Role of claudins in electrolyte transports through paracellular pathways of the renal tubule

紧密连接蛋白-2在小鼠近端小管的紧密连接中高度表达，所述小鼠近端小管由具有独特的细胞旁渗透性质的渗漏上皮组成，所述细胞旁渗透性质是其高NaCl重吸收率的基础。为了研究密蛋白-2在该肾单位区段中的细胞旁NaCl转运中的功能作用，我们通过基因靶向破坏产生了缺乏密蛋白-2的小鼠[Cld 2 ^<-/->]，并将它们的形态和功能特性与它们的野生型同窝仔[Cld 2 ^<+/+>]进行了比较。Cld 2 ^<-/->小鼠具有大体上正常的外观、活动、生长和行为。光学显微镜检查结果显示Cld 2 ^<-/->肾中无大体组织学异常。超薄切片和冷冻断裂复型电子显微镜显示，与Cld 2 ^<+/+>小鼠相似，Cld 2 ^<-/->小鼠的近端小管的特征在于具有一条或两条连续链的发育不良的紧密连接。与此形成鲜明对比的是，对近端小管S2节段的离体灌注功能研究显示，Cld 2 ^<-/->小鼠的Na^+、Cl^-和水的净跨上皮吸收显著减少，claudin-2-缺陷导致细胞旁分流阻力增加，而不影响顶端或基底侧膜阻力。此外，Cld 2 ^<+/+>小鼠近端小管细胞旁通透性的Na^+选择性在Cld 2 ^<-/->小鼠中转化为Cl^-选择性。与Cld 2 ^<+/+>小鼠相比，静脉给予2%NaCl的Cld 2 ^<-/->小鼠表现出更大的尿NaCl损失，尽管在自由获取水和食物的情况下，Cld 2 ^<-/->小鼠表现出与Cld 2 ^<+/+>小鼠相似幅度的NaCl尿排泄。我们的结论是claudin-2在小鼠近曲小管紧密连接中构成了渗漏性和Na^+选择性的细胞旁通道。

Claudin-2 is highly expressed in tight junctions of mouse proximal tubule composing of a leaky epithelium with unique paracellular permeability properties that underlies its high rate of NaCl reabsorption. To investigate the functional role of claudin-2 in the paracellular NaCl transport in this nephron segment, we generated mice lacking claudin-2 [Cld2^<-/->] by gene targeting disruption and compared their morphological and functional properties with their wild-type littermates [Cld2^<+/+>]. The Cld2^<-/-> mice had grossly normal appearance, activity, growth, and behavior. Light microscopic findings exhibited no gross histological abnormalities in the Cld2^<-/-> kidney. Ultrathin section and freeze-fracture replica electron microscopy revealed that, similar to Cld2^<+/+> mice, the proximal tubule of Cld2^<-/-> mice was characterized by poorly-developed tight junctions with one or two continuous strands. In sharp contrast, the functional studies in isolated perfused S2 segments of proximal tubules showed that net transepithelial absorptions of Na^+, Cl^-, and water were significantly decreased in Cld2^<-/-> mice and that the claudin-2-deficiency led to an increase in paracellular shunt resistance without affecting apical or basolateral membrane resistances. Moreover, the Na^+-selectivity of paracellular permeability of proximal tubules in Cld2^<+/+> mice was converted to be Cl^--selective in Cld2^<-/-> mice. Cld2^<-/-> mice intravenously given 2% NaCl exhibited exaggerated urinary NaCl loss compared with Cld2^<+/+> mice, although Cld2^<-/-> mice under free access to water and food showed urinary excretions of NaCl with a similar magnitude to Cld2^<+/+> mice. We conclude that claudin-2 constitutes leaky and Na^+-selective paracellular channels within tight junctions of mouse proximal tubule.