Expression of a cell adhesion molecule, neuropilin, in the developing chick nervous system

Transmembrane signaling proteoglycans

Virtually all metazoan cells contain at least one and usually several types of transmembrane proteoglycans. These are varied in protein structure and type of polysaccharide, but the total number of vertebrate genes encoding transmembrane proteoglycan core proteins is less than 10. Some core proteins, including those of the syndecans, always possess covalently coupled glycosaminoglycans; others do not. Syndecan has a long evolutionary history, as it is present in invertebrates, but many other transmembrane proteoglycans are vertebrate inventions. The variety of proteins and their glycosaminoglycan chains is matched by diverse functions. However, all assume roles as coreceptors, often working alongside high-affinity growth factor receptors or adhesion receptors such as integrins. Other common themes are an ability to signal through their cytoplasmic domains, often to the actin cytoskeleton, and linkage to PDZ protein networks. Many transmembrane proteoglycans associate on the cell surface with metzincin proteases and can be shed by them. Work with model systems in vivo and in vitro reveals roles in growth, adhesion, migration, and metabolism. Furthermore, a wide range of phenotypes for the core proteins has been obtained in mouse knockout experiments. Here some of the latest developments in the field are examined in hopes of stimulating further interest in this fascinating group of molecules.

Neuropilin-1 antagonism in human carcinoma cells inhibits migration and enhances chemosensitivity

BACKGROUND

Neuropilin-1 (NRP1) is a non-tyrosine kinase receptor for vascular endothelial growth factor (VEGF) recently implicated in tumour functions.

METHODS

In this study we used a specific antagonist of VEGF binding to the NRP1 b1 domain, EG3287, to investigate the functional roles of NRP1 in human carcinoma cell lines, non-small-cell lung A549, kidney ACHN, and prostate DU145 cells expressing NRP1, and the underlying mechanisms involved.

RESULTS

EG3287 potently displaced the specific binding of VEGF to NRP1 in carcinoma cell lines and significantly inhibited the migration of A549 and ACHN cells. Neuropilin-1 downregulation by siRNA also decreased cell migration. EG3287 reduced the adhesion of A549 and ACHN cells to extracellular matrix (ECM), and enhanced the anti-adhesive effects of a beta1-integrin function-blocking antibody. EG3287 increased the cytotoxic effects of the chemotherapeutic agents 5-FU, paclitaxel, or cisplatin on A549 and DU145 cells, through inhibition of integrin-dependent cell interaction with the ECM.

CONCLUSIONS

These findings indicate that NRP1 is important for tumour cell migration and adhesion, and that NRP1 antagonism enhances chemosensitivity, at least in part, by interfering with integrin-dependent survival pathways. A major implication of this study is that therapeutic strategies targeting NRP1 in tumour cells may be particularly useful in combination with other drugs for combating tumour survival, growth, and metastatic spread independently of an antiangiogenic effect of blocking NRP1.

Neuropilin ligands in vascular and neuronal patterning

Blood vessels and neurons share guidance cues and cell-surface receptors to control their behaviour during embryogenesis. The transmembrane protein NRP1 (neuropilin 1) is present on both blood vessels and nerves and binds two structurally diverse ligands, the class 3 semaphorin SEMA3A and an isoform of the vascular endothelial growth factor VEGF-A termed VEGF(165) (VEGF(164) in mice). In vitro, SEMA3A competes with VEGF(164) for binding to NRP1 to modulate the migration of endothelial cells and neuronal progenitors. It was therefore hypothesized that NRP1 signalling controls neurovascular co-patterning by integrating competing VEGF(164) and SEMA3A signals. However, SEMA3A, but not VEGF(164), is required for axon patterning of motor and sensory nerves, and, vice versa, VEGF(164) rather than SEMA3A is required for blood vessel development. Ligand competition for NRP1 therefore does not explain neurovascular congruence. Instead, these ligands control different aspects of neurovascular patterning that have an impact on cardiovascular function. Thus SEMA3A/NRP1 signalling guides the NCC (neural crest cell) precursors of sympathetic neurons as well as their axonal projections. In addition, VEGF(164) and a second class 3 semaphorin termed SEMA3C contribute to the remodelling of the embryonic pharyngeal arch arteries and primitive heart outflow tract by acting on endothelium and NCCs respectively. Consequently, loss of either of these NRP1 ligands disrupts blood flow into and out of the heart. Multiple NRP1 ligands therefore co-operate to orchestrate cardiovascular morphogenesis.

Pulmonary epithelial neuropilin-1 deletion enhances development of cigarette smoke-induced emphysema

RATIONALE

Cigarette smoke (CS) exposure is an important risk factor for chronic obstructive pulmonary disease; however, not all smokers develop disease, suggesting that other factors influence disease development.

OBJECTIVES

We sought to determine whether neuropilin-1 (Nrp1), an integral component of receptor complexes mediating alveolar septation and vascular development, was involved in maintenance of normal alveolar structure, and/or altered susceptibility to the effects of CS.

METHODS

Transgenic mice were generated to achieve inducible lung-specific deletion of epithelial Nrp1. We determined whether conditional Nrp1 deletion altered airspace size, then compared the effects of chronic CS or filtered air exposure on airspace size, inflammation, and the balance between cell death and proliferation in conditionally Nrp1-deficient adult mice and littermate controls. Finally, we evaluated the effects of Nrp1 silencing on cell death after acute exposure of A549 cells to cigarette smoke extract or short chain ceramides.

MEASUREMENTS AND MAIN RESULTS

Genetic deletion of epithelial Nrp1 in either postnatal or adult lungs resulted in a small increase in airspace size. More notably, both airspace enlargement and apoptosis of type I and type II alveolar epithelial cells were significantly enhanced following chronic CS exposure in conditionally Nrp1-deficient adult mice. Silencing of Nrp1 in A549 cells did not alter cell survival after vehicle treatment but significantly augmented apoptosis after exposure to cigarette smoke extract or ceramide.

CONCLUSIONS

These data support a role for epithelial Nrp1 in the maintenance of normal alveolar structure and suggest that dysregulation of Nrp1 expression may promote epithelial cell death in response to CS exposure, thereby enhancing emphysema development.

Pre-target axon sorting establishes the neural map topography

Sensory information detected by the peripheral nervous system is represented as a topographic map in the brain. It has long been thought that the topography of the map is determined by graded positional cues that are expressed by the target. Here, we 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, are expressed in a complementary manner. We 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, pre-target axon sorting plays an important role in establishing the topographic order based on the relative levels of guidance molecules expressed by axons.

In vitro guidance of retinal axons by a tectal lamina-specific glycoprotein Nel

Nel is a glycoprotein containing five chordin-like and six epidermal growth factor-like domains and is strongly expressed in the nervous system. In this study, we have examined expression patterns and in vitro functions of Nel in the chicken retinotectal system. We have found that in the developing tectum, expression of Nel is localized in specific laminae that retinal axons normally do not enter, including the border between the retinorecipient and non-retinorecipient laminae. Nel-binding activity is detected on retinal axons both in vivo and in vitro, suggesting that retinal axons express a receptor for Nel. In vitro, Nel inhibits retinal axon outgrowth and induces growth cone collapse and axon retraction. These results indicate that Nel acts as an inhibitory guidance cue for retinal axons, and suggest its roles in the establishment of the lamina-specificity in the retinotectal projection.

Extracellular protein kinase CK2 is a novel associating protein of neuropilin-1

Neuropilin-1 (NRP1) is a multifunctional transmembrane protein which has a short cytoplasmic region with no particular functional domain, and is considered to act as a co-receptor for both VEGFs and semaphorins. However, the molecular mechanisms by which NRP1 carries out such versatile functions are still poorly understood. Here we identified protein kinase CK2 holoenzyme as a novel NRP1 binding protein by our combined purification strategy using epitope-tag immunoprecipitation followed by reverse-phase column chromatography. Further we showed that CK2 binds to the extracellular domain of NRP1 which is also phosphorylated by CK2 both in vitro and in vivo. Our findings of novel molecular interactions and modification of NRP1 may provide a new clue to understand the diverse functions of NRP1.

