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.     

Integrins team up with tyrosine kinase receptors and plexins to control angiogenesis

PURPOSE OF REVIEW: Understanding the role of integrins in the formation of vascular bed is important for designing new therapeutic approaches to ameliorate or inhibit pathological vascularization. Besides regulating cell adhesion and migration, integrins dynamically participate in a network with soluble molecules and their receptors. This study summarizes recent progress in the understanding of the reciprocal interactions between integrins, tyrosine kinase, and semaphorin receptors. RECENT FINDINGS: During angiogenic remodeling, endothelial cells that line blood vessel walls dynamically modify their integrin-mediated adhesive contacts with the surrounding extracellular matrix. During angiogenesis, opposing autocrine and paracrine loops of growth factors and semaphorins regulate endothelial integrin activation and function through tyrosine kinase receptors and the neuropilin/plexins system. Moreover, proangiogenic and antiangiogenic factors can directly bind integrins and regulate endothelial cell behavior. Studies describing these intense research areas are discussed. SUMMARY: Alteration in the balance between the angiogenic growth factors and semaphorins results in an impairment of integrin functions and could account for cardiovascular malformation and structural and functional abnormalities of the tumor vasculature.     

The ErbB receptor tyrosine family as signal integrators

ErbB receptor tyrosine kinases (RTKs) and their ligands have important roles in normal development and in human cancer. Among the ErbB receptors only ErbB2 has no direct ligand; however, ErbB2 acts as a co-receptor for the other family members, promoting high affinity ligand binding and enhancement of ligand-induced biological responses. These characteristics demonstrate the central role of ErbB2 in the receptor family, which likely explains why it is involved in the development of many human malignancies, including breast cancer. ErbB RTKs also function as signal integrators, cross-regulating different classes of membrane receptors including receptors of the cytokine family. Cross-regulation of ErbB RTKs and cytokines receptors represents another mechanism for controlling and enhancing tumor cell proliferation.     

Semaphorin3A signaling controls Fas (CD95)-mediated apoptosis by promoting Fas translocation into lipid rafts

Semaphorins and their receptors (plexins) have pleiotropic biologic functions, including regulation of immune responses. However, the role of these molecules inside the immune system and the signal transduction mechanism(s) they use are largely unknown. Here, we show that Semaphorin3A (Sema3A) triggers a proapoptotic program that sensitizes leukemic T cells to Fas (CD95)-mediated apoptosis. We found that Sema3A stimulation provoked Fas translocation into lipid raft microdomains before binding with agonistic antibody or FasL (CD95L). Disruption of lipid rafts reduced sensitivity to Fas-mediated apoptosis in the presence of Sema3A. Furthermore, we show that plexin-A1, together with Sema3A-binding neuropilin-1, was rapidly incorporated into membrane rafts after ligand stimulation, resulting in the transport of actin-linking proteins into Fas-enriched rafts. Cells expressing a dominant-negative mutant of plexin-A1 did not show Fas clustering and apoptosis on Sema3A/Fas costimulation. This work identifies a novel biologic function of semaphorins and presents an unexpected signaling mechanism linking semaphorin to the tumor necrosis factor family receptors.     

Sulfated polysaccharides identified as inducers of neuropilin-1 internalization and functional inhibition of VEGF165 and semaphorin3A

Neuropilin-1 (NRP1) and NRP2 are cell surface receptors shared by class 3 semaphorins and vascular endothelial growth factor (VEGF). Ligand interaction with NRPs selects the specific signal transducer, plexins for semaphorins or VEGF receptors for VEGF, and promotes NRP internalization, which effectively shuts down receptor-mediated signaling by a second ligand. Here, we show that the sulfated polysaccharides dextran sulfate and fucoidan, but not others, reduce endothelial cell-surface levels of NRP1, NRP2, and to a lesser extent VEGFR-1 and VEGFR-2, and block the binding and in vitro function of semaphorin3A and VEGF(165). Administration of fucoidan to mice reduces VEGF(165)-induced angiogenesis and tumor neovascularization in vivo. We find that dextran sulfate and fucoidan can bridge the extracellular domain of NRP1 to that of the scavenger receptor expressed by endothelial cells I (SREC-I), and induce NRP1 and SREC-I coordinate internalization and trafficking to the lysosomes. Overexpression of SREC-I in SREC-I-negative cells specifically reduces cell-surface levels of NRP1, indicating that SREC-I mediates NRP1 internalization. These results demonstrate that engineered receptor internalization is an effective strategy for reducing levels and function of cell-surface receptors, and identify certain sulfated polysaccharides as "internalization inducers."     

