Non-muscle myosin II and myosin light chain kinase are downstream targets for vasopressin signaling in the renal collecting duct

The knock-out of paracingulin attenuates hypertension through modulation of kidney ion transport

Hypertension is a major risk factor for human morbidity and mortality, and the junctional protein paracingulin (CGNL1, JACOP) is required for the development of hypertension in a Dahl salt-sensitive rat model and is linked to human hypertension in genome wide association studies. However, the mechanism through which CGNL1 may regulate hypertension is unknown. Here, we address this question using a mouse model, where hypertension is induced by unilateral nephrectomy and angiotensin II infusion (N+A protocol). Although untreated WT and CGNL1-KO mice showed similar blood pressure, the N+A protocol induced hypertension in WT mice but not in CGNL1-KO mice. We show by immunolocalization and transcriptomic analysis that CGNL1 is expressed throughout the kidney tubules and in the endothelium of blood vessels, but not in smooth muscle. The N+A protocol induced decreased potassium urinary excretion in wild-type (WT), but not in CGNL1-KO mice. Immunoblot analysis shows that the KO of CGNL1 blunted the N+A-induced changes in the expression levels and activation of tubular ion transporters, including the Na/H exchanger 3 (NHE3) and the thiazide-sensitive Na-Cl cotransporter (NCC), and blunted the angiotensin II-dependent changes in the levels and/or activation of AMP-activated protein kinase (AMPK), ERK and myosin light chain. In contrast, myography showed comparable vascular reactivity in thoracic aortas and mesenteric arteries isolated from WT or CGNL1-KO mice. Together, these results suggest the KO of CGNL1 attenuates hypertension by uncoupling angiotensin II signaling in kidney tubule cells, indicating a novel pathway of regulation of signaling by a junctional protein.NEW & NOTEWORTHY The knock-out of paracingulin (CGNL1) prevents the development of hypertension in a unilateral nephrectomy/angiotensin II infusion model (N+A) in mice and this antihypertensive effect likely depends on uncoupling of angiotensin II from stimulation of sodium transporter activity in kidney tubules rather than on alteration of resistance blood vessel contractility.

Actin cytoskeleton and associated myosin motors within the renal epithelium

This review highlights the complexity of renal epithelial cell membrane architectures and organelles through careful review of ultrastructural and physiological studies published over the past several decades. We also showcase the vital roles played by the actin cytoskeleton and actin-associated myosin motor proteins in regulating cell type-specific physiological functions within the cells of the renal epithelium. The purpose of this review is to provide a fresh conceptual framework to explain the structure-function relationships that exist between the actin cytoskeleton, organelle structure, and cargo transport within the mammalian kidney. With recent advances in technologies to visualize the actin cytoskeleton and associated proteins within intact kidneys, it has become increasingly imperative to reimagine the functional roles of these proteins in situ to provide a rationale for their unique, cell type-specific functions that are necessary to establish and maintain complex physiological processes.

Using CRISPR-Cas9/phosphoproteomics to identify substrates of calcium/calmodulin-dependent kinase 2δ

Ca2+/Calmodulin-dependent protein kinase 2 (CAMK2) family proteins are involved in the regulation of cellular processes in a variety of tissues including brain, heart, liver, and kidney. One member, CAMK2δ (CAMK2D), has been proposed to be involved in vasopressin signaling in the renal collecting duct, which controls water excretion through regulation of the water channel aquaporin-2 (AQP2). To identify CAMK2D target proteins in renal collecting duct cells (mpkCCD), we deleted Camk2d and carried out LC-MS/MS-based quantitative phosphoproteomics. Specifically, we used CRISPR/Cas9 with two different guide RNAs targeting the CAMK2D catalytic domain to create multiple CAMK2D KO cell lines. AQP2 protein abundance was lower in the CAMK2D KO cells than in CAMK2D-intact controls. AQP2 phosphorylation at Ser256 and Ser269 (normalized for total AQP2) was decreased. However, trafficking of AQP2 to and from the apical plasma membrane was sustained. Large-scale quantitative phosphoproteomic analysis (TMT-labeling) in the presence of the vasopressin analog dDAVP (0.1 nM, 30 min) allowed quantification of 11,570 phosphosites of which 169 were significantly decreased, while 206 were increased in abundance in CAMK2D KO clones. These data are available for browsing or download at https://esbl.nhlbi.nih.gov/Databases/CAMK2D-proteome/. Motif analysis of the decreased phosphorylation sites revealed a target preference of -(R/K)-X-X-p(S/T)-X-(D/E), matching the motif identified in previous in vitro phosphorylation studies using recombinant CAMK2D. Thirty five of the significantly downregulated phosphorylation sites in CAMK2D KO cells had exactly this motif and are judged to be likely direct CAMK2D targets. This adds to the list of known CAMK2D target proteins found in prior reductionist studies.

Data resource: vasopressin-regulated protein phosphorylation sites in the collecting duct

Vasopressin controls renal water excretion through actions to regulate aquaporin-2 (AQP2) trafficking, transcription, and degradation. These actions are in part dependent on vasopressin-induced phosphorylation changes in collecting duct cells. Although most efforts have focused on the phosphorylation of AQP2 itself, phosphoproteomic studies have identified many vasopressin-regulated phosphorylation sites in proteins other than AQP2. The goal of this bioinformatics-based review is to create a compendium of vasopressin-regulated phosphorylation sites with a focus on those that are seen in both native rat inner medullary collecting ducts and cultured collecting duct cells from the mouse (mpkCCD), arguing that these sites are the best candidates for roles in AQP2 regulation. This analysis identified 51 vasopressin-regulated phosphorylation sites in 45 proteins. We provide resource web pages at https://esbl.nhlbi.nih.gov/Databases/AVP-Phos/ and https://esbl.nhlbi.nih.gov/AVP-Network/, listing the phosphorylation sites and describing annotated functions of each of the vasopressin-targeted phosphoproteins. Among these sites are 23 consensus protein kinase A (PKA) sites that are increased in response to vasopressin, consistent with a central role for PKA in vasopressin signaling. The remaining sites are predicted to be phosphorylated by other kinases, most notably ERK1/2, which accounts for decreased phosphorylation at sites with a X-p(S/T)-P-X motif. Additional protein kinases that undergo vasopressin-induced changes in phosphorylation are Camkk2, Cdk18, Erbb3, Mink1, and Src, which also may be activated directly or indirectly by PKA. The regulated phosphoproteins are mapped to processes that hypothetically can account for vasopressin-mediated control of AQP2 trafficking, cytoskeletal alterations, and Aqp2 gene expression, providing grist for future studies.NEW & NOTEWORTHY Vasopressin regulates renal water excretion through control of the aquaporin-2 water channel in collecting duct cells. Studies of vasopressin-induced protein phosphorylation have focused mainly on the phosphorylation of aquaporin-2. This study describes 44 phosphoproteins other than aquaporin-2 that undergo vasopressin-mediated phosphorylation changes and summarizes potential physiological roles of each.

Bayesian analysis of dynamic phosphoproteomic data identifies protein kinases mediating GPCR responses

Background

A major goal in the discovery of cellular signaling networks is to identify regulated phosphorylation sites (“phosphosites”) and map them to the responsible protein kinases. The V2 vasopressin receptor is a G-protein coupled receptor (GPCR) that is responsible for regulation of renal water excretion through control of aquaporin-2-mediated osmotic water transport in kidney collecting duct cells. Genome editing experiments have demonstrated that virtually all vasopressin-triggered phosphorylation changes are dependent on protein kinase A (PKA), but events downstream from PKA are still obscure.

