In the kidney, the actions of vasopressin on its type-2 receptor (V2R) induce increased water reabsorption alongside polyphosphorylation and membrane targeting of the water channel aquaporin-2 (AQP2). Loss-of-function mutations in the V2R cause Xlinked nephrogenic diabetes insipidus. Treatment of this condition would require bypassing the V2R to increase AQP2 membrane targeting, but currently no specific pharmacological therapy is available. The present study examined specific E-prostanoid receptors for this purpose. In vitro, prostaglandin E2 (PGE2) and selective agonists for the E-prostanoid receptors EP2 (butaprost) or EP4 (CAY10580) all increased trafficking and ser-264 phosphorylation of AQP2 in Madin-Darby canine kidney cells. Only PGE2 and butaprost increased cAMP and ser-269 phosphorylation of AQP2. Ex vivo, PGE2, butaprost, or CAY10580 increased AQP2 phosphorylation in isolated cortical tubules, whereas PGE2 and butaprost selectively increased AQP2 membrane accumulation in kidney slices. In vivo, a V2R antagonist caused a severe urinary concentrating defect in rats, which was greatly alleviated by treatment with butaprost. In conclusion, EP2 and EP4 agonists increase AQP2 phosphorylation and trafficking, likely through different signaling pathways. Furthermore, EP2 selective agonists can partially compensate for a nonfunctional V2R, providing a rationale for new treatment strategies for hereditary nephrogenic diabetes insipidus.T he anti-diuretic hormone vasopressin (VP) controls wholebody water balance mainly by regulating the water permeability of the kidney collecting duct. VP binds to the Gs-proteincoupled VP type 2 receptor (V2R) in collecting duct principal cells, stimulating the accumulation of aquaporin-2 (AQP2) in the apical plasma membrane (1). This process increases the water permeability of the epithelium, allowing water to be reabsorbed from the collecting-duct lumen and increasing the concentration of the urine. The intracellular signaling cascades of VP in the collecting duct involve increased cAMP and protein kinasedependent phosphorylation of AQP2 at ser-256, ser-264, and ser-269. The ser-256 and ser-269 sites appear to be essential for apical membrane accumulation of AQP2 (2). Although VP's role is well characterized, there is evidence that alternative mechanisms may also regulate water permeability. For example, functional studies on collecting ducts have shown an EC 50 of 10 −11 M for VP-induced increases in water permeability and a requirement of 10 −8 M for maximal effect (3). Water restriction does not increase plasma VP to these levels (4-6). This leaves room for additional mechanisms to be involved in modulating collecting-duct water permeability, supported by evidence that the urinary concentrating ability of the kidney can increase in the presence of a V2R antagonist during water restriction (7).Prostanoids are a family of arachidonic acid derivatives produced in most cell types. The biological actions of one class of prostanoids, prostaglandin E2 (PGE2), are diverse. This may...

The cellular functions of many eukaryotic membrane proteins, including the vasopressin-regulated water channel aquaporin-2 (AQP2), are regulated by posttranslational modifications. In this article, we discuss the experimental discoveries that have advanced our understanding of how posttranslational modifications affect AQP2 function, especially as they relate to the role of AQP2 in the kidney. We review the most recent data demonstrating that glycosylation and, in particular, phosphorylation and ubiquitination are mechanisms that regulate AQP2 activity, subcellular sorting and distribution, degradation, and protein interactions. From a clinical perspective, posttranslational modification resulting in protein misrouting or degradation may explain certain forms of nephrogenic diabetes insipidus. In addition to providing major insight into the function and dynamics of renal AQP2 regulation, the analysis of AQP2 posttranslational modification may provide general clues as to the role of posttranslational modification for regulation of other membrane proteins.

