NLRX1 is a regulator of mitochondrial antiviral immunity

Abstract

The RIG-like helicase (RLH) family of intracellular receptors detect viral nucleic acid and signal through the mitochondrial antiviral signalling adaptor MAVS (also known as Cardif, VISA and IPS-1) during a viral infection [1][2][3][4][5][6] . MAVS activation leads to the rapid production of antiviral cytokines, including type 1 interferons. Although MAVS is vital to antiviral immunity, its regulation from within the mitochondria remains unknown. Here we describe human NLRX1, a highly conserved nucleotide-binding domain (NBD)-and leucine-rich-repeat (LRR)-containing family member (known as NLR) that localizes to the mitochondrial outer membrane and interacts with MAVS. Expression of NLRX1 results in the potent inhibition of RLH-and MAVS-mediated interferon-b promoter activity and in the disruption of virus-induced RLH-MAVS interactions. Depletion of NLRX1 with small interference RNA promotes virus-induced type I interferon production and decreases viral replication. This work identifies NLRX1 as a check against mitochondrial antiviral responses and represents an intersection of three ancient cellular processes: NLR signalling, intracellular virus detection and the use of mitochondria as a platform for anti-pathogen signalling. This represents a conceptual advance, in that NLRX1 is a modulator of pathogen-associated molecular pattern receptors rather than a receptor, and identifies a key therapeutic target for enhancing antiviral responses.Mammalian members of the nucleotide-binding domain (NBD) and leucine-rich-repeat-containing (LRR) (known as NLR, see http://www.genenames.org/genefamily/nacht.html) family of proteins are indispensable for cellular responses to pathogens. This NBD-LRR protein structure is ancient and highly conserved, as shown by its initial identification among plant disease-resistance proteins 7-12 . Current dogma posits that NLRs function as cytoplasmic surveillance molecules that sense intracellular pathogen-associated molecular patterns (PAMPs), or as regulators of pathogen-initiated signalling cascades 13,14 . Viral PAMPs are detected by the cytoplasmic RLH receptors RIG-I (also known as DDX58) and MDA-5 (also known as IFIH1), which signal through the mitochondrial protein MAVS, resulting in the activation of interferon regulatory factor 3 (IRF3) and NF-kB and type-1 interferon transcription [1][2][3][4][5][6] . Abrogation of MAVS expression or function leads to reduced type 1 interferon production and antiviral protection 15 .To study the potential role of NLR proteins in regulating mitochondrial antiviral signalling, we used bioinformatics to identify NLRs localized to the mitochondria. We identified one putative mitochondrial NLR called NLRX1 (previously known as CLR11.3 and NOD9) 9,16 (Fig. 1a). The predicted peptide sequence and distinct domains of NLRX1 are shown in Supplementary Fig. 1. Consistent with the conserved motif structure of the NLR family, NLRX1 contains a central putative NBD and carboxy-terminal LRRs. The assignment of the amino-terminal effector domain to a subclass i...

Inflammasomes and its importance in viral infections

“…Additionally, Sendai virus infection has been shown to induce MAVS and RIG-1 interaction, which further mediates IRF3 dimer formation. This enhanced interaction between MAVS and RIG-1 and IRF3 dimer formation is attenuated in the presence of NLRX1, further supporting the negative regulatory role of NLRX1 in MAVSmediated antiviral responses [102]. However, the interaction between NLRX1 and MAVS remains doubtful owing to the localisation of these two proteins in distinct mitochondrial subdomains that are separated by two membranes.…”

“…NLRX1 belongs to the family of NLR that contains the N-terminal domain with mitochondrion-targeting sequence, followed by NACHT and LRRs [102,103]. NLRXI is unique among NLRs due to its localisation to mitochondria that probably makes the NLRX1 as a link between mitochondria function and innate immunity.…”

Cellular and molecular regulation of innate inflammatory responses

“…Upon RNA viral stimulation, RIG-I could interact with adaptor ASC to trigger caspase-1-dependent inflammasome activation and IL-1β maturation. 34 On the contrary, some NLR members play negative roles in regulating RLR-mediated type I IFN responses via targeting distinct signaling molecules: for NLR family CARD domaincontaining protein 5 (NLRC5) via interaction with RIG-I and MDA5; 96 for NLR family member X1 (NLRX1) via interaction with MAVS and disruption of RLR-MAVS interactions; 97,98 for NLRC3 via impeding the interaction between STING and TBK1 interaction; 99 for NLRP4 via targeting TBK1 for degradation. 100 NOD proteins and TLRs are both critical for host defense against bacterial infection.…”

Beyond the inflammasome: regulatory NOD-like receptor modulation of the host immune response following virus exposure

“…While some ligands have been identified, it is highly likely that the full repertoire of viral PAMPs that signal through NLRX1 have yet to be fully elucidated. It is considered unique among the NLR family as it has been found to be associated with the mitochondria in multiple studies (Arnoult et al, 2009;Moore et al, 2008;Tattoli et al, 2008). Initially, Moore et al (2008) showed that NLRX1 is most likely associated with the mitochondrial outer membrane (Moore et al, 2008).…”

“…This subgroup is currently composed of three members, NLRP12, NLRX1 and NLRC3. Each of these NLRs modulates diverse signalling pathways, including NF-k B and mitogen-activate protein kinase (MAPK) signalling, the type I IFN response, autophagy and the generation of reactive oxygen species (ROS) (Allen et al, 2011(Allen et al, , 2012bLei et al, 2013;Moore et al, 2008;Tattoli et al, 2008).…”