Interaction of the guidance molecule Slit with cellular receptors

SLIT3-mediated fibroblast signaling: a promising target for antifibrotic therapies

The SLIT family (SLIT1-3) of highly conserved glycoproteins was originally identified as ligands for the Roundabout (ROBO) family of single-pass transmembrane receptors, serving to provide repulsive axon guidance cues in the nervous system. Intriguingly, studies involving SLIT3 mutant mice suggest that SLIT3 might have crucial biological functions outside the neural context. Although these mutant mice display no noticeable neurological abnormalities, they present pronounced connective tissue defects, including congenital central diaphragmatic hernia, membranous ventricular septal defect, and osteopenia. We recently hypothesized that the phenotype observed in SLIT3-deficient mice may be tied to abnormalities in fibrillar collagen-rich connective tissue. Further research by our group indicates that both SLIT3 and its primary receptor, ROBO1, are expressed in fibrillar collagen-producing cells across various nonneural tissues. Global and constitutive SLIT3 deficiency not only reduces the synthesis and content of fibrillar collagen in various organs but also alleviates pressure overload-induced fibrosis in both the left and right ventricles. This review delves into the known phenotypes of SLIT3 mutants and the debated role of SLIT3 in vasculature and bone. Present evidence hints at SLIT3 acting as an autocrine regulator of fibrillar collagen synthesis, suggesting it as a potential antifibrotic treatment. However, the precise pathway and mechanisms through which SLIT3 regulates fibrillar collagen synthesis remain uncertain, presenting an intriguing avenue for future research.

Brain-wide correspondence of neuronal epigenomics and distant projections

Single-cell analyses parse the brain's billions of neurons into thousands of 'cell-type' clusters residing in different brain structures1. Many cell types mediate their functions through targeted long-distance projections allowing interactions between specific cell types. Here we used epi-retro-seq2 to link single-cell epigenomes and cell types to long-distance projections for 33,034 neurons dissected from 32 different regions projecting to 24 different targets (225 source-to-target combinations) across the whole mouse brain. We highlight uses of these data for interrogating principles relating projection types to transcriptomics and epigenomics, and for addressing hypotheses about cell types and connections related to genetics. We provide an overall synthesis with 926 statistical comparisons of discriminability of neurons projecting to each target for every source. We integrate this dataset into the larger BRAIN Initiative Cell Census Network atlas, composed of millions of neurons, to link projection cell types to consensus clusters. Integration with spatial transcriptomics further assigns projection-enriched clusters to smaller source regions than the original dissections. We exemplify this by presenting in-depth analyses of projection neurons from the hypothalamus, thalamus, hindbrain, amygdala and midbrain to provide insights into properties of those cell types, including differentially expressed genes, their associated cis-regulatory elements and transcription-factor-binding motifs, and neurotransmitter use.

Axon guidance molecules in liver pathology: Journeys on a damaged passport

BACKGROUND AND AIMS

The liver is an innervated organ that develops a variety of chronic liver disease (CLD). Axon guidance cues (AGCs), of which ephrins, netrins, semaphorins and slits are the main representative, are secreted or membrane-bound proteins that can attract or repel axons through interactions with their growth cones that contain receptors recognizing these messengers. While fundamentally implicated in the physiological development of the nervous system, the expression of AGCs can also be reinduced under acute or chronic conditions, such as CLD, that necessitate redeployment of neural networks.

METHODS

This review considers the ad hoc literature through the neglected canonical neural function of these proteins that is also applicable to the diseased liver (and not solely their observed parenchymal impact).

RESULTS

AGCs impact fibrosis regulation, immune functions, viral/host interactions, angiogenesis, and cell growth, both at the CLD and HCC levels. Special attention has been paid to distinguishing correlative and causal data in such datasets in order to streamline data interpretation. While hepatic mechanistic insights are to date limited, bioinformatic evidence for the identification of AGCs mRNAs positive cells, protein expression, quantitative regulation, and prognostic data have been provided. Liver-pertinent clinical studies based on the US Clinical Trials database are listed. Future research directions derived from AGC targeting are proposed.

CONCLUSION

This review highlights frequent implication of AGCs in CLD, linking traits of liver disorders and the local autonomic nervous system. Such data should contribute to diversifying current parameters of patient stratification and our understanding of CLD.

The CNS/PNS Extracellular Matrix Provides Instructive Guidance Cues to Neural Cells and Neuroregulatory Proteins in Neural Development and Repair

BACKGROUND

The extracellular matrix of the PNS/CNS is unusual in that it is dominated by glycosaminoglycans, especially hyaluronan, whose space filling and hydrating properties make essential contributions to the functional properties of this tissue. Hyaluronan has a relatively simple structure but its space-filling properties ensure micro-compartments are maintained in the brain ultrastructure, ensuring ionic niches and gradients are maintained for optimal cellular function. Hyaluronan has cell-instructive, anti-inflammatory properties and forms macro-molecular aggregates with the lectican CS-proteoglycans, forming dense protective perineuronal net structures that provide neural and synaptic plasticity and support cognitive learning.

