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      RIG-I-like receptors: their regulation and roles in RNA sensing

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          Abstract

          Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are key sensors of virus infection, mediating the transcriptional induction of type I interferons and other genes that collectively establish an antiviral host response. Recent studies have revealed that both viral and host-derived RNAs can trigger RLR activation; this can lead to an effective antiviral response but also immunopathology if RLR activities are uncontrolled. In this Review, we discuss recent advances in our understanding of the types of RNA sensed by RLRs in the contexts of viral infection, malignancies and autoimmune diseases. We further describe how the activity of RLRs is controlled by host regulatory mechanisms, including RLR-interacting proteins, post-translational modifications and non-coding RNAs. Finally, we discuss key outstanding questions in the RLR field, including how our knowledge of RLR biology could be translated into new therapeutics.

          Abstract

          The RNA-sensing retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) are important inducers of type I interferons and other antiviral immune mediators. Here, Jan Rehwinkel and Michaela Gack explain how members of the RLR family are regulated and reflect on the importance of the RLRs in viral infection, autoimmunity and cancer.

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          Most cited references156

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          RIG-I-mediated antiviral responses to single-stranded RNA bearing 5'-phosphates.

          Double-stranded RNA (dsRNA) produced during viral replication is believed to be the critical trigger for activation of antiviral immunity mediated by the RNA helicase enzymes retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). We showed that influenza A virus infection does not generate dsRNA and that RIG-I is activated by viral genomic single-stranded RNA (ssRNA) bearing 5'-phosphates. This is blocked by the influenza protein nonstructured protein 1 (NS1), which is found in a complex with RIG-I in infected cells. These results identify RIG-I as a ssRNA sensor and potential target of viral immune evasion and suggest that its ability to sense 5'-phosphorylated RNA evolved in the innate immune system as a means of discriminating between self and nonself.
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            Influenza A virus NS1 targets the ubiquitin ligase TRIM25 to evade recognition by the host viral RNA sensor RIG-I.

            The ubiquitin ligase TRIM25 mediates Lysine 63-linked ubiquitination of the N-terminal CARD domains of the viral RNA sensor RIG-I to facilitate type I interferon (IFN) production and antiviral immunity. Here, we report that the influenza A virus nonstructural protein 1 (NS1) specifically inhibits TRIM25-mediated RIG-I CARD ubiquitination, thereby suppressing RIG-I signal transduction. A novel domain in NS1 comprising E96/E97 residues mediates its interaction with the coiled-coil domain of TRIM25, thus blocking TRIM25 multimerization and RIG-I CARD domain ubiquitination. Furthermore, a recombinant influenza A virus expressing an E96A/E97A NS1 mutant is defective in blocking TRIM25-mediated antiviral IFN response and loses virulence in mice. Our findings reveal a mechanism by which influenza virus inhibits host IFN response and also emphasize the vital role of TRIM25 in modulating antiviral defenses.
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              Structural basis for the activation of innate immune pattern-recognition receptor RIG-I by viral RNA.

              RIG-I is a key innate immune pattern-recognition receptor that triggers interferon expression upon detection of intracellular 5'triphosphate double-stranded RNA (5'ppp-dsRNA) of viral origin. RIG-I comprises N-terminal caspase activation and recruitment domains (CARDs), a DECH helicase, and a C-terminal domain (CTD). We present crystal structures of the ligand-free, autorepressed, and RNA-bound, activated states of RIG-I. Inactive RIG-I has an open conformation with the CARDs sequestered by a helical domain inserted between the two helicase moieties. ATP and dsRNA binding induce a major rearrangement to a closed conformation in which the helicase and CTD bind the blunt end 5'ppp-dsRNA with perfect complementarity but incompatibly with continued CARD binding. We propose that after initial binding of 5'ppp-dsRNA to the flexibly linked CTD, co-operative tight binding of ATP and RNA to the helicase domain liberates the CARDs for downstream signaling. These findings significantly advance our molecular understanding of the activation of innate immune signaling helicases. Copyright © 2011 Elsevier Inc. All rights reserved.
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                Author and article information

                Contributors
                jan.rehwinkel@imm.ox.ac.uk
                mgack@uchicago.edu
                Journal
                Nat Rev Immunol
                Nat. Rev. Immunol
                Nature Reviews. Immunology
                Nature Publishing Group UK (London )
                1474-1733
                1474-1741
                13 March 2020
                : 1-15
                Affiliations
                [1 ]ISNI 0000 0004 1936 8948, GRID grid.4991.5, Medical Research Council Human Immunology Unit, Medical Research Council Weatherall Institute of Molecular Medicine, Radcliffe Department of Medicine, , University of Oxford, ; Oxford, UK
                [2 ]ISNI 0000 0004 1936 7822, GRID grid.170205.1, Department of Microbiology, , The University of Chicago, ; Chicago, IL USA
                Author information
                http://orcid.org/0000-0003-3841-835X
                http://orcid.org/0000-0002-2163-2598
                Article
                288
                10.1038/s41577-020-0288-3
                7094958
                31792373
                5fde3bfe-6705-4376-8b67-95eaa10072e5
                © Springer Nature Limited 2020

                This article is made available via the PMC Open Access Subset for unrestricted research re-use and secondary analysis in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the World Health Organization (WHO) declaration of COVID-19 as a global pandemic.

                History
                : 4 February 2020
                Categories
                Review Article

                viral infection,innate immunity,rig-i-like receptors

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