Abstract
BackgroundGlobal activation of the embryonic genome (EGA), one of the most critical steps in early mammalian embryo development, is recognized as the time when interspecies somatic cell nuclear transfer (iSCNT) embryos fail to thrive.Methodology/Principal FindingsIn this study, we analyzed the EGA-related transcriptome of rhesus-bovine iSCNT 8- to 16-cell embryos and dissected the reprogramming process in terms of embryonic gene activation, somatic gene silencing, and maternal RNA degradation. Compared with fibroblast donor cells, two thousand and seven genes were activated in iSCNT embryos, one quarter of them reaching expression levels comparable to those found in in vitro fertilized (IVF) rhesus embryos. This suggested that EGA in iSCNT embryos had partially recapitulated rhesus embryonic development. Eight hundred and sixty somatic genes were not silenced properly and continued to be expressed in iSCNT embryos, which indicated incomplete nuclear reprogramming. We compared maternal RNA degradation in bovine oocytes between bovine-bovine SCNT and iSCNT embryos. While maternal RNA degradation occurred in both SCNT and iSCNT embryos, we saw more limited overall degradation of maternal RNA in iSCNT embryos than in SCNT embryos. Several important maternal RNAs, like GPF9, were not properly processed in SCNT embryos.Conclusions/SignificanceOur data suggested that iSCNT embryos are capable of triggering EGA, while a portion of somatic cell-associated genes maintain their expression. Maternal RNA degradation seems to be impaired in iSCNT embryos. Further understanding of the biological roles of these genes, networks, and pathways revealed by iSCNT may expand our knowledge about cell reprogramming, pluripotency, and differentiation.
Highlights
In mammals, the maternal RNA and the proteins present in the oocyte’s cytosol are responsible for early embryonic development
We identified 3,438 up-regulated transcripts in the interspecies somatic cell nuclear transfer (iSCNT) embryo compared to bovine oocyte
When we compared this list with the reprogrammed iSCNT transcriptome — 2,007 genes — we found 443 genes shared in both datasets (Fig. 3 A,B), indicating that the reprogrammed transcriptome in the iSCNT embryo partially resembled normal development or normal embryonic genome activation (EGA) gene expression in the in vitro fertilized (IVF) embryo
Summary
The maternal RNA and the proteins present in the oocyte’s cytosol are responsible for early embryonic development These maternal components govern the first embryo cleavages and as they drop by degradation or usage, the zygote nuclei start transcription and protein translation, taking control of embryonic development. This process is called embryonic genome activation (EGA). Compared to the development of a fertilized preimplantation embryo, a somatic cell nuclear transfer (SCNT)derived embryo has the added challenge of silencing its somaticspecific genes while reactivating all the embryo-related genes In doing so, it must shed its differentiated phenotype and gain a new pluripotent state. Global activation of the embryonic genome (EGA), one of the most critical steps in early mammalian embryo development, is recognized as the time when interspecies somatic cell nuclear transfer (iSCNT) embryos fail to thrive
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