Factors involved in nuclear reprogramming during early development in the rabbit

Mechanisms of nuclear reprogramming and assessment of potential malfunctions that could be deleterious for development were evaluated in rabbit zygotes, parthenotes, and nuclear transfer embryos by analysis of DNA replication, nucleolar fibrillarin label, and localization of nuclear material reactive to the MPM-2 antibody. Nuclear transfer embryos were derived from G1/early S-phase donor nuclei and MII oocytes. In nuclear transfer embryos, DNA rereplication was likely to have occurred because label was incorporated, possibly in the centromeric regions of the chromosomes, prior to premature chromosome condensation and again following pronuclear formation. In parthenotes, DNA replication began very late in the cell cycle, which may be due to deficiencies in the artificial activation stimulus. The presence of fibrillarin label in the nucleolus was used as an indication of nucleolar transcriptional activity. Fibrillarin label was absent in embryos of all types up to the 16-32-cell stage. Although fibrillarin reappeared in nuclear transfer and parthenote embryos at the appropriate stage, not all blastomeres showed label indicating impaired development in these embryos. Labelling of phosphorylated epitopes by MPM-2 antibody showed a change in pattern of labelling during early development. Early cleavage stage embryos did not exhibit labelling over the spindle poles as did blastomeres from 32-cell embryos and tissue culture cells. All cell types exhibited labelling during interphase as dots located primarily over the nucleus in blastomeres from 32-cell embryos and in tissue culture cells, together with cytoplasmic label in embryos at early cleavage stages. Nuclear transplant embryos had a normal pattern of MPM-2 label. In contrast, the appearance of MPM-2 label in parthenotes depended on the type of calcium stimulation. These results demonstrate defects in DNA synthesis, nucleolar activity, and specific phosphorylation events, likely resulting from an improper activation stimulus and chromosome condensation in the transplanted nucleus.