Phosphorylation-dependent regulation of T-cell activation by PAG/Cbp, a lipid raft-associated transmembrane adaptor

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

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

Lck, Membrane Microdomains, and TCR Triggering Machinery: Defining the New Rules of Engagement

In spite of a comprehensive understanding of the schematics of T cell receptor (TCR) signaling, the mechanisms regulating compartmentalization of signaling molecules, their transient interactions, and rearrangement of membrane structures initiated upon TCR engagement remain an outstanding problem. These gaps in our knowledge are exemplified by recent data demonstrating that TCR triggering is largely dependent on a preactivated pool of Lck concentrated in T cells in a specific type of membrane microdomains. Our current model posits that in resting T cells all critical components of TCR triggering machinery including TCR/CD3, Lck, Fyn, CD45, PAG, and LAT are associated with distinct types of lipid-based microdomains which represent the smallest structural and functional units of membrane confinement able to negatively control enzymatic activities and substrate availability that is required for the initiation of TCR signaling. In addition, the microdomains based segregation spatially limits the interaction of components of TCR triggering machinery prior to the onset of TCR signaling and allows their rapid communication and signal amplification after TCR engagement, via the process of their coalescence. Microdomains mediated compartmentalization thus represents an essential membrane organizing principle in resting T cells. The integration of these structural and functional aspects of signaling into a unified model of TCR triggering will require a deeper understanding of membrane biology, novel interdisciplinary approaches and the generation of specific reagents. We believe that the fully integrated model of TCR signaling must be based on membrane structural network which provides a proper environment for regulatory processes controlling TCR triggering.

Channel catfish leukocyte immune-type receptor mediated inhibition of cellular cytotoxicity is facilitated by SHP-1-dependent and -independent mechanisms

Channel catfish (Ictalurus punctatus) leukocyte immune-type receptors (IpLITRs) are immunoregulatory proteins belonging to the immunoglobulin superfamily that likely play an important role in the regulation of teleost immune cell effector responses. IpLITRs are expressed by myeloid and lymphoid subsets and based on their structural features can be classified as either putative stimulatory or inhibitory forms. We have recently demonstrated at the biochemical and functional levels that stimulatory IpLITR-types induced intracellular signaling cascades resulting in immune cell activation. Alternatively, we have shown that putative inhibitory IpLITRs may abrogate immune cell responses by recruiting teleost Src homology 2 (SH2) domain-containing cytoplasmic phosphatases (SHP) to their tyrosine-containing cytoplasmic tails. In the present study, we used vaccinia virus to express recombinant chimeric proteins encoding the extracellular and transmembrane regions of human KIR2DL3 fused with the cytoplasmic tails of two putative inhibitory IpLITRs (i.e. IpLITR1.2a and IpLITR1.1b) in mouse spleen-derived cytotoxic lymphocytes. This approach allowed us to study the specific effects of IpLITR-induced signaling on lymphocyte killing of B cell targets (e.g. 721.221 cells) using a standard chromium release assay. Our results suggest that both IpLITR1.2a and IpLITR1.1b are potent inhibitors of lymphocyte-mediated cellular cytotoxicity. Furthermore, using a catalytically inactive SHP-1 mutant in combination with site-directed mutagenesis and co-immunoprecipitations, we also demonstrate that the IpLITR1.2a-mediated functional inhibitory response is SHP-1-dependent. Alternatively, IpLITR1.1b-mediated inhibition of cellular cytotoxicity is facilitated by both SHP-1-dependent and independent mechanisms, possibly involving the C-terminal Src kinase (Csk). The involvement of this inhibitory kinase requires binding to a tyrosine residue encoded in the unique membrane proximal cytoplasmic tail region of IpLITR1.1b. Overall, this represents the first functional information for inhibitory IpLITR-types and reveals that catfish LITRs engage SHP-dependent and -independent inhibitory signaling pathways to abrogate lymphocyte-mediated killing.

A specific type of membrane microdomains is involved in the maintenance and translocation of kinase active Lck to lipid rafts

Lck is the principal signal-generating tyrosine kinase of the T cell activation mechanism. We have previously demonstrated that induced Lck activation outside of lipid rafts (LR) results in the rapid translocation of a fraction of Lck to LR. While this translocation predicates the subsequent production of IL-2, the mechanism underpinning this process is unknown. Here, we describe the main attributes of this translocating pool of Lck. Using fractionation of Brij58 lysates, derived from primary naive non-activated CD4(+) T cells, we show that a significant portion of Lck is associated with high molecular weight complexes representing a special type of detergent-resistant membranes (DRMs) of relatively high density and sensitivity to laurylmaltoside, thus called heavy DRMs. TcR/CD4 coaggregation-mediated activation resulted in the redistribution of more than 50% of heavy DRM-associated Lck to LR in a microtubular network-dependent fashion. Remarkably, in non-activated CD4(+) T-cells, only heavy DRM-associated Lck is phosphorylated on its activatory tyrosine 394 and this pool of Lck is found to be membrane confined with CD45 phosphatase. These data are the first to illustrate a lipid microdomain-based mechanism concentrating the preactivated pool of cellular Lck and supporting its high stoichiometry of colocalization with CD45 in CD4(+) T cells. They also provide a new structural framework to assess the mechanism underpinning the compartmentalization of critical signaling elements and regulation of spatio-temporal delivery of Lck function during the T cell proximal signaling.

