Neuropilins as semaphorin receptors

The Sympathetic-Immune Milieu in Metabolic Health and Diseases: Insights from Pancreas, Liver, Intestine, and Adipose Tissues

Sympathetic innervation plays a crucial role in maintaining energy balance and contributes to metabolic pathophysiology. Recent evidence has begun to uncover the innervation landscape of sympathetic projections and sheds light on their important functions in metabolic activities. Additionally, the immune system has long been studied for its essential roles in metabolic health and diseases. In this review, the aim is to provide an overview of the current research progress on the sympathetic regulation of key metabolic organs, including the pancreas, liver, intestine, and adipose tissues. In particular, efforts are made to highlight the critical roles of the peripheral nervous system and its potential interplay with immune components. Overall, it is hoped to underscore the importance of studying metabolic organs from a comprehensive and interconnected perspective, which will provide valuable insights into the complex mechanisms underlying metabolic regulation and may lead to novel therapeutic strategies for metabolic diseases.

Autocrine S100B in astrocytes promotes VEGF-dependent inflammation and oxidative stress and causes impaired neuroprotection

Minimal hepatic encephalopathy (MHE) is strongly associated with neuroinflammation. Nevertheless, the underlying mechanism of the induction of inflammatory response in MHE astrocytes remains not fully understood. In the present study, we investigated the effect and mechanism of S100B, a predominant isoform expressed and released from mature astrocytes, on MHE-like neuropathology in the MHE rat model. We discovered that S100B expressions and autocrine were significantly increased in MHE rat brains and MHE rat brain-derived astrocytes. Furthermore, S100B stimulates VEGF expression via the interaction between TLR2 and RAGE in an autocrine manner. S100B-facilitated VEGF autocrine expression further led to a VEGFR2 and COX-2 interaction, which in turn induced the activation of NFƙB, eventually resulting in inflammation and oxidative stress in MHE astrocytes. MHE astrocytes supported impairment of neuronal survival and growth in a co-culture system. To sum up, a comprehensive understanding of the role of S100B-overexpressed MHE astrocyte in MHE pathogenesis may provide insights into the etiology of MHE.

HNF4G accelerates glioma progression by facilitating NRP1 transcription

Hepatocyte nuclear factor 4γ (HNF4G) is considered to be a transcription factor and functions as an oncogene in certain types of human cancer. However, the precise functions and the potential molecular mechanisms of HNF4G in glioma remain unclear. Therefore, the present study aimed to elucidate the role of HNF4G in glioma and the underlying mechanism. Western blotting and reverse transcription-quantitative PCR (RT-qPCR) demonstrated that HNF4G was highly expressed in glioma tissues and cell lines. The overexpression of HNF4G in LN229 and U251 glioma cells promoted cell proliferation and cell cycle progression, and inhibited apoptosis, while the knockdown of HNF4G suppressed cell proliferation, cell cycle progression and tumor growth, and induced apoptosis. A significant positive association was detected between HNF4G and neuropilin-1 (NRP1) mRNA expression in glioma tissues. Bioinformatics analysis, chromatin immunoprecipitation-RT-qPCR and promoter reporter assays confirmed that HNF4G promoted NRP1 transcription in glioma by binding to its promoter. NRP1 overexpression facilitated glioma cell proliferation and cell cycle progression, and suppressed apoptosis in vitro, while the knockdown of NRP1 inhibited cell proliferation and cell cycle progression, and facilitated apoptosis. NRP1 overexpression reversed the effects induced by HNF4G knockdown on glioma cell proliferation, cell cycle progression and apoptosis. In summary, the present study demonstrated that HNF4G promotes glioma cell proliferation and suppresses apoptosis by activating NRP1 transcription. These findings indicate that HNF4G acts as an oncogene in glioma and may thus be an effective therapeutic target for glioma.

Wiring the ocular surface: A focus on the comparative anatomy and molecular regulation of sensory innervation of the cornea

The cornea is richly innervated with sensory nerves that function to detect and clear harmful debris from the surface of the eye, promote growth and survival of the corneal epithelium and hasten wound healing following ocular disease or trauma. Given their importance to eye health, the neuroanatomy of the cornea has for many years been a source of intense investigation. Resultantly, complete nerve architecture maps exist for adult human and many animal models and these maps reveal few major differences across species. Interestingly, recent work has revealed considerable variation across species in how sensory nerves are acquired during developmental innervation of the cornea. Highlighting such species-distinct key differences, but also similarities, this review provides a full, comparative anatomy analysis of sensory innervation of the cornea for all species studied to date. Further, this article comprehensively describes the molecules that have been shown to guide and direct nerves toward, into and through developing corneal tissue as the final architectural pattern of the cornea's neuroanatomy is established. Such knowledge is useful for researchers and clinicians seeking to better understand the anatomical and molecular basis of corneal nerve pathologies and to hasten neuro-regeneration following infection, trauma or surgery that damage the ocular surface and its corneal nerves.

Neuroimmune regulation of white adipose tissues

The white adipose tissues (WAT) are located in distinct depots throughout the body. They serve as an energy reserve, providing fatty acids for other tissues via lipolysis when needed, and function as an endocrine organ to regulate systemic metabolism. Their activities are coordinated through intercellular communications among adipocytes and other cell types such as residential and infiltrating immune cells, which are collectively under neuronal control. The adipocytes and immune subtypes including macrophages/monocytes, eosinophils, neutrophils, group 2 innate lymphoid cells (ILC2s), T and B cells, dendritic cells (DCs), and natural killer (NK) cells display cellular and functional diversity in response to the energy states and contribute to metabolic homeostasis and pathological conditions. Accumulating evidence reveals that neuronal innervations control lipid deposition and mobilization via regulating lipolysis, adipocyte size, and cellularity. Vice versa, the neuronal innervations and activity are influenced by cellular factors in the WAT. Though the literature describing adipose tissue cells is too extensive to cover in detail, we strive to highlight a selected list of neuronal and immune components in this review. The cell-to-cell communications and the perspective of neuroimmune regulation are emphasized to enlighten the potential therapeutic opportunities for treating metabolic disorders.

