Inhibition of P‐TEFb disrupts global transcription,oocyte maturation,and embryo development in the mouse

Positive transcription elongation factor b (P‐TEFb) is an RNA polymerase II kinase that phosphorylates Ser2 of the carboxyl‐terminal domain and promotes the elongation phase of transcription. Despite the fact that P‐TEFb has role in many cellular processes, the role of this kinase complex remains to be understood in early developmental events. In this study, using immunocytochemical analyses, we find that the P‐TEFb components, Cyclin T1, CDK9, and its T‐loop phosphorylated form, are localized to nuclear speckles, as well as in nucleoli in mouse germinal vesicle oocytes. Moreover, using fluorescence in situ hybridization, we show that in absence of CDK9 activity, nucleolar integration, as well as production of 28S rRNA is impaired in oocytes and embryos. We also present evidence indicating that P‐TEFb kinase activity is essential for completion of mouse oocyte maturation and embryo development. Treatment with CDK9 inhibitor, flavopiridol resulted in metaphase I arrest in maturing oocytes. Inhibition of CDK9 kinase activity did not interfere with in vitro fertilization and pronuclear formation. However, when zygotes or 2‐cell embryos were treated with flavopiridol only in their G2 phase of the cell cycle, development to the blastocyst stage was impaired. Inhibition of the CDK9 activity after embryonic genome activation resulted in failure to form normal blastocysts and aberrant phosphorylation of RNA polymerase II CTD. In all stages analyzed, treatment with flavopiridol abrogated global transcriptional activity. Collectively, our data suggest that P‐TEFb kinase activity is crucial for oocyte maturation, embryo development, and regulation of global RNA transcription in mouse early development.