The kinase, SH3, and SH2 domains of Lck play critical roles in T-cell activation after ZAP-70 membrane localization

Three new compounds with nitric oxide inhibitory activity from Tirpitzia sinensis, an ethnomedicinal plant from Southwest China

The medicinal plant Tirpitzia sinensis has been used by the Zhuang ethnic people in mountainous areas of Southwest China to stop bleeding, invigorate blood circulation, and treat inflammation and wounds. In order to further explore its traditional medicinal uses, the phytochemical constituents of this species were examined. Three new compounds, the lignan tirpitzin (1), the flavonoid tirpitzoside (2), and the furan-glycoside tirpitziol (3), along with five known compounds were isolated from the aerial part of T. sinensis for the first time. The structures of these compounds were elucidated by 1D and 2D NMR, LC/MS, IR spectrometric methods and compared with published data. The results of an in silico pharmacophore-based analysis showed potential targets of the new compounds, including ERBB2, IRAK4, LCK, JAK2, MAPK14, and MMP-12. These targets suggested that 1-3 may be involved with wound-healing and/or inflammation, leading to an in vitro assay of nitric oxide (NO) inhibition assays with lipopolysaccharide-induced BV-2 cells. All three new compounds displayed moderate NO inhibitory activity with the IC₅₀ values of 14.97 ± 0.87, 26.63 ± 1.32, and 17.09 ± 2.3 μM, respectively.

Regulation of T cell development by c-Cbl: essential role of Lck

A canonical pre-TCR/TCR signaling pathway critical for thymic T cell development involves sequential phosphorylation and signaling through Lck, Zap70, Lat and Slp76. However, we and others have previously reported that genomic deletion of c-Cbl (Cbl) partially or completely reverses the defects in thymic development in mice deficient in Zap70, Slp76, Lat or Vav1, indicating the presence of alternative pathways normally suppressed by Cbl. To further elucidate pre-TCR/TCR signaling pathways involved in thymic development, we characterized the effect of Cbl inactivation on developmental and signaling defects in mice deficient in proximal signaling molecules Lck and Zap70. Inactivation of Cbl partially reversed defective T cell development in Zap70 (-/-) mice and reversed defects in phosphorylation of Erk, Plc-γ1, Vav1 and Akt, in TCR-stimulated Cbl (-/-) Zap70 (-/-) thymocytes. Recent reports identified an essential role of Lck in associating with CD4 and CD8 coreceptors and mediating the requirement for MHC restriction in TCR recognition. Since TCR recognition has been shown to be MHC-restricted in Cbl (-/-) mice, it was of interest to determine whether the requirement for Lck remained unmodified by Cbl deletion. Indeed, in contrast to the effect of Cbl inactivation in partially or fully bypassing requirements for other TCR signaling components, inactivation of Cbl did not reverse either defective T cell development or defective phosphorylation of TCR signaling molecules in Lck (-/-) mice. Thus, Lck, which plays a unique role in enforcing MHC restriction, is essential for thymic development in presence or absence of Cbl, ensuring MHC restriction of T cells derived from either pathway.

How does the kinase Lck phosphorylate the T cell receptor? Spatial organization as a regulatory mechanism

T cell signaling begins with the ligation of the T cell antigen receptor (TCR) by a cognate peptide and the phosphorylation of the receptor’s immunoreceptor tyrosine-based activation motif domains by the kinase Lck. However, the canonical receptor model is insufficient to explain how the constitutively active kinase Lck can discriminate between non-ligated and ligated TCRs. Here, we discuss the factors that are thought to regulate the spatial distribution of the TCR and Lck, and therefore critically influence TCR signaling initiation.

Molecular characterization, tissue expression, polymorphism and association of porcine LCK gene

Lymphocyte-specific protein tyrosine kinase (LCK) is an important reproduction related gene. In this study, the full-length cDNA sequence of porcine LCK gene was cloned through the rapid amplification of cDNA ends (RACE) method. The porcine LCK gene encodes a protein of 509 amino acids which shares high homology with the LCK of nine species: giant panda (97%), dog (97%), cattle (96%), sheep (95%), rabbit (95%), human (96%), rat (94%), mouse (94%) and horse (94%). This novel porcine gene was assigned to GeneID: 100233188. The phylogenetic analysis revealed that the porcine LCK gene has a closer genetic relationship with the LCK gene of dog. This gene is structured in twelve exons and eleven introns as revealed by computer-assisted analysis. The tissue transcription profile analysis indicated that the porcine LCK gene is generally but differentially expressed in the detected tissues including large intestine, spleen, lung, muscle, fat, liver, heart, kidney and ovary. PCR-Alu I-RFLP was established to detect an A/G substitution at the position of 1127-bp of mRNA and eight pig breeds displayed obvious genotype and allele frequency differences at this mutation locus. Association of this single-nucleotide polymorphism with litter size traits was assessed in Large White (n = 100) and Landrace (n = 100) pig populations, and results demonstrated that this polymorphic locus was significantly associated with the litter size of all parities in Large White sows and Landrace sows (P < 0.01). Therefore, LCK gene could be an useful candidate gene in selection for increasing litter size in pigs. These data serve as a foundation for further insight into this novel porcine gene.

