Factors controlling the loss of immunoreactive somatic histone H1 from blastomere nuclei in oocyte cytoplasm: a potential marker of nuclear reprogramming.

Nuclei of differentiated cells can acquire totipotency following transfer into the cytoplasm of oocytes. While the molecular basis of this nuclear reprogramming remains unknown, the developmental potential of nuclear-transfer embryos is influenced by the cell-cycle stage of both donor and recipient. As somatic H1 becomes immunologically undetectable on bovine embryonic nuclei following transfer into ooplasm and reappears during development of the reconstructed embryo, suggesting that it may act as a marker of nuclear reprogramming, we investigated the link between cell-cycle state and depletion of immunoreactive H1 following nuclear transplantation. Blastomere nuclei at M-, G1-, or G2-phase were introduced into ooplasts at metaphase II, telophase II, or interphase, and the reconstructed embryos were processed for immunofluorescent detection of somatic histone H1. Immunoreactivity was lost more quickly from donor nuclei at metaphase than at G1 or G2. Regardless of the stage of the donor nucleus, immunoreactivity was lost most rapidly when the recipient cytoplast was at metaphase and most slowly when the recipient was at interphase. When the recipient oocyte was not enucleated, however, immunoreactive H1 remained in the donor nucleus. The phosphorylation inhibitors 6-DMAP, roscovitine, and H89 inhibited the depletion of immunoreactive H1 from G2, but not G1, donor nuclei. In addition, immunoreactive H1 was depleted from mouse blastomere nuclei following transfer into bovine oocytes. Finally, expression of the developmentally regulated gene, eIF-1A, but not of Gapdh, was extinguished in metaphase recipients but not in interphase recipients. These results indicate that evolutionarily conserved cell-cycle-regulated activities, nuclear elements, and phosphorylation-linked events participate in the depletion of immunoreactive histone H1 from blastomere nuclei transferred in oocyte cytoplasm and that this is linked to changes in gene expression in the transferred nucleus.