The role of erk1 and erk2 in multiple stages of T cell development

Erk1 positively regulates osteoclast differentiation and bone resorptive activity

The extracellular signal-regulated kinases (ERK1 and 2) are widely-expressed and they modulate proliferation, survival, differentiation, and protein synthesis in multiple cell lineages. Altered ERK1/2 signaling is found in several genetic diseases with skeletal phenotypes, including Noonan syndrome, Neurofibromatosis type 1, and Cardio-facio-cutaneous syndrome, suggesting that MEK-ERK signals regulate human skeletal development. Here, we examine the consequence of Erk1 and Erk2 disruption in multiple functions of osteoclasts, specialized macrophage/monocyte lineage-derived cells that resorb bone. We demonstrate that Erk1 positively regulates osteoclast development and bone resorptive activity, as genetic disruption of Erk1 reduced osteoclast progenitor cell numbers, compromised pit formation, and diminished M-CSF-mediated adhesion and migration. Moreover, WT mice reconstituted long-term with Erk1^−/− bone marrow mononuclear cells (BMMNCs) demonstrated increased bone mineral density as compared to recipients transplanted with WT and Erk2^−/− BMMNCs, implicating marrow autonomous, Erk1-dependent osteoclast function. These data demonstrate Erk1 plays an important role in osteoclast functions while providing rationale for the development of Erk1-specific inhibitors for experimental investigation and/or therapeutic modulation of aberrant osteoclast function.

HIV-1 infection abrogates CD8+ T cell mitogen-activated protein kinase signaling responses

Mitogen-activated protein kinase (MAPK) signaling pathways are dynamic and sensitive regulators of T cell function and differentiation. Altered MAPK signaling has been associated with the inflammatory and autoimmune diseases lupus and arthritis and with some pathogenic viral infections. HIV-1 infection is characterized by chronic immune inflammation, aberrantly heightened CD8(+) T cell activation levels, and altered T cell function. The relationship between MAPK pathway function, HIV-1-induced activation (CD38 and HLA-DR), and exhaustion (Tim-3) markers in circulating CD8(+) T cells remains unknown. Phosphorylation of the MAPK effector proteins ERK and p38 was examined by "phosflow" flow cytometry in 79 recently HIV-1-infected, antiretroviral-treatment-naïve adults and 21 risk-matched HIV-1-negative controls. We identified a subset of CD8(+) T cells refractory to phorbol 12-myristate 13-acetate plus ionomycin-induced ERK1/2 phosphorylation (referred to as p-ERK1/2-refractory cells) that was greatly expanded in HIV-1-infected adults. The CD8(+) p-ERK1/2-refractory cells were highly activated (CD38(+) HLA-DR(+)) but not exhausted (Tim-3 negative), tended to have low CD8 expression, and were enriched in intermediate and late transitional memory states of differentiation (CD45RA(-) CD28(-) CD27(+/-)). Targeting MAPK pathways to restore ERK1/2 signaling may normalize immune inflammation levels and restore CD8(+) T cell function during HIV-1 infection.

Role of Ras/Raf/MEK/ERK signaling in physiological hematopoiesis and leukemia development

Recent research on hematological malignancies has shown that malignant cells often co-opt physiological pathways to promote their growth and development. Bone marrow homeostasis requires a fine balance between cellular differentiation and self-renewal; cell survival and apoptosis; and cellular proliferation and senescence. The Ras/Raf/MEK/ERK pathway has been shown to be important in regulating these biological functions. Moreover, the Ras/Raf/MEK/ERK pathway has been estimated to be mutated in 30% of all cancers, thus making it the focus of many scientific studies which have lead to a deeper understanding of cancer development and help to elucidate potential weaknesses that can be targeted by pharmacological agents [1]. In this review, we specifically focus on the role of this pathway in physiological hematopoiesis and how augmentation of the pathway may lead to hematopoietic malignancies. We also discuss the challenges and success of targeting this pathway.

The Raf/MEK/extracellular signal-regulated kinase 1/2 pathway can mediate growth inhibitory and differentiation signaling via androgen receptor downregulation in prostate cancer cells

Upregulated ERK1/2 activity is correlated with androgen receptor (AR) downregulation in certain prostate cancer (PCa) that exhibits androgen deprivation-induced neuroendocrine differentiation, but its functional relevance requires elucidation. We found that sustained ERK1/2 activation using active Raf or MEK1/2 mutants is sufficient to induce AR downregulation at mRNA and protein levels in LNCaP. Downregulation of AR protein, but not mRNA, was blocked by proteasome inhibitors, MG132 and bortezomib, indicating that the pathway regulation is mediated at multiple points. Ectopic expression of a constitutively active AR inhibited Raf/MEK/ERK-mediated regulation of the differentiation markers, neuron-specific enolase and neutral endopeptidase, and the cyclin-dependent kinase inhibitors, p16(INK4A) and p21(CIP1), but not Rb phosphorylation and E2F1 expression, indicating that AR has a specific role in the pathway-mediated differentiation and growth inhibitory signaling. However, despite the sufficient role of Raf/MEK/ERK, its inhibition using U0126 or ERK1/2 knockdown could not block androgen deprivation-induced AR downregulation in an LNCaP neuroendocrine differentiation model, suggesting that additional signaling pathways are involved in the regulation. We additionally report that sustained Raf/MEK/ERK activity can downregulate full length as well as hormone binding domain-deficient AR isoforms in androgen-refractory C4-2 and CWR22Rv1, but not in LAPC4 and MDA-PCa-2b. Our study demonstrates a novel role of the Raf/MEK/ERK pathway in regulating AR expression in certain PCa types and provides an insight into PCa responses to its aberrant activation.

