Itk/Emt: a link between T cell antigen receptor-mediated Ca2+ events and cytoskeletal reorganization

Cyclophilin A associates with and regulates the activity of ZAP70 in TCR/CD3-stimulated T cells

The ZAP70 protein tyrosine kinase (PTK) couples stimulated T cell antigen receptors (TCRs) to their downstream signal transduction pathways and is sine qua non for T cell activation and differentiation. TCR engagement leads to activation-induced post-translational modifications of ZAP70, predominantly by kinases, which modulate its conformation, leading to activation of its catalytic domain. Here, we demonstrate that ZAP70 in TCR/CD3-activated mouse spleen and thymus cells, as well as human Jurkat T cells, is regulated by the peptidyl-prolyl cis-trans isomerase (PPIase), cyclophilin A (CypA) and that this regulation is abrogated by cyclosporin A (CsA), a CypA inhibitor. We found that TCR crosslinking promoted a rapid and transient, Lck-dependent association of CypA with the interdomain B region, at the ZAP70 regulatory domain. CsA inhibited CypA binding to ZAP70 and prevented the colocalization of CypA and ZAP70 at the cell membrane. In addition, imaging analyses of antigen-specific T cells stimulated by MHC-restricted antigen-fed antigen-presenting cells revealed the recruitment of ZAP70-bound CypA to the immunological synapse. Enzymatically active CypA downregulated the catalytic activity of ZAP70 in vitro, an effect that was reversed by CsA in TCR/CD3-activated normal T cells but not in CypA-deficient T cells, and further confirmed in vivo by FRET-based studies. We suggest that CypA plays a role in determining the activity of ZAP70 in TCR-engaged T cells and impact on T cell activation by intervening with the activity of multiple downstream effector molecules.

Itk: the rheostat of the T cell response

The nonreceptor tyrosine kinase Itk plays a key role in TCR-initiated signaling that directly and significantly affects the regulation of PLCγ1 and the consequent mobilization of Ca²⁺. Itk also participates in the regulation of cytoskeletal reorganization as well as cellular adhesion, which is necessary for a productive T cell response. The functional cellular outcome of these molecular regulations by Itk renders it an important mediator of T cell development and differentiation. This paper encompasses the structure of Itk, the signaling parameters leading to Itk activation, and Itk effects on molecular pathways resulting in functional cellular outcomes. The incorporation of these factors persuades one to believe that Itk serves as a modulator, or rheostat, critically fine-tuning the T cell response.

Inhibition of the IL-2-inducible tyrosine kinase (Itk) activity: a new concept for the therapy of inflammatory skin diseases

T-cell-mediated processes play an essential role in the pathogenesis of several inflammatory skin diseases such as atopic dermatitis, allergic contact dermatitis and psoriasis. The aim of this study was to investigate the role of the IL-2-inducible tyrosine kinase (Itk), an enzyme acting downstream of the T-cell receptor (TCR), in T-cell-dependent skin inflammation using three approaches. Itk knockout mice display significantly reduced inflammatory symptoms in mouse models of acute and subacute contact hypersensitivity (CHS) reactions. Systemic administration of a novel small molecule Itk inhibitor, Compound 44, created by chemical optimization of an initial high-throughput screening hit, inhibited Itk's activity with an IC50 in the nanomolar range. Compound 44 substantially reduced proinflammatory immune responses in vitro and in vivo after systemic administration in two acute CHS models. In addition, our data reveal that human Itk, comparable to its murine homologue, is expressed mainly in T cells and is increased in lesional skin from patients with atopic dermatitis and allergic contact dermatitis. Finally, silencing of Itk by RNA interference in primary human T cells efficiently blocks TCR-induced lymphokine secretion. In conclusion, Itk represents an interesting new target for the therapy of T-cell-mediated inflammatory skin diseases.

