The astrocyte-expressed integrin αvβ8 governs blood vessel sprouting in the developing retina

The mouse retina is vascularized after birth when angiogenic blood vessels grow and sprout along a pre-formed latticework of astrocytes. How astrocyte-derived cues control patterns of blood vessel growth and sprouting, however, remains enigmatic. Here, we have used molecular genetic strategies in mice to demonstrate that αvβ8 integrin expressed in astrocytes is essential for neovascularization of the developing retina. Selective ablation of αv or β8 integrin gene expression in astrocytes leads to impaired blood vessel sprouting and intraretinal hemorrhage, particularly during formation of the secondary vascular plexus. These pathologies correlate, in part, with diminished αvβ8 integrin-mediated activation of extracellular matrix-bound latent transforming growth factor βs (TGFβs) and defective TGFβ signaling in vascular endothelial cells, but not astrocytes. Collectively, our data demonstrate that αvβ8 integrin is a component of a paracrine signaling axis that links astrocytes to blood vessels and is essential for proper regulation of retinal angiogenesis.     

GPR126 Protein Regulates Developmental and Pathological Angiogenesis through Modulation of VEGFR2 Receptor Signaling

Angiogenesis, the formation of new blood vessels from pre-existing ones, is essential for development, wound healing, and tumor progression. The VEGF pathway plays irreplaceable roles during angiogenesis, but how other signals cross-talk with and modulate VEGF cascades is not clearly elucidated. Here, we identified that Gpr126, an endothelial cell-enriched gene, plays an important role in angiogenesis by regulating endothelial cell proliferation, migration, and tube formation. Knockdown of Gpr126 in the mouse retina resulted in the inhibition of hypoxia-induced angiogenesis. Interference of Gpr126 expression in zebrafish embryos led to defects in intersegmental vessel formation. Finally, we identified that GPR126 regulated the expression of VEGFR2 by targeting STAT5 and GATA2 through the cAMP-PKA-cAMP-response element-binding protein signaling pathway during angiogenesis. Our findings illustrate that GPR126 modulates both physiological and pathological angiogenesis through VEGF signaling, providing a potential target for the treatment of angiogenesis-related diseases.     

Rasip1 is required for endothelial cell motility, angiogenesis and vessel formation

Ras proteins are small GTPases that regulate cellular growth and differentiation. Components of the Ras signaling pathway have been shown to be important during embryonic vasculogenesis and angiogenesis. Here, we report that Rasip1, which encodes a novel Ras-interacting protein, is strongly expressed in vascular endothelial cells throughout development, in both mouse and frog. Similar to the well-characterized vascular markers VEGFR2 and PECAM, Rasip1 is specifically expressed in angioblasts prior to vessel formation, in the initial embryonic vascular plexus, in the growing blood vessels during angiogenesis and in the endothelium of mature blood vessels into the postnatal period. Rasip1 expression is undetectable in VEGFR2 null embryos, which lack endothelial cells, suggesting that Rasip1 is endothelial specific. siRNA-mediated reduction of Rasip1 severely impairs angiogenesis and motility in endothelial cell cultures, and morpholino knockdown experiments in frog embryos demonstrate that Rasip1 is required for embryonic vessel formation in vivo. Together, these data identify Rasip1 as a novel endothelial factor that plays an essential role in vascular development.     

Convected element method for simulation of angiogenesis

We describe a novel Convected Element Method (CEM) for simulation of formation of functional blood vessels induced by tumor-generated growth factors in a process called angiogenesis. Angiogenesis is typically modeled by a convection-diffusion-reaction equation defined on a continuous domain. A difficulty arises when a continuum approach is used to represent the formation of discrete blood vessel structures. CEM solves this difficulty by using a hybrid continuous/discrete solution method allowing lattice-free tracking of blood vessel tips that trace out paths that subsequently are used to define compact vessel elements. In contrast to more conventional angiogenesis modeling, the new branches form evolving grids that are capable of simulating transport of biological and chemical factors such as nutrition and anti-angiogenic agents. The method is demonstrated on expository vessel growth and tumor response simulations for a selected set of conditions, and include effects of nutrient delivery and inhibition of vessel branching. Initial results show that CEM can predict qualitatively the development of biologically reasonable and fully functional vascular structures. Research is being carried out to generalize the approach which will allow quantitative predictions.     

