NRP1 is a membrane-bound coreceptor to a tyrosine kinase receptor for both vascular endothelial growth factor (VEGF; 192240) and semaphorin (see SEMA3A; 603961) family members. NRP1 plays versatile roles in angiogenesis, axon guidance, cell survival, migration, and invasion.

Neuropilin is a type I transmembrane protein initially identified by Takagi et al. (1987) and Fujisawa et al. (1989) as an epitope recognized by a monoclonal antibody that labels specific subsets of axons in the developing Xenopus nervous system. Neuropilin comprises in its extracellular domain several distinctive motifs; its cytoplasmic domain is short (40 amino acids) and is highly conserved among Xenopus, mouse, and chick. He and Tessier-Lavigne (1997) cloned the gene encoding human neuropilin and characterized the structure of the protein product.

Using VEGF165 to probe a breast carcinoma cell line expression library, Soker et al. (1998) cloned NRP1. The deduced 923-amino acid protein contains an N-terminal signal sequence, an ectodomain, a transmembrane region, and a cytoplasmic domain, consistent with the structure of a cell surface receptor. Northern blot analysis detected a 7.0-kb transcript. Expression was high in heart and placenta, moderate in lung, liver, skeletal muscle, kidney, and pancreas, and relatively low in brain.

Gagnon et al. (2000) cloned a 2.2-kb truncated NRP1 cDNA from a prostate carcinoma cell line cDNA library. The 3-prime end contains a unique intron-derived sequence that is absent in full-length NRP1 cDNA. The truncated cDNA encodes a deduced 664-amino acid soluble protein, designated sNRP1, that contains only the N-terminal extracellular CUB and coagulation factor homology domains. Western blot analysis detected sNRP1 secreted by cells at an apparent molecular mass of 90 kD. In situ hybridization of human tissues detected differential expression of full-length NRP1 and sNRP1 mRNA in liver, kidney, skin, and breast. Full-length NRP1, but not sNRP1, was detected in blood vessels.

Rossignol et al. (2000) identified another truncated soluble NRP1 isoform. The deduced 704-amino acid protein lacks the MAM homology, transmembrane, and C-terminal cytoplasmic domains. RT-PCR analysis detected full-length NRP1 and both soluble isoforms in all tissues examined.

Cackowski et al. (2004) identified 2 novel splice variants that encode soluble isoforms of NRP1. These isoforms, which contain 551 and 609 amino acids, are distinct from 2 previously characterized soluble isoforms containing 644 and 704 amino acids (Gagnon et al., 2000; Rossignol et al., 2000). All NRP1 soluble isoforms are identical to the full-length 923-amino acid membrane-bound protein through the N-terminal SEMA3A- and VEGF165-binding domains, but they lack the C-terminal MAM domain, transmembrane region, and cytoplasmic domain of the full-length protein.

Rossignol et al. (2000) determined that the NRP1 gene contains 17 exons and spans 120 kb. Cackowski et al. (2004) found that soluble NRP1 isoforms are derived from transcripts that are alternatively spliced after exon 9.

By somatic cell hybrid analysis, Rossignol et al. (1999) mapped the NRP1 gene to chromosome 10. They localized the gene to 10p12 by radiation hybrid mapping.

Extending axons in the developing nervous system are guided to their targets through the coordinate actions of attractive and repulsive guidance cues. The semaphorin family of guidance cues comprises several members that can function as diffusible axonal chemorepellents. He and Tessier-Lavigne (1997) set out to elucidate the mechanisms that mediate the repulsive actions of collapsin-1/semaphorin III/D (SEMA3A), referred to as 'Sema III'. By expression cloning they searched for sema III-binding proteins in embryonic rat sensory neurons. They found that sema III binds with high affinity to the transmembrane protein neuropilin, and that antibodies to neuropilin blocks the ability of sema III to repel sensory axons and to induce collapse of their growth cones. He and Tessier-Lavigne (1997) concluded that neuropilin is a receptor or a component of a receptor complex that mediates the effects of sema III on these axons.

Kolodkin et al. (1997) likewise showed that neuropilin is a semaphorin III receptor. They also identified rat neuropilin-2 (NRP2; 602070), a related neuropilin family member, and showed that neuropilin and neuropilin-2 are expressed in overlapping, yet distinct, populations of neurons in the rat embryonic nervous system.