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.     

Benzodiazepines and anterior pituitary function

Benzodiazepines (BDZ) are one of the most prescribed classes of drugs because of their marked anxiolytic, anticonvulsant, muscle relaxant and hypnotic effects. The pharmacological actions of BDZ depend on the activation of 2 specific receptors. The central BDZ receptor, present in several areas of the central nervous system (CNS), is a component of the GABA-A receptor, the activation of which increases GABAergic neurotransmission and is followed by remarkable neuroendocrine effects. The peripheral benzodiazepine receptors (PBR), structurally and functionally different from the GABA-A receptor, have been shown in peripheral tissues but also in the CNS, in both neurones and glial cells, and in the pituitary gland. BDZ receptors bind to a family of natural peptides called endozepines, firstly isolated from neurons and glial cells in the brain and then in several peripheral tissues as well. Endozepines modulate several central and peripheral biological activities, including some neuroendocrine functions and synthetic BDZ are likely to mimic them, at least partially. BZD, especially alprazolam (AL), possess a clear inhibitory influence on the activity of the HPA axis in both animals and humans. This effect seems to be mediated at the hypothalamic and/or suprahypothalamic level via suppression of CRH. The strong negative influence of AL on hypothalamicpituitary-adrenal (HPA) axis agrees with its peculiar efficacy in the treatment of panic disorders and depression. BZD have also been shown to increase GH secretion via mechanisms mediated at the hypothalamic or supra-hypothalamic level, though a pituitary action cannot be ruled out. Besides the impact on HPA and somatotrope function, BDZ also significantly affect the secretion of other pituitary hormones, such as gonadotropins and PRL, probably acting through GABAergic mediation in the hypothalamus and/or in the pituitary gland. In all, BDZ are likely to represent a useful tool to investigate GABAergic activity and clarify its role in the neuroendocrine control of anterior pituitary function; their usefulness probably overrides what had been supposed before.     

The role of semaphorins and their receptors in platelets: Lessons learned from neuronal and immune synapses

During thrombus formation, activated platelets come into close and increasingly stable contact with each other. This produces a microenvironment in which soluble agonists can accumulate, and proteins on the surface of adjacent platelets can directly interact with each other, potentially modulating subsequent thrombus growth and stability. In some ways, this microenvironment resembles the synapses that support signal propagation between neurons and the exchange of information between T-cells, B-cells, and dendritic cells. Drawing on this analogy, this brief review discusses the role of semaphorins and their receptors in platelets, two protein families that have previously been defined by their role at cell:cell contacts, in both the developing nervous system and adaptive immunity.     

The role of semaphorins and their receptors in platelets: Lessons learned from neuronal and immune synapses

During thrombus formation, activated platelets come into close and increasingly stable contact with each other. This produces a microenvironment in which soluble agonists can accumulate, and proteins on the surface of adjacent platelets can directly interact with each other, potentially modulating subsequent thrombus growth and stability. In some ways, this microenvironment resembles the synapses that support signal propagation between neurons and the exchange of information between T-cells, B-cells, and dendritic cells. Drawing on this analogy, this brief review discusses the role of semaphorins and their receptors in platelets, two protein families that have previously been defined by their role at cell:cell contacts, in both the developing nervous system and adaptive immunity.     

