Differential localization of VE- and N-cadherins in human endothelial cells: VE-cadherin competes with N-cadherin for junctional localization

SHP2 association with VE-cadherin complexes in human endothelial cells is regulated by thrombin

Thrombin-mediated changes in endothelial cell adherens junctions modulate vascular permeability. We demonstrate that the nonreceptor protein-tyrosine phosphatase SHP2 co-precipitates with VE-cadherin complexes in confluent, quiescent human umbilical vein endothelial cells. Ligand-binding blots using a SHP2-glutathione S-transferase fusion peptide established that SHP2 associates selectively with beta-catenin in VE-cadherin complexes. Thrombin treatment of human umbilical vein endothelial cells promotes SHP2 tyrosine phosphorylation and dissociation from VE-cadherin complexes. The loss of SHP2 from the cadherin complexes correlates with a dramatic increase in the tyrosine phosphorylation of beta-catenin, gamma-catenin, and p120-catenin complexed with VE-cadherin. We propose that thrombin regulates the tyrosine phosphorylation of VE-cadherin-associated beta-catenin, gamma-catenin, and p120-catenin by modulating the quantity of SHP2 associated with VE-cadherin complexes. Such changes in adherens junction complex composition likely underlie thrombin-elicited alterations in endothelial monolayer permeability.

Molecular mechanisms that control endothelial cell contacts

Endothelial cell contacts control the permeability of the blood vessel wall. This allows the endothelium to form a barrier for solutes, macromolecules, and leukocytes between the vessel lumen and the interstitial space. Loss of this barrier function in pathophysiological situations can lead to extracellular oedema. The ability of leukocytes to enter tissue at sites of inflammation is dependent on molecular mechanisms that allow leukocytes to adhere to the endothelium and to migrate through the endothelial cell layer and the underlying basal lamina. It is a commonly accepted working hypothesis that inter-endothelial cell contacts are actively opened and closed during this process. Angiogenesis is another important process that requires well-controlled regulation of inter-endothelial cell contacts. The formation of new blood vessels by sprouting from pre-existing vessels depends on the loosening of established endothelial cell contacts and the migration of endothelial cells that form the outgrowing sprouts. This review focuses on the molecular composition of endothelial cell surface proteins and proteins of the cytoskeletal undercoat of the plasma membrane at sites of inter-endothelial cell contacts and discusses the current knowledge about the potential role of such molecules in the regulation of endothelial cell contacts.

Selective uncoupling of p120(ctn) from E-cadherin disrupts strong adhesion

p120(ctn) is a catenin whose direct binding to the juxtamembrane domain of classical cadherins suggests a role in regulating cell-cell adhesion. The juxtamembrane domain has been implicated in a variety of roles including cadherin clustering, cell motility, and neuronal outgrowth, raising the possibility that p120 mediates these activities. We have generated minimal mutations in this region that uncouple the E-cadherin-p120 interaction, but do not affect interactions with other catenins. By stable transfection into E-cadherin-deficient cell lines, we show that cadherins are both necessary and sufficient for recruitment of p120 to junctions. Detergent-free subcellular fractionation studies indicated that, in contrast to previous reports, the stoichiometry of the interaction is extremely high. Unlike alpha- and beta-catenins, p120 was metabolically stable in cadherin-deficient cells, and was present at high levels in the cytoplasm. Analysis of cells expressing E-cadherin mutant constructs indicated that p120 is required for the E-cadherin-mediated transition from weak to strong adhesion. In aggregation assays, cells expressing p120-uncoupled E-cadherin formed only weak cell aggregates, which immediately dispersed into single cells upon pipetting. As an apparent consequence, the actin cytoskeleton failed to insert properly into peripheral E-cadherin plaques, resulting in the inability to form a continuous circumferential ring around cell colonies. Our data suggest that p120 directly or indirectly regulates the E-cadherin-mediated transition to tight cell-cell adhesion, possibly blocking subsequent events necessary for reorganization of the actin cytoskeleton and compaction.

