Protein phosphatase 2C is responsible for VP-induced dephosphorylation of AQP2 serine 261

Identification of protein kinase A signalling molecules in renal collecting ducts

Body water homeostasis is maintained by the correct balance between water intake and water loss through urine, faeces, sweat and breath. It is known that elevated circulating levels of the antidiuretic hormone vasopressin decrease urine volume to prevent excessive water loss from the body. Vasopressin/cAMP/protein kinase A (PKA) signalling is the canonical pathway in renal collecting ducts for phosphorylating aquaporin-2 (AQP2) water channels, which leads to the reabsorption of water from urine via AQP2. Although recent omics data have verified various downstream targets of PKA, crucial regulators that mediate PKA-induced AQP2 phosphorylation remain unknown, mainly because vasopressin is usually used to activate PKA as a positive control. Vasopressin is extremely potent and phosphorylates various PKA substrates non-specifically, making it difficult to narrow down the candidate mediators responsible for AQP2 phosphorylation. The intracellular localization of PKA is tightly regulated by its scaffold proteins, also known as A-kinase anchoring proteins (AKAPs). Furthermore, each AKAP has a target domain that determines its intracellular localization, enabling the creation of a local PKA signalling network. Although vasopressin activates most PKAs independently of their intracellular localization, some chemical compounds preferentially act on PKAs localized on AQP2-containing vesicles while simultaneously phosphorylating AQP2 and its surrounding PKA substrates. Immunoprecipitation with antibodies against phosphorylated PKA substrates followed by mass spectrometry analysis revealed that the PKA substrate in proximity to AQP2 was lipopolysaccharide-responsive and beige-like anchor (LRBA). Furthermore, Lrba knockout studies revealed that LRBA was required for vasopressin-induced AQP2 phosphorylation.

Many kinases for controlling the water channel aquaporin-2

Aquaporin-2 (AQP2) is a member of the aquaporin water channel family. In the kidney, AQP2 is expressed in collecting duct principal cells where it facilitates water reabsorption in response to antidiuretic hormone (arginine vasopressin, AVP). AVP induces the redistribution of AQP2 from intracellular vesicles and its incorporation into the plasma membrane. The plasma membrane insertion of AQP2 represents the crucial step in AVP-mediated water reabsorption. Dysregulation of the system preventing the AQP2 plasma membrane insertion causes diabetes insipidus (DI), a disease characterised by an impaired urine concentrating ability and polydipsia. There is no satisfactory treatment of DI available. This review discusses kinases that control the localisation of AQP2 and points out potential kinase-directed targets for the treatment of DI.

Regulation of the water channel aquaporin-2 by cullin E3 ubiquitin ligases

Aquaporin 2 (AQP2) is a vasopressin (VP)-regulated water channel in the renal collecting duct. Phosphorylation and ubiquitylation of AQP2 play an essential role in controlling the cellular abundance of AQP2 and its accumulation on the plasma membrane in response to VP. Cullin-RING ubiquitin ligases (CRLs) are multisubunit E3 ligases involved in ubiquitylation and degradation of their target proteins, eight of which are expressed in the collecting duct. Here, we used an established cell model of the collecting duct (mpkCCD14 cells) to study the role of cullins in modulating AQP2. Western blotting identified Cul-1 to Cul-5 in mpkCCD14 cells. Treatment of cells for 4 h with a pan-cullin inhibitor (MLN4924) decreased AQP2 abundance, prevented a VP-induced reduction in AQP2 Ser261 phosphorylation, and attenuated VP-induced plasma membrane accumulation of AQP2 relative to the vehicle. AQP2 ubiquitylation levels were significantly higher after MLN4924 treatment compared with controls, and they remained higher despite VP treatment. Cullin inhibition increased ERK1/2 activity, a kinase that regulates AQP2 Ser261 phosphorylation, and VP-induced reductions in ERK1/2 phosphorylation were absent during MLN4924 treatment. Furthermore, the greater Ser261 phosphorylation and reduction in AQP2 abundance during MLN4924 treatment were attenuated during ERK1/2 inhibition. MLN4924 increased intracellular calcium levels via calcium release-activated calcium channels, inhibition of which abolished MLN4924 effects on Ser261 phosphorylation and AQP2 abundance. In conclusion, CRLs play a vital role in mediating some of the effects of VP to increase AQP2 plasma membrane accumulation and AQP2 abundance. Whether modulation of cullin activity can contribute to body water homeostasis requires further studies.NEW & NOTEWORTHY Aquaporin 2 (AQP2) is essential for body water homeostasis and is regulated by the antidiuretic hormone vasopressin. The posttranslational modification ubiquitylation is a key regulator of AQP2 abundance and plasma membrane localization. Here we demonstrate that cullin-RING E3 ligases play a vital role in mediating some of the effects of vasopressin to increase AQP2 abundance and plasma membrane accumulation. The results suggest that manipulating cullin activity could be a novel strategy to alter kidney water handling.

