The prostaglandin E2 analogue sulprostone antagonizes vasopressin-induced antidiuresis through activation of Rho

Dysregulation of renal aquaporins and epithelial sodium channel in lithium-induced nephrogenic diabetes insipidus

Lithium is used commonly to treat bipolar mood disorders. In addition to its primary therapeutic effects in the central nervous system lithium has a number of side effects in the kidney. The side effects include nephrogenic diabetes insipidus with polyuria, mild sodium wasting, and changes in acid/base balance. These functional changes are associated with marked structural changes in collecting duct cell composition and morphology, likely contributing to the functional changes. Over the past few years, investigations of lithium-induced renal changes have provided novel insight into the molecular mechanisms that are responsible for the disturbances in water, sodium, and acid/base metabolism. This includes dysregulation of renal aquaporins, epithelial sodium channel, and acid/base transporters. This review focuses on these issues with the aim to present this in context with clinically relevant features.

Involvement of non-conserved residues important for PGE2 binding to the constrained EP3 eLP2 using NMR and site-directed mutagenesis

A peptide constrained to a conformation of second extracellular loop of human prostaglandin-E(2) (PGE(2)) receptor subtype3 (hEP3) was synthesized. The contacts between the peptide residues at S211 and R214, and PGE(2) were first identified by NMR spectroscopy. The results were used as a guide for site-directed mutagenesis of the hEP3 protein. The S211L and R214L mutants expressed in HEK293 cells lost binding to [(3)H]PGE(2). This study found that the non-conserved S211 and R214 of the hEP3 are involved in PGE(2) recognition, and implied that the corresponding residues in other subtype receptors could be important to distinguish the different configurations of PGE(2) ligand recognition sites.

Reciprocal interaction with G-actin and tropomyosin is essential for aquaporin-2 trafficking

Trafficking of water channel aquaporin-2 (AQP2) to the apical membrane and its vasopressin and protein kinase A (PKA)–dependent regulation in renal collecting ducts is critical for body water homeostasis. We previously identified an AQP2 binding protein complex including actin and tropomyosin-5b (TM5b). We show that dynamic interactions between AQP2 and the actin cytoskeleton are critical for initiating AQP2 apical targeting. Specific binding of AQP2 to G-actin in reconstituted liposomes is negatively regulated by PKA phosphorylation. Dual color fluorescence cross-correlation spectroscopy reveals local AQP2 interaction with G-actin in live epithelial cells at single-molecule resolution. Cyclic adenosine monophosphate signaling and AQP2 phosphorylation release AQP2 from G-actin. In turn, AQP2 phosphorylation increases its affinity to TM5b, resulting in reduction of TM5b bound to F-actin, subsequently inducing F-actin destabilization. RNA interference–mediated knockdown and overexpression of TM5b confirm its inhibitory role in apical trafficking of AQP2. These findings indicate a novel mechanism of channel protein trafficking, in which the channel protein itself critically regulates local actin reorganization to initiate its movement.

Mechanisms of the urinary concentration defect and effect of desmopressin during endotoxemia in rats

Acute renal failure during human sepsis is often nonoliguric. To study the underlying mechanisms, renal function was assessed in endotoxic and control male Wistar rats during and after saline loading and treatment with the selective V2 receptor agonist desmopressin. Escherichia coli endotoxin (dose, 8 mg/kg) was administered from time (t)=0 to t=60 min; saline loading (rate, 5 mL/100 g per hour) was administered from t=0 to t=120 min. Thereafter, half of each group received desmopressin (dose, 10 microg) for 1 h. The inner medullary (IM) osmolality, hematocrit, plasma, and urinary concentrations of sodium, potassium, urea, and osmolality were measured; then, aquaporin 2 (AQP2) immunohistochemistry was performed. Plasma vasopressin concentrations were measured at t=180 min. Saline loading increased urine volume in all rats. In the endotoxic group, mean arterial pressure decreased when saline loading was stopped. Despite increased hematocrit and vasopressin levels (>16 pg/mL), the endotoxin group had a low IM osmolality (mean +/- SEM, 412+/-0.04 mOsm/kg H2O) in comparison with the control group (mean +/- SEM, 1,094+/-0.17 mOsm/kg H2O) and was not able to either decrease urine volume or raise urine osmolality. Desmopressin treatment in endotoxin-treated rats maintained mean arterial pressure, increased sodium reabsorption, IM osmolality, and urine osmolality, and decreased urine flow. The AQP2 intensity decreased in the endotoxin group, and the apical localization disappeared; both were not affected by desmopressin. Our results indicate that endotoxemia in rats acutely diminishes renal urinary concentration capacity and is associated with a decreased IM osmolality and diminished apical AQP2 localization. These findings may help to explain nonoliguric acute renal failure in human septic shock.

Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption

To prevent dehydration, terrestrial animals and humans have developed a sensitive and versatile system to maintain their water homeostasis. In states of hypernatremia or hypovolemia, the antidiuretic hormone vasopressin (AVP) is released from the pituitary and binds its type-2 receptor in renal principal cells. This triggers an intracellular cAMP signaling cascade, which phosphorylates aquaporin-2 (AQP2) and targets the channel to the apical plasma membrane. Driven by an osmotic gradient, pro-urinary water then passes the membrane through AQP2 and leaves the cell on the basolateral side via AQP3 and AQP4 water channels. When water homeostasis is restored, AVP levels decline, and AQP2 is internalized from the plasma membrane, leaving the plasma membrane watertight again. The action of AVP is counterbalanced by several hormones like prostaglandin E2, bradykinin, dopamine, endothelin-1, acetylcholine, epidermal growth factor, and purines. Moreover, AQP2 is strongly involved in the pathophysiology of disorders characterized by renal concentrating defects, as well as conditions associated with severe water retention. This review focuses on our recent increase in understanding of the molecular mechanisms underlying AVP-regulated renal water transport in both health and disease.

Dysregulation of Renal Cyclooxygenase-2 in Rats with Lithium-induced Nephrogenic Diabetes Insipidus

This study aimed to examine whether the expression of major prostaglandin E₂ (PGE₂) synthesis enzyme, cyclooxygenase-2 (COX-2), is changed in the kidneys of the rats with lithium-induced nephrogenic diabetes insipidus (Li-NDI). Sprague-Dawley rats treated with lithium for 4 weeks were used as the NDI model and expression of renal COX-2 was determined by immunoblotting and immunohistochemistry. In Li-NDI where urine output was markedly increased and urine osmolality was significantly decreased, COX-2 expression in the inner medulla was decreased (28% of control), while it increased 18-fold in the cortex and outer medulla. Consistent with this, labeling intensity of COX-2 in macula densa region was increased, whereas it was decreased in the interstitial cells in the inner medulla, indicating a differential regulation of COX-2 between the cortex and inner medulla in Li-NDI. Accordingly, urinary PGE₂ excretion was significantly increased in Li-NDI. In conclusion, there is a differential regulation of COX-2 between cortex and inner medulla in Li-NDI and urinary PGE₂ excretion is increased in Li-NDI, possibly due to an increased renal production. This may suggest that increased renal production of PGE₂ could play a role in modulating water reabsorption in the renal collecting duct in Li-NDI.

Structural and functional changes in epitheliocytes of collecting tubes in renal papilla of Brattleboro rats treated with vasopressin

The functional response of the kidney to desmopressin and morphological changes in epitheliocytes of collecting tubes were studied on homozygotic Brattleboro rats. Redistribution of beta-glucuronidase fractions and increase in the number of osmiophilic granules reflecting increased production of vasopressin-dependent proteins and hyaluronate hydrolase exocytosis were typical structural correlates of the effect of vasopressin.

Microtubules are needed for the perinuclear positioning of aquaporin-2 after its endocytic retrieval in renal principal cells

Water reabsorption in the renal collecting duct is regulated by arginine vasopressin (AVP). AVP induces the insertion of the water channel aquaporin-2 (AQP2) into the plasma membrane of principal cells, thereby increasing the osmotic water permeability. The redistribution of AQP2 to the plasma membrane is a cAMP-dependent process and thus a paradigm for cAMP-controlled exocytic processes. Using primary cultured rat inner medullary collecting duct cells, we show that the redistribution of AQP2 to the plasma membrane is accompanied by the reorganization of microtubules and the redistribution of the small GTPase Rab11. In resting cells, AQP2 is colocalized with Rab11 perinuclearly. AVP induced the redistribution of AQP2 to the plasma membrane and of Rab11 to the cell periphery. The redistribution of both proteins was increased when microtubules were depolymerized by nocodazole. In addition, the depolymerization of microtubules prevented the perinuclear positioning of AQP2 and Rab11 in resting cells, which was restored if nocodazole was washed out and microtubules repolymerized. After internalization of AQP2, induced by removal of AVP, forskolin triggered the AQP2 redistribution to the plasma membrane even if microtubules were depolymerized and without the previous positioning of AQP2 in the perinuclear recycling compartment. Collectively, the data indicate that microtubule-dependent transport of AQP2 is predominantly responsible for trafficking and localization of AQP2 inside the cell after its internalization but not for the exocytic transport of the water channel. We also demonstrate that cAMP-signaling regulates the localization of Rab11-positive recycling endosomes in renal principal cells.

