Cyclic AMP is sufficient for triggering the exocytic recruitment of aquaporin-2 in renal epithelial cells

Expression Regulation and Trafficking of Aquaporins

Aquaporins (AQPs) mediate the bidirectional water flow driven by an osmotic gradient. Either gating or trafficking allows for rapid and specific AQP regulation in a tissue-dependent manner. The regulatory mechanisms of AQP2 are discussed mainly in this chapter, as the mechanisms controlling the regulation and trafficking of AQP2 have been very well studied. The targeting of AQP2 to the apical plasma membrane of collecting duct principal cells is mainly regulated by the action of arginine vasopressin (AVP) on the type 2 AVP receptor (V2R), which cause increased intracellular cAMP or elevated intracellular calcium levels. Activation of these intracellular signaling pathways results in vesicles bearing AQP2 transport, docking and fusion with the apical membrane, which increase density of AQP2 on the membrane. The removal of AQP2 from the membrane requires dynamic cytoskeletal remodeling. AQP2 is degraded through the ubiquitin proteasome pathway and lysosomal proteolysis pathway. Finally, we review updated findings in transcriptional and epigenetic regulation of AQP2.

The microbiota-gut-kidney axis mediates host osmoregulation in a small desert mammal

Regulating sodium and water balances is crucial for survival of small, desert mammals. Studies demonstrate that the gut microbiota and their metabolites are involved in host energy homeostasis, but little is known on the interactions among salt loading, gut microbiota, and osmoregulation. The aim of this study was to fill this gap. Mongolian gerbils (Meriones unguiculatus) were offered drinking water (Con) and either water containing moderate (4%, MS) or high NaCl (8%, HS) ad libitum. Intake of HS reduced α diversity of the microbial community and, at the genus level, reduced the relative abundances of Rikenella and Christensenella but increased Atopobium. To confirm the function of gut microbiota in host osmoregulation, we transplanted caecal microbiota in HS gerbils. To cope with salt loading, the gerbils concentrated urine, resulting in negative energy balance and systemic inflammation. The HS gerbils increased hypothalamic arginine vasopressin and intestinal and renal aquaporin 2 to support water retention, and reduced intestinal and renal epithelial sodium channel α to promote sodium excretion. However, HS gerbils with caecal microbiota transplant (CMT) from Con donors maintained energy balance and osmoregulation, and had a much reduced systemic inflammation. Further, CMT from Con donors to HS recipients reshaped the gut microbiota, particularly by reducing Parabacteroides distasonis and Prevotella copri, and increasing Lactobacillus reuteri abundances, with a resulting increase in bacterial metabolites such as butyrate. These findings highlight a vital role of the microbiota-gut-kidney axis in mediating salt-related osmoregulation, allowing small mammals to adapt to high salt loads in a desert habitat.

The Role of Apelin/Apelin Receptor in Energy Metabolism and Water Homeostasis: A Comprehensive Narrative Review

The apelin receptor (APJ) is a member of the family A of G-protein-coupled receptors (GPCRs) and is involved in range of physiological and pathological functions, including fluid homeostasis, anxiety, and depression, as well as cardiovascular and metabolic disorders. APJ was classically described as a monomeric transmembrane receptor that forms a ternary complex together with its ligand and associated G proteins. More recently, increasing evidence indicates that APJ may interact with other GPCRs to form heterodimers, which may selectively modulate distinct intracellular signal transduction pathways. Besides, the apelin/APJ system plays important roles in the physiology and pathophysiology of several organs, including regulation of blood pressure, cardiac contractility, angiogenesis, metabolic balance, and cell proliferation, apoptosis, or inflammation. Additionally, the apelin/APJ system is widely expressed in the central nervous system, especially in neurons and oligodendrocytes. This article reviews the role of apelin/APJ in energy metabolism and water homeostasis. Compared with the traditional diuretics, apelin exerts a positive inotropic effect on the heart, while increases water excretion. Therefore, drugs targeting apelin/APJ system undoubtedly provide more therapeutic options for patients with congestive heart failure accompanied with hyponatremia. To provide more precise guidance for the development of clinical drugs, further in-depth studies are warranted on the metabolism and signaling pathways associated with apelin/APJ system.

The physiological and molecular mechanisms to maintain water and salt homeostasis in response to high salt intake in Mongolian gerbils (Meriones unguiculatus)

Desert rodents are faced with many challenges such as high dietary salt in their natural habitats and they have evolved abilities to conserve water and tolerate salt. However, the physiological and molecular mechanisms involved in water and salt balances in desert rodents are unknown. We hypothesized that desert rodents regulated water and salt balances by altering the expression of AQP2 and α-ENaC in the kidney. Mongolian gerbils (Meriones unguiculatus), a desert species, were acclimated to drinking water with different salt contents: (0, control; 4% NaCl, moderate salt, MS; 8% NaCl, high salt, HS) for 4 weeks. The gerbils drinking salty water had lower body mass, food intake, water intake, metabolic water production and urine volume. The HS gerbils increased the expression of arginine vasopressin (AVP) in the hypothalamus, and also enhanced the expression of AQP2 and cAMP/PKA/CREB signaling pathway in the kidney. In addition, these gerbils reduced serum aldosterone levels and α-ENaC expression in the kidney. Creatinine clearance was lower in the HS group than that in the control group, but serum and urine creatinine levels did not change. These data indicate that desert rodents rely on AVP-dependent upregulation of AQP2 and aldosterone-dependent downregulation of α-ENaC in the kidney to promote water reabsorption and sodium excretion under high salt intake.

