Claudin-1 contributes to the epithelial barrier function in MDCK cells

Tight junctions (TJs) create a paracellular permeability barrier and also a ct as a fence preventing intermixing of proteins and lipids between the api cal and basolateral plasma membranes. Recently, claudin-1 has been identifi ed as an integral membrane protein localizing at TJs, and introduced claudi n-1 can form TJ-like networks in fibroblasts. To investigate the function o f claudin-1, MDCK cells were transfected with a mammalian expression vector containing myc-tagged mouse claudin-1, and four stable clones were obtaine d. The myc-tagged claudin-1 precisely colocalized with both occludin and ZO -1 at cell-cell contact sites, indicating that exogenous claudin-1 was prop erly targeted to the TJs, Immunoblot analysis revealed that overexpression of claudin-1 increased expression of ZO-1 but not of occludin or ZO-2. The barrier functions of these cells were evaluated by transepithelial electric al resistance (TER) and paracellular nux. Claudin-1-expressing cells exhibi ted about four times higher TER than wild-type MDCK cells, Consistent with the increase of TER, the cells overexpressing claudin-1 showed reduced para cellular flux, estimated at 4 and 40 kD FITC-dextrans. These results sugges t that claudin-1 is involved in the barrier function at TJs.