Regulatory and signaling properties of the Vav family

Src kinase regulates the activation of a novel FGD-1-related Cdc42 guanine nucleotide exchange factor in the signaling pathway from the endothelin A receptor to JNK

Small GTPases act as binary switches by cycling between an inactive (GDP-bound) and an active (GTP-bound) state. Upon stimulation with extracellular signals, guanine-nucleotide exchange factors (GEFs) stimulate the exchange of GDP to GTP to shift toward the active forms of small GTPases, recognizing the downstream targets. Here we show that KIAA0793, containing substantial sequence homology with the catalytic Dbl homology domain of the faciogenital dysplasia gene product (FGD1), is a specific GEF for Cdc42. We, therefore, tentatively named it FRG (FGD1-related Cdc42-GEF). Src kinase directly phosphorylates and activates FRG, as Vav family GEFs. Additionally, FRG is involved in the signaling pathway from the endothelin A receptor to c-Jun N-terminal kinase, resulting in the inhibition of cell motility. These results suggest that FRG is a member of Cdc42-GEF and plays an important role in the signaling pathway downstream of G protein-coupled receptors.

Vav1 phosphorylation is induced by beta2 integrin engagement on natural killer cells upstream of actin cytoskeleton and lipid raft reorganization

The guanine nucleotide exchange factor Vav1 regulates actin polymerization and contributes to cytotoxicity by natural killer (NK) cells. An open question is how Vav1 becomes activated and what receptor can signal upstream of actin cytoskeleton rearrangement upon NK cell contact with target cells. Using transfected insect cells that express ligands of human NK cell receptors, we show that engagement of the beta2 integrin LFA-1 on NK cells by intercellular adhesion molecule (ICAM)-1 led to a tyrosine phosphorylation of Vav1 that was not sensitive to cholesterol depletion and to inhibition of actin polymerization. Vav1 phosphorylation was blocked by an inhibitor of Src-family kinases, and correlated with activation of its downstream effector PAK. Binding of activation receptor 2B4 to its ligand CD48 was not sufficient for Vav1 phosphorylation. However, coengagement of 2B4 with LFA-1 resulted in an enhancement of Vav1 phosphorylation that was sensitive to cholesterol depletion and to inhibition of actin polymerization. Vav1 was recruited to a detergent-resistant membrane (DRM) fraction only when 2B4 and LFA-1 were coengaged, but not after LFA-1 engagement. Therefore, binding of LFA-1 to ICAM-1 on target cells may initiate an early signaling cascade in NK cells through activation of Vav1, leading to cytoskeleton reorganization and amplification of signals from other activation receptors.

SLP-76 coordinates Nck-dependent Wiskott-Aldrich syndrome protein recruitment with Vav-1/Cdc42-dependent Wiskott-Aldrich syndrome protein activation at the T cell-APC contact site

We have shown previously that Wiskott-Aldrich syndrome protein (WASP) activation at the site of T cell-APC interaction is a two-step process, with recruitment dependent on the proline-rich domain and activation dependent on binding of Cdc42-GTP to the GTPase binding domain. Here, we show that WASP recruitment occurs through binding to the C-terminal Src homology 3 domain of Nck. In contrast, WASP activation requires Vav-1. In Vav-1-deficient T cells, WASP recruitment proceeds normally, but localized activation of Cdc42 and WASP is disrupted. The recruitment and activation of WASP are coordinated by tyrosine-phosphorylated Src homology 2 domain-containing leukocyte protein of 76 kDa, which functions as a scaffold, bringing Nck and WASP into proximity with Vav-1 and Cdc42-GTP. Taken together, these findings reconstruct the signaling pathway leading from TCR ligation to localized WASP activation.

Evolution of key cell signaling and adhesion protein families predates animal origins

The evolution of animals from a unicellular ancestor involved many innovations. Choanoflagellates, unicellular and colonial protozoa closely related to Metazoa, provide a potential window into early animal evolution. We have found that choanoflagellates express representatives of a surprising number of cell signaling and adhesion protein families that have not previously been isolated from nonmetazoans, including cadherins, C-type lectins, several tyrosine kinases, and tyrosine kinase signaling pathway components. Choanoflagellates have a complex and dynamic tyrosine phosphoprotein profile, and cell proliferation is selectively affected by tyrosine kinase inhibitors. The expression in choanoflagellates of proteins involved in cell interactions in Metazoa demonstrates that these proteins evolved before the origin of animals and were later co-opted for development.

Vav mediates Ras stimulation by direct activation of the GDP/GTP exchange factor Ras GRP1

Here we describe a new signaling cross-talk between the Vav/Rac1 and Ras pathways that is established through the stimulation of RasGRP1, an exchange factor for Ras subfamily GTPases. This interaction is crucial for Ras activation in lymphoid cells, since this GTPase cannot become activated in the absence of Vav proteins. The activation of RasGRP1 requires both the generation of diacylglycerol via phospho lipase C-gamma and the induction of actin polymerization, two responses induced by Vav and Rac1 that facilitate the translocation of RasGRP1 to juxtamembrane areas of the cell. Consistent with this, the cross-talk can be activated by tyrosine-phosphorylated wild-type Vav, oncogenic Vav and constitutively active Rac1. Conversely, Ras activation can be blocked in lymphocytes and ectopic systems using inhibitors affecting either phospholipase C-gamma or F-actin polymerization. These results indicate that a relay mechanism exists in lymphoid and other cells helping in the generation of robust signaling responses by the Rac/Rho and Ras pathways upon receptor engagement.

Unifying concepts in CD28, ICOS and CTLA4 co-receptor signalling

Many studies have shown the central importance of the co-receptors CD28, inducible costimulatory molecule (ICOS) and cytotoxic T lymphocyte antigen 4 (CTLA4) in the regulation of many aspects of T-cell function. CD28 and ICOS have both overlapping and distinct functions in the positive regulation of T-cell responses, whereas CTLA4 negatively regulates the response. The signalling pathways that underlie the function of each of the co-receptors indicate their shared and unique properties and provide compelling hints of functions that are as yet uncovered. Here, we outline the shared and distinct signalling events that are associated with each of the co-receptors and provide unifying concepts that are related to signalling functions of these co-receptors.

Cbl-b negatively regulates B cell antigen receptor signaling in mature B cells through ubiquitination of the tyrosine kinase Syk

Members of the Cbl family of molecular adaptors play key roles in regulating tyrosine kinase-dependent signaling in a variety of cellular systems. Here we provide evidence that in B cells Cbl-b functions as a negative regulator of B cell antigen receptor (BCR) signaling during the normal course of a response. In B cells from Cbl-b-deficient mice cross-linking the BCRs resulted in sustained phosphorylation of Igalpha, Syk, and phospholipase C (PLC)-gamma2, leading to prolonged Ca2+ mobilization, and increases in extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal protein kinase (JNK) phosphorylation and surface expression of the activation marker, CD69. Image analysis following BCR cross-linking showed sustained polarization of the BCRs into large signaling-active caps associated with phosphorylated Syk in Cbl-b-deficient B cells in contrast to the BCRs in Cbl-b-expressing B cells that rapidly proceeded to form small, condensed, signaling inactive caps. Significantly, prolonged phosphorylation of Syk correlated with reduced ubiquitination of Syk indicating that Cbl-b negatively regulates BCR signaling by targeting Syk for ubiquitination.

