The role of SLP-76 and LAT in lymphocyte development

Copy number variations affecting growth curve parameters in a crossbred chicken population

Copy number variations (CNVs) are key structural variations in the genome and may contribute to phenotypic differences. In this study, we used a F2 chicken population created from reciprocal crossing between fast-growing Arian broiler line and Urmia native chickens. The chickens were genotyped by 60 K SNP BeadChip, and PennCNV algorithm was used to detect genome-wide CNVs. The growth curve parameters of W0, k, L, Wf, Wi, ti and average GR were used as phenotypic data. The association between CNV and growth curve parameters was carried out using the CNVRanger R/Bioconductor package. Five CNV regions (CNVRs) were chosen for the validation experiment using qPCR. Gene enrichment analysis was done using WebGestalt. The STRING database was used to search for significant pathways. The results identified 966 CNVs and 600 CNVRs including 468 gains, 67 losses, and 65 both events on autosomal chromosomes. Validation of the CNVRs obtained from the qPCR assay were 79 % consistent with the prediction by PennCNV. A total of 43 significant CNVs were obtained for the seven growth curve parameters. The 416 genes annotated for significant CNVs. Six genes out of 416 genes were most related to growth curve parameters. These genes were LCP2, Dock2, CD80, CYFIP1, NIPA1 and NIPA2. Some of these genes in their biological process were associated with the growth, reproduction and development of cells or organs that ultimately lead to the growth of the body. The results of the study could pave the way for better understanding the molecular process of CNVs and growth curve parameters in birds.

Role for Ets-2(Thr-72) transcription factor in stage-specific thymocyte development and survival

Interference of Ras signaling deregulates thymocyte development in mouse models. However, the role of Ets-2, a transcription factor that is phosphorylated on a critical threonine residue (Thr-72) by the Ras/MAPK pathway in thymocyte development, has not been defined. Transgenic mice overexpressing a phosphomutant Ets-2 (T72A) in the thymus displayed reduced thymus size associated with a 60-80% reduction in thymocyte populations. The transgenic mice exhibited a 20-fold increase in a c-Kit(+) CD4(+) CD8(+) CD3(-) population and a 5-fold increase in a unique CD5(low) population associated with a partial developmental block at the DN2-DN3 stage of thymocytes. Transgenic thymocytes exhibited increased apoptosis, and overexpression of Bcl-2 rescued the hypocellularity and associated thymocyte developmental block in double transgenic mice. The observed defects in these mice are not dependent on Ets-1 expression. These studies implicate for the first time a stage-specific Ets-1-independent regulatory role for Ets-2 in early thymocyte development and survival.

Genomic expression profiling of TNF-alpha-treated BDC2.5 diabetogenic CD4+ T cells

TNF-alpha plays an important role in immune regulation, inflammation, and autoimmunity. Chronic TNF exposure has been shown to down-modulate T cell responses. In a mouse T cell hybridoma model, TNF attenuated T cell receptor (TCR) signaling. We have confirmed that chronic TNF and anti-TNF exposure suppressed and increased T cell responses, respectively. In adult TCR (BDC2.5) transgenic nonobese diabetic mice, DNA microarray analysis of global gene expression in BDC2.5 CD4(+) T cells in response to chronic TNF or anti-TNF exposure showed that genes involved in functional categories including T cell signaling, cell cycle, proliferation, ubiquitination, cytokine synthesis, calcium signaling, and apoptosis were modulated. Genes such as ubiquitin family genes, cytokine inducible Src homology 2-containing genes, cyclin-dependent kinase inhibitors p21, p57, calmodulin family genes (calmodulin-1, -2, and -3) and calcium channel voltage-dependent, N type alpha1B subunit (CaV2.2) were induced by TNF, whereas Vav2, Rho GTPase-activating protein, calcium channel voltage-dependent, L type alpha1C subunit (CaV1.2), IL-1 receptor-associated kinase-1 and -2, and IL enhancer binding factor 3 were reduced by TNF. Genes such as CaV1.2 and proliferating cell nuclear antigen, repressed by TNF, were induced by anti-TNF treatment. Further, we showed that chronic TNF exposure impaired NF-kappaB and adaptor protein 1 transactivation activity, leading to T cell unresponsiveness. Thus, our results present a detailed picture of transcriptional programs affected by chronic TNF exposure and provide candidate target genes that may function to mediate TNF-induced T cell unresponsiveness.

