Proper oocyte maturation is a prerequisite for successful reproduction and requires the resumption of meiosis to the metaphase II stage (MII). In bovine oocytes, nuclear maturation has been shown to occur in in vitro maturing cumulus-enclosed oocytes (COCs) in the absence of transcription, but their developmental capacity is reduced compared to transcriptionally competent COCs. To assess the impact of transcription during in vitro maturation of bovine COCs on the quantitative oocyte proteome, a holistic nano-LC–MS/MS analysis of germinal vesicle oocytes and MII oocytes matured with or without addition of the transcription inhibitor actinomycin D (ActD) was carried out. Analyzing eight biological replicates for each of the three groups, a total of 2018 proteins was identified. These could be clearly classified into proteins depending or not depending on transcription during oocyte maturation. Proteins whose abundance increased after maturation irrespective of transcription inhibition - and hence independent of transcription - were related to the cell cycle, reflecting the progression of meiosis, and to cellular component organization, which is crucial for cytoplasmic maturation. In contrast, transcription-dependent proteins were associated with cell–cell adhesion and translation. Since a high rate of protein synthesis in oocytes has been shown to correlate with their developmental competence, oocyte maturation in transcriptionally impaired COCs is apparently disturbed. Our experiments reveal that impaired transcription during in vitro maturation of COCs has a substantial effect on specific components of the oocyte proteome, and that transcription is required for specific classes of oocyte proteins predominantly involved in translation.

Summary sentence

For oocyte maturation, selective functional classes of proteins are dependent on newly synthesized messenger RNAs.