AIMS

To highlight the central nervous system/peripheral nervous system (CNS/PNS) and its diverse extracellular and cell-associated proteoglycans that have cell-instructive properties regulating neural repair processes and functional recovery through interactions with cell adhesive molecules, receptors and neuroregulatory proteins. Despite a general lack of stabilising fibrillar collagenous and elastic structures in the CNS/PNS, a sophisticated dynamic extracellular matrix is nevertheless important in tissue form and function.

CONCLUSIONS

This review provides examples of the sophistication of the CNS/PNS extracellular matrix, showing how it maintains homeostasis and regulates neural repair and regeneration.

Controlled Delivery of Slit3 Proteins from Alginate Microbeads Inhibits In Vitro Angiogenesis

BACKGROUND

The Slit-Robo pathway is a key regulator of angiogenesis and cellular function in experimental models. Slit3 proteins exhibit both proangiogenic and antiangiogenic properties, but the exact mechanism remains unclear. It is theorized that Slit3 may be a potential treatment for vascular diseases and cancer.

METHODS

Slit3 labeled with I-125 was encapsulated in microbeads composed of low-viscosity alginate of high-glucuronic acid content, first coated with poly-L-ornithine for various durations and finally with low-viscosity high mannuronic acid. Gamma counter was used to measure microbead encapsulation efficiency and Slit3 release. Markers of angiogenesis were assessed with Boyden chamber, scratch wound, and Matrigel tube formation assays using human umbilical vein and mouse endothelial cells.

RESULTS

On incubation of Slit3-loaded microbeads, there was an initial burst phase release of Slit3 for the first 24 h followed by sustained release for 6 to 12 d. Microbead composition determined encapsulation efficiency and rate of release; Slit3 encapsulation was most efficient in microbeads with lower low-viscosity alginate of high-glucuronic acid content concentrations (1.5%) and no poly-L-ornithine coating. Compared with controls (media alone), Slit3 microbeads significantly inhibited in vitro cellular migration, endothelial cell migration for wound closure at 24 and 48 h and endothelial tube formation (P < 0.001, respectively).

CONCLUSIONS

Slit3 can be effectively encapsulated and delivered via a controlled release pattern using alginate microbeads. Microbead encapsulation reduces in vitro endothelial tube formation and inhibits cellular migration to impair angiogenesis. Thus, Slit3 microparticles may be explored as a therapeutic option to mitigate tumor proliferation.

Molecular targets for tivantinib (ARQ 197) and vasculogenic mimicry in human melanoma cells

Tivantinib (TivB) was reported previously to target MET and microtubule assembly in different cells resulting in cytotoxicity. However, its other cellular targets remain unknown, especially the proteins involved in focal adhesion and cytoskeletal organization. We studied the effect of TivB on vinculin a focal adhesion protein, and RhoC, a GTPase which promote the reorganization of cytoskeleton. Biomolecules involved in vasculogenic mimicry (VM) previously not reported in melanoma, and their susceptibility to TivB was also evaluated. TivB affects the viability and apoptosis of human melanoma cells depending on the cell type. Vinculin and RhoC were increased in the presence of TivB and affected the integrity of actin filaments and altered the cellular morphology. TivB disrupts the VM exhibited by melanoma cells in 3D matrix. Roundabout Guidance Receptor 4 (Robo4), a receptor protein implicated in axonal guidance and angiogenesis and its ligand Slit2 are expressed in human C8161 and WM793 melanoma cells, but absent in other melanoma cells including normal melanocytes. VM is more prominent in C8161 cells and could be blocked by siRNA mediated silencing of Robo4 mRNA, but TivB does not affect Robo4 in C8161 cells. Immunoblot analysis indicated no changes in Robo4 and Slit2 protein expression, however, both vinculin and RhoC protein increased in TivB treated melanoma cells. These results suggest that TivB affects cell cytoskeleton and morphology by altering proteins such as vinculin and RhoC. Our studies indicate TivB could target molecules other than MET in melanoma cells, which may provide insight into its alternate mechanism of action.

Common cues wire the spinal cord: Axon guidance molecules in spinal neuron migration

Topographic arrangement of neuronal cell bodies and axonal tracts are crucial for proper wiring of the nervous system. This involves often-coordinated neuronal migration and axon guidance during development. Most neurons migrate from their birthplace to specific topographic coordinates as they adopt the final cell fates and extend axons. The axons follow temporospatial specific guidance cues to reach the appropriate targets. When neuronal or axonal migration or their coordination is disrupted, severe consequences including neurodevelopmental disorders and neurological diseases, can arise. Neuronal and axonal migration shares some molecular mechanisms, as genes originally identified as axon guidance molecules have been increasingly shown to direct both navigation processes. This review focuses on axon guidance pathways that are shown to also direct neuronal migration in the vertebrate spinal cord.

