Semaphorins A and E act as antagonists of neuropilin-1 and agonists of neuropilin-2 receptors

The Versatile Roles of “Axon Guidance” Cues in Tissue Morphogenesis

The Netrin, Slit, Semaphorin, and Ephrin families of secreted proteins were originally characterized in the nervous system as guidance cues for axons; however, recent studies demonstrate that many members of these families contribute to the development of a variety of organs. Here, the current knowledge of their roles is discussed with a focus on four tissues: lung, mammary, cardiovascular, and kidney. While many studies indicate a role for "axon guidance" cues in regulating cell-cell and cell-extracellular matrix (ECM) interactions during organogenesis, there is accumulating evidence that they also contribute to tissue development by regulating the transcription and translation of genes encoding key morphogenetic factors.

A screen for downstream effectors of Neurogenin2 in the embryonic neocortex

Neurogenin (Ngn) 1 and Ngn2 encode basic-helix-loop-helix transcription factors expressed in the developing neocortex. Like other proneural genes, Ngns participate in the specification of neural fates and neuronal identities, but downstream effectors remain poorly defined. We set out to identify Ngn2 effectors in the cortex using a subtractive hybridization screen and identified several regionally expressed genes that were misregulated in Ngn2 and Ngn1;Ngn2 mutants. Included were genes down-regulated in germinal zone progenitors (e.g., Nlgn1, Unc5H4, and Dcc) and in postmitotic neurons in the cortical plate (e.g., Bhlhb5 and NFIB) and subplate (e.g., Mef2c, srGAP3, and protocadherin 9). Further analysis revealed that Ngn2 mutant subplate neurons were misspecified and that thalamocortical afferents (TCAs) that normally target this layer instead inappropriately projected towards the germinal zone. Strikingly, EphA5 and Sema3c, which encode repulsive guidance cues, were down-regulated in the Ngn2 and Ngn1;Ngn2 mutant germinal zones, providing a possible molecular basis for axonal targeting defects. Thus, we identified several new components of the differentiation cascade(s) activated downstream of Ngn1 and Ngn2 and provided novel insights into a new developmental process controlled by these proneural genes. Further analysis of the genes isolated in our screen should provide a fertile basis for understanding the molecular mechanisms underlying corticogenesis.

The vascular endothelial growth factor family and its receptors

This article focuses on describing the biology of vascular endothelial growth factor (VEGF) and its receptors as well as the regulation of their expression. A thorough understanding of the VEGF system is paramount in optimizing antiangiogenic therapies as a component of antineoplastic regimens.

PlexinD1 and semaphorin signaling are required in endothelial cells for cardiovascular development

The identification of new signaling pathways critical for cardiac morphogenesis will contribute to our understanding of congenital heart disease (CHD), which remains a leading cause of mortality in newborn children worldwide. Signals mediated by semaphorin ligands and plexin receptors contribute to the intricate patterning of axons in the central nervous system. Here, we describe a related signaling pathway involving secreted class 3 semaphorins, neuropilins, and a plexin receptor, PlexinD1, expressed by endothelial cells. Interruption of this pathway in mice results in CHD and vascular patterning defects. The type of CHD caused by inactivation of PlexinD1 has previously been attributed to abnormalities of neural crest. Here, we show that this form of CHD can be caused by cell-autonomous endothelial defects. Thus, molecular programs that mediate axon guidance in the central nervous system also function in endothelial cells to orchestrate critical aspects of cardiac morphogenesis.

Current progress in neural crest cell motility and migration and future prospects for the zebrafish model system

The neural crest is a unique population of cells that contributes to the formation of diverse cell types, including craniofacial cartilage, peripheral neurons, the cardiac outflow tract, and pigment cells. Neural crest cells (NCCs) are specified within the neuroepithelium, undergo an epithelial-to-mesenchymal transition, and migrate to target destinations throughout the embryo. Here, we review current understanding of two steps in NCC development, both of which involve NCC motility. The first is NCC delamination from the neuroepithelium and the changes in cell adhesion and the cytoskeleton necessary for the initiation of migration. The second is NCC migration and the signals that guide NCCs along specific migratory pathways. We illustrate the strength of the zebrafish, Danio rerio, as a model organism to study NCC motility. The zebrafish is particularly well suited for the study of neural crest motility because of the ability to combine genetic manipulation with live imaging of migrating NCCs.

Neuropilin-1 in human colon cancer: expression, regulation, and role in induction of angiogenesis

Neuropilin-1 (NRP-1), a recently identified co-receptor for vascular endothelial growth factor, is expressed by several nongastrointestinal tumor types and enhances prostate cancer angiogenesis and growth in preclinical models. We investigated the expression and regulation of NRP-1 and the effect of NRP-1 overexpression on angiogenesis and growth of human colon adenocarcinoma by immunohistochemistry and in situ hybridization. NRP-1 was expressed in 20 of 20 human colon adenocarcinoma specimens but not in the adjacent nonmalignant colonic mucosa. By reverse transcriptase-polymerase chain reaction analysis, NRP-1 mRNA was expressed in seven of seven colon adenocarcinoma cell lines. Subcutaneous xenografts of stably transfected KM12SM/LM2 human colon cancer cells overexpressing NRP-1 led to increased tumor growth and angiogenesis in nude mice. In in vitro assays, conditioned medium from NRP-1-transfected cell lines led to an increase in endothelial cell migration, but did not affect endothelial cell growth. Epidermal growth factor (EGF) led to induction of NRP-1 in human colon adenocarcinoma cells and selective blockade of the epidermal growth factor receptor (EGFR) decreased constitutive and EGF-induced NRP-1 expression. Blockade of the Erk 1/2 and P38 mitogen-activated protein kinase signaling pathways also led to a decrease in constitutive and EGF-induced NRP-1 expression. These findings demonstrate the ubiquitous expression of NRP-1 in human colon cancer and suggest that NRP-1 may contribute to colon cancer angiogenesis and growth. This study also suggests that EGF and mitogen-activated protein kinase signaling pathways play an important role in NRP-1 regulation in colon cancer cells.

