The role of wnt signaling in physiological and pathological angiogenesis

ETS transcription factor ETV2 directly converts human fibroblasts into functional endothelial cells

Transplantation of endothelial cells (ECs) is a promising therapeutic approach for ischemic disorders. In addition, the generation of ECs has become increasingly important for providing vascular plexus to regenerated organs, such as the liver. Although many attempts have been made to generate ECs from pluripotent stem cells and nonvascular cells, the minimum number of transcription factors that specialize in directly inducing vascular ECs remains undefined. Here, by screening 18 transcription factors that are important for both endothelial and hematopoietic development, we demonstrate that ets variant 2 (ETV2) alone directly converts primary human adult skin fibroblasts into functional vascular endothelial cells (ETVECs). In coordination with endogenous FOXC2 in fibroblasts, transduced ETV2 elicits expression of multiple key endothelial development factors, including FLI1, ERG, and TAL1, and induces expression of endothelial functional molecules, including EGFL7 and von Willebrand factor. Consequently, ETVECs exhibits EC characteristics in vitro and forms mature functional vasculature in Matrigel plugs transplanted in NOD SCID mice. Furthermore, ETVECs significantly improve blood flow recovery in a hind limb ischemic model using BALB/c-nu mice. Our study indicates that the creation of ETVECs provides further understanding of human EC development induced by ETV2.

The contribution of CXCL12-expressing radial glia cells to neuro-vascular patterning during human cerebral cortex development

This study was conducted on human developing brain by laser confocal and transmission electron microscopy (TEM) to make a detailed analysis of important features of blood-brain barrier (BBB) microvessels and possible control mechanisms of vessel growth and differentiation during cerebral cortex vascularization. The BBB status of cortex microvessels was examined at a defined stage of cortex development, at the end of neuroblast waves of migration, and before cortex lamination, with BBB-endothelial cell markers, namely tight junction (TJ) proteins (occludin and claudin-5) and influx and efflux transporters (Glut-1 and P-glycoprotein), the latter supporting evidence for functional effectiveness of the fetal BBB. According to the well-known roles of astroglia cells on microvessel growth and differentiation, the early composition of astroglia/endothelial cell relationships was analyzed by detecting the appropriate astroglia, endothelial, and pericyte markers. GFAP, chemokine CXCL12, and connexin 43 (Cx43) were utilized as markers of radial glia cells, CD105 (endoglin) as a marker of angiogenically activated endothelial cells (ECs), and proteoglycan NG2 as a marker of immature pericytes. Immunolabeling for CXCL12 showed the highest level of the ligand in radial glial (RG) fibers in contact with the growing cortex microvessels. These specialized contacts, recognizable on both perforating radial vessels and growing collaterals, appeared as CXCL12-reactive en passant, symmetrical and asymmetrical, vessel-specific RG fiber swellings. At the highest confocal resolution, these RG varicosities showed a CXCL12-reactive dot-like content whose microvesicular nature was confirmed by ultrastructural observations. A further analysis of RG varicosities reveals colocalization of CXCL12 with Cx43, which is possibly implicated in vessel-specific chemokine signaling.

The role of intrinsic apoptotic signaling in hemorrhagic shock-induced microvascular endothelial cell barrier dysfunction

Hemorrhagic shock leads to endothelial cell barrier dysfunction resulting in microvascular hyperpermeability. Hemorrhagic shock-induced microvascular hyperpermeability is associated with worse clinical outcomes in patients with traumatic injuries. The results from our laboratory have illustrated a possible pathophysiological mechanism showing involvement of mitochondria-mediated "intrinsic" apoptotic signaling in regulating hemorrhagic shock-induced microvascular hyperpermeability. Hemorrhagic shock results in overexpression of Bcl-2 family of pro-apoptotic protein, BAK, in the microvascular endothelial cells. The increase in BAK initiates "intrinsic" apoptotic signaling cascade with the release of mitochondrial cytochrome c in the cytoplasm and activation of downstream effector caspase-3, leading to loss of endothelial cell barrier integrity. Thus, this review article offers a brief overview of important findings from our past and present research work along with new leads for future research. The summary of our research work will provide information leading to different avenues in developing novel strategies against microvascular hyperpermeability following hemorrhagic shock.

Endostar, a modified recombinant human endostatin, suppresses angiogenesis through inhibition of Wnt/β-catenin signaling pathway

Endostar, a novel modified recombinant human endostatin, is now widely studied for the treatment of diseases that are characterized or caused by pathological angiogenesis. However, its molecular mechanism remains unclear. In this study, we investigated the effects of Endostar on the Wnt/β-catenin signaling pathway, which has emerged as an important aspect of angiogenesis. We showed that Endostar significantly inhibited the proliferation, migration, invasion, and capillary-like tube formation of human umbilical vascular endothelial cells in a dose-dependent manner. Using a luciferase reporter assay, we also demonstrated that Endostar suppressed β-catenin-dependent T cell factor transcriptional activity in increasing doses. Moreover, we found that Endostar treatment also restricted the stabilized mutant β-catenin-mediated increase in transcriptional activity, suggesting that Endostar exerts its inhibitory influence on Wnt/β-catenin signaling by targeting β-catenin or its downstream molecules. Western blot and immunofluorescence results revealed that Endostar significantly decreased nuclear and total β-catenin levels. Finally, we discovered that Endostar down-regulated cyclin D1 and VEGF, two proteins that are known as the downstream targets of Wnt/β-catenin signaling and that also play important roles in angiogenesis. Our findings suggested that Endostar inhibits angiogenesis and that the downregulation of the Wnt/β-catenin signaling pathway may be involved in the inhibition of angiogenesis by Endostar. These results support the use of Endostar in further clinical applications.

Diegel C, Lang RA, Williams BO, Monga SP. β-catenin signaling in murine liver zonation and regeneration: a Wnt-Wnt situation!

Liver-specific β-catenin knockout (β-Catenin-LKO) mice have revealed an essential role of β-catenin in metabolic zonation where it regulates pericentral gene expression and in initiating liver regeneration (LR) after partial hepatectomy (PH), by regulating expression of Cyclin-D1. However, what regulates β-catenin activity in these events remains an enigma. Here we investigate to what extent β-catenin activation is Wnt-signaling-dependent and the potential cell source of Wnts. We studied liver-specific Lrp5/6 KO (Lrp-LKO) mice where Wnt-signaling was abolished in hepatocytes while the β-catenin gene remained intact. Intriguingly, like β-catenin-LKO mice, Lrp-LKO exhibited a defect in metabolic zonation observed as a lack of glutamine synthetase (GS), Cyp1a2, and Cyp2e1. Lrp-LKO also displayed a significant delay in initiation of LR due to the absence of β-catenin-TCF4 association and lack of Cyclin-D1. To address the source of Wnt proteins in liver, we investigated conditional Wntless (Wls) KO mice, which lacked the ability to secrete Wnts from either liver epithelial cells (Wls-LKO), or macrophages including Kupffer cells (Wls-MKO), or endothelial cells (Wls-EKO). While Wls-EKO was embryonic lethal precluding further analysis in adult hepatic homeostasis and growth, Wls-LKO and Wls-MKO were viable but did not show any defect in hepatic zonation. Wls-LKO showed normal initiation of LR; however, Wls-MKO showed a significant but temporal deficit in LR that was associated with decreased β-catenin-TCF4 association and diminished Cyclin-D1 expression.