EP2 receptor mediates PGE2-induced cystogenesis of human renal epithelial cells

Autosomal-dominant polycystic kidney disease (ADPKD) is characterized by formation of cysts from tubular epithelial cells. Previous studies indicate that secretion of prostaglandin E2 (PGE2) into cyst fluid and production of cAMP underlie cyst expansion. However, the mechanism by which PGE2 directly stimulates cAMP formation and modulates cystogenesis is still unclear, because the particular E-prostanoid (EP) receptor mediating the PGE2 effect has not been characterized. Our goal is to define the PGE2 receptor subtype involved in ADPKD. We used a three-dimensional cell-culture system of human epithelial cells from normal and ADPKD kidneys in primary cultures to demonstrate that PGE2 induces cyst formation. Biochemical evidence gathered by using real-time RT-PCR mRNA analysis and immunodetection indicate the presence of EP2 receptor in cystic epithelial cells in ADPKD kidney. Pharmacological evidence obtained by using PGE2-selective analogs further demonstrates that EP2 mediates cAMP formation and cystogenesis. Functional evidence for a role of EP2 receptor in mediating cAMP signaling was also provided by inhibiting EP2 receptor expression with transfection of small interfering RNA in cystic epithelial cells. Our results indicate that PGE2 produced in cyst fluid binds to adjacent EP2 receptors located on the apical side of cysts and stimulates EP2 receptor expression. PGE2 binding to EP2 receptor leads to cAMP signaling and cystogenesis by a mechanism that involves protection of cystic epithelial cells from apoptosis. The role of EP2 receptor in mediating the PGE2 effect on stimulating cyst formation may have direct pharmacological implications for the treatment of polycystic kidney disease.

Prostaglandin E2 inhibits vasotocin-induced osmotic water permeability in the frog urinary bladder by EP1-receptor-mediated activation of NO/cGMP pathway

PGE(2) is a well-known inhibitor of the antidiuretic hormone-induced increase of osmotic water permeability (OWP) in different osmoregulatory epithelia; however, the mechanisms underlying this effect of PGE(2) are not completely understood. Here, we report that, in the frog Rana temporaria urinary bladder, EP(1)-receptor-mediated inhibition of arginine-vasotocin (AVT)-induced OWP by PGE(2) is attributed to increased generation of nitric oxide (NO) in epithelial cells. It was shown that the inhibitory effect of 17-phenyl-trinor-PGE(2) (17-ph-PGE(2)), an EP(1) agonist, on AVT-induced OWP was significantly reduced in the presence of 7-nitroindazole (7-NI), a neuronal NO synthase (nNOS) inhibitor. NO synthase (NOS) activity in both lysed and intact epithelial cells measured as a rate of conversion of l-[(3)H]arginine to l-[(3)H]citrulline was Ca(2+) dependent and inhibited by 7-NI. PGE(2) and 17-ph-PGE(2), but not M&B-28767 (EP(3) agonist) or butaprost (EP(2) agonist), stimulated NOS activity in epithelial cells. The above effect of PGE(2) was abolished in the presence of SC-19220, an EP(1) antagonist. 7-NI reduced the stimulatory effect of 17-ph-PGE(2) on NOS activity. 17-ph-PGE(2) increased intracellular Ca(2+) concentration and cGMP in epithelial cells. Western blot analysis revealed an nNOS expression in epithelial cells. These results show that the inhibitory effect of PGE(2) on AVT-induced OWP in the frog urinary bladder is based at least partly on EP(1)-receptor-mediated activation of the NO/cGMP pathway, suggesting a novel cross talk between AVT, PGE(2), and nNOS that may be important in the regulation of water transport.

Urine concentrating defect in prostaglandin EP1-deficient mice

We investigated the role of the prostaglandin E2(PGE2) EP1receptor in modulating urine concentration as it is expressed along the renal collecting duct where arginine-vasopressin (AVP) exerts its anti-diuretic activity, and in the paraventricular and supraoptic nuclei of the hypothalamus where AVP is synthesized. The urine osmolality of EP1-null mice (EP1−/−) failed to match levels achieved by wild-type (WT) counterparts upon water deprivation (WD) for 24 h. This difference was reflected by higher plasma osmolality in WD EP1−/−mice. Along the collecting duct, the induction and subapical to plasma membrane translocation of the aquaporin-2 water channel in WD EP1−/−mice appeared equivalent to that of WD WT mice as determined by quantitative RT-PCR and immunohistochemistry. However, medullary interstitial osmolalities dropped significantly in EP1−/−mice following WD. Furthermore, urinary AVP levels of WD EP1−/−mice were significantly lower than those of WD WT mice. This deficit could be traced back to a blunted induction of hypothalamic AVP mRNA expression in WD EP1−/−mice as determined by quantitative RT-PCR. Administration of the AVP mimetic [deamino-Cys1,d-Arg8]-vasopressin restored a significant proportion of the urine concentrating ability of WD EP1−/−mice. When mice were water loaded to suppress endogenous AVP production, urine osmolalities increased equally for WT and EP1−/−mice. These data suggest that PGE2modulates urine concentration by acting at EP1receptors, not in the collecting duct, but within the hypothalamus to promote AVP synthesis in response to acute WD.

