Role of vascular endothelial-cadherin in vascular morphogenesis

Microbial induction of vascular pathology in the CNS

The central nervous system (CNS) is a finely tuned organ that participates in nearly every aspect of our day-to-day function. Neurons lie at the core of this functional unit and maintain an active dialogue with one another as well as their fellow CNS residents (e.g. astrocytes, oligodendrocytes, microglia). Because of this complex dialogue, it is essential that the CNS milieu be tightly regulated in order to permit uninterrupted and efficient neural chemistry. This is accomplished in part by anatomical barriers that segregate vascular components from the cerebral spinal fluid (CSF) and brain parenchyma. These barriers impede entry of noxious materials and enable the CNS to maintain requisite protein and ionic balances for constant electrochemical signaling. Under homeostatic conditions, the CNS is protected by the presence of specialized endothelium/epithelium, the blood brain barrier (BBB), and the blood-CSF barrier. However, following CNS infection these protective barriers can be comprised, sometimes resulting in severe neurological complications triggered by an imbalance or blockage of neural chemistry. In some instances, these disruptions are severe enough to be fatal. This review focuses on a selection of microbes (both viruses and parasites) that compromise vascular barriers and induce neurological complications upon gaining access to the CNS. Emphasis is placed on CNS diseases that result from a pathogenic interplay between host immune defenses and the invading microbe.

VE-cadherin: at the front, center, and sides of endothelial cell organization and function

Endothelial cells form cell-cell adhesive structures, called adherens and tight junctions, which maintain tissue integrity, but must be dynamic for leukocyte transmigration during the inflammatory response and cellular remodeling during angiogenesis. This review will focus on Vascular Endothelial (VE)-cadherin, an endothelial-specific cell-cell adhesion protein of the adherens junction complex. VE-cadherin plays a key role in endothelial barrier function and angiogenesis, and consequently VE-cadherin availability and function are tightly regulated. VE-cadherin also participates directly and indirectly in intracellular signaling pathways that control cell dynamics and cell cycle progression. Here we highlight recent work that has advanced our understanding of multiple regulatory and signaling mechanisms that converge on VE-cadherin and have consequences for endothelial barrier function and angiogenic remodeling.

Role of Afadin in Vascular Endothelial Growth Factor– and Sphingosine 1-Phosphate–Induced Angiogenesis

Rationale : Angiogenesis contributes to physiological and pathological conditions, including atherosclerosis. The Rap1 small G protein regulates vascular integrity and angiogenesis. However, little is known about the effectors of Rap1 involved in angiogenesis. It is not known whether afadin, an adherens junction protein that connects immunoglobulin-like adhesion molecule nectins to the actin cytoskeleton and binds activated Rap1, plays a role in angiogenesis.

Objective : We investigated the role of endothelial afadin in angiogenesis and attempted to clarify the underlying molecular mechanism.

Methods and Results : Treatment of human umbilical vein endothelial cells (HUVECs) with vascular endothelial growth factor (VEGF) and sphingosine 1-phosphate (S1P) induced the activation of Rap1. Activated Rap1 regulated intracellular localization of afadin. Knockdown of Rap1 or afadin by small interfering RNA inhibited the VEGF- and S1P-induced capillary-like network formation, migration, and proliferation, and increased the serum deprivation-induced apoptosis of HUVECs. Knockdown of Rap1 or afadin decreased the accumulation of adherens and tight junction proteins to the cell–cell contact sites. Rap1 regulated the interaction between afadin and phosphatidylinositol 3-kinase (PI3K), recruitment of the afadin–PI3K complex to the leading edge, and the activation of Akt, indicating the involvement of Rap1 and afadin in the PI3K–Akt signaling pathway. Binding of afadin to Rap1 regulated the activity of Rap1 in a positive-feedback manner. In vivo, conditional deletion of afadin in mouse vascular endothelium using a Cre-loxP system impaired the VEGF- and S1P-induced angiogenesis.

Conclusions : These results demonstrate a novel molecular mechanism by which Rap1 and afadin regulate the VEGF- and S1P-induced angiogenesis.

Regulation of endothelial permeability via paracellular and transcellular transport pathways

The endothelium functions as a semipermeable barrier regulating tissue fluid homeostasis and transmigration of leukocytes and providing essential nutrients across the vessel wall. Transport of plasma proteins and solutes across the endothelium involves two different routes: one transcellular, via caveolae-mediated vesicular transport, and the other paracellular, through interendothelial junctions. The permeability of the endothelial barrier is an exquisitely regulated process in the resting state and in response to extracellular stimuli and mediators. The focus of this review is to provide a comprehensive overview of molecular and signaling mechanisms regulating endothelial barrier permeability with emphasis on the cross-talk between paracellular and transcellular transport pathways.

Protocadherin of the liver, kidney, and colon associates with detergent-resistant membranes during cellular differentiation

Protocadherin LKC (PLKC) is a member of the heterogeneous subgroup of protocadherins that was identified and described as a potential tumor-suppressor gene involved in contact inhibition (Okazaki, N., Takahashi, N., Kojima, S., Masuho, Y., and Koga, H. (2002) Carcinogenesis 23, 1139-1148 and Ose, R., Yanagawa, T., Ikeda, S., Ohara, O., and Koga, H. (2009) Mol. Oncol. 3, 54-66). Several aspects of the structure, posttranslational processing, targeting, and function of this new protocadherin are still not known. Here, we demonstrate that the expression of PLKC at the apical membrane domain and its concentration at regions of cell-cell contacts occur concomitantly with significant elevation of PLKC-mRNA levels. Furthermore, it can be found within the adherens junctions, but it does not colocalize with tight junctions proteins ZO-1 and occludin, respectively. Additionally, unlike E-cadherin, PLKC is not redistributed upon Ca(2+) removal. Biosynthetic labeling revealed N- and O-glycosylation as posttranslational modifications as well as a fast transport to the cell surface and a low turnover rate. During differentiation, PLKC associates with detergent-resistant membranes that trigger its redistribution from intracellular membranes to the cell surface. This association occurs concomitant with alterations in the glycosylation pattern. We propose a role for PLKC in the establishment of a proper epithelial cell polarity that requires O-linked glycosylation and association of the protein with detergent-resistant membranes.

