Scientists in California have turned human skin cells directly into functioning neurons or brain cells, bypassing the pluripotent stem cell stage, according to a study published in the journal Nature this week.

The process took four to five weeks and only needed the addition of four genes to effect the transformation.

Last year, the same team, from Stanford University School of Medicine, showed it was possible to convert mouse skin cells directly into neurons.

They used a similar combination of proteins in the human cell conversion as the ones they used in the mouse cell conversion, except the human process was less efficient and took longer.

The finding is significant because it avoids having to make induced pluripotent stem cells (iPS), and may make it much easier to create and study neurons that are patient or disease-specific in the laboratory.

iPS cells take months to create, and recent reports have raised other problems. Studies in lab mice suggest they reject genetically identical iPS cells because of the genes used to induce them. There are also concerns that they may trigger tumors.

Senior author Dr Marius Wernig, assistant professor of pathology and a member of Stanford’s Institute for Stem Cell Biology and Regenerative Medicine, told the press that:

“We are now much closer to being able to mimic brain or neurological diseases in the laboratory.”

“We may perhaps even be able to one day use these cells for human therapies,” he added.

The study is the latest in the fast-moving new field of transdifferentiation, where cells are forced to take on new identities. In the last 12 months, scientists have converted connective tissue from the skin into heart cells, blood cells and liver cells, reports Nature News.

Following their success with the mouse skin cells, the team went through a staged process to repeat it with human cells.

First they proved they could convert human embryonic stem cells into neurons by infecting them with a virus expressing the same three genes they used in the mouse experiment. They called the treatment “BAM”, short for the three genes, the transcription factors Brn2, Ascl1 and Myt1.

The BAM treatment took only six days to convert human embryonic stem cells into fuctioning neurons and it also worked on iPS cells.

But when they moved to the next stage, and used the BAM treatment to do with human skin cells what they achieved with the mouse skin cells, they ran into problems.

Using fibroblast skin cells from the skin of aborted fetuses and the foreskin of newborns, they found the BAM treatment was able successfully to create cells that looked like neurons, but they lacked one important function: they could not fire the electrical pulses needed to communicate with each other.

So the researchers guessed an ingredient was missing, and through trial and error they eventually found it, a fourth transcription factor, the gene called NeuroD.

With NeuroD added using the virus carrier, Wernig and colleagues managed to coax many of the neurons to respond to electrical stimulation so that within a few weeks, they were making synaptic connections with the mouse neurons that were growing next to them.

There are problems: although in the case of mice, about 20% of skins can be converted directly into neurons, with human skin cells this efficiency goes down to around 2% or 4%. And while the mouse process only takes days, for human skin, it is a matter of several weeks, and the electrical pulses of the resulting neurons are not as strong as those of naturally derived neurons.

The team is now working to try and overcome these problems, and perfect a growing culture that increases the speed and efficiency of the direct conversion process.

“Clearly mice and humans are different in significant ways,” said Wernig.

Another team at Stanford recently converted skin cells to iPS cells and then coaxed them to become patient-specific neurons to treat a woman with Parkinson’s disease. But the process is very labor-intensive and relies on cell lines that have a smaller range of diversity compared to naturally created neurons.

Comparing the direct conversion method with the iPS method, Wernig said it was important to keep working on both:

“The iPS cell approach is doable and has been shown to work.”

“It’s possible that the best approach may vary depending on the disease or the type of research being done,” he added.

The National Institutes of Health, the Stanford Institute for Stem Cell Biology and Regenerative Medicine, the Donald E. and Delia B. Baxter Foundation, the Ellison Medical Foundation, the Stinehard-Reed Foundation, and the New York Stem Cell Foundation provided funds and support for the research, as did Stanford’s Department of Pathology.

“Induction of human neuronal cells by defined transcription factors.”
Zhiping P. Pang, Nan Yang, Thomas Vierbuchen, Austin Ostermeier, Daniel R. Fuentes, Troy Q. Yang, Ami Citri, Vittorio Sebastiano, Samuele Marro, Thomas C. Südhof, and Marius Wernig
Nature Published online 26 May 2011

Related article (Jan 2010): Mouse Skin Cells Turned Directly Into Neurons, Skipping IPS Stage

Additional sources: Stanford School of Medicine, Nature News.

Written by: Catharine Paddock, PhD