Cbp deficiency alters Csk localization in lipid rafts but does not affect T-cell development

CSK-mediated signalling by integrins in cancer

Cancer progression and metastasis are processes heavily controlled by the integrin receptor family. Integrins are cell adhesion molecules that constitute the central components of mechanosensing complexes called focal adhesions, which connect the extracellular environment with the cell interior. Focal adhesions act as key players in cancer progression by regulating biological processes, such as cell migration, invasion, proliferation, and survival. Src family kinases (SFKs) can interplay with integrins and their downstream effectors. SFKs also integrate extracellular cues sensed by integrins and growth factor receptors (GFR), transducing them to coordinate metastasis and cell survival in cancer. The non-receptor tyrosine kinase CSK is a well-known SFK member that suppresses SFK activity by phosphorylating its specific negative regulatory loop (C-terminal Y⁵²⁷ residue). Consequently, CSK may play a pivotal role in tumour progression and suppression by inhibiting SFK oncogenic effects in several cancer types. Remarkably, CSK can localise near focal adhesions when SFKs are activated and even interact with focal adhesion components, such as phosphorylated FAK and Paxillin, among others, suggesting that CSK may regulate focal adhesion dynamics and structure. Even though SFK oncogenic signalling has been extensively described before, the specific role of CSK and its crosstalk with integrins in cancer progression, for example, in mechanosensing, remain veiled. Here, we review how CSK, by regulating SFKs, can regulate integrin signalling, and focus on recent discoveries of mechanotransduction. We additionally examine the cross talk of integrins and GFR as well as the membrane availability of these receptors in cancer. We also explore new pharmaceutical approaches to these signalling pathways and analyse them as future therapeutic targets.

Regulation, targets and functions of CSK

The Src family kinases (SFK) plays an important role in multiple signal transduction pathways. Aberrant activation of SFKs leads to diseases such as cancer, blood disorders, and bone pathologies. By phosphorylating and inactivating SFKs, the C-terminal Src kinase (CSK) serves as the key negative regulator of SFKs. Similar to Src, CSK is composed of SH3, SH2, and a catalytic kinase domain. However, while the Src kinase domain is intrinsically active, the CSK kinase domain is intrinsically inactive. Multiple lines of evidence indicate that CSK is involved in various physiological processes including DNA repair, permeability of intestinal epithelial cells (IECs), synaptic activity, astrocyte-to-neuron communication, erythropoiesis, platelet homeostasis, mast cell activation, immune and inflammation responses. As a result, dysregulation of CSK may lead to many diseases with different underlying molecular mechanisms. Furthermore, recent findings suggest that in addition to the well-established CSK-SFK axis, novel CSK-related targets and modes of CSK regulation also exist. This review focuses on the recent progress in this field for an up-to-date understanding of CSK.

Dissection of the catalytic and regulatory structure-function relationships of Csk protein tyrosine kinase

Protein tyrosine kinases (PTKs) are a large enzyme family that regulates many cellular processes. The key to their broad role in signaling is their tunable substrate specificity and regulatory mechanisms that allow each to respond to appropriate regulatory signals and phosphorylate the correct physiological protein substrates. Thus, in addition to the general PTK catalytic platform, each PTK acquires unique structural motifs that confer a unique combination of catalytic and regulatory properties. Understanding the structural basis for these properties is essential for understanding and manipulating the PTK-based signaling networks in normal and cancer cells. C-terminal Src kinase (Csk) and its homolog, Csk-homologous kinase (Chk), phosphorylate Src family kinases on a C-terminal Tyr residue and negatively regulate their kinase activity. While this regulatory function is biologically essential, Csk and Chk have also been excellent model PTKs for dissecting the structural basis of PTK catalysis and regulation. In this article, we review the structure-function studies of Csk and Chk that shed light on the regulatory and catalytic mechanisms of protein tyrosine kinases in general.

Transmembrane adaptor protein PAG is a mediator of PD-1 inhibitory signaling in human T cells

The inhibitory receptor PD-1 is expressed on T cells to inhibit select functions when ligated. The complete signaling mechanism downstream of PD-1 has yet to be uncovered. Here, we discovered phosphoprotein associated with glycosphingolipid-enriched microdomains 1 (PAG) is phosphorylated following PD-1 ligation and associate this with inhibitory T cell function. Clinical cohort analysis correlates low PAG expression with increased survival from numerous tumor types. PAG knockdown in T cells prevents PD-1-mediated inhibition of cytokine secretion, cell adhesion, CD69 expression, and ERK204/187 phosphorylation, and enhances phosphorylation of SRC527 following PD-1 ligation. PAG overexpression rescues these effects. In vivo, PAG contributes greatly to the growth of two murine tumors, MC38 and B16, and limits T cell presence within the tumor. Moreover, PAG deletion sensitizes tumors to PD-1 blockade. Here PAG is established as a critical mediator of PD-1 signaling and as a potential target to enhance T cell activation in tumors.

Multi-color Molecular Visualization of Signaling Proteins Reveals How C-Terminal Src Kinase Nanoclusters Regulate T Cell Receptor Activation

Elucidating the mechanisms that controlled T cell activation requires visualization of the spatial organization of multiple proteins on the submicron scale. Here, we use stoichiometrically accurate, multiplexed, single-molecule super-resolution microscopy (DNA-PAINT) to image the nanoscale spatial architecture of the primary inhibitor of the T cell signaling pathway, Csk, and two binding partners implicated in its membrane association, PAG and TRAF3. Combined with a newly developed co-clustering analysis framework, we find that Csk forms nanoscale clusters proximal to the plasma membrane that are lost post-stimulation and are re-recruited at later time points. Unexpectedly, these clusters do not co-localize with PAG at the membrane but instead provide a ready pool of monomers to downregulate signaling. By generating CRISPR-Cas9 knockout T cells, our data also identify that a major risk factor for autoimmune diseases, the protein tyrosine phosphatase non-receptor type 22 (PTPN22) locus, is essential for Csk nanocluster re-recruitment and for maintenance of the synaptic PAG population.

Multiple Signaling Roles of CD3ε and Its Application in CAR-T Cell Therapy

A T cell receptor (TCR) mediates antigen-induced signaling through its associated CD3ε, δ, γ, and ζ, but the contributions of different CD3 chains remain elusive. Using quantitative mass spectrometry, we simultaneously quantitated the phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) of all CD3 chains upon TCR stimulation. A subpopulation of CD3ε ITAMs was mono-phosphorylated, owing to Lck kinase selectivity, and specifically recruited the inhibitory Csk kinase to attenuate TCR signaling, suggesting that TCR is a self-restrained signaling machinery containing both activating and inhibitory motifs. Moreover, we found that incorporation of the CD3ε cytoplasmic domain into a second-generation chimeric antigen receptor (CAR) improved antitumor activity of CAR-T cells. Mechanistically, the Csk-recruiting ITAM of CD3ε reduced CAR-T cytokine production whereas the basic residue rich sequence (BRS) of CD3ε promoted CAR-T persistence via p85 recruitment. Collectively, CD3ε is a built-in multifunctional signal tuner, and increasing CD3 diversity represents a strategy to design next-generation CAR.