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.

A role for p120 RasGAP in thymocyte positive selection and survival of naive T cells

Activation of the Ras small GTP-binding protein is necessary for normal T cell development and function. However, it is unknown which Ras GTPase-activating proteins (RasGAPs) inactivate Ras in T cells. We used a T cell-specific RASA1-deficient mouse model to investigate the role of the p120 RasGAP (RASA1) in T cells. Death of CD4(+)CD8(+) double-positive thymocytes was increased in RASA1-deficient mice. Despite this finding, on an MHC class II-restricted TCR transgenic background, evidence was obtained for increased positive selection of thymocytes associated with augmented activation of the Ras-MAPK pathway. In the periphery, RASA1 was found to be dispensable as a regulator of Ras-MAPK activation and T cell functional responses induced by full agonist peptides. However, numbers of naive T cells were substantially reduced in RASA1-deficient mice. Loss of naive T cells in the absence of RASA1 could be attributed in part to impaired responsiveness to the IL-7 prosurvival cytokine. These findings reveal an important role for RASA1 as a regulator of double-positive survival and positive selection in the thymus as well as naive T cell survival in the periphery.

Dampening of death pathways by schnurri-2 is essential for T-cell development

Generation of a diverse and self-tolerant T cell repertoire requires appropriate interpretation of T cell receptor (TCR) signals by CD4⁺CD8⁺ double positive (DP) thymocytes. Thymocyte cell fate is dictated by the nature of TCR:MHC-peptide interactions, with signals of higher strength leading to death (negative selection) and signals of intermediate strength leading to differentiation (positive selection)1. Molecules that regulate T cell development by modulating TCR signal strength have been described but components that specifically define the boundaries between positive and negative selection remain unknown. Here we show that repression of TCR-induced death pathways is critical for proper interpretation of positive selecting signals in vivo, and identify Schnurri2 (Shn2) as a crucial death dampener. Our results indicate that Shn2^−/− DP thymocytes inappropriately undergo negative selection in response to positive selecting signals, thus leading to disrupted T cell development. Shn2^−/− DP thymocytes are more sensitive to TCR-induced death in vitro and die in response to positive selection interactions in vivo. However, Shn2-deficient thymocytes can be positively selected when TCR-induced death is genetically-ablated. Shn2 levels increase after TCR stimulation suggesting that integration of multiple TCR:MHC-peptide interactions may fine tune the death threshold. Mechanistically, Shn2 functions downstream of TCR proximal signaling compenents to dampen Bax activation and the mitochondrial death pathway. Our findings uncover a critical regulator of T cell development that controls the balance between death and differentiation.

c-Raf, but not B-Raf, is essential for development of K-Ras oncogene-driven non-small cell lung carcinoma

We have investigated the role of individual members of the Raf/Mek/Erk cascade in the onset of K-Ras oncogene-driven non-small cell lung carcinoma (NSCLC). Ablation of Erk1 or Erk2 in K-Ras oncogene-expressing lung cells had no significant effect due to compensatory activities. Yet, elimination of both Erk kinases completely blocked tumor development. Similar results were obtained with Mek kinases. Ablation of B-Raf had no significant effect on tumor development. However, c-Raf expression was absolutely essential for the onset of NSCLC. Interestingly, concomitant elimination of c-Raf and B-Raf in adult mice had no deleterious consequences for normal homeostasis. These results indicate that c-Raf plays a unique role in mediating K-Ras signaling and makes it a suitable target for therapeutic intervention.

Expanding roles for ThPOK in thymic development

The role of the zinc finger transcription factor ThPOK (T-helper-inducing POZ-Kruppel-like factor) in promoting commitment of αβ T cells to the CD4 lineage is now well established. New results indicate that ThPOK is also important for the development and/or acquisition of effector functions by other T cell subsets, including several not marked by CD4 expression, i.e. double-negative invariant natural killer T (iNKT) cells, γδ cells, and even memory CD8(+) T cells. There is compelling evidence that ThPOK expression in most or all of these cases is dependent on T-cell receptor signaling and that differences in relative TCR signal strength/length may induce different levels of ThPOK expression. The developmental consequences of ThPOK expression vary according to cell type, which may partly reflect differences in ThPOK levels and/or in transcriptional networks between cell types.

Transcription factor Foxp1 exerts essential cell-intrinsic regulation of the quiescence of naive T cells

The molecular mechanisms that underlie T cell quiescence are poorly understood. Here we report that mature naive CD8(+) T cells lacking the transcription factor Foxp1 gained effector phenotype and function and proliferated directly in response to interleukin 7 (IL-7) in vitro. Foxp1 repressed expression of the IL-7 receptor α-chain (IL-7Rα) by antagonizing Foxo1 and negatively regulated signaling by the kinases MEK and Erk. Acute deletion of Foxp1 induced naive T cells to gain an effector phenotype and proliferate in lympho-replete mice. Foxp1-deficient naive CD8(+) T cells proliferated even in lymphopenic mice deficient in major histocompatibility complex class I. Our results demonstrate that Foxp1 exerts essential cell-intrinsic regulation of naive T cell quiescence, providing direct evidence that lymphocyte quiescence is achieved through actively maintained mechanisms that include transcriptional regulation.

Itk: the rheostat of the T cell response

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

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.

