A functional T-cell receptor signaling pathway is required for p95vav activity

A model system for activation-induced alternative splicing of CD45 pre-mRNA in T cells implicates protein kinase C and Ras

Multiple isoforms of the protein tyrosine phosphatase CD45 are expressed on the surface of human T cells. Interestingly, the expression of these isoforms has been shown to vary significantly upon T-cell activation. In this report, we describe a novel cell line-based model system in which we can mimic the activation-induced alternative splicing of CD45 observed in primary T cells. Of the many proximal signaling events induced by T-cell stimulation, we show that activation of protein kinase C and activation of Ras are important for the switch toward the exclusion of CD45 variable exons, whereas events related to Ca(2+) flux are not. In addition, the ability of cycloheximide to block the activation-induced alternative splicing of CD45 suggests a requirement for de novo protein synthesis. We further demonstrate that sequences which have previously been implicated in the tissue-specific regulation of CD45 variable exons are likewise necessary and sufficient for activation-induced splicing. These results provide an initial understanding of the requirements for CD45 alternative splicing upon T-cell activation, and they confirm the importance of this novel cell line in facilitating a more detailed analysis of the activation-induced regulation of CD45 than has been previously possible.

Age-sensitive and -insensitive pathways leading to JNK activation in mouse CD4(+) T-cells

The c-Jun N-terminal kinase (JNK) can be activated in T-cells either by the combination of TCR and CD28 costimulation or by a variety of stress-related stimuli including UV light, H(2)O(2), and hyperosmolar sorbitol solutions. In T-lymphocytes, TCR/CD28 stimulation of JNK leads to induction of new gene expression via c-Jun, ATF-2, and Elk-1. Phosphorylation of c-Jun in CD4(+) T-cells stimulated by CD3/CD4/CD28 cross-linking declines with age, due to diminished activation of JNK. Here we show that the age-related decline in TCR/CD28 activation of JNK reflects two effects of age: the accumulation of memory cells (in which JNK stimulation is poor regardless of donor age) and age-dependent declines in JNK activation within the naive subset. Cyclosporin A inhibits induction of JNK function by TCR/CD28, PMA/ionomycin, ceramide, or H(2)O(2), but not induction by UV light or hyperosmolar sorbitol. Although aging impairs JNK induction by UV light, it has no effect on JNK activation by ceramide, H(2)O(2), or sorbitol. The data as a whole indicate that there are at least four pathways that activate JNK in CD4(+) T-cells, of which two are age-sensitive and two others unaffected by aging. Two of the pathways (UV and hyperosmolar sorbitol) are insensitive to cyclosporin inhibition. Finally, we show that the alterations in JNK function are not due to changes in the expression of MKK4, an upstream activator of JNK, and that another JNK kinase, MKK7, is not expressed in splenic T-cells.

T cell receptor-mediated signs and signals governing T cell development

The developmental fate of T cells is largely controlled by the nature and success of signals mediated by the pre-T cell receptor (TCR) and TCR complexes. These intracellular signals are regulated by cascades of protein tyrosine phosphorylations initiated following ligand binding to the pre-TCR or TCR complexes. The phosphorylation cascades are primarily orchestrated by two distinct families of protein tyrosine kinases (PTKs), the Src- and the Syk/ZAP-70-families. Germline gene targeting experiments, several human immunodeficiencies, and somatic cell mutants have all contributed to our understanding of how these families of kinases coordinate their actions to promote signaling. Upon activation, the PTKs transmit their signals to a number of newly described adaptor proteins including LAT, SLP-76, and vav, among others. The following review combines results derived from different experimental strategies to examine the contributions of the PTKs and the adaptor molecules to pre-TCR and TCR signaling processes.

TCR and CD28 Are Coupled Via ZAP-70 to the Activation of the Vav/Rac-1-/PAK-1/p38 MAPK Signaling Pathway

CD28 costimulation amplifies TCR-dependent signaling in activated T cells, however, the biochemical mechanism(s) by which this occurs is not precisely understood. The small GTPase Rac-1 controls the catalytic activity of the mitogen-activated protein kinases (MAPKs) and cell cycle progression through G1. Rac-1 activation requires the phospho-tyrosine (p-Tyr)-dependent recruitment of the Vav GDP releasing factor (GRF) to the plasma membrane and assembly of GTPase/GRF complexes, an event critical for Ag receptor-triggered T cell activation. Here, we show that TCR/CD28 costimulation synergistically induces Rac-1 GDP/GTP exchange. Our findings, obtained by using ZAP-70-negative Jurkat T cells, indicate that CD28 costimulation augments TCR-mediated T cell activation by increasing the ZAP-70-mediated Tyr phosphorylation of Vav. This event regulates the Rac-1-associated GTP/GDP exchange activity of Vav and downstream pathway(s) leading to PAK-1 and p38 MAPK activation. CD28 amplifies TCR-induced ZAP-70 activity and association of Vav with ZAP-70 and linker for activation of T cells (LAT). These results favor a model in which ZAP-70 regulates the intersection of the TCR and CD28 signaling pathways, which elicits the coupling of TCR and CD28 to the Rac-1, PAK-1, and p38 MAPK effector molecules.

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.

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.

