Endothelial Claudin

TLDR

Tight junctions in endothelial cells regulate vascular permeability, and claudin‑5, a ubiquitously expressed claudin, is hypothesized to be a key endothelial TJ component. The authors generated a claudin‑6–specific polyclonal antibody and a cross‑reactive antibody against claudin‑5/TMVCF and claudin‑6 to probe their localization. Immunofluorescence and electron microscopy revealed claudin‑5/TMVCF localized exclusively to endothelial cell borders of blood vessels, formed TJ strands, was absent in epithelial cells, and its ectopic expression in fibroblasts reconstituted endothelial‑like TJs, confirming it as an endothelial‑specific TJ component.

Abstract

Tight junctions (TJs) in endothelial cells are thought to determine vascular permeability. Recently, claudin-1 to -15 were identified as major components of TJ strands. Among these, claudin-5 (also called transmembrane protein deleted in velo-cardio-facial syndrome [TMVCF]) was expressed ubiquitously, even in organs lacking epithelial tissues, suggesting the possible involvement of this claudin species in endothelial TJs. We then obtained a claudin-6-specific polyclonal antibody and a polyclonal antibody that recognized both claudin-5/TMVCF and claudin-6. In the brain and lung, immunofluorescence microscopy with these polyclonal antibodies showed that claudin-5/TMVCF was exclusively concentrated at cell-cell borders of endothelial cells of all segments of blood vessels, but not at those of epithelial cells. Immunoreplica electron microscopy revealed that claudin-5/TMVCF was a component of TJ strands. In contrast, in the kidney, the claudin-5/TMVCF signal was restricted to endothelial cells of arteries, but was undetectable in those of veins and capillaries. In addition, in all other tissues we examined, claudin-5/TMVCF was specifically detected in endothelial cells of some segments of blood vessels, but not in epithelial cells. Furthermore, when claudin-5/TMVCF cDNA was introduced into mouse L fibroblasts, TJ strands were reconstituted that resembled those in endothelial cells in vivo, i.e., the extracellular face-associated TJs. These findings indicated that claudin-5/TMVCF is an endothelial cell-specific component of TJ strands.