Hypertonicity is involved in redirecting the aquaporin-2 water channel into the basolateral, instead of the apical, plasma membrane of renal epithelial cells

Differentiated mouse kidney tubuloids as a novel in vitro model to study collecting duct physiology

Kidney tubuloids are cell models that are derived from human or mouse renal epithelial cells and show high similarities with their in vivo counterparts. Tubuloids grow polarized in 3D, allow for long-term expansion, and represent multiple segments of the nephron, as shown by their gene expression pattern. In addition, human tubuloids form tight, functional barriers and have been succesfully used for drug testing. Our knowledge of mouse tubuloids, on the other hand, is only minimal. In this study, we further characterized mouse tubuloids and differentiated them towards the collecting duct, which led to a significant upregulation of collecting duct-specific mRNAs of genes and protein expression, including the water channel AQP2 and the sodium channel ENaC. Differentiation resulted in polarized expression of collecting duct water channels AQP2 and AQP3. Also, a physiological response to desmopressin and forskolin stimulation by translocation of AQP2 to the apical membrane was demonstrated. Furthermore, amiloride-sensitive ENaC-mediated sodium uptake was shown in differentiated tubuloids using radioactive tracer sodium. This study demonstrates that mouse tubuloids can be differentiated towards the collecting duct and exhibit collecting duct-specific function. This illustrates the potential use of mouse kidney tubuloids as novel in vitro models to study (patho)physiology of kidney diseases.

Updates and Perspectives on Aquaporin-2 and Water Balance Disorders

Ensuring the proper amount of water inside the body is essential for survival. One of the key factors in the maintenance of body water balance is water reabsorption in the collecting ducts of the kidney, a process that is regulated by aquaporin-2 (AQP2). AQP2 is a channel that is exclusively selective for water molecules and impermeable to ions or other small molecules. Impairments of AQP2 result in various water balance disorders, including nephrogenic diabetes insipidus (NDI), which is a disease characterized by a massive loss of water through the kidney and consequent severe dehydration. Dysregulation of AQP2 is also a cause of water retention with hyponatremia in heart failure, hepatic cirrhosis, and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Antidiuretic hormone vasopressin is an upstream regulator of AQP2. Its binding to the vasopressin V2 receptor promotes AQP2 targeting to the apical membrane and thus enables water reabsorption. Tolvaptan, a vasopressin V2 receptor antagonist, is effective and widely used for water retention with hyponatremia. However, there are no studies showing improvement in hard outcomes or long-term prognosis. A possible reason is that vasopressin receptors have many downstream effects other than AQP2 function. It is expected that the development of drugs that directly target AQP2 may result in increased treatment specificity and effectiveness for water balance disorders. This review summarizes recent progress in studies of AQP2 and drug development challenges for water balance disorders.

Urinary reabsorption in the rat kidney by anticholinergics

Anticholinergics, therapeutic agents for overactive bladder, are clinically suggested to reduce urine output. We investigated whether this effect is due to bladder or kidney urine reabsorption. Various solutions were injected into the bladder of urethane-anesthetized SD rats. The absorption rate for 2 h was examined following the intravenous administration of the anticholinergics imidafenacin (IM), atropine (AT), and tolterodine (TO). The bilateral ureter was then canulated and saline was administered to obtain a diuretic state. Anticholinergics or 1-deamino-[8-D-arginine]-vasopressin (dDAVP) were intravenously administered. After the IM and dDAVP administrations, the rat kidneys were immunostained with AQP2 antibody, and intracellular cAMP was measured. The absorption rate was ~ 10% of the saline injected into the bladder and constant even when anticholinergics were administered. The renal urine among peaked 2 h after the saline administration. Each of the anticholinergics significantly suppressed the urine production in a dose-dependent manner, as did dDAVP. IM and dDAVP increased the intracellular cAMP levels and caused the AQP2 molecule to localize to the collecting duct cells' luminal side. The urinary reabsorption mechanism through the bladder epithelium was not activated by anticholinergic administration. Thus, anticholinergics suppress urine production via an increase in urine reabsorption in the kidneys' collecting duct cells via AQP2.

Chlorpromazine Induces Basolateral Aquaporin-2 Accumulation via F-Actin Depolymerization and Blockade of Endocytosis in Renal Epithelial Cells

We previously showed that in polarized Madin-Darby canine kidney (MDCK) cells, aquaporin-2 (AQP2) is continuously targeted to the basolateral plasma membrane from which it is rapidly retrieved by clathrin-mediated endocytosis. It then undertakes microtubule-dependent transcytosis toward the apical plasma membrane. In this study, we found that treatment with chlorpromazine (CPZ, an inhibitor of clathrin-mediated endocytosis) results in AQP2 accumulation in the basolateral, but not the apical plasma membrane of epithelial cells. In MDCK cells, both AQP2 and clathrin were concentrated in the basolateral plasma membrane after CPZ treatment (100 µM for 15 min), and endocytosis was reduced. Then, using rhodamine phalloidin staining, we found that basolateral, but not apical, F-actin was selectively reduced by CPZ treatment. After incubation of rat kidney slices in situ with CPZ (200 µM for 15 min), basolateral AQP2 and clathrin were increased in principal cells, which simultaneously showed a significant decrease of basolateral compared to apical F-actin staining. These results indicate that clathrin-dependent transcytosis of AQP2 is an essential part of its trafficking pathway in renal epithelial cells and that this process can be inhibited by selectively depolymerizing the basolateral actin pool using CPZ.

A versatile aquaporin-2 cell system for quantitative temporal expression and live cell imaging

Aquaporin-2 (AQP2) fine tunes urine concentration in response to the antidiuretic hormone vasopressin. In addition, AQP2 has been suggested to promote cell migration and epithelial morphogenesis. A cell system allowing temporal and quantitative control of expression levels of AQP2 and phospho-mimicking mutants has been missing, as has a system allowing expression of fluorescently tagged AQP2 for time-lapse imaging. In the present study, we generated and validated a Flp-In T-REx Madin-Darby canine kidney cell system for temporal and quantitative control of AQP2 and phospho-mimicking mutants. We verified that expression levels can be temporally and quantitatively controlled and that AQP2 translocated to the plasma membrane in response to elevated cAMP, which also induced S256 phosphorylation. The phospho-mimicking mutants AQP2-S256A and AQP2-S256D localized as previously described, primarily intracellular and to the plasma membrane, respectively. Induction of AQP2 expression in combination with transient, low expression of enhanced green fluorescent protein-tagged AQP2 enabled expression without aggregation and correct translocation in response to elevated cAMP. Interestingly, time-lapse imaging revealed AQP2-containing tubulating endosomes and that tubulation significantly decreased 30 min after cAMP elevation. This was mirrored by the phospho-mimicking mutants AQP2-S256A and AQP2-S256D, where AQP2-S256A-containing endosomes tubulated, whereas AQP2-S256D-containing endosomes did not. Thus, this cell system enables a multitude of cell-based assays warranted to provide deeper insights into the mechanisms of AQP2 regulation and effects on cell migration and epithelial morphogenesis.