Lipid phosphate phosphatase 3 stabilization of beta-catenin induces endothelial cell migration and formation of branching point structures

Endothelial cell (EC) migration, cell-cell adhesion, and the formation of branching point structures are considered hallmarks of angiogenesis; however, the underlying mechanisms of these processes are not well understood. Lipid phosphate phosphatase 3 (LPP3) is a recently described p120-catenin-associated integrin ligand localized in adherens junctions (AJs) of ECs. Here, we tested the hypothesis that LPP3 stimulates beta-catenin/lymphoid enhancer binding factor 1 (beta-catenin/LEF-1) to induce EC migration and formation of branching point structures. In subconfluent ECs, LPP3 induced expression of fibronectin via beta-catenin/LEF-1 signaling in a phosphatase and tensin homologue (PTEN)-dependent manner. In confluent ECs, depletion of p120-catenin restored LPP3-mediated beta-catenin/LEF-1 signaling. Depletion of LPP3 resulted in destabilization of beta-catenin, which in turn reduced fibronectin synthesis and deposition, which resulted in inhibition of EC migration. Accordingly, reexpression of beta-catenin but not p120-catenin in LPP3-depleted ECs restored de novo synthesis of fibronectin, which mediated EC migration and formation of branching point structures. In confluent ECs, however, a fraction of p120-catenin associated and colocalized with LPP3 at the plasma membrane, via the C-terminal cytoplasmic domain, thereby limiting the ability of LPP3 to stimulate beta-catenin/LEF-1 signaling. Thus, our study identified a key role for LPP3 in orchestrating PTEN-mediated beta-catenin/LEF-1 signaling in EC migration, cell-cell adhesion, and formation of branching point structures.

p120-Catenin is required for mouse vascular development

RATIONALE

p120-catenin (p120) is an armadillo family protein that binds to the cytoplasmic domain of classical cadherins and prevents cadherin endocytosis. The role of p120 in vascular development is unknown.

OBJECTIVE

The purpose of this study is to examine the role of p120 in mammalian vascular development by generating a conditionally mutant mouse lacking endothelial p120 and determining the effects of the knockout on vasculogenesis, angiogenic remodeling, and the regulation of endothelial cadherin levels.

METHODS AND RESULTS

A conditional Cre/loxP gene deletion strategy was used to ablate p120 expression, using the Tie2 promoter to drive endothelial Cre recombinase expression. Mice lacking endothelial p120 died embryonically beginning at embryonic day 11.5. Major blood vessels appeared normal at embryonic day 9.5. However, both embryonic and extraembryonic vasculature of mutant animals were disorganized and displayed decreased microvascular density by embryonic day 11.5. Importantly, both vascular endothelial cadherin and N-cadherin levels were significantly reduced in vessels lacking p120. This decrease in cadherin expression was accompanied by reduced pericyte recruitment and hemorrhaging. Furthermore, p120-null cultured endothelial cells exhibited proliferation defects that could be rescued by exogenous expression of vascular endothelial cadherin.

CONCLUSIONS

These findings reveal a fundamental role for p120 in regulating endothelial cadherin levels during vascular development, as well as microvascular patterning, vessel integrity, and endothelial cell proliferation. Loss of endothelial p120 results in lethality attributable to decreased microvascular density and hemorrhages.

Signalling pathways in vasculogenic mimicry

Solid tumour growth is dependent on the development of an adequate blood supply. For years, sprouting angiogenesis has been considered an exclusive mechanism of tumour vascularization. However, over the last years, several other mechanisms have been identified, including vessel-co-option, intussusception, recruitment of endothelial precursor cells (EPCs) and even mechanisms that do not involve endothelial cells, a process called vasculogenic mimicry (VM). The latter describes a mechanism by which highly aggressive tumour cells can form vessel-like structures themselves, by virtue of their high plasticity. VM has been observed in several tumour types and its occurrence is strongly associated with a poor prognosis. This review will focus on signalling molecules and cascades involved in VM. In addition, we will discuss the presence of VM in relation to ongoing cancer research. Finally, we describe the clinical significance of VM regarding anti-angiogenesis treatment modalities.

Rap1, a mercenary among the Ras-like GTPases

The small Ras-like GTPase Rap1 is an evolutionary conserved protein that originally gained interest because of its capacity to revert the morphological phenotype of Ras-transformed fibroblasts. Rap1 is regulated by a large number of stimuli that include growth factors and cytokines, but also physical force and osmotic stress. Downstream of Rap1, a plethora of effector molecules has been proposed on the basis of biochemical studies. Here, we present an overview of genetic studies on Rap1 in various model organisms and relate the observed phenotypes to in vitro studies. The picture that emerges is one in which Rap1 is a versatile regulator of morphogenesis, by regulating diverse processes that include establishment of cellular polarity, cell-matrix interactions and cell-cell adhesion. Surprisingly, genetic experiments indicate that in the various model organisms, Rap1 uses distinct effector molecules that impinge upon the actin cytoskeleton and adhesion molecules.

Mathematical modeling of the capillary-like pattern generated by adrenomedullin-treated human vascular endothelial cells in vitro

A recently proposed approach was used to model the self-organization into capillary-like structures of human vascular endothelial cells cultured on Matrigel. The model combines a Cellular Potts Model, considering cell adhesion, cytoskeletal rearrangement and chemotaxis, and a Partial Differential Equation model describing the release and the diffusion of a chemoattractant. The results were compared with the data from real in vitro experiments to establish the capability of the model to accurately reproduce both the spontaneous self-assembly of unstimulated cells and their self-organization in the presence of the pro-angiogenic factor adrenomedullin. The results showed that the model can accurately reproduce the self-assembly of unstimulated cells, but it failed in reproducing the adrenomedullin-induced self-organization of the cells. The extension of the model to include cell proliferation led to a good match between simulated and experimental patterns in both cases with predicted proliferation rates in agreement with the data of cell proliferation experiments.

Cadherins as novel targets for anti-cancer therapy

The cell adhesion molecules N-, VE- and OB-cadherin have been implicated as regulators of tumor growth and metastasis. We discuss evidence that N- and VE-cadherin play a key role in promoting blood vessel formation and stability, processes which are essential for tumor growth. Secondly, we describe the potential involvement of N- and OB-cadherin in the metastatic process. Finally, studies concerning the effects of the N-cadherin antagonist designated ADH-1 on tumor growth are presented. Collectively, these observations suggest that antagonists of N-, VE- and OB-cadherin would be useful as anti-cancer agents.

