Regulation of ZAP-70 intracellular localization: visualization with the green fluorescent protein

Novel Compound Heterozygous ZAP70 R37G A507T Mutations in Infant with Severe Immunodeficiency

Zeta-chain associated protein kinase 70 kDa (ZAP70) combined immunodeficiency (CID) is an autosomal recessive severe immunodeficiency that is characterized by abnormal T-cell receptor signaling. Children with the disorder typically present during the first year of life with diarrhea, failure to thrive, and recurrent bacterial, viral, or opportunistic infections. To date, the only potential cure is hematopoietic stem cell transplant (HSCT). The majority of described mutations causing disease occur in the homozygous state, though heterozygotes are reported without a clear understanding as to how the individual mutations interact to cause disease. This case describes an infant with novel ZAP-70 deficiency mutations involving the SH2 and kinase domains cured with allogeneic HSCT utilizing a reduced-intensity conditioning regimen and graft manipulation. We then were able to further elucidate the molecular signaling alterations imparted by these mutations that lead to altered immune function. In order to examine the effect of these novel compound ZAP70 heterozygous mutations on T cells, Jurkat CD4+ T cells were transfected with either wild type, or with individual ZAP70 R37G and A507T mutant constructs. Downstream TCR signaling events and protein localization results link these novel mutations to the expected immunological outcome as seen in the patient's primary cells. This study further characterizes mutations in the ZAP70 gene as combined immunodeficiency and the clinical phenotype.

Regulating the discriminatory response to antigen by T-cell receptor

The cell-mediated immune response constitutes a robust host defense mechanism to eliminate pathogens and oncogenic cells. T cells play a central role in such a defense mechanism and creating memories to prevent any potential infection. T cell recognizes foreign antigen by its surface receptors when presented through antigen-presenting cells (APCs) and calibrates its cellular response by a network of intracellular signaling events. Activation of T-cell receptor (TCR) leads to changes in gene expression and metabolic networks regulating cell development, proliferation, and migration. TCR does not possess any catalytic activity, and the signaling initiates with the colocalization of several enzymes and scaffold proteins. Deregulation of T cell signaling is often linked to autoimmune disorders like severe combined immunodeficiency (SCID), rheumatoid arthritis, and multiple sclerosis. The TCR remarkably distinguishes the minor difference between self and non-self antigen through a kinetic proofreading mechanism. The output of TCR signaling is determined by the half-life of the receptor antigen complex and the time taken to recruit and activate the downstream enzymes. A longer half-life of a non-self antigen receptor complex could initiate downstream signaling by activating associated enzymes. Whereas, the short-lived, self-peptide receptor complex disassembles before the downstream enzymes are activated. Activation of TCR rewires the cellular metabolic response to aerobic glycolysis from oxidative phosphorylation. How does the early event in the TCR signaling cross-talk with the cellular metabolism is an open question. In this review, we have discussed the recent developments in understanding the regulation of TCR signaling, and then we reviewed the emerging role of metabolism in regulating T cell function.

T and NK cell lymphoma cell lines do not rely on ZAP-70 for survival

B-cell receptor (BCR) signalling is critical for the survival of B-cell lymphomas and is a therapeutic target of drugs such as Ibrutinib. However, the role of T-cell receptor (TCR) signalling in the survival of T/Natural Killer (NK) lymphomas is not clear. ZAP-70 (zeta associated protein-70) is a cytoplasmic tyrosine kinase with a critical role in T-cell receptor (TCR) signalling. It has also been shown to play a role in normal NK cell signalling and activation. High ZAP-70 expression has been detected by immunohistochemistry in peripheral T cell lymphoma (PTCL) and NK cell lymphomas (NKTCL). We therefore, studied the role of TCR pathways in mediating the proliferation and survival of these malignancies through ZAP-70 signalling. ZAP-70 protein was highly expressed in T cell lymphoma cell lines (JURKAT and KARPAS-299) and NKTCL cell lines (KHYG-1, HANK-1, NK-YS, SNK-1 and SNK-6), but not in multiple B-cell lymphoma cell lines. siRNA depletion of ZAP-70 suppressed the phosphorylation of ZAP-70 substrates, SLP76, LAT and p38MAPK, but did not affect cell viability or induce apoptosis in these cell lines. Similarly, while stable overexpression of ZAP-70 mediates increased phosphorylation of target substrates in the TCR pathway, it does not promote increased survival or growth of NKTCL cell lines. The epidermal growth factor receptor (EGFR) inhibitor Gefitinib, which has off-target activity against ZAP-70, also did not show any differential cell kill between ZAP-70 overexpressing (OE) or knockdown (KD) cell lines. Whole transcriptome RNA sequencing highlighted that there was very minimal differential gene expression in three different T/NK cell lines induced by ZAP-70 KD. Importantly, ZAP-70 KD did not significantly enrich for any downstream TCR related genes and pathways. Altogether, this suggests that high expression and constitutive signalling of ZAP-70 in T/NK lymphoma is not critical for cell survival or downstream TCR-mediated signalling and gene expression. ZAP-70 therefore may not be a suitable therapeutic target in T/NK cell malignancies.

