Embryonic development is disrupted by modest increases in vascular endothelial growth factor gene expression

Role of TIF1alpha as a modulator of embryonic transcription in the mouse zygote

The first events of the development of any embryo are under maternal control until the zygotic genome becomes activated. In the mouse embryo, the major wave of transcription activation occurs at the 2-cell stage, but transcription starts already at the zygote (1-cell) stage. Very little is known about the molecules involved in this process. We show that the transcription intermediary factor 1 α (TIF1α) is involved in modulating gene expression during the first wave of transcription activation. At the onset of genome activation, TIF1α translocates from the cytoplasm into the pronuclei to sites of active transcription. These sites are enriched with the chromatin remodelers BRG-1 and SNF2H. When we ablate TIF1α through either RNA interference (RNAi) or microinjection of specific antibodies into zygotes, most of the embryos arrest their development at the 2–4-cell stage transition. The ablation of TIF1α leads to mislocalization of RNA polymerase II and the chromatin remodelers SNF2H and BRG-1. Using a chromatin immunoprecipitation cloning approach, we identify genes that are regulated by TIF1α in the zygote and find that transcription of these genes is misregulated upon TIF1α ablation. We further show that the expression of some of these genes is dependent on SNF2H and that RNAi for SNF2H compromises development, suggesting that TIF1α mediates activation of gene expression in the zygote via SNF2H. These studies indicate that TIF1α is a factor that modulates the expression of a set of genes during the first wave of genome activation in the mouse embryo.

Depolymerized Hyaluronan Induces Vascular Endothelial Growth Factor, a Negative Regulator of Developmental Epithelial-to-Mesenchymal Transformation

Cardiac malformations constitute the most common birth defects, of which heart septal and valve defects are the most frequent forms diagnosed in infancy. These cardiac structures arise from the endocardial cushions through dynamic interactions between cells and the extracellular matrix (cardiac jelly). Targeted deletion of the hyaluronan synthase-2 ( Has2 ) gene in mice results in an absence of hyaluronan (HA), cardiac jelly, and endocardial cushions, a loss of vascular integrity, and death at embryonic day 9.5. Despite the requirements for Has2 and its product, HA, in the developing heart, little is known about the normal processing and removal of HA during development. Cell culture studies show that HA obtains new bioactivity after depolymerization into small oligosaccharides. We previously showed reduction in Has2 expression and diminished presence of HA at later stages of heart development as tissue remodeling formed the leaflets of the cardiac valves. Here we show that small oligosaccharide forms of HA (o-HA) act antagonistically to developmental epithelial-to-mesenchymal transformation (EMT), which is required to generate the progenitor cells that populate the endocardial cushions. We further show that o-HA induces vascular endothelial growth factor (VEGF), which acts as a negative regulator of EMT. This is the first report illustrating a functional link between oligosaccharide HA and VEGF. Collectively, our data indicate that following endocardial cell EMT, native HA is likely processed to o-HA, which stimulates VEGF activity to attenuate cardiac developmental EMT.

Human pulmonary valve progenitor cells exhibit endothelial/mesenchymal plasticity in response to vascular endothelial growth factor-A and transforming growth factor-beta2

In situ analysis of fetal semilunar valve leaflets has revealed cells coexpressing endothelial and mesenchymal markers along the endothelium, with diminished frequency seen in adult valves. To determine whether such cells are progenitor cells, we isolated clonal populations from human pulmonary valves. The clones expressed endothelial markers but showed potential to further differentiate into endothelium in response to vascular endothelial growth factor (VEGF)-A. When exposed to transforming growth factor (TGF)-beta2, individual clones adopted a mesenchymal phenotype to varying degrees and expressed markers of endothelial to mesenchymal transformation (EMT). Both VEGF- and TGFbeta2-induced phenotypic changes were partially reversible, indicating the plasticity of these cells. When challenged with VEGF or TGFbeta2, a hierarchy of endothelial/mesenchymal potential could be seen among the clonal populations: cells initially closer to an endothelial phenotype showed a strong response to TGFbeta2 that could be inhibited by VEGF, whereas cells closer to a mesenchymal phenotype responded to TGFbeta2 but were resistant to endothelial-inducing effects of VEGF. These findings suggest the presence of bipotential valve progenitor cells with ability to differentiate into either endothelial or interstitial cells of the valve leaflet. Understanding the differentiation potential and function of these cells may be important for understanding heart valve disease and may also be applied to current paradigms for creating tissue-engineered heart valves.

Partial rescue of defects in Cited2-deficient embryos by HIF-1alpha heterozygosity

Hypoxia inducible factor-1 (HIF-1) initiates key cellular and tissue responses to physiological and pathological hypoxia. Evidence from in vitro and structural analyses supports a critical role for Cited2 in down-regulating HIF-1-mediated transcription by competing for binding with oxygen-sensitive HIF-1alpha to transcriptional co-activators CBP/p300. We previously detected elevated expression of HIF-1 target genes in Cited2(-/-) embryonic hearts, indicating that Cited2 inhibits HIF-1 transactivation in vivo. In this study, we show for the first time that highly hypoxic cardiac regions in mouse embryos corresponded to the sites of defects in Cited2(-/-) embryos and that defects of the outflow tract, interventricular septum, cardiac vasculature, and hyposplenia were largely rescued by HIF-1alpha haploinsufficiency. The hypoxia of the outflow tract and interventricular septum peaked at E13.5 and dissipated by E15.5 in wild-type hearts, but persisted in E15.5 Cited2(-/-) hearts. The persistent hypoxia and abnormal vasculature in the myocardium of interventricular septum in E15.5 Cited2(-/-) hearts were rescued with decreased HIF-1alpha gene dosage. Accordingly, mRNA levels of HIF-1-responsive genes were reduced in Cited2(-/-) embryonic hearts by HIF-1alpha heterozygosity. These findings suggest that a precise level of HIF-1 transcriptional activity critical for normal development is triggered by differential hypoxia and regulated through feedback inhibition by Cited2.

Rosiglitazone antagonizes vascular endothelial growth factor signaling and nuclear factor of activated T cells activation in cardiac valve endothelium

Nuclear factor of activated T cells, Cytoplasmic 1 (NFATc1) is required for heart valve formation. Vascular endothelial growth factor (VEGF) signaling, mediated by NFATc1 activation, positively regulates growth of valvular endothelial cells. However, regulators of VEGF/NFATc1 signaling in valve endothelium are poorly understood. Peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits NFATc1 activity in T cells and cardiomyocytes, but it is not known if PPARgamma controls NFATc1 function in endothelial cells. The authors hypothesize PPARgamma antagonizes VEGF signaling in valve endothelium by inhibiting NFATc1. Endothelial cells isolated from human valve leaflet tissue were shown by immunocytochemistry to express the endothelial-specific markers von Willebrand factor (vWF) and platelet endothelial cell adhesion molecule (PECAM)-1. VEGF-induced proliferation and migration of human pulmonary valve endothelial cells (HPVECs) were inhibited by rosiglitazone (ROSI), a specific ligand of PPARgamma activation, suggesting that PPARgamma disrupts VEGF signaling in the valve endothelium. ROSI also antagonized VEGF-mediated NFATc1 nuclear translocation in HPVECs, suggesting that PPARgamma inhibits VEGF signaling of NFATc1 activation in the valve. The effect of ROSI on nonvalve human umbilical vein endothelial cells (HUVECs) was tested in parallel and a similar inhibition of NFATc1 activation was observed. These data provide the first demonstration that ROSI negatively regulates VEGF signaling in the valve endothelium by a mechanism involving NFATc1 activation and nuclear translocation.

