New eukaryotic semaphorins with close homology to semaphorins of DNA viruses

Semaphorins and Their Receptors: From Axonal Guidance to Atherosclerosis

Semaphorins are a large family of secreted, transmembrane, or GPI-anchored proteins initially identified as axon guidance cues signaling through their receptors, neuropilins, and plexins. Emerging evidence suggests that beyond the guidance, they also function in a broad spectrum of pathophysiological conditions, including atherosclerosis, a vascular inflammatory disease. Particular semaphorin members have been demonstrated to participate in atherosclerosis via eliciting endothelial dysfunction, leukocyte infiltration, monocyte-macrophage retention, platelet hyperreactivity, and neovascularization. In this review, we focus on the role of those semaphorin family members in the development of atherosclerosis and highlight the mechanistic relevance of semaphorins to atherogenesis.

Neuroimmune semaphorins as costimulatory molecules and beyond

Several neuronal guidance proteins, known as semaphorin molecules, function in the immune system. This dual tissue performance has led to them being defined as "neuroimmune semaphorins". They have been shown to regulate T cell activation by serving as costimulatory molecules. Similar to classical costimulatory molecules, neuroimmune semaphorins are either constitutively or inducibly expressed on immune cells. In contrast to the classical costimulatory molecule function, the action of neuroimmune semaphorins requires the presence of two signals, the first one provided by TCR/MHC engagement, and the second one provided by B7/CD28 interaction. Thus, neuroimmune semaphorins serve as a "signal three" for immune cell activation and regulate the overall intensity of immune response. The current knowledge on their structures, multiple receptors, specific cell/tissue/organ expression, and distinct functions in different diseases are summarized and discussed in this review.

Semaphorin7A Promotion of Tumoral Growth and Metastasis in Human Oral Cancer by Regulation of G1 Cell Cycle and Matrix Metalloproteases: Possible Contribution to Tumoral Angiogenesis

Background

Semaphorins (SEMAs) consist of a large family of secreted and membrane-anchored proteins that are important in neuronal pathfinding and axon guidance in selected areas of the developing nervous system. Of them, SEMA7A has been reported to have a chemotactic activity in neurogenesis and to be an immunomodulator; however, little is known about the relevance of SEMA7A in the behaviors of oral squamous cell carcinoma (OSCC).

Methods

We evaluated SEMA7A expression in OSCC-derived cell lines and primary OSCC samples using quantitative reverse transcriptase-polymerase chain reaction, immunoblotting, and semiquantitative immunohistochemistry (sq-IHC). In addition, SEMA7A knockdown cells (shSEMA7A cells) were used for functional experiments, including cellular proliferation, invasiveness, and migration assays. We also analyzed the clinical correlation between SEMA7A status and clinical behaviors in patients with OSCC.

Results

SEMA7A mRNA and protein were up-regulated significantly (P<0.05) in OSCC-derived cell lines compared with human normal oral keratinocytes. The shSEMA7A cells showed decreased cellular growth by cell-cycle arrest at the G1 phase, resulting from up-regulation of cyclin-dependent kinase inhibitors (p21^Cip1 and p27^Kip1) and down-regulation of cyclins (cyclin D1, cyclin E) and cyclin-dependent kinases (CDK2, CDK4, and CDK6); and decreased invasiveness and migration activities by reduced secretion of matrix metalloproteases (MMPs) (MMP-2, proMMP-2, pro-MMP-9), and expression of membrane type 1- MMP (MT1-MMP). We also found inactivation of the extracellular regulated kinase 1/2 and AKT pathways, an upstream molecule of cell-cycle arrest at the G1 phase, and reduced secretion of MMPs in shSEMA7A cells. sq-IHC showed that SEMA7A expression in the primary OSCCs was significantly (P = 0.001) greater than that in normal counterparts and was correlated with primary tumoral size (P = 0.0254) and regional lymph node metastasis (P = 0.0002).

Conclusion

Our data provide evidence for an essential role of SEMA7A in tumoral growth and metastasis in OSCC and indicated that SEMA7A may play a potential diagnostic/therapeutic target for use in patients with OSCC.

Semaphorin 7A plays a critical role in IgE-mediated airway inflammation in mice

Elevated allergen-specific IgE levels are a hallmark of allergic asthma, a disease involving chronic airway inflammation characterized by airway hyperresponsiveness (AHR); neutrophilic airway inflammation is found in patients with severe asthma. Furthermore, we have reported that interleukin (IL)-33 and IL-17A contribute to IgE-mediated AHR through neutrophilic inflammation in mice. Meanwhile, semaphorins regulating neuronal and immune function have been focused on in several diseases. Here, we investigated whether semaphorin 7A (SEMA7A) is related to IgE-mediated neutrophilic inflammation in mice. BALB/c mice sensitized with antigen-specific IgE monoclonal antibody were repeatedly challenged by the antigen. When anti-SEMA7A antibody was administered during the fourth to seventh challenges, the infiltration by macrophages, lymphocytes, neutrophils, and eosinophils in the lungs was reduced at the seventh challenge (P<0.05, 0.05, 0.01, and 0.05, respectively). However, the increased production of IL-4, IL-5, IL-13, IL-33, IL-17A, IL-6, and CXCL1 in the lungs was not suppressed. In histological analysis, the epithelial cells, blood vessels, and inflammatory cells in the lungs of IgE-sensitized mice showed SEMA7A expression; plexin C1 for the receptor was expressed in the inflammatory cells. Meanwhile, we examined the effect of anti-SEMA7A antibody on AHR and neutrophilic inflammation enhanced by the collaborative action of IL-33 and IL-17A in normal mice, resulting in the suppression of these responses (P<0.05 and 0.01, respectively). Collectively, we demonstrated that SEMA7A plays a critical role in IgE-mediated neutrophilic airway inflammation. Therefore, SEMA7A may be a potential therapeutic target for severe allergic asthma showing neutrophilic airway inflammation.