Tyrosine kinases Btk and Tec regulate osteoclast differentiation by linking RANK and ITAM signals

Certain autoimmune diseases result in abnormal bone homeostasis, but association of immunodeficiency with bone is poorly understood. Osteoclasts, which derive from bone marrow cells, are under the control of the immune system. Differentiation of osteoclasts is mainly regulated by signaling pathways activated by RANK and immune receptors linked to ITAM-harboring adaptors. However, it is unclear how the two signals merge to cooperate in osteoclast differentiation. Here we report that mice lacking the tyrosine kinases Btk and Tec show severe osteopetrosis caused by a defect in bone resorption. RANK and ITAM signaling results in formation of a Btk(Tec)/BLNK(SLP-76)-containing complex and PLCgamma-mediated activation of an essential calcium signal. Furthermore, Tec kinase inhibition reduces osteoclastic bone resorption in models of osteoporosis and inflammation-induced bone destruction. Thus, this study reveals the importance of the osteoclastogenic signaling complex composed of tyrosine kinases, which may provide the molecular basis for a new therapeutic strategy.

c-Myc mediates pre-TCR-induced proliferation but not developmental progression

Constitutive and cell-autonomous signals emanating from the pre-T-cell receptor (pre-TCR) promote proliferation, survival and differentiation of immature thymocytes. We show here that induction of pre-TCR signaling resulted in rapid elevation of c-Myc protein levels. Cre-mediated thymocyte-specific ablation of c-Myc in CD25(+)CD44(-) thymocytes reduced proliferation and cell growth at the pre-TCR checkpoint, resulting in thymic hypocellularity and a severe reduction in CD4(+)CD8(+) thymocytes. In contrast, c-Myc deficiency did not inhibit pre-TCR-mediated differentiation or survival. Myc(-/-) double-negative (DN) 3 cells progressed to the double-positive (DP) stage and up-regulated TCRalphabeta surface expression in the absence of cell proliferation, in vivo as well as in vitro. These observations indicate that distinct signals downstream of the pre-TCR are responsible for proliferation versus differentiation, and demonstrate that c-Myc is only required for pre-TCR-induced proliferation but is dispensable for developmental progression from the DN to the DP stage.

Active Ca2+/calmodulin-dependent protein kinase II gamma B impairs positive selection of T cells by modulating TCR signaling

T cell development is regulated at two critical checkpoints that involve signaling events through the TCR. These signals are propagated by kinases of the Src and Syk families, which activate several adaptor molecules to trigger Ca(2+) release and, in turn, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation. In this study, we show that a constitutively active form of CaMKII antagonizes TCR signaling and impairs positive selection of thymocytes in mice. Following TCR engagement, active CaMKII decreases TCR-mediated CD3zeta chain phosphorylation and ZAP70 recruitment, preventing further downstream events. Therefore, we propose that CaMKII belongs to a negative-feedback loop that modulates the strength of the TCR signal through the tyrosine phosphatase Src homology 2 domain-containing phosphatase 2 (SHP-2).

Modulation of T-cell receptor signal transduction by herpesvirus signaling adaptor protein

Because of its central regulatory role, T-cell receptor (TCR) signal transduction is a common target of viruses. We report here the identification of a small signaling protein, ORF5, of the T-lymphotropic tumor virus herpesvirus saimiri (HVS). ORF5 is predicted to contain 89 to 91 amino acids with an amino-terminal myristoylation site and six SH2 binding motifs, showing structural similarity to cellular LAT (linker for activation of T cells). Sequence analysis showed that, despite extensive sequence variation, the myristoylation site and SH2 binding motifs were completely conserved among 13 different ORF5 isolates. Upon TCR stimulation, ORF5 was efficiently tyrosine phosphorylated and subsequently interacted with cellular SH2-containing signaling proteins Lck, Fyn, SLP-76, and p85 through its tyrosine residues. ORF5 expression resulted in the marked augmentation of TCR signal transduction activity, evidenced by increased cellular tyrosine phosphorylation, intracellular calcium mobilization, CD69 surface expression, interleukin-2 production, and activation of the NF-AT, NF-kappa B, and AP-1 transcription factors. Despite its structural similarity to cellular LAT, however, ORF5 could only partially substitute for LAT function in TCR signal transduction. These results demonstrate that HVS utilizes a novel signaling protein, ORF5, to activate TCR signal transduction. This activation probably facilitates viral gene expression and, thereby, persistent infection.

