Slit-2/Robo-1 modulates the CXCL12/CXCR4-induced chemotaxis of T cells

An autophagy program that promotes T cell egress from the lymph node controls responses to immune checkpoint blockade

Lymphatic endothelial cells (LECs) of the lymph node (LN) parenchyma orchestrate leukocyte trafficking and peripheral T cell dynamics. T cell responses to immunotherapy largely rely on peripheral T cell recruitment in tumors. Yet, a systematic and molecular understanding of how LECs within the LNs control T cell dynamics under steady-state and tumor-bearing conditions is lacking. Intravital imaging combined with immune phenotyping shows that LEC-specific deletion of the essential autophagy gene Atg5 alters intranodal positioning of lymphocytes and accrues their persistence in the LNs by increasing the availability of the main egress signal sphingosine-1-phosphate. Single-cell RNA sequencing of tumor-draining LNs shows that loss of ATG5 remodels niche-specific LEC phenotypes involved in molecular pathways regulating lymphocyte trafficking and LEC-T cell interactions. Functionally, loss of LEC autophagy prevents recruitment of tumor-infiltrating T and natural killer cells and abrogates response to immunotherapy. Thus, an LEC-autophagy program boosts immune-checkpoint responses by guiding systemic T cell dynamics.

Recombinant Slit2 suppresses neuroinflammation and Cdc42-mediated brain infiltration of peripheral immune cells via Robo1-srGAP1 pathway in a rat model of germinal matrix hemorrhage

BACKGROUND

Germinal matrix hemorrhage (GMH) is a devastating neonatal stroke, in which neuroinflammation is a critical pathological contributor. Slit2, a secreted extracellular matrix protein, plays a repulsive role in axon guidance and leukocyte chemotaxis via the roundabout1 (Robo1) receptor. This study aimed to explore effects of recombinant Slit2 on neuroinflammation and the underlying mechanism in a rat model of GMH.

METHODS

GMH was induced by stereotactically infusing 0.3 U of bacterial collagenase into the germinal matrix of 7-day-old Sprague Dawley rats. Recombinant Slit2 or its vehicle was administered intranasally at 1 h after GMH and daily for 3 consecutive days. A decoy receptor recombinant Robo1 was co-administered with recombinant Slit2 after GMH. Slit2 siRNA, srGAP1 siRNA or the scrambled sequences were administered intracerebroventricularly 24 h before GMH. Neurobehavior, brain water content, Western blotting, immunofluorescence staining and Cdc42 activity assays were performed.

RESULTS

The endogenous brain Slit2 and Robo1 expressions were increased after GMH. Robo1 was expressed on neuron, astrocytes and infiltrated peripheral immune cells in the brain. Endogenous Slit2 knockdown by Slit2 siRNA exacerbated brain edema and neurological deficits following GMH. Recombinant Slit2 (rSlit2) reduced neurological deficits, proinflammatory cytokines, intercellular adhesion molecules, peripheral immune cell markers, neuronal apoptosis and Cdc42 activity in the brain tissue after GMH. The anti-neuroinflammation effects were reversed by recombinant Robo1 co-administration or srGAP1 siRNA.

CONCLUSIONS

Recombinant Slit2 reduced neuroinflammation and neuron apoptosis after GMH. Its anti-neuroinflammation effects by suppressing onCdc42-mediated brain peripheral immune cells infiltration was at least in part via Robo1-srGAP1 pathway. These results imply that recombinant Slit2 may have potentials as a therapeutic option for neonatal brain injuries.

Role of the SLIT-ROBO signaling pathway in renal pathophysiology and various renal diseases

SLIT ligand and its receptor ROBO were initially recognized for their role in axon guidance in central nervous system development. In recent years, as research has advanced, the role of the SLIT-ROBO signaling pathway has gradually expanded from axonal repulsion to cell migration, tumor development, angiogenesis, and bone metabolism. As a secreted protein, SLIT regulates various pathophysiological processes in the kidney, such as proinflammatory responses and fibrosis progression. Many studies have shown that SLIT-ROBO is extensively involved in various aspects of kidney development and maintenance of structure and function. The SLIT-ROBO signaling pathway also plays an important role in different types of kidney disease. This article reviews the advances in the study of the SLIT-ROBO pathway in various renal pathophysiological and kidney disorders and proposes new directions for further research in this field.

Platelet formation and activation are influenced by neuronal guidance proteins

Platelets are anucleate blood cells derived from megakaryocytes. They link the fundamental functions of hemostasis, inflammation and host defense. They undergo intracellular calcium flux, negatively charged phospholipid translocation, granule release and shape change to adhere to collagen, fibrin and each other, forming aggregates, which are key to several of their functions. In all these dynamic processes, the cytoskeleton plays a crucial role. Neuronal guidance proteins (NGPs) form attractive and repulsive signals to drive neuronal axon navigation and thus refine neuronal circuits. By binding to their target receptors, NGPs rearrange the cytoskeleton to mediate neuron motility. In recent decades, evidence has indicated that NGPs perform important immunomodulatory functions and influence platelet function. In this review, we highlight the roles of NGPs in platelet formation and activation.

Genomic evolution and the impact of SLIT2 mutation in relapsed intrahepatic cholangiocarcinoma

BACKGROUND AND AIMS

Intrahepatic cholangiocarcinoma (ICC) is aggressive and has high rates of relapse, conferring poor long-term survival after curative resection. Little is known about the genomic evolution that occurs during ICC relapse.

APPROACH AND RESULTS

We conducted whole-exome sequencing of 30 paired primary and relapsed tumors from 10 patients with ICC who received curative resection. We sought to identify frequently altered genes, infer tumor subclonal architectures, and track genomic evolution from primary to relapsed tumors. We examined functional effects and the mechanism of action of SLIT2, a gene specifically mutated in relapsed tumors, on tumor growth and metastasis and the tumor microenvironment. Our results indicated that relapsed ICCs were genetically derived from intrahepatic dissemination of primary tumors. However, they acquired additional mutations while maintaining most drivers, such as TP53 and IDH1. Multiregion sequencing suggested polyclonal seeding of ICC dissemination. Four of 10 relapsed ICCs acquired SLIT2 mutations that were not present in the corresponding primary tumors. Validation in an expanded sample revealed SLIT2 mutations in 2.3% (1/44) of primary ICCs and 29.5% (13/44) of relapsed ICCs. Biofunctional investigations revealed that inactivating mutation of SLIT2 resulted in activation of PI3K-Akt signaling in ICC cells, directly enhanced neutrophil chemotaxis, mediated tumor-associated neutrophil infiltration, and contributed to ICC growth and metastasis.

CONCLUSIONS

We characterized genomic evolution during ICC relapse and identified SLIT2 as a driver of tumor dissemination and tumor-associated neutrophil infiltration.

The expression of Slit2 and Robo1 increased during retinoic acid syndrome in acute promyelocytic leukemia and impacted differentiated cell migration

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The upregulation of Robo1 and Slit2 was first found in APL patients.

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The positive correlation between Robo1/Slit2 and retinoic acid syndrome was first demonstrated.

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It was demonstrated for the first time that Slit2 induces the migration of differentiated cells.

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Slit2 did not inhibit il8-induced differentiated cell migration.

Retinoic acid syndrome (RAS) is a serious complication developed during the induction therapy of acute promyelocytic leukemia (APL). Cytokines and differentiated cells migration play important roles in the development of RAS. Slit guidance ligand 2 (Slit2) and roundabout 1 (Robo1) involve in cell migration. Our study aimed to investigate the expression of Slit2 and Robo1 in APL and check whether they affected promyelocytes migration. 62 cases of newly diagnosed APL patients were involved and received all-trans retinoic acid (ATRA) and arsenic trioxide as induction therapy. Bone marrow cells (BMCs) were obtained on days 0 and 28, and promyelocytes and plasma were collected from day 1 to day 21. The expression of Robo1 in promyelocytes, and that of Slit2 and cytokines, including IL-8,IL-1β and others, in serum were monitored. 20 healthy individuals donated their cells as control. Of the 62 APL patients, 16 (25.81%) patients developed RAS. The expression of Robo1, Slit2 and IL-8 increased significantly with the development of RAS. In the 16 patients with RAS, levels of Slit2, Robo1 and IL-8 were higher during the development of RAS than before or after the RAS (P < 0.05). RhSlit2-N and rhIL-8 induced cells migration, and the migration induced by IL-8 was not inhibited by rhSlit2-N. Elevated Slit2 and Robo1 levels might be useful markers for the diagnosis and treatment of RAS. The levels of Slit2, Robo1 and IL-8 showed a positive correlation with the severity of RAS. Slit2 and IL-8 promoted the migration of differentiated cells.

