Inosine-Mediated Modulation of RNA Sensing by TLR7/8.

Centre for Cancer Research, Monash Institute of Medical Research, Monash University, Clayton, Victoria 3168, Australia.
Journal of Virology (Impact Factor: 4.65). 01/2014; 88(2):799-810. DOI: 10.1128/JVI.01571-13
Source: PubMed

ABSTRACT RNA-specific adenosine-deaminase (ADAR)-mediated adenosine to inosine (A-to-I) editing is a critical arm of the antiviral response. However, mechanistic insights into how A-to-I RNA editing affects viral infection are lacking. We posited that inosine incorporation into RNA facilitates sensing of non-self RNA by innate immune sensors, and accordingly investigated the impact of inosine-modified RNA on Toll-like receptor (TLR) 7/8 sensing. Inosine incorporation into synthetic single-stranded (ss) RNA potentiated tumor necrosis factor α (TNF-α) or interferon α (IFN-α) production in human peripheral blood mononuclear cells (PBMCs), in a sequence-dependent manner, indicative of TLR7/8 recruitment. The effect of inosine incorporation on TLR7/8 sensing was restricted to immunostimulatory ssRNAs, and was not seen with inosine-containing short double-stranded RNAs, nor with a deoxy-inosine-modified ssRNA. Inosine-mediated increase of self-secondary structure of an ssRNA resulted in potentiated IFN-α production in human PBMCs through TLR7 recruitment, as established through the use of a TLR7 antagonist and Tlr7-deficient cells. There was a correlation between hyper-editing of influenza A viral ssRNA and its ability to stimulate TNF-α, independent of 5' -triphosphate residues, and involving Adar-1. Furthermore, A-to-I editing of viral ssRNA directly enhanced mouse Tlr7 sensing, when present in proportions reproducing biologically relevant levels of RNA editing. Thus we demonstrate for the first time that inosine incorporation into immunostimulatory ssRNA can potentiate TLR7/8 activation. Our results suggest a novel function of A-to-I RNA editing, which is to facilitate TLR7/8 sensing of phagocytosed viral RNA.

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    ABSTRACT: Adenosine deaminase acting on RNA1 (ADAR1) catalyzes the C6 deamination of adenosine (A) to produce inosine (I) in regions of RNA with double-stranded (ds) character. This process is known as A-to-I RNA editing. Alternative promoters drive the expression of the Adar1 gene and alternative splicing gives rise to transcripts that encode 2 ADAR1 protein size isoforms. ADAR1 p150 is an interferon (IFN)-inducible dsRNA adenosine deaminase found in the cytoplasm and nucleus, whereas ADAR1 p110 is constitutively expressed and nuclear in localization. Dependent on the duplex structure of the dsRNA substrate, deamination of adenosine by ADAR can be either highly site-selective or nonspecific. A-to-I editing can alter the stability of RNA structures and the coding of RNA as I is read as G instead of A by ribosomes during mRNA translation and by polymerases during RNA replication. A-to-I editing is of broad physiologic significance. Both the production and the action of IFNs, and hence the subsequent interaction of viruses with their hosts, are among the processes affected by A-to-I editing.
    Journal of Interferon & Cytokine Research 06/2014; 34(6):437-446. DOI:10.1089/jir.2014.0001 · 3.90 Impact Factor


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Jun 3, 2014