Uncoupling of nonreceptor tyrosine kinases from PLC-gamma1 in an SLP-76-deficient T cell

Signal transduction mediated by the T cell antigen receptor: the role of adapter proteins

Engagement of the T cell antigen receptor (TCR) leads to a complex series of molecular changes at the plasma membrane, in the cytoplasm, and at the nucleus that lead ultimately to T cell effector function. Activation at the TCR of a set of protein tyrosine kinases (PTKs) is an early event in this process. This chapter reviews some of the critical substrates of these PTKs, the adapter proteins that, following phosphorylation on tyrosine residues, serve as binding sites for many of the critical effector enzymes and other adapter proteins required for T cell activation. The role of these adapters in binding various proteins, the interaction of adapters with plasma membrane microdomains, and the function of adapter proteins in control of the cytoskeleton are discussed.

The T-cell receptor signalosome: a dynamic structure with expanding complexity

Signal transduction in T cells is a dynamic process involving a large number of membrane and cytosolic proteins. The TCR macromolecular complex (signalosome) is initiated by receptor occupancy and becomes more elaborate over time. This review describes how 'vertical' displacement mechanisms and lateral coalescence of lipid-raft-associated scaffold proteins combine to form distinct signalosomes, which control signal specificity.

Communication between the TCR and integrins: role of the molecular adapter ADAP/Fyb/Slap

TCR stimulation induces integrin-mediated adhesion, facilitating stabilization of conjugates between T cells and antigen-presenting cells and thereby contributing to T cell activation. Integrin activation has been shown to require cytoskeletal reorganization; however, the molecular mechanisms mediating communication between the TCR and integrins remain unclear. Recently the adapter protein ADAP/Fyb/Slap has been shown to couple TCR stimulation to integrin activation by mediating increased integrin avidity. ADAP may also play a role in transduction of external signals by integrins. Like other adapters, ADAP is a multifunctional protein and interacts with molecules such as Fyn, Slp-76, Ena/VASP proteins, Vav1, WASP and the Arp2/3 complex.

Mechanisms of signaling by the hematopoietic-specific adaptor proteins, SLP-76 and LAT and their B cell counterpart, BLNK/SLP-65

Adaptor proteins lack catalytic activity and contain only protein-protein interaction domains. They have been shown to interact with an ever-growing number of signaling proteins and to play essential roles in many signaling pathways. SLP-76 and LAT are cell-type-specific adaptor proteins expressed in T cells, NK cells, platelets, and mast cells. In these cell types, SLP-76 and LAT are required for signaling by immunoreceptor tyrosine-based activation motif(ITAM)-containing receptors, including the T cell receptor (TCR), the pre-TCR, the high-affinity Fc epsilon receptor, and the platelet GPVI collagen receptor. In B cells, an analogous adaptor, BLNK/SLP-65, is required for signaling by the ITAM-containing B cell receptor. This review summarizes recent research on SLP-76, LAT, and BLNK. A major challenge in understanding adaptor protein function has been to sort out the many interactions mediated by adaptor proteins and to define the mechanisms by which adaptors mediate critical signaling events. In the case of LAT, SLP-76, and BLNK, the availability of tractable genetic systems, deficient in expression of each of these adaptor proteins, has facilitated in-depth investigation of their signaling functions and mechanisms of action. The picture that has emerged is one in which multiple adaptor proteins cooperate to bring about the formation of a large signaling complex, localized to specialized lipid microdomains within the cell membrane and known as GEMs. Adaptors not only recruit signaling proteins, but also play an active role in regulating the conformation and activation of many of the proteins recruited to the complex. In particular, recent research has shed light on the mechanisms by which multiple adaptor proteins cooperate to bring about the recruitment and activation of phospholipase C gamma in response to the activation of ITAM-containing receptors.

ADAP-ting TCR signaling to integrins

Adaptor proteins are essential components of T cell receptor (TCR) signaling cascades regulating gene transcription and cytoskeletal reorganization. The molecular adaptor adhesion- and degranulation-promoting adaptor protein (ADAP), also known as Fyn binding protein (FYB) or Slp-76-associated protein of 130 kilodaltons (SLAP-130), interacts with a number of signaling intermediates including Slp-76, the Src family tyrosine kinase Fyn, vasodilator-stimulated phosphoprotein (VASP), and the actin-nucleating protein WASP. Recently ADAP was shown genetically to positively regulate T cell activation, TCR-induced integrin clustering, and T cell adhesion. The mechanism by which ADAP couples TCR stimulation to integrin clustering remains unclear; however, studies of ADAP, the exchange factor Vav1, and WASP suggest that TCR and integrin clustering may be controlled by distinct signaling pathways.

Differential SLP-76 expression and TCR-mediated signaling in effector and memory CD4 T cells

We present in this study novel findings on TCR-mediated signaling in naive, effector, and memory CD4 T cells that identify critical biochemical markers to distinguish these subsets. We demonstrate that relative to naive CD4 T cells, memory CD4 T cells exhibit a profound decrease in expression of the linker/adapter molecule SLP-76, while effector T cells express normal to elevated levels of SLP-76. The reduced level of SLP-76 is memory CD4 T cells is coincident with reduced phosphorylation overall, yet the residual SLP-76 couples to a subset of TCR-associated linker molecules, leading to downstream mitogen-activated protein (MAP) kinase activation. By contrast, effector CD4 T cells strongly phosphorylate SLP-76, linker for activation of T cells, and additional Grb2-coupled proteins, exhibit increased associations of SLP-76 to phosphorylated linkers, and hyperphosphorylate downstream Erk1/2 MAP kinases. Our results suggest distinct coupling of signaling intermediates to the TCR in naive, effector, and memory CD4 T cells. Whereas effector CD4 T cells amplify existing TCR signaling events accounting for rapid effector responses, memory T cells engage fewer signaling intermediates to efficiently link TCR triggering directly to downstream MAP kinase activation.

