Structure of the semaphorin-3A receptor binding module

Role of Semaphorin 3A in common psychiatric illnesses such as schizophrenia, depression, and anxiety

With societal development and an ageing population, psychiatric disorders have become a common cause of severe and long-term disability and socioeconomic burdens worldwide. Semaphorin 3A (Sema-3A) is a secreted glycoprotein belonging to the semaphorin family. Sema-3A is well known as an axon guidance factor in the neuronal system and a potent immunoregulator at all stages of the immune response. It is reported to have various biological functions and is involved in many human diseases, including autoimmune diseases, angiocardiopathy, osteoporosis, and tumorigenesis. The signals of sema-3A involved in the pathogenesis of these conditions, are transduced through its cognate receptors and diverse downstream signalling pathways. An increasing number of studies show that sema-3A plays important roles in synaptic and dendritic development, which are closely associated with the pathophysiological mechanisms of psychiatric disorders, including schizophrenia, depression, and autism, suggesting the involvement of sema-3A in the pathogenesis of mental diseases. This indicates that mutations in sema-3A and alterations in its receptors and signalling may compromise neurodevelopment and predispose patients to these disorders. However, the role of sema-3A in psychiatric disorders, particularly in regulating neurodevelopment, remains elusive. In this review, we summarise the recent progress in understanding sema-3A in the pathogenesis of mental diseases and highlight sema-3A as a potential target for the prevention and treatment of these diseases.

Truncated-semaphorin3A is a potential regulatory molecule to restore immune homeostasis in immune-mediated diseases

Regulatory molecules have recently been recognized for their beneficial effects in the treatment of immune-mediated diseases, rather than using cytotoxic immune-suppressing drugs, which are associated with many unwanted side effects. Semaphorin3A (sema3A), a unique regulatory master of the immune system, was shown to be decreased in the serum of systemic lupus erythematosus (SLE) patients, in association with disease severity. Later, we were able to show its extremely beneficial effect in treating lupus nephritis in the NZB/W mice model. The mechanisms by which sema3A maintains its regulatory effect is by binding the regulatory receptor CD72 on B cells, thereby reducing the threshold of BCR signaling on B cells and reducing the production of pro-inflammatory cytokines. The aim of this study was to generate a stable sema3A molecule, easy to produce with a higher binding capacity to CD72 receptor rather than to Neuropilin-1 (NRP-1) receptor, which is expressed in many cell types. Using the crystallographic structure of parental sema3A, we synthesized a new secreted (shorter) sema3A derivative, which we called truncated sema3A (T-sema3A). The new molecule lacked the NRP-1 binding domain (the C-terminal site) and has an artificial dimerization site at position 257 (serine residue was exchanged with a cysteine residue). To facilitate the purification of this molecule we added Histidine epitope tag in frame upstream to a stop codon. This construct was transfected using a viral vector to 293HEK cells to generate cells stably expressing T-sema3A. T-sema3A is shown to be with a higher binding ability to CD72 than to NRP-1 as demonstrated by a homemade ELISA. In addition, T-sema3A was shown to be a regulatory agent which can induce the expression of IL-10 and TGF-β and reduce the secretion of pro-inflammatory cytokines such as IL-6, IFN-γ, and IL-17A from human T and B-lymphocytes. Keeping this in mind, T-sema3A is highly effective in maintaining immune homeostasis, therefore, becoming a potential agent in restoring the regulatory status of the immune system in immune-mediated diseases.

Motor neurons use push-pull signals to direct vascular remodeling critical for their connectivity

The nervous system requires metabolites and oxygen supplied by the neurovascular network, but this necessitates close apposition of neurons and endothelial cells. We find motor neurons attract vessels with long-range VEGF signaling, but endothelial cells in the axonal pathway are an obstacle for establishing connections with muscles. It is unclear how this paradoxical interference from heterotypic neurovascular contacts is averted. Through a mouse mutagenesis screen, we show that Plexin-D1 receptor is required in endothelial cells for development of neuromuscular connectivity. Motor neurons release Sema3C to elicit short-range repulsion via Plexin-D1, thus displacing endothelial cells that obstruct axon growth. When this signaling pathway is disrupted, epaxial motor neurons are blocked from reaching their muscle targets and concomitantly vascular patterning in the spinal cord is altered. Thus, an integrative system of opposing push-pull cues ensures detrimental axon-endothelial encounters are avoided while enabling vascularization within the nervous system and along peripheral nerves.

Hybrid in vitro/in silico analysis of low-affinity protein-protein interactions that regulate signal transduction by Sema6D

Semaphorins constitute a large family of secreted and membrane-bound proteins that signal through cell-surface receptors, plexins. Semaphorins generally use low-affinity protein-protein interactions to bind with their specific plexin(s) and regulate distinct cellular processes such as neurogenesis, immune response, and organogenesis. Sema6D is a membrane-bound semaphorin that interacts with class A plexins. Sema6D exhibited differential binding affinities to class A plexins in prior cell-based assays, but the molecular mechanism underlying this selectivity is not well understood. Therefore, we performed hybrid in vitro/in silico analysis to examine the binding mode of Sema6D to class A plexins and to identify residues that give rise to the differential affinities and thus contribute to the selectivity within the same class of semaphorins. Our biophysical binding analysis indeed confirmed that Sema6D has a higher affinity for Plexin-A1 than for other class A plexins, consistent with the binding selectivity observed in the previous cell-based assays. Unexpectedly, our present crystallographic analysis of the Sema6D-Plexin-A1 complex showed that the pattern of polar interactions is not interaction-specific because it matches the pattern in the prior structure of the Sema6A-Plexin-A2 complex. Thus, we performed in silico alanine scanning analysis and discovered hotspot residues that selectively stabilized the Sema6D-Plexin-A1 pair via Van der Waals interactions. We then validated the contribution of these hotspot residues to the variation in binding affinity with biophysical binding analysis and molecular dynamics simulations on the mutants. Ultimately, our present results suggest that shape complementarity in the binding interfaces is a determinant for binding selectivity.

Cryo-electron Microscopic Analysis of Single-Pass Transmembrane Receptors

Single-pass transmembrane receptors (SPTMRs) represent a diverse group of integral membrane proteins that are involved in many essential cellular processes, including signal transduction, cell adhesion, and transmembrane transport of materials. Dysregulation of the SPTMRs is linked with many human diseases. Despite extensive efforts in past decades, the mechanisms of action of the SPTMRs remain incompletely understood. One major hurdle is the lack of structures of the full-length SPTMRs in different functional states. Such structural information is difficult to obtain by traditional structural biology methods such as X-ray crystallography and nuclear magnetic resonance (NMR). The recent rapid development of single-particle cryo-electron microscopy (cryo-EM) has led to an exponential surge in the number of high-resolution structures of integral membrane proteins, including SPTMRs. Cryo-EM structures of SPTMRs solved in the past few years have tremendously improved our understanding of how SPTMRs function. In this review, we will highlight these progresses in the structural studies of SPTMRs by single-particle cryo-EM, analyze important structural details of each protein involved, and discuss their implications on the underlying mechanisms. Finally, we also briefly discuss remaining challenges and exciting opportunities in the field.

