Department of Physics and Biophysics, University of San Diego, San Diego, California.
Department of Physics, University of Massachusetts Amherst, Amherst, Massachusetts.
Biophys J. 2018 Sep 18;115(6):1055-1067. doi: 10.1016/j.bpj.2018.08.010. Epub 2018 Aug 16.
We use optical tweezers microrheology and fluorescence microscopy to characterize the nonlinear mesoscale mechanics and mobility of in vitro co-entangled actin-microtubule composites. We create a suite of randomly oriented, well-mixed networks of actin and microtubules by co-polymerizing varying ratios of actin and tubulin in situ. To perturb each composite far from equilibrium, we use optical tweezers to displace an embedded microsphere a distance greater than the lengths of the filaments at a speed much faster than their intrinsic relaxation rates. We simultaneously measure the force the filaments exert on the bead and the subsequent force relaxation. We find that the presence of a large fraction of microtubules (>0.7) is needed to substantially increase the measured force, which is accompanied by large heterogeneities in force response. Actin minimizes these heterogeneities by reducing the mesh size of the composites and supporting microtubules against buckling. Composites also undergo a sharp transition from strain softening to stiffening when the fraction of microtubules (ϕ) exceeds 0.5, which we show arises from faster poroelastic relaxation and suppressed actin bending fluctuations. The force after bead displacement relaxes via power-law decay after an initial period of minimal relaxation. The short-time relaxation profiles (t < 0.06 s) arise from poroelastic and bending contributions, whereas the long-time power-law relaxation is indicative of filaments reptating out of deformed entanglement constraints. The scaling exponents for the long-time relaxation exhibit a nonmonotonic dependence on ϕ, reaching a maximum for equimolar composites (ϕ = 0.5), suggesting that reptation is fastest in ϕ = 0.5 composites. Corresponding mobility measurements of steady-state actin and microtubules show that both filaments are indeed the most mobile in ϕ = 0.5 composites. This nonmonotonic dependence of mobility on ϕ demonstrates the important interplay between mesh size and filament rigidity in polymer networks and highlights the surprising emergent properties that can arise in composites.
我们使用光镊微流变学和荧光显微镜来表征体外纠缠的肌动蛋白-微管复合材料的非线性介观力学和迁移率。我们通过原位共聚不同比例的肌动蛋白和微管蛋白，创建了一系列随机取向、混合良好的肌动蛋白和微管网络。为了使每个复合材料远离平衡状态，我们使用光镊将嵌入的微球位移超过纤维的长度，速度远快于它们的固有松弛率。我们同时测量纤维对珠子施加的力和随后的力松弛。我们发现，需要存在大量的微管蛋白（> 0.7）才能显著增加测量的力，这伴随着力响应的大异质性。肌动蛋白通过减小复合材料的网格尺寸并支撑微管蛋白防止屈曲，从而最小化这些异质性。当微管蛋白的分数（ϕ）超过 0.5 时，复合材料还会经历从应变软化到硬化的急剧转变，我们表明这是由于更快的多孔弹性松弛和抑制的肌动蛋白弯曲波动所致。珠子位移后的力在初始最小松弛期后通过幂律衰减来松弛。短时间松弛曲线（t < 0.06 s）源于多孔弹性和弯曲贡献，而长时间幂律松弛则表明纤维从变形缠结约束中蠕动出来。长时间松弛的标度指数表现出对 ϕ 的非单调依赖性，在等摩尔复合材料（ϕ = 0.5）处达到最大值，表明在 ϕ = 0.5 复合材料中蠕动最快。稳态肌动蛋白和微管蛋白的相应迁移率测量表明，两种纤维实际上在 ϕ = 0.5 复合材料中最具迁移性。迁移率对 ϕ 的这种非单调依赖性表明了网格尺寸和纤维刚性在聚合物网络中的重要相互作用，并突出了复合材料中可能出现的惊人涌现特性。
