The Trafficking of the Water Channel Aquaporin-2 in Renal Principal Cells—a Potential Target for Pharmacological Intervention in Cardiovascular Diseases

Arginine-vasopressin (AVP) stimulates the redistribution of water channels, aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. By this AVP directs 10% of the water reabsorption from the 170 L of primary urine that the human kidneys produce each day. This review discusses molecular mechanisms underlying the AVP-induced redistribution of AQP2; in particular, it provides an overview over the proteins participating in the control of its localization. Defects preventing the insertion of AQP2 into the plasma membrane cause diabetes insipidus. The disease can be acquired or inherited, and is characterized by polyuria and polydipsia. Vice versa, up-regulation of the system causing a predominant localization of AQP2 in the plasma membrane leads to excessive water retention and hyponatremia as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure or liver cirrhosis. This article briefly summarizes the currently available pharmacotherapies for the treatment of such water balance disorders, and discusses the value of newly identified mechanisms controlling AQP2 for developing novel pharmacological strategies. Innovative concepts for the therapy of water balance disorders are required as there is a medical need due to the lack of causal treatments.


INTRODUCTION
The kidney has multiple functions including excretion of toxins, control of blood pressure and pH. One of its main functions is water reabsorption from primary urine. The kidney generates more than 170 L of primary urine per day by ultrafiltration of the blood. This huge volume, mainly water, of course needs to be reduced to prevent dehydration. Indeed, only around 1% of the volume is excreted as urine. How does the kidney achieve this?
The functional unit of the kidney is the nephron, of which there are around 1-2 million per kidney. A nephron is a tubular system consisting of several functionally distinct segments. The initial unit is the glomerulus where the blood is filtered at a rate of ∼120 ml/min into the surrounding Bowman's capsule. The filtrate flows from there through the proximal convoluted tubule, the descending and ascending limb of Henle, the distal convoluted tubule, and the collecting duct into the bladder. During the passage the volume of the filtrate is reduced and its composition altered. As the final urine, the filtrate reaches the bladder where it is stored until it is cleared.

RENAL WATER CHANNELS: EXPRESSION PATTERN AND FUNCTIONS
The concentration of the filtrate is achieved mainly by aquaporins (AQPs) that allow diffusion of water across membranes. The water flow is driven by an osmotic gradient established by sodium across the kidney.
There are 13 mammalian AQPs, AQP0-AQP12, that show high structural similarity. They all contain six transmembrane domains, which form a water pore characterized by a dual NPA motif (Preston et al., 1992;Jung et al., 1994;Kosinska Eriksson et al., 2013). All AQPs assemble into homotetramers. Nine AQPs (AQP1-8 and AQP11) are expressed along the nephron with distinct localization to particular segments ( Table 1). Crucial roles in water reabsorption from primary urine play AQP1-4. AQP1 in the plasma membrane of epithelial cells facing the lumen of the proximal tubule and descending limb of Henle facilitates reabsorption of 80-90% of water from the filtrate (Nielsen et al., 1993;Schnermann et al., 1998). The ascending limb of Henle is impermeable for water due to the lack of AQPs. From the distal convoluted tubule throughout the collecting duct AQP3 and 4 are constitutively expressed in the basolateral plasma membrane Terris et al., 1995). Thus the apical plasma membrane is water impermeable. However, in the collecting duct the principal cells additionally express AQP2. It resides on intracellular vesicles under resting conditions. Stimulation of vasopressin V2 receptors (V2R) on the basolateral surface of the cells with arginine-vasopressin (AVP) causes a redistribution of AQP2 predominantly into the apical plasma membrane (Figure 1). The insertion facilitates water reabsorption of around 10% of the water from the remaining filtrate (Fushimi et al., 1993;Katsura et al., 1995;Nielsen et al., 2002). In total, the reabsorption of water by AQPs accounts for the reduction of the volume of the initial filtrate from 170 L to around 2-4 L/day of final urine that is on average excreted by an adult.
This review focuses on mechanisms controlling the localization of AQP2 and the value of such control mechanisms as pharmacological targets in cardiovascular diseases.

AVP STIMULATES THE REDISTRIBUTION OF AQP2 FROM INTRACELLULAR VESICLES INTO THE PLASMA MEMBRANE
The modulation of the final 10% of water reabsorption in the collecting duct fine-tunes body water homeostasis. Osmoreceptors in the hypothalamus detect changes in blood osmolality that arise, for example, from hypernatremia and hypovolemia, and induce the release of the peptide hormone AVP (antidiuretic hormone) (Verney, 1947;Hayashi et al., 1994). AVP is synthesized in the hypothalamus as a preprohormone. It is released to the pituitary gland and secreted from there to stimulate renal collecting duct principal cells to retain water. As a result, blood volume increases, osmolality decreases, and osmoreceptor signaling declines.
The primary regulator of blood pressure is the reninangiotensin-aldosterone system (RAAS). Hypotension or hypovolemia induce renin secretion from renal juxtaglomerular cells. Renin converts angiotensin I (Ang I) to angiotensin II (Ang II). AngII acts as a vasoconstrictor, and induces aldosterone and AVP secretion to stimulate renal sodium and water reabsorption. This increases blood volume and blood pressure. As a counterpart natriuretic peptides are secreted by cardiac myocytes upon hypervolemia-induced mechanical stretching of the atrial (atrial natriuretic peptide, ANP) and ventricular (brain natriuretic peptide, BNP) walls. The peptides have vasodilative, natriuretic and diuretic effects to lower blood volume and pressure.
At the molecular level, AVP binds to its cognate G proteincoupled receptor, the V2R, which is located in the basolateral plasma membrane of collecting duct principal cells (Figure 1). The binding of AVP to V2R activates the stimulatory G protein, G s . G s , in turn activates adenylyl cyclases AC3 and AC6 to convert ATP to cyclic adenosine monophosphate (cAMP). One of the effectors of cAMP is protein kinase A (PKA). It consists of a dimer of regulatory subunits (RIα, RIβ, RIIα, or RIIβ) and two catalytic subunits (Cα, Cβ, Cγ, or PrKX) each bound to one R subunit. Upon binding of cAMP to two sites in the R subunits PKA undergoes a conformational change and the C subunits are released. The free, and now active C subunits phosphorylate nearby targets. PKA phosphorylates serine 256 (S256) in the C terminus of AQP2.
AQP2 undergoes a constitutive recycling: its trafficking from intracellular vesicles into the plasma membrane and endocytic retrieval to its intracellular storage site is in dynamic equilibrium under resting conditions. The equilibrium is shifted toward plasma membrane insertion by the AVP-induced phosphorylation of S256 (Klussmann et al., 2000;Nedvetsky et al., 2009;Moeller et al., 2011). The finding that upon inhibition of endocytosis AQP2 accumulates in the apical plasma membrane of cultured cells regardless of AVP stimulation or S256 phosphorylation emphasizes the importance of the recycling pathway (Lu et al., 2004). It also shows that the redistribution of AQP2 can be induced independently of PKA activation (Nejsum et al., 2005). In addition to the phosphorylation status of S256 that of at least three further C-terminal residues of AQP2 changes in response to AVP stimulation (Fushimi et al., 1997;Katsura et al., 1997;Kamsteeg et al., 2000;Tamma et al., 2010): S261, S264, and S269. S261 is phosphorylated under resting conditions and found intracellularly; AVP triggers its dephosphorylation (Hoffert et al., 2007). In contrast, AVP induces the phosphorylation of S264 and S269 and thereby confers plasma membrane retention properties to AQP2 Hoffert et al., 2008;Moeller et al., 2009).
FIGURE 1 | Model of the arginine-vasopressin (AVP)-stimulated AQP2 translocation from intracellular vesicles into the plasma membrane of renal collecting duct principal cells. The plasma membrane insertion of AQP2 facilitates water reabsorption from hypoosmotic primary urine. The exocytosis-like process requires PKA phosphorylation of AQP2 at S256. Water exits the cells through water channels AQP3 and 4 constitutively expressed in the basolateral plasma membrane. For details please refer to section Proteins controlling AQP2 trafficking.
exo-and endocytic trafficking, cytoskeletal proteins as the tracks for the trafficking, proteins controlling cAMP signaling such as A-kinase anchoring proteins (AKAPs) and phosphodiestereases (PDEs). A few examples are discussed below.

