiogenesis and in vascular permeability. The regulation of its biological activity may be a central mechanism in normal or pathological angiogenesis. VE-cadherin has been shown to be phosphorylated on tyrosine in vitro under various conditions, including stimulation by VEGF. In the present study, we addressed the question of the existence of a tyrosine phosphorylated form of VE-cadherin in vivo, in correlation with the quiescent versus angiogenic state of adult tissues. Phosphorylated VE-cadherin was detected in mouse lung, uterus, and ovary but not in other tissues unless mice were injected with peroxovanadate to block protein phosphatases. Remarkably, VE-cadherin tyrosine phosphorylation was dramatically increased in uterus and ovary, and not in other organs, during PMSG/hCG-induced angiogenesis. In parallel, we observed an increased association of VE-cadherin with Flk1 (VEGF receptor 2) during hormonal angiogenesis. Additionally, Src kinase was constitutively associated with VE-cadherin in both quiescent and angiogenic tissues and increased phosphorylation of VE-cadherin-associated Src was detected in uterus and ovary after hormonal treatment. Src-VE-cadherin association was detected in cultured endothelial cells, independent of VE-cadherin phosphorylation state and Src activation level. In this model, Src inhibition impaired VEGF-induced VE-cadherin phosphorylation, indicating that VE-cadherin phosphorylation was dependent on Src activation. We conclude that VE-cadherin is a substrate for tyrosine kinases in vivo and that its phosphorylation, together with that of associated Src, is increased by angiogenic stimulation. Physical association between Flk1, Src, and VE-cadherin may thus provide an efficient mechanism for amplification and perpetuation of VEGF-stimulated angiogenic processes.

Periodic adhesive fingers between contacting cells

We study in detail the properties of fingers, a particular type of cell-cell adhesive structures appearing in adherens junctions. These periodic patterns break the symmetry of cell-cell contacts. We show that finger formation is driven by cadherin interactions and actin growth. A theoretical model is introduced in which the growth of fingers is limited by membrane tension. The steady shape and formation kinetics of fingers are experimentally measured and compared with the theoretical predictions.

Sorting of furin in polarized epithelial and endothelial cells: expression beyond the Golgi apparatus

The conversion of proteins into their mature forms underlies the functionality of many fundamental cellular pathways. One posttranslational modification leading to maturation of precursor proteins consists of the cleavage of their prodomain at pairs of basic amino acids by enzymes of the subtilisin-like mammalian proprotein convertase family. One of these enzymes, furin, acts in the constitutive secretory pathway of almost every cell type. However, in spite of furin's major roles in many pathophysiological processes, the exact subcellular sites of processing and activation of its substrates remain elusive. In this study, furin antigenic sites were tracked in subcellular compartments of various tissues and corresponding cell lines by high-resolution immunogold electron microscopy, Western blotting, cell transfection, and in vivo gene delivery of the furin cDNA. In addition to the Golgi apparatus, furin was assigned to endosomes and plasma membranes of polarized intestinal and renal epithelial cells and endothelial cells of the continuous, fenestrated, and discontinuous capillaries. Roles of furin in endothelial permeability, basement membrane turnover, and shedding of transmembrane proteins are supported by our data.

Transference of recombinant VE-cadherin cytoplasmic domain alters endothelial junctional integrity and porcine microvascular permeability

VE-cadherin constitutes endothelial adherens junctions through a homophilic binding of its extracellular domain and by the anchoring of its intracellular domain to actin cytoskeleton via catenins. The aim of this study was to determine the functional importance of VE-cadherin-cytoskeleton association in the maintenance of endothelial junctional integrity. A recombinant VE-cadherin cytoplasmic domain (rVE-cad CPD) was expressed in E. coli and purified through Ni-NTA spin columns. Immunoprecipitation assays showed that rVE-cad CPD was able to bind beta-catenin in vitro and to compete with endogenous VE-cadherin for binding of beta-catenin in human umbilical vein endothelial cells. A significant increase in the transendothelial flux of albumin was observed in the endothelial cell monolayers transfected with rVE-cad CPD. Importantly, transfection of rVE-cad CPD into intact isolated coronary venules markedly elevated the albumin permeability of the venular endothelium. In addition, immunofluorescence microscopic analysis revealed a conformational change of VE-cadherin from a uniform, continuous distribution along the cell membrane under control conditions to a diffuse, stitch-like pattern after rVE-cad CPD transfection. The effects were likely due to an attenuated anchorage of endogenous VE-cadherin to the cytoskeleton, as evidenced by a decreased partitioning of VE-cadherin in the detergent-insoluble cytoskeletal pool. The results suggest that the intracellular association of VE-cadherin with beta-catenin-linked cytoskeleton is essential to the maintenance of endothelial junctional integrity and microvascular permeability.

