Embryonic angiogenesis factors

Endothelial Progenitor Cells in Moyamoya Disease: Current Situation and Controversial Issues

Due to the lack of animal models and difficulty in obtaining specimens, the study of pathogenesis of moyamoya disease (MMD) almost stagnated. In recent years, endothelial progenitor cells (EPCs) have attracted more and more attention in vascular diseases due to their important role in neovascularization. With the aid of paradigms and methods in cardiovascular diseases research, people began to explore the role of EPCs in the processing of MMD. In the past decade, studies have shown that abnormalities in cell amounts and functions of EPCs were closely related to the vascular pathological changes in MMD. However, the lack of consistent criteria, such as isolation, cultivation, and identification standards, is also blocking the way forward. The goal of this review is to provide an overview of the current situation and controversial issues relevant to studies about EPCs in the pathogenesis and etiology of MMD.

Neural crest cell-derived pericytes act as pro-angiogenic cells in human neocortex development and gliomas

Central nervous system diseases involving the parenchymal microvessels are frequently associated with a ‘microvasculopathy’, which includes different levels of neurovascular unit (NVU) dysfunction, including blood–brain barrier alterations. To contribute to the understanding of NVU responses to pathological noxae, we have focused on one of its cellular components, the microvascular pericytes, highlighting unique features of brain pericytes with the aid of the analyses carried out during vascularization of human developing neocortex and in human gliomas. Thanks to their position, centred within the endothelial/glial partition of the vessel basal lamina and therefore inserted between endothelial cells and the perivascular and vessel-associated components (astrocytes, oligodendrocyte precursor cells (OPCs)/NG2-glia, microglia, macrophages, nerve terminals), pericytes fulfil a central role within the microvessel NVU. Indeed, at this critical site, pericytes have a number of direct and extracellular matrix molecule- and soluble factor-mediated functions, displaying marked phenotypical and functional heterogeneity and carrying out multitasking services. This pericytes heterogeneity is primarily linked to their position in specific tissue and organ microenvironments and, most importantly, to their ontogeny. During ontogenesis, pericyte subtypes belong to two main embryonic germ layers, mesoderm and (neuro)ectoderm, and are therefore expected to be found in organs ontogenetically different, nonetheless, pericytes of different origin may converge and colonize neighbouring areas of the same organ/apparatus. Here, we provide a brief overview of the unusual roles played by forebrain pericytes in the processes of angiogenesis and barriergenesis by virtue of their origin from midbrain neural crest stem cells. A better knowledge of the ontogenetic subpopulations may support the understanding of specific interactions and mechanisms involved in pericyte function/dysfunction, including normal and pathological angiogenesis, thereby offering an alternative perspective on cell subtype-specific therapeutic approaches.

Neural crest cell-derived pericytes act as pro-angiogenic cells in human neocortex development and gliomas

Central nervous system diseases involving the parenchymal microvessels are frequently associated with a ‘microvasculopathy’, which includes different levels of neurovascular unit (NVU) dysfunction, including blood–brain barrier alterations. To contribute to the understanding of NVU responses to pathological noxae, we have focused on one of its cellular components, the microvascular pericytes, highlighting unique features of brain pericytes with the aid of the analyses carried out during vascularization of human developing neocortex and in human gliomas. Thanks to their position, centred within the endothelial/glial partition of the vessel basal lamina and therefore inserted between endothelial cells and the perivascular and vessel-associated components (astrocytes, oligodendrocyte precursor cells (OPCs)/NG2-glia, microglia, macrophages, nerve terminals), pericytes fulfil a central role within the microvessel NVU. Indeed, at this critical site, pericytes have a number of direct and extracellular matrix molecule- and soluble factor-mediated functions, displaying marked phenotypical and functional heterogeneity and carrying out multitasking services. This pericytes heterogeneity is primarily linked to their position in specific tissue and organ microenvironments and, most importantly, to their ontogeny. During ontogenesis, pericyte subtypes belong to two main embryonic germ layers, mesoderm and (neuro)ectoderm, and are therefore expected to be found in organs ontogenetically different, nonetheless, pericytes of different origin may converge and colonize neighbouring areas of the same organ/apparatus. Here, we provide a brief overview of the unusual roles played by forebrain pericytes in the processes of angiogenesis and barriergenesis by virtue of their origin from midbrain neural crest stem cells. A better knowledge of the ontogenetic subpopulations may support the understanding of specific interactions and mechanisms involved in pericyte function/dysfunction, including normal and pathological angiogenesis, thereby offering an alternative perspective on cell subtype-specific therapeutic approaches.

Historical Overview of In Vivo and In Vitro Angiogenesis Assays

Most of angiogenesis assays were designed and developed during Folkman's era. But the growth of new blood vessels in several pathologic conditions as tumor development or inflammation were observed long time ago.The development of new blood vessels was early observed by ancient Egyptians who tried to destroy them by applying empirical methods. From the first observations regarding angiogenesis to a personalized therapy targeting newly formed blood vessels a lot of experimental in vitro and in vivo angiogenesis assays have been developed. The present work will overview the oldest and less known part of angiogenesis assays development, and in addition, it will present the newest data in the experimental field of angiogenesis which is rapidly improved by the needs of new antiangiogenic and antivascular therapy development.

Isolation and culture of endothelial cells from embryonic rat yolk sac

Yolk sac blood islands are the first morphologic evidence of hematopoietic development during mammalian embryogenesis, and visseral yolk sac mesoderm gives rise to the first embryonic blood cells within a rich endothelial network. Present study reports the isolation and culture of endothelial cells from 11.5 days old embryonic rat yolk sac. The embryos were dissected from 11.5 days pregnant Wistar rat (Rattus norvegicus) and the external yolk sac membrane and embryos were removed under aseptic condition. After washing three times with Calcium-Magnesium free Hank’s balanced salt solution (CMF-HBSS), the tissue was minced, and fragments were incubated in CMF-HBSS containing 2mg/ml Trypsin, 100mg/ml collagenase I and 40mg/ml DNAse at 37°C until the tissue was completely dispersed. The digestion effect was then neutralized by fetal bovine serum at 1:3 (v/v). The cell suspension was centrifuged at 1000 rpm for 10 min., the supernatants were discarded and the cell pellets resuspended in Dulbecco modified Eagle medium containing 15% fetal bovine serum, 1.25mg/ml amphotericin B, 25mg/ml gentamycin sulphate and 100mg/ml endothelial cell growth supplement. The resuspended cells were plated in two diverse 25cm2 culture flasks for overnight differential adherence at 37°C. The non-adherent cells were removed by gentle aspiration and adherent cells refed with fresh medium. The cells were transferred using 1ml of 0.2% Trypsin when cultures reached near-confluence. The cultured yolk sac endothelial cells had characteristic cobblestone appearence and positive immunofluorescent staining for von Willebrand Factor (vWF). Weibel-Palade bodies, the major ultrastructural marker for endothelium, were also detected in cultured cells by electron microscopy.