Sema3A Antibody BI-X Prevents Cell Permeability and Cytoskeletal Collapse in HRMECs and Increases Tip Cell Density in Mouse Oxygen-Induced Retinopathy

Purpose

Semaphorin 3A (Sema3A) is an axonal guidance molecule that inhibits angiogenesis by vasorepulsion and blocks revascularization in the ischemic retina. BI-X is an intravitreal anti-Sema3A agent under clinical investigation in patients with proliferative diabetic retinopathy (PDR) and diabetic macular ischemia (DMI).

Methods

Surface plasmon resonance was used to determine binding affinity of BI-X to human and murine Sema3A. In vitro, human retinal microvascular endothelial cells (HRMECs) were used to assess effects of BI-X on cell permeability and cytoskeletal collapse induced by Sema3A. In vivo, intravitreal BI-X or an anti-trinitrophenol control antibody was administered in both eyes in mice with oxygen-induced retinopathy (OIR). Retinal flat mounts were prepared, and avascular area and tip cell density were determined using confocal laser-scanning microscopy.

Results

Dissociation constants for BI-X binding to human and murine Sema3A were 29 pM and 27 pM, respectively. In vitro, BI-X prevented HRMEC permeability and cytoskeletal collapse induced by Sema3A. In vivo, BI-X increased tip cell density by 33% (P < 0.001) and reduced avascular area by 12% (not significant). A significant negative correlation was evident between avascular area and tip cell density (r² = 0.4205, P < 0.0001).

Conclusions

BI-X binds to human Sema3A with picomolar affinity and prevents cell permeability and cytoskeletal collapse in HRMECs. BI-X also enhances revascularization in mice with OIR.

Translational Relevance

BI-X is a potent inhibitor of human Sema3A that improves revascularization in a murine model of OIR; BI-X is currently being investigated in patients with laser-treated PDR and DMI.

Identifying Function Determining Residues in Neuroimmune Semaphorin 4A

Semaphorin 4A (Sema4A) exerts a stabilizing effect on human Treg cells in PBMC and CD4+ T cell cultures by engaging Plexin B1. Sema4A deficient mice display enhanced allergic airway inflammation accompanied by fewer Treg cells, while Sema4D deficient mice displayed reduced inflammation and increased Treg cell numbers even though both Sema4 subfamily members engage Plexin B1. The main objectives of this study were: 1. To compare the in vitro effects of Sema4A and Sema4D proteins on human Treg cells; and 2. To identify function-determining residues in Sema4A critical for binding to Plexin B1 based on Sema4D homology modeling. We report here that Sema4A and Sema4D display opposite effects on human Treg cells in in vitro PBMC cultures; Sema4D inhibited the CD4+CD25+Foxp3+ cell numbers and CD25/Foxp3 expression. Sema4A and Sema4D competitively bind to Plexin B1 in vitro and hence may be doing so in vivo as well. Bayesian Partitioning with Pattern Selection (BPPS) partitioned 4505 Sema domains from diverse organisms into subgroups based on distinguishing sequence patterns that are likely responsible for functional differences. BPPS groups Sema3 and Sema4 into one family and further separates Sema4A and Sema4D into distinct subfamilies. Residues distinctive of the Sema3,4 family and of Sema4A (and by homology of Sema4D) tend to cluster around the Plexin B1 binding site. This suggests that the residues both common to and distinctive of Sema4A and Sema4D may mediate binding to Plexin B1, with subfamily residues mediating functional specificity. We mutated the Sema4A-specific residues M198 and F223 to alanine; notably, F223 in Sema4A corresponds to alanine in Sema4D. Mutant proteins were assayed for Plexin B1-binding and Treg stimulation activities. The F223A mutant was unable to stimulate Treg stability in in vitro PBMC cultures despite binding Plexin B1 with an affinity similar to the WT protein. This research is a first step in generating potent mutant Sema4A molecules with stimulatory function for Treg cells with a view to designing immunotherapeutics for asthma.

Identification and Functional Characterization of a Novel Variant in the SEMA3A Gene in a Chinese Family with Kallmann Syndrome

BACKGROUND

Kallmann syndrome (KS) is a rare genetic disease characterized by the reproductive system and olfactory dysplasia due to the defective migration of gonadotropin-releasing hormone (GnRH) neurons. However, this disorder is clinically heterogeneous and the genotype-phenotype relationship has not been determined.

OBJECTIVE

The present study aimed to identify the variant causing KS in a Chinese family and evaluate the functional consequences and phenotypes associated with the novel variant.

METHODS

A Chinese family with KS was screened for pathogenic variants by whole-exome sequencing (WES). Bioinformatic analysis was performed to predict the consequences of the identified variant. The expression of the mutant protein was examined in vitro.

RESULTS

A novel heterozygous variant (NM_006080.2 : c.814G > T) in SEMA3A was identified in the patient and his father, which caused the substitution of aspartic acid with tyrosine in codon 272. It was predicted to result in pathogenic significance with a high damaging score and seriously affect protein structure by bioinformatic analysis. In vitro experiments revealed this variant could significantly decrease the expression of SEMA3A. Furthermore, it may cause the disease by failing to induce the phosphorylation of focal adhesion kinase (FAK) in GnRH neurons.

CONCLUSION

Identification and functional characterization of this novel variant in the SEMA3A gene in a Chinese family with Kallmann syndrome extend the genetic variant spectrum of SEMA3A and provide more data about the heterogeneity of KS, which may provide further insights into the diagnosis of KS and help patients get additional data in genetic counseling and timely treatment.

Interactions between semaphorins and plexin-neuropilin receptor complexes in the membranes of live cells

Signaling of semaphorin ligands via their plexin–neuropilin receptors is involved in tissue patterning in the developing embryo. These proteins play roles in cell migration and adhesion but are also important in disease etiology, including in cancer angiogenesis and metastasis. While some structures of the soluble domains of these receptors have been determined, the conformations of the full-length receptor complexes are just beginning to be elucidated, especially within the context of the plasma membrane. Pulsed-interleaved excitation fluorescence cross-correlation spectroscopy allows direct insight into the formation of protein–protein interactions in the membranes of live cells. Here, we investigated the homodimerization of neuropilin-1 (Nrp1), plexin A2, plexin A4, and plexin D1 using pulsed-interleaved excitation fluorescence cross-correlation spectroscopy. Consistent with previous studies, we found that Nrp1, plexin A2, and plexin A4 are present as dimers in the absence of exogenous ligand. Plexin D1, on the other hand, was monomeric under similar conditions, which had not been previously reported. We also found that plexin A2 and A4 assemble into a heteromeric complex. Stimulation with semaphorin 3A or semaphorin 3C neither disrupts nor enhances the dimerization of the receptors when expressed alone, suggesting that activation involves a conformational change rather than a shift in the monomer–dimer equilibrium. However, upon stimulation with semaphorin 3C, plexin D1 and Nrp1 form a heteromeric complex. This analysis of interactions provides a complementary approach to the existing structural and biochemical data that will aid in the development of new therapeutic strategies to target these receptors in cancer.

