Palmer A, Xu J, Kuo S C, Wirtz D
Department of Chemical Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA.
Biophys J. 1999 Feb;76(2):1063-71. doi: 10.1016/S0006-3495(99)77271-1.
Filamentous actin (F-actin), one of the constituents of the cytoskeleton, is believed to be the most important participant in the motion and mechanical integrity of eukaryotic cells. Traditionally, the viscoelastic moduli of F-actin networks have been measured by imposing a small mechanical strain and quantifying the resulting stress. The magnitude of the viscoelastic moduli, their concentration dependence and strain dependence, as well as the viscoelastic nature (solid-like or liquid-like) of networks of uncross-linked F-actin, have been the subjects of debate. Although this paper helps to resolve the debate and establishes the extent of the linear regime of F-actin networks' rheology, we report novel measurements of the high-frequency behavior of networks of F-actin, using a noninvasive light-scattering based technique, diffusing wave spectroscopy (DWS). Because no external strain is applied, our optical assay generates measurements of the mechanical properties of F-actin networks that avoid many ambiguities inherent in mechanical measurements. We observe that the elastic modulus has a small magnitude, no strain dependence, and a weak concentration dependence. Therefore, F-actin alone is not sufficient to generate the elastic modulus necessary to sustain the structural rigidity of most cells or support new cellular protrusions. Unlike previous studies, our measurements show that the mechanical properties of F-actin are highly dependent on the frequency content of the deformation. We show that the loss modulus unexpectedly dominates the elastic modulus at high frequencies, which are key for fast transitions. Finally, the measured mean square displacement of the optical probes, which is also generated by DWS measurements, offers new insight into the local bending fluctuations of the individual actin filaments and shows how they generate enhanced dissipation at short time scales.
丝状肌动蛋白（F-肌动蛋白）是细胞骨架的组成成分之一，被认为是真核细胞运动和机械完整性的最重要参与者。传统上，F-肌动蛋白网络的粘弹性模量是通过施加小的机械应变并量化由此产生的应力来测量的。F-肌动蛋白网络的粘弹性模量大小、其浓度依赖性和应变依赖性，以及未交联F-肌动蛋白网络的粘弹性性质（类固体或类液体），一直是争论的主题。尽管本文有助于解决这一争论并确定F-肌动蛋白网络流变学线性区域的范围，但我们报告了使用基于非侵入性光散射技术的扩散波谱（DWS）对F-肌动蛋白网络高频行为的新测量结果。由于未施加外部应变，我们的光学检测方法对F-肌动蛋白网络机械性能的测量避免了机械测量中固有的许多模糊性。我们观察到弹性模量大小较小，无应变依赖性，浓度依赖性较弱。因此，仅F-肌动蛋白不足以产生维持大多数细胞结构刚性或支持新的细胞突起所需的弹性模量。与先前的研究不同，我们的测量结果表明F-肌动蛋白的机械性能高度依赖于变形的频率成分。我们表明，在高频下损耗模量意外地主导了弹性模量，而高频对于快速转变至关重要。最后，通过DWS测量产生的光学探针的测量均方位移，为单个肌动蛋白丝的局部弯曲波动提供了新的见解，并展示了它们如何在短时间尺度上产生增强的耗散。
