Silencing of microRNAs in vivo with ‘antagomirs’

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Abstract
The discovery of microRNAs (miRNAs), the non-coding RNAs thought to be involved in many biological processes, is changing our perception of gene regulation. Little is known about their function in mammalian systems in vivo, but a newly developed group of compounds that silences miRNAs in mice should provide a powerful tool for the study of their function — and a potential therapeutic strategy for silencing miRNAs in disease. These ‘antagomirs’ are chemically engineered oligonucleotides with sequences that complement natural miRNAs. Intravenous administration of antagomirs to miR-16, -122, -192 and -194 in mice caused a marked reduction of corresponding miRNA expression in liver, lung, kidney, heart, muscle, intestine, fat, skin, bone marrow, ovaries and adrenals. In worms and flies, miRNAs play important developmental roles in the embryo. In vertebrates, various developmental genes have been shown to be targets of miRNA regulation, but there were no examples of miRNAs playing specific roles in known developmental processes. Now one such example has been found: miR-196 acts in mouse embryos as a mechanism to ensure accurate expression of genes primarily regulated by Hoxb8 and Shh transcription factors. This supports the idea that vertebrate miRNAs may function as a secondary level of gene regulation. MicroRNAs (miRNAs) are an abundant class of non-coding RNAs that are believed to be important in many biological processes through regulation of gene expression1,2,3. The precise molecular function of miRNAs in mammals is largely unknown and a better understanding will require loss-of-function studies in vivo. Here we show that a novel class of chemically engineered oligonucleotides, termed ‘antagomirs’, are efficient and specific silencers of endogenous miRNAs in mice. Intravenous administration of antagomirs against miR-16, miR-122, miR-192 and miR-194 resulted in a marked reduction of corresponding miRNA levels in liver, lung, kidney, heart, intestine, fat, skin, bone marrow, muscle, ovaries and adrenals. The silencing of endogenous miRNAs by this novel method is specific, efficient and long-lasting. The biological significance of silencing miRNAs with the use of antagomirs was studied for miR-122, an abundant liver-specific miRNA. Gene expression and bioinformatic analysis of messenger RNA from antagomir-treated animals revealed that the 3′ untranslated regions of upregulated genes are strongly enriched in miR-122 recognition motifs, whereas downregulated genes are depleted in these motifs. Analysis of the functional annotation of downregulated genes specifically predicted that cholesterol biosynthesis genes would be affected by miR-122, and plasma cholesterol measurements showed reduced levels in antagomir-122-treated mice. Our findings show that antagomirs are powerful tools to silence specific miRNAs in vivo and may represent a therapeutic strategy for silencing miRNAs in disease.