Collecting duct water permeability inhibition by EGF is associated with decreased cAMP, PKA activity, and AQP2 phosphorylation at Ser269

Prior studies showed that epidermal growth factor (EGF) inhibits vasopressin-stimulated osmotic water permeability in the renal collecting duct. Here, we investigated the underlying mechanism. Using isolated perfused rat inner medullary collecting ducts (IMCDs), we found that the addition of EGF to the peritubular bath significantly decreased 1-deamino-8-d-arginine vasopressin (dDAVP)-stimulated water permeability, confirming prior observations. The inhibitory effect of EGF on water permeability was associated with a reduction in intracellular cAMP levels and protein kinase A (PKA) activity. Using phospho-specific antibodies and immunoblotting in IMCD suspensions, we showed that EGF significantly reduces phosphorylation of AQP2 at Ser264 and Ser269. This effect was absent when 8-cpt-cAMP was used to induce AQP2 phosphorylation, suggesting that EGF's inhibitory effect was at a pre-cAMP step. Immunofluorescence labeling of microdissected IMCDs showed that EGF significantly reduced apical AQP2 abundance in the presence of dDAVP. To address what protein kinase might be responsible for Ser269 phosphorylation, we used Bayesian analysis to integrate multiple-omic datasets. Thirteen top-ranked protein kinases were subsequently tested by in vitro phosphorylation experiments for their ability to phosphorylate AQP2 peptides using a mass spectrometry readout. The results show that the PKA catalytic-α subunit increased phosphorylation at Ser256, Ser264, and Ser269. None of the other kinases tested phosphorylated Ser269. In addition, H-89 and PKI strongly inhibited dDAVP-stimulated AQP2 phosphorylation at Ser269. These results indicate that EGF decreases the water permeability of the IMCD by inhibiting cAMP production, thereby inhibiting PKA and decreasing AQP2 phosphorylation at Ser269, a site previously shown to regulate AQP2 endocytosis.NEW & NOTEWORTHY The authors used native rat collecting ducts to show that inhibition of vasopressin-stimulated water permeability by epidermal growth factor involves a reduction of aquaporin 2 phosphorylation at Ser269, a consequence of reduced cAMP production and PKA activity.

Vasopressin V2 receptor, tolvaptan, and ERK1/2 phosphorylation in the renal collecting duct

Tolvaptan, a vasopressin antagonist selective for the V2-subtype vasopressin receptor (V2R), is widely used in the treatment of hyponatremia and autosomal-dominant polycystic kidney disease (ADPKD). Its effects on signaling in collecting duct cells have not been fully characterized. Here, we perform RNA-seq in a collecting duct cell line (mpkCCD). The data show that tolvaptan inhibits the expression of mRNAs that were previously shown to be increased in response to vasopressin including aquaporin-2, but also reveals mRNA changes that were not readily predictable and suggest off-target actions of tolvaptan. One such action is activation of the MAPK kinase (ERK1/ERK2) pathway. Prior studies have shown that ERK1/ERK2 activation is essential in the regulation of a variety of cellular and physiological processes and can be associated with cell proliferation. In immunoblotting experiments, we demonstrated that ERK1/ERK2 phosphorylation in mpkCCD cells was significantly reduced by vasopressin, in contrast to the increases seen in non-collecting-duct cells overexpressing V2R in prior studies. We also found that tolvaptan has a strong effect to increase ERK1/ERK2 phosphorylation in the presence of vasopressin and that tolvaptan's effect to increase ERK1/ERK2 phosphorylation is absent in mpkCCD cells in which both protein kinase A (PKA)-catalytic subunits have been deleted. Thus, it appears that the tolvaptan effect to increase ERK activation is PKA-dependent and is not due to an off-target effect of tolvaptan. We conclude that in cells expressing V2R at endogenous levels: 1) vasopressin decreases ERK1/ERK2 activation; 2) in the presence of vasopressin, tolvaptan increases ERK1/ERK2 activation; and 3) these effects are PKA-dependent.NEW & NOTEWORTHY Vasopressin is a key hormone that regulates the function of the collecting duct of the kidney. ERK1 and ERK2 are enzymes that play key roles in physiological regulation in all cells. The authors used collecting duct cell cultures to investigate the effects of vasopressin and the vasopressin receptor antagonist tolvaptan on ERK1 and ERK2 phosphorylation and activation.