Regulation of Vav localization in membrane rafts by adaptor molecules Grb2 and BLNK

Despite the importance of the Vav family proteins for B cell receptor (BCR) signaling, their activation mechanisms remain poorly understood. We demonstrate here that adaptor molecules Grb2 and BLNK, in addition to Vav, are required for efficient Rac1 activation in response to BCR stimulation. Loss of either Grb2 or BLNK results in decreased translocation of Vav3 to membrane rafts. By expression of Vav3 as a raft-targeted construct, the defective Rac1 activation in Grb2- or BLNK-deficient B cells is restored. Hence, our findings suggest that Grb2 and BLNK cooperate to localize Vav into membrane rafts, thereby contributing to optimal activation of Vav in B cells.

Contingent phosphorylation/dephosphorylation provides a mechanism of molecular memory in WASP

Cells can retain information about previous stimuli to produce distinct future responses. The biochemical mechanisms by which this is achieved are not well understood. The Wiskott-Aldrich syndrome protein (WASP) is an effector of the Rho-family GTPase Cdc42, whose activation leads to stimulation of the actin nucleating assembly, Arp2/3 complex. We demonstrate that efficient phosphorylation and dephosphorylation of WASP at Y291 are both contingent on binding to activated Cdc42. Y291 phosphorylation increases the basal activity of WASP toward Arp2/3 complex and enables WASP activation by new stimuli, SH2 domains of Src-family kinases. The requirement for contingency in both phosphorylation and dephosphorylation enables long-term storage of information by WASP following decay of GTPase signals. This biochemical circuitry allows WASP to respond to the levels and timing of GTPase and kinase signals. It provides mechanisms to specifically achieve transient or persistent actin remodeling, as well as long-lasting potentiation of actin-based responses to kinases.

The p38 mitogen-activated protein kinase pathway and its role in interferon signaling

Interferons (IFNs) are pleiotropic cytokines that exhibit multiple biological effects on cells and tissues. IFN receptors are expressed widely in mammalian cells and virtually all different cell types express them on their surface. The Type I IFN receptor has a multichain structure, composed of at least two distinct receptor subunits, IFNalphaR1 and IFNalphaR2. Two Jak-kinases, Tyk-2 and Jak-1, associate with the different receptor subunits and are activated in response to IFNalpha or IFNbeta to regulate engagement of multiple downstream signaling cascades. These include the Stat-pathway, whose function is essential for transcriptional activation of IFN-sensitive genes, and the insulin receptor substrate pathway, which regulates downstream activation of the phosphatidyl-inositol-3' kinase. Members of the Map family of kinases are also activated by the Type I IFN receptor and participate in the generation of IFN signals. The p38 Map kinase pathway appears to play a very important role in the induction of IFN responses. p38 is rapidly activated during engagement of the Type I IFN receptor, and such an activation is regulated by the small G-protein Rac1, which functions as its upstream effector in a tyrosine kinase-dependent manner. The activated form of p38 regulates downstream activation of other serine kinases, notably MapKapK-2 and MapKapK-3, indicating the existence of Type I IFN-dependent signaling cascades activated downstream of p38. Extensive studies have shown that p38 plays a critical role in Type I IFN-dependent transcriptional regulation, without modifying activation of the Stat-pathway. It is now well established that the function of p38 is essential for gene transcription via ISRE or GAS elements, but has no effects on the phosphorylation of Stat-proteins, the formation of Stat-complexes, and their binding to the promoters of IFN-sensitive genes. As Type I IFNs regulate gene expression for proteins with antiviral properties, it is not surprising that pharmacological inhibition of the p38 pathway blocks induction of IFNalpha-antiviral responses. In addition, pharmacological inhibition of p38 abrogates the suppressive effects of Type I IFNs on normal human hematopoietic progenitors, indicating a critical role for this signaling cascade in the induction of the regulatory effects of Type I IFNs on hematopoiesis. p38 is also activated during IFNalpha-treatment of primary leukemia cells from patients with chronic myelogenous leukemia. Such activation is required for IFNalpha-dependent suppression of leukemic cell progenitor growth, indicating that this pathway plays a critical role in the induction of the antileukemic effects of IFNalpha.

Human mammary epithelial cell transformation through the activation of phosphatidylinositol 3-kinase

Recent studies have demonstrated that introduction of hTERT in combination with SV40 large T antigen (LT), small t antigen (st), and H-rasV12 suffices to transform many primary human cells. In human mammary epithelial cells (HMECs) expressing elevated c-Myc, activated H-Ras is dispensable for anchorage-independent growth. Using this system, we show that st activates the PI3K pathway and that constitutive PI3K signaling substitutes for st in transformation. Moreover, using constitutively active versions of Akt1 and Rac1, we show that these downstream pathways of PI3K synergize to achieve anchorage-independent growth. At lower levels of c-myc expression, activated PI3K also replaces st to complement H-rasV12 and LT and confers both soft agar growth and tumorigenicity. However, elevated c-myc expression cannot replace H-rasV12 for tumorigenesis. These observations begin to define the pathways perturbed during the transformation of HMECs.

AML1 interconnected pathways of leukemogenesis

The AML1 transcription factor, identified by the cloning of the translocation t(8;21) breakpoint, is one of the most frequent targets for chromosomal translocations in leukemia. Furthermore, polysomies and point mutations can also alter AML1 function. AML1, also called CBF alpha 2, PEBP alpha 2 or RUNX1, is thus implicated in a great number of acute leukemias via a variety of pathogenic mechanisms and seems to act either as an oncogene or a tumor suppressor gene. Characterization of AML1 knockout mice has shown that AML1 is necessary for normal development of all hematopoietic lineages and alterations in the overal functional level of AML1 can have a profound effect on hematopoiesis. Numerous studies have shown that AML1 plays a vital role in the regulation of expression of many genes involved in hematopoietic cell development, and the impairment of AML1 function disregulates the pathways leading to cellular proliferation and differentiation. However, heterozygous AML1 mutations alone may not be sufficient for the development of leukemia. A cumulative process of mutagenesis involving additional genetic events in functionally related molecules, may be necessary for the development of leukemia and may determine the leukemic phenotype. We review the known AML1 target genes, AML1 interacting proteins, AML1 gene alterations and their effects on AML1 function, and mutations in AML1-related genes associated with leukemia. We discuss the interconnections between all these genes in cell signaling pathways and their importance for future therapeutic developments.

