Requirement for tyrosine residues 315 and 319 within zeta chain-associated protein 70 for T cell development

A Story of Kinases and Adaptors: The Role of Lck, ZAP-70 and LAT in Switch Panel Governing T-Cell Development and Activation

Specific antigen recognition is one of the immune system's features that allows it to mount intense yet controlled responses to an infinity of potential threats. T cells play a relevant role in the host defense and the clearance of pathogens by means of the specific recognition of peptide antigens presented by antigen-presenting cells (APCs), and, to do so, they are equipped with a clonally distributed antigen receptor called the T-cell receptor (TCR). Upon the specific engagement of the TCR, multiple intracellular signals are triggered, which lead to the activation, proliferation and differentiation of T lymphocytes into effector cells. In addition, this signaling cascade also operates during T-cell development, allowing for the generation of cells that can be helpful in the defense against threats, as well as preventing the generation of autoreactive cells. Early TCR signals include phosphorylation events in which the tyrosine kinases Lck and ZAP70 are involved. The sequential activation of these kinases leads to the phosphorylation of the transmembrane adaptor LAT, which constitutes a signaling hub for the generation of a signalosome, finally resulting in T-cell activation. These early signals play a relevant role in triggering the development, activation, proliferation and apoptosis of T cells, and the negative regulation of these signals is key to avoid aberrant processes that could generate inappropriate cellular responses and disease. In this review, we will examine and discuss the roles of the tyrosine kinases Lck and ZAP70 and the membrane adaptor LAT in these cellular processes.

Negative Regulation of Zap70 by Lck Forms the Mechanistic Basis of Differential Expression in CD4 and CD8 T Cells

Lck and Zap70, two non-receptor tyrosine kinases, play a crucial role in the regulation of membrane proximal TCR signaling critical for thymic selection, CD4/CD8 lineage choice and mature T cell function. Signal initiation upon TCR/CD3 and peptide/MHC interaction induces Lck-mediated phosphorylation of CD3 ITAMs. This is necessary for Zap70 recruitment and its phosphorylation by Lck leading to full Zap70 activation. In its native state Zap70 maintains a closed conformation creating an auto-inhibitory loop, which is relieved by Lck-mediated phosphorylation of Y315/Y319. Zap70 is differentially expressed in thymic subsets and mature T cells with CD8 T cells expressing the highest amount compared to CD4 T cells. However, the mechanistic basis of differential Zap70 expression in thymic subsets and mature T cells is not well understood. Here, we show that Zap70 is degraded relatively faster in DP and mature CD4 T cells compared to CD8 T cells, and inversely correlated with relative level of activated Zap70. Importantly, we found that Zap70 expression is negatively regulated by Lck activity: augmented Lck activity resulting in severe diminution in total Zap70. Moreover, Lck-mediated phosphorylation of Y315/Y319 was essential for Zap70 degradation. Together, these data shed light on the underlying mechanism of Lck-mediated differential modulation of Zap70 expression in thymic subsets and mature T cells.

TULA proteins as signaling regulators

Two members of the UBASH3/STS/TULA family exhibit a unique protein domain structure, which includes a histidine phosphatase domain, and play a key role in regulating cellular signaling. UBASH3A/STS-2/TULA is mostly a lymphoid protein, while UBASH3B/STS-1/TULA-2 is expressed ubiquitously. Dephosphorylation of tyrosine-phosphorylated proteins by TULA-2 and, probably to a lesser extent, by TULA critically contribute to the molecular basis of their regulatory effect. The notable differences between the effects of the two family members on cellular signaling and activation are likely to be linked to the difference between their specific enzymatic activities. However, these differences might also be related to the functions of their domains other than the phosphatase domain and independent of their phosphatase activity. The down-regulation of the Syk/Zap-70-mediated signaling, which to-date appears to be the best-studied regulatory effect of TULA family, is discussed in detail in this publication.

T cell receptor dwell times control the kinase activity of Zap70

Kinase recruitment to membrane receptors is essential for signal transduction. However, the underlying regulatory mechanisms are poorly understood. We investigated how conformational changes control T cell receptor (TCR) association and activity of the kinase Zap70. Structural analysis showed that TCR binding or phosphorylation of Zap70 triggers a transition from a closed, autoinhibited conformation to an open conformation. Using Zap70 mutants with defined conformations, we found that TCR dwell times controlled Zap70 activity. The closed conformation minimized TCR dwell times and thereby prevented activation by membrane-associated kinases. Parallel recruitment of coreceptor-associated Lck kinase to the TCR ensured Zap70 phosphorylation and stabilized Zap70 TCR binding. Our study suggests that the dynamics of cytosolic enzyme recruitment to the plasma membrane regulate the activity and function of receptors lacking intrinsic catalytic activity.