Negative regulation of mTOR activation by diacylglycerol kinases

The engagement of TCR induces T-cell activation, which initiates multiple characteristic changes such as increase in cell size, cell division, and the production of cytokines and other effector molecules. The mammalian target of rapamycin (mTOR) regulates protein synthesis, transcription, cell survival, and autophagy. Critical roles of mTOR in T-cell activation and effector/memory differentiation have been revealed using chemical inhibitors or by genetic ablation of mTOR in T cells. However, the connection between mTOR signaling and other signaling cascades downstream of TCR is unclear. We demonstrate that diacylglycerol (DAG) and TCR engagement activate signaling in both mTOR complexes 1 and 2 through the activation of the Ras-mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 (Mek1/2)-extracellular signal-regulated kinase 1/2 (Erk1/2)-activator protein 1 (AP-1), known collectively as the Ras-Mek1/2-Erk1/2-AP-1 pathway. Deficiency of RasGRP1 or inhibition of Mek1/2 activity drastically decreases TCR-induced mTOR activation, whereas constitutively active Ras or Mek1 promotes mTOR activation. Although constitutively active Akt promotes TCR-induced mTOR activation, such activation is attenuated by Mek1/2 inhibition. We demonstrated further that DAG kinases (DGKs) α and ζ, which terminate DAG-mediated signaling, synergistically inhibit TCR-induced mTOR activation by inhibiting the Ras-Mek1/2-Erk/12 pathway. These observations provide novel insights into the regulation of mTOR activation.

Signaling in thymic selection

T cell receptor signaling allows the developing thymocyte to undergo positive or negative selection, which is required for the formation of a useful mature T cell repertoire. Recent developments include the finding that much of the Lck kinase (required to initiate T cell signaling) is already in an active configuration before signaling. The analog strength of antigen binding to the T cell receptor binding may be translated into a digital signal by the amount of time the TCR is paired with a co-receptor carrying Lck. Downstream, the cellular localization of MAP kinase signaling is determined by the strength of the signal and in turn predicts positive or negative selection. A novel protein, Themis, is important in crossing the positive selection developmental checkpoint, but its mode of action is still uncertain. Commitment to the CD4 or CD8 lineage is influenced by the amount of ZAP-70 signaling and also by closely regulated responsiveness to intrathymic cytokines such as IL7.

CCAAT/enhancer-binding proteins (C/EBP)-α and -β are essential for ovulation, luteinization, and the expression of key target genes

LH activation of the epidermal growth factor receptor/RAS/ERK1/2 pathway is essential for ovulation and luteinization because granulosa cell (GC) depletion of ERK1/2 (ERK1/2(gc)(-/-) mice) renders mice infertile. As mediators of ERK1/2-dependent GC differentiation, the CCAAT/enhancer-binding proteins, (C/EBP)α and C/EBPβ, were also disrupted. Female Cebpb(gc)(-/-) mutant mice, but not Cebpa(gc)(-/-) mice, were subfertile whereas Cebpa/b(gc)(-/-) double-mutant females were sterile. Follicles failed to ovulate, ovaries were devoid of corpora lutea, luteal cell marker genes (Lhcgr, Prlr, Ptgfr, Cyp11a1, and Star) were absent, and serum progesterone levels were low. Microarray analyses identified numerous C/EBPα/β target genes in equine chorionic gonadotropin (eCG)-human (h)CG-treated mice. At 4 h post-hCG, a subset (19%) of genes altered in the Cebpa/b-depleted cells was also altered in ERK1/2-depleted cells; hence they are common effectors of ERK1/2. Additional genes down-regulated in the Cebpa/b-depleted cells at 8 and 24 h post-hCG include known (Akr1b7, Runx2, Star, Saa3) and novel (Abcb1b, Apln, Igfbp4, Prlr, Ptgfr Timp4) C/EBP targets and effectors of luteal and vascular cell development. Bhmt, a gene controlling methionine metabolism and thought to be expressed exclusively in liver and kidney, was high in wild-type luteal cells but totally absent in Cebpa/b mutant cells. Because numerous genes potentially associated with vascular development were suppressed in the mutant cells, C/EBPα/β appear to dictate the luteinization process by also controlling genes that regulate the formation of the extensive vascular network required to sustain luteal cells. Thus, C/EBPα/β mediate the terminal differentiation of GCs during the complex process of luteinization.

Increased peripheral blood expression of electron transport chain genes in bipolar depression

OBJECTIVE

  To identify specific genetic pathways showing altered expression in peripheral blood of depressed subjects with bipolar disorder (BPD).

METHODS

  Illumina Sentrix BeadChip (Human-6v2) microarrays containing >48,000 transcript probes were used to measure levels of gene expression in peripheral blood from 20 depressed subjects with BPD and in 15 healthy control subjects. Quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) was used to confirm a subset of these differences.

RESULTS

  A total of 1,180 genes were differentially expressed between subjects with BPD and healthy controls (fold-change >1.3, false discovery rate-corrected p < 0.05, covaried for age and sex). Of these, 559 genes were up-regulated in BPD subjects and 621 were down-regulated. Surprisingly, there was no difference between medicated (n = 11) and unmedicated (n = 9) subjects with BPD for any of these genes. Pathway analysis using GeneGo MetaCore software showed that the most significantly affected pathway was the mitochondrial electron transport chain (ETC). Of the 85 objects (genes or proteins) in this pathway, 22 were up-regulated and 2 down-regulated in subjects with BPD. qRT-PCR confirmed up-regulation of nuclear encoded ETC genes in complexes I, III, IV, and V and, in addition, demonstrated up-regulation of mitochondrially encoded genes in each of these complexes.

CONCLUSION

  These results suggest that increased expression of multiple components of the mitochondrial ETC may be a primary deficit in bipolar depression, rather than an effect of medication.

