ZAP-70 is essential for the T cell antigen receptor-induced plasma membrane targeting of SOS and Vav in T cells

The intrinsic substrate specificity of the human tyrosine kinome

Phosphorylation of proteins on tyrosine (Tyr) residues evolved in metazoan organisms as a mechanism of coordinating tissue growth1. Multicellular eukaryotes typically have more than 50 distinct protein Tyr kinases that catalyse the phosphorylation of thousands of Tyr residues throughout the proteome1-3. How a given Tyr kinase can phosphorylate a specific subset of proteins at unique Tyr sites is only partially understood4-7. Here we used combinatorial peptide arrays to profile the substrate sequence specificity of all human Tyr kinases. Globally, the Tyr kinases demonstrate considerable diversity in optimal patterns of residues surrounding the site of phosphorylation, revealing the functional organization of the human Tyr kinome by substrate motif preference. Using this information, Tyr kinases that are most compatible with phosphorylating any Tyr site can be identified. Analysis of mass spectrometry phosphoproteomic datasets using this compendium of kinase specificities accurately identifies specific Tyr kinases that are dysregulated in cells after stimulation with growth factors, treatment with anti-cancer drugs or expression of oncogenic variants. Furthermore, the topology of known Tyr signalling networks naturally emerged from a comparison of the sequence specificities of the Tyr kinases and the SH2 phosphotyrosine (pTyr)-binding domains. Finally we show that the intrinsic substrate specificity of Tyr kinases has remained fundamentally unchanged from worms to humans, suggesting that the fidelity between Tyr kinases and their protein substrate sequences has been maintained across hundreds of millions of years of evolution.

WZ3146 inhibits mast cell Lyn and Fyn to reduce IgE-mediated allergic responses in vitro and in vivo

Mast cells (MCs) play an important role as effector cells that cause allergic responses in allergic diseases. For these reasons, MC is considered an attractive therapeutic target for allergic disease treatment. In this study, we investigated the inhibitory effect of WZ3146, N-[3-[5-chloro-2-[4-(4-methylpiperazin-1-yl)anilino]pyrimidin-4-yl]oxyphenyl]prop-2-enamide, and the mechanisms of its actions on the MC activation and IgE-mediated allergic response by using three types of MCs such as rat basophilic leukemia (RBL)-2H3 cells, mouse bone marrow mast cells (BMMCs), and human Laboratory of Allergic Diseases 2 (LAD2) cells. WZ3146 inhibited antigen-stimulated degranulation in a dose-dependent manner (IC₅₀, ~ 0.35 μM for RBL-2H3 cells; ~ 0.39 μM for BMMCs; ~ 0.41 for LAD2 cells). WZ3146 also suppressed the production of histamine, tumor necrosis factor (TNF)-α and interleukin (IL)-6, which mediate various allergic responses, in a dose-dependent manner. As the mechanism of WZ3146 to inhibit MCs, it inhibited the activation of spleen tyrosine kinase (Syk) and the downstream signaling proteins of Syk such as linker for activation of T cell (LAT) and phospholipase (PL) Cγ1 in the signaling pathway of FcεRI. In addition, WZ3146 inhibited the activation of Akt, extracellular signal-regulated kinase (ERK)1/2, p38, and c-Jun N-terminal kinase (JNK). However, WZ3146 did not inhibit degranulation of MCs by thapsigargin or ionomycin, which increase calcium concentration in cytosol. Notably, WZ3146 inhibited the activity of Lyn and Fyn, but not Syk. In an following animal experiment, WZ3146 inhibited IgE-dependent passive cutaneous anaphylaxis (PCA) in a dose-dependent manner (ED₅₀, ~ 20 mg/kg). Taken together, in this study we show that the pyrimidine derivative, WZ3146, inhibits the IgE-mediated allergic response by inhibiting Lyn and Fyn Src-family kinases, which are initially activated by antigen stimulation in MCs. Therefore, we propose that WZ3146 could be used as a new therapeutic agent for the treatment of allergic diseases.

Alternative ZAP70-p38 signals prime a classical p38 pathway through LAT and SOS to support regulatory T cell differentiation

T cell receptor (TCR) stimulation activates diverse kinase pathways, which include the mitogen-activated protein kinases (MAPKs) ERK and p38, the phosphoinositide 3-kinases (PI3Ks), and the kinase mTOR. Although TCR stimulation activates the p38 pathway through a "classical" MAPK cascade that is mediated by the adaptor protein LAT, it also stimulates an "alternative" pathway in which p38 is activated by the kinase ZAP70. Here, we used dual-parameter, phosphoflow cytometry and in silico computation to investigate how both classical and alternative p38 pathways contribute to T cell activation. We found that basal ZAP70 activation in resting T cell lines reduced the threshold ("primed") TCR-stimulated activation of the classical p38 pathway. Classical p38 signals were reduced after T cell-specific deletion of the guanine nucleotide exchange factors Sos1 and Sos2, which are essential LAT signalosome components. As a consequence of Sos1/2 deficiency, production of the cytokine IL-2 was impaired, differentiation into regulatory T cells was reduced, and the autoimmune disease EAE was exacerbated in mice. These data suggest that the classical and alternative p38 activation pathways exist to generate immune balance.

