Interaction of Shc with the zeta chain of the T cell receptor upon T cell activation

pYtags enable spatiotemporal measurements of receptor tyrosine kinase signaling in living cells

Receptor tyrosine kinases (RTKs) are major signaling hubs in metazoans, playing crucial roles in cell proliferation, migration, and differentiation. However, few tools are available to measure the activity of a specific RTK in individual living cells. Here, we present pYtags, a modular approach for monitoring the activity of a user-defined RTK by live-cell microscopy. pYtags consist of an RTK modified with a tyrosine activation motif that, when phosphorylated, recruits a fluorescently labeled tandem SH2 domain with high specificity. We show that pYtags enable the monitoring of a specific RTK on seconds-to-minutes time scales and across subcellular and multicellular length scales. Using a pYtag biosensor for epidermal growth factor receptor (EGFR), we quantitatively characterize how signaling dynamics vary with the identity and dose of activating ligand. We show that orthogonal pYtags can be used to monitor the dynamics of EGFR and ErbB2 activity in the same cell, revealing distinct phases of activation for each RTK. The specificity and modularity of pYtags open the door to robust biosensors of multiple tyrosine kinases and may enable engineering of synthetic receptors with orthogonal response programs.

Bridging the Gap: Modulatory Roles of the Grb2-Family Adaptor, Gads, in Cellular and Allergic Immune Responses

Antigen receptor signaling pathways are organized by adaptor proteins. Three adaptors, LAT, Gads, and SLP-76, form a heterotrimeric complex that mediates signaling by the T cell antigen receptor (TCR) and by the mast cell high affinity receptor for IgE (FcεRI). In both pathways, antigen recognition triggers tyrosine phosphorylation of LAT and SLP-76. The recruitment of SLP-76 to phospho-LAT is bridged by Gads, a Grb2 family adaptor composed of two SH3 domains flanking a central SH2 domain and an unstructured linker region. The LAT-Gads-SLP-76 complex is further incorporated into larger microclusters that mediate antigen receptor signaling. Gads is positively regulated by dimerization, which promotes its cooperative binding to LAT. Negative regulation occurs via phosphorylation or caspase-mediated cleavage of the linker region of Gads. FcεRI-mediated mast cell activation is profoundly impaired in LAT- Gads- or SLP-76-deficient mice. Unexpectedly, the thymic developmental phenotype of Gads-deficient mice is much milder than the phenotype of LAT- or SLP-76-deficient mice. This distinction suggests that Gads is not absolutely required for TCR signaling, but may modulate its sensitivity, or regulate a particular branch of the TCR signaling pathway; indeed, the phenotypic similarity of Gads- and Itk-deficient mice suggests a functional connection between Gads and Itk. Additional Gads binding partners include costimulatory proteins such as CD28 and CD6, adaptors such as Shc, ubiquitin regulatory proteins such as USP8 and AMSH, and kinases such as HPK1 and BCR-ABL, but the functional implications of these interactions are not yet fully understood. No interacting proteins or function have been ascribed to the evolutionarily conserved N-terminal SH3 of Gads. Here we explore the biochemical and functional properties of Gads, and its role in regulating allergy, T cell development and T-cell mediated immunity.

Bridging the Gap: Modulatory Roles of the Grb2-Family Adaptor, Gads, in Cellular and Allergic Immune Responses

Antigen receptor signaling pathways are organized by adaptor proteins. Three adaptors, LAT, Gads, and SLP-76, form a heterotrimeric complex that mediates signaling by the T cell antigen receptor (TCR) and by the mast cell high affinity receptor for IgE (FcεRI). In both pathways, antigen recognition triggers tyrosine phosphorylation of LAT and SLP-76. The recruitment of SLP-76 to phospho-LAT is bridged by Gads, a Grb2 family adaptor composed of two SH3 domains flanking a central SH2 domain and an unstructured linker region. The LAT-Gads-SLP-76 complex is further incorporated into larger microclusters that mediate antigen receptor signaling. Gads is positively regulated by dimerization, which promotes its cooperative binding to LAT. Negative regulation occurs via phosphorylation or caspase-mediated cleavage of the linker region of Gads. FcεRI-mediated mast cell activation is profoundly impaired in LAT- Gads- or SLP-76-deficient mice. Unexpectedly, the thymic developmental phenotype of Gads-deficient mice is much milder than the phenotype of LAT- or SLP-76-deficient mice. This distinction suggests that Gads is not absolutely required for TCR signaling, but may modulate its sensitivity, or regulate a particular branch of the TCR signaling pathway; indeed, the phenotypic similarity of Gads- and Itk-deficient mice suggests a functional connection between Gads and Itk. Additional Gads binding partners include costimulatory proteins such as CD28 and CD6, adaptors such as Shc, ubiquitin regulatory proteins such as USP8 and AMSH, and kinases such as HPK1 and BCR-ABL, but the functional implications of these interactions are not yet fully understood. No interacting proteins or function have been ascribed to the evolutionarily conserved N-terminal SH3 of Gads. Here we explore the biochemical and functional properties of Gads, and its role in regulating allergy, T cell development and T-cell mediated immunity.

Lck promotes Zap70-dependent LAT phosphorylation by bridging Zap70 to LAT

T cell-antigen receptor (TCR) signaling requires the sequential activities of the kinases Lck and Zap70. Upon TCR stimulation, Lck phosphorylates the TCR, thus leading to the recruitment, phosphorylation, and activation of Zap70. Lck binds and stabilizes phosho-Zap70 by using its SH2 domain, and Zap70 phosphorylates the critical adaptors LAT and SLP76, which coordinate downstream signaling. It is unclear whether phosphorylation of these adaptors occurs through passive diffusion or active recruitment. We report the discovery of a conserved proline-rich motif in LAT that mediates efficient LAT phosphorylation. Lck associates with this motif via its SH3 domain, and with phospho-Zap70 via its SH2 domain, thereby acting as a molecular bridge that facilitates the colocalization of Zap70 and LAT. Elimination of this proline-rich motif compromises TCR signaling and T cell development. These results demonstrate the remarkable multifunctionality of Lck, wherein each of its domains has evolved to orchestrate a distinct step in TCR signaling.

Crk adaptor proteins regulate CD3ζ chain phosphorylation and TCR/CD3 down-modulation in activated T cells

T cell receptor (TCR) recognition of a peptide antigen in the context of MHC molecules initiates positive and negative cascades that regulate T cell activation, proliferation and differentiation, and culminate in the acquisition of effector T cell functions. These processes are a prerequisite for the induction of specific T cell-mediated adaptive immune responses. A key event in the activation of TCR-coupled signaling pathways is the phosphorylation of tyrosine residues within the cytoplasmic tails of the CD3 subunits, predominantly CD3ζ. These transiently formed phosphotyrosyl epitopes serve as docking sites for SH2-domain containing effector molecules, predominantly the ZAP70 protein tyrosine kinase, which is critical for signal propagation. We found that CrkI and CrkII adaptor proteins also interact with CD3ζ in TCR activated-, but not in resting-, T cells. Crk binding to CD3ζ was independent of ZAP70 and also occurred in ZAP70-deficient T cells. Binding was mediated by Crk-SH2 domain interaction with phosphotyrosine-containing motifs on CD3ζ, via a direct physical interaction, as demonstrated by Far-Western blot. CrkII binding to CD3ζ could also be demonstrated in a heterologous system, where coexpression of a catalytically active Lck was used to phosphorylate the CD3ζ chain. TCR activation-induced Crk binding to CD3ζ resulted in increased and prolonged phosphorylation of CD3ζ, as well as ZAP70 and LAT, suggesting a positive role for CrkI/II binding to CD3ζ in regulation of TCR-coupled signaling pathways. Furthermore, Crk-dependent increased phosphorylation of CD3ζ coincided with inhibition of TCR downmodulation, supporting a positive role for Crk adaptor proteins in TCR-mediated signal amplification.

