Retinoic acid-induced gene-I (RIG-I) associates with nucleotide-binding oligomerization domain-2 (NOD2) to negatively regulate inflammatory signaling

Cytoplasmic caspase recruiting domain (CARD)-containing molecules often function in the induction of potent antimicrobial responses in order to protect mammalian cells from invading pathogens. Retinoic acid-induced gene-I (RIG-I) and nucleotide binding oligomerization domain 2 (NOD2) serve as key factors in the detection of viral and bacterial pathogens, and in the subsequent initiation of innate immune signals to combat infection. RIG-I and NOD2 share striking similarities in their cellular localization, both localize to membrane ruffles in non-polarized epithelial cells and both exhibit a close association with the junctional complex of polarized epithelia. Here we show that RIG-I and NOD2 not only colocalize to cellular ruffles and cell-cell junctions, but that they also form a direct interaction that is mediated by the CARDs of RIG-I and multiple regions of NOD2. Moreover, we show that RIG-I negatively regulates ligand-induced nuclear factor-κB (NF-κB) signaling mediated by NOD2, and that NOD2 negatively regulates type I interferon induction by RIG-I. We also show that the three main Crohn disease-associated mutants of NOD2 (1007fs, R702W, G908R) form an interaction with RIG-I and negatively regulate its signaling to a greater extent than wild-type NOD2. Our results show that in addition to their role in innate immune recognition, RIG-I and NOD2 form a direct interaction at actin-enriched sites within cells and suggest that this interaction may impact RIG-I- and NOD2-dependent innate immune signaling.     

Microbial induction of CARD15 expression in intestinal epithelial cells via toll-like receptor 5 triggers an antibacterial response loop

With the discovery of CARD15 as susceptibility gene for Crohn's disease (CD) a first link to a potential defect in the innate immune system was made. In this work we aimed to analyze enterocyte NOD2/CARD15 expression and regulation in response to bacterial motifs and the consequences of the most common CD-specific CARD15 mutation on antibacterial responses of normal intestinal epithelial cells (IEC). Under normal conditions, IEC lines and ileal enterocytes did not express NOD2/CARD15 mRNA or protein, contrary to IEC derived from inflammatory CD sections. In vitro analyses revealed that the simple contact with non-pathogenic commensal E. Coli K12 was sufficient to induced NOD2/CARD15 mRNA and protein in human IEC (HIEC). We identified bacterial flagellin interacting with TLR5 as major motif in this regulation of NOD2/CARD15. E. Coli mutants not expressing flagellin (DeltaFliC) failed to induce CARD15. Similarly, in HIEC transfected with a plasmid encoding dominant negative TLR5, no CARD15 induction was observed after K12 contact. Isolated TLR2 or TLR4 stimulation had no or only a marginal effect on NOD2/CARD15 expression. NOD2/CARD15 negative HIEC were unresponsive to muramyl dipeptide (MDP), but once NOD2/CARD15 was induced, HIEC and Caco2 cells responded to intra or extracellular MDP presentation with the activation of the NFkB pathway. IEC transfected with the Crohn-specific CARD15 mutant (F3020insC, FS) failed to activate NFkB after MDP-challenge, in contrast to CARD15WT IEC. In response to MDP, IEC induced a massive antibacterial peptide (ABP) response, seen in the apical release of CCL20. This was completely abolished in IEC carrying CARD15FS. These data suggest a critical role of NOD2/CARD15 in the bacterial clearance of the intestinal epithelium while CD-specific mutated NOD2/CARD15 causes an impaired epithelial barrier.     

Engagement of Nucleotide-binding Oligomerization Domain-containing Protein 1 (NOD1) by Receptor-interacting Protein 2 (RIP2) Is Insufficient for Signal Transduction