Semaphorin signaling in angiogenesis, lymphangiogenesis and cancer

Angiogenesis, the formation of new blood vessels from preexisting vasculature, is essential for many physiological processes, and aberrant angiogenesis contributes to some of the most prevalent human diseases, including cancer. Angiogenesis is controlled by delicate balance between pro- and anti-angiogenic signals. While pro-angiogenic signaling has been extensively investigated, how developmentally regulated, naturally occurring anti-angiogenic molecules prevent the excessive growth of vascular and lymphatic vessels is still poorly understood. In this review, we summarize the current knowledge on how semaphorins and their receptors, plexins and neuropilins, control normal and pathological angiogenesis, with an emphasis on semaphorin-regulated anti-angiogenic signaling circuitries in vascular and lymphatic endothelial cells. This emerging body of information may afford the opportunity to develop novel anti-angiogenic therapeutic strategies.     

Neovascularization and Hematopoietic Stem Cells

Vasculogenesis and angiogenesis are the major forms of blood vessel formation. Angiogenesis is the process where new vessels grow from pre-existing blood vessels, and is very important in the functional recovery of pathological conditions, such as wound healing and ischemic heart diseases. The development of better animal model and imaging technologies in past decades has greatly enriched our understanding on vasculogenesis and angiogenesis processes. Hypoxia turned out to be an important driving force for angiogenesis in various ischemic conditions. It stimulates expression of many growth factors like vascular endothelial growth factor, platelet-derived growth factor, insulin-like growth factor, and fibroblast growth factor, which play critical role in induction of angiogenesis. Other cellular components like monocytes, T cells, neutrophils, and platelets also play significant role in induction and regulation of angiogenesis. Various stem/progenitor cells also being recruited to the ischemic sites play crucial role in the angiogenesis process. Pre-clinical studies showed that stem/progenitor cells with/without combination of growth factors induce neovascularization in the ischemic tissues in various animal models. In this review, we will discuss about the fundamental factors that regulate the angiogenesis process and the use of stem cells as therapeutic regime for the treatment of ischemic diseases.     

Angiogenesis in the mouse retina: a model system for experimental manipulation

The vascular system of the mouse retina provides a useful model for analyzing the molecular and cellular mechanisms regulating angiogenesis because (1) hierarchical vascular networks are newly formed only after birth, (2) the cellular components involved in angiogenesis are well characterized, and (3) all the processes are accessible for monitoring and manipulation. In this article, we present an overview of our current understanding of the process of retinal angiogenesis and describe a number of methodologies applicable to experimental manipulation of the retinal vascular system.     

Numerical evaluation reveals the effect of branching morphology on vessel transport properties during angiogenesis

Blood flow governs transport of oxygen and nutrients into tissues. Hypoxic tissues secrete VEGFs to promote angiogenesis during development and in tissue homeostasis. In contrast, tumors enhance pathologic angiogenesis during growth and metastasis, suggesting suppression of tumor angiogenesis could limit tumor growth. In line with these observations, various factors have been identified to control vessel formation in the last decades. However, their impacts on the vascular transport properties of oxygen remain elusive. Here, we take a computational approach to examine the effects of vascular branching on blood flow in the growing vasculature. First of all, we reconstruct a 3D vascular model from the 2D confocal images of the growing vasculature at postnatal day 5 (P5) mouse retina, then simulate blood flow in the vasculatures, which are obtained from the gene targeting mouse models causing hypo- or hyper-branching vascular formation. Interestingly, hyper-branching morphology attenuates effective blood flow at the angiogenic front, likely promoting tissue hypoxia. In contrast, vascular hypo-branching enhances blood supply at the angiogenic front of the growing vasculature. Oxygen supply by newly formed blood vessels improves local hypoxia and decreases VEGF expression at the angiogenic front during angiogenesis. Consistent with the simulation results indicating improved blood flow in the hypo-branching vasculature, VEGF expression around the angiogenic front is reduced in those mouse retinas. Conversely, VEGF expression is enhanced in the angiogenic front of hyper-branching vasculature. Our results indicate the importance of detailed flow analysis in evaluating the vascular transport properties of branching morphology of the blood vessels.     

Dissecting coronary angiogenesis: 3D co-culture of cardiomyocytes with endothelial or mesenchymal cells

