Tight Junction Proteins Claudin-2 and -12 Are Critical for Vitamin D-dependent Ca2+ Absorption between Enterocytes

Abstract

Ca²⁺ is absorbed across intestinal epithelial monolayers via transcellular and paracellular pathways, and an active form of vitamin D₃, 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃], is known to promote intestinal Ca²⁺ absorption. However, the molecules driving the paracellular Ca²⁺ absorption and its vitamin D dependency remain obscure. Because the tight junction proteins claudins are suggested to form paracellular channels for selective ions between neighboring cells, we hypothesized that specific intestinal claudins might facilitate paracellular Ca²⁺ transport and that expression of these claudins could be induced by 1α,25(OH)₂D₃. Herein, we show, by using RNA interference and overexpression strategies, that claudin-2 and claudin-12 contribute to Ca²⁺ absorption in intestinal epithelial cells. We also provide evidence showing that expression of claudins-2 and -12 is up-regulated in enterocytes in vitro and in vivo by 1α,25(OH)₂D₃ through the vitamin D receptor. These findings strongly suggest that claudin-2- and/or claudin-12-based tight junctions form paracellular Ca²⁺ channels in intestinal epithelia, and they highlight a novel mechanism behind vitamin D-dependent calcium homeostasis.