Placing a nucleus in a cell extract causes the DNA to fragment, as during apoptosis.


William Earnshaw and colleagues wanted to plumb the intricacies of cell birth, but they ended up discovering a vital tool for studying cell death in vitro (Lazebnik et al., 1993). The sequence of events in apoptosis remained uncertain at the time, recalls Earnshaw (now at the University of Edinburgh in the UK), because suicidal cells die asynchronously. “You could never have a tube of cells all undergoing apoptosis at the same time,” he says. This made it difficult to pinpoint the biochemical details of each step.

Earnshaw's group was hoping to crack a different question: how the cell's chromatin condenses during mitosis. To study the process, they had devised a cell-free system containing cytoplasm from dividing liver cancer cells. Nuclei bathed in these extracts appeared to begin mitosis—their DNA clumped against the nuclear membrane, for example.

But Earnshaw's post-doc Yuri Lazebnik (now at Cold Spring Harbor Laboratory in New York) happened to attend a seminar on apoptosis and recognized similarities between dying cells and his isolated nuclei. Back in the lab, experiments confirmed that nuclei incubated in the cell extracts were following the script for apoptosis, not girding for division. Just as in apoptosis, DNA within the nuclei got minced into pieces that were multiples of 200 base pairs in length—the result of enzymes cutting between the nucleosomes. As in a dying cell, the nuclear membrane blebbed and extruded dense balls of chromatin. And the researchers found that zinc, which can stall apoptosis, prevented the nuclei from deteriorating.

The results were important because they allowed researchers to create synchronized systems to study how protein-slicing enzymes such as the caspases orchestrate apoptosis, says Earnshaw. His group was the first to capitalize on this new ability (Lazebnik et al., 1994), identifying the specific amino acid sequence where caspases clip the DNA repair protein PARP. But if Lazebnik hadn't gone to that seminar, says Earnshaw, the researchers might still think that they had been looking at mitosis.

Lazebnik, Y.A., et al.
J. Cell Biol.

Lazebnik, Y.A., et al.