Gating of Sema3E/PlexinD1 signaling by neuropilin-1 switches axonal repulsion to attraction during brain development

Semaphorin 3E-Plexin-D1 signaling controls pathway-specific synapse formation in the striatum

The proper formation of synaptic connectivity in the mammalian brain is critical for complex behavior. In the striatum, balanced excitatory synaptic transmission from multiple sources onto two classes of principal neurons is required for coordinated and voluntary motor control. Here we show that the interaction between the secreted semaphorin 3E (Sema3E) and its receptor Plexin-D1 is a critical determinant of synaptic specificity in cortico-thalamo-striatal circuits in mice. We find that sema3E is highly expressed in thalamostriatal projection neurons whereas, in the striatum, plexin-D1 is selectively expressed in direct pathway medium spiny neurons (MSNs). Despite physical intermingling of the MSNs, genetic ablation of plexin-D1 or sema3E results in functional and anatomical rearrangement of thalamostriatal synapses specifically in direct pathway MSNs without effects on corticostriatal synapses. Thus, our results demonstrate that Sema3E and Plexin-D1 specify the degree of glutamatergic connectivity between a specific source and target within the complex circuitry of the basal ganglia.

Mutation of Semaphorin-6A disrupts limbic and cortical connectivity and models neurodevelopmental psychopathology

Psychiatric disorders such as schizophrenia and autism are characterised by cellular disorganisation and dysconnectivity across the brain and can be caused by mutations in genes that control neurodevelopmental processes. To examine how neurodevelopmental defects can affect brain function and behaviour, we have comprehensively investigated the consequences of mutation of one such gene, Semaphorin-6A, on cellular organisation, axonal projection patterns, behaviour and physiology in mice. These analyses reveal a spectrum of widespread but subtle anatomical defects in Sema6A mutants, notably in limbic and cortical cellular organisation, lamination and connectivity. These mutants display concomitant alterations in the electroencephalogram and hyper-exploratory behaviour, which are characteristic of models of psychosis and reversible by the antipsychotic clozapine. They also show altered social interaction and deficits in object recognition and working memory. Mice with mutations in Sema6A or the interacting genes may thus represent a highly informative model for how neurodevelopmental defects can lead to anatomical dysconnectivity, resulting, either directly or through reactive mechanisms, in dysfunction at the level of neuronal networks with associated behavioural phenotypes of relevance to psychiatric disorders. The biological data presented here also make these genes plausible candidates to explain human linkage findings for schizophrenia and autism.

More than nervous: the emerging roles of plexins

Plexins are the receptors for semaphorins, a large family of axon guidance cues. Accordingly, the role of plexins in the development of the nervous system was the first to be acknowledged. However, the expression of plexins is not restricted to neuronal cells, and recent research has been increasingly focused on the roles of plexin-semaphorin signalling outside of the nervous system. During embryogenesis, plexins regulate the development of many organs, including the cardiovascular system, skeleton and kidney. They have also been shown to be involved in immune system functions and tumour progression. Analyses of the plexin signalling in different tissues and cell types have provided new insight to the versatility of plexin interactions with semaphorins and other cell-surface receptors. In this review we try to summarise the current understanding of the roles of plexins in non-neural development and immunity.

Semaphorin 3E-Plexin-D1 signaling regulates VEGF function in developmental angiogenesis via a feedback mechanism

Blood vessel networks are typically formed by angiogenesis, a process in which new vessels form by sprouting of endothelial cells from pre-existing vessels. This process is initiated by vascular endothelial growth factor (VEGF)-mediated tip cell selection and subsequent angiogenic sprouting. Surprisingly, we found that VEGF directly controls the expression of Plexin-D1, the receptor for the traditional repulsive axon guidance cue, semaphorin 3E (Sema3E). Sema3E-Plexin-D1 signaling then negatively regulates the activity of the VEGF-induced Delta-like 4 (Dll4)-Notch signaling pathway, which controls the cell fate decision between tip and stalk cells. Using the mouse retina as a model system, we show that Plexin-D1 is selectively expressed in endothelial cells at the front of actively sprouting blood vessels and its expression is tightly controlled by VEGF secreted by surrounding tissues. Therefore, although the Sema3E secreted by retinal neurons is evenly distributed throughout the retina, Sema3E-Plexin-D1 signaling is spatially controlled by VEGF through its regulation of Plexin-D1. Moreover, we show that gain and loss of function of Sema3E and Plexin-D1 disrupts normal Dll4 expression, Notch activity, and tip/stalk cell distribution in the retinal vasculature. Finally, the retinal vasculature of mice lacking sema3E or plexin-D1 has an uneven growing front, a less-branched vascular network, and abnormal distribution of dll4-positive cells. Lowering Notch activity in the mutant mice can reverse this defect, solidifying the observation that Dll4-Notch signaling is regulated by Sema3E-Plexin-D1 and is required for its function in vivo. Together, these data reveal a novel role of Sema3E-Plexin-D1 function in modulating angiogenesis via a VEGF-induced feedback mechanism.

A role for class 3 semaphorins in prostate cancer

BACKGROUND

Class 3 semaphorins are secreted proteins that act as guidance cues for migrating cells via their transmembrane receptors plexins and neuropilins. Semaphorins have a role in cancer affecting tumor progression both directly, and indirectly by affecting angiogenesis.