Mitogen-activated protein kinase signalling and ERK1/2 bistability in asthma

Mitogen-activated protein kinases (MAPKs) integrate signals from numerous receptors and translate these signals into cell functions. MAPKs are critical for immune cell metabolism, migration, production of pro-inflammatory mediators, survival and differentiation. We provide a concise review of the involvement of MAPK in important cells of the immune system. Certain cell functions, e.g. production of pro-inflammatory mediators resolve quickly and may require a transient MAPK activation, other processes such as cell differentiation and long-term survival may require persistent MAPK signal. The persistent MAPK signal is frequently a consequence of positive feedback loops or double negative feedback loops which perpetuate the signal after removal of an external cell stimulus. This self-perpetuated activation of a signalling circuit is a manifestation of its bistability. Bistable systems can exist in 'on' and 'off' states and both states are stable. We have demonstrated the existence of self-perpetuated activation mechanism for ERK1/2 in bronchial epithelial cells. This sustained activation of ERK1/2 supports long-term survival of these cells and primes them for cytokine transcription. ERK1/2 bistability arises from repetitive stimulation of the cell. The repeated stimulation (e.g. repeated viral infection or repeated allergen exposure) seems to be a common theme in asthma and other chronic illnesses. We thus hypothesize that the self-perpetuated ERK1/2 signal plays an important role in the pathogenesis of asthma.

The essential function for serum response factor in T-cell development reflects its specific coupling to extracellular signal-regulated kinase signaling

Serum response factor (SRF) recruits members of two families of signal-regulated coactivators, the extracellular signal-regulated kinase (ERK)-regulated ternary complex factors (TCFs) and the actin-regulated myocardin-related transcription factors (MRTFs), to its target genes through its DNA-binding domain. Whether coactivator association is required for SRF function in vivo and whether particular SRF functions reflect specific coupling to one or the other signal pathway have remained largely unexplored. We show that SRF is essential for thymocyte positive selection and thymic T(reg) and NK T-cell development but dispensable for early thymocyte development and negative selection. Expression of wild-type SRF, or mutants lacking the N-terminal phosphorylation sites or C-terminal transcriptional activation domain, restores positive selection in SRF null thymocytes. In contrast, SRF.V194E, which cannot recruit TCF or MRTF family members, is inactive, although it is recruited to target genes. Fusion of a TCF C-terminal activation domain to SRF.V194E effectively restores ERK-dependent single-positive (SP) thymocyte development. The resulting SP thymocytes exhibit normal surface marker expression and proliferation following T-cell receptor cross-linking. Thus, ERK signaling through the TCF pathway to SRF is necessary and sufficient for SRF function in thymocyte positive selection.

Protein kinase C regulates mitochondrial targeting of Nur77 and its family member Nor-1 in thymocytes undergoing apoptosis

Nur77 orphan steroid receptor and its family member Nor-1 are required for apoptosis of developing T cells. In thymocytes, signals from the TCR complex induce Nur77 and Nor-1 expression followed by translocation from the nucleus to mitochondria. Nur77 and Nor-1 associate with Bcl-2 in the mitochondria, resulting in a conformation change that exposes the Bcl-2 BH3 domain, a presumed pro-apoptotic molecule of Bcl-2. As Nur77 and Nor-1 are heavily phosphorylated, we examined the requirement of Nur77 and Nor-1 phosphorylation in mitochondria translocation and Bcl-2 BH3 exposure. We found that HK434, a PKC agonist, in combination with calcium ionophore, can induce Nur77 and Nor-1 phosphorylation, translocation, Bcl-2 BH3 exposure and thymocyte apoptosis. Inhibitors of both classical and novel forms of PKC were able to block this process. In contrast, only the general but not classical PKC-specific inhibitors were able to block the same process initiated by PMA, a commonly used PKC agonist. These data demonstrate a differential activation of PKC isoforms by PMA and HK434 in thymocytes, and show the importance of PKC in mitochondria translocation of Nur77/Nor-1 and Bcl-2 conformation change during TCR-induced thymocyte apoptosis.

Kinase suppressor of Ras 1 is required for full ERK activation in thymocytes but not for thymocyte selection

The scaffold protein kinase suppressor of Ras 1 (KSR1) is critical for efficient activation of ERK in a number of cell types. Consistent with this, we observed a defect in ERK activation in thymocytes that lack KSR1. Interestingly, we found that the defect was much greater after PMA stimulation than by CD3 activation. Since ERK activation is believed to be important for thymocyte development, we analyzed thymocyte selection in KSR1-deficient (KSR1(-/-) ) mice. We found that positive selection in two different TCR transgenic models, HY and AND, was normal. On the other hand, negative selection in the HY model was slightly impaired in KSR1(-/-) mice. However, a defect in negative selection was not apparent in the AND TCR model system or in an endogenous superantigen-mediated model of negative selection. These results suggest that, despite a requirement for KSR1 for full ERK activation in thymocytes, full and efficient ERK activation is not essential for the majority of thymocyte selection events.

Deletion of alloreactive T cells by veto cytotoxic T lymphocytes is mediated through extracellular signal-regulated kinase phosphorylation

BACKGROUND

Anti third-party cytotoxic T lymphocytes (CTLs) were shown to exhibit marked veto activity, thereby inducing transplantation tolerance across major histocompatibility antigens. Elimination of effector cells requires co-expression of CD8 and FasL on the veto cells and is mediated through CD8-major histocompatibility complex (MHC) class I interaction and Fas-Fas ligand signaling.

METHODS

To further interrogate the signaling events induced in the effector cells on their interaction with veto cell populations, effector cells from 2C transgenic mice were preincubated with different signaling inhibitors and were subject to fluorescence-activated cell sorting and western blot analysis.