A Soluble VE-cadherin Fragment Forms 2D Arrays of Dimers upon Binding to a Lipid Monolayer

A high concentration of cadherin molecules at cell-cell adhesion sites is believed to be essential for generating strong intercellular junctions. In order to determine the interactions of cadherin domains involved in the early stages of lateral cluster formation on the cell surface, a recombinant fragment encompassing the first four domains of human VE-cadherin with a His-tag at the C terminus (VE-EC1-4-His) was produced. Two-dimensional crystals of VE-EC1-4-His were formed at the air-water interface using conventional lipids modified to contain a Ni(2+)-chelating group, which provides a specific site for interaction with the polyhistidine tag. The VE-EC1-4-His was monomeric at the concentration employed for crystal formation; however, the crystals exhibited a p2 symmetry and the presence of cis-dimer interactions between symmetry-related molecules. The VE-EC1-4-His molecules in the crystalline array have a remarkably compact conformation in contrast to the elongated "string of pearls" conformation seen in the hexameric assembly of VE-EC1-4-His in solution, and as seen in the crystal structure of C-cadherin. These results indicate that VE-cadherin can exist in at least two oligomeric states with different interactions between domains and can adopt highly different conformational states. We suggest that the compact cis-dimeric state may occur on isolated cells and that the compact form may serve to protect the molecule from degradation. As previously proposed we suppose that the trans-hexameric form is involved in intercellular adhesion.

Identification of proteases involved in the proteolysis of vascular endothelium cadherin during neutrophil transmigration

Transmigration of neutrophils across the endothelium occurs at the cell-cell junctions where the vascular endothelium cadherin (VE cadherin) is expressed. This adhesive receptor was previously demonstrated to be involved in the maintenance of endothelium integrity. We propose that neutrophil transmigration across the vascular endothelium goes in parallel with cleavage of VE cadherin by elastase and cathepsin G present on the surface of neutrophils. This hypothesis is supported by the following lines of evidence. 1) Proteolytic fragments of VE cadherin are released into the culture medium upon adhesion of neutrophils to endothelial cell monolayers; 2) conditioned culture medium, obtained after neutrophil adhesion to endothelial monolayers, cleaves the recombinantly expressed VE cadherin extracellular domain; 3) these cleavages are inhibited by inhibitors of elastase; 4) VE cadherin fragments produced by conditioned culture medium or by exogenously added elastase are identical as shown by N-terminal sequencing and mass spectrometry analysis; 5) both elastase- and cathepsin G-specific VE cadherin cleavage patterns are produced upon incubation with tumor necrosis factor alpha-stimulated and fixed neutrophils; 6) transendothelial permeability increases in vitro upon addition of either elastase or cathepsin G; and 7) neutrophil transmigration is reduced in vitro in the presence of elastase and cathepsin G inhibitors. Our results suggest that cleavage of VE cadherin by neutrophil surface-bound proteases induces formation of gaps through which neutrophils transmigrate.

Neutrophils induce sequential focal changes in endothelial adherens junction components: role of elastase

OBJECTIVE

In vitro studies have indicated that polymorphonuclear leukocytes (PMNs) traverse endothelial cell monolayers via the paracellular pathway (i.e., through endothelial cell-cell junctions. Herein, we assessed whether the adherens junctions (AJs) are disrupted during PMN transendothelial cell migration.

