Disruption of T cell signaling networks and development by Grb2 haploid insufficiency

A stepwise and digital pattern of RSK phosphorylation determines the outcome of thymic selection

Developing CD4+CD8+ double-positive (DP) thymocytes with randomly generated T cell receptors (TCRs) undergo positive (maturation) or negative (apoptosis) selection on the basis of the strength of TCR stimulation. Selection fate is determined by engagement of TCR ligands with a subtle difference in affinity, but the molecular details of TCR signaling leading to the different selection outcomes have remained unclear. We performed phosphoproteome analysis of DP thymocytes and found that p90 ribosomal protein kinase (RSK) phosphorylation at Thr562 was induced specifically by high-affinity peptide ligands. Such phosphorylation of RSK triggered its translocation to the nucleus, where it phosphorylated the nuclear receptor Nur77 and thereby promoted its mitochondrial translocation for apoptosis induction. Inhibition of RSK activity protected DP thymocytes from antigen-induced cell death. We propose that RSK phosphorylation constitutes a mechanism by which DP thymocytes generate a stepwise and binary signal in response to exposure to TCR ligands with a graded affinity.

GRB2 promotes thymocyte positive selection by facilitating THEMIS-mediated inactivation of SHP1

The T-lineage restricted protein THEMIS has been shown to play a critical role in T cell development. THEMIS, via its distinctive CABIT domains, inhibits the catalytic activity of the tyrosine phosphatase SHP1 (PTPN6). SHP1 and THEMIS bind to the ubiquitous cytosolic adapter GRB2, and the purported formation of a tri-molecular THEMIS-GRB2-SHP1 complex facilitates inactivation of SHP1 by THEMIS. The importance of this function of GRB2 among its numerous documented activities is unclear as GRB2 binds to multiple proteins and participates in several signaling responses in thymocytes. Here, we show that similar to Themis-/- thymocytes, the primary molecular defect in GRB2-deficient thymocytes is increased catalytically active SHP1 and the developmental block in GRB2-deficient thymocytes is alleviated by deletion or inhibition of SHP1 and is exacerbated by SHP1 overexpression. Thus, the principal role of GRB2 during T cell development is to promote THEMIS-mediated inactivation of SHP1 thereby enhancing the sensitivity of TCR signaling in CD4+CD8+ thymocytes to low affinity positively selecting self-ligands.

Isoform-specific functions of Ras in T-cell development and differentiation

Ras GTPases, well characterized for their role in oncogenesis, are the cells' molecular switches that signal to maintain immune homeostasis through cellular development, proliferation, differentiation, survival, and apoptosis. In the immune system, T cells are the central players that cause autoimmunity if dysregulated. Antigen-specific T-cell receptor (TCR) stimulation activates Ras-isoforms, which exhibit isoform-specific activator and effector requirements, functional specificities, and a selective role in T-cell development and differentiation. Recent studies show the role of Ras in T-cell-mediated autoimmune diseases; however, there is a scarcity of knowledge about the role of Ras in T-cell development and differentiation. To date, limited studies have demonstrated Ras activation in response to positive and negative selection signals and Ras isoform-specific signaling, including subcellular signaling, in immune cells. The knowledge of isoform-specific functions of Ras in T cells is essential, but still inadequate to develop the T-cell-targeted Ras isoform-specific treatment strategies for the diseases caused by altered Ras-isoform expression and activation in T cells. In this review, we discuss the role of Ras in T-cell development and differentiation, critically analyzing the isoform-specific functions.

Allosteric regulation of GRB2 modulates RAS activation

RAS activation is a multiple-step process in which linkage of the extracellular stimuli to the RAS activator SOS1 is the main step in RAS activation. GRB2 adaptor protein is the main modulator in SOS1 recruitment to the plasma membrane and its activation. This interaction is well studied but the exact mechanism of GRB2-SOS1 complex formation and SOS1 activation has yet remained obscure. Here, a new allosteric mechanism for the GRB2 regulation is described as a prerequisite for the modulation of SOS1 activation. This regulatory mechanism comprises a series of intramolecular interactions which are potentiated by GRB2 interaction with upstream ligands.Abbreviations: GRB2, growth factor receptor-bound protein 2; SOS1, son of sevenless 1; RAS, Rat Sarcoma; GEF, guanine nucleotide exchange factor; GAP, GTPase-activating protein; HER2, human epidermal growth factor receptor; SH3, SRC Homology 3; SH2, SRC Homology 2; PRD, proline-rich domain; PRM, proline-rich motif; PRP, proline-rich peptide; RTK, receptor tyrosine kinases.

Efficacy and Safety of AM-111 in the Treatment of Acute Unilateral Sudden Deafness-A Double-blind, Randomized, Placebo-controlled Phase 3 Study

Objective:

To confirm the efficacy and safety of AM-111 (brimapitide), a cell-penetrating c-Jun N-terminal Kinase (JNK) inhibitor, in patients suffering from severe to profound acute unilateral idiopathic sudden sensorineural hearing loss (ISSNHL).

Study design:

Prospective, double-blind, randomized, placebo-controlled phase 3 study with follow-up visits on Days 3, 7, 28, and 91.

Setting:

Fifty-one European and Asian sites (tertiary referral centers, private ENT practices).

Patients:

Two hundred fifty-six patients aged 18 to 65 years presenting within 72 hours following ISSNHL onset with mean hearing loss ≥ 40 dB and mean threshold ≥ 60 dB at the 3 worst affected contiguous test frequencies.

Interventions:

Single-dose intratympanic injection of AM-111 (0.4 or 0.8 mg/ml) or placebo; oral prednisolone as reserve therapy if hearing improvement < 10 dB at Day 7.

Main outcome measures:

Hearing improvement to Day 28 was the primary efficacy endpoint; complete hearing recovery, frequency of reserve therapy used, complete tinnitus remission, improvement in word recognition were secondary endpoints. Safety was evaluated by the frequency of clinically relevant hearing deterioration and adverse events.

Results:

While the primary efficacy endpoint was not met in the overall study population, post-hoc analysis showed a clinically relevant and nominally significant treatment effect for AM-111 0.4 mg/ml in patients with profound ISSNHL. The study drug and the administration procedure were well tolerated.

Conclusions:

AM-111 provides effective otoprotection in case of profound ISSNHL. Activation of the JNK stress kinase, AM-111's pharmacologic target, seems to set in only following pronounced acute cochlear injury associated with large hearing threshold shifts.

