Expression and cellular distribution of cyclin-dependent kinase 4 (Cdk4) and connexin 43 (Cx43) in porcine oocytes before and after in vitro maturation.

It is recognised that connexin 43 (Cx43) and cyclin-dependent kinase 4 (Cdk4) are involved in the cumulus cell-oocyte communication via gap junctions and the control of cell cycle progress. However, little is known about their mRNA expression pattern and encoded proteins distribution in porcine oocytes during in vitro maturation (IVM). Cumulus-oocyte complexes (COCs) were collected from 31 puberal crossbred Landrace gilts and analysed for their Cdk4 and Cx43 mRNA expression using RQ-PCR and for the respective protein expression by confocal microscopic observations. An increased Cdk4 and Cx43 mRNA expression was found in oocytes after IVM (P < 0.001 and P < 0.05, respectively). Confocal microscopic observations revealed a significant increase of Cdk4 protein expression in the cytoplasm of oocytes during the maturation process. The localisation of Cx43 changed from zona pellucida before to cytoplasm of oocytes after IVM. It is supposed that the increased expression of Cdk4 and Cx43 mRNA in oocytes after IVM is linked with the accumulation of a large amount of templates during the process of oocyte maturation. The translocation especially of Cx43 from the zona pellucida into the cytoplasm may be associated with a decrease in gap junction activity in fully grown porcine oocytes. Both Cdk4 and Cx43 can be used as 'checkpoints' of oocyte maturation.