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Event: 110

Key Event Title

A descriptive phrase which defines a discrete biological change that can be measured. More help

Impairment, Endothelial network

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. More help
Impairment, Endothelial network
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Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. More help
Level of Biological Organization

Cell term

The location/biological environment in which the event takes place.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable.  Further information on Event Components and Biological Context may be viewed on the attached pdf. More help

Organ term

The location/biological environment in which the event takes place.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable.  Further information on Event Components and Biological Context may be viewed on the attached pdf. More help
Organ term

Key Event Components

The KE, as defined by a set structured ontology terms consisting of a biological process, object, and action with each term originating from one of 14 biological ontologies (Ives, et al., 2017; Biological process describes dynamics of the underlying biological system (e.g., receptor signalling).Biological process describes dynamics of the underlying biological system (e.g., receptor signaling).  The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signaling by that receptor).  Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description.  To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons.  If a desired term does not exist, a new term request may be made via Term Requests.  Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add.  Further information on Event Components and Biological Context may be viewed on the attached pdf. More help
Process Object Action
endothelium development abnormal

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE.Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
Developmental Vascular Toxicity KeyEvent Tom Knudsen (send email) Open for citation & comment WPHA/WNT Endorsed
AHR activation to ELS mortality, via VEGF KeyEvent Amani Farhat (send email) Open for citation & comment WPHA/WNT Endorsed

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KE.In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
Vertebrates Vertebrates High NCBI

Life Stages

An indication of the the relevant life stage(s) for this KE. More help
Life stage Evidence
Embryo High
Development High

Sex Applicability

An indication of the the relevant sex for this KE. More help
Term Evidence
Unspecific High

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. More help

In embryological terms, the angiogenic cycle entails a stepwise progression of formation, maturation, and stabilization of the microvasculature [Hanahan, 1997; Drake et al. 2007; Chung and Ferrara 2011; Knudsen and Kleinstreuer, 2011; Coultas et al. 2005; Huang, 2020]. This level of impairment of blood vessel morphogenesis best maps to Gene Ontology (GO) annotations: GO:001885 for ‘endothelial cell development’, which is defined as “The progression of an endothelial cell over time, from its formation to the mature structure”; and/or GO:0045601 for ‘regulation of endothelial cell differentiation’, defined as “Any process that stops, prevents, or reduces the frequency, rate or extent of endothelial cell differentiation”. The numbers of curated genes associated with these categories in the MGI database ( are 75 genes and 44 genes, respectively, for a total of 97 genes altogether. In addition, pericyte-endothelial interactions are indispensable for maturation and stabilization via broader signaling pathways (eg, VEGFA, PDGFB, Notch-DLL4, AGPNT, Norrin, TGF-β) that have been characterized during patterning neovascularization [Azam et al. 2018; Huang, 2020]. Neovascular stabilization is an active process that requires specific cellular signaling, including pro-angiogenic pathways such as VEGF and FGF, angiopoietin-Tie2 for endothelial cell survival and junction stabilization, PDGF and TGF-β signaling that modify mural cell (pericytes, vascular smooth muscle cells) functions to fortify vessel integrity [Murakami, 2012]. Breakdown of these signaling systems results in pathological hyperpermeability and/or genetic vascular abnormalities such as vascular malformations, ultimately progressing to hemorrhage and edema. Vascular mural cells are recruited to the endothelial network by endothelial cell signals [Sinha and Santoro, 2018]. A number of anti-angiogenic compounds, including Vatalanib and Thalidomide, have been shown to impair neovascularization during developmental angiogenesis [Tran et al. 2007; Therapontos et al. 2009; Jang et al. 2009; Rutland et al. 2009; Tal et al. 2014; Vargesson, 2015; Beedie et al. 2016; Ellis-Hutchings et al. 2017; Kotini et al. 2020]. In exposed zebrafish embryos, early effects of potential vascular disrupting chemicals (pVDCs) invoke changes to the anatomical development of intersegmental vessels from the dorsal aorta [Tran et al. 2007; Tal et al. 2014; McCollum et al. 2017]. Thalidomide, for example, has been shown to primarily disrupt immature vascular networks versus more mature vasculature in the embryo [Therapontos et al. 2009; Beedie et al. 2016a, 2016b, 2017]. Evidence for KE:110 in human studies is indirect, based on the association of malformations with altered vascular patterns and exposure to anti-angiogenic drugs in women of reproductive potential or during pregnancy [Husain et al. 2008; van Gelder et al. 2010; Gold et al. 2011; Ligi et al. 2014; Vargesson and Hootnick, 2017]. Key nodes in the ontogenetic regulation of angiogenesis have been investigated with human cell-based high-throughput assay (HTS) platforms in ToxCast to screen for pVDCs acting on the formation, maturation and/or stabilization of endothelial networks [Houck et al. 2009; Knudsen et al. 2011; Kleinstreuer et al. 2014; Saili et al. 2019; Zurlinden et al. 2020].