Biophysical analysis of anopheles gambiae leucine-rich repeat proteins APL1A1, APL1B [corrected] and APL1C and their interaction with LRIM1

Natural infection of Anopheles gambiae by malaria-causing Plasmodium parasites is significantly influenced by the APL1 genetic locus. The locus contains three closely related leucine-rich repeat (LRR) genes, APL1A, APL1B and APL1C. Multiple studies have reported the participation of APL1A-C in the immune response of A. gambiae to invasion by both rodent and human Plasmodium isolates. APL1C forms a heterodimer with the related LRR protein LRIM1 via a C-terminal coiled-coil domain that is also present in APL1A and APL1B. The LRIM1/APL1C heterodimer protects A. gambiae from infection by binding the complement-like protein TEP1 to form a stable and active immune complex. Here we report solution x-ray scatting data for the LRIM1/APL1C heterodimer, the oligomeric state of LRIM1/APL1 LRR domains in solution and the crystal structure of the APL1B LRR domain. The LRIM1/APL1C heterodimeric complex has a flexible and extended structure in solution. In contrast to the APL1A, APL1C and LRIM1 LRR domains, the APL1B LRR domain is a homodimer. The crystal structure of APL1B-LRR shows that the homodimer is formed by an N-terminal helix that complements for the absence of an N-terminal capping motif in APL1B, which is a unique distinction within the LRIM1/APL1 protein family. Full-length APL1A1 and APL1B form a stable complex with LRIM1. These results support a model in which APL1A1, APL1B and APL1C can all form an extended, flexible heterodimer with LRIM1, providing a repertoire of functional innate immune complexes to protect A. gambiae from a diverse array of pathogens.

Netrin-DCC signaling regulates corpus callosum formation through attraction of pioneering axons and by modulating Slit2-mediated repulsion

The left and right sides of the nervous system communicate via commissural axons that cross the midline during development using evolutionarily conserved molecules. These guidance cues have been particularly well studied in the mammalian spinal cord, but it remains unclear whether these guidance mechanisms for commissural axons are similar in the developing forebrain, in particular for the corpus callosum, the largest and most important commissure for cortical function. Here, we show that Netrin1 initially attracts callosal pioneering axons derived from the cingulate cortex, but surprisingly is not attractive for the neocortical callosal axons that make up the bulk of the projection. Instead, we show that Netrin-deleted in colorectal cancer signaling acts in a fundamentally different manner, to prevent the Slit2-mediated repulsion of precrossing axons thereby allowing them to approach and cross the midline. These results provide the first evidence for how callosal axons integrate multiple guidance cues to navigate the midline.

Characterization of the interaction between Robo1 and heparin and other glycosaminoglycans

Roundabout 1 (Robo1) is the cognate receptor for secreted axon guidance molecule, Slits, which function to direct cellular migration during neuronal development and angiogenesis. The Slit2-Robo1 signaling is modulated by heparan sulfate, a sulfated linear polysaccharide that is abundantly expressed on the cell surface and in the extracellular matrix. Biochemical studies have further shown that heparan sulfate binds to both Slit2 and Robo1 facilitating the ligand-receptor interaction. The structural requirements for heparan sulfate interaction with Robo1 remain unknown. In this report, surface plasmon resonance (SPR) spectroscopy was used to examine the interaction between Robo1 and heparin and other GAGs and determined that heparin binds to Robo1 with an affinity of ~650 nM. SPR solution competition studies with chemically modified heparins further determined that although all sulfo groups on heparin are important for the Robo1-heparin interaction, the N-sulfo and 6-O-sulfo groups are essential for the Robo1-heparin binding. Examination of differently sized heparin oligosaccharides and different GAGs also demonstrated that Robo1 prefers to bind full-length heparin chains and that GAGs with higher sulfation levels show increased Robo1 binding affinities.

Slit2N/Robo1 inhibit HIV-gp120-induced migration and podosome formation in immature dendritic cells by sequestering LSP1 and WASp

Cell-mediated transmission and dissemination of sexually-acquired human immunodeficiency virus 1 (HIV-1) in the host involves the migration of immature dendritic cells (iDCs). iDCs migrate in response to the HIV-1 envelope protein, gp120, and inhibiting such migration may limit the mucosal transmission of HIV-1. In this study, we elucidated the mechanism of HIV-1-gp120-induced transendothelial migration of iDCs. We found that gp120 enhanced the binding of Wiskott-Aldrich Syndrome protein (WASp) and the Actin-Related Protein 2/3 (Arp2/3) complex with β-actin, an interaction essential for the proper formation of podosomes, specialized adhesion structures required for the migration of iDCs through different tissues. We further identified Leukocyte-Specific Protein 1 (LSP1) as a novel component of the WASp-Arp2/3-β-actin complex. Pretreating iDCs with an active fragment of the secretory glycoprotein Slit2 (Slit2N) inhibited HIV-1-gp120-mediated migration and podosome formation, by inducing the cognate receptor Roundabout 1 (Robo1) to bind to and sequester WASp and LSP1 from β-actin. Slit2N treatment also inhibited Src signaling and the activation of several downstream molecules, including Rac1, Pyk2, paxillin, and CDC42, a major regulator of podosome formation. Taken together, our results support a novel mechanism by which Slit2/Robo1 may inhibit the HIV-1-gp120-induced migration of iDCs, thereby restricting dissemination of HIV-1 from mucosal surfaces in the host.

