Plexin A3 and plexin A4 convey semaphorin signals during facial nerve development

Plexina4 and cell survival in the developing zebrafish hindbrain

BACKGROUND

Growth factors are important in the developing and mature nervous system to support the survival of neurons. Developmental signaling molecules are known for their roles in controlling neurogenesis and neural circuit formation. Whether or not these molecules also have roles in cell survival in the developing nervous system is poorly understood. Plexins are a family of transmembrane receptors that bind Semaphorin ligands and are known to function in the guidance of developing axons and blood vessels.

RESULTS

In embryonic zebrafish, plexina4 is expressed widely in the brain, becoming largely restricted to the hindbrain as neurogenesis and differentiation proceed. Apoptosis is increased in the embryonic hindbrain of a plexina4ca307/ca307 CRISPR mutant. Based on the literature, we tested the secreted heat shock protein, Clusterin, as a candidate ligand to mediate cell survival through Plexina4. clusterin is expressed by the floor plate of the embryonic zebrafish hindbrain, in proximity to plexina4-expressing hindbrain cells. Morpholino-mediated knockdown of Clusterin increases cell apoptosis in the hindbrain, with additional cell death observed in epistasis experiments where Clusterin is knocked down in a plexina4 mutant background.

CONCLUSIONS

Our data suggest that Plexina4 promotes cell survival in the developing zebrafish hindbrain, likely through a pathway independent of Clusterin.

A balance of noncanonical Semaphorin signaling from the cerebrospinal fluid regulates apical cell dynamics during corticogenesis

During corticogenesis, dynamic regulation of apical adhesion is fundamental to generate correct numbers and cell identities. While radial glial cells (RGCs) maintain basal and apical anchors, basal progenitors and neurons detach and settle at distal positions from the apical border. Whether diffusible signals delivered from the cerebrospinal fluid (CSF) contribute to the regulation of apical adhesion dynamics remains fully unknown. Secreted class 3 Semaphorins (Semas) trigger cell responses via Plexin-Neuropilin (Nrp) membrane receptor complexes. Here, we report that unconventional Sema3-Nrp preformed complexes are delivered by the CSF from sources including the choroid plexus to Plexin-expressing RGCs via their apical endfeet. Through analysis of mutant mouse models and various ex vivo assays mimicking ventricular delivery to RGCs, we found that two different complexes, Sema3B/Nrp2 and Sema3F/Nrp1, exert dual effects on apical endfeet dynamics, nuclei positioning, and RGC progeny. This reveals unexpected balance of CSF-delivered guidance molecules during cortical development.

Neuropilin 2/Plexin-A3 Receptors Regulate the Functional Connectivity and the Excitability in the Layers 4 and 5 of the Cerebral Cortex

The functions of cortical networks are progressively established during development by series of events shaping the neuronal connectivity. Synaptic elimination, which consists of removing the supernumerary connections generated during the earlier stages of cortical development, is one of the latest stages in neuronal network maturation. The semaphorin 3F coreceptors neuropilin 2 (Nrp2) and plexin-A3 (PlxnA3) may play an important role in the functional maturation of the cerebral cortex by regulating the excess dendritic spines on cortical excitatory neurons. Yet, the identity of the connections eliminated under the control of Nrp2/PlxnA3 signaling is debated, and the importance of this synaptic refinement for cortical functions remains poorly understood. Here, we show that Nrp2/PlxnA3 controls the spine densities in layer 4 (L4) and on the apical dendrite of L5 neurons of the sensory and motor cortices. Using a combination of neuroanatomical, ex vivo electrophysiology, and in vivo functional imaging techniques in Nrp2 and PlxnA3 KO mice of both sexes, we disprove the hypothesis that Nrp2/PlxnA3 signaling is required to maintain the ectopic thalamocortical connections observed during embryonic development. We also show that the absence of Nrp2/PlxnA3 signaling leads to the hyperexcitability and excessive synchronization of the neuronal activity in L5 and L4 neuronal networks, suggesting that this system could participate in the refinement of the recurrent corticocortical connectivity in those layers. Altogether, our results argue for a role of semaphorin-Nrp2/PlxnA3 signaling in the proper maturation and functional connectivity of the cerebral cortex, likely by controlling the refinement of recurrent corticocortical connections.SIGNIFICANCE STATEMENT The function of a neuronal circuit is mainly determined by the connections that neurons establish with one another during development. Understanding the mechanisms underlying the establishment of the functional connectivity is fundamental to comprehend how network functions are implemented, and to design treatments aiming at restoring damaged neuronal circuits. Here, we show that the cell surface receptors for the family of semaphorin guidance cues neuropilin 2 (Nrp2) and plexin-A3 (PlxnA3) play an important role in shaping the functional connectivity of the cerebral cortex likely by trimming the recurrent connections in layers 4 and 5. By removing the supernumerary inputs generated during early development, Nrp2/PlxnA3 signaling reduces the neuronal excitability and participates in the maturation of the cortical network functions.

Crosstalk with keratinocytes causes GNAQ oncogene specificity in melanoma

Different melanoma subtypes exhibit specific and non-overlapping sets of oncogene and tumor suppressor mutations, despite a common cell of origin in melanocytes. For example, activation of the Gα_q/11 signaling pathway is a characteristic initiating event in primary melanomas that arise in the dermis, uveal tract, or central nervous system. It is rare in melanomas arising in the epidermis. The mechanism for this specificity is unknown. Here, we present evidence that in the mouse, crosstalk with the epidermal microenvironment actively impairs the survival of melanocytes expressing the GNAQ^Q209L oncogene. We found that GNAQ^Q209L, in combination with signaling from the interfollicular epidermis (IFE), stimulates dendrite extension, leads to actin cytoskeleton disorganization, inhibits proliferation, and promotes apoptosis in melanocytes. The effect was reversible and paracrine. In contrast, the epidermal environment increased the survival of wildtype and Braf^V600E expressing melanocytes. Hence, our studies reveal the flip side of Gα_q/11 signaling, which was hitherto unsuspected. In the future, the identification of the epidermal signals that restrain the GNAQ^Q209L oncogene could suggest novel therapies for GNAQ and GNA11 mutant melanomas.

Neuropathological Findings in a Case of Parkinsonism and Developmental Delay Associated with a Monoallelic Variant in PLXNA1

BACKGROUND

PLXNA1 encodes for Plexin-A, a transmembrane protein expressed in the developing nervous system. Mutations in this gene have been associated with developmental delay but have not been previously associated with the development of parkinsonism.

OBJECTIVES

To describe the case of a 38-year-old patient with developmental delay who developed parkinsonism later in life.

METHODS

Post-mortem exome sequencing was performed with confirmation by Sanger sequencing. Brain autopsy was also performed.

RESULTS

Post-mortem exome sequencing on the proband identified a heterozygous predicted nonsense PLXNA1 variant (c.G3361T:p.Glu1121Ter). Pathology demonstrated arhinencephaly with brainstem heterotopia, diffuse Lewy body disease, and frontotemporal lobar dementia-tau.