RESULTS

Screening with inhibitors revealed specific inhibition only with the map kinase (MEK)/extracellular signal regulated kinase (ERK) inhibitor, U0126. Accordingly, fluorescence-activated cell sorting and western blot analysis showed that ERK phosphorylation is induced in the effector cells within 1 hr of incubation with the veto cells. ERK phosphorylation had no effect on the Fas expression level, nor was it reduced when using effector cells from Fas KO mice. Examination of ERK phosphorylation in high and low MHC-I expressing effectors revealed marked differences, suggesting that the interaction between CD8 on the veto CTL, and MHC-I on the effector cells is likely responsible for ERK phosphorylation. Furthermore, XIAP in 2C cells is specifically reduced on binding to the cognate veto cells during the mixed lymphocyte reaction but before the appearance of Annexin V reactivity.

CONCLUSIONS

These results suggest that the interaction between CD8 on veto CTL and the MHC class I alpha3 domain on the effector cell, leads to phosphorylation of MEK/ERK in the latter cell, associated with a significant reduction of XIAP levels which, in turn, enables potent triggering of Fas-FasL mediated apoptosis on cognate binding of the veto CTLs.

Tenuous paths in unexplored territory: From T cell receptor signaling to effector gene expression during thymocyte selection

During the last step of alphabeta T cell development, thymocytes that have rearranged genes encoding TCR chains and express CD4 and CD8 coreceptors are selected on the basis of their TCR reactivity to escape programmed cell death and become mature CD4 or CD8 T cells. This process is triggered by intrathymic TCR signaling, that activates 'sensor' transcription factors 'constitutively' expressed in DP thymocytes. Eventually, TCR-signaled thymocytes evolve effector transcriptional circuits that control basal metabolism, migration, survival and initiation of lineage-specific gene expression. This review examines how components of the 'sensing' transcription apparatus responds to positive selection signals, and highlights important differences with mature T cell responses. In a second part, we evaluate current observations and hypotheses on the connections between sensing transcription factors and effector circuitries.

Towards a molecular understanding of the differential signals regulating alphabeta/gammadelta T lineage choice

While insights into the molecular processes that specify adoption of the alphabeta and gammadelta fates are beginning to emerge, the basis for control of specification remains highly controversial. This review highlights the current models attempting to explain T lineage commitment. Recent observations support the hypothesis that the T cell receptor (TCR) provides instructive cues through differences in TCR signaling intensity and/or longevity. Accordingly, we review evidence addressing the importance of differences in signal strength/longevity, how signals differing in intensity/longevity may be generated, and finally how such signals modulate the activity of downstream effectors to promote the opposing developmental fates.

Decision checkpoints in the thymus

The development of T cells in the thymus involves several differentiation and proliferation events, during which hematopoietic precursors give rise to T cells ready to respond to antigen stimulation and undergo effector differentiation. This review addresses signaling and transcriptional checkpoints that control the intrathymic journey of T cell precursors. We focus on the divergence of alphabeta and gammadelta lineage cells and the elaboration of the alphabeta T cell repertoire, with special emphasis on the emergence of transcriptional programs that direct lineage decisions.

The role of ThPOK in control of CD4/CD8 lineage commitment

During alphabeta T cell development, cells diverge into alternate CD4 helper and CD8(+) cytotoxic T cell lineages. The precise correlation between a T cell's CD8 and CD4 choice and its TCR specificity to class I or class II MHC was noted more than 20 years ago, and establishing the underlying mechanism has remained a focus of intense study since then. This review deals with three formerly discrete topics that are gradually becoming interconnected: the role of TCR signaling in lineage commitment, the regulation of expression of the CD4 and CD8 genes, and transcriptional regulation of lineage commitment. It is widely accepted that TCR signaling exerts a decisive influence on lineage choice, although the underlying mechanism remains intensely debated. Current evidence suggests that both duration and intensity of TCR signaling may control lineage choice, as proposed by the kinetic signaling and quantitative instructive models, respectively. Alternate expression of the CD4 and CD8 genes is the most visible manifestation of lineage choice, and much progress has been made in defining the responsible cis elements and transcription factors. Finally, important clues to the molecular basis of lineage commitment have been provided by the recent identification of the transcription factor ThPOK as a key regulator of lineage choice. ThPOK is selectively expressed in class II-restricted cells at the CD4(+)8(lo) stage and is necessary and sufficient for development to the CD4 lineage. Given the central role of ThPOK in lineage commitment, understanding its upstream regulation and downstream gene targets is expected to reveal further important aspects of the molecular machinery underlying lineage commitment.

PKCtheta is necessary for efficient activation of NFkappaB, NFAT, and AP-1 during positive selection of thymocytes

While it has been shown in several publications that the serine-threonine kinase PKCθ is required for efficient activation of mature T lymphocytes, the role of PKCθ in T cell development in the thymus is somewhat controversial. In this study, using knockout mice, we show that PKCθ is important in positive selection. The thymus of PKCθ^−/− animals contains significantly less mature single positive T cells compared to wild-type controls. Biochemically, PKCθ deficient thymocytes show defective activation of the transcription factors AP-1, NFAT and NFκB as well as impaired phosphorylation of the MAP kinase ERK after T cell receptor stimulation in vitro. Together, these results reveal a crucial role of PKCθ in positive selection of thymocytes in a pathway leading to the activation of ERK, AP-1, NFAT, and NFκB.

