Polo-like kinase 1 (PLK1) is a novel CARD14-binding protein in keratinocytes.

Caspase recruitment domain (CARD)-containing protein 14 (CARD14) is an intracellular protein that mediates nuclear factor-kappa B (NF-ĸB) signaling and proinflammatory gene expression in skin keratinocytes. Several hyperactivating CARD14 mutations have been associated with psoriasis and other inflammatory skin diseases. CARD14-induced NF-ĸB signaling is dependent on the formation of a CARD14-BCL10-MALT1 (CBM) signaling complex, but upstream receptors and molecular mechanisms that activate and regulate CARD14 signaling are still largely unclear. Using unbiased affinity purification and mass spectrometry (AP-MS) screening, we discover polo-like kinase 1 (PLK1) as a novel CARD14-binding protein. CARD14-PLK1 binding is independent of the CARD14 CARD domain but involves a consensus phospho-dependent PLK1-binding motif in the CARD14 linker region (LR). Expression of the psoriasis-associated CARD14(E138A) variant in human keratinocytes induces the recruitment of PLK1 to CARD14-containing signalosomes in interphase cells, but does not affect the specific location of PLK1 in mitotic cells. Finally, disruption of the PLK1-binding motif in CARD14(E138A) increases CARD14-induced proinflammatory signaling and gene expression. Together, our data identify PLK1 as a novel CARD14-binding protein and indicate a negative regulatory role for PLK1 in CARD14 signaling.

A20 is a master switch of IL-33 signaling in macrophages and determines IL-33-induced lung immunity.

BACKGROUND: IL-33 plays a major role in the pathogenesis of allergic diseases such as asthma and atopic dermatitis. On its release from lung epithelial cells, IL-33 primarily drives type 2 immune responses, accompanied by eosinophilia and robust production of IL-4, IL-5, and IL-13. However, several studies show that IL-33 can also drive a type 1 immune response. OBJECTIVE: We sought to determine the role of A20 in the regulation of IL-33 signaling in macrophages and IL-33-induced lung immunity. METHODS: We studied the immunologic response in lungs of IL-33-treated mice that specifically lack A20 in myeloid cells. We also analyzed IL-33 signaling in A20-deficient bone marrow-derived macrophages. RESULTS: IL-33-induced lung innate lymphoid cell type 2 expansion, type 2 cytokine production, and eosinophilia were drastically reduced in the absence of macrophage A20 expression, whereas neutrophils and interstitial macrophages in lungs were increased. In vitro, IL-33-mediated nuclear factor kappa B activation was only weakly affected in A20-deficient macrophages. However, in the absence of A20, IL-33 gained the ability to activate signal transducer and activator of transcription 1 (STAT1) signaling and STAT1-dependent gene expression. Surprisingly, A20-deficient macrophages produced IFN-γ in response to IL-33, which was fully STAT1-dependent. Furthermore, STAT1 deficiency partially restored the ability of IL-33 to induce ILC2 expansion and eosinophilia in myeloid cell-specific A20 knockout mice. CONCLUSIONS: We reveal a novel role for A20 as a negative regulator of IL-33-induced STAT1 signaling and IFN-γ production in macrophages, which determines lung immune responses.

Normal lymphocyte homeostasis and function in MALT1 protease-resistant HOIL-1 knock-in mice.

