Nature: Eight Proteins Transform Mouse Stem Cells into Oocyte-like Cells

A combination of eight proteins can transform mouse stem cells into cells that look much like immature eggs, oocytes. The resulting ovomatrix-like cells cannot undergo meiosis, but can be fertilized by sperm and then divide until they reach the eight-cell stage of embryonic development, the researchers reported.

"This suggests that there can be a direct transition from stem cells to oocytes. I think this is exciting," says Petra Hajkova, a developmental epigeneticist at Imperial College London, which will help researchers explore the basic biology of oocyte development.

In the future, the study could help clone endangered animals or help women with mitochondrial diseases have healthy children, said Yoshihiko Yamasaki (from Kyushu University), one of the authors of the article.

Oocytes are a relatively scarce cell type in the human body and are not well understood, which has attracted him and colleagues to explore how they develop, Yamasaki said. Past studies have demonstrated that changes in gene expression are essential for very early primordial germ cell migration into oocytes. On this basis, Hamazaki and colleagues performed more gene expression analysis and identified 27 candidate transcription factors that function during migratory transformation. To test the function of each transcription factor, the team used pluripotent embryonic stem cells (ESC) that knock out or inactivate the genes encoding transcription factors one by one. "It is much easier to construct knockout pluripotent embryonic stem cells than to construct knockout mice, but despite this, considerable work needs to be done to prepare 27 knockout pluripotent embryonic stem cells," Yamasaki said.

The experimental results revealed eight transcription factors essential for oocyte development: NOBOX, FIGLA, TBPL2, SOHLH1, STAT3, DYNLL1, SUB1 and LHX8. The team then cultured another group of embryonic stem cells and "overexpressed" the genes that produce these eight transcription factors, which as a result forced these embryonic stem cells into an oocyte-like state.

He explained: "It is thought that oocytes develop from germ cells, but we can successfully prepare oocytes from non-germ cells." "At first, I was so surprised that I could not believe my results, so I repeated the experiment again and again, and when I got the same results, I was finally sure."

In subsequent experiments, ovoid cells did not continue to divide into daughter cells with half of the chromosomes—meiosis is an important part of reproduction. But the researchers said that when the team introduced wild-type mouse sperm into the culture, the oocytes did divide into cell clusters of 8 cells and then could not survive, possibly because these cells had too many chromosomes.

Richard Schultz, a cell biologist at the University of California, Davis, commented that the work was impressive in identifying core transcription factors that can push embryonic stem cells to look like oocyte states. But ovoid cells do not undergo meiosis, so they are not functional. This was a great improvement, but only 95 percent was completed. We haven't got 100 percent information yet "— to understand which essential key factors can promote germline oocytes to develop and mature into oocytes, which then mature into eggs with half of their chromosomes.

Although no pathway was found to direct meiosis, this work "allowed us to produce large numbers of oocytes. We believe that this technology can accelerate basic biological research in oocytes, which remain one of the most mysterious cell types." Yamasaki explained that due to the large number of oocytes produced by the team's technology, this work may help improve animal cloning. The cytoplasm of the induced oocyte-like cells did not show any abnormalities and therefore may also be valuable for reproductive problems in women with mitochondrial diseases. Children inherit the disease from their mothers, but the use of enucleated stem cells to induce oocytes (which may have healthy mitochondria) as carriers of the nuclei of oocytes from affected mothers may be a solution to this problem.

"This shows that these oocytes may be very useful," Hajkova said.