Fragments of SLIT3 inhibit cellular migration

The repellent factor family of Slit molecules has been described as having a repulsive function in the developing nervous system on growing axons expressing the Roundabout (Robo) receptors. Recent studies determined the effects of Slit molecules on the migratory and invasive potential of several types of tumor cells but also on synovial fibroblasts (SFs) derived from rheumatoid arthritis (RA) patients. To optimize a potential therapeutic application we aimed at generatingfragments of Slit3 showing the same functional ability as the full-length molecule but having the advantage of a smaller size. Recombinant Slit3 proteins were expressed and analyzed by western blotting. Their activity was defined by functional assays such as migration assays with RASF and melanoma cells. Recombinant Slit3 containing only leucine rich repeat domain 2 (D2), the domain important for Robo binding and the minimal functional unit D2 dNC were both able to inhibit migration of RASFs as effectively as Slit3 with all 4 repeats. Collectively, our data showed that the ability of Slit3 to reduce the migratory activity of synovial cells from patients with RA and melanoma cells can be mimicked by small protein fragments derived from Slit3. Slit3 fragments may be helpful in therapeutic attempts; however, further studies are necessary in order to elucidate their activity in vivo.

Hypoxia-induced alternative splicing in endothelial cells

Background

Adaptation to low oxygen by changing gene expression is vitally important for cell survival and tissue development. The sprouting of new blood vessels, initiated from endothelial cells, restores the oxygen supply of ischemic tissues. In contrast to the transcriptional response induced by hypoxia, which is mainly mediated by members of the HIF family, there are only few studies investigating alternative splicing events. Therefore, we performed an exon array for the genome-wide analysis of hypoxia-related changes of alternative splicing in endothelial cells.

Methodology/Principal findings

Human umbilical vein endothelial cells (HUVECs) were incubated under hypoxic conditions (1% O₂) for 48 h. Genome-wide transcript and exon expression levels were assessed using the Affymetrix GeneChip Human Exon 1.0 ST Array. We found altered expression of 294 genes after hypoxia treatment. Upregulated genes are highly enriched in glucose metabolism and angiogenesis related processes, whereas downregulated genes are mainly connected to cell cycle and DNA repair. Thus, gene expression patterns recapitulate known adaptations to low oxygen supply. Alternative splicing events, until now not related to hypoxia, are shown for nine genes: six which are implicated in angiogenesis-mediated cytoskeleton remodeling (cask, itsn1, larp6, sptan1, tpm1 and robo1); one, which is involved in the synthesis of membrane-anchors (pign) and two universal regulators of gene expression (cugbp1 and max).

Conclusions/Significance

For the first time, this study investigates changes in splicing in the physiological response to hypoxia on a genome-wide scale. Nine alternative splicing events, until now not related to hypoxia, are reported, considerably expanding the information on splicing changes due to low oxygen supply. Therefore, this study provides further knowledge on hypoxia induced gene expression changes and presents new starting points to study the hypoxia adaptation of endothelial cells.

A roundabout way to cancer

The Slit family of secreted proteins and their transmembrane receptor, Robo, were originally identified in the nervous system where they function as axon guidance cues and branching factors during development. Since their discovery, a great number of additional roles have been attributed to Slit/Robo signaling, including regulating the critical processes of cell proliferation and cell motility in a variety of cell and tissue types. These processes are often deregulated during cancer progression, allowing tumor cells to bypass safeguarding mechanisms in the cell and the environment in order to grow and escape to new tissues. In the past decade, it has been shown that the expression of Slit and Robo is altered in a wide variety of cancer types, identifying them as potential therapeutic targets. Further, studies have demonstrated dual roles for Slits and Robos in cancer, acting as both oncogenes and tumor suppressors. This bifunctionality is also observed in their roles as axon guidance cues in the developing nervous system, where they both attract and repel neuronal migration. The fact that this signaling axis can have opposite functions depending on the cellular circumstance make its actions challenging to define. Here, we summarize our current understanding of the dual roles that Slit/Robo signaling play in development, epithelial tumor progression, and tumor angiogenesis.