Maturation of cortical circuits requires Semaphorin 7A

Abnormal cortical circuits underlie some cognitive and psychiatric disorders, yet the molecular signals that generate normal cortical networks remain poorly understood. Semaphorin 7A (Sema7A) is an atypical member of the semaphorin family that is GPI-linked, expressed principally postnatally, and enriched in sensory cortex. Significantly, SEMA7A is deleted in individuals with 15q24 microdeletion syndrome, characterized by developmental delay, autism, and sensory perceptual deficits. We studied the role that Sema7A plays in establishing functional cortical circuitry in mouse somatosensory barrel cortex. We found that Sema7A is expressed in spiny stellate cells and GABAergic interneurons and that its absence disrupts barrel cytoarchitecture, reduces asymmetrical orientation of spiny stellate cell dendrites, and functionally impairs thalamocortically evoked synaptic responses, with reduced feed-forward GABAergic inhibition. These data identify Sema7A as a regulator of thalamocortical and local circuit development in layer 4 and provide a molecular handle that can be used to explore the coordinated generation of excitatory and inhibitory cortical circuits.

Semaphorin7A promotes tumor growth and exerts a pro-angiogenic effect in macrophages of mammary tumor-bearing mice

Semaphorins are a large family of molecules involved in axonal guidance during the development of the nervous system and have been recently shown to have both angiogenic and anti-angiogenic properties. Specifically, semaphorin 7A (SEMA7A) has been reported to have a chemotactic activity in neurogenesis and to be an immune modulator through α1β1integrins. SEMA7A has been shown to promote monocyte chemotaxis and induce them to produce proinflammatory mediators. In this study we explored the role of SEMA7A in a murine model of breast cancer. We show that SEMA7A is highly expressed by DA-3 murine mammary tumor cells in comparison to normal mammary cells (EpH4), and that peritoneal elicited macrophages from mammary tumor-bearing mice also express SEMA7A at higher levels compared to those derived from normal mice. We also show that murine macrophages treated with recombinant murine SEMA7A significantly increased their expression of proangiogenic molecule CXCL2/MIP-2. Gene silencing of SEMA7A in peritoneal elicited macrophages from DA-3 tumor-bearing mice resulted in decreased CXCL2/MIP-2 expression. Mice implanted with SEMA7A silenced tumor cells showed decreased angiogenesis in the tumors compared to the wild type tumors. Furthermore, peritoneal elicited macrophages from mice bearing SEMA7A-silenced tumors produce significantly (p < 0.01) lower levels of angiogenic proteins, such as CXCL2/MIP-2, CXCL1, and MMP-9, compared to those from control DA-3 mammary tumors. We postulate that SEMA7A in mammary carcinomas may skew monocytes into a pro-tumorigenic phenotype to support tumor growth. SEMA7A could prove to be valuable in establishing new research avenues toward unraveling important tumor-host immune interactions in breast cancer patients.

Semaphorins 3A and 7A: potential immune and neuroregenerative targets in multiple sclerosis

Semaphorins have been classically defined as axonal signalling cues involved in central nervous system (CNS) development, but in adults these molecules are expressed in distinct tissues and exert various functions under several physiological and pathological contexts. Semaphorins capable of modulating the immune system are particularly relevant in autoimmune diseases, especially multiple sclerosis (MS), which is a demyelinating, neurodegenerative disease. In this article, we compile recent insights into the specific roles of semaphorin (sema)3A and sema7A to clarify the details of their possible participation in the inflammatory and neurodegenerative phases of MS.

Semaphorin7A and its receptors: pleiotropic regulators of immune cell function, bone homeostasis, and neural development

Semaphorins form a large, evolutionary conserved family of cellular guidance signals. The semaphorin family contains several secreted and transmembrane proteins, but only one GPI-anchored member, Semaphorin7A (Sema7A). Although originally identified in immune cells, as CDw108, Sema7A displays widespread expression outside the immune system. It is therefore not surprising that accumulating evidence supports roles for this protein in a wide variety of biological processes in different organ systems and in disease. Well-characterized biological effects of Sema7A include those during bone and immune cell regulation, neuron migration and neurite growth. These effects are mediated by two receptors, plexinC1 and integrins. However, most of what is known today about Sema7A signaling concerns Sema7A-integrin interactions. Here, we review our current knowledge of Sema7A function and signaling in different organ systems, highlighting commonalities between the cellular effects and signaling pathways activated by Sema7A in different cell types. Furthermore, we discuss a potential role for Sema7A in disease and provide directions for further research.

Semaphorins in the development, homeostasis and disease of hormone systems

Semaphorin proteins are among the best-studied families of guidance cues. Initially characterized as repulsive neuronal guidance cues, during the last decade, significant progress has been made in defining their involvement in the regulation of dynamic changes in the cellular cytoskeleton during embryonic and postnatal neuronal development, under both physiological and pathological conditions. However, semaphorins are not restricted to the nervous system but widely expressed in other tissues, where they play key roles in angiogenesis and organogenesis. In recent years, there has been an increasing emphasis on the potential influence of semaphorins on the development and homeostasis of hormone systems, and conversely, how circulating reproductive hormones regulate semaphorin expression. In this review, we summarize recent studies analyzing the contribution of semaphorin signaling to the morphogenesis, differentiation and plasticity of fundamental neuroendocrine and endocrine systems that regulate key physiological processes, such as reproduction, bone formation and the control of energy homeostasis.