Semaphorin 6D regulates the late phase of CD4+ T cell primary immune responses

The semaphorin and plexin family of ligand and receptor proteins provides important axon guidance cues required for development. Recent studies have expanded the role of semaphorins and plexins in the regulation of cardiac, circulatory and immune system function. Within the immune system, semaphorins and plexins regulate cell–cell interactions through a complex network of receptor and ligand pairs. Immune cells at different stages of development often express multiple semaphorins and plexins, leading to multivariate interactions, involving more than one ligand and receptor within each functional group. Because of this complexity, the significance of semaphorin and plexin regulation on individual immune cell types has yet to be fully appreciated. In this work, we examined the regulation of T cells by semaphorin 6D. Both in vitro and in vivo T cell stimulation enhanced semaphorin 6D expression. However, semaphorin 6D was only expressed by a majority of T cells during the late phases of activation. Consequently, the targeted disruption of semaphorin 6D receptor–ligand interactions inhibited T cell proliferation at late but not early phases of activation. This proliferation defect was associated with reduced linker of activated T cells protein phosphorylation, which may reflect semaphorin 6D regulation of c-Abl kinase activity. Semaphorin 6D disruption also inhibited expression of CD127, which is required during the multiphase antigen-presenting cell and T cell interactions leading to selection of long-lived lymphocytes. This work reveals a role for semaphorin 6D as a regulator of the late phase of primary immune responses.     

Le brain-derived neurotrophic factor (BDNF) : rôle de cette neurotrophine dans la physiopathologie cardiovasculaire

Neurotrophic-factors research is dominated by neurotrophins (NT): a family of polypeptides which includes molecules such as Nerve Growth Factor (NGF) and the Brain-Derived Neurotrophic Factor (BDNF). They are homodimeric polypeptides. NTs interact with classes of receptors on responsive cells: protein-tyrosine kinase-type receptors (Trk family). It is well established that the levels of NT determine the balance between cell survival and apoptosis during neural development. Recently, it has been shown that BDNF played a role in the etiology of some cardiovascular diseases: induction of angiogenesis in ischemic issues. Plasma BDNF was increased in the circulation in patients with unstable angina. BDNF was expressed in atheromatous intima and adventitia in human coronary artery. Our own studies suggest that BDNF serum levels in patients with acute myocardial infarction or under cardiopulmonary bypass could related to platelet activation, oxidative stress and inflammatory response. Thus, investigations of this new factor: BDNF will help to better understand vascular development and may lead to new therapeutic strategies for some cardiovascular diseases.     

Imidazoline antihypertensive drugs: selective i(1) -imidazoline receptors activation

Involvement of imidazoline receptors (IR) in the regulation of vasomotor tone as well as in the mechanism of action of some centrally acting antihypertensives has received tremendous attention. To date, pharmacological studies have allowed the characterization of three main imidazoline receptor classes, the I(1) -imidazoline receptor which is involved in central inhibition of sympathetic tone to lower blood pressure, the I(2) -imidazoline receptor which is an allosteric binding site of monoamine oxidase B (MAO-B), and the I(3) -imidazoline receptor which regulates insulin secretion from pancreatic β-cells. All three imidazoline receptors represent important targets for cardiovascular research. The hypotensive effect of clonidine-like centrally acting antihypertensives was attributed both to α(2) -adrenergic receptors and nonadrenergic I(1) -imidazoline receptors, whereas their sedative action involves activation of only α(2) -adrenergic receptors located in the locus coeruleus. Since more selective I(1) -imidazoline receptors ligands reduced incidence of typical side effects of other centrally acting antihypertensives, there is significant interest in developing new agents with higher selectivity and affinity for I(1) -imidazoline receptors. The selective imidazoline receptors agents are also more effective in regulation of body fat, neuroprotection, inflammation, cell proliferation, epilepsy, depression, stress, cell adhesion, and pain. New agonists and antagonists with high selectivity for imidazoline receptor subtypes have been recently developed. In the present review we provide a brief update to the field of imidazoline research, highlighting some of the chemical diversity and progress made in the theoretical studies of imidazoline receptor ligands.     