Microglia prevent peripheral immune cell invasion and promote an anti-inflammatory environment in the brain of APP-PS1 transgenic mice

Background

Undoubtedly, neuroinflammation is a major contributor to Alzheimer’s disease (AD) progression. Neuroinflammation is characterized by the activity of brain resident glial cells, in particular microglia, but also by peripheral immune cells, which infiltrate the brain at certain stages of disease progression. The specific role of microglia in shaping AD pathology is still controversially discussed. Moreover, a possible role of microglia in the interaction and recruitment of peripheral immune cells has so far been completely ignored.

Methods

We ablated microglia cells in 12-month-old WT and APP-PS1 transgenic mice for 4 weeks using the CSF1R inhibitor PLX5622 and analyzed its consequences to AD pathology and in particular to peripheral immune cell infiltration.

Results

PLX5622 treatment successfully reduced microglia numbers. Interestingly, it uncovered a treatment-resistant macrophage population (Iba1⁺/TMEM119⁻). These cells strongly expressed the phagocytosis marker CD68 and the lymphocyte activation, homing, and adhesion molecule CD44, specifically at sites of amyloid-beta plaques in the brains of APP-PS1 mice. In consequence, ablation of microglia significantly raised the number of CD3⁺/CD8⁺ T-cells and reduced the expression of anti-inflammatory genes in the brains of APP-PS1 mice.

Conclusion

We conclude that in neurodegenerative conditions, chronically activated microglia might limit CD3⁺/CD8⁺ T-cell recruitment to the brain and that local macrophages connect innate with adaptive immune responses. Investigating the role of peripheral immune cells, their interaction with microglia, and understanding the link between innate and adaptive immune responses in the brain might be a future directive in treating AD pathology.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1304-4) contains supplementary material, which is available to authorized users.

Imaging Spatiotemporal Activities of ZAP-70 in Live T Cells Using a FRET-Based Biosensor

The zeta-chain-associated protein kinase 70 kDa (ZAP-70), a member of the spleen tyrosine kinase (Syk) family, plays an essential role in early T cell receptor (TCR) signaling. Defects in ZAP-70 lead to impaired thymocyte development and peripheral T cell activation. To better understand its activation dynamics and regulation, we visualized ZAP-70 activities in single live T cells with a Förster resonance energy transfer (FRET)-based biosensor, which was designed for probing kinase activities of the Syk family. We observed in Jurkat E6.1 T cells rapid and specific FRET changes following anti-CD3 stimulation and subsequent piceatannol inhibition. The initiation of ZAP-70 activation was prompt (within 10 s) and correlates with the accompanied intracellular calcium elevation, as revealed by simultaneous imaging of the biosensor and calcium. Different from the previously reported ZAP-70 activation in the immunological synapse and the opposite pole (anti-synapse), we have observed rapid and sustained ZAP-70 activation only at the synapse with superantigen-pulsed Raji B cells. Furthermore, ZAP-70 signaling was impaired by cholesterol depletion, further supporting the importance of membrane organization in TCR signaling. Together our results provide a direct characterization of the spatiotemporal features of ZAP-70 activity in real time at subcellular levels.

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

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

Quantitative detection of zeta-chain-associated protein 70 expression in chronic lymphocytic leukemia

Overexpression of zeta-chain-associated protein 70 (ZAP-70) was recently recognized as an independent prognostic marker for the aggressive form of chronic lymphocytic leukemia (CLL). The objective of this study was to demonstrate the feasibility and implementation of quantitative detection of ZAP-70 protein in B cells to clearly distinguish patients with CLL with the aggressive form of the disease. B cells were isolated from patient blood and lysed. Released ZAP-70 protein was detected using an immunomagnetic fluorescence assay. The assay protocol was developed using Jurkat cells and recombinant ZAP-70 (rZAP-70). The limit of detection was determined to be lower than 125 Jurkat cells and 39 pg of rZAP-70 protein. The signal response was linear over a wide dynamic range, from 125 to 40 000 Jurkat cells per test (R(2) = 0.9987) and from 0 to 40 000 pg rZAP-70 protein per test (R(2) = 0.9928). The results from 20 patients with CLL correlated strongly with flow cytometry analysis. Concordance between the two methods for positive and negative results was 100% (7/7) and 92% (12/13), respectively, while the overall concordance between the two methods was 95%. The assay reported here is a simple, reliable and reproducible method for quantitative detection of ZAP-70 in patient leukemic cells, without the need for cell fixation or permeabilization. The ZAP-70 signal was linear over a wide dynamic range, which we believe enables quantitative assessment of small changes in ZAP-70 expression over the course of the disease and in response to therapeutic intervention.