SARS-CoV-2 Infection in the Central and Peripheral Nervous System-Associated Morbidities and Their Potential Mechanism

The recent outbreak of SARS-CoV-2 infections that causes coronavirus-induced disease of 2019 (COVID-19) is the defining and unprecedented global health crisis of our time in both the scale and magnitude. Although the respiratory tract is the primary target of SARS-CoV-2, accumulating evidence suggests that the virus may also invade both the central nervous system (CNS) and the peripheral nervous system (PNS) leading to numerous neurological issues including some serious complications such as seizures, encephalitis, and loss of consciousness. Here, we present a comprehensive review of the currently known role of SARS-CoV-2 and identify all the neurological problems reported among the COVID-19 case reports throughout the world. The virus might gain entry into the CNS either through the trans-synaptic route via the olfactory neurons or through the damaged endothelium in the brain microvasculature using the ACE2 receptor potentiated by neuropilin-1 (NRP-1). The most critical of all symptoms appear to be the spontaneous loss of breathing in some COVID-19 patients. This might be indicative of a dysfunction within the cardiopulmonary regulatory centers in the brainstem. These pioneering studies, thus, lay a strong foundation for more in-depth basic and clinical research required to confirm the role of SARS-CoV-2 infection in neurodegeneration of critical brain regulatory centers.

Plasmacytoid dendritic cells in the eye

Plasmacytoid dendritic cells (pDCs) are a unique subpopulation of immune cells, distinct from classical dendritic cells. pDCs are generated in the bone marrow and following development, they typically home to secondary lymphoid tissues. While peripheral tissues are generally devoid of pDCs during steady state, few tissues, including the lung, kidney, vagina, and in particular ocular tissues harbor resident pDCs. pDCs were originally appreciated for their potential to produce large quantities of type I interferons in viral immunity. Subsequent studies have now unraveled their pivotal role in mediating immune responses, in particular in the induction of tolerance. In this review, we summarize our current knowledge on pDCs in ocular tissues in both mice and humans, in particular in the cornea, limbus, conjunctiva, choroid, retina, and lacrimal gland. Further, we will review our current understanding on the significance of pDCs in ameliorating inflammatory responses during herpes simplex virus keratitis, sterile inflammation, and corneal transplantation. Moreover, we describe their novel and pivotal neuroprotective role, their key function in preserving corneal angiogenic privilege, as well as their potential application as a cell-based therapy for ocular diseases.

Macrophage-Derived Slit3 Controls Cell Migration and Axon Pathfinding in the Peripheral Nerve Bridge

Slit-Robo signaling has been characterized as a repulsive signal for precise axon pathfinding and cell migration during embryonic development. Here, we describe a role for Sox2 in the regulation of Robo1 in Schwann cells and for Slit3-Robo1 signaling in controlling axon guidance within the newly formed nerve bridge following peripheral nerve transection injury. In particular, we show that macrophages form the outermost layer of the nerve bridge and secrete high levels of Slit3, while migratory Schwann cells and fibroblasts inside the nerve bridge express the Robo1 receptor. In line with this pattern of Slit3 and Robo1 expression, we observed multiple axon regeneration and cell migration defects in the nerve bridge of Sox2-, Slit3-, and Robo1-mutant mice. Our findings have revealed important functions for macrophages in the peripheral nervous system, utilizing Slit3-Robo1 signaling to control correct peripheral nerve bridge formation and precise axon targeting to the distal nerve stump following injury.

The Neuropilin-1 Ligand, Sema3A, Acts as a Tumor Suppressor in the Pathogenesis of Acute Leukemia

Semaphorin-3A (Sema3A) and vascular endothelial growth factor (VEGF165) are ligands of neuropilin-1 (NRP-1 or CD304) and are related to immunoregulation and tumor angiogenesis, respectively. However, possible interactions between NRP-1 and Sema3A and VEGF165 in acute leukemia remain unclear, especially whether Sema3A plays a role in acute leukemia. In this study, both of the proportion of regulatory T cells (Tregs) and their expression of NRP-1 were found to increase in acute leukemia patients compared with healthy controls. In contrast, lower mRNA and plasma levels of Sema3A were detected in the acute leukemia patients. In vitro, the addition of exogenous Sema3A inhibited the expression of NRP-1 on Tregs and it promoted apoptosis of leukemia cells. However, in the presence of anti-Sema3A antibody, the effect of rhSema3A on NRP-1 expression was reversed. These results suggest that Sema3A promotes apoptosis in leukemia cells by inhibiting expression of NRP-1, and thus, represents a tumor suppressor protein with a role in the pathogenesis of acute leukemia. Consequently, NRP-1/Sema3A signaling may represent a novel target for the treatment of acute leukemia and should be further studied. Anat Rec, 302:1127-1135, 2019. © 2018 Wiley Periodicals, Inc.

The role of placental growth factor (PlGF) and its receptor system in retinal vascular diseases

Placental growth factor (PlGF) is a member of the vascular endothelial growth factor (VEGF) family. Upon binding to VEGF- and neuropilin-receptor sub-types, PlGF modulates a range of neural, glial and vascular cell responses that are distinct from VEGF-A. As PlGF expression is selectively associated with pathological angiogenesis and inflammation, its blockade does not affect the healthy vasculature. PlGF actions have been extensively described in tumor biology but more recently there has been accumulating preclinical evidence that indicates that this growth factor could have an important role in retinal diseases. High levels of PlGF have been found in aqueous humor, vitreous and/or retina of patients exhibiting retinopathies, especially those with diabetic retinopathy (DR) and neovascular age-related macular degeneration (nvAMD). Expression of this growth factor seems to correlate closely with many of the key pathogenic features of early and late retinopathy in preclinical models. For example, studies using genetic modification and/or pharmacological treatment to block PlGF in the laser-induced choroidal neovascularization (CNV) model, oxygen-induced retinopathy model, as well as various murine diabetic models, have shown that PlGF deletion or inhibition can reduce neovascularization, retinal leakage, inflammation and gliosis, without affecting vascular development or inducing neuronal degeneration. Moreover, an inhibitory effect of PlGF blockade on retinal scarring in the mouse CNV model has also been recently demonstrated and was found to be unique for PlGF inhibition, as compared to various VEGF inhibition strategies. Together, these preclinical results suggest that anti-PlGF therapy might have advantages over anti-VEGF treatment, and that it may have clinical applications as a standalone treatment or in combination with anti-VEGF. Additional clinical studies are clearly needed to further elucidate the role of PlGF and its potential as a therapeutic target in ocular diseases.

Development of the renal vasculature

The kidney vasculature has a unique and complex architecture that is central for the kidney to exert its multiple and essential physiological functions with the ultimate goal of maintaining homeostasis. An appropriate development and coordinated assembly of the different vascular cell types and their association with the corresponding nephrons is crucial for the generation of a functioning kidney. In this review we provide an overview of the renal vascular anatomy, histology, and current knowledge of the embryological origin and molecular pathways involved in its development. Understanding the cellular and molecular mechanisms involved in renal vascular development is the first step to advance the field of regenerative medicine.

