Cell-Free Extracts from Mammalian Oocytes Partially Induce Nuclear Reprogramming in Somatic Cells1

Nuclear transfer has been regarded as the only reliable tool for studying nuclear reprogramming of mammalian somatic cells by oocytes. However, nuclear transfer is not well suited for biochemical analyses of the molecular mechanisms of reprogramming. A cell-free system from oocytes is an attractive alternative way to mimic reprogramming in vitro, since a large number of cells can be treated and analyzed. Nevertheless, a cell-free system using oocytes has not been developed in mammals. Here, cell extracts from porcine oocytes were prepared and their ability to induce nuclear reprogramming was evaluated. Extracts from metaphase II (MII) oocytes erased the machinery for regulating gene expression in reversibly permeabilized somatic cells. For example, the extracts caused histone deacetylation and the disappearance of TATA box-binding protein from the nuclei. However, MII-extract-treated cells did not show any obvious changes after cell culture. In contrast, extracts from germinal vesicle (GV) oocytes activated pluripotent marker genes, especially NANOG, and induced partial dedifferentiation after cell culture. The activation of pluripotent marker genes by GV extracts was associated with histone acetylation that was induced during extract treatment. These results indicate that GV- and MII-oocyte extracts have different roles on nuclear reprogramming. Furthermore, both oocyte extracts induced site-specific demethylation in the upstream region of NANOG. These results indicate that cell-free extracts derived from GV- and MII-oocytes could be useful for studying the mechanisms involved in nuclear reprogramming.