Apoptosis of thymic lymphoma clones by thymic epithelial cells: a putative model for 'death by neglect'

We have previously described an in vitro system in which thymic epithelial cells induce apoptosis in CD4+ 8+ thymocytes or thymic lymphoma cells, in the absence of an exogenous antigen. A thymic epithelial cell line (TEC) recapitulated the response, by inducing apoptosis in CD4+ 8+ thymocytes of the thymic lymphoma clone, PD1.6. The present study pursues the involvement of the T-cell receptor (TcR) in the response of PD1.6 to TEC. TcR cross-linking did not cause apoptosis of PD1.6, although it induced tyrosine phosphorylation of p95vav. In contrast, TEC did not induce phosphorylation of p95vav but induced apoptosis of PD1.6 cells. These results suggest that TcR-evoked signals are not involved in TEC-induced apoptosis of PD1.6. Intracellular calcium chelation, using BAPTA-loaded PD1.6 cells, diminished TEC-induced apoptosis. Protein kinase C depletion in PD1.6 cells augmented their apoptotic response to TEC. Thus, the response of PD1.6 to TEC is calcium-dependent and inhibited by PKC. Likewise, the apoptotic response of PD1.6 to A23187 was abrogated by PKC activation. PD1.6 cells may represent an immature double positive thymocyte population, which does not undergo negative selection. The interaction of PD1.6 with TEC may thus serve as a model for the TcR-independent 'Death by Neglect', which takes place in the thymus during thymocyte development.

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.

Activation of Vav and Ras through the nerve growth factor and B cell receptors by different kinases

Engagement of the B-cell antigen receptor (BCR) or the nerve growth factor receptor (NGFR/TrkA) induces activation of multiple tyrosine kinases, resulting in phosphorylation of numerous intracellular substrates. We show that addition of NGF or anti-IgM antibody leads to the early tyrosine phosphorylation of p95(vav), which is expressed exclusively in hematopoietic cells; NGF, similar to crosslinking the BCR, also results in the rapid activation of Ras. The phosphorylation of Vav and activation of Ras triggered by NGF is mediated through Trk tyrosine kinase, whereas signaling through the BCR uses a different tyrosine kinase. We also show that NGF induces tyrosine phosphorylation of Shc and its association with Grb2. Vav and Ras with the adaptor proteins Shc and Grb2 appear to serve as a link between different receptor-mediated signaling pathways and, in human B cells, may play an important regulatory role in neuroimmune interactions.

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.

Interdomain B in ZAP-70 regulates but is not required for ZAP-70 signaling function in lymphocytes

The protein tyrosine kinase ZAP-70 plays an important role in T-cell activation and development. After T-cell receptor stimulation, ZAP-70 associates with the receptor and is phosphorylated on many tyrosines, including Y292, Y315, and Y319 within interdomain B. Previously, we demonstrated that Y292 negatively regulates ZAP-70 function and that Y315 positively regulates ZAP-70 function by interacting with Vav. Recent studies have suggested that Y319 also positively regulate ZAP-70 function. Paradoxically, removal of interdomain B (to create the construct designated Delta), containing the Y292, Y315, and Y319 sites, did not eliminate the ability of ZAP-70 to induce multiple gene reporters in Syk-deficient DT-40 B cells and ZAP-70/Syk-deficient Jurkat cells. Here we show that Delta still utilizes the same pathways as wild-type ZAP-70 to mediate NF-AT induction. This is manifested by the ability of Delta to restore induction of calcium fluxes and mitogen-activated protein kinase activation and by the ability of dominant negative Ras and FK506 to block the induction of NF-AT activity mediated by Delta. Biochemically we show that the stimulated tyrosine phosphorylation of Vav, Shc, and ZAP-70 itself is diminished, whereas that of Slp-76 is increased in cells reconstituted with Delta. Deletion of interdomain B did not affect the ability of ZAP-70 to bind to the receptor. The in vitro kinase activity of ZAP-70 lacking interdomain B was markedly reduced, but the kinase activity was still required for the protein's in vivo activity. Based on these data, we concluded that interdomain B regulates but is not required for ZAP-70 signaling function leading to cellular responses.

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.

Cutting Edge: Negative Regulation of Human T Cell Activation by the Receptor-Type Protein Tyrosine Phosphatase CD148

T cell activation represents a balance between positive and negative signals delivered via distinct cell surface molecules. Many cytoplasmic protein tyrosine phosphatases are involved in regulating cellular responses by antagonizing the action of protein tyrosine kinases. CD148 is a receptor-type protein tyrosine phosphatase expressed by all human mononuclear cells. We have investigated the effect of CD148 on TCR-mediated activation of human T cells. Overexpression of wild-type, but not a phosphatase-deficient, CD148 in Jurkat T cells inhibited TCR-mediated activation, evidenced by reduced expression of the early activation Ag CD69, inhibition of tyrosine phosphorylation of many intracellular proteins including the critical protein tyrosine kinase ZAP-70, and impairment of mitogen-activated protein kinase activation. Taken together, these results suggest that CD148 is an important phosphatase involved in negatively regulating the proximal signaling events during activation of Ag-specific T cells.

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.

Differential roles of ERK and p38 MAP kinase pathways in positive and negative selection of T lymphocytes

Clonal selection of T lymphocytes is essential for establishing self/non-self discrimination of immune recognition. It is known that cell surface signals such as avidity and valency of TCR-ligand interactions influence the fate of individual thymocytes, founding a primary repertoire of T cells. However, intracellular signals that govern positive and negative selection in the thymus have been unclear. The present study using the retroviral gene transfer technique shows that MKK1 activation in developing T cells is sufficient for providing positive selection signals. We also show that the MKK6-p38 signaling pathway is critically involved in inducing negative selection of thymocytes. These results suggest that intracellular signals through different MAP kinase cascades selectively guide positive and negative selection of T lymphocytes.

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.

Role of the cyclosporin-sensitive transcription factor NFAT1 in the allergic response

Proteins belonging to the NFAT (nuclear factor of activated T cells) family of transcription factors are expressed in most immune cell types, and play a central role in the transcription of cytokine genes, such as IL-2, IL-4, IL-5, IL-13, IFN-gamma, TNF-alpha, and GM-CSF. The activity NFAT proteins is regulated by the calcium/calmodulin-dependent phosphatase calcineurin, a target for inhibition by CsA and FK506. Recently, two different groups have described that mice lacking the NFAT1 transcription factor show an enhanced immune response, with tendency towards the development of a late Th2-like response. This review evaluates the possible role of NFAT proteins in the Th2 immune response and in the eosinophil-mediated allergic response.