PAG1 limits allergen-induced type 2 inflammation in the murine lung

BACKGROUND

Phosphoprotein associated with glycosphingolipid-enriched microdomains 1 (PAG1) is a transmembrane adaptor protein that affects immune receptor signaling in T and B cells. Evidence from genome-wide association studies of asthma suggests that genetic variants that regulate the expression of PAG1 are associated with asthma risk. However, it is not known whether PAG1 expression is causally related to asthma pathophysiology. Here, we investigated the role of PAG1 in a preclinical mouse model of house dust mite (HDM)-induced allergic sensitization and allergic airway inflammation.

METHODS

Pag1-deficient (Pag1^-/- ) and wild-type (WT) mice were sensitized or sensitized/challenged to HDM, and hallmark features of allergic inflammation were assessed. The contribution of T cells was assessed through depletion (anti-CD4 antibody) and adoptive transfer studies.

RESULTS

Type 2 inflammation (eosinophilia, eotaxin-2 expression, IL-4/IL-5/IL-13 production, mucus production) in the airways and lungs was significantly increased in HDM sensitized/challenged Pag1^-/- mice compared to WT mice. The predisposition to allergic sensitization was associated with increased airway epithelial high-mobility group box 1 (HMGB1) translocation and release, increased type 2 innate lymphoid cells (ILC2s) and monocyte-derived dendritic cell numbers in the mediastinal lymph nodes, and increased T-helper type 2 (T_H 2)-cell differentiation. CD4⁺ T-cell depletion studies or the adoptive transfer of WT OVA-specific CD4⁺ T cells to WT or Pag1^-/- recipients demonstrated that the heightened propensity for T_H 2-cell differentiation was both T cell intrinsic and extrinsic.

CONCLUSION

PAG1 deficiency increased airway epithelial activation, ILC2 expansion, and T_H 2 differentiation. As a consequence, PAG1 deficiency predisposed toward allergic sensitization and increased the severity of experimental asthma.

Beyond the Cell Surface: Targeting Intracellular Negative Regulators to Enhance T cell Anti-Tumor Activity

It is well established that extracellular proteins that negatively regulate T cell function, such as Cytotoxic T-Lymphocyte-Associated protein 4 (CTLA-4) and Programmed Cell Death protein 1 (PD-1), can be effectively targeted to enhance cancer immunotherapies and Chimeric Antigen Receptor T cells (CAR-T cells). Intracellular proteins that inhibit T cell receptor (TCR) signal transduction, though less well studied, are also potentially useful therapeutic targets to enhance T cell activity against tumor. Four major classes of enzymes that attenuate TCR signaling include E3 ubiquitin kinases such as the Casitas B-lineage lymphoma proteins (Cbl-b and c-Cbl), and Itchy (Itch), inhibitory tyrosine phosphatases, such as Src homology region 2 domain-containing phosphatases (SHP-1 and SHP-2), inhibitory protein kinases, such as C-terminal Src kinase (Csk), and inhibitory lipid kinases such as Src homology 2 (SH2) domain-containing inositol polyphosphate 5-phosphatase (SHIP) and Diacylglycerol kinases (DGKs). This review describes the mechanism of action of eighteen intracellular inhibitory regulatory proteins in T cells within these four classes, and assesses their potential value as clinical targets to enhance the anti-tumor activity of endogenous T cells and CAR-T cells.

Carboxyl-Terminal Src Kinase Binds CD28 upon Activation and Mutes Downstream Signaling

Full T cell activation depends on stimulation of the TCR in conjunction with a costimulatory receptor. The involvement of costimulatory molecules is potent, and a mechanistic understanding of how downstream signaling is regulated is required to fully understand T cell responsiveness. In this study, a proteomic approach was taken to identify the interactomes of the coreceptors CD2 and CD28. These coreceptors are both positive regulators of T cell activation, but CD28 less potently induces TCR-proximal signaling. C-terminal Src kinase (CSK), a negative regulator of TCR signaling, was identified as a specific and direct interactor only of activated CD28. CSK is recruited to CD28 upon T cell activation, and the in vitro kinase activity of CSK is enhanced in the presence of phosphorylated CD28. Interruption of the CSK/CD28 interaction prior to TCR/CD28 costimulation induces a signaling response which mimics the more potent CD2-induced TCR-proximal pathway activation. Thus, CD28 functions as a novel adaptor protein for CSK, and CSK regulates signaling downstream of CD28.

The Csk-Associated Adaptor PAG Inhibits Effector T Cell Activation in Cooperation with Phosphatase PTPN22 and Dok Adaptors

The transmembrane adaptor PAG (Cbp) has been proposed to mediate membrane recruitment of Csk, a cytoplasmic protein tyrosine kinase playing a critical inhibitory role during T cell activation, by inactivating membrane-associated Src kinases. However, this model has not been validated by genetic evidence. Here, we demonstrate that PAG-deficient mice display enhanced T cell activation responses in effector, but not in naive, T cells. PAG-deficient mice also have augmented T cell-dependent autoimmunity and greater resistance to T cell anergy. Interestingly, in the absence of PAG, Csk becomes more associated with alternative partners; i.e., phosphatase PTPN22 and Dok adaptors. Combining PAG deficiency with PTPN22 or Dok adaptor deficiency further enhances effector T cell responses. Unlike PAG, Cbl ubiquitin ligases inhibit the activation of naive, but not of effector, T cells. Thus, Csk-associating PAG is a critical component of the inhibitory machinery controlling effector T cell activation in cooperation with PTPN22 and Dok adaptors.

Src-family kinases negatively regulate NFAT signaling in resting human T cells

T cell signaling is required for activation of both natural and therapeutic T cells including chimeric antigen receptor (CAR) T cells. Identification of novel factors and pathways regulating T cell signaling may aid in development of effective T cell therapies. In resting human T cells, the majority of Src-family of tyrosine kinases (SFKs) are inactive due to phosphorylation of a conserved carboxy-terminal tyrosine residue. Recently, a pool of enzymatically active SFKs has been identified in resting T cells; however, the significance of these is incompletely understood. Here, we characterized the role of active SFKs in resting human T cells. Pharmacologic inhibition of active SFKs enhanced distal TCR signaling as measured by IL-2 release and CD25 surface expression following TCR-independent activation. Mechanistically, inhibition of the active pool of SFKs induced nuclear translocation of NFAT1, and enhanced NFAT1-dependent signaling in resting T cells. The negative regulation of NFAT1 signaling was in part mediated by the Src-kinase Lck as human T cells lacking Lck had increased levels of nuclear NFAT1 and demonstrated enhanced NFAT1-dependent gene expression. Inhibition of active SFKs in resting primary human T cells also increased nuclear NFAT1 and enhanced NFAT1-dependent signaling. Finally, the calcineurin inhibitor FK506 and Cyclosporin A reversed the effect of SFKs inhibition on NFAT1. Together, these data identified a novel role of SFKs in preventing aberrant NFAT1 activation in resting T cells, and suggest that maintaining this pool of active SFKs in therapeutic T cells may increase the efficacy of T cell therapies.