How It Is Measured or Detected

A description of the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements.These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA). Do not provide detailed protocols. More help

Microvascular structure: Endothelial network formation can be monitored quantitatively in vitro using different human cell-based angiogenesis assays that score endothelial cell migration, cell counts, tubule counts, tubule length, tubule area, tubule intensity, and node counts [Muller et al. 2002; Masckauchan et al. 2005; Sarkanen et al. 2010; Knudsen et al. 2016; Nguyen et al. 2017; Toimela et al. 2017; Saili et al. 2019; Zurlinden et al. 2020]. Cell types commonly employed are human umbilical endothelial cells (HUVECs) and more recently endothelial cells derived from human induced pluripotent stem cells (iPSC-ECs) through various differentiation and purification protocols [Belair et al. 2015 and 2016; Iwata et al. 2017; Bezenah et al. 2018; van Duinen et al. 2019 and 2020]. Synthetic hydrogels are shown to promote robust in vitro network formation by HUVEC or iPSC-ECs in response to angiogenic factors as superior sensitivity and reproducibility to detect pVDCs [Nguyen et al. 2017]. Although endothelial cell models of migration, proliferation, apoptosis, and tube formation are popular due to their simplicity and throughput, these assays lack the biological complexity of an in vivo system. Animal models, including the chick chorioallantoic membrane assay, corneal neovascularization assay, and 3D embedded matrices preserve biological complexity but are costly and low throughput [Tran et al. 2007]. Endothelial-specific transgenic zebrafish reporter embryos thus provide a test system that combines the biological complexity of in vivo models with automated high-throughput screening (HTS).

Maturation and stabilization: Chemical effects may be detected by HTS assays for phenotypic profiling in endothelial co-culture systems based on specific biomarker protein readouts [Kleinstreuer et al. 2014]. The ToxCast portfolio includes eight human cell-based systems for screening chemicals that disrupt physiologically important cell-cell signaling pathways, including vascular biology. The endpoints measured can be closely linked to in vivo outcomes. Local signals may act through several receptor modalities, including receptor tyrosine kinases (RTKs), G-protein coupled receptors (GPCRs), and glycosyl phosphatidyl-inositol (GPI)-anchored receptors as part of a ToxCast in vitro signature for profiling potential vascular disrupting compounds (pVDCs) [Knudsen and Kleinstreuer, 2011; Kleinstreuer et al. 2013; Tal et al. 2017; Saili et al. 2019].

Assessing weight of evidence with a ToxCast pVDC predictive signature assays for KE:110:

ToxCast HTS predictions for 38 potential pVDCs and non-pVDCs were tested across ten in vitro platforms from laboratories addressing different aspects of the vasculogenic/angiogenic cycle. Three tubulogenesis platforms used traditional HUVECs [Sarkanen et al. 2010; Toimela et al. 2017]; 3D endothelial sprouting and network assays used endothelial cells derived from human induced pluripotent stem cells (iPSCs) [Belair et al. 2016b; Nguyen et al. 2017; Zurlinden et al. 2020]; microvessel outgrowth in rat fetal aortic explants [Ellis-Hutchings et al. 2017] and transgenic endothelial reporter zebrafish lines [Tal et al. 2017; McCollum et al. 2017] rounded out the panel. While no single study confirmed all of the pVDC predictions, the combined vascular disrupting effects across all studies aligned well with the in silico predictions (87% accuracy; positive predictive value of 93%, negative predictive value of 73%) [Saili et al. 2019]. ToxCast assay features input to the prediction model were detected as follows.