AKAPs
AKAPs are scaffolding proteins. They tether PKA in close proximity of its substrates Langeberg and Scott, 2015). Global inhibition of AKAP-PKA interactions in a cell culture model of renal principal cells, primary inner medullary collecting duct (IMCD) cells, revealed that AKAP-PKA interactions are crucial for the AVP-stimulated translocation of AQP2 (Klussmann et al., 1999;Klussmann and Rosenthal, 2001). AKAP18δ and AKAP220 are located on AQP2bearing vesicles to tether PKA in close proximity of AQP2. They apparently facilitate the phosphorylation of S256 (Henn et al., 2004;McSorley et al., 2006;Okutsu et al., 2008).

PDEs
Termination of cAMP signaling is achieved through hydrolysis of cAMP by PDEs. There are 11 families of PDEs hydrolyzing cAMP, cGMP, or both. PDE4 family isozymes are cAMPspecific, constitutively active and shape local cAMP gradients, thus regulating its availability for PKA activation (Houslay, 2010;Klussmann, 2015;Mika and Conti, 2015). PDE4D directly
Frontiers in Pharmacology | www.frontiersin.org interacts with AKAP18δ on AQP2-bearing vesicles . The presence of PDE4D on AQP2-bearing vesicles prevents an inappropriate plasma membrane insertion of AQP2 and thus water reabsorption in resting IMCD cells. Specific PDE4 inhibition with rolipram did not induce, but enhance the cAMPstimulated AQP2 translocation into the plasma membrane Szaszák et al., 2008). On the other hand, hyperactive PDE4 diminishes intracellular cAMP and causes AVP resistance in a mouse model (Valtin et al., 1990;Takeda et al., 1991).

Cytoskeletal Components
AQP2 is folded and assembled into homotetramers in the endoplasmic reticulum (ER), passes through the Golgi and is then stored in intracellular vesicles in the perinuclear region. Under resting conditions, AQP2 binds monomeric G-actin (Noda et al., 2004b(Noda et al., , 2008. PKA-mediated AQP2 phosphorylation of S256 weakens the interaction. This increases the affinity of AQP2 to the actin-stabilizing protein tropomyosin-5b, withdrawing it from F-actin. Consequently, actin depolymerizes, enabling the vesicle redistribution to the plasma membrane (Noda et al., 2008). AVPstimulated AQP2 vesicle trafficking to the plasma membrane depends on F-actin depolymerization (Simon et al., 1993).
Activation of the small GTPase RhoA induces F-actin-containing stress fibers and tonically inhibits AQP2 trafficking Tamma et al., 2001). Activated PKA phosphorylates and thereby inhibits RhoA (Lang et al., 1996;Dong et al., 1998), contributing to the AVP-induced destabilization of the actin network. A role of actin in the redistribution of AQP2 is supported by observations in cultured renal collecting duct cells (CD8 cells) treated with the serine/threonine phosphatase 1 and 2a inhibitor, okadaic acid. The agent increased the AQP2 phosphorylation of AQP2 at S256 by around 60% and led to a depolymerization of the actin network. These findings also supported the notion that the redistribution of AQP2 is not exclusively controlled by PKA (Valenti et al., 2000).

SNAREs
The plasma membrane insertion of AQP2 occurs in an exocytosis-like manner involving the SNARE (soluble Nethylmaleimide sensitive factor attachment protein receptor) machinery (Liebenhoff and Rosenthal, 1995;Gouraud, 2002). The vesicle-associated membrane proteins (VAMP) 2 and 3 reside on AQP2-bearing vesicles. They interact with apical plasma membrane-located syntaxin 3 and the synaptosomeassociated protein (SNAP) 23 to achieve vesicle fusion and the insertion of AQP2 into plasma membrane (Barile et al., 2005;Procino et al., 2008).

Proteins for AQP2 Endocytosis and Recycling
Adjustment of body water balance by AQP2-mediated water reabsorption leads to a decline of AVP, followed by internalization of AQP2. AQP2 retrieval from the plasma membrane proceeds via clathrin-mediated endocytosis (Sun et al., 2002). AVP stimulates co-accumulation of Hsc70 and AQP2 in the apical plasma membrane (rat kidney sections), and functional Hsc70 is required for AQP2 endocytosis in LLC-PK 1 cells stably expressing AQP2. AQP2 directly interacts with the endocytotic proteins clathrin, dynamin, and the adaptor protein AP2 (Lu et al., 2007). The phosphomimic mutants S256D and S269D of AQP2 show reduced interactions with the endocytotic machinery, which reduces internalization and increases half-life due to decreased proteasomal degradation (Moeller et al., 2010). Myelin and lymphocyte-associated protein (MAL), a protein found in glycosphingolipid-enriched membranes and potentially involved in the apical transport machinery forms a complex with AQP2 in LLC-PK 1 cells (Frank et al., 1998;Martín-Belmonte et al., 2000;Kamsteeg et al., 2007). As phosphorylation of S256 strengthens the interaction with MAL and MAL attenuates AQP2 internalization, this is in line with the above-described reduced endocytosis to explain the stabilization and accumulation of pS256 in the apical plasma membrane.
From the plasma membrane AQP2 is recycled or directed for degradation. The motor protein myosin Vb, its receptor on AQP2-bearing vesicles, Rab11, and the adaptor protein Rab11-FIP2 are essential for recycling through the Rab11-dependent recycling pathway . Positioning of AQP2bearing vesicles in their perinuclear storage region involves microtubules (Vossenkämper et al., 2007).