A preliminary study of the effect of a high-salt diet on transcriptome dynamics in rat hypothalamic forebrain and brainstem cardiovascular control centers

BACKGROUND

High dietary salt intake is strongly correlated with cardiovascular (CV) diseases and it is regarded as a major risk factor associated with the pathogenesis of hypertension. The CV control centres in the brainstem (the nucleus tractus solitarii (NTS) and the rostral ventrolateral medulla (RVLM)) and hypothalamic forebrain (the subfornical organ, SFO; the supraoptic nucleus, SON and the paraventricular nucleus, PVN) have critical roles in regulating CV autonomic motor outflows, and thus maintaining blood pressure (BP). Growing evidence has implicated autonomic regulatory networks in salt-sensitive HPN (SSH), but the genetic basis remains to be delineated. We hypothesized that the development and/ or maintenance of SSH is reliant on the change in the expression of genes in brain regions controlling the CV system.

METHODOLOGY

We used RNA-Sequencing (RNA-Seq) to describe the differential expression of genes in SFO, SON, PVN, NTS and RVLM of rats being chronically fed with high-salt (HS) diet. Subsequently, a selection of putatively regulated genes was validated with quantitative reverse transcription polymerase chain reaction (qRT-PCR) in both Spontaneously Hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats.

RESULTS

The findings enabled us to identify number of differentially expressed genes in SFO, SON, PVN, NTS and RVLM; that are either up-regulated in both strains of rats (SON- Caprin2, Sctr), down-regulated in both strains of rats (PVN- Orc, Gkap1), up-regulated only in SHRs (SFO- Apopt1, Lin52, AVP, OXT; SON- AVP, OXT; PVN- Caprin2, Sclt; RVLM- A4galt, Slc29a4, Cmc1) or down-regulated only in SHRs (SON- Ndufaf2, Kcnv1; PVN- Pi4k2a; NTS- Snrpd2l, Ankrd29, St6galnac6, Rnf157, Iglon5, Csrnp3, Rprd1a; RVLM- Ttr, Faim).

CONCLUSIONS

These findings demonstrated the adverse effects of HS diet on BP, which may be mediated via modulating the signaling systems in CV centers in the hypothalamic forebrain and brainstem.

The underlying physiological basis of the desert rodent Meriones shawi's survival to prolonged water deprivation: Central vasopressin regulation on peripheral kidney water channels AQPs-2

Meriones shawi (M. shawi) is a particular semi-desert rodent known by its resistance to long periods of thirst. The aim of the present investigation is to clarify the underlying mechanisms allowing M. shawi to resist to hard conditions of dehydration. For this reason we used two different approaches: i) a morphometric study, which consists in measuring the effect of dehydration on body and kidneys weights as well as the report kidney weight/body weight, ii) By immunohistochemistry, we proceed to study the effect of dehydration on the immunoreactivity of central vasopressin (AVP) and the kidney aquaporin-2 (AQP-2) which is a channel protein that allows water to permeate across cell membranes. Our results showed both a body mass decrease accompanied by a remarkable kidneys hypertrophy. The immunohistochemical study showed a significant increase of AQP-2 immunoreactivity in the medullar part of Meriones kidneys allowing probably to Meriones a great ability to water retention. Consistently, we demonstrate that the increased AQP-2 expression occurred together with an increase in vasopressin (AVP) expression in both hypothalamic supraoptic (SON) and paraventricular nucleus (PVN), which are a major hub in the osmotic control circuitry. These various changes seen either in body weight and kidneys or at the cellular level might be the basis of peripheral control of body water homeostasis, providing to M. shawia strong resistance against chronic dehydration.

Molecular Biology of Aquaporins

Aquaporins (AQPs ) are a family of membrane water channels that basically function as regulators of intracellular and intercellular water flow. To date, thirteen AQPs , which are distributed widely in specific cell types in various organs and tissues, have been characterized in humans. Four AQP monomers, each of which consists of six membrane-spanning alpha-helices that have a central water-transporting pore, assemble to form tetramers, forming the functional units in the membrane. AQP facilitates osmotic water transport across plasma membranes and thus transcellular fluid movement. The cellular functions of aquaporins are regulated by posttranslational modifications , e.g. phosphorylation, ubiquitination, glycosylation, subcellular distribution, degradation, and protein interactions. Insight into the molecular mechanisms responsible for regulated aquaporin trafficking and synthesis is proving to be fundamental for development of novel therapeutic targets or reliable diagnostic and prognostic biomarkers.

Signaling Modification by GPCR Heteromer and Its Implication on X-Linked Nephrogenic Diabetes Insipidus

The involvement of secretin (SCT) and secretin receptor (SCTR) in regulating body water homeostasis is well established. Identified as one of the vasopressin (Vp)-independent mechanisms in fluid balance, SCT regulates aquaporin 2 (AQP2) in the kidney distal collecting duct cells through activating intracellular cAMP production. This ability to bypass Vp-mediated water reabsorption in kidney implicates SCT’s potential to treat nephrogenic diabetes insipidus (NDI). Research on NDI in the past has largely been focused on the searching for mutations in vasopressin receptor 2 (AVPR2), while the functional relationship between SCTR, AVPR2 and NDI remains unclear. Here, we demonstrate the interaction between SCTR and AVPR2 to modulate cellular signaling in vitro. Interestingly, we show in this report that upon heteromer formation with SCTR, R137H, a NDI-causing AVPR2 mutant that is defective in trafficking to cell surface, can functionally be rescued. Our data may provide an explanation for this clinically mild case of NDI, and insights into the pathological development of NDI in the future.