METHODS

The expression of semaphorins and their receptors in prostate cancer cell lines and tissue was determined by RT-PCR, Western blotting and immunohistochemistry. The effect of Sema3E on prostate cancer cell lines was determined by adhesion assays and transwell migration assays.

RESULTS

Semaphorins and their receptors, plexins and neuropilins, are widely co-expressed in prostate cancer cell lines and tissue with a significant overexpression of Sema3E in tumor tissue. Sema3E affected integrin-mediated adhesion to fibronectin of prostate cancer cells, and inhibited their motility. Expression of Sema3C was upregulated and Sema3A and Sema3E were down regulated in prostate cells by hypoxia, consistent with an additional role for Sema3A and 3E as anti-angiogenic factors in prostate cancer.

CONCLUSIONS

Semaphorin 3E is aberrantly expressed in prostate cancer and affects adhesion and motility of prostate cancer cells, indicating a role for the Sema3E/PlexinD1 signaling pathway in prostate cancer and identifying a new possible target for therapy.

Sema3E/plexin-D1 mediated epithelial-to-mesenchymal transition in ovarian endometrioid cancer

Cancer cells often employ developmental cues for advantageous growth and metastasis. Here, we report that an axon guidance molecule, Sema3E, is highly expressed in human high-grade ovarian endometrioid carcinoma, but not low-grade or other ovarian epithelial tumors, and facilitates tumor progression. Unlike its known angiogenic activity, Sema3E acted through Plexin-D1 receptors to augment cell migratory ability and concomitant epithelial-to-mesenchymal transition (EMT). Sema3E-induced EMT in ovarian endometrioid cancer cells was dependent on nuclear localization of Snail1 through activation of phosphatidylinositol-3-kinase and ERK/MAPK. RNAi-mediated knockdown of Sema3E, Plexin-D1 or Snail1 in Sema3E-expressing tumor cells resulted in compromised cell motility, concurrent reversion of EMT and diminished nuclear localization of Snail1. By contrast, forced retention of Snail1 within the nucleus of Sema3E-negative tumor cells induced EMT and enhanced cell motility. These results show that in addition to the angiogenic effects of Sema3E on tumor vascular endothelium, an EMT strategy could be exploited by Sema3E/Plexin-D1 signaling in tumor cells to promote cellular invasion/migration.

Sema3E-PlexinD1 signaling selectively suppresses disoriented angiogenesis in ischemic retinopathy in mice

During development, the retinal vasculature grows toward hypoxic areas in an organized fashion. By contrast, in ischemic retinopathies, new blood vessels grow out of the retinal surfaces without ameliorating retinal hypoxia. Restoration of proper angiogenic directionality would be of great benefit to reoxygenize the ischemic retina and resolve disease pathogenesis. Here, we show that binding of the semaphorin 3E (Sema3E) ligand to the transmembrane PlexinD1 receptor initiates a signaling pathway that normalizes angiogenic directionality in both developing retinas and ischemic retinopathy. In developing mouse retinas, inhibition of VEGF signaling resulted in downregulation of endothelial PlexinD1 expression, suggesting that astrocyte-derived VEGF normally promotes PlexinD1 expression in growing blood vessels. Neuron-derived Sema3E signaled to PlexinD1 and activated the small GTPase RhoJ in ECs, thereby counteracting VEGF-induced filopodia projections and defining the retinal vascular pathfinding. In a mouse model of ischemic retinopathy, enhanced expression of PlexinD1 and RhoJ in extraretinal vessels prevented VEGF-induced disoriented projections of the endothelial filopodia. Remarkably, intravitreal administration of Sema3E protein selectively suppressed extraretinal vascular outgrowth without affecting the desired regeneration of the retinal vasculature. Our study suggests a new paradigm for vascular regeneration therapy that guides angiogenesis precisely toward the ischemic retina.

Plexin-D1 is a novel regulator of germinal centers and humoral immune responses

Long-lived humoral immune responses depend upon the generation of memory B cells and long-lived plasma cells during the germinal center (GC) reaction. These memory compartments, characterized by class-switched IgG and high-affinity Abs, are the basis for successful vaccination. We report that a new member of the plexin family of molecules, plexin-D1, controls the GC reaction and is required for secondary humoral immune responses. Plexin-D1 was not required for B cell maturation, marginal zone precursor development, dark and light zone formation, Igλ(+) and Igκ(+) B cell skewing, B1/B2 development, and the initial extrafollicular response. Plexin-D1 expression was increased following B cell activation, and PlxnD1(-/-) mice exhibited defective GC reactions during T-dependent immune activation. PlxnD1(-/-) B cells showed a defect in migration toward the GC chemokines, CXCL12, CXCL13, and CCL19. Accordingly, PlxnD1(-/-) mice exhibited defective production of IgG1 and IgG2b, but not IgG3 serum Ab, accompanied by reductions in long-lived bone marrow plasmacytes and recall humoral memory responses. These data show a new role for immune plexins in the GC reaction and generation of immunologic memory.