Modelling and Refining Neuronal Circuits with Guidance Cues: Involvement of Semaphorins

The establishment of neuronal circuits requires neurons to develop and maintain appropriate connections with cellular partners in and out the central nervous system. These phenomena include elaboration of dendritic arborization and formation of synaptic contacts, initially made in excess. Subsequently, refinement occurs, and pruning takes places both at axonal and synaptic level, defining a homeostatic balance maintained throughout the lifespan. All these events require genetic regulations which happens cell-autonomously and are strongly influenced by environmental factors. This review aims to discuss the involvement of guidance cues from the Semaphorin family.

Architecture of the Sema3A/PlexinA4/Neuropilin tripartite complex

Secreted class 3 semaphorins (Sema3s) form tripartite complexes with the plexin receptor and neuropilin coreceptor, which are both transmembrane proteins that together mediate semaphorin signal for neuronal axon guidance and other processes. Despite extensive investigations, the overall architecture of and the molecular interactions in the Sema3/plexin/neuropilin complex are incompletely understood. Here we present the cryo-EM structure of a near intact extracellular region complex of Sema3A, PlexinA4 and Neuropilin 1 (Nrp1) at 3.7 Å resolution. The structure shows a large symmetric 2:2:2 assembly in which each subunit makes multiple interactions with others. The two PlexinA4 molecules in the complex do not interact directly, but their membrane proximal regions are close to each other and poised to promote the formation of the intracellular active dimer for signaling. The structure reveals a previously unknown interface between the a2b1b2 module in Nrp1 and the Sema domain of Sema3A. This interaction places the a2b1b2 module at the top of the complex, far away from the plasma membrane where the transmembrane regions of Nrp1 and PlexinA4 embed. As a result, the region following the a2b1b2 module in Nrp1 must span a large distance to allow the connection to the transmembrane region, suggesting an essential role for the long non-conserved linkers and the MAM domain in neuropilin in the semaphorin/plexin/neuropilin complex.

Myeloid-resident neuropilin-1 promotes choroidal neovascularization while mitigating inflammation

Age-related macular degeneration (AMD) in its various forms is a leading cause of blindness in industrialized countries. Here, we provide evidence that ligands for neuropilin-1 (NRP1), such as Semaphorin 3A and VEGF-A, are elevated in the vitreous of patients with AMD at times of active choroidal neovascularization (CNV). We further demonstrate that NRP1-expressing myeloid cells promote and maintain CNV. Expression of NRP1 on cells of myeloid lineage is critical for mitigating production of inflammatory factors such as IL6 and IL1β. Therapeutically trapping ligands of NRP1 with an NRP1-derived trap reduces CNV. Collectively, our findings identify a role for NRP1-expressing myeloid cells in promoting pathological angiogenesis during CNV and introduce a therapeutic approach to counter neovascular AMD.

State of the structure address on MET receptor activation by HGF

The MET receptor tyrosine kinase (RTK) and its cognate ligand hepatocyte growth factor (HGF) comprise a signaling axis essential for development, wound healing and tissue homeostasis. Aberrant HGF/MET signaling is a driver of many cancers and contributes to drug resistance to several approved therapeutics targeting other RTKs, making MET itself an important drug target. In RTKs, homeostatic receptor signaling is dependent on autoinhibition in the absence of ligand binding and orchestrated set of conformational changes induced by ligand-mediated receptor dimerization that result in activation of the intracellular kinase domains. A fundamental understanding of these mechanisms in the MET receptor remains incomplete, despite decades of research. This is due in part to the complex structure of the HGF ligand, which remains unknown in its full-length form, and a lack of high-resolution structures of the complete MET extracellular portion in an apo or ligand-bound state. A current view of HGF-dependent MET activation has evolved from biochemical and structural studies of HGF and MET fragments and here we review what these findings have thus far revealed.

Semaphorin 3A promotes the osteogenic differentiation of rat bone marrow-derived mesenchymal stem cells in inflammatory environments by suppressing the Wnt/β-catenin signaling pathway

After periodontal treatment, the local inflammatory environment surrounding periodontal tissues cannot be entirely eliminated. The means by which alveolar bone repair and regeneration are promoted in inflammatory environments have important clinical significance. As a powerful protein that promotes the differentiation of osteocytes, semaphorin 3A (Sema3A) shows potential for bone regeneration therapy. However, the effect of Sema3A on osteogenic differentiation in an inflammatory environment, as well as the underlying mechanism, have not yet been explored. We used lentivirus to transduce rat bone marrow-derived mesenchymal stem cells (rBMSCs) to stably overexpress Sema3A. Lipopolysaccharide from Escherichia coli (E. coli LPS) was used to stimulate rBMSCs to establish an inflammatory environment. ALP staining, Alizarin red staining, ALP activity tests, quantitative RT-PCR (qRT-PCR), and Western blotting were used to elucidate the effect of Sema3A on the osteogenesis of rBMSCs in inflammatory environments. XAV939 and LiCl were used to determine whether the Wnt/β-catenin signaling pathway was involved in attenuating the inhibition of Sema3A-induced osteogenic differentiation by LPS. The qRT-PCR and Western blot results demonstrated that the lentiviral vector (LV-NC) and lentiviral-Sema3A (LV-Sema3A) were successfully transduced into rBMSCs. An inflammatory environment could be established by stimulating rBMSCs with 1 μg/ml E. coli LPS. After Sema3A overexpression, mineral deposition was exacerbated, and the BSP and Runx2 gene and protein expression levels were increased. Furthermore, E. coli LPS activated the Wnt/β-catenin signaling pathway and decreased rBMSC osteogenesis, but these effects were attenuated by Sema3A. In conclusion, Sema3A could protect BMSCs from LPS-mediated inhibition of osteogenic differentiation in inflammatory environments by suppressing the Wnt/β-catenin pathway.

Structural basis of semaphorin-plexin cis interaction

Semaphorin ligands interact with plexin receptors to contribute to functions in the development of myriad tissues including neurite guidance and synaptic organisation within the nervous system. Cell-attached semaphorins interact in trans with plexins on opposing cells, but also in cis on the same cell. The interplay between trans and cis interactions is crucial for the regulated development of complex neural circuitry, but the underlying molecular mechanisms are uncharacterised. We have discovered a distinct mode of interaction through which the Drosophila semaphorin Sema1b and mouse Sema6A mediate binding in cis to their cognate plexin receptors. Our high-resolution structural, biophysical and in vitro analyses demonstrate that monomeric semaphorins can mediate a distinctive plexin binding mode. These findings suggest the interplay between monomeric vs dimeric states has a hereto unappreciated role in semaphorin biology, providing a mechanism by which Sema6s may balance cis and trans functionalities.

