Differential intrinsic enzymatic activity of Syk and Zap-70 protein-tyrosine kinases

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.

Bruton's tyrosine kinase (Btk): function, regulation, and transformation with special emphasis on the PH domain

Bruton's agammaglobulinemia tyrosine kinase (Btk) is a cytoplasmic tyrosine kinase important in B-lymphocyte development, differentiation, and signaling. Btk is a member of the Tec family of kinases. Mutations in the Btk gene lead to X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (Xid) in mice. Activation of Btk triggers a cascade of signaling events that culminates in the generation of calcium mobilization and fluxes, cytoskeletal rearrangements, and transcriptional regulation involving nuclear factor-kappaB (NF-kappaB) and nuclear factor of activated T cells (NFAT). In B cells, NF-kappaB was shown to bind to the Btk promoter and induce transcription, whereas the B-cell receptor-dependent NF-kappaB signaling pathway requires functional Btk. Moreover, Btk activation is tightly regulated by a plethora of other signaling proteins including protein kinase C (PKC), Sab/SH3BP5, and caveolin-1. For example, the prolyl isomerase Pin1 negatively regulates Btk by decreasing tyrosine phosphorylation and steady state levels of Btk. It is intriguing that PKC and Pin1, both of which are negative regulators, bind to the pleckstrin homology domain of Btk. To this end, we describe here novel mutations in the pleckstrin homology domain investigated for their transforming capacity. In particular, we show that the mutant D43R behaves similar to E41K, already known to possess such activity.

Steroid effects on ZAP-70 and SYK in relation to apoptosis in poor prognosis chronic lymphocytic leukemia

There is resurgent interest in glucocorticoids (GCs) in the treatment of poor prognosis chronic lymphocytic leukemia (CLL). Little is known however on how GCs induce apoptosis in CLL. Methylprednisolone (MP) induces apoptosis in ZAP-70 positive CLL more readily than in ZAP-70 negative CLL, which is in contrast to the effects of radiation and chemotherapy. The increased GC sensitivity of ZAP-70+ CLL was studied in relation to the expression status of ZAP-70 and the related signal transducing tyrosine kinase SYK. Both ZAP-70 and SYK were downregulated by GC treatment. Moreover, SYK was dephosphorylated by the phosphatase PTP1B of which the expression and translation levels were induced by GCs. Inhibition of PTP1B successfully restored ZAP-70 expression and SYK phosphorylation but did not interfere with GC-induced apoptosis. Therefore, the downregulation of ZAP-70 and P-SYK per se during treatment with GCs is not sufficient to induce apoptosis, and different mechanisms must therefore be responsible for the increased steroid sensitivity of ZAP-70+ CLL.

The c-Cbl proto-oncoprotein downregulates EBV LMP2A signaling

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) plays a key role in regulating viral latency and EBV pathogenesis by functionally mimicking signals induced by the B-cell receptor (BCR) altering normal B cell development. As c-Cbl ubiquitin ligase (E3) is a critical negative regulator in the BCR signal pathway, the role of c-Cbl in the function and formation of the LMP2A signalosome was examined. c-Cbl promoted LMP2A degradation through ubiquitination, specifically degraded the Syk protein tyrosine kinase in the presence of LMP2A, and inhibited LMP2A induction of the EBV lytic cycle. Our earlier studies indicated that LMP2A-dependent Lyn degradation was mediated by Nedd4-family E3s in LMP2A expressing cells. Combine with these new findings, we propose a model in which c-Cbl and Nedd4-family E3s cooperate to degrade target proteins at discrete steps in the function of the LMP2A signalosome.

Analysis of the linker for activation of T cells and the linker for activation of B cells in natural killer cells reveals a novel signaling cassette, dual usage in ITAM signaling, and influence on development of the Ly49 repertoire

The linker for activation of T cells (LAT) and the linker for activation of B cells (LAB/NTAL/LAT2) are integral proteins in receptor coupling to downstream events. Both proteins are expressed in natural killer (NK) cells and LAT is phosphorylated during target cell interactions or ligation of the immunoreceptor tyrosine-based activation motif (ITAM)-coupled CD16. Regardless, Lat(-/-) mice exhibit normal natural and antibody-mediated killing. Here we place both LAT and LAB in the DAP12 pathway of NK cells. Moreover, we unveil a LAT-independent pathway that requires expression of Syk. Mice lacking either LAT or LAB have a skewed Ly49 repertoire, and activated NK cells from Lat(-/-) mice have reduced responses to the ITAM-coupled receptor NK1.1. In contrast, resting Lat(-/-) NK cells show intact NK1.1 responses, whereas NK cells without LAB are hyperactive. Elimination of both adaptors severely reduces NK1.1 signaling under both conditions. Together these data show that NK ITAMs preferentially use a signaling cassette regulated by interplay between LAT and LAB. Activation by interleukin-2 causes a shift to greater dependency on LAT due to suppression of Syk signaling. The overlapping use of multiple adaptors permits fine-tuning of NK-cell ITAM responses over the course of an immune response.

The impact of Arabidopsis on human health: diversifying our portfolio

Studies of the model plant Arabidopsis thaliana may seem to have little impact on advances in medical research, yet a survey of the scientific literature shows that this is a misconception. Many discoveries with direct relevance to human health and disease have been elaborated using Arabidopsis, and several processes important to human biology are more easily studied in this versatile model plant.

Redundant role for Zap70 in B cell development and activation

Expression of the Syk family tyrosine kinase Zap70 is strongly correlated with poor clinical outcome in chronic lymphocytic leukemia, the most common human leukemia characterized by B cell accumulation. The expression of Zap70 may reflect the specific cell of origin of the tumor or may contribute to pathology. Thus, the normal role of Zap70 in B cell physiology is of great interest. While initial studies reported that Zap70 expression in the mouse was limited to T and NK cells, more recent work has shown expression in early B cell progenitors and in splenic B cells, suggesting that the kinase may play a role in the development or activation of B cells. In this study, we show that Zap70 is expressed in all developing subsets of B cells as well as in recirculating B cells, marginal zone B cells and peritoneal B1 cells. Analysis of Zap70-deficient mice shows no unique role for Zap70 in either the development of B cells or in their in vitro and in vivo activation. However, we show that Zap70 can rescue the defective positive selection of immature B cells into the recirculating pool in Syk-deficient mice, demonstrating functional redundancy between these two kinases.

Unveiling ZAP-70's plan B

In this issue of Blood, Chen and colleagues demonstrate that ZAP-70 enhances BCR signaling in B-CLL cells by promoting phosphorylation of the ITAMs in the Ig signaling subunit independently of its kinase activity.

