cAMP-dependent phosphorylation stimulates water permeability of aquaporin-collecting duct water channel protein expressed in Xenopus oocytes

The subcellular localization of an aquaporin-2 tetramer depends on the stoichiometry of phosphorylated and nonphosphorylated monomers

In renal principal cells, vasopressin regulates the shuttling of the aquaporin (AQP)2 water channel between intracellular vesicles and the apical plasma membrane. Vasopressin-induced phosphorylation of AQP2 at serine 256 (S256) by protein kinase A (PKA) is essential for its localization in the membrane. However, phosphorylated AQP2 (p-AQP2) has also been detected in intracellular vesicles of noninduced principal cells. As AQP2 is expressed as homotetramers, we hypothesized that the number of p-AQP2 monomers in a tetramer might be critical for the its steady state distribution. Expressed in oocytes, AQP2-S256D and AQP2-S256A mimicked p-AQP2 and non-p-AQP2, respectively, as routing and function of AQP2-S256D and wild-type AQP2 (wt-AQP2) were identical, whereas AQP2-S256A was retained intracellularly. In coinjection experiments, AQP2-S256A and AQP2-S256D formed heterotetramers. Coinjection of different ratios of AQP2-S256A and AQP2-S256D cRNAs revealed that minimally three AQP2-S256D monomers in an AQP2 tetramer were essential for its plasma membrane localization. Therefore, our results suggest that in principal cells, minimally three monomers per AQP2 tetramer have to be phosphorylated for its steady state localization in the apical membrane. As other multisubunit channels are also regulated by phosphorylation, it is anticipated that the stoichiometry of their phosphorylated and nonphosphorylated subunits may fine-tune the activity or subcellular localization of these complexes.

Nitric oxide and atrial natriuretic factor stimulate cGMP-dependent membrane insertion of aquaporin 2 in renal epithelial cells

In collecting duct principal cells, aquaporin 2 (AQP2) is shuttled from intracellular vesicles to the plasma membrane upon vasopressin (VP) stimulation. VP activates adenylyl cyclase, increases intracellular cAMP, activating protein kinase A (PKA) to phosphorylate AQP2 on the COOH-terminal residue, serine 256. Using rat kidney slices and LLC-PK1 cells stably expressing AQP2 (LLC-AQP2 cells), we now show that AQP2 trafficking can be stimulated by cAMP-independent pathways. In these systems, the nitric oxide (NO) donors sodium nitroprusside (SNP) and NONOate and the NO synthase substrate L-arginine mimicked the effect of VP, stimulating relocation of AQP2 from cytoplasmic vesicles to the plasma membrane. Unlike VP, these other agents did not increase intracellular cAMP. However, SNP increased intracellular cGMP, and exogenous cGMP stimulated AQP2-membrane insertion. Atrial natriuretic factor, which signals via cGMP, also stimulated AQP2 translocation. The VP and SNP effects were blocked by the kinase inhibitor H89. SNP did not stimulate membrane insertion of AQP2 in LLC-PK1 cells expressing the phosphorylation-deficient mutant 256SerAla-AQP2, indicating that phosphorylation of Ser256 is required for signaling. Both PKA and cGMP-dependent protein kinase G phosphorylated AQP2 on this COOH-terminal residue in vitro. These results demonstrate a novel, cAMP-independent and cGMP-dependent pathway for AQP2 membrane insertion in renal epithelial cells.

Effects of ethanol on plasma chloroquine, arginine vasopressin (AVP) concentrations and renal hydro-electrolyte handling in the rat

Current evidence in literature suggests that acute effects of either chloroquine or ethanol on kidney function partly depend on influencing plasma concentrations of arginine vasopressin (AVP). Therefore, the goal of the current study was to explore the effects of chloroquine and/or various doses of ethanol on plasma AVP levels and associated effects on renal hydro-electrolyte handling. Separate groups of male anaesthetized Sprague-Dawley rats were placed on a continuous jugular infusion of 0.077 M NaCl at 150 microL/min(-1). After 3 h equilibration period, consecutive 20 min urine collections were made over the subsequent 4 h of 1 h control, 1 h 20 min treatment and 1 h 40 min postequilibration periods for measurements of urine flow and Na+ and K+ excretion rates. Chloroquine (0.06 microg/min(-1)) and/or ethanol at either 2.4, 6, 18 or 24 microg/min(-1) were added to the infusate during the treatment period. Trunk blood was collected after the treatment period from parallel groups for AVP, ethanol and chloroquine measurements. Vehicle infused animals acted as control animals. Infusion of ethanol at low rate of 2.4 microg/min(-1) increased Na+ excretion rates, but high rates (6-24 microg/min(-1)) did not elicit such effects. Plasma ethanol concentrations were undetectable following administration of ethanol alone at 2.4 or 6 microg/min(-1). However, ethanols were measurable following co-infusion of chloroquine and ethanol at 6 microg/min(-1) (6+/-1 mg/dL(-1)). Concurrent chloroquine and ethanol (24 microg/min(-1)) administration elevated plasma ethanol concentrations by 26% by comparison with that of ethanol alone at the same dose. Chloroquine and ethanol infusion at all doses significantly (p < 0.01) increased plasma chloroquine concentrations. Intravenous infusion of ethanol increased plasma AVP concentrations in a dose-dependent manner. The observations of this study suggest that acute ethanol increases plasma AVP levels in a dose-dependent manner to affect hydro-electrolyte balance.