SHC1 sensitizes cancer cells to the 8-Cl-cAMP treatment

8-Chloro-cyclic AMP (8-Cl-cAMP) is a cyclic AMP analog that induces growth inhibition and apoptosis in a broad spectrum of cancer cells. Previously, we found that 8-Cl-cAMP-induced growth inhibition is mediated by AMP-activated protein kinase (AMPK) as well as p38 mitogen-activated protein kinase (p38 MAPK). To identify downstream mediators of the 8-Cl-cAMP signaling, we performed co-immunoprecipitation combined with mass spectrometry using the anti-AMPK or p38 MAPK antibodies. Through this approach, SHC1 was identified as one of the binding partners of p38 MAPK. SHC1 phosphorylation was suppressed by 8-Cl-cAMP in HeLa and MCF7 cancer cells, which was mediated by its metabolites, 8-Cl-adenosine and 8-Cl-ATP; however, 8-Cl-cAMP showed no effect on SHC1 phosphorylation in normal human fibroblasts. SHC1 siRNA induced AMPK and p38 MAPK phosphorylation and growth inhibition in cancer cells, and SHC1 overexpression re-sensitized human foreskin fibroblasts to the 8-Cl-cAMP treatment. SHC1 phosphorylation was unaffected by Compound C (an AMPK inhibitor) and SB203580 (a p38 MAPK inhibitor), which suggests that SHC1 is upstream of AMPK and p38 MAPK in the 8-Cl-cAMP-stimulated signaling cascade. On the basis of these findings, we conclude that SHC1 functions as a sensor during the 8-Cl-cAMP-induced growth inhibition in SHC1-overexpressing cancer cells.

The catalytic activity of the kinase ZAP-70 mediates basal signaling and negative feedback of the T cell receptor pathway

T cell activation by antigens binding to the T cell receptor (TCR) must be properly regulated to ensure normal T cell development and effective immune responses to pathogens and transformed cells while avoiding autoimmunity. The Src family kinase Lck and the Syk family kinase ZAP-70 (ζ chain-associated protein kinase of 70 kD) are sequentially activated in response to TCR engagement and serve as critical components of the TCR signaling machinery that leads to T cell activation. We performed a mass spectrometry-based phosphoproteomic study comparing the quantitative differences in the temporal dynamics of phosphorylation in stimulated and unstimulated T cells with or without inhibition of ZAP-70 catalytic activity. The data indicated that the kinase activity of ZAP-70 stimulates negative feedback pathways that target Lck and thereby modulate the phosphorylation patterns of the immunoreceptor tyrosine-based activation motifs (ITAMs) of the CD3 and ζ chain components of the TCR and of signaling molecules downstream of Lck, including ZAP-70. We developed a computational model that provides a mechanistic explanation for the experimental findings on ITAM phosphorylation in wild-type cells, ZAP-70-deficient cells, and cells with inhibited ZAP-70 catalytic activity. This model incorporated negative feedback regulation of Lck activity by the kinase activity of ZAP-70 and predicted the order in which tyrosines in the ITAMs of TCR ζ chains must be phosphorylated to be consistent with the experimental data.

TCR ITAM multiplicity is required for the generation of follicular helper T-cells

The T-cell antigen receptor (TCR) complex contains 10 copies of a di-tyrosine Immunoreceptor-Tyrosine-based-Activation-Motif (ITAM) that initiates TCR signalling by recruiting protein tyrosine kinases. ITAM multiplicity amplifies TCR signals, but the importance of this capability for T-cell responses remains undefined. Most TCR ITAMs (6 of 10) are contributed by the CD3ζ subunits. We generated 'knock-in' mice that express non-signalling CD3ζ chains in lieu of wild-type CD3ζ. Here we demonstrate that ITAM multiplicity is important for the development of innate-like T-cells and follicular helper T-cells, events that are known to require strong/sustained TCR-ligand interactions, but is not essential for 'general' T-cell responses including proliferation and cytokine production or for the generation of a diverse antigen-reactive TCR repertoire.

GRB2 Nucleates T Cell Receptor-Mediated LAT Clusters That Control PLC-γ1 Activation and Cytokine Production

GRB2 is a ubiquitously expressed adaptor protein required for signaling downstream of multiple receptors. To address the role of GRB2 in receptor-mediated signaling, the expression of GRB2 was suppressed in human CD4+ T cells and its role downstream of the T cell receptor (TCR) was examined. Interestingly, GRB2 deficient T cells had enhanced signaling from complexes containing the TCR. However, GRB2 deficient T cells had substantially reduced production of IL-2 and IFN-γ. This defect was attributed to diminished formation of linker for activation of T cells (LAT) signaling clusters, which resulted in reduced MAP kinase activation, calcium flux, and PLC-γ1 recruitment to LAT signaling clusters. Add back of wild-type GRB2, but not a novel N-terminal SH3 domain mutant, rescued LAT microcluster formation, calcium mobilization, and cytokine release, providing the first direct evidence that GRB2, and its ability to bind to SH3 domain ligands, is required for establishing LAT microclusters. Our data demonstrate that the ability of GRB2 to facilitate protein clusters is equally important in regulating TCR-mediated functions as its capacity to recruit effector proteins. This highlights that GRB2 regulates signaling downstream of adaptors and receptors by both recruiting effector proteins and regulating the formation of signaling complexes.

ShcA regulates thymocyte proliferation through specific transcription factors and a c-Abl-dependent signaling axis

Signaling via the pre-T-cell receptor (pre-TCR), along with associated signals from Notch and chemokine receptors, regulates the β-selection checkpoint that operates on CD4(-) CD8(-) doubly negative (DN) thymocytes. Since many hematopoietic malignancies arise at the immature developmental stages of lymphocytes, understanding the signal integration and how specific signaling molecules and distal transcription factors regulate cellular outcomes is of importance. Here, a series of molecular and genetic approaches revealed that the ShcA adapter protein critically influences proliferation and differentiation during β-selection. We found that ShcA functions downstream of the pre-TCR and p56(Lck) and show that ShcA is important for extracellular signal-regulated kinase (ERK)-dependent upregulation of transcription factors early growth factor 1 (Egr1) and Egr3 in immature thymocytes and, in turn, of the expression and function of the Id3 and E2A helix-loop-helix (HLH) proteins. ShcA also contributes to pre-TCR-mediated induction of c-Myc and additional cell cycle regulators. Moreover, using an unbiased Saccharomyces cerevisiae (yeast) screen, we identified c-Abl as a binding partner of phosphorylated ShcA and demonstrated the relevance of the ShcA-c-Abl interaction in immature thymocytes. Collectively, these data identify multiple modes by which ShcA can fine-tune the development of early thymocytes, including a previously unappreciated ShcA-c-Abl axis that regulates thymocyte proliferation.

