Counting the molecules that pull cells apart
Scientists at the Max Planck Institute for Cell Biology and Genetics in Dresden and the European Molecular Biology Laboratory (EMBL) in Heidelberg map forces that help cells divide
"Cells obey the laws of physics and chemistry," begins a famous biology textbook, and one of the main goals of molecular biology is to link the properties of single molecules to the behavior of cells and the lives of organisms. So it is probably no surprise that an important new discovery about the physical forces that underlie cell division comes from a physics student-turned biologist, using math and a laser microscope. The findings appear in the current issue of the journal Science (Science, 26 July 2003).
Stephan Grill, Joe Howard, Erik Schäffer, Ernst Stelzer and Tony Hyman - in a collaboration between the Max-Planck Institute of Molecular Cell Biology and Genetics in Dresden and EMBL in Heidelberg - have done something few scientists have managed: they have counted the number of proteins that help an egg cell divide. This initial division happens in a special way in the roundworm C. elegans, one of biology's most important model organisms.
"The fertilized egg splits into one large and one smaller cell," Grill says. "That difference in size is crucial to the development of the whole roundworm body. Normally people think of cells as dividing into two identical daughters; if they don't, there must be forces at work that create an imbalance. We wanted to map them."
As a PhD student at EMBL, working in the research groups of Tony Hyman and the microscopist Ernst Stelzer, Grill pursued an intriguing lead. A cable-like network of proteins called microtubules tows freshly-copied DNA off to opposing sides of the cell. The identical sets of genetic material are then sealed off in their own cells. Normally the anchors that the tow-lines are attached to, called centrosomes, remain near the center of the cell. But in the roundworm egg, one centrosome wanders off towards the outer rim of the cell. Either it was being pulled there or pushed there, Grill reasoned, so he began zapping parts of the cell with a laser, trying to disrupt the mechanism.
Grill followed Hyman and the laser microscope to Dresden, maintaining the collaboration with Stelzer. In the latest round of experiments, he used the laser to punch a hole in the core of the centrosome. As the structure disintegrated, he tracked what happened to the fragments. By measuring the rate at which they drifted apart, he could put exact numbers on the forces pulling them.
"The 'force-generators' are molecules called motors; their job is to pull cargoes down microtubules," Grill says. Here they pull on the microtubule aster to position it. But if there is more motors on one side, or stronger motors on one side we could not distinguish. At this point, Joe Howard came into play: He just looked at the data, and suggested that we should look at the variance in fragment speeds from experiment to experiment. This was possible because we had performed a large enough number of experiments for a thorough statistical analysis. Indeed, this variance displayed an intriguing feature, which, after a bit of mathematical exercise, showed the researchers that there are more motors pulling on the posterior aster: about 25, compare to roughly 15 on the other side. Even though a small number of motors are involved, that's enough to pull the centrosome off-center. This has dramatic consequences it permits the proper development of the body of the embryo."
The measurements will now permit Grill and his colleagues to understand how other molecules change cellular forces and influence cell division. They have already shown that a signal passed along by the protein G-alpha is necessary to activate motors and pull the centrosome off-center.
"Cell division is a very complex process, whether the result is identical daughters or asymmetric ones," Grill says. "Having precise numbers will let us fine-tune the mathematical models and use them to look for molecules that help orchestrate this process in many other types of cells."