Vav family proteins couple to diverse cell surface receptors

The guanine nucleotide exchange factor Vav3 intervenes in the migration pathway of oligodendrocyte precursor cells on tenascin-C

Oligodendrocyte precursor cells (OPCs) are the exclusive source of myelination in the central nervous system (CNS). Prior to myelination, OPCs migrate to target areas and mature into myelinating oligodendrocytes. This process is underpinned by drastic changes of the cytoskeleton and partially driven by pathways involving small GTPases of the Rho subfamily. In general, the myelination process requires migration, proliferation and differentiation of OPCs. Presently, these processes are only partially understood. In this study, we analyzed the impact of the guanine nucleotide exchange factor (GEF) Vav3 on the migration behavior of OPCs. Vav3 is known to regulate RhoA, Rac1 and RhoG activity and is therefore a promising candidate with regard to a regulatory role concerning the rearrangement of the cytoskeleton. Our study focused on the Vav3 knockout mouse and revealed an enhanced migration capacity of Vav3−/− OPCs on the extracellular matrix (ECM) glycoprotein tenascin-C (TnC). The migration behavior of individual OPCs on further ECM molecules such as laminin-1 (Ln1), laminin-2 (Ln2) and tenascin-R (TnR) was not affected by the elimination of Vav3. The migration process was further investigated with regard to intracellular signal transmission by pharmacological blockade of downstream pathways of specific Rho GTPases. Our data suggest that activation of RhoA GTPase signaling compromises migration, as inhibition of RhoA-signaling promoted migration behavior. This study provides novel insights into the control of OPC migration, which could be useful for further understanding of the complex differentiation and myelination process.

Vav Proteins in Development of the Brain: A Potential Relationship to the Pathogenesis of Congenital Zika Syndrome?

Zika virus (ZIKV) infection during pregnancy can result in a significant impact on the brain and eye of the developing fetus, termed congenital zika syndrome (CZS). At a morphological level, the main serious presentations of CZS are microcephaly and retinal scarring. At a cellular level, many cell types of the brain may be involved, but primarily neuronal progenitor cells (NPC) and developing neurons. Vav proteins have guanine exchange activity in converting GDP to GTP on proteins such as Rac1, Cdc42 and RhoA to stimulate intracellular signaling pathways. These signaling pathways are known to play important roles in maintaining the polarity and self-renewal of NPC pools by coordinating the formation of adherens junctions with cytoskeletal rearrangements. In developing neurons, these same pathways are adopted to control the formation and growth of neurites and mediate axonal guidance and targeting in the brain and retina. This review describes the role of Vavs in these processes and highlights the points of potential ZIKV interaction, such as (i) the binding and entry of ZIKV in cells via TAM receptors, which may activate Vav/Rac/RhoA signaling; (ii) the functional convergence of ZIKV NS2A with Vav in modulating adherens junctions; (iii) ZIKV NS4A/4B protein effects on PI3K/AKT in a regulatory loop via PPI3 to influence Vav/Rac1 signaling in neurite outgrowth; and (iv) the induction of SOCS1 and USP9X following ZIKV infection to regulate Vav protein degradation or activation, respectively, and impact Vav/Rac/RhoA signaling in NPC and neurons. Experiments to define these interactions will further our understanding of the molecular basis of CZS and potentially other developmental disorders stemming from in utero infections. Additionally, Vav/Rac/RhoA signaling pathways may present tractable targets for therapeutic intervention or molecular rationale for disease severity in CZS.

Hematopoietic stem cell-derived functional osteoblasts exhibit therapeutic efficacy in a murine model of osteogenesis imperfecta

Currently, there is no cure for osteogenesis imperfecta (OI)-a debilitating pediatric skeletal dysplasia. Herein we show that hematopoietic stem cell (HSC) therapy holds promise in treating OI. Using single-cell HSC transplantation in lethally irradiated oim/oim mice, we demonstrate significant improvements in bone morphometric, mechanics, and turnover parameters. Importantly, we highlight that HSCs cause these improvements due to their unique property of differentiating into osteoblasts/osteocytes, depositing normal collagen-an attribute thus far assigned only to mesenchymal stem/stromal cells. To confirm HSC plasticity, lineage tracing was done by transplanting oim/oim with HSCs from two specific transgenic mice-VavR, in which all hematopoietic cells are GFP+ and pOBCol2.3GFP, where GFP is expressed only in osteoblasts/osteocytes. In both models, transplanted oim/oim mice demonstrated GFP+ HSC-derived osteoblasts/osteocytes in bones. These studies unequivocally establish that HSCs differentiate into osteoblasts/osteocytes, and HSC transplantation can provide a new translational approach for OI.

Vav3 Mediates Pseudomonas aeruginosa Adhesion to the Cystic Fibrosis Airway Epithelium

Pseudomonas aeruginosa (Pa) represents the leading cause of airway infection in cystic fibrosis (CF). Early airways colonization can be explained by enhanced adhesion of Pa to the respiratory epithelium. RNA sequencing (RNA-seq) on fully differentiated primary cultures of airway epithelial cells from CF and non-CF donors predict that VAV3, β1 INTEGRIN, and FIBRONECTIN genes are significantly enriched in CF. Indeed, Vav3 is apically overexpressed in CF, associates with active β1 integrin luminally exposed, and increases fibronectin deposition. These luminal microdomains, rich in fibronectin and β1 integrin and regulated by Vav3, mediate the increased Pa adhesion to the CF epithelium. Interestingly, Vav3 inhibition normalizes the CF-dependent fibronectin and β1-integrin ectopic expression, improves the CF epithelial integrity, and prevents the enhanced Pa trapping to the CF epithelium. Through its capacity to promote a luminal complex with active β1 integrin and fibronectin that favors bacteria trapping, Vav3 may represent a new target in CF.

The guanine nucleotide exchange factor VAV3 participates in ERBB4-mediated cancer cell migration

ERBB4 is a member of the epidermal growth factor receptor (EGFR)/ERBB subfamily of receptor tyrosine kinases that regulates cellular processes including proliferation, migration, and survival. ERBB4 signaling is involved in embryogenesis and homeostasis of healthy adult tissues, but also in human pathologies such as cancer, neurological disorders, and cardiovascular diseases. Here, an MS-based analysis revealed the Vav guanine nucleotide exchange factor 3 (VAV3), an activator of Rho family GTPases, as a critical ERBB4-interacting protein in breast cancer cells. We confirmed the ERBB4-VAV3 interaction by targeted MS and coimmunoprecipitation experiments and further defined it by demonstrating that kinase activity and Tyr-1022 and Tyr-1162 of ERBB4, as well as the intact phosphotyrosine-interacting SH2 domain of VAV3, are necessary for this interaction. We found that ERBB4 stimulates tyrosine phosphorylation of the VAV3 activation domain, known to be required for guanine nucleotide exchange factor (GEF) activity of VAV proteins. In addition to VAV3, the other members of the VAV family, VAV1 and VAV2, also coprecipitated with ERBB4. Analyses of the effects of overexpression of dominant-negative VAV3 constructs or shRNA-mediated down-regulation of VAV3 expression in breast cancer cells indicated that active VAV3 is involved in ERBB4-stimulated cell migration. These results define the VAV GEFs as effectors of ERBB4 activity in a signaling pathway relevant for cancer cell migration.

Mesenchymal stromal/stem cells modulate response to experimental sepsis-induced lung injury via regulation of miR-27a-5p in recipient mice

Introduction

Mesenchymal stromal cell (MSC) therapy mitigates lung injury and improves survival in murine models of sepsis. Precise mechanisms of therapeutic benefit remain poorly understood.

Objectives

To identify host-derived regulatory elements that may contribute to the therapeutic effects of MSCs, we profiled the microRNAome (miRNAome) and transcriptome of lungs from mice randomised to experimental polymicrobial sepsis-induced lung injury treated with either placebo or MSCs.

Methods and results

A total of 11 997 genes and 357 microRNAs (miRNAs) expressed in lungs were used to generate a statistical estimate of association between miRNAs and their putative mRNA targets; 1395 miRNA:mRNA significant association pairs were found to be differentially expressed (false discovery rate ≤0.05). MSC administration resulted in the downregulation of miR-27a-5p and upregulation of its putative target gene VAV3 (adjusted p=1.272E-161) in septic lungs. In human pulmonary microvascular endothelial cells, miR-27a-5p expression levels were increased while VAV3 was decreased following lipopolysaccharide (LPS) or tumour necrosis factor (TNF) stimulation. Transfection of miR-27a-5p mimic or inhibitor resulted in increased or decreased VAV3 message, respectively. Luciferase reporter assay demonstrated specific binding of miR-27a-5p to the 3′UTR of VAV3. miR27a-5p inhibition mitigated TNF-induced (1) delayed wound closure, increased (2) adhesion and (3) transendothelial migration but did not alter permeability. In vivo, cell infiltration was attenuated by intratracheal coinstillation of the miR-27a-5p inhibitor, but this did not protect against endotoxin-induced oedema formation.