CONCLUSION

Wnt-signaling is the major upstream effector of β-catenin activity in pericentral hepatocytes and during LR. Hepatocytes, cholangiocytes, or macrophages are not the source of Wnts in regulating hepatic zonation. However, Kupffer cells are a major contributing source of Wnt secretion necessary for β-catenin activation during LR.

WIF1 can effectively co-regulate pro-apoptotic activity through the combination with DKK1

Wnt inhibitory factor-1 (WIF1) is a conserved lipid-binding protein that interrupts Wnt ligands by interacting with their Frizzled receptors. Thus, they may suppress the activation of the Wnt/β-catenin triggered signaling cascade. Recently, we found that WIF1 can effectively co-regulate pro-apoptotic activity through the combination with Dickkopf-1 (DKK1). The tumor suppressor p53 protein expression was remarkably increased in the WIF1- and DKK1-transfected cells, along with p21. In contrast, expressions of the anti-apoptotic proteins, c-Myc and Bcl-2, were noticeably reduced. In addition, WIF1 and/or DKK1 significantly activated the transcription of p21 and p53, whereas c-Myc and Bcl-2 activities were remarkably reduced. The tumor suppressor WIF1 was also found to be capable of suppressing tumor growth through the inhibition of tumor angiogenesis in the cellular biological/physiological condition through the targeting of the PI3K/Akt/mTOR signaling pathway, while also being recognized as a Wnt antagonist factor in the Wnt cascade. Consistently, WIF1 conspicuously decreased the VEGF-induced phosphorylation of the PI3K/Akt signaling cascade components, including PDK1, mTOR, TSC-2, GSK-3β, and the p70S6K protein. Collectively, our results indicate for the first time that the tumor suppressor WIF1 is involved in angiogenesis and supplies a possible molecular target for the treatment of distinct malignant cancers, as well as several other associated diseases.

A novel signaling pathway regulates colon cancer angiogenesis through Norrin

Norrin binds to the frizzled-4 receptor, stimulating canonical Wnt signaling. We investigate here the role of colorectal cancer (CRC) produced Norrin in endothelial cell growth, motility, and blood vessel formation, as well as the expression of the Norrin signaling pathway components in the CRC tumor microenvironment. Norrin conditioned medium produced by CRC cell line CaCO2 transfected with Norrin expression construct increased endothelial cell motility. Blocking Norrin signaling reduced endothelial cell motility, branch point number (1/mm(2)), and the network length (mm/mm(2)) during in vitro angiogenesis. Colorectal tumors express Norrin protein. Endothelial cells in the colorectal tumor microenvironment contain all of the components of the Norrin signaling pathway needed to respond to Norrin protein. This study presents data that Norrin may play a role in the regulation of angiogenesis in the colorectal cancer tumor microenvironment.

Glycosylation pattern and axin expression in normal and IUGR placentae

OBJECTIVE

The aim of this study was to investigate the protein glycosylation pattern and AXIN1 protein expression in human placentae of normal pregnancies and compare them with placentae of pregnancies complicated with intrauterine growth restriction (IUGR).

METHODS

A total of 38 placentae (17 placentae of IUGR fetuses from singleton pregnancies and gestational age-matched 21 control placentae from normal singleton pregnancies) were collected from the Clinical Hospital Sveti Duh, Department of Gynecology and Obstetrics, Zagreb, Croatia. Gestational age was determined according to the last menstrual period (LMP) and by ultrasound measurements. Expression of glycoproteins was measured by Western blotting with SNA, UEA-I, PHA-E and DBA lectins as probes whereas expression of AXIN1 was determined by immunohistochemistry.

RESULTS

Comparison of detected sugars revealed differences in protein glycosylation between normal and IUGR placentae. Higher expression of AXIN1 protein located mostly in the cytoplasm of syncytiotrophoblast and to a lesser extent in its nuclei was found in IUGR placentae.

CONCLUSION

Results of our study suggest that changes in glycoprotein content may contribute to restricted placenta growth and development. Higher expression of AXIN1 protein in IUGR placentae indicates a role of Wnt/β-catenin signaling pathway in pathology of placental development.

Sox17 is indispensable for acquisition and maintenance of arterial identity

The functional diversity of the arterial and venous endothelia is regulated through a complex system of signalling pathways and downstream transcription factors. Here we report that the transcription factor Sox17, which is known as a regulator of endoderm and hemopoietic differentiation, is selectively expressed in arteries, and not in veins, in the mouse embryo and in mouse postnatal retina and adult. Endothelial cell-specific inactivation of Sox17 in the mouse embryo is accompanied by a lack of arterial differentiation and vascular remodelling that results in embryo death in utero. In mouse postnatal retina, abrogation of Sox17 expression in endothelial cells leads to strong vascular hypersprouting, loss of arterial identity and large arteriovenous malformations. Mechanistically, Sox17 acts upstream of the Notch system and downstream of the canonical Wnt system. These data introduce Sox17 as a component of the complex signalling network that orchestrates arterial/venous specification.

The transcription factor Sox17 is required for the development of the vasculature in vertebrates. Here Corada et al. show that Sox17 acts downstream of Wnt signalling and upstream of Notch signalling in the regulation of artery and vein differentiation in mice.

Frizzled7 controls vascular permeability through the Wnt-canonical pathway and cross-talk with endothelial cell junction complexes

AIMS

Vascular permeability is essential for the health of normal tissues and is an important characteristic of many disease states. The role of the Wnt/frizzled pathway in vascular biology has recently been reported. The objectives of this study are to analyse the role of Frizzled7 (Fzd7) receptor in the control of vascular integrity.

METHODS AND RESULTS

Fzd7 is expressed in endothelial cells and accumulates at the points of cell-cell contact in association with VE-cadherin and β-catenin, two major adherens junction molecules. To selectively delete fzd7 in the vasculature, we developed gene targeting approaches using CreLox strategy in mice. Genetic fzd7 inhibition in the endothelium increases vascular permeability in basal and factor-induced conditions. On the cellular level, fzd7 knockdown or depletion leads to an increase in paracellular permeability with a loss of adherens junction organization. These impairments are associated with a decrease in both VE-Cadherin and β-catenin expression, a decrease in their association and an increase of tyrosine phosphorylation of VE-cadherin/β-catenin. Fzd7 transduces a Wnt/β-catenin signalling cascade that is required to regulate β-catenin and canonical target gene expression. Finally, LiCl, a GSK3 inhibitor, and β-catenin overexpression rescued endothelial integrity and adherens junction organization, induced by fzd7 deletion.