Defective Store-Operated Calcium Entry Causes Partial Nephrogenic Diabetes Insipidus

Store-operated calcium entry (SOCE) is the mechanism by which extracellular signals elicit prolonged intracellular calcium elevation to drive changes in fundamental cellular processes. Here, we investigated the role of SOCE in the regulation of renal water reabsorption, using the inbred rat strain SHR-A3 as an animal model with disrupted SOCE. We found that SHR-A3, but not SHR-B2, have a novel truncating mutation in the gene encoding stromal interaction molecule 1 (STIM1), the endoplasmic reticulum calcium (Ca(2+)) sensor that triggers SOCE. Balance studies revealed increased urine volume, hypertonic plasma, polydipsia, and impaired urinary concentrating ability accompanied by elevated circulating arginine vasopressin (AVP) levels in SHR-A3 compared with SHR-B2. Isolated, split-open collecting ducts (CD) from SHR-A3 displayed decreased basal intracellular Ca(2+) levels and a major defect in SOCE. Consequently, AVP failed to induce the sustained intracellular Ca(2+) mobilization that requires SOCE in CD cells from SHR-A3. This effect decreased the abundance of aquaporin 2 and enhanced its intracellular retention, suggesting impaired sensitivity of the CD to AVP in SHR-A3. Stim1 knockdown in cultured mpkCCDc14 cells reduced SOCE and basal intracellular Ca(2+) levels and prevented AVP-induced translocation of aquaporin 2, further suggesting the effects in SHR-A3 result from the expression of truncated STIM1. Overall, these results identify a novel mechanism of nephrogenic diabetes insipidus and uncover a role of SOCE in renal water handling.

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.

Apelin counteracts vasopressin-induced water reabsorption via cross talk between apelin and vasopressin receptor signaling pathways in the rat collecting duct

Apelin receptors (ApelinRs) are expressed along an increasing cortico-medullary gradient in collecting ducts (CDs). We showed here that iv injection of apelin 17 (K17F) in lactating rats characterized by increases in both synthesis and release of arginine vasopressin (AVP) increased diuresis concomitantly with a significant decrease in urine osmolality and no change in Na(+) and K(+) excretion. Under these conditions, we also observed a significant decrease in apical aquaporin-2 immunolabeling in CD, with a cortico-medullary gradient, suggesting that K17F-induced diuresis could be linked to a direct action of apelin on CD. We then examined the potential cross talk between V1a AVP receptor (V1a-R), V2 AVP receptor (V2-R) and ApelinR signaling pathways in outer medullary CD (OMCD) and inner medullary CD microdissected rat CD. In OMCD, expressing the 3 receptors, K17F inhibited cAMP production and Ca(2+) influx induced by 1-desamino-8-D-arginine vasopressin a V2-R agonist. Similar effects were observed in inner medullary CD expressing only V2-R and ApelinR. In contrast, in OMCD, K17F increased by 51% the Ca(2+) influx induced by the stimulation of V1a-R by AVP in the presence of the V2-R antagonist SR121463B, possibly enhancing the physiological antagonist effect of V1a-R on V2-R. Thus, the diuretic effect of apelin is not only due to a central effect by inhibiting AVP release in the blood circulation as previously shown but also to a direct action of apelin on CD, by counteracting the antidiuretic effect of AVP occurring via V2-R.

Culturing primary rat inner medullary collecting duct cells

Arginine-vasopressin (AVP) facilitates water reabsorption by renal collecting duct principal cells and thereby fine-tunes body water homeostasis. AVP binds to vasopressin V2 receptors (V2R) on the surface of the cells and thereby induces synthesis of cAMP. This stimulates cellular signaling processes leading to changes in the phosphorylation of the water channel aquaporin-2 (AQP2). Protein kinase A phoshorylates AQP2 and thereby triggers the translocation of AQP2 from intracellular vesicles into the plasma membrane facilitating water reabsorption from primary urine. Aberrations of AVP release from the pituitary or AVP-activated signaling in principal cells can cause central or nephrogenic diabetes insipidus, respectively; an elevated blood plasma AVP level is associated with cardiovascular diseases such as chronic heart failure and the syndrome of inappropriate antidiuretic hormone secretion. Here, we present a protocol for cultivation of primary rat inner medullary collecting duct (IMCD) cells, which express V2R and AQP2 endogenously. The cells are suitable for elucidating molecular mechanisms underlying the control of AQP2 and thus to discover novel drug targets for the treatment of diseases associated with dysregulation of AVP-mediated water reabsorption. IMCD cells are obtained from rat renal inner medullae and are used for experiments six to eight days after seeding. IMCD cells can be cultured in regular cell culture dishes, flasks and micro-titer plates of different formats, the procedure only requires a few hours, and is appropriate for standard cell culture laboratories.

Mechanism for targeting the A-kinase anchoring protein AKAP18δ to the membrane

A-kinase anchoring proteins (AKAPs) are a family of scaffolding proteins that target PKA and other signaling molecules to cellular compartments and thereby spatiotemporally define cellular signaling events. The AKAP18 family comprises AKAP18α, AKAP18β, AKAP18γ, and AKAP18δ. The δ isoform targets PKA and phosphodiesterase PDE4D to AQP2 (aquaporin-2)-bearing vesicles to orchestrate the acute regulation of body water balance. Therefore, AKAP18δ must adopt a membrane localization that seems at odds with (i) its lack of palmitoylation or myristoylation sites that tailor its isoforms AKAP18α and AKAP18β to membrane compartments and (ii) the high sequence identity to the preferentially cytoplasmic AKAP18γ. Here, we show that the electrostatic attraction of the positively charged amino acids of AKAP18δ to negatively charged lipids explains its membrane targeting. As revealed by fluorescence correlation spectroscopy, the binding constant of purified AKAP18δ fragments to large unilamellar vesicles correlates (i) with the fraction of net negatively charged lipids in the bilayer and (ii) with the total amount of basic residues in the protein. Although distantly located on the sequence, these positively charged residues concentrate in the tertiary structure and form a clear binding surface. Thus, specific recruitment of the AKAP18δ-based signaling module to membranes such as those of AQP2-bearing vesicles must be achieved by additional mechanisms, most likely compartment-specific protein-protein interactions.