Identification of UT-A1- and AQP2-interacting proteins in rat inner medullary collecting duct

The urea channel UT-A1 and the water channel aquaporin-2 (AQP2) mediate vasopressin-regulated transport in the renal inner medullary collecting duct (IMCD). To identify the proteins that interact with UT-A1 and AQP2 in native rat IMCD cells, we carried out chemical cross-linking followed by detergent solubilization, immunoprecipitation, and LC-MS/MS analysis of the immunoprecipitated material. The analyses revealed 133 UT-A1-interacting proteins and 139 AQP2-interacting proteins, each identified in multiple replicates. Fifty-three proteins that were present in both the UT-A1 and the AQP2 interactomes can be considered as mediators of housekeeping interactions, likely common to all plasma membrane proteins. Among proteins unique to the UT-A1 list were those involved in posttranslational modifications: phosphorylation (protein kinases Cdc42bpb, Phkb, Camk2d, and Mtor), ubiquitylation/deubiquitylation (Uba1, Usp9x), and neddylation (Nae1 and Uba3). Among the proteins unique to the AQP2 list were several Rab proteins (Rab1a, Rab2a, Rab5b, Rab5c, Rab7a, Rab11a, Rab11b, Rab14, Rab17) involved in membrane trafficking. UT-A1 was found to interact with UT-A3, although quantitative proteomics revealed that most UT-A1 molecules in the cell are not bound to UT-A3. In vitro incubation of UT-A1 peptides with the protein kinases identified in the UT-A1 interactome revealed that all except Mtor were capable of phosphorylating known sites in UT-A1. Overall, the UT-A1 and AQP2 interactomes provide a snapshot of a dynamic process in which UT-A1 and AQP2 are produced in the rough endoplasmic reticulum, processed through the Golgi apparatus, delivered to endosomes that move into and out of the plasma membrane, and are regulated in the plasma membrane.

Characterization of urinary exosomal release of aquaporin-1 and -2 after renal ischemia-reperfusion in rats

Acute kidney injury (AKI) is an important risk factor for the development of chronic kidney disease (CKD), and an alteration in renal water handling has been observed during the transition of AKI to CKD. Urinary exosomal release of aquaporin-1 (AQP1) and AQP2, important proteins for renal water handling, has recently been reported to predict their levels of renal expression. Therefore, we examined the patterns of urinary exosomal release of AQP1 and AQP2, and the exosomal marker proteins tumor susceptibility 101 protein (TSG101) and ALG-2 interacting protein X (Alix), in the acute and chronic phases following induction of AKI by renal bilateral ischemia/reperfusion (I/R) in rats. Blood tests and histological examinations indicated that AKI occurred before at 7 days after renal I/R ( day 7) and that renal fibrosis developed progressively thereafter. Immunoblotting demonstrated significant decreases in the urinary exosomal release of AQP1 and AQP2 during severe AKI. Urinary exosomal release of Alix and TSG101 was significantly increased on day 7. These data were also confirmed in rats with unilateral renal I/R causing more serious AKI. Urinary exosomal release of either the Ser-256- or Ser-269-phosphorylated form of AQP2, both of which are involved in apical trafficking of AQP2, was positively correlated with that of total AQP2. These results suggest that urinary exosomal release of AQP1 and AQP2 is reduced in I/R-induced AKI, whereas that of Alix and TSG101 is increased in the initial phase of renal fibrosis. Furthermore, apical trafficking of AQP2 appears to be related to urinary exosomal release of AQP2.

Wiryeongtang regulates hypertonicity-induced expression of aquaporin-2 water channels in mIMCD-3 cells

The kidneys have a key role in the homeostasis of water excretion and reabsorption. Water channels, particularly aquaporin-2 (AQP2), are important proteins in water homeostasis in the body through the short‑term and long-term regulation of water permeability. Wiryeongtang (WRT) is a well-known traditional oriental medicine, which is used for the treatment of chronic edema and dysuresia. The aim of the present study was to evaluate the inhibitory effect of WRT on the hypertonicity-induced expression of AQP2 in the inner medullary collecting duct cell line (IMCD‑3). Western blotting, reverse transcription‑polymerase chain reaction and immunofluorescence analysis were performed to determine the effect of WRT under hypertonic stress. WRT attenuated the 175 mM NaCl hypertonic stress‑induced increases in protein and mRNA levels of AQP2 and apical membrane insertion in a concentration‑dependent manner. However, no differences were observed in the levels of AQP1, AQP3 or AQP4 between the hypertonic stress and WRT groups. WRT attenuated the hypertonicity-induced phosphorylation of glucocorticoid-inducible protein kinase 1. In addition, the mRNA expression of tonicity‑responsive enhancer binding protein was attenuated by WRT under hypertonic stress. Pretreatment with WRT also decreased the hypertonic stress‑induced expression of AQP2, as with KT5720, a protein kinase A inhibitor. These results provided evidence of the beneficial effect of the traditional formula WRT in regulating water balance in hypertonic stress of the renal collecting ducts.

Quantitative evaluation of Oryeongsan and its action on water regulation in renal inner medullary collecting duct cells

ETHNOPHARMACOLOGICAL RELEVANCE

Oryeongsan (ORS, Wulingsan) has been reported to possess renal protective effects from renal diseases such as diabetes-induced renal damage, and nephrocalcinosis.

AIM OF THE STUDY

This study was conducted to evaluate the quantitative analysis and the inhibitory effect of ORS on hypertonic stress-induced water channel and apoptosis in murine inner medullary collecting duct cell line (mIMCD-3).

MATERIALS AND METHODS

Chromatographic and NMR spectroscopic analysis were performed and water balance regulation was determined by Western blot, RT-PCR, and immunofluorescnece.

RESULTS

Seven active principles (5-hydroxymethylfurfural, alismoxide, methyl(-)trans-cinnamate, adenine, guanosine, adenosine, and ferulic acid) in ORS were isolated and the structures were identified mainly by NMR spectroscopic analysis. In addition, contents of these metabolites in ORS were evaluated by HPLC analysis. Pretreatment with ORS significantly attenuated the hypertonic stress (175mM NaCl)-induced increase in protein levels of AQP2 and apical membrane insertion. ORS also attenuated osmolyte sodium-myo-inositol transporter (SMIT) expression and tonicity-responsive enhancer binding protein (TonEBP) mRNA under hypertonic stress. Those actions of ORS presented the similar effect of PKA inhibitor which AQP2 expression throughout the inhibition of vasopressin-mediated cAMP/PKA signal pathway. On the other hand, pretreatment with ORS attenuated hypertonic stress-induced cell death. Hypertonic stress-induced Bax or caspase-3 expression was decreased by ORS, resulting in anti-apoptotic effect.

CONCLUSIONS

The present data suggest that the beneficial effect of ORS in water balance and apoptosis against hypertonic stress of renal collecting ducts.

Aquaporins in desert rodent physiology

Desert rodents face a sizeable challenge in maintaining salt and water homeostasis due to their life in an arid environment. A number of their organ systems exhibit functional characteristics that limit water loss above that which occurs in non-desert species under similar conditions. These systems include renal, pulmonary, gastrointestinal, nasal, and skin epithelia. The desert rodent kidney preserves body water by producing a highly concentrated urine that reaches a maximum osmolality nearly three times that of the common laboratory rat. The precise mechanism by which urine is concentrated in any mammal is unknown. Insights into the process may be more apparent in species that produce highly concentrated urine. Aquaporin water channels play a fundamental role in water transport in several desert rodent organ systems. The role of aquaporins in facilitating highly effective water preservation in desert rodents is only beginning to be explored. The organ systems of desert rodents and their associated AQPs are described.

