The dynamic morphology and mechanics of the cytoskeleton is determined by interacting networks of semiflexible actin filaments and rigid microtubules. Active rearrangement of networks of actin and microtubules can not only be driven by motor proteins but by changes to ionic conditions. For example, high concentrations of multivalent ions can induce bundling and crosslinking of both filaments. Yet, how cytoskeleton networks respond in real-time to changing ion concentrations, and how actin-microtubule interactions impact network response to these changing conditions remains unknown. Here, we use microfluidic perfusion chambers and two-color confocal fluorescence microscopy to show that increasing magnesium ions trigger contraction of both actin and actin-microtubule networks. Specifically, we use microfluidics to vary the Mg²⁺ concentration between 2 and 20 mM while simultaneously visualizing the triggered changes to the overall network size. We find that as Mg²⁺ concentration increases both actin and actin-microtubule networks undergo bulk contraction, which we measure as the shrinking width of each network. However, surprisingly, lowering the Mg²⁺concentration back to 2 mM does not stop or reverse the contraction but rather causes both networks to contract further. Further, actin networks begin to contract at lower Mg²⁺ concentrations and shorter times than actin-microtubule networks. In fact, actin-microtubule networks only undergo substantial contraction once the Mg²⁺ concentration begins to lower from 20 mM back to 2 mM. Our intriguing findings shed new light on how varying environmental conditions can dynamically tune the morphology of cytoskeleton networks and trigger active contraction without the use of motor proteins.

细胞骨架的动态形态和力学由半柔性肌动蛋白丝和刚性微管的相互作用网络决定。肌动蛋白和微管网络的主动重排不仅可以由运动蛋白驱动，还可以由离子条件的变化驱动。例如，高浓度的多价离子可以诱导两种细丝的集束和交联。然而，细胞骨架网络如何实时响应不断变化的离子浓度，以及肌动蛋白-微管相互作用如何影响网络对这些变化条件的响应仍然未知。在这里，我们使用微流体灌注室和双色共聚焦荧光显微镜来显示增加的镁离子会引发肌动蛋白和肌动蛋白-微管网络的收缩。具体来说，我们使用微流体来改变 Mg²⁺浓度介于 2 和 20 mM 之间，同时可视化触发的整个网络大小的变化。我们发现随着 Mg ²⁺浓度的增加，肌动蛋白和肌动蛋白-微管网络都会发生体积收缩，我们将其测量为每个网络的收缩宽度。然而，令人惊讶的是，将 Mg ²⁺浓度降低回 2 mM 并不会停止或逆转收缩，而是会导致两个网络进一步收缩。此外，与肌动蛋白-微管网络相比，肌动蛋白网络在较低的 Mg ²⁺浓度和较短的时间内开始收缩。事实上，肌动蛋白-微管网络只有在 Mg ²⁺浓度开始从 20 mM 降至 2 mM。我们有趣的发现揭示了不同的环境条件如何在不使用运动蛋白的情况下动态调整细胞骨架网络的形态并触发主动收缩。