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

MicroRNA (miRNA) and RNA interference (RNAi) pathways rely on small RNAs produced by Dicer endonucleases. Mammalian Dicer primarily supports the essential gene-regulating miRNA pathway, but how it is specifically adapted to miRNA biogenesis is unknown. We show that the adaptation entails a unique structural role of Dicer's DExD/H helicase domain. Although mice tolerate loss of its putative ATPase function, the complete absence of the domain is lethal because it assures high-fidelity miRNA biogenesis. Structures of murine Dicer•-miRNA precursor complexes revealed that the DExD/H domain has a helicase-unrelated structural function. It locks Dicer in a closed state, which facilitates miRNA precursor selection. Transition to a cleavage-competent open state is stimulated by Dicer-binding protein TARBP2. Absence of the DExD/H domain or its mutations unlocks the closed state, reduces substrate selectivity, and activates RNAi. Thus, the DExD/H domain structurally contributes to mammalian miRNA biogenesis and underlies mechanistical partitioning of miRNA and RNAi pathways.

• Keywords
• DExD, Dicer, PKR, RNAi, TARBP2, cryo-EM, dsRBD, dsRNA, helicase, miRNA, mirtron,
• MeSH
• MicroRNAs * genetics   metabolism   MeSH
• Mice MeSH
• Ribonuclease III * metabolism   MeSH
• RNA Interference MeSH
• Mammals metabolism   MeSH
• Carrier Proteins metabolism   MeSH
• Animals MeSH
• Check Tag
• Mice MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Comment MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• MicroRNAs * MeSH
• Ribonuclease III * MeSH
• Carrier Proteins MeSH

PIWI-interacting RNAs (piRNAs) support the germline by suppressing retrotransposons. Studies of the pathway in mice have strongly shaped the view that mammalian piRNAs are essential for male but not for female fertility. Here, we report that the role of the piRNA pathway substantially differs in golden hamsters (Mesocricetus auratus), the piRNA pathway setup of which more closely resembles that of other mammals, including humans. The loss of the Mov10l1 RNA helicase-an essential piRNA biogenesis factor-leads to striking phenotypes in both sexes. In contrast to mice, female Mov10l1-/- hamsters are sterile because their oocytes do not sustain zygotic development. Furthermore, Mov10l1-/- male hamsters have impaired establishment of spermatogonia accompanied by transcriptome dysregulation and an expression surge of a young retrotransposon subfamily. Our results show that the mammalian piRNA pathway has essential roles in both sexes and its adaptive nature allows it to manage emerging genomic threats and acquire new critical roles in the germline.

• MeSH
• Cricetinae MeSH
• Mesocricetus metabolism   MeSH
• RNA, Small Interfering genetics   MeSH
• Oocytes metabolism   pathology   MeSH
• Retroelements physiology   MeSH
• RNA Helicases genetics   MeSH
• Spermatogenesis genetics   physiology   MeSH
• Spermatogonia metabolism   pathology   MeSH
• Testis metabolism   MeSH
• Gene Silencing physiology   MeSH
• Animals MeSH
• Check Tag
• Cricetinae MeSH
• Male MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• RNA, Small Interfering MeSH
• Retroelements MeSH
• RNA Helicases MeSH