Environmental and intrinsic modulations of venous differentiation.

Endothelial cells in veins differ in morphology, function and gene expression from those in arteries and lymphatics. Understanding how venous and arterial identities are induced during development is required to understand how arterio-venous malformations occur, and to improve the outcome of vein grafts in surgery by promoting arterialization of veins. To identify factors that promote venous endothelial cell fate in vivo, we isolated veins from quail embryos, at different developmental stages, that were grafted into the coelom of chick embryos. Endothelial cells migrated out from the grafted vein and their colonization of host veins and/or arteries was quantified. We show that venous fate is promoted by sympathetic vessel innervation at embryonic day 11. Removal of sympathetic innervation decreased vein colonization, while norepinephrine enhanced venous colonization. BMP treatment or inhibition of ERK enhanced venous fate, revealing environmental neurotransmitter and BMP signaling and intrinsic ERK inhibition as actors in venous fate acquisition. We also identify the BMP antagonist Noggin as a potent mediator of venous arterialization.

Sympathetic Innervation Promotes Arterial Fate by Enhancing Endothelial ERK Activity.

RATIONALE: Arterial endothelial cells are morphologically, functionally, and molecularly distinct from those found in veins and lymphatic vessels. How arterial fate is acquired during development and maintained in adult vessels is incompletely understood. OBJECTIVE: We set out to identify factors that promote arterial endothelial cell fate in vivo. METHODS AND RESULTS: We developed a functional assay, allowing us to monitor and manipulate arterial fate in vivo, using arteries isolated from quails that are grafted into the coelom of chick embryos. Endothelial cells migrate out from the grafted artery, and their colonization of host arteries and veins is quantified. Here we show that sympathetic innervation promotes arterial endothelial cell fate in vivo. Removal of sympathetic nerves decreases arterial fate and leads to colonization of veins, whereas exposure to sympathetic nerves or norepinephrine imposes arterial fate. Mechanistically, sympathetic nerves increase endothelial ERK (extracellular signal-regulated kinase) activity via adrenergic α1 and α2 receptors. CONCLUSIONS: These findings show that sympathetic innervation promotes arterial endothelial fate and may lead to novel approaches to improve arterialization in human disease.

The H2.0-like homeobox transcription factor modulates yolk sac vascular remodeling in mouse embryos.

OBJECTIVE: The H2.0-like homeobox transcription factor (HLX) plays an essential role in visceral organogenesis in mice and has been shown to regulate angiogenic sprouting in vitro and in zebrafish embryos. We therefore examined the role of HLX in vascular development in mouse and avian embryos. APPROACH AND RESULTS: In situ hybridization showed that Hlx is expressed in a subset of sprouting blood vessels in postnatal mouse retinas and embryos. Hlx expression was conserved in quail embryos and upregulated in blood vessels at the onset of circulation. In vitro assays showed that Hlx is dynamically regulated by growth factors and shear stress alterations. Proangiogenic vascular endothelial growth factor induces Hlx expression in cultured endothelial cells, whereas signals that induce stalk cell identity lead to a reduction in Hlx expression. HLX was also downregulated in embryos in which flow was ablated, whereas injection of a starch solution, which increases blood viscosity and therefore shear stress, causes an upregulation in HLX. HLX knockdown in vitro resulted in a reduction in tip cell marker expression and in reduced angiogenic sprouting, but Hlx(-/-) embryos showed no defect in vascular sprouting at E8.5, E9.5, or E11.5 in vivo. Vascular remodeling of the capillary plexus was altered in Hlx(-/-) embryos, with a modestly enlarged venous plexus and reduction of the arterial plexus. CONCLUSIONS: Our findings indicate not only that Hlx regulates sprouting in vitro, but that its role in sprouting is nonessential in vivo. We find HLX is regulated by shear stress and a subtle defect in vascular remodeling is present in knockout embryos.

[Remember: Françoise Dieterlen]. / Je me souviens de Françoise Dieterlen.

This Françoise Dieterlen's homage gathers scientific and personal memories between 1984 and 2000, when I worked in her laboratory at Nogent-sur-Marne (France). I describe a clever woman who took care of her students and taught me all fundamental qualities to become a researcher: discipline, rigor and patience.

Title: Je me souviens de Françoise Dieterlen. Abstract: Cette revue est un hommage à Françoise Dieterlen qui fut ma directrice de recherche entre 1984 et 2000. J'y rassemble des souvenirs de science mais aussi personnels et essaie de dresser le portrait d'une femme éclairée, proche de ses étudiants, qui a su m'inculquer les principes de base primordiaux pour faire de la recherche fondamentale et façonner le jeune étudiant que j'étais en un chercheur conscient de la discipline, de la rigueur et de la patience inhérentes à ce métier.

Three-dimensional imaging of vascular development in the mouse epididymis.