Aquaporin-1 expression in visceral smooth muscle cells of female rat reproductive tract

The aquaporins (AQ-s) are a group of intrinsic membrane proteins which facilitate movement of water across cell membranes; their recent identification in the kidney has led to the reappraisal of the mechanisms and pathways of water movement across epithelia. Aquaporin-1, (CHIP-28) is reported distributed in cardiac myocytes and vascular smooth muscle cells of large arteries. A related protein, AQ-4, has been identified in the sarcolemma of skeletal muscle fibres. We report aquaporin expression in the cell membrane of smooth muscle cells of the rat genital tract; fluorescence immunohistochemistry of rat uterine (fallopian) tube and vagina demonstrated AQ-1 in visceral smooth muscle of these tissues. In the uterine tube, AQ-1 labelling is most pronounced in the innermost longitudinal and the inner cells of the circular muscle layer and is absent from the outer longitudinal muscle layer of the myosalpinx. The possibility of a specific role for AQ-1 in tubal transport by altering the tubal luminal diameter during the estrus cycle is suggested.

The mechanisms of aquaporin control in the renal collecting duct

The antidiuretic hormone arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Central to its antidiuretic action in mammals is the exocytotic insertion of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the apical membrane of principal cells, an event initiated by an increase in cAMP and activation of protein kinase A. Water is then reabsorbed from the hypotonic urine of the collecting duct. The water channels aquaporin-3 (AQP3) and aquaporin-4 (AQP4), which are constitutively present in the basolateral membrane, allow the exit of water from the cell into the hypertonic interstitium. Withdrawal of the hormone leads to endocytotic retrieval of AQP2 from the cell membrane. The hormone-induced rapid redistribution between the interior of the cell and the cell membrane establishes the basis for the short term regulation of water permeability. In addition water channels (AQP2 and 3) of principal cells are regulated at the level of expression (long term regulation). This review summarizes the current knowledge on the molecular mechanisms underlying the short and long term regulation of water channels in principal cells. In the first part special emphasis is placed on the proteins involved in short term regulation of AQP2 (SNARE proteins, Rab proteins, cytoskeletal proteins, G proteins, protein kinase A anchoring proteins and endocytotic proteins). In the second part, physiological and pathophysiological stimuli determining the long term regulation are discussed.

Aquaporin adipose, a putative glycerol channel in adipocytes

Adipose tissue is a major site of glycerol production in response to energy balance. However, molecular basis of glycerol release from adipocytes has not yet been elucidated. We recently cloned a novel member of the aquaporin family, aquaporin adipose (AQPap), which has glycerol permeability. The current study was designed to examine the hypothesis that AQPap serves as a glycerol channel in adipocytes. Adipose tissue expressed AQPap mRNA in high abundance, but not the mRNAs for the other aquaglyceroporins, AQP3 and AQP9, indicating that AQPap is the only known aquaglyceroporin expressed in adipose tissue. Glycerol release from 3T3-L1 cells was increased during differentiation in parallel with AQPap mRNA levels and suppressed by mercury ion, which inhibits the function of AQPs, supporting AQPap functions as a glycerol channel in adipocytes. Fasting increased and refeeding suppressed adipose AQPap mRNA levels in accordance with plasma glycerol levels and oppositely to plasma insulin levels in mice. Insulin dose-dependently suppressed AQPap mRNA expression in 3T3-L1 cells. AQPap mRNA levels and adipose glycerol concentrations measured by the microdialysis technique were increased in obese mice with insulin resistance. Accordingly, negative regulation of AQPap expression by insulin was impaired in the insulin-resistant state. Exposure of epinephrine translocated AQPap protein from perinuclear cytoplasm to the plasma membrane in 3T3-L1 adipocytes. These results strongly suggest that AQPap plays an important role in glycerol release from adipocytes.

The phosphatase inhibitor okadaic acid induces AQP2 translocation independently from AQP2 phosphorylation in renal collecting duct cells

Phosphorylation by kinases and dephosphorylation by phosphatase markedly affect the biological activity of proteins involved in intracellular signaling. In this study we investigated the effect of the serine/threonine phosphatase inhibitor okadaic acid on water permeability properties and on aquaporin2 (AQP2) translocation in AQP2-transfected renal CD8 cells. In CD8 cells both forskolin alone and okadaic acid alone increased the osmotic water permeability coefficient P(f) by about 4- to 5-fold. In intact cells, in vivo phosphorylation studies revealed that forskolin stimulation resulted in a threefold increase in AQP2 phosphorylation. In contrast, okadaic acid treatment promoted only a 60% increase in AQP2 phosphorylation which was abolished when this treatment was performed in the presence of 1 μM H89, a specific protein kinase A (PKA) inhibitor. Nevertheless, in this latter condition, confocal microscopy analysis revealed that AQP2 translocated and fused to the apical membrane. Okadaic acid-induced AQP2 translocation was dose dependent having its maximal effect at a concentration of 1 μM. In conclusion, our results clearly indicate that okadaic acid exerts a full forskolin-like effect independent from AQP2 phosphorylation. Thus AQP2 phosphorylation is not essential for water channel translocation in renal cells, indicating that different pathways might exist leading to AQP2 apical insertion and increase in P(f).