Comparative physiology of the piscine natriuretic peptide system

The natriuretic peptide (NP) family is a seemingly ubiquitous sodium and volume reducing endocrine system of predominantly cardiac origin. Members of the NP system include ANP, BNP, CNP, VNP, their guanylate cyclase (GC)-linked receptors (NPR-A and NPR-B), and clearance receptor (NPR-C). Through the activation of their membrane-bound GC receptors, these small peptides modulate cellular functions that affect both salt and water balance. The elucidation of piscine NP sequences, structure, and functions has steadily advanced over the past 15 years spearheaded by research from Dr. Yoshio Takei’s laboratory. The development of these homologous NPs has led to extensive research into both the evolutionary and physiological significance of NPs in fishes. One outcome has been the development of two seemingly disparate hypotheses of NP function; a role in salt excretion, the osmoregulatory hypothesis, versus a role in protecting the heart, the cardioprotective hypotheses. In the osmoregulatory hypothesis NPs are released in response to elevated ambient salinity and inhibit drinking and intestinal uptake of salt, thereby effectively reducing plasma sodium levels. In contrast, the cardioprotective theory depicts NPs acting to prevent debilitating cardiodilation from an excess of either venous or arterial pressure through vasodilation and a reduction of blood volume. These seemingly distinct hypotheses may be elements of a more general regulatory system and certainly require further investigation. Undoubtedly their resolution will not only give us a better perspective of the evolutionary basis of the NP system but will provide us with a greater appreciation of salt and water homeostasis in vertebrates.     

MicroRNAs bind to Toll-like receptors to induce prometastatic inflammatory response

MicroRNAs (miRNAs) are small noncoding RNAs, 19-24 nucleotides in length, that regulate gene expression and are expressed aberrantly in most types of cancer. MiRNAs also have been detected in the blood of cancer patients and can serve as circulating biomarkers. It has been shown that secreted miRNAs within exosomes can be transferred from cell to cell and can regulate gene expression in the receiving cells by canonical binding to their target messenger RNAs. Here we show that tumor-secreted miR-21 and miR-29a also can function by another mechanism, by binding as ligands to receptors of the Toll-like receptor (TLR) family, murine TLR7 and human TLR8, in immune cells, triggering a TLR-mediated prometastatic inflammatory response that ultimately may lead to tumor growth and metastasis. Thus, by acting as paracrine agonists of TLRs, secreted miRNAs are key regulators of the tumor microenvironment. This mechanism of action of miRNAs is implicated in tumor-immune system communication and is important in tumor growth and spread, thus representing a possible target for cancer treatment.     

ABC de los «Toll-like receptors»: relación con el desarrollo y progresión de enfermedades autoinmunes

The Toll-like receptor family is an important group of pattern-recognition receptors whose ligands include a wide range of molecules with strong adjuvant activity (such as lipopolysaccharide, lipopeptides and bacterial DNA). These ligands can activate dendritic cells, macrophages and other antigen presenting cells that allow the effective presentation of microbial antigens to cells of the adaptive immune system. Nowadays, the identification and characterization of endogenous ligands for these receptors has provided a novel perspective for examining the etiology of some autoimmune diseases. Instead of being considered as an aberrant response to host antigens by the adaptive immune system, autoimmunity can be viewed as arising from a response to exogenous or endogenous ligands by the innate immune system, at least in some cases. This review summarizes recently published data that indicate an important connection between DNA-and RNA-containing immune complexes, activation of Toll-like receptors, production of type I interferons (INF-α, INF-β) and the development of some systemic autoimmune diseases.     

