Scientists identify embryonic stem cells by appearance alone
CAMBRIDGE, Mass. (August 27, 2007) Some scientific results are hard to spot,
especially in genetic research. Often scientists are unable to physically
see if the gene they inserted into a cell has produced the desired trait.
To overcome this problem researchers use various genetic markers that
contain pieces of foreign DNA that cause cells to, for example,
glow when exposed to ultraviolet light.
But scientists in the lab of Whitehead Member Rudolf Jaenisch
didn’t have to resort to these genetic markers in their latest
experiment because the results were easy to see. Building on
their widely publicized June Nature paper, which demonstrated
that it’s possible to convert specialized mouse skin cells into
unspecialized stem cells, Whitehead postdoctoral researchers
Alexander Meissner and Marius Wernig have now identified successfully
reprogrammed cells by looks alone.
Their findings, which appear online in the journal Nature Biotechnology
on Aug. 27, bring human stem cell therapies a step closer to reality.
Before reprogramming can be applied to our own species to generate custom
embryonic stem cells, scientists must be able to accomplish it
without altering the DNA of the cells involved.
“This eliminates one of the major hurdles to reprogramming human cells,” says Jaenisch, who is also an MIT professor of biology.
“If we overcome the other obstacles, this approach could one day provide custom human embryonic stem cells for use in therapy.” |
“This eliminates one of the major hurdles to reprogramming
human cells,” says Jaenisch, who is also an MIT professor of biology.
“If we overcome the other obstacles, this approach could
one day provide custom human embryonic stem cells for use in therapy.”
Last spring, Wernig and Meissner relied on genetic markers
to identify successfully reprogrammed cells. This required them
to work with fibroblasts from a genetically modified mouse.
The mouse was grown from embryonic stem cells that contained foreign DNA
coding for antibiotic resistance. The scientists had strategically
inserted these foreign DNA “markers” at particular points along the genome,
next to genes expressed only in embryonic stem cells. All of the cells
(including fibroblasts) in the resulting mouse contained the markers.
In the original experiment, the researchers took fibroblasts
from the tail of this mouse and infected them with a special virus
containing four genes (Oct4, Sox2, c-myc, and Klf4) capable of converting
the cells to an embryonic state. Genes typically active in embryonic
stem cells roared to life, triggering the adjacent foreign DNA to provide
antibiotic resistance. Thus only fully reprogrammed cells survived exposure
to an antibiotic, which allowed the scientists to isolate them.
“When we conducted the original experiment, we noticed that many of
the infected cells had already started to change shape before
the markers were activated,” says Wernig.
So they set up a new experiment to test if visual identification
alone would work. Indeed, they were able to separate the reprogrammed
cells from ordinary fibroblasts under a microscope, based on several
physical differences. Fibroblasts are big and flat. Embryonic stem
cells are small, round and form tight colonies.
“We’ve shown that there’s no need to use markers to isolate successfully
reprogrammed cells,” says Meissner. “This significantly simplifies
this approach in mice, as we can now work with ordinary fibroblasts.”
But another hurdle remains before the technique can be applied to human cells.
The scientists are now working to eliminate the virus from the
reprogramming process. Jaenisch believes they will eventually
succeed and points out that the technique could eventually yield
a bountiful supply of custom human embryonic stem cells for use in therapy.
Meissner and Wernig successfully reprogrammed about 0.5 percent
of the fibroblasts. Given that there are millions of cells in a
typical skin biopsy (researchers used skin from either the end of
the tail or from the ear of the mouse), that translates into thousands
of stem cells, each one capable of developing into any cell type of the body
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