Regulation of NF-kappaB-dependent T cell activation and development by MEKK3

The regulatory effect of NF-κB signaling pathway on biomineralization and shell regeneration in pearl oyster, Pinctada fucata

Pinctada fucata is an important pearl production shellfish in aquaculture. The formation of shells and pearls is a hot research topic in biomineralization, and matrix proteins secreted by the mantle tissues play the key role in this process. However, upstream regulatory mechanisms of transcription factors on the matrix protein genes remain unclear. Previous studies have shown that NF-κB signaling pathway regulated biomineralization process through expression regulation of specific matrix proteins, including Nacrein, Prismalin-14 and MSI60. In this study, we systematically investigated the regulatory effect of the NF-κB signaling pathway key factor Pf-Rel and inhibitory protein poI-κB on the biomineralization and shell regeneration process. We applied RNA interference and antibody injection assays to study in vivo function of transcription factor Pf-Rel and characterized shell morphology changes using scanning electron microscopy and Raman spectroscopy. We found that transcription factor Pf-Rel plays a positive regulatory role in the growth regulation of the prismatic and nacreous layers, while the function of inhibitory protein poI-κB is to prevent excessive growth and accumulation of both layers. RNA-seq was conducted based on RNA interference animal model to identify potential regulatory genes by transcription factor Pf-Rel. Shell damage repair experiments were performed to simulate shell regeneration process, and observations of newly formed shells revealed that NF-κB signaling pathway had different functions at different times. This study provides us with a more macroscopic perspective based on transcription factors to investigate biomineralization and shell regeneration.

MEKK3 in hybrid snakehead (Channa maculate ♀ ×Channa argus ♂): Molecular characterization and immune response to infection with Nocardia seriolae and Aeromonas schubertii

Mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 3 (MEKK3) is a serine/threonine protein kinase that acts as a key regulator and is widely involved in various innate and acquired immune signaling pathways. In this study, we first cloned the complete open reading frame (ORF) of the MEKK3 gene (named CcMEKK3) in a hybrid snakehead (Channa maculate ♀ × Channa argus ♂). The full-length ORF of CcMEKK3 is 1851 bp, and encodes a putative protein of 616 amino acids containing a serine/threonine kinase catalytic (S-TKc) domain and a Phox and Bem1p (PB1) domain. A sequence alignment and phylogenetic tree analysis showed that CcMEKK3 is highly conserved relative to the MEKK3 proteins of other teleost species. CcMEKK3 was constitutively expressed in all the healthy hybrid snakehead tissues tested, with greatest expression in the immune tissues, such as the head kidney and spleen. The expression of CcMEKK3 was usually upregulated in the head kidney, spleen, and liver at different time points after infection with Nocardia seriolae or Aeromonas schubertii. Similarly, the dynamic expression levels of CcMEKK3 in head kidney leukocytes after stimulation revealed that CcMEKK3 was induced by LTA, LPS, and poly(I:C). In the subcellular localization analysis, CcMEKK3 was evenly distributed in the cytoplasm of HEK293T cells, and its overexpression significantly promoted the activities of NF-κB and AP-1. These results suggest that CcMEKK3 is involved in the immune defense against these two pathogens, and plays a crucial role in activating the NF-κB and MAPK signaling pathways.

The Many Roles of Ubiquitin in NF-κB Signaling

The nuclear factor κB (NF-κB) signaling pathway ubiquitously controls cell growth and survival in basic conditions as well as rapid resetting of cellular functions following environment changes or pathogenic insults. Moreover, its deregulation is frequently observed during cell transformation, chronic inflammation or autoimmunity. Understanding how it is properly regulated therefore is a prerequisite to managing these adverse situations. Over the last years evidence has accumulated showing that ubiquitination is a key process in NF-κB activation and its resolution. Here, we examine the various functions of ubiquitin in NF-κB signaling and more specifically, how it controls signal transduction at the molecular level and impacts in vivo on NF-κB regulated cellular processes.