Methods

Here, we used: 1) Tandem mass tag-based quantitative phosphoproteomics to experimentally track phosphorylation changes over time in native collecting ducts isolated from rat kidneys; 2) a clustering algorithm to classify time course data based on abundance changes and the amino acid sequences surrounding the phosphosites; and 3) Bayes’ Theorem to integrate the dynamic phosphorylation data with multiple prior “omic” data sets covering expression, subcellular location, known kinase activity, and characteristic surrounding sequences to identify a set of protein kinases that are regulated secondary to PKA activation.

Results

Phosphoproteomic studies revealed 185 phosphosites regulated by vasopressin over 15 min. The resulting groups from the cluster algorithm were integrated with Bayes’ Theorem to produce corresponding ranked lists of kinases likely responsible for each group. The top kinases establish three PKA-dependent protein kinase modules whose regulation mediate the physiological effects of vasopressin at a cellular level. The three modules are 1) a pathway involving several Rho/Rac/Cdc42-dependent protein kinases that control actin cytoskeleton dynamics; 2) mitogen-activated protein kinase and cyclin-dependent kinase pathways that control cell proliferation; and 3) calcium/calmodulin-dependent signaling.

Conclusions

Our findings identify a novel set of downstream small GTPase effectors and calcium/calmodulin-dependent kinases with potential roles in the regulation of water permeability through actin cytoskeleton rearrangement and aquaporin-2 trafficking. The proposed signaling network provides a stronger hypothesis for the kinases mediating V2 vasopressin receptor responses, encouraging future targeted examination via reductionist approaches. Furthermore, the Bayesian analysis described here provides a template for investigating signaling via other biological systems and GPCRs.

Vasopressin and Its Analogues: From Natural Hormones to Multitasking Peptides

Human neurohormone vasopressin (AVP) is synthesized in overlapping regions in the hypothalamus. It is mainly known for its vasoconstricting abilities, and it is responsible for the regulation of plasma osmolality by maintaining fluid homeostasis. Over years, many attempts have been made to modify this hormone and find AVP analogues with different pharmacological profiles that could overcome its limitations. Non-peptide AVP analogues with low molecular weight presented good affinity to AVP receptors. Natural peptide counterparts, found in animals, are successfully applied as therapeutics; for instance, lypressin used in treatment of diabetes insipidus. Synthetic peptide analogues compensate for the shortcomings of AVP. Desmopressin is more resistant to proteolysis and presents mainly antidiuretic effects, while terlipressin is a long-acting AVP analogue and a drug recommended in the treatment of varicose bleeding in patients with liver cirrhosis. Recently published results on diverse applications of AVP analogues in medicinal practice, including potential lypressin, terlipressin and ornipressin in the treatment of SARS-CoV-2, are discussed.

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.

Subcellular distribution of α2-adrenoceptor subtypes in the rodent kidney

Renal α2-adrenoceptors have been reported to play a role in the regulation of urinary output, renin secretion, and water and sodium excretion in the kidneys. However, the distribution of α2-adrenoceptor subtypes in the kidneys remains unclear. In this study, we aimed to investigate the localization of α2-adrenoceptor subtypes in rat kidneys using 8-week-old Sprague-Dawley rats. Immunofluorescence imaging revealed that both α2A- and α2B-adrenoceptors were expressed in the basolateral, but not apical, membrane of the epithelial cells of the proximal tubules. We also found that α2A- and α2B-adrenoceptors were not expressed in the glomeruli, collecting ducts, or the descending limb of the loop of Henle and vasa recta. In contrast, α2C-adrenoceptors were found to be localized in the glomeruli and lumen of the cortical and medullary collecting ducts. These results suggest that noradrenaline may act on the basement membrane of the proximal tubules through α2A- and α2B-adrenoceptors. Moreover, noradrenaline may be involved in the regulation of glomerular filtration and proteinuria through the induction of morphological changes in mesangial cells and podocytes via α2C-adrenoceptors. In the collecting ducts, urinary noradrenaline may regulate morphological changes of the microvilli through α2C-adrenoceptors. Our findings provide an immunohistochemical basis for understanding the cellular targets of α2-adrenergic regulation in the kidneys. This may be used to devise therapeutic strategies targeting α2-adrenoceptors.

Aquaporin 2 regulation: implications for water balance and polycystic kidney diseases

Targeting the collecting duct water channel aquaporin 2 (AQP2) to the plasma membrane is essential for the maintenance of mammalian water homeostasis. The vasopressin V2 receptor (V2R), which is a G_S protein-coupled receptor that increases intracellular cAMP levels, has a major role in this targeting process. Although a rise in cAMP levels and activation of protein kinase A are involved in facilitating the actions of V2R, studies in knockout mice and cell models have suggested that cAMP signalling pathways are not an absolute requirement for V2R-mediated AQP2 trafficking to the plasma membrane. In addition, although AQP2 phosphorylation is a known prerequisite for V2R-mediated plasma membrane targeting, none of the known AQP2 phosphorylation events appears to be rate-limiting in this process, which suggests the involvement of other factors; cytoskeletal remodelling has also been implicated. Notably, several regulatory processes and signalling pathways involved in AQP2 trafficking also have a role in the pathophysiology of autosomal dominant polycystic kidney disease, although the role of AQP2 in cyst progression is unknown. Here, we highlight advances in the field of AQP2 regulation that might be exploited for the treatment of water balance disorders and provide a rationale for targeting these pathways in autosomal dominant polycystic kidney disease.

MYH9 facilitates autoregulation of adipose tissue depot development

White adipose tissue not only serves as a reservoir for energy storage but also secretes a variety of hormonal signals and modulates systemic metabolism. A substantial amount of adipose tissue develops in early postnatal life, providing exceptional access to the formation of this important tissue. Although a number of factors have been identified that can modulate the differentiation of progenitor cells into mature adipocytes in cell-autonomous assays, it remains unclear which are connected to physiological extracellular inputs and are most relevant to tissue formation in vivo. Here, we elucidate that mature adipocytes themselves signal to adipose depot–resident progenitor cells to direct depot formation in early postnatal life and gate adipogenesis when the tissue matures. Our studies revealed that as the adipose depot matures, a signal generated in mature adipocytes is produced, converges on progenitor cells to regulate the cytoskeletal protein MYH9, and attenuates the rate of adipogenesis in vivo.

Phosphoproteomic Identification of Vasopressin/cAMP/Protein Kinase A-Dependent Signaling in Kidney

Water excretion by the kidney is regulated by the neurohypophyseal peptide hormone vasopressin through actions in renal collecting duct cells to regulate the water channel protein aquaporin-2. Vasopressin signaling is initiated by binding to a G-protein-coupled receptor called V2R, which signals through heterotrimeric G-protein subunit Gs α, adenylyl cyclase 6, and activation of the cAMP-regulated protein kinase (PKA). Signaling events coupling PKA activation and aquaporin-2 regulation were largely unknown until the advent of modern protein mass spectrometry techniques that allow proteome-wide quantification of protein phosphorylation changes (phosphoproteomics). This short review documents phosphoproteomic findings in collecting duct cells describing the response to V2R-selective vasopressin agonists and antagonists, the response to CRISPR-mediated deletion of PKA, results from in vitro phosphorylation studies using recombinant PKA, the response to the broad-spectrum kinase inhibitor H89 (N-[2-p-bromocinnamylamino-ethyl]-5-isoquinolinesulphonamide), and the responses underlying lithium-induced nephrogenic diabetes insipidus. These phosphoproteomic data sets have been made available online for modeling vasopressin signaling and signaling downstream from other G-protein-coupled receptors. SIGNIFICANCE STATEMENT: New developments in protein mass spectrometry are facilitating progress in identification of signaling networks. Using mass spectrometry, it is now possible to identify and quantify thousands of phosphorylation sites in a given cell type (phosphoproteomics). The authors describe the use of phosphoproteomics technology to identify signaling mechanisms downstream from a G-protein-coupled receptor, the vasopressin V2 subtype receptor, and its role of the regulation and dysregulation of water excretion in the kidney. Data from multiple phosphoproteomic data sets are provided as web-based resources.

