30 Jan 2014

This paper was retracted on 3 July 2014.

Until now, scientists turned mature mammalian cells into pluripotent ones by modifying their genomes (see Takahashi and Yamanaka, 2006). However, two papers in the January 30 Nature outline a faster, cheaper, and easier method. Scientists led by Teruhiko Wakayama, RIKEN Center for Developmental Biology, Kobe, Japan, and Charles Vacanti, Brigham and Women's Hospital, Boston, found that submerging white blood cells from a mouse in acid for half an hour reset them to a naïve, undifferentiated state. These turned out to be even more developmentally flexible than embryonic stem cells. "It was surprising to see that such a remarkable transformation could be triggered simply by stimuli from outside of the cell," said first author Haruko Obokata of RIKEN. She added that the finding may upend the canonical view of cellular differentiation and pluripotency.

Differentiated cells from a plant can be isolated and made pluripotent with chemicals (for a review, see Thorpe, 2007). However, no study has yet shown that animal cells already committed to a specific cell fate can return to their original naïve condition in a similar fashion. Obokata and colleagues wondered if external, stressful stimuli could do the trick.

To find out, they bombarded white blood cells from newborn mice with various stressors that nearly killed them, such as heat, acid, squeezing, and excess calcium exposure. They then cultured the survivors in a medium that supports stem cells. To the authors' surprise, several physical stressors brought about the expression of pluripotent markers. An acidic solution with a pH of about 5.7 for 30 minutes worked particularly well. After a week, half of the surviving cells stopped expressing their own blood cell-specific markers and instead pumped out fluorescently labeled Oct4 (see movie below)—a protein marker of pluripotency—as well as other markers seen in embryonic stem cells, such as Nanog (see May 2003 news story).

Stressed out

Lowering the pH included expression of Oct4 (green) and other markers of pluripotency in lymphocytes. Click to view video. *Movie courtesy of Haruko Obokata*

The genes for those two proteins were demethylated, implying that the epigenome had been altered. Authors termed this phenomenon "stimulus-triggered acquisition of pluripotency," or STAP. Similar acid-conversion experiments worked in cells from other tissues, including skin, muscle, fat, and brain. Obokata said she did not isolate a specific cell type from the brain, but instead used a mixture.

Injected into embryos, these STAP cells showed all the hallmarks of pluripotency. They successfully integrated into chimeras—mice comprised of a mixture of native and donor cells. Some even made their way into the chimeras' germlines, giving rise to STAP-cell-derived offspring all on their own (see image below). In culture, the cells differentiated into representatives of all three germ layers, and formed teratomas when injected into mice. While STAP cells lasted only a short time in culture, they self-renewed for at least four months if the scientists grew them in media typically used to cultivate embryonic stem cells. They called these embryonic stem cell-like specimens STAP "stem" cells.

STAP cells can create an entire fetus. *Image courtesy of Haruko Obokata*

In the second paper, the researchers note that in chimeric mice, STAP cells contributed not only to the embryo, but also to the placenta, something that embryonic stem cells cannot usually do. In fact, if they were grown in a placenta-friendly medium, STAP cells started turning into placental precursors. This suggests that STAP cells represent an even earlier developmental state than the embryonic variety, wrote the authors. After fertilization, the blastocyst gives rise to both placental and embryonic lineages.

This work implies, though it does not prove, that STAP cells are as potent as the fertilized egg, said Lorenz Studer, Memorial Sloan Kettering Cancer Center, New York. "Somatic cells latently possess a surprising plasticity," wrote the authors. They hypothesized that regulatory molecules activated by stressors release the epigenetic shackles that hold a cell to one identity, and free it up to take on another.

Do STAP cells form in vivo? They may be blocked somehow, suggested the authors. In preliminary experiments, they looked at cells from the esophagus of mice with acid reflux. They found several that expressed Oct4, but not Nanog, suggesting a barricade early on the path to pluripotency.

This finding "opens up the possibility of obtaining patient-specific stem cells by a simple procedure," wrote Austin Smith, University of Cambridge, U.K., in an accompanying News & Views article. He pointed out that STAP cells have yet to be produced from other species, including humans. In addition, since these scientists used samples from newborn animals, it remains to be seen if adult cells can undergo the same conversion. In preliminary experiments, Obokata found that STAP capability dwindles with age, but may eventually be easier even in old cells, with tweaks to the protocol. Despite these limitations, the scientists "have established a new principle: that a physical stimulus can … dismember gene-control circuitry and create a ‘plastic’ state," Smith wrote.

Obokata would not comment on the relevance of these findings to human research, as she and her colleagues have only looked at mouse cells so far. However, she said that experiments on samples from newborn babies are now underway. Senior author Vacanti is noted for developing techniques in tissue engineering and for pilot human experiments. Outside scientists interviewed for this article were cautiously optimistic. Asa Abeliovich, Columbia University, New York, said that while the finding is mechanistically exciting in terms of basic reprogramming, it is too early to speculate about how applicable it will be, for instance, for the study of human disease. Studies of human cells in vitro will help clarify, he said.

Studer agreed. Until it is clear what STAP cells represent and how they are made, it is difficult to predict whether the technique will replace current technologies to induce pluripotent stem cells, he said. Nevertheless, "the data look quite convincing," he told Alzforum. "They offer a new way to make cells that can differentiate into all the cells types of the body, which is pretty remarkable because there haven't been many ways to do that so far."

Others reserved judgment until the findings are replicated. Obokata mentioned that collaborators on the project from separate labs were able to replicate the findings.—Gwyneth Dickey Zakaib

Acid Bath Turns Cells Pluripotent

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