Long considered an accessory tubule of the male reproductive system, the epididymis is proving to be a key determinant of male fertility. In addition to its secretory role in ensuring functional maturation and survival of spermatozoa, the epididymis has a complex immune function. Indeed, it must manage both peripheral tolerance to sperm antigens foreign to the immune system and the protection of spermatozoa as well as the organ itself against pathogens ascending the epididymal tubule. Although our knowledge of the immunobiology of this organ is beginning to accumulate at the molecular and cellular levels, the organization of blood and lymphatic networks of this tissue, important players in the immune response, remains largely unknown. In the present report, we have taken advantage of a VEGFR3:YFP transgenic mouse model. Using high-resolution three-dimensional (3D) imaging and organ clearing coupled with multiplex immunodetections of lymphatic (LYVE1, PDPN, PROX1) and/or blood (PLVAP/Meca32) markers, we provide a simultaneous deep 3D view of the lymphatic and blood epididymal vasculature in the mature adult mouse as well as during postnatal development.

Alphastatin-C a new inhibitor of endothelial cell activation is a pro-arteriogenic agent in vivo and retards B16-F10 melanoma growth in a preclinical model.

Most characterized angiogenic modulators are proteolytic fragments of structural plasma and/or matrix components. Herein, we have identified a novel anti-angiogenic peptide generated by the in vitro hydrolysis of the C-terminal moiety of the fibrinogen alpha chain, produced by the snake venom metalloprotease bothropasin (SVMP), a hemorrhagic proteinase in Bothrops jararaca venom. The 14-amino acids peptide (alphastatin-C) is a potent antagonist of basic fibroblast growth factor, induced endothelial cell (HUVEC-CS) proliferation, migration and capillary tube formation in matrigel. It also inhibits cell adhesion to fibronectin. The basis of the antagonism between bFGF and alphastatin-C is elucidated by the inhibition of various bFGF induced signaling pathways and their molecular components modification, whenever the combination of the stimuli is provided, in comparison to the treatment with bFGF only. To corroborate to the potential therapeutic use of alphastatin-C, we have chosen to perform in vivo assays in two distinct angiogenic settings. In chick model, alphastatin-C inhibits chorioallantoic membrane angiogenesis. In mouse, it efficiently reduces tumor number and volume in a melanoma model, due to the impairment of tumor neovascularization in treated mice. In contrast, we show that the alphastatin-C peptide induces arteriogenesis, increasing pial collateral density in neonate mice. alphastatin-C is an efficient new antiangiogenic FGF-associated agent in vitro, it is an inhibitor of embryonic and tumor vascularization in vivo while, it is an arteriogenic agent. The results also suggest that SVMPs can be used as in vitro biochemical tools to process plasma and/or matrix macromolecular components unraveling new angiostatic peptides.

Netrin-1 inhibits sprouting angiogenesis in developing avian embryos.

Netrin-1 is a bifunctional axonal guidance cue, capable of attracting or repelling developing axons via activation of receptors of the deleted in colorectal cancer (DCC) and uncoordinated 5 (UNC5) families, respectively. In addition to its role in axon guidance, Netrin-1 has been implicated in angiogenesis, where it may also act as a bifunctional cue. Attractive effects of Netrin-1 on endothelial cells appear to be mediated by an as yet unknown receptor, while repulsion of developing blood vessels in mouse embryos is mediated by the UNC5B receptor. To explore evolutionary conservation of vascular UNC5B expression and function, we have cloned the chick unc5b homologue. Chick and quail embryos showed unc5b expression in arterial EC and sprouting angiogenic capillaries. To test if Netrin-1 displayed pro- or anti-angiogenic activities in the avian embryo, we grafted cell lines expressing recombinant chick or human Netrin-1 at different stages of development. Netrin-1 expressing cells inhibited angiogenic sprouting of unc5b expressing blood vessels, but had no pro-angiogenic activity at any stage of development examined. Netrin-1 also had no effect on the recruitment of circulating endothelial precursor cells. Taken together, these data indicate that vascular unc5b expression and function is conserved between chick and mice.

Vasculogenesis and angiogenesis in the mouse embryo studied using quail/mouse chimeras.

