In RNA interference (RNAi), long double-stranded RNA is cleaved by the Dicer endonuclease into small interfering RNAs (siRNAs), which guide degradation of complementary RNAs. While RNAi mediates antiviral innate immunity in plants and many invertebrates, vertebrates have adopted a sequence-independent response and their Dicer produces siRNAs inefficiently because it is adapted to process small hairpin microRNA precursors in the gene-regulating microRNA pathway. Mammalian endogenous RNAi is thus a rudimentary pathway of unclear significance. To investigate its antiviral potential, we modified the mouse Dicer locus to express a truncated variant (DicerΔHEL1) known to stimulate RNAi and we analyzed how DicerΔHEL1/wt mice respond to four RNA viruses: coxsackievirus B3 and encephalomyocarditis virus from Picornaviridae; tick-borne encephalitis virus from Flaviviridae; and lymphocytic choriomeningitis virus (LCMV) from Arenaviridae. Increased Dicer activity in DicerΔHEL1/wt mice did not elicit any antiviral effect, supporting an insignificant antiviral function of endogenous mammalian RNAi in vivo. However, we also observed that sufficiently high expression of DicerΔHEL1 suppressed LCMV in embryonic stem cells and in a transgenic mouse model. Altogether, mice with increased Dicer activity offer a new benchmark for identifying and studying viruses susceptible to mammalian RNAi in vivo.

In RNA interference (RNAi), the enzyme Dicer cuts long double-stranded RNA into small interfering RNAs that degrade matching RNAs. RNAi is a key antiviral defense in plants and invertebrates but vertebrates evolved a principally different antiviral defense. The authors genetically modified Dicer in mice to activate RNAi in mammals. These modified mice were tested against four RNA viruses but showed no significant antiviral response. However, further increased expression of modified Dicer did suppress one virus (lymphocytic choriomeningitis virus) in embryonic stem cells and in a transgenic mouse model, suggesting that some viruses might be sensitive to increased RNAi activity in mammals.

Bioinformatics Group Department of Biology Faculty of Science University of Zagreb Horvatovac 102a 10000 Zagreb Croatia

Department of Experimental Biology Faculty of Science Masaryk University Kamenice 5 CZ 62500Brno Czech Republic

Faculty of Medicine Enterovirus Laboratory Institute of Microbiology Slovak Medical University Limbova 12 83303Bratislava Slovakia

Faculty of Science University of South Bohemia Branisovska 1645 31a CZ 37005Ceske Budejovice Czech Republic

Institute of Parasitology Biology Centre of the Czech Academy of Sciences Branisovska 31 CZ 37005 Ceske Budejovice Czech Republic

Laboratory of Adaptive Immunity Institute of Molecular Genetics of the Czech Academy of Sciences Videnska 1083 142 20 Prague Czech Republic

Laboratory of Emerging Viral Infections Veterinary Research Institute Hudcova 70 CZ 62100 Brno Czech Republic

Laboratory of Epigenetic Regulations Institute of Molecular Genetics of the Czech Academy of Sciences Videnska 1083 142 20 Prague Czech Republic

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