Chimeric VEGF-E(NZ7)/PlGF promotes angiogenesis via VEGFR-2 without significant enhancement of vascular permeability and inflammation

OBJECTIVE

Vascular endothelial growth factor (VEGF) plays critical roles in the regulation of angiogenesis and lymphangiogenesis. However, tissue edema, hemorrhage, and inflammation occur when VEGF-A is used for angiogenic therapy. To design a novel angiogenic factor without severe side effects, we examined the biological function of chimeric VEGF-E(NZ7)/placental growth factor (PlGF), which is composed of Orf-Virus(NZ7)-derived VEGF-E(NZ7) and human PlGF1, in a transgenic (Tg) mouse model.

METHODS AND RESULTS

A strong angiogenic response was observed in both VEGF-E(NZ7)/PlGF and VEGF-A165 Tg mice. Notably, the vascular leakage of VEGF-E(NZ7)/PlGF-induced blood vessels was 4-fold lower than that of VEGF-A165-induced blood vessels. Furthermore, the monocyte/macrophage recruitment in the skin of VEGF-E(NZ7)/PlGF Tg mice was approximately 8-fold decreased compared with that of VEGF-A165 Tg mice. In addition, the lymphatic vessels in VEGF-E(NZ7)/PlGF Tg mice were structurally normal, whereas they were markedly dilated in VEGF-A165 Tg mice, possibly because of the high vascular leakage. Receptor binding assay demonstrated that VEGF-E(NZ7)/PlGF was the ligand only activating VEGF receptor (VEGFR)-2.

CONCLUSIONS

These results indicated that neither the hyperpermeability in response to simultaneous stimulation of VEGFR-1 and VEGFR-2 nor VEGFR-1-mediated severe inflammation was associated with VEGF-E(NZ7)/PlGF-induced angiogenesis. The unique receptor binding property may shed light on VEGF-E(NZ7)/PlGF as a novel candidate for therapeutic angiogenesis.

Endothelial-cardiomyocyte interactions in cardiac development and repair

Communication between endothelial cells and cardiomyocytes regulates not only early cardiac development but also adult cardiomyocyte function, including the contractile state. In the normal mammalian myocardium, each cardiomyocyte is surrounded by an intricate network of capillaries and is next to endothelial cells. Cardiomyocytes depend on endothelial cells not only for oxygenated blood supply but also for local protective signals that promote cardiomyocyte organization and survival. While endothelial cells direct cardiomyocytes, cardiomyocytes reciprocally secrete factors that impact endothelial cell function. Understanding how endothelial cells communicate with cardiomyocytes will be critical for cardiac regeneration, in which the ultimate goal is not simply to improve systolic function transiently but to establish new myocardium that is both structurally and functionally normal in the long term.

VEGF receptor signalling - in control of vascular function

Vascular endothelial growth-factor receptors (VEGFRs) regulate the cardiovascular system. VEGFR1 is required for the recruitment of haematopoietic precursors and migration of monocytes and macrophages, whereas VEGFR2 and VEGFR3 are essential for the functions of vascular endothelial and lymphendothelial cells, respectively. Recent insights have shed light onto VEGFR signal transduction and the interplay between different VEGFRs and VEGF co-receptors in development, adult physiology and disease.

Vascular endothelial growth factor and kinase domain region receptor are involved in both seminiferous cord formation and vascular development during testis morphogenesis in the rat

Morphological male sex determination is dependent on migration of endothelial and preperitubular cells from the adjacent mesonephros into the developing testis. Our hypothesis is that VEGFA and its receptor KDR are necessary for both testicular cord formation and neovascularization. The Vegfa gene has 8 exons with many splice variants. Vegfa120, Vegfa164, and Vegfa188 mRNA isoforms were detected on Embryonic Day (E) 13.5 (plug date=E0) in the rat. Vegfa120, Vegfa144, Vegfa164, Vegfa188, and Vegfa205 mRNA were detected at E18 and Postnatal Day 3 (P3). Kdr mRNA was present on E13.5, whereas Fms-like tyrosine kinase 1 receptor (Flt1) mRNA was not detected until E18. VEGFA protein was localized to Sertoli cells at cord formation and KDR to germ and interstitial cells. The VEGFA signaling inhibitors SU1498 (40 microM) and VEGFR-TKI (8 microM) inhibited cord formation in E13 testis cultures with 90% reduced vascular density (P<0.01) in VEGFR-TKI-treated organs. Furthermore, Je-11 (10 microM), an antagonist to VEGFA, also perturbed cord formation and inhibited vascular density by more than 50% (P<0.01). To determine signal transduction pathways involved in VEGFA's regulation of testis morphogenesis, E13 testis were treated with LY 294002 (15 microM), a phosphoinositide 3-kinase (PI3K) pathway inhibitor, resulting in inhibition of both vascular density (46%) and cord formation. Thus, we support our hypothesis and conclude that VEGFA, secreted by the Sertoli cell, is involved in both neovascularization and cord formation and potentially acts through the PI3K pathway during testis morphogenesis to elicit its effects.

Single-nucleotide polymorphisms of VEGF gene are associated with risk of congenital valvuloseptal heart defects

BACKGROUND

Disturbed vascular endothelial growth factor (VEGF) production during early heart morphogenesis causes endocardial cushion malformation, which results in congenital heart disease (CHD). We tested whether functional VEGF -460T/C and +405G/C polymorphisms that have an impact on VEGF levels were associated with CHD.

METHODS

Dried blood samples were collected from 102 CHD children and 112 healthy control neonates. Genotyping was done with polymerase chain reaction-restriction fragment length polymorphism (VEGF +405G/C) and real-time polymerase chain reaction methods (VEGF -460T/C).

RESULTS

VEGF -460C allele frequency was similar in control and CHD subjects. VEGF +405C allele was less prevalent in controls than in CHD subjects (0.21 vs 0.42, P < .001). Having VEGF +405C presented increased risk for CHD (odds ratio [OR] 1.72, 95% CI 1.32-2.26). VEGF -460CT/+405CC allele associations did not occur in controls but in CHD patients (0% vs 13%, OR 2.26, 95% CI 1.93-2.64), whereas -460CT/+405GG allele association was more prevalent in controls (32% vs 16%, OR 0.58, 95% CI 0.37-0.89).

CONCLUSIONS

VEGF gene and allele associations may be associated with increased risk of CHD.

Plasmin modulates vascular endothelial growth factor-A-mediated angiogenesis during wound repair

Plasmin-catalyzed cleavage of the vascular endothelial growth factor (VEGF)-A isoform VEGF165 results in loss of its carboxyl-terminal heparin-binding domain and significant loss in its bioactivity. Little is known about the in vivo significance of this process. To investigate the biological relevance of the protease sensitivity of VEGF165 in wound healing we assessed the activity of a VEGF165 mutant resistant to plasmin proteolysis (VEGF165(A111P)) in a genetic mouse model of impaired wound healing (db/db mouse). In the present study we demonstrate that in this mouse model plasmin activity is increased at the wound site. The stability of the mutant VEGF165 was substantially increased in wound tissue lysates in comparison to wild-type VEGF165, thus indicating a prolonged activity of the plasmin-resistant VEGF165 mutant. The db/db delayed healing phenotype could be reversed by topical application of wild-type VEGF165 or VEGF165(A111P). However, resistance of VEGF165 to plasmin cleavage resulted in the increased stability of vascular structures during the late phase of healing due to increased recruitment of perivascular cells and delayed and reduced endothelial cell apoptosis. Our data provide the first indication that plasmin-catalyzed cleavage regulates VEGF165-mediated angiogenesis in vivo. Inactivation of the plasmin cleavage site Arg110/Ala111 may preserve the biological function of VEGF165 in therapeutic angiogenesis under conditions in which proteases are highly active, such as wound repair and inflammation.

