Defects in actin-cap formation in Vav-deficient mice implicate an actin requirement for lymphocyte signal transduction

Identification and characterization of hPEM-2, a guanine nucleotide exchange factor specific for Cdc42

Guanine nucleotide exchange factors of the Dbl family regulate the actin cytoskeleton through activation of Rho-like GTPases. At present the Dbl family consists of more than thirty members; many have not been phenotypically or biochemically characterized. Guanine nucleotide exchange factors universally feature a Dbl homology domain followed by a pleckstrin homology domain. Employing data base screening we identified a recently cloned cDNA, KIAA0424, showing substantial sequence homology with Rac activators such as Tiam1, Sos, Vav, and PIX within the catalytic domain. This cDNA appears to be the human homologue of the Ascidian protein Posterior End Mark-2 (PEM-2). We refer to this exchanger as hPEM-2. hPEM-2 encodes a protein of 70 kDa and features an N-terminal src homology 3 domain, followed by tandem Dbl homology and pleckstrin homology domains. The gene is highly expressed in brain and is localized on the human X-chromosome. Employing biochemical activity assays for Rho-like GTPases we found that hPEM-2 specifically activates Cdc42 and not Rac or RhoA. Ectopic expression of hPEM-2 in NIH3T3 fibroblasts revealed a Cdc42 phenotype featuring filopodia formation, followed by cortical actin polymerization and cell rounding. hPEM-2 represents an exchange factor, which may have a role in the regulation of a number of cellular processes through Cdc42.

Biological and regulatory properties of Vav-3, a new member of the Vav family of oncoproteins

We report here the identification and characterization of a novel Vav family member, Vav-3. Signaling experiments demonstrate that Vav-3 participates in pathways activated by protein tyrosine kinases. Vav-3 promotes the exchange of nucleotides on RhoA, on RhoG and, to a lesser extent, on Rac-1. During this reaction, Vav-3 binds physically to the nucleotide-free states of those GTPases. These functions are stimulated by tyrosine phosphorylation in wild-type Vav-3 and become constitutively activated upon deletion of the entire calponin-homology region. Expression of truncated versions of Vav-3 leads to drastic actin relocalization and to the induction of stress fibers, lamellipodia, and membrane ruffles. Moreover, expression of Vav-3 alters cytokinesis, resulting in the formation of binucleated cells. All of these responses need only the expression of the central region of Vav-3 encompassing the Dbl homology (DH), pleckstrin homology (PH), and zinc finger (ZF) domains but do not require the presence of the C-terminal SH3-SH2-SH3 regions. Studies conducted with Vav-3 proteins containing loss-of-function mutations in the DH, PH, and ZF regions indicate that only the DH and ZF regions are essential for Vav-3 biological activity. Finally, we show that one of the functions of the Vav-3 ZF region is to work coordinately with the catalytic DH region to promote both the binding to GTP-hydrolases and their GDP-GTP nucleotide exchange. These results highlight the role of Vav-3 in signaling and cytoskeletal pathways and identify a novel functional cross-talk between the DH and ZF domains of Vav proteins that is imperative for the binding to, and activation of, Rho GTP-binding proteins.

Signaling scaffolds in immune cells

Of the past several years progress in understanding TCR signal transduction has led to the discovery of new kinases, adapter molecules and multiple signaling pathways. The study of molecules such as LAT, SLP-76, FYB, SKAP-55 and VAV have revealed multiple mechanisms with which to control the activation of downstream signaling pathways through RAS, PLC gamma-1 and ERK/MAPK. Signaling through SLP-76 can play a role in TCR-induced cytoskeleton changes through activation of effector molecules in the RAC/RHO-family of GTPases. In addition, SLP-76 through its association with FYB/FYN-T appears to play a role in IL-2 gene transcription following TCR activation. Finally, these newly identified adaptor molecules, such as LAT, may be crucial in T-cell activation by enhancing the recruitment of critical kinases to glycolipid-enriched microdomains of the activated T-cell receptor complex.

A thermodynamic model for receptor clustering

Intracellular signaling often arises from ligand-induced oligomerization of cell surface receptors. This oligomerization or clustering process is fundamentally a cooperative behavior between near-neighbor receptor molecules; the properties of this cooperative process clearly affect the signal transduction. Recent investigations have revealed the molecular basis of receptor-receptor interactions, but a simple theoretical framework for using these data to predict cluster formation has been lacking. Here, we propose a simple, coarse-grained, phenomenological model for ligand-modulated receptor interactions and discuss its equilibrium properties via mean-field theory. The existence of a first-order transition for this model has immediate implications for the robustness of the cellular signaling response.

Signals involved in thymocyte positive and negative selection

Extensive research has focused upon understanding how thymocytes distinguish between interactions that lead to positive or negative selection. Various intracellular pathways that are activated after TCR engagement are outlined in this review, and their contribution to thymocyte selection is discussed. Although thymocyte fate is generally governed by a quantitative/avidity model, this largely reflects the interactions that occur at the cell surface. Therefore, we outline possible models of how different intercellular interactions are translated into intracellular signals that diverge and lead to thymocyte survival or death.

