Department of Cellular Physiology and Metabolism, University of Geneva, University Medical Center, Geneva, Switzerland.
National Center of Competence in Research Kidney Control of Homeostasis (Kidney.CH), Zurich, Switzerland.
J Am Soc Nephrol. 2020 May;31(5):1009-1023. doi: 10.1681/ASN.2019080790. Epub 2020 Apr 3.
Water and solute transport across epithelia can occur the transcellular or paracellular pathways. Tight junctions play a key role in mediating paracellular ion reabsorption in the kidney. In the renal collecting duct, which is a typical absorptive tight epithelium, coordination between transcellular sodium reabsorption and paracellular permeability may prevent the backflow of reabsorbed sodium to the tubular lumen along a steep electrochemical gradient.
To investigate whether transcellular sodium transport controls tight-junction composition and paracellular permeability modulating expression of the transmembrane protein claudin-8, we used cultured mouse cortical collecting duct cells to see how overexpression or silencing of epithelial sodium channel (ENaC) subunits and claudin-8 affect paracellular permeability. We also used conditional kidney tubule-specific knockout mice lacking ENaC subunits to assess the ENaC's effect on claudin-8 expression.
Overexpression or silencing of the ENaC -subunit was associated with parallel and specific changes in claudin-8 abundance. Increased claudin-8 abundance was associated with a reduction in paracellular permeability to sodium, whereas decreased claudin-8 abundance was associated with the opposite effect. Claudin-8 overexpression and silencing reproduced these functional effects on paracellular ion permeability. Conditional kidney tubule-specific ENaC -subunit knockout mice displayed decreased claudin-8 expression, confirming the cell culture experiments' findings. Importantly, ENaC -subunit or -subunit silencing or kidney tubule-specific -ENaC or -ENaC knockout mice did not alter claudin-8 abundance.
Our data reveal the specific coupling between ENaC -subunit and claudin-8 expression. This coupling may play an important role in preventing the backflow of reabsorbed solutes and water to the tubular lumen, as well as in coupling paracellular and transcellular sodium permeability.
水和溶质可以通过上皮细胞的细胞内或细胞旁途径进行转运。紧密连接在介导肾脏细胞旁离子重吸收方面起着关键作用。在肾集合管中，这是一种典型的吸收性紧密上皮细胞，细胞内钠重吸收和细胞旁通透性之间的协调可能会防止沿着陡峭的电化学梯度将重吸收的钠回流到管腔中。
为了研究细胞内钠转运是否控制紧密连接的组成和细胞旁通透性，我们使用培养的小鼠皮质集合管细胞来观察上皮钠通道 (ENaC) 亚基和紧密连接蛋白 8 (claudin-8) 的表达上调或沉默如何影响细胞旁通透性。我们还使用条件性肾管状特异性敲除 ENaC 亚基的小鼠来评估 ENaC 对 claudin-8 表达的影响。
ENaC 亚基的过表达或沉默与 claudin-8 丰度的平行和特异性变化相关。claudin-8 丰度增加与钠的细胞旁通透性降低有关，而 claudin-8 丰度降低则与相反的效果相关。claudin-8 的过表达和沉默复制了这些对细胞旁离子通透性的功能影响。条件性肾管状特异性 ENaC 亚基敲除小鼠显示出 claudin-8 表达减少，证实了细胞培养实验的结果。重要的是，ENaC 亚基或亚基沉默或肾管状特异性 ENaC 或 ENaC 敲除小鼠不会改变 claudin-8 的丰度。
我们的数据揭示了 ENaC 亚基和 claudin-8 表达之间的特定偶联。这种偶联可能在防止重吸收溶质和水回流到管腔中以及在偶联细胞旁和细胞内钠通透性方面发挥重要作用。
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