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.

• MeSH
• DEAD-box RNA Helicases genetics   metabolism   MeSH
• RNA, Small Interfering genetics   MeSH
• Mice, Inbred C57BL MeSH
• Mice MeSH
• Immunity, Innate * genetics   MeSH
• Ribonuclease III * genetics   metabolism   MeSH
• RNA Interference * MeSH
• Encephalomyocarditis virus genetics   immunology   MeSH
• Lymphocytic choriomeningitis virus immunology   genetics   MeSH
• Encephalitis Viruses, Tick-Borne genetics   immunology   MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• DEAD-box RNA Helicases MeSH
• Dicer1 protein, mouse MeSH Browser
• RNA, Small Interfering MeSH
• Ribonuclease III * MeSH

Canonical RNA interference (RNAi) is sequence-specific mRNA degradation guided by small interfering RNAs (siRNAs) made by RNase III Dicer from long double-stranded RNA (dsRNA). RNAi roles include gene regulation, antiviral immunity or defense against transposable elements. In mammals, RNAi is constrained by Dicer's adaptation to produce another small RNA class-microRNAs. However, a truncated Dicer isoform (ΔHEL1) supporting RNAi exists in mouse oocytes. A homozygous mutation to express only the truncated ΔHEL1 variant causes dysregulation of microRNAs and perinatal lethality in mice. Here, we report the phenotype and canonical RNAi activity in DicerΔHEL1/wt mice, which are viable, show minimal miRNome changes, but their endogenous siRNA levels are an order of magnitude higher. We show that siRNA production in vivo is limited by available dsRNA, but not by Protein kinase R, a dsRNA sensor of innate immunity. dsRNA expression from a transgene yields sufficient siRNA levels to induce efficient RNAi in heart and muscle. DicerΔHEL1/wt mice with enhanced canonical RNAi offer a platform for examining potential and limits of mammalian RNAi in vivo.

• Keywords
• Dicer, Mirtron, PKR, dsRNA, siRNA,
• MeSH
• DEAD-box RNA Helicases genetics   metabolism   MeSH
• RNA, Double-Stranded * metabolism   genetics   MeSH
• RNA, Small Interfering * genetics   metabolism   MeSH
• MicroRNAs genetics   metabolism   MeSH
• Mice MeSH
• Protein Isoforms genetics   metabolism   MeSH
• Ribonuclease III * genetics   metabolism   MeSH
• RNA Interference * MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• DEAD-box RNA Helicases MeSH
• Dicer1 protein, mouse MeSH Browser
• RNA, Double-Stranded * MeSH
• RNA, Small Interfering * MeSH
• MicroRNAs MeSH
• Protein Isoforms MeSH
• Ribonuclease III * MeSH