Compartmentalization of cAMP-dependent signaling by phosphodiesterase-4D is involved in the regulation of vasopressin-mediated water reabsorption in renal principal cells

The cAMP/protein kinase A (PKA)-dependent insertion of water channel aquaporin-2 (AQP2)-bearing vesicles into the plasma membrane in renal collecting duct principal cells (AQP2 shuttle) constitutes the molecular basis of arginine vasopressin (AVP)-regulated water reabsorption. cAMP/PKA signaling systems are compartmentalized by A kinase anchoring proteins (AKAP) that tether PKA to subcellular sites and by phosphodiesterases (PDE) that terminate PKA signaling through hydrolysis of localized cAMP. In primary cultured principal cells, AVP causes focal activation of PKA. PKA and cAMP-specific phosphodiesterase-4D (PDE4D) are located on AQP2-bearing vesicles. The selective PDE4 inhibitor rolipram increases AKAP-tethered PKA activity on AQP2-bearing vesicles and enhances the AQP2 shuttle and thereby the osmotic water permeability. AKAP18delta, which is located on AQP2-bearing vesicles, directly interacts with PDE4D and PKA. In response to AVP, PDE4D and AQP2 translocate to the plasma membrane. Here PDE4D is activated through PKA phosphorylation and reduces the osmotic water permeability. Taken together, a novel, compartmentalized, and physiologically relevant cAMP-dependent signal transduction module on AQP2-bearing vesicles, comprising anchored PDE4D, AKAP18delta, and PKA, has been identified.

A Role of myosin Vb and Rab11-FIP2 in the aquaporin-2 shuttle

Arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Its binding to Gs-coupled vasopressin V2 receptors increases cyclic AMP (cAMP) and subsequently elicits the redistribution of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the plasma membrane (AQP2 shuttle), thereby facilitating water reabsorption from primary urine. The AQP2 shuttle is a paradigm for cAMP-dependent exocytic processes. Using sections of rat kidney, the AQP2-expressing cell line CD8, and primary principal cells, we studied the role of the motor protein myosin Vb, its vesicular receptor Rab11, and the myosin Vb- and Rab11-binding protein Rab11-FIP2 in the AQP2 shuttle. Myosin Vb colocalized with AQP2 intracellularly in resting and at the plasma membrane in AVP-treated cells. Rab11 was found on AQP2-bearing vesicles. A dominant-negative myosin Vb tail construct and Rab11-FIP2 lacking the C2 domain (Rab11-FIP2-DeltaC2), which disrupt recycling, caused condensation of AQP2 in a Rab11-positive compartment and abolished the AQP2 shuttle. This effect was dependent on binding of myosin Vb tail and Rab11-FIP2-DeltaC2 to Rab11. In summary, we identified myosin Vb as a motor protein involved in AQP2 recycling and show that myosin Vb- and Rab11-FIP2-dependent recycling of AQP2 is an integral part of the AQP2 shuttle.

Prostaglandin E2 receptor distribution and function in the gastrointestinal tract

Prostaglandin E2 (PGE2) is one of the most important biologically active prostanoids found throughout the gastrointestinal tract. Despite the fact that PGE2 regulates many physiological functions of the gut including mucosal protection, gastrointestinal secretion and motility, it is implicated in the pathophysiology of inflammatory bowel diseases (IBD) and colorectal neoplasia. The varied biological functions exerted by PGE2 are through the pharmacologically distinct, G-protein coupled plasma membrane receptors termed EP receptors. Disruptions of various prostanoid receptor genes have helped in unravelling the physiological functions of these receptors. To date, all four subtypes of EP receptors have been individually knocked out in mice and various phenotypes have been reported for each subtype. Similarly, in vitro and in vivo studies using EP receptor agonists and antagonists have helped in uncoupling the diverse functions of PGE2 signalling involving distinct EP receptors in the gut. In this review, we will summarize and conceptualize the salient features of EP receptor subtypes, their regional functions in the gut and how expressions of EP receptors are altered during disease states.

Renal prostaglandin E2 receptor (EP) expression profile is altered in streptozotocin and B6-Ins2Akita type I diabetic mice

The homeostatic function of prostaglandin E(2) (PGE(2)) is dependent on a balance of EP receptor-mediated events. A disruption in this balance may contribute to the progression of renal injury. Although PGE(2) excretion is elevated in diabetes, the expression of specific EP receptor subtypes has not been studied in the diabetic kidney. Therefore, the purpose of this study was to characterize the expression profile of four EP receptor subtypes (EP(1-4)) in 16-wk streptozotocin (STZ) and B6-Ins2(Akita) type I diabetic mice. In diabetic mice, the ratio of kidney weight to body weight was increased twofold compared with controls, blood glucose was elevated, but urine albumin was only increased in B6-Ins2(Akita) mice. The excretion of PGE(2) and its metabolite was augmented two- to fourfold as determined by enzyme immunoassay. Accordingly, renal cyclooxygenases were also increased in diabetic mice, with isoform-specific and regional differences in each model. Finally, there was altered EP(1-4) receptor expression in diabetic kidneys, with significant differences between STZ and B6-Ins2(Akita) mice (fold-control). In STZ mice, cortical EP(1) increased by 1.6, EP(3) increased by 2.3, and EP(4) decreased by 0.63; yet in B6-Ins2(Akita) mice, cortical EP(1) increased by 2.4, but there was a general decrease in the remaining subtypes. Similarly, in the STZ medulla EP(3) increased by 3.6, but both EP(1) and EP(3) increased by 5.5 and 1.95, respectively, in B6-Ins2(Akita) mice. Therefore, knowing the pattern of change in relative EP receptor expression in the kidney could be useful in identifying specific EP targets for the prevention of various components of diabetic kidney disease.