Differential expression of endothelial beta-catenin and plakoglobin during development and maturation of the blood-brain and blood-retina barrier in the chicken

The development of the blood-brain barrier depends upon the formation of a closely regulated system of adherens and tight junctions. A prerequisite for a functional junction system is the linkage of transmembrane adhesion receptors (cadherins) to the cytoskeleton via catenins. The localization of specific catenins at the adherens junction correlates with the stability of interendothelial contacts in vitro, but in vivo data are lacking thus far. Investigating brain angiogenesis in the chicken, we demonstrated that beta-catenin, but not plakoglobin, initially codistributed with N-cadherin at the ablumenal endothelial membrane at contact sites to perivascular cells, from where both antigens disappeared during blood-brain barrier maturation. In contrast, plakoglobin was most prominent at the interendothelial junction where only small amounts of beta-catenin were present. Western-blot analysis revealed a stronger developmental decrease of beta-catenin than plakoglobin, whereas N-cadherin was completely lost. beta-Catenin but not N-cadherin was reinduced in brain endothelial cells during dedifferentiation in vitro and localized to the interendothelial junctions. These first in vivo data support the hypothesis that endothelial beta-catenin and N-cadherin are transiently relevant for the contact of brain endothelial to perivascular cells. Plakoglobin seems not to interact with N-cadherin but is exclusively localized at interendothelial junctions providing evidence for its role in the formation of stable adherens junctions, which may play a role for the initiation, and/or stabilization of tight junctions. Dev Dyn 2000;217:86-98.

Histamine alters cadherin-mediated sites of endothelial adhesion

We tested the hypothesis that histamine alters the focal apposition of endothelial cells by acting on sites of cadherin-mediated cell-cell adhesion. Focal apposition was measured as the impedance of a cell-covered electrode, which was partitioned into a cell-matrix resistance, a cell-cell resistance, and membrane capacitance. Histamine causes an immediate, short-lived decrease in the impedance of an electrode covered with human umbilical vein endothelial (HUVE) cells. ECV304 cells are a line of spontaneously transformed HUVE cells that do not express the endothelial cadherin, cadherin-5. Histamine increased ECV304 cell calcium to 600 nM. Histamine did not increase myosin light chain phosphorylation of control or transfected ECV304 cells. ECV304 cells transfected with either E-cadherin or cadherin-5 on a dexamethasone-responsive plasmid (pLKneo) increased their cell-cell resistance when stimulated with dexamethasone, whereas ECV304 cells transfected with pLKneo-lacZ did not. Histamine did not affect the impedance of ECV304 cells transfected with pLKneo-lacZ. In contrast, histamine decreased the cell-cell resistance of ECV304 cells transfected with either pLKneo-E-cadherin or pLKneo-cadherin-5. From these data, we conclude that histamine acts on sites of cadherin-mediated cell-cell apposition.

Vascular endothelial (VE)-cadherin: only an intercellular glue?

Data collected during the past years indicate that AJ- and more specifically VE-cadherin play an important role in endothelial cell biology. VE-cadherin may transfer information intracellularly through interaction with a complex network of cytoskeletal and signaling molecules. Expression of VE-cadherin is required for the control of vascular permeability and vascular integrity. In addition, the molecule may exert a morphogenetic role modulating the capacity of endothelial cells to organize into tubular-like structures. VE-cadherin presents many structural and sequence homologies to the other members of the family and apparently binds to the same intracellular molecules. However, remarkably, VE-cadherin may transfer specific signals to endothelial cells to modulate their functional reactivity.

Endothelial adhesion molecules in the development of the vascular tree: the garden of forking paths

In the past, year targeted null mutation studies have further supported the concept that endothelial cell-matrix and cell-cell adhesion is involved in the formation and maintenance of the network of branched tubes within the vascular tree. In addition, recent results derived from the closely related experimental system of branching tubulogenesis in epithelial cells may provide an appealing model for endothelial biology.

A role for cadherin-5 in regulation of vascular endothelial growth factor receptor 2 activity in endothelial cells