Disrupting the intermolecular self-association of Itk enhances T cell signaling

The Tec family tyrosine kinase (Itk), is a key component of the TCR signaling pathway. Biochemical studies have shown that Itk activation requires recruitment of Itk to the membrane via its pleckstrin homology domain, phosphorylation of Itk by the Src kinase, Lck, and binding of Itk to the SLP-76/LAT adapter complex. However, the regulation of Itk enzymatic activity by Itk domain interactions is not yet well understood. In this study, we show that full-length Itk self-associates in an intermolecular fashion. Using this information, we have designed an Itk variant that exhibits reduced self-association but maintains normal binding to exogenous ligands via each of its regulatory domains. When expressed in insect cells, the Itk substrate phospholipase Cgamma1 is phosphorylated more efficiently by the Itk variant than by wild-type Itk. Furthermore, expression of the Itk variant in primary murine T cells induced higher ERK activation and increased calcium flux following TCR stimulation compared with that of wild-type Itk. Our results indicate that the Tec kinase Itk is negatively regulated by intermolecular clustering and that disruption of this clustering leads to increased Itk kinase activity following TCR stimulation.

Development of a platelet-activating factor antagonist for HIV-1 associated neurocognitive disorders

The neuroregulatory activities of PMS-601, a platelet activating factor antagonist, were investigated in laboratory and animal models of HIV-1 encephalitis (HIVE). For the former, PMS-601 reduced monocyte-derived macrophage pro-inflammatory secretions, multinucleated giant cell (MGC) formation, and neuronal loss independent of antiretroviral responses. PMS-601 treatment of HIVE severe combined immunodeficient mice showed reduced microgliosis, MGCs and neurodegeneration. These observations support the further development of PMS-601 as an adjunctive therapy for HIV-1 associated neurocognitive disorders.

The cellular response to heat stress in the gobyGillichthys mirabilis: a cDNA microarray and protein-level analysis

The cellular response to stress relies on the rapid induction of genes encoding proteins involved in preventing and repairing macromolecular damage incurred as a consequence of environmental insult. To increase our understanding of the scope of this response, a cDNA microarray, consisting of 9207 cDNA clones, was used to monitor gene expression changes in the gill and white muscle tissues of a eurythermic fish, Gillichthys mirabilis(Gobiidae) exposed to ecologically relevant heat stress. In each tissue, the induction or repression of over 200 genes was observed. These genes are associated with numerous biological processes, including the maintenance of protein homeostasis, cell cycle control, cytoskeletal reorganization,metabolic regulation and signal transduction, among many others. In both tissues, the molecular chaperones, certain transcription factors and a set of additional genes with various functions were induced in a similar manner;however, the majority of genes displayed tissue-specific responses. In gill,thermal stress induced the expression of the major structural components of the cytoskeleton, whereas these same genes did not respond to heat in muscle. In muscle, many genes involved in promoting cell growth and proliferation were repressed, perhaps to conserve energy for repair and replacement of damaged macromolecules, but a similar repression was not observed in the gill. Many of the observed changes in gene expression were similar to those described in model species whereas many others were unexpected. Measurements of the concentrations of the protein products of selected genes revealed that in each case an induction in mRNA synthesis correlated with an increase in protein production, though the timing and magnitude of the increase in protein was not consistently predicted by mRNA concentration, an important consideration in assessing the condition of the stressed cell using transcriptomic analysis.

Molecular details of Itk activation by prolyl isomerization and phospholigand binding: the NMR structure of the Itk SH2 domain bound to a phosphopeptide

The Src homology 2 (SH2) domain of interleukin-2 tyrosine kinase (Itk) is a critical component of the regulatory apparatus controlling the activity of this immunologically important enzyme. To gain insight into the structural features associated with the activated form of Itk, we have solved the NMR structure of the SH2 domain bound to a phosphotyrosine-containing peptide (pY) and analyzed changes in trans-hydrogen bond scalar couplings ((3h)J(NC')) that result from pY binding. Isomerization of a single prolyl imide bond in this domain is responsible for simultaneous existence of two distinct SH2 conformers. Prolyl isomerization directs ligand recognition: the trans conformer preferentially binds pY. The structure of the SH2/pY complex provides insight into the ligand specificity; the BG loop in the ligand-free trans SH2 conformer is pre-arranged for optimal contacts with the pY+3 residue of the ligand. Analysis of (3h)J(NC') couplings arising from hydrogen bonds has revealed propagation of structural changes from the pY binding pocket to the CD loop containing conformationally heterogeneous proline as well as to the alphaB helix, on the opposite site of the domain. These findings offer a structural framework for understanding the roles of prolyl isomerization and pY binding in Itk regulation.