Conclusions

Our data support involvement of miR-27a-5p and VAV3 in cellular adhesion and infiltration during acute lung injury and a potential role for miR-27a-based therapeutics for acute respiratory distress syndrome.

Targeting Rac and Cdc42 GEFs in Metastatic Cancer

The Rho family GTPases Rho, Rac, and Cdc42 have emerged as key players in cancer metastasis, due to their essential roles in regulating cell division and actin cytoskeletal rearrangements; and thus, cell growth, migration/invasion, polarity, and adhesion. This review will focus on the close homologs Rac and Cdc42, which have been established as drivers of metastasis and therapy resistance in multiple cancer types. Rac and Cdc42 are often dysregulated in cancer due to hyperactivation by guanine nucleotide exchange factors (GEFs), belonging to both the diffuse B-cell lymphoma (Dbl) and dedicator of cytokinesis (DOCK) families. Rac/Cdc42 GEFs are activated by a myriad of oncogenic cell surface receptors, such as growth factor receptors, G-protein coupled receptors, cytokine receptors, and integrins; consequently, a number of Rac/Cdc42 GEFs have been implicated in metastatic cancer. Hence, inhibiting GEF-mediated Rac/Cdc42 activation represents a promising strategy for targeted metastatic cancer therapy. Herein, we focus on the role of oncogenic Rac/Cdc42 GEFs and discuss the recent advancements in the development of Rac and Cdc42 GEF-interacting inhibitors as targeted therapy for metastatic cancer, as well as their potential for overcoming cancer therapy resistance.

Vav2 lacks Ca2+ entry-promoting scaffolding functions unique to Vav1 and inhibits T cell activation via Cdc42

Vav family guanine nucleotide exchange factors (GEFs) are essential regulators of immune function. Despite their structural similarity, Vav1 promotes and Vav2 opposes T cell receptor (TCR)-induced Ca2+ entry. By using a Vav1-deficient Jurkat T cell line, we find that Vav1 facilitates Ca2+ entry via non-catalytic scaffolding functions that are encoded by the catalytic core of Vav1 and flanking linker regions. We implicate, in this scaffolding function, a previously undescribed polybasic motif that is strictly conserved in Vav1 and absent from Vav2 in tetrapods. Conversely, the catalytic activity of Vav2 contributes to the suppression of TCR-mediated Ca2+ entry. By performing an in vivo 'GEF trapping' assay in intact cells, we demonstrate that Cdc42 interacts with the catalytic surface of Vav2 but not Vav1, and that Vav1 discriminates Cdc42 from Rac1 via F56 (W56 in Rac1). Finally, the Cdc42-specific inhibitor ZCL278 and the shRNA-mediated suppression of Cdc42 each prevent the inhibition of TCR-induced Ca2+ entry by Vav2. These findings define stark differences in the functions of Vav1 and Vav2, and provide an explanation for the differential usage of these Vav isoforms by immune subpopulations.

Differential Subcellular Distribution and Translocation of Seven 14-3-3 Isoforms in Response to EGF and During the Cell Cycle

Multiple isoforms of 14-3-3 proteins exist in different organisms. In mammalian cells, 14-3-3 protein has seven isoforms (α/β, ε, η, γ, σ, θ/τ, and δ/ζ), with α and δ representing the phosphorylated versions of β and ζ, respectively. While the existence of multiple isoforms may represent one more level of regulation in 14-3-3 signaling, our knowledge regarding the isoform-specific functions of 14-3-3 proteins is very limited. Determination of the subcellular localization of the different 14-3-3 isoforms could give us important clues of their specific functions. In this study, by using indirect immunofluorescence, subcellular fractionation, and immunoblotting, we studied the subcellular localization of the total 14-3-3 protein and each of the seven 14-3-3 isoforms; their redistribution throughout the cell cycle; and their translocation in response to EGF in Cos-7 cells. We showed that 14-3-3 proteins are broadly distributed throughout the cell and associated with many subcellular structures/organelles, including the plasma membrane (PM), mitochondria, ER, nucleus, microtubules, and actin fibers. This broad distribution underlines the multiple functions identified for 14-3-3 proteins. The different isoforms of 14-3-3 proteins have distinctive subcellular localizations, which suggest their distinctive cellular functions. Most notably, 14-3-3ƞ is almost exclusively localized to the mitochondria, 14-3-3γ is only localized to the nucleus, and 14-3-3σ strongly and specifically associated with the centrosome during mitosis. We also examined the subcellular localization of the seven 14-3-3 isoforms in other cells, including HEK-293, MDA-MB-231, and MCF-7 cells, which largely confirmed our findings with Cos-7 cells.

Rac1 S71 Mediates the Interaction between Rac1 and 14-3-3 Proteins

Both 14-3-3 proteins (14-3-3s) and Rho proteins regulate cytoskeleton remodeling and cell migration, which suggests a possible interaction between the signaling pathways regulated by these two groups of proteins. Indeed, more and more emerging evidence indicates the mutual regulation of these two signaling pathways. However, all of the data regarding the interaction between Rac1 signaling pathways and 14-3-3 signaling pathways are through either the upstream regulators or downstream substrates. It is not clear if Rac1 could interact with 14-3-3s directly. It is interesting to notice that the Rac1 sequence 68RPLSYP73 is likely a 14-3-3 protein binding motif following the phosphorylation of S71 by Akt. Thus, we hypothesize that Rac1 directly interacts with 14-3-3s. We tested this hypothesis in this research. By using mutagenesis, co-immunoprecipitation (co-IP), Rac1 activity assay, immunoblotting, and indirect immunofluorescence, we demonstrate that 14-3-3s interact with Rac1. This interaction is mediated by Rac1 S71 in both phosphorylation-dependent and -independent manners, but the phosphorylation-dependent interaction is much stronger. Epidermal growth factor (EGF) strongly stimulates the phosphorylation of Rac1 S71 and the interaction between 14-3-3s and Rac1. Mutating S71 to A completely abolishes both phosphorylation-dependent and -independent interactions between 14-3-3s and Rac1. The interaction between 14-3-3s and Rac1 mostly serve to regulate the activity and subcellular localization of Rac1. Among the seven 14-3-3 isoforms, 14-3-3η, -σ, and -θ showed interactions with Rac1 in both Cos-7 and HEK 293 cells. 14-3-3γ also binds to Rac1 in HEK 293 cells, but not in Cos-7 cells. We conclude that 14-3-3s interact with Rac1. This interaction is mediated by Rac1 S71 in both phosphorylation-dependent and -independent manners. The interaction between 14-3-3 and Rac1 mostly serves to regulate the activity and subcellular localization of Rac1. Among the seven 14-3-3 isoforms, 14-3-3η, -γ, -σ, and -θ interact with Rac1.

Hematopoietic Stem Cells as a Novel Source of Dental Tissue Cells

While earlier studies have suggested that cells positive for hematopoietic markers can be found in dental tissues, it has yet to be confirmed. To conclusively demonstrate this, we utilized a unique transgenic model in which all hematopoietic cells are green fluorescent protein⁺ (GFP⁺). Pulp, periodontal ligament (PDL) and alveolar bone (AvB) cell culture analysis demonstrated numerous GFP⁺ cells, which were also CD45⁺ (indicating hematopoietic origin) and co-expressed markers of cellular populations in pulp (dentin matrix protein-1, dentin sialophosphoprotein, alpha smooth muscle actin [ASMA], osteocalcin), in PDL (periostin, ASMA, vimentin, osteocalcin) and in AvB (Runx-2, bone sialoprotein, alkaline phosphatase, osteocalcin). Transplantation of clonal population derived from a single GFP⁺ hematopoietic stem cell (HSC), into lethally irradiated recipient mice, demonstrated numerous GFP⁺ cells within dental tissues of recipient mice, which also stained for markers of cell populations in pulp, PDL and AvB (used above), indicating that transplanted HSCs can differentiate into cells in dental tissues. These hematopoietic-derived cells deposited collagen and can differentiate in osteogenic media, indicating that they are functional. Thus, our studies demonstrate, for the first time, that cells in pulp, PDL and AvB can have a hematopoietic origin, thereby opening new avenues of therapy for dental diseases and injuries.