Atorvastatin reduces the expression of prostaglandin E2 receptors in human carotid atherosclerotic plaques and monocytic cells: potential implications for plaque stabilization

Prostaglandin E2 (PGE2), the product of cyclooxygenase-2 (COX-2) and prostaglandin E synthase-1 (mPGES-1), acts through its receptors (EPs) and induces matrix metalloproteinase (MMP) expression, which may favor the instability of atherosclerotic plaques. The effect of statins on EPs expression has not been previously studied. The aim of this study was to investigate the effect of atorvastatin (ATV, 80 mg/d, for one month) on EP expression in plaques and peripheral blood mononuclear cells (PBMC) of patients with carotid atherosclerosis. In addition, we studied the mechanisms by which statins could modulate EPs expression on cultured monocytic cells (THP-1) stimulated with proinflammatory cytokines (IL-1beta and TNF-alpha). Patients treated with atorvastatin showed reduced EP-1 (14 +/- 1.8% versus 26 +/- 2%; P < 0.01), EP-3 (10 +/- 1.5% versus 26 +/- 1.5%; P < 0.05), and EP-4 expression (10 +/- 4.1% versus 26.6 +/- 4.9%; P < 0.05) in atherosclerotic plaques (immunohistochemistry), and EP-3 and EP-4 mRNA expression in PBMC (real time PCR) in relation to non-treated patients. In cultured monocytic cells, atorvastatin (10 micromol/L) reduced EP-1/-3/-4 expression, along with COX-2, mPGES-1, MMP-9, and PGE2 levels elicited by IL-1beta and TNF-alpha. Similar results were noted with aspirin (100 micromol/L), dexamethasone (1 micromol/L), and the Rho kinase inhibitors Y-27632 and fasudil (10 micromol/L both). The effect of atorvastatin was reversed by mevalonate, farnesyl pyrophosphate, and geranylgeranyl pyrophosphate. On the whole, we have shown that atorvastatin reduces EPs expression in atherosclerotic plaques and blood mononuclear cells of patients with carotid stenosis and in cultured monocytic cells. The inhibition of EP receptors could explain, at least in part, some of the mechanisms by which statins could modulate the COX-2/mPGES-1 proinflammatory pathway and favor plaque stabilization in humans.

Regulation of aquaporin-2 trafficking and its binding protein complex

Trafficking of water channel aquaporin-2 (AQP2) to the apical membrane is critical to water reabsorption in renal collecting ducts and its regulation maintains body water homeostasis. However, exact molecular mechanisms which recruit AQP2 are unknown. Recent studies highlighted a key role for spatial and temporal regulation of actin dynamics in AQP2 trafficking. We have recently identified AQP2-binding proteins which directly regulate this trafficking: SPA-1, a GTPase-activating protein (GAP) for Rap1, and cytoskeletal protein actin. In addition, a multiprotein "force generator" complex which directly binds to AQP2 has been discovered. This review summarizes recent advances related to the mechanism for AQP2 trafficking.

Aldosterone increases urine production and decreases apical AQP2 expression in rats with diabetes insipidus

Vasopressin and aldosterone are essential hormones in the regulation of water and sodium balance. Aldosterone regulates sodium reabsorption, although synergistic effects on collecting duct water permeability have been shown. We investigated the effects of 7-day aldosterone infusion or oral spironolactone treatment on water balance and aquaporin (AQP) 2 expression in rats with 21 days of lithium-induced nephrogenic diabetes insipidus (Li-NDI). In rats with Li-NDI, aldosterone markedly increased (271 +/- 14 ml/24 h), whereas spironolactone decreased (74 +/- 11 ml/24 h) urine production compared with rats treated with lithium only (120 +/- 11 ml/24 h). Aldosterone increased free-water clearance and creatinine clearance, whereas spironolactone caused a decreased creatinine clearance but unchanged free-water clearance. Immunoblotting showed unchanged AQP2 expression in cortex/outer stripe of the outer medulla and inner medulla. In the inner stripe of the outer medulla aldosterone caused a decreased AQP2 expression, whereas spironolactone caused an increase compared with rats treated with lithium only. Semiquantitative confocal immunofluorescence microscopy of AQP2 immunolabeling showed reduced AQP2 expression in the apical plasma membrane domain in connecting tubule (CNT) and initial cortical collecting ducts (iCCD) in response to aldosterone-treated rats compared with rats treated with lithium only. Spironolactone significantly increased apical AQP2 expression in the iCCD compared with rats treated with lithium only. We also tested whether similar changes could be observed in vasopressin-deficient BB rats and found similar changes in urine production and subcellular AQP2 expression in the CNT and iCCD in response to aldosterone and spironolactone. This study shows that aldosterone treatment perturbs diabetes insipidus and is associated with AQP2 redistribution in CNT and iCCD likely mediated by the spironolactone-sensitive mineralocorticoid receptor.