CONCLUSION

These findings establish that Fzd7 is a new partner of adherens junctional complex and represents a novel molecular switch for the control of vascular permeability via activation of the Wnt-canonical pathway.

Cell-cell interaction in the heart via Wnt/β-catenin pathway after cardiac injury

The adult mammalian heart predominantly comprises myocytes, fibroblasts, endothelial cells, smooth muscle cells, and epicardial cells arranged in a precise three-dimensional framework. Following cardiac injury, the spatial arrangement of cells is disrupted as different populations of cells are recruited to the heart in a temporally regulated manner. The alteration of the cellular composition of the heart after cardiac injury thus enables different phenotypes of cells to interact with each other in a spatio-temporal-dependent manner. It can be argued that the integrated study of such cellular interactions rather than the examination of single populations of cells can provide more insights into the biology of cardiac repair especially at an organ-wide level. Many signalling systems undoubtedly mediate such cross talk between cells after cardiac injury. The Wnt/β-catenin system plays an important role during cardiac development and disease. Here, we describe how cell populations in the heart after cardiac injury mediate their interactions via the Wnt/β-catenin pathway, determine how such interactions can affect a cardiac repair response and finally suggest an integrated approach to study cardiac cellular interactions.

The Wnt/planar cell polarity signaling pathway contributes to the integrity of tight junctions in brain endothelial cells

Wnt morphogens released by neural precursor cells were recently reported to control blood-brain barrier (BBB) formation during development. Indeed, in mouse brain endothelial cells, activation of the Wnt/β-catenin signaling pathway, also known as the canonical Wnt pathway, was shown to stabilize endothelial tight junctions (TJs) through transcriptional regulation of the expression of TJ proteins. Because Wnt proteins activate several distinct β-catenin-dependent and independent signaling pathways, this study was designed to assess whether the noncanonical Wnt/Par/aPKC planar cell polarity (PCP) pathway might also control TJ integrity in brain endothelial cells. First we established, in the hCMEC/D3 human brain endothelial cell line, that the Par/aPKC PCP complex colocalizes with TJs and controls apicobasal polarization. Second, using an siRNA approach, we showed that the Par/aPKC PCP complex regulates TJ stability and reassembling after osmotic shock. Finally, we provided evidence that Wnt5a signals in hCMEC/D3 cells through activation of the Par/aPKC PCP complex, independently of the Wnt canonical β-catenin-dependent pathway and significantly contributes to TJ integrity and endothelial apicobasal polarity. In conclusion, this study suggests that the Wnt/Par/aPKC PCP pathway, in addition to the Wnt/β-catenin canonical pathway, is a key regulator of the BBB.

Elevated circulation levels of an antiangiogenic SERPIN in patients with diabetic microvascular complications impair wound healing through suppression of Wnt signaling

Wound healing, angiogenesis and hair follicle maintenance are often impaired in the skin of diabetic patients, but the pathogenesis has not been well understood. Here, we report that circulation levels of kallistatin, a member of the serine proteinase inhibitor (SERPIN) superfamily with anti-angiogenic activities, were elevated in Type 2 diabetic patients with diabetic vascular complications. To test the hypothesis that elevated kallistatin levels could contribute to a wound healing deficiency via inhibition of Wnt/β-catenin signaling, we generated kallistatin-transgenic (KS-TG) mice. KS-TG mice had reduced cutaneous hair follicle density, microvascular density, and panniculus adiposus layer thickness as well as altered skin microvascular hemodynamics and delayed cutaneous wound healing. Using Wnt reporter mice, our results showed that Wnt/β-catenin signaling is suppressed in dermal endothelium and hair follicles in KS-TG mice. Lithium, a known activator of β-catenin via inhibition of glycogen synthase kinase-3β, reversed the inhibition of Wnt/β-catenin signaling by kallistatin and rescued the wound healing deficiency in KS-TG mice. These observations suggest that elevated circulating anti-angiogenic serpins in diabetic patients may contribute to impaired wound healing through inhibition of Wnt/β-catenin signaling. Activation of Wnt/β-catenin signaling, at a level downstream of Wnt receptors, may ameliorate the wound healing deficiency in diabetic patients.

Functional analysis LRP6 novel mutations in patients with coronary artery disease

Background

Genetic architecture of coronary artery disease (CAD) is still to be defined. Since low density lipoprotein receptor-related protein 6 (LRP6) gene play critical roles in Wnt signal transduction which are important for vascular development and endodermis specification, we therefore resequenced it to search for mutations in CAD patients.

Methods

We systemically sequenced all the exons and promoter region of LRP6 gene in a sample of 380 early onset CAD patients and 380 control subjects in Chinese.

Results

In total, we identified 5 patient-specific mutations including K82N (two patients), S488Y (one patient), P1066T (two patients), P1206H (two patients) and I1264V (one patient) All these mutations located at the extracellular domain of LRP6 gene. In vitro functional analysis of patient-specific mutations demonstrated that these mutations resulted in a significant reduction in both protein level transporting to cell membrane and downstream Wnt signal activity. Furthermore, we found that LRP6 novel mutations attenuated proliferation and migration of human umbilical vein endothelial cells (HUVECs) when compared with wild type (WT) LRP6.

Conclusion

Our results demonstrated that these loss-of-function variants might contribute to disease liability in a subset of CAD and defects in Wnt signal activation might be important contributing factors for the onset of CAD.

Systems biology approach to identify alterations in the stem cell reservoir of subcutaneous adipose tissue in a rat model of diabetes: effects on differentiation potential and function

AIMS/HYPOTHESIS

Autologous progenitor cells represent a promising option for regenerative cell-based therapies. Nevertheless, it has been shown that ageing and cardiovascular risk factors such as diabetes affect circulating endothelial and bone marrow-derived progenitor cells, limiting their therapeutic potential. However, their impact on other stem cell populations remains unclear. We therefore investigated the effects of diabetes on adipose-derived stem cells (ASCs) and whether these effects might limit the therapeutic potential of autologous ASCs.

METHODS

A systems biology approach was used to analyse the expression of genes related to stem cell identification in subcutaneous adipose tissue (SAT), the stromal vascular fraction and isolated ASCs from Zucker diabetic fatty rats and their non-diabetic controls. An additional model of type 2 diabetes without obesity was also investigated. Bioinformatic approaches were used to investigate the biological significance of these changes. In addition, functional studies on cell viability and differentiation potential were performed.