Ubiquitin
Under resting conditions AQP2 is phosphorylated at S261 by, amongst others p38 MAP kinase (Nedvetsky et al., 2010). The phosphorylation is a signal for AQP2's mono-and poly-ubiquitination. The short-chain ubiquitination of AQP2 at K270 increases its endocytosis under resting conditions (Kamsteeg et al., 2006). In primary IMCD cells, AVP causes a decrease in the phosphorylation of S261 within 30 min of exposure. This reduces polyubiquitination and prevents AQP2's proteasomal degradation resulting in an increase in its abundance. Thus, AQP2 expression adapts quickly to water availability: thirsting increases, whereas water overload reduces AQP2 levels (Nedvetsky et al., 2010). The AVP-induced reduction of polyubiquitination of AQP2 implies that its recycling via the Rab11 pathway requires its deubiquitination.

DYSREGULATION OF AVP-MEDIATED WATER REABSORPTION CAUSES WATER BALANCE DISORDERS
Several diseases are associated or caused by dysregulation of AVP-mediated water reabsorption (Figure 2). Elevated levels of The antidiuretic peptide hormone arginine-vasopressin (AVP) is synthesized in the hypothalamus as a prohormone. Mature AVP is secreted from the neurohypophysis to stimulate the kidneys to retain water. Mutations in the AVP gene impede correct processing or reduce its affinity for the V2 receptor, which precludes AVP-mediated water reabsorption from primary urine and is termed central diabetes insipidus (CDI). In the rare case of gestational diabetes insipidus, placenta-released vasopressinase degrades AVP, counteracting antidiuretic signaling. Similarly, mutations in the gene encoding the V2 receptor (AVPR2) render renal principal cells (Continued) FIGURE 2 | Continued AVP-resistant. Defects in AQP2 protein expression, channel function, intracellular sorting, or membrane translocation disrupt the responsiveness of principal cells to antidiuretic stimuli and maintains low water permeability of the kidney collecting duct. (B) Deregulated accumulation of AVP enhances antidiuretic stimulation of renal principal cells through the V2 receptor and triggers enhanced AQP2-conducted water reabsorption from the primary urine urine (SIADH, heart failure, liver cirrhosis). The same effect is caused by a mutation in the AVPR2 gene constitutively activating the receptor. Both defects abnormally elevate water retention.
AVP as in the syndrome of inappropriate antidiuretic hormone secretion (SIADH), late stage heart failure, and liver cirrhosis cause excessive water retention. Defects preventing the insertion of AQP2 into the plasma membrane lead to diabetes insipidus.

