Department of Biomedical Engineering, University of Illinois Chicago, Chicago, Illinois, USA.
Department of Physics and Center for Gene Regulation in Health and Disease, Cleveland State University, Cleveland, Ohio, USA.
Cytoskeleton (Hoboken). 2024 Aug;81(8):369-381. doi: 10.1002/cm.21860. Epub 2024 Apr 27.
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.
一个初级纤毛由九组微管二联体组成，被纤毛膜包裹，是一种机械感受器，在外力作用下弯曲，并通过纤毛弯曲激活的跨膜蛋白向细胞内发送信号。九组微管二联体是主要的承载结构组成部分，而纤毛膜上的跨膜蛋白则是主要的感应组成部分。在所有现有的模型中，都没有对这两个组成部分进行区分，其中从结构组成部分（九组微管二联体）计算出的应力被用于解释感应位置，这可能会产生完全误导。我们首次通过分别考虑这两个组成部分来开发基于微观结构的初级纤毛模型。首先，我们细化了单个微管弯曲的各向异性圆柱壳的解析解，并通过有限元模拟与微管弯曲的理论预测进行了很好的对比，验证了模型中结构组成部分的准确性。其次，通过将纤毛膜与九组微管二联体集成，并在我们的计算模型上模拟尖端固定的光镊实验，我们发现当整个纤毛弯曲时，微管二联体可能会发生显著的扭曲。第三，除了与纤毛长度有关外，我们还发现纤毛的力学性能也高度依赖于变形。更重要的是，我们发现以前认为基底附近的纤毛膜不受纯面内张力或压缩的作用，而是具有显著的局部弯曲应力。这对传统的纤毛机械感觉模型提出了挑战，表明跨膜蛋白的激活可能更多地与膜曲率有关，而不是与膜拉伸有关。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中，发现与具有均匀微管长度的理想模型相比，基于成像的模型显示九组微管二联体与纤毛膜的相互作用更加均匀，它们的接触位置可以在纤毛膜中引起比基底附近更高的弯曲曲率。