Extracellular signal-regulated kinase 1/2 (ERK1/2) signaling in cardiac hypertrophy

Cardiac hypertrophy results from increased mechanical load on the heart and through the action of neurohumoral mediators. ERK1/2 are known to be activated in response to almost every stress- and agonist-induced hypertrophic stimulus examined to date, suggesting the straightforward hypothesis that these kinases are required for promoting the cardiac growth response. However, recent data from genetically modified mouse models suggest a more complicated picture. For example, inducible expression of dual-specificity phosphatase 6, an ERK1/2-inactivating phosphatase, eliminated ERK1/2 phosphorylation in transgenic mice, but it did not diminish the hypertrophic response to pressure overload. Similarly, Erk1-/- and Erk2+/- mice showed no reduction in stimulus-induced cardiac growth in vivo. However, blockade or deletion of cardiac ERK1/2 did predispose the heart to decompensation and failure after long-term pressure overload. Thus, ERK1/2 signaling is not to be absolutely necessary for mediating cardiac hypertrophy, although it does appear to provide critical protective effects/signals during stress-stimulation.

Genetic demonstration of a redundant role of extracellular signal-regulated kinase 1 (ERK1) and ERK2 mitogen-activated protein kinases in promoting fibroblast proliferation

The extracellular signal-regulated kinase 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinase signaling pathway plays an important role in the proliferative response of mammalian cells to mitogens. However, the individual contribution of the isoforms ERK1 and ERK2 to cell proliferation control is unclear. The two ERK isoforms have similar biochemical properties and recognize the same primary sequence determinants on substrates. On the other hand, analysis of mice lacking individual ERK genes suggests that ERK1 and ERK2 may have evolved unique functions. In this study, we used a robust genetic approach to analyze the individual functions of ERK1 and ERK2 in cell proliferation using genetically matched primary embryonic fibroblasts. We show that individual loss of either ERK1 or ERK2 slows down the proliferation rate of fibroblasts to an extent reflecting the expression level of the kinase. Moreover, RNA interference-mediated silencing of ERK1 or ERK2 expression in cells genetically disrupted for the other isoform similarly reduces cell proliferation. We generated fibroblasts genetically deficient in both Erk1 and Erk2. Combined loss of ERK1 and ERK2 resulted in a complete arrest of cell proliferation associated with G(1) arrest and premature replicative senescence. Together, our findings provide compelling genetic evidence for a redundant role of ERK1 and ERK2 in promoting cell proliferation.

The death effector domain protein PEA-15 negatively regulates T-cell receptor signaling

PEA-15 is a death effector domain-containing phosphoprotein that binds ERK and restricts it to the cytoplasm. PEA-15 also binds to FADD and thereby blocks apoptosis induced by death receptors. Abnormal expression of PEA-15 is associated with type II diabetes and some cancers; however, its physiological function remains unclear. To determine the function of PEA-15 in vivo, we used C57BL/6 mice in which the PEA-15 coding region was deleted. We thereby found that PEA-15 regulates T-cell proliferation. PEA-15-null mice did not have altered thymic or splenic lymphocyte cellularity or differentiation. However, PEA-15 deficient T cells had increased CD3/CD28-induced nuclear translocation of ERK and increased activation of IL-2 transcription and secretion in comparison to control wild-type littermates. Indeed, activation of the T-cell receptor in wild-type mice caused PEA-15 release of ERK. In contrast, overexpression of PEA-15 in Jurkat T cells blocked nuclear translocation of ERK and IL-2 transcription. Finally, PEA-15-null T cells showed increased IL-2 dependent proliferation on stimulation. No differences in T cell susceptibility to apoptosis were found. Thus, PEA-15 is a novel player in T-cell homeostasis. As such this work may have far reaching implications in understanding how the immune response is controlled.

The MAPK ERK1 is a negative regulator of the adult steady-state splenic erythropoiesis

The mitogen-activated protein kinases (MAPKs) extracellular signal-regulated kinase 1 (ERK1) and ERK2 are among the main signal transduction molecules, but little is known about their isoform-specific functions in vivo. We have examined the role of ERK1 in adult hematopoiesis with ERK1(-/-) mice. Loss of ERK1 resulted in an enhanced splenic erythropoiesis, characterized by an accumulation of erythroid progenitors in the spleen, without any effect on the other lineages or on bone marrow erythropoiesis. This result suggests that the ablation of ERK1 induces a splenic stress erythropoiesis phenotype. However, the mice display no anemia. Deletion of ERK1 did not affect erythropoietin (EPO) serum levels or EPO/EPO receptor signaling and was not compensated by ERK2. Splenic stress erythropoiesis response has been shown to require bone morphogenetic protein 4 (BMP4)-dependent signaling in vivo and to rely on the expansion of a resident specialized population of erythroid progenitors, termed stress erythroid burst-forming units (BFU-Es). A great expansion of stress BFU-Es and increased levels of BMP4 mRNA were found in ERK1(-/-) spleens. The ERK1(-/-) phenotype can be transferred by bone marrow cells. These findings show that ERK1 controls a BMP4-dependent step, regulating the steady state of splenic erythropoiesis.

Genetic analysis of Ras signalling pathways in cell proliferation, migration and survival

We have used mouse embryonic fibroblasts (MEFs) devoid of Ras proteins to illustrate that they are essential for proliferation and migration, but not for survival, at least in these cells. These properties are unique to the Ras subfamily of proteins because ectopic expression of other Ras-like small GTPases, even when constitutively active, could not compensate for the absence of Ras proteins. Only constitutive activation of components of the Raf/Mek/Erk pathway was sufficient to sustain normal proliferation and migration of MEFs devoid of Ras proteins. Activation of the phosphatidylinositol 3-kinase (PI3K)/PTEN/Akt and Ral guanine exchange factor (RalGEF)/Ral pathways, either alone or in combination, failed to induce proliferation or migration of Rasless cells, although they cooperated with Raf/Mek/Erk signalling to reproduce the full response mediated by Ras signalling. In contrast to current hypotheses, Ras signalling did not induce proliferation by inducing expression of D-type Cyclins. Rasless MEFs had normal levels of Cyclin D1/Cdk4 and Cyclin E/Cdk2. However, these complexes were inactive. Inactivation of the pocket proteins or knock down of pRb relieved MEFs from their dependence on Ras signalling to proliferate.

