Abstract:
The differential epigenetic changes in embryos from different species provide a model to study how the nucleus from one species interacts with cytoplasm from another species. In this study we examined histone methylation at lysine 9 of histone 3 (K9H3) and lysine 20 of histone H4 (K20H4) and the expression levels of three early development-related genes (Oct-4, Hsp 70.1 and Hprt) in individual intra- and inter-species cloned and control embryos at the 1-, 2-, 4- and 8-cell stages. Mouse fetal fibroblast (MFF) nuclei were transferred into mouse, bovine, or rabbit oocytes. As control, we used in vivo derived (mouse and rabbit) or in vitro-produced (bovine) embryos. Histone methylation was detected by anti-MeK9H3 and anti-MeK20H4 antibodies. Gene expression analysis was performed using a quantitative RT-PCR technique (Daniels et al. 2000 Biol. Reprod. 63, 1034-1040). Data were analyzed by Student's t-test. No embryos from inter-species cloning (MFF-bovine and MFF-rabbit) survived beyond the 8-12 cell stage. MFF-mouse and MFF-bovine embryos exhibited demethylation of K9H3 and K20H4 at the 2-cell stage and the methylation level was increased at the 4-cell stage, but no demethylation was observed at the 2-cell stage of MFF-rabbit embryos and the methylation level in these embryos was significantly higher than that of in vivo rabbit embryos. The level of Oct-4 mRNA was low at the 1- and 2-cell stages of in vivo mouse embryos and increased at the 8-cell stage. No significant increase in Oct-4 transcript was detected at the 8-cell stage of inter-species cloned embryos. The expression of Hsp 70.1 in in vivo mouse embryos was increased at the 2-cell stage and decreased to a level similar to that in the zygote at the 8-cell stage. In cloned embryos, Hsp 70.1 transcripts were also increased at the 2-cell stage, but there was no significant decrease of Hsp70.1 mRNA abundance at the 8-cell stage of inter-species embryos as compared to the corresponding 2-cell stage. For MFF-mouse embryos, Hsp 70.1 expression was increased at the 2-cell stage, but at the 8-cell stage the transcript level was at the level similar to that in inter-species clones. Hprt expression was increased at the 8-cell stage of in vivo mouse embryos. The dynamic change of Hprt transcript in MFF-mouse embryos was not significantly different from that of in vivo mouse embryos, but no significant change of Hprt expression occurred in the development of MFF-bovine and MFF-rabbit embryos. Differential epigenetic characteristics of mouse somatic nucleus after transfer into oocytes from different species suggest the existence of incompatibilities of nuclear-cytoplasm interaction between distantly related species. This abnormal interaction at the time of genome activation may affect normal development.<fn_group><fn>This work was supported by the Bayerische Forschungsstiftung and by Therapeutic Human Polyclonals, Inc.</fn></fn_group>