The κ opioid system regulates endothelial cell differentiation and pathfinding in vascular development

The opioid system (opioid peptides and receptors) regulates a variety of neurophysiologic functions, including pain control. Here we show novel roles of the κ opioid system in vascular development. Previously, we revealed that cAMP/protein kinase A (PKA) signaling enhanced differentiation of vascular progenitors expressing VEGF receptor-2 (fetal liver kinase 1; Flk1) into endothelial cells (ECs) through dual up-regulation of Flk1 and Neuropilin1 (NRP1), which form a selective and sensitive VEGF(164) receptor. Kappa opioid receptor (KOR), an inhibitory G protein-coupled receptor, was highly expressed in embryonic stem cell-derived Flk1(+) vascular progenitors. The addition of KOR agonists to Flk1(+) vascular progenitors inhibited EC differentiation and 3-dimensional vascular formation. Activation of KOR decreased expression of Flk1 and NRP1 in vascular progenitors. The inhibitory effects of KOR were reversed by 8-bromoadenosine-3',5'-cAMP or a PKA agonist, N(6)-benzoyl-cAMP, indicating that KOR inhibits cAMP/PKA signaling. Furthermore, KOR-null or dynorphin (an endogenous KOR agonist)-null mice showed a significant increase in overall vascular formation and ectopic vascular invasion into somites at embryonic day -10.5. ECs in these null mice showed significant increase in Flk1 and NRP1, along with reciprocal decrease in plexinD1, which regulates vascular pathfinding. The opioid system is, thus, a new regulator of vascular development that simultaneously modifies 2 distinct vascular properties, EC differentiation and vascular pathfinding.

Diverse functions for the semaphorin receptor PlexinD1 in development and disease

Plexins are a family of single-pass transmembrane proteins that serve as cell surface receptors for Semaphorins during the embryonic development of animals. Semaphorin-Plexin signaling is critical for many cellular aspects of organogenesis, including cell migration, proliferation and survival. Until recently, little was known about the function of PlexinD1, the sole member of the vertebrate-specific PlexinD (PlxnD1) subfamily. Here we review novel findings about PlxnD1's roles in the development of the cardiovascular, nervous and immune systems and salivary gland branching morphogenesis and discuss new insights concerning the molecular mechanisms of PlxnD1 activity.

Flotillin-mediated endocytic events dictate cell type-specific responses to semaphorin 3A

Cortical efferents growing in the same environment diverge early in development. The expression of particular transcription factors dictates the trajectories taken, presumably by regulating responsiveness to guidance cues via cellular mechanisms that are not yet known. Here, we show that cortical neurons that are dissociated and grown in culture maintain their cell type-specific identities defined by the expression of transcription factors. Using this model system, we sought to identify and characterize mechanisms that are recruited to produce cell type-specific responses to Semaphorin 3A (Sema3A), a guidance cue that would be presented similarly to cortical axons in vivo. Axons from presumptive corticofugal neurons lacking the transcription factor Satb2 and expressing Ctip2 or Tbr1 respond far more robustly to Sema3A than those from presumptive callosal neurons expressing Satb2. Both populations of axons express similar levels of Sema3A receptors (neuropilin-1, cell adhesion molecule L1, and plexinA4), but significantly, axons from neurons lacking Satb2 internalize more Sema3A, and they do so via a raft-mediated endocytic pathway. We used an in silico approach to identify the endocytosis effector flotillin-1 as a Sema3A signaling candidate. We tested the contributions of flotillin-1 to Sema3A endocytosis and signaling, and show that raft-mediated Sema3A endocytosis is defined by and depends on the recruitment of flotillin-1, which mediates LIM domain kinase activation and regulates axon responsiveness to Sema3A in presumptive corticofugal axons.

Common factors regulating patterning of the nervous and vascular systems

The vascular and the nervous systems of vertebrates share many features with similar and often overlapping anatomy. The parallels between these two systems extend to the molecular level, where recent work has identified ever-increasing similarities between the molecular mechanisms employed in the specification, differentiation, and patterning of both systems. This review discusses some of the most recent literature on this subject, with particular emphasis on the roles that the Ephrin, Semaphorin, Netrin, and Slit signaling pathways play in vascular development.

Spatial organization of transmembrane receptor signalling

The spatial organization of transmembrane receptors is a critical step in signal transduction and receptor trafficking in cells. Transmembrane receptors engage in lateral homotypic and heterotypic cis-interactions as well as intercellular trans-interactions that result in the formation of signalling foci for the initiation of different signalling networks. Several aspects of ligand-induced receptor clustering and association with signalling proteins are also influenced by the lipid composition of membranes. Thus, lipid microdomains have a function in tuning the activity of many transmembrane receptors by positively or negatively affecting receptor clustering and signal transduction. We review the current knowledge about the functions of clustering of transmembrane receptors and lipid-protein interactions important for the spatial organization of signalling at the membrane.

Sema3E-Plexin D1 signaling drives human cancer cell invasiveness and metastatic spreading in mice

Semaphorin 3E (Sema3E) is a secreted molecule implicated in axonal path finding and inhibition of developmental and postischemic angiogenesis. Sema3E is also highly expressed in metastatic cancer cells, but its mechanistic role in tumor progression was not understood. Here we show that expression of Sema3E and its receptor Plexin D1 correlates with the metastatic progression of human tumors. Consistent with the clinical data, knocking down endogenous expression of either Sema3E or Plexin D1 in human metastatic carcinoma cells hampered their metastatic potential when injected into mice, while tumor growth was not markedly affected. Conversely, overexpression of exogenous Sema3E in cancer cells increased their invasiveness, transendothelial migration, and metastatic spreading, although it inhibited tumor vessel formation, resulting in reduced tumor growth in mice. The proinvasive and metastatic activity of Sema3E in tumor cells was dependent on transactivation of the Plexin D1-associated ErbB2/Neu oncogenic kinase. In sum, Sema3E-Plexin D1 signaling in cancer cells is crucially implicated in their metastatic behavior and may therefore be a promising target for strategies aimed at blocking tumor metastasis.