In embryogenesis, coronary blood vessels are formed by vasculogenesis from epicardium-derived progenitors. Subsequently, growing or regenerating myocardium increases its vasculature by angiogenesis, forming new vessels from the pre-existing ones. Recently, cell therapies for myocardium ischemia that used different protocols have given promising results, using either extra-cardiac blood vessel cell progenitors or stimulating the cardiac angiogenesis. We have questioned whether cardiomyocytes could sustain both vasculogenesis and angiogenesis. We used a 3D culture model of tissue-like spheroids in co-cultures of cardiomyocytes supplemented either with endothelial cells or with bone marrow-derived mesenchymal stroma cells. Murine foetal cardiomyocytes introduced into non-adherent U-wells formed 3D contractile structures. They were coupled by gap junctions. Cardiomyocytes segregated inside the 3D structure into clumps separated by connective tissue septa, rich in fibronectin. Three vascular endothelial growth factor isoforms were produced (VEGF 120, 164 and 188). When co-cultured with human umbilical cord endothelial cells, vascular structures were produced in fibronectin-rich external layer and in radial septa, followed by angiogenic sprouting into the cardiomyocyte microtissue. Presence of vascular structures led to the maintenance of long-term survival and contractile capacity of cardiac microtissues. Conversely, bone marrow mesenchymal cells formed isolated cell aggregates, which progressively expressed the endothelial markers von Willebrand's antigen and CD31. They proceeded to typical vasculogenesis forming new blood vessels organised in radial pattern. Our results indicate that the in vitro 3D model of cardiomyocyte spheroids provides the two basic elements for formation of new blood vessels: fibronectin and VEGF. Within the myocardial environment, endothelial and mesenchymal cells can proceed to formation of new blood vessels either through angiogenesis or vasculogenesis, respectively.     

reg6 is required for branching morphogenesis during blood vessel regeneration in zebrafish caudal fins

Postnatal neovascularization is essential for wound healing, cancer progression, and many other physiological functions. However, its genetic mechanism is largely unknown. In this report, we study neovascularization in regenerating adult zebrafish fins using transgenic fish that express EGFP in blood vessel endothelial cells. We first describe the morphogenesis of regenerating vessels in wild-type animals and then the phenotypic analysis of a genetic mutation that disrupts blood vessel regeneration. In wild-type zebrafish caudal fins, amputated blood vessels heal their ends by 24 h postamputation (hpa) and then reconnect arteries and veins via anastomosis, to resume blood flow at wound sites by 48 hpa. The truncated vessels regenerate by first growing excess vessels to form unstructured plexuses, resembling the primary capillary plexuses formed during embryonic vasculogenesis. Interestingly, this mode of vessel growth switches by 8 days postamputation (dpa) to growth without a plexus intermediate. During blood vessel regeneration, vessel remodeling begins during early plexus formation and continues until the original vasculature pattern is reestablished at approximately 35 dpa. Temperature-sensitive mutants for reg6 have profound defects in blood vessel regeneration. At the restrictive temperature, reg6 regenerating blood vessels first fail to make reconnections between severed arteries and veins, and then form enlarged vascular sinuses rather than branched vascular plexuses. Reciprocal temperature-shift experiments show that reg6 function is required throughout plexus formation, but not during later growth. Our results suggest that the reg6 mutation causes defects in branch formation and/or angiogenic sprouting.     

Mathematical modelling of dynamic adaptive tumour-induced angiogenesis: clinical implications and therapeutic targeting strategies

Angiogenesis, the growth of a network of blood vessels, is a crucial component of solid tumour growth, linking the relatively harmless avascular growth phase and the potentially fatal vascular growth phase. As a process, angiogenesis is a well-orchestrated sequence of events involving endothelial cell migration, proliferation; degradation of tissue; new capillary vessel (sprout) formation; loop formation (anastomosis) and, crucially, blood flow through the network. Once there is blood flow associated with the nascent network, the subsequent growth of the network evolves both temporally and spatially in response to the combined effects of angiogenic factors, migratory cues via the extracellular matrix and perfusion-related haemodynamic forces in a manner that may be described as both adaptive and dynamic. In this paper we present a mathematical model which simultaneously couples vessel growth with blood flow through the vessels--dynamic adaptive tumour-induced angiogenesis (DATIA). This new mathematical model presents a theoretical and computational investigation of the process and highlights a number of important new targets for therapeutic intervention. In contrast to earlier flow models, where the effects of perfusion (blood flow) were essentially evaluated a posteriori, i.e. after generating a hollow network, blood flow in the model described in this paper has a direct impact during capillary growth, with radial adaptations and network remodelling occurring as immediate consequences of primary anastomoses. Capillary network architectures resulting from the dynamically adaptive model are found to differ radically from those obtained using earlier models. The DATIA model is used to examine the effects of changing various physical and biological model parameters on the developing vascular architecture and the delivery of chemotherapeutic drugs to the tumour. Subsequent simulations of chemotherapeutic treatments under different parameter regimes lead to the identification of a number of new therapeutic targets for tumour management.     

