MicroRNA Expression and Regulation in the Cumulus-Oocyte Complex and Preimplantation Embryo

Abstract

This thesis is an investigation of the regulatory mechanisms that control microRNA (miRNA) abundance in the mammalian oocyte and preimplantation embryo. MiRNAs are highly conserved, short non-coding RNAs that act as potent effectors of post-transcriptional gene regulation. They are abundantly expressed in the oocyte and early embryo during critical developmental periods when transcription is very low or absent and post-transcriptional mechanisms for gene regulation predominate. Based on these characteristics miRNAs have emerged as strong candidate regulators of RNA stability during oocyte maturation and during the transition from maternal to zygotic control of the genome. The hypothesis addressed by the studies described here is that specific miRNAs represent key regulators of oocyte maturation and subsequent embryo development, are transcribed and processed in cumulus-oocyte complexes (COCs), and have the potential to reflect gamete and embryo quality. To test this hypothesis, miR-21 and miR-34 miRNAs were examined in an in vitro model of bovine oocyte maturation and preimplantation embryo development. The dynamics of these two important miRNA families were used to evaluate the following potential mechanisms of miRNA regulation: 1) transcriptional control in the oocyte and surrounding cumulus oophorus cells, 2) post-transcriptional regulation by availability of the miRNA-processing enzyme Drosha, 3) delivery of sperm-borne miRNA to the zygote upon fertilization. Evidence supported roles for transcriptional and post-transcriptional regulation of miRNA abundance in the cumulus and oocyte compartments of the COC, however the miRNAs analysed in this thesis did not appear subject to regulation from the paternal gamete. Importantly, the transcriptional regulation of miR-21 in cumulus cells during in vitro maturation was sensitive to an inhibitor of the transcription factor STAT3. Ligands that act predominantly through the JAK/STAT pathway play well-established roles in the ovary and are known to improve oocyte competency in vitro, suggesting that miR-21 has the potential to be used as an informative marker of COC quality and oocyte competence. The studies presented here have explored several distinct aspects of miRNA biology in the oocyte and strongly support the overall importance of miRNAs in bovine gamete and embryo biology, as well as their potential utility in fertility assessment.