RhoH regulates subcellular localization of ZAP-70 and Lck in T cell receptor signaling

RhoH is an hematopoietic-specific, GTPase-deficient Rho GTPase that plays a role in T development. We investigated the mechanisms of RhoH function in TCR signaling. We found that the association between Lck and CD3ζ was impaired in RhoH-deficient T cells, due to defective translocation of both Lck and ZAP-70 to the immunological synapse. RhoH with Lck and ZAP-70 localizes in the detergent-soluble membrane fraction where the complex is associated with CD3ζ phosphorylation. To determine if impaired translocation of ZAP-70 was a major determinant of defective T cell development, Rhoh(-/-) bone marrow cells were transduced with a chimeric myristoylation-tagged ZAP-70. Myr-ZAP-70 transduced cells partially reversed the in vivo defects of RhoH-associated thymic development and TCR signaling. Together, our results suggest that RhoH regulates TCR signaling via recruitment of ZAP-70 and Lck to CD3ζ in the immunological synapse. Thus, we define a new function for a RhoH GTPase as an adaptor molecule in TCR signaling pathway.

T-cell receptor signaling is mediated by transient Lck activity, which is inhibited by inorganic mercury

Genetically susceptible rodents exposed to low nontoxic levels of inorganic mercury (Hg(2+)) develop idiosyncratic autoimmune disease associated with defective T-cell function. However, the molecular mechanisms underlying this phenomenon remain mostly unexplained. Brief exposure of T cells to micromolar concentrations of Hg(2+) leads to physiologically relevant nontoxic cellular mercury burdens, and as we have previously reported, attenuates T-cell receptor (TCR) signal strength by approximately 50%. We have found this to be the result of an inadequate activation of the tyrosine kinase ZAP-70, which is hypophosphorylated following TCR stimulation in Hg(2+) burdened cells when compared to untreated controls. In T cells, ZAP-70 phosphorylation is dependent on lymphocyte-specific protein tyrosine kinase (Lck) activity, which in turn is either positively or negatively regulated by the phosphorylation of specific Lck tyrosine residues. In particular, the general belief is that Lck is negatively regulated by phosphorylation of tyrosine 192 (Y192). We now demonstrate by Western blotting that, in Jurkat T cells, TCR signal transduction (and ZAP-70 phosphorylation) was positively associated with a rapid transient phosphorylation of Y192, which was inhibited in cells that were briefly (5 min) exposed to 5 microM Hg(2+). Thus, Hg(2+) inhibits a critical activating role played by Lck Y192 during the most proximal events of the TCR-induced cell signaling.

Enrichment of regulatory CD4(+)CD25(+) T cells by inhibition of phospholipase D signaling

Antigen stimulation of lymphocytes induces upregulation of phospholipase D (PLD) activity, but the biological significance of PLD-mediated signaling in T cells has not been well established. Here we demonstrate that PLD signaling is essential for proliferation of mouse CD8(+) T cells and CD4(+)CD25(-) T cells, but is not required for proliferation of CD4(+)CD25(+) regulatory T cells. We exploited this observation to develop an efficient method to enrich for regulatory T cells starting from preparations of total CD4(+) T lymphocytes. Inhibition of PLD signaling blocked effector T-cell proliferation after T cell-antigen receptor (TCR) engagement, but had no significant effect on the proliferation of CD4(+)CD25(+) T cells with regulatory functions. Consequently, cells expanded in vitro for one week by antigen receptor stimulation with PLD signal inhibition were markedly enriched for regulatory T cells.

Lck couples Shc to TCR signaling

Recent genetic evidence demonstrated that Shc is a critical molecule for T cell activation and differentiation. However, how Shc is coupled to the T cell antigen receptor (TCR) has not been clearly characterized. Here we report that the tyrosine kinase Lck functions as a connecting molecule for TCR and Shc. Lck plays a critical role in TCR signal transduction by phosphorylating the immuno-receptor tyrosine based activation motif (ITAM). Our data shows that the PTB domain of Shc binds the SH2/3 domains of Lck in a phosphotyrosine-independent manner. Inhibition of the Lck/Shc interaction led to the loss of IL-2 promoter activation, confirming that the role of Shc in IL-2 production requires its interaction with Lck. Together, the data show that Shc is connected to the activated TCR via direct interaction with Lck.

IkB-α-specific transcript regulation by the C-terminal end of c-Rel

The NF-kB family transcription factor c-Rel is a critical molecule for inducing expression of cytokine genes by T cells. Here, we report that a deletion of the C-terminal end, similar to the deletion in the highly oncogenic chicken v-Rel gene, renders c-Rel hyperactive toward cytokine gene promoters. At the same time, this mutation dramatically reduced c-Rel activity in induction of IkB-alpha mRNA expression. Moreover, ectopic expression of IkB-alpha, along with the C-terminal truncated c-Rel, abrogates hyperactivity of this mutant. IkB-alpha co-expression did not affect the function of wild-type c-Rel. The data demonstrate that the C-terminal end of c-Rel has specific activity for IkB-alpha mRNA expression and is dispensable for IL-2 gene expression.