Rac3 regulates breast cancer invasion and metastasis by controlling adhesion and matrix degradation

The initial step of metastasis is the local invasion of tumor cells into the surrounding tissue. Invadopodia are actin-based protrusions that mediate the matrix degradation necessary for invasion and metastasis of tumor cells. We demonstrate that Rac3 GTPase is critical for integrating the adhesion of invadopodia to the extracellular matrix (ECM) with their ability to degrade the ECM in breast tumor cells. We identify two pathways at invadopodia important for integrin activation and delivery of matrix metalloproteinases: through the upstream recruiter CIB1 as well as the downstream effector GIT1. Rac3 activity, at and surrounding invadopodia, is controlled by Vav2 and βPIX. These guanine nucleotide exchange factors regulate the spatiotemporal dynamics of Rac3 activity, impacting GIT1 localization. Moreover, the GTPase-activating function of GIT1 toward the vesicular trafficking regulator Arf6 GTPase is required for matrix degradation. Importantly, Rac3 regulates the ability of tumor cells to metastasize in vivo. The Rac3-dependent mechanisms we show in this study are critical for balancing proteolytic activity and adhesive activity to achieve a maximally invasive phenotype.

Interaction between Rho GTPases and 14-3-3 Proteins

The Rho GTPase family accounts for as many as 20 members. Among them, the archetypes RhoA, Rac1, and Cdc42 have been the most well-characterized. Like all members of the small GTPases superfamily, Rho proteins act as molecular switches to control cellular processes by cycling between active, GTP-bound and inactive, GDP-bound states. The 14-3-3 family proteins comprise seven isoforms. They exist as dimers (homo- or hetero-dimer) in cells. They function by binding to Ser/Thr phosphorylated intracellular proteins, which alters the conformation, activity, and subcellular localization of their binding partners. Both 14-3-3 proteins and Rho GTPases regulate cell cytoskeleton remodeling and cell migration, which suggests a possible interaction between the signaling pathways regulated by these two groups of proteins. Indeed, more and more emerging evidence indicates the mutual regulation of these two signaling pathways. There have been many documented reviews of 14-3-3 protein and Rac1 separately, but there is no review regarding the interaction and mutual regulation of these two groups of proteins. Thus, in this article we thoroughly review all the reported interactions between the signaling pathways regulated by 14-3-3 proteins and Rho GTPases (mostly Rac1).

A Novel Vav3 Homolog Identified in Lamprey, Lampetra japonica, with Roles in Lipopolysaccharide-Mediated Immune Response

Vav guanine nucleotide exchange factor 3 (Vav3), a Rho family GTPase, regulates multiple cell signaling pathways including those of T- and B-cell receptors in vertebrates through mediating the activities of the Rho family members. Whether the lamprey possesses Vav3 homolog and what role it plays in immune response remain unknown. Gene cloning, recombinant expression, antibody production and expression pattern analyses were performed to characterize the lamprey Vav3 in the current study. The lamprey Vav3 is closer to jawed vertebrates' Vav3 molecules (about 53% identities in general) than to Vav2 molecules of jawless and jawed vertebrates (about 51% identities in general) in sequence similarity. Conserved motif analysis showed that the most distinguished parts between Vav3 and Vav2 proteins are their two Src-homology 3 domains. The relative expression levels of lamprey vav3 mRNA and protein were significantly up-regulated in lamprey lymphocytes and supraneural myeloid bodies after mixed-antigens stimulation, respectively. In addition, lamprey Vav3 were up-regulated drastically in lymphocytes and supraneural myeloid bodies after lipopolysaccharide (LPS) rather than phytohemagglutinin (PHA) stimulation. Lamprey Vav3 distributed in the cytoplasm of variable lymphocyte receptor B positive (VLRB⁺) lymphocytes, and the number of plasmacytes (VLRB and lamprey Vav3 double positive) in blood lymphocytes also increased after LPS stimulation. Our results proved that lamprey Vav3 was involved in the LPS-mediated immune reaction of lamprey and provided a clue for the further study of the precise role lamprey Vav3 played in the signaling pathway of lamprey VLRB⁺ lymphocytes.

Binding of CIB1 to the αIIb tail of αIIbβ3 is required for FAK recruitment and activation in platelets

Background

It is believed that activation of c-Src bound to the integrin β₃ subunit initiates outside-in signaling. The involvement of α_IIb in outside-in signaling is poorly understood.

Objectives

We have previously shown that CIB1 specifically interacts with the cytoplasmic domain of α_IIb and is required for α_IIbβ₃ outside-in signaling. Here we evaluated the role of CIB1 in regulating outside-in signaling in the absence of inside-out signaling.

Methods

We used α_IIb cytoplasmic domain peptide and CIB1-function blocking antibody to inhibit interaction of CIB1 with α_IIb subunit as well as Cib1^-/- platelets to evaluate the consequence of CIB1 interaction with α_IIb on outside-in signaling.

Results

Fibrinogen binding to α_IIbβ₃ results in calcium-dependent interaction of CIB1 with α_IIb, which is not required for filopodia formation. Dynamic rearrangement of cytoskeleton results in CIB1-dependent recruitment of FAK to the α_IIb complex and its activation. Disruption of the association of CIB1 and α_IIb by incorporation of α_IIb peptide or anti-CIB1 inhibited both FAK association and activation. Furthermore, FAK recruitment to the integrin complex was required for c-Src activation. Inhibition of c-Src had no effect on CIB1 accumulation with the integrin at the filopodia, suggesting that c-Src activity is not required for the formation of CIB1-α_IIb-FAK complex.

Conclusion

Our results suggest that interaction of CIB1 with α_IIb is one of the early events occurring during outside-in signaling. Furthermore, CIB1 recruits FAK to the α_IIbβ₃ complex at the filopodia where FAK is activated, which in turn activates c-Src, resulting in propagation of outside-in signaling leading to platelet spreading.

Involvement of the guanine nucleotide exchange factor Vav3 in central nervous system development and plasticity

Small GTP-hydrolyzing enzymes (GTPases) of the RhoA family play manifold roles in cell biology and are regulated by upstream guanine nucleotide exchange factors (GEFs). Herein, we focus on the GEFs of the Vav subfamily. Vav1 was originally described as a proto-oncogene of the hematopoietic lineage. The GEFs Vav2 and Vav3 are more broadly expressed in various tissues. In particular, the GEF Vav3 may play important roles in the developing nervous system during the differentiation of neural stem cells into the major lineages, namely neurons, oligodendrocytes and astrocytes. We discuss its putative regulatory roles for progenitor differentiation in the developing retina, polarization of neurons and formation of synapses, migration of oligodendrocyte progenitors and establishment of myelin sheaths. We propose that Vav3 mediates the response of various neural cell types to environmental cues.

Gene expression profile after activation of RIG-I in 5'ppp-dsRNA challenged DF1

Retinoic acid inducible gene I (RIG-I) can recognize influenza viruses and evoke the innate immune response. RIG-I is absent in the chicken genome, but is conserved in the genome of ducks. Lack of RIG-I renders chickens more susceptible to avian influenza infection, and the clinical symptoms are more prominent than in other poultry. It is unknown whether introduction of duck RIG-I into chicken cells can establish the immunity as is seen in ducks and the role of RIG-I in established immunity is unknown. In this study, a chicken cell strain with stable expression of duRIG-I was established by lentiviral infection, giving DF1/LV5-RIG-I, and a control strain DF1/LV5 was established in parallel. To verify stable, high level expression of duRIG-I in DF1 cells, the levels of duRIG-I mRNA and protein were determined by real-time RT-PCR and Western blot, respectively. Further, 5'triphosphate double stranded RNA (5'ppp-dsRNA) was used to mimic an RNA virus infection and the infected DF1/LV5-RIG-I and DF1/LV5 cells were subjected to high-throughput RNA-sequencing, which yielded 193.46 M reads and 39.07 G bases. A total of 278 differentially expressed genes (DEGs), i.e., duRIG-I-mediated responsive genes, were identified by RNA-seq. Among the 278 genes, 120 DEGs are annotated in the KEGG database, and the most reliable KEGG pathways are likely to be the signaling pathways of RIG-I like receptors. Functional analysis by Gene ontology (GO) indicates that the functions of these DEGs are primarily related to Type I interferon (IFN) signaling, IFN-β-mediated cellular responses and up-regulation of the RIG-I signaling pathway. Based on the shared genes among different pathways, a network representing crosstalk between RIG-I and other signaling pathways was constructed using Cytoscape software. The network suggests that RIG-mediated pathway may crosstalk with the Jak-STAT signaling pathway, Toll-like receptor signaling pathway, Wnt signaling pathway, ubiquitin-mediated proteolysis and MAPK signaling pathway during the transduction of antiviral signals. After screening, a group of key responsive genes in RIG-I-mediated signaling pathways, such as ISG12-2, Mx1, IFIT5, TRIM25, USP18, STAT1, STAT2, IRF1, IRF7 and IRF8, were tested for differential expression by real-time RT-PCR. In summary, by combining our results and the current literature, we propose a RIG-I-mediated signaling network in chickens.