Minireview: aquaporin 2 trafficking

In the kidney aquaporin-2 (AQP2) provides a target for hormonal regulation of water transport by vasopressin. Short-term control of water permeability occurs via vesicular trafficking of AQP2 and long-term control through changes in the abundance of AQP2 and AQP3 water channels. Defective AQP2 trafficking causes nephrogenic diabetes insipidus, a condition characterized by the kidney inability to produce concentrated urine because of the insensitivity of the distal nephron to vasopressin. AQP2 is redistributed to the apical membrane of collecting duct cells through activation of a cAMP signaling cascade initiated by the binding of vasopressin to its V2-receptor. Protein kinase A-mediated phosphorylation of AQP2 has been proposed to be essential in regulating AQP2-containing vesicle exocytosis. Cessation of the stimulus is followed by endocytosis of the AQP2 proteins exposed on the plasma membrane and their recycling to the original stores, in which they are retained. Soluble N-ethylmaleimide sensitive fusion factor attachment protein receptors (SNARE) and actin cytoskeleton organization regulated by small GTPase of the Rho family were also proved to be essential for AQP2 trafficking. Data for functional involvement of the SNARE vesicle-associated membrane protein 2 in AQP2 targeting has recently been provided. Changes in AQP2 expression/trafficking are of particular importance in pathological conditions characterized by both dilutional and concentrating defects. One of these conditions, hypercalciuria, has shown to be associated with alteration of AQP2 urinary excretion. More precisely, recent data support the hypothesis that, in vivo external calcium, through activation of calcium-sensing receptors, modulates the expression/trafficking of AQP2. Together these findings underscore the importance of AQP2 in kidney pathophysiology.

Adipocytes support cAMP-dependent translocation of aquaporin-2 from intracellular sites distinct from the insulin-responsive GLUT4 storage compartment

Aquaporin-2 (AQP2), when expressed in fully differentiated 3T3-L1 adipocytes, displays cAMP-dependent plasma membrane translocation in a manner similar to its behavior in renal epithelial cells. The translocation of AQP2 required phosphorylation at serine 256, as the expression of AQP2/S256D was constitutively plasma membrane localized, whereas AQP2/S256A was refractory to forskolin stimulation. Unlike GLUT4, this property is not inhibited by depolymerization of cortical actin. In addition, coexpression with the dominant negative form of TC10 (TC10/T31N) or inhibition of phosphatidylinositol 3-kinase did not abrogate the cAMP-mediated response. Under basal conditions, AQP2 is localized in both the perinuclear region and in punctate vesicles scattered within the periphery of the cell. Two- and three-dimensional confocal immunofluorescence microscopy demonstrated that the adipocyte AQP2 cAMP-responsive compartment was distinct from the GLUT4 insulin-responsive compartment. Consistent with this conclusion, insulin was an effective stimulator of GLUT4 translocation but had no effect on AQP2. Conversely, forskolin induced AQP2 translocation but not GLUT4. Colocalization studies with the early endosomal marker EEA1 and transferrin receptor suggested that the AQP2 compartment is mostly distinct from endosomal vesicles. Interestingly, however, the peripheral AQP2 vesicles significantly overlapped vesicle-associated membrane protein-2, underscoring the role of the latter in hormone-regulated exocytosis. To acquire insulin responsiveness following biosynthesis, GLUT4 undergoes a slow sorting step that requires 6-9 h. In contrast, AQP2 rapidly acquires forskolin responsiveness (3 h following biosynthesis) and directly enters the cAMP-regulated compartment without transiting the plasma membrane. Together, these data demonstrate that adipocytes display two different intracellular sorting mechanisms that direct distinct hormone-sensitive partitioning of GLUT4 and AQP2.

Clinical update on renal aquaporins

Following the discovery of the aquaporin-1 water channel over a decade ago, molecular techniques have been developed to examine the role of renal aquaporin water channels under numerous physiological and pathological conditions. The present article reviews current knowledge regarding the function and dysfunction of renal aquaporins in disorders of water metabolism.

Bradykinin signaling counteracts cAMP-elicited aquaporin 2 translocation in renal cells

Bradykinin (BK) is one of the most important peptides regulating vascular tone, water, and ionic balance in the body, playing a key role in controlling BP. It is interesting that patients with essential hypertension excrete less BK than normotensive individuals. For elucidating the mechanism by which BK regulates renal water transport that contributes to its antihypertensive effect, aquaporin 2 (AQP2)-transfected collecting duct CD8 cells, expressing the BK type II receptor (BK2R), were used as an experimental model. In CD8 cells, BK pretreatment impaired forskolin-induced AQP2 translocation to the apical plasma membrane. For clarifying the signal transduction cascade associated with this effect, whether BK induced an increase in cytosolic calcium, via the G protein Gq, known to be coupled to BK2R, first was investigated. Spectrofluorometry using fura-2-AM revealed that 100 nM BK elicited a significant increase in Ca(i), which was abolished by the receptor antagonist HOE-140. BK acts through BK2R coupled to both Gq and Galpha13, a known upstream effector of Rho protein. In CD8 cells, BK causes an increase in Rho activity, likely as a result of Galpha13 activation. This results in stabilization of the cortical F-actin network, thus impairing AQP2 trafficking. These effects counteract physiologic vasopressin stimulation, which instead has an opposite effect on actin network organization through Rho inactivation.

