Traction Forces Control Cell-Edge Dynamics and Mediate Distance Sensitivity during Cell Polarization.

Cell movement and shape changes rely on traction forces generated by actin-myosin systems. These forces are transmitted through adhesions to the environment. Most studies have focused on stationary cells, analyzing forces at focal adhesions or linking them to static cell features like area or edge curvature. However, the relationship between traction forces and dynamic shape changes or motion remains unclear. Some studies report conflicting results about whether forces increase or decrease before cell retraction. This uncertainty has limited understanding of how forces influence edge dynamics during polarization. Prior research has shown that traction forces are essential for cell motion and matrix remodeling, but how they coordinate with protrusion and retraction is not fully explained. The role of force fluctuations during dynamic cycles is still debated. No prior work had resolved how forces might mediate distance sensitivity in edge behavior. This gap motivated the current investigation into force dynamics during keratocyte polarization.

The study aimed to analyze traction force dynamics during the protrusion-retraction cycle of polarizing keratocytes. Researchers sought to determine how forces fluctuate with cell-edge behavior and whether they mediate distance sensitivity. They focused on fish epidermal keratocytes, known for their rapid polarization. The goal was to test a recently proposed rule linking edge behavior to distance from the cell center. The study also aimed to assess how force inhibition affects polarization and whether external forces could trigger retraction. Researchers wanted to clarify whether actin flow rate contributes to edge dynamics. Finally, they aimed to model actin-myosin networks to explore if force-distance relationships are emergent properties of such systems.

The researchers used traction force analysis to track force dynamics during keratocyte polarization. They monitored protrusion-retraction cycles and measured force fluctuations. A phenomenological rule was tested, relating edge behavior to distance from the cell center. Cell contractility was inhibited using specific drugs to observe effects on force and polarization. External forces were applied to test if they could trigger retraction. Actin flow rate was measured and compared to traction stress patterns. A computational model of actin-myosin networks was developed to simulate force-distance relationships. The model aimed to determine if such relationships emerge naturally from network properties.

Traction forces fluctuated with the protrusion-retraction cycle of keratocytes, peaking at the start of retraction. Forces increased during protrusion and reached maximum at the onset of retraction. Both traction forces and the probability of switching from protrusion to retraction increased with distance from the cell center. Inhibiting contractility reduced edge fluctuations and disrupted polarization. Externally applied forces could induce protrusion-retraction switches. Actin flow rate did not show the same distance dependence as traction stress. This suggests actin flow is not the organizing factor for edge dynamics. The model showed that force-distance relationships might emerge from actin-myosin network properties.

The findings suggest that traction forces mediate distance sensitivity in keratocyte edge dynamics. Forces fluctuate with protrusion-retraction cycles and increase with distance from the cell center. Force inhibition disrupts polarization, but external forces can trigger retraction. Actin flow rate does not exhibit the same distance dependence as traction stress, arguing against its role in organizing edge behavior. The model supports the idea that force-distance relationships may be emergent features of actin-myosin networks. These results align with the authors' claim that forces organize edge dynamics during polarization. The study does not propose new mechanisms beyond those stated in the abstract. The authors do not suggest broader implications beyond the specific findings on keratocyte polarization.