Dynamic actin polymerization drives T cell receptor-induced spreading: a role for the signal transduction adaptor LAT.

T cell activation induces functional changes in cell shape and cytoskeletal architecture. To facilitate the collection of dynamic, high-resolution images of activated T cells, we plated T cells on coverslips coated with antibodies to the T cell receptor (TCR). Using these images, we were able to quantitate the morphological responses of individual cells over time. Here, we show that TCR engagement triggers the formation and expansion of contacts bounded by continuously remodeled actin-rich rings. These processes are associated with the extension of lamellipodia and require actin polymerization, tyrosine kinase activation, cytoplasmic calcium increases, and LAT, an important hematopoietic adaptor. In addition, the maintenance of the resulting contact requires sustained calcium influxes, an intact microtubule cytoskeleton, and functional LAT.

Deficiency of PTEN in Jurkat T cells causes constitutive localization of Itk to the plasma membrane and hyperresponsiveness to CD3 stimulation.

Pleckstrin homology (PH) domain binding to D3-phosphorylated phosphatidylinositides (PI) provides a reversible means of recruiting proteins to the plasma membrane, with the resultant change in subcellular localization playing a key role in the activation of multiple intracellular signaling pathways. Previously we found that the T-cell-specific PH domain-containing kinase Itk is constitutively membrane associated in Jurkat T cells. This distribution was unexpected given that the closely related B-cell kinase, Btk, is almost exclusively cytosolic. In addition to constitutive membrane association of Itk, unstimulated JTAg T cells also exhibited constitutive phosphorylation of Akt on Ser-473, an indication of elevated basal levels of the phosphatidylinositol 3-kinase (PI3K) products PI-3,4-P(2) and PI-3,4,5-P(3) in the plasma membrane. Here we describe a defect in expression of the D3 phosphoinositide phosphatase, PTEN, in Jurkat and JTAg T cells that leads to unregulated PH domain interactions with the plasma membrane. Inhibition of D3 phosphorylation by PI3K inhibitors, or by expression of PTEN, blocked constitutive phosphorylation of Akt on Ser-473 and caused Itk to redistribute to the cytosol. The PTEN-deficient cells were also hyperresponsive to T-cell receptor (TCR) stimulation, as measured by Itk kinase activity, tyrosine phosphorylation of phospholipase C-gamma1, and activation of Erk compared to those in PTEN-replete cells. These data support the idea that PH domain-mediated association with the plasma membrane is required for Itk activation, provide evidence for a negative regulatory role of PTEN in TCR stimulation, and suggest that signaling models based on results from Jurkat T-cell lines may underestimate the role of PI3K in TCR signaling.

Biochemical interactions integrating Itk with the T cell receptor-initiated signaling cascade.

Itk, a Tec family tyrosine kinase, acts downstream of Lck and phosphatidylinositol 3'-kinase to facilitate T cell receptor (TCR)-dependent calcium influxes and increases in extracellular-regulated kinase activity. Here we demonstrate interactions between Itk and crucial components of TCR-dependent signaling pathways. First, the inositide-binding pocket of the Itk pleckstrin homology domain directs the constitutive association of Itk with buoyant membranes that are the primary site of TCR activation and are enriched in both Lck and LAT. This association is required for the transphosphorylation of Itk. Second, the Itk proline-rich region binds to Grb2 and LAT. Third, the Itk Src homology (SH3) 3 and SH2 domains interact cooperatively with Syk-phosphorylated SLP-76. Notably, SLP-76 contains a predicted binding motif for the Itk SH2 domain and binds to full-length Itk in vitro. Finally, we show that kinase-inactive Itk can antagonize the SLP-76-dependent activation of NF-AT. The inhibition of NF-AT activation depends on the Itk pleckstrin homology domain, proline-rich region, and SH2 domain. Together, these observations suggest that multivalent interactions recruit Itk to LAT-nucleated signaling complexes and facilitate the activation of LAT-associated phospholipase Cgamma1 by Itk.

T cell receptor-initiated calcium release is uncoupled from capacitative calcium entry in Itk-deficient T cells.

Itk, a Tec family tyrosine kinase, plays an important but as yet undefined role in T cell receptor (TCR) signaling. Here we show that T cells from Itk-deficient mice have a TCR-proximal signaling defect, resulting in defective interleukin 2 secretion. Upon TCR stimulation, Itk-/- T cells release normal amounts of calcium from intracellular stores, but fail to open plasma membrane calcium channels. Since thapsigargin-induced store depletion triggers normal calcium entry in Itk-/- T cells, an impaired biochemical link between store depletion and channel opening is unlikely to be responsible for this defect. Biochemical studies indicate that TCR-induced inositol 1,4,5 tris-phosphate (IP3) generation and phospholipase C gamma1 tyrosine phosphorylation are substantially reduced in Itk-/- T cells. In contrast, TCR-zeta and ZAP-70 are phosphorylated normally, suggesting that Itk functions downstream of, or in parallel to, ZAP-70 to facilitate TCR-induced IP3 production. These findings support a model in which quantitative differences in cytosolic IP3 trigger distinct responses, and in which only high concentrations of IP3 trigger the influx of extracellular calcium.

Lck phosphorylates the activation loop tyrosine of the Itk kinase domain and activates Itk kinase activity.

