Reciprocal regulation of SH3 and SH2 domain binding via tyrosine phosphorylation of a common site in CD3epsilon

The SCHOOL of nature: I. Transmembrane signaling

Receptor-mediated transmembrane signaling plays an important role in health and disease. Recent significant advances in our understanding of the molecular mechanisms linking ligand binding to receptor activation revealed previously unrecognized striking similarities in the basic structural principles of function of numerous cell surface receptors. In this work, I demonstrate that the Signaling Chain Homooligomerization (SCHOOL)-based mechanism represents a general biological mechanism of transmembrane signal transduction mediated by a variety of functionally unrelated single- and multichain activating receptors. within the SCHOOL platform, ligand binding-induced receptor clustering is translated across the membrane into protein oligomerization in cytoplasmic milieu. This platform resolves a long-standing puzzle in transmembrane signal transduction and reveals the major driving forces coupling recognition and activation functions at the level of protein-protein interactions-biochemical processes that can be influenced and controlled. The basic principles of transmembrane signaling learned from the SCHOOL model can be used in different fields of immunology, virology, molecular and cell biology and others to describe, explain and predict various phenomena and processes mediated by a variety of functionally diverse and unrelated receptors. Beyond providing novel perspectives for fundamental research, the platform opens new avenues for drug discovery and development.

Cloning and expression analysis of Atlantic halibut (Hippoglossus hippoglossus) CD3 genes

The CD3 complex is in higher vertebrates shown to be important for the activation of T-cells. The T-cell system in fish is believed to be similar to that in higher vertebrates, and the CD3 chains could therefore be an important marker for identification of T-cells in fish. Here, we report the cDNA and corresponding gene sequence of Atlantic halibut (Hippoglossus hippoglossus) CD3gammadelta, CD3varepsilon, and CD3zeta chains, and the tissue-specific expression pattern of CD3 and T- cell receptor (TCR) genes. Important structural characteristics defining the CD3 genes seemed to be conserved in the halibut CD3 chains, such as a signal peptide, an extracellular region, a transmembrane helix having a negatively charged residue, and an ITAM bearing cytoplasmic tail. The extracellular domain of halibut CD3gammadelta and CD3varepsilon included two cysteines presumably involved in Ig-fold stabilisation and the CxxCxE motif important for dimerization. A spliced variant of CD3varepsilon was identified, lacking the Ig-fold, but with the CxxCxE motif intact. The real time RT-PCR analysis revealed a highly similar expression pattern of the CD3 genes and the TCRalpha and TCRbeta genes, indicating that the functional relationship between the TCR and the CD3 genes are preserved in teleosts.

Expression of fully assembled TCR-CD3 complex on double positive thymocytes: synergistic role for the PRS and ER retention motifs in the intra-cytoplasmic tail of CD3epsilon

TCR expression on double-positive (DP) thymocytes is a prerequisite for thymic selection that results in the generation of mature CD4(+) and CD8(+) single-positive T cells. TCR is expressed at very low level on preselection DP thymocytes and is dramatically up-regulated on positively selected thymocytes. However, mechanism governing TCR expression on developing thymocytes is not understood. In the present report, we demonstrate that the intra-cytoplasmic (IC) domain of CD3epsilon plays a critical role in regulating TCR expression on DP thymocytes. We provide genetic and biochemical evidence to show that the CD3epsilon IC domain mutations result in elevated expression of fully assembled TCR on DP thymocytes. We also demonstrate that TCR up-regulation on DP thymocytes in these transgenic mice occurs in a ligand-independent manner. Further, we show that the proline-rich sequence and endoplasmic reticulum (ER) retention motifs in the IC domain of CD3epsilon play synergistic role in regulating TCR surface expression on DP thymocytes.

Nck adapter proteins: functional versatility in T cells

Nck is a ubiquitously expressed adapter protein that is almost exclusively built of one SH2 domain and three SH3 domains. The two isoproteins of Nck are functionally redundant in many aspects and differ in only few amino acids that are mostly located in the linker regions between the interaction modules. Nck proteins connect receptor and non-receptor tyrosine kinases to the machinery of actin reorganisation. Thereby, Nck regulates activation-dependent processes during cell polarisation and migration and plays a crucial role in the signal transduction of a variety of receptors including for instance PDGF-, HGF-, VEGF- and Ephrin receptors. In most cases, the SH2 domain mediates binding to the phosphorylated receptor or associated phosphoproteins, while SH3 domain interactions lead to the formation of larger protein complexes. In T lymphocytes, Nck plays a pivotal role in the T cell receptor (TCR)-induced reorganisation of the actin cytoskeleton and the formation of the immunological synapse. However, in this context, two different mechanisms and adapter complexes are discussed. In the first scenario, dependent on an activation-induced conformational change in the CD3epsilon subunits, a direct binding of Nck to components of the TCR/CD3 complex was shown. In the second scenario, Nck is recruited to the TCR complex via phosphorylated Slp76, another central constituent of the membrane proximal activation complex. Over the past years, a large number of putative Nck interactors have been identified in different cellular systems that point to diverse additional functions of the adapter protein, e.g. in the control of gene expression and proliferation.