Integrins as "functional hubs" in the regulation of pathological angiogenesis

It is well accepted that complex biological processes such as angiogenesis are not controlled by a single family of molecules or individually isolated signaling pathways. In this regard, new insight into the interconnected mechanisms that regulate angiogenesis might be gained by examining this process from a more global network perspective. The coordination of signaling cues from both outside and inside many different cell types is required for the successful completion of angiogenesis. Evidence is accumulating that the multifunctional integrin family of cell adhesion receptors represent an important group of molecules that play active roles in sensing, integrating, and distributing a diverse set of signals that regulate many cellular events required for angiogenesis. Given the ability of integrins to bind numerous extracellular ligands and transmit signals in a bi-directional fashion, we will discuss the multiple ways by which integrins may serve as a functional hub during pathological angiogenesis. In addition, we will highlight potential imaging and therapeutic strategies based on the expanding new insight into integrin function.

Impaired vascular development in the yolk sac and allantois in mice lacking RA-GEF-1

RA-GEF-1 is a guanine nucleotide exchange factor for the small GTPase Rap1. RA-GEF-1 knockout mice show defects in vascular development starting around 7.5days post coitum and die by 9.5days post coitum. Here, we employed in vitro culture systems for allantois explants and endothelial cells to gain insights into the mechanism for RA-GEF-1-mediated regulation of embryonic vascular network formation. The development of the vascular plexus and the accumulation of VE-cadherin at cell-cell junctions were significantly impaired in the RA-GEF-1 knockout allantois and yolk sac. Rap1 activation as visualized by an activation-specific probe was also diminished by RA-GEF-1 knockout. Reduced accumulation of VE-cadherin at cell-cell junctions and defects in blood vessel formation in vitro due to the lack of RA-GEF-1 were suppressed by ectopic expression of constitutively activated Rap1. Overall, these results suggest the involvement of Rap1 downstream of RA-GEF-1 in the regulation of vascular network formation in mouse embryos.

Statins and angiogenesis: is it about connections?

Statins, inhibitors of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase, have been shown to induce both angiogenic and angiostatic responses. We attempted to resolve this controversy by studying the effects of two different statins, rosuvastatin and simvastatin, in two different assay systems. In the matrigel angiogenesis assay, both statins enhanced tube formation by human umbilical vein endothelial cells (HUVECs, p<0.01 vs. control). In the ex vivo mouse aortic ring sprouting assay, both statins virtually abolished new vessel formation (p<0.01). As a basic difference between the two models of angiogenesis is dispersed state of endothelial cells vs. compact monolayer, we analyzed influence of statins on endothelial junction proteins. RT-PCR analysis and cytoimmunostaining of HUVECs treated with simvastatin revealed increased expression of VE-cadherin (p<0.05). The blockade of VE-cadherin with a specific antibody reversed simvastatin-induced tube formation (p<0.002). These data suggest that statins through VE-cadherin stimulation modulate cell-cell adhesion and diminish the ability of cells to proliferate and migrate. The observations of reduced angiogenesis in the intact vessel may relate to anti-atherosclerotic and anti-cancer effects of statins, and provide a feasible explanation for conflicting data under different experimental conditions.

[Endothelial cell-cell junctions in vessel formation]

The endothelium, lining the inner side of all vessel types, is constituted of a monolayer of endothelial cells with cobblestone morphology. Endothelial cell-cell contacts contain numerous transmembrane adhesive proteins that are either clustered in junctional structures or located along the intercellular cleft. These proteins promote cell-cell adhesion and control vascular permeability to fluids and molecules, as well as transmigration of various types of leukocytes. In addition, recent findings showed that constituents of the junctions might be part of the vascular invasion machinery by activating cell protrusions. Such activities may thus be considered as markers of pathological angiogenesis or targets of antiangiogenic therapy.

Stable vascular connections and remodeling require full expression of VE-cadherin in zebrafish embryos

Background

VE-cadherin is an endothelial specific, transmembrane protein, that clusters at adherens junctions where it promotes homotypic cell-cell adhesion. VE-cadherin null mutation in the mouse results in early fetal lethality due to altered vascular development. However, the mechanism of action of VE-cadherin is complex and, in the mouse embryo, it is difficult to define the specific steps of vascular development in which this protein is involved.

Methodology and Principal Findings

In order to study the role VE-cadherin in the development of the vascular system in a more suitable model, we knocked down the expression of the coding gene in zebrafish. The novel findings reported here are: 1) partial reduction of VE-cadherin expression using low doses of morpholinos causes vascular fragility, head hemorrhages and increase in permeability; this has not been described before and suggests that the total amount of the protein expressed is an important determinant of vascular stability; 2) concentrations of morpholinos which abrogate VE-cadherin expression prevent vessels to establish successful reciprocal contacts and, as a consequence, vascular sprouting activity is not inhibited. This likely explains the observed vascular hyper-sprouting and the presence of several small, collapsing vessels; 3) the common cardinal vein lacks a correct connection with the endocardium leaving the heart separated from the rest of the circulatory system. The lack of closure of the circulatory loop has never been described before and may explain some downstream defects of the phenotype such as the lack of a correct vascular remodeling.

Conclusions and Significance

Our observations identify several steps of vascular development in which VE-cadherin plays an essential role. While it does not appear to regulate vascular patterning it is implicated in vascular connection and inhibition of sprouting activity. These processes require stable cell-cell junctions which are defective in absence of VE-cadherin. Notably, also partial modifications in VE-cadherin expression prevent the formation of a stable vasculature. This suggests that partial internalization or change of function of this protein may strongly affect vascular stability and organization.

Endothelial differentiation of Wharton's jelly-derived mesenchymal stem cells in comparison with bone marrow-derived mesenchymal stem cells

OBJECTIVE

Mesenchymal stem cells (MSCs) can be isolated from umbilical cord Wharton's jelly (UC-MSC) and UC can be easily obtained, representing a noncontroversial source of MSCs. UC-MSCs are more primitive than other tissue sources. Previous studies showed that UC-MSCs were still viable and were not rejected 4 months after transplantation as xenografts without the need for immune suppression, indicating that they are favorable cell source for transplantation. In this study, UC-MSCs were induced to differentiate into endothelial-like cells and compared with bone marrow (BM)-MSCs for their endothelial differentiation potential.

MATERIALS AND METHODS

UC-MSCs and BM-MSCs were characterized for expression of MSC-specific markers and osteogenic, adipogenic, and chondrogenic differentiation. They were induced to differentiate into endothelial-like cells and analyzed for expression of the endothelial-specific markers and functions.

