Factors engaged in reactivation of DNA replication in the nuclei of growing mouse oocytes introduced into the cytoplasm of parthenogenetic one-cell embryos

Abstract

Mammalian primary oocytes are arrested in the post-replicative G2 phase of the cell cycle. In contrast to other G2 nuclei, the nucleus of the growing mouse oocyte can reinitiate DNA synthesis after transfer by cell fusion under favorable cytoplasmic conditions, created by the parthenogenetic one-cell embryo. In the present study, we used the cell hybrid system to analyze the distribution of proteins involved in DNA re-replication in the oocyte nucleus. We show that this process is preceded by an extensive rearrangement of the insoluble fractions of minichromosome maintenance (MCM) proteins (Mcm2, -6 and 7). We also demonstrate that Cdc6 protein is present in primary growing mouse oocytes freshly isolated from the ovary, in a soluble and insoluble form. In contrast to MCM proteins, the insoluble fraction of Cdc6 was not rearranged in oocyte nuclei reinitiating DNA replication in hybrid cells. The rearrangement of MCM proteins and reinitiation of DNA synthesis occurred in the nuclei, in which the nuclear envelope remained intact. Reinitiation of DNA replication in the oocyte nucleus was sensitive to the inhibition of both CDK activity and polyadenylation of maternal mRNAs, indicating a role of proteins synthesized de novo by the embryo. These results allow us to understand better the mechanisms involved in the reinitiation of DNA replication in growing oocytes.