Regulation of epithelial paracellular permeability by apical transport processes

上皮细胞是一种特殊的细胞，排列在肾脏和肠道的体腔内，阻止有机体与环境之间的分子自由交换。精确控制通过这一层的运输对维持生命至关重要：当屏障功能失效时，随之而来的液体不受控制的运动会导致肠道和肾脏功能的严重障碍。在从肠道吸收或在肾脏形成尿液的过程中，营养分子被吸收并通过这一层释放。有两条主要的运输途径用于跨细胞层的分子交换：通过细胞(跨细胞)和细胞之间(细胞旁)。跨细胞摄取营养物质可以通过细胞旁途径促进分子的运输，但这种耦合的机制，即细胞层正常功能的基础，目前尚不清楚。我们认为以下一系列事件与跨细胞和细胞旁运输途径的运作有关：跨细胞摄取葡萄糖或氨基酸与改变细胞膜物理性质的Na+运动有关，即改变膜上的电压。由于结构蛋白肌球蛋白的化学修饰，电压变化启动了一系列蛋白质(Rho，Rho激酶)的激活，导致细胞收缩。肌球蛋白与肌动蛋白一起形成能够在激活时缩短的细丝，从而导致细胞收缩，从而通过一种尚不清楚的机制导致细胞旁通透性的增加。我们将使用细胞模型来测试这一建议的事件序列，这些细胞模型显示了肠道和肾腔上皮层的特性。我们将使用生化和显微镜方法跟踪细胞旁运输过程、调节肌球蛋白的蛋白质的活性状态以及细胞收缩和结构的变化。我们希望这些实验能够更好地了解上皮层的重要运输功能。这一知识最终将导致改进的方法，以加强上皮的运作。

Epithelial cells are specialized cells that line body cavities in the kidney and gut and prevent free exchange of molecules between the organism and its environment. Precise control of transport through this layer is vital for maintaining life: when the barrier function fails, the ensuing uncontrolled movement of fluids causes serious disturbances in gut and kidney functions. Nutrient molecules are taken up and released through the layer during absorption from the gut or formation of the urine in the kidney. Two major transport pathways operate for exchange of molecules across the layer: through the cells (transcellular) and between the cells (paracellular). Transcellular uptake of nutrients can enhance transport of molecules through the paracellular pathway however the mechanism of this coupling, that is the basis of normal function of the layer, is not known. We propose that the following sequence of events couples operation of the transcellular and paracellular transport pathways: transcellular uptake of glucose or amino acids is associated with movement of Na+ that alters the physical properties of the cell membrane, namely it changes the voltage across the membrane. The voltage change initiates the activation of a series of proteins (Rho, Rho kinase) leading to contraction of the cells due to chemical modification of the structural protein myosin. Myosin, together with actin, forms filaments capable of shortening upon activation, thereby causing cells to contract, that leads to increased paracellular permeability by a mechanism that is not well understood. We will test this suggested sequence of events using cell models that show properties of epithelial layers lining gut and kidney cavities. We will follow paracellular transport processes, activity state of proteins regulating myosin, and changes in cell contraction and structure using biochemical and microscopy methods. We expect these experiments to provide a better understanding of the vital transporting functions of epithelial layers. This knowledge eventually will lead to improved methods to enhance the operation of epithelium.