Etienne Meylan

Cardif is an adaptor protein in the RIG-I antiviral pathway and is targeted by hepatitis C virus.

Antiviral immunity against a pathogen is mounted upon recognition by the host of virally associated structures. One of these viral ‘signatures’, double-stranded (ds) RNA, is a replication product of most viruses within infected cells and is sensed by Toll-like receptor 3 (TLR3) and the recently identified cytosolic RNA helicases RIG-I (retinoic acid inducible gene I, also known as Ddx58) and Mda5 (melanoma differentiation-associated gene 5, also known as Ifih1 or Helicard). Both helicases detect dsRNA, and through their protein-interacting CARD domains, relay an undefined signal resulting in the activation of the transcription factors interferon regulatory factor 3 (IRF3) and NF-κB. Here we describe Cardif, a new CARD-containing adaptor protein that interacts with RIG-I and recruits IKKα, IKKβ and IKKɛ kinases by means of its C-terminal region, leading to the activation of NF-κB and IRF3. Overexpression of Cardif results in interferon-β and NF-κB promoter activation, and knockdown of Cardif by short interfering RNA inhibits RIG-I-dependent antiviral responses. Cardif is targeted and inactivated by NS3-4A, a serine protease from hepatitis C virus known to block interferon-β production. Cardif thus functions as an adaptor, linking the cytoplasmic dsRNA receptor RIG-I to the initiation of antiviral programmes.

Intracellular pattern recognition receptors in the host response

The innate immune system relies on its capacity to rapidly detect invading pathogenic microbes as foreign and eliminate them. Indeed, Toll-like receptors are a class of membrane receptors that sense extracellular microbes and trigger anti-pathogen signalling cascades. Recently, intracellular microbial sensors have also been identified, including NOD-like receptors and the helicase-domain-containing antiviral proteins RIG-I and MDA5. Some of these cytoplasmic molecules sense microbial, as well as non-microbial, danger signals, but the mechanisms of recognition used by these sensors remain poorly understood. Nonetheless, it is apparent that these proteins are likely to have critical roles in health and disease.

Systematic RNA interference reveals that oncogenic KRAS -driven cancers require TBK1

The proto-oncogene KRAS is mutated in a wide array of human cancers, most of which are aggressive and respond poorly to standard therapies. Although the identification of specific oncogenes has led to the development of clinically effective, molecularly targeted therapies in some cases, KRAS has remained refractory to this approach. A complementary strategy for targeting KRAS is to identify gene products that, when inhibited, result in cell death only in the presence of an oncogenic allele. Here we have used systematic RNA interference to detect synthetic lethal partners of oncogenic KRAS and found that the non-canonical IκB kinase TBK1 was selectively essential in cells that contain mutant KRAS. Suppression of TBK1 induced apoptosis specifically in human cancer cell lines that depend on oncogenic KRAS expression. In these cells, TBK1 activated NF-κB anti-apoptotic signals involving c-Rel and BCL-XL (also known as BCL2L1) that were essential for survival, providing mechanistic insights into this synthetic lethal interaction. These observations indicate that TBK1 and NF-κB signalling are essential in KRAS mutant tumours, and establish a general approach for the rational identification of co-dependent pathways in cancer.

Identification of Bacterial Muramyl Dipeptide as Activator of the NALP3/Cryopyrin Inflammasome

Activation of caspase-1 and subsequent processing and secretion of the pro-inflammatory cytokine IL-1beta is triggered upon assembly of the inflammasome complex. It is generally believed that bacterial lipopolysaccharides (LPS) are activators of the inflammasome through stimulation of Toll-like receptor 4 (TLR4). Like TLRs, NALP3/Cryopyrin, which is a key component of the inflammasome, contains Leucine-Rich-Repeats (LRRs). LRRs are frequently used to sense bacterial components, thus raising the possibility that bacteria directly activate the inflammasome. Here, we show that bacterial peptidoglycans (PGN), but surprisingly not LPS, induce NALP3-mediated activation of caspase-1 and maturation of proIL-1beta. Activation is independent of TLRs because the PGN degradation product muramyl dipeptide (MDP), which is not sensed by TLRs, is the minimal-activating structure. Macrophages from a patient with Muckle-Wells syndrome, an autoinflammatory disease associated with mutations in the NALP3/Cryopyrin gene, show increased IL-1beta secretion in the presence of MDP. The activation of the NALP3-inflammasome by MDP may be the basis of the potent adjuvant activity of MDP.