Neuropilin: Handyman and Power Broker in the Tumor Microenvironment

Neuropilin-1 and neuropilin-2 form a small family of transmembrane receptors, which, due to the lack of a cytosolic protein kinase domain, act primarily as co-receptors for various ligands. Performing at the molecular level both the executive and organizing functions of a handyman as well as of a power broker, they are instrumental in controlling the signaling of various receptor tyrosine kinases, integrins, and other molecules involved in the regulation of physiological and pathological angiogenic processes. In this setting, the various neuropilin ligands and interaction partners on various cells of the tumor microenvironment, such as cancer cells, endothelial cells, cancer-associated fibroblasts, and immune cells, are surveyed. The suitability of various neuropilin-targeting substances and the intervention in neuropilin-mediated interactions is considered as a possible building block of tumor therapy.

A rationally designed NRP1-independent superagonist SEMA3A mutant is an effective anticancer agent

Vascular normalizing strategies, aimed at ameliorating blood vessel perfusion and lessening tissue hypoxia, are treatments that may improve the outcome of cancer patients. Secreted class 3 semaphorins (SEMA3), which are thought to directly bind neuropilin (NRP) co-receptors that, in turn, associate with and elicit plexin (PLXN) receptor signaling, are effective normalizing agents of the cancer vasculature. Yet, SEMA3A was also reported to trigger adverse side effects via NRP1. We rationally designed and generated a safe, parenterally deliverable, and NRP1-independent SEMA3A point mutant isoform that, unlike its wild-type counterpart, binds PLXNA4 with nanomolar affinity and has much greater biochemical and biological activities in cultured endothelial cells. In vivo, when parenterally administered in mouse models of pancreatic cancer, the NRP1-independent SEMA3A point mutant successfully normalized the vasculature, inhibited tumor growth, curbed metastatic dissemination, and effectively improved the supply and anticancer activity of chemotherapy. Mutant SEMA3A also inhibited retinal neovascularization in a mouse model of age-related macular degeneration. In summary, mutant SEMA3A is a vascular normalizing agent that can be exploited to treat cancer and, potentially, other diseases characterized by pathological angiogenesis.

Diversity of oligomerization in Drosophila semaphorins suggests a mechanism of functional fine-tuning

Semaphorin ligands and their plexin receptors are one of the major cell guidance factors that trigger localised changes in the cytoskeleton. Binding of semaphorin homodimer to plexin brings two plexins in close proximity which is a prerequisite for plexin signalling. This model appears to be too simplistic to explain the complexity and functional versatility of these molecules. Here, we determine crystal structures for all members of Drosophila class 1 and 2 semaphorins. Unlike previously reported semaphorin structures, Sema1a, Sema2a and Sema2b show stabilisation of sema domain dimer formation via a disulfide bond. Unexpectedly, our structural and biophysical data show Sema1b is a monomer suggesting that semaphorin function may not be restricted to dimers. We demonstrate that semaphorins can form heterodimers with members of the same semaphorin class. This heterodimerization provides a potential mechanism for cross-talk between different plexins and co-receptors to allow fine-tuning of cell signalling.

Characterization of a sandwich ELISA for quantification of total human soluble neuropilin-1

Background

Neuropilin‐1 (NRP1) is a highly interactive molecule that exists as transmembrane and soluble isoforms. Measurement of circulating levels of soluble NRP1 (sNRP1) in human serum and plasma has proven to be difficult due to present matrix interferences and due to the lack of a reliable technique.

Methods

We developed a highly specific and sensitive sandwich ELISA assay for total sNRP1 quantification in peripheral blood, and we validated the test according to ICH guidelines. The linear epitopes of the employed polyclonal and monoclonal anti‐human NRP1 antibodies were mapped with microarray technology. We included a sample pre‐treatment step with guanidine hydrochloride (GuHCl) to release sNRP1 from existing interferants.

Results

The ELISA assay which is calibrated with sNRP1 isoform 2 and covers a calibration range from 0.375 to 12 nmol/L detects sNRP1 in human serum and plasma (heparin, EDTA, and citrate). Multiple linear epitopes recognized by the polyclonal coating antibody are distributed over the whole sNRP1 sequence. The monoclonal detection antibody binds to a linear epitope which is in the N‐terminal region of the a1 domain of human sNRP1. Assay parameters like precision (intra‐assay: 6%), dilution linearity (95%‐115%), specificity (98%), and spike recovery (81%‐109%) meet the international standards of acceptance.

Conclusion

Our novel sandwich ELISA provides a reliable tool for the quantitative determination of total human sNRP1. The assay detects free and previous ligand‐bound total NRP1.

Neuropilins in the Context of Tumor Vasculature

Neuropilin-1 and Neuropilin-2 form a small family of plasma membrane spanning receptors originally identified by the binding of semaphorin and vascular endothelial growth factor. Having no cytosolic protein kinase domain, they function predominantly as co-receptors of other receptors for various ligands. As such, they critically modulate the signaling of various receptor tyrosine kinases, integrins, and other molecules involved in the regulation of physiological and pathological angiogenic processes. This review highlights the diverse neuropilin ligands and interacting partners on endothelial cells, which are relevant in the context of the tumor vasculature and the tumor microenvironment. In addition to tumor cells, the latter contains cancer-associated fibroblasts, immune cells, and endothelial cells. Based on the prevalent neuropilin-mediated interactions, the suitability of various neuropilin-targeted substances for influencing tumor angiogenesis as a possible building block of a tumor therapy is discussed.

Class-3 Semaphorins and Their Receptors: Potent Multifunctional Modulators of Tumor Progression

Semaphorins are the products of a large gene family containing 28 genes of which 21 are found in vertebrates. Class-3 semaphorins constitute a subfamily of seven vertebrate semaphorins which differ from the other vertebrate semaphorins in that they are the only secreted semaphorins and are distinguished from other semaphorins by the presence of a basic domain at their C termini. Class-3 semaphorins were initially characterized as axon guidance factors, but have subsequently been found to regulate immune responses, angiogenesis, lymphangiogenesis, and a variety of additional physiological and developmental functions. Most class-3 semaphorins transduce their signals by binding to receptors belonging to the neuropilin family which subsequently associate with receptors of the plexin family to form functional class-3 semaphorin receptors. Recent evidence suggests that class-3 semaphorins also fulfill important regulatory roles in multiple forms of cancer. Several class-3 semaphorins function as endogenous inhibitors of tumor angiogenesis. Others were found to inhibit tumor metastasis by inhibition of tumor lymphangiogenesis, by direct effects on the behavior of tumor cells, or by modulation of immune responses. Notably, some semaphorins such as sema3C and sema3E have also been found to potentiate tumor progression using various mechanisms. This review focuses on the roles of the different class-3 semaphorins in tumor progression.