Elevated vascular endothelial cell growth factor affects mesocardial morphogenesis and inhibits normal heart bending

Signaling by means of vascular endothelial cell growth factor (VEGF) and its receptors (VEGFRs) is required for cardiovascular development. To examine how VEGF/VEGFR receptor signaling affects early endocardial cell behavior, embryonic quail hearts were subjected to elevated VEGF165 levels (five- to nine-somite stage). Primitive embryonic hearts microinjected with recombinant human (rh)VEGF165 exhibit several distinct malformations compared with hearts in untreated embryos: the endocardial tube is malformed with tortuous cords and folds surrounded by a diminished cardiac jelly space, and the lumens of affected hearts are conspicuously reduced. Furthermore, the embryonic heart fails to loop properly. Inhibition of bending is accompanied by an apparent failure of the dorsal mesocardium to atrophy--an event thought to be necessary for heart bending. Instead of atrophy, VEGF-treated mesocardia exhibit a marked increased in the number of resident endothelial cells. Collectively, the data suggest that the abnormally robust mesocardia in VEGF-treated hearts impede the mechanical deformation required for normal heart bending. We conclude that the excessive VEGF signaling culminates in a physical or biomechanical mechanism that acts over a wide, tissue-level, length scale to cause a severe developmental defect--failure of heart bending.

Experimental hypoxia and embryonic angiogenesis

We examined the role of hypoxia and HIF factors in embryonic angiogenesis and correlated the degree of hypoxia with the level of HIF and VEGF expression and blood vessel formation. Quail eggs were incubated in normoxic and hypoxic (16% O(2)) conditions. Tissue hypoxia marker, pimonidazol hydrochloride, was applied in vivo for 1 hr and detected in sections with Hypoxyprobe-1 Ab. VEGF and HIF expression was detected by in situ hybridization. HIF-1alpha protein was detected in sections and by Western blot. Endothelial cells were visualized with QH-1 antibody. Hypoxic regions were detected even in normoxic control embryos, mainly in brain, neural tube, branchial arches, limb primordia, and mesonephros. The expression patterns of HIF-1alpha and HIF-1beta factors followed, in general, the Hypoxyprobe-1 marked regions. HIF-2alpha was predominantly expressed in endothelial cells. Diffuse VEGF expression was detected in hypoxic areas of neural tube, myocardium, digestive tube, and most prominently in mesonephros. Growing capillaries were directed to areas of VEGF positivity. Hypoxic regions in hypoxic embryos were larger and stained more intensely. VEGF and HIF-1 factors were proportionately elevated in Hypoxyprobe-1 marked regions without being expressed at new sites and were followed by increased angiogenesis. Our results demonstrate that normal embryonic vascular development involves the HIF-VEGF regulatory cascade. Experimentally increasing the level of hypoxia to a moderate level resulted in over-expression of HIF-1 factors and VEGF followed by an increase in the density of developing vessels. These data indicate that embryonic angiogenesis is responsive to environmental oxygen tension and, therefore, is not entirely genetically controlled.

The roles of nitric oxide in murine cardiovascular development

Nitric oxide (NO) participates in a diverse array of biological functions in mammalian organ systems. Depending on the biochemical environment, the production of NO may result in cytoprotection or cytotoxicity. The paradoxical actions of NO arise from the complexities generated by the redox milieu, NO concentration/bioavailability, and tissue/cell context, which ultimately result in the wide range of regulatory roles observed. Additionally, in physiological versus pathological states, NO often displays diametrically opposing affects in several organ systems. Here, we will discuss the roles of NO during reproduction, organ system development, in particular, the cardiovascular system, and its potential implications in diabetes-induced fetal defects.

VEGF-A signaling through Flk-1 is a critical facilitator of early embryonic lung epithelial to endothelial crosstalk and branching morphogenesis

Vascular endothelial growth factor-A (VEGF-A) signaling directs both vasculogenesis and angiogenesis. However, the role of VEGF-A ligand signaling in the regulation of epithelial-mesenchymal interactions during early mouse lung morphogenesis remains incompletely characterized. Fetal liver kinase-1 (Flk-1) is a VEGF cognate receptor (VEGF-R2) expressed in the embryonic lung mesenchyme. VEGF-A, expressed in the epithelium, is a high affinity ligand for Flk-1. We have used both gain and loss of function approaches to investigate the role of this VEGF-A signaling pathway during lung morphogenesis. Herein, we demonstrate that exogenous VEGF 164, one of the 3 isoforms generated by alternative splicing of the Vegf-A gene, stimulates mouse embryonic lung branching morphogenesis in culture and increases the index of proliferation in both epithelium and mesenchyme. In addition, it induces differential gene and protein expression among several key lung morphogenetic genes, including up-regulation of BMP-4 and Sp-c expression as well as an increase in Flk-1-positive mesenchymal cells. Conversely, embryonic lung culture with an antisense oligodeoxynucleotide (ODN) to the Flk-1 receptor led to reduced epithelial branching, decreased epithelial and mesenchymal proliferation index as well as downregulating BMP-4 expression. These results demonstrate that the VEGF pathway is involved in driving epithelial to endothelial crosstalk in embryonic mouse lung morphogenesis.

The vascular endothelial growth factor (VEGF)/VEGF receptor system and its role under physiological and pathological conditions

The VEGF (vascular endothelial growth factor) family and its receptors are essential regulators of angiogenesis and vascular permeability. Currently, the VEGF family consists of VEGF-A, PlGF (placenta growth factor), VEGF-B, VEGF-C, VEGF-D, VEGF-E and snake venom VEGF. VEGF-A has at least nine subtypes due to the alternative splicing of a single gene. Although the VEGF165 isoform plays a central role in vascular development, recent studies have demonstrated that each VEGF isoform plays distinct roles in vascular patterning and arterial development. VEGF-A binds to and activates two tyrosine kinase receptors, VEGFR (VEGF receptor)-1 and VEGFR-2. VEGFR-2 mediates most of the endothelial growth and survival signals, but VEGFR-1-mediated signalling plays important roles in pathological conditions such as cancer, ischaemia and inflammation. In solid tumours, VEGF-A and its receptor are involved in carcinogenesis, invasion and distant metastasis as well as tumour angiogenesis. VEGF-A also has a neuroprotective effect on hypoxic motor neurons, and is a modifier of ALS (amyotrophic lateral sclerosis). Recent progress in the molecular and biological understanding of the VEGF/VEGFR system provides us with novel and promising therapeutic strategies and target proteins for overcoming a variety of diseases.

Understanding heart development and congenital heart defects through developmental biology: a segmental approach

ABSTRACT The heart is the first organ to form and function during development. In the pregastrula chick embryo, cells contributing to the heart are found in the postero-lateral epiblast. During the pregastrula stages, interaction between the posterior epiblast and hypoblast is required for the anterior lateral plate mesoderm (ALM) to form, from which the heart will later develop. This tissue interaction is replaced by an Activin-like signal in culture. During gastrulation, the ALM is committed to the heart lineage by endoderm-secreted BMP and subsequently differentiates into cardiomyocyte. The right and left precardiac mesoderms migrate toward the ventral midline to form the beating primitive heart tube. Then, the heart tube generates a right-side bend, and the d-loop and presumptive heart segments begin to appear segmentally: outflow tract (OT), right ventricle, left ventricle, atrioventricular (AV) canal, atrium and sinus venosus. T-box transcription factors are involved in the formation of the heart segments: Tbx5 identifies the left ventricle and Tbx20 the right ventricle. After the formation of the heart segments, endothelial cells in the OT and AV regions transform into mesenchyme and generate valvuloseptal endocardial cushion tissue. This phenomenon is called endocardial EMT (epithelial-mesenchymal transformation) and is regulated mainly by BMP and TGFbeta. Finally, heart septa that have developed in the OT, ventricle, AV canal and atrium come into alignment and fuse, resulting in the completion of the four-chambered heart. Altered development seen in the cardiogenetic process is involved in the pathogenesis of congenital heart defects. Therefore, understanding the molecular nature regulating the 'nodal point' during heart development is important in order to understand the etiology of congenital heart defects, as well as normal heart development.

