Abstract

Adherent cells use actomyosin contractility to generate mechanical force and to sense the physical properties of their environment, with dramatic consequences for migration, division, differentiation and fate. However, the organization of the actomyosin system within cells is highly variable, with its assembly and function being controlled by small GTPases from the Rho-family. How activation of these regulators translates into cell-scale force generation and the corresponding sensing capabilities in the context of different physical environments is not understood. Here we probe this relationship combining recent advances in non-neuronal optogenetics with micropatterning and traction force microscopy on soft elastic substrates. We find that after whole-cell RhoA-activation by the CRY2/CIBN optogenetic system with a short pulse of 100 milliseconds, single cells contract before returning to their original tension setpoint with near perfect precision on a time scale of several minutes. To decouple the biochemical and mechanical elements of this response, we introduce a mathematical model that is parametrized by fits to the dynamics of the substrate deformation energy. We find that the RhoA-response builds up quickly on a time scale of 20 seconds, but decays slowly on a time scale of 50 seconds. The larger the cells and the more polarized their actin cytoskeleton, the more substrate deformation energy is generated. RhoA-activation starts to saturate if optogenetic pulse length exceeds 50 milliseconds, revealing the intrinsic limits of biochemical activation. Together our results suggest that adherent cells establish tensional homeostasis by the RhoA-system, but that the setpoint and the dynamics around it are strongly determined by cell size and the architecture of the actin cytoskeleton, which both are controlled by the extracellular environment.