Regulation of cytokine receptor signaling by SOCS1

The suppressor of cytokine signaling (SOCS) family of proteins is a novel class of negative feedback regulators of cytokine receptor signaling. SOCS1 is rapidly induced following stimulation by several type I and type II cytokines, and it attenuates their signaling by its ability to bind and inhibit all four of the Janus family of intracellular tyrosine kinases (JAKs). Studies from our own and other laboratories have documented another important function of SOCS1 in facilitating ubiquitination of protein substrates and their subsequent proteasomal degradation. SOCS1 also functions as a potential tumor suppressor by inhibiting several hematopoietic oncogenes. In addition to these negative regulatory functions, we have recently shown a positive regulatory role for SOCS1 in increasing the stability of major histocompatibility complex (MHC) class II proteins by preventing their degradation. These findings illustrate multiple roles for SOCS1 in cytokine receptor signaling, and provide groundwork for detailed analysis of the role of SOCS1 in pre-T cell receptor (TCR) and TCR signaling, and regulation of T helper (Th)1 and Th2 differentiation.

The haematopoietic specific signal transducer Vav1 is expressed in a subset of human neuroblastomas

Vav1 is a signal transducer protein expressed exclusively in the haematopoietic system, where it plays a pivotal role in growth factor-induced differentiation and proliferation. Vav1 couples tyrosine kinase signals with the activation of the Rho/Rac GTPases, leading to cell differentiation and/or proliferation. Vav1 was originally detected as an oncogene, but its involvement in human malignancies has not been reported thus far. We report here that Vav1 is expressed in a neuroblastoma cell line, SK-N-MC. Molecular analysis indicated that there are no gross rearrangements or mutations in the Vav1 gene in SK-N-MC cells. Vav1 protein from SK-N-MC cells was similar to wild-type Vav1 in apparent molecular weight, phosphorylation state, and ability to associate with active EGFR. We also analysed the expression of Vav1 in 42 specimens of human neuroblastoma. Vav1 was expressed in the majority of these tumours. Our results suggest that Vav1 may play a role in the neoplastic process in a subset of neuroblastomas.

GTPases and T cell activation

Guanine nucleotide binding proteins rapidly cycle between a guanosine diphosphate (GDP)-bound and guanosine triphosphate (GTP)-bound state, and they operate as binary switches that control cell activation in response to environmental cues. GTPases adopt different conformations when binding GTP vs. GDP. The GTP-bound state is generally considered to be the active conformation that allows GTPases to interact with downstream effectors and thereby initiate downstream signaling pathways, which regulate many important biological processes. Many members of the Ras family of GTPases, notably Ras and Rap1A, and the Rho family GTPases, Cdc42Hs, Rac1, Rac2 and RhoA, are important components of signal transduction pathways used by antigen receptors, costimulatory, cytokine and chemokine receptors to regulate the immune response. This review discusses current knowledge and ideas about the regulation and function of these GTPases in lymphocytes.

Detection and identification of Vav1 protein in primary cultured murine cerebellar neurons and in neuroblastoma cells (SH-SY5Y and Neuro-2a)

Vav1 was detected in neuronal cells during a screening for 1-methylthiodihydroceramide (1-MSDH-Cer) binding proteins. 1-MSDH-Cer is a metabolically stable analogue of dihydroceramide that was reported to strongly interfere with the formation of ceramide and hence the biosynthesis of all sphingolipids in neuronal cells. To identify target proteins that function as putative mediators of this molecule, a 1-MSDH-Cer affinity chromatography was utilised. When the cytosolic fraction of human neuroblastoma SH-SY5Y cells was subjected to 1-MSDH-Cer affinity chromatography, the sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the eluted protein fraction revealed an about 2-fold enrichment of the 98 kD protein band. Tryptic digestion of the excised band in combination with MALDI mass spectrometry strongly suggested that this band contained Vav1 protein. This was surprising since Vav1 in contrast to the other two isoforms Vav2 and Vav3 is believed to be exclusively expressed in hematopoietic cells and has not been detected in neuronal cells until now. The expression of Vav1 was confirmed in human SH-SY5Y neuroblastoma cells and additionally in murine Neuro-2A neuroblastoma cells as well as in primary cultured murine cerebellar neurons by Western blot analysis and reverse transcription polymerase chain reaction.

Regulation of lymphocyte-mediated killing by GTP-binding proteins

Exocytosis of granules containing apoptosis-inducing proteins is one mechanism of target cell killing by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Granules containing perforin and granzymes are redistributed to the area of cell contact initiated by specific interactions between surface ligands on a target cell and receptors on an effector lymphocyte. The formation of a stable conjugate between a cytotoxic lymphocyte and its potential target cell, followed by the directed delivery of granule components to the target cell are prerequisites of lymphocyte-mediated killing. Critical to understanding the development of cytotoxic function by CTLs and NK cells is the delineation of the second messenger pathways that specifically control the reorganization of the actin cytoskeleton during cell-mediated cytotoxicity. The low molecular weight guanosine 5'-triphosphate-binding proteins of the Rho family play a central role in these regulatory events controlling cytotoxic lymphocyte activation.

Effects of large clostridial cytotoxins on activation of RBL 2H3-hm1 mast cells indicate common and different roles of Rac in FcepsilonRI and M1-receptor signaling

Using Rho GTPases-inhibiting clostridial cytotoxins, we showed recently in RBL cells that the GTPase Rac is involved in FcepsilonRI (high-affinity receptor for IgE) signaling and receptor-mediated calcium mobilization, including influx via calcium release-activated calcium channels. Here, we studied the role of Rho GTPases in muscarinic M1 receptor signaling in RBL 2H3-hm1 cells. Clostridium difficile toxin B, which inactivates Rho, Rac, and Cdc42, and Clostridium sordellii lethal toxin, which inhibits Rac but not Rho, blocked M1-mediated exocytosis, indicating that Rac but not Rho is involved in the regulation of receptor-mediated exocytosis. Although antigen-induced FcepsilonRI stimulation caused tyrosine phosphorylation of the Rac guanine nucleotide exchange factor Vav, M1 stimulation by carbachol activated Rac independently of Vav. The Rac-inactivating toxins blocked M1 receptor-induced membrane translocation of the pleckstrin homology domain of protein kinase B, which is a phosphoinositide 3-kinase effector. The M1-induced calcium release from internal stores was not affected by toxin B; however, the subsequent calcium influx from the extracellular space was inhibited. The data suggest that besides capacitative calcium entry, the M1 signaling pathway activates further calcium entry channels with mechanisms that are not affected by the inhibition of Rac.