A novel ERK-dependent signaling process that regulates interleukin-2 expression in a late phase of T cell activation

Engagement of the T cell antigen receptor (TCR) rapidly induces multiple signal transduction pathways, including ERK activation. Here, we report a critical role for ERK at a late stage of T cell activation. Inhibition of the ERK pathway 2-6 h after the start of TCR stimulation significantly impaired interleukin-2 (IL-2) production, whereas the same treatment during the first 2 h had no effect. ERK inhibition significantly impaired nuclear translocation of c-Rel with a minimum reduction of NF-AT activity. Requirement for sustained ERK activation was also confirmed using primary T cells. To induce sustained activation of ERK, T cells required continuous engagement of TCR. Stimulation of T cells with soluble anti-TCR antibody resulted in activation of ERK lasting for 60 min, but failed to induce IL-2 production. In contrast, plate-bound anti-TCR antibody activated ERK over 4 h and induced IL-2. Furthermore, T cells treated with soluble anti-TCR antibody produced IL-2 when phorbol 12-myristate 13-acetate, which activates ERK, was present in the culture medium 2-6 h after the start of stimulation. Together, the data demonstrate the presence of a novel activation process following TCR stimulation that requires ERK-dependent regulation of c-Rel, a member of the NF-kappaB family.

Selective interaction of LAT (linker of activated T cells) with the open-active form of Lck in lipid rafts reveals a new mechanism for the regulation of Lck in T cells

In T cells, the lipid raft-associated Lck is strongly tyrosine phosphorylated and has reduced enzymic activity in contrast with the detergent-soluble pool, which has substantial activity. Lck tagged at the C-terminus (Lck/V5-His) was efficiently captured by epitope-specific reagents from the detergent-soluble fraction but not from lipid rafts. Binding was restored following urea denaturation, suggesting that Lck/V5-His is in a 'closed' conformation in these domains. In agreement with this hypothesis, the Tyr(505) --> Phe/V5-His and Arg(154) --> Lys/V5-His mutants, which disrupt the SH2-Tyr(505) intramolecular interaction, were efficiently precipitated from lipid rafts. In contrast to Lck, Fyn/V5-His was precipitated equally well from both fractions. In the LAT(linker of activated T cells)-deficient J.CaM2 cells, Tyr(505) phosphorylation of raft-associated Lck was reduced whereas its enzymic activity was elevated. This correlated with decreased levels of raft-localized Csk (C-terminal Src kinase) kinase. Increased tyrosine phosphorylation of Lck was restored in LAT-reconstituted J.CaM2 cells suggesting that LAT negatively regulates Lck activity in lipid rafts. Co-immunoprecipitation experiments from Tyr(505) --> Phe/V5-His-expressing cells revealed that LAT preferentially interacts with the 'open' form of Lck in T cell raft domains. These results demonstrate that, unlike the non-raft pool, Lck in lipid rafts has a 'closed'-inactive structure, and that LAT plays a role in maintaining this conformation, possibly by facilitating critical associations within lipid rafts via its capacity to interact with the 'open' form of the kinase.

Optimization of 2-phenylaminoimidazo[4,5-h]isoquinolin-9-ones: orally active inhibitors of lck kinase

The tyrosine kinase p56lck (lck) is essential for T cell activation; thus, inhibitors of lck have potential utility as autoimmune agents. Our initial disclosure of a new class of lck inhibitors based on the phenylaminoimidazoisoquinolin-9-one showed reasonable cellular activity but did not work in vivo upon oral administration. Our current work highlights the further use of rational drug design and molecular modeling to produce a series of lck inhibitors that demonstrate cellular activity below 100 nM and are as efficacious as cyclosporin A in an in vivo mouse model of anti-CD3-induced IL-2 production.

Negative feedback of T cell activation through inhibitory adapters and costimulatory receptors

Antigen recognition by the T cell receptor (TCR) complex induces the formation of a TCR signalosome by recruiting various signaling molecules, generating the recognition signals for T cell activation. The activation status and functional outcome are positively and negatively regulated by dynamic organization of the signalosome and by costimulation signals. We have studied the negative regulation of T cell activation, particularly through inhibitory adapters and costimulation receptors that are little expressed in resting cells but are induced upon T cell activation. We described Grb-associated binder 2 (Gab2) and cytotoxic T lymphocyte antigen-4 (CTLA-4) as a representative inhibitory adapter and a negative costimulation receptor, respectively, both of which exhibit negative feedback. Gab2 functions as a signal branch for activation vs. inhibition, as phosphorylation of either Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) or Gab2 by zeta-associated protein of 70 kDa (ZAP-70) determines the fate of the response. As a professional inhibitory receptor, CTLA-4 inhibits T cell response by competition of ligand binding with positive costimulator receptor CD28, and also induces inhibitory signaling. The trafficking and the cell surface expression of CTLA-4 are dynamically regulated and induced. CTLA-4 is accumulated in lysosomes and secreted to the T cell-APC contact site upon TCR stimulation. As T cell activation proceeds, these inhibitory adapters and costimulation receptors are induced and suppress/regulate the responses as negative feedback.

Genetic evidence for Shc requirement in TCR-induced c-Rel nuclear translocation and IL-2 expression

Shc, a prototypic adapter molecule, has been implicated in T cell receptor (TCR) signal transduction, but its role has not been identified clearly. Here we report that Shc is essential for TCR-induced IL-2 production but is dispensable for CD69 or CD25 expression. Engagement of TCR in mutant Jurkat T cells lacking Shc fails to produce IL-2 because of impaired mitogen-activated protein kinase activation. Activation of c-Rel, a transcription factor essential for IL-2 expression, was impaired also. In contrast, activation of nuclear factor of activated T cell and expression of CD69/CD25 were comparable between the mutant and wild-type Jurkat cells. These defects were rescued by expression of exogenous Shc. Activation of c-Rel using the estrogen receptor fusion protein restored the activation of the IL-2 promoter in an estrogen-dependent manner. These results show that Shc plays an essential role in the TCR-induced activation of c-Rel and the IL-2 promoter.

