Role of vascular endothelial-cadherin in vascular morphogenesis

Gαs Relays Sphingosine-1-Phosphate Receptor 1 Signaling to Stabilize Vascular Endothelial-Cadherin at Endothelial Junctions to Control Mouse Embryonic Vascular Integrity

Sphingosine-1-phosphate receptor 1 (S1PR1), a G protein-coupled receptor (GPCR), controls vascular stability by stabilizing vascular endothelial (VE)-cadherin junctional localization and inhibiting vascular endothelial growth factor receptor 2 (VEGFR2) signaling. However, the molecular mechanisms that link S1PR1 signaling to intracellular effectors remain unknown. In this study, we demonstrate that the heterotrimeric G protein subfamily member Gαs, encoded by GNAS, acts as a relay mediator of S1PR1 signaling to control vascular integrity by stabilizing VE-cadherin at endothelial junctions. The endothelial cell-specific deletion of Gαs in mice causes early embryonic lethality with massive hemorrhage and a disorganized vasculature. The immunostaining results revealed that Gαs deletion remarkably reduces the junctional localization of VE-cadherin, whereas the mural cell coverage of the vessels is not impaired. In addition, we found that Gαs depletion blocks the S1PR1-activation induced VE-cadherin stabilization at junctions, supporting that Gαs acts downstream of S1PR1 signaling. Thus, our results demonstrate that Gαs is an essential mediator to relay S1PR1 signaling and maintain vascular integrity.

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

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

Histone acetyltransferase p300 promotes MRTF-A-mediates transactivation of VE-cadherin gene in human umbilical vein endothelial cells

Vascular endothelial cadherin (VE-cadherin) is the major determinant of endothelial cell contact integrity and is required in vascular development and angiogenesis. Serum response factor (SRF) plays essential roles in postnatal retinal angiogenesis and adult neovascularization. It is unclear whether transcription of VE-cadherin is mediated by a SRF co-activator, myocardin-related transcription factor-A (MRTF-A). Here we have demonstrated that MRTF-A is a key regulatory factor to activate the transcription of VE-cadherin in human umbilical vein endothelial cells (HUVECs). siRNA-mediated knockdown of MRTF-A decreased the level of VE-cadherin in HUVECs. Vascular endothelial growth factor (VEGF) induced MRTF-A binding to the SRF-binding site (CArG box) within VE-cadherin promoter. Histone acetyltransferase p300 and MRTF-A could synergistically augment the expression of VE-cadherin by enhancing acetylation of histone3K9 (H3K9Ac), histone3K14 (H3K14Ac) and histone4 at the SRF-binding site within VE-cadherin promoter. Taken together, these data identified a detailed regulatory mechanism of VE-cadherin gene expression.

Investigating in vitro angiogenesis by computer-assisted image analysis and computational simulation

In vitro assays that stimulate the formation of capillary-like structures by EC have become increasingly popular, because they allow the study of the EC's intrinsic ability to self-organize to form vascular-like patterns. Here we describe a widely applied protocol involving the use of basement membrane matrix (Matrigel) as a suitable environment to induce an angiogenic phenotype in cultured EC. EC differentiation on basement membrane matrix is a highly specific process, which recapitulates many steps in blood vessel formation and for this reason it is presently considered as a reliable in vitro tool to identify factors with potential antiangiogenic or pro-angiogenic properties. The morphological features of the obtained cell patterns can also be accurately quantified by computer-assisted image analysis and the main steps of such a procedure will be here outlined and discussed. The dynamics of in vitro EC self-organization is a complex biological process, involving a network of interactions between a high number of cells. For this reason, the combined use of in vitro experiments and computational modeling can represent a key approach to unravel how mechanical and chemical signaling by EC coordinates their organization into capillary-like tubes. Thus, a particularly helpful approach to modeling is also briefly described together with examples of its application.

Cdh5/VE-cadherin promotes endothelial cell interface elongation via cortical actin polymerization during angiogenic sprouting

Organ morphogenesis requires the coordination of cell behaviors. Here, we have analyzed dynamic endothelial cell behaviors underlying sprouting angiogenesis in vivo. Two different mechanisms contribute to sprout outgrowth: tip cells show strong migratory behavior, whereas extension of the stalk is dependent upon cell elongation. To investigate the function of Cdh5 in sprout outgrowth, we generated null mutations in the zebrafish cdh5 gene, and we found that junctional remodeling and cell elongation are impaired in mutant embryos. The defects are associated with a disorganization of the actin cytoskeleton and cannot be rescued by expression of a truncated version of Cdh5. Finally, the defects in junctional remodeling can be phenocopied by pharmacological inhibition of actin polymerization, but not by inhibiting actin-myosin contractility. Taken together, our results support a model in which Cdh5 organizes junctional and cortical actin cytoskeletons, as well as provides structural support for polymerizing F-actin cables during endothelial cell elongation.

Vascular development and hemodynamic force in the mouse yolk sac

Vascular remodeling of the mouse embryonic yolk sac is a highly dynamic process dependent on multiple genetic signaling pathways as well as biomechanical factors regulating proliferation, differentiation, migration, cell-cell, and cell-matrix interactions. During this early developmental window, the initial primitive vascular network of the yolk sac undergoes a dynamic remodeling process concurrent with the onset of blood flow, in which endothelial cells establish a branched, hierarchical structure of large vessels and smaller capillary beds. In this review, we will describe the molecular and biomechanical regulators which guide vascular remodeling in the mouse embryonic yolk sac, as well as live imaging methods for characterizing endothelial cell and hemodynamic function in cultured embryos.

Redundant role of protein kinase C delta and epsilon during mouse embryonic development

Protein Kinase C delta and epsilon are mediators of important cellular events, such as cell proliferation, migration or apoptosis. The formation of blood vessels, i.e., vasculo- and angiogenesis, is a process where these isoforms have also been shown to participate. However, mice deficient in either Protein Kinase C delta or epsilon are viable and therefore their individual contribution to the formation of the vasculature appeared so far dispensable. In this study, we show that double null mutation of Protein Kinase C delta and epsilon causes embryonic lethality at approximately E9.5. At this stage, whole mount staining of the endothelial marker CD31 in double null embryos revealed defective blood vessel formation. Moreover, culture of double deficient mouse allantois showed impaired endothelial cell organization, and analyses of double deficient embryo sections showed dilated vessels, decreased endothelial-specific adherent junctions, and decreased contact of endothelial cells with mural cells. Protein kinase C delta and epsilon also appeared essential for vascular smooth muscle cell differentiation, since α-smooth muscle actin, a classical marker for vascular smooth muscle cells, was almost undetectable in double deficient embryonic aorta at E9.5. Subsequent qPCR analyses showed decreased VE-cadherin, Vegfr2, Cd31, Cdh2, Ets1, and Fli-1, among other angiogenesis related transcripts in double deficient embryos. Taken together, these data suggest for the first time an in vivo redundant role between members of the novel Protein Kinase C subfamily that allows for mutual compensation during mouse embryonic development, with vasculogenesis/angiogenesis as an obvious common function of these two Protein Kinase Cs. Protein Kinase C delta and epsilon might therefore be useful targets for inhibiting vasculo- and/or angiogenesis.