Sorting Nexin 27 Regulates the Lysosomal Degradation of Aquaporin-2 Protein in the Kidney Collecting Duct

Sorting nexin 27 (SNX27), a PDZ (Postsynaptic density-95/Discs large/Zonula occludens 1) domain-containing protein, cooperates with a retromer complex, which regulates intracellular trafficking and the abundance of membrane proteins. Since the carboxyl terminus of aquaporin-2 (AQP2c) has a class I PDZ-interacting motif (X-T/S-X-Φ), the role of SNX27 in the regulation of AQP2 was studied. Co-immunoprecipitation assay of the rat kidney demonstrated an interaction of SNX27 with AQP2. Glutathione S-transferase (GST) pull-down assays revealed an interaction of the PDZ domain of SNX27 with AQP2c. Immunocytochemistry of HeLa cells co-transfected with FLAG-SNX27 and hemagglutinin (HA)-AQP2 also revealed co-localization throughout the cytoplasm. When the PDZ domain was deleted, punctate HA-AQP2 labeling was localized in the perinuclear region. The labeling was intensively overlaid by Lysotracker staining but not by GM130 labeling, a cis-Golgi marker. In rat kidneys and primary cultured inner medullary collecting duct cells, the subcellular redistribution of SNX27 was similar to AQP2 under 1-deamino-8-D-arginine vasopressin (dDAVP) stimulation/withdrawal. Cell surface biotinylation assay showed that dDAVP-induced AQP2 translocation to the apical plasma membrane was unaffected after SNX27 knockdown in mpkCCD cells. In contrast, the dDAVP-induced AQP2 protein abundance was significantly attenuated without changes in AQP2 mRNA expression. Moreover, the AQP2 protein abundance was markedly declined during the dDAVP withdrawal period after stimulation under SNX27 knockdown, which was inhibited by lysosome inhibitors. Autophagy was induced after SNX27 knockdown in mpkCCD cells. Lithium-induced nephrogenic diabetes insipidus in rats revealed a significant downregulation of SNX27 in the kidney inner medulla. Taken together, the PDZ domain-containing SNX27 interacts with AQP2 and depletion of SNX27 contributes to the autophagy-lysosomal degradation of AQP2.

CRISPR-Cas9/phosphoproteomics identifies multiple noncanonical targets of myosin light chain kinase

Prior studies have implicated myosin light chain kinase (MLCK) in the regulation of aquaporin-2 (AQP2) in the renal collecting duct. To discover signaling targets of MLCK, we used CRISPR-Cas9 to delete the MLCK gene (Mylk) to obtain MLCK-null mpkCCD cells and carried out comprehensive phosphoproteomics using stable isotope labeling with amino acids in cell culture for quantification. Immunocytochemistry and electron microscopy demonstrated a defect in the processing of AQP2-containing early endosomes to late endosomes. The phosphoproteomics experiments revealed that, of the 1,743 phosphopeptides quantified over multiple replicates, 107 were changed in abundance by MLCK deletion (29 decreased and 78 increased). One of the decreased phosphopeptides corresponded to the canonical target site in myosin regulatory light chain. Network analysis indicated that targeted phosphoproteins clustered into distinct structural/functional groups: actomyosin, signaling, nuclear envelope, gene transcription, mRNA processing, energy metabolism, intermediate filaments, adherens junctions, and tight junctions. There was significant overlap between the derived MLCK signaling network and a previously determined PKA signaling network. The presence of multiple proteins in the actomyosin category prompted experiments showing that MLCK deletion inhibits the normal effect of vasopressin to depolymerize F-actin, providing a potential explanation for the AQP2 trafficking defect. Changes in phosphorylation of multiple proteins in the nuclear envelope prompted measurement of nuclear size, showing a significant increase in average nuclear volume. We conclude that MLCK is part of a multicomponent signaling pathway in both the cytoplasm and nucleus that includes much more than simple regulation of conventional nonmuscle myosins through myosin regulatory light chain phosphorylation.

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.

Vasopressin-Independent Regulation of Aquaporin-2 by Tamoxifen in Kidney Collecting Ducts

Arginine vasopressin (AVP) mediates water reabsorption in the kidney collecting ducts through regulation of aquaporin-2 (AQP2). Also, estrogen has been known to regulate AQP2. Consistently, we previously demonstrated that tamoxifen (TAM), a selective estrogen receptor modulator, attenuates the downregulation of AQP2 in lithium-induced nephrogenic diabetes insipidus (NDI). In this study, we investigated the AVP-independent regulation of AQP2 by TAM and the therapeutic effect of TAM on the dysregulation of AQP2 and impaired urinary concentration in a unilateral ureteral obstruction (UUO) model. Primary cultured inner medullary collecting duct (IMCD) cells from kidneys of male Sprague-Dawley rats were treated with TAM. Rats subjected to 7 days of UUO were treated with TAM by oral gavage. Changes of intracellular trafficking and expression of AQP2 were evaluated by quantitative PCR, Western blotting, and immunohistochemistry. TAM induced AQP2 protein expression and intracellular trafficking in primary cultured IMCD cells, which were independent of the vasopressin V2 receptor (V2R) and cAMP activation, the critical pathways involved in AVP-stimulated regulation of AQP2. TAM attenuated the downregulation of AQP2 in TGF-β treated IMCD cells and IMCD suspensions prepared from UUO rats. TAM administration in vivo attenuated the downregulation of AQP2, associated with an improvement of urinary concentration in UUO rats. In addition, TAM increased CaMKII expression, suggesting that calmodulin signaling pathway is likely to be involved in the TAM-mediated AQP2 regulation. In conclusion, TAM is involved in AQP2 regulation in a vasopressin-independent manner and improves urinary concentration by attenuating the downregulation of AQP2 and maintaining intracellular trafficking in UUO.

Sensory primary cilium is a responsive cAMP microdomain in renal epithelia

Primary cilia are hair-like cellular extensions that sense microenvironmental signals surrounding cells. The role of adenylyl cyclases in ciliary function has been of interest because the product of adenylyl cyclase activity, cAMP, is relevant to cilia-related diseases. In the present study, we show that vasopressin receptor type-2 (V2R) is localized to cilia in kidney epithelial cells. Pharmacologic inhibition of V2R with tolvaptan increases ciliary length and mechanosensory function. Genetic knockdown of V2R, however, does not have any effect on ciliary length, although the effect of tolvaptan on ciliary length is dampened. Our study reveals that tolvaptan may have a cilia-specific effect independent of V2R or verapamil-sensitive calcium channels. Live-imaging of single cilia shows that V2R activation increases cilioplasmic and cytoplasmic cAMP levels, whereas tolvaptan mediates cAMP changes only in a cilia-specific manner. Furthermore, fluid-shear stress decreases cilioplasmic, but not cytoplasmic cAMP levels. Our data indicate that cilioplasmic and cytoplasmic cAMP levels are differentially modulated. We propose that the cilium is a critical sensor acting as a responsive cAMP microcompartment during physiologically relevant stimuli.

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.