FYN-T-FYB-SLP-76 interactions define a T-cell receptor zeta/CD3-mediated tyrosine phosphorylation pathway that up-regulates interleukin 2 transcription in T-cells

Protein-tyrosine kinases p56(Lck), SYK, and ZAP-70 and downstream adaptors LAT and SLP-76 have been implicated as essential components in T-cell activation. Another lymphoid-specific adaptor FYB/SLAP has also been identified as a predominant binding partner of SLP-76 and the Src kinase FYN-T, although its role in the activation process has been unclear. In this study, we demonstrate that FYN-T selectively phosphorylates FYB providing a template for the recruitment of FYN-T and SLP-76 SH2 domain binding. This interaction is unusual in its distinct cytoplasmic localization and its long term stable kinetics of phosphorylation. Furthermore, we demonstrate for the first time that the co-expression of all three components of the FYN-T-FYB-SLP-76 matrix can synergistically up-regulate T-cell receptor-driven interleukin 2 transcription activity. These findings document the existence of a T-cell receptor-regulated FYN-T-FYB pathway that interfaces with the adaptor SLP-76 and up-regulates lymphokine production in T-cells.

Tyrosine Phosphorylation of Vav Stimulates IL-6 Production in Mast Cells by a Rac/c-Jun N-Terminal Kinase-Dependent Pathway

This study investigates whether the guanine nucleotide exchange activity of Vav is linked to cytokine production in mast cells. Overexpression of Vav in the RBL-2H3 mast cell line resulted in the constitutive tyrosine phosphorylation and activation of Vav. We analyzed the functional effect of Vav overexpression on cytokine production. IL-2 and IL-6 mRNA levels were dramatically increased in Vav-overexpressing cells and correlated with increased NF-AT activity. Little or no effect was observed on the mRNA levels of IL-3, IL-4, GM-CSF, TNF-α, and TGF-β. FcεRI engagement did not further enhance IL-2 and IL-6 mRNA levels and only slightly enhanced NF-AT activity, but dramatically increased the mRNA levels of other tested cytokines. To understand the signal transduction required, we focused primarily on IL-6 induction by measuring mitogen-activated protein kinase activity and analyzing the effects of mutant or dominant negative forms of Vav, Rac1, and c-Jun N-terminal kinase-1 (JNK1). Vav overexpression resulted in the constitutive activation of JNK1 with little or no effect on p38 mitogen-activated protein kinase and ERK2. This was dependent on Vav-mediated activation of Rac1 as a Dbl domain-mutated Vav, inactive Rac N17, and inactive JNK1 down-regulated the Vav-induced JNK1 or IL-6 responses. Vav expression, but not expression of domain-mutated Vav, increased IL-6 secretion from nonimmortalized bone marrow-derived mast cells upon FcεRI engagement. We conclude that Vav phosphorylation contributes to IL-6 induction in mast cells.

T-cell activation and the dynamic world of rafts

Recent evidence, indicates that T-cell receptor (TCR) triggering and T-cell activation are dynamic processes that involve various aspects of T-cell organization. In addition to the interaction between the TCR molecule and its ligand, T-cell activation depends on a combination of many other events involving coreceptor molecules, actin cytoskeleton and plasma membrane lipids. Altogether, these cell structures organize the formation of a specialized junction between the T cell and the antigen-presenting cell (APC), that plays a critical role in sustaining and amplifying TCR signalling.

The Guanine-Nucleotide Exchange Factor Vav Is a Crucial Regulator of B Cell Receptor Activation and B Cell Responses to Nonrepetitive Antigens

The proto-oncogene product Vav is required for receptor clustering, proliferation, and differentiation of T cells, and Vav was identified as a substrate in the TCR and B cell receptor signaling pathway. The role of Vav in B cell responses to Ag challenge in vivo is not known. In this study, we show that Vav regulates B cell proliferation following in vitro activation of Ag receptors, but Vav has no apparent role in CD40-, IL-4-, or LPS-induced B cell activation. Increased degrees of Ag receptor cross-linking can partially reverse the proliferative defect in the anti-IgM response of vav−/− B cells. In vivo, vav−/− mice mounted protective antiviral IgM and IgG responses to infections with vesicular stomatitis virus and recombinant vaccinia virus expressing the vesicular stomatitis virus glycoprotein, which harbor repetitive surface epitopes that directly cross-link the Ag receptor and activate B cells in the absence of T cell help. vav−/− B cells also responded normally to the polyvalent, repetitive hapten Ag trinitrophenyl (TNP)-Ficoll that effectively cross-links B cell receptors. However, vav−/− mice failed to mount immune responses to the nonrepetitive, T cell-dependent hapten Ag (4-hydroxy-5-iodo-3-nitrophenyl)acetyl (NIP)-OVA. These results provide the first genetic evidence on the role of the guanine exchange factor Vav in immune responses to viral infections and antigenic challenge in vivo, and suggest that Vav adjusts the threshold for Ag receptor-mediated B cell activation depending on the nature of the Ag.