Loss of Ly-6A.2 expression on immature developing T cells in the thymus is necessary for their normal growth and generation of the Vbeta T-cell repertoire

Stage-specific expression of a number of cell-surface and signaling proteins is critical for normal development of T cells in the thymus. Equally important may be the loss of expression/signaling of developmentally regulated proteins for proper transitioning of developing T cells into thymic subsets. Ly-6A.2 exhibits a regulated pattern of expression on T cells maturing in the thymus, and dysregulating its expression results in arrest of developing T cells within the CD3-CD4-CD8- triple negative (TN) stage where the normal expression of Ly-6A.2 is extinguished. To further characterize the mechanisms underlying this block, we examined whether cell signaling and/or cell adhesion properties of the Ly-6A.2 molecule influenced the block in T-cell development. Analysis of bone marrow chimeras generated by injecting CFSE-labeled Ly-6A.2 transgenic bone marrow cells into irradiated syngeneic non-transgenic mice revealed normal trafficking of developing T cells from the cortex into the medulla. Production of LAT but not p56lck was diminished in CD4-CD8- DN cells from Ly-6A.2 dysregulated mice when compared with control littermates. Dysregulated expression of Ly-6A.2 did not suppress endogenous TCR-Vbeta expression. Finally, dysregulated expression of Ly-6A.2 enhanced apoptosis of an immature CD4+CD8+ (DP) subset of developing cells and altered the selected TCR-Vbeta repertoire. Taken together, these observations indicate that the termination of Ly-6A.2 expression and signaling within the CD4-CD8-CD3- subset of developing T cells is an important checkpoint during normal thymic development.

LAT links the pre-BCR to calcium signaling

SLP-65(-/-) mice display a partial block at the pre-B cell stage of development. Here, we show that LAT is required for the differentiation of SLP-65(-/-) pre-B cells. We show that LAT and SLP-76 are recruited to the pre-BCR and associated with Ig-alpha upon pre-BCR engagement, whereas LAT interaction with SLP-76 is already detected in untreated pre-B cells. Reconstitution of LAT or SLP-65 expression in SLP-65/LAT(-/-) pre-B cells restored their calcium (Ca2+) mobilization capacity, led to downregulation of surface pre-BCR, and induced differentiation to BCR+ cells. Together, our results suggest that the adaptor proteins LAT and SLP-76 are involved in pre-BCR signaling, thereby rescuing arrested murine SLP-65(-/-) pre-B cells.

Dok-related protein negatively regulates T cell development via its RasGTPase-activating protein and Nck docking sites

Downstream of kinase (Dok)-related protein (DokR, also known as p56(dok)/FRIP/Dok-R) is implicated in cytokine and immunoreceptor signaling in myeloid and T cells. Tyrosine phosphorylation induces DokR to bind the signal relay molecules, RasGTPase-activating protein (RasGAP) and Nck. Here, we have examined the function of DokR during hematopoietic development and the requirement for RasGAP and Nck binding sites in its biological function. Retroviral-mediated expression of DokR in bone marrow cells dramatically inhibited their capacity to form colonies in vitro in response to the cytokines macrophage colony-stimulating factor and stem cell factor, whereas responses to interleukin-3 and granulocyte macrophage colony-stimulating factor were only weakly affected. When introduced into lethally irradiated mice, hematopoietic cells expressing DokR showed a drastically reduced capacity to repopulate lymphoid tissues. Most notably, DokR dramatically reduced repopulation of the thymus, in part by reducing the number of T cell precursors seeding in the thymus, but equally, through inhibiting the transition of CD4(-)CD8(-) to CD4(+)CD8(+) T cells. Consequently, the number of mature peripheral T cells was markedly reduced. In contrast, a minimal effect on B cell and myeloid lineage development was observed. Importantly, functional RasGAP and Nck binding sites were found to be essential for the biological effects of DokR in vitro and in vivo.

Scaffolds, adaptors and linkers of TCR signaling: theory and practice

Four non-enzymatic proteins form the structural core of the TCR signaling machinery, linking antigen-receptor activation to signaling. These four proteins, each with well defined protein-protein interaction domains, include three 'scaffolds' (LAT, SLP-76 and SLAP-130/Fyb/ADAP and a 'pure adaptor' (GADS). The biological functions of many distinct protein-protein interaction domains have been dissected through a methodological series of knockout and reconstitution experiments. In reviewing these recent advances, we attempt to address two questions often asked by immunologists not familiar with the field: what do scaffolds/adaptors/linkers do; and what do these terms mean?

