Reprogramming of the higher order chromatin organization in bovine nuclear transfer embryos.

Abstract

Reprogramming of nuclei after somatic cell nuclear transfer is a crucial process to reset the expression profile of the donor cell to that of a developing embryo. Reprogramming affects the epigenome of the donor nucleus, which is the complete set of epigenetic determinants that establishes and maintains a given transcriptome. These epigenetic determinants include e.g. DNA methylation, modifications of histone tails, histone variants, and other chromatin components. A less well characterized level of epigenetic gene regulation is the spatial higher order chromatin organization. In somatic cells chromosomes are organized as individual entities called chromosome territories (CTs), which localize according to their gene density with gene-rich chromosomes being in the nuclear interior, gene-poor ones at the nuclear periphery. We have shown previously that this gene density related distribution is established during major genome activation (MGA) in bovine in vitro fertilized (IVF) embryos, while embryos up to the 8-cell stage do not show a difference in their CT localization. In the present study, we investigated whether the gene density related distribution of chromosomes present in bovine fibroblasts is reprogrammed after nuclear transfer (NT), i.e. erased to restore the situation found in IVF embryos. Moreover, we analyzed if a gene density related CT arrangement is re-established at the same time point as in IVF embryos, i.e. during MGA. Therefore we analyzed the nuclear distribution of bovine chromosome 19 (19 genes Mb-1) and 20 (5 genes Mb-1) in one-, 2- and 4-cell NT embryos as pre-MGA stages and in embryos with 10&#x2013;20 nuclei cells-1, representing embryos during MGA. CTs were visualized via a recently developed protocol utilizing fluorescence in situ hybridization (FISH) on three dimensionally preserved embryos. In 1- and 2-cell embryos we could not detect a gene density related CT distribution arguing for a rapid reprogramming, i.e. erasure of the positional information previously present in the donor nuclei. Surprisingly, we observed a precocious establishment of a gene density related distribution already at the 4-cell stage that became more pronounced in 10&#x2013;20-cell embryos, when this specific CT distribution is established in IVF embryos. We conclude that the spatial arrangement of chromosomes is an epigenetic parameter that is rapidly reprogrammed upon nuclear transfer of a somatic nucleus, exposed to the oocyte cytoplasm. Moreover, the gene density related positioning of chromosomes in bovine NT embryos is re-established earlier than in IVF embryos, suggesting a possible involvement in early developmental failures of NT embryos.<fn-group><fn>Funded by the Deutsche Forschungsgemeinschaft (ZA 425/1-2, CR 59/26-1).</fn></fn-group>