CD47 Signals T Cell Death

Activation-induced death of T cells regulates immune responses and is considered to involve apoptosis induced by ligation of Fas and TNF receptors. The role of other receptors in signaling T cell death is less clear. In this study we demonstrate that activation of specific epitopes on the Ig variable domain of CD47 rapidly induces apoptosis of T cells. A new mAb, Ad22, to this site induces apoptosis of Jurkat cells and CD3ε-stimulated PBMC, as determined by morphological changes, phosphatidylserine exposure on the cell surface, uptake of propidium iodide, and true counts by flow cytometry. In contrast, apoptosis was not observed following culture with anti-CD47 mAbs 2D3 or B6H12 directed to a distant or closely adjacent region, respectively. CD47-mediated cell death was independent of CD3, CD4, CD45, or p56lck involvement as demonstrated by studies with variant Jurkat cell lines deficient in these signaling pathways. However, coligation of CD3ε and CD47 enhanced phosphatidylserine externalization on Jurkat cells with functional CD3. Furthermore, normal T cells required preactivation to respond with CD47-induced apoptosis. CD47-mediated cell death appeared to proceed independent of Fas or TNF receptor signaling and did not involve characteristic DNA fragmentation or requirement for IL-1β-converting enzyme-like proteases or CPP32. Taken together, our data demonstrate that under appropriate conditions, CD47 activation results in very rapid T cell death, apparently mediated by a novel apoptotic pathway. Thus, CD47 may be critically involved in controlling the fate of activated T cells.

Genetic analysis of B cell antigen receptor signaling

In B lymphocytes, a signaling complex that contributes to cell fate decisions is the B cell antigen receptor (BCR). Data from knockout experiments in cell lines and mice have revealed distinct functions for the intracellular protein tyrosine kinases (Lyn, Syk, Btk) in BCR signaling and B cell development. Combinations of intracellular signaling pathways downstream of these PTKs determine the quality and quantity of BCR signaling. For example, concerted actions of the PLC-gamma 2 and PI3-K pathways are required for proper calcium responses. Similarly, the regulation of ERK and JNK responses involves both PLC-gamma 2 and GTPases pathways. Since the immune response in vivo is regulated by alteration of these signaling outcomes, achieving a precise understanding of intracellular molecular events leading to B lymphocyte proliferation, deletion, anergy, receptor editing, and survival still remains a challenge for the future.

Selection of the T cell repertoire

Advances in gene technology have allowed the manipulation of molecular interactions that shape the T cell repertoire. Although recognized as fundamental aspects of T lymphocyte development, only recently have the mechanisms governing positive and negative selection been examined at a molecular level. Positive selection refers to the active process of rescuing MHC-restricted thymocytes from programmed cell death. Negative selection refers to the deletion or inactivation of potentially autoreactive thymocytes. This review focuses on interactions during thymocyte maturation that define the T cell repertoire, with an emphasis placed on current literature within this field.

The Wiskott-Aldrich syndrome protein (WASP): roles in signaling and cytoskeletal organization

The Wiskott-Aldrich Syndrome (WAS) is a rare X-linked primary immunodeficiency that is characterized by recurrent infections, hematopoietic malignancies, eczema, and thrombocytopenia. A variety of hematopoietic cells are affected by the genetic defect, including lymphocytes, neutrophils, monocytes, and platelets. Early studies noted both signaling and cytoskeletal abnormalities in lymphocytes from WAS patients. Following the identification of WASP, the gene mutated in patients with this syndrome, and the more generally expressed WASP homologue N-WASP, studies have demonstrated that WASP-family molecules associate with numerous signaling molecules known to alter the actin cytoskeleton. WASP/N-WASP may depolymerize actin directly and/or serve as an adaptor or scaffold for these signaling molecules in a complex cascade that regulates the cytoskeleton.

SLP-76 and Vav function in separate, but overlapping pathways to augment interleukin-2 promoter activity

SLP-76 and Vav, two hematopoietic cell specific molecules, are critical for T cell development and activation. Following T cell antigen receptor stimulation, SLP-76 and Vav both undergo tyrosine phosphorylation and associate with each other via the SH2 domain of Vav and phosphorylated tyrosines of SLP-76. Furthermore, SLP-76 and Vav have a synergistic effect on interleukin (IL)-2 promoter activity in T cells. In this report, we show that two tyrosines, Tyr-113 and Tyr-128, of SLP-76 are required for its binding to Vav, both in vitro and in intact cells. Surprisingly, we find also that the interaction between SLP-76 and Vav is not required for their cooperation in augmenting IL-2 promoter activity, as the two molecules appear to function in different signaling pathways upstream of IL-2 gene expression. Overexpression of SLP-76 in the Jurkat T cell line potentiates the activities of both nuclear factor of activated T cells and AP-1 transcription factors. In contrast, overexpression of Vav leads to enhanced nuclear factor of activated T cells activity without affecting AP-1. Additionally, overexpression of Vav, but not SLP-76, augments CD28-induced IL-2 promoter activity. These findings suggest that the synergy between SLP-76 and Vav in regulating IL-2 gene expression reflects the cooperation between different signaling pathways.