Tamoxifen attenuates development of lithium-induced nephrogenic diabetes insipidus in rats

Lithium is widely used in treatment of bipolar affective disorders but often causes nephrogenic diabetes insipidus (NDI), a disorder characterized by severe urinary-concentrating defects. Lithium-induced NDI is caused by lithium uptake by collecting duct principal cells and altered expression of aquaporin-2 (AQP2), which are essential for water reabsorption of tubular fluid in the collecting duct. Sex hormones have previously been shown to affect the regulation of AQP2, so we tested whether tamoxifen (TAM), a selective estrogen receptor modulator, would attenuate lithium-induced alterations on renal water homeostasis. Rats were treated for 14 days with lithium, and TAM treatment was initiated 1 wk after onset of lithium administration. Lithium treatment resulted in severe polyuria and reduced AQP2 expression, which were ameliorated by TAM. Consistent with this, TAM attenuated downregulation of AQP2 and increased phosphorylation of the cAMP-responsive element-binding protein, which induced AQP2 expression in freshly isolated inner-medullary collecting duct suspension prepared from lithium-treated rats. In conclusion, TAM attenuated polyuria dose dependently and impaired urine concentration and downregulation of AQP2 protein expression in rats with lithium-induced NDI. These findings suggest that TAM is likely to be a novel therapeutic option for lithium-induced NDI.

Biphasic myosin II light chain activation during clot retraction

Clot retraction is an essential step during primary haemostasis, thereby promoting thrombus stability and wound healing. Integrin αIIbβ3 plays a critical role in clot retraction, by inducing acto-myosin interactions that allow platelet cytoskeleton reorganisation. However, the signalling pathways that lead to clot retraction are still misunderstood. In this study, we report the first data on the kinetics of myosin II light chain (MLC) phosphorylation during clot retraction. We found an early phosphorylation peak followed by a second peak. By using specific inhibitors of kinases and small G proteins, we showed that MLC kinase (MLCK), RhoA/ROCK, and Rac-1 were involved in clot retraction and in the early MLC phosphorylation peak. Only Rac-1 and actin polymerisation, controlled by outside-in signalling, were crucial to the second MLC phosphorylation peak.

Systems-level identification of PKA-dependent signaling in epithelial cells

G protein stimulatory α-subunit (G_αs)-coupled heptahelical receptors regulate cell processes largely through activation of protein kinase A (PKA). To identify signaling processes downstream of PKA, we deleted both PKA catalytic subunits using CRISPR-Cas9, followed by a "multiomic" analysis in mouse kidney epithelial cells expressing the G_αs-coupled V2 vasopressin receptor. RNA-seq (sequencing)-based transcriptomics and SILAC (stable isotope labeling of amino acids in cell culture)-based quantitative proteomics revealed a complete loss of expression of the water-channel gene Aqp2 in PKA knockout cells. SILAC-based quantitative phosphoproteomics identified 229 PKA phosphorylation sites. Most of these PKA targets are thus far unannotated in public databases. Surprisingly, 1,915 phosphorylation sites with the motif x-(S/T)-P showed increased phosphooccupancy, pointing to increased activity of one or more MAP kinases in PKA knockout cells. Indeed, phosphorylation changes associated with activation of ERK2 were seen in PKA knockout cells. The ERK2 site is downstream of a direct PKA site in the Rap1GAP, Sipa1l1, that indirectly inhibits Raf1. In addition, a direct PKA site that inhibits the MAP kinase kinase kinase Map3k5 (ASK1) is upstream of JNK1 activation. The datasets were integrated to identify a causal network describing PKA signaling that explains vasopressin-mediated regulation of membrane trafficking and gene transcription. The model predicts that, through PKA activation, vasopressin stimulates AQP2 exocytosis by inhibiting MAP kinase signaling. The model also predicts that, through PKA activation, vasopressin stimulates Aqp2 transcription through induction of nuclear translocation of the acetyltransferase EP300, which increases histone H3K27 acetylation of vasopressin-responsive genes (confirmed by ChIP-seq).

Vasopressin-induced serine 269 phosphorylation reduces Sipa1l1 (signal-induced proliferation-associated 1 like 1)-mediated aquaporin-2 endocytosis

The abundance of integral membrane proteins in the plasma membrane is determined by a dynamic balance between exocytosis and endocytosis, which can often be regulated by physiological stimuli. Here, we describe a mechanism that accounts for the ability of the peptide hormone vasopressin to regulate water excretion via a phosphorylation-dependent modulation of the PDZ domain-ligand interaction involving the water channel protein aquaporin-2. We discovered that the PDZ domain-containing protein Sipa1l1 (signal-induced proliferation-associated 1 like 1) binds to the cytoplasmic PDZ-ligand motif of aquaporin-2 and accelerates its endocytosis in the absence of vasopressin. Vasopressin-induced aquaporin-2 phosphorylation within the type I PDZ-ligand motif disrupted the interaction, in association with reduced aquaporin-2 endocytosis and prolonged plasma membrane aquaporin-2 retention. This phosphorylation-dependent alteration in the PDZ domain-ligand interaction was explained by 3D structural models, which showed a hormone-regulated mechanism that controls osmotic water transport and systemic water balance in mammals.

Depletion of vacuolar protein sorting-associated protein 35 is associated with increased lysosomal degradation of aquaporin-2

The carboxyl terminus of aquaporin-2 (AQP2c) undergoes posttranslational modifications, including phosphorylation and ubiquitination, in the process of regulating aquaporin-2 (AQP2) translocation and protein abundance. We aimed to identify novel proteins interacting with AQP2c. Recombinant AQP2c protein was made in Escherichia coli BL21 (DE3) cells by exploiting the pET32 TrxA fusion system. Lysates of rat kidney inner medullary collecting duct (IMCD) tubule suspensions interacted with rat AQP2c bound to Ni²⁺-resin were subjected to LC-MS/MS proteomic analysis. Potential interacting proteins were identified, including vacuolar protein sorting-associated protein 35 (Vps35). Coimmunoprecipitation assay demonstrated that Vps35 interacted with AQP2c. Immunohistochemistry of rat kidney revealed that AQP2 and Vps35 were partly colocalized at the intracellular vesicles in collecting duct cells. The role of Vps35 in AQP2 regulation induced by 1-deamino-8D-arginine vasopressin (dDAVP) was examined in mpkCCDc14 cells. Cell surface biotinylation assay demonstrated that dDAVP-induced apical translocation of AQP2 was significantly decreased under siRNA-mediated Vps35 knockdown. dDAVP-induced AQP2 upregulation was less prominent in the cells with Vps35 knockdown. Moreover, AQP2 protein abundance was decreased to a greater extent during the withdrawal period after dDAVP stimulation under Vps35 knockdown, which was significantly inhibited by chloroquine (a blocker of the lysosomal pathway) but not by MG132 (a proteasome inhibitor). Immunocytochemistry demonstrated that internalized AQP2 was more associated with lysosomal-associated membrane protein 1 (LAMP-1) in primary cultured IMCD cells under a Vps35 knockdown situation. Taken together, our results show that Vps35 interacts with AQP2c, and depletion of Vps35 is likely to be associated with decreased AQP2 trafficking and increased lysosomal degradation of AQP2 protein.