Aquaporin 2 promotes cell migration and epithelial morphogenesis

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

Monitoring single-channel water permeability in polarized cells

So far the determination of unitary permeability (p_f) of water channels that are expressed in polarized cells is subject to large errors because the opening of a single water channel does not noticeably increase the water permeability of a membrane patch above the background. That is, in contrast to the patch clamp technique, where the single ion channel conductance may be derived from a single experiment, two experiments separated in time and/or space are required to obtain the single-channel water permeability p_f as a function of the incremental water permeability (P_f,c) and the number (n) of water channels that contributed to P_f,c. Although the unitary conductance of ion channels is measured in the native environment of the channel, p_f is so far derived from reconstituted channels or channels expressed in oocytes. To determine the p_f of channels from live epithelial monolayers, we exploit the fact that osmotic volume flow alters the concentration of aqueous reporter dyes adjacent to the epithelia. We measure these changes by fluorescence correlation spectroscopy, which allows the calculation of both P_f,c and osmolyte dilution within the unstirred layer. Shifting the focus of the laser from the aqueous solution to the apical and basolateral membranes allowed the FCS-based determination of n. Here we validate the new technique by determining the p_f of aquaporin 5 in Madin-Darby canine kidney cell monolayers. Because inhibition and subsequent activity rescue are monitored on the same sample, drug effects on exocytosis or endocytosis can be dissected from those on p_f.

Cytokine-induced paracrystals prolong the activity of signal transducers and activators of transcription (STAT) and provide a model for the regulation of protein solubility by small ubiquitin-like modifier (SUMO)

The biological effects of cytokines are mediated by STAT proteins, a family of dimeric transcription factors. In order to elicit transcriptional activity, the STATs require activation by phosphorylation of a single tyrosine residue. Our experiments revealed that fully tyrosine-phosphorylated STAT dimers polymerize via Tyr(P)-Src homology 2 domain interactions and assemble into paracrystalline arrays in the nucleus of cytokine-stimulated cells. Paracrystals are demonstrated to be dynamic reservoirs that protect STATs from dephosphorylation. Activated STAT3 forms such paracrystals in acute phase liver cells. Activated STAT1, in contrast, does not normally form paracrystals. By reversing the abilities of STAT1 and STAT3 to be sumoylated, we show that this is due to the unique ability of STAT1 among the STATs to conjugate to small ubiquitin-like modifier (SUMO). Sumoylation had one direct effect; it obstructed proximal tyrosine phosphorylation, which led to semiphosphorylated STAT dimers. These competed with their fully phosphorylated counterparts and interfered with their polymerization into paracrystals. Consequently, sumoylation, by preventing paracrystal formation, profoundly curtailed signal duration and reporter gene activation in response to cytokine stimulation of cells. The study thus identifies polymerization of activated STAT transcription factors as a positive regulatory mechanism in cytokine signaling. It provides a unifying explanation for the different subnuclear distributions of STAT transcription factors and reconciles the conflicting results as to the role of SUMO modification in STAT1 functioning. We present a generally applicable system in which protein solubility is maintained by a disproportionately small SUMO-modified fraction, whereby modification by SUMO partially prevents formation of polymerization interfaces, thus generating competitive polymerization inhibitors.

Mechanisms of vasopressin-induced intracellular Ca2+ oscillations in rat inner medullary collecting duct

Arginine vasopressin (AVP) causes increase in intracellular Ca(2+) concentration with an oscillatory pattern. Ca(2+) mobilization is required for AVP-stimulated apical exocytosis in inner medullary collecting duct (IMCD). The mechanistic basis of these Ca(2+) oscillations was investigated by confocal fluorescence microscopy and flash photolysis of caged molecules in perfused IMCD. Photorelease of caged cAMP and direct activation of ryanodine receptors (RyRs) by photorelease of caged cyclic ADP-ribose (cADPR) both mimicked the AVP-induced Ca(2+) oscillations. Preincubation of IMCD with 100 μM 8-bromo-cADPR (a competitive inhibitor of cADPR) delayed the onset and attenuated the magnitude of AVP-induced Ca(2+) oscillations. These observations indicate that the cADPR/RyR pathway is capable of supporting Ca(2+) oscillations and endogenous cADPR plays a major role in the AVP-induced Ca(2+) oscillations in IMCD. In contrast, photorelease of caged inositol 1,4,5-trisphosphate (IP(3)) induced Ca(2+) release but did not maintain sustained Ca(2+) oscillations. Removal of extracellular Ca(2+) halted ongoing AVP-mediated Ca(2+) oscillation, suggesting that it requires extracellular Ca(2+) entry. AVP-induced Ca(2+) oscillation was unaffected by nifedipine. Intracellular Ca(2+) store depletion induced by 20 μM thapsigargin in Ca(2+)-free medium triggered store-operated Ca(2+) entry (SOCE) in IMCD, which was attenuated by 1 μM GdCl(3) and 50 μM SKF-96365. After incubation of IMCD with 1 nM AVP in Ca(2+)-free medium, application of extracellular Ca(2+) also triggered Ca(2+) influx, which was sensitive to GdCl(3) and SKF-96365. In summary, our observations are consistent with the notion that AVP-induced Ca(2+) oscillations in IMCD are mediated by the interplay of Ca(2+) release from RyRs and a Ca(2+) influx mechanism involving nonselective cation channels that resembles SOCE.

Routes of epithelial water flow: aquaporins versus cotransporters

The routes water takes through membrane barriers is still a matter of debate. Although aquaporins only allow transmembrane water movement along an osmotic gradient, cotransporters are believed to be capable of water transport against the osmotic gradient. Here we show that the renal potassium-chloride-cotransporter (KCC1) does not pump a fixed amount of water molecules per movement of one K(+) and one Cl(-), as was reported for the analogous transporter in the choroid plexus. We monitored water and potassium fluxes through monolayers of primary cultured renal epithelial cells by detecting tiny solute concentration changes in the immediate vicinity of the monolayer. KCC1 extruded K(+) ions in the presence of a transepithelial K(+) gradient, but did not transport water. KCC1 inhibition reduced epithelial osmotic water permeability P(f) by roughly one-third, i.e., the effect of inhibitors was small in resting cells and substantial in hormonal stimulated cells that contained high concentrations of aquaporin-2 in their apical membranes. The furosemide or DIOA (dihydroindenyl-oxy-alkanoic acid)-sensitive water flux was much larger than expected when water passively followed the KCC1-mediated ion flow. The inhibitory effect of these drugs on water flux was reversed by the K(+)-H(+) exchanger nigericin, indicating that KCC1 affects water transport solely by K(+) extrusion. Intracellular K(+) retention conceivably leads to cell swelling, followed by an increased rate of endocytic AQP2 retrieval from the apical membrane.

