Abstract

The products of the neuropilin-1 (Np-1) and neuropilin-2 (Np-2) genes are receptors for factors belonging to the class 3 semaphorin family and participate in the guidance of growing axons to their targets. In the presence of heparin-like molecules, both receptors also function as receptors for the heparin-binding 165-amino acid isoform of vascular endothelial growth factor (VEGF(165)). Both receptors are unable to bind to the 121-amino acid isoform of vascular endothelial growth factor (VEGF(121)), which lacks a heparin-binding domain. Interestingly, complexes corresponding in size to (125)I-VEGF(121).neuropilin complexes are formed when (125)I-VEGF(121) is bound and cross-linked to porcine aortic endothelial cells co-expressing VEGFR-1 and either Np-1 or Np-2. These complexes do not seem to represent complexes of (125)I-VEGF(121) with a truncated form of VEGFR-1, presumably formed as a result of the presence of Np-1 or Np-2 in the cells, because such truncated forms could not be detected with anti-VEGFR-1 antibodies. Antibodies directed against VEGFR-1 co-immunoprecipitated the (125)I-VEGF(121).Np-2 sized cross-linked complex along with (125)I-VEGF(121).VEGFR-1 complexes from cells expressing both VEGFR-1 and Np-2 but not from control cells, indicating that VEGFR-1 and Np-2 associate with each other. To perform the reciprocal experiment we have expressed in porcine aortic endothelial cells a Np-2 receptor containing an in-frame myc epitope at the C terminus. Surprisingly, the myc-tagged Np-2 receptor lost most of its VEGF(165) binding capacity but not its semaphorin-3F binding ability. Nevertheless, when Np-2myc was co-expressed in cells with VEGFR-1, it partially regained its VEGF(165) binding ability. Antibodies directed against the myc epitope co-immunoprecipitated (125)I-VEGF(165).Np-2myc and (125)I- VEGF(165).VEGFR-1 complexes from cells co-expressing VEGFR-1 and Np-2myc, indicating again that VEGFR-1 associates with Np-2. Our experiments therefore indicate that Np-2, and possibly also Np-1, associate with VEGFR-1 and that such complexes may be part of a cell membrane-associated signaling complex.