Semaphorins: Their Dual Role in Regulating Immune-Mediated Diseases
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Nonspecific low back pain is closely associated with afferent nerve ingrowth into degenerated IVDs and increasing the inflammatory response. Members of the class 3 semaphorins signal their response through two prominent receptors; the NRP (Neuropilin-1) and the Plexin A. Sema3A (Semaphorin3A) is primarily known for their role in modulating neuronal survival as well as neurite outgrowth and guidance via regulation of Sema3A-NRP-1-plexinA signal pathway. Also, sema3A is shown to be conductive to innervate the inner painful degenerated IVDs (Intervertebral discs). Furthermore, sema3A is thought to act as a barrier to endothelial cells survival and migration on vascular endothelial growth factor (VEGF) and inhibition of KLF5-induced (Kruppel-like factor 5) inflammatory mediators within degenerated IVDs. Therefore, Sema3A produce a new perspective of dual-action therapeutic agent for attenuating the regulator of innervation and angiogenesis into degenerated IVDs and inhibition of KLF5-induced inflammation.
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Class-3 semaphorins are secreted axon guidance factors. Some of these semaphorins have recently been characterized as suppressors of tumor progression. To determine if class-3 semaphorins can be used to inhibit the development of glioblastoma-multiforme tumors, we expressed recombinant sema-3A, 3B, 3D, 3E, 3F or 3G in U87MG glioblastoma cells. Sema3A and sema3B expressing cells contracted and changed shape persistently while cells expressing other semaphorins did not. Sema3A and sema3F differed from other semaphorins including sema3B as they also inhibited the proliferation of the cells and the formation of soft agar colonies. With the exception of sema3G and sema3B, expression of these semaphorins in U87MG cells inhibited significantly tumor development from subcutaneously implanted cells. Strong inhibition of tumor development was also observed following implantation of U87MG cells expressing each of the class-3 semaphorins in the cortex of mouse brains. Sema3D and sema3E displayed the strongest inhibitory effects and their expression in U373MG or in U87MG glioblastoma cells implanted in the brains of mice prolonged the survival of the mice by more then two folds. Furthermore, most of the mice that died prior to the end of the experiment did not develop detectable tumors and many of the mice survived to the end of the experiment. Most of the semaphorins that we have used here with the exception of sema3D were characterized previously as inhibitors of angiogenesis. Our results indicate that sema3D also functions as an inhibitor of angiogenesis and suggest that the anti-tumorigenic effects are due primarily to inhibition of tumor angiogenesis. These results indicate that class-3 semaphorins such as sema3D and sema3E could perhaps be used to treat glioblastoma patients.
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Background Previously, we reported that Sema3A, one of the secreted repulsive axon guidance molecules, CRMP (collapsin response mediator protein)‐2, a putative intracellular signalling molecule for Sema3A and Sema3A receptor neuropilin‐1 are expressed in the developing lung. Sema3A inhibits branching morphogenesis of embryonic lung in organ culture. Results We examined the gene expression of Sema3A, Sema3C, Sema3F and their receptors, NP‐1, NP‐2 and plexin‐A1 by in situ hybridization. Transcripts of all six genes were detected in mouse lung from embryonic day E11.5 to E17.5, and displayed highly specific spatiotemporal distributions. The distribution of the receptor genes was detected in patterns which were consistent with known receptor usage of the semaphorins. In contrast to Sema3A, we found that the other class 3 semaphorins, Sema3C and Sema3F, stimulated branching morphogenesis. This stimulatory effect of Sema3C or Sema3F was accompanied by a moderate increase in the incorporation of bromodeoxyuridine (BrdU) into DNA in the terminal epithelial cells. Conclusion The coordinated expression patterns of different semaphorins and their receptors, together with the specific activities affecting branching morphogenesis, suggest that the semaphorins act as both positive and negative regulators of branching morphogenesis in the developing lung.
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Neural alterations and aberrantly expressed nerve-specific factors promoting tumor progression are known to contribute to pancreatic cancer's extremely poor prognosis. Despite hints that axon guidance factor semaphorin 3A (SEMA3A) may function as a tumor inhibitor, its clinical importance and therapeutic potential have not yet been explored. The present study investigated the role of SEMA3A and its receptors-plexins A1-A4 (PLXNA1-A4) and neuropilin-1 (NRP1)-in pancreatic cancer. QRT-PCR and immunohistochemical analyses revealed overexpression of SEMA3A, NRP1 and PLXNA1 in metaplastic ducts, malignant cells and nerves of cancerous specimens, and showed that elevated levels of corresponding mRNA (6.8-fold, 2.0-fold and 1.5-fold, respectively) clearly correlated with negative clinicopathological manifestations such as shorter survival (SEMA3A and PLXNA1) and a lesser degree of tumor differentiation (NRP1) in Stages I-III patients. High SEMA3A expression in pancreata of Stage IV M1 patients and in peritoneal metastases, and consequent functional studies indicated that poor clinical outcome might be related to the ability of SEMA3A to promote dissemination and invasiveness of pancreatic cancer cells through activation of multiple pathways involving Rac1, GSK3b or p42/p44 MAPK, but not E- to N-cadherin switch, MMP-9 or VEGF induction. Thus, this study is the first to quantify expression of the SEMA3A system in human malignancy and to show that overexpression of SEMA3A by nerves and transformed cells leads to a SEMA3A-rich environment which may favor malignant activities of tumor cells. Furthermore, negative clinicopathological correlations suggest that SEMA3A might represent a novel intervention target but not a treatment option for pancreatic cancer patients.