Anti-chemorepulsive effects of vascular endothelial growth factor and placental growth factor-2 in dorsal root ganglion neurons are mediated via neuropilin-1 and cyclooxygenase-derived prostanoid production

Vascular endothelial growth factor (VEGF) displays neurotrophic and neuroprotective activities, but the mechanisms underlying these effects have not been defined. Neuropilin-1 (NP-1) is a receptor for VEGF165 and placental growth factor-2 (PlGF-2), but the role of NP-1 in VEGF-dependent neurotrophic actions is unclear. Dorsal root ganglion (DRG) neurons expressed high levels of NP-1 mRNA and protein, much lower levels of KDR, and no detectable Flt-1. VEGF165 and PlGF-2 promoted DRG growth cone formation with an effect similar to that of nerve growth factor, whereas the Flt-1-specific ligand, PlGF-1, and the KDR/Flt-4 ligand, VEGF-D, had no effect. The chemorepellent NP-1 ligand, semaphorin 3A, antagonized the response to VEGF and PlGF-2. The specific KDR inhibitor, SU5614, did not affect the anti-chemorepellent effects of VEGF and PlGF-2, whereas a novel, specific antagonist of VEGF binding to NP-1, called EG3287, prevented inhibition of growth cone collapse. VEGF stimulated prostacyclin and prostaglandin E2 production in DRG cultures that was blocked by inhibitors of cyclooxygenases; the anti-chemorepellent activities of VEGF and PlGF-2 were abrogated by cyclooxygenase inhibitors, and a variety of prostacyclin analogues and prostaglandins strikingly inhibited growth cone collapse. These findings support a specific role for NP-1 in mediating neurotrophic actions of VEGF family members and also identify a novel role for prostanoids in the inhibition of neuronal chemorepulsion.

Semaphorin 3C preserves survival and induces neuritogenesis of cerebellar granule neurons in culture

Semaphorins (sema) constitute a family of molecules sharing a common extracellular domain (semaphorin domain). This family includes several types of secreted and membrane-associated molecules that are grouped into eight subclasses (subclasses 1-7 and viral semaphorins). Subclass 3 semaphorins are secreted molecules involved in axonal guidance, mainly through repulsive gradients and induction of growth cone collapse. More recently sema 3 molecules have been identified as positive factors in dependence of the type of neurons. Besides their axonal guidance function, some semaphorins have been implicated in apoptosis and survival. We investigated the effect of sema3C on survival and neurite outgrowth of rat cerebellar granule neurons (CGNs) in culture. 3T3 cells were stably transfected with sema3C. Several clonal lines were established and tested for their neuritogenic activity and one, S3C-8, was selected for the bulk of experiments. S3C-8 was co-cultured with CGNs. Sema3C enhanced CGN viability as assessed in co-cultures of CGNs with monolayers of S3C-8 in comparison with co-cultures of CGNs with control mock-transfected 3T3 cells. Moreover sema3C induced neuritogenesis of cultured CGNs, which express neuropilin-1 and -2. S3C-8 cells, overexpressing sema3C, were significantly more neuritogenic for CGN than poly l-lysine (PLL), a positive substrate for CGNs, as assessed by the measurement of the length of neurites and confirmed by Tau expression along the time of culture. CGNs co-cultured with S3C-8, showed up-regulation of the expression of axonal microtubule-associated proteins (MAPs) such as Tau, phosphorylated MAP2C and mode I-phosphorylated MAP1B compared with neurons cultured on control 3T3 cells. We also found increased expression of a specific marker of neuronal cell bodies and dendrites, high molecular weight MAP2 (HMW-MAP2). Interestingly, there was no accompanying up-regulation of a marker enriched within the neuronal somatodendritic domain, mode II-phosphorylated MAP1B. These data support the idea that secreted sema3C favors survival and neuritogenesis of cultured CGNs.

Vascular Endothelial Growth Factor and Its Receptors in Embryonic Zebrafish Blood Vessel Development

There is intense interest in how blood vessel development is regulated. A number of vascular growth factors and their receptors have been described. The vascular endothelial growth factor (VEGF) and its receptors are major contributors to normal mammalian vascular development. These receptors include VEGFR-1, VEGFR-2, VEGFR-3, neuropilin-1 (NRP1), and NRP2. The function of these genes have been determined to some degree in mouse gene targeting studies. These knockouts are embryonically lethal, and early death can be attributed in part to lack of normal blood and lymphatic vessel development. More recently, it has been demonstrated that zebrafish are an excellent model for studying the genes and proteins that regulate embryonic vascular development. Zebrafish have a number of advantages compared to mice, including rapid embryonic development and the ability to examine and manipulate embryos outside of the animal. In this review, we describe some of the earlier mouse VEGF/receptor functional studies and emphasize the development of the zebrafish vasculature. We describe the zebrafish vasculature, zebrafish VEGF and VEGF receptors, advantages of the zebrafish model, resources, and methods of determining growth factor and receptor function.

Genetics in zebrafish, mice, and humans to dissect congenital heart disease: insights in the role of VEGF

Heart development and the establishment of a functional circulatory circuit are complex biological processes in which subtle perturbations may result in catastrophic consequences of cardiovascular birth defects. Studies in model organisms, most notably the mouse and the zebrafish, have identified genes that also cause these life-threatening defects when mutated in humans. Gradually, a framework for the genetic pathway controlling these events is now beginning to emerge. However, the puzzling phenotypic variability of the cardiovascular disease phenotype in humans and the recent identification of phenotypic modifiers using model organisms indicates that other genetic loci might interact to modify the disease phenotype. To illustrate this, we review the role of vascular endothelial growth factor (VEGF) during vascular and cardiac development and stress how zebrafish and mouse genetic studies have helped us to understand the role this growth factor has in human disease, in particular in the Di-George syndrome.

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.

Fibroblast growth factor-inducible-14 is induced in axotomized neurons and promotes neurite outgrowth

For successful nerve regeneration, a coordinated shift in gene expression pattern must occur in axotomized neurons. To identify genes participating in axonal regeneration, we characterized mRNA expression profiles in dorsal root ganglia (DRG) before and after sciatic nerve transection. Dozens of genes are differentially expressed after sciatic nerve injury by microarray analysis. Induction of SOX11, FLRT3, myosin-X, and fibroblast growth factor-inducible-14 (Fn14) mRNA in axotomized DRG neurons was verified by Northern analysis and in situ hybridization. The Fn14 gene encodes a tumor necrosis-like weak inducer of apoptosis (TWEAK) receptor and is dramatically induced in DRG neurons after nerve damage, despite low expression in developing DRG neurons. Fn14 expression in PC12 cells is also upregulated by nerve growth factor treatment. Overexpression of Fn14 promotes growth cone lamelipodial formation and increases neurite outgrowth in PC12 cells. These Fn14 effects are independent of the ligand, TWEAK. Fn14 colocalizes with the Rho family GTPases, Cdc42 and Rac1. Furthermore, Fn14 physically associates with Rac1 GTPase in immunoprecipitation studies. The neurite outgrowth-promoting effect of Fn14 is enhanced by Rac1 activation and suppressed by Rac1 inactivation. These findings suggest that Fn14 contributes to nerve regeneration via a Rac1 GTPase-dependent mechanism.