Requirement for NF-kappaB signalling in a mouse model of lung adenocarcinoma

NF-κB transcription factors function as crucial regulators of inflammatory and immune responses as well as of cell survival. They have also been implicated in cellular transformation and tumorigenesis. However, despite extensive biochemical characterization of NF-κB signalling during the past twenty years, the requirement for NF-κB in tumour development in vivo, particularly in solid tumours, is not completely understood. Here we show that the NF-κB pathway is required for the development of tumours in a mouse model of lung adenocarcinoma. Concomitant loss of p53 (also known as Trp53) and expression of oncogenic Kras(G12D) resulted in NF-κB activation in primary mouse embryonic fibroblasts. Conversely, in lung tumour cell lines expressing Kras(G12D) and lacking p53, p53 restoration led to NF-κB inhibition. Furthermore, the inhibition of NF-κB signalling induced apoptosis in p53-null lung cancer cell lines. Inhibition of the pathway in lung tumours in vivo, from the time of tumour initiation or after tumour progression, resulted in significantly reduced tumour development. Together, these results indicate a critical function for NF-κB signalling in lung tumour development and, further, that this requirement depends on p53 status. These findings also provide support for the development of NF-κB inhibitory drugs as targeted therapies for the treatment of patients with defined mutations in Kras and p53.

TRADD Protein Is an Essential Component of the RIG-like Helicase Antiviral Pathway

Upon detection of viral RNA, the helicases RIG-I and/or MDA5 trigger, via their adaptor Cardif (also known as IPS-1, MAVS, or VISA), the activation of the transcription factors NF-kappaB and IRF3, which collaborate to induce an antiviral type I interferon (IFN) response. FADD and RIP1, known as mediators of death-receptor signaling, are implicated in this antiviral pathway; however, the link between death-receptor and antiviral signaling is not known. Here we showed that TRADD, a crucial adaptor of tumor necrosis factor receptor (TNFRI), was important in RIG-like helicase (RLH)-mediated signal transduction. TRADD is recruited to Cardif and orchestrated complex formation with the E3 ubiquitin ligase TRAF3 and TANK and with FADD and RIP1, leading to the activation of IRF3 and NF-kappaB. Loss of TRADD prevented Cardif-dependent activation of IFN-beta, reduced the production of IFN-beta in response to RNA viruses, and enhanced vesicular stomatitis virus replication. Thus, TRADD is not only an essential component of proinflammatory TNFRI signaling, but is also required for RLH-Cardif-dependent antiviral immune responses.

DAI/ZBP1 recruits RIP1 and RIP3 through RIP homotypic interaction motifs to activate NF-κB

Detection of viral nucleic acids is central to antiviral immunity. Recently, DAI/ZBP1 (DNA‐dependent activator of IRFs/Z‐DNA binding protein 1) was identified as a cytoplasmic DNA sensor and shown to activate the interferon regulatory factor (IRF) and nuclear factor‐kappa B (NF‐κB) transcription factors, leading to type‐I interferon production. DAI‐induced IRF activation depends on TANK‐binding kinase 1 (TBK1), whereas signalling pathways and molecular components involved in NF‐κB activation remain elusive. Here, we report the identification of two receptor‐interacting protein (RIP) homotypic interaction motifs (RHIMs) in the DAI protein sequence, and show that these domains relay DAI‐induced NF‐κB signals through the recruitment of the RHIM‐containing kinases RIP1 and RIP3. We show that knockdown of not only RIP1, but also RIP3 affects DAI‐induced NF‐κB activation. Importantly, RIP recruitment to DAI is inhibited by the RHIM‐containing murine cytomegalovirus (MCMV) protein M45. These findings delineate the DAI signalling pathway to NF‐κB and suggest a possible new immune modulation strategy of the MCMV.

