Known and potential functions for the SLP-76 adapter protein in regulating T-cell activation and development

Heterogeneity of Signaling Complex Nanostructure in T Cells Activated Via the T Cell Antigen Receptor

Activation of the T cell antigen receptor (TCR) is a key step in initiating the adaptive immune response. Single-molecule localization techniques have been used to investigate the arrangement of proteins within the signaling complexes formed around activated TCRs, but a clear picture of nanoscale organization in stimulated T cells has not emerged. Here, we have improved the examination of T cell nanostructure by visualizing individual molecules of six different proteins in a single sample of activated Jurkat T cells using the multiplexed antibody-size limited direct stochastic optical reconstruction microscopy (madSTORM) technique. We formally define irregularly shaped regions of interest, compare areas where signaling complexes are concentrated with other areas, and improve the statistical analyses of the locations of molecules. We show that nanoscale organization of proteins is mainly confined to the areas with dense concentrations of TCR-based signaling complexes. However, randomly distributed molecules are also found in some areas containing concentrated signaling complexes. These results are consistent with the view that the proteins within signaling complexes are connected by numerous weak interactions, leading to flexible, dynamic, and mutable structures which produce large variations in the nanostructure found in activated T cells.

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.

Development of nanoscale structure in LAT-based signaling complexes

The adapter molecule linker for activation of T cells (LAT) plays a crucial role in forming signaling complexes induced by stimulation of the T cell receptor (TCR). These multi-molecular complexes are dynamic structures that activate highly regulated signaling pathways. Previously, we have demonstrated nanoscale structure in LAT-based complexes where the adapter SLP-76 (also known as LCP2) localizes to the periphery of LAT clusters. In this study, we show that initially LAT and SLP-76 are randomly dispersed throughout the clusters that form upon TCR engagement. The segregation of LAT and SLP-76 develops near the end of the spreading process. The local concentration of LAT also increases at the same time. Both changes require TCR activation and an intact actin cytoskeleton. These results demonstrate that the nanoscale organization of LAT-based signaling complexes is dynamic and indicates that different kinds of LAT-based complexes appear at different times during T cell activation.

Intervention of PKC-θ as an immunosuppressive regimen

PKC-θ is selectively enriched in T cells and specifically translocates to immunological synapse where it mediates critical T cell receptor signals required for T cell activation, differentiation, and survival. T cells deficient in PKC-θ are defective in their ability to differentiate into inflammatory effector cells that mediate actual immune responses whereas, their differentiation into regulatory T cells (Treg) that inhibits the inflammatory T cells is enhanced. Therefore, the manipulation of PKC-θ activity can shift the ratio between inflammatory effector T cells and inhibitory Tregs, to control T cell-mediated immune responses that are responsible for autoimmunity and allograft rejection. Indeed, PKC-θ-deficient mice are resistant to the development of several Th2 and Th17-dependent autoimmune diseases and are defective in mounting alloimmune responses required for rejection of transplanted allografts and graft-versus-host disease. Selective inhibition of PKC-θ is therefore considered as a potential treatment for prevention of autoimmune diseases and allograft rejection.

Dok-1 overexpression promotes development of γδ natural killer T cells

In T cells, two members of the Dok family, Dok-1 and Dok-2, are predominantly expressed. Recent evidence suggests that they play a negative role in T-cell signaling. In order to define whether Dok proteins regulate T-cell development, we have generated transgenic mice overexpressing Dok-1 in thymocytes and peripheral T cells. We show that overexpression of Dok-1 retards the transition from the CD4(-) CD8(-) to CD4(+) CD8(+) stage. Moreover, there is a specific expansion of PLZF-expressing Vγ1.1(+) Vδ6.3(+) T cells. This subset of γδ T cells acquires innate characteristics including rapid IL-4 production following stimulation and requiring SLAM-associated adaptor protein (SAP) for their development. Moreover, Dok-1 overexpression promotes the generation of an innate-like CD8(+) T-cell population that expresses Eomesodermin. Altogether, these findings identify a novel role for Dok-1 in the regulation of thymic differentiation and in particular, in the development of PLZF(+) γδ T cells.

Cooperative interactions at the SLP-76 complex are critical for actin polymerization

T-cell antigen receptor (TCR) engagement induces formation of multi-protein signalling complexes essential for regulating T-cell functions. Generation of a complex of SLP-76, Nck and VAV1 is crucial for regulation of the actin machinery. We define the composition, stoichiometry and specificity of interactions in the SLP-76, Nck and VAV1 complex. Our data reveal that this complex can contain one SLP-76 molecule, two Nck and two VAV1 molecules. A direct interaction between Nck and VAV1 is mediated by binding between the C-terminal SH3 domain of Nck and the VAV1 N-terminal SH3 domain. Disruption of the VAV1:Nck interaction deleteriously affected actin polymerization. These novel findings shed new light on the mechanism of actin polymerization after T-cell activation.