Concise Review: Paracrine Functions of Vascular Niche Cells in Regulating Hematopoietic Stem Cell Fate

The functions of endothelial cells (ECs) in regulating oxygen delivery, nutrient exchange, coagulation, and transit of inflammatory cells throughout the body are well‐‐established. ECs have also been shown to regulate the maintenance and regeneration of organ‐specific stem cells in mammals. In the hematopoietic system, hematopoietic stem cells (HSCs) are dependent on signals from the bone marrow (BM) vascular niche for their maintenance and regeneration after myelosuppressive injury. Recent studies have demonstrated the essential functions of BM ECs and perivascular stromal cells in regulating these processes. In the present study, we summarize the current understanding of the role of BM ECs and perivascular cells in regulating HSC maintenance and regeneration and highlight the contribution of newly discovered EC‐derived paracrine factors that regulate HSC fate. Stem Cells Translational Medicine2017;6:482–489

Role of VEGF in organogenesis

The cardiovascular system, consisting of the heart, blood vessels and hematopoietic cells, is the first organ system to develop in vertebrates and is essential for providing oxygen and nutrients to the embryo and adult organs. Work done predominantly using the mouse and zebrafish as model systems has demonstrated that Vascular Endothelial Growth Factor (VEGF, also known as VEGFA) and its receptors KDR (FLK1/VEGFR2), FLT1 (VEGFR1), NRP1 and NRP2 play essential roles in many different aspects of cardiovascular development, including endothelial cell differentiation, migration and survival as well as heart formation and hematopoiesis. This review will summarize the approaches taken and conclusions reached in dissecting the role of VEGF signalling in vivo during the development of the early cardiovasculature and other organ systems. The VEGF-mediated assembly of a functional vasculature is also a prerequisite for the proper formation of other organs and for tissue homeostasis, because blood vessels deliver oxygen and nutrients and vascular endothelium provides inductive signals to other tissues. Particular emphasis will therefore be placed in this review on the cellular interactions between vascular endothelium and developing organ systems, in addition to a discussion of the role of VEGF in modulating the behavior of nonendothelial cell populations.

S1P1 inhibits sprouting angiogenesis during vascular development

Coordination between the vascular system and forming organs is essential for proper embryonic development. The vasculature expands by sprouting angiogenesis, during which tip cells form filopodia that incorporate into capillary loops. Although several molecules, such as vascular endothelial growth factor A (Vegfa), are known to induce sprouting, the mechanism that terminates this process to ensure neovessel stability is still unknown. Sphingosine-1-phosphate receptor 1 (S1P(1)) has been shown to mediate interaction between endothelial and mural cells during vascular maturation. In vitro studies have identified S1P(1) as a pro-angiogenic factor. Here, we show that S1P(1) acts as an endothelial cell (EC)-autonomous negative regulator of sprouting angiogenesis during vascular development. Severe aberrations in vessel size and excessive sprouting found in limbs of S1P(1)-null mouse embryos before vessel maturation imply a previously unknown, mural cell-independent role for S1P(1) as an anti-angiogenic factor. A similar phenotype observed when S1P(1) expression was blocked specifically in ECs indicates that the effect of S1P(1) on sprouting is EC-autonomous. Comparable vascular abnormalities in S1p(1) knockdown zebrafish embryos suggest cross-species evolutionary conservation of this mechanism. Finally, genetic interaction between S1P(1) and Vegfa suggests that these factors interplay to regulate vascular development, as Vegfa promotes sprouting whereas S1P(1) inhibits it to prevent excessive sprouting and fusion of neovessels. More broadly, because S1P, the ligand of S1P(1), is blood-borne, our findings suggest a new mode of regulation of angiogenesis, whereby blood flow closes a negative feedback loop that inhibits sprouting angiogenesis once the vascular bed is established and functional.

Bioactive scaffolds for engineering vascularized cardiac tissues

Functional vascularization is a key requirement for the development and function of most tissues, and most critically cardiac muscle. Rapid and irreversible loss of cardiomyocytes during cardiac infarction directly results from the lack of blood supply. Contractile cardiac grafts, engineered using cardiovascular cells in conjunction with biomaterial scaffolds, are an actively studied method for cardiac repair. In this article, we focus on biomaterial scaffolds designed to mediate the development and maturation of vascular networks, by immobilized growth factors. The interactive effects of multiple vasculogenic factors are discussed in the context of cardiac tissue engineering.

Lysophosphatidic acid in vascular development and disease

Lysophosphatidic acid (LPA) is a small signaling lipid that is capable of stimulating a plethora of different cellular responses through the activation of its family of cognate G protein-coupled receptors. LPA mediates a wide range of biological effects in many tissue types that have been recently reviewed; however, its effects on vasculature development and function have received comparatively less examination. In this review, literature on the actions of LPA in three main aspects of vascular development (vasculogenesis, angiogenesis, and vascular maturation) is discussed. In addition, evidence for the roles of LPA signaling in the formation of secondary vascular structures, such as the blood brain barrier, is considered, consistent with significant roles for LPA signaling in vascular development, function, and disease.

The blood-brain barrier in neurodegenerative disease: a rhetorical perspective

Recent studies suggest that the function of the blood-brain barrier (BBB) is not static under normal physiologic conditions and is likely altered in neurodegenerative disease. Prevailing thinking about CNS function, and neurodegenerative disease in particular, is neurocentric excluding the impact of factors outside the CNS. This review challenges this perspective and discusses recent reports suggesting the involvement of peripheral factors including toxins and elements of adaptive immunity that may not only play a role in pathogenesis, but also progression of neurodegenerative diseases. Central to this view is neuroinflammation. Several studies indicate that the neuroinflammatory changes that accompany neurodegeneration affect the BBB or its function by altering transport systems, enhancing immune cell entry, or influencing the BBB's role as a signaling interface. Such changes impair the BBB's normal homeostatic function and affect neural activity. Moreover, recent studies reveal that alterations in BBB and its transporters affect the entry of drugs used to treat neurodegenerative diseases. Incorporating BBB compromise and dysfunction into our view of neurodegenerative disease leads to the inclusion of peripheral mediators in its pathogenesis and progression. In addition, this changing view of the BBB raises interesting new therapeutic possibilities for drug delivery as well as treatment strategies designed to reinstate normal barrier function.

Endothelial and hematopoietic progenitor cells (EPCs and HPCs): hand in hand fate determining partners for cancer cells

Tumor growth and metastasis need new vessel formation by angiogenesis provided by mature endothelial cells and postnatal vasculogenesis provided by endothelial progenitor cells (EPCs). Emerging data suggest a coordinated interaction between EPCs and hematopoietic progenitor cells (HPCs) in these processes. The complexity of the mechanisms governing the new vessel formation by postnatal vasculogenesis has increased by new evidence that not only bone marrow derived EPCs and HPCs seem to be involved in this process but also local progenitors residing within the vascular wall are mobilized and activated to new vessel formation by tumor cells. This review attempts to bring these systemic and local players of postnatal vasculogenesis together and to highlight their role in tumor growth and mestastasis.

