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

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release date: 30-Jan-2012
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Contact: Krista Conger
kristac@stanford.edu
650-725-5371
Stanford
University Medical Center

STANFORD, Calif. ? Mouse skin cells can be converted directly
into cells that become the three main parts of the nervous
system, according to researchers at the Stanford University
School of Medicine. The finding is an extension of a previous
study by the same group showing that mouse and human skin cells
can be directly converted into functional neurons.

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.

###

In addition to Wernig and Lujan, other Stanford researchers
involved in the study include postdoctoral scholars Soham
Chanda, PhD, and Henrik Ahlenius, PhD; and professor of
molecular and cellular physiology Thomas Sudhof, MD.

The research was supported by the California Institute for
Regenerative Medicine, the New York Stem Cell Foundation, the
Ellison Medical Foundation, the Stinehart-Reed Foundation and
the National Institutes of Health.

The Stanford University School of Medicine consistently ranks
among the nation's top medical schools, integrating research,
medical education, patient care and community service. For more
news about the school, please visit http://mednews.stanford.edu.
The medical school is part of Stanford Medicine, which includes
Stanford Hospital & Clinics and Lucile Packard Children's
Hospital. For information about all three, please visit
http://stanfordmedicine.org/about/news.html.

PRINT MEDIA CONTACT: Krista Conger at (650) 725-5371 (kristac@stanford.edu)
BROADCAST MEDIA CONTACT: M.A. Malone at (650) 723-6912
(mamalone@stanford.edu)

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