Signal transduction and co-stimulatory pathways

Using specific cell surface receptors lymphocytes continuously sample their environment. Maturation of the immune system and initiation of a specific immune response rely on an array of extracellular cues that elicit complex intracellular biochemical signals. Essential molecules involved in signal transduction from immunoreceptors have emerged. After immunoreceptor engagement a core signaling complex is assembled comprising cytoplasmic immunoreceptor chains, kinases of the Src and ZAP70 families and various cytoplasmic and transmembrane adaptor molecules. Further effectors nucleate onto this complex evoking the characteristic responses of lymphocyte activation. Successful maturation of T cells into effector cells relies on the presence of a persistent stimulus presented in an appropriate extracellular environment. Encounter of MHC presented antigenic peptides and their cognate T cell receptors (TCRs) results in the formation of a nanometer intercellular gap between T cells and antigen presenting cells, which is now commonly referred to as the immunological synapse. The synapse is believed to sustain persistent TCR engagement. Its formation requires massive changes in T cell cytoskeletal architecture which essentially relies on signals provided by costimulatory molecules. The well orchestrated interplay between TCR and costimulatory signals decides about successful immune response and tolerance induction or immune failure and autoimmunity.

The transmembrane adapter LAT plays a central role in immune receptor signalling

The transmembrane adapter LAT (linker for activation of T cells) plays a central role in signalling by ITAM bearing receptors expressed on T cells, natural killer cells, mast cells and platelets. Receptor engagement leads to the phosphorylation of tyrosine residues present in the intracellular domain of LAT and formation of a multiprotein complex with other adapter molecules and enzymes including Grb2, Gads/SLP-76 and PLCgamma isoforms. These signalling events predominantly take place in glycolipid-enriched membrane domains. The constitutive presence of LAT in GEMs enables its function as the main scaffolding protein for the organization of GEM-localized signalling. The study of LAT-deficient mice and LAT-deficient cell lines further emphasizes the importance of LAT for these signalling cascades but also defines the existence of LAT-independent events downstream of the Syk-family kinase-ITAM complex.

p38 MAP kinase activity modulates alpha beta T cell development

Reciprocal interactions between bone marrow derived precursor cells and the thymic environment lead to the generation of the complete repertoire of diverse and functional T cells. We have previously shown that p38 MAP kinase is activated in response to intrathymic signals in thymocytes. In this report we provide evidence that p38 MAP kinase activity is essential for pre-TCR-mediated transition of thymocytes from the CD4- CD8- to the CD4+ CD8+ stage of development. In the absence of p38 MAP kinase activity differentiation of alpha beta T cells, but not gamma delta T cells, was blocked.

High resolution mapping of mast cell membranes reveals primary and secondary domains of Fc(epsilon)RI and LAT

In mast cells, cross-linking the high-affinity IgE receptor (Fc(epsilon)RI) initiates the Lyn-mediated phosphorylation of receptor ITAMs, forming phospho-ITAM binding sites for Syk. Previous immunogold labeling of membrane sheets showed that resting Fc(epsilon)RI colocalize loosely with Lyn, whereas cross-linked Fc(epsilon)RI redistribute into specialized domains (osmiophilic patches) that exclude Lyn, accumulate Syk, and are often bordered by coated pits. Here, the distribution of Fc(epsilon)RI beta is mapped relative to linker for activation of T cells (LAT), Grb2-binding protein 2 (Gab2), two PLCgamma isoforms, and the p85 subunit of phosphatidylinositol 3-kinase (PI3-kinase), all implicated in the remodeling of membrane inositol phospholipids. Before activation, PLCgamma1 and Gab2 are not strongly membrane associated, LAT occurs in small membrane clusters separate from receptor, and PLCgamma2, that coprecipitates with LAT, occurs in clusters and along cytoskeletal cables. After activation, PLCgamma2, Gab2, and a portion of p85 colocalize with Fc(epsilon)RI beta in osmiophilic patches. LAT clusters enlarge within 30 s of receptor activation, forming elongated complexes that can intersect osmiophilic patches without mixing. PLCgamma1 and another portion of p85 associate preferentially with activated LAT. Supporting multiple distributions of PI3-kinase, Fc(epsilon)RI cross-linking increases PI3-kinase activity in anti-LAT, anti-Fc(epsilon)RIbeta, and anti-Gab2 immune complexes. We propose that activated mast cells propagate signals from primary domains organized around Fc(epsilon)RIbeta and from secondary domains, including one organized around LAT.

