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The ‘Sleeper’ project
A mouse produced by cloning from a single neuron yields answers about the genetics of olfaction
BY JOSEPH BONNER
Like your nose, your immune system is
primarily a detection device. Its chief job is to detect and
identify molecules. Just as there is an almost infinite number of
smells, there’s a nearly infinite number of microbial
invaders that must be properly distinguished from one another in
order for your body to resist infection.
Yet both the olfactory and immune systems can
detect this infinite number of substances by using a finite number
of genes.
Scientists know how the immune system
accomplishes this task: it combines pieces of genes in a process
called DNA rearrangement, which results in irreversible changes to
the genetic material of the cell. What they didn’t know was
whether or not the smell-detecting nerve cells in the nose do the
same thing.
To find out, scientists led by
Rockefel-ler’s Peter Mombaerts did something amazing: they used cloning
technology to produce an entire mouse from the DNA in just one of the animal’s neurons. They
then traced the spread of that neuron’s nucleus as the mouse
embryo developed and eventually as the mouse itself grew.
Harvey, as they called their mouse, gave them their answer.
“We and many other scientists have been
looking for changes in genetic material of olfactory neurons that,
it now turns out, do not occur,” says Mombaerts, who is head
of the Laboratory of Developmental Biology and Neurogenetics.
To conduct their study, Mombaerts, along with
postdocs Jinsong Li and Tomohiro Ishii and research associate Paul
Feinstein, chose an odorant receptor gene called M71, one of the
1,000 odorant receptor genes in the mouse genome. They
permanently linked a green fluorescent protein to M71 so that every
cell in which M71 is active lights up green under the microscope.
After inserting the nucleus of the M71 nerve
cell into a mouse egg, the scientists were able to literally watch
the green spread as the DNA housed in the nucleus was replicated
with each cell division.
“Our cells were labeled with fluorescent protein continuously, so even after the
nucleus was pulled out and put into an egg, the egg is green, the
embryo’s stem cells are green, everything else in the mouse
embryo is green, because it’s driven from a promoter in the
DNA that’s active in all cells,” says Mombaerts.
“We have an irreversible marker that shows us that we started
with that particular cell.”
Then, using a technique called in situ
hybridization, the scientists showed that in addition to producing
M71 neurons, cells expressing the green protein also generate other
types of olfactory neurons in different parts of the lining of the
nose. Therefore, Mombaerts says, olfactory sensory neurons that
contain the same DNA that once belonged to an M71-expressing cell
are able to express other receptor genes — not just the M71
gene from which they are derived.
The findings, therefore, do not support the
hypothesis that irreversible changes in DNA occur when an olfactory
neuron’s DNA is expressed.
Perhaps just as important, the scientists
found that a neuron that no longer divided could still be coaxed
into cell division. “This may prove to be important for the
nascent field of therapeutic cloning to produce custom-tailored
cellular therapies for people with diabetes and other
disorders,” says Mombaerts.
“It shatters the concern that we cannot
use post-mitotic cells for nuclear transfer.”
Although therapeutic cloning is not the main
line of Mombaerts’ research, his lab is at the frontier in
their research with mice. With the publication of 16 cell lines in
this study, the total of published mouse cell lines produced by
cloning technology has reached 65. Of this worldwide production, 41
— two-thirds — were established in Mombaerts’
Rockefeller lab.
March 26, 2004
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