Prostanoids are prominent, yet complex, components in the maintenance of body water homeostasis. Recent functional and molecular studies have revealed that the local lipid mediator PGE2 is involved both in water excretion and absorption. The biologic actions of PGE2 are exerted through four different G-protein-coupled receptors; designated EP1-4, which couple to separate intracellular signaling pathways. Here, we discuss new developments in our understanding of the actions of PGE2 that have been uncovered utilizing receptor specific agonists and antagonists, EP receptor and PG synthase knockout mice, polyuric animal models, and the new understanding of the molecular regulation of collecting duct water permeability. The role of PGE2 in urinary concentration comprises a variety of mechanisms, which are not fully understood and likely depend on which receptor is activated under a particular physiologic condition. EP3 and microsomal PG synthase type 1 play a role in decreasing collecting duct water permeability and increasing water excretion, whereas EP2 and EP4 can bypass vasopressin signaling and increase water reabsorption through two different intracellular signaling pathways. PGE2 has an intricate role in urinary concentration, and we now suggest how targeting specific prostanoid receptor signaling pathways could be exploited for the treatment of disorders in water balance. The neurohypophyseal hormone, vasopressin (VP), is a major regulator of renal water excretion, predominantly through the seven trans-membrane receptor (7TMR) V2R. It binds to Gas, inducing the cAMP second messenger system, resulting in increased NaCl absorption by the thick ascending limb and increased urea transport in the inner medullary collecting ducts (IMCDs). [1][2][3] In addition, VP induces accumulation of the water channel aquaporin-2 (AQP2) in the rate-limiting apical plasma membrane of collecting duct (CD) principal cells, 4 a process that involves a diverse range of post-translational modifications including phosphorylation of AQP2. 5 Thus, VP increases the osmotic gradient for water transport and the water permeability (Pf) of the CD simultaneously.In addition to systemic hormones, local regulatory factors play a role in the CD, of which PGE2 (Figure 1) is the focus of this review. 6-9 PGE2 has a diverse range of biologic actions elicited by four different 7TMRs. The renal localization of EP receptors and intracellular signaling pathways are diverse (Table 1). Here, we propose that urinary concentration does not rely exclusively on fluctuations in plasma VP concentrations, but that the locally derived lipid mediator, PGE2, plays an important role in regulating water excretion. PGE2 IN THE MODULATION OF URINARY CONCENTRATING MECHANISMSThe ability of PGE2 to modulate the effects of VP in the CD has been described in a range of experimental studies and discussed in previous reviews. [36][37][38][39] The present focus is on understanding the mechanisms for this effect, and on recent in vivo studies elucidating the role of P...

pose of the present studies was to determine the effects of high-dose aldosterone and dDAVP treatment on renal aquaporin-2 (AQP2) regulation and urinary concentration. Rats were treated for 6 days with either vehicle (CON; n ϭ 8), dDAVP (0.5 ng/h, dDAVP, n ϭ 10), aldosterone (Aldo, 150 g/day, n ϭ 10) or combined dDAVP and aldosterone treatment (dDAVPϩAldo, n ϭ 10) and had free access to water with a fixed food intake. Aldosterone treatment induced hypokalemia, decreased urine osmolality, and increased the urine volume and water intake in ALDO compared with CON and dDAVPϩAldo compared with dDAVP. Immunohistochemistry and semiquantitative laser confocal microscopy revealed a distinct increase in basolateral domain AQP2 labeling in cortical collecting duct (CCD) principal cells and a reduction in apical domain labeling in Aldo compared with CON rats. Given the presence of hypokalemia in aldosterone-treated rats, we studied dietary-induced hypokalemia in rats, which also reduced apical AQP2 expression in the CCD but did not induce any increase in basolateral AQP2 expression in the CCD as observed with aldosterone treatment. The aldosterone-induced basolateral AQP2 expression in the CCD was thus independent of hypokalemia but was dependent on the presence of sodium and aldosterone. This redistribution was clearly blocked by mineralocorticoid receptor blockade. The increased basolateral expression of AQP2 induced by aldosterone may play a significant role in water metabolism in conditions with increased sodium reabsorption in the CCD.aquaporin-2; mineralocorticoid; water; salt; trafficking; cortical collecting duct ALDOSTERONE AND VASOPRESSIN are major hormones in the regulation of extracellular fluid volume. A potential direct role of aldosterone in water metabolism is still unclear. As previously demonstrated, arginine vasopressin (AVP) increases the osmotic water permeability (P f ) in isolated, perfused collecting ducts from normal rabbits and rats (3, 23). The increased P f by AVP is mediated by the vasopressin-regulated water channel aquaporin-2 (AQP2) expressed in connecting tubule (CNT) cells and collecting duct principal cells (10,27,28). Pretreatment of rabbits with mineralocorticoid (in vivo, deoxycorticosterone) enhanced the AVP-induced increase in P f by AVP in the isolated, perfused cortical collecting duct (CCD) (3), suggesting that mineralocorticoids may play some role in the regulation of water balance in some species.Previous studies have demonstrated effects of aldosterone on AQP2 regulation. In vitro studies show that aldosterone has a synergistic effect with vasopressin on stimulation of AQP2 expression in a mouse CCD cell line after Ͼ24 h of exposure to the hormone (13). Animal models of aldosterone-deficient vs. aldosterone-replaced adrenalectomized rats presented no changes in whole-kidney AQP2 expression (18), although segmental differences cannot be excluded. In addition to regulation of protein expression, AQP2 is regulated by trafficking from intracellular vesicles to the apical plasma memb...

Early downregulation of AQP2 and sodium transporters takes place segmentally in the kidney after LPS administration. In addition, an infiltration of activated macrophages and increased iNOS expression may play a role in the urinary concentrating defect in acute LPS-induced entotoxaemia.