RESULTS

UC-MSCs and BM-MSCs showed similarities in expression of the MSC-specific markers and osteogenic, adipogenic, and chondrogenic differentiation. They showed similar low-density lipoprotein-uptaking capacity following endothelial differentiation. However, UC-MSCs had higher proliferative potential than BM-MSCs. Both real-time reverse transcription polymerase chain reaction and immunocytochemical analyses demonstrated that UC-MSCs had higher expression of the endothelial-specific markers than BM-MSCs following endothelial differentiation. Both Matrigel and coculture angiogenesis assays showed that UC-MSCs and BM-MSCs after endothelial differentiation were able to form the capillary network and differentiated UC-MSCs had significantly higher total tubule length, diameter, and area than differentiated BM-MSCs.

CONCLUSION

These results showed that UC-MSCs had higher endothelial differentiation potential than BM-MSCs. Therefore, UC-MSCs are more favorable choice than BM-MSCs for neovascularization of engineered tissues.

p120-catenin inhibits VE-cadherin internalization through a Rho-independent mechanism

p120-catenin is a cytoplasmic binding partner of cadherins and functions as a set point for cadherin expression by preventing cadherin endocytosis, and degradation. p120 is known to regulate cell motility and invasiveness by inhibiting RhoA activity. However, the relationship between these functions of p120 is not understood. Here, we provide evidence that p120 functions as part of a plasma membrane retention mechanism for VE-cadherin by preventing the recruitment of VE-cadherin into membrane domains enriched in components of the endocytic machinery, including clathrin and the adaptor complex AP-2. The mechanism by which p120 regulates VE-cadherin entry into endocytic compartments is dependent on p120's interaction with the cadherin juxtamembrane domain, but occurs independently of p120's prevention of Rho GTPase activity. These findings clarify the mechanism for p120's function in stabilizing VE-cadherin at the plasma membrane and demonstrate a novel role for p120 in modulating the availability of cadherins for entry into a clathrin-dependent endocytic pathway.

Expression of RAC2 in endothelial cells is required for the postnatal neovascular response

Herein, we describe an obligate role for the hematopoietic specific GTPase, RAC2 in endothelial integrin signaling and the postnatal neovascularization response in vivo. Using a Rac2 knockout mouse model, we discovered that despite the presence of both RAC1 and RAC2 protein in endothelial cells, RAC2 is obligately required for the postnatal neovascular response and alphavbeta3/ alpha4beta1/alpha5beta1 integrin-directed migration on vitronectin, H296 and CH271, fibronectin fragments, respectively. The molecular basis for RAC2 specificity was explored. A genetic analysis of Syk -/+ or Syk-/+;Rac2 -/+ mice revealed that SYK kinase is required for the integrin induced activation of RAC2. The analysis of endothelial cells from Rac2-/+ versus Syk-/+;Rac2-/+ mice provided genetic evidence that SYK-RAC2 signaling axis regulates integrin (alphavbeta3, alpha4beta1 and alpha5beta1) dependent migration. Our results provide evidence that a specific region of the nonreceptor protein tyrosine kinase, SYK, the B linker region containing Y342 and Y346 is required for SYK's regulation of RAC2 and integrin dependent migration. Moreover, the capacity of mice to vascularize the ischemic hindlimb following femoral artery ligation or matrigel plugs was markedly reduced in mice homozygous deficient for the Rac2 gene. These findings identify a novel signaling axis for the induction and potential modulation of postnatal angiogenesis.

Endothelium as master regulator of organ development and growth

Development of the vasculature is one of the earliest events during embryogenesis, preceding organ formation. Organogenesis requires a complex set of paracrine signals between the vasculature and the developing nonvascular tissues to support differentiation and organ growth. However, the role of endothelium in controlling organ growth and, ultimately, size is little-understood. In this review, we summarize new data regarding the endothelium function in order to provide a more comprehensive understanding of the communication between the endothelium and the organ's tissue.

Fate tracing reveals the endothelial origin of hematopoietic stem cells

Hematopoietic stem cells (HSCs) originate within the aortic-gonado-mesonephros (AGM) region of the midgestation embryo, but the cell type responsible for their emergence is unknown since critical hematopoietic factors are expressed in both the AGM endothelium and its underlying mesenchyme. Here we employ a temporally restricted genetic tracing strategy to selectively label the endothelium, and separately its underlying mesenchyme, during AGM development. Lineage tracing endothelium, via an inducible VE-cadherin Cre line, reveals that the endothelium is capable of HSC emergence. The endothelial progeny migrate to the fetal liver, and later to the bone marrow, and are capable of expansion, self-renewal, and multilineage hematopoietic differentiation. HSC capacity is exclusively endothelial, as ex vivo analyses demonstrate lack of VE-cadherin Cre induction in circulating and fetal liver hematopoietic populations. Moreover, AGM mesenchyme, as selectively traced via a myocardin Cre line, is incapable of hematopoiesis. Our genetic tracing strategy therefore reveals an endothelial origin of HSCs.

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

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

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

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

Endothelial cell migration directs testis cord formation

While the molecular cues initiating testis determination have been identified in mammals, the cellular interactions involved in generating a functional testis with cord and interstitial compartments remain poorly understood. Previous studies have shown that testis cord formation relies on cell migration from the adjacent mesonephros, and have implicated immigrant peritubular myoid cells in this process. Here, we used recombinant organ culture experiments to show that immigrant cells are endothelial, not peritubular myoid or other interstitial cells. Inhibition of endothelial cell migration and vascular organisation using a blocking antibody to VE-cadherin, also disrupted the development of testis cords. Our data reveal that migration of endothelial cells is required for testis cord formation, consistent with increasing evidence of a broader role for endothelial cells in establishing tissue architecture during organogenesis.

Expression profiling of Galectin-3-depleted melanoma cells reveals its major role in melanoma cell plasticity and vasculogenic mimicry

Galectin-3 (Gal-3) is a beta-galactoside-binding protein that is involved in cancer progression and metastasis. Using a progressive human melanoma tissue microarray, we previously demonstrated that melanocytes accumulate Gal-3 during the progression from benign to dysplastic nevi to melanoma and further to metastatic melanoma. Herein, we show that silencing of Gal-3 expression with small hairpin RNA results in a loss of tumorigenic and metastatic potential of melanoma cells. In vitro, Gal-3 silencing resulted in loss of tumor cell invasiveness and capacity to form tube-like structures on collagen ("vasculogenic mimicry"). cDNA microarray analysis after Gal-3 silencing revealed that Gal-3 regulates the expression of multiple genes, including endothelial cell markers that appear to be aberrantly expressed in highly aggressive melanoma cells, causing melanoma cell plasticity. These genes included vascular endothelial-cadherin, which plays a pivotal role in vasculogenic mimicry, as well as interleukin-8, fibronectin-1, endothelial differentiation sphingolipid G-protein receptor-1, and matrix metalloproteinase-2. Chromatin immunoprecipitation assays and promoter analyses revealed that Gal-3 silencing resulted in a decrease of vascular endothelial-cadherin and interleukin-8 promoter activities due to enhanced recruitment of transcription factor early growth response-1. Moreover, transient overexpression of early growth response-1 in C8161-c9 cells resulted in a loss of vascular endothelial-cadherin and interleukin-8 promoter activities and protein expression. Thus, Gal-3 plays an essential role during the acquisition of vasculogenic mimicry and angiogenic properties associated with melanoma progression.