Vascular cell adhesion molecule 1 (VCAM1): the pVDC signature aggregates assays from the BioMAP Systems Predictive Toxicology panel [Kunkel et al., 2004; Houck et al., 2009] focusing here on chemical disruption of endothelial VCAM1 expression following stimulation by cytokines-growth factors. This assay endpoint is an in vitro surrogate for inflammatory cell recruitment per endothelial dysfunction and has been probed across five different cell systems: 4H (HUVECs stimulated with IL-4 + histamine); 3C (HUVECs stimulated with IL-1β + TNFα + IFNϒ); CASM3C (primary human coronary artery smooth muscle cells stimulated with IL-1β + TNFα + IFNϒ); LPS (HUVECs co-cultured with monocytes and stimulated with bacterial endotoxin); and hDFCGF (human dermal fibroblasts stimulated with IL-1β + TNFα + IFNϒ and EGF + bFGF + PDGF-BB)[Knudsen and Kleinstreuer, 2011, Kleinstreuer et al., 2014].

Angiogenic cytokines and chemokines: the pVDC signature aggregates features for LPS-induced TNFα protein expression (see BioMAP descriptor above), nuclear factor-kappa B (NFkB) mediated reporter gene activation (Attagene; cis- configuration), and caspase 8 enzymatic activity (NovaScreen; inhibition or activation). TNFα is a proinflammatory cytokine that can promote angiogenesis indirectly through NFkB-mediated expression of angiogenic growth factors or inhibit angiogenesis by direct effects on endothelial proliferation and survival. The pVDC signature also aggregates features for signaling activity of the pro-angiogenic cytokines interleukin-1 alpha (IL1a, a macrophage-derived activator of TNFα) and interleukin 6 (IL6). These cytokines act through the G-protein coupled receptors (GPCRs) IL1R and IL6R, respectively. CXCL8 (chemokine (C-X-C motif) ligand 8), formerly known as interleukin 8 (IL8), is angiogenic through its cognate GPCRs (CXCR1, CXCR2). In contrast to CXCL8, the chemokines CXCL9 (alias MIG, monokine induced by IFNϒ) and CXCL10 (alias IP10, interferon-inducible cytokine IP-10) are considered anti-angiogenic through their cognate receptor, CXCR3 [Knudsen et al. 2011; Kleinstreuer et al. 2013; Tal et al. 2017; Saili et al. 2019; Zurlinden et al. 2020].

Angiogenic growth factors: FGFs and VEGFs exert their effects on endothelial cell proliferation, migration, and differentiation via specific binding to receptor tyrosine kinases VEGFR and FGFR. The pVDC signature has features for liganding VEGFR1, VEGFR2, and VEGFR3 based on receptor kinase activity (RTK, inhibition or activation) from the NovaScreen biochemical profile [Sipes et al. 2013] and for down-regulation of VEGFR2 expression in the 4H BioMAP system (HUVECs stimulated with IL-4 + histamine, B). VEGFR1 is a non-signaling VEGF-A decoy receptor that can be cleaved from the cell surface; VEGFR2 is the most important VEGF-A receptor and a master switch for developmental angiogenesis; and VEGFR3 is a VEGF-C receptor up-regulated by Notch signals. The pVDC signature includes features for the basic helix-loop-helix transcription factors Aryl Hydrocarbon Receptor (AhR) and Hypoxia Inducible Factor-1 alpha (HIF1a) that are upstream regulators of VEGF gene expression during ischemia or hypoxia. HIF1a and AhR are measured in reporter assays (Attagene). In addition to HIF1a and AhR, the pVDC signature has features for the estrogen receptor alpha (ERa), also a trans-activator of VEGF expression. This included human ERa binding activity (NovaScreen), ERa reporter trans-activation (Attagene) and ERE (estrogen responsive element) reporter cis-activation (Attagene).