Reciprocal regulation of aquaporin-2 abundance and degradation by protein kinase A and p38-MAP kinase

Arginine-vasopressin (AVP) modulates the water channel aquaporin-2 (AQP2) in the renal collecting duct to maintain homeostasis of body water. AVP binds to vasopressin V2 receptors (V2R), increasing cAMP, which promotes the redistribution of AQP2 from intracellular vesicles into the plasma membrane. cAMP also increases AQP2 transcription, but whether altered degradation also modulates AQP2 protein levels is not well understood. Here, elevation of cAMP increased AQP2 protein levels within 30 minutes in primary inner medullary collecting duct (IMCD) cells, in human embryonic kidney (HEK) 293 cells ectopically expressing AQP2, and in mouse kidneys. Accelerated transcription or translation did not explain this increase in AQP2 abundance. In IMCD cells, cAMP inhibited p38-mitogen-activated protein kinase (p38-MAPK) via activation of protein kinase A (PKA). Inhibition of p38-MAPK associated with decreased phosphorylation (serine 261) and polyubiquitination of AQP2, preventing proteasomal degradation. Our results demonstrate that AVP enhances AQP2 protein abundance by altering its proteasomal degradation through a PKA- and p38-MAPK-dependent pathway.

Intrarenal purinergic signaling in the control of renal tubular transport

Renal tubular epithelial cells receive hormonal input that regulates volume and electrolyte homeostasis. In addition, numerous intrarenal, local signaling agonists have appeared on the stage of renal physiology. One such system is that of intrarenal purinergic signaling. This system involves all the elements necessary for agonist-mediated intercellular communication. ATP is released from epithelial cells, which activates P2 receptors in the apical and basolateral membrane and thereby modulates tubular transport. Termination of the signal is conducted via the breakdown of ATP to adenosine. Recent far-reaching advances indicate that ATP is often used as a local transmitter for classical sensory transduction. This transmission apparently also applies to sensory functions in the kidney. Locally released ATP is involved in sensing of renal tubular flow or in detecting the distal tubular load of NaCl at the macula densa. This review describes the relevant aspects of local, intrarenal purinergic signaling and outlines its integrative concepts.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.

Controlled aquaporin-2 expression in the hypertonic environment

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

Interaction of angiotensin-converting enzyme (ACE) with membrane-bound carboxypeptidase M (CPM) - a new function of ACE

Angiotensin-converting enzyme (ACE) demonstrates, besides its typical dipeptidyl-carboxypeptidase activity, several unusual functions. Here, we demonstrate with molecular, biochemical, and cellular techniques that the somatic wild-type murine ACE (mACE), stably transfected in Chinese Hamster Ovary (CHO) or Madin-Darby Canine Kidney (MDCK) cells, interacts with endogenous membranal co-localized carboxypeptidase M (CPM). CPM belongs to the group of glycosylphosphatidylinositol (GPI)-anchored proteins. Here we report that ACE, completely independent of its known dipeptidase activities, has GPI-targeted properties. Our results indicate that the spatial proximity between mACE and the endogenous CPM enables an ACE-evoked release of CPM. These results are discussed with respect to the recently proposed GPI-ase activity and function of sperm-bound ACE.

Regulation of aquaporin-2 trafficking

Principal cells lining renal collecting ducts control the fine-tuning of body water homeostasis by regulating water reabsorption through the water channels aquaporin-2 (AQP2), aquaporin-3 (AQP3), and aquaporin-4 (AQP4). While the localization of AQP2 is subject to regulation by arginine-vasopressin (AVP), AQP3 and AQP4 are constitutively expressed in the basolateral plasma membrane. AVP adjusts the amount of AQP2 in the plasma membrane by triggering its redistribution from intracellular vesicles into the plasma membrane. This permits water entry into the cells and water exit through AQP3 and AQP4. The translocation of AQP2 is initiated by an increase in cAMP following V2R activation through AVP. The AVP-induced rise in cAMP activates protein kinase A (PKA), which in turn phosphorylates AQP2, and thereby triggers the redistribution of AQP2. Several proteins participating in the control of cAMP-dependent AQP2 trafficking have been identified; for example, A kinase anchoring proteins (AKAPs) tethering PKA to cellular compartments; phosphodiesterases (PDEs) regulating the local cAMP level; cytoskeletal components such as F-actin and microtubules; small GTPases of the Rho family controlling cytoskeletal dynamics; motor proteins transporting AQP2-bearing vesicles to and from the plasma membrane for exocytic insertion and endocytic retrieval; SNAREs inducing membrane fusions, hsc70, a chaperone, important for endocytic retrieval. In addition, cAMP-independent mechanisms of translocation mainly involving the F-actin cytoskeleton have been uncovered. Defects of AQP2 trafficking cause diseases such as nephrogenic diabetes insipidus (NDI), a disorder characterized by a massive loss of hypoosmotic urine.This review summarizes recent data elucidating molecular mechanisms underlying the trafficking of AQP2. In particular, we focus on proteins involved in the regulation of trafficking, and physiological and pathophysiological stimuli determining the cellular localization of AQP2. The identification of proteins and protein-protein interactions may lead to the development of drugs targeting AQP2 trafficking. Such drugs may be suitable for the treatment of diseases associated with dysregulation of body water homeostasis, including NDI or cardiovascular diseases (e.g., chronic heart failure) where the AVP level is elevated, inducing excessive water retention.

