Figure 163

The tight junction. The tight junction, the most apical component of the junctional complex of epithelial cells, serves two main functions in epithelial cells: 1) It separates the apical and basolateral plasma membrane domains of the cells, allowing for vectorial transport of ions and molecules; 2) it provides the major framework for the paracellular permeability barrier, allowing for generation of chemical and electrical gradients [31]. These functions are critically important to the proper functioning of renal tubules. The tight junction is comprised of a number of proteins (cytoplasmic and transmembrane) that interact with a similar group of proteins between adjacent cells to form the permeability barrier [16, 32-37]. These proteins include the transmembrane protein occludin [35, 38] and the cytosolic proteins zonula occludens 1 (ZO-1), ZO-2 [36], p130, [39], cingulin [33, 40], 7H6 antigen [34] and symplekin [41], although other as yet unidentified components likely exist. The tight junction also appears to interact with the actin-based cytoskeleton, probably in part through ZO-1-fodrin interactions.

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