Cells might be able to rapidly adjust their stiffness by contracting myosin motors, according to a new cytoskeleton model from Daisuke Mizuno, Christoph Schmidt (Georg-August-Universität, Göttingen, Germany), and colleagues.
Cells adjust their rigidity when they interact with the extracellular matrix, for example, and when an external force acts on them. To mimic cytoskeleton dynamics, the researchers sandwiched a gel of cross-linked actin fibers and myosin motors between a coverslip and a microscope slide. Using a laser to jiggle tiny beads embedded in the gel, they could gauge the gel's stiffness and measure the motions generated by the motors. Myosin's action increased tension in the actin fibers and raised the gel's stiffness by up to 100 times. Per actin filament, it only required a fraction of a piconewton to cause this dramatic effect, less force than a single myosin molecule produces, Schmidt notes.
The model captures several other aspects of cell dynamics. Random competition between motor clusters can trigger local contractions within the gel, for instance, and the slow rise of tension followed by sudden release matches behavior in real cells.
The results imply that to become more rigid, “the cell simply contracts its muscles,” says Schmidt. A cell may change its stiffness through this flexing alone, without altering actin polymerization or other properties. The researchers are now using embedded beads to measure forces within living cells.