hSiah2 is a new Vav binding protein which inhibits Vav-mediated signaling pathways

The hematopoietic proto-oncogene vav has been characterized as a Rac1-GDP/GTP exchanger protein which regulates cytoskeletal reorganization as well as signaling pathways leading to the activation of stress-activated protein kinases (SAPK/JNKs). Furthermore, vav overexpression enhances basal and T-cell receptor (TCR)-mediated stimulation of the nuclear factor of activated T cells (NFAT). We report here the interaction between Vav and hSiah2, a mammalian homolog of Drosophila Seven in absentia (Sina) that has been implicated in R7 photoreceptor cell formation during Drosophila eye development via the proteasome degradation pathway. Vav and hSiah2 interact in vitro and in vivo and colocalize in the cytoplasm of hematopoietic cells. The Src homology domain of Vav and the C-terminal region of hSiah2 are required for this interaction. We provide evidence for a negative regulation by hSiah2 of Vav-induced basal and TCR-mediated NFAT-dependent transcription. Overexpression of hSiah2 also inhibits the onco-Vav-induced JNK activation. Although the Vav-interacting domain is located in the C-terminal portion of hSiah2, the N-terminal region of hSiah2 is necessary for the inhibitory role that seems to be independent of the proteasome degradation.

T cell signaling: a decision of life and death

T lymphocytes constitute an essential part of the immune system. Their generation, activation, proliferation but also survival is subject to tight regulation by several extracellular factors including cytokines, MHC-antigen complexes and co-stimulatory ligands. The balanced interplay between these factors determines the fate of the T cell. Both in thymic development and in a peripheral immune response, triggering of the T cell antigen receptor (TCR) through interaction with the MHC-antigen complex can result in T cell proliferation. However, in the absence of co-stimulatory signals from antigen-presenting cells a state of non-responsiveness is induced that is called anergy. In addition, stimulation of the TCR on activated T cells or thymocytes can lead to the induction of apoptosis. Here we will give an overview of the intracellular signal transduction pathways that are activated by the stimuli that dictate the fate of a T cell as they were presented at the International Symposium on soluble HLA antigens held in 1997 in Brussels.

The oncogene product Vav is a crucial regulator of primary cytotoxic T cell responses but has no apparent role in CD28-mediated co-stimulation

The guanine nucleotide-exchange factor Vav is a regulator of antigen-mediated cytoskeletal reorganization required for receptor clustering, proliferation and thymic selection. Moreover, Vav has been identified as a major substrate in the CD28 signal transduction pathway and overexpression of Vav enhances TCR-mediated IL-2 secretion in T cells. Here we show that CD3- plus CD28-mediated proliferation and IL-2 production were reduced in vav gene-deficient T cells. However, Vav had no apparent role in phorbol 12-myristate 13-acetate-plus CD28-mediated proliferation and IL-2 production, suggesting that Vav acts downstream of the TCR/CD3 complex. In vivo, Vav expression was crucial to generate primary vesicular stomatitis virus (VSV)-specific cytotoxic T cell responses. In contrast, vav-/- mice exhibited a reduced but significant footpad swelling after lymphocytic choriomeningitis virus (LCMV) infections and mounted a measurable primary cytotoxic T cell response to LCMV. Upon in vitro restimulation, cytotoxic T cell responses of both VSV- and LCMV-infected mice reached near normal levels. Our data provide the first genetic evidence that Vav is an important effector molecule that relays antigen receptor signaling to IL-2 production and activation of cytotoxic T cells.

The Rho-family GTP exchange factor Vav is a critical transducer of T cell receptor signals to the calcium, ERK, and NF-kappaB pathways

Vav is a GTP/GDP exchange factor (GEF) for members of the Rho-family of GTPases that is rapidly tyrosine-phosphorylated after engagement of the T cell receptor (TCR), suggesting that it may transduce signals from the receptor. T cells from mice made Vav-deficient by gene targeting (Vav-/-) fail to proliferate in response to TCR stimulation because they fail to secrete IL-2. We now show that this is due at least in part to the failure to initiate IL-2 gene transcription. Furthermore, we analyze TCR-proximal signaling pathways in Vav-/- T cells and show that despite normal activation of the Lck and ZAP-70 tyrosine kinases, the mutant cells have specific defects in TCR-induced intracellular calcium fluxes, in the activation of extracellular signal-regulated mitogen-activated protein kinases and in the activation of the NF-kappaB transcription factor. Finally, we show that the greatly reduced TCR-induced calcium flux of Vav-deficient T cells is an important cause of their proliferative defect, because restoration of the calcium flux with a calcium ionophore reverses the phenotype.