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.

Differential role of SLP-76 domains in T cell development and function

The adapter SLP-76 is essential for thymocyte development. SLP-76(-/-) mice were reconstituted with SLP-76 deletion mutant transgenes to examine the role of SLP-76 domains in T cell development and function. The N-terminal domain deletion mutant completely failed to restore thymocyte development. Mice reconstituted with Gads-binding site and SH2 domain deletion mutants had decreased thymic cellularity, impaired transition from double to single positive thymocytes, and decreased numbers of mature T cells in the spleen. Calcium mobilization and extracellular signal-regulated protein kinase activation were decreased in the Gads-binding site mutant but almost normal in the SH2 domain mutant. T cells from both mutants failed to proliferate following T cell antigen receptor ligation. Nevertheless, both mutants mounted partial cutaneous hypersensitivity responses and normal T cell dependent IgG1 antibody responses. These results indicate differential roles for SLP-76 domains in T cell development, proliferation and effector functions.

Signalling pathways induced by protease-activated receptors and integrins in T cells

Recent characterization of the thrombin receptor indicates that it plays a role in T-cell signalling pathways. However, little is known regarding the signalling events following stimulation of additional members of the protease-activated receptor (PAR) family, i.e. PAR2 and PAR3. Most of the postligand cascades are largely unknown. Here, we illustrate that in Jurkat T-leukaemic cells, activation of PAR1, PAR2 and PAR3 induce tyrosine phosphorylation of Vav1. This response was impaired in Jurkat T cells deficient in p56lck (JCaM1.6). Activation of PARs also led to an increase in tyrosine phosphorylation of ZAP-70 and SLP-76, two key proteins in T-cell receptor (TCR) signalling. We also demonstrated that p56lck is meaningful for integrin signalling. Thus, JCaM1.6 cells exhibited a marked reduction in their adherence to fibronectin-coated plates, as compared to the level of adherence of Jurkat T cells. While the phosphorylation of Vav1 in T cells is augmented following adhesion, no additional increase was noted following treatment of the adhered cells with PARs. Altogether, we have identified key components in the postligand-signalling cascade of PARs and integrins. Furthermore, we have identified Lck as a critical and possibly upstream component of PAR-induced Vav1 phosphorylation, as well as integrin activation, in Jurkat T cells.

CD28 signaling via VAV/SLP-76 adaptors: regulation of cytokine transcription independent of TCR ligation

Since CD28 provides cosignals in T cell responses, a key question is whether the coreceptor operates exclusively via TCRzeta/CD3 or also operates as an independent signaling unit. In this study, we show that CD28 can cooperate with VAV/SLP-76 adaptors to upregulate interleukin 2/4 transcription independently of TCR ligation. CD28 signaling is dependent on VAV/SLP-76 complex formation and induces membrane localization of these complexes. CD28-VAV/SLP-76 also functions in nonlymphoid cells to promote nuclear entry of NFAT, indicating that these adaptors are the only lymphoid components needed for this pathway. Further downstream, CD28-VAV/SLP-76 synergizes with Rac1 and causes F-actin remodelling proximal to receptor. Autonomous CD28 signaling may account for the distinct nature of the second signal and in trans amplification of T cell responses.

Differential requirement for SLP-76 domains in T cell development and function

The hematopoietic cell-specific adaptor protein, SLP-76, is critical for T cell development and mature T cell receptor (TCR) signaling; however, the structural requirements of SLP-76 for mediating thymopoiesis and mature T cell function remain largely unknown. In this study, transgenic mice were generated to examine the requirements for specific domains of SLP-76 in thymocytes and peripheral T cells in vivo. Examination of mice expressing various mutants of SLP-76 on the null background demonstrates a differential requirement for specific domains of SLP-76 in thymocytes and T cells and provides new insight into the molecular mechanisms underlying SLP-76 function.

Hematopoietic progenitor kinase 1 associates physically and functionally with the adaptor proteins B cell linker protein and SLP-76 in lymphocytes

B cell linker protein (BLNK) is a SLP-76-related adaptor protein essential for signal transduction from the BCR. To identify components of BLNK-associated signaling pathways, we performed a phosphorylation-dependent yeast two-hybrid analysis using BLNK probes. Here we report that the serine/threonine kinase hematopoietic progenitor kinase 1 (HPK1), which is activated upon antigen-receptor stimulation and which has been implicated in the regulation of MAP kinase pathways, interacts physically and functionally with BLNK in B cells and with SLP-76 in T cells. This interaction requires Tyr(379) of HPK1 and the Src homology 2 (SH2) domain of BLNK/SLP-76. Via homology modeling, we defined a consensus binding site within ligands for SLP family SH2 domains. We further demonstrate that the SH2 domain of SLP-76 participates in the regulation of AP-1 and NFAT activation in response to T cell receptor (TCR) stimulation and that HPK1 inhibits AP-1 activation in a manner partially dependent on its interaction with SLP-76. Our data are consistent with a model in which full activation of HPK1 requires its own phosphorylation on tyrosine and subsequent interaction with adaptors of the SLP family, providing a mechanistic basis for the integration of this kinase into antigen receptor signaling cascades.