Regulation of Syk by phosphorylation on serine in the linker insert

The Syk protein-tyrosine kinase is phosphorylated on multiple tyrosines after the aggregation of the B cell antigen receptor. However, metabolic labeling experiments indicate that Syk is inducibly phosphorylated to an even greater extent on serine after receptor ligation. A combination of phosphopeptide mapping and mass spectrometric analyses indicates that serine 291 is a major site of phosphorylation. Serine 291 lies within a 23-amino acid insert located within the linker B region that distinguishes Syk from SykB and Zap-70. The phosphorylation of serine-291 by protein kinase C enhances the ability of Syk to couple the antigen receptor to the activation of the transcription factors NFAT and Elk-1. Protein interaction studies indicate a role for the phosphorylated linker insert in promoting an interaction between Syk and the chaperone protein, prohibitin.

Spatiotemporal basis of CTLA-4 costimulatory molecule-mediated negative regulation of T cell activation

T cell activation is positively and negatively regulated by a pair of costimulatory receptors, CD28 and CTLA-4, respectively. Because these receptors share common ligands, CD80 and CD86, the expression and behavior of CTLA-4 is critical for T cell costimulation regulation. However, in vivo blocking of CD28-mediated costimulation by CTLA-4 and its mechanisms still remain elusive. Here, we demonstrate the dynamic behavior of CTLA-4 in its real-time competition with CD28 at the central-supramolecular activation cluster (cSMAC), resulting in the dislocalization of protein kinase C-θ and CARMA1 scaffolding protein. CTLA-4 translocation to the T cell receptor microclusters and the cSMAC is tightly regulated by its ectodomain size, and its accumulation at the cSMAC is required for its inhibitory function. The CTLA-4-mediated suppression was demonstrated by the in vitro anergy induction in regulatory T cells constitutively expressing CTLA-4. These results show the dynamic mechanism of CTLA-4-mediated T cell suppression at the cSMAC.

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

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

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.

Tyr130 phosphorylation triggers Syk release from antigen receptor by long-distance conformational uncoupling

The Syk protein-tyrosine kinase plays a major role in signaling through the B cell receptor for antigen (BCR). Syk binds the receptor via its tandem pair of SH2 domains interacting with a doubly phosphorylated immunoreceptor tyrosine-based activation motif (dp-ITAM) of the BCR complex. Upon phosphorylation of Tyr-130, which lies between the two SH2 domains distant to the phosphotyrosine binding sites, Syk dissociates from the receptor. To understand the structural basis for this dissociation, we investigated the structural and dynamic characteristics of the wild type tandem SH2 region (tSH2) and a variant tandem SH2 region (tSH2(pm)) with Tyr-130 substituted by Glu to permanently introduce a negative charge at this position. NMR heteronuclear relaxation experiments, residual dipolar coupling measurements and analytical ultracentrifugation revealed substantial differences in the hydrodynamic behavior of tSH2 and tSH2(pm). Although the two SH2 domains in tSH2 are tightly associated, the two domains in tSH2(pm) are partly uncoupled and tumble in solution with a faster correlation time. In addition, the equilibrium dissociation constant for the binding of tSH2(pm) to dp-ITAM (1.8 microM) is significantly higher than that for the interaction between dp-ITAM and tSH2 but is close to that for a singly tyrosine-phosphorylated peptide binding to a single SH2 domain. Experimental data and hydrodynamic calculations both suggest a loss of domain-domain contacts and change in relative orientation upon the introduction of a negative charge on residue 130. A long-distance structural mechanism by which the phosphorylation of Y130 negatively regulates the interaction of Syk with immune receptors is proposed.

Efficient gene transfer in CLL by mRNA electroporation

Chronic lymphocytic leukemia (CLL) consists of at least two major prognostic subgroups, characterized by different cellular and molecular markers. This observation sparked studies on the function and clinical importance of these markers. In order to address their function adequately, an efficient and reliable method for gene transfer is needed. In this study, we compared efficiency and utility of different gene transfer techniques in CLL. Lenti-, retro- and adenoviral transduction did not yield appreciable numbers of marker gene enhanced green fluorescent protein (EGFP) positive CLL cells, despite various prestimulation protocols. Efficient transgene expression was observed after nucleofection of CLL cells with plasmid DNA, at the expense of low survival rates. After optimization, electroporation of in vitro transcribed mRNA yielded up to 90% EGFP+CLL cells without affecting survival. Transgene expression remained detectable for at least 2 weeks after electroporation. Furthermore, we could demonstrate overexpression of ZAP70 and of a ZAP70-EGFP fusion protein after electroporation with ZAP70 or ZAP70-EGFP mRNA. We conclude that mRNA electroporation is a novel and straightforward method for highly efficient gene transfer in CLL. The application of this technique should facilitate functional studies on CLL cells, as well as clinical research.