IGSF4 is a novel TCR ζ-chain-interacting protein that enhances TCR-mediated signaling

Immunoglobulin superfamily member 4 (IGSF4) is a known ligand of CRTAM, a receptor expressed in activated NKT and CD8(+) T cells, but its function in T cell immunity has not been elucidated. In this study, we show that IGSF4 directly interacts with the T cell receptor (TCR) ζ-chain and enhances TCR signaling by enhancing ζ-chain phosphorylation. Ectopic overexpression of IGSF4 enhances TCR-mediated T cell activation. In contrast, IGSF4 knockdown shows a dramatic decrease in markers associated with T cell activation compared with those in control small interfering RNA. The transmembrane domain is essential for TCR ζ-chain association and clustering to the immunological synapse, and the ectodomain is associated with T cell interaction with antigen-presenting cells (APCs). IGSF4-deficient mice have impaired TCR-mediated thymocyte selection and maturation. Furthermore, these mice reveal attenuated effector T cell functions accompanied by defective TCR signaling. Collectively, the results indicate that IGSF4 plays a central role in T cell functioning by dual independent mechanisms, control of TCR signaling and control of T cell-APC interaction.

Phosphodiesterases as targets for modulating T-cell responses

The cAMP-protein kinase A (PKA) signaling pathway is strongly involved in the regulation and modulation of immune responses, and cAMP is the most potent and acute inhibitor of T-cell activation. Thus, cAMP levels in the cell must be tightly regulated. Cyclic AMP-specific phosphodiesterases (PDEs) provide the only mechanism for degrading cAMP in cells, thereby functioning as key regulators of signaling. To obtain a complete immune response with optimal cytokine production and T-cell proliferation, ligation of both the T-cell receptor (TCR) and the CD28 receptor is required. However, engagement of the TCR in primary T cells is followed by rapid cAMP production in lipid rafts and activation of the cAMP- PKA-Csk pathway inhibiting proximal T-cell signaling. In contrast, TCR/CD28 costimulation leads to the recruitment of a PDE4/β-arrestin complex to rafts in a phosphatidylinositol 3-kinase (PI3K)-dependent manner, resulting in the downregulation of cAMP levels. Thus, the activities of both PKA and PDE4 seem to be important for regulation of TCR-induced signaling and T-cell function. The use of selective inhibitors has revealed that PDEs are important drug targets in several diseases with an inflammatory component where immune function is important such as asthma, chronic obstructive pulmonary disease (COPD), cardiovascular diseases, and neurological disorders. PDEs are also interesting drug targets in immunosuppression following transplantation and for modulation of immune responses.

A tale of two TRAPs: LAT and LAB in the regulation of lymphocyte development, activation, and autoimmunity

Transmembrane adaptor proteins (TRAPs) link antigen receptor engagement to downstream cellular processes. Although these proteins typically lack intrinsic enzymatic activity, they are phosphorylated on multiple tyrosine residues following lymphocyte activation, allowing them to function as scaffolds for the assembly of multi-molecular signaling complexes. Among the many TRAPs that have been discovered in recent years, the LAT (linker for activation of T cells) family of adaptor proteins plays an important role in the positive and negative regulation of lymphocyte maturation, activation, and differentiation. Of the two members in this family, LAT is an indispensable component controlling T cell and mast cell activation and function; LAB (linker for activation of B cells), also called NTAL, is necessary to fine-tune lymphocyte activation and may be a key regulator of innate immune responses. Here, we review recent advances on the function of LAT and LAB in the regulation of development and activation of immune cells.

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.