CONCLUSIONS

This case of a patient with developmental delay and parkinsonism with PLXNA1 mutation highlights a need for assessing long-term outcomes of individuals with neurodevelopmental disorders, as well as the need for genetic testing in adults. It also suggests that the link between PLXNA1 and α-synuclein should be explored in the future. © 2021 International Parkinson and Movement Disorder Society.

Semaphorin Regulation by the Chromatin Remodeler CHD7: An Emerging Genetic Interaction Shaping Neural Cells and Neural Crest in Development and Cancer

CHD7 is a chromatin remodeler protein that controls gene expression via the formation of multi-protein complexes with specific transcription factors. During development, CHD7 controls several differentiation programs, mainly by acting on neural progenitors and neural crest (NC) cells. Thus, its roles range from the central nervous system to the peripheral nervous system and the organs colonized by NC cells, including the heart. Accordingly, mutated CHD7 is linked to CHARGE syndrome, which is characterized by several neuronal dysfunctions and by malformations of NC-derived/populated organs. Altered CHD7 has also been associated with different neoplastic transformations. Interestingly, recent evidence revealed that semaphorins, a class of molecules involved in developmental and pathological processes similar to those controlled by CHD7, are regulated by CHD7 in a context-specific manner. In this article, we will review the recent insights that support the existence of genetic interactions between these pathways, both during developmental processes and cancer progression.

PlexinA4-Semaphorin3A-mediated crosstalk between main cortical interneuron classes is required for superficial interneuron lamination

In the mammalian cerebral cortex, the developmental events governing allocation of different classes of inhibitory interneurons (INs) to distinct cortical layers are poorly understood. Here we report that the guidance receptor PlexinA4 (PLXNA4) is upregulated in serotonin receptor 3a-expressing (HTR3A⁺) cortical INs (hINs) as they invade the cortical plate, and that it regulates their laminar allocation to superficial cortical layers. We find that the PLXNA4 ligand Semaphorin3A (SEMA3A) acts as a chemorepulsive factor on hINs migrating into the nascent cortex and demonstrate that SEMA3A specifically controls their laminar positioning through PLXNA4. We identify deep-layer INs as a major source of SEMA3A in the developing cortex and demonstrate that targeted genetic deletion of Sema3a in these INs specifically affects laminar allocation of hINs. These data show that, in the neocortex, deep-layer INs control laminar allocation of hINs into superficial layers.

Congenital Oculonasal Synkinesis

A 4-year-old girl with maxillary hypoplasia, intermittent exotropia, and high myopia displayed congenital oculonasal synkinesis. We examine the implications for pathogenesis of these disparate craniofacial findings.

CRMP2 mediates Sema3F-dependent axon pruning and dendritic spine remodeling

Regulation of axon guidance and pruning of inappropriate synapses by class 3 semaphorins are key to the development of neural circuits. Collapsin response mediator protein 2 (CRMP2) has been shown to regulate axon guidance by mediating semaphorin 3A (Sema3A) signaling; however, nothing is known about its role in synapse pruning. Here, using newly generated crmp2^-/- mice we demonstrate that CRMP2 has a moderate effect on Sema3A-dependent axon guidance in vivo, and its deficiency leads to a mild defect in axon guidance in peripheral nerves and the corpus callosum. Surprisingly, crmp2^-/- mice display prominent defects in stereotyped axon pruning in hippocampus and visual cortex and altered dendritic spine remodeling, which is consistent with impaired Sema3F signaling and with models of autism spectrum disorder (ASD). We demonstrate that CRMP2 mediates Sema3F signaling in primary neurons and that crmp2^-/- mice display ASD-related social behavior changes in the early postnatal period as well as in adults. Together, we demonstrate that CRMP2 mediates Sema3F-dependent synapse pruning and its dysfunction shares histological and behavioral features of ASD.

PLXNA1 and PLXNA3 cooperate to pattern the nasal axons that guide gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons regulate puberty onset and sexual reproduction by secreting GnRH to activate and maintain the hypothalamic-pituitary-gonadal axis. During embryonic development, GnRH neurons migrate along olfactory and vomeronasal axons through the nose into the brain, where they project to the median eminence to release GnRH. The secreted glycoprotein SEMA3A binds its receptors neuropilin (NRP) 1 or NRP2 to position these axons for correct GnRH neuron migration, with an additional role for the NRP co-receptor PLXNA1. Accordingly, mutations in SEMA3A, NRP1, NRP2 and PLXNA1 have been linked to defective GnRH neuron development in mice and inherited GnRH deficiency in humans. Here, we show that only the combined loss of PLXNA1 and PLXNA3 phenocopied the full spectrum of nasal axon and GnRH neuron defects of SEMA3A knockout mice. Together with Plxna1, the human orthologue of Plxna3 should therefore be investigated as a candidate gene for inherited GnRH deficiency.

Semaphorin-5B Controls Spiral Ganglion Neuron Branch Refinement during Development

During nervous system development, axons often undergo elaborate changes in branching patterns before circuits have achieved their mature patterns of innervation. In the auditory system, type I spiral ganglion neurons (SGNs) project their peripheral axons into the cochlear epithelium and then undergo a process of branch refinement before forming synapses with sensory hair cells. Here, we report that Semaphorin-5B (Sema5B) acts as an important mediator of this process. During cochlear development in mouse, immature hair cells express Sema5B, whereas the SGNs express both PlexinA1 and PlexinA3, which are known Sema5B receptors. In these studies, genetic sparse labeling and three-dimensional reconstruction techniques were leveraged to determine the morphologies of individual type I SGNs after manipulations of Sema5B signaling. Treating cultured mouse cochleae with Sema5B-Fc (to activate Plexin-As) led to type I SGNs with less numerous, but longer terminal branches. Conversely, cochleae from Sema5b knock-out mice showed type I SGNs with more numerous, but shorter terminal branches. In addition, conditional loss of Plxna1 in SGNs (using Bhlhb5Cre) led to increased type I SGN branching, suggesting that PlexinA1 normally responds to Sema5B in this process. In these studies, mice of either sex were used. The data presented here suggest that Sema5B-PlexinA1 signaling limits SGN terminal branch numbers without causing axonal repulsion, which is a role that distinguishes Sema5B from other Semaphorins in cochlear development.SIGNIFICANCE STATEMENT The sensorineural components of the cochlea include hair cells, which respond mechanically to sound waves, and afferent spiral ganglion neurons (SGNs), which respond to glutamate released by hair cells and transmit auditory information into the CNS. An important component of synapse formation in the cochlea is a process of SGN "debranching" whereby SGNs lose extraneous branches before developing unramified bouton endings that contact the hair cells. In this work, we have found that the transmembrane ligand Semaphorin-5B and its receptor PlexinA1 regulate the debranching process. The results in this report provide new knowledge regarding the molecular control of cochlear afferent innervation.