Docking protein Gab2 is phosphorylated by ZAP-70 and negatively regulates T cell receptor signaling by recruitment of inhibitory molecules

To maintain various T cell responses and immune equilibrium, activation signals triggered by T cell antigen receptor (TCR) must be regulated by inhibitory signals. Gab2, an adaptor protein of the insulin receptor substrate-1 family, has been shown to be involved in the downstream signaling from cytokine receptors. We investigated the functional role of Gab2 in TCR-mediated signal transduction. Gab2 was phosphorylated by ZAP-70 and co-precipitated with phosphoproteins, such as ZAP-70, LAT, and CD3zeta, upon TCR stimulation. Overexpression of Gab2 in Jurkat cells or antigen-specific T cell hybridomas resulted in the inhibition of NF-AT activation, interleukin-2 production, and tyrosine phosphorylation. The structure-function relationship of Gab2 was analyzed by mutants of Gab2. The Gab2 mutants lacking SHP-2-binding sites mostly abrogated the inhibitory activity of Gab2, but its inhibitory function was restored by fusing to active SHP-2 as a chimeric protein. A mutant with defective phosphatidylinositol 3-kinase binding capacity also impaired the inhibitory activity, and the pleckstrin homology domain-deletion mutant revealed a crucial function of the pleckstrin homology domain for localization to the plasma membrane. These results suggest that Gab2 is a substrate of ZAP-70 and functions as a switch molecule toward inhibition of TCR signal transduction by mediating the recruitment of inhibitory molecules to the TCR signaling complex.

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

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

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

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

SH2 domain-mediated targeting, but not localization, of Syk in the plasma membrane is critical for FcepsilonRI signaling

Aggregation of the high-affinity IgE receptor induces the tyrosine phosphorylation of subunits of the receptor and the subsequent association with the receptor of the cytosolic protein tyrosine kinase Syk. The current experiments examined the functional importance of membrane association of Syk and the role of the SH2 domain in receptor-mediated signal transduction. Wild-type Syk and chimeric Syk molecules with the c-Src myristylation sequence at the amino-terminus were expressed in a Syk-negative mast cell line. Chimeric Syk with the myristylation sequence was membrane associated, and a small fraction was constitutively colocalized with FcepsilonRI, Lyn, and LAT (linker for T-cell activation) in the glycolipid-enriched microdomains or rafts. However, even under these conditions, the tyrosine phosphorylation of Syk and the downstream propagation of signals required FcepsilonRI aggregation. This chimeric Syk was less active than wild-type Syk in FcepsilonRI-mediated signal transduction. In contrast, a truncated membrane-associated form of Syk that lacked the SH2 domains was not tyrosine phosphorylated by receptor aggregation and failed to transduce intracellular signals. These findings suggest that SH2 domain-mediated membrane translocation of Syk is essential for the FcepsilonRI-mediated activation of Syk for downstream signaling events leading to histamine release. Furthermore, the localization of Syk in glycolipid-enriched microdomains by itself is not enough to generate or enhance signaling events.

A molecular framework for two-step T cell signaling: Lck Src homology 3 mutations discriminate distinctly regulated lipid raft reorganization events

Costimulation by CD28 or lipid-raft-associated CD48 potentiate TCR-induced signals, cytoskeletal reorganization, and IL-2 production. We and others have proposed that costimulators function to construct a raft-based platform(s) especially suited for TCR engagement and sustained and processive signal transduction. Here, we characterize TCR/CD48 and TCR/CD28 costimulation in T cells expressing Lck Src homology 3 (SH3) mutants. We demonstrate that Lck SH3 functions after initiation of TCR-induced tyrosine phosphorylation and concentration of transducers within rafts, to regulate the costimulation-dependent migration of rafts to the TCR contact site. Expression of kinase-active/SH3-impaired Lck mutants disrupts costimulation-dependent raft recruitment, sustained TCR protein tyrosine phosphorylation, and IL-2 production. However, TCR-induced apoptosis, shown only to require "partial" TCR signals, is unaffected by expression of kinase-active/SH3-impaired Lck mutants. Therefore, two distinctly regulated raft reorganization events are required for processive and sustained "complete" TCR signal transduction and T cell activation. Together with recent characterization of CD28 and CD48 costimulatory activities, these findings provide a molecular framework for two signal models of T cell activation.

Lck protein tyrosine kinase is a key regulator of T-cell activation and a target for signal intervention by Herpesvirus saimiri and other viral gene products

Protein tyrosine kinases (PTKs) are critically involved in signaling pathways that regulate cell growth, differentiation, activation, and transformation. It is not surprising, therefore, that viruses acquire effector molecules targeting these kinases to ensure their own replication and/or persistence. This review summarizes our current knowledge on Lck, a member of the Src family of PTK, and its viral interaction partners. Lck plays a key role in T lymphocyte activation and differentiation. It is associated with a variety of cell surface receptors and is critical for signal transduction from the T-cell antigen receptor (TCR). Consequently, Lck is targeted by regulatory proteins of T-lymphotropic viruses, especially by the Herpesvirus saimiri (HVS) tyrosine kinase interacting protein (Tip). This oncoprotein physically interacts with Lck in HVS transformed T cells and has an impact on its catalytic activity. However, while Tip inhibits Lck activity in stably expressing cell lines, opposite effects were observed in several in vitro systems. At least in part, this complex situation may be related to the bipartite nature of the interaction surface of the two proteins. Studies on the interrelationships between Lck and its viral partners contribute to the understanding of the mechanisms of T-cell growth regulation, in general, and of viral pathogenicity in particular. In addition, understanding the regulation of Lck activity by viral proteins may serve as a basis for the development of new drugs capable of modifying Lck activity in different pathological situations.