Structural and functional analysis of aquaporin-2 mutants involved in nephrogenic diabetes insipidus

Aquaporins are water channels found in the cell membrane, where they allow the passage of water molecules in and out of the cells. In the kidney collecting duct, arginine vasopressin-dependent trafficking of aquaporin-2 (AQP2) fine-tunes reabsorption of water from pre-urine, allowing precise regulation of the final urine volume. Point mutations in the gene for AQP2 may disturb this process and lead to nephrogenic diabetes insipidus (NDI), whereby patients void large volumes of highly hypo-osmotic urine. In recessive NDI, mutants of AQP2 are retained in the endoplasmic reticulum due to misfolding. Here we describe the structural and functional characterization of three AQP2 mutations associated with recessive NDI: T125M and T126M, situated close to a glycosylation site and A147T in the transmembrane region. Using a proteoliposome assay, we show that all three mutants permit the transport of water. The crystal structures of T125M and T126M together with biophysical characterization of all three mutants support that they retain the native structure, but that there is a significant destabilization of A147T. Our work provides unique molecular insights into the mechanisms behind recessive NDI as well as deepens our understanding of how misfolded proteins are recognized by the ER quality control system.

Intracellular sites of AQP2 S256 phosphorylation identified using inhibitors of the AQP2 recycling itinerary

Vasopressin (VP)-regulated aquaporin-2 (AQP2) trafficking between cytoplasmic vesicles and the plasma membrane of kidney principal cells is essential for water homeostasis. VP affects AQP2 phosphorylation at several serine residues in the COOH-terminus; among them, serine 256 (S256) appears to be a major regulator of AQP2 trafficking. Mutation of this serine to aspartic acid, which mimics phosphorylation, induces constitutive membrane expression of AQP2. However, the intracellular location(s) at which S256 phosphorylation occurs remains elusive. Here, we used strategies to block AQP2 trafficking at different cellular locations in LLC-PK1 cells and monitored VP-stimulated phosphorylation of S256 at these sites by immunofluorescence and Western blot analysis with phospho-specific antibodies. Using methyl-β-cyclodextrin, cold block or bafilomycin, and taxol, we blocked AQP2 at the plasma membrane, in the perinuclear trans-Golgi network, and in scattered cytoplasmic vesicles, respectively. Regardless of its cellular location, VP induced a significant increase in S256 phosphorylation, and this effect was not dependent on a functional microtubule cytoskeleton. To further investigate whether protein kinase A (PKA) was responsible for S256 phosphorylation in these cellular compartments, we created PKA-null cells and blocked AQP2 trafficking using the same procedures. We found that S256 phosphorylation was no longer increased compared with baseline, regardless of AQP2 localization. Taken together, our data indicate that AQP2 S256 phosphorylation can occur at the plasma membrane, in the trans-Golgi network, or in cytoplasmic vesicles and that this event is dependent on the expression of PKA in these cells.NEW & NOTEWORTHY Phosphorylation of aquaporin-2 by PKA at serine 256 (S256) occurs in various subcellular locations during its recycling itinerary, suggesting that the protein complex necessary for AQP2 S256 phosphorylation is present in these different recycling stations. Furthermore, we showed, using PKA-null cells, that PKA activity is required for vasopressin-induced AQP2 phosphorylation. Our data reveal a complex spatial pattern of intracellular AQP2 phosphorylation at S256, shedding new light on the role of phosphorylation in AQP2 membrane accumulation.

Comparative quantitation of aquaporin-2 and arginine vasopressin receptor-2 localizations among chronic kidney disease and healthy kidney in dogs

BACKGROUND AND AIM

Aquaporin-2 (AQP2) and arginine vasopressin receptor-2 (AVPR2) are proteins that control water homeostasis in principal cells. Chronic kidney disease (CKD) is defined as the impairment and irreversible loss of kidney function and/or structure, which causes water imbalances and polyuria. The study aimed to know the expression of AQPs and AVPR2 in the kidneys of a canine with CKD.

