Regulation of epithelial paracellular permeability by apical transport processes

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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.