The Vav-Rac1 pathway in cytotoxic lymphocytes regulates the generation of cell-mediated killing

The Rac1 guanine nucleotide exchange factor, Vav, is activated in hematopoietic cells in response to a large variety of stimuli. The downstream signaling events derived from Vav have been primarily characterized as leading to transcription or transformation. However, we report here that Vav and Rac1 in natural killer (NK) cells regulate the development of cell-mediated killing. There is a rapid increase in Vav tyrosine phosphorylation during the development of antibody-dependent cellular cytotoxicity and natural killing. In addition, overexpression of Vav, but not of a mutant lacking exchange factor activity, enhances both forms of killing by NK cells. Furthermore, dominant-negative Rac1 inhibits the development of NK cell-mediated cytotoxicity by two mechanisms: (a) conjugate formation between NK cells and target cells is decreased; and (b) those NK cells that do form conjugates have decreased ability to polarize their granules toward the target cell. Therefore, our results suggest that in addition to participating in the regulation of transcription, Vav and Rac1 are pivotal regulators of adhesion, granule exocytosis, and cellular cytotoxicity.

Histamine induces nuclear factor of activated T cell-mediated transcription and cyclosporin A-sensitive interleukin-8 mRNA expression in human umbilical vein endothelial cells

The nuclear factor of activated T cells (NFAT) mediates a cyclosporin A (CsA)- and FK506-suppressible transcriptional program in lymphocytes after antigen-stimulated phospholipase C activation. Nonlymphoid cells also express NFAT isoforms, raising the possibility that these isoforms can be regulated by other extracellular stimuli. This study sought to determine whether histamine can trigger NFAT-mediated transcription in human umbilical vein endothelial cells (HUVEC), using a retrovirus-based luciferase reporter driven by a well characterized, NFAT-specific enhancer. Luciferase levels are induced up to 60-fold over basal levels after costimulation of HUVEC with Ca2+-mobilizing drugs and a phorbol ester, a response that is 20-fold greater than that observed when HUVEC are stimulated with either drug alone. These synergistic responses are inhibited in cells treated with CsA. CsA and FK506 also inhibit the luciferase response to histamine, indicating that histamine can induce NFAT-mediated transcription in HUVEC. To identify candidate genes in HUVEC that might be regulated by NFAT, the expression of several chemokine mRNAs was measured after histamine treatment. Of the mRNAs tested, only those encoding monocyte chemotactic protein-1 (approximately 2-fold over basal) and interleukin-8 (approximately 6-fold over basal) are induced by histamine; both of these responses are suppressed by CsA and FK506. The H1 histamine receptor antagonist chlorpheniramine, but not the H2 receptor antagonist ranitidine, blocks the effects of histamine in this preparation. These data provide the first evidence for a physiological inducer of NFAT-mediated transcription in endothelial cells and support the hypothesis that NFAT participates in H1 histamine receptor-induced interleukin-8 gene expression.

The cyclosporin A-sensitive nuclear factor of activated T cells (NFAT) proteins are expressed in vascular smooth muscle cells. Differential localization of NFAT isoforms and induction of NFAT-mediated transcription by phospholipase C-coupled cell surface receptors

Expression of the antigen-regulated, cyclosporin A-sensitive nuclear factor of activated T cells (NFAT) is not restricted to lymphoid cells, as thought initially, but the physiological inducers of NFAT-mediated transcription in non-lymphoid cells are unknown. Here, cultured vascular smooth muscle cells (VSMC) are shown to express two isoforms of the NFAT family endogenously, which are localized differentially in cells under resting conditions. Using a retroviral NFAT-specific luciferase reporter, we show that VSMC support previously unrecognized complexities in NFAT-mediated transcription, including evidence for negative regulation by Ca2+ signaling and positive regulation through co-activation of adenylyl cyclase and Ca2+ mobilization. The VSMC mitogen platelet derived growth factor-BB (PDGF-BB) induces NFAT-mediated transcription in VSMC. Thrombin and angiotensin II, which activate Galphaq-coupled receptors, are significantly weaker inducers of NFAT-mediated luciferase expression than is PDGF-BB. However, co-stimulation studies show that Galphaq receptor agonists augment the NFAT-mediated transcriptional response to PDGF-BB. This synergy can be explained in part by augmented intracellular Ca2+ transients elicited by multiple agonist challenges. These data indicate that agonists for phospholipase C-coupled receptors stimulate NFAT-mediated transcription in VSMC differentially, and that NFAT can function to integrate co-activating signals in the extracellular environment.

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

BACKGROUND

Antigen-receptor interactions on lymphocytes result in local clustering of actin, receptors and signaling molecules into an asymmetric membrane structure termed a cap. Although actin polymerization is known to be required, the mechanisms underlying cap formation are unclear. We have studied the events underlying cap formation using mice bearing a null mutation in vav (vav-/-), a gene that encodes a guanine-nucleotide exchange factor for the GTPase Rac.