Multigenic truncation of the semaphorin-plexin pathway by a germline chromothriptic rearrangement associated with Moebius syndrome

Moebius syndrome (MBS) is a congenital disorder caused by paralysis of the facial and abducens nerves. Although a number of candidate genes have been suspected, so far only mutations in PLXND1 and REV3L are confirmed to cause MBS. Here, we fine mapped the breakpoints of a complex chromosomal rearrangement (CCR) 46,XY,t(7;8;11;13) in a patient with MBS, which revealed 41 clustered breakpoints with typical hallmarks of chromothripsis. Among 12 truncated protein-coding genes, SEMA3A is known to bind to the MBS-associated PLXND1. Intriguingly, the CCR also truncated PIK3CG, which in silico interacts with REVL3 encoded by the other known MBS-gene REV3L, and with the SEMA3A/PLXND1 complex via FLT1. Additional studies of other complex rearrangements may reveal whether the multiple breakpoints in germline chromothripsis may predispose to complex multigenic disorders.

Class-3 Semaphorins and Their Receptors: Potent Multifunctional Modulators of Tumor Progression

Semaphorins are the products of a large gene family containing 28 genes of which 21 are found in vertebrates. Class-3 semaphorins constitute a subfamily of seven vertebrate semaphorins which differ from the other vertebrate semaphorins in that they are the only secreted semaphorins and are distinguished from other semaphorins by the presence of a basic domain at their C termini. Class-3 semaphorins were initially characterized as axon guidance factors, but have subsequently been found to regulate immune responses, angiogenesis, lymphangiogenesis, and a variety of additional physiological and developmental functions. Most class-3 semaphorins transduce their signals by binding to receptors belonging to the neuropilin family which subsequently associate with receptors of the plexin family to form functional class-3 semaphorin receptors. Recent evidence suggests that class-3 semaphorins also fulfill important regulatory roles in multiple forms of cancer. Several class-3 semaphorins function as endogenous inhibitors of tumor angiogenesis. Others were found to inhibit tumor metastasis by inhibition of tumor lymphangiogenesis, by direct effects on the behavior of tumor cells, or by modulation of immune responses. Notably, some semaphorins such as sema3C and sema3E have also been found to potentiate tumor progression using various mechanisms. This review focuses on the roles of the different class-3 semaphorins in tumor progression.

Keratin 13 expression reprograms bone and brain metastases of human prostate cancer cells

Lethal progression of prostate cancer metastasis can be improved by developing animal models that recapitulate the clinical conditions. We report here that cytokeratin 13 (KRT13), an intermediate filament protein, plays a directive role in prostate cancer bone, brain, and soft tissue metastases. KRT13 expression was elevated in bone, brain, and soft tissue metastatic prostate cancer cell lines and in primary and metastatic clinical prostate, lung, and breast cancer specimens. When KRT13 expression was determined at a single cell level in primary tumor tissues of 44 prostate cancer cases, KRT13 level predicted bone metastasis and the overall survival of prostate cancer patients. Genetically enforced KRT13 expression in human prostate cancer cell lines drove metastases toward mouse bone, brain and soft tissues through a RANKL-independent mechanism, as KRT13 altered the expression of genes associated with EMT, stemness, neuroendocrine/neuromimicry, osteomimicry, development, and extracellular matrices, but not receptor activator NF-κB ligand (RANKL) signaling networks in prostate cancer cells. Our results suggest new inhibitors targeting RANKL-independent pathways should be developed for the treatment of prostate cancer bone and soft tissue metastases.

Molecular specification of facial branchial motor neurons in vertebrates

Orofacial muscles are critical for life-sustaining behaviors, such as feeding and breathing. Centuries of work by neuroanatomists and surgeons resulted in the mapping of bulbar motor neurons in the brainstem and the course of the cranial nerves that carry their axons. Despite the sophisticated understanding of the anatomy of the region, the molecular mechanisms that dictate the development and maturation of facial motor neurons remain poorly understood. This fundamental problem has been recently revisited by physiologists with novel techniques of studying the rhythmic contraction of orofacial muscles in relationship to breathing. The molecular understanding of facial motor neuron development will not only lead to the comprehension of the neural basis of facial expression but may also unlock new avenues to generate stem cell-derived replacements. This review summarizes the current understanding of molecular programs involved in facial motor neuron generation, migration, and maturation, including neural circuit assembly.

Neuronal expression patterns of the PlexinA family during zebrafish development

Plexins (Plxns) and Semaphorins (Semas) are key signaling molecules that regulate many aspects of development. Plxns are a family of transmembrane protein receptors that are activated upon extracellular binding by Semas. Activated Plxns trigger intracellular signaling cascades, which regulate a range of developmental processes, including axon guidance, neuronal positioning and vasculogenesis. Semas are a large family of both transmembrane and secreted signaling molecules, and show subtype specific binding to different Plxn family members. Each Plxn can play different roles in development, and so tightly regulated temporal and spatial expression of receptor subtypes is critical to ensure appropriate signaling. Here we elucidate the expression profiles of the plxnA family, plxnA1a, A1b, A2, A3 and A4 at 18, 24, 36, 48, 60 and 72 h post fertilization in the developing zebrafish. We show that PlxnA family members are expressed in neuronal tissues during zebrafish development, but exhibit key differences in expression within these tissues. We also highlight that plxnA1 has two genes in zebrafish, A1a and A1b, which show divergences in expression patterns during early development.

Semaphorin-Plexin signaling influences early ventral telencephalic development and thalamocortical axon guidance

Background

Sensory processing relies on projections from the thalamus to the neocortex being established during development. Information from different sensory modalities reaching the thalamus is segregated into specialized nuclei, whose neurons then send inputs to cognate cortical areas through topographically defined axonal connections.

Developing thalamocortical axons (TCAs) normally approach the cortex by extending through the subpallium; here, axonal navigation is aided by distributed guidance cues and discrete cell populations, such as the corridor neurons and the internal capsule (IC) guidepost cells. In mice lacking Semaphorin-6A, axons from the dorsal lateral geniculate nucleus (dLGN) bypass the IC and extend aberrantly in the ventral subpallium. The functions normally mediated by Semaphorin-6A in this system remain unknown, but might depend on interactions with Plexin-A2 and Plexin-A4, which have been implicated in other neurodevelopmental processes.

Methods

We performed immunohistochemical and neuroanatomical analyses of thalamocortical wiring and subpallial development in Sema6a and Plxna2; Plxna4 null mutant mice and analyzed the expression of these genes in relevant structures.

Results

In Plxna2; Plxna4 double mutants we discovered TCA pathfinding defects that mirrored those observed in Sema6a mutants, suggesting that Semaphorin-6A − Plexin-A2/Plexin-A4 signaling might mediate dLGN axon guidance at subpallial level.