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

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

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

Vasopressin-regulated miRNAs and AQP2-targeting miRNAs in kidney collecting duct cells

Mature microRNA (miRNA) acts as an important posttranscriptional regulator. We aimed to profile vasopressin-responsive miRNAs in kidney inner medullary collecting duct cells and to identify aquaporin-2 (AQP2)-targeting miRNAs. Microarray chip assay was carried out in inner medullary collecting duct tubule suspensions from rat kidneys in the absence or presence of desmopressin (dDAVP) stimulation (10−9 M, 2 h). The results demonstrated 19 miRNAs, including both precursor and mature miRNAs, as potential candidates that showed significant changes in expression after dDAVP stimulation ( P < 0.05). Nine mature miRNAs exhibiting >1.3-fold changes in expression on the microarray (miR-127, miR-1, miR-873, miR-16, miR-206, miR-678, miR-496, miR-298, and miR-463) were further examined by quantitative real-time PCR, and target genes of the selected miRNAs were predicted. Next, to identify AQP2-targeting miRNAs, in silico analysis was performed. Four miRNAs (miR-32, miR-137, miR-216a, and miR-216b) target the 3′-untranslated region of rat AQP2 mRNA. Target seed regions of miR-32 and miR-137 were also conserved in the 3′-untranslated region of mouse AQP2 mRNA. Quantitative real-time PCR and immunoblot analysis demonstrated that dDAVP-induced AQP2 expression was significantly attenuated in mpkCCDc14 cells when cells were transfected with miRNA mimics of miR-32 or miR-137. Moreover, luciferase reporter assay demonstrated a significant decrease of AQP2 translation in mpkCCDc14 cells transfected with miRNA mimics of miR-32 or miR-137. The present study provides novel insights into the regulation of AQP2 by RNA interference; however, vasopressin-regulated miRNAs did not include miR-32 or miR-137, indicating that the interaction of miRNAs with the AQP2 regulatory pathway requires further analysis.

Extracellular pH affects phosphorylation and intracellular trafficking of AQP2 in inner medullary collecting duct cells

Kidney collecting duct cells are continuously exposed to the changes of extracellular pH (pHe). We aimed to study the effects of altered pHe on desmopressin (dDAVP)-induced phosphorylation (Ser256, Ser261, Ser264, and Ser269) and apical targeting of aquaporin-2 (AQP2) in rat kidney inner medullary collecting duct (IMCD) cells. When freshly prepared IMCD tubule suspensions exposed to HEPES buffer with pH 5.4, 6.4, 7.4, or 8.4 for 1 h were stimulated with dDAVP (10−10 M, 3 min), AQP2 phosphorylation at Ser256, Ser264, and Ser269 was significantly attenuated under acidic conditions. Next, IMCD cells primary cultured in transwell chambers were exposed to a transepithelial pH gradient for 1 h (apical pH 6.4, 7.4, or 8.4 vs. basolateral pH 7.4 and vice versa). Immunocytochemistry and cell surface biotinylation assay revealed that exposure to either apical pH 6.4 or basolateral pH 6.4 for 1 h was associated with decreased dDAVP (10−9 M, 15 min, basolateral)-induced apical targeting of AQP2 and surface expression of AQP2. Fluorescence resonance energy transfer analysis revealed that the dDAVP (10−9 M)-induced increase of PKA activity was significantly attenuated when LLC-PK1 cells were exposed to pHe 6.4 compared with pHe 7.4 and 8.4. In contrast, forskolin (10−7 M)-induced PKA activation and dDAVP (10−9 M)-induced increases of intracellular Ca2+ were not affected. Taken together, dDAVP-induced phosphorylation and apical targeting of AQP2 are attenuated in IMCD cells under acidic pHe, likely via an inhibition of vasopressin V2 receptor-G protein-cAMP-PKA actions.

A knowledge base of vasopressin actions in the kidney

Biological information is growing at a rapid pace, making it difficult for individual investigators to be familiar with all information that is relevant to their own research. Computers are beginning to be used to extract and curate biological information; however, the complexity of human language used in research papers continues to be a critical barrier to full automation of knowledge extraction. Here, we report a manually curated knowledge base of vasopressin actions in renal epithelial cells that is designed to be readable either by humans or by computer programs using natural language processing algorithms. The knowledge base consists of three related databases accessible at https://helixweb.nih.gov/ESBL/TinyUrls/Vaso_portal.html. One of the component databases reports vasopressin actions on individual proteins expressed in renal epithelia, including effects on phosphorylation, protein abundances, protein translocation from one subcellular compartment to another, protein-protein binding interactions, etc. The second database reports vasopressin actions on physiological measures in renal epithelia, and the third reports specific mRNA species whose abundances change in response to vasopressin. We illustrate the application of the knowledge base by using it to generate a protein kinase network that connects vasopressin binding in collecting duct cells to physiological effects to regulate the water channel protein aquaporin-2.

Vasopressin-2 receptor signaling and autosomal dominant polycystic kidney disease: from bench to bedside and back again

Blockade of the vasopressin-2 receptor (V2R) in the kidney has recently emerged as a promising therapeutic strategy in autosomal dominant polycystic kidney disease. The pathophysiologic basis of V2R-dependent cyst proliferation and disease progression, however, is not fully understood. Recent evidence suggests that polycystic kidney disease is characterized by defects in urinary concentrating mechanisms and subsequent deregulation of vasopressin excretion by the neurohypophysis. On the cellular level, several recent studies revealed unexpected crosstalk of signaling pathways downstream of V2R activation in the kidney epithelium. This review summarizes some of the unexpected roles of V2R signaling and suggests that vasopressin signaling itself may contribute crucially to loss of polarity and enhanced proliferation in cystic kidney epithelium.

Integrin-linked kinase regulates tubular aquaporin-2 content and intracellular location: a link between the extracellular matrix and water reabsorption

One of the clinical alterations observed in chronic renal disease (CRD) is the impaired urine concentration, known as diabetes insipidus (DI). Tubulointerstitial fibrosis of the kidney is also a pathological finding observed in CRD and involves composition of extracellular matrix (ECM). However, an association between these two events has not been elucidated. In this study, we showed that the extracellular-to-intracellular scaffold protein integrin-linked kinase (ILK) regulates expression of tubular water channel aquaporin-2 (AQP2) and its apical membrane presence in the renal tubule. Basally, polyuria and decreased urine osmolality were present in ILK conditional-knockdown (cKD-ILK) adult mice compared with nondepleted ILK littermates. No changes were observed in arginine-vasopressin (AVP) blood levels, renal receptor (V2R), or AQP3 expression. However, tubular AQP2 was decreased in expression and apical membrane presence in cKD-ILK mice, where the canonical V2R/cAMP axis activation is still functional, but independent of the absence of ILK. Thus, cKD-ILK constitutes a nephrogenic diabetes insipidus (NDI) model. AQP2 and ILK colocalize in cultured inner medullary collecting duct (mIMCD3) cells. Specific ILK siRNAs and collagen I (Col) decrease ILK and AQP2 levels and AQP2 presence on the membrane of tubular mIMCD3 cells, which impairs the capacity of the cells to transport water under hypotonic stress. The present work points to ILK as a therapeutic target in NDI.

Myosin IIB and F-actin control apical vacuolar morphology and histamine-induced trafficking of H-K-ATPase-containing tubulovesicles in gastric parietal cells

Selective inhibitors of myosin or actin function and confocal microscopy were used to test the role of an actomyosin complex in controlling morphology, trafficking, and fusion of tubulovesicles (TV) containing H-K-ATPase with the apical secretory canaliculus (ASC) of primary-cultured rabbit gastric parietal cells. In resting cells, myosin IIB and IIC, ezrin, and F-actin were associated with ASC, whereas H-K-ATPase localized to intracellular TV. Histamine caused fusion of TV with ASC and subsequent expansion resulting from HCl and water secretion; F-actin and ezrin remained associated with ASC whereas myosin IIB and IIC appeared to dissociate from ASC and relocalize to the cytoplasm. ML-7 (inhibits myosin light chain kinase) caused ASC of resting cells to collapse and most myosin IIB, F-actin, and ezrin to dissociate from ASC. TV were unaffected by ML-7. Jasplakinolide (stabilizes F-actin) caused ASC to develop large blebs to which actin, myosin II, and ezrin, as well as tubulin, were prominently localized. When added prior to stimulation, ML-7 and jasplakinolide prevented normal histamine-stimulated transformations of ASC/TV and the cytoskeleton, but they did not affect cells that had been previously stimulated with histamine. These results indicate that dynamic pools of actomyosin are required for maintenance of ASC structure in resting cells and for trafficking of TV to ASC during histamine stimulation. However, the dynamic pools of actomyosin are not required once the histamine-stimulated transformation of TV/ASC and cytoskeleton has occurred. These results also show that vesicle trafficking in parietal cells shares mechanisms with similar processes in renal collecting duct cells, neuronal synapses, and skeletal muscle.