FLK-1/vascular endothelial growth factor receptor 2 (VEGFR-2) is one of the receptors for VEGF. In this study we examined the effect of cell density on activation of VEGFR-2. VEGF induces only very slight tyrosine phosphorylation of VEGFR-2 in confluent (95-100% confluent) pig aortic endothelial (PAE) cells. In contrast, robust VEGF-dependent tyrosine phosphorylation of VEGFR-2 was observed in cells plated in sparse culture conditions (60-65% confluent). A similar cell density-dependent phenomenon was observed in different endothelial cells but not in NIH-3T3 fibroblast cells expressing VEGFR-2. Stimulating cells with high concentrations of VEGF or replacing the extracellular domain of VEGFR-2 with that of the colony-stimulating factor 1 receptor did not alleviate the sensitivity of VEGFR-2 to cell density, indicating that the confluent cells were probably not secreting an antagonist to VEGF. Furthermore, in PAE cells, ectopically introduced platelet-derived growth factor alpha receptor could be activated at both high and low cell density conditions, indicating that the density effect was not universal for all receptor tyrosine kinases expressed in endothelial cells. In addition to lowering the density of cells, removing divalent cations from the medium of confluent cells potentiated VEGFR-2 phosphorylation in response to VEGF. These findings suggested that cell-cell contact may be playing a role in regulating the activation of VEGFR-2. To this end, pretreatment of confluent PAE cells with a neutralizing anti-cadherin-5 antibody potentiated the response of VEGFR-2 to VEGF. Our data demonstrate that endothelial cell density plays a critical role in regulating VEGFR-2 activity, and that the underlying mechanism appears to involve cadherin-5.

Histamine induces tyrosine phosphorylation of endothelial cell-to-cell adherens junctions

Endothelial adherens junctions (AJ) promote intercellular adhesion and may contribute to the control of vascular permeability. These structures are formed by a transmembrane and cell-specific adhesive protein, vascular endothelial (VE)-cadherin, which is linked by its cytoplasmic tail to intracellular proteins called catenins (alpha-catenin, beta-catenin, and plakoglobin) and to the actin cytoskeleton. Little is known about the functional regulation of AJ in endothelial cells. In this study, we analyzed the effect of histamine on AJ organization in cultured endothelial cells. We first observed that histamine induced detectable intercellular gaps only in loosely-confluent cells, whereas this effect was strongly reduced or absent in long-confluent cultures. Despite this difference, in vitro permeability was augmented by histamine in both conditions. In resting conditions, tyrosine phosphorylation of AJ components and permeability values were higher in recently-confluent cells as compared with long-confluent cells. Histamine did not affect the phosphorylation state of AJ in recently-confluent cells but strongly increased this parameter in long-confluent cultures. In addition, in long-confluent cells, histamine caused dissociation of VE-cadherin from the actin cytoskeleton measured by a decrease of the amount of the molecule in the detergent-insoluble fraction of the cell extracts. Dibutyryl cAMP was able to prevent the effect of histamine on both tyrosine phosphorylation of AJ components and on endothelial permeability. The effect of histamine was specific for VE-cadherin because the phosphorylation state of neural (N)-cadherin, the other major endothelial cadherin, was unchanged by this agent. Hence AJ components are a target of histamine activation cascade; we suggest that induction of tyrosine phosphorylation of VE-cadherin and catenins contributes to the histamine effect on permeability, even in absence of frank intercellular gaps and cell retraction.

Cadherins and their connections: adhesion junctions have broader functions

Cadherins - a family of cell-cell adhesion molecules - are linked to the actin cytoskeleton via intervening proteins. Recent results address molecular explanations for observed cadherin behavior, point to signals that regulate adhesion by modulating elements of the cadherin-associated complex, challenge the belief that different cadherins generally cannot cross-adhere, and highlight instructive roles for cadherins in cell signaling and differentiation.

Transient and steady-state effects of shear stress on endothelial cell adherens junctions

Endothelial cells exhibit profound changes in cell shape in response to altered shear stress that may require disassembly/reassembly of adherens junction protein complexes that mediate cell-cell adhesion. To test this hypothesis, we exposed confluent porcine aortic endothelial cells to 15 dyne/cm(2) of shear stress for 0, 8.5, 24, or 48 hours, using a parallel plate flow chamber. Cells were fixed and stained with antibodies to vascular endothelial (VE) cadherin, alpha-catenin, beta-catenin, or plakoglobin. Under static conditions, staining for all proteins was intense and peripheral, forming a nearly continuous band around the cells at cell-cell junctions. After 8.5 hours of shear stress, staining was punctate and occurred only at sites of continuous cell attachment. After 24 or 48 hours of shear, staining for VE-cadherin, alpha-catenin, and beta-catenin was intense and peripheral, forming a band of "dashes" (adherens plaques) that colocalized with the ends of stress fibers that inserted along the lateral membranes of cells. Staining for plakoglobin was not observed after 24 hours of shear stress, but returned after 48 hours. Western blot analysis indicated that protein levels of VE-cadherin, alpha-catenin, and plakoglobin decreased, whereas beta-catenin levels increased after 8.5 hours of shear stress. As cell shape change reached completion (24 to 48 hours), all protein levels were upregulated except for plakoglobin, which remained below control levels. The partial disassembly of adherens junctions we have observed during shear induced changes in endothelial cell shape may have important implications for control of the endothelial permeability barrier and other aspects of endothelial cell function.