Lymphocyte signaling and activation by the CARMA1-BCL10-MALT1 signalosome

The CARMA1-BCL10-MALT1 (CBM) signalosome triggers canonical NF-κB signaling and lymphocyte activation upon antigen-receptor stimulation. Genetic studies in mice and the analysis of human immune pathologies unveiled a critical role of the CBM complex in adaptive immune responses. Great progress has been made in elucidating the fundamental mechanisms that dictate CBM assembly and disassembly. By bridging proximal antigen-receptor signaling to downstream signaling pathways, the CBM complex exerts a crucial scaffolding function. Moreover, the MALT1 subunit confers a unique proteolytic activity that is key for lymphocyte activation. Deregulated 'chronic' CBM signaling drives constitutive NF-κB signaling and MALT1 activation, which contribute to the development of autoimmune and inflammatory diseases as well as lymphomagenesis. Thus, the processes that govern CBM activation and function are promising targets for the treatment of immune disorders. Here, we summarize the current knowledge on the functions and mechanisms of CBM signaling in lymphocytes and how CBM deregulations contribute to aberrant signaling in malignant lymphomas.

Negative role of TAK1 in marginal zone B-cell development incidental to NF-κB noncanonical pathway activation

The transcription factor nuclear factor-κB (NF-κB) signaling pathway is crucial in B-cell physiology. One key molecule regulating this pathway is the serine/threonine kinase TAK1 (MAP3K7). TAK1 is responsible for positive feedback mechanisms in B-cell receptor signaling that serve as an NF-κB activation threshold. This study aimed to better understand the correlation between TAK1-mediated signaling and B-cell development and humoral immune responses. Here we showed that a B-cell conditional deletion of TAK1 using mb1-cre resulted in a dramatic elimination of the humoral immune response, consistent with the absence of the B-1 B-cell subset. When monitoring the self-reactive B-cell system (the immunoglobulin hen egg lysozyme/soluble hen egg lysozyme double-transgenic mouse model), we found that TAK1-deficient B cells exhibited an enhanced susceptibility to cell death that might explain the disappearance of the B1 subset. In contrast, these mice gained numerous marginal zone (MZ) B cells. We consequently examined the basal and B-cell receptor-induced activity of NF-κB2 that is reported to regulate MZ B-cell development, and demonstrated that the activity of NF-κB2 increased in TAK1-deficient B cells. Thus, our results present a novel in vivo function, the negative role of TAK1 in MZ B-cell development that is likely associated with NF-κB2 activation.

Expression and prognostic role of MEKK3 and pERK in patients with renal clear cell carcinoma

Mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 3 (MEKK3) is an important serine/threonine protein kinase and a member of the MAPK family. MEKK3 can effectively activate the MEK/ERK signaling pathway and promote an autocrine growth loop critical for tumor genesis, cell proliferation, terminal differentiation, apoptosis and survival. To explore the relationship between MEKK3 and cell apoptosis, clinicopathology and prognosis, we characterize the expression of MEKK3, pERK and FoxP3 in the renal clear cell carcinoma (RCCC). Protein expression was detected by tissue microarray and immunochemistry in 46 cases of RCCC and 28 control cases. Expression levels of CD3+ ,CD3+CD4+,CD3+CD8+,CD4+CD25+, CD4+CD25+ FoxP3+ were assessed by flow cytometry and analyzed for their association with pathological factors, correlation and prognosis in RCCC. Expression of MEKK3, pERK and FoxP3 was significantly up-regulated in RCCC as compared to control levels (p<0.01), associated with pathological grade (p<0.05)and clinical stage (p<0.05). CD4+CD25+ Foxp3+ Treg cells were also significantly increased in RCCC patients (p<0.05). Cox multivariate regression analysis showed that MEKK3, pERK expression and patholigical stage were independent prognostic factors in patients with RCCC (p<0.05). MEKK3 can be used as an important marker of early diagnosis and prognostic evaluation in RCCC. It may be associated with imbalance of anti-tumor immunity and overexpression of pERK. Expression of MEKK3 and pERK are significantly increased in RCCC, with protein expression and clinical stage acting as independent prognostic factors.