RESULTS

Lymphocytes from vav-/- mice failed to form T-cell receptor caps following activation and had a defective actin cytoskeleton. The vav-/- T cells were deficient in interleukin-2 (IL-2) production and proliferation, and the peak of Ca2+ mobilization was reduced although of normal duration. Activation of Jun N-terminal kinase or stress-activated kinase (JNK or SAPK) and mitogen-activated protein kinase (MAPK) and the induction of the transcription factor NF-ATc1 and egr-1 genes was normal. Despite the reduced Ca2+ mobilization, translocation of cytoplasmic NF-ATc to the nucleus was normal, reflecting that the lower levels of Ca2+ in vav-/- cells were still sufficient to activate calcineurin. Treatment of lymphocytes with cytochalasin D, which blocks actin polymerization, inhibited cap formation and produced defects in signaling and IL-2 transcriptional induction in response to antigen-receptor signaling that were nearly identical to those seen in vav-/- cells. In transfection studies, either constitutively active Vav or Rac could complement constitutively active calcineurin to activate NF-AT-dependent transcription.

CONCLUSIONS

These results indicate that Vav is required for cap formation in lymphocytes. Furthermore, the correlation between cap formation, IL-2 production and proliferation supports the hypothesis that an actin-dependent pathway is a source of specialized growth regulatory signals.

Signaling through CD5 activates a pathway involving phosphatidylinositol 3-kinase, Vav, and Rac1 in human mature T lymphocytes

CD5 acts as a coreceptor on T lymphocytes and plays an important role in T-cell signaling and T-cell-B-cell interactions. Costimulation of T lymphocytes with anti-CD5 antibodies results in an increase of the intracellular Ca2+ levels, and subsequently in the activation of Ca2+/calmodulin-dependent (CaM) kinase type IV. In the present study, we have characterized the initial signaling pathway induced by anti-CD5 costimulation. The activation of phosphatidylinositol (PI) 3-kinase through tyrosine phosphorylation of its p85 subunit is a proximal event in the CD5-signaling pathway and leads to the activation of the lipid kinase activity of the p110 subunit. The PI 3-kinase inhibitors wortmannin and LY294002 inhibit the CD5-induced response as assessed in interleukin-2 (IL-2) secretion experiments. The expression of an inactivated Rac1 mutant (Rac1.N17) in T lymphocytes transfected with an IL-2 promoter-driven reporter construct also abrogates the response to CD5 costimulation, while the expression of a constitutively active Rac1 mutant (Rac1-V12) completely replaces the CD5 costimulatory signal. The Rac1-specific guanine nucleotide exchange factor Vav is heavily phosphorylated on tyrosine residues upon CD5 costimulation, which is a prerequisite for its activation. A role for Vav in the CD5-induced signaling pathway is further supported by the findings that the expression of a dominant negative Vav mutant (Vav-C) completely abolishes the response to CD5 costimulation while the expression of a constitutively active Vav mutant [Vav(delta1-65)] makes the CD5 costimulation signal superfluous. Wortmannin is unable to block the Vav(delta1-65)- or Rac1.V12-induced signals, indicating that both Vav and Rac1 function downstream from PI 3-kinase. Vav and Rac1 both act upstream from the CD5-induced activation of CaM kinase IV, since KN-62, an inhibitor of CaM kinases, and a dominant negative CaM kinase IV mutant block the Vav(delta1-65)-and Rac1.V12-mediated signals. We propose a model for the CD5-induced signaling pathway in which the PI 3-kinase lipid products, together with tyrosine phosphorylation, activate Vav, resulting in the activation of Rac1 by the Vav-mediated exchange of GDP for GTP.

Immunoreceptor DAP12 bearing a tyrosine-based activation motif is involved in activating NK cells

Natural killer (NK) cells express cell-surface receptors of the immunoglobulin and C-type lectin superfamilies that recognize major histocompatibility complex (MHC) class I peptides and inhibit NK-cell-mediated cytotoxicity. These inhibitory receptors possess ITIM sequences (for immunoreceptor tyrosine-based inhibitory motifs) in their cytoplasmic domains that recruit SH2-domain-containing protein tyrosine phosphatases, resulting in inactivation of NK cells. Certain isoforms of these NK-cell receptors lack ITIM sequences and it has been proposed that these 'non-inhibitory' receptors may activate, rather than inhibit, NK cells. Here we show that DAP12, a disulphide-bonded homodimer containing an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain, non-covalently associates with membrane glycoproteins of the killer-cell inhibitory receptor (KIR) family without an ITIM in their cytoplasmic domain. Crosslinking of KIR-DAP12 complexes results in cellular activation, as demonstrated by tyrosine phosphorylation of cellular proteins and upregulation of early-activation antigens. Phosphorylated DAP12 peptides bind ZAP-70 and Syk protein tyrosine kinases, suggesting that the activation pathway is similar to that of the T- and B-cell antigen receptors.

Activation of the T-cell receptor signaling pathway by Nef from an aggressive strain of simian immunodeficiency virus

The Nef from a highly virulent strain of simian immunodeficiency virus (SIV), SIVpbj14, and a Nef from the traditional strain SIVmac239 bearing the mutation from RQ to YE (YE-Nef) both induce an acute lethal disease in monkeys. The YE mutation and its surrounding sequence resemble the immunoreceptor tyrosine-based activation motif (ITAM), which is present in the cytoplasmic tail of T- and B-cell antigen receptors and mediates signaling during lymphocyte activation. We show here that the ITAM from YE-Nef performs the same function. First, not only does YE-Nef increase the activity of the transcription factor NFAT, which is one of the downstream targets of T-cell activation, but the ITAM from the YE-Nef by itself also activates NFAT. Second, the ITAM from YE-Nef is phosphorylated on tyrosine residues by Lck and associates with ZAP-70, a T-cell-specific tyrosine kinase. The phosphorylation of both conserved tyrosine residues on the ITAM is required for the recruitment of ZAP-70. Finally, Lck is required for the activation of NFAT by YE-Nef. These results demonstrate that YE-Nef contains a functional ITAM and elucidate the molecular mechanisms underlying the pathogenesis of SIVpbj14.

