Abstract
Although microRNAs are commonly known to function as a component of RNA-induced silencing complexes in the cytoplasm, they have been detected in other organelles, notably the nucleus and the nucleolus, of mammalian cells. We have conducted a systematic search for miRNAs in HeLa cell nucleoli, and identified 11 abundant miRNAs with a high level of nucleolar accumulation. Through in situ hybridisation, we have localised these miRNAs, including miR-191 and miR-484, in the nucleolus of a diversity of human and rodent cell lines. The nucleolar association of these miRNAs is resistant to various cellular stresses, but highly sensitive to the presence of exogenous nucleic acids. Introduction of both single- and double-stranded DNA as well as double stranded RNA rapidly induce the redistribution of nucleolar miRNAs to the cytoplasm. A similar change in subcellular distribution is also observed in cells infected with the influenza A virus. The partition of miRNAs between the nucleolus and the cytoplasm is affected by Leptomycin B, suggesting a role of Exportin-1 in the intracellular shuttling of miRNAs. This study reveals a previously unknown aspect of miRNA biology, and suggests a possible link between these small noncoding RNAs and the cellular management of foreign genetic materials.
Highlights
MicroRNAs have emerged as ubiquitous regulators of gene expression
By using an established protocol, we separated HeLa cells into cytoplasmic and nuclear fractions, which were further separated into nucleoli and nucleoplasm (Figure 1A)
We restricted our further analyses to miRNAs with normalized nucleolus/cell ratios of above 0.30, i.e., those with nucleolar distributions more than 30% of RNU44 and are significantly highly than those of typical mRNAs
Summary
MicroRNAs (miRNAs) have emerged as ubiquitous regulators of gene expression. The latest version of miRbase reveals 1600 unique human miRNAs (Release 19: August 2012); many of them are highly cell type- or disease-specific [1]. Biogenesis of miRNAs requires multiple enzymatic complexes located in different subcellular compartments. MiRNAs are transcribed and processed in the nucleus, and matured in the cytoplasm, where they are incorporated into an RNA-induced silencing complex (RISC). Engaged RISCs affect the stability and/or translational potential of mRNA targets [2,3]. Bioinformatic estimations suggest that up to one-third of all protein-coding genes in the human genome are under the regulation of at least one miRNA [4]
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