Contractile Forces During Ecm Rigidity Sensing Are Regulated By Tropomyosin-1

Extracellular matrix (ECM) rigidity is a critical regulator of important cellular processes such as survival, proliferation, and differentiation. To measure ECM rigidity, cells form actomyosin-based contractile units at the lamellipodium that pull on the matrix with forces proportional to its stiffness. However, the mechanisms that regulate force production are unknown. Here we show that generation of force involves constant step-wise movements of myosin that are controlled by tropomyosin-1. Nanometer- and millisecond-scale measurements of fibroblasts pulling on elastomeric PDMS pillars show that pillar displacement occurs by discrete steps of ∼1 nm. In contractile pairs, simultaneous steps of opposing pillars give a net movement of ∼2.2 nm, independent of rigidity. Changes in the stepping patterns on different rigidities indicate that the level of force is critical for sensing pillar stiffness. Importantly, knockdown of tropomyosin-1 causes larger steps and increased forces that result in aberrant rigidity sensing. These results indicate that tropomyosin-1 is critical for rigidity sensing by controlling myosin movements on actin.