Cell motion mediated by friction forces: Understanding the major principles

Normally, cell motion is driven by forces, which are generated within the cell by polymerization of actin filaments and transmitted to external substrates by specialized protein complexes called cell adhesions. Recent experiments demonstrated the ability of cells subjected to spatial constraints to move in the absence of adhesions or upon the adhesion decoupling from the actin filaments. Here we consider the physical principles of this phenomenon. We suggest that the cell translocation accompanied by the actin retrograde flow can be understood based on the actin-substrate mechanical coupling through the friction between the substrate and the proteins anchoring the actin network to the cell membrane. We consider a cell as a membrane folded between two flat substrates around a system of actin filaments polymerizing against the cell edge. The filaments are connected to the membrane by mobile anchors. We show that the direction of the cell translocation is set by the relationship between the friction coefficients of the anchors and the membrane with the substrates. We demonstrate that the filament actions are essentially non-additive. Finally, we show that the model recovers the observed velocities of motion of spatially confined cells without cell adhesions and the related rates of the actin retrograde flow.