Semaphorin 3A receptor inhibitor as a novel therapeutic to promote innervation of bioengineered teeth

The sensory innervation of the dental pulp is essential for tooth function and protection. It is mediated by axons originating from the trigeminal ganglia and is spatio-temporally regulated. We have previously shown that the innervation of bioengineered teeth can be achieved only under immunosuppressive conditions. The aim of this study was to develop a model to determine the role of Semaphorin 3A (Sema3A) in the innervation of bioengineered teeth. We first analysed innervation of the dental pulp of mandibular first molars in newborn (postnatal day 0: PN0) mice deficient for Sema3A (Sema3A^-/- ), a strong inhibitor of axon growth. While at PN0, axons detected by immunostaining for peripherin and NF200 were restricted to the peridental mesenchyme in Sema3A^+/+ mice, they entered the dental pulp in Sema3A^-/- mice. Then, we have implanted cultured teeth obtained from embryonic day-14 (E14) molar germs of Sema3A^-/- mice together with trigeminal ganglia. The dental pulps of E14 cultured and implanted Sema3A^-/- teeth were innervated, whereas the axons did not enter the pulp of E14 Sema3A^+/+ cultured and implanted teeth. A "Membrane Targeting Peptide NRP1," suppressing the inhibitory effect of Sema3A, has been previously identified. The injection of this peptide at the site of implantation allowed the innervation of the dental pulp of bioengineered teeth obtained from E14 dental dissociated mesenchymal and epithelial cells reassociations of ICR mice. In conclusion, these data show that inhibition of only one axon repellent molecule, Sema3A, allows for pulp innervation of bioengineered teeth.

Common cues wire the spinal cord: Axon guidance molecules in spinal neuron migration

Topographic arrangement of neuronal cell bodies and axonal tracts are crucial for proper wiring of the nervous system. This involves often-coordinated neuronal migration and axon guidance during development. Most neurons migrate from their birthplace to specific topographic coordinates as they adopt the final cell fates and extend axons. The axons follow temporospatial specific guidance cues to reach the appropriate targets. When neuronal or axonal migration or their coordination is disrupted, severe consequences including neurodevelopmental disorders and neurological diseases, can arise. Neuronal and axonal migration shares some molecular mechanisms, as genes originally identified as axon guidance molecules have been increasingly shown to direct both navigation processes. This review focuses on axon guidance pathways that are shown to also direct neuronal migration in the vertebrate spinal cord.

Inhibition of primary breast tumor growth and metastasis using a neuropilin-1 transmembrane domain interfering peptide

The transmembrane domains (TMD) in membrane receptors play a key role in cell signaling. As previously shown by us a peptide targeting the TMD of neuropilin-1 (MTP-NRP1), blocks cell proliferation, cell migration and angiogenesis in vitro, and decreases glioblastoma growth in vivo. We now explored the clinical potential of MTP-NRP1 on breast cancer models and demonstrate that MTP-NRP1 blocks proliferation of several breast cancer lines including the MDA-MB-231, a triple negative human breast cancer cell line. In models with long term in vivo administration of the peptide, MTP-NRP1 not only reduced tumor volume but also decreased number and size of breast cancer metastases. Strikingly, treating mice before tumors developed protected from metastasis establishment/formation. Overall, our results report that targeting the TMD of NRP1 in breast cancer is a potent new strategy to fight against breast cancer and related metastasis.

New functions of Semaphorin 3E and its receptor PlexinD1 during developing and adult hippocampal formation

The development and maturation of cortical circuits relies on the coordinated actions of long and short range axonal guidance cues. In this regard, the class 3 semaphorins and their receptors have been seen to be involved in the development and maturation of the hippocampal connections. However, although the role of most of their family members have been described, very few data about the participation of Semaphorin 3E (Sema3E) and its receptor PlexinD1 during the development and maturation of the entorhino-hippocampal (EH) connection are available. In the present study, we focused on determining their roles both during development and in adulthood. We determined a relevant role for Sema3E/PlexinD1 in the layer-specific development of the EH connection. Indeed, mice lacking Sema3E/PlexinD1 signalling showed aberrant layering of entorhinal axons in the hippocampus during embryonic and perinatal stages. In addition, absence of Sema3E/PlexinD1 signalling results in further changes in postnatal and adult hippocampal formation, such as numerous misrouted ectopic mossy fibers. More relevantly, we describe how subgranular cells express PlexinD1 and how the absence of Sema3E induces a dysregulation of the proliferation of dentate gyrus progenitors leading to the presence of ectopic cells in the molecular layer. Lastly, Sema3E mutant mice displayed increased network excitability both in the dentate gyrus and the hippocampus proper.

A mutation within the SH2 domain of slp-76 regulates the tissue distribution and cytokine production of iNKT cells in mice

TCR ligation is critical for the selection, activation, and integrin expression of T lymphocytes. Here, we explored the role of the TCR adaptor protein slp-76 on iNKT-cell biology. Compared to B6 controls, slp-76(ace/ace) mice carrying a missense mutation (Thr428Ile) within the SH2-domain of slp-76 showed an increase in iNKT cells in the thymus and lymph nodes, but a decrease in iNKT cells in spleens and livers, along with reduced ADAP expression and cytokine response. A comparable reduction in iNKT cells was observed in the livers and spleens of ADAP-deficient mice. Like ADAP(-/-) iNKT cells, slp-76(ace/ace) iNKT cells were characterized by enhanced CD11b expression, correlating with an impaired induction of the TCR immediate-early gene Nur77 and a decreased adhesion to ICAM-1. Furthermore, CD11b-intrinsic effects inhibited cytokine release, concanavalin A-mediated inflammation, and iNKT-cell accumulation in the liver. Unlike B6 and ADAP(-/-) mice, the expression of the transcription factors Id3 and PLZF was reduced, whereas NP-1-expression was enhanced in slp-76(ace/ace) mice. Blockade of NP-1 decreased the recovery of iNKT cells from peripheral lymph nodes, identifying NP-1 as an iNKT-cell-specific adhesion factor. Thus, slp-76 contributes to the regulation of the tissue distribution, PLZF, and cytokine expression of iNKT cells via ADAP-dependent and -independent mechanisms.