A requirement for the Rho-family GTP exchange factor Vav in positive and negative selection of thymocytes

The T cell repertoire is shaped by positive and negative selection of thymocytes that express low levels of T cell receptor (TCR) and both CD4 and CD8. TCR-mediated signals that determine these selection processes are only partly understood. Vav, a GDP-GTP exchange factor for Rho-family proteins, is tyrosine phosphorylated following TCR stimulation, suggesting that it may transduce TCR signals. We now demonstrate that mice lacking Vav are viable and display a profound defect in the positive selection of both class I- and class II-restricted T cells. In contrast, Vav is not essential for negative selection, though in its absence negative selection is much less effective. Vav may influence the efficiency of TCR-induced selection events by regulating the intracellular calcium flux of thymocytes.

The complexity of signaling pathways activated by the BCR

Cross-linking of the B cell antigen receptor (BCR) leads to the activation of three types of intracellular protein tyrosine kinases. These tyrosine kinases then phosphorylate signaling components to activate a variety of signaling reactions, including phosphatidylinositol 4,5-bisphosphate hydrolysis, Ras activation, and phosphatidylinositol 3-kinase activation. Each of these signaling reactions, and also the signaling molecules Vav and HS1, appears to be important for at least some of the many types of B cell responses to antigen. The complexity of BCR signaling reactions may be required to allow the B cell to respond in a number of distinct ways to antigen (proliferation, survival, apoptosis, maturational arrest, etc.) depending on the maturation state of the B cell, the location in the body, the physical nature of the antigen, and the possible presence of the antigen in complex with antibody or complement components.

The Vav binding site (Y315) in ZAP-70 is critical for antigen receptor-mediated signal transduction

Stimulation of antigen receptors in T and B cells leads to the activation of the Src and Syk families of protein tyrosine kinases (PTK). These PTKs subsequently phosphorylate numerous intracellular substrates, including the 95-kD protooncogene product Vav. Vav is essential for both T and B cell development and T and B cell antigen receptor-mediated signal transduction. After receptor ligation, Vav associates with phosphorylated Syk and ZAP-70 PTKs, an interaction that depends upon its SH2 domain. Here we demonstrate that a point mutation of tyrosine 315 (Y315F) in ZAP-70, a putative Vav SH2 domain binding site, eliminated the Vav- ZAP-70 interaction. Moreover, the Y315 mutation impaired the function of ZAP-70 in antigen receptor signaling. Surprisingly, this mutation also resulted in marked reduction in the tyrosine phosphorylation of ZAP-70, Vav, SLP-76, and Shc. These data demonstrate that the Vav binding site in ZAP-70 plays a critical role in antigen receptor-mediated signal transduction.

T cell antigen receptor signal transduction

The T cell antigen receptor (TCR) initiates signal transduction by activating multiple cytoplasmic protein tyrosine kinases (PTKs). Considerable progress in the field of TCR signal transduction has been made in three areas recently: first, in understanding the structure and function of the PTK ZAP-70; second, in the elucidation of the function of the substrates and pathways downstream of the PTKs; and third, in the identification of molecules that negatively regulate TCR signalling.

Phosphotyrosine-dependent activation of Rac-1 GDP/GTP exchange by the vav proto-oncogene product

The oncogenic protein Vav harbours a complex array of structural motifs, including leucine-rich, Dbl-homology, pleckstrin-homology, zinc-finger, SH2 and SH3 domains. Upon stimulation by antigens or mitogens, Vav becomes phosphorylated on key tyrosine residues and associates with other signalling proteins, including the mitogen receptors Zap-70 (ref. 6), Vap-1 (ref. 5) and Slp-76 (ref. 7). Disruption of the vav locus by homologous recombination causes severe defects in signalling by primary antigen receptors, leading to abnormal lymphocyte proliferation and lymphopenia. Despite the importance of Vav cell signalling, the function of this protein remains unknown. Here we show that tyrosine-phosphorylated Vav, but not the non-phosphorylated protein, catalyses GDP/GTP exchange on Rac-1, a protein implicated in cell proliferation and cytoskeletal organization, causing this GTPase to switch from its inactive to its active state. Transfection experiments also show that phosphorylation of Vav on tyrosine residues leads to nucleotide exchange on Rac-1 in vivo and stimulates c-Jun kinase, a downstream element in the signalling pathway involving this GTPase. Our results have identified a function for Vav and define a mechanism in which engaged membrane receptors activate its signalling pathway.

Transcription factors of the NFAT family: regulation and function

As targets for the immunosuppressive drugs cyclosporin A and FK506, transcription factors of the NFAT (nuclear factor of activated T cells) family have been the focus of much attention. NFAT proteins, which are expressed in most immune-system cells, play a pivotal role in the transcription of cytokine genes and other genes critical for the immune response. The activity of NFAT proteins is tightly regulated by the calcium/calmodulin-dependent phosphatase calcineurin, a primary target for inhibition by cyclosporin A and FK506. Calcineurin controls the translocation of NFAT proteins from the cytoplasm to the nucleus of activated cells by interacting with an N-terminal regulatory domain conserved in the NFAT family. The DNA-binding domains of NFAT proteins resemble those of Rel-family proteins, and Rel and NFAT proteins show some overlap in their ability to bind to certain regulatory elements in cytokine genes. NFAT is also notable for its ability to bind cooperatively with transcription factors of the AP-1 (Fos/Jun) family to composite NFAT:AP-1 sites, found in the regulatory regions of many genes that are inducibly transcribed by immune-system cells. This review discusses recent data on the diversity of the NFAT family of transcription factors, the regulation of NFAT proteins within cells, and the cooperation of NFAT proteins with other transcription factors to regulate the expression of inducible genes.