Deficiency of neuropilin 2 suppresses VEGF-induced retinal neovascularization

Vascular endothelial growth factor (VEGF) plays a central role in the development of ocular neovascularization (NV) and is an excellent target for therapeutic intervention. VEGF acts through several receptors, including VEGF receptor 1, VEGF receptor 2, neuropilin-1 (Npn1), and Npn2, but the exact role of these receptors in the development of retinal NV is unknown. In this study, we investigated the expression of npn2 mRNA during new blood vessel growth in the retina and used npn2 knockout mice to assess the impact of deficiency of Npn2 on retinal NV. The level of npn2 mRNA in the retina increased during retinal vascular development, after exposure to hyperoxia, and after the onset of retinal ischemia. Immunohistochemistry showed colocalization of Npn2 with a vascular marker in retinal NV. Compared with littermate controls, mice deficient in Npn2 had significantly less ischemia-induced retinal NV and very little subretinal NV due to expression of a Vegf transgene. These data suggest that Npn2 facilitates VEGF-induced retinal NV and may constitute a useful target for therapeutic intervention in ocular diseases complicated by NV.

Critical role of microenvironmental factors in angiogenesis

Therapeutic angiogenesis, which entails the induction of new blood vessels by the delivery of angiogenic growth factors, is a highly attractive approach to the treatment of ischemic diseases. However, it is becoming increasingly clear that this is not easily achieved, as the effects of angiogenic growth factors can differ markedly depending on the timing of their expression, on the shape of the concentration gradients they form in vivo, and the inter-actions between endothelial cells and pericytes they induce. In fact, the same dose of vascular endothelial growth factor can induce stable, nonleaky, pericyte-covered normal capillaries or aberrant vascular structures that develop into hemangiomas. This difference in outcome can be due solely to the spatial characteristics of the delivery method. If delivery allows a homogeneous spatial distribution of VEGF in the microenvironment around each producing cell, angiogenesis can be therapeutic, whereas if the total dose is the average of diverse spatial levels, aberrant angiogenesis cannot be avoided. To achieve therapeutic angiogenesis, a means of regulating the microenvironmental levels of angiogenic factors will be critical to the generation of effective new treatment strategies.

Connexin43 deficiency causes dysregulation of coronary vasculogenesis

The connexin43 knockout (Cx43alpha1 KO) mouse dies at birth from outflow obstruction associated with infundibular pouches. To elucidate the origin of the infundibular pouches, we used microarray analysis to investigate gene expression changes in the pouch tissue. We found elevated expression of many genes encoding markers for vascular smooth muscle (VSM), endothelial cells, and fibroblasts, cell types that are epicardially derived and essential for coronary vasculogenesis. This was accompanied by increased expression of VEGF and genes in the TGFbeta and VEGF/Notch/Eph cell-signaling pathways known to regulate vasculogenesis/angiogenesis. Using immunohistochemistry and a VSM lacZ reporter gene, we confirmed an abundance of ectopic VSM and endothelial cells in the infundibular pouch and in some regions of the right ventricle forming secondary pouches. This was associated with distinct thinning of the compact myocardium. TUNEL labeling showed increased apoptosis in the pouch tissue, in agreement with the finding of altered expression of many apoptotic genes. Defects in vascular remodeling were indicated by a marked reduction in the branching complexity of the distal coronary arteries. In the near term KO mouse, we also observed a profusion of large coronary vascular plexuses subepicardially. This was associated with elevated epicardial expression of VEGF and abnormal epicardial cell morphology. Together, these observations indicate that dysregulated coronary vasculogenesis plays a pivotal role in formation of the infundibular pouches and suggests an essential role for Cx43alpha1 gap junctions in coronary vasculogenesis and vascular remodeling.

Stepwise commitment from embryonic stem to hematopoietic and endothelial cells

There is great excitement in generating different types of somatic cells from in vitro differentiated embryonic stem (ES) cells, because they can potentially be utilized for therapies for human diseases for which there are currently no effective treatments. Successful generation and application of ES-derived somatic cells requires better understanding of molecular mechanisms that regulate self-renewal and lineage commitment. Accordingly, many studies are aimed toward understanding mechanisms for maintaining the stem cell state and pathways leading to lineage specification. In this chapter we discuss recent studies that examine molecules that are critical for ES cell self-renewal, as well as hematopoietic and endothelial cell lineage differentiation from ES cells.

The PTEN/PI3K pathway governs normal vascular development and tumor angiogenesis

PTEN is an important tumor suppressor gene. Hereditary mutation of PTEN causes tumor-susceptibility diseases such as Cowden disease. We used the Cre-loxP system to generate an endothelial cell-specific mutation of Pten (Tie2CrePten) in mice. Tie2CrePten(flox/+) mice displayed enhanced tumorigenesis due to an increase in angiogenesis driven by vascular growth factors. This effect was partially dependent on the PI3K subunits p85alpha and p110gamma. In vitro, Tie2CrePten(flox/+) endothelial cells showed enhanced proliferation/migration. Tie2CrePten(flox/flox) mice died before embryonic day 11.5 (E11.5) due to bleeding and cardiac failure caused by impaired recruitment of pericytes and vascular smooth muscle cells to blood vessels, and of cardiomyocytes to the endocardium. These phenotypes depend strongly on p110gamma rather than on p85alpha and were associated with decreased expression of Ang-1, VCAM-1, connexin 40, and ephrinB2 but increased expression of Ang-2, VEGF-A, VEGFR1, and VEGFR2. Pten is thus indispensable for normal cardiovascular morphogenesis and post-natal angiogenesis, including tumor angiogenesis.

Processing of VEGF-A by matrix metalloproteinases regulates bioavailability and vascular patterning in tumors

Vascular endothelial growth factor (VEGF) is a critical mediator of blood vessel formation during development and in pathological conditions. In this study, we demonstrate that VEGF bioavailability is regulated extracellularly by matrix metalloproteinases (MMPs) through intramolecular processing. Specifically, we show that a subset of MMPs can cleave matrix-bound isoforms of VEGF, releasing soluble fragments. We have mapped the region of MMP processing, have generated recombinant forms that mimic MMP-cleaved and MMP-resistant VEGF, and have explored their biological impact in tumors. Although all forms induced similar VEGF receptor 2 phosphorylation levels, the angiogenic outcomes were distinct. MMP-cleaved VEGF promoted the capillary dilation of existent vessels but mediated a marginal neovascular response within the tumor. In contrast, MMP-resistant VEGF supported extensive growth of thin vessels with multiple and frequent branch points. Our findings support the view that matrix-bound VEGF and nontethered VEGF provide different signaling outcomes. These findings reveal a novel aspect in the regulation of extracellular VEGF that holds significance for vascular patterning.

Heart valve development: endothelial cell signaling and differentiation

During the past decade, single gene disruption in mice and large-scale mutagenesis screens in zebrafish have elucidated many fundamental genetic pathways that govern early heart patterning and differentiation. Specifically, a number of genes have been revealed serendipitously to play important and selective roles in cardiac valve development. These initially surprising results have now converged on a finite number of signaling pathways that regulate endothelial proliferation and differentiation in developing and postnatal heart valves. This review highlights the roles of the most well-established ligands and signaling pathways, including VEGF, NFATc1, Notch, Wnt/beta-catenin, BMP/TGF-beta, ErbB, and NF1/Ras. Based on the interactions among and relative timing of these pathways, a signaling network model for heart valve development is proposed.