Phosphorylation of SOS1 on tyrosine 1196 promotes its RAC GEF activity and contributes to BCR-ABL leukemogenesis

Son of Sevenless 1 (SOS1) is a dual guanine nucleotide exchange factor (GEF) that activates the small GTPases RAC and RAS. Although the molecular mechanisms of RAS GEF catalysis have been unveiled, how SOS1 acquires RAC GEF activity and what is the physio-pathological relevance of this activity is much less understood. Here we show that SOS1 is tyrosine phosphorylated on Y1196 by ABL. Phosphorylation of Y1196 controls SOS1 inter-molecular interaction, is required to promote the exchange of nucleotides on RAC in vitro and for platelet-derived growth factor (PDGF) activation of RAC- and RAC-dependent actin remodeling and cell migration. SOS1 is also phosphorylated on Y1196 by BCR-ABL in chronic myelogenous leukemic cells. Importantly, in these cells, SOS1 is required for BCR-ABL-mediated activation of RAC, cell proliferation and transformation in vitro and in a xenograft mouse model. Finally, genetic removal of Sos1 in the bone marrow-derived cells (BMDCs) from Sos1^fl/fl mice and infected with BCR-ABL causes a significant delay in the onset of leukemogenesis once BMDCs are injected into recipient, lethally irradiated mice. Thus, SOS1 is required for full transformation and critically contribute to the leukemogenic potential of BCR-ABL.

Interchangeability of Themis1 and Themis2 in thymocyte development reveals two related proteins with conserved molecular function

Themis1, a recently identified T cell protein, has a critical function in the generation of mature CD4(+)CD8(-) and CD4(-)CD8(+) (CD4 and CD8 single-positive [SP]) thymocytes and T cells. Although Themis1 has been shown to bind to the adaptor proteins LAT and Grb2, previous studies have yielded conflicting results regarding whether thymocytes from Themis1(-/-) mice exhibit TCR-mediated signaling defects. In this study, we demonstrate that, in the absence of Themis1, TCR-mediated signaling is selectively impaired in CD4 SP and CD8 SP thymocytes but is not affected in CD4(+)CD8(+) double-positive thymocytes despite high expression of Themis1 in double-positive thymocytes. Like Themis1, Themis2, a related member of the Themis family, which is expressed in B cells and macrophages, contains two conserved cysteine-based domains, a proline-rich region, and a nuclear localization signal. To determine whether Themis1 and Themis2 can perform similar functions in vivo, we analyzed T cell development and TCR-mediated signaling in Themis1(-/-) mice reconstituted with either Themis1 or Themis2 transgenes. Notably, Themis1 and Themis2 exhibited the same potential to restore T cell development and TCR-mediated signaling in Themis1(-/-) mice. Both proteins were tyrosine phosphorylated and were recruited within Grb2 signaling complexes to LAT following TCR engagement. These results suggest that conserved molecular features of the Themis1 and Themis2 proteins are important for their biological activity and predict that Themis1 and Themis2 may perform similar functions in T and B cells, respectively.

Understanding SOS (Son of Sevenless)

Son of Sevenless (SOS) was discovered in Drosophila melanogaster. Essential for normal eye development in Drosophila, SOS has two human homologues, SOS1 and SOS2. The SOS1 gene encodes the Son of Sevenless 1 protein, a Ras and Rac guanine nucleotide exchange factor. This protein is composed of several important domains. The CDC25 and REM domains provide the catalytic activity of SOS1 towards Ras and the histone fold DH/PH (Dbl homology and Pleckstrin homology) domains function, in tandem, to stimulate GTP/GDP exchange for Rac. In contrast to Ras, there have been few studies that implicate SOS1 in human disease and, initially, less attention was given to this gene. However, mutations in SOS1 have been reported recently in Noonan syndrome and in type 1 hereditary gingival fibromatosis. Although, there have been very few studies that focus on the regulation of this important gene by physiological or exogenous factors, we recently found that the SOS1 gene was induced by the environmental toxin, dioxin, and that this effect was mediated by the aryl hydrocarbon receptor (AhR). These recent observations raise the possibility that alterations in the expression of the SOS1 gene and, consequently, in the activity of the SOS1 protein may affect toxicological endpoints and lead to clinical disease. These possibilities, thus, have stimulated much interest in SOS1 recently. In this article, we review the functions of SOS1 and the evidence for its roles in physiology and pathology across species.