[Lymphocytes: what is "Vav"]

Guanine exchange factors (GEF) of the Vav family are critical activators of Rho GTPases, which control actin cytoskeletal reorganization and gene transcription. Among all GEFs identified, Vav proteins are the only GEFs regulated by tyrosine phosphorylation. Moreover, their structure contains several protein-protein or protein-lipid interaction domains. These domains are involved in the formation of multimolecular signalling complexes, highlighting the adaptor role of Vav proteins. The unique combination of these properties makes Vav proteins privileged integrators of multiple signalling pathways in a broad range of tissues and cells. Lymphocyte function during inflammatory and immune responses requires a dynamic remodeling of cellular architecture. Thus, it is not surprising that Vav proteins have been found to play a central role in the regulation of physiologic and pathologic lymphocyte responses.

RhoG regulates gene expression and the actin cytoskeleton in lymphocytes

RhoG, a member of the Rho family of GTPases, has been implicated as a regulator of the actin cytoskeleton. In this study, we show a novel function for the small GTPase RhoG on the regulation of the interferon-gamma promoter and nuclear factor of activated T cells (NFAT) gene transcription in lymphocytes. Optimal function of RhoG for the expression of these genes requires a calcium signal, normally provided by the antigen receptor. In addition, RhoG potentiation of NFAT requires the indirect activity of Rac and Cdc42; however, pathways distinct from those activated by Rac and Cdc42 mediate RhoG activation of NFAT-dependent transcription. Using effector domain mutants of RhoG we found that its ability to potentiate NFAT-dependent transcription correlates with its capacity to increase actin polymerization, supporting the suggestion that NFAT-dependent transcription is an actin-dependent process. RhoG also promotes T-cell spreading on fibronectin, a property that is independent of its ability to enhance NFAT-dependent transcription. Hence, these results implicate RhoG in leukocyte trafficking and the control of gene expression induced in response to antigen encounter.

Actin cytoskeletal dynamics in T lymphocyte activation and migration

Dynamic rearrangements of the actin cytoskeleton are crucial for the function of numerous cellular elements including T lymphocytes. They are required for migration of T lymphocytes through the body to scan for the presence of antigens, as well as for the formation and stabilization of the immunological synapse at the interface between antigen-presenting cells and T lymphocytes. Supramolecular activation clusters within the immunological synapse play an important role for the initiation of T cell responses and for the execution of T cell effector functions. In addition to the T cell receptor/CD3 induced actin nucleation via Wasp/Arp2/3-activation, signals through accessory receptors of the T cell (i.e., costimulation) regulate actin cytoskeletal dynamics. In this regard, the actin-binding proteins cofilin and L-plastin represent prominent candidates linking accessory receptor stimulation to the rearrangement of the actin cytoskeleton. Cofilin enhances actin polymerization via its actin-severing activity, and as a long-lasting effect, cofilin generates novel actin monomers through F-actin depolymerization. L-plastin stabilizes actin filament structures by means of its actin-bundling activity.

Guanine exchange-dependent and -independent effects of Vav1 on integrin-induced T cell spreading

Vav1 is a 95-kDa member of the Dbl family of guanine exchange factors and a prominent hemopoietic cell-specific protein tyrosine kinase substrate, the involvement of which in cytoskeletal rearrangements has been linked to its ability to activate Rho family small GTPases. Beta1 integrin ligation by fibronectin induced Vav1 phosphorylation in peripheral blood lymphocytes and in two different T cell lines. Vav1 overexpression led to massive T cell spreading on beta1 integrin ligands, and, conversely, two dominant negative mutants blocked integrin-induced spreading. Vav1 and beta1 integrin ligation synergistically activated Pak, but not Rac, Cdc42, or c-Jun N-terminal kinase. In addition, Vav1 cooperated with constitutively active V12Rac mutant, but not with V12Cdc42, to induce T cell spreading after integrin occupancy. More importantly, a Vav1 mutant that lacked guanine exchange factor activity still cooperated with V12Rac. In contrast, a point mutation in the SH2 domain of Vav1 abolished this synergistic effect. Therefore, our results suggest a new regulatory effect of Vav1 in T cell spreading, which is independent of its guanine exchange factor activity.

Vav1 and Ly-GDI two regulators of Rho GTPases, function cooperatively as signal transducers in T cell antigen receptor-induced pathways

The Rho family GTPases are pivotal for T cell signaling; however, the regulation of these proteins is not fully known. One well studied regulator of Rho GTPases is Vav1; a hematopoietic cell-specific guanine nucleotide exchange factor critical for signaling in T cells, including stimulation of the nuclear factor of activated T cells (NFAT). Surprisingly, Vav1 associates with Ly-GDI, a hematopoietic cell-specific guanine nucleotide dissociation inhibitor of Rac. Here, we studied the functional significance of the interaction between Vav1 and Ly-GDI in T cells. Upon organization of the immunological synapse, both Ly-GDI and Vav1 relocalize to T cell extensions in contact with the antigen-presenting cell. Ly-GDI is phosphorylated on tyrosine residues following T cell receptor stimulation, and it associates with the Src homology 2 region of an adapter protein, Shc. In addition, the interaction between Ly-GDI and Vav1 requires tyrosine phosphorylation. Overexpression of Ly-GDI alone is inhibitory to NFAT stimulation and calcium mobilization. However, when co-expressed with Vav1, Ly-GDI enhances Vav1 induction of NFAT activation, phospholipase Cgamma phosphorylation, and calcium mobilization. Moreover, Ly-GDI does not alter the regulation of these phenomena when coexpressed with oncogenic Vav1. Since oncogenic Vav1 does not bind Ly-GDI, this suggests that the functional cooperativity of Ly-GDI and Vav1 is dependent upon their association. Thus, our data suggest that the interaction of Vav1 and Ly-GDI creates a fine tuning mechanism for the regulation of intracellular signaling pathways leading to NFAT stimulation.

Alternative splicing variants of the human DBL (MCF-2) proto-oncogene

The DBL (MCF-2) proto-oncogene is a prototype guanine nucleotide exchange factor (GEF) that modulates the Rho family of GTPases. In this communication we describe the isolation of three novel splicing variants of Dbl. The prototype Dbl gene (designated var.1 here) contains 25 exons, while splicing variant 2 (var.2) lacks exons 23 and 24. Var.3 contains additional 3 exons from 5(')-UTR in place of exon 1, while var.4, var.2, and var.3 contain a 48bp insertion between exons 10 and 11, resulting in the insertion of 16 amino acids. We found that var.1 was expressed only in brain, whereas var.3 was expressed in heart, kidney, spleen, liver, and testis, and var.4 in brain, heart, kidney, testis, placenta, stomach, and peripheral blood. The Dbl protein was detectable in brain, heart, kidney, intestine, muscle, lung, and testis. An assay for GEF activity revealed that the var.2 exhibits decreased GEF activity towards Cdc42, var.3 exhibits a weak but significant activity toward Rac1 and Cdc42, var.4 exhibits significant activity toward RhoA and Cdc42, while var.1 exhibits no activity toward RhoA, Rac1, or Cdc42. In summary, we describe 4 splicing variants of the human DBL proto-oncogene that show different tissue distributions and GEF specificities.