Structural basis for activation of ZAP-70 by phosphorylation of the SH2-kinase linker

Serial activation of the tyrosine kinases Lck and ZAP-70 initiates signaling downstream of the T cell receptor. We previously reported the structure of an autoinhibited ZAP-70 variant in which two regulatory tyrosine residues (315 and 319) in the SH2-kinase linker were replaced by phenylalanine. We now present a crystal structure of ZAP-70 in which Tyr 315 and Tyr 319 are not mutated, leading to the recognition of a five-residue sequence register error in the SH2-kinase linker of the original crystallographic model. The revised model identifies distinct roles for these two tyrosines. As seen in a recently reported structure of the related tyrosine kinase Syk, Tyr 315 of ZAP-70 is part of a hydrophobic interface between the regulatory apparatus and the kinase domain, and the integrity of this interface would be lost upon engagement of doubly phosphorylated peptides by the SH2 domains. Tyr 319 is not necessarily dislodged by SH2 engagement, which activates ZAP-70 only ∼5-fold in vitro. In contrast, phosphorylation by Lck activates ZAP-70 ∼100-fold. This difference is due to the ability of Tyr 319 to suppress ZAP-70 activity even when the SH2 domains are dislodged from the kinase domain, providing stringent control of ZAP-70 activity downstream of Lck.

LCK is an important mediator of B-cell receptor signaling in chronic lymphocytic leukemia cells

B-cell receptor (BCR) signals promote survival of chronic lymphocytic leukemia (CLL) cells, and it is believed that overexpressed and constitutively active Lyn mediates this signaling. Here, we show that CLL cells express lymphocyte-specific protein tyrosine kinase (LCK) and that inhibition of this Src family tyrosine kinase with the specific inhibitor [4-amino-5-(4-phenoxyphenyl)-7H-pyrrolo[3,2-d]pyrimidin-7-yl-cyclopentane (Lck-i)], or reduction of its expression with siRNA, blocks the induction of CD79a, Syk, inhibitor of IκB kinase (IKK), Akt, and extracellular signal-regulated kinase (ERK) phosphorylation by BCR cross-linking in these cells. Furthermore, we show that CLL cells with high levels of LCK expression have higher levels of BCR-mediated IKK, Akt, and ERK phosphorylation as well as cell survival than CLL cells with low levels of LCK expression. We also show that treatment of CLL cells with Lck-i inhibits BCR cross-linking-induced cell survival. Taken together, these data show a major role for LCK in proximal and distal BCR-mediated signaling in CLL cells and suggest that LCK expression is important in the pathogenesis of this disease. On a clinical level, these studies advocate the use of specific LCK inhibitors in the treatment of progressive CLL.

Fine-tuning of proximal TCR signaling by ZAP-70 tyrosine residues in Jurkat cells

Zeta-chain-associated protein kinase of 70kDa (ZAP-70) kinase is a key regulator in the early steps of TCR signaling but some aspects of its fine regulation are still unclear. From its 31 tyrosine (Y) residues, 11 phosphorylation sites have been identified, some with activator (Y315 and Y493) or inhibitory (Y292 and Y492) and others with unknown function (Y069, Y126 and Y178). In our present work, we aimed to elucidate the role of different Y residues of ZAP-70, especially those with unknown function, in calcium signaling and the autoregulation of the kinase. ZAP-70-deficient Jurkat cells (P116) were stably reconstituted with point-mutated ZAP-70 constructs where tyrosine residues 069, 126, 178, 238, 292, 315, 492 or 493 were replaced with phenylalanine (F). The anti-CD3-elicited calcium signal increased in F069-, F292- and F492-ZAP-70-expressing cell lines but decreased in the F126-, F315- and F493-ZAP-70-expressing cell lines. ZAP-70 point mutations led to phosphorylation changes predominantly in SH2 domain containing leukocyte protein of 76kDa (SLP-76) but not linker of activated T cells (LAT) during CD3-activation; moreover, we detected basal hyperphosphorylation of SLP-76 Y128 in the F126-, F178- and F492-ZAP-70-expressing cell lines. In summary, Y069, Y178, Y292 and Y492 have inhibitory, while Y126, Y315 and Y493 activator role in anti-CD3-induced T-cell activation. Phosphorylation changes in LAT and SLP-76 suggest that fine regulation of ZAP-70 on calcium signaling is rather transmitted through SLP-76 not LAT. Additionally, negative or positive autoregulatory function of Y292 and Y493 or Y315, respectively, was revealed in ZAP-70. These data indicate that previously not characterized Y069, Y126 and Y178 in ZAP-70 participate in the fine regulation of TCR signaling.

