Novel isoform of lymphoid adaptor FYN-T-binding protein (FYB-130) interacts with SLP-76 and up-regulates interleukin 2 production

Mapping the splicing landscape of the human immune system

Most human genes code for more than one transcript. Different ratios of transcripts of the same gene can be found in different cell types or states, indicating differential use of transcription start sites or differential splicing. Such differential transcript use (DTUs) events provide an additional layer of regulation and protein diversity. With the exceptions of PTPRC and CIITA, there are very few reported cases of DTU events in the immune system. To rigorously map DTUs between different human immune cell types, we leveraged four publicly available RNA sequencing datasets. We identified 282 DTU events between five human healthy immune cell types that appear in at least two datasets. The patterns of the DTU events were mostly cell-type-specific or lineage-specific, in the context of the five cell types tested. DTUs correlated with the expression pattern of potential regulators, namely, splicing factors and transcription factors. Of the several immune related conditions studied, only sepsis affected the splicing of more than a few genes and only in innate immune cells. Taken together, we map the DTUs landscape in human peripheral blood immune cell types, and present hundreds of genes whose transcript use changes between cell types or upon activation.

Multi-functional adaptor SKAP1: regulator of integrin activation, the stop-signal, and the proliferation of T cells

T-cell activation is a complex process involving a network of kinases and downstream molecular scaffolds or adaptors that integrate surface signals with effector functions. One key immune-specific adaptor is Src kinase-associated phosphoprotein 1 (SKAP1), which is also known as src kinase-associated protein of 55 kDa (SKAP55). This mini-review explains how SKAP1 plays multiple roles in regulating integrin activation, the "stop-signal", and the optimization of the cell cycling of proliferating T cells through interactions with various mediators, including the Polo-like kinase 1 (PLK1). Ongoing research on SKAP1 and its binding partners will likely provide important insights into the regulation of immune function and have implications for the development of new treatments for disease states such as cancer and autoimmunity.

Optimal CD8+ T cell effector function requires costimulation-induced RNA-binding proteins that reprogram the transcript isoform landscape

Boosting T cell activation through costimulation directs defense against cancer and viral infections. Despite multiple studies targeting costimulation in clinical trials, the increased potency and reprogramming of T cells endowed by costimulation is poorly understood. Canonical dogma states that transcription mediates T cell activation. Here, we show that the spliceosome, controlling post-transcriptional alternative splicing and alternative polyadenylation, is the most enriched pathway in T cells after CD134/CD137 costimulation. Costimulation of CD8+ T cells significantly increases expression of 29 RNA-binding proteins while RNA-seq uncovers over 1000 differential alternative splicing and polyadenylation events. Using in vivo mouse and in vitro human models, we demonstrate that RNA-binding protein Tardbp is required for effector cytokine production, CD8+ T cell clonal expansion, and isoform regulation after costimulation. The prospect of immune response optimization through reprogramming of mRNA isoform production offered herein opens new avenues for experimentally and therapeutically tuning the activities of T cells.

Association between host genetics of sheep and the rumen microbial composition

A synergy between the rumen microbiota and the host genetics has created a symbiotic relationship, beneficial to the host's health. In this study, the association between the host genetics and rumen microbiome of Damara and Meatmaster sheep was investigated. The composition of rumen microbiota was estimated through the analysis of the V3-V4 region of the 16S rRNA gene, while the sheep blood DNA was genotyped with Illumina OvineSNP50 BeadChip and the genome-wide association (GWA) was analyzed. Sixty significant SNPs dispersed in 21 regions across the Ovis aries genome were found to be associated with the relative abundance of seven genera: Acinetobacter, Bacillus, Clostridium, Flavobacterium, Prevotella, Pseudomonas, and Streptobacillus. A total of eighty-four candidate genes were identified, and their functional annotations were mainly associated with immunity responses and function, metabolism, and signal transduction. Our results propose that those candidate genes identified in the study may be modulating the composition of rumen microbiota and further indicating the significance of comprehending the interactions between the host and rumen microbiota to gain better insight into the health of sheep.

Enhanced Susceptibility of ADAP-Deficient Mice to Listeria monocytogenes Infection Is Associated With an Altered Phagocyte Phenotype and Function

The adhesion and degranulation-promoting adaptor protein (ADAP) serves as a multifunctional scaffold and is involved in the formation of immune signaling complexes. To date, only limited data exist regarding the role of ADAP in pathogen-specific immunity during in vivo infection, and its contribution in phagocyte-mediated antibacterial immunity remains elusive. Here, we show that mice lacking ADAP (ADAPko) are highly susceptible to the infection with the intracellular pathogen Listeria monocytogenes (Lm) by showing enhanced immunopathology in infected tissues together with increased morbidity, mortality, and excessive infiltration of neutrophils and monocytes. Despite high phagocyte numbers in the spleen and liver, ADAPko mice only inefficiently controlled pathogen growth, hinting at a functional impairment of infection-primed phagocytes in the ADAP-deficient host. Flow cytometric analysis of hallmark pro-inflammatory mediators and unbiased whole genome transcriptional profiling of neutrophils and inflammatory monocytes uncovered broad molecular alterations in the inflammatory program in both phagocyte subsets following their activation in the ADAP-deficient host. Strikingly, ex vivo phagocytosis assay revealed impaired phagocytic capacity of neutrophils derived from Lm-infected ADAPko mice. Together, our data suggest that an alternative priming of phagocytes in ADAP-deficient mice during Lm infection induces marked alterations in the inflammatory profile of neutrophils and inflammatory monocytes that contribute to enhanced immunopathology while limiting their capacity to eliminate the pathogen and to prevent the fatal outcome of the infection.

The Multiple Roles of the Cytosolic Adapter Proteins ADAP, SKAP1 and SKAP2 for TCR/CD3 -Mediated Signaling Events

T cells are the key players of the adaptive immune response. They coordinate the activation of other immune cells and kill malignant and virus-infected cells. For full activation T cells require at least two signals. Signal 1 is induced after recognition of MHC/peptide complexes presented on antigen presenting cells (APCs) by the clonotypic TCR (T-cell receptor)/CD3 complex whereas Signal 2 is mediated via the co-stimulatory receptor CD28, which binds to CD80/CD86 molecules that are present on APCs. These signaling events control the activation, proliferation and differentiation of T cells. In addition, triggering of the TCR/CD3 complex induces the activation of the integrin LFA-1 (leukocyte function associated antigen 1) leading to increased ligand binding (affinity regulation) and LFA-1 clustering (avidity regulation). This process is termed "inside-out signaling". Subsequently, ligand bound LFA-1 transmits a signal into the T cells ("outside-in signaling") which enhances T-cell interaction with APCs (adhesion), T-cell activation and T-cell proliferation. After triggering of signal transducing receptors, adapter proteins organize the proper processing of membrane proximal and intracellular signals as well as the activation of downstream effector molecules. Adapter proteins are molecules that lack enzymatic or transcriptional activity and are composed of protein-protein and protein-lipid interacting domains/motifs. They organize and assemble macromolecular complexes (signalosomes) in space and time. Here, we review recent findings regarding three cytosolic adapter proteins, ADAP (Adhesion and Degranulation-promoting Adapter Protein), SKAP1 and SKAP2 (Src Kinase Associated Protein 1 and 2) with respect to their role in TCR/CD3-mediated activation, proliferation and integrin regulation.