In order to understand where and when Semaphorin-6A, Plexin-A2 and Plexin-A4 may be required for proper subpallial TCA guidance, we then characterized their spatiotemporal expression dynamics during early TCA development. We observed that the thalamic neurons whose axons are misrouted in these mutants normally express Semaphorin-6A but not Plexin-A2 or Plexin-A4. By contrast, all three proteins are expressed in corridor cells and other structures in the developing basal ganglia.

This finding could be consistent with an hypothetical action of Plexins as guidance signals through Sema6A as a receptor on dLGN axons, and/or with their indirect effect on TCA guidance due to functions in the morphogenesis of subpallial intermediate targets. In support of the latter possibility, we observed that in both Plxna2; Plxna4 and Sema6a mutants some IC guidepost cells abnormally localize in correspondence of the ventral path misrouted TCAs elongate into.

Conclusions

These findings implicate Semaphorin-6A − Plexin-A2/Plexin-A4 interactions in dLGN axon guidance and in the spatiotemporal organization of guidepost cell populations in the mammalian subpallium.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-017-0083-4) contains supplementary material, which is available to authorized users.

Transcriptome profiling reveals expression signatures of cranial neural crest cells arising from different axial levels

Background

Cranial neural crest cells (NCCs) are a unique embryonic cell type which give rise to a diverse array of derivatives extending from neurons and glia through to bone and cartilage. Depending on their point of origin along the antero-posterior axis cranial NCCs are rapidly sorted into distinct migratory streams that give rise to axial specific structures. These migratory streams mirror the underlying segmentation of the brain with NCCs exiting the diencephalon and midbrain following distinct paths compared to those exiting the hindbrain rhombomeres (r). The genetic landscape of cranial NCCs arising at different axial levels remains unknown.

Results

Here we have used RNA sequencing to uncover the transcriptional profiles of mouse cranial NCCs arising at different axial levels. Whole transcriptome analysis identified over 120 transcripts differentially expressed between NCCs arising anterior to r3 (referred to as r1-r2 migratory stream for simplicity) and the r4 migratory stream. Eight of the genes differentially expressed between these populations were validated by RT-PCR with 2 being further validated by in situ hybridisation. We also explored the expression of the Neuropilins (Nrp1 and Nrp2) and their co-receptors and show that the A-type Plexins are differentially expressed in different cranial NCC streams.

Conclusions

Our analyses identify a large number of genes differentially regulated between cranial NCCs arising at different axial levels. This data provides a comprehensive description of the genetic landscape driving diversity of distinct cranial NCC streams and provides novel insight into the regulatory networks controlling the formation of specific skeletal elements and the mechanisms promoting migration along different paths.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0147-z) contains supplementary material, which is available to authorized users.

The role of the semaphorins in cancer

The semaphorins were initially characterized as axon guidance factors, but have subsequently been implicated also in the regulation of immune responses, angiogenesis, organ formation, and a variety of additional physiological and developmental functions. The semaphorin family contains more then 20 genes divided into 7 subfamilies, all of which contain the signature sema domain. The semaphorins transduce signals by binding to receptors belonging to the neuropilin or plexin families. Additional receptors which form complexes with these primary semaphorin receptors are also frequently involved in semaphorin signaling. Recent evidence suggests that semaphorins also fulfill important roles in the etiology of multiple forms of cancer. Some semaphorins have been found to function as bona-fide tumor suppressors and to inhibit tumor progression by various mechanisms while other semaphorins function as inducers and promoters of tumor progression.

Plexin a4 expression in adult rat cranial nerves

PlexinsA1-A4 participate in class 3 semaphorin signaling as co-receptors to neuropilin 1 and 2. PlexinA4 is the latest member of the PlexinA subfamily to be identified. In previous studies, we described the expression of PlexinA4 in the brain and spinal cord of the adult rat. Here, antibodies to PlexinA4 were used to reveal immunolabeling in most of the cranial nerve surveyed. Labeling was found in the olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, and hypoglossal nerves. This is the first detailed description of the cellular and subcellular distribution of PlexinA4 in the adult cranial nerves. The findings will set the basis for future studies on the potential role of PlexinA4 in regeneration and repair of the adult central and peripheral nervous system.

Semaphorin signalling during development

Semaphorins are secreted and membrane-associated proteins that regulate many different developmental processes, including neural circuit assembly, bone formation and angiogenesis. Trans and cis interactions between semaphorins and their multimeric receptors trigger intracellular signal transduction networks that regulate cytoskeletal dynamics and influence cell shape, differentiation, motility and survival. Here and in the accompanying poster we provide an overview of the molecular biology of semaphorin signalling within the context of specific cell and developmental processes, highlighting the mechanisms that act to fine-tune, diversify and spatiotemporally control the effects of semaphorins.

The membrane proteome of sensory cilia to the depth of olfactory receptors

In the nasal cavity, the nonmotile cilium of olfactory sensory neurons (OSNs) constitutes the chemosensory interface between the ambient environment and the brain. The unique sensory organelle facilitates odor detection for which it includes all necessary components of initial and downstream olfactory signal transduction. In addition to its function in olfaction, a more universal role in modulating different signaling pathways is implicated, for example, in neurogenesis, apoptosis, and neural regeneration. To further extend our knowledge about this multifunctional signaling organelle, it is of high importance to establish a most detailed proteome map of the ciliary membrane compartment down to the level of transmembrane receptors. We detached cilia from mouse olfactory epithelia via Ca(2+)/K(+) shock followed by the enrichment of ciliary membrane proteins at alkaline pH, and we identified a total of 4,403 proteins by gel-based and gel-free methods in conjunction with high resolution LC/MS. This study is the first to report the detection of 62 native olfactory receptor proteins and to provide evidence for their heterogeneous expression at the protein level. Quantitative data evaluation revealed four ciliary membrane-associated candidate proteins (the annexins ANXA1, ANXA2, ANXA5, and S100A5) with a suggested function in the regulation of olfactory signal transduction, and their presence in ciliary structures was confirmed by immunohistochemistry. Moreover, we corroborated the ciliary localization of the potassium-dependent Na(+)/Ca(2+) exchanger (NCKX) 4 and the plasma membrane Ca(2+)-ATPase 1 (PMCA1) involved in olfactory signal termination, and we detected for the first time NCKX2 in olfactory cilia. Through comparison with transcriptome data specific for mature, ciliated OSNs, we finally delineated the membrane ciliome of OSNs. The membrane proteome of olfactory cilia established here is the most complete today, thus allowing us to pave new avenues for the study of diverse molecular functions and signaling pathways in and out of olfactory cilia and thus to advance our understanding of the biology of sensory organelles in general.

