Molecular cloning of SLP-76, a 76-kDa tyrosine phosphoprotein associated with Grb2 in T cells

LAT encodes T cell activation pathway balance

Immune cells transduce environmental stimuli into responses essential for host health via complex signaling cascades. T cells, in particular, leverage their unique T cell receptors (TCRs) to detect specific Human Leukocyte Antigen (HLA)-presented peptides. TCR activation is then relayed via linker for activation of T cells (LAT), a TCR-proximal disordered adapter protein, which organizes protein partners and mediates the propagation of signals down diverse pathways including NFAT and AP-1. Here, we studied how balanced downstream pathway activation is encoded in the amino acid sequence of LAT. To comprehensively profile the sequence-function relationship of LAT, we developed a pooled, single-cell, high-content screening approach in which a large series of mutants in the LAT protein were analyzed to characterize their effects on T cell activation. Measuring epigenetic, transcriptomic, and cell surface protein dynamics of single cells harboring distinct LAT mutants, we found functional regions spanning over 40% of the LAT amino acid sequence. Conserved sequence motifs for protein interactions along with charge distribution are critical sequence features, and contribute to interpretation of human genetic variation in LAT. While mutant defect severity spans from moderate to complete loss of function, nearly all defective mutants, irrespective of their position in LAT, confer balanced defects across all downstream pathways. To understand the molecular basis for this observation, we performed proximal protein labeling which demonstrated that disruption of LAT interaction with a single partner protein indirectly disrupts other partner interactions, likely through the dual roles of these proteins as effectors of downstream pathways and bridging factors between LAT molecules. Overall, we report widely distributed functional regions throughout a disordered adapter and a precise physical organization of LAT and interacting molecules which constrains signaling outputs. More broadly, we describe an approach for interrogating sequence-function relationships for proteins with complex activities across regulatory layers of the cell.

Peeking Into the Black Box of T Cell Receptor Signaling

I have spent more than the last 40 years at the University of California, San Francisco (UCSF), studying T cell receptor (TCR) signaling. I was blessed with supportive mentors, an exceptionally talented group of trainees, and wonderful collaborators and colleagues during my journey who have enabled me to make significant contributions to our understanding of how the TCR initiates signaling. TCR signaling events contribute to T cell development as well as to mature T cell activation and differentiation.

GRB2 dimerization mediated by SH2 domain-swapping is critical for T cell signaling and cytokine production

GRB2 is an adaptor protein required for facilitating cytoplasmic signaling complexes from a wide array of binding partners. GRB2 has been reported to exist in either a monomeric or dimeric state in crystal and solution. GRB2 dimers are formed by the exchange of protein segments between domains, otherwise known as "domain-swapping". Swapping has been described between SH2 and C-terminal SH3 domains in the full-length structure of GRB2 (SH2/C-SH3 domain-swapped dimer), as well as between α-helixes in isolated GRB2 SH2 domains (SH2/SH2 domain-swapped dimer). Interestingly, SH2/SH2 domain-swapping has not been observed within the full-length protein, nor have the functional influences of this novel oligomeric conformation been explored. We herein generated a model of full-length GRB2 dimer with an SH2/SH2 domain-swapped conformation supported by in-line SEC-MALS-SAXS analyses. This conformation is consistent with the previously reported truncated GRB2 SH2/SH2 domain-swapped dimer but different from the previously reported, full-length SH2/C-terminal SH3 (C-SH3) domain-swapped dimer. Our model is also validated by several novel full-length GRB2 mutants that favor either a monomeric or a dimeric state through mutations within the SH2 domain that abrogate or promote SH2/SH2 domain-swapping. GRB2 knockdown and re-expression of selected monomeric and dimeric mutants in a T cell lymphoma cell line led to notable defects in clustering of the adaptor protein LAT and IL-2 release in response to TCR stimulation. These results mirrored similarly-impaired IL-2 release in GRB2-deficient cells. These studies show that a novel dimeric GRB2 conformation with domain-swapping between SH2 domains and monomer/dimer transitions are critical for GRB2 to facilitate early signaling complexes in human T cells.

Transcriptome analysis and protein-protein interaction in resistant and susceptible families of Japanese flounder (Paralichthys olivaceus) to understand the mechanism against Edwardsiella tarda

Edwardsiella tarda is one of the most harmful bacterial pathogens for aquaculture flatfish. After artificial infection of 47 Japanese flounder (Paralichthys olivaceus) families, resistant and susceptible families were identified in this study. High-throughput sequencing was performed on the liver transcriptome of uninfected groups (PoRU and PoSU) and infected groups (PoRC and PoSC). Through assembly and annotation, a total of 3012 and 1386 differentially expressed genes (DEGs) were identified in PoRU vs. PoSU and PoRC vs. PoSC. The significant enrichment pathways between PoRU and PoSU were mainly in metabolic and biosynthesis pathways. A total of thirty dominant enrichment pathways between PoRC and PoSC mainly focused on some immune-related pathways, including the hematopoietic cell lineage, cytokine-cytokine receptor interaction, complement and coagulation cascades, antigen processing and presentation, the intestinal immune network for immunoglobulin A (IgA) production and T/B cell receptor signaling pathway. Under the protein-protein interaction (PPI) analysis, hub genes, including CD molecules, complement component factors and chemokines, were identified in the network, and 16 core genes were differentially expressed in resistant and sustainable families in quantitative polymerase chain reaction (qPCR) validation. This study represents the first transcriptome analysis based on resistant and susceptible families and provides resistant genes to understand the potential molecular mechanisms of antibacterial function in marine fish. The results obtained in this study provide crucial information on gene markers for resistant breeding of Japanese flounder.

A rapid CRISPR competitive assay for in vitro and in vivo discovery of potential drug targets affecting the hematopoietic system

CRISPR/Cas9 can be used as an experimental tool to inactivate genes in cells. However, a CRISPR-targeted cell population will not show a uniform genotype of the targeted gene. Instead, a mix of genotypes is generated - from wild type to different forms of insertions and deletions. Such mixed genotypes complicate analysis of the role of the targeted gene in the studied cell population. Here, we present a rapid and universal experimental approach to functionally analyze a CRISPR-targeted cell population that does not involve generating clonal lines. As a simple readout, we leverage the CRISPR-induced genetic heterogeneity and use sequencing to identify how different genotypes are enriched or depleted in relation to the studied cellular behavior or phenotype. The approach uses standard PCR, Sanger sequencing, and a simple sequence deconvoluting software, enabling laboratories without specific in-depth experience to perform these experiments. As proof of principle, we present examples studying various aspects related to hematopoietic cells (T cell development in vivo and activation in vitro, differentiation of macrophages and dendritic cells, as well as a leukemia-like phenotype induced by overexpressing a proto-oncogene). In conclusion, we present a rapid experimental approach to identify potential drug targets related to mature immune cells, as well as normal and malignant hematopoiesis.