Presenilin1/gamma-secretase promotes the EphB2-induced phosphorylation of ephrinB2 by regulating phosphoprotein associated with glycosphingolipid-enriched microdomains/Csk binding protein

Reverse signaling through the ephrinB ligands is important for several morphogenetic events, such as axon guidance, neuronal plasticity, spine maturation, and synaptogenesis. Signaling is initiated by binding of EphB receptors to ephrinB ligands, stimulating their tyrosine phosphorylation via an unclear mechanism. Here we show that this mechanism involves presenilin1 (PS1)/γ-secretase regulation of phosphoprotein associated with glycosphingolipid-enriched microdomains/Csk binding protein (PAG/Cbp), an adaptor protein that controls the activity of Src kinases. Using immunoprecipitation and Western blot of mouse primary neuronal and human embryonic kidney (HEK293) cell extracts overexpressing PAG/Cbp, we show that EphB2 induces tyrosine dephosphorylation of PAG/Cbp in a γ-secretase-dependent manner. In these cells, PAG/Cbp dephosphorylation is promoted by the PS1/γ-secretase-produced fragment of ephrinB2 cleavage (ephrinB2/CTF2), which forms complexes with PAG/Cbp when introduced exogenously. EphB2-induced tyrosine phosphorylation of ephrinB2 depends on PAG/Cbp because EphB2 cannot increase ephrinB2 phosphorylation in cells treated with anti-PAG siRNA or in PAG/Cbp-knockout (KO) cells. Furthermore, in contrast to WT PS1, familial Alzheimer disease (FAD) PS1 mutants expressed in PS1-KO mouse embryonic fibroblasts inhibited both the EphB2-induced dephosphorylation of PAG/Cbp and the phosphorylation of ephrinB2. PS1 FAD mutations may thus inhibit the function of ephrinB in the brain, promoting neurodegeneration in Alzheimer disease.

A new pathway of CD5 glycoprotein-mediated T cell inhibition dependent on inhibitory phosphorylation of Fyn kinase

Triggering of the T cell receptor initiates a signaling cascade resulting in the activation of the T cell. These signals are integrated alongside those resulting from the triggering of other receptors whose function is to modulate the overall response. CD5 is an immunotyrosine-based inhibition motif-bearing receptor that antagonizes the overt T cell receptor activation response by recruiting inhibitory intracellular mediators such as SHP-1, RasGAP, or Cbl. We now propose that the inhibitory effects of CD5 are also mediated by a parallel pathway that functions at the level of inhibition of Fyn, a kinase generally associated with T cell receptor-mediated activation. After CD5 ligation, phosphorylation of the negative regulatory tyrosine (Tyr(531)) of Fyn increases, and this correlates with a substantial reduction in the kinase activity of Fyn and a profound inhibition of ZAP-70 activation. The effect requires the last 23 amino acids of the cytoplasmic domain of the receptor, strongly implying the involvement of a new CD5-interacting signaling or adaptor protein. Furthermore, we show that upon CD5 ligation there is a profound shift in its distribution from the bulk fluid phase to the lipid raft environment, where it associates with Fyn, Lck, and PAG. We suggest that the relocation of CD5, which we also show is capable of forming homodimers, to the proximity of raft-resident molecules enables CD5 to inhibit membrane proximal signaling by controlling the phosphorylation and activity of Fyn, possibly by interfering with the disassembly of C-terminal Src kinase (Csk)-PAG-Fyn complexes during T cell activation.

Down-regulation of the tumor suppressor C-terminal Src kinase (Csk)-binding protein (Cbp)/PAG1 is mediated by epigenetic histone modifications via the mitogen-activated protein kinase (MAPK)/phosphatidylinositol 3-kinase (PI3K) pathway

The transmembrane adaptor protein Cbp (or PAG1) functions as a suppressor of Src-mediated tumor progression by promoting the inactivation of Src. The expression of Cbp is down-regulated in Src-transformed cells and in various human cancer cells, suggesting a potential role for Cbp as a tumor suppressor. However, the mechanisms underlying the down-regulation of Cbp remain unknown. The present study shows that Cbp expression is down-regulated by epigenetic histone modifications via the MAPK/PI3K pathway. In mouse embryonic fibroblasts, transformation by oncogenic Src and Ras induced a marked down-regulation of Cbp expression. The levels of Cbp expression were inversely correlated with the activity of MEK and Akt, and Cbp down-regulation was suppressed by inhibiting MEK and PI3K. Src transformation did not affect the stability of Cbp mRNA, the transcriptional activity of the cbp promoter, or the DNA methylation status of the cbp promoter CpG islands. However, Cbp expression was restored by treatment with histone deacetylase (HDAC) inhibitors and by siRNA-mediated knockdown of HDAC1/2. Src transformation significantly decreased the acetylation levels of histone H4 and increased the trimethylation levels of histone H3 lysine 27 in the cbp promoter. EGF-induced Cbp down-regulation was also suppressed by inhibiting MEK and HDAC. Furthermore, the inhibition of MEK or HDAC restored Cbp expression in human cancer cells harboring Cbp down-regulation through promoter hypomethylation. These findings suggest that Cbp down-regulation is primarily mediated by epigenetic histone modifications via oncogenic MAPK/PI3K pathways in a subset of cancer cells.