Polarized Trafficking of AQP2 Revealed in Three Dimensional Epithelial Culture

In renal collecting duct (CD) principal cells (PCs), vasopressin (VP) acts through its receptor, V2R, to increase intracellular cAMP leading to phosphorylation and apical membrane accumulation of the water channel aquaporin 2 (AQP2). The trafficking and function of basolaterally located AQP2 is, however, poorly understood. Here we report the successful application of a 3-dimensional Madin-Darby canine kidney (MDCK) epithelial model to study polarized AQP2 trafficking. This model recapitulates the luminal architecture of the CD and bi-polarized distribution of AQP2 as seen in kidney. Without stimulation, AQP2 is located in the subapical and basolateral regions. Treatment with VP, forskolin (FK), or 8-(4-Chlorophenylthio)-2′-O-methyladenosine 3′,5′-cyclic monophosphate monosodium hydrate (CPT-cAMP) leads to translocation of cytosolic AQP2 to the apical membrane, but not to the basolateral membrane. Treating cells with methyl-β-cyclodextrin (mβCD) to acutely block endocytosis causes accumulation of AQP2 on the basolateral membrane, but not on the apical membrane. Our data suggest that AQP2 may traffic differently at the apical and basolateral domains in this 3D epithelial model. In addition, application of a panel of phosphorylation specific AQP2 antibodies reveals the polarized, subcellular localization of differentially phosphorylated AQP2 at S256, S261, S264 and S269 in the 3D culture model, which is consistent with observations made in the CDs of VP treated animals, suggesting the preservation of phosphorylation dependent regulatory mechanism of AQP2 trafficking in this model. Therefore we have established a 3D culture model for the study of trafficking and regulation of both the apical and basolaterally targeted AQP2. The new model will enable further characterization of the complex mechanism regulating bi-polarized trafficking of AQP2 in vitro.

AVP-induced increase in AQP2 and p-AQP2 is blunted in heart failure during cardiac remodeling and is associated with decreased AT1R abundance in rat kidney

AIM

The objective was to examine the renal effects of long-term increased angiotensin II and vasopressin plasma levels in early-stage heart failure (HF). We investigated the regulations of the V2 vasopressin receptor, the type 1A angiotensin II receptor, the (pro)renin receptor, and the water channels AQP2, AQP1, AQP3, and AQP4 in the inner medulla of rat kidney.

METHODS

HF was induced by coronary artery ligation. Sixty-eight rats were allocated to six groups: Sham (N = 11), HF (N = 11), sodium restricted sham (N = 11), sodium restricted HF (N = 11), sodium restricted sham + DDAVP (N = 12), and sodium restricted HF + DDAVP (N = 12). 1-desamino-8-D-arginine vasopressin (0.5 ng h-1 for 7 days) or vehicle was administered. Pre- and post-treatment echocardiographic evaluation was performed. The rats were sacrificed at day 17 after surgery, before cardiac remodeling in rat is known to be completed.

RESULTS

HF rats on standard sodium diet and sodium restriction displayed biochemical markers of HF. These rats developed hyponatremia, hypo-osmolality, and decreased fractional excretion of sodium. Increase of AQP2 and p(Ser256)-AQP2 abundance in all HF groups was blunted compared with control groups even when infused with DDAVP and despite increased vasopressin V2 receptor and Gsα abundance. This was associated with decreased protein abundance of the AT1A receptor in HF groups vs. controls.

CONCLUSION

Early-stage HF is associated with blunted increase in AQP2 and p(Ser256)-AQP2 despite of hyponatremia, hypo-osmolality, and increased inner medullary vasopressin V2 receptor expression. Decreased type 1A angiotensin II receptor abundance likely plays a role in the transduction of these effects.

A novel therapeutic effect of statins on nephrogenic diabetes insipidus

Statins competitively inhibit hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase, resulting in reduced plasma total and low-density lipoprotein cholesterol levels. Recently, it has been shown that statins exert additional ‘pleiotropic’ effects by increasing expression levels of the membrane water channels aquaporin 2 (AQP2). AQP2 is localized mainly in the kidney and plays a critical role in determining cellular water content. This additional effect is independent of cholesterol homoeostasis, and depends on depletion of mevalonate-derived intermediates of sterol synthetic pathways, i.e. farnesylpyrophosphate and geranylgeranylpyrophosphate. By up-regulating the expression levels of AQP2, statins increase water reabsorption by the kidney, thus opening up a new avenue in treating patients with nephrogenic diabetes insipidus (NDI), a hereditary disease that yet lacks high-powered and limited side effects therapy. Aspects related to water balance determined by AQP2 in the kidney, as well as standard and novel therapeutic strategies of NDI are discussed.

Regulation of proximal tubule vacuolar H(+)-ATPase by PKA and AMP-activated protein kinase

The vacuolar H(+)-ATPase (V-ATPase) mediates ATP-driven H(+) transport across membranes. This pump is present at the apical membrane of kidney proximal tubule cells and intercalated cells. Defects in the V-ATPase and in proximal tubule function can cause renal tubular acidosis. We examined the role of protein kinase A (PKA) and AMP-activated protein kinase (AMPK) in the regulation of the V-ATPase in the proximal tubule as these two kinases coregulate the V-ATPase in the collecting duct. As the proximal tubule V-ATPases have different subunit compositions from other nephron segments, we postulated that V-ATPase regulation in the proximal tubule could differ from other kidney tubule segments. Immunofluorescence labeling of rat ex vivo kidney slices revealed that the V-ATPase was present in the proximal tubule both at the apical pole, colocalizing with the brush-border marker wheat germ agglutinin, and in the cytosol when slices were incubated in buffer alone. When slices were incubated with a cAMP analog and a phosphodiesterase inhibitor, the V-ATPase accumulated at the apical pole of S3 segment cells. These PKA activators also increased V-ATPase apical membrane expression as well as the rate of V-ATPase-dependent extracellular acidification in S3 cell monolayers relative to untreated cells. However, the AMPK activator AICAR decreased PKA-induced V-ATPase apical accumulation in proximal tubules of kidney slices and decreased V-ATPase activity in S3 cell monolayers. Our results suggest that in proximal tubule the V-ATPase subcellular localization and activity are acutely coregulated via PKA downstream of hormonal signals and via AMPK downstream of metabolic stress.

The physiological and pathophysiological functions of renal and extrarenal vasopressin V2 receptors

The arginine vasopressin (AVP) type 2 receptor (V2R) is unique among AVP receptor subtypes in signaling through cAMP. Its key function is in the kidneys, facilitating the urine concentrating mechanism through the AVP/V2 type receptor/aquaporin 2 system in the medullary and cortical collecting ducts. Recent clinical and research observations strongly support the existence of an extrarenal V2R. The clinical importance of the extrarenal V2R spans widely from stimulation of coagulation factor in the endothelium to as yet untested potential therapeutic targets. These include V2R-regulated membranous fluid turnover in the inner ear, V2R-regulated mitogensis and apoptosis in certain tumor tissues, and numerous other cell types where the physiological role of V2Rs still requires further research. Here, we review current evidence on the physiological and pathophysiological functions of renal and extrarenal V2Rs. These functions of V2R are important, not only in rare diseases with loss or gain of function of V2R but also in relation to the recent use of nonpeptide V2R antagonists to treat hyponatremia and possibly retard the growth of cysts and development of renal failure in autosomal dominant polycystic kidney disease. The main functions of V2R in principal cells of the collecting duct are water, salt, and urea transport by modifying the trafficking of aquaporin 2, epithelial Na(+) channels, and urea transporters and vasodilation and stimulation of coagulation factor properties, mainly seen with pharmacological doses of 1-desamino-8-D-AVP. The AVPR2 gene is located on the X chromosome, in a region with high probability of escape from inactivation; this may lead to phenotypic sex differences, with females expressing higher levels of transcript than males.