Alternative ZAP70-p38 signals prime a classical p38 pathway through LAT and SOS to support regulatory T cell differentiation

T cell receptor (TCR) stimulation activates diverse kinase pathways, which include the mitogen-activated protein kinases (MAPKs) ERK and p38, the phosphoinositide 3-kinases (PI3Ks), and the kinase mTOR. Although TCR stimulation activates the p38 pathway through a "classical" MAPK cascade that is mediated by the adaptor protein LAT, it also stimulates an "alternative" pathway in which p38 is activated by the kinase ZAP70. Here, we used dual-parameter, phosphoflow cytometry and in silico computation to investigate how both classical and alternative p38 pathways contribute to T cell activation. We found that basal ZAP70 activation in resting T cell lines reduced the threshold ("primed") TCR-stimulated activation of the classical p38 pathway. Classical p38 signals were reduced after T cell-specific deletion of the guanine nucleotide exchange factors Sos1 and Sos2, which are essential LAT signalosome components. As a consequence of Sos1/2 deficiency, production of the cytokine IL-2 was impaired, differentiation into regulatory T cells was reduced, and the autoimmune disease EAE was exacerbated in mice. These data suggest that the classical and alternative p38 activation pathways exist to generate immune balance.

Regulation of T-Cell Signaling by Post-Translational Modifications in Autoimmune Disease

The adaptive immune system involves antigen-specific host defense mechanisms mediated by T and B cells. In particular, CD4⁺ T cells play a central role in the elimination of pathogens. Immunological tolerance in the thymus regulates T lymphocytes to avoid self-components, including induction of cell death in immature T cells expressing the self-reactive T-cell receptor repertoire. In the periphery, mature T cells are also regulated by tolerance, e.g., via induction of anergy or regulatory T cells. Thus, T cells strictly control intrinsic signal transduction to prevent excessive responses or self-reactions. If the inhibitory effects of T cells on these mechanisms are disrupted, T cells may incorrectly attack self-components, which can lead to autoimmune disease. The functions of T cells are supported by post-translational modifications, particularly phosphorylation, of signaling molecules, the proper regulation of which is controlled by endogenous mechanisms within the T cells themselves. In recent years, molecular targeted agents against kinases have been developed for treatment of autoimmune diseases. In this review, we discuss T-cell signal transduction in autoimmune disease and provide an overview of acetylation-mediated regulation of T-cell signaling pathways.

Adenine Nucleotides Attenuate Murine T Cell Activation Induced by Concanavalin A or T Cell Receptor Stimulation

Extracellular ATP and its metabolites affect various cellular immune responses, including T cell function, but there are apparently conflicting reports concerning the effects of adenine nucleotides on T cells. For example, it has been reported that ATP-mediated activation of P2 receptor is involved in T cell activation; activation of adenosine receptors suppresses T cell function; and 1 mM ATP induces T cell death via activation of P2X7 receptor. Therefore, in this work we investigated in detail the effects of 100–250 μM ATP, ADP, or AMP on murine T cell activation. First, an in vitro study showed that pretreatment of murine splenic T cells with 100–250 μM ATP, ADP, or AMP significantly suppressed the concanavalin A (ConA)-induced release of cytokines, including IL-2. This suppression was not due to induction of cell death via the P2X7 receptor or to an immunosuppressive effect of adenosine. ATP attenuated the expression of CD25, and decreased the cell proliferation ability of activated T cells. The release of IL-2 by ConA-stimulated lymphocytes was suppressed by post-treatment with ATP, as well as by pretreatment. These results suggest that exogenous ATP suppresses the activation of T cells. Secondly, we evaluated the effect of ATP in a ConA-treated mice. Treatment with ATP attenuated the increase of IL-2 concentration in the blood. Overall, these results suggest that adenine nucleotides might have potential as supplemental therapeutic agents for T cell-mediated immune diseases, by suppressing T cell activation.

Grb2-Mediated Recruitment of USP9X to LAT Enhances Themis Stability following Thymic Selection

Themis is a new component of the TCR signaling machinery that plays a critical role during T cell development. The positive selection of immature CD4⁺CD8⁺ double-positive thymocytes and their commitment to the CD4⁺CD8^- single-positive stage are impaired in Themis^-/- mice, suggesting that Themis might be important to sustain TCR signals during these key developmental processes. However, the analysis of Themis mRNA levels revealed that Themis gene expression is rapidly extinguished during positive selection. We show in this article that Themis protein expression is increased in double-positive thymocytes undergoing positive selection and is sustained in immature single-positive thymocytes, despite the strong decrease in Themis mRNA levels in these subsets. We found that Themis stability is controlled by the ubiquitin-specific protease USP9X, which removes ubiquitin K48-linked chains on Themis following TCR engagement. Biochemical analyses indicate that USP9X binds directly to the N-terminal CABIT domain of Themis and indirectly to the adaptor protein Grb2, with the latter interaction enabling recruitment of Themis/USP9X complexes to LAT, thereby sustaining Themis expression following positive selection. Together, these data suggest that TCR-mediated signals enhance Themis stability upon T cell development and identify USP9X as a key regulator of Themis protein turnover.

p38 (Mapk14/11) occupies a regulatory node governing entry into primitive endoderm differentiation during preimplantation mouse embryo development

During mouse preimplantation embryo development, the classically described second cell-fate decision involves the specification and segregation, in blastocyst inner cell mass (ICM), of primitive endoderm (PrE) from pluripotent epiblast (EPI). The active role of fibroblast growth factor (Fgf) signalling during PrE differentiation, particularly in the context of Erk1/2 pathway activation, is well described. However, we report that p38 family mitogen-activated protein kinases (namely p38α/Mapk14 and p38β/Mapk11; referred to as p38-Mapk14/11) also participate in PrE formation. Specifically, functional p38-Mapk14/11 are required, during early-blastocyst maturation, to assist uncommitted ICM cells, expressing both EPI and earlier PrE markers, to fully commit to PrE differentiation. Moreover, functional activation of p38-Mapk14/11 is, as reported for Erk1/2, under the control of Fgf-receptor signalling, plus active Tak1 kinase (involved in non-canonical bone morphogenetic protein (Bmp)-receptor-mediated PrE differentiation). However, we demonstrate that the critical window of p38-Mapk14/11 activation precedes the E3.75 timepoint (defined by the initiation of the classical ‘salt and pepper’ expression pattern of mutually exclusive EPI and PrE markers), whereas appropriate lineage maturation is still achievable when Erk1/2 activity (via Mek1/2 inhibition) is limited to a period after E3.75. We propose that active p38-Mapk14/11 act as enablers, and Erk1/2 as drivers, of PrE differentiation during ICM lineage specification and segregation.

Cellular and Molecular Mechanisms of Autoimmunity and Lupus Nephritis

Autoimmunity involves immune responses directed against self, which are a result of defective self/foreign distinction of the immune system, leading to proliferation of self-reactive lymphocytes, and is characterized by systemic, as well as tissue-specific, inflammation. Numerous mechanisms operate to ensure the immune tolerance to self-antigens. However, monogenetic defects or genetic variants that weaken immune tolerance render susceptibility to the loss of immune tolerance, which is further triggered by environmental factors. In this review, we discuss the phenomenon of immune tolerance, genetic and environmental factors that influence the immune tolerance, factors that induce autoimmunity such as epigenetic and transcription factors, neutrophil extracellular trap formation, extracellular vesicles, ion channels, and lipid mediators, as well as costimulatory or coinhibitory molecules that contribute to an autoimmune response. Further, we discuss the cellular and molecular mechanisms of autoimmune tissue injury and inflammation during systemic lupus erythematosus and lupus nephritis.

