Department of Physics, Lehigh University, Bethlehem, Pennsylvania, USA.
Biophys J. 2012 Apr 4;102(7):1493-502. doi: 10.1016/j.bpj.2012.03.005. Epub 2012 Apr 3.
Many animal cells initiate crawling by protruding lamellipodia, consisting of a dense network of actin filaments, at their leading edge. We imaged XTC cells that exhibit flat lamellipodia on poly-L-lysine-coated coverslips. Using active contours, we tracked the leading edge and measured the total amount of F-actin by summing the pixel intensities within a 5-μm band. We observed protrusion and retraction with period 130-200 s and local wavelike features. Positive (negative) velocities correlated with minimum (maximum) integrated actin concentration. Approximately constant retrograde flow indicated that protrusions and retractions were driven by fluctuations of the actin polymerization rate. We present a model of these actin dynamics as an excitable system in which a diffusive, autocatalytic activator causes actin polymerization; F-actin accumulation in turn inhibits further activator accumulation. Simulations of the model reproduced the pattern of actin polymerization seen in experiments. To explore the model's assumption of an autocatalytic activation mechanism, we imaged cells expressing markers for both F-actin and the p21 subunit of the Arp2/3 complex. We found that integrated Arp2/3-complex concentrations spike several seconds before spikes of F-actin concentration. This suggests that the Arp2/3 complex participates in an activation mechanism that includes additional diffuse components. Response of cells to stimulation by fetal calf serum could be reproduced by the model, further supporting the proposed dynamical picture.
许多动物细胞在其前缘伸出由肌动蛋白丝组成的密集网络的片状伪足，从而开始爬行。我们对在聚-L-赖氨酸包被的盖玻片上表现出扁平片状伪足的 XTC 细胞进行成像。使用活动轮廓，我们跟踪前缘并通过在 5μm 带宽内对像素强度求和来测量 F-肌动蛋白的总量。我们观察到突起和缩回的周期为 130-200s，具有局部波浪状特征。正（负）向速度与最小（最大）整合肌动蛋白浓度相关。大约恒定的逆行流表明，突起和缩回是由肌动蛋白聚合速率的波动驱动的。我们提出了一个肌动蛋白动力学模型，作为一个兴奋系统，其中扩散的自催化激活剂引起肌动蛋白聚合；反过来，F-肌动蛋白的积累又抑制了进一步的激活剂积累。该模型的模拟再现了实验中观察到的肌动蛋白聚合模式。为了探索模型中自催化激活机制的假设，我们对表达 F-肌动蛋白和 Arp2/3 复合物的 p21 亚基的细胞进行成像。我们发现，整合的 Arp2/3 复合物浓度在 F-肌动蛋白浓度的尖峰前几秒钟就会出现尖峰。这表明 Arp2/3 复合物参与了包括额外扩散成分的激活机制。该模型可以再现细胞对胎牛血清刺激的反应，进一步支持了所提出的动力学图像。