Different effects of angiopoietin-2 in different vascular beds: new vessels are most sensitive

In this study, we used double transgenic mice with inducible expression of angiopoietin-2 (Ang2) to investigate the role of Ang2 in the retinal and choroidal circulations and in three models of ocular neovascularization (NV). Mice with induced expression of Ang2 ubiquitously, or specifically in the retina, survived and appeared grossly normal. They also had normal-appearing retinal and choroidal circulations, demonstrating that high levels of Ang2 did not induce regression of mature retinal or choroidal vessels. When Ang2 expression was induced soon after birth, there was increased density of the deep capillary bed on postnatal day (P) 11 that returned to normal by P18, the time that retinal vascular development is usually completed. In mice with ischemic retinopathy, induction of Ang2 during the ischemic period resulted in a significant increase in retinal NV, but induction of Ang2 at a later time point when ischemia (and vascular endothelial growth factor [VEGF]) was less, hastened regression of NV. In triple transgenic mice that coexpressed VEGF and Ang2, the increased expression of Ang2 inhibited VEGF-induced NV in the retina. Increased expression of Ang2 also resulted in regression of choroidal neovascularization. These data suggest that ocular neovascularization, but not mature retinal or choroidal vessels, is sensitive to Ang2; a high Ang2/VEGF ratio promotes regression, while high Ang2 in the setting of hypoxia and/or concomitantly high Ang2 and VEGF stimulate neovascularization.

The role of vascular endothelial growth factor in ocular health and disease

The leading causes of blindness are retinal and choroidal diseases manifesting abnormal vessel permeability and growth. Scientists have sought to understand the mechanisms underlying these pathologic conditions with the hope of developing directed and effective pharmacologic therapies. Research has yielded important new mechanistic data, making possible the development of new drugs. One of the most important targets to have emerged is a secreted protein named vascular endothelial growth factor. This review will summarize the current state of our knowledge regarding this growth factor and outline some important questions that remain to be answered.

ErbB2 overexpression in mammary cells upregulates VEGF through the core promoter

The angiogenic molecule, vascular endothelial growth factor (VEGF), is a critical regulator of normal and pathologic angiogenesis. ErbB2, an epidermal growth factor receptor family member whose overexpression in mammary tumors is correlated with poor patient prognosis, has been implicated as a positive modulator of VEGF expression. Mammary tumor cells overexpressing ErbB2 (NAFA cells) and a normal mouse mammary cell line (HC11) transfected with ErbB2 expression vectors were used to study the effects of ErbB2 overexpression on VEGF regulation. We found that ErbB2 overexpression led to an increase in endogenous VEGF mRNA as well as ErbB3 protein levels in HC11 cells. Additionally, we determined that ErbB2 overexpression-mediated upregulation of VEGF involves at least two distinct promoter elements, one previously identified as the hypoxia responsive element and the other the core promoter region (-161 to -51bp), which is specifically controlled via two adjacent SP1 binding sites (-80 to -60bp).

Angiogenesis--a self-adapting principle in hypoxia

Normal tissue function depends on adequate supply of oxygen through blood vessels. Reduced oxygen supply (hypoxia) induces a variety of specific adaptation mechanisms in mammals that occur at the cellular, local and systemic level. These mechanisms are in part governed by the activation of the hypoxia-inducible transcription factors HIF-1 and HIF-2. Prolyl and asparaginyl hydroxylases as recently characterized oxygen sensors allow the regulation of HIFs that in turn modulate expression of hypoxically regulated genes such as VEGF. VEGF plays a key role in the formation of a functional and integrated vascular network required during physiological processes such as embryogenesis or female reproductive cycle as well as during a variety of pathological processes such tumor growth, wound healing, retinopathy and ischemic diseases (myocardial infarction, cerebral ischemia). However, other angiogenic factors, such as angiopoietins, PDGF, ephrins and erythropoietin are additionally needed to enable the formation of a functional vascular network. Many of these factors are activated during hypoxia although no HIF binding sites have yet been identified in the regulatory sequences of theses genes. Hypoxia-induced gene products that result in new vessel growth may be part of a self-regulated physiological protection mechanism preventing cell injury, especially under conditions of chronically reduced blood blow (chronic ischemia).

Differential levels of tissue hypoxia in the developing chicken heart

Tissue hypoxia plays a critical role in normal development, including cardiogenesis. Previously, we showed that oxygen concentration, as assessed by the hypoxia indicator EF5, is lowest in the outflow tract (OFT) myocardium of the developing chicken heart and may be regulating events in OFT morphogenesis. In this study, we identified additional areas of the embryonic chicken heart that were intensely positive for EF5 within the myocardium in discrete regions of the atrial wall and the interventricular septum (IVS). The region of the IVS that is EF5-positive includes a portion of the developing central conduction system identified by HNK-1 co-immunostaining. The EF5 positive tissues were also specifically positive for nuclear-localized hypoxia inducible factor 1alpha (HIF-1alpha), the oxygen-sensitive component of the hypoxia inducible factor 1 (HIF-1) heterodimer. The pattern of the most intensely EF5-stained myocardial regions of the atria and IVS resemble the pattern of the major coronary vessels that form in later stages within or immediately adjacent to these particular regions. These vessels include the sinoatrial nodal artery that is a branch of the right coronary artery within the atrial wall and the anterior/posterior interventricular vessels of the IVS. These findings indicate that a portion of the developing central conduction system and the patterning of coronary vessels may be subject to a level of regulation that is dependent on differential oxygen concentration within cardiac tissues and subsequent HIF-1 regulation of gene expression.

Genetic heterogeneity of the vasculogenic phenotype parallels angiogenesis; Implications for cellular surrogate marker analysis of antiangiogenesis

Development of antiangiogenic therapies would be significantly facilitated by quantitative surrogate pharmacodynamic markers. Circulating peripheral blood endothelial cells (CECs) and/or their putative progenitor subset (CEPs) have been proposed but not yet fully validated for this purpose. Herein, we provide such validation by showing a striking correlation between highly genetically heterogeneous bFGF- or VEGF-induced angiogenesis and intrinsic CEC or CEP levels measured by flow cytometry, among eight different inbred mouse strains. Moreover, studies using genetically altered mice showed that levels of these cells are affected by regulators of angiogenesis, including VEGF, Tie-2, and thrombospondin-1. Finally, treatment with a targeted VEGFR-2 antibody caused a dose-dependent reduction in viable CEPs that precisely paralleled its previously and empirically determined antitumor activity.

VEGF trap as a novel antiangiogenic treatment currently in clinical trials for cancer and eye diseases, and VelociGene- based discovery of the next generation of angiogenesis targets

The concept that tumors can be controlled by directly targeting their vascular supply has finally come of age, because clinical trials using a humanized monoclonal antibody that blocks VEGF have demonstrated exciting efficacy in cancer patients, as well as in vascular eye diseases that can lead to blindness. However, data suggest that these current regimens may not provide complete VEGF inhibition and, thus, that the maximum therapeutic potential of VEGF blockade has not yet been achieved. We describe the status of a very potent and high-affinity VEGF blocker, termed the VEGF Trap, that may provide the opportunity to maximize the potential of VEGF blockade in cancer as well as in vascular eye diseases. We also describe use of the VEGF Trap as a research tool, when coupled to high-throughput mouse genetics approaches such as VelociGene that can be exploited in strategies to discover and validate the next generation of angiogenesis targets.