Nucleocytoplasmic trafficking of the Syk protein tyrosine kinase

The protein tyrosine kinase Syk couples the B-cell receptor (BCR) for antigen to multiple intracellular signaling pathways and also modulates cellular responses to inducers of oxidative stress in a receptor-independent fashion. In B cells, Syk is found in both the nuclear and cytoplasmic compartments but contains no recognizable nuclear localization or export signals. Through the analysis of a series of deletion mutants, we identified the presence of an unconventional shuttling sequence near the junction of the catalytic domain and the linker B region that accounts for Syk's subcellular localization. This localization is altered following prolonged engagement of the BCR, which causes Syk to be excluded from the nucleus. Nuclear exclusion requires the receptor-mediated activation of protein kinase C and new protein synthesis. Both of these processes also potentiate the activation of caspase 3 in cells in response to oxidative stress in a manner that is dependent on the localization of Syk outside of the nucleus. In contrast, restriction of Syk to the nucleus greatly diminishes the stress-induced activation of caspase 3.

Dexamethasone induces rapid tyrosine-phosphorylation of ZAP-70 in Jurkat cells

Steroid hormones are known to mediate rapid non-genomic effects occurring within minutes, besides the classical genomic actions mediated by the nuclear translocation of the cytoplasmic glucocorticoid receptor (GR). The glucocorticoid hormone (GC) has significant role in the regulation of T-cell activation; however, the cross-talk between the GC and T-cell receptor (TcR) signal transducing pathways are still to be elucidated. We examined the rapid effects of GC exposure on in vitro cultured human T-cells. Our results showed that Dexamethasone (DX), a GC analogue, when applied at high dose (10 microM), induced rapid (within 5 min) tyrosine-phosphorylation events in Jurkat cells. Short DX pre-treatment strongly inhibited the tyrosine-phosphorylation stimulated by CD3 cross-linking. Furthermore, we also investigated the phosphorylation status of ZAP-70, an important member of tyrosine kinase mediated signalling pathway of TcR-elicited T-cell activation. Here, we demonstrate that high dose DX induced a rapid ZAP-70 tyrosine-phosphorylation in Jurkat T-cells. DX-induced ZAP-70 phosphorylation could be inhibited by RU486 (GR antagonist), suggesting that this process was GR mediated. DX-induced ZAP-70 phosphorylation did not occur in the absence of active p56-lck as examined in the p56-lck kinase-deficient Jurkat cell line JCaM1.6. Our results show that DX, at a high dose, can rapidly influence the initial tyrosine-phosphorylation events of the CD3 signalling pathway in Jurkat cells, thereby modifying TcR-derived signals. Lck and ZAP-70 represent an important molecular link between the TcR and GC signalling pathways.

Evidence for the Involvement of Tyrosine Kinase ZAP 70 in Nuclear Retinoid Receptor-dependent Transactivation in T Lymphocytes

Retinoic acid receptors (RARs) and retinoid X receptors (RXRs) are transcription factors that control diverse cellular functions during development and homeostasis. The biochemical role of these proteins in T lymphocytes is not well known. Here we have studied the role of protein-tyrosine kinase ZAP 70, a key enzyme involved in the proximal signaling events during T cell activation, in the modulation of RXRE- and RARE-dependent activation in T lymphocytes. Surprisingly, ZAP 70-negative Jurkat T cells showed considerable loss of both RXRE- and RARE-mediated transactivation as compared with wild type Jurkat cells. In addition, ZAP 70-negative cells failed to exhibit normal protein kinase C and calcineurin-induced transcriptional activity. ZAP 70-negative cells that were reconstituted with active ZAP 70 regained the transactivation function, whereas cells expressing kinase-dead form of ZAP 70 failed to do so. Defective transcriptional activation was also observed in actively proliferating human peripheral blood T lymphocytes in which RNA interference was used to induce loss of ZAP 70 expression. In addition, an Lck-deficient Jurkat cell line that cannot efficiently activate ZAP 70 was also found defective in RXRE-mediated transcription. Finally, RNA interference-induced loss of ZAP 70 or Lck protein in Jurkat cells resulted in significant decrease in the RXRE-dependent activation. Together, these results suggest a novel functional role for ZAP 70 in nuclear receptor-driven transactivation in T lymphocytes.