Semaphorins in bone development, homeostasis, and disease

Semaphorins were originally identified as axon guidance cues in the development of the nervous system. In recent years, numerous studies have determined that they are also involved in organogenesis, vascularization/angiogenesis, oncogenesis, and immune responses. In addition, the mechanisms underlying the diverse functions of semaphorins and their receptors have been identified. Recently, significant advances have been made in our understanding of the roles of semaphorins in bone remodeling, particularly the regulation of osteoclast and osteoblast differentiation and migration. Moreover, dysregulated semaphorin expression causes severe bone diseases, including osteoporosis and osteopetrosis. This review focuses on advanced findings on the role of semaphorins/receptors and their intracellular signaling in the regulation of bone homeostasis.

Semaphorin 7A contributes to West Nile virus pathogenesis through TGF-β1/Smad6 signaling

Semaphorin 7A (Sema7A) is a membrane-associated/secreted protein that plays an essential role in connecting the vertebrate neuronal and immune systems. However, the role of Sema7A has not been elucidated in viral pathogenesis. In this study, we show that abrogation of Sema7A protects mice from lethal West Nile virus (WNV) infection. Mice lacking Sema7A showed increased survival, reduced viral burden, and less blood-brain barrier permeability upon WNV infection. Increased Sema7A levels were evident in murine tissues, as well as in murine cortical neurons and primary human macrophages upon WNV infection. Treatment with Sema7A Ab blocked WNV infection in both of these cell types. Furthermore, Sema7A positively regulates the production of TGF-β1 and Smad6 to facilitate WNV pathogenesis in mice. Collectively, these data elucidate the role of Sema7A in shared signaling pathways used by the immune and nervous systems during viral pathogenesis that may lead to the development of Sema7A-blocking therapies for WNV and possibly other flaviviral infections.

Semaphorin-7a reverses the ERF-induced inhibition of EMT in Ras-dependent mouse mammary epithelial cells

Epithelial-to-mesenchymal transition (EMT) is a key process in cancer progression and metastasis, requiring cooperation of the epidermal growth factor/Ras with the transforming growth factor-β (TGF-β) signaling pathway in a multistep process. The molecular mechanisms by which Ras signaling contributes to EMT, however, remain elusive to a large extent. We therefore examined the transcriptional repressor Ets2-repressor factor (ERF)-a bona fide Ras-extracellular signal-regulated kinase/mitogen-activated protein kinase effector-for its ability to interfere with TGF-β-induced EMT in mammary epithelial cells (EpH4) expressing oncogenic Ras (EpRas). ERF-overexpressing EpRas cells failed to undergo TGF-β-induced EMT, formed three-dimensional tubular structures in collagen gels, and retained expression of epithelial markers. Transcriptome analysis indicated that TGF-β signaling through Smads was mostly unaffected, and ERF suppressed the TGF-β-induced EMT via Semaphorin-7a repression. Forced expression of Semaphorin-7a in ERF-overexpressing EpRas cells reestablished their ability to undergo EMT. In contrast, inhibition of Semaphorin-7a in the parental EpRas cells inhibited their ability to undergo TGF-β-induced EMT. Our data suggest that oncogenic Ras may play an additional role in EMT via the ERF, regulating Semaphorin-7a and providing a new interconnection between the Ras- and the TGF-β-signaling pathways.

Developing T-cell migration: role of semaphorins and ephrins

Cell migration is a crucial event for normal T-cell development, and various ligand/receptor pairs have been implicated. Most of them, including chemokines and extracellular matrix proteins, have attractant properties on thymocytes. We discuss herein two further groups of ligand/receptor pairs, semaphorins/neuropilins and ephs/ephrins, which are constitutively expressed by thymocytes and thymic microenvironmental cells. Evidence shows that the corresponding interactions are relevant for developing T-cell migration, including the entry of bone marrow progenitor cells, migration of CD4/CD8-defined thymocyte subpopulations triggered by chemokines and/or extracellular matrix proteins, and thymocyte export. Conceptually, the data summarized here show that thymocyte migration results from a complex network of molecular interactions, which generate not only attraction, but also repulsion of migrating T-cell precursors.

Dysregulation of Semaphorin7A/β1-integrin signaling leads to defective GnRH-1 cell migration, abnormal gonadal development and altered fertility

Reproduction in mammals is dependent on the function of specific neurons that secrete gonadotropin-releasing hormone-1 (GnRH-1). These neurons originate prenatally in the nasal placode and migrate into the forebrain along the olfactory-vomeronasal nerves. Alterations in this migratory process lead to defective GnRH-1 secretion, resulting in heterogeneous genetic disorders such as idiopathic hypogonadotropic hypogonadism (IHH), and other reproductive diseases characterized by the reduction or failure of sexual competence. Combining mouse genetics with in vitro models, we demonstrate that Semaphorin 7A (Sema7A) is essential for the development of the GnRH-1 neuronal system. Loss of Sema7A signaling alters the migration of GnRH-1 neurons, resulting in significantly reduced numbers of these neurons in the adult brain as well as in reduced gonadal size and subfertility. We also show that GnRH-1 cells differentially express the Sema7 receptors β1-integrin and Plexin C1 as a function of their migratory stage, whereas the ligand is robustly expressed along developing olfactory/vomeronasal fibers. Disruption of Sema7A function in vitro inhibits β1-integrin-mediated migration. Analysis of Plexin C1(-/-) mice did not reveal any difference in the migratory process of GnRH-1 neurons, indicating that Sema7A mainly signals through β1-integrin to regulate GnRH-1 cell motility. In conclusion, we have identified Sema7A as a gene implicated in the normal development of the GnRH-1 system in mice and as a genetic marker for the elucidation of some forms of GnRH-1 deficiency in humans.

