Study Reveals Dynamic Interface of Molecular Clutch in Cell Migration

By Eric Sauter

Using a remarkable new technology, scientists at The Scripps Research Institute have identified a number of key protein interactions that allow cells to migrate throughout the body. These findings, which describe in detail how cells transmit molecular information into physical movement, provide the first direct analysis of cellular movement and may point the way to potential treatments for a variety of diseases including cancer.

The study, led by Scripps Research Associate Professor Clare Waterman-Storer, was published in the January 5 issue of the journal Science.

The study describes the interaction among focal adhesion proteins and actin filaments. Actin is a protein that forms a cytoskeleton inside the cell enabling movement and polarity; focal adhesions provide a link between actin and the extracellular matrix, the non-cell connective tissue that provides cell support.

The long-term implication of these results is significant in light of the new levels of understanding—much of it generated by the ongoing work of the Waterman-Storer laboratory—of the phenomenon of cell migration.

"It's becoming more apparent that signals that control growth and motion can be mediated by mechanical stimuli," she said. "To offer just one important example, new blood vessels crawl along the more rigid areas of the extracellular matrix and that has potential application in oncology. Consequently, expanding our understanding of how molecules transmit information into physical force is now recognized as being just as important as how one molecule transmits information to another through biochemical signaling. The technology we pioneered for this study—Correlational Fluorescent Speckle Microscopy—allows us to visualize for the first time the molecules that mediate that molecular clutch."

The new technology is a unique combination of total internal reflection fluorescence microscopy (TIRFM) and fluorescent speckle microscopy (FSM). The former optimizes the image contrast of the interface between actin and focal adhesions, while the latter marks the focal adhesion assemblies with fluorescent clusters called speckles. The technology, which has been in development for the last two years, is important for understanding cell structures and processes in general, Waterman-Storer said.

Cells migrate frequently within the body, often according to function, and the process is integral to a number of important biological functions. Some of those functions like tissue repair and cell differentiation are both necessary and normal. Others, like metastatic cancer, atherosclerosis and arthritis are pathological.

However, until this study, predicting the various interactions of focal adhesion proteins in vivo was virtually impossible because of the complexity of those interactions.

In discussing the new findings, Waterman-Storer compared the inner workings of cell motility (migration) to that of an automobile.

"For the cell to move, the motion inside the cell has to be translated onto its exterior," she said. "The best metaphor is the way a car works. Inside the cell, the engine is running constantly but in order for the cell to decide which direction to go, it has to engage the motor to exert that force on the extracellular environment. When you drive a car, you engage the engine through the clutch to a transmission and that in turn engages the wheels. In the cell there are motors everywhere and the cell engages these different motors to coordinate its movement."

The study's analysis of the dynamic interactions between various focal adhesion components and actin filaments reveals that the efficiency of motion transmission from the actin filaments to proteins within focal adhesions decreases from actin-binding proteins to core focal adhesion proteins and then to integrin, a membrane protein that helps in cell attachment to the extracellular matrix, creating a hierarchical molecular clutch. This is likely the result of differential transmission of F-actin-based motion through a network of transient protein-to-protein interactions in focal adhesions, the study concluded.

The degree of molecular motion transmission through the focal adhesions was correlated with protrusion and retraction events during cell migration.

"The technique developed by Dr. Waterman-Storer has given us amazing insights into a very complex but fundamental type of cell movement," said Richard Rodewald, a cell biologist at the National Institutes of Health, which helped support the research. "With remarkable resolution, Dr. Waterman-Storer has visualized in real time and in living cells the movements of individual molecules that interact with focal adhesions, the main traction sites of the mobile cell. Her analysis offers the most detailed model to date for how mechanical force is generated and regulated at these sites."

The other authors of the study, Differential Transmission of Actin Motion within Focal Adhesions, are Ke Hu, Lin Ji, Kathryn T. Applegate, and Gaudenz Danuser of The Scripps Research Institute. For more information on the paper, see the journal Science at: http://www.sciencemag.org/cgi/content/abstract/315/5808/111

In addition to the NIH, the study was supported by the Leukemia & Lymphoma Society and The National Science Foundation.

Send comments to: mikaono[at]scripps.edu

"It's becoming more apparent that signals that control growth and motion can be mediated by mechanical stimuli."

—Clare Waterman-Storer