The role of the plexin-A2 receptor in semaphorin-3A and semaphorin-3B signal transduction

Class-3 semaphorins are anti-angiogenic and anti-tumorigenic guidance factors that bind to neuropilins which in turn associate with class-A plexins to transduce semaphorin signals. To study the role of the plexin-A2 receptor in semaphorin signaling, we silenced its expression in endothelial cells and in glioblastoma cells. The silencing did not affect sema3A signaling which depended on neuropilin-1, plexin-A1 and plexin-A4, but abolished completely sema3B signaling which required in addition plexin-A4 and one of the two neuropilins. Interestingly, over-expression of plexin-A2 in plexin-A1 or plexin-A4 silenced cells restored responses to both semaphorins although it nullified their ability to differentiate between them, suggesting that when over-expressed plexin-A2 is functionally interchangeable with other class-A plexins. In-contrast, although plexin-A4 over-expression restored sema3A signaling in plexin-A1 silenced cells, it failed to restore sema3B signaling in plexin-A2 silenced cells. It follows that the identity of plexins in functional semaphorin receptors can be flexible depending on their expression level. Our results suggest that changes in the expression of plexins induced by microenvironmental cues can trigger differential responses of different populations of migrating cells to encountered gradients of semaphorins.

cAMP-induced expression of neuropilin1 promotes retinal axon crossing in the zebrafish optic chiasm

Growing axons navigate a complex environment as they respond to attractive and repellent guidance cues. Axons can modulate their responses to cues through a G-protein-coupled, cAMP-dependent signaling pathway. To examine the role of G-protein signaling in axon guidance in vivo, we used the GAL4/UAS system to drive expression of dominant-negative heterotrimeric G-proteins (DNG) in retinal ganglion cells (RGCs) of embryonic zebrafish. Retinal axons normally cross at the ventral midline and project to the contralateral tectum. Expression of DNGα(S) in RGCs causes retinal axons to misproject to the ipsilateral tectum. These errors resemble misprojections in adcy1, adcy8, nrp1a, sema3D, or sema3E morphant embryos, as well as in sema3D mutant embryos. nrp1a is expressed in RGCs as their axons extend toward and across the midline. sema3D and sema3E are expressed adjacent to the chiasm, suggesting that they facilitate retinal midline crossing. We demonstrate synergistic induction of ipsilateral misprojections between adcy8 knockdown and transgenic DNGα(S) expression, adcy8 and nrp1a morphants, or nrp1a morphants and transgenic DNGα(S) expression. Using qPCR analysis, we show that either transgenic DNGα(S)-expressing embryos or adcy8 morphant embryos have decreased levels of nrp1a and nrp1b mRNA. Ipsilateral misprojections in adcy8 morphants are corrected by the expression of an nrp1a rescue construct expressed in RGCs. These findings are consistent with the idea that elevated cAMP levels promote Neuropilin1a expression in RGCs, increasing the sensitivity of retinal axons to Sema3D, Sema3E, or other neuropilin ligands at the midline, and consequently facilitate retinal axon crossing in the chiasm.

Deletion of Sema3a or plexinA1/plexinA3 causes defects in sensory afferent projections of statoacoustic ganglion neurons

Statoacoustic ganglion (SAG) neurons project sensory afferents to appropriate targets in the inner ear to form functional vestibular and auditory circuits. Neuropilin1 (Npn1), a receptor for class 3 semaphorins, is required to generate appropriate afferent projections in SAG neurons; however, the ligands and coreceptors involved in Npn1 functioning remain unknown. Here we show that both plexinA1 and plexinA3 are expressed by SAG neurons, and plexinA1/plexinA3 double mutant mice show defects in afferent projections of SAG neurons in the inner ear. In control mice, sensory afferents of SAG neurons terminate at the vestibular sensory patches, whereas in plexinA1/plexinA3 double mutants, they extend more dorsally in the inner ear beyond normal vestibular target areas. Moreover, we find that semaphorin3a (Sema3a) is expressed in the dorsal otocyst, and Sema3a mutant mice show defects in afferent projections of SAG neurons similar to those observed in plexinA1/plexinA3 double mutants and in mice lacking a functional Npn1 receptor. Taken together, these genetic findings demonstrate that Sema3a repellent signaling plays a role in the establishment of proper afferent projections in SAG neurons, and this signaling likely occurs through a receptor complex involving Npn1 and either plexinA1 or plexinA3.

VEGF ligands and receptors: implications in neurodevelopment and neurodegeneration

Intensive research in the last decade shows that the prototypic angiogenic factor vascular endothelial growth factor (VEGF) can have direct effects in neurons and modulate processes such as neuronal migration, axon outgrowth, axon guidance and neuronal survival. Depending on the neuronal cell type and the process, VEGF seems to exert these effects by signaling via different receptors. It is also becoming clear that other VEGF ligands such as VEGF-B, -C and -D can act in various neuronal cell types as well. Moreover, apart from playing a role in physiological conditions, VEGF and VEGF-B have been related to different neurological disorders. We give an update on how VEGF controls different processes during neurodevelopment as well as on its role in several neurodegenerative disorders. We also discuss recent findings demonstrating that other VEGF ligands influence processes such as neurogenesis and dendrite arborization and participate in neurodegeneration.

Neuropilin signalling in vessels, neurons and tumours

The neuropilins NRP1 and NRP2 are transmembrane proteins that regulate many different aspects of vascular and neural development. Even though they were originally identified as adhesion molecules, they are most commonly studied as co-receptors for secreted signalling molecules of the class 3 semaphorin (SEMA) and vascular endothelial growth factor (VEGF) families. During nervous system development, both classes of ligands control soma migration, axon patterning and synaptogenesis in the central nervous system, and they additionally help to guide the neural crest cell precursors of neurons and glia in the peripheral nervous system. Both classes of neuropilin ligands also control endothelial cell behaviour, with NRP1 acting as a VEGF-A isoform receptor in blood vascular endothelium and as a semaphorin receptor in lymphatic valve endothelium, and NRP2 promoting lymphatic vessel growth induced by VEGF-C. Here we provide an overview of neuropilin function in neurons and neural crest cells, discuss current knowledge of neuropilin signalling in the vasculature and conclude with a summary of neuropilin roles in cancer.

NRP1 and NRP2 cooperate to regulate gangliogenesis, axon guidance and target innervation in the sympathetic nervous system

The sympathetic nervous system (SNS) arises from neural crest (NC) cells during embryonic development and innervates the internal organs of vertebrates to modulate their stress response. NRP1 and NRP2 are receptors for guidance cues of the class 3 semaphorin (SEMA) family and are expressed in partially overlapping patterns in sympathetic NC cells and their progeny. By comparing the phenotypes of mice lacking NRP1 or its ligand SEMA3A with mice lacking NRP1 in the sympathetic versus vascular endothelial cell lineages, we demonstrate that SEMA3A signalling through NRP1 has multiple cell-autonomous roles in SNS development. These roles include neuronal cell body positioning, neuronal aggregation and axon guidance, first during sympathetic chain assembly and then to regulate the innervation of the heart and aorta. Loss of NRP2 or its ligand SEMA3F impaired sympathetic gangliogenesis more mildly than loss of SEMA3A/NRP1 signalling, but caused ectopic neurite extension along the embryonic aorta. The analysis of compound mutants lacking SEMA3A and SEMA3F or NRP1 and NRP2 in the SNS demonstrated that both signalling pathways cooperate to organise the SNS. We further show that abnormal sympathetic development in mice lacking NRP1 in the sympathetic lineage has functional consequences, as it causes sinus bradycardia, similar to mice lacking SEMA3A.

