It’s a kind of magic!

Gene Regulation, Stem Cells and Cancer

CRG researchers discover a faster and more efficient mechanism for reprogramming induced pluripotent stem cells

A pair of hands, a top hat, a spotlight and a wand. That’s all a magician needs to pull out a rabbit out of a hat and receive a standing ovation from the public. We humans have always felt attracted to the supernatural, including magic. In fact, we love it, as the Harry Potter phenomenon has shown. We feel attracted to the world of wizards, mystical creatures and magical spells, although we all know magic does not exist in the real world, don’t we?

But, although magic and magicians are part of fantasy worlds, in reality there are sometimes tricks or facts that seem magical. Things we do not fully comprehend -which supposedly obey physical laws- and which are attributed some kind of superpower. This might the case of a protein with a very unfriendly and hard-to-remember name, C/EBPα, but which has an amazing ability: it can convert adult B lymphocytes into another cell type and facilitate their being reprogrammed into embryonic stem cells. And in a flash.

No more lottery

But, let’s start from the beginning. Last year, Dr. Shinya Yamanaka, together with Dr. John Gurdon, were awarded the Nobel Prize in Medicine for discovering that it was possible to transform tissue cells into induced pluripotent stem cells (iPS). These iPS cells behave in a similar way to embryonic stem cells, so they are really powerful, but can be created from differentiated adult cells.

Although Yamanaka’s discovery was groundbreaking and has great potential for regenerative medicine, the truth is it was also enormously problematic: the mechanism described was mainly a lottery; the reprogramming took weeks and a very low proportion of the cells were reprogrammed.

“The success rate was very low and, in addition, cells accumulated mutations and errors”, says Bruno Di Stefano, a PhD student in Thomas Graf’s lab at the Centre for Genomic Regulation (CRG), in Barcelona.

It was assumed that the reason why so many cells were resistant to the Yamanaka factors was what is known as a stochastic mechanism, where one cell out of hundred behaves a little bit differently, purely by chance. And that single cell was the only one susceptible. “But we have discovered this is not true. If you know how to prepare the cells, in this case with another transcription factor, you can make all of them become iPS cells within days. There is no more lottery. Every cell responds”, explains Thomas Graf, senior group leader at the CRG and ICREA research professor.

A transcription factor named C/EBPα

CRG researchers employed a specific transcription factor, C/EBPα, which is normally used to turn blood cells into macrophages. This factor is a protein that is able to recognise double-stranded DNA and attach to only certain sections of the genetic sequence. When bound, it can activate the transcription of the DNA into RNA, which is then translated into proteins. In a still poorly-understood way, it seems this factor opens up the chromatin.

Genetic information is compacted into the nucleus like a skein of wool and to access the genes, it is necessary to untangle the ball in the region where the relevant information is found. What the factor does is open the area containing the genes responsible for pluripotency. So there is no longer room for chance: the genes involved are ready to be activated and enable the successful reprogramming of all the cells. So blood cells can be converted into pluripotent cells in a controlled way.

“We have been working on this transcription factor for years and we know a lot about its mechanism. We were interested in finding a link between C/EBPα and its possible role in assisting the Yamanaka factors”, points out Graf.

An earlier paper gave them a clue. It had already been reported that combining Yamanaka factors with C/EBPα made it possible to obtain more iPS cells, although it was only a modest increase. CRG researchers had the idea it was important not to express both factors together, so they experimented by applying first one, the C/EBPα factor, and then the other, the Yamanaka factors. And abracadabra! It worked!

“If you do it the opposite way, there is no effect”, adds Di Stefano, a PhD student and main author of the paper published in the prestigious journal Nature. To a certain extent this process was, in Thomas Graf’s words, “a kind of magic”, in the sense that it is highly unexpected that a transcription factor that normally works in macrophages, as does C/EBPα, has anything to do with pluripotent cells.

“Although it is still very speculative, we think the reason this magic factor works in such an unexpected context is because we are seeing a replica of what happens earlier, in embryonic development”, hypotheses Graf.

This work sheds light on embryonic stem cells and the transdifferentiation mechanism and leads the way to new advances in regenerative medicine and their medical applications.

Reference work:

Di Stefano B, Sardina JL, van Oevelen C, Collombet S, Kallin EM, Vicent GP, Lu J, Thieffry D, Beato M, Graf T.
“C/EBPα poises B cells for rapid reprogramming into induced pluripotent stem cells.”
Nature, doi: 10.1038/nature12885. Epub 2013 Dec 15.