Genome-wide DNA methylation profiling and its involved molecular pathways from one individual with thyroid malignant/benign tumor and hyperplasia: A case report

BACKGROUND

During development, methylation permanently changes gene activity, while aberrant gene methylation is key to human tumorigenesis. Gene methylation is an epigenetic event leading to gene silencing and some tumor suppressor genes that are aberrantly methylated in both thyroid cancer and benign thyroid tumor, suggesting a role for methylation in early thyroid tumorigenesis. Specific gene methylation occurs in certain types of thyroid cancer and depends on particular signaling pathways. Most reports rely on data from varied samples that vary tremendously with respect to methylation.

RESULTS

We observed that hyperplastic/malignant (H/M) thyroid tissue and benign/manligant (B/M) tissue had the most profoundly methylated loci compared to hyperplastic/benign (H/B) tissue. These loci are mapped to 863 genes (|Δβ value| > 0.15) in B/M and 1082 genes (|Δβ value| > 0.15) in H/M. After bioinformatic analysis, these genes were found to be involved in T-cell receptor signaling pathway (B/M) and Jak-Stat signaling pathways (H/M).

CONCLUSION

Our study offers the most comprehensive DNA methylation data for thyroid disease to date, using 1 patient with 3 tissue types and high-resolution 450K arrays. Our data may lay the foundation for future identification of novel epigenetic targets or diagnosis of thyroid cancer.

Expression of Twist2 is controlled by T-cell receptor signaling and determines the survival and death of thymocytes

Self-reactive thymocytes are eliminated by negative selection, whereas competent thymocytes survive by positive selection. The strength of the T-cell receptor (TCR) signal determines the fate of thymocytes undergoing either positive or negative selection. The TCR signal strength is relatively higher in negative selection than in positive selection and induces pro-apoptotic molecules such as Nur77 and Nor-1, which are members of the orphan nuclear receptor family, that then cause TCR-mediated apoptosis. However, at the molecular level, it remains unclear how positive or negative selection is distinguished based on the TCR signal. We found that the expression of Twist2 is differentially regulated in positively and negatively selected thymocytes. In particular, TCR signal strength that elicits positive selection induces Twist2 expression via the Ca²⁺-Cacineurin-NFATc3 pathway, whereas strength of the TCR signal that results in negative selection abolishes NFATc3-dependent Twist2 induction via specific activation of the JNK pathway. Using Twist2-deficient and Twist2 transgenic mice, we also found that Twist2 determines thymocyte sensitivity to TCR-mediated apoptosis by regulating the expression of Nur77 and Nor-1. Twist2 partially retains histone deacetylase 7 (HDAC7) in the nucleus and recruits it to the Nur77 promoter region to repress Nur77 in positively selected thymocytes. Thus our results suggest a molecular mechanism of how thymocytes interpret the strength of the TCR signal and how TCR sensitivity is controlled during thymic selection.

Themis1 enhances T cell receptor signaling during thymocyte development by promoting Vav1 activity and Grb2 stability

The T cell signaling protein Themis1 is essential for the positive and negative selection of thymocytes in the thymus. Although the developmental defect that results from the loss of Themis1 suggests that it enhances T cell receptor (TCR) signaling, Themis1 also recruits Src homology 2 domain-containing phosphatase-1 (SHP-1) to the vicinity of TCR signaling complexes, suggesting that it has an inhibitory role in TCR signaling. We used TCR signaling reporter mice and quantitative proteomics to explore the role of Themis1 in developing T cells. We found that Themis1 acted mostly as a positive regulator of TCR signaling in vivo when receptors were activated by positively selecting ligands. Proteomic analysis of the Themis1 interactome identified SHP-1, the TCR-associated adaptor protein Grb2, and the guanine nucleotide exchange factor Vav1 as the principal interacting partners of Themis1 in isolated mouse thymocytes. Analysis of TCR signaling in Themis1-deficient and Themis1-overexpressing mouse thymocytes demonstrated that Themis1 promoted Vav1 activity both in vitro and in vivo. The reduced activity of Vav1 and the impaired T cell development in Themis1(-/-) mice were due in part to increased degradation of Grb2, which suggests that Themis1 is required to maintain the steady-state abundance of Grb2 in thymocytes. Together, these data suggest that Themis1 acts as a positive regulator of TCR signaling in developing T cells, and identify a mechanism by which Themis1 regulates thymic selection.

Grb2 Is Important for T Cell Development, Th Cell Differentiation, and Induction of Experimental Autoimmune Encephalomyelitis

The small adaptor protein growth factor receptor-bound protein 2 (Grb2) modulates and integrates signals from receptors on cellular surfaces in inner signaling pathways. In murine T cells, Grb2 is crucial for amplification of TCR signaling. T cell-specific Grb2(fl/fl) Lckcre(tg) Grb2-deficient mice show reduced T cell numbers due to impaired negative and positive selection. In this study, we found that T cell numbers in Grb2(fl/fl) CD4cre(tg) mice were normal in the thymus and were only slightly affected in the periphery. Ex vivo analysis of CD4(+) Th cell populations revealed an increased amount of Th1 cells within the CD4(+) population of Grb2(fl/fl) CD4cre(tg) mice. Additionally, Grb2-deficient T cells showed a greater potential to differentiate into Th17 cells in vitro. To test whether these changes in Th cell differentiation potential rendered Grb2(fl/fl) CD4cre(tg) mice more prone to inflammatory diseases, we used the murine Th1 cell- and Th17 cell-driven model of experimental autoimmune encephalomyelitis (EAE). In contrast to our expectations, Grb2(fl/fl) CD4cre(tg) mice developed a milder form of EAE. The impaired EAE disease can be explained by the reduced proliferation rate of Grb2-deficient CD4(+) T cells upon stimulation with IL-2 or upon activation by allogeneic dendritic cells, because the activation of T cells by dendritic cells and the subsequent T cell proliferation are known to be crucial factors for the induction of EAE. In summary, Grb2-deficient T cells show defects in T cell development, increased Th1 and Th17 cell differentiation capacities, and impaired proliferation after activation by dendritic cells, which likely reduce the clinical symptoms of EAE.