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.

Early growth response 2 regulates the survival of thymocytes during positive selection

The early growth response (Egr) transcription factor family regulates multiple steps during T-cell development. We examine here the role played by Egr2 in positive selection. In double-positive cells, Egr2 is upregulated immediately following TCR ligation, and its expression requires both the MAPK and calcineurin signaling pathways. Inducible transgenic and knockout mice were generated to cause gain- or loss-of-function of Egr2 in double-positive cells, and had reciprocal effects; more mature single-positive cells were made when Egr2 was overexpressed, and fewer when Egr2 was absent. These defects were associated with changes in the survival of positively selected cells rather than perturbation of positive selection or immediate post-selection signaling. The survival function of Egr2 at least partly depends upon its ability to activate the cytokine-mediated survival pathway, likely through negative regulation of both the IL-7R and suppressor of cytokine signaling 1 (Socs1), the molecular switch whose downregulation normally results in restored responsiveness to cytokine signaling following selection. While gain of Egr2 caused a decrease in Socs1 mRNA, loss of Egr2 resulted in downregulation of IL-7R, upregulation of Socs1, and inhibition of Stat5 phosphorylation and IL-7-mediated survival post-selection. Therefore, expression of Egr2 following positive selection links the initial TCR signaling event to subsequent survival of signaled cells.

Studies on MAP Kinase signaling in the immune system

The primary function of the immune system is to protect the organism from invading pathogens. In response to pathogen invasion, multiple signaling pathways are activated in immune cells, leading to diverse immune defense mechanisms. Chief among these pathways is the activation of MAPKs, which are crucial for transcriptional and nontranscriptional responses of the immune system. Here we describe protocols to study the roles of MAPKs in T lymphocytes, a cell type central for immune regulation. Specifically, we describe flow cytometry-based assays to analyze the roles of MAPKs in the development, homeostasis, proliferation, and apoptosis of murine T cells. We also describe methods to examine the activation of MAPKs in T cells.

Lentiviral vectors to study the differential function of ERK1 and ERK2 MAP kinases

Accumulating evidence indicates that p44(ERK1) and p42(ERK2) mitogen-activated protein kinases (MAPKs) have distinct quantitative roles in cell signaling. In our recently proposed model of regulation of ERK1 and ERK2, p42 plays a major role in delivering signals from the cell membrane to the nucleus, while p44 acts as a partial agonist of ERK2 toward effectors and downstream activators, thus providing a fine tuning system of the global signaling output. Here, we describe systems to modulate MAPK signaling in vitro and in vivo via lentiviral vector (LV)-mediated gene transfer, using three systems: RNAi with small hairpin RNAs, microRNA-mediated gene knockdown, and expression of signaling-interfering mutants of MEK1. We show, by using proliferation assays in mouse embryo fibroblasts (MEF) and NIH 3T3 cells, that gene knockdown of ERK1 promotes cell proliferation in a manner indistinguishable from a constitutively active MEK1 construct, while ERK2 RNAi causes a significant growth arrest, similar to that observed with the ectopic expression of a dominant negative MEK1 mutant.

Thymic nurse cells exhibit epithelial progenitor phenotype and create unique extra-cytoplasmic membrane space for thymocyte selection

Thymic nurse cells (TNCs) are epithelial cells in the thymic cortex that contain as many as 50 thymocytes within specialized cytoplasmic vacuoles. The function of this cell-in-cell interaction has created controversy since their discovery in 1980. Further, some skepticism exists about the idea that apoptotic thymocytes within the TNC complex result from negative selection, a process believed to occur exclusively within the medulla. In this report, we have microscopic evidence that defines a unique membranous environment wherein lipid raft aggregates around the alphabetaTCR expressed on captured thymocytes and class II MHC molecules expressed on TNCs. Further, immunohistological examination of thymic sections show TNCs located within the cortico-medullary junction to express cytokeratins five and eight (K5 and K8), and the transcription factor Trp-63, the phenotype defined elsewhere as the thymic epithelial progenitor subset. Our results suggest that the microenvironment provided by TNCs plays an important role in thymocyte selection as well as the potential for TNCs to be involved in the maintenance of thymic epithelia.

Efficient inhibition of the formation of joint adhesions by ERK2 small interfering RNAs

Transforming growth factor-beta1 and fibroblast growth factor-2 play very important roles in fibroblast proliferation and collagen expression. These processes lead to the formation of joint adhesions through the SMAD and MAPK pathways, in which extracellular signal-regulated kinase (ERK)2 is considered to be crucial. Based on these theories, we examined the effects of a lentivirus-mediated small interfering RNA (siRNA) targeting ERK2 on the suppression of joint adhesion formation in vivo. The effects were assessed in vivo from different aspects including the adhesion score, histology and joint contracture angle. We found that the adhesions in the ERK2 siRNA group became soft and weak, and were easily stretched. Accordingly, the flexion contracture angles in the ERK2 siRNA group were also reduced (P<0.05 compared with the control group). The animals appeared healthy, with no signs of impaired wound healing. In conclusion, local delivery of a lentivirus-mediated siRNA targeting ERK2 can ameliorate joint adhesion formation effectively and safely.

