No Rest for Cellular Motors

Cells that appear to be parked may have their engines idling. The new findings suggest that so-called focal adhesions--clumps of proteins on the cell's surface that can grab on to another surface--can move around quite a lot when a cell is stationary. The discovery, reported in today's Science, suggests that cells may have a molecular clutch that controls the amount of traction they have. If so, new drugs might be developed someday that could jam the clutch and prevent tumors from spreading.

Grabbing on with focal adhesions enables a cell to pull itself from place by contracting attached actin and myosin fibers. Fibroblast cells use this mobility to help repair wounded connective tissue, and cancer cells use it to break away from tumors. Scientists thought these adhesions settled down when a cell was at rest.

To observe the movements of focal adhesions, cell biologist Gregg Gundersen and his colleagues at Columbia University in New York City took mouse fibroblast cells and transfected them with a gene for a fluorescent version of integrin, one of the sticky proteins in the adhesion. The researchers took pictures every 10 to 15 minutes. To their surprise, they found focal adhesions constantly moving inside stationary cells.

The adhesions either migrated along the surface in the direction of the nucleus or in a few instances drifted toward one end of the cell. Gundersen says this shifting may allow the cell to prepare for its next movement. "If [the focal adhesions] were fixed down really tight, the cell would have to pull up all these tent stakes to get going," he says.

Other cell biologists, such as Rick Horwitz of the University of Virginia in Charlottesville, are intrigued by the findings. Horwitz says he's curious whether one or more molecules acts as a clutch that decides whether the focal adhesions grab on to their surroundings to obtain traction or slide freely. Whatever the mechanism, it could allow researchers to understand and control the ability of cells, including cancerous ones, to migrate.