E-cadherin can replace N-cadherin during secretory-stage enamel development

BACKGROUND

N-cadherin is a cell-cell adhesion molecule and deletion of N-cadherin in mice is embryonic lethal. During the secretory stage of enamel development, E-cadherin is down-regulated and N-cadherin is specifically up-regulated in ameloblasts when groups of ameloblasts slide by one another to form the rodent decussating enamel rod pattern. Since N-cadherin promotes cell migration, we asked if N-cadherin is essential for ameloblast cell movement during enamel development.

METHODOLOGY/PRINCIPAL FINDINGS

The enamel organ, including its ameloblasts, is an epithelial tissue and for this study a mouse strain with N-cadherin ablated from epithelium was generated. Enamel from wild-type (WT) and N-cadherin conditional knockout (cKO) mice was analyzed. μCT and scanning electron microscopy showed that thickness, surface structure, and prism pattern of the cKO enamel looked identical to WT. No significant difference in hardness was observed between WT and cKO enamel. Interestingly, immunohistochemistry revealed the WT and N-cadherin cKO secretory stage ameloblasts expressed approximately equal amounts of total cadherins. Strikingly, E-cadherin was not normally down-regulated during the secretory stage in the cKO mice suggesting that E-cadherin can compensate for the loss of N-cadherin. Previously it was demonstrated that bone morphogenetic protein-2 (BMP2) induces E- and N-cadherin expression in human calvaria osteoblasts and we show that the N-cadherin cKO enamel organ expressed significantly more BMP2 and significantly less of the BMP antagonist Noggin than did WT enamel organ.

CONCLUSIONS/SIGNIFICANCE

The E- to N-cadherin switch at the secretory stage is not essential for enamel development or for forming the decussating enamel rod pattern. E-cadherin can substitute for N-cadherin during these developmental processes. Bmp2 expression may compensate for the loss of N-cadherin by inducing or maintaining E-cadherin expression when E-cadherin is normally down-regulated. Notably, this is the first demonstration of a natural endogenous increase in E-cadherin expression due to N-cadherin ablation in a healthy developing tissue.

VE-cadherin Y685F knock-in mouse is sensitive to vascular permeability in recurrent angiogenic organs

Covalent modifications such as tyrosine phosphorylation are associated with the breakdown of endothelial cell junctions and increased vascular permeability. We previously showed that vascular endothelial (VE)-cadherin was tyrosine phosphorylated in vivo in the mouse reproductive tract and that Y685 was a target site for Src in response to vascular endothelial growth factor in vitro. In the present study, we aimed to understand the implication of VE-cadherin phosphorylation at site Y685 in cyclic angiogenic organs. To achieve this aim, we generated a knock-in mouse carrying a tyrosine-to-phenylalanine point mutation of VE-cadherin Y685 (VE-Y685F). Although homozygous VE-Y685F mice were viable and fertile, the nulliparous knock-in female mice exhibited enlarged uteri with edema. This phenotype was observed in 30% of females between 4 to 14 mo old. Histological examination of longitudinal sections of the VE-Y685F uterus showed an extensive disorganization of myometrium and endometrium with highly edematous uterine glands, numerous areas with sparse cells, and increased accumulation of collagen fibers around blood vessels, indicating a fibrotic state. Analysis of cross section of ovaries showed the appearance of spontaneous cysts, which suggested increased vascular hyperpermeability. Electron microscopy analysis of capillaries in the ovary showed a slight but significant increase in the gap size between two adjacent endothelial cell membranes in the junctions of VE-Y685F mice (wild-type, 11.5 ± 0.3, n = 78; and VE-Y685F, 12.48 ± 0.3, n = 65; P = 0.045), as well as collagen fiber accumulation around capillaries. Miles assay revealed that either basal or vascular endothelial growth factor-stimulated permeability in the skin was increased in VE-Y685F mice. Since edema and fibrotic appearance have been identified as hallmarks of initial increased vascular permeability, we conclude that the site Y685 in VE-cadherin is involved in the physiological regulation of capillary permeability. Furthermore, this knock-in mouse model is of potential interest for further studies of diseases that are associated with abnormal vascular permeability.

AmotL2 links VE-cadherin to contractile actin fibres necessary for aortic lumen expansion

The assembly of individual endothelial cells into multicellular tubes is a complex morphogenetic event in vascular development. Extracellular matrix cues and cell-cell junctional communication are fundamental to tube formation. Together they determine the shape of endothelial cells and the tubular structures that they ultimately form. Little is known regarding how mechanical signals are transmitted between cells to control cell shape changes during morphogenesis. Here we provide evidence that the scaffold protein amotL2 is needed for aortic vessel lumen expansion. Using gene inactivation strategies in zebrafish, mouse and endothelial cell culture systems, we show that amotL2 associates to the VE-cadherin adhesion complex where it couples adherens junctions to contractile actin fibres. Inactivation of amotL2 dissociates VE-cadherin from cytoskeletal tensile forces that affect endothelial cell shape. We propose that the VE-cadherin/amotL2 complex is responsible for transmitting mechanical force between endothelial cells for the coordination of cellular morphogenesis consistent with aortic lumen expansion and function.

Altered transendothelial transport of hormones as a contributor to diabetes

The vascular endothelium is a dynamic structure responsible for the separation and regulated movement of biological material between circulation and interstitial fluid. Hormones and nutrients can move across the endothelium either via a transcellular or paracellular route. Transcellular endothelial transport is well understood and broadly acknowledged to play an important role in the normal and abnormal physiology of endothelial function. However, less is known about the role of the paracellular route. Although the concept of endothelial dysfunction in diabetes is now widely accepted, we suggest that alterations in paracellular transport should be studied in greater detail and incorporated into this model. In this review we provide an overview of endothelial paracellular permeability and discuss its potential importance in contributing to the development of diabetes and associated complications. Accordingly, we also contend that if better understood, altered endothelial paracellular permeability could be considered as a potential therapeutic target for diabetes.

Deconvoluting the ontogeny of hematopoietic stem cells

Two different models describe the development of definitive hematopoiesis and hematopoietic stem cells (HSCs). In one of these, the visceral yolk sac serves as a starting point of relatively lengthy developmental process culminating in the fetal liver hematopoiesis. In another, the origin of adult hematopoiesis is split between the yolk sac and the dorsal aorta, which has a peculiar capacity to generate definitive HSCs. Despite a large amount of experimental data consistent with the latter view, it becomes increasingly unsustainable in the light of recent cell tracing studies. Moreover, analysis of the published studies supporting the aorta-centered version uncovers significant caveats in standard experimental approach and argumentation. As a result, the theory cannot offer feasible cellular mechanisms of the HSC emergence. This review summarizes key efforts to discern the developmental pathway of the adult-type HSCs and attempts to put forward a hypothesis on the inflammatory mechanisms of hematopoietic ontogenesis.