Genomic structure and immunological response of an STAT4 family member from rock bream (Oplegnathus fasciatus)

The Janus tyrosine kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathway plays a critical role in host defense against viral and bacterial infections. STAT proteins are a group of transcription factors that translocate into the nucleus and are critical for the induction of many genes crucial for the allergic cascade and immune defense. In the present study, a member of the STAT4 family was identified from rock bream (RbSTAT4) at the genomic level, and its transcriptional regulation in response to different pathological stimuli under in vivo conditions was investigated. The genomic sequence of RbSTAT4 is approximately 15.6 kb in length, including a putative core promoter region and 24 exons interrupted by 23 introns. Bioinformatics analysis of RbSTAT4 identified the presence of typical and conserved features of the STAT4 family, including the STAT_int domain, STAT alpha domain, STAT bind domain, linker domain, SH2 domain, and transcriptional activation domain. According to the phylogenetic analysis, RbSTAT4 exhibited the closest evolutionary proximity with the STAT4 member from mandarin fish (Siniperca chuatsi). The RbSTAT4 transcript in healthy rock breams was detected to have ubiquitous expression in 11 different tissues examined, where liver and spleen tissues showed moderate expressions compared with the highest expression level detected in gill tissue. The time-course in vivo immune stimulation of rock bream with lipopolysaccharide, poly I:C, live Edwardsiella tarda, and rock bream iridovirus caused significant transcriptional regulation of the RbSTAT4 expression in gill, head kidney, and spleen tissues, suggesting that RbSTAT4 is involved in immune regulation mechanisms and/or signaling cascades, orchestrating against both bacterial and viral pathogens.

MEKK3 regulates IFN-gamma production in T cells through the Rac1/2-dependent MAPK cascades

MEKK3 is a conserved Ser/Thr protein kinase belonging to the MAPK kinase kinase (MAP3K) family. MEKK3 is constitutively expressed in T cells, but its function in T cell immunity has not been fully elucidated. Using Mekk3 T cell conditional knockout (T-cKO) mice, we show that MEKK3 is required for T cell immunity in vivo. Mekk3 T-cKO mice had reduced T cell response to bacterial infection and were defective in clearing bacterial infections. The Ag-induced cytokine production, especially IFN-γ production, was impaired in Mekk3-deficient CD4 T cells. The TCR-induced ERK1/2, JNK, and p38 MAPKs activation was also defective in Mekk3-deficient CD4 T cells. In vitro, MEKK3 is not required for Th1 and Th2 cell differentiation. Notably, under a nonpolarizing condition (Th0), Mekk3 deficiency led to a significant reduction of IFN-γ production in CD4 T cells. Furthermore, the IL-12/IL-18-driven IFN-γ production and MAPK activation in Mekk3-deficient T cells was not affected suggesting that MEKK3 may selectively mediate the TCR-induced MAPK signals for IFN-γ production. Finally, we found that MEKK3 activation by TCR stimulation requires Rac1/2. Taken together, our study reveals a specific role of MEKK3 in mediating the TCR signals for IFN-γ production.

The kinases MEKK2 and MEKK3 regulate transforming growth factor-β-mediated helper T cell differentiation

Mitogen-activated protein kinases (MAPKs) are key mediators of the T cell receptor (TCR) signals but their roles in T helper (Th) cell differentiation are unclear. Here we showed that the MAPK kinase kinases MEKK2 (encoded by Map3k2) and MEKK3 (encoded by Map3k3) negatively regulated transforming growth factor-β (TGF-β)-mediated Th cell differentiation. Map3k2(-/-)Map3k3(Lck-Cre/-) mice showed an abnormal accumulation of regulatory T (Treg) and Th17 cells in the periphery, consistent with Map3k2(-/-)Map3k3(Lck-Cre/-) naive CD4(+) T cells' differentiation into Treg and Th17 cells with a higher frequency than wild-type (WT) cells after TGF-β stimulation in vitro. In addition, Map3k2(-/-)Map3k3(Lck-Cre/-) mice developed more severe experimental autoimmune encephalomyelitis. Map3k2(-/-)Map3k3(Lck-Cre/-) T cells exhibited impaired phosphorylation of SMAD2 and SMAD3 proteins at their linker regions, which negatively regulated the TGF-β responses in T cells. Thus, the crosstalk between TCR-induced MAPK and the TGF-β signaling pathways is important in regulating Th cell differentiation.