Prostaglandin E(2) interaction with AVP: effects on AQP2 phosphorylation and distribution

Prostaglandin E(2) (PGE(2)) antagonizes the action of arginine vasopressin (AVP) on collecting duct water permeability. To investigate the mechanism of this antagonism, rat renal inner medulla (IM) was incubated with the two hormones, and the phosphorylation and subcellular distribution of the water channel, aquaporin-2 (AQP2) were studied. Using a phosphorylation state-specific AQP2 antibody, we demonstrated that AVP stimulates AQP2 phosphorylation at the Ser(256) protein kinase A consensus site in a time- and dose-dependent manner. In parallel studies using a differential centrifugation technique, we demonstrated that AVP induced translocation of AQP2 from an intracellular vesicle-enriched fraction to a plasma membrane-enriched fraction. PGE(2) (10(-7) M) added after AVP (10(-8) M) did not decrease AQP2 phosphorylation but reversed AVP-induced translocation of AQP2 to the plasma membrane. Preincubation of IM with PGE(2) did not prevent the effects of AVP on AQP2 phosphorylation and trafficking. PGE(2) alone did not influence AQP2 phosphorylation and subcellular distribution. Our data indicate that 1) recruitment of AQP2 to the plasma membrane and its retrieval to a pool of intracellular vesicles may be regulated independently, 2) PGE(2) may counteract AVP action by activation of AQP2 retrieval, 3) dephosphorylation of AQP2 is not a prerequisite for its internalization.

Chloroquine inhibits arginine vasopressin production in isolated rat inner medullary segments induced cAMP collecting duct

Previous studies showed that acute chloroquine administration increases plasma arginine vasopressin (AVP) concentration in the rat without influencing urine flow rate. The present study was designed to investigate whether chloroquine inhibits the AVP-induced cAMP production that mediates the antidiuretic effects of vasopressin. Single inner medullary collecting duct (IMCD) segments were pre-incubated at 35 degrees C for 10 min followed by 4 min at 37 degrees C with combinations of AVP and/or chloroquine with 1 mM 3-isobutyl-I-methylxanthine (IBMX) and cAMP concentrations were measured by radioimmunoassay. To establish the possible site of interference in cAMP production IMCD segments were incubated in the presence of chloroquine and forskolin. Chloroquine at concentrations ranging from 10(-9) M to 10(-6) M did not affect cAMP production by comparison with control. However, AVP (10(-8) M) and forskolin (10(-6) M) significantly (p < 0.01) increased cAMP accumulation. Chloroquine at all concentrations significantly suppressed the AVP stimulated cAMP production (e.g., chloroquine (10(-8) M) + AVP (10(-8) M) 41 +/- 12 fmol/4 mm (n = 9 tubules) vs. AVP (10(-8) M) alone 82 +/- 9 fmol/4 min/mm (n = 37 tubules). Chloroquine at all concentrations tested did not have any effect an forskolin-induced cAMP production. The data suggest that chloroquine inhibits the AVP induced cAMP production at the level of hormone/receptor complex. This possibly explains the previously reported lack of the normal antidiuretic responses of AVP in rats following chloroquine administration.

Localization and regulation of PKA-phosphorylated AQP2 in response to V(2)-receptor agonist/antagonist treatment

Phosphorylation of Ser(256), in a PKA consensus site, in AQP2 (p-AQP2) appears to be critically involved in the vasopressin-induced trafficking of AQP2. In the present study, affinity-purified antibodies that selectively recognize AQP2 phosphorylated at Ser(256) were developed. These antibodies were used to determine 1) the subcellular localization of p-AQP2 in rat kidney and 2) changes in distribution and/or levels of p-AQP2 in response to [desamino-Cys(1),D-Arg(8)]vasopressin (DDAVP) treatment or V(2)-receptor blockade. Immunoelectron microscopy revealed that p-AQP2 was localized in both the apical plasma membrane and in intracellular vesicles of collecting duct principal cells. Treatment of rats with V(2)-receptor antagonist for 30 min resulted in almost complete disappearance of p-AQP2 labeling of the apical plasma membrane with only marginal labeling of intracellular vesicles remaining. Immunoblotting confirmed a marked decrease in p-AQP2 levels. In control Brattleboro rats (BB), lacking vasopressin secretion, p-AQP2 labeling was almost exclusively present in intracellular vesicles. Treatment of BB rats with DDAVP for 2 h induced a 10-fold increase in p-AQP2 labeling of the apical plasma membrane. The overall abundance of p-AQP2, however, was not increased, as determined both by immunoelectron microscopy and immunoblotting. Consistent with this, 2 h of DDAVP treatment of normal rats also resulted in unchanged p-AQP2 levels. Thus the results demonstrate that AQP2 phosphorylated in Ser(256) is present in the apical plasma membrane and in intracellular vesicles and that both the intracellular distribution/trafficking, as well as the abundance of p-AQP2, are regulated via V(2) receptors by altering phosphorylation and/or dephosphorylation of Ser(256) in AQP2.

Topographic distribution of aquaporin 2 mRNA in the kidney of dehydrated rats

BACKGROUND

Stimulation of arginine vasopressin results in an immediate redistribution of water channels (aquaporin 2; AQP2) in the apical membrane of the collecting ducts, leading to water reabsorption. Water restriction for >/=24 h increases AQP2 proteins in the whole collecting duct which is highest in the inner medulla of the kidney, indicating that dehydration enhances synthesis of this protein. Although increased expression of AQP2 mRNA under this condition has been reported, the increased ratio of mRNA expression in the three regions of the kidney, cortex, outer medulla, and inner medulla, during the dehydration is still unclear.

METHODS

We investigated the AQP2 transcripts using male Sprague-Dawley rats deprived of water for 24 h. Mimic cDNA for competitive polymerase chain reaction (PCR) was constructed by deleting 180 bp at the middle of a 780-bp partial PCR product for rat AQP2 cDNA. In situ hybridization of the kidney and Northern blotting of inner medulla were performed using a 60-bp oligo-cDNA probe which identified rat AQP2 transcripts in the collecting duct.