Interaction between the N-terminal SH3 domain of Nck-alpha and CD3-epsilon-derived peptides: non-canonical and canonical recognition motifs

The first SH3 domain (SH3.1) of Nckalpha specifically recognizes the proline-rich region of CD3varepsilon, a subunit of the T cell receptor complex. We have solved the NMR structure of Nckalpha SH3.1 that shows the characteristic SH3 fold consisting of two antiparallel beta-sheets tightly packed against each other. According to chemical shift mapping analysis, a peptide encompassing residues 150-166 of CD3varepsilon binds at the canonical SH3 binding site. An exhaustive comparison with the structures of other SH3 domains able and unable to bind CD3varepsilon reveals that Nckalpha SH3.1 recognises a non-canonical PxxPxxDY motif that orientates at the binding site as a class II ligand. A positively charged residue (K/R) at position -2 relative to the WW sequence at the beginning of strand beta3 is crucial for PxxDY recognition. A 14-mer optimised Nckalpha SH3.1 ligand was found using a multi-substitution approach. Based on NMR data, this improved ligand binds Nckalpha SH3.1 through a PxxPxRDY motif that combines specific stabilising interactions corresponding to both canonical class II, PxxPx(K/R), and non-canonical PxxPxxDY motifs. This explains its higher capacity for Nckalpha SH3.1 binding relative to the wild type sequence.

The proline-rich sequence of CD3epsilon as an amplifier of low-avidity TCR signaling

Engagement of peptide-MHC by the TCR induces a conformational change in CD3epsilon that exposes a proline-rich sequence (PRS) and recruits the cytoskeletal adaptor Nck. This event, which precedes phosphorylation of the CD3epsilon ITAM, has been implicated in synapse formation and T cell function. However, there is compelling evidence that responsiveness to TCR ligation is CD3epsilon PRS independent. In this study, we show that the CD3epsilon PRS is necessary for peptide-MHC-induced phosphorylation of CD3epsilon and for recruitment of protein kinase Ctheta to the immune synapse in differentiated CD8+ T lymphocytes. However, whereas these two events are dispensable for functional T cell responsiveness to high-avidity ligands, they are required for responsiveness to low-avidity ones. Thus, in at least certain T cell clonotypes, the CD3epsilon PRS amplifies weak TCR signals by promoting synapse formation and CD3epsilon phosphorylation.

Structural and functional evidence that Nck interaction with CD3epsilon regulates T-cell receptor activity

Recruitment of signaling molecules to the cytoplasmic domains of the CD3 subunits of the T-cell receptor (TCR) is crucial for early T-cell activation. These transient associations either do or do not require tyrosine phosphorylation of CD3 immune tyrosine activation motifs (ITAMs). Here we show that the non-ITAM-requiring adaptor protein Nck forms a complex with an atypical PxxDY motif of the CD3epsilon tail, which encompasses Tyr166 within the ITAM and a TCR endocytosis signal. As suggested by the structure of the complex, we find that Nck binding inhibits phosphorylation of the CD3epsilon ITAM by Fyn and Lck kinases in vitro. Moreover, the CD3epsilon-Nck interaction downregulates TCR surface expression upon physiological stimulation in mouse primary lymph node cells. This indicates that Nck performs an important regulatory function in T lymphocytes by inhibiting ITAM phosphorylation and/or removing cell surface TCR via CD3epsilon interaction.

The proline-rich sequence of CD3epsilon controls T cell antigen receptor expression on and signaling potency in preselection CD4+CD8+ thymocytes

Antigen recognition by T cell antigen receptors (TCRs) is thought to 'unmask' a proline-rich sequence (PRS) present in the CD3epsilon cytosolic segment, which allows it to trigger T cell activation. Using 'knock-in' mice with deletion of the PRS, we demonstrate here that elimination of the CD3epsilon PRS had no effect on mature T cell responsiveness. In contrast, in preselection CD4+CD8+ thymocytes, the CD3epsilon PRS acted together with the adaptor protein SLAP to promote CD3zeta degradation, thereby contributing to downregulation of TCR expression on the cell surface. In addition, analysis of CD4+CD8+ thymocytes of TCR-transgenic mice showed that the CD3epsilon PRS enhanced TCR sensitivity to weak ligands. Our results identify previously unknown functions for the evolutionarily conserved CD3epsilon PRS at the CD4+CD8+ developmental stage and suggest a rather limited function in mature T cells.