Functional cloning of Src-like adapter protein-2 (SLAP-2), a novel inhibitor of antigen receptor signaling

In an effort to identify novel therapeutic targets for autoimmunity and transplant rejection, we developed and performed a large-scale retroviral-based functional screen to select for proteins that inhibit antigen receptor-mediated activation of lymphocytes. In addition to known regulators of antigen receptor signaling, we identified a novel adaptor protein, SLAP-2 which shares 36% sequence similarity with the known Src-like adaptor protein, SLAP. Similar to SLAP, SLAP-2 is predominantly expressed in hematopoietic cells. Overexpression of SLAP-2 in B and T cell lines specifically impaired antigen receptor-mediated signaling events, including CD69 surface marker upregulation, nuclear factor of activated T cells (NFAT) promoter activation and calcium influx. Signaling induced by phorbol myristate acetate (PMA) and ionomycin was not significantly reduced, suggesting SLAP-2 functions proximally in the antigen receptor signaling cascade. The SLAP-2 protein contains an NH2-terminal myristoylation consensus sequence and SH3 and SH2 Src homology domains, but lacks a tyrosine kinase domain. In antigen receptor-stimulated cells, SLAP-2 associated with several tyrosine phosphorylated proteins, including the ubiquitin ligase Cbl. Deletion of the COOH terminus of SLAP-2 blocked function and abrogated its association with Cbl. Mutation of the putative myristoylation site of SLAP-2 compromised its inhibitory activity and impaired its localization to the membrane compartment. Our identification of the negative regulator SLAP-2 demonstrates that a retroviral-based screening strategy may be an efficient way to identify and characterize the function of key components of many signal transduction systems.

Zap-70-independent Ca(2+) mobilization and Erk activation in Jurkat T cells in response to T-cell antigen receptor ligation

The tyrosine kinase ZAP-70 has been implicated as a critical intermediary between T-cell antigen receptor (TCR) stimulation and Erk activation on the basis of the ability of dominant negative ZAP-70 to inhibit TCR-stimulated Erk activation, and the reported inability of anti-CD3 antibodies to activate Erk in ZAP-70-negative Jurkat cells. However, Erk is activated in T cells receiving a partial agonist signal, despite failing to activate ZAP-70. This discrepancy led us to reanalyze the ZAP-70-negative Jurkat T-cell line P116 for its ability to support Erk activation in response to TCR/CD3 stimulation. Erk was activated by CD3 cross-linking in P116 cells. However, this response required a higher concentration of anti-CD3 antibody and was delayed and transient compared to that in Jurkat T cells. Activation of Raf-1 and MEK-1 was coincident with Erk activation. Remarkably, the time course of Ras activation was comparable in the two cell lines, despite proceeding in the absence of LAT tyrosine phosphorylation in the P116 cells. CD3 stimulation of P116 cells also induced tyrosine phosphorylation of phospholipase C-gamma1 (PLCgamma1) and increased the intracellular Ca(2+) concentration. Protein kinase C (PKC) inhibitors blocked CD3-stimulated Erk activation in P116 cells, while parental Jurkat cells were refractory to PKC inhibition. The physiologic relevance of these signaling events is further supported by the finding of PLCgamma1 tyrosine phosphorylation, Erk activation, and CD69 upregulation in P116 cells on stimulation with superantigen and antigen-presenting cells. These results demonstrate the existence of two pathways leading to TCR-stimulated Erk activation in Jurkat T cells: a ZAP-70-independent pathway requiring PKC and a ZAP-70-dependent pathway that is PKC independent.

Positive and negative regulation of T-cell activation by adaptor proteins

Adaptor proteins, molecules that mediate intermolecular interactions, are now known to be as crucial for lymphocyte activation as are receptors and effectors. Extensive work from numerous laboratories has identified and characterized many of these adaptors, demonstrating their roles as both positive and negative regulators. Studies into the molecular basis for the actions of these molecules shows that they function in various ways, including: recruitment of positive or negative regulators into signalling networks, modulation of effector function by allosteric regulation of enzymatic activity, and by targeting other proteins for degradation. This review will focus on a number of adaptors that are important for lymphocyte function and emphasize the various ways in which these proteins carry out their essential roles.

The transmembrane adapter LAT plays a central role in immune receptor signalling

The transmembrane adapter LAT (linker for activation of T cells) plays a central role in signalling by ITAM bearing receptors expressed on T cells, natural killer cells, mast cells and platelets. Receptor engagement leads to the phosphorylation of tyrosine residues present in the intracellular domain of LAT and formation of a multiprotein complex with other adapter molecules and enzymes including Grb2, Gads/SLP-76 and PLCgamma isoforms. These signalling events predominantly take place in glycolipid-enriched membrane domains. The constitutive presence of LAT in GEMs enables its function as the main scaffolding protein for the organization of GEM-localized signalling. The study of LAT-deficient mice and LAT-deficient cell lines further emphasizes the importance of LAT for these signalling cascades but also defines the existence of LAT-independent events downstream of the Syk-family kinase-ITAM complex.

The role of the adapter molecule SLP-76 in platelet function

Following vascular injury, one of the most critical initial events is activation of platelets followed by formation of a hemostatic plug. Platelets are capable of responding to a diverse array of agonists resulting in adhesion and granule release. The biochemical events underlying platelet activation are just beginning to be understood. One class of molecules shown to play important roles in this process is adapters. Adapter molecules contain distinct modular domains which mediate protein-protein or protein-lipid interactions giving these proteins the ability to nucleate signal transduction complexes. In this review we will discuss the function of the hematopoietic cell specific adapter molecule, SLP-76 in both platelet activation and hemostasis. Because many parallels exist between signal transduction pathways in platelets and lymphocytes, we will also review the function of SLP-76 in coordinating signal transduction pathways following antigen bind to the T cell receptor.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

Membrane raft-dependent regulation of phospholipase Cgamma-1 activation in T lymphocytes

Numerous signaling molecules associate with lipid rafts, either constitutively or after engagement of surface receptors. One such molecule, phospholipase Cgamma-1 (PLCgamma1), translocates from the cytosol to lipid rafts during T-cell receptor (TCR) signaling. To investigate the role played by lipid rafts in the activation of this molecule in T cells, an influenza virus hemagglutinin A (HA)-tagged PLCgamma1 was ectopically expressed in Jurkat T cells and targeted to these microdomains by the addition of a dual-acylation signal. Raft-targeted PLCgamma1 was constitutively tyrosine phosphorylated and induced constitutive NF-AT-dependent transcription and interleukin-2 secretion in Jurkat cells. Tyrosine phosphorylation of raft-targeted PLCgamma1 did not require Zap-70 or the interaction with the adapters Lat and Slp-76, molecules that are necessary for TCR signaling. In contrast, the Src family kinase Lck was required. Coexpression in HEK 293T cells of PLCgamma1-HA with Lck or the Tec family kinase Rlk resulted in preferential phosphorylation of raft-targeted PLCgamma1 over wild-type PLCgamma1. These data show that localization of PLCgamma1 in lipid rafts is sufficient for its activation and demonstrate a role for lipid rafts as microdomains that dynamically segregate and integrate PLCgamma1 with other signaling components.

