DLL1-mediated Notch activation regulates endothelial identity in mouse fetal arteries

Notch signaling has been shown to regulate various aspects of vascular development. However, a specific role of the ligand Delta-like 1 (DLL1) has not been shown thus far. Here, we demonstrate that during fetal development, DLL1 is an essential Notch ligand in the vascular endothelium of large arteries to activate Notch1 and maintain arterial identity. DLL1 was detected in fetal arterial endothelial cells beginning at embryonic day 13.5. While DLL4-mediated activation has been shown to suppress vascular endothelial growth factor (VEGF) pathway components in growing capillary beds, DLL1-Notch signaling was required for VEGF receptor expression in fetal arteries. In the absence of DLL1 function, VEGF receptor 2 (VEGFR2) and its coreceptor, neuropilin-1 (NRP1), were down-regulatedin mutant arteries, which was followed by up-regulation of chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII), a repressor of arterial differentiation and Nrp1 expression in veins. Consistent with a positive modulation of the VEGF pathway by DLL1, the Nrp1 promoter contains several recombinant signal-binding protein 1 for J kappa (RBPJkappa)-binding sites and was responsive to Notch activity in cell culture. Our results establish DLL1 as a critical endothelial Notch ligand required for maintaining arterial identity during mouse fetal development and suggest context-dependent interrelations of the VEGFA and Notch signaling pathways.