February 6, 2012

How molecules alter cell’s skeletal shape, drive cell’s movement

Cell biologists at Johns Hopkins have identified key steps in how certain molecules alter a cell’s skeletal shape and drive the cell’s movement.

Results of their research, published in the Dec. 13 issue of Science Signaling, have implications for figuring out what triggers the metastatic spread of cancer cells and wound healing.

“Essentially we are figuring out how cells crawl,” said Takanari Inoue, an assistant professor of cell biology and a member of the Center for Cell Dynamics in the Johns Hopkins University School of Medicine’s Institute for Basic Biomedical Sciences. “With work like ours, scientists can reveal what happens when cells move when they aren’t supposed to.”

The new discovery highlights the role of the cell’s skeleton, or cytoskeleton, in situations where “shape shifting” can rapidly change a cell’s motion and function in response to differing environmental conditions.

When a cell such as a fibroblast, which gathers with others to heal wounds, moves from one place to another, its cytoskeleton forms ripplelike waves or ruffles across its surface that move toward the front of the cell and down, helping pull the cell across a surface. Researchers have shown that these ruffles form when a small molecule, PIP2, appears on the inside surface of the membrane at the front edge of a cell. Until now, however, they have been unable to re-create cell ruffles simply by directing PIP2 to the cell’s front edge. Manipulations have instead led the cytoskeleton to form completely different structures, squiggles that zip across the inside of the cell like shooting stars across the sky, which the researchers call comets.

In their experiments, Inoue and his group looked for factors that determined whether a cell forms ruffles or comets. The researchers tried to create ruffles on the cell by sending to the cell membrane an enzyme that converts another small molecule into PIP2. Using a microscope and cytoskeleton building blocks marked to glow, the researchers watched the cytoskeleton assembling itself and saw that this approach caused the cytoskeleton to form comets, not the ruffles that they had predicted.

The team suspected that comets formed because of a fall in levels of another small molecule, PI4P, used to make PIP2.

To test this idea, the researchers tried to make ruffles on cells by increasing PIP2 at the membrane, rather than by changing the quantities of any other molecules. Using molecular tricks that hid existing PIP2 and then revealed it, the researchers effectively increased the amount of available PIP2 at the membrane. This time, the researchers saw ruffles.

“Now that we’ve figured out this part of how cells make ruffles, we hope to continue teasing apart the mechanism of cell movement to someday understand metastasis,” Inoue said. “It will be interesting to manipulate other molecules at the cell surface to see what other types of cytoskeletal conformations we can control.”

Additional authors of the study were Tasuku Ueno and Christopher Pohlmeyer, both of Johns Hopkins; and Bjorn Falkenburger, of the University of Washington.

The study was funded by grants from the National Institutes of Health and the Japan Society for the Promotion of Science.