ZAP-70 enhances IgM signaling independent of its kinase activity in chronic lymphocytic leukemia

We transduced chronic lymphocytic leukemia (CLL) cells lacking ZAP-70 with vectors encoding ZAP-70 or various mutant forms of ZAP-70 and monitored the response of transduced CLL cells to treatment with F(ab)(2) anti-IgM (anti-mu). CLL cells made to express ZAP-70, a kinase-defective ZAP-70 (ZAP-70-KA(369)), or a ZAP-70 unable to bind c-Cbl (ZAP-YF(292)) experienced greater intracellular calcium flux and had greater increases in the levels of phosphorylated p72(Syk), B-cell linker protein (BLNK), and phospholipase C-gamma, and greater activation of the Ig accessory molecule CD79b in response to treatment with anti-mu than did mock-transfected CLL cells lacking ZAP-70. Transfection of CLL cells with vectors encoding truncated forms of ZAP-70 revealed that the SH2 domain, but not the SH1 domain, was necessary to enhance intracellular calcium flux in response to treatment with anti-mu. We conclude that ZAP-70 most likely acts as an adapter protein that facilitates B-cell receptor (BCR) signaling in CLL cells independent of its tyrosine kinase activity or its ability to interact with c-Cbl.

The B-cell receptor and ZAP-70 in chronic lymphocytic leukemia

In addition to the important observations relating immunoglobulin (Ig) mutation status to clinical behavior, studies on the Ig expressed in chronic lymphocytic leukemia (CLL) have revealed compelling evidence that antigen selection contributes to the pathogenesis of this disease. CLL cells that use unmutated Ig can generally be distinguished from CLL cells that use Ig with somatic mutations by expression of the 70-kD zeta-associated protein (ZAP-70). ZAP-70 apparently enhances the capacity of CLL cells to respond to antigen, and therefore might play a causal role in the relatively aggressive clinical behavior noted for patients who have CLL cells that use unmutated Ig. Clinical surveys have found that expression of ZAP-70 by CLL cells is apparently a stronger predictor of early disease progression than is the use by CLL cells of unmutated Ig. As such, strategies that respectively monitor or target Ig-receptor signaling in CLL might be very useful in the risk assessment or treatment of this disease.

Spleen tyrosine kinase (Syk) as a novel target for allergic asthma and rhinitis

Allergic asthma and rhinitis are prevalent diseases in the modern world, both marked by inflammation of the airways. The spleen tyrosine kinase (Syk) plays a critical role in the regulation of such immune and inflammatory responses. Although Syk is best known as a key component of immunoreceptor signalling complexes in leukocytes, recent studies demonstrated Syk expression in cells outside the haematopoietic lineage. Moreover, in recent years, it has been established that Syk is involved in various signalling cascades including those originating from integrin and cytokine receptors. Thus, Syk likely has a much wider biological role than previously recognised. Specific inhibition of Syk using aerosolised antisense oligonucleotides in liposome complexes significantly decreased lung inflammatory responses in experimental asthma and acute lung injury models. In addition, pharmacological inhibitors of Syk have been recently developed with potential for use as therapeutics. However, in the development and the rational delivery of drugs targeting Syk, it is important to consider the multiple cell types that express this kinase and the potential effects of its inhibition on various physiological functions. This review focuses on the recent data and the emerging ideas about Syk as a therapeutic target.

ZAP-70 enhances B-cell-receptor signaling despite absent or inefficient tyrosine kinase activation in chronic lymphocytic leukemia and lymphoma B cells

Expression of ZAP-70 is an important negative prognostic factor in chronic lymphocytic leukemia (CLL). This protein tyrosine kinase is a key mediator of T-cell receptor (TCR) signaling and is structurally homologous to Syk, which plays an analogous role in B-cell receptor (BCR) signaling. Recent studies indicate that ZAP-70 may participate in BCR signaling as well, but the mechanism of action is not completely understood. We have now compared antigen receptor-induced activation of ZAP-70 in B cells and T cells by analyzing phosphorylation of critical regulatory tyrosine residues. We show that BCR-mediated activation of ZAP-70 is very inefficient in CLL and lymphoma B cells and is negligible when compared to activation of Syk. Despite the inefficient catalytic activation, the ability of ZAP-70 to recruit downstream signaling molecules in response to antigen receptor stimulation appeared relatively preserved. Moreover, ectopic expression of ZAP-70 enhanced and prolonged activation of several key mediators of BCR signaling, such as the Syk, ERK, and Akt kinases, and decreased the rate of ligand-mediated BCR internalization. We conclude that the role of ZAP-70 in BCR signaling is quite distinct from its role in TCR signaling and is likely mediated by inhibition of events that terminate the signaling response.

Differential expression of spleen tyrosine kinase Syk isoforms in tissues: effects of the microbial flora

Spleen tyrosine kinase (Syk) is expressed widely in hematopoietic and non-hematopoietic cells. The widespread distribution of Syk and its involvement in host defense and allergic reactions, prompted us analyze the influence of microbial exposure on Syk expression. We compared the distribution of Syk in various tissues of germ-free and conventional mice using immunohistochemistry, Western blot analysis and real time RT-PCR. Total Syk expression was similar between germ-free and conventional mice. Since it has been claimed that Syk isoforms are differentially expressed, we studied the distribution and abundance of Syk (L) and Syk (S) isoforms in tissues from these mice. In contrast to previous reports, we found broad tissue expression of Syk (S). Interestingly, in germ-free mice the amount of Syk (S) but not Syk L protein was selectively increased in lung and spleen. In summary, our study reveals new and broad tissue expression of both Syk isoforms and demonstrates that lack of microbial flora results in selectively increased expression of Syk (S) isoform in lung and spleen.

Multiplex Analysis of Intracellular Signaling Pathways in Lymphoid Cells by Microbead Suspension Arrays

Phosphorylation analysis of signaling proteins is key for examining intracellular signaling pathways. Conventional biochemical approaches, e.g. immunoprecipitation, Western blot, and ELISA, have played a major role in elucidation of individual signaling events. However, these methods are laborious, time-consuming, and difficult to adapt for high throughput analysis. A multiplex approach to measure phosphorylation state of multiple signaling proteins simultaneously would significantly enhance the efficiency and scope of signaling pathway analysis for mechanistic studies and clinical application. This report describes a novel multiplex microbead suspension array approach to examine phosphoproteomic profiles in lymphoid cells. In the Jurkat T-cell leukemia line, the multiplex assay enabled targeted investigation of phosphorylation kinetics of signal transduction from receptor proximal events (tyrosine phosphoproteins CD3, Lck, Zap-70, and linker for T-cell activation) to cytosolic events (serine/threonine phosphoproteins Erk and Akt) to transcription factors (serine/threonine phosphorylated Rsk, cyclic AMP-response element-binding protein, and STAT3). To broaden the application of the multiplex analysis, signaling pathways were also studied in B-cell lymphoid tumor lines that included chronic lymphocytic leukemia lines. In these cell lines, multiplex suspension array enabled phosphoproteomic analysis of signaling cascade mediated by Syk, a homolog of Zap-70. Results obtained by multiplex analysis were confirmed by immunoprecipitation and Western blot methods. The examples of T-cell and B-cell signaling pathway analyses in this report demonstrate the utility of the multiplex suspension arrays to investigate phosphorylation dynamics and kinetics of several signaling proteins simultaneously in signal transduction pathways.

