The tight junction is not a constitutional junctional apparatus in endothelial cells, but develops in a particular lineage of endothelia, such as the capillary endothelia in the brain and retina, and thus is considered to be pivotal for the maintenance of the blood-tissue barrier. Occludin is an integral membrane component of tight junctions, but the role of occludin in the endothelial cell function remains to be elucidated. We have cloned and transfected rat full-length occludin cDNA into a rat endothelial cell line (RLE) that expressed only a trace amount of occludin with no fine circumferential actin bundles at the cell border in native conditions. Occludin was expressed at the cell border of RLE cells, and circumferential fine actin bundles developed in close relation to the sites of occludin localization. Even under subconfluent culture conditions, fine circumferential actin bundles were formed at the sites where occludin-positive cell-cell contact was achieved. In immunoelectron microscopy, occludin was localized at distinct areas of the plasma membrane, always in association with the cytoplasmic actin filaments. On the other hand, actin bundles were not seen in occludin-negative juxtaposing plasma membranes. Collectively, these data strongly suggested a possible determinant function of occludin for the organization of actin in endothelial cells.
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