Soker et al. (1998) confirmed that NRP1 binds VEGF165 but not VEGF121. They showed that when coexpressed in cells with KDR (VEGFR2; 191306), neuropilin-1 enhances the binding of VEGF165 to KDR and VEGF165-mediated chemotaxis. Conversely, inhibition of VEGF165 binding to neuropilin-1 inhibits its binding to KDR and its mitogenic activity for endothelial cells. Soker et al. (1998) proposed that neuropilin-1 is a novel VEGF receptor that modulates VEGF binding to KDR and subsequent bioactivity and therefore may regulate VEGF-induced angiogenesis.

There are 3 isoforms of placenta growth factor (PGF; 601121), designated PGF1, PGF2, and PGF3. Only PGF2 is able to bind heparin. Migdal et al. (1998) found that PGF2 bound NRP1 in human umbilical vein endothelial cells in a heparin-dependent fashion. Sulfation of the glucosamine-O-6 and iduronic acid-O-2 groups of heparin potentiated PGF2 binding to NRP1. NRP1 also bound PGF1 with lower affinity.

Takahashi et al. (1999) found that the 2 semaphorin-binding proteins, plexin-1 (PLXN1; 601055) and neuropilin-1 (NRP1), form a stable complex. PLXN1 alone did not bind SEMA3A, but the NRP1/PLXN1 complex had a higher affinity for SEMA3A than did NRP1 alone. While SEMA3A binding to NRP1 did not alter nonneuronal cell morphology, SEMA3A interaction with NRP1/PLXN1 complexes induced adherent cells to round up. Expression of a dominant-negative PLXN1 in sensory neurons blocked SEMA3A-induced growth cone collapse. SEMA3A treatment led to the redistribution of growth cone NRP1 and PLXN1 into clusters. Thus, the authors concluded that physiologic SEMA3A receptors consist of NRP1/PLXN1 complexes.

Gagnon et al. (2000) determined that the 644-amino acid soluble NRP1 isoform (sNRP1) bound VEGF165, but not VEGF121. It inhibited VEGF165 binding to endothelial and tumor cells and VEGF165-induced tyrosine phosphorylation of KDR in endothelial cells. Rat prostate carcinoma cells expressing recombinant sNRP1 showed extensive hemorrhage, damaged vessels, and apoptotic tumor cells. Gagnon et al. (2000) concluded that sNRP1 appears to be a VEGF165 antagonist.

By raising monoclonal antibodies against immunomagnetically purified CD4 (186940)-positive blood dendritic cells (BDCs), Dzionek et al. (2000) identified 3 BDC antigens: BDCA2 (CLEC4C; 606677), BDCA3, and BDCA4. In fresh human blood, expression of BDCA2 and BDCA4 was strictly confined to plasmacytoid CD123 (IL3RA; 308385)-bright/CD11C (ITGAX; 151510)-negative BDCs, whereas expression of BDCA3 was restricted to a small population of CD123-negative/CD11C-positive BDCs. This small population of BDCA3-positive BDCs shared many immunophenotypic features with classical CD123-dim/CD11C-bright BDCs, but unlike those BDCs, BDCA3-positive BDCs lacked expression of BDCA1 (CD1C; 188340), CD2 (186990), and several Fc receptors (see 146790).

Most striatal and cortical interneurons arise from the basal telencephalon, later segregating to their respective targets. Marin et al. (2001) demonstrated that migrating cortical interneurons avoid entering the striatum because of a chemorepulsive signal composed at least in part of semaphorin-3A (603961) and semaphorin-3F (601124). Migrating interneurons expressing neuropilins, receptors for semaphorins, are directed to the cortex; those lacking them go to the striatum. Loss of neuropilin function increases the number of interneurons that migrate into the striatum. Marin et al. (2001) concluded that their observations reveal a mechanism by which neuropilins mediate sorting of distinct neuronal populations into different brain structures, and provide evidence that, in addition to guiding axons, these receptors also control neuronal migration in the central nervous system.