SLIT2/ROBO signaling in tumor-associated microglia and macrophages drives glioblastoma immunosuppression and vascular dysmorphia

SLIT2 is a secreted polypeptide that guides migration of cells expressing Roundabout 1 and 2 (ROBO1 and ROBO2) receptors. Herein, we investigated SLIT2/ROBO signaling effects in gliomas. In patients with glioblastoma (GBM), SLIT2 expression increased with malignant progression and correlated with poor survival and immunosuppression. Knockdown of SLIT2 in mouse glioma cells and patient-derived GBM xenografts reduced tumor growth and rendered tumors sensitive to immunotherapy. Tumor cell SLIT2 knockdown inhibited macrophage invasion and promoted a cytotoxic gene expression profile, which improved tumor vessel function and enhanced efficacy of chemotherapy and immunotherapy. Mechanistically, SLIT2 promoted microglia/macrophage chemotaxis and tumor-supportive polarization via ROBO1- and ROBO2-mediated PI3K-γ activation. Macrophage Robo1 and Robo2 deletion and systemic SLIT2 trap delivery mimicked SLIT2 knockdown effects on tumor growth and the tumor microenvironment (TME), revealing SLIT2 signaling through macrophage ROBOs as a potentially novel regulator of the GBM microenvironment and immunotherapeutic target for brain tumors.

Natural Killer Cell Interactions With Myeloid Derived Suppressor Cells in the Tumor Microenvironment and Implications for Cancer Immunotherapy

The tumor microenvironment (TME) is a complex and heterogeneous environment composed of cancer cells, tumor stroma, a mixture of tissue-resident and infiltrating immune cells, secreted factors, and extracellular matrix proteins. Natural killer (NK) cells play a vital role in fighting tumors, but chronic stimulation and immunosuppression in the TME lead to NK cell exhaustion and limited antitumor functions. Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of myeloid cells with potent immunosuppressive activity that gradually accumulate in tumor tissues. MDSCs interact with innate and adaptive immune cells and play a crucial role in negatively regulating the immune response to tumors. This review discusses MDSC-mediated NK cell regulation within the TME, focusing on critical cellular and molecular interactions. We review current strategies that target MDSC-mediated immunosuppression to enhance NK cell cytotoxic antitumor activity. We also speculate on how NK cell-based antitumor immunotherapy could be improved.

TIPS: trajectory inference of pathway significance through pseudotime comparison for functional assessment of single-cell RNAseq data

Recent advances in bioinformatics analyses have led to the development of novel tools enabling the capture and trajectory mapping of single-cell RNA sequencing (scRNAseq) data. However, there is a lack of methods to assess the contributions of biological pathways and transcription factors to an overall developmental trajectory mapped from scRNAseq data. In this manuscript, we present a simplified approach for trajectory inference of pathway significance (TIPS) that leverages existing knowledgebases of functional pathways and other gene lists to provide further mechanistic insights into a biological process. TIPS identifies key pathways which contribute to a process of interest, as well as the individual genes that best reflect these changes. TIPS also provides insight into the relative timing of pathway changes, as well as a suite of visualizations to enable simplified data interpretation of scRNAseq libraries generated using a wide range of techniques. The TIPS package can be run through either a web server or downloaded as a user-friendly GUI run in R, and may serve as a useful tool to help biologists perform deeper functional analyses and visualization of their single-cell data.

The neurorepellent, Slit2, prevents macrophage lipid loading by inhibiting CD36-dependent binding and internalization of oxidized low-density lipoprotein

Atherosclerosis is characterized by retention of modified lipoproteins, especially oxidized low density lipoprotein (oxLDL) within the sub-endothelial space of affected blood vessels. Recruited monocyte-derived and tissue-resident macrophages subsequently ingest oxLDL by binding and internalizing oxLDL via scavenger receptors, particularly CD36. The secreted neurorepellent, Slit2, acting through its transmembrane receptor, Roundabout-1 (Robo-1), was previously shown to inhibit recruitment of monocytes into nascent atherosclerotic lesions. The effects of Slit2 on oxLDL uptake by macrophages have not been explored. We report here that Slit2 inhibits uptake of oxLDL by human and murine macrophages, and the resulting formation of foam cells, in a Rac1-dependent and CD36-dependent manner. Exposure of macrophages to Slit2 prevented binding of oxLDL to the surface of cells. Using super-resolution microscopy, we observed that exposure of macrophages to Slit2 induced profound cytoskeletal remodeling with formation of a thick ring of cortical actin within which clusters of CD36 could not aggregate, thereby attenuating binding of oxLDL to the surface of cells. By inhibiting recruitment of monocytes into early atherosclerotic lesions, and the subsequent binding and internalization of oxLDL by macrophages, Slit2 could represent a potent new tool to combat individual steps that collectively result in progression of atherosclerosis.

Structural insights and evaluation of the potential impact of missense variants on the interactions of SLIT2 with ROBO1/4 in cancer progression

The cognate interaction of ROBO1/4 with its ligand SLIT2 is known to be involved in lung cancer progression. However, the precise role of genetic variants, disrupting the molecular interactions is less understood. All cancer-associated missense variants of ROBO1/4 and SLIT2 from COSMIC were screened for their pathogenicity. Homology modelling was done in Modeller 9.17, followed by molecular simulation in GROMACS. Rigid docking was performed for the cognate partners in PatchDock with refinement in HADDOCK server. Post-docking alterations in conformational, stoichiometric, as well as structural parameters, were assessed. The disruptive variants were ranked using a weighted scoring scheme. In silico prioritisation of 825 variants revealed 379 to be potentially pathogenic out of which, about 12% of the variants, i.e. ROBO1 (14), ROBO4 (8), and SLIT2 (23) altered the cognate docking. Six variants of ROBO1 and 5 variants of ROBO4 were identified as "high disruptors" of interactions with SLIT2 wild type. Likewise, 17 and 13 variants of SLIT2 were found to be "high disruptors" of its interaction with ROBO1 and ROBO4, respectively. Our study is the first report on the impact of cancer-associated missense variants on ROBO1/4 and SLIT2 interactions that might be the drivers of lung cancer progression.

Placental multipotent mesenchymal stromal cell-derived Slit2 may regulate macrophage motility during placental infection

Slit proteins have been reported to act as axonal repellents in Drosophila; however, their role in the placental microenvironment has not been explored. In this study, we found that human placental multipotent mesenchymal stromal cells (hPMSCs) constitutively express Slit2. Therefore, we hypothesized that Slit2 expressed by hPMSCs could be involved in macrophage migration during placental inflammation through membrane cognate Roundabout (Robo) receptor signaling. In order to develop a preclinical in vitro mouse model of hPMSCs in treatment of perinatal infection, RAW 264.7 cells were used in this study. Slit2 interacted with Robo4 that was highly expressed in RAW 264.7 macrophages: their interaction increased the adhesive ability of RAW 264.7 cells and inhibited migration. Lipopolysaccharide (LPS)-induced CD11bCD18 expression could be inhibited by Slit2 and by hPMSC-conditioned medium (CM). LPS-induced activation of p38 and Rap1 was also attenuated by Slit2 and by hPMSC-CM. Noticeably, these inhibitory effects of hPMSC-CM decreased after depletion of Slit2 from the CM. Furthermore, we found that p38 siRNA inhibited LPS-induced Rap1 expression in RAW 264.7 cells, indicating that Rap1 functions downstream of p38 signaling. p38 siRNA increased cell adhesion and inhibited migration through reducing LPS-stimulated CD11bCD18 expression in RAW 264.7 cells. Thus, hPMSC-derived Slit2 may inhibit LPS-induced CD11bCD18 expression to decrease cell migration and increase adhesion through modulating the activity and motility of inflammatory macrophages in placenta. This may represent a novel mechanism for LPS-induced placental infection.

