Calcium Inhibits Paracellular Sodium Conductance through Claudin-2 by Competitive Binding

Claudins form paracellular pores at the tight junction in epithelial cells. Profound depletion of extracellular calcium is well known to cause loosening of the tight junction with loss of transepithelial resistance. However, moderate variations in calcium concentrations within the physiological range can also regulate transepithelial permeability. To investigate the underlying molecular mechanisms, we studied the effects of calcium on the permeability of claudin-2, expressed in an inducible MDCK I cell line. We found that in the physiological range, calcium acts as a reversible inhibitor of the total conductance and Na⁺ permeability of claudin-2, without causing changes in tight junction structure. The effect of calcium is enhanced at low Na⁺ concentrations, consistent with a competitive effect. Furthermore, mutation of an intrapore negatively charged binding site, Asp-65, to asparagine partially abrogated the inhibitory effect of calcium. This suggests that calcium competes with Na⁺ for binding to Asp-65. Other polyvalent cations had similar effects, including La³⁺, which caused severe and irreversible inhibition of conductance. Brownian dynamics simulations demonstrated that such inhibition can be explained if Asp-65 has a relatively high charge density, thus favoring binding of Ca²⁺ over that of Na⁺, reducing Ca²⁺ permeation by inhibiting its dissociation from this site, and decreasing Na⁺ conductance through repulsive electrostatic interaction with Ca²⁺. These findings may explain why hypercalcemia inhibits Na⁺ reabsorption in the proximal tubule of the kidney.