NF-κB signaling pathways regulated by CARMA family of scaffold proteins

The NF-κB family of transcription factors plays a crucial role in cell activation, survival and proliferation. Its aberrant activity results in cancer, immunodeficiency or autoimmune disorders. Over the past two decades, tremendous progress has been made in our understanding of the signals that regulate NF-κB activation, especially how scaffold proteins link different receptors to the NF-κB-activating complex, the IκB kinase complex. The growing number of these scaffolds underscores the complexity of the signaling networks in different cell types. In this review, we discuss the role of scaffold molecules in signaling cascades induced by stimulation of antigen receptors, G-protein-coupled receptors and C-type Lectin receptors, resulting in NF-κB activation. Especially, we focus on the family of Caspase recruitment domain (CARD)-containing proteins known as CARMA and their function in activation of NF-κB, as well as the link of these scaffolds to the development of various neoplastic diseases through regulation of NF-κB.

Dephosphorylation of Carma1 by PP2A negatively regulates T-cell activation

The Carma1-Bcl10-Malt1 (CBM) complex bridges T-cell receptor (TCR) signalling to the canonical IκB kinase (IKK)/NF-κB pathway. NF-κB activation is triggered by PKCθ-dependent phosphorylation of Carma1 after TCR/CD28 co-stimulation. PKCθ-phosphorylated Carma1 was suggested to function as a molecular scaffold that recruits preassembled Bcl10-Malt1 complexes to the membrane. We have identified the serine-threonine protein phosphatase PP2A regulatory subunit Aα (PPP2R1A) as a novel interaction partner of Carma1. PPP2R1A is associated with Carma1 in resting as well as activated T cells in the context of the active CBM complex. By siRNA-mediated knockdown and in vitro dephosphorylation, we demonstrate that PP2A removes PKCθ-dependent phosphorylation of Ser645 in Carma1, and show that maintenance of this phosphorylation is correlated with increased T-cell activation. As a result of PP2A inactivation, we find that enhanced Carma1 S645 phosphorylation augments CBM complex formation, NF-κB activation and IL-2 or IFN-γ production after stimulation of Jurkat T cells or murine Th1 cells. Thus, our data define PP2A-mediated dephosphorylation of Carma1 as a critical step to limit T-cell activation and effector cytokine production.

Molecular basis of lysophosphatidic acid-induced NF-κB activation

PKC, β-arrestin 2, CARMA3, BCL10, MALT1, TRAF6 and MEKK3 are signaling proteins that have a key role in G protein-coupled receptor (GPCR)-mediated activation of nuclear factor-κB (NF-κB) pathway in nonhematopoietic cells in response to lysophosphatidic acid (LPA) stimulation. The PKC, β-arrestin 2, CARMA3-BCL10-MALT1-TRAF6 signalosome, and MEKK3 functions as a link between GPCR signaling and IKK-NF-κB activation. Here we briefly summarize recent progress in the understanding of the molecular and biological functions of these proteins in GPCR-mediated NF-κB activation in nonhematopoietic cells.

The persistence of T cell memory

T cell memory is a crucial feature of the adaptive immune system in the defense against pathogens. During the last years, numerous studies have focused their efforts on uncovering the signals, inflammatory cues, and extracellular factors that support memory differentiation. This research is beginning to decipher the complex gene network that controls memory programming. However, how the different signals, that a T cell receives during the process of differentiation, interplay to trigger memory programming is still poorly defined. In this review, we focus on the most recent advances in the field and discuss how T cell receptor signaling and inflammation control CD8 memory differentiation.

Regulation of JNK and p38 MAPK in the immune system: signal integration, propagation and termination

Stress-activated MAP kinases (MAPKs), comprised of JNK and p38, play prominent roles in the innate and adaptive immune systems. Activation of MAPKs is mediated by a three-tiered kinase module comprised of MAPK kinase kinases (MAP3Ks), MAPK kinases (MAP2Ks) and MAPKs through sequential protein phosphorylation. Activated MAPKs, in turn, phosphorylate transcription factors and other targets to regulate gene transcription and immune responses. Recent studies have provided new insight into the upstream and downstream components of the MAPK pathway that facilitate the activation and propagation of MAPK signaling in immune responses. Moreover, MAPK activity is negatively regulated by MAPK phosphatases (MKPs), a group of dual-specificity phosphatases that dephosphorylate and inactivate the MAPKs. Here we discuss the recent advances in our understanding of these regulatory processes in MAPK signaling with a focus on their impacts on immune function.