Regulation of aquaporin-2 in the kidney: A molecular mechanism of body-water homeostasis

The kidneys play a key role in the homeostasis of body water and electrolyte balance. Aquaporin-2 (AQP2) is the vasopressin-regulated water-channel protein expressed at the connecting tubule and collecting duct, and plays a key role in urine concentration and body-water homeostasis through short-term and long-term regulation of collecting duct water permeability. The signaling transduction pathways resulting in the AQP2 trafficking to the apical plasma membrane of the collecting duct principal cells, including AQP2 phosphorylation, RhoA phosphorylation, actin depolymerization, and calcium mobilization, and the changes of AQP2 abundance in water-balance disorders have been extensively studied. Dysregulation of AQP2 has been shown to be importantly associated with a number of clinical conditions characterized by body-water balance disturbances, including hereditary nephrogenic diabetes insipidus (NDI), lithium-induced NDI, electrolytes disturbance, acute and chronic renal failure, ureteral obstruction, nephrotic syndrome, congestive heart failure, and hepatic cirrhosis. Recent studies exploiting omics technology further demonstrated the comprehensive vasopressin signaling pathways in the collecting ducts. Taken together, these studies elucidate the underlying molecular mechanisms of body-water homeostasis and provide the basis for the treatment of body-water balance disorders.

Comparative physiology and architecture associated with the mammalian urine concentrating mechanism: role of inner medullary water and urea transport pathways in the rodent medulla

Comparative studies of renal structure and function have potential to provide insights into the urine-concentrating mechanism of the mammalian kidney. This review focuses on the tubular transport pathways for water and urea that play key roles in fluid and solute movements between various compartments of the rodent renal inner medulla. Information on aquaporin water channel and urea transporter expression has increased our understanding of functional segmentation of medullary thin limbs of Henle's loops, collecting ducts, and vasa recta. A more complete understanding of membrane transporters and medullary architecture has identified new and potentially significant interactions between these structures and the interstitium. These interactions are now being introduced into our concept of how the inner medullary urine-concentrating mechanism works. A variety of regulatory pathways lead directly or indirectly to variable patterns of fluid and solute movements among the interstitial and tissue compartments. Animals with the ability to produce highly concentrated urine, such as desert species, are considered to exemplify tubular structure and function that optimize urine concentration. These species may provide unique insights into the urine-concentrating process.(1)

Effect of Atractylodes macrocephala on Hypertonic Stress-Induced Water Channel Protein Expression in Renal Collecting Duct Cells

Edema is a symptom that results from the abnormal accumulation of fluid in the body. The cause of edema is related to the level of aquaporin (AQP)2 protein expression, which regulates the reabsorption of water in the kidney. Edema is caused by overexpression of the AQP2 protein when the concentration of Na⁺ in the blood increases. The rhizome of Atractylodes macrocephala has been used in traditional oriental medicine as a diuretic drug; however, the mechanism responsible for the diuretic effect of the aqueous extract from A. macrocephala rhizomes (AAMs) has not yet been identified. We examined the effect of the AAM on the regulation of water channels in the mouse inner medullary collecting duct (mIMCD)-3 cells under hypertonic stress. Pretreatment of AAM attenuates a hypertonicity-induced increase in AQP2 expression as well as the trafficking of AQP2 to the apical plasma membrane. Tonicity-responsive enhancer binding protein (TonEBP) is a transcription factor known to play a central role in cellular homeostasis by regulating the expression of some proteins, including AQP2. Western immunoblot analysis demonstrated that the protein and mRNA expression levels of TonEBP also decrease after AAM treatment. These results suggest that the AAM has a diuretic effect by suppressing water reabsorption via the downregulation of the TonEBP-AQP2 signaling pathway.

Basolateral targeting and microtubule-dependent transcytosis of the aquaporin-2 water channel

The aquaporin-2 (AQP2) water channel relocates mainly to the apical plasma membrane of collecting duct principal cells after vasopressin (VP) stimulation. AQP2 transport to this membrane domain is assumed to be a direct route involving recycling of intracellular vesicles. However, basolateral plasma membrane expression of AQP2 is observed in vivo in principal cells. Here, we asked whether there is a transcytotic pathway of AQP2 trafficking between apical and basolateral membranes. We used MDCK cells in which AQP2 normally accumulates apically after VP exposure. In contrast, both site-specific biotinylation and immunofluorescence showed that AQP2 is strongly accumulated in the basolateral membrane, along with the endocytic protein clathrin, after a brief cold shock (4°C). This suggests that AQP2 may be constitutively targeted to basolateral membranes and then retrieved by clathrin-mediated endocytosis at physiological temperatures. Rab11 does not accumulate in basolateral membranes after cold shock, suggesting that the AQP2 in this location is not associated with Rab11-positive vesicles. After rewarming (37°C), basolateral AQP2 staining is diminished and it subsequently accumulates at the apical membrane in the presence of VP/forskolin, suggesting that transcytosis can be followed by apical insertion of AQP2. This process is inhibited by treatment with colchicine. Our data suggest that the cold shock procedure reveals the presence of microtubule-dependent AQP2 transcytosis, which represents an indirect pathway of apical AQP2 delivery in these cells. Furthermore, our data indicate that protein polarity data obtained from biotinylation assays, which require cells to be cooled to 4°C during the labeling procedure, should be interpreted with caution.

Aquaporin 2 promotes cell migration and epithelial morphogenesis

The aquaporin 2 (AQP2) water channel, expressed in kidney collecting ducts, contributes critically to water homeostasis in mammals. Animals lacking or having significantly reduced levels of AQP2, however, have not only urinary concentrating abnormalities but also renal tubular defects that lead to neonatal mortality from renal failure. Here, we show that AQP2 is not only a water channel but also an integrin-binding membrane protein that promotes cell migration and epithelial morphogenesis. AQP2 expression modulates the trafficking and internalization of integrin β1, facilitating its turnover at focal adhesions. In vitro, disturbing the interaction between AQP2 and integrin β1 by mutating the RGD motif led to reduced endocytosis, retention of integrin β1 at the cell surface, and defective cell migration and tubulogenesis. Similarly, in vivo, AQP2-null mice exhibited significant retention of integrin β1 at the basolateral membrane and had tubular abnormalities. In summary, these data suggest that the water channel AQP2 interacts with integrins to promote renal epithelial cell migration, contributing to the structural and functional integrity of the mammalian kidney.