Altered thymic selection by overexpressing cellular FLICE inhibitory protein in T cells causes lupus-like syndrome in a BALB/c but not C57BL/6 strain

Cellular FLICE inhibitory protein (c-FLIP) is an endogenous inhibitor of the caspase-8 pro-apoptotic signaling pathway downstream of death receptors. Recent evidence indicates that the long form of c-FLIP (c-FLIP_L) is required for proliferation and effector T cell development. However, the role of c-FLIP_L in triggering autoimmunity has not been carefully investigated. We now report that c-FLIP_L transgenic (Tg) mice develop splenomegaly, lymphadenopathy, multi-organ infiltration, high titers of autoantibodies, and proliferative glomerulonephritis with immune complex deposition in a strain-dependent fashion. The development of autoimmunity requires CD4⁺ T cells and may result from impaired thymic selection. At the molecular level, c-FLIP_L over-expression inhibits the ZAP-70 activation, thus impairing the signaling pathway derived from ZAP-70 required for thymic selection. Therefore, we have identified c-FLIP_L as a susceptibility factor under the influence of epistatic modifiers for the development of autoimmunity.

The structure, regulation, and function of ZAP-70

The tyrosine ZAP-70 (zeta-associated protein of 70 kDa) kinase plays a critical role in activating many downstream signal transduction pathways in T cells following T-cell receptor (TCR) engagement. The importance of ZAP-70 is evidenced by the severe combined immunodeficiency that occurs in ZAP-70-deficient mice and humans. In this review, we describe recent analyses of the ZAP-70 crystal structure, revealing a complex regulatory mechanism of ZAP-70 activity, the differential requirements for ZAP-70 and spleen tyrosine kinase (SyK) in early T-cell development, as well as the role of ZAP-70 in chronic lymphocytic leukemia and autoimmunity. Thus, the critical importance of ZAP-70 in TCR signaling and its predominantly T-cell-restricted expression pattern make ZAP-70 an attractive drug target for the inhibition of pathological T-cell responses in disease.

T-cell receptor-induced NF-kappaB activation is negatively regulated by E3 ubiquitin ligase Cbl-b

It has previously been shown that E3 ubiquitin ligase Casitas B-lineage lymphoma-b (Cbl-b) negatively regulates T-cell activation, but the molecular mechanism(s) underlying this inhibition is not completely defined. In this study, we report that the loss of Cbl-b selectively results in aberrant activation of NF-kappaB upon T-cell antigen receptor (TCR) ligation, which is mediated by phosphatidylinositol 3-kinase (PI3-K)/Akt and protein kinase C-theta (PKC-theta). TCR-induced hyperactivation of Akt in the absence of Cbl-b may potentiate the formation of caspase recruitment domain-containing membrane-associated guanylate kinase protein 1 (CARMA1)-B-cell lymphoma/leukemia 10 (Bcl10)-mucosa-associated lymphatic tissue 1(MALT1) (CBM) complex, which appears to be independent of PKC-theta. Cbl-b associates with PKC-theta upon TCR stimulation and regulates TCR-induced PKC-theta activation via Vav-1, which couples PKC-theta to PI3-K and allows it to be phosphorylated. PKC-theta then couples IkappaB kinases (IKKs) to the CBM complex, resulting in the activation of the IKK complex. Therefore, our data provide the first evidence to demonstrate that the down-regulation of TCR-induced NF-kappaB activation by Cbl-b is mediated coordinately by both Akt-dependent and PKC-theta-dependent signaling pathways in primary T cells.

Auto-inhibition of the Dbl family protein Tim by an N-terminal helical motif

Dbl-related oncoproteins are guanine nucleotide exchange factors specific for Rho-family GTPases and typically possess tandem Dbl homology (DH) and pleckstrin homology domains that act in concert to catalyze exchange. Because the ability of many Dbl-family proteins to catalyze exchange is constitutively activated by truncations N-terminal to their DH domains, it has been proposed that the activity of Dbl-family proteins is regulated by auto-inhibition. However, the exact mechanisms of regulation of Dbl-family proteins remain poorly understood. Here we show that the Dbl-family protein, Tim, is auto-inhibited by a short, helical motif immediately N-terminal to its DH domain, which directly occludes the catalytic surface of the DH domain to prevent GTPase activation. Similar to the distantly related Vav isozymes, auto-inhibition of Tim is relieved by truncation, mutation, or phosphorylation of the auto-inhibitory helix. A peptide comprising the helical motif inhibits the exchange activity of Tim in vitro. Furthermore, substitutions within the most highly conserved surface of the DH domain designed to disrupt interactions with the auto-inhibitory helix also activate the exchange process.