Tumor cell-organ microenvironment interactions in the pathogenesis of cancer metastasis

The process of cancer metastasis is sequential and selective and contains stochastic elements. The growth of metastases represents the endpoint of many lethal events that few tumor cells can survive. Primary tumors consist of multiple subpopulations of cells with heterogeneous metastatic properties, and the outcome of metastasis depends on the interplay of tumor cells with various host factors. The findings that different metastases can originate from different progenitor cells account for the biological diversity that exists among various metastases. Even within a solitary metastasis of proven clonal origin, however, heterogeneity of biological characteristics can develop rapidly. The pathogenesis of metastasis depends on multiple interactions of metastatic cells with favorable host homeostatic mechanisms. Interruption of one or more of these interactions can lead to the inhibition or eradication of cancer metastasis. For many years, all of our efforts to treat cancer have concentrated on the inhibition or destruction of tumor cells. Strategies both to treat tumor cells (such as chemotherapy and immunotherapy) and to modulate the host microenvironment (including the tumor vasculature) should offer additional approaches for cancer treatment. The recent advances in our understanding of the biological basis of cancer metastasis present unprecedented possibilities for translating basic research to the clinical reality of cancer treatment.

[Vascular development in inflammatory bowel disease]

There are 4 major concepts in vascular development: vasculogenesis (formation of blood vessels from angioblasts), angiogenesis (formation of vascular sprouts from preexisting vessels), arteriogenesis (thickening and development of vessels) and lymphangiogenesis (formation of lymphatic vessels). In the last decade, these concepts, especially angiogenesis and lymphangiogenesis, have acquired major importance due to their role in tumoral growth and metastatic dissemination. Moreover, the activity of various diseases that involve chronic inflammation, such as asthma, psoriasis and rheumatoid arthritis, has been associated with vascular development. Several growth factors and cytokines are involved in this process and consequently investigation into these elements, both in peripheral blood and their expression in affected tissues, could elucidate the role of vascular development in diseases whose pathogenesis involves chronic inflammation, such as inflammatory bowel disease. The presence of distinct molecules involved in vascular development processes, such as vascular endothelial growth factor (VEGF), basic fibroblastic growth factor and placental growth factor, among others, has been studied in both ulcerative colitis and Crohn's disease, although not extensively. It has been suggested that the phenomena of vasculogenesis, angiogenesis and lymphangiogenesis play a critical, although not exclusive, role in the inflammation that characterizes inflammatory bowel disease. In general, the results obtained to date suggest that new vascular formation is involved in the pathogenesis of these diseases.

Model of vasculogenesis from embryonic stem cells for vascular research and regenerative medicine

Embryonic stem (ES) cells are highlighted as promising cell sources for regenerative medicine. Here, we focused on providing the platform that forced ES cells to reproduce the vascular organization process, leading to efficiency and safety evaluation as preclinical testing of biological agents. Murine ES cell-derived embryoid bodies on matrigel, but not collagen or gelatin, could be differentiated into sprouting blood vessels without the addition of growth factors. The expression of endothelial cell marker CD31 and smooth muscle marker alpha-smooth muscle actin was partially colocalized and started to increase 7 days after culture on matrigel, accompanied by the induction of a number of growth factors, such as vascular endothelial growth factor, fibroblast growth factor-2, hepatocyte growth factor, transforming growth factor-beta, and angiopoietin-1. Moreover, notch-related genes, such as Del1 or Del4 (delta-like 1/4) and hey1 or hey2 (hairy/enhancer of split related TRPW motif 1/2), were upregulated in a similar time course. The treatment of neutralizing antibodies against these growth factors failed to inhibit the differentiation into the sprouting blood vessels, whereas arginine-glycine-aspartic peptide, a selective inhibitor for the alphavbeta3-integrins, did inhibit differentiation. An anticancer drug to inhibit angiogenesis, TNP-470, also blocked the vascular formation in this model. ES cells could reproduce the vascular organization process on the biosynthetic scaffolds, such as matrigel, without the addition of growth factors. In the future, a human ES-based tissue model would be an optional tool for the screening of pharmaceutical drugs for vascular disease.

Vascularization in tissue remodeling after rat hepatic necrosis induced by dimethylnitrosamine

We observed postnecrotic tissue remodeling to examine vascularization in adult rat livers. Livers, bone marrow, and peripheral blood from rats at 24 h to 14 days after an injection of dimethylnitrosamine (DMN) were examined by light microscopic, immunohistochemical, and ultrastructural methods. Numerous ED-1 (a marker for rat monocytes/macrophages)-positive round mononuclear cells infiltrated in the necrotic areas at 36 h after DMN treatment. On day 5, when necrotic tissues were removed, some of the cells were transformed from round to spindle in shape. On day 7, these cells were contacted with residual reticulin fibers and became positive for SE-1, a marker of hepatic sinusoidal endothelial cells and Tie-1, an endothelial cell-specific surface receptor, associated with frequent occurrence of ED-1/SE-1 and ED-1/Tie-1 double-positive spindle cells. Ultrastructurally, the spindle cells simultaneously showed phagocytosis and endothelial cell-like morphology. With time necrotic areas diminished, and on day 14, the necrotic tissues were almost replaced by regenerated liver tissues and thin bundles of central-to-central bridging fibrosis. Bone marrow from 12 h to day 2 showed an increase of BrdU-positive mononuclear cells. Some of them were positive for ED-1. The BrdU-labeled and ED-1-positive cells appeared as early as 12 h after DMN injection and reached a peak in number at 36 h. They were similar in structure to ED-1-positive cells in necrotic liver tissues. These findings suggest that round mononuclear ED-1-positive cells proliferate first in bone marrow after DMN treatment, reach necrotic areas of the liver through the circulation, and differentiate to sinusoidal endothelial cells. Namely, hepatic sinusoids in DMN-induced necrotic areas may partly be reorganized possibly by vasculogenesis.

The role of erythropoietin in regulating angiogenesis

Erythropoietin (EPO) is an essential growth factor that regulates erythrocyte production in mammals. In this study, we demonstrate a novel role of EPO in regulating angiogenesis in vivo. Epo and Epo receptor (EpoR) are expressed in the vasculature during embryogenesis. Deletion of Epo or EpoR leads to angiogenic defects starting at E10.5, 2 days before ventricular hypoplasia and 3 days before the onset of the embryonic lethal phenotype. Overall, angiogenesis was severely affected in the mutant embryos: vascular anomalies included decreased complexity of the vessel networks. However, de novo vasculogenesis remained intact, consistent with the differential expression of Epo and EpoR during the early stages of embryonic development. The aforementioned angiogenesis defect can be partially rescued by expressing human EPO during embryogenesis. Moreover, Ang-1 expression is regulated by EPO/EPOR under normoxic conditions. Taken together, our results suggest important roles of EPO and EPOR in angiogenesis.