Semaphorin signaling: molecular switches at the midline

To establish axonal connections growth cones must navigate multiple intermediate targets before reaching their final target. During this journey growth cones are guided by extracellular repulsive and attractive signals. Although initially identified as repulsive molecules, members of the semaphorin family include both attractants and repellents. How a navigating growth cone responds to a specific semaphorin is not absolute but instead depends on the biological context in which this cue is encountered. Here we review recent breakthroughs in our understanding of the extrinsic signals and molecular processes that control growth cone responses to class 3 semaphorins (Sema3s) at a well-characterized intermediate target, the spinal cord midline.

Axon responses of embryonic stem cell-derived dopaminergic neurons to semaphorins 3A and 3C

Class 3 Semaphorins are a subfamily of chemotropic molecules implicated in the projection of dopaminergic neurons from the ventral mesencephalon and in the formation of the nigrostriatal pathway (NSP) during embryonic development. In humans, loss of mesencephalic dopaminergic neurons leads to Parkinson's disease (PD). Cell replacement therapy with dopaminergic neurons generated from embryonic stem cells (ES-TH(+)) is being actively explored in models of PD. Among several requisites for this approach to work are adequate reconstruction of the NSP and correct innervation of normal striatal targets by dopaminergic axons. In this work, we characterized the response of ES-TH(+) neurons to semaphorins 3A, 3C, and 3F and compared it with that of tyrosine hidroxylase-positive neurons (TH(+)) obtained from embryonic ventral mesencephalon (VM-TH(+)). We observed that similar proportions of ES-TH(+) and VM-TH(+) neurons express semaphorin receptors neuropilins 1 and 2. Furthermore, the axons of both populations responded very similarly to semaphorin exposure: semaphorin 3A increased axon length, and semaphorin 3C attracted axons and increased their length. These effects were mediated by neuropilins, insofar as addition of blocking antibodies against these proteins reduced the effects on axonal growth and attraction, and only TH(+) axons expressing neuropilins responded to the semaphorins analyzed. The observations reported here show phenotypic similarities between VM-TH(+) and ES-TH(+) neurons and suggest that semaphorins 3A and 3C could be employed to guide axons of grafted ES-TH(+) in therapeutic protocols for PD.

VEGFR2 (KDR/Flk1) signaling mediates axon growth in response to semaphorin 3E in the developing brain

Common factors are thought to control vascular and neuronal patterning. Here we report an in vivo requirement for the vascular endothelial growth factor receptor type 2 (VEGFR2) in axon tract formation in the mouse brain. We show that VEGFR2 is expressed by neurons of the subiculum and mediates axonal elongation in response to the semaphorin (Sema) family molecule, Sema3E. We further show that VEGFR2 associates with the PlexinD1/Neuropilin-1 (Nrp1) receptor complex for Sema3E and becomes tyrosine-phosphorylated upon Sema3E stimulation. In subicular neurons, Sema3E triggers VEGFR2-dependent activation of the phosphatidylinositol-3 kinase (PI3K)/Akt pathway that is required for the increase in axonal growth. These results implicate VEGFR2 in axonal wiring through a mechanism dependent on Sema3E and independent of vascular endothelial growth factor (VEGF) ligands. This mechanism provides an explanation as to how a semaphorin can activate an axon growth promoting response in developing neurons.

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.

Inhibition of semaphorin as a novel strategy for therapeutic angiogenesis

RATIONALE

The axon-guiding molecules known as semaphorins and their receptors (plexins) regulate the vascular pattern and play an important role in the development of vascular network during embryogenesis. Semaphorin (Sema)3E is one of the class 3 semaphorins, and plexinD1 is known to be its receptor. Although these molecules have a role in embryonic vascular development, it remains unclear whether the Sema3E/plexinD1 axis is involved in postnatal angiogenesis.

OBJECTIVE

The objective of this study was to elucidate the role of Sema3E/plexinD1 in postnatal angiogenesis.

METHODS AND RESULTS

Sema3E inhibited cell growth and tube formation by suppressing the vascular endothelial growth factor (VEGF) signaling pathway. Expression of Sema3E and plexinD1 was markedly upregulated in ischemic limbs of mice (2.5- and 4.5-fold increase for Sema3E and plexinD1, respectively), and inhibition of this pathway by introduction of the plexinD1-Fc gene or disruption of Sema3E led to a significant increase of blood flow recovery (1.6- and 1.5-fold increase for the plexinD1-Fc gene treatment and Sema3E disruption, respectively). Hypoxia activated the tumor suppressor protein p53, thereby upregulating Sema3E expression. Expression of p53 and Sema3E was enhanced in diabetic mice compared with normal mice (2- and 1.3-fold increase for p53 and Sema3E, respectively). Consequently, neovascularization after VEGF treatment was poor in the ischemic tissues of diabetic mice, whereas treatment with VEGF plus plexinD1-Fc markedly improved neovascularization.