MATERIALS AND METHODS

The kidneys were collected from two dog carcasses from Small Animal Teaching Hospital, Faculty of Veterinary Medicine, Chiang Mai University. The kidney tissue was prepared for immunohistochemistry and investigated the expression and localization of tissue's AQP2 and AVPR2. For statistical analysis, the Mann-Whitney U-test was applied to the data.

RESULTS

By immunohistochemistry, AQP2 was expressed strongly in the basolateral and apical membranes of the principal cells, whereas AVPR2 was localized in the principal cell's basolateral membrane in both renal cortex and renal medulla. In the normal kidney, the semi-quantitative immunohistochemistry for the percentage of protein expression of AQP2 and AVPR2 was 5.062±0.4587 and 4.306±0.7695, respectively. In contrast, protein expression of AQP2 and AVPR2 in CKD was found to be 1.218±0.1719 and 0.8536±0.1396, respectively. The data shows that the percentage of AQP2 and AVPR2 expression was decreased, corresponding to a 4-fold and 5-fold in CKD (p<0.001).

CONCLUSION

Our findings revealed that CKD was a marked decrease in AQP2 and AVPR2 expression. The central role of specific AQP2 and AVPR2 in regulating water homeostasis will provide correlations in case of CKD with polyuria.

Activation of AQP2 water channels by protein kinase A: therapeutic strategies for congenital nephrogenic diabetes insipidus

Background

Congenital nephrogenic diabetes insipidus (NDI) is primarily caused by loss-of-function mutations in the vasopressin type 2 receptor (V2R). Renal unresponsiveness to the antidiuretic hormone vasopressin impairs aquaporin-2 (AQP2) water channel activity and water reabsorption from urine, resulting in polyuria. Currently available symptomatic treatments inadequately reduce patients’ excessive amounts of urine excretion, threatening their quality of life. In the past 25 years, vasopressin/cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) has been believed to be the most important signaling pathway for AQP2 activation. Although cAMP production without vasopressin is the reasonable therapeutic strategy for congenital NDI caused by V2R mutations, the efficacy of candidate drugs on AQP2 activation is far less than that of vasopressin.

Results

Intracellular distribution and activity of PKA are largely controlled by its scaffold proteins, A-kinase anchoring proteins (AKAPs). Dissociating the binding of AKAPs and PKA significantly increased PKA activity in the renal collecting ducts and activated AQP2 phosphorylation and trafficking. Remarkably, the AKAPs–PKA disruptor FMP-API-1 increased transcellular water permeability in isolated renal collecting ducts to the same extent as vasopressin. Moreover, derivatives of FMP-API-1 possessed much more high potency. FMP-API-1/27 is the first low-molecular-weight compound to be discovered that can phosphorylate AQP2 more effectively than preexisting drug candidates.

Conclusion

AKAP-PKA disruptors are a promising therapeutic target for congenital NDI. In this article, we shall discuss the pathophysiological roles of PKA and novel strategies to activate PKA in renal collecting ducts.

The Antiangiogenic Effect of Sanguinarine Chloride on Experimental Choroidal Neovacularization in Mice via Inhibiting Vascular Endothelial Growth Factor

Background: The purpose of this study is to investigate the antiangiogenic effect of Sanguinarine chloride (SC) on models of age-related macular degeneration (AMD) both in vivo and in vitro.

Methods: Choroidal neovascularization (CNV) was conducted by laser photocoagulation in C57BL6/J mice. SC (2.5 μM, 2 μl/eye) was intravitreally injected immediately after laser injury. The control group received an equal amount of PBS. 7 days after laser injury, CNV severity was evaluated using fundus fluorescein angiography, hematoxylin and eosin (H&E) staining, and choroid flat-mount staining. Vascular endothelial growth factor (VEGF) expression in the retina/choroid complex was measured by western blot analysis and ELISA kit. In vitro, human retinal microvascular endothelial cells (HRMECs) were used to investigate the effects of SC on cell tube formation, migration, and cytotoxicity. The expression of VEGF-induced expression of extracellular signal-regulated kinase (ERK)1/2, protein kinase B (AKT), mitogen-activated protein kinases (p38-MAPK) in vitro and laser induced VEGF expression in vivo were also analyzed.

