The complex kinase Cdk-activating kinase (CAK) consists of the catalytic subunit Cdk7, regulatory subunit Cyclin H, and assembly factor Mat1. The CAK is essential for maturation-promoting factor (MPF) activation by phosphorylating threonine 161 (T161) of Cdc2 in mitosis. Although it is known that meiotic resumption of oocytes is regulated by MPF activity, the role of CAK in meiosis is still unclear. In the present study, we attempted to confirm the involvement of CAK in meiotic resumption of porcine immature oocyte. Cumulus–oocyte complexes (COC) were collected from antral follicles of gilts and cultured up to 48 h in TYH medium containing 20% porcine follicular fluid, 3.2 mg/mL of BSA, and 1.0 IU mL–1 of pregnant mare serum gonadotropin. The T161 phosphorylation level of Cdc2 in cultured oocytes was analysed by Western blot analysis. The transcripts were collected from noncultured or cultured oocytes, and Cdk7, Cyclin H, and Mat1 expression were detected by RT-PCR. Overexpression of Cdc2 or inhibition of Cdk7, Cyclin H, and Mat1 during oocyte maturation was performed by microinjection of mRNA or antisense RNA into ooplasm of immature COC and verified by Western blot or semiquantitative RT-PCR. Maturation-promoting factor kinase activity was assayed by Histone H1 kinase activity assay. Statistical analyses in this study were carried out by Student’s t-test. The T161 phosphorylation of Cdc2 was found during the culture period from 18 h to 48h, which was after germinal vesicle breakdown (GVB). Overexpression of Cdc2 increased the incidence of GVB at 18 h, but overexpression of mutant Cdc2 (replaced T161 by alanine) had no influence on GVB. These results indicate that T161 phosphorylation of Cdc2 is important for meiotic resumption. Next, we attempted to confirm the CAK function during oocyte maturation. Transcripts of Cdk7, Cyclin H, and Mat1 were detectable throughout the culture period. Inhibition of Cdk7 and Cyclin H caused a decrease in T161 phosphorylation and MPF activity, and the incidence of GVB was significantly lower than in nontreated oocytes. In contrast, Mat1-inhibited oocytes resumed meiosis and developed to the metaphase II stage, and the incidence was not different between Mat1-inhibited oocytes and nontreated oocytes. These results suggest that Cdk7 and Cyclin H are working as CAK and activate Cdc2 by T161 phosphorylation, although Mat1 is dispensable during oocyte maturation.
Introduction of Cyclin B Induces Activation of the Maturation-Promoting Factor and Breakdown of Germinal Vesicle in Growing Zebrafish Oocytes Unresponsive to the Maturation-Inducing Hormone