RasGRP1 Is an Essential Signaling Molecule for Development of B1a Cells with Autoantigen Receptors

B1a cells, particularly the PD-L2(+) B1a cell subset, are enriched with autoantigen-specific receptors. However, the underlying molecular mechanism responsible for the skewed selection of autoreactive B1a cells remains unclear. In this study, we find that B1 cells express only Ras guanyl nucleotide-releasing protein (RasGRP) 1, whereas B2 cells express mostly RasGRP3 and little RasGRP1. RasGRP1 is indispensable for transduction of weak signals. RasGRP1 deficiency markedly impairs B1a cell development and reduces serum natural IgM production; in particular, B1a cells that express autoantigen receptors, such as anti-phosphatidylcholine B1a cells, are virtually eliminated. Thus, unlike Btk and other signalosome components, RasGRP1 deficiency selectively affects only the B1a cell population with autoantigen receptors rather than the entire pool of B1a cells.

GRB2 Nucleates T Cell Receptor-Mediated LAT Clusters That Control PLC-γ1 Activation and Cytokine Production

GRB2 is a ubiquitously expressed adaptor protein required for signaling downstream of multiple receptors. To address the role of GRB2 in receptor-mediated signaling, the expression of GRB2 was suppressed in human CD4+ T cells and its role downstream of the T cell receptor (TCR) was examined. Interestingly, GRB2 deficient T cells had enhanced signaling from complexes containing the TCR. However, GRB2 deficient T cells had substantially reduced production of IL-2 and IFN-γ. This defect was attributed to diminished formation of linker for activation of T cells (LAT) signaling clusters, which resulted in reduced MAP kinase activation, calcium flux, and PLC-γ1 recruitment to LAT signaling clusters. Add back of wild-type GRB2, but not a novel N-terminal SH3 domain mutant, rescued LAT microcluster formation, calcium mobilization, and cytokine release, providing the first direct evidence that GRB2, and its ability to bind to SH3 domain ligands, is required for establishing LAT microclusters. Our data demonstrate that the ability of GRB2 to facilitate protein clusters is equally important in regulating TCR-mediated functions as its capacity to recruit effector proteins. This highlights that GRB2 regulates signaling downstream of adaptors and receptors by both recruiting effector proteins and regulating the formation of signaling complexes.

CBAP promotes thymocyte negative selection by facilitating T-cell receptor proximal signaling

T-cell receptor (TCR)-transduced signaling is critical to thymocyte development at the CD4/CD8 double-positive stage, but the molecules involved in this process are not yet fully characterized. We previously demonstrated that GM-CSF/IL-3/IL-5 receptor common β-chain-associated protein (CBAP) modulates ZAP70-mediated T-cell migration and adhesion. On the basis of the high expression of CBAP during thymocyte development, we investigated the function of CBAP in thymocyte development using a CBAP knockout mouse. CBAP-deficient mice showed normal early thymocyte development and positive selection. In contrast, several negative selection models (including TCR transgene, superantigen staphylococcal enterotoxin B, and anti-CD3 antibody treatment) revealed an attenuation of TCR-induced thymocyte deletion in CBAP knockout mice. This phenotype correlated with a reduced accumulation of BIM upon TCR crosslinking in CBAP-deficient thymocytes. Loss of CBAP led to reduced TCR-induced phosphorylation of proteins involved in both proximal and distal signaling events, including ZAP70, LAT, PLCγ1, and JNK1/2. Moreover, TCR-induced association of LAT signalosome components was reduced in CBAP-deficient thymocytes. Our data demonstrate that CBAP is a novel component in the TCR signaling pathway and modulates thymocyte apoptosis during negative selection.

Fine-tuning T cell receptor signaling to control T cell development

T cell development from immature CD4(+)CD8(+) double-positive (DP) thymocytes to the mature CD4 or CD8 single-positive (SP) stage requires proper T cell receptor (TCR) signaling. The current working model of thymocyte development is that the strength of the TCR-mediated signal - from little-or-none, through intermediate, to strong - received by the immature cells determines whether they will undergo death by neglect, positive selection, or negative selection, respectively. In recent years, several developmentally regulated, stage-specifically expressed proteins and miRNAs have been found that act like fine-tuners for signal transduction and propagation downstream of the TCR. This allows them to govern thymocyte positive selection. Here, we summarize recent findings on these molecules and suggest new concepts of TCR positive-selection signaling.

Alternative capture of noncoding RNAs or protein-coding genes by herpesviruses to alter host T cell function

In marmoset T cells transformed by Herpesvirus saimiri (HVS), a viral U-rich noncoding (nc) RNA, HSUR 1, specifically mediates degradation of host microRNA-27 (miR-27). High-throughput sequencing of RNA after crosslinking immunoprecipitation (HITS-CLIP) identified mRNAs targeted by miR-27 as enriched in the T cell receptor (TCR) signaling pathway, including GRB2. Accordingly, transfection of miR-27 into human T cells attenuates TCR-induced activation of mitogen-activated protein kinases (MAPKs) and induction of CD69. MiR-27 also robustly regulates SEMA7A and IFN-γ, key modulators and effectors of T cell function. Knockdown or ectopic expression of HSUR 1 alters levels of these proteins in virally transformed cells. Two other T-lymphotropic γ-herpesviruses, AlHV-1 and OvHV-2, do not produce a noncoding RNA to downregulate miR-27 but instead encode homologs of miR-27 target genes. Thus, oncogenic γ-herpesviruses have evolved diverse strategies to converge on common targets in host T cells.

Strength of TCR signal from self-peptide modulates autoreactive thymocyte deletion and Foxp3(+) Treg-cell formation

Autoreactive CD4(+) CD8(-) (CD4SP) thymocytes can be subjected to deletion when they encounter self-peptide during their development, but they can also undergo selection to become CD4SPFoxp3(+) Treg cells. We have analyzed the relationship between these distinct developmental fates using mice in which signals transmitted by the TCR have been attenuated by mutation of a critical tyrosine residue of the adapter protein SLP-76. In mice containing polyclonal TCR repertoires, the mutation caused increased frequencies of CD4SPFoxp3(+) thymocytes. CD4SP thymocytes expressing TCR Vβ-chains that are subjected to deletion by endogenous retroviral superantigens were also present at increased frequencies, particularly among Foxp3(+) thymocytes. In transgenic mice in which CD4SP thymocytes expressing an autoreactive TCR undergo both deletion and Treg-cell formation in response to a defined self-peptide, SLP-76 mutation abrogated deletion of autoreactive CD4SP thymocytes. Notably, Foxp3(+) Treg-cell formation still occurred, albeit with a reduced efficiency, and the mutation was also associated with decreased Nur77 expression by the autoreactive CD4SP thymocytes. These studies provide evidence that the strength of the TCR signal can play a direct role in directing the extent of both thymocyte deletion and Treg-cell differentiation, and suggest that distinct TCR signaling thresholds and/or pathways can promote CD4SP thymocyte deletion versus Treg-cell formation.