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.

Inherited SLP76 deficiency in humans causes severe combined immunodeficiency, neutrophil and platelet defects

The T cell receptor (TCR) signaling pathway is an ensemble of numerous proteins that are crucial for an adequate immune response. Disruption of any protein involved in this pathway leads to severe immunodeficiency and unfavorable clinical outcomes. Here, we describe an infant with severe immunodeficiency who was found to have novel biallelic mutations in SLP76. SLP76 is a key protein involved in TCR signaling and in other hematopoietic pathways. Previous studies of this protein were performed using Jurkat-derived human leukemic T cell lines and SLP76-deficient mice. Our current study links this gene, for the first time, to a human immunodeficiency characterized by early-onset life-threatening infections, combined T and B cell immunodeficiency, severe neutrophil defects, and impaired platelet aggregation. Hereby, we characterized aspects of the patient's immune phenotype, modeled them with an SLP76-deficient Jurkat-derived T cell line, and rescued some consequences using ectopic expression of wild-type SLP76. Understanding human diseases due to SLP76 deficiency is helpful in explaining the mixed T cell and neutrophil defects, providing a guide for exploring human SLP76 biology.

The 14-3-3/SLP76 protein-protein interaction in T-cell receptor signalling: a structural and biophysical characterization

The SH2 domain-containing protein of 76 kDa, SLP76, is an important adaptor protein that coordinates a complex protein network downstream of T-cell receptors (TCR), ultimately regulating the immune response. Upon phosphorylation on Ser376, SLP76 interacts with 14-3-3 adaptor proteins, which leads to its proteolytic degradation. This provides a negative feedback mechanism by which TCR signalling can be controlled. To gain insight into the 14-3-3/SLP76 protein-protein interaction (PPI), we have determined a high-resolution crystal structure of a SLP76 synthetic peptide containing Ser376 with 14-3-3σ. We then characterized its binding to 14-3-3 proteins biophysically by means of fluorescence polarization and isothermal titration calorimetry. Furthermore, we generated two recombinant SLP76 protein constructs and characterized their binding to 14-3-3. Our work lays the foundation for drug design efforts aimed at targeting the 14-3-3/SLP76 interaction and, thereby, TCR signalling.

Glycogen synthase kinase 3 (GSK-3) controls T-cell motility and interactions with antigen presenting cells

Objective

The threonine/serine kinase glycogen synthase kinase 3 (GSK-3) targets multiple substrates in T-cells, regulating the expression of Tbet and PD-1 on T-cells. However, it has been unclear whether GSK-3 can affect the motility of T-cells and their interactions with antigen presenting cells.

Results

Here, we show that GSK-3 controls T-cell motility and interactions with other cells. Inhibition of GSK-3, using structurally distinct inhibitors, reduced T-cell motility in terms of distance and displacement. While SB415286 reduced the number of cell-cell contacts, the dwell times of cells that established contacts with other cells did not differ for T-cells treated with SB415286. Further, the increase in cytolytic T-cell (CTL) function in killing tumor targets was not affected by the inhibition of motility. This data shows that the inhibition of GSK-3 has differential effects on T-cell motility and CTL function where the negative effects on cell–cell interactions is overridden by the increased cytolytic potential of CTLs.

Decoding communication patterns of the innate immune system by quantitative proteomics

The innate immune system is a collective network of cell types involved in cell recruitment and activation using a robust and refined communication system. Engagement of receptor-mediated intracellular signaling initiates communication cascades by conveying information about the host cell status to surrounding cells for surveillance and protection. Comprehensive profiling of innate immune cells is challenging due to low cell numbers, high dynamic range of the cellular proteome, low abundance of secreted proteins, and the release of degradative enzymes (e.g., proteases). However, recent advances in mass spectrometry-based proteomics provides the capability to overcome these limitations through profiling the dynamics of cellular processes, signaling cascades, post-translational modifications, and interaction networks. Moreover, integration of technologies and molecular datasets provide a holistic view of a complex and intricate network of communications underscoring host defense and tissue homeostasis mechanisms. In this Review, we explore the diverse applications of mass spectrometry-based proteomics in innate immunity to define communication patterns of the innate immune cells during health and disease. We also provide a technical overview of mass spectrometry-based proteomic workflows, with a focus on bottom-up approaches, and we present the emerging role of proteomics in immune-based drug discovery while providing a perspective on new applications in the future.

Ubc9 Interacts with and SUMOylates the TCR Adaptor SLP-76 for NFAT Transcription in T Cells

Although the immune adaptor SH2 domain containing leukocyte phosphoprotein of 76 kDa (SLP-76) integrates and propagates the TCR signaling, the regulation of SLP-76 during the TCR signaling is incompletely studied. In this article, we report that SLP-76 interacts with the small ubiquitin-like modifier (SUMO) E2 conjugase Ubc9 and is a substrate for Ubc9-mediated SUMOylation in human and mouse T cells. TCR stimulation promotes SLP-76-Ubc9 binding, accompanied by an increase in SLP-76 SUMOylation. Ubc9 binds to the extreme C terminus of SLP-76 spanning residues 516-533 and SUMOylates SLP-76 at two conserved residues K266 and K284. In addition, SLP-76 and Ubc9 synergizes to augment the TCR-mediated IL-2 transcription by NFAT in a manner dependent of SUMOylation of SLP-76. Moreover, although not affecting the TCR proximal signaling events, the Ubc9-mediated SUMOylation of SLP-76 is required for TCR-induced assembly of Ubc9-NFAT complex for IL-2 transcription. Together, these results suggest that Ubc9 modulates the function of SLP-76 in T cell activation both by direct interaction and by SUMOylation of SLP-76 and that the Ubc9-SLP-76 module acts as a novel regulatory complex in the control of T cell activation.