Molecular adapters in Fc(epsilon)RI signaling and the allergic response

IgE-dependent activation of mast cells is central to the allergic response. The engagement of IgE-occupied receptors initiates a series of molecular events that cause the release of preformed, and de novo synthesis of, allergic mediators. Recent investigations demonstrate a critical role for non-enzymatic proteins that facilitate the activation and coordination of biochemical signals required for mast cell activation. Among these LAT, SLP-76 and Gab2 are critically important as adapters that facilitate events initiated by IgE receptor-dependent activation of Src family protein tyrosine kinases, Lyn and Fyn. An evaluation of the role of these adapters points to complementary but independent steps in early signaling and the possibility that preference for one or another adaptor complex may result in selective mast cell responses.

Structural determinants for the biological activity of Vav proteins

We have used an extensive mutagenesis approach to study the specific role of the eight structural domains of Vav during both the activation and signaling steps of this Rac1 exchange factor. Our results indicate that several Vav domains (Dbl homology, pleckstrin homology, and zinc finger) are essential for all the biological activities tested, whereas others are required for discrete, cell type-specific biological effects. Interestingly, we have found that Vav domains have no unique functions. Thus, the calponin homology domain mediates the inhibition of Vav both in vitro and in vivo but, at the same time, exerts effector functions in lymphocytes upon receptor activation. The Vav SH2 and SH3 regions play regulatory roles in the activation of Vav in fibroblasts, mediating both its phosphorylation and translocation to the plasma membrane. In contrast, the Vav SH2 and SH3 regions act as scaffolding platforms in T-cells, ensuring the proper phosphorylation of Vav and the subsequent engagement of downstream effectors. We also provide evidence indicating that the zinc finger region exerts at least three different functional roles in Vav, aiding in the down-regulation of its basal activity, the engagement of substrates, and the induction of ancillary pathways required for cell transformation. Finally, the results obtained are consistent with a new regulatory model for Vav, in which the calponin homology region inhibits the basal activity of Vav through interactions with the zinc finger region.

CD19 signaling pathways play a major role for murine AIDS induction and progression

Infection of genetically susceptible mice with the LP-BM5 mixture of murine leukemia viruses including an etiologic defective virus (BM5def) causes an immunodeficiency syndrome called murine AIDS (MAIDS). The disease is characterized by interactions between B cells and CD4(+) T cells resulting in polyclonal activation of both cell types. It is known that BM5def is expressed at highest levels in B cells and that B cells serve as viral APC. The CD19-CD21 complex and CD22 on the surface of B cells play critical roles as regulators of B cell responses to a variety of stimuli, influencing cell activation, differentiation, and survival. CD19 integrates positive signals induced by B cell receptor ligation by interacting with the protooncogene Vav, which leads to subsequent tyrosine phosphorylation of this molecule. In contrast, CD22 negatively regulates Vav phosphorylation. To analyze the role of CD19, CD21, Vav, and CD22 in MAIDS, we infected mice deficient in CD19, CD21 (CR2), Vav-1, or CD22 with LP-BM5 murine leukemia viruses. Infected CR2(-/-) mice developed MAIDS with a time course and severity indistinguishable from that of wild-type mice. In contrast, CD19 as well as Vav-1 deficiency restricted viral replication and suppressed the development of typical signs of MAIDS including splenomegaly, lymphadenopathy, and hypergammaglobulinemia. Finally, CD22 deficiency was found to accelerate MAIDS development. These results provide novel insights into the B cell signaling pathways required for normal induction and progression of MAIDS.

Vav1, but not Vav2, contributes to platelet aggregation by CRP and thrombin, but neither is required for regulation of phospholipase C

We have investigated the role of the Rho and Rac family small guanine triphosphate (GTP) exchange factors (RhoGEFs), Vav1 and Vav2, in the activation of platelets by the immunoreceptor tyrosine-based activation motif (ITAM)-coupled collagen receptor GPVI and by the G protein-coupled receptor agonist thrombin. The glycoprotein VI (GPVI)-specific agonist collagen-related peptide (CRP) and thrombin stimulated tyrosine phosphorylation of Vav1 but not Vav2 in human platelets. Surprisingly, however, CRP did not activate the low-molecular-weight G protein Rac and stimulated only a small increase in activity of p21-associated kinase 2 (PAK2), despite the fact that both proteins are regulated downstream of Vav1 in other cells. Further, activation of Rac and PAK2 by thrombin was maintained in platelets from mice deficient in Vav1. Activation of phospholipase C (PLC) by GPVI and thrombin was unaltered in Vav1-, Vav2-, and Vav1/Vav2-deficient platelets. A weak inhibition of late-stage aggregation to CRP and thrombin was observed in platelets deficient in Vav1 but not Vav2, whereas spreading on fibrinogen was not changed. The present results demonstrate that neither Vav1 nor Vav2 lie upstream of PLC or Rac in platelets, highlighting an important difference in their role in signaling by ITAM-coupled receptors in other cell types. The present study has provided evidence for a possible role of Vav1 but not Vav2 in the later stages of platelet aggregation.

Phosphorylation of p85 beta PIX, a Rac/Cdc42-specific guanine nucleotide exchange factor, via the Ras/ERK/PAK2 pathway is required for basic fibroblast growth factor-induced neurite outgrowth

Guanine nucleotide exchange factors (GEFs) have been implicated in growth factor-induced neuronal differentiation through the activation of small GTPases. Although phosphorylation of these GEFs is considered an activation mechanism, little is known about the upstream of PAK-interacting exchange factor (PIX), a member of the Dbl family of GEFs. We report here that phosphorylation of p85 betaPIX/Cool/p85SPR is mediated via the Ras/ERK/PAK2 pathway. To understand the role of p85 betaPIX in basic fibroblast growth factor (bFGF)-induced neurite outgrowth, we established PC12 cell lines that overexpress the fibroblast growth factor receptor-1 in a tetracycline-inducible manner. Treatment with bFGF induces the phosphorylation of p85 betaPIX, as determined by metabolic labeling and mobility shift upon gel electrophoresis. Interestingly, phosphorylation of p85 betaPIX is inhibited by PD98059, a specific MEK inhibitor, suggesting the involvement of the ERK cascade. PAK2, a major PAK isoform in PC12 cells as well as a binding partner of p85 betaPIX, also functions upstream of p85 betaPIX phosphorylation. Surprisingly, PAK2 directly binds to ERK, and its activation is dependent on ERK. p85 betaPIX specifically localizes to the lamellipodia at neuronal growth cones in response to bFGF. A mutant form of p85 betaPIX (S525A/T526A), in which the major phosphorylation sites are replaced by alanine, shows significant defect in targeting. Moreover, expression of the mutant p85 betaPIX efficiently blocks PC12 cell neurite outgrowth. Our study defines a novel signaling pathway for bFGF-induced neurite outgrowth that involves activation of the PAK2-p85 betaPIX complex via the ERK cascade and subsequent translocation of this complex.