Konjac ceramide (kCer) regulates keratinocyte migration by Sema3A-like repulsion mechanism

Previously, we proposed the following mechanism for konjac ceramide (kCer)-mediated neurite outgrowth inhibition: kCer binds to Nrp as a Sema3A agonist, resulting in Nrp1/PlexA complex formation and activation of the Sema3A signaling pathway to induce phosphorylation of CRMP2 and microtubule depolymerization. The Sema3A/Nrp1 signaling pathway is known to be also expressed in normal human keratinocytes. To determine whether kCer can function in human keratinocytes as it does in neurites, that is, if it can bind to Nrp1 in place of Sema3A, we studied the effect of kCer on HaCaT cell migration activity. Using a trans-well chamber assay, we compared the effects of Sema3A and kCer on serum-derived cell migration activity. kCer showed Sema3A-like suppression of cell migration activity and induction of cellular Cofilin phosphorylation. In addition, kCer and Sema3A inhibited histamine (His)-enhanced migration of immature HaCaT cells. We have demonstrated that kCer does not interact with histaime receptors H1R or H4R directly, but we speculate that kCer may transduce a signal downstream of the His signaling pathway.

Targeting Semaphorin 3C in Prostate Cancer With Small Molecules

Despite the amenability of early-stage prostate cancer to surgery and radiation therapy, locally advanced and metastatic prostate cancer is clinically problematic. Chemical castration is often used as a first-line therapy for advanced disease, but progression to the castration-resistant prostate cancer phase occurs with dependable frequency, largely through mutations to the androgen receptor (AR), aberrant AR signaling, and AR-independent mechanisms, among other causes. Semaphorin 3C (SEMA3C) is a secreted signaling protein that is essential for cardiac and neuronal development and has been shown to be regulated by the AR, to drive epithelial-to-mesenchymal transition and stem features in prostate cells, to activate receptor tyrosine kinases, and to promote cancer progression. Given that SEMA3C is linked to several key aspects of prostate cancer progression, we set out to explore SEMA3C inhibition by small molecules as a prospective cancer therapy. A homology-based SEMA3C protein structure was created, and its interaction with the neuropilin (NRP)-1 receptor was modeled to guide the development of the corresponding disrupting compounds. Experimental screening of 146 in silico‒identified molecules from the National Cancer Institute library led to the discovery of four promising candidates that effectively bind to SEMA3C, inhibit its association with NRP1, and attenuate prostate cancer growth. These findings provide proof of concept for the feasibility of inhibiting SEMA3C with small molecules as a therapeutic approach for prostate cancer.

Making Connections: Guidance Cues and Receptors at Nonneural Cell-Cell Junctions

The field of axon guidance was revolutionized over the past three decades by the identification of highly conserved families of guidance cues and receptors. These proteins are essential for normal neural development and function, directing cell and axon migration, neuron-glial interactions, and synapse formation and plasticity. Many of these genes are also expressed outside the nervous system in which they influence cell migration, adhesion and proliferation. Because the nervous system develops from neural epithelium, it is perhaps not surprising that these guidance cues have significant nonneural roles in governing the specialized junctional connections between cells in polarized epithelia. The following review addresses roles for ephrins, semaphorins, netrins, slits and their receptors in regulating adherens, tight, and gap junctions in nonneural epithelia and endothelia.

Matrilysin/MMP-7 Cleavage of Perlecan/HSPG2 Complexed with Semaphorin 3A Supports FAK-Mediated Stromal Invasion by Prostate Cancer Cells

Interrupting the interplay between cancer cells and extracellular matrix (ECM) is a strategy to halt tumor progression and stromal invasion. Perlecan/heparan sulfate proteoglycan 2 (HSPG2) is an extracellular proteoglycan that orchestrates tumor angiogenesis, proliferation, differentiation and invasion. Metastatic prostate cancer (PCa) cells degrade perlecan-rich tissue borders to reach bone, including the basement membrane, vasculature, reactive stromal matrix and bone marrow. Domain IV-3, perlecan’s last 7 immunoglobulin repeats, mimics native proteoglycan by promoting tumoroid formation. This is reversed by matrilysin/matrix metalloproteinase-7 (MMP-7) cleavage to favor cell dispersion and tumoroid dyscohesion. Both perlecan and Domain IV-3 induced a strong focal adhesion kinase (FAK) dephosphorylation/deactivation. MMP-7 cleavage of perlecan reversed this, with FAK in dispersed tumoroids becoming phosphorylated/activated with metastatic phenotype. We demonstrated Domain IV-3 interacts with the axon guidance protein semaphorin 3A (Sema3A) on PCa cells to deactivate pro-metastatic FAK. Sema3A antibody mimicked the Domain IV-3 clustering activity. Direct binding experiments showed Domain IV-3 binds Sema3A. Knockdown of Sema3A prevented Domain IV-3-induced tumoroid formation and Sema3A was sensitive to MMP-7 proteolysis. The perlecan-Sema3A complex abrogates FAK activity and stabilizes PCa cell interactions. MMP-7 expressing cells destroy the complex to initiate metastasis, destroy perlecan-rich borders, and favor invasion and progression to lethal bone disease.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

Characterization of Konjac Ceramide (kCer) Binding to Sema3A Receptor Nrp1

Konjac ceramide (kCer) can be prepared by a chemoenzymatic method as previously published (Usuki, S.; Tamura, N.; Sakai, S.; Tamura, T.; Mukai, K.; Igarashi, Y. Biochem. Biophys. Rep. 5, 160-167 (2016)). Thus prepared kCer showed an activation effect on Sema3A signaling pathway to induce phosphorylation of CRMP2 and microtubule depolymerizaion, resulting in opposing NGF-induced neurite outgrowth. In the present study, we have shown that kCer is a potential Sema3A-like ligand that has a competitive effect on Sema3A binding to a cell surface receptor Nrp1, but animal-type ceramides have no effect on Sema3A binding to Nrp1. In addition, kCer showed a direct molecular interaction with Nrp1, but animal-type ceramides, C16Cer, C18Cer, and C24Cer show no specific bindings to Nrp1. Further, kCer showed an additive effect to activate the Sema3A signaling pathway together with low-dose Sema3A but a reversed effect to inhibit this pathway when combined with high-dose Sema3A.

Konjac Ceramide (kCer) Regulates NGF-Induced Neurite Outgrowth via the Sema3A Signaling Pathway

The tuber of the konjac plant is a source enriched with GlcCer (kGlcCer), and has been used as a dietary supplement to improve the dry skin and itching that are caused by a deficiency of epidermal ceramide. Previously, we showed chemoenzymatically prepared konjac ceramide has a neurite-outgrowth inhibitory effect that is very similar to that of Sema3A and is not seen with animal-type ceramides. While, it has been unclear whether kCer may act on Sema3A or TrkA signaling pathway. In the present study, we showed kCer induces phosphorylation of CRMP2 and microtubules depolymerization via Sema3A signaling pathway not TrkA. It is concluded that kCer may be a potential Sema3A-like agonist that activates Sema3A signaling pathway directly.