The Effect of Vascular Endothelial Growth Factor on in vitro Embryonic Heart Development in Rats

In vitro effects of vascular endothelial growth factor (VEGF) on heart development and total embryonic growth were investigated in 84 rat embryos (obtained from nine pregnant females) at 9.5 days of gestation that were cultured in whole rat serum (WRS), in <30 kDa + >50 kDa serum fractions [retenate (R)], and in R + VEGF. After 24-h culture, the embryos from each group were harvested and divided into two groups. One group was analysed morphologically and biochemically to obtain embryo protein content, the second group was serially sectioned and examined by light microscopy. Morphological score, embryo protein content, somite number and crown-rump length of embryos indicated that embryos cultured in R had significant embryonic retardation, whereas the addition of VEGF to R increased embryonic growth and development. The morphological scores for WRS, R and R + VEGF were 57.7 +/- 0.87, 46.6 +/- 1.90 and 52.1 +/- 0.97, somite numbers were 26.5 +/- 0.47, 20.1 +/- 0.63 and 24.4 +/- 0.46, crown-rump lengths were 3 +/- 0.07, 2.4 +/- 0.06 and 2.7 +/- 0.06 mm, and embryo protein contents were 160.5 +/- 7.41, 98.2 +/- 4.81 and 141.1 +/- 10.96 mug per embryo, respectively. The results of histological examination of heart development were similar. The hearts of embryos grown in R were unseptated and tubular. The atrioventricular endocardial cushions were incompletely developed. The addition of VEGF to R improved heart development. There were no gross morphological differences in the cardiac development between embryos grown in WRS and R + VEGF. In both groups, development of the muscular interventricular septum had begun. Development of the atrioventricular cushions was also similar in both groups and had caused narrowing of the atrioventricular canals, but the atrial septation was not observed.

Glucocorticoid-induced apoptosis in CNS microvascular pericytes

Pericyte loss or migration from its vascular location may be an important step in microvascular remodeling. Decreased pericyte to endothelial ratios are characteristics of newly formed vessels as well as microvessels undergoing regression, and may be due to selective degeneration via necrotic cell death or via programmed cell death. In this study, we have examined glucocorticoid-induced apoptosis in primary rat CNS pericytes. Characterization of apoptosis was determined using five independent criteria: (1) the translocation of receptors for annexin V from the inner to the outer surface of the plasma membrane, (2) the translocation of cytochrome C from the mitochondria to the cytosol, (3) the induction of DNA fragmentation, (4) the induction of classic changes in cell morphology, and (5) the appearance of TUNEL-positive cells. Incubation of CNS pericytes with dexamethasone induced the appearance of apoptotic cells in a time- and dose-dependent manner. Pericytes express immunologically detectable glucocorticoid receptors, and addition of the glucocorticoid receptor antagonist mifepristone inhibited dexamethasone-induced pericyte apoptosis. That pericytes undergo apoptosis in response to dexamethasone suggests that the regulatory function of this steroid may be important in vascular development and that pericyte apoptotic cell death may accompany vascular regression. Deregulation of pericyte involvement in vascular homeostasis and hemostasis may result in clinical disease.

Long-term culture and differentiation of rat embryonic stem cell-like cells into neuronal, glial, endothelial, and hepatic lineages

The in vitro differentiation of mouse embryonic stem cells into different somatic cell types such as neurons, endothelial cells, or myocytes is a well-established procedure. Long-term culture of rat embryonic stem cells is known to be hazardous, and attempts to differentiate these cells in vitro so far have been unsuccessful. We herein describe stable long-term culture of an alkaline phosphatase-positive rat embryonic stem cell-like cell line (RESC) and its differentiation into neuronal, endothelial, and hepatic lineages. RESCs were characterized by typical growth in single cells as well as in embryoid bodies when cultured in the presence of leukemia inhibitory factor. RESC expressed stage-specific-embryonic antigen-1 and the major histocompatibility complex class I molecule. For neuronal differentiation, cells were incubated with medium containing 10(-6) M retinoic acid for 14 days. For endothelial differentiation, RESCs were grown on Matrigel for 14 days, and for induction of hepatocyte-specific antigen expression, RESCs were grown in medium supplemented with fibroblast growth factor-4. Differentiated cells exhibited typical morphological changes and expressed neuronal (nestin, mitogen-activated protein-2, synaptophysin), glial (S100, glial fibrillary acid protein), endothelial (panendothelial antibody, CD31) and hepatocyte-specific (alpha-fetoprotein [alphaFP], albumin, alpha-1-antitrypsin, CK18) antigens. In addition, expression of hepatocyte-specific genes (alphaFP, transthyretin, carbamoyl-phosphate synthetase, and coagulation factor-2) was detected by reverse transcription polymerase chain reaction. We were able to culture RESCs under stable, long-term conditions and to initiate programmed differentiation of RESCs to endothelial, neuronal, glial, and hepatic lineages in the rat species.

Activated Fps/Fes partially rescues the in vivo developmental potential of Flk1-deficient vascular progenitor cells

Relatively little is known about the modulators of the vascular endothelial growth factor A (VEGF-A)/Flk1 signaling cascade. To functionally characterize this pathway, VEGF-A stimulation of endothelial cells was performed. VEGF-A–mediated Flk1 activation resulted in increased translocation of the endogenous Fps/Fes cytoplasmic tyrosine kinase to the plasma membrane and increased tyrosine phosphorylation, suggesting a role for Fps/Fes in VEGF-A/Flk1 signaling events. Addition of a myristoylation consensus sequence to Fps/Fes resulted in VEGF-A–independent membrane localization of Fps/Fes in endothelial cells. Expression of the activated Fps/Fes protein in Flk1-deficient embryonic stem (ES) cells rescued their contribution to the developing vascular endothelium in vivo by using ES cell–derived chimeras. Activated Fps/Fes contributed to this rescue event by restoring the migratory potential to Flk1 null progenitors, which is required for movement of hemangioblasts from the primitive streak region into the yolk sac proper. Activated Fps/Fes in the presence of Flk1 increased the number of hemangioblast colonies in vitro and increased the number of mesodermal progenitors in vivo. These results suggest that Fps/Fes may act synergistically with Flk1 to modulate hemangioblast differentiation into the endothelium. We have also demonstrated that activated Fps/Fes causes hemangioma formation in vivo, independently of Flk1, as a result of increasing vascular progenitor density.

Angiogenesis in Nervous System Disorders

Angiogenesis is a crucial requirement for embryonal development and new vessel formation during adult life. Various disease processes such as cancer, ischemia, vascular malformations, and inflammatory processes also depend on pathological angiogenesis. A better understanding of the complex and coordinated interactions among various angiogenic pathways involved in pathological angiogenesis is necessary to improve our therapeutic approaches to the various disease processes observed in the central nervous system. This review summarizes the current understanding of the role of principal angiogenic factors relevant to neurosurgical abnormalities.

Tenascin in meningioma: expression is correlated with anaplasia, vascular endothelial growth factor expression, and peritumoral edema but not with tumor border shape

OBJECTIVE

Tenascin is an extracellular matrix glycoprotein that is expressed during embryogenesis, inflammation, angiogenesis, and carcinogenesis. The aim of this study was to investigate how tenascin expression relates to histological grade, angiogenesis, and radiological findings in meningiomas.