心脏血管的形成

心脏的血 管 形成 是 血管 发生 (vasculogenesis)、血 管 生成 (angiogenesis)及 动 脉生 成 (arteriogenesis)三种 机制 共同 作 用的 结 果 .血管 发 生是 指在 胚 胎期 ,来 源 于中 胚 层的 干细 胞增 殖 和分 化 ,形 成 内皮 细胞 ,进而 与其 他细 胞形 成 原始 的 心血 管系 统 .血 管生 成 出现 在血 管 发生 之后 ,是指 通过 内 皮细 胞的 增 殖由 原始 血 管丛 或已 存在 的血 管 形成 无 完好 血管 的 膜中 的毛 细 血管 .而 动 脉生 成是 指 具有 完好 的 动脉 中膜 的 小动 脉 的生 成,也包 括原 有的 侧 支循 环 的改 建及 成 熟 .总结 了 出生 前后 心 脏脉 管系 统 形成 的细 胞 及分 子机 理 ,并 从生 物 学及 临床 治疗 上就 一 些内 皮 前体 细胞 及 其它 脉管 起 源相 关问 题 进行 简单 的 介绍 .     

Nicotinic acid inhibits angiogenesis likely through cytoskeleton remodeling

Angiogenesis is a physiological procedure during which the new blood vessels develop from the pre-existing vessels. Uncontrolled angiogenesis is related to various diseases including cancers. Clinical inhibition of undesired angiogenesis is still under investigation. We utilized nicotinic acid, a family member of the B-vitamin niacin (vitamin B3) that has been used in the prevention and treatment of atherosclerosis or other lipid-metabolic disorders, to treat human umbilical vein endothelial cells (HUVECs) and chick chorioallantoic membrane (CAM), and investigated its influence on angiogenesis in vitro and in vivo. We found that nicotinic acid could obviously inhibit HUVEC proliferation induced by vascular endothelial growth factor. Both the in vitro and in vivo assays showed that nicotinic acid could significantly inhibit the process of angiogenesis. To further investigate the mechanism underlying the effect of nicotinic acid on angiogenesis, we found that it might function via regulating the cytoskeleton arrangements, especially the rearranging the structures of F-actin and paxillin. In summary, we discovered that nicotinic acid could obviously inhibit the process of angiogenesis by changing the angiogenesis factor expression levels and inducing the cytoskeleton rearrangement of endothelial cells.     

VEGF inhibition: insights from preclinical and clinical studies

Angiogenesis, the growth of new blood vessels, is required for a variety of normal proliferative processes. Furthermore, angiogenesis is well established as also playing an important role in neoplastic growth and metastasis. Numerous regulators of angiogenesis have been identified and characterized over the last few decades. Among these, vascular endothelial growth factor (VEGF)-A appears especially important in several pathophysiological processes. Several VEGF inhibitors have been approved, by the US Food and Drug Administration, for the treatment of tumors or age-releted macular degeneration. This review examines the various mouse tumor models in which VEGF inhibitors have been tested and the lessons learned from these studies.     

PAKing up to the endothelium

Angiogenesis recapitulates the growth of blood vessels that progressively expand and remodel into a highly organized and stereotyped vascular network. During adulthood, endothelial cells that formed the vascular wall retain their plasticity and can be engaged in neo-vascularization in response to physiological stimuli, such as hypoxia, wound healing and tissue repair, ovarian cycle and pregnancy. In addition, numerous human diseases and pathological conditions are characterized by an excessive, uncontrolled and aberrant angiogenesis. The signalling pathways involving the small Rho GTPase, Rac and its downstream effector the p21-activated serine/threonine kinase (PAK) had recently emerged as pleiotropic modulators in these processes. Indeed, Rac and PAK were found to modulate endothelial cell biology, such as sprouting, migration, polarity, proliferation, lumen formation, and maturation. Elucidating the Rac/PAK molecular circuitry will provide essential information for the development of new therapeutic agents designed to normalize the blood vasculature in human diseases.     

Capillary plexus development in the day five to day six chick chorioallantoic membrane is inhibited by cytochalasin D and suramin