Genome-wide analysis reveals NRP1 as a direct HIF1α-E2F7 target in the regulation of motorneuron guidance in vivo

In this study, we explored the existence of a transcriptional network co-regulated by E2F7 and HIF1α, as we show that expression of E2F7, like HIF1α, is induced in hypoxia, and because of the previously reported ability of E2F7 to interact with HIF1α. Our genome-wide analysis uncovers a transcriptional network that is directly controlled by HIF1α and E2F7, and demonstrates both stimulatory and repressive functions of the HIF1α -E2F7 complex. Among this network we reveal Neuropilin 1 (NRP1) as a HIF1α-E2F7 repressed gene. By performing in vitro and in vivo reporter assays we demonstrate that the HIF1α-E2F7 mediated NRP1 repression depends on a 41 base pairs ‘E2F-binding site hub’, providing a molecular mechanism for a previously unanticipated role for HIF1α in transcriptional repression. To explore the biological significance of this regulation we performed in situ hybridizations and observed enhanced nrp1a expression in spinal motorneurons (MN) of zebrafish embryos, upon morpholino-inhibition of e2f7/8 or hif1α. Consistent with the chemo-repellent role of nrp1a, morpholino-inhibition of e2f7/8 or hif1α caused MN truncations, which was rescued in TALEN-induced nrp1a^hu10012 mutants, and phenocopied in e2f7/8 mutant zebrafish. Therefore, we conclude that repression of NRP1 by the HIF1α-E2F7 complex regulates MN axon guidance in vivo.

Tangential migration of glutamatergic neurons and cortical patterning during development: Lessons from Cajal-Retzius cells

Tangential migration is a mode of cell movement, which in the developing cerebral cortex, is defined by displacement parallel to the ventricular surface and orthogonal to the radial glial fibers. This mode of long-range migration is a strategy by which distinct neuronal classes generated from spatially and molecularly distinct origins can integrate to form appropriate neural circuits within the cortical plate. While it was previously believed that only GABAergic cortical interneurons migrate tangentially from their origins in the subpallial ganglionic eminences to integrate in the cortical plate, it is now known that transient populations of glutamatergic neurons also adopt this mode of migration. These include Cajal-Retzius cells (CRs), subplate neurons (SPs), and cortical plate transient neurons (CPTs), which have crucial roles in orchestrating the radial and tangential development of the embryonic cerebral cortex in a noncell-autonomous manner. While CRs have been extensively studied, it is only in the last decade that the molecular mechanisms governing their tangential migration have begun to be elucidated. To date, the mechanisms of SPs and CPTs tangential migration remain unknown. We therefore review the known signaling pathways, which regulate parameters of CRs migration including their motility, contact-redistribution and adhesion to the pial surface, and discuss this in the context of how CR migration may regulate their signaling activity in a spatial and temporal manner. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 847-881, 2016.

Functional synergy between cholecystokinin receptors CCKAR and CCKBR in mammalian brain development

Cholecystokinin (CCK), a peptide hormone and one of the most abundant neuropeptides in vertebrate brain, mediates its actions via two G-protein coupled receptors, CCKAR and CCKBR, respectively active in peripheral organs and the central nervous system. Here, we demonstrate that the CCK receptors have a dynamic and largely reciprocal expression in embryonic and postnatal brain. Using compound homozygous mutant mice lacking the activity of both CCK receptors, we uncover their additive, functionally synergistic effects in brain development and demonstrate that CCK receptor loss leads to abnormalities of cortical development, including defects in the formation of the midline and corpus callosum, and cortical interneuron migration. Using comparative transcriptome analysis of embryonic neocortex, we define the molecular mechanisms underlying these defects. Thus we demonstrate a developmental, hitherto unappreciated, role of the two CCK receptors in mammalian neocortical development.

Chronic allograft rejection: a fresh look

PURPOSE OF REVIEW

New developments suggest that the graft itself and molecules expressed within the graft microenvironment dictate the phenotype and evolution of chronic rejection.

RECENT FINDINGS

Once ischemia-reperfusion injury, cellular and humoral immune responses target the microvasculature, the associated local tissue hypoxia results in hypoxia-inducible factor 1α-dependent expression of pro-inflammatory and proangiogenic growth factors including vascular endothelial growth factor (VEGF) as a physiological response to injury. Local expression of VEGF can promote the recruitment of alloimune T cells into the graft. mTOR/Akt signaling within endothelial cells regulates cytokine- and alloantibody-induced activation and proliferation and their proinflammatory phenotype. Inhibition of mTOR and/or Akt results in an anti-inflammatory phenotype and enables the expression of coinhibitory molecules that limit local T cell reactivation and promotes immunoregulation. Semaphorin family molecules may bind to neuropilin-1 on regulatory T cell subsets to stabilize functional responses. Ligation of neuropilin-1 on Tregs also inhibits Akt-induced responses suggesting common theme for enhancing local immunoregulation and long-term graft survival.

SUMMARY

Events within the graft initiated by mTOR/Akt-induced signaling promote the development of chronic rejection. Semaphorin-neuropilin biology represents a novel avenue for targeting this biology and warrants further investigation.

Roles of vascular endothelial growth factor in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal devastating neurodegenerative disorder, involving progressive degeneration of motor neurons in spinal cord, brainstem, and motor cortex. Riluzole is the only drug approved in ALS but it only confers a modest improvement in survival. In spite of a high number of clinical trials no other drug has proved effectiveness. Recent studies support that vascular endothelial growth factor (VEGF), originally described as a key angiogenic factor, also plays a key role in the nervous system, including neurogenesis, neuronal survival, neuronal migration, and axon guidance. VEGF has been used in exploratory clinical studies with promising results in ALS and other neurological disorders. Although VEGF is a very promising compound, translating the basic science breakthroughs into clinical practice is the major challenge ahead. VEGF-B, presenting a single safety profile, protects motor neurons from degeneration in ALS animal models and, therefore, it will be particularly interesting to test its effects in ALS patients. In the present paper the authors make a brief description of the molecular properties of VEGF and its receptors and review its different features and therapeutic potential in the nervous system/neurodegenerative disease, particularly in ALS.