DECREASED AVP-MEDIATED WATER REABSORPTION CAUSES DIABETES INSIPIDUS
Patients suffering from diabetes insipidus display a limited ability to concentrate urine. This leads to polyuria and polydipsia (Knoers, 1993;Bockenhauer and Bichet, 2015), and if untreated to severe dehydration, hypernatremia, and hyperchloremia (Multari et al., 2001;Qureshi et al., 2014). Diabetes insipidus may be inherited or acquired. In central diabetes insipidus (CDI) mutations in the gene encoding AVP lead to insufficient amounts of AVP released from the pituitary gland. The consequence is a compromised concentrating ability of the collecting duct due to a low level of V2R stimulation and abnormally low AQP2 levels, resulting from the lack of AVP-dependent AQP2 gene expression (Ishikawa, 2000). The prevalence of CDI is 1 in 25,000 (www.orpha.net). An animal model reproducing CDI is the Brattleboro rat. The rats lack AVP and the expression levels of AQP2 are decreased (Sokol and Zimmerman, 1982;Kishore et al., 1996).
Mutations in the V2R gene (X-chromosome, Xq28) cause Xlinked NDI, which accounts for ∼90% of all congenital NDI forms (Peters et al., 2007;Heinke and Labudde, 2012). Due to the lack of a functional V2R gene, the AQP2 level is decreased and AQP2 does not translocate into the plasma membrane. The prevalence of X-linked NDI is 3.7 in 1 million males (Bichet et al., 1992). Interestingly, in the Canadian province Nova Scotia the incidence is much higher with 58 in 1 million male live births, a founder effect discussed as the "Hopewell hypothesis" (Bichet et al., 1993;Arthus et al., 2000). The X-linked inheritance is obviously more apparent in males due to the lack of a second X chromosome. Even after administration of dDAVP (see below) the male patients do not show signs of improvement (Knoers and Deen, 2001). An animal model reproducing the phenotype is the inducible V2R knockout mouse model (Li et al., 2009a).
The remaining ∼10% of the persons with congenital autosomal recessive and autosomal dominant forms of NDI carry mutations in the AQP2 gene (Deen et al., 1994;Park et al., 2014). Autosomal dominant forms were observed in patients with frame shift mutations producing a C-terminally elongated AQP2 protein (721delG/727delG) or a mutation leading to an E258K substitution (Mulders et al., 1998;Kuwahara et al., 2001;Marr et al., 2002b). When overexpressed in Xenopus oocytes, the E258K, and AQP2-727delG versions were misrouted to the Golgi and late endosomes, respectively (Mulders et al., 1998;Marr et al., 2002b). The dominant-negative effect seems to be conferred by the formation of heterotetramers with wild type AQP2 that fail to translocate to the plasma membrane in response to AVP (Marr et al., 2002b). Disruption of the PKA consensus sequence by the substitution R254L interferes with the AVP-induced PKA phosphorylation of S256 and also leads to autosomal dominant NDI (de Mattia et al., 2005). The AQP2 version S256L, associated with congenital progressive hydronephrosis, cannot be phosphorylated and causes a severe urine concentration defect in mice, underpinning the importance of S256 for controlling water homeostasis (McDill et al., 2006).
Most of the more than 30 known AQP2 mutations associated with hereditary nephrogenic diabetes insipidus cause autosomal recessive NDI (Fujiwara and Bichet, 2005;Park et al., 2014;Cen et al., 2015). The mutations predominantly cause ER retention and impair protein stability and proper channel function (Deen et al., 1995;Mulders et al., 1997;Goji et al., 1998;Marr et al., 2002a). In comparison to the autosomal dominant mutants, the recessive ones such as AQP2-R187C do not form heterotetramers with wild type AQP2 (Kamsteeg et al., 1999).
There are several animal models available for studying AQP2 in the pathophysiology of NDI (Boone and Deen, 2009). Mice globally lacking AQP2 or mice expressing the T126M mutant die within 2 weeks of age (Yang et al., 2001;Rojek et al., 2006). The collecting duct-specific knockout of AQP2 enabled survival. In addition, mouse models with an amino acid substitution in the PKA phosphorylation site (S256A; McDill et al., 2006), an F204V substitution (Lloyd et al., 2005) or a complete deletion of the C-terminus were established to study recessive NDI (Shi et al., 2007). Sohara et al. presented the first mouse model for autosomal-dominant NDI, in which the assembly of mutant with wild type AQP2 into heterotetramers supported the in vitro findings (Sohara et al., 2006b). Using this model the PDE inhibitor rolipram was identified as a potential drug to treat autosomal-dominant NDI.
The most common form of NDI is acquired NDI. One of the main causes is the long-term treatment of bipolar disorders with lithium. This causes acquired NDI in up to 40% of the patients (Sim et al., 2014). In elderly patients a 6-year prevalence rate of 3% was observed (Rej et al., 2014). The uptake route of lithium into collecting duct principal cells is through the epithelial sodium channel (ENaC). Lithium inhibits adenylyl cyclases and thereby cAMP synthesis and, consequently, the cAMP-dependent signaling that controls AQP2 trafficking and expression. A marked decrease in AQP2 expression in human and rat medullary collecting ducts has been reported with longterm lithium carbonate treatment (Blount et al., 2010).
Gestational DI is another form of acquired NDI. It manifests during pregnancy in the third trimester and in the early postpartum period. During pregnancy, the threshold level of serum osmolality to induce AVP secretion is lowered. The metabolic AVP clearance is increased due to increased levels of the trophoblast-produced aminopeptidase vasopressinase, which can lead to a transient form of DI (Barron et al., 1984;Durr et al., 1987;Davison et al., 1989). Gestational DI has a prevalence of 2-4 in 100,000 pregnancies and can be treated with dDAVP (see below), as this AVP analog is resistant to degradation by vasopressinase (Ananthakrishnan, 2009). Placental abruption can cause acute postpartum DI owing to high amounts of placental vasopressinase released into the circulation (Wallia et al., 2013). Occurrence of gestational DI is elevated in women with preeclampsia, HELLP syndrome (Hemolysis, Elevated Liver enzymes, and Low Platelets) or liver steatosis. These conditions present defective hepatic vasopressinase degradation (Katz and Bowes, 1987;Usta et al., 1994;Ellidokuz et al., 2006).
Bilateral ureteral obstruction (BUO) can cause acquired NDI with a marked decrease in AQP2 abundance (Jensen et al., 2009;Radin et al., 2012). In a rat model, the AQP2 level drops to a fourth of the control within 24 h upon induction of BUO and the phosphorylation of S256 is decreased (Stødkilde et al., 2011). Even 1 week after the release of BUO and when the urine output had become normal, AQP2 levels reached only half those of the controls. Upon dehydration, the affected rats were not able to efficiently concentrate urine (Frøkiaer et al., 1996), indicating that reduced levels of AQP2 are the main limiting factor during recovery from polyuria caused by BUO.

INCREASED AVP-MEDIATED WATER REABSORPTION CAUSES WATER RETENTION AND HYPONATREMIA DISORDERS
In SIADH, late stage heart failure, and liver cirrhosis elevated levels of AVP lead to diluted plasma with hyponatremia with or without hypervolemia. Hyponatremia is a physiological response to maintain an adequate circulating blood volume but turns into an electrolyte disorder when serum sodium drops to less than 135 mEq/L. SIADH is the most frequent cause of hyponatremia, accounting for approximately one-third of all cases (Ishikawa, 2015). The abundance and plasma membrane localization of AQP2 is increased (Kortenoeven et al., 2013), which leads to excessive water retention and hypervolemic (dilutional) hyponatremia (Fenske and Allolio, 2010;Braun et al., 2015). A mutation in the V2R gene that causes nephrogenic syndrome of inappropriate antidiuresis (NSIAD) leads to constitutively active receptors triggering SIADH-like symptoms without elevated AVP levels (Feldman et al., 2005).
In humans with heart failure and in several animal models of heart failure, RAAS system activity, the non-osmotic release of AVP and the expression level and membrane localization of AQP2 are significantly increased. The consequence is water retention with increased circulatory blood volume (Radin et al., 2012;Cui et al., 2015;Ishikawa, 2015). An excessive circulatory volume results in hypervolemic hyponatremia. These patients have a poor prognosis (Noveanu et al., 2011). However, recent studies of heart failure in rat models revealed increased AQP2 protein abundance even in the presence of normal AVP and unchanged plasma sodium levels . Thus in heart failure AQP2 may be up-regulated more generally than previously thought (Brønd et al., 2013).
Liver cirrhosis is associated with a non-osmotic AVP release, water retention and hyponatremia (John and Thuluvath, 2015). The NO concentration in the plasma of cirrhosis patients correlates with the extent to which AQP2 levels are increased (Chung et al., 2010). However, the underlying mechanisms modulating water reabsorption through AQP2, and sodium excretion are unknown (Wilson et al., 2013).
Nephrotic syndrome is similar to heart failure and liver cirrhosis with regard to increased AVP levels and water retention (Apostol et al., 1997;Bou Matar et al., 2012). In contrast to heart failure and liver cirrhosis the nephrotic syndrome shows an "escape" from AVP in the renal collecting ducts. This "escape phenomena" is characterized by a notable down-regulation of AQP2 expression despite high AVP levels. Water balance disorders associated with "escape" phenomena are characterized by an ability to prevent water retention and hyponatremia via secondary natriuresis Verbalis, 2006). A possible mechanism involved in the AVP "escape" includes reduced V2 receptor binding and uncoupling of the V2 receptor from its cognate G protein resulting in abnormally low cytosolic cAMP concentrations upon dDAVP stimulation (Ecelbarger et al., 1998;Tian et al., 2000;Jonassen et al., 2003;Brønd et al., 2004).

