Dynamics and Mechanics of Stress Fiber Sarcomeres

Actin filaments inside endothelial cells assemble into tensed bundles called stress fibers. Stress fibers are observed in vivo in a variety of tissues including the aorta, the heart, the spleen, the eye, and hair follicles. Sarcomeres are the force generating units of stress fibers and are responsible for generating intracellular tension. Little is known about how the sarcomeres are assembled in living cells. The mechanical properties of sarcomeres in living cells are also unknown. Here we used femtosecond laser ablation to sever individual stress fibers in living capillary endothelial cells. Sarcomeres contracted over a time scale of seconds, and the contraction depended on myosin activity. The response can be described with mechanical models consisting of active force generation and springs. Our initial results suggest that sarcomeres are assembled close to focal adhesions, and this involves actin flow from the adhesion sites. Preliminary results on measuring actin exchange kinetics in sarcomeres will be presented.