Encephalomyocarditis Virus Viroporin 2B Activates NLRP3 Inflammasome Minako Ito, Yusuke Yanagi, Takeshi Ichinohe* Department of Virology, Faculty of Medicine, Kyushu University, Maidashi, Higashi-ku, Fukuoka, Japan Abstract Nod-like receptors (NLRs) comprise a large family of intracellular pattern- recognition receptors. Members of the NLR family assemble into large multiprotein complexes, termed the inflammasomes. The NLR family, pyrin domain-containing 3 (NLRP3) is triggered by a diverse set of molecules and signals, and forms the NLRP3 inflammasome. Recent studies have indicated that both DNA and RNA viruses stimulate the NLRP3 inflammasome, leading to the secretion of interleukin 1 beta (IL-1b) and IL-18 following the activation of caspase-1. We previously demonstrated that the proton-selective ion channel M2 protein of influenza virus activates the NLRP3 inflammasome. However, the precise mechanism by which NLRP3 recognizes viral infections remains to be defined. Here, we demonstrate that encephalomyocarditis virus (EMCV), a positive strand RNA virus of the family Picornaviridae, activates the NLRP3 inflammasome in mouse dendritic cells and macrophages. Although transfection with RNA from EMCV virions or EMCV-infected cells induced robust expression of type I interferons in macrophages, it failed to stimulate secretion of IL-1b. Instead, the EMCV viroporin 2B was sufficient to cause inflammasome activation in lipopolysaccharide-primed macrophages. While cells untransfected or transfected with the gene encoding the EMCV non-structural protein 2A or 2C expressed NLRP3 uniformly throughout the cytoplasm, NLRP3 was redistributed to the perinuclear space in cells transfected with the gene encoding the EMCV 2B or influenza virus M2 protein. 2B proteins of other picornaviruses, poliovirus and enterovirus 71, also caused the NLRP3 redistribution. Elevation of the intracellular Ca2+ level, but not mitochondrial reactive oxygen species and lysosomal cathepsin B, was important in EMCV-induced NLRP3 inflammasome activation. Chelation of extracellular Ca2+ did not reduce virus-induced IL-1b secretion. These results indicate that EMCV activates the NLRP3 inflammasome by stimulating Ca2+ flux from intracellular storages to the cytosol, and highlight the importance of viroporins, transmembrane pore-forming viral proteins, in virus-induced NLRP3 inflammasome activation. Citation: Ito M, Yanagi Y, Ichinohe T (2012) Encephalomyocarditis Virus Viroporin 2B Activates NLRP3 Inflammasome. PLoS Pathog 8(8): e1002857. doi:10.1371/ journal.ppat.1002857 Editor: Blossom Damania, University of North Carolina at Chapel Hill, United States of America Received December 22, 2011; Accepted June 26, 2012; Published August 9, 2012 Copyright: ß 2012 Ito et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by grants from the Ministry of Education, Culture, Sports, Science, and Technology, and the Ministry of Health, Labor, and Welfare of Japan; the Kanae Foundation for the Promotion of Medical Science; and Takeda Science Foundation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: [email protected] Introduction receptor or tumor necrosis factor receptor (signal 1), in addition to the activation of caspase-1 through inflammasome activation The innate immune system, the first line of defense against (signal 2) [7,8]. In influenza virus infection, the signal 1 is provided pathogens, utilizes pattern-recognition receptors (PRRs) to detect by TLR7 that recognizes influenza virus RNA, whereas the signal pathogen-associated molecular patterns (PAMPs). RNA viruses are 2 comes from the function of the virus-encoded proton-selective detected by host PRRs including Toll-like receptors (TLRs), ion channel M2 protein, but not from viral RNA [9]. retinoic acid-inducible gene-I (RIG-I)-like helicases (RLHs), and Encephalomyocarditis virus (EMCV), a member of the genus Nod-like receptor (NLR) family, pyrin domain-containing 3 Cardiovirus within the family Picornaviridae, is a nonenveloped, (NLRP3) [1,2,3,4]. positive single-stranded RNA virus. This virus has a ,7.8 kb viral NLRP3 plays an important role in the secretion of proin- genome covalently linked to a viral protein VPg at its 59 end that flammatory cytokines interleukin 1 beta (IL-1b) and IL-18 after serves as a primer for viral RNA synthesis. The viral genome viral infections. Upon activation, NLRP3 forms the protein encodes a polyprotein precursor, which is divided into the P1, P2 complex termed ‘‘NLRP3 inflammasome’’ by recruiting the and P3 regions and processed mainly by the virus-encoded 3C