Molecular basis of blood group expression

Antigen diversity arises from changes at the gene level that range from single nucleotide polymorphisms (SNPs) to intra- and inter-genic exchanges, inversions, insertions, and deletions. Nucleotide changes often result in amino acid difference from the wild-type gene product and with those changes new blood group antigens arise. Alternatively, there is loss of expression altogether, which is deemed the 'null' phenotype. Near complete knowledge of the genetic changes underlying the expression of blood group antigens will lead to the reality that red cell genotyping as a test-of-record. The importance of molecular testing in immunohematology necessitates appropriate training and competency programs to ensure that the highly skilled staff has the appropriate knowledge background. This review summarizes the core mechanisms for gene expression and provides a compilation of the molecular basis for blood group expression.

Roles of semaphorins in the immune and hematopoietic system

Semaphorins were originally discovered in the nervous system, and have been implicated in repulsive axon guidance during the development of nervous system. Semaphorins are also implicated in tumor progression, by affecting adhesion, migration of malignant cells and angiogenesis, and are involved in normal cardiovascular development. Recently, several semaphorins and their receptors are expressed in a variety of lymphoid and myeloid cells, and affect immune cell functions, including cell proliferation, differentiation, chemotaxis, and cytokine production. This review focuses on recent work on the functions of semaphorins in the immune system and autoimmune diseases.

Expression patterns of semaphorin7A and plexinC1 during rat neural development suggest roles in axon guidance and neuronal migration

Background

Although originally identified as embryonic axon guidance cues, semaphorins are now known to regulate multiple, distinct, processes crucial for neuronal network formation including axon growth and branching, dendritic morphology, and neuronal migration. Semaphorin7A (Sema7A), the only glycosylphosphatidylinositol-anchored semaphorin, promotes axon growth in vitro and is required for the proper growth of the mouse lateral olfactory tract in vivo. Sema7A has been postulated to signal through two unrelated receptors, an RGD-dependent α1β1-integrin and a member of the plexin family, plexinC1. β1-integrins underlie Sema7A-mediated axon growth and Sema7A function in the immune system. Sema7A-plexinC1 interactions have also been implicated in immune system function, but the neuronal role of this ligand-receptor pair remains to be explored. To gain further insight into the function(s) of Sema7A and plexinC1 during neural development, we present here a detailed analysis of Sema7A and plexinC1 expression in the developing rat nervous system.

Results

In situ hybridization revealed select expression of Sema7A and plexinC1 in multiple neuronal systems including: the olfactory system, the hypothalamo-hypophysial system, the hippocampus, the meso-diencephalic dopamine system, and the spinal cord. Within these systems, Sema7A and plexinC1 are often expressed in specific neuronal subsets. In general, Sema7A transcript levels increase significantly towards adulthood, whereas plexinC1 expression decreases as development proceeds.

PlexinC1, but not Sema7A, is strongly expressed by distinct populations of migrating neurons. In addition to neuronal expression, Sema7A and plexinC1 transcripts were detected in oligodendrocytes and ependymal cells, respectively.

Conclusion

Sema7A and plexinC1 expression patterns are consistent with these proteins serving both cooperative and separate functions during neural development. The prominent expression of plexinC1 in several distinct populations of migrating neurons suggests a novel role for this plexin family member in neuronal migration.

Semaphorin 7A plays a critical role in TGF-beta1-induced pulmonary fibrosis

Semaphorin (SEMA) 7A regulates neuronal and immune function. In these studies, we tested the hypothesis that SEMA 7A is also a critical regulator of tissue remodeling. These studies demonstrate that SEMA 7A and its receptors, plexin C1 and β1 integrins, are stimulated by transforming growth factor (TGF)-β₁ in the murine lung. They also demonstrate that SEMA 7A plays a critical role in TGF-β₁–induced fibrosis, myofibroblast hyperplasia, alveolar remodeling, and apoptosis. TGF-β₁ stimulated SEMA 7A via a largely Smad 3–independent mechanism and stimulated SEMA 7A receptors, matrix proteins, CCN proteins, fibroblast growth factor 2, interleukin 13 receptor components, proteases, antiprotease, and apoptosis regulators via Smad 2/3–independent and SEMA 7A–dependent mechanisms. SEMA 7A also played an important role in the pathogenesis of bleomycin-induced pulmonary fibrosis. TGF-β₁ and bleomycin also activated phosphatidylinositol 3-kinase (PI3K) and protein kinase B (PKB)/AKT via SEMA 7A–dependent mechanisms, and PKB/AKT inhibition diminished TGF-β₁–induced fibrosis. These observations demonstrate that SEMA 7A and its receptors are induced by TGF-β₁ and that SEMA 7A plays a central role in a PI3K/PKB/AKT-dependent pathway that contributes to TGF-β₁–induced fibrosis and remodeling. They also demonstrate that the effects of SEMA 7A are not specific for transgenic TGF-β₁, highlighting the importance of these findings for other fibrotic stimuli.