RESULTS

Dehydration resulted in a significant increase in plasma osmolality and arginine vasopressin concentration and urinary osmolality. Competitive PCR demonstrated that dehydration increased AQP2 transcripts in all parts of the kidney, but was highest in the inner medulla. Northern blotting confirmed the high increased rate of AQP2 transcription in the inner medulla. In situ hybridization showed markedly intensified signals in the inner medulla of dehydrated rats.

CONCLUSIONS

Our data indicate that dehydration increases the abundance of AQP2 transcripts which may be closely associated with enhancement in AQP2 protein synthesis reported previously. This topographically variable increase in transcription is considered to be one of the mechanisms involved in long-term regulation of water permeability in the collecting duct.

Modulation of vasopressin-elicited water transport by trafficking of aquaporin2-containing vesicles

Vasopressin or AVP regulates water reabsorption by the kidney inner medullary collecting duct (IMCD) through the insertion and removal of aquaporin (AQP) 2 water channels into the IMCD apical membrane. AVP-elicited trafficking of AQP2 with the apical membrane occurs via a specialized population of vesicles that resemble synaptic vesicles in neurons. AQP2 vesicles and the IMCD apical membrane contain homologs of vesicle-targeting and signal transduction proteins found in neurons. Expression studies of AQP2, including human AQP2 mutants, suggest that the carboxyl-terminal domain of AQP2 is important in AQP2 trafficking, particularly as a site for cAMP-dependent protein kinase phosphorylation. These present data reveal that IMCD cells possess a complex integrated-signaling and vesicle-trafficking machinery that provides integration of AVP-elicited water transport with many other parameters within the IMCD cell as well as kidney.

ETB receptor activation leads to activation and phosphorylation of NHE3

In OKP cells expressing ETB endothelin receptors, activation of Na+/H+ antiporter activity by endothelin-1 (ET-1) was resistant to low concentrations of ethylisopropyl amiloride, indicating regulation of Na+/H+ exchanger isoform 3 (NHE3). ET-1 increased NHE3 phosphorylation in cells expressing ETB receptors but not in cells expressing ETA receptors. Receptor specificity was not due to demonstrable differences in receptor-specific activation of tyrosine phosphorylation pathways or inhibition of adenylyl cyclase. Phosphorylation was associated with a decrease in mobility on SDS-PAGE, which was reversed by treating immunoprecipitated NHE3 with alkaline phosphatase. Phosphorylation was first seen at 5 min and was maximal at 15-30 min. Phosphorylation was maximal with 10(-9) M ET-1. Phosphorylation occurred on threonine and serine residues at multiple sites. In summary, ET-1 induces NHE3 phosphorylation in OKP cells on multiple threonine and serine residues. ETB receptor specificity, time course, and concentration dependence are all similar between ET-1-induced increases in NHE3 activity and phosphorylation, suggesting that phosphorylation plays a key role in activation.

Physiology and pathophysiology of renal aquaporins

The discovery of aquaporin membrane water channels by Agre and coworkers answered a long-standing biophysical question of how water specifically crosses biologic membranes, and provided insight, at the molecular level, into the fundamental physiology of water balance and the pathophysiology of water balance disorders. Of nine aquaporin isoforms, at least six are known to be present in the kidney at distinct sites along the nephron and collecting duct. Aquaporin-1 (AQP1) is extremely abundant in the proximal tubule and descending thin limb, where it appears to provide the chief route for proximal nephron water reabsorption. AQP2 is abundant in the collecting duct principal cells and is the chief target for vasopressin to regulate collecting duct water reabsorption. Acute regulation involves vasopressin-regulated trafficking of AQP2 between an intracellular reservoir and the apical plasma membrane. In addition, AQP2 is involved in chronic/adaptational regulation of body water balance achieved through regulation of AQP2 expression. Importantly, multiple studies have now identified a critical role of AQP2 in several inherited and acquired water balance disorders. This concerns inherited forms of nephrogenic diabetes insipidus and several, much more common acquired types of nephrogenic diabetes insipidus where AQP2 expression and/or targeting are affected. Conversely, AQP2 expression and targeting appear to be increased in some conditions with water retention such as pregnancy and congestive heart failure. AQP3 and AQP4 are basolateral water channels located in the kidney collecting duct, and AQP6 and AQP7 appear to be expressed at lower abundance at several sites including the proximal tubule. This review focuses mainly on the role of AQP2 in water balance regulation and in the pathophysiology of water balance disorders.

Protein kinase A anchoring proteins are required for vasopressin-mediated translocation of aquaporin-2 into cell membranes of renal principal cells

The antidiuretic hormone arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells by inducing a cAMP-dependent translocation of water channels (aquaporin-2, AQP-2) from intracellular vesicles into the apical cell membranes. In subcellular fractions from primary cultured rat inner medullary collecting duct (IMCD) cells, enriched for intracellular AQP-2-bearing vesicles, catalytic protein kinase A (PKA) subunits and several protein kinase A anchoring proteins (AKAPs) were detected. In nonstimulated IMCD cells the majority of AQP-2 staining was detected intracellularly but became mainly localized within the cell membrane after stimulation with AVP or forskolin. Quantitative analysis revealed that preincubation of the cells with the synthetic peptide S-Ht31, which prevents the binding between AKAPs and regulatory subunits of PKA, strongly inhibited AQP-2 translocation in response to forskolin. Preincubation of the cells with the PKA inhibitor H89 prior to forskolin stimulation abolished AQP-2 translocation. In contrast to H89, S-Ht31 did not affect the catalytic activity of PKA. These data demonstrate that not only the activity of PKA, but also its tethering to subcellular compartments, are prerequisites for cAMP-dependent AQP-2 translocation.

