Annual Report [2018]

Toward a Grand Unified Theory of Spindles

Flatiron Institute

Biologists in the 20th century broke down the cell into parts, and now 21st-century researchers are figuring out how to put those components back together.

Take the spindle, which lines up chromosomes during cell division before pulling them apart, ensuring that each daughter cell inherits the parent cell’s genes.

“The spindle has an infinite number of varieties because it’s in all these different types of eukaryotic cells,” says Michael Shelley, group leader for biophysical modeling and director of the Flatiron Institute’s Center for Computational Biology (CCB). “It’s made up of microtubules and motors and crosslinkers but has different structures in different cells.”

Exactly how rigid rods and microscopic motors choreograph this fundamental line dance remains unknown, but Shelley and his collaborators aim to find out.

One line of research asks how motor proteins link and move microtubules, the building blocks of the spindle. A question the team is pursuing involves the protein kinesin, which has two heads that can grab one microtubule each, creating a linked pair. The kinesin then ‘walks’ along these microtubules. If the microtubules have opposite orientations, the walking motion pulls them in opposite directions. If they are aligned, the kinesin’s walk has no effect on the pair.

“Fluorescent microscopy revealed that although microscopically the rods pointed different ways, macroscopically half flowed to the left, while half went right — a result that suggested the calculations had assumed too much.”

Earlier calculations of interactions in a dilute mixture predicted that a given microtubule’s movement would depend on which way its neighbors pointed: Those aligned would stay put, but rebels would be ejected. And yet, real spindles don’t display such fickle behavior. Microtubule hordes tens of thousands strong appear to move in lockstep, despite different regions having different average orientations. 

Shelley, along with CCB research scientist Sebastian Fürthauer, CCB visiting scholar and Harvard University applied physicist Daniel Needleman, and their colleagues, investigated what was going on by skipping the complexity of real spindles and using a simple test-tube system with just microtubules and kinesin. In the experiment, the motors linked the rods into a gel. Fluorescent microscopy revealed that although microscopically the rods pointed different ways, macroscopically half flowed to the left, while half went right — a result that suggested the calculations had assumed too much. “What if they’re not dilute?” says Needleman. “What if they’re really heavily cross-linked?”

In this computer animation, a kinesin motor protein drags a blue vesicle while walking along a microtubule. Video excerpt from BioVisions at Harvard University

The research, published as an arXiv preprint in December 2018, presents the group’s experimental observations as well as a mathematical framework for the microtubule-kinesin interaction. That framework assumes that both microtubules and motors are more concentrated and therefore more interlinked, which generates predictions that match what happened in the test tube. “It’s a natural hypothesis that the same thing is happening in the spindle itself,” Needleman says. 

A better understanding of how spindles behave could lead to new treatments for cell-division-related health problems, such as infertility. The group has been working on a comprehensive spindle model for several years and hopes to complete it soon. 

“What we’re working toward is a grand unified theory of the spindle,” Needleman says. “This was one of the missing pieces that we needed to understand how we go from the behavior of motors and microtubules to large-scale, collective self-organization.”

Kinesin molecular motors (black) are part of the microscopic machinery inside cells. The kinesin molecules have pairs of “feet” at either end that latch on to microtubules (blue). The feet march from the negative to the positive end of each microtubule. If the microtubules are aligned, the feet move in the same direction, and the microtubules stay put. If the microtubules are anti-aligned, the feet move in opposing directions, causing the microtubules to slide past one another.