Characterization of human vascular endothelial cadherin glycans

The glycosylation pattern of human vascular endothelial cadherin (VE-cadherin), purified from cultured human umbilical cord vein endothelial cells, was analyzed. VE-cadherin was metabolically radiolabeled with d-[6-(3)H]glucosamine, isolated by immunoprecipitation, purified by SDS-PAGE and in-gel digested with endoproteinase Asp N. Oligosaccharides were sequentially released from resulting glycopeptides and analyzed by chromatographic profiling. The results revealed that VE-cadherin carries predominantly sialylated diantennary and hybrid-type glycans in addition to some triantennary and high mannose-type species. Highly branched, tetraantennary oligosaccharides were found in trace amounts only. Immunohistochemical labeling of VE-cadherin and sialic acids displayed a codistribution along the intercellular junctions in endothelial cells of human umbilical arteries, veins, and cultured endothelial monolayers. Ca(2+)-depletion, performed on cultured endothelial cells, resulted in a reversible complete disappearance of VE-cadherin and of almost all sialic acid staining from the junctions. Sialidase treatment of whole cells caused a change of VE-cadherin immunofluorescence from a continuous and netlike superstructural organization to a scattered inconsistent one. Hence, cell surface sialic acids might play a role in VE-cadherin organization.

Targeted deficiency or cytosolic truncation of the VE-cadherin gene in mice impairs VEGF-mediated endothelial survival and angiogenesis

Vascular endothelial cadherin, VE-cadherin, mediates adhesion between endothelial cells and may affect vascular morphogenesis via intracellular signaling, but the nature of these signals remains unknown. Here, targeted inactivation (VEC-/-) or truncation of the beta-catenin-binding cytosolic domain (VECdeltaC/deltaC) of the VE-cadherin gene was found not to affect assembly of endothelial cells in vascular plexi, but to impair their subsequent remodeling and maturation, causing lethality at 9.5 days of gestation. Deficiency or truncation of VE-cadherin induced endothelial apoptosis and abolished transmission of the endothelial survival signal by VEGF-A to Akt kinase and Bcl2 via reduced complex formation with VEGF receptor-2, beta-catenin, and phosphoinositide 3 (PI3)-kinase. Thus, VE-cadherin/ beta-catenin signaling controls endothelial survival.

Role of vascular endothelial-cadherin in vascular morphogenesis

Vascular endothelial (VE)-cadherin is an adhesive transmembrane protein specifically expressed at interendothelial junctions. Its extracellular domain exhibits Ca2+-dependent homophilic reactivity, promoting cell-cell recognition. Mice deficient in VE-cadherin die at mid-gestation resulting from severe vascular defects. At the early phases of vascular development (E8.5) of VE-cadherin-deficient embryos, in situ differentiation of endothelial cells was delayed although their differentiation program appeared normal. Vascularization was defective in the anterior part of the embryo, while dorsal aortae and vitelline and umbilical arteries formed normally in the caudal part. At E9.25, organization of endothelial cells into large vessels was incomplete and angiogenesis was impaired in mutant embryos. Defects were more severe in extraembryonic vasculature. Blood islands of the yolk sac and clusters of angioblasts in allantois failed to establish a capillary plexus and remained isolated. This was not due to defective cell-cell recognition as endothelial cells formed intercellular junctions, as shown by electron microscopy. These data indicate that VE-cadherin is dispensable for endothelial homophilic adhesion but is required for vascular morphogenesis.

Inhibition of the expression of VE-cadherin/catenin complex by gamma linolenic acid in human vascular endothelial cells, and its impact on angiogenesis

Gamma linolenic acid (GLA) has been recently shown to inhibit tumour-induced angiogenesis. The present study investigated the effects of GLA on the HUVEC-specific adhesion. After treatment with GLA, HUVECs decreased the amounts of Triton soluble and insoluble VE-cadherin and beta-catenin and reduced tube formation in matrix in a concentration-dependent manner. An anti-VE-cadherin antibody dissociated HUVECs' colonies and exerted similar inhibitory effects on tube formation of HUVECs. These data indicate that the VE-cadherin/catenins complex is essential for formation and maintenance of new capillaries. It is concluded, therefore, that GLA inhibits tumour-induced angiogenesis partly via the decrease in the expression of VE-cadherin and beta-catenin.