A vascular gene trap screen defines RasGRP3 as an angiogenesis-regulated gene required for the endothelial response to phorbol esters

We identified Ras guanine-releasing protein 3 (RasGRP3) as a guanine exchange factor expressed in blood vessels via an embryonic stem (ES) cell-based gene trap screen to identify novel vascular genes. RasGRP3 is expressed in embryonic blood vessels, down-regulated in mature adult vessels, and reexpressed in newly formed vessels during pregnancy and tumorigenesis. This expression pattern is consistent with an angiogenic function for RasGRP3. Although a loss-of-function mutation in RasGRP3 did not affect viability, RasGRP3 was up-regulated in response to vascular endothelial growth factor (VEGF) stimulation of human umbilical vein endothelial cells, placing RasGRP3 regulation downstream of VEGF signaling. Phorbol esters mimic the second messenger diacylglycerol (DAG) in activating both protein kinase C (PKC) and non-PKC phorbol ester receptors such as RasGRP3. ES cell-derived wild-type blood vessels exposed to phorbol myristate acetate (PMA) underwent extensive aberrant morphogenesis that resulted in the formation of large endothelial sheets rather than properly branched vessels. This response to PMA was completely dependent on the presence of RasGRP3, as mutant vessels were refractory to the treatment. Taken together, these findings show that endothelial RasGRP3 is up-regulated in response to VEGF stimulation and that RasGRP3 functions as an endothelial cell phorbol ester receptor in a pathway whose stimulation perturbs normal angiogenesis. This suggests that RasGRP3 activity may exacerbate vascular complications in diseases characterized by excess DAG, such as diabetes.

Angiogenic modulators in valve development and disease: does valvular disease recapitulate developmental signaling pathways?

PURPOSE OF REVIEW

Neovascularization is a recognized feature of many valvular diseases and is established by numerous angiogenic modulators. Less known is that angiogenic modulators are multifunctional and have additional roles in valve development and disease. Recent advancements in this area are described.

RECENT FINDINGS

Initiation of epithelial to mesenchymal transformation, a developmental induction that specifies primordial interstitial cells (mesenchymal cells), requires vascular endothelial growth factor A, which stimulates matrix metalloproteinase 2 production and the invasive migration of mesenchymal cells. Epithelial to mesenchymal transformation also requires the matrix component hyaluronan to facilitate signaling through ErbB2/ErbB3 receptors and then is terminated by an increase in vascular endothelial growth factor A expression. Fibroblast growth factor 4 has been implicated in stimulating the following stage of proliferative expansion. Subsequently, in the remodeling phase, heparin-binding epidermal growth factor-like growth factor limits mesenchymal cell proliferation by signaling through the EGFR/ErbB1 receptor. Many adult valvular lesions appear similar to the embryonic proliferative expansion phase as they exhibit accumulations of extracellular matrix and myofibroblasts (a mesenchyme-like interstitial cell). The origins of such lesions may involve transforming growth factor beta 1. Similar to epithelial to mesenchymal transformation, tumor growth factor beta1 can induce cultured valvular endothelial cells to transdifferentiate to a myofibroblast-like phenotype. This scenario may occur in carcinoid valve disease because serotonin can induce interstitial cell expression of tumor growth factor beta1. Additionally, prolonged tumor growth factor beta1 activity may predispose to calcific degeneration. Calcific leaflets also exhibit tenascin-C, which may facilitate inflammatory cell migration through upregulation of pro-matrix metalloproteinase 2.

SUMMARY

Numerous angiogenic modulators control multiple stages of valvulogenesis and in the context of adult valvular disease may recapitulate their embryonic roles. Thus, lessons learned from valvulogenesis may provide insights into the molecular basis of adult valvular disease.

Hypoxia and hypoxia-inducible factor-1 target genes in central nervous system radiation injury: a role for vascular endothelial growth factor

PURPOSE

Microvascular permeability changes and loss of blood-brain barrier integrity are important features of central nervous system (CNS) radiation injury. Expression of vascular endothelial growth factor (VEGF), an important determinant of microvascular permeability, was examined to assess its role in CNS radiation damage. Because hypoxia mediates VEGF up-regulation through hypoxia-inducible factor-1alpha (HIF1alpha) induction, we studied the relationships of hypoxia, HIF1alpha expression, and expression of VEGF in this damage pathway.

EXPERIMENTAL DESIGN

Expression of HIF1alpha, VEGF, and another hypoxia-responsive gene, glucose transporter-1, was assessed in the irradiated rat spinal cord using immunohistochemistry and in situ hybridization. Hypoxic areas were identified using the nitroimidazole 2-(2-nitro-1H-imidazole-L-yl)-N-(2,2,3,3,3,-pentafluoropropyl) acetamide. To determine the causal importance of VEGF expression in radiation myelopathy, we studied the response of transgenic mice with greater (VEGF-A(hi/+)), reduced (VEGF-A(lo/+)), and wild-type VEGF activity to thoracolumbar irradiation.

RESULTS

In rat spinal cord, the number of cells expressing HIF1alpha and VEGF increased rapidly from 16 to 20 weeks after radiation, before white matter necrosis and forelimb paralysis. A steep dose response was observed in expression of HIF1alpha and VEGF. HIF1alpha and VEGF expressing cells were identified as astrocytes. Hypoxia was present in regions where up-regulation of VEGF and glucose transporter-1 and increased permeability was observed. VEGF-A(lo/+) mice had a longer latency to development of hindlimb weakness and paralysis compared with wild-type or VEGF-A(hi/+) mice.

CONCLUSIONS

VEGF expression appears to play an important role in CNS radiation injury. This focuses attention on VEGF and other genes induced in response to hypoxia as targets for therapy to reduce or prevent CNS radiation damage.

A field of myocardial-endocardial NFAT signaling underlies heart valve morphogenesis

The delicate leaflets that make up vertebrate heart valves are essential for our moment-to-moment existence. Abnormalities of valve formation are the most common serious human congenital defect. Despite their importance, relatively little is known about valve development. We show that the initiation of heart valve morphogenesis in mice requires calcineurin/NFAT to repress VEGF expression in the myocardium underlying the site of prospective valve formation. This repression of VEGF at E9 is essential for endocardial cells to transform into mesenchymal cells. Later, at E11, a second wave of calcineurin/NFAT signaling is required in the endocardium, adjacent to the earlier myocardial site of NFAT action, to direct valvular elongation and refinement. Thus, NFAT signaling functions sequentially from myocardium to endocardium within a valvular morphogenetic field to initiate and perpetuate embryonic valve formation. This mechanism also operates in zebrafish, indicating a conserved role for calcineurin/NFAT signaling in vertebrate heart valve morphogenesis.

Hypoxia-responsive signaling regulates the apoptosis-dependent remodeling of the embryonic avian cardiac outflow tract

We proposed a model in which myocardial hypoxia triggers the apoptosis-dependent remodeling of the avian outflow tract (OFT) in the transition of the embryo to a dual circulation. In this study, we examined hypoxia-dependent signaling in cardiomyocyte apoptosis and outflow tract remodeling. The hypoxia-inducible transcription factor HIF-1alpha was specifically present in the nuclei of OFT cardiomyocytes from stages 25-32, the period of hypoxia-dependent OFT remodeling. HIF-1alpha expression was sensitive to changes in ambient oxygen concentrations, while its dimerization partner HIF-1beta was constitutively expressed. There was not a simple relationship between HIF-1alpha expression and apoptosis. Apoptotic cardiomyocytes were detected in HIF-1alpha-positive and -negative regions, and a hypoxic stimulus sufficient to induce nuclear accumulation of HIF-1alpha did not induce cardiomyocyte apoptosis. The hypoxia-dependent expression of the vascular endothelial growth factor receptor (VEGFR2) in the distal OFT myocardium may be protective as cardiomyocyte apoptosis in the early stages (25-30) of OFT remodeling was absent from this region. Furthermore, recombinant adenoviral-mediated expression of dominant negative Akt, an inhibitor of tyrosine kinase receptor signaling, augmented cardiomyocyte apoptosis in the OFT and constitutively active Akt suppressed it. Adenovirus-mediated forced expression of VEGF165 induced conotruncal malformation such as double outlet right ventricle (DORV) and ventricular septal defect (VSD), similar to defects observed when apoptosis-dependent remodeling of the OFT was specifically targeted. We conclude that normal developmental remodeling of the embryonic avian cardiac OFT involves hypoxia/HIF-1-dependent signaling and cardiomyocyte apoptosis. Autocrine signaling through VEGF/VEGFR2 and Akt provides survival signals for the hypoxic OFT cardiomyocytes, and regulated VEGF signaling is required for the normal development of the OFT.

