The fibers that give cells the ability to flatten themselves as they crawl along a surface are laid out like a tent, researchers have discovered. One network of fibers connects the base and top of the cells, while another set—like a canvas stretched over tent poles—controls the shape, angles, and placement of those fibers from above. By using super-resolution live-cell imaging, a team of scientists was able to see the arrangement of these molecules in the front edge of a crawling cell in three dimensions for the first time (1).
Previous methods used to visualize the flat leading edge of a crawling cell—called the lamella—not only had low resolution but also required fixing cells before microscopy. “It artificially flattened cells, and you lost all the three-dimensional architecture,” said Jennifer Lippincott-Schwartz of the National Institute of Child Health and Human Development’s Section on Organelle Biology. “It was really the ability to do this new imaging in both live cells and gently fixed cells that gave us the ability to see this beautiful three-dimensional arrangement of the filaments.”
Scientists knew that the lamella contained actin arcs and dorsal and ventral stress fibers. But they’d only been able to view the filaments from above, looking at a single plane, so how the filaments interacted to flatten a crawling cell wasn't clear.
Lippincott-Schwartz and her colleagues turned to structured illumination microscopy (SIM) to look at the lamella in new detail. They not only saw how the fibers were arranged but also could visualize the molecules of myosin-2 that apply force to the system. What they found was that the actin arcs coupled with myosin are arranged linearly on the top of the cell. Stiff dorsal stress fibers (the tent poles) connect the actin arcs with adhesions on the bottom of the cell that interact with the surface below.
“The stress fiber is playing the important role of connecting an actin contractile system in one part of the cell with an adhesion system in another part,” Lippincott-Schwartz said.
When the actin arcs on the top of the cell contract, they tug on the tops of the dorsal stress fibers—as the fibers tilt, the cell’s top and bottom get closer. The newly discovered system of fibers could be relevant not only for cells moving along a flat epithelium—a bone or the lining of an organ—but also for cells moving through three-dimensional meshwork, Lippincott-Schwartz hypothesized.
Her team next plans to investigate how the flattening of a cell influences the rest of its physiology—whether water moves out of the cell when it flattens and crawls, for example—as well as which pathways initiate crawling to begin with.
Burnette DT, Shao L, Ott C, Pasapera AM, Fischer RS, Baird MA, Der Loughian C, Delanoe-Ayari H, Paszek MJ, Davidson MW, Betzig E, Lippincott-Schwartz J. A contractile and counterbalancing adhesion system controls the 3D shape of crawling cells. J Cell Biol. 2014 Apr 14;205(1):83-96.