Visualization of APP dimerization and APP-Notch2 heterodimerization in living cells using bimolecular fluorescence complementation

We previously demonstrated that the amyloid precursor protein (APP) interacts with Notch receptors. Here, we confirmed the APP/Notch1 endogenous interaction in embryonic day 17 rat brain tissue, suggesting the interaction was not as a result of over-expression artifacts. To investigate potential homodimeric and heterodimeric interactions of APP and Notch2 (N2), we have visualized the subcellular localization of the APP/N2 complexes formed in living cells using bimolecular fluorescence complementation (BiFC) analysis. BiFC was accomplished by fusing the N-terminal fragment or the C-terminal fragment of yellow fluorescent protein (YFP) to APP, N2, and a C-terminally truncated form of N2. When expressed in COS-7 cells, these tagged proteins alone did not produce a fluorescent signal. The tagged APP homodimer produced a weak fluorescent signal, while neither full-length N2, nor a truncated N2 alone, produced a visible signal, suggesting that N2 receptors do not form homodimers. The strongest fluorescent signal was obtained with co-expression of the C-terminal fragment of YFP fused to APP and the N-terminal fragment of YFP fused to the truncated form of N2. This heterodimer localized to plasma membrane, endoplasmic reticulum (ER), Golgi and other compartments. The results were confirmed and quantified by flow cytometry. The BiFC method of specifically visualizing APP/Notch interactions can be applied to study APP and Notch signaling during development, aging and neurodegeneration.

Functional and prognostic role of ZAP-70 in chronic lymphocytic leukaemia

It has become clear that the heterogeneity of chronic lymphocytic leukaemia (CLL) is not a continuous spectrum, but is bipolar. Originally distinguished by the mutational status of the immunoglobulin variable region genes, the two poles are perhaps better identified by the expression of ZAP-70, a signalling molecule normally utilised by T cells rather than B cells, but anomalously expressed in the more aggressive subtype of CLL. Assaying ZAP-70 expression has become progressively simplified so that a directly stained flow cytometric test is currently being evaluated, and a version of this should shortly be available to routine laboratories. In addition, the understanding of the nature of CLL has been advanced rapidly and this should lead to new, better targeted therapies, which in contrast to the current armoury, will work better for the more malignant variants of CLL than for the more benign. In particular, ZAP-70 is especially attractive because its aberrant expression in tumour cells from the more aggressive forms of CLL requires the chaperoning action of activated heat-shock protein 90, which may be specifically inhibited.

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.

ZAP-70 is expressed by a subset of normal human B-lymphocytes displaying an activated phenotype

The Syk family tyrosine kinase ZAP-70 is essential for normal T-cell development and signaling. Recently, leukemic cells from some patients with B-cell chronic lymphocytic leukemia (B-CLL) were shown to express ZAP-70. Owing to the prognostic value of B-CLL ZAP-70 expression, this phenotype may reflect intrinsic biological differences between the two subsets of disease. However, it remains unclear whether CLL-B cells aberrantly acquire ZAP-70 expression during the transformation process or whether ZAP-70 may be expressed under certain conditions in normal human B-lymphocytes. To discriminate between these two possibilities, we assessed ZAP-70 expression in normal human B-lymphocytes. Our data demonstrate that ZAP-70 is expressed in a subpopulation of tonsillar and splenic normal B-lymphocytes that express an activated phenotype. Furthermore, ZAP-70 expression can be induced in vitro upon stimulation of blood and tonsillar B cells. Finally, we show that phosphorylation of ZAP-70 occurs in tonsillar B cells with stimulation through the B-cell receptor. These results provide new insight into normal human B-cell biology as well as provide clues about the transformed cell in B-CLL.

A Novel Mode of Gleevec Binding Is Revealed by the Structure of Spleen Tyrosine Kinase

Spleen tyrosine kinase (Syk) is a non-receptor tyrosine kinase required for signaling from immunoreceptors in various hematopoietic cells. Phosphorylation of two tyrosine residues in the activation loop of the Syk kinase catalytic domain is necessary for signaling, a phenomenon typical of tyrosine kinase family members. Syk in vitro enzyme activity, however, does not depend on phosphorylation (activation loop tyrosine --> phenylalanine mutants retain catalytic activity). We have determined the x-ray structure of the unphosphorylated form of the kinase catalytic domain of Syk. The enzyme adopts a conformation of the activation loop typically seen only in activated, phosphorylated tyrosine kinases, explaining why Syk does not require phosphorylation for activation. We also demonstrate that Gleevec (STI-571, Imatinib) inhibits the isolated kinase domains of both unphosphorylated Syk and phosphorylated Abl with comparable potency. Gleevec binds Syk in a novel, compact cis-conformation that differs dramatically from the binding mode observed with unphosphorylated Abl, the more Gleevec-sensitive form of Abl. This finding suggests the existence of two distinct Gleevec binding modes: an extended, trans-conformation characteristic of tight binding to the inactive conformation of a protein kinase and a second compact, cis-conformation characteristic of weaker binding to the active conformation. Finally, the Syk-bound cis-conformation of Gleevec bears a striking resemblance to the rigid structure of the nonspecific, natural product kinase inhibitor staurosporine.

ZAP-70 directly enhances IgM signaling in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) B cells that express unmutated immunoglobulin heavy-chain variable region genes (IgV(H)) generally express ZAP-70, in contrast to normal B cells or most CLL cases with mutated IgV(H). Following IgM ligation, ZAP-70+ CLL cells had significantly higher levels of phosphorylated p72(Syk), BLNK, and phospholipase-Cgamma (PLCgamma) and had greater[Ca2+]i flux than did ZAP-70-negative CLL cases, including unusual ZAP-70-negative cases with unmutated IgV(H). IgM ligation of ZAP-70-negative CLL B cells infected with an adenovirus vector encoding ZAP-70 induced significantly greater levels of phosphorylated p72(Syk), BLNK, and PLCgamma and had greater[Ca2+]i flux than did similarly stimulated, noninfected CLL cells or CLL cells infected with a control adenovirus vector. We conclude that expression of ZAP-70 in CLL allows for more effective IgM signaling in CLL B cells, a feature that could contribute to the relatively aggressive clinical behavior generally associated with CLL cells that express unmutated IgV(H).

