Adenosine Deaminase Acting on RNA 1 Limits RIG-I RNA Detection and Suppresses IFN Production Responding to Viral and Endogenous RNAs

Abstract

Type I interferons (IFNs) play central roles in innate immunity; however, overproduction of IFN can lead to immunopathology. Here, we demonstrate that adenosine deaminase acting on RNA 1 (ADAR1), an RNA-editing enzyme induced by interferon, is essential for cells to avoid inappropriate sensing of cytosolic RNA in an inducible knockout cell model – the primary mouse embryo fibroblast (MEF) derived from ADAR1 lox/lox & Cre-ER mice, as well as in HEK293 cells. ADAR1 suppresses viral and cellular RNA detection by RIG-I through its RNA binding rather than its RNA editing activity. DsRNA binds to both ADAR1 and RIG-I, but ADAR1 reduces RIG-I RNA binding. In the absence of ADAR1, cellular RNA stimulates type I IFN production without viral infection or exogenous RNA stimulation. Moreover, we showed in the ADAR1 inducible knockout mice that ADAR1 gene disruption results in a high level IFN production in neuronal tissues – the hallmark of Aicardi-Goutières Syndrome (AGS), a heritable autoimmune disease recently found to be associated with ADAR1 gene mutations. In summary, this study found that ADAR1 limits cytosolic RNA sensing by RIG-I through its RNA binding activity; therefore, ADAR1 suppresses type I IFN production stimulated by viral and cellular RNAs. These results explain why loss of ADARA1 causes IFN induction and also indicates a mechanism for the involvement of ADAR1 in autoimmune diseases such as AGS.

A-to-I editing of coding and non-coding RNAs by ADARs

“…AGS is an autosomal-recessive inflammatory disorder that affects the brain and skin and is characterized by an aberrant immune response and increased interferon-α expression 84 . Failed editing of certain endogenous dsRNAs such as Alu by the mutant ADAR1, and the consequent activation of interferon signalling, seem to underlie the pathogenesis of AGS 83,85 . Indeed, the overproduction of interferons is detected in the brains and spinal cords of conditional Adar1 -null mice 85 .…”

Evading innate immunity in nonviral mRNA delivery: don’t shoot the messenger

“…Another feasible explanation is that the RNA binding activity of ADAR1 is involved in the suppression of IFN signaling. Recently, Yang et al demonstrated that ADAR1 binds dsRNA, thereby limiting cytosolic dsRNA sensing by RLRs [65]. In addition, ADAR1 suppresses activation of both PKR and IRF3, by a mechanism still to be resolved [66].…”

ADAR1 Prevents Liver Injury from Inflammation and Suppresses Interferon Production in Hepatocytes

“…28,46 Sera from the conditional KO mice for ADAR1 and littermate controls were assessed for alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase, and alkaline phosphatase levels with a DRI-CHEM 4000 Chemistry Analyzer (HESKA Co., Loveland, CO).…”

“…46,55 Hepatocytes free of nonparenchymal cells were isolated from the inducible ADAR1 KO mice by two-step liver perfusion, and the cells were cultured under 56,57 Tamoxifen was added to the culture medium to induce ADAR1 gene deletion ( Figure 6A). ADAR1 gene deletion was effectively achieved by tamoxifen induction in the hepatocytes, as verified by the PCR analysis of the ADAR1 gene alleles and diminishing levels of ADAR1 protein ( Figure 6, B and C).…”

“…Recently, we found that ADAR1 inhibits the cytosolic RNA sensing pathway and suppresses type I IFN production in response to viral and endogenous RNAs. 46 Hepatocyte is one of the cell types with most abundant RNAs because of its active metabolism. It is thus conceivable that cellular RNA in hepatocyte stimulates the RNA sensing pathway in the absence of ADAR1 that lead to inflammatory cytokine productions.…”