CONCLUSIONS

These results indicate that inhibition of Sema3E may be a novel strategy for therapeutic angiogenesis, especially when VEGF is ineffective.

Transient neuronal populations are required to guide callosal axons: a role for semaphorin 3C

The corpus callosum (CC) is the main pathway responsible for interhemispheric communication. CC agenesis is associated with numerous human pathologies, suggesting that a range of developmental defects can result in abnormalities in this structure. Midline glial cells are known to play a role in CC development, but we here show that two transient populations of midline neurons also make major contributions to the formation of this commissure. We report that these two neuronal populations enter the CC midline prior to the arrival of callosal pioneer axons. Using a combination of mutant analysis and in vitro assays, we demonstrate that CC neurons are necessary for normal callosal axon navigation. They exert an attractive influence on callosal axons, in part via Semaphorin 3C and its receptor Neuropilin-1. By revealing a novel and essential role for these neuronal populations in the pathfinding of a major cerebral commissure, our study brings new perspectives to pathophysiological mechanisms altering CC formation.

Semaphorin 3F is a bifunctional guidance cue for dopaminergic axons and controls their fasciculation, channeling, rostral growth, and intracortical targeting

Dopaminergic neurons in the mesodiencephalon (mdDA neurons) make precise synaptic connections with targets in the forebrain via the mesostriatal, mesolimbic, and mesoprefrontal pathways. Because of the functional importance of these remarkably complex ascending axon pathways and their implication in human disease, the mechanisms underlying the development of these connections are of considerable interest. Despite extensive in vitro studies, the molecular determinants that ensure the perfect formation of these pathways in vivo remain mostly unknown. Here, we determine the embryonic origin and ontogeny of the mouse mesoprefrontal pathway and use these data to reveal an unexpected requirement for semaphorin 3F (Sema3F) and its receptor neuropilin-2 (Npn-2) during mdDA pathway development using tissue culture approaches and analysis of sema3F(-/-), npn-2(-/-), and npn-2(-/-);TH-Cre mice. We show that Sema3F is a bifunctional guidance cue for mdDA axons, some of which have the remarkable ability to regulate their responsiveness to Sema3F as they develop. During early developmental stages, Sema3F chemorepulsion controls previously uncharacterized aspects of mdDA pathway development through both Npn-2-dependent (axon fasciculation and channeling) and Npn-2-independent (rostral growth) mechanisms. Later on, chemoattraction mediated by Sema3F and Npn-2 is required to orient mdDA axon projections in the cortical plate of the medial prefrontal cortex. This latter finding demonstrates that regulation of axon orientation in the target field occurs by chemoattractive mechanisms, and this is likely to also apply to other neural systems. In all, this study provides a framework for additional dissection of the molecular basis of mdDA pathway development and disease.

Semaphorin signaling in cancer cells and in cells of the tumor microenvironment--two sides of a coin

Semaphorins are a large family of secreted and membrane-bound molecules that were initially implicated in the development of the nervous system and in axon guidance. More recently, they have been found to regulate cell adhesion and motility, angiogenesis, immune responses, and tumor progression. Semaphorin receptors, the neuropilins and the plexins, are expressed by a wide variety of cell types, including endothelial cells, bone-marrow-derived cells and cancer cells. Interestingly, a growing body of evidence indicates that semaphorins also have an important role in cancer. It is now known that cancer progression, invasion and metastasis involve not only genetic changes in the tumor cells but also crosstalk between tumor cells and their surrounding non-tumor cells. Through the recruitment of endothelial cells, leukocytes, pericytes and fibroblasts, and the local release of growth factors and cytokines, the tumor microenvironment can mediate tumor-cell survival, tumor proliferation and regulation of the immune response. Moreover, by conferring cancer cells with an enhanced ability to migrate and invade adjacent tissues, extracellular regulatory signals can play a major role in the metastatic process. In this Commentary, we focus on the emerging role of semaphorins in mediating the crosstalk between tumor cells and multiple stromal cell types in the surrounding microenvironment.

Deal breaker: semaphorin and specificity in the spinal stretch reflex circuit

Stretch reflex circuits are a prime example of wiring specificity in the vertebrate spinal cord. Homonymous sensory afferents and motoneurons typically form monosynaptic connections, while neurons innervating antagonistic or unrelated muscles do not. Pecho-Vrieseling et al. now show that the semaphorin Sema3E and its receptor Plexin-D1 prevent monosynaptic connectivity in the cutaneous maximus muscle stretch reflex circuit.

Specificity of sensory-motor connections encoded by Sema3e-Plxnd1 recognition

Spinal reflexes are mediated by synaptic connections between sensory afferents and motor neurons. The organization of these circuits shows several levels of specificity. Only certain classes of proprioceptive sensory neurons make direct, monosynaptic connections with motor neurons. Those that do are bound by rules of motor pool specificity: they form strong connections with motor neurons supplying the same muscle, but avoid motor pools supplying antagonistic muscles. This pattern of connectivity is initially accurate and is maintained in the absence of activity, implying that wiring specificity relies on the matching of recognition molecules on the surface of sensory and motor neurons. However, determinants of fine synaptic specificity here, as in most regions of the central nervous system, have yet to be defined. To address the origins of synaptic specificity in these reflex circuits we have used molecular genetic methods to manipulate recognition proteins expressed by subsets of sensory and motor neurons. We show here that a recognition system involving expression of the class 3 semaphorin Sema3e by selected motor neuron pools, and its high-affinity receptor plexin D1 (Plxnd1) by proprioceptive sensory neurons, is a critical determinant of synaptic specificity in sensory-motor circuits in mice. Changing the profile of Sema3e-Plxnd1 signalling in sensory or motor neurons results in functional and anatomical rewiring of monosynaptic connections, but does not alter motor pool specificity. Our findings indicate that patterns of monosynaptic connectivity in this prototypic central nervous system circuit are constructed through a recognition program based on repellent signalling.

