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A mouse, a mouse made with rat stem cells, a rat with mouse stem cells, and a rat.

A mouse, a mouse made with rat stem cells, a rat with mouse stem cells, and a rat.

Nakauchi et al./The university of tokyo

Major grant in limbo, NIH revisits ethics of animal-human chimeras

Few grants are more coveted than a Pioneer Award from the U.S. National Institutes of Health (NIH), which provides up to $500,000 annually for 
5 years to a researcher pursuing innovative, potentially ground-breaking research. So when Juan Carlos Izpisúa 
Belmonte, a developmental biologist at the Salk Institute for Biological Studies in San Diego, California, heard from NIH  earlier this year that his was among the top-ranked applications, he was thrilled—but there was a catch. The application is on hold, the agency has told him, as NIH reconsiders its rules for the kind of experiments he wants to do: mixing human stem cells into very early animal embryos and letting them develop, a strategy that could produce tissues or organs for transplantation.

On 23 September, NIH issued a notice saying that it will not fund such research “while the Agency considers a possible policy revision in this area.” And it has invited scientists and bioethicists to a meeting on 6 November to discuss the ethical questions raised by such experiments. Izpisúa Belmonte took the news in stride. “I’m not upset. Quite the opposite. I think it is great that we openly discuss this and hope that a conclusion is reached,” he says.

The mixtures of cells under debate are called chimeras, named for a monster of Greek mythology that had the body and head of a lion, a fire-breathing goat’s head on its back, and a snakelike tail. In ancient Greece, the chimera was a bad omen, appearing before shipwrecks, volcanoes, and other disasters.

Scientists see the term in a more positive light, using animal chimeras in a range of developmental biology and stem cell experiments. Chimeras that combine animal and human cells, especially those that involve pluripotent human cells, raise ethical questions, however. Pluripotent cells are a powerful type of stem cell that can become any cell type in the body. Some worry that such human cells, when combined with animal embryos, could develop into brain cells, sperm, or egg cells in the chimeric offspring.

The U.S. National Research Council and the Institute of Medicine recommended limits on such research in 2005, among them that no human stem cells be added to primate embryos and that animal-
human chimeras not be allowed to breed. Current NIH funding guidelines, finalized in 2009, reflect those recommendations. They prohibit breeding animals in which human stem cells might have become sperm or eggs, and they rule out primate-human experiments. They do not, however, prohibit injecting human pluripotent cells into the embryos of other animals and letting the chimeras develop.

That is what several groups of researchers are now trying to do. Their goal is to learn how to coax stem cells to become specific tissues or organs. Doing that in the lab, by recreating the 3D environment of a developing organ and reproducing all the signals it receives, is very difficult. “We don’t know how to guide the cells to become the cells we want,” Izpisúa Belmonte says. Instead, he and his colleagues want “to use the animals as an incubator. We don’t know how they do it, but every day [developing animals] produce perfect organs.”


So, Izpisúa Belmonte and others have set out to knock out the genes that drive the creation of specific organs or tissues, such as the pancreas, in an animal embryo and then inject human pluripotent stem cells into it. They hope that the human cells will preferentially fill the void left by the animal’s missing pancreatic cells, forming a human pancreas in the developing animal.

Earlier this year he and his colleagues identified a new type of human pluripotent stem cell that seems to be especially good at contributing to animal embryos. By injecting these cells into pig embryos, they have now made chimeras that have developed for 2 to 
3 weeks. (He uses non-NIH funds for the work.) So far, Izpisúa Belmonte says, no human cells have been seen in the nervous system. The cells do, however, contribute to the developing pancreas 
and heart.

The Pioneer Award would have enabled him to take the effort further, but last month Izpisúa Belmonte received a letter from NIH saying that the application had been put on hold and inviting him to the upcoming meeting on chimeras. Izpisúa Belmonte will be there. “I applaud this workshop. It’s important to have guidelines so that researchers have a clear path in this promising and fast-moving area.”

An NIH spokesperson tells Science that the agency wants to “evaluate the state of the science in this area, the ethical issues that should be considered, and the relevant animal welfare concerns.” NIH says that no current grant has been halted by the funding pause, but other scientists fear that the workshop is a step toward broader restrictions, says Steve Goldman, a neuroscientist at the University of Rochester in New York whose work involves injecting human stem cells into mouse brains. That approach is not affected by the new rule, since he doesn’t work with early embryos, but NIH has asked him to speak at the November meeting.  

Sean Wu, who studies heart stem cells at Stanford University in Palo Alto, California,  says the NIH notice prompted Stanford to urge scientists to be sure their experiments didn’t violate the new rules. “We were bombarded” with emailed warnings, says Wu, who received an NIH New Innovator Award in 2008 to study interspecies chimeras. The university announcement came “out of the blue”, he notes, and has spread confusion and concern. An NIH decision not to fund the work would be a big blow to the field, Wu says. “No one wants to do it if NIH won’t fund it.”

For Hiromitsu Nakauchi, a stem cell scientist at Stanford University, and the University of Tokyo, the debate is familiar. He showed in 2010 that by adding rat stem cells to mice embryos lacking a pancreas gene, he could grow a rat pancreas in a mouse. The technique also enabled his team to grow a pancreas from one pig species in the body of another. But the follow-up experiments he wanted to do with human stem cells in goat or pig embryos were forbidden in Japan. An ethics commission decided that the experiments should be allowed, but official regulations still aren’t in place, Nakauchi says. In part because of the restrictions in Japan, he accepted a position at Stanford University, where he received $6.2 million for the work from the California Institute for Regenerative Medicine (CIRM).

Nakauchi’s project is proceeding with colleagues at the University of California, Davis, and at Stanford, where they have injected human induced pluripotent stem cells into sheep embryos. He  was planning to apply for NIH funding to 
continue the work, and the agency’s announcement surprised him.

Using chimeras to grow human tissues is a long way from the clinic, Izpisúa Belmonte stresses, but his initial results have him optimistic that the approach is a viable one. “I am very excited about this project,” he says. “We need to do the experiments and see.” Whether NIH will support that work may be determined next month. 

With reporting by Jocelyn Kaiser.