A gene expression atlas of the central nervous system based on bacterial artificial chromosomes

The mammalian central nervous system (CNS) contains a remarkable array of neural cells, each with a complex pattern of connections that together generate perceptions and higher brain functions. Here we describe a large-scale screen to create an atlas of CNS gene expression at the cellular level, and to provide a library of verified bacterial artificial chromosome (BAC) vectors and transgenic mouse lines that offer experimental access to CNS regions, cell classes and pathways. We illustrate the use of this atlas to derive novel insights into gene function in neural cells, and into principal steps of CNS development. The atlas, library of BAC vectors and BAC transgenic mice generated in this screen provide a rich resource that allows a broad array of investigations not previously available to the neuroscience community.

Semaphorin 3F is critical for development of limbic system circuitry and is required in neurons for selective CNS axon guidance events

Little is known about the role of class 3 semaphorins in the development of CNS circuitry. Several class 3 semaphorins, including semaphorin 3F (Sema3F) bind to the receptor neuropilin-2 to confer chemorepulsive responses in vitro. To understand the role of Sema3F in the establishment of neural circuitry in vivo, we have generated sema3F null and sema3F conditional mutant mice. Inspection of the peripheral nervous system in sema3F null mice reveals that Sema3F is essential for the proper organization of specific cranial nerve projections. Analysis of the CNS in sema3F null mice reveals a crucial role for Sema3F in the rostral forebrain, midbrain, and hippocampus in establishing specific Npn-2 (neuropilin-2)-expressing limbic tracts. Furthermore, we identify Sema3F and Npn-2 as the first guidance cue-receptor pair shown to be essential for controlling the development of amygdaloid circuitry. In addition, we provide genetic evidence in vertebrates for a neuronal requirement of a soluble axon guidance cue in CNS axon guidance. Our data reveal a requirement for neuronal Sema3F in the normal development of the anterior commissure in the ventral forebrain and infrapyramidal tract in the hippocampus. Thus, our results show that Sema3F is the principal ligand for Npn-2-mediated axon guidance events in vivo and is a critical determinant of limbic and peripheral nervous system circuitry.

Expression and regulation of the novel vascular endothelial growth factor receptor neuropilin-1 by epidermal growth factor in human pancreatic carcinoma

BACKGROUND

It was recently shown that neuropilin-1 (NRP-1), which was described originally as a receptor for the semaphorins/collapsins (ligands involved in neuronal guidance), is a coreceptor for vascular endothelial growth factor (VEGF) and increases the affinity of specific isoforms of VEGF to its receptor, VEGF-R2.

METHODS

The authors investigated the expression and regulation of NRP-1 in human pancreatic adenocarcinoma specimens and cell lines.

RESULTS

Immunohistochemical analysis revealed that NRP-1 was expressed in 12 of 12 human pancreatic adenocarcinoma specimens but was absent in nonmalignant pancreatic tissue. Northern blot analysis revealed NRP-1 mRNA expression in 8 of 11 human pancreatic adenocarcinoma cell lines. NRP-1 mRNA expression was increased by epidermal growth factor (EGF) but not by tumor necrosis factor alpha in several of the human pancreatic adenocarcinoma cell lines studied. Treating human Panc-48 adenocarcinoma cells with EGF activated Akt and Erk but not P-38. Blockade of the phosphatidylinositol-3 kinase (PI-3K)/Akt, mitogen-activated protein kinase (MAPK)/Erk, or P-38 pathways abrogated EGF-induced NRP-1 expression. Finally, EGF receptor blockade in vivo led to a decrease in NRP-1 expression in an orthotopic model of human pancreatic carcinoma.

CONCLUSIONS

NRP-1 is expressed in most human pancreatic adenocarcinomas and cell lines but not in nonmalignant pancreatic tissue. EGF regulates NRP-1 expression through the PI-3K/Akt and MAPK/Erk signaling pathways, and blockade of the EGF receptor is associated with decreased expression of NRP-1 in vivo. NRP-1 may act as a coreceptor for VEGF in pancreatic carcinoma, as it does in other tumor systems, thereby enhancing angiogenesis and the effect of VEGF on the growth of pancreatic adenocarcinoma.

TRPC5 is a regulator of hippocampal neurite length and growth cone morphology

Growth cone motility is regulated by both fast voltage-dependent Ca2+ channels and by unknown receptor-operated Ca2+ entry mechanisms. Transient receptor potential (TRP) homomeric TRPC5 ion channels are receptor-operated, Ca2+-permeable channels predominantly expressed in the brain. Here we show that TRPC5 is expressed in growth cones of young rat hippocampal neurons. Our results indicate that TRPC5 channel subunits interact with the growth cone-enriched protein stathmin 2, are packaged into vesicles and are carried to newly forming growth cones and synapses. Once in the growth cone, TRPC5 channels regulate neurite extension and growth-cone morphology. Dominant-negative TRPC5 expression allowed significantly longer neurites and filopodia to form. We conclude that TRPC5 channels are important components of the mechanism controlling neurite extension and growth cone morphology.

Neuropilin-1 conveys semaphorin and VEGF signaling during neural and cardiovascular development

Neuropilin-1 (Npn-1) is a receptor that binds multiple ligands from structurally distinct families, including secreted semaphorins (Sema) and vascular endothelial growth factors (VEGF). We generated npn-1 knockin mice, which express an altered ligand binding site variant of Npn-1, and npn-1 conditional null mice to establish the cell-type- and ligand specificity of Npn-1 function in the developing cardiovascular and nervous systems. Our results show that VEGF-Npn-1 signaling in endothelial cells is required for angiogenesis. In striking contrast, Sema-Npn-1 signaling is not essential for general vascular development but is required for axonal pathfinding by several populations of neurons in the CNS and PNS. Remarkably, both Sema-Npn-1 signaling and VEGF-Npn-1 signaling are critical for heart development. Therefore, Npn-1 is a multifunctional receptor that mediates the activities of structurally distinct ligands during development of the heart, vasculature, and nervous system.