Desmopressin (dDAVP)
CDI is commonly treated by the administration of an AVP analog, desmopressin (dDAVP), for example, via a nasal spray or infusion (van Balkom et al., 2004;Kortenoeven and Fenton, 2014). Chronic treatment with dDAVP increases the AQP2 level in apical plasma membranes of both cortical and inner medullary collecting duct principal cells, and decreases urine output usually drastically (Agarwal and Gupta, 2008;Kortenoeven and Fenton, 2014).

Diuretics
The main strategy for the treatment of X-linked NDI is to bypass non-functional V2R and restore physiological expression levels of AQP2 as well as AQP2 trafficking. Thiazides are widely used in patients with NDI. They decrease urine volume, but the mechanism by which they exhibit their paradoxical antidiuretic effect is poorly understood (Kim et al., 2004a). It was proposed that thiazides inhibit reabsorption of sodium and chloride in the distal collecting duct and thus increase the osmolality of the urine (Bockenhauer and Bichet, 2014).
Hydrochlorothiazide (HCTZ) combined with amiloride is the main therapy in patients with lithium-induced NDI (Sinke et al., 2014). Treatment with thiazides can decrease the glomerular filtration rate, which causes a reduced excretion of lithium, resulting in lithium intoxication. Amiloride, a potassium sparing diuretic, interferes with lithium uptake by blocking ENaC, and for this reason is combined with thiazides (Bedford et al., 2008;Kishore et al., 2015). Also a combination of HCTZ with the NSAID, indomethacin (see below) reduces urine output more effectively than the administration of thiazides alone (Kortenoeven and Fenton, 2014). The expression of AQP2 was substantially reduced in the collecting ducts of lithium-treated rats and was increased in the presence of amiloride (Bedford et al., 2008).

Statins
Statins are lipid-lowering drugs used to reduce cardiovascular risks. They competitively inhibit hepatic 3-hydroxy-3methylglutaryl-coenzyme A reductase catalyzing the conversion of HMG-CoA to mevalonate. Thereby statins inhibit the production of isoprenoid intermediates such as farnesyl pyrophosphate (FPP), geranylgeranyl pyrophosphate (GGPP). A range of proteins undergoes post-translational modifications by addition of isoprenoid pyrophosphates (Verhulst et al., 2004). FPP and GGPP act as lipid anchors required for membrane tethering and activation of heterotrimeric G proteins and small GTPases, amongst them RhoA (Li et al., 2011;Szygula-Jurkiewicz et al., 2014). Statins inhibit RhoA isoprenylation, leading to the accumulation of inactive RhoA in the cytoplasm (Liao and Laufs, 2005).
In MCD4 cells, mouse collecting duct cells stably expressing AQP2, statins inhibit the isoprenylation and thus activation of RhoA (Procino et al., 2010). This causes F-actin depolymerization and an AVP-independent increase in AQP2 plasma membrane localization Procino et al., 2010Procino et al., , 2011aLi et al., 2011;Wade, 2011). Statins also activate the NO/cGMP pathway (Tamma et al., 2003a;Rikitake and Liao, 2005;Maher et al., 2009) and thus may induce AQP2 phosphorylation at S256 and thereby induce the insertion into the plasma membrane. Incubation of LLC-PK 1 and MCD4 cells with simvastatin resulted in AQP2 plasma membrane accumulation in a dosedependent manner due to reduced clathrin-mediated endocytosis (Procino et al., 2012). Short-term exposure to simvastatin produced no change in cholesterol plasma membrane levels, but increased AQP2 accumulation in the apical membrane of principal cells in Brattleboro rats. This increased water reabsorption and urine concentration (Li et al., 2011;Bonfrate et al., 2015). In adult Munich-Wistar male rats subjected to BUO for 2 h atorvastatin improved urinary concentrating ability by reversing ureteral obstruction-induced down-regulation of AQP2 (Danilovic et al., 2012). Fluvastatin increased AQP2 plasma membrane abundance in wild-type mice. In the same study when X-linked NDI mice were treated with secretin plus fluvastatin, urine volume was reduced by nearly 90% and the urine osmolality was doubled. Fluvastatin promoted AQP2 trafficking to the plasma membrane whereas secretin increased intracellular stores of AQP2 (Procino et al., 2014). Additional experiments in mice indicated that fluvastatin increases AQP2 membrane accumulation by altering the prenylation status of key proteins regulating AQP2 trafficking such as RhoA (Procino et al., 2011a).
In a recent study, 24 naïve hypercholesterolemic patients were treated with simvastatin (20 mg/day for 12 weeks). This rapidly and significantly enhanced urine excretion of AQP2, and decreased the 24 h diuresis while urine osmolality increased. This occurred similarly and persistently in patients who were treated with statins in the long-term, i.e., for at least 1 year .
Collectively, the data suggest that statins provide novel opportunities for the treatment of X-linked NDI as their target is independent from V2R.