Distinct spatial and temporal distribution of ZAP70 and Lck following stimulation of interferon and T-cell receptors

T-cell receptor (TCR) stimulation results in the recruitment and activation of the proteins ZAP70 and Lck. These two proteins have been implicated in signalling derived from interferon receptors, although their precise role in this independent pathway has not been determined fully. These observations raise a fundamental question of how a given protein in a cell can be involved in more than one signalling pathway, yet each pathway is able to produce a highly specific downstream response to its own stimulant. To maintain exclusivity of response, each pathway must isolate its component molecules chemically, spatially or dynamically from other prevailing pathways. To address this question, the proteins ZAP70 and Lck were investigated following stimulation of the interferon-alpha receptor and the TCR in T cells by two different extracellular stimulants: interferon-alpha and the anti-CD3 antibody, OKT3, respectively. We first demonstrate that ZAP70 plays a pivotal role in interferon-stimulated MAPK activation, and that the tyrosine residue at position 319 of ZAP70 is important for interferon-stimulated ERK activation. Translocation of both ZAP70 and Lck to the nucleus following interferon receptor stimulation is demonstrated for the first time. Fluorescence resonance energy transfer microscopy revealed a high degree of spatial localization of the ZAP70/Lck complex within the cell following IFNalpha stimulation, in contrast to a diffuse presence following the application of OKT3. The difference in the spatio-temporal localization of these proteins following stimulation may eliminate signal crosstalk, and could explain the differentiation of the specific downstream responses of these pathways.

Peptide microarrays for the detection of molecular interactions in cellular signal transduction

The formation of protein complexes is a hallmark of cellular signal transduction. Here, we show that peptide microarrays provide a robust and quantitative means to detect signalling-dependent changes of molecular interactions. Recruitment of a protein into a complex upon stimulation of a cell leads to the masking of an otherwise exposed binding site. In cell lysates this masking can be detected by reduced binding to a microarray carrying a peptide that corresponds to the binding motif of the respective interaction domain. The method is exemplified for the lymphocyte-specific tyrosine kinase 70 kDa zeta-associated protein binding to a bis-phosphotyrosine-motif of the activated T-cell receptor via its tandem SH2 domain. Compared to established techniques, the method provides a significant shortcut to the detection of molecular interactions.

Hydrogen peroxide triggers nuclear export of telomerase reverse transcriptase via Src kinase family-dependent phosphorylation of tyrosine 707

The regulation of telomerase reverse transcriptase (TERT) plays an important role in the proliferative capacity and survival of cells. Here, we report that exogenously as well as endogenously induced oxidative stress leads to translocation of endogenous as well as overexpressed human TERT from the nucleus into the cytosol. TERT is transported through the nuclear pores in a leptomycin-sensitive and Ran GTPase-dependent process. H(2)O(2)-induced nuclear export of TERT is preceded by TERT tyrosine phosphorylation at position 707 and prevented by the Src kinase family inhibitor PP1. Oxidative stress-induced nuclear export of TERT depends on association with the Ran GTPase. In contrast, mutation of tyrosine 707 inhibits phosphorylation induced by oxidative stress and prevents association with Ran and nuclear export of TERT. Moreover, inhibition of tyrosine phosphorylation at 707 increases the antiapoptotic capacity of TERT. Taken together, depletion of nuclear TERT by tyrosine phosphorylation-dependent nuclear export of TERT is a novel mechanism for regulation of TERT localization, which reduces the antiapoptotic activity of TERT.

Nuclear localization of the tyrosine kinase Itk and interaction of its SH3 domain with karyopherin alpha (Rch1alpha)

We report a physical and functional association between the Tec-family tyrosine kinase Itk (Emt/Tsk) and the nuclear import chaperone karyopherin alpha (Rch1alpha) in human T cells. The Itk-SH3 domain and the Rch1alpha proline-rich (PR) motif were crucial for the Itk/Rch1alpha constitutive interaction as demonstrated by directed mutagenesis of the Rch1alpha PR motif (proline 242 to alanine, P242A). TCR-CD3 stimulation of Jurkat T cells resulted in increased Itk/Rch1alpha complex formation, recruitment of karyopherin beta to the protein complex and Rch1alpha tyrosine phosphorylation. Analysis of in vitro kinase reactions with a panel of recombinant glutathione-S-transferase (GST) fusion tyrosine kinases (Itk, Lck, ZAP-70 and Jak3) revealed that only GST-Itk efficiently phosphorylated a recombinant GST-Rch1alpha fusion. We observed constitutive nuclear localization of Itk that was up-regulated following either TCR-CD3 stimulation or over-expression of wild-type Rch1alpha in T cells. Further, nuclear localization of Itk and TCR-CD3-mediated IL-2 production were significantly down-regulated following expression of the Rch1alpha-P242A mutant, implicating a role for Rch1alpha in the nuclear translocation of Itk.

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.