Angiogenic outgrowth: the ephrins (EFNA1 and EFNB2 in particular) couple VEGF signaling to angiogenic sprouting during early development of the embryonic vasculature (vasculogenesis, angiogenesis). The ToxCast pVDC signature included features for EPH-receptor tyrosine kinase biochemical activity (increased or decreased) for receptors EPHA1, EPHA2, EPHB1 and EPHB2 via their cognate cell membrane-anchored ligands (EFNAs). In contrast to the ephrin system, a number of chemicals had activity on diverse assays for urokinase-type plasminogen activator (uPA). That system, consisting of uPA (4 features) and its GPI-anchored receptor, uPAR (8 features) - both assayed in the BioMAP System [Kleinstreuer et al. 2014], functions in VEGFR2-induced changes to focal adhesion and extracellular matrix (ECM) degradation at the leading edge of endothelial cells during angiogenic sprouting. Binding of uPA to uPAR results in serine-protease conversion of plasminogen to plasmin that initiates a proteolytic cascade leading to degradation of the basement membrane and angiogenic sprouting. The uPA proteolytic cascade is suppressed by the serine protease inhibitor, endothelial plasminogen activator inhibitor type 1 (PAI1). The PAI1/uPA/uPAR assays report chemical effects on the system (up or down) across diverse cellular platforms: 4H, 3C, CASM3C, and hDFCGF noted above; BE3C (human bronchial epithelial cells stimulated with IL-1β + TNFα + IFNϒ); and KF3T (human keratinocytes + fibroblasts stimulated with IL-1β + TNFα + IFNϒ + TGF-β). The pVDC signature has features for thrombomodulin (THBD) and the thromboxane A2 (TBXA2) receptor that participate in the regulation of endothelial migration during angiogenic sprouting. THBD is a type I transmembrane glycoprotein that mediates regulator of uPA/uPAR and TBXA2 is an angiogenic eicosanoid generated by endothelial cyclooxygenase-2 (COX-2) following VEGF- or bFGF stimulation. THBD protein expression was monitored in the 3C and CASM3C BioMAP systems (up, down) and TBXA2 was assayed for ligand binding in the NovaScreen platform.

Endothelial cell migration and proliferation: the pVDC signature includes assays for human primary vascular cultures (endothelial and vascular smooth muscle cells). Assays for nuclear localization of beta-catenin (CTNB) are based on the principle that nuclear translocation activates pathways important for endothelial cell migration, proliferation and survival during capillary network formation in HUVEC cells [Muller et al. 2002; Masckauchan et al. 2005].

Vascular stabilization: The signature has features for transforming growth factor-beta 1 (TGF-b), which regulates vascular morphogenesis and integrity, and for Tie2 - a receptor tyrosine kinase activated by the angiopoietins (ANG1, ANG2) that function stabilize nascent vasculature. The pVDC signature has features for the anti-angiogenic phosphatases PTEN (phosphatase and tensin homolog), PTPN11 (tyrosine-protein phosphatase non-receptor type 11) and PTPN12, and endothelial-specific receptor tyrosine protein phosphatase beta (PTPRB). Matrix metalloproteinases (MMPs) 1/2/9 aggregate features on biochemical activity and cellular function of zinc-dependent endopeptidases MMP1, MMP2 and MMP9 that facilitate angiogenesis through ECM degradation by activated endothelial cells.

Domain of Applicability

A description of the scientific basis for the indicated domains of applicability and the WoE calls (if provided).  More help

Endothelial networks are necessary components of normal development. Direct evidence comes from the observation of severe dysmorphogenesis and embryolethality in genetic mouse models lacking a functional VEGF signaling pathway [Fong et al. 1995; Shalaby et al. 1995; Carmeliet et al. 1996; Maltepe et al. 1997; Abbott and Buckalew, 2000; Chan et al. 2002; Coultas et al. 2005; van den Akker et al. 2007; Eberlein et al. 2021]. These alterations may follow impairment of the primitive capillary network in the early embryo and extraembryonic membranes (vasculogenesis) or its subsequent expansion and patterning of the embryonic and placental vasculature (angiogenesis). Several anti-angiogenic compounds are known to impair these stages of vascular development across multiple vertebrate species (e.g., zebrafish, frog, chick, mouse, rat) [Tran et al. 2007; Therapontos et al. 2009; Jang et al. 2009; Rutland et al. 2009; Tal et al. 2014; Vargesson, 2015; Beedie et al. 2016; Ellis-Hutchings et al. 2017; Kotini et al. 2020]. Vascular patterning is known to be sensitive event in human pregnancy as well [Husain et al. 2008; van Gelder et al. 2010; Gold et al. 2011; Vargesson and Hootnick, 2017]. Anatomically, the stabilization and has varied themes for arterial, venous, and lymphatic channels [Beedie et al. 2017; Tal et al. 2017]. These events are mediated by angiogenic factors through receptor tyrosine kinases (RTKs), G-protein coupled receptors (GPCRs), and glycosyl phosphatidyl-inositol (GPI)-anchored receptors, and later vascular flow-mediated signals [Drake et al. 2007; Knudsen and Kleinstreuer, 2011]. These provide assayable targets for high-throughput screening (HTS) assays, and an open source of data screening hundreds of chemicals for impairment to the angiogenic cycle [Tran et al. 2007; Houck et al. 2009; Kleinstreuer et al. 2011; Knudsen et al. 2011 and 2013; Kleinstreuer et al. 2014; Tal et al. 2014 and 2017; McCollum et al. 2017; Saili et al. 2019; Zurlinden et al. 2020].