Netrins & Semaphorins: Novel regulators of the immune response

Netrins and semaphorins, members of the neuronal guidance cue family, exhibit a rich biology with significant roles that extend beyond chemotactic guidance of the axons to build the neuronal patterns of the body. Screening of adult tissues and specific cellular subsets have illuminated that these proteins are also abundantly expressed under both steady state and pathological scenarios. This observation suggests that, in addition to their role in the development of the axonal tree, these proteins possess additional novel functions in adult physiopathology. Notably, a series of striking evidence has emerged in the literature describing their roles as potent regulators of both innate and adaptive immunity, providing extra dimension to our knowledge of neuronal guidance cues. In this review, we summarize the key complex roles of netrins and semaphorins outside the central nervous system (CNS) with focus on their immunomodulatory functions that impact pathophysiological conditions.

Targeting the hepatocyte growth factor/Met pathway in cancer

Hepatocyte growth factor (HGF)-induced activation of its cell surface receptor, the Met tyrosine kinase, drives mitogenesis, motogenesis and morphogenesis in a wide spectrum of target cell types and embryologic, developmental and homeostatic contexts. Typical paracrine HGF/Met signaling is regulated by HGF activation at target cell surfaces, HGF binding-induced receptor activation, internalization and degradation. Despite these controls, HGF/Met signaling contributes to oncogenesis, tumor angiogenesis and invasiveness, and tumor metastasis in many types of cancer, leading to the rapid growth of pathway-targeted anticancer drug development programs. We review here HGF and Met structure and function, basic properties of HGF/Met pathway antagonists now in clinical development, and recent clinical trial results. Presently, the main challenges facing the effective use of HGF/Met-targeted antagonists for cancer treatment include optimal patient selection, diagnostic and pharmacodynamic biomarker development, and the identification and testing of effective therapy combinations. The wealth of basic information, analytical reagents and model systems available regarding normal and oncogenic HGF/Met signaling will continue to be invaluable in meeting these challenges and moving expeditiously toward more effective cancer treatment.

The Role of Semaphorin 3A in Bone Remodeling

Bone remodeling occurs at the bone surface throughout adult life and associates bony quantity and quality. This process is a balance between the osteoblastic bone formation and osteoclastic bone resorption, which cross-talks together. Semaphorin 3A is a membrane-associated secreted protein and regarded as a diffusible axonal chemorepellent, which has been identified in the involvement of bone resorption and formation synchronously. However, the role of Semaphorin 3A in bone homeostasis and diseases remains elusive, in particular the association to osteoblasts and osteoclasts. In this review article, we summarize recent progress of Semaphorin 3A in the bone mass, homeostasis, and diseases and discuss the novel application of nerve-based bone regeneration. This will facilitate the understanding of Semaphorin 3A in skeletal biology and shed new light on the modulation and potential treatment in the bone disorders.

Plate-Based Assay for Measuring Direct Semaphorin-Neuropilin Interactions

The semaphorins are an essential family of axon guidance molecules that can be either secreted or are transmembrane proteins. Class 3 semaphorin (Sema3) family members are secreted and provide long-range guidance cues through two receptor families: neuropilins (Nrp) and plexins. Nrp is uniquely required for high-affinity Sema3 binding and signaling. Therefore, characterizing the molecular details of the Sema3/Nrp interaction is important for understanding the broader physiological and pathological role of the Sema3 family of proteins. Here we describe an in vitro plate-based binding assay for characterization of the Sema3/Nrp interaction. This assay utilizes Nrp-affinity plates and an alkaline phosphatase (AP)-Sema3 fusion to rapidly measure direct Sema3/Nrp binding. This assay can be used to measure receptor-ligand binding, the contribution of different domains, and exogenous factors, and to characterize competitive ligand binding.

The role of the semaphorins in cancer

The semaphorins were initially characterized as axon guidance factors, but have subsequently been implicated also in the regulation of immune responses, angiogenesis, organ formation, and a variety of additional physiological and developmental functions. The semaphorin family contains more then 20 genes divided into 7 subfamilies, all of which contain the signature sema domain. The semaphorins transduce signals by binding to receptors belonging to the neuropilin or plexin families. Additional receptors which form complexes with these primary semaphorin receptors are also frequently involved in semaphorin signaling. Recent evidence suggests that semaphorins also fulfill important roles in the etiology of multiple forms of cancer. Some semaphorins have been found to function as bona-fide tumor suppressors and to inhibit tumor progression by various mechanisms while other semaphorins function as inducers and promoters of tumor progression.

Characterizing Plexin GTPase Interactions Using Gel Filtration, Surface Plasmon Resonance Spectrometry, and Isothermal Titration Calorimetry

Plexins are unique, as they are the first example of a transmembrane receptor that interacts directly with small GTPases, a family of proteins that are essential for cell motility and proliferation/survival. We and other laboratories have determined the structure of the Rho GTPase-binding domain (RBD) of several plexins and also of the entire intracellular region of plexin-B1. Structures of plexin complexes with Rho GTPases, Rac1 and Rnd1, and a structure with a Ras GTPase, Rap1b, have also been solved. The relationship between plexin-Rho and plexin-Ras interactions is still unclear and in vitro biophysical experiments that characterize the protein interactions of purified components play an important role in advancing our understanding of the molecular mechanisms that underlie the function of plexin. This chapter describes the use of gel filtration (also known as size-exclusion chromatography or SEC), surface plasmon resonance (SPR), and isothermal titration calorimetry (ITC) in studies of plexin-small GTPase interactions with plexin-B1:Rac1 as an example. Together with other assays and manipulations (e.g., by mutagenesis or protein domain truncation/deletion), these in vitro measurements provide an important reference for the role and extent of the interactions.

Structural Perspectives on Axon Guidance

Axon guidance relies on a combinatorial code of receptor and ligand interactions that direct adhesive/attractive and repulsive cellular responses. Recent structural data have revealed many of the molecular mechanisms that govern these interactions and enabled the design of sophisticated mutant tools to dissect their biological functions. Here, we discuss the structure/function relationships of four major classes of guidance cues (ephrins, semaphorins, slits, netrins) and examples of morphogens (Wnt, Shh) and of cell adhesion molecules (FLRT). These cell signaling systems rely on specific modes of receptor-ligand binding that are determined by selective binding sites; however, defined structure-encoded receptor promiscuity also enables cross talk between different receptor/ligand families and can also involve extracellular matrix components. A picture emerges in which a multitude of highly context-dependent structural assemblies determines the finely tuned cellular behavior required for nervous system development.