Visualization of Syk-antigen receptor interactions using green fluorescent protein: differential roles for Syk and Lyn in the regulation of receptor capping and internalization

The cross-linking of the B cell Ag receptor (BCR) is coupled to the stimulation of multiple intracellular signal transduction cascades via receptor-associated, protein tyrosine kinases of both the Src and Syk families. To monitor changes in the subcellular distribution of Syk in B cells responding to BCR cross-linking, we expressed in Syk-deficient DT40 B cells a fusion protein consisting of Syk coupled to green fluorescent protein. Treatment of these cells with anti-IgM Abs leads to the recruitment of the kinase from cytoplasmic and nuclear compartments to the site of the cross-linked receptor at the plasma membrane. The Syk-receptor complexes aggregate into membrane patches that redistribute to form a cap at one pole of the cell. Syk is not demonstrably associated with the internalized receptor. Catalytically active Syk promotes and stabilizes the formation of tightly capped BCR complexes at the plasma membrane. Lyn is not required for the recruitment of Syk to the cross-linked receptor, but is required for the internalization of the clustered BCR complexes. In the absence of Lyn, receptor-Syk complexes at the plasma membrane are long lived, and the receptor-mediated activation of the NF-AT transcription factor is enhanced. Thus, Lyn appears to function to negatively regulate aspects of BCR-dependent signaling by stimulating receptor internalization and down-regulation.

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.

Non-CD28 costimulatory molecules present in T cell rafts induce T cell costimulation by enhancing the association of TCR with rafts

While CD28 functions as the major T cell costimulatory receptor, a number of other T cell molecules have also been described to induce T cell costimulation. Here, we investigated the mechanisms by which costimulatory molecules other than CD28 contribute to T cell activation. Non-CD28 costimulatory molecules such as CD5, CD9, CD2, and CD44 were present in the detergent-insoluble glycolipid-enriched (DIG) fraction/raft of the T cell surface, which is rich in TCR signaling molecules and generates a TCR signal upon recruitment of the TCR complex. Compared with CD3 ligation, coligation of CD3 and CD5 as an example of DIG-resident costimulatory molecules led to an enhanced association of CD3 and DIG. Such a DIG redistribution markedly up-regulated TCR signaling as observed by ZAP-70/LAT activation and Ca2+ influx. Disruption of DIG structure using an agent capable of altering cholesterol organization potently diminished Ca2+ mobilization induced by the coligation of CD3 and CD5. This was associated with the inhibition of the redistribution of DIG although the association of CD3 and CD5 was not affected. Thus, the DIG-resident costimulatory molecules exert their costimulatory effects by contributing to an enhanced association of TCR/CD3 and DIG.

T cell activation stimulates the association of enzymatically active tyrosine-phosphorylated ZAP-70 with the Crk adapter proteins

Engagement of the T cell antigen receptor initiates signal transduction involving tyrosine phosphorylation of multiple effector molecules and the formation of multimolecular complexes at the receptor site. Adapter proteins that possess SH2 and SH3 protein-protein interaction domains are implicated in the assembly of cell activation-induced signaling complexes. We found that Crk adapter proteins undergo activation-induced interaction with the zeta-chain associated protein (ZAP-70) tyrosine kinase in the human T cell line, Jurkat. Incubation of various glutathione S-transferase fusion proteins with a lysate of activated Jurkat cells resulted in selective association of ZAP-70 with Crk, but not Grb2 or Nck, adapter proteins. In addition, tyrosine-phosphorylated ZAP-70 co-immunoprecipitated with Crk from a lysate of activated Jurkat cells, and ZAP-70 association with GST-Crk was observed in a lysate of activated human peripheral blood T cells. Association between the two molecules was mediated by direct physical interaction and involved the Crk-SH2 domain and phosphotyrosyl-containing sequences on ZAP-70. The association required intact Lck, considered to be an upstream regulator of ZAP-70, because it could not take place in activated JCaM1 cells, which express normal levels of ZAP-70 but are devoid of Lck. Finally, glutathione S-transferase-Crk fusion proteins were found to interact predominantly with membrane-residing tyrosine-phosphorylated ZAP-70 that exhibited autophosphorylation activity as well as phosphorylation of an exogenous substrate, CFB3. These findings suggest that Crk adapter proteins play a role in the early activation events of T lymphocytes, apparently, by direct interaction with, and regulation of, the membrane-residing ZAP-70 protein tyrosine kinase.

FYN-T-FYB-SLP-76 interactions define a T-cell receptor zeta/CD3-mediated tyrosine phosphorylation pathway that up-regulates interleukin 2 transcription in T-cells

Protein-tyrosine kinases p56(Lck), SYK, and ZAP-70 and downstream adaptors LAT and SLP-76 have been implicated as essential components in T-cell activation. Another lymphoid-specific adaptor FYB/SLAP has also been identified as a predominant binding partner of SLP-76 and the Src kinase FYN-T, although its role in the activation process has been unclear. In this study, we demonstrate that FYN-T selectively phosphorylates FYB providing a template for the recruitment of FYN-T and SLP-76 SH2 domain binding. This interaction is unusual in its distinct cytoplasmic localization and its long term stable kinetics of phosphorylation. Furthermore, we demonstrate for the first time that the co-expression of all three components of the FYN-T-FYB-SLP-76 matrix can synergistically up-regulate T-cell receptor-driven interleukin 2 transcription activity. These findings document the existence of a T-cell receptor-regulated FYN-T-FYB pathway that interfaces with the adaptor SLP-76 and up-regulates lymphokine production in T-cells.