SLIT2/ROBO1-signaling inhibits macropinocytosis by opposing cortical cytoskeletal remodeling

Macropinocytosis is essential for myeloid cells to survey their environment and for growth of RAS-transformed cancer cells. Several growth factors and inflammatory stimuli are known to induce macropinocytosis, but its endogenous inhibitors have remained elusive. Stimulation of Roundabout receptors by Slit ligands inhibits directional migration of many cell types, including immune cells and cancer cells. We report that SLIT2 inhibits macropinocytosis in vitro and in vivo by inducing cytoskeletal changes in macrophages. In mice, SLIT2 attenuates the uptake of muramyl dipeptide, thereby preventing NOD2-dependent activation of NF-κB and consequent secretion of pro-inflammatory chemokine, CXCL1. Conversely, blocking the action of endogenous SLIT2 enhances CXCL1 secretion. SLIT2 also inhibits macropinocytosis in RAS-transformed cancer cells, thereby decreasing their survival in nutrient-deficient conditions which resemble tumor microenvironment. Our results identify SLIT2 as a physiological inhibitor of macropinocytosis and challenge the conventional notion that signals that enhance macropinocytosis negatively regulate cell migration, and vice versa.

The potential of Slit2 as a therapeutic target for central nervous system disorders

Introduction: Slit2 is an extracellular matrix protein that regulates migration of developing axons during central nervous system (CNS) development. Roundabout (Robo) receptors expressed by various cell types in the CNS, mediate intracellular signal transduction pathways for Slit2. Recent studies indicate that Slit2 plays important protective roles in a myriad of processes such as cell migration, immune response, vascular permeability, and angiogenesis in CNS pathologies. Areas covered: This review provides an overview of the diverse functions of Slit2 in CNS disorders and discusses the potential of Slit2 as a therapeutic target. We reviewed preclinical studies reporting the role of Slit2 in various CNS disease models, transgenic animal research, and rodent models that utilized Slit2 as a therapy. Expert opinion: Slit2 exerts a wide array of beneficial effects ranging from anti-migration, blood-brain barrier (BBB) protection, inhibition of peripheral immune cell infiltration, and anti-apoptosis in various disease models. However, a dual role of Slit2 in endothelial permeability has been observed in transgenic animals. Further research on Slit2 will be crucial including key issues such as effects of transgenic overexpression versus exogenous Slit2, function of Slit2 dependent on cellular expression of Robo receptors and the underlying pathology for potential clinical translation.

Silymarin inhibited DU145 cells by activating SLIT2 protein and suppressing expression of CXCR4

Among other cancers, prostate cancer is globally the second most rampant one with the incidence of 29.4% among men. SLIT2/ROBO1 signaling is very crucial pathway causally implicated in many cancers and reported to inhibit a variety of cancer cell types. CXCR4 is a chemokine receptor implicated in cancer progression. Silymarin is a phytochemical, of which anti-carcinogenic activity was suggested in various cancers, including prostate cancer. However, there are no studies examining the effect of silymarin on SLIT2-Robo1-CXCR4 axis. Herein, our goal is to explore cytotoxic and morphological effects of silymarin on DU145 cells and to reveal its role in Slit2/Robo and CXCR1 pathway. First, 24, 48 and 72 h-long cytotoxicity tests were performed for dose analysis of silymarin, followed H-E stain for morphological evaluation with varying doses of silymarin. Afterward, western blot and immunocytochemistry analyses were carried out for SLIT2, ROBO1 and CXCR4 proteins. According to MTT analysis, IC50 concentrations for silymarin were 315, 126 and 70 µM against DU145 cells for 24, 48 and 72 h treatments. In H-E, several apoptotic hallmarks, including, condensed, kidney-shaped and eccentric nuclei, membrane blebbings and apoptotic body formations were observed. Silymarin increased the expressions of SLIT2 and ROBO1 while decreased CXCR4 when compared to control group in immunocytochemistry and Western blot. To summarize, silymarin inhibited DU145 cells dose-dependently by activating SLIT2 protein and inhibiting expression of CXCR4. This study is the first examining the interplay between Slit2-Robo1-CXCR4 proteins and silymarin in DU145 cells. We believe that our study will provide new insights for future studies.

Recombinant Slit2 attenuates neuronal apoptosis via the Robo1-srGAP1 pathway in a rat model of neonatal HIE

Apoptosis following hypoxic-ischemic injury to the brain plays a major role in neuronal cell death. The neonatal brain is more susceptible to injury as the cortical neurons are immature and there are lower levels of antioxidants. Slit2, an extracellular matrix protein, has been shown to be neuroprotective in various models of neurological diseases. However, there is no information about the role of Slit2 in neonatal hypoxia-ischemia. In this study, we evaluated the effect of Slit2 and its receptor Robo1 in a rat model with neonatal HIE. 10-day old rat pups were used to create the neonatal HIE model. The right common carotid artery was ligated followed by 2.5 h of hypoxia. Recombinant Slit2 was administered intranasally 1 h post HI, recombinant Robo1 was used as a decoy receptor and administered intranasally 1h before HI and srGAP1-siRNA was administered intracerebroventricularly 24 h before HI. Brain infarct area measurement, short-term and long-term neurological function tests, Western blot, immunofluorescence staining, Fluoro-Jade C staining, Nissl staining and TUNEL staining were the assessments done following drug administration. Recombinant Slit2 administration reduced neuronal apoptosis and neurological deficits after neonatal HIE which were reversed by co-administration of recombinant Robo1 and srGAP1-siRNA administration. Recombinant Slit2 showed improved outcomes possibly via the robo1-srGAP1 pathway which mediated the inhibition of RhoA. In this study, the results suggest that Slit2 may help in attenuation of apoptosis and could be a therapeutic agent for treatment of neonatal hypoxic ischemic encephalopathy.

Targeting the SLIT/ROBO pathway in tumor progression: molecular mechanisms and therapeutic perspectives

The SLITs (SLIT1, SLIT2, and SLIT3) are a family of secreted proteins that mediate positional interactions between cells and their environment during development by signaling through ROBO receptors (ROBO1, ROBO2, ROBO3, and ROBO4). The SLIT/ROBO signaling pathway has been shown to participate in axonal repulsion, axon guidance, and neuronal migration in the nervous system and the formation of the vascular system. However, the role of the SLIT/ROBO pathway has not been thoroughly clarified in tumor development. The SLIT/ROBO pathway can produce both beneficial and detrimental effects in the growth of malignant cells. It has been confirmed that SLIT/ROBO play contradictory roles in tumorigenesis. Here, we discuss the tumor promotion and tumor suppression roles of the SLIT/ROBO pathway in tumor growth, angiogenesis, migration, and the tumor microenvironment. Understanding these roles will help us develop more effective cancer therapies.

Lung Tumorigenesis Alters the Expression of Slit2-exon15 Splicing Variants in Tumor Microenvironment

Slit2 expression is downregulated in various cancers, including lung cancer. We identified two Slit2 splicing variants at exon15—Slit2-WT and Slit2-ΔE15. In the RT-PCR analyses, the Slit2-WT isoform was predominantly expressed in all the lung cancer specimens and in their normal lung counterparts, whereas Slit2-ΔE15 was equivalently or predominantly expressed in 41% of the pneumothorax specimens. A kRas^G12D transgenic mice system was used to study the effects of tumorigenesis on the expressions of the Slit2-exon15 isoforms. The results revealed that a kRas^G12D-induced lung tumor increased the Slit2-WT/Slit2-ΔE15 ratio and total Slit2 expression level. However, the lung tumors generated via a tail vein injection of lung cancer cells decreased the Slit2-WT/Slit2-ΔE15 ratio and total Slit2 expression level. Interestingly, the lipopolysaccharide (LPS)-induced lung inflammation also decreased the Slit2-WT/Slit2-ΔE15 ratio. Since Slit2 functions as an anti-inflammatory factor, the expression of Slit2 increases in kRas^G12D lungs, which indicates that Slit2 suppresses immunity during tumorigenesis. However, an injection of lung cancer cells via the tail vein and the LPS-induced lung inflammation both decreased the Slit2 expression. The increased Slit2 in the tumor microenvironment was mostly Slit2-WT, which lacks growth inhibitory activity. Thus, the results of our study suggested that the upregulation of Slit2-WT, but not Slit2-ΔE15, in a cancer microenvironment is an important factor in suppressing immunity while not interfering with cancer growth.