Viral induction of the zinc finger antiviral protein is IRF3-dependent but NF-kappaB-independent

The zinc finger antiviral protein (ZAP) is an interferon-stimulated gene that restricts the replication of retroviruses, alphaviruses, and filoviruses. Relatively little is known, however, regarding the detailed mechanism of ZAP induction during viral infections. We show that, although being inducible by either interferon or virus, expression of ZAP is more efficiently activated by virus than are several other classical interferon-stimulated genes and that viral induction of ZAP occurs under the direct control of interferon regulatory factor 3 (IRF3) independent of interferon paracrine/autocrine signaling. ZAP was up-regulated in cells unresponsive to type I and III interferons upon engagement of TLR3, retinoic inducible gene I/melanoma differentiation-associated gene 5 pathways, or ectopic expression of a constitutively active IRF3 mutant. Conversely, induction of ZAP by virus or dsRNA was severely impaired in cells expressing a dominant-negative mutant IRF3 and completely abrogated in cells lacking IRF3. In contrast to IRF3, ZAP induction was independent of NF-kappaB activity. Mutational analysis of the human ZAP promoter revealed that multiple interferon-stimulated response elements far distal to the transcription start site serve redundantly to control IRF3-dependent induction of ZAP transcription. Chromatin immunoprecipitation assays demonstrated that IRF3 selectively binds the distal interferon-stimulated response elements in human ZAP promoter following viral infection. Collectively, these data suggest that ZAP is a direct target gene of IRF3 action in cellular antiviral responses.

Endocytic events in TCR signaling: focus on adapters in microclusters

Although the critical role of T-cell receptor (TCR) microclusters in T-cell activation is now widely accepted, the mechanisms of regulation of these TCR-rich structures, which also contain enzymes, adapters, and effectors, remain poorly defined. Soon after microcluster formation, several signaling proteins rapidly dissociate from the TCR. Recent studies from our laboratory demonstrated that the movement of the adapters linker for activation of T cells (LAT) and Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76) away from initial microcluster formation sites represents endocytic events. Ubiquitylation, Cbl proteins, and multiple endocytic pathways are involved in the internalization events that disassemble signaling microclusters. Several recent studies have indicated that microcluster movement and centralization plays an important role in signal termination. We suggest that microcluster movement is directly linked to endocytic events, thus implicating endocytosis of microclusters as a means to regulate signaling output of the T cell.

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.

T-Cell Antigen Receptor-Induced Signaling Complexes: Internalization Via a Cholesterol-Dependent Endocytic Pathway

T-cell antigen receptor engagement causes the rapid assembly of signaling complexes. The adapter protein SLP-76, detected as SLP-yellow fluorescent protein, initially clustered with the TCR and other proteins, then translocated medially on microtubules. As shown by total internal reflection fluorescence microscopy and the inhibition of SLP-76 movement at 16 degrees C, this movement required endocytosis. Immunoelectron microscopy showed SLP-76 staining of smooth pits and tubules. Cholesterol depletion decreased the movement of SLP-76 clusters, as did coexpression of the ubiquitin-interacting motif domain from eps15. These data are consistent with the internalization of SLP-76 via a lipid raft-dependent pathway that requires interaction of the endocytic machinery with ubiquitinylated proteins. The endocytosed SLP-76 clusters contained phosphorylated SLP-76 and phosphorylated LAT. The raft-associated, transmembrane protein LAT likely targets SLP-76 to endocytic vesicles. The endocytosis of active SLP-76 and LAT complexes suggests a possible mechanism for downregulation of signaling complexes induced by TCR activation.

Dual effects of Sprouty1 on TCR signaling depending on the differentiation state of the T cell

Sprouty (Spry) is known to be a negative feedback inhibitor of growth factor receptor signaling through inhibition of the Ras/MAPK pathway. Several groups, however, have reported a positive role for Spry involving sequestration of the inhibitory protein c-Cbl. Thus, Spry may have various functions in the regulation of receptor-mediated signaling depending on the context. In the immune system, the function of Spry is unknown. In this study, we investigated the role of Spry1 in T cell activation. Spry1, among the four mammalian homologs, was specifically induced by TCR signaling of CD4(+) murine T cells. In fully differentiated Th1 clones, overexpressed Spry1 inhibited TCR signaling and decreased IL-2 production while reducing expression with specific siRNA transfection had the opposite effect, increasing IL-2 production. In contrast, in naive T cells, Spry1 overexpression enhanced TCR signaling, and increased proliferation and IL-2 production, while siRNA transfection again had the opposite effect, reducing IL-2 production following activation. The enhancing effect in naive cells was abrogated by preactivation of the T cells with Ag and APC, indicating that the history of exposure to Ag is correlated with a hierarchy of T cell responsiveness to Spry1. Furthermore, both the NF-AT and MAPK pathways were influenced by Spry1, implying a different molecular mechanism from that for growth factor receptor signaling. Thus, Spry1 uses a novel mechanism to bring about differential effects on TCR signaling through the same receptor, depending on the differentiation state of the T cell.

