Ezrin directly interacts with AQP2 and promotes its endocytosis.

The water channel aquaporin-2 (AQP2) is a major regulator of water homeostasis in response to vasopressin (VP). Dynamic trafficking of AQP2 relies on its close interaction with trafficking machinery proteins and the actin cytoskeleton. Here, we report the identification of ezrin, an actin-binding protein from the ezrin/radixin/moesin (ERM) family as an AQP2-interacting protein. Ezrin was first detected in a co-immunoprecipitation (co-IP) complex using an anti-AQP2 antibody in a proteomic analysis. Immunofluorescence staining revealed the co-expression of ezrin and AQP2 in collecting duct principal cells, and VP treatment caused redistribution of both proteins to the apical membrane. The ezrin-AQP2 interaction was confirmed by co-IP experiments with an anti-ezrin antibody, and by pulldown assays using purified full-length and FERM domain-containing recombinant ezrin. By using purified recombinant proteins, we showed that ezrin directly interacts with AQP2 C-terminus through its N-terminal FERM domain. Knocking down ezrin expression with shRNA resulted in increased membrane accumulation of AQP2 and reduced AQP2 endocytosis. Therefore, through direct interaction with AQP2, ezrin facilitates AQP2 endocytosis, thus linking the dynamic actin cytoskeleton network with AQP2 trafficking.

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.

Vasopressin induces apical expression of caveolin in rat kidney collecting duct principal cells.

Caveolin (Cav)1 is expressed in the basolateral membrane domain of renal collecting duct (CD) principal cells (PCs), where it is associated with caveolae. To reveal any potential involvement of Cav1 in vasopressin signaling, we used specific monoclonal and polyclonal antibodies to examine its localization in CD PCs of Brattleboro (BB) rats treated with vasopressin (DDAVP). Compared with controls, immunofluorescence revealed a time-dependent increase in Cav1 expression in the apical membrane domain of PCs, where it overlapped with aquaporin-2 (AQP2). After 24 h of DDAVP treatment, Cav1 was visible as an increased number of small apical spots. The staining gradually became more extensive, and, after 2 wk of DDAVP, it occupied the majority of the apical membrane domain of many PCs. Cav1 also assumed an apical localization in PCs of DDAVP-treated Sprague-Dawley and Long-Evans rats. Similarly, Cav2 appeared at the apical pole of PCs after DDAVP treatment of BB, Sprague-Dawley, and Long-Evans rats. Immunogold electron microscopy confirmed bipolar Cav1 membrane expression in DDAVP-treated BB rats, whereas caveolae were only detected on the basolateral membrane. Immunoblot analysis of BB rat whole kidney homogenates revealed no significant increase in Cav1 levels in DDAVP-treated rats, suggesting that DDAVP induces Cav1 relocalization or modifies its targeting. We conclude that Cav1 and Cav2 trafficking and membrane localization are dramatically altered by the action of DDAVP. Importantly, the absence of apical caveolae indicates that while Cavs may have an as yet undetermined role in vasopressin-regulated signaling processes, this is probably unrelated to AQP2 internalization by caveolae.

A zebrafish model of conditional targeted podocyte ablation and regeneration.

Podocytes are specialized cells that contribute critically to the normal structure and function of the glomerular filtration barrier. Their depletion plays an important role in the pathogenesis of glomerulosclerosis. Here, we report generation of a genetic model of conditional podocyte ablation and regeneration in zebrafish using a bacterial nitroreductase strategy to convert a prodrug, metronidazole, into a cytotoxic metabolite. A transgenic zebrafish line was generated that expresses green fluorescence protein (GFP) and the nitroreductase fusion protein under the control of the podocin promoter Tg(podocin:nitroreductase-GFP). Treatment of these transgenic zebrafish with metronidazole results in podocyte apoptosis, a loss of nephrin and podocin expression, foot process effacement, and a leaky glomerular filtration barrier. Following metronidazole washout, proliferating cells were detected in the glomeruli of recovering transgenic fish with a restoration of nitroreductase-GFP fluorescence, nephrin and podocin expression, a reestablishment of normal foot process architecture, and glomerular barrier function. Thus, our studies show that zebrafish podocytes are capable of regenerating following depletion, and establish the Tg(podocin:NTR-GFP) fish as a new model to study podocyte injury and repair.