Receptor complexes for each of the Class 3 Semaphorins

The Class 3 Semaphorins (Sema3s) are a sub-family of proteins whose known biological roles are varied and growing. The mechanism of action of the Sema3s requires binding to transmembrane receptors that comprise heteromeric complexes of Neuropilins, Plexins and cell adhesion molecules (CAMs). However, knowledge of the receptor components of the Sema3s remains incomplete, and there may be receptor components which are as yet undiscovered. The receptor complexes of the Sema3s share receptor components with each other, and it is the specific combination of these components within a heteromeric complex that is thought to give rise to selective binding and signalling for individual Sema3s. This crosstalk makes it experimentally difficult to define a single holoreceptor for each Sema3. Furthermore, the receptor composition for a given Sema3 may differ between cell types, and change as a function of developmental state or pathological situation. Nevertheless, there are at least some known differences in the constitutive structure of the receptors for the Sema3s. For example in neural cells, Sema3a and Sema3f signal through different Neuropilins (Nrp1 and Nrp2 respectively) and L1cam only appears important for Sema3a signaling, while Nrcam forms a complex with Nrp2. Further complexity arises from crosstalk of other families of ligands (e.g., VEGF) with Sema3 receptor components. Thus the Sema3s, which have been shown as antagonists for each other, can also act as antagonists for other families of molecules. This review compiles experimental evidence describing the receptor components for the Sema3s, detailing the current state of knowledge of which components are important for signaling of each Sema3 before going on to consider possible future directions for the field.

Diverse roles for VEGF-A in the nervous system

Vascular endothelial growth factor A (VEGF-A) is best known for its essential roles in blood vessel growth. However, evidence has emerged that VEGF-A also promotes a wide range of neuronal functions, both in vitro and in vivo, including neurogenesis, neuronal migration, neuronal survival and axon guidance. Recent studies have employed mouse models to distinguish the direct effects of VEGF on neurons from its indirect, vessel-mediated effects. Ultimately, refining our knowledge of VEGF signalling pathways in neurons should help us to understand how the current use of therapeutics targeting the VEGF pathway in cancer and eye disease might be expanded to promote neuronal health and nerve repair.

Intrinsic properties guide proximal abducens and oculomotor nerve outgrowth in avian embryos

Proper movement of the vertebrate eye requires the formation of precisely patterned axonal connections linking cranial somatic motoneurons, located at defined positions in the ventral midbrain and hindbrain, with extraocular muscles. The aim of this research was to assess the relative contributions of intrinsic, population-specific properties and extrinsic, outgrowth site-specific cues during the early stages of abducens and oculomotor nerve development in avian embryos. This was accomplished by surgically transposing midbrain and caudal hindbrain segments, which had been pre-labeled by electroporation with an EGFP construct. Graft-derived EGFP+ oculomotor axons entering a hindbrain microenvironment often mimicked an abducens initial pathway and coursed cranially. Similarly, some EGFP+ abducens axons entering a midbrain microenvironment mimicked an oculomotor initial pathway and coursed ventrally. Many but not all of these axons subsequently projected to extraocular muscles that they would not normally innervate. Strikingly, EGFP+ axons also took initial paths atypical for their new location. Upon exiting from a hindbrain position, most EGFP+ oculomotor axons actually coursed ventrally and joined host branchiomotor nerves, whose neurons share molecular features with oculomotor neurons. Similarly, upon exiting from a midbrain position, some EGFP+ abducens axons turned caudally, elongated parallel to the brainstem, and contacted the lateral rectus muscle, their originally correct target. These data reveal an interplay between intrinsic properties that are unique to oculomotor and abducens populations and shared ability to recognize and respond to extrinsic directional cues. The former play a prominent role in initial pathway choices, whereas the latter appear more instructive during subsequent directional choices.

PlexinA4 distribution in the adult rat spinal cord and dorsal root ganglia

PlexinsA1-A4 participate in class 3 semaphorin signaling as co-receptors to neuropilin 1 and 2, PlexinA4 being the latest member of the PlexinA subfamily to be identified. Little is known about the cellular distribution of PlexinA4 in the spinal cord and dorsal root ganglion (DRG). Here, immunohistochemical studies using antibodies to PlexinA4 revealed immunolabeling in neurons in both dorsal and, to a greater extent, ventral horns of the spinal cord. Ventral horn PlexinA4 positive neurons exhibited morphology, size, and location consistent with both motor neurons and interneurons. Labeling was found in motor axons exiting through the ventral roots, and more widespread labeling was observed in ascending and descending white matter tracts. Within the DRG, immunostaining was observed in neuronal cell bodies as well as the central and peripheral processes of these cells. PlexinA4 is expressed in the peripheral nervous system where its expression is regulated upon nerve injury. This is the first detailed description of the cellular and subcellular distribution of PlexinA4 in the adult spinal cord and DRG, and it will set the basis for future studies on the potential role of PlexinA4 in regeneration and repair of the adult central and peripheral nervous system.

Expression patterns of Sema3F, PlexinA4, -A3, Neuropilin1 and -2 in the postnatal mouse molar suggest roles in tooth innervation and organogenesis

OBJECTIVE

Semaphorins form a family of axon wiring molecules but still little is known about their role in tooth formation. A class 3 semaphorin, Semaphorin3F (Sema3F), besides acting as a chemorepellant for different types of axons, controls a variety of non-neuronal developmental processes.

MATERIALS AND METHODS

Cellular mRNA expression patterns of Sema3F as well as neuropilin 1 (Npn1), neuropilin 2 (Npn2), plexinA3 and plexinA4 receptors were analyzed by sectional in situ hybridization in the mouse molar tooth during postnatal days 0-7. The expression of the receptors was studied in PN5 trigeminal ganglia.

RESULTS

Sema3F, Npn1, -2 and plexinA4 exhibited distinct, spatiotemporally changing expression patterns, whereas plexinA3 was not observed in the tooth germs. Besides being expressed in the base of the dental mesenchyme Sema3F, like plexinA4, Npn1 and -2, was present in the ameloblast cell lineage. Npn1 and Npn2 were additionally seen in the pulp horns and endothelial cells and like PlexinA4 in the developing alveolar bone. Npn1, plexinA3 and -A4 were observed in trigeminal ganglion neurons.