Ethanol inhibits lipid raft-mediated TCR signaling and IL-2 expression: potential mechanism of alcohol-induced immune suppression

BACKGROUND

Alcohol abuse has long-term deleterious effects on the immune system, and results in a depletion and loss of function of CD4(+) T lymphocytes, which regulate both innate and adaptive immunity. T-lymphocyte activation via T-cell receptor (TCR) involves the lipid raft colocalization and aggregation of proteins into the immunological signalosome, which triggers a signaling cascade resulting in the production of interleukin-2 (IL-2). IL-2 regulates the proliferation and clonal expansion of activated T cells and is essential for an effective immune response. The present work examines the mechanisms underlying ethanol-induced dysfunction of CD4(+) T lymphocytes based on the hypothesis that ethanol downregulates lipid raft-mediated TCR signal transduction and resultant IL-2 production.

METHODS

Primary or cultured human T lymphocytes were exposed to ethanol for 24 hours prior to stimulation with anti-CD3/anti-CD28 antibodies or phytohemagglutinin. Effects of ethanol exposure on TCR-signaling (including activation of Lck, ZAP70, LAT, and PLCγ1) and IL-2 gene expression were examined.

RESULTS

Exposure of both primary and cultured human CD4(+) T lymphocytes to physiologically relevant concentrations of ethanol leads to down-regulation of IL-2 mRNA and protein via inhibition of DNA-binding activity of NFAT, the essential transcription factor for IL-2. Ethanol decreases tyrosine phosphorylation and activation of upstream signaling proteins PLCγ1, LAT, ZAP70, and Lck. These effects are prevented by inhibition of metabolism of ethanol. Sucrose density gradient fractionation and confocal microscopy revealed that ethanol inhibited essential upstream lipid raft-mediated TCR-dependent signaling events, namely colocalization of Lck, ZAP70, LAT, and PLCγ1 with plasma membrane lipid rafts.

CONCLUSIONS

Overall, our data demonstrate that ethanol inhibits lipid raft-mediated TCR-signaling in CD4(+) T lymphocytes, resulting in suppression of IL-2 production. These findings may represent a novel mechanism underlying alcohol abuse-associated immune suppression and may be particularly relevant in diseases such as HIV/AIDS and hepatitis C virus infection where alcohol abuse is a known comorbidity.

ZAP-70: an essential kinase in T-cell signaling

ZAP-70 is a cytoplasmic protein tyrosine kinase that plays a critical role in the events involved in initiating T-cell responses by the antigen receptor. Here we review the structure of ZAP-70, its regulation, its role in development and in disease. We also describe a model experimental system in which ZAP-70 function can be interrupted by a small chemical inhibitor.

Autoinhibition and adapter function of Syk

Development, survival, and activation of B lymphocytes are controlled by signals emanating from the B-cell antigen receptor (BCR). The BCR has an autonomous signaling function also known as tonic signaling that allows for long-term survival of B cells in the immune system. Upon binding of antigen to the BCR, the tonic signal is amplified and diversified, leading to alteration in gene expression and B-cell activation. The spleen tyrosine kinase (Syk) intimately cooperates with the signaling subunits of the BCR and plays a central role in the amplification and diversification of BCR signals. In this review, we discuss the molecular mechanisms by which Syk activity is inhibited and activated at the BCR. Importantly, Syk acts not only as a kinase that phosphorylates downstream substrates but also as an adapter that can bind to a diverse set of signaling proteins. Depending on its interactions and localization, Syk can signal opposing cell fate decisions such as proliferation or differentiation of B cells.

