The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

ZO-1 regulates VE-cadherin–dependent endothelial junctions and actomyosin organization, thereby influencing cell–cell tension, migration, angiogenesis, and barrier formation

Endothelial progenitor cell (EPC) nomenclature remains ambiguous and there is a general lack of concordance in the stem cell field with many distinct cell sub-types continually grouped under the term “EPC”. It would be highly advantageous to agree standards to confirm an endothelial progenitor phenotype and this should include detailed immunophenotyping, potency assays, and clear separation from haematopoietic angiogenic cells which are not endothelial progenitors. In this review, we seek to discourage the indiscriminate use of ‘EPCs’, and instead propose precise terminology based on defining cellular phenotype and function. Endothelial colony forming cells (ECFCs) and myeloid angiogenic cells (MACs) are examples of two distinct and well-defined cell types that have been considered ‘EPCs’ because they both promote vascular repair, albeit by completely different mechanisms of action. It is acknowledged that scientific nomenclature should be a dynamic process driven by technological and conceptual advances; ergo the ongoing ‘EPC’ nomenclature ought not to be permanent and should become more precise in the light of strong scientific evidence. This is especially important as these cells become recognised for their role in vascular repair in health and disease; and, in some cases, progress towards use in cell therapy.

The regulation of blood vessel formation is of fundamental importance to many physiological processes, and angiogenesis is a major area for novel therapeutic approaches to diseases from ischemia to cancer. A poorly understood clinical manifestation of pathological angiogenesis is angiodysplasia, vascular malformations that cause severe gastrointestinal bleeding. Angiodysplasia can be associated with von Willebrand disease (VWD), the most common bleeding disorder in man. VWD is caused by a defect or deficiency in von Willebrand factor (VWF), a glycoprotein essential for normal hemostasis that is involved in inflammation. We hypothesized that VWF regulates angiogenesis. Inhibition of VWF expression by short interfering RNA (siRNA) in endothelial cells (ECs) caused increased in vitro angiogenesis and increased vascular endothelial growth factor (VEGF) receptor-2 (VEGFR-2)-dependent proliferation and migration, coupled to decreased integrin ␣v␤3 levels and increased angiopoietin (Ang)-2 release. ECs expanded from blood- IntroductionAngiogenesis, the formation of new vessels from pre-existing ones, occurs physiologically in specific circumstances such as wound healing and the menstrual cycle. Dysregulated angiogenesis contributes to the pathogenesis of many disorders, including diabetes, cancer, and macular degeneration (reviewed in Carmeliet 1 ). Angiogenic factors such as vascular endothelial growth factor (VEGF) and the angiopoietins (Ang) orchestrate signaling pathways that promote endothelial cell (EC) migration, proliferation, and ultimately the formation of a new vessel. VEGF-A is a major regulator of angiogenesis (reviewed in Grothey and Galanis 2 ) and acts on ECs mainly through VEGF receptor-2 (VEGFR-2), a tyrosine kinase receptor (reviewed in Olsson 3 ), to promote endothelial proliferation, migration, and sprouting of tip cells in the early stages of angiogenesis (reviewed in Gerhardt 4 ). Ang-1 and Ang-2, which bind to the endothelial Tie-2 receptor, act in the later stages of blood vessel formation and are essential for the maturation of a stable vascular network and for the maintenance of endothelial integrity (reviewed in Thomas and Augustin 5 ). Ang-1 and Ang-2 were originally identified as agonist and antagonist of Tie-2 signaling, respectively, with Ang-1 supporting EC survival and endothelial integrity 6 and Ang-2 promoting blood vessel destabilization and regression. 7 However, recent data suggest a more complex picture that includes cross-talk between the VEGF and the Ang pathways. 8 Growth factor signaling pathways are influenced by surface adhesion molecules that mediate cell-cell or cell-matrix interactions, particularly by members of the integrin superfamily. The integrin that has received most attention in ECs is ␣v␤3 (reviewed in Hodivala-Dilke 9 ), which mediates binding to several extracellular matrix proteins and growth factor receptors including VEGFR-2, thus influencing VEGFR-2 signaling (reviewed in Somanath et al 10 ). ␣v␤3 plays a complex role in angiogenesis. Although the origina...