The chick chorioallantoic membrane (CAM) is a valuable model for evaluating angiogenesis and vasculogenesis. Our purpose was to characterize the formation of the CAM vasculature, in particular the capillary plexus, between days five and six after fertilization and to examine the mode of action of cytochalasin D and suramin on vascular development during this interval. The CAM increased 20-fold in size between days five and six, during which time the capillary plexus forms by both migration of mesodermal blood vessels toward the ectoderm and by the formation of new vessels from angioblasts near the ectoderm. Between days five and six, the CAM becomes thinner, and the density of the mesodermal cells decreases. To determine the mode of action of anti-angiogenic drugs on the day five to day six CAM, various concentrations of cytochalasin D or suramin were added directly to day five CAMs, and their effects were evaluated on day six. Both drugs significantly inhibited CAM growth, altered branching patterns of the major vessels, decreased area of the major vessels, and inhibited the formation of the capillary plexus by inhibiting both vasculogenesis and the migration of mesodermal blood vessels to the ectoderm. Cytochalasin D also inhibited compartmentalization of the plexus. Cytochalasin D and suramin were inhibitory at similar doses. This study provides new information on early CAM development, establishes the mode of action and dose dependency of cytochalasin D and suramin on day five to day six CAMs, and demonstrates that the day five to day six CAM provides a useful assay to examine the effect of anti-angiogenic drugs on blood vessel development, including capillary plexus formation.     

基质衍生的血管生成抑制剂研究进展

血管的生成与肿瘤密切相关,抑制肿瘤血管生成可以调节肿瘤的生长。体内存在着内源性的促血管生成因子和抑制因子的平衡,当促血管形成因子增强就会产生新生血管供肿瘤生长,而当抑制因子增强则会抑制肿瘤的生长。本文即对细胞外基质衍生的内源性血管生成抑制因子TSP、内皮他丁、Arresten;Canstatin、Endorepellin、Fibulin、Tumstatin等的特性、应用和作用机制等作一总结。     

Distinct signalling pathways regulate sprouting angiogenesis from the dorsal aorta and the axial vein

Angiogenesis, the formation of new blood vessels from pre-existing vessels, is critical to most physiological processes and many pathological conditions. During zebrafish development, angiogenesis expands the axial vessels into a complex vascular network that is necessary for efficient oxygen delivery. Although the dorsal aorta and the axial vein are spatially juxtaposed, the initial angiogenic sprouts from these vessels extend in opposite directions, indicating that distinct cues may regulate angiogenesis of the axial vessels. We found that angiogenic sprouts from the dorsal aorta are dependent on vascular endothelial growth factor A (Vegf-A) signalling, and do not respond to bone morphogenetic protein (Bmp) signals. In contrast, sprouts from the axial vein are regulated by Bmp signalling independently of Vegf-A signals, indicating that Bmp is a vein-specific angiogenic cue during early vascular development. Our results support a paradigm whereby different signals regulate distinct programmes of sprouting angiogenesis from the axial vein and dorsal aorta, and indicate that signalling heterogeneity contributes to the complexity of vascular networks.     

Nitric oxide signaling during myocardial angiogenesis

Ischemic heart disease develops as a consequence of coronary atherosclerotic lesion formation. Coronary collateral vessels and microvascular angiogenesis develop as an adaptive response to myocardial ischemia, which ameliorates the function of the damaged heart. Angiogenesis, the formation of new blood vessels from pre-existing vascular bed, is of paramount importance in the maintenance of vascular integrity both in the repair process of damaged tissue and in the formation of collateral vessels in response to tissue ischemia. Angiogenesis is modulated by a multitude of cytokines/chemokines and growth factors. In this regard, angiogenesis cannot be viewed as a single process. It is likely that different mediators are involved in different phases of angiogenesis. Vascular endothelial cells (ECs) produce nitric oxide (NO), an endothelium-derived labile molecule, which maintains vascular homeostasis and thereby prevents vascular atherosclerotic changes. In patients with ischemic heart disease, the release of endothelium-derived NO is decreased, which plays an important role in the atherosclerotic disease progression. In recent years, endothelium-derived NO has been shown to modulate angiogenesis in vitro and in vivo. In this review, we summarize recent progress in the field of the NO-mediated regulation of postnatal angiogenesis, particularly in response to myocardial ischemia.     

Postnatal vasculogenesis

It is generally accepted that vasculogenesis is limited to early embryogenesis and is believed not to occur in adult, whereas angiogenesis occurs in both the developing embryo and postnatal life. However, the distinction between them is not absolute, because both require endothelial cell proliferation and migration and three-dimensional reorganization of newly formed blood vessels, nor are they mutually exclusive, inasmuch as angioblasts can be incorporated into expanding pre-existing blood vessels. Recent observations indicate that vasculogenesis may not be restricted to early embryogenesis, but may also have a physiological role or contribute to the pathology of vascular diseases in adults. The major evidence in favor of this new view comes from: (i) demonstration of the presence of circulating endothelial cells and endothelial precursor cells; (ii) newly described mechanisms of blood vessel formation in tumor growth. The potential biomedical applications of endothelial precursor cells and the new opportunities for the development of new forms of tumor-targeted treatments are discussed.