MDA5 and LGP2: accomplices and antagonists of antiviral signal transduction

Abstract

Mammalian cells have the ability to recognize virus infection and mount a powerful antiviral transcriptional response that provides an initial barrier to replication and impacts both innate and adaptive immune responses. Retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) proteins mediate intracellular virus recognition and are activated by viral RNA ligands to induce antiviral signal transduction. While the mechanisms of RIG-I regulation are already well understood, less is known about the more enigmatic melanoma differentiation-associated 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2). Emerging evidence suggests that these two RLRs are intimately associated as both accomplices and antagonists of antiviral signal transduction.

FIG 1

RLR family proteins in antiviral signaling. (A) Diagram illustrating key features of RIG-I, MDA5, and LGP2 mentioned in the text. The three RLRs are composed of a central DECH box helicase domain containing conserved domain 1 (encompassing helicase motifs Q, I, II, and III) and domain 2 (encompassing helicase motifs IV, V, and VI), both of which are essential to coordinate RNA binding and ATP hydrolysis activities. A C-terminal domain (CTD) is required for RNA terminus recognition and is involved in autoregulation of RIG-I. RIG-I and MDA5 contain tandem caspase activation and recruitment domain (CARD) regions at their N termini, which are essential for downstream signaling activity. The position of the minimal V protein binding region (MVBR), the target site for paramyxovirus V protein antagonism of MDA5 and LGP2, coincides with the boundaries of domain 2. (B) Simplified overview of RLR signal transduction leading to IFN-β gene transcription. Virus infection causes accumulation of cytosolic RNA species with non-self features, including 5′-triphosphates and double-stranded regions (for RIG-I recognition) or long RNAs with structural features (for MDA5 recognition). These RNAs trigger derepression of RIG-I and activate exposure of RLR CARDs to allow interaction with the CARD of the signaling adaptor, IPS-1/MAVS, that is localized on the mitochondria (Mito). Activation of IPS-1/MAVS polymerization stimulates assembly of a signaling scaffold for TRAF2, TRAF5, and TRAF6 and their associated kinases (e.g., TBK, IKKα, IKKβ, and IKKε) that result in phosphorylation-mediated activation of latent IRF3 and NF-κB transcription factors. These factors, along with ATF/JUN, assemble on the IFN-β gene proximal enhancer, resolve nucleosome-mediated repression, and recruit RNA polymerase to induce transcription. Transcribed IFN-β mRNA is translated and subsequently secreted from the cell.

FIG 2

Unifying model to reconcile dual LGP2 functions in antiviral signaling. The panels illustrate known aspects of MDA5 and LGP2 before and after virus infection. For greater clarity, RIG-I is not depicted. (A) Depiction of an uninfected cell at steady state. Blue spheres represent MDA5 CARD motifs, colored ellipses represent MDA5 (red) and LGP2 (light blue) helicase domains, and open ellipses represent CTDs. Both RLRs are present at steady state at modest concentrations, and LGP2 uses its RNA ligand-independent ATP hydrolysis to scan the cytoplasm for dsRNA molecules. IFN-β gene expression is in the repressed, nucleosome (white-hatched black circle)-occupied state, and basal LGP2 transcription is low. In the absence of signaling, IPS-1/MAVS is inactive, and transcription factors IRF3 and NF-κB are in their quiescent, latent cytoplasmic state. (B) Depiction of an acutely infected cell. After virus entry, potential dsRNA ligands accumulate (red line) and are bound by LGP2, which facilitates MDA5 recognition, triggering MDA5-RNA association and seeding filament formation. In turn, this activates IPS-1/MAVS polymerization and downstream signal transduction that induces IFN-β and other antiviral genes, including the LGP2 gene itself. (C) Depiction of an infected cell during the resolution of RLR signaling. LGP2 accumulation as a result of new protein synthesis reaches sufficient concentration to negatively regulate RLR signaling, which occurs downstream from both MDA5 and RIG-I by ATP- and RNA-independent mechanisms. Interference with IPS-1/MAVS signaling by LGP2 is illustrated as a block to downstream signaling, allowing the IFN-β promoter and LGP2 transcription to return to their steady-state configurations.