Prostaglandins
Prostaglandin E 2 (PGE 2 ) is synthesized and released in the collecting duct . It is one of the major cyclooxygenated metabolites of arachidonic acid that stimulates all four G protein-coupled E-prostanoid receptors, EP 1 -EP 4 (Hao and Breyer, 2008). PGE 2 /EP 1 signaling elicits a diuretic response via the Gα q -mediated activation of PKC, triggering AQP2 internalization (Bachteeva et al., 2007). However, an EP 1 -mediated decrease in AQP2-induced water permeability was only observed in frog urinary bladder, but not in collecting ducts (Bachteeva et al., 2007). EP 3 is coupled to Gα i and reduces cAMP synthesis upon activation (Sugimoto et al., 1992), thus decreasing AQP2 expression and membrane localization (Zelenina et al., 2000). EP 3 -signaling also stimulates RhoA (Tamma et al., 2003b), probably mediated by the Gα 12/13 -dependent activation of Rho guanine nucleotide exchange factors (GEFs) (Yamaguchi et al., 2000;Wells et al., 2002). GEFs directly activate RhoA and thus attenuate AQP2 membrane trafficking Tamma et al., 2001). The activation of the different signaling pathways is most likely due to different EP 3 splice variants (Namba et al., 1993;Hatae et al., 2002). The observation of various cellular responses might also be the result of the differential expression of PGE 2 receptors in model systems derived from different renal sections of diverse species (Olesen and Fenton, 2013). EP 2 and EP 4 couple to Gα s (Alexander et al., 2011). The stimulation of each receptor results in the increased phosphorylation of AQP2-S264, whose effect is presently unknown . Only EP 2 signaling increases the cytosolic cAMP concentration AVP-independently (Breyer and Breyer, 2001) and stimulates the phosphorylation of AQP2 at S256 and S269 . However, there are contradictory data on whether EP 2 is expressed in the collecting duct at all (Sugimoto et al., 1994;Breyer et al., 1996;Morath et al., 1999;Jensen et al., 2001;Regan, 2003;Hao and Breyer, 2008). EP 4 is widely spread in the collecting duct (Sugimoto and Narumiya, 2007). Its activation enhances AQP2 membrane trafficking, although the cytosolic cAMP concentration remains unaffected . This might result from promiscuous G protein coupling (Fujino and Regan, 2006) which may occur upon PKA phosphorylation of EP 4 (Olesen and Fenton, 2013). In line, EP 4 activation stimulates phosphatidylinositol 3-kinase (PI3K) (Fujino et al., 2003), which is suggested to regulate AQP2 expression and endosomal retrieval (Tajika et al., 2004;Bustamante et al., 2005). Altogether, the molecular details of PGE 2 signaling remain to be completely understood.
Non-steroidal anti-inflammatory drugs (NSAIDs) block the production of PGE 2 , other prostaglandins and thromboxanes by inhibition of the cyclooxygenases, COX-1 and COX-2 (Jin, 2015). NSAIDs are commonly used in the treatment of pain and inflammation. Their use in cell culture and animal models has shed some light on the role of prostaglandins such as PGE 2 in the control of water homeostasis (Baggaley et al., 2010;Jia et al., 2012;Kortenoeven et al., 2012a).
Recently, effects of the chronic use of the NSAIDs, ibuprofen and meloxicam, on AQP2 were investigated in kidneys from Sprague-Dawley rats. The daily application of the drugs enhanced urine output and significantly reduced urine osmolality after 7-14 days. In line, both drugs significantly reduced the AQP2 protein abundance by 60-70%. Meloxicam significantly increased AQP2 phosphorylated at S256 and S261, whereas ibuprofen increased the AQP2 phosphorylated at S256 but did not affect the phosphorylation of AQP2 at S261. Both ibuprofen and meloxicam increased the phosphorylation of AQP2 at S264 and S269. The observed alterations in the C-terminal phosphorylations of AQP2 may partially compensate for the decrease in AQP2 abundance by promoting its membrane localization and thereby permitting only a limited effect on urine osmolality (Ren et al., 2015).
Indomethacin enhances the antidiuretic activity of AVP and markedly reduces urine output by increasing the AQP2 plasma membrane insertion. This effect of indomethacin is additive with that of thiazide diuretics (Kim et al., 2004b).
In a study where male Wistar rats were treated with indomethacin, ibuprofen and meloxicam AQP2 expression was significantly decreased in water-restricted rats but PGE 2 excretion did not significantly change. This effect was less pronounced in water-loaded rats and meloxicam had no significant effect. The stronger effect of NSAIDs on the AQP2 protein level during water restriction seems most relevant for elderly and severely ill patients who are prone to dehydration (Baggaley et al., 2010;Lauridsen et al., 2010).
Stimulation of EP 2 and EP 4 increases AQP2 trafficking via increased cAMP. The EP 2 agonist, butaprost relieved symptoms of NDI in a rat model (Gao et al., 2015;Olesen et al., 2011). Pharmacological blocking of V2R in rats results in NDI, which could be greatly mitigated via the administration of an EP 2 agonist. EP 2 agonists are already used in the treatment of dysmenorrhea (Moeller et al., 2013). Moreover, selective EP 4 agonists relieved NDI symptoms developed in V2R genedeficient mice (Bockenhauer and Bichet, 2014). In a mouse model for X-linked NDI, an EP 4 receptor agonist, [ONO-AE-329 (ONO)] efficiently increased urine osmolality and reduced polyuria.
Taken together a possible strategy for the treatment of NDI might be the stimulation of EP 2 and/or EP 4 and simultaneous blocking of EP 3 . PGE 2 is particularly important for lithium-induced diabetes insipidus. Increased expression of COX-2 and abnormally high urinary excretion of PGE 2 has been noted in animal models suffering from lithium-induced NDI. Lithium-treated patients have elevated urine PGE 2 excretion as well (Kim et al., 2008). Administration of COX-2 inhibitors to adult mice and rats relieved the lithium-associated polyuria. Indomethacin blocks prostaglandin production in lithium-treated mpkCCD cells. Additionally, PGE 2 synthase-1 (mPGES-1)-deficient mice did not develop lithium-associated polyuria (Jia et al., 2009;Baggaley et al., 2010;Kjaersgaard et al., 2014). In clinical practice inhibition of prostaglandin synthesis with NSAIDs remains an effective therapy for lithium-induced NDI (Kortenoeven et al., 2012a).

PDE Inhibitors
An increase in cGMP levels, e.g., by ANP or the nitric oxide (NO) donor sodium nitroprusside activates protein kinase G (PKG). PKG can phosphorylate AQP2 at S256 to induce its redistribution into the plasma membrane (Bouley et al., 2000). PDE5 specifically hydrolyses cGMP and thereby prevents cGMP/PKG signaling. PDE5 is present in the principal cells of the collecting duct along with further PDEs, including PDE1 and PDE4 (Dousa, 1999). Inhibition of PDE5, in particular with sildenafil (Viagra) causes AQP2 plasma membrane accumulation in LLC-PK 1 cells. This effect was attributed to the activation of the NO/cGMP pathway (Bouley et al., 2000(Bouley et al., , 2005 as also sodium nitroprusside, L-arginine (a precursor of NO) and ANP induce plasma membrane accumulation of AQP2 in vitro, and ANP also in vivo . In line, mice deficient in NO synthase isoforms developed NDI (Morishita et al., 2005a). PDE5 inhibitors and ANP combined cause natriuresis and decreased urine output (Bouley et al., 2005;Assadi and Ghane Sharbaf, 2015). Therefore, the activation of AVP-independent cGMP signaling with sildenafil could be an option in the treatment of X-linked NDI as it bypasses the defective V2R/cAMP axis . Sildenafil is already used for the treatment of erectile dysfunction, pulmonary hypertension and subarachnoid hemorrhage.
The PDE4 family of enzymes (PDE4A, PDE4B, PDE4C, and PDE4D) encodes more than 20 isozymes (Houslay, 2010;Klussmann, 2015). Rolipram selectively inhibits the PDE4 isozymes by targeting the catalytic domain of the enzymes. Rolipram enhances the AVP-induced accumulation of cAMP, the translocation of AQP2 into the plasma membrane and the osmotic water permeability of cultured IMCD cells . Moreover, rolipram increased urine osmolality in a mouse model of autosomal dominant NDI (Sohara et al., 2006b). However, the clinical trial with rolipram failed (Bichet et al., 1990) because of its intolerable side effects, mainly nausea and vomiting. Recently, roflumilast, and apremilast, both nonselective inhibitors of PDE4A-D, with fewer side effects reached the market. Roflumilast is approved for the treatment of chronic obstructive pulmonary disease (COPD) and apremilast for psoriatic arthritis (Klussmann, 2015). These new PDE4 inhibitors may provide a new opportunity for the therapy of X-linked NDI.