N-cadherin levels in endothelial cells are regulated by monolayer maturity and p120 availability

Endothelial cells (ECs) express VE-cadherin and N-cadherin, and recent data suggest that VE-cadherin levels are dependent on N-cadherin expression. While investigating changes in N-cadherin levels during endothelial monolayer maturation, the authors found that VE-cadherin levels are maintained in ECs despite a decrease in N-cadherin, suggesting that VE-cadherin levels may not depend on N-cadherin. Knockdown of N-cadherin did not affect VE-cadherin levels in ECs with low endogenous N-cadherin expression. Surprisingly, however, knockdown of N-cadherin in ECs with high endogenous N-cadherin expression increased VE-cadherin levels, suggesting an inverse relationship between the two. This was further supported by a decrease in VE-cadherin following overexpression of N-cadherin. Experiments in which p120, a catenin that binds N- and VE-cadherin, was knocked down or overexpressed indicate that these two cadherins compete for p120. These data demonstrate that VE-cadherin levels are not directly related to N-cadherin levels but may be inversely related due to competition for p120.

Contact-inhibited chemotaxis in de novo and sprouting blood-vessel growth

Blood vessels form either when dispersed endothelial cells (the cells lining the inner walls of fully formed blood vessels) organize into a vessel network (vasculogenesis), or by sprouting or splitting of existing blood vessels (angiogenesis). Although they are closely related biologically, no current model explains both phenomena with a single biophysical mechanism. Most computational models describe sprouting at the level of the blood vessel, ignoring how cell behavior drives branch splitting during sprouting. We present a cell-based, Glazier-Graner-Hogeweg model (also called Cellular Potts Model) simulation of the initial patterning before the vascular cords form lumens, based on plausible behaviors of endothelial cells. The endothelial cells secrete a chemoattractant, which attracts other endothelial cells. As in the classic Keller-Segel model, chemotaxis by itself causes cells to aggregate into isolated clusters. However, including experimentally observed VE-cadherin-mediated contact inhibition of chemotaxis in the simulation causes randomly distributed cells to organize into networks and cell aggregates to sprout, reproducing aspects of both de novo and sprouting blood-vessel growth. We discuss two branching instabilities responsible for our results. Cells at the surfaces of cell clusters attempting to migrate to the centers of the clusters produce a buckling instability. In a model variant that eliminates the surface-normal force, a dissipative mechanism drives sprouting, with the secreted chemical acting both as a chemoattractant and as an inhibitor of pseudopod extension. Both mechanisms would also apply if force transmission through the extracellular matrix rather than chemical signaling mediated cell-cell interactions. The branching instabilities responsible for our results, which result from contact inhibition of chemotaxis, are both generic developmental mechanisms and interesting examples of unusual patterning instabilities.

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.

Deciphering the functional role of endothelial junctions by using in vivo models

Endothelial cell-to-cell junctions are vital for the formation and integrity of blood vessels. The main adhesive junctional complexes in endothelial cells, adherens junctions and tight junctions, are formed by transmembrane adhesive proteins that are linked to intracellular signalling partners and cytoskeletal-binding proteins. Gene inactivation and blocking antibodies in mouse models have revealed some of the functions of the individual junctional components in vivo, and are increasing our understanding of the functional role of endothelial cell junctions in angiogenesis and vascular homeostasis. Adherens-junction organization is required for correct vascular morphogenesis during embryo development. By contrast, the data available suggest that tight-junction proteins are not essential for vascular development but are necessary for endothelial barrier function.

Overexpression of delta-like 4 induces arterialization and attenuates vessel formation in developing mouse embryos

The importance of Notch signaling pathway in the regulation of vascular development and angiogenesis is suggested by the expression of Notch receptors and ligands in vascular endothelial cells (ECs) and the observed vascular phenotypes in mutants of Notch receptors or ligands, especially Dll4. DLL4 is specifically expressed in arterial ECs during development, and haplo-insufficiency is embryonically lethal in mice. To address the role of Dll4 in vascular development, we produced mDll4 conditionally overexpressed transgenic mice that were crossed with constitutive recombinase cre lines. Double transgenic embryos displayed grossly enlarged dorsal aortae (DA) and died before embryonic day 10.5 (E10.5), showing a variable degree of premature arteriovenous fusion. Veins displayed ectopic expression of arterial markers. Other defects included reduced vascular sprouting, EC proliferation, and migration. mDll4 overexpression also inhibited VEGF signaling and increased fibronectin accumulation around the vessels. In vitro and in vivo studies of DLL4-FL (Dll4-full-length) in ECs recapitulate many of the mDll4 transgenics findings, including decreased tube formation, reduced vascular branching, fewer vessels, increased pericyte recruitment, and increased fibronectin expression. These results establish the role of Dll4 in arterial identity determination, and regulation of angiogenesis subject to dose and location.

Identification of a core set of 58 gene transcripts with broad and specific expression in the microvasculature

OBJECTIVE

Pathological angiogenesis is an integral component of many diseases. Antiangiogenesis and vascular targeting are therefore promising new therapeutic principles. However, few endothelial-specific putative drug targets have been identified, and information is still limited about endothelial-specific molecular processes. Here we aimed at determining the endothelial cell-specific core transcriptome in vivo.

METHODS AND RESULTS

Analysis of publicly available microarray data identified a mixed vascular/lung cluster of 132 genes that correlated with known endothelial markers. Filtering against kidney glomerular/nonglomerular and brain vascular/nonvascular microarray profiles separated contaminating lung markers, leaving 58 genes with broad and specific microvascular expression. More than half of these have not previously been linked to endothelial functions or studied in detail before. The endothelial cell-specific expression of a selected subset of these, Eltd1, Gpr116, Ramp2, Slc9a3r2, Slc43a3, Rasip1, and NM_023516, was confirmed by real-time quantitative polymerase chain reaction and/or immunohistochemistry.