Different Isoforms of the Neuronal Guidance Molecule Slit2 Directly Cause Chemoattraction or Chemorepulsion of Human Neutrophils

The movement of neutrophils between blood and tissues appears to be regulated by chemoattractants and chemorepellents. Compared with neutrophil chemoattractants, relatively little is known about neutrophil chemorepellents. Slit proteins are endogenously cleaved into a variety of N- and C-terminal fragments, and these fragments are neuronal chemorepellents and inhibit chemoattraction of many cell types, including neutrophils. In this report, we show that the ∼140-kDa N-terminal Slit2 fragment (Slit2-N) is a chemoattractant and the ∼110-kDa N-terminal Slit2 fragment (Slit2-S) is a chemorepellent for human neutrophils. The effects of both Slit2 fragments were blocked by Abs to the Slit2 receptor Roundabout homolog 1 or the Slit2 coreceptor Syndecan-4. Slit2-N did not appear to activate Ras but increased phosphatidylinositol 3,4,5-triphosphate levels. Slit2-N-induced chemoattraction was unaffected by Ras inhibitors, reversed by PI3K inhibitors, and blocked by Cdc42 and Rac inhibitors. In contrast, Slit2-S activated Ras but did not increase phosphatidylinositol 3,4,5-triphosphate levels. Slit2-S-induced chemorepulsion was blocked by Ras and Rac inhibitors, not affected by PI3K inhibitors, and reversed by Cdc42 inhibitors. Slit2-N, but not Slit2-S, increased neutrophil adhesion, myosin L chain 2 phosphorylation, and polarized actin formation and single pseudopods at the leading edge of cells. Slit2-S induced multiple pseudopods. These data suggest that Slit2 isoforms use similar receptors but different intracellular signaling pathways and have different effects on the cytoskeleton and pseudopods to induce neutrophil chemoattraction or chemorepulsion.

Slit-Robo signalling in heart development

The Slit ligands and their Robo receptors are well-known for their roles during axon guidance in the central nervous system but are still relatively unknown in the cardiac field. However, data from different animal models suggest a broad involvement of the pathway in many aspects of heart development, from cardiac cell migration and alignment, lumen formation, chamber formation, to the formation of the ventricular septum, semilunar and atrioventricular valves, caval veins, and pericardium. Absence of one or more of the genes in the pathway results in defects ranging from bicuspid aortic valves to ventricular septal defects and abnormal venous connections to the heart. Congenital heart defects are the most common congenital malformations found in life new-born babies and progress in methods for large scale human genetic testing has significantly enhanced the identification of new causative genes involved in human congenital heart disease. Recently, loss of function variants in ROBO1 have also been linked to ventricular septal defects and tetralogy of Fallot in patients. Here, we will give an overview of the role of the Slit-Robo signalling pathway in Drosophila, zebrafish, and mouse heart development. The extent of these data warrant further attention on the SLIT-ROBO signalling pathway as a candidate for an array of human congenital heart defects.

SUV39H2 promotes colorectal cancer proliferation and metastasis via tri-methylation of the SLIT1 promoter

Suppressor of variegation 3-9 homolog 2 (SUV39H2) is a member of the SUV39H subfamily of lysine methyltransferases. Its role in colorectal cancer (CRC) proliferation and metastasis has remained unexplored. Here, we determined that SUV39H2 was upregulated in CRC tissues compared with that in adjacent non-neoplastic tissues. Further statistical analysis revealed that high SUV39H2 expression was strongly associated with distant metastasis (P = 0.016) and TNM stage (P = 0.038) and predicted a shorter overall survival (OS; P = 0.018) and progression-free survival (PFS; P = 0.018) time for CRC patients. Both in vitro and in vivo assays demonstrated that ectopically expressed SUV39H2 enhanced CRC proliferation and metastasis, while SUV39H2 knockdown inhibited CRC proliferation and metastasis. A molecular screen of SUV39H2 targets found that SUV39H2 negatively regulated the expression of SLIT guidance ligand 1 (SLIT1). Moreover, rescue assays suggested that SLIT1 could antagonize the function of SUV39H2 in CRC. Mechanistic studies indicated that SUV39H2 can directly bind to the SLIT1 promoter, suppressing SLIT1 transcription by catalyzing histone H3 lysine 9 (H3K9) tri-methylation. In summary, we propose that SUV39H2 can predict CRC patient prognosis and stimulate CRC malignant phenotypes via SLIT1 promoter tri-methylation.

miR-218 suppresses epithelial-to-mesenchymal transition by targeting Robo1 and Ecop in lung adenocarcinoma cells

AIM

Although, miR-218 has been implicated in epithelial-to-mesenchymal transition process, the detailed mechanisms of miR-218 involvement in epithelial-to-mesenchymal transition in human lung adenocarcinoma cell are still unclear.

MATERIALS & METHODS

miR-218 function assays and its target gene analysis were performed.

RESULTS

miR-218 suppresses human lung adenocarcinoma cell migration and invasion and inhibits its target gene, Ecop and Robo1 expression, which subsequently suppresses NF-κB activity and its downstream targets.

CONCLUSION

miR-218 inhibits human lung adenocarcinoma cell migration and invasion via the suppression of Ecop and Robo1 expression, thus suggesting that miR-218 could serve as a potential therapeutic target.

Proteotranscriptomics Reveal Signaling Networks in the Ovarian Cancer Microenvironment

Ovarian cancer is characterized by early transcoelomic metastatic spread via the peritoneal fluid, where tumor cell spheroids (TU), tumor-associated T cells (TAT), and macrophages (TAM) create a unique microenvironment promoting cancer progression, chemoresistance, and immunosuppression. However, the underlying signaling mechanisms remain largely obscure. To chart these signaling networks, we performed comprehensive proteomic and transcriptomic analyses of TU, TAT, and TAM from ascites of ovarian cancer patients. We identify multiple intercellular signaling pathways driven by protein or lipid mediators that are associated with clinical outcome. Beyond cytokines, chemokines and growth factors, these include proteins of the extracellular matrix, immune checkpoint regulators, complement factors, and a prominent network of axon guidance molecules of the ephrin, semaphorin, and slit families. Intriguingly, both TU and TAM from patients with a predicted short survival selectively produce mediators supporting prometastatic events, including matrix remodeling, stemness, invasion, angiogenesis, and immunosuppression, whereas TAM associated with a longer survival express cytokines linked to effector T-cell chemoattraction and activation. In summary, our study uncovers previously unrecognized signaling networks in the ovarian cancer microenvironment that are of potential clinical relevance.

A map of human circular RNAs in clinically relevant tissues

Abstract

Cellular circular RNAs (circRNAs) are generated by head-to-tail splicing and are present in all multicellular organisms studied so far. Recently, circRNAs have emerged as a large class of RNA which can function as post-transcriptional regulators. It has also been shown that many circRNAs are tissue- and stage-specifically expressed. Moreover, the unusual stability and expression specificity make circRNAs important candidates for clinical biomarker research. Here, we present a circRNA expression resource of 20 human tissues highly relevant to disease-related research: vascular smooth muscle cells (VSMCs), human umbilical vein cells (HUVECs), artery endothelial cells (HUAECs), atrium, vena cava, neutrophils, platelets, cerebral cortex, placenta, and samples from mesenchymal stem cell differentiation. In eight different samples from a single donor, we found highly tissue-specific circRNA expression. Circular-to-linear RNA ratios revealed that many circRNAs were expressed higher than their linear host transcripts. Among the 71 validated circRNAs, we noticed potential biomarkers. In adenosine deaminase-deficient, severe combined immunodeficiency (ADA-SCID) patients and in Wiskott-Aldrich-Syndrome (WAS) patients’ samples, we found evidence for differential circRNA expression of genes that are involved in the molecular pathogenesis of both phenotypes. Our findings underscore the need to assess circRNAs in mechanisms of human disease.

Key messages

circRNA resource catalog of 20 clinically relevant tissues.

circRNA expression is highly tissue-specific.

circRNA transcripts are often more abundant than their linear host RNAs.

circRNAs can be differentially expressed in disease-associated genes.

Electronic supplementary material

The online version of this article (10.1007/s00109-017-1582-9) contains supplementary material, which is available to authorized users.