apoptosis-associated speck-like protein containing a caspase protease. Processing of the P1 region produces the structural recruitment domain (ASC) and pro-caspase-1, which is activated capsid proteins 1A (VP4), 1B (VP2), 1C (VP3), and 1D (VP1), by autocatalytic cleavage within the complex [5]. The active whereas the P2 and P3 regions are processed into the nonstruc- caspase-1 catalyzes proteolytic processing of pro-IL-1b and pro- tural proteins 2A, 2B, 2C, 3A, 3B (VPg), 3C and 3D as well as IL-18 into active cytokines that are then released across the plasma cleavage intermediates (2BC, 3AB, and 3CD) [10]. The EMCV membrane by poorly understood mechanisms [6]. Secretion of 2A protein has been implicated in the shutoff of host protein these two cytokines requires upregulations of pro-IL-1b, pro-IL- synthesis and viral pathogenesis [11,12], but little is known about 18, and NLRP3, which are induced by signals from TLRs, IL-1 the roles of the EMCV 2B and 2C proteins. Picornavirus 2B PLoS Pathogens | www.plospathogens.org 1 August 2012 | Volume 8 | Issue 8 | e1002857 NLRP3 Inflammasome Activation by EMCV Author Summary peptide inhibitor of caspase-1, without affecting the secretion of IL-6 in both bone marrow-derived dendritic cells (BMDCs) and The innate immune system, the first line of defense against BMMs (Figure 1D and Figure S1). To dissect the importance of invading pathogens, plays a key role not only in limiting various components of the inflammasome complex in IL-1b microbe replications at early stages of infection, but also in secretion in response to EMCV infection, we generated initiating and orchestrating antigen-specific adaptive RAW264.7 cells stably expressing short hairpin RNA (shRNA) immune responses. The innate immune responses against against murine NLRP3, ASC or caspase-1. Quantitative RT-PCR viruses usually rely on recognition of viral nucleic acids by and Western blot analysis confirmed knockdown of NLRP3, ASC, host pattern-recognition receptors such as Toll-like recep- and caspase-1 at the mRNA and protein levels (Figure S2A–B). tors and cytosolic helicases. In addition, recent studies Furthermore, these cells produced comparable levels of IL-6 after have indicated that certain viruses activate the NLRP3 LPS stimulation to EGFP-knockdown cells (Figure S2C), indicat- inflammasome, a multiprotein complex containing the ing that innate immunological responses are not generally affected intracellular pattern-recognition receptor NLRP3, which in by the knockdown. Like that after influenza virus infection [9], IL- turn induces secretion of proinflammatory cytokines. We b have previously revealed the role of the NLRP3 inflamma- 1 secretion after EMCV infection was found to be dependent on some in innate recognition of influenza virus, in which the NLRP3, ASC and caspase-1 (Figure 1E), indicating that EMCV influenza virus proton-selective ion channel M2 protein, infection activates the NLRP3 inflammasome. This is consistent with previous studies demonstrating that EMCV-induced IL-1b but not viral RNA, is required. Here, we demonstrate that 2/2 2/2 another RNA virus, encephalomyocarditis virus (EMCV), production was abrogated in NLRP3 , ASC , and caspase- 2/2 also activates the NLRP3 inflammasome in a viral RNA- 1 BMDCs [19,20]. independent manner. Instead, the EMCV viroporin 2B, which is involved in Ca2+ flux from intracellular storages Viral RNA is insufficient to trigger NLRP3-mediated IL-1b into the cytosol, activates the NLRP3 inflammasome. Our secretion results highlight the importance of viroporins, virus- We next examined whether viral replication is required for encoded transmembrane pore-forming proteins, in recog- NLRP3 inflammasome activation by EMCV. Unlike untreated nition of virus infections by NLRP3. virions, UV-irradiated EMCV failed to induce IL-1b secretion from LPS-primed BMDCs (Figure 2A), indicating that the viral proteins have been reported to act as viroporins, transmembrane particles or genomic RNAs per se are insufficient to activate the pore-forming viral proteins that alter the membrane permeability NLRP3 inflammasome. To test whether viral RNA translation is to ions by forming membrane channels, and participate in a range needed to elicit EMCV-induced NLRP3 inflammasome activa- of viral functions [13]. The avian encephalomyelitis virus (AEV) tion, LPS- primed BMDCs were treated with a translation 2C protein is known to induce apoptosis [10,14]. inhibitor cycloheximide (CHX) prior to stimulation with ATP or EMCV is detected by at least three classes of PRRs in the host infection