Structure and function of vav

The proto-oncogene vav is expressed solely in cells of hematopoietic origin regardless of their differentiation lineage. However, recently an homologue of vav, which is widely expressed (vav2) has been identified. Vav is a complicated and interesting molecule that contains a number of structural features found in proteins involved in cell signaling. Vav has a leucine-rich region, a leucine-zipper, a calponin homology domain, an acidic domain, a Dbl-homology domain, a pleckstrin homology domain, a cysteine-rich domain, two Src homology 3 domains, with a proline-rich region in the amino-SH3 domain, and finally an Src homology 2 domain. These domains have been implicated in protein protein interactions and strongly suggest that vav is involved in signaling events. vav is also rapidly and transiently tyrosine phosphorylated through the activation of multiple receptors on hematopoietic cells. Furthermore, vav is tyrosine phosphorylated upon the activation of several cytokines and growths factors. Recently, the generation of nice vav-/- showed that vav has an essential role in proliferation/activation of T and B cells. The purpose of this review is to summarize the current knowledge on vav and to evaluate the roles of vav in cellular functions.

Enhancement of lymphocyte responsiveness by a gain-of-function mutation of ZAP-70

The protein tyrosine kinase ZAP-70 plays an essential role in T-cell activation and development. After T-cell receptor stimulation, ZAP-70 is associated with the receptor and is phosphorylated on many tyrosine residues, including tyrosine 292 (Y-292), in the region between the C-terminal SH2 domain and the kinase domain (interdomain B). Here we show that a mutation of Y-292 (292F) or deletion of interdomain B enhanced the ability of ZAP-70 to reconstitute B-cell receptor stimulation-dependent NF-AT induction in a B-cell line deficient in Syk. In contrast, in a T-cell line, expression of 292F led to basal NF-AT induction independent of T-cell receptor stimulation. These results demonstrate that the role of Y-292 is to negatively regulate the function of ZAP-70 in lymphocytes. This appears to be a dominant function of interdomain B because deletion of most of interdomain B also resulted in a mutant of ZAP-70 with enhanced ability to reconstitute Syk-deficient DT-40 B cells. Since our biochemical studies did not reveal an effect of the 292F mutation on either the kinase activity of ZAP-70 or on the ability of ZAP-70 to bind to the receptor, we propose a model in which Y-292 interacts with an inhibitory protein to negatively regulate ZAP-70 function.

Association of a p95 Vav-containing signaling complex with the FcepsilonRI gamma chain in the RBL-2H3 mast cell line. Evidence for a constitutive in vivo association of Vav with Grb2, Raf-1, and ERK2 in an active complex

Aggregation of the high affinity receptor for IgE (FcepsilonRI) on the mucosal mast cell line, RBL-2H3, results in the rapid and persistent tyrosine phosphorylation of Vav. Immunoprecipitation of Vav from activated cells revealed co-immunoprecipitated phosphoproteins of molecular weights identical to the FcepsilonRI beta and gamma chains, and the former was reactive with antibody to the FcepsilonRI beta chain. Conversely, Western blots revealed the presence of p95 Vav in FcepsilonRI immunoprecipitates. The association of Vav and of Grb2 with the receptor was found to be regulated by aggregation of the receptor, and the interaction of Vav with the FcepsilonRI was localized to the gamma chain. To gain insight on the signaling pathway in which Vav participates, we investigated the in vivo associations of Vav with other molecules. A reducible chemical cross-linking agent was used to covalently maintain protein interactions under nonreducing conditions. A fraction of Vav increased in mass to form a complex of >300 kDa in molecular mass. Under reducing conditions the cross-linked Vav immunoprecipitates showed the presence of Grb2, Raf-1, and p42(mapk) (ERK2). In vitro kinase assays of Raf-1 activity associated with Vav revealed that this complex had an activity greater than that of Raf-1 derived from nonactivated cells, and aggregation of the FcepsilonRI did not modulate this activity. In contrast, aggregation of the FcepsilonRI increased the total Raf-1 activity by 2-5-fold. These results demonstrate that Vav associates constitutively with components of the mitogen-activated protein kinase pathway to form an active multimeric signaling complex whose in vivo activity and associations may be directed by aggregation of the FcepsilonRI. The findings of this study may also be relevant to other members of the immune recognition receptor family that share the T-cell antigen receptor zeta/gamma chains.

Differential regulation of activation-induced tyrosine phosphorylation and recruitment of SLP-76 to Vav by distinct isoforms of the CD45 protein-tyrosine phosphatase

The CD45 family of transmembrane protein-tyrosine phosphatases plays a critical role in T cell activation signaling by regulating the tyrosine phosphorylation of protein-tyrosine kinases and their substrates. Multiple alternatively spliced CD45 isoforms, differing only in their extracellular domains, are differentially expressed by subsets of T cells with distinct functional repertoires. However, the physiological function of the various isoforms remains elusive. Using a novel panel of Jurkat T cell clones that uniquely express either the smallest (CD45(0)) or the largest (CD45(ABC)) isoform, we previously demonstrated CD45 isoform-specific differences in interleukin-2 secretion and tyrosine phosphorylation of Vav. We now demonstrate differential activation-induced tyrosine phosphorylation of a 76-kDa Vav-associated protein (pp76) by cells expressing distinct CD45 isoforms. The tyrosine phosphorylation of Vav and associated pp76 follow parallel kinetics. pp76 interacts with the SH2 and SH3 domains of Vav. We have identified pp76 as SLP-76, a recently cloned Grb2-binding protein. After activation with anti-CD3, CD45(ABC) transfectants demonstrate increased tyrosine phosphorylation and physical association of SLP-76 with Vav compared to transfectants expressing CD45(0). These results establish a novel physical link between Vav and SLP-76 that is differentially regulated by CD45 isoform expression.