Vav proteins, adaptors and cell signaling

The Vav family is a group of signal transduction molecules with oncogenic potential that play important roles in development and cell signaling. Members of this family are distributed in all animal metazoans but not in unicellular organisms. Recent genomic studies suggest that the function of Vav proteins co-evolved with tyrosine kinase pathways, probably to assure the optimal conversion of extracellular signals into biological responses coupled to the cytoskeleton and gene transcription. To date, the best-known function of Vav proteins is their role as GDP/GTP exchange factors for Rho/Rac molecules, a function strictly controlled by tyrosine phosphorylation. Recent publications indicate that this function is highly dependent on the interaction of adaptor proteins that aid in the proper phosphorylation of Vav proteins, their interaction with other signaling molecules, and in modulating the strength of their signal outputs. In addition to the function of Vav proteins as exchange factors, there is increasing evidence suggesting that Vav proteins can mediate other cellular functions independently of their exchange activities, probably by working themselves as adaptor molecules. In this review, we will give a summary of the recent advances in this field, placing special emphasis on the non-catalytic roles of Vav and its interaction with other adaptor molecules.

MIST functions through distinct domains in immunoreceptor signaling in the presence and absence of LAT

MIST (also termed Clnk) is an adaptor protein structurally related to SLP-76 and BLNK/BASH/SLP-65 hematopoietic cell-specific adaptor proteins. By using the BLNK-deficient DT40 chicken B cell system, we demonstrated MIST functions through distinct intramolecular domains in immunoreceptor signaling depending on the availability of linker for activation of T cells (LAT). MIST can partially restore the B cell antigen receptor (BCR) signaling in the BLNK-deficient cells, which requires phosphorylation of the two N-terminal tyrosine residues. Co-expression of LAT with MIST fully restored the BCR signaling and dispenses with the requirement of the two tyrosines in MIST for BCR signaling. However, some other tyrosine(s), as well as the Src homology (SH) 2 domain and the two proline-rich regions in MIST, is still required for full reconstitution of the BCR signaling, in cooperation with LAT. The C-terminal proline-rich region of MIST is dispensable for the LAT-aided full restoration of MAP kinase activation, although it is responsible for the interaction with LAT and for the localization in glycolipid-enriched microdomains. On the other hand, the N-terminal proline-rich region, which is a binding site of the SH3 domain of phospholipase Cgamma, is essential for BCR signaling. These results revealed a marked plasticity of MIST function as an adaptor in the cell contexts with or without LAT.

Positive regulation of T cell activation and integrin adhesion by the adapter Fyb/Slap

The molecular adapter Fyb/Slap regulates signaling downstream of the T cell receptor (TCR), but whether it plays a positive or negative role is controversial. We demonstrate that Fyb/Slap-deficient T cells exhibit defective proliferation and cytokine production in response to TCR stimulation. Fyb/Slap is also required in vivo for T cell-dependent immune responses. Functionally, Fyb/Slap has no apparent role in the activation of known TCR signaling pathways, F-actin polymerization, or TCR clustering. Rather, Fyb/Slap regulates TCR-induced integrin clustering and adhesion. Thus, Fyb/Slap is the first molecular adapter to be identified that couples TCR stimulation to the avidity modulation of integrins governing T cell adhesion.

Coupling of the TCR to integrin activation by Slap-130/Fyb

SLAP-130/Fyb (SLP-76-associated phosphoprotein or Fyn-binding protein; also known as Fyb/Slap) is a hematopoietic-specific adapter, which associates with and modulates function of SH2-containing leukocyte phosphoprotein of 76 kilodaltons (SLP-76). T cells from mice lacking SLAP-130/Fyb show markedly impaired proliferation following CD3 engagement. In addition, the T cell receptor (TCR) in SLAP-130/Fyb mutant cells fails to enhance integrin-dependent adhesion. Although TCR-induced actin polymerization is normal, TCR-stimulated clustering of the integrin LFA-1 is defective in SLAP-130/Fyb-deficient cells. These data indicate that SLAP-130/Fyb is important for coupling TCR-mediated actin cytoskeletal rearrangement with activation of integrin function, and for T cells to respond fully to activating signals.

Synergistic regulation of immunoreceptor signaling by SLP-76-related adaptor Clnk and serine/threonine protein kinase HPK-1

