Elsevier

Current Opinion in Immunology

Volume 32, February 2015, Pages 48-53
Current Opinion in Immunology

Viral RNA detection by RIG-I-like receptors

https://doi.org/10.1016/j.coi.2014.12.012Get rights and content

Highlights

  • RLRs recognize viral non-self RNA with differential molecular machinery.

  • Cytoplasmic aggregates containing RLRs are formed in response to viral dsRNA.

  • Ubiquitin-chains are a critical regulator of RLR-mediated signaling.

In higher vertebrates, recognition of the non-self signature of invading viruses by genome-encoded pattern recognition receptors initiates antiviral innate immunity. Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) detect viral RNA as a non-self pattern in the cytoplasm and activate downstream signaling. Detection of viral RNA also activates stress responses resulting in stress granule-like aggregates, which facilitate RLR-mediated antiviral immunity. Among the three RLR family members RIG-I and melanoma differentiation-associated gene 5 (MDA5) recognize distinct viral RNA species with differential molecular machinery and activate signaling through mitochondrial antiviral signaling (MAVS, also known as IPS-1/VISA/Cardif), which leads to the expression of cytokines including type I and III interferons (IFNs) to restrict viral propagation. In this review, we summarize recent knowledge regarding RNA recognition and signal transduction by RLRs and MAVS/IPS-1.

Introduction

In 2004, RLRs were identified as RNA sensors to trigger innate immune responses against viral infection [1]. Mammalian RLRs are composed of three family members; RIG-I (DDX58), MDA5 (IFIH1) and laboratory of genetics and physiology 2 (LGP2; DHX58), and all are expressed in the cytoplasm of ubiquitous types of cells [2]. These RLRs all share a DExD/H-box RNA helicase domain and a C-terminal domain (CTD), while RIG-I and MDA5, but not LGP2, have a N-terminal caspase recruitment domain (CARD), which is responsible for interacting with a downstream adaptor molecule, MAVS/IPS-1 (Figure 1). The C-terminal RNA helicase and CTD are implicated in the detection of viral RNA, and ATP-dependent conformational change allows CARDs to interact with MAVS/IPS-1. For RIG-I activation, conjugation of Lys63-linked ubiquitin chain (Ubs) by tripartite motif protein 25 (TRIM25) and/or association of unanchored Ubs with CARD are required [3]. A recent in vitro study revealed that MDA5 also interacts with unanchored Ubs [4]. The CARDs accumulated on the mitochondrial surface recruits signaling adaptors and kinases, including IκB kinase (IKK) family kinases, IKKα/β/γ, TBK1 and IKKɛ⋅ IKKα/β/γ activates NF-κB and TBK1 and IKKɛ activates IFN regulatory factor (IRF)-3 and 7. The activated NF-κB and IRF-3/7 can translocate into the nucleus, and interact with the promoter regions of target genes, including IFNs and inflammatory cytokines. Secreted IFNs transmit a signal via cognate receptors and induce the expression of hundreds of IFN-stimulated genes (ISGs), including, double-stranded RNA dependent protein kinase (PKR), 2′-5′-oligoadenylate synthetase (OAS) and RLRs, leading to the establishment of an antiviral state [5]. Here we review recent advances in our knowledge of the molecular machinery for RLR activation and RLR-mediated signal transduction.

Section snippets

RNA recognition and signal activation by RLRs

RIG-I is activated by infection by a variety of RNA viruses, such as influenza A virus (IAV), Newcastle disease virus, Sendai virus, vesicular stomatitis virus (VSV), measles virus (MV), and hepatitis C virus [2, 6]. The non-self signature of these viruses is a 5′-triphosphate (5′ppp)-containing short double-stranded (ds) structure with a complementary end and/or a poly-U/UC rich ds-stretch. Recently, it has been demonstrated that incoming 5′ppp-containing viral RNA with nucleocapsid proteins

Signal activation via aggregate formation of RLRs

Previously, it was suggested that dimerization or oligomerization of RLRs is required for signal activation [14]. Recent in vitro studies proposed a model in which RIG-I, MDA5 and MAVS/IPS-1 signal through multimolecular aggregates (Figure 2). RIG-I forms a complex with dsRNA in a 5′ppp and ATP-dependent, but also 2CARD-independent manner [30, 31]. Since sliding RIG-I on dsRNA was reported [32], ATP hydrolysis-driven translocation may allow RIG-I to form a beads-on-a-string complex on viral

Stress response and RLR signaling

Recent studies demonstrate that infection by various viruses induces the formation of stress granule (SG)-like aggregates, termed antiviral SG (avSG), in the cytoplasm [40]. In many cases, PKR is responsible for sensing viral infection and initiating avSG formation (Figure 2). An avSG contains RLRs and several antiviral molecules, including PKR and OAS as well as viral ribonucleoprotein complex (RNP) [41]. Inhibition of the avSG formation impairs the virus-induced activation of IFN genes,

Regulation of RLRs-mediated signaling by ubiquitin chains

In vitro experiments showed that Lys-63-linked Ub chains are critical for the oligomerization of RIG-I and MDA5. However, the involvement of Lys63-Ubs has been controversial. Although the initial report indicated that ubiquitination of 2CARD at Lys172 by TRIM25 is required for RIG-I activation [3], subsequent studies have shown that RIG-I with Arg172 is fully active [43] and that the interaction between 2CARD and unanchored Ubs is important for the signaling-competent tetramer formation of

Regulation by phosphorylation of RLRs

It was suggested that RIG-I signaling is attenuated by phosphorylation at the 2CARD and CTD of RIG-I by protein kinase C (PKC) α/β and casein kinase II (CKII) respectively [55]. On the other hand, protein phosphatase 1 (PP1), PP1α and PP1γ, directly interact with and dephosphorylate RIG-I and MDA5 and virus-induced signaling [56••]. In the case of MDA5, phosphorylation at the Ser88 residue in 2CARD attenuates MDA5 signaling and it is dephosphorylated by PP1, suggesting a role for the

Concluding remarks

Recent advances in biochemical and structural analysis elegantly elucidates the molecular machinery underlying non-self RNA recognition and signal activation by RLRs and MAVS/IPS-1. Although RIG-I and MDA5 differentially recognize distinct RNA species, both induce filamentous aggregates on dsRNA and the relieved 2CARDs form an oligomer to interact with the prion-like aggregates of MAVS/IPS-1 CARDs. Furthermore, virus-induced SG-like aggregates might also be involved in some parts of these

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

This work was supported by grants from The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, including Innovative Area “Infection competency” (No. 24115004 and 25115503), Scientific Research “A” (No. 23249023) and “B” (No. 26293101), The Ministry of Health, Labor and Welfare (MHLW) of Japan, the Uehara Memorial Foundation, the Mochida Memorial Foundation for Medical and Pharmaceutical Research, the Takeda Science Foundation and the Naito Foundation.

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