Involvement of Tiam1, RhoG and ELMO2/ILK in Rac1-mediated phagocytosis in human trabecular meshwork cells

We previously demonstrated that an αvβ5 integrin/FAK- mediated pathway regulated the phagocytic properties of human trabecular meshwork (HTM) cells. Here we demonstrate that this process is mediated by Rac-1 and a previously unreported signaling pathway that utilizes the Tiam1 as well as a novel ILK/RhoG/ELMO2 signaling pathway. Phagocytosis in both a TM-1 cell line and normal HTM cells was mediated by Rac1 and could be significantly decreased by >75% using the Rac1 inhibitor EHop-016. Knockdown of Rac1 in TM-1 cells also inhibited phagocytosis by 40% whereas overexpression of a constitutively active Rac1 or stimulation with PDGF increased phagocytosis by 83% and 32% respectively. Tiam1 was involved in regulating phagocytosis. Knockdown of Tiam1 inhibited phagocytosis by 72% while overexpression of Tiam1 C1199 increased phagocytosis by 75%. Other upstream effectors of Rac1 found to be involved included ELMO2, RhoG, and ILK. Knockdowns of ELMO2, ILK, and RhoG caused a reduction in phagocytosis by 51%, 55% and 46% respectively. In contrast, knockdown of Vav2 and Dock1 or overexpression of Vav2 Y159/172F did not cause a significant change in phagocytosis. These data suggest a novel link between Tiam1 and RhoG/ILK /ELMO2 pathway as upstream effectors of the Rac1-mediated phagocytic process in TM cells.

Vav3 is linked to poor prognosis of pancreatic cancers and promotes the motility and invasiveness of pancreatic cancer cells

BACKGROUND/OBJECTIVES

The aim of this study was to investigate the role of the guanine nucleotide exchange factor Vav3 in the motility and invasiveness of pancreatic ductal adenocarcinoma (PDAC) cells.

METHODS

Immunohistochemistry was used to determine whether high Vav3 expression in human PDAC tissues is correlated with poor prognosis. Immunocytochemistry was used to determine the association and intracellular distribution of Vav3, Rac1 and Akt in PDAC cells. Phosphoprotein array analysis was performed to determine the Vav3-associated intracellular signaling pathways. Immunocytochemistry and Matrigel invasion assays were used to examine the effects of Vav3 on the formation of cell protrusions and PDAC cell invasion.

RESULTS

Expression of Vav3 in PDAC tissue was significantly correlated with overall survival. Vav3 was localized in cell protrusions of migrating PDAC cells. Knockdown of Vav3 inhibited the motility and invasiveness of PDAC cells through a decrease in cell protrusions. The levels of active Rac1 or active Akt were not associated with the concentration of Vav3 in cell protrusions. The Vav3-dependent promotion of motility and invasiveness was not modulated by Rac1 or Akt. Additionally, knockdown of Vav3 increased phosphorylated WNK1 in PDAC cells, and knockdown of WNK1 inhibited the motility and invasiveness. This study suggests that Vav3 can be a useful marker for predicting the outcome of patients with PDAC and that Vav3 can promote PDAC cell motility and invasion through association with dephosphorylation of WNK1.

CONCLUSIONS

Vav3 was accumulated in cell protrusions, contributed to the formation of membrane protrusions, and thereby increased the motility and invasiveness of PDAC cells.

Differential recognition of syk-binding sites by each of the two phosphotyrosine-binding pockets of the Vav SH2 domain

The association of spleen tyrosine kinase (Syk), a central tyrosine kinase in B cell signaling, with Vav SH2 domain is controlled by phosphorylation of two closely spaced tyrosines in Syk linker B: Y342 and Y346. Previous studies established both singly phosphorylated and doubly phosphorylated forms play a role in signaling. The structure of the doubly phosphorylated form identified a new recognition of phosphotyrosine whereby two phosphotyrosines bind simultaneously to the Vav SH2 domain, one in the canonical pTyr pocket and one in the specificity pocket on the opposite side of the central β-sheet. It is unknown if the specificity pocket can bind phosphotyrosine independent of phosphotyrosine binding the pTyr pocket. To address this gap in knowledge, we determined the structure of the complex between Vav1 SH2 and a peptide (SykLB-YpY) modeling the singly phosphorylated-Y346 form of Syk with unphosphorylated Y342. The nuclear magnetic resonance (NMR) data conclusively establish that recognition of phosphotyrosine is swapped between the two pockets; phosphorylated pY346 binds the specificity pocket of Vav1 SH2, and unphosphorylated Y342 occupies what is normally the pTyr binding pocket. Nearly identical changes in chemical shifts occurred upon binding all three forms of singly and doubly phosphorylated peptides; however, somewhat smaller shift perturbations for SykLB-YpY from residues in regions of high internal mobility suggest that internal motions are coupled to binding affinity. The differential recognition that includes this swapped binding of phosphotyrosine to the specificity pocket of Vav SH2 increases the repertoire of possible phosphotyrosine binding by SH2 domains in regulating protein-protein interactions in cellular signaling.

Mutation of Vav1 adaptor region reveals a new oncogenic activation

Vav family members function as remarkable scaffold proteins that exhibit both GDP/GTP exchange activity for Rho/Rac GTPases and numerous protein-protein interactions via three adaptor Src-homology domains. The exchange activity is under the unique regulation by phosphorylation of tyrosine residues hidden by intra-molecular interactions. Deletion of the autoinhibitory N-terminal region results in an oncogenic protein, onco-Vav, leading to a potent activation of Rac GTPases whereas the proto-oncogene barely leads to transformation. Substitution of conserved residues of the SH2-SH3 adaptor region in onco-Vav reverses oncogenicity. While a unique substitution D797N did not affect transformation induced by onco-Vav, we demonstrate that this single substitution leads to transformation in the Vav1 proto-oncogene highlighting the pivotal role of the adaptor region. Moreover, we identified the cell junction protein β-catenin as a new Vav1 interacting partner. We show that the oncogenicity of activated Vav1 proto-oncogene is associated with a non-degradative phosphorylation of β-catenin at residues important for its functions and its redistribution along the cell membrane in fibroblasts. In addition, a similar interaction is evidenced in epithelial lung cancer cells expressing ectopically Vav1. In these cells, Vav1 is also involved in the modulation of β-catenin phosphorylation. Altogether, our data highlight that only a single mutation in the proto-oncogene Vav1 enhances tumorigenicity.

Vav family exchange factors: an integrated regulatory and functional view

The Vav family is a group of tyrosine phosphorylation-regulated signal transduction molecules hierarchically located downstream of protein tyrosine kinases. The main function of these proteins is to work as guanosine nucleotide exchange factors (GEFs) for members of the Rho GTPase family. In addition, they can exhibit a variety of catalysis-independent roles in specific signaling contexts. Vav proteins play essential signaling roles for both the development and/or effector functions of a large variety of cell lineages, including those belonging to the immune, nervous, and cardiovascular systems. They also contribute to pathological states such as cancer, immune-related dysfunctions, and atherosclerosis. Here, I will provide an integrated view about the evolution, regulation, and effector properties of these signaling molecules. In addition, I will discuss the pros and cons for their potential consideration as therapeutic targets.

SH2-PLA: a sensitive in-solution approach for quantification of modular domain binding by proximity ligation and real-time PCR

BACKGROUND

There is a great interest in studying phosphotyrosine dependent protein-protein interactions in tyrosine kinase pathways that play a critical role in many aspects of cellular function. We previously established SH2 profiling, a phosphoproteomic approach based on membrane binding assays that utilizes purified Src Homology 2 (SH2) domains as a molecular tool to profile the global tyrosine phosphorylation state of cells. However, in order to use this method to investigate SH2 binding sites on a specific target in cell lysate, additional procedures such as pull-down or immunoprecipitation which consume large amounts of sample are required.