Insufficient deactivation of the protein tyrosine kinase lck amplifies T-cell responsiveness in acute coronary syndrome

RATIONALE

In the vulnerable atherosclerotic plaque, T cells may destabilize the tissue structure through direct cell-injurious effector functions. T cells transmit environmental signals, such as recognition of antigen, into cellular responses through regulated phosphorylation of cytoplasmic proteins, with the Src family kinase Lck (lymphocyte-specific protein tyrosine kinase) in critical membrane-proximal position of the T-cell receptor (TCR) signaling cascade. The balance between protein phosphorylation and dephosphorylation defines the signal transduction threshold and determines appropriate T-cell responses.

OBJECTIVE

We have examined whether abnormal calibration of intracellular signaling pathways renders acute coronary syndrome (ACS) patients susceptible to disproportionate T-cell responses.

METHODS AND RESULTS

Intracellular signaling cascades were quantified in CD4 T cells from ACS patients and control individuals after stimulation with major histocompatibility complex class II-superantigen complexes. ACS T cells mobilized more intracellular calcium and accumulated higher levels of phosphotyrosine than control T cells. Proximal steps in TCR signaling, such as recruitment of ZAP-70 and clustering of TCR complexes in the immune synapse, were abnormally enhanced in ACS T cells. Acceleration of the signaling cascade derived from a proximal defect in ACS T cells, which failed to phosphorylate Lck at Tyr505, extending activation of the Src kinase. Abnormalities in TCR signaling did not correlate with systemic inflammation as measured by C-reactive protein.

CONCLUSIONS

An intrinsic abnormality in the signaling machinery of ACS T cells resulting in the accumulation of active Lck lowers the TCR threshold and renders lymphocytes hyperreactive and capable of unwanted immune responses.

Stability of an autoinhibitory interface in the structure of the tyrosine kinase ZAP-70 impacts T cell receptor response

The delivery of signals from the activated T cell antigen receptor (TCR) inside the cell relies on the protein tyrosine kinase ZAP-70 (zeta-associated protein of 70 kDa). A recent crystal structure of inactive full-length ZAP-70 suggests that a central interface formed by the docking of the two SH2 domains of ZAP-70 onto the kinase domain is crucial for suppressing catalytic activity. Here we validate the significance of this autoinhibitory interface for the regulation of ZAP-70 catalytic activity and the T cell response. For this purpose, we perform in vitro catalytic activity assays and binding experiments using ZAP-70 proteins purified from insect cells to examine activation of ZAP-70. Furthermore, we use cell lines stably expressing wild-type or mutant ZAP-70 to monitor proximal events in T cell signaling, including TCR-induced phosphorylation of ZAP-70 substrates, activation of the MAP kinase pathway, and intracellular Ca(2+) levels. Taken together, our results directly correlate the stability of the autoinhibitory interface with the activation of these key events in the T cell response.

Organization of proximal signal initiation at the TCR:CD3 complex

The series of events leading to T-cell activation following antigen recognition has been extensively investigated. Although the exact mechanisms of ligand binding and transmission of this extracellular interaction into a productive intracellular signaling sequence remains incomplete, it has been known for many years that the immunoreceptor tyrosine activation motifs (ITAMs) of the T-cell receptor (TCR):CD3 complex are required for initiation of this signaling cascade because of the recruitment and activation of multiple protein tyrosine kinases, signaling intermediates, and adapter molecules. It however remains unclear why the TCR:CD3 complex requires 10 ITAMs, while many other ITAM-containing immune receptors, such as Fc receptors (FcRs) and the B cell receptor (BCR), contain far fewer ITAMs. We have recently demonstrated that various parameters of T cell development and activation are influenced by the number, as well as location and type, of ITAMs within the TCR:CD3 complex and hence propose that the TCR is capable of 'scalable signaling' that facilitates the initiation and orchestration of diverse T-cell functions. While many of the underlying mechanisms remain hypothetical, this review intends to amalgamate what we have learned from conventional biochemical analyses regarding initiation and diversification of T-cell signaling, with more recent evidence from molecular and fluorescent microscopic analyses, to propose a broader purpose for the TCR:CD3 ITAMs. Rather than simply signal initiation, individual ITAMs may also be responsible for the differential recruitment of signaling and regulatory molecules which ultimately affects T-cell development, activation and differentiation.

