Researchers turn skin cells into neural precusors, bypassing stem-cell stage

The multiple successes of the direct conversion method could
refute the idea that pluripotency (a term that describes the
ability of stem cells to become nearly any
cell in the body) is necessary for a cell to transform from one
cell type to another. Together, the results raise the
possibility that embryonic stem cell research and
another technique called "induced pluripotency" could be
supplanted by a more direct way of generating specific types of
cells for therapy or research.

This new study, which will be published online Jan. 30 in the
Proceedings of the National Academy
of Sciences, is a substantial advance over the previous
paper in that it transforms the skin cells into neural precursor cells, as opposed to neurons. While
neural precursor cells can differentiate into neurons, they can
also become the two other main cell types in the nervous
system: astrocytes and oligodendrocytes. In addition to their
greater versatility, the newly derived neural precursor cells
offer another advantage over neurons because they can be
cultivated to large numbers in the laboratory — a feature
critical for their long-term usefulness in transplantation or
drug screening.

In the study, the switch from skin to neural precursor cells
occurred with high efficiency over a period of about three
weeks after the addition of just three transcription factors.
(In the previous study, a different combination of three
transcription factors was used to generate mature neurons.) The
finding implies that it may one day be possible to generate a
variety of neural-system cells for transplantation that would
perfectly match a human patient.

"We are thrilled about the prospects for potential medical use
of these cells," said Marius Wernig, MD, assistant professor of
pathology and a member of Stanford's Institute for Stem Cell
Biology and Regenerative Medicine. "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. 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."

Wernig is the senior author of the research. Graduate student
Ernesto Lujan is the first author.

While much research has been devoted to harnessing the
pluripotency of embryonic stem cells, taking those cells
from an embryo and then implanting them in a patient could
prove difficult because they would not match genetically. An
alternative technique involves a concept called induced
pluripotency, first described in 2006. In this approach,
transcription factors are added to specialized cells like those
found in skin to first drive them back along the developmental
timeline to an undifferentiated stem-cell-like state. These
"iPS cells" are then grown under a variety of conditions to
induce them to re-specialize into many different cell types.

Scientists had thought that it was necessary for a cell to
first enter an induced pluripotent state or for researchers to
start with an embryonic stem cell, which is pluripotent by
nature, before it could go on to become a new cell type.
However, research from Wernig's laboratory in early 2010 showed
that it was possible to directly convert one "adult" cell type
to another with the application of specialized transcription
factors, a process known as transdifferentiation.

Wernig and his colleagues first converted skin cells from an
adult mouse to functional neurons (which they termed induced
neuronal, or iN, cells), and then replicated the feat with
human cells. In 2011 they showed that they could also directly
convert liver cells into iN cells.

"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," said pediatric cardiologist Deepak Srivastava, MD,
who was not involved in these studies. "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.

"Direct conversion has a number of advantages," said Lujan. "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 cause cancers
when transplanted into animals or humans.

The lab's previous success converting skin cells into neurons
spurred Wernig and Lujan to see if they could also generate the
more-versatile neural precursor cells, or NPCs. To do so, they
infected embryonic mouse skin cells — a commonly used
laboratory cell line — with a virus encoding 11 transcription
factors known to be expressed at high levels in NPCs. A little
more than three weeks later, they saw that about 10 percent of
the cells had begun to look and act like NPCs.

Repeated experiments allowed them to winnow the original panel
of 11 transcription factors to just three: Brn2, Sox2 and
FoxG1. (In contrast, the conversion of skin cells directly to
functional neurons requires the transcription factors Brn2,
Ascl1 and Myt1l.) Skin cells expressing these three transcription factors became neural
precursor cells that were able to differentiate into not just
neurons and astrocytes, but also oligodendrocytes, which make
the myelin that insulates nerve fibers and allows them to
transmit signals. The scientists dubbed the newly converted
population "induced neural precursor cells," or iNPCs.

In addition to confirming that the astrocytes, neurons and oligodendrocytes were expressing the
appropriate genes and that they resembled their naturally
derived peers in both shape and function when grown in the
laboratory, the researchers wanted to know how the iNPCs would
react when transplanted into an animal. They injected them into
the brains of newborn laboratory mice bred to lack the ability
to myelinate neurons. After 10 weeks, Lujan 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," said Lujan.

The scientists are now working to replicate the work with
skin cells from adult mice and humans, but
Lujan emphasized that much more research is needed before any
human transplantation experiments could be conducted. In the
meantime, however, the ability to quickly and efficiently
generate neural precursor cells that can be grown in the
laboratory to mass quantities and maintained over time will be
valuable in 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,"
said Wernig.

Provided by Stanford University Medical Center (news : web)

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Researchers turn skin cells into neural precusors, bypassing stem-cell stage