Slit2 suppresses endothelial cell proliferation and migration by inhibiting the VEGF-Notch signaling pathway

Slit homolog 2 (Slit2) is distributed in various tissues and participates in numerous cellular processes; however, the role of Slit2 in the regulation of angiogenesis remains controversial, since it has previously been reported to exert proangiogenic and antiangiogenic activities. The present study aimed to investigate the effects of Slit2 on vascular endothelial cell proliferation and migration in vitro, and to reveal the possible underlying signaling pathway. Aortic endothelial cells were isolated from Sprague Dawley rats and cultured. Cell proliferation assay, cell migration assay, immunocytochemistry and small interfering RNA transfection were subsequently performed. The results demonstrated that exogenous Slit2 administration markedly suppressed TNF-α-induced endothelial cell proliferation and migration in vitro. In addition, TNF-α application upregulated the protein expression levels of vascular endothelial growth factor (VEGF) and Notch in RAECs, whereas Slit2 administration downregulated VEGF and Notch expression in RAECs cultured in TNF-α conditioned medium. Further studies indicated that knockdown of VEGF suppressed the effects of TNF-α on the induction of RAEC proliferation and migration. VEGF knockdown-induced inhibition of RAEC proliferation and migration in TNF-α conditioned medium was also achieved without Slit2 administration. Furthermore, VEGF knockdown markedly decreased Notch1 and Notch2 expression. These results indicated that Slit2 suppresses TNF-α-induced vascular endothelial cell proliferation and migration in vitro by inhibiting the VEGF-Notch signaling pathway. Therefore, Slit2 may inhibit the proliferation and migration of endothelial cells during vascular development.

Panels of chemically-modified heparin polysaccharides and natural heparan sulfate saccharides both exhibit differences in binding to Slit and Robo, as well as variation between protein binding and cellular activity

Heparin/heparan sulfate (HS) glycosaminoglycans are required for Slit-Robo cellular responses. Evidence exists for interactions between each combination of Slit, Robo and heparin/HS and for formation of a ternary complex. Heparin/HS are complex mixtures displaying extensive structural diversity. The relevance of this diversity has been studied to a limited extent using a few select chemically-modified heparins as models of HS diversity. Here we extend these studies by parallel screening of structurally diverse panels of eight chemically-modified heparin polysaccharides and numerous natural HS oligosaccharide chromatographic fractions for binding to both Drosophila Slit and Robo N-terminal domains and for activation of a chick retina axon response to the Slit fragment. Both the polysaccharides and oligosaccharide fractions displayed variability in binding and cellular activity that could not be attributed solely to increasing sulfation, extending evidence for the importance of structural diversity to natural HS as well as model modified heparins. They also displayed differences in their interactions with Slit compared to Robo, with Robo preferring compounds with higher sulfation. Furthermore, the patterns of cellular activity across compounds were different to those for binding to each protein, suggesting that biological outcomes are selectively determined in a subtle manner that does not simply reflect the sum of the separate interactions of heparin/HS with Slit and Robo.

Slit-Robo signaling

Slits are secreted proteins that bind to Roundabout (Robo) receptors. Slit-Robo signaling is best known for mediating axon repulsion in the developing nervous system. However, in recent years the functional repertoire of Slits and Robo has expanded tremendously and Slit-Robo signaling has been linked to roles in neurogenesis, angiogenesis and cancer progression among other processes. Likewise, our mechanistic understanding of Slit-Robo signaling has progressed enormously. Here, we summarize new insights into Slit-Robo evolutionary and system-dependent diversity, receptor-ligand interactions, signaling crosstalk and receptor activation.

Recombinant N-Terminal Slit2 Inhibits TGF-β-Induced Fibroblast Activation and Renal Fibrosis

Fibrosis and inflammation are closely intertwined injury pathways present in nearly all forms of CKD for which few safe and effective therapies exist. Slit glycoproteins signaling through Roundabout (Robo) receptors have been described to have anti-inflammatory effects through regulation of leukocyte cytoskeletal organization. Notably, cytoskeletal reorganization is also required for fibroblast responses to TGF-β Here, we examined whether Slit2 also controls TGF-β-induced renal fibrosis. In cultured renal fibroblasts, which we found to express Slit2 and Robo-1, the bioactive N-terminal fragment of Slit2 inhibited TGF-β-induced collagen synthesis, actin cytoskeletal reorganization, and Smad2/3 transcriptional activity, but the inactive C-terminal fragment of Slit2 did not. In mouse models of postischemic renal fibrosis and obstructive uropathy, treatment with N-terminal Slit2 before or after injury inhibited the development of renal fibrosis and preserved renal function, whereas the C-terminal Slit2 had no effect. Our data suggest that administration of recombinant Slit2 may be a new treatment strategy to arrest chronic injury progression after ischemic and obstructive renal insults by not only attenuating inflammation but also, directly inhibiting renal fibrosis.

Simultaneous Activation of Induced Heterodimerization between CXCR4 Chemokine Receptor and Cannabinoid Receptor 2 (CB2) Reveals a Mechanism for Regulation of Tumor Progression

The G-protein-coupled chemokine receptor CXCR4 generates signals that lead to cell migration, cell proliferation, and other survival mechanisms that result in the metastatic spread of primary tumor cells to distal organs. Numerous studies have demonstrated that CXCR4 can form homodimers or can heterodimerize with other G-protein-coupled receptors to form receptor complexes that can amplify or decrease the signaling capacity of each individual receptor. Using biophysical and biochemical approaches, we found that CXCR4 can form an induced heterodimer with cannabinoid receptor 2 (CB2) in human breast and prostate cancer cells. Simultaneous, agonist-dependent activation of CXCR4 and CB2 resulted in reduced CXCR4-mediated expression of phosphorylated ERK1/2 and ultimately reduced cancer cell functions such as calcium mobilization and cellular chemotaxis. Given that treatment with cannabinoids has been shown to reduce invasiveness of cancer cells as well as CXCR4-mediated migration of immune cells, it is plausible that CXCR4 signaling can be silenced through a physical heterodimeric association with CB2, thereby inhibiting subsequent functions of CXCR4. Taken together, the data illustrate a mechanism by which the cannabinoid system can negatively modulate CXCR4 receptor function and perhaps tumor progression.

Recombinant Slit2 attenuates neuroinflammation after surgical brain injury by inhibiting peripheral immune cell infiltration via Robo1-srGAP1 pathway in a rat model

BACKGROUND AND PURPOSE

Peripheral immune cell infiltration to the brain tissue at the perisurgical site can promote neuroinflammation after surgical brain injury (SBI). Slit2, an extracellular matrix protein, has been reported to reduce leukocyte migration. This study evaluated the effect of recombinant Slit2 and the role of its receptor roundabout1 (Robo1) and its downstream mediator Slit-Robo GTPase activating protein 1 (srGAP1)-Cdc42 on peripheral immune cell infiltration after SBI in a rat model.

METHODS

One hundred and fifty-three adult male Sprague-Dawley rats (280-350 g) were used. Partial resection of right frontal lobe was performed to induce SBI. Slit2 siRNA was administered by intracerebroventricular injection 24h before SBI. Recombinant Slit2 was injected intraperitoneally 1h before SBI. Recombinant Robo1 used as a decoy receptor was co-administered with recombinant Slit2. srGAP1 siRNA was administered by intracerebroventricular injection 24h before SBI. Post-assessments included brain water content measurement, neurological tests, ELISA, Western blot, immunohistochemistry, and Cdc42 activity assay.

RESULTS

Endogenous Slit2 was increased after SBI. Robo1 was expressed by peripheral immune cells. Endogenous Slit2 knockdown worsened brain edema after SBI. Recombinant Slit2 administration reduced brain edema, neurological deficits, and pro-inflammatory cytokines after SBI. Recombinant Slit2 reduced peripheral immune cell markers cluster of differentiation 45 (CD45) and myeloperoxidase (MPO), as well as Cdc42 activity in the perisurgical brain tissue which was reversed by recombinant Robo1 co-administration and srGAP1 siRNA.

CONCLUSIONS

Recombinant Slit2 improved outcomes by reducing neuroinflammation after SBI, possibly by decreasing peripheral immune cell infiltration to the perisurgical site through Robo1-srGAP1 mediated inhibition of Cdc42 activity. These results suggest that Slit2 may be beneficial to reduce SBI-induced neuroinflammation.