Cyclic AMP-mediated immune regulation--overview of mechanisms of action in T cells

The canonical second messenger cAMP is well established as a potent negative regulator of T cell immune function. Through protein kinase A (PKA) it regulates T cell function at the level of transcription factors, members of the mitogen-activated protein kinase pathway, phospholipases (PLs), Ras homolog (Rho)A and proteins involved in the control of cell cycle progression. Type I PKA is the predominant PKA isoform in T cells. Furthermore, whereas type II PKA is located at the centrosome, type I PKA is anchored close to the T cell receptor (TCR) in lipid rafts by the Ezrin-ERM-binding phosphoprotein of 50 kDa (EBP50)-phosphoprotein associated with glycosphingolipid-enriched microdomains (PAG) scaffold complex. The most TCR-proximal target for type I PKA is C-terminal Src kinase (Csk), which upon activation by raft recruitment and phosphorylation inhibits the Src family tyrosine kinases Lck and Fyn and thus functions to maintain T cell homeostasis. Recently, induction of cAMP levels in responder T cells has emerged as one of the mechanisms by which regulatory T (T(R)) cells execute their suppressive action. Thus, the cAMP-type I PKA-Csk pathway emerges as a putative target for therapeutic intervention in autoimmune disorders as well as in cancer, where T(R) cell-mediated suppression contributes to suboptimal local immune responses.

Quantitative phosphoproteomic analysis of T cell receptor signaling in diabetes prone and resistant mice

Type 1 diabetes, in human patients and NOD mice, results from an immune attack on insulin-producing beta-cells of the pancreas by autoreactive T lymphocytes. In NOD mice, genetically controlled perturbations in the signaling pathways downstream of the antigen-specific T cell receptor (TCR) may be instrumental in the altered responses of T cells, manifest as inefficient induction of apoptosis after recognition of self-antigens in the thymus or as perturbed reactivity of mature T cells in peripheral organs. To map this signaling difference(s), we have used mass spectrometry-based quantitative phosphoproteomics to compare the activation of primary CD4(+) T cells of diabetes-prone NOD and -resistant B6.H2g7 mice. Immunoprecipitation and IMAC purification of tyrosine-phosphorylated peptides, combined with a stable-isotope iTRAQ labeling, enabled us to identify and quantify over 77 phosphorylation events in 54 different proteins downstream of TCR stimulation of primary CD4(+) T cells. This analysis showed a generally higher level of phosphotyrosine in activated NOD cells, as well as several phosphorylation sites that appeared to be differentially regulated in these two strains (involving TXK, CD5, PAG1, and ZAP-70). These data highlight the differences in signaling between CD4(+) T cell compartments of NOD and B6g7 mice and may underlie the dysregulation of T cells in NOD mice.

Regulation of lymphocyte development and activation by the LAT family of adapter proteins

Transmembrane adapter proteins (TRAPs) are critical components of signaling pathways in lymphocytes, linking antigen receptor engagement to downstream cellular processes. While these proteins lack intrinsic enzymatic activity, their phosphorylation following receptor ligation allows them to function as scaffolds for the assembly of multi-molecular signaling complexes. Many TRAPs have recently been discovered, and numerous studies demonstrate their roles in the positive and negative regulation of lymphocyte maturation, activation, and differentiation. One such example is the linker for activation of T cells (LAT) family of adapter proteins. While LAT has been shown to play an indispensable role in T-cell and mast cell function, the other family members, linker for activation of B cells (LAB) and linker for activation of X cells (LAX), are necessary to fine-tune immune responses. In addition to its well-established role in the positive regulation of lymphocyte activation, LAT exerts an inhibitory effect on T-cell receptor-mediated signaling. Furthermore, LAT, along with LAB and LAX, plays a crucial role in establishing and maintaining tolerance. Here, we review recent data concerning the regulation of lymphocyte development and activation by the LAT family of proteins.

Lymphoid tyrosine phosphatase and autoimmunity: human genetics rediscovers tyrosine phosphatases

A relatively large number of protein tyrosine phosphatases (PTPs) are known to regulate signaling through the T cell receptor (TCR). Recent human genetics studies have shown that several of these PTPs are encoded by major autoimmunity genes. Here, we will focus on the lymphoid tyrosine phosphatase (LYP), a critical negative modulator of TCR signaling encoded by the PTPN22 gene. The functional analysis of autoimmune-associated PTPN22 genetic variants suggests that genetic variability of TCR signal transduction contributes to the pathogenesis of autoimmunity in humans.

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.

Modulation of immune cell signalling by the leukocyte common tyrosine phosphatase, CD45

CD45 is a leukocyte specific transmembrane glycoprotein and a receptor-like protein tyrosine phosphatase (PTP). CD45 can be expressed as several alternatively spliced isoforms that differ in the extracellular domain. The isoforms are regulated in a cell type and activation state-dependent manner, yet their function has remained elusive. The Src family kinase members Lck and Lyn are key substrates for CD45 in T and B lymphocytes, respectively. CD45 lowers the threshold of antigen receptor signalling, which impacts T and B cell activation and development. CD45 also regulates antigen triggered Fc receptor signalling in mast cells and Toll-like receptor (TLR) signalling in dendritic cells, thus broadening the role of CD45 to other recognition receptors involved in adaptive and innate immunity. In addition, CD45 can affect immune cell adhesion and migration and can modulate cytokine production and signalling. Here we review what is known about the substrate specificity and regulation of CD45 and summarise its effect on immune cell signalling pathways, from its established role in T and B antigen receptor signalling to its emerging role regulating innate immune cell recognition and cytokine production.

