Scientists Discover Synthetic Chemicals that Create Pluripotent Stem Cells from Adult Cells

By Renee Twombly

Scientists at The Scripps Research Institute report that they have significantly improved upon a revolutionary technique that uses genes to turn skin cells from an adult back into pluripotent stem cells.

In the June 5, 2008 issue of the journal Cell Stem Cell, investigators describe for the first time how they identified and used small, drug-like chemicals to help coax mouse brain cells back into pluripotent stem cells in a way that reduced some of the major drawbacks of the technique developed two years ago by Japanese researcher Shinya Yamanaka to produce pluripotent stem cells, once derived only from embryos, from adult cells. The new findings provide a safer, more efficient method to reprogram cells, paving the way for clinical testing of reprogrammed stem cells.

"This shows that we can make cell reprogramming technology much more practical than it has been," says the study's lead author, Sheng Ding, an associate professor at Scripps Research and a chemist who studies stem cell biology. "These advances bring us closer to the day when we can use these powerful cells to make any kind of human tissue that we need to help patients."

Ding has been able to use just two of the four genes (some of which are cancer causing genes) that were initially used to reprogram the cell, and by using synthetic drug-like chemical compounds he has also reduced use of viruses that inserted those problematic genes permanently into the new cell's genome.

He believes that, someday, chemical cocktails might be used to reprogram cells for cell-based therapy. One cocktail of small molecules would be used to revert specialized adult cells back to an earlier developmental stage, and then a second cocktail would differentiate the cell into the type needed to replace diseased cells in any organ or tissue.

"This study is a proof of principle that this kind of approach is possible," he says.

A Different Technique

Last December, two labs, one in Japan and another in Wisconsin, announced they had transformed human skin cells into cells that act like embryonic stem cells, which they dubbed "induced pluripotent stem (iPS) cells." In both cases, the teams used viruses to insert multiple copies of four genes (e.g. c-Myc, Oct4, Sox2, Klf4) permanently into the genome of the cells, and these inserted genes produced transcription factors that turned on and off other genes, pushing the cell to dedifferentiate. These genes are expressed in embryonic stem cells but are generally "silent" in adult cells, and for reasons that are not well understood, once activated, they can reprogram a cell. But cancers can be one result of over-expression and re-activation of these genes, especially c-Myc and Oct4, which are known oncogenes (cancer causing genes). Scientists have in fact seen cancer development when these transformed/reprogrammed cells contributed to adult tissues in animals, Ding says.

While the previous method created a true breakthrough in stem cell science, it can't simply be used for cell therapy, Ding says. "People won't use cells that are permanently modified to contain multiple copies of cancer causing genes."

Researchers have since announced that iPS cells can be produced without use of c-Myc, but the technique is slow and inefficient, according to Ding.

To solve these problems, Ding and his research team, which included investigators from Scripps Research and the Max Planck Institute for Molecular Biomedicine in Germany, searched for cells that naturally express any of the four genes and selected neural progenitor cells from the brains of mice for the proof-of-concept studies. These cells, like many adult tissue-specific stem and progenitor cells, exist in adults and are more developmentally advanced than embryonic stem cells, but still have the capability of morphing into specialized cells, and they expressed Sox genes. Using such cells, the scientists showed that only two genes (in comparison to the original four genes) are sufficient to generate iPS cells, demonstrating the huge advantage of using adult existing progenitor cells for such application with less exogenous genetic manipulation.

Furthermore, the scientists then went on and screened a wide array of small molecules and found different chemicals that could accomplish two critical improvements: one acted to replace viral delivery and over-expression of oncogenic transcription factors to create iPS cells with less genetic modification in cells and another increased the overall efficiency of the cell reprogramming system.

So, with use of the small molecules, researchers only needed to use viruses to add Oct4 and Klf4, or even without Oct4 in another setting, to the neural progenitor cell to push reprogramming.

The study offers a number of important implications, Ding says. It shows that researchers can advantageously exploit adult cells in our body that already express one or some of the four critical genes, and demonstrates that small molecules can functionally replace viral transduction of transcription factors, thus reducing risks associated with genetic manipulation.

Human progenitor cells harvested from the skin could be used in such a system, Ding says.

The small molecules also increased the efficiency by which induced pluripotent stem cells were created, helped researchers select the cells that had successfully undergone transformation, and provided mechanistic targets. "This makes it possible to study the process as it unfolds," Ding says.

Co-authors of the study, titled "A combined chemical and genetic approach for the generation of induced pluripotent stem cells," include: Yan Shi, Caroline Desponts, and Heung Sik Hahm from Scripps Research; and Jeong Tae Do and Hans Schöler from the Max Planck Institute. For more information, see http://www.cellstemcell.com/content/article
/abstract?uid=PIIS1934590908002257&highlight=ding

The study was funded by The Scripps Research Institute.

Send comments to: mikaono[at]scripps.edu

"This shows that we can make cell reprogramming technology much more practical than it has been."

—Sheng Ding