METHODS

Human umbilical vein endothelial cells (HUVECs) were grown to confluence on porous membranes and activated with interleukin-1beta, and PMN transendothelial migration was facilitated by formyl-methionyl-leucyl-phenylalanine. Using dual immunofluorescence staining and laser scanning confocal microscopy, we assessed the effects of PMN-endothelial cell adhesive interactions (i.e., adhesion to and emigration across monolayers) on the AJ components vascular endothelial (VE)-cadherin, beta-catenin, alpha-catenin, and gamma-catenin.

RESULTS

In the AJ immediately adjacent to the adherent PMN, there was a loss of staining for some of the AJ components. AJ components further away from HUVEC-PMN adhesive interactions were unaffected. An iterative approach indicated that the four components were sequentially lost from the AJ. beta-catenin was lost first, followed by VE-cadherin, alpha-catenin, and, finally, gamma-catenin. In the absence of PMNs, the cross-linking of VE-cadherin, but not platelet endothelial cell adhesion molecule-1 or intercellular adhesion molecule-1, increased the cytoplasmic accumulation of beta-catenin. During PMN transendothelial migration, all of the junctional components under study were lost at the immediate site of monolayer penetration. Again, at regions removed from the actual site of PMN penetration of the monolayers, the AJ components were unaffected. PMN-induced disorganization of the AJs was partially prevented by an elastase inhibitor.

CONCLUSIONS

These findings suggest that adherent PMNs induce a localized, sequential disassembly of AJs, which is partially mediated by PMN-derived elastase and involves the initial loss of an intracellular component of AJs (i.e., beta-catenin).

Transmission of human cytomegalovirus from infected uterine microvascular endothelial cells to differentiating/invasive placental cytotrophoblasts

Analysis of placentas infected with human cytomegalovirus (CMV) suggested that viral transmission could involve differentiating/invasive cytotrophoblasts in villi that attach the placenta to the uterine wall. To parse the cellular components in this process, we developed a coculture system of polarized uterine microvascular endothelial cell (UtMVEC) infection with an endothelial cell-tropic pathogenic strain of CMV. Then we evaluated the potential role of neutrophils and endothelial cells in the spread of infection to differentiating cytotrophoblasts. As shown by immunocytochemistry and analysis of viral replication, CMV preferentially infected endothelial cells via apical membranes and disrupted cell junction proteins, thereby altering paracellular permeability and cell polarity. Neutralizing antibodies to CMV glycoprotein B, an envelope component that facilitates virion penetration, blocked plaque formation in polarized UtMVEC. Neutrophils transmitted CMV infection to UtMVEC, which in turn infected cytotrophoblasts. However, neutrophils did not directly infect cytotrophoblasts. These findings implicate endothelial cells from the uterine microvasculature as a potential source for CMV infection of endovascular cytotrophoblasts of the anchoring villi. Possibly the cytokine/chemokine milieu in the pregnant uterus could attract immune cells that infect endothelial cells in hybrid fetal-maternal vessels. In turn, these cells could infect endovascular cytotrophoblasts, one possible initiation point of a cascade that results in retrograde placental CMV infection.

Wound closure in sheared endothelial cells is enhanced by modulation of vascular endothelial-cadherin expression and localization

