Polymerizing actin regulates myosin-independent mechanosensing by modulating actin elasticity and flow fluctuation

The stiffness of the extracellular matrix induces differential tension within integrin-based adhesions. However, it has been unclear if the stiffness-dependent differential tension is induced solely by myosin activity. Here, we report that in the absence of myosin contractility, 3T3 fibroblasts still transmit stiffness-dependent differential levels of traction. This myosin-independent differential traction is regulated by polymerizing actin assisted by actin nucleators Arp2/3 and formin where formin has stronger contribution than Arp2/3. Interestingly, we report a four-fold reduction in traction of cells when both Arp2/3 and myosin were inhibited, compared to cells with only myosin inhibition, while there was only a slight reduction in F-actin flow speed in those cells. We show that the conventional rigid-actin-based clutch model is insufficient to explain this force-flow behavior and requires the inclusion of F-actin’s own elasticity into consideration. Our model prediction suggests that Arp2/3 and formin modulate stiffness sensing via stiffening F-actin network with stronger effect from formin. Analysis of F-actin flow reveals stiffness-dependent fluctuation frequency in the flow speed, which is predictable only via the model considering actin elasticity. Our data and model provide a potential role of the polymerizing actin and its elasticity in myosin-independent mechanosensing.