Csk-binding protein controls red blood cell development via regulation of Lyn tyrosine kinase activity

Erythropoiesis is controlled principally through erythropoietin (Epo) receptor signaling, which involves Janus kinase 2 (JAK2) and Lyn tyrosine kinase, both of which are important for regulating red blood cell (RBC) development. Negative regulation of Lyn involves C-Src kinase (Csk)-mediated phosphorylation of its C-terminal tyrosine, which is facilitated by the transmembrane adaptor Csk-binding protein (Cbp). Although Cbp has significant functions in controlling Lyn levels and activity in erythroid cells in vitro, its importance to primary erythroid cell development and signaling has remained unclear. To address this, we assessed the consequence of loss of Cbp on the erythroid compartment in vivo and whether Epo-responsive cells isolated from Cbp-knockout mice exhibited altered signaling. Our data show that male Cbp^-/- mice display a modest but significant alteration to late erythroid development in bone marrow with evidence of increased erythrocytes in the spleen, whereas female Cbp^-/- mice exhibit a moderate elevation in early erythroid progenitors (not seen in male mice) that does not influence the later steps in RBC development. In isolated primary erythroid cells and cell lines generated from Cbp^-/- mice, survival signaling through Lyn/Akt/FoxO3 was elevated, resulting in sustained viability during differentiation. The high Akt activity disrupted GAB2/SHP-2 feedback inhibition of Lyn; however, the elevated Lyn activity also increased inhibitory signaling via SHP-1 to restrict the Erk1/2 pathway. Interestingly, whereas loss of Cbp led to mild changes to late RBC development in male mice, this was not apparent in female Cbp^-/- mice, possibly due to their elevated estrogen, which is known to facilitate early progenitor self-renewal.

Normal Development and Function of T Cells in Proline Rich 7 (Prr7) Deficient Mice

Transmembrane adaptor proteins (TRAPs) are important organisers for the transduction of immunoreceptor-mediated signals. Prr7 is a TRAP that regulates T cell receptor (TCR) signalling and potently induces cell death when overexpressed in human Jurkat T cells. Whether endogenous Prr7 has a similar functional role is currently unknown. To address this issue, we analysed the development and function of the immune system in Prr7 knockout mice. We found that loss of Prr7 partially impairs development of single positive CD4+ T cells in the thymus but has no effect on the development of other T cell subpopulations, B cells, NK cells, or NKT cells. Moreover, Prr7 does not affect the TCR signalling pathway as T cells derived from Prr7 knockout and wild-type animals and stimulated in vitro express the same levels of the activation marker CD69, and retain their ability to proliferate and activate induced cell death programs. Importantly, Prr7 knockout mice retained the capacity to mount a protective immune response when challenged with Listeria monocytogenes infection in vivo. In addition, T cell effector functions (activation, migration, and reactivation) were normal following induction of experimental autoimmune encephalomyelitis (EAE) in Prr7 knockout mice. Collectively, our work shows that loss of Prr7 does not result in a major immune system phenotype and suggests that Prr7 has a dispensable function for TCR signalling.

Co-recruitment analysis of the CBL and CBLB signalosomes in primary T cells identifies CD5 as a key regulator of TCR-induced ubiquitylation

T-cell receptor (TCR) signaling is essential for the function of T cells and negatively regulated by the E3 ubiquitin-protein ligases CBL and CBLB Here, we combined mouse genetics and affinity purification coupled to quantitative mass spectrometry to monitor the dynamics of the CBL and CBLB signaling complexes that assemble in normal T cells over 600 seconds of TCR stimulation. We identify most previously known CBL and CBLB interacting partners, as well as a majority of proteins that have not yet been implicated in those signaling complexes. We exploit correlations in protein association with CBL and CBLB as a function of time of TCR stimulation for predicting the occurrence of direct physical association between them. By combining co-recruitment analysis with biochemical analysis, we demonstrated that the CD5 transmembrane receptor constitutes a key scaffold for CBL- and CBLB-mediated ubiquitylation following TCR engagement. Our results offer an integrated view of the CBL and CBLB signaling complexes induced by TCR stimulation and provide a molecular basis for their negative regulatory function in normal T cells.

Small molecule inhibition of Csk alters affinity recognition by T cells

The C-terminal Src kinase (Csk), the primary negative regulator of Src-family kinases (SFK), plays a crucial role in controlling basal and inducible receptor signaling. To investigate how Csk activity regulates T cell antigen receptor (TCR) signaling, we utilized a mouse expressing mutated Csk (Csk^AS) whose catalytic activity is specifically and rapidly inhibited by a small molecule. Inhibition of Csk^AS during TCR stimulation led to stronger and more prolonged TCR signaling and to increased proliferation. Inhibition of Csk^AS enhanced activation by weak but strictly cognate agonists. Titration of Csk inhibition revealed that a very small increase in SFK activity was sufficient to potentiate T cell responses to weak agonists. Csk plays an important role, not only in basal signaling, but also in setting the TCR signaling threshold and affinity recognition.

DOI:http://dx.doi.org/10.7554/eLife.08088.001

The immune system has ‘T’ cells that recognize when the body is infected with a virus or bacterium and mount an immune response that is targeted to that microbe. They can also find and eliminate cancer cells. The microbes and cancer cells produce molecules called antigens that are detected by proteins on the surface of the T cell called T cell receptors (TCR). Antigen recognition causes the TCRs to transmit signals to the inside of the T cell that trigger an immune response. The degree to which the TCRs are active, and the length of time that they transmit signals regulates the size of the immune response. Therefore, developing new drugs that manipulate the activity of TCRs could be useful to treat many diseases.

An enzyme called Csk inhibits the activities of a small family of proteins involved in a variety of different processes. One protein that Csk targets is called Lck and is required for the activation of immune responses in T cells. However, it is not clear whether Csk interacting with this protein stops the release of signals from TCRs, or whether it alters the level to which TCRs need to be activated before they transmit the signals.

Manz, Tan et al. studied T cells from mice that had a mutant form of Csk that is inhibited by a drug called 3-iodo-benzyl-PP1 (3-IB-PP1), but otherwise works normally. When these cells detected an antigen in the presence of the drug, its TCRs were more highly activated and transmitted signals for a longer period of time than cells not exposed to the drug. The drug especially enhanced immune responses to very weak antigens, ones that might not activate T cells under normal circumstances.

Manz, Tan et al.'s findings confirm that Csk plays a negative role in the activation of T cells and suggest that Csk may be a useful target for drug therapies that aim to fine-tune immune responses. The next challenge is to find a drug that can inhibit normal Csk enzymes and test this in mice and other animals.