The uniqueness of MALT1 protease activity in controlling several aspects of immunity in humans has made it a very attractive therapeutic target for multiple autoimmune diseases and lymphoid malignancies. Despite several encouraging preclinical studies with MALT1 inhibitors, severe reduction in regulatory T cells and immune-mediated pathology seen in MALT1 protease-dead (MALT1-PD) mice and some, but not all, studies analysing the effect of prolonged pharmacological MALT1 protease inhibition, indicates the need to further unravel the mechanism of MALT1 protease function. Notably, the contribution of individual MALT1 substrates to the immune defects seen in MALT1-PD mice is still unclear. Previous in vitro studies indicated a role for MALT1-mediated cleavage of the E3 ubiquitin ligase HOIL-1 in the modulation of nuclear factor-κB (NF-κB) signalling and inflammatory gene expression in lymphocytes. Here, we addressed the immunological consequences of inhibition of HOIL-1 cleavage by generating and immunophenotyping MALT1 cleavage-resistant HOIL-1 knock-in (KI) mice. HOIL-1 KI mice appear healthy and have no overt phenotype. NF-κB activation in T or B cells, as well as IL-2 production and in vitro T-cell proliferation, is comparable between control and HOIL-1 KI cells. Inhibition of HOIL-1 cleavage in mice has no effect on thymic T-cell development and conventional T-cell homeostasis. Likewise, B-cell development and humoral immune responses are not affected. Together, these data exclude an important role of MALT1-mediated HOIL-1 cleavage in T- and B-cell development and function in mice.

Defining the combinatorial space of PKC::CARD-CC signal transduction nodes.

Signal transduction typically displays a so-called bow-tie topology: Multiple receptors lead to multiple cellular responses but the signals all pass through a narrow waist of central signaling nodes. One such signaling node for several inflammatory and oncogenic signaling pathways is the CARD-CC/BCL10/MALT1 (CBM) complexes, which get activated by protein kinase C (PKC)-mediated phosphorylation of the caspase activation and recruitment domain (CARD)-coiled-coil domain (CC) component. In humans, there are four CARD-CC family proteins (CARD9, CARD10, CARD11, and CARD14) and 9 true PKC isozymes (α to ι). At this moment, less than a handful of PKC::CARD-CC relationships are known. In order to explore the biologically relevant combinatorial space out of all 36 potential permutations in this two-component signaling event, we made use of CARD10-deficient human embryonic kidney 293T cells for subsequent pairwise cotransfections of all CARD-CC family members and all activated PKCs. Upon analysis of NF-κB-dependent reporter gene expression, we could define specific PKC::CARD-CC relationships. Surprisingly, as many as 21 PKC::CARD-CC functional combinations were identified. CARD10 was responsive to most PKCs, while CARD14 was mainly activated by PKCδ. The CARD11 activation profile was most similar to that of CARD9. We also discovered the existence of mixed protein complexes between different CARD-CC proteins, which was shown to influence their PKC response profile. Finally, multiple PKCs were found to use a common phosphorylation site to activate CARD9, while additional phosphorylation sites contribute to CARD14 activation. Together, these data reveal the combinatorial space of PKC::CARD-CC signal transduction nodes, which will be valuable for future studies on the regulation of CBM signaling.

MALT1-Deficient Mice Develop Atopic-Like Dermatitis Upon Aging.

MALT1 plays an important role in innate and adaptive immune signaling by acting as a scaffold protein that mediates NF-κB signaling. In addition, MALT1 is a cysteine protease that further fine tunes proinflammatory signaling by cleaving specific substrates. Deregulated MALT1 activity has been associated with immunodeficiency, autoimmunity, and cancer in mice and humans. Genetically engineered mice expressing catalytically inactive MALT1, still exerting its scaffold function, were previously shown to spontaneously develop autoimmunity due to a decrease in Tregs associated with increased effector T cell activation. In contrast, complete absence of MALT1 does not lead to autoimmunity, which has been explained by the impaired effector T cell activation due to the absence of MALT1-mediated signaling. However, here we report that MALT1-deficient mice develop atopic-like dermatitis upon aging, which is preceded by Th2 skewing, an increase in serum IgE, and a decrease in Treg frequency and surface expression of the Treg functionality marker CTLA-4.

MALT1 Proteolytic Activity Suppresses Autoimmunity in a T Cell Intrinsic Manner.