Results: SC (≤2.5 μM) was safe both in vitro and in vivo. Intravitreal injection of SC restrained the formation of laser induced CNV in mice and decreased VEGF expression in the laser site of the retina/choroid complex. In vitro, SC inhibited VEGF-induced tube formation and endothelial cell migration by decreasing the phosphorylation of AKT, ERK1/2, and p38-MAPK in HRMECs.

Conclusions: SC could inhibit laser-induced CNV formation via down-regulating VEGF expression and restrain the VEGF-induced tube formation and endothelial migration. Therefore, SC could be a potential candidate for the treatment of wet AMD.

Human Pluripotent Stem Cell-Derived Kidney Organoids with Improved Collecting Duct Maturation and Injury Modeling

Maximizing the potential of human kidney organoids for drug testing and regenerative medicine and to model development and disease requires addressing cell immaturity, the lack of a mature collecting system, and off-target cell types. By independently generating two kidney progenitor cell populations-metanephric mesenchyme and ureteric bud (UB)-like cells-we could generate kidney organoids with a collecting system. We also identify the hormones aldosterone and arginine vasopressin (AVP) as critical to promote differentiation of collecting duct cell types including both principal cells (PCs) and intercalated cells (ICs). The resulting PCs express aquaporin-2 (AQP2) protein, which undergoes translocation to the apical membrane after vasopressin or forskolin stimulation. By single-cell RNA sequencing (scRNA-seq), we demonstrate improved proximal tubule maturation and reduced off-target cell populations. We also show appropriate downregulation of progenitor cell types, improved modeling of tubular injury, the presence of urothelium (Uro), and the ability of Notch pathway modulation to regulate PC:IC ratios during organoid development.

Advances in aquaporin-2 trafficking mechanisms and their implications for treatment of water balance disorders

In mammals, conservation of body water is critical for survival and is dependent on the kidneys' ability to minimize water loss in the urine during periods of water deprivation. The collecting duct water channel aquaporin-2 (AQP2) plays an essential role in this homeostatic response by facilitating water reabsorption along osmotic gradients. The ability to increase the levels of AQP2 in the apical plasma membrane following an increase in plasma osmolality is a rate-limiting step in water reabsorption, a process that is tightly regulated by the antidiuretic hormone arginine vasopressin (AVP). In this review, the focus is on the role of the carboxyl-terminus of AQP2 as a key regulatory point for AQP2 trafficking. We provide an overview of AQP2 structure, disease-causing mutations in the AQP2 carboxyl-terminus, the role of posttranslational modifications such as phosphorylation and ubiquitylation in the tail domain, and their implications for balanced trafficking of AQP2. Finally, we discuss how various modifications of the AQP2 tail facilitate selective protein-protein interactions that modulate the AQP2 trafficking mechanism.

Direct effect of protein kinase A on four putative phosphorylation sites of aquaporin 2 in vitro

The water channel aquaporin 2 (AQP2) has four phosphorylation sites at Ser256, Ser261, Ser264, and Ser269 in the C-terminus and these sites are important for AQP2 bioactivity. However, the exact role of each phosphorylation site still remains unclear. In this study, we generated unique AQP2 mutants in which we eliminated three phosphorylation sites but maintained only one site at the C-terminal end. The AQP2 phosphorylation of each single site by protein kinase A (PKA) was examined by in vitro translation and 32P incorporation. The ability of AQP2 trafficking to the cell membrane was evaluated by cell surface biotinylation. Among the four phosphorylation sites, AQP2 mutant with only S256 preserved the most ability of AQP2 to cell membrane expression. The AQP2 water permeability was measured in oocyte. Ser256 is the most important site for AQP2 function. Interestingly, Ser261 and Ser264 significantly inhibit AQP2 activity. Ser269 slightly but not statistically reduced AQP2 activity. Our data suggest that the four phosphorylation sites execute differential roles in concert in AQP2 functional regulation. AQP2 activity regulated by phosphorylation at Ser256 can be counterbalanced by phosphorylation at Ser261 and Ser264.

