Cell response to substrate energy dissipation outweighs rigidity sensing

The mechanical properties of the extracellular matrix (ECM) are essential for tissue homeostasis, as they determine cell spreading, differentiation, proliferation and migration through the concerted action of mechanoresponsive proteins such as YAP. However, interpretation of the mechanical crosstalk between cells and the ECM using hydrogel mimetics has been limited by the intricacies to engineer viscoelasticity typical of the ECM, the difficulty in discerning viscoelastic from viscoplastic behaviors, and the convolution of their mechanical and non-mechanical properties. As a result, whether cells sense ECM energy dissipation (i.e. viscoelasticity) independently of stiffness remains unknown. Here, we exploit fundamental polypeptide conformational changes under force to produce titin-based protein hydrogels with tailored viscoelasticity and preserved chemical composition, ultrastructure, crosslinking density, and biochemical environment. Using our system, we demonstrate that cells respond to changes in viscoelastic cues only, as nuclear YAP decreases with substrate energy dissipation. Furthermore, the rate of YAP translocation to the nucleus is more sensitive to changes in energy dissipation than in stiffness; indicating that the response to the viscous component engineered into our ECM mimetics supersedes rigidity effects. This observation implies that cells perceive ECM rigidity and energy dissipation by non-overlapping mechanisms, illustrating the relevance of ECM viscoelasticity in the biology of the cell.