ReviewRIG-I-like receptors: Sensing and responding to RNA virus infection
Introduction
The past decade has witnessed tremendous progress in our understanding of the innate host response to infection and the pattern recognition receptors (PRRs) that sense and respond to infectious pathogens. Three major classes of PRRs are involved in the detection of invading pathogens: nucleotide-oligomerization domain (NOD)-like receptors, Toll-like receptors (TLRs), and retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) [1], [2], [3]. These sensors are part of a first line of immune defense that sense exogenous pathogens via pathogen-associated molecular patterns (PAMPs), leading to the production of pro-inflammatory cytokines and type I interferons (IFNs). Type I IFN production provokes swift eradication of invading pathogens by stimulating both antiviral immunity and triggering adaptive immunity.
TLRs are transmembrane proteins implicated in the detection of a vast range of microbial pathogens including viruses, bacteria, protozoa, and fungi. TLR signaling is achieved by the recognition of specific PAMPs via extracellular leucine-rich repeat (LRR) motifs that transmit signals through the cytoplasmic Toll-interleukin (IL)-1 receptor (TIR) domain [4], [5]. Among the TLR family, a subgroup of endosome localized TLRs (TLR3, 7, 8, 9) detect nucleic acids particularly viral DNA/RNA and can be distinguished from surface expressed TLRs (TLR1, 2, 4, 5, 6, 10) that recognize bacterial and fungal cell wall components, as well as some viral proteins [6]. TLR3 is known to detect double-stranded (ds) RNA, while TLR7 and TLR8 recognize single-stranded (ss) RNA and TLR9 recognizes unmethylated DNA with CpG motifs [7], [8], [9], [10]. Endosomal TLRs, upon binding to their respective ligands, recruit downstream adaptor molecules such as MyD88 and TIR domain-containing adaptor inducing IFN-β (TRIF) leading to NF-κB and IFN activation [11]. Among the cytoplasmic PRRs, NLRs are known to detect cytosolic microbial components and “danger” signals (such as ATP and toxins) through their characteristic C-terminal LRR and internal nucleotide-binding domain (NBD), leading to the activation of the inflammasome, a large multiprotein complex whose assembly activates caspase-1, promoting the maturation of pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18 [1], [12].
Recently, several groups have identified another cytoplasmic sensor, the PYHIN (pyrin and HIN domain-containing protein) family member absent in melanoma 2 (AIM2) as the key sensor for cytoplasmic dsDNA [13], [14], [15], [16]. Cytoplasmic DNA triggers formation of the AIM2 inflammasome by inducing AIM2 oligomerization, leading to activation of the (apoptosis-associated speck-like protein containing a CARD) ASC pyroptosome and caspase-1 [14].
Distinct from the TLR pathways, RIG-I-like receptors (RLRs) – the retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene-5 (MDA-5) – are novel cytoplasmic RNA helicases that recognize viral RNA present within the cytoplasm. Although both TLR7 and TLR9 are critical for recognition of viral nucleic acids in the endosomes of plasmacytoid dendritic cells (pDCs), most other cell types recognize viral RNA intermediates through the RLR arm of the innate immune response [17], [18], [19]. This review will focus on how RLRs detect viral RNA, discriminate between exogenous viral and endogenous self-RNA, and how the host regulates the antiviral response by modulation of RLR-mediated signaling.
Section snippets
Recognition of RNA viruses by RLRs
RIG-I and MDA-5 are closely related proteins that belong to the DExD/H box RNA helicase family and contain two amino (N)-terminal caspase recruitment domains (CARDs), a central ATPase and helicase domain and a carboxy C-terminal regulatory domain [17] (Fig. 1). The importance of the RIG-I pathway in antiviral immunity was confirmed with the generation of RIG-I-deficient mice [20], revealing that RIG-I and not the TLR system played an essential role in the IFN antiviral response in most cell
Molecular structure of RIG-I
RIG-I protein is present in the cytoplasm in an inactive form and is activated by viral infection or transfection of dsRNA. A mutant of RIG-I containing only the N-terminal CARD domain functions as a constitutive activator that induces IFN production in the absence of viral infection, whereas a CARD-deficient mutant of RIG-I functions as a dominant negative inhibitor [17], [35]. Mutagenesis of the adenosine triphosphate (ATP)-binding residue of the helicase domain from lysine to alanine (K270A)
RLR family members MDA-5 and LGP2
Structurally, MDA-5 shows a 23% aa homology to N-terminal tandem CARD domains of RIG-I and a 35% aa homology to the helicase domain of RIG-I (Fig. 1). In contrast, the C-terminal end of MDA-5 does not contain a RD and shows no autoinhibitory function [18], [36]. As mentioned previously, generation of RIG-I and MDA-5 knockout mice demonstrated that these sensors detect different viruses [20], [21]. In addition, a genome-wide association study of nonsynonymous SNPs identified MDA-5 as a gene
RIG-I signal transduction through the mitochondrial MAVS adaptor
The adaptor molecule that provides a link between RIG-I sensing of incoming viral RNA and downstream activation events was independently elucidated as mitochondrial antiviral signaling adaptor (MAVS), also known as (IPS-1/VISA/Cardif) [38], [39], [40], [41]. MAVS consists of an amino-terminal CARD domain, a proline-rich region (PRR) in the middle of the protein, and a C-terminal transmembrane domain (TM) that localized MAVS to the mitochondrial membrane, suggesting a critical function for the
TRAF3
The activation of RIG-I/MDA-5 ultimately leads to the TM-dependent dimerization of the MAVS N-terminal CARD domains, thereby providing an interface for direct binding to and activation of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) family members that are involved in both the IFN and NF-κB arms of the innate immune response [56], [57]. MAVS regulation of type I IFN induction is achieved by direct and specific interaction between the TRAF domain of TRAF3 and a
Regulation of RLR signaling by ubiquitination
Activation of RLRs results in the dissemination of an antiviral cascade necessary to combat invading pathogens [76]. Thus, limiting the intensity and duration RLR signaling is essential to prevent this protective response from causing injury to the host. Recent studies have highlighted the importance of ubiquitination in modulating the innate immune response in response to invading pathogens in both the TLR and RLR signaling pathways. TRIM25α, a member of the tripartite motif (TRIM) protein
Conclusions and perspectives
Studies in this rapidly moving field over the past 5 years have defined a dynamic cystolic recognition pathway that triggers a robust innate immune response against RNA virus infection. Also emerging as rapidly are details of the strategies used by different viruses to thwart these defense mechanisms. Many questions still remain unanswered: what are the molecular structures of RNAs recognized by MDA-5; how does RLR signaling access viral ribonucleoprotein complexes following infection; what is
Acknowledgements
This research was supported by grants from Canadian Institutes of Health Research, the National Cancer Institute of Canada, with the support of the Canadian Cancer Society and CANFAR, the Canadian Foundation for AIDS Research. JH was supported by a CIHR Senior Investigator award and PN by a Doctoral Fellowship from FRSQ.
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