CONCLUSIONS

We have used a combination of publicly available and own microarray data to identify 58 gene transcripts with broad yet specific expression in microvascular endothelium. Most of these have unknown functions, but many of them are predicted to be cell surface expressed or implicated in cell signaling processes and should therefore be explored as putative microvascular drug targets.

Vascular endothelial cadherin promotes breast cancer progression via transforming growth factor beta signaling

Epithelial-to-mesenchymal transition (EMT) is an important event during carcinoma progression and leads to increased tumor cell malignancy. Here, we show that vascular endothelial (VE)-cadherin is induced during EMT in mammary tumor cells and is aberrantly expressed in invasive human breast carcinomas. VE-cadherin enhanced the capacity of fibroblastoid tumor cells to proliferate, form cord-like invasive structures, and adhere to endothelial cells, characteristics that are key contributors to their increased malignancy and metastatic potential. Consistently, VE-cadherin expression in malignant fibroblastoid tumor cells promoted the growth of experimental mammary carcinomas in vivo. Analysis of the signaling mechanisms involved revealed that VE-cadherin expression influences the levels of Smad2 phosphorylation and expression of target genes of transforming growth factor-beta (TGF-beta), a major mediator of advanced tumor progression and malignant tumor cell proliferation. VE-cadherin might thus promote tumor progression not only by contributing to tumor angiogenesis but also by enhancing tumor cell proliferation via the TGF-beta signaling pathway. This article provides evidence for a novel function of VE-cadherin in tumor progression and reveals a previously unknown molecular link between VE-cadherin expression and TGF-beta signaling. Our findings may have important implications for the clinical application of anti-VE-cadherin strategies.

Deficiency of zonula occludens-1 causes embryonic lethal phenotype associated with defected yolk sac angiogenesis and apoptosis of embryonic cells

Zonula occludens (ZO)-1/2/3 are the members of the TJ-MAGUK family of membrane-associated guanylate kinases associated with tight junctions. To investigate the role of ZO-1 (encoded by Tjp1) in vivo, ZO-1 knockout (Tjp1(-/-)) mice were generated by gene targeting. Although heterozygous mice showed normal development and fertility, delayed growth and development were evident from E8.5 onward in Tjp1(-/-) embryos, and no viable Tjp1(-/-) embryos were observed beyond E11.5. Tjp1(-/-) embryos exhibited massive apoptosis in the notochord, neural tube area, and allantois at embryonic day (E)9.5. In the yolk sac, the ZO-1 deficiency induced defects in vascular development, with impaired formation of vascular trees, along with defective chorioallantoic fusion. Immunostaining of wild-type embryos at E8.5 for ZO-1/2/3 revealed that ZO-1/2 were expressed in almost all embryonic cells, showing tight junction-localizing patterns, with or without ZO-3, which was confined to the epithelial cells. ZO-1 deficiency depleted ZO-1-expression without influence on ZO-2/3 expression. In Tjp1(+/+) yolk sac extraembryonic mesoderm, ZO-1 was dominant without ZO-2/3 expression. Thus, ZO-1 deficiency resulted in mesoderms with no ZO-1/2/3, associated with mislocalization of endothelial junctional adhesion molecules. As a result, angiogenesis was defected in Tjp1(-/-) yolk sac, although differentiation of endothelial cells seemed to be normal. In conclusion, ZO-1 may be functionally important for cell remodeling and tissue organization in both the embryonic and extraembryonic regions, thus playing an essential role in embryonic development.

Transcription factor Erg regulates angiogenesis and endothelial apoptosis through VE-cadherin

Tight regulation of the balance between apoptosis and survival is essential in angiogenesis. The ETS transcription factor Erg is required for endothelial tube formation in vitro. Inhibition of Erg expression in human umbilical vein endothelial cells (HUVECs), using antisense oligonucleotides, resulted in detachment of cell-cell contacts and increased cell death. Inhibition of Erg expression by antisense in HUVECs also lowered expression of the adhesion molecule vascular endothelial (VE)-cadherin, a key regulator of endothelial intercellular junctions and survival. Using chromatin immunoprecipitation, we showed that Erg binds to the VE-cadherin promoter. Furthermore, Erg was found to enhance VE-cadherin promoter activity in a transactivation assay. Apoptosis induced by inhibition of Erg was partly rescued by overexpression of VE-cadherin-GFP, suggesting that VE-cadherin is involved in the Erg-dependent survival signals. To show the role of Erg in angiogenesis in vivo, we used siRNA against Erg in a Matrigel plug model. Erg inhibition resulted in a significant decrease in vascularization, with increase in caspase-positive endothelial cells (ECs). These results identify a new pathway regulating angiogenesis and endothelial survival, via the transcription factor Erg and the adhesion molecule VE-cadherin.

Transgenic mouse models of angiogenesis and lymphangiogenesis

The development of transgenic technologies in mice has allowed the study of the consequences of genetic alterations on angiogenesis and lymphangiogenesis. This review summarizes the murine models currently available for studies involving the manipulation of angiogenesis and lymphangiogenesis. Abnormal embryonic vascular development, resulting from defects in the formation of a primitive vascular plexus, has been observed in mice lacking vascular endothelial growth factor, vascular endothelial growth factor receptor-1 and -2, transforming growth factor-beta, fibronectin, or vascular endothelial cadherin. Defects in the expansion and remodeling of the embryonic vasculature occur in mice deficient in Tie-1, Tie-2, or angiopoietin-1, and in mice overexpressing neuropilin or angiopoietin-2. Impaired recruitment and investment of mural cells have been observed in mice with disruption of the genes encoding platelet-derived growth factor-B, platelet-derived growth factor-B receptor, and tissue factor. Gene-targeting experiments in mice have identified the EphB/ephrinB system as a critical and rate-limiting determinant of arteriovenous differentiation during embryonic vascular development. Vascular endothelial growth factor-C is necessary for the initial sprouting and migration of lymphatic endothelial cells from embryonic veins, and mice lacking vascular endothelial growth factor-C die prenatally, whereas vascular endothelial growth factor-D is dispensable for embryonic lymphatic development.

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.

