Functions of the cytoplasmic RNA sensors RIG-I and MDA-5: key regulators of innate immunity

Abstract

The innate immune system responds within minutes of infection to produce type I interferons and pro-inflammatory cytokines. Interferons induce the synthesis of cell proteins with antiviral activity, and also shape the adaptive immune response by priming T cells. Despite the discovery of interferons over 50 years ago, only recently have we begun to understand how cells sense the presence of a virus infection. Two families of pattern recognition receptors have been shown to distinguish unique molecules present in pathogens, such as bacterial and fungal cell wall components, viral RNA and DNA, and lipoproteins. The first family includes the membrane-bound toll-like receptors (TLRs). Studies of the signaling pathways that lead from pattern recognition to cytokine induction have revealed extensive and overlapping cascades that involve protein-protein interactions and phosphorylation, and culminate in activation of transcription proteins that control the transcription of genes encoding interferons and other cytokines. A second family of pattern recognition receptors has recently been identified, which comprises the cytoplasmic sensors of viral nucleic acids, including MDA-5, RIG-I, and LGP2. In this review we summarize the discovery of these cytoplasmic sensors, how they recognize nucleic acids, the signaling pathways leading to cytokine synthesis, and viral countermeasures that have evolved to antagonize the functions of these proteins. We also consider the function of these cytoplasmic sensors in apoptosis, development and differentiation, and diabetes.

Fig. 1

Schematic representation of the primary structure and functional domains of MDA-5, RIG-I, RIG-I-SV (splice variant) and LGP2. CARD: caspase activating recruitment domain. RD: repressor domain. CTD: C terminal domain.

Fig. 2. Model for activation of RIG-I and MDA-5 by RNA

In the absence of RNA, the sensor molecule is folded in an inactive state, with the CARD domain occluded by the CTD. The latter also contains the binding sites for RNA. When RNAs are produced in virus-infected cells (either dsRNA or ssRNA with 5’-phosphates), these bind the CTD and cause a conformational change that exposes the CARD domain. ATP is required for the conformational change. The CARD domain then interacts with downstream signaling molecules, leading to IFN transcription.

Fig. 3. The RIG-I and MDA-5 signaling pathway

Short dsRNA’s or 5’-triphosphate ssRNA’s activate RIG-I which associates with mitochondrial IPS-1 to trigger FADD/RIP1 (innateosome) controlled activation of the type I interferon genes. Longer RNA’s generated from picornaviruses trigger MDA5-mediated activation of type I IFN. The RIG-I/IPS-1 pathway also requires the assistance of STING to activate type I IFN, which resides in the endoplasmic reticulum (ER). STING may regulate the translocon composed of the TRAP complex and Sec61, which in turn may regulate exocyst-mediated stimulation of the TBK-1 pathway. STING also appears pivotal in regulating DNA-mediated triggering of type I IFN production.

Fig. 4. Viral countermeasures in the RNA sensing and signaling pathway

The steps leading from detection of RNA by RIG-I and MDA-5 through signaling and transcription of IFN genes are depicted. Viral gene products that interfere at different steps of this pathway are listed. An arrow indicates stimulation, and a bar indicates inhibition of the pathways

Fig. 5

A. Schematic diagram showing the domain structure of RIG-I and MDA-5. The numbers above the cartoon indicate the size of the domains. Abbrevitions: C1= CARD domain 1; C2 = CARD domain 2; hel = helicase and DEAH= DEAH box domain. B. Phylogentic tree showing the overall topology of the helicase/DEAH domains. C. Stepwise diagram showing the duplication events involved in the evolution of the helicase/DEAH linked domains leading to RIGI and MDA5. Each double-headed arrow represents a gene duplication event. D. Phylogenetic tree showing the topology for the CARD domains.