Fig. 1: Model set-up.
a, Integrins and adaptor proteins such as talin are the main components of cell-adhesion linkages that form between the actin cytoskeleton and the substrate. The myosin II-generated retrograde flow of actin filaments leads to mechanical stretching of these links, causing local substrate deformations (shown in red). Force-transmitting units, each including an intracellular part (integrin and adaptor proteins) and an extracellular part (locally deformed substrate), are referred to in this study as ‘molecular clutches’. b, In the model, molecular clutches are represented by composite springs consisting of two parts: an intracellular one (kc) and an extracellular one (\({k}_{{\rm{s}}}^{{\prime} }\)). Although we assume the latter has a linear response to the applied load, we consider the former to be a linear spring in the linear clutch model and a nonlinear spring with a talin-like force response in the talin clutch model. The pulling force (Fm) generated by myosin II motors causes actin filaments to move at a speed v. This leads to a gradual stretching of molecular clutches along the x axis parallel to the substrate surface (contact angle θ ≈ 15°)65, resulting in their extension, l. This, in turn, allows transmission of the myosin II-generated force (Fm) to the substrate, leading to the cell traction force (Fc), which causes deformation of the substrate (xs) over the entire area of cell-adhesion complexes. The efficiency of the force transmission is mainly determined by the kinetic behaviour of molecular clutches under a mechanical load, which is described by their formation and dissociation rates, kon and koff, respectively. The viscoelastic behaviour of the substrate at the scale of cell-adhesion complexes is described in the study using the Kelvin–Voigt model, which represents substrate deformations as a purely elastic spring with stiffness ks, connected in parallel to a purely viscous damper characterized by viscosity constant ξs. In general, by replacing the Kelvin–Voigt model with the Maxwell model or the standard linear solid model, it is possible to describe the mechanosensitive behaviour of cell-adhesion complexes on liquid-like materials and on materials with more complex viscoelastic behaviour, respectively. SLS model, standard linear solid model.