Arginine vasopressin stimulates phosphorylation of aquaporin-2 in rat renal tissue

Aquaporin-2 (AQP2), the protein that mediates arginine vasopressin (AVP)-regulated apical water transport in the renal collecting duct, possesses a single consensus phosphorylation site for cAMP-dependent protein kinase A (PKA) at Ser256. The aim of this study was to examine whether AVP, and other agents that increase cAMP levels, could stimulate the phosphorylation of AQP2 in intact rat renal tissue. Rat renal papillae were prelabeled with 32P and incubated with vehicle or drugs, and then AQP2 was immunoprecipitated. Two polypeptides corresponding to nonglycosylated (29 kDa) and glycosylated (35-48 kDa) AQP2 were identified by SDS-PAGE. AVP caused a time- and dose-dependent increase in phosphorylation of both glycosylated and nonglycosylated AQP2. The threshold dose for a significant increase in phosphorylation was 10 pM, which corresponds to a physiological serum concentration of AVP. Maximal phosphorylation was reached within 1 min of AVP incubation. This effect on AQP2 phosphorylation was mimicked by the vasopressin (V2) agonist, 1-desamino-[8-D-arginine]vasopressin (DDAVP), or forskolin. Two-dimensional phosphopeptide mapping indicated that AVP and forskolin stimulated the phosphorylation of the same site in AQP2. Immunoblot analysis using a phosphorylation state-specific antiserum revealed an increase in phosphorylation of Ser256 after incubation of papillae with AVP. The results indicate that AVP stimulates phosphorylation of AQP2 at Ser256 via activation of PKA, supporting the idea that this is one of the first steps leading to increased water permeability in collecting duct cells.

Low aquaporin-2 levels in polyuric DI +/+ severe mice with constitutively high cAMP-phosphodiesterase activity

In the renal collecting duct, vasopressin acutely activates cAMP production, resulting in trafficking of aquaporin-2 water channels (AQP2) to the apical plasma membrane, thereby increasing water permeability. This acute response is modulated by long-term changes in AQP2 expression. Recently, a cAMP-responsive element has been identified in the AQP2 gene, raising the possibility that changes in cAMP levels may control AQP2 expression. To investigate this possibility, we determined AQP2 protein levels in a strain of mice, DI +/+ severe (DI), which have genetically high levels of cAMP-phosphodiesterase activity, and hence low cellular cAMP levels, and severe polyuria. Semiquantitative immunoblotting of membrane fractions prepared from whole kidneys revealed that AQP2 levels in DI mice were only 26 +/- 7% (+/-SE) of those in control mice (n = 10, P < 0.01). In addition, semiquantitative Northern blotting revealed a significantly lower AQP2 mRNA expression in kidneys from DI mice compared with control mice (43 +/- 6% vs. 100 +/- 10%; n = 6 in each group, P < 0.05). AQP3 levels were also reduced. The mice were polyuric and urine osmolalities were accordingly substantially lower in the DI mice than in controls (496 +/- 53 vs. 1,696 +/- 105 mosmol/kgH2O, respectively). Moreover, there was a linear correlation between urine osmolalities and AQP2 levels (P < 0.05). Immunoelectron microscopy confirmed the markedly lower expression of AQP2 in collecting duct principal cells in kidneys of DI mice and, furthermore, demonstrated that AQP2 was almost completely absent from the apical plasma membrane. Thus expression of AQP2 and AQP2 trafficking were severely impaired in DI mice. These results are consistent with the view that in vivo regulation of AQP2 expression by vasopressin is mediated by cAMP.

A water channel of the nematode C. elegans and its implications for channel selectivity of MIP proteins

A genome project focusing on the nematode Caenorhabditis elegans has demonstrated the presence of eight cDNAs belonging to the major intrinsic protein superfamily. We functionally characterized one of these cDNAs named C01G6.1. Injection of C01G6.1 cRNA increased the osmotic water permeability (Pf) of Xenopus oocytes 11-fold and the urea permeability 4.5-fold but failed to increase the glycerol permeability. It has been speculated that the MIP family may be separated into two large subfamilies based on the presence or absence of two segments of extra amino acid residues ( approximately 15 amino acids) at the second and third extracellular loops. Because C01G6.1 (designated AQP-CE1), AQP3, and glycerol facilitator (GlpF) all have these two segments, we replaced the segments of AQP-CE1 with those of AQP3 and GlpF to identify their roles. The functional characteristics of these mutants were principally similar to that of wild-type AQP-CE1, although the values of Pf and urea permeability were decreased by 39-74% and 28-65%, respectively. These results suggest that the two segments of extra amino acid residues may not contribute to channel selectivity or formation of the route for small solutes.

Novel mutations in aquaporin-2 gene in female siblings with nephrogenic diabetes insipidus: evidence of disrupted water channel function

Novel mutations of the aquaporin-2 (AQP2) gene have been detected in Japanese female siblings with autosomal-recessive nephrogenic diabetes insipidus. The patients were compound heterozygote for point mutations at nucleotide position 374 (C374T) and at position 523 (G523A) in exon 2 of the AQP2 gene, resulting in substitution of methionine for threonine at codon 125 (T125M) and arginine for glycine at codon 175 (G175R). The water permeability (Pf) of oocytes injected with wild-type complementary RNA increased 9.0-fold compared with the Pf of water-injected oocytes, whereas the increases in the Pf of oocytes injected with T125M and G175R complementary RNA were only 1.7-fold and 1.5-fold, respectively. Immunoblot and immunocytochemistry indicated that the plasma membrane expressions of T125M and G175R AQP2 proteins were comparable to that of the wild-type, suggesting that although neither the T125M nor G175R mutation had a significant effect on plasma membrane expression, they both distorted the structure and function of the aqueous pore of AQP2. These results provide evidence that the nephrogenic diabetes insipidus in patients with T125M and G175R mutations is attributable not to the misrouting of AQP2, but to the disrupted water channel function.