RESULTS

Widespread downregulation of mesenchymal stem cell markers was observed in SAT of diabetic rats. Gene expression and in silico analysis revealed a significant effect on molecules involved in the maintenance of pluripotency and self-renewal, and on the alteration of main signalling pathways important for stem cell maintenance. The viability and differentiation potential of ASCs from diabetic rats was impaired in in vitro models and in in vivo angiogenesis.

CONCLUSIONS/INTERPRETATION

The impact of type 2 diabetes on ASCs might compromise the efficiency of spontaneous self-repair and direct autologous stem cell therapy.

Endothelial lineage differentiation from induced pluripotent stem cells is regulated by microRNA-21 and transforming growth factor β2 (TGF-β2) pathways

Finding a suitable cell source for endothelial cells (ECs) for cardiovascular regeneration is a challenging issue for regenerative medicine. In this paper, we describe a novel mechanism regulating induced pluripotent stem cells (iPSC) differentiation into ECs, with a particular focus on miRNAs and their targets. We first established a protocol using collagen IV and VEGF to drive the functional differentiation of iPSCs into ECs and compared the miRNA signature of differentiated and undifferentiated cells. Among the miRNAs overrepresented in differentiated cells, we focused on microRNA-21 (miR-21) and studied its role in iPSC differentiation. Overexpression of miR-21 in predifferentiated iPSCs induced EC marker up-regulation and in vitro and in vivo capillary formation; accordingly, inhibition of miR-21 produced the opposite effects. Importantly, miR-21 overexpression increased TGF-β2 mRNA and secreted protein level, consistent with the strong up-regulation of TGF-β2 during iPSC differentiation. Indeed, treatment of iPSCs with TGFβ-2 induced EC marker expression and in vitro tube formation. Inhibition of SMAD3, a downstream effector of TGFβ-2, strongly decreased VE-cadherin expression. Furthermore, TGFβ-2 neutralization and knockdown inhibited miR-21-induced EC marker expression. Finally, we confirmed the PTEN/Akt pathway as a direct target of miR-21, and we showed that PTEN knockdown is required for miR-21-mediated endothelial differentiation. In conclusion, we elucidated a novel signaling pathway that promotes the differentiation of iPSC into functional ECs suitable for regenerative medicine applications.

VE-cadherin and endothelial adherens junctions: active guardians of vascular integrity

VE-cadherin is a component of endothelial cell-to-cell adherens junctions, and it has a key role in the maintenance of vascular integrity. During embryo development, VE-cadherin is required for the organization of a stable vascular system, and in the adult it controls vascular permeability and inhibits unrestrained vascular growth. The mechanisms of action of VE-cadherin are complex and include reshaping and organization of the endothelial cell cytoskeleton and modulation of gene transcription. Here we review some of the most important pathways through which VE-cadherin modulates vascular homeostasis and discuss the emerging concepts in the overall biological role of this protein.

Regulation of angiogenesis via Notch signaling in breast cancer and cancer stem cells

Breast cancer angiogenesis is elicited and regulated by a number of factors including the Notch signaling. Notch receptors and ligands are expressed in breast cancer cells as well as in the stromal compartment and have been implicated in carcinogenesis. Signals exchanged between neighboring cells through the Notch pathway can amplify and consolidate molecular differences, which eventually dictate cell fates. Notch signaling and its crosstalk with many signaling pathways play an important role in breast cancer cell growth, migration, invasion, metastasis and angiogenesis, as well as cancer stem cell (CSC) self-renewal. Therefore, significant attention has been paid in recent years toward the development of clinically useful antagonists of Notch signaling. Better understanding of the structure, function and regulation of Notch intracellular signaling pathways, as well as its complex crosstalk with other oncogenic signals in breast cancer cells will be essential to ensure rational design and application of new combinatory therapeutic strategies. Novel opportunities have emerged from the discovery of Notch crosstalk with inflammatory and angiogenic cytokines and their links to CSCs. Combinatory treatments with drugs designed to prevent Notch oncogenic signal crosstalk may be advantageous over λ secretase inhibitors (GSIs) alone. In this review, we focus on the more recent advancements in our knowledge of aberrant Notch signaling contributing to breast cancer angiogenesis, as well as its crosstalk with other factors contributing to angiogenesis and CSCs.

Transcription profiles of endothelial cells in the rat ductus arteriosus during a perinatal period

Endothelial cells (ECs) lining the blood vessels serve a variety of functions and play a central role in the homeostasis of the circulatory system. Since the ductus arteriosus (DA) has different arterial characteristics from its connecting vessels, we hypothesized that ECs of the DA exhibited a unique gene profile involved in the regulation of DA-specific morphology and function. Using a fluorescence-activated cell sorter, we isolated ECs from pooled tissues from the DA or the descending aorta of Wistar rat fetuses at full-term of gestation (F group) or neonates 30 minutes after birth (N group). Using anti-CD31 and anti-CD45 antibodies as cell surface markers for ECs and hematopoietic derived cells, respectively, cDNAs from the CD31-positive and CD45-negative cells were hybridized to the Affymetrix GeneChip® Rat Gene 1.0 ST Array. Among 26,469 gene-level probe sets, 82 genes in the F group and 81 genes in the N group were expressed at higher levels in DA ECs than in aortic ECs (p<0.05, fold change>2.0). In addition to well-known endothelium-enriched genes such as Tgfb2 and Vegfa, novel DA endothelium-dominant genes including Slc38a1, Capn6, and Lrat were discovered. Enrichment analysis using GeneGo MetaCore software showed that DA endothelium-related biological processes were involved in morphogenesis and development. We identified many overlapping genes in each process including neural crest-related genes (Hoxa1, Hoxa4, and Hand2, etc) and the second heart field-related genes (Tbx1, Isl1, and Fgf10, etc). Moreover, we found that regulation of epithelial-to-mesenchymal transition, cell adhesion, and retinol metabolism are the active pathways involved in the network via potential interactions with many of the identified genes to form DA-specific endothelia. In conclusion, the present study uncovered several significant differences of the transcriptional profile between the DA and aortic ECs. Newly identified DA endothelium-dominant genes may play an important role in DA-specific functional and morphologic characteristics.

Building blood vessels in development and disease

PURPOSE OF REVIEW

This review will examine developmental angiogenesis and tumor-related changes to endothelial cells.

RECENT FINDINGS

Processes that govern developmental angiogenesis become dysfunctional in the tumor environment, leading to abnormal tumor endothelial cells and blood vessels. Recent findings suggest that tumor endothelial cells are permanently modified compared with normal counterparts.

SUMMARY

Coordination of numerous intracellular and extracellular programs promotes the formation of new blood vessels that are necessary for both development and certain diseases. Developmental angiogenesis uses canonical signaling modalities to effectively assemble endothelial cells into predictable vessel structures, and disruption of critical signaling factors has dramatic effects on blood vessel development. Solid tumors co-opt developmental cues to promote formation of tumor vessels that sustain their growth, but these angiogenic signals are not well regulated and produce endothelial cell dysfunction. Aberrant growth factor signaling contributes to phenotypic changes and acquired irreversible intracellular signaling, cytoskeletal and genetic modifications in endothelial cells of tumor vessels. Permanently altered tumor endothelial cells may represent a significant population.