Class A Plexins Are Organized as Preformed Inactive Dimers on the Cell Surface

Plexins are single-pass transmembrane receptors that bind the axon guidance molecules semaphorins. Single-pass transmembrane proteins are an important class of receptors that display a wide variety of activation mechanisms, often involving ligand-dependent dimerization or conformational changes. Resolving the activation mechanism and dimerization state of these receptors is extremely challenging, especially in a live-cell environment. Here, we report on the dimerization state of PlexinA4 and its response to activation by semaphorin binding. Semaphorins are dimeric molecules that activate plexin by binding two copies of plexin simultaneously and inducing formation of a specific active dimer of plexin. An open question is whether there are preexisting plexin dimers that could act as autoinhibitory complexes. We address these questions with pulsed interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). PIE-FCCS is a two-color fluorescence microscopy method that is directly sensitive to protein dimerization in a live-cell environment. With PIE-FCCS, we show that inactive PlexinA4 is dimerized in the live-cell plasma membrane. By comparing the cross correlation of full-length PlexinA4 to control proteins and plexin mutants, we show that dimerization of inactive PlexinA4 requires the Sema domain, but not the cytoplasmic domain. Ligand stimulation with Sema6A does not change the degree of cross correlation, indicating that plexin activation does not lead to higher-order oligomerization. Together, the results suggest that semaphorin activates plexin by disrupting an inhibitory plexin dimer and inducing the active dimer.

Structural Basis for Plexin Activation and Regulation

Class A plexins (PlxnAs) act as semaphorin receptors and control diverse aspects of nervous system development and plasticity, ranging from axon guidance and neuron migration to synaptic organization. PlxnA signaling requires cytoplasmic domain dimerization, but extracellular regulation and activation mechanisms remain unclear. Here we present crystal structures of PlxnA (PlxnA1, PlxnA2, and PlxnA4) full ectodomains. Domains 1-9 form a ring-like conformation from which the C-terminal domain 10 points away. All our PlxnA ectodomain structures show autoinhibitory, intermolecular "head-to-stalk" (domain 1 to domain 4-5) interactions, which are confirmed by biophysical assays, live cell fluorescence microscopy, and cell-based and neuronal growth cone collapse assays. This work reveals a 2-fold role of the PlxnA ectodomains: imposing a pre-signaling autoinhibitory separation for the cytoplasmic domains via intermolecular head-to-stalk interactions and supporting dimerization-based PlxnA activation upon ligand binding. More generally, our data identify a novel molecular mechanism for preventing premature activation of axon guidance receptors.

Class 3 semaphorins in cardiovascular development

Secreted class 3 semaphorins (Sema3), which signal through holoreceptor complexes that are formed by different subunits, such as neuropilins (Nrps), proteoglycans, and plexins, were initially characterized as fundamental regulators of axon guidance during embryogenesis. Subsequently, Sema3A, Sema3C, Sema3D, and Sema3E were discovered to play crucial roles in cardiovascular development, mainly acting through Nrp1 and Plexin D1, which funnels the signal of multiple Sema3 in vascular endothelial cells. Mechanistically, Sema3 proteins control cardiovascular patterning through the enzymatic GTPase-activating-protein activity of the cytodomain of Plexin D1, which negatively regulates the function of Rap1, a small GTPase that is well-known for its ability to drive vascular morphogenesis and to elicit the conformational activation of integrin adhesion receptors.

Transcriptomic Profiling Discloses Molecular and Cellular Events Related to Neuronal Differentiation in SH-SY5Y Neuroblastoma Cells

Human SH-SY5Y neuroblastoma cells are widely utilized in in vitro studies to dissect out pathogenetic mechanisms of neurodegenerative disorders. These cells are considered as neuronal precursors and differentiate into more mature neuronal phenotypes under selected growth conditions. In this study, in order to decipher the pathways and cellular processes underlying neuroblastoma cell differentiation in vitro, we performed systematic transcriptomic (RNA-seq) and bioinformatic analysis of SH-SY5Y cells differentiated according to a two-step paradigm: retinoic acid treatment followed by enriched neurobasal medium. Categorization of 1989 differentially expressed genes (DEGs) identified in differentiated cells functionally linked them to changes in cell morphology including remodelling of plasma membrane and cytoskeleton, and neuritogenesis. Seventy-three DEGs were assigned to axonal guidance signalling pathway, and the expression of selected gene products such as neurotrophin receptors, the functionally related SLITRK6, and semaphorins, was validated by immunoblotting. Along with these findings, the differentiated cells exhibited an ability to elongate longer axonal process as assessed by the neuronal cytoskeletal markers biochemical characterization and morphometric evaluation. Recognition of molecular events occurring in differentiated SH-SY5Y cells is critical to accurately interpret the cellular responses to specific stimuli in studies on disease pathogenesis.

Ligand-mediated Galectin-1 endocytosis prevents intraneural H2O2 production promoting F-actin dynamics reactivation and axonal re-growth

Axonal growth cone collapse following spinal cord injury (SCI) is promoted by semaphorin3A (Sema3A) signaling via PlexinA4 surface receptor. This interaction triggers intracellular signaling events leading to increased hydrogen peroxide levels which in turn promote filamentous actin (F-actin) destabilization and subsequent inhibition of axonal re-growth. In the current study, we demonstrated that treatment with galectin-1 (Gal-1), in its dimeric form, promotes a decrease in hydrogen peroxide (H2O2) levels and F-actin repolimerization in the growth cone and in the filopodium of neuron surfaces. This effect was dependent on the carbohydrate recognition activity of Gal-1, as it was prevented using a Gal-1 mutant lacking carbohydrate-binding activity. Furthermore, Gal-1 promoted its own active ligand-mediated endocytosis together with the PlexinA4 receptor, through mechanisms involving complex branched N-glycans. In summary, our results suggest that Gal-1, mainly in its dimeric form, promotes re-activation of actin cytoskeleton dynamics via internalization of the PlexinA4/Gal-1 complex. This mechanism could explain, at least in part, critical events in axonal regeneration including the full axonal re-growth process, de novo formation of synapse clustering, axonal re-myelination and functional recovery of coordinated locomotor activities in an in vivo acute and chronic SCI model.

SIGNIFICANCE STATEMENT

Axonal regeneration is a response of injured nerve cells critical for nerve repair in human spinal cord injury. Understanding the molecular mechanisms controlling nerve repair by Galectin-1, may be critical for therapeutic intervention. Our results show that Galectin-1; in its dimeric form, interferes with hydrogen peroxide production triggered by Semaphorin3A. The high levels of this reactive oxygen species (ROS) seem to be the main factor preventing axonal regeneration due to promotion of actin depolymerization at the axonal growth cone. Thus, Galectin-1 administration emerges as a novel therapeutic modality for promoting nerve repair and preventing axonal loss.

An Overview of VEGF-Mediated Signal Transduction

Vascular endothelial growth factor (VEGF) plays a fundamental role in angiogenesis and endothelial cell biology, and has been the subject of intense study as a result. VEGF acts via a diverse and complex range of signaling pathways, with new targets constantly being discovered. This review attempts to summarize the current state of knowledge regarding VEGF cell signaling in endothelial and cardiovascular biology, with a particular emphasis on its role in angiogenesis.

