Mechanotransduction at tight junctions and epithelial differentiation and dynamics

上皮是覆盖身体表面和排列内部器官的细胞层。它们形成了功能性屏障，保护我们免受环境的影响，使我们的器官能够产生和维持不同成分的隔室，比如将视网膜与后面的血液或眼睛分开的屏障。为了使单个上皮细胞相互作用并形成上皮组织，它们需要与邻近细胞组装粘附复合物。其中一种粘合复合物称为紧密结，在相邻细胞之间形成屏障；因此，紧密连接对于上皮形成组织屏障至关重要，因为它们可以防止邻近细胞之间的随机扩散。因此，必须保持紧密连接的完整性，以防止上皮屏障破坏和组织衰竭。然而，上皮细胞在细胞分裂或器官和组织发育过程中经常受到物理压力并发生细胞形状变化。因此，可能存在允许紧密连接适应细胞形状变化的机制，并可能帮助细胞感知和适应作用于紧密连接的外部物理力。在这里，我们关注这样的机制是否存在以及这样的分子桥是如何建立的问题。紧密连接是由许多不同的蛋白质组成的，这些蛋白质形成了一个分子网络，从细胞表面的细胞粘附蛋白开始，细胞通过它相互作用。这些细胞-细胞粘附蛋白与细胞内调节各种连接功能的大量蛋白质相互作用，这些蛋白质被认为是支持紧密连接结构的分子支架。其中一些蛋白质还可以与细胞骨架相互作用，细胞骨架是一种支持细胞结构和形状的蛋白质纤维网络。然而，这些相互作用的功能相关性还没有得到很好的理解。我们假设可以与细胞表面和内部细胞骨架上的细胞粘附蛋白相互作用的成分可能作为力传导连接体。因此，我们基于这种蛋白质构建了一个传感器，使我们能够确定分子是否确实处于张力状态。初步实验表明，传感器是功能性的，紧密连接确实是一个受力结构。我们现在的目标是确定连接结构原则，使紧密连接能够承受力，并在细胞骨架和细胞表面之间传导它们，并利用功能测定来确定这些原则对上皮组织形成和发育的生理功能。预期的结果将有助于我们了解与生物体发育、组织功能和再生相关的生理重要过程。它们将有助于我们理解破坏上皮组织的常见疾病，如癌症、病毒和细菌感染，以及常见的慢性炎症和与年龄相关的疾病。我们也期望发现的结果和原理将支持组织工程和再生医学方法。

Epithelia are layers of cells that cover body surfaces and line internal organs. They form functional barriers that protect us from the environment and enable our organs to generate and maintain compartments of different compositions, such as the barrier that separates the retina from the blood at the back or the eye. For individual epithelial cells to interact and form epithelial tissues, they need to assemble adhesive complexes with neighbouring cells. One of these adhesive complexes is called tight junction and forms a barrier in between neighbouring cells; hence, tight junctions are essential for epithelia to form tissue barriers as they prevent random diffusion along the space in between neighbouring cells. Consequently, the integrity of tight junctions must be maintained in order to prevent epithelial barrier breakdown and tissue failure. However, epithelial cells are often under physical strain and undergo cell shape changes during cell division or during the development of our organs and tissues. Therefore, mechanisms are likely to exist that allow tight junctions to adapt to changing cell shapes and, possibly, help cells sense and adapt to external physical forces that act on tight junctions. Here, we focus on the questions of whether such mechanisms exist and how such molecular bridges are built. Tight junctions are composed of many different proteins that form a molecular network that starts with cell-cell adhesion proteins at the cell surface by which cells interact with each other. These cell-cell adhesion proteins interact with a large range of proteins inside the cells that regulate the various junctional functions and that are thought to function as molecular scaffolds that support the structure of tight junctions. Some of these proteins can also interact with the cytoskeleton, a network of protein fibres that supports the cell's structure and shape. However, the functional relevance of these interactions is not well understood. We hypothesized that components that can interact with the cell-cell adhesion proteins at the cell surface and the internal cytoskeleton might work as force transducing linkers. Hence, we have constructed a sensor based on such a protein that allows us to determine whether the molecule is indeed under tension. Pilot experiments indicate that the sensor is functional and that tight junctions are indeed a force-bearing structure. Our objectives now are to determine the junctional architectural principles that enable tight junctions to bear forces and transduce them between the cytoskeleton and the cell surface, and to make use of functional assays to determine the physiological function of these principles for epithelial tissue formation and development. The expected results will help us to understand physiologically important processes relevant for organism development, and tissue function and regeneration. They will contribute to our understanding of common diseases that disrupt epithelial tissues such as cancer, viral and bacterial infections, and common chronic inflammatory and age-related conditions. We also expect that the results and principles to be discovered will support tissue engineering and regenerative medicine approaches.