MALT1 is a central signaling component in innate and adaptive immunity by regulating NF-κB and other key signaling pathways in different cell types. Activities of MALT1 are mediated by its scaffold and protease functions. Because of its role in lymphocyte activation and proliferation, inhibition of MALT1 proteolytic activity is of high interest for therapeutic targeting in autoimmunity and certain lymphomas. However, recent studies showing that Malt1 protease-dead knock-in (Malt1-PD) mice suffer from autoimmune disease have somewhat tempered the initial enthusiasm. Although it has been proposed that an imbalance between immune suppressive regulatory T cells (Tregs) and activated effector CD4+ T cells plays a key role in the autoimmune phenotype of Malt1-PD mice, the specific contribution of MALT1 proteolytic activity in T cells remains unclear. Using T cell-conditional Malt1 protease-dead knock-in (Malt1-PDT) mice, we here demonstrate that MALT1 has a T cell-intrinsic role in regulating the homeostasis and function of thymic and peripheral T cells. T cell-specific ablation of MALT1 proteolytic activity phenocopies mice in which MALT1 proteolytic activity has been genetically inactivated in all cell types. The Malt1-PDT mice have a reduced number of Tregs in the thymus and periphery, although the effect in the periphery is less pronounced compared to full-body Malt1-PD mice, indicating that also other cell types may promote Treg induction in a MALT1 protease-dependent manner. Despite the difference in peripheral Treg number, both T cell-specific and full-body Malt1-PD mice develop ataxia and multi-organ inflammation to a similar extent. Furthermore, reconstitution of the full-body Malt1-PD mice with T cell-specific expression of wild-type human MALT1 eliminated all signs of autoimmunity. Together, these findings establish an important T cell-intrinsic role of MALT1 proteolytic activity in the suppression of autoimmune responses.

Mepazine Inhibits RANK-Induced Osteoclastogenesis Independent of Its MALT1 Inhibitory Function.

Mucosa-associated lymphoid tissue lymphoma translocation protein 1 (MALT1) is an intracellular cysteine protease (paracaspase) that plays an integral role in innate and adaptive immunity. The phenothiazine mepazine has been shown to inhibit the proteolytic activity of MALT1 and is frequently used to study its biological role. MALT1 has recently been suggested as a therapeutic target in rheumatoid arthritis. Here, we analyzed the effect of mepazine on the receptor activator of nuclear factor κ-B (RANK)-induced osteoclastogenesis. The treatment of mouse bone marrow precursor cells with mepazine strongly inhibited the RANK ligand (RANKL)-induced formation of osteoclasts, as well as the expression of several osteoclast markers, such as TRAP, cathepsin K, and calcitonin. However, RANKL induced osteoclastogenesis equally well in bone marrow cells derived from wild-type and Malt1 knock-out mice. Furthermore, the protective effect of mepazine was not affected by MALT1 deficiency. Additionally, the absence of MALT1 did not affect RANK-induced nuclear factor κB (NF-κB) and activator protein 1 (AP-1) activation. Overall, these studies demonstrate that MALT1 is not essential for RANK-induced osteoclastogenesis, and implicate a MALT1-independent mechanism of action of mepazine that should be taken into account in future studies using this compound.

Importance of Validating Antibodies and Small Compound Inhibitors Using Genetic Knockout Studies-T Cell Receptor-Induced CYLD Phosphorylation by IKKε/TBK1 as a Case Study.