A hypomorphic allele of ZAP-70 reveals a distinct thymic threshold for autoimmune disease versus autoimmune reactivity

ZAP-70 is critical for T cell receptor (TCR) signaling. Tyrosine to phenylalanine mutations of Y315 and Y319 in ZAP-70 suggest these residues function to recruit downstream effector molecules, but mutagenesis and crystallization studies reveal that these residues also play an important role in autoinhibition ZAP-70. To address the importance of the scaffolding function, we generated a zap70 mutant mouse (YYAA mouse) with Y315 and Y319 both mutated to alanines. These YYAA mice reveal that the scaffolding function is important for normal development and function. Moreover, the YYAA mice have many similarities to a previously identified ZAP-70 mutant mouse, SKG, which harbors a distinct hypomorphic mutation. Both YYAA and SKG mice have impaired T cell development and hyporesponsiveness to TCR stimulation, markedly reduced numbers of thymic T regulatory cells and defective positive and negative selection. YYAA mice, like SKG mice, develop rheumatoid factor antibodies, but fail to develop autoimmune arthritis. Signaling differences that result from ZAP-70 mutations appear to skew the TCR repertoire in ways that differentially influence propensity to autoimmunity versus autoimmune disease susceptibility. By uncoupling the relative contribution from T regulatory cells and TCR repertoire during thymic selection, our data help to identify events that may be important, but alone are insufficient, for the development of autoimmune disease.

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.

Syk and pTyr'd: Signaling through the B cell antigen receptor

The B cell receptor (BCR) transduces antigen binding into alterations in the activity of intracellular signaling pathways through its ability to recruit and activate the cytoplasmic protein-tyrosine kinase Syk. The recruitment of Syk to the receptor, its activation and its subsequent interactions with downstream effectors are all regulated by its phosphorylation on tyrosine. This review discusses our current understanding of how this phosphorylation regulates the activity of Syk and its participation in signaling through the BCR.

Structural basis for the inhibition of tyrosine kinase activity of ZAP-70

ZAP-70, a cytoplasmic tyrosine kinase required for T cell antigen receptor signaling, is controlled by a regulatory segment that includes a tandem SH2 unit responsible for binding to immunoreceptor tyrosine-based activation motifs (ITAMs). The crystal structure of autoinhibited ZAP-70 reveals that the inactive kinase domain adopts a conformation similar to that of cyclin-dependent kinases and Src kinases. The autoinhibitory mechanism of ZAP-70 is, however, distinct and involves interactions between the regulatory segment and the hinge region of the kinase domain that reduce its flexibility. Two tyrosine residues in the SH2-kinase linker that activate ZAP-70 when phosphorylated are involved in aromatic-aromatic interactions that connect the linker to the kinase domain. These interactions are inconsistent with ITAM binding, suggesting that destabilization of this autoinhibited ZAP-70 conformation is the first step in kinase activation.

T cell activation-induced CrkII binding to the Zap70 protein tyrosine kinase is mediated by Lck-dependent phosphorylation of Zap70 tyrosine 315

The Zap70 protein tyrosine kinase controls TCR-linked signal transduction pathways and is critical for T cell development and responsiveness. Following engagement of TCR, the Zap70 undergoes phosphorylation on multiple tyrosine residues that are implicated in the regulation of its catalytic activity and interaction with signaling effector molecules downstream of the TCR. We have shown previously that the CT10 regulator of kinase II (CrkII) adapter protein interacts with tyrosine-phosphorylated Zap70 in TCR-engaged T cells, and now extend these studies to show that Tyr315 in the Zap70 interdomain B region is the site of interaction with CrkII. A point mutation of Tyr315 (Y315F) eliminated the CrkII-Zap70 interaction capacity. Phosphorylation of Tyr315 and Zap70 association with CrkII were both dependent upon the Lck protein tyrosine kinase. Previous studies demonstrated the Tyr315 is the Vav-Src homology 2 (SH2) binding site, and that replacement of Tyr315 by Phe impaired the function of Zap70 in TCR signaling. However, fluorescence polarization-based binding studies revealed that the CrkII-SH2 and the Vav-SH2 bind a phosphorylated Tyr315-Zap70-derived peptide with affinities of a similar order of magnitude (Kd of 2.5 and 1.02 microM, respectively). The results suggest therefore that the biological functions attributed to the association of Zap70 with Vav following T cell activation may equally reflect the association of Zap70 with CrkII, and further support a regulatory role for CrkII in the TCR-linked signal transduction pathway.