Slit2N Inhibits Transmission of HIV-1 from Dendritic Cells to T-cells by Modulating Novel Cytoskeletal Elements

Dendritic cells are among the first cells to encounter sexually acquired human immunodeficiency virus (HIV-1), in the mucosa, and they can transmit HIV-1 to CD4(+) T-cells via an infectious synapse. Recent studies reveal that actin-rich membrane extensions establish direct contact between cells at this synapse and facilitate virus transmission. Genesis of these contacts involves signaling through c-Src and Cdc42, which modulate actin polymerization and filopodia formation via the Arp2/3 complex and Diaphanous 2 (Diaph2). We found that Slit2N, a ligand for the Roundabout (Robo) receptors, blocked HIV-1-induced signaling through Arp2/3 and Diaph2, decreased filopodial extensions on dendritic cells, and inhibited cell-to-cell transmission of HIV-1 in a Robo1-dependent manner. Employing proteomic analysis, we identified Flightless-1 as a novel, Robo1-interacting protein. Treatment with shRNAs reduced levels of Flightless-1 and demonstrated its role in efficient cell-to-cell transfer of HIV-1. These results suggest a novel strategy to limit viral infection in the host by targeting the Slit/Robo pathway with modulation of cytoskeletal elements previously unrecognized in HIV-1 transmission.

Myo9b is a key player in SLIT/ROBO-mediated lung tumor suppression

Emerging evidence indicates that the neuronal guidance molecule SLIT plays a role in tumor suppression, as SLIT-encoding genes are inactivated in several types of cancer, including lung cancer; however, it is not clear how SLIT functions in lung cancer. Here, our data show that SLIT inhibits cancer cell migration by activating RhoA and that myosin 9b (Myo9b) is a ROBO-interacting protein that suppresses RhoA activity in lung cancer cells. Structural analyses revealed that the RhoGAP domain of Myo9b contains a unique patch that specifically recognizes RhoA. We also determined that the ROBO intracellular domain interacts with the Myo9b RhoGAP domain and inhibits its activity; therefore, SLIT-dependent activation of RhoA is mediated by ROBO inhibition of Myo9b. In a murine model, compared with control lung cancer cells, SLIT-expressing cells had a decreased capacity for tumor formation and lung metastasis. Evaluation of human lung cancer and adjacent nontumor tissues revealed that Myo9b is upregulated in the cancer tissue. Moreover, elevated Myo9b expression was associated with lung cancer progression and poor prognosis. Together, our data identify Myo9b as a key player in lung cancer and as a ROBO-interacting protein in what is, to the best of our knowledge, a newly defined SLIT/ROBO/Myo9b/RhoA signaling pathway that restricts lung cancer progression and metastasis. Additionally, our work suggests that targeting the SLIT/ROBO/Myo9b/RhoA pathway has potential as a diagnostic and therapeutic strategy for lung cancer.

The Neurorepellent Slit2 Inhibits Postadhesion Stabilization of Monocytes Tethered to Vascular Endothelial Cells

The secreted neurorepellent Slit2, acting through its transmembrane receptor, Roundabout (Robo)-1, inhibits chemotaxis of varied cell types, including leukocytes, endothelial cells, and vascular smooth muscle cells, toward diverse attractants. The role of Slit2 in regulating the steps involved in recruitment of monocytes in vascular inflammation is not well understood. In this study, we showed that Slit2 inhibited adhesion of monocytic cells to activated human endothelial cells, as well as to immobilized ICAM-1 and VCAM-1. Microfluidic live cell imaging showed that Slit2 inhibited the ability of monocytes tethered to endothelial cells to stabilize their actin-associated anchors and to resist detachment in response to increasing shear forces. Transfection of constitutively active plasmids revealed that Slit2 inhibited postadhesion stabilization of monocytes on endothelial cells by preventing activation of Rac1. We further found that Slit2 inhibited chemotaxis of monocytes toward CXCL12 and CCL2. To determine whether Slit2 and Robo-1 modulate pathologic monocyte recruitment associated with vascular inflammation and cardiovascular disease, we tested PBMC from patients with coronary artery disease. PBMC from these patients had reduced surface levels of Robo-1 compared with healthy age- and sex-matched subjects, and Slit2 failed to inhibit chemotaxis of PBMC of affected patients, but not healthy control subjects, toward CCL2. Furthermore, administration of Slit2 to atherosclerosis-prone LDL receptor-deficient mice inhibited monocyte recruitment to nascent atherosclerotic lesions. These results demonstrate that Slit2 inhibits chemotaxis of monocytes, as well as their ability to stabilize adhesions and resist detachment forces. Slit2 may represent a powerful new tool to inhibit pathologic monocyte recruitment in vascular inflammation and atherosclerosis.

Slit2-Robo signaling in inflammation and kidney injury

Acute kidney injury is an increasingly common global health problem and is associated with severe morbidity and mortality. In addition to facing high mortality rates, the survivors of acute kidney injury are at increased risk of developing chronic kidney disease and end-stage renal disease. Renal ischemia-reperfusion injury (IRI) is the most common cause of acute kidney injury, and results from impaired delivery of oxygen and nutrients to the kidney. Massive leukocyte influx into the post-ischemic kidney is one of the hallmarks of IRI. The recruited leukocytes exacerbate tissue damage and, if uncontrolled, initiate the progressive changes that lead to renal fibrosis and chronic kidney disease. Early on, recruitment and activation of platelets promotes microthrombosis in the injured kidney, further exacerbating kidney damage. The diversity, complexity, and multiplicity of pathways involved in leukocyte recruitment and platelet activation make it extremely challenging to control these processes, and past efforts have met with limited success in human trials. A generalized strategy to inhibit infiltration of inflammatory leukocytes and platelets, thereby reducing inflammation and injury, may prove to be more beneficial. In this review, we summarize recent findings demonstrating that the neuronal guidance cues, Slit and Roundabout (Robo), prevent the migration of multiple leukocyte subsets towards diverse inflammatory chemoattractants, and have potent anti-platelet functions in vitro and in vivo. These properties uniquely position Slit2 as a novel therapeutic that could be used to prevent acute kidney injury associated with IRI.

Somatic and Germline Diversification of a Putative Immunoreceptor within One Phylum: Dscam in Arthropods

Arthropod Dscam, the homologue of the human Down Syndrome cell adhesion molecule, is a receptor used by the nervous and immune systems. Unlike in vertebrates, evolutionary pressure has selected and maintained a vast Dscam diversity of isoforms, known to specifying neuronal identity during the nervous system differentiation. This chapter examines the different modes of Dscam diversification in the context of arthropods' evolution and that of their immune system, where its role is controversial. In the single Dscam gene of insects and crustaceans, mutually exclusive alternative splicing affects three clusters of duplicated exons encoding the variable parts of the receptor. The Dscam gene produces over 10,000 isoforms. In the more basal arthropods such as centipedes, Dscam diversity results from a combination of many germline genes (over 80) with, in about half of those, the possibility of alternative splicing affecting only one exon cluster. In the even more basal arthropods, such as chelicerates, no splicing possibility is detected, but there exist dozens of germline Dscam genes. Compared to controlling the expression of multiple germline genes, the somatic mutually alternative splicing within a single gene may offer a simplified way of expressing a large Dscam repertoire. Expressed by hemocytes, Dscam is considered a phagocytic receptor but is also encountered in solution. More information is necessary about its binding to pathogens, its role in phagocytosis, its possible role in specifying hemocyte identity, its kinetics of expression, and the regulation of its RNA splicing to understand how its diversity is linked to immunity.

Fibroblasts secrete Slit2 to inhibit fibrocyte differentiation and fibrosis

Monocytes leave the blood and enter tissues. In healing wounds and fibrotic lesions, some of the monocytes differentiate into fibroblast-like cells called fibrocytes. In healthy tissues, even though monocytes enter the tissue, for unknown reasons, very few monocytes differentiate into fibrocytes. In this report, we show that fibroblasts from healthy human tissues secrete the neuronal guidance protein Slit2 and that Slit2 inhibits human fibrocyte differentiation. In mice, injections of Slit2 inhibit bleomycin-induced lung fibrosis. In lung tissue from pulmonary fibrosis patients with relatively normal lung function, Slit2 has a widespread distribution whereas, in patients with advanced disease, there is less Slit2 in the fibrotic lesions. These data may explain why fibrocytes are rarely observed in healthy tissues, may suggest that the relative levels of Slit2 present in healthy tissue and at sites of fibrosis may have a significant effect on the decision of monocytes to differentiate into fibrocytes, and may indicate that modulating Slit2 signaling may be useful as a therapeutic for fibrosis.