SummaryBlood vessel stability is essential for embryonic development; in the adult, many diseases are associated with loss of vascular integrity. The ETS transcription factor ERG drives expression of VE-cadherin and controls junctional integrity. We show that constitutive endothelial deletion of ERG (ErgcEC-KO) in mice causes embryonic lethality with vascular defects. Inducible endothelial deletion of ERG (ErgiEC-KO) results in defective physiological and pathological angiogenesis in the postnatal retina and tumors, with decreased vascular stability. ERG controls the Wnt/β-catenin pathway by promoting β-catenin stability, through signals mediated by VE-cadherin and the Wnt receptor Frizzled-4. Wnt signaling is decreased in ERG-deficient endothelial cells; activation of Wnt signaling with lithium chloride, which stabilizes β-catenin levels, corrects vascular defects in ErgcEC-KO embryos. Finally, overexpression of ERG in vivo reduces permeability and increases stability of VEGF-induced blood vessels. These data demonstrate that ERG is an essential regulator of angiogenesis and vascular stability through Wnt signaling.

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IntroductionAngiogenesis, namely the formation of new vessels from preexisting ones, is essential for normal development, as well as for pathologic conditions, including tumor development, diabetic retinopathy, atherosclerosis, and rheumatoid arthritis. In recent years, endothelial apoptosis was shown to play an important role in remodeling the vascular network. Endothelial apoptosis is observed at the initiation of angiogenesis and at the branching and regression of neovessels. [1][2][3] Angiogenesis is controlled by the balance between proangiogenic factors and angiogenesis inhibitors, which also regulate apoptosis of endothelial cells (ECs). Indeed, the mechanism underlying many antiangiogenic therapies appears to be the induction of endothelial cell death. 4 Among the transcription factors involved in the regulation of angiogenesis and vascular development are members of the ETS family. This large family of transcription factors contains approximately 30 members that share a highly conserved DNA-binding domain (ETS domain) and are involved in regulating a wide variety of biologic processes. Many ETS proteins are expressed early in the developing vasculature in several organisms, and loss-of-function studies in mice and zebrafish have shown a critical role for ETS factors in vascular development. 5 In the adult, several endothelial ETS transcription factors were shown to regulate angiogenesis. 6 Among these, the ETS family member Erg is constitutively expressed in ECs, 7 and its expression is apparently restricted to ECs, megakaryocytes, 8 and chondrocytes. 9 Erg drives transcription of genes involved in endothelial homeostasis and angiogenesis, such as eNOS, 10,11 Data from animal models indicate that Erg is involved in endothelial differentiation and vascular development 12 ; for example, Erg overexpression in Xenopus embryos resulted in ectopic endothelial differentiation. 13 We have previously shown that Erg is required for angiogenesis in human ECs in vitro 14 ; however, no data are available on the role of Erg in angiogenesis in the adult in vivo.Endothelial junctions are crucial for the maintenance and regulation of vascular homeostasis and function and mediate a complex signaling network. 15 A major regulator of adherent junctions is vascular endothelial (VE)-cadherin, a Ca 2ϩ -dependent cell-surface adhesion molecule that forms homophilic interactions and is required for the integrity of the endothelial monolayer, endothelial permeability, and the control of cell growth. 16,17 VE-cadherin was clearly shown to regulate vascular development and angiogenesis: genetic inactivation of the VE-cadherin gene leads to early embryonic death because of vascular defects 18,19 and antibodies to VE-cadherin inhibit angiogenesis both in vitro and in vivo (reviewed in Dejana et al 20 ). VE-cadherin regulates a number of signaling events, by intracellular interaction with proteins of the armadillo family, including ␤-catenin and plakoglobin, as well as by clustering signaling molecules and growth factor receptors. 1...