While significant progress has been made in understanding the biochemical and biophysical interactions of a wide variety of individual molecules, we know surprisingly little about how the cell coordinates hundreds and thousands of simultaneously active molecules across both time and space. For instance, we know in great detail the mechanochemical cycle of a non-muscle myosin II motor, but that doesn’t tell us how a cell determines where to put down an adhesion or how hard to pull on it as it polarizes and migrates. These types of questions are further complicated by the fact that many suspected regulatory factors are tightly coupled, making it difficult to establish distinct regulatory roles.
In this case, we wanted to know how the cell determines the magnitude and distribution of traction stresses when adhered to a surface. When it became apparent that cell geometry was important we turned to micropatterning as a way to tightly control cell shape. In this way we could decouple the roles of the suspected regulatory parameters: spread area, cell shape, number of adhesions and the stiffness of the substrate. By combining our traction force experiments with micropatterning, we could isolate single parameters to vary. The image selected for the cover demonstrates the beauty and precision of this approach. It depicts the actin cytoskeleton, stained by a fluorescent phalloidin, in an experiment where we held the spread area constant while altering the cell shape.
Using these techniques we were able to establish that the amount of work done by the cell was regulated by the spread area alone, and was independent of the substrate stiffness or the number of focal adhesions. Changes in cell shape served to regulate the distribution of stresses on the substrate, but did not change the overall contractility of the cell. These results enabled us to build a simple yet accurate physical model of the cell that worked for all cell shapes, not just the micropatterened ones. We can now use this model to investigate the molecular mechanisms driving these physical processes. Our aim is to highlight the interesting mechanical means that a cell can use to regulate molecular interactions at the cellular scale.
– Patrick Oakes, Shiladitya Banerjee, Cristina Marchetti & Margaret Gardel