Fusing VE-cadherin to α-catenin impairs fetal liver hematopoiesis and lymph but not blood vessel formation

We have recently shown that genetic replacement of VE-cadherin by a VE-cadherin-α-catenin fusion construct strongly impairs opening of endothelial cell contacts during leukocyte extravasation and induction of vascular permeability in adult mice. Here we show that this mutation leads to lethality at midgestation on a clean C57BL/6 background. Investigating the reasons for embryonic lethality, we observed a lack of fetal liver hematopoiesis and severe lymphedema but no detectable defects in blood vessel formation and remodeling. As for the hematopoiesis defect, VE-cadherin-α-catenin affected neither the generation of hematopoietic stem and progenitor cells (HSPCs) from hemogenic endothelium nor their differentiation into multiple hematopoietic lineages. Instead, HSPCs accumulated in the fetal circulation, suggesting that their entry into the fetal liver was blocked. Edema formation was caused by disturbed lymphatic vessel development. Lymphatic progenitor cells of VE-cadherin-α-catenin-expressing embryos were able to leave the cardinal vein and migrate to the site of the first lymphatic vessel formation, yet subsequently, these cells failed to form large lumenized lymphatic vessels. Thus, stabilizing endothelial cell contacts by a covalent link between VE-cadherin and α-catenin affects recruitment of hematopoietic progenitors into the fetal liver and the development of lymph but not blood vessels.

Fluid shear stress pre-conditioning promotes endothelial morphogenesis of embryonic stem cells within embryoid bodies

Pluripotent embryonic stem cells (ESCs) are capable of differentiating into all mesoderm-derived cell lineages, including endothelial, hematopoietic, and cardiac cell types. Common strategies to direct mesoderm differentiation of ESCs rely on exposing the cells to a series of biochemical and biophysical cues at different stages of differentiation to promote maturation toward specific cell phenotypes. Shear forces that mimic cardiovascular physiological forces can evoke a myriad of responses in somatic and stem cell populations, and have, thus, been studied as a means to direct stem cell differentiation. However, elucidating the effects of shear pre-conditioning on the subsequent vascular differentiation and morphogenesis of ESCs has yet to be examined. In this study, ESC monolayers were subjected to physiological shear (5 dyn/cm(2)) or static conditions for 2 days on collagen IV-coated substrates before initiating embryoid body (EB) differentiation. Immediately after the pre-conditioning period, shear pre-conditioned and statically cultured ESCs exhibited similar morphologies and largely retained a pluripotent phenotype; however, ESCs exposed to fluid shear expressed increased levels of endothelial marker genes Flk-1 (∼3-fold), VE-cadherin (∼3-fold), and PECAM (∼2-fold), compared with statically cultured ESCs. After 7 days of EB culture, ∼70% of EBs formed from shear pre-conditioned ESCs expressed significantly higher levels of endothelial marker genes compared with EBs formed from statically cultured ESCs. Interestingly, unlike EBs formed from statically cultured ESCs, EBs formed from fluid shear stress pre-conditioned ESCs exhibited a centrally localized region of VE-cadherin(+) cells that persisted for at least 10 days of differentiation. These results demonstrate that fluid shear stress pre-conditioning not only promotes ESC endothelial gene expression but also subsequently impacts the organization of endothelial cells within EBs. Together, these studies highlight a novel approach to promote in vitro morphogenesis of developmental vasculogenic models and potentially promote pre-vascularization of tissue-engineered constructs derived from pluripotent stem cells.

Cellular and molecular mechanisms underlying blood vessel lumen formation

The establishment of a functional vascular system requires multiple complex steps throughout embryogenesis, from endothelial cell (EC) specification to vascular patterning into venous and arterial hierarchies. Following the initial assembly of ECs into a network of cord-like structures, vascular expansion and remodeling occur rapidly through morphogenetic events including vessel sprouting, fusion, and pruning. In addition, vascular morphogenesis encompasses the process of lumen formation, critical for the transformation of cords into perfusable vascular tubes. Studies in mouse, zebrafish, frog, and human endothelial cells have begun to outline the cellular and molecular requirements underlying lumen formation. Although the lumen can be generated through diverse mechanisms, the coordinated participation of multiple conserved molecules including transcription factors, small GTPases, and adhesion and polarity proteins remains a fundamental principle, leading us closer to a more thorough understanding of this complex event.

Decreased interleukin-20 level in patients with systemic sclerosis: are they related with angiogenesis?

In this study, we aimed to evaluate the relation between angiogenesis indicators and T helper 17 cytokine group in patients with systemic sclerosis (SSc) which is a disease characterized by impaired angiogenesis and autoimmune response. In our study, patients with SSc are compared with patients with primary Raynaud's phenomenon (RP) and healthy controls. Forty SSc patients, 18 primary RP cases, and 20 healthy controls were included in our study. The demographic and clinical features of patients with SSc were recorded. The serum levels of vascular endothelial growth factor (VEGF), vascular endothelial (VE)-cadherin, interleukin (IL)-20, IL-22, and IL-23 were assessed. In the SSc group, IL-20 level was significantly lower than in both primary RP group and controls (p values <0.001). VE-cadherin level in SSc was significantly higher than in primary RP (p = 0.016). The IL-22 and IL-23 and VEGF levels of SSc, primary RP, and control groups were similar (p values >0.05). In SSc patients, IL-23 correlated negatively with VEGF (r = -0.36, p = 0.025) and positively with VE-cadherin (r = 0.55, p < 0.001). IL-20 levels in SSc patients correlated with disease duration (r = 0.32, p = 0.044). SSc patients with limited involvement had significantly higher VE-cadherin levels than SSc patients with diffuse involvement (p = 0.044). We observed that IL-20 which is an IL-10 group angiogenesis indicator was observed to be suppressed in SSc, suggesting abnormal angiogenesis.

Stanniocalcin-1 and -2 promote angiogenic sprouting in HUVECs via VEGF/VEGFR2 and angiopoietin signaling pathways

The members of stanniocalcins (STCs: STC-1 and STC-2) family are known to be involved in tumor progression and metastasis. Although current evidences suggest the involvement of STCs in vascular biology, the functional roles of STCs in angiogenesis have not yet been elucidated. The objective of this study was to decipher the roles of STCs in angiogenesis of human umbilical vascular endothelial cells (HUVECs). We prepared STC1 or STC2 lentiviral particles to transduce the cells to reveal their effects on the processes of cell proliferation, migration and tube formation. The stimulatory effects of STCs on these processes were demonstrated, supporting the notion of STCs in angiogenesis. To dissect the molecular components involved, STC1 or STC2 transduction led to significant increases in the expression levels of cell cycle regulators (i.e. cyclin-D and phospho-retinoblastoma), matrix metalloproteinase (MMP)-2 but a decrease of tissue inhibitors of metalloproteases (TIMP)-1. The expression levels of the cell adhesion/junctional proteins vimentin and VE-cadherin, were significantly induced. Moreover the transduction induced both mRNA and protein levels of eNOS, VEGF and VEGFR2 (KDR mRNA and pKDR), highlighting the stimulatory effects of STCs on VEGF-signaling pathway. Furthermore STC2 transduction but not STC1, activated angiopoietin (Ang)-2 pathway. Taken together, STC1 and STC2 play positive roles in angiogenic sprouting. The action of STC1 was mediated via VEGF/VEGFR2 pathway while STC2 were mediated via VEGF/VEGFR2 and Ang-2 pathways.