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.

Calcitonin has a vasopressin-like effect on aquaporin-2 trafficking and urinary concentration.

The most common cause of hereditary nephrogenic diabetes insipidus is a nonfunctional vasopressin (VP) receptor type 2 (V2R). Calcitonin, another ligand of G-protein-coupled receptors, has a VP-like effect on electrolytes and water reabsorption, suggesting that it may affect AQP2 trafficking. Here, calcitonin increased intracellular cAMP and stimulated the membrane accumulation of AQP2 in LLC-PK1 cells. Pharmacologic inhibition of protein kinase A (PKA) and deficiency of a critical PKA phosphorylation site on AQP2 both prevented calcitonin-induced membrane accumulation of AQP2. Fluorescence assays showed that calcitonin led to a 70% increase in exocytosis and a 20% decrease in endocytosis of AQP2. Immunostaining of rat kidney slices demonstrated that calcitonin induced a significant redistribution of AQP2 to the apical membrane of principal cells in cortical collecting ducts and connecting segments but not in the inner stripe or inner medulla. Calcitonin-treated VP-deficient Brattleboro rats had a reduced urine flow and two-fold higher urine osmolality during the first 12 hours of treatment compared with control groups. Although this VP-like effect of calcitonin diminished over the following 72 hours, the tachyphylaxis was reversible. Taken together, these data show that calcitonin induces cAMP-dependent AQP2 trafficking in cortical collecting and connecting tubules in parallel with an increase in urine concentration. This suggests that calcitonin has a potential therapeutic use in nephrogenic diabetes insipidus.

Bypassing vasopressin receptor signaling pathways in nephrogenic diabetes insipidus.

Water reabsorption in the kidney represents a critical physiological event in the maintenance of body water homeostasis. This highly regulated process relies largely on vasopressin (VP) action and on the VP-sensitive water channel (AQP2) that is expressed in principal cells of the kidney collecting duct. Defects in the VP signaling pathway and/or in AQP2 cell surface expression can lead to an inappropriate reduction in renal water reabsorption and the development of nephrogenic diabetes insipidus, a disease characterized by polyuria and polydipsia. This review focuses on the major regulatory steps that are involved in AQP2 trafficking and function. Specifically, we begin with a discussion on VP-receptor-independent mechanisms of AQP2 trafficking, with special emphasis on the nitric oxide-cyclic guanosine monophosphate signaling pathway, followed by a review of the mechanisms that govern AQP2 endocytosis and exocytosis. We then discuss emerging data illustrating roles played by the actin cytoskeleton on AQP2 trafficking, and lastly we consider elements that affect AQP2 protein expression in cells. Recent advances in each topic are summarized and are presented in the context of their potential to serve as a basis for the development of novel therapies that may ultimately improve life quality of nephrogenic diabetes insipidus patients.

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.

Phosphorylation events and the modulation of aquaporin 2 cell surface expression.

PURPOSE OF REVIEW: This review highlights the role of phosphorylation in the trafficking and targeting of aquaporin 2. Current knowledge will be put into the context of modulating the cell surface expression of aquaporin 2 by vasopressin in renal epithelial cells, which is critical for regulation of urinary concentration and control of fluid and electrolyte homeostasis. RECENT FINDINGS: In addition to previously identified phosphorylation sites on aquaporin 2, new data have revealed three other serine residues in the C-terminus whose phosphorylation is altered by vasopressin. Several steps in aquaporin 2 recycling, including exocytosis and endocytosis, are coordinated by phosphorylation and dephosphorylation to regulate cell surface accumulation. Aquaporin 2 phosphorylation on serine 256 regulates aquaporin 2 association with proteins that are involved in trafficking, including hsc/hsp70 and myelin and lymphocyte-associated protein. SUMMARY: Aquaporin 2 trafficking is regulated by phosphorylation of serine 256 and other amino acid residues in its cytoplasmic domain. These events increase or decrease interaction of aquaporin 2 with key regulatory proteins to determine the cellular distribution and fate of aquaporin 2, both after vasopressin addition and under baseline conditions. Better understanding of these mechanisms may provide new therapeutic avenues for patients with X-linked nephrogenic diabetes insipidus, as well as providing basic cell biological information relevant to membrane trafficking processes in general.

