Claudin extracellular domains determine paracellular charge selectivity and resistance but not tight junction fibril architecture

Tight junctions (TJs) regulateparacellular permeability across epithelia and vary widely in theirtransepithelial electrical resistance (TER) and charge selectivity. Theclaudin family of transmembrane proteins influences these properties.We previously reported that claudin-4 increased TER ~300% whenexpressed in low-resistance Madin-Darby canine kidney (MDCK) II cellsand decreased the paracellular permeability for Na⁺ morethan Cl (Van Itallie C, Rahner C, and Anderson JM.J Clin Invest 107: 1319-1327, 2001). Incomparison, we report here that expression of claudin-2 increases TERby only ~20% and does not change the ionic selectivity of MDCK IIcells from their cation-selective background. To test whether theextracellular domains of claudins-4 and -2 determine their uniqueparacellular properties, we determined the effects of interchangingthese domains between claudins-4 and -2. Inducible expression ofwild-type claudins and extracellular domain chimeras increased both thenumber and depth of fibrils, but the characteristic fibril morphologiesof claudin-4 or -2 were not altered by switching extracellular domains.Like claudin-4, chimeras expressing the first or both extracellulardomains of claudin-4 on claudin-2 increased TER severalfold andprofoundly decreased the permeability of Na⁺ relative toCl. In contrast, chimeras expressing the first or bothextracellular domains of claudin-2 on claudin-4 increased the TER byonly ~60 and ~40%, respectively, and only modestly altered chargeselectivity. These results support a model in which the claudins createparacellular channels and the first extracellular domain is sufficientto determine both paracellular charge selectivity and TER.