T-cell receptor–induced phosphorylation of the ζ chain is efficiently promoted by ZAP-70 but not Syk

Engagement of the T-cell receptor (TCR) results in the activation of Lck/Fyn and ZAP-70/Syk tyrosine kinases. Lck-mediated tyrosine phosphorylation of signaling motifs (ITAMs) in the CD3-ζ subunits of the TCR is an initial step in the transduction of signaling cascades. However, ζ phosphorylation is also promoted by ZAP-70, as TCR-induced ζ phosphorylation is defective in ZAP-70–deficient T cells. We show that this defect is corrected by stable expression of ZAP-70, but not Syk, in primary and transformed T cells. Indeed, these proteins are differentially coupled to the TCR with a 5- to 10-fold higher association of ZAP-70 with ζ as compared to Syk. Low-level Syk-ζ binding is associated with significantly less Lck coupled to the TCR. Moreover, diminished coupling of Lck to ζ correlates with a poor phosphorylation of the positive regulatory tyr352 residue of Syk. Thus, recruitment of Lck into the TCR complex with subsequent ζ chain phosphorylation is promoted by ZAP-70 but not Syk. Importantly, the presence of ZAP-70 positively regulates the TCR-induced tyrosine phosphorylation of Syk. The interplay between Syk and ZAP-70 in thymocytes, certain T cells, and B-chronic lymphocytic leukemia cells, in which they are coexpressed, will therefore modulate the amplitude of antigen-mediated receptor signaling.

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.

Hypermethylation of the spleen tyrosine kinase promoter in T-lineage acute lymphoblastic leukemia

Sequence analysis of the noncoding first exon (exon 1) of the Syk gene demonstrated the presence of a previously cloned CpG island (GenBank #Z 65706). Transient transfection analysis in Daudi cells demonstrated promoter activity (18-fold increase over parental luciferase plasmid) for a 348 bp BstXI-BsrBI fragment containing this island. This region exhibits a high GC content (approximately 75%), contains several SP1 binding sites and a potential initiator sequence, but lacks a strong TATA consensus. Bisulfite sequencing and methylation-specific PCR (MSP) of this region demonstrated that the Syk promoter CpG island was largely unmethylated in B-lineage leukemia cell lines, control peripheral blood cells, human thymocytes and CD3(+) T lymphocytes. However, dense methylation was seen in four T-lineage leukemia cell lines, Jurkat, H9, Molt 3 and HUT 78. MSP screening of leukemia cells from six T-lineage acute lymphoblastic leukemia (ALL) patients demonstrated methylation of the Syk promoter CpG island in one T-lineage ALL patient. Promoter methylation was correlated with reduced to absent expression of Syk mRNA and SYK protein in the T-lineage leukemia cell lines. Treatment of the leukemia lines Ha and Molt 3, with the methylation inhibitor, 5-aza-2'-deoxycytidine (5-aza-CdR) resulted in increased Syk mRNA expression. The presence of a methylated promoter sequence in these T-lineage leukemia cell lines and in one T-lineage patient suggests a potential role for SYK as a tumor suppressor in T-ALL.

Positive and negative regulation of T-cell activation through kinases and phosphatases

The sequence of events in T-cell antigen receptor (TCR) signalling leading to T-cell activation involves regulation of a number of protein tyrosine kinases (PTKs) and the phosphorylation status of many of their substrates. Proximal signalling pathways involve PTKs of the Src, Syk, Csk and Tec families, adapter proteins and effector enzymes in a highly organized tyrosine-phosphorylation cascade. In intact cells, tyrosine phosphorylation is rapidly reversible and generally of a very low stoichiometry even under induced conditions due to the fact that the enzymes removing phosphate from tyrosine-phosphorylated substrates, the protein tyrosine phosphatases (PTPases), have a capacity that is several orders of magnitude higher than that of the PTKs. It follows that a relatively minor change in the PTK/PTPase balance can have a major impact on net tyrosine phosphorylation and thereby on activation and proliferation of T-cells. This review focuses on the involvement of PTKs and PTPases in positive and negative regulation of T-cell activation, the emerging theme of reciprocal regulation of each type of enzyme by the other, as well as regulation of phosphotyrosine turnover by Ser/Thr phosphorylation and regulation of localization of signal components.

The formation and functions of the 21- and 23-kDa tyrosine-phosphorylated TCR zeta subunits

The interaction between the T cell receptor (TCR) and its cognate antigen/major histocompatibility complex (MHC) complex activates a cascade of intracellular protein phosphorylations within the T cell. The signals are initiated by the specific phosphorylation of two tyrosine residues located in a conserved sequence motif termed an ITAM (immune receptor-based tyrosine activation motif). There are 10 ITAMs in the TCR complex, and 6 of these ITAMs are present in the TCR zeta homodimer. Following TCR stimulation, the TCR zeta subunit forms two tyrosine-phosphorylated intermediates of 21- and 23-kDa, respectively. The dramatic and diverse biological responses of T cells are proposed to be partly regulated by the relative ratios of the 21- vs. 23-kDa phosphorylated forms of TCR zeta that are induced following TCR ligation. In this review, we describe a stepwise model of zeta phosphorylation required for the formation of these two phosphorylated derivatives. We describe the kinases and phosphatases controlling these phosphorylation processes. In addition, we present some preliminary findings from ongoing studies that discuss the contributions of each phosphorylated form of zeta on T cell development, TCR signaling, T cell anergy induction, and T cell survival.

