In vitro maturation of bovine oocytes requires polyadenylation of mRNAs coding proteins for chromatin condensation, spindle assembly, MPF and MAP kinase activation

In the oocyte mRNA molecules are stored in order to be used during the maturation process when transcription is silenced. Translational activation of stored mRNA templates commonly is correlated with their cytoplasmic polyadenylation. In the present study, the effects of cordycepin, a potent polyadenylation inhibitor, on the in vitro maturation MPF and MAP kinase activation of bovine cumulus oocyte complexes (COCs) were examined. The presence of cordycepin (5μg/ml) during the whole culture period (24 h) prevents chromatin condensation and germinal vesicle breakdown (GVBD) as well as MPF and MAP kinase activation. When COCs were first cultivated in inhibitor-free medium for 6 or 9 h and subsequently transferred to cordycepin supplemented medium for further 18 or 15 h neither MPF nor MAP kinase became activated and 86 and 85%, respectively, of the oocytes underwent GVBD but failed to form a spindle and hypercondensed their chromatin. Extending the first culture period in inhibitor-free medium to 12 or 15 h before transferring the COCs to cordycepin supplemented medium for a further 12 or 9 h allowed 48 and 79%, respectively of oocytes to reach the metaphase 2 (M 2) stage. From these data, it is concluded that mRNA molecules coding for proteins required for chromatin condensation and GVBD become polyadenylated during the first 6 h following the onset of culture whereas mRNA molecules coding for proteins required for spindle assembly of metaphase 1 (M 1) and MPF and MAP kinase activation become polyadenylated between 9 and 12 h following initiation of culture.