Role for the Rac1 Exchange Factor Vav in the Signaling Pathways Leading to NK Cell Cytotoxicity

Here we investigate the activation of and a possible role for the hematopoietic Rac1 exchange factor, Vav, in the signaling mechanisms leading to NK cell-mediated cytotoxicity. Our data show that direct contact of NK cells with a panel of sensitive tumor targets leads to a rapid and transient tyrosine phosphorylation of Vav and to its association with tyrosine-phosphorylated Syk. Vav tyrosine phosphorylation is also observed following the activation of NK cells through the low-affinity Fc receptor for IgG (FcγRIII). In addition, we demonstrate that both direct and Ab-mediated NK cell binding to target cells result in the activation of nucleotide exchange on endogenous Rac1. Furthermore, Vav antisense oligodeoxynucleotide treatment leads to an impairment of NK cytotoxicity, with FcγRIII-mediated killing being more sensitive to the abrogation of Vav expression. These results provide new insight into the signaling pathways leading to cytotoxic effector function and define a role for Vav in the activation of NK cell-mediated killing.

Socs1 binds to multiple signalling proteins and suppresses steel factor-dependent proliferation

We have identified Socs1 as a downstream component of the Kit receptor tyrosine kinase signalling pathway. We show that the expression of Socs1 mRNA is rapidly increased in primary bone marrow-derived mast cells following exposure to Steel factor, and Socs1 inducibly binds to the Kit receptor tyrosine kinase via its Src homology 2 (SH2) domain. Previous studies have shown that Socs1 suppresses cytokine-mediated differentiation in M1 cells inhibiting Janus family kinases. In contrast, constitutive expression of Socs1 suppresses the mitogenic potential of Kit while maintaining Steel factor-dependent cell survival signals. Unlike Janus kinases, Socs1 does not inhibit the catalytic activity of the Kit tyrosine kinase. In order to define the mechanism by which Socs1-mediated suppression of Kit-dependent mitogenesis occurs, we demonstrate that Socs1 binds to the signalling proteins Grb-2 and the Rho-family guanine nucleotide exchange factors Vav. We show that Grb2 binds Socs1 via its SH3 domains to putative diproline determinants located in the N-terminus of Socs1, and Socs1 binds to the N-terminal regulatory region of Vav. These data suggest that Socs1 is an inducible switch which modulates proliferative signals in favour of cell survival signals and functions as an adaptor protein in receptor tyrosine kinase signalling pathways.

SH3P7 is a cytoskeleton adapter protein and is coupled to signal transduction from lymphocyte antigen receptors

Lymphocytes respond to antigen receptor engagement with tyrosine phosphorylation of many cellular proteins, some of which have been identified and functionally characterized. Here we describe SH3P7, a novel substrate protein for Src and Syk family kinases. SH3P7 migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as a 55-kDa protein that is preferentially expressed in brain, thymus, and spleen. It contains multiple amino acid sequence motifs, including two consensus tyrosine phosphorylation sites of the YXXP type and one SH3 domain. A region of sequence similarity, which we named SCAD, was found in SH3P7 and three actin-binding proteins. The SCAD region may represent a new type of protein-protein interaction domain that mediates binding to actin. Consistent with this possibility, SH3P7 colocalizes with actin filaments of the cytoskeleton. Altogether, our data implicate SH3P7 as an adapter protein which links antigen receptor signaling to components of the cytoskeleton.

Defective immunoglobulin class switching in Vav-deficient mice is attributable to compromised T cell help

Vav, a guanine nucleotide exchange factor for members of the Rho family of small GTPases, is activated through engagement of B and T lymphocyte antigen receptors. It is important for establishing the signaling threshold of the TCR, as mice lacking Vav display defective thymocyte selection. Here, conventional B cells are shown to develop normally in Vav-deficient mice but these mice have few B-1 B cells. The threshold for inducing B cell proliferation through BCR engagement in vitro is greater in Vav-deficient B cells. Nevertheless, in vivo the mutant mice have normal antibody responses to haptenated Ficoll. In contrast, Vav-/- mice show defective class switching to IgG and germinal center formation when immunized with haptenated protein. Interestingly, this defect is reversed in chimeras where normal T cells are present. Antigen-specific proliferation of T cells in the T zone was found to be similar in wild-type and Vav-/- mice but the induction of IL-4 mRNA and switch transcripts was specifically impaired. These results suggest that defective immunoglobulin class switching in Vav-deficient mice is attributable to compromised T cell help.