CYLD is a deubiquitinating enzyme that plays a crucial role in immunity and inflammation as a negative regulator of NF-κB transcription factor and JNK kinase signaling. Defects in either of these pathways contribute to the progression of numerous inflammatory and autoimmune disorders. Therefore, we set out to unravel molecular mechanisms that control CYLD activity in the context of T cell receptor (TCR) signaling. More specifically, we focused on CYLD phosphorylation at Ser418, which can be detected upon immunoblotting of cell extracts with phospho(Ser418)-CYLD specific antibodies. Jurkat T cells stimulated with either anti-CD3/anti-CD28 or PMA/Ionomycin (to mimic TCR signaling) were used as a model system. The role of specific kinases was analyzed using pharmacological as well as genetic approaches. Our initial data indicated that CYLD is directly phosphorylated by the noncanonical IκB kinases (IKKs) IKKε and TANK Binding Kinase 1 (TBK1) at Ser418 upon TCR stimulation. Treatment with MRT67307, a small compound inhibitor for IKKε and TBK1, inhibited TCR-induced CYLD phosphorylation. However, the phospho(Ser418)-CYLD immunoreactive band was still present in CRISPR/Cas9 generated IKKε/TBK1 double knockout cell lines, where it could still be prevented by MRT67307, indicating that the initially observed inhibitory effect of MRT67307 on TCR-induced CYLD phosphorylation is IKKε/TBK1-independent. Most surprisingly, the phospho(Ser418)-CYLD immunoreactive band was still detectable upon immunoblotting of cell extracts obtained from CYLD deficient cells. These data demonstrate the non-specificity of MRT67307 and phospho(Ser418)-CYLD specific antibodies, implying that previously published results based on these tools may also have led to wrong conclusions. We therefore advise to use genetic knockout studies or alternative approaches for a better validation of antibodies and small compound inhibitors. Interestingly, immunoprecipitation with the phospho(Ser418)-CYLD antibody, followed by immunoblotting with anti-CYLD, revealed that CYLD is phosphorylated by IKKε/TBK1 at Ser418 upon T cell stimulation, but that its direct detection with the phospho(Ser418)-CYLD-specific antibody in a western blot is masked by another inducible protein of the same size that is recognized by the same antibody.

The paracaspase MALT1 mediates CARD14-induced signaling in keratinocytes.

Mutations in CARD14 have recently been linked to psoriasis susceptibility. CARD14 is an epidermal regulator of NF-κB activation. However, the ability of CARD14 to activate other signaling pathways as well as the biochemical mechanisms that mediate and regulate its function remain to be determined. Here, we report that in addition to NF-κB signaling, CARD14 activates p38 and JNK MAP kinase pathways, all of which are dependent on the paracaspase MALT1. Mechanistically, we demonstrate that CARD14 physically interacts with paracaspase MALT1 and activates MALT1 proteolytic activity and inflammatory gene expression, which are enhanced by psoriasis-associated CARD14 mutations. Moreover, we show that MALT1 deficiency or pharmacological inhibition of MALT1 catalytic activity inhibits pathogenic mutant CARD14-induced cytokine and chemokine expression in human primary keratinocytes. Collectively, our findings demonstrate a novel role for MALT1 in CARD14-induced signaling and indicate MALT1 as a valuable therapeutic target in psoriasis.

XEDAR activates the non-canonical NF-κB pathway.

Members of the tumor necrosis factor receptor (TNFR) superfamily are involved in a number of physiological and pathological responses by activating a wide variety of intracellular signaling pathways. The X-linked ectodermal dysplasia receptor (XEDAR; also known as EDA2R or TNFRSF27) is a member of the TNFR superfamily that is highly expressed in ectodermal derivatives during embryonic development and binds to ectodysplasin-A2 (EDA-A2), a member of the TNF family that is encoded by the anhidrotic ectodermal dysplasia (EDA) gene. Although XEDAR was first described in the year 2000, its function and molecular mechanism of action is still largely unclear. XEDAR has been reported to activate canonical nuclear factor κB (NF-κB) signaling and mitogen-activated protein (MAP) kinases. Here we report that XEDAR is also able to trigger the non-canonical NF-κB pathway, characterized by the processing of p100 (NF-κB2) into p52, followed by nuclear translocation of p52 and RelB. We provide evidence that XEDAR-induced p100 processing relies on the binding of XEDAR to TRAF3 and TRAF6, and requires the kinase activity of NIK and IKKα. We also show that XEDAR stimulation results in NIK accumulation and that p100 processing is negatively regulated by TRAF3, cIAP1 and A20.

A20 inhibits LUBAC-mediated NF-κB activation by binding linear polyubiquitin chains via its zinc finger 7.