The transcriptional co-repressor TLE3 regulates development of trophoblast giant cells lining maternal blood spaces in the mouse placenta

TLE3 is a transcriptional co-repressor that interacts with several DNA-binding repressors, including downstream effectors of the Notch signaling pathway. We generated Tle3-deficient mice and found that they die in utero and their death is associated with abnormal development of the placenta with major defects in the maternal vasculature. In the normal placenta, maternal blood spaces are lined, not as usual in the mammalian circulation by endothelial cells, but rather by specialized embryo-derived cells of the trophoblast cell lineage named trophoblast giant cells (TGC). Tle3 mRNA is expressed in those specialized TGC and Tle3 mutants show severe defects in differentiation of TGC-lined channels and lacunar spaces that take blood out of the labyrinth zone of the placenta and into the uterine veins. The mutants also show somewhat milder defects on the arterial-side of the maternal vascular circuit in spiral arteries and canals that take blood into the labyrinth. Notch2 and Tle3 expression patterns overlap in several TGC subtypes and we found that Tle3 and Notch2 mutants have some overlapping features. However, they also show differences implying that TLE3 may mediate some but not all of the effects of Notch2 signaling during placenta development. Therefore, formation of the different types of maternal blood spaces by different TGC subtypes is regulated by distinct molecular mechanisms.

Aberrant expression and regulation of NR2F2 and CTNNB1 in uterine fibroids

Uterine fibroids are the most common benign tumour afflicting women of reproductive age. Despite the large healthcare burden caused by fibroids, there is only limited understanding of the molecular mechanisms that drive fibroid pathophysiology. Although a large number of genes are differentially expressed in fibroids compared with myometrium, it is likely that most of these differences are a consequence of the fibroid presence and are not causal. The aim of this study was to investigate the expression and regulation of NR2F2 and CTNNB1 based on their potential causal role in uterine fibroid pathophysiology. We used real-time quantitative RT-PCR, western blotting and immunohistochemistry to describe the expression of NR2F2 and CTNNB1 in matched human uterine fibroid and myometrial tissues. Primary myometrial and fibroid smooth muscle cell cultures were treated with progesterone and/or retinoic acid (RA) and sonic hedgehog (SHH) conditioned media to investigate regulatory pathways for these proteins. We showed that NR2F2 and CTNNB1 are aberrantly expressed in fibroid tissue compared with matched myometrium, with strong blood vessel-specific localisation. Although the SHH pathway was shown to be active in myometrial and fibroid primary cultures, it did not regulateNR2F2orCTNNB1mRNA expression. However, progesterone and RA combined regulatedNR2F2mRNA, but notCTNNB1, in myometrial but not fibroid primary cultures. In conclusion, we demonstrate aberrant expression and regulation of NR2F2 and CTNNB1 in uterine fibroids compared with normal myometrium, consistent with the hypothesis that these factors may play a causal role uterine fibroid development.

Dkk1 and MSX2-Wnt7b signaling reciprocally regulate the endothelial-mesenchymal transition in aortic endothelial cells

OBJECTIVE

Endothelial cells (ECs) can undergo an endothelial-mesenchymal transition with tissue fibrosis. Wnt- and Msx2-regulated signals participate in arteriosclerotic fibrosis and calcification. We studied the impact of Wnt7, Msx2, and Dkk1, a Wnt7 antagonist, on endothelial-mesenchymal transition in primary aortic ECs.

APPROACH AND RESULTS

Transduction of aortic ECs with vectors expressing Dkk1 suppressed EC differentiation and induced a mineralizing myofibroblast phenotype. Dkk1 suppressed claudin 5, PECAM, cadherin 5 (Cdh5), Tie1, and Tie2. Dkk1 converted the cuboidal cell monolayer into a spindle-shaped multilayer and inhibited EC cord formation. Myofibroblast and osteogenic markers, SM22, type I collagen, Osx, Runx2, and alkaline phosphatase, were upregulated by Dkk1 via activin-like kinase/Smad pathways. Dkk1 increased fibrotic mineralization of aortic ECs cultured under osteogenic conditions--the opposite of mesenchymal cell responses. Msx2 and Wnt7b maintained morphology and upregulated markers of differentiated ECs. Deleting EC Wnt7b with the Cdh5-Cre transgene in Wnt7b(fl/fl);LDLR(-/-) mice upregulated aortic osteogenic genes (Osx, Sox9, Runx2, and Msx2) and nuclear phospho-Smad1/5, and increased collagen and calcium accumulation.

CONCLUSIONS

Dkk1 enhances endothelial-mesenchymal transition in aortic ECs, whereas Wnt7b and Msx2 signals preserve EC phenotype. EC responses to Dkk1, Wnt7b, and Msx2 are the opposite of mesenchymal responses, coupling EC phenotypic stability with osteofibrogenic predilection during arteriosclerosis.

Transdifferentiation of fast skeletal muscle into functional endothelium in vivo by transcription factor Etv2

Etv2, a master regulator of endothelial cell fate, can induce fast skeletal muscle cells to transdifferentiate into endothelial cells in the zebrafish embryo.

Etsrp/Etv2 (Etv2) is an evolutionarily conserved master regulator of vascular development in vertebrates. Etv2 deficiency prevents the proper specification of the endothelial cell lineage, while its overexpression causes expansion of the endothelial cell lineage in the early embryo or in embryonic stem cells. We hypothesized that Etv2 alone is capable of transdifferentiating later somatic cells into endothelial cells. Using heat shock inducible Etv2 transgenic zebrafish, we demonstrate that Etv2 expression alone is sufficient to transdifferentiate fast skeletal muscle cells into functional blood vessels. Following heat treatment, fast skeletal muscle cells turn on vascular genes and repress muscle genes. Time-lapse imaging clearly shows that muscle cells turn on vascular gene expression, undergo dramatic morphological changes, and integrate into the existing vascular network. Lineage tracing and immunostaining confirm that fast skeletal muscle cells are the source of these newly generated vessels. Microangiography and observed blood flow demonstrated that this new vasculature is capable of supporting circulation. Using pharmacological, transgenic, and morpholino approaches, we further establish that the canonical Wnt pathway is important for induction of the transdifferentiation process, whereas the VEGF pathway provides a maturation signal for the endothelial fate. Additionally, overexpression of Etv2 in mammalian myoblast cells, but not in other cell types examined, induced expression of vascular genes. We have demonstrated in zebrafish that expression of Etv2 alone is sufficient to transdifferentiate fast skeletal muscle into functional endothelial cells in vivo. Given the evolutionarily conserved function of this transcription factor and the responsiveness of mammalian myoblasts to Etv2, it is likely that mammalian muscle cells will respond similarly.