Waltzing with WASP

Wiskott-Aldrich syndrome (WAS) is an inherited immune deficiency that is marked by eczema, bleeding and recurrent infections. The lymphocytes and platelets of WAS patients display cytoskeletal abnormalities, and their T lymphocytes show a diminished proliferative response to stimulation through the T-cell receptor-CD3 complex (TCR-CD3). The product of the WAS gene, WAS protein (WASP), binds to the small GTPase Cdc42. Small GTPases of the Rho family are crucial for the regulation of the actin-based cytoskeleton. WASP and its relative NWASP might play an important role in regulating the actin cytoskeleton. Since both WASP and NWASP have the potential to bind to multiple proteins, they might serve as a hub to coordinate the redistribution of many cellular signals to the actin cytoskeleton. In this review, the authors discuss the possible role of WASP/NWASP and of the newly described protein WIP, which interacts with WASP and NWASP, in coupling signals from the T-cell receptor to the actin-based cytoskeleton.

A receptor/cytoskeletal movement triggered by costimulation during T cell activation

During T cell activation, the engagement of costimulatory molecules is often crucial to the development of an effective immune response, but the mechanism by which this is achieved is not known. Here, it is shown that beads attached to the surface of a T cell translocate toward the interface shortly after the start of T cell activation. This movement appears to depend on myosin motor proteins and requires the engagement of the major costimulatory receptor pairs, B7-CD28 and ICAM-1-LFA-1. This suggests that the engagement of costimulatory receptors triggers an active accumulation of molecules at the interface of the T cell and the antigen-presenting cell, which then increases the overall amplitude and duration of T cell signaling.

The role of PI 3-kinase in insulin action

This review focuses on the recent advances made in our understanding of the mechanism by which insulin induces the activation of PI 3-kinase(s) whose role is to generate 3-phosphoinositide lipids which are the second messenger of the insulin signalling pathway. The mechanism by which these signalling molecules induce the activation of downstream signalling pathways leading to the activation of protein kinase B (PKB, also known as Akt) and other kinases is also discussed. PKB is likely to be a major mediator of many of the physiological responses of a cell to insulin and likely physiological cellular targets of this enzyme are highlighted.

Vav regulates peptide-specific apoptosis in thymocytes

The protooncogene Vav functions as a GDP/GTP exchange factor (GEF) for Rho-like small GTPases involved in cytoskeletal reorganization and cytokine production in T cells. Gene-targeted mice lacking Vav have a severe defect in positive and negative selection of T cell antigen receptor transgenic thymocytes in vivo, and vav-/- thymocytes are completely resistant to peptide-specific and anti-CD3/anti-CD28-mediated apoptosis. Vav acts upstream of mitochondrial pore opening and caspase activation. Biochemically, Vav regulates peptide-specific Ca2+ mobilization and actin polymerization. Peptide-specific cell death was blocked both by cytochalasin D inhibition of actin polymerization and by inhibition of protein kinase C (PKC). Activation of PKC with phorbol ester restored peptide-specific apoptosis in vav-/- thymocytes. Vav was found to bind constitutively to PKC-theta in thymocytes. Our results indicate that peptide-triggered thymocyte apoptosis is mediated via Vav activation, changes in the actin cytoskeleton, and subsequent activation of a PKC isoform.

Degree of TCR Internalization and Ca2+ Flux Correlates with Thymocyte Selection

Recent evidence suggests that TCR down-regulation directly reflects the number of TCRs that have engaged MHC/peptide ligand complexes. Here, we examined the influence of defined peptides on thymic selection based on their ability to induce differential TCR internalization. Our results demonstrate that there is a direct correlation: peptides that induce strong TCR down-regulation are most efficient at mediating negative selection, whereas peptides that induce suboptimal TCR internalization are more efficient at triggering positive selection. As a consequence of suboptimal TCR internalization, a proportion of TCR complexes that remain on the cell surface may be able to relay continual signals required for survival and differentiation. In addition, we show that the magnitude of Ca2+ influx set by these peptides reflects the hierarchy of TCR down-regulation and correlates with positive vs negative selection of transgenic thymocytes. Together, our data suggest that T cell selection is mediated by differing intensities of the same TCR-mediated signal, rather than by distinct signals.

Fyn and ZAP-70 are required for Vav phosphorylation in T cells stimulated by antigen-presenting cells

In T cells, triggering of the T cell antigen receptor or of the co-stimulatory receptor CD28 can direct tyrosine phosphorylation of the signaling protein Vav. We investigated the role played by the protein tyrosine kinases Fyn, Lck, and ZAP-70 in these processes in a T cell hybridoma after physiological stimulation of the T cell receptor (TCR) and CD28. A dominant-negative mutant approach based on overexpression of catalytically inactive alleles of these kinases showed that CD28-induced Vav phosphorylation preferentially requires Fyn, whereas ZAP-70 had no role. Consistently, Vav was strongly phosphorylated in Lck-deficient JCAM-1 cells after CD28 ligation. In contrast, ZAP-70 appeared to control TCR-directed Vav phosphorylation. However, overexpression of ZAP-70 carrying a mutated Tyr315, contained within a motif previously suggested to be a Vav Src homology 2 domain binding site, had little or no effect. Immunoprecipitation assays showed that phosphorylated Vav associated with Fyn after CD28 triggering and that this interaction, likely to involve binding of Fyn Src homology 2 domain to Vav, was more strongly detectable after concomitant CD28 and TCR stimulation. These data suggest that Fyn plays a major role in controlling Vav phosphorylation upon T cell activation and that the mechanism implicating ZAP-70 in this process may be more complex than previously anticipated.