Following activation, the cytoplasmic pattern recognition receptor nucleotide-binding oligomerization domain-containing protein 1 (NOD1) interacts with its adaptor protein receptor-interacting protein 2 (RIP2) to propagate immune signaling and initiate a proinflammatory immune response. This interaction is mediated by the caspase recruitment domain (CARD) of both proteins. Polymorphisms in immune proteins can affect receptor function and predispose individuals to specific autoinflammatory disorders. In this report, we show that mutations in helix 2 of the CARD of NOD1 disrupted receptor function but did not interfere with RIP2 interaction. In particular, N43S, a rare polymorphism, resulted in receptor dysfunction despite retaining normal cellular localization, protein folding, and an ability to interact with RIP2. Mutation of Asn-43 resulted in an increased tendency to form dimers, which we propose is the source of this dysfunction. We also demonstrate that mutation of Lys-443 and Tyr-474 in RIP2 disrupted the interaction with NOD1. Mapping the key residues involved in the interaction between NOD1 and RIP2 to the known structures of CARD complexes revealed the likely involvement of both type I and type III interfaces in the NOD1·RIP2 complex. Overall we demonstrate that the NOD1-RIP2 signaling axis is more complex than previously assumed, that simple engagement of RIP2 is insufficient to mediate signaling, and that the interaction between NOD1 and RIP2 constitutes multiple CARD-CARD interfaces.     

Interaction between NOD2 and CARD9 involves the NOD2 NACHT and the linker region between the NOD2 CARDs and NACHT domain

NOD2 activation by muramyl dipeptide causes a proinflammatory immune response in which the adaptor protein CARD9 works synergistically with NOD2 to drive p38 and c-Jun N-terminal kinase (JNK) signalling. To date the nature of the interaction between NOD2 and CARD9 remains undetermined. Here we show that this interaction is not mediated by the CARDs of NOD2 and CARD9 as previously suggested, but that NOD2 possesses two interaction sites for CARD9; one in the CARD–NACHT linker and one in the NACHT itself.     

Regulation of DMBT1 via NOD2 and TLR4 in intestinal epithelial cells modulates bacterial recognition and invasion

Mucosal epithelial cell layers are constantly exposed to a complex resident microflora. Deleted in malignant brain tumors 1 (DMBT1) belongs to the group of secreted scavenger receptor cysteine-rich proteins and is considered to be involved in host defense by pathogen binding. This report describes the regulation and function of DMBT1 in intestinal epithelial cells, which form the primary immunological barrier for invading pathogens. We report that intestinal epithelial cells up-regulate DMBT1 upon proinflammatory stimuli (e.g., TNF-alpha, LPS). We demonstrate that DMBT1 is a target gene for the intracellular pathogen receptor NOD2 via NF-kappaB activation. DMBT1 is strongly up-regulated in the inflamed intestinal mucosa of Crohn's disease patients with wild-type, but not with mutant NOD2. We show that DMBT1 inhibits cytoinvasion of Salmonella enterica and LPS- and muramyl dipeptide-induced NF-kappaB activation and cytokine secretion in vitro. Thus, DMBT1 may play an important role in the first line of mucosal defense conferring immune exclusion of bacterial cell wall components. Dysregulated intestinal DMBT1 expression due to mutations in the NOD2/CARD15 gene may be part of the complex pathophysiology of barrier dysfunction in Crohn's disease.     

Structural insights into the MDP binding and CARD–CARD interaction in zebrafish (Danio rerio) NOD2: a molecular dynamics approach

Nucleotide binding and oligomerization domain (NOD2) is a key component of innate immunity that is highly specific for muramyl dipeptide (MDP)—a peptidoglycan component of bacterial cell wall. MDP recognition by NOD2–leucine rich repeat (LRR) domain activates NF‐κB signaling through a protein–protein interaction between caspase activating and recruitment domains (CARDs) of NOD2 and downstream receptor interacting and activating protein kinase 2 (RIP2). Due to the lack of crystal/NMR structures, MDP recognition and CARD–CARD interaction are poorly understood. Herein, we have predicted the probable MDP and CARD–CARD binding surfaces in zebrafish NOD2 (zNOD2) using various in silico methodologies. The results show that the conserved residues Phe819, Phe871, Trp875, Trp929, Trp899, and Arg845 located at the concave face of zNOD2–LRR confer MDP recognition by hydrophobic and hydrogen bond (H‐bond) interactions. Molecular dynamics simulations reveal a stable association between the electropositive surface on zNOD2–CARDa and the electronegative surface on zRIP2–CARD reinforced mostly by H‐bonds and electrostatic interactions. Importantly, a 3.5 Å salt bridge is observed between Arg60 of zNOD2–CARDa and Asp494 of zRIP2–CARD. Arg11 and Lys53 of zNOD2–CARDa and Ser498 and Glu508 of zRIP2–CARD are critical residues for CARD–CARD interaction and NOD2 signaling. The 2.7 Å H‐bond between Lys104 of the linker and Glu508 of zRIP2–CARD suggests a possible role of the linker for shaping CARD–CARD interaction. These findings are consistent with existing mutagenesis data. We provide first insight into MDP recognition and CARD–CARD interaction in the zebrafish that will be useful to understand the molecular basis of NOD signaling in a broader perspective. Copyright © 2014 John Wiley & Sons, Ltd.     