Semaphorin 7A initiates T-cell-mediated inflammatory responses through alpha1beta1 integrin

Semaphorins are axon guidance factors that assist growing axons in finding appropriate targets and forming synapses. Emerging evidence suggests that semaphorins are involved not only in embryonic development but also in immune responses. Semaphorin 7A (Sema7A; also known as CD108), which is a glycosylphosphatidylinositol-anchored semaphorin, promotes axon outgrowth through beta1-integrin receptors and contributes to the formation of the lateral olfactory tract. Although Sema7A has been shown to stimulate human monocytes, its function as a negative regulator of T-cell responses has also been reported. Thus, the precise function of Sema7A in the immune system remains unclear. Here we show that Sema7A, which is expressed on activated T cells, stimulates cytokine production in monocytes and macrophages through alpha1beta1 integrin (also known as very late antigen-1) as a component of the immunological synapse, and is critical for the effector phase of the inflammatory immune response. Sema7A-deficient (Sema7a-/-) mice are defective in cell-mediated immune responses such as contact hypersensitivity and experimental autoimmune encephalomyelitis. Although antigen-specific and cytokine-producing effector T cells can develop and migrate into antigen-challenged sites in Sema7a-/- mice, Sema7a-/- T cells fail to induce contact hypersensitivity even when directly injected into the antigen-challenged sites. Thus, the interaction between Sema7A and alpha1beta1 integrin is crucial at the site of inflammation. These findings not only identify a function of Sema7A as an effector molecule in T-cell-mediated inflammation, but also reveal a mechanism of integrin-mediated immune regulation.

The molecular diversity of Sema7A, the semaphorin that carries the JMH blood group antigens

BACKGROUND

Semaphorin 7A (Sema7A), the protein that carries the JMH blood group antigen, is involved in immune responses and plays an important role in axon growth and guidance. Because previous serologic studies on red blood cells (RBCs) suggested a considerable diversity of Sema7A, the present study was designed to elucidate the complex picture of the molecular diversity of this protein.

STUDY DESIGN AND METHODS

The JMH antigen status was determined by serology, flow cytometry, and Western blot. Genomic and transcript analysis of SEMA7A was performed by nucleotide sequencing. Recombinant Sema7A proteins were used for genotype-phenotype correlation. A three-dimensional model of Sema7A was generated for topologic analyses.

RESULTS

Our studies on 44 individuals with unusual JMH phenotypes and their family members revealed that aberrant Sema7A expression can be an inherited or an acquired phenomenon and is based on reduced surface expression or qualitative changes in Sema7A. These different phenotypes are caused by variations of the SEMA7A gene or seem to be generated by autoimmune-related or RBC lineage-specific mechanisms. The variant JMH phenotypes were related to the presence of missense mutations in SEMA7A, predicting amino acid changes in the semaphorin domain of Sema7A. Sequence analysis of the variant SEMA7A alleles revealed mutations affecting codons 207 and 460/461. Topologic analyses showed that Sema7A polymorphisms were prominently located on the top and bottom of the semaphorin domain, suggesting a functional relevance of these sites.

CONCLUSION

These findings provide a basis with which to delineate the various ligand-binding surfaces of Sema7A.

Immune Semaphorins: Increasing Members and Their Diverse Roles

The semaphorin family consists of soluble and membrane-bound proteins originally identified as axonal guidance cues functioning during neuronal development. However, it is becoming increasingly clear that semaphorins play diverse roles in organogenesis, vascular growth, and tumor progression. In addition, emerging evidence indicates that several semaphorins, called "immune semaphorins," play crucial roles also during immune responses. Extensive studies on the immune semaphorins have revealed not only parallels but also differences in the semaphorin functions between the immune and nervous systems, providing unexpected but meaningful insights into the biological activities of these molecules. This chapter focuses on our current understanding of the roles of semaphorins and their receptors in the immune system.

Semaphorin 7A is a negative regulator of T cell responses

Semaphorins play an essential role in axonal guidance, and emerging evidence points to diverse functions of several Semaphorin family members in the immune system. Semaphorin 7A (Sema7A) promotes axonal growth in the central nervous system. Here, we show that Sema7A also plays a critical role in negative regulation of T cell activation and function. T cells deficient in Sema7A exhibit enhanced homeostatic and antigen-induced proliferative response. Moreover, autoreactive Sema7A-deficient T cells mediate aggressive autoimmune disease. The deficiency in Sema7A leads to defective TCR downmodulation and T cell hyperresponsiveness. These results demonstrate an important role of Sema7A in limiting autoimmune responses and add to growing evidence of shared signaling pathways used by the immune and nervous systems.

Semaphorins: a new class of immunoregulatory molecules

The immune and nervous systems play distinct roles in maintaining physiological homeostasis. Recent data indicates that these systems influence one another and share many proteins and pathways that are essential for their normal function and development. Molecules originally shown to be critical for the development of proper immune responses have recently been found to function in the nervous system. Conversely, neuronal guidance cues can modulate immune functions. Although semaphorins were originally identified as axon guidance factors active during neuronal development, several recent studies have identified indispensable functions for these molecules in the immune system. This review provides an overview of the rapidly emerging functions of semaphorins and their receptors in the immune system.

Association study of semaphorin 7a (sema7a) polymorphisms with bone mineral density and fracture risk in postmenopausal Korean women

Bone mineral density (BMD), the major factor determining bone strength, is closely related to osteoporotic fracture risk and is determined largely by multiple genetic factors. Semaphorin 7a (SEMA7A), a recently described member of the semaphorin family, has been shown to play a critical role in the activation of monocyte/macrophages that share progenitors with bone-resorbing osteoclasts and thus might contribute to osteoclast development. In the present study, we directly sequenced the SEMA7A gene in 24 Korean individuals, and identified 15 sequence variants. Five polymorphisms (+15667G > A, +15775C > G, +16285C > T, +19317C > T, +22331A > G) were selected and genotyped in postmenopausal Korean women (n = 560) together with measurement of the areal BMD (g/cm2) of the anterior-posterior lumbar spine and the non-dominant proximal femur using dual-energy X-ray absorptiometry. We found that polymorphisms of the SEMA7A gene were associated with the BMD of the lumbar spine and femoral neck. SEMA7A + 15775C > G and SEMA7A+22331A > G were associated with low BMD of the femoral neck (P = 0.02) and lumbar spine (P = 0.04) in a recessive model. SEMA7A-ht4 also showed an association with risk of vertebral fracture (OR = 1.87-1.93, P = 0.02-0.03). Our results suggest that variations in SEMA7A may play a role in decreased BMD and risk of vertebral fracture.