Angiopoietin 1 expression levels in the myocardium direct coronary vessel development

Mutational studies in genetically engineered mice have shown that the angiopoietin/Tie2(Tek) signaling pathway is indispensable for vascular development. To further investigate the role of Angiopoietin 1 in heart development, we developed transgenic mice that express Angiopoietin 1 under control of doxycycline in cardiac myocytes. Ninety percent of all transgenic mice die between embryonic day 12.5 and 15.5. Beginning at embryonic day 12.5, transgenic mice exhibit dilated atria, a significant thinning of the myocardial wall, and eventual outflow tract collapse. In addition, hearts of the most severely affected transgenic embryos have no coronary arteries as a result of the defective development and maintenance of the epicardium. The subsequent lack of blood and nutrient delivery to the developing heart may account for decreases in N-cadherin expression and subsequent loss of cell-cell contact leading to cell death, and ultimately the collapse and hemorrhage of the heart. These results suggest a pivotal role for Angiopoietin 1 in cardiovascular development, specifically the development of the epicardium and coronary vasculature.

Sculpting Heart Valves with NFATc and VEGF

Heart valves are of vital importance for our moment-to-moment existence, but how they form remains a mystery. In this issue of Cell, Chang et al. reveal a novel role for calcineurin, NFATs, and VEGF in valve formation. Dynamic changes in NFAT/VEGF expression in regional myocardial and endocardial fields and developmental windows orchestrate this complex process.

Endothelial cells promote cardiac myocyte survival and spatial reorganization: implications for cardiac regeneration

BACKGROUND

Endothelial-cardiac myocyte (CM) interactions play a key role in regulating cardiac function, but the role of these interactions in CM survival is unknown. This study tested the hypothesis that endothelial cells (ECs) promote CM survival and enhance spatial organization in a 3-dimensional configuration.

METHODS AND RESULTS

Microvascular ECs and neonatal CMs were seeded on peptide hydrogels in 1 of 3 experimental configurations: CMs alone, CMs mixed with ECs (coculture), or CMs seeded on preformed EC networks (prevascularized). Capillary-like networks formed by ECs promoted marked CM reorganization along the EC structures, in contrast to limited organization of CMs cultured alone. The presence of ECs markedly inhibited CM apoptosis and necrosis at all time points. In addition, CMs on preformed EC networks resulted in significantly less CM apoptosis and necrosis compared with simultaneous EC-CM seeding (P<0.01, ANOVA). Furthermore, ECs promoted synchronized contraction of CMs as well as connexin 43 expression.

CONCLUSIONS

These results provide direct evidence for a novel role of endothelium in survival and organization of nearby CMs. Successful strategies for cardiac regeneration may therefore depend on establishing functional CM-endothelium interactions.

Vascular endothelial growth factor: basic science and clinical progress

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. Hypoxia has been shown to be a major inducer of VEGF gene transcription. The tyrosine kinases Flt-1 (VEGFR-1) and Flk-1/KDR (VEGFR-2) are high-affinity VEGF receptors. The role of VEGF in developmental angiogenesis is emphasized by the finding that loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF is critical also for reproductive and bone angiogenesis. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Anti-VEGF monoclonal antibodies and other VEGF inhibitors block the growth of several tumor cell lines in nude mice. Clinical trials with various VEGF inhibitors in a variety of malignancies are ongoing. Very recently, an anti-VEGF monoclonal antibody (bevacizumab; Avastin) has been approved by the Food and Drug Administration as a first-line treatment for metastatic colorectal cancer in combination with chemotherapy. Furthermore, VEGF is implicated in intraocular neovascularization associated with diabetic retinopathy and age-related macular degeneration.

PECAM-1: old friend, new partners

The maintenance of vascular function is of paramount importance to an organism's existence. PECAM-1 (CD31), first thought of as a marker for endothelia, has been shown to be an important scaffolding molecule involved in several signaling pathways. Recent studies have demonstrated an even wider range of functions for this versatile molecule including participation in maintenance of adherens junction integrity and permeability, organization of the intermediate filament cytoskeleton, regulation of catenin localization and transcriptional activities, participation in STAT isoform signaling, control of apoptotic events, and modulation of cardiac cushion development.

Vascular morphogenesis and the formation of vascular networks

A recent workshop organized by Luisa Iruela-Arispe and Brant Weinstein and sponsored by the North American Vascular Biology Organization (NAVBO) brought 200 developmental biologists together at the Asilomar Conference Center in Pacific Grove, California, to share some of their latest findings. This superb meeting synthesized data from a variety of model systems ranging from Urbilateria (a common ancestor to vertebrates and invertebrates), fruit flies, frogs, zebrafish, and mice to human genetic disorders. Participants enjoyed lively discussions on developmental vascular biology while experiencing the natural beauty of the Pacific Coast.

The vascular endothelial growth factor (VEGF) receptor Flt-1 (VEGFR-1) modulates Flk-1 (VEGFR-2) signaling during blood vessel formation

Mice lacking the vascular endothelial growth factor (VEGF) receptor flt-1 (VEGFR-1) die from vascular overgrowth, caused primarily by aberrant endothelial cell division (Kearney JB, Ambler CA, Monaco KA, Johnson N, Rapoport RG, Bautch VL: Vascular endothelial growth factor receptor Flt-1 negatively regulates developmental blood vessel formation by modulating endothelial cell division. Blood 2002, 99:2397-2407). Because a second high-affinity VEGF receptor, flk-1, produces a positive endothelial proliferation signal, it was logical to ask whether flt-1 affects developmental blood vessel formation by modulating signaling through flk-1. Differentiated embryonic stem cell cultures lacking flt-1 (flt-1-/-) had increased flk-1 tyrosine phosphorylation, indicating that flk-1 signaling is up-regulated in the mutant background. The selective flk-1 inhibitor SU5416 partially rescued the flt-1-/- mutant phenotype, and this rescue was accompanied by a decrease in the relative amount of flk-1 tyrosine phosphorylation. Thus reduced flk-1 signal transduction can partially compensate for the lack of flt-1. The flt-1-/- mutant phenotype was also partially rescued by Flt-1/Fc, a truncated flt-1 that binds and sequesters the VEGF ligand. Taken together, these data show that down-regulation of flk-1 signaling by two different strategies partially rescues the developmental vascular overgrowth seen in the absence of flt-1, and they support a model whereby flt-1 modulates the flk-1 signal at an early point in the pathway.

Vascular defects in gain-of-function fps/fes transgenic mice correlate with PDGF- and VEGF-induced activation of mutant Fps/Fes kinase in endothelial cells

BACKGROUND

Fps/Fes is a cytoplasmic tyrosine kinase that is abundantly expressed in the myeloid, endothelial, epithelial, neuronal and platelet lineages. Genetic manipulation in mice has uncovered potential roles for this kinase in hematopoiesis, innate immunity, inflammation and angiogenesis.

OBJECTIVE

We have utilized a genetic approach to explore the role of Fps/Fes in angiogenesis.

