Dephosphorylation of Bcl‐10 by calcineurin is essential for canonical NF‐κB activation in Th cells

Abstract

Antigen-specific stimulation of T helper (Th) cells initiates signaling cascades that ultimately result in the activation of the transcription factors NF-jB, NFAT, and AP-1 which regulate, together with other factors, many T-cell functions such as cytokine production, proliferation, and differentiation. Ordered assembly and different phosphorylation events, along with subcellular translocation of the CARMA1/Bcl-10/MALT1 complex, determine NF-jB activation after T-cell receptor (TCR) triggering. We now provide evidence that inhibition of the Ser/Thr phosphatase calcineurin (CaN) prevents dephosphorylation of Bcl-10. CaN, in constant interaction with the Bcl-10/MALT1 complex, is able to dephosphorylate Bcl-10. The CaN inhibitor cyclosporine A (CsA) converts a transient phosphorylation of Bcl-10 Ser138 during the immediate early phase of T-cell activation into a persistent state. Thus, subsequent processes such as IKKb phosphorylation, IjBa degradation, p65 nuclear translocation, and DNA binding are diminished. Consistently, CsA treatment does not affect the phosphorylation pattern of the upstream kinase PKCh. Together, our findings demonstrate that CaN functions as a critical signaling molecule during Th cell activation, regulating Bcl-10 phosphorylation and thereby NF-jB activation.Keywords: Calcineurin . Cell activation . T cell . TCR signaling . Transcription factors Supporting Information available online IntroductionAntigen-specific stimulation of T helper (Th) cells induces activation of the transcription factors nuclear factor of activated T cells (NFAT) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) via a complex signaling network. The cooperative action of these and other transcription factors results in gene expression profiles leading to the activation of Th cells, production of growth factors, in particular cytokines, and proliferation [1][2][3]. T-cell receptor (TCR) engagement triggers phosphorylation of membrane-associated tyrosine kinases that activate PLCg, which in turn generates the second messengers IP 3 and DAG. The IP 3 -induced Ca 21 influx triggers the Ser/Thr phosphatase calcineurin (CaN) which dephosphorylates NFAT. This dephosphorylation step is crucial for the nuclear translocation of NFAT and binding to promoters of target genes. The second messenger, DAG, activates the NF-kB signaling cascade by binding to the protein kinase C-y (PKCy) leading to a change in PKCy conformation into an open, active state. The precise nature of these changes is not entirely clear, but phosphorylation at different sites, subcellular translocation, and assembly of assorted molecules contributes to this process (reviewed in [4,5] [6,7]. The importance of Bcl-10 for NF-kB activation is underlined by studies in Bcl-10 knockout mice showing that Bcl-10 is essential for NF-kB-driven cytokine production, cell proliferation, and B-as well as T-cell development [8,9]. Furthermore, Bcl-10 overexpression in MALT B-cell lymphomas is characterized by constant NF-kB activation [10,11]. Bcl-1...

Dependence on nuclear factor of activated T-cells (NFAT) levels discriminates conventional T cells from Foxp3 + regulatory T cells

“…Second, additional pathways such as for AP1 and NF-κB induction are Ca 2+ /calcineurin-dependent (36). For example, calcineurin controls the formation of the CARMA1/BCL10/MALT1 complex during TCR-induced NF-κB activation (37,38), whereas mathematical calculations provide evidence that NF-κB is activated at lower Ca 2+ oscillation frequencies than NFAT (39). This is consistent with the observation that severely reduced calcium flux as observed in Stim1 fl/fl × Cd4-cre allows regular development of nTreg, albeit Stim1/2 double-deficiency conducting an almost complete block of calcium flux abrogates thymic Treg development (40).…”

NFATc1 releases BCL6‐dependent repression of CCR2 agonist expression in peritoneal macrophages fromSaccharomyces cerevisiaeinfected mice

“…This might be due to the strong pleiotropic activity of these CN inhibitors. However, in contrast to the situation in T cells [4,5], the translocation of RELA protein remained insensitive to CsA/FK506 in peritoneal resident macrophages, and the noncanonical NF-κB pathway was not activated by yeast infection. These data are consistent with the normal clearance of peritoneal yeast infection in mice deficient for RELA.…”

“…However, severe side-effects including a high susceptibility to opportunistic infections, hamper the clinical applications of CsA-and FK506-based regimens. CN is a ubiquitously expressed phosphatase with multiple downstream targets, such as the Ca ++ /CN/NFAT, Ca ++ /CN/cofilin, Ca ++ /CN/BAD and NF-κB networks [4,5]. Although ablation of CN-B compromises neutrophil-dependent killing of fungal pathogens ex vivo [6], it is still not completely clear how CsA and FK506 facilitate fungal and other opportunistic infections.…”

“…We show that, upon binding to a composite NFAT/BCL6 regulatory element within the Ccl2 promoter, NFATc1/β proteins release the BCL6-dependent repression of Ccl2 gene in macrophages. These findings suggest a novel CN-dependent cross-talk between NFAT and BCL6 transcription factors, which may affect the outcome of opportunistic fungal infections in immunocompromised patients.Keywords: BCL6 r CCL2 r CCR2 r Inflammatory monocytes r NFATc1 r Opportunistic infection Additional supporting information may be found in the online version of this article at the publisher's web-site [4,5]. Although ablation of CN-B compromises neutrophil-dependent killing of fungal pathogens ex vivo [6], it is still not completely clear how CsA and FK506 facilitate fungal and other opportunistic infections.…”

“…Keywords: BCL6 r CCL2 r CCR2 r Inflammatory monocytes r NFATc1 r Opportunistic infection Additional supporting information may be found in the online version of this article at the publisher's web-site [4,5]. Although ablation of CN-B compromises neutrophil-dependent killing of fungal pathogens ex vivo [6], it is still not completely clear how CsA and FK506 facilitate fungal and other opportunistic infections.…”