Distinct roles of neuropilin 1 signaling for radial and tangential extension of callosal axons

Cortical excitatory neurons migrate from their origin in the ventricular zone (VZ) toward the pial surface. During migration, these neurons exhibit a stellate shape in the intermediate zone (IZ), transform into bipolar cells, and then initiate radial migration, extending a trailing process, which may lead to an axon. Here we examined the role of neuropilin 1 (NRP1) in these developmental events. Both NRP1 mRNA and protein were highly expressed in the IZ, where stellate-shaped cells were located. DiI labeling experiments showed that neuronal migration occurred normally in Nrp1 mutant mice up to embryonic day (E) 14.5, the latest day to which the mutant survives, with only subtle axonal defasciculation. However, interference with Nrp1 signaling at a later stage caused pathfinding errors: when a dominant negative form of Nrp1 was electroporated into the cortical VZ cells at E12.5 or E15.5 and examined perinatally, guidance errors were found in tangential axonal extension toward the midline. In contrast, no significant effect was noted on the migration of cortical excitatory neurons. These findings indicate that NRP1 plays an important role in the guidance of callosal axons originating from cortical excitatory neurons but does not support a role in their migration. Moreover, insofar as radial axonal extension within the cortical plate was unaffected, the present findings imply that molecular mechanisms for the axonal extension of excitatory neurons within the cortical plate are distinct from those in the white matter.

Vocal control area-related expression of neuropilin-1, plexin-A4, and the ligand semaphorin-3A has implications for the evolution of the avian vocal system

The avian vocal system is a good model for exploring the molecular basis of neural circuit evolution related to behavioral diversity. Previously, we conducted a comparative gene expression analysis among two different families of vocal learner, the Bengalese finch (Lonchura striata var. domestica), a songbird, and the budgerigar (Melopsittacus undulatus), a parrot; and a non-learner, the quail (Coturnix coturnix), to identify various axon guidance molecules such as cadherin and neuropilin-1 as vocal control area-related genes. Here, we continue with this study and examine the expression of neuropilin and related genes in these species in more detail. We found that neuropilin-1 and its coreceptor, plexin-A4, were expressed in several vocal control areas in both Bengalese finch and budgerigar brains. In addition, semaphorin-3A, the ligand of neuropilin-1, expression was not detected in vocal control areas in both species. Furthermore, there was some similar gene expression in the quail brain. These results suggest the possibility that a change in the expression of a combination of semaphorin/neuropilin/plexin was involved in the acquisition of vocal learning ability during evolution.

Reprogramming multipotent tumor cells with the embryonic neural crest microenvironment

The embryonic microenvironment is an important source of signals that program multipotent cells to adopt a particular fate and migratory path, yet its potential to reprogram and restrict multipotent tumor cell fate and invasion is unrealized. Aggressive tumor cells share many characteristics with multipotent, invasive embryonic progenitors, contributing to the paradigm of tumor cell plasticity. In the vertebrate embryo, multiple cell types originate from a highly invasive cell population called the neural crest. The neural crest and the embryonic microenvironments they migrate through represent an excellent model system to study cell diversification during embryogenesis and phenotype determination. Recent exciting studies of tumor cells transplanted into various embryo models, including the neural crest rich chick microenvironment, have revealed the potential to control and revert the metastatic phenotype, suggesting further work may help to identify new targets for therapeutic intervention derived from a convergence of tumorigenic and embryonic signals. In this mini-review, we summarize markers that are common to the neural crest and highly aggressive human melanoma cells. We highlight advances in our understanding of tumor cell behaviors and plasticity studied within the chick neural crest rich microenvironment. In so doing, we honor the tremendous contributions of Professor Elizabeth D. Hay toward this important interface of developmental and cancer biology.

Neuropilin-1-VEGFR-2 complexing requires the PDZ-binding domain of neuropilin-1

Vascular endothelial growth factor (VEGF) acts as a hierarchically high switch of the angiogenic cascade by interacting with its high affinity VEGF receptors and with neuropilin co-receptors. VEGF(165) binds to both Neuropilin-1 (NP-1) and VEGFR-2, and it is believed that ligand binding forms an extracellular bridge between both molecules. This leads to complex formation, thereby enhancing VEGFR-2 phosphorylation and subsequent signaling. We found that inhibition of VEGF receptor (VEGFR) phosphorylation reduced complex formation between NP-1 and VEGFR-2, suggesting a functional role of the cytoplasmic domain of VEGFR-2 for complex formation. Correspondingly, deleting the PDZ-binding domain of NP-1 decreased complex formation, indicating that extracellular VEGF(165) binding is not sufficient for VEGFR-2-NP-1 interaction. Synectin is an NP-1 PDZ-binding domain-interacting molecule. Experiments in Synectin-deficient endothelial cells revealed reduced VEGFR-2-NP-1 complex formation, suggesting a role for Synectin in VEGFR-2-NP-1 signaling. Taken together, the experiments have identified a novel mechanism of NP-1 interaction with VEGFR-2, which involves the cytoplasmic domain of NP-1.

Lamina-specific axonal projections in the zebrafish tectum require the type IV collagen Dragnet

The mechanisms underlying the precise targeting of tectal layers by ingrowing retinal axons are largely unknown. In zebrafish, individual axons choose one of four retinorecipient layers upon entering the tectum and remain restricted to this layer, despite continual remodeling and shifting of their terminal arbors. In dragnet mutants, by contrast, a large fraction of retinal axons aberrantly trespass between layers or form terminal arbors that span two layers. The dragnet gene, drg, encodes collagen IV(alpha5) (Col4a5), a basement membrane component lining the surface of the tectum. Heparan sulfate proteoglycans (HSPGs) are normally associated with the tectal basement membrane but are dispersed in the dragnet mutant tectum. Zebrafish boxer (extl3) mutants, which are deficient in HSPG synthesis, show laminar targeting defects similar to those in dragnet. Our results show that the collagen IV sheet anchors secreted factors at the surface of the tectum, which serve as guidance cues for retinal axons.

Targeting neuropilin 1 as an antitumor strategy in lung cancer

PURPOSE

Neuropilin 1 (NRP1) is a mediator of lung branching and angiogenesis in embryonic development and angiogenesis in cancer. The role of NRP1 in cancer progression is not fully elucidated. We investigated the role of NRP1 in cancer invasion and tumor angiogenesis, its signaling pathways, prognostic significance, and therapeutic implications.

EXPERIMENTAL DESIGN

Sixty patients with non-small cell lung cancer (NSCLC) were studied. NRP1 mRNA expression was measured using real-time quantitative reverse-transcription PCR. NRP1 and cancer cell invasion, angiogenesis, and signaling pathways were studied using NRP1 stimulation by vascular endothelial growth factor 165 (VEGF(165)) and NRP1 inhibition by small interfering RNAs (siRNA), soluble NRP1 (sNRP1), and NRP1-inhibition peptides. The NRP1-inhibition peptides were identified using a phage display peptide library.