Constitutive activation of the Ras/MAP kinase pathway and enhanced TCR signaling by targeting the Shc adaptor to membrane rafts

The Shc adaptor is responsible for coupling receptor tyrosine kinases and tyrosine kinase-associated receptors to the Ras/MAP kinase pathway. Shc is believed to be regulated by a change in subcellular localization from the cytosol to the plasma membrane, where it recruits Grb-2/Sos complexes and hence permits juxtaposition of the guanine nucleotide exchange factor Sos to Ras, resulting in GDP/GTP exchange and Ras activation. Shc has been recently shown to inducibly colocalize in detergent-resistant membrane rafts together with the activated TCR and associated signaling molecules. To understand whether Shc localization in membrane rafts is sufficient to regulate Shc function, we constructed a Shc chimera containing the Ras membrane localization motif at the C-terminus. We show that membrane targeted Shc was constitutively localized in the plasma membrane of T-cells, and was mostly compartmentalized in lipid rafts. Membrane targeted Shc was phosphorylated on tyrosine residues and bound Grb-2/Sos in the absence of TCR engagement. Furthermore, expression of membrane targeted Shc resulted in constitutive downstream signaling, including Erk2 activation and enhancement of TCR dependent activation of the TCR responsive transcription factor NF-AT. Hence localization of Shc in membrane rafts is sufficient for Shc to acquire a signaling competent state. Interestingly, a membrane targeted Shc mutant lacking both Grb-2 binding sites was not only incapable of signaling in the absence of TCR triggering, but transdominantly inhibited endogenous Shc, supporting a non redundant role for Shc in the activation of the Ras/MAP kinase pathway in T-cells.

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

Translocation of the SOS and Vav GDP/GTP exchange factors proximal to Ras and Rac GTPases localized in the plasma membrane glycolipid-enriched microdomains is a pivotal step required for T cell antigen receptor-induced T cell activation. Here we demonstrate that the T cell antigen receptor zeta-chain-associated ZAP-70 kinase and T cell antigen receptor zeta-chain immunoreceptor tyrosine-based activation motifs are essential for the membrane recruitment of SOS and Vav. Plasma membrane targeting of SOS or Vav begins with the assembly of ZAP-70 with Grb-2 and SOS. The subsequent tyrosine phosphorylation of LAT (linker for activation of T cell) by ZAP-70 leads to a shift in equilibrium from the ZAP-70.Grb-2.SOS(Vav) complex to the (Vav)SOS.Grb-2.LAT complex. This shift results in the targeting of SOS and Vav into glycolipid-enriched microdomains and initiation of the Ras and Rac signaling cascades involved in T cell activation, proliferation, and cytokine production.

Adhesion to fibronectin promotes the activation of the p125(FAK)/Zap-70complex in human T cells

The beta1 integrins are a family of heterodimeric adhesion receptors involved in cell-to-cell contacts and cell-to-extracellular matrix interactions. Through their adhesive role, integrins participate in transduction of outside/inside signals and contribute to trigger a multitude of cellular events such as differentiation, cell activation, and motility. The fibronectin integrin receptors, alpha4beta1 and alpha5beta1, can function as costimulatory molecules in T-cell receptor (TCR)-dependent T-cell activation. In the current study the Jurkat T-cell line was used as a model system to investigate the TCR-independent role of cell adhesion to fibronectin in the activation of Zap-70, a central molecule in the signalling events in T cells. Upon adhesion to plastic immobilized fibronectin but not to bovine serum albumin (BSA) the phosphorylation of p125FAK, a protein kinase that localizes to focal adhesion sites, was induced. Moreover, clustering of fibronectin receptors led to the detection of a p125FAK/Zap-70 complex. Finally, while the complex between fak-B, another protein kinase localized to focal adhesion sites, and Zap-70 was detected in cells plated either on BSA or on fibronectin, the formation of the p125FAK/Zap-70 complex appeared specifically induced following fibronectin-mediated integrin clustering. These data suggest the existence of a high degree of specificity when the members of the beta1 integrin family mediate signalling pathways in T cells.

Age-related impairments in TCR/CD3 activation of ZAP-70 are associated with reduced tyrosine phosphorylations of zeta-chains and p59fyn/p56lck in human T cells

The expression and catalytic activity of the protein tyrosine kinase (PTK) ZAP-70 are needed for normal intracellular signaling through the T-cell receptor (TCR)/CD3 complex. However, the possible effect of aging on the catalytic activity of ZAP-70 in human peripheral blood T cells stimulated via the TCR/CD3 complex is unknown. The current studies show that T cells from a substantial proportion of elderly humans (12) exhibit significant reductions in the catalytic activity, but not expression of ZAP-70 when stimulated by ligation of the TCR/CD3 with cross-linked anti-CD3epsilon monoclonal antibody OKT3. In addition, the reduced catalytic activity of ZAP-70 in T cells from elderly subjects was not restored to the normal levels in response to ligation of CD4 receptors, suggesting defects in PTKs linked to both CD3 and CD4 receptors. Other experiments demonstrated that the age-related impairments of ZAP-70 activation in anti-CD3-stimulated T cells were accompanied by decreased tyrosine phosphorylations of zeta-chains and autophosphorylations of the PTKs p561ck/p59fyn. Moreover, the age-related defects in these early TCR/CD3-mediated phosphorylation events were readily detectable in both CD45RO+ memory and CD45RA+ naive T cells. Thus, these results suggest that defects in early TCR/CD3-mediated phosphorylation events among CD45RO+ memory and CD45RA+ naive T cells from certain elderly humans may con tribute to impaired induction of ZAP-70 catalytic activity.