DOI:http://dx.doi.org/10.7554/eLife.08088.002

Long-Range Modulation of PAG1 Expression by 8q21 Allergy Risk Variants

The gene(s) whose expression is regulated by allergy risk variants is unknown for many loci identified through genome-wide association studies. Addressing this knowledge gap might point to new therapeutic targets for allergic disease. The aim of this study was to identify the target gene(s) and the functional variant(s) underlying the association between rs7009110 on chromosome 8q21 and allergies. Eight genes are located within 1 Mb of rs7009110. Multivariate association analysis of publicly available exon expression levels from lymphoblastoid cell lines (LCLs) identified a significant association between rs7009110 and the expression of a single gene, PAG1 (p = 0.0017), 732 kb away. Analysis of histone modifications and DNase I hypersensitive sites in LCLs identified four putative regulatory elements (PREs) in the region. Chromosome conformation capture confirmed that two PREs interacted with the PAG1 promoter, one in allele-specific fashion. To determine whether these PREs were functional, LCLs were transfected with PAG1 promoter-driven luciferase reporter constructs. PRE3 acted as a transcriptional enhancer for PAG1 exclusively when it carried the rs2370615:C allergy predisposing allele, a variant in complete linkage disequilibrium with rs7009110. As such, rs2370615, which overlaps RelA transcription factor (TF) binding in LCLs and was found to disrupt Foxo3a binding to PRE3, represents the putative functional variant in this locus. Our studies suggest that the risk-associated allele of rs2370615 predisposes to allergic disease by increasing PAG1 expression, which might promote B cell activation and have a pro-inflammatory effect. Inhibition of PAG1 expression or function might have therapeutic potential for allergic diseases.

Destruction of a distal hypoxia response element abolishes trans-activation of the PAG1 gene mediated by HIF-independent chromatin looping

A crucial step in the cellular adaptation to oxygen deficiency is the binding of hypoxia-inducible factors (HIFs) to hypoxia response elements (HREs) of oxygen-regulated genes. Genome-wide HIF-1α/2α/β DNA-binding studies revealed that the majority of HREs reside distant to the promoter regions, but the function of these distal HREs has only been marginally studied in the genomic context. We used chromatin immunoprecipitation (ChIP), gene editing (TALEN) and chromosome conformation capture (3C) to localize and functionally characterize a 82 kb upstream HRE that solely drives oxygen-regulated expression of the newly identified HIF target gene PAG1. PAG1, a transmembrane adaptor protein involved in Src signalling, was hypoxically induced in various cell lines and mouse tissues. ChIP and reporter gene assays demonstrated that the -82 kb HRE regulates PAG1, but not an equally distant gene further upstream, by direct interaction with HIF. Ablation of the consensus HRE motif abolished the hypoxic induction of PAG1 but not general oxygen signalling. 3C assays revealed that the -82 kb HRE physically associates with the PAG1 promoter region, independent of HIF-DNA interaction. These results demonstrate a constitutive interaction between the -82 kb HRE and the PAG1 promoter, suggesting a physiologically important rapid response to hypoxia.

Transmembrane adaptor protein PAG/CBP is involved in both positive and negative regulation of mast cell signaling

The transmembrane adaptor protein PAG/CBP (here, PAG) is expressed in multiple cell types. Tyrosine-phosphorylated PAG serves as an anchor for C-terminal SRC kinase, an inhibitor of SRC-family kinases. The role of PAG as a negative regulator of immunoreceptor signaling has been examined in several model systems, but no functions in vivo have been determined. Here, we examined the activation of bone marrow-derived mast cells (BMMCs) with PAG knockout and PAG knockdown and the corresponding controls. Our data show that PAG-deficient BMMCs exhibit impaired antigen-induced degranulation, extracellular calcium uptake, tyrosine phosphorylation of several key signaling proteins (including the high-affinity IgE receptor subunits, spleen tyrosine kinase, and phospholipase C), production of several cytokines and chemokines, and chemotaxis. The enzymatic activities of the LYN and FYN kinases were increased in nonactivated cells, suggesting the involvement of a LYN- and/or a FYN-dependent negative regulatory loop. When BMMCs from PAG-knockout mice were activated via the KIT receptor, enhanced degranulation and tyrosine phosphorylation of the receptor were observed. In vivo experiments showed that PAG is a positive regulator of passive systemic anaphylaxis. The combined data indicate that PAG can function as both a positive and a negative regulator of mast cell signaling, depending upon the signaling pathway involved.

Sequencing and expression analyses of the synaptic lipid raft adapter gene PAG1 in schizophrenia

Disruption of synaptic networks has been advocated in the pathogenesis of psychiatric diseases like schizophrenia. The majority of synaptic proteins involved in neuronal communications are localized in lipid rafts. These rafts form the platform for coordinating neuronal signal transduction, by clustering interacting partners. The PAG1 protein is a transmembrane adaptor protein in the lipid raft signaling cluster that regulates Src family kinases (SFKs), a convergent point for multiple pathways regulating N-methyl-D-aspartate (NMDA) receptors. Reports of de novo missense mutations in PAG1 and SFK mediated reductions in tyrosine phosphorylation of NMDA receptor subunit proteins in schizophrenia patients, point to a putative role in schizophrenia pathogenesis. To evaluate this, we resequenced the entire coding region of PAG1 in Japanese schizophrenia patients (n = 1,140) and controls (n = 1,140). We identified eight missense variants, of which four were previously unreported. Case-control genetic association analysis of these variants in a larger cohort (n = 4,182) showed neither a statistically significant association of the individual variants with schizophrenia, nor any increased burden of the rare alleles in the patient group. Expression levels of PAG1 in post-mortem brain samples from schizophrenia patients and controls also showed no significant differences. To assess the precise role of PAG1 in schizophrenia, future studies with larger sample sizes are needed.

Inhibition of the kinase Csk in thymocytes reveals a requirement for actin remodeling in the initiation of full TCR signaling

Signaling via the T cell antigen receptor (TCR) is initiated by Src-family kinases (SFKs). To understand how the kinase Csk, a negative regulator of SFKs, controls the basal state and the initiation of TCR signaling, we generated mice that express a Csk variant sensitive to an analog of the common kinase inhibitor PP1 (Csk(AS)). Inhibition of Csk(AS) in thymocytes, without engagement of the TCR, induced potent activation of SFKs and proximal TCR signaling up to phospholipase C-γ1 (PLC-γ1). Unexpectedly, increases in inositol phosphates, intracellular calcium and phosphorylation of the kinase Erk were impaired. Altering the actin cytoskeleton pharmacologically or providing costimulation via CD28 'rescued' those defects. Thus, Csk has a critical role in preventing TCR signaling. However, our studies also revealed a requirement for actin remodeling, initiated by costimulation, for full TCR signaling.