Cell culture models and animal models for studying the patho-physiological role of renal aquaporins

Aquaporins (AQPs) are key players regulating urinary-concentrating ability. To date, eight aquaporins have been characterized and localized along the nephron, namely, AQP1 located in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2, AQP3 and AQP4 in collecting duct principal cells; AQP5 in intercalated cell type B; AQP6 in intercalated cells type A in the papilla; AQP7, AQP8 and AQP11 in the proximal tubule. AQP2, whose expression and cellular distribution is dependent on vasopressin stimulation, is involved in hereditary and acquired diseases affecting urine-concentrating mechanisms. Due to the lack of selective aquaporin inhibitors, the patho-physiological role of renal aquaporins has not yet been completely clarified, and despite extensive studies, several questions remain unanswered. Until the recent and large-scale development of genetic manipulation technology, which has led to the generation of transgenic mice models, our knowledge on renal aquaporin regulation was mainly based on in vitro studies with suitable renal cell models. Transgenic and knockout technology approaches are providing pivotal information on the role of aquaporins in health and disease. The main goal of this review is to update and summarize what we can learn from cell and animal models that will shed more light on our understanding of aquaporin-dependent renal water regulation.

Effect of Poria cocos on hypertonic stress-induced water channel expression and apoptosis in renal collecting duct cells

ETHNOPHARMACOLOGICAL RELEVANCE

A major physiological role of the kidney is to regulate body water and urine concentration. Aquaporin-2 (AQP2), a family of water channels, plays an important role in the urinary concentrating process and regulation of water balance in the kidney. The dried sclerotia of Poria cocos Wolf has been known to have a diuretic effect and used for the treatment of chronic edema and nephrosis.

AIM OF THE STUDY

This study was conducted to evaluate the inhibitory effect of the sclerotia of Poria cocos (WPC) on hypertonic stress-induced AQP2 expression and apoptosis in inner medullary collecting duct cell lines (IMCD-3).

MATERIALS AND METHODS

Hypertonic stress was induced by 175mM NaCl. Inhibitory effect of WPC on hypertonic stress-induced AQP2 expression and apoptosis were determined by western blot, RT-PCR, and immunofluorescence.

RESULTS

Hypertonic stress (175mM NaCl) increased in the levels of AQP2 expression by hypertonicity in IMCD-3 cells. WPC attenuated the hypertonicity-induced increase in protein and mRNA levels of AQP2 in a concentration-dependent manner. Pretreatment with WPC attenuated hypertonicity-induced cell death. Hypertonicity increased serum- and glucocorticoid-inducible protein kinase (Sgk1) phosphorylation, however, WPC attenuated the hypertonicity-induced Sgk1 activation. Tonicity-responsive enhancer binding protein (TonEBP) mRNA was also recovered by WPC under hypertonic stress. Pretreatment with WPC presented the similar effect of PKA inhibitor which decreased hypertonic stress-induced AQP2 expression. Hypertonicity increased cAMP levels and the changes were blocked by WPC. On the other hand, hypertonic stress-induced Bax or caspase-3 expression was decreased by WPC, resulting in anti-apoptotic effect.

CONCLUSIONS

These results provided evidence that the beneficial effect of WPC in water balance against in vitro hypertonic stress of renal collecting ducts. In addition, WPC exhibits anti-apoptotic property response to hypertonic stress. Thus, these data suggests that WPC has benefit for the therapeutic approach to the inhibition of renal disorder.

New insights into the dynamic regulation of water and acid-base balance by renal epithelial cells

Maintaining tight control over body fluid and acid-base homeostasis is essential for human health and is a major function of the kidney. The collecting duct is a mosaic of two cell populations that are highly specialized to perform these two distinct processes. The antidiuretic hormone vasopressin (VP) and its receptor, the V2R, play a central role in regulating the urinary concentrating mechanism by stimulating accumulation of the aquaporin 2 (AQP2) water channel in the apical membrane of collecting duct principal cells. This increases epithelial water permeability and allows osmotic water reabsorption to occur. An understanding of the basic cell biology/physiology of AQP2 regulation and trafficking has informed the development of new potential treatments for diseases such as nephrogenic diabetes insipidus, in which the VP/V2R/AQP2 signaling axis is defective. Tubule acidification due to the activation of intercalated cells is also critical to organ function, and defects lead to several pathological conditions in humans. Therefore, it is important to understand how these "professional" proton-secreting cells respond to environmental and cellular cues. Using epididymal proton-secreting cells as a model system, we identified the soluble adenylate cyclase (sAC) as a sensor that detects luminal bicarbonate and activates the vacuolar proton-pumping ATPase (V-ATPase) via cAMP to regulate tubular pH. Renal intercalated cells also express sAC and respond to cAMP by increasing proton secretion, supporting the hypothesis that sAC could function as a luminal sensor in renal tubules to regulate acid-base balance. This review summarizes recent advances in our understanding of these fundamental processes.

Vasopressin-dependent coupling between sodium transport and water flow in a mouse cortical collecting duct cell line

Water balance is achieved through the ability of the kidney to control water reabsorption in the connecting tubule and the collecting duct. In a mouse cortical collecting duct cell line (mCCD(c11)), physiological concentrations of arginine vasopressin increased both electrogenic, amiloride-sensitive, epithelial sodium channel (ENaC)-mediated sodium transport measured by the short-circuit current (Isc) method and water flow (Jv apical to basal) measured by gravimetry with similar activation coefficient K(1/2) (6 and 12 pM, respectively). Jv increased linearly according to the osmotic gradient across the monolayer. A small but highly significant Jv was also measured under isoosmotic conditions. To test the coupling between sodium reabsorption and water flow, mCCD(c11) cells were treated for 24 h under isoosmotic condition with either diluent, amiloride, vasopressin or vasopressin and amiloride. Isc, Jv, and net chemical sodium fluxes were measured across the same monolayers. Around 30% of baseline and 50% of vasopressin-induced water flow is coupled to an amiloride-sensitive, ENaC-mediated, electrogenic sodium transport, whereas the remaining flow is coupled to an amiloride-insensitive, nonelectrogenic sodium transport mediated by an unknown electroneutral transporter. The mCCD(c11) cell line is a first example of a mammalian tight epithelium allowing quantitative study of the coupling between sodium and water transport. Our data are consistent with the 'near isoosmotic' fluid transport model.

AMP-activated protein kinase inhibits KCNQ1 channels through regulation of the ubiquitin ligase Nedd4-2 in renal epithelial cells

The KCNQ1 K(+) channel plays a key role in the regulation of several physiological functions, including cardiac excitability, cardiovascular tone, and body electrolyte homeostasis. The metabolic sensor AMP-activated protein kinase (AMPK) has been shown to regulate a growing number of ion transport proteins. To determine whether AMPK regulates KCNQ1, we studied the effects of AMPK activation on KCNQ1 currents in Xenopus laevis oocytes and collecting duct epithelial cells. AMPK activation decreased KCNQ1 currents and channel surface expression in X. laevis oocytes, but AMPK did not phosphorylate KCNQ1 in vitro, suggesting an indirect regulatory mechanism. As it has been recently shown that the ubiquitin-protein ligase Nedd4-2 inhibits KCNQ1 plasma membrane expression and that AMPK regulates epithelial Na(+) channels via Nedd4-2, we examined the role of Nedd4-2 in the AMPK-dependent regulation of KCNQ1. Channel inhibition by AMPK was blocked in oocytes coexpressing either a dominant-negative or constitutively active Nedd4-2 mutant, or a Nedd4-2 interaction-deficient KCNQ1 mutant, suggesting that Nedd4-2 participates in the regulation of KCNQ1 by AMPK. KCNQ1 is expressed at the basolateral membrane in mouse polarized kidney cortical collecting duct (mpkCCD(c14)) cells and in rat kidney. Treatment with the AMPK activators AICAR (2 mM) or metformin (1 mM) reduced basolateral KCNQ1 currents in apically permeabilized polarized mpkCCD(c14) cells. Moreover, AICAR treatment of rat kidney slices ex vivo induced AMPK activation and intracellular redistribution of KCNQ1 from the basolateral membrane in collecting duct principal cells. AICAR treatment also induced increased ubiquitination of KCNQ1 immunoprecipitated from kidney slice homogenates. These results indicate that AMPK inhibits KCNQ1 activity by promoting Nedd4-2-dependent channel ubiquitination and retrieval from the plasma membrane.

