Scientists turn skin into neural ‘precursors’ cells. First, they showed that skin from mice and humans could be directly converted into functional neurons, or brain cells. Going a step further, researchers from the Stanford University School of Medicine have now shown that mouse skin cells can be converted directly into the “precursor” cells that become the three main parts of the nervous system.
“We are thrilled about the prospects for potential medical use of these cells,” says Dr. Marius Wernig, senior study author, assistant professor of pathology and member of Stanford’s Institute for Stem Cell Biology and Regenerative Medicine.
It’s more advantageous to create neural precursor cells than neurons because these precursor cells are more versatile, the researchers explain. Precursors can turn, not only into neurons, but also into the two other main cell types in the nervous system—astrocytes and oligodendrocytes.
Apart from greater flexibility, another advantage of the newly derived neural precursor cells — dubbed by their creators as “induced neural precursor cells” or iNPCs — is that they can be cultivated in large numbers in the lab. That’s critical if they are to be useful in transplantation or drug screening, the researchers explain. Their findings were published online Jan. 30 in the Proceedings of the National Academy of Sciences.
Still not satisfied, the California researchers took the iNPCs they created and implanted them in mice genetically bred to lack the ability to coat their neurons with myelin that insulates nerve fibers and allows brain cells to transmit signals.
After 10 weeks, the team found that the cells had differentiated into oligodendrocytes and had begun to coat the mice’s neurons with myelin.
Previously, much research was devoted to embryonic stem cells because these pluripotent cells could differentiate into a variety of specialist cell types, from brain to bone—opening the possibility that they can be used to treat or regenerate damaged cells. Currently, many trials are taking place, for stroke patients or specific forms of blindness.
But taking those cells from an embryo has proven to be a controversial concept and then, implanting them in a patient has proven to be just as difficult when donor stem cells do not match patients genetically. Patients then have to take immunosuppressant drugs to keep their bodies from rejecting the stem cells.
In 2006, an alternative technique was developed in a concept called induced pluripotency. This involved adding “transcription factors” to specialized cells like those found in skin to drive them back along the developmental timeline to an undifferentiated stem-cell-like state.
These “induced pluripotent stem cells” or “iPS cells” or “iPSCs” are then grown under various specific conditions to induce them to re-specialize into many different cell types. However, the process often results in cancer-causing genes being activated and pluripotent cells often cause cancers when transplanted into animals or humans.
Since 2006, scientists thought that cells had to be either embryonic stem cells of induced pluripotent cells in order for them to develop into new cell types.
But in early 2010, research from Dr. Wernig’s lab showed that applying specialized “transcription factors” in a process known as “transdifferentiation,” allowed researchers to convert one “adult” cell type to another.
Dr. Wernig’s team first converted skin cells from an adult mouse to functional neurons, which they termed induced neuronal, or iN, cells. Then, they replicated the feat with human cells. In 2011, they showed that they could also convert liver cells directly into iN cells.
And now, they’ve created the “induced neural precursor cells”.
The team’s double success in being able to convert directly one type of cells to another disproves the concept that pluripotency, or the ability of stem cells to become nearly any cell in the body, is needed for a cell to transform from one cell type to another.
This may mean that embryonic stem cell research—surrounded by controversy—and the other technique, “induced pluripotency,” could be replaced by a more direct way of generating specific types of cells for therapy or research.
“Direct conversion has a number of advantages,” says first author Ernesto Lujan, a graduate student at Stanford University.
“It occurs with relatively high efficiency and it generates a fairly homogenous population of cells. In contrast, cells derived from iPS cells must be carefully screened to eliminate any remaining pluripotent cells or cells that can differentiate into different lineages.” Pluripotent cells can differentiate into cancers cells when transplanted into animals or humans.
How they made it happen
Brain cells and skin cells contain the same genetic information. But in each, the genetic code is interpreted differently, controlled by “transcription factors.”
Following success in converting skin cells into neurons, Dr. Wernig and Lujan wanted to see if they could also generate the more-versatile neural precursor cells, or NPCs.
To do so, they infected embryonic mouse skin cells with a virus encoding 11 transcription factors known to be expressed at high levels in NPCs. Over three weeks later, they saw that about 1 in 10 of the cells had transformed into neural precursor cells.
Repeated experiments allowed them to winnow the original panel of 11 transcription factors to just three: Brn2, Sox2 and FoxG1. Skin cells expressing these three transcription factors became NPCs that were able to differentiate into the different part of the nervous system: neurons, astrocytes and oligodendrocytes.
For disease and drug studies
The team’s success in discovering the ability to grow neural precursor cells in the lab quickly and efficiently and in large quantities that can be maintained over time, is valuable for disease and drug-targeting studies.
“In addition to direct therapeutic application, these cells may be very useful to study human diseases in a laboratory dish or even following transplantation into a developing rodent brain,” says Dr. Wernig.
In the previous study, Dr. Wernig and his colleagues used three transcription factors to generate mature neurons. In this new study, skin cells converted to neural precursor cells with high efficiency over a period of about three weeks after the researchers added another combination of just three transcription factors.
The findings imply that one day, it may be possible to generate a variety of neural-system cells for transplantation that would perfectly match a human patient.
Integrating into mouse brain
After confirming that the lab-grown astrocytes, neurons and oligodendrocytes were expressing the appropriate genes and that they resembled their naturally derived peers in both shape and function, Dr. Wernig and colleagues wanted to know how the iNPCs would react when transplanted into an animal.
So they injected them into the brains of newborn laboratory mice bred to lack the ability to myelinate neurons. After 10 weeks, they found that the cells had differentiated into oligodendroytes and had begun to coat the animals’ neurons with myelin.
“Not only do these cells appear functional in the laboratory, they also seem to be able to integrate appropriately in an in vivo animal model,” says co-author Lujan.
“We’ve shown the cells can integrate into a mouse brain and produce a missing protein important for the conduction of electrical signal by the neurons,” says Dr. Wernig. “This is important because the mouse model we used mimics that of a human genetic brain disease.”
“However, more work needs to be done to generate similar cells from human skin cells and assess their safety and efficacy,” he admits.
Commenting on the new research, pediatric cardiologist Dr. Deepak Srivastava, who was not involved in the studies says: “Dr. Wernig’s demonstration that fibroblasts can be converted into functional nerve cells opens the door to consider new ways to regenerate damaged neurons using cells surrounding the area of injury.”
“It also suggests that we may be able to transdifferentiate cells into other cell types.” Srivastava is the director of cardiovascular research at the Gladstone Institutes at the University of California-San Francisco. In 2010, Srivastava transdifferentiated mouse heart fibroblasts into beating heart muscle cells.
The next step for the eager scientists is to replicate their work with skin cells from adult mice and humans, but Lujan emphasizes that much more research is needed before any human transplantation experiments can be conducted.