Altered expression of COX-1, COX-2, and mPGES in rats with nephrogenic and central diabetes insipidus

Prostaglandins have an important role in renal salt and water reabsorption. PGE2is the main kidney prostaglandin and is thought to be mainly produced in the kidney inner medulla (IM). There are indications that PGE2synthesis in nephrogenic (NDI) and central (CDI) diabetes insipidus is altered. We hypothesize that the expression of the major PGE2synthesis enzymes cyclooxygenases 1 and 2 (COX-1, COX-2) and membrane-associated PGE2synthase (mPGES) is altered in the kidneys of rats with NDI and CDI. Wistar rats treated with lithium for 4 wk were used as the NDI model. One-half of the NDI model rats were additionally dehydrated for 48 h. Brattleboro (BB) rats that lack endogenous antidiuretic hormone were used as the CDI model. Expression and localization of COX-1, COX-2, and mPGES in IM, inner stripe of outer medulla (ISOM), and cortex were determined by immunoblotting and immunohistochemistry. In lithium-induced NDI, expression of COX-1, COX-2, and mPGES was markedly decreased in IM. In ISOM and cortex, COX-1 expression was marginally reduced and mPGES expression was unaltered. COX-2 expression was undetected in ISOM and marginally increased in cortex. Consistent with this, the density of COX-2-expressing cells in macula densa was significantly increased, indicating differential regulation of COX-2 in IM and cortex. Dehydration of NDI rats resulted in a marked increase in COX-2 immunolabeling in IM interstitial cells, and there was no significant change in COX-1 and mPGES expression in any kidney zone. Treatment of DDAVP in BB rats for 6 days resulted in a markedly increased expression of COX-1, COX-2, and mPGES in IM. In the cortex, there were no changes in the expression of COX-1 and mPGES, whereas COX-2 expression was decreased. These results identify markedly reduced expression of COX-1, COX-2, and mPGES in IM in lithium-induced NDI. Furthermore, there were major changes in the expression of COX-1, COX-2, and mPGES in rats with CDI.

Extracellular calcium antagonizes forskolin-induced aquaporin 2 trafficking in collecting duct cells

BACKGROUND

Urinary concentrating defects and polyuria are the most important renal manifestations of hypercalcemia and the resulting hypercalciuria. In this study, we tested the hypothesis that hypercalciuria-associated polyuria in kidney collecting duct occurs through an impairment of the vasopressin-dependent aquaporin 2 (AQP2) water channel targeting to the apical membrane possibly involving calcium-sensing receptor (CaR) signaling.

METHODS

AQP2-transfected collecting duct CD8 cells were used as experimental model. Quantitation of cell surface AQP2 immunoreactivity was performed using an antibody recognizing the extracellular AQP2 C loop. Intracellular cyclic adenosine monophosphate (cAMP) accumulation was measured in CD8 cells using a cAMP enzyme immunoassay kit. To study the translocation of protein kinase C (PKC), membranes or cytosol fractions from CD8 cells were subjected to Western blotting using anti-PKC isozymes antibodies. The amount of F-actin was determined by spectrofluorometric techniques. Intracellular calcium measurements were performed by spectrofluorometric analysis with Fura-2/AM.

RESULTS

We demonstrated that extracellular calcium (Ca2+ o) (5 mmol/L) strongly inhibited forskolin-stimulated increase in AQP2 expression in the apical plasma membrane. At least three intracellular pathways activated by extracellular calcium were found to contribute to this effect. Firstly, the increase in cAMP levels in response to forskolin stimulation was drastically reduced in cells pretreated with Ca2+ o compared to untreated cells. Second, Ca2+ o activated PKC, known to counteract vasopressin response. Third, quantification of F-actin demonstrated that Ca2+ o caused a nearly twofold increase in F-actin content compared with basal conditions. All these effects were mimicked by a nonmembrane permeable agonist of the extracellular CaR, Gd3+.

CONCLUSION

Together, these data demonstrate that extracellular calcium, possibly acting through the endogenous CaR, antagonizes forskolin-induced AQP2 translocation to the apical plasma membrane in CD8 cells. In hypercalciuria, this mechanism might blunt water reabsorption and prevent further calcium concentration, thus protecting against a potential risk of urinary calcium-containing stone formation.

Down-regulation of urinary AQP2 and unaffected response to hypertonic saline after 24 hours of fasting in humans

BACKGROUND

In rats, 24 hours of fasting impairs urinary concentrating ability by down-regulation of aquaporin-2 (AQP2). We tested the hypothesis that 24 hours of fasting in humans reduces the capability to form AQP2 and impairs the antidiuretic response to hypertonic saline infusion.