Noncatalytic function of ERK1/2 can promote Raf/MEK/ERK-mediated growth arrest signaling

Kinase activity is known as the key biochemical property of MAPKs. Here, we report that ERK1/2 also utilizes its noncatalytic function to mediate certain signal transductions. Sustained activation of the Raf/MEK/ERK pathway induces growth arrest, accompanied by changes in cell cycle regulators (decreased retinoblastoma phosphorylation, E2F1 down-regulation, and/or p21(CIP1) up-regulation) and cell type-specific changes in morphology and expression of c-Myc or RET in the human tumor lines LNCaP, U251, and TT. Ablation of ERK1/2 by RNA interference abrogated all these effects. However, active site-disabled ERK mutants (ERK1-K71R, ERK2-K52R, and ERK2-D147A), which competitively inhibit activation of endogenous ERK1/2, could not block Raf/MEK-induced growth arrest as well as changes in the cell cycle regulators, although they effectively blocked phosphorylation of the ERK1/2 catalytic activity readouts, p90(RSK) and ELK1, as well as the cell type-specific changes. Because this indicated a potential noncatalytic ERK1/2 function, we generated stable lines of the tumor cells in which both ERK1 and ERK2 were significantly knocked down, and we further investigated the possibility using rat-derived kinase-deficient ERK mutants (ERK2-K52R and ERK2-T183A/Y185F) that were not targeted by human small hairpin RNA. Indeed, ERK2-K52R selectively restored Raf-induced growth inhibitory signaling in ERK1/2-depleted cells, as manifested by regained cellular ability to undergo growth arrest and to control the cell cycle regulators without affecting c-Myc and morphology. However, ERK2-T183A/Y185F was less effective, indicating the requirement of TEY site phosphorylation. Our study suggests that functions of ERK1/2 other than its "canonical" kinase activity are also involved in the pathway-mediated growth arrest signaling.

Prosurvival and proapoptotic functions of ERK1/2 activation in murine thymocytes in vitro

The extracellular signal-regulated kinases 1/2 (ERK1/2) are serine/threonine-selective protein kinases involved in proliferation and differentiation of cells, including thymocytes. The requirement of ERK1/2 for thymocyte differentiation and maturation has been well established; however, their role in regulating thymocyte survival and apoptosis has not been resolved. Here, we asked whether ERK1/2 affected thymocyte survival in vitro in response to apoptotic stimuli. The results show that phorbol 12-myristate 13-acetate (PMA) treatment (with or without ionomycin) and serum starvation (s/s) induced sustained ERK1/2 activation in murine thymocytes. Importantly, pharmacological treatment of thymocytes with the MEK inhibitor UO126 revealed that PMA-induced ERK1/2 activation was proapoptotic, whereas serum starvation-induced ERK1/2 activation inhibited apoptosis and promoted cell survival. While basal MEK activity was required for both s/s- and PMA-induced ERK1/2 activation, MEK activity increased only in response to PMA. The results show that the suppression of ERK1/2 phosphatases was responsible for s/s-induced sustained ERK1/2 activation. Unexpectedly, neither s/s-induced proapoptotic nor PMA-induced anti-apoptotic functions of ERK1/2 depended on the Bcl-2 family phosphoprotein Bim(EL), which was previously implicated in thymocyte apoptosis. Lastly, etoposide treatment of immature thymocytes induced both p53 and ERK1/2 activation, but ERK1/2 activity did not affect the phosphorylation and stabilization of p53. Thus, ERK1/2 has a dual role in promoting cell survival and cell death in thymocytes in the context of different stimuli.

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.

Protein-tyrosine phosphatase-kappa regulates CD4+ T cell development through ERK1/2-mediated signaling

T cells express diverse antigen-specific receptors and are required for eradicating pathogens and transformed cells. T cells expressing CD4 acquire helper effector functions and those expressing CD8 exert cytotoxic activity after antigen recognition. The protein-tyrosine phosphatase, receptor type kappa (PTPRKappa) is mutated in LEC rats, resulting in impaired CD4(+) T cell development in the thymus. However, the molecular mechanism of PTPRK controlling CD4(+) T cell development remains unclear. We demonstrate herein that inhibition of PTPRK by transducing a dominant negative form of the intracellular domain of PTPRK (PTPRK-ICD-DN) in bone marrow-derived stem cells suppresses the development of CD4(+) T cells. The inhibition of PTPRK by PTPRK-ICD-DN or short-hairpin RNA for PTPRK attenuates ERK1/2 phosphorylation in T cells after PMA and ionomycin stimulation. Total thymocytes from LEC rats also showed weaker phosphorylation of ERK1/2 after PMA and ionomycin stimulation than control thymocytes. Furthermore, inhibition of PTPRK by PTPRK-ICD-DN suppressed MEK1/2 and c-Raf phosphorylation, which is required for ERK1/2 phosphorylation. These data indicate that PPTRK positively regulates ERK1/2 phosphorylation, which impacts CD4(+) T cell development.

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.

MKP-1 is necessary for T cell activation and function

MAPKs are evolutionarily conserved immune regulators. MAPK phosphatases (MKPs) that negatively regulate MAPK activities have recently emerged as critical players in both innate and adaptive immune responses. MKP-1, also known as DUSP1, was previously shown to negatively regulate innate immunity by inhibiting pro-inflammatory cytokine production. Here, we found that MKP-1 is necessary in T cell activation and function. MKP-1 deficiency in T cells impaired the activation, proliferation, and function of T cells in vitro, associated with enhanced activation of JNK and reduced NFATc1 translocation into the nucleus. Consistently, MKP-1(-/-) mice were defective in anti-influenza immunity in vivo and resistant to experimental autoimmune encephalomyelitis. Our results thus demonstrate that MKP-1 is a critical positive regulator of T cell activation and function and may be targeted in treatment of autoimmune diseases.

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.