Neuropilin-1 mediates vascular permeability independently of vascular endothelial growth factor receptor-2 activation

Neuropilin-1 (NRP1) regulates developmental and pathological angiogenesis, arteriogenesis, and vascular permeability, acting as a coreceptor for semaphorin 3A (Sema3A) and the 165-amino acid isoform of vascular endothelial growth factor A (VEGF-A165). NRP1 is also the receptor for the CendR peptides, a class of cell- and tissue-penetrating peptides with a specific R-x-x-R carboxyl-terminal motif. Because the cytoplasmic domain of NRP1 lacks catalytic activity, NRP1 is mainly thought to act through the recruitment and binding to other receptors. We report here that the NRP1 intracellular domain mediates vascular permeability. Stimulation with VEGF-A165, a ligand-blocking antibody, and a CendR peptide led to NRP1 accumulation at cell-cell contacts in endothelial cell monolayers, increased cellular permeability in vitro and vascular leakage in vivo. Biochemical analyses, VEGF receptor-2 (VEGFR-2) silencing, and the use of a specific VEGFR blocker established that the effects induced by the CendR peptide and the antibody were independent of VEGFR-2. Moreover, leakage assays in mice expressing a mutant NRP1 lacking the cytoplasmic domain revealed that this domain was required for NRP1-induced vascular permeability in vivo. Hence, these data define a vascular permeability pathway mediated by NRP1 but independent of VEGFR-2 activation.

Viral glycoproteins: biological role and application in diagnosis

The viruses that infect humans cause a huge global disease burden and produce immense challenge towards healthcare system. Glycoproteins are one of the major components of human pathogenic viruses. They have been demonstrated to have important role(s) in infection and immunity. Concomitantly high titres of antibodies against these antigenic viral glycoproteins have paved the way for development of novel diagnostics. Availability of appropriate biomarkers is necessary for advance diagnosis of infectious diseases especially in case of outbreaks. As human mobilization has increased manifold nowadays, dissemination of infectious agents became quicker that paves the need of rapid diagnostic system. In case of viral infection it is an emergency as virus spreads and mutates very fast. This review encircles the vast arena of viral glycoproteins, their importance in health and disease and their diagnostic applications.

Chemoenzymatically prepared konjac ceramide inhibits NGF-induced neurite outgrowth by a semaphorin 3A-like action

Dietary sphingolipids such as glucosylceramide (GlcCer) are potential nutritional factors associated with prevention of metabolic syndrome. Our current understanding is that dietary GlcCer is degraded to ceramide and further metabolized to sphingoid bases in the intestine. However, ceramide is only found in trace amounts in food plants and thus is frequently taken as GlcCer in a health supplement. In the present study, we successfully prepared konjac ceramide (kCer) using endoglycoceramidase I (EGCase I). Konjac, a plant tuber, is an enriched source of GlcCer (kGlcCer), and has been commercialized as a dietary supplement to improve dry skin and itching that are caused by a deficiency of epidermal ceramide. Nerve growth factor (NGF) produced by skin cells is one of the itch factors in the stratum corneum of the skin. Semaphorin 3A (Sema 3A) has been known to inhibit NGF-induced neurite outgrowth of epidermal nerve fibers. It is well known that the itch sensation is regulated by the balance between NGF and Sema 3A. In the present study, while kGlcCer did not show an in vitro inhibitory effect on NGF-induced neurite outgrowth of PC12 cells, kCer was demonstrated to inhibit a remarkable neurite outgrowth. In addition, the effect of kCer was similar to that of Sema 3A in cell morphological changes and neurite retractions, but different from C2-Ceramide. kCer showed a Sema 3A-like action, causing CRMP2 phosphorylation, which results in a collapse of neurite growth cones. Thus, it is expected that kCer is an advanced konjac ceramide material that may have neurite outgrowth-specific action to relieve uncontrolled and serious itching, in particular, from atopic eczema.

Structural mechanisms of plexin signaling

Signaling through plexin, the major cell surface receptor for semaphorin, plays critical roles in regulating processes such as neuronal axon guidance, angiogenesis and immune response. Plexin is normally kept inactive in the absence of semaphorin. Upon binding of semaphorin to the extracellular region, plexin is activated and transduces signal to the inside of the cell through its cytoplasmic region. The GTPase Activating Protein (GAP) domain in the plexin cytoplasmic region mediates the major intracellular signaling pathway. The substrate specificity and regulation mechanisms of the GAP domain have only been revealed recently. Many intracellular proteins serve as either upstream regulators or downstream transducers by directly interacting with plexin. The mechanisms of action for some of these proteins also start to emerge from recent studies. We review here these advances in the mechanistic understanding of plexin intracellular signaling from a structural perspective.

MicroRNA-30b promotes axon outgrowth of retinal ganglion cells by inhibiting Semaphorin3A expression

Semaphorin3A (Sema3A) is a major inhibitory factor of optic nerve (ON) regeneration post-injury. Many microRNAs (miRNAs) are expressed specifically in the mammalian brain and retina and are dynamically regulated during development, suggesting that this group of miRNAs may be associated with neural development. We found that microRNA-30b (miR-30b) bound to the three prime untranslated region (3' UTR) of Sema3A and inhibited the expression of Sema3A mRNA. The mRNA expression level of miR-30b and the protein expression levels of Sema3A, Neuropilin1 (NRP1), PlexinA1 (PlexA1), phosphorylated p38MAPK (p-p38MAPK), and active caspase-3 were all upregulated in retinas from rats with a damaged ON relative to those with an intact ON. Transfection of cultured retinal ganglion cells (RGCs) with an miR-30b mimic led to decreased levels of Sema3A, NRP1, PlexA1, p-p38MAPK, and active caspase-3 protein expression, as well as axon elongation and reduced levels of apoptosis. These findings provide evidence that miR-30b inhibits Sema3A expression. Decreased Sema3A expression promotes axon outgrowth in RGCs due to reduced levels of Sema3A binding to NRP1 and PlexA1 and simultaneously reduces apoptosis by inhibiting the p38MAPK and caspase-3 pathways. Our findings provide the first evidence that miR-30b-mediated Sema3A downregulation may serve as a new strategy for the clinical treatment of ON injury.

Understanding cell signalling systems: paving the way for new therapies

The cell-to-cell signalling mechanisms of multi-cellular organisms orchestrate human development during embryogenesis and control homeostasis in adult tissues. These are mechanisms vital to human health and perturbation of cell-to-cell signalling is a contributing factor in many pathologies including cancer. The semaphorin cell guidance cues and their cognate plexin receptors exemplify a cell-to-cell signalling system for which insights into mechanistic principles are emerging. X-ray crystallographic data from Diamond beam lines have enabled us to probe the inner workings of semaphorin-plexin signalling to atomic-level resolutions. Importantly, we can complement protein crystallographic results with biophysical and cellular studies to dovetail structural information with functional impact. The signature seven-bladed β propeller 'sema' domain of the semaphorins forms a dimer; in contrast the equivalent domain in the plexins is monomeric. The generic architecture of a semaphorin-plexin complex is characterized by the dimeric semaphorin cross-linking two copies of the plexin receptor. For specific family members, the co-receptor neuropilin serves to bolster this architecture, but in all cases, the dimeric interaction lies at the core of the ligand receptor complex, providing the essential trigger for signalling.

Chronic allograft rejection: a fresh look

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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