Ubiquitin Regulates Caspase Recruitment Domain-mediated Signaling by Nucleotide-binding Oligomerization Domain-containing Proteins NOD1 and NOD2

NOD1 and NOD2 (nucleotide-binding oligomerization domain-containing proteins) are intracellular pattern recognition receptors that activate inflammation and autophagy. These pathways rely on the caspase recruitment domains (CARDs) within the receptors, which serve as protein interaction platforms that coordinately regulate immune signaling. We show that NOD1 CARD binds ubiquitin (Ub), in addition to directly binding its downstream targets receptor-interacting protein kinase 2 (RIP2) and autophagy-related protein 16-1 (ATG16L1). NMR spectroscopy and structure-guided mutagenesis identified a small hydrophobic surface of NOD1 CARD that binds Ub. In vitro, Ub competes with RIP2 for association with NOD1 CARD. In vivo, we found that the ligand-stimulated activity of NOD1 with a mutant CARD lacking Ub binding but retaining ATG16L1 and RIP2 binding is increased relative to wild-type NOD1. Likewise, point mutations in the tandem NOD2 CARDs at positions analogous to the surface residues defining the Ub interface on NOD1 resulted in loss of Ub binding and increased ligand-stimulated NOD2 signaling. These data suggest that Ub binding provides a negative feedback loop upon NOD-dependent activation of RIP2.     

Solution structure of NOD1 CARD and mutational analysis of its interaction with the CARD of downstream kinase RICK

NOD1 is a cytosolic signalling host pattern-recognition receptor composed of a caspase-activating and recruitment domain (CARD), a nucleotide-binding and oligomerization domain (NOD) and leucine-rich repeats. It plays a crucial role in innate immunity by activating the NF-kappaB pathway via its downstream effector the kinase RICK (RIP2) following the recognition of a specific bacterial ligand. RICK is recruited by NOD1 through interaction of their respective CARDs. Here we present the high resolution NMR structure of the NOD1 CARD. It is generally similar to other CARDs of known structure, consisting of six tightly packed helices, although the length and orientation of the last helix is unusual. Mutations in both the NOD1 and RICK CARD domains were assayed by immuno-precipitation of cell lysates and in vivo NF-kappaB activation in order to define residues important for CARD-CARD interaction and downstream signalling. The results show that the interaction is critically dependent on three acidic residues on NOD1 CARD and three basic residues on RICK CARD and thus is likely to have a strong electrostatic component, similar to other characterised CARD-CARD interactions.     

Carrageenan induces interleukin-8 production through distinct Bcl10 pathway in normal human colonic epithelial cells

Carrageenan is a high molecular weight sulfated polygalactan used to improve the texture of commercial food products. Its use increased markedly during the last half century, although carrageenan is known to induce inflammation in rheumatological models and in intestinal models of colitis. We performed studies to determine its direct effects on human intestinal cells, including normal human intestinal epithelial cells from colonic surgeries, the normal intestinal epithelial cell line NCM460, and normal rat ileal epithelial cells. Cells were treated with high molecular weight lambda-carrageenan at a concentration of 1 mug/ml for 1-96 h. IL-8, IL-8 promoter activity, total and nuclear NF-kappaB, IkappaBalpha, phospho-IkappaBalpha, and Bcl10 were assessed by immunohistochemistry, Western blot, ELISA, and cDNA microarray. Increased Bcl10, nuclear and cytoplasmic NF-kappaB, IL-8 promoter activation, and IL-8 secretion were detected following carrageenan exposure. Knockdown of Bcl10 by siRNA markedly reduced the increase in IL-8 that followed carrageenan exposure in the NCM460 cells. These results show, for the first time, that exposure of human intestinal epithelial cells to carrageenan triggers a distinct inflammatory pathway via activation of Bcl10 with NF-kappaB activation and upregulation of IL-8 secretion. Since Bcl10 contains a caspase-recruitment domain, similar to that found in NOD2/CARD15 and associated with genetic predisposition to Crohn's disease, the study findings may represent a link between genetic and environmental etiologies of inflammatory bowel disease. Because of the high use of carrageenan as a food additive in the diet, the findings may have clinical significance.     

