Deciphering Regulation of Transcript Abundance in Maturing Bovine Oocytes

Oocytes acquire the capacity to undergo fertilization and early embryo development during maturation from the germinal vesicle (GV) to the metaphase II (MII) stage. GV oocytes become transcriptionally silent upon resumption of meiosis necessitating mechanisms to supply oocytes with essential proteins that drive meiosis, fertilization, and pre-implantation embryogenesis up to embryonic genome activation in a spatio-temporal manner. One such mechanism involves activation (readenylation) of stable/translationally repressed (deadenylated) transcripts stored in the cytoplasm modulated by regulatory elements in the 3’UTRs. To examine dynamic changes in transcript abundance and the effects of posttranscriptional regulation during oocyte maturation we conducted polyadenylated mRNA sequencing (RNA-Seq) using individual bovine GV and MII oocytes. Analysis of RNA-Seq data allowed quantification of transcript abundance and extension of the bovine genome annotation enabling a more thorough analysis of 3’UTRs. Polyadenylated transcript levels increased and decreased for 503 and 1952 genes, respectively. 3’UTR sequence analysis determined the presence or absence of sequence motif combinations that have been shown to be associated with transcript posttranscriptional regulation by de- and re-adenylation mechanisms. Among genes with a complete 3’UTR, most of the upregulated genes (85.66%) presented these motifs, suggesting that the increase in transcript abundance results from transcript readenylation and not de novo transcription. Alternatively, only a small proportion of downregulated genes (18.03%) contained these motifs, implying transcript degradation as a cause of decreased abundance. RNA-Seq enabled quantification of polyadenylated transcript abundance and subsequent UTR analysis providing insight into the mechanisms facilitating posttranscriptional regulation during oocyte maturation and pre-implantation embryogenesis.