Truncation of C-mip (Tc-mip), a new proximal signaling protein, induces c-maf Th2 transcription factor and cytoskeleton reorganization

Several arguments suggest that minimal change nephrotic syndrome (MCNS) results from yet unknown systemic disorder of T cell function. By screening a cDNA library from T cell relapse, we identified a new pleckstrin homology (PH) domain-containing protein encoded by a gene located on chromosome 16q24. Two alternative transcripts were identified. The first species (c-mip) was expressed in fetal liver, kidney, and peripheral blood mononuclear cells (PBMCs), but weakly detected in PBMCs from MCNS patients. The second form (Tc-mip, standing for truncated c-maf inducing protein), corresponds to subtracted transcript and lacks the NH2-terminal PH domain. The expression of Tc-mip was restricted to fetal liver, thymus, and MCNS PBMCs where it was specifically recruited in CD4+ T cells subset. Overexpression of Tc-mip in T cell Jurkat induced c-maf, transactivated the interleukin 4 gene and down-regulated the interferon gamma expression, characteristic of a Th2 commitment. Moreover, the overexpression of Tc-mip induced Src phosphorylation, T cell clustering, and a cellular redistribution of the cytoskeleton-associated L-plastin, by a PI3 kinase independent pathway. Tc-mip represents therefore the first identified protein, which links proximal signaling to c-maf induction.

Inducible T cell tyrosine kinase regulates actin-dependent cytoskeletal events induced by the T cell antigen receptor

The tec family kinase, inducible T cell tyrosine kinase (Itk), is critical for both development and activation of T lymphocytes. We have found that Itk regulates TCR/CD3-induced actin-dependent cytoskeletal events. Expression of Src homology (SH) 2 domain mutant Itk transgenes into Jurkat T cells inhibits these events. Furthermore, Itk(-/-) murine T cells display significant defects in TCR/CD3-induced actin polymerization. In addition, Jurkat cells deficient in linker for activation of T cells expression, an adaptor critical for Itk activation, display impaired cytoskeletal events and expression of SH3 mutant Itk transgenes reconstitutes this impairment. Interestingly, expression of an Itk kinase-dead mutant transgene into Jurkat cells has no effect on cytoskeletal events. Collectively, these data suggest that Itk regulates TCR/CD3-induced actin-dependent cytoskeletal events, possibly in a kinase-independent fashion.

Lipid phosphatases in the regulation of T cell activation: living up to their PTEN-tial

The initiating events associated with T activation in response to stimulation of the T cell antigen receptor (TCR) and costimulatory receptors, such as CD28, are intimately associated with the enzymatically catalyzed addition of phosphate not only to key tyrosine, threonine and serine residues in proteins but also to the D3 position of the myo-inositol ring of phosphatidylinositol (PtdIns). This latter event is catalyzed by the lipid kinase phosphoinositide 3-kinase (PI3K). The consequent production of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 serves both to recruit signaling proteins to the plasma membrane and to induce activating conformational changes in proteins that contain specialized domains for the binding of these phospholipids. The TCR signaling proteins that are subject to regulation by PI3K include Akt, phospholipase Cgamma1 (PLCgamma1), protein kinase C zeta (PKC-zeta), Itk, Tec and Vav, all of which play critical roles in T cell activation. As is the case for phosphorylation of protein substrates, the phosphorylation of PtdIns is under dynamic regulation, with the D3 phosphate being subject to hydrolysis by the 3-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10), thereby placing PTEN in direct opposition to PI3K. In this review we consider recent data concerning how PTEN may act in regulating the process of T cell activation.