Linear polyubiquitination of proteins has recently been implicated in NF-κB signalling and is mediated by the linear ubiquitin chain assembly complex (LUBAC), consisting of HOIL-1, HOIP and Sharpin. However, the mechanisms that regulate linear ubiquitination are still unknown. Here, we show that A20 is rapidly recruited to NEMO and LUBAC upon TNF stimulation and that A20 inhibits LUBAC-induced NF-κB activation via its C-terminal zinc-finger 7 (ZF7) domain. Expression of a polypeptide corresponding to only ZF7 was sufficient to inhibit TNF-induced NF-κB activation. Both A20 and ZF7 can form a complex with NEMO and LUBAC, and are able to prevent the TNF-induced binding of NEMO to LUBAC. Finally, we show that ZF7 preferentially binds linear polyubiquitin chains in vitro, indicating A20-ZF7 as a novel linear ubiquitin-binding domain (LUBID). We thus propose a model in which A20 inhibits TNF- and LUBAC-induced NF-κB signalling by binding to linear polyubiquitin chains via its seventh zinc finger, which prevents the TNF-induced interaction between LUBAC and NEMO.

Expression of the NF-kappaB inhibitor ABIN-3 in response to TNF and toll-like receptor 4 stimulation is itself regulated by NF-kappaB.

Although the nuclear factor-kappaB (NF-kappaB)-dependent gene expression is critical to the induction of an efficient immune response to infection or tissue injury, excessive or prolonged NF-kappaB signalling can contribute to the development of several inflammatory diseases. Therefore, the NF-kappaB signal transduction pathway is tightly regulated by several intracellular proteins. We have previously identified A20-binding inhibitor of NF-kappaB activation (ABIN)-3 as an lipopolysaccharide (LPS)-inducible protein in monocytes that negatively regulates NF-B activation in response to tumour necrosis factor (TNF) and LPS. Here we report that ABIN-3 expression is also up-regulated upon TNF treatment of monocytes and other non-myeloid cell types. We also found a significantly enhanced expression of ABIN-3 in monocytes of sepsis patients, which is restored to control levels by corticotherapy. To further understand the transcriptional regulation of ABIN-3 expression, we isolated the human ABIN-3 promoter and investigated its activation in response to TNF and LPS. This revealed that the LPS- and TNF-inducible expression of ABIN-3 is dependent on the binding of NF-kappaB to a specific B site in the ABIN-3 promoter. Altogether, these data indicate an important role for NF-kappaB-dependent gene expression of ABIN-3 in the negative feedback regulation of TNF receptor and toll-like receptor 4 induced NF-kappaB activation.

LIND/ABIN-3 is a novel lipopolysaccharide-inducible inhibitor of NF-kappaB activation.

Recognition of lipopolysaccharide (LPS) by Toll-like receptor (TLR)4 initiates an intracellular signaling pathway leading to the activation of nuclear factor-kappaB (NF-kappaB). Although LPS-induced activation of NF-kappaB is critical to the induction of an efficient immune response, excessive or prolonged signaling from TLR4 can be harmful to the host. Therefore, the NF-kappaB signal transduction pathway demands tight regulation. In the present study, we describe the human protein Listeria INDuced (LIND) as a novel A20-binding inhibitor of NF-kappaB activation (ABIN) that is related to ABIN-1 and -2 and, therefore, is further referred to as ABIN-3. Similar to the other ABINs, ABIN-3 binds to A20 and inhibits NF-kappaB activation induced by tumor necrosis factor, interleukin-1, and 12-O-tetradecanoylphorbol-13-acetate. However, unlike the other ABINs, constitutive expression of ABIN-3 could not be detected in different human cells. Treatment of human monocytic cells with LPS strongly induced ABIN-3 mRNA and protein expression, suggesting a role for ABIN-3 in the LPS/TLR4 pathway. Indeed, ABIN-3 overexpression was found to inhibit NF-kappaB-dependent gene expression in response to LPS/TLR4 at a level downstream of TRAF6 and upstream of IKKbeta. NF-kappaB inhibition was mediated by the ABIN-homology domain 2 and was independent of A20 binding. Moreover, in vivo adenoviral gene transfer of ABIN-3 in mice reduced LPS-induced NF-kappaB activity in the liver, thereby partially protecting mice against LPS/D-(+)-galactosamine-induced mortality. Taken together, these results implicate ABIN-3 as a novel negative feedback regulator of LPS-induced NF-kappaB activation.