CKIP-1 is an intrinsic negative regulator of T-cell activation through an interaction with CARMA1

The transcription factor NF-κB plays a key regulatory role in lymphocyte activation and generation of immune response. Stimulation of T cell receptor (TCR) induces phosphorylation of CARMA1 by PKCθ, resulting in formation of CARMA1-Bcl10-MALT1 (CBM) complex at lipid rafts and subsequently leading to NF-κB activation. While many molecular events leading to NF-κB activation have been reported, it is less understood how this activation is negatively regulated. We performed a cell-based screening for negative regulators of TCR-mediated NF-κB activation, using mutagenesis and complementation cloning strategies. Here we show that casein kinase-2 interacting protein-1 (CKIP-1) suppresses PKCθ-CBM-NF-κB signaling. We found that CKIP-1 interacts with CARMA1 and competes with PKCθ for association. We further confirmed that a PH domain of CKIP-1 is required for association with CARMA1 and its inhibitory effect. CKIP-1 represses NF-κB activity in unstimulated cells, and inhibits NF-κB activation induced by stimulation with PMA or constitutively active PKCθ, but not by stimulation with TNFα. Interestingly, CKIP-1 does not inhibit NF-κB activation induced by CD3/CD28 costimulation, which caused dissociation of CKIP-1 from lipid rafts. These data suggest that CKIP-1 contributes maintenance of a resting state on NF-κB activity or prevents T cells from being activated by inadequate signaling. In conclusion, we demonstrate that CKIP-1 interacts with CARMA1 and has an inhibitory effect on PKCθ-CBM-NF-κB signaling.

Palmitoylated transmembrane adaptor proteins in leukocyte signaling

Transmembrane adaptor proteins (TRAPs) are structurally related proteins that have no enzymatic function, but enable inducible recruitment of effector molecules to the plasma membrane, usually in a phosphorylation dependent manner. Numerous surface receptors employ TRAPs for either propagation or negative regulation of the signal transduction. Several TRAPs (LAT, NTAL, PAG, LIME, PRR7, SCIMP, LST1/A, and putatively GAPT) are known to be palmitoylated that could facilitate their localization in lipid rafts or tetraspanin enriched microdomains. This review summarizes expression patterns, binding partners, signaling pathways, and biological functions of particular palmitoylated TRAPs with an emphasis on the three most recently discovered members, PRR7, SCIMP, and LST1/A. Moreover, we discuss in silico methodology used for discovery of new family members, nature of their binding partners, and microdomain localization.

Focal adhesion kinase negatively regulates Lck function downstream of the T cell antigen receptor

Focal adhesion kinase (FAK) is a critical regulator of signal transduction in multiple cell types. Although this protein is activated upon TCR engagement, the cellular function that FAK plays in mature human T cells is unknown. By suppressing the function of FAK, we revealed that FAK inhibits TCR-mediated signaling by recruiting C-terminal Src kinase to the membrane and/or receptor complex following TCR activation. Thus, in the absence of FAK, the inhibitory phosphorylation of Lck and/or Fyn is impaired. Together, these data highlight a novel role for FAK as a negative regulator TCR function in human T cells. These results also suggest that changes in FAK expression could modulate sensitivity to TCR stimulation and contribute to the progression of T cell malignancies and autoimmune diseases.

The role of membrane rafts in Lck transport, regulation and signalling in T-cells

Tyrosine phosphorylation is one of the key covalent modifications that occur in multicellular organisms. Since its discovery more than 30 years ago, tyrosine phosphorylation has come to be understood as a fundamentally important mechanism of signal transduction and regulation in all eukaryotic cells. The tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase) plays a crucial role in the T-cell response by transducing early activation signals triggered by TCR (T-cell receptor) engagement. These signals result in the phosphorylation of immunoreceptor tyrosine-based activation motifs present within the cytosolic tails of the TCR-associated CD3 subunits that, once phosphorylated, serve as scaffolds for the assembly of a large supramolecular signalling complex responsible for T-cell activation. The existence of membrane nano- or micro-domains or rafts as specialized platforms for protein transport and cell signalling has been proposed. The present review discusses the signals that target Lck to membrane rafts and the importance of these specialized membranes in the transport of Lck to the plasma membrane, the regulation of Lck activity and the phosphorylation of the TCR.

Novel tools to dissect the dynamic regulation of TCR signaling by the kinase Csk and the phosphatase CD45

Although the biochemical events induced by T-cell receptor (TCR) triggering have been well studied, both the mediators and function of basal signaling in T cells remain poorly understood. Furthermore, the precise mechanisms by which MHC-peptide interaction with the TCR disrupt the basal equilibrium to induce downstream signaling are also unclear. Here we describe novel approaches to understand the basal state of T cells and the mechanisms of TCR triggering by perturbing regulation of the Src family kinases (SFKs). The SFKs are critical proximal mediators of TCR signaling that are in turn tightly regulated by the tyrosine kinase Csk and the receptor-like tyrosine phosphatase CD45. We have developed a small-molecule analog-sensitive allele of Csk and an allelic series of mice in which expression of CD45 is varied across a broad range. Our studies have unmasked contributions of Csk and CD45 to maintain the basal state of T cells and also suggest that dynamic regulation of Csk may be involved in TCR triggering.

A deficient translocation of CD3ζ, ZAP-70 and Grb2 to lipid raft, as a hallmark of defective adaptive immune response during chronic hepatitis B infection

Hepatitis B is considered to be a worldwide public health problem. An immunosuppressor microenvironment has been proposed to contribute to viral persistence during chronic disease. Understanding the intracellular signaling cascade in T-cells from HBV-infected patients, will contribute to unravel the mechanisms that control the development of immune response during hepatitis B. We analyze lipid rafts formation and early activation signals in chronic HBV infected patients, compared to naturally immune subjects (NIS). Patients show: (1) diminished GM1 clustering, (2) A deficient lipid rafts recruitment of CD3ζ/ZAP-70/Grb2, and (3) these proteins do not merge with GM1 within the lipid rafts. Finally, immunoprecipitation assays proved that ZAP-70 does not associate to CD3ζ. These results show for the first time, defects regarding early key events in T-cell activation, in chronically infected HBV patients, which may contribute not only to understand HBV immune tolerance, but to reveal new potential therapeutic targets to control the infection.

T cell receptor signalling networks: branched, diversified and bounded

Engagement of antigen-specific T cell receptors (TCRs) is a prerequisite for T cell activation. Acquisition of appropriate effector T cell function requires the participation of multiple signals from the T cell microenvironment. Trying to understand how these signals integrate to achieve specific functional outcomes while maintaining tolerance to self is a major challenge in lymphocyte biology. Several recent publications have provided important insights into how dysregulation of T cell signalling and the development of autoreactivity can result if the branching and integration of signalling pathways are perturbed. We discuss how these findings highlight the importance of spatial segregation of individual signalling components as a way of regulating T cell responsiveness and immune tolerance.

PAG/Cbp suppression reveals a contribution of CTLA-4 to setting the activation threshold in T cells

Background

PAG/Cbp represents a ubiquitous mechanism for regulating Src family kinases by recruiting Csk to the plasma membrane, thereby controlling cellular activation. Since Src kinases are known oncogenes, we used RNA interference in primary human T cells to test whether the loss of PAG resulted in lymphocyte transformation.

Results

PAG-depletion enhanced Src kinase activity and augmented proximal T-cell receptor signaling; exactly the phenotype expected for loss of this negative regulator. Surprisingly, rather than becoming hyper-proliferative, PAG-suppressed T cells became unresponsive. This was mediated by a Fyn-dependent hyper-phosphorylation of the inhibitory receptor CTLA-4, which recruited the protein tyrosine phosphatase Shp-1 to lipid rafts. Co-suppression of CTLA-4 abrogates this inhibition and restores proliferation to T cells.