RESULTS

NSCLC patients with high expression of NRP1 had shorter disease-free (P = 0.0162) and overall survival (P = 0.0164; log-rank test). Multivariate analyses showed NRP1 is an independent prognostic factor in overall (HR, 2.37, 95% CI = 1.15 to 4.9, P = 0.0196) and disease-free survival [hazard ratio (HR), 2.38; 95% confidence interval (95% CI), 1.15-4.91; P = 0.0195] of NSCLC patients. Knockdown of NRP1 suppressed cancer cell migration, invasion, filopodia formation, tumorigenesis, angiogenesis, and in vivo metastasis. NRP1 signaling pathways involved VEGF receptor 2 and phosphoinositide-3-kinase (PI3K) and Akt activation. Two potent synthetic anti-NRP1 peptides, DG1 and DG2, which block NRP1 signaling pathways and suppress tumorigenesis, cancer invasion, and angiogenesis, were identified.

CONCLUSIONS

NRP1 is a cancer invasion and angiogenesis enhancer. NRP1 expression is an independent predictor of cancer relapse and poor survival in NSCLC patients. NRP1 plays a critical role in tumorigenesis, cancer invasion, and angiogenesis through VEGF, PI3K, and Akt pathways. NRP1 may have potential as a new therapeutic target in NSCLC.

Structural similarities and functional diversity of eukaryotic discoidin-like domains

The discoidin domain is a approximately 150 amino acid motif common in both eukaryotic and prokaryotic proteins. It is found in a variety of extracellular, intracellular and transmembrane multidomain proteins characterized by a considerable functional diversity, mostly involved in developmental processes. The biological role of the domain depends on its interactions with different molecules, including growth factors, phospholipids and lipids, galactose or its derivatives, and collagen. The conservation of the motif, as well as the serious physiological consequences of discoidin domain disorders underscore the importance of the fold, while the ability to accommodate such an extraordinarily broad range of ligand molecules makes it a fascinating research target. In present review we characterize the distinctive features of discoidin domains and briefly outline the biological role of this module in various eukaryotic proteins.

Semaphorins in development and adult brain: Implication for neurological diseases

As a group, Semaphorins are expressed in most tissues and this distribution varies considerably with age. Semaphorins are dynamically expressed during embryonic development and their expression is often associated with growing axons. This expression decreases with maturity and several observations support the idea that in adult brain the expression of secreted Semaphorins is sensitive to electrical activity and experience. The functional role of Semaphorins in guiding axonal projections is well established and more recent evidence points to additional roles in the development, function and reorganization of synaptic complexes. Semaphorins exert the majority of their effects by binding to cognate receptor proteins through their extracellular domains. A common theme is that Semaphorin-triggered signalling induces the rearrangement of the actin and microtubule cytoskeleton. Mutations in Semaphorin genes are linked to several human diseases associated with neurological changes, but their actual influence in the pathogenesis of these diseases remains to be demonstrated. In addition, Semaphorins and their receptors are likely to mediate cross-talk between neurons and other cell types, including in pathological situations where their influence can be damaging or favourable depending on the context. We discuss how the manipulation of Semaphorin function might be crucial for future clinical studies.

Vascular endothelial growth factor (VEGF) signaling in tumor progression

Vascular endothelial cells are ordinarily quiescent in adult humans and divide less than once per decade. When tumors reach a size of about 0.2-2.0mm in diameter, they become hypoxic and limited in size in the absence of angiogenesis. There are about 30 endogenous pro-angiogenic factors and about 30 endogenous anti-angiogenic factors. In order to increase in size, tumors undergo an angiogenic switch where the action of pro-angiogenic factors predominates, resulting in angiogenesis and tumor progression. One mechanism for driving angiogenesis results from the increased production of vascular endothelial growth factor (VEGF) following up-regulation of the hypoxia-inducible transcription factor. The human VEGF family consists of VEGF (VEGF-A), VEGF-B, VEGF-C, VEGF-D, and placental growth factor (PlGF). The VEGF family of receptors consists of three protein-tyrosine kinases and two non-protein kinase receptors (neuropilin-1 and -2). Owing to the importance of angiogenesis in tumor progression, inhibition of VEGF signaling represents an attractive cancer treatment.

Semaphorin signals in cell adhesion and cell migration: functional role and molecular mechanisms

Cell migration is pivotal in embryo development and in the adult. During development a wide range of progenitor cells travel over long distances before undergoing terminal differentiation. Moreover, the morphogenesis of epithelial tissues and of the cardiovascular system involves remodelling compact cell layers and sprouting of new tubular branches. In the adult, cell migration is essential for leucocytes involved in immune response. Furthermore, invasive and metastatic cancer cells have the distinctive ability to overcome normal tissue boundaries, travel in and out of blood vessels, and settle down in heterologous tissues. Cell migration normally follows strict guidance cues, either attractive, or inhibitory and repulsive. Semaphorins are a wide family of signals guiding cell migration during development and in the adult. Recent findings have established that semaphorin receptors, the plexins, govern cell migration by regulating integrin-based cell substrate adhesion and actin cytoskeleton dynamics, via specific monomeric GTPases. Plexins furthermore recruit tyrosine kinases in receptor complexes, which allows switching between multiple signaling pathways and functional outcomes. In this article, we will review the functional role of semaphorins in cell migration and the implicated molecular mechanisms controlling cell adhesion.

Inhibition of vascular endothelial growth factor (VEGF)-165 and semaphorin 3A-mediated cellular invasion and tumor growth by the VEGF signaling inhibitor ZD4190 in human colon cancer cells and xenografts

We recently showed by DNA microarray analysis that vascular endothelial growth factor (VEGF) receptor (VEGFR) is expressed in HCT8/S11 human colon cancer cells, suggesting that several angiogenic factors may target colon cancer cells themselves. In this study, transcripts encoding the VEGF-165 and semaphorin 3A (Sema3A) receptors and coreceptors Flt-1, KDR/Flk-1, plexin A1, and neuropilins NP-1 and NP-2 were identified by reverse transcription-PCR in the human colon cancer cell lines HCT8/S11, HT29, HCT116, and PCmsrc. Collagen invasion induced by VEGF-165 and Sema3A in HCT8/S11 cells (EC(50), 0.4-1 nmol/L) required p42/44 mitogen-activated protein kinase and signaling through RhoA/Rho-kinase-dependent and -independent pathways, respectively. As expected, the VEGFR signaling inhibitor ZD4190 selectively abrogated the proinvasive activity of VEGF in collagen gels (IC(50), 10 nmol/L) and chick heart fragments. We identify a novel function for VEGF-165 and Sema3A as proinvasive factors for human colorectal cancer cells. Interestingly, oral administration of the single drug ZD4190 to athymic mice (50 mg/kg/d, once daily) inhibited by 70% the growth of HCT8/S11 tumor cell xenografts. Combinations between the src tyrosine kinase inhibitor M475271 and ZD4190 or cisplatin resulted in additive therapeutic activity against LNM35 human lung tumor xenografts. Our data have significant implications for new therapeutic approaches and individualized treatment targeting VEGFR and src signaling pathways in combination with established clinical drugs at primary tumors and distant metastases in colon and lung cancer patients.

Labeled lines in the retinotectal system: markers for retinorecipient sublaminae and the retinal ganglion cell subsets that innervate them

Axons of retinal ganglion cells (RGCs) carry visual information to the brain. In most vertebrates, the major synaptic target of RGCs is the optic tectum. In the chick, RGC axons form synapses in just 4 of 16 histologically recognizable laminae (the retinorecipient laminae [RRLs]), and arbors of individual RGCs are confined to a single RRL. To analyze the development and function of these parallel pathways, markers are required that selectively label them. Here, we have identified molecular markers for individual RRLs and for RGCs that project to them. Some of the markers may mediate or modulate signaling through the separate pathways: neuropeptides (substance P, neuromedin B, somatostatin-I and -II) and their receptors (substance P receptor), neurotransmitter synthetic enzymes (choline acetyltransferase) and the corresponding receptors (acetylcholine receptor beta2) and calcium-binding proteins (parvalbumin and calbindin). Other markers are adhesive proteins that could mediate selective connectivity of RGC subsets within specific RRLs (cadherin-7, cadherin-11, reelin and neuropilin-1). We further show that RGC subsets whose axons project to specific RRLs are heterogeneous with respect to the retinal sublaminae within which their dendrites arborize. Our results define laminar-specified circuits from retina to brain and support a model in which RGCs transmit information from multiple sources to single central laminae, where it can be integrated.

