FUSION WITH ACTIVATED MOUSE OOCYTES MODULATES THE TRANSCRIPTIONAL ACTIVITY OF INTRODUCED SOMATIC-CELL NUCLEI

We have analyzed the transcriptional activity of somatic cell nuclei f used with artificially activated mouse oocytes. Two types of somatic c ells have been used: transcriptionally silent thymocytes, obtained fro m the thymus of newborn mice, and transcriptionally active murine eryt hroleukemia cells (MEL) from in vitro culture. Cells were fused with a ctivated oocytes, either less than 1 h, or 3 h post-ethanol-treatment. When the fusion occurred early after activation (1 h or less), the tr ansferred somatic cell nuclei reacted by nuclear envelope breakdown (N EBD), which exposed their chromatin to the cytoplasmic environment of the oocytes, and the reconstituted nuclei underwent significant ultras tructural remodeling. No transcriptional activity was ever detected in these reconstituted nuclei during the subsequent 3-4 h of culture of the resulting hybrid cells. In the case of MEL, this means that transc riptional activity ceased as soon as they entered the cytoplasm. In co ntrast, somatic nuclei which entered the cytoplasm of activated oocyte s 3 h postactivation did not undergo NEED and their remodeling was les s pronounced. In contrast to the first group, these nuclei were transc riptionally active during the following 3-4 h of culture. In both case s, the female pronucleus remained transcriptionally silent. (C) 1996 A cademic Press, Inc.