Conclusion

We have identified a fail-safe mechanism as well as a novel contribution of CTLA-4 to setting the activation threshold in T cells.

Differential polarization of C-terminal Src kinase between naive and antigen-experienced CD8+ T cells

In CD8(+) T cells, engagement of the TCR with agonist peptide:MHC molecules causes dynamic redistribution of surface molecules including the CD8 coreceptor to the immunological synapse. CD8 associates with the Src-family kinase (SFK) Lck, which, in turn, initiates the rapid tyrosine phosphorylation events that drive cellular activation. Compared with naive T cells, Ag-experienced CD8(+) T cells make shorter contacts with APC, are less dependent on costimulation, and are triggered by lower concentrations of Ag, yet the molecular basis of this more efficient response of memory T cells is not fully understood. In this article, we show differences between naive and Ag-experienced CD8(+) T cells in colocalization of the SFKs and their negative regulator, C-terminal Src kinase (Csk). In naive CD8(+) T cells, there was pronounced colocalization of SFKs and Csk at the site of TCR triggering, whereas in Ag-experienced cells, Csk displayed a bipolar distribution with a proportion of the molecules sequestered within a cytosolic area in the distal pole of the cell. The data show that there is differential redistribution of a key negative regulator away from the site of TCR engagement in Ag-experienced CD8(+) T cells, which might be associated with the more efficient responses of these cells on re-exposure to Ag.

The autoimmunity risk variant LYP-W620 cooperates with CSK in the regulation of TCR signaling

The protein tyrosine phosphatase LYP, a key regulator of TCR signaling, presents a single nucleotide polymorphism, C1858T, associated with several autoimmune diseases such as type I diabetes, rheumatoid arthritis, and lupus. This polymorphism changes an R by a W in the P1 Pro rich motif of LYP, which binds to CSK SH3 domain, another negative regulator of TCR signaling. Based on the analysis of the mouse homologue, Pep, it was proposed that LYP and CSK bind constitutively to inhibit LCK and subsequently TCR signaling. The detailed study of LYP/CSK interaction, here presented, showed that LYP/CSK interaction was inducible upon TCR stimulation, and involved LYP P1 and P2 motifs, and CSK SH3 and SH2 domains. Abrogating LYP/CSK interaction did not preclude the regulation of TCR signaling by these proteins.

Phosphoprotein associated with glycosphingolipid-enriched microdomains differentially modulates SRC kinase activity in brain maturation

Src family kinases (SFK) control multiple processes during brain development and function. We show here that the phosphoprotein associated with glycosphigolipid-enriched microdomains (PAG)/Csk binding protein (Cbp) modulates SFK activity in the brain. The timing and localization of PAG expression overlap with Fyn and Src, both of which we find associated to PAG. We demonstrate in newborn (P1) mice that PAG negatively regulates Src family kinases (SFK). P1 Pag1^-/- mouse brains show decreased recruitment of Csk into lipid rafts, reduced phosphorylation of the inhibitory tyrosines within SFKs, and an increase in SFK activity of >/ = 50%. While in brain of P1 mice, PAG and Csk are highly and ubiquitously expressed, little Csk is found in adult brain suggesting altered modes of SFK regulation. In adult brain Pag1-deficiency has no effect upon Csk-distribution or inhibitory tyrosine phosphorylation, but kinase activity is now reduced (−20–30%), pointing to the development of a compensatory mechanism that may involve PSD93. The distribution of the Csk-homologous kinase CHK is not altered. Importantly, since the activities of Fyn and Src are decreased in adult Pag1^-/- mice, thus presenting the reversed phenotype of P1, this provides the first in vivo evidence for a Csk-independent positive regulatory function for PAG in the brain.

The effects of membrane compartmentalization of csk on TCR signaling

The TCR signal transduction is initiated by the activation of Src-family kinases (SFK) which phosphorylate Immunoreceptor tyrosine-based activation motifs (ITAM) present in the intracellular parts of the T-cell receptor (TCR) signaling subunits. Numerous data suggest that after stimulation TCR interacts with membrane rafts and thus it gains access to SFK and other important molecules involved in signal transduction. However, the precise mechanism of this process is unclear. One of the key questions is how SFK access TCR and what is the importance of non-raft and membrane raft-associated SFK for the initiation and maintenance of the TCR signaling. To answer this question we targeted a negative regulator of SFK, C-terminal Src kinase (Csk) to membrane rafts, recently described "heavy rafts" or non-raft membrane. Our data show that only Csk targeted into "classical" raft but not to "heavy raft" or non-raft membrane effectively inhibits TCR signaling, demonstrating the critical role of membrane raft-associated SFK in this process.

The interaction of LCK and the CD4 co-receptor alters the dose response of T-cells to interleukin-7

CD8 and CD4 T-cells grow optimally under different concentrations of the cytokine, interleukin-7 (IL-7). While CD8 T-cells expand at high doses of IL-7, CD4 T-cells favor low doses. To examine the reason for the preference of CD4 T-cells for lower doses of the cytokine, we used IL-7 dependent T-cells to study signal transduction upon a range of IL-7 concentrations. We found that the high dose responsiveness of CD8 T-cells to IL-7 could be altered if these cells also expressed CD4. Using the phosphorylation of STAT5 as an indicator of growth, we found that the co-receptor associated kinase, LCK, contributed to phospho-STAT5 levels. Phospho-STAT5 was elevated at high dose IL-7 for CD8 T-cells and at low dose IL-7 for CD4 T-cells, which was reversed upon LCK inhibition. Examining the direct association of LCK with CD4 using a T- cell line that over-expresses CD4, we determined that CD4 could directly sequester LCK. Non-CD4 T-cells were not restricted in this manner and levels of phospho-STAT5 increased proportionally to the IL-7 dose. Our studies, therefore, show that the response of a T-cell to IL-7 can be modulated by the availability of LCK.

Spatiotemporal control of cyclic AMP immunomodulation through the PKA-Csk inhibitory pathway is achieved by anchoring to an Ezrin-EBP50-PAG scaffold in effector T cells

A variety of immunoregulatory signals to effector T cells from monocytes, macrophages and regulatory T cells act through cyclic adenosine monophosphate. In the effector T cell, the protein kinase A (PKA) type I isoenzyme localizes to lipid rafts during T cell activation and modulates directly the proximal events that take place after engagement of the T cell receptor. The most proximal target for PKA phosphorylation is C-terminal Src kinase (Csk), which initiates a negative signal pathway that fine-tunes the T cell activation process. The A kinase anchoring protein Ezrin colocalizes PKA and Csk by forming a supramolecular signaling complex consisting of PKA, Ezrin, Ezrin/radixin/moesin (ERM) binding protein of 50 kDa (EBP50), phosphoprotein associated with glycosphingolipid-enriched membrane microdomains (GEMs) (PAG) and Csk.