CD8+ T cells induce thyroid epithelial cell hyperplasia and fibrosis

CD8(+) T cells can be important effector cells in autoimmune inflammation, generally because they can damage target cells by cytotoxicity. This study shows that activated CD8(+) T cells induce thyroid epithelial cell hyperplasia and proliferation and fibrosis in IFN-γ(-/-) NOD.H-2h4 SCID mice in the absence of CD4(+) T cells. Because CD8(+) T cells induce proliferation rather than cytotoxicity of target cells, these results describe a novel function for CD8(+) T cells in autoimmune disease. In contrast to the ability of purified CD8(+) T cells to induce thyrocyte proliferation, CD4(+) T cells or CD8 T cell-depleted splenocytes induced only mild thyroid lesions in SCID recipients. T cells in both spleens and thyroids highly produce TNF-α. TNF-α promotes proliferation of thyrocytes in vitro, and anti-TNF-α inhibits development of thyroid epithelial cell hyperplasia and proliferation in SCID recipients of IFN-γ(-/-) splenocytes. This suggests that targeting CD8(+) T cells and/or TNF-α may be effective for treating epithelial cell hyperplasia and fibrosis.     

NOD2 mutation and mice: no Crohn's disease but many lessons to learn

Many patients with ileal Crohn's disease, a chronic intestinal inflammation, carry mutations in the gene encoding NOD2 (CARD15), but the mechanistic details of how this mutation leads to disease are not fully understood. NOD2 is expressed in antigen-presenting cells and Paneth cells, which are secretory epithelial cells of the small intestine. Two complementary studies using genetically engineered murine models help to explain the association of NOD2 malfunction and mucosal disease. One study observes a dysregulation of proinflammatory responses, suggesting that the most common NOD2 mutation in humans results in a gain of function. The other study determined that NOD2-null mutations impair the Paneth-cell antimicrobial response, which is consistent with recent findings in humans. Together, these studies fuel optimism that new therapeutic directions might emerge to better treat this severe mucosal disease.     

A CARD9 Polymorphism Is Associated with Decreased Likelihood of Persistent Conjugated Hyperbilirubinemia in Intestinal Failure

Recently, genetic associations have been described in intestinal transplants. Namely, Crohn''s disease susceptibility gene NOD2 polymorphisms have been reported to be more prevalent in patients with graft failure following intestinal transplantation (IT). Therefore, we sought to determine if polymorphisms in the NOD2 signaling cascade, including NOD2, CARD9, RAC1 and ATG16L1 are associated with intestinal failure (IF) or its complications. We carried out a cross-sectional study of 59 children with IF and 500 healthy Caucasian controls. Using the Taqman platform we determined the prevalence of NOD2 as well as ATG16L1, RAC1 and CARD9 SNPs. NOD2 pathway polymorphisms were evaluated in relation to outcomes of episodes of sepsis, ICU admissions, hyperbilirubinemia and need for IT. We found that the minor allele of a CARD9 SNP was associated with protection from developing IF when compared to healthy controls and was also associated with decreased odds of sustained conjugated hyperbilirubinemia. Therefore, IF patients with CARD9 polymorphism are less likely to develop progressive liver disease and suggests that host innate immunity may play a role in IF associated liver disease.     

The tandem CARDs of NOD2: intramolecular interactions and recognition of RIP2

Caspase recruitment domains (CARDs) are homotypic protein interaction modules that link the stimulus-dependent assembly of large signaling platforms such as inflammasomes to the activation of downstream effectors that often include caspases and kinases and thereby play an important role in the regulation of inflammatory and apoptotic signaling pathways. NOD2 belongs to the NOD-like (NLR) family of intracellular pattern recognition receptors (PRR) and induces activation of the NF-κB pathway in response to the recognition of bacterial components. This process requires the specific recognition of the CARD of the protein kinase RIP2 by the tandem CARDs of NOD2. Here we demonstrate that the tandem CARDs of NOD2 are engaged in an intramolecular interaction that is important for the structural stability of this region. Using a combination of ITC and pull-down experiments we identify distinct surface areas that are involved in the intramolecular tandem CARD interaction and the interaction with the downstream effector RIP2. Our findings indicate that while CARDa of NOD2 might be the primary binding partner of RIP2 the two CARDs of NOD2 do not act independently of one another but may cooperate to from a binding surface that is distinct from that of single CARDs.     