Antagonistic interactions among Plexins regulate the timing of intersegmental vessel formation

The angioblast is an embryonic endothelial cell precursor that migrates long distances to reach its final position, navigating by sensing attractive and repulsive cues from the environment. Members of the semaphorin family have been implicated in controlling the behaviour of angioblast tip cells through repulsive signalling in vitro, but their in vivo roles are less clear. Here we show that zebrafish semaphorin3e (sema3e) is expressed by endothelial cells of the dorsal aorta, primary motoneurons, and endodermal cells. Further, loss of Sema3e leads to delayed exit of angioblasts from the dorsal aorta in ISV formation. Through transplant analysis, we show that Sema3e acts autonomously and non-autonomously in angioblasts to modulate interactions among themselves. The semaphorin receptors, PlexinD1 and PlexinB2, are expressed by zebrafish angioblasts. Loss of plxnB2 results in delayed ISV sprouting identical to that seen in sema3e morphants, while loss of plexinD1 in out of bounds (obd) mutants results in precocious ISV sprouting. Loss of either sema3e or plxnB2 in obd mutants generates an intermediate phenotype, suggesting that PlxnD1 and Sema3e/PlxnB2 antagonize each other to control timing of ISV sprouting. Consistent with this observation, we show that PlxnB2 acts cell autonomously in endothelial cells. This suggests a model where multiple semaphorin-plexin interactions control angioblast sprouting behaviour.

The growth cone: an integrator of unique cues into refined axon guidance

One of the challenges to understanding nervous system development is to establish how a fairly limited number of axon guidance cues can set up the patterning of very complex nervous systems. Most of the recent insights relevant to guidance mechanisms have come from cell biologists focusing on processes and molecular machinery controlling the guidance responses in the growth cone.

Guidance from above: common cues direct distinct signaling outcomes in vascular and neural patterning

The nervous and vascular systems are both exquisitely branched and complicated systems and their proper development requires careful guidance of nerves and vessels. The recent realization that common ligand-receptor pairs are used in guiding the patterning of both systems has prompted the question of whether similar signaling pathways are used in both systems. This review highlights recent progress in our understanding of the similarities and differences in the intracellular signaling mechanisms downstream of semaphorins, ephrins and vascular endothelial growth factor in neurons and endothelial cells during neural and vascular development. We present evidence that similar intracellular signaling principles underlying cytoskeletal regulation are used to control neural and vascular guidance, although the specific molecules used in neurons and endothelial cells are often different.

Different requirement for Rnd GTPases of R-Ras GAP activity of Plexin-C1 and Plexin-D1

Plexins, comprising Plexin-A, -B, -C, and -D subfamilies, are receptors for semaphorins governing cell adhesion, migration, and axon guidance. Among plexin subfamilies, Plexin-A1 and Plexin-B1 have been shown to function as an R-Ras GAP, inducing repulsive responses, and the expression of R-Ras GAP activity requires the binding of Rnd1, a member of Rnd subfamily of Rho GTPases. However, signaling pathways of Plexin-D1 and Plexin-C1 still remain obscure. Here, we found that Plexin-D1 displayed R-Ras GAP activity and inhibited migration of COS-7 cells, and these actions required Rnd2, another Rnd subfamily GTPase. Rnd2 bound to Plexin-D1 in cortical neurons, and Sema3E/Plexin-D1-induced inhibition of axon outgrowth of cortical neurons required Rnd2 and down-regulation of R-Ras activity. On the other hand, Plexin-C1 displayed R-Ras GAP activity and inhibited cell migration of COS-7 cells without Rnd proteins. Therefore, R-Ras GAP activity is a common function of plexin subfamilies but the regulation of R-Ras GAP activity of plexins by Rnd proteins is different among plexin subfamilies.

Semaphorin-6A controls guidance of corticospinal tract axons at multiple choice points

Background

The trajectory of corticospinal tract (CST) axons from cortex to spinal cord involves a succession of choice points, each of which is controlled by multiple guidance molecules. To assess the involvement of transmembrane semaphorins and their plexin receptors in the guidance of CST axons, we have examined this tract in mutants of Semaphorin-6A (Sema6A), Plexin-A2 (PlxnA2) and Plexin-A4 (PlxnA4).

Results

We describe defects in CST guidance in Sema6A mutants at choice points at the mid-hindbrain boundary (MHB) and in navigation through the pons that dramatically affect how many axons arrive to the hindbrain and spinal cord and result in hypoplasia of the CST. We also observe defects in guidance within the hindbrain where a proportion of axons aberrantly adopt a ventrolateral position and fail to decussate. This function in the hindbrain seems to be mediated by the known Sema6A receptor PlxnA4, which is expressed by CST axons. Guidance at the MHB, however, appears independent of this and of the other known receptor, PlxnA2, and may depend instead on Sema6A expression on CST axons themselves at embryonic stages.

