A mRNA and Cognate MicroRNAs Localize in the Nucleolus

Abstract

We previously discovered that a set of 5 microRNAs are concentrated in the nucleolus of rat myoblasts. We now report that several mRNAs are also localized in the nucleoli of these cells as determined by microarray analysis of RNA from purified nucleoli. Among the most abundant of these nucleolus-localized mRNAs is that encoding insulin-like growth factor 2 (IGF2), a regulator of myoblast proliferation and differentiation. The presence of IGF2 mRNA in nucleoli was confirmed by fluorescence in situ hybridization, and RT-PCR experiments demonstrated that these nucleolar transcripts are spliced, thus arriving from the nucleoplasm. Bioinformatics analysis predicted canonically structured, highly thermodynamically stable interactions between IGF2 mRNA and all 5 of the nucleolus-localized microRNAs. These results raise the possibility that the nucleolus is a staging site for setting up particular mRNA-microRNA interactions prior to export to the cytoplasm.

Intracellular and extracellular microRNA: An update on localization and biological role

“…Finally, miRNAs were also found in the nucleolus, where their function remains to be explained (Politz et al, 2006;Marcon et al, 2008;Politz et al, 2009;Li et al, 2013c;Bai et al, 2014). Interestingly, the nucleolus contains a potential target for several such miRNAs, a spliced mRNA encoding insulin-like growth factor 2 (IGF2) (Reyes-Gutierrez et al, 2014). It is possible that miRNAs can associate with some mRNA targets and ribosomes already in the nucleus before their export to the cytoplasm.…”

MicroRNA in Control of Gene Expression: An Overview of Nuclear Functions

“…Based on these findings, it is possible to speculate that miRNA-targeted rRNAs may confer onto the mature ribosomes some specific properties that help to define their interaction with accessory proteins. Reyes-Gutierrez et al [67] have also proposed that miRNAs bind to some mRNA targets in the nucleolus before their export to the cytoplasm where mRNAs arrive in a pre-silenced status. Alternatively, nucleoli may constitute the site of storage for miRNAs, which are redistributed into the nucleoplasm and/or cytoplasm following genotoxic stress [64], thus evoking the ancestral role of the RISC as a genome defence system.…”

The Whereabouts of microRNA Actions: Cytoplasm and Beyond

“…It remains unclear whether this nucleolar miRNA can bind to other proteins or bind directly to its targets without AGO (as proposed by [40]). Interestingly, a fraction of spliced IGF2 mRNAs, which have predicted binding sites for miR-206 and four other nucleolus-localized miRNAs at the 3′ untranslated regions (UTR), were also found in the nucleolus [78]. Given that the protocol to isolate highly purified nucleoli from mammalian cells is well established, future experiments can rigorously test whether insulin growth factor 2 (IGF2) or other mRNA targets are specifically bound by AGO2 and/or miRNAs.…”