Cannabinoid receptor type 1- and 2-mediated increase in cyclic AMP inhibits T cell receptor-triggered signaling

The aim of this study was to characterize inhibitory mechanisms on T cell receptor signaling mediated by the cannabinoid receptors CB1 and CB2. Both receptors are coupled to G(i/o) proteins, which are associated with inhibition of cyclic AMP formation. In human primary and Jurkat T lymphocytes, activation of CB1 by R(+)-methanandamide, CB2 by JWH015, and both by Delta9-tetrahydrocannabinol induced a short decrease in cyclic AMP lasting less than 1 h. However, this decrease was followed by a massive (up to 10-fold) and sustained (at least up to 48 h) increase in cyclic AMP. Mediated by the cyclic AMP-activated protein kinase A and C-terminal Src kinase, the cannabinoids induced a stable phosphorylation of the inhibitory Tyr-505 of the leukocyte-specific protein tyrosine kinase (Lck). By thus arresting Lck in its inhibited form, the cannabinoids prevented the dephosphorylation of Lck at Tyr-505 in response to T cell receptor activation, which is necessary for the subsequent initiation of T cell receptor signaling. In this way the cannabinoids inhibited the T cell receptor-triggered signaling, i.e. the activation of the zeta-chain-associated protein kinase of 70 kDa, the linker for activation of T cells, MAPK, the induction of interleukin-2, and T cell proliferation. All of the effects of the cannabinoids were blocked by the CB1 and CB2 antagonists AM281 and AM630. These findings help to better understand the immunosuppressive effects of cannabinoids and explain the beneficial effects of these drugs in the treatment of T cell-mediated autoimmune disorders like multiple sclerosis.

Cholesterol-binding toxins and anti-cholesterol antibodies as structural probes for cholesterol localization

Cholesterol is one of the major constituents of mammalian cell membranes. It plays an indispensable role in regulating the structure and function of cell membranes and affects the pathology of various diseases. In recent decades much attention has been paid to the existence of membrane microdomains, generally termed lipid "rafts", and cholesterol, along with sphingolipids, is thought to play a critical role in raft structural organization and function. Cholesterol-binding probes are likely to provide useful tools for analyzing the distribution and dynamics of membrane cholesterol, as a structural element of raft microdomains, and elsewhere within the cell. Among the probes, non-toxic derivatives of perfringolysin O, a cholesterol-binding cytolysin, bind cholesterol in a concentration-dependent fashion with a strict threshold. They selectively recognize cholesterol in cholesterol-enriched membranes, and have been used in many studies to detect microdomains in plasma and intracellular membranes. Anti-cholesterol antibodies that recognize cholesterol in domain structures have been developed in recent years. In this chapter, we describe the characteristics of these cholesterol-binding proteins and their applications to studies on membrane cholesterol localization.

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

RATIONALE

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

OBJECTIVE

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

METHODS AND RESULTS

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

CONCLUSIONS

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

A new approach for quantitative phosphoproteomic dissection of signaling pathways applied to T cell receptor activation

Reversible protein phosphorylation plays a pivotal role in the regulation of cellular signaling pathways. Current approaches in phosphoproteomics focus on analysis of the global phosphoproteome in a single cellular state or of receptor stimulation time course experiments, often with a restricted number of time points. Although these studies have provided some insights into newly discovered phosphorylation sites that may be involved in pathways, they alone do not provide enough information to make precise predictions of the placement of individual phosphorylation events within a signaling pathway. Protein disruption and site-directed mutagenesis are essential to clearly define the precise biological roles of the hundreds of newly discovered phosphorylation sites uncovered in modern proteomics experiments. We have combined genetic analysis with quantitative proteomic methods and recently developed visual analysis tools to dissect the tyrosine phosphoproteome of isogenic Zap-70 tyrosine kinase null and reconstituted Jurkat T cells. In our approach, label-free quantitation using normalization to copurified phosphopeptide standards is applied to assemble high density temporal data within a single cell type, either Zap-70 null or reconstituted cells, providing a list of candidate phosphorylation sites that change in abundance after T cell stimulation. Stable isotopic labeling of amino acids in cell culture (SILAC) ratios are then used to compare Zap-70 null and reconstituted cells across a time course of receptor stimulation, providing direct information about the placement of newly observed phosphorylation sites relative to Zap-70. These methods are adaptable to any cell culture signaling system in which isogenic wild type and mutant cells have been or can be derived using any available phosphopeptide enrichment strategy.