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

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

Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption

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

Translocation of aquaporin-containing vesicles to the plasma membrane is facilitated by actomyosin relaxation

Docking and fusion of vesicles to the plasma membrane is a fundamental process in living cells. An established model for the trafficking of vesicles is based on primary epithelial cells from the collecting duct of the nephron. Upon stimulation with the signaling peptide arginine-vasopressin (AVP), aquaporin-containing vesicles are directed to the plasma membrane. Since aquaporin selectively enhances the water permeability of plasma membranes, this process helps to balance the water content of the organism. A mechanism has been suggested involving local depolymerization of F-actin to facilitate the movement of vesicles to the membrane. Since F-actin is the major component of cytoskeletal restoring forces, AVP-stimulated cells can be expected to lose rigidity. Here, we used atomic force microscopy force mapping to test whether AVP alters cell stiffness. The Young's modulus of living epithelial cells at 37 degrees C was continuously monitored, yielding a 51% decrease of Young's modulus after the addition of AVP. The data demonstrate that not the depolymerization of actin but a relaxation of actomyosin interaction facilitates vesicle translocation.

Compartmentalized cAMP signalling in regulated exocytic processes in non-neuronal cells

Cyclic adenosine monophosphate (cAMP) is a central second messenger controlling a plethora of vital functions. Studies of cAMP dynamics in living cells have revealed markedly inhomogeneous concentrations of the second messenger in different compartments. Moreover, cAMP effectors such as cAMP-dependent protein kinase (PKA) and cAMP-activated GTP-exchange factors (Epacs) are tethered to specific cellular sites. Both the tailoring of cAMP concentrations, and the activities of cAMP-dependent signalling systems at specific cellular locations are prerequisites for most, if not all, cAMP-dependent processes. This review focuses on the role of compartmentalized cAMP signalling in exocytic processes in non-neuronal cells. Particularly, the insertion of aquaporin-2 into the plasma membrane of renal principal cells as an example for a cAMP-dependent exocytic process in a non-secretory cell type, renin secretion from juxtaglomerular cells as a cAMP-triggered exocytosis from an endocrine cell, insulin release from pancreatic beta-cells as a Ca2+-mediated and cAMP-potentiated exocytic processes in an endocrine cell, and cAMP- or Ca2+ -triggered H+ secretion from gastric parietal cells as an exocytic process in an exocrine cell are discussed. The selected examples of cAMP-regulated exocytic pathways are reviewed with regard to key proteins involved: adenylyl cyclases, phosphodiesterases, PKA, A kinase anchoring proteins (AKAPs) and Epacs.

Calcium signaling in vasopressin-induced aquaporin-2 trafficking

It has been the general consensus that cAMP-mediated PKA-dependent phosphorylation of aquaporin-2 is the primary mechanism of vasopressin to regulate osmotic water permeability in kidney collecting duct. By using laser scanning confocal microscopy to monitor [Ca2+]i and apical exocytosis in individual cells of inner medullary collecting duct, we have demonstrated that vasopressin also triggers intracellular Ca2+ mobilization, which is coupled to apical exocytotic insertion of aquaporin-2. Vasopressin-induced Ca2+ mobilization is in the form of oscillations, which involves both intracellular Ca2+ release from ryanodine-gated Ca2+ stores and extracellular Ca2+ influx via capacitative calcium entry. Each individual cell operates as an independent calcium oscillator with time variance in frequency and amplitude. Vasopressin-induced Ca2+ mobilization is mediated by cAMP, but is independent of PKA. Exogenous cAMP analog (8-pCPT-2'-O-Me-cAMP), which activates Epac (exchange protein directly activated by cAMP), but not PKA, triggers Ca2+ mobilization and apical exocytosis. These observations suggest that activation of Epac by cAMP may also contribute to the action of vasopressin in regulating osmotic water permeability. There are multiple plausible candidates for downstream effectors of vasopressin-induced Ca2+ signal including calmodulin, myosin light chain kinase, calmodulin kinase II, and calcineurin. All of them have been implicated in the regulation of aquaporin-2 trafficking and/or water permeability.

Vasopressin-induced membrane trafficking of TRPC3 and AQP2 channels in cells of the rat renal collecting duct

The canonical transient receptor potential channels TRPC3 and TRPC6 are abundantly expressed along with the water channel aquaporin-2 (AQP2) in principal cells of the cortical and medullary collecting duct. Although TRPC3 is selectively localized to the apical membrane and TRPC6 is found in both the apical and basolateral domains, immunofluorescence is often observed in the cytoplasm, suggesting that TRPC3 and TRPC6 may exist in intracellular vesicles and may shuttle to and from the membrane in response to receptor stimulation. To test this hypothesis, the effect of arginine-vasopressin (AVP) on the subcellular distribution of TRPC3, TRPC6, and AQP2 was examined in the rat kidney and in cultured cell lines from the cortical (M1) and inner medullary (IMCD-3) collecting duct. Immunofluorescence analysis revealed that TRPC3, but not TRPC6, colocalized with AQP2 in intracellular vesicles. AVP caused the insertion and accumulation of TRPC3 and AQP2 in the apical membrane but had no effect on the subcellular distribution of TRPC6. TRPC3, but not TRPC6, coimmunoprecipitated with AQP2 from both medulla and M1 and IMCD-3 cell lysates. Apical-to-basolateral transepithelial 45Ca2+ flux in polarized IMCD-3 cell monolayers was stimulated by diacylglycerol analogs or by the purinergic receptor agonist ATP but not by thapsigargin. Stimulated 45Ca2+ flux was increased by overexpression of TRPC3 and attenuated by a dominant-negative TRPC3 construct. Furthermore, 45Ca2+ flux was greatly reduced by the pyrazole-derivative BTP2, a known inhibitor of TRPC3 channels. These results demonstrate that 1) TRPC3 and TRPC6 exist in different vesicle populations, 2) TRPC3 physically associates with APQ2 and shuttles to the apical membrane in response to AVP, and 3) TRPC3 is responsible for transepithelial Ca2+ flux in principal cells of the renal collecting duct.