Cyclin-Dependent Kinase 18 Controls Trafficking of Aquaporin-2 and Its Abundance through Ubiquitin Ligase STUB1, Which Functions as an AKAP

Arginine-vasopressin (AVP) facilitates water reabsorption in renal collecting duct principal cells through regulation of the water channel aquaporin-2 (AQP2). The hormone binds to vasopressin V2 receptors (V2R) on the surface of the cells and stimulates cAMP synthesis. The cAMP activates protein kinase A (PKA), which initiates signaling that causes an accumulation of AQP2 in the plasma membrane of the cells facilitating water reabsorption from primary urine and fine-tuning of body water homeostasis. AVP-mediated PKA activation also causes an increase in the AQP2 protein abundance through a mechanism that involves dephosphorylation of AQP2 at serine 261 and a decrease in its poly-ubiquitination. However, the signaling downstream of PKA that controls the localization and abundance of AQP2 is incompletely understood. We carried out an siRNA screen targeting 719 kinase-related genes, representing the majority of the kinases of the human genome and analyzed the effect of the knockdown on AQP2 by high-content imaging and biochemical approaches. The screening identified 13 hits whose knockdown inhibited the AQP2 accumulation in the plasma membrane. Amongst the candidates was the so far hardly characterized cyclin-dependent kinase 18 (CDK18). Our further analysis revealed a hitherto unrecognized signalosome comprising CDK18, an E3 ubiquitin ligase, STUB1 (CHIP), PKA and AQP2 that controls the localization and abundance of AQP2. CDK18 controls AQP2 through phosphorylation at serine 261 and STUB1-mediated ubiquitination. STUB1 functions as an A-kinase anchoring protein (AKAP) tethering PKA to the protein complex and bridging AQP2 and CDK18. The modulation of the protein complex may lead to novel concepts for the treatment of disorders which are caused or are associated with dysregulated AQP2 and for which a satisfactory treatment is not available, e.g., hyponatremia, liver cirrhosis, diabetes insipidus, ADPKD or heart failure.

Targeting the Trafficking of Kidney Water Channels for Therapeutic Benefit

The ability to regulate water movement is vital for the survival of cells and organisms. In addition to passively crossing lipid bilayers by diffusion, water transport is also driven across cell membranes by osmotic gradients through aquaporin water channels. There are 13 aquaporins in human tissues, and of these, aquaporin-2 (AQP2) is the most highly regulated water channel in the kidney: The expression and trafficking of AQP2 respond to body volume status and plasma osmolality via the antidiuretic hormone, vasopressin (VP). Dysfunctional VP signaling in renal epithelial cells contributes to disorders of water balance, and research initially focused on regulating the major cAMP/PKA pathway to normalize urine concentrating ability. With the discovery of novel and more complex signaling networks that regulate AQP2 trafficking, promising therapeutic targets have since been identified. Several strategies based on data from preclinical studies may ultimately translate to the care of patients with defective water homeostasis.

Phosphoproteomic identification of vasopressin V2 receptor-dependent signaling in the renal collecting duct

Vasopressin controls water balance largely through PKA-dependent effects to regulate the collecting duct water channel aquaporin-2 (AQP2). Although considerable information has accrued regarding the regulation of water and solute transport in collecting duct cells, information is sparse regarding the signaling connections between PKA and transport responses. Here, we exploited recent advancements in protein mass spectrometry to perform a comprehensive, multiple-replicate analysis of changes in the phosphoproteome of native rat inner medullary collecting duct cells in response to the vasopressin V2 receptor-selective agonist 1-desamino-8D-arginine vasopressin. Of the 10,738 phosphopeptides quantified, only 156 phosphopeptides were significantly increased in abundance, and only 63 phosphopeptides were decreased, indicative of a highly selective response to vasopressin. The list of upregulated phosphosites showed several general characteristics: 1) a preponderance of sites with basic (positively charged) amino acids arginine (R) and lysine (K) in position -2 and -3 relative to the phosphorylated amino acid, consistent with phosphorylation by PKA and/or other basophilic kinases; 2) a greater-than-random likelihood of sites previously demonstrated to be phosphorylated by PKA; 3) a preponderance of sites in membrane proteins, consistent with regulation by membrane association; and 4) a greater-than-random likelihood of sites in proteins with class I COOH-terminal PDZ ligand motifs. The list of downregulated phosphosites showed a preponderance of those with proline in position +1 relative to the phosphorylated amino acid, consistent with either downregulation of proline-directed kinases (e.g., MAPKs or cyclin-dependent kinases) or upregulation of one or more protein phosphatases that selectively dephosphorylate such sites (e.g., protein phosphatase 2A). The phosphoproteomic data were used to create a web resource for the investigation of G protein-coupled receptor signaling and regulation of AQP2-mediated water transport.