ShcA regulates late stages of T cell development and peripheral CD4+ T cell numbers

T cell development in the thymus is a highly regulated process that critically depends upon productive signaling via the preTCR at the β-selection stage, as well as via the TCR for selection from the CD4(+)CD8(+) double-positive stage to the CD4 or CD8 single-positive stage. ShcA is an adapter protein expressed in thymocytes, and it is required for productive signaling through the preTCR, with impaired signaling via ShcA leading to a developmental block at the β-selection checkpoint. However, the role of ShcA in subsequent stages of T cell development has not been addressed. In this study, we generated transgenic mice (CD4-Cre/ShcFFF mice) that specifically express a phosphorylation-defective dominant-negative ShcA mutant (ShcFFF) in late T cell development. Thymocytes in CD4-Cre/ShcFFF mice progressed normally through the β-selection checkpoint, but displayed a significant reduction in the numbers of single-positive CD4(+) and CD8(+) thymocytes. Furthermore, CD4-Cre/ShcFFF mice, when bred with transgenic TCR mouse strains, had impaired signaling through the transgenic TCRs. Consistent with defective progression to the single-positive stage, CD4-Cre/ShcFFF mice also had significant peripheral lymphopenia. Moreover, these CD4-Cre/ShcFFF mice develop attenuated disease in CD4(+) T cell-dependent experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis. Collectively, these data identify an important role for the adapter protein ShcA in later stages of thymic T cell development and in peripheral T cell-dependent events.

Development of unique antibodies directed against each of the six different phosphotyrosine residues within the T cell receptor CD3ζ chain

Signal transduction from the T cell antigen receptor (TCR)/CD3 complex involves six different immunoreceptor tyrosine-based activation motifs (ITAM) located within the cytoplasmic tails of the CD3 chains. Each ITAM possesses two conserved tyrosine residues that can undergo phosphorylation upon TCR/CD3 crosslinking and become a docking site for SH2-containing effector molecules. Specificity of the SH2 domains is determined by their ability to bind a phosphorylated tyrosine in the context of a longer peptide motif within the target protein. As a result, phosphorylation of different tyrosines within the CD3 cytoplasmic tails creates docking sites for distinct SH2-containing signaling proteins that differentially impact on the quality of the T cell response. In the present study, we prepared antibodies specific for each of the six different phosphotyrosines of the mouse CD3ζ chain. The antibodies were characterized with respect to their cross-reactivity, ability to recognize the phosphorylated versus non-phosphorylated forms of tyrosine-containing motifs, and cross-reactivity with the homologous phospho-motifs on the human CD3ζ protein. The antibodies were found to be specific and selective for phospho-CD3ζ. They can serve as useful tools for distinguishing between the six potential tyrosine phosphorylation sites on the CD3ζ chain, and for correlating the phosphorylation of specific CD3ζ tyrosine residues with activation of signaling pathways that dictate T cell differentiation into responding, anergic, or apoptotic cells.

Positive and negative regulation of antigen receptor signaling by the Shc family of protein adapters

The Shc adapter family includes four members that are expressed as multiple isoforms and participate in signaling by a variety of cell-surface receptors. The biological relevance of Shc proteins as well as their variegated function, which relies on their highly conserved modular structure, is underscored by the distinct and dramatic phenotypic alterations resulting from deletion of individual Shc isoforms both in the mouse and in two model organisms, Drosophila melanogaster and Caenorhabditis elegans. The p52 isoform of ShcA couples antigen and cytokine receptors to Ras activation in both lymphoid and myeloid cells. However, the recognition of the spectrum of activities of p52ShcA in the immune system has been steadily expanding in recent years to other fundamental processes both at the cell and organism levels. Two other Shc family members, p66ShcA and p52ShcC/Rai, have been identified recently in T and B lymphocytes, where they antagonize survival and attenuate antigen receptor signaling. These developments reveal an unexpected and complex interplay of multiple Shc proteins in lymphocytes.

The PTB domain of ShcA couples receptor activation to the cytoskeletal regulator IQGAP1

Adaptor proteins respond to stimuli and recruit downstream complexes using interactions conferred by associated protein domains and linear motifs. The ShcA adaptor contains two phosphotyrosine recognition modules responsible for binding activated receptors, resulting in the subsequent recruitment of Grb2 and activation of Ras/MAPK. However, there is evidence that Grb2-independent signalling from ShcA has an important role in development. Using mass spectrometry, we identified the multidomain scaffold IQGAP1 as a ShcA-interacting protein. IQGAP1 and ShcA co-precipitate and are co-recruited to membrane ruffles induced by activated receptors of the ErbB family, and a reduction in ShcA protein levels inhibits the formation of lamellipodia. We used NMR to characterize a direct, non-canonical ShcA PTB domain interaction with a helical fragment from the IQGAP1 N-terminal region that is pTyr-independent. This interaction is mutually exclusive with binding to a more conventional PTB domain peptide ligand from PTP-PEST. ShcA-mediated recruitment of IQGAP1 may have an important role in cytoskeletal reorganization downstream of activated receptors at the cell surface.

CD247 can bind SHC1, no matter if CD247 is phosphorylated

On T cell receptor (TCR) stimulation, src homology 2 domain-containing transforming protein C1 (SHC1) had been found to bind the tyrosine-phosphorylated CD247 chain of the receptor via its src homology 2 (SH2) domain, delivering signals that control T cell development and activation. However, how the phosphorylation of CD247 led to the instant binding has not been characterized clearly. To study the binding process in detail, we simulated and compared the interaction processes of the SH2 domain with CD247 and phosphorylated CD247, respectively. Unexpectedly, the simulation revealed that SHC1 can also bind the nonphosphorylated CD247 peptide, which was further validated to be a weak binding by affinity pull-down experiment. The molecular dynamics (MD) simulation also revealed that the CD247 peptide formed a folding conformation with its Leu209 inserted into the hydrophobic binding pocket in SHC1. And on phosphorylation, it was the electrostatic attraction between the CD247 Tyr(P)206 and the SHC1 Tyr(P)-binding pocket that destroyed the folding conformation of the nonphosphorylated CD247 and, aided by the electrostatic attraction between SHC1 and the Asp203 of CD247, led to the extended conformation of the phosphorylated CD247 binding to SHC1 strongly. The results suggest that nonphosphorylated CD247 can recruit SHC1 in advance to prepare for the instant needs for SHC1 on TCR stimulation. In view of the ubiquity of phosphorylation in protein interaction regulation, we think this study also exemplified the usefulness of MD in more interactome research involving phosphorylation.

Indirect recruitment of the signalling adaptor Shc to the fibroblast growth factor receptor 2 (FGFR2)

The adaptor protein Shc (Src homology and collagen-containing protein) plays an important role in the activation of signalling pathways downstream of RTKs (receptor tyrosine kinases) regulating diverse cellular functions, such as differentiation, adhesion, migration and mitogenesis. Despite being phosphorylated downstream of members of the FGFR (fibroblast growth factor receptor) family, a direct interaction of Shc with this receptor family has not been described to date. Various studies have suggested potential binding sites for the Shc PTB domain (phosphotyrosine-binding domain) and/or the SH2 (Src homology 2) domain on FGFR1, but no interaction of full-length Shc with these sites has been reported in vivo. In the present study, we investigated the importance of the SH2 domain and the PTB domain in recruitment of Shc to FGFR2(IIIc) to characterize the interaction of these two proteins. Confocal microscopy revealed extensive co-localization of Shc with FGFR2. The PTB domain was identified as the critical component of Shc which mediates membrane localization. Results from FLIM (fluorescence lifetime imaging microscopy) revealed that the interaction between Shc and FGFR2 is indirect, suggesting that the adaptor protein forms part of a signalling complex containing the receptor. We identified the non-RTK Src as a protein which potentially mediates the formation of such a ternary complex. Although an interaction between Src and Shc has been described previously, in the present study we implicate the Shc SH2 domain as a novel mediator of this association. The recruitment of Shc to FGFR2 via an indirect mechanism provides new insight into the regulation of protein assembly and activation of various signalling pathways downstream of this RTK.