Desmoglein-2-integrin Beta-8 interaction regulates actin assembly in endothelial cells: deregulation in systemic sclerosis

Background

The inability of endothelial cells of patients affected by the diffuse form of Systemic sclerosis (SSc) to perform angiogenesis is a marker of the disease. We previously demonstrated that desmoglein-2 reduction is a major difference between (SSc)-microvascular endothelial cells (MVECs) and normal (N)-MVECs. Here we investigated the role of desmoglein-2 in human N-MVECs and SSc-MVECs angiogenesis.

Methodology/principal findings

Angiogenesis was studied by Matrigel invasion, capillary morphogenesis in vitro and Matrigel plug assay in vivo. Gene profiling was studied by Affymetrix technology and signal transduction by Western blotting. Colocalization was validated by immunoprecipitation and confocal microscopy. SiRNAs were used to validate the roles of specific molecules. We observed that desmoglein-2 co-localizes with integrin-beta8 in N-MVECs. This complex is required to signal through Rac, FAK, SMAD1/5 and MAP-kinases, promoting an angiogenic program. Inhibition of desmoglein-2 by DSG2-siRNA impaired actin stress fibres formation, capillary morphogenesis in vitro and angiogenesis in vivo. Transcriptome profiling after DSG2 inhibition revealed alterations of several genes involved in actin organization. siRNA inhibition of integrin-beta8 and RAC2 also resulted into capillary morphogenesis impairment in N-MVECs, due to reduced expression of the same actin-assembly genes that were down-regulated by DSG2 silencing. SSc-MVECs showed down-regulation of the same genes in DSG2-siRNA treated N-MVECs, suggesting that impairment of desmoglein-2/integrin-beta8 complex contributes to angiogenesis derangement in SSc. Transfection of DSG2 in SSc-MVEC partially restored their angiogenic properties in vitro.

Conclusions/significance

We have shown that impairment of actin assembly as a result of desmoglein-2/integrin-beta8 complex formation is a major factor contributing to angiogenesis deregulation in Systemic sclerosis.

Role of differential adhesion in cell cluster evolution: from vasculogenesis to cancer metastasis

Cell-cell and cell-matrix adhesions are fundamental to numerous physiological processes, including angiogenesis, tumourigenesis, metastatic spreading and wound healing. We use cellular potts model to computationally predict the organisation of cells within a 3D matrix. The energy potentials regulating cell-cell (JCC) and cell-matrix (JMC) adhesive interactions are systematically varied to represent different, biologically relevant adhesive conditions. Chemotactically induced cell migration is also addressed. Starting from a cluster of cells, variations in relative cell adhesion alone lead to different cellular patterns such as spreading of metastatic tumours and angiogenesis. The combination of low cell-cell adhesion (high JCC) and high heterotypic adhesion (low JMC) favours the fragmentation of the original cluster into multiple, smaller cell clusters (metastasis). Conversely, cellular systems exhibiting high-homotypic affinity (low JCC) preserve their original configuration, avoiding fragmentation (organogenesis). For intermediate values of JCC and JMC (i.e. JCC/JMC ∼ 1), tubular and corrugated structures form. Fully developed vascular trees are assembled only in systems in which contact-inhibited chemotaxis is activated upon cell contact. Also, the rate of secretion, diffusion and sequestration of chemotactic factors, cell deformability and motility do not significantly affect these trends. Further developments of this computational model will predict the efficacy of therapeutic interventions to modulate the diseased microenvironment by directly altering cell cohesion.

Erectile dysfunction precedes other systemic vascular diseases due to incompetent cavernous endothelial cell-cell junctions

PURPOSE

Erectile dysfunction is often a harbinger of cardiovascular disease. We sought to gain mechanistic insight at the cellular and molecular levels into why erectile dysfunction precedes the clinical consequences of cardiovascular disease.

MATERIALS AND METHODS

Diabetes was induced by intraperitoneal streptozotocin injection in 8-week-old C57BL/6J mice. At 8 weeks after diabetes induction, we determined the expression of endothelial cell-cell junction proteins and vascular endothelial permeability in the penis, heart and hind limb by systemic injection of various vascular space markers (350 Da to 2,000 kDa) or by immunohistochemical staining with antibody to oxidized low density lipoprotein. We also investigated the effect of recombinant Ang1 protein on cavernous endothelial permeability.

RESULTS

Alterations in the integrity of the endothelial cell-cell junction, including a decrease in endothelial cell-cell junction proteins and an increase in vascular permeability to fluorescent tracers or oxidized low density lipoprotein, were prominent in the cavernous tissue of diabetic mice. In contrast, no significant changes in endothelial cell-cell junction proteins or vascular permeability were noted in heart or hind limb tissue according to the diabetic condition. Intracavernous injection of Ang1 protein, an anti-permeability factor, significantly decreased cavernous endothelial permeability to oxidized low density lipoprotein by restoring endothelial cell-cell junction proteins in diabetic mice.

CONCLUSIONS

The incompetent cavernous endothelial cell-cell junction in the diabetic condition provides an important clue to why erectile dysfunction is highly prevalent and often precedes other systemic vascular diseases.

p120-catenin and β-catenin differentially regulate cadherin adhesive function

β-Catenin and p120-catenin bind to cadherin cytoplasmic tails and are believed to modulate cadherin function and adhesion. This study shows that p120-catenin and β-catenin function in a distinct but complementary manner to regulate the size and strength of cadherin adhesive contacts.

Vascular endothelial (VE)-cadherin, the major adherens junction adhesion molecule in endothelial cells, interacts with p120-catenin and β-catenin through its cytoplasmic tail. However, the specific functional contributions of the catenins to the establishment of strong adhesion are not fully understood. Here we use bioengineering approaches to identify the roles of cadherin–catenin interactions in promoting strong cellular adhesion and the ability of the cells to spread on an adhesive surface. Our results demonstrate that the domain of VE-cadherin that binds to β-catenin is required for the establishment of strong steady-state adhesion strength. Surprisingly, p120 binding to the cadherin tail had no effect on the strength of adhesion when the available adhesive area was limited. Instead, the binding of VE-cadherin to p120 regulates adhesive contact area in a Rac1-dependent manner. These findings reveal that p120 and β-catenin have distinct but complementary roles in strengthening cadherin-mediated adhesion.