Using quail/chick chimeras, we have previously shown that different embryonic territories are vascularized through two distinct mecanisms, angiogenesis and vasculogenesis. Angiogenesis occurs in tissues of somatopleural origin, vasculogenesis occurs in territories of splanchnopleural origin. The aim of this work was to establish if these modes of vascularization were conserved in the mammalian embryo. Since in vivo manipulations with mammalian embryos are difficult to perform, we used a quail/mouse chimera approach. Mouse limb buds of somatopleural origin, and visceral organ rudiments of splanchnopleural origin, were grafted into the coelomic cavity of 2.5 day-old quail embryos. After four to seven days, the hosts were killed and the origin of the endothelial cells in the mouse tissues was determined by double staining with the quail endothelial and hematopoietic cell-specific marker, QH1 and mouse-specific VEGFR2 and VEGFR3 probes. Our findings show that the great majority of vessels which developed in the mouse limbs was QH1+, indicating that these tissues were vascularized by angiogenesis. Conversely, visceral organs were vascularized through the vasculogenesis process by mouse endothelial cells which differentiated in situ. These results demonstrate for the first time that in the mouse embryo, as previously shown in avian species, the tissues from somatopleural origin are vascularized by angiogenesis, while rudiments of a splanchnopleural origin are vascularized by vasculogenesis, both at vascular and lymphatic levels.

Vascular development: from precursor cells to branched arterial and venous networks.

The adult vascular system is composed of an arterial, a venous and a lymphatic compartment. These different compartments respectively provide oxygen and nutrients to peripheral organs, remove carbon dioxide and waste products and maintain an immune barrier to defend the host against foreign organisms. Malfunctions of the vascular system represent a major cause of mortality and disease in developed countries. Understanding of the molecular mechanisms regulating vascular system development and maintenance is thus crucial for the design of therapies to cure vascular diseases. The molecules implicated in the control of physiological and pathological angiogenesis in the adult already function during embryonic development. Indeed, the survival of the embryo also critically depends on the establishment of a functional circulatory loop. Here we review our current knowledge about the emergence of endothelial precursor cells in the embryo, of their assembly into the primary vascular plexus and of the remodeling of this plexus into arteries and veins. We also focus on the molecular mechanisms controlling the development of arteries, veins and lymphatic vessels.

Vascular Mural Cells Promote Noradrenergic Differentiation of Embryonic Sympathetic Neurons.

The sympathetic nervous system controls smooth muscle tone and heart rate in the cardiovascular system. Postganglionic sympathetic neurons (SNs) develop in close proximity to the dorsal aorta (DA) and innervate visceral smooth muscle targets. Here, we use the zebrafish embryo to ask whether the DA is required for SN development. We show that noradrenergic (NA) differentiation of SN precursors temporally coincides with vascular mural cell (VMC) recruitment to the DA and vascular maturation. Blocking vascular maturation inhibits VMC recruitment and blocks NA differentiation of SN precursors. Inhibition of platelet-derived growth factor receptor (PDGFR) signaling prevents VMC differentiation and also blocks NA differentiation of SN precursors. NA differentiation is normal in cloche mutants that are devoid of endothelial cells but have VMCs. Thus, PDGFR-mediated mural cell recruitment mediates neurovascular interactions between the aorta and sympathetic precursors and promotes their noradrenergic differentiation.

Hemangioblasts and hemopoietic stem cells during ontogeny.

This review focuses on the emergence of hemopoietic stem cells (HSC) in the embryonic aorta, which was analysed in the avian model. Intraaortic clusters, a characteristic vertebrate anatomical feature, were shown to derive from the splanchnopleural (ventral) mesoderm, which has the potential to give rise to both angioblasts and hemopoietic cells. In contrast, the somitic mesoderm was shown to give rise to angioblasts only. The derivation of hemopoietic progenitors from endothelial cells in the floor of the aorta was followed by means of in vivo labelling experiments. Finally, the expression of gene-encoding transcription factors involved in the emergence of HSC was restricted to the floor of the aorta immediately prior to and during the appearance of intraaortic clusters.

Netrin-1 controls sympathetic arterial innervation.

Autonomic sympathetic nerves innervate peripheral resistance arteries, thereby regulating vascular tone and controlling blood supply to organs. Despite the fundamental importance of blood flow control, how sympathetic arterial innervation develops remains largely unknown. Here, we identified the axon guidance cue netrin-1 as an essential factor required for development of arterial innervation in mice. Netrin-1 was produced by arterial smooth muscle cells (SMCs) at the onset of innervation, and arterial innervation required the interaction of netrin-1 with its receptor, deleted in colorectal cancer (DCC), on sympathetic growth cones. Function-blocking approaches, including cell type-specific deletion of the genes encoding Ntn1 in SMCs and Dcc in sympathetic neurons, led to severe and selective reduction of sympathetic innervation and to defective vasoconstriction in resistance arteries. These findings indicate that netrin-1 and DCC are critical for the control of arterial innervation and blood flow regulation in peripheral organs.

Extraembryonic origin of circulating endothelial cells.