Association of soluble adenylyl cyclase with the V-ATPase in renal epithelial cells.

Activation of soluble adenylyl cyclase (sAC) by bicarbonate causes local cAMP generation, indicating that sAC might act as a pH and/or bicarbonate sensor in kidney cells involved in acid-base homeostasis. Therefore, we examined the expression of sAC in renal acid-base transporting intercalated cells (IC) and compared its distribution to that of the vacuolar proton pumping ATPase (V-ATPase) under different conditions. In all IC, sAC and V-ATPase showed considerable overlap under basal conditions, but sAC staining was also found in other cellular locations in the absence of V-ATPase. In type A-IC, both sAC and V-ATPase were apically and subapically located, whereas in type B-IC, significant basolateral colocalization of sAC and the V-ATPase was seen. When apical membrane insertion of the V-ATPase was stimulated by treatment of rats with acetazolamide, sAC was also concentrated in the apical membrane of A-IC. In mice that lack a functional B1 subunit of the V-ATPase, sAC was colocalized apically in A-IC along with V-ATPase containing the alternative B2 subunit isoform. The close association between these two enzymes was confirmed by coimmunoprecipitation of sAC from kidney homogenates using anti-V-ATPase antibodies. Our data show that sAC and the V-ATPase colocalize in IC, that they are concentrated in the IC plasma membrane under conditions that "activate" these proton secretory cells, and that they are both present in an immunoprecipitated complex. This suggests that these enzymes have a close association and could be part of a protein complex that is involved in regulating renal distal proton secretion.

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.

Alix (AIP1) is a vasopressin receptor (V2R)-interacting protein that increases lysosomal degradation of the V2R.

The vasopressin type 2 receptor (V2R) is a G protein-coupled receptor that plays a central role in renal water reabsorption. Termination of ligand (vasopressin) stimulation is an important physiological regulatory event, but few proteins that interact with the V2R during downregulation after vasopressin (VP) binding have been identified. Using yeast two-hybrid screening of a human kidney cDNA library, we show that a 100-kDa protein called ALG-2-interacting protein X (Alix) interacts with the last 29 amino acids of the V2R COOH terminus. This was confirmed by pull-down assays using a GST-V2R-COOH-tail fusion protein. Alix was immunolocalized in principal cells of the kidney, which also express the V2R. The function of the Alix-V2R interaction was studied by transfecting Alix into LLC-PK(1) epithelial cells expressing V2R-green fluorescent protein (GFP). Under basal conditions, V2R-GFP localized mainly at the plasma membrane. On VP treatment, V2R-GFP was internalized into perinuclear vesicles in the nontransfected cells. In contrast, V2R-GFP fluorescence was virtually undetectable 2 h after exposure to VP in cells that coexpressed Alix. Western blotting using an anti-GFP antibody showed marked degradation of the V2R after 2 h in the presence of VP and Alix, a time point at which little or no degradation was detected in the absence of Alix. In contrast, little or no degradation of the parathyroid hormone receptor was detectable in the presence or absence of Alix and/or the PTH ligand. The VP-induced disappearance of V2R-GFP was abolished by chloroquine, a lysosomal degradation inhibitor, but not by MG132, a proteosome inhibitor. These data suggest that Alix increases the rate of lysosomal degradation of V2R and may play an important regulatory role in the VP response by modulating V2R downregulation.