N-cadherin-mediated adhesion and signaling from development to disease: lessons from mice

Of the 20 classical cadherin subtypes identified in mammals, the functions of the two initially identified family members E- (epithelial) and N- (neural) cadherin have been most extensively studied. E- and N-Cadherin have mostly mutually exclusive expression patterns, with E-cadherin expressed primarily in epithelial cells, whereas N-cadherin is found in a variety of cells, including neural, muscle, and mesenchymal cells. N-Cadherin function, in particular, appears to be cell context-dependent, as it can mediate strong cell-cell adhesion in the heart but induces changes in cell behavior in favor of a migratory phenotype in the context of epithelial-mesenchymal transition (EMT). The ability of tumor cells to alter their cadherin expression profile, for example, E- to N-cadherin, is critical for malignant progression. Recent advances in mouse molecular genetics, and specifically tissue-specific knockout and knockin alleles of N-cadherin, have provided some unexpected results. This chapter highlights some of the genetic studies that explored the complex role of N-cadherin in embryonic development and disease.

Reverse genetics screen in zebrafish identifies a role of miR-142a-3p in vascular development and integrity

MicroRNAs are a well-studied class of non-coding RNA and are known to regulate developmental processes in eukaryotes. Their role in key biological processes such as vasculature development has attracted interest. However, a comprehensive understanding of molecular regulation of angiogenesis and vascular integrity during development remains less explored. Here we identified miRNAs involved in the development and maintenance of vasculature in zebrafish embryos using a reverse genetics approach. Using a combination of bioinformatics predictions and literature based evidences we mined over 701 Human and 329 Zebrafish miRNAs to derive a list of 29 miRNAs targeting vascular specific genes in zebrafish. We shortlisted eight miRNAs and investigated their potential role in regulating vascular development in zebrafish transgenic model. In this screen we identified three miRNAs, namely miR-1, miR-144 and miR-142a-3p that have the potential to influence vascular development in zebrafish. We show that miR-142a-3p mediates vascular integrity and developmental angiogenesis in vivo. Overexpression of miR-142a-3p results in loss of vascular integrity, hemorrhage and vascular remodeling during zebrafish embryonic development, while loss of function of miR-142a-3p causes abnormal vascular remodeling. MiR-142a-3p functions in part by directly repressing cdh5 (VE-cadherin). The vascular abnormalities that results from modulation of miR-142a-3p are reminiscent of cdh5 perturbation in zebrafish embryos. We also demonstrate that the action of miR-142a on cdh5 is potentially regulated by Lmo2, an important transcription factor, known for its role in vasculature development. The miR142a-3p mediated control of cdh5 constitutes an additional layer of regulation for maintaining vascular integrity and developmental angiogenesis. These findings have implications in development, wound repair and tumor growth.

A continuum of transcriptional identities visualized by combinatorial fluorescent in situ hybridization

Oligonucleotide-based fluorescent in situ hybridization (FISH) coupled with high-resolution high-sensitivity microscopy allows the visualization of single RNA molecules within fixed cells and tissues as distinct foci. We show here that combinatorial labeling of RNA molecules with several fluorescent dyes extends the number of genes that can be targeted simultaneously beyond the number of fluorophores used. This approach also inherently validates the identification of transcripts reducing false positive counts. We have used combinatorial FISH and image analysis to measure the transcript densities of six genes using three fluorophores. This has allowed us to visualize the endothelial maturation of lateral mesoderm in an in vitro ES differentiation assay from a single snapshot of molecular identities. Our observations show that, under these specific conditions, endothelial maturation follows a homogeneous course with a gradual increase in expression of Cdh5 and a concomitant loss of early transcription factors, arguing that maturation is governed in a generally deterministic manner. This methodology is limited by the number of fluorophores that can be used and by the available microscopic resolution, but currently available equipment should allow the visualization of transcripts from 10 or more genes simultaneously.

The phosphatase PTP-PEST/PTPN12 regulates endothelial cell migration and adhesion, but not permeability, and controls vascular development and embryonic viability

Protein-tyrosine phosphatase (PTP)-PEST (PTPN12) is ubiquitously expressed. It is essential for normal embryonic development and embryonic viability in mice. Herein we addressed the involvement of PTP-PEST in endothelial cell functions using a combination of genetic and biochemical approaches. By generating primary endothelial cells from an inducible PTP-PEST-deficient mouse, we found that PTP-PEST is not needed for endothelial cell differentiation and proliferation or for the control of endothelial cell permeability. Nevertheless, it is required for integrin-mediated adhesion and migration of endothelial cells. PTP-PEST-deficient endothelial cells displayed increased tyrosine phosphorylation of Cas, paxillin, and Pyk2, which were previously also implicated in integrin functions. By eliminating PTP-PEST in endothelial cells in vivo, we obtained evidence that expression of PTP-PEST in endothelial cells is required for normal vascular development and embryonic viability. Therefore, PTP-PEST is a key regulator of integrin-mediated functions in endothelial cells seemingly through its capacity to control Cas, paxillin, and Pyk2. This function explains at least in part the essential role of PTP-PEST in embryonic development and viability.

Small GTPase R-Ras regulates integrity and functionality of tumor blood vessels

We show that R-Ras, a small GTPase of the Ras family, is essential for the establishment of mature, functional blood vessels in tumors. The genetic disruption of R-Ras severely impaired the maturation processes of tumor vessels in mice. Conversely, the gain of function of R-Ras improved vessel structure and blood perfusion and blocked plasma leakage by enhanced endothelial barrier function and pericyte association with nascent blood vessels. Thus, R-Ras promotes normalization of the tumor vasculature. These findings identify R-Ras as a critical regulator of vessel integrity and function during tumor vascularization.

The murine allantois: a model system for the study of blood vessel formation

The allantois is the embryonic precursor of the umbilical cord in mammals and is one of several embryonic regions, including the yolk sac and dorsal aorta, that undergoes vasculogenesis, the de novo formation of blood vessels. Despite its importance in establishing the chorioallantoic placenta and umbilical circulation, the allantois frequently is overlooked in embryologic studies. Nonetheless, recent studies demonstrate that vasculogenesis, vascular remodeling, and angiogenesis are essential allantois functions in the establishment of the chorioallantoic placenta. Here, we review blood vessel formation in the murine allantois, highlighting the expression of genes and involvement of pathways common to vasculogenesis or angiogenesis in other parts of the embryo. We discuss experimental techniques available for manipulation of the allantois that are unavailable for yolk sac or dorsal aorta, and review how this system has been used as a model system to discover new genes and mechanisms involved in vessel formation. Finally, we discuss the potential of the allantois as a model system to provide insights into disease and therapeutics.

