Neuropilin-1 on hematopoietic cells as a source of vascular development

Neuropilin-1 (NP-1) is a receptor for vascular endothelial growth factor-165 (VEGF₁₆₅) and acts as a coreceptor that enhances the function of VEGF165 through VEGF receptor-2 (VEGFR-2). Studies using transgenic and knock-out mice of NP-1 indicated that this molecule is important for vascular development as well as neuronal development. We recently reported that clustered soluble NP-1 phosphorylates VEGFR-2 on endothelial cells with a low dose of VEGF₁₆₅ and rescues the defective vascularity of the NP-1^-/- embryo in vitro and in vivo. Here we show that NP-1 is expressed by CD45⁺ hematopoietic cells in the fetal liver, can bind VEGF₁₆₅, and phosphorylates VEGFR-2 on endothelial cells. CD45⁺NP-1⁺ cells rescued the defective vasculogenesis and angiogenesis in the NP-1^-/- P-Sp (para-aortic splanchnopleural mesodermal region) culture, although CD45⁺NP-1⁺ cells did not. Moreover, CD45⁺NP-1⁺ cells together with VEGF₁₆₅ induced angiogenesis in an in vivo Matrigel assay and cornea neovascularization assay. The extracellular domain of NP-1 consists of "a," "b," and "c" domains, and it is known that the "a" and "c" domains are necessary for dimerization of NP-1. We found that both the "a" and "c" domains are essential for such rescue of defective vascularities in the NP-1 mutant. These results suggest that NP-1 enhances vasculogenesis and angiogenesis exogenously and that dimerization of NP-1 is important for enhancing vascular development. In NP-1^-/- embryos, vascular sprouting is impaired at the central nervous system (CNS) and pericardium where VEGF is not abundant, indicating that NP-1-expressing cells are required for normal vascular development.