RESULTS

We have developed PLA-SH2, an alternative in-solution modular domain binding assay that takes advantage of Proximity Ligation Assay and real-time PCR. The SH2-PLA assay utilizes oligonucleotide-conjugated anti-GST and anti-EGFR antibodies recognizing a GST-SH2 probe and cellular EGFR, respectively. If the GST-SH2 and EGFR are in close proximity as a result of SH2-phosphotyrosine interactions, the two oligonucleotides are brought within a suitable distance for ligation to occur, allowing for efficient complex amplification via real-time PCR. The assay detected signal across at least 3 orders of magnitude of lysate input with a linear range spanning 1-2 orders and a low femtomole limit of detection for EGFR phosphotyrosine. SH2 binding kinetics determined by PLA-SH2 showed good agreement with established far-Western analyses for A431 and Cos1 cells stimulated with EGF at various times and doses. Further, we showed that PLA-SH2 can survey lung cancer tissues using 1 μl lysate without requiring phospho-enrichment.

CONCLUSIONS

We showed for the first time that interactions between SH2 domain probes and EGFR in cell lysate can be determined in a microliter-scale assay using SH2-PLA. The obvious benefit of this method is that the low sample requirement allows detection of SH2 binding in samples which are difficult to analyze using traditional protein interaction assays. This feature along with short assay runtime makes this method a useful platform for the development of high throughput assays to determine modular domain-ligand interactions which could have wide-ranging applications in both basic and translational cancer research.

The guanine nucleotide exchange factor Vav3 regulates differentiation of progenitor cells in the developing mouse retina

The seven main cell types in the mammalian retina arise from multipotent retinal progenitor cells, a process that is tightly regulated by intrinsic and extrinsic signals. However, the molecular mechanisms that control proliferation, differentiation and cell-fate decisions of retinal progenitor cells are not fully understood yet. Here, we report that the guanine nucleotide exchange factor Vav3, a regulator of Rho-GTPases, is involved in retinal development. We demonstrate that Vav3 is expressed in the mouse retina during the embryonic period. In order to study the role of Vav3 in the developing retina, we generate Vav3-deficient mice. The loss of Vav3 results in an accelerated differentiation of retinal ganglion cells and cone photoreceptors during early and late embryonic development. We provide evidence that more retinal progenitor cells express the late progenitor marker Sox9 in Vav3-deficient mice than in wild-types. This premature differentiation is compensated during the postnatal period and late-born cell types such as bipolar cells and Müller glia display normal numbers. Taken together, our data imply that Vav3 is a regulator of retinal progenitor cell differentiation, thus highlighting a novel role for guanine nucleotide exchange factors in retinogenesis.

Vav3, a GEF for RhoA, Plays a Critical Role under High Glucose Conditions

BACKGROUND

The role of small GTPase molecules is poorly understood under high glucose conditions.

METHODS

We analyzed the expression pattern of Vav3 in skeletal muscle C2C12 cells under high glucose culture condition with reverse transcription-polymerase chain reaction and Western blot analysis. We also measured glucose uptake using isotope-labelled glucose.

RESULTS

We showed that expression of Vav3 (a guanine nucleotide exchange factor for RhoA) increased. mRNA and protein levels in skeletal muscle C2C12 cells under high glucose conditions. The AMP-activated protein kinase (AMPK) activator AMPK agonist 5-aminoimidazole-4-carboxy-amide-1-d-ribofuranoside (AICAR) suppressed high glucose-induced Vav3 induction. In addition, exposure of cells to high glucose concentration increased the phosphorylation of PAK-1, a molecule downstream of RhoA. The phosphorylation of paxillin, a downstream molecule of PAK-1, was also increased by exposure to high glucose. Phosphorylation of these molecules was not observed in the presence of AICAR, indicating that AMPK is involved in the RhoA signal pathway under high glucose conditions. Knock down of Vav3 enhances metformin-mediated glucose uptake. Inhibition of AMPK blocked the increases of Vav3 knock down-induced glucose uptake. Metformin-mediated Glut4 translocation was also increased by Vav3 knock-down, suggesting that Vav3 is involved in metformin-mediated glucose uptake.

CONCLUSION

These results demonstrate that Vav3 is involved in the process of metformin-mediated glucose regulation.

Rho protein GTPases and their interactions with NFκB: crossroads of inflammation and matrix biology

The RhoGTPases, with RhoA, Cdc42 and Rac being major members, are a group of key ubiquitous proteins present in all eukaryotic organisms that subserve such important functions as cell migration, adhesion and differentiation. The NFκB (nuclear factor κB) is a family of constitutive and inducible transcription factors that through their diverse target genes, play a major role in processes such as cytokine expression, stress regulation, cell division and transformation. Research over the past decade has uncovered new molecular links between the RhoGTPases and the NFκB pathway, with the RhoGTPases playing a positive or negative regulatory role on NFκB activation depending on the context. The RhoA–NFκB interaction has been shown to be important in cytokine-activated NFκB processes, such as those induced by TNFα (tumour necrosis factor α). On the other hand, Rac is important for activating the NFκB response downstream of integrin activation, such as after phagocytosis. Specific residues of Rac1 are important for triggering NFκB activation, and mutations do obliterate this response. Other upstream triggers of the RhoGTPase–NFκB interactions include the suppressive p120 catenin, with implications for skin inflammation. The networks described here are not only important areas for further research, but are also significant for discovery of targets for translational medicine.

Tracking the elusive fibrocyte: identification and characterization of collagen-producing hematopoietic lineage cells during murine wound healing

Fibrocytes are a unique population of circulating cells reported to exhibit characteristics of both hematopoietic and mesenchymal cells, and play an important role in wound healing. However, putative fibrocytes have been found to lose expression of hematopoietic surface markers such as CD45 during differentiation, making it difficult to track these cells in vivo with conventional methodologies. In this study, to distinguish hematopoietic and nonhematopoietic cells without surface markers, we took advantage of the gene vav 1, which is expressed solely on hematopoietic cells but not on other cell types, and established a novel transgenic mouse, in which hematopoietic cells are irreversibly labeled with green fluorescent protein and nonhematopoietic cells with red fluorescent protein. Use of single-cell transcriptional analysis in this mouse model revealed two discrete types of collagen I (Col I) expressing cells of hematopoietic lineage recruited into excisional skin wounds. We confirmed this finding on a protein level, with one subset of these Col I synthesizing cells being CD45+ and CD11b+, consistent with the traditional definition of a fibrocyte, while another was CD45- and Cd11b-, representing a previously unidentified population. Both cell types were found to initially peak, then reduce posthealing, consistent with a disappearance from the wound site and not a loss of identifying surface marker expression. Taken together, we have unambiguously identified two cells of hematopoietic origin that are recruited to the wound site and deposit collagen, definitively confirming the existence and natural time course of fibrocytes in cutaneous healing.

The autoimmune-associated genetic variant PTPN22 R620W enhances neutrophil activation and function in patients with rheumatoid arthritis and healthy individuals

OBJECTIVES

A genetic variant of the leukocyte phosphatase PTPN22 (R620W) is strongly associated with autoimmune diseases including rheumatoid arthritis (RA). Functional studies on the variant have focussed on lymphocytes, but it is most highly expressed in neutrophils. We have investigated the effects of the variant on neutrophil function in health and in patients with RA.

METHODS

Healthy individuals and patients with RA were genotyped for PTPN22 (R620W) and neutrophils isolated from peripheral blood. Neutrophil adhesion and migration across inflamed endothelium were measured. Calcium (Ca(2+)) release and reactive oxygen species (ROS) production in response to fMLP stimulation were also assessed.

RESULTS

Expression of R620W enhanced neutrophil migration through cytokine activated endothelium (non-R620W=24%, R620W=45% migrating cells, p<0.001). Following fMLP stimulation, neutrophils that were heterozygous and homozygous for R620W released significantly more Ca(2+) when compared to non-R620W neutrophils, in healthy individuals and patients with RA. fMLP stimulation, after TNF-α priming, provoked more ROS from neutrophils heterozygous for R620W in patients with RA (non-R620W vs R620W=∼1.75-fold increase) and healthy individuals (non-R620W vs R620W=fourfold increase) and this increase was statistically significant in healthy individuals (p<0.001) but not in patients with RA (p<0.25).

CONCLUSIONS

Expression of PTPN22 (R620W) enhanced neutrophil effector functions in health and RA, with migration, Ca(2+) release and production of ROS increased. Neutrophils are found in large numbers in the RA joint, and this hyperactivity of R620W cells may directly contribute to the joint damage, as well as to the initiation and perpetuation of the chronic immune-mediated inflammatory processes driving the disease.

