Architectural defects in pronuclei of mouse nuclear transplant embryos.

Reprogramming somatic nuclear function by transplantation of nuclei into recipient oocytes is associated with a morphological remodeling of the somatic nucleus. Successful cloning of animals by nuclear transplantation (NT) demonstrates that reprogramming somatic cell function is possible. However, low pregnancy rates and high frequencies of lethal abnormalities in animals born suggest that reprogramming is rarely complete. To address this issue, we tested the hypothesis that nuclear transplantation leads to nuclear remodeling deficiencies. We report the identification of several markers of morphological remodeling, or lack thereof, of mouse cumulus cell nuclei after transplantation into oocytes. Notably, nuclear transplant mouse embryos exhibit nuclear assembly of the differentiated cell-specific A-type lamins at the one-cell stage, as a result of misregulation of lamin A gene expression. The transplanted nuclei also display enhanced concentration of the nuclear matrix-associated protein NuMA as a result of translation from maternal mRNA and de novo transcription. The A-kinase anchoring protein 95 (AKAP95), a marker of the nuclear envelope-chromatin interface, is of somatic origin. Furthermore, greater resistance of AKAP95 and DNA to in situ extractions of one-cell stage NT embryos with non-ionic detergent, DNase, RNase and NaCl reflects an enhanced proportion of heterochromatin in these embryos. Passage through first embryonic mitosis does not rescue the defects detected in one-cell stage embryos. We propose that somatic nuclear reprogramming deficiencies by NT might emanate from, at least in part, failure to remodel the somatic nucleus morphologically into a functional embryonic nucleus.