Regulation of nuclear localization and transcriptional activity of TFII-I by Bruton's tyrosine kinase

Bruton's tyrosine kinase (Btk) is required for normal B-cell development, as defects in Btk lead to X-linked immunodeficiency (xid) in mice and X-linked agammaglobulinemia (XLA) in humans. Here we demonstrate a functional interaction between the multifunctional transcription factor TFII-I and Btk. Ectopic expression of wild-type Btk enhances TFII-I-mediated transcriptional activation and its tyrosine phosphorylation in transient-transfection assays. Mutation of Btk in either the PH domain (R28C, as in the murine xid mutation) or the kinase domain (K430E) compromises its ability to enhance both the tyrosine phosphorylation and the transcriptional activity of TFII-I. TFII-I associates constitutively in vivo with wild-type Btk and kinase-inactive Btk but not xid Btk. However, membrane immunoglobulin M cross-linking in B cells leads to dissociation of TFII-I from Btk. We further show that while TFII-I is found in both the nucleus and cytoplasm of wild-type and xid primary resting B cells, nuclear TFII-I is greater in xid B cells. Most strikingly, receptor cross-linking of wild-type (but not xid) B cells results in increased nuclear import of TFII-I. Taken together, these data suggest that although the PH domain of Btk is primarily responsible for its physical interaction with TFII-I, an intact kinase domain of Btk is required to enhance transcriptional activity of TFII-I in the nucleus. Thus, mutations impairing the physical and/or functional association between TFII-I and Btk may result in diminished TFII-I-dependent transcription and contribute to defective B-cell development and/or function.

Degradation of ZAP-70 Following Antigenic Stimulation in Human T Lymphocytes: Role of Calpain Proteolytic Pathway

T cell activation by the specific Ag results in dramatic changes of the T cell phenotype that include a rapid and profound down-regulation and degradation of triggered TCRs. In this work, we investigated the fate of the TCR-associated ZAP-70 kinase in Ag-stimulated T cells. T cells stimulated by peptide-pulsed APCs undergo an Ag dose-dependent decrease of the total cellular content of ZAP-70, as detected by FACS analysis and confocal microscopy on fixed and permeabilized T cell-APC conjugates and by Western blot on total cell lysates. The time course of ZAP-70 consumption overlaps with that of ζ-chain degradation, indicating that ZAP-70 is degraded in parallel with TCR internalization and degradation. Pharmacological activation of protein kinase C (PKC) does not induce ZAP-70 degradation, which, on the contrary, requires activation of protein tyrosine kinases. Two lines of evidence indicate that the Ca2+-dependent cysteine protease calpain plays a major role in initiating ZAP-70 degradation: 1) treatment of T cells with cell-permeating inhibitors of calpain markedly reduces ZAP-70 degradation; 2) ZAP-70 is cleaved in vitro by calpain. Our results show that, in the course of T cell-APC cognate interaction, ZAP-70 is rapidly degraded via a calpain-dependent mechanism.

Prolactin receptor signaling: shared components with the T-cell antigen receptor in Nb2 lymphoma cells

Previously, we reported that activation of the human prolactin receptor (PRLR) produced a protein phosphorylation pattern strikingly similar to that provoked by Concanavalin A (Con A), an activator of the T-cell antigen receptor (TCR). These results suggested that certain signaling components of the TCR may be shared by the activated PRLR. Additional studies here assessed the levels of TCR expression following PRLR stimulation and the effect of TCR activation on PRL-stimulated proliferation in lactogen-dependent pre-T Nb2-11 lymphoma cells. The results indicated that the TCR was expressed on the surface of approx 4% of exponentially proliferating and prolactin- (PRL) treated cells. In contrast, approx 45% of quiescent cells, cultured in the absence of PRL for 24 h, expressed the TCR at the cell surface, suggesting that lactogen withdrawal may up-regulate TCR cell-surface expression. Moreover, TCR activation with anti-CD3 antibodies attenuated PRL-stimulated Nb2-11 cell proliferation in a concentration-dependent manner. In other experiments, immunoprecipitation and immunoblotting of Nb2-11 lysates revealed that activation of the PRLR resulted in rapid tyrosyl phosphorylation of ZAP-70, a critical TCR-associated tyrosine kinase. In addition, ZAP-70 was found to associate transiently with the putative guanine nucleotide exchange factor and substrate, Vav, in PRL-treated cells. ZAP-70 was also found to associate constitutively with the PRLR; PRL stimulation provoked the transient recruitment of Vav to the complex. These observations suggest that PRL signaling reflects the transient formation of a PRLR-ZAP-70-Vav complex and its immunomodulatory actions involve diverse interactions that affect TCR expression and signaling mechanisms.

