Formation of nucleoli in interspecies nuclear transfer embryos derived from bovine, porcine, and rabbit oocytes and nuclear donor cells of various species

Abstract

The most successful development of interspecies somatic cell nuclear transfer (iSCNT) embryos has been achieved in closely related species. The analyses of embryonic gene activity in iSCNT embryos of different species combinations have revealed the existence of significant aberrations in expression of housekeeping genes and genes dependent on the major embryonic genome activation (EGA). However, there are many studies with successful blastocyst (BL) development of iSCNT embryos derived from donor cells and oocytes of animal species with distant taxonomical relations (inter-family/inter-class) that should indicate proper EGA at least in terms of RNA polymerase I activation, nucleoli formation, and activation of genes engaged in morula and BL formation. We investigated the ability of bovine, porcine, and rabbit oocytes to activate embryonic nucleoli formation in the nuclei of somatic cells of different mammalian species. In iSCNT embryos, nucleoli precursor bodies originate from the oocyte, while most proteins engaged in the formation of mature nucleoli should be transcribed from genes de novo in the donor nucleus at the time of EGA. Thus, the success of nucleoli formation depends on species compatibility of many components of this complex process. We demonstrate that the time and cell stage of nucleoli formation are under the control of recipient ooplasm. Oocytes of the studied species possess different abilities to support nucleoli formation. Formation of nucleoli, which is a complex but small part of the whole process of EGA, is essential but not absolutely sufficient for the development of iSCNT embryos to the morula and BL stages.

Absence of nucleolus formation in raccoon dog-porcine interspecies somatic cell nuclear transfer embryos results in embryonic developmental failure

“…It has been reported that, in some iSCNT embryos, nucleoli were not formed and development was arrested depending on the species of the cell donor and oocyte [13]. We examined the formation of nucleoli in raccoon dog iSCNT embryos by anti-nucleolin (C23) antibody labeling.…”

“…It is present primarily in nucleoli with active RNA polymerase I. All these attributes make nucleolin an ideal marker of nucleoli [13]. …”

“…The demethylation of repetitive sequences of the donor genome is determined by recipient ooplasm and not by intrinsic properties of the donor nucleus [24, 25]. Ooplasms with different demethylation capacities have different abilities in supporting the initiation of nucleolus formation in various iSCNT embryos [13]. …”

“…Nucleoli are closely related to embryonic genome activation (EGA). Nucleolus formation in IVF bovine and porcine embryos occurs at the time of EGA [13]. …”

“…It was observed that successfully developed iSCNT embryos showed nucleolus formation, whereas unsuccessfully developed iSCNT embryos had no nucleolus formation. Indeed, formation of nucleoli is essential for iSCNT embryo development [13]. …”