Cellular mechanisms of aquaporin trafficking

Aquaporins (AQPs) are a family of functionally important water channel proteins that are of special cell biological interest because of their diverse intracellular targeting and trafficking properties. AQPs have been found in many different cells and tissues. This short review summarizes recent work that addresses the regulation of AQP2 trafficking in response to vasopressin.

Membrane trafficking of the cystic fibrosis gene product, cystic fibrosis transmembrane conductance regulator, tagged with green fluorescent protein in madin-darby canine kidney cells

The mechanism by which cAMP stimulates cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride (Cl-) secretion is cell type-specific. By using Madin-Darby canine kidney (MDCK) type I epithelial cells as a model, we tested the hypothesis that cAMP stimulates Cl- secretion by stimulating CFTR Cl- channel trafficking from an intracellular pool to the apical plasma membrane. To this end, we generated a green fluorescent protein (GFP)-CFTR expression vector in which GFP was linked to the N terminus of CFTR. GFP did not alter CFTR function in whole cell patch-clamp or planar lipid bilayer experiments. In stably transfected MDCK type I cells, GFP-CFTR localization was substratum-dependent. In cells grown on glass coverslips, GFP-CFTR was polarized to the basolateral membrane, whereas in cells grown on permeable supports, GFP-CFTR was polarized to the apical membrane. Quantitative confocal fluorescence microscopy and surface biotinylation experiments demonstrated that cAMP did not stimulate detectable GFP-CFTR translocation from an intracellular pool to the apical membrane or regulate GFP-CFTR endocytosis. Disruption of the microtubular cytoskeleton with colchicine did not affect cAMP-stimulated Cl- secretion or GFP-CFTR expression in the apical membrane. We conclude that cAMP stimulates CFTR-mediated Cl- secretion in MDCK type I cells by activating channels resident in the apical plasma membrane.

Acute effects of vasopressin V2-receptor antagonist on kidney AQP2 expression and subcellular distribution

The acute effect of treatment with the vasopressin V2-receptor antagonist OPC-31260 (OPC) on aquaporin-2 (AQP2) distribution and expression in rat kidney was examined. Immunofluorescence and semi-quantitative immunoelectron microscopy revealed that 15 and 30 min of OPC treatment resulted in significant reduction in apical plasma membrane labeling of AQP2, with a concomitant increase in labeling of vesicles and multivesicular bodies. In parallel, OPC treatment induced a large increase in urine output [0.6 +/- 0.2 vs. 8.3 +/- 1.0 ml/h (n = 4)]. Northern blotting using a 32P-labeled AQP2 cDNA probe and a digoxigenin-labeled AQP2 RNA probe revealed a band of approximately 1.6 kb corresponding to the predicted size of AQP2 mRNA. In control experiments, thirsting increased, whereas water loading decreased AQP2 mRNA levels. Treatment of rats with OPC caused a significant reduction in AQP2 mRNA within 30 min (52 +/- 21%, n = 8, P < 0.025) and 60 min (56 +/- 7%, n = 4, P < 0.001) of treatment compared with intravenous saline-injected controls. Thus a very rapid reduction in AQP2 mRNA was observed in response to vasopressin-receptor antagonist treatment. The reduction in AQP2 mRNA persisted after 24 h (40 +/- 17%, n = 5, P < 0.05) of OPC treatment. There was a parallel increase in diuresis and reduction in urine osmolality. In conclusion, V2-receptor blockade produced a rapid internalization of AQP2 parallel with a rapid increase in urine output. Furthermore, OPC treatment caused a rapid and significant reduction in AQP2 mRNA expression, demonstrating that for rapid regulation of AQP2 expression, modulation of AQP2 mRNA levels is regulated via vasopressin-receptor signaling pathways.

Evidence against a role of mouse, rat, and two cloned human t1alpha isoforms as a water channel or a regulator of aquaporin-type water channels

T1alpha is a protein of unknown function that is expressed at the plasma membrane in epithelia involved in fluid transport, including type I alveolar epithelial cells, choroid plexus, and ciliary epithelium. The purpose of this study was to test the hypothesis that T1alpha functions as a water channel or a regulator of aquaporin-type water channels that are coexpressed with T1alpha. Two complementary DNAs (cDNAs) (hT1alpha-1 and hT1alpha-2) encoding human isoforms of T1alpha were cloned by homology to the rat T1alpha coding sequence. The cDNAs encoded 164 (hT1alpha-1) and 162 (hT1alpha-2) amino acid proteins with high homology to rat T1alpha in a putative membrane-spanning domain. hT1alpha-1 transcripts of 2. 6 and 1.4 kb were detected in human lung, heart, and skeletal muscle, and a single hT1alpha-2 transcript of 1.2 kb was detected in human lung. Rat and mouse T1alpha were isolated by reverse transcription-polymerase chain reaction and confirmed by DNA sequence analysis. Expression of mouse, rat, and human T1alpha isoforms in Xenopus oocytes did not increase osmotic water permeability (Pf) above that in water-injected oocytes, nor was there an effect of protein kinase A or C activation; Pf was increased > 10-fold in positive control oocytes expressing aquaporin (AQP)1 or AQP5. Coexpression of AQP1 or AQP5 with excess T1alpha gave Pf not different from that in oocytes expressing AQP1 or AQP5 alone. Oocyte plasma membrane localization of epitope-tagged T1alpha protein was confirmed and quantified by immunoprecipitation of microdissected plasma membranes. Quantitative densitometry indicated that the single-channel water permeability of T1alpha is under 2 x 10(-16) cm3/s, suggesting that T1alpha is not involved in the high transalveolar water permeability in intact lung. The cloning of hT1alpha isoforms may permit the development of an assay of type I cell antigen in airspace fluid as a marker of human lung injury.