How the Discovery of the CD4/CD8-p56lck Complexes Changed Immunology and Immunotherapy

The past 25 years have seen enormous progress in uncovering the receptors and signaling mechanisms on T-cells that activate their various effecter functions. Until the late 1980s, most studies on T-cells had focused on the influx of calcium and the levels of cAMP/GMP in T-cells. My laboratory then uncovered the interaction of CD4 and CD8 co-receptors with the protein-tyrosine kinase p56lck which are now widely accepted as the initiators of the tyrosine phosphorylation cascade leading to T-cell activation. The finding explained how immune recognition receptors expressed by many immune cells, which lack intrinsic catalytic activity, can transduce activation signals via non-covalent association with non-receptor tyrosine kinases. The discovery also established the concept that a protein tyrosine phosphorylation cascade operated in T-cells. In this vein, we and others then showed that the CD4- and CD8-p56lck complexes phosphorylate the TCR complexes which led to the identification of other protein-tyrosine kinases such as ZAP-70 and an array of substrates that are now central to studies in T-cell immunity. Other receptors such as B-cell receptor, Fc receptors and others were also subsequently found to use src kinases to control cell growth. In T-cells, p56lck driven phosphorylation targets include co-receptors such as CD28 and CTLA-4 and immune cell-specific adaptor proteins such as LAT and SLP-76 which act to integrate signals proximal to surface receptors. CD4/CD8-p56lck regulated events in T-cells include intracellular calcium mobilization, integrin activation and the induction of transcription factors for gene expression. Lastly, the identification of the targets of p56lck in the TCR and CD28 provided the framework for the development of chimeric antigen receptor (CAR) therapy in the treatment of cancer. In this review, I outline a history of the development of events that led to the development of the "TCR signaling paradigm" and its implications to immunology and immunotherapy.

ADAP Y571 Phosphorylation Is Required to Prime STAT3 for Activation in TLR4-Stimulated Macrophages

Adhesion and degranulation-promoting adapter protein (ADAP), originally identified as an essential adaptor molecule in TCR signaling and T cell adhesion, has emerged as a critical regulator in innate immune cells such as macrophages; however, its role in macrophage polarization and inflammatory responses remains unknown. In this study, we show that ADAP plays an essential role in TLR4-mediated mouse macrophage polarization via modulation of STAT3 activity. Macrophages from ADAP-deficient mice exhibit enhanced M1 polarization, expression of proinflammatory cytokines and capacity in inducing Th1 responses, but decreased levels of anti-inflammatory cytokines in response to TLR4 activation by LPS. Furthermore, overexpression of ADAP enhances, whereas loss of ADAP reduces, the LPS-mediated phosphorylation and activity of STAT3, suggesting ADAP acts as a coactivator of STAT3 activity and function. Furthermore, the coactivator function of ADAP mostly depends on the tyrosine phosphorylation at Y571 in the motif YDSL induced by LPS. Mutation of Y571 to F severely impairs the stimulating effect of ADAP on STAT3 activity and the ability of ADAP to inhibit M1-like polarization in TLR4-activated mouse macrophages. Moreover, ADAP interacts with STAT3, and loss of ADAP renders mouse macrophages less sensitive to IL-6 stimulation for STAT3 phosphorylation. Collectively, our findings revealed an additional layer of regulation of TLR4-mediated mouse macrophage plasticity whereby ADAP phosphorylation on Y571 is required to prime STAT3 for activation in TLR4-stimulated mouse macrophages.

Identification of gene modules associated with survival of diffuse large B-cell lymphoma treated with CHOP-based chemotherapy

Diffuse Large B-cell Lymphoma (DLBCL), a heterogeneous disease, is influenced by complex network of gene interactions. Most previous studies focused on individual genes, but ignored the importance of intergenic correlations. In current study, we aimed to explore the association between gene networks and overall survival (OS) of DLBCL patients treated with CHOP-based chemotherapy (cyclophosphamide combination with doxorubicin, vincristine and prednisone). Weighted gene co-expression network analysis was conducted to obtain insights into the molecular characteristics of DLBCL. Ten co-expression gene networks (modules) were identified in training dataset (n = 470), and their associations with patients' OS after chemotherapy were tested. The results were validated in four independent datasets (n = 802). Gene ontology (GO) biological function enrichment analysis was conducted with Metascape. Three modules (purple, brown and red), which were enriched in T-cell immune, cell-cell adhesion and extracellular matrix (ECM), respectively, were found to be related to longer OS. Higher expression of several hub genes within these three co-expression modules, for example, LCP2 (HR = 0.77, p = 5.40 × 10^-2), CD2 (HR = 0.87, p = 6.31 × 10^-2), CD3D (HR = 0.83, p = 6.94 × 10^-3), FYB (HR = 0.82, p = 1.40 × 10^-2), GZMK (HR = 0.92, p = 1.19 × 10^-1), FN1 (HR = 0.88, p = 7.06 × 10^-2), SPARC (HR = 0.82, p = 2.06 × 10^-2), were found to be associated with favourable survival. Moreover, the associations of the modules and hub genes with OS in different molecular subtypes and different chemotherapy groups were also revealed. In general, our research revealed the key gene modules and several hub genes were upregulated correlated with good survival of DLBCL patients, which might provide potential therapeutic targets for future clinical research.

ADAP Promotes Degranulation and Migration of NK Cells Primed During in vivo Listeria monocytogenes Infection in Mice

The adhesion and degranulation-promoting adaptor protein (ADAP) serves as a multifunctional scaffold and is involved in the formation of immune signaling complexes. To date only limited and moreover conflicting data exist regarding the role of ADAP in NK cells. To extend existing knowledge we investigated ADAP-dependency of NK cells in the context of in vivo infection with the intracellular pathogen Listeria monocytogenes (Lm). Ex vivo analysis of infection-primed NK cells revealed impaired cytotoxic capacity in NK cells lacking ADAP as indicated by reduced CD107a surface expression and inefficient perforin production. However, ADAP-deficiency had no global effect on NK cell morphology or intracellular distribution of CD107a-containing vesicles. Proteomic definition of ADAPko and wild type NK cells did not uncover obvious differences in protein composition during the steady state and moreover, similar early response patterns were induced in NK cells upon infection independent of the genotype. In line with protein network analyses that suggested an altered migration phenotype in naïve ADAPko NK cells, in vitro migration assays uncovered significantly reduced migration of both naïve as well as infection-primed ADAPko NK cells compared to wild type NK cells. Notably, this migration defect was associated with a significantly reduced expression of the integrin CD11a on the surface of splenic ADAP-deficient NK cells 1 day post-Lm infection. We propose that ADAP-dependent alterations in integrin expression might account at least in part for the fact that during in vivo infection significantly lower numbers of ADAPko NK cells accumulate in the spleen i.e., the site of infection. In conclusion, we show here that during systemic Lm infection in mice ADAP is essential for efficient cytotoxic capacity and migration of NK cells.