Vacuolar H+-ATPase apical accumulation in kidney intercalated cells is regulated by PKA and AMP-activated protein kinase

The vacuolar H(+)-ATPase (V-ATPase) in type A kidney intercalated cells is a major contributor to acid excretion in the collecting duct. The mechanisms of V-ATPase-trafficking regulation in kidney intercalated cells have not been well-characterized. In developmentally related epididymal clear cells, we showed previously that PKA, acting downstream of soluble adenylyl cyclase (sAC), induces V-ATPase apical membrane accumulation. These PKA-mediated effects were inhibited by activators of the metabolic sensor AMP-activated kinase (AMPK) in clear cells. Here, we examined the regulation of V-ATPase subcellular localization in intercalated cells by PKA and AMPK in rat kidney tissue slices ex vivo. Immunofluorescence labeling of kidney slices revealed that the PKA activator N(6)-monobutyryl cAMP (6-MB-cAMP) induced V-ATPase apical membrane accumulation in collecting duct intercalated cells, whereas the V-ATPase had a more cytosolic distribution when incubated in Ringer buffer alone for 30 min. V-ATPase accumulated at the apical membrane in intercalated cells in kidney slices incubated in Ringer buffer for 75 min, an effect that was prevented by treatment with PKA inhibitor (mPKI). The V-ATPase distribution was cytosolic in intercalated cells treated with the carbonic anhydrase inhibitor acetazolamide or the sAC inhibitor KH7, effects that were overridden by 6-MB-cAMP. Preincubation of kidney slices with an AMPK activator blocked V-ATPase apical membrane accumulation induced by 6-MB-cAMP, suggesting that AMPK antagonizes cAMP/PKA effects on V-ATPase distribution. Taken together, our results suggest that in intercalated cells V-ATPase subcellular localization and therefore its activity may be coupled to acid-base status via PKA, and metabolic status via AMPK.

Lovastatin-induced cholesterol depletion affects both apical sorting and endocytosis of aquaporin-2 in renal cells

Vasopressin causes the redistribution of the water channel aquaporin-2 (AQP2) from cytoplasmic storage vesicles to the apical plasma membrane of collecting duct principal cells, leading to urine concentration. The molecular mechanisms regulating the selective apical sorting of AQP2 are only partially uncovered. In this work, we investigate whether AQP2 sorting/trafficking is regulated by its association with membrane rafts. In both MCD4 cells and rat kidney, AQP2 preferentially associated with Lubrol WX-insoluble membranes regardless of its presence in the storage compartment or at the apical membrane. Block-and-release experiments indicate that 1) AQP2 associates with detergent-resistant membranes early in the biosynthetic pathway; 2) strong cholesterol depletion delays the exit of AQP2 from the trans-Golgi network. Interestingly, mild cholesterol depletion promoted a dramatic accumulation of AQP2 at the apical plasma membrane in MCD4 cells in the absence of forskolin stimulation. An internalization assay showed that AQP2 endocytosis was clearly reduced under this experimental condition. Taken together, these data suggest that association with membrane rafts may regulate both AQP2 apical sorting and endocytosis.

Aquaporin-2 abundance in the renal collecting duct: new insights from cultured cell models

The renal cortico-papillary osmotic gradient is generated by sodium reabsorption in the thick ascending limb. The antidiuretic hormone arginine vasopressin (AVP) increases collecting duct water permeability by enhancing aquaporin-2 (AQP2) water channel insertion in the apical membrane of principal cells, allowing water to passively flow along the osmotic gradient from the tubule lumen to the interstitium. In addition to short-term AQP2 redistribution between intracellular compartments and the cell surface, AQP2 whole cell abundance is tightly regulated. AVP is a major transcriptional activator of the AQP2 gene, and stimulation of insulin- and calcium-sensing receptors respectively potentiate and reduce its action. Extracellular tonicity is another key factor that determines the levels of AQP2 abundance. Its effect is dependent on activation of the tonicity-responsive enhancer binding protein that reinforces AVP-induced AQP2 transcriptional activation. Conversely, activation of the NF-κB transcriptional factor by proinflammatory factors reduces AQP2 gene transcription. Aldosterone additionally regulates AQP2 whole cell abundance by simultaneously reducing AQP2 gene transcription and stimulating AQP2 mRNA translation. These examples illustrate how cross talk between various stimuli regulates AQP2 abundance in collecting duct principal cells and consequently contributes to maintenance of body water homeostasis.

Characterization of D150E and G196D aquaporin-2 mutations responsible for nephrogenic diabetes insipidus: importance of a mild phenotype

Aquaporin-2 (AQP2) is a water channel responsible for the final water reabsorption in renal collecting ducts. Alterations in AQP2 function induce nephrogenic diabetes insipidus (NDI), a condition characterized by severe polyuria and polydipsia. Three patients affected with severe NDI, who were compound heterozygous for the AQP2 mutations D150E and G196D, are presented here along with a mildly affected D150E homozygous patient from another family. Using Xenopus oocytes as an expression system, these two mutations (G196D and D150E) were compared with the wild-type protein (AQP2-wt) for functional activity (water flux analysis), protein maturation, and plasma membrane targeting. AQP2-wt induces a major increase in water permeability (P(f) = 47.4 +/- 12.2 x 10(-4) cm/s) whereas D150E displays intermediate P(f) values (P(f) = 12.5 +/- 3.0 x 10(-4) cm/s) and G196D presents no specific water flux, similar to controls (P(f) = 2.1 +/- 0.8 x 10(-4) cm/s and 2.2 +/- 0.7 x 10(-4) cm/s, respectively). Western blot and immunocytochemical evaluations show protein targeting that parallels activity levels with AQP2-wt adequately targeted to the plasma membrane, partial targeting for D150E, and complete sequestration of G196D within intracellular compartments. When coinjecting AQP2-wt with mutants, no (AQP2-wt + D150E) or partial (AQP2-wt + G196D) reduction of water flux were observed compared with AQP2-wt alone, whereas complete loss of function was found when both mutants were coinjected. These results essentially recapitulate the clinical profiles of the family members, showing a typical dominant negative effect when G196D is coinjected with either AQP2-wt or D150E but not between AQP2-wt and D150E mutant.