The GEF Vav regulates guided cell migration by coupling guidance receptor signalling to local Rac activation

Guided cell migration is a key mechanism for cell positioning in morphogenesis. The current model suggests that the spatially controlled activation of receptor tyrosine kinases (RTKs) by guidance cues would limit Rac activity at the leading edge, which is critical for establishing and maintaining polarized cell protrusions at the front. However, little is known about the mechanisms by which RTKs control the local activation of Rac. Here, using a multidisciplinary approach, we identify the GTP exchange factor (GEF) vav as a key regulator of Rac activity downstream of RTKs in a developmentally regulated cell migration event, that of the Drosophila border cells (BCs). We show that elimination of vav impairs BC migration. Live imaging analysis reveals that vav is required for the stabilization and maintenance of protrusions at the front of the BC cluster. In addition, activation of the PDGF/VEGF-related receptor (PVR) by its ligand the PDGF/PVF1 factor brings about Vav activation by direct interaction with the intracellular domain of PVR. Finally, FRET analyses demonstrate that Vav is required in BCs for the asymmetric distribution of Rac activity at the front. Our results unravel an important role for the Vav proteins as signal transducers that couple signalling downstream of RTKs with local Rac activation during morphogenetic movements.

The N-terminal 20-amino acid region of guanine nucleotide exchange factor Vav1 plays a distinguished role in T cell receptor-mediated calcium signaling

Vav1 is a guanine nucleotide exchange factor (GEF) specifically expressed in hematopoietic cells. It consists of multiple structural domains and plays important roles in T cell activation. The other highly conserved isoforms of Vav family, Vav2 and Vav3, are ubiquitously expressed in human tissues including lymphocytes. All three Vav proteins activate Rho family small GTPases, which are involved in a variety of biological processes during T cell activation. Intensive studies have demonstrated that Vav1 is indispensable for T cell receptor (TCR)-mediated signal transduction, whereas Vav2 and Vav3 function as GEFs that overlap with Vav1 on TCR-induced cytoskeleton reorganization. T cells lacking Vav1 exhibited severe defect in TCR-mediated calcium elevation, indicating that the co-existing Vav2 and Vav3 did not compensate Vav1 in calcium signaling. What is the functional particularity of Vav1 in lymphocytes? In this study, we identified the N-terminal 20 amino acids of Vav1 in the calponin homology (CH) domain to be essential for its interaction with calmodulin (CaM) that leads to TCR-induced calcium mobilization. Substitution of the 1-20 amino acids of Vav1 with those of Vav2 or Vav3 abolished the association with CaM, and the N-terminal mutations of Vav1 failed to potentiate normal TCR-induced calcium mobilization, that in turn, suspended nuclear factor of activated T cells (NFAT) activation and IL-2 production. This study highlights the importance of the N-terminal 20 aa of Vav1 for CaM binding, and provides new insights into the distinguished and irreplaceable role of Vav1 in T cell activation and signal transduction.

Modulators of sensitivity and resistance to inhibition of PI3K identified in a pharmacogenomic screen of the NCI-60 human tumor cell line collection

The phosphoinositide 3-kinase (PI3K) signaling pathway is significantly altered in a wide variety of human cancers, driving cancer cell growth and survival. Consequently, a large number of PI3K inhibitors are now in clinical development. To begin to improve the selection of patients for treatment with PI3K inhibitors and to identify de novo determinants of patient response, we sought to identify and characterize candidate genomic and phosphoproteomic biomarkers predictive of response to the selective PI3K inhibitor, GDC-0941, using the NCI-60 human tumor cell line collection. In this study, sixty diverse tumor cell lines were exposed to GDC-0941 and classified by GI(50) value as sensitive or resistant. The most sensitive and resistant cell lines were analyzed for their baseline levels of gene expression and phosphorylation of key signaling nodes. Phosphorylation or activation status of both the PI3K-Akt signaling axis and PARP were correlated with in vitro response to GDC-0941. A gene expression signature associated with in vitro sensitivity to GDC-0941 was also identified. Furthermore, in vitro siRNA-mediated silencing of two genes in this signature, OGT and DDN, validated their role in modulating sensitivity to GDC-0941 in numerous cell lines and begins to provide biological insights into their role as chemosensitizers. These candidate biomarkers will offer useful tools to begin a more thorough understanding of determinants of patient response to PI3K inhibitors and merit exploration in human cancer patients treated with PI3K inhibitors.

SRC Homology 2 Domain Binding Sites in Insulin, IGF-1 and FGF receptor mediated signaling networks reveal an extensive potential interactome

Specific peptide ligand recognition by modular interaction domains is essential for the fidelity of information flow through the signal transduction networks that control cell behavior in response to extrinsic and intrinsic stimuli. Src homology 2 (SH2) domains recognize distinct phosphotyrosine peptide motifs, but the specific sites that are phosphorylated and the complement of available SH2 domains varies considerably in individual cell types. Such differences are the basis for a wide range of available protein interaction microstates from which signaling can evolve in highly divergent ways. This underlying complexity suggests the need to broadly map the signaling potential of systems as a prerequisite for understanding signaling in specific cell types as well as various pathologies that involve signal transduction such as cancer, developmental defects and metabolic disorders. This report describes interactions between SH2 domains and potential binding partners that comprise initial signaling downstream of activated fibroblast growth factor (FGF), insulin (Ins), and insulin-like growth factor-1 (IGF-1) receptors. A panel of 50 SH2 domains screened against a set of 192 phosphotyrosine peptides defines an extensive potential interactome while demonstrating the selectivity of individual SH2 domains. The interactions described confirm virtually all previously reported associations while describing a large set of potential novel interactions that imply additional complexity in the signaling networks initiated from activated receptors. This study of pTyr ligand binding by SH2 domains provides valuable insight into the selectivity that underpins complex signaling networks that are assembled using modular protein interaction domains.

The actin regulatory protein HS1 interacts with Arp2/3 and mediates efficient neutrophil chemotaxis

HS1 is an actin regulatory protein and cortactin homolog that is expressed in hematopoietic cells. Antigen receptor stimulation induces HS1 phosphorylation, and HS1 is essential for T cell activation. HS1 is also expressed in neutrophils; however, the function of HS1 in neutrophils is not known. Here we show that HS1 localizes to the neutrophil leading edge, and is phosphorylated in response to the chemoattractant formyl-Met-Leu-Phe (fMLP) in adherent cells. Using live imaging in microchannels, we show that depletion of endogenous HS1 in the neutrophil-like PLB-985 cell line impairs chemotaxis. We also find that HS1 is necessary for chemoattractant-induced activation of Rac GTPase signaling and Vav1 phosphorylation, suggesting that HS1-mediated Rac activation is necessary for efficient neutrophil chemotaxis. We identify specific phosphorylation sites that mediate HS1-dependent neutrophil motility. Expression of HS1 Y378F, Y397F is sufficient to rescue migration of HS1-deficient neutrophils, however, a triple phospho-mutant Y222F, Y378F, Y397F did not rescue migration of HS1-deficient neutrophils. Moreover, HS1 phosphorylation on Y222, Y378, and Y397 regulates its interaction with Arp2/3. Collectively, our findings identify a novel role for HS1 and its phosphorylation during neutrophil directed migration.

Vav3-rac1 signaling regulates prostate cancer metastasis with elevated Vav3 expression correlating with prostate cancer progression and posttreatment recurrence

Prostate cancer remains the second leading cause of cancer death in men in the Western world. Yet current therapies do not significantly improve the long-term survival of patients with distant metastasis. In this study, we investigated the role of the guanine nucleotide exchange factor Vav3 in prostate cancer progression and metastasis and found that Vav3 expression correlated positively with prostate cancer cell migration and invasion. Stimulation of the receptor tyrosine kinase EphA2 by ephrinA1 resulted in recruitment and tyrosine phosphorylation of Vav3, leading to Rac1 activation as well as increased migration and invasion in vitro. Reduction of Vav3 resulted in fewer para-aortic lymph nodes and bone metastasis in vivo. Clinically, expression of Vav3 and EphA2 was elevated in late-stage and metastatic prostate cancers. Among patients with stage IIB or earlier prostate cancer, higher Vav3 expression correlated with lower cumulative biochemical failure-free survival, suggesting that Vav3 may represent a prognostic marker for posttreatment recurrence of prostate cancer. Together, our findings provide evidence that the Vav3-mediated signaling pathway may serve as a therapeutic target for prostate cancer metastasis.

Inhibition and Termination of Physiological Responses by GTPase Activating Proteins

Physiological processes are strictly organized in space and time. However, in cell physiology research, more attention is given to the question of space rather than to time. To function as a signal, environmental changes must be restricted in time; they need not only be initiated but also terminated. In this review, we concentrate on the role of one specific protein family involved in biological signal termination. GTPase activating proteins (GAPs) accelerate the endogenously low GTP hydrolysis rate of monomeric guanine nucleotide-binding proteins (GNBPs), limiting thereby their prevalence in the active, GTP-bound form. We discuss cases where defective or excessive GAP activity of specific proteins causes significant alteration in the function of the nervous, endocrine, and hemopoietic systems, or contributes to development of infections and tumors. Biochemical and genetic data as well as observations from human pathology support the notion that GAPs represent vital elements in the spatiotemporal fine tuning of physiological processes.