Recently, the identification of Clnk, a third member of the SLP-76 family of adaptors expressed exclusively in cytokine-stimulated hemopoietic cells, has been reported by us and by others. Like SLP-76 and Blnk, Clnk was shown to act as a positive regulator of immunoreceptor signaling. Interestingly, however, it did not detectably associate with known binding partners of SLP-76, including Vav, Nck, and GADS. In contrast, it became complexed in activated T cells and myeloid cells with an as yet unknown tyrosine-phosphorylated polypeptide of approximately 92 kDa (p92). In order to understand better the function of Clnk, we sought to identify the Clnk-associated p92. Using a yeast two-hybrid screen and cotransfection experiments with Cos-1 cells, evidence was adduced that p92 is HPK-1, a serine/threonine-specific protein kinase expressed in hemopoietic cells. Further studies showed that Clnk and HPK-1 were also associated in hemopoietic cells and that their interaction was augmented by immunoreceptor stimulation. A much weaker association was detected between HPK-1 and SLP-76. Transient transfections in Jurkat T cells revealed that Clnk and HPK-1 cooperated to increase immunoreceptor-mediated activation of the interleukin 2 (IL-2) promoter. Moreover, the ability of Clnk to stimulate IL-2 promoter activity could be blocked by expression of a kinase-defective version of HPK-1. Lastly we found that in spite of the differential ability of Clnk and SLP-76 to bind cellular proteins, Clnk was apt at rescuing immunoreceptor signaling in a Jurkat T-cell variant lacking SLP-76. Taken together, these results show that Clnk physically and functionally interacts with HPK-1 in hemopoietic cells. Moreover, they suggest that Clnk is capable of functionally substituting for SLP-76 in immunoreceptor signaling, albeit by using a distinct set of intracellular effectors.

Cutting edge: T cell migration regulated by CXCR4 chemokine receptor signaling to ZAP-70 tyrosine kinase

Chemokines regulate the homeostatic trafficking of lymphocytes and lymphocyte influx into sites of injury and inflammation. The signaling pathways by which chemokine receptors regulate lymphocyte migration remain incompletely characterized. We demonstrate that Jurkat T cells lacking the ZAP-70 tyrosine kinase exhibit reduced migration in response to the CXCR4 ligand CXCL12 when compared with wild-type Jurkat T cells. Expression of wild-type, but not kinase-inactive, ZAP-70 resulted in enhanced migration of ZAP-70-deficient Jurkat T cells. The tyrosine residue at position 292 in the interdomain B region of ZAP-70 exerts a negative regulatory effect on ZAP-70-dependent migration. Stimulation of Jurkat T cells with CXCL12 also resulted in ZAP-70-dependent tyrosine phosphorylation of the Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76) adapter protein. Although CXCL12-dependent migration of SLP-76-deficient Jurkat T cells was impaired, re-expression of SLP-76 did not enhance migration. These results suggest a novel function for ZAP-70, but not SLP-76, in CXCR4 chemokine receptor signaling in human T cells.

Identification of the minimal tyrosine residues required for linker for activation of T cell function

The linker for activation of T cells (LAT) is essential for signaling through the T cell receptor (TCR). Following TCR stimulation, LAT becomes tyrosine-phosphorylated, creating docking sites for other signaling proteins such as phospholipase C-gamma(1) (PLC-gamma(1)), Grb2, and Gads. In this study, we have attempted to identify the critical tyrosine residues in LAT that mediate TCR activation-induced mobilization of intracellular Ca(2+) and activation of the MAP kinase Erk2. By using the LAT-deficient Jurkat derivative, J.CaM2, stable cell lines were established expressing various tyrosine mutants of LAT. We show that three specific tyrosine residues (Tyr(132), Tyr(171), and Tyr(191)) are necessary and sufficient to achieve a Ca(2+) flux following TCR stimulation. These tyrosine residues function by reconstituting PLC-gamma(1) phosphorylation and recruitment to LAT. However, these same tyrosines can only partially reconstitute Erk activation. Full reconstitution of Erk requires two additional tyrosine residues (Tyr(110) and Tyr(226)), both of which have the Grb2-binding motif YXN. This reconstitution of Erk activation requires that the critical tyrosine residues be on the same molecule of LAT, suggesting that a single LAT molecule nucleates multiple protein-protein interactions required for optimal signal transduction.

Mapping the Zap-70 phosphorylation sites on LAT (linker for activation of T cells) required for recruitment and activation of signalling proteins in T cells

T-cell-receptor (TCR)-mediated LAT (linker for activation of T cells) phosphorylation is critical for the membrane recruitment of signalling complexes required for T-cell activation. Although tyrosine phosphorylation of LAT is required for recruitment and activation of signalling proteins, the molecular mechanism associated with this event is unclear. In the present study we reconstituted the LAT signalling pathway by demonstrating that a direct tyrosine phosphorylation of LAT with activated protein-tyrosine kinase Zap70 is necessary and sufficient for the association and activation of signalling proteins. Zap-70 efficiently phosphorylates LAT on tyrosine residues at positions 226, 191, 171, 132 and 127. By substituting these tyrosine residues in LAT with phenylalanine and by utilizing phosphorylated peptides derived from these sites, we mapped the tyrosine residues in LAT required for the direct interaction and activation of Vav, p85/p110alpha and phospholipase Cgamma1 (PLCgamma1). Our results indicate that Tyr(226) and Tyr(191) are required for Vav binding, whereas Tyr(171) and Tyr(132) are necessary for association and activation of phosphoinositide 3-kinase activity and PLCgamma1 respectively. Furthermore, by expression of LAT mutants in LAT-deficient T cells, we demonstrate that Tyr(191) and Tyr(171) are required for T-cell activation and Tyr(132) is required for the activation of PLCgamma1 and Ras signalling pathways.

Identification of a phospholipase C-gamma1 (PLC-gamma1) SH3 domain-binding site in SLP-76 required for T-cell receptor-mediated activation of PLC-gamma1 and NFAT

SLP-76 is an adapter protein required for T-cell receptor (TCR) signaling. In particular, TCR-induced tyrosine phosphorylation and activation of phospholipase C-gamma1 (PLC-gamma1), and the resultant TCR-inducible gene expression, depend on SLP-76. Nonetheless, the mechanisms by which SLP-76 mediates PLC-gamma1 activation are not well understood. We now demonstrate that SLP-76 directly interacts with the Src homology 3 (SH3) domain of PLC-gamma1. Structure-function analysis of SLP-76 revealed that each of the previously defined protein-protein interaction domains can be individually deleted without completely disrupting SLP-76 function. Additional deletion mutations revealed a new, 67-amino-acid functional domain within the proline-rich region of SLP-76, which we have termed the P-1 domain. The P-1 domain mediates a constitutive interaction of SLP-76 with the SH3 domain of PLC-gamma1 and is required for TCR-mediated activation of Erk, PLC-gamma1, and NFAT (nuclear factor of activated T cells). The adjacent Gads-binding domain of SLP-76, also within the proline-rich region, mediates inducible recruitment of SLP-76 to a PLC-gamma1-containing complex via the recruitment of both PLC-gamma1 and Gads to another cell-type-specific adapter, LAT. Thus, TCR-induced activation of PLC-gamma1 entails the binding of PLC-gamma1 to both LAT and SLP-76, a finding that may underlie the requirement for both LAT and SLP-76 to mediate the optimal activation of PLC-gamma1.