Cytoskeleton-membrane interactions in membrane raft structure

Cell membranes are structurally heterogeneous, composed of discrete domains with unique physical and biological properties. Membrane domains can form through a number of mechanisms involving lipid-lipid and protein-lipid interactions. One type of membrane domain is the cholesterol-dependent membrane raft. How rafts form remains a current topic in membrane biology. We review here evidence of structuring of rafts by the cortical actin cytoskeleton. This includes evidence that the actin cytoskeleton associates with rafts, and that many of the structural and functional properties of rafts require an intact actin cytoskeleton. We discuss the mechanisms of the actin-dependent raft organization, and the properties of the actin cytoskeleton in regulating raft-associated signaling events. We end with a discussion of membrane rafts and the actin cytoskeleton in T cell activation, which function synergistically to initiate the adaptive immune response.

Sequential cooperation of CD2 and CD48 in the buildup of the early TCR signalosome

The buildup of TCR signaling microclusters containing adaptor proteins and kinases is prerequisite for T cell activation. One hallmark in this process is association of the TCR with lipid raft microdomains enriched in GPI-proteins that have potential to act as accessory molecules for TCR signaling. In this study, we show that GPI-anchored CD48 but not CD59 was recruited to the immobilized TCR/CD3 complex upon activation of T cells. CD48 reorganization was vital for T cell IL-2 production by mediating lateral association of the early signaling component linker for activated T cells (LAT) to the TCR/CD3 complex. Furthermore, we identified CD2 as an adaptor linking the Src protein tyrosine kinase Lck and the CD48/LAT complex to TCR/CD3: CD2 associated with TCR/CD3 upon T cell activation irrespective of CD48 expression, while association of CD48 and LAT with the TCR/CD3 complex depended on CD2. Consequently, our data indicate that CD2 and CD48 cooperate hierarchically in the buildup of the early TCR signalosome; CD2 functions as the master switch recruiting CD48 and Lck. CD48 in turn shuttles the transmembrane adapter molecule LAT.

CD45, CD148, and Lyp/Pep: critical phosphatases regulating Src family kinase signaling networks in immune cells

Reciprocal regulation of tyrosine phosphorylation by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) is central to normal immune cell function. Disruption of the equilibrium between PTK and PTP activity can result in immunodeficiency, autoimmunity, or malignancy. Src family kinases (SFKs) play a central role in both immune cell function and disease due to their proximal position in numerous signal transduction cascades including those emanating from integrin, T and B-cell antigen receptors, Fc, growth factor, and cytokine receptors. Given that tight regulation of SFKs activity is critical for appropriate responses to stimulation of these various signaling pathways, it is perhaps not surprising that multiple PTPs are involved in their regulation. Here, we focus on the role of three phosphatases, CD45, CD148, and LYP/PEP, which are critical regulators of SFKs in hematopoietic cells. We review our current understanding of their structures, expression, functions in different hematopoietic cell subsets, regulation, and putative roles in disease. Finally, we discuss remaining questions that must be addressed if we are to have a clearer understanding of the coordinated regulation of tyrosine phosphorylation and signaling networks in hematopoietic cells and how they could potentially be manipulated therapeutically in disease.

Signaling defects in anti-tumor T cells

The immune response to cancer has been long recognized, including both innate and adaptive responses, showing that the immune system can recognize protein products of genetic and epigenetic changes in transformed cells. The accumulation of antigen-specific T cells within the tumor, the draining lymph node, and the circulation, either in newly diagnosed patients or resultant from experimental immunotherapy, proves that tumors produce antigens and that priming occurs. Unfortunately, just as obviously, tumors grow, implying that anti-tumor immune responses are either not sufficiently vigorous to eliminate the cancer or that anti-tumor immunity is suppressed. Both possibilities are supported by current data. In experimental animal models of cancer and also in patients, systemic immunity is usually not dramatically suppressed, because tumor-bearing animals and patients develop T-cell-dependent immune responses to microbes and to either model antigens or experimental cancer vaccines. However, inhibition of specific anti-tumor immunity is common, and several possible explanations of tolerance to tumor antigens or tumor-induced immunesuppression have been proposed. Inhibition of effective anti-tumor immunity results from the tumor or the host response to tumor growth, inhibiting the activation, differentiation, or function of anti-tumor immune cells. As a consequence, anti-tumor T cells cannot respond productively to developmental, targeting, or activation cues. While able to enhance the number and phenotype of anti-tumor T cells, the modest success of immunotherapy has shown the necessity to attempt to reverse tolerance in anti-tumor T cells, and the vanguard of experimental therapy now focuses on vaccination in combination with blockade of immunosuppressive mechanisms. This review discusses several potential mechanisms by which anti-tumor T cells may be inhibited in function.