Phosphorylation of human AQP2 and its role in trafficking

Human Aquaporin 2 (AQP2) is a membrane-bound water channel found in the kidney collecting duct whose regulation by trafficking plays a key role in regulating urine volume. AQP2 trafficking is tightly controlled by the pituitary hormone arginine vasopressin (AVP), which stimulates translocation of AQP2 residing in storage vesicles to the apical membrane. The AVP-dependent translocation of AQP2 to and from the apical membrane is controlled by multiple phosphorylation sites in the AQP2 C-terminus, the phosphorylation of which alters its affinity to proteins within the cellular membrane protein trafficking machinery. The aim of this chapter is to provide a summary of what is currently known about AVP-mediated AQP2 trafficking, dissecting the roles of individual phosphorylation sites, kinases and phosphatases and interacting proteins. From this, the picture of an immensely complex process emerges, of which many structural and molecular details remains to be elucidated.

Inhibition of non-receptor tyrosine kinase Src induces phosphoserine 256-independent aquaporin-2 membrane accumulation

KEY POINTS

Aquaporin-2 (AQP2) is crucial for water homeostasis, and vasopressin (VP) induces AQP2 membrane trafficking by increasing intracellular cAMP, activating PKA and causing phosphorylation of AQP2 at serine 256, 264 and 269 residues and dephosphorylation of serine 261 residue on the AQP2 C-terminus. It is thought that serine 256 is the master regulator of AQP2 trafficking, and its phosphorylation has to precede the change of phosphorylation state of other serine residues. We found that Src inhibition causes serine 256-independent AQP2 membrane trafficking and induces phosphorylation of serine 269 independently of serine 256. This targeted phosphorylation of serine 269 is important for Src inhibition-induced AQP2 membrane accumulation; without serine 269, Src inhibition exerts no effect on AQP2 trafficking. This result helps us better understand the independent pathways that can target different AQP2 residues, and design new strategies to induce or sustain AQP2 membrane expression when VP signalling is defective.

ABSTRACT

Aquaporin-2 (AQP2) is essential for water homeostasis. Upon stimulation by vasopressin, AQP2 is phosphorylated at serine 256 (S256), S264 and S269, and dephosphorylated at S261. It is thought that S256 is the master regulator of AQP2 trafficking and membrane accumulation, and that its phosphorylation has to precede phosphorylation of other serine residues. In this study, we found that VP reduces Src kinase phosphorylation: by suppressing Src using the inhibitor dasatinib and siRNA, we could increase AQP2 membrane accumulation in cultured AQP2-expressing cells and in kidney collecting duct principal cells. Src inhibition increased exocytosis and inhibited clathrin-mediated endocytosis of AQP2, but exerted its effect in a cAMP, PKA and S256 phosphorylation (pS256)-independent manner. Despite the lack of S256 phosphorylation, dasatinib increased phosphorylation of S269, even in S256A mutant cells in which S256 phosphorylation cannot occur. To confirm the importance of pS269 in AQP2 re-distribution, we expressed an AQP2 S269A mutant in LLC-PK1 cells, and found that dasatinib no longer induced AQP2 membrane accumulation. In conclusion, Src inhibition causes phosphorylation of S269 independently of pS256, and induces AQP2 membrane accumulation by inhibiting clathrin-mediated endocytosis and increasing exocytosis. We conclude that S269 can be phosphorylated without pS256, and pS269 alone is important for AQP2 apical membrane accumulation under some conditions. These data increase our understanding of the independent pathways that can phosphorylate different residues in the AQP2 C-terminus, and suggest new strategies to target distinct AQP2 serine residues to induce membrane expression of this water channel when VP signalling is defective.

Phosphorylation-Dependent Regulation of Mammalian Aquaporins

Water homeostasis is fundamental for cell survival. Transport of water across cellular membranes is governed by aquaporins—tetrameric integral membrane channels that are highly conserved throughout the prokaryotic and eukaryotic kingdoms. In eukaryotes, specific regulation of these channels is required and is most commonly carried out by shuttling the protein between cellular compartments (trafficking) or by opening and closing the channel (gating). Structural and functional studies have revealed phosphorylation as a ubiquitous mechanism in aquaporin regulation by both regulatory processes. In this review we summarize what is currently known about the phosphorylation-dependent regulation of mammalian aquaporins. Focusing on the water-specific aquaporins (AQP0–AQP5), we discuss how gating and trafficking are controlled by phosphorylation and how phosphorylation affects the binding of aquaporins to regulatory proteins, thereby highlighting structural details and dissecting the contribution of individual phosphorylated residues when possible. Our aim is to provide an overview of the mechanisms behind how aquaporin phosphorylation controls cellular water balance and to identify key areas where further studies are needed.