Pharmacological Chaperones
Cell-permeable pharmacological chaperons have proven their efficacy in restoring the plasma membrane localization and functional rescue of V2R. Vaptans (see below) not only block the V2R but can also act as such chaperones. The cellpermeable V2R antagonists S121463 and VPA-985 stabilize ERretained V2R mutants, so that the receptor can be released from the ER and inserted into the plasma membrane (Morello et al., 2000;Wüller et al., 2004). These positive effects of pharmacological chaperons were reproducible in Madin-Darby canine kidney (MDCK) cells overexpressing nine ER-retained V2R mutants involved in NDI (Robben et al., 2006). SR49059 (Conivaptan), a non-selective blocker of V2R and vasopressin V1a receptors (Bernier et al., 2004), has been tested in NDI patients where it decreased urine output and was thus suggested for the treatment of X-linked NDI (Bernier et al., 2006). The chaperones are active in nanomolar concentrations on cultured cells. Regarding NDI treatment the high affinity OPC31260 (mozavaptan) and OPC41061 (tolvaptan) nonpeptidic antagonists would promise beneficial effects in terms of V2R rescue, low doses and efficient substitution for AVP Wesche et al., 2012). On the other hand, since low affinity antagonists require higher doses for clinical efficacy this could lead to hepatic toxicity because of potential interference with cytochrome P450 systems. Hence, further studies using modified antagonists or treatment regimes are required.
Vice versa, agonists binding to V2R mutants induced the production of sufficient amounts of cAMP and the translocation of AQP2 into the apical plasma membrane of renal cells. Biased agonists of the V2R represent a novel therapeutic strategy for the treatment of congenital NDI. The non-peptidic agonists MCF14 (OPC23h), MCF18 (VNA932), and MCF57 are full agonist of the V2R but do not induce receptor internalization and arrestin-associated signaling. From a clinical perspective these properties can be of great importance as they can have long-lasting effects (Mouillac and Mendre, 2014).
Both strategies with antagonists or agonists are not applicable if V2R is not at least partially functional or if it is insensitive to such agents (Moeller et al., 2013;Bockenhauer and Bichet, 2014).

Activators of the AQP2 Shuttle
An unbiased screening of 3646 compounds for stimulators of the redistribution of AQP2 to the apical plasma membrane of LLC-PK 1 cells identified the JAK-2 and EGFR inhibitor AG-490. The molecule induced the AQP2 accumulation at the plasma membranes of LLC-PK 1 and Madin-Darby canine kidney (MDCK) cells and at that of principal cells in rat kidney sections. AG-490 treatment of AVPdeficient Brattleboro rats improved their urine concentrating capacity within 2 h Sands and Blount, 2014).

PHARMACOLOGICAL TREATMENTS OF WATER BALANCE DISORDERS WITH ENHANCED AVP-MEDIATED WATER REABSORPTION AND NOVEL AVENUES FOR THERAPY Vaptans
Vaptans, also termed aquaretics, comprise a family of V2R antagonists. They include intravenously administered conivaptan and oral vaptans like tolvaptan, lixivaptan, and satavaptan (Narayen and Mandal, 2012). Tolvaptan, the most extensively used vaptan, selectively inhibits the binding of AVP to the V2R with an almost two times higher affinity than AVP, and interferes with the AQP2 redistribution into the plasma membrane of collecting duct principal cells (Sato et al., 2014). Patients treated with tolvaptan have increased urine AQP2 levels (Sato et al., 2014), indicative of shedding of AQP2 from the apical plasma membrane into the urine.
The aquaretic effect is achieved by enhanced free water clearance without induction of natriuresis, kaliuresis or changes of the glomerular filtration rate (Graziani et al., 2014). Vaptans are approved for the therapy of hypervolemic (heart failure, liver cirrhosis, SIADH) (Narayen and Mandal, 2012) and normovolemic hyponatremia, and since recently for autosomal dominant polycystic kidney disease (Lin et al., 2014;Blair and Keating, 2015). In particular, the adjustment of sodium levels in patients suffering from SIADH and hyponatremia can be easier controlled with vaptans than with conventional hypertonic saline infusions. This translates into a better safety profile, and a lower risk of side effects (Nathan, 2007). Heart failure patients are treated with conventional anti-hypertensive drugs and only since recently additionally with V2 receptor antagonists (vaptans, see below) if hospitalized (Gilotra and Russell, 2014). More than half of the cirrhosis patients suffer from ascites, a major complication associated with sodium and AVP-dependent water retention that is characterized by the accumulation of fluid in the abdominal cavity. Hence, V2R antagonists could prove beneficial in the symptomatic treatment of this condition by increasing urine output (Yan et al., 2015).

Demeclocycline
Demeclocycline, a bacteriostatic antibiotic of the tetracycline group, is used for the treatment of sustained hyponatremia in patients with SIADH. It is the first choice in this case because of its aquaretic effect. Demeclocycline decreases the AVP-induced cAMP production and abundance of adenylyl cyclases 3 and 5/6, and reduces the expression of AQP2 in the apical plasma membrane of mpkCCD cells without affecting the V2R. In a rat model of SIADH, demeclocycline increased urine volume, decreased urine osmolality and alleviated hyponatremia as has been shown in patients (Kortenoeven et al., 2013). Other tetracycline antibiotics including minocycline, doxycycline and tetracycline have similar effects with regard to reducing urine volume in toads (Feldman and Singer, 1974;Hirji and Mucklow, 1991). Tetracycline reduces urinary concentrating ability also in humans (Wilson et al., 1973). However, demeclocycline (600-1200 mg/day) may also cause NDI in some patients, and that is the reason for the limited application of these drugs in clinical practice (Gross, 2012).