A model of an integrated immune system pathway in Homo sapiens and its interaction with superantigen producing expression regulatory pathway in Staphylococcus aureus: comparing behavior of pathogen perturbed and unperturbed pathway

Response of an immune system to a pathogen attack depends on the balance between the host immune defense and the virulence of the pathogen. Investigation of molecular interactions between the proteins of a host and a pathogen helps in identifying the pathogenic proteins. It is necessary to understand the dynamics of a normally behaved host system to evaluate the capacity of its immune system upon pathogen attack. In this study, we have compared the behavior of an unperturbed and pathogen perturbed host system. Moreover, we have developed a formalism under Flux Balance Analysis (FBA) for the optimization of conflicting objective functions. We have constructed an integrated pathway system, which includes Staphylococcal Superantigen (SAg) expression regulatory pathway and TCR signaling pathway of Homo sapiens. We have implemented the method on this pathway system and observed the behavior of host signaling molecules upon pathogen attack. The entire study has been divided into six different cases, based on the perturbed/unperturbed conditions. In other words, we have investigated unperturbed and pathogen perturbed human TCR signaling pathway, with different combinations of optimization of concentrations of regulatory and signaling molecules. One of these cases has aimed at finding out whether minimization of the toxin production in a pathogen leads to the change in the concentration levels of the proteins coded by TCR signaling pathway genes in the infected host. Based on the computed results, we have hypothesized that the balance between TCR signaling inhibitory and stimulatory molecules can keep TCR signaling system into resting/stimulating state, depending upon the perturbation. The proposed integrated host-pathogen interaction pathway model has accurately reflected the experimental evidences, which we have used for validation purpose. The significance of this kind of investigation lies in revealing the susceptible interaction points that can take back the Staphylococcal Enterotoxin (SE)-challenged system within the range of normal behavior.

Growth factor receptor-bound protein 2 contributes to (hem)immunoreceptor tyrosine-based activation motif-mediated signaling in platelets

RATIONALE

Platelets are anuclear cell fragments derived from bone marrow megakaryocytes (MKs) that safeguard vascular integrity but may also cause pathological vessel occlusion. One major pathway of platelet activation is triggered by 2 receptors that signal through an (hem)immunoreceptor tyrosine-based activation motif (ITAM), the activating collagen receptor glycoprotein (GP) VI and the C-type lectin-like receptor 2 (CLEC-2). Growth factor receptor-bound protein 2 (Grb2) is a ubiquitously expressed adapter molecule involved in signaling processes of numerous receptors in different cell types, but its function in platelets and MKs is unknown.

OBJECTIVE

We tested the hypothesis that Grb2 is a crucial adapter protein in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets.

METHODS AND RESULTS

Here, we show that genetic ablation of Grb2 in MKs and platelets did not interfere with MK differentiation or platelet production. However, Grb2-deficiency severely impaired glycoprotein VI-mediated platelet activation because of defective stabilization of the linker of activated T-cell (LAT) signalosome and activation of downstream signaling proteins that resulted in reduced adhesion, aggregation, and coagulant activity on collagen in vitro. Similarly, CLEC-2-mediated signaling was impaired in Grb2-deficient platelets, whereas the cells responded normally to stimulation of G protein-coupled receptors. In vivo, this selective (hem)immunoreceptor tyrosine-based activation motif signaling defect resulted in prolonged bleeding times but affected arterial thrombus formation only after concomitant treatment with acetylsalicylic acid, indicating that defective glycoprotein VI signaling in the absence of Grb2 can be compensated through thromboxane A2-induced G protein-coupled receptor signaling pathways.

CONCLUSIONS

These results reveal an important contribution of Grb2 in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets in hemostasis and thrombosis by stabilizing the LAT signalosome.

Regulation of ras exchange factors and cellular localization of ras activation by lipid messengers in T cells

The Ras-MAPK signaling pathway is highly conserved throughout evolution and is activated downstream of a wide range of receptor stimuli. Ras guanine nucleotide exchange factors (RasGEFs) catalyze GTP loading of Ras and play a pivotal role in regulating receptor-ligand induced Ras activity. In T cells, three families of functionally important RasGEFs are expressed: RasGRF, RasGRP, and Son of Sevenless (SOS)-family GEFs. Early on it was recognized that Ras activation is critical for T cell development and that the RasGEFs play an important role herein. More recent work has revealed that nuances in Ras activation appear to significantly impact T cell development and selection. These nuances include distinct biochemical patterns of analog versus digital Ras activation, differences in cellular localization of Ras activation, and intricate interplays between the RasGEFs during distinct T cell developmental stages as revealed by various new mouse models. In many instances, the exact nature of these nuances in Ras activation or how these may result from fine-tuning of the RasGEFs is not understood. One large group of biomolecules critically involved in the control of RasGEFs functions are lipid second messengers. Multiple, yet distinct lipid products are generated following T cell receptor (TCR) stimulation and bind to different domains in the RasGRP and SOS RasGEFs to facilitate the activation of the membrane-anchored Ras GTPases. In this review we highlight how different lipid-based elements are generated by various enzymes downstream of the TCR and other receptors and how these dynamic and interrelated lipid products may fine-tune Ras activation by RasGEFs in developing T cells.

Activation of Extracellular Signal-Regulated Kinase but Not of p38 Mitogen-Activated Protein Kinase Pathways in Lymphocytes Requires Allosteric Activation of SOS

Thymocytes convert graded T cell receptor (TCR) signals into positive selection or deletion, and activation of extracellular signal-related kinase (ERK), p38, and Jun N-terminal protein kinase (JNK) mitogen-activated protein kinases (MAPKs) has been postulated to play a discriminatory role. Two families of Ras guanine nucleotide exchange factors (RasGEFs), SOS and RasGRP, activate Ras and the downstream RAF-MEK-ERK pathway. The pathways leading to lymphocyte p38 and JNK activation are less well defined. We previously described how RasGRP alone induces analog Ras-ERK activation while SOS and RasGRP cooperate to establish bimodal ERK activation. Here we employed computational modeling and biochemical experiments with model cell lines and thymocytes to show that TCR-induced ERK activation grows exponentially in thymocytes and that a W729E allosteric pocket mutant, SOS1, can only reconstitute analog ERK signaling. In agreement with RasGRP allosterically priming SOS, exponential ERK activation is severely decreased by pharmacological or genetic perturbation of the phospholipase Cγ (PLCγ)-diacylglycerol-RasGRP1 pathway. In contrast, p38 activation is not sharply thresholded and requires high-level TCR signal input. Rac and p38 activation depends on SOS1 expression but not allosteric activation. Based on computational predictions and experiments exploring whether SOS functions as a RacGEF or adaptor in Rac-p38 activation, we established that the presence of SOS1, but not its enzymatic activity, is critical for p38 activation.