Coordinate regulation of stem cell competition by Slit-Robo and JAK-STAT signaling in the Drosophila testis

Stem cells in tissues reside in and receive signals from local microenvironments called niches. Understanding how multiple signals within niches integrate to control stem cell function is challenging. The Drosophila testis stem cell niche consists of somatic hub cells that maintain both germline stem cells and somatic cyst stem cells (CySCs). Here, we show a role for the axon guidance pathway Slit-Roundabout (Robo) in the testis niche. The ligand Slit is expressed specifically in hub cells while its receptor, Roundabout 2 (Robo2), is required in CySCs in order for them to compete for occupancy in the niche. CySCs also require the Slit-Robo effector Abelson tyrosine kinase (Abl) to prevent over-adhesion of CySCs to the niche, and CySCs mutant for Abl outcompete wild type CySCs for niche occupancy. Both Robo2 and Abl phenotypes can be rescued through modulation of adherens junction components, suggesting that the two work together to balance CySC adhesion levels. Interestingly, expression of Robo2 requires JAK-STAT signaling, an important maintenance pathway for both germline and cyst stem cells in the testis. Our work indicates that Slit-Robo signaling affects stem cell function downstream of the JAK-STAT pathway by controlling the ability of stem cells to compete for occupancy in their niche.

MicroRNA-218 inhibits cell invasion and migration of pancreatic cancer via regulating ROBO1

miRNA-218 is a highlighted tumor suppressor and its underlying role in tumor progression is still unknown. Here, we restored the expression of miRNA-218 in pancreatic cancer to clarify the function and potent downstream pathway of miRNA-218. The expressions of both miRNA-218 and its potent target gene ROBO1 were revealed by RT-PCR and western blotting analysis. Transfection of miRNA-218 precursor mimics and luciferase assay were performed to elucidate the regulation mechanism between miRNA-218 and ROBO1. Cells, stably expressing miRNA-218 followed by forced expression of mutant ROBO1, were established through co-transfections of both lentivirus vector and plasmid vector. The cell migration and invasion abilities were evaluated by migration assay and invasion assay respectively. An increased expression of ROBO1 was revealed in cell BxPC-3-LN compared with cell BxPC-3. Elevated expression of miRNA-218 would suppress the expression of ROBO1 via complementary binding to a specific region within 3'UTR of ROBO1 mRNA (sites 971-978) in pancreatic cancer cells. Stably restoring the expression of miRNA-218 in pancreatic cancer significantly downregulated the expression of ROBO1 and effectively inhibited cell migration and invasion. Forced expression of mutant ROBO1 could reverse the repression effects of miRNA-218 on cell migration and invasion. Consequently, miRNA-218 acted as a tumor suppressor in pancreatic cancer by inhibiting cell invasion and migration. ROBO1 was a functional target of miRNA-218's downstream pathway involving in cell invasion and migration of pancreatic cancer.

MicroRNA and signaling pathways in gastric cancer

MicroRNAs (miRNAs) function as either oncogenes or tumor suppressors by inhibiting the expression of target genes, some of which are either directly or indirectly involved with canonical signaling pathways. The relationship between miRNAs and signaling pathways in gastric cancer is extremely complicated. In this paper, we determined the pathogenic mechanism of gastric cancer related to miRNA expression based on recent high-quality studies and then clarified the regulation network of miRNA expression and the correlated functions of these miRNAs during the progression of gastric cancer. We try to illustrate the correlation between the expression of miRNAs and outcomes of patients with gastric cancer. Understanding this will allow us to take a big step forward in the treatment of gastric cancer.

Role of ROBO4 signalling in developmental and pathological angiogenesis

Transmembrane roundabout receptor family members (ROBO1-ROBO4) principally orchestrate the neuronal guidance mechanism of the nervous system. Secreted glycoprotein SLITs are the most appreciated ligands for ROBOs. Recently identified ROBO4 is the key mediator of SLIT-ROBO mediated developmental and pathological angiogenesis. Although SLIT2 has been shown to interact with ROBO4 as ligand, it remains an open question whether this protein is the physiologic partner of ROBO4. The purpose of this review is to summarise how reliable SLIT2 as ligand for ROBO4 is, if not what the other possible mechanisms demonstrated till date for ROBO4 mediated developmental and pathological angiogenesis are. We conclude that ROBO4 is expressed specially in vascular endothelial cells and maintains the vascular integrity via either SLIT2 dependent or SLIT2 independent manner. On the contrary, it promotes the pathological angiogenesis by involving different signalling arm(s)/unknown ligand(s). This review explores the interactions SLIT2/ROBO1, SLIT2/ROBO1-ROBO4, ROBO1/ROBO4, and ROBO4/UNC5B which can be promising and potential therapeutic targets for developmental angiogenesis defects and pathological angiogenesis. Finally we have reviewed the ROBO4 signalling pathways and made an effort to elaborate the insight of this signalling as therapeutic target of pathological angiogenesis.

Roundabout receptors

Roundabout receptors (Robo) and their Slit ligands were discovered in the 1990s and found to be key players in axon guidance. Slit was initially described s an extracellular matrix protein that was expressed by midline glia in Drosophila. A few years later, it was shown that, in vertebrates and invertebrates, Slits acted as chemorepellents for axons crossing the midline. Robo proteins were originally discovered in Drosophila in a mutant screen for genes involved in the regulation of midline crossing. This ligand-receptor pair has since been implicated in a variety of other neuronal and non-neuronal processes ranging from cell migration to angiogenesis, tumourigenesis and even organogenesis of tissues such as kidneys, lungs and breasts.

Slit2-Robo4 pathway modulates lipopolysaccharide-induced endothelial inflammation and its expression is dysregulated during endotoxemia

The secretory protein Slit2 and its receptors Robo1 and Robo4 are considered to regulate mobility and permeability of endothelial cells and other cell types. However, the roles of Slit2 and its two receptors in endothelial inflammatory responses remain to be clarified. In this study, we show that, in primary HUVECs, Slit2 represses LPS-induced secretion of certain inflammatory cytokines/chemokines, cell adhesion molecule ICAM-1 upregulation, and monocyte adhesion. Slit2's anti-inflammatory effect is mediated by its dominant endothelial-specific receptor Robo4. However, the minor receptor Robo1 has proinflammatory properties and is downregulated by Slit2 via targeting of miR-218. Elucidation of molecular mechanism reveals that Slit2 represses inflammatory responses by inhibiting the Pyk2-NF-κB pathway downstream of LPS-TLR4. Further studies reveal that LPS enhances endothelial inflammation by downregulating the anti-inflammatory Slit2 and Robo4 in HUVECs in vitro, as well as in arterial endothelial cells and liver in vivo during endotoxemia. These results suggest that Slit2-Robo4 signaling is important in regulating LPS-induced endothelial inflammation, and LPS, in turn, enhances inflammation by interfering with the expression of the anti-inflammatory Slit2-Robo4 during the disease state. This implies that Slit2-Robo4 is a key regulator of endothelial inflammation, and its dysregulation during endotoxemia is a novel mechanism for LPS-induced vascular pathogenesis.

SDF-1-CXCR4 differentially regulates autoimmune diabetogenic T cell adhesion through ROBO1-SLIT2 interactions in mice

AIMS/HYPOTHESIS

We had previously reported that stromal cell-derived factor 1 (SDF-1) mediates chemorepulsion of diabetogenic T cell adhesion to islet microvascular endothelium through unknown mechanisms in NOD mice. Here we report that SDF-1-mediated chemorepulsion occurs through slit homologue (SLIT)2-roundabout, axon guidance receptor, homologue 1 (Drosophila) (ROBO1) interactions.

METHODS

C-X-C receptor (CXCR)4 and ROBO1 protein expression was measured in mouse and human T cells. Parallel plate flow chamber adhesion and detachment studies were performed to examine the molecular importance of ROBO1 and SLIT2 for SDF-1-mediated T cell chemorepulsion. Diabetogenic splenocyte transfer was performed in NOD/LtSz Rag1(-/-) mice to examine the effect of the SDF-1 mimetic CTCE-0214 on adoptive transfer of diabetes.