Two specific populations of GABAergic neurons originating from the medial and the caudal ganglionic eminences aid in proper navigation of callosal axons

The corpus callosum (CC) plays a crucial role in interhemispheric communication. It has been shown that CC formation relies on the guidepost cells located in the midline region that include glutamatergic and GABAergic neurons as well as glial cells. However, the origin of these guidepost GABAergic neurons and their precise function in callosal axon pathfinding remain to be investigated. Here, we show that two distinct GABAergic neuronal subpopulations converge toward the midline prior to the arrival of callosal axons. Using in vivo and ex vivo fate mapping we show that CC GABAergic neurons originate in the caudal and medial ganglionic eminences (CGE and MGE) but not in the lateral ganglionic eminence (LGE). Time lapse imaging on organotypic slices and in vivo analyses further revealed that CC GABAergic neurons contribute to the normal navigation of callosal axons. The use of Nkx2.1 knockout (KO) mice confirmed a role of these neurons in the maintenance of proper behavior of callosal axons while growing through the CC. Indeed, using in vitro transplantation assays, we demonstrated that both MGE- and CGE-derived GABAergic neurons exert an attractive activity on callosal axons. Furthermore, by combining a sensitive RT-PCR technique with in situ hybridization, we demonstrate that CC neurons express multiple short and long range guidance cues. This study strongly suggests that MGE- and CGE-derived interneurons may guide CC axons by multiple guidance mechanisms and signaling pathways.

Neuropilin-1-dependent regulation of EGF-receptor signaling

Neuropilin-1 (NRP1) is a coreceptor for multiple extracellular ligands. NRP1 is widely expressed in cancer cells and in advanced human tumors; however, its functional relevance and signaling mechanisms are unclear. Here, we show that NRP1 expression controls viability and proliferation of different cancer cells, independent of its short intracellular tail. We found that the extracellular domain of NRP1 interacts with the EGF receptor (EGFR) and promotes its signaling cascade elicited upon EGF or TGF-α stimulation. Upon NRP1 silencing, the ability of ligand-bound EGFR to cluster on the cell surface, internalize, and activate the downstream AKT pathway is severely impaired. EGFR is frequently activated in human tumors due to overexpression, mutation, or sustained autocrine/paracrine stimulation. Here we show that NRP1-blocking antibodies and NRP1 silencing can counteract ligand-induced EGFR activation in cancer cells. Thus our findings unveil a novel molecular mechanism by which NRP1 can control EGFR signaling and tumor growth.

Extracellular matrix functions during neuronal migration and lamination in the mammalian central nervous system

Extracellular matrix (ECM) glycoproteins are expressed in the central nervous system (CNS) in complex and developmentally regulated patterns. The ECM provides a number of critical functions in the CNS, contributing both to the overall structural organization of the CNS and to control of individual cells. At the cellular level, the ECM affects its functions by a wide range of mechanisms, including providing structural support to cells, regulating the activity of second messenger systems, and controlling the distribution and local concentration of growth and differentiation factors. Perhaps the most well known role of the ECM is as a substrate on which motile cells can migrate. Genetic, cell biological, and biochemical studies provide strong evidence that ECM glycoproteins such as laminins, tenascins, and proteoglycans control neuronal migration and positioning in several regions of the developing and adult brain. Recent findings have also shed important new insights into the cellular and molecular mechanisms by which reelin regulates migration. Here we will summarize these findings, emphasizing the emerging concept that ECM glycoproteins promote different modes of neuronal migration such as radial, tangential, and chain migration. We also discuss several studies demonstrating that mutations in ECM glycoproteins can alter neuronal positioning by cell nonautonomous mechanisms that secondarily affect migrating neurons.

Increased smooth muscle contractility in mice deficient for neuropilin 2

Neuropilins (NRPs) are transmembrane receptors that bind class 3 semaphorins and VEGF family members to regulate axon guidance and angiogenesis. Although expression of NRP1 by vascular smooth muscle cells (SMCs) has been reported, NRP function in smooth muscle (SM) in vivo is unexplored. Using Nrp2(+/LacZ) and Nrp2(+/gfp) transgenic mice, we observed robust and sustained expression of Nrp2 in the SM compartments of the bladder and gut, but no expression in vascular SM, skeletal muscle, or cardiac muscle. This expression pattern was recapitulated in vitro using primary human SM cell lines. Alterations in cell morphology after treatment of primary visceral SMCs with the NRP2 ligand semaphorin-3F (SEMA3F) were accompanied by inhibition of RhoA activity and myosin light chain phosphorylation, as well as decreased cytoskeletal stiffness. Ex vivo contractility testing of bladder muscle strips exposed to electrical stimulation or soluble agonists revealed enhanced tension generation of tissues from mice with constitutive or SM-specific knockout of Nrp2, compared with controls. Mice lacking Nrp2 also displayed increased bladder filling pressures, as assessed by cystometry in conscious mice. Together, these findings identify Nrp2 as a mediator of prorelaxant stimuli in SMCs and suggest a novel function for Nrp2 as a regulator of visceral SM contractility.

Neuronal action on the developing blood vessel pattern

The nervous system relies on a highly specialized network of blood vessels for development and neuronal survival. Recent evidence suggests that both the central and peripheral nervous systems (CNS and PNS) employ multiple mechanisms to shape the vascular tree to meet its specific metabolic demands, such as promoting nerve-artery alignment in the PNS or the development the blood brain barrier in the CNS. In this article we discuss how the nervous system directly influences blood vessel patterning resulting in neuro-vascular congruence that is maintained throughout development and in the adult.

Secreted human amyloid precursor protein binds semaphorin 3a and prevents semaphorin-induced growth cone collapse

The amyloid precursor protein (APP) is well known for giving rise to the amyloid-β peptide and for its role in Alzheimer's disease. Much less is known, however, on the physiological roles of APP in the development and plasticity of the central nervous system. We have used phage display of a peptide library to identify high-affinity ligands of purified recombinant human sAPPα₆₉₅ (the soluble, secreted ectodomain from the main neuronal APP isoform). Two peptides thus selected exhibited significant homologies with the conserved extracellular domain of several members of the semaphorin (Sema) family of axon guidance proteins. We show that sAPPα₆₉₅ binds both purified recombinant Sema3A and Sema3A secreted by transfected HEK293 cells. Interestingly, sAPPα₆₉₅ inhibited the collapse of embryonic chicken (Gallus gallus domesticus) dorsal root ganglia growth cones promoted by Sema3A (K_d≤8·10⁻⁹ M). Two Sema3A-derived peptides homologous to the peptides isolated by phage display blocked sAPPα binding and its inhibitory action on Sema3A function. These two peptides are comprised within a domain previously shown to be involved in binding of Sema3A to its cellular receptor, suggesting a competitive mechanism by which sAPPα modulates the biological action of semaphorins.

