Regulation of epithelial paracellular permeability by apical transport processes

上皮细胞是排列在肾脏和肠道中的体腔中的特化细胞，并防止生物体与其环境之间的分子自由交换。精确控制通过这一层的运输对于维持生命至关重要：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紊乱。营养分子在从肠道吸收或在肾脏中形成尿液的过程中被吸收并通过该层释放。两种主要的转运途径用于跨层的分子交换：通过细胞（跨细胞）和细胞之间（细胞旁）。跨细胞摄取营养素可以增强分子通过细胞旁途径的运输，然而这种偶联的机制，即层的正常功能的基础，是未知的。我们建议，以下事件的顺序耦合操作的跨细胞和细胞旁的运输途径：跨细胞摄取葡萄糖或氨基酸与Na+的运动，改变细胞膜的物理性质，即它改变跨膜的电压。电压变化引发一系列蛋白质（Rho，Rho激酶）的激活，导致细胞由于结构蛋白肌球蛋白的化学修饰而收缩。肌球蛋白与肌动蛋白一起形成能够在激活时缩短的细丝，从而引起细胞收缩，这通过尚不清楚的机制导致细胞旁通透性增加。我们将使用显示肠腔和肾腔内衬上皮层特性的细胞模型来测试这一建议的事件序列。我们将使用生物化学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。这些知识最终将导致改进的方法，以提高上皮的操作。

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.