Actin moves chromosomes

Surprise findings from starfish oocytes address key questions about cell division

By | July 14, 2005

An actin network can move chromosomes during cell division, scientists report in this week's Nature. Their surprise findings are the first indication that the protein filaments play a role in chromosome movement, introducing a novel mechanism that seems necessary for delivering distal chromosomes to within the capture distance of centrosomal microtubules.

"The mechanism described [by the group] is so unexpected and beautiful. I never saw even a hint of something like that," said Alex Mogilner, from University of California, Davis, who was not involved in the research. He told The Scientist that the results address central questions in cell division—how the mitotic spindle is assembled from microtubules and other proteins, and how astral microtubules connect to chromosomes. "This has great importance for the field," Mogilner said.

Jan Ellenberg and colleagues at the European Molecular Biology Laboratory in Heidelberg followed meiosis in starfish (Asterina miniata) oocytes, and showed for the first time in vivo that in large oocyte nuclei, chromosome capture by microtubules is inefficient.

"We could visualize for the first time with 4-D images that chromosomes nucleate actin filaments directly, and that this process is essential for chromosome movement and, thus, to prevent chromosome loss," first author Péter Lénárt told The Scientist. Until now actin was found to be involved in diverse cellular processes, but not chromosome dynamics.

"We were interested in understanding how microtubules reorganize at the time of nuclear envelope breakdown [NEBD]. By labeling microtubules in live cells, we noted that after NEBD, microtubule asters were much too short to capture distal chromosomes more than 40 m away from centrosomes," continued Lénárt. "Since most cellular movements are mediated by either the microtubule or the actin cytoskeleton, it wasn't difficult to guess that, if not microtubules, then actin must drive chromosome congression."

"I think the data are very solid and convincing," Claire Walczak, from Indiana University, Bloomington, who did not participate in this study, told The Scientist. "However, I would not call what they are examining congression, but rather capture and translocation of the chromosomes. To me, the definition of congression is the alignment of the chromosomes at the metaphase plate and it is unclear whether this long-range capture is true congression or a mechanism to bring the chromosomes to the spindle proper so that the canonical congression process can occur."

"No one has ever shown that actin moves chromosomes," said senior author Jan Ellenberg. "We were able to do so because our group is one of the few that studies cell division in starfish–an ideal model for observing division in living animal eggs–in a very fruitful collaboration with Mark Terasaki at the Marine Biological Laboratory."

Indeed, a key point of the work was the choice of organism. Starfish oocytes are transparent and develop simply in seawater, which renders them an excellent specimen for live cell microscopy. In addition, starfish are evolutionary close to vertebrates.

"Using starfish oocytes, Lénárt et al. have discovered a fundamental biological mechanism that is likely to apply to mammalian oocytes as well. This is another great example of a centrally important concept emerging from the study of echinoderm oocytes," Laurinda Jaffe from University of Connecticut Health Center in Farmington, who did not participate in this study, told The Scientist.

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