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 [1-3]. 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 [4-10]. 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 [11-15]. Here, we analyze the dynamics of traction forces during the protrusion-retraction cycle of polarizing fish epidermal keratocytes and find that forces fluctuate together with the cycle, increasing during protrusion and reaching maximum at the beginning of retraction. We relate force dynamics to the recently discovered phenomenological rule [16] that governs cell-edge behavior during keratocyte polarization: both traction forces and probability of switch from protrusion to retraction increase with the distance from the cell center. Diminishing forces with cell contractility inhibitor leads to decreased edge fluctuations and abnormal polarization, although externally applied force can induce protrusion-retraction switch. These results suggest that forces mediate distance sensitivity of the edge dynamics and organize cell-edge behavior, leading to spontaneous polarization. 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 might be an emergent feature of such networks.

中文翻译：

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牵引力控制细胞边缘动力学并在细胞极化期间调节距离敏感性。

牵引力由细胞肌动蛋白-肌球蛋白系统产生，并通过粘连传递到环境中。它们被认为可以驱动细胞运动、形状变化和细胞外基质重塑 [1-3]。然而，大多数牵引力分析都是在固定细胞上进行的，研究单个粘着斑水平的力或将它们与静态细胞参数（如面积和边缘曲率）联系起来 [4-10]。目前尚不清楚牵引力如何与形状变化和运动相关，例如，据报道，在细胞收缩之前，力会增加或减少 [11-15]。在这里，我们分析了极化鱼表皮角质细胞的突出-收缩循环过程中牵引力的动力学，发现力随着循环而波动，在前伸时增加并在收缩开始时达到最大值。我们将力动力学与最近发现的现象学规则 [16] 联系起来，该规则控制着角质细胞极化期间的细胞边缘行为：牵引力和从突出到收缩的转换概率都随着距细胞中心的距离而增加。用细胞收缩性抑制剂减少力会导致边缘波动减少和异常极化，尽管外部施加的力会导致突起-收缩转换。这些结果表明，力介导边缘动力学的距离敏感性并组织细胞边缘行为，导致自发极化。肌动蛋白流速没有表现出与牵引应力相同的距离依赖性，反对其在组织边缘动力学中的作用。最后，