Expression and function of semaphorin 7A in bone cells

BACKGROUND INFORMATION

Sema-7A is a glycosylphosphatidylinositol-anchored semaphorin that was first identified in the immune system. It is a member of a large family of proteins involved in axon guidance signalling. Sema-7A is expressed in the myeloid and the lymphoid lineage and seems to be involved in cytokine expression and chemotaxy through its receptor Plexin C1. However, it can promote axon outgrowth, acting through a beta1 subunit-containing integrin receptor.

RESULTS AND CONCLUSIONS

In the present study, we have investigated its regulation and function in bone cells. Semiquantitative reverse transcriptase-PCR demonstrated that Sema-7A mRNA is present during all stages of osteoblast differentiation and maturation in mouse calvaria cells and in MC3T3 cell line in vitro. Its expression is also regulated during primary osteoclast differentiation in vitro. We report that Sema-7A is capable of increasing the migration of MC3T3 cells and that this process is mediated by the mitogen-activated protein kinase pathway in osteoblasts, probably through the integrin subunit beta1. Moreover, the addition of recombinant soluble Sema-7A to the culture enhances osteoclast fusion. These findings indicate for the first time the possible involvement of Sema-7A in bone cell differentiation.

Mécanismes cellulaires et moléculaires de la croissance axonale

INTRODUCTION

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

STATE OF THE ART

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

PERSPECTIVES

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

CONCLUSION

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

Cocaine-induced expression changes of axon guidance molecules in the adult rat brain

Administration of drugs of abuse induces strong molecular adaptations and plasticity within the mesolimbic dopamine (DA) system, a pathway essential for reward-seeking behavior. Little is known about the specific targets involved in this neuroadaptation process, but there are indications that cocaine and other drugs of abuse share the ability to alter the morphology of neuronal dendrites and spines, the primary site of excitatory synapses in the brain. Axon guidance molecules, the very molecular cues that regulate the formation of axon-target connections during development, may mediate these alterations. To test this hypothesis, we investigated mRNA expression changes of 39 axon guidance molecules, including 17 Semaphorins, 12 Ephs, 8 Ephrins, and 2 neuropilins in the mesolimbic dopamine system of cocaine-treated animals under different paradigms by mean of DNA-Microarray and quantitative real-time PCR. In all cases, strong changes in gene expression are observed, yielding to up or downregulation of these axon guidance molecules. Our data suggest that cocaine treatment induces activation of a complex program of synaptic rearrangements, which may partly recapitulate the plastic changes occurring during development, and may underlie the important neuroplastic adaptations that occur in the reward- and memory-related brain centers following drug action. We conclude that in some brain regions, exposure to psychomotor-stimulant drugs produce expression changes in axon guidance molecules, which may contribute to cognitive deficits associated with drug abuse.

Semaphorin 7A promotes axon outgrowth through integrins and MAPKs

Striking parallels exist between immune and nervous system cellular signalling mechanisms. Molecules originally shown to be critical for immune responses also serve neuronal functions, and similarly neural guidance cues can modulate immune function. We show here that semaphorin 7A (Sema7A), a membrane-anchored member of the semaphorin family of guidance proteins previously known for its immunomodulatory effects, can also mediate neuronal functions. Unlike many other semaphorins, which act as repulsive guidance cues, Sema7A enhances central and peripheral axon growth and is required for proper axon tract formation during embryonic development. Unexpectedly, Sema7A enhancement of axon outgrowth requires integrin receptors and activation of MAPK signalling pathways. These findings define a previously unknown biological function for semaphorins, identify an unexpected role for integrins and integrin-dependent intracellular signalling in mediating semaphorin responses, and provide a framework for understanding and interfering with Sema7A function in both immune and nervous systems.

Sema7A is a potent monocyte stimulator

Sema7A is a recently described member of the semaphorin family that is associated with the cell surface via a glycophosphatidylinositol linkage. This study examined the mRNA expression and biological properties of this protein. Although the expression of Sema7A was demonstrated in lymphoid and myeloid cells, no stimulation of cytokine production or proliferation was evident in B or T cells. In contrast, Sema7A is an extremely potent monocyte activator, stimulating chemotaxis at 0.1 pm and inflammatory cytokine production (interleukin-1 (IL-1beta), tumour necrosis factor-alpha (TNF-alpha), IL-6 and IL-8) and superoxide release at 1-10 pm. Sema7A is less effective at stimulating neutrophils. Sema7A also significantly increases granulocyte-macrophage colony-stimulating factor (GM-CSF) production from monocytes but has no consistent effect on IL-10, IL-12 or IL-18. Sema7A can also induce monocytes toward a dendritic cell morphology. Sema7A is expressed in monocytes and probably released through proteolysis and acts as a very potent autocrine activator of these cells.