RhoB links PDGF signaling to cell migration by coordinating activation and localization of Cdc42 and Rac

The small GTPase RhoB regulates endocytic trafficking of receptor tyrosine kinases (RTKs) and the non-receptor kinases Src and Akt. While receptor-mediated endocytosis is critical for signaling processes driving cell migration, mechanisms that coordinate endocytosis with the propagation of migratory signals remain relatively poorly understood. In this study, we show that RhoB is essential for activation and trafficking of the key migratory effectors Cdc42 and Rac in mediating the ability of platelet-derived growth factor (PDGF) to stimulate cell movement. Stimulation of the PDGF receptor-β on primary vascular smooth muscle cells (VSMCs) results in RhoB-dependent trafficking of endosome-bound Cdc42 from the perinuclear region to the cell periphery, where the RhoGEF Vav2 and Rac are also recruited to drive formation of circular dorsal and peripheral ruffles necessary for cell migration. Our findings identify a novel RhoB-dependent endosomal trafficking pathway that integrates RTK endocytosis with Cdc42/Rac localization and cell movement.

Two closely spaced tyrosines regulate NFAT signaling in B cells via Syk association with Vav

Activated Syk, an essential tyrosine kinase in B cell signaling, interacts with Vav guanine nucleotide exchange factors and regulates Vav activity through tyrosine phosphorylation. The Vav SH2 domain binds Syk linker B by an unusual recognition of two closely spaced Syk tyrosines: Y342 and Y346. The binding affinity is highest when both Y342 and Y346 are phosphorylated. An investigation in B cells of the dependence of Vav phosphorylation and NFAT activation on phosphorylation of Y342 and Y346 finds that cellular response levels match the relative binding affinities of the Vav1 SH2 domain for singly and doubly phosphorylated linker B peptides. This key result suggests that the uncommon recognition determinant of these two closely spaced tyrosines is a limiting factor in signaling. Interestingly, differences in affinities for binding singly and doubly phosphorylated peptides are reflected in the on rate, not the off rate. Such a control mechanism would be highly effective for regulating binding among competing Syk binding partners. The nuclear magnetic resonance (NMR) structure of Vav1 SH2 in complex with a doubly phosphorylated linker B peptide reveals diverse conformations associated with the unusual SH2 recognition of two phosphotyrosines. NMR relaxation indicates compensatory changes in loop fluctuations upon binding, with implications for nonphosphotyrosine interactions of Vav1 SH2.

Guanine nucleotide exchange factors for RhoGTPases: good therapeutic targets for cancer therapy?

Rho guanosine triphosphatases (GTPases) are a family of small proteins which function as molecular switches in a variety of signaling pathways following stimulation of cell surface receptors. RhoGTPases regulate numerous cellular processes including cytoskeleton organization, gene transcription, cell proliferation, migration, growth and cell survival. Because of their central role in regulating processes that are dysregulated in cancer, it seems reasonable that defects in the RhoGTPase pathway may be involved in the development of cancer. RhoGTPase activity is regulated by a number of protein families: guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs) and guanine nucleotide-dissociation inhibitors (GDIs). This review discusses the participation of RhoGTPases and their regulators, especially GEFs in human cancers. In particular, we focus on the involvement of the RhoGTPase GEF, Vav1, a hematopoietic specific signal transducer which is involved in human neuroblastoma, pancreatic ductal carcinoma and lung cancer. Finally, we summarize recent advances in the design and application of a number of molecules that specifically target individual RhoGTPases or their regulators or effectors, and discuss their potential for cancer therapy.

Bypass mechanisms of the androgen receptor pathway in therapy-resistant prostate cancer cell models

BACKGROUND

Prostate cancer is initially dependent on androgens for survival and growth, making hormonal therapy the cornerstone treatment for late-stage tumors. However, despite initial remission, the cancer will inevitably recur. The present study was designed to investigate how androgen-dependent prostate cancer cells eventually survive and resume growth under androgen-deprived and antiandrogen supplemented conditions. As model system, we used the androgen-responsive PC346C cell line and its therapy-resistant sublines: PC346DCC, PC346Flu1 and PC346Flu2.

METHODOLOGY/PRINCIPAL FINDINGS

Microarray technology was used to analyze differences in gene expression between the androgen-responsive and therapy-resistant PC346 cell lines. Microarray analysis revealed 487 transcripts differentially-expressed between the androgen-responsive and the therapy-resistant cell lines. Most of these genes were common to all three therapy-resistant sublines and only a minority (∼5%) was androgen-regulated. Pathway analysis revealed enrichment in functions involving cellular movement, cell growth and cell death, as well as association with cancer and reproductive system disease. PC346DCC expressed residual levels of androgen receptor (AR) and showed significant down-regulation of androgen-regulated genes (p-value = 10(-7)). Up-regulation of VAV3 and TWIST1 oncogenes and repression of the DKK3 tumor-suppressor was observed in PC346DCC, suggesting a potential AR bypass mechanism. Subsequent validation of these three genes in patient samples confirmed that expression was deregulated during prostate cancer progression.

CONCLUSIONS/SIGNIFICANCE

Therapy-resistant growth may result from adaptations in the AR pathway, but androgen-independence may also be achieved by alternative survival mechanisms. Here we identified TWIST1, VAV3 and DKK3 as potential players in the bypassing of the AR pathway, making them good candidates as biomarkers and novel therapeutical targets.

Vav3-deficient mice exhibit a transient delay in cerebellar development

Vav3 is a guanosine diphosphate/guanosine triphosphate exchange factor for Rho/Rac GTPases that has been involved in functions related to the hematopoietic system, bone formation, cardiovascular regulation, angiogenesis, and axon guidance. We report here that Vav3 is expressed at high levels in Purkinje and granule cells, suggesting additional roles for this protein in the cerebellum. Consistent with this hypothesis, we demonstrate using Vav3-deficient mice that this protein contributes to Purkinje cell dendritogenesis, the survival of granule cells of the internal granular layer, the timely migration of granule cells of the external granular layer, and to the formation of the cerebellar intercrural fissure. With the exception of the latter defect, the dysfunctions found in Vav3(-/-) mice only occur at well-defined postnatal developmental stages and disappear, or become ameliorated, in older animals. Vav2-deficient mice do not show any of those defects. Using primary neuronal cultures, we show that Vav3 is important for dendrite branching, but not for primary dendritogenesis, in Purkinje and granule cells. Vav3 function in the cerebellum is functionally relevant, because Vav3(-/-) mice show marked motor coordination and gaiting deficiencies in the postnatal period. These results indicate that Vav3 function contributes to the timely developmental progression of the cerebellum.

A Pyk2-Vav1 complex is recruited to beta3-adhesion sites to initiate Rho activation

Integrin alphavbeta3-mediated adhesion of haemopoietic cells to vitronectin results in beta3 tyrosine phosphorylation and Rho activation which is necessary for adhesion. Previously, we have shown that the RhoGEF (Rho guanine-nucleotide-exchange factor) Vav1 could associate indirectly with alphavbeta3 during leucocyte adhesion to vitronectin. In the present study, we have identified the non-receptor tyrosine kinase Pyk2 (proline-rich tyrosine kinase 2) as the adaptor protein that links Vav1 with alphavbeta3. The association of Pyk2 and Vav1 with beta3 relies on the presence of Tyr747 in beta3, the primary site of beta3 phosphorylation. However, association of Pyk2 with Vav1 is independent of beta3 tyrosine phosphorylation. Formation of a Pyk2-Vav1 complex occurs upon cell adhesion and Pro717 of Pyk2 plays a key role in Pyk2 interaction with Vav1. Utilizing purified recombinant proteins, we confirmed the direct interaction between Pyk2 and Vav1 In vitro. Cells transfected with GFP (green fluorescent protein)-Pyk2-P717A demonstrated severely suppressed cytoskeletal reorganization, impaired Vav1 recruitment, decreased Rho GTPase activation and loss of cell adhesion. Using siRNA (small interfering RNA) to specifically reduce Pyk2 levels in cells resulted in disrupted association between Vav1 and beta3 and impaired cell adhesion. These results indicate that Pyk2 is a critical signalling molecule downstream of beta3 integrin tyrosine phosphorylation and mediates Vav1 recruitment to accomplish actin reorganization necessary for adhesion.

uPAR promotes formation of the p130Cas-Crk complex to activate Rac through DOCK180

The urokinase-type plasminogen activator receptor (uPAR) drives tumor cell membrane protrusion and motility through activation of Rac; however, the pathway leading from uPAR to Rac activation has not been described. In this study we identify DOCK180 as the guanine nucleotide exchange factor acting downstream of uPAR. We show that uPAR cooperates with integrin complexes containing β₃ integrin to drive formation of the p130Cas–CrkII signaling complex and activation of Rac, resulting in a Rac-driven elongated-mesenchymal morphology, cell motility, and invasion. Our findings identify a signaling pathway underlying the morphological changes and increased cell motility associated with uPAR expression.

