The microRNA miR-196 acts upstream of Hoxb8 and Shh in limb development

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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 gene regulatory molecules (reviewed in refs 1, 2). Although computational work indicates that miRNAs repress more than a third of human genes3, their roles in vertebrate development are only now beginning to be determined. Here we show that miR-196 acts upstream of Hoxb8 and Sonic hedgehog (Shh) in vivo in the context of limb development, thereby identifying a previously observed but uncharacterized inhibitory activity that operates specifically in the hindlimb. Our data indicate that miR-196 functions in a fail-safe mechanism to assure the fidelity of expression domains that are primarily regulated at the transcriptional level, supporting the idea that many vertebrate miRNAs may function as a secondary level of gene regulation.