Structural basis for the requirement of two phosphotyrosine residues in signaling mediated by Syk tyrosine kinase

The protein-tyrosine kinase Syk couples immune recognition receptors to multiple signal transduction pathways, including the mobilization of calcium and the activation of NFAT. The ability of Syk to regulate signaling is influenced by its phosphorylation on tyrosine residues within the linker B region. The phosphorylation of both Y342 and Y346 is necessary for optimal signaling from the B cell receptor for antigen. The SH2 domains of multiple signaling proteins share the ability to bind this doubly phosphorylated site. The NMR structure of the C-terminal SH2 domain of PLCgamma (PLCC) bound to a doubly phosphorylated Syk peptide reveals a novel mode of phosphotyrosine recognition. PLCC undergoes extensive conformational changes upon binding to form a second phosphotyrosine-binding pocket in which pY346 is largely desolvated and stabilized through electrostatic interactions. The formation of the second binding pocket is distinct from other modes of phosphotyrosine recognition in SH2-protein association. The dependence of signaling on simultaneous phosphorylation of these two tyrosine residues offers a new mechanism to fine-tune the cellular response to external stimulation.

CD8 T Cell Sensory Adaptation Dependent on TCR Avidity for Self-Antigens

Adaptation of the T cell activation threshold may be one mechanism to control autoreactivity. To investigate its occurrence in vivo, we engineered a transgenic mouse model with increased TCR-dependent excitability by expressing a Zap70 gain-of-function mutant (ZAP-YEEI) in postselection CD8 thymocytes and T cells. Increased basal phosphorylation of the Zap70 substrate linker for activation of T cells was detected in ZAP-YEEI-bearing CD8 T cells. However, these cells were not activated, but had reduced levels of TCR and CD5. Moreover, they produced lower cytokine amounts and showed faster dephosphorylation of linker for activation of T cells and ERK upon activation. Normal TCR levels and cytokine production were restored by culturing cells in the absence of TCR/spMHC interaction, demonstrating dynamic tuning of peripheral T cell responses. The effect of avidity for self-ligand(s) on this sensory adaptation was studied by expressing ZAP-YEEI in P14 or HY TCR transgenic backgrounds. Unexpectedly, double-transgenic animals expressed ZAP-YEEI prematurely in double-positive thymocytes, but no overt alteration of selection processes was observed. Instead, modifications of TCR and CD5 expression due to ZAP-YEEI suggested that signal tuning occurred during thymic maturation. Importantly, although P14 x ZAP-YEEI peripheral CD8 T cells were reduced in number and showed lower Ag-induced cytokine production and limited lymphopenia-driven proliferation, the peripheral survival/expansion and Ag responsiveness of HY x ZAP-YEEI cells were enhanced. Our data provide support for central and peripheral sensory T cell adaptation induced as a function of TCR avidity for self-ligands and signaling level. This may contribute to buffer excessive autoreactivity while optimizing TCR repertoire usage.

Intramolecular regulatory switch in ZAP-70: analogy with receptor tyrosine kinases

ZAP-70, a Syk family cytoplasmic protein tyrosine kinase (PTK), is required to couple the activated T-cell antigen receptor (TCR) to downstream signaling pathways. It contains two tandem SH2 domains that bind to phosphorylated TCR subunits and a C-terminal catalytic domain. The region connecting the SH2 domains with the kinase domain, termed interdomain B, has previously been shown to have striking regulatory effects on ZAP-70 function, presumed to be due to the recruitment of key substrates. Paradoxically, deletion of interdomain B preserves ZAP-70 function. Recent structural studies of several receptor tyrosine kinases (RTKs) revealed that their juxtamembrane regions negatively regulate their catalytic activities. In EphB2 and several other RTKs, this autoinhibition depends upon interaction between the kinase domain and tyrosine residues within the juxtamembrane region. Autoinhibition is released when these tyrosines become phosphorylated following receptor stimulation. Sequence homology suggested analogous regulation for ZAP-70. Based on mutagenesis analysis of ZAP-70 interdomain B, we find that this region downregulates ZAP-70 catalytic activity in a similar manner as the juxtamembrane region of EphB2. Similar regulation was also noted for the related Syk kinase. These findings suggest that a general autoinhibitory mechanism employed by RTKs is also used by some cytoplasmic tyrosine kinases.