A novel role for Slit2/Robo1 axis in modulating HIV-1 replication in T cells

OBJECTIVE

Slit2 is a secreted glycoprotein that has been shown to possess anti-inflammatory properties. In addition, Slit2 has been shown to modulate CXCR4-mediated functional effects in T cells. However, its role in HIV-1 pathogenesis is not known. The objective of the current study is to analyze the role of Slit2 in modulating HIV-1 replication in T cells.

METHODS

The effect of endogenous Slit2 expression of HIV-1 replication in T cells was studied by transient overexpression or downregulation of Slit2. The role of exogenous Slit2 was studied by analyzing the effect of soluble Slit2 protein on HIV-1 replication in T-cell lines and peripheral blood mononuclear cells (PBMCs).

RESULTS

Studies on T-cell lines revealed a higher expression of Slit2 in Jurkat T cells compared with MT4 cells. We observed that downregulation of Slit2 in Jurkat T cells using Slit2-specific small inhibitor RNA enhanced HIV-1 replication. However, overexpression of Slit2 in MT4 cells and PBMCs reduced HIV-1 replication. As Slit2 is a secretory protein, we further analyzed the role of soluble Slit2 on HIV-1 virus replication using various cell lines and PBMCs. Our data indicated that exogenous Slit2 inhibited replication of both X4-tropic and R5-tropic HIV-1 viruses. Further studies revealed that Slit2 mediated its functional effects by binding to Robo1 receptor.

CONCLUSION

Taken together, our results describe Slit2/Robo1 axis as a novel modulator of HIV-1 replication in T cells. These studies may contribute to the understanding of the immunopathogenesis of HIV-1 infection.

The membrane bound LRR lipoprotein Slr, and the cell wall-anchored M1 protein from Streptococcus pyogenes both interact with type I collagen

Streptococcus pyogenes is an important human pathogen and surface structures allow it to adhere to, colonize and invade the human host. Proteins containing leucine rich repeats (LRR) have been indentified in mammals, viruses, archaea and several bacterial species. The LRRs are often involved in protein-protein interaction, are typically 20–30 amino acids long and the defining feature of the LRR motif is an 11-residue sequence LxxLxLxxNxL (x being any amino acid). The streptococcal leucine rich (Slr) protein is a hypothetical lipoprotein that has been shown to be involved in virulence, but at present no ligands for Slr have been identified. We could establish that Slr is a membrane attached horseshoe shaped lipoprotein by homology modeling, signal peptidase II inhibition, electron microscopy (of bacteria and purified protein) and immunoblotting. Based on our previous knowledge of LRR proteins we hypothesized that Slr could mediate binding to collagen. We could show by surface plasmon resonance that recombinant Slr and purified M1 protein bind with high affinity to collagen I. Isogenic slr mutant strain (MB1) and emm1 mutant strain (MC25) had reduced binding to collagen type I as shown by slot blot and surface plasmon resonance. Electron microscopy using gold labeled Slr showed multiple binding sites to collagen I, both to the monomeric and the fibrillar structure, and most binding occurred in the overlap region of the collagen I fibril. In conclusion, we show that Slr is an abundant membrane bound lipoprotein that is co-expressed on the surface with M1, and that both these proteins are involved in recruiting collagen type I to the bacterial surface. This underlines the importance of S. pyogenes interaction with extracellular matrix molecules, especially since both Slr and M1 have been shown to be virulence factors.

Syndecan contributes to heart cell specification and lumen formation during Drosophila cardiogenesis

The transmembrane proteoglycan Syndecan contributes to cell surface signaling of diverse ligands in mammals, yet in Drosophila, genetic evidence links Syndecan only to the Slit receptor Roundabout and to the receptor tyrosine phosphatase LAR. Here we characterize the requirement for syndecan in the determination and morphogenesis of the Drosophila heart, and reveal two phases of activity, indicating that Syndecan is a co-factor in at least two signaling events in this tissue. There is a stochastic failure to determine heart cell progenitors in a subset of abdominal hemisegments in embryos mutant for syndecan, and subsequent to Syndecan depletion by RNA interference. This phenotype is sensitive to gene dosage in the FGF receptor (Heartless), its ligand, Pyramus, as well as BMP-ligand Decapentaplegic (Dpp) and co-factor Sara. Syndecan is also required for lumen formation during assembly of the heart vessel, a phenotype shared with mutations in the Slit and Integrin signaling pathways. Phenotypic interactions of syndecan with slit and Integrin mutants suggest intersecting function, consistent with Syndecan acting as a co-receptor for Slit in the Drosophila heart.