The role of neuropilins in cancer

Neuropilins are multifunctional non-tyrosine kinase receptors that bind to class 3 semaphorins and vascular endothelial growth factor. NRP-1 and NRP-2 were first identified for their key role in mediating axonal guidance in the developing nervous system through their interactions with class 3 semaphorins. Growing evidence supports a critical role for these receptors in tumor progression. Neuropilin expression is up-regulated in multiple tumor types, and correlates with tumor progression and prognosis in specific tumors. Neuropilins may indirectly mediate effects on tumor progression by affecting angiogenesis or directly through effects on tumor cells. This article reviews emerging evidence for the role of neuropilins in tumor biology. The therapeutic implications of these data are far-reaching and suggest that neuropilin-targeted interventions may be useful as a component of antineoplastic therapy.

C terminus of RGS-GAIP-interacting protein conveys neuropilin-1-mediated signaling during angiogenesis

Initially, it was thought that there was no intracellular signaling mediated by NRP-1 alone in response to its ligands. However, the emerging data from our group as well as others suggest that the signaling through NRP-1 actually promotes angiogenesis and is mediated through its C-terminal domain and downstream molecules such as phosphoinositide 3-kinase. Hence, understanding the signal transduction pathways mediated by NRP-1 and identification of its downstream molecules are of importance. By using both in vivo zebrafish model and in vitro tissue culture system, we have shown that the C-terminal three amino acids of NRP-1 (SEA-COOH) are required for NRP-1-mediated angiogenesis. Furthermore, knocking down of RGS-GAIP-interacting protein C terminus (GIPC) in zebrafish, which is associated with C-terminal domain of NRP-1, exhibits similar vasculature phenotypes to those from NRP-1 null. Specific and effective silencing of GIPC in vascular endothelium results in inhibition of NRP-1-mediated migration. In both cases as described, PDZ domain of GIPC is responsible for its function. Taken together, our data suggest a novel role of GIPC in angiogenesis and vessel formation and also support our hypothesis that NRP-1 can facilitate downstream signaling to promote angiogenesis through GIPC.

Uterine angiogenesis during implantation and decidualization in mice

Increased uterine vascular permeability and angiogenesis are hallmarks of implantation and placentation. These events are profoundly influenced by vascular endothelial growth factor (VEGF). Although VEGF and its receptor Flk-1 are primarily important for uterine vascular permeability and angiogenesis before and during the attachment phase of the implantation process, VEGF together with the angiopoietins and their receptor Tie-2 directs angiogenesis during decidualization after implantation. Uterine expression of HIF and ARNT follows the localization of VEGF expression with increasing angiogenesis during the postimplantation period, although their expression does not correlate with VEGF expression during the pre-implantation period. Upstream of VEGF, estrogen promotes uterine vascular permeability but inhibits angiogenesis, whereas progesterone stimulates angiogenesis with little effect on vascular permeability. Furthermore, COX-2-derived prostaglandins participate in uterine vascular permeability and angiogenesis during implantation and decidualization. (Reprod Med Biol 2006; 5: 81-86).

Uterine angiogenesis during implantation and decidualization in mice

Increased uterine vascular permeability and angiogenesis are hallmarks of implantation and placentation. These events are profoundly influenced by vascular endothelial growth factor (VEGF). Although VEGF and its receptor Flk-1 are primarily important for uterine vascular permeability and angiogenesis before and during the attachment phase of the implantation process, VEGF together with the angiopoietins and their receptor Tie-2 directs angiogenesis during decidualization after implantation. Uterine expression of HIF and ARNT follows the localization of VEGF expression with increasing angiogenesis during the postimplantation period, although their expression does not correlate with VEGF expression during the pre-implantation period. Upstream of VEGF, estrogen promotes uterine vascular permeability but inhibits angiogenesis, whereas progesterone stimulates angiogenesis with little effect on vascular permeability. Furthermore, COX-2-derived prostaglandins participate in uterine vascular permeability and angiogenesis during implantation and decidualization. (Reprod Med Biol 2006; 5: 81-86).

Molecular regulation of tumor angiogenesis: mechanisms and therapeutic implications

Angiogenesis, the process of new capillary formation from a pre-existing vessel plays an essential role in both embryonic and postnatal development, in the remodeling of various organ systems, and in several pathologies, particularly cancer. In the last 20 years of angiogenesis research, a variety of angiogenic regulators, both positive and negative, have been identified. The discovery of several anti-angiogenic factors has led to the development of novel cancer therapies based on targeting a tumor's vascular supply. A number of these new therapies are currently being tested in clinical trials in the U.S.A. and elsewhere. A major advance in the field of anti-angiogenic therapy occurred recently when the FDA approved Avastin (bevacizumab), the first solely anti-angiogenesis therapy approved for treatment of human cancer. While it has long been appreciated that tumor growth and progression are dependent on angiogenesis, it is only recently that progress has been made in elucidating the molecular mechanisms that regulate the earliest stage in the angiogenic program, the angiogenic switch. This checkpoint is characterized by the transition of a dormant, avascular tumor into an active, vascular one. Anti-angiogenic therapies to date have essentially been designed to suppress the neovasculature in established tumors. However, identifying the mechanisms that cause a tumor to acquire an angiogenic phenotype may lead to the discovery of new therapeutic modalities and complementary diagnostics that could be used to block the angiogenic switch, thereby preventing subsequent tumor progression. In this chapter on the role of angiogenesis in cancer, we (1) provide an overview of the process of angiogenesis with special regard to the molecules and physiological conditions that regulate this process, (2) review recent studies describing the use of anti-angiogenic approaches in the treatment of a variety of human cancers, and (3) discuss the recent literature focused on the study of the molecules and molecular mechanisms that may be regulating the initiation of the angiogenic phenotype in tumors, and the clinical impact that this knowledge may have in the future.

Ligand-induced internalization selects use of common receptor neuropilin-1 by VEGF165 and semaphorin3A

Neuropilin-1 (Npn-1) is a receptor shared by class 3 semaphorins and heparin-binding forms of vascular endothelial growth factor (VEGF), protein families that regulate endothelial and neuronal-cell function. Ligand interaction with Npn-1 dictates the choice of signal transducer; plexins transduce semaphorin signals, and VEGF receptors transduce VEGF signals. It is not clear how class 3 semaphorins affect endothelial-cell function and how the shared receptor Npn-1 selects its ligand. We report that semaphorin3A (Sema3A) inhibits endothelial-cell lamellipodia formation, adhesion, survival, proliferation, and cord formation. VEGF(165), but not VEGF(121), could block all these effects of Sema3A. VEGF(165) competed with Sema3A for binding to endothelial cells, effectively reduced cell-surface Npn-1, and promoted its internalization. Use of soluble forms of Npn-1 or VEGF receptor-1 to block VEGF(165) binding to Npn-1 or to VEGF receptors provided evidence that surface Npn-1 and VEGF receptors are required for VEGF(165)-induced Npn-1 internalization. Sema3A also reduced cell-surface Npn-1 in endothelial cells and promoted its internalization, but required a higher concentration than VEGF(165). These results demonstrate that preferential receptor binding and internalization by a ligand are mechanisms by which the common receptor Npn-1 can play an essential role in prioritizing conflicting signals.