Rescue of a Nephrogenic Diabetes Insipidus-causing Vasopressin V2 Receptor Mutant by Cell-penetrating Peptides

Mutant membrane proteins are frequently retained in the early secretory pathway by a quality control system, thereby causing disease. An example are mutants of the vasopressin V(2) receptor (V(2)R) leading to nephrogenic diabetes insipidus. Transport-defective V(2)Rs fall into two classes: those retained exclusively in the endoplasmic reticulum (ER) and those reaching post-ER compartments such as the ER/Golgi intermediate compartment. Although numerous chemical or pharmacological chaperones that rescue the transport of ER-retained membrane proteins are known, substances acting specifically in post-ER compartments have not been described as yet. Using the L62P (ER-retained) and Y205C (reaching post-ER compartments) mutants of the V(2)R as a model, we show here that the cell-penetrating peptide penetratin and its synthetic analog KLAL rescue the transport of the Y205C mutant. In contrast, the location of the L62P mutant is not influenced by either peptide because the peptides are unable to enter the ER. We also show data indicating that the peptide-mediated transport rescue is associated with an increase in cytosolic Ca(2+) concentrations. Thus, we describe a new class of substances influencing protein transport specifically in post-ER compartments.

Regulation of AQP2 in Collecting Duct : An emphasis on the Effects of Angiotensin II or Aldosterone

Vasopressin, angiotensin II (AngII), and aldosterone are essential hormones in the regulation of body fluid homeostatsis. We examined the effects of AngII or aldosterone on the regulation of body water balance. We demonstrated that 1) short-term treatment with AngII in the primary cultured inner medullary collecting duct cells played a role in the regulation of AQP2 targeting to the plasma membrane through AT1 receptor activation. This potentiated the effects of dDAVP on cAMP accumulation, AQP2 phosphorylation, and AQP2 plasma membrane targeting; 2) pharmacological blockade of the AngII AT1 receptor in rats co-treated with dDAVP and dietary NaCl-restriction (to induce high plasma endogenous AngII) resulted in an increase in urine production, a decrease in urine osmolality, and blunted the dDAVP-induced upregulation of AQP2; 3) long-term aldosterone infusion in normal rats or in rats with diabetes insipidus was associated with polyuria and decreased urine concentration, accompanied by decreased apical but increased basolateral AQP2 labeling intensity in the connecting tubule and cortical collecting duct; and 4) in contrast to the effects of dDAVP and AngII, short-term aldosterone treatment does not alter the intracellular distribution of AQP2. In conclusion, angiotensin II, and aldosterone could play a role in the regulation of renal water reabsorption by changing intracellular AQP2 targeting and/or AQP2 abundance, in addition to the vasopressin.

Phenotypes developed in secretin receptor-null mice indicated a role for secretin in regulating renal water reabsorption

Aquaporin 2 (AQP2) is responsible for regulating the concentration of urine in the collecting tubules of the kidney under the control of vasopressin (Vp). Studies using Vp-deficient Brattleboro rats, however, indicated the existence of substantial Vp-independent mechanisms for membrane insertion, as well as transcriptional regulation, of this water channel. The Vp-independent mechanism(s) is clinically relevant to patients with X-linked nephrogenic diabetes insipidus (NDI) by therapeutically bypassing the dysfunctional Vp receptor. On the basis of studies with secretin receptor-null (SCTR(-/-)) mice, we report here for the first time that mutation of the SCTR gene could lead to mild polydipsia and polyuria. Additionally, SCTR(-/-) mice were shown to have reduced renal expression of AQP2 and AQP4, as well as altered glomerular and tubular morphology, suggesting possible disturbances in the filtration and/or water reabsorption process in these animals. By using SCTR(-/-) mice as controls and comparing them with wild-type animals, we performed both in vivo and in vitro studies that demonstrated a role for secretin in stimulating (i) AQP2 translocation from intracellular vesicles to the plasma membrane in renal medullary tubules and (ii) expression of this water channel under hyperosmotic conditions. The present study therefore provides information for at least one of the Vp-independent mechanisms that modulate the process of renal water reabsorption. Future investigations in this direction should be important in developing therapeutic means for treating NDI patients.

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

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

Increased AQP2 targeting in primary cultured IMCD cells in response to angiotensin II through AT1 receptor

Vasopressin and angiotensin II (ANG II) play a major role in renal water and Na(+) reabsorption. We previously demonstrated that ANG II AT(1) receptor blockade decreases dDAVP-induced water reabsorption and AQP2 levels in rats, suggesting cross talk between these two peptide hormones (Am J Physiol Renal Physiol 288: F673-F684, 2005). To directly address this issue, primary cultured inner medullary collecting duct (IMCD) cells from male Sprague-Dawley rats were treated for 15 min with 1) vehicle, 2) ANG II, 3) ANG II + the AT(1) receptor blocker candesartan, 4) dDAVP, 5) ANG II + dDAVP, or 6) ANG II + dDAVP + candesartan. Immunofluorescence microscopy revealed that 10(-8) M ANG II or 10(-11) M dDAVP (protocol 1) was associated with increased AQP2 labeling of the plasma membrane and decreased cytoplasmic labeling, respectively. cAMP levels increased significantly in response to 10(-8) M ANG II and were potentiated by cotreatment with 10(-11) M dDAVP. Consistent with this finding, immunoblotting revealed that this cotreatment significantly increased expression of phosphorylated AQP2. ANG II-induced AQP2 targeting was blocked by 10(-5) M candesartan. In protocol 2, treatment with a lower concentration of dDAVP (10(-12) M) or ANG II (10(-9) M) did not change subcellular AQP2 distribution, whereas 10(-12) M dDAVP + 10(-9) M ANG II enhanced AQP2 targeting. This effect was inhibited by cotreatment with 10(-5) M candesartan. ANG II-induced cAMP accumulation and AQP2 targeting were inhibited by inhibition of PKC activity. In conclusion, ANG II plays a role in the regulation of AQP2 targeting to the plasma membrane in IMCD cells through AT(1) receptor activation and potentiates the effect of dDAVP on AQP2 plasma membrane targeting.