The HIV-1 Nef protein and phagocyte NADPH oxidase activation

Nef, a multifunctional HIV protein, activates the Vav/Rac/p21-activated kinase (PAK) signaling pathway. Given the potential role of this pathway in the activation of the phagocyte NADPH oxidase, we have investigated the effect of the HIV-1 Nef protein on the phagocyte respiratory burst. Microglia (cell line and primary culture) were transduced with lentiviral expression vectors. Expression of Nef did not activate the NADPH oxidase by itself but led to a massive enhancement of the responses to a variety of stimuli (Ca(2+) ionophore, formyl peptide, endotoxin). These effects were not caused by up-regulation of phagocyte NADPH oxidase subunits. Nef mutants lacking motifs involved in the interaction with Vav and PAK failed to reproduce the effects of wild type Nef, suggesting a role for the Vav/Rac/PAK signaling pathway. The following results suggest a key role for Rac in the priming effect of Nef. (i) Inactivation of Rac by Clostridium difficile toxin B abolished the Nef effect. (ii) The fraction of activated Rac1 was increased in Nef-transduced cells, and (iii) the dominant positive Rac1(V12) mutant mimicked the effect of Nef. These results are to our knowledge the first analysis of the effect of Rac activation on the NADPH oxidase in intact phagocytes. Rac activation is not sufficient to stimulate the phagocyte NADPH oxidase; however, it markedly enhances the NADPH oxidase response to other stimuli.

Adaptor protein APS binds the NH2-terminal autoinhibitory domain of guanine nucleotide exchange factor Vav3 and augments its activity

The N-terminal calponin homology (CH) domain of Vav guanine nucleotide exchange factor is thought to serve a regulatory role in the autoinhibition, however, its precise function is not entirely clear. We found that the adaptor molecule APS could bind the CH domain of Vav3, a member of the vav proto-oncogene family. The binding of Vav3 and APS was apparently stabilized by the tyrosine phosphorylation of Vav3 by Lck, and the association of APS with Vav3 in turn enhanced the Lck-mediated phosphorylation of Vav3. Focus formation assays demonstrated that APS could increase the transforming activity of proto-Vav3. Further analyses revealed that the Vav3 CH domain could bind the pleckstrin homology (PH) domain of APS and that this binding was indispensable to enhance the transforming activity of Vav3. We present here a novel stimulatory mechanism of Vav3 in which APS directly relieves the autoinhibitory CH domain and furthermore enhances its tyrosine phosphorylation by Lck.

Critical role of the pleckstrin homology and cysteine-rich domains in Vav signaling and transforming activity

Vav family proteins are members of the Dbl family of guanine nucleotide exchange factors and activators of Rho family small GTPases. In addition to the Dbl homology (DH) domain important for guanine nucleotide exchange factor catalytic function, all Dbl family proteins contain an adjacent pleckstrin homology (PH) domain that serves to regulate DH domain activity. Although the role of the PH domain in Vav function has been evaluated extensively, its precise role and whether it serves a distinct role in different Vav proteins remain unresolved. Additionally, the precise role of an adjacent cysteine-rich domain (CRD) in regulating DH domain function is also unclear. In this study, we evaluated the contribution of these putative protein-protein or protein-lipid interaction domains to Vav signaling and transforming activity. In contrast to previous observations, we found that the PH domain is critical for Vav transforming activity. Similarly, the CRD was also essential and served a function distinct from that of the PH domain. Although mutation of either domain reduced Vav membrane association, addition of plasma membrane targeting sequences to either the CRD or PH domain mutant proteins did not restore Vav transforming activity. This result contrasts with other Dbl family proteins, where a membrane targeting sequence alone was sufficient to restore the loss of function caused by mutation of the PH domain. Furthermore, green fluorescent protein fusion proteins containing the PH domain or CRD, or both, failed to target to the plasma membrane, suggesting that these two domains also serve regulatory functions independent of promoting membrane localization. Finally, we found that phosphatidylinositol 3-kinase activation may promote Vav membrane association via phosphatidylinositol 3,4,5-triphosphate binding to the PH domain.

Pleiotropic defects in TCR signaling in a Vav-1-null Jurkat T-cell line

The Rac/Rho-specific guanine nucleotide exchange factor, Vav-1, is a key component of the T-cell antigen receptor (TCR)-linked signaling machinery. Here we have used somatic cell gene-targeting technology to generate a Vav-1-deficient Jurkat T-cell line. The J.Vav1 cell line exhibits dramatic defects in TCR-dependent interleukin (IL)-2 promoter activation, accompanied by significant reductions in the activities of the NFAT(IL-2), NFkappaB, AP-1 and REAP transcription factors that bind to the IL-2 promoter region. In contrast, loss of Vav-1 had variable effects on early TCR-stimulated signaling events. J.Vav1 cells display a selective defect in sustained Ca(2+) signaling during TCR stimulation, and complementation of this abnormality by exogenously introduced Vav-1 is dependent on the Vav-1 calponin homology domain. While JNK activation was severely impaired, the stimulation of Ras, ERK and protein kinase C-theta activities, as well as the mobilization of lipid rafts, appeared normal in the J.Vav1 cells. Finally, evidence is presented to suggest that the alternative Vav family members, Vav-2 and Vav-3, are activated during TCR ligation, and partially compensate for the loss of Vav-1 in Jurkat T cells.

Role of Dbl's big sister in the anti-mitogenic pathway from alpha1B-adrenergic receptor to c-Jun N-terminal kinase

We previously reported that the alpha1B-adrenergic receptor leads to activation of Rho family small GTPases, and in turn, c-Jun N-terminal kinase (JNK), which results in the inhibition of cell proliferation. Here, we show the involvement of the Rho family guanine nucleotide exchange factor (GEF) Dbl's Big Sister (Dbs) in the signaling pathway. Transfection of a Dbl-homology (DH) and pleckstrin-homology (PH) domain-deficient form of Dbs into cells blocked the alpha1B-adrenergic receptor-induced activation of JNK. Conversely, transfection of an isolated DH domain of Dbs induced JNK activation. Stimulation of the alpha1B-adrenergic receptor enhanced an intrinsic Cdc42-GEF activity of Dbs in a manner dependent on Src family tyrosine kinases. Additionally, DH and PH domain deficient Dbs blocked the receptor-induced inhibition of cell proliferation, while DH domain of Dbs inhibited cell proliferation via the JNK-dependent pathway. Taken together, Dbs may play an important role in the anti-mitogenic JNK pathway downstream of the alpha1B-adrenergic receptor.

CD28 costimulation: a source of Vav-1 for TCR signaling with the help of SLP-76?