GTPase-activating protein Rasal1 associates with ZAP-70 of the TCR and negatively regulates T-cell tumor immunity

Immunotherapy involving checkpoint blockades of inhibitory co-receptors is effective in combating cancer. Despite this, the full range of mediators that inhibit T-cell activation and influence anti-tumor immunity is unclear. Here, we identify the GTPase-activating protein (GAP) Rasal1 as a novel TCR-ZAP-70 binding protein that negatively regulates T-cell activation and tumor immunity. Rasal1 inhibits via two pathways, the binding and inhibition of the kinase domain of ZAP-70, and GAP inhibition of the p21^ras-ERK pathway. It is expressed in activated CD4 + and CD8 + T-cells, and inhibits CD4 + T-cell responses to antigenic peptides presented by dendritic cells as well as CD4 + T-cell responses to peptide antigens in vivo. Furthermore, siRNA reduction of Rasal1 expression in T-cells shrinks B16 melanoma and EL-4 lymphoma tumors, concurrent with an increase in CD8 + tumor-infiltrating T-cells expressing granzyme B and interferon γ-1. Our findings identify ZAP-70-associated Rasal1 as a new negative regulator of T-cell activation and tumor immunity.

Activation of T cells in the tumor microenvironment can be inhibited through a variety of mechanisms. Here, the authors show that Rasal1, a GTPase-activating protein, binds and inhibits signaling downstream of the T Cell Receptor complex and that consistently, its reduced expression enhances anti-tumor T-cell responses in two syngeneic cancer mouse models.

Quantitative interactomics in primary T cells unveils TCR signal diversification extent and dynamics

The activation of T cells by the T cell antigen receptor (TCR) results in the formation of signaling protein complexes (signalosomes), the composition of which has not been analyzed at a systems level. Here, we isolated primary CD4⁺ T cells from 15 gene-targeted mice, each expressing one tagged form of a canonical protein of the TCR-signaling pathway. Using affinity purification coupled with mass spectrometry, we analyzed the composition and dynamics of the signalosomes assembling around each of the tagged proteins over 600 s of TCR engagement. We showed that the TCR signal-transduction network comprises at least 277 unique proteins involved in 366 high-confidence interactions, and that TCR signals diversify extensively at the level of the plasma membrane. Integrating the cellular abundance of the interacting proteins and their interaction stoichiometry provided a quantitative and contextual view of each documented interaction, permitting anticipation of whether ablation of a single interacting protein can impinge on the whole TCR signal-transduction network.

Immune adaptor SKAP1 acts a scaffold for Polo-like kinase 1 (PLK1) for the optimal cell cycling of T-cells

While the immune cell adaptor protein SKAP1 mediates LFA-1 activation induced by antigen-receptor (TCR/CD3) ligation on T-cells, it is unclear whether the adaptor interacts with other mediators of T-cell function. In this context, the serine/threonine kinase, polo-like kinase (PLK1) regulates multiple steps in the mitotic and cell cycle progression of mammalian cells. Here, we show that SKAP1 is phosphorylated by and binds to PLK1 for the optimal cycling of T-cells. PLK1 binds to the N-terminal residue serine 31 (S31) of SKAP1 and the interaction is needed for optimal PLK1 kinase activity. Further, siRNA knock-down of SKAP1 reduced the rate of T-cell division concurrent with a delay in the expression of PLK1, Cyclin A and pH3. Reconstitution of these KD cells with WT SKAP1, but not the SKAP1 S31 mutant, restored normal cell division. SKAP1-PLK1 binding is dynamically regulated during the cell cycle of T-cells. Our findings identify a novel role for SKAP1 in the regulation of PLK1 and optimal cell cycling needed for T-cell clonal expansion in response to antigenic activation.

Characterization of Mice with a Platelet-Specific Deletion of the Adapter Molecule ADAP

The adhesion and degranulation-promoting adapter protein (ADAP) is expressed in T cells, NK cells, myeloid cells, and platelets. The involvement of ADAP in the regulation of receptor-mediated inside-out signaling leading to integrin activation is well characterized, especially in T cells and in platelets. Due to the fact that animal studies using conventional knockout mice are limited by the overlapping effects of the different ADAP-expressing cells, we generated conditional ADAP knockout mice (ADAP^fl/fl PF4-Cre^tg) (PF4, platelet factor 4). We observed that loss of ADAP restricted to the megakaryocytic lineage has no impact on other hematopoietic cells even under stimulation conditions. ADAP^fl/fl PF4-Cre^tg mice showed thrombocytopenia in combination with reduced plasma levels of PF4 and transforming growth factor β1 (TGF-β1). In vitro, platelets from these mice revealed reduced P-selectin expression, lower levels of TGF-β1 release, diminished integrin αIIbβ3 activation, and decreased fibrinogen binding after stimulation with podoplanin, the ligand of C-type lectin-like receptor 2 (CLEC-2). Furthermore, loss of ADAP was associated with impaired CLEC-2-mediated activation of phospholipase Cγ2 (PLCγ2) and extracellular signal-regulated kinase 1/2 (ERK1/2). Induction of experimental autoimmune encephalomyelitis (EAE) in mice lacking ADAP expression in platelets caused a more severe disease. In vivo administration of TGF-β1 early after T cell transfer reduced EAE severity in mice with loss of ADAP restricted to platelets. Our results reveal a regulatory function of ADAP in platelets in vitro and during autoimmune disease EAE in vivo.

ADAP is an upstream regulator that precedes SLP-76 at sites of TCR engagement and stabilizes signaling microclusters

Antigen recognition by the T cell receptor (TCR) directs the assembly of essential signaling complexes known as SLP-76 (also known as LCP2) microclusters. Here, we show that the interaction of the adhesion and degranulation-promoting adaptor protein (ADAP; also known as FYB1) with SLP-76 enables the formation of persistent microclusters and the stabilization of T cell contacts, promotes integrin-independent adhesion and enables the upregulation of CD69. By analyzing point mutants and using a novel phospho-specific antibody, we show that Y595 is essential for normal ADAP function, that virtually all tyrosine phosphorylation of ADAP is restricted to a Y595-phosphorylated (pY595) pool, and that multivalent interactions between the SLP-76 SH2 domain and its binding sites in ADAP are required to sustain ADAP phosphorylation. Although pY595 ADAP enters SLP-76 microclusters, non-phosphorylated ADAP is enriched in protrusive actin-rich structures. The pre-positioning of ADAP at the contact sites generated by these structures favors the retention of nascent SLP-76 oligomers and their assembly into persistent microclusters. Although ADAP is frequently depicted as an effector of SLP-76, our findings reveal that ADAP acts upstream of SLP-76 to convert labile, Ca2+-competent microclusters into stable adhesive junctions with enhanced signaling potential.

Tyrosine-phosphorylation of the scaffold protein ADAP and its role in T cell signaling

INTRODUCTION

The Adhesion and Degranulation promoting Adaptor Protein (ADAP) is phosphorylated upon T cell activation and acts as a scaffold for the formation of a signaling complex that integrates molecular interactions between T cell or chemokine receptors, the actin cytoskeleton, and integrin-mediated cellular adhesion and migration.