Expression of ZAP-70 is associated with increased B-cell receptor signaling in chronic lymphocytic leukemia

We examined isolated leukemia B cells of patients with chronic lymphocytic leukemia (CLL) for expression of zeta-associated protein 70 (ZAP-70). CLL B cells that have nonmutated immunoglobulin variable region genes (V genes) expressed levels of ZAP-70 protein that were comparable to those expressed by normal blood T cells. In contrast, CLL B cells that had mutated immunoglobulin variable V genes, or that had low-level expression of CD38, generally did not express detectable amounts of ZAP-70 protein. Leukemia cells from identical twins with CLL were found discordant for expression of ZAP-70, suggesting that B-cell expression of ZAP-70 is not genetically predetermined. Ligation of the B-cell receptor (BCR) complex on CLL cells that expressed ZAP-70 induced significantly greater tyrosine phosphorylation of cytosolic proteins, including p72(Syk), than did similar stimulation of CLL cells that did not express ZAP-70. Also, exceptional cases of CLL cells that expressed mutated immunoglobulin V genes and ZAP-70 also experienced higher levels tyrosine phosphorylation of such cytosolic proteins following BCR ligation. Following BCR ligation, ZAP-70 underwent tyrosine phosphorylation and became associated with surface immunoglobulin and CD79b, arguing for the involvement of ZAP-70 in BCR signaling. These data indicate that expression of ZAP-70 is associated with enhanced signal transduction via the BCR complex, which may contribute to the more aggressive clinical course associated with CLL cells that express nonmutated immunoglobulin receptors.

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.

Genomic studies of the spleen protein tyrosine kinase locus reveal a complex promoter structure and several genetic variants

Here we show that the gene of the cytoplasmic tyrosine kinase SYK spans a region of 90kb with 13 coding exons, an alternative exon 14 and at least two 5' untranslated regions exons 1a and 1b. 5' RACE (Rapid amplification of cDNA ends) of human Syk cDNAs demonstrated a complex promoter usage and splicing pattern. We identified three common single nucleotide polymorphisms in the exon la promoter region of the Syk gene as well as a variant Syk cDNA haplotype. This haplotype was characterized by a constellation of 5 silent mutations in the Syk cDNA: 1065(C-T), 1302(G-C), 1338(G-A), 1521(C-T) and 1545(T-C). A hypervariable CATATA(n) repeat polymorphism was also localized to the intron between exons 11 and 12. These novel insights into the genomic organization, promoter structure and genetic variability of Syk will serve as a foundation for detailed molecular epidemiological investigation of its potential role in human cancer biology.

The non-receptor tyrosine kinase Syk is a target of Cbl-mediated ubiquitylation upon B-cell receptor stimulation

The negative regulator Cbl functions as a ubiquitin ligase towards activated receptor tyrosine kinases and facilitates their transport to lysosomes. Whether Cbl ubiquitin ligase activity mediates its negative regulatory effects on cytoplasmic tyrosine kinases of the Syk/ZAP-70 family has not been addressed, nor is it known whether these kinases are regulated via ubiquitylation during lymphocyte B-cell receptor engagement. Here we show that B-cell receptor stimulation in Ramos cells induces the ubiquitylation of Syk tyrosine kinase which is inhibited by a dominant-negative mutant of Cbl. Intact tyrosine kinase-binding and RING finger domains of Cbl were found to be essential for Syk ubiquitylation in 293T cells and for in vitro Syk ubiquitylation. These same domains were also essential for Cbl-mediated negative regulation of Syk as measured using an NFAT-luciferase reporter in a lymphoid cell. Association with Cbl did not alter the kinase activity of Syk. Altogether, our results support an essential role for Cbl ubiquitin ligase activity in the negative regulation of Syk, and establish that ubiquitylation provides a mechanism of Cbl-mediated negative regulation of cytoplasmic targets.

Beyond the RING: CBL proteins as multivalent adapters

Following discovery of c-Cbl, a cellular form of the transforming retroviral protein v-Cbl, multiple Cbl-related proteins have been identified in vertebrate and invertebrate organisms. c-Cbl and its homologues are capable of interacting with numerous proteins involved in cell signaling, including various molecular adapters and protein tyrosine kinases. It appears that Cbl proteins play several functional roles, acting both as multivalent adapters and inhibitors of various protein tyrosine kinases. The latter function is linked, to a substantial extent, to the E3 ubiquitin-ligase activity of Cbl proteins. Experimental evidence for these functions, interrelations between them, and their biological significance are addressed in this review, with the main accent placed on the adapter functions of Cbl proteins.

Spleen tyrosine kinase (Syk) deficiency in childhood pro-B cell acute lymphoblastic leukemia

The cytoplasmic spleen tyrosine kinase (SYK) is a key regulator of signal transduction events, apoptosis and orderly cell cycle progression in B-lineage lymphoid cells. Although SYK has not been linked to a human disease, defective expression of the closely related T-cell tyrosine kinase ZAP-70 has been associated with severe combined immunodeficiency. Childhood CD19(+)CD10(-) pro-B cell acute lymphoblastic leukemia (ALL) is thought to originate from B-cell precursors with a maturational arrest at the pro-B cell stage and it is associated with poor prognosis. Since lethally irradiated mice reconstituted with SYK-deficient fetal liver-derived lymphohematopoietic progenitor cells show a block in B-cell ontogeny at the pro-B to pre-B cell transition, we examined the SYK expression profiles of primary leukemic cells from children with pro-B cell ALL. Here we report that leukemic cells from pediatric CD19(+)CD10(-) pro-B cell ALL patients (but not leukemic cells from patients with CD19(+)CD10(+) common pre-pre-B cell ALL) have markedly reduced SYK activity. Sequencing of the reverse transcriptase-polymerase chain reaction (RT-PCR) products of the Syk mRNA in these pro-B leukemia cells revealed profoundly aberrant coding sequences with deletions or insertions. These mRNA species encode abnormal SYK proteins with a missing or truncated catalytic kinase domain. In contrast to pro-B leukemia cells, pre-pre-B leukemia cells from children with CD19(+)CD10(+) common B-lineage ALL and EBV-transformed B-cell lines from healthy volunteers expressed wild-type Syk coding sequences. Examination of the genomic structure of the Syk gene by inter-exonic PCR and genomic cloning demonstrated that the deletions and insertions in the abnormal mRNA species of pro-B leukemia cells are caused by aberrant splicing resulting in either mis-splicing, exon skipping or inclusion of alternative exons, consistent with an abnormal posttranscriptional regulation of alternative splicing of Syk pre-mRNA. Our findings link for the first time specific molecular defects involving the Syk gene to an immunophenotypically distinct category of childhood ALL. To our knowledge, this is the first discovery of a specific tyrosine kinase deficiency in a human hematologic malignancy.

Proximal protein tyrosine kinases in immunoreceptor signaling

Immunoreceptor engagement results in the sequential activation of several classes of protein tyrosine kinases, including the Src and Syk/Zap-70 families. Recent progress has been made in our understanding of the regulation and function of these molecules. First, it was revealed that membrane compartmentation of protein tyrosine kinases may be essential for their proper biological function. Second, Src family kinases were found to act not only as positive regulators, but also as inhibitors of cell activation. Third, it was appreciated that Csk, a potent inhibitor of Src kinases, is regulated by an assortment of protein-protein interactions. Fourth, differences in the regulation of Syk and Zap-70 were observed, suggesting significant distinctions in the purpose of these two kinases in immunoreceptor signaling. And fifth, it was suggested that proximal kinases implicated in immunoreceptor-mediated signal transduction may be regulated by protein degradation via binding to c-Cbl, a ubiquitin ligase.