Csk-binding protein can regulate Lyn signals controlling cell morphology

The Src family kinase Lyn is involved in differentiation signals emanating from activated erythropoietin (Epo) receptors, it interacts with COOH-terminal Src kinase-binding protein (Cbp), an adaptor protein that recruits negative regulators COOH-terminal Src kinase (Csk) and suppressor of cytokine signaling-1 (SOCS1). Lyn phosphorylates Cbp on several tyrosine residues, including Tyr314, which recruits Csk/SOCS1, as well as Tyr381 and Tyr409 that bind Lyns own SH2 domain. We show that Cbp alters not only the ability of erythroid cells to differentiate but also their colony morphology. Consequently, we detailed the ability of Cbp to interact with and influence Lyns ability to initiate changes in cellular architecture, which affect cell-cell and cell-substratum interactions. Over-expression of active Lyn promotes filopodia formation while inactive Lyn promotes lamellipodia formation. Conversely, Cbp over-expression, which inhibits Lyn activity, promotes lamellipodia formation, while Cbp mutants preventing its interaction/signaling consequently allow Lyn to promote filopodia formation. Thus, the Lyn-Cbp pathway and subsequent regulation of Lyn signaling and cell morphology involves a dynamic and complex series of interactions.

Lipid rafts and T-lymphocyte function: implications for autoimmunity

Experimental evidence indicates that the mammalian cell membrane is compartmentalized. A structural feature that supports membrane segmentation implicates assemblies of selected lipids broadly referred to as lipid rafts. In T-lymphocytes, lipid rafts are implicated in signalling from the T-cell antigen receptor (TCR) and in localization and function of proteins residing proximal to the receptor. This review summarizes the current literature that deals with lipid raft involvement in T-cell activation and places particular emphasis in recent studies investigating lipid rafts in autoimmunity. The potential of lipid rafts as targets for the development of a new class of immune-modulating compounds is discussed.

Normal development and activation but altered cytokine production of Fyn-deficient CD4+ T cells

The Src family kinase Fyn is expressed in T cells and has been shown to phosphorylate proteins involved in TCR signaling, cytoskeletal reorganization, and IL-4 production. Fyn-deficient mice have greatly decreased numbers of NKT cells and have thymocytes and T cells with compromised responses following Ab crosslinking of their TCRs. Herein we have addressed the role of Fyn in peptide/MHC class II-induced CD4(+) T cell responses. In Fyn-deficient mice, CD4(+) T cells expressing the DO11.10 TCR transgene developed normally, and the number and phenotype of naive and regulatory DO11.10(+)CD4(+) T cells in the periphery were comparable with their wild-type counterparts. Conjugation with chicken OVA peptide 323-339-loaded APCs, and the subsequent proliferation in vitro or in vivo of DO11.10(+) Fyn-deficient CD4(+) T cells, was virtually indistinguishable from the response of DO11.10(+) wild-type CD4(+) T cells. Proliferation of Fyn-deficient T cells was not more dependent on costimulation through CD28. Additionally, we have found that differentiation, in vitro or in vivo, of transgenic CD4(+) Fyn-deficient T cells into IL-4-secreting effector cells was unimpaired, and under certain conditions DO11.10(+) Fyn-deficient CD4(+) T cells were more potent cytokine-producing cells than DO11.10(+) wild-type CD4(+) T cells. These data demonstrate that ablation of Fyn expression does not alter most Ag-driven CD4(+) T cell responses, with the exception of cytokine production, which under some circumstances is enhanced in Fyn-deficient CD4(+) T cells.

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.

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.

Protein tyrosine phosphatases in autoimmunity

Protein tyrosine phosphatases (PTPs) are important regulators of many cellular functions and a growing number of PTPs have been implicated in human disease conditions, such as developmental defects, neoplastic disorders, and immunodeficiency. Here, we review the involvement of PTPs in human autoimmunity. The leading examples include the allelic variant of the lymphoid tyrosine phosphatase (PTPN22), which is associated with multiple autoimmune diseases, and mutations that affect the exon-intron splicing of CD45 (PTPRC). We also find it likely that additional PTPs are involved in susceptibility to autoimmune and inflammatory diseases. Finally, we discuss the possibility that PTPs regulating the immune system may serve as therapeutic targets.

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).

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.

SRC family kinases and receptors: analysis of three activation mechanisms by dynamic systems modeling

Src family kinases (SFKs) interact with a number of cellular receptors. They participate in diverse signaling pathways and cellular functions. Most of the receptors involved in SFK signaling are characterized by similar modes of regulation. This computational study discusses a general kinetic model of SFK-receptor interaction. The analysis of the model reveals three major ways of SFK activation: release of inhibition by C-terminal Src kinase, weakening of the inhibitory intramolecular phosphotyrosine-SH2 interaction, and amplification of a stimulating kinase activity. The SFK model was then extended to simulate interaction with growth factor and T-cell receptors. The modular SFK signaling system was shown to adapt to the requirements of specific signaling contexts and yield qualitatively different responses in the different simulated environments. The model also provides a systematic overview of the major interactions between SFKs and various cellular signaling systems and identifies their common properties.