Targeting Robo4-dependent Slit signaling to survive the cytokine storm in sepsis and influenza

The innate immune system provides a first line of defense against invading pathogens by releasing multiple inflammatory cytokines, such as interleukin-1beta and tumor necrosis factor-alpha, which directly combat the infectious agent and recruit additional immune responses. This exuberant cytokine release paradoxically injures the host by triggering leakage from capillaries, tissue edema, organ failure, and shock. Current medical therapies target individual pathogens with antimicrobial agents or directly either blunt or boost the host's immune system. We explored a third approach: activating with the soluble ligand Slit an endothelium-specific, Robo4-dependent signaling pathway that strengthens the vascular barrier, diminishing deleterious aspects of the host's response to the pathogen-induced cytokine storm. This approach reduced vascular permeability in the lung and other organs and increased survival in animal models of bacterial endotoxin exposure, polymicrobial sepsis, and H5N1 influenza. Thus, enhancing the resilience of the host vascular system to the host's innate immune response may provide a therapeutic strategy for treating multiple infectious agents.

Slit3 inhibits Robo3-induced invasion of synovial fibroblasts in rheumatoid arthritis

Introduction

The repellent factor family of Slit molecules has been described to have repulsive function in the developing nervous system on growing axons expressing the Robo receptors. However, until today no data are available on whether these repellent factors are involved in the regulation of synovial fibroblast (SF) activity in rheumatoid arthritis (RA).

Methods

mRNA expression in primary synovial fibroblasts was quantified by quantitative reverse transcription PCR and protein expression was measured by fluorescence activated cell sorting (FACS) analysis. Different functional assays were performed with rheumatoid arthritis synovial fibroblasts (RASF): proliferation, migration and a novel in-vitro cartilage destruction assay.

Results

First, we found increased expression of Robo3 expression in RASF compared to normal SF. Interestingly, analysis of data from a recently published genome-wide association study suggests a contribution of ROBO3 gene polymorphisms to susceptibility of RA. Functional assays performed with RASF revealed induction of migration and cartilage destruction by Robo3 and increased matrix metalloproteinase (MMP)1 and MMP3 expression. Treatment of RASF in early passages with Slit3 led to inhibition of migration whereas RASF in later passages, having reduced Robo3 expression in cell culture, were not inhibited by Slit3 treatment. Here, reduction of Robo3 expression from passage 3 to 10 might reflect an important step in losing repulsive activity of Slit3.

Conclusions

Taken together, our data showed that deregulation of the Robo3 receptor in synovial fibroblasts in RA correlates with aggressiveness of the fibroblasts. Slit3 reduces the migratory activity of synovial cells from patients with RA, potentially by repulsion of the cells in analogy to the neuronal system. Further studies will be necessary to prove Slit activity in vivo.

CLERC and centrosomal leucine-rich repeat proteins

The centrosome functions as the microtubule-organizing center and plays a vital role in organizing spindle poles during mitosis. Recently, we identified a centrosomal protein called CLERC (Centrosomal leucine-rich repeat and coiled-coil containing protein) which is a human ortholog of Chlamydomonas Vfl1 protein. The bibliography as well as database searches provided evidence that the human proteome contains at least seven centrosomal leucine-rich repeat proteins including CLERC. CLERC and four other centrosomal leucine-rich repeat proteins contain the SDS22-like leucine-rich repeat motifs, whereas the remaining two proteins contain the RI-like and the cysteine-containing leucine-rich repeat motifs. Individual leucine-rich repeat motifs are highly conserved and present in evolutionarily diverse organisms. Here, we provide an overview of CLERC and other centrosomal leucine-rich repeat proteins, their structures, their evolutionary relationships, and their functional properties.

Slit-Robo signaling in ocular angiogenesis

Slit-Robo signaling was firstly discovered as a major repellent pathway at the midline of the central nervous system. Intense investigation found that this pathway also plays an important role in other biological process including angiogenesis. Robo4 is the vascular endothelial cell specific member of Robo family. It was found that Slit-Robo signaling can inhibit endothelial cell migration, tube formation and vascular permeability. Slit-Robo signaling also plays an important role in embryonic and tumor angiogenesis. In animal model of ocular angiogenesis, addition of Slit inhibited laser induced choroidal neovascularization, oxygen induced retinopathy and VEGF induced retinal permeability in a Robo4 dependent manner. Recent data demonstrates that Robo1 and Robo4 form a heterodimer in endothelial cells, The role of this heterodimer in counteracting VEGF signaling is unknown. Further investigation is required to better understand Slit-Robo signaling and develop novel therapy for angiogenesis.