Regulation of PAK activation and the T cell cytoskeleton by the linker protein SLP-76

Tyrosine phosphorylation of linker proteins enables the T cell antigen receptor (TCR)-associated protein tyrosine kinases to phosphorylate and regulate effector molecules that generate second messengers. We demonstrate here that the SLP-76 linker protein interacts with both nck, an adaptor protein, and Vav, a guanine nucleotide exchange factor for Rho-family GTPases. The assembly of this tri-molecular complex permits the activated Rho-family GTPases to regulate target effectors that interact through nck. In turn, assembly of this complex mediates the enzymatic activation of the p21-activated protein kinase 1 and facilitates actin polymerization. Hence, phosphorylation of linker proteins not only bridges the TCR-associated PTK, ZAP-70, with downstream effector proteins, but also provides a scaffold to integrate distinct signaling complexes to regulate T cell function.

LAT is required for TCR-mediated activation of PLCgamma1 and the Ras pathway

In this study, we present the further characterization of a mutant Jurkat T cell line, J.CaM2, that is defective in TCR-mediated signal transduction. Although initial TCR-mediated signaling events such as the inducible tyrosine phosphorylation of the TCR-zeta chain and ZAP-70 are intact in J.CaM2, subsequent events, including increases in intracellular calcium, Ras activation, and IL-2 gene expression are defective. Subsequent analysis of J.CaM2 demonstrated a severe deficiency in pp36/LAT expression, a recently cloned adaptor protein implicated in TCR signaling. Importantly, reexpression of LAT in J.CaM2 restored all aspects of TCR signaling. These results demonstrate a necessary and exclusive role for LAT in T cell activation.

Thymocyte selection: not by TCR alone

During development of T cells in the thymus, T-cell receptor (TCR)-mediated recognition of self-MHC/self-peptide complexes on thymic stroma dictates the developmental fate of immature CD4+CD8+ (double positive) thymocytes. Intriguingly, TCR-generated intracellular signals can elicit two entirely different cellular responses in such thymocytes: apoptosis or further differentiation. The critical issue in understanding end-stage T-cell development is how TCR occupancy can be perceived in such markedly different ways by the TCR. Here, we review the cytoplasmic and nuclear events that result from TCR signaling during thymocyte selection. Studies aimed at distinguishing molecular components involved in positive selection (resulting in signals for further differentiation) and negative selection (resulting in apoptosis) will help solve this fascinating feature of T-lymphocyte biology. We also discuss how non-TCR-derived signaling might serve to fine tune the TCR-driven selection events in thymocytes. Central to this aspect of the conceptual framework needed to explain thymocyte selection is the observation that thymic antigen-presenting cells appear to be specialized in the induction of either positive or negative selection. Finally, we suggest a hypothesis that integrates the facts currently available on developing thymocytes, and which may serve to refine our exploration of unresolved issues in thymocyte selection. This hypothesis expands our focus to include signals from receptors other than TCRs as modulating and amplifying factors in thymocyte signaling.

Thymocyte selection in Vav and IRF-1 gene-deficient mice

T cells undergo a defined program of phenotypic and genetic changes during differentiation within the thymus. These changes define commitment of T-cell receptor (TCR) gamma delta and TCR alpha beta cells and lineage differentiation into CD4+ T helper and CD8+ cytotoxic T cells. T-cell differentiation and selection in the thymus constitute a tightly co-ordinated multistep journey through a network that can be envisaged as a three-dimensional informational highway made up of stromal cells and extracellular matrix molecules. This intrathymic journey is controlled by information exchange, with thymocytes depending on two-way cellular interactions with thymic stromal cells in order to receive essential signals for maturation and selection. Genetic inactivation of surface receptors, signal transduction molecules, and transcription factors using homologous recombination has provided novel insight into the signaling cascades that relay surface receptor engagement to gene transcription and subsequent progression of the developmental program. In this review we discuss molecular mechanisms of T lymphocyte development in mice that harbour genetic mutations in the guanine nucleotide exchange factor Vav and the interferon regulatory transcription factor 1 (IRF-1). We also propose a novel model of T-cell selection based on TCR alpha chain-directed signals for allelic exclusion and TCR alpha-based selection for single receptor usage.