Differential impact of semaphorin 3E and 3A on CNS axons

During the development of the central nervous system (CNS), the correct wiring of outgrowing neurites is mediated by antagonistic mechanisms. Aberrant growth is prevented by repulsive factors such as semaphorins. Expression of the ligands Sema3A and -3E and the receptors neuropilin Npn-1, -2a and -2b in the chick visual system were analyzed by RT-PCR. Whereas Sema3A and its major receptor Npn-1 were abundant, Sema3E and Npn-2 isoform expression was highly restricted and developmentally regulated. Peak expression occurred during retinal axon innervation of the tectum. Functional in vitro assays with recombinant proteins revealed a topography-specific growth cone collapsing activity of Sema3A for tectal axons. Interestingly, whereas tectal axons collapsed in a topographic-specific manner only in the presence of Sema3A, retinal axons responded only to Sema3E. The collapsing activity was intracellularly mediated by cGMP. For a detailed analysis of neuronal responses to sempahorins, time lapse video recording was performed. When tectal and retinal axons were pre-exposed to brain-derived neurotrophic factor (BDNF), a protective effect was evident only in the case of retinal axons. Our results suggest a molecular mechanism whereby ingrowth of retinal axons into the tectum can be regulated by Sema3E/BDNF modulation without disturbing tectal axon growth out of the tectum mediated by Sema3A.

Roles of neuropilins in neuronal development, angiogenesis, and cancers

Neuropilin-1 (NRP1) and neuropilin-2 (NRP2) are transmembrane glycoproteins that have been characterized as receptors for both semaphorins for neuronal guidance and vascular endothelial growth factor (VEGF) for angiogenesis. Biologic properties of NRPs have been linked to their unique domain structures. However, molecular interaction among NRPs, VEGF, and VEGF receptors is still not clear. Although several types of cancer cells can express NRPs, the role of NRPs in tumor pathogenesis is largely unknown. Thus, future investigations should include determining the effects and mechanisms of NRPs on proliferation, apoptosis, and migration of neuronal , endothelial, and cancer cells. Study of protein-protein interaction, signal transduction pathways, and NRP-mediated gene expression is particularly important to understand NRPs functions, which may have significant clinical applications in the treatment of neurological disorders, cardiovascular diseases, and certain cancers.

A GFP-based genetic screen reveals mutations that disrupt the architecture of the zebrafish retinotectal projection

The retinotectal projection is a premier model system for the investigation of molecular mechanisms that underlie axon pathfinding and map formation. Other important features, such as the laminar targeting of retinal axons, the control of axon fasciculation and the intrinsic organization of the tectal neuropil, have been less accessible to investigation. In order to visualize these processes in vivo, we generated a transgenic zebrafish line expressing membrane-targeted GFP under control of the brn3c promoter/enhancer. The GFP reporter labels a distinct subset of retinal ganglion cells (RGCs), which project mainly into one of the four retinorecipient layers of the tectum and into a small subset of the extratectal arborization fields. In this transgenic line, we carried out an ENU-mutagenesis screen by scoring live zebrafish larvae for anatomical phenotypes. Thirteen recessive mutations in 12 genes were discovered. In one mutant, ddl, the majority of RGCs fail to differentiate. Three of the mutations, vrt, late and tard, delay the orderly ingrowth of retinal axons into the tectum. Two alleles of drg disrupt the layer-specific targeting of retinal axons. Three genes, fuzz, beyo and brek, are required for confinement of the tectal neuropil. Fasciculation within the optic tract and adhesion within the tectal neuropil are regulated by vrt, coma, bluk, clew and blin. The mutated genes are predicted to encode molecules essential for building the intricate neural architecture of the visual system.

Mécanismes cellulaires et moléculaires de la croissance axonale

INTRODUCTION

During embryonic and post-natal development, numerous axonal connections are formed establishing a functional nervous system. Knowledge of the underlying molecular and cellular mechanisms controlling this phenomenon is improving.

STATE OF THE ART

In this review, we present the general principles of axon guidance together with the major families of guidance signals. This includes the tyrosine kinase receptors Eph and their ligands Ephrins, the netrins, the semaphorins, the slits and other major components of the extracellular matrix. These types of guidance signals share common functional properties leading to actin cytoskeleton remodelling. The direct or indirect interactions between the receptors of these guidance cues and actin modulators is the final step of the signalling cascade constituting the fundamental mechanism defining the orientation and extension of the axonal growth cone. These factors are involved in the formation of many, if not all, axonal projections for which they act as repulsive (inhibitory) or attractive (promoting) signals.

PERSPECTIVES

the knowledge of these mechanisms is particularly interesting since the inhibition of axonal outgrowth is considered to be one of the major obstacles to nerve regeneration in the central nervous system. Indeed, most of the guidance signals expressed during brain development are up-regulated in lesion sites where they contribute to the lack of nerve re-growth. Here, we present the nature of the mechanical barrier, the so called glial scar, and we describe the major inhibitory molecules preventing axonal extension.

CONCLUSION

the comprehension of the molecular mechanisms involved in axon growth and guidance represents a major advance towards the definition of novel therapeutic strategies improving nerve regeneration. The path to the clinical application of these molecular factors remains long. Nevertheless, the next decade will undoubtedly provide challenging data that will modify the current therapeutic approaches.

Schwann cell-autonomous role of neuropilin-2

Neuropilins and group A plexins are components of receptor complexes for class 3 semaphorins, gradients of which help to guide migration of neural progenitor cells and axonal growth cones during development. We demonstrated previously that neuropilins and class 3 semaphorins are induced in sciatic nerve by crush or transection. We now report that in cultured rat Schwann cells, expression of mRNA encoding neuropilin-2 (NRP2) and plexin-A3 (PlexA3), proteins involved in semaphorin-3F (Sema3F) signal transduction, is diminished markedly by forskolin, an adenylate cyclase activator that, like axonal contact, induces Schwann cell synthesis of myelin lipids and proteins. Interestingly, Schwann cell expression of mRNA encoding NRP1, which participates in Sema3A signaling, is not downregulated by forskolin. Antibodies that recognize ectodomains of NRP2 but not control antibodies prevented cultured Schwann cells from aligning in parallel and forming columns. These results are consistent with the view that in nerves undergoing Wallerian degeneration, Schwann cell NRP2 facilitates assembly of Schwann cells into the tubular aggregates (bands of Büngner) that guide regenerating axons.

Neuropilin-1 regulates attachment in human endothelial cells independently of vascular endothelial growth factor receptor-2

Neuropilin-1 (NRP-1) is a type 1 membrane protein that binds the axon guidance factors belonging to the class-3 semaforin family. In endothelial cells, NRP-1 serves as a co-receptor for vascular endothelial growth factor (VEGF) and regulates VEGF receptor 2 (VEGFR-2)-dependent angiogenesis. Although gene-targeting studies documenting embryonic lethality in NRP-1 null mice have demonstrated a critical role for NRP-1 in vascular development, the activities of NRP-1 in mature endothelial cells have been incompletely defined. Using RNA interference-mediated silencing of NRP-1 or VEGFR-2 in primary human endothelial cells, we confirm that NRP-1 modulates VEGFR-2 signaling-dependent mitogenic functions of VEGF. Importantly, we now show that NRP-1 regulates endothelial cell adhesion to extracellular matrix proteins independently of VEGFR-2. Based on its dual role as an enhancer of VEGF activity and a mediator of endothelial cell adhesiveness described here, NRP-1 emerges as a promising molecular target for the development of antiangiogenic drugs.