ZAP-70 association with T cell receptor zeta (TCRzeta): fluorescence imaging of dynamic changes upon cellular stimulation

The nonreceptor protein tyrosine kinase ZAP-70 is a critical enzyme required for successful T lymphocyte activation. After antigenic stimulation, ZAP-70 rapidly associates with T cell receptor (TCR) subunits. The kinetics of its translocation to the cell surface, the properties of its specific interaction with the TCRzeta chain expressed as a chimeric protein (TTzeta and Tzetazeta), and its mobility in different intracellular compartments were studied in individual live HeLa cells, using ZAP-70 and Tzetazeta fused to green fluorescent protein (ZAP-70 GFP and Tzetazeta-GFP, respectively). Time-lapse imaging using confocal microscopy indicated that the activation-induced redistribution of ZAP-70 to the plasma membrane, after a delayed onset, is of long duration. The presence of the TCRzeta chain is critical for the redistribution, which is enhanced when an active form of the protein tyrosine kinase Lck is coexpressed. Binding specificity to TTzeta was indicated using mutant ZAP-70 GFPs and a truncated zeta chimera. Photobleaching techniques revealed that ZAP-70 GFP has decreased mobility at the plasma membrane, in contrast to its rapid mobility in the cytosol and nucleus. Tzetazeta- GFP is relatively immobile, while peripherally located ZAP-70 in stimulated cells is less mobile than cytosolic ZAP-70 in unstimulated cells, a phenotype confirmed by determining the respective diffusion constants. Examination of the specific molecular association of signaling proteins using these approaches has provided new insights into the TCRzeta-ZAP-70 interaction and will be a powerful tool for continuing studies of lymphocyte activation.

Adaptor function for the Syk kinases-interacting protein 3BP2 in IL-2 gene activation

Syk-family tyrosine kinases are essential for lymphocyte development and activation. Using a yeast two-hybrid screen to identify Syk kinases-interacting proteins (SKIPs), we isolated 3BP2, an Abl SH3-interacting protein of unknown function. 3BP2 was selectively expressed in hematopoietic/lymphoid tissues and bound via its SH2 domain activated Syk-family kinases in mammalian cells, including in antigen receptor-stimulated T cells. In addition to Zap-70, the 3BP2 SH2 domain associated in vitro with LAT, Grb2, PLCgamma1, and Cbl from activated T cell lysates. Transient 3BP2 overexpression induced transcriptional activation of the IL-2 promoter and its NFAT or AP-1 elements. This activity was dependent on the SH2 and pleckstrin-homology domains of 3BP2, and required functional Syk kinases, Ras, and calcineurin. Thus, 3BP2 is an important adaptor that may couple activated Zap-70/Syk to a LAT-containing signaling complex involved in TCR-mediated gene transcription.

ZAP-70 tyrosine kinase is required for LFA-1-dependent T cell migration

The ZAP-70 tyrosine kinase is essential for T cell activation by the T cell receptor. We show that ZAP-70 is also required for migration of T cells that is dependent on the integrin LFA-1. Invasion of TAM2D2 T cell hybridoma cells into fibroblast monolayers, which is LFA-1-dependent, was blocked by overexpression of dominant-negative ZAP-70 and by piceatannol but not by herbimycin A. The Syk inhibitor piceatannol blocks the Syk homologue ZAP-70, which is expressed by TAM2D2 cells, with the same dose dependence as the inhibition of invasion. Dominant-negative ZAP-70 completely inhibited the extensive metastasis formation of TAM2D2 cells to multiple organs upon i.v. injection into mice. Migration of TAM2D2 cells through filters coated with the LFA-1 ligand ICAM-1, induced by 1 ng/ml of the chemokine SDF-1, was blocked by anti-LFA-1 mAb and also abrogated by dominant-negative ZAP-70 and piceatannol. In contrast, migration induced by 100 ng/ml SDF-1 was independent of both LFA-1 and ZAP-70. LFA-1 cross-linking induced tyrosine phosphorylation, which was blocked by dominant-negative ZAP-70 and piceatannol. We conclude that LFA-1 engagement triggers ZAP-70 activity that is essential for LFA-1-dependent migration.

LAT: the ZAP-70 tyrosine kinase substrate that links T cell receptor to cellular activation

Despite extensive study, several of the major components involved in T cell receptor-mediated signaling remain unidentified. Here we report the cloning of the cDNA for a highly tyrosine-phosphorylated 36-38 kDa protein, previously characterized by its association with Grb2, phospholipase C-gamma1, and the p85 subunit of phosphoinositide 3-kinase. Deduced amino acid sequence identifies a novel integral membrane protein containing multiple potential tyrosine phosphorylation sites. We show that this protein is phosphorylated by ZAP-70/Syk protein tyrosine kinases leading to recruitment of multiple signaling molecules. Its function is demonstrated by inhibition of T cell activation following overexpression of a mutant form lacking critical tyrosine residues. Therefore, we propose to name the molecule LAT-linker for activation of T cells.