Vaccinia virus semaphorin A39R is a 50-55 kDa secreted glycoprotein that affects the outcome of infection in a murine intradermal model

Vaccinia virus (VV) protein A39R has amino acid similarity to the extracellular domain of a glycosylphosphatidylinositol-linked cell surface semaphorin (SEMA7A/CDw108) that has an immunological expression profile and binding properties, thereby implicating A39R as an immunomodulator. Previously, a closely related A39R protein expressed by ectromelia virus was shown to induce cytokine production and up-regulate ICAM-1 expression in mouse monocytes in vitro. In this study, we show that the A39R gene of VV strain Copenhagen (COP) encodes a 50-55 kDa secreted glycoprotein and is expressed late during infection. The A39R protein was secreted by eight of 15 strains of VV, but not by strain Western Reserve (WR). To analyse the VV A39R function, several recombinant viruses were made, including an A39R deletion mutant of VV COP and a WR mutant containing the A39R sequence from COP. Loss of the gene from COP did not affect virus growth in vitro, or VV virulence in a mouse intranasal model, and had only a slight effect on lesion size in an intradermal model. In contrast, expression of COP A39R by VV WR was associated with an increase in the severity and persistence of skin lesions after intradermal infection of mice. Finally, a histological examination of mouse skin infected with recombinant viruses suggested that A39R has direct or indirect pro-inflammatory properties.

Structural and functional diversity of blood group antigens

Biochemical and molecular genetic studies have revealed that blood group antigens are present on cell surface molecules of wide structural diversity, including carbohydrate epitopes on glycoproteins and/or glycolipids, and peptide antigens on proteins inserted within the membrane via single or multi-pass transmembrane domains, or via glycosylphosphatidylinositol linkages. These studies have also shown that some blood group antigens are carried by complexes consisting of several membrane components which may be lacking or severely deficient in rare blood group 'null' phenotypes. In addition, although all blood group antigens are serologically detectable on red blood cells (RBCs), most of them are also expressed in non-erythroid tissues, raising further questions on their physiological function under normal and pathological conditions. In addition to their structural diversity, blood group antigens also possess wide functional diversity, and can be schematically subdivided into five classes: i) transporters and channels; ii) receptors for ligands, viruses, bacteria and parasites; iii) adhesion molecules; iv) enzymes; and v) structural proteins. The purpose of this review is to summarize recent findings on these molecules, and in particular to illustrate the existing structure-function relationships.

Molecular basis of semaphorin-mediated axon guidance

The semaphorin family of proteins constitute one of the major cues for axonal guidance. The prototypic member of this family is Sema3A, previously designated semD/III or collapsin-1. Sema3A acts as a diffusible, repulsive guidance cue in vivo for the peripheral projections of embryonic dorsal root ganglion neurons. Sema3A binds with high affinity to neuropilin-1 on growth cone filopodial tips. Although neuropilin-1 is required for Sema3A action, it is incapable of transmitting a Sema3A signal to the growth cone interior. Instead, the Sema3A/neuropilin-1 complex interacts with another transmembrane protein, plexin, on the surface of growth cones. Certain semaphorins, other than Sema3A, can bind directly to plexins. The intracellular domain of plexin is responsible for initiating the signal transduction cascade leading to growth cone collapse, axon repulsion, or growth cone turning. This intracellular cascade involves the monomeric G-protein, Rac1, and a family of neuronal proteins, the CRMPs. Rac1 is likely to be involved in semaphorin-induced rearrangements of the actin cytoskeleton, but how plexin controls Rac1 activity is not known. Vertebrate CRMPs are homologous to the Caenorhabditis elegans unc-33 protein, which is required for proper axon morphology in worms. CRMPs are essential for Sema3A-induced, neuropilin-plexin-mediated growth cone collapse, but the molecular interactions of growth cone CRMPs are not well defined. Mechanistic aspects of plexin-based signaling for semaphorin guidance cues may have implications for other axon guidance events and for the basis of growth cone motility.

A high-resolution radiation hybrid map of the proximal portion of mouse chromosome 5

Radiation hybrid (RH) mapping of the mouse genome provides a useful tool in the integration of existing genetic and physical maps, as well as in the ongoing effort to generate a dense map of expressed sequence tags. To facilitate functional analysis of mouse Chromosome 5, we have constructed a high-resolution RH map spanning 75 cM of the chromosome. During the course of these studies, we have developed RHBase, an RH data management program that provides data storage and an interface to several RH mapping programs and databases. We have typed 95 markers on the T31 RH panel and generated an integrated map, pooling data from several sources. The integrated RH map ranges from the most proximal marker, D5Mit331 (Chromosome Committee offset, 3 cM), to D5Mit326, 74.5 cM distal on our genetic map (Chromosome Committee offset, 80 cM), and consists of 138 markers, including 89 simple sequence length polymorphic markers, 11 sequence-tagged sites generated from BAC end sequence, and 38 gene loci, and represents average coverage of approximately one locus per 0.5 cM with some regions more densely mapped. In addition to the RH mapping of markers and genes previously localized on mouse Chromosome 5, this RH map places the alpha-4 GABA(A) receptor subunit gene (Gabra4) in the central portion of the chromosome, in the vicinity of the cluster of three other GABA(A) receptor subunit genes (Gabrg1-Gabra2-Gabrb1). Our mapping effort has also defined a new cluster of four genes in the semaphorin gene family (Sema3a, Sema3c, Sema3d, and Sema3e) and the Wolfram syndrome gene (Wfs1) in this region of the chromosome.

