New research discovers details involved in reprogramming stem cells into motor neurons.
Somatic stem cells - also called adult stem cells - are undifferentiated cells that can be found throughout the human body in a tissue or organ. Their role is to maintain, renew, and repair the tissue in which they are found.
Through epigenetic reprogramming techniques - first introduced in 2006 by Nobel Prize winner Shinya Yamanaka and colleagues - these cells have been artificially transformed into neural stem cells.
This process involved transforming fibroblasts - a type of cell found in connective tissue - first into pluripotent cells, then into neural stem cells, and finally into neurons.
In direct reprogramming, however, the pluripotency stage is skipped. This allows for the transformation to take place in a more timely manner, and it also bypasses other limitations and risks of tumor formation found in the regular reprogramming technique.
Direct reprogramming has been used before to regenerate missing or damaged motor neurons. New research uncovers details of the transformation process that could one day enable researchers to create new types of cells.
The findings have been published in the journal Cell Stem Cell.
Studying direct reprogramming of stem cells
The researchers analyzed the changes that occur in the cells during the direct reprogramming process.
The transformation takes around 2 days.
The process involves three transcription factors. These are genes that control the expression of other genes.
In order to understand the cellular and genetic mechanisms behind the transformation, researchers analyzed how these transcription factors bind to the genome, how the genes expression changes, and how chromatin changes every 6 hours.
Uwe Ohler, senior researcher at the Max Delbrück Center for Molecular Medicine in Berlin and one of the lead authors of the study, explains why researchers were interested in these changes.
"A cell in an embryo develops by passing through several intermediate stages. But in direct programming we do not have that: we replace the gene transcription network of the cell with a completely new one at once, without the progression through intermediate stages. We asked, what are the timing and kinetics of chromatin changes and transcription events that directly lead to the final cell fate?"
Uwe Ohler, lead author
Shaun Mahony, assistant professor of biochemistry and molecular biology at Penn State and one of the lead authors of the paper, explains what enabled them to study these changes in such minute detail:
"We have a very efficient system in which we can transform stem cells into motor neurons with something like a 90 to 95 percent success rate by adding the cocktail of transcription factors. Because of that efficiency, we were able to use our system to tease out the details of what actually happens in the cell during this transformation."
Changing stem cells into motor neurons
Researchers uncovered a series of highly complex, independent changes that together converge to change the stem cells into motor neurons.
Early in the transformation process, two of the transcription factors - Isl1 and Lhx3 - together bind to the genome and trigger a chain reaction of events that includes changes in the chromatin and gene expression in cells.
The third transcription factor - Ngn2 - acts on its own, also making changes to the gene expression.
Later in the process, Isl1 and Lhx3 use the changes made by Ngn2 to complete the transformation.
For the direct programming to succeed, the two parallel processes must successfully converge.
Cell replacement may help treat neurodegenerative diseases
The study not only details the challenges of cell-replacement technology, but it also leads the way to developing new, more efficient methods of replacing damaged cells. This could prove invaluable in the treatment of some neurodegenerative diseases.
"By detailing the mechanisms underlying the direct programming of motor neurons from stem cells, our study not only informs the study of motor neuron development and its associated diseases, but also informs our understanding of the direct programming process and may help with the development of techniques to generate other cell types."
Shaun Mahony, lead author
The advantages of direct reprogramming include the fact that it can be done either in vitro or in vivo. Performing the reprogramming inside the human body - in vivo - has the advantage of being localized, at the site of cellular damage.
However, Esteban Mazzoni, an assistant professor in New York University's Department of Biology and one of the lead authors of the new study, explains that reprogramming is not always efficient, and there are still a lot of unknown factors given the complexity of biological processes.
"Despite having a great therapeutic potential, direct programming is generally inefficient and doesn't fully take into account molecular complexity," Mazzoni says. But their study highlights new, more viable methods to replace cells.
"Our findings point to possible new avenues for enhanced gene-therapy methods. Looking ahead, we think it is reasonable to use this newly gained knowledge to, for instance, manipulate cells in the spinal cord to replace the neurons required for voluntary movement that are destroyed by afflictions such as ALS."
Esteban Mazzoni, lead author