Speculation on the lineage relationships among CD4(-)8(+) gut-derived T cells and their role(s)

The thymus-independent T lymphopoietic capacity of the murine intestinal mucosa has been established. Cryptopatches have now been identified as the location of the elusive precursors for gut-derived T cells. These cryptopatch cells have been shown to give rise to intestinal T cells expressing either TCRgammadelta or TCRalphabeta. Here we discuss the role of MHC in the development and selection of gut-derived T cells. Through the analysis of iIEL selection in animals expressing a transgenic TCRalphabeta, in the presence or absence of p56(lck), we discuss lineage relationships among CD4(-)8(+) iIEL subsets, and their possible function(s).

Interaction between CD45-AP and protein-tyrosine kinases involved in T cell receptor signaling

CD45-AP associates specifically with CD45, a protein-tyrosine phosphatase essential for antigen receptor-mediated signal transduction. CD45 modulates the activity of Src family protein-tyrosine kinases involved at the onset of antigen receptor-mediated signaling by dephosphorylating their regulatory tyrosyl residues. We have shown that lymphocyte responses to antigen receptor stimulation are impaired in CD45-AP-null mice. To examine the possibility that CD45-AP coordinates the interaction between CD45 and its substrates, we investigated the associations of CD45-AP with several protein-tyrosine kinases. Endogenous CD45-AP coimmunoprecipitated with Lck and ZAP-70 in both CD45-positive T cells and their CD45-negative variants after stimulation by antigen receptor ligation. Concomitantly, CD45 coimmunoprecipitated with Lck and ZAP-70 after T cell receptor-mediated stimulation of CD45-positive cells. Recombinant CD45-AP exhibited specific binding to Lck and ZAP-70 protein-tyrosine kinases, but not to Fyn or Csk, in lysates of both CD45-positive and -negative T cells. Specific interactions were demonstrated between the respective recombinant proteins as well. These results demonstrate that CD45-AP associates directly and selectively with Lck and ZAP-70 in response to T cell receptor-mediated stimulation. The associations of CD45-AP with Lck and ZAP-70 may mediate the functional interactions of these kinases with CD45 during antigen receptor stimulation.

Interleukin 2-mediated uncoupling of T cell receptor alpha/beta from CD3 signaling

T cell activation and clonal expansion is the result of the coordinated functions of the receptors for antigen and interleukin (IL)-2. The protein tyrosine kinase p56(lck) is critical for the generation of signals emanating from the T cell antigen receptor (TCR) and has also been demonstrated to play a role in IL-2 receptor signaling. We demonstrate that an IL-2-dependent, antigen-specific CD4(+) T cell clone is not responsive to anti-TCR induced growth when propagated in IL-2, but remains responsive to both antigen and CD3epsilon-specific monoclonal antibody. Survival of this IL-2-dependent clone in the absence of IL-2 was supported by overexpression of exogenous Bcl-xL. Culture of this clonal variant in the absence of IL-2 rendered it susceptible to anti-TCR-induced signaling, and correlated with the presence of kinase-active Lck associated with the plasma membrane. The same phenotype is observed in primary, resting CD4(+) T cells. Furthermore, the presence of kinase active Lck associated with the plasma membrane correlates with the presence of ZAP 70-pp21zeta complexes in both primary T cells and T cell clones in circumstances of responsive anti-TCR signaling. The results presented demonstrate that IL-2 signal transduction results in the functional uncoupling of the TCR complex through altering the subcellular distribution of kinase-active Lck.

Mutations in the Activation Loop Tyrosines of Protein Tyrosine Kinase Syk Abrogate Intracellular Signaling But Not Kinase Activity

The protein tyrosine kinase Syk plays a pivotal role in mediating the high-affinity IgE receptor (FcεRI)-induced degranulation of mast cells. To examine the mechanism of Syk regulation, the two tyrosine residues at 519 and 520 in the putative activation loop of rat Syk were mutated to phenylalanine either singly or in combination. The various mutants were expressed in a Syk-negative variant of the RBL-2H3 (rat basophilic leukemia 2H3) mast cell line. In these transfected cell lines, mutant Syk did show increased tyrosine phosphorylation in vivo and increased enzymatic activity in vitro after FcεRI aggregation. There were conformational changes detected by an Ab when the wild-type and mutant Syk were either tyrosine phosphorylated or bound to tyrosine-phosphorylated immunoreceptor tyrosine-based activation motif peptides. However, these mutant Syk were incapable of transducing FcεRI signaling. In cells in which the expression level of mutant Syk was similar to that of the wild-type Syk, FcεRI cross-linking induced no increase in cellular protein tyrosine phosphorylation, no increase in tyrosine phosphorylation of phospholipase C-γ2 and mitogen-activated protein kinase, and no histamine release. Overexpression of Y519F or Y520F Syk mutants partially reconstituted the signaling pathways. These results indicate that these tyrosines in the putative activation loop are not essential for the enzymatic activity of Syk or for the conformational changes induced by binding of tyrosine-phosphorylated immunoreceptor tyrosine-based activation motif peptides. However, these tyrosines are necessary for Syk-mediated propagation of FcεRI signaling.