METHODS

A hypervascular line of mice generated by expression of a 'gain-of-function' human fps/fes transgene (fps(MF)) encoding a myristoylated variant of Fps (MFps) was used in these studies. The hypervascular phenotype of this line was extensively characterized by intravital microscopy and biochemical approaches.

RESULTS

fps(MF) mice exhibited 1.6-1.7-fold increases in vascularity which was attributable to increases in the number of secondary vessels. Vessels were larger, exhibited varicosities and disorganized patterning, and were found to have defects in histamine-induced permeability. Biochemical characterization of endothelial cell (EC) lines derived from fps(MF) mice revealed that MFps was hypersensitive to activation by vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF).

CONCLUSIONS

MFps mediates enhanced sensitization to VEGF and PDGF signaling in ECs. We propose that this hypersensitization contributes to excessive angiogenic signaling and that this underlies the observed hypervascular phenotype of fps(MF) mice. These phenotypes recapitulate important aspects of the vascular defects observed in both VEGF and angiopoietin-1 transgenic mice. The fps/fes proto-oncogene product therefore represents a novel player in the regulation of angiogenesis, and the fps(MF) line of mice constitutes a unique new murine model for the study of this process.

Calcineurin signaling and NFAT activation in cardiovascular and skeletal muscle development

Calcineurin signaling has been implicated in a broad spectrum of developmental processes in a variety of organ systems. Calcineurin is a calmodulin-dependent, calcium-activated protein phosphatase composed of catalytic and regulatory subunits. The serine/threonine-specific phosphatase functions within a signal transduction pathway that regulates gene expression and biological responses in many developmentally important cell types. Calcineurin signaling was first defined in T lymphocytes as a regulator of nuclear factor of activated T cells (NFAT) transcription factor nuclear translocation and activation. Recent studies have demonstrated the vital nature of calcium/calcineurin/NFAT signaling in cardiovascular and skeletal muscle development in vertebrates. Inhibition, mutation, or forced expression of calcineurin pathway genes result in defects or alterations in cardiomyocyte maturation, heart valve formation, vascular development, skeletal muscle differentiation and fiber-type switching, and cardiac and skeletal muscle hypertrophy. Conserved calcineurin genes are found in invertebrates such as Drosophila and Caenorhabditis elegans, and genetic studies have demonstrated specific myogenic functions for the phosphatase in their development. The ability to investigate calcineurin signaling pathways in vertebrates and model genetic organisms provides a great potential to more fully comprehend the functions of calcineurin and its interacting genes in heart, blood vessel, and muscle development.

The VEGF receptor flt-1 (VEGFR-1) is a positive modulator of vascular sprout formation and branching morphogenesis

Sprouting angiogenesis is critical to blood vessel formation, but the cellular and molecular controls of this process are poorly understood. We used time-lapse imaging of green fluorescent protein (GFP)-expressing vessels derived from stem cells to analyze dynamic aspects of vascular sprout formation and to determine how the vascular endothelial growth factor (VEGF) receptor flt-1 affects sprouting. Surprisingly, loss of flt-1 led to decreased sprout formation and migration, which resulted in reduced vascular branching. This phenotype was also seen in vivo, as flt-1(-/-) embryos had defective sprouting from the dorsal aorta. We previously showed that loss of flt-1 increases the rate of endothelial cell division. However, the timing of division versus morphogenetic effects suggested that these phenotypes were not causally linked, and in fact mitoses were prevalent in the sprout field of both wild-type and flt-1(-/-) mutant vessels. Rather, rescue of the branching defect by a soluble flt-1 (sflt-1) transgene supports a model whereby flt-1 normally positively regulates sprout formation by production of sflt-1, a soluble form of the receptor that antagonizes VEGF signaling. Thus precise levels of bioactive VEGF-A and perhaps spatial localization of the VEGF signal are likely modulated by flt-1 to ensure proper sprout formation during blood vessel formation.

Dual roles of Sema6D in cardiac morphogenesis through region-specific association of its receptor, Plexin-A1, with off-track and vascular endothelial growth factor receptor type 2

Semaphorins, originally identified as axon guidance facto s in the nervous system, play integral roles in organogenesis. Here, we demonstrate a critical involvement of Sema6D in cardiac morphogenesis. Ectopic expression of Sema6D o RNA interference against Sema6D induces expansion or narrowing of the ventricular chamber, respectively, during chick embryonic development. Sema6D also exerts region-specific activities on cardiac explants, a migration-promoting activity on outgrowing cells from the conotruncal segment, and a migration-inhibitory activity on those from the ventricle. Plexin-A1 mediates these activities as the major Sema6D-binding receptor. Plexin-A1 forms a receptor complex with vascular endothelial growth factor receptor type 2 in the conotruncal segment or with Off-track in the ventricle segment; these complexes are responsible for the effects of Sema6D on the respective regions. Thus, the differential association of Plexin-A1 with additional receptor components entitles Sema6D to exert distinct biological activities at adjacent regions. This is crucial for complex cardiac morphogenesis.

Microenvironmental VEGF concentration, not total dose, determines a threshold between normal and aberrant angiogenesis

Use of long-term constitutive expression of VEGF for therapeutic angiogenesis may be limited by the growth of abnormal blood vessels and hemangiomas. We investigated the relationship between VEGF dosage and the morphology and function of newly formed blood vessels by implanting retrovirally transduced myoblasts that constitutively express VEGF164 into muscles of adult mice. Reducing VEGF dosage by decreasing the total number of VEGF myoblasts implanted did not prevent vascular abnormalities. However, when clonal populations of myoblasts homogeneously expressing different levels of VEGF were implanted, a threshold between normal and aberrant angiogenesis was found. Clonal myoblasts that expressed low to medium levels of VEGF induced growth of stable, pericyte-coated capillaries of uniform size that were not leaky and became VEGF independent, as shown by treatment with the potent VEGF blocker VEGF-TrapR1R2. In contrast, clones that expressed high levels of VEGF induced hemangiomas. Remarkably, when different clonal populations were mixed, even a small proportion of cells with high production of VEGF was sufficient to cause hemangioma growth. These results show for the first time to our knowledge that the key determinant of whether VEGF-induced angiogenesis is normal or aberrant is the microenvironmental amount of growth factor secreted, rather than the overall dose. Long-term continuous delivery of VEGF, when maintained below a threshold microenvironmental level, can lead to normal angiogenesis without other exogenous growth factors.

Vasculogenesis and angiogenesis

Two distinct mechanisms, vasculogenesis and angiogenesis implement the formation of the vascular network in the embryo. Vasculogenesis gives rise to the heart and the first primitive vascular plexus inside the embryo and in its surrounding membranes, as the yolk sac circulation. Angiogenesis is responsible for the remodeling and expansion of this network. While vasculogenesis refers to in situ differentiation and growth of blood vessels from mesodermal derived hemangioblasts, angiogenesis comprises two different mechanisms: endothelial sprouting and intussusceptive microvascular growth (IMG). The sprouting process is based on endothelial cell migration, proliferation and tube formation. IMG divides existing vessel lumens by formation and insertion of tissue folds and columns of interstitial tissue into the vessel lumen. The latter are termed interstitial or intervascular tissue structures (ITSs) and tissue pillars or posts. Intussusception also includes the establishment of new vessels by in situ loop formation in the wall of large veins. The molecular regulation of these distinct mechanisms is discussed in respect to the most important positive regulators, VEGF and its receptors flk-1 (KDR) and flt-1, the Angiopoietin/tie system and the ephrin-B/EpH-B system. The cellular mechanisms and the molecular regulation of angiogenesis in the pathological state are summarized and the differences of physiological and pathological angiogenesis elaborated.