David Cyranoski in Nature:
In 2006, Japanese researchers reported1 a technique for creating cells that have the embryonic ability to turn into almost any cell type in the mammalian body — the now-famous induced pluripotent stem (iPS) cells. In papers published this week in Nature2, 3, another Japanese team says that it has come up with a surprisingly simple method — exposure to stress, including a low pH — that can make cells that are even more malleable than iPS cells, and do it faster and more efficiently. “It’s amazing. I would have never thought external stress could have this effect,” says Yoshiki Sasai, a stem-cell researcher at the RIKEN Center for Developmental Biology in Kobe, Japan, and a co-author of the latest studies. It took Haruko Obokata, a young stem-cell biologist at the same centre, five years to develop the method and persuade Sasai and others that it works. “Everyone said it was an artefact — there were some really hard days,” says Obokata.
Obokata says that the idea that stressing cells might make them pluripotent came to her when she was culturing cells and noticed that some, after being squeezed through a capillary tube, would shrink to a size similar to that of stem cells. She decided to try applying different kinds of stress, including heat, starvation and a high-calcium environment. Three stressors — a bacterial toxin that perforates the cell membrane, exposure to low pH and physical squeezing — were each able to coax the cells to show markers of pluripotency. But to earn the name pluripotent, the cells had to show that they could turn into all cell types — demonstrated by injecting fluorescently tagged cells into a mouse embryo. If the introduced cells are pluripotent, the glowing cells show up in every tissue of the resultant mouse. This test proved tricky and required a change in strategy. Hundreds of mice made with help from mouse-cloning pioneer Teruhiko Wakayama at the University of Yamanashi, Japan, were only faintly fluorescent. Wakayama, who had initially thought that the project would probably be a “huge effort in vain”, suggested stressing fully differentiated cells from newborn mice instead of those from adult mice. This worked to produce a fully green mouse embryo.