METHODS

Twenty typical, 20 atypical, and 5 malignant meningiomas were studied retrospectively. Tenascin expression and vascular endothelial growth factor (VEGF) expression in the tumor tissue were investigated by immunohistochemistry. Tenascin messenger ribonucleic acid expression was also studied by comparative reverse transcriptase-polymerase chain reaction. Magnetic resonance images from each case were assessed for peritumoral edema and tumor border shape.

RESULTS

The atypical and malignant meningiomas showed higher levels of tenascin expression than the typical meningiomas. The more sensitive messenger ribonucleic acid-based methods confirmed this finding. Tenascin expression was correlated with peritumoral edema and VEGF expression but not with tumor border shape. In the 13 tumors with marked tenascin expression, peritumoral edema was Grade 0 in one, Grade 1 in three, and Grade 2 in nine specimens. In the same 13 tumors, VEGF expression was Grade 1 in five and Grade 2 in eight specimens, and the findings for tumor border shape were Grade 0 in seven, Grade 1 in four, and Grade 2 in two specimens.

CONCLUSION

In meningiomas, tenascin expression is correlated with anaplasia, tumor-associated edema, and VEGF expression but not with tumor border shape. This protein may play a role in the neoplastic and/or angiogenic processes in atypical and malignant meningiomas and may thus be a potential target for meningioma therapy.

The significance of immunohistochemistry in the diagnosis and therapy of neoplasms

This review article details the diagnostical significance of immunohistochemistry, which has developed during the last quarter of the century. Certainly, the advancement of monoclonal antibody technology has been of great significance in assuring the place of immunohistochemistry in the modern accurate microscopic diagnosis of human neoplasms, as a method of choice in histopathology. The fact still remains that in order to properly assess any immunohistochemical reactivity used for differential diagnostic purposes, the target cells have to be identified as neoplastically transformed cells by routine histopathological techniques. Selected groups of target molecules of great significance in cancer biology are discussed. The discovery of neoplasm-associated antigens has not only made the more accurate diagnosis of human cancer feasible but has also shed light on the extensive immunophenotypical heterogeneity of even the most closely linked human malignancies. The identification of disseminated neoplastically transformed cells by immunohistochemistry has allowed for a clearer picture of cancer invasion and metastasis, as well as the evolution of the tumour cell associated immunophenotype towards increased malignancy. Some possibilities of neoplasm-associated antigen targeted, receptor-directed immunotherapy are discussed and reviewed in this manuscript. Future antineoplastic therapeutical approaches should see the inclusion of a variety of immunotherapies, in the form of an individualised 'cocktail' specific for the particular immunophenotypical pattern associated with each individual patient's neoplastic disease.

Changes in the temporal and spatial expression of H beta 58 during formation and maturation of the chorioallantoic placenta in the Rat

Cloning and sequencing of a cDNA amplified by RNA fingerprinting at the implantation site of pregnant rats revealed 80% similarity with H beta 58, previously shown to be essential for formation of the chorioallantoic placenta in the mouse. H beta 58 mRNA was detected in the endometrium of hormonally sensitized rats stimulated to undergo decidualization and in the contralateral uterine horns lacking a decidual stimulus, indicating that uterine expression of H beta 58 mRNA did not require decidualization or the presence of a blastocyst. Immunodetection in the early postimplantation uterus (Days 6-8 of pregnancy) showed H beta 58 localized in the luminal and glandular epithelia and some stromal cells. Decidual cells at Day 6 of pregnancy expressed H beta 58, and by Day 9 of pregnancy, the protein localized throughout the maternal decidua. The temporal and spatial distribution of H beta 58 in the developing chorioallantoic placenta was assessed at Days 10, 12, and 14 of pregnancy. Immunoreactive H beta 58 localized to erythroid cells within the developing fetal vasculature of the chorioallantoic primordia at Day 10 of pregnancy. By Day 12, the fetal vasculature extended into the placental labyrinth, and the erythroid stem cells continued to strongly express H beta 58. At Day 14 of pregnancy, immunoreactivity became evident in the trophoblast giant cells and syncytiotrophoblast of the fetal placenta. As the chorioallantoic placenta matured (Day 18), H beta 58 mRNA was 3.6-fold higher in the labyrinth compared with the junctional region. Stable cell lines (HRP/LRP) isolated from the rat labyrinthine placenta expressed H beta 58 mRNA and protein. The expression pattern of H beta maternal and fetal placental tissues and its early expression in fetal erythroid stem cells during formation and maturation of the chorioallantoic placenta suggest that H beta 58 plays key roles in the regulatory networks that control hematopoietic development and placentation.

Embryonic and early fetal development of human lung vasculature and its functional implications

Recently, we have identified in the mouse three processes involved in the early development of pulmonary vasculature: angiogenesis for branching of central vessels, vasculogenesis (lakes in the mesenchyme) for peripheral vessels, and a lytic process to establish luminal connection between the two. We have established that these three processes also operate in the human by studying serial sections of human embryos and early fetuses. Vascular lakes of hematopoietic cells appear at stage 13, i.e., 4+ weeks gestational age (GA), the first intrapulmonary vascular structure to appear. At stage 20 (50.5 days GA), a venous network with luminal connections to central pulmonary veins (PV) is present. Airways have not yet reached these regions of lung. At its first intrapulmonary appearance, the pulmonary artery (PA) is small and thick walled: it runs with the airway but its branching is slower, so many peripheral airways are not accompanied by a PA branch. By contrast, the PV has a peripheral patent network well before the PA. In the pseudoglandular phase, airway branching continues, and the PA catches up so that small PA branches are found with all airways. Later in this phase small nonmuscular vessels lie in the mesenchyme close to airway epithelium. By the early canalicular phase and the age of viability, continuity between pulmonary artery and the peripheral capillary network must be established. In a 10-week fetus several structures suggesting a breakthrough site were seen. Air-blood barrier structure is first seen at 19 weeks. Thus in the lung, the PA and PV are dissociated in their timing and pattern of branching. Early veins are present diffusely through the mesenchyme and establish central luminal connection to the main pulmonary vein before airway or artery are present at this level.

Angiogenesis in the development of head and neck cancer and its inhibition by chemopreventive agents

Squamous cell carcinoma is an aggressive malignancy that often develops as multiple independent lesions throughout the mucosa of the upper aerodigestive tract. Therefore, the comprehensive treatment of this disease must not only address the initial primary neoplasm, but also prevent the progression of the premalignant lesions lurking throughout the rest of the mucosal surfaces. The need to treat these lesions has resulted in a search for chemopreventive agents that can halt or even reverse their malignant progression. The biologic and molecular mechanisms by which most chemopreventive agents act have remained unclear and controversial. Recent work from several laboratories has demonstrated that some drugs may act in part by inhibiting the ability of tumors to induce blood vessel growth. Angiogenesis, the growth of new blood vessels from pre-existing ones, is absolutely required for solid neoplasms to grow beyond 2-3 mm in diameter. Therefore, chemopreventive agents that act to inhibit angiogenesis may provide a very powerful modality by which one may limit the growth of both pre-malignant lesions and small nests of tumor cells. This review will outline the basic changes that occur in tumor cells that result in the switch from an anti-angiogenic to an angiogenic phenotype. In addition, it will discuss the mechanisms by which some chemopreventive agents, presently under clinical investigation, inhibit tumor angiogenesis. Finally, this paper will present a rationale for the use of multiple anti-angiogenic agents as a means of developing new chemopreventive protocols that result in reduced patient toxicity while maintaining similar clinical efficacies.

