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Polarized epithelial cells create a highly efficient and selective barrier between different tissue compartments. Two junctional complexes constitute this barrier—the most apically-located tight junction (TJ) and the closely adjacent adherens junction (AJ). TJs and AJs play important roles in regulating paracellular permeability, cell differentiation, and proliferation. Dynamic cell deadhesion/readhesion during embryonic morphogenesis, as well as extrusion of aptototic cells from mature epithelial layers, require rapid and effective disassembly of TJs and AJs. Recently, it has been suggested that such diassembly involves clathrin-mediated endocytosis of junctional proteins (Ivanov et al., Mol. Biol. Cell. [2004], 15, 176-188).

The cover image captures an early stage of TJ endocytosis in T84 intestinal epithelial cells. Disassembly of apical junctions was induced by removal of extracellular calcium in epithelilal monolayers. Calcium depletion induces rapid translocation of TJ protein occludin (green labeling) from areas of cell-cell contact into centrally located ring-like structures. In these structures, occludin colocalizes with α-adaptin (red labeling), an adaptor protein that is crucial for formation of clathrin-coated pits. Other TJ and AJ proteins have been shown to translocate into similar α-adaptin and clathrin-containing ring-like structures in calcium-depleted cells. At later stages of endocytosis, occludin and other TJ/AJ proteins transiently enter early endosomes and accumulate within a unique sub-apical cytosolic storage compartment. We suggest that clathrin-mediated endocytosis may represent a common mechanism for disruption of epithelial junctions during normal tissue remodeling and in pathology. —A. I. Ivanov, A. Nusrat, and C. A. Parkos
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