LIP5 interacts with aquaporin 2 and facilitates its lysosomal degradation

Vasopressin binding to the V2 receptor in renal principal cells leads to activation of protein kinase A, phosphorylation of aquaporin 2 (AQP2) at Ser256, and the translocation of AQP2 to the apical membrane, resulting in concentration of the urine. In contrast, phorbol ester-induced activation of protein kinase C pathway leads to ubiquitination of AQP2 at Lys270 and its internalization to multivesicular bodies, where it is targeted for lysosomal degradation or stored for recycling. Because little is known about the regulation of AQP2 trafficking, we used the carboxy-terminal tail of constitutively nonphosphorylated AQP2 (S256A) as a bait for interacting proteins in a yeast two-hybrid assay. We isolated lysosomal trafficking regulator-interacting protein 5 (LIP5) and found that LIP5 interacted with the proximal carboxy-terminal tail (L230-D243) of AQP2 in vitro but not with AQP3 or AQP4, which are also expressed in principal cells. Immunohistochemistry revealed that LIP5 co-localized with AQP2 in principal cells. LIP5 binding occurred independent of the state of Ser256 phosphorylation or Lys270 ubiquitination. LIP5 has been shown to facilitate degradation of the EGF receptor; here, LIP5 seemed to bind this receptor. Knockdown of LIP5 in mouse renal cells (mpkCCD) reduced the phorbol ester-induced degradation of AQP2 approximately two-fold. In summary, LIP5 binds cargo proteins and, considering the role of LIP5 in protein sorting to multivesicular bodies, plays a role in the degradation of AQP2, possibly by reducing the formation of late endosomes.

Sensing, signaling and sorting events in kidney epithelial cell physiology

The kidney regulates body fluid, ion and acid/base homeostasis through the interaction of a host of channels, transporters and pumps within specific tubule segments, specific cell types and specific plasma membrane domains. Furthermore, renal epithelial cells have adapted to function in an often unique and challenging environment that includes high medullary osmolality, acidic pHs, variable blood flow and constantly changing apical and basolateral 'bathing' solutions. In this review, we focus on selected protein trafficking events by which kidney epithelial cells regulate body fluid, ion and acid-base homeostasis in response to changes in physiological conditions. We discuss aquaporin 2 and G-protein-coupled receptors in fluid and ion balance, the vacuolar H(+)-adenosine triphosphatase (V-ATPase) and intercalated cells in acid/base regulation and acidification events in the proximal tubule degradation pathway. Finally, in view of its direct role in vesicle trafficking that we outline in this study, we propose that the V-ATPase itself should, under some circumstances, be considered a fourth category of vesicle 'coat' protein (COP), alongside clathrin, caveolin and COPs.

Controlled aquaporin-2 expression in the hypertonic environment

The corticomedullary osmolality gradient is the driving force for water reabsorption occurring in the kidney. In the collecting duct, this gradient allows luminal water to move across aquaporin (AQP) water channels, thereby increasing urine concentration. However, this same gradient exposes renal cells to great osmotic challenges. These cells must constantly adapt to fluctuations of environmental osmolality that challenge cell volume and incite functional change. This implies profound alterations of cell phenotype regarding water permeability. AQP2 is an essential component of the urine concentration mechanism whose controlled expression dictates apical water permeability of collecting duct principal cells. This review focuses on changes of AQP2 abundance and trafficking in hypertonicity-challenged cells. Intracellular mechanisms governing these events are discussed and the biological relevance of altered AQP2 expression by hypertonicity is outlined.

A fluorimetry-based ssYFP secretion assay to monitor vasopressin-induced exocytosis in LLC-PK1 cells expressing aquaporin-2

Vasopressin (VP)-induced exocytosis was dissected in native and aquaporin-2 (AQP2)-expressing renal LLC-PK(1) cells by a fluorimetric exocytosis assay based on soluble secreted yellow fluorescent protein (ssYFP). YFP was targeted to the secretory pathway by addition of an 18-amino acid signal peptide from hen egg white lysozyme. Immunofluorescence labeling, together with analysis of Alexa 555-dextran internalization, revealed that ssYFP is exclusively located in the secretory pathway. Immunofluorescence and immunogold electron microscopy showed significant colocalization of ssYFP and AQP2. Fluorimetry and Western blot analysis demonstrated similar constitutive ssYFP secretion in native LLC-PK(1) and AQP2-expressing cells. In AQP2-expressing cells, a twofold increase in ssYFP secretion was observed within 15 min of VP stimulation. This transient burst of ssYFP secretion was abolished by the PKA inhibitor H-89 and was not observed in native cells. The endocytotic inhibitor methyl-beta-cyclodextrin, which also promotes membrane accumulation of AQP2, had no effect on ssYFP secretion. Although cells expressing phosphorylation-deficient AQP2-S256A showed significantly lower baseline levels of constitutive secretion, VP induced a significant increase in exocytosis. Our data indicate that 1) this assay can monitor exocytosis in cultured epithelial cells, 2) VP has an acute stimulatory effect on ssYFP secretion in AQP2-expressing, but not native, cells, and 3) phosphorylation of AQP2 at S256 may be involved in the regulation of constitutive AQP2 exocytosis and play only a minor role in the VP-induced burst. These results support the idea that, in addition to its role in reducing AQP2 endocytosis, VP increases AQP2 exocytosis.

Acute hypertonicity alters aquaporin-2 trafficking and induces a MAPK-dependent accumulation at the plasma membrane of renal epithelial cells

The unique phenotype of renal medullary cells allows them to survive and functionally adapt to changes of interstitial osmolality/tonicity. We investigated the effects of acute hypertonic challenge on AQP2 (aquaporin-2) water channel trafficking. In the absence of vasopressin, hypertonicity alone induced rapid (<10 min) plasma membrane accumulation of AQP2 in rat kidney collecting duct principal cells in situ, and in several kidney epithelial lines. Confocal microscopy revealed that AQP2 also accumulated in the trans-Golgi network (TGN) following hypertonic challenge. AQP2 mutants that mimic the Ser(256)-phosphorylated and -nonphosphorylated state accumulated at the cell surface and TGN, respectively. Hypertonicity did not induce a change in cytosolic cAMP concentration, but inhibition of either calmodulin or cAMP-dependent protein kinase A activity blunted the hypertonicity-induced increase of AQP2 cell surface expression. Hypertonicity increased p38, ERK1/2, and JNK MAPK activity. Inhibiting MAPK activity abolished hypertonicity-induced accumulation of AQP2 at the cell surface but did not affect either vasopressin-dependent AQP2 trafficking or hypertonicity-induced AQP2 accumulation in the TGN. Finally, increased AQP2 cell surface expression induced by hypertonicity largely resulted from a reduction in endocytosis but not from an increase in exocytosis. These data indicate that acute hypertonicity profoundly alters AQP2 trafficking and that hypertonicity-induced AQP2 accumulation at the cell surface depends on MAP kinase activity. This may have important implications on adaptational processes governing transcellular water flux and/or cell survival under extreme conditions of hypertonicity.

