Regulation of the Micromechanical Properties of Cells by Intermediate Filaments

VIF独特的物理特性，包括高柔韧性和高抗破碎性的结合，导致大应变范围内的应变硬化流变学，可以与更硬但更脆的交联肌动蛋白网络结合，特别是在细胞皮层，以高度局部化和快速可调的方式控制细胞力学（1,2）。图1显示了由丝蛋白交联的肌动蛋白网络（4mg /ml）在小菌株时变硬，但在菌株小于20%时仍然断裂。维门汀网在大应变下继续应变硬化并抵抗断裂。像聚丙烯酰胺这样的合成水凝胶只表现出线性弹性。VIF网络的非线性弹性意味着当丝链受到外部或内部产生的应力施加张力时，这些网络会变硬。由动力蛋白和动力蛋白马达介导的VIF和微管之间的活性界面(3)可以产生内部预应力，使VIF网络和整体细胞骨架硬化。大多数关于皮质细胞硬度的研究都强调了肌动蛋白网络的贡献，因为在低应变下，体外交联肌动蛋白形成了最坚硬的网络，因为肌动蛋白最集中在黏着点和细胞表面，这里是光学和磁性操纵珠、原子力显微镜尖端和校准玻璃纤维等探针附着的地方。即使在设计用于测试细胞表面或片层基附近的刚度的测量中，肌动蛋白和微管被拉runculin和秋水仙碱解聚也不会使细胞的弹性模量降低超过一个因素

The unique physical properties of VIF, including the combination of high flexibility with very high resistance to breakage, lead to strain-stiffening rheology over a large strain range that can combine with the stiffer, but more brittle, crosslinked actin network, especially at the cell cortex, to control cell mechanics in a highly localized and rapidly tunable manner (1,2). Figure 1 shows how actin networks (4 mg/ml) crosslinked by filamin stiffen at small strains, but still break at strains less than 20%. Vimentin networks continue to strain-stiffen at large strains and resist breakage. Synthetic hydrogels like polyacrylamide exhibit only linear elasticity. The nonlinear elasticity of VIF networks means that these networks stiffen when tension is applied to the filament strands either by externally or internally generated stresses. The active interface between VIF and microtubules mediated by both dynein and kinesin motors (3) can generate internal pre-stress that stiffens VIF networks and the integrated cytoskeleton. Most studies of cortical cell stiffness have emphasized the contribution of actin networks, because, at low strains, in vitro crosslinked actin forms the stiffest networks and because actin is most concentrated at focal adhesion sites and the cell surface where probes such as optically and magnetically manipulated beads, atomic force microscope tips, and calibrated glass fibers attach. Even in measurements designed to test the stiffness near the cell surface or lamellipodium, depolymerization of actin and microtubules by latrunculin and colchicine does not decrease the cell's elastic modulus more than a factor of 3. Cells from vimentin null mice are 40% softer than corresponding cells from wild type animals when measured by magnetic twisting rheometry (4). Consistent with the strain-stiffening effect of VIF, stiffness differences between wt and vim''' cells increase with increasing deformation (5). This effect on global cell stiffness is consistent with a recent multi-scale simulation study (6). The finding that oxidized LDL increases human endothelial cell stiffness coincident with a reorganization of the VIF network (7) suggests that local and temporal changes in VIF contribute to the stiffness-related changes in human disease. The contribution of VIF to cell stiffness need not result simply from the rheology of pure VIF networks since in the cell VIFs interdigitate with actin filaments and microtubules and, in the crowded context of the cytoskeleton, both steric and biochemical interactions contribute to the mechanical response. An example of the synergistic rheological response of composite networks formed by both VIF and F-actin is shown in Figure 2. Here the stress resulting from increasing strain is plotted for equal weight concentrations (1 mg/ml) of purified F-actin filament types. At these relatively low concentrations F-actin and vimentin form weak networks, but their combination is much stronger than the sum of its parts (8), and the upward curvature of the stress-strain plot illustrates the stiffening with increasing deformation characteristic of IF networks (1). At the higher cellular concentrations of F-actin and vimentin (generally more than 10 mg/ml) the local stiffening, especially at large.