Primary immune responses require contact between dendritic cells (DC) and resting naive T cells in secondary lymphoid organs. Noting that in 1868 Paul Langerhans described the analogy between DC (subsequently termed Langerhans cells) and neurons in studies of the nerves in human skin, Tordjman et al. (2002) proposed that NRP1 may be expressed by DC. Using immunofluorescence microscopy, RT-PCR, and immunoblot analysis, as well as flow cytometry, Tordjman et al. (2002) detected NRP1 on dendritic cells and resting T lymphocytes. After T-cell contact with DC, T-cell NRP1 colocalized with CD3 (see 186780) in the immunologic synapse and, sometimes, also at the opposite pole of the T cell. Soluble NRP1 interacts in a homophilic fashion with NRP1 on both DC and T cells, and this binding can be inhibited by blocking antibodies to NRP1. Ectopic expression of NRP1 in COS cells induced cluster formation with resting T cells. In the presence of blocking antibody to NRP1, cluster formation between DC and resting, but not activated, T cells is partially inhibited as is T cell proliferation, reflecting the presence of numerous other adhesion or integrin molecules such as CD58 (LFA3; 153420) involved in the stabilization of DC-T cell contact. Tordjman et al. (2002) concluded that NRP1-mediated interactions are a necessary element in the initiation of the primary immune response and offer another example, like that of agrin (103320), of a molecule shared by neurologic and immunologic synapses.

Neuropilin-1, earlier identified as a neuronal receptor that mediates repulsive growth cone guidance, functions also in endothelial cells as an isoform-specific receptor for vascular endothelial growth factor VEGF165 and as a coreceptor in vitro of VEGFR2. Oh et al. (2002) showed that VEGF selectively upregulates NRP1 but not NRP2 via the VEGF receptor 2-dependent pathway. In a murine model of VEGF-dependent angioproliferative retinopathy, intense NRP1 mRNA expression was observed in the newly formed vessels. Furthermore, selective NRP1 inhibition in this model suppressed neovascular formation substantially. These results suggested that VEGF cannot only activate endothelial cells directly but also contribute to robust angiogenesis in vivo by a mechanism that involves upregulation of its cognate receptor expression.

Cackowski et al. (2004) found that the 551- and 609-amino acid soluble NRP1 isoforms showed binding capacities for SEMA3A and VEGF165 similar to that of the 644-amino acid soluble NRP1 isoform. In addition, all 3 of these isoforms inhibited full-length NRP1-mediated migration in a breast carcinoma cell line. Cackowski et al. (2004) concluded that the soluble NRP1 isoforms antagonize NRP1-mediated cellular activities.

Using immunohistochemistry, Lepelletier et al. (2007) found that NP1 and SEMA3A were expressed in thymic epithelial cells (TECs) and CD4/CD8 (see 186910) thymocytes. Both IL7 (146660), which is constitutively secreted by TECs, and T-cell receptor (TCR) engagement upregulated NP1 expression in thymocytes. SEMA3A blocked adhesion of NP1-positive thymocytes to TECs and induced thymocyte repulsive migration, partially by inhibiting binding of very late antigens (see ITGA4; 192975) to laminin (see LAMA1; 150320). Lepelletier et al. (2007) concluded that NP1 and SEMA3A interactions are important in regulation of migration and adhesion of thymocytes.

Sarris et al. (2008) found that mouse Nrp1, which is expressed by most regulatory T cells (Tregs), but not by naive T-helper cells, promoted prolonged interactions with immature dendritic cells, resulting in higher sensitivity to limiting amounts of antigen. They proposed that Treg cells have an advantage over naive Th cells, with the same specificity leading to a default suppression of immune responses in the absence of proinflammatory 'danger signals.'

Imai et al. (2009) analyzed the pre-target axon sorting for olfactory map formation in mice. In olfactory sensory neurons, an axon guidance receptor, neuropilin-1, and its repulsive ligand, semaphorin-3A (SEMA3A; 603961), are expressed in a complementary manner. Imai et al. (2009) found that expression levels of neuropilin-1 determined both pre-target sorting and projection sites of axons. Olfactory sensory neuron-specific knockout of semaphorin-3A perturbed axon sorting and altered the olfactory map topography. Thus, Imai et al. (2009) concluded that pre-target axon sorting plays an important role in establishing the topographic order based on the relative levels of guidance molecules expressed by axons.