We previously demonstrated that laminar shear stress enhances human coronary artery endothelial cell (HCAEC) wound closure via the mechanisms of cell spreading and migration. Because cell-cell junctional proteins such as vascular endothelial cell cadherin (VE-cadherin) are critical to cell-cell adhesion and motility, we tested the hypothesis that modulation of VE-cadherin expression under shear stress may be linked to this enhancement in wound closure. HCAEC monolayers were preconditioned to attain cellular alignment by shearing at 12 dynes/cm(2) for 18 hr in a parallel-plate flow chamber. Subsequently, they were divided into the following three groups: (i) control; (ii) treated with anti-cadherin-5 antibody; or (iii) treated with the calcium chelating agent EGTA. Next, the monolayers were wounded with a metal spatula and resheared at 20 dynes/cm(2) or left static. Time-lapse imaging was performed during the first 3 hr after imposition of these conditions. Immunocytochemistry or Western blot analyses for VE-cadherin expression were performed on all wounded monolayers. Deconvolution microscopy, three-dimensional cell-cell junctional reconstruction images, and histogram analyses of interendothelial junction signal intensities were performed on cells at the wound edge of a monolayer. Under shear, HCAEC demonstrated increased VE-cadherin immunofluorescence and protein expression despite an enhancement in wound closure compared with static conditions. In separate experiments, application with anti-cadherin-5 antibody or treatment with EGTA attenuated VE-cadherin expression and further enhanced wound closure compared with control shear and all static conditions. In addition, the pattern of VE-cadherin localization with these treatments became more intracellular and nuclear in appearance. These findings of changes in this junctional adhesion protein expression and localization may further our understanding of laminar shear stress-induced endothelial repair in the coronary circulation.

Protein kinase signaling in the modulation of microvascular permeability

The permeability of exchange microvessels is regulated through complex interactions between signaling molecules and structural proteins in the endothelium. Endothelial barrier integrity is maintained by adhesive interactions occurring at the cell-cell and cell-matrix contacts via junctional proteins and focal adhesion complexes that are anchored to the cytoskeleton. Cyclic AMP (cAMP) and cAMP-dependent kinase counteract with the nitric oxide (NO)-cyclic GMP (cGMP) pathway to protect the basal barrier function. Upon stimulation by physical stress, growth factors, or inflammatory agents, endothelial cells undergo a series of intracellular signaling reactions involving activation of protein kinase C (PKC), protein kinase G (PKG), mitogen-activated protein kinases (MAPK), and/or protein tyrosine kinases. The phosphorylation cascades trigger biochemical and conformational changes in the barrier structure and ultimately lead to an opening of the paracellular pathway. In particular, myosin light chain kinase (MLCK) activation and subsequent myosin light chain (MLC) phosphorylation in endothelial cells directly result in cell contraction and shape changes. The phosphorylation of beta-catenin may cause disorganization of adherens junctions or dissociation of vascular endothelial (VE)-cadherin-catenin complex from its cytoskeletal anchor, leading to loose or opened intercellular junctions. Additionally, focal adhesion kinase (FAK) phosphorylation-coupled focal adhesion assembly and redistribution provide an anchorage support for the conformational changes occurring in the cells and at the cell junctions. The Src family tyrosine kinases may serve as common signals that coordinate these molecular events to facilitate the paracellular transport of macromolecules. The critical roles of protein kinases in endothelial hyperpermeability implicate the therapeutic significance of protein kinase inhibitors in the prevention and treatment of diseases and injuries that are associated with microvascular barrier dysfunction.

Induction of permeability across endothelial cell monolayers by tumor necrosis factor (TNF) occurs via a tissue factor–dependent mechanism: relationship between the procoagulant and permeability effects of TNF

Tumor necrosis factor (TNF) has marked effects on permeability and procoagulant activity on tumor-associated neovasculature when used in isolation perfusion, the latter effect primarily mediated via induction of cell surface expression of tissue factor (TF) on endothelial tissue. However, the cellular events that result in rapid alterations in endothelial cell (EC) permeability after intravascular TNF administration in isolation perfusion are not well characterized. We demonstrate that short exposure intervals to TNF induces TF expression on ECs but has no effect on permeability as assessed by flux of Evans blue–bound albumin across confluent EC monolayers using a 2-compartment model under basal culture conditions. However, a rapid and significant increase in EC permeability occurred with TNF in the presence of factor VIII–deficient plasma. Permeability was induced only with luminal versus abluminal TNF exposure and was blocked by antithrombin III, TF pathway inhibitor, or anti-TF antibody cotreatment. These data indicate that EC surface expression of TF and extrinsic clotting factors are critical in augmenting capillary leak following intravascular TNF administration. Alterations in permeability were associated with intercellular gap formation at sites of down-regulation of vascular endothelial (VE)–cadherin expression, the primary endothelial intercellular adhesion molecule, and intracellular contraction and alignment of F-actin cytoskeletal elements. Rapid induction of TF by TNF may be the primary EC response that results in alterations in permeability and procoagulant activity observed following intravascular TNF administration in isolation perfusion.