The Role of Adaptor Proteins in the Biology of Natural Killer T (NKT) Cells

Adaptor proteins contribute to the selection, differentiation and activation of natural killer T (NKT) cells, an innate(-like) lymphocyte population endowed with powerful immunomodulatory properties. Distinct from conventional T lymphocytes NKT cells preferentially home to the liver, undergo a thymic maturation and differentiation process and recognize glycolipid antigens presented by the MHC class I-like molecule CD1d on antigen presenting cells. NKT cells express a semi-invariant T cell receptor (TCR), which combines the Vα14-Jα18 chain with a Vβ2, Vβ7, or Vβ8 chain in mice and the Vα24 chain with the Vβ11 chain in humans. The avidity of interactions between their TCR, the presented glycolipid antigen and CD1d govern the selection and differentiation of NKT cells. Compared to TCR ligation on conventional T cells engagement of the NKT cell TCR delivers substantially stronger signals, which trigger the unique NKT cell developmental program. Furthermore, NKT cells express a panoply of primarily inhibitory NK cell receptors (NKRs) that control their self-reactivity and avoid autoimmune activation. Adaptor proteins influence NKT cell biology through the integration of TCR, NKR and/or SLAM (signaling lymphocyte-activation molecule) receptor signals or the variation of CD1d-restricted antigen presentation. TCR and NKR ligation engage the SH2 domain-containing leukocyte protein of 76kDa slp-76 whereas the SLAM associated protein SAP serves as adaptor for the SLAM receptor family. Indeed, the selection and differentiation of NKT cells selectively requires co-stimulation via SLAM receptors. Furthermore, SAP deficiency causes X-linked lymphoproliferative disease with multiple immune defects including a lack of circulating NKT cells. While a deletion of slp-76 leads to a complete loss of all peripheral T cell populations, mutations in the SH2 domain of slp-76 selectively affect NKT cell biology. Furthermore, adaptor proteins influence the expression and trafficking of CD1d in antigen presenting cells and subsequently selection and activation of NKT cells. Adaptor protein complex 3 (AP-3), for example, is required for the efficient presentation of glycolipid antigens which require internalization and processing. Thus, our review will focus on the complex contribution of adaptor proteins to the delivery of TCR, NKR and SLAM receptor signals in the unique biology of NKT cells and CD1d-restricted antigen presentation.

Disparate effects of Shb gene deficiency on disease characteristics in murine models of myeloid, B-cell, and T-cell leukemia

The Src homology-2 domain protein B is an adaptor protein operating downstream of tyrosine kinases. The Shb gene knockout has been found to accelerate p210 Breakpoint cluster region-cAbl oncogene 1 tyrosine kinase-induced leukemia. In human myeloid leukemia were tumors with high Src homology-2 domain protein B mRNA content, tumors were, however, associated with decreased latency and myeloid leukemia exhibiting immune cell characteristics. Thus, the aim of this study was to investigate the effects of Shb knockout on the development of leukemia in three additional models, that is, colony stimulating factor 3 receptor-T618I–induced neutrophilic leukemia, p190 Breakpoint cluster region-cAbl oncogene 1 tyrosine kinase-induced B-cell leukemia, and G12D-Kras-induced T-cell leukemia/thymic lymphoma. Wild-type or Shb knockout bone marrow cells expressing the oncogenes were transplanted to bone marrow–deficient recipients. Organs from moribund mice were collected and further analyzed. Shb knockout increased the development of CSF3RT618I-induced leukemia and increased the white blood cell count at the time of death. In the p190 Breakpoint cluster region-cAbl oncogene 1 tyrosine kinase B-cell model, Shb knockout reduced white blood cell counts without affecting latency, whereas in the G12D-Kras T-cell model, thymus size was increased without major effects on latency, suggesting that Shb knockout accelerates the development thymic lymphoma. Cytokine secretion plays a role in the progression of leukemia, and consequently Shb knockout bone marrows exhibited lower expression of granulocyte colony stimulating factor and interleukin 6 in the neutrophilic model and interleukin 7 and chemokine C-X-C motif ligand 12 (C-X-C motif chemokine 12) in the B-cell model. It is concluded that in experimental mouse models, the absence of the Shb gene exacerbates the disease in myeloid leukemia, whereas it alters the disease characteristics without affecting latency in B- and T-cell leukemia. The results suggest a role of Shb in modulating the disease characteristics depending on the oncogenic insult operating on hematopoietic cells. These findings help explain the outcome of human disease in relation to Src homology-2 domain protein B mRNA content.

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).

Multifaceted Role of BTLA in the Control of CD8+ T-cell Fate after Antigen Encounter

Purpose: Adoptive T-cell therapy using autologous tumor-infiltrating lymphocytes (TIL) has shown an overall clinical response rate 40%-50% in metastatic melanoma patients. BTLA (B-and-T lymphocyte associated) expression on transferred CD8⁺ TILs was associated with better clinical outcome. The suppressive function of the ITIM and ITSM motifs of BTLA is well described. Here, we sought to determine the functional characteristics of the CD8⁺BTLA⁺TIL subset and define the contribution of the Grb2 motif of BTLA in T-cell costimulation.Experimental Design: We determined the functional role and downstream signal of BTLA in both human CD8⁺ TILs and mouse CD8⁺ T cells. Functional assays were used including single-cell analysis, reverse-phase protein array (RPPA), antigen-specific vaccination models with adoptively transferred TCR-transgenic T cells as well as patient-derived xenograft (PDX) model using immunodeficient NOD-scid IL2Rgammanull (NSG) tumor-bearing mice treated with autologous TILs.Results: CD8⁺BTLA^- TILs could not control tumor growth in vivo as well as their BTLA⁺ counterpart and antigen-specific CD8⁺BTLA^- T cells had impaired recall response to a vaccine. However, CD8⁺BTLA⁺ TILs displayed improved survival following the killing of a tumor target and heightened "serial killing" capacity. Using mutants of BTLA signaling motifs, we uncovered a costimulatory function mediated by Grb2 through enhancing the secretion of IL-2 and the activation of Src after TCR stimulation.Conclusions: Our data portrays BTLA as a molecule with the singular ability to provide both costimulatory and coinhibitory signals to activated CD8⁺ T cells, resulting in extended survival, improved tumor control, and the development of a functional recall response. Clin Cancer Res; 23(20); 6151-64. ©2017 AACR.

