Figure 4. Molecular mechanisms of endothelial tip cell selection.

a | Sprouting endothelial cells are hierarchically organized into leading ‘tip cells’ (TCs) and trailing ‘stalk cells’ (SCs) that exhibit very distinct and specialized cell behaviours. TC formation is induced by vascular endothelial growth factor (VEGF) signalling, whereas Delta-like 4 (DLL4)–Notch signalling represses VEGF receptor (VEGFR) signalling and TC fate in SCs. b | VEGFA and VEGFC signalling via VEGFR2 and VEGFR3 induce the invasive and motile TC behaviour that drives angiogenesis. VEGFR2 activation induces DLL4 expression in TCs, which activates Notch on adjacent SCs. VEGF-mediated disruption of a repressive translocation ETS leukaemia (TEL) and carboxy-terminal-binding protein (CtBP) complex that binds theDLL4 promoter may partially account for the ability of VEGF to induceDLL4 expression. Notch signalling in SCs downregulates the expression of VEGFR3 (encoded by FLT4) and upregulates the expression of VEGFR1 (encoded by FLT1) and soluble VEGFR1 (sVEGFR1), which represses VEGFR2 function and blocks TC behaviour. Notch activation also inducesDLL4 expression in SCs to propagate the DLL4 Notch-mediated lateral inhibition of VEGFR2 and VEGFR3 along developing vessels. Notch-induced Notch-regulated ankyrin repeat-containing protein (NRARP) expression enhances WNT signalling in SCs, which maintains EC–EC junctions, promotes proliferation and may augment DLL4 expression via β-catenin. NRARP also promotes feedback inhibition of Notch signalling. Notch signalling in TCs is blocked by Jagged 1, which is expressed in SCs and impedes DLL4–Notch interactions on TCs when Notch is glycosylated (Gl) by Fringe family glycosyltransferases in the Golgi apparatus. Deacetylation of the Notch intracellular domain (NICD) by sirtuin 1 (SIRT1) may also negatively influence TC Notch signalling. CLDN5, claudin 5; LRP, low-density lipoprotein receptor-related protein.