Vav2 activates c-fos serum response element and CD69 expression but negatively regulates nuclear factor of activated T cells and interleukin-2 gene activation in T lymphocyte

Vav1 and Vav2 are members of the Dbl family of guanine nucleotide exchange factors for the Rho family of small GTPases. Although the role of Vav1 during lymphocyte development and activation is well characterized, the function of Vav2 is still unclear. In this study, we compared the signaling pathways regulated by Vav1 and Vav2 following engagement of the T cell receptor (TCR). We show that Vav2 is tyrosine-phosphorylated upon TCR stimulation and by co-expressed Src and Syk family kinases. Using glutathione S-transferase fusion proteins, we observed that the Src homology 2 domain of Vav2 binds tyrosine-phosphorylated proteins from TCR-stimulated Jurkat T cell lysates, including c-Cbl and SLP-76. Like Vav1, Vav2 cooperated with TCR stimulation to increase extracellular signal-regulated kinase activation and to promote c-fos serum response element transcriptional activity. Moreover, both proteins displayed a similar action in increasing the expression of the early activation marker CD69 in Jurkat T cells. However, in contrast to Vav1, Vav2 dramatically suppressed TCR signals leading to nuclear factor of activated T cells (NF-AT)-dependent transcription and induction of the interleukin-2 promoter. Vav2 appears to act upstream of the phosphatase calcineurin because a constitutively active form of calcineurin rescued the effect of Vav2 by restoring TCR-induced NF-AT activation. Interestingly, the Dbl homology and Src homology 2 domains of Vav2 were necessary for its inhibitory effect on NF-AT activation and for induction of serum response element transcriptional activity. Taken together, our results indicate that Vav1 and Vav2 exert overlapping but nonidentical functions in T cells. The negative regulatory pathway elicited by Vav2 might play an important role in regulating lymphocyte activation processes.

LAT, the linker for activation of T cells: a bridge between T cell-specific and general signaling pathways

A key event in the regulation of the adaptive immune response is the binding of major histocompatibility complex-bound foreign peptides to T cell antigen receptors (TCRs) that are present on the cell surface of T lymphocytes. Recognition of the presence of cognate antigen in the host animal induces a series of biochemical changes within the T cell; these changes, in the context of additional signals from other surface receptors, ultimately result in massive proliferation of receptor-engaged T cells and the acquisition of effector and memory functions. Early studies established the importance of the activation of the enzymes phospholipase C-gamma1 (PLC-gamma1) and phosphatidylinositol 3-kinase (PI3K), as well as the small molecular weight heterotrimeric guanine nucleotide binding protein (G protein) Ras, in this process. These biochemical events are dependent on the activity of several protein tyrosine kinases that become activated immediately upon TCR engagement. An unresolved question in the field has been which molecules and what sequence of events tie together the early tyrosine phosphorylation events with the activation of these downstream signaling molecules. A likely candidate for linking the proximal and distal portions of the TCR signaling pathway is the recently described protein, LAT. LAT is a 36-kD transmembrane protein that becomes rapidly tyrosine-phosphorylated after TCR engagement. Phosphorylation of LAT creates binding sites for the Src homology 2 (SH2) domains of other proteins, including PLC-gamma1, Grb2, Gads, Grap, 3BP2, and Shb, and indirectly binds SOS, c-Cbl, Vav, SLP-76, and Itk. LAT is localized to the glycolipid-enriched membrane (GEM) subdomains of the plasma membrane by virtue of palmitoylation of two cysteine residues positioned near the endofacial side of the plasma membrane. Notably, in the absence of LAT, TCR engagement does not lead to activation of distal signaling events. This review examines the circumstances surrounding the discovery of LAT and our current understanding of its properties, and discusses current models for how LAT may be functioning to support the transduction of TCR-initiated, T cell-specific signaling events to the distal, general signaling machinery.

TCR-independent T cell development mediated by gain-of-oncogene function or loss-of-tumor-suppressor gene function

The mechanisms that govern differentiation of T cell precursors during intrathymic development bridge an interdisciplinary research field of immunology, oncology and developmental biology. Critical checkpoints controlling early thymic T cell development and homeostasis are set by the proper signaling function of the IL-7 receptor, c-Kit receptor, and the pre-T cell antigen receptor (pre-TCR). Given the intimate link between cell cycle control and differentiation in T cell development, proto-oncogenes and tumor suppressors participate as physiological effectors downstream of these receptors not only to influence the cell cycle but also to determine differentiation and survival. Gain- or loss-of-function mutations of these downstream effectors uncouples partially or completely T cell precursors from these checkpoints, providing a selective advantage and enabling aberrant development. These effectors can be identified by provirus tagging in normal mice and more readily by complementation tagging in mice with a predefined block in T cell differentiation.