Mostafazadeh Nima, Resnick Andrew, Young Y-N, Peng Zhangli
bioRxiv. 2023 Jul 15:2023.07.14.549117. doi: 10.1101/2023.07.14.549117.
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, 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.
Factors regulating the mechanical response of a primary cilium to fluid flow remain unclear. Modeling the microtubule doublet as a composite of two orthotropic shells and the ciliary axoneme as an elastic shell enclosing nine such microtubule doublets, we found that the length distribution of microtubule doublets (inferred from cryogenic electron tomography images) is the primary determining factor in the bending stiffness of primary cilia, rather than just the ciliary length. This implies ciliary-associated transmembrane proteins may be activated by membrane curvature changes rather than just membrane stretching. These insights challenge the traditional view of ciliary mechanosensation and expands our understanding of the different ways in which cells perceive and respond to mechanical stimuli.
初级纤毛由包裹在纤毛膜内的九组微管二联体组成，是一种机械传感细胞器，在外部机械负荷下会弯曲，并通过纤毛弯曲激活的跨膜蛋白发送细胞内信号。九组微管二联体是主要的承重结构成分，而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，这两种成分没有区分，其中根据结构成分（九组微管二联体）计算的应力被用来解释传感位置，这可能会产生完全误导。我们首次通过分别考虑这两种成分，开发了一种基于微观结构的初级纤毛模型。首先，我们细化了单个微管的正交各向异性圆柱壳弯曲的解析解，并在有限元模拟和微管弯曲的理论预测之间取得了极好的一致性，以此验证模型中的结构成分。其次，通过将纤毛膜与九组微管二联体整合，我们发现微管二联体可能会随着整个纤毛弯曲而发生显著扭转。第三，我们发现纤毛的力学性能不仅取决于纤毛长度，还高度依赖于变形。更重要的是，我们发现纤毛基部附近的纤毛膜并非如之前所认为的那样仅承受平面内的拉伸或压缩，而是存在显著的局部弯曲应力。这对传统的纤毛机械传感模型提出了挑战，表明跨膜蛋白可能更多地是由膜曲率而非膜拉伸激活。最后，我们将初级纤毛的成像数据纳入基于微观结构的纤毛模型，发现与微管长度均匀的理想模型相比，基于成像信息的模型显示九组微管二联体与纤毛膜的相互作用更均匀，并且它们的接触位置会使纤毛膜产生比基部附近更高的弯曲曲率。
调节初级纤毛对流体流动的机械响应的因素仍不清楚。将微管二联体建模为两个正交各向异性壳的复合体，将纤毛轴丝建模为包裹九组此类微管二联体的弹性壳，我们发现微管二联体的长度分布（从低温电子断层扫描图像推断）是初级纤毛弯曲刚度的主要决定因素，而不仅仅是纤毛长度。这意味着与纤毛相关的跨膜蛋白可能是由膜曲率变化而非仅仅膜拉伸激活。这些见解挑战了传统的纤毛机械传感观点，并扩展了我们对细胞感知和响应机械刺激的不同方式的理解。