PHARMACOLOGICAL TARGETING OF PROTEIN-PROTEIN INTERACTIONS FOR INTERFERENCE WITH AQP2 TRAFFICKING
Protein-protein interactions play key roles in all biological processes. They are diverse and highly specific and, therefore, provide ample opportunities for the development of novel drugs (Jones and Thornton, 1996;Klussmann and Rosenthal, 2008;Cho et al., 2016). Since AKAP-PKA interactions are involved in AQP2 trafficking (Klussmann et al., 1999;Klussmann and Rosenthal, 2001), their disruption may lead to a new concept for the treatment of diseases characterized by excessive AVPmediated water retention such as heart failure (Nedvetsky et al., 2009;Tröger et al., 2012;Deak and Klussmann, 2015;Dema et al., 2015). Indeed, peptides, peptidomimetics, and small molecules for the non-selective disruption of AKAP-PKA interactions are available (Hundsrucker et al., 2006a,b;Hundsrucker and Klussmann, 2008;Christian et al., 2011;Schäfer et al., 2013;Schächterle et al., 2015). All of these agents target the AKAP-binding domain of regulatory R subunits of PKA, the dimerization/docking (D/D) domain. They bind competitively and thereby prevent the interactions of R subunits with AKAPs (Deak and Klussmann, 2015). The peptides such as Ht31, AKAP18-derived peptides or AKAP IS all bind R subunits with high affinity and thereby effectively disrupt AKAP-PKA interactions. However, short half-life, low bioavailability, and membrane permeability limit the development of peptide-based drugs, although several peptide-drugs reached the market (Otvos and Wade, 2014). The available peptidomimetics are low affinity binders and require a multistep synthesis. They would require further optimization prior to becoming candidates for drug development (Dema et al., 2015). Most promising are small molecule inhibitors of protein-protein interactions. With good bioavailability and enhanced plasma membrane permeability they can be optimized toward effective inhibitors. However, the development is challenging, because protein-protein interaction interfaces are generally large and shallow and can hardly be completely covered by small molecules. Despite this, in the past years remarkable progress has been made and several small molecule inhibitors of protein-protein interactions were discovered (Sheng et al., 2015). The only small molecule inhibiting AKAP-PKA interactions, FMP-API-1, is not suitable for further development toward a drug candidate as it also activates PKA (Christian et al., 2011;Yu et al., 2014).
In order to identify small molecule inhibitors of the AVPinduced redistribution of AQP2, high-throughput screening of small molecule libraries provides ample opportunities. A prototypical example is a screening of 17,700 small molecules for inhibitors of the cAMP-induced redistribution of AQP2 using a cell-based assay (Bogum et al., 2013). This approach yielded 17 inhibitors, among them, 4-acetyldiphyllin (4-AD) a selective blocker of V-ATPase. 4AD increased pH levels of intracellular vesicles and caused an accumulation of AQP2 in the Golgi. The compound prevented the PKA-induced phosphorylation of AQP2 at S256 without affecting cAMP levels, PKA activity or the phosphorylations of AQP2 at S261 and S269. Although the screening approach did not yield an inhibitor of PPIs, it is suitable for identification of small molecules that may serve as a starting point for the development of novel drug candidates for the therapy of disorders associated with increased AQP2 levels (Bogum et al., 2013).

SUMMARY AND OUTLOOK
The localization of AQP2 is critical for the reabsorption of water. If located in the plasma membrane of renal collecting duct principal cells AQP2 facilitates water reabsorption from primary urine. A predominant localization in the plasma membrane contributes to excessive water retention as in heart failure, SIADH or liver cirrhosis; vice versa a predominant intracellular localization causes diabetes insipidus, a disease characterized by a massive loss of hypotonic urine. A variety of cAMP-dependent and independent signaling pathways and defined protein-protein interactions have been identified that control the localization of AQP2. Considering the still unmet medical need in both classes of diseases, i.e., diseases with a high-or a low-level plasma membrane localization of AQP2, pharmacological targeting of the mechanisms controlling AQP2 trafficking provides ample opportunities for the development of novel drugs.
Several established drugs modulate the localization of AQP2 in animal models, and first data from patients confirmed these effects. Examples are statins that promote or NSAIDs and vaptans that prevent the plasma membrane localization of AQP2. Further clinical trials will have to confirm the value of repurposing such drugs toward the treatment of water balance disorders. However, all of these established drugs cause unwanted side effects, which may limit their use. Therefore, it will be necessary to continue with the identification of molecular mechanisms underlying the control of the localization of AQP2 to provide novel drug targets. In particular, identifying protein-protein interactions that govern the control of AQP2 is relevant as protein-protein interactions are highly specific and diverse and therefore represent a unique class of drug targets. AKAP-PKA interactions that are essential for the AVP-induced redistribution of AQP2 are an example (Christian et al., 2011;Schächterle et al., 2015). It is important to define "hot spots" within the interacting surfaces, i.e., crucial amino acids that mediate the interaction and that can be targeted with small molecules for effective disruption (Buchwald, 2010;Villoutreix et al., 2014;Sheng et al., 2015). For identifying small molecule inhibitors of protein-protein interactions highthroughput screening may be used. This approach has led to the identification of FMP-API-1, the first small molecule disruptor of AKAP-PKA interactions (Keskin et al., 2008;Christian et al., 2011).
Other unbiased screening approaches have identified small molecule modulators of the AVP-induced redistribution of AQP2. The identification of the inhibitor 4AD for example provided new insight into the regulation of AQP2. It identified V-ATPase as a protein involved in the control of AQP2 (Bogum et al., 2013). Another example is the identification of a small molecule activator of the AQP2 redistribution, AG-490. AG-490 inhibits the EGF receptor and JAK-2 kinase . Even though targeting a ubiquitous protein such as V-ATPase or the EGF receptor may not be an option for the treatment of patients, the screening approaches are valuable for the identification of new molecules and targets and may provide new starting points for the development of novel drugs for the treatment of water balance disorders.
Taken together, many questions surrounding water balance disorders have been answered but even more remain unanswered. A particular topic in the future will be to translate the insights into molecular mechanisms underlying the trafficking of AQP2 into novel therapies for these diseases with their unmet medical needs.

AUTHOR CONTRIBUTIONS
TV and MS contributed equally. TV wrote most of the pharmacology section, MS most of the section relating to molecular mechanisms underlying AQP2 trafficking and the figures. DF contributed the tables and to sections about AQP2 trafficking. EK designed the study wrote parts of all sections and edited the manuscript.