Regulation of stage-specific nuclear translocation of Dnmt1o during preimplantation mouse development

DNA methylation of CpG dinucleotides by DNA methyltransferase 1 is implicated in the regulation of transcription and, in particular, the transcription of imprinted genes. Although the oocyte-specific form of Dnmt1 (Dnmt1o) possesses a functional nuclear localization signal, it is predominantly localized in the cytoplasm of the oocyte and preimplantation mouse embryo but undergoes a transient nuclear localization during the eight-cell stage, when the embryos undergo compaction. We report here that Dnmt1o is likely retained in the cytoplasm by an active process, since approximately 70% of DNA methyltransferase activity is retained following permeabilization procedures that result in the release of approximately 75% of oocyte/embryo protein. Treatment of the embryos with agents that disrupt either microfilaments or microtubules has little, if any, effect on the retention of Dnmt1o in permeabilized embryos. While Dnmt1o does not colocalize with either mitochondria or endoplasmic reticulum, it does colocalize with annexin V, which is known to interact with Dnmt1o. We also report that the timing of nuclear entry of Dnmt1o during the eight-cell stage is independent of DNA replication, transcription, and protein synthesis, as well as compaction, cell contact, and cytokinesis. The time of nuclear entry, therefore, appears linked to the time following fertilization, which suggests that a molecular clock governs the time of nuclear import.