Abstract
BackgroundOocyte-derived maternal RNAs drive early embryogenesis when the newly formed embryo is transcriptionally inactive. Recent studies in zebrafish have identified the role of microRNAs during the maternal-to-embryonic transition (MET). MicroRNAs are short RNAs that bind to the 3' UTR of target mRNAs to repress their translation and accelerate their decay. Newborn ovary homeobox gene (NOBOX) is a transcription factor that is preferentially expressed in oocytes and essential for folliculogenesis in mice. NOBOX knockout mice are infertile and lack of NOBOX disrupts expression of many germ-cell specific genes and microRNAs. We recently reported the cloning and expression of bovine NOBOX during early embryonic development and our gene knockdown studies indicate that NOBOX is a maternal effect gene essential for early embryonic development. As NOBOX is a maternal transcript critical for development and NOBOX is depleted during early embryogenesis, we hypothesized that NOBOX is targeted by microRNAs for silencing and/or degradation.ResultsUsing an algorithm "MicroInspector", a potential microRNA recognition element (MRE) for miR-196a was identified in the 3' UTR of the bovine NOBOX mRNA. Expression analysis of miR-196a in bovine oocytes and during early embryonic development indicated that it is expressed both in oocytes and embryos and tends to increase at the four-cell and eight-cell stages. Ectopic expression of NOBOX and miR-196a in HeLa cells inhibited the expression of NOBOX protein compared to the control cells without miR-196a. Similarly, the activity of a luciferase construct containing the entire 3' UTR of bovine NOBOX was suppressed, and the regulation was abolished by mutations in the miR-196a binding site indicating that the predicted MRE is critical for the direct and specific binding of miR-196a to the NOBOX mRNA. Furthermore, ectopic expression of miR-196a mimic in bovine early embryos significantly reduced the NOBOX expression at the both mRNA and protein levels.ConclusionCollectively, our results demonstrate that miR-196a is a bona fide negative regulator of NOBOX during bovine early embryogenesis.
Highlights
Oocyte-derived maternal RNAs drive early embryogenesis when the newly formed embryo is transcriptionally inactive
To identify miRNAs that potentially regulate Newborn ovary homeobox gene (NOBOX) expression, we analyzed the 3’ untranslated region (3’ UTR) sequence of bovine NOBOX using the “Microinspector” algorithm to predict potential miRNA target sites [32]. miR-196a was chosen for further studies, because the predicted microRNA recognition element (MRE) in the bovine NOBOX 3’ UTR had a low predicted free energy of hybridization with the cognate miRNA (-19.8 kcal/mol), suggesting a stable miRNA: mRNA duplex within the 9 nucleotide seed region at the 5’ end of the miRNA (Figure 1)
Collectively, our results demonstrate the ability of miR196a to negatively regulate NOBOX expression in a sequence specific fashion and the ability of miR-196a to suppress NOBOX mRNA and protein in early embryos
Summary
Using an algorithm “MicroInspector”, a potential microRNA recognition element (MRE) for miR-196a was identified in the 3’ UTR of the bovine NOBOX mRNA. Expression analysis of miR-196a in bovine oocytes and during early embryonic development indicated that it is expressed both in oocytes and embryos and tends to increase at the four-cell and eight-cell stages. The activity of a luciferase construct containing the entire 3’ UTR of bovine NOBOX was suppressed, and the regulation was abolished by mutations in the miR-196a binding site indicating that the predicted MRE is critical for the direct and specific binding of miR-196a to the NOBOX mRNA. Ectopic expression of miR-196a mimic in bovine early embryos significantly reduced the NOBOX expression at the both mRNA and protein levels
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