From Molecules to Mechanisms: Functional Proteomics and Its Application to Renal Tubule Physiology

Classical physiological studies using electrophysiological, biophysical, biochemical, and molecular techniques have created a detailed picture of molecular transport, bioenergetics, contractility and movement, and growth, as well as the regulation of these processes by external stimuli in cells and organisms. Newer systems biology approaches are beginning to provide deeper and broader understanding of these complex biological processes and their dynamic responses to a variety of environmental cues. In the past decade, advances in mass spectrometry-based proteomic technologies have provided invaluable tools to further elucidate these complex cellular processes, thereby confirming, complementing, and advancing common views of physiology. As one notable example, the application of proteomics to study the regulation of kidney function has yielded novel insights into the chemical and physical processes that tightly control body fluids, electrolytes, and metabolites to provide optimal microenvironments for various cellular and organ functions. Here, we systematically review, summarize, and discuss the most significant key findings from functional proteomic studies in renal epithelial physiology. We also identify further improvements in technological and bioinformatics methods that will be essential to advance precision medicine in nephrology.

Aquaporin Membrane Channels in Oxidative Stress, Cell Signaling, and Aging: Recent Advances and Research Trends

Reactive oxygen species (ROS) are produced as a result of aerobic metabolism and as by-products through numerous physiological and biochemical processes. While ROS-dependent modifications are fundamental in transducing intracellular signals controlling pleiotropic functions, imbalanced ROS can cause oxidative damage, eventually leading to many chronic diseases. Moreover, increased ROS and reduced nitric oxide (NO) bioavailability are main key factors in dysfunctions underlying aging, frailty, hypertension, and atherosclerosis. Extensive investigation aims to elucidate the beneficial effects of ROS and NO, providing novel insights into the current medical treatment of oxidative stress-related diseases of high epidemiological impact. This review focuses on emerging topics encompassing the functional involvement of aquaporin channel proteins (AQPs) and membrane transport systems, also allowing permeation of NO and hydrogen peroxide, a major ROS, in oxidative stress physiology and pathophysiology. The most recent advances regarding the modulation exerted by food phytocompounds with antioxidant action on AQPs are also reviewed.

Impaired AQP2 trafficking in Fxyd1 knockout mice: A role for FXYD1 in regulated vesicular transport

The final adjustment of urine volume occurs in the inner medullary collecting duct (IMCD), chiefly mediated by the water channel aquaporin 2 (AQP2). With vasopressin stimulation, AQP2 accumulation in the apical plasma membrane of principal cells allows water reabsorption from the lumen. We report that FXYD1 (phospholemman), better known as a regulator of Na,K-ATPase, has a role in AQP2 trafficking. Daytime urine of Fxyd1 knockout mice was more dilute than WT despite similar serum vasopressin, but both genotypes could concentrate urine during water deprivation. FXYD1 was found in IMCD. In WT mice, phosphorylated FXYD1 was detected intracellularly, and vasopressin induced its dephosphorylation. We tested the hypothesis that the dilute urine in knockouts was caused by alteration of AQP2 trafficking. In WT mice at baseline, FXYD1 and AQP2 were not strongly co-localized, but elevation of vasopressin produced translocation of both FXYD1 and AQP2 to the apical plasma membrane. In kidney slices, baseline AQP2 distribution was more scattered in the Fxyd1 knockout than in WT. Apical recruitment of AQP2 occurred in vasopressin-treated Fxyd1 knockout slices, but upon vasopressin washout, there was more rapid reversal of apical AQP2 localization and more heterogeneous cytoplasmic distribution of AQP2. Notably, in sucrose gradients, AQP2 was present in a detergent-resistant membrane domain that had lower sedimentation density in the knockout than in WT, and vasopressin treatment normalized its density. We propose that FXYD1 plays a role in regulating AQP2 retention in apical membrane, and that this involves transfers between raft-like membrane domains in endosomes and plasma membranes.