Abstract
Traction forces are generated by cellular actin-myosin system and transmitted to the environment through adhesions. They are believed to drive cell motion, shape changes, and extracellular matrix remodeling. However, most of the traction force analysis has been performed on stationary cells, investigating forces at the level of individual focal adhesions or linking them to static cell parameters such as area and edge curvature. It is not well understood how traction forces are related to shape changes and motion, e.g. forces were reported to either increase or drop prior to cell retraction. Here, we analyze the dynamics of traction forces during the protrusion-retraction cycle of polarizing fish epidermal keratocytes and find that forces fluctuate in concert with the cycle, increasing during the protrusion phase and reaching maximum at the beginning of retraction. We relate force dynamics to the recently discovered phenomenological rule that governs cell edge behavior during keratocyte polarization: both traction forces and the probability of switch from protrusion to retraction increase with the distance from the cell center. Diminishing traction forces with cell contractility inhibitor leads to decreased edge fluctuations and abnormal polarization, while externally applied force can induce protrusion-retraction switch. These results suggest that forces mediate distance-sensitivity of the edge dynamics and ultimately organize cell-edge behavior leading to spontaneouspolarization. Actin flow rate did not exhibit the same distance-dependence as traction stress, arguing against its role in organizing edge dynamics. Finally, using a simple model of actin-myosin network, we show that force-distance relationship may be an emergent feature of such networks.
Highlights
This was accompanied by a change in the distribution of maximal cell extension: the cells generally extended and fluctuated less than in control before polarization but extended much more once polarized. This is reflected in the asymmetric extension distribution with large number of very high extension values (Figure 2B). These results suggest that cell extension and traction stresses are controlled by multiple factors involving the balance of adhesion strength, actin protrusion, and contractility
To get more insight into the relationship between traction stress and edge dynamics, we investigated how stress and cell edge position changed with time
In other words, when the stress increased, the velocity was most likely to be positive a few seconds before, and if the stress decreased, the velocity was most likely to be negative a few seconds before. This finding reinforces the previous result that stress increases during protrusion and for a short time after the retraction onset
Summary
Traction forces are generated by cellular actinmyosin system and transmitted to the environment through adhesions. This finding reinforces the previous result that stress increases during protrusion and for a short time after the retraction onset This analysis is consistent with a previous study where force dynamics during the protrusion-retraction cycles in fibroblasts was deduced from the patterns of actin flow [13] but is at odds with the idea that retraction is triggered by weakening of the adhesions at the cell edge [15]. If this were the case, one would expect the traction stress to decrease prior to the onset of retraction. B Epidermal Keratocytes B Cell Culture d METHOD DETAILS B Microscopy B Polyacrilamide Gel Preparation B Traction Force Microscopy B Cell Outlines and Switches B Stress, Distance and Curvature Analysis
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