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The corrections

For cells to replicate, chromosomes must segregate precisely. Sometimes, that doesn’t happen.

BY JOSEPH BONNER

For nearly as long as scientists have been studying cells, they have been studying cell division. Yet despite over 150 years of research, plenty of details have yet to be understood.

In Tarun Kapoor’s laboratory, which studies how dividing cells segregate their genetic material, a new element of the process has now emerged that demonstrates how one type of error in the progression of cell division is avoided.

When cells divide, the parent cell must replicate and segregate — with exquisite precision — each of its 46 chromosomes so that two “daughter” cells inherit all of its genetic information. During normal cell division, each replicated chromosome pair attaches to a bipolar structure called the mitotic spindle, made up of protein polymers known as microtubules. One sister chromosome attaches to one pole of the spindle, and the other sister attaches to the opposite pole. As the cell divides, sister chromosomes split up and are pulled in opposite directions so that each daughter cell receives a copy of each chromosome.

When there are errors in this process, the daughter cells don’t receive all their genes, leading to developmental defects and diseases such as cancer. One such error occurs when both sister chromosomes of a replicated pair attach to the same pole of the mitotic spindle in a dividing cell. The result is then one daughter cell with an extra copy of a chromosome and one daughter cell with one chromosome missing.

The question Kapoor and his colleagues wanted to answer was: How does the cell avoid this?

In general, researchers in Kapoor’s lab use small organic molecules to interrupt the process of cell division. These inhibitors work by permeating the cell membrane and blocking the action of various proteins.

Enter an enzyme called Aurora kinase. Without Aurora kinase, chromosomes become very prone to improper attachments in dividing cells. But its role in correcting improper chromosome attachments was largely unknown. Michael Lampson, a postdoc in Kapoor’s Laboratory of Chemistry and Cell Biology, used a small molecule called Aurora kinase inhibitor-1 (AKI-1), originally developed by AstraZeneca as a potential cancer drug. AKI-1, as its name implies, inhibits Aurora kinase activity.

According to Lampson, scientists at the Research Institute of Molecular Pathology in Vienna had proposed a model that suggested Aurora kinase activity would cause some of the incorrect attachments to break and lead to the formation of new, correct attachments. Lampson devised a strategy to test this hypothesis, incubating cells with the Aurora kinase inhibitor to accumulate attachment errors. The kinase could then be reactivated by simply removing the inhibitor. Using real-time, high-resolution microscopy in Rockefeller’s Bio-Imaging Resource Center, Lampson and his colleagues activated Aurora kinase and watched.

Within a few minutes, something unexpected happened. “Once we washed out the Aurora kinase inhibitor, the microtubule fibers shortened and the chromosomes moved directly to the pole,” says Lampson.

According to Lampson, Aurora kinase eliminates the microtubule fiber by causing it to disassemble. As it disassembles, it pulls the chromosomes into the pole, enabling them to also attach to a microtubule from the opposite pole. Once the chromosome has an attachment to both poles, it can become correctly aligned.

“It’s an unexpected mechanism. It shows how Aurora kinases are regulating the microtubules to correct errors,” says Lampson.

But what’s even more exciting, the researchers showed that it’s possible to use small organic molecules to switch proteins both off and on in living cells while observing the effects. “The technique has generally not been possible experimentally because once you turn a protein off you can’t turn it back on,” says Lampson. “These experiments demonstrate how activation and deactivation of protein function, through chemical genetics, can be a valuable approach to understanding cellular mechanisms.”

February 27, 2004

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