Cell Suicide

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American Society for Cell Biology Meeting, Washington, December 2001 Holes barred by protein purse-string Suicidal cells are squeezed out of the way. 13 December 2001 HELEN PEARSON Cells jostle each other like crowds at a concert ?Corbis Dying cells are squeezed out by their neighbours to keep the body's barriers intact, researchers announced at the meeting of the American Society for Cell Biology in Washington. Lining the lung and gut, layers one cell thick divide our insides from the outside, controlling the movement of molecules and preventing the entry of bacteria. Cell death and division constantly renews this 'epithelial' tissue. "When you depend on a one-cell layer and cell death occurs, you're faced with a problem," says Jody Rosenblatt of University College London. Dying cells must be carted off without creating holes, yet the removal mechanism has been a mystery. Surrounding cells squeeze out a sick cell like toothpaste from a tube, by forming a contracting ring of protein, Rosenblatt and her colleagues have found. Cell 'extrusion' occurs before there are any obvious signs that the cell is in trouble; early signals released by the dying cell are thought to tell others to close in. Overcrowded areas are more likely to oust their neighbours. "It's like being at a concert with some jostling going on," says Rosenblatt. "Elbows are out." Tight-packed cells may actually push others over the edge to cell suicide, she speculates. Actin up The team exposed dog epithelial cells to UV light to trigger cell suicide. By measuring the sheet's electrical resistance, they found that, even when half of them are dying, the layer remains intact. Actin, a protein that makes the cell's skeleton, forms a circular cable around the dying cell and closes it down within an hour. Rosenblatt filmed the process using actin attached to green fluorescent protein. "It makes sense for it to work this way," agrees Bill Bement of the University of Wisconsin in Madison. The pressure of the ring keeps the seal between cells, he explains. Bement studies a similar 'purse-string' mechanism thought to close epithelial wounds. Researchers had assumed that neighbours or passing blood immune cells devoured dead cells. But this idea cannot explain how the cell layer is kept intact, he says. By attacking the closure mechanism, diarrhoea-causing gut bacteria may get into the body, he suggests. ?Nature News Service / Macmillan Magazines Ltd 2001