An aquaporin-2 water channel mutant which causes autosomal dominant nephrogenic diabetes insipidus is retained in the Golgi complex

Mutations in the aquaporin-2 (AQP2) water channel gene cause autosomal recessive nephrogenic diabetes insipidus (NDI). Here we report the first patient with an autosomal dominant form of NDI, which is caused by a G866A transition in the AQP2 gene of one allele, resulting in a E258K substitution in the C-tail of AQP2. To define the molecular cause of NDI in this patient, AQP2-E258K was studied in Xenopus oocytes. In contrast to wild-type AQP2, AQP2-E258K conferred a small increase in water permeability, caused by a reduced expression at the plasma membrane. Coexpression of wild-type AQP2 with AQP2-E258K, but not with an AQP2 mutant in recessive NDI (AQP2-R187C), revealed a dominant-negative effect on the water permeability conferred by wild-type AQP2. The physiologically important phosphorylation of S256 by protein kinase A was not affected by the E258K mutation. Immunoblot and microscopic analyses revealed that AQP2-E258K was, in contrast to AQP2 mutants in recessive NDI, not retarded in the endoplasmic reticulum, but retained in the Golgi compartment. Since AQPs are thought to tetramerize, the retention of AQP2-E258K together with wild-type AQP2 in mixed tetramers in the Golgi compartment is a likely explanation for the dominant inheritance of NDI in this patient.

The Dichotomy of MIP Family Suggests Two Separate Origins of Water Channels

MIP family proteins can be divided into two groups according to their primary sequences. The CHIP group is predominant in the plant and animal kingdoms and functions primarily as water channels. The GLP group is a minor group with limited prevalence and functions primarily as glycerol transporters. Both prototypes are present in bacteria and may have evolved separately.

Aquaporin-2 and -3: representatives of two subgroups of the aquaporin family colocalized in the kidney collecting duct

Since the molecular identification of the first aquaporin in 1992, the number of proteins known to belong to this family has been rapidly increasing. These members may be separated into two subgroups based on gene structure, sequence homology, and function. Regulation of the water permeability of the collecting ducts of the kidney is essential for urinary concentration. Aquaporin-2 and -3, which are representative of these subgroups, are colocalized in the collecting ducts. Understanding these subgroups will elucidate the differences between aquaporin-2 and -3. Aquaporin-2 is a vasopressin-regulated water channel located in the apical membrane, and aquaporin-3 is a constitutive water channel located in the basolateral membrane. In contrast to aquaporin-3, which appears to be less well regulated, many studies have now identified multiple regulational mechanisms at the gene, protein, and cell levels for aquaporin-2, thus reflecting its physiological importance. Evidence of the participation of aquaporin-2 in the pathophysiology of water-balance disorders is accumulating.

Regulation of aquaporin-4 water channels by phorbol ester-dependent protein phosphorylation

The molecular mechanisms for regulating water balance in many tissues are unknown. Like the kidney, the eye contains multiple water channel proteins (aquaporins) that transport water through membranes, including two (AQP1 and AQP4) in the ciliary body, the site of aqueous humor production. However, because humans with defective AQP1 are phenotypically normal and because the ocular application of phorbol esters reduce intraocular pressure, we postulated that the water channel activity of AQP4 may be regulated by these agents. We now report that protein kinase C activators, phorbol 12,13-dibutyrate, and phorbol 12-myristate 13-acetate strongly stimulate the phosphorylation of AQP4 and inhibit its activity in a dose-dependent manner. Phorbol 12,13-dibutyrate (10 microM) and phorbol 12-myristate 13-acetate (10 nM) reduced the rate of AQP4-expressing oocyte swelling by 87 and 92%, respectively. Further, phorbol 12,13-dibutyrate significantly increased the amount of phosphorylated AQP4. These results demonstrate that protein kinase C can regulate the activity of AQP4 through a mechanism involving protein phosphorylation. Moreover, they suggest important potential roles for AQP4 in several clinical disorders involving rapid water transport such as glaucoma, brain edema, and swelling of premature infant lungs.

Upregulation of aquaporin 2 water channel expression in pregnant rats

Water retention is characteristic of pregnancy but the mechanism(s) of the altered water metabolism has yet to be elucidated. The collecting duct water channel, aquaporin 2 (AQP2), plays a pivotal role in the renal water regulation, and we hypothesized that AQP2 expression could be modified during pregnancy. Sprague-Dawley female rats were studied on days 7 (P7), 14 (P14), and 20 (P20) of pregnancy, and expression of AQP2 in papillae was examined. Nonpregnant (NP) littermates were used as controls. Plasma osmolalities were significantly lower in pregnant rats by day 7 of gestation (P7 283.8+/-1.82, P14 284.3+/-1.64, P < 0.001, P20 282. 4+/-1.32, P < 0.0001, vs. NP 291.8+/-1.06 mosmol/kgH2O). However, plasma vasopressin concentrations in pregnant rats were not significantly different than in nonpregnant rats (NP 1.03+/-0.14, P7 1.11+/-0.21, P14 1.15+/-0.21, P20 1.36+/-0.24 pg/ml, NS). The mRNA of AQP2 was increased early during pregnancy: AQP2/beta actin: P7 196+/-17.9, P14 200+/-6.8, and P20 208+/-15.5%, P < 0.005 vs. NP (100+/-11.1%). AQP2 protein was also increased during pregnancy: AQP2 protein: P7 269+/-10.0, P14 251+/-12.0, P < 0.0001, and P20 250+/-13.6%, P < 0.001 vs. NP (100+/-12.5%). The effect of V2 vasopressin receptor antagonist, OPC-31260, was then investigated. AQP2 mRNA was suppressed significantly by OPC-31260 administration to P14 rats (AQP2/beta actin: P14 with OPC-31260 39.6+/-1.7%, P < 0.001 vs. P14 with vehicle) and was decreased to the same level of expression as NP rats receiving OPC-31260. Similar findings were found with the analysis of AQP2 protein. The decreased plasma osmolality of P14 rats was not modified by OPC-31260. The results of the study indicate that upregulation of AQP2 contributes to the water retention in pregnancy through a V2 receptor-mediated effect. In addition to vasopressin, other factors may be involved in this upregulation.