Direct visualization of renal vascular morphogenesis in Flk1 heterozygous mutant mice

Flk1, a receptor tyrosine kinase for vascular endothelial growth factor (VEGF), is the earliest known marker for endothelial precursors (angioblasts). We examined heterozygous mice in which the Flk1 gene was partially replaced by a promoter-less LacZ insert and used beta-galactosidase histochemistry to view cells transcribing Flk1. In day 10 (E10) embryos, a Flk1-positive network surrounded the metanephric blastema, and, at E11, a vessel entered the metanephros from its ventral aspect alongside the ingrowing ureteric bud. However, aortic branches did not engage embryonic kidneys at these time points. In newborns, beta-galactosidase was localized exclusively and intensely to endothelial cells of all vessels and glomeruli. In contrast, when E12 kidneys grown in organ culture for 6 days were examined, only scattered Flk1-positive cells were seen, glomeruli were unlabeled, and vessels were absent. When organ-cultured kidneys were then grafted into wild-type anterior eye chambers, numerous Flk1-positive endothelial cells in vessels and glomeruli were found, all stemming from the graft. Image analysis showed that grafts with the most abundant glomerulo- and tubulogenesis were also those with the richest expression of Flk1. We conclude that 1) kidney microvessels precede renal artery development, 2) angioblast differentiation is arrested in organ culture but released on grafting when vasculogenesis resumes, and 3) nephrogenesis and microvessel assembly are tightly coupled in vivo.

Ontogeny of the opioid growth factor, [Met5]-enkephalin, preproenkephalin gene expression, and the zeta opioid receptor in the developing and adult aorta of rat

Opioid peptides are known to play a role in the function of the mammalian cardiovascular system in the newborn. To learn about mechanisms underlying the morphogenesis of the developing vasculature and the principles contributing to the organization and maintenance of adult blood vessels, we have investigated the expression of the opioid growth factor (OGF), [Met5]-enkephalin, and its receptor, zeta. Moreover, gene expression for preproenkephalin mRNA, which encodes OGF, was studied to determine the source(s) of this inhibitory growth factor. By using immunocytochemistry, both OGF and the zeta opioid receptor were detected at embryonic day (E) 16 in the mesenchymal cells of the aortic wall. Staining appeared to be abundant in endothelial cells, smooth muscle cells, and fibroblasts at E20 and in the neonate. Immunoreactivity was noted to decrease progressively from day 5 to 10, but by weaning (day 21) and continuing into adulthood intense staining for both the peptide and receptor were observed. Preproenkephalin mRNA was detected throughout the aortic wall at E16, and the number of silver grains increased up today. Message was progressively reduced at days 5, 10, and 21, but signal in the adult aorta was comparable to that observed at day 5. These results indicate that components related to an endogenous opioid system regulating growth are present in the embryo, display a distinct spatial and temporal pattern of ontogeny, and persist into adulthood. In addition, these data indicate that OGF is an autocrine produced growth factor that is related to the emergence of vascular architecture and the maintenance of homeostasis in blood vessels.

Transient cranial hemorrhage does not cause depressed contractility in cardiac neural crest-ablated chick embryos

Ablation of cardiac neural crest results in cardiovascular malformations. Depressed ventricular contractility has been noted in cardiac neural crest-ablated embryos several hours before the time when neural crest cells would have arrived in the cardiac outflow tract and several days before the appearance of any malformations. The reason for this depressed heart function is not known. Recently, transient cranial hemorrhages were found in chick embryos during the third day of incubation, several hours before depressed ventricular contractility can be measured. We sought to determine whether depressed ventricular contractility could be associated with these transient hemorrhages. We were also interested in defining some of the factors that influence the incidence and severity of hemorrhaging. Three groups of embryos were used: cardiac neural crest-ablated, sham-operated, and unopened controls. All groups were found to experience transient hemorrhage from a common origin in the forebrain. However, the incidence and degree of hemorrhage were higher and more severe in embryos with cardiac neural crest ablation than in the other two groups. The cardiac ejection and shortening fractions were measured at stage 18 in embryos with and without hemorrhaging, and it was found that a decrease in these parameters was associated solely with ablation of cardiac neural crest and not with hemorrhage. By altering the incubation conditions, we determined that conditions that increase the oxygen in the air space are associated with increased severity and occurrence of hemorrhage and decreased viability in the first 3 days of incubation. Our results indicate that transient cranial hemorrhages does not cause depressed contractility in cardiac neural crest-ablated embryos, and increased severity of hemorrhaging is most likely due to an increase in oxygen availability.

A requirement for Flk1 in primitive and definitive hematopoiesis and vasculogenesis

Mouse embryos lacking the receptor tyrosine kinase, Flk1, die without mature endothelial and hematopoietic cells. To investigate the role of Flk1 during vasculogenesis and hematopoiesis, we examined the developmental potential of Flk1-/- embryonic stem cells in chimeras. We show that Flk1 is required cell autonomously for endothelial development. Furthermore, Flk1-/- cells do not contribute to primitive hematopoiesis in chimeric yolk sacs or definitive hematopoiesis in adult chimeras and chimeric fetal livers. We also demonstrate that cells lacking Flk1 are unable to reach the correct location to form blood islands, suggesting that Flk1 is involved in the movement of cells from the posterior primitive streak to the yolk sac and, possibly, to the intraembryonic sites of early hematopoiesis.

A role for notochord in axial vascular development revealed by analysis of phenotype and the expression of VEGR-2 in zebrafish flh and ntl mutant embryos

The notochord is required for the differentiation of nearby tissues, including the neural tube and the floor plate. Because the dorsal aorta and axial vein are midline structures, their development might also be influenced by the notochord. To investigate this possibility, we cloned zebrafish VEGR-2, homologous to the earliest known marker of endothelial cells in mammals. In flh and ntl mutant embryos, which lack a notochord, we found a defect in axial blood vessel formation, and a delay in the fusion of VEGR-2 positive endothelial progenitor cells into the primary vascular plexus and a block in the establishment of mature vessels. Differences in the vascular phenotype between the two mutations correlated with the severity of their axial mesodermal defects. These observations support a role for the notochord in vasculogenesis.

The regulation of neovascularization of matrix metalloproteinases and their inhibitors

The process of new capillary formation from preexisting vessels, angiogenesis, is a complex physiological event which is strictly controlled, occurring only very rarely under normal conditions. In contrast, there are a number of serious diseases, among them solid tumor growth, rheumatoid arthritis and several eye diseases, which are characterized by unrestricted new capillary growth and which are described as "angiogenic diseases." One of the key events required for successful angiogenesis is extracellular proteolysis. Increased attention has been focused on matrix metalloproteinase (MMP) family of enzymes whose activity is a rate-limiting step in extracellular matrix remodeling. This review will present the accumulating body of evidence, from a number of laboratories, which documents the important role of MMP activity in the regulation of angiogenesis. Taken together, these data suggest that one strategy for controlling the deregulated angiogenesis characteristic of these serious angiogenic diseases may be one which is operative at the level of the control of MMP activity.

