Abstract：
A primary cilium, made of nine microtubule doublets enclosed in a cilium membrane, is a mechanosensing organelle that bends under an external mechanical load and sends an intracellular signal through transmembrane proteins activated by cilium bending. The nine microtubule doublets are the main load-bearing structural component, while the transmembrane proteins on the cilium membrane are the main sensing component. No distinction was made between these two components in all existing models, where the stress calculated from the structural component (nine microtubule doublets) was used to explain the sensing location, which may be totally misleading. For the first time, we developed a microstructure-based primary cilium model by considering these two components separately. First, we refined the analytical solution of bending an orthotropic cylindrical shell for individual microtubule, and obtained excellent agreement between finite element simulations and the theoretical predictions of a microtubule bending as a validation of the structural component in the model. Second, by integrating the cilium membrane with nine microtubule doublets and simulating the tip-anchored optical tweezer experiment on our computational model, we found that the microtubule doublets may twist significantly as the whole cilium bends. Third, besides being cilium-length-dependent, we found the mechanical properties of the cilium are also highly deformation-dependent. More important, we found that the cilium membrane near the base is not under pure in-plane tension or compression as previously thought, but has significant local bending stress. This challenges the traditional model of cilium mechanosensing, indicating that transmembrane proteins may be activated more by membrane curvature than membrane stretching. Finally, we incorporated imaging data of primary cilia into our microstructure-based cilium model, and found that comparing to the ideal model with uniform microtubule length, the imaging-informed model shows the nine microtubule doublets interact more evenly with the cilium membrane, and their contact locations can cause even higher bending curvature in the cilium membrane than near the base.
摘要：
初级纤毛，由封闭在纤毛膜中的九个微管双峰组成，是一种机械传感细胞器，在外部机械负荷下弯曲，并通过纤毛弯曲激活的跨膜蛋白发送细胞内信号。九个微管双峰是主要的承重结构部件，纤毛膜上的跨膜蛋白是主要的传感成分。在所有现有模型中，这两个组件之间没有区别，从结构部件（九个微管双峰）计算的应力用于解释传感位置，这可能是完全误导。第一次,通过分别考虑这两个成分，我们开发了基于微观结构的初级纤毛模型。首先,我们改进了单个微管弯曲正交各向异性圆柱壳的解析解，并在有限元模拟与微管弯曲的理论预测之间获得了极好的一致性，以验证模型中的结构部件。第二,通过将纤毛膜与九个微管双峰整合在一起，并在我们的计算模型上模拟尖端锚定的光镊子实验，我们发现，当整个纤毛弯曲时，微管双峰可能会明显扭曲。第三，除了依赖于纤毛长度，我们发现纤毛的机械性能也高度依赖于变形。更重要的是,我们发现靠近基部的纤毛膜并不像以前认为的那样受到纯的平面内拉伸或压缩，但具有显著的局部弯曲应力。这挑战了传统的纤毛机械传感模型，表明跨膜蛋白可能比膜拉伸更多地被膜弯曲激活。最后,我们将初级纤毛的成像数据纳入我们基于微结构的纤毛模型，并发现与具有均匀微管长度的理想模型相比，成像模型显示九个微管双峰与纤毛膜更均匀地相互作用，它们的接触位置会导致纤毛膜比基部附近更高的弯曲曲率。