Negative regulation of T cell receptor signaling by Siglec-7 (p70/AIRM) and Siglec-9

Siglec-7 (p70/AIRM) and Siglec-9 are "CD33"-related siglecs expressed on natural killer (NK) cells and subsets of peripheral T cells. Like other inhibitory NK cell receptors, they contain immunoglobulin receptor family tyrosine-based inhibitory motifs in their cytoplasmic domains, and Siglec-7 has been demonstrated to negatively regulate NK cell activation. Based on reports of the presence of these siglecs on T cells, we sought to determine if they are capable of modulating T cell receptor (TCR) signaling using Jurkat T cells stably and transiently transfected with Siglec-7 or Siglec-9. Following either pervanadate stimulation or TCR engagement, both Siglecs exhibited increased tyrosine phosphorylation and recruitment of SHP-1. Effects of Siglec-7 and -9 were also evident in downstream events in the signaling pathway. Both siglecs reduced phosphorylation of Tyr319 on ZAP-70, known to play a pivotal role in up-regulation of gene transcription following TCR stimulation. There was also a corresponding decreased transcriptional activity of nuclear factor of activated T cells (NFAT) as determined using a luciferase reporter gene. Like all siglecs, Siglec-7 and -9 recognize sialic acid-containing glycans of glycoproteins and glycolipids as ligands. Mutation of the conserved Arg in the ligand binding site of Siglec-7 (Arg124) or Siglec-9 (Arg120) resulted in reduced inhibitory function in the NFAT/luciferase transcription assay, suggesting that ligand binding is required for optimal inhibition of TCR signaling. The combined results demonstrate that both Siglec-7 and Siglec-9 are capable of negative regulation of TCR signaling and that ligand binding is required for optimal activity.

ζ-Associated Protein of 70 kDa (ZAP-70), but Not Syk, Tyrosine Kinase Can Mediate Apoptosis of T Cells through the Fas/Fas Ligand, Caspase-8 and Caspase-3 Pathways

The TCR zeta-chain-associated protein of 70 kDA (ZAP-70) and Syk tyrosine kinases play critical roles in regulating TCR-mediated signal transduction. They not only share some overlapped functions but also may play unique roles in regulating the function and development of T cells. However, it is not known whether they have different effects on the activation and activation-induced cell death of T cells. To address this question, we generated cDNAs encoding chimeric molecules that a tailless TCR zeta-chain was directly linked to truncated ZAP-70 (Z/ZAP) or Syk (Z/Syk) molecules lacking the two Src homology 2 domains. Transfection of these molecules into zeta-chain-deficient cells restored their TCR expression. In addition, Z/ZAP and Z/Syk transfectants but not control cells demonstrated kinase activities in phosphorylating an exogenous substrate specific for ZAP-70 and Syk kinases. Z/ZAP transfectants activated through TCRs underwent a faster time course of apoptosis and had a greater percentage of apoptotic cells than that of Z/Syk and control cells. Activated Z/ZAP transfectants increased Fas and Fas ligand (FasL) expression 3- and 40-fold, respectively. Blocking of the Fas/FasL interaction could inhibit the apoptosis of Z/ZAP transfectants. In contrast, although activated Z/Syk transfectants could increase FasL expression, their Fas expression actually decreased and the percentage of apoptotic cells did not increase. Further studies of the mechanisms revealed that activation of Z/ZAP but not Z/Syk transfectants resulted in rapid activation of caspase-3 and caspase-8 that could also be inhibited by blocking Fas/FasL interaction. These results demonstrated that ZAP-70 and Syk play distinct roles in T cell activation and activation-induced cell death.

Restricting Zap70 expression to CD4+CD8+ thymocytes reveals a T cell receptor-dependent proofreading mechanism controlling the completion of positive selection

Although T cell receptor (TCR) signals are essential for intrathymic T cell-positive selection, it remains controversial whether they only serve to initiate this process, or whether they are required throughout to promote thymocyte differentiation and survival. To address this issue, we have devised a novel approach to interfere with thymocyte TCR signaling in a developmental stage-specific manner in vivo. We have reconstituted mice deficient for Zap70, a tyrosine kinase required for TCR signaling and normally expressed throughout T cell development, with a Zap70 transgene driven by the adenosine deaminase (ADA) gene enhancer, which is active in CD4(+)CD8(+) thymocytes but inactive in CD4(+) or CD8(+) single-positive (SP) thymocytes. In such mice, termination of Zap70 expression impaired TCR signal transduction and arrested thymocyte development after the initiation, but before the completion, of positive selection. Arrested thymocytes had terminated Rag gene expression and up-regulated TCR and Bcl-2 expression, but failed to differentiate into mature CD4 or CD8 SP thymocytes, to be rescued from death by neglect or to sustain interleukin 7R alpha expression. These observations identify a TCR-dependent proofreading mechanism that verifies thymocyte TCR specificity and differentiation choices before the completion of positive selection.

