Laser slices mitotic spindle, unraveling theory of its structure

Using laser ablation to slice through microtubules in the mitotic spindle, researchers have developed a clearer picture of how cell division occurs. Image courtesy of Julie Eichhorn.

The
mitotic spindle, an apparatus that segregates chromosomes during cell
division, may be more complex than the standard textbook picture
suggests, according to researchers at the Harvard School of Engineering
and Applied Sciences (SEAS).

The findings, which result from quantitative measurements of the mitotic spindle, will appear tomorrow in the journal Cell.

The
researchers used a femtosecond laser to slice through the strands of
the organelle and then performed a mathematical analysis to infer the
microscopic structure of the spindle from its response to this damage.

“We’ve
been using this nanosurgery technique to understand the architecture
and assembly of the spindle in a way that was never possible before,”
says Eric Mazur, Balkanski Professor of Physics and Applied Physics at
Harvard, who co-authored the study. “It’s very exciting.”

The
spindle, which is made of protein strands called microtubules, forms
during cell division and segregates chromosomes into the daughter cells.
It was previously unclear how microtubules are organized in the
spindles of animal cells, and it was often assumed that the microtubules
stretch along the length of the entire structure, pole to pole.

Mazur
and his colleagues demonstrated that the microtubules can begin to form
throughout the spindle. They also vary in length, with the shortest
ones close to the poles.

“We
wondered whether this size difference might result from a gradient of
microtubule stabilization across the spindle, but it actually results
from transport,” says lead author Jan Brugués, a postdoctoral fellow at
SEAS. “The microtubules generally nucleate and grow from the center of
the spindle, from which point they are transported towards the poles.
They disassemble over the course of their lifespan, resulting in long,
young microtubules close to the midline and older, short microtubules
closer to the poles.”

An artistic representation of the cutting method used to measure the microtubules’ lengths. Image courtesy of Julie Eichhorn.

“This research provides concrete evidence for something that we’ve only been able to estimate until now,” Brugués adds.

Mazur
and Brugués worked with principal investigator Daniel Needleman,
Assistant Professor of Applied Physics and Molecular and Cellular
Biology at Harvard, and Valeria Nuzzo, a former postdoctoral fellow in
Mazur’s lab at SEAS, to bring the tools of applied physics to bear on a
biological question.

The
team used a femtosecond laser to make two small slices perpendicular to
the plane of growth of the spindle apparatus in egg extracts of the
frog species Xenopus laevis.

They
were then able to collect quantitative data on the reconstruction of
the spindle following this disruption and precisely determine the length
and polarity of individual microtubules. Observing the speed and extent
of depolymerization (unraveling) of the spindle, the team worked
backwards to compile a complete picture of the beginning and end points
of each microtubule. Finally, additional experiments and a numerical
model confirmed the role of transport.

“The
laser allowed us to make precise cuts and perform experiments that
simply were not possible using previous techniques,” says Mazur.

With
further inquiries into spindle architecture, the researchers hope that
scientists will one day have a complete understanding, and possibly even
control over, the formation of the spindle.

click to enlarge Top: A series of fluorescent images of a spindle taken before the cut and at 5 seconds and 10 seconds after the cut. Scale bar, 10 µm. Bottom: A graphical representation of the cut microtubules. The cut generates new ‘plus’ ends (red) and new ‘minus’ ends (green). The newly generated minus ends remain stable, whereas the new plus ends depolymerize, which creates two depolymerization fronts of opposed polarity. Image courtesy of Jan Brugués.

“Understanding
the spindle means understanding cell division,” notes Brugués. “With a
better understanding of how the spindle is supposed to operate, we have
more hope of tackling the range of conditions—from cancer to birth
defects—that result from disruptions to the cell cycle or from improper
chromosomal segregation.”

The research was supported by the National Science Foundation and by a fellowship from the Human Frontiers Science Program.

Video 1

A
pulse from the femtosecond laser slices through the mitotic spindle,
causing the damaged microtubules to depolymerize. Meanwhile, undamaged
and newly nucleated microtubules continue growing and soon fill the gap.
Courtesy of Jan Brugués.

Video 2

To conduct the experiments, the research team induced spindle formation in Xenopus egg extracts. This video shows the initial formation of the spindle. Courtesy of Jan Brugués.

Source: Harvard University