Endothelial adherens and tight junctions in vascular homeostasis, inflammation and angiogenesis

Endothelial cells lining the vessel wall are connected by adherens, tight and gap junctions. These junctional complexes are related to those found at epithelial junctions but with notable changes in terms of specific molecules and organization. Endothelial junctional proteins play important roles in tissue integrity but also in vascular permeability, leukocyte extravasation and angiogenesis. In this review, we will focus on specific mechanisms of endothelial tight and adherens junctions.

Defective vascular morphogenesis and mid-gestation embryonic death in mice lacking RA-GEF-1

A multitude of guanine nucleotide exchange factors (GEFs) regulate Rap1 small GTPases, however, their individual functions remain obscure. Here, we investigate the in vivo function of the Rap1 GEF RA-GEF-1. The expression of RA-GEF-1 in wild-type mice starts at embryonic day (E) 8.5, and continues thereafter. RA-GEF-1(-/-) mice appear normal until E7.5, but become grossly abnormal and dead by E9.5. This mid-gestation death appears to be closely associated with severe defects in yolk sac blood vessel formation. RA-GEF-1(-/-) yolk sacs form apparently normal blood islands by E8.5, but the blood islands fail to coalesce into a primary vascular plexus, indicating that vasculogenesis is impaired. Furthermore, RA-GEF-1(-/-) embryos proper show severe defects in the formation of major blood vessels. These results suggest that deficient Rap1 signaling may lead to defective vascular morphogenesis in the yolk sac and embryos proper.

An in vitro embryotoxicity assay based on the disturbance of the differentiation of murine embryonic stem cells into endothelial cells. I: Establishment of the differentiation protocol

The aim of the present study was to establish an experimental protocol to differentiate murine embryonic stem (ES) cells into endothelial cells in vitro. The spinner flask technique as well as the hanging drop method were used to generate so-called embryoid bodies (EBs). In order to find out the optimal differentiation environment, EBs were cultured under various experimental conditions for up to 14 days. The influence of basic fibroblast growth factor (bFGF) alone, vascular endothelial growth factor (VEGF) alone, bFGF and VEGF together and a cocktail consisting of bFGF, VEGF, interleukin-6 (IL-6) and erythropoietin (Epo) on the induction of differentiation of ES cells into endothelial cells was studied. Different concentrations of growth factors and times of treatment were applied. Endothelial cells were characterized by analyzing the expression of platelet-endothelial cell adhesion molecule (PECAM-1), the endothelial-specific vascular endothelial cadherin (VE-Cadherin), the angiopoietin receptor Tie-2, VEGF receptors 1 and 2 (Flt-1 and Flk-1, respectively) and the soluble form of Flt-1 (sFlt) at the mRNA level. PECAM-1 and VE-Cadherin were also studied at the protein level. The data clearly showed that EBs generated by the hanging drop method, followed by their transfer into suspension culture on day 3 of differentiation and their subsequent plating on day 5 is the best of the studied methods to differentiate ES cells into endothelial cells. Addition of VEGF alone or a cocktail consisting of VEGF, bFGF, IL-6 and Epo resulted in the strongest gene expression levels of the above mentioned endothelial cell markers in the differentiated ES cells.

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.

Expression of vascular endothelial-cadherin in human hepatocellular carcinoma tissues

AIM

Angiogenesis is important in tumor growth and progression to metastasis. Vascular endothelial (VE)-cadherin is an endothelial cell-specific cadherin required for angiogenesis, but its expression in hepatocellular carcinoma (HCC) tissues has not been examined.

METHODS

Expression of VE-cadherin was analyzed in 31 HCC frozen tissue specimens by immunohistochemical and immunoelectron microscopic procedures. In addition, the association of its expression with clinicopathological parameters was investigated to determine the possible diagnostic or predictive value of VE-cadherin expression in neoplastic and non-neoplastic liver lesions.

RESULTS

Immunoreactive VE-cadherin expression was faint or barely detectable on sinusoidal endothelial cells of normal liver but was evident on sinusoidal or capillary endothelium of chronic hepatitis, cirrhosis, and HCC tissues. VE-cadherin expression was more intense on capillary endothelium of HCC tissues in 26 (84%) of 31 patients than on sinusoidal endothelium of surrounding non-tumorous liver tissues with chronic liver diseases. The intensity or intracapillary extent of positive stain for VE-cadherin on capillary endothelium of HCC tissues was significantly associated with tumor size, capsular invasion and tumor cell differentiation in HCC.

CONCLUSIONS

Intense VE-cadherin expression was evident in capillary endothelium of HCC tissues, giving the first indication of association with clinicopathological features of HCC patients.

Endothelial cell motility is compatible with junctional integrity

Confluent endothelial cells in culture are generally regarded as a model of resting endothelium in blood vessels (i.e., forming junctions at points of cell-cell contact, losing ability to proliferate in response to growth factors, and remaining stationary). However, incompatibility between junctional integrity and endothelial cell motility remains uncertain. The aim of this study was to determine whether endothelial cells (in colonies generated from differentiating embryonic stem cells in contact with OP9 stromal cell layer) have a resting endothelial phenotype (i.e., lack motility). Time-lapse analyses showed that though endothelial cells were connected to each other through adherens junctions and tight junctions, they were moving continuously within the colonies. Endothelial cell movement was accompanied by formation of lamellipodia, which transiently accumulated green fluorescent protein-tagged beta-actin and p41-Arc (a subunit of the actin-related protein 2/3 complex) at their anterior tips, suggesting that the movement is an active behavior of endothelial cells. Endothelial cell-specific expression of yellow fluorescent protein-tagged vascular endothelial-cadherin and claudin-5 revealed that adherens junctions and tight junctions persisted during endothelial cell migration. Furthermore, intercellular junctions underwent dynamic remodeling at the leading edge of moving endothelial cells. These results suggest that endothelial cells can remain highly motile without losing intercellular junctions.

