Claudin-2 expression induces cation-selective channels in tight junctions of epithelial cells

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Abstract
Tight junctions seal the paracellular pathway of epithelia but, in leaky tissues, also exhibit specific permeability. In order to characterize the contribution of claudin-2 to barrier and permeability properties of the tight junction in detail, we studied two strains of Madin-Darby canine kidney cells (MDCK-C7 and MDCK-C11) with different tight junctional permeabilities. Monolayers of C7 cells exhibited a high transepithelial resistance (>1 kΩ cm2), compared with C11 cells (2). Genuine expression of claudin-1 and claudin-2, but not of occludin or claudin-3, was reciprocal to transepithelial resistance. However, confocal microscopy revealed a marked subjunctional localization of claudin-1 in C11 cells, indicating that claudin-1 is not functionally related to the low tight junctional resistance of C11 cells. Strain MDCK-C7, which endogenously does not express junctional claudin-2, was transfected with claudin-2 cDNA. In transfected cells, but not in vector controls, the protein was detected in colocalization with junctional occludin by means of immunohistochemical analyses. Overexpression of claudin-2 in the originally tight epithelium with claudin-2 cDNA resulted in a 5.6-fold higher paracellular conductivity and relative ion permeabilities of Na+≡1, K+=1.02, NMDG+=0.79, choline+=0.71, Cl-=0.12, Br-=0.10 (vector control, 1:1.04:0.95:0.94:0.85:0.83). By contrast, fluxes of (radioactively labeled) mannitol and lactulose and (fluorescence labeled) 4 kDa dextran were not changed. Hence, with regular Ringer9s, Na+ conductivity was 0.2 mS cm-2 in vector controls and 1.7 mS cm-2 in claudin-2-transfected cells, while Cl- conductivity was 0.2 mS cm-2 in both cells. Thus, presence of junctional claudin-2 causes the formation of cation-selective channels sufficient to transform a `tight9 tight junction into a leaky one.