A role for semaphorins and neuropilins in oligodendrocyte guidance

Oligodendrocytes develop in defined CNS regions as progenitor cells, which migrate to their final destinations, encountering soluble and membrane-bound signals that influence their differentiation and potential to myelinate axonal projections. To identify the regulatory genes that may be involved in this process, microarray analysis of developing oligodendroglia was performed. Several neural guidance genes, including members of the neuropilin (NP) and semaphorin families were detected. These findings were verified and expanded upon using RT-PCR with RNA from fluorescent activated cell sorted A2B5+ oligodendrocyte progenitors and O4+ pro-oligodendrocytes isolated from in vitro and in vivo sources. RT-PCR, western and immunocytochemical analyses revealed that oligodendrocytes expressed NP1, several alternatively spliced isoforms of NP2, and a broad spectrum of both soluble (Class 3), membrane-spanning (Class 4-6), and membrane-tethered (Class 7) semaphorin ligands. Class 3 semaphorins, in a modified stripe assay, caused the collapse of oligodendrocyte progenitor growth cones, redirection of processes, and altered progenitor migration. Our data support a role for neuropilins and semaphorins in orchestrating the migration patterns of developing oligodendrocytes in the CNS.

Human malignant glioma cells express semaphorins and their receptors, neuropilins and plexins

Semaphorins comprise a family of molecules implicated in the guiding of growing axons and neuronal progenitor cells. Further, semaphorins have been suggested to play a role in cancer metastasis. Neuropilins 1 and 2 are cell surface receptors for soluble class 3 semaphorins. Plexins are direct receptors for membrane-bound semaphorins and, by binding to neuropilins, coreceptors necessary for class 3 semaphorin signaling. We here report that human malignant glioma cell lines express neuropilins 1 and 2 mRNA and protein, as well as either plexin A1, A2, or B1. Further, all glioma cell lines express SEMA3A and SEMA3C and exhibit SEMA3A binding sites. Exogenous SEMA3A expressed in 293 or U87MG cells has no collapsing or chemorepulsive activities on glioma cells as determined by F-actin staining and collagen coculture assays. In summary, human glioma cells express class 3 semaphorins and receptors for soluble and membrane-bound semaphorins, suggesting a possible role of the semaphorin/neuropilin system in the interactions of human malignant glioma with the host's central nervous and immune systems.

Temporal specific patterns of semaphorin gene expression in rat brain after kainic acid-induced status epilepticus

Mossy fiber sprouting and other forms of synaptic reorganization may form the basis for a recurrent excitatory network in epileptic foci. Four major classes of axon guidance molecules--the ephrins, netrins, slits, and semaphorins--provide targeting information to outgrowing axons along predetermined pathways during development. These molecules may also play a role in synaptic reorganization in the adult brain and thereby promote epileptogenesis. We studied semaphorin gene expression, as assessed by in situ hybridization, using riboprobes generated from rat cDNA in an adult model of synaptic reorganization, kainic acid (KA)-induced status epilepticus (SE). Within the first week after KA-induced SE, semaphorin 3C, a class III semaphorin, mRNA content is decreased in the CA1 area of the hippocampus and is increased in the upper layers of cerebral cortex. Another class III semaphorin, semaphorin 3F, is also decreased in CA1 and CA3 of hippocampus within the first week after KA-SE. These changes in gene expression are principally confined to neurons. By contrast, there was little change in the semaphorin 4C mRNA content of CA1 neurons at this time. No changes in expression of semaphorin 3A and 4C genes were detected 28 days after KA-induced SE. Regulation of semaphorin gene expression after KA-induced SE suggests that neurons may regulate the expression of axonal guidance molecules and thereby contribute to synaptic reorganization after injury of the mature brain. The anatomic locale of the altered semaphorin gene expression may serve as a marker for specific networks undergoing synaptic reorganization in the epileptic brain.

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.

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.

The interaction of Neuropilin-1 and Neuropilin-2 with tyrosine-kinase receptors for VEGF

The Neuropilin-1 (NRP1) and Neuropilin-2 (NRP2) receptors were initially described as receptors for axon guidance factors belonging to the class-3 Semaphorin sub-family. Subsequently, it was found the Neuropilins also function as receptors for some forms of vascular endothelial growth factor (VEGF). VEGF165 binds to both NRP1 and to NRP2 but VEGF121, does not bind to either of these receptors. VEGF145 on the other hand, binds to NRP2 but not to NRP1. Additional VEGF family members such as the heparin binding form of placenta growth factor (PlGF-2) and VEGF-B bind to NRP1, and it was also shown that both PlGF-2 and VEGF-C bind to NRP2. The intracellular domains of the Neuropilins are short, and do not suffice for independent transduction of biological signals subsequent to Semaphorin or VEGF binding. It was shown that both Neuropilins can form complexes with receptors belonging to the Plexin family, and that such Plexin/Neuropilin complexes are able to transduce signals following the binding of class-3 Semaphorins to Neuropilins. The VEGF165 induced proliferation and migration of cells that express the VEGF tyrosine-kinase receptor VEGFR2 is enhanced in the presence of NRP1, suggesting that Neuropilins may also form complexes with VEGF tyrosine-kinase receptors such as VEGFR2. However, it is not yet clear whether VEGFR2 and NRPI form complexes and contrasting results have been reported with regard to this issue. In contrast, it was recently reported by two laboratories that Neuropilins can form complexes with the second tyrosine-kinase receptor of VEGF, VEGFR1. However, the biological function of these complexes is still unclear.

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.

Semaphorin junction: making tracks toward neural connectivity

Semaphorins constitute one of the largest families of repulsive and attractive growth cone guidance proteins. They affect the growth cone's actin cytoskeleton through interactions with receptor complexes composed of ligand-binding, signal-transducing, and modulatory subunits. Our understanding of the intracellular signal transduction machinery linking semaphorins to actin dynamics is limited; however, recent advances provide a more comprehensive view of the molecular basis of neuronal semaphorin signaling.