N-cadherin expression in endothelial cells during early angiogenesis in the eye and brain of the chicken: relation to blood-retina and blood-brain barrier development

The factors responsible for the induction and maintenance of blood-brain barrier properties are still undefined. The process of blood-brain barrier formation is thought to take place in a two-stage manner: the initial commitment of vascular sprouts by neuroectodermal cells may be followed by the stabilization of barrier properties. In the present study, we investigated the expression pattern of neural (N)-cadherin during early angiogenesis in the brain and the pecten oculi of the chicken. The pecten has been introduced previously as a model for the investigation of the formation and maturation of barrier properties in the central nervous system. Whereas perineural and choroid vessels remained immunonegative for N-cadherin, vascular sprouts invading both the brain and the pecten primordium acquired anti-N-cadherin immunoreactivity. Confocal laser scanning and immunoelectron microscopy indicated that the antigen was located at the ablumenal endothelial membrane in contact with subendothelial cells. With the onset of barrier differentiation as determined by junctional restriction of the tight junction protein occludin, N-cadherin labelling rapidly decreased. Specific intraneuroectodermal upregulation and decline of endothelial N-cadherin was confirmed by in situ hybridization and suggests that N-cadherin expression by cerebral and pecteneal endothelial cells represents an initial and transient signal which may be involved in the commitment of early blood vessels to express blood-brain and blood-retina barrier properties.

Adhesion-guided in vitro morphogenesis in pure and mixed cell cultures

The ability to understand and control the morphogenesis of mammalian cells is a fundamental objective of cell and developmental biology and tissue engineering research. Numerous processes, both biochemical and biophysical in nature, have been studied in an attempt to elucidate the mechanisms underlying this behavior. We focus here on the contributions of biophysical phenomena to the morphogenetic behavior of pure and mixed cell populations on solid surfaces in vitro. These principles are illustrated using characteristic liver tissue cells as a model system. The studies discussed demonstrate that cell-substratum and cell-cell adhesive forces are critical determinants of the ultimate morphology, cytoarchitecture, and organization achieved by these cells in vitro.

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

Although cadherins appear to be necessary for proper cell-cell contacts, the physiological role of VE-cadherin (vascular endothelium cadherin) in adult tissue has not been clearly determined. To shed some light on this question, we have disturbed the adhesive function of VE-cadherin in human endothelial cell culture using a polyclonal anti-VE-cadherin antibody. This antibody disrupts confluent endothelial cell monolayers in vitro and transiently generates numerous gaps at cell-cell junctions. The formation of these gaps correlates with a reversible increase in the monolayer permeability. We present evidence that destruction of the homotypic interactions between the extracellular domains of VE-cadherin induces a rapid resynthesis of VE-cadherin, leading to restoration of endothelial cell-cell contacts. The expression of new molecules of VE-cadherin correlates with a modest but significant increase in VE-cadherin mRNA synthesis. Altogether, these results establish a critical role for VE-cadherin in the maintenance and restoration of endothelium integrity.

Cell-cell interactions during transendothelial migration of tumor cells

A key event in cancer metastasis is the transendothelial migration of tumor cells. This process involves multiple adhesive interactions between tumor cells and the endothelium. After adhering to the surface of endothelial cells, tumor cells must penetrate the endothelial junction, which contains high concentrations of the cell adhesion molecules VE-cadherin and PECAM-1. Studies using an in vitro model system, consisting of melanoma cells which are seeded onto a monolayer of endothelial cells cultured on Matrigel, have revealed reorganization of the cytoskeleton and dynamic changes in the cell shape of both tumor and endothelial cells. The initial stages of transmigration are characterized by numerous membrane blebs protruding from the basolateral surfaces of the melanoma cells. Contact regions also show an abundance of microfilaments arising from the underlying endothelial cells. These adhesive interactions lead to the redistribution of both VE-cadherin and PECAM-1 and, consequently, a localized dissolution of the endothelial junction. The penetration of the endothelial junction is initiated by melanoma pseudopods. Despite the disappearance of VE-cadherin from the retracting endothelial junction, heterotypic contacts between the tumor cell and its surrounding endothelial cells show a high concentration of pan-cadherin staining, suggesting that transmigration of melanoma cells might yet be facilitated by interactions with another member of the cadherin family. Upon adhesion to the Matrigel, melanoma cells begin to spread and invade the matrix material, while the endothelial cells extend processes over the melanoma cells to reform the monolayer. Interestingly, the leading margins of these endothelial processes contain a high concentration ofN-cadherin. VE-cadherin and PECAM-1 reappear only when the advancing endothelial processes meet to reform the endothelial junction. Together, these observations suggest that endothelial cells actively participate in the transmigration of tumor cells and specific cadherins are involved in different steps of this complex process.