T cells require dual stimulation to become activated. When T cells encounter antigen-presenting cells, both the T cell receptor (TCR) and the CD28 coreceptor are ligated and activated. Michel and Acuto discuss how the adaptor SLP-76, which is recruited to the activated TCR complex, and the Rho family guanosine triphosphatase exchanger Vav-1, which is recruited by the CD28 receptor and TCR, may form a macromolecular complex that results in T cells activation. Vav-1 may serve as a central integrator between CD28 signaling and TCR signaling through its indirect effects on phosphoinositide 3-kinase-dependent signaling.

Genetic analysis of integrin activation in T lymphocytes

Among the myriad receptors expressed by T cells, the sine qua non is the CD3/T cell receptor (CD3/TCR) complex, because it is uniquely capable of translating the presence of a specific antigen into intracellular signals necessary to trigger an immune response against a pathogen or tumor. Much work over the past 2 decades has attempted to define the signaling pathways leading from the CD3/TCR complex that culminate ultimately in the functions necessary for effective T cell immune responses, such as cytokine production. Here, we summarize recent advances in our understanding of the mechanisms by which the CD3/TCR complex controls integrin-mediated T cell adhesion, and discuss new information that suggests that there may be unexpected facets to this pathway that distinguish it from those previously defined.

The mouse Arhgef6 gene: cDNA sequence, expression analysis, and chromosome assignment

Mutations in ARHGEF6, encoding a guanine nucleotide exchange factor for Rho GTPases, have been shown to cause X-chromosomal non-specific mental retardation (MRX) in human. Here we report the identification and characterization of the orthologous mouse gene, Arhgef6. The gene transcript of approximately 4.4 kb is present in various mouse tissues indicating ubiquitous expression as found for human ARHGEF6. The open reading frame encodes a polypeptide of 771 amino acids with high homology to human ARHGEF6. The structural motifs of both proteins are conserved including an N-terminal CH domain, followed by an SH3 domain, and a tandem organization of the DH and PH domains. Analysis of the temporal expression pattern revealed that Arhgef6 is strongly expressed in the very early embryo and somewhat less in later stages. A genomic cosmid clone containing Arhgef6 exons 1 and 2 has been isolated and used for mapping the gene on the mouse X chromosome.

VAV proteins as signal integrators for multi-subunit immune-recognition receptors

In recent years, substantial progress has been made towards the identification of intracellular signalling molecules that couple multi-subunit immune-recognition receptors (MIRRs) to their various effector functions. Among these, the VAV proteins have been observed to have a crucial role in regulating some of the earliest events in receptor signalling. VAV proteins function, in part, as guanine-nucleotide exchange factors (GEFs) for the RHO/RAC family of GTPases. This review focuses on the role of VAV proteins in the regulation of lymphocyte development and function, and emphasizes the regulatory roles that these proteins have through both GEF-dependent and -independent mechanisms.

Src-dependent Syk activation controls CD69-mediated signaling and function on human NK cells

CD69 C-type lectin receptor represents a functional triggering molecule on activated NK cells, capable of directing their natural killing function. The receptor-proximal signaling pathways activated by CD69 cross-linking and involved in CD69-mediated cytotoxic activity are still poorly understood. Here we show that CD69 engagement leads to the rapid and selective activation of the tyrosine kinase Syk, but not of the closely related member of the same family, ZAP70, in IL-2-activated human NK cells. Our results indicate the requirement for Src family kinases in the CD69-triggered activation of Syk and suggest a role for Lck in this event. We also demonstrate that Syk and Src family tyrosine kinases control the CD69-triggered tyrosine phosphorylation and activation of phospholipase Cgamma2 and the Rho family-specific exchange factor Vav1 and are responsible for CD69-triggered cytotoxicity of activated NK cells. The same CD69-activated signaling pathways are also observed in an RBL transfectant clone, constitutively expressing the receptor. These data demonstrate for the first time that the CD69 receptor functionally couples to the activation of Src family tyrosine kinases, which, by inducing Syk activation, initiate downstream signaling pathways and regulate CD69-triggered functions on human NK cells.

Understanding Rho/Rac biology in T-cells using animal models

Experiments with cell lines have unveiled the implication of the Rho/Rac family of GTPases in cytoskeletal organization, mitogenesis, and cell migration. However, there have not been adequate animal models to investigate the role of these proteins in more physiological settings. This scenario has changed recently in the case of the T-cell lineage after the generation of animal models for Rho/Rac family members, their regulators, and effectors. These studies have revealed the implication of these GTPases on multiple regulatory layers of T-cells, including the coordination of cytoskeletal change, activation of kinase cascades, stimulation of calcium fluxes, and the induction of gene expression. These pathways affect the transition of different T-cell maturation stages, the positive/negative selection of thymocytes, T-cell responses to antigens, and the homeostasis of peripheral T-lymphocytes. Moreover, these animals have revealed interesting cross-talks between Rho/Rac pathways and other signal transduction routes that participate in lymphocyte responses.

CD4-Lck through TCR and in the absence of Vav exchange factor induces Bax increase and mitochondrial damage

In the present study, we aimed to demonstrate that CD4 may represent a critical turning point that governs the apoptotic and survival programs in T cells, without modifying the physical association with the TCR-CD3 complex. To address this issue, we have explored the possibility that the activation of CD4 may transduce apoptotic signals unless signaling effectors neutralize them. Our data show that in Jurkat T cells CD4 engagement by Leu3a mAb results in a rapid and strong increase of Lck kinase activity, subsequent alterations of mitochondrial membrane potential, and apoptosis. Critical parameters are coassociation of CD4/Lck with TCR/CD3 and up-regulation of the proapoptotic protein Bax. Indeed, Leu3a-mediated Lck activation failed to induce apoptotic features in Jurkat cells either defective for TCR/CD3 or overexpressing the antiapoptotic protein Bcl-2. Furthermore, we demonstrate that Leu3a treatment of Jurkat cells overexpressing Vav results in the inhibition of mitochondrial damage and apoptosis; this rescue effect is accompanied with a significant decrease of Bax expression observed in apoptotic cells. Our evidence that the activation of Lck activates in T cells apoptotic pathways which are counteracted by Vav, a signaling molecule that cooperates with CD28 to boost TCR signals, suggests a novel role for costimulation in protecting T cells from CD4-mediated cell death.