Compartmentalised MAPK pathways

The mitogen-activated protein kinase (MAPK) pathway provides cells with the means to interpret external signal cues or conditions, and respond accordingly. This cascade regulates many cell functions such as differentiation, proliferation and migration. Through modulation of both the amplitude and duration of MAPK signalling, cells can control their responses to the multiple activators of the pathway. In addition, recent work has highlighted the importance of the cellular compartment from which the signalling occurs. Cells have developed intricate systems that enable them to localise MAPK components to specific subcellular domains in response to a particular stimulus. Consequently, different factors can activate the same kinase in separate locations. Crucial to this ability are molecular scaffolds, which act as signalling modules for MAPKs, confining them to the desired compartment. The participation of the MAPK network in fundamental physiological processes, such as cell proliferation and inflammation, and the derangement of the homeostasis that occurs in disease processes, renders MAPK a highly desirable target for therapeutic intervention. As we enhance our comprehension of scaffolds and other regulatory molecules, novel targets for drug design may be discovered that will afford selective and specific MAPK modulation.

Key molecules involved in receptor-mediated lymphocyte activation

This unit, along with Unit 11.9B, provides a summary of our current knowledge about various signaling pathways critical to the function of immune cells. Here, our understanding of T cell receptor (TCR)- and B cell receptor (BCR)-mediated signaling is summarized. A schematic representation of immunologically relevant cytokine receptors and the Janus Family Kinases (JAKs) that is activated through these receptors is provided, along with details about molecules involved in interleukin 2 mediated signal transduction.

Inhibition of ZAP-70 kinase activity via an analog-sensitive allele blocks T cell receptor and CD28 superagonist signaling

ZAP-70 is a cytoplasmic protein tyrosine kinase that is required for T cell antigen receptor (TCR) signaling. Both mice and humans deficient in ZAP-70 fail to develop functional T cells, thus demonstrating its necessity for T cell development and function. There is currently no highly specific, cell-permeable, small molecule inhibitor for ZAP-70; therefore, we generated a mutant ZAP-70 allele that retains kinase activity but is sensitive to inhibition by a mutant-specific inhibitor. We validated the chemical genetic inhibitor system in Jurkat T cell lines, where the inhibitor blocked ZAP-70-dependent TCR signaling in cells expressing the analog-sensitive allele. Interestingly, the inhibitor also ablated CD28 superagonist signaling, thereby demonstrating the utility of this system in dissecting the requirement for ZAP-70 in alternative mechanisms of T cell activation. Thus, we have developed the first specific chemical means of inhibiting ZAP-70 in cells, which serves as a valuable tool for studying the function of ZAP-70 in T cells.

Defective T cell development and function in the absence of Abelson kinases

Thymocyte proliferation, survival, and differentiation are tightly controlled by signaling from the pre-TCR. In this study, we show for the first time that the Abelson (Abl) kinases regulate proximal signaling downstream of the pre-TCR. Conditional deletion of Abl kinases in thymocytes reveals a cell-autonomous role for these proteins in T cell development. The conditional knockout mice have reduced numbers of thymocytes, exhibit an increase in the percentage of the CD4(-)CD8(-) double-negative population, and are partially blocked in the transition to the CD4(+)CD8(+) double-positive stage. Moreover, the total number of T cells is greatly reduced in the Abl mutant mice, and the null T cells exhibit impaired TCR-induced signaling, proliferation, and cytokine production. Notably, Abl mutant mice are compromised in their ability to produce IFN-positive CD8 T cells and exhibit impaired CD8(+) T cell expansion in vivo upon Listeria monocytogenes infection. Furthermore, Ab production in response to T cell-dependent Ag is severely impaired in the Abl mutant mice. Together these findings reveal cell-autonomous roles for the Abl family kinases in both T cell development and mature T cell function, and show that loss of these kinases specifically in T cells results in compromised immunity.

PIP3 pathway in regulatory T cells and autoimmunity

Regulatory T cells (Tregs) play an important role in preventing both autoimmune and inflammatory diseases. Many recent studies have focused on defining the signal transduction pathways essential for the development and the function of Tregs. Increasing evidence suggest that T-cell receptor (TCR), interleukin-2 (IL-2) receptor (IL-2R), and co-stimulatory receptor signaling are important in the early development, peripheral homeostasis, and function of Tregs. The phosphoinositide-3 kinase (PI3K)-regulated pathway (PIP3 pathway) is one of the major signaling pathways activated upon TCR, IL-2R, and CD28 stimulation, leading to T-cell activation, proliferation, and cell survival. Activation of the PIP3 pathway is also negatively regulated by two phosphatidylinositol phosphatases SHIP and PTEN. Several mouse models deficient for the molecules involved in PIP3 pathway suggest that impairment of PIP3 signaling leads to dysregulation of immune responses and, in some cases, autoimmunity. This review will summarize the current understanding of the importance of the PIP3 pathway in T-cell signaling and the possible roles this pathway performs in the development and the function of Tregs.

A phosphorylation-dependent gating mechanism controls the SH2 domain interactions of the Shc adaptor protein

The Shc (Src homology collagen-like) adaptor protein plays a crucial role in linking stimulated receptors to mitogen-activated protein kinase activation through the formation of dynamic signalling complexes. Shc comprises an N-terminal phosphotyrosine binding (PTB) domain, a C-terminal Src homology 2 (SH2) domain and a central proline-rich collagen homology 1 domain. The latter domain contains three tyrosine residues that are known to become phosphorylated. We have expressed and purified the human p52Shc isoform and characterised its binding to different ligands. CD spectra revealed that some parts of the Shc protein are not fully folded, remaining largely unaffected by the binding of ligands. The PTB domain binds peptide and Ins-1,4,5-P(3) (but not Ins-1,3,5-P(3)) independently, suggesting two distinct sites of interaction. In the unphosphorylated Shc, the SH2 domain is non-functional. Ligand binding to the PTB domain does not affect this. However, phosphorylation of the three tyrosine residues promotes binding to the SH2 domain. Thus, Shc has an intrinsic phosphorylation-dependent gating mechanism where the SH2 domain adopts an open conformation only when tyrosine phosphorylation has occurred.

Hypophosphorylated TCR/CD3zeta signals through a Grb2-SOS1-Ras pathway in Lck knockdown cells

Despite the loss of proximal TCR-dependent signaling events, downstream T cell responses are paradoxically augmented in T cells with siRNA-mediated Lck knockdown (Methi et al., J. Immunol. 2005. 175: 7398-7406). This indicates that alternative Lck-independent pathways of T cell activation exist or that low levels of Lck elicit other signals than normal T cell activation. Here we report the recruitment of Grb2-SOS1 to CD3zeta of the TCR complex after prolonged anti-CD3 (OKT3) stimulation in T cells with Lck knockdown. Grb2 bound to incompletely phosphorylated ITAM1 with the pY-Y configuration in a solid-phase assay, but was excluded by ZAP-70 in the doubly phosphorylated pY-pY conformation. Ras and ERK1/2 activation was augmented after prolonged stimulation in T cells with Lck knockdown compared to control, leading to increased activation of the proximal IL-2 promoter (NFAT-AP-1). Finally, the phosphorylation of Ras-GAP was strongly suppressed in Lck knockdown cells, indicating that a Ras negative feedback mechanism is dependent on Lck.