Trio is a key guanine nucleotide exchange factor coordinating regulation of the migration and morphogenesis of granule cells in the developing cerebellum

Orchestrated regulation of neuronal migration and morphogenesis is critical for neuronal development and establishment of functional circuits, but its regulatory mechanism is incompletely defined. We established and analyzed mice with neural-specific knock-out of Trio, a guanine nucleotide exchange factor with multiple guanine nucleotide exchange factor domains. Knock-out mice showed defective cerebella and severe signs of ataxia. Mutant cerebella had no granule cells in the internal granule cell layer due to aberrant granule cell migration as well as abnormal neurite growth. Trio-deficient granule cells showed reduced extension of neurites and highly branched and misguided processes with perturbed stabilization of actin and microtubules. Trio deletion caused down-regulation of the activation of Rac1, RhoA, and Cdc42, and mutant granule cells appeared to be unresponsive to neurite growth-promoting molecules such as Netrin-1 and Semaphorin 6A. These results suggest that Trio may be a key signal module for the orchestrated regulation of neuronal migration and morphogenesis during cerebellar development. Trio may serve as a signal integrator decoding extrinsic signals to Rho GTPases for cytoskeleton organization.

Vasculogenesis and angiogenesis in the endometrium during menstrual cycle and implantation

Blood vessels develop via two subsequent processes, vasculogenesis and angiogenesis, both being of crucial importance during menstrual cycle and implantation. These processes are also involved in the development of the fetal and placental vasculatures. During vasculogenesis, formation of the earliest primitive capillaries is achieved by in situ differentiation of hemangiogenic stem cells that are derived from pluripotent mesenchymal cells. The subsequent process, angiogenesis, is characterized by development of new vessels from already existing vessels, and is a well coordinated process initiated by stimulation of various growth factors. Vasculogenesis and angiogenesis are important and complex processes involving extensive interplay between cells and growth factors. The development, maturation and maintenance of the vascular network are necessary for successful hemochorial placentation as well as normal embryonic development and growth. In this review, we outline the basic mechanisms of vasculogenesis and angiogenesis in the endometrium during the menstrual cycle and different stages of implantation, and consider how this data can be applied to human pregnancy. Recent studies have shown that during the initiation steps of implantation, angiogenic factors trigger vasculogenesis and angiogenesis. Different inducers and stimulators affect angiogenesis and vasculogenesis by directly or indirectly stimulating proliferation, differentiation and migration of endothelial or respective precursor cells. As a conclusion, understanding the mechanisms of angiogenesis and the roles of angiogenic factors during the menstrual cycle and implantation may provide new insights and possible approaches for embryo implantation and healthy pregnancy.

Downregulation of genes with a function in axon outgrowth and synapse formation in motor neurones of the VEGFdelta/delta mouse model of amyotrophic lateral sclerosis

Background

Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen that stimulates vasculogenesis. It has also been shown to act as a neurotrophic factor in vitro and in vivo. Deletion of the hypoxia response element of the promoter region of the gene encoding VEGF in mice causes a reduction in neural VEGF expression, and results in adult-onset motor neurone degeneration that resembles amyotrophic lateral sclerosis (ALS). Investigating the molecular pathways to neurodegeneration in the VEGF^δ/δ mouse model of ALS may improve understanding of the mechanisms of motor neurone death in the human disease.

Results

Microarray analysis was used to determine the transcriptional profile of laser captured spinal motor neurones of transgenic and wild-type littermates at 3 time points of disease. 324 genes were significantly differentially expressed in motor neurones of presymptomatic VEGF^δ/δ mice, 382 at disease onset, and 689 at late stage disease. Massive transcriptional downregulation occurred with disease progression, associated with downregulation of genes involved in RNA processing at late stage disease. VEGF^δ/δ mice showed reduction in expression, from symptom onset, of the cholesterol synthesis pathway, and genes involved in nervous system development, including axonogenesis, synapse formation, growth factor signalling pathways, cell adhesion and microtubule-based processes. These changes may reflect a reduced capacity of VEGF^δ/δ mice for maintenance and remodelling of neuronal processes in the face of demands of neural plasticity. The findings are supported by the demonstration that in primary motor neurone cultures from VEGF^δ/δ mice, axon outgrowth is significantly reduced compared to wild-type littermates.

Conclusions

Downregulation of these genes involved in axon outgrowth and synapse formation in adult mice suggests a hitherto unrecognized role of VEGF in the maintenance of neuronal circuitry. Dysregulation of VEGF may lead to neurodegeneration through synaptic regression and dying-back axonopathy.

Expression by midbrain dopamine neurons of Sema3A and 3F receptors is associated with chemorepulsion in vitro but a mild in vivo phenotype

Here we explore the role of semaphorin 3A and 3F (Sema3A, Sema3F) in the formation of the mesotelencephalic pathway. We show that Sema3A and 3F are expressed in the ventral mesencephalon (VM) of E13.5 rat embryos; the receptors Neuropilin 1 and Neuropilin 2, and co-receptors L1CAM, NrCAM, and Plexins A1 and A3 but not A4 are expressed by VM dopaminergic neurons; these neurons bind Sema3A and 3F in vitro which induces collapse of their growth cones and elicits, with different potencies, a repulsive response; and this response is absent in axons from Nrp1 and Nrp2 null embryos. Despite these in vitro effects, only very mild anatomical defects were detected in the organization of the mesotelencephalic pathway in embryonic and adult Nrp1 or Nrp2 null mice. However, the dopaminergic meso-habenular pathway and catecholaminergic neurons in the parafascicular and paraventricular nuclei of the thalamus were significantly affected in Nrp2 null mice. These data are consistent with a model whereby Sema3A and 3F, in combination with other guidance molecules, contributes to the navigation of DA axons to their final synaptic targets.

Anti-neuropilin-1 peptide inhibition of synoviocyte survival, angiogenesis, and experimental arthritis

OBJECTIVE

To delineate the role of neuropilin-1 (NP-1), a vascular endothelial growth factor receptor (VEGFR), in rheumatoid inflammation and to determine whether blockade of NP-1 could suppress synoviocyte survival and angiogenesis.

METHODS

VEGF(111-165) peptide, which encompasses the NP-1 binding domain of VEGF(165), was generated by cleaving VEGF(165) with plasmin. The effect of this peptide on the interaction between VEGF(165) and its receptor was determined by (125)I-VEGFR binding assay. Assays to determine synoviocyte apoptosis, adhesion, and migration were performed in the presence of VEGF(165) and/or the peptide. VEGF(165)-induced angiogenesis was assessed by measuring the proliferation, tube formation, and wounding migration of endothelial cells (ECs). Mice were immunized with type II collagen to induce experimental arthritis.