METHODS

In a crossover study of 14 healthy subjects, the effect of 24 hours of fasting was compared to a nonfasting control experiment on urinary excretion of AQP2 (u-AQP2), free water clearance (C(H(2)O)), plasma arginine vasopressin (AVP), urinary cyclic AMP (u-cAMP), and natriuretic peptides. The following response to 3% sodium infusion was measured using the same effect variables. U-AQP2, AVP, and u-cAMP were determined by radioimmunoassays.

RESULTS

Fasting during 24 hours reduced u-AQP2 (14%), increased AVP (30%) despite a reduction in serum osmolality (P < 0.05), and depleted volume. C(H(2)O) and urine volume were not reduced, thus relatively increased after fasting. u-cAMP was not significantly different between the two procedures. Three percent saline resulted in the same relative increases in AVP, serum osmolality, u-AQP2, and u-cAMP and decreases in C(H(2)O) and urine volume independently of fasting. The reduced u-AQP2 and increased AVP after fasting were maintained during and after saline infusion.

CONCLUSION

Twenty-four hours of fasting decreased u-AQP2 and reduced urine osmolality likely as a result of decreased sensitivity of collecting duct cells to AVP. Fasting-related insensitivity of collecting duct cells to AVP was restored by 3% saline infusion. Finally, after saline infusion, other factors such as the increased plasma atrial natriuretic peptide (p-ANP) levels could contribute to the u-AQP2 regulation.

Bidirectional regulation of AQP2 trafficking and recycling: involvement of AQP2-S256 phosphorylation

The present study examined the role of PKA and serine256 (S256) phosphorylation for AQP2 trafficking and recycling using cells transfected with wild-type AQP2 (AQP2-WT) or mutant AQP2 and high-resolution confocal microscopic techniques. In transiently transfected MDCK-C7 cells, stimulation with forskolin induced translocation of AQP2-WT to the plasma membrane. Treatment of AQP2-WT cells with the PKA inhibitor H-89 following forskolin stimulation resulted in internalization of AQP2-WT. Moreover, H-89 treatment of AQP2-S256D (mimicking constitutively phosphorylated AQP2 and hence localized to the plasma membrane) resulted in redistribution of AQP2-S256D to intracellular vesicles, even in the presence of forskolin. Both PGE2 and dopamine stimulation induced endocytosis of AQP2-WT and AQP2-S256D, respectively, in forskolin-stimulated cells. Consistent with this, dopamine in the presence of vasopressin stimulated endocytosis of AQP2 in slices of rat kidney inner medulla without substantial dephosphorylation. In conclusion, these results strongly suggest that 1) S256 phosphorylation is necessary but not sufficient for AQP2 plasma membrane expression, 2) active PKA is required for AQP2 plasma membrane expression, 3) PGE2 and dopamine induce internalization of AQP2 independently of AQP2 dephosphorylation, and 4) preceding activation of cAMP production is necessary for PGE2 and dopamine to cause AQP2 internalization.

Identification of a novel A-kinase anchoring protein 18 isoform and evidence for its role in the vasopressin-induced aquaporin-2 shuttle in renal principal cells

Arginine vasopressin (AVP) increases the water permeability of renal collecting duct principal cells by inducing the fusion of vesicles containing the water channel aquaporin-2 (AQP2) with the plasma membrane (AQP2 shuttle). This event is initiated by activation of vasopressin V2 receptors, followed by an elevation of cAMP and the activation of protein kinase A (PKA). The tethering of PKA to subcellular compartments by protein kinase A anchoring proteins (AKAPs) is a prerequisite for the AQP2 shuttle. During the search for AKAP(s) involved in the shuttle, a new splice variant of AKAP18, AKAP18delta, was identified. AKAP18delta functions as an AKAP in vitro and in vivo. In the kidney, it is mainly expressed in principal cells of the inner medullary collecting duct, closely resembling the distribution of AQP2. It is present in both the soluble and particulate fractions derived from renal inner medullary tissue. Within the particulate fraction, AKAP18delta was identified on the same intracellular vesicles as AQP2 and PKA. AVP not only recruited AQP2, but also AKAP18delta to the plasma membrane. The elevation of cAMP caused the dissociation of AKAP18delta and PKA. The data suggest that AKAP18delta is involved in the AQP2 shuttle.

Molecular Mechanisms and Drug Development in Aquaporin Water Channel Diseases: Molecular Mechanism of Water Channel Aquaporin-2 Trafficking

Targeted positioning of water channel aquaporin-2 (AQP2) strictly regulates body water homeostasis. Trafficking of AQP2 to the apical membrane is critical for the reabsorption of water in renal collecting ducts. Besides the cAMP-mediated effect of vasopressin on AQP2 trafficking to the apical membrane, other signaling cascades also induce this sorting. Recently, AQP2-binding proteins that directly regulate this trafficking have been uncovered: SPA-1, a GTPase-activating protein (GAP) for Rap1, and cytoskeletal protein actin. This review summarizes recent advances related to the trafficking mechanism of AQP2 and its defect causing nephrogenic diabetes insipidus (NDI).