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.

Contributions to our understanding of T cell physiology through unveiling the T cell proteome

Since the sequencing of the human genome was completed, attention has turned to examining the functionality of the molecular machinery, in particular of protein expression. Differential proteome analysis by two-dimensional electrophoresis has been adopted to study changes in T cell proteomes during T cell activation, and this work is increasing our understanding of the complexity of signals elicited across multiple pathways. The purpose of this review is to summarize the available evidence in the application of proteomic techniques and methodologies to understand T cell receptor activation from lipid raft and cytoskeletal rearrangements, through to signalling cascades, transcription factor modulation and changes in protein expression patterns. These include post-translational modifications, which are not encoded by the genome.

Importance of the CD3γ Ectodomain Terminal β-Strand and Membrane Proximal Stalk in Thymic Development and Receptor Assembly

CD3epsilongamma and CD3epsilondelta are noncovalent heterodimers; each consists of Ig-like extracellular domains associated side-to-side via paired terminal beta-strands that are linked to individual subunit membrane proximal stalk segments. CD3epsilon, CD3gamma, and CD3delta stalks contain the RxCxxCxE motif. To investigate the functional importance of a CD3 stalk and terminal beta-strand, we created a CD3gamma double mutant CD3gamma(C82S/C85S) and a CD3gamma beta-strand triple mutant CD3gamma(Q76S/Y78A/Y79A) for use in retroviral transduction of lymphoid progenitors for comparison with CD3gammawt. Although both mutant CD3gamma molecules reduced association with CD3epsilon in CD3epsilongamma heterodimers, CD3gamma(Q76S/Y78A/Y79A) abrogated surface TCR expression whereas CD3gamma(C82S/C85S) did not. Furthermore, CD3gamma(C82S/C85S) rescued thymic development in CD3gamma(-/-) fetal thymic organ culture. However, the numbers of double-positive and single-positive thymocytes after CD3gamma(C82S/C85S) transduction were significantly reduced despite surface pre-TCR and TCR expression comparable to that of CD3gamma(-/-) thymocytes transduced in fetal thymic organ culture with a retrovirus harboring CD3gammawt cDNA. Furthermore, double-negative thymocyte development was perturbed with attenuated double-negative 3/double-negative 4 maturation and altered surface-expressed CD3epsilongamma, as evidenced by the loss of reactivity with CD3gamma N terminus-specific antisera. Single histidine substitution of either CD3gamma stalk cysteine failed to restore CD3epsilongamma association and conformation in transient COS-7 cell transfection studies. Thus, CD3gamma(C82) and CD3gamma(C85) residues likely are either reduced or form a tight intrachain disulfide loop rather than contribute to a metal coordination site in conjunction with CD3epsilon(C80) and CD3epsilon(C83). The implications of these results for CD3epsilongamma and TCR structure and signaling function are discussed.

ShcA mediates the dominant pathway to extracellular signal-regulated kinase activation during early thymic development

During thymic development, the beta selection checkpoint is regulated by pre-T-cell receptor-initiated signals. Progression through this checkpoint is influenced by phosphorylation and activation of the serine/threonine kinases extracellular signal-regulated kinase 1 (ERK1) and ERK2, but the in vivo relevance of specific upstream players leading to ERK activation is not known. Here, using mice with a conditional loss of the shc1 gene or expressing mutants of ShcA, we demonstrate that the adapter protein ShcA is responsible for up to 70% of ERK activation in double-negative (DN) thymocytes in vivo and ex vivo. We also identify two specific tyrosines on ShcA that promote ERK phosphorylation in vivo, and mice expressing ShcA with mutations of these tyrosines show impaired DN thymocyte development. This work provides the first in vivo demonstration of the relative requirement of upstream adapters in controlling ERK activation during beta selection and suggests a dominant role for ShcA.

Lck couples Shc to TCR signaling

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

The CD3 gamma epsilon/delta epsilon signaling module provides normal T cell functions in the absence of the TCR zeta immunoreceptor tyrosine-based activation motifs

T cell receptor (TCR) signal transduction is mediated by the immunoreceptor tyrosine-based activation motifs (ITAM). The ten ITAM in the TCR complex are distributed in two distinct signaling modules termed TCR zetazeta and CD3 gammaepsilon/deltaepsilon. To delineate the specific role of the zeta ITAM in T cell development and TCR signal transmission, we compared the properties of T cells from different TCR zeta-transgenic lines wherein tyrosine-to-phenylalanine substitutions had been introduced in the zeta subunit. These lines lack selected phosphorylated forms of TCR zeta including just p23, both p21 and p23, or all phospho-zeta derivatives. We report herein that the efficiency of positive selection in HY TCR-transgenic female mice was directly related to the number of zeta ITAM in the TCR. In contrast, TCR-mediated signal transmission and T cell proliferative responses following agonist peptide stimulation were similar and independent of the zeta ITAM. Only the duration of MAPK activation was affected by multiple zeta ITAM substitutions. These results strongly suggest that the ITAM in the CD3 gammaepsilon/deltaepsilon module can provide normal TCR signal transmission, with zeta ITAM providing a secondary function facilitating MAPK activation and positive selection.

Quantitative analysis of phosphotyrosine signaling networks triggered by CD3 and CD28 costimulation in Jurkat cells

The mechanism by which stimulation of coreceptors such as CD28 contributes to full activation of TCR signaling pathways has been intensively studied, yet quantitative measurement of costimulation effects on functional TCR signaling networks has been lacking. In this study, phosphotyrosine networks triggered by CD3, CD28, or CD3 and CD28 costimulation were analyzed by site-specific quantitative phosphoproteomics, resulting in identification of 101 tyrosine and 3 threonine phosphorylation sites and quantification of 87 sites across four cell states. As expected, CD3 stimulation induced phosphorylation of CD3 chains and upstream components of TCR pathways such as Zap70, while CD28 stimulation induced phosphorylation of CD28, Vav-1, and other adaptor proteins including downstream of tyrosine kinase 1, Grb2-associated protein 2 (Grap2), and Wiskott-Aldrich syndrome protein. CD3 and CD28 costimulation induced a complex response including decreased threonine phosphorylation in the ERK1 and ERK2 activation loops and increased phosphorylation of selected tyrosine sites on ERK1/2, p38, phospholipase C-gamma, Src homology 2 domain-containing transforming protein 1, Grap2, and Vav-1, potentiating T cell activation. Hierarchical clustering and self-organizing maps were used to identify modules of coregulated phosphorylation sites within the network. Quantitative information in our study suggests quantitative and qualitative contribution by costimulation of CD28 on CD3-stimulated TCR signaling networks via enhanced phosphorylation of phospholipase C-gamma/Src homology 2 domain-containing transforming protein 1/Grap2/Vav-1 and their effects on downstream components including MAPKs.