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Class III semaphorins (SemaIIIs) are intercellular cues secreted by surrounding tissues to guide migrating cells and axons in the developing organism. This chemotropic activity is crucial for the formation of nerves and vasculature. Intriguingly, SemaIIIs are also synthesized by neurons during axon pathfinding, but their function as intrinsic cues remains unknown. We have explored the role of Sema3A expression in motoneurons during spinal nerve development. Loss- and gain-of-function in the neural tube of the chick embryo were undertaken to target Sema3A expression in motoneurons while preserving Sema3A sources localized in peripheral tissues, known to provide important repulsive information for delineating the routes of motor axons towards their ventral or dorsal targets. Strikingly, Sema3A overexpression induced defasciculation and exuberant growth of motor axon projections into these normally non-permissive territories. Moreover, knockdown studies showed that motoneuronal Sema3A is required for correct spinal nerve compaction and dorsal motor axon extension. Further analysis of Sema3A gain- and loss-of-function in ex vivo models revealed that Sema3A in motoneurons sets the level of sensitivity of their growth cones to exogenous Sema3A exposure. This regulation is associated with post-transcriptional and local control of the availability of the Sema3A receptor neuropilin 1 at the growth cone surface. Thus, by modulating the strength of Sema3A-mediated environmental repulsive constraints, Sema3A in motoneurons enables axons to extend more or less far away from these repulsive sources. Such interplay between intrinsic and extrinsic Sema3A may represent a fundamental mechanism in the accurate specification of axon pathways.
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Abstract The extracellular molecule semaphorin 3A (Sema3A) is proposed to be a negative guidance cue that participates in patterning DRG sensory axons in the developing chick spinal cord. During development Sema3A is first expressed throughout the spinal cord gray matter, but Sema3A expression later disappears from the dorsal horn, where small‐caliber cutaneous afferents terminate. Sema3A expression remains in the ventral horn, where large‐muscle proprioceptive afferents terminate. It has been proposed that temporal changes in the sensitivity of different classes of sensory afferents to Sema3A contribute to the different pathfinding of these sensory afferents. This study compared the expression of the semaphorin 3A receptor subunit, neuropilin‐1, and the collapse response of growth cones to semaphorin 3A for NGF (cutaneous)‐ and NT3 (proprioceptive)‐dependent sensory axons extended from E6‐E10 chick embryos. Growth cones extended from E6 DRGs in NT3‐containing medium expressed neuropilin‐1 and collapsed in response to Sema3A. From E7 until E10 NT3‐responsive growth cones expressed progressively lower levels of neuropilin‐1, and were less sensitive to Sema3A. On the other hand, growth cones extended from DRGs in NGF‐containing medium expressed progressively higher levels of neuropilin‐1 and higher levels of collapse response to Sema3A over the period from E6–E10. Thus, developmental patterning of sensory terminals in the chick spinal cord may arise from changes in both Sema3A expression in the developing spinal cord and accompanying changes in neuronal expression of the Sema3A receptor subunit, neuropilin‐1. © 2002 Wiley Periodicals, Inc. J Neurobiol 51: 43–53, 2002
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Class 1 and 3 semaphorins repulse axons but bind to different cell surface proteins. We find that the two known semaphorin-binding proteins, plexin 1 (Plex 1) and neuropilin-1 (NP-1), form a stable complex. Plex 1 alone does not bind semaphorin-3A (Sema3A), but the NP-1/Plex 1 complex has a higher affinity for Sema3A than does NP-1 alone. While Sema3A binding to NP-1 does not alter nonneuronal cell morphology, Sema3A interaction with NP-1/Plex 1 complexes induces adherent cells to round up. Expression of a dominant-negative Plex 1 in sensory neurons blocks Sema3A-induced growth cone collapse. Sema3A treatment leads to the redistribution of growth cone NP-1 and plexin into clusters. Thus, physiologic Sema3A receptors consist of NP-1/plexin complexes.
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Differing Semaphorin 3A Concentrations Trigger Distinct Signaling Mechanisms in Growth Cone Collapse
Semaphorin-3A (Sema3A) is a major guidance cue in the developing nervous system. Previous studies have revealed a dependence of responses to Sema3A on local protein synthesis (PS) in axonal growth cones, but a recent study has called this dependence into question. To understand the basis of this discrepancy we used the growth cone collapse assay on chick dorsal root ganglion neurons. We show that the dependence of growth cone collapse on protein synthesis varies according to Sema3A concentration, from near-total at low concentration (<100 ng/ml) to minimal at high concentration (>625 ng/ml). Further, we show that neuropilin-1 (NP-1) mediates both PS-dependent and PS-independent collapse. Our findings are consistent with the operation of at least two distinct Sema3A signaling pathways: one that is PS-dependent, involving mammalian target of rapamycin, and one that is PS-independent, involving GSK-3β activation and operative at all concentrations of Sema3A examined. The results provide a plausible explanation for the discrepancy in PS-dependence reported in the literature, and indicate that different signaling pathways activated within growth cones can be modulated by changing the concentration of the same guidance cue.
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