A Nck-Pak1 signaling module is required for T-cell receptor-mediated activation of NFAT, but not of JNK

The T-cell antigen receptor (TCR) triggers a signaling cascade initiated by the tyrosine kinase Lck and requiring the proto-oncogene p95(vav). Vav is activated by Lck and can function as a guanine nucleotide exchange factor for the Rho-family GTPases, Rac1 and Cdc42. To investigate the involvement of these GTPases in TCR signaling, we focused on their well characterized effector, Pak1. This serine/threonine kinase is activated by GTP-bound Rac1 or Cdc42. However, its role in mediating downstream signaling events is controversial. We observed rapid, TCR-dependent activation of Pak1 and TCR-inducible association of Pak1 with Nck, which was tyrosine phosphorylated following stimulation. Pak1 activation occurred independently of Ras activation or calcium flux, but was dependent on the Lck tyrosine kinase, and was downstream of Vav and Cdc42. Dominant negative Pak1 or Nck specifically inhibited TCR-mediated activation of the nuclear factor of activated T cells (NFAT) transcription factor. TCR-mediated activation of Erk2 was also inhibited by dominant negative Pak. However, Pak1 activation was neither necessary nor sufficient for TCR-dependent c-Jun N-terminal kinase (JNK) activation. Therefore, Pak1 acts downstream of Vav and is required for activation of Erk2 and NFAT by a JNK-independent pathway. This is the first demonstration of a requirement for Pak to mediate the regulation of gene expression by an extracellular ligand.

The transcription factor NFAT4 is involved in the generation and survival of T cells

Nuclear factor of activated T cells (NFAT) is a family of four related transcription factors implicated in cytokine and early response gene expression in activated lymphocytes. Here we report that NFAT4, in contrast to NFATp and NFATc, is preferentially expressed in DP thymocytes. Mice lacking NFAT4 have impaired development of CD4 and CD8 SP thymocytes and peripheral T cells as well as hyperactivation of peripheral T cells. The thymic defect is characterized by increased apoptosis of DP thymocytes. The increased apoptosis and hyperactivation may reflect heightened sensitivity to TcR-mediated signaling. Further, mice lacking NFAT4 have impaired production of Bcl-2 mRNA and protein. NFAT4 thus plays an important role in the successful generation and survival of T cells.

Lymphocyte signalling: a coordinating role for Vav?

Vav1 is a guanine nucleotide exchange factor that selectively activates the Rac1 GTPase and is expressed specifically in haematopoietic cells. Recent work has revealed how Vav1 integrates signals from lymphocyte antigen receptors and costimulatory receptors to control immune function.

Wiskott-Aldrich syndrome protein-deficient mice reveal a role for WASP in T but not B cell activation

The Wiskott-Aldrich syndrome (WAS) is a human X-linked immunodeficiency resulting from mutations in a gene (WASP) encoding a cytoplasmic protein implicated in regulating the actin cytoskeleton. To elucidate WASP function, we disrupted the WASP gene in mice by gene-targeted mutation. WASP-deficient mice showed apparently normal lymphocyte development, normal serum immunoglobulin levels, and the capacity to respond to both T-dependent and T-independent type II antigens. However, these mice did have decreased peripheral blood lymphocyte and platelet numbers and developed chronic colitis. Moreover, purified WASP-deficient T cells showed markedly impaired proliferation and antigen receptor cap formation in response to anti-CD3epsilon stimulation. Yet, purified WASP-deficient B cells showed normal responses to anti-Ig stimulation. We discuss the implications of our findings regarding WASP function in receptor signaling and cytoskeletal reorganization in T and B cells and compare the effects of WASP deficiency in mice and humans.

Vav is a regulator of cytoskeletal reorganization mediated by the T-cell receptor

BACKGROUND

Vav is a guanine-nucleotide exchange factor for the Rho-like small GTPases RhoA, Rac1 and Cdc42, which regulate cytoskeletal reorganization and activation of stress-activated protein kinases (SAPK/JNKs). Vav is expressed in hematopoietic cells and is phosphorylated in T and B cells following activation of various growth factor or antigen receptors. Vav interacts with several signaling molecules in T cells, but the functional relevance of these interactions is established only for Slp76: they cooperate to induce activity of the transcription factor NF-AT and interleukin-2 expression. We have investigated the role of Vav in T cells by generating vav-/- mice.

RESULTS

Mice deficient for vav were viable and healthy, but had impaired T-cell development. In vav-/- T cells, in response to activation of the T-cell receptor (TCR), cell cycle progression, induction of NF-ATc1 activity, downregulation of the cell-cycle inhibitor p27Kip1, interleukin-2 production, actin polymerization and the clustering of TCRs into patches and caps--a cytoskeletal reorganization process--were defective. TCR-mediated activation of mitogen-activated protein kinase and SAPK/JNK was unaffected. Ca2+ mobilization was impaired in vav-/- thymocytes and T cells. In wild-type cells, Vav constitutively associated with the cytoskeletal membrane anchors talin and vinculin. In the absence of Vav, phosphorylation of Slp76, Slp76-talin interactions, and recruitment of the actin cytoskeleton to the CD3 zeta chain of the TCR co-receptor were impaired.

CONCLUSIONS

Vav is a crucial regulator of TCR-mediated Ca2+ flux, cytoskeletal reorganization and TCR clustering, and these are required for T-cell maturation, interleukin-2 production and cell cycle progression.