p95vav associates with tyrosine-phosphorylated SLP-76 in antigen-stimulated T cells

p95vav, the product of the vav protooncogene, has been implicated in the T cell receptor (TCR)-mediated signaling cascade p95vav is phosphorylated on tyrosine residues after TCR stimulation by anti-TCR/CD3 antibodies and possesses a number of landmark features of signaling molecules such as a putative guanine nucleotide exchange factor domain, a pleckstrin homology domain, and an Sre homology (SH) 2 and two SH3 domains, which provide the capacity to form multimeric signaling complexes. However, the precise role of p95vav in TCR signaling remains unclear. In this work we show that physiological stimulation of T cell hybridomas with antigen presented by major histocompatibility complex class II molecules leads to a strong tyrosine phosphorylation of p95vav and its association with tyrosine-phosphorylated SLP-76. SLP-76 is a newly described SH2-containing protein that has been previously found to bind to the adapter molecule Grb2. Moreover, we provide evidence that p95vav-SI P-76 association is SH2-mediated by demonstrating that this interaction can be inhibited by a phosphopeptide containing a putative p95vav-SH2-binding motif (pYESP) present in SLP-76. Furthermore, in vitro experiments show that after antigen stimulation, phosphorylated p95vav-SLP-76 can bind to Grb2 in a complex that contains pp36/38 and pp116 proteins. Our data provide a clue to explain recent independent observations that overexpression of p95vav or SLP-76 enhances TCR-mediated gene activation.

Phosphotyrosines in the killer cell inhibitory receptor motif of NKB1 are required for negative signaling and for association with protein tyrosine phosphatase 1C

NKB1 is one member of a growing family of killer cell inhibitory receptors (KIR). It is expressed on natural killer (NK) cells and T cells, and has been shown to inhibit cytolytic functions of these cells upon interacting with its ligand, HLA-B (Bw4). We demonstrate here that the cytoplasmic region of NKB1 is capable of inhibiting T cell activation in Jurkat cells. The tyrosine phosphorylation of the NKB1 KIR consensus motif, YxxL(x)26 YxxL, induces an association with the protein tyrosine phosphatase 1C (PTP1C). Importantly, mutation of both tyrosines in the motif abolished the inhibitory functions of NKB1 and abrogated PTP1C association. Mutational analysis of the individual tyrosines suggest that the membrane proximal tyrosine may play a crucial role in mediating the inhibitory signal. These results demonstrate that KIR can not only inhibit cytolytic activity, but can also negatively regulate T cell receptor activation events that lead to downstream gene activation, and further supports a model that implicates PTP1C as a mediator in the KIR inhibitory signal.

IQGAP1, a calmodulin-binding protein with a rasGAP-related domain, is a potential effector for cdc42Hs

Proteins that associate with the GTP-bound forms of the Ras superfamily of proteins are potential effector targets for these molecular switches. A 195 kDa protein was purified from cell lysates by affinity chromatography on immobilized cdc42Hs-GTP and a corresponding cDNA was isolated. Sequence analysis revealed localized identities to calponin, the WW domain, unconventional myosins and to the rasGAP-related domain (GRD) contained in IRA, NF-1, SAR1 and rasGAP. p195 was found to be identical to IQGAP1, a protein previously reported to bind ras. Purified recombinant p195/IQGAP1 bound to and inhibited the GTPase activity of cdc42Hs and rac whereas no interaction with ras was detected. The C-terminal half of IQGAP1 containing the GRD bound to cdc42 and rac in a GRD-dependent fashion, but a smaller fragment containing only the GRD did not. Cdc42 was also co-immunoprecipitated from cell lysates with antibody specific to p195/IQGAP1. Calmodulin also co-immunoprecipitated with p195/IQGAP1 and was found to associate with fragments containing the IQ domain. Expression of a cDNA fragment encoding the GRD inhibited the CDC24/CDC42 pathway in yeast, but no effect on ras was observed. In mammalian cells, both endogenous and ectopically expressed p195/IQGAP1 were localized to lamellipodia and ruffling cell membranes, where co-localization with actin was apparent. These results suggest that IQGAP1 is an effector target for cdc42Hs and may mediate the effects of this GTPase on cell morphology.

Vav and SLP-76 interact and functionally cooperate in IL-2 gene activation

T cell antigen receptor (TCR) stimulation induces tyrosine phosphorylation of many intracellular proteins, including the proto-oncogene Vav, which is expressed exclusively in hematopoietic and trophoblast cells. Vav is critical for lymphocyte development and activation. Overexpression of Vav in Jurkat T cells leads to potentiation of TCR-mediated IL-2 gene activation. However, the biochemical function of Vav is unknown. Here, we demonstrate that the major induced tyrosine phosphoprotein associated with Vav is the hematopoietic cell-specific SLP-76. The Vav SH2 domain is required for this interaction and for TCR-mediated Vav tyrosine phosphorylation. Similar to Vav, overexpression of SLP-76 markedly potentiates TCR-mediated NF-AT and IL-2 gene activation. Furthermore, overexpression of both Vav and SLP-76 synergistically induces basal and TCR-stimulated NF-AT activation. These results suggest that a signaling complex containing Vav and SLP-76 plays an important role in lymphocyte activation.

Regulation of antigen receptor signal transduction by protein tyrosine kinases

The past two years have seen further clarification of the early events occurring in antigen receptor signal transduction that are mediated by the immunoreceptor tyrosine-based activation motif (ITAM). The ITAM was shown to be a specific binding site for the ZAP-70/Syk protein tyrosine kinases and the structure of this complex was solved. In addition, possible mechanisms of activation and functions for these kinases were reported. Lastly, genetic studies established the critical importance of these kinases in antigen-receptor signaling and lymphocyte development.

