Acid bath offers easy path to stem cells
Just squeezing or bathing cells in acidic conditions can readily reprogram them into an embryonic state. In papers published this week in Nature, 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.
Haruko 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.
To convince sceptics, Obokata had to prove that the pluripotent cells were converted mature cells and not pre-existing pluripotent cells. So she made pluripotent cells by stressing T cells, a type of white blood cell whose maturity is clear from a rearrangement that its genes undergo during development. She also caught the conversion of T cells to pluripotent cells. Obokata called the phenomenon stimulus-triggered acquisition of pluripotency (STAP).
One of the most surprising findings is that the STAP cells can also form placental tissue, something that neither iPS cells nor embryonic stem cells can do. That could make cloning dramatically easier, says Wakayama. Currently, cloning requires extraction of unfertilized eggs, transfer of a donor nucleus into the egg, in vitro cultivation of an embryo and then transfer of the embryo to a surrogate. If STAP cells can create their own placenta, they could be transferred directly to the surrogate.
Obokata has already reprogrammed a dozen cell types, including those from the brain, skin, lung and liver, hinting that the method will work with most, if not all, cell types. On average, she says, 25% of the cells survive the stress and 30% of those convert to pluripotent cells — already a higher proportion than the roughly 1% conversion rate of iPS cells, which take several weeks to become pluripotent. She now wants to use these results to examine how reprogramming in the body is related to the activity of stem cells. Obokata is also trying to make the method work with cells from adult mice and humans.
Nature 505, 596 (30 January 2014)