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

Claudin-2在小鼠近端小管的紧密连接处高度表达，该小管由具有独特的细胞旁通透性的渗漏上皮组成，这是其高盐重吸收速率的基础。为了研究claudin-2在肾单位段细胞旁氯化钠转运中的功能作用，我们通过基因打靶干扰的方法产生了缺乏claudin-2[Cld2^&lt；-/-&gt；]的小鼠，并将它们的形态和功能特性与它们的野生型窝种[Cld2^&lt；+/+&gt；]进行了比较。Cld2^&lt；-/-&gt；小鼠的外表、活动、生长和行为都非常正常。光镜下发现Cld2^&lt；-/-&gt；肾脏没有大体组织学异常。超薄切片和冷冻断裂复制品电子显微镜显示，与Cld2^&lt；+/+&gt；小鼠相似，Cld2^&lt；-/-&gt；小鼠的近端小管具有发育不良的紧密连接，具有一条或两条连续的链。与此形成鲜明对比的是，近端肾小管灌流S2节段的功能研究显示，Cld2^-/-&gt；小鼠跨上皮净吸收Na~+、Cl~+和水的能力显著降低，而Claudin-2-缺乏导致细胞旁分流阻力增加，但不影响顶端或基底侧膜阻力。此外，Cld2^&lt；+/+&gt；小鼠近端小管细胞旁通透性的Na~+-选择性在Cld2^&lt；-/-&gt；小鼠中转为氯离子选择性。与Cld2^&lt；+/+&gt；小鼠相比，静脉注射2%氯化钠的Cld2^&lt；-/-&gt；小鼠表现出夸大的尿盐丢失，尽管自由饮水和食物的Cld2^&lt；-/-&gt；小鼠的尿氯化钠排泄量与Cld2^&lt；+/+&gt；小鼠相似。我们的结论是，Claudin-2在小鼠近端小管的紧密连接中构成了泄漏的和对Na~+有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.