Induction of NFAT-mediated transcription by Gq-coupled receptors in lymphoid and non-lymphoid cells

The nuclear factor of activated T cells (NFAT) was discovered as an inducible transcription factor activated by antigen stimulation of the T cell receptor in lymphocytes. Stimulation of NFAT-mediated transcription is now reported in both lymphoid and non-lymphoid cells following activation of a neurotransmitter receptor. Carbachol induces robust luciferase responses in Jurkat and pheochromocytoma PC12 cells expressing an NFAT-luciferase reporter construct and a Gq-coupled m3 muscarinic receptor. Cyclosporin blocks this response in PC12 cells, as in Jurkat cells. In PC12 cells expressing a Gi-coupled m2 muscarinic receptor, carbachol induces NFAT-mediated luciferase activity that is strictly dependent upon co-expression of a chimeric G alpha q/alpha i subunit, which confers Gq-effector coupling on Gi-linked receptors. These findings suggest that neurotransmitters, autacoids, or hormones acting on Gq-protein-coupled receptors may serve as physiological stimulators of NFAT in lymphoid and non-lymphoid cells.

Vav: function and regulation in hematopoietic cell signaling

Vav, a 95 kDa proto-oncogene product expressed specifically in hematopoietic cells, was originally isolated as a transforming human oncogene. Vav contains an array of functional domains that are involved in interactions with other proteins and, possibly, with lipids. These include, among others, a putative guanine nucleotide exchange domain, a cysteine-rich region similar to the phorbol ester/diacylglycerol-binding domain of protein kinase C, a pleckstrin-homology domain, and Src-homology 2 and 3 (SH2 and SH3, respectively) domains. The presence of these domains, the transforming activity of the vav oncogene, and the rapid increase in tyrosine phosphorylation of Vav induced by triggering of diverse receptors indicate that it plays an important role in hematopoietic cell signaling pathways. Such a role is supported by recent studies using "knockout" mice and transiently transfected T cells, in which Vav deletion or overexpression, respectively, had marked effects on lymphocyte development or activation. The presence of a putative guanine nucleotide exchange domain, the prototype of which is found in the dbl oncogene product, implies that Vav functions as a guanine nucleotide exchange factor (GEF) for one (or more) members of the Ras-like family of small GTP-binding proteins. In support of such a role, Vav preparations were found in some (but not other) studies to mediate in vitro-specific GEF activity for Ras. Additional studies are required to identify the physiological regulators and targets of Vav, and its exact role in hematopoietic cell development and signaling.

Sos, Vav, and C3G participate in B cell receptor-induced signaling pathways and differentially associate with Shc-Grb2, Crk, and Crk-L adaptors

B cell antigen receptor (BCR)-mediated signal transduction controls B cell proliferation and differentiation. The BCR activates Ras, presumably by the formation of a Shc-Grb2 adaptor complex, which recruits the Grb2-associated guanine nucleotide exchange factor Sos to the plasma membrane. In order to reveal additional BCR-induced signaling events involving the Grb2 adaptor, we undertook the isolation of Grb2-binding proteins. Using the yeast two-hybrid system and bacterial fusion proteins, Vav and C3G were identified as Grb2 binders. Vav is a putative nucleotide exchange factor and a target for BCR-induced tyrosine phosphorylation. C3G exerts nucleotide exchange activity on the Ras-related Rap1 protein. While Sos binds to both Grb2 Src homology-3 (SH3) domains, Vav was found to associate selectively with the carboxyl-terminal SH3 domain, while C3G bound selectively to the amino-terminal SH3 domain of bacterially expressed Grb2. Despite the association of Vav with Grb2 in vitro, we could not demonstrate an interaction between endogenous Vav and Grb2 molecules in primary B cells. Instead, Vav was found to inducibly associate with the Grb2-related adaptor protein Crk upon BCR stimulation. C3G did not bind to either Grb2, Shc, or Crk in vivo. Instead, C3G was found in association with the Crk-L adaptor, both before and after BCR stimulation. We show that Crk-L also participates in BCR signaling, since it inducibly interacts with tyrosine-phosphorylated Cbl. We conclude that, in addition to Sos, Vav and C3G play a role in BCR-mediated signal transduction. These guanine nucleotide exchange factors selectively associate with Grb2, Crk, and Crk-L, respectively, which may serve to direct them to different target molecules. Since Cbl binds to Grb2, Crk, as well as Crk-L, we hypothesize that Cbl may affect the function of all three exchangers.

Implication of the GRB2-associated phosphoprotein SLP-76 in T cell receptor-mediated interleukin 2 production

Recently we described the molecular cloning of SLP-76, a hematopoietic cell-specific 76-kD protein that was first identified through its association with GST/Grb2 fusion proteins. The primary sequence of SLP-76 predicts a protein of 533 amino acids comprising an amino-terminal region with numerous potential tyrosine phosphorylation sites, a central region rich in proline residues, and a single carboxy-terminal SH2 domain. Here we demonstrate formally that Grb2 associates with unphosphorylated SLP-76 and map the Grb2 binding site on SLP-76 undergoes rapid tyrosine phosphorylation and associates with tyrosine phosphoproteins of 36, 62, and 130 kD. In vitro experiments show that the SH2 domain of SLP-76 associates with the 62- and 130-kD proteins and additionally with a serine/threonine kinase. Finally, we demonstrate that transient overexpression of SLP-76 results in dramatically enhanced TCR-mediated induction of nuclear factor of activated T cells (NFAT) and interleukin (IL) 2 promoter activity; and we provide evidence that a functional SLP-76 SH2 domain is required for this effect. Our data document the in vivo associations of SLP-76 with several proteins that potentially participate in T cell activation and implicate SLP-76 itself as an important molecule in TCR-mediated IL-2 production.