The Tec family tyrosine kinase Itk has been implicated in T cell receptor (TCR) signaling, yet its precise role and mechanism of activation remain undefined. To investigate these issues, we examined the biochemical response of Itk to TCR stimulation. We found that Itk is tyrosine-phosphorylated after TCR cross-linking and that this phosphorylation depends on the presence of functional Lck. To determine if this Lck dependence results from direct phosphorylation of Itk by Lck, we generated recombinant Itk and Lck using a baculovirus expression system and used these proteins in subsequent biochemical analyses. We found that Lck phosphorylates Itk upon co-expression in insect cells and, further, that this phosphorylation of Itk results in increased Itk in vitro kinase activity. The major site of Lck phosphorylation on Itk was mapped to the conserved tyrosine (Tyr511) in the activation loop of the Itk kinase domain. Substitution of this tyrosine with phenylalanine abolishes Itk kinase activity in insect cells, indicating that phosphorylation at this site plays a critical role in regulating Itk function.

Regulatory intramolecular association in a tyrosine kinase of the Tec family.

The T-cell-specific tyrosine kinase Itk is a member of the Tec family of non-receptor tyrosine kinases, and is required for signalling through the T-cell antigen receptor (TCR). The role of Itk in TCR signalling and the manner in which Itk activity is regulated are not well understood. Substrate binding and enzymatic activity of the structurally related Src kinases are regulated by an intramolecular interaction between the Src-homology-2 (SH2) domain and a phosphotyrosine. Although Itk also contains SH3, SH2 and tyrosine kinase domains, it lacks the corresponding regulatory phosphorylation site, and therefore must be regulated by an alternative mechanism. The proline-rich sequence adjacent to the SH3 domain of Tec family kinases contains an SH3 ligand, potentially allowing a different intramolecular interaction. By using multidimensional nuclear magnetic resonance we have determined the structure of a fragment of Itk, confirming that these domains interact intramolecularly. Formation of this intramolecular SH3-ligand complex prevents the Itk SH3 domain and proline-rich region from interacting with their respective protein ligands, Sam68 and Grb-2. We believe that this structure represents the first example of an intramolecular interaction between an SH3 domain and a proline-rich ligand, and has implications for the regulation of Tec family kinases.

Identification of Itk/Tsk Src homology 3 domain ligands.

The tyrosine kinase Itk/Tsk is a T cell specific analog of Btk, the tyrosine kinase defective in the human immunodeficiency X-linked agammaglobulinemia and in xid mice. T lymphocytes from Itk-deficient mice are refractory to mitogenic stimuli delivered through the T cell receptor (TCR). To gain insights into the biochemical role of Itk, the binding properties of the Itk SH3 domain were examined. An optimal Itk SH3 binding motif was derived by screening biased phage display libraries; peptides based on this motif bound with high affinity and selectivity to the Itk SH3 domain. Initial studies with T cell lysates indicated that the Itk SH3 domain bound Cbl, Fyn, and other tyrosine phosphoproteins from TCR-stimulated Jurkat cells. Under conditions of increased detergent stringency Sam 68, Wiskott-Aldrich Syndrome protein, and hnRNP-K, but not Cbl and Fyn, were bound to the Itk SH3 domain. By examining the ability of different SH3 domains to interact with deletion variants of Sam 68 and WASP, we demonstrated that the Itk-SH3 domain and the SH3 domains of Src family kinases bind to overlapping but distinct sets of proline-rich regions in Sam 68 and WASP.

p56Lck and p59Fyn regulate CD28 binding to phosphatidylinositol 3-kinase, growth factor receptor-bound protein GRB-2, and T cell-specific protein-tyrosine kinase ITK: implications for T-cell costimulation.

T-cell activation requires cooperative signals generated by the T-cell antigen receptor zeta-chain complex (TCR zeta-CD3) and the costimulatory antigen CD28. CD28 interacts with three intracellular proteins-phosphatidylinositol 3-kinase (PI 3-kinase), T cell-specific protein-tyrosine kinase ITK (formerly TSK or EMT), and the complex between growth factor receptor-bound protein 2 and son of sevenless guanine nucleotide exchange protein (GRB-2-SOS). PI 3-kinase and GRB-2 bind to the CD28 phosphotyrosine-based Tyr-Met-Asn-Met motif by means of intrinsic Src-homology 2 (SH2) domains. The requirement for tyrosine phosphorylation of the Tyr-Met-Asn-Met motif for SH2 domain binding implicates an intervening protein-tyrosine kinase in the recruitment of PI 3-kinase and GRB-2 by CD28. Candidate kinases include p56Lck, p59Fyn, zeta-chain-associated 70-kDa protein (ZAP-70), and ITK. In this study, we demonstrate in coexpression studies that p56Lck and p59Fyn phosphorylate CD28 primarily at Tyr-191 of the Tyr-Met-Asn-Met motif, inducing a 3- to 8-fold increase in p85 (subunit of PI 3-kinase) and GRB-2 SH2 binding to CD28. Phosphatase digestion of CD28 eliminated binding. In contrast to Src kinases, ZAP-70 and ITK failed to induce these events. Further, ITK binding to CD28 was dependent on the presence of p56Lck and is thus likely to act downstream of p56Lck/p59Fyn in a signaling cascade. p56Lck is therefore likely to be a central switch in T-cell activation, with the dual function of regulating CD28-mediated costimulation as well as TCR-CD3-CD4 signaling.