Activated Cdc42-associated kinase 1 (ACK1) binds the sterile α motif (SAM) domain of the adaptor SLP-76 and phosphorylates proximal tyrosines

The adaptor protein Src homology 2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) plays a crucial role in T cell activation by linking antigen receptor (T cell receptor, TCR) signals to downstream pathways. At its N terminus, SLP-76 has three key tyrosines (Tyr-113, Tyr-128, and Tyr-145, "3Y") as well as a sterile α motif (SAM) domain whose function is unclear. We showed previously that the SAM domain has two binding regions that mediate dimer and oligomer formation. In this study, we have identified SAM domain-carrying non-receptor tyrosine kinase, activated Cdc42-associated tyrosine kinase 1 (ACK1; also known as Tnk2, tyrosine kinase non-receptor 2) as a novel binding partner of SLP-76. Co-precipitation, laser-scanning confocal microscopy, and in situ proximity analysis confirmed the binding of ACK1 to SLP-76. Further, the interaction was induced in response to the anti-TCR ligation and abrogated by the deletion of SLP-76 SAM domain (ΔSAM) or mutation of Tyr-113, Tyr-128, and Tyr-145 to phenylalanine (3Y3F). ACK1 induced phosphorylation of the SLP-76 N-terminal tyrosines (3Y) dependent on the SAM domain. Further, ACK1 promoted calcium flux and NFAT-AP1 promoter activity and decreased the motility of murine CD4+ primary T cells on ICAM-1-coated plates, an event reversed by a small molecule inhibitor of ACK1 (AIM-100). These findings identify ACK1 as a novel SLP-76-associated protein-tyrosine kinase that modulates early activation events in T cells.

A mutation within the SH2 domain of slp-76 regulates the tissue distribution and cytokine production of iNKT cells in mice

TCR ligation is critical for the selection, activation, and integrin expression of T lymphocytes. Here, we explored the role of the TCR adaptor protein slp-76 on iNKT-cell biology. Compared to B6 controls, slp-76(ace/ace) mice carrying a missense mutation (Thr428Ile) within the SH2-domain of slp-76 showed an increase in iNKT cells in the thymus and lymph nodes, but a decrease in iNKT cells in spleens and livers, along with reduced ADAP expression and cytokine response. A comparable reduction in iNKT cells was observed in the livers and spleens of ADAP-deficient mice. Like ADAP(-/-) iNKT cells, slp-76(ace/ace) iNKT cells were characterized by enhanced CD11b expression, correlating with an impaired induction of the TCR immediate-early gene Nur77 and a decreased adhesion to ICAM-1. Furthermore, CD11b-intrinsic effects inhibited cytokine release, concanavalin A-mediated inflammation, and iNKT-cell accumulation in the liver. Unlike B6 and ADAP(-/-) mice, the expression of the transcription factors Id3 and PLZF was reduced, whereas NP-1-expression was enhanced in slp-76(ace/ace) mice. Blockade of NP-1 decreased the recovery of iNKT cells from peripheral lymph nodes, identifying NP-1 as an iNKT-cell-specific adhesion factor. Thus, slp-76 contributes to the regulation of the tissue distribution, PLZF, and cytokine expression of iNKT cells via ADAP-dependent and -independent mechanisms.

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

Scaffold Protein SLP-76 Primes PLCγ1 for Activation by ITK-Mediated Phosphorylation

Activation of the phospholipase, PLCγ1, is critical for proper T cell signaling following antigen receptor engagement. In T cells, the Tec family kinase, interleukin-2-induced tyrosine kinase (ITK), phosphorylates PLCγ1 at tyrosine 783 (Y783) leading to activation of phospholipase function and subsequent production of the second messengers inositol 1,4,5-trisphosphate and diacylglycerol. In this work, we demonstrate that PLCγ1 can be primed for ITK-mediated phosphorylation on Y783 by a specific region of the adaptor protein, SLP-76. The SLP-76 phosphotyrosine-containing sequence, pY(173)IDR, does not conform to the canonical recognition motif for an SH2 domain yet binds with significant affinity to the C-terminal SH2 domain of PLCγ1 (SH2C). The SLP-76 pY(173) motif competes with the autoinhibited conformation surrounding the SH2C domain of PLCγ1 leading to exposure of the ITK recognition element on the PLCγ1 SH2 domain and release of the target tyrosine, Y783. These data contribute to the evolving model for the molecular events occurring early in the T cell activation process.

Modulation of TCR responsiveness by the Grb2-family adaptor, Gads

T cell antigen receptor (TCR) signaling depends on three interacting adaptor proteins: SLP-76, Gads, and LAT. Their mechanisms of signaling have been extensively explored, with the aid of fortuitously isolated LAT- and SLP-76-deficient T cell lines, but no such tools were available for Gads, a Grb2-family adaptor that bridges the TCR-inducible interaction between SLP-76 and LAT. TALEN-directed genome editing was applied to disrupt the first coding exon of human Gads in the Jurkat T cell line. Gads was dispensable for TCR-induced phosphorylation of SLP-76, but was a dose-dependent amplifier of TCR-induced CD69 expression. Gads conferred responsiveness to weak TCR stimuli, leading to PLC-γ1 phosphorylation and calcium flux. TALEN-derived, Gads-deficient T cell lines provide a uniquely tractable genetic platform for exploring its regulatory features, such as Gads phosphorylation at T262, which we observed by mass spectrometry. Upon mutation of this site, TCR responsiveness and sensitivity to weak TCR stimuli were increased. This study demonstrates the feasibility of TALEN-based reverse genetics in Jurkat T cells, while enriching our understanding of Gads as a regulated modulator of TCR sensitivity.

