Balkovetz Daniel F, Chumley Phillip, Amlal Hassane
Dept. of Medicine, 1530 3rd. Ave. South, LHRB 642, Univ. of Alabama at Birmingham, Birmingham, AL 35294-0007, USA.
Am J Physiol Renal Physiol. 2009 Sep;297(3):F604-11. doi: 10.1152/ajprenal.00043.2009. Epub 2009 Jul 8.
Chronic metabolic acidosis (CMA) is associated with an inhibition of fluid reabsorption in the renal proximal tubule. The effects of CMA on paracellular transport across the renal epithelial tight junction (TJ) is unknown. Claudin-2 is a transmembrane TJ-associated protein which confers TJ paracellular permeability to Na(+). We examined the effects of CMA on the expression of TJ transport proteins using both in vivo and in vitro models of CMA. The results showed downregulation of claudin-2 mRNA and protein expression in the cortex of rats subjected to the NH(4)Cl loading model of CMA. Madin-Darby canine kidney (MDCK) and HK-2 cells are models of renal epithelial cells and express claudin-2 protein in their TJ. We examined the effects of acidic pH exposure on the expression of claudin-2 in MDCK and HK-2 renal epithelial cells. Exposure of MDCK cells to pH 6.96 medium caused a significant and reversible decrease in claudin-2 protein abundance. A dose-response analysis of acidic medium exposure of MDCK and HK-2 cells demonstrated a downregulation of claudin-2 protein. The downregulation effect of acidic pH is specific to claudin-2 expression as the expression of other TJ-associated proteins (i.e., claudin-1, -3, -4, and -7, occludin, and zonula occludens-1) remained unchanged compared with control pH (7.40). Collectively, these data demonstrate that CMA downregulates the expression of claudin-2 likely through a direct effect of acidic pH. Potential physiological significance of these changes is discussed.
慢性代谢性酸中毒（CMA）与肾近端小管中液体重吸收的抑制有关。CMA对经肾上皮紧密连接（TJ）的细胞旁转运的影响尚不清楚。Claudin-2是一种跨膜TJ相关蛋白，赋予TJ对Na⁺的细胞旁通透性。我们使用CMA的体内和体外模型研究了CMA对TJ转运蛋白表达的影响。结果显示，在采用氯化铵负荷模型诱导CMA的大鼠皮质中，claudin-2 mRNA和蛋白表达下调。Madin-Darby犬肾（MDCK）细胞和HK-2细胞是肾上皮细胞模型，在其TJ中表达claudin-2蛋白。我们研究了酸性pH暴露对MDCK和HK-2肾上皮细胞中claudin-2表达的影响。将MDCK细胞暴露于pH 6.96的培养基中会导致claudin-2蛋白丰度显著且可逆地降低。对MDCK和HK-2细胞酸性培养基暴露的剂量反应分析表明claudin-2蛋白表达下调。酸性pH的下调作用对claudin-2表达具有特异性，因为与对照pH（7.40）相比，其他TJ相关蛋白（即claudin-1、-3、-4和-7、闭合蛋白和紧密连接蛋白-1）的表达保持不变。总体而言，这些数据表明CMA可能通过酸性pH的直接作用下调claudin-2的表达。讨论了这些变化潜在的生理意义。