Molecular cues to implantation

Successful implantation is the result of reciprocal interactions between the implantation-competent blastocyst and receptive uterus. Although various cellular aspects and molecular pathways of this dialogue have been identified, a comprehensive understanding of the implantation process is still missing. The receptive state of the uterus, which lasts for a limited period, is defined as the time when the uterine environment is conducive to blastocyst acceptance and implantation. A better understanding of the molecular signals that regulate uterine receptivity and implantation competency of the blastocyst is of clinical relevance because unraveling the nature of these signals may lead to strategies to correct implantation failure and improve pregnancy rates. Gene expression studies and genetically engineered mouse models have provided valuable clues to the implantation process with respect to specific growth factors, cytokines, lipid mediators, adhesion molecules, and transcription factors. However, a staggering amount of information from microarray experiments is also being generated at a rapid pace. If properly annotated and explored, this information will expand our knowledge regarding yet-to-be-identified unique, complementary, and/or redundant molecular pathways in implantation. It is hoped that the forthcoming information will generate new ideas and concepts for a process that is essential for maintaining procreation and solving major reproductive health issues in women.

Discovery of semaphorin receptors, neuropilin and plexin, and their functions in neural development

The semaphorin receptors neuropilin and plexin were initially identified as antigens for monoclonal antibodies MAb-A5 and MAb-B2, which bind to specific neuropiles and plexiform layers within the Xenopus tadpole nervous systems, several years before the discovery of the first semaphorin. This article provides an overview of how neuropilin and plexin were discovered. In addition, it describes the functions of neuropilin in the signaling of chemorepulsive activities of class 3 semaphorins and roles of neuropilin-mediated semaphorin activities in the directional guidance of the peripheral nervous system (PNS) and integration of the peripheral ganglia.

X-linked retinoschisis: A clinical and molecular genetic review

X-linked retinoschisis is a leading cause of macular degeneration in male children. It is characterized by a high degree of clinical variability. Clinical features include a stellate foveal retinoschisis, with or without peripheral retinoschisis. The schisis occurs within the inner retina, primarily at the level of the nerve fiber layer. The disease-causing gene, X-linked retinoschisis 1, has recently been identified, and is expressed in photoreceptor and bipolar cells. This gene codes for retinoschisin, a secreted protein containing a discoidin domain which may be involved in cellular adhesion or cell-cell interactions. The identification of this gene allows for improved diagnosis and contributes to the understanding of this condition. Visual prognosis is variable, as X-linked retinoschisis exhibits a high degree of phenotypic variability. Although there is no treatment to halt the progressive maculopathy, clinical management is directed toward treatment of amblyopia and surgical correction of certain complications. X-linked retinoschisis is an important condition to study, both to improve the clinical management of this disorder, and to better understand retinal function and development. Herein, we review the clinical, histopathologic, and molecular genetic and treatment options of X-linked retinoschisis.

Semaphorin3D guides retinal axons along the dorsoventral axis of the tectum

We examined the role of Sema3D, a semaphorin of previously unknown function, in guiding retinal ganglion cell (RGC) axons to the optic tectum in the developing zebrafish. Sema3D is expressed more strongly in the ventral versus dorsal tectum, suggesting that it may participate in guiding RGC axons along the dorsoventral axis of the tectum. Ubiquitous misexpression of Sema3D in transgenic zebrafish inhibits ventral but not dorsal RGC axon growth. In addition, ventral RGC axons avoid or stop at individual cells misexpressing Sema3D along their pathway. Sema3D ubiquitous misexpression at later stages also causes ventral RGC axon arbors to spread more widely along the dorsoventral axis of the tectum. Knock-down of Sema3D with morpholino antisense causes ventral RGC axons to extend aberrantly into the ventral tectum. These results suggest that Sema3D in the ventral tectum normally acts to inhibit ventral RGCs from extending into ventral tectum, ensuring their correct innervation of dorsal tectum.

Cranial expression of class 3 secreted semaphorins and their neuropilin receptors

The semaphorin family of chemorepellents and their receptors the neuropilins are implicated in a variety of cellular processes, including axon guidance and cell migration. Semaphorins may bind more than one neuropilin or a heterodimer of both, thus a detailed knowledge of their expression patterns may reveal possible cases of redundancy or mutual antagonism. To assess their involvement in cranial development, we cloned fragments of the chick orthologues of Sema3B and Sema3F. We then carried out mRNA in situ hybridisation of all six class 3 semaphorins and both neuropilins in the embryonic chick head. We present evidence for spatiotemporal regulation of these molecules in the brainstem and developing head, including the eye, ear, and branchial arches. These expression patterns provide a basis for functional analysis of semaphorins and neuropilins in the development of axon projections and the morphogenesis of cranial structures.

Neuropilin-1-mediated vascular permeability factor/vascular endothelial growth factor-dependent endothelial cell migration

Neuropilin-1 (NRP-1) has been found to be expressed by endothelial cells and tumor cells as an isoform-specific receptor for vascular permeability factor/vascular endothelial growth factor (VEGF). Previous studies were mainly focused on the extracellular domain of NRP-1 that can bind to VEGF165 and, thus, enables NRP-1 to act as a co-receptor for VEGF165, which enhances its binding to VEGFR-2 and its bioactivity. However, the exact functional roles and related signaling mechanisms of NRP-1 in angiogenesis are not well understood. In this study we constructed a chimeric receptor, EGNP-1, by fusing the extracellular domain of epidermal growth factor receptor to the transmembrane and intracellular domains of NRP-1 and transduced it into HUVECs with a retroviral expression vector. We observed that NRP-1/EGNP-1 mediates ligand-stimulated migration of human umbilical vein endothelial cells (HUVECs) but not proliferation. Our results show that NRP-1 alone can mediate HUVEC migration through its intracellular domain, and its C-terminal three amino acids (SEA-COOH) are essential for the process. We demonstrate that phosphatidylinositol 3-kinase inhibitor Ly294002 and the p85 dominant negative mutant can block NRP-1-mediated HUVEC migration. NRP-1-mediated migration can be significantly reduced by overexpression of the dominant negative mutant of RhoA (RhoA-19N). In addition, Gq family proteins and Gbetagamma subunits are also required for NRP-1-mediated HUVEC migration. These results show for the first time that NRP-1 can independently promote cell signaling in endothelial cells and also demonstrate the importance of last three amino acids of NRP-1 for its function.

Structural and functional relation of neuropilins

Neuropilin is a type I transmembrane protein and the molecular mass is 120 kDa. Two homologues, Neuropilin-1 and -2, are identified. The primary structure of Neuropilin-1 and Neuropilin-2 is well conserved and is divided into four domains, CUB (a1/a2) domain, FV/FVIII (b1/b2) domain, MAM (c) domain, and (d) domain that contains a transmembrane and a short cytoplasmic region. Both Neuropilin-1 and Neuropilin-2 have truncated and secreted form of splice variants. Neuropilins act as a receptor for two different extracellular ligands, class 3 semaphorins and specific isoforms of vascular endothelial growth factor. In both cases, neuropilin requires an additional transmembrane molecule to exhibit biological activity. Plexin-A is essential for class 3 semaphorin signaling. Vascular endothelial cell growth factor (VEGF) receptor is the major receptor for VEGF and neuropilin acts as isoform specific co-receptor for VEGF. The CUB and FV/FVIII domains of Neuropilin are the binding sites of semaphorin and VEGF. The MAM domain mediates semaphorin signaling to Plexin-A. Cross talk between semaphorin and VEGF on neuropilin suggests that class 3 semaphorins and the secreted forms of neuropilin act as antagonists to VEGF and its related growth factors.

From the discovery of neuropilin to the determination of its adhesion sites

Neuropilin (NRP) and plexin (Plex) that are now known to be semaphorin receptors were initially identified as antigens for monoclonal antibodies (MAbs) that bound to particular neuropiles and plexiform layers of the Xenopus tadpole optic tectum, several years before the discovery of semaphorin. The extracellular segment of the NRP protein is a mosaic of 3 functionally different protein motifs that are thought to be involved in molecular and/or cellular interactions, suggesting that NRP serves in a various cell-cell interaction by binding a variety of molecules. The first identified function of NRP was the cell adhesion activity; Cell reaggregation study using NRP-expressing cell lines revealed that NRP can mediate cell adhesion via heterophilic molecular interaction. Later, NRP was shown to bind semaphorins and vascular endothelial growth factor (VEGF). It was also shown that NRP makes receptor complexes with Plex to propagate semaphorin signals.