AREAS COVERED

This article reviews current knowledge of the functions of the adapter protein ADAP in T cell signaling with a focus on the role of individual phosphotyrosine (pY) motifs for SH2 domain mediated interactions. The data presented was obtained from literature searches (PubMed) as well as the authors own research on the topic. Expert commentary: ADAP can be regarded as a paradigmatic example of how tyrosine phosphorylation sites serve as dynamic interaction hubs. Molecular crowding at unstructured and redundant sites (pY595, pY651) is contrasted by more specific interactions enabled by the three-dimensional environment of a particular phosphotyrosine motif (pY571).

SHP-1 Acts as a Key Regulator of Alloresponses by Modulating LFA-1-Mediated Adhesion in Primary Murine T Cells

The clinical potential of transplantation is often reduced by T cell-mediated alloresponses that cause graft rejection or graft-versus-host disease. Integrin-mediated adhesion between alloreactive T cells and antigen-presenting cells is essential for allorejection. The identity of the signaling events needed for the activation of integrins such as LFA-1 is poorly understood. Here, we identified a novel role of the protein tyrosine phosphatase SHP-1 in the regulation of murine LFA-1-mediated adhesion in an allograft setting. Upon alloactivation, SHP-1 activity is reduced, resulting in an increase in LFA-1 adhesion compared to that for syngeneically activated T cells. The importance of these differential activation properties was further indicated by small interfering RNA (siRNA) knockdown of SHP-1 in syngeneically and allogeneically stimulated T cells. Mechanistically, SHP-1 modulated the binding of SLP-76 to ADAP by dephosphorylation of the YDGI tyrosine motif of ADAP, a known docking site for the Src family kinase Fyn. This novel key role of SHP-1 in the regulation of LFA-1-mediated adhesion may provide a new insight into T cell-mediated alloresponses and may pave the way to the development of new immunosuppressive pharmaceutical agents.

Glycogen Synthase Kinase 3 Inactivation Drives T-bet-Mediated Downregulation of Co-receptor PD-1 to Enhance CD8(+) Cytolytic T Cell Responses

Despite the importance of the co-receptor PD-1 in T cell immunity, the upstream signaling pathway that regulates PD-1 expression has not been defined. Glycogen synthase kinase 3 (GSK-3, isoforms α and β) is a serine-threonine kinase implicated in cellular processes. Here, we identified GSK-3 as a key upstream kinase that regulated PD-1 expression in CD8(+) T cells. GSK-3 siRNA downregulation, or inhibition by small molecules, blocked PD-1 expression, resulting in increased CD8(+) cytotoxic T lymphocyte (CTL) function. Mechanistically, GSK-3 inactivation increased Tbx21 transcription, promoting enhanced T-bet expression and subsequent suppression of Pdcd1 (encodes PD-1) transcription in CD8(+) CTLs. Injection of GSK-3 inhibitors in mice increased in vivo CD8(+) OT-I CTL function and the clearance of murine gamma-herpesvirus 68 and lymphocytic choriomeningitis clone 13 and reversed T cell exhaustion. Our findings identify GSK-3 as a regulator of PD-1 expression and demonstrate the applicability of GSK-3 inhibitors in the modulation of PD-1 in immunotherapy.

The Fyn-ADAP Axis: Cytotoxicity Versus Cytokine Production in Killer Cells

Lymphocyte signaling cascades responsible for anti-tumor cytotoxicity and inflammatory cytokine production must be tightly regulated in order to control an immune response. Disruption of these cascades can cause immune suppression as seen in a tumor microenvironment, and loss of signaling integrity can lead to autoimmunity and other forms of host-tissue damage. Therefore, understanding the distinct signaling events that exclusively control specific effector functions of “killer” lymphocytes (T and NK cells) is critical for understanding disease progression and formulating successful immunotherapy. Elucidation of divergent signaling pathways involved in receptor-mediated activation has provided insights into the independent regulation of cytotoxicity and cytokine production in lymphocytes. Specifically, the Fyn signaling axis represents a branch point for killer cell effector functions and provides a model for how cytotoxicity and cytokine production are differentially regulated. While the Fyn–PI(3)K pathway controls multiple functions, including cytotoxicity, cell development, and cytokine production, the Fyn–ADAP pathway preferentially regulates cytokine production in NK and T cells. In this review, we discuss how the structure of Fyn controls its function in lymphocytes and the role this plays in mediating two facets of lymphocyte effector function, cytotoxicity and production of inflammatory cytokines. This offers a model for using mechanistic and structural approaches to understand clinically relevant lymphocyte signaling.

The Immune Adaptor SLP-76 Binds to SUMO-RANGAP1 at Nuclear Pore Complex Filaments to Regulate Nuclear Import of Transcription Factors in T Cells

While immune cell adaptors regulate proximal T cell signaling, direct regulation of the nuclear pore complex (NPC) has not been reported. NPC has cytoplasmic filaments composed of RanGAP1 and RanBP2 with the potential to interact with cytoplasmic mediators. Here, we show that the immune cell adaptor SLP-76 binds directly to SUMO-RanGAP1 of cytoplasmic fibrils of the NPC, and that this interaction is needed for optimal NFATc1 and NF-κB p65 nuclear entry in T cells. Transmission electron microscopy showed anti-SLP-76 cytoplasmic labeling of the majority of NPCs in anti-CD3 activated T cells. Further, SUMO-RanGAP1 bound to the N-terminal lysine 56 of SLP-76 where the interaction was needed for optimal RanGAP1-NPC localization and GAP exchange activity. While the SLP-76-RanGAP1 (K56E) mutant had no effect on proximal signaling, it impaired NF-ATc1 and p65/RelA nuclear entry and in vivo responses to OVA peptide. Overall, we have identified SLP-76 as a direct regulator of nuclear pore function in T cells.

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Immune adaptor SLP-76 binds to SUMO-RanGAP1 of cytoplasmic fibrils of the NPC

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SLP-76 K-56 binding needed for optimal RanGAP1 localization and exchange activity

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SLP-76 K56E mutant impaired NF-ATc1 and NFκB p65 (RelA) nuclear entry

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Immune adaptors directly regulate nuclear entry of transcription factors in T cells

The adapter proteins ADAP and Nck cooperate in T cell adhesion

Nck adapter proteins link receptor and receptor-associated tyrosine kinases with proteins implicated in the regulation of the actin cytoskeleton. Nck is involved in a multitude of receptor-initiated signaling pathways and its physiological role thus covers aspects of tissue development and homeostasis, malignant transformation/invasiveness of tumour cells and also immune cell function. In T cells, changes of cell polarity and morphology associated with cellular activation and effector function crucially rely on the T cell receptor-mediated recruitment and activation of different actin-regulatory proteins to orchestrate and drive cytoskeletal reorganization at the immunological synapse. In a former approach to determine the interactome of Nck in human T cells, we identified the adapter protein ADAP as a Nck-interacting protein. This adhesion and degranulation-promoting adapter protein had already been implicated in the inside-out activation of integrins. Employing co-immunoprecipitations, we demonstrate that both Nck family members Nck1 and Nck2 coprecipitate with ADAP. Specifically, Nck interacts via its Src homology 2 domain with phosphorylated tyrosine Y595DDV and Y651DDV sites of ADAP. Moreover, we show that endogenous ADAP is phosphorylated in primary human T cell blasts and thus associates with Nck. At the functional level, ADAP and Nck adapter proteins cooperatively facilitate T cell adhesion to the LFA-1 ligand ICAM-1. Our data indicate that the ADAP/Nck complex might provide a means to link integrin activation with the actin cytoskeleton.