CONCLUSIONS

Sema3F may act as a tooth target-derived axonal chemorepellant controlling establishment of the tooth nerve supply. Furthermore, Sema3F, like Npn1, -2 and plexinA4 may serve non-neuronal functions by controlling the development of the ameloblast cell lineage. Moreover, Npn1 and Npn2 may regulate dental vasculogenesis and, together with PlexinA4, alveolar bone formation. Further analyses such as investigation of transgenic mouse models will be required to elucidate in vivo signaling functions of Sema3F and the receptors in odontogenesis.

Cranial nerve fasciculation and Schwann cell migration are impaired after loss of Npn-1

Interaction of the axon guidance receptor Neuropilin-1 (Npn-1) with its repulsive ligand Semaphorin 3A (Sema3A) is crucial for guidance decisions, fasciculation, timing of growth and axon-axon interactions of sensory and motor projections in the embryonic limb. At cranial levels, Npn-1 is expressed in motor neurons and sensory ganglia and loss of Sema3A-Npn-1 signaling leads to defasciculation of the superficial projections to the head and neck. The molecular mechanisms that govern the initial fasciculation and growth of the purely motor projections of the hypoglossal and abducens nerves in general, and the role of Npn-1 during these events in particular are, however, not well understood. We show here that selective removal of Npn-1 from somatic motor neurons impairs initial fasciculation and assembly of hypoglossal rootlets and leads to reduced numbers of abducens and hypoglossal fibers. Ablation of Npn-1 specifically from cranial neural crest and placodally derived sensory tissues recapitulates the distal defasciculation of mixed sensory-motor nerves of trigeminal, facial, glossopharyngeal and vagal projections, which was observed in Npn-1(-/-) and Npn-1(Sema-) mutants. Surprisingly, the assembly and fasciculation of the purely motor hypoglossal nerve are also impaired and the number of Schwann cells migrating along the defasciculated axonal projections is reduced. These findings are corroborated by partial genetic elimination of cranial neural crest and embryonic placodes, where loss of Schwann cell precursors leads to aberrant growth patterns of the hypoglossal nerve. Interestingly, rostral turning of hypoglossal axons is not perturbed in any of the investigated genotypes. Thus, initial hypoglossal nerve assembly and fasciculation, but not later guidance decisions depend on Npn-1 expression and axon-Schwann cell interactions.

VEGF mediates commissural axon chemoattraction through its receptor Flk1

Growing axons are guided to their targets by attractive and repulsive cues. In the developing spinal cord, Netrin-1 and Shh guide commissural axons toward the midline. However, the combined inhibition of their activity in commissural axon turning assays does not completely abrogate turning toward floor plate tissue, suggesting that additional guidance cues are present. Here we show that the prototypic angiogenic factor VEGF is secreted by the floor plate and is a chemoattractant for commissural axons in vitro and in vivo. Inactivation of Vegf in the floor plate or of its receptor Flk1 in commissural neurons causes axon guidance defects, whereas Flk1 blockade inhibits turning of axons to VEGF in vitro. Similar to Shh and Netrin-1, VEGF-mediated commissural axon guidance requires the activity of Src family kinases. Our results identify VEGF and Flk1 as a novel ligand/receptor pair controlling commissural axon guidance.

Plexin-A4 promotes tumor progression and tumor angiogenesis by enhancement of VEGF and bFGF signaling

Plexin-A4 is a receptor for sema6A and sema6B and associates with neuropilins to transduce signals of class-3 semaphorins. We observed that plexin-A1 and plexin-A4 are required simultaneously for transduction of inhibitory sema3A signals and that they form complexes. Unexpectedly, inhibition of plexin-A1 or plexin-A4 expression in endothelial cells using specific shRNAs resulted in prominent plexin type specific rearrangements of the actin cytoskeleton that were accompanied by inhibition of bFGF and VEGF-induced cell proliferation. The two responses were not interdependent since silencing plexin-A4 in U87MG glioblastoma cells inhibited cell proliferation and strongly inhibited the formation of tumors from these cells without affecting cytoskeletal organization. Plexin-A4 formed stable complexes with the FGFR1 and VEGFR-2 tyrosine-kinase receptors and enhanced VEGF-induced VEGFR-2 phosphorylation in endothelial cells as well as bFGF-induced cell proliferation. We also obtained evidence suggesting that some of the pro-proliferative effects of plexin-A4 are due to transduction of autocrine sema6B-induced pro-proliferative signals, since silencing sema6B expression in endothelial cells and in U87MG cells mimicked the effects of plexin-A4 silencing and also inhibited tumor formation from the U87MG cells. Our results suggest that plexin-A4 may represent a target for the development of novel anti-angiogenic and anti-tumorigenic drugs.

Matrix-binding vascular endothelial growth factor (VEGF) isoforms guide granule cell migration in the cerebellum via VEGF receptor Flk1

Vascular endothelial growth factor (VEGF) regulates angiogenesis, but also has important, yet poorly characterized roles in neuronal wiring. Using several genetic and in vitro approaches, we discovered a novel role for VEGF in the control of cerebellar granule cell (GC) migration from the external granule cell layer (EGL) toward the Purkinje cell layer (PCL). GCs express the VEGF receptor Flk1, and are chemoattracted by VEGF, whose levels are higher in the PCL than EGL. Lowering VEGF levels in mice in vivo or ectopic VEGF expression in the EGL ex vivo perturbs GC migration. Using GC-specific Flk1 knock-out mice, we provide for the first time in vivo evidence for a direct chemoattractive effect of VEGF on neurons via Flk1 signaling. Finally, using knock-in mice expressing single VEGF isoforms, we show that pericellular deposition of matrix-bound VEGF isoforms around PC dendrites is necessary for proper GC migration in vivo. These findings identify a previously unknown role for VEGF in neuronal migration.

Defective gonadotropin-releasing hormone neuron migration in mice lacking SEMA3A signalling through NRP1 and NRP2: implications for the aetiology of hypogonadotropic hypogonadism

Kallmann syndrome (KS) is a genetic disease characterized by hypogonadotropic hypogonadism and impaired sense of smell. The genetic causes underlying this syndrome are still largely unknown, but are thought to be due to a developmental defect in the migration of gonadotropin-releasing hormone (GnRH) neurons. Understanding the causes of the disease is hampered by lack of appropriate mouse models. GnRH neurons are hypothalamic cells that centrally control reproduction in mammals by secreting the GnRH decapeptide into the portal blood vessels of the pituitary to stimulate the production of gonadotropins. During development, these cells are born in the nasal placode outside the brain and migrate in association with olfactory/vomeronasal axons to reach the forebrain and position themselves in the hypothalamus. By combining the analysis of genetically altered mice with in vitro models, we demonstrate here that a secreted guidance cue of the class 3 semaphorin family, SEMA3A, is essential for the development of the GnRH neuron system: loss of SEMA3A signalling alters the targeting of vomeronasal nerves and the migration of GnRH neurons into the brain, resulting in reduced gonadal size. We found that SEMA3A signals redundantly through both its classical receptors neuropilin (NRP) 1 and, unconventionally, NRP2, while the usual NRP2 ligand SEMA3F is dispensable for this process. Strikingly, mice lacking SEMA3A or semaphorin signalling through both NRP1 and NRP2 recapitulate the anatomical features of a single case of KS analysed so far, and may therefore be used as genetic models to elucidate the pathogenesis of KS.