The function of neuropilin/plexin complexes

Neuropilins bind the secreted class 3 semaphorins with high affinity but require a member of the plexin family to form receptors that are able to activate downstream signal transduction cascades. In this receptor complex neuropilins act as the ligand-binding subunit while plexins function as the signal-transducing subunit in the induction of cytoskeletal collapse by semaphorins. The cytoplasmic domain is highly conserved within the plexin family and interacts with Rho-like GTPases.

The role of neuropilin in vascular and tumor biology

Neuropilin-1 (NRP1) and NRP2 are related transmembrane receptors that function as mediators of neuronal guidance and angiogenesis. NRPs bind members of the class 3 semaphorin family, regulators of neuronal guidance, and of the vascular endothelial growth factor (VEGF) family of angiogenesis factors. There is substantial evidence that NRPs serve as mediators of developmental and tumor angiogenesis. NRPs are expressed in endothelial cells (EC) and bind VEGF165. NRP1 is a co-receptor for VEGF receptor-2 (VEGFR2) that enhances the binding of VEGF165 to VEGFR2 and VEGF165-mediated chemotaxis. NRP1 expression is regulated in EC by tumor necrosis factor-alpha, the transcription factors dHAND and Ets-1, and vascular injury. During avian blood vessel development NRP1 is expressed only in arteries whereas NRP2 is expressed in veins. Transgenic mouse models demonstrate that NRP1 plays a critical role in embryonic vascular development. Overexpression of NRP1 results in the formation of excess capillaries and hemorrhaging. NRP1 knockouts have defects in yolk sac, embryo and neuronal vascularization, and in development of large vessels in the heart. Tumor cells express NRPs and bind VEGF165. NRP1 upregulation is positively correlated with the progression of various tumors. Overexpression of NRPI in rat tumor cells results in enlarged tumors and substantially enhanced tumor angiogenesis. On the other hand, soluble NRP1 (sNRP1) is an antagonist of tumor angiogenesis. Semaphorin 3A binds to EC and tumor cells. It also inhibits EC motility and capillary sprouting in vitro. VEGF165 and Sema3A are competitive inhibitors for NRP1 mediated functions in EC and neurons. These results suggest that NRP1 is a novel regulator of the vascular system.

Molecular analysis of axon repulsion by the notochord

During development of the amniote peripheral nervous system, the initial trajectory of primary sensory axons is determined largely by the action of axon repellents. We have shown previously that tissues flanking dorsal root ganglia, the notochord lying medially and the dermamyotomes lying laterally, are sources of secreted molecules that prevent axons from entering inappropriate territories. Although there is evidence suggesting that SEMA3A contributes to the repellent activity of the dermamyotome, the nature of the activity secreted by the notochord remains undetermined. We have employed an expression cloning strategy to search for axon repellents secreted by the notochord, and have identified SEMA3A as a candidate repellent. Moreover, using a spectrum of different axon populations to assay the notochord activity, together with neuropilin/Fc receptor reagents to block semaphorin activity in collagen gel assays, we show that SEMA3A probably contributes to notochord-mediated repulsion. Sympathetic axons that normally avoid the midline in vivo are also repelled, in part, by a semaphorin-based notochord activity. Although our results implicate semaphorin signalling in mediating repulsion by the notochord, repulsion of early dorsal root ganglion axons is only partially blocked when using neuropilin/Fc reagents. Moreover, retinal axons, which are insensitive to SEMA3A, are also repelled by the notochord. We conclude that multiple factors act in concert to guide axons in this system, and that further notochord repellents remain to be identified.

Identification and characterization of a novel mouse plexin, plexin-A4

Plexins belonging to the plexin-A subfamily form complexes with neuropilins and propagate signals of class 3 semaphorins into neurons, even though they do not directly bind the semaphorins. In this study, we identified a new member of the plexin-A subfamily in the mice, plexin-A4, and showed that it was expressed in the developing nervous system with a pattern different to that of other members of the plexin-A subfamily (plexin-A1, plexin-A2 and plexin-A3). COS-7 cells coexpressing plexin-A4 with neuropilin-1 were induced to contract by Sema3A, a member of the class 3 semaphorin. Ectopic expression of plexin-A4 in mitral cells that are originally insensitive to Sema3A resulted in the collapse of growth cones in the presence of Sema3A. These results suggest that plexin-A4 plays a role in the propagation of Sema3A activities.

Differential non-target-derived repulsive signals play a critical role in shaping initial axonal growth of dorsal root ganglion neurons

Initial trajectories of dorsal root ganglion (DRG) axons are shaped by chemorepulsive signals from surrounding tissues. Although we have previously shown that axonin-1/SC2 expression on DRG axons is required to mediate a notochord-derived chemorepulsive signal, Dev. Biol. 224, 112-121), other molecules involved in the non-target-derived repulsive signals are largely unknown. Using coculture assays composed of tissues derived from the chick embryo or mutant mice treated with function-blocking antibodies and phosphatidylinositol-specific phospholipase C, we report here that the chemorepellent semaphorin 3A (Sema3A) and its receptor neuropilin-1 are required for mediating the dermamyotome- and notochord-derived, but not the ventral spinal cord-derived, chemorepulsive signal for DRG axons. The dermamyotome-derived chemorepulsion is exclusively dependent on Sema3A/neuropilin-1, whereas other molecules are also involved in the notochord-derived chemorepulsion. Chemorepulsion from the ventral spinal cord does not depend on Sema3A/neuropilin-1 but requires axonin-1/SC2 to repel DRG axons. Thus, differential chemorepulsive signals help shape the initial trajectories of DRG axons and are critical for the proper wiring of the nervous system.

Neuropilin 1 and neuropilin 2 co-expression is significantly correlated with increased vascularity and poor prognosis in nonsmall cell lung carcinoma

BACKGROUND

Cell-retained isoforms of vascular endothelial growth factor A (VEGF-A) have been reported to play an essential role in tumor progression through stromal neovascularization in malignant solid tumors. While more than 95% of nonsmall cell lung carcinoma (NSCLC) expresses cell-retained VEGF-A isoform, the clinicopathologic implications of neuropilin (NRP), considered the specific receptor for limited types of VEGF-A isoform, are not well understood.

METHODS

The authors examined NRP1 and NRP2 mRNA expression in 68 NSCLCs and 15 extraneoplastic tissues by a densitometry-assisted, semi-quantitative reverse transcription-polymerase chain reaction. The authors determined the distinct expression of NRPs using the expression level of NRPs relative by optical density to beta2-microglobulin. The authors also investigated VEGF-A isoforms, their receptors, and the clinical implications. Vascularity of NSCLC was morphologically estimated on sections immunostained with anti-CD34 antibody.

RESULTS

Eleven of 15 extraneoplastic specimens showed NRP1 expression (73.3%) and 8 showed NRP2 expression (53.3%). The expression level of NRP1 or NRP2 of neoplasmic tissue was higher than that of extraneoplastic tissues (P < 0.01, Mann-Whitney U test). Fifty-five and 44 NSCLCs expressed NRP1 and NRP2, respectively. Forty patients co-expressing NRP1 and NRP2 showed significantly poorer prognosis and increased vessel counts as compared to those 28 cases without co-expression (P < 0.05, log-rank test; P < 0.05, Mann-Whitney U test).

CONCLUSIONS

The co-expression of NRP1 and NRP2 genes is significantly correlated with tumor progression through neovascularization in NSCLC. These results suggest that both NRP1 and NRP2 are key molecules for stromal vascularization by cell-retained VEGF in NSCLC.