Synergy between extracellular modules of vascular endothelial cadherin promotes homotypic hexameric interactions

Vascular endothelial (VE) cadherin is an endothelial specific cadherin that plays a major role in remodeling and maturation of vascular vessels. Recently, we presented evidence that the extracellular part of VE cadherin, which consists of five homologous modules, associates as a Ca(2+)-dependent hexamer in solution (Legrand, P., Bibert, S., Jaquinod, M., Ebel, C., Hewat, E., Vincent, F., Vanbelle, C., Concord, E., Vernet, T., and Gulino, D. (2001) J. Biol. Chem. 276, 3581-3588). In an effort to identify which extracellular modules are involved in the elaboration and stability of this hexameric structure, we expressed various VE cadherin-derived fragments overlapping individual or multiple successive modules as soluble proteins, purified each to homogeneity, and tested their propensity to self-associate. Altogether, the results demonstrate that, as their length increases, VE cadherin recombinant fragments generate increasingly complex self-associating structures; although single module fragments do not oligomerize, some two or three module-containing fragments self-assemble as dimers, and four module-containing fragments associate as hexamers. Our results also suggest that, before elaborating a hexameric structure, molecules of VE cadherin self-assemble as intermediate dimers. A synergy between the extracellular modules of VE cadherin is thus required to build homotypic interactions. Placed in a cellular context, this particular self-association mode may reflect the distinctive biological requirements imposed on VE cadherin at adherens junctions in the vascular endothelium.

Interaction of fibrin(ogen) with the endothelial cell receptor VE-cadherin: mapping of the receptor-binding site in the NH2-terminal portions of the fibrin beta chains

Interaction of fibrin with endothelial cells stimulates capillary tube formation thus promoting angiogenesis. This interaction occurs via endothelial cell receptor VE-cadherin and fibrin beta chain 15-42 regions [Bach, T. L., et al. (1998) J. Biol. Chem. 272, 30719-30728]. To clarify the mechanism of this interaction, we expressed in Escherichia coli a number of recombinant fibrin(ogen) fragments containing the beta15-42 region or the VE-cad(1-2) and VE-cad(1-4) fragments encompassing two and four extracellular NH2-terminal domains of VE-cadherin, respectively, and tested interaction between them by surface plasmon resonance and ELISA. Neither the recombinant Bbeta1-57 or Bbeta1-64 fragments, nor beta15-57 or beta15-64 prepared from the latter fragments by thrombin treatment to remove fibrinopeptides B, bound the recombinant VE-cadherin fragments. At the same time, a dimeric recombinant thrombin-treated (beta15-66)2 fragment, which had been disulfide-linked via Cys65 to mimic the dimeric arrangement of the beta chains in fibrin, bound VE-cad(1-4) well, but not VE-cad(1-2); no binding was observed with the untreated (Bbeta1-66)2 dimer. We next mutated several residues in the dimer, His16, Arg17, Pro18, and Asp20, and tested the interaction of the thrombin-treated mutants with VE-cad(1-4) by ligand blotting and surface plasmon resonance. No binding was observed with the H16A and R17Q single mutants and the H16P, P18V double mutant while the P18A and D20N single mutants bound VE-cad(1-4) with the same affinity as the thrombin-treated wild-type dimer. These results indicate that the VE-cadherin binding site in fibrin includes NH2-terminal regions of both fibrin beta-chains, that His16 and Arg17 are critical for the binding, and that the third and/or fourth extracellular domains of VE-cadherin are required for the binding to occur.