Regulation of NK cell-mediated cytotoxicity by the adaptor protein 3BP2

Stimulation of lymphocytes through multichain immune recognition receptors activates multiple signaling pathways. Adaptor proteins play an important role in integrating these pathways by their ability to simultaneously bind multiple signaling components. Recently, the 3BP2 adaptor protein has been shown to positively regulate the transcriptional activity of T cells. However, the mechanisms by which signaling components are involved in this regulation remain unclear, as does a potential role for 3BP2 in the regulation of other cellular functions. Here we describe a positive regulatory role for 3BP2 in NK cell-mediated cytotoxicity. We also identify p95(vav) and phospholipase C-gamma isoforms as binding partners of 3BP2. Our results show that tyrosine-183 of 3BP2 is specifically involved in this interaction and that this residue critically influences 3BP2-dependent function. Therefore, 3BP2 regulates NK cell-mediated cytotoxicity by mobilizing key downstream signaling effectors.

CD95/Fas induces cleavage of the GrpL/Gads adaptor and desensitization of antigen receptor signaling

The balance between cell survival and cell death is critical for normal lymphoid development. This balance is maintained by signals through lymphocyte antigen receptors and death receptors such as CD95/Fas. In some cells, ligating the B cell antigen receptor can protect the cell from apoptosis induced by CD95. Here we report that ligation of CD95 inhibits antigen receptor-mediated signaling. Pretreating CD40-stimulated tonsillar B cells with anti-CD95 abolished B cell antigen receptor-mediated calcium mobilization. Furthermore, CD95 ligation led to the caspase-dependent inhibition of antigen receptor-induced calcium mobilization and to the activation of mitogen-activated protein kinase pathways in B and T cell lines. A target of CD95-mediated caspase 3-like activity early in the apoptotic process is the adaptor protein GrpL/Gads. GrpL constitutively interacts with SLP-76 via its C-terminal SH3 domain to regulate transcription factors such as NF-AT. Cleavage of GrpL removes the C-terminal SH3 domain so that it is no longer capable of recruiting SLP-76 to the membrane. Transfection of a truncated form of GrpL into Jurkat T cells blocked T cell antigen receptor-induced activation of NF-AT. These results suggest that CD95 signaling can desensitize antigen receptors, in part via cleavage of the GrpL adaptor.

Suppression of thymic development by the dominant-negative form of Gads

Gads, a hematopoietic-lineage-specific Grb2 family member, is involved in the signaling mediated by the TCR through its interactions with SLP-76 and LAT. Here, we generated transgenic mice expressing Grf40-dSH2, an SH2-deleted dominant-negative form of Gads, which is driven by the lck proximal promoter. The total number of thymocytes was profoundly reduced in the transgenic mice, whereas in the double-negative (CD4(-)CD8(-)) thymocyte subset, in particular the CD25(+)CD44(-) pre-T cell population, it was significantly increased. However, CD5 expression, which is mediated by pre-TCR stimulation, was significantly suppressed on the CD4(-)CD8(-) thymocytes of the transgenic mice. Furthermore, the SLP-76-dependent signaling was markedly suppressed as well. These data suggest that Gads plays an important role in the pre-TCR as well as TCR signaling in thymocytes.

Adapters in lymphocyte signalling

Adapter proteins are well recognised as important molecular switches connecting immunoreceptors with intracellular signalling pathways. However, recent data suggest that homeostasis within the lymphatic system also depends on the coordinated activities of negative regulatory adapter proteins. These prevent activation of lymphocytes in the absence of externally applied signals and regulate termination/limitation of ongoing immune responses via different mechanisms.

Prolonged exposure of T cells to TNF down-regulates TCR zeta and expression of the TCR/CD3 complex at the cell surface

A role for TNF-alpha in the pathogenesis of chronic inflammatory disease is now firmly established. Paradoxically, TNF also has potent immunomodulatory effects on CD4(+) T lymphocytes, because Ag-specific proliferative and cytokine responses are suppressed following prolonged exposure to TNF. We explored whether TNF attenuated T cell activation by uncoupling proximal TCR signal transduction pathways using a mouse T cell hybridoma model. Chronic TNF exposure induced profound, but reversible, T cell hyporesponsiveness, with TNF-treated T cells requiring TCR engagement with higher peptide concentrations for longer periods of time for commitment to IL-2 production. Subsequent experiments revealed that chronic TNF exposure led to a reversible loss of TCRzeta chain expression, in part through a reduction in gene transcription. Down-regulation of TCRzeta expression impaired TCR/CD3 assembly and expression at the cell surface and uncoupled membrane-proximal tyrosine phosphorylation events, including phosphorylation of the TCRzeta chain itself, CD3epsilon, ZAP-70 protein tyrosine kinase, and linker for activation of T cells (LAT). Intracellular Ca(2+) mobilization was also suppressed in TNF-treated T cells. We propose that TNF may contribute to T cell hyporesponsiveness in chronic inflammatory and infectious diseases by mechanisms that include down-regulation of TCRzeta expression. We speculate that by uncoupling proximal TCR signals TNF could also interrupt mechanisms of peripheral tolerance that are dependent upon intact TCR signal transduction pathways.

