Force Loading During Mechanosensing Emerges from Non-Mechanosensitive Active Displacements

Abstract

It is widely accepted that during mechanosensing cells apply higher forces with increased rigidity of the extracellular matrix (ECM). This is explained by the integrin clutch model through rigidity-dependent changes in the lifetimes of the bonds formed within the actin-integrins-ECM link, and the reinforcement of adhesions through talin stretching and vinculin recruitment, which increases the number of clutches. This allows more efficient force loading on the adhesions and transmission of higher forces into the matrix when it is rigid. However, although plenty of evidence exists on the role of forces in mechanosensing, and although force generation is a highly dynamic process, the rigidity- and time-dependency of force loading during mechanosensing remain poorly understood. In particular, it is unclear whether, and if so how, sensing of the rigidity feeds back to the active contractile forces as deformation of the matrix proceeds in time. Here, based on a mathematical model of cellular contractility and extensive experiments using cells adhering to micro-pillar arrays, we show that for a broad range of physiologically relevant rigidities, the contractile force is proportional to the rigidity of the matrix, where the proportionality factor is an intrinsic, cell type-specific, time-dependent contractile displacement that is independent of the rigidity. Namely, we show that active contractions are non-mechanosensitive. Additionally, we show that the contractile displacements are highly correlated in space and time with local concentrations of F-actin near the adhesion sites. Using super-resolution microscopy, we further show an increase in the density and organization of the actin fibers associated with the contractile displacements when cells are plated on increased rigidities, indicating that the primary mechanosensitive factor regulating force transmission during mechanosensing is the actin network associated with the integrin adhesions.