Proteomic analysis of the SH2 domain-containing leukocyte protein of 76 kDa (SLP76) interactome in resting and activated primary mast cells [corrected]

We report the first proteomic analysis of the SLP76 interactome in resting and activated primary mouse mast cells. This was made possible by a novel genetic approach used for the first time here. It consists in generating knock-in mice that express signaling molecules bearing a C-terminal tag that has a high affinity for a streptavidin analog. Tagged molecules can be used as molecular baits to affinity-purify the molecular complex in which they are engaged, which can then be studied by mass spectrometry. We examined first SLP76 because, although this cytosolic adapter is critical for both T cell and mast cell activation, its role is well known in T cells but not in mast cells. Tagged SLP76 was expressed in physiological amounts and fully functional in mast cells. We unexpectedly found that SLP76 is exquisitely sensitive to mast cell granular proteases, that Zn(2+)-dependent metalloproteases are especially abundant in mast cells and that they were responsible for SLP76 degradation. Adding a Zn(2+) chelator fully protected SLP76 in mast cell lysates, thereby enabling an efficient affinity-purification of this adapter with its partners. Label-free quantitative mass spectrometry analysis of affinity-purified SLP76 interactomes uncovered both partners already described in T cells and novel partners seen in mast cells only. Noticeably, molecules inducibly recruited in both cell types primarily concur to activation signals, whereas molecules recruited in activated mast cells only are mostly associated with inhibition signals. The transmembrane adapter LAT2, and the serine/threonine kinase with an exchange factor activity Bcr were the most recruited molecules. Biochemical and functional validations established the unexpected finding that Bcr is recruited by SLP76 and positively regulates antigen-induced mast cell activation. Knock-in mice expressing tagged molecules with a normal tissue distribution and expression therefore provide potent novel tools to investigate signalosomes and to uncover novel signaling molecules in mast cells.

SLP-76 sterile α motif (SAM) and individual H5 α helix mediate oligomer formation for microclusters and T-cell activation

Despite the importance of the immune adaptor SLP-76 in T-cell immunity, it has been unclear whether SLP-76 directly self-associates to form higher order oligomers for T-cell activation. In this study, we show that SLP-76 self-associates in response to T-cell receptor ligation as mediated by the N-terminal sterile α motif (SAM) domain. SLP-76 co-precipitated alternately tagged SLP-76 in response to anti-CD3 ligation. Dynamic light scattering and fluorescent microscale thermophoresis of the isolated SAM domain (residues 1–78) revealed evidence of dimers and tetramers. Consistently, deletion of the SAM region eliminated SLP-76 co-precipitation of itself, concurrent with a loss of microcluster formation, nuclear factor of activated T-cells (NFAT) transcription, and interleukin-2 production in Jurkat or primary T-cells. Furthermore, the H5 α helix within the SAM domain contributed to self-association. Retention of H5 in the absence of H1–4 sufficed to support SLP-76 self-association with smaller microclusters that nevertheless enhanced anti-CD3-driven AP1/NFAT transcription and IL-2 production. By contrast, deletion of the H5 α helix impaired self-association and anti-CD3 induced AP1/NFAT transcription. Our data identified for the first time a role for the SAM domain in mediating SLP-76 self-association for T-cell function.

SLP-76 is required for high-affinity IgE receptor- and IL-3 receptor-mediated activation of basophils

Basophils have been reported to play a critical role in allergic inflammation by secreting IL-4 in response to IL-3 or high-affinity IgE receptor (FcεRI)-cross-linking. However, the signaling pathways downstream of FcεRI and the IL-3 receptor in basophils have yet to be determined. In the present study, we used mice deficient in SLP-76 (Src homology 2 domain-containing leukocyte phosphoprotein of 76kDa) to demonstrate critical functions of this adaptor molecule in transducing FcεRI- and IL-3 receptor-mediated signals that induce basophil activation. Although SLP-76 was dispensable for in vivo differentiation, as well as IL-3-induced in vitro proliferation of basophils, IL-4 production induced by both stimuli was completely ablated by SLP-76 deficiency. Biochemical analyses revealed that IL-3-induced phosphorylation of phospholipase C (PLC) γ2 and Akt, but not STAT5, was severely reduced in SLP-76-deficient basophils, whereas FcεRI cross-linking phosphorylation of PLCγ2, but not Akt, was abrogated by SLP-76 deficiency, suggesting important differences in the requirement of SLP-76 for Akt activation between FcεRI- and IL-3 receptor-mediated signaling pathways in basophils. Because IL-3-induced IL-4 production was sensitive to calcineurin inhibitors and an intracellular calcium chelator, in addition to PI3K inhibitors, SLP-76 appears to regulate FcεRI- and IL-3 receptor-induced IL-4 production via mediating PLCγ2 activation in basophils. Taken together, these findings indicate that SLP-76 is an essential signaling component for basophil activation downstream of both FcεRI and the IL-3 receptor.

The adaptor protein SLP-76 regulates HIV-1 release and cell-to-cell transmission in T cells

HIV-1 infection in T cells is regulated by TCR activation. However, the cellular proteins of the TCR pathway that regulate HIV-1 infection are poorly characterized. In this study, in HIV-1 infection, we observed a significant reduction of HIV-1 virus production in Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76)-deficient Jurkat T cells compared with wild-type and SLP-76-reconstituted Jurkat T cells. We further confirmed the role of SLP-76 in HIV-1 infection by small interfering RNA-mediated knockdown in MT4 cells and PBMCs. Structural-functional analysis revealed that the N-terminal domain of SLP-76 was important for regulating HIV-1 infection. Further mechanistic studies revealed that lack of SLP-76 impaired virus release, but did not affect viral entry, integration, and transcription. We also showed that SLP-76 plays a critical role in cell-to-cell transmission of HIV-1. Signaling studies revealed that SLP-76 associated with viral negative regulatory factor protein and multiple signaling molecules during HIV-1 infection. Furthermore, SLP-76 facilitated the association of negative regulatory factor and F-actin, suggesting that SLP-76 mediates the formation of a signaling complex that may regulate viral release via cytoskeletal changes. Taken together, our studies demonstrate a novel role for the adaptor molecule SLP-76 in regulating HIV-1 infection in T cells with the potential to develop innovative strategies against HIV-1.

SCIMP, a transmembrane adaptor protein involved in major histocompatibility complex class II signaling

Formation of the immunological synapse between an antigen-presenting cell (APC) and a T cell leads to signal generation in both cells involved. In T cells, the lipid raft-associated transmembrane adaptor protein LAT plays a central role. Its phosphorylation is a crucial step in signal propagation, including the calcium response and mitogen-activated protein kinase activation, and largely depends on its association with the SLP76 adaptor protein. Here we report the discovery of a new palmitoylated transmembrane adaptor protein, termed SCIMP. SCIMP is expressed in B cells and other professional APCs and is localized in the immunological synapse due to its association with tetraspanin-enriched microdomains. In B cells, it is constitutively associated with Lyn kinase and becomes tyrosine phosphorylated after major histocompatibility complex type II (MHC-II) stimulation. When phosphorylated, SCIMP binds to the SLP65 adaptor protein and also to the inhibitory kinase Csk. While the association with SLP65 initiates the downstream signaling cascades, Csk binding functions as a negative regulatory loop. The results suggest that SCIMP is involved in signal transduction after MHC-II stimulation and therefore serves as a regulator of antigen presentation and other APC functions.