Crystal structure of the human neuropilin-1 b1 domain

Neuropilin-1 (Npn-1) is a type I cell surface receptor involved in a broad range of developmental processes, including axon guidance, angiogenesis, and heterophilic cell adhesion. We have determined the crystal structure of the human Npn-1 b1 domain to 1.9 A. The overall structure resembles coagulation factor V and VIII (F5/8) C1 and C2 domains, exhibiting a distorted jellyroll fold. Details of the structure provide insight to b1 domain regions responsible for ligand binding and facilitate rationalization of existing biochemical binding data. A polar cleft formed by adjacent loops at one end of the molecule in conjunction with flanking electronegative surfaces may represent the binding site for the positively charged tails of semaphorins and VEGF(165). The nature of the cell adhesion binding site of the b1 domain can be visualized in context of the structure.

Truncated soluble Nogo receptor binds Nogo-66 and blocks inhibition of axon growth by myelin

CNS myelin contains axon outgrowth inhibitors, such as Nogo, that restrict regenerative growth after injury. An understanding of the mechanism of Nogo signaling through its receptor (NgR) is critical to developing strategies for overcoming Nogo-mediated inhibition. Here we analyze the function of NgR domains in outgrowth inhibition. Analysis of alkaline phosphatase (AP)-Nogo binding in COS-7 cells reveals that the leucine-rich repeat domain is necessary and sufficient for Nogo binding and NgR multimerization. Viral infection of embryonic day 7 chick retinal ganglion cells with mutated NgR demonstrates that the NgR C-terminal domain is required for inhibitory signaling but not ligand binding. The NgR glycosylphosphatidylinositol domain is not essential for inhibitory signaling but may facilitate Nogo responses. From this analysis, we have developed a soluble, truncated version of the Nogo receptor that antagonizes outgrowth inhibition on both myelin and Nogo substrates. These data suggest that NgR mediates a significant fraction of myelin inhibition of axon outgrowth.

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.

Oligodendrocyte-myelin glycoprotein is a Nogo receptor ligand that inhibits neurite outgrowth

The inhibitory activity associated with myelin is a major obstacle for successful axon regeneration in the adult mammalian central nervous system (CNS). In addition to myelin-associated glycoprotein (MAG) and Nogo-A, available evidence suggests the existence of additional inhibitors in CNS myelin. We show here that a glycosylphosphatidylinositol (GPI)-anchored CNS myelin protein, oligodendrocyte-myelin glycoprotein (OMgp), is a potent inhibitor of neurite outgrowth in cultured neurons. Like Nogo-A, OMgp contributes significantly to the inhibitory activity associated with CNS myelin. To further elucidate the mechanisms that mediate this inhibitory activity of OMgp, we screened an expression library and identified the Nogo receptor (NgR) as a high-affinity OMgp-binding protein. Cleavage of NgR and other GPI-linked proteins from the cell surface renders axons of dorsal root ganglia insensitive to OMgp. Introduction of exogenous NgR confers OMgp responsiveness to otherwise insensitive neurons. Thus, OMgp is an important inhibitor of neurite outgrowth that acts through NgR and its associated receptor complex. Interfering with the OMgp/NgR pathway may allow lesioned axons to regenerate after injury in vivo.

Aberrant sensory innervation of the olfactory bulb in neuropilin-2 mutant mice

The mammalian olfactory system consists of two anatomically segregated structures, the main olfactory system and the vomeronasal system, which each detect distinct types of chemical stimuli in the environment. During development, sensory neurons establish precise axonal connections with their respective targets within the olfactory bulb. The specificity of the odorant or vomeronasal receptor expressed by the sensory neuron is crucial in this process, yet it is less clear which of the more conventional axon guidance molecules are involved. Here, we show that neuropilin-2, a coreceptor for some of the class 3 semaphorins, is expressed in subpopulations of olfactory and vomeronasal sensory neurons. We generated a knock-out mutation in the neuropilin-2 gene by gene targeting in embryonic stem cells. Neuropilin-2 mutant mice exhibit profound and distinct effects on target innervation within the olfactory bulb. In the main olfactory system, axons of olfactory sensory neurons penetrate into the deeper layers of the main olfactory bulb. In the vomeronasal system, axonal fasciculation within the vomeronasal nerve is affected; some axons are misrouted and innervate glomeruli in an ectopic domain of the accessory olfactory bulb.

Injury-induced class 3 semaphorin expression in the rat spinal cord

In this study we evaluate the expression of all members of the class 3 semaphorins and their receptor components following complete transection and contusion lesions of the adult rat spinal cord. Following both types of lesions the expression of all class 3 semaphorins is induced in fibroblast in the neural scar. The distribution of semaphorin-positive fibroblasts differs markedly in scars formed after transection or contusion lesion. In contusion lesions semaphorin expression is restricted to fibroblasts of the meningeal sheet surrounding the lesion, while after transection semaphorin-positive fibroblast penetrate deep into the center of the lesion. Two major descending spinal cord motor pathways, the cortico- and rubrospinal tract, continue to express receptor components for class 3 semaphorins following injury, rendering them potentially sensitive to scar-derived semaphorins. In line with this we observed that most descending spinal cord fibers were not able to penetrate the semaphorin positive portion of the neural scar formed at the lesion site. These results suggest that the full range of secreted semaphorins contributes to the inhibitory nature of the neural scar and thereby may inhibit successful regeneration in the injured spinal cord. Future studies will focus on the neutralization of class 3 semaphorins, in order to reveal whether this creates a more permissive environment for regeneration of injured spinal cord axons.

Neuropilin-2 mediates axonal fasciculation, zonal segregation, but not axonal convergence, of primary accessory olfactory neurons

The mechanisms that underlie axonal pathfinding of vomeronasal neurons from the vomeronasal organ (VNO) in the periphery to select glomeruli in the accessory olfactory bulb (AOB) are not well understood. Neuropilin-2, a receptor for secreted semaphorins, is expressed in V1R- and V3R-expressing, but not V2R-expressing, postnatal vomeronasal neurons. Analysis of the vomeronasal nerve in neuropilin-2 (npn-2) mutant mice reveals pathfinding defects at multiple choice points. Vomeronasal sensory axons are severely defasciculated and a subset innervates the main olfactory bulb (MOB). While most axons of V1R-expressing neurons reach the AOB and converge into distinct glomeruli in stereotypic locations, they are no longer restricted to their normal anterior AOB target zone. Thus, Npn-2 and candidate pheromone receptors play distinct and complementary roles in promoting the wiring and patterning of sensory neurons in the accessory olfactory system.

