核受体LXRβ对肾脏集合管上皮水通道蛋白AQP2的调节机制

The kidney play important role in regulating body water homeostasis. Water channels (Aquaporins, AQPs) located in the apical and basolateral membrane of epithelial cells along renal tubules are essential for water reabsorption and urine concentration. Apical accumulation of Aquaporin2 (AQP2) of renal collecting duct will increases the water permeability and lead to urine concentration. Among all renal water channels, AQP2 expression is regulated by circulating arginine vasopressin (AVP). Liver X Receptors (LXRs) are well-known for their role in lipid homeostasis. Our previous study suggests nuclear receptor LXRβ is also a key receptor in controlling water balance with targets in AVP system. LXRβ can induce AVP expression in hypothalamus and lead to water reabsorption in kidney. Our present data showed that activation of LXRβ increased the protein level of AQP2 via affecting it’s ubiquitination in primary cultured mouse inner medullary duct cells. It suggests that beside the above mentioned central AVP mechanisms, LXRβ possess a peripheral nephrogenic mechanism in regulating water balance through direct regulating AQP2 in renal collecting duct. In contrast to the well-established AVP mediated regulation in protein level and protein phosphorylation, LXRβ regulation of AQPs via post-transcriptional modification is poorly understood. To improve our understanding of basic mechanism, we propose to: (1) characterized how LXRβ regulating AQP2 protein level in collecting duct principle cell; (2) identified the pathway responsible for LXRβ activation induced AQPs ubiquitination stasis change. Detailed knowledge of the molecular mechanisms for AQP2 regulation would provide new therapeutic targets for water balance disorders in patients.

肾脏集合管上皮细胞管腔面分布的水通道Aquaporin2(AQP2)对尿液浓缩和机体水平衡的维持发挥至关重要的作用。AQP2蛋白水平及其在管腔面的分布量直接影响机体产生的尿量。细胞通过出胞和胞吞改变管腔面AQP2的数量来调节集合管对水的通透性，而AQP2泛素化在其膜转运过程中发挥重要作用。前期研究发现核受体LXRβ可以通过增加中枢抗利尿激素的释放促进肾对水的重吸收，其机制可能和集合管上皮细胞中AQP2蛋白增加相关。但近期发现，离体原代细胞培养下LXRβ激活可以对AQP2进行转录后调节，且此调节很可能通过影响其泛素化来实现。鉴此，本课题将重点探讨：(1) 核受体LXRβ上调肾脏集合管上皮细胞中AQP2蛋白水平的机制；(2) LXRβ如何通过影响AQP2泛素化来改变细胞对水的通透性。本课题的开展将对阐明肾脏水重吸收调控机制提供重要的实验依据，并为水代谢异常疾病的治疗及药物开发提供相关思路。

真核细胞表面的水通道Aquaporin对细胞体积, 细胞内水平衡的维持发挥至关重要的作用。在哺乳动物体内已发现的水通道蛋白有13个成员,分别命名为AQP1-AQP12, 水通道蛋白在机体内选择性地表达于多种组织和器官,并且发挥重要的生理功能。AQP2蛋白水平及其在管腔面的分布量直接影响机体产生的尿量, 在眼睛里睫状体以及小梁网内皮细胞膜上的水通道AQP1及AQP4控制着房水的产生及回收。细胞通过出胞和胞吞改变管腔面AQP的数量来调节细胞对水的通透性，而AQP蛋白的数量由合成量及降解量共同控制，转录水平调控以及降解调控组织内对水平衡（尿液产生及重吸收，房水的产生及回流）中发挥重要作用。本研究发现核受体LXRβ可以通过增加中枢抗利尿激素的释放促进肾对水的重吸收，其机制可能和集合管上皮细胞中AQP2蛋白增加相关离体原代细胞培养下LXRβ激活可以对AQP2进行转录后调节，经研究发现: 核受体LXRβ上调肾脏集合管上皮细胞中AQP2转录是间接作用, 没有发现LXRβ可以直接作用于AQP2的promoter；AQP2泛素并非预期假设所想是由Nedd3/ARDD3共同完成；但是，我们利用人小梁网细胞，发现水通道AQP1在细胞膜的分布收内皮素ET-1的影响，并证明这种影响是由转录因子ATF4调控完成，而且这种调控是转录负调控机制。本课题的开展将对阐明肾脏水重吸收及小梁网房水回流调控机制提供重要的实验依据，并为水代谢异常疾病的治疗及药物开发提供相关思路。