Defining the Contribution of ENaC to ADH-mediated Water and Sodium Excretion

 描述(由申请人提供)：抗利尿激素(ADH)通过控制肾脏的水分运输，在调节水分平衡方面发挥关键作用。ADH与血管加压素-2受体(V2R)结合，增加集合管细胞水通道AQP2的表达，从而增加肾小管水的通透性。髓间质中陡峭的溶质皮质髓质梯度提供了从肾小管液体中吸收水分的驱动力。 远端肾单位。我们和其他人已经证明，ADH还刺激集合管上皮钠通道(ENaC)的活动，ENaC可能有助于尿液浓缩。然而，ENaC传统上被认为是肾素-血管紧张素-醛固酮系统(RAAS)的末端效应器，是控制肾脏Na+排泄从而控制血压的最终元件。然而，我们已经证明，在某些低钠血症的情况下，例如肾上腺切除后，ENaC的活动出人意料地强劲。肾上腺切除后，血清ADH水平升高，导致ADH介导的ENaC活性增加。这种ENaC活性的增加是在没有醛固酮的情况下发生的，这提出了许多关于ENaC的ADH调节的新问题。ENaC可以同时对ADH和RAAS作出反应的观点与公认的仅控制肾脏Na+重吸收和血压的通道的作用不一致。为了调和ADH在调节水分和平衡方面明显相互冲突的角色，我们提出了一个统一的范式，将ENaC的贡献定义为水和Na+平衡的调节。我们假设ADH同时激活ENaC和AQP2可促进尿液浓度和血浆稀释度。如果平行的信号通路刺激ENaC活性达到足够高的水平，那么ADH介导的ENaC激活将导致Na+滞留超过尿液浓度所需，并导致高血压。我们将使用最先进的方法(例如，对小鼠分离开放集合管的膜片钳研究)和新型试剂(连接段/集合管特异的Nedd4-2基因敲除小鼠)来测试三个特定目标：1)测试ENaC是否有助于病理性肾脏水的重吸收和低钠血症；2)测试V2R激活是否可导致肾脏Na+滞留和高血压；以及3)测试V2R信号是否通过Nedd4-2在体内刺激ENaC。这项提议将提供ADH、AQP2和ENaC如何相互作用并最终控制水和Na+稳态的机械性见解。这一知识具有重要的临床意义，因为它可能会提出新的治疗低钠血症和高血压的策略，目前临床医生已经可以使用这些策略。

 DESCRIPTION (provided by applicant): Antidiuretic hormone (ADH) plays a key role in regulating water balance by controlling renal water transport. ADH binds to the vasopressin-2 receptor (V2R) and increases expression of aquaporin 2 (AQP2) water channels in collecting duct cells to increase renal tubular water permeability. A steep solute corticomedullary gradient in the medullary interstitium provides the driving force for water absorption from tubular fluid in the distal nephron. We and others have demonstrated that ADH also stimulates activity of the epithelial sodium channel (ENaC) in the collecting duct and that ENaC possibly facilitates urine concentration. However, ENaC is traditionally regarded as the end-effector of the renin-angiotensin-aldosterone system (RAAS) and the final element that controls renal Na+ excretion and thus blood pressure. Yet we have shown that in some cases of hyponatremia, such as after adrenalectomy, ENaC activity is surprisingly robust. After adrenalectomy, serum ADH levels rise, leading to an increase in ADH-mediated ENaC activity. This increase in ENaC activity occurs in the absence of aldosterone, raising many new questions about ADH regulation of ENaC. The notion that ENaC can respond to both ADH and RAAS is at odds with the accepted role of this channel in merely controlling renal Na+ reabsorption and blood pressure. To reconcile the apparent conflicting roles of ADH in regulating both water and balance, we propose a unifying paradigm that defines the contribution of ENaC as a regulator of water and Na+ balance. We hypothesize that simultaneous activation of ENaC and AQP2 by ADH promotes urine concentration and plasma dilution. If parallel signaling pathways stimulate ENaC activity to a sufficiently high level, then ADH-mediated ENaC activation will induce Na+ retention beyond what is needed for urine concentration and lead to hypertension. We will use state-of-the-art methodologies (e.g., patch clamp studies of isolated split-open collecting duct in mice) and novel reagents (connecting segment/collecting duct-specific Nedd4-2 knockout mice) to test three Specific Aims: 1) Test whether ENaC contributes to pathologic renal water reabsorption and hyponatremia, 2) Test whether V2R activation can induce renal Na+ retention and high blood pressure, and 3) Test whether V2R signals through Nedd4-2 to stimulate ENaC in vivo. This proposal will provide mechanistic insights into how ADH, AQP2, and ENaC interact and ultimately control water and Na+ homeostasis. This knowledge is clinically significant because it may suggest new strategies, which are already currently available to clinicians, for the treatment of hyponatremia and hypertension.