An induction gene trap screen in neural stem cells reveals an instructive function of the niche and identifies the splicing regulator sam68 as a tenascin-C-regulated target gene

Neural stem cells (NSCs) reside in a niche that abounds in extracellular matrix (ECM) molecules. The ECM glycoprotein tenascin-C (Tnc) that occurs in more than 25 isoforms represents a major constituent of the privileged NSC milieu. To understand its role for NSCs, the induction gene trap technology was successfully applied to mouse embryonic NSCs, and a library of more than 500 NSC lines with independent gene trap vector integrations was established. Our pilot screen identified Sam68 as a target of Tnc signaling in NSCs. The Tnc-mediated downregulation of Sam68, which we found expressed at low levels in the niche along with Tnc, was independently confirmed on the protein level. Sam68 is a multifunctional RNA-binding protein, and its potential significance for cultured NSCs was studied by overexpression. Increased Sam68 levels caused a marked reduction in NSC cell proliferation. In addition, Sam68 is a signal-dependent regulator of alternative splicing, and its overexpression selectively increased the larger Tnc isoforms, whereas a mutated phosphorylation-deficient Sam68 variant did not. This emphasizes the importance of Sam68 for NSC biology and implicates an instructive rather than a purely permissive role for Tnc in the neural stem cell niche.

Vav3 oncogene activates estrogen receptor and its overexpression may be involved in human breast cancer

BACKGROUND

Our previous study revealed that Vav3 oncogene is overexpressed in human prostate cancer, activates androgen receptor, and stimulates growth in prostate cancer cells. The current study is to determine a potential role of Vav3 oncogene in human breast cancer and impact on estrogen receptor a (ERalpha)-mediated signaling axis.

METHODS

Immunohistochemistry analysis was performed in 43 breast cancer specimens and western blot analysis was used for human breast cancer cell lines to determine the expression level of Vav3 protein. The impact of Vav3 on breast cancer cell growth was determined by siRNA knockdown of Vav3 expression. The role of Vav3 in ERalpha activation was examined in luciferase reporter assays. Deletion mutation analysis of Vav3 protein was performed to localize the functional domain involved in ERalpha activation. Finally, the interaction of Vav3 and ERalpha was assessed by GST pull-down analysis.

RESULTS

We found that Vav3 was overexpressed in 81% of human breast cancer specimens, particularly in poorly differentiated lesions. Vav3 activated ERalpha partially via PI3K-Akt signaling and stimulated growth of breast cancer cells. Vav3 also potentiated EGF activity for cell growth and ERalpha activation in breast cancer cells. More interestingly, we found that Vav3 complexed with ERalpha. Consistent with its function for AR, the DH domain of Vav3 was essential for ERalpha activation.

CONCLUSION

Vav3 oncogene is overexpressed in human breast cancer. Vav3 complexes with ERalpha and enhances ERalpha activity. These findings suggest that Vav3 overexpression may aberrantly enhance ERalpha-mediated signaling axis and play a role in breast cancer development and/or progression.

Collagen phagocytosis is regulated by the guanine nucleotide exchange factor Vav2

Collagen phagocytosis is a crucial alpha2beta1-integrin-dependent process that mediates extracellular matrix remodeling by fibroblasts. We showed previously that after initial contact with collagen, activated Rac1 accelerates collagen phagocytosis but the Rac guanine nucleotide exchange factors (GEFs) that regulate Rac are not defined. We examined here the GEFs that regulate collagen phagocytosis in mouse fibroblasts. Collagen binding enhanced Rac1 activity (5-20 min) but not Cdc42 or RhoA activity. Analysis of collagen bead-associated proteins showed enrichment with Vav2, which correlated temporally with increased Rac1 activity. Knockdown of Vav2 prevented Rac activation, recruitment of Rac1 to collagen bead binding sites, and collagen bead binding, but knockdown of Sos-1 or beta-Pix had no effect on Rac activation or collagen binding. Vav2 was associated with the nucleotide-free Rac1 mutant (G15ARac1) after collagen binding. Collagen bead binding promoted phosphorylation of Vav2, which temporally correlated with Rac1 activation and which required Src kinase activity. Blockage of Src activity prevented collagen bead-induced Rac activation and collagen bead binding. Collectively these data indicate that Vav2 regulates the Rac1 activity associated with the binding step of collagen phagocytosis.

RNA interference screen identifies Abl kinase and PDGFR signaling in Chlamydia trachomatis entry

To elucidate the mechanisms involved in early events in Chlamydia trachomatis infection, we conducted a large scale unbiased RNA interference screen in Drosophila melanogaster S2 cells. This allowed identification of candidate host factors in a simple non-redundant, genetically tractable system. From a library of 7,216 double stranded RNAs (dsRNA), we identified ∼226 host genes, including two tyrosine kinases, Abelson (Abl) kinase and PDGF- and VEGF-receptor related (Pvr), a homolog of the Platelet-derived growth factor receptor (PDGFR). We further examined the role of these two kinases in C. trachomatis binding and internalization into mammalian cells. Both kinases are phosphorylated upon infection and recruited to the site of bacterial attachment, but their roles in the infectious process are distinct. We provide evidence that PDGFRβ may function as a receptor, as inhibition of PDGFRβ by RNA interference or by PDGFRβ neutralizing antibodies significantly reduces bacterial binding, whereas depletion of Abl kinase has no effect on binding. Bacterial internalization can occur through activation of PDGFRβ or through independent activation of Abl kinase, culminating in phosphorylation of the Rac guanine nucleotide exchange factor (GEF), Vav2, and two actin nucleators, WAVE2 and Cortactin. Finally, we show that TARP, a bacterial type III secreted actin nucleator implicated in entry, is a target of Abl kinase. Together, our results demonstrate that PDGFRβ and Abl kinases function redundantly to promote efficient uptake of this obligate intracellular parasite.

Chlamydia trachomatis infections are a worldwide problem; they are the leading cause of preventable blindness in developing nations and the most common cause of sexually transmitted disease in the Western world. Binding and entry into host cells are critical steps to the pathogenesis of this obligate intracellular parasite; however little is known regarding the mechanism of these processes. In this work, we describe a large scale RNA interference screen to identify host factors essential for early steps in C. trachomatis infection. We discover that the Platelet Derived Growth Factor Receptor β (PDGFRβ) can function as a receptor for C. trachomatis, and that activation of both PDGFRβ and Abl kinase signaling pathways by C. trachomatis leads to phosphorylation of a Rac guanine nucleotide exchange factor, Vav2, and several actin nucleators, including WAVE2, Cortactin, and TARP, a Chlamydia type III secreted effector. Our work suggests a model of redundant activation of PDGFRβ and Abl kinase upon C. trachomatis binding that culminates in cytoskeletal rearrangements that modulate efficient uptake of this obligate intracellular parasite.

Gene expression profiling of the rat endometriosis model

PROBLEM

To investigate the molecular mechanism of endometriosis, gene expression profiling was analyzed in a rat endometriosis model.

METHOD OF STUDY

An endometriosis model was induced by uterine autotransplantation in the peritoneal cavity on a female-SD rat (8 weeks old). As control samples, the normal uterine tissues were used. The gene expression was compared between endometriotic lesions and normal uterine tissues by cDNA microarray analysis, quantitative real time RT-PCR and immunohistochemistry.

RESULTS

The expression of 71 genes was upregulated and that of 45 genes was downregulated in the endometriotic lesions compared to normal uterine tissues. The upregulated genes included genes encoding cytokines, chemokines, growth factors and cell adhesion molecules. The levels of transcripts of osteopontin, Lyn, Vav1, Runx1, and l-selectin in the endometriotic lesions were 130, 10, 10, 12 and 46-fold higher than the respective levels in the eutopic endometrial samples.

CONCLUSION

The results suggest that osteopontin, Lyn, Vav1, Runx1, and l-selectin play important roles in the pathogenesis of endometriosis.