Integrated signalling pathways for mast-cell activation

Mast-cell activation mediated by the high-affinity receptor for IgE (FcepsilonRI) is considered to be a key event in the allergic inflammatory response. However, in a physiological setting, other receptors, such as KIT, might also markedly influence the release of mediators by mast cells. Recent studies have provided evidence that FcepsilonRI-dependent degranulation is regulated by two complementary signalling pathways, one of which activates phospholipase Cgamma and the other of which activates phosphatidylinositol 3-kinase, using specific transmembrane and cytosolic adaptor molecules. In this Review, we discuss the evidence for these interacting pathways and describe how the capacity of KIT, and other receptors, to influence FcepsilonRI-dependent mast-cell-mediator release might be a function of the relative abilities of these receptors to activate these alternative pathways.

The Ras and Rho GTPases genetically interact to co-ordinately regulate cell polarity during development in Penicillium marneffei

Ras and Rho GTPases have been examined in a wide variety of eukaryotes and play varied and often overlapping roles in cell polarization and development. Studies in Saccharomyces cerevisiae and mammalian cells have defined some of the central activities of these GTPases. However, these paradigms do not explain the role of these proteins in all eukaryotes. Unlike yeast, but like more complex eukaryotes, filamentous fungi have Rac-like proteins in addition to Ras and Cdc42. To investigate the unique functions of these proteins and determine how they interact to co-ordinately regulate morphogenesis during growth and development we undertook a genetic analysis of GTPase function by generating double mutants of the Rho GTPases cflA and cflB and the newly isolated Ras GTPase rasA from the dimorphic pathogenic fungus, Penicillium marneffei. P. marneffei growth at 25 degrees C is as multinucleate, septate, branched hyphae which are capable of undergoing asexual development (conidiation), while at 37 degrees C, uninucleate pathogenic yeast cells which divide by fission are produced. Here we show that RasA (Ras) acts upstream of CflA (Cdc42) to regulate germination of spores and polarized growth of both hyphal and yeast cells, while also exhibiting CflA-independent activities. CflA (Cdc42) and CflB (Rac) co-ordinately control hyphal cell polarization despite also having unique roles in regulating conidial germination and polarized growth of yeast cells (CflA) and polarized growth of conidiophore cell types and hyphal branching (CflB).

Tyrosine phosphorylation of adaptor protein 3BP2 induces T cell receptor-mediated activation of transcription factor

Molecular adaptors/scaffolds have indispensable roles in the activation of lymphocytes. In this report, we have demonstrated the role of tyrosine phosphorylation of an adaptor protein 3BP2 (c-Abl-SH3 domain binding protein-2, also known as SH3BP2) in T cell receptor (TCR)-mediated activation of transcription factor. Short interfering RNA for 3BP2 suppresses the expression level of endogenous 3BP2 and inhibits TCR-mediated activation of interleukin (IL)-2 promoter and nuclear factor of activated T cells (NFAT) element. Engagement of TCR induces tyrosine phosphorylation and lipid raft translocation of 3BP2. The overexpression studies reveal that substitution of 3BP2-Tyr(183), Tyr(446), or Arg(486) in the SH2 domain suppresses TCR-mediated activation of NFAT. Point mutations of 3BP2 cannot affect the translocation of 3BP2 into the lipid raft. Phosphorylation of Tyr(183) is required for the interaction with Vav1, the guanine nucleotide exchanging factor of Rac1. In fact, overexpression of dominant-negative form of Rac1 inhibits TCR-mediated activation of NFAT. Phosphorylation of Tyr(446) recruits the SH2 domain of Lck for the optimal activation of transcription factors. Furthermore, point mutation of Arg(486) in the 3BP2-SH2 domain that couples ZAP-70 to LAT dramatically reduces NFAT activation. These results suggest that the site-directed functions of 3BP2 induce the activation of transcription factors.

Membrane localization and function of Vav3 in T cells depend on its association with the adapter SLP-76

The Vav family of guanine exchange factors plays a critical role in lymphocyte proliferation, cytoskeletal reorganization, and gene transcription upon immunoreceptor engagement. Although the role of Vav1 in T cells is well documented, the role of Vav3 is less clear. We investigated the subcellular localization of Vav3 during T cell activation. We report here that phosphorylation of Vav3 on tyrosine residue Tyr(173) is not required for T cell receptor (TCR)-induced Vav3 membrane translocation or immunological synapse (IS) recruitment, but mutation of this residue enhanced TCR-induced nuclear factor of activated T cells (NFAT) activation. However, Vav3 mutants either containing an Src homology 2 (SH2)-disabled point mutation (R697L) or lacking its SH3-SH2-SH3 domains were unable to bind SLP-76 did not translocate to the membrane or to the IS and furthermore failed to activate NFAT. Importantly, the membrane translocation of Vav3 was abrogated in Lck, ZAP-70, LAT, and SLP-76-deficient T cells, where Vav3 binding to SLP-76 was disrupted. Finally, we confirmed and underlined the critical role of Vav3 in NFAT activation by knocking down Vav3 expression in Vav1-deficient T cells. Altogether, our data show that TCR-induced association of Vav3 with SLP-76 is required for its membrane/IS localization and function.

