Department of Physiology, University of Alberta, Edmonton, AB, T6G 2H7, Canada.
Women's and Children's Health Research Institute, Edmonton, AB, T6G 1C9, Canada.
Proc Natl Acad Sci U S A. 2021 Nov 30;118(48). doi: 10.1073/pnas.2111247118.
Calcium (Ca) 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 Ca pore remains to be delineated. Claudins ()-2 and -12 confer Ca permeability, but deletion of either claudin does not result in a negative Ca 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 double knockout mouse (DKO). These animals have reduced intestinal Ca absorption. Colonic Ca permeability is also reduced in DKO mice and significantly lower than single-null animals, while small intestine Ca permeability is unaltered. The DKO mice display significantly greater urinary Ca wasting than 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 Ca permeability in cell models with endogenous expression of the other claudin. We find claudin-2 and claudin-12 form partially redundant, independent Ca permeable pores in renal and colonic epithelia that enable paracellular Ca (re)absorption in these segments, with either one sufficient to maintain Ca balance.
钙（Ca）稳态通过肠道吸收、肾脏重吸收和骨重塑之间的协调来维持。肠道和肾脏（重）吸收通过细胞内和细胞旁途径进行。在后一种情况下，在摄入充足的情况下，大部分（重）吸收发生。上皮细胞旁通透性由称为紧密连接蛋白的紧密连接蛋白赋予。然而，细胞旁 Ca 孔的分子身份仍有待描绘。Claudin（）-2 和 -12 赋予 Ca 通透性，但敲除任何一种 Claudin 都不会导致负钙平衡或增加钙调激素水平，这表明存在其他运输途径或两种 Claudin 的平行作用。为了验证这一点，我们生成了一个 Claudin-2 和 Claudin-12 双敲除小鼠（DKO）。这些动物的肠道 Ca 吸收减少。DKO 小鼠的结肠 Ca 通透性也降低，明显低于单敲除动物，而小肠 Ca 通透性不变。DKO 小鼠的尿钙排泄量明显高于 Claudin-2 敲除动物。这些变化导致低钙血症和骨密度降低，而在单敲除动物中没有观察到。两种 Claudin 都定位于结肠上皮隐窝和肾脏近端小管细胞，但它们在体外没有物理相互作用。在具有内源性另一种 Claudin 表达的细胞模型中过表达任一 Claudin 均可增加 Ca 通透性。我们发现 Claudin-2 和 Claudin-12 在肾和结肠上皮中形成部分冗余的、独立的 Ca 通透孔，使这些节段中的细胞旁 Ca（重）吸收成为可能，其中任何一种都足以维持 Ca 平衡。