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.

Global analysis of the effects of the V2 receptor antagonist satavaptan on protein phosphorylation in collecting duct

Satavaptan (SR121463) is a vasopressin V2 receptor antagonist that has been shown to improve hyponatremia in patients with cirrhosis, congestive heart failure, and syndrome of inappropriate antidiuresis. While known to inhibit adenylyl cyclase-mediated accumulation of intracellular cyclic AMP and potentially recruit β-arrestin in kidney cell lines, very little is known regarding the signaling pathways that are affected by this drug. To this end, we carried out a global quantitative phosphoproteomic analysis of native rat inner medullary collecting duct cells pretreated with satavaptan or vehicle control followed by the V2 receptor agonist desmopressin (dDAVP) for 0.5, 2, 5, or 15 min. A total of 2,449 unique phosphopeptides from 1,160 proteins were identified. Phosphopeptides significantly changed by satavaptan included many of the same kinases [protein kinase A, phosphoinositide 3-kinase, mitogen-activated protein kinase kinase kinase 7 (TAK1), and calcium/calmodulin-dependent kinase kinase 2] and channels (aquaporin-2 and urea transporter UT-A1) regulated by vasopressin. Time course clustering and kinase motif analysis suggest that satavaptan blocks dDAVP-mediated activation of basophilic kinases, while also blocking dDAVP-mediated inhibition of proline-directed kinases. Satavaptan affects a variety of dDAVP-mediated processes including regulation of cell-cell junctions, actin cytoskeleton dynamics, and signaling through Rho GTPases. These results demonstrate that, overall, satavaptan acts as a selective V2 receptor antagonist and affects many of the same signaling pathways regulated by vasopressin. This study represents the first "systems-wide" analysis of a "vaptan"-class drug and provides a wealth of new data regarding the effects of satavaptan on vasopressin-mediated phosphorylation events.

Vasopressin and the regulation of aquaporin-2

Water excretion is regulated in large part through the regulation of osmotic water permeability of the renal collecting duct epithelium. Water permeability is controlled by vasopressin through regulation of the water channel, aquaporin-2 (AQP2). Two processes contribute: (1) regulation of AQP2 trafficking to the apical plasma membrane; and (2) regulation of the total amount of the AQP2 protein in the cells. Regulation of AQP2 abundance is defective in several water-balance disorders, including many polyuric disorders and the syndrome of inappropriate antidiuresis. Here we review vasopressin signaling in the renal collecting duct that is relevant to the two modes of water permeability regulation.

Quantitative apical membrane proteomics reveals vasopressin-induced actin dynamics in collecting duct cells

In kidney collecting duct cells, filamentous actin (F-actin) depolymerization is a critical step in vasopressin-induced trafficking of aquaporin-2 to the apical plasma membrane. However, the molecular components of this response are largely unknown. Using stable isotope-based quantitative protein mass spectrometry and surface biotinylation, we identified 100 proteins that showed significant abundance changes in the apical plasma membrane of mouse cortical collecting duct cells in response to vasopressin. Fourteen of these proteins are involved in actin cytoskeleton regulation, including actin itself, 10 actin-associated proteins, and 3 regulatory proteins. Identified were two integral membrane proteins (Clmn, Nckap1) and one actin-binding protein (Mpp5) that link F-actin to the plasma membrane, five F-actin end-binding proteins (Arpc2, Arpc4, Gsn, Scin, and Capzb) involved in F-actin reorganization, and two actin adaptor proteins (Dbn1, Lasp1) that regulate actin cytoskeleton organization. There were also protease (Capn1), protein kinase (Cdc42bpb), and Rho guanine nucleotide exchange factor 2 (Arhgef2) that mediate signal-induced F-actin changes. Based on these findings, we devised a live-cell imaging method to observe vasopressin-induced F-actin dynamics in polarized mouse cortical collecting duct cells. In response to vasopressin, F-actin gradually disappeared near the center of the apical plasma membrane while consolidating laterally near the tight junction. This F-actin peripheralization was blocked by calcium ion chelation. Vasopressin-induced apical aquaporin-2 trafficking and forskolin-induced water permeability increase were blocked by F-actin disruption. In conclusion, we identified a vasopressin-regulated actin network potentially responsible for vasopressin-induced apical F-actin dynamics that could explain regulation of apical aquaporin-2 trafficking and water permeability increase.

Aquaporin-2 regulation in health and disease

Aquaporin-2 (AQP2), the vasopressin-regulated water channel of the renal collecting duct, is dysregulated in numerous disorders of water balance in people and animals, including those associated with polyuria (urinary tract obstruction, hypokalemia, inflammation, and lithium toxicity) and with dilutional hyponatremia (syndrome of inappropriate antidiuresis, congestive heart failure, cirrhosis). Normal regulation of AQP2 by vasopressin involves 2 independent regulatory mechanisms: (1) short-term regulation of AQP2 trafficking to and from the apical plasma membrane, and (2) long-term regulation of the total abundance of the AQP2 protein in the cells. Most disorders of water balance are the result of dysregulation of processes that regulate the total abundance of AQP2 in collecting duct cells. In general, the level of AQP2 in a collecting duct cell is determined by a balance between production via translation of AQP2 mRNA and removal via degradation or secretion into the urine in exosomes. AQP2 abundance increases in response to vasopressin chiefly due to increased translation subsequent to increases in AQP2 mRNA. Vasopressin-mediated regulation of AQP2 gene transcription is poorly understood, although several transcription factor-binding elements in the 5' flanking region of the AQP2 gene have been identified, and candidate transcription factors corresponding to these elements have been discovered in proteomics studies. Here, we review progress in this area and discuss elements of vasopressin signaling in the collecting duct that may impinge on regulation of AQP2 in health and in the context of examples of polyuric diseases.

Quantitative phosphoproteomics in nuclei of vasopressin-sensitive renal collecting duct cells

Vasopressin regulates transport across the collecting duct epithelium in part via effects on gene transcription. Transcriptional regulation occurs partially via changes in phosphorylation of transcription factors, transcriptional coactivators, and protein kinases in the nucleus. To test whether vasopressin alters the nuclear phosphoproteome of vasopressin-sensitive cultured mouse mpkCCD cells, we used stable isotope labeling and mass spectrometry to quantify thousands of phosphorylation sites in nuclear extracts and nuclear pellet fractions. Measurements were made in the presence and absence of the vasopressin analog dDAVP. Of the 1,251 sites quantified, 39 changed significantly in response to dDAVP. Network analysis of the regulated proteins revealed two major clusters ("cell-cell adhesion" and "transcriptional regulation") that were connected to known elements of the vasopressin signaling pathway. The hub proteins for these two clusters were the transcriptional coactivator β-catenin and the transcription factor c-Jun. Phosphorylation of β-catenin at Ser552 was increased by dDAVP [log(2)(dDAVP/vehicle) = 1.79], and phosphorylation of c-Jun at Ser73 was decreased [log(2)(dDAVP/vehicle) = -0.53]. The β-catenin site is known to be targeted by either protein kinase A or Akt, both of which are activated in response to vasopressin. The c-Jun site is a canonical target for the MAP kinase Jnk2, which is downregulated in response to vasopressin in the collecting duct. The data support the idea that vasopressin-mediated control of transcription in collecting duct cells involves selective changes in the nuclear phosphoproteome. All data are available to users at http://helixweb.nih.gov/ESBL/Database/mNPPD/.