Neovascularization in hyperplastic, metaplastic and potentially preneoplastic lesions of the bronchial mucosa

Angiogenesis is important in a large number of normal and pathological processes including tumour growth and development, inflammation and in wound healing. We investigated whether neovascularization exists in hyperplastic, metaplastic and potentially preneoplastic lesions of the bronchial mucosa as prestages for lung cancer. Biopsy specimens from 86 patients were investigated light microscopically. Formalin-fixed and paraffin-embedded specimens of regular bronchial mucosa including epithelium, basement membrane zone and tunica propria (n = 12) without inflammation were compared with specimens with inflammatory reaction (n = 9), basal cell- and goblet cell hyperplasia (n = 24), squamous cell metaplasia (n = 9), squamous cell metaplasia with different degrees of dysplasia (n = 11), specimens of micropapillomatosis (n = 9) and 13 cases with carcinoma in situ. The grade of neovascularization was assessed by the microvessel density, which was obtained by an immunohistochemical staining of endothelial cells using factor VIII-related antigen and determined by an automatic image-analysing-system. Microvessels were counted in selected areas of highest neovascularization on a x 100 field 0.4 mm underneath the basement membrane zone in the tunica propria. Microvessel count, minimal and maximal diameter of the vessels were chosen as morphological variables. A significantly increased microvessel count with 33 vessels/0.6 mm2 was found in specimens with inflammation of the tunica mucosa (regular bronchial mucosa: 20 vessels/0.6 mm2). Microvessel diameter (surface of cut section) increased in specimens of bronchial mucosa with inflammation to 11.3 x 10(-4) mm2 (regular bronchial mucosa: 9.04 x 10(-4) mm2). Microvessel count increased in cases of squamous cell metaplasia (33 vessels/0.6 mm2) squamous cell metaplasia with different degrees of dysplasia (50 vessels/0.6 mm2) and carcinoma in situ with 61 vessels/0.6 mm2. With increasing dysplasia, increasing neo-vascularization was found in close vicinity to the basement membrane zone. Simultaneously, interepithelial sprouts of endothelial cells were seen. Qualitative and quantitative differences were thus found in potentially preneoplastic lesions.

Developmental changes in endothelin A and endothelin B receptor subtypes in embryonic chick brains

The kD and Bmax values for [125I]endothelin-1 ([125I]ET-1) binding to the brain membrane preparations of 3- to 20-day-old chick embryos were compared. The kD values differed only slightly during development. In contrast, the Bmax values varied six-fold during the embryonic stages examined and displayed two distinct peaks, on day 5 (204 +/- 13 fmol/mg protein, n = 3) and day 13 (110 +/- 4 fmol/mg protein, n = 7) of incubation. Displacement of [125I]ET-1 binding to 5- and 13-day-old embryonic brain membrane preparations was carried out using ET-3 and BQ-123, a selective ETA receptor antagonist, as competing ligands. The results suggest that the ETA receptor subtype is primarily synthesized at the beginning of development followed by the production of non-selective ETB receptor subtype at a later time.

Embryonic angiogenesis: a review

Supply with nutrients is essential from early embryonic stages onwards. Therefore, circulatory organs form the first functioning organ system. With the exception of the heart, this system is at first formed by only one cell type, the endothelial cell. Emergence, behavior, and differentiation of endothelial cells are discussed in this review. At first, endothelial cells develop from angioblasts (primary angiogenesis/angioblastic development), later they develop from preexisting endothelial cells (secondary angiogenesis/angiotrophic growth). The composition of the extracellular matrix may promote or inhibit angiogenesis. Various growth factors which can be bound to the extracellular matrix may have been found, but only two of them (VEGF, P1GF) seem to influence endothelial cell behavior directly. Heterogeneity and organ-typical differentiation of endothelial cells seem to be dependent on cell-cell signaling within each organ.

Immunohistochemical localization of vascular endothelial growth factor in the human placenta

To understand the role of vascular endothelial growth factor (VEGF) in placental development, we examined immunohistochemically 56 placentae ranging from 6--41-weeks gestation using rabbit antibody to a synthetic multiple antigen peptide (MAP), composed of N-terminal amino acid residues 1--20 of human VEGF. In the present study, syncytiotrophoblast and invading extravillous trophoblasts ubiquitously expressed VEGF throughout gestation. However, the expression of VEGF in syncytiotrophoblasts was uneven in the first trimester and most intense at the sprouting sites. In addition, some stromal cells in the villi and decidual cells were also positive in the first trimester. A morphometrical analysis of the ratio of the VEGF-positive cell area to the capillary area in the terminal villi statistically revealed a critical point of change at 16 weeks' gestation. These results provide further evidence to support the hypothesis that VEGF, locally expressed by trophoblasts, stromal cells in villi and decidual cells, may play an important role in the physiological growth and function of the vascular system in the villous stroma and basal plate during placental development and maturation.

Postnatal remodeling of the leptomeningeal vascular network as assessed by intravital fluorescence video-microscopy in the rat

An intriguing characteristic of the ontogenic development of the cerebral vasculature is the rapid differentiation of the neonatal leptomeningeal vascular plexus into the mature, adult network form. The physiological and cellular mechanisms of this cerebrovascular remodeling process are unclear. The objective of this work was to determine and correlate changes in vascular density, network pattern and flow velocity in leptomeningeal microvessels of the rat during postnatal development in vivo. To this end, microvascular diameter, segment length, and vascular density of reconstructed leptomeningeal networks were measured from video-recordings of the microcirculation visualized through a cranial window in 0-15-day-old Sprague-Dawley rats. The velocity of erythrocytes in the microvessels was measured by frame to frame tracking of fluorescently labeled red blood cells. We found that surface vascular density (total vessel length per area), node density and segment density (object per area) decreased significantly by the second week after birth. Anastomosing vascular polygons, characteristic to newborn networks, became less numerous and larger in diameter during the postnatal 2-week period, indicating progressive rarefaction of the networks. Vessel diameter and red cell velocity showed transient increases at 1.5 weeks. The velocity/diameter ratio (V/D), an index of wall shear rate, increased by the age of 1.5 weeks and remained unchanged afterwards. There was a negative correlation between V/D and diameter at 1 week; this relationship was reversed to a positive correlation at 2 weeks. We conclude that postnatal remodeling of the leptomeningeal vascular network is associated with rarefaction and an adaptation of vessel caliber to wall shear rate. These changes may contribute to arterio-venous differentiation and redistribution of blood flow from the superficial to the intracortical vasculature in the developing brain.

Hemangioma, supraumbilical midline raphé, and coarctation of the aorta with a right aortic arch: single causal entity?