Repulsive axon guidance molecule Slit3 is a novel angiogenic factor

Slits are large, secreted repulsive axon guidance molecules. Recent genetic studies revealed that the Slit3 is dispensable for neural development but required for non-neuron-related developmental processes, such as the genesis of the diaphragm and kidney. Here we report that Slit3 potently promotes angiogenesis, a process essential for proper organogenesis during embryonic development. We observed that Slit3 is expressed and secreted by both endothelial cells and vascular smooth muscle cells in vasculature and that the Slit cognate receptors Robo1 and Robo4 are universally expressed by endothelial cells, suggesting that Slit3 may act in paracrine and autocrine manners to regulate endothelial cells. Cellular function studies revealed that Slit3 stimulates endothelial-cell proliferation, promotes endothelial-cell motility and chemotaxis via interaction with Robo4, and accelerates endothelial-cell vascular network formation in vitro with a specific activity comparable with vascular endothelial growth factor. Furthermore, Slit3 stimulates neovessel sprouting ex vivo and new blood vessel growth in vivo. Consistent with these observations, the Slit3 knockout mice display disrupted angiogenesis during embryogenesis. Taken together, our studies reveal that the repulsive axon guidance molecule Slit3 is a novel and potent angiogenic factor and functions to promote angiogenesis in coordinating organogenesis during embryonic development.

Slit2-Robo4 signalling promotes vascular stability by blocking Arf6 activity

Slit-Roundabout (Robo) signalling has a well-understood role in axon guidance. Unlike in the nervous system, however, Slit-dependent activation of an endothelial-specific Robo, Robo4, does not initiate a guidance program. Instead, Robo4 maintains the barrier function of the mature vascular network by inhibiting neovascular tuft formation and endothelial hyperpermeability induced by pro-angiogenic factors. In this study, we used cell biological and biochemical techniques to elucidate the molecular mechanism underlying the maintenance of vascular stability by Robo4. Here, we demonstrate that Robo4 mediates Slit2-dependent suppression of cellular protrusive activity through direct interaction with the intracellular adaptor protein paxillin and its paralogue, Hic-5. Formation of a Robo4-paxillin complex at the cell surface blocks activation of the small GTPase Arf6 and, consequently, Rac by recruitment of Arf-GAPs (ADP-ribosylation factor- directed GTPase-activating proteins) such as GIT1. Consistent with these in vitro studies, inhibition of Arf6 activity in vivo phenocopies Robo4 activation by reducing pathologic angiogenesis in choroidal and retinal vascular disease and VEGF-165 (vascular endothelial growth factor-165)-induced retinal hyperpermeability. These data reveal that a Slit2-Robo4-paxillin-GIT1 network inhibits the cellular protrusive activity underlying neovascularization and vascular leak, and identify a new therapeutic target for ameliorating diseases involving the vascular system.

LRRC19, a novel member of the leucine-rich repeat protein family, activates NF-kappaB and induces expression of proinflammatory cytokines

We have identified a new functional transmembrane receptor, LRRC19 (leucine-rich repeat containing 19), that belongs to the LRR protein family. LRRC19's central core has four analogous LRR repeating modules in a juxtaposed array and a casein kinase (CK2) phosphorylation site in the cytoplasmic domain. LRRC19 mRNA was found in the kidney, spleen and intestine of adult mice using both RT-PCR and in situ hybridization. LRRC19 does not contain a cytoplasmic Toll/IL-1 receptor (TIR) domain but was able to activate NF-kappaB and induce production of proinflammatory cytokines. LRRC19 shares a close evolutionary relationship with multiple Toll-like receptors (TLRs), especially TLR3. Importantly, the TLR3 ligand, as well as other TLR ligands, significantly promoted the expression of proinflammatory cytokines and the activation of NF-kappaB by LRRC19. Thus, LRRC19 may play an important role in inducing innate immune responses in certain tissues such as the kidney.

Active involvement of Robo1 and Robo4 in filopodia formation and endothelial cell motility mediated via WASP and other actin nucleation-promoting factors

This study aimed to further elucidate the function of Roundabout proteins in endothelium. We show that both Robo1 and Robo4 are present in human umbilical vein endothelial cells (HUVECs) and have knocked expression down using small interfering RNA (siRNA) technology. Roundabout knockout endothelial cells were then studied in a variety of in vitro assays. We also performed a yeast 2-hybrid analysis using the intracellular domain of Robo4 as bait to identify interacting proteins and downstream signaling. Both Robo1 and Robo4 siRNA knockdown and transfection of Robo4-green fluorescent protein inhibited endothelial cell movement and disrupted tube formation on Matrigel. Consistent with a role in regulating cell movement, yeast 2-hybrid and glutathione-S-transferase pulldown analyses show Robo4 binding to a Wiskott-Aldrich syndrome protein (WASP), neural Wiskott-Aldrich syndrome protein, and WASP-interacting protein actin-nucleating complex. We have further shown that Robo1 forms a heterodimeric complex with Robo4, and that transfection of Robo4GFP into HUVECs induces filopodia formation. We finally show using Robo1 knockdown cells that Robo1 is essential for Robo4-mediated filopodia induction. Our results favor a model whereby Slit2 binding to a Robo1/Robo4 heterodimer activates actin nucleation-promoting factors to promote endothelial cell migration.