Fas apoptosis inhibitory molecule regulates T cell receptor-mediated apoptosis of thymocytes by modulating Akt activation and Nur77 expression

Fas apoptosis inhibitory molecule (FAIM) has been demonstrated to confer resistance to Fas-induced apoptosis of lymphocytes and hepatocytes in vitro and in vivo. Here, we show that FAIM is up-regulated in thymocytes upon T cell receptor (TCR) engagement and that faim(-/-) thymocytes are highly susceptible to TCR-mediated apoptosis with increased activation of caspase-8 and -9. Furthermore, injection of anti-CD3 antibodies leads to augmented depletion of CD4(+)CD8(+) T cells in the thymus of faim(-/-) mice compared with wild-type control, suggesting that FAIM plays a role in thymocyte apoptosis. Cross-linking of the TCR on faim(-/-) thymocytes leads to an elevated protein level of the orphan nuclear receptor Nur77, which plays a role in thymocyte apoptosis. Interestingly, in the absence of FAIM, there are reduced ubiquitination and degradation of the Nur77 protein. Faim(-/-) thymocytes also exhibit a defective TCR-induced activation of Akt whose activity we now show is required for Nur77 ubiquitination. Further analyses utilizing FAIM-deficient primary thymocytes and FAIM-overexpressing DO-11.10 T cells indicate that FAIM acts upstream of Akt during TCR signaling and influences the localization of Akt to lipid rafts, hence affecting its activation. Taken together, our study defined a TCR-induced FAIM/Akt/Nur77 signaling axis that is critical for modulating the apoptosis of developing thymocytes.

Activated dectin-1 localizes to lipid raft microdomains for signaling and activation of phagocytosis and cytokine production in dendritic cells

Lipid rafts are plasma membrane microdomains that are enriched in cholesterol, glycosphingolipids, and glycosylphosphatidylinositol-anchored proteins and play an important role in the signaling of ITAM-bearing lymphocyte antigen receptors. Dectin-1 is a C-type lectin receptor (CLR) that recognizes beta-glucan in the cell walls of fungi and triggers signal transduction via its cytoplasmic hemi-ITAM. However, it is not known if similar to antigen receptors, Dectin-1 would also signal via lipid rafts and if the integrity of lipid raft microdomains is important for the physiological functions mediated by Dectin-1. We demonstrate here using sucrose gradient ultracentrifugation and confocal microscopy that Dectin-1 translocates to lipid rafts upon stimulation of dendritic cells (DCs) with the yeast derivative zymosan or beta-glucan. In addition, two key signaling molecules, Syk and PLCgamma2 are also recruited to lipid rafts upon the activation of Dectin-1, suggesting that lipid raft microdomains facilitate Dectin-1 signaling. Disruption of lipid raft integrity with the synthetic drug, methyl-beta-cyclodextrin (betamD) leads to reduced intracellular Ca2+ flux and defective Syk and ERK phosphorylation in Dectin-1-activated DCs. Furthermore, betamD-treated DCs have significantly attenuated production of IL-2, IL-10, and TNFalpha upon Dectin-1 engagement, and they also exhibit impaired phagocytosis of zymosan particles. Taken together, the data indicate that Dectin-1 and perhaps also other CLRs are recruited to lipid rafts upon activation and that the integrity of lipid rafts is important for the signaling and cellular functions initiated by this class of innate receptors.

T-cell receptor proximal signaling via the Src-family kinases, Lck and Fyn, influences T-cell activation, differentiation, and tolerance

T-cell development in the thymus and activation of mature T cells in secondary lymphoid organs requires the ability of cells to respond appropriately to environmental signals at multiple stages of their development. The process of thymocyte selection insures a functional T-cell repertoire, while activation of naive peripheral T cells induces proliferation, gain of effector function, and, ultimately, long-lived T-cell memory. The T-cell immune response is initiated upon engagement of the T-cell receptor (TCR) and coreceptor, CD4 or CD8, by cognate antigen/major histocompatibility complexes presented by antigen-presenting cells. TCR/coreceptor engagement induces the activation of biochemical signaling pathways that, in combination with signals from costimulator molecules and cytokine receptors, direct the outcome of the response. Activation of the src-family kinases p56(lck) (Lck) and p59(fyn) (Fyn) is central to the initiation of TCR signaling pathways. This review focuses on our current understanding of the mechanisms by which these two proteins orchestrate T-cell function.

Genetic deletion of faim reveals its role in modulating c-FLIP expression during CD95-mediated apoptosis of lymphocytes and hepatocytes

Fas-apoptosis inhibitory molecule (FAIM) is inducibly expressed in B lymphocytes and had been shown to antagonize Fas-mediated killing of B-cell lines in vitro. However, its mechanism and role in vivo are unknown. We have generated faim(-/-) mice and found these mutants to be viable. In contrast to fas(-/-) mice, faim(-/-) mice have normal B- and T-cell populations. However, faim(-/-) B cells and thymocytes show increased sensitivity to Fas-triggered apoptosis in vitro, and faim(-/-) mice suffer greater mortality and exhibit exacerbated liver damage in response to Fas (CD95) engagement in vivo. The lack of FAIM results in greater activation of caspase-8 and -3 in Fas-stimulated thymocytes. Detailed biochemical analyses further reveal the decreased expression of c-FLIP(L) and c-FLIP(R) in faim(-/-) thymocytes and increased association of caspase-8 with Fas in Fas-activated mutant cells. Decreased levels of c-FLIP(L) and c-FLIP(R) are also evident in faim(-/-) liver. Thus, FAIM plays a novel role in modulating Fas-mediated apoptosis and acts through influencing the expression of c-FLIP and regulating the physical binding of caspase-8 to Fas.

Isoform-specific regulation of immune cell reactivity by the catalytic subunit of protein kinase A (PKA)

There are two major genes encoding the catalytic subunits of protein kinase A, Calpha and Cbeta. The functional significance of these isoforms is enigmatic. Lymphoid cells of the immune system express both Calpha and Cbeta. In this study we tested the role of Calpha and Cbeta in regulating immune cell reactivity to antigens using mice carrying a targeted disruption of the Calpha and Cbeta gene respectively. Calpha and Cbeta ablation both resulted in a 50% reduction in PKA-specific kinase activity and the level of PKA type I but not PKA type II. Moreover, despite that C subunit ablation did not affect immune cell development and homeostasis, Calpha but not Cbeta ablation augmented expression of the activation marker CD69 on lymphocytes. CD69 induction coincided with immune cell hyperresponsiveness and was associated with reduced sensitivity to cAMP-mediated inhibition of anti-CD3 induced T cell proliferation. Our results imply that Calpha is required for normal immune cell reactivity and demonstrates isoform-specific effects and non-redundant functions of C subunit isoforms expressed in the same cell.

The lipid raft-anchored adaptor protein Cbp controls the oncogenic potential of c-Src

The tyrosine kinase c-Src is upregulated in various human cancers irrespective of its negative regulator Csk, but the regulatory mechanisms remain unclear. Here, we show that a lipid raft-anchored Csk adaptor, Cbp/PAG, is directly involved in controlling the oncogenicity of c-Src. Using Csk-deficient cells that can be transformed by c-Src overexpression, we found that Cbp expression is markedly downregulated by c-Src activation and re-expression of Cbp efficiently suppresses c-Src transformation as well as tumorigenesis. Cbp-deficient cells are more susceptible to v-Src transformation than their parental cells. Upon phosphorylation, Cbp specifically binds to activated c-Src and sequesters it in lipid rafts, resulting in an efficient suppression of c-Src function independent of Csk. In some human cancer cells and tumors, Cbp is downregulated and the introduction of Cbp significantly suppresses tumorigenesis. These findings indicate a potential role for Cbp as a suppressor of c-Src-mediated tumor progression.