Role of two adaptor molecules SLP-76 and LAT in the PI3K signaling pathway in activated T cells

Previously, we identified p85, a subunit of PI3K, as one of the molecules that interacts with the N-terminal region of Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76). We also demonstrated that tyrosine phosphorylation either at the 113 and/or 128 position is sufficient for the association of SLP-76 with the Src homology 2 domain near the N terminus of p85. The present study further examines the role of the association of these two molecules on the activation of PI3K signaling cascade. Experiments were done to determine the role of SLP-76, either wild-type, tyrosine mutants, or membrane-targeted forms of various SLP-76 constructs, on the membrane localization and phosphorylation of Akt, which is an event downstream of PI3K activation. Reconstitution studies with these various SLP-76 constructs in a Jurkat variant cell line that lacks SLP-76 or linker for activation of T cells (LAT) show that the activation of PI3K pathway following TCR ligation requires both SLP-76 and LAT adaptor proteins. The results suggest that SLP-76 associates with p85 after T cell activation and that LAT recruits this complex to the membrane, leading to Akt activation.

Shb deficient mice display an augmented TH2 response in peripheral CD4+ T cells

Background

Shb, a ubiquitously expressed Src homology 2 domain-containing adaptor protein has previously been implicated in the signaling of various tyrosine kinase receptors including the TCR. Shb associates with SLP76, LAT and Vav, all important components in the signaling cascade governing T cell function and development. A Shb knockout mouse was recently generated and the aim of the current study was to address the importance of Shb deficiency on T cell development and function.

Results

Shb knockout mice did not display any major changes in thymocyte development despite an aberrant TCR signaling pattern, including increased basal activation and reduced stimulation-induced phosphorylation. The loss of Shb expression did however affect peripheral CD4+ T_H cells resulting in an increased proliferative response to TCR stimulation and an elevated IL-4 production of naïve T_H cells. This suggests a T_H2 skewing of the Shb knockout immune system, seemingly caused by an altered TCR signaling pattern.

Conclusion

Our results indicate that Shb appears to play an important modulating role on TCR signaling, thus regulating the peripheral CD4+ T_H2 cell response.

Binding of the cSH3 domain of Grb2 adaptor to two distinct RXXK motifs within Gab1 docker employs differential mechanisms

A ubiquitous component of cellular signaling machinery, Gab1 docker plays a pivotal role in routing extracellular information in the form of growth factors and cytokines to downstream targets such as transcription factors within the nucleus. Here, using isothermal titration calorimetry (ITC) in combination with macromolecular modeling (MM), we show that although Gab1 contains four distinct RXXK motifs, designated G1, G2, G3, and G4, only G1 and G2 motifs bind to the cSH3 domain of Grb2 adaptor and do so with distinct mechanisms. Thus, while the G1 motif strictly requires the PPRPPKP consensus sequence for high-affinity binding to the cSH3 domain, the G2 motif displays preference for the PXVXRXLKPXR consensus. Such sequential differences in the binding of G1 and G2 motifs arise from their ability to adopt distinct polyproline type II (PPII)- and 3(10) -helical conformations upon binding to the cSH3 domain, respectively. Collectively, our study provides detailed biophysical insights into a key protein-protein interaction involved in a diverse array of signaling cascades central to health and disease.

Understanding signal integration through targeted mutations of an adapter protein

Immunoreceptor engagement leads to the activation of multiple second messenger cascades, and integration of these pathways requires proper function of a number of adapter proteins. Although adapters possess no intrinsic enzymatic function, they nucleate the formation of multi-molecular protein complexes to support downstream signaling. Since adapters contain functionally distinct domains, intense investigation has been devoted to understanding how these regions act to integrate signals. This review describes the evolution of studies investigating one of these adapters, the SH2 domain-containing leukocyte protein of 76 kDa. Through utilizing biochemical, genetic and imaging techniques, a model has emerged describing how this adapter regulates signals resulting in complex immune responses.

Coordination of receptor signaling in multiple hematopoietic cell lineages by the adaptor protein SLP-76

The adaptor protein SLP-76 is expressed in multiple hematopoietic lineages including T cells, platelets, and neutrophils. SLP-76 mediated signaling is dependent on its multiple protein interaction domains, as it creates a scaffold on which key signaling complexes are built. SLP-76 is critical for supporting signaling downstream of both immunoreceptors and integrins. The signaling molecules used both upstream and downstream of SLP-76 are similar among these receptors and across cell types; however, important differences exist. Appreciating how SLP-76 coordinates signal transduction across different cell and receptor types provides insights into the complex interplay of pathways critical for activation of cells of the immune system that are essential for host defense.

Genome-based expression profiling study of Hunner's ulcer type interstitial cystitis: an array of 40-gene model

INTRODUCTION AND HYPOTHESIS

The aim of this study was to explore potential molecular mechanisms contributing to the pathogenesis of Hunner's ulcer type interstitial cystitis (IC).

METHODS

Dataset acquisitions from Gene Expression Omnibus under platform accession no GSE 11783. We compared global gene expression profiles in bladder epithelial cells from IC patients with Hunner's ulcer corresponding to normal controls. We re-sampling and exploit the correlation structure presented in the dataset through the transcriptional response. For each patient, two bladder biopsies were studied, one from an ulcer area and one from a non-ulcer area. RNA was extracted, and all labeled samples were hybridized to Human Genome U133 Plus 2.0 Array (Affymetrix, CA, USA).

RESULTS

The Mahalanobis distance in hierarchical cluster analysis revealed a model of 40 genes expression which is increased in IC and ulcerated IC. Our results can be summarized as follows: First, the expressions of major histocompatibility complex (MHC) class IF and II molecules, leukocyte immunoglobulin-like receptors, hepatitis A virus cellular receptor 2, and interleukin 32 were increased in bladder epithelial from IC and ulcerative IC area. Next, there is an indication of antigen-mediated aggregation of the high-affinity Fc epsilon and gamma RI leading to allergic inflammation through the disease status. Third, the high-affinity Fc gamma RI subunit facilitated T-cell-mediated immune response through the disease status. Such changes, jointly termed "bladder remodeling," can constitute an important long-term consequence of Hunner's ulcer type IC.

