G蛋白耦联膜受体TGR5与核受体FXR在肾脏尿浓缩功能中的作用及机制

Nephrogenic diabetes insipidus (NDI) is caused by the inability of the kidney to respond to arginine vasopressin (AVP, also called antidiuretic hormone, ADH) stimulation. NDI is characterized by an impaired renal water reabsorption and urine concentration, leading to polyuria and consequently, polydipsia. The most common cause of NDI is lithium-induced NDI, which coincides with downregulation of aquaporin 2 (AQP2) protein in the collecting ducts of the kidney. Recent evidence shows that misfolded protein trapped in the endoplasmic reticulum (ER) cause ER stress, preventing the translocation of intracellular protein to the plasma membrane. Inability of trapped AQP2 in ER to translocate to apical plasma membrane of principal cells contributes to several types of hereditary NDI. Bile acid G protein-coupled membrane receptor TGR5 and nuclear farnesoid X receptor (FXR) abundantly express in the kidney where they exert beneficial effects on renal function. Whether FXR and TGR5 are involved in renal water metabolism is still unknown. Our preliminary data demonstrated that the compound INT767, a dual agonist for TGR5 and FXR, or the compound INT-747, a FXR agonist, increased AQP2 protein expression in primary cultured renal inner medullary collecting duct (IMCD) cells. However, it is INT767, but not INT747, that significantly upregulated protein expression of renal AQP2 and improved urinary concentrating defect in lithium-treated rats, likely via ameliorating ER stress in the principal cells. It suggests that activation of both TGR5 and FXR is necessary in regulating renal AQP2 in lithium-induced NDI. We therefore hypothesize that activation of both TGR5 and FXR is involved in renal water metabolism and urine concentration via regulating AQP2 in the collecting ducts. In the current project, we are going to investigate 1) whether TGR5 and FXR activated by specific agonists regulates AQP2 expression and trafficking in IMCD cells and mpkCCDc14 cells as well as relevant signaling pathways; 2) whether TGR5 or FXR gene knockout mice develops urinary concentrating defect; whether the activation of TGR5 or FXR receptor in knockout mice potentially regulates water metabolism and improves urine concentration; 3) whether activation of TGR5 and FXR by specific agonists coordinately regulates expression of AQP2 via attenuating ER stress and prevents urinary concentration defect in lithium-induced NDI. The current project is expected to provide evidence for therapeutic intervention of NDI targeting to TGR5 and/or FXR.

肾性尿崩症是肾脏不能浓缩尿液而持续排出大量稀释性尿的病理状态。肾脏水通道蛋白2（AQP2）表达下调是尿浓缩功能障碍的重要分子基础。我们前期工作发现，胆汁酸G蛋白耦联膜受体TGR5和核受体FXR共同激活缓解锂处理所致肾性尿崩症大鼠集合管主细胞内质网应激，显著增加肾脏AQP2表达，改善尿浓缩功能。因此提出假说：TGR5和FXR共同激活通过增加AQP2表达参与肾脏水代谢调节，改善肾性尿崩症相关的尿浓缩功能障碍。本项目拟：1）利用体外培养的肾脏集合管细胞在细胞水平阐明TGR5或FXR调节AQP2表达的信号通路；2）利用基因敲除小鼠和特异性TGR5、FXR受体激动剂，明确TGR5和FXR对肾脏AQP2的调节及尿浓缩能力的影响及机制；3）利用锂处理诱导的肾性尿崩症模型，揭示TGR5和FXR共同激活通过缓解内质网应激，增加AQP2表达，改善尿浓缩功能障碍的分子机制。 本课题将为肾性尿崩症治疗提供新靶点。

G蛋白偶联的胆汁酸膜受体TGR5在肾脏生理与病理生理过程中起重要作用。本研究通过体外细胞实验及动物实验发现，TGR5 的激活可通过cAMP-PKA信号通路，促进肾脏集合管主细胞中水通道AQP2 蛋白表达和水的跨细胞膜转运。TGR5 激活增加锂盐处理小鼠肾脏集合管主细胞中AQP2 的蛋白表达，缓解小鼠的尿液浓缩功能障碍。与野生型小鼠相比，tgr5-/-小鼠肾脏AQP2 的蛋白和基因表达下调，尿浓缩能力减弱。这些发现揭示了 TGR5在肾脏尿液浓缩和水转运过程中的作用及调节机制，提示TGR5 的激活在某些肝脏疾病（如肝硬化）伴有水潴留情况的潜在作用。缺血再灌注诱导的急性肾脏损害伴随肾脏尿浓缩功能障碍，本研究发现，TGR5受体激活通过激活缺氧诱导因子HIF1诱导的炎症反应抑制，增加AQP2蛋白表达，改善缺血再灌注大鼠尿浓缩功能；在缺氧-复氧条件下，TGR5受体激活增加HIF1蛋白表达，减轻炎症反应，促进肾脏集合管主细胞AQP2蛋白表达。这些发现提示TGR5受体激活可能减轻急性肾脏损害。肾脏集合管钠的重吸收对于维持体液稳态和血压调控方面起其重要作用，本研究发现，TGR5受体激活通过PKC/ERK等细胞信号途径降低集合管上皮钠通道（ENaC）蛋白表达，降低DOCA盐诱导的高血压。这些研究在一定程度上促进了对胆汁酸病理生理作用的了解，TGR5信号通路可能是某些肾脏疾病的作用靶点。