Tran et al. (2009) found that a Sema3A/Npn1/PlexA4 (604280) signaling cascade controls basal dendritic arborization in layer V cortical neurons, but does not influence spine morphogenesis or distribution. In contrast, they demonstrated that the secreted semaphorin Sema3F (601124) is a negative regulator of spine development and synaptic structure. Mice with null mutations in genes encoding Sema3F and its holoreceptor components Npn2 (602070) and plexin A3 (PLEXA3; 300022) exhibit increased dentate gyrus granule cell and cortical layer V pyramidal neuron spine number and size, and also aberrant spine distribution. Moreover, Sema3F promotes loss of spines and excitatory synapses in dissociated neurons in vitro, and in Npn2-null brain slices cortical layer V and dentate gyrus granule cells exhibit increased miniature excitatory postsynaptic current frequency. These disparate effects of secreted semaphorins are reflected in the restricted dendritic localization of Npn2 to apical dendrites and of Npn1 to all dendrites of cortical pyramidal neurons.

Beck et al. (2011) used a mouse model of skin tumors to investigate the impact of the vascular niche and VEGF (VEGFA; 192240) signaling on controlling the stemness of squamous skin tumors during the early stages of tumor progression. They showed that cancer stem cells of skin papillomas are localized in a perivascular niche, in the immediate vicinity of endothelial cells. Furthermore, blocking Vegfr2 (191306) caused tumor regression not only by decreasing the microvascular density, but also by reducing cancer stem cell pool size and impairing cancer stem cell renewal properties. Conditional deletion of Vegfa in tumor epithelial cells caused tumors to regress, whereas Vegf overexpression by tumor epithelial cells accelerated tumor growth. In addition to its well-known effect on angiogenesis, Vegf affected skin tumor growth by promoting cancer stemness and symmetric cancer stem cell division, leading to cancer stem cell expansion. Moreover, deletion of Nrp1, a VEGF coreceptor expressed in cutaneous cancer stem cells, blocked Vegf's ability to promote cancer stemness and renewal. Beck et al. (2011) concluded that their results identified a dual role for tumor cell-derived VEGF in promoting cancer stemness: by stimulating angiogenesis in a paracrine manner, VEGF creates a perivascular niche for cancer stem cells, and by directly affecting cancer stem cells through NRP1 in an autocrine loop, VEGF stimulates cancer stemness and renewal. Finally, deletion of NRP1 in normal epidermis prevents skin tumor initiation.

Hayashi et al. (2012) showed that Sema3A exerts an osteoprotective effect by both suppressing osteoclastic bone resorption and increasing osteoblastic bone formation. The binding of Sema3A to Nrp1 inhibited RANKL (602642)-induced osteoclast differentiation by inhibiting ITAM (608740) and RhoA (165390) signaling pathways. In addition, Sema3A and Nrp1 binding stimulated osteoblast and inhibited adipocyte differentiation through the canonical Wnt/beta-catenin signaling pathway (see 116806). The osteopenic phenotype in Sema3a-null mice was recapitulated by mice in which the Sema3A-binding site of Nrp1 had been genetically disrupted. Intravenous Sema3A administration in mice increased bone volume and expedited bone regeneration.

Delgoffe et al. (2013) showed that the immune cell-expressed ligand semaphorin 4A (SEMA4A; 607292) and the Treg cell-expressed receptor Nrp1 interact both in vitro, to potentiate Treg-cell function and survival, and in vivo, at inflammatory sites. Using mice with a Treg cell-restricted deletion of Nrp1, Delgoffe et al. (2013) showed that Nrp1 is dispensable for suppression of autoimmunity and maintenance of immune homeostasis, but is required by Treg cells to limit antitumor immune responses and to cure established inflammatory colitis. Sema4a ligation of Nrp1 restrained Akt (164730) phosphorylation cellularly and at the immunologic synapse by Pten (601728), which increased nuclear localization of the transcription factor Foxo3a (602681). The Nrp1-induced transcriptome promoted Treg cell stability by enhancing quiescence and survival factors while inhibiting programs that promote differentiation. Importantly, this Nrp1-dependent molecular program is evident in intratumoral Treg cells. Delgoffe et al. (2013) concluded that their data supported a model in which Treg-cell stability can be subverted in certain inflammatory sites, but is maintained by a Sema4a-Nrp1 axis.