Phagocytosis and the actin cytoskeleton

The process of engulfing a foreign particle - phagocytosis - is of fundamental importance for a wide diversity of organisms. From simple unicellular organisms that use phagocytosis to obtain their next meal, to complex metazoans in which phagocytic cells represent an essential branch of the immune system, evolution has armed cells with a fantastic repertoire of molecules that serve to bring about this complex event. Regardless of the organism or specific molecules concerned, however, all phagocytic processes are driven by a finely controlled rearrangement of the actin cytoskeleton. A variety of signals can converge to locally reorganise the actin cytoskeleton at a phagosome, and there are significant similarities and differences between different organisms and between different engulfment processes within the same organism. Recent advances have demonstrated the complexity of phagocytic signalling, such as the involvement of phosphoinostide lipids and multicomponent signalling complexes in transducing signals from phagocytic receptors to the cytoskeleton. Similarly, a wide diversity of ‘effector molecules’ are now implicated in actin-remodelling downstream of these receptors.

Alternative antigen receptor (TCR) signaling in T cells derived from ZAP-70-deficient patients expressing high levels of Syk

ZAP-70-deficient patients present with nonfunctional CD4+ T cells in the periphery. We find that a subset of primary ZAP-70-deficient T cells, expressing high levels of the related protein-tyrosine kinase Syk, can proliferate in vitro. These cells (denoted herein as Syk(hi)/ZAP-70(-) T cells) provide a unique model in which the contribution of Syk to TCR-mediated responses can be explored in a nontransformed background. Importantly, CD3-induced responses, such as tyrosine phosphorylation of cellular substrates (LAT, SLP76, and PLC-gamma1), as well as calcium mobilization, which are defective in T cells expressing neither ZAP-70 nor Syk, are observed in Syk(hi)/ZAP-70(-) T cells. However, Syk(hi)/ZAP-70(-) T cells differ from control T cells with respect to the T cell antigen receptor (TCR)-mediated activation of the MAPK cascades: extracellular signal-regulated kinase activity and recruitment of the JNK and p38 stress-related MAPK pathways are diminished. This distinct phenotype of Syk(hi)/ZAP-70(-) T cells is associated with a profound decrease in CD3-mediated interleukin 2 secretion and proliferation relative to control T cells. Thus, ZAP-70 and Syk appear to play distinct roles in transducing a TCR-mediated signal.

The linker phosphorylation site Tyr292 mediates the negative regulatory effect of Cbl on ZAP-70 in T cells

The protooncogene product Cbl has emerged as a negative regulator of tyrosine kinases. We have shown previously that Cbl binds to ZAP-70 through its N-terminal tyrosine kinase binding (TKB) domain. In this study, we demonstrate that overexpression of Cbl in Jurkat T cells decreases the TCR-induced phosphorylation of ZAP-70 and other cellular phosphoproteins. Coexpression of Cbl with ZAP-70 in COS cells reproduced the Cbl-induced reduction in the level of phosphorylated ZAP-70. The effect of Cbl was eliminated by the TKB-inactivating G306E mutation in Cbl as well as by a phenylalanine mutation of Tyr292 within the TKB domain binding site on ZAP-70. Notably, the oncogenic Cbl-70Z/3 mutant associated with ZAP-70, but did not reduce the levels of phosphorylated ZAP-70. Overexpression of Cbl, but not Cbl-G306E, in Jurkat T cells led to a decrease in the TCR-induced NF-AT luciferase reporter activity. Overexpression of the TKB domain itself, but not its G306E mutant, functioned in a dominant-negative manner and led to an increase in NF-AT reporter activity. Cbl-70Z/3-overexpressing cells exhibited an increase in both basal and TCR-induced NF-AT luciferase reporter activity, and this trend was reversed by the G306E mutation. Finally, by reconstituting a ZAP-70-deficient Jurkat T cell line, p116, we demonstrate that wild-type ZAP-70 is susceptible to the negative regulatory effect of Cbl, whereas the ZAP-70-Y292F mutant is resistant. Together, our results establish that the linker phosphorylation site Tyr292 mediates the negative regulatory effect of Cbl on ZAP-70 in T cells.

Fcgamma receptor-mediated phagocytosis in macrophages lacking the Src family tyrosine kinases Hck, Fgr, and Lyn

Macrophage Fcgamma receptors (FcgammaRs) mediate the uptake and destruction of antibody-coated viruses, bacteria, and parasites. We examined FcgammaR signaling and phagocytic function in bone marrow-derived macrophages from mutant mice lacking the major Src family kinases expressed in these cells, Hck, Fgr, and Lyn. Many FcgammaR-induced functional responses and signaling events were diminished or delayed in these macrophages, including immunoglobulin (Ig)G-coated erythrocyte phagocytosis, respiratory burst, actin cup formation, and activation of Syk, phosphatidylinositol 3-kinase, and extracellular signal-regulated kinases 1 and 2. Significant reduction of IgG-dependent phagocytosis was not seen in hck(-)(/)-fgr(-)(/)- or lyn(-)(/)- cells, although the single mutant lyn(-)(/)- macrophages did manifest signaling defects. Thus, Src family kinases clearly have roles in two events leading to FcgammaR-mediated phagocytosis, one involving initiation of actin polymerization and the second involving activation of Syk and subsequent internalization. Since FcgammaR-mediated phagocytosis did occur at modest levels in a delayed fashion in triple mutant macrophages, these Src family kinases are not absolutely required for uptake of IgG-opsonized particles.

The role of membrane-associated adaptors in T cell receptor signalling

Engagement of the T cell receptor leads to activation of several tyrosine kinases and phosphorylation of many intracellular proteins. This is followed by Ca2+ mobilization and activation of multiple biochemical pathways, including the Ras/MAPK cascade, and several downstream serine/threonine kinases. Membrane-associated adaptor proteins play an important role in T cell activation by coupling TCR ligation at the membrane to distal signalling cascades. Several new membrane associated adaptors have been identified in recent years. LAT (linker for activation of T cells) is an adaptor molecule, which following its phosphorylation associates with Grb2, Gads, PLC-gamma 1, and other signalling molecules. The functional importance of this molecule has been demonstrated by the study of LAT-deficient cell lines and LAT-deficient mice. Two other recently identified adaptor proteins, TRIM (T cell receptor interacting molecule) and SIT (SHP2-interacting transmembrane adaptor protein), which constitutively associate with several surface molecules, bind to PI3K and SHP2, respectively, after T cell activation and might also function in the TCR signalling pathway.