Placodal sensory ganglia coordinate the formation of the cranial visceral motor pathway

The parasympathetic reflex circuit is controlled by three basic neurons. In the vertebrate head, the sensory, and pre- and postganglionic neurons that comprise each circuit have stereotypic positions along the anteroposterior (AP) axis, suggesting that the circuit arises from a common developmental plan. Here, we show that precursors of the VIIth circuit are initially aligned along the AP axis, where the placode-derived sensory neurons provide a critical "guidepost" through which preganglionic axons and their neural crest-derived postganglionic targets navigate before reaching their distant target sites. In the absence of the placodal sensory ganglion, preganglionic axons terminate and the neural crest fated for postganglionic neurons undergo apoptosis at the site normally occupied by the placodal sensory ganglion. The stereotypic organization of the parasympathetic cranial sensory-motor circuit thus emerges from the initial alignment of its precursors along the AP axis, with the placodal sensory ganglion coordinating the formation of the motor pathway.

Immunohistochemical Distribution of PlexinA4 in the Adult Rat Central Nervous System

PlexinA4 is the latest member to be identified of the PlexinA subfamily, critical transducers of class 3 semaphorin signaling as co-receptors to neuropilins 1 and 2. Despite functional information regarding the role of PlexinA4 in development and guidance of specific neuronal pathways, little is known about its distribution in the adult central nervous system (CNS). Here we report an in depth immunohistochemical analysis of PlexinA4 expression in the adult rat CNS. PlexinA4 staining was present in neurons and fibers throughout the brain and spinal cord, including neocortex, hippocampus, lateral hypothalamus, red nucleus, facial nucleus, and the mesencephalic trigeminal nucleus. PlexinA4 antibodies labeled fibers in the lateral septum, nucleus accumbens, several thalamic nuclei, substantia nigra pars reticulata, zona incerta, pontine reticular region, as well as in several cranial nerve nuclei. This constitutes the first detailed description of the topographic distribution of PlexinA4 in the adult CNS and will set the basis for future studies on the functional implications of PlexinA4 in adult brain physiology.

Neuropilin, you gotta let me know: should I stay or should I go?

Neuropilins are highly conserved single pass transmembrane proteins specific to vertebrates. They were originally identified as adhesion molecules in the nervous system, but were subsequently rediscovered as the ligand binding subunit of the class 3 semaphorin receptor in neurons and then as blood vessel receptors for the vascular endothelial growth factor VEGF. More recently they have also been implicated as mediators of the T-cell immune response and as key prognostic markers in several types of cancer. Because neuropilins bind multiple ligands and associate with several different types of co-receptors, they variably promote cell adhesion, repulsion or attraction. Which response they ultimately invoke is decided by the cellular and even subcellular context the neuropilins find themselves in. Here, we review how the developmental functions of the neuropilins are influenced by such different contexts.

Neurotrophin NT3 promotes ovarian primordial to primary follicle transition

Neurotrophins are growth factors that are known to have a role in promoting cell survival and differentiation. The focus of the current study is to examine the role of neurotrophins in regulating ovarian primordial follicle development. Ovaries from 4-day old rats were placed into organ culture and cultured for 10 days in the absence or presence of neurotrophin-3 (NT3), brain-derived neurotrophic factor (BDNF), or nerve growth factor (NGF). Treatment of ovaries with NT3 resulted in a significant (P<0.01) increase in primordial follicle development (i.e. primordial to primary follicle transition). Treatment with BDNF at high doses of 100-250 ng/ml also significantly (P<0.01) increased primordial follicle development, but NGF had no effect. Immunohistochemical studies determined that NT3 was present in granulosa cells, interstitial tissue, and in the oocytes of primordial and primary follicles. The NT3 receptor NTRK3 was present in oocytes at all stages of development. Analysis of ovaries that contain predominantly primordial follicles demonstrated the transcripts for NT3, NTRK3, NGF, and the BDNF/neurotrophin-4 (NT4) receptor NTRK2 are expressed, while BDNF, NT4, and the NGF receptor NTRK1 are not detectable. Inhibition of the NTRK3 receptor with the tyrphostin AG 879 resulted in oocyte death and a significant (P<0.01) reduction in follicle pool size. Inhibition of the NTRK receptors with K252a slowed primordial to primary follicle transition. A microarray analysis demonstrated that a small number of genes were differentially expressed after NT3 treatment. Observations indicate that the neurotrophin NT3, acting through the NTRK3 receptor in oocytes, promotes the primordial to primary follicle transition.

Role and therapeutic potential of VEGF in the nervous system

The development of the nervous and vascular systems constitutes primary events in the evolution of the animal kingdom; the former provides electrical stimuli and coordination, while the latter supplies oxygen and nutrients. Both systems have more in common than originally anticipated. Perhaps the most striking observation is that angiogenic factors, when deregulated, contribute to various neurological disorders, such as neurodegeneration, and might be useful for the treatment of some of these pathologies. The prototypic example of this cross-talk between nerves and vessels is the vascular endothelial growth factor or VEGF. Although originally described as a key angiogenic factor, it is now well established that VEGF also plays a crucial role in the nervous system. We describe the molecular properties of VEGF and its receptors and review the current knowledge of its different functions and therapeutic potential in the nervous system during development, health, disease and in medicine.

Neuropilin-mediated neural crest cell guidance is essential to organise sensory neurons into segmented dorsal root ganglia

The peripheral nervous system (PNS) of higher vertebrates is segmented to align the spinal nerve roots with the vertebrae. This co-patterning is set up during embryogenesis, when vertebrae develop from the sclerotome layer of the metameric somites, and PNS neurons and glia differentiate from neural crest cells (NCCs) that preferentially migrate into the anterior sclerotome halves. Previous analyses of mice deficient in the class 3 semaphorin (SEMA3) receptors neuropilin (NRP) 1 or 2 raised the possibility that each controlled a distinct aspect of trunk NCC migration. We now demonstrate that both pathways act sequentially in distinct NCC subpopulations and thereby cooperate to enforce segmental NCC migration. Specifically, SEMA3A/NRP1 signalling first directs one population of NCCs from the intersomitic path into the sclerotome, and SEMA3F/NRP2 signalling acts subsequently to restrict a second population to the anterior half of the sclerotome. NCC exclusion from either the posterior sclerotome or the intersomitic boundary is sufficient to enforce the separation of neighbouring NCC streams and the segregation of sensory NCC progeny into metameric dorsal root ganglia (DRG). By contrast, the combined loss of both guidance pathways leads to ectopic invasion of the intersomitic furrows and posterior sclerotome halves, disrupting metameric NCC streaming and DRG segmentation.