SOX7 regulates the expression of VE-cadherin in the haemogenic endothelium at the onset of haematopoietic development

At early stages of vertebrate ontogeny, blood and endothelial cells develop from a common mesodermal progenitor, the haemangioblast. Upon haematopoietic commitment, the haemangioblast generates blood precursors through populations of endothelial cells with haemogenic properties. Although several transcription factors have been implicated in haemangioblast differentiation, the precise mechanisms governing cell fate decisions towards the generation of haemogenic endothelium precursors remain largely unknown. Under defined conditions, embryonic stem (ES) cells can be differentiated into haemangioblast-like progenitors that faithfully recapitulate early embryonic haematopoiesis. Here, we made use of mouse ES cells as a model system to understand the role of SOX7, a member of a large family of transcription factors involved in a wide range of developmental processes. During haemangioblast differentiation, SOX7 is expressed in haemogenic endothelium cells and is downregulated in nascent blood precursors. Gain-of-function assays revealed that the enforced expression of Sox7 in haemangioblast-derived blast colonies blocks further differentiation and sustains the expression of endothelial markers. Thus, to explore the transcriptional activity of SOX7, we focused on the endothelial-specific adhesion molecule VE-cadherin. Similar to SOX7, VE-cadherin is expressed in haemogenic endothelium and is downregulated during blood cell formation. We show that SOX7 binds and activates the promoter of VE-cadherin, demonstrating that this gene is a novel downstream transcriptional target of SOX7. Altogether, our findings suggest that SOX7 is involved in the transcriptional regulation of genes expressed in the haemogenic endothelium and provide new clues to decipher the molecular pathways that drive early embryonic haematopoiesis.

Endothelial differentiation of human stem cells seeded onto electrospun polyhydroxybutyrate/polyhydroxybutyrate-co-hydroxyvalerate fiber mesh

Tissue engineering is based on the association of cultured cells with structural matrices and the incorporation of signaling molecules for inducing tissue regeneration. Despite its enormous potential, tissue engineering faces a major challenge concerning the maintenance of cell viability after the implantation of the constructs. The lack of a functional vasculature within the implant compromises the delivery of nutrients to and removal of metabolites from the cells, which can lead to implant failure. In this sense, our investigation aims to develop a new strategy for enhancing vascularization in tissue engineering constructs. This study's aim was to establish a culture of human adipose tissue-derived stem cells (hASCs) to evaluate the biocompatibility of electrospun fiber mesh made of polyhydroxybutyrate (PHB) and its copolymer poly-3-hydroxybutyrate-co-3-hydroxyvalerate (PHB-HV) and to promote the differentiation of hASCs into the endothelial lineage. Fiber mesh was produced by blending 30% PHB with 70% PHB-HV and its physical characterization was conducted using scanning electron microscopy analysis (SEM). Using electrospinning, fiber mesh was obtained with diameters ranging 300 nm to 1.3 µm. To assess the biological performance, hASCs were extracted, cultured, characterized by flow cytometry, expanded and seeded onto electrospun PHB/PHB-HV fiber mesh. Various aspects of the cells were analyzed in vitro using SEM, MTT assay and Calcein-AM staining. The in vitro evaluation demonstrated good adhesion and a normal morphology of the hASCs. After 7, 14 and 21 days of seeding hASCs onto electrospun PHB/PHB-HV fiber mesh, the cells remained viable and proliferative. Moreover, when cultured with endothelial differentiation medium (i.e., medium containing VEGF and bFGF), the hASCs expressed endothelial markers such as VE-Cadherin and the vWF factor. Therefore, the electrospun PHB/PHB-HV fiber mesh appears to be a suitable material that can be used in combination with endothelial-differentiated cells to improve vascularization in engineered bone tissues.

Identification of IGPR-1 as a novel adhesion molecule involved in angiogenesis

IGPR-1 is a novel adhesion molecule that regulates cell–cell interaction. IGPR-1 associates with several SH3-containing proteins, including SPIN90/WISH, and regulates capillary tube formation of primary endothelial cells.

Angiogenesis—the growth of new blood vessels from preexisting vessels—is an important physiological process and is considered to play a key role in tumor growth and metastasis. We identified the immunoglobulin-containing and proline-rich receptor-1 (IGPR-1, also called TMIGD2) gene as a novel cell adhesion receptor that is expressed in various human organs and tissues, mainly in cells with epithelium and endothelium origins. IGPR-1 regulates cellular morphology, homophilic cell aggregation, and cell–cell interaction. IGPR-1 activity also modulates actin stress fiber formation and focal adhesion and reduces cell migration. Silencing of expression of IGPR-1 by small interfering RNA (siRNA) and by ectopic overexpression in endothelial cells showed that IGPR-1 regulates capillary tube formation in vitro, and B16F melanoma cells engineered to express IGPR-1 displayed extensive angiogenesis in the mouse Matrigel angiogenesis model. Moreover, IGPR-1, through its proline-rich cytoplasmic domain, associates with multiple Src homology 3 (SH3)–containing signaling proteins, including SH3 protein interacting with Nck (SPIN90/WISH), bullous pemphigoid antigen-1, and calcium channel β2. Silencing of expression of SPIN90/WISH by siRNA in endothelial cells showed that SPIN90/WISH is required for capillary tube formation. These features of IGPR-1 suggest that IGPR-1 is a novel receptor that plays an important role in cell–cell interaction, cell migration, and angiogenesis.

microRNA-125b inhibits tube formation of blood vessels through translational suppression of VE-cadherin

Angiogenesis is controlled positively or negatively by extrinsic and intrinsic molecular cues in endothelial cells (ECs); in the tumor microenvironment, the action of positive regulators exceeds that of negative regulators. Thus, overinduction of negative regulators may inhibit tumor angiogenesis. MicroRNAs (miRNAs or miRs) are endogenous short noncoding RNAs regulating gene expression either through translational inhibition or destabilization of target mRNA. Here, we show that miR-125b expression is transiently induced in ECs on stimulation with vascular endothelial growth factor or by ischemia. miR-125b inhibits translation of vascular endothelial (VE)-cadherin mRNA and in vitro tube formation by ECs. Injection of miR-125b into the tumor inhibited VE-cadherin expression by ECs and induced nonfunctional blood vessel formation, resulting in inhibition of tumor growth. It has been suggested that pro-angiogenic signals in ECs also upregulate anti-angiogenic molecules simultaneously via negative feedback. Because miR-125b induction in ECs is transient after pro-angiogenic stimulation, prolonged overexpression of miR-125b could result in blood vessel regression. Thus, miR-125b may be useful in cancer therapy by causing the collapse of the lumen of ECs.

Discovery and characterization of novel microRNAs during endothelial differentiation of human embryonic stem cells

MicroRNAs (miRNAs) are small RNAs that participate in the regulation of genes associated with the differentiation and proliferation. In this study, 5 novel miRNAs were identified from human mesenchymal stem cells and characterized using various analyses. To investigate the potential functions associated with the regulation of cell differentiation, the differences in miRNA expression were examined in undifferentiated and differentiated human embryonic stem (ES) cells using reverse transcription (RT)-PCR analysis. Specifically, 3 miRNAs exhibited decreased expression levels in human umbilical vein endothelial cells (HUVECs) and endothelial cells derived from human ES cells. Putative target genes related to differentiation or maturation of endothelial cells were predicted by seed sequences of 2 novel miRNAs and analyzed for their expression via miRNA-mediated regulation using a luciferase assay. In HUVECs, CDH5 gene expression was directly repressed by hsa-miR-6086. Similarly, hsa-miR-6087 significantly downregulated endoglin expression. Therefore, the roles of these 2 miRNAs may be to directly suppress their target genes, popularly known as endothelial cell markers. Taken together, our results demonstrate that several novel miRNAs perform critical roles in human endothelial cell development.