SCHOOL model and new targeting strategies

Protein-protein interactions play a central role in biological processes and thus are an appealing target for innovative drug design a nd development. They can be targeted bysmall molecule inhibitors, peptides and peptidomimetics, which represent an alternative to protein therapeutics that carry many disadvantages. In this chapter, I describe specific protein-protein interactions suggested by a novel model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, to be critical for cell activation mediated by multichain immune recognition receptors (MIRRs) expressed on different cells of the hematopoietic system. Unraveling a long-standing mystery of MIRR triggering and transmembrane signaling, the SCHOOL model reveals the intrareceptor transmembrane interactions and interreceptor cytoplasmic homointeractions as universal therapeutic targets for a diverse variety of disorders mediated by immune cells. Further, assuming that the general principles underlying MIRR-mediated transmembrane signaling mechanisms are similar, the SCHOOL model can be applied to any particular receptor of the MIRR family. Thus, an important application of the SCHOOL model is that global therapeutic strategies targeting key protein-protein interactions involved in MIRR triggering and transmembrane signal transduction may be used to treat a diverse set of immune-mediated diseases. This assumes that clinical knowledge and therapeutic strategies can be transferred between seemingly disparate disorders, such as T-cell-mediated skin diseases and platelet disorders, or combined to develop novel pharmacological approaches. Intriguingly, the SCHOOL model unravels the molecular mechanisms underlying ability of different human viruses such as human immunodeficiency virus, cytomegalovirus and severe acute respiratory syndrome coronavirus to modulate and/or escape the host immune response. It also demonstrates how the lessons learned from viral pathogenesis can be used practically for rational drug design. Application of this model to platelet collagen receptor signaling has already led to the development of a novel concept of platelet inhibition and the invention of new platelet inhibitors, thus proving the suggested hypothesis and highlighting the importance and broad perspectives of the SCHOOL model in the development of new targeting strategies.

T cell activation and the cytoskeleton: you can't have one without the other

More than a quarter of a century has passed since the observation that T cells rapidly polarize their actin and microtubule cytoskeletal systems toward antigen-presenting cells during activation. Since this initial discovery, several receptors on T cells (e.g., T cell receptor [TCR], co-receptors, integrins, and chemokine receptors) have been identified to regulate these two cytoskeletal networks through complex signaling pathways, which are still being elucidated. There is now an undeniable body of biochemical, pharmacological, and genetic evidence indicating that regulators of actin and microtubule dynamics are crucial for T cell activation and effector functions. In fact, the actin cytoskeleton participates in the initial clustering of TCR-major histocompatibility complex or peptide complexes, formation and stabilization of the immune synapse, integrin-mediated adhesion, and receptor sequestration, whereas both the actin and microtubule cytoskeletons regulate the establishment of cell polarity, cell migration, and directed secretion of cytokines and cytolytic granules. Over the past several years, we have begun to more thoroughly understand the contributions of specific actin-regulatory and actin-nucleating proteins that govern these processes. Herein, we discuss our current understanding of how activating receptors on T lymphocytes regulate the actin and microtubule cytoskeletons, and how in turn, these distinct but integrated cytoskeletal networks coordinate T cell immune responses.

Avian and 1918 Spanish influenza a virus NS1 proteins bind to Crk/CrkL Src homology 3 domains to activate host cell signaling

NS1 (nonstructural protein 1) is an important virulence factor of the influenza A virus. We observed that NS1 proteins of the 1918 pandemic virus (A/Brevig Mission/1/18) and many avian influenza A viruses contain a consensus Src homology 3 (SH3) domain-binding motif. Screening of a comprehensive human SH3 phage library revealed the N-terminal SH3 of Crk and CrkL as the preferred binding partners. Studies with recombinant proteins confirmed avid binding of NS1 proteins of the 1918 virus and a representative avian H7N3 strain to Crk/CrkL SH3 but not to other SH3 domains tested, including p85alpha and p85beta. Endogenous CrkL readily co-precipitated NS1 from cells infected with the H7N3 virus. In transfected cells association with CrkL was observed for NS1 of the 1918 and H7N3 viruses but not A/Udorn/72 or A/WSN/33 NS1 lacking this sequence motif. SH3 binding was dispensable for suppression of interferon-induced gene expression by NS1 but was associated with enhanced phosphatidylinositol 3-kinase signaling, as evidenced by increased Akt phosphorylation. Thus, the Spanish Flu virus resembles avian influenza A viruses in its ability to recruit Crk/CrkL to modulate host cell signaling.