The endothelial cell is a specialized cell type that lines blood vessels. These cells are involved in normal cardiovascular function and become damaged in cardiovascular disease states such as atherosclerosis and stroke. We have discovered that developing muscle cells in the zebrafish embryo can be converted into endothelial cells by the expression of a transcription factor called Etv2. Etv2 normally functions during embryonic development to specify blood and blood vessels. When expressed in muscle cells, Etv2 induces the expression of genes that are normally expressed in endothelial cells; it also represses muscle gene expression. On expressing Etv2, muscle cells change shape and go on to form lumenized blood vessels that connect to the existing circulatory system and support blood flow. The Wnt and VEGF signaling pathways are required for this fate transformation. Our results suggest that muscle cells may be a viable source for the de novo generation of endothelial cells for use in transplantation therapies and they highlight signalling pathways that might be manipulated to improve the efficiency of this process in mammalian cells.

c-Cbl, a ubiquitin E3 ligase that targets active β-catenin: a novel layer of Wnt signaling regulation

Regulation of transcriptionally active nuclear β-catenin during the Wnt-on phase is crucial to ensure controlled induction of Wnt target genes. Several ubiquitin E3 ligases are known to regulate cytosolic β-catenin during the Wnt-off phase, but little is known about the fate of active nuclear β-catenin in the Wnt-on phase. We now describe ubiquitination of active β-catenin in the Wnt-on phase by a RING finger ubiquitin E3 ligase, Casitas B-lineage lymphoma (c-Cbl) in endothelial cells. c-Cbl binds preferentially to nuclearly active β-catenin in the Wnt-on phase via the armadillo repeat region. Wild-type c-Cbl suppresses and E3 ligase-deficient c-Cbl-70Z increases Wnt signaling. Wnt induces nuclear translocation of c-Cbl where it ubiquitinates nuclear β-catenin. Deletion of the c-Cbl UBA domain abrogates its dimerization, binding to β-catenin, Wnt-induced c-Cbl nuclear translocation, and ubiquitination of nuclear β-catenin. c-Cbl activity inhibits pro-angiogenic Wnt targets IL-8 and VEGF levels and angiogenesis in a β-catenin-dependent manner. This study defines for the first time c-Cbl as a ubiquitin E3 ligase that targets nuclearly active β-catenin in the Wnt-on phase and uncovers a novel layer of regulation of Wnt signaling.

Using the past to inform the future: anti-VEGF therapy as a road map to develop novel therapies for diabetic retinopathy

Therapies targeting vascular endothelial growth factor (VEGF) are revolutionizing the treatment of diabetic retinopathy (DR) and diabetic macular edema (DME). In August 2012, ranibizumab, a monoclonal antibody fragment targeting VEGF designed for ocular use, became the first and only U.S. Food and Drug Administration-approved medical therapy for DME and the first approved treatment in over 25 years. This approval was based on strong preclinical data followed by numerous clinical trials that demonstrate an essential role of VEGF in vascular permeability and angiogenesis in both normal physiology and disease pathology. In this Perspective, we will examine the experimental studies and scientific data that aided in the success of the development of therapies targeting VEGF and consider how these approaches may inform the development of future therapeutics for diabetic eye disease. A multipoint model is proposed, based on well-established drug development principles, with the goal of improving the success of clinical drug development. This model suggests that to provide a validated preclinical target, investigators should demonstrate the following: the role of the target in normal physiology, a causal link to disease pathogenesis, correlation to human disease, and the ability to elicit clinically relevant improvements of disease phenotypes in animal models with multiple, chemically diverse interventions. This model will provide a framework to validate the current preclinical targets and identify novel targets to improve drug development success for DR.

Inhibition of LRP5/6-mediated Wnt/β-catenin signaling by Mesd attenuates hyperoxia-induced pulmonary hypertension in neonatal rats

BACKGROUND

Hyperoxia-induced neonatal lung injury is associated with activation of Wnt/β-catenin signaling. Low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) are Wnt coreceptors that bind to Wnt ligands and mediate canonical Wnt/β-catenin signaling. We hypothesized that inhibition of LRP5/6 by their universal inhibitor, Mesd, would attenuate hyperoxia-induced lung injury.

METHODS

Newborn rat pups were randomly exposed to normoxia or hyperoxia at 90% FiO2 and injected intraperitoneally with placebo or Mesd every other day for 14 d. On day 15, phosphorylation of LRP5/6 (pLRP5/6), expression of Wnt/β-catenin target genes, cyclin D1 and Wnt-induced signaling protein-1 (WISP-1), right-ventricular systolic pressure (RVSP), right-ventricular hypertrophy (RVH), pulmonary vascular remodeling, alveolarization, and vascularization were measured.

RESULTS

Hyperoxia exposure markedly induced pLRP5/6, cyclin D1, and WISP-1 expression in the lungs of placebo animals, but they were significantly attenuated by the administration of Mesd. Mesd also significantly attenuated hyperoxia-induced pulmonary hypertension (PH) and pulmonary vascular remodeling. However, there was no effect on alveolarization or vascularization after Mesd administration.

CONCLUSION

This study demonstrates that LRP5/6 mediates pulmonary vascular remodeling and PH in hyperoxia-induced neonatal lung injury, thereby suggesting a potential therapeutic target to alleviate PH in neonates with severe bronchopulmonary dysplasia.

Dysfunctional resident lung mesenchymal stem cells contribute to pulmonary microvascular remodeling

Pulmonary vascular remodeling and oxidative stress are common to many adult lung diseases. However, little is known about the relevance of lung mesenchymal stem cells (MSCs) in these processes. We tested the hypothesis that dysfunctional lung MSCs directly participate in remodeling of the microcirculation. We employed a genetic model to deplete extracellular superoxide dismutase (EC-SOD) in lung MSCs coupled with lineage tracing analysis. We crossed ^floxpsod3 and mT/mG reporter mice to a strain expressing Cre recombinase under the control of the ABCG2 promoter. We demonstrated In vivo that depletion of EC-SOD in lung MSCs resulted in their contribution to microvascular remodeling in the smooth muscle actin positive layer. We further characterized lung MSCs to be multipotent vascular precursors, capable of myofibroblast, endothelial and pericyte differentiation in vitro. EC-SOD deficiency in cultured lung MSCs accelerated proliferation and apoptosis, restricted colony-forming ability, multilineage differentiation potential and promoted the transition to a contractile phenotype. Further studies correlated cell dysfunction to alterations in canonical Wnt/β-catenin signaling, which were more evident under conditions of oxidative stress. Our data establish that lung MSCs are a multipotent vascular precursor population, a population which has the capacity to participate in vascular remodeling and their function is likely regulated in part by the Wnt/β-catenin signaling pathway. These studies highlight an important role for microenviromental regulation of multipotent MSC function as well as their potential to contribute to tissue remodeling.