Epac-mediated Ca(2+) mobilization and exocytosis in inner medullary collecting duct

PKA has traditionally been thought as the binding protein of cAMP for mediating arginine vasopressin (AVP)-regulated osmotic water permeability in kidney collecting duct. It is now known that cAMP also exerts its effects via Epac (exchange protein directly activated by cAMP) and that intracellular Ca(2+) mobilization is necessary for AVP-induced apical exocytosis in inner medullary collecting duct (IMCD). The role of Epac as an effector of cAMP action in addition to PKA was investigated using confocal fluorescence microscopy in perfused IMCD. PKA inhibitors (1 microM H-89 or 10 microM KT-5720) at concentrations known to inhibit aquaporin-2 (AQP2) phosphorylation did not prevent AVP-induced Ca(2+) mobilization and oscillations. Epac-selective cAMP agonist (8-pCPT-2'-O-Me-cAMP) mimicked AVP in triggering Ca(2+) mobilization and oscillations, which was blocked by ryanodine but not by Rp-cAMP (a competitive antagonist of cAMP binding to PKA). 8-pCPT-2'-O-Me-cAMP also triggered apical exocytosis in the presence of a PKA inhibitor. Immunolocalization of AQP2 in perfused IMCD demonstrated that 8-pCPT-2'-O-Me-cAMP induces apical targeting of AQP2 and that AQP2 is abundant in junctional regions of basolateral membrane. Immunofluorescence study also confirmed the presence of Epac (isoform I) in IMCD. These results indicate that activation of Epac by an exogenous cAMP analog triggers intracellular Ca(2+) mobilization and apical exocytotic insertion of AQP2 in IMCD.

Regulation of aquaporin-2 trafficking and its binding protein complex

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

Minireview: aquaporin 2 trafficking

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

Clinical update on renal aquaporins

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

Immunocytochemical studies on translocation of phosphorylated aquaporin-h2 protein in granular cells of the frog urinary bladder before and after stimulation with vasotocin

We have generated a specific antibody against phosphorylated aquaporin-h2 (pAQP-h2) protein to investigate the role of phosphorylation in the translocation of AQP-h2 protein within the granule cells of the urinary bladder of the frog (Hyla japonica). The antibody was generated against a synthetic peptide (ST-160) corresponding to amino acids 255-268, with a phosphorylated Ser-262, a residue that is putatively phosphorylated by protein A kinase. Using this antibody, we found, by Western blot analysis, that phosphorylation of the AQP-h2 protein rapidly increased within 2 min after vasotocin (AVT) stimulation and remained at a higher than normal level for 15 min. Moreover, quantitative immunoelectron microscopy indicated that the location of the AQP-h2 protein dramatically changed after AVT stimulation. Before stimulation, pAQP-h2 protein was localized in only a small number of intracellular vesicles near the nucleus of the granular cells, whereas the labeling density of the intracellular vesicles and the apical membrane rapidly increased after stimulation. This finding was also confirmed by the results of an immunofluorescence study. Thus, phosphorylation of AQP-h2 protein seems to be essential for translocation of the protein from the cytoplasmic pool to the apical plasma membrane of the granular cells in frog urinary bladder.

Disease-causing V(2) vasopressin receptors are retained in different compartments of the early secretory pathway

The G protein-coupled V(2) vasopressin receptor is crucially involved in water reabsorption in the renal collecting duct. Mutations in the human V(2) vasopressin receptor gene cause nephrogenic diabetes insipidus. Many of the disease-causing mutants are retained intracellularly by the quality control system of the early secretory pathway. It was previously thought that quality control system is restricted to the endoplasmic reticulum (ER). Here, we have examined the retention mechanisms of eight V(2) vasopressin receptor mutants. We show that mutants L62P, DeltaL62-R64 and S167L are trapped exclusively in the ER. In contrast, mutants R143P, Y205C, InsQ292, V226E and R337X reach the ER/Golgi intermediate compartment (ERGIC) and are rerouted to the ER. The ability of the mutant receptors to reach the ERGIC is independent of their expression levels. Instead, it is determined by their folding state. Mutant receptors in the ERGIC may be sorted into retrograde transport vesicles by an interaction of an RXR motif in the third intracellular loop with the coatomer complex I. Our data show that disease-causing mutants of a particular membrane protein may be retained in different compartments of the early secretory pathway and that the folding states of the proteins determine their retention mechanism.

The mitochondrial uncoupling-protein homologues

Uncoupling protein(UCP)1 is an integral membrane protein that is located in the mitochondrial inner membrane of brown adipocytes. Its physiological role is to mediate a regulated, thermogenic proton leak. UCP2 and UCP3 are recently identified UCP1 homologues. They also mediate regulated proton leak, and might function to control the production of superoxide and other downstream reactive oxygen species. However, their role in normal physiology remains unknown. Recent studies have shown that UCP2 has an important part in the pathogenesis of type-2 diabetes. The obscure roles of the UCP homologues in normal physiology, together with their emerging role in pathophysiology, provide exciting potential for further investigation.

Combined transport of water and ions through membrane channels

The coupling of ion and water flow through membrane channels is under dispute. Among all human aquaporins only aquaporin-6 exhibits ion channel activity. Whether aquaporin-6 functions also as a water channel cannot yet be determined with confidence. Similarly, a comparison of single-channel water permeabilities from ion channels and aquaporins suggests that ion channels may play a secondary role as water channels. However, the fraction of absorbed fluid that crosses epithelial ion channels still remains to be determined.