Sarcomere Mechanics in Endothelial Cells

Tension generation in cells is necessary for normal cell function. In vivo tension-generation in endothelial cells of the aorta, the spleen and the eye occurs in actin stress fibers. Sarcomeres are the tension generating units of actin stress fibers in endothelial cells. How sarcomeres generate tension is not well understood due to the lack of experimental measurements of sarcomere contraction in the living endothelial cells. Using femtosecond laser ablation, we severed living stress fibers and measured sarcomere contraction under zero tension. The length of the sarcomere decreased in two phases with an instantaneous, initial response followed by a slower change in length. The latter phase ceased abruptly after a minimum sarcomere length was reached, suggesting a rigid resistance that prevents further contraction. These novel measurements combined with modeling suggest that myosin generated forces in adjacent sarcomeres are directly in balance, and argue against sarcomere models with spring-like elements in parallel with myosin contractile elements. We propose a new model for tension generation where the force in the stress fiber is balanced by actomyosin forces in each sarcomere. This provides a mechanistic interpretation for our observations, previous observations of inhomogeneous sarcomere contraction and previously reported visco-elastic behavior of stress fibers.