Neuropilin-1 is differentially expressed in myoepithelial cells and vascular smooth muscle cells in preneoplastic and neoplastic human breast: a possible marker for the progression of breast cancer

The expression and distribution of neuropilin-1 (NRP-1) was examined in the samples of normal human breast tissues and in non-neoplastic and neoplastic areas of breast tissue removed for carcinoma using RT-PCR as well as conventional and tissue microarrays immunohistochemical analyses. The NRP-1 mRNA expression was significantly higher in neoplastic tissues as compared to normal breast samples. Immunohistochemically, the myoepithelial cells of the mammary ducts and lobules display positive reactions for NRP-1, whereas the inner ductal and lobular epithelial cell layers failed to react. The myoepithelial cells of ducts and lobules in both neoplastic and non-neoplastic tissue specimens displayed a stronger positive reaction for NRP-1 than those in the normal breast. A positive reaction for NRP-1, but with a gradual reduction in intensity, was observed in the myoepithelial cells of ducts with atypical epithelial hyperplasia and ductal carcinoma in situ (DCIS). The reaction was undetected or minimally detected in the areas of invasive carcinoma. NRP-1 positive immunolabeling was also localized in the vascular smooth muscle cells and in some endothelial cells of the blood vessels in normal, non-neoplastic and neoplastic breast tissue samples. In areas of breast carcinoma, NRP-1 immunolabeling was more prominent in both vascular smooth muscle cells and in some endothelial cells than in similar cells in normal breast. The specificity of the newly developed antibody for NRP-1 was confirmed by in situ hybridization with DIG-labeled PCR generated probe. These results suggest that NRP-1 may be a multiple function protein in human breast and may be involved in the induction of local invasiveness of neoplasia and angiogenesis and have direct relevance to the progression of breast cancer.

Nerve growth factor and semaphorin 3A signaling pathways interact in regulating sensory neuronal growth cone motility

Neurotrophins and semaphorin 3A are present along pathways and in targets of developing axons of dorsal root ganglion (DRG) sensory neurons. Growth cones of sensory axons are probably regulated by interaction of cytoplasmic signaling triggered coincidentally by both types of guidance molecules. We investigated the in vitro interactions of neurotrophins and semaphorin 3A (Sema3A) in modulating growth cone behaviors of axons extended from DRGs of embryonic day 7 chick embryos. Growth cones of DRGs raised in media containing 10(-9) m NGF or BDNF were more resistant to Sema3A-induced growth cone collapse than when DRGs were raised in 10(-11) m NGF. After overnight culture in 10(-11) m NGF, a 1 hr treatment with 10(-9) m NGF or BDNF was sufficient to increase growth cone resistance to Sema3A-induced collapse. This neurotrophin-mediated decrease in the collapse response of DRG growth cones was not associated with reduced expression on growth cones of the Sema3A-binding protein neuropilin-1. A series of pharmacological studies followed. Phosphatidylinositol 3 kinase activity is not required for these effects of NGF. The effects of inhibitors and activators of protein kinase A (PKA) indicate that PKA activity is involved in NGF modulation of Sema3A-induced growth cone collapse. The effects of inhibitors and activators of PKG indicate that PKG activity is involved in Sema3A-induced growth cone collapse. The effects of inhibitors also indicate that Rho-kinase activity is involved in Sema3A-induced growth cone collapse. These results are consistent with the idea that growth cone responses to an individual guidance cue depend on coincident signaling by other guidance cues and by other regulatory pathways.

Characterization of neuropilin-1 structural features that confer binding to semaphorin 3A and vascular endothelial growth factor 165

Neuropilin-1 (Npn-1) is a receptor for both semaphorin 3A (Sema3A) and vascular endothelial growth factor 165 (VEGF(165)). To understand the role Npn-1 plays as a receptor for these structurally and functionally unrelated ligands, we set out to identify structural features of Npn-1 that confer binding to Sema3A or VEGF(165). We constructed Npn-1 variants containing deletions within the "a" and "b" domains of Npn-1. More than 16 variants were expressed in COS-1 cells and tested for alkaline phosphatase-Sema3A as well as alkaline phosphatase-VEGF(165) binding. Our results indicate that each of the two Npn-1 CUB domains and the amino-terminal coagulation factor V/VIII domain (CF V/VIII) are essential for Sema3A binding, but only the amino-terminal Npn-1 CF V/VIII domain is required for binding to VEGF(165). Guided by the structure of the bovine spermadhesin CUB domain, point mutants targeting defined surfaces of the Npn-1 a1 CUB domain were generated and tested for Sema3A and VEGF(165) binding. One Npn-1 variant, Npn-1(2ABC), exhibits complete loss of Sema3A binding while retaining normal VEGF(165) binding. Moreover, co-immunoprecipitation experiments show that Npn-1(2ABC) can form a signaling complex with the VEGF(165) signaling receptor KDR/VEGFR-2. These results establish the identity of contact sites between Npn-1 and its semaphorin ligands, and they provide a foundation for understanding how Npn-1 functions as a receptor for distinct classes of ligands in vivo.

Pathological significance of vascular endothelial growth factor A isoform expression in human cancer

Vascular endothelial growth factor (VEGF) is a highly specific factor for vascular endothelial cells. Five VEGF-A isoforms (splice variants 121, 145, 165, 189 and 206) are generated as a result of alternative splicing from a single VEGF-A gene. These differ in their molecular weights and in biological properties such as their ability to bind to cell-surface heparan sulfate proteoglycans. Deregulated VEGF-A expression contributes to the development of solid tumors by promoting tumor angiogenesis. More specifically, VEGF-A189 expression is related to angiogenesis and prognosis in certain human solid tumors. VEGF-A189 expression is also related to the xenotransplantability of human cancers into immunodeficient mice in vivo. Consequently, inhibition of VEGF-A or VEGF-A189 signaling regulates the development and metastasis of a variety of tumors. This review focuses on recent studies of the mechanisms by which VEGF-A regulates angiogenesis in the cancer stroma and on our recent findings concerning the potential mechanisms of VEGF-A189 expression on tumor growth and metastasis.

Temporal regulation of neuropilin-1 expression and sensitivity to semaphorin 3A in NGF- and NT3-responsive chick sensory neurons

The extracellular molecule semaphorin 3A (Sema3A) is proposed to be a negative guidance cue that participates in patterning DRG sensory axons in the developing chick spinal cord. During development Sema3A is first expressed throughout the spinal cord gray matter, but Sema3A expression later disappears from the dorsal horn, where small-caliber cutaneous afferents terminate. Sema3A expression remains in the ventral horn, where large-muscle proprioceptive afferents terminate. It has been proposed that temporal changes in the sensitivity of different classes of sensory afferents to Sema3A contribute to the different pathfinding of these sensory afferents. This study compared the expression of the semaphorin 3A receptor subunit, neuropilin-1, and the collapse response of growth cones to semaphorin 3A for NGF (cutaneous)- and NT3 (proprioceptive)-dependent sensory axons extended from E6-E10 chick embryos. Growth cones extended from E6 DRGs in NT3-containing medium expressed neuropilin-1 and collapsed in response to Sema3A. From E7 until E10 NT3-responsive growth cones expressed progressively lower levels of neuropilin-1, and were less sensitive to Sema3A. On the other hand, growth cones extended from DRGs in NGF-containing medium expressed progressively higher levels of neuropilin-1 and higher levels of collapse response to Sema3A over the period from E6-E10. Thus, developmental patterning of sensory terminals in the chick spinal cord may arise from changes in both Sema3A expression in the developing spinal cord and accompanying changes in neuronal expression of the Sema3A receptor subunit, neuropilin-1.