NOD1 Cooperates with TLR2 to Enhance T Cell Receptor-Mediated Activation in CD8 T Cells

Pattern recognition receptors (PRR), like Toll-like receptors (TLR) and NOD-like receptors (NLR), are involved in the detection of microbial infections and tissue damage by cells of the innate immune system. Recently, we and others have demonstrated that TLR2 can additionally function as a costimulatory receptor on CD8 T cells. Here, we establish that the intracytosolic receptor NOD1 is expressed and functional in CD8 T cells. We show that C12-iEDAP, a synthetic ligand for NOD1, has a direct impact on both murine and human CD8 T cells, increasing proliferation and effector functions of cells activated via their T cell receptor (TCR). This effect is dependent on the adaptor molecule RIP2 and is associated with an increased activation of the NF-κB, JNK and p38 signaling pathways. Furthermore, we demonstrate that NOD1 stimulation can cooperate with TLR2 engagement on CD8 T cells to enhance TCR-mediated activation. Altogether our results indicate that NOD1 might function as an alternative costimulatory receptor in CD8 T cells. Our study provides new insights into the function of NLR in T cells and extends to NOD1 the recent concept that PRR stimulation can directly control T cell functions.     

CARD6 is interferon inducible but not involved in nucleotide-binding oligomerization domain protein signaling leading to NF-kappaB activation

We have previously reported the cloning and characterization of CARD6, a caspase recruitment domain (CARD)-containing protein that is structurally related to the interferon (IFN)-inducible GTPases. CARD6 associates with microtubules and with receptor-interacting protein 2 (RIP2). RIP2 mediates NF-κB activation induced by the intracellular nucleotide-binding oligomerization domain (NOD) receptors that sense bacterial peptidoglycan. Here we report that the expression of CARD6 and RIP2 in bone marrow-derived macrophages is rapidly induced by beta IFN and gamma IFN. This IFN-induced upregulation of CARD6 is suppressed by lipopolysaccharide (LPS), in contrast to LPS's enhancement of IFN-induced RIP2 upregulation. We generated CARD6-deficient (CARD6^−/−) mice and carried out extensive analyses of signaling pathways mediating innate and adaptive immune responses, including the NOD pathways, but did not detect any abnormalities. Moreover, CARD6^−/− mice were just as susceptible as wild-type mice to infection by Salmonella enterica serovar Typhimurium, Listeria monocytogenes, Candida albicans, lymphocytic choriomeningitis virus, or mouse adenovirus type 1. Thus, although structural and in vitro analyses strongly suggest an important role for CARD6 in immune defense, the physiological function of CARD6 remains obscure.     

miR-122 targets NOD2 to decrease intestinal epithelial cell injury in Crohn’s disease

Crohn’s disease (CD) is one of the two major types of inflammatory bowel disease (IBD) thought to be caused by genetic and environmental factors. Recently, miR-122 was found to be deregulated in association with CD progression. However, the underlying molecular mechanisms remain unclear. In the present study, the gene nucleotide-binding oligomerization domain 2 (NOD2/CARD15), which is strongly associated with susceptibility to CD, was identified as a functional target of miR-122. MiR-122 inhibited LPS-induced apoptosis by suppressing NOD2 in HT-29 cells. NOD2 interaction with LPS initiates signal transduction mechanisms resulting in the activation of nuclear factor κB (NF-κB) and the stimulation of downstream pro-inflammatory events. The activation of NF-κB was inhibited in LPS-stimulated HT-29 cells pretreated with miR-122 precursor or NOD2 shRNA. The expression of the pro-inflammatory cytokines TNF-α and IFN-γ was significantly decreased, whereas therelease of the anti-inflammatory cytokines IL-4 and IL-10 was increased in LPS-stimulated HT-29 cells pretreated with miR-122 precursor, NOD2 shRNA or the NF-κB inhibitor QNZ. Taken together, these results indicate that miR-122 and its target gene NOD2 may play an important role in the injury of intestinal epithelial cells induced by LPS.