VE-cadherin and desmoplakin are assembled into dermal microvascular endothelial intercellular junctions: a pivotal role for plakoglobin in the recruitment of desmoplakin to intercellular junctions

Vascular endothelial cells assemble adhesive intercellular junctions comprising a unique cadherin, VE-cadherin, which is coupled to the actin cytoskeleton through cytoplasmic interactions with plakoglobin, beta-catenin and alpha -catenin. However, the potential linkage between VE-cadherin and the vimentin intermediate filament cytoskeleton is not well characterized. Recent evidence indicates that lymphatic and vascular endothelial cells express desmoplakin, a cytoplasmic desmosomal protein that attaches intermediate filaments to the plasma membrane in epithelial cells. In the present study, desmoplakin was localized to intercellular junctions in human dermal microvascular endothelial cells. To determine if VE-cadherin could associate with desmoplakin, VE-cadherin, plakoglobin, and a desmoplakin amino-terminal polypeptide (DP-NTP) were co-expressed in L-cell fibroblasts. In the presence of VE-cadherin, both plakoglobin and DP-NTP were recruited to cell-cell borders. Interestingly, beta-catenin could not substitute for plakoglobin in the recruitment of DP-NTP to cell borders, and DP-NTP bound to plakoglobin but not beta-catenin in the yeast two-hybrid system. In addition, DP-NTP colocalized at cell-cell borders with alpha-catenin in the L-cell lines, and endogenous desmoplakin and alpha-catenin colocalized in cultured dermal microvascular endothelial cells. This is in striking contrast to epithelial cells, where desmoplakin and alpha -+catenin are restricted to desmosomes and adherens junctions, respectively. These results suggest that endothelial cells assemble unique junctional complexes that couple VE-cadherin to both the actin and intermediate filament cytoskeleton.

Trophoblast pseudo-vasculogenesis: faking it with endothelial adhesion receptors

During early development, a subset of fetal (placental) cytotrophoblasts exhibits tumor-like behavior and invades the uterus. To access a supply of maternal blood, they invade arterioles and form heterotypic interactions with, and replace, resident maternal endothelium, creating a hybrid uterine vasculature. Recently, it has become clear that invading cytotrophoblasts transform their adhesion receptor phenotype to resemble the endothelial cells they replace. Furthermore, they express vasculogenic factors and receptors. Is this a form of vasculogenesis?

The membrane-proximal region of the E-cadherin cytoplasmic domain prevents dimerization and negatively regulates adhesion activity

Cadherins are transmembrane glycoproteins involved in Ca2+-dependent cell-cell adhesion. Deletion of the COOH-terminal residues of the E-cadherin cytoplasmic domain has been shown to abolish its cell adhesive activity, which has been ascribed to the failure of the deletion mutants to associate with catenins. Based on our present results, this concept needs revision. As was reported previously, leukemia cells (K562) expressing E-cadherin with COOH-terminal deletion of 37 or 71 amino acid residues showed almost no aggregation. Cells expressing E-cadherin with further deletion of 144 or 151 amino acid residues, which eliminates the membrane-proximal region of the cytoplasmic domain, showed E-cadherin-dependent aggregation. Thus, deletion of the membrane-proximal region results in activation of the nonfunctional E-cadherin polypeptides. However, these cells did not show compaction. Chemical cross-linking revealed that the activated E-cadherin polypeptides can be cross-linked to a dimer on the surface of cells, whereas the inactive polypeptides, as well as the wild-type E-cadherin polypeptide containing the membrane-proximal region, can not. Therefore, the membrane-proximal region participates in regulation of the adhesive activity by preventing lateral dimerization of the extracellular domain.