Point mutation of a tyrosine in the linker region of Syk results in a gain of function

The protein tyrosine kinase Syk plays an essential role in Fc epsilon RI-mediated histamine release in mast cells by regulating the phosphorylation of other proteins. We investigated the functional role of a putative Syk phosphorylation site, Tyr317. This tyrosine in the linker region of Syk is a possible site for binding by the negative regulator Cbl. Syk with Tyr317 mutated to Phe (Y317F) was expressed in a Syk-negative variant of the RBL-2H3 mast cells. Compared with cells expressing wild-type Syk, expression of the Y317F mutant resulted in an increase in the Fc epsilon RI-mediated tyrosine phosphorylation of phospholipase C-gamma and a dramatic enhancement of histamine release. The in vivo Fc epsilon RI-induced tyrosine phosphorylation of wild-type Syk and that of the Y317F mutant were similar. Although the Fc epsilon RI-induced tyrosine phosphorylation of total cellular proteins was enhanced in the cells expressing the Y317F Syk, the phosphorylation of some other molecules, including the receptor subunits, Vav and mitogen-activated protein kinase, was not increased. The Fc epsilon RI-induced phosphorylation of Cbl was downstream of Syk kinase activity and was unchanged by expression of the Y317F mutation. These data indicate that Tyr317 in the linker region of Syk functions to negatively regulate the signals leading to degranulation.

Enhancement of Fcγ Receptor-Mediated Phagocytosis by Transforming Mutants of Cbl

Phagocytosis mediated by FcγR plays an important role in host defense. The molecular events involved in this process have not been completely defined. The adapter protein Cbl has been implicated in FcγR signaling, but the function of Cbl in phagocytosis is unknown. Here we show that overexpression of the transforming mutants of Cbl, Cbl-70Z, and v-Cbl, but not wild-type (wt) Cbl, enhance phagocytosis mediated by FcγR in COS cells. Cbl-70Z, but not Cbl-wt, also enhanced FcγR-mediated phagocytosis in P388D1 murine macrophage cells. Cbl-70Z did not affect tyrosine phosphorylation or in vitro kinase activity of Syk, indicating that Syk may not be the direct target of Cbl-70Z in the enhancement of phagocytosis. A point mutation (G306E) in the phosphotyrosine domain of Cbl-70Z, as well as a C-terminal 67-aa deletion, partially abolished the enhancing effect on FcγR-mediated phagocytosis. A double mutant of Cbl-70Z containing both the G306E mutation and the C-terminal deletion completely lacked the ability to enhance phagocytosis. Thus, both the phosphotyrosine binding domain and the carboxyl-terminal tail were required for optimal enhancement of phagocytosis by Cbl-70Z. Functional phosphatidylinositol 3-kinase was required for Cbl-70Z to enhance phagocytosis, since wortmannin, a phosphatidylinositol 3-kinase inhibitor, inhibited FcγR-mediated phagocytosis in the presence of Cbl-70Z. These studies demonstrate that mutants of Cbl can modulate the phagocytic pathway mediated by FcγR and imply a functional involvement of c-Cbl in Fcγ receptor-mediated phagocytosis.

CD45 Is Essential for FcεRI Signaling by ZAP70, But Not Syk, in Syk-Negative Mast Cells

The ZAP70/Syk family of protein tyrosine kinases plays an important role in Ag receptor signaling. Structural similarity of Syk and ZAP70 suggests their functional overlap. Previously, it was observed that expression of either ZAP70 or Syk reconstitutes Ag receptor signaling in Syk-negative B cells. However, in CD45-deficient T cells, Syk, but not ZAP70, restores T cell receptor-signaling pathway. To study the function of Syk, ZAP70, and CD45 in mast cells, a Syk/CD45 double-deficient variant of RBL-2H3 cells was characterized. After transfection, stable cell lines were isolated that expressed ZAP70, Syk, CD45, ZAP70 plus CD45, and Syk plus CD45. IgE stimulation did not induce degranulation in parental double-deficient cells, nor in the cells expressing only CD45. ZAP70 expression did not restore FcεRI signaling unless CD45 was coexpressed in the cells. However, Syk alone restored the IgE signal transduction pathway. The coexpression of CD45 with Syk had no significant effects on the responses to FcεRI-aggregation. There was much better binding of Syk than ZAP70 to the phosphorylated FcεRIγ-ITAM. Furthermore, unlike Syk, ZAP70 required CD45 to display receptor-induced increase in kinase activity. Therefore, in mast cells, ZAP70, but not Syk, requires CD45 for Ag receptor-induced signaling.

The Cbl protooncoprotein: a negative regulator of immune receptor signal transduction

The Cbl protooncoprotein has recently emerged as a component of tyrosine kinase-mediated signal transduction in a variety of cell types. Here, we discuss evidence that supports a role for Cbl as a novel negative regulator of immune receptor signaling, and present models for its mode of function.

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

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

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

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

Interactions of CD45-associated protein with the antigen receptor signaling machinery in T-lymphocytes

CD45 is a transmembrane protein tyrosine phosphatase playing an essential role during T-cell activation. This function relates to the ability of CD45 to regulate p56(lck), a cytoplasmic protein tyrosine kinase necessary for T-cell antigen receptor (TCR) signaling. Previous studies have demonstrated that CD45 is constitutively associated in T-lymphocytes with a transmembrane molecule termed CD45-AP (or lymphocyte phosphatase-associated phosphoprotein). Even though the exact role of this polypeptide is unclear, recent analyses of mice lacking CD45-AP have indicated that its expression is also required for optimal T-cell activation. Herein, we wished to understand better the function of CD45-AP. The results of our studies showed that in T-cells, CD45-AP is part of a multimolecular complex that includes not only CD45, but also TCR, the CD4 and CD8 coreceptors, and p56(lck). The association of CD45-AP with TCR, CD4, and CD8 seemed to occur via the shared ability of these molecules to bind CD45. However, binding of CD45-AP to p56(lck) could take place in the absence of other lymphoid-specific components, suggesting that it can be direct. Structure-function analyses demonstrated that such an interaction was mediated by an acidic segment in the cytoplasmic region of CD45-AP and by the kinase domain of p56(lck). Interestingly, the ability of CD45-AP to interact with Lck in the absence of other lymphoid-specific molecules was proportional to the degree of catalytic activation of p56(lck). Together, these findings suggest that CD45-AP is an adaptor molecule involved in orchestrating interactions among components of the antigen receptor signaling machinery. Moreover, they raise the possibility that one of the functions of CD45-AP is to recognize activated Lck molecules and bring them into the vicinity of CD45.