Crucial role of SLP-76 and ADAP for neutrophil recruitment in mouse kidney ischemia-reperfusion injury

Leukocyte recruitment to the kidney during acute injury is mediated by E-selectin–mediated rolling and requires SLP-76 and the adaptor protein ADAP.

Neutrophils trigger inflammation-induced acute kidney injury (AKI), a frequent and potentially lethal occurrence in humans. Molecular mechanisms underlying neutrophil recruitment to sites of inflammation have proved elusive. In this study, we demonstrate that SLP-76 (SH2 domain–containing leukocyte phosphoprotein of 76 kD) and ADAP (adhesion and degranulation promoting adaptor protein) are involved in E-selectin–mediated integrin activation and slow leukocyte rolling, which promotes ischemia-reperfusion–induced AKI in mice. By using genetically engineered mice and transduced Slp76^−/− primary leukocytes, we demonstrate that ADAP as well as two N-terminal–located tyrosines and the SH2 domain of SLP-76 are required for downstream signaling and slow leukocyte rolling. The Tec family kinase Bruton tyrosine kinase is downstream of SLP-76 and, together with ADAP, regulates PI3Kγ (phosphoinositide 3-kinase–γ)- and PLCγ2 (phospholipase Cγ2)-dependent pathways. Blocking both pathways completely abolishes integrin affinity and avidity regulation. Thus, SLP-76 and ADAP are involved in E-selectin–mediated integrin activation and neutrophil recruitment to inflamed kidneys, which may underlie the development of life-threatening ischemia-reperfusion–induced AKI in humans.

SKAP1 protein PH domain determines RapL membrane localization and Rap1 protein complex formation for T cell receptor (TCR) activation of LFA-1

Although essential for T cell function, the identity of the T cell receptor (TCR) "inside-out" pathway for the activation of lymphocyte function-associated antigen 1 (LFA-1) is unclear. SKAP1 (SKAP-55) is the upstream regulator needed for TCR-induced RapL-Rap1 complex formation and LFA-1 activation. In this paper, we show that SKAP1 is needed for RapL binding to membranes in a manner dependent on the PH domain of SKAP1 and the PI3K pathway. A SKAP1 PH domain-inactivating mutation (i.e. R131M) markedly impaired RapL translocation to membranes for Rap1 and LFA-1 binding and the up-regulation of LFA-1-intercellular adhesion molecule 1 (ICAM-1) binding. Further, N-terminal myr-tagged SKAP1 for membrane binding facilitated constitutive RapL membrane and Rap1 binding and effectively substituted for PI3K and TCR ligation in the activation of LFA-1 in T cells.

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.

T cell receptor "inside-out" pathway via signaling module SKAP1-RapL regulates T cell motility and interactions in lymph nodes

Although essential for T cell function, the identity of the T cell receptor "inside-out" pathway for lymphocyte function-associated antigen 1 (LFA-1) adhesion has proved elusive. Here, we define the "inside-out" pathway mediated by N-terminal SKAP1 (SKAP-55) domain binding to the C-terminal SARAH domain of RapL. TcR induced Rap1-RapL complex formation and LFA-1 binding failed to occur in Skap1(-/-) primary T cells. SKAP1 generated a SKAP1-RapL-Rap1 complex that bound to LFA-1, whereas a RapL mutation (L224A) that abrogated SKAP1 binding without affecting MST1 disrupted component colocalization in vesicles as well as T cell-dendritic cell (DC) conjugation. RapL expression also "slowed" T cell motility in D011.10 transgenic T cells in lymph nodes (LNs), an effect reversed by the L224A mutation with reduced dwell times between T cells and DCs. Overall, our findings define a TCR "inside-out" pathway via N-SKAP1-C-RapL that regulates T cell adhesion, motility, and arrest times with DCs in LNs.

SLP-76-ADAP adaptor module regulates LFA-1 mediated costimulation and T cell motility

Although adaptor ADAP (FYB) and its binding to SLP-76 has been implicated in TcR-induced "inside-out" signaling for LFA-1 activation in T cells, little is known regarding its role in LFA-1-mediated "outside-in" signaling. In this study, we demonstrate that ADAP and SLP-76-ADAP binding are coupled to LFA-1 costimulation of IL-2 production, F-actin clustering, cell polarization, and T cell motility. LFA-1 enhancement of anti-CD3-induced IL-2 production was completely dependent on SLP-76-ADAP binding. Further, anti-CD3 was found to require CD11a ligation by antibody or ICAM1 to cause T cell polarization. ADAP augmented this polarization induced by anti-CD3/CD11a, but not by anti-CD3 alone. ADAP expression with LFA-1 ligation alone was sufficient to polarize T cells directly and to increase T cell motility whereas the loss of ADAP in ADAP-/- primary T cells reduced motility. A mutant lacking SLP-76-binding sites (M12) blocked LFA-1 costimulation of IL-2 production, polarization, and motility. LFA-1-ADAP polarization was also dependent on src kinases, Rho GTPases, phospholipase C, and phosphoinositol 3-kinase. Our findings provide evidence of an obligatory role for the SLP-76-ADAP module in LFA-1-mediated costimulation in T cells.

SKAP-55, SKAP-55-related and ADAP adaptors modulate integrin-mediated immune-cell adhesion

Integrin adhesion is essential for aspects of immune function, including antigen presentation and migration in lymph nodes, germinal centers and sites of inflammation. Antigen receptors on B and T cells generate 'inside-out' signals for increased integrin clustering and adhesion. Although upstream components of B-cell-receptor or T-cell-receptor signaling are needed, the identity of key downstream effectors that mediate integrin adhesion is only just emerging. New candidates include immune-cell-specific adaptor proteins ADAP, SKAP-55 and SKAP-55-related (SKAP-55R). SKAP-55 has recently been identified as an effector in T cells in SKAP-55-deficient mice, whereas SKAP-55R is needed for B-cell adhesion. ADAP is required for SKAP-55 and SKAP-55R protein stability. SKAP-55 and SKAP-55R have unexpectedly specialized roles in T- and B-cell adhesion of the immune system.