LRRCE: a leucine-rich repeat cysteine capping motif unique to the chordate lineage

Background

The small leucine-rich repeat proteins and proteoglycans (SLRPs) form an important family of regulatory molecules that participate in many essential functions. They typically control the correct assembly of collagen fibrils, regulate mineral deposition in bone, and modulate the activity of potent cellular growth factors through many signalling cascades. SLRPs belong to the group of extracellular leucine-rich repeat proteins that are flanked at both ends by disulphide-bonded caps that protect the hydrophobic core of the terminal repeats. A capping motif specific to SLRPs has been recently described in the crystal structures of the core proteins of decorin and biglycan. This motif, designated as LRRCE, differs in both sequence and structure from other, more widespread leucine-rich capping motifs. To investigate if the LRRCE motif is a common structural feature found in other leucine-rich repeat proteins, we have defined characteristic sequence patterns and used them in genome-wide searches.

Results

The LRRCE motif is a structural element exclusive to the main group of SLRPs. It appears to have evolved during early chordate evolution and is not found in protein sequences from non-chordate genomes. Our search has expanded the family of SLRPs to include new predicted protein sequences, mainly in fishes but with intriguing putative orthologs in mammals. The chromosomal locations of the newly predicted SLRP genes would support the large-scale genome or gene duplications that are thought to have occurred during vertebrate evolution. From this expanded list we describe a new class of SLRP sequences that could be representative of an ancestral SLRP gene.

Conclusion

Given its exclusivity the LRRCE motif is a useful annotation tool for the identification and classification of new SLRP sequences in genome databases. The expanded list of members of the SLRP family offers interesting insights into early vertebrate evolution and suggests an early chordate evolutionary origin for the LRRCE capping motif.

Pursuing a 'turning point' in growth cone research

Growth cones are highly motile structures found at the leading edge of developing and regenerating nerve processes. Their role in axonal pathfinding has been well established and many guidance cues that influence growth cone behavior have now been identified. Many studies are now providing insights into the transduction and integration of signals in the growth cone, though a full understanding of growth cone behavior still eludes us. This review focuses on recent studies adding to the growing body of literature on growth cone behavior, focusing particularly on the level of autonomy the growth cone possesses and the role of local protein synthesis.

Slits and Roundabouts in cancer, tumour angiogenesis and endothelial cell migration

Angiogenesis describes the development of new blood vessels from pre-existing vessels. The hijacking of this physiological process by tumours allows them to develop their own supplies of nutrients and oxygen, enabling their growth and metastasis. A large body of literature has accumulated over the last 20 years relating to angiogenesis, including signalling pathways involved in this process. One such pathway uses Slit-Roundabout proteins that are implicated in the development of cancers and tumour angiogenesis. The Roundabout family of receptors are large, single-pass transmembrane cell surface receptors involved in directing cell migration in response to their cognate Slit ligands. Although best known for their role in neuronal development, Slits and Roundabouts have now been implicated in myogenesis, leukocyte chemotaxis and tumour angiogenesis, confirming that the Robo signalling pathway functions across multiple cell types. We review here the evidence for a role for Slits and Roundabouts in cancer. In particular, we focus on the role of Robo1 and Robo4 in tumour angiogenesis and discuss the signalling pathways downstream of these proteins mediating endothelial cell migration.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

Slit1a inhibits retinal ganglion cell arborization and synaptogenesis via Robo2-dependent and -independent pathways

Upon arriving at their targets, developing axons cease pathfinding and begin instead to arborize and form synapses. To test whether CNS arborization and synaptogenesis are controlled by Slit-Robo signaling, we followed single retinal ganglion cell (RGC) arbors over time. ast (robo2) mutant and slit1a morphant arbors had more branch tips and greater arbor area and complexity compared to wild-type and concomitantly more presumptive presynaptic sites labeled with YFP-Rab3. Increased arborization in ast was phenocopied by dominant-negative Robo2 expressed in single RGCs and rescued by full-length Robo2, indicating that Robo2 acts cell-autonomously. Time-lapse imaging revealed that ast and slit1a morphant arbors stabilized earlier than wild-type, suggesting a role for Slit-Robo signaling in preventing arbor maturation. Genetic analysis showed that Slit1a acts both through Robo2 and Robo2-independent mechanisms. Unlike previous PNS studies showing that Slits promote branching, our results show that Slits inhibit arborization and synaptogenesis in the CNS.

Common cues regulate neural and vascular patterning

Nerves and blood vessels often follow parallel trajectories as they course through the body to their distal targets. Proteins that regulate the process of axon guidance have likewise been shown to play a crucial role in blood vessel migration. With the recent description of the endothelial tip cell as an analog of the axonal growth cone, the nerve-vessel analogy seems complete. Notwithstanding these considerable similarities, one crucial difference remains between neural and vascular guidance. While a navigating axon is but a single cell, a sprouting vessel is composed of multiple cells that must be co-ordinately regulated. Recent studies of the Dll4-Notch1 signaling pathway have provided valuable insight into how the vasculature accomplishes this crucial task.