Zhang et al. (2018) showed that preventing lacteal chylomicron uptake by inducible endothelial genetic deletion of Nrp1 and Vegfr1 (FLT1; 165070) renders mice resistant to diet-induced obesity. Absence of Nrp1 and Flt1 receptors increased Vegfa bioavailability and signaling through Vegfr2 inducing lacteal junction zippering and chylomicron malabsorption. Restoring permeable lacteal junctions by Vegfr2 and vascular endothelial cadherin (VE-cadherin; 601120) signaling inhibition rescued chylomicron transport in mutant mice. Zippering of lacteal junctions by disassembly of cytoskeletal VE-cadherin anchors prevented chylomicron uptake in wildtype mice.

Because mutant mice lacking a functional SEMA3A-binding domain in NRP1 have a Kallmann syndrome-like phenotype (see HH16, 614897), Hanchate et al. (2012) analyzed the NRP1 gene in 24 patients with Kallmann syndrome carrying heterozygous mutations in the SEMA3A gene and in 100 Kallmann syndrome patients without SEMA3A mutations, but found no mutations. The authors concluded that mutations in NRP1 are rare or not present in patients with Kallmann syndrome.

Associations Pending Confirmation

For discussion of a possible relationship between variation in the NRP1 gene and truncus arteriosus, see 602069.0001.

Takashima et al. (2002) showed that transgenic mice died in utero at embryonic day 8.5 when both Nrp1 and Nrp2, which they called Np1 and Np2, respectively, were knocked out. The yolk sacs of these mice were totally avascular. Mice deficient for Nrp2 but heterozygous for Nrp1 or deficient for Nrp1 but heterozygous for Nrp2 were also embryonic lethal and survived to embryonic days 10 to 10.5. Other details of the abnormal vascular phenotype resembled those of Vegf and Vefgr2 knockouts. The results suggested that neuropilins are early genes in embryonic vessel development and that both NRP1 and NRP2 are required.

NRP1 is a cell surface receptor for both VEGF and SEMA3A and is expressed by both neurons and endothelial cells. Lee et al. (2002) showed that in zebrafish the Nrp1 protein was a functional receptor for human VEGF165. Whole-mount in situ hybridization showed that transcripts of the zebrafish NRP1 gene during embryonic and early larval development were detected mainly in neuronal and vascular tissues. A knockdown of the gene in embryos resulted in vascular defects. Embryos treated with VEGFR2 kinase inhibitor had a similar vessel defect, suggesting that knockdown of zebrafish NRP1 reduces VEGF activity. To determine whether NRP1 and VEGF activities are interdependent in vivo, zebrafish NRP1 and VEGF morpholinos were coinjected into embryos at concentrations that individually did not significantly inhibit blood vessel development. The result was a potent inhibition of blood cell circulation via both intersegmental and axial vessels, demonstrating that VEGF and NRP1 act synergistically to promote a functional circulatory system. These results provided the first physiologic demonstration that NRP1 regulated angiogenesis through a VEGF-dependent pathway.

Gu et al. (2003) generated Npn1 knockin mice, which expressed a variant of Npn1 with altered ligand binding, and conditional Npn1-null mice. They determined that Vegf-Npn1 signaling in endothelial cells was required for angiogenesis. Sema-Npn1 signaling was dispensable for angiogenesis, but it was required for axonal pathfinding by several populations of neurons in the central and peripheral nervous systems. Both Vegf-Npn1 and Sema-Npn1 signaling were critical for heart development.

Young et al. (2012) generated mice lacking a functional semaphorin (see 603961)-binding domain in Nrp1 and observed the development of a Kallmann syndrome-like phenotype (see HH16, 614897). Pathohistologic analysis of the mutant mice showed abnormal development of the peripheral olfactory system and defective embryonic migration of the neuroendocrine GnRH cells to the basal forebrain, which resulted in increased mortality of newborn mice and reduced fertility in adults.