CONCLUSIONS

Our results indicate that genome-based expression profiling can be used for the diagnostic tests of Hunner's ulcer type IC in clinical practice.

Gene expression patterns associated with chicken jejunal development

Jejunal development occurs in a spatio-temporal pattern and is characterized by morphological and functional changes. To investigate jejunal development at the transcriptomic level, we performed microarray studies in 1-21-day-old chickens. Nine gene clusters were identified, each with a specific gene expression pattern. Subsequently, groups of genes with similar functions could be identified. Genes involved in morphological and functional development were highly expressed immediately after hatch with declining expression patterns afterwards. Immunological development can be roughly divided based on expression patterns into three processes over time; first innate response and immigration of immune cells, secondly differentiation and specialization, and thirdly maturation and immune regulation. We conclude that specific gene expression patterns coincide with the immunological, morphological, and functional development as measured by other methods. Our data show that transcriptomic approaches provide more detailed information on the biological processes underlying jejunal development.

SLP-76 couples Syk to the osteoclast cytoskeleton

The capacity of the osteoclast (OC) to resorb bone is dictated by cytoskeletal organization, which in turn emanates from signals derived from the alpha(v)beta(3) integrin and c-Fms. Syk is key to these signals and, in other cells, this tyrosine kinase exerts its effects via intermediaries including the SLP adaptors, SLP-76 and BLNK (B cell linker). Thus, we asked whether these two SLP proteins regulate OC function. We find BLNK-deficient OCs are normal, whereas cytoskeletal organization of those lacking SLP-76 is delayed, thus modestly reducing bone resorption in vitro. Cytoskeletal organization and bone resorption are more profoundly arrested in cultured OCs deficient in BLNK and SLP-76 double knockout (DKO) phenotypes. In contrast, stimulated bone resorption in vivo is inhibited approximately 40% in either SLP-76(-/-) or DKO mice. This observation, taken with the fact that DKO OCs are rescued by retroviral transduction of only SLP-76, indicates that SLP-76 is the dominant SLP family member in the resorptive process. We also find SLP-76 is phosphorylated in a Syk-dependent manner. Furthermore, in the absence of the adaptor protein, integrin-mediated phosphorylation of Vav3, the OC cytoskeleton-organizing guanine nucleotide exchange factor, is abrogated. In keeping with a central role of SLP-76/Vav3 association in osteoclastic resorption, retroviral transduction of SLP-76, in which the Vav binding site is disrupted (3YF), fails to normalize the cytoskeleton of DKO OCs and the resorptive capacity of the cells. Finally, c-Fms-activated Syk also exerts its OC cytoskeleton-organizing effect in a SLP-76/Vav3-dependent manner.

The Role of SH2 Domain-containing Leukocyte Phosphoprotein of 76 kDa in the Regulation of Immune Cell Development and Function

Recent years have seen an explosion of new knowledge defining the molecular events that are critical for development and activation of immune cells. Much of this new information has come from a careful molecular dissection of key signal transduction pathways that are initiated when immune cell receptors are engaged. In addition to the receptors themselves and critical effector molecules, these signaling pathways depend on adapters, proteins that have no intrinsic effector function but serve instead as scaffolds to nucleate multimolecular complexes. This review summarizes some of what has been learned about one such adapter protein, SH2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76), and how it regulates and integrates signals after engagement of immunoreceptors and integrins on various immune cell lineages.

Biochemical signaling pathways for memory T cell recall

Memory T cells exhibit low activation thresholds and rapid effector responses following antigen stimulation, contrasting naive T cells with high activation thresholds and no effector responses. Signaling mechanisms for the distinct properties of naive and memory T cells remain poorly understood. Here, I will discuss new results on signal transduction in naive and memory T cells that suggest proximal control of activation threshold and a distinct biochemical pathway to rapid recall. The signaling and transcriptional pathways controlling immediate effector function in memory T cells closely resemble pathways for rapid effector cytokine production in innate immune cells, suggesting memory T cells use innate pathways for efficacious responses.

Signal transduction during natural killer cell activation

Understanding of transmembrane signaling during NK-cell activation has greatly expanded during the past few years. The discovery and characterization of novel triggering and inhibitory receptors have revealed the complexity of these processes. This unit focuses on receptor-initiated signaling pathways that modulate NK functions. Establishing the roles of different signaling pathways in NK cells is a crucial step in the design of therapeutic approaches for selective enhancement or suppression of NK-cell activation.

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.

Animal models for the molecular and mechanistic study of lymphatic biology and disease

The development of animal model systems for the study of the lymphatic system has resulted in an explosion of information regarding the mechanisms governing lymphatic development and the diseases associated with lymphatic dysfunction. Animal studies have led to a new molecular model of embryonic lymphatic vascular development, and have provided insight into the pathophysiology of both inherited and acquired lymphatic insufficiency. It has become apparent, however, that the importance of the lymphatic system to human disease extends, beyond its role in lymphedema, to many other diverse pathologic processes, including, very notably, inflammation and tumor lymphangiogenesis. Here, we have undertaken a systematic review of the models as they relate to molecular and functional characterization of the development, maturation, genetics, heritable and acquired diseases, and neoplastic implications of the lymphatic system. The translation of these advances into therapies for human diseases associated with lymphatic dysfunction will require the continued study of the lymphatic system through robust animal disease models that simulate their human counterparts.

Signal transduction during natural killer cell activation

Natural killer (NK) cells are a subpopulation of lymphocytes that can mediate cytotoxicity of certain tumor cells, virus-infected cells, and normal cells. In addition to their cytotoxic potential, NK cells secrete a variety of cytokines and chemokines that can modulate the function, growth, and differentiation of other immune cells. These different responses are initiated by the interaction of specific NK surface receptors with defined soluble or cell-associated ligands. There are several different types of receptors on the NK cell surface including "triggering" receptors, adhesion molecules, cytokine receptors, and MHC-recognizing killer-cell inhibitory receptors. The functional response of an NK cell is the result of the integration of signals transduced by these different types of receptors. Some of these signaling pathways are similar to other lymphoid cells, but there are also unique features employed by NK cells. This overview focuses on receptor-initiated signaling pathways that modulate NK functions.