MicroRNA control of vascular endothelial growth factor signaling output during vascular development

The regulated response of endothelial cells to signals in their environment is not only critical for the de novo formation of primordial vascular networks during early development (ie, vasculogenesis), but is also required for the subsequent growth and remodeling of new blood vessels from preexisting ones (ie, angiogenesis). Vascular endothelial growth factors (Vegfs) and their endothelial cell-specific receptors play a crucial role in nearly all aspects of blood vessel growth. How the outputs from these pathways affect and coordinate endothelial behavior is an area of intense research. Recently, numerous studies have highlighted roles for microRNAs in modulating Vegf signaling output in several different contexts. In this review, we will provide an overview of how small RNAs regulate multiple aspects of the Vegf signaling pathway. In particular, we highlight areas where identification of microRNAs and their targets has provided new insight into the role of downstream effectors in modulating Vegf output during development. As Vegf plays a broad role in multiple aspects of endothelial biology and has become a target for therapeutic manipulation of pathological blood vessel growth, microRNAs that affect Vegf signaling output will undoubtedly be major targets of clinical value.

Wnt5a inhibits hypoxia-induced pulmonary arterial smooth muscle cell proliferation by downregulation of β-catenin

Chronic hypoxia-induced pulmonary arterial hypertension (HPH) is closely associated with profound vascular remodeling, especially pulmonary arterial medial hypertrophy and muscularization due to hyperplasia of pulmonary artery smooth muscle cells (PASMCs). Aberrant Wnt signaling has been associated with lung diseases, but its role in pulmonary hypertension is unclear. This study evaluated the effect of Wnt5a on hypoxia-induced proliferation of human PASMCs and its possible mechanism. The results show that hypoxia (3% O2, 48 h) induced proliferation of human PASMCs, accompanied with a significant decrease in Wnt5a gene expression, increase in β-catenin and Cyclin D1 expression, as well as β-catenin nuclear translocation. Treatment with recombinant mouse Wnt5a significantly inhibited hypoxia-induced proliferation of human PASMCs, upregulation of Cyclin D1 and β-catenin expression, as well as the nuclear translocation of β-catenin. These effects were inhibited by Wnt5a antibody. Knocking down β-catenin or Cyclin D1 gene expression inhibited hypoxia-induced human PASMC proliferation, whereas overexpression of β-catenin increased hypoxia-induced human PASMC proliferation and counteracted the inhibitory effect of Wnt5a. These results suggest that Wnt5a has an antiproliferative effect on hypoxia-induced human PASMC proliferation by downregulation of β-catenin and its target gene Cyclin D1. Hypoxia-induced downregulation of Wnt5a may be a way to facilitate hypoxia-induced human PASMC proliferation. The results of this study will help to understand the novel strategies for PH treatment involving Wnt signaling.

Molecular parallels between neural and vascular development

The human central nervous system (CNS) features a network of ~400 miles of blood vessels that receives >20% of the body's cardiac output and uses most of its blood glucose. Many human diseases, including stroke, retinopathy, and cancer, are associated with the biology of CNS blood vessels. These vessels originate from extrinsic cell populations, including endothelial cells and pericytes that colonize the CNS and interact with glia and neurons to establish the blood-brain barrier and control cerebrovascular exchanges. Neurovascular interactions also play important roles in adult neurogenic niches, which harbor a unique population of neural stem cells that are intimately associated with blood vessels. We here review the cellular and molecular mechanisms required to establish the CNS vascular network, with a special focus on neurovascular interactions and the functions of vascular endothelial growth factors.

The role of VE-cadherin in vascular morphogenesis and permeability control

VE-cadherin is an endothelial-specific cadherin that is essential for the formation and regulation of endothelial cell junctions. The adhesive function and expression levels of VE-cadherin at endothelial contacts are central determinants of the control of vascular permeability and leukocyte recruitment into tissue. In addition to controlling junctional integrity, VE-cadherin modulates a multitude of signaling processes that influence the behavior of endothelial cells, such as proliferation, survival, migration, polarity, expression of other junctional components, and tube and lumen formation of blood vessels. This chapter highlights recent progress in understanding how VE-cadherin modulates these various cellular processes. In addition, the current knowledge about how VE-cadhern participates in the regulation of the endothelial barrier in the adult organism is discussed.

Circulation Research Most Read: 2010–2011

The following represent a selection of the most read Circulation Research articles published between January 2010 and December 2011, presented in reverse order of publication. Articles were selected based on the number of Full Text/PDF downloads, adjusted to compensate for differences in the length of time articles have been available online.

As stated in the past, our motivation in compiling such lists of most read articles is multifarious. By highlighting these articles, we wish to direct the attention of our readers to new information that may be of particular interest to a large fraction of the community of cardiovascular scholars. In addition, a synopsis of the most popular articles can be a useful indicator of burgeoning areas of research that are likely to dominate the landscape for years to come. This “honor roll” is also meant to acknowledge the outstanding work of the authors and their efforts in advancing the frontiers of cardiovascular science. Furthermore, we believe that the articles highlighted below represent paradigms of scientific excellence, particularly with respect to the three criteria that we value most at Circulation Research : conceptual and/or mechanistic novelty, scientific impact, and methodological rigor. Finally, we hope that this list will provide tangible evidence of the high (and rising) level of scientific excellence of the work published in Circulation Research .

Wnt signaling in cardiovascular physiology

Wnt signaling pathways play a key role in cardiac development, angiogenesis, and cardiac hypertrophy; emerging evidence suggests that they are also involved in the pathophysiology of atherosclerosis. Specifically, an important role for Wnts has been described in the regulation of endothelial inflammation, vascular calcification, and mesenchymal stem cell differentiation. Wnt signaling also induces monocyte adhesion to endothelial cells and is crucial for the regulation of vascular smooth-muscle cell (VSMC) behavior. We discuss how the Wnt pathways are implicated in vascular biology and outline the role of Wnt signaling in atherosclerosis. Dissecting Wnt pathways involved in atherogenesis and cardiovascular disease may provide crucial insights into novel mechanisms with therapeutic potential for atherosclerosis.

Endothelial cell-to-cell junctions: adhesion and signaling in physiology and pathology

Besides intercellular recognition and adhesion, which are primarily performed by the transmembrane components, many of the molecules associated in endothelial cell-to-cell junctions initiate or regulate signal transmission. Clustering of molecules at junctions has the consequence of allowing new local interactions to direct specific cellular responses with crucial effects on the physiology and pathology of the endothelium and, more generally, of the vascular system. The implication is that cell-to-cell junctions could be envisaged as molecular targets for different types of therapeutic intervention. These could be directed to "cure" the defects of endothelial junctions that accompany several pathologies or to reversibly open them in a controlled way for the efficient delivery of drugs to the tissues. These aims can become more and more approachable as the knowledge of the molecular organization and function of endothelial junctions increases and their organ and tissue specificities become understood.