SH3 domain of spectrin participates in the activation of Rac in specialized calpain-induced integrin signaling complexes

In this study, we used cultured cells spreading on beta3 integrin substrates to examine the possibility that spectrin is involved in signal transduction. Spectrin clustered with specialized calpain-induced beta3 integrin signaling complexes that mediate the initial attachment of cells and initiate Rac activation and lamellipodia extension. It was absent from focal complexes and focal adhesions, the integrin complexes that mediate adhesion in lamellipodia and fully spread cells. Spectrin contains a Src homology (SH3) domain of unknown function. Cells overexpressing this domain adhered and calpain-induced integrin signaling complexes formed. However, Rac activation, lamellipodia extension and cell spreading were inhibited. Spreading was restored by overexpression of constitutively active Rac. These studies point to a previously unrecognized role for spectrin and its SH3 domain in initiating Rac activation in the specialized integrin clusters that initiate cell adhesion and spreading. Thus, spectrin may have a pivotal role in initiating integrin-induced physiological and pathological events such as development, proliferation, cell survival, wound healing, metastasis and atherosclerosis.

CD38 Signaling in T Cells Is Initiated within a Subset of Membrane Rafts Containing Lck and the CD3-ζ Subunit of the T Cell Antigen Receptor

In this study we present data supporting that most CD38 is pre-assembled in a subset of Brij 98-resistant raft vesicles, which were stable at 37 degrees C, and have relatively high levels of Lck and the CD3-zeta subunit of T cell antigen receptor-CD3 complex in contrast with a Brij 98-soluble pool, where CD38 is associated with CD3-zeta, and Lck is not detected. Our data further indicate that following CD38 engagement, LAT and Lck are tyrosine phosphorylated exclusively in Brij 98-resistant rafts, and some key signaling components translocate into rafts (i.e. Sos and p85-phosphatidylinositol 3-kinase). Moreover, N-Ras results activated within rafts immediately upon CD38 ligation, whereas activated Erk was mainly found in soluble fractions with delayed kinetics respective to Ras activation. Furthermore, full phosphorylation of CD3-zeta and CD3-epsilon only occurs in rafts, whereas partial CD3-zeta tyrosine phosphorylation occurs exclusively in the soluble pool, which correlated with increased levels of c-Cbl tyrosine phosphorylation in the non-raft fractions. Taken together, these results suggest that, unlike the non-raft pool, CD38 in rafts is able to initiate and propagate several activating signaling pathways, possibly by facilitating critical associations within other raft subsets, for example, LAT rafts via its capacity to interact with Lck and CD3-zeta. Overall, these findings provide the first evidence that CD38 operates in two functionally distinct microdomains of the plasma membrane.

Defective activity of ERK-1 and ERK-2 mitogen-activated protein kinases in peripheral blood T lymphocytes from patients with systemic lupus erythematosus: potential role of altered coupling of Ras guanine nucleotide exchange factor hSos to adapter protein Grb2 in lupus T cells

The integrity of the Ras/Raf/mitogen-activated protein kinase (MAPK) cascade is critical for maintenance of T cell tolerance, a process that fails in patients with systemic lupus erythematosus (SLE). In this study we have examined the activity of mitogen-activated protein kinases ERK-1 and ERK-2 in resting and TCR-activated peripheral blood T lymphocytes from patients with SLE. We also examined the binding of Ras guanine nucleotide exchange factor, human Son of Sevenless (hSos), to cytosolic adapter protein growth factor receptor-bound protein 2. T cells from lupus patients showed diminished catalytic activity and TCR-driven dual phosphorylation of ERK-1 and ERK-2 upon stimulation through the TCR/CD3 receptor, a defect that may be related to altered translocation of hSos to the Ras/Raf membrane complex and diminished nuclear translocation of trans-acting factor AP-1. Defective MAPK activity triggered by TCR/ CD3 activation may alter the coordination of signals needed for normal interleukin-2 production and maintenance of tolerance in lupus T cells.

Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa and phospholipase C gamma 1 are required for NF-kappa B activation and lipid raft recruitment of protein kinase C theta induced by T cell costimulation

The NF-kappaB activation pathway induced by T cell costimulation uses various molecules including Vav1 and protein kinase C (PKC)theta. Because Vav1 inducibly associates with further proteins including phospholipase C (PLC)gamma1 and Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76), we investigated their role for NF-kappaB activation in Jurkat leukemia T cell lines deficient for expression of these two proteins. Cells lacking SLP-76 or PLCgamma1 failed to activate NF-kappaB in response to T cell costimulation. In contrast, replenishment of SLP-76 or PLCgamma1 expression restored CD3/CD28-induced IkappaB kinase (IKK) activity as well as NF-kappaB DNA binding and transactivation. PKCtheta activated NF-kappaB in SLP-76- and PLCgamma1-deficient cells, showing that PKCtheta is acting further downstream. In contrast, Vav1-induced NF-kappaB activation was normal in SLP-76(-) cells, but absent in PLCgamma1(-) cells. CD3/CD28-stimulated recruitment of PKCtheta and IKKgamma to lipid rafts was lost in SLP-76- or PLCgamma1-negative cells, while translocation of Vav1 remained unaffected. Accordingly, recruitment of PKCtheta to the immunological synapse strictly relied on the presence of SLP-76 and PLCgamma1, but synapse translocation of Vav1 identified in this study was independent from both proteins. These results show the importance of SLP-76 and PLCgamma1 for NF-kappaB activation and raft translocation of PKCtheta and IKKgamma.

Differentially expressed genes in HIV-1 tat-expressing CD4(+) T-cell line

Several studies have indicated that human immunodeficiency virus type-1 (HIV-1) transactivating Tat protein is essential for proviral DNA transcription and virus replication. In addition, it is actively released from acutely HIV-1-infected cells and interacts either with the same virus-infected and virus producing cell, or with bystander uninfected cells, influencing the expression of several genes and related cellular functions. The main goal of this paper was to determine the Tat-related expression of basic cellular genes in a permanently tat transfected CD4+ cell line, to identify the cellular genes influenced by the presence of endogenous-exogenous Tat protein. For this purpose, we analyzed, by a cDNA-membrane-array assay, cellular mRNAs expressed in serum-free cultures of lymphoblastoid CD4(+) Jurkat cells, stably transfected with a plasmid constitutively expressing tat gene, in comparison with Jurkat cells transfected with the backbone plasmid only, and parental Jurkat cells. The expression of mRNAs in permanently tat-transfected Jurkat cells showed significant differences in 24 out of 1176 analyzed genes in comparison with parental or backbone plasmid transfected cells. Most of the genes overexpressed in permanently tat-transfected Jurkat cells, belong to transcription factors, or to receptors, adaptors, and mediators of signal transduction pathways, and to factors involved in response to oxidative stress, suggesting a complex regulation of CD4(+) T-lymphoid cell survival and proliferation by HIV-1 Tat protein.

Shb links SLP-76 and Vav with the CD3 complex in Jurkat T cells

This study addresses the interactions between the adaptor protein Shb and components involved in T cell signalling, including SLP-76, Gads, Vav and ZAP70. We show that both SLP-76 and ZAP70 co-immunoprecipitate with Shb in Jurkat T cells and that Shb and Vav co-immunoprecipitate when cotransfected in COS cells. We also demonstrate, utilizing fusion protein constructs, that SLP-76, Gads and Vav associate independently of each other to different domains or regions, of Shb. Overexpression of an SH2 domain-defective Shb causes diminished phosphorylation of SLP-76 and Vav and consequently decreased activation of c-Jun kinase upon T cell receptor (TCR) stimulation. Shb was also found to localize to glycolipid-enriched membrane microdomains (GEMs), also called lipid rafts, after TCR stimulation. Our results indicate that upon TCR stimulation, Shb is targeted to these lipid rafts where Shb aids in recruiting the SLP-76-Gads-Vav complex to the T cell receptor zeta-chain and ZAP70.

Vav regulates activation of Rac but not Cdc42 during FcgammaR-mediated phagocytosis

Phagocytosis is the process whereby cells direct the spatially localized, receptor-driven engulfment of particulate materials. It proceeds via remodeling of the actin cytoskeleton and shares many of the core cytoskeletal components involved in adhesion and migration. Small GTPases of the Rho family have been widely implicated in coordinating actin dynamics in response to extracellular signals and during diverse cellular processes, including phagocytosis, yet the mechanisms controlling their recruitment and activation are not known. We show herein that in response to ligation of Fc receptors for IgG (FcgammaR), the guanine nucleotide exchange factor Vav translocates to nascent phagosomes and catalyzes GTP loading on Rac, but not Cdc42. The Vav-induced Rac activation proceeds independently of Cdc42 function, suggesting distinct roles for each GTPase during engulfment. Moreover, inhibition of Vav exchange activity or of Cdc42 activity does not prevent Rac recruitment to sites of particle attachment. We conclude that Rac is recruited to Fcgamma membrane receptors in its inactive, GDP-bound state and that Vav regulates phagocytosis through subsequent catalysis of GDP/GTP exchange on Rac.

