Dynamics of Telomeres and MicroRNAs During Oocyte Maturation and Embryo Development in the Cow
Successful fertilization and subsequent embryo development rely on complex molecular processes including genetic reprogramming and chromatin remodelling. During the embryonic genome activation through the early cleavage stages of embryo development, telomere-reprogramming events occur to maintain genomic stability for the organism through continuous mitotic divisions. Telomeres are nucleoprotein complexes that consist of a repeat nucleotide sequence and proteins that make up the shelterin complex that bind to telomeric repeats and increase genomic stability at the chromosome ends. The mechanisms that regulate telomere biology in bovine oocytes and embryos remain uncharacterized. MicroRNAs are small non-coding RNA molecules that post-transcriptionally regulate gene expression by either repressing the translation, or targeting transcripts for degradation. MicroRNAs have been previously reported to regulate important molecular functions in various cell types and have been shown to be present in bovine oocytes and embryos. It was therefore hypothesized that small RNAs expressed during maturation and development in bovine oocytes and embryos participate in the regulation of telomere biology. Examining the dynamics of telomere reprogramming in the developing bovine embryo through qPCR assays revealed an increase in telomerase activity and telomere length in blastocyst stage embryos and a decrease in the expression of one of the primary telomere related proteins, TERF2. Small RNA sequencing was then employed to characterize the miRNA population present in oocytes but did not reveal miRNA candidates with the potential to participate in telomere regulation in GV or MII oocytes or zygote stages. However, microRNA expression patterns were observed to be markedly different between stages, to correlate well with the bovine proteome at the same stages and with predicted transcript targets involved in other important signalling pathways required for oocyte competence and fertilization. Significant increases in the expression of miR-155, miR-222, miR-21, and let-7d were noted, while decreases in miR-190 and several other miRNAs were observed, although these did not correlate with telomere biology in the oocyte. Importantly, miR-148a was both abundant and relatively static throughout oocyte maturation. Finally, selective transcriptional activation of several primary-microRNAs highlights the importance of miRNAs in the molecular control of oocyte and embryo biology in cattle.