Structural requirements of Syk kinase for Fcgamma receptor-mediated phagocytosis

The tyrosine kinase Syk plays a critical role in the phagocytic pathway mediated by Fcgamma receptors (FcgammaR). In transfected COS1 cells co-expression of Syk enhances FcgammaR mediated phagocytosis. The other member of the Syk kinase family, the highly homologous tyrosine kinase Zap70, also plays a role in signaling by immunoglobulin gene family receptors, but does not increase the phagocytic efficiency of FcgammaRs. The homologous tandem SH2 and kinase domains of Syk and Zap70 are separated by a nonhomologous region referred to as the unique domain. Zap70's inability to enhance phagocytosis was not due to unique domain tyrosine 292, previously implicated in negative regulation of Zap70 function. We determined the regions of Syk important for its interaction with the phagocytic pathway. An intact kinase domain was required for Syk's effect on phagocytosis. Furthermore, the Syk variant SykB, lacking 23 amino acids in the unique region, signaled for phagocytosis as efficiently as did Syk. We then constructed exchange chimeras between Syk and Zap70 and determined the contributions of the SH2, unique and kinase domains to phagocytic signaling. Our data suggest that the Syk kinase domain, which has high intrinsic kinase activity, is important for facilitating phagocytic signaling by FcgammaRI and FcgammaRIIIA.

Altered expression of tyrosine kinases of the Src and Syk families in human T-cell leukemia virus type 1-infected T-cell lines

During the late phase of adult T-cell leukemia/lymphoma, a severe lymphoproliferative disorder caused by human T-cell leukemia virus type 1 (HTLV-1), leukemic cells no longer produce interleukin-2. Several studies have reported the lack of the Src-like protein tyrosine kinase Lck and overexpression of Lyn and Fyn in these cells. In this report we demonstrate that, in addition to the downregulation of TCR, CD45, and Lck (which are key components of T-cell activation), HTLV-1-infected cell lines demonstrate a large increase of FynB, a Fyn isoform usually poorly expressed in T cells. Furthermore, similar to anergic T cells, Fyn is hyperactive in one of these HTLV-1-infected T-cell lines, probably as a consequence of Csk downregulation. A second family of two proteins, Zap-70 and Syk, relay the signal of T-cell activation. We demonstrate that in contrast to uninfected T cells, Zap-70 is absent in HTLV-1-infected T cells, whereas Syk is overexpressed. In searching for the mechanism responsible for FynB overexpression and Zap-70 downregulation, we have investigated the ability of the Tax and Rex proteins to modulate Zap-70 expression and the alternative splicing mechanism which gives rise to either FynB or FynT. By using Jurkat T cells stably transfected with the tax and rex genes or inducibly expressing the tax gene, we found that the expression of Rex was necessary to increase fynB expression, suggesting that Rex controls fyn gene splicing. Conversely, with the same Jurkat clones, we found that the expression of Tax but not Rex could downregulate Zap-70 expression. These results suggest that the effect of Tax and Rex must cooperate to deregulate the pathway of T-cell activation in HTLV-1-infected T cells.

Phosphorylation of Tyr319 in ZAP-70 is required for T-cell antigen receptor-dependent phospholipase C-gamma1 and Ras activation

Accumulating evidence indicates that the interdomain B regions of ZAP-70 and Syk play pivotal roles in the coupling of T-cell antigen receptor (TCR) stimulation to the activation of downstream signaling pathways. The interdomain B region of ZAP-70 contains at least three candidate sites of tyrosine phosphorylation. In this report, we identify Tyr319 as a functionally important phosphorylation site in the ZAP-70 interdomain B region. TCR crosslinkage triggered a rapid increase in the phosphorylation of Tyr319 in Jurkat T cells. Although mutation of Tyr319 to Phe had no effect on the tyrosine kinase activity of ZAP-70, the resulting ZAP(Y319-->F) mutant failed to reconstitute TCR-dependent Ca2+ mobilization, Ras activation, CD69 expression and NFAT-dependent transcription in ZAP-70-deficient Jurkat cells. These defects were correlated with reduced tyrosine phosphorylation of phospholipase C (PLC)-gamma1 and the LAT adapter protein in the ZAP(Y319-->F)-expressing cells. On the other hand, ZAP(Y319-->F)-expressing cells displayed normal increases in SLP-76 phosphorylation and ERK activation during TCR stimulation. Phosphorylation of Tyr319 promoted the association of ZAP-70 with the SH2 domains of two key signaling molecules, Lck and PLC-gamma1. These studies suggest that Tyr319 phosphorylation is required for the assembly of a ZAP-70-containing signaling complex that leads to the activation of the PLC-gamma1- and Ras-dependent signaling cascades in antigen-stimulated T cells.

Volume expansion stimulates p72(syk) and p56(lyn) in skate erythrocytes

Hypotonic volume expansion of skate erythrocytes rapidly stimulates the tyrosine phosphorylation of band 3, the membrane protein thought to mediate the osmotically sensitive taurine efflux. Skate erythrocytes possess numerous tyrosine kinases including p59fyn, p56lyn, pp60(src), and p72(syk), demonstrated by immune complex assays measuring autocatalytic kinase activity. Inclusion of the cytoplasmic domain of band 3 in this assay showed that only Syk and Lyn can directly phosphorylate the cytoplasmic domain of band 3. Upon cell volume expansion, Syk activity was increased as assessed by three different assays (immune complex assay measuring autophosphorylation, assay of the level of phosphotyrosine of the immunoprecipitated kinase, and assay of level of 32P in the kinase immunoprecipitated from cells prelabeled with 32PO4 and then volume-expanded). The tyrosine kinase Lyn was also stimulated by volume expansion, most notably when analyzed by the latter two methods. Volume expansion stimulated a large increase in the ability of Syk to phosphorylate band 3 at times that coincide with the stimulation of taurine flux. The stilbene piceatannol inhibited Syk preferentially over Lyn and other tyrosine kinases and inhibited volume-stimulated taurine efflux in a concentration-dependent manner similar to that for the inhibition of Syk. Two major phosphorylation peaks were detected in tryptic digests of cdb3 separated by reverse phase HPLC. Edman degradation demonstrated a phosphotyrosine in a YXXL motif. In conclusion, p72(syk) appears to be a strong candidate as a pivotal signal-transducing step in the volume-activated taurine efflux in skate red cells. The level of band-3 phosphorylation may be regulated, in addition, by a protein-tyrosine phosphatase of the 1B variety.