Conclusion

These data identify Sema6A as a major contributor to the guidance of CST axons at multiple choice points. They highlight the active control of guidance at the MHB and also implicate the inferior olive as an important structure in the guidance of CST axons within the hindbrain. They also suggest that Sema6A, which is strongly expressed by oligodendrocytes, may affect CST regeneration in adults.

Functional diversity and mechanisms of action of the semaphorins

The second EMBO workshop on ;Semaphorin function and mechanisms of action', held in the gorgeous surroundings of the 12th Century Abbaye des Vaulx de Cernay near Paris, France this May, brought together a wide range of scientists working in diverse systems with a common interest: the semaphorins. Emerging new themes discussed at the meeting included the recognition of an increasingly complex way in which different cells regulate responsiveness, and the significance of considering semaphorins in the pathology of various diseases.

Semaphorin 3E expression correlates inversely with Plexin D1 during tumor progression

Plexin D1 (PLXND1) is broadly expressed on tumor vessels and tumor cells in a number of different human tumor types. Little is known, however, about the potential functional contribution of PLXND1 expression to tumor development. Expression of semaphorin 3E (Sema3E), one of the ligands for PLXND1, has previously been correlated with invasive behavior and metastasis, suggesting that the PLXND1-Sema3E interaction may play a role in tumor progression. Here we investigated PLXND1 and Sema3E expression during tumor progression in cases of melanoma. PLXND1 was not expressed by melanocytic cells in either naevi or melanomas in situ, whereas expression increased with invasion level, according to Clark's criteria. Furthermore, 89% of the metastatic melanomas examined showed membranous PLXND1-staining of tumor cells. Surprisingly, expression of Sema3E was inversely correlated with tumor progression, with no detectable staining in melanoma metastasis. To functionally assess the effects of Sema3E expression on tumor development, we overexpressed Sema3E in a xenograft model of metastatic melanoma. Sema3E expression dramatically decreased metastatic potential. These results show that PLXND1 expression during tumor development is strongly correlated with both invasive behavior and metastasis, but exclude Sema3E as an activating ligand.

Tie2Cre-mediated inactivation of plexinD1 results in congenital heart, vascular and skeletal defects

PlexinD1 is a membrane-bound receptor that mediates signals derived from class 3 secreted semaphorins. Although semaphorin signaling in axon guidance in the nervous system has been extensively studied, functions outside the nervous system including important roles in vascular patterning have also been demonstrated. Inactivation of plexinD1 leads to neo-natal lethality, structural defects of the cardiac outflow tract, peripheral vascular abnormalities, and axial skeletal morphogenesis defects. PlexinD1 is expressed by vascular endothelial cells, but additional domains of expression have also been demonstrated including in lymphocytes, osteoblasts, neural crest and the central nervous system. Hence, the cell-type specific functions of plexinD1 have remained unclear. Here, we describe the results of tissue-specific gene inactivation of plexinD1 in Tie2 expressing precursors, which recapitulates the null phenotype with respect to congenital heart, vascular, and skeletal abnormalities resulting in neonatal lethality. Interestingly, these mutants also have myocardial defects not previously reported. In addition, we demonstrate functions for plexinD1 in post-natal retinal vasculogenesis and adult angiogenesis through the use of inducible cre-mediated deletion. These results demonstrate an important role for PlexinD1 in embryonic and adult vasculature.

Urothelial expression of neuropilins and VEGF receptors in control and interstitial cystitis patients

Interstitial cystitis (IC) is a chronic and painful bladder syndrome of unknown cause with no reliable biological marker or effective therapy. Vascular endothelial growth factor (VEGF), which plays a key role in bladder inflammation, is closely associated with the vascular alterations observed in patients with IC. However, our recent findings of VEGF receptors (VEGF-Rs) and VEGF coreceptors on nonendothelial cells in human and mouse urothelium suggest that additional VEGF targets and functions are possible in IC bladders. We report here that VEGF-Rs and coreceptors (neuropilins; NRP) are strongly expressed in both the human bladder urothelium and in the human bladder cancer cell line (J82) and that the expression of NRP2 and VEGF-R1 is significantly downregulated in IC compared with control subjects. In addition, treatment of J82 cells with bacillus Calmette-Guérin (BCG), a novel treatment strategy for IC, upregulates the messages for NRPs and VEGF-Rs. Furthermore, intravesical instillation of an internalizable VEGF fluorescent tracer (scVEGF/Cy5.5) into mouse urinary bladders results in a marked ligand accumulation in the urothelium and bladder parenchyma, indicating that urothelial VEGF-Rs are functionally active and capable of ligand interaction and internalization. Our results suggest that the VEGF pathway is altered in IC, that urinary VEGF may gain access to the bladder wall via these receptors, and that BCG treatment may replenish the missing VEGF-Rs/NRP receptors. Together, these results suggest that levels of NRPs, VEGF-Rs, and VEGF are new putative markers for the diagnosis of IC and that modulating these receptors can be exploited as therapeutic strategies.

Dorsal turning of motor corticospinal axons at the pyramidal decussation requires plexin signaling

BACKGROUND

The development of the corticospinal tract (CST) in higher vertebrates relies on a series of axon guidance decisions along its long projection pathway. Several guidance molecules are known to be involved at various decision points to regulate the projection of CST axons. However, previous analyses of the CST guidance defects in mutant mice lacking these molecules have suggested that there are other molecules involved in CST axon guidance that are yet to be identified. In this study, we investigate the role of plexin signaling in the guidance of motor CST axons in vivo.