RESULTS

CXCR4 and ROBO1 protein expression was elevated in diabetic NOD/ShiLtJ T cells over time and coincided with the onset of hyperglycaemia. CXCR4 and ROBO1 expression was also increased in human type 1 diabetic T cells, with ROBO1 expression maximal at less than 1 year post diagnosis. Cell detachment studies revealed that immunoneutralisation of ROBO1 prevented SDF-1-mediated chemorepulsion of NOD T cell firm adhesion to TNFα-stimulated islet endothelial cells. SDF-1 increased NOD T cell adhesion to recombinant adhesion molecules, a phenomenon that was reversed by recombinant SLIT2. Finally, we found that an SDF-1 peptide mimetic prevented NOD T cell adhesion in vitro and significantly delayed adoptive transfer of autoimmune diabetes in vivo.

CONCLUSIONS/INTERPRETATION

These data reveal a novel molecular pathway, which regulates diabetogenic T cell recruitment and may be useful in modulating autoimmune diabetes.

More than one way to produce protein diversity: duplication and limited alternative splicing of an adhesion molecule gene in basal arthropods

Exon duplication and alternative splicing evolved multiple times in metazoa and are of overall importance in shaping genomes and allowing organisms to produce many fold more proteins than there are genes in the genome. No other example is as striking as the one of the Down syndrome cell adhesion molecule (Dscam) of insects and crustaceans (pancrustaceans) involved in the nervous system differentiation and in the immune system. To elucidate the evolutionary history of this extraordinary gene, we investigated Dscam homologs in two basal arthropods, the myriapod Strigamia maritima and the chelicerate Ixodes scapularis. In both, Dscam diversified extensively by whole gene duplications resulting in multigene expansions. Within some of the S. maritima genes, exons coding for one of the immunoglobulin domains (Ig7) duplicated and are mutually exclusively alternatively spliced. Our results suggest that Dscam diversification was selected independently in chelicerates, myriapods, and pancrustaceans and that the usage of Dscam diversity by immune cells evolved for the first time in basal arthropods. We propose an evolutionary scenario for the appearance of the highly variable Dscam gene of pancrustaceans, adding to the understanding of how alternative splicing, exon, and gene duplication contribute to create molecular diversity associated with potentially new cellular functions.

Slit2-Robo signaling: a novel regulator of vascular injury

PURPOSE OF REVIEW

Vascular injury is a common contributor to, and complication of, kidney disease. Given the prevalence and importance of vascular injury in renal disease, interest has grown in a novel signaling pathway first identified in developing neurons that also has widespread effects on vascular structure and function, comprising the secreted ligand Slit2 and its cognate Roundabout (Robo) receptors.

RECENT FINDINGS

Although initially discovered as a modulator of neuronal migration during development, the Slit2-Robo signaling pathway has recently been found to regulate the structure and function of various subsets of vascular cells and circulating hematopoietic cells that interact with the vessel wall. Through the regulation of intermediate signaling enzymes that control the organization of the actin cytoskeleton, Slit2 and its Robo receptors regulate such diverse processes as angiogenesis, endothelial permeability, vascular smooth muscle cell migration, and thrombosis.

SUMMARY

Recent advances in our understanding of Slit2-Robo signaling have provided novel insights into the pathophysiology of vascular injury that is commonly associated with renal disease. These insights have created potential opportunities for the development of new therapies targeting vascular injury associated with renal disease.

Slit2 prevents neutrophil recruitment and renal ischemia-reperfusion injury

Neutrophils recruited to the postischemic kidney contribute to the pathogenesis of ischemia-reperfusion injury (IRI), which is the most common cause of renal failure among hospitalized patients. The Slit family of secreted proteins inhibits chemotaxis of leukocytes by preventing activation of Rho-family GTPases, suggesting that members of this family might modulate the recruitment of neutrophils and the resulting IRI. Here, in static and microfluidic shear assays, Slit2 inhibited multiple steps required for the infiltration of neutrophils into tissue. Specifically, Slit2 blocked the capture and firm adhesion of human neutrophils to inflamed vascular endothelial barriers as well as their subsequent transmigration. To examine whether these observations were relevant to renal IRI, we administered Slit2 to mice before bilateral clamping of the renal pedicles. Assessed at 18 hours after reperfusion, Slit2 significantly inhibited renal tubular necrosis, neutrophil and macrophage infiltration, and rise in plasma creatinine. In vitro, Slit2 did not impair the protective functions of neutrophils, including phagocytosis and superoxide production, and did not inhibit neutrophils from killing the extracellular pathogen Staphylococcus aureus. In vivo, administration of Slit2 did not attenuate neutrophil recruitment or bacterial clearance in mice with ascending Escherichia coli urinary tract infections and did not increase the bacterial load in the livers of mice infected with the intracellular pathogen Listeria monocytogenes. Collectively, these results suggest that Slit2 may hold promise as a strategy to combat renal IRI without compromising the protective innate immune response.

Loss of expression and promoter methylation of SLIT2 are associated with sessile serrated adenoma formation

Serrated adenomas form a distinct subtype of colorectal pre-malignant lesions that may progress to malignancy along a different molecular pathway than the conventional adenoma-carcinoma pathway. Previous studies have hypothesised that BRAF mutation and promoter hypermethylation plays a role, but the evidence for this is not robust. We aimed to carry out a whole-genome loss of heterozygosity analysis, followed by targeted promoter methylation and expression analysis to identify potential pathways in serrated adenomas. An initial panel of 9 sessile serrated adenomas (SSA) and one TSA were analysed using Illumina Goldengate HumanLinkage panel arrays to ascertain regions of loss of heterozygosity. This was verified via molecular inversion probe analysis and microsatellite analysis of a further 32 samples. Methylation analysis of genes of interest was carried out using methylation specific PCR (verified by pyrosequencing) and immunohistochemistry used to correlate loss of expression of genes of interest. All experiments used adenoma samples and normal tissue samples as control. SSA samples were found on whole-genome analysis to have consistent loss of heterozygosity at 4p15.1-4p15.31, which was not found in the sole TSA, adenomas, or normal tissues. Genes of interest in this region were PDCH7 and SLIT2, and combined MSP/IHC analysis of these genes revealed significant loss of SLIT2 expression associated with promoter methylation of SLIT2. Loss of expression of SLIT2 by promoter hypermethylation and loss of heterozygosity events is significantly associated with serrated adenoma development, and SLIT2 may represent a epimutated tumour suppressor gene according to the Knudson "two hit" hypothesis.

Slit/Robo1 signaling regulates neural tube development by balancing neuroepithelial cell proliferation and differentiation

Formation of the neural tube is the morphological hallmark for development of the embryonic central nervous system (CNS). Therefore, neural tube development is a crucial step in the neurulation process. Slit/Robo signaling was initially identified as a chemo-repellent that regulated axon growth cone elongation, but its role in controlling neural tube development is currently unknown. To address this issue, we investigated Slit/Robo1 signaling in the development of chick neCollege of Life Sciences Biocentre, University of Dundee, Dundee DD1 5EH, UKural tube and transgenic mice over-expressing Slit2. We disrupted Slit/Robo1 signaling by injecting R5 monoclonal antibodies into HH10 neural tubes to block the Robo1 receptor. This inhibited the normal development of the ventral body curvature and caused the spinal cord to curl up into a S-shape. Next, Slit/Robo1 signaling on one half-side of the chick embryo neural tube was disturbed by electroporation in ovo. We found that the morphology of the neural tube was dramatically abnormal after we interfered with Slit/Robo1 signaling. Furthermore, we established that silencing Robo1 inhibited cell proliferation while over-expressing Robo1 enhanced cell proliferation. We also investigated the effects of altering Slit/Robo1 expression on Sonic Hedgehog (Shh) and Pax7 expression in the developing neural tube. We demonstrated that over-expressing Robo1 down-regulated Shh expression in the ventral neural tube and resulted in the production of fewer HNK-1(+) migrating neural crest cells (NCCs). In addition, Robo1 over-expression enhanced Pax7 expression in the dorsal neural tube and increased the number of Slug(+) pre-migratory NCCs. Conversely, silencing Robo1 expression resulted in an enhanced Shh expression and more HNK-1(+) migrating NCCs but reduced Pax7 expression and fewer Slug(+) pre-migratory NCCs were observed. In conclusion, we propose that Slit/Robo1 signaling is involved in regulating neural tube development by tightly coordinating cell proliferation and differentiation during neurulation.