Ras and extracellular signal-regulated kinase signaling in thymocytes and T cells

Extracellular signal-regulated kinase (ERK) activation is important for both thymocyte development and T cell function. Classically, signal transduction from the T cell antigen receptor (TCR) to ERK is thought to be regulated by signaling from Ras guanine nucleotide exchange factors (GEFs), through the small G protein Ras, to the three-tiered Raf-MAPK/ERK kinase (MEK)-ERK kinase cascade. Developing and mature T cells express four members of two RasGEF families, RasGRP1, RasGRP4, son of sevenless 1 (Sos1), and Sos2, and several models describing combined signaling from these RasGEFs have been proposed. However, recent studies suggest that existing models need revision to include both distinct and overlapping roles of multiple RasGEFs during thymocyte development and novel, Ras-independent signals to ERK that have been identified in peripheral T cells.

TCR-mediated Erk activation does not depend on Sos and Grb2 in peripheral human T cells

Sos proteins are ubiquitously expressed activators of Ras. Lymphoid cells also express RasGRP1, another Ras activator. Sos and RasGRP1 are thought to cooperatively control full Ras activation upon T-cell receptor triggering. Using RNA interference, we evaluated whether this mechanism operates in primary human T cells. We found that T-cell antigen receptor (TCR)-mediated Erk activation requires RasGRP1, but not Grb2/Sos. Conversely, Grb2/Sos—but not RasGRP1—are required for IL2-mediated Erk activation. Thus, RasGRP1 and Grb2/Sos are insulators of signals that lead to Ras activation induced by different stimuli, rather than cooperating downstream of the TCR.

Piecing together the humoral and cellular mechanisms of immune thrombocytopenia

The precise mechanisms leading to platelet-targeted autoimmunity in immune thrombocytopenia (ITP) are not known. Cellular checkpoints normally regulate immunological self-reactivity during the development of B and T cells through cell deletion, receptor editing, induction of anergy, and extrinsic cellular suppression. When these checkpoints fail, tolerance to self-antigens may be lost. In this review, we summarize the various immune mechanisms contributing to the development of ITP and relate them back to the checkpoint model of autoimmunity. These mechanisms, including increased levels of lymphocyte growth factors, resistance to death signals, and loss of T-regulatory function, result in an environment permissive to the development of platelet-reactive B and T cells. The mechanisms that lead to thrombocytopenia once tolerance for platelet antigens is lost are examined, including complement-dependent and apoptotic pathways. An improved understanding of ITP pathogenesis will ultimately guide the development of better therapies.

A perspective on non-catalytic Src homology (SH) adaptor signalling proteins

Intracellular adaptor signalling proteins are members of a large family of mediators crucial for signal transduction pathways. Structurally, these molecules contain one Src Homology 2 (SH2) domain and one or more Src Homology 3 (SH3) domain(s); with either a catalytic subunit, or with other non-catalytic modular subunits. Cells depend on these regulatory signalling molecules to transmit information to the nucleus from both external and internal cues including growth factors, cytokines and steroids. Although there is a vast library of adaptor signalling proteins expressed ubiquitously in cells, the vital role these SH containing proteins play in regulating cellular signalling lacks the recognition they deserve. Their target selection method via the SH domains is simple yet highly effective. The SH3 domain(s) interact with proteins that contain proline-rich motifs, whereas the SH2 domain only binds to proteins containing phosphotyrosine residues. This unique characteristic physically enables proteins from a diverse range of networks to assemble for amplification of a signalling event. The biological consequence generated from these adaptor signalling proteins in a constantly changing microenvironment have profound regulatory effect on cell fate decision particularly when this is involved in the progression of a diseased state.

Targeted Sos1 deletion reveals its critical role in early T-cell development

Activation of the small G protein Ras is required for thymocyte differentiation. In thymocytes, Ras is activated by the Ras guanine exchange factors (RasGEFs) Sos1, Sos2, and RasGRP1. We report the development of a floxed allele of sos1 to assess the role of Sos1 during thymocyte development. Sos1 was required for pre-T-cell receptor (pre-TCR)- but not TCR-stimulated developmental signals. Sos1 deletion led to a partial block at the DN-to-DP transition. Sos1-deficient thymocytes showed reduced pre-TCR-stimulated proliferation, differentiation, and ERK phosphorylation. In contrast, TCR-stimulated positive selection, and negative selection under strong stimulatory conditions, remained intact in Sos1-deficient mice. Comparison of RasGEF expression at different developmental stages showed that relative to Sos2 and RasGRP1, Sos1 is most abundant in DN thymocytes, but least abundant in DP thymocytes. These data reveal that Sos1 is uniquely positioned to affect signal transduction early in thymocyte development.

Grb2 regulates B-cell maturation, B-cell memory responses and inhibits B-cell Ca2+ signalling

Grb2 is a ubiquitously expressed adaptor protein, which activates Ras and MAP kinases in growth factor receptor signalling, while in B-cell receptor (BCR) signalling this role is controversial. In B cell lines it was shown that Grb2 can inhibit BCR-induced Ca(2+) signalling. Nonetheless, the physiological role of Grb2 in primary B cells is still unknown. We generated a B-cell-specific Grb2-deficient mouse line, which had a severe reduction of mature follicular B cells in the periphery due to a differentiation block and decreased B-cell survival. Moreover, we found several changes in important signalling pathways: enhanced BCR-induced Ca(2+) signalling, alterations in mitogen-activated protein kinase activation patterns and strongly impaired Akt activation, the latter pointing towards a defect in PI3K signalling. Interestingly, B-cell-specific Grb2-deficient mice showed impaired IgG and B-cell memory responses, and impaired germinal centre formation. Thus, Grb2-dependent signalling pathways are crucial for lymphocyte differentiation processes, as well as for control of secondary humoral immune responses.

Life and death in the thymus--cell death signaling during T cell development

The thymus is an organ vital to proper T cell development, and the regulation of cell survival and death contributes significantly to its efficient function. Vital to many of the developmental processes that occur in the thymus, control over cell survival and death is orchestrated by several signaling processes. In this review, we focus on the regulation of death in early thymocytes known as CD4/CD8 double negative cells, including the roles of interleukin-7 and Bcl-2 family members in this developmental stage. We next consider the survival and death of later thymocytes that express both CD4 and CD8, the 'double-positive' thymocytes. These findings are discussed within the context of recent studies demonstrating the existence of caspase-independent cell death pathways.