RESULTS

Expression pattern studies show that plexin-A3, plexin-A4, and neuropilin-1 are expressed in the developing cerebral cortex when the motor CST axons originating from layer V cortical neurons are guided down to the spinal cord. By analyzing mutant mice, we show that motor CST axons that turn dorsally to cross the midline at the pyramidal decussation require plexin-A3 and plexin-A4 signaling. Although other CST guidance defects are found in neuropilin-1 mutants, this dorsal turning defect is not observed in either neuropilin-1 or neuropilin-2 mutants, suggesting that the local cues that activate plexin signaling at the dorsal turning point are membrane-bound semaphorins. Further expression pattern study and mutant analysis indicate that Sema6A is one of the local cues for motor CST axon turning at the pyramidal decussation.

CONCLUSION

Dorsal turning and midline crossing at the pyramidal decussation is a crucial step to properly direct CST axons into the dorsal spinal cord. We show that the signaling of plexin-A3, plexin-A4, and Sema6A is at least partially required for dorsal turning of the CST axons, while neuropilin-1 is required for proper fasciculation of the tract at midline crossing. Together with previous reports, these results demonstrate that several guidance cues are specifically utilized to regulate the dorsal turning and midline crossing of developing CST axons.

The semaphorins: versatile regulators of tumour progression and tumour angiogenesis

The semaphorins and their receptors, the neuropilins and the plexins, were originally characterized as constituents of the complex regulatory system responsible for the guidance of axons during the development of the central nervous system. However, a growing body of evidence indicates that various semaphorins can either promote or inhibit tumour progression through the promotion or inhibition of processes such as tumour angiogenesis, tumour metastasis and tumour cell survival. This Review focuses on the emerging role of the semaphorins in cancer.

ABL2/ARG tyrosine kinase mediates SEMA3F-induced RhoA inactivation and cytoskeleton collapse in human glioma cells

Class three semaphorins (SEMAs) were originally shown to be mediators of axon guidance that repelled axons and collapsed growth cones, but it is now evident that SEMA3F, for example, has similar effects on tumor cells and endothelial cells (EC). In both human U87MG glioma cells and human umbilical vein EC, SEMA3F induced rapid cytoskeletal collapse, suppressed cell contractility, decreased phosphorylation of cofilin, and inhibited cell migration in culture. Analysis of the signaling pathways showed that SEMA3F formed a complex with NRP2 (neuropilin-2) and plexin A1. These interactions eventually led to inactivation of the small GTPase, RhoA, which is necessary for stress fiber formation and cytoskeleton integrity. A novel upstream RhoA mediator was shown to be ABL2, also known as ARG, a membrane-anchored nonreceptor tyrosine kinase. Within minutes after the addition of SEMA3F, ABL2 directly bound plexin A1 but not to a plexin A1 mutant lacking the cytoplasmic domain. In addition, ABL2 phosphorylated and thereby activated p190RhoGAP, which inactivated RhoA (GTP to GDP), resulting in cytoskeleton collapse and inhibition of cell migration. On the other hand, cells overexpressing an ABL2 inactive kinase mutant or treated with ABL2 small interfering RNA did not inactivate RhoA. Cells treated with p190RhoGAP small interfering RNA also did not inactivate RhoA. Together, these results suggested that ABL2/ARG is a novel mediator of SEMA3F-induced RhoA inactivation and collapsing activity.

Semaphorins and their receptors in lung cancer

Semaphorins are a large family of secreted, transmembrane and GPI-linked proteins initially characterized in the development of the nervous system and axonal guidance. Semaphorins are expressed in many tissues where they regulate normal development, organ morphogenesis, immunity and angiogenesis. They affect the cytoskeleton, actin filament organization, microtubules and cell adhesion. Semaphorin signaling is transduced by plexins, which in the case of most class-3 semaphorins requires high-affinity neuropilin receptors. The neuropilins also function as receptors for VEGF and other growth factors, and their expression is often abnormal in tumors. In cancer, semaphorins have both tumor suppressor and tumor promoting functions. We review here the current status of semaphorins and their receptors in tumor development with a focus on lung cancer.

Semaphorin signaling: progress made and promises ahead

Semaphorins were initially characterized according to their role in repulsive axon guidance but are now recognized as crucial regulators of morphogenesis and homeostasis over a wide range of organ systems. The pleiotropic nature of semaphorin signaling and its implication in human disease has triggered an enormous interest in the receptor and intracellular signaling mechanisms that direct the cell-type-specific and diverse biological effects of semaphorins. Recent breakthroughs in our understanding of semaphorin signaling link integrin and semaphorin signaling pathways, identify novel ligand-receptor interactions and provide insight into the cellular and molecular bases of bifunctional and reverse signaling events. These discoveries could lead to therapeutic advances in axonal regeneration, cancer and other diseases.

Semaphorins deployed to repel cell migrants at spinal cord borders

In the spinal cord, developing motor neurons extend their axons into the periphery while their cell bodies remain within the motor columns in the spinal cord. Two recent papers show that this partitioning involves forward and reverse semaphorin-plexin signaling between motor neurons and neural crest boundary cap cells.