Wnt activation by wild type and mutant myocilin in cultured human trabecular meshwork cells

BACKGROUND

Myocilin is a gene linked to the most prevalent form of glaucoma, a major blinding disease. The trabecular meshwork (TM), a specialized eye tissue, is believed to be involved, at least in part, in the development of glaucoma. The Pro³⁷⁰ to Leu (P370L) mutation of myocilin is associated with severe glaucoma phenotypes and Gln³⁶⁸ stop (Q368X) is the most common myocilin mutation reported. Myocilin, upon overexpression, has been shown to induce phenotypes that include a loss of actin stress fibers, an increase in the cAMP level and protein kinase A (PKA) activity, as well as a reduction in the RhoA activity. We examined herein whether Wnt signaling pathway is involved in the myocilin phenotypes and whether P370L and Q368X mutants also display biological effects similar to those of the wild type myocilin.

METHODOLOGY/PRINCIPAL FINDINGS

Wild type myocilin, when transfected into cultured human TM cells, induced a loss of actin stress fibers as judged by phalloidin staining. Such a loss was averted by treatment of secreted Frizzled-related protein 1 (sFRP1), an inhibitor of Wnt signaling. Consistent with the notion that Wnt pathway mediates the myocilin phenotype, Wnt activation was demonstrated by TOP/FOP-Flash reporter assays. Treatment of human TM cells of a Wnt activator, SB216763, as well as transfection of myocilin P370L and Q368X mutants all resulted in actin stress fiber loss, PKA activation and RhoA inactivation. The PKA elevation was obviated by the sFRP1 treatment, indicating that Wnt signaling was upstream that of PKA.

CONCLUSIONS/SIGNIFICANCE

The present study demonstrated that following forced expression of wild type myocilin, Wnt was activated, triggering in turn other myocilin-related alterations. P370L and Q368X mutations induced similar phenotypes, suggesting one possible mechanism how the mutants may lead to TM cell damage and pathology.

Macrophages and angiogenesis: a role for Wnt signaling

Macrophages regulate many developmental and pathological processes in both embryonic and adult tissues, and recent studies have shown a significant role in angiogenesis. Similarly, Wnt signaling is fundamental to tissue morphogenesis and also has a role in vascular development. In this review, we summarize recent advances in the field of macrophage-regulated angiogenesis, with a focus on the role of macrophage-derived Wnt ligands. We review data that provide both direct and indirect evidence for macrophage-derived Wnt regulation of physiologic and pathologic angiogenesis. Finally, we propose that Wnt signaling plays a central role in differentiation of tumor associated and wound infiltrating macrophages to a proangiogenic phenotype.

The abluminal endothelial membrane in neurovascular remodeling in health and disease

After brain injury, blood-brain barrier (BBB) integrity can be compromised as a consequence of the breakdown of cell-cell interactions in the neurovascular unit, resulting in the loss of the characteristic endothelial luminal-to-abluminal structure. During the process of restoration of the BBB and vascularization, the endothelial cells are continuously reshaped, with both the luminal and abluminal membranes serving as sites of signaling. Here, we focus on the bidirectional signaling processes that are rapidly initiated between endothelial and perivascular cells and occur in certain brain diseases or in response to injury. The goal of these processes is (i) the reemergence of endothelial cell polarity, (ii) the remodeling of extracellular matrix interactions, (iii) the realignment of pericytes and astrocytic endfeet with endothelial cells, and (iv) the restitution of a well-organized and stable BBB. This abluminal membrane exemplifies how the brain vasculature responds to stressors and may represent promising targets for therapeutic interventions of brain diseases.

Novel targets against retinal angiogenesis in diabetic retinopathy

Proliferative diabetic retinopathy (PDR), characterized by pathologic retinal angiogenesis, is a major cause of blindness in the USA and globally. Treatments targeting vascular endothelial growth factor (VEGF) have emerged as a beneficial part of the therapeutic armamentarium for this condition, highlighting the utility of identifying and targeting specific pathogenic molecules. There continues to be active research into the molecular players regulating retinal angiogenesis, including pro-angiogenic factors, anti-angiogenic factors, and integrins and matrix proteinases. New insights have been especially prominent regarding molecules which regulate specialized endothelial cells called tip cells, which play a lead role in endothelial sprouting. Together, these research efforts are uncovering new, important molecular regulators of retinal angiogenesis, which provide fertile areas for therapeutic exploration. This review discusses potential molecular targets, with an emphasis towards newer targets.

Vascular endothelial-cadherin and vascular stability

PURPOSE OF REVIEW

Vascular integrity is characterized by a tight control of permeability to cells and solutes and by resistance to blood flow. In several pathologies including tumor angiogenesis, vascular malformations, hemorrhagic stroke and others, there is the need to stabilize the vessels and prevent undesired bleeding or edema. Here, we discuss the current knowledge on the role of endothelial cell-to-cell adherens junctions in maintaining vascular integrity.

RECENT FINDINGS

The identification of several components of adherens junctions in endothelial cells helped understanding of the complex role of these structures not only in maintaining cell-to-cell adhesion but also in transferring intracellular signals. Vascular endothelial (VE)-cadherin, an endothelial-specific adhesion protein at adherens junctions, was found to interact with several signaling partners that induce contact inhibition of growth, decrease in permeability, tight junction organization and others. Changes in VE-cadherin levels in vivo may significantly affect vascular permeability, and induce uncontrolled growth and vascular fragility.

SUMMARY

In the past years, the research on angiogenesis was mostly directed to the definition of the mechanisms able to modulate vascular growth. We now understand that in many pathological conditions we do not simply need to increase or inhibit vascularization but we also need to develop tools able to stabilize organ perfusion and to avoid hemorrhages or edema.

The Wnt pathway and the roles for its antagonists, DKKS, in angiogenesis

The Wnt signaling pathway is involved in a wide range of developmental and physiological processes, such as cell fate specification, tissue morphogenesis, and homeostasis. Thus, its dysregulation has been found in multiple diseases, including some cardiovascular disorders. The loss or gain of function of Wnt pathway components results in abnormal vascular development and angiogenesis. Further study has revealed that Wnt signaling in endothelial cells appears to contribute to vascular morphogenesis and endothelial cell specification. Owing to the significance of Wnt signaling in angiogenesis, Wnt antagonists have been considered potential treatments for neovascular disorders. In line with this, members of the Dkk protein family (Dkks), well-known Wnt antagonists, have been recently found to regulate angiogenesis. This review summarizes our present knowledge of the roles of Wnt signaling and Wnt antagonists, particularly Dkks, in angiogenic regulation and explores the therapeutic potential of Wnt antagonists.