Adherens junctions in mammalian development, homeostasis and disease: lessons from mice

Mice have proven to be a particularly powerful model to study molecular mechanisms of development and disease. The reason for this is the close evolutionary relationship between rodents and humans, similarities in physiological mechanisms in mice and human, and the large number of techniques available to study gene functions in mice. A large number of mice mutations, either germ line, conditional or inducible, have been generated in the past years for adherens junctions components, and the number is still increasing. In this review we will discuss mice models that have contributed to understanding the developmental and physiological role of adherens junctions and their components in mammals and have revealed novel mechanistic aspects of how adherens junctions regulate morphogenesis and tissue homeostasis.

Protocadherin-12 deficiency leads to modifications in the structure and function of arteries in mice

We studied the role of protocadherin-12 on arterial function. This protein belongs to the cadherin superfamily and is located at the intercellular junctions of endothelial cells where it promotes homotypic cellular adhesion. We previously showed that mice deficient for PCDH12 exhibited developmental growth retardation owing to placenta defects without altering neither survival nor fertility. Here, we investigated the effects of PCDH12 deficiency on the structural, mechanical properties and functionality of arteries from adult mice. Histological studies of the PCDH12(-/-) mouse arteries have shown age-independent modifications such as ramifications of medial elastic lamellae, accompanied by the appearance of radial fibers linking together two successive concentric elastic lamellae. Mechanical studies also revealed some age-independent modifications in the PCDH12(-/-) mice arteries such as an increase in inner-diameter and circumferential mid-wall stress. Moreover, the PCDH12(-/-) mice exhibited a mild reduction of blood pressure, thus maintaining the inner-diameter close to its normal value and a normal circumferential wall stress for vascular cells. This is likely a compensation mechanism enabling normal blood flow in the arteries. The vascular phenotypic differences observed between PCDH12(-/-) and wild type mice arteries did not seem to be age-dependent, except for some results regarding the carotid artery: the reactivity to acetylcholine and the circumferential mid-wall stress decreased with ageing in the PCDH12(-/-) mice, as opposed to the increase observed in the wild types. In conclusion, deficiency in one specific interendothelial junction component leads to significant changes in the structure and function of the vascular wall. Possible explanations for the observed modifications are discussed.

Endogenous vascular endothelial growth factor-A (VEGF-A) maintains endothelial cell homeostasis by regulating VEGF receptor-2 transcription

Vascular endothelial growth factor A (VEGF-A) is one of the most important factors controlling angiogenesis. Although the functions of exogenous VEGF-A have been widely studied, the roles of endogenous VEGF-A remain unclear. Here we focused on the mechanistic functions of endogenous VEGF-A in endothelial cells. We found that it is complexed with VEGF receptor 2 (VEGFR-2) and maintains a basal expression level for VEGFR-2 and its downstream signaling activation. Endogenous VEGF-A also controls expression of key endothelial specific genes including VEGFR-2, Tie-2, and vascular endothelial cadherin. Of importance, endogenous VEGF-A differs from exogenous VEGF-A by regulating VEGFR-2 transcription through mediation of FoxC2 binding to the FOX:ETS motif, and the complex formed by endogenous VEGF-A with VEGFR-2 is localized within the EEA1 (early endosome antigen 1) endosomal compartment. Taken together, our results emphasize the importance of endogenous VEGF-A in endothelial cells by regulating key vascular proteins and maintaining the endothelial homeostasis.

Tubulogenesis during blood vessel formation

The ability to form and maintain a functional system of contiguous hollow tubes is a critical feature of vascular endothelial cells (ECs). Lumen formation, or tubulogenesis, occurs in blood vessels during both vasculogenesis and angiogenesis in the embryo. Formation of vascular lumens takes place prior to the establishment of blood flow and to vascular remodeling which results in a characteristic hierarchical vessel organization. While epithelial lumen formation has received intense attention in past decades, more recent work has only just begun to elucidate the mechanisms controlling the initiation and morphogenesis of endothelial lumens. Studies using in vitro and in vivo models, including zebrafish and mammals, are beginning to paint an emerging picture of how blood vessels establish their characteristic morphology and become patent. In this article, we review and discuss the molecular and cellular mechanisms driving the formation of vascular tubes, primarily in vivo, and we compare and contrast proposed models for blood vessel lumen formation.

Fusobacterium nucleatum adhesin FadA binds vascular endothelial cadherin and alters endothelial integrity

Fusobacterium nucleatum is a Gram-negative oral anaerobe, capable of systemic dissemination causing infections and abscesses, often in mixed-species, at different body sites. We have shown previously that F. nucleatum adheres to and invades host epithelial and endothelial cells via a novel FadA adhesin. In this study, vascular endothelial (VE)-cadherin, a member of the cadherin family and a cell-cell junction molecule, was identified as the endothelial receptor for FadA, required for F. nucleatum binding to the cells. FadA colocalized with VE-cadherin on endothelial cells, causing relocation of VE-cadherin away from the cell-cell junctions. As a result, the endothelial permeability was increased, allowing the bacteria to cross the endothelium through loosened junctions. This crossing mechanism may explain why the organism is able to disseminate systemically to colonize in different body sites and even overcome the placental and blood-brain barriers. Co-incubation of F. nucleatum and Escherichia coli enhanced penetration of the endothelial cells by the latter in the transwell assays, suggesting F. nucleatum may serve as an 'enabler' for other microorganisms to spread systemically. This may explain why F. nucleatum is often found in mixed infections. This study reveals a possible novel dissemination mechanism utilized by pathogens.

Hemogenic endothelium: origins, regulation, and implications for vascular biology

The study of endothelial development has been intertwined with hematopoiesis since the early 20th century when a bi-potential cell (hemangioblast) was noted to produce both endothelial and hematopoietic cells. Since then, ideas regarding the nature of connection between the vascular and hematopoietic systems have ranged from a tenuous association to direct lineage origination. In this review, historical data that spans hematopoietic development is examined within the context of hemogenic endothelium. Hemogenic endothelium, a specialized endothelial population capable of hematopoiesis, is an emerging theory that has recently gained momentum. Evidence across species and decades are reviewed, as are the possible modulators of the phenomenon, which include pathways that specify definitive hematopoiesis (Runx1), arterial identity (Notch1), as well as physiological and developmental factors.