T cell receptor-mediated signal transduction controlled by the beta chain transmembrane domain: apoptosis-deficient cells display unbalanced mitogen-activated protein kinases activities upon T cell receptor engagement

The bases that support the versatility of the T cell receptor (TCR) to generate distinct T cell responses remain unclear. We have previously shown that mutant cells in the transmembrane domain of TCRbeta chain are impaired in TCR-induced apoptosis but are not affected in other functions. Here we describe the biochemical mechanisms by which this mutant receptor supports some T cell responses but fails to induce apoptosis. Extracellular signal-regulated protein kinase (ERK) is activated at higher and more sustained levels in TCRbeta-mutated than in wild type cells. Conversely, activation of both c-Jun N-terminal kinase and p38 mitogen-activated protein kinase is severely reduced in mutant cells. By attempting to link this unbalanced induction to altered upstream events, we found that ZAP-70 is normally activated. However, although SLP-76 phosphorylation is normally induced, TCR engagement of mutant cells results in lower tyrosine phosphorylation of LAT but in higher tyrosine phosphorylation of Vav than in wild type cells. The results suggest that an altered signaling cascade leading to an imbalance in mitogen-activated protein kinase activities is involved in the selective impairment of apoptosis in these mutant cells. Furthermore, they also provide new insights in the contribution of TCR to decipher the signals that mediate apoptosis distinctly from proliferation.

Dynamic polarization of the microtubule cytoskeleton during CTL-mediated killing

Efficient unidirectional killing by cytotoxic T lymphocytes (CTL) requires translocation of the microtubule organizing center (MTOC) to the target cell contact site. Here we utilize modulated polarization microscopy and computerized 3D reconstruction of tubulin and LFA-1 immunofluorescence images to investigate how this is accomplished. The results show that the MTOC is drawn vectorially to the contact site by a microtubule sliding mechanism. Once the MTOC arrives at the contact site, it oscillates laterally. Microtubules loop through and anchor to a ring-shaped zone (pSMAC) defined by the dense clustering of LFA-1 at the target contact site. Microtubules that run straight between the MTOC and pSMAC and then turn sharply may indicate the action of a microtubule motor such as dynein.

Effect of redox balance alterations on cellular localization of LAT and downstream T-cell receptor signaling pathways

The integral membrane protein linker for activation of T cells (LAT) is a central adapter protein in the T-cell receptor (TCR)-mediated signaling pathways. The cellular localization of LAT is extremely sensitive to intracellular redox balance alterations. Reduced intracellular levels of the antioxidant glutathione (GSH), a hallmark of chronic oxidative stress, resulted in the membrane displacement of LAT, abrogated TCR-mediated signaling and consequently hyporesponsiveness of T lymphocytes. The membrane displacement of LAT is accompanied by a considerable difference in the mobility of LAT upon native and nonreducing denaturing polyacrylamide gel electrophoresis analysis, a finding indicative of a conformational change. Targeted mutation of redox-sensitive cysteine residues within LAT created LAT mutants which remain membrane anchored under conditions of chronic oxidative stress. The expression of redox-insensitive LAT mutants allows for restoration of TCR-mediated signal transduction, whereas CD28-mediated signaling pathways remained impaired. These results are indicative that the membrane displacement of LAT as a result of redox balance alterations is a consequence of a conformational change interfering with the insertion of LAT into the plasma membrane. Conclusively, the data suggest a role for LAT as a crucial intermediate in the sensitivity of TCR signaling and hence T lymphocytes toward chronic oxidative stress.

CD46/CD3 costimulation induces morphological changes of human T cells and activation of Vav, Rac, and extracellular signal-regulated kinase mitogen-activated protein kinase

Efficient T cell activation requires at least two signals, one mediated by the engagement of the TCR-CD3 complex and another one mediated by a costimulatory molecule. We recently showed that CD46, a complement regulatory receptor for C3b as well as a receptor for several pathogens, could act as a potent costimulatory molecule for human T cells, highly promoting T cell proliferation. Indeed, we show in this study that CD46/CD3 costimulation induces a synergistic activation of extracellular signal-related kinase mitogen-activated protein kinase. Furthermore, whereas T lymphocytes primarily circulate within the bloodstream, activation may induce their migration toward secondary lymphoid organs or other tissues to encounter APCs or target cells. In this study, we show that CD46/CD3 costimulation also induces drastic morphological changes of primary human T cells, as well as actin relocalization. Moreover, we show that the GTP/GDP exchange factor Vav is phosphorylated upon CD46 stimulation alone, and that CD46/CD3 costimulation induces a synergistic increase of Vav phosphorylation. These results prompted us to investigate whether CD46/CD3 costimulation induced the activation of GTPases from the Rho family. Indeed, we report that the small GTPase Rac is also activated upon CD46/CD3 costimulation, whereas no change of Rho and Cdc42 activity could be detected. Therefore, CD46 costimulation profoundly affects T cell behavior, and these results provide important data concerning the biology of primary human T cells.

