Abstract
Neuropilin-1 (NRP-1) is present on the cell surface of endothelial cells, or as a soluble truncated variant. Membrane NRP-1 is proposed to enhance angiogenesis by promoting the formation of a signaling complex between vascular endothelial growth factor-A(165) (VEGF-A(165)), VEGF receptor-2 (VEGFR-2) and heparan sulfate, whereas the soluble NRP-1 is thought to act as an antagonist of signaling complex formation. We have analyzed the angiogenic potential of a chimera comprising the entire extracellular NRP-1 region dimerized through an Fc IgG domain and a monomeric truncated NRP-1 variant. Both NRP-1 proteins stimulated tubular morphogenesis and cell migration in HDMECs and HUVECs. Fc rNRP-1 was able to induce VEGFR-2 phosphorylation and expression of the VEGFR-2 specific target, regulator of calcineurin-1 (RCAN1.4). siRNA mediated gene silencing of VEGFR-2 revealed that VEGFR-2 was required for Fc rNRP-1 mediated activation of the intracellular signaling proteins PLC-γ, AKT, and MAPK and tubular morphogenesis. The stimulatory activity was independent of VEGF-A(165). This was evidenced by depleting the cell culture of exogenous VEGF-A(165), and using instead for routine culture VEGF-A(121), which does not interact with NRP-1, and by the inability of VEGF-A sequestering antibodies to inhibit the angiogenic activity of the NRP proteins. Analysis of angiogenesis over a period of 6 days in an in vitro fibroblast/endothelial co-culture model revealed that Fc rNRP-1 could induce endothelial cell tubular morphogenesis. Thus, we conclude that soluble Fc rNRP-1 is a VEGF-A(165)-independent agonist of VEGFR-2 and stimulates angiogenesis in endothelial cells.