TAL-1/SCL and its partners E47 and LMO2 up-regulate VE-cadherin expression in endothelial cells

The basic helix-loop-helix TAL-1/SCL essential for hematopoietic development is also required during vascular development for embryonic angiogenesis. We reported that TAL-1 acts positively on postnatal angiogenesis by stimulating endothelial morphogenesis. Here, we investigated the functional consequences of TAL-1 silencing in human primary endothelial cells. We found that TAL-1 knockdown caused the inhibition of in vitro tubulomorphogenesis, which was associated with a dramatic reduction in vascular endothelial cadherin (VE-cadherin) at intercellular junctions. Consistently, silencing of TAL-1 as well as of its cofactors E47 and LMO2 down-regulated VE-cadherin at both the mRNA and the protein level. Endogenous VE-cadherin transcription could be activated in nonendothelial HEK-293 cells by the sole concomitant ectopic expression of TAL-1, E47, and LMO2. Transient transfections in human primary endothelial cells derived from umbilical vein (HUVECs) demonstrated that VE-cadherin promoter activity was dependent on the integrity of a specialized E-box associated with a GATA motif and was maximal with the coexpression of the different components of the TAL-1 complex. Finally, chromatin immunoprecipitation assays showed that TAL-1 and its cofactors occupied the VE-cadherin promoter in HUVECs. Together, these data identify VE-cadherin as a bona fide target gene of the TAL-1 complex in the endothelial lineage, providing a first clue to TAL-1 function in angiogenesis.

Gene expression profiles of endothelial progenitor cells by oligonucleotide microarray analysis

Among the many tissue stem or progenitor cells recently being unveiled, endothelial progenitor cells (EPCs) have attracted particular attention, not only because of their cardinal role in vascular biology and embryology but also because of their potential use in the therapeutic development of a variety of postnatal diseases, including cardiovascular and peripheral vascular disorders and cancer. The aim of this study is to provide some basic and comprehensive information on gene expression of EPCs to characterize the cells in molecular terms. Here, we focus on EPCs derived from CD34-positive mononuclear cells of human umbilical cord blood. The EPCs were purified and expanded in culture and analyzed by a high-density oligonucleotide microarray and real-time RT-PCR analysis. We identified 169 up-regulated and 107 down-regulated genes in the EPCs compared with three differentiated endothelial cells of human umbilical vein endothelial cells (HUVEC), human lung microvascular endothelial cells (LMEC) and human aortic endothelial cells (AoEC). It is expected that the obtained list include key genes which are critical for EPC function and survival and thus potential targets of EPC recognition in vivo and therapeutic modulation of vasculogenesis in cancer as well as other diseases, in which de novo vasculogenesis plays a crucial role. For instance, the list includes Syk and galectin-3, which encode protein tyrosine kinase and beta-galactoside-binding protein, respectively, and are expressed higher in EPCs than the three control endothelial cells. In situ hybridization showed that the genes were expressed in isolated cells in the fetal liver at E11.5 and E14.5 of mouse development.

Cell-cell adhesion in lung endothelium

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

Cadherin:catenin complex: A novel regulator of vascular smooth muscle cell behaviour

Dysfunctional vascular smooth muscle cell (VSMC) behaviour contributes to the pathogenesis of atherosclerosis and restenosis. Increased rates of VSMC apoptosis are thought to lead to thinning of the fibrous atherosclerotic plaque and thereby instability, while migration of VSMCs to the intima, and inappropriate VSMC proliferation, contribute to intimal thickening that occurs in atherosclerosis and restenosis. Studies, mainly in cancer and neuronal cells, have demonstrated that cell-cell adhesion by the cadherin:catenin complex modulates apoptosis, migration and proliferation. In contrast, until recently the involvement of this complex in the regulation of VSMC behaviour was relatively unstudied. In this review, evidence for the regulation of VSMC apoptosis, migration and proliferation by the cadherin:catenin complex will be discussed.

Genetic and epigenetic mechanisms in the early development of the vascular system

The cardiovascular system plays a critical role in vertebrate development and homeostasis. Vascular development is a highly organized sequence of events that requires the correct spatial and temporal expression of specific sets of genes leading to the development of a primary vascular network. There have been intensive efforts to determine the molecular mechanisms regulating vascular growth and development, and much of the rationale for this has stemmed from the increasing clinical importance and therapeutic potential of modulating vascular formation during various disease states.

Src kinase phosphorylates vascular endothelial-cadherin in response to vascular endothelial growth factor: identification of tyrosine 685 as the unique target site

Src-family tyrosine kinases are regulatory proteins that play a pivotal role in the disorganization of cadherin-dependent cell-cell contacts. We previously showed that Src was associated with vascular endothelial (VE)-cadherin and that tyrosine phosphorylation level of VE-cadherin was dramatically increased in angiogenic tissues as compared to quiescent tissues. Here, we examined whether VE-cadherin was a direct substrate for Src in vascular endothelial growth factor (VEGF)-induced VE-cadherin phosphorylation, and we identified the target tyrosine sites. Co-transfections of Chinese hamster ovary cells (CHO) cells with VE-cadherin and constitutively active Src (Y530F) resulted in a robust tyrosine phosphorylation of VE-cadherin that was not detected with kinase-dead Src (K298M). In an in vitro Src assay, the VE-cadherin cytoplasmic domain is directly phosphorylated by purified Src as well as the tyrosine residue 685 (Tyr)685-containing peptide RPSLY(685)AQVQ. VE-cadherin peptide mapping from human umbilical vein endothelial cells stimulated by VEGF and VE-cadherin-CHO cells transfected with active Src revealed that Y685 was the unique phosphorylated site. The presence of PhosphoY685 was confirmed by its ability to bind to C-terminal Src kinase-SH2 domain in a pull-down assay. Finally, we found that in a VEGF-induced wound-healing assay, cadherin adhesive activity was impaired by Src kinase inhibitors. These data identify that VEGF-induced-VE-cadherin tyrosine phosphorylation is mediated by Src on Y685, a process that appears to be critical for VEGF-induced endothelial cell migration.

VE-Cadherin-Cre-recombinase transgenic mouse: a tool for lineage analysis and gene deletion in endothelial cells

The ability to target gene deletion to a specific cellular compartment via the Cre/loxP system has been a powerful tool in the analysis of broadly expressed genes. Here, we report the generation of a transgenic mouse line in which expression of Cre-recombinase is under the regulatory control of the VE-Cadherin promoter. Temporal distribution and activity of the enzyme was evaluated with two independent Cre reporter lines. Histological analysis was performed throughout development and in the adult. Recombination of lox P sites with subsequent expression of beta-galactosidase or GFP was detected as early as E7.5 in endothelial cells of the yolk sac. Progressive staining of the embryonic vasculature was noted from E8.5-13.5; however, more contiguous reporter expression was only seen by E14.5 onward in all endothelial compartments including arteries, veins, and capillaries. In addition, we found Cre activity in lymphatic endothelial cells. Unlike other endothelial-specific Cre mice, this model showed expression in the adult quiescent vasculature. Furthermore, the constitutive nature of the VE-Cadherin promoter in the adult can be advantageous for analysis of gene deletion in pathological settings.