Rac activation induces NADPH oxidase activity in transgenic COSphox cells, and the level of superoxide production is exchange factor-dependent

Transient expression of constitutively active Rac1 derivatives, (G12V) or (Q61L), was sufficient to induce phagocyte NADPH oxidase activity in a COS-7 cell model in which human cDNAs for essential oxidase components, gp91(phox), p22(phox), p47(phox), and p67(phox), were expressed as stable transgenes. Expression of constitutively active Rac1 in "COS(phox)" cells induced translocation of p47(phox) and p67(phox) to the membrane. Furthermore, translocation of p47(phox) was induced in the absence of p67(phox) expression, even though Rac does not directly bind p47(phox). Rac effector domain point substitutions (A27K, G30S, D38A, Y40C), which can selectively eliminate interaction with different effector proteins, impaired Rac1V12-induced superoxide production. Activation of endogenous Rac1 by expression of constitutively active Rac-guanine nucleotide exchange factor (GEF) derivatives was sufficient to induce high level NADPH oxidase activity in COS(phox) cells. The constitutively active form of the hematopoietic-specific GEF, Vav1, was the most effective at activating superoxide production, despite detection of higher levels of Rac1-GTP upon expression of constitutively active Vav2 or Tiam1 derivatives. These data suggest that Rac can play a dual role in NADPH oxidase activation, both by directly participating in the oxidase complex and by activating signaling events leading to oxidase assembly, and that Vav1 may be the physiologically relevant GEF responsible for activating this Rac-regulated complex.

Distinct role of phosphatidylinositol 3-kinase and Rho family GTPases in Vav3-induced cell transformation, cell motility, and morphological changes

Vav3 is a member of the Vav family of guanine nucleotide exchange factors (GEFs) for the Rho family GTPases. Deleting the N-terminal calponin homology (CH) domain to generate Vav3-(5-10) or deleting both the CH and the acidic domain to generate Vav3-(6-10) results in activating the transforming potential of Vav3. Expression of either the full-length Vav3 or its truncation mutants led to activation of phosphatidylinositol 3-kinase (PI3K), mitogen-activated protein kinase (MAPK), focal adhesion kinase (FAK), and Stat3. We investigated the requirement of these signaling molecules for Vav3-induced focus formation and found that PI3K and its downstream signaling molecules, Akt and p70 S6 kinase, are required, albeit to varying degrees. Inhibition of PI3K had a more dramatic effect than inhibition of MAPK on Vav3-(6-10)-induced focus formation. Activated PI3K enhanced the focus-forming activity of Vav3-(6-10). Wild type FAK but not Y397F mutant FAK enhanced Vav3-(6-10)-induced focus formation. Dominant negative (dn) mutant of Stat3 resulted in a 60% inhibition of the focus-forming activity of Vav3-(6-10). Moreover, Rac1, RhoA, and to a lesser extent, Cdc42, are important for Vav3-(6-10)-induced focus formation. Constitutively activated (ca) Rac synergizes with Vav3-(6-10) in focus formation. This synergy requires signaling via Rho-associated kinase (ROK) and p21-activated kinase (PAK), downstream effectors of Rac. Consistently, a ca PAK mutant enhanced, whereas a dn PAK mutant inhibited the focus-forming ability of Vav3-(6-10). Despite having potent focus-forming ability, Vav3-(6-10) has very weak colony-forming ability. This colony-forming ability of Vav3-(6-10) can be enhanced dramatically by co-expressing an activated PI3K and to some extent by co-expressing an activated PAK mutant or c-Myc. Interestingly, inhibition of PI3K and MAPK had no effect on the ability of either wild type or Vav3-(6-10) to induce cytoskeletal changes including formation of lamellipodia and filopodia in NIH 3T3 cells. Over expression of Vav3 or Vav3-(6-10) resulted in an enhancement of cell motility. This enhancement was dependent on PI3K, Rac1, and Cdc42 but not on Rho. Overall, our results show that signaling pathways of PI3K, MAPK, and Rho family GTPases are differentially required for Vav3-induced focus formation, colony formation, morphological changes, and cell motility.

PU.1 regulates the expression of the vav proto-oncogene

Vav is a guanine nucleotide exchange factor for the rho/rac GTPases that is upregulated in the embryo during the transition from primitive to definitive hematopoiesis. It is one of several genetic markers that correlates with the differentiation of the intraembryonic definitive hematopoietic stem cell. Subsequently, in the adult, vav is expressed predominantly in cells of the hematopoietic system. A heat-resistant protein complex that bound to a 23-bp segment, which is essential for vav promoter activity, was found to be present in myeloid cells but not T-cells. The complex was absent in non-hematopoietic cells which normally do not express vav. Using a saturation mutagenesis method, Mutex, a "footprint" of the protein binding site (AGAGGAAGT) was obtained that was consistent with the consensus binding site for PU.1. A specific antibody to PU.1 supershifted the complex and identified the presence of PU.1 within the complex. A GST fusion protein of the human PU.1 bound to the same consensus sequence as the heat-resistant complex from myeloid lineages. Specific mutation of the GGAA PU.1 core binding site silenced vav promoter activity and a dominant negative PU.1 inhibited the transactivation of PU.1 at the vav promoter as measured by the expression of the EGFP reporter gene. In addition, PCR analysis of immunoprecipitated chromatin using specific antibodies for PU.1 detected the co-immunoprecipitation of DNA containing the vav promoter. These results suggest that PU.1 is essential for transcriptional activity of the vav promoter in myeloid cells.

Vav1 is a component of transcriptionally active complexes

The importance of the hematopoietic protooncogene Vav1 in immune cell function is widely recognized, although its regulatory mechanisms are not completely understood. Here, we examined whether Vav1 has a nuclear function, as past studies have reported its nuclear localization. Our findings provide a definitive demonstration of Vav1 nuclear localization in a receptor stimulation-dependent manner and reveal a critical role for the COOH-terminal Src homology 3 (SH3) domain and a nuclear localization sequence within the pleckstrin homology domain. Analysis of DNA-bound transcription factor complexes revealed nuclear Vav1 as an integral component of transcriptionally active nuclear factor of activated T cells (NFAT)- and nuclear factor (NF)kappaB-like complexes, and the COOH-terminal SH3 domain as being critical in their formation. Thus, we describe a novel nuclear role for Vav1 as a component and facilitator of NFAT and NFkappaB-like transcriptional activity.

Vav1 transduces T cell receptor signals to the activation of phospholipase C-gamma1 via phosphoinositide 3-kinase-dependent and -independent pathways

Vav1 is a signal transducing protein required for T cell receptor (TCR) signals that drive positive and negative selection in the thymus. Furthermore, Vav1-deficient thymocytes show greatly reduced TCR-induced intracellular calcium flux. Using a novel genetic system which allows the study of signaling in highly enriched populations of CD4(+)CD8(+) double positive thymocytes, we have studied the mechanism by which Vav1 regulates TCR-induced calcium flux. We show that in Vav1-deficient double positive thymocytes, phosphorylation, and activation of phospholipase C-gamma1 (PLCgamma1) is defective. Furthermore, we demonstrate that Vav1 regulates PLCgamma1 phosphorylation by at least two distinct pathways. First, in the absence of Vav1 the Tec-family kinases Itk and Tec are no longer activated, most likely as a result of a defect in phosphoinositide 3-kinase (PI3K) activation. Second, Vav1-deficient thymocytes show defective assembly of a signaling complex containing PLCgamma1 and the adaptor molecule Src homology 2 domain-containing leukocyte phosphoprotein 76. We show that this latter function is independent of PI3K.