Center for Medical Physics and Technology, University of Erlangen-Nuremberg, Erlangen, Germany.
J Phys Condens Matter. 2010 May 19;22(19):194105. doi: 10.1088/0953-8984/22/19/194105. Epub 2010 Apr 26.
The cytoskeleton (CSK) of living cells is a crosslinked fiber network, subject to ongoing biochemical remodeling processes that can be visualized by tracking the spontaneous motion of CSK-bound microbeads. The bead motion is characterized by anomalous diffusion with a power-law time evolution of the mean square displacement (MSD), and can be described as a stochastic transport process with apparent diffusivity D and power-law exponent β: MSD ∼ D (t/t(0))(β). Here we studied whether D and β change with the time that has passed after the initial bead-cell contact, and whether they are sensitive to bead coating (fibronectin, integrin antibodies, poly-L-lysine, albumin) and bead size (0.5-4.5 µm). The measurements are interpreted in the framework of a simple model that describes the bead as an overdamped particle coupled to the fluctuating CSK network by an elastic spring. The viscous damping coefficient characterizes the degree of bead internalization into the cell, and the spring constant characterizes the strength of the binding of the bead to the CSK. The model predicts distinctive signatures of the MSD that change with time as the bead couples more tightly to the CSK and becomes internalized. Experimental data show that the transition from the unbound to the tightly bound state occurs in an all-or-nothing manner. The time point of this transition shows considerable variability between individual cells (2-30 min) and depends on the bead size and bead coating. On average, this transition occurs later for smaller beads and beads coated with ligands that trigger the formation of adhesion complexes (fibronectin, integrin antibodies). Once the bead is linked to the CSK, however, the ligand type and bead size have little effect on the MSD. On longer timescales of several hours after bead addition, smaller beads are internalized into the cell more readily, leading to characteristic changes in the MSD that are consistent with increased viscous damping by the cytoplasm and reduced binding strength.
活细胞的细胞骨架（CSK）是交联纤维网络，不断进行生化重塑过程，通过跟踪 CSK 结合微珠的自发运动可以观察到这些过程。微珠的运动表现为具有幂律时变均方位移（MSD）的反常扩散，可以描述为具有表观扩散系数 D 和幂律指数β的随机输运过程：MSD∼D(t/t0)β。在这里，我们研究了 D 和β是否随初始珠-细胞接触后经过的时间而变化，以及它们是否对珠的涂层（纤连蛋白、整合素抗体、聚-L-赖氨酸、白蛋白）和珠的大小（0.5-4.5μm）敏感。该测量是在一个简单模型的框架内进行解释的，该模型将微珠描述为一个受粘滞阻尼的粒子，通过弹性弹簧与波动的 CSK 网络耦合。粘性阻尼系数表征了珠内化到细胞中的程度，而弹簧常数则表征了珠与 CSK 的结合强度。该模型预测了 MSD 的独特特征，这些特征随时间而变化，因为珠与 CSK 的结合越来越紧密并被内化。实验数据表明，从无约束到紧密约束状态的转变以全或无的方式发生。这种转变的时间点在个体细胞之间表现出很大的可变性（2-30 分钟），并且取决于珠的大小和珠的涂层。平均而言，对于较小的珠和涂有触发粘附复合物形成的配体的珠（纤连蛋白、整合素抗体），这种转变发生得更晚。然而，一旦珠与 CSK 相连，配体类型和珠的大小对 MSD 的影响就很小。在添加珠后数小时的更长时间尺度上，较小的珠更容易被内化到细胞中，导致 MSD 发生特征性变化，这与细胞质的粘性阻尼增加和结合强度降低一致。