Angiopoietins-1 and -2 play opposing roles in endothelial sprouting of embryoid bodies in 3D culture and their receptor Tie-2 associates with the cell-cell adhesion molecule PECAM1

Angiopoietins 1 and 2, ligands for the receptor kinase Tie-2, have been proposed to play critical but opposing roles in vascular development. Since signaling by Tie-2 is likely affected by other endothelial cell receptors such as Flk-1, the receptor for VEGF, and cell-cell adhesion receptors PECAM1 and VE-cad, we explored their interactions in a 3D model of vasculogenesis. When murine embryoid bodies (EBs) were treated with VEGF in Matrigel in the presence or absence of Ang-1 or Ang-2 for eight days, Ang-1 abrogated vascular sprouting for treatments started at days 0 or 3. In contrast, Ang-2 greatly accelerated vascular sprouting compared to untreated EBs. These results were confirmed in a second model system where VEGF treated HUVECs were grown in Matrigel in the presence or absence of Ang-1 or Ang-2. Since vascular sprouting must be precisely controlled in the developing embryo, it is likely that cell-cell adhesion molecules play a role in sensing the density of vascular sprouts. In this respect, we have shown that PECAM1 and CEACAM1 play essential roles in vascular sprouting. We now show that PECAM1 is associated with Tie-2, becomes phosphorylated on its ITIMs, and recruits the inhibitory phosphatases SHP-1 and SHP-2. In addition, PECAM1 is associated with VE-cad and may similarly regulate its signaling via recruitment of SHP-1/2.

Endothelial progenitor cell biology in disease and tissue regeneration

Endothelial progenitor cells are increasingly being studied in various diseases ranging from ischemia, diabetic retinopathy, and in cancer. The discovery that these cells can be mobilized from their bone marrow niche to sites of inflammation and tumor to induce neovasculogenesis has afforded a novel opportunity to understand the tissue microenvironment and specific cell-cell interactive pathways. This review provides a comprehensive up-to-date understanding of the physiological function and therapeutic utility of these cells. The emphasis is on the systemic factors that modulate their differentiation/mobilization and survival and presents the challenges of its potential therapeutic clinical utility as a diagnostic and prognostic reagent.

An anteroposterior wave of vascular inhibitor downregulation signals aortae fusion along the embryonic midline axis

Paracrine signals, both positive and negative, regulate the positioning and remodeling of embryonic blood vessels. In the embryos of mammals and birds, the first major remodeling event is the fusion of bilateral dorsal aortae at the midline to form the dorsal aorta. Although the original bilaterality of the dorsal aortae occurs as the result of inhibitory factors (antagonists of BMP signaling) secreted from the midline by the notochord, it is unknown how fusion is later signaled. Here, we report that dorsal aortae fusion is tightly regulated by a change in signaling by the notochord along the anteroposterior axis. During aortae fusion, the notochord ceases to exert its negative influence on vessel formation. This is achieved by a transcriptional downregulation of negative regulators while positive regulators are maintained at pre-fusion levels. In particular, Chordin, the most abundant BMP antagonist expressed in the notochord prior to fusion, undergoes a dramatic downregulation in an anterior to posterior wave. With inhibitory signals diminished and sustained expression of the positive factors SHH and VEGF at the midline, fusion of the dorsal aortae is signaled. These results demonstrate a novel mechanism by which major modifications of the vascular pattern can occur through modulation of vascular inhibitors without changes in the levels of positive vascular regulators.

CCM1 regulates vascular-lumen organization by inducing endothelial polarity

Little is known about the molecular mechanisms that regulate the organization of vascular lumen. In this paper we show that lumen formation correlates with endothelial polarization. Adherens junctions (AJs) and VE-cadherin (VEC, encoded by CDH5) are required for endothelial apicobasal polarity in vitro and during embryonic development. Silencing of CDH5 gene expression leads to abrogation of endothelial polarity accompanied by strong alterations in lumenal structure. VEC co-distributes with members of the Par polarity complex (Par3 and PKCzeta) and is needed for activation of PKCzeta. CCM1 is encoded by the CCM1 gene, which is mutated in 60% of patients affected by cerebral cavernous malformation (CCM). The protein interacts with VEC and directs AJ organization and AJ association with the polarity complex, both in cell-culture models and in human CCM1 lesions. Both VEC and CCM1 control Rap1 concentration at cell-cell junctions. We propose that VEC, CCM1 and Rap1 form a signaling complex. In the absence of any of these proteins, AJs are dismantled, cell polarity is lost and vascular lumenal structure is severely altered.

Wilms' tumour protein Wt1 stimulates transcription of the gene encoding vascular endothelial cadherin

The Wilms' tumour gene, Wt1, encodes a zinc finger protein, which is mutated in a subset of paediatric renal carcinomas known as Wilms' tumours (nephroblastomas). Recent findings indicate that Wt1, beside its role in genitourinary development, is also necessary for normal vascularisation of the embryonic heart, and may even be involved in tumour angiogenesis. The original purpose of this study was to decipher potential downstream signalling pathways of Wt1 for blood vessel formation. We found that the Wt1(-KTS) protein, which functions as a transcription factor, stimulated the expression of cadherin 5 (CDH5, vascular endothelial (VE) cadherin) and other vascular genes, i.e. those encoding vascular endothelial growth factor receptors 1 and 2, and angiopoietin-2. Furthermore, an enhancer element was identified in the first intron of the CDH5 gene, which bound to the Wt1(-KTS) protein and was necessary for reporter gene activation by Wt1(-KTS) in transiently transfected cell lines. Wt1 and VE-cadherin proteins could be co-localised by double immunofluorescence staining in maturating glomeruli of embryonic murine kidneys. VE-cadherin transcripts were reduced in some but not all tissues of Wt1-deficient mouse embryos. These results indicate that Wt1 can stimulate vascular gene transcription. By demonstrating that Wt1(-KTS) protein trans-activates an enhancer element in the first intron we identified CDH5 as a novel target gene of Wt1. It is suggested that transcriptional activation of CDH5 by Wt1 fulfils regulatory functions during vascular development and kidney formation.

Moesin1 and Ve-cadherin are required in endothelial cells during in vivo tubulogenesis

Endothelial tubulogenesis is a crucial step in the formation of functional blood vessels during angiogenesis and vasculogenesis. Here, we use in vivo imaging of living zebrafish embryos expressing fluorescent fusion proteins of beta-Actin, alpha-Catenin, and the ERM family member Moesin1 (Moesin a), to define a novel cord hollowing process that occurs during the initial stages of tubulogenesis in intersegmental vessels (ISVs) in the embryo. We show that the primary lumen elongates along cell junctions between at least two endothelial cells during embryonic angiogenesis. Moesin1-EGFP is enriched around structures that resemble intracellular vacuoles, which fuse with the luminal membrane during expansion of the primary lumen. Analysis of silent heart mutant embryos shows that initial lumen formation in the ISVs is not dependent on blood flow; however, stabilization of a newly formed lumen is dependent upon blood flow. Zebrafish moesin1 knockdown and cell transplantation experiments demonstrate that Moesin1 is required in the endothelial cells of the ISVs for in vivo lumen formation. Our analyses suggest that Moesin1 contributes to the maintenance of apical/basal cell polarity of the ISVs as defined by adherens junctions. Knockdown of the adherens junction protein Ve-cadherin disrupts formation of the apical membrane and lumen in a cell-autonomous manner. We suggest that Ve-cadherin and Moesin1 function to establish and maintain apical/basal polarity during multicellular lumen formation in the ISVs.