Claudin-8 modulates paracellular permeability to acidic and basic ions in MDCK II cells
Claudin-8 modulates paracellular permeability to acidic and basic ions in MDCK II cells
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DOI：
10.1113/jphysiol.2005.099135
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发表时间：
2006-02-15
影响因子：
5.5
通讯作者：
Yu, ASL
中科院分区：
文献类型：
--
作者：
Angelow, S;Kim, KJ;Yu, ASL
肾净酸排泄需要肾小管重吸收过滤后的碳酸氢盐，然后在集合管中分泌质子和铵，从而为这些离子产生陡峭的肾小管梯度。为了防止这些离子的被动回漏，集合管中的紧密连接必须对这些离子具有高度的不可渗透性。我们之前生成了可诱导表达claudin-8（一种在集合管中表达的紧密连接蛋白）的Madin-Darby犬肾（MDCK II）细胞系。在这些细胞中，claudin-8 显示出作为碱金属和二价阳离子的细胞旁屏障的功能。我们现在使用这个模型来检验这样的假设：claudin-8 还可以作为参与肾脏酸排泄的酸性或碱性离子的细胞旁屏障。我们开发了一系列精确且无偏差的方法，基于扩散电位、短路电流和 pH 统计测量的组合，来估计 MDCK II 细胞中质子、铵和碳酸氢盐的细胞旁通透性。我们发现，在对照条件下（即，在没有claudin-8的情况下），这些细胞对测试的酸性和碱性离子具有高度渗透性。有趣的是，质子渗透表现出与本体溶液相似的异常低的活化能。这表明细胞旁质子转移可能通过 Grotthuss 机制发生，这意味着细胞旁孔足够宽，可以容纳自由移动状态的水分子。诱导claudin-8表达不仅降低了对质子的渗透性，而且还降低了对铵和碳酸氢盐的渗透性。我们得出的结论是，claudin-8 的功能可能是限制这三种离子通过细胞旁途径的被动渗漏，从而在尿净酸排泄中发挥许可作用。
Renal net acid excretion requires tubular reabsorption of filtered bicarbonate, followed by secretion of protons and ammonium in the collecting duct, generating steep transtubular gradients for these ions. To prevent passive backleak of these ions, the tight junctions in the collecting duct must be highly impermeable to these ions. We previously generated a Madin-Darby canine kidney (MDCK II) cell line with inducible expression of claudin-8, a tight junction protein expressed in the collecting duct. In these cells, claudin-8 was shown to function as a paracellular barrier to alkali metal and divalent cations. We have now used this model to test the hypothesis that claudin-8 also functions as a paracellular barrier to acidic or basic ions involved in renal acid excretion. We developed a series of precise and unbiased methods, based on a combination of diffusion potential, short-circuit current, and pH stat measurements, to estimate paracellular permeability to protons, ammonium and bicarbonate in MDCK II cells. We found that under control conditions (i.e. in the absence of claudin-8), these cells are highly permeable to the acidic and basic ions tested. Interestingly, proton permeation exhibited an unusually low activation energy similar to that in bulk solution. This suggests that paracellular proton transfer may occur by a Grotthuss mechanism, implying that the paracellular pores are sufficiently wide to accommodate water molecules in a freely mobile state. Induction of claudin-8 expression reduces permeability not only to protons, but also to ammonium and bicarbonate. We conclude that claudin-8 probably functions to limit the passive leak of these three ions via paracellular routes, thereby playing a permissive role in urinary net acid excretion.