Rho kinase regulation of vasopressin-induced calcium entry in vascular smooth muscle cell: comparison between rat isolated aorta and cultured aortic cells

In addition to its role in artery contraction, Rho kinase (ROCK) is reported to be involved in the Ca(2+) response to vasoconstrictor agonist in rat aorta. However the signaling pathway mediated by ROCK had not been investigated so far and it was not known whether ROCK also contributed to Ca(2+) signaling in cultured vascular smooth muscle cells (VSMC), which undergo profound phenotypic changes. Our results showed that in VSMC, ROCK inhibition by Y-27632 or H-1152 had no effect on the Ca(2+) response to vasopressin, while in aorta the vasopressin-induced Ca(2+) entry was significantly decreased. The inhibition of myosin light chain kinase (MLCK) by ML-7 depressed the vasopressin-induced Ca(2+) signal in aorta but not in VSMC. The difference in ROCK sensitivity of vasopressin-induced Ca(2+) entry between aorta and VSMC was not related to an alteration of the RhoA/ROCK pathway. However, MLCK expression and activity were depressed in cultured cells compared to aorta. We concluded that the regulation of vasopressin-induced Ca(2+) entry by ROCK in aorta could involve the myosin cytoskeleton and could be prevented by the downregulation of MLCK in VSMC. These results underline the important differences in Ca(2+) regulation between whole tissue and cultured cells.

Quantitative proteomics identifies vasopressin-responsive nuclear proteins in collecting duct cells

Vasopressin controls transport in the renal collecting duct, in part, by regulating transcription. This complex process, which can involve translocation and/or modification of transcriptional regulators, is not completely understood. Here, we applied a method for large-scale profiling of nuclear proteins to quantify vasopressin-induced changes in the nuclear proteome of cortical collecting duct (mpkCCD) cells. Using stable isotope labeling and tandem mass spectrometry, we quantified 3987 nuclear proteins and identified significant changes in the abundance of 65, including previously established targets of vasopressin signaling in the collecting duct. Vasopressin-induced changes in the abundance of the transcription factors JunB, Elf3, Gatad2b, and Hmbox1; transcriptional co-regulators Ctnnb1 (β-catenin) and Crebbp; subunits of the Mediator complex; E3 ubiquitin ligase Nedd4; nuclear transport regulator RanGap1; and several proteins associated with tight junctions and adherens junctions. Bioinformatic analysis showed that many of the quantified transcription factors have putative binding sites in the 5'-flanking regions of genes coding for the channel proteins Aqp2, Aqp3, Scnn1b (ENaCβ), and Scnn1g (ENaCγ), which are known targets of vasopressin. Immunoblotting demonstrated that the increase in β-catenin in nuclear fractions was accompanied by an even larger increase in its phosphorylated form (pSer552). The findings provide a new online database resource for nuclear proteomics (http://helixweb.nih.gov/ESBL/Database/mNPD/) and generate new hypotheses regarding vasopressin-mediated transcriptional regulation in the collecting duct.

Molecular biology of water and salt regulation in the kidney

The kidney plays a central role in the regulation of the salt and water balance, which depends upon an array of solute and water transporters in the renal tubules and upon vascular elements in the various regions of the kidney. Many recent studies have improved our understanding of this process. In this review, we summarize the current data on the molecules involved in sodium and water transport in the renal tubules, focusing in particular on aquaporins and renal sodium transporters and channels.

Dynamics of the G protein-coupled vasopressin V2 receptor signaling network revealed by quantitative phosphoproteomics

G protein-coupled receptors (GPCRs) regulate diverse physiological processes, and many human diseases are due to defects in GPCR signaling. To identify the dynamic response of a signaling network downstream from a prototypical G(s)-coupled GPCR, the vasopressin V2 receptor, we have carried out multireplicate, quantitative phosphoproteomics with iTRAQ labeling at four time points following vasopressin exposure at a physiological concentration in cells isolated from rat kidney. A total of 12,167 phosphopeptides were identified from 2,783 proteins, with 273 changing significantly in abundance with vasopressin. Two-dimensional clustering of phosphopeptide time courses and Gene Ontology terms revealed that ligand binding to the V2 receptor affects more than simply the canonical cyclic adenosine monophosphate-protein kinase A and arrestin pathways under physiological conditions. The regulated proteins included key components of actin cytoskeleton remodeling, cell-cell adhesion, mitogen-activated protein kinase signaling, Wnt/β-catenin signaling, and apoptosis pathways. These data suggest that vasopressin can regulate an array of cellular functions well beyond its classical role in regulating water and solute transport. These results greatly expand the current view of GPCR signaling in a physiological context and shed new light on potential roles for this signaling network in disorders such as polycystic kidney disease. Finally, we provide an online resource of physiologically regulated phosphorylation sites with dynamic quantitative data (http://helixweb.nih.gov/ESBL/Database/TiPD/index.html).

Phosphoproteomics of vasopressin signaling in the kidney

Protein phosphorylation plays a critical role in the signaling pathways regulating water and solute transport in the distal renal tubule (i.e., renal collecting duct). A central mediator in this process is the antidiuretic peptide hormone arginine vasopressin, which regulates a number of transport proteins including water channel aquaporin-2 and urea transporters (UT-A1 and UT-A3). Within the past few years, tandem mass spectrometry-based proteomics has played a pivotal role in revealing global changes in the phosphoproteome in response to vasopressin signaling in the renal collecting duct. This type of large-scale 'shotgun' approach has resulted in an exponential increase in the number of phosphoproteins known to be regulated by vasopressin and has expanded on the established signaling mechanisms and kinase pathways regulating collecting duct physiology. This article will provide a brief background on vasopressin action, will highlight a number of recent quantitative phosphoproteomic studies in both native rat kidney and cultured collecting duct cells, and will conclude with a perspective focused on emerging trends in the field of phosphoproteomics.

N-methyl-D-aspartate receptor subunit NR3a expression and function in principal cells of the collecting duct

N-methyl-D-aspartate receptors (NMDARs) are Ca(2+)-permeable, ligand-gated, nonselective cation channels that function as neuronal synaptic receptors but which are also expressed in multiple peripheral tissues. Here, we show for the first time that NMDAR subunits NR3a and NR3b are highly expressed in the neonatal kidney and that there is continued expression of NR3a in the renal medulla and papilla of the adult mouse. NR3a was also expressed in mIMCD-3 cells, where it was found that hypoxia and hypertonicity upregulated NR3a expression. Using short-hairpin (sh) RNA-based knockdown, a stable inner medullary collecting duct (IMCD) cell line was established that had ∼80% decrease in NR3a. Knockdown cells exhibited an increased basal intracellular calcium concentration, reduced cell proliferation, and increased cell death. In addition, NR3a knockdown cells exhibited reduced water transport in response to the addition of vasopressin, suggesting an alteration in aquaporin-2 (AQP2) expression/function. Consistent with this notion, we demonstrate decreased surface expression of glycosylated AQP2 in IMCD cells transfected with NR3a shRNA. To determine whether this also occurred in vivo, we compared AQP2 levels in wild-type vs. in NR3a(-/-) mice. Total AQP2 protein levels in the outer and inner medulla were significantly reduced in knockout mice compared with control mice. Finally, NR3a(-/-) mice showed a significant delay in their ability to increase urine osmolality during water restriction. Thus NR3a may play a renoprotective role in collecting duct cells. Therefore, under conditions that are associated with high vasopressin levels, NR3a, by maintaining low intracellular calcium levels, protects the function of the principal cells to reabsorb water and thereby increase medullary osmolality.