List of the literature that was cited for this KE description. More help

Bishop ET, Bell GT, Bloor S, Broom IJ, Hendry NFK and Wheatley DN. An in vitro model of angiogenesis: Basic features. Angiogenesis. 1999 3(4): 335-344.

Chung AS, Ferrara N. Developmental and pathological angiogenesis. Annual review of cell and developmental biology. 2011;27:563-84. PubMed PMID: 21756109.

Coultas L, Chawengsaksophak K, Rossant J. Endothelial cells and VEGF in vascular development. Nature. 2005 Dec 15;438(7070):937-45. PubMed PMID: 16355211.

Hanahan D. Signaling vascular morphogenesis and maintenance. Science. 1997 Jul 4;277(5322):48-50. PubMed PMID: 9229772.

Knudsen TB, Kleinstreuer NC. Disruption of embryonic vascular development in predictive toxicology. Birth defects research Part C, Embryo today : reviews. 2011;93(4):312-23.

Masckauchan TN, Shawber CJ, Funahashi Y, Li CM, Kitajewski J. Wnt/beta-catenin signaling induces proliferation, survival and interleukin-8 in human endothelial cells. Angiogenesis. 2005;8(1):43-51. PubMed PMID: 16132617.

McCollum CW, Conde-Vancells J, Hans C, Vazquez-Chantada M, Kleinstreuer N, Tal T, Knudsen T, Shah SS, Merchant FA, Finnell RH, Gustafsson JA, Cabrera R and Bondesson M. Identification of vascular disruptor compounds by analysis in zebrafish embryos and mouse embryonic endothelial cells. Reprod Toxicol. 2017; 70: 60-69. PMID:27838387.

Muller T, Bain G, Wang X, Papkoff J. Regulation of epithelial cell migration and tumor formation by beta-catenin signaling. Experimental cell research. 2002 Oct 15;280(1):119-33. PubMed PMID: 12372345.

Nguyen EH, Daly WT, Le NNT, Farnoodian M, Belair DG, Schwartz MP, Lebakken CS, Ananiev GE, Saghiri MA, Knudsen TB, Sheibani N and Murphy WL. Versatile synthetic alternatives to Matrigel for vascular toxicity screening and stem cell expansion. Nat Biomed Eng. 2017; 1 PMID:29104816.

Saili KS, Franzosa JA, Baker NC, Ellis-Hutchings RG, Settivari RS, Carney EW, Spencer R, Zurlinden TJ, Kleinstreuer NC, Li S, Xia M and Knudsen TB. Systems Modeling of Developmental Vascular Toxicity. Curr Opin Toxicol. 2019; 15(1): 55-63. PMID:32030360.

Sarkanen JR, Mannerstrom M, Vuorenpaa H, Uotila J, Ylikomi T, Heinonen T. Intra-Laboratory Pre-Validation of a Human Cell Based in vitro Angiogenesis Assay for Testing Angiogenesis Modulators. Frontiers in pharmacology. 2010;1:147.

Tal T, Kilty C, Smith A, LaLone C, Kennedy B, Tennant A, McCollum CW, Bondesson M, Knudsen T, Padilla S and Kleinstreuer N. Screening for angiogenic inhibitors in zebrafish to evaluate a predictive model for developmental vascular toxicity. Reprod Toxicol. 2017; 70: 70-81. PMID:28007540.

Vargesson N. Vascularization of the developing chick limb bud: role of the TGFβ signalling pathway. J Anat. 2016 Jan, 202(1): 93-103. PMCID: PMC1571066.

Zurlinden TJ, Saili KS, Baker NC, Toimela T, Heinonen T and Knudsen TB. A cross-platform approach to characterize and screen potential neurovascular unit toxicants. Reprod Toxicol. 2020; 96: 300-315. PMID:32590145.