Phosphorylation and coupling of zeta-chains to activated T-cell receptor (TCR)/CD3 complexes from peripheral blood T-cells of elderly humans

Aging is often accompanied by altered T-cell signaling and functions. Signals mediated through the T-cell receptor (TCR)/CD3 complex are associated with tyrosine phosphorylations of zeta-chains by the regulated activities of protein tyrosine kinases p56(lck) and p59(fyn) as well as protein tyrosine phosphatases. In the present investigation, the coupling and phosphorylation of zeta-chains to TCR/CD3 immunocomplexes were examined in peripheral blood T-cells from 13 elderly and young humans stimulated by ligation of the TCR/CD3 with cross-linked anti-CD3epsilon monoclonal antibody OKT3. Western blots analyzing the non-covalent coupling of zeta-chains to TCR/CD3 immunocomplexes from Brij-96 detergent lysates of anti-CD3 ligated T-cells showed that the levels of zeta-chains within TCR/CD3 immunocomplexes from T-cells of elderly and young subjects did not significantly differ. By contrast, the levels of phosphorylated zeta-chains generated during in vitro phosphorylations of TCR/CD3 immunocomplexes from elderly subjects were significantly reduced and averaged 44% of those observed for anti-CD3epsilon ligated T-cells from young subjects. Analyses of the levels of zeta-chain coupling and phosphorylations in T-cells from each of the 13 elderly individuals also showed that the reductions in zeta-chain phosphorylations were heterogeneous and unrelated to modest reductions in coupling. Furthermore, the age-related decreases in zeta-chain phosphorylations were not due to diminished frequencies of CD3epsilon+ cells or densities of CD3epsilon surface receptors and could be observed without reductions in epsilon-chain phosphorylations. These results suggest that aberrancies of zeta-chain phosphorylations can occur in T-cells of elderly humans independent from any uncoupling of zeta-chains to activated TCR/CD3 complexes.

Tyrosine 474 of ZAP-70 is required for association with the Shc adaptor and for T-cell antigen receptor-dependent gene activation

The protein tyrosine kinase ZAP-70 plays a central role in T-cell activation. Following receptor engagement, ZAP-70 is recruited to the phosphorylated subunits of the T-cell antigen receptor (TCR). This event results in ZAP-70 activation and in association of ZAP-70 with a number of signaling proteins. Among these is the Shc adaptor, which couples the activated TCR to Ras. Shc interaction with ZAP-70 is mediated by the Shc PTB domain. The inhibitory effect of a Shc mutant containing the isolated PTB domain suggests that Shc interaction with ZAP-70 might be required for TCR signaling. Here, we show that a point mutation (Phe474) of the putative Shc binding site on ZAP-70, spanning tyrosine 474, prevented ZAP-70 interaction with Shc and the subsequent binding of Shc to phospho-zeta. Neither ZAP-70 catalytic activity nor the pattern of protein phosphorylation induced by TCR triggering was affected by this mutation. However expression of the Phe474 ZAP-70 mutant resulted in impaired TCR-dependent gene activation. ZAP-70 could effectively phosphorylate Shc in vitro. Only the CH domain, which contains the two Grb2 binding sites on Shc, was phosphorylated by ZAP-70. Both Grb2 binding sites were excellent substrates for ZAP-70. The data show that Tyr474 on ZAP-70 is required for TCR signaling and suggest that Shc association with ZAP-70 and the resulting phosphorylation of Shc might be an obligatory step in linking the activated TCR to the Ras pathway.

Genetic evidence of a role for Lck in T-cell receptor function independent or downstream of ZAP-70/Syk protein tyrosine kinases

T-cell antigen receptor (TCR) engagement results in sequential activation of the Src protein tyrosine kinases (PTKs) Lck and Fyn and the Syk PTKs, ZAP-70 and Syk. While the Src PTKs mediate the phosphorylation of TCR-associated signaling subunits and the phosphorylation and activation of the Syk PTKs, the lack of a constitutively active Syk PTK has prohibited the analysis of Lck function downstream of these initiating signaling events. We describe here the generation of an activated Syk family PTK by substituting the kinase domain of Syk for the homologous region in ZAP-70 (designated as KS for kinase swap). Expression of the KS chimera resulted in its autophosphorylation, the phosphorylation of cellular proteins, the upregulation of T-cell activation markers, and the induction of interleukin-2 gene synthesis in a TCR-independent fashion. The KS chimera and downstream ZAP-70 or Syk substrates, such as SLP-76, were still phosphorylated when expressed in Lck-deficient JCaM1.6 T cells. However, expression of the KS chimera in JCaM1.6 cells failed to rescue downstream signaling events, demonstrating a functional role for Lck beyond the activation of the ZAP-70 and Syk PTKs. These results indicate that downstream TCR signaling pathways may be differentially regulated by ZAP-70 and Lck PTKs and provide a mechanism by which effector functions may be selectively activated in response to TCR stimulation.