Novel Src homology 3 domain-binding motifs identified from proteomic screen of a Pro-rich region

The Src homology (SH) 3 domain has been shown recently to bind peptide sequences that lack the canonical PXXP motif. The diverse specificity in ligand recognition for a group of 15 SH3 domains has now been investigated using arrays of peptides derived from the proline-rich region of the SH2 domain-containing leukocyte protein of 76 kDa (SLP-76). A screen of the peptide arrays using individual or mixed SH3 domains has allowed the identification of a number of candidate SH3-binding peptides. Although some peptides contain the conventional PXXP motif, most are devoid of such a motif and are instead enriched in basic residues. Fluorescent polarization measurements using soluble peptides and purified SH3 domains demonstrated that several SH3 domains, including those from growth factor receptor-bound protein 2 (Grb2), NCK, and phospholipase C (PLC)-gamma1, bound with moderate affinities (10-100 microm) to a group of non-conventional peptides. Of particular interest, the PLC-gamma1 SH3 domain was found to associate with SLP-76 through at least three distinct sites, two of which bore a novel KKPP motif and the other contained the classic PXXP sequence. Intriguingly mutation of critical residues for the three sites not only affected binding of SLP-76 to the PLC-gamma1 SH3 domain but also to the Grb2 C-terminal SH3 domain, indicating that the binding sites in SLP-76 for the two SH3 domains are overlapped. Our studies suggest that the SH3 domain is an inherently promiscuous interaction module capable of binding to peptides that may or may not contain a PXXP motif. Furthermore the identification of numerous non-conventional SH3-binding peptides in SLP-76 implies that the global ligand pool for SH3 domains in a mammalian proteome may be significantly greater than previously acknowledged.

Mutation of the phospholipase C-gamma1-binding site of LAT affects both positive and negative thymocyte selection

Linker for activation of T cells (LAT) is a scaffolding adaptor protein that is critical for T cell development and function. A mutation of LAT (Y136F) that disrupts phospholipase C-γ1 activation and subsequent calcium influx causes a partial block in T cell development and leads to a severe lymphoproliferative disease in homozygous knock-in mice. One possible contribution to the fatal disease of LAT Y136F knock-in mice could be from autoreactive T cells generated in these mice because of altered thymocyte selection. To examine the impact of the LAT Y136F mutation on thymocyte positive and negative selection, we bred this mutation onto the HY T cell receptor (TCR) transgenic, recombination activating gene-2 knockout background. Female mice with this genotype showed a severe defect in positive selection, whereas male mice exhibited a phenotype resembling positive selection (i.e., development and survival of CD8^hi HY TCR-specific T cells) instead of negative selection. These results support the hypothesis that in non-TCR transgenic, LAT Y136F knock-in mice, altered thymocyte selection leads to the survival and proliferation of autoreactive T cells that would otherwise be negatively selected in the thymus.

Development of the lymphatic vascular system: a mystery unravels

The blood vascular and the lymphatic system play complementary roles in tissue perfusion and fluid reabsorption. Despite its critical role in mediating tissue fluid homeostasis, intestinal lipid absorption, and the immune response, the lymphatic system has not received as much attention as the blood vascular system, largely due to a lack of lymphatic-specific markers and to the dearth of knowledge about the molecular regulation of lymphatic development and function. A series of recent landmark studies now significantly has advanced our understanding of the lymphatic system. Based upon the discovery and characterization of lymphatic-specific growth factors, receptors, and transcriptional regulators, the mystery of lymphatic vascular system development begins to be unraveled. The successful isolation and cultivation of blood vascular and lymphatic endothelial cells has enabled comparative molecular and cellular analyses of these two genetically and developmentally closely related cell lineages. Moreover, studies of several genetic mouse models have set the framework for a new molecular model of embryonic lymphatic vascular development and have identified molecular pathways whose mutational inactivation leads to human diseases associated with lymphedema. Although these rapid advances already have led to development of the first lymphatic-targeted molecular therapies, there still remain many unanswered questions regarding almost every aspect of lymphatic vascular biology, making the lymphatic system a highly exciting and rewarding field of study.