Novel recognition mode between Vav and Grb2 SH3 domains

Vav is a guanine nucleotide exchange factor for the Rho/Rac family that is expressed exclusively in hematopoietic cells. Growth factor receptor-bound protein 2 (Grb2) has been proposed to play important roles in the membrane localization and activation of Vav through dimerization of its C-terminal Src-homology 3 (SH3) domain (GrbS) and the N-terminal SH3 domain of Vav (VavS). The crystal structure of VavS complexed with GrbS has been solved. VavS is distinct from other SH3 domain proteins in that its binding site for proline-rich peptides is blocked by its own RT loop. One of the ends of the VavS beta-barrel forms a concave hydrophobic surface. The GrbS components make a contiguous complementary interface with the VavS surface. The binding site of GrbS for VavS partially overlaps with the canonical binding site for proline-rich peptides, but is definitely different. Mutations at the interface caused a decrease in the binding affinity of VavS for GrbS by 4- to 40-fold. The structure reveals how GrbS discriminates VavS specifically from other signaling molecules without binding to the proline-rich motif.

A novel function for the Tec family tyrosine kinase Itk in activation of beta 1 integrins by the T-cell receptor

Stimulation of T cells via the CD3--T-cell receptor (TCR) complex results in rapid increases in beta 1 integrin-mediated adhesion via poorly defined intracellular signaling events. We demonstrate that TCR-mediated activation of beta 1 integrins requires activation of the Tec family tyrosine kinase Itk and phosphatidylinositol 3-kinase (PI 3-K)-dependent recruitment of Itk to detergent-insoluble glycosphingolipid-enriched microdomains (DIGs) via binding of the pleckstrin homology domain of Itk to the PI 3-K product PI(3,4,5)-P(3). Activation of PI 3-K and the src family kinase Lck, via stimulation of the CD4 co-receptor, can initiate beta 1 integrin activation that is dependent on Itk function. Targeting of Itk specifically to DIGs, coupled with CD4 stimulation, can also activate beta 1 integrin function independently of TCR stimulation. Changes in beta 1 integrin function mediated by TCR activation of Itk are also accompanied by Itk-dependent modulation of the actin cytoskeleton. Thus, TCR-mediated activation of beta 1 integrins involves membrane relocalization and activation of Itk via coordinate action of PI 3-K and a src family tyrosine kinase.

Regulation of beta 1 integrin-mediated adhesion by T cell receptor signaling involves ZAP-70 but differs from signaling events that regulate transcriptional activity

Stimulation of the CD3/TCR results within minutes in an increase in T cell adhesion mediated by beta(1) integrins. The biochemical pathways that control CD3-mediated increases in beta(1) integrin-mediated adhesion remain poorly characterized. In this study, the role of the tyrosine kinase ZAP-70 in the regulation of beta(1) integrin activity by the CD3/TCR was investigated. CD3 stimulation did not increase beta(1) integrin-mediated adhesion of the ZAP-70-deficient Jurkat T cell line, P116, to the beta(1) integrin ligand fibronectin. Reintroduction of wild-type ZAP-70, but not a kinase-inactive variant, K369R, corrected the adhesive defect observed in P116 T cells. In addition, the kinase-inactive ZAP-70 mutant inhibited CD3-induced adhesion of primary human T cell blasts. Interestingly, a ZAP-70 mutant with a tyrosine to phenylalanine substitution at position 319 (Y319F) restored the adhesive defect in P116 T cells, even though Y319F ZAP-70 failed to fully reconstitute CD3-initiated NF-AT-dependent transcription and tyrosine phosphorylation of the LAT adapter protein. Finally, expression of mutants of LAT and the SLP-76 adapter protein that modulate CD3-mediated activation of an NF-AT reporter gene failed to block CD3-induced increases in beta(1) integrin-mediated adhesion. These observations support a model in which the tyrosine kinase activity of ZAP-70 kinase is critical for regulation of beta(1) integrin activity by CD3/TCR. However, the signaling events downstream of ZAP-70 that regulate CD3/TCR-mediated activation of beta(1) integrin function exhibit key differences when compared with the signaling pathways that regulate transcriptional events initiated by CD3/TCR stimulation.

RasGRP is essential for mouse thymocyte differentiation and TCR signaling

The Ras signaling pathway plays a critical role in thymopoiesis and T cell activation, but the mechanism of Ras regulation is controversial. At least one mode of Ras regulation in T cells involves the messenger diacylglycerol (DAG). RasGRP, a Ras activator with a DAG-binding C1 domain, is expressed in T cells and thymocytes. Here we show that thymi of RasGRP-null mutant mice have approximately normal numbers of immature thymocytes but a marked deficiency of mature, single-positive (CD4+CD8- and CD4-CD8+) thymocytes. In Ras signaling and proliferation assays, mutant thymocytes showed a complete lack of response to DAG analogs or T cell receptor (TCR) stimulation by antibodies. Thus, TCR and DAG are linked through RasGRP to Ras signaling.