RESULTS

VEGF(111-165) peptide specifically inhibited the binding of (125)I-VEGF(165) to NP-1 on rheumatoid synoviocytes and ECs. The peptide eliminated the VEGF(165)-mediated increase in synoviocyte survival and activation of p-ERK and Bcl-2. The peptide also completely inhibited a VEGF(165)-induced increase in synoviocyte adhesion and migration. In addition, the anti-NP-1 peptide blocked VEGF(165)-stimulated proliferation, capillary tube formation, and wounding migration of ECs in vitro. VEGF(165)-induced neovascularization in a Matrigel plug in mice was also blocked by treatment with the peptide. Finally, subcutaneous injection of anti-NP-1 peptide suppressed arthritis severity and autoantibody formation in mice with experimental arthritis and inhibited synoviocyte hyperplasia and angiogenesis in arthritic joints.

CONCLUSION

Anti-NP-1 peptide suppressed VEGF(165)-induced increases in synoviocyte survival and angiogenesis, and thereby blocked experimental arthritis. Our findings suggest that anti-NP-1 peptide could be useful in alleviating chronic arthritis.

Peptide-based interference of the transmembrane domain of neuropilin-1 inhibits glioma growth in vivo

Angiogenesis in glioblastoma is largely dependent on vascular endothelial growth factor (VEGF) signalling. Consistently, the VEGF coreceptor NRP1 promotes angiogenesis and tumour growth in gliomas. Here, we provide data showing that an innovative peptidic tool targeting the transmembrane domain of NRP1 efficiently blocks rat and human glioma growth in vivo. We show both in vivo and in vitro that the antitumour effect results from the anti-proliferative, anti-migratory and anti-angiogenic properties of the compound. The proposed NRP1 antagonizing peptide is therefore a promising novel class of anti-angiogenic drugs that might prolong glioma patient survival. Our results finally show for the first time that the transmembrane domain of important signalling receptors can be antagonized in vivo thereby providing a new avenue towards the development of atypical antagonists with strong therapeutic potential.

Chondroitin sulfate acts in concert with semaphorin 3A to guide tangential migration of cortical interneurons in the ventral telencephalon

Chondroitin sulfate (CS) carrying proteoglycans (PGs) are widely expressed in the nervous system, and there is increasing evidence that they regulate developmental mechanisms like neurite outgrowth, axonal guidance and neuronal migration. Moreover, they can also act indirectly by organizing and/or modulating growth factors and guidance molecules. We found that chondroitin-4-sulfate is coexpressed with semaphorin 3A (Sema 3A) in the striatal mantle zone (SMZ), a nontarget region of neuropilin (Nrp)-1-expressing cortical interneurons flanking their migratory route in the subpallium. Using in vitro assays, we showed that CS PGs exert a repulsive effect on cortical interneurons, independently of Sema 3A, due to the CS side chains. We further showed that extracellular Sema 3A binds to CS. Disrupting Sema 3A-Nrp-1 signaling led migrating medial ganglionic eminence neurons to inappropriately invade the SMZ and even more so after removal of the CS side chains. Moreover, we found that soluble Sema 3A enhances the CS-induced repulsion in vitro. We concluded that CS acts as a repellent for cortical interneurons and that, in addition, CS restricts secreted Sema 3A within SMZ. Thus, both molecules act in concert to repel cortical interneurons from the SMZ during tangential migration toward the cerebral cortex.

Neuropilin-2 acts as a modulator of Sema3A-dependent glioma cell migration

Semaphorin 3A (Sema3A) is a secreted guidance molecule initially described in the nervous system. This protein is able to control axon growth but also effects on endothelial cells migration. Here, we report that Sema3A acts as a chemorepellent factor for the rat C6 glioma cells and three different human glioma cell lines. Interestingly, Sema3A triggered a chemoattractive response in a fourth human glioma cell line. The nature of the receptor complex ensuring the appropriate signaling was dissected in C6 cells by using function blocking antibodies and gain- or loss-of function experiments using recombinant receptors. Our results demonstrate that neuropilin-1, neuropilin-2 and PlexinA1 are necessary to trigger cell repulsion. The selective blockade of neuropilin-1 or Plexin-A1 switched the chemorepulsive effect of Sema3A into a chemoattractive one. Strikingly, blocking Neuropilin-2 suppressed Sema3A-induced cell migration while overexpression of neuropilin-2 was able to convert the chemorepulsive effect of Sema3A into a chemoattractive one. Our results not only provide additional evidence for a biological function of Sema3A in glioma migration but also reveal part of the receptor complex involved. Hence, our study describes a receptor-based plasticity in cancer cells leading to opposite migration behavior in response to the same extracellular signal.

Implications of vascular endothelial growth factor for postischemic neurovascular remodeling

Neurovascular remodeling has been recently recognized as a promising target for neurologic therapies. Hopes have emerged that, by stimulating vessel growth, it may be possible to stabilize brain perfusion, and at the same time promote neuronal survival, brain plasticity, and neurologic recovery. In this review, we outline the role of vascular endothelial growth factor (VEGF) in the ischemic brain, analyzing how this growth factor contributes to brain remodeling. Studies with therapeutic VEGF administration resulted in quite variable results depending on the route and time point of delivery. Local VEGF administration consistently enhanced neurologic recovery, whereas acute intravenous delivery exacerbated brain infarcts due to enhanced brain edema. Future studies should answer the following questions: (1) whether increased vessel density translates into improvements in blood flow in the hemodynamically compromised brain; (2) how VEGF influences brain plasticity and contributes to motor and nonmotor recovery; (3) what are the actions of VEGF not only in young animals with preserved vasculature, on which previous studies have been conducted, but also in aged animals and in animals with preexisting atherosclerosis; and (4) whether the effects of VEGF can be mimicked by pharmacological compounds or by cell-based therapies. Only on the basis of such information can more definite conclusions be made with regard to whether the translation of therapeutic angiogenesis into clinics is promising.

Angiogenic signalling pathways

Hypoxia is widely recognised as a key driving force for tumor angiogenesis by its induction of vascular endothelial growth factor (VEGF) and other direct-acting angiogenic factors. We describe the effect of hypoxia on gene expression and downstream angiogenic signalling; however, the angiogenic process is complex, and many other signalling pathways beyond VEGF are implicated in the formation of new vessels. These include extra-cellular signalling pathways such as the notch/delta, ephrin/Eph receptor, roundabout/slit, and netrin/UNC (uncoordinated) receptor families as well as intracellular proteins such as hedgehog and sprouty. The remarkable diversity in angiogenic signalling pathways provides many opportunities for therapeutic intervention, and anti-angiogenesis is currently a major area of oncology research.