Quantitative protein and mRNA profiling shows selective post-transcriptional control of protein expression by vasopressin in kidney cells

Previous studies in yeast have supported the view that post-transcriptional regulation of protein abundances may be more important than previously believed. Here we ask the question: "In a physiological regulatory process (the response of mammalian kidney cells to the hormone vasopressin), what fraction of the expressed proteome undergoes a change in abundance and what fraction of the regulated proteins have corresponding changes in mRNA levels?" In humans and other mammals, vasopressin fulfills a vital homeostatic role (viz. regulation of renal water excretion) by regulating the water channel aquaporin-2 in collecting duct cells. To address the question posed, we utilized large-scale quantitative protein mass spectrometry (LC-MS/MS) employing stable isotopic labeling in cultured mpkCCD cells ('SILAC') coupled with transcriptomic profiling using oligonucleotide expression arrays (Affymetrix). Preliminary studies analyzing two nominally identical control samples by SILAC LC-MS/MS yielded a relative S.D. of 13% (for ratios), establishing the precision of the SILAC approach in our hands. We quantified nearly 3000 proteins with nontargeted SILAC LC-MS/MS, comparing vasopressin- versus vehicle-treated samples. Of these proteins 786 of them were quantified in each of 3 experiments, allowing statistical analysis and 188 of these showed significant vasopressin-induced changes in abundance, including aquaporin-2 (20-fold increase). Among the proteins with statistically significant abundance changes, a large fraction (at least one-third) was found to lack changes in the corresponding mRNA species (despite sufficient statistical power), indicating that post-transcriptional regulation of protein abundance plays an important role in the vasopressin response. Bioinformatic analysis of the regulated proteins (versus all transcripts) shows enrichment of glutathione S-transferase isoforms as well as proteins involved in organization of the actin cytoskeleton. The latter suggests that long-term regulatory processes may contribute to actomyosin-dependent trafficking of the water channel aquaporin-2. The results provide impetus for increased focus on translational regulation and regulation of protein degradation in physiological control in mammalian epithelial cells.

Flotillin microdomains interact with the cortical cytoskeleton to control uropod formation and neutrophil recruitment

We studied the function of plasma membrane microdomains defined by the proteins flotillin 1 and flotillin 2 in uropod formation and neutrophil chemotaxis. Flotillins become concentrated in the uropod of neutrophils after exposure to chemoattractants such as N-formyl-Met-Leu-Phe (fMLP). Here, we show that mice lacking flotillin 1 do not have flotillin microdomains, and that recruitment of neutrophils toward fMLP in vivo is reduced in these mice. Ex vivo, migration of neutrophils through a resistive matrix is reduced in the absence of flotillin microdomains, but the machinery required for sensing chemoattractant functions normally. Flotillin microdomains specifically associate with myosin IIa, and spectrins. Both uropod formation and myosin IIa activity are compromised in flotillin 1 knockout neutrophils. We conclude that the association between flotillin microdomains and cortical cytoskeleton has important functions during neutrophil migration, in uropod formation, and in the regulation of myosin IIa.

Vasopressin increases phosphorylation of Ser84 and Ser486 in Slc14a2 collecting duct urea transporters

Vasopressin-regulated urea transport in the renal inner medullary collecting duct (IMCD) is mediated by two urea channel proteins, UT-A1 and UT-A3, derived from the same gene (Slc14a2) by alternative splicing. The NH(2)-terminal 459 amino acids are the same in both proteins. To study UT-A1/3 phosphorylation, we made phospho-specific antibodies to UT-A sequences targeting phospho-serines at positions 84 and 486, sites identified previously by protein mass spectrometry. Both antibodies proved specific, recognizing only the phosphorylated forms of UT-A1 and -A3. Immunoblotting of rat IMCD suspensions or whole inner medullas showed that the V2R-selective vasopressin analog 1-deamino-8-d-arginine vasopressin (dDAVP) increases phosphorylation at Ser84 (in UT-A1 and UT-A3) and Ser486 (in UT-A1) by about eightfold. Time course studies in rat IMCD suspensions showed maximum phosphorylation within 1 min of dDAVP exposure, consistent with the time course of vasopressin-stimulated phosphorylation of the vasopressin-sensitive water channel aquaporin-2 at Ser256. Confocal immunofluorescence in Brattleboro rat medullary tissue showed labeling limited to the IMCD, which increased markedly in response to dDAVP. Immuno-electron microscopy studies showed that both phosphorylated forms were present mainly in intracellular compartments in the presence of vasopressin. These studies demonstrate regulated phosphorylation of both UT-A1 and UT-A3 in response to vasopressin in a manner consistent with coordinate regulation of UT-A and aquaporin-2 in the renal IMCD. The findings add to prior evidence for vasopressin-induced phosphorylation of UT-A1, providing evidence that UT-A3 may be regulated by phosphorylation as well.

Planar polarity of multiciliated ependymal cells involves the anterior migration of basal bodies regulated by non-muscle myosin II

Motile cilia generate constant fluid flow over epithelial tissue, and thereby influence diverse physiological processes. Such functions of ciliated cells depend on the planar polarity of the cilia and on their basal bodies being oriented in the downstream direction of fluid flow. Recently, another type of basal body planar polarity, characterized by the anterior localization of the basal bodies in individual cells, was reported in the multiciliated ependymal cells that line the surface of brain ventricles. However, little is known about the cellular and molecular mechanisms by which this polarity is established. Here, we report in mice that basal bodies move in the apical cell membrane during differentiation to accumulate in the anterior region of ependymal cells. The planar cell polarity signaling pathway influences basal body orientation, but not their anterior migration, in the neonatal brain. Moreover, we show by pharmacological and genetic studies that non-muscle myosin II is a key regulator of this distribution of basal bodies. This study demonstrates that the orientation and distribution of basal bodies occur by distinct mechanisms.

Quantitative phosphoproteomic analysis reveals vasopressin V2-receptor-dependent signaling pathways in renal collecting duct cells

Vasopressin's action in renal cells to regulate water transport depends on protein phosphorylation. Here we used mass spectrometry-based quantitative phosphoproteomics to identify signaling pathways involved in the short-term V2-receptor-mediated response in cultured collecting duct cells (mpkCCD) from mouse. Using Stable Isotope Labeling by Amino acids in Cell culture (SILAC) with two treatment groups (0.1 nM dDAVP or vehicle for 30 min), we carried out quantification of 2884 phosphopeptides. The majority (82%) of quantified phosphopeptides did not change in abundance in response to dDAVP. Analysis of the 273 phosphopeptides increased by dDAVP showed a predominance of so-called "basophilic" motifs consistent with activation of kinases of the AGC family. Increases in phosphorylation of several known protein kinase A targets were found. In addition, increased phosphorylation of targets of the calmodulin-dependent kinase family was seen, including autophosphorylation of calmodulin-dependent kinase 2 at T286. Analysis of the 254 phosphopeptides decreased in abundance by dDAVP showed a predominance of so-called "proline-directed" motifs, consistent with down-regulation of mitogen-activated or cyclin-dependent kinases. dDAVP decreased phosphorylation of both JNK1/2 (T183/Y185) and ERK1/2 (T183/Y185; T203/Y205), consistent with a decrease in activation of these proline-directed kinases in response to dDAVP. Both ERK and JNK were able to phosphorylate residue S261of aquaporin-2 in vitro, a site showing a decrease in phosphorylation in response to dDAVP in vivo. The data support roles for multiple vasopressin V2-receptor-dependent signaling pathways in the vasopressin signaling network of collecting duct cells, involving several kinases not generally accepted to regulate collecting duct function.