The Ras/MAPK cascade and the control of positive selection

Immature double positive (DP) thymocytes bearing a T cell receptor (TCR) that interacts with self-major histocompatibility complex (MHC) molecules receive signals that induce either their differentiation (positive selection) or apoptosis (negative selection). Furthermore, those cells that are positively selected develop into two different lineages, CD4 or CD8, depending on whether their TCRs bind to MHC class II or I, respectively. Positive selection therefore involves rescue from the default fate (death), lineage commitment, and progression to the single positive (SP) stage. These are probably temporally distinct events that may require both unique and overlapping signals. Work in the past several years has started to unravel the signaling networks that control these processes. One of the first pathways identified as important for positive selection was Ras and its downstream effector, the Erk mitogen-activated protein kinase (MAPK) cascade. In this review we examine the factors that connect the TCR to the Ras/Erk cascade in DP thymocytes, as well as what we know about the downstream effectors of the Ras/Erk cascade important for positive selection. We also consider the possible role of this cascade in CD4/CD8 lineage development, and the possible interactions of the Ras/Erk cascade with Notch during these cell fate determination processes.

Crystal structure and NMR studies of the apo SH2 domains of ZAP-70: two bikes rather than a tandem

The protein kinase ZAP-70 is involved in T-cell activation, and interacts with tyrosine-phosphorylated peptide sequences known as immunoreceptor tyrosine activation motifs (ITAMs), which are present in three of the subunits of the T-cell receptor. We have studied the tandem SH2 (tSH2) domains of ZAP-70, by both X-ray and NMR. Here, we present the crystal structure of the apoprotein, i.e., the tSH2 domain in the absence of ITAM. Comparison with the previously reported complex structure reveals that binding to the ITAM peptide induces surprisingly large movements between the two SH2 domains and within the actual binding sites. The conformation of the ITAM-free protein is partly governed by a hydrophobic cluster between the linker region and the C-terminal SH2 domain. Our data suggest that the two SH2 domains are able to undergo large interdomain movements. The proposed relative flexibility of the SH2 domains is further supported by the finding that no NMR signals could be detected for the two helices connecting the SH2 domains; these are likely to be broadened beyond detection due to conformational exchange. It is likely that this conformational reorientation induced by ITAM binding is the main signaling event activating the kinase domain in ZAP-70. Another NMR observation was that the N-terminal SH2 domain could bind tetrapeptides derived from the ITAM sequence, apparently without the need to interact with the C-terminal domain. In contrast, the C-terminal domain has little affinity for tetrapeptides. The opposite situation is true for binding to plain phosphotyrosine, where the C-terminal domain has a higher affinity. Distinct features in the crystal structure, showing the interdependence of both domains, explain these binding data.

Phosphorylation of Tyr342 in the linker region of Syk is critical for Fc epsilon RI signaling in mast cells

The linker region of Syk and ZAP70 tyrosine kinases plays an important role in regulating their function. There are three conserved tyrosines in this linker region; Tyr317 of Syk and its equivalent residue in ZAP70 were previously shown to negatively regulate the function of Syk and ZAP70. Here we studied the roles of the other two tyrosines, Tyr342 and Tyr346 of Syk, in Fc epsilon RI-mediated signaling. Antigen stimulation resulted in Tyr342 phosphorylation in mast cells. Syk with Y342F mutation failed to reconstitute Fc epsilon RI-initiated histamine release. In the Syk Y342F-expressing cells there was dramatically impaired receptor-induced phosphorylation of multiple signaling molecules, including LAT, SLP-76, phospholipase C-gamma2, but not Vav. Compared to wild-type Syk, Y342F Syk had decreased binding to phosphorylated immunoreceptor tyrosine-based activation motifs and reduced kinase activity. Surprisingly, mutation of Tyr346 had much less effect on Fc epsilon RI-dependent mast cell degranulation. An anti-Syk-phospho-346 tyrosine antibody indicated that antigen stimulation induced only a very minor increase in the phosphorylation of this tyrosine. Therefore, Tyr342, but not Tyr346, is critical for regulating Syk in mast cells and the function of these tyrosines in immune receptor signaling appears to be different from what has been previously reported for the equivalent residues of ZAP70.

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.