Activation of p38 mitogen-activated protein kinase by norepinephrine in T-lineage cells

The catecholamine norepinephrine (NE) stimulates T lymphocytes through a beta-adrenergic receptor (βAR)/adenylyl cyclase (AC)/cyclic AMP (cAMP)/protein kinase A (PKA) pathway, leading to altered cell responsiveness and apoptosis. p38 Mitogen-activated protein kinase (MAPK), a major intracellular signalling mediator for cellular and environmental stressors, is involved in the production of immune modulators and in the regulation of T-cell development, survival and death. In these studies we investigated the relationship among NE signalling, p38 MAPK activity and T-cell death. We showed that NE stimulation of BALB/c mouse thymocytes and S49 thymoma cells selectively increases the dual phosphorylation and activity of p38α MAPK. p38 MAPK activation involves the βAR, Gs protein, AC, cAMP and PKA, as determined through the use of a βAR antagonist, activators of AC and cAMP, and S49 clonal mutants deficient in Gs and PKA. Dual phosphorylation of p38 MAPK is also dependent on its own catalytic activity. Inhibition of p38 MAPK activity revealed its involvement in cAMP-mediated activating transcription factor-2 (ATF-2) phosphorylation, Fas ligand messenger RNA (mRNA) up-regulation, and cell death. These results identify a mechanism through which NE stimulation of the βAR/Gs/PKA pathway activates p38 MAPK, which can be potentiated by autophosphorylation, and leads to changes in T-cell dynamics, in part through the regulation of Fas ligand mRNA expression.

Themis2/ICB1 is a signaling scaffold that selectively regulates macrophage Toll-like receptor signaling and cytokine production

Background

Thymocyte expressed molecule involved in selection 1 (Themis1, SwissProt accession number Q8BGW0) is the recently characterised founder member of a novel family of proteins. A second member of this family, Themis2 (Q91YX0), also known as ICB1 (Induced on contact with basement membrane 1), remains unreported at the protein level despite microarray and EST databases reporting Themis2 mRNA expression in B cells and macrophages.

Methodology/Principal Findings

Here we characterise Themis2 protein for the first time and show that it acts as a macrophage signalling scaffold, exerting a receptor-, mediator- and signalling pathway-specific effect on TLR responses in RAW 264.7 macrophages. Themis2 over-expression enhanced the LPS-induced production of TNF but not IL-6 or Cox-2, nor TNF production induced by ligands for TLR2 (PAM3) or TLR3 (poly I∶C). Moreover, LPS-induced activation of the MAP kinases ERK and p38 was enhanced in cells over-expressing Themis2 whereas the activation of JNK, IRF3 or NF-κB p65, was unaffected. Depletion of Themis2 protein by RNA inteference inhibited LPS-induced TNF production in primary human macrophages demonstrating a requirement for Themis2 in this event. Themis2 was inducibly tyrosine phosphorylated upon LPS challenge and interacted with Lyn kinase (P25911), the Rho guanine nucleotide exchange factor, Vav (P27870), and the adaptor protein Grb2 (Q60631). Mutation of either tyrosine 660 or a proline-rich sequence (PPPRPPK) simultaneously interrupted this complex and reduced by approximately 50% the capacity of Themis2 to promote LPS-induced TNF production. Finally, Themis2 protein expression was induced during macrophage development from murine bone marrow precursors and was regulated by inflammatory stimuli both in vitro and in vivo.

Conclusions/Significance

We hypothesise that Themis2 may constitute a novel, physiological control point in macrophage inflammatory responses.

Grb2, a simple adapter with complex roles in lymphocyte development, function, and signaling

Lymphocyte development, activation, and tolerance depend on antigen receptor signaling transduced via multiple intracellular signalosomes. These signalosomes are assembled by different adapters. Given that signaling molecules can be either positive or negative regulators for a biochemical target, the complex of a target with different regulator may dictate the final signaling outcome. Grb2 is a simple adapter known to be involved in a variety of growth factor receptor signaling. However, its role in antigen receptor signaling as well as lymphocyte development and function has emerged only recently. Despite its simple molecular structure, recent experiments show that Grb2 may play a complex role in T and B-cell antigen receptor signaling. In this article, we review recent findings about the physiological role of Grb2 in T and B-cell development and activation and summarize the current mechanistic understanding of how Grb2 exerts its function following T and B-cell antigen receptor stimulation.

Grb2 functions at the top of the T-cell antigen receptor-induced tyrosine kinase cascade to control thymic selection

Grb2 is an adaptor molecule that mediates Ras-MAPK activation induced by various receptors. Here we show that conditional ablation of Grb2 in thymocytes severely impairs both thymic positive and negative selections. Strikingly, the mutation attenuates T-cell antigen receptor (TCR) proximal signaling, including tyrosine phosphorylation of multiple signaling proteins and Ca(2+) influx. The defective TCR signaling can be attributed to a marked impairment in Lck activation. Ectopic expression of a mutant Grb2 composed of the central SH2 and the C-terminal SH3 domains in Grb2(-/-) thymocytes fully restores thymocyte development. Thus, Grb2 plays a pivotal role in both thymic positive and negative selection. It amplifies TCR signaling at the top end of the tyrosine phosphorylation cascade via a scaffolding function.

Fas apoptosis inhibitory molecule regulates T cell receptor-mediated apoptosis of thymocytes by modulating Akt activation and Nur77 expression

Fas apoptosis inhibitory molecule (FAIM) has been demonstrated to confer resistance to Fas-induced apoptosis of lymphocytes and hepatocytes in vitro and in vivo. Here, we show that FAIM is up-regulated in thymocytes upon T cell receptor (TCR) engagement and that faim(-/-) thymocytes are highly susceptible to TCR-mediated apoptosis with increased activation of caspase-8 and -9. Furthermore, injection of anti-CD3 antibodies leads to augmented depletion of CD4(+)CD8(+) T cells in the thymus of faim(-/-) mice compared with wild-type control, suggesting that FAIM plays a role in thymocyte apoptosis. Cross-linking of the TCR on faim(-/-) thymocytes leads to an elevated protein level of the orphan nuclear receptor Nur77, which plays a role in thymocyte apoptosis. Interestingly, in the absence of FAIM, there are reduced ubiquitination and degradation of the Nur77 protein. Faim(-/-) thymocytes also exhibit a defective TCR-induced activation of Akt whose activity we now show is required for Nur77 ubiquitination. Further analyses utilizing FAIM-deficient primary thymocytes and FAIM-overexpressing DO-11.10 T cells indicate that FAIM acts upstream of Akt during TCR signaling and influences the localization of Akt to lipid rafts, hence affecting its activation. Taken together, our study defined a TCR-induced FAIM/Akt/Nur77 signaling axis that is critical for modulating the apoptosis of developing thymocytes.

Nuclear factor-kappaB modulates regulatory T cell development by directly regulating expression of Foxp3 transcription factor

Naturally derived regulatory T (Treg) cells are characterized by stable expression of the transcription factor Foxp3 and characteristic epigenetic imprinting at the Foxp3 gene locus. Here, we found that enhancing nuclear factor (NF)-kappaB activity via a constitutive active inhibitor of kappaB kinase beta (IKKbeta) transgene in T cells led to increased number of Foxp3(+) cells in the thymus and can rescue Foxp3 expression in thymocytes deficient in other pleiotropic signaling molecules. Enhancing the signal strength of the NF-kappaB pathway also induced Foxp3 expression in otherwise conventionally selected T cells. NF-kappaB directly promoted the transcription of Foxp3, and upon T cell receptor (TCR) stimulation, c-Rel, a NF-kappaB family member, bound to Foxp3 enhancer region, which is specifically demethylated in natural Treg cells. Hence, NF-kappaB signaling pathway is a key regulator of Foxp3 expression during natural Treg cell development.