Molecular architecture of signal complexes regulating immune cell function

Signals transmitted via multichain immunoreceptors control the development, differentiation and activation of hematopoetic cells. The cytoplasmic parts of these receptors contain immunoreceptor tyrosine-based activation motifs (ITAMs) that upon phosphorylation by members of the Src tyrosine kinase family orchestrate a complex set of signaling events involving tyrosine phosphorylation, generation of second messengers like DAG, IP3 and Ca2+, activation of effector molecules like Ras and MAPKs and the translocation and activation of transcription factors like NFAT, API and NF-kB. Spatial and temporal organization of these signaling events is essential both to connect the receptors to downstream cascades as well as to control the functional outcome of the immune activation. Throughout this process control and fine-tuning of the different signals are necessary both for effective immune function and in order to avoid inappropriate or exaggerated immune activation and autoimmunity. This control includes modulating mechanisms that set the threshold for activation and reset the activation status after an immune response has been launched. One immunomodulating pathway is the cAMP-protein kinase A-Csk pathway scaffolded by a supramolecular complex residing in lipid rafts with the A kinase-anchoring protein (AKAP) ezrin, the Csk-binding protein PAG and a linker between the two, EBP50. Failure of correct scaffolding and loss of spatiotemporal control can potentially have severe consequences, leading to immune failure or autoimmunity. The clinical relevance of supramolecular complexes specifically organized by scaffolding proteins in regulating immune activity and the specter of genetic diseases linked to different signaling components suggest that protein-protein contact surfaces can be potential targets for drug intervention. It is also of interest to note that different pathogens have evolved strategies to specifically modulate signal integration, thereby rewiring the signal in a way beneficial for their survival. In addition to demonstrating the importance of different signal processes, these adaptations are elegant illustrations of the potential for drug targeting of protein assembly. This chapter reviews some of the important scaffolding events downstream of immunoreceptors with focus on signaling transduction through the T-cell receptor (TCR).

Deletion of the LIME adaptor protein minimally affects T and B cell development and function

LIME (Lck-interacting membrane protein) is a transmembrane adaptor that associates with the Lck and Fyn protein tyrosine kinases and with the C-terminal Src kinase (Csk). To delineate the role of LIME in vivo, LIME-deficient mice were generated. Although Lime transcripts were expressed in immature and mature B and T cells, the absence of LIME impeded neither the development nor the function of B and T cells. TCR transgenic mice deprived of LIME showed, however, a 1.8-fold enhancement in positive selection. Since B cells and activated T cells express LIME and the related adaptor NTAL, mice lacking both adaptors were generated. Double-deficient mice showed no defect in the development and function of B and T cells, and the lack of LIME had no effect on the autoimmune syndrome that develops in aged NTAL-deficient mice. In contrast to a previous report, we further showed that this autoimmune syndrome develops in the absence of T cells. Therefore, our in vivo results refute all the previous roles postulated for LIME on the basis of studies of transformed B and T cells and demonstrate that LIME has no seminal role in the signaling cassette operated by antigen receptors and coreceptors.

Protein tyrosine phosphatase PTPN22 in human autoimmunity

The discovery that a single-nucleotide polymorphism (SNP) in lymphoid tyrosine phosphatase (LYP), encoded by the PTPN22 gene, is associated with type 1 diabetes (T1D) has now been verified by numerous studies and has been expanded to rheumatoid arthritis, juvenile rheumatoid arthritis (JRA), systemic lupus erythematosus, Graves' disease, generalized vitiligo and other human autoimmune diseases. In this paper, we discuss the association of PTPN22 with autoimmunity, the biochemistry of the PTPN22-encoded phosphatase, and the molecular mechanism(s) by which the disease-predisposing allele contributes to the development of human disease.

Oncogenic association of the Cbp/PAG adaptor protein with the Lyn tyrosine kinase in human B-NHL rafts

B-non-Hodgkin lymphomas (B-NHLs) use a raft-associated signalosome made of the constitutively active Lyn kinase, the tyrosine phosphorylated Cbp/PAG adaptor, and tyrosine phosphorylated STAT3 transcription factor. No such "signalosome" is found in rafts of ALK(+) T lymphoma and Hodgkin-derived cell lines, despite similar Cbp/PAG, Lyn, and STAT3 expression and similar amounts of raft sphingolipids. Stable association of the signalosome with B-NHL rafts requires (1) a Lyn kinase (auto)phosphorylated in its regulatory and active site tyrosines, (2) a Cbp/PAG adaptor phosphorylated at tyrosine 317 and bound to Lyn SH2 via phosphotyrosine 299 and neighboring residues, and (3) a tyrosine phosphorylated STAT3 linked via SH2 to the regulatory, C-terminal tyrosine of Lyn. No Csk appears to be part of this B-NHL signalosome. An oncogenic role for Lyn was shown after exposure of B-NHL lines to Lyn inhibitors that prevented Lyn and Cbp/PAG phosphorylation, dissociated the signalosome from rafts, and eventually induced death. Cell death followed decreases in Lyn or Cbp/PAG expression levels in one mantle cell lymphoma line, but not in a Hodgkin-derived one. The Lyn-Cbp/PAG signalosome appears to control proliferation and survival in most B-NHLs and constitutes a therapeutic target in B-NHL cells that exhibit oncogenic "addiction" to the Lyn kinase.

Inhibition of T cell activation by cyclic adenosine 5'-monophosphate requires lipid raft targeting of protein kinase A type I by the A-kinase anchoring protein ezrin

cAMP negatively regulates T cell immune responses by activation of type I protein kinase A (PKA), which in turn phosphorylates and activates C-terminal Src kinase (Csk) in T cell lipid rafts. Using yeast two-hybrid screening, far-Western blot, immunoprecipitation and immunofluorescense analyses, and small interfering RNA-mediated knockdown, we identified Ezrin as the A-kinase anchoring protein that targets PKA type I to lipid rafts. Furthermore, Ezrin brings PKA in proximity to its downstream substrate Csk in lipid rafts by forming a multiprotein complex consisting of PKA/Ezrin/Ezrin-binding protein 50, Csk, and Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains. The complex is initially present in immunological synapses when T cells contact APCs and subsequently exits to the distal pole. Introduction of an anchoring disruptor peptide (Ht31) into T cells competes with Ezrin binding to PKA and thereby releases the cAMP/PKA type I-mediated inhibition of T cell proliferation. Finally, small interfering RNA-mediated knockdown of Ezrin abrogates cAMP regulation of IL-2. We propose that Ezrin is essential in the assembly of the cAMP-mediated regulatory pathway that modulates T cell immune responses.