Most congenital cutaneous hemangiomas are a sporadic occurrence. Hemangiomas have been found in association with coarctation of the aorta and a right aortic arch. A separate association has been noted of midline ventral defects with hemangiomas. We report on a patient with multiple hemangiomas, coarctation of the aorta and a right aortic arch, a superaumbilical midabdominal raphé and sternal cleft. Our patient represents an overlap between these two conditions. Review of the literature identified four additional patients with a similar combination of anomalies. The clinical overlap between these 5 patients suggests that they are variants of the same conditions and represent a spectrum of defects that includes hemangiomas, midline ventral defects, aortic arch abnormalities and brain malformation.

The pathophysiology of angiogenesis

The formation of new capillary blood vessels, a process termed "angiogenesis", is one of the most pervasive and fundamentally essential biological processes encountered in mammalian organizations. Angiogenesis is an important event in a variety of physiological settings, such as embryonic development, chronic inflammation, and wound repair. It is a process that is tightly regulated in both time and space. Angiogenesis is driven by a cocktail of growth factors and pro-angiogenic cytokines and is tempered by an equally diverse group of inhibitors of neovascularization. Angiogenesis is also central to the etiology and pathogenesis of a number of pathological processes that include, among others, solid tumors, diseases of the eye, and chronic inflammatory disorders such as rheumatoid arthritis, psoriasis, and periodontitis. Based on recent work from several laboratories, it is now eminently clear that most if not all angiogenesis and vasoproliferative-dependent disease processes are not only a consequence of the unrestricted production of normal or aberrant forms of pro-angiogenic mediators but also the result of a relative deficiency in angiogenic-inhibitory molecules. In this review, I will describe how these multifunctional mediator systems function to coordinate and regulate the angiogenic response, and how disruption in the molecular controls that regulate the production of pro-angiogenic and angiostatic mediators leads to aberrant angiogenesis and disease. The implications of these findings in the development of novel therapeutic strategies for the treatment of diseases characterized by disregulated angiogenesis will also be discussed.

Angiogenesis: mechanistic insights, neovascular diseases, and therapeutic prospects

This review of angiogenesis aims to describe (a) stimuli that either elicit or antagonize angiogenesis, (b) the response of the vasculature to angiogenic or anti-angiogenic stimuli, i.e., processes required for the formation of new vessels, (c) aspects of angiogenesis relating to tissue remodeling and disease, and (d) the potential of angiogenic or antiangiogenic therapeutic measures. Angiogenesis, the formation of new vessels from existing microvessels, is important in embryogenesis, wound healing, diabetic retinopathy, tumor growth, and other diseases. Hypoxia and other as yet ill-defined stimuli drive tumor, inflammatory, and connective tissue cells to generate angiogenic molecules such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), transforming growth factor-beta (TGF-beta), platelet-derived growth factor (PDGF), and others. Natural and synthetic angiogenesis inhibitors such as angiostatin and thalidomide can repress angiogenesis. Angiogenic and antiangiogenic molecules control the formation of new vessels via different mechanisms. VEGF and FGF elicit their effects mainly via direct action on relevant endothelial cells. TGF-beta and PDGF can attract inflammatory or connective tissue cells which in turn control angiogenesis. Additionally, PDGF may act differently on specific phenotypes of endothelial cells that are engaged in angiogenesis or that are of microvascular origin. Thus phenotypic traits of endothelial cells committed to angiogenesis may determine their cellular responses to given stimuli. Processes necessary for new vessel formation and regulated by angiogenic/antiangiogenic molecules include the migration and proliferation of endothelial cells from the microvasculature, the controlled expression of proteolytic enzymes, the breakdown and reassembly of extracellular matrix, and the morphogenic process of endothelial tube formation. In animal models some angiogenesis-dependent diseases can be controlled via induction or inhibition of new vessel formation. Life-threatening infantile hemangiomas are a first established indication for antiangiogenic therapy in humans. Treatment of other diseases by modulation of angiogenesis are currently tested in clinical trials. Thus the manipulation of new vessel formation in angiogenesis-dependent conditions such as wound healing, inflammatory diseases, ischemic heart and peripheral vascular disease, myocardial infarction, diabetic retinopathy, and cancer is likely to create new therapeutic options.

Vascular endothelial cell lineage-specific promoter in transgenic mice

Vascular endothelial cells play essential roles in the function and development of the cardiovascular system. However, due to the lack of lineage-specific markers suitable for molecular and biochemical analyses, very little is known about the molecular mechanisms that regulate endothelial cell differentiation. We report the first vascular endothelial cell lineage-specific (including angioblastic precursor cells) 1.2 kb promoter in transgenic mice. Moreover, deletion analysis of this promoter region in transgenic embryos revealed multiple elements that are required for the maximum endothelial cell lineage-specific expression. This is a powerful molecular tool that will enable us to identify factors and cellular signals essential for the establishment of vascular endothelial cell lineage. It will also allow us to deliver genes specifically into this cell type in vivo to test specifically molecules that have been implicated in cardiovascular development. Furthermore, we have established embryonic stem (ES) cells from the blastocysts of the transgenic mouse that carry the 1.2 kb promoter-LacZ reporter transgene. These ES cells were able to differentiate in vitro to form cystic embryoid bodies (CEB) that contain endothelial cells determined by PECAM immunohistochemistry. However, these in vitro differentiated endothelial cells did not express the LacZ reporter gene. This indicates the lack of factors and/or cellular interactions which are required to induce the expression of the reporter gene mediated by this 1.2 kb promoter in this in vitro differentiation system. Thus this system will allow us to screen for the putative inducers that exist in vivo but not in vitro. These putative inducers are presumably important for in vivo differentiation of vascular endothelial cells.

Interactions between the Flk-1 receptor, vascular endothelial growth factor, and cell surface proteoglycan identified with a soluble receptor reagent

Fetal liver kinase-1 (Flk-1) is a transmembrane tyrosine kinase that was identified in endothelial cells and populations of cells enriched in hematopoietic progenitors. To characterize the interaction of Flk-1 with potential ligands the receptor extracellular domain was genetically fused to an alkaline phosphatase (AP) tag. A soluble ligand for Flk-1 was identified in the supernatants of numerous mesenchymal cell lines by co-immunoprecipitation with the Flk1-AP fusion protein. This polypeptide was shown by N-terminal sequencing to be vascular endothelial growth factor (VEGF). Receptor-AP fusion proteins can thus be used to identify soluble ligands as well as transmembrane ligands, and this approach is therefore likely to be widely applicable to many types of orphan receptor. The Flk1-AP soluble receptor was also found to bind to cell surfaces, showing two apparent classes of binding site with different affinities. This interaction could be reconstructed by introducing a VEGF expression plasmid into cells. These results indicate that VEGF presented at the cell surface can bind to the Flk-1 receptor, and could mediate a direct cell-cell interaction. The Flk1-AP fusion protein was also found to bind heparin, implying that ligand binding by the Flk-1 receptor may involve a three way interaction between the Flk-1 receptor, VEGF, and heparin-like cell surface proteoglycans.