Upstream eventInhibition, VegfR2
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Disruption of VEGFR Signaling Leading to Developmental Defects||adjacent||High||High|
Life Stage Applicability
Key Event Relationship Description
VEGFR2 is the master switch of angiogenesis, which is triggered by binding to its cognate ligand (VEGF-A). In the induction of an angiogenic sprout, VEGF-A triggers a response in endothelial cells (EC). VEGF-A has a mitogenic effect on EC 'stalk cells' and promotes exploratory and behavior and motility of the pioneering ‘tip cells’. The latter extend filopodial processes toward the VEGF-A gradient to pioneer sprouting, while stalk cells proliferate to extend the microvessel and lumenize it for blood transport. Angiogenic sprouting is a complex molecular process [Herbert and Stanier 2011]. An early step is tip cell selection. Endothelial cells are normally suppressed in their tip cell behaviors due to lateral inhibition by Notch-Delta. Lateral inhibition is broken when VEGFR2 is activated by VEGF-A by an uncertain mechanism. Next, EC tip cells extend filopodial processes and migrate along VEGF-A corridors. Their branching to a tree-like network is patterned by the EphrinB2–EPHB4 cell adhesion system. Proliferating EC stalk cells meanwhile follow the pioneering sprout.
Evidence Supporting this KER
VEGFR2 is the most important VEGF-A receptor and is the 'master switch' for angiogenic sprouting [Herbert and Stanier 2011].
Vascular endothelial growth factor-A (VEGF-A), in particular the VEGF165 splice variant, plays a key role in the regulation of angiogenesis during early embryogenesis. This is evidenced by immature blood vessel formation and embryonic lethality in mutant mouse embryos heterozygous for the Vegfa-null allele [Ferrara et al. 1996; Carmellet et al. 1996]. Targeted disruption of genes encoding VEGFR1 or VEGFR2 are early embryonic lethal; however, the vascular phenotypes differ in either case. Whereas VEGFR1-mutant (Flt1-null) embryos display excessive endothelial cell growth and disorganization of the vascular network [Fong et al. 1995], VEGFR2-mutant (Flk1-null) embryos die from a lack blood vessel network formation [Shalaby et al. 1995].
Uncertainties and Inconsistencies
Many physiological signals influence VEGF-A production (e.g., hypoxia, estrogen) and post-VEGFR2 signaling. For example, VEGFR2 signals may be influenced by crosstalk with VEGFR1 and VEGFR3, other receptor tyrosine kinases (FGFR, EGFR), G-protein coupled receptors (CXCRs and CCRs), and GPI-linked surface receptors (uPAR) [Kleinstreuer et al. 2011]. The ToxCast pVDC signature includes assays for many of these targets and shows that environmental chemicals perturbing VEGFR2 also affect molecular targets in some other signaling system [Knudsen et al. 2016]. As such, quantitative linkages to VEGF signaling must consider the uncertainties from effects to other MIEs.
Quantitative Understanding of the Linkage
Studies with pharmacological VEGFR2 inhibitors have shown a concentration dependent effect on angiogenic sprouting in zebrafish embryos [Tal et al. 2014] and human endothelial cells [Belair et al. 2016].
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Studies have demonstrated a quantitative relationship between VEGFR2 signaling and angiogenic sprouting dynamics in human endothelial cells [Belair et al. 2016] and zebrafish embryos [Shirinifard et al. 2013].
Belair D, Schwartz MP, Knudsen T and Murphy WL. Human iPSC-Derived Endothelial Cell Sprouting Assay in Synthetic Hydrogel Arrays. Acta Biomaterialia 2016. (in press).
Carmellet P, Ferreira V, Breier G, Pollefeyt S, Kleckens L, Gertsenstein M, Fahrig M, Vandenhoeck A, Harpal K, Eberhardt C, Declercq C, Pawlling J, Moons L, Collen D, Resau W, Nagy A (1996) Abnormal blood vessel development and lethality in embryos lacking a single VEGF allele. Nature 380: 435–439.
Ferrara N, Carver-Moore K, Chen H, Dowd M, Lu L, O’Shea KS, Powell-Braxton L, Hillan KJ, Moore MW (1996) Heterozygous embryonic lethality induced by targeted inactivation of the VEGF gene. Nature. 380: 439–442.
Fong GH, Rossant J, Gertsenstein M, Breitman ML (1995) Role of the Flt-1 receptor tyrosine kinase in regulating the assembly of vascular endothelium. Nature 376: 66-70.
Herbert SP and Stainier DY. Molecular control of endothelial cell behaviour during blood vessel morphogenesis. Nat Rev Mol Cell Biol. 2011 Aug 23;12(9):551-564. doi: 10.1038/nrm3176.
Shalaby F, Rossant J, Yamaguchi TP, Gertsenstein M, Wu XF, Breitman ML, Schuh AC (1995) Failure of blood-island formation and vasculogenesis in Flk-1-deficient mice. Nature 376: 62-66.
Shirinifard A, McCollum CW, Bolin MB, Gustafsson JA, Glazier JA, Clendenon SG. 3D quantitative analyses of angiogenic sprout growth dynamics. Developmental dynamics : an official publication of the American Association of Anatomists. 2013;242(5):518-26.
Tal TL, McCollum CW, Harris PS, Olin J, Kleinstreuer N, Wood CE, Hans C, Shah S, Merchant FA, Bondesson M, Knudsen TB, Padilla S and Hemmer MJ (2014) Immediate and long-term consequences of vascular toxicity during zebrafish development. Reproductive Toxicology 48:51-61.