Glycerophosphoinositol-4-phosphate enhances SDF-1alpha-stimulated T-cell chemotaxis through PTK-dependent activation of Vav

Glycerophosphoinositols (GPIs) are water-soluble phosphoinosite metabolites produced by all cell types, whose levels increase in response to a variety of extracellular stimuli, and are particularly high in Ras-transformed cells. GPIs are released to the extracellular space, wherefrom they can be taken up by other cells through a specific transporter. Exogenous GPIs affect a plethora of cellular functions. Among these compounds the most active is GroPIns4P, which affects cAMP levels and PKA-dependent functions through the inhibition of heterotrimeric Gs proteins. GroPIns4P has also recently been found to promote actin cytoskeleton reorganization by inducing Rho and Rac activation through an as yet unidentified mechanism. Here we have assessed the potential effects of GroPIns4P on T-cells. We found that GroPIns4P enhances CXCR4-dependent chemotaxis. This activity results from the capacity of GroPIns4P to activate the Rho GTPase exchange factor, Vav, through an Lck-dependent pathway which also results in activation of the stress kinases JNK and p38. GroPIns4P was also found to activate with a delayed kinetics the Lck-dependent activation of ZAP-70, Shc and Erk1/2. The activities of GroPIns4P were found to be dependent on its capacity to inhibit cAMP production and PKA activation. Collectively, the data provide the first evidence of a role of glycerophosphoinositols as modulators of T-cell signaling and establish a mechanistic basis for the effects of this phosphoinositide derivative on F-actin dynamics.

Clustering of membrane raft proteins by the actin cytoskeleton

Cell membranes are laterally organized into functionally discrete domains that include the cholesterol-dependent membrane "rafts." However, how membrane domains are established and maintained remains unresolved and controversial but often requires the actin cytoskeleton. In this study, we used fluorescence resonance energy transfer to measure the role of the actin cytoskeleton in the co-clustering of membrane raft-associated fluorescent proteins (FPs) and FPs targeted to the nonraft membrane fraction. By fitting the fluorescence resonance energy transfer data to an isothermal binding equation, we observed a specific co-clustering of raft-associated donor and acceptor probes that was sensitive to latrunculin B (Lat B), which disrupts the actin cytoskeleton. Conversely, treating with jasplakinolide to enhance actin polymerization increased co-clustering of the raft-associated FPs over that of the nonraft probes. We also observed by immunoblotting experiments that the actin-dependent co-clustering coincided with regulation of the raft-associated Src family kinase Lck. Specifically, Lat B decreased the phosphorylation of the C-terminal regulatory tyrosine of Lck (Tyr505), and combining the Lat B with filipin further decreased the Tyr505 phosphorylation. Furthermore, the Lat B-dependent changes in Lck regulation required CD45 because no significant changes occurred in treated T cells lacking CD45 expression. These data define a role for the actin cytoskeleton in promoting co-clustering of raft-associated proteins and show that this property is important toward regulating raft-associated signaling proteins such as Lck.

Lipid rafts in T cell signalling and disease

Lipid rafts is a blanket term used to describe distinct areas in the plasma membrane rich in certain lipids and proteins and which are thought to perform diverse functions. A large number of studies report on lipid rafts having a key role in receptor signalling and activation of lymphocytes. In T cells, lipid raft involvement was demonstrated in the early steps during T cell receptor (TCR) stimulation. Interestingly, recent evidence has shown that signalling in these domains differs in T cells isolated from patients with autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Here, we discuss these findings and explore the potential of lipid rafts as targets for the development of a new class of agents to downmodulate immune responses and for the treatment of autoimmune diseases.

Fyn regulates the duration of TCR engagement needed for commitment to effector function

In naive T cells, engagement of the TCR with agonist peptide:MHC molecules leads to phosphorylation of key intracellular signaling intermediates within seconds and this peaks within minutes. However, the cell does not commit to proliferation and IL-2 cytokine production unless receptor contact is sustained for several hours. The biochemical basis for this transition to full activation may underlie how T cells receive survival signals while maintaining tolerance, and is currently not well understood. We show here that for CD8 T cells commitment to proliferation and cytokine production requires sustained activation of the Src family kinase Lck and is opposed by the action of Fyn. Thus, in the absence of Fyn, commitment to activation occurs more rapidly, the cells produce more IL-2, and undergo more rounds of division. Our data demonstrate a role for Fyn in modulating the response to Ag in primary T cells.