Stanford researchers
discover vital role of centrosome in cell
BY KRISTIN WEIDENBACH
When an animal cell
reproduces itself, there are only two essential
components that must be duplicated the DNA that
carries all the genetic information and an unassuming
little cell body called the centrosome. Scientists have
known for many years about the process of DNA
replication, but they are only just beginning to learn
about how the centrosome replicates itself.
Now, Stanford researchers
have discovered that the centrosome begins to duplicate
when a pair of even smaller bodies inside the centrosome
separates. The separation of these tiny cylindrical
bodies the centrioles is the first in a series of
steps that culminate in an animal cell dividing into two
perfect replicas, according to Tim Stearns, PhD,
assistant professor in the departments of biological
sciences and genetics; Peter Jackson, PhD, assistant
professor in the departments of pathology and
microbiology and immunology; and Kathleen Lacey, a
graduate student in Stearns' lab.
The research team was able
to further demonstrate in their study that the ringmaster
of this intricate performance is a protein complex called
cyclin E/cdk2.
Cyclin E/cdk2 was already
known to play a crucial role in DNA replication but its
influence over centrosome division was not known until
the Stanford team and another group from the University
of Massachusetts Medical School announced their findings.
The Stanford scientists describe their research in the
March 16 issue of the Proceedings of the National Academy
of Sciences.
"It's a key
discovery," said William Brinkley, a cell biologist
at Baylor College of Medicine in Houston, commenting on
the achievements of both groups in a "News of the
Week" article that accompanied publication of the
Massachusetts research in Science in February.
The two teams used a
similar experimental approach in their investigations,
with a jelly-like mass of frog eggs playing a key role in
both. The eggs, which are large and divide rapidly
following fertilization, provide a perfect vehicle for
studying the early events of cell division.
The Stanford team used
living eggs to study how the cells normally divide and
used eggs they had crushed into an extract to study the
chemical details of different cell cycle events. "We
can use frog eggs to serve as a little in vitro test
tube," said Jackson.
In their study, the
Stanford team was able to show that cyclin E/cdk2 causes
the centrosome to divide. When they added a specific
inhibitor of cyclin E the centrosome sat sedately and
refrained from dividing.
To confirm the crucial
role of cyclin E/cdk2, the researchers delved deeper into
the centrosome and studied the tiny centrioles housed
inside. Most of the time these two bundles of fibers sit
serenely side-by-side but in a cell that is preparing to
divide, the bundles move apart the first sign that
cell division is about to occur.
Using fluorescent
microscopy and colored fluorescent dyes, the researchers
were able to look inside the centrosome and see the
centrioles as two tiny dots. When they incubated the
centrosomes in egg extract rich in cyclin E/cdk2, the
researchers saw that the two dots separated, indicating
that the twin centrioles had split apart. When the
researchers added the cyclin E inhibitor the two dots
remained tightly coupled.
The results of this test
confirmed the finding that the first step of centrosome
duplication is the separation of the centrioles, and that
initiation of this event requires cyclin E/cdk2.
The researchers are now
trying to determine what holds the centrioles together
until cyclin E/cdk2 dictates that they should come apart.
Centriole separation is
the key regulated event in centrosome duplication and
that implies that something is holding the centrioles
together until just the right time, said Stearns.
"The image would be that you have some sort of glue
that holds these things together and you have to dissolve
the glue in order to get them to separate," added
Jackson.
The researchers are also
interested in looking more closely at centrosome
replication in cancer cells because multiple centrosomes
and abnormal numbers of chromosomes are characteristic of
cancerous cells. Having the correct number of centrosomes
is critical for accurate cell division, the researchers
explained, as they outlined the highly regulated process.
When the cell prepares to
divide, the centrosome duplicates and the two freshly
minted sibling centrosomes migrate to opposite ends of
the cell. A web of fibers connects the two centrosomes
forming a spindle upon which the chromosomes gather in
pairs at the cell center. When the time is right, the
chromosome pairs separate and retract along the spindle
fibers towards the centrosomes located at the poles of
the cell. A nuclear membrane forms around each
chromosomal clump and eventually two new cells are born
each with an identical set of chromosomes.
If there are too many
centrosomes a mutant, multi-cornered spindle will form
and the chromosomes will be pulled in random directions
when the cell begins to divide. Daughter cells can end up
with too many or too few chromosomes, resulting in the
gain or loss of genetic information.
Stearns concedes that
there is no causal link as yet, but his team believes
that defects in the control of centrosome duplication are
an obvious mechanism by which cells with an abnormal
number of chromosomes can be produced.
"If you don't set up
the spindle properly then the chromosomes are not going
to end up in the right place," said Jackson.
"It's clear that some cancers lose chromosomes at a
high rate and that chromosome loss may be a reflection of
problems in this mechanism."
Funding for the study was
provided by the National Institutes of Health. SR
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