A single gene product, claudin-1 or -2, reconstitutes tight junction strands and recruits occludin in fibroblasts

Transepithelial electrical resistance and tight junctions of human gingival keratinocytes

Human gingival keratinocytes (HGKs) were studied by means of freeze-fracture technique, conventional electron microscopy and the transepithelial electrical resistance for the investigation of intercellular contacts. For the purpose of comparison, MDCK cells and HaCat cells were also included. An unexpected finding was the presence of tight junctions in the HGKs. In vivo the tight junctions, which were of low complexity and P-face-associated, were co-distributed with desmosomes; in one case, the strands ran directly through desmosomal plaques. Where tight junctions and desmosomes occurred together, no gap junctions were seen. In contrast, where no tight junctions were present, gap junctions and desmosomes were co-localized. However, the unfavourable fracture planes through the tissue did not allow a clearcut allocation of gap junction/tight junction occurrence to certain strata. In vitro, HGKs also expressed tight junctions which formed networks of low complexity and high P-face association. Whereas desmosomes were highly expressed, gap junctions were not observed in cultured keratinocytes. Transepithelial electrical resistances (TEER) of cultured HGKs were higher than the values in low resistance-MDCK cells and HaCat cells but considerably lower than the values in high resistance MDCK cells, supporting the fundamental correlation between tight junction morphology and TEER. The results with this cell culture model of the human gingiva provide some valuable information about in vitro differentation and concommittent changes in cellular contacts of human gingival keratinocytes.

Claudin multigene family encoding four-transmembrane domain protein components of tight junction strands

Two related integral membrane proteins, claudin-1 and -2, recently were identified as novel components of tight junction (TJ) strands. Here, we report six more claudin-like proteins, indicating the existence of a claudin gene family. Three of these were reported previously as RVP1, Clostridium perfringens enterotoxin receptor, and TMVCF, but their physiological functions were not determined. Through similarity searches followed by PCR, we isolated full length cDNAs of mouse RVP1, Clostridium perfringens enterotoxin receptor, and TMVCF as well as three mouse claudin-like proteins and designated them as claudin-3 to -8, respectively. All of these claudin family members showed similar patterns on hydrophilicity plots, which predicted four transmembrane domains in each molecule. Northern blotting showed that the tissue distribution pattern varied significantly, depending on claudin species. Similarly to claudin-1 and -2, when these claudins were HA-tagged and introduced into cultured Madin-Darby canine kidney cells, all showed a tendency to concentrate at TJs. Immunofluorescence and immunoelectron microscopy with polyclonal antibodies specific for claudin-3, -4, or -8 revealed that these molecules were exclusively concentrated at TJs in the liver and/or kidney. These findings indicated that multiple claudin family members are involved in the formation of TJ strands in various tissues.