Small proline-rich repeat protein 1A is expressed by axotomized neurons and promotes axonal outgrowth

The ability of neurons to regenerate an axon after injury is determined by both the surrounding environment and factors intrinsic to the damaged neuron. We have used cDNA microarrays to survey those genes induced during successful sciatic nerve regeneration. The small proline-rich repeat protein 1A (SPRR1A) is not detectable in uninjured neurons but is induced by >60-fold after peripheral axonal damage. The protein is localized to injured neurons and axons. sprr1a is one of a group of epithelial differentiation genes, including s100c and p21/waf, that are coinduced in neurons by axotomy. Overexpressed SPRR1A colocalizes with F-actin in membrane ruffles and augments axonal outgrowth on a range of substrates. In axotomized sensory neurons, reduction of SPRR1A function restricts axonal outgrowth. Neuronal SPRR1A may be a significant contributor to successful nerve regeneration.

Small proline-rich repeat protein 1A is expressed by axotomized neurons and promotes axonal outgrowth

The ability of neurons to regenerate an axon after injury is determined by both the surrounding environment and factors intrinsic to the damaged neuron. We have used cDNA microarrays to survey those genes induced during successful sciatic nerve regeneration. The small proline-rich repeat protein 1A (SPRR1A) is not detectable in uninjured neurons but is induced by >60-fold after peripheral axonal damage. The protein is localized to injured neurons and axons. sprr1a is one of a group of epithelial differentiation genes, including s100c and p21/waf, that are coinduced in neurons by axotomy. Overexpressed SPRR1A colocalizes with F-actin in membrane ruffles and augments axonal outgrowth on a range of substrates. In axotomized sensory neurons, reduction of SPRR1A function restricts axonal outgrowth. Neuronal SPRR1A may be a significant contributor to successful nerve regeneration.

Identification of semaphorin3B as a direct target of p53

A cDNA microarray analysis indicated that Semaphorin3B (Sema3B), a gene whose product is involved in axon guidance and axonal repulsion, is inducible by p53. Introduction of exogenous p53 into a glioblastoma cell line lacking wild-type p53 (U373MG) dramatically induced expression of Sema3B mRNA. An electrophoretic mobility shift assay and a reporter assay confirmed that a potential p53 binding site present in the promoter region had p53-dependent transcriptional activity. Expression of endogenous Sema3B was induced in response to genotoxic stresses caused by adriamycin treatment or UV irradiation in a p53-dependent manner. Ectopic expression of Sema3B in p53-defective cells reduced the number of colonies in colony formation assays. These results suggest that Sema3B might play some role in regulating cell growth as a mediator of p53 tumor-suppressor activity.

Semaphorin 3A elicits stage-dependent collapse, turning, and branching in Xenopus retinal growth cones

The semaphorin receptor, neuropilin-1 (NP-1), was first identified in Xenopus as the A5 antigen and is expressed abundantly in developing retinal ganglion cells (RGCs). Here we show that growth cones acquire responsiveness to semaphorin 3A (Sema 3A) with age and that the onset of responsiveness correlates with the appearance of NP-1 immunoreactivity. Growth cones from "old" (stage 35/36) retinal explants collapse rapidly (5-10 min) in response to Sema 3A and turn away from a gradient of Sema 3A, whereas "young" growth cones (stage 24) are insensitive to Sema 3A. Moreover, transfection of full-length NP-1 into young neurons confers premature Sema 3A sensitivity. When young neurons are aged in culture they develop Sema 3A sensitivity in parallel with those in vivo, suggesting that an intrinsic mechanism of NP-1 regulation mediates this age-dependent change. Sema 3A-induced collapse is transient, and after recovery approximately 30% of growth cones extend new branches within 1 hr, implicating Sema 3A as a branching factor. Pharmacological inhibitors were used to investigate whether these three Sema 3A-induced behaviors (collapse, turning, and branching) use distinct second messenger signaling pathways. All three behaviors were found to be mediated via cGMP. In situ hybridization shows that Sema 3A is expressed in the tectum and at the anterior boundary of the optic tract where axons bend caudally, suggesting that Sema 3A/NP-1 interactions play a role in guiding axons in the optic tract and in stimulating terminal branching in the tectum.

Cell signalling cascades regulating neuronal growth-promoting and inhibitory cues

During development of the nervous system, neurons extend axons over considerable distances in a highly stereospecific fashion in order to innervate their targets in an appropriate manner. This involves the recognition, by the axonal growth cone, of guidance cues that determine the pathway taken by the axons. These guidance cues can act to promote and/or repel growth cone advance, and they can act either locally or at a distance from their place of synthesis. The directed growth of axons is partly governed by cell adhesion molecules (CAMs) on the neuronal growth cone that bind to CAMs on the surface of other axons or non-neuronal cells. In vitro assays have established the importance of the CAMs (N-CAM, N-cadherin and the L1 glycoprotein) in promoting axonal growth over cells, such as Schwann cells, astrocytes and muscle cells. Strong evidence now exists implicating the fibroblast growth factor receptor tyrosine kinase as the primary signal transduction molecule in the CAM pathway. Cell adhesion molecules are important constituents of synapses, and CAMs appear to play important and diverse roles in regulating synaptic plasticity associated with learning and memory. Negative extracellular signals which physically direct neurite growth have also been described. The latter include the neuronal growth inhibitory proteins Nogo and myelin-associated glycoprotein, as well as the growth cone collapsing Semaphorins/neuropilins. Although less well characterised, evidence is now beginning to emerge describing a role for Rho kinase-mediated signalling in inhibition of neurite outgrowth. This review focuses on some of the major themes and ideas associated with this fast-moving field of neuroscience.

Plexin-A3 mediates semaphorin signaling and regulates the development of hippocampal axonal projections

Plexins are receptors implicated in mediating signaling by semaphorins, a family of axonal chemorepellents. The role of specific plexins in mediating semaphorin function in vivo has not, however, yet been examined in vertebrates. Here, we show that plexin-A3 is the most ubiquitously expressed plexin family member within regions of the developing mammalian nervous system known to contain semaphorin-responsive neurons. Using a chimeric receptor construct, we provide evidence that plexin-A3 can transduce a repulsive signal in growth cones in vitro. Analysis of plexin-A3 knockout mice shows that plexin-A3 contributes to Sema3F and Sema3A signaling and that plexin-A3 regulates the development of hippocampal axonal projections in vivo.