Rate of endothelial expansion is controlled by cell:cell adhesion

Procedures used to alleviate blood vessel occlusion result in varying degrees of damage to the vascular wall and endothelial denudation. The presence of intact, functioning endothelium is thought to be important in controlling smooth muscle cell growth, and limiting the intimal thickening which results from damage to the vessel wall. Recovery of the endothelium is commonly slow and incomplete, due in part to endothelial lateral cell:cell adhesion, which limits cell migration and proliferation. We have investigated the effect of fibroblast growth factor 2 and vascular/endothelial growth factor on the relationship between the temporal distribution of the junctional adhesion proteins, platelet/endothelial cell adhesion molecule, vascular/endothelial cadherin and plakoglobin, and cellular migration and proliferation in an in vitro model of endothelial expansion. We found that whereas cell:cell junctions were initially disturbed to similar extents by single applications of the growth factors, outward cell migration and proliferation rates were inversely correlated with the speed at which cell:cell junctions were re-established. This occurred very rapidly with vascular/endothelial growth factor treatment and more slowly with fibroblast growth factor-2, resulting in more extensive outward migration and proliferation in response to the latter. Platelet/endothelial cell adhesion molecule and vascular/endothelial cadherin appeared to be associated with cell:cell junctional control of migration and proliferation, while plakoglobin did not contribute. It was concluded that the rate of endothelial expansion in response to growth factors, is limited by the rate of re-association of junctional complexes following initial disruption.

Albumin-derived advanced glycation end-products trigger the disruption of the vascular endothelial cadherin complex in cultured human and murine endothelial cells

Endothelial cell (EC) junctions regulate in large part the integrity and barrier function of the vascular endothelium. Advanced glycation end-products (AGEs), the irreversibly formed reactive derivatives of non-enzymic glucose-protein condensation reactions, are strongly implicated in endothelial dysfunction that distinguishes diabetes- and aging-associated vascular complications. The aim of the present study was to determine whether AGEs affect EC lateral junction proteins, with particular regard to the vascular endothelial cadherin (VE-cadherin) complex. Our results indicate that AGE-modified BSA (AGE-BSA), a prototype of advanced glycated proteins, disrupts the VE-cadherin complex when administered to ECs. AGE-BSA, but not unmodified BSA, was found to induce decreases in the levels of VE-cadherin, beta-catenin and gamma-catenin in the complex and in total cell extracts, as well as a marked reduction in the amount of VE-cadherin present at the cell surface. In contrast, the level of platelet endothelial cell adhesion molecule-1 (PECAM-1), which is located at lateral junctions, was not altered. Supplementation of the cellular antioxidative defences abolished these effects. Finally, the loss of components of the VE-cadherin complex was correlated with increases in vascular permeability and in EC migration. These findings suggest that some of the AGE-induced biological effects on the endothelium could be mediated, at least in part, by the weakening of intercellular contacts caused by decreases in the amount of VE-cadherin present.

Albumin-derived advanced glycation end-products trigger the disruption of the vascular endothelial cadherin complex in cultured human and murine endothelial cells

Endothelial cell (EC) junctions regulate in large part the integrity and barrier function of the vascular endothelium. Advanced glycation end-products (AGEs), the irreversibly formed reactive derivatives of non-enzymic glucose-protein condensation reactions, are strongly implicated in endothelial dysfunction that distinguishes diabetes- and aging-associated vascular complications. The aim of the present study was to determine whether AGEs affect EC lateral junction proteins, with particular regard to the vascular endothelial cadherin (VE-cadherin) complex. Our results indicate that AGE-modified BSA (AGE-BSA), a prototype of advanced glycated proteins, disrupts the VE-cadherin complex when administered to ECs. AGE-BSA, but not unmodified BSA, was found to induce decreases in the levels of VE-cadherin, beta-catenin and gamma-catenin in the complex and in total cell extracts, as well as a marked reduction in the amount of VE-cadherin present at the cell surface. In contrast, the level of platelet endothelial cell adhesion molecule-1 (PECAM-1), which is located at lateral junctions, was not altered. Supplementation of the cellular antioxidative defences abolished these effects. Finally, the loss of components of the VE-cadherin complex was correlated with increases in vascular permeability and in EC migration. These findings suggest that some of the AGE-induced biological effects on the endothelium could be mediated, at least in part, by the weakening of intercellular contacts caused by decreases in the amount of VE-cadherin present.