ZAP-70 and SLP-76 regulate protein kinase C-theta and NF-kappa B activation in response to engagement of CD3 and CD28

The transcription factor NF-kappaB is a critical regulator of T cell function that becomes strongly activated in response to coengagement of TCR and CD28. Although events immediately proximal to NF-kappaB activation are well understood, uncertainty remains over which upstream signaling pathways engaged by TCR and CD28 lead to NF-kappaB activation. By using Jurkat T cell lines that are deficient or replete for either the protein tyrosine kinase ZAP-70 or the cytosolic adapter molecule SLP-76, the role of these proteins in modulating NF-kappaB activation was examined. NF-kappaB was not activated in response to coengagement of TCR and CD28 in either the ZAP-70- or SLP-76-negative cells, whereas stimuli that bypass these receptors (PMA plus A23187, or TNF-alpha) activated NF-kappaB normally. Protein kinase C (PKC) theta activation, which is required for NF-kappaB activation, also was defective in these cells. Reexpression of ZAP-70 restored PKCtheta and NF-kappaB activation in response to TCR and CD28 coengagement. p95(vav) (Vav)-1 tyrosine phosphorylation was largely unperturbed in the ZAP-70-negative cells; however, receptor-stimulated SLP-76/Vav-1 coassociation was greatly reduced. Wild-type SLP-76 fully restored PKCtheta and NF-kappaB activation in the SLP-76-negative cells, whereas 3YF-SLP-76, which lacks the sites of tyrosine phosphorylation required for Vav-1 binding, only partially rescued signaling. These data illustrate the importance of the ZAP-70/SLP-76 signaling pathway in CD3/CD28-stimulated activation of PKC theta and NF-kappaB, and suggest that Vav-1 association with SLP-76 may be important in this pathway.

Protein tyrosine phosphatase CD148-mediated inhibition of T-cell receptor signal transduction is associated with reduced LAT and phospholipase Cgamma1 phosphorylation

In this study, we investigate the role of the receptor-like protein tyrosine phosphatase CD148 in T-cell activation. Overexpression of CD148 in the Jurkat T-cell line inhibited activation of the transcription factor nuclear factor of activated T cells following T-cell receptor (TCR) stimulation but not following stimulation through a heterologously expressed G protein-coupled receptor, the human muscarinic receptor subtype 1. Using a tetracycline-inducible expression system, we show that the TCR-mediated activation of both the Ras and calcium pathways was inhibited by expression of CD148 at levels that approximate those found in activated primary T cells. These effects were dependent on the phosphatase activity of CD148. Analysis of TCR-induced protein tyrosine phosphorylation demonstrated that most phosphoproteins were unaffected by CD148 expression. However, phospholipase Cgamma1 (PLCgamma1) and LAT were strikingly hypophosphorylated in CD148-expressing cells following TCR stimulation, whereas the phosphorylation levels of Slp-76 and Itk were modestly reduced. Based on these results, we propose that CD148 negatively regulates TCR signaling by interfering with the phosphorylation and function of PLCgamma1 and LAT.

Caspase-dependent cleavage of the hematopoietic specific adaptor protein Gads alters signalling from the T cell receptor

Gads is a SH2 and SH3 domain-containing, hematopoietic-specific adaptor protein that functions in signalling from the T cell receptor. Gads acts by linking SLP-76, bound by the carboxy-terminal Gads SH3 domain, to tyrosine phosphorylated LAT which contains binding sites for the Gads SH2 domain. Gads is distinguished from Grb2 and the closely related Grap protein by the presence of a 120 amino acid unique region between the SH2 domain and the carboxy terminal SH3 domain. Here we demonstrate that the unique region of Gads contains a capase cleavage site. Induction of apoptosis in lymphocytes results in detectable Gads cleavage by 60 min. Gads cleavage is blocked in vivo by treating cells with a caspase 3 inhibitor. A putative caspase 3 cleavage site was identified within the unique region and mutation of this site prevented Gads cleavage in vitro, and in vivo. The Gads cleavage products retained the predicted binding specificity for SLP-76 and LAT. Expression of the Gads cleavage products in Jurkat T cells inhibited NFAT activation following TCR cross linking. These findings indicate that cleavage of Gads in vivo could function to alter signalling downstream of the T cell receptor by disrupting cross talk between SLP-76 and LAT.

Dynamic actin polymerization drives T cell receptor-induced spreading: a role for the signal transduction adaptor LAT

T cell activation induces functional changes in cell shape and cytoskeletal architecture. To facilitate the collection of dynamic, high-resolution images of activated T cells, we plated T cells on coverslips coated with antibodies to the T cell receptor (TCR). Using these images, we were able to quantitate the morphological responses of individual cells over time. Here, we show that TCR engagement triggers the formation and expansion of contacts bounded by continuously remodeled actin-rich rings. These processes are associated with the extension of lamellipodia and require actin polymerization, tyrosine kinase activation, cytoplasmic calcium increases, and LAT, an important hematopoietic adaptor. In addition, the maintenance of the resulting contact requires sustained calcium influxes, an intact microtubule cytoskeleton, and functional LAT.

A PAK1-PIX-PKL complex is activated by the T-cell receptor independent of Nck, Slp-76 and LAT

Given the importance of the Rho GTPase family member Rac1 and the Rac1/Cdc42 effector PAK1 in T-cell activation, we investigated the requirements for their activation by the T-cell receptor (TCR). Rac1 and PAK1 activation required the tyrosine kinases ZAP-70 and Syk, but not the cytoplasmic adaptor Slp-76. Surprisingly, PAK1 was activated in the absence of the transmembrane adaptor LAT while Rac1 was not. However, efficient PAK1 activation required its binding sites for Rho GTPases and for PIX, a guanine nucleotide exchange factor for Rho GTPases. The overexpression of ssPIX that either cannot bind PAK1 or lacks GEF function blocked PAK1 activation. These data suggest that a PAK1-PIX complex is recruited to appropriate sites for activation and that PIX is required for Rho family GTPase activation upstream of PAK1. Furthermore, we detected a stable trimolecular complex of PAK1, PIX and the paxillin kinase linker p95PKL. Taken together, these data show that PAK1 contained in this trimolecular complex is activated by a novel LAT- and Slp-76-independent pathway following TCR stimulation.