Identification of phosphorylation sites within the SH3 domains of Tec family tyrosine kinases

Tec family protein tyrosine kinases (TFKs) play a central role in hematopoietic cellular signaling. Initial activation takes place through specific tyrosine phosphorylation situated in the activation loop. Further activation occurs within the SH3 domain via a transphosphorylation mechanism, which for Bruton's tyrosine kinase (Btk) affects tyrosine 223. We found that TFKs phosphorylate preferentially their own SH3 domains, but differentially phosphorylate other member family SH3 domains, whereas non-related SH3 domains are not phosphorylated. We demonstrate that SH3 domains are good and reliable substrates. We observe that transphosphorylation is selective not only for SH3 domains, but also for dual SH3SH2 domains. However, the dual domain is phosphorylated more effectively. The major phosphorylation sites were identified as conserved tyrosines, for Itk Y180 and for Bmx Y215, both sites being homologous to the Y223 site in Btk. There is, however, one exception because the Tec-SH3 domain is phosphorylated at a non-homologous site, nevertheless a conserved tyrosine, Y206. Consistent with these findings, the 3D structures for SH3 domains point out that these phosphorylated tyrosines are located on the ligand-binding surface. Because a number of Tec family kinases are coexpressed in cells, it is possible that they could regulate the activity of each other through transphosphorylation.

Beyond calcium: new signaling pathways for Tec family kinases

The Tec kinases represent the second largest family of mammalian non-receptor tyrosine kinases and are distinguished by the presence of distinct proline-rich regions and pleckstrin homology domains that are required for proper regulation and activation. Best studied in lymphocyte and mast cells, these kinases are critical for the full activation of phospholipase-C γ (PLC-γ) and Ca2+ mobilization downstream of antigen receptors. However, it has become increasingly clear that these kinases are activated downstream of many cell-surface receptors,including receptor tyrosine kinases, cytokine receptors, integrins and G-protein-coupled receptors. Evidence suggests that the Tec kinases influence a wide range of signaling pathways controlling activation of MAP kinases,actin reorganization, transcriptional regulation, cell survival and cellular transformation. Their impact on cellular physiology suggests that the Tec kinases help regulate multiple cellular processes beyond Ca2+mobilization.

New insights into the regulation and functions of Tec family tyrosine kinases in the immune system

The Tec family of protein tyrosine kinases play an important role in signaling through antigen-receptors such as the TCR, BCR and Fcepsilon receptor. Recent studies have generated new insights into the domains in Tec kinases that take part in intramolecular and intermolecular binding. Furthermore, the consequences of these domain interactions for Tec activation and downregulation have been better defined. Genetic studies of kinase-knockout mice have emphasized the importance of Tec kinases in lymphocyte development, differentiation and apoptosis.

Regulation of the tyrosine kinase Itk by the peptidyl-prolyl isomerase cyclophilin A

Interleukin-2 tyrosine kinase (Itk) is a nonreceptor protein tyrosine kinase of the Tec family that participates in the intracellular signaling events leading to T cell activation. Tec family members contain the conserved SH3, SH2, and catalytic domains common to many kinase families, but they are distinguished by unique sequences outside of this region. The mechanism by which Itk and related Tec kinases are regulated is not well understood. Our studies indicate that Itk catalytic activity is inhibited by the peptidyl prolyl isomerase activity of cyclophilin A (CypA). NMR structural studies combined with mutational analysis show that a proline-dependent conformational switch within the Itk SH2 domain regulates substrate recognition and mediates regulatory interactions with the active site of CypA. CypA and Itk form a stable complex in Jurkat T cells that is disrupted by treatment with cyclosporin A. Moreover, the phosphorylation levels of Itk and a downstream substrate of Itk, PLCgamma1, are increased in Jurkat T cells that have been treated with cyclosporin A. These findings support a novel mode of tyrosine kinase regulation for a Tec family member and provide a molecular basis for understanding a cellular function of the ubiquitous peptidyl prolyl isomerase, CypA.

Signaling pathways of D3-phosphoinositide-binding kinases in T cells and their regulation by PTEN

Phosphoinositide 3-kinases (PI3Ks) phosphorylate the D3 position of the myo -inositol ring of inositol phospholipids, producing, amongst others, phosphatidylinositol-(3,4,5)-trisphosphate. This activity is opposed by the lipid phosphatase PTEN, which catalyzes the removal of this phosphate. Stimulation of PI3Ks is elicited by engagement of receptors for antigen, cytokines and chemokines, and by co-stimulatory molecules. Kinases and other enzymes containing pleckstrin homology domains are activated by binding to these phospholipids, affecting a variety of cellular processes that control lymphocyte function, including cell survival, proliferation, chemotaxis and cytoskeletal reorganization. This review highlights the signaling pathways of these kinases and other enzymes in T cells, their biological effects, and their regulation by PTEN.