Circulating endothelial cells (CEC) are contained in the bone marrow and peripheral blood of adult humans and participate to the revascularization of ischemic tissues. These cells represent attractive targets for cell or gene therapy aimed at improving ischemic revascularization or inhibition of tumor angiogenesis. The embryonic origin of CEC has not been addressed previously. Here we use quail-chick chimeras to study CEC origin and participation to the developing vasculature. CEC are traced with different markers, in particular the QH1 antibody recognizing only quail endothelial cells. Using yolk-sac chimeras, where quail embryos are grafted onto chick yolk sacs and vice-versa, we show that CEC are generated in the yolk sac. These cells are mobilized during wound healing, demonstrating their participation to angiogenic repair processes. Furthermore, we found that the allantois is also able to give rise to CEC in situ. In contrast to the yolk sac and allantois, the embryo proper does not produce CEC. Our results show that CEC exclusively originate from extra-embryonic territories made with splanchnopleural mesoderm and endoderm, while definitive hematopoietic stem cells and endothelial cells are of intra-embryonic origin.

Neuropilin-2 mediates VEGF-C-induced lymphatic sprouting together with VEGFR3.

Vascular sprouting is a key process-driving development of the vascular system. In this study, we show that neuropilin-2 (Nrp2), a transmembrane receptor for the lymphangiogenic vascular endothelial growth factor C (VEGF-C), plays an important role in lymphatic vessel sprouting. Blocking VEGF-C binding to Nrp2 using antibodies specifically inhibits sprouting of developing lymphatic endothelial tip cells in vivo. In vitro analyses show that Nrp2 modulates lymphatic endothelial tip cell extension and prevents tip cell stalling and retraction during vascular sprout formation. Genetic deletion of Nrp2 reproduces the sprouting defects seen after antibody treatment. To investigate whether this defect depends on Nrp2 interaction with VEGF receptor 2 (VEGFR2) and/or 3, we intercrossed heterozygous mice lacking one allele of these receptors. Double-heterozygous nrp2vegfr2 mice develop normally without detectable lymphatic sprouting defects. In contrast, double-heterozygote nrp2vegfr3 mice show a reduction of lymphatic vessel sprouting and decreased lymph vessel branching in adult organs. Thus, interaction between Nrp2 and VEGFR3 mediates proper lymphatic vessel sprouting in response to VEGF-C.

Identification, emergence and mobilization of circulating endothelial cells or progenitors in the embryo.

Using quail-chick parabiosis and QH1 monoclonal antibody analysis, we have identified circulating endothelial cells and/or progenitors in the embryo. These cells were already present early in ontogeny, before the third embryonic day. Under normal conditions, they integrated into most tissues but remained scarce. When experimental angiogenic responses were induced by wounding or grafts onto the chorioallantoic membrane, circulating endothelial cells were rapidly mobilized and selectively integrated sites of neoangiogenesis. Their mobilization was not dependent on the presence of the bone marrow as it was effective before its differentiation. Surprisingly, mobilization was not effective during sprouting angiogenesis following VEGF treatment of chorioallantoic membrane. Thus, embryonic circulating endothelial cells were efficiently mobilized during the establishment of an initial vascular supply to ischemic tissues following wounding or grafting, but were not involved during classical sprouting angiogenesis.

[Identification, emergence and mobilization of circulating endothelial cells in the embryo]. / Identification, emergence et mobilisation des cellules endothéliales circulantes chez l'embryon.

Using quail-chick parabiosis and the QH1 monoclonal antibody, specific for the endothelial and hematopoietic cells of the quail species, as a marker, we identified circulating endothelial cells in the embryo. In normal conditions, these cells could integrate endothelia in many tissues but their number remained low. When artificial angiogenic responses were created, i.e., in grafting experiments on the chorioallantoic membrane or wound healing, the circulating endothelial cells were rapidly mobilized to reach the embryonic regions submitted to these processes and their number dramatically increased. Interestingly, 1) on one hand, these circulating endothelial cells were present early in ontogeny, before the third embryonic day in the quail embryo; 2) on the other hand, their mobilization was not dependent on the presence of the bone marrow since it was effective before the differentiation of this tissue.

Vasculogenesis and the search for the hemangioblast.

Embryonic endothelial cells (EC) are generated by two mechanisms, vasculogenesis and angiogenesis (1). The term vasculogenesis describes the de novo emergence of EC progenitors from the mesoderm, whereas angiogenesis corresponds to the generation of EC by sprouting from the pre-existing vascular network. Until recently, it was thought that vasculogenesis was restricted to the period of embryonic development, whereas in the adult, only angiogenesis contributed to EC proliferation. The discovery of circulating EC progenitors in adult bone marrow and peripheral blood has suggested that additional mechanisms besides angiogenesis can occur in the adult, and therefore have renewed interest in the embryonic origin and the development of these progenitor cells. Vasculogenesis in the chick embryo has been studied since the beginning of the 20th century. During early development, vasculogenesis is intimately linked to the emergence of hematopoietic cells (HC). The existence of a common precursor for both EC and HC, termed "hemangioblast," was postulated (2). The purpose of this review is to summarize the experimental evidence concerning the emergence of EC and HC during embryonic life.