RasGRP1 and RasGRP3 regulate B cell proliferation by facilitating B cell receptor-Ras signaling

The RasGRPs are a family of Ras activators that possess diacylglycerol-binding C1 domains. In T cells, RasGRP1 links TCR signaling to Ras. B cells coexpress RasGRP1 and RasGRP3. Using Rasgrp1 and Rasgrp3 single and double null mutant mice, we analyzed the role of these proteins in signaling to Ras and Erk in B cells. RasGRP1 and RasGRP3 both contribute to BCR-induced Ras activation, although RasGRP3 alone is responsible for maintaining basal Ras-GTP levels in unstimulated cells. Surprisingly, RasGRP-mediated Ras activation is not essential for B cell development because this process occurs normally in double-mutant mice. However, RasGRP-deficient mice do exhibit humoral defects. Loss of RasGRP3 led to isotype-specific deficiencies in Ab induction in immunized young mice. As reported previously, older Rasgrp1-/- mice develop splenomegaly and antinuclear Abs as a result of a T cell defect. We find that such mice have elevated serum Ig levels of several isotypes. In contrast, Rasgrp3-/- mice exhibit hypogammaglobulinemia and show no signs of splenomegaly or autoimmunity. Double-mutant mice exhibit intermediate serum Ab titers, albeit higher than wild-type mice. Remarkably, double-mutant mice exhibit no signs of autoimmunity or splenomegaly. B cell proliferation induced by BCR ligation with or without IL-4 was found to be RasGRP1- and RasGRP3-dependent. However, the RasGRPs are not required for B cell proliferation per se, because LPS-induced proliferation is unaffected in double-mutant mice.

Cooperation and selectivity of the two Grb2 binding sites of p52Shc in T-cell antigen receptor signaling to Ras family GTPases and Myc-dependent survival

Shc proteins participate in a variety of processes regulating cell proliferation, survival and apoptosis. The two ubiquitously expressed isoforms, p52Shc/p46Shc, couple tyrosine kinase receptors to Ras by recruiting Grb2/Sos complexes to a membrane-proximal localization. Tyrosine residues 239/240 and 317 become phosphorylated following receptor engagement and, as such, form two Grb2 binding sites, which have been proposed to be differentially coupled to Myc-dependent survival and to fos-dependent proliferation, respectively. Here, we have addressed the individual function of YY239/240 and Y317 in T-cell antigen receptor (TCR) signaling. We show that p52Shc is phosphorylated on both YY239/240 and Y317 following TCR engagement. Mutation of either YY239/240 or Y317 results in impaired interaction with Grb2 and inhibition of Ras/MAP kinase activation and CD69 induction, supporting a role for both Grb2 binding sites in this function. Substitution of either YY239/240 or Y317 also results in a defective activation of Rac and the coupled stress kinases JNK and p38. Furthermore, mutation of Y317 or, to a larger extent, of YY239/240, results in increased activation-induced cell death, which in cells expressing the FF239/240 mutant is accompanied by impaired TCR-dependent c-myc transcription. The data underline a pleiotropic and nonredundant role of Shc, mediated by both YY239/240 and Y317, in T-cell activation and survival.

Solution structure of the CD3epsilondelta ectodomain and comparison with CD3epsilongamma as a basis for modeling T cell receptor topology and signaling

Invariant CD3 subunit dimers (CD3epsilongamma, CD3epsilondelta, and CD3zetazeta) are the signaling components of the alphabeta T cell receptor (TCR). The recently solved structure of murine CD3epsilongamma revealed a unique side-to-side interface and central beta-sheets conjoined between the two C2-set Ig-like ectodomains, with the pairing of the parallel G strands implying a potential concerted piston-type movement for signal transduction. Although CD3gamma and CD3delta each dimerize with CD3epsilon, there are differential CD3 subunit requirements for receptor assembly and signaling among T lineage subpopulations, presumably mandated by structural differences. Here we present the solution structure of the heterodimeric CD3epsilondelta complex. Whereas the CD3epsilon subunit conformation is virtually identical to that in CD3epsilongamma, the CD3delta ectodomain adopts a C1-set Ig fold, with a narrower GFC front face beta-sheet that is more parallel to the ABED back face than those beta-sheets in CD3epsilon and CD3gamma. The dimer interface between CD3delta and CD3epsilon is highly conserved among species and of similar character to that in CD3epsilongamma. Glycosylation sites in CD3delta are arranged such that the glycans may point away from the membrane, consistent with a model of TCR assembly that allows the CD3delta chain to be in close contact with the TCR alpha-chain. This and many other structural and biological features provide a basis for modeling putative TCR/CD3 extracellular domain associations. The fact that the two clusters of transmembrane helices, namely, the three CD3epsilon-CD3gamma-TCRbeta segments and the five CD3epsilon-CD3delta-TCRalpha-CD3zeta-CD3zeta segments, are presumably centered beneath the G strand-paired CD3 heterodimers has important implications for TCR signaling.

p66SHC promotes apoptosis and antagonizes mitogenic signaling in T cells

Of the three Shc isoforms, p66Shc is responsible for fine-tuning p52/p46Shc signaling to Ras and has been implicated in apoptotic responses to oxidative stress. Here we show that human peripheral blood lymphocytes and mouse thymocytes and splenic T cells acquire the capacity to express p66Shc in response to apoptogenic stimulation. Using a panel of T-cell transfectants and p66Shc(-/-) T cells, we show that p66Shc expression results in increased susceptibility to apoptogenic stimuli, which depends on Ser36 phosphorylation and correlates with an altered balance in apoptosis-regulating gene expression. Furthermore, p66Shc blunts mitogenic responses to T-cell receptor engagement, at least in part by transdominant inhibition of p52Shc signaling to Ras/mitogen-activated protein kinases, in an S36-dependent manner. The data highlight a novel interplay between p66Shc and p52Shc in the control of T-cell fate.

T-cell receptor signal transmission: who gives an ITAM?

T cells have an amazing ability to discern and differentially respond to MHC-embedded peptides that can differ by only a single amino acid. This potential involves a combination of the precise ligand-binding specificities of the T-cell receptor (TCR) and the distinct intracellular signaling processes it transmits. Signaling processes are controlled by the ten immunoreceptor tyrosine-based activation motifs (ITAMs) present in the invariant chains of the TCR complex (TCR zeta and CD3-gamma, -delta and -epsilon ). Here, we discuss recent studies of the functions of TCR invariant chains and the contribution of the ten ITAMs to T-cell signal transmission. We incorporate these results into two non-exclusive models of TCR signal transduction: the ITAM multiplicity model, which describes a functional redundancy within the TCR zeta and CD3 ITAMs; and the differential signaling model, which proposes distinct functions for the CD3-gamma, -delta and -epsilon and TCR zeta modules.

T cell receptor-independent CD4 signalling: CD4-MHC class II interactions regulate intracellular calcium and cyclic AMP

CD4 is a coreceptor on T helper (Th) cells that interacts with MHC class II molecules (MHCII). The mechanisms mediating the effects of CD4 on responses by T helper cells to stimulation of the antigen-specific T cell receptor (TCR) are still poorly understood. Here, we demonstrate T cell costimulation via CD4 signalling independent of T cell receptor-mediated signals. Incubation of T helper cells with peptide mimetics of the CD4-binding region on the MHC class II beta2 domain caused intracellular calcium mobilization in the absence of antigen or other T cell receptor stimuli. Engagement of CD4 by peptide mimetics or wild-type MHC class II, but not by mutant MHC class II molecules incapable of engaging CD4, inhibited the T cell receptor-mediated increase in cyclic AMP (cAMP) concentrations in T helper cells. CD4-mediated signals activated cyclic AMP phosphodiesterases (PDEs) and inhibited adenylyl cyclase. Full activation and clonal expansion of antigen-stimulated T helper cells required the CD4-mediated regulation of cyclic AMP. Our results suggest a costimulatory mechanism of CD4 function that acts on the second messengers, calcium and cyclic AMP.