RIP4 (DIK/PKK), a novel member of the RIP kinase family, activates NF-κB and is processed during apoptosis

RIP1 and its homologs, RIP2 and RIP3, form part of a family of Ser/Thr kinases that regulate signal transduction processes leading to NF‐κB activation. Here, we identify RIP4 (DIK/PKK) as a novel member of the RIP kinase family. RIP4 contains an N‐terminal RIP‐like kinase domain and a C‐terminal region characterized by the presence of 11 ankyrin repeats. Overexpression of RIP4 leads to activation of NF‐κB and JNK. Kinase inactive RIP4 or a truncated version containing the ankyrin repeats have a dominant negative (DN) effect on NF‐κB induction by multiple stimuli. RIP4 binds to several members of the TRAF protein family, and DN versions of TRAF1, TRAF3 and TRAF6 inhibit RIP4‐induced NF‐κB activation. Moreover, RIP4 is cleaved after Asp340 and Asp378 during Fas‐induced apoptosis. These data suggest that RIP4 is involved in NF‐κB and JNK signaling and that caspase‐dependent processing of RIP4 may negatively regulate NF‐κB‐dependent pro‐survival or pro‐inflammatory signals.

Nonstructural protein 3‐4A: the Swiss army knife of hepatitis C virus

Summary.  Hepatitis C virus (HCV) nonstructural protein 3‐4A (NS3‐4A) is a complex composed of NS3 and its cofactor NS4A. It harbours serine protease as well as NTPase/RNA helicase activities and is essential for viral polyprotein processing, RNA replication and virion formation. Specific inhibitors of the NS3‐4A protease significantly improve sustained virological response rates in patients with chronic hepatitis C when combined with pegylated interferon‐α and ribavirin. The NS3‐4A protease can also target selected cellular proteins, thereby blocking innate immune pathways and modulating growth factor signalling. Hence, NS3‐4A is not only an essential component of the viral replication complex and prime target for antiviral intervention but also a key player in the persistence and pathogenesis of HCV. This review provides a concise update on the biochemical and structural aspects of NS3‐4A, its role in the pathogenesis of chronic hepatitis C and the clinical development of NS3‐4A protease inhibitors.

Intracellular Trafficking of Interleukin-1 Receptor I Requires Tollip

Interleukin-1 receptor (IL-1RI) is a master regulator of inflammation and innate immunity. When triggered by IL-1beta, IL-1RI aggregates with IL-1R-associated protein (IL-1RAcP) and forms a membrane proximal signalosome that potently activates downstream signaling cascades. IL-1beta also rapidly triggers endocytosis of IL-1RI. Although internalization of IL-1RI significantly impacts signaling, very little is known about trafficking of IL-1RI and therefore about precisely how endocytosis modulates the overall cellular response to IL-1beta. Upon internalization, activated receptors are often sorted through endosomes and delivered to lysosomes for degradation. This is a highly regulated process that requires ubiquitination of cargo proteins as well as protein-sorting complexes that specifically recognize ubiquitinated cargo. Here, we show that IL-1beta induces ubiquitination of IL-1RI and that via these attached ubiquitin groups, IL-1RI interacts with the ubiquitin-binding protein Tollip. By using an assay to follow trafficking of IL-1RI from the cell surface to late endosomes and lysosomes, we demonstrate that Tollip is required for sorting of IL-1RI at late endosomes. In Tollip-deficient cells and cells expressing only mutated Tollip (incapable of binding IL-1RI and ubiquitin), IL-1RI accumulates on late endosomes and is not efficiently degraded. Furthermore, we show that IL-1RI interacts with Tom1, an ubiquitin-, clathrin-, and Tollip-binding protein, and that Tom1 knockdown also results in the accumulation of IL-1RI at late endosomes. Our findings suggest that Tollip functions as an endosomal adaptor linking IL-1RI, via Tom1, to the endosomal degradation machinery.