When historians chronicle the stem cell research wars, Shinya Yamanaka will likely go down as a peacemaker. The Japanese scientist has helped send the field on a surprising end run around the moral debate surrounding embryonic stem cells, the creation of which requires the destruction of embryos. Last year Yamanaka led one of two teams that showed that normal human skin cells can be genetically reprogrammed into the equivalent of stem cells. These so-called induced pluripotent stem cells (iPS cells) seem to be essentially identical to embryonic stem cells and possess the ability to become any cell.

The 46-year-old Yamanaka is a clean-cut, almost military figure. His small office in an aging wing of Kyoto University’s Institute for Frontier Medical Sciences is spotlessly tidy, with nothing to mark his achievement in producing iPS cells. A Nobel Prize may one day adorn his shelf space. As Yamanaka glances around, he remarks, “About 10 meters beneath us is a room that I have never entered. I’m not allowed to enter because I don’t have permission from the government. It contains the only stem cells derived from human embryos in the country.”

Though permissive in spirit, Japan in practice imposes strict rules on the production and (unlike in the U.S.) the use of stem cells derived from human embryos. Researchers can spend up to a year in paperwork submissions before gaining access to them.

It was Japan’s rule-bound, often stifling scientific culture that made Yamanaka an accidental pioneer. Originally an orthopedic surgeon in Osaka, he decided in the mid-1990s to do postdoctoral work on genetic reprogramming of cancer-related genes in mice at San Francisco’s Gladstone Institute of Cardiovascular Disease. There he found ready access to existing lines of embryonic stem cells, as well as an environment with solid funding and exchanges among leading researchers worldwide. At home, though, he went into a funk. “When I went back to Japan, I lost all those stimuli,” Yamanaka recalls. “I had only a little funding and a few good scientists around me, and I had to take care of almost 1,000 mice by myself.”

Fighting despair, he was about to quit and return to surgery. But two things galvanized him to continue: an invitation to head a small lab at the Nara Institute of Science and Technology and the creation of the first generation of human embryonic stem cells, which was made by the University of Wisconsin–Madison’s James A. Thomson (who last year led the other team that produced human iPS cells).

After Thomson’s achievement in isolating embryonic stem cells, many researchers began trying to control the differentiation of those cells into specific cell types that might replace diseased or damaged tissues, thereby revolutionizing clinical care. “That was too competitive for our small lab,” Yamanaka recounts, “so I thought I should do the opposite—instead of making embryonic stem cells into something, I would make embryonic stem cells from something else.” From Ian Wilmut’s success in cloning animals such as Dolly the sheep, he says, “we knew that even completely differentiated cells can go back to an embryonic­like status. But we also thought it would be a very, very long project,” one that might take 20 or 30 years.

It took fewer than 10. Yamanaka became highly motivated to solve two main problems surrounding embryonic stem cells. One was their source. He tells of visiting a friend’s fertility lab and observing early embryos under a microscope. The sight of fragile, nascent life moved him, although he emphasizes that he is not against using embryonic cells “to save patients.” The other problem is the threat of immune rejection if cells derived from an embryo are transplanted into a person. Differentiated cells created from a patient’s iPS cells would pose no such danger.

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At first, then, Yamanaka set about to determine how mouse embryonic cells maintain their pluripotency, the ability to differentiate into any body cell type. He hypothesized that certain proteins would be evident in mouse embryonic cells but not in differentiated cells. He also thought that introducing the genes for these proteins—specifically, transcription factors, which control the activity of other genes—into a normal skin cell’s chromosomes would transform it into an embryonic cell.

After four years of experimentation, he uncovered 24 factors that, when added to ordinary mouse fibroblast cells and subjected to the correct culturing procedures, could create pluripotent cells virtually identical to stem cells. Yamanaka kept examining each factor and found that none could do the job alone; instead a combination of four particular genes did the trick. In 2006 he published a landmark article in Cell identifying them: Oct3/4, Sox2, c-Myc and Klf4.

News of the stunning feat prompted scientists around the world to try to reproduce it using human, rather than mouse, cells. In 2007 Yamanaka reported that triumph with the four transcription factors at the same time as Thomson’s team. “It is actually fairly straightforward to repeat what we have done,” Thomson told the press at the time—still, researchers have likened the breakthrough to turning lead into gold.

The achievement sparked many investigators to switch their efforts from embryonic stem cells to the induced versions. Yamanaka and others have now derived iPS cells from a variety of tissue types, including liver, stomach and brain, and turned the iPS cells into skin, muscle, gut and cartilage, as well as neural cells that can secrete the neurotransmitter dopamine and heart cells that can beat in sync.

Two big safety issues, though, will keep iPS cells out of the clinic for a while. One is that the transcription factor c-Myc happens to be a powerful cancer gene, and the cells produced by Yamanaka’s team tended to become cancerous. “Making iPS cells is very similar to making cancer,” he explains. In principle, c-Myc may not be necessary: in mice, Yamanaka and a group led by Rudolf Jaenisch at the Massachusetts Institute of Technology found a way to avoid using c-Myc, in part, by optimizing culture conditions. Out of 100 mice implanted with iPS cells created without c-Myc in Yamanaka’s lab, none died after 100 days, compared with six out of 100 that died of tumors when c-Myc was used.*

The other risk is the vector used to deliver the genes into target cells—namely, retroviruses. The process results in stem cells full of viruses. Moreover, retroviruses can induce mutations in the cells that lead to cancer. Researchers may soon overcome this hurdle, too. In September a team at the Harvard Stem Cell Institute announced the creation of mouse iPS cells using as a vector the adenovirus, which is safer than retroviruses. In October, Yamanaka’s lab reported success using plasmids, or circular pieces of DNA. Other retrovirus alternatives include proteins and lipid molecules.

Although the surge in interest has led to rapid developments and much competition among labs, Yamanaka and others do not think that iPS cells can replace their embryonic counterparts yet. “We don’t yet know if embryonic stem cells and iPS cells are truly equivalent,” says Konrad Hochedlinger of Massachusetts General Hospital’s Center for Regenerative Medicine. He adds that “at this point, iPS cells are a powerful additional source of pluripotent cells. Time will tell if iPS cells will at some point replace embryonic stem cells. It would be premature to make such a decision now.”

But while insisting iPS cell work remains far from being clinic-ready, Yamanaka trumpets its vast potential for conditions such as diabetes, spinal cord injury, Parkinson’s and even, he chuckles, baldness. “This enormous and striking finding provides a clear framework for regenerative medicine and cell therapy,” says Shin­ichi Nishikawa, director of the Laboratory for Stem Cell Biology at Japan’s RIKEN Center for Developmental Biology.

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Over the next five years, Yamanaka’s group of about 20 investigators will focus on how iPS cells can help predict a drug’s side effects and elucidate problems in toxicology and disease mechanisms. For all the excitement, possibilities and rivalry bubbling in the wake of his findings, the one-time physician tempers his expectations with firm caution. “We still need a lot of basic research in terms of the safety of iPS cells,” Yamanaka reiterates. “This is a not an international competition like the Olympic Games. It should be international collaboration. This is the beginning of a long process.”

Note: This story was originally published with the title, "Turning Back the Cellular Clock."

*Erratum (10/8/12): The number of mice used was incorrect as published. The sentence should have read: "Out of 26 mice implanted with iPS cells created without c-Myc in Yamanaka's lab, none died after 100 days, compared with six out of 37 that died of tumors when c-Myc was used."