Autonomous turning of cerebellar granule cells in vitro by intrinsic programs

External guidance cues play a role in controlling neuronal cell turning in the developing brain, but little is known about whether intrinsic programs are also involved in controlling the turning. In this study, we examined whether granule cells undergo autonomous changes in the direction of migration in the microexplant cultures of the early postnatal mouse cerebellum. We found that granule cells exhibit spontaneous and periodical turning without cell-cell contact and in the absence of external guidance cues. The frequency of turning was increased by stimulating the Ca(2+) influx and the internal Ca(2+) release, or inhibiting the cAMP signaling pathway, while the frequency was reduced by inhibiting the Ca(2+) influx. Granule cell turning in vitro was classified into four distinct modes, which were characterized by the morphological changes in the leading process and the trailing process, such as bifurcating, turning, withdrawing, and changing the polarity. The occurrence of the 1st and 2nd modes of turning was differentially affected by altering the Ca(2+) and cAMP signaling pathways. Collectively, the results demonstrate that intrinsic programs regulate the autonomous turning of cerebellar granule cells in vitro. Furthermore, the results suggest that extrinsic signals play a role as essential modulators of intrinsic programs.

Postmitotic Nkx2-1 controls the migration of telencephalic interneurons by direct repression of guidance receptors

The homeodomain transcription factor Nkx2-1 plays key roles in the developing telencephalon, where it regulates the identity of progenitor cells in the medial ganglionic eminence (MGE) and mediates the specification of several classes of GABAergic and cholinergic neurons. Here, we have investigated the postmitotic function of Nkx2-1 in the migration of interneurons originating in the MGE. Experimental manipulations and mouse genetics show that downregulation of Nkx2-1 expression in postmitotic cells is necessary for the migration of interneurons to the cortex, whereas maintenance of Nkx2-1 expression is required for interneuron migration to the striatum. Nkx2-1 exerts this role in the migration of MGE-derived interneurons by directly regulating the expression of a guidance receptor, Neuropilin-2, which enables interneurons to invade the developing striatum. Our results demonstrate a role for the cell-fate determinant Nkx2-1 in regulating neuronal migration by direct transcriptional regulation of guidance receptors in postmitotic cells.

Enhanced L1CAM expression on pancreatic tumor endothelium mediates selective tumor cell transmigration

L1 cell adhesion molecule (L1CAM) is a transmembrane cell adhesion molecule initially defined as a promigratory molecule in the developing nervous system that appears to be also expressed in some endothelial cells. However, little is known about the functional role of L1CAM on endothelial cells. We observed that L1CAM expression was selectively enhanced on endothelium associated with pancreatic adenocarcinoma in situ and on cultured pancreatic tumor-derived endothelial cells in vitro. L1CAM expression of endothelial cells could be augmented by incubation with immunomodulatory cytokines such as tumor necrosis factor alpha, interferon gamma, or transforming growth factor beta 1. Antibodies to L1CAM and the respective ligand neuropilin-1 blocked tube formation and stromal cell-derived factor 1beta induced transmigration of tumor endothelial cells in vitro. L1CAM expression on tumor-derived-endothelial cells enhanced Panc1 carcinoma cell adhesion to endothelial cell monolayers and transendothelial migration. Our data demonstrate a functional role of L1CAM expression on tumor endothelium that could favor metastasis and angiogenesis during tumor progression.

Expression of Robo/Slit and Semaphorin/Plexin/Neuropilin family members in the developing hypothalamic paraventricular and supraoptic nuclei

The hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON) contain neuroendocrine cells that modulate pituitary secretion to maintain homeostasis. These two nuclei have a common developmental origin but they eventually form at locations distant from each other. Little is known about the molecular cues that direct the segregation of PVN and SON. As a means to identify potential factors, we have documented expression patterns of genes with known guidance roles in neural migration. Here, we focus on two groups of ligand/receptor families classified to mediate chemo-repulsion of neurons and their axons: the Slit/Robo and the Semaphorin/Plexin/Neuropilin families. Their dynamic expression patterns within and around the common PVN/SON progenitor as well as the mature PVN and SON may provide a framework for understanding the formation of these two important nuclei.

Different isoforms of the C. elegans FGF receptor are required for attraction and repulsion of the migrating sex myoblasts

The Caenorhabditis elegans FGF receptor, EGL-15, is alternatively-spliced to yield two major isoforms that differ in their extracellular domains. The EGL-15(5A) isoform is necessary for the gonadal chemoattraction of the migrating sex myoblasts (SMs), while the EGL-15(5B) isoform is required for viability. Here we show that 5A is predominantly expressed in the M lineage, which gives rise to the migrating SMs and their sex muscle descendants, while 5B is predominantly expressed in the hypodermis. Tissue-specific expression, however, explains only part of the functional differences between these two receptor isoforms. 5A can carry out the reciprocal essential function of 5B when expressed in the hypodermis, but 5B is incapable of carrying out SM chemoattraction. Our data, therefore, indicate that the structural differences in these two isoforms contribute to their functional differences. Two lines of evidence indicate that the 5B isoform also plays a role in SM migration, implicating it in the repulsion that is observed when the chemoattraction is compromised. Thus, structural differences in the extracellular domains of these two isoforms can specify either attraction to or repulsion from the gonad.

Plexin-A3 and plexin-A4 restrict the migration of sympathetic neurons but not their neural crest precursors

During development, the semaphorin family of guidance molecules is required for proper formation of the sympathetic nervous system. Plexins are receptors that mediate semaphorin signaling, but how plexins function during sympathetic development is not fully understood. Using phenotypic analyses of mutant mice in vivo, expression pattern studies, and in vitro assays, we show that plexin-A3 and plexin-A4 are essential for normal sympathetic development. This study confirms our previous in vitro findings that the two plexins differentially regulate the guidance of sympathetic axons. In addition, we find that semaphorin signaling through plexin-A3 and plexin-A4 restricts the migration of sympathetic neurons, but these two plexins function redundantly since migration defects are only observed in plexin-A3/-A4 double mutants. Surprisingly, our analysis also indicates that plexin-A3 and plexin-A4 are not required for guiding neural crest precursors prior to reaching the sympathetic anlagen. Immunoprecipitation studies suggest that these two plexins independently mediate secreted semaphorin signaling. Thus, plexin-A3 and plexin-A4 are expressed in newly-differentiated sympathetic neurons, but not their neural crest precursors. They function cooperatively to regulate the migration of sympathetic neurons and then differentially to guide the sympathetic axons.