Factors controlling axonal and dendritic arbors

The sculpting and maintenance of axonal and dendritic arbors is largely under the control of molecules external to the cell. These factors include both substratum-associated and soluble factors that can enhance or inhibit the outgrowth of axons and dendrites. A large number of factors that modulate axonal outgrowth have been identified, and the first stages of the intracellular signaling pathways by which they modify process outgrowth have been characterized. Relatively fewer factors and pathways that affect dendritic outgrowth have been described. The factors that affect axonal arbors form an incompletely overlapping set with those that affect dendritic arbors, allowing selective control of the development and maintenance of these critical aspects of neuronal morphology.

Evidence for gradients of gene expression correlating with zonal topography of the olfactory sensory map

Signals regulating diversification of olfactory sensory neurons to express odorant receptors and other genes necessary for correct assembly of the olfactory sensory map persist in the olfactory epithelium of adult mouse. We have screened for genes with an expression pattern correlating with the topography odorant receptor-expression zones. The Msx1 homeobox gene and a semaphorin receptor (Neuropilin-2) showed graded expression patterns in the olfactory epithelium. The gradients of Msx1 and Neuropilin-2 expression in basal cells and neurons, respectively, correlated with expression of a retinoic acid-synthesizing enzyme (RALDH2) in lamina propria. A BMP-type I receptor (Alk6) showed a reverse gradient of expression in the supporting cells of the epithelium. Considering known functions of identified genes in cell specification and axon guidance this suggests that zonal division of the olfactory sensory map is maintained, during continuous neurogenesis, as a consequence of topographic counter gradients of positional information.

Development of the visual system of the chick. II. Mechanisms of axonal guidance

The quest to understand axonal guidance mechanisms requires exact and multidisciplinary analyses of axon navigation. This review is the second part of an attempt to synthesise experimental data with theoretical models of the development of the topographic connection of the chick retina with the tectum. The first part included classic ideas from developmental biology and recent achievements on the molecular level in understanding cytodifferentiation and histogenesis [J. Mey, S. Thanos, Development of the visual system of the chick. (I) Cell differentiation and histogenesis, Brain Res. Rev. 32 (2000) 343-379]. The present part deals with the question of how millions of fibres exit from the eye, traverse over several millimetres and spread over the optic tectum to assemble a topographic map, whose precision accounts for the sensory performance of the visual system. The following topics gained special attention in this review. (i) A remarkable conceptual continuity between classic embryology and recent molecular biology has revealed that positional cellular specification precedes and determines the formation of the retinotectal map. (ii) Graded expression of asymmetric genes, transcriptional factors and receptors for signal transduction during early development seem to play a crucial role in determining the spatial identity of neurons within surface areas of retina and optic tectum. (iii) The chemoaffinity hypothesis constitutes the conceptual framework for development of the retinotopic organisation of the primary visual pathway. Studies of repulsive factors in vitro developed the original hypothesis from a theoretical postulate of chemoattraction to an empirically supported concept based on chemorepulsion. (iv) The independent but synchronous development of retina and optic tectum in topo-chronologically corresponding patterns ensures that ingrowing retinal axons encounter receptive target tissue at appropriate locations, and at the time when connections are due to be formed. (v) The growth cones of the retino-fugal axons seem to be guided both by local cues on glial endfeet and within the extracellular matrix. On the molecular level, the ephrins and their receptors have emerged as the most likely candidates for the material substrate of a topographic projection along the anterior-posterior axis of the optic tectum. Yet, since a number of alternative molecules have been proposed for the same function, it remains the challenge for the near future to define the proportional contribution of each one of the individual mechanisms proposed by matching theoretical predictions with the experimental evidence.

Diverse gene expression and function of semaphorins in developing lung: positive and negative regulatory roles of semaphorins in lung branching morphogenesis

BACKGROUND

Previously, we reported that Sema3A, one of the secreted repulsive axon guidance molecules, CRMP (collapsin response mediator protein)-2, a putative intracellular signalling molecule for Sema3A and Sema3A receptor neuropilin-1 are expressed in the developing lung. Sema3A inhibits branching morphogenesis of embryonic lung in organ culture.

RESULTS

We examined the gene expression of Sema3A, Sema3C, Sema3F and their receptors, NP-1, NP-2 and plexin-A1 by in situ hybridization. Transcripts of all six genes were detected in mouse lung from embryonic day E11.5 to E17.5, and displayed highly specific spatiotemporal distributions. The distribution of the receptor genes was detected in patterns which were consistent with known receptor usage of the semaphorins. In contrast to Sema3A, we found that the other class 3 semaphorins, Sema3C and Sema3F, stimulated branching morphogenesis. This stimulatory effect of Sema3C or Sema3F was accompanied by a moderate increase in the incorporation of bromodeoxyuridine (BrdU) into DNA in the terminal epithelial cells.

CONCLUSION

The coordinated expression patterns of different semaphorins and their receptors, together with the specific activities affecting branching morphogenesis, suggest that the semaphorins act as both positive and negative regulators of branching morphogenesis in the developing lung.

Cell migration from the corticostriatal angle to the basal telencephalon in rat embryos

Lateral cortical stream (LCS) has been described as a flow of cell migration found in the lateral cortex of the embryonic telencephalon of mammals. The destinations of the cell migration were reported as the ventrolateral neocortex, the pyriform cortex, endopyriform nucleus and the claustrum. At the same time, however, other destinations of LCS have been suggested in the ventral telencephalon. Therefore, we investigated the additional destinations of the LCS using a combination of several molecular biological techniques. Using an expression vector of modified green fluorescent protein (GFP) introduced into the ventricular zone (VZ) around the corticostriatal angle, both tangential and radial cell migration was revealed in the LCS. The radial cell migration in the LCS supplied cells to the ventro-lateral neocortex, pyriform cortex and to the level of the lateral olfactory tract. In the second experiment, we injected COS-1 cells transfected with a Sema3A expression vector into one side of the neocortex. The cell supply to the destination of the LCS ceased due to the formation of a large necrosis in the LCS, which was triggered by the Sema3A-COS cell injection, and the dense cell layer in the olfactory tubercle shrunk on the side where COS cells were injected. These data indicated that the majority of neurons in the dense cell layer of the olfactory tubercle reached this point through the LCS. One of the origins of the cells in the LCS would be in the corticostriatal angle.