Immature hematopoietic cells display selective requirements for adhesion- and degranulation-promoting adaptor protein in development and homeostatsis

Adhesion- and degranulation-promoting adaptor protein (ADAP) modulates T cell development and function and promotes TCR signaling. Regulation of ADAP protein expression during thymopoiesis and in development of other hematopoietic lineages has not been explored. Using intracellular staining, we detected ADAP protein in bone marrow lymphocyte precursors. Like its binding partner SH2-containing leukocyte phosphoprotein of 76 kDa, ADAP is dynamically regulated during thymocyte positive selection. ADAP is also found in unconventional thymocytes, including NKT, CD8alphaalpha, and TCRgammadelta T cells. In peripheral T cells, ADAP is up-regulated after TCR stimulation and with acquisition of memory status. Although absent in splenic B cells, ADAP is present in pro-B cells, as well as in BM erythrocyte and myeloid progenitors. Studies with radiation chimeras show that ADAP is dispensable for NKT, CD8alphaalpha and TCRgammadelta T cell development, while confirming that ADAP is required for optimal development of conventional TCRalphabeta T cells in the thymus. Interestingly, ADAP is necessary for CD8alphaalpha homeostasis in the small intestinal epithelium, yet is dispensable for optimal reconstitution of splenic B cell populations. Our observations highlight the dynamic regulation of ADAP during T cell maturation and document expression patterns that suggest a possible role for ADAP in development of non-T hematopoietic lineages.

Compartmentalization of ITAM and integrin signaling by adapter molecules

Adapters are multidomain molecules that recruit effector proteins during signal transduction by immunoreceptors and integrins. The absence of these scaffolding molecules profoundly affects development and function of various hematopoietic lineages, underscoring their importance as regulators of signaling cascades. An emerging aspect of the mechanism by which engaged immunoreceptors and integrins transmit signals within the cell is by differential usage of various adapters that function to nucleate formation of distinct signaling complexes in a specific location within the cell. In this review, we discuss the mechanisms by which adapter proteins coordinate signal transduction with an emphasis on the role of subcellular compartmentalization in adapter function.

Regulation of T-cell antigen receptor-mediated inside-out signaling by cytosolic adapter proteins and Rap1 effector molecules

Integrins are critical for the migration of T cells to lymphoid organs and to sites of inflammation and are also necessary for productive interactions between T cells and antigen-presenting cells. Integrin activation is enhanced following T-cell receptor (TCR) engagement, as signals initiated by the TCR increase affinity and avidity of integrins for their ligands. This process, known as inside-out signaling, has been shown to require several molecular components including the cytosolic adapter proteins adhesion and degranulation-promoting adapter protein and Src homology 2 domain-containing adapter protein of 55 kDa, the low molecular weight guanosine triphosphatase Rap1, and the Rap1 effector proteins Rap1 guanosine triphosphate-interacting adapter molecule, regulator of adhesion and cell polarization enriched in lymphoid tissues, and protein kinase D1. Herein, we review recent findings about how the TCR is linked to integrin activation through inside-out signaling.

The ADAP/SKAP55 signaling module regulates T-cell receptor-mediated integrin activation through plasma membrane targeting of Rap1

Adhesion of T cells after stimulation of the T-cell receptor (TCR) is mediated via signaling processes that have collectively been termed inside-out signaling. The molecular basis for inside-out signaling is not yet completely understood. Here, we show that a signaling module comprising the cytosolic adapter proteins ADAP and SKAP55 is involved in TCR-mediated inside-out signaling and, moreover, that the interaction between ADAP and SKAP55 is mandatory for integrin activation. Disruption of the ADAP/SKAP55 module leads to displacement of the small GTPase Rap1 from the plasma membrane without influencing its GTPase activity. These findings suggest that the ADAP/SKAP55 complex serves to recruit activated Rap1 to the plasma membrane. In line with this hypothesis is the finding that membrane targeting of the ADAP/SKAP55 module induces T-cell adhesion in the absence of TCR-mediated stimuli. However, it appears as if the ADAP/SKAP55 module can exert its signaling function outside of the classical raft fraction of the cell membrane.

ADAP is required for normal alphaIIbbeta3 activation by VWF/GP Ib-IX-V and other agonists

Interaction between von Willebrand factor (VWF) and platelet GP Ib-IX-V is required for hemostasis, in part because intracellular signals from VWF/GP Ib-IX-V activate the ligand-binding function of integrin alphaIIbbeta3. Because they also induce tyrosine phosphorylation of the ADAP adapter, we investigated ADAP's role in GP Ib-IX-V signal transduction. Fibrinogen or ligand-mimetic POW-2 Fab binding to alphaIIbbeta3 was stimulated by adhesion of ADAP+/+ murine platelets to dimeric VWF A1A2 but was significantly reduced in ADAP-/- platelets (P<.01). alphaIIbbeta3 activation by ADP or a Par4 thrombin receptor agonist was also decreased in ADAP-/- platelets. ADAP stabilized the expression of another adapter, SKAP-HOM, via interaction with the latter's SH3 domain. However, no abnormalities in alphaIIbbeta3 activation were observed in SKAP-HOM-/- platelets, which express normal ADAP levels, further implicating ADAP as a modulator of alphaIIbbeta3 function. Under shear flow conditions over a combined surface of VWF A1A2 and fibronectin to test interactions involving GP Ib-IX-V and alphaIIbbeta3, respectively, ADAP-/- platelets displayed reduced alphaIIbbeta3-dependent stable adhesion. Furthermore, ADAP-/- mice demonstrated increased rebleeding from tail wounds. These studies establish ADAP as a component of inside-out signaling pathways that couple GP Ib-IX-V and other platelet agonist receptors to alphaIIbbeta3 activation.

Adhesion- and degranulation-promoting adapter protein is required for efficient thymocyte development and selection

Adhesion- and degranulation-promoting adapter protein (ADAP) is required in TCR-induced activation and proliferation of peripheral T cells. Loss of ADAP also impairs TCR-initiated inside-out activation of the integrin LFA-1 (CD11a/CD18, alphaLbeta2). In this study, we demonstrate that ADAP-deficient CD4/CD8 double-positive (DP) cells have a diminished ability to proliferate, and that these DP thymocytes up-regulate CD69 poorly in vivo. Moreover, in both MHC class I- and class II-restricted TCR transgenic models, loss of ADAP interferes with both positive and negative selection. ADAP deficiency also impairs the ability of transgene-bearing DP thymocytes to form conjugates with Ag-loaded presenting cells. These findings suggest that ADAP is critical for thymocyte development and selection.

Regulation and Function of SKAP-55 Non-canonical Motif Binding to the SH3c Domain of Adhesion and Degranulation-promoting Adaptor Protein

The immune cell adaptor adhesion and degranulation promoting adaptor protein (ADAP) and its binding to T-cell adaptor Src kinase-associated protein of 55 kDa (SKAP-55) play a key role in the modulation of T-cell adhesion. While primary binding occurs via SKAP-55 SH3 domain binding to a proline-rich region in ADAP, a second interaction occurs between the ADAP C-terminal SH3 domain (ADAP-SH3c) and a non-canonical RKXXY294XXY297 motif in SKAP-55. Increasing numbers of non-canonical SH3 domain binding motifs have been identified in a number of biological systems. The presence of tyrosine residues in the SKAP-55 RKXXY294XXY297 motif suggested that phosphorylation might influence this unusual SH3 domain interaction. Here, we show that the Src kinase p59fyn can induce the in vivo phosphorylation of the motif, and this event blocks ADAP-SH3c domain binding to the peptide motif. The importance of tyrosine phosphorylation was confirmed by plasmon resonance interaction analysis showing that phosphorylation of Tyr294 residue plays a central role in mediating dissociation, whereas phosphorylation of the second Tyr297 had no effect. Although loss of this secondary interaction did not result in the disruption of the complex, the Y294F mutation blocked T-cell receptor-induced up-regulation of lymphocyte function-associated antigen-1-mediated adhesion to intercellular adhesion molecule-1 and interleukin-2 promoter activity. Our findings identify a RKXXY294 motif in SKAP-55 that mediates unique ADAP SH3c domain binding and is needed for LFA-1-mediated adhesion and cytokine production.