Semaphorin 4D cooperates with VEGF to promote angiogenesis and tumor progression

The semaphorins and plexins comprise a family of cysteine-rich proteins implicated in control of nerve growth and development and regulation of the immune response. Our group and others have found that Semaphorin 4D (SEMA4D) and its receptor, Plexin-B1, play an important role in tumor-induced angiogenesis, with some neoplasms producing SEMA4D in a manner analogous to vascular endothelial growth factor (VEGF) in order to attract Plexin-B1-expressing endothelial cells into the tumor for the purpose of promoting growth and vascularity. While anti-VEGF strategies have been the focus of most angiogenesis inhibition research, such treatment can lead to upregulation of pro-angiogenic factors that can compensate for the loss of VEGF, eventually leading to failure of therapy. Here, we demonstrate that SEMA4D cooperates with VEGF to promote angiogenesis in malignancies and can perform the same function in a setting of VEGF blockade. We also show the potential value of inhibiting SEMA4D/Plexin-B1 signaling as a complementary mechanism to anti-VEGF treatment, particularly in VEGF inhibitor-resistant tumors, suggesting that this may represent a novel treatment for some cancers.     

Growth hormone-releasing hormone and growth hormone secretagogue-receptor ligands: focus on reproductive system

Growth hormone-releasing hormone (GHRH) and somatostatin are the most important hypothalamic neurohormones controlling growth hormone (GH) secretion. Several neurotransmitters and neuropeptides also play an important role in the control of GH secretion, mainly acting via modulation of GHRH and somatostatin. In the past two decades, particular attention has been given to a new family of substances showing a strong GH-releasing effect: GH secretagogues (GHSs). GHSs increase GH secretion in a dose-and age-related manner after iv and even oral administration. The endocrine effects of GHSs, are not fully specific for GH; they show, in fact, prolactin- (PRL), adenocorticotropic hormone- and cortisol-releasing effects. Specific GHS receptors are present in both the central nervous system and peripheral tissues, where they mediate several extraendocrine effects of GHSs. The isolation of these “orphan” receptors suggested the existence of an endogenous GHS-like ligand that could be represented by a recently discovered gastric peptide, named ghrelin. The interaction between GHSs and GHRH at the central level and in the pituitary gland, but not at peripheral level, has clearly been shown. Because GHRH and GHS receptors share the same localization in some peripheral tissues, they may have some interactions even at this level.     

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

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

The dependence receptor DCC requires lipid raft localization for cell death signaling

DCC (deleted in colorectal cancer) is a putative tumor suppressor gene whose expression is lost in numerous cancers. DCC also encodes the main receptor for the neuronal navigation cue netrin-1. It has been shown that DCC belongs to the so-called family of dependence receptors. Such receptors induce apoptosis when their ligand is absent, thus conferring a state of cellular dependence on ligand availability. We recently proposed that DCC is a tumor suppressor because it induces the death of tumor cells that grow in settings of ligand unavailability. Moreover, it seems that the DCC/netrin-1 pair may also regulate neuron survival during nervous system development. However, the mechanisms by which DCC triggers cell death are still unknown. We show here that the localization of DCC to lipid rafts is a prerequisite for its proapoptotic activity, both in immortalized cells and in primary neurons. The presence of DCC in lipid rafts probably allows the formation of an adequate submembrane complex, because the interaction of caspase-9 with DCC is inhibited by the disorganization of lipid rafts. Thus, dependence receptors may require lipid raft localization for cell death signaling.     

Insights into the functions of type 1 (AT1) angiotensin II receptors provided by gene targeting.

The renin-angiotensin system (RAS) has a wide range of actions in biological processes ranging from development and reproduction to cardiovascular and renal functions. Most of these actions are mediated by the octapeptide hormone angiotensin II. The identified family of angiotensin II receptors is divided into two pharmacological classes: type 1 (AT1) and type 2 (AT2). The classically recognized actions of the RAS are primarily mediated by the AT1 subtype of angiotensin receptors, and these receptors are the targets of a new class of anti-hypertensive agents. In recent years, our understanding of the physiological functions of AT1 receptors has been advanced through the use of gene-targeting technology. In this review, we will summarize the emerging picture of AT1 receptor functions that has been provided by gene-targeting experiments.