Foxp1 is an essential transcriptional regulator for the generation of quiescent naive T cells during thymocyte development

Proper thymocyte development is required to establish T-cell central tolerance and to generate naive T cells, both of which are essential for T-cell homeostasis and a functional immune system. Here we demonstrate that the loss of transcription factor Foxp1 results in the abnormal development of T cells. Instead of generating naive T cells, Foxp1-deficient single-positive thymocytes acquire an activated phenotype prematurely in the thymus and lead to the generation of peripheral CD4(+) T and CD8(+) T cells that exhibit an activated phenotype and increased apoptosis and readily produce cytokines upon T-cell receptor engagement. These results identify Foxp1 as an essential transcriptional regulator for thymocyte development and the generation of quiescent naive T cells.

RasGRP1 regulates antigen-induced developmental programming by naive CD8 T cells

Ag encounter by naive CD8 T cells initiates a developmental program consisting of cellular proliferation, changes in gene expression, and the formation of effector and memory T cells. The strength and duration of TCR signaling are known to be important parameters regulating the differentiation of naive CD8 T cells, although the molecular signals arbitrating these processes remain poorly defined. The Ras-guanyl nucleotide exchange factor RasGRP1 has been shown to transduce TCR-mediated signals critically required for the maturation of developing thymocytes. To elucidate the role of RasGRP1 in CD8 T cell differentiation, in vitro and in vivo experiments were performed with 2C TCR transgenic CD8 T cells lacking RasGRP1. In this study, we report that RasGRP1 regulates the threshold of T cell activation and Ag-induced expansion, at least in part, through the regulation of IL-2 production. Moreover, RasGRP1(-/-) 2C CD8 T cells exhibit an anergic phenotype in response to cognate Ag stimulation that is partially reversible upon the addition of exogenous IL-2. By contrast, the capacity of IL-2/IL-2R interactions to mediate Ras activation and CD8 T cell expansion and differentiation appears to be largely RasGRP1-independent. Collectively, our results demonstrate that RasGRP1 plays a selective role in T cell signaling, controlling the initiation and duration of CD8 T cell immune responses.

Integration of the beta-catenin-dependent Wnt pathway with integrin signaling through the adaptor molecule Grb2

BACKGROUND

THE COMPLEXITY OF WNT SIGNALING LIKELY STEMS FROM TWO SOURCES: multiple pathways emanating from frizzled receptors in response to wnt binding, and modulation of those pathways and target gene responsiveness by context-dependent signals downstream of growth factor and matrix receptors. Both rac1 and c-jun have recently been implicated in wnt signaling, however their upstream activators have not been identified.

METHODOLOGY/PRINCIPAL FINDINGS

Here we identify the adapter protein Grb2, which is itself an integrator of multiple signaling pathways, as a modifier of beta-catenin-dependent wnt signaling. Grb2 synergizes with wnt3A, constitutively active (CA) LRP6, Dvl2 or CA-beta-catenin to drive a LEF/TCF-responsive reporter, and dominant negative (DN) Grb2 or siRNA to Grb2 block wnt3A-mediated reporter activity. MMP9 is a target of beta-catenin-dependent wnt signaling, and an MMP9 promoter reporter is also responsive to signals downstream of Grb2. Both a jnk inhibitor and DN-c-jun block transcriptional activation downstream of Dvl2 and Grb2, as does DN-rac1. Integrin ligation by collagen also synergizes with wnt signaling as does overexpression of Focal Adhesion Kinase (FAK), and this is blocked by DN-Grb2.

CONCLUSIONS/SIGNIFICANCE

These data suggest that integrin ligation and FAK activation synergize with wnt signaling through a Grb2-rac-jnk-c-jun pathway, providing a context-dependent mechanism for modulation of wnt signaling.

Cutting edge: Extracellular signal-related kinase is not required for negative selection of developing T cells

Signals initiated through the TCR during development can result in either survival and differentiation or cell death. High affinity signals that induce death elicit a robust yet transient activation of signaling pathways, including Erk, whereas low affinity ligands, which promote survival, generate a gradual and weaker activation of the same pathways. It was recently demonstrated that Erk localizes to distinct cellular locations in response to high and low affinity ligands. Although a requirement for Erk in positive selection is well established, its role in negative selection is controversial and, thus, the importance of Erk relocalization during development is not understood. In this study, we examined the role of Erk in negative selection using mice that are genetically deficient in both Erk1 and Erk2 in T cells. Results from three different models reveal that thymocyte deletion remains intact in the absence of Erk.

Gasp, a Grb2-associating protein, is critical for positive selection of thymocytes

T cells develop in the thymus through positive and negative selection, which are responsible for shaping the T cell receptor (TCR) repertoire. To elucidate the molecular mechanisms involved in selection remains an area of intense interest. Here, we identified and characterized a gene product Gasp (Grb2-associating protein, also called Themis) that is critically required for positive selection. Gasp is a cytosolic protein with no known functional motifs that is expressed only in T cells, especially immature CD4/CD8 double positive (DP) thymocytes. In the absence of Gasp, differentiation of both CD4 and CD8 single positive cells in the thymus was severely inhibited, whereas all other TCR-induced events such as beta-selection, negative selection, peripheral activation, and homeostatic proliferation were unaffected. We found that Gasp constitutively associates with Grb2 via its N-terminal Src homology 3 domain, suggesting that Gasp acts as a thymocyte-specific adaptor for Grb2 or regulates Ras signaling in DP thymocytes. Collectively, we have described a gene called Gasp that is critical for positive selection.

Beta-catenin/Tcf determines the outcome of thymic selection in response to alphabetaTCR signaling

Thymic maturation of T cells depends on the intracellular interpretation of alphabetaTCR signals by processes that are poorly understood. In this study, we report that beta-catenin/Tcf signaling was activated in double-positive thymocytes in response to alphabetaTCR engagement and impacted thymocyte selection. TCR engagement combined with activation of beta-catenin signaled thymocyte deletion, whereas Tcf-1 deficiency rescued from negative selection. Survival/apoptotis mediators including Bim, Bcl-2, and Bcl-x(L) were alternatively influenced by stabilization of beta-catenin or ablation of Tcf-1, and Bim-mediated beta-catenin induced thymocyte deletion. TCR activation in double-positive cells with stabilized beta-catenin triggered signaling associated with negative selection, including sustained overactivation of Lat and Jnk and a transient activation of Erk. These observations are consistent with beta-catenin/Tcf signaling acting as a switch that determines the outcome of thymic selection downstream the alphabetaTCR cascade.