Allelic exclusion and differentiation by protein kinase C-mediated signals in immature thymocytes

Pre-T cell receptor (preTCR)-derived signals mediate the transition of thymocytes from the CD4(-) CD8(-) double-negative (DN) to CD4(+) CD8(+) double-positive stage of T lymphocyte development. This progression, termed beta-selection, is limited to thymocytes that have generated a functional TCR-beta chain able to associate with pTalpha to form the preTCR complex. Formation of the preTCR complex not only induces differentiation, survival, and proliferation of DN thymocytes; it also inhibits further TCR-beta gene rearrangement through an ill-defined process known as allelic exclusion. The signaling pathways controlling this critical developmental checkpoint have not been characterized. Here we demonstrate that formation of the preTCR complex leads to the activation of protein kinase C (PKC), and that activation of PKC is necessary for the differentiation and expansion of DN thymocytes. Importantly, we also show that allelic exclusion at the TCR-beta gene loci is enforced by PKC-mediated signals. These results define PKC as a central mediator of both differentiation and allelic exclusion during thymocyte development.

Allelic exclusion and differentiation by protein kinase C-mediated signals in immature thymocytes

Pre-T cell receptor (preTCR)-derived signals mediate the transition of thymocytes from the CD4(-) CD8(-) double-negative (DN) to CD4(+) CD8(+) double-positive stage of T lymphocyte development. This progression, termed beta-selection, is limited to thymocytes that have generated a functional TCR-beta chain able to associate with pTalpha to form the preTCR complex. Formation of the preTCR complex not only induces differentiation, survival, and proliferation of DN thymocytes; it also inhibits further TCR-beta gene rearrangement through an ill-defined process known as allelic exclusion. The signaling pathways controlling this critical developmental checkpoint have not been characterized. Here we demonstrate that formation of the preTCR complex leads to the activation of protein kinase C (PKC), and that activation of PKC is necessary for the differentiation and expansion of DN thymocytes. Importantly, we also show that allelic exclusion at the TCR-beta gene loci is enforced by PKC-mediated signals. These results define PKC as a central mediator of both differentiation and allelic exclusion during thymocyte development.

Altered T-cell receptor + CD28-mediated signaling and blocked cell cycle progression in interleukin 10 and transforming growth factor-beta-treated alloreactive T cells that do not induce graft-versus-host disease

The induction of anergy in T cells, although widely accepted as critical for the maintenance of tolerance, is still poorly understood at the molecular level. Recent evidence demonstrates that in addition to blockade of costimulation using monoclonal antibodies (mAbs) directed against cell surface determinants, treatment of mixed lymphocyte reaction (MLR) cultures with interleukin 10 (IL-10) and transforming growth factor-beta (TGF-beta) results in induction of tolerance, rendering alloreactive murine CD4(+) T cells incapable of inducing graft-versus-host disease (GVHD) after in vivo transfer to histoincompatible recipients. The present study, using these cells prior to adoptive transfer, determined that IL-10 + TGF-beta-tolerant CD4(+) T cells exhibit an altered pattern of T-cell receptor (TCR) + CD28-mediated signaling and are incapable of progressing out of the G(1) phase of the cell cycle during stimulation with HLA class II disparate antigen-presenting cells. TGFbeta + IL-10-tolerant cells were incapable of phosphorylating TCR-zeta, or activating ZAP-70, Ras, and MAPK, similarly to T-cell tolerized by blockade of B7/CD28 and CD40/CD40L pathways. Moreover, these cells were incapable of clonal expansion due to defective synthesis of cyclin D3 and cyclin A, and defective activation of cyclin-dependent kinase (cdk)4, cdk6, and cdk2. These cells also exhibited defective down-regulation of p27(kip1) cdk inhibitor and lack of cyclin D2-cdk4 activation, Rb hyperphosphorylation, and progression to the S phase of the cell cycle. These data link anergy-specific proximal biochemical alterations and the downstream nuclear pathways that control T-cell expansion and provide a biochemical profile of IL-10 + TGF-beta-tolerant alloreactive T cells that do not induce GVHD when transferred into MHC class II disparate recipients in vivo.

Distinct T cell developmental consequences in humans and mice expressing identical mutations in the DLAARN motif of ZAP-70

The protein tyrosine kinase, ZAP-70, is pivotally involved in transduction of Ag-binding signals from the TCR required for T cell activation and development. Defects in ZAP-70 result in SCID in humans and mice. We describe an infant with SCID due to a novel ZAP-70 mutation, comparable with that which arose spontaneously in an inbred mouse colony. The patient inherited a homozygous missense mutation within the highly conserved DLAARN motif in the ZAP-70 kinase domain. Although the mutation only modestly affected protein stability, catalytic function was absent. Despite identical changes in the amino acid sequence of ZAP-70, the peripheral T cell phenotypes of our patient and affected mice are distinct. ZAP-70 deficiency in this patient, as in other humans, is characterized by abundant nonfunctional CD4(+) T cells and absent CD8(+) T cells. In contrast, ZAP-70-deficient mice lack both major T cell subsets. Although levels of the ZAP-70-related protein tyrosine kinase, Syk, may be sufficiently increased in human thymocytes to rescue CD4 development, survival of ZAP-70-deficient T cells in the periphery does not appear to be dependent on persistent up-regulation of Syk expression.