solve a mystery, sometimes a great detective need only study the clues
in front of him. Like Agatha Christie’s Hercule Poirot and Arthur Conan
Doyle’s Sherlock Holmes, Tomomi Kiyomitsu used his keen powers of
observation to solve a puzzle that had mystified researchers for years:
in a cell undergoing mitotic cell division, what internal signals cause
its chromosomes to align on a center axis?
have been looking at these proteins and players in mitosis for decades,
and no one ever saw what Tomomi observed,” says Whitehead Institute
Member Iain Cheeseman. “And it’s very clear that these things are
happening. These are very strong regulatory paradigms that are setting
down these cell division axes. And careful cell biology allowed him to
see that this was occurring. People have been looking at this for a long
time, but never with the careful eyes he brought to it.”
Kiyomitsu, a postdoctoral researcher in Cheeseman’s lab, published his work in this week’s issue of the journal Nature Cell Biology.
process of mitotic cell division has been studied intensely for more
than 50 years. Using fluorescence microscopy, today’s scientists can see
the tug-of-war cells undergo as they move through mitosis. Thread-like
proteins, called microtubules, extend from one of two spindle poles on
either side of the cell and attempt to latch onto the duplicated
chromosomes. This entire “spindle” structure acts to physically
distribute the chromosomes, but it is not free floating in the cell. In
addition to microtubules from both spindle poles that attach to all of
the chromosomes, astral microtubules that are connected to the cell
cortex—a protein layer lining the cell membrane—act to pull the spindle
poles back and forth within the cell until the spindle and chromosomes
align down the center axis of the cell. Then the microtubules tear the
duplicated chromosomes in half, so that ultimately one copy of each
chromosome ends up in each of the new daughter cells.
process of mitosis is extremely precise; when it comes to manipulating
DNA, cells verge on being obsessive and with good reason. Gaining or
losing a chromosome during cell division can lead to cell death,
developmental disorders, or cancer.
Kiyomitsu watched mitosis unfold in symmetrically dividing human cells,
he noticed that when the spindle oscillates toward the cell’s center, a
partial halo of the protein dynein lines the cell cortex on the side
farther away from the spindle. As the spindle swings to the left, dynein
appears on the right, but when the spindle swing to the right, dynein
vanishes and reappears on the left side.
Kiyomitsu, the key to the alignment mystery was dynein, which is known
as a motor protein that “walks” molecular cargoes along microtubules.
Kiyomitsu determined that in this case, dynein is anchored to the cell
cortex by a complex that includes the protein LGN, short for
leucine-glycine-asparagine-enriched protein. Instead of moving along an
astral microtubule, the stationary dynein acts as a winch to pull on the
spindle pole, and the microtubules and chromosomes attached to it,
toward the cell cortex.
found that when a spindle pole comes within close proximity to the cell
cortex, a signal from a protein called Polo-like kinase 1 (Plk1)
emanates from the spindle pole, knocking dynein off of LGN and the cell
cortex, stopping the spindle pole’s forward motion, and freeing dynein
to move to the opposite side of the cell. These oscillations continue
with decreasing amplitude until the spindle settles along the cell’s
he was deciphering dynein’s role in spindle alignment, Kiyomitsu
noticed that a layer of LGN extends all around the cell cortex, except
in the areas that are closest to the chromosomes. As the chromosomes
swing back and forth, the area cleared of LGN changes in response.
Because dynein needs to anchor to LGN, this cleared area ensures that
dynein can only attach and pull to the right and left of the aligning
chromosomes, rather than from above and below.
testing a couple of signaling molecules associated with chromosomes,
Kiyomitsu determined that a signal from the chromosomes, involving the
ras-related nuclear protein (Ran), blocks LGN, and therefore dynein,
from attaching to the cell cortex closest to the chromosomes. Ran bound
to guanosine-5′-triphosphate (Ran-GTP), which controls nuclear import in
the interphase stage of mitosis, had previously been suggested to
control spindle assembly during mitosis in germ cells, but roles for the
Ran gradient in mitotic non-germ cells were unclear. Kiyomitsu’s work
suggests a key role for Ran in directing spindle orientation.
Kiyomitsu says the axis that the spindle poles travel along is crucial to cells.
spindle orientation is critical for maintaining the balance between
stem cells and mature cells during development,” he notes. “And if this
orientation becomes dysregulated or misregulated, it is reported that
this may contribute to causing cancer even if chromosomes are properly
work was supported by the Massachusetts Life Sciences Center, the
Searle Scholars Program, and the Human Frontiers Science Foundation, the
National Institutes of Health (NIH)/National Institute of General
Medical Sciences, and the American Cancer Society.
“Chromosome and spindle pole-derived signals generate an intrinsic code for spindle position and orientation”