Physical and Functional Association of c-Src and Adhesion and Degranulation Promoting Adaptor Protein (ADAP) in Osteoclastogenesis in Vitro

c-Src plays a crucial role in osteoclastogenesis. In this study, we searched for c-Src-binding proteins using a combination of pull-down assays and mass spectrometric analysis, and identified the association of adhesion and degranulation promoting adaptor protein (ADAP) with c-Src in RAW264 cells and osteoclast precursors prepared from bone marrow cells. The kinase activity and the SH2 domain of c-Src were required for this association and Tyr807 in the extreme carboxyl terminus of ADAP was identified as a major recognition site. ADAP was found to be expressed in cells at the prefusion stage and localized mainly in the leading edge of lamellipodia and in pseudopodia. Tyrosine phosphorylation of ADAP was induced in an integrin-dependent manner, and the level was Src kinase-dependent. ADAP-knockdown RAW264 cells showed retarded migration and formed few multinucleated cells. Cas, known to be phosphorylated by c-Src, was identified as a major tyrosine-phosphorylated protein in differentiating RAW264 cells and the phosphorylation appeared to be decreased in ADAP-knockdown cells. ADAP thus may play an important role as a partner of c-Src for cell migration and progression to the multinucleated cell stage in osteoclastogenesis in vitro.

The SLP-76 family of adapter proteins

Adapter molecules are multidomain proteins lacking intrinsic catalytic activity, functioning instead by nucleating molecular complexes during signal transduction. The SLP-76 family of adapters includes SH2 domain-containing leukocyte phosphoprotein of 76kDa (SLP-76), B cell linker protein (BLNK), and cytokine-dependent hematopoietic cell linker (Clnk). These proteins are critical for integration of numerous signaling cascades downstream of immunotyrosine-based activation motif (ITAM)-bearing receptors and integrins in diverse hematopoietic cell types. Mutations in genes encoding SLP-76 family adapters result in severe phenotypes, underscoring the critical role these proteins play in cellular development and function by directing formation of signaling complexes in a temporally- and spatially-specific manner.

Deficiency of ADAP/Fyb/SLAP-130 destabilizes SKAP55 in Jurkat T cells

ADAP (adhesion and degranulation-promoting adaptor protein) and SKAP55 (Src kinase-associated phosphoprotein of 55 kDa) are T cell adaptors that mediate inside-out signaling from the T cell antigen receptor to integrins, giving rise to increased integrin affinity/avidity and formation of the immunological synapse between the T cell and the antigen-presenting cell. These two proteins are tightly and constitutively associated with one another, and their ability to interact is required for inside-out signaling. Here we show in an ADAP-deficient Jurkat T cell line that the co-dependence of ADAP and SKAP55 extends beyond their functional and physical interactions and show that SKAP55 protein is unstable in the absence of ADAP. Restoration of ADAP to the ADAP-deficient Jurkat T cell line restores SKAP55 expression by causing a 5-fold decrease in the rate of SKAP55 proteolysis. Inactivation of the Src homology 3 domain of SKAP55, which mediates the association between SKAP55 with ADAP, blocks the protective effect of ADAP. The half-life of SKAP55, in the absence of ADAP, is approximately 15-20 min, increasing to 90 min in the presence of ADAP. This is a remarkably rapid rate of turnover for a signaling protein and suggests the possibility that stimuli that signal for the stabilization of SKAP55 may play an important role in T cell adhesion and conjugate formation.

Fyn binding protein, Fyb, interacts with mammalian actin binding protein, mAbp1

The immune cell specific protein Fyn-T binding protein (Fyb) has been identified as a target of the Yersinia antiphagocytic effector Yersinia outer protein H (YopH), but its role in macrophages is unknown. By using Fyb domains as bait to screen a mouse lymphoma cDNA library, we identified a novel interaction partner, mammalian actin binding protein 1 (mAbp1). We show that mAbp1 binds the Fyb N-terminal via its C-terminally located src homology 3 domain. The interaction between Fyb and mAbp1 is detected in macrophage lysates and the proteins co-localize with F-actin in the leading edge. Hence, mAbp1 is likely to constitute a downstream effector of Fyb involved in F-actin dynamics.

Cellular mechanisms of bacterial internalization counteracted by Yersinia

Upon host-cell contact, human pathogenic Yersinia species inject Yop virulence effectors into the host through a Type III secretion-and-translocation system. These virulence effectors cause a block in phagocytosis (YopE, YopT, YpkA, and YopH) and suppression of inflammatory mediators (YopJ). The Yops that block phagocytosis either interfere with the host cell actin regulation of Rho GTPases (YopE, YopT, and YpkA) or specifically and rapidly inactivate host proteins involved in signaling from the receptor to actin (YopH). The block in uptake has been shown to be activated following binding to Fc, Complement, and beta1-integrin receptors in virtually any kind of host cell. Thus, the use of Yersinia as a model system to study Yersinia-host cell interactions provides a good tool to explore signaling pathways involved in phagocytosis.

PRAM-1 is required for optimal integrin-dependent neutrophil function

PML-retinoic acid receptor alpha (RARalpha) regulated adaptor molecule 1 (PRAM-1) is an intracellular adaptor molecule that is upregulated during the induced granulocytic differentiation of promyelocytic leukemic cells and during normal human myelopoiesis. This report describes the generation of PRAM-1-deficient mice and an analysis of the function of this adaptor in neutrophil differentiation and mature neutrophil function. We demonstrate here that neutrophil differentiation is not impaired in PRAM-1-deficient mice and that PRAM-1-deficient neutrophils function normally following engagement of Fcgamma receptors. In contrast, mature PRAM-1-null neutrophils exhibit significant defects in adhesion-dependent reactive oxygen intermediate production and degranulation. Surprisingly, other integrin-dependent responses, such as cell spreading and activation of several signaling pathways, are normal. Together, these findings demonstrate the uncoupling of key integrin-dependent responses in the absence of PRAM-1 and show this adaptor to be critical for select integrin functions in neutrophils.

ADAP-SLP-76 binding differentially regulates supramolecular activation cluster (SMAC) formation relative to T cell-APC conjugation

T cell–APC conjugation as mediated by leukocyte function-associated antigen-1 (LFA-1)–intercellular adhesion molecule (ICAM)-1 binding is followed by formation of the supramolecular activation cluster (SMAC) at the immunological synapse. The intracellular processes that regulate SMAC formation and its influence on T cell function are important questions to be addressed. Here, using a mutational approach, we demonstrate that binding of adaptor adhesion and degranulation promoting adaptor protein (ADAP) to SLP-76 differentially regulates peripheral SMAC (pSMAC) formation relative to conjugation. Although mutation of the YDDV sites (termed M12) disrupted SLP-76 SH2 domain binding and prevented the ability of ADAP to increase conjugation and LFA-1 clustering, M12 acted selectively as a dominant negative (DN) inhibitor of pSMAC formation, an effect that was paralleled by a DN effect on interleukin-2 production. ADAP also colocalized with LFA-1 at the immunological synapse. Our findings identify ADAP–SLP-76 binding as a signaling event that differentially regulates SMAC formation, and support a role for SMAC formation in T cell cytokine production.