The Ras/MAPK cascade and the control of positive selection

Immature double positive (DP) thymocytes bearing a T cell receptor (TCR) that interacts with self-major histocompatibility complex (MHC) molecules receive signals that induce either their differentiation (positive selection) or apoptosis (negative selection). Furthermore, those cells that are positively selected develop into two different lineages, CD4 or CD8, depending on whether their TCRs bind to MHC class II or I, respectively. Positive selection therefore involves rescue from the default fate (death), lineage commitment, and progression to the single positive (SP) stage. These are probably temporally distinct events that may require both unique and overlapping signals. Work in the past several years has started to unravel the signaling networks that control these processes. One of the first pathways identified as important for positive selection was Ras and its downstream effector, the Erk mitogen-activated protein kinase (MAPK) cascade. In this review we examine the factors that connect the TCR to the Ras/Erk cascade in DP thymocytes, as well as what we know about the downstream effectors of the Ras/Erk cascade important for positive selection. We also consider the possible role of this cascade in CD4/CD8 lineage development, and the possible interactions of the Ras/Erk cascade with Notch during these cell fate determination processes.

Role of Shc in T-cell development and function

Shc is a prototype adapter protein that is expressed from the earliest stages of T-cell development. Shc becomes rapidly tyrosine phosphorylated after T-cell receptor (TCR) engagement. Expression of dominant negative forms of Shc in T-cell lines had also suggested a role for this adapter downstream of the TCR. However, until recently, the relative significance of Shc compared to several other adapters in T cells was unclear. Mice lacking Shc expression specifically in the T-cell lineage together with inducible expression of dominant negative Shc in transgenic mice have revealed an essential and nonredundant role for Shc in thymic T-cell development. Functional defects in a Jurkat T-cell line lacking Shc expression also suggest a role for Shc in mature T-cell functions. While the requirement of Shc in T-cell signaling is now established, precisely what signaling pathways downstream of Shc make this adapter unique are less clear. Although the Shc-mediated activation of the extracellular signal regulated kinase (Erk)/mitogen-activated protein kinase (MAPK) pathway could be one component, Shc likely signals to other pathways in T cells that are not yet discovered. A better molecular understanding of Shc function in the future could provide insights into how multiple adapters coordinate the various outcomes downstream of the TCR.

Kinetic perspectives of T cell antigen receptor signaling. A two-tier model for T cell full activation

T-cell activation consists of multiple layers of signaling events. Interleukin-2 production is of interest for many, since its expression determines a critical difference between partial and full T-cell activation. To achieve full activation of T cells, it is necessary for the T-cell antigen receptor (TCR) to be engaged for an extended period of time. However, why extended stimulation is required for full T-cell activation is not understood at the molecular level. In this review, orchestrated events of TCR signal transduction will be analyzed in a kinetic manner and connected toward the understanding of the mechanism of T-cell activation. Based on recent results, a model of the mechanism that dictates the threshold between partial and full T-cell activation is proposed.

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

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

Low proliferation capacity of lymphocytes from alloxan-diabetic rats: involvement of high glucose and tyrosine phosphorylation of Shc and IRS-1

The proliferation capacity of lymphocytes obtained from mesenteric lymph nodes of control and alloxan-diabetic (40 mg/kg) rats in response to concanavalin A (ConA) and lipopolysaccharide (LPS) stimuli was examined. Proliferation response of lymphocytes from diabetic rats was significantly reduced under Con A (43%) and LPS (46%) stimulation as compared with the control group. Insulin (166 microM) promoted a marked increase of lymphocyte proliferation (7.5-fold) in the control group and this response was much lower (2.6-fold) in lymphocyte from diabetic rats. Cells were also cultured in medium containing glucose at 5, 10 or 20 mM. High glucose concentration (20 mM) caused a marked inhibition of lymphocyte proliferation reaching the values of the diabetic group. In lymphocytes from control rats, the degree of Shc tyrosine phosphorylation was gradually increased, whereas that of cells from diabetic rats was much lower in response to insulin. In lymphocytes obtained from control rats, the tyrosine phosphorylation of IRS-1 was time-dependent on insulin. In cells from diabetic rats, the basal tyrosine phosphorylation of IRS-1 was higher than that of control rats, however, there was no further phosphorylation after insulin addition. We conclude that the response of lymphocyte proliferation from diabetic rats to Con A and LPS stimuli is decreased but insulin was able to promote a significant proliferative effect on these cells. Also, high glycemia in addition to the lack of insulin participates in the reduced proliferation capacity of lymphocytes from diabetic rats.

A nonredundant role for the adapter protein Shc in thymic T cell development

Signaling via the pre-T cell receptor (pre-TCR) regulates survival, proliferation, allelic exclusion and differentiation of thymocytes. The role played by the adapter protein Shc in T cells has remained controversial, and its role in pre-TCR signaling has not been addressed. We examined Shc function in thymic T cell development using two genetic approaches. Cre-loxP-mediated inducible expression in transgenic mice of a phosphorylation-defective mutant of Shc impaired signaling through the pre-TCR as well as subsequent proliferation and differentiation. Conditional deletion of the Shc locus in thymocytes also affected thymic maturation at the same pre-TCR developmental stage. Thus, both Shc expression and its tyrosine phosphorylation play an essential and nonredundant role in thymic T cell development.

Contributions of the T cell receptor-associated CD3gamma-ITAM to thymocyte selection

The immunoreceptor tyrosine-based activation motifs (ITAMs) in the CD3 chains associated with the T cell receptor (TCR) are crucial for TCR signaling. To probe the role of the CD3gamma-ITAM in T cell development, we created knock-in mice in which the CD3gamma chain of the TCR complex is replaced by a mutant signaling-deficient CD3gamma chain, lacking the CD3gamma-ITAM. This mutation results in considerable impairment in positive selection in the polyclonal TCR repertoire. When CD3gamma-deltaITAM mice are crossed to mice expressing transgenic F5 TCRs, their thymocytes are completely unable to perform positive selection in vivo in response to intrathymic ligands. Also, the in vitro positive selection response of double-positive (DP) thymocytes with F5-CD3gamma-deltaITAM mutant receptors to their agonist ligand and many of its variants is severely impaired or abrogated. Yet, the binding and dissociation constants of agonist ligands for the F5 receptor are not affected by the CD3gamma-deltaITAM mutation. Furthermore, DP thymocytes with mutant receptors can respond to agonist ligand with normal antigen sensitivity and to normal levels, as shown by their ability to induce CD69 up-regulation, TCR down-regulation, negative selection, and ZAP70 and c-Jun NH2-terminal kinase activation. In sharp contrast, induction of extracellular signal-regulated kinase (ERK) activation and linker for activation of T cells (LAT) phosphorylation are severely impaired in these cells. Together, these findings underscore that intrinsic properties of the TCR-CD3 complex regulate selection at the DP checkpoint. More importantly, this analysis provides the first direct genetic evidence for a role of the CD3gamma-ITAM in TCR-driven thymocyte selection.