Expression of class 3 semaphorins and neuropilin receptors in the developing mouse tooth

The semaphorins are a large family of secreted or cell-bound signals needed for the development of the nervous system. We compared mRNA expression of class 3 semaphorins (Sema3A, 3B, 3C and 3F) and their two receptors (Neuropilin-1 and -2) in the embryonic mouse first molar tooth germ (E10-18) by radioactive in situ hybridization. All genes showed distinct developmentally regulated expression patterns during tooth organogenesis. Interestingly, Sema3A and 3C were first detected in the early dental epithelium, and later both genes were present in the epithelial primary enamel knot, a putative signaling center of the embryonic tooth regulating tooth morphogenesis. Prior to birth, Sema3A was also observed in tooth-specific cells, preodontoblasts, which later differentiate into odontoblasts secreting dentin, and in the mesenchymal dental follicle cells surrounding the tooth germ. Sema3B appeared transiently in the dental mesenchyme in the bud and cap stage tooth while Sema3F was expressed in both epithelial and mesenchymal components of the tooth. Of note, Npn-1 expression pattern was largely complementary to that of Sema3A, and transcripts were restricted to the dental mesenchymal cells. Npn-1 expression was first seen in the developing dental follicle, and later transcripts also appeared in the dental papilla mesenchyme. In contrast, Npn-2 signal was seen in both epithelial and mesenchymal tissues such as in the primary enamel knot and preodontoblasts.

Plexina1 autoinhibition by the plexin sema domain

Semaphorin 3A (Sema3A) binds to neuropilin-1 (NP1) and activates the transmembrane Plexin to transduce a repulsive axon guidance signal. Here, we show that Sema3 signals are transduced equally effectively by PlexinA1 or PlexinA2, but not by PlexinA3. Deletion analysis of the PlexinA1 ectodomain demonstrates that the sema domain prevents PlexinA1 activation in the basal state. Sema-deleted PlexinA1 is constitutively active, producing cell contraction, growth cone collapse, and inhibition of neurite outgrowth. The sema domain of PlexinA1 physically associates with the remainder of the PlexinA1 ectodomain and can reverse constitutive activation. Both the sema portion and the remainder of the ectodomain of PlexinA1 associate with NP1 in a Sema3A-independent fashion. Plexin A1 is autoinhibited by its sema domain, and Sema3A/NP1 releases this inhibition.

Peripheral nerve injury fails to induce growth of lesioned ascending dorsal column axons into spinal cord scar tissue expressing the axon repellent Semaphorin3A

We have investigated the hypothesis that the chemorepellent Semaphorin3A may be involved in the failure of axonal regeneration after injury to the ascending dorsal columns of adult rats. Following transection of the thoracic dorsal columns, fibroblasts in the dorsolateral parts of the lesion site showed robust expression of Semaphorin3A mRNA. In addition, dorsal root ganglion (DRG) neurons with projections through the dorsal columns to the injury site persistently expressed both Semaphorin3A receptor components, neuropilin-1 and plexin-A1. These ascending DRG collaterals failed to invade scar regions occupied by Semaphorin3A-positive fibroblasts, even in animals which had received conditioning lesions of the sciatic nerve to enhance regeneration. Other axon populations in the dorsal spinal cord were similarly unable to penetrate Semaphorin3A-positive scar tissue. These data suggest that Semaphorin3A may create an exclusion zone for regenerating dorsal column fibres and that enhancing the intrinsic regenerative response of DRG neurons has only limited effects on axonal regrowth. Tenascin-C and chondroitin sulphate proteoglycans were also detected at the injury site, which was largely devoid of central nervous system (CNS) myelin, showing that several classes of inhibitory factors, including semaphorins, with only partially overlapping spatial and temporal patterns of expression are in a position to participate in preventing regenerative axonal growth in the injured dorsal columns. Interestingly, conditioning nerve injuries enabled numerous ascending DRG axons to regrow across areas of strong tenascin-C and chondroitin sulphate proteoglycan expression, while areas containing Semaphorin3A and CNS myelin were selectively avoided by (pre)primed axonal sprouts.

Identification of a receptor mediating Nogo-66 inhibition of axonal regeneration

Nogo has been identified as a component of the central nervous system (CNS) myelin that prevents axonal regeneration in the adult vertebrate CNS. Analysis of Nogo-A has shown that an axon-inhibiting domain of 66 amino acids is expressed at the extracellular surface and at the endoplasmic reticulum lumen of transfected cells and oligodendrocytes. The acidic amino terminus of Nogo-A is detected at the cytosolic face of cellular membranes and may contribute to inhibition of axon regeneration at sites of oligodendrocyte injury. Here we show that the extracellular domain of Nogo (Nogo-66) inhibits axonal extension, but does not alter non-neuronal cell morphology. In contrast, a multivalent form of the N terminus of Nogo-A affects the morphology of both neurons and other cell types. Here we identify a brain-specific, leucine-rich-repeat protein with high affinity for soluble Nogo-66. Cleavage of the Nogo-66 receptor and other glycophosphatidylinositol-linked proteins from axonal surfaces renders neurons insensitive to Nogo-66. Nogo-66 receptor expression is sufficient to impart Nogo-66 axonal inhibition to unresponsive neurons. Disruption of the interaction between Nogo-66 and its receptor provides the potential for enhanced recovery after human CNS injury.

The semaphorin 3A receptor may directly regulate the activity of small GTPases

The axon guidance signal semaphorin 3A induces the rapid collapse of growth cones by activating a receptor complex that contains neuropilin-1 as the ligand-binding and a plexin as the signal-transducing subunit. Here we show that plexins bind Rho-like GTPases and may directly regulate their activity. The cytoplasmic domain of plexins shows sequence similarity to GTPase activating proteins (GAPs) and mutation of two arginines that correspond to the catalytic residues of Ras GAPs inactivates plexin-A1. Our data suggest that plexins may be integral membrane proteins with an intrinsic GAP activity that is essential for their ability to induce growth cone collapse.