Junctional adhesion molecule, a novel member of the immunoglobulin superfamily that distributes at intercellular junctions and modulates monocyte transmigration

Tight junctions are the most apical components of endothelial and epithelial intercellular cleft. In the endothelium these structures play an important role in the control of paracellular permeability to circulating cells and solutes. The only known integral membrane protein localized at sites of membrane-membrane interaction of tight junctions is occludin, which is linked inside the cells to a complex network of cytoskeletal and signaling proteins. We report here the identification of a novel protein (junctional adhesion molecule [JAM]) that is selectively concentrated at intercellular junctions of endothelial and epithelial cells of different origins. Confocal and immunoelectron microscopy shows that JAM codistributes with tight junction components at the apical region of the intercellular cleft. A cDNA clone encoding JAM defines a novel immunoglobulin gene superfamily member that consists of two V-type Ig domains. An mAb directed to JAM (BV11) was found to inhibit spontaneous and chemokine-induced monocyte transmigration through an endothelial cell monolayer in vitro. Systemic treatment of mice with BV11 mAb blocked monocyte infiltration upon chemokine administration in subcutaneous air pouches. Thus, JAM is a new component of endothelial and epithelial junctions that play a role in regulating monocyte transmigration.

Identification of a novel cadherin (vascular endothelial cadherin-2) located at intercellular junctions in endothelial cells

Endothelial cells express two major cadherins, VE- and N-cadherins, but only the former consistently participates in adherens junction organization. In heart microvascular endothelial cells, we identified a new member of the cadherin superfamily using polymerase chain reaction. The entire putative coding sequence was determined. Similarly to protocadherins, while the extracellular domain presented homology with other members of the cadherin superfamily, the intracellular region was unrelated either to cadherins or to any other known protein. We propose for this new protein the name of vascular endothelial cadherin-2. By Northern blot analysis, the mRNA was present only in cultured endothelial cell lines but not in other cell types such as NIH 3T3, Chinese hamster ovary, or L cells. In addition, mRNA was particularly abundant in highly vascularized organs such as lung or kidney. In endothelial cells and transfectants, this cadherin was unable to bind catenins and presented a weak association with the cytoskeleton. This new molecule shares some functional properties with VE-cadherin and other members of the cadherin family. In Chinese hamster ovary transfectants it promoted homotypic Ca2+ dependent aggregation and adhesion and clustered at intercellular junctions. However, in contrast to VE-cadherin, it did not modify paracellular permeability, cell migration, and density-dependent cell growth. These observations suggest that different cadherins may promote homophilic cell-to-cell adhesion but that the functional consequences of this interaction depend on their binding to specific intracellular signaling/cytoskeletal proteins.

In vitro generation of lymphohematopoietic cells from endothelial cells purified from murine embryos

We have investigated the lymphohematopoietic potentials of endothelial cells (EC) and hematopoietic cells (HPC) sorted from embryos. Expression of VE-cadherin, CD45, and Ter119 was used to distinguish EC (VE-cadherin+CD45-Ter119-) from HPC (VE-cadherin-CD45+). Thus defined, EC population takes up acetylated LDL and coexpresses CD31, Flk1, and CD34. In E9.5 embryos, EC from yolk sac (YS) and the embryo proper generate blood cells, including lymphocytes. Thus, lymphohematopoietic EC do exist in the embryo, and they are generated both in YS and the embryo proper. On the other hand, HPC with lymphopoietic potency appear first in the embryo proper. These findings implicate involvement of multiple environmental cues for acquiring lymphopoietic competency during differentiation of HPC.

Molecular distinction and angiogenic interaction between embryonic arteries and veins revealed by ephrin-B2 and its receptor Eph-B4

The vertebrate circulatory system is composed of arteries and veins. The functional and pathological differences between these vessels have been assumed to reflect physiological differences such as oxygenation and blood pressure. Here we show that ephrin-B2, an Eph family transmembrane ligand, marks arterial but not venous endothelial cells from the onset of angiogenesis. Conversely, Eph-B4, a receptor for ephrin-B2, marks veins but not arteries. ephrin-B2 knockout mice display defects in angiogenesis by both arteries and veins in the capillary networks of the head and yolk sac as well as in myocardial trabeculation. These results provide evidence that differences between arteries and veins are in part genetically determined and suggest that reciprocal signaling between these two types of vessels is crucial for morphogenesis of the capillary beds.