Cell migration is achieved by the dynamic feedback interactions between traction forces generated by the cell and exerted onto the underlying extracellular matrix (ECM), and intracellular mechano-chemical signaling pathways, e.g., Rho GTPase (RhoA, Rac1, and Cdc42) activities [1,2,3]. These components are differentially distributed within a cell, and thus the coordination between tractions and mechanotransduction (i.e, RhoA and Rac1 activities) must be implemented at a precise spatial and temporal order to achieve optimized, directed cell migration [4,5]. Recent studies have shown that focal adhesions at the leading edge exert strong tractions [6], and these traction sites are co-localized with focal adhesion sites [7]. Further, by using the fluorescence resonance energy transfer (FRET) technology coupled with genetically encoded biosensors, researchers reported that Rho GTPases, such as RhoA [8], Rac1 [9], and Cdc42 [10] are maximally activated at the leading edge, suggesting the leading edge of the cell as its common functional site for Rho GTPase activities. All these works, however, were done separately, and the relationship between tractions and mechanotransduction during cell migration has not been demonstrated directly because of the difficulty in simultaneously recording tractions and mechanotransduction in migrating cells, precluding direct comparison between these results. Furthermore, these studies have been conducted by monitoring cells on glass coverslips, the stiffness of which is ∼ 65 giga pascal (GPa), at least three to six order higher than the physiological range of ECM stiffness. Although it is increasingly accepted that ECM stiffness influences cell migration, it is not known exactly how physiologically relevant ECM stiffness (order of kPa range) affects the dynamics of RhoA and Rac1 activities. For a complete understanding of the mechanism of mechano-chemical signaling in the context of cell migration, the dynamics and interplay between biomechanical (e.g., tractions) and biochemical (e.g., Rho GTPase) activities should be visualized within the physiologically relevant range of ECM stiffness.
Force Fluctuations within Focal Adhesions Mediate ECM-Rigidity Sensing to Guide Directed Cell Migration