Yeast two-hybrid screening for proteins interacting with the anti-apoptotic protein A20.

The yeast two-hybrid system is a powerful technique for identifying proteins that interact with a specific protein of interest. The rationale of the yeast two-hybrid system relies on the physical separation of the DNA-binding domain from the transcriptional activation domain of several transcription factors. Therefore, the protein of interest (bait) is fused to a DNA-binding domain, and complimentary DNA (cDNA) library-encoded proteins are fused to a transcriptional activation domain. When a protein encoded by the cDNA library binds to the bait, both activities of the transcription factor are rejoined and transcription from a reporter gene is started. Here, we will give a comprehensive guide for the GAL4-based two-hybrid system, exemplified by the detection of binding partners for the zinc finger protein A20. The latter is an inducible cellular inhibitor of tumor necrosis factor (TNF)-induced apoptosis and nuclear factor (NF)-kappaB-dependent gene expression. Yeast two-hybrid screening with A20 as bait revealed several A20-binding proteins, including A20 itself, members of the 14-3-3 family, as well as three novel proteins ABIN-1, ABIN-2, and TXBP151. The latter protein was subsequently shown to mediate at least part of the anti-apoptotic activities of A20, whereas ABIN-1 and -2 are more likely to be involved in the NF-kappaB inhibitory effects of A20.

A novel type of deubiquitinating enzyme.

A previous report from this laboratory described two novel proteins that have sequence similarity to A20, a negative regulator of NF-kappaB (Evans, P. C., Taylor, E. R., Coadwell, J., Heyninck, K., Beyaert, R., and Kilshaw, P. J. (2001) Biochem. J. 357, 617-623). One of these molecules, cellular zinc finger anti-NF-kappaB (Cezanne), a 100-kDa cytoplasmic protein, also suppressed NF-kappaB. Here we demonstrate that Cezanne is a novel deubiquitinating enzyme, distinct from the two known families of deubiquitinases, Types I and II. We show that Cezanne contains an N-terminal catalytic domain that belongs to the recently discovered ovarian tumor protein (OTU) superfamily, a group of proteins displaying structural similarity to cysteine proteases but having no previously described function. Recombinant Cezanne cleaved ubiquitin monomers from linear or branched synthetic ubiquitin chains and from ubiquitinated proteins. Mutation of a conserved cysteine residue in the catalytic site of the proteolytic domain caused Cezanne to co-precipitate polyubiquitinated cellular proteins. We also provide evidence for an additional ubiquitin binding site in the C-terminal part of the molecule. Our data provide the first demonstration of functional activity among OTU proteins.

Structure-function analysis of the A20-binding inhibitor of NF-kappa B activation, ABIN-1.

Nuclear factor kappa B (NF-kappa B)-dependent gene expression plays an important role in numerous cellular processes including stress responses, inflammation and cell proliferation. Therefore, the activity of this transcription factor needs to be tightly regulated. Among others, the NF-kappa B-dependent zinc finger protein A20 is involved in the negative feedback regulation of NF-kappa B activation in response to tumor necrosis factor (TNF). We previously demonstrated that A20 can interact with A20-binding inhibitors of NF-kappa B activation (ABINs), which have the potential to inhibit TNF-induced activation of NF-kappa B upon overexpression. The ABIN proteins were therefore proposed to mediate the NF-kappa B inhibiting function of A20. Here we demonstrate the presence of a short homologous region in ABINs and I kappa B kinase gamma, the regulatory subunit of the I kappa B kinase complex. Site-specific mutagenesis of this region abolished the NF-kappa B inhibiting function of ABIN-1, without affecting the interaction with A20. Furthermore, coexpression of these ABIN-1 mutants interfered in a dominant negative manner with the NF-kappa B inhibiting function of ABIN-1, whereas the A20-mediated inhibition was unaffected. These results suggest that A20 and ABIN-1 probably act independently of their mutual interaction.