Localization and trafficking of aquaporin 2 in the kidney

Aquaporins (AQPs) are membrane proteins serving in the transfer of water and small solutes across cellular membranes. AQPs play a variety of roles in the body such as urine formation, prevention from dehydration in covering epithelia, water handling in the blood–brain barrier, secretion, conditioning of the sensory system, cell motility and metastasis, formation of cell junctions, and fat metabolism. The kidney plays a central role in water homeostasis in the body. At least seven isoforms, namely AQP1, AQP2, AQP3, AQP4, AQP6, AQP7, and AQP11, are expressed. Among them, AQP2, the anti-diuretic hormone (ADH)-regulated water channel, plays a critical role in water reabsorption. AQP2 is expressed in principal cells of connecting tubules and collecting ducts, where it is stored in Rab11-positive storage vesicles in the basal state. Upon ADH stimulation, AQP2 is translocated to the apical plasma membrane, where it serves in the influx of water. The translocation process is regulated through the phosphorylation of AQP2 by protein kinase A. As soon as the stimulation is terminated, AQP2 is retrieved to early endosomes, and then transferred back to the Rab 11-positive storage compartment. Some AQP2 is secreted via multivesicular bodies into the urine as exosomes. Actin plays an important role in the intracellular trafficking of AQP2. Recent findings have shed light on the molecular basis that controls the trafficking of AQP2.

Early upregulation in nasal epithelium and strong expression in olfactory bulb glomeruli suggest a role for Aquaporin-4 in olfaction

Aquaporin-4 (AQP4) has been reported to be upregulated post-partum in pregnancy and in early lung development. Several technical challenges exist in measuring AQP4 protein levels, among them sensitivity to detergent solubilization, sample heating and gel composition. Here we have optimized quantification of AQP4 using immuno-blots. Using improved methodology we find no evidence for AQP4 upregulation post-partum or in the early lung development. However, in the nasal epithelium AQP4 is upregulated as early as in the brain. Furthermore, AQP4 is strongly expressed in the glomerulus, the synaptic unit of the olfactory bulb, suggesting a role for AQP4 in olfactory function.

Heat shock protein 70 interacts with aquaporin-2 and regulates its trafficking

The trafficking of aquaporin-2 (AQP2) involves multiple complex pathways, including regulated, cAMP-, and cGMP-mediated pathways, as well as a constitutive recycling pathway. Although several accessory proteins have been indirectly implicated in AQP2 recycling, the direct protein-protein interactions that regulate this process remain largely unknown. Using yeast two-hybrid screening of a human kidney cDNA library, we have identified the 70-kDa heat shock proteins as AQP2-interacting proteins. Interaction was confirmed by mass spectrometry of proteins pulled down from rat kidney papilla extract using a GST-AQP2 C-terminal fusion protein (GST-A2C) as a bait, by co-immunoprecipitation (IP) assays, and by direct binding assays using purified hsc70 and the GST-A2C. The direct interaction of AQP2 with hsc70 is partially inhibited by ATP, and the Ser-256 residue in the AQP2 C terminus is important for this direct interaction. Vasopressin stimulation in cells enhances the interaction of hsc70 with AQP2 in IP assays, and vasopressin stimulation in vivo induces an increased co-localization of hsc70 and AQP2 on the apical membrane of principal cells in rat kidney collecting ducts. Functional knockdown of hsc70 activity in AQP2 expressing cells results in membrane accumulation of AQP2 and reduced endocytosis of rhodamine-transferrin. Our data also show that AQP2 interacts with hsp70 in multiple in vitro binding assays. Finally, in addition to hsc70 and hsp70, AQP2 interacts with several other key components of the endocytotic machinery in co-IP assays, including clathrin, dynamin, and AP2. To summarize, we have identified the 70-kDa heat shock proteins as a AQP2 interactors and have shown for hsc70 that this interaction is involved in AQP2 trafficking.

Long-term aldosterone treatment induces decreased apical but increased basolateral expression of AQP2 in CCD of rat kidney

The purpose of the present studies was to determine the effects of high-dose aldosterone and dDAVP treatment on renal aquaporin-2 (AQP2) regulation and urinary concentration. Rats were treated for 6 days with either vehicle (CON; n = 8), dDAVP (0.5 ng/h, dDAVP, n = 10), aldosterone (Aldo, 150 μg/day, n = 10) or combined dDAVP and aldosterone treatment (dDAVP+Aldo, n = 10) and had free access to water with a fixed food intake. Aldosterone treatment induced hypokalemia, decreased urine osmolality, and increased the urine volume and water intake in ALDO compared with CON and dDAVP+Aldo compared with dDAVP. Immunohistochemistry and semiquantitative laser confocal microscopy revealed a distinct increase in basolateral domain AQP2 labeling in cortical collecting duct (CCD) principal cells and a reduction in apical domain labeling in Aldo compared with CON rats. Given the presence of hypokalemia in aldosterone-treated rats, we studied dietary-induced hypokalemia in rats, which also reduced apical AQP2 expression in the CCD but did not induce any increase in basolateral AQP2 expression in the CCD as observed with aldosterone treatment. The aldosterone-induced basolateral AQP2 expression in the CCD was thus independent of hypokalemia but was dependent on the presence of sodium and aldosterone. This redistribution was clearly blocked by mineralocorticoid receptor blockade. The increased basolateral expression of AQP2 induced by aldosterone may play a significant role in water metabolism in conditions with increased sodium reabsorption in the CCD.

Aquaporin 2 (AQP2) and vasopressin type 2 receptor (V2R) endocytosis in kidney epithelial cells: AQP2 is located in 'endocytosis-resistant' membrane domains after vasopressin treatment

BACKGROUND INFORMATION

Aquaporin 2 (AQP2) plays an important, VP (vasopressin)-regulated role in water reabsorption by the kidney. The amount of AQP2 expressed at the surface of principal cells results from an equilibrium between the AQP2 in intracellular vesicles and the AQP2 on the plasma membrane. VP shifts the equilibrium in favour of the plasma membrane and this allows osmotic equilibration to occur between the collecting duct lumen and the interstitial space. Membrane accumulation of AQP2 could result from a VP-induced increase in exocytosis, a decrease in endocytosis, or both. In the present study, we further investigated AQP2 accumulation at the cell surface, and compared it with V2R (VP type 2 receptor) trafficking using cells that express epitope-tagged AQP2 and V2R.

RESULTS

Endocytosis of V2R and of AQP2 are independent events that can be separated temporally and spatially. The burst of endocytosis seen after VP addition to target cells, when AQP2 accumulates at the cell surface, is primarily due to internalization of the V2R. Increased endocytosis is not induced by forskolin, which also induces membrane accumulation of AQP2 by direct stimulation of adenylate cyclase. This indicates that cAMP elevation is not the primary cause of the initial, VP-induced endocytic process. After VP exposure, AQP2 is not located in endosomes with internalized V2R. Instead, it remains at the cell surface in 'endocytosis-resistant' membrane domains, visualized by confocal imaging. After VP washout, AQP2 is progressively internalized with the fluid-phase marker FITC-dextran, indicating that VP washout releases an endocytotic block that maintains AQP2 at the cell surface. Finally, polarized application of VP to filter-grown cells shows that apical VP can induce basolateral endocytosis and V2R down-regulation, and vice versa.

CONCLUSIONS

After VP stimulation of renal epithelial cells, AQP2 accumulates at the cell surface, while the V2R is actively internalized. This endocytotic block may involve a reduced capacity of phosphorylated AQP2 to interact with components of the endocytotic machinery. In addition, a complex cross-talk exists between the apical and basolateral plasma-membrane domains with respect to endocytosis and V2R down-regulation. This may be of physiological significance in down-regulating the VP response in the kidney in vivo.