Claudin-2 and claudin-12 form independent, complementary pores required to maintain calcium homeostasis.

Abstract

Calcium (Ca²⁺) homeostasis is maintained through coordination between intestinal absorption, renal reabsorption, and bone remodeling. Intestinal and renal (re)absorption occurs via transcellular and paracellular pathways. The latter contributes the bulk of (re)absorption under conditions of adequate intake. Epithelial paracellular permeability is conferred by tight-junction proteins called claudins. However, the molecular identity of the paracellular Ca²⁺ pore remains to be delineated. Claudins (<i>Cldn</i>)-2 and -12 confer Ca²⁺ permeability, but deletion of either claudin does not result in a negative Ca²⁺ balance or increased calciotropic hormone levels, suggesting the existence of additional transport pathways or parallel roles for the two claudins. To test this, we generated a <i>Cldn2/12</i> double knockout mouse (DKO). These animals have reduced intestinal Ca²⁺ absorption. Colonic Ca²⁺ permeability is also reduced in DKO mice and significantly lower than single-null animals, while small intestine Ca²⁺ permeability is unaltered. The DKO mice display significantly greater urinary Ca²⁺ wasting than <i>Cldn2</i> null animals. These perturbations lead to hypocalcemia and reduced bone mineral density, which was not observed in single-KO animals. Both claudins were localized to colonic epithelial crypts and renal proximal tubule cells, but they do not physically interact in vitro. Overexpression of either claudin increased Ca²⁺ permeability in cell models with endogenous expression of the other claudin. We find claudin-2 and claudin-12 form partially redundant, independent Ca²⁺ permeable pores in renal and colonic epithelia that enable paracellular Ca²⁺ (re)absorption in these segments, with either one sufficient to maintain Ca²⁺ balance.