Physiology and pathophysiology of the aquaporin-2 water channel

Aquaporins are integral membrane proteins, which function as specialized water channels to facilitate the passage of water through the cell membrane. In mammals six different aquaporins have been identified up to now, four of which (aquaporin-1 to aquaporin-4) are expressed in the kidney. Because of its importance for normal water homeostasis and its involvement in many water balance disorders, aquaporin-2, the predominant vasopressin-regulated water channel of the renal collecting duct, is discussed in detail.

Mercury-sensitive residues and pore site in AQP3 water channel

Water channel function of all aquaporins (AQPs) but AQP4 can be inhibited by mercurial reagents. Mercurial reagents are believed to bind specifically to cysteine residues and block the aqueous pore of AQPs. Because of the low homology of AQP3 to other AQPs, it is not certain whether the pore structure of AQP3 is similar to that of the others. Determination of mercury-sensitive cysteine residues in AQP3 and comparison with those in other AQPs will help to resolve this question. When AQP3 was expressed in Xenopus oocytes, incubation with 0.3 mM HgCl2 decreased its osmotic water permability (Pf) by approximately 30%. To identify the mercury-sensitive site, six individual cysteine residues in human AQP3 (at positions 11, 29, 40, 91, 174, and 267) were altered by site-directed mutagenesis. Mutants of C11S and C11A had a similar basal Pf to wild-type but acquired mercury resistance. Replacement of Cys-11 with Trp, which possesses a large side chain, did not change Pf. Mercurial inhibition of Pf was still observed in five other Cys-to-Ser mutants. These results suggest that Cys-11 is the mercury-sensitive residue in AQP3 and that this residue might be independent of water channel function. Mutation of Tyr-212, a position corresponding to the mercury-sensitive residues in AQP1 and AQP2, to cysteine enhanced the mercurial inhibition of Pf. Y212W had no water channel activity. Expression of AQP3 increased glycerol permeability (Pgly) 3.1-fold, whereas Pgly of Y212W-expressing oocytes was similar to Pgly of control oocytes. Cysteine mutation at Tyr-212 increased the inhibitory effect of mercury on Pgly. These results suggest that the structure of the aqueous pore of AQP3 resembles those of AQP1 and AQP2 and support the hypothesis that water and small molecules share a common pore in AQP3.

Regulation of water channel activity of aquaporin 1 by arginine vasopressin and atrial natriuretic peptide

Aquaporin 1 (AQP1), a six-transmembrane domain protein that functions as a water channel, is present in many fluid secreting and absorbing tissues such as kidney, brain, heart, and eye. It is believed that among the five known mammalian aquaporins, kidney aquaporin (AQP2) is the only water channel that is regulated by arginine vasopressin (AVP). The present data suggest that AQP1 may also be regulated by AVP. The application of AVP to Xenopus oocytes injected with AQP1 cRNA increased the membrane permeability to water. In addition, our data reveal that atrial natriuretic peptide (ANP), a peptide hormone that plays an important role in the regulation of body fluid homeostasis, blocks the AQP1-mediated increase in water permeability. Incubation with 8-bromo-cAMP or direct 8-bromo-cAMP injection into oocytes expressing AQP1 cRNA significantly increased membrane permeability to water, suggesting that stimulation of AQP1 activity by AVP may involve a cAMP-dependent mechanism. Regulation of water permeability by AVP and ANP has potential relevance to active water transport in a variety of tissues that express AQP1 including kidney, brain, and eye.

Regulation of aquaporin-2 water channel trafficking by vasopressin

Vasopressin regulates water excretion from the kidney by increasing the osmotic water permeability of the renal collecting duct. The aquaporin-2 water channel has been demonstrated to be the target for this action of vasopressin. Recent studies have demonstrated that vasopressin, acting through cyclic AMP, triggers fusion of aquaporin-2-bearing vesicles with the apical plasma membrane of the collecting duct principal cells. The vesicle-targeting proteins synaptobrevin-2 and syntaxin-4 are proposed to play roles in this process.

Water and glycerol permeabilities of aquaporins 1-5 and MIP determined quantitatively by expression of epitope-tagged constructs in Xenopus oocytes

The goal of this study was to compare single channel water and glycerol permeabilities of mammalian aquaporins (AQP) 1-5 and the major intrinsic protein of lens fiber (MIP). Each of the six cloned cDNAs from rat was left untagged or was epitope-tagged with c-Myc or FLAG at either the N or C terminus so that results would not depend on epitope identity or location. The constructs were expressed in Xenopus oocytes for measurement of osmotic water permeability (Pf), [3H]glycerol uptake, and protein expression. Each of the 30 epitope-tagged constructs was expressed strongly at the oocyte plasma membrane. The 10-min uptake of [3H]glycerol was increased significantly (range of 4.5-8-fold over control) in oocytes