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.

SLP-65 signal transduction requires Src homology 2 domain-mediated membrane anchoring and a kinase-independent adaptor function of Syk

The family of SLPs (Src homology 2 domain-containing leukocyte adaptor proteins) are cytoplasmic signal effectors of lymphocyte antigen receptors. A main function of SLP is to orchestrate the assembly of Ca(2+)-mobilizing enzymes at the inner leaflet of the plasma membrane. For this purpose, SLP-76 in T cells utilizes the transmembrane adaptor LAT, but the mechanism of SLP-65 membrane anchoring in B cells remains an enigma. We now employed two genetic reconstitution systems to unravel structural requirements of SLP-65 for the initiation of Ca(2+) mobilization and subsequent activation of gene transcription. First, mutational analysis of SLP-65 in DT40 B cells revealed that its C-terminal Src homology 2 domain controls efficient tyrosine phosphorylation by the kinase Syk, plasma membrane recruitment, as well as downstream signaling to NFAT activation. Second, we dissected these processes by expressing SLP-65 in SLP-76-deficient T cells and found that a kinase-independent adaptor function of Syk is required to link phosphorylated SLP-65 to Ca(2+) mobilization. These approaches unmask a mechanistic complexity of SLP-65 activation and coupling to signaling cascades in that Syk is upstream as well as downstream of SLP-65. Moreover, membrane anchoring of the SLP-65-assembled Ca(2+) initiation complex, which appears to be fundamentally different from that of closely related SLP-76, does not necessarily involve a B cell-specific component.

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.

Keeping the (kinase) party going: SLP-76 and ITK dance to the beat

The Tec-family protein tyrosine kinase IL-2-inducible T cell kinase (ITK) mediates T cell activation, as does the adaptor protein SLP-76 (SH2-domain-containing leukocyte protein of 76 kD), which forms a complex with ITK and other intracellular signaling enzymes. One of these enzymes is phospholipase C-gamma1 (PLC-gamma1), which mediates T cell receptor (TCR)-stimulated intracellular calcium mobilization leading to the activation of transcription factors such as nuclear factor of activated T cells. The Src-family tyrosine kinase Lck and the Syk-family tyrosine kinase zeta chain-associated protein kinase of 70 kD (ZAP-70), together with ITK, are necessary for the phosphorylation of PLC-gamma1 in response to TCR stimulation. ITK is thought to phosphorylate a specific tyrosine residue of PLC-gamma1 that is required for its activation. The mechanism of activation of ITK appears to involve the interaction between SLP-76 and ITK, which not only initiates ITK activity but is also important to maintain the kinase activity of ITK. This suggests that SLP-76 acts as more than a neutral adaptor in mediating T cell activation; SLP-76 also directly influences the kinase activity of ITK, allowing ITK to phosphorylate PLC-gamma1.

A novel pathway down-modulating T cell activation involves HPK-1-dependent recruitment of 14-3-3 proteins on SLP-76

The SH2 domain-containing leukocyte protein of 76 kD (SLP-76) is a pivotal element of the signaling machinery controlling T cell receptor (TCR)-mediated activation. Here, we identify 14-3-3epsilon and zeta proteins as SLP-76 binding partners. This interaction was induced by TCR ligation and required phosphorylation of SLP-76 at serine 376. Ribonucleic acid interference and in vitro phosphorylation experiments showed that serine 376 is the target of the hematopoietic progenitor kinase 1 (HPK-1). Interestingly, either S376A mutation or HPK-1 knockdown resulted in increased TCR-induced tyrosine phosphorylation of SLP-76 and phospholipase C-gamma1. Moreover, an SLP-76-S376A mutant induced higher interleukin 2 gene transcription than wild-type SLP-76. These data reveal a novel negative feedback loop involving HPK-1-dependent serine phosphorylation of SLP-76 and 14-3-3 protein recruitment, which tunes T cell activation.

No Life Without Death

Apoptosis-programed cell death-is the most common form of death in the body. Once apoptosis is induced, proper execution of the cell death program requires the coordinated activation and execution of multiple molecular processes. Here, we describe the pathways and the basic components of the death-inducing machinery. Since apoptosis is a key regulator of tissue homeostasis, an imbalance of apoptosis results in severe diseases like cancer, autoimmunity, and AIDS.

Syk and Slp-76 mutant mice reveal a cell-autonomous hematopoietic cell contribution to vascular development

Developmental studies support a common origin for blood and endothelial cells, while studies of adult angiogenic responses suggest that the hematopoietic system can be a source of endothelial cells later in life. Whether hematopoietic tissue is a source of endothelial cells during normal vascular development is unknown. Mouse embryos lacking the signaling proteins Syk and Slp-76 develop abnormal blood-lymphatic endothelial connections. Here we demonstrate that expression of GFPSlp-76 in a subset of hematopoietic cells rescues this phenotype, and that deficient cells confer focal vascular phenotypes in chimeric embryos consistent with a cell-autonomous mechanism. Endogenous Syk and Slp-76, as well as transgenic GFPSlp-76, are expressed in circulating cells previously proposed to be endothelial precursors, supporting a causal role for these cells. These studies provide genetic evidence for hematopoietic contribution to vascular development and suggest that hematopoietic tissue can provide a source of vascular endothelial progenitor cells throughout life.

Crystal structure of the C-terminal SH3 domain of the adaptor protein GADS in complex with SLP-76 motif peptide reveals a unique SH3-SH3 interaction

The Grb2-like adaptor protein GADS is essential for tyrosine kinase-dependent signaling in T lymphocytes. Following T cell receptor ligation, GADS interacts through its C-terminal SH3 domain with the adaptors SLP-76 and LAT, to form a multiprotein signaling complex that is crucial for T cell activation. To understand the structural basis for the selective recognition of GADS by SLP-76, herein is reported the crystal structure at 1.54 Angstrom of the C-terminal SH3 domain of GADS bound to the SLP-76 motif 233-PSIDRSTKP-241, which represents the minimal binding site. In addition to the unique structural features adopted by the bound SLP-76 peptide, the complex structure reveals a unique SH3-SH3 interaction. This homophilic interaction, which is observed in presence of the SLP-76 peptide and is present in solution, extends our understanding of the molecular mechanisms that could be employed by modular proteins to increase their signaling transduction specificity.