Hematopoietic adaptors in T-cell signaling: potential applications to transplantation

Recent advances have been made in understanding the basis of T-cell signaling with the identification of hematopoeitic-specific adaptor proteins, or molecular scaffolds that facilitate protein complex formation and the integration of signals from the surface of T cells. Their potential relevance as targets in the modulation of transplantation relates to their immune-cell-specific expression and their ability to integrate signals needed for T-cell/APC conjugate formation, cytokine production and the clonal expansion of T cells. While LAT, GADS and SLP-76 are needed for TcR-induced cytokine production, the adaptors ADAP, VAV and SKAP-55 play specialized roles in the regulation of integrin adhesion and conjugation. Given the importance of these functions to the reactivity of T cells to allodeterminants of tissue grafts (GvH), and in the recognition and destruction of leukemic cells (GvL), these adaptors represent a new generation of potential targets in the modulation of transplantation.

Cytoskeletal remodeling in lymphocyte activation

The formerly distinct fields of lymphocyte signal transduction and cytoskeletal remodeling have recently become linked, as proteins involved in transducing signals downstream of lymphocyte antigen receptors have also been implicated in actin cytoskeleton remodeling, microtubule dynamics and regulation of cell polarity. These discoveries have fuelled interest in understanding both the role of the actin cytoskeleton as an integral component of lymphocyte activation and the interplay between lymphoid cell-cell contact sites (immunological synapse), retractile pole structures (uropod, distal pole complex), and Rho-family GTPases (Rac, Rho, Cdc42), their upstream activators (Dbl-family guanine nucleotide exchange factors) and their downstream effectors (WASp, Arp2/3, ADAP). To understand how these complex regulatory networks are wired, a new breed of computational biologists uses mathematical language to reproduce and simulate signaling circuits 'in silico'.

SKAP-55 regulates integrin adhesion and formation of T cell-APC conjugates

Src kinase-associated phosphoprotein of 55 kDa (SKAP-55; encoded by SCAP1) is a T cell adaptor protein of unknown function that contains a pleckstrin homology and an SH3 domain. Here we show that SKAP-55 regulates integrin-mediated adhesion and conjugate formation between T cells and antigen-presenting cells (APCs). SKAP-55 enhances adhesion to fibronectin and intercellular adhesion molecule-1 (ICAM-1), colocalizes with actin at the T cell-APC synapse and promotes the clustering of lymphocyte-associated antigen-1 (LFA-1). Enhanced conjugation is comparable to that induced by adhesion and degranulation-promoting adaptor protein (ADAP), a binding partner of SKAP-55, and is abrogated by deletion of the SKAP-55 SH3 domain. Conjugate formation is accompanied by the translocation of SKAP-55 to membrane rafts, an event that is regulated by both LFA-1 and T cell receptor ligation. Our findings identify a mechanism by which SKAP-55 modulates T cell responses to antigen.

The TCR ADAPts to integrin-mediated cell adhesion

Adapter proteins regulate leukocyte signal transduction through recruitment of effector molecules to multiprotein complexes. Recent studies in Adhesion and Degranulation promoting Adapter Protein (ADAP)-deficient mice have established that the cytoplasmic phosphoprotein ADAP is required for optimal, mature T-cell proliferation. Furthermore, ADAP plays a key role in T-cell antigen receptor (TCR)-mediated 'inside out' signaling leading to integrin activation and to enhanced cellular adhesion to integrin ligands. ADAP associates physically with molecules known to play roles in the regulation of TCR-stimulated actin polymerization. These associations support the hypothesis that ADAP functions in actin cytoskeletal reorganization leading to cellular adhesion and activation.

Communication between the TCR and integrins: role of the molecular adapter ADAP/Fyb/Slap

TCR stimulation induces integrin-mediated adhesion, facilitating stabilization of conjugates between T cells and antigen-presenting cells and thereby contributing to T cell activation. Integrin activation has been shown to require cytoskeletal reorganization; however, the molecular mechanisms mediating communication between the TCR and integrins remain unclear. Recently the adapter protein ADAP/Fyb/Slap has been shown to couple TCR stimulation to integrin activation by mediating increased integrin avidity. ADAP may also play a role in transduction of external signals by integrins. Like other adapters, ADAP is a multifunctional protein and interacts with molecules such as Fyn, Slp-76, Ena/VASP proteins, Vav1, WASP and the Arp2/3 complex.

Scaffolds, adaptors and linkers of TCR signaling: theory and practice

Four non-enzymatic proteins form the structural core of the TCR signaling machinery, linking antigen-receptor activation to signaling. These four proteins, each with well defined protein-protein interaction domains, include three 'scaffolds' (LAT, SLP-76 and SLAP-130/Fyb/ADAP and a 'pure adaptor' (GADS). The biological functions of many distinct protein-protein interaction domains have been dissected through a methodological series of knockout and reconstitution experiments. In reviewing these recent advances, we attempt to address two questions often asked by immunologists not familiar with the field: what do scaffolds/adaptors/linkers do; and what do these terms mean?

Mechanisms of signaling by the hematopoietic-specific adaptor proteins, SLP-76 and LAT and their B cell counterpart, BLNK/SLP-65

Adaptor proteins lack catalytic activity and contain only protein-protein interaction domains. They have been shown to interact with an ever-growing number of signaling proteins and to play essential roles in many signaling pathways. SLP-76 and LAT are cell-type-specific adaptor proteins expressed in T cells, NK cells, platelets, and mast cells. In these cell types, SLP-76 and LAT are required for signaling by immunoreceptor tyrosine-based activation motif(ITAM)-containing receptors, including the T cell receptor (TCR), the pre-TCR, the high-affinity Fc epsilon receptor, and the platelet GPVI collagen receptor. In B cells, an analogous adaptor, BLNK/SLP-65, is required for signaling by the ITAM-containing B cell receptor. This review summarizes recent research on SLP-76, LAT, and BLNK. A major challenge in understanding adaptor protein function has been to sort out the many interactions mediated by adaptor proteins and to define the mechanisms by which adaptors mediate critical signaling events. In the case of LAT, SLP-76, and BLNK, the availability of tractable genetic systems, deficient in expression of each of these adaptor proteins, has facilitated in-depth investigation of their signaling functions and mechanisms of action. The picture that has emerged is one in which multiple adaptor proteins cooperate to bring about the formation of a large signaling complex, localized to specialized lipid microdomains within the cell membrane and known as GEMs. Adaptors not only recruit signaling proteins, but also play an active role in regulating the conformation and activation of many of the proteins recruited to the complex. In particular, recent research has shed light on the mechanisms by which multiple adaptor proteins cooperate to bring about the recruitment and activation of phospholipase C gamma in response to the activation of ITAM-containing receptors.