Involvement of the TCR Cbeta FG loop in thymic selection and T cell function

The asymmetric disposition of T cell receptor (TCR) Cbeta and Calpha ectodomains creates a cavity with a side-wall formed by the rigid Cbeta FG loop. To investigate the significance of this conserved structure, we generated loop deletion (betaDeltaFG) and betawt transgenic (tg) mice using the TCR beta subunit of the N15 CTL. N15betawt and N15betaDeltaFG H-2(b) animals have comparable numbers of thymocytes in S phase and manifest developmental progression through the CD4(-)CD8(-) double-negative (DN) compartment. N15betaDeltaFG facilitates transition from DN to CD4(+)8(+) double-positive (DP) thymocytes in recombinase activating gene (RAG)-2(-/-) mice, showing that pre-TCR function remains. N15betaDeltaFG animals possess approximately twofold more CD8(+) single-positive (SP) thymocytes and lymph node T cells, consistent with enhanced positive selection. As an altered Valpha repertoire observed in N15betaDeltaFG mice may confound the deletion's effect, we crossed N15alphabeta TCR tg RAG-2(-/-) with N15betaDeltaFG tg RAG-2(-/-) H-2(b) mice to generate N15alphabeta RAG-2(-/-) and N15alphabeta.betaDeltaFG RAG-2(-/-) littermates. N15alphabeta.betaDeltaFG RAG-2(-/-) mice show an 8-10-fold increase in DP thymocytes due to reduced negative selection, as evidenced by diminished constitutive and cognate peptide-induced apoptosis. Compared with N15alphabeta, N15alphabeta.betaDeltaFG T cells respond poorly to cognate antigens and weak agonists. Thus, the Cbeta FG loop facilitates negative selection of thymocytes and activation of T cells.

T-cell tolerance and autoimmune diabetes

Herein we describe the major signaling events that occur in T-cells upon T-cell receptor (TCR) engagement, and the mechanisms responsible for the induction of T-cell anergy that may ultimately lead to the development of immunospecific therapies in T-cell mediated autoimmune diseases. A new type of antigen presenting molecule (dimeric MHC class-II/peptide, DEF) endowed with antigen-specific immunomodulatory effects such as induction of Th2 polarization and T-cell anergy is also described as a potential antidiabetogenic agent. According to our preliminary results, the MHC II/peptide-based approach may provide rational grounds for further development of antigen-specific immunotherapeutic agents such as human-like MHC lI/peptide chimeras endowed with efficient down-regulatory effects in CD4 T-cell-mediated autoimmune diseases such as Type 1 diabetes, multiple sclerosis, primary biliary cirrhosis, and rheumatoid arthritis.

A novel Src homology 2 domain-containing molecule, Src-like adapter protein-2 (SLAP-2), which negatively regulates T cell receptor signaling

We have cloned a novel adapter protein containing Src homology 2 and Src homology 3 domains similar to the Src family of tyrosine kinases. This molecule lacks a catalytic tyrosine kinase domain and is related to a previously identified protein, Src-like adapter protein (SLAP), and is therefore designated SLAP-2. Northern blot analysis indicates that SLAP-2 is predominantly expressed in the immune system. Jurkat T cells express SLAP-2 protein and overexpression of SLAP-2 in these cells negatively regulates T cell receptor signaling as assessed by interleukin-2 promoter or NF-AT promoter reporter constructs. Mutational analysis revealed that an intact SH2 domain of SLAP-2 is essential for this inhibitory effect, whereas mutation of the SH3 domain alone has no effect. This inhibitory effect is upstream of the activation of Ras and increase of intracellular calcium levels, as no inhibition was observed when the cells were activated by phorbol ester plus ionomycin. SLAP-2 interacts with Cbl in vivo in a phosphorylation independent manner and with ZAP-70 and T cell receptor zeta chain upon T cell receptor activation. Finally, we show that the mutation of a predicted myristoylation site within the NH(2)-terminal of SLAP-2 is essential for its inhibitory effect. This report therefore implicates SLAP and SLAP-2 as a family of adapter proteins that negatively regulate T cell receptor signaling.

Downstream of kinase, p62(dok), is a mediator of Fc gamma IIB inhibition of Fc epsilon RI signaling

The low-affinity receptor for IgG, Fc gamma RIIB, is expressed widely in the immune system and functions to attenuate Ag-induced immune responses. In mast cells, coaggregation of Fc gamma RIIB with the high-affinity IgE receptor, Fc epsilon RI, leads to inhibition of Ag-induced degranulation and cytokine production. Fc gamma RIIB inhibitory activity requires a conserved motif within the Fc gamma RIIB cytoplasmic domain termed the immunoreceptor tyrosine-based inhibition motif. When coaggregated with an activating receptor (e.g., Fc epsilon RI, B cell Ag receptor), Fc gamma RIIB is rapidly phosphorylated on tyrosine and recruits the SH2 domain-containing inositol 5-phosphatase (SHIP). However, the mechanisms by which SHIP mediates Fc gamma RIIB inhibitory function in mast cells remain poorly defined. In this report we demonstrate that Fc gamma RIIB coaggregation with Fc epsilon RI stimulates enhanced SHIP tyrosine phosphorylation and association with Shc and p62(dok). Concurrently, enhanced p62(dok) tyrosine phosphorylation and association with RasGAP are observed, suggesting that SHIP may mediate Fc gamma RIIB inhibitory function in mast cells via recruitment of p62(dok) and RasGAP. Supporting this hypothesis, recruitment of p62(dok) to Fc epsilon RI is sufficient to inhibit Fc epsilon RI-induced calcium mobilization and extracellular signal-regulated kinase 1/2 activation. Interestingly, both the amino-terminal pleckstrin homology and phosphotyrosine binding domains and the carboxyl-terminal proline/tyrosine-rich region of p62(dok) can mediate inhibition, suggesting activation of parallel downstream signaling pathways that converge at extracellular signal-regulated kinase 1/2 activation. Finally, studies using gene-ablated mice indicate that p62(dok) is dispensable for Fc gamma RIIB inhibitory signaling in mast cells. Taken together, these data suggest a role for p62(dok) as a mediator of Fc gamma RIIB inhibition of Fc epsilon RI signal transduction in mast cells.

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

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

Regulation of RAS in human platelets. Evidence that activation of RAS is not sufficient to lead to ERK1-2 phosphorylation

In this study, we show that the G protein-coupled receptor agonist thrombin, the glycoprotein VI agonist convulxin, and the cytokine receptor Mpl agonist thrombopoietin (TPO) are able to induce activation of RAS in human platelets. Recruitment of GRB2 by tyrosine-phosphorylated proteins in response to TPO and convulxin but not by thrombin occurred with a similar time-course to RAS activation, consistent with a causal relationship. On the other hand, activation of ERK2 by thrombin and convulxin is delayed and also inhibited by the protein kinase C inhibitor Ro-31 8220, whereas RAS activation is unaffected. Further evidence for differential regulation of RAS and ERK is provided by the observations that TPO, which activates RAS but not protein kinase C, does not activate ERK, and that the inhibitor of SRC kinases PP1 inhibits activation of RAS but not ERK2 in response to thrombin. Our results demonstrate that activation of RAS is not necessarily coupled to ERK in human platelets.