CDw108 expression during T-cell development

We recently reported a gene encoding the human CDw108, a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein that is preferentially expressed on activated T lymphocytes and erythrocytes. The present study investigated the expression of CDw108 on various tissues and cells, particularly on T cells during development. The murine CDw108 cDNA was cloned initially, and it was highly homologous to the human CDw108 (88.0% or 89.3% similarity at the nucleotide or amino acid level, respectively) or identical to the murine semaphorin K1/Sema7A. The CDw108 mRNA was demonstrated in a few tissues including thymus and brain with the highest expression coming on day 7 in whole embryo followed by relatively consistent expression during development. Cell-surface expression of the CDw108 during T-cell development was further examined by flow cytometry in the human umbilical cord blood and thymus. It was preferentially expressed on a CD34+ stem cell population of umbilical cord blood, and CD3dull CD34+/- CD117 (c-kit)+ CD4bright CDbright cells in the thymus that are involved in the stage of positive selection. These results suggest the contribution of CDw108 in T-cell development, especially in the stage of positive selection in the thymus.

The genome of fowlpox virus

Here we present the genomic sequence, with analysis, of a pathogenic fowlpox virus (FPV). The 288-kbp FPV genome consists of a central coding region bounded by identical 9.5-kbp inverted terminal repeats and contains 260 open reading frames, of which 101 exhibit similarity to genes of known function. Comparison of the FPV genome with those of other chordopoxviruses (ChPVs) revealed 65 conserved gene homologues, encoding proteins involved in transcription and mRNA biogenesis, nucleotide metabolism, DNA replication and repair, protein processing, and virion structure. Comparison of the FPV genome with those of other ChPVs revealed extensive genome colinearity which is interrupted in FPV by a translocation and a major inversion, the presence of multiple and in some cases large gene families, and novel cellular homologues. Large numbers of cellular homologues together with 10 multigene families largely account for the marked size difference between the FPV genome (260 to 309 kbp) and other known ChPV genomes (178 to 191 kbp). Predicted proteins with putative functions involving immune evasion included eight natural killer cell receptors, four CC chemokines, three G-protein-coupled receptors, two beta nerve growth factors, transforming growth factor beta, interleukin-18-binding protein, semaphorin, and five serine proteinase inhibitors (serpins). Other potential FPV host range proteins included homologues of those involved in apoptosis (e.g., Bcl-2 protein), cell growth (e.g., epidermal growth factor domain protein), tissue tropism (e.g., ankyrin repeat-containing gene family, N1R/p28 gene family, and a T10 homologue), and avian host range (e.g., a protein present in both fowl adenovirus and Marek's disease virus). The presence of homologues of genes encoding proteins involved in steroid biogenesis (e.g., hydroxysteroid dehydrogenase), antioxidant functions (e.g., glutathione peroxidase), vesicle trafficking (e.g., two alpha-type soluble NSF attachment proteins), and other, unknown conserved cellular processes (e.g., Hal3 domain protein and GSN1/SUR4) suggests that significant modification of host cell function occurs upon viral infection. The presence of a cyclobutane pyrimidine dimer photolyase homologue in FPV suggests the presence of a photoreactivation DNA repair pathway. This diverse complement of genes with likely host range functions in FPV suggests significant viral adaptation to the avian host.

Shared resources between the neural and immune systems: semaphorins join the ranks

The semaphorin family of molecules contains members known to deliver guidance cues to migrating axons during development. Semaphorins also have been identified on the surface of hematopoietic cells and, interestingly, in the genomes of certain lytic viruses. Recent studies indicate that semaphorins bind with high affinity to at least two different receptor families and are biologically active on immune cells as well as neuronal cells.

Cloning, expression, and genetic mapping of Sema W, a member of the semaphorin family

The semaphorins comprise a large family of membrane-bound and secreted proteins, some of which have been shown to function in axon guidance. We have cloned a transmembrane semaphorin, Sema W, that belongs to the class IV subgroup of the semaphorin family. The mouse and rat forms of Sema W show 97% amino acid sequence identity with each other, and each shows about 91% identity with the human form. The gene for Sema W is divided into 15 exons, up to 4 of which are absent in the human cDNAs that we sequenced. Unlike many other semaphorins, Sema W is expressed at low levels in the developing embryo but was found to be expressed at high levels in the adult central nervous system and lung. Functional studies with purified membrane fractions from COS7 cells transfected with a Sema W expression plasmid showed that Sema W has growth-cone collapse activity against retinal ganglion-cell axons, indicating that vertebrate transmembrane semaphorins, like secreted semaphorins, can collapse growth cones. Genetic mapping of human SEMAW with human/hamster radiation hybrids localized the gene to chromosome 2p13. Genetic mapping of mouse Semaw with mouse/hamster radiation hybrids localized the gene to chromosome 6, and physical mapping placed the gene on bacteria artificial chromosomes carrying microsatellite markers D6Mit70 and D6Mit189. This localization places Semaw within the locus for motor neuron degeneration 2, making it an attractive candidate gene for this disease.

Molecular cloning and expression of murine homologue of semaphorin K1 gene

We have cloned a mouse homologue of semaphorin K1 cDNA. Sequence analysis has predicted that the mouse semaphorin K1 encodes a 664 amino acid protein and contains a characteristic semaphorin domain in the N terminus followed by an immunoglobulin-like domain and a glycosylphosphatidylinositol anchoring sequence in the C terminus. In addition, this molecule has a RGD motif within the semaphorin domain. There is a high degree of amino acid identity between mouse and human semaphorin K1(89.5%). Northern blot analysis has revealed that the highest expression is detected in the nervous system and the expression level increases during neural development. Moreover, moderate expression is also detected in the immune system such as thymus and spleen. Thus, semaphorin K1 may play essential roles not only in the nervous system but also in the immune system.