T cell receptor (TCR) engagement in apoptosis-defective, but interleukin 2 (IL-2)-producing, T cells results in impaired ZAP70/CD3-zeta association

We have previously shown that a tyrosine to leucine replacement in the transmembrane region of T cell receptor (TCR)-beta results in a deficient induction of CD95-L and apoptosis upon TCR triggering in a transfected T cell line. By contrast, interleukin (IL)-2 production and the expression of CD25 and CD69 were normally induced. Since the mutation in TCR-beta also resulted in impaired association of CD3-zeta, it was proposed that this chain is specifically required for the induction of apoptosis. We now show that the deficient induction of CD95-L and apoptosis does not derive from a general lower production of second messengers, since intracellular Ca2+ fluxes and tyrosine phosphorylation of total proteins were elicited at wild-type levels. Unlike in T cell clones stimulated with partial agonists, both p21 and p18 forms of tyrosine-phosphorylated CD3-zeta were detected, although the overall level of tyrosine-phosphorylated CD3-zeta was low. More strikingly, inducible association of ZAP70 to CD3-zeta was strongly inhibited, despite a normal induction of ZAP70 tyrosine phosphorylation. Finally, ZAP70 was not concentrated near the plasma membrane in the apoptosis-deficient cells. These results suggest that CD3-zeta is necessary for engagement of a specific signaling pathway leading to CD95-L expression that also needs the recruitment of ZAP70.

Tyrosine kinase chimeras for antigen-selective T-body therapy

Protein tyrosine kinases (PTKs) transmit activation signals in almost every cell type, including immune effector cells. The aberrant or constitutive activation of PTKs can often cause neoplastic transformation. The use of chimeric receptors based on PTKs may enable us to elucidate the signaling pathways of normal immune cells and other cell types, and the abnormal events that can lead to malignant transformation. In this review, we focus on antigen specific chimeric PTKs in which antibody-derived scFv are joined to the Syk family of PTKs. These chimeric receptors yielded reagents that can selectively redirect immune effector cells and specifically activate them to produce cytokines or lyse their target. The advantages of using such PTK-based chimeras to redirect lymphocytes to tumor targets and their potential as an immunotherapeutic approach to malignant disease is discussed.

ZAP-70 protein tyrosine kinase is constitutively targeted to the T cell cortex independently of its SH2 domains

ZAP-70 is a nonreceptor protein tyrosine kinase that is essential for signaling via the T cell antigen receptor (TCR). ZAP-70 becomes phosphorylated and activated by LCK protein tyrosine kinase after interaction of its two NH2-terminal SH2 domains with tyrosine-phosphorylated subunits of the activated TCR. In this study, the localization of ZAP-70 was investigated by immunofluorescence and confocal microscopy. ZAP-70 was found to be localized to the cell cortex in a diffuse band under the plasma membrane in unstimulated T cells, and this localization was not detectably altered by TCR stimulation. Analysis of mutants indicated that ZAP-70 targeting was independent of its SH2 domains but required its active kinase domain. The specific compartmentalization of ZAP-70 suggests that it may interact with an anchoring protein in the cell cortex via its hinge or kinase domains. It is likely that the maintenance of high concentrations of ZAP-70 at the cell cortex, that only has to move a short distance to interact with phophorylated TCR subunits, facilitates rapid initiation of signaling by the TCR. In addition, as the major increase in tyrosine phosphorylation induced by the TCR also occurs at the cell cortex (Ley, S.C., M. Marsh, C.R. Bebbington, K. Proudfoot, and P. Jordan. 1994. J. Cell. Biol. 125:639-649), ZAP-70 may be localized close to its downstream targets.

Lck-independent triggering of T-cell antigen receptor signal transduction by staphylococcal enterotoxins

Superantigens (SAgs) activate T-cells in a manner specific to the Vbeta region of the T-cell antigen receptor. Stimulations by SAgs provoke drastic T-cell activation that leads to programmed cell death or the anergic state of responding cells. To characterize the signal transduction pathway initiated by SAgs, mutant lines derived from the human leukemic T-cell line Jurkat were tested for their reactivities against prototypic SAgs, staphylococcal enterotoxins. The J.CaM1.6 cell line, which lacks Lck expression and lost reactivity against T-cell antigen receptor-mediated stimulation, was activated by staphylococcal enterotoxins in a manner indistinguishable from the Jurkat cell line. In contrast, the J.45. 01 cell line, which lacks expression of functional CD45, showed severely impaired reactivity. The role of Lck appears to be replaced by another Src family protein-tyrosine kinase, Fyn. In J.CaM1.6 cells, Fyn was rapidly phosphorylated and activated after staphylococcal enterotoxin treatment. The kinase-inactive mutant of Fyn significantly suppressed the reactivity against staphylococcal enterotoxin E in J.CaM1.6 cells, and the expression of the active form of Fyn reconstituted reactivity against staphylococcal enterotoxin E in J.45.01 cells. These results demonstrate that SAgs activate T-cells in an Lck-independent pathway and that Fyn plays a critical role in the process.

Molecular mechanisms in B cell antigen receptor signaling

Recent gene-targeting experiments have highlighted the importance of the intracellular protein tyrosine kinases Lyn, Syk, and Btk in BCR signal transduction and B cell development. In addition, the interactions of these kinases and their regulatory mechanisms have been reported. Activation loop phosphorylation of these kinases is critical for their participation in signal propagation. Several substrates have been identified for these kinases and this has led to elucidation of the mechanisms by which these kinases mediate the downstream signaling events that lead to cellular responses of B cells.