Durotaxis is the process of directed cell migration towards areas of increased extracellular matrix (ECM) stiffness. Waterman and colleagues now report that cells use force fluctuations in individual focal adhesions to sample different ECM rigidities and migrate towards stiff ECM (Cell 151, 1513–1527; 2012).

Using high-resolution time-lapse traction force microscopy, the authors analysed mouse embryonic fibroblasts plated on matrices of different rigidities, and observed that single focal adhesions exhibited asymmetrically distributed traction stress and were not mechanically coupled. They found that individual focal adhesions adopted a state of either stable or fluctuating traction, similarly to a pattern of intermittently 'tugging' at the ECM. Tugging and stable focal adhesion tractions were shown to be supported by soft and rigid ECMs, respectively, in a mechanosensitive response dependent on Rho kinase (ROCK). The authors showed that force transmission and tugging traction dynamics in focal adhesions were regulated by a previously identified signalling axis involving FAK-dependent phosphorylation of paxillin and its subsequent interaction with vinculin. Manipulation of paxillin and ROCK activities in cells plated on ECMs of different rigidities demonstrated that focal adhesion traction dynamics did not affect their maturation or directed migration towards diffuse or substrate-bound cues (PDGF and fibronectin, respectively). In contrast, following the generation of a local rigidity gradient, focal-adhesion-mediated sensing of ECM stiffness by tugging traction dynamics was shown to be necessary for durotaxis.