Researchers have created live mice from induced pluripotent stem (iPS) cell lines, demonstrating iPS cells are as pluripotent as embryonic stem (ES) cells, according to two studies published online today.

iPS mouse Image: Qi Zhou, Chinese Academy of Sciences

"This ability to make a mouse from the cells of a Petri dish is the most stringent standard that mouse embryonic cells are held to, and iPS cells now meet that standard, which has been a lingering doubt," said stem cell biologist George Daley of the Harvard Stem Cell Institute and Children's Hospital Boston, who was not involved in the work. "It assures us that iPS cells can do all the things that ES cells can do."

In one study, published in Nature, researchers created 37 iPS cell lines from mouse embryonic fibroblasts (MEFs) by exposing them to the four 'Yamanaka factors' -- the cocktail of transcription factors that most robustly induce pluripotency. They then subjected six of these cells lines to tetraploid complementation, a technique involving the creation of a four-cell blastocyst. These four-cell blastocysts can only contribute to extraembryonic tissues such as the placenta and thus fail to develop, but injected stem cells can develop into young if the stem cells are truly pluripotent. It's "the gold standard of pluripotency," said Fanyi Zeng of Shanghai Jiao Tong University, a coauthor on the study, in a telephone press briefing.

Using this technique with their iPS cell lines, the researchers succeeded in producing 27 live mice composed entirely from iPS cells. The first generation mice are now more than 9 months old. The researchers said they have some abnormalities (not described in the paper), but declined to specify what they were in the briefing. They did say they had also produced more than 200 second generation iPS mice and 100 third generation offspring, which seem normal and healthy so far. "[This is the] first time to confirm [that] iPS cells have full potency," said study coauthor Qi Zhou of the Chinese Academy of Sciences in the briefing.

In the second study, published in Cell Stem Cell, Shaorong Gao and his colleagues at the National Institute of Biological Sciences in Beijing used the same technique to create and test five iPS cell lines from MEFs. They successfully created four full term pups, one of which lived to adulthood and appears to be healthy, according to the paper.

Not all the cell lines the researchers created were able to generate viable offspring. In fact, only half of the iPS cell lines tested in Zeng and Zhou's group produced live mice, and only one of five lines from Gao's group worked. One possible explanation for this failure is the way they induced pluripotency, said stem cell researcher Rudolf Jaenisch of the Massachusetts Institute of Technology, who was not involved in the work. The viruses used to introduce the Yamanaka factors to the MEFs can integrate unpredictably into the genome, and alternative methods of inducing pluripotency are a hot area of stem cell research. "These viral vectors they used to make the iPS cells are not fully silent," Jaenisch said. "Even low levels of vector expression change the cells in a molecular way." Thus, it is possible that varying levels of vector expression can affect some iPS lines more than others.

Achieving higher efficiency in this process is certainly a future goal, Zeng said. Her team is now looking into why the iPS cell lines derived at 14 days after being infected with the Yamanaka factors seem to be more efficient than those derived at 20 or 36 days. It will also be informative to conduct similar tests of pluripotency with adult fibroblasts and other types of tissue, Zeng said, in order to help identify "the fundamental mechanisms of cellular reprogramming."

The demonstration that iPS cells can generate healthy embryos does not mean stem cell researchers should abandon ES cell studies, said Daley. "iPS cells still, in most instances, have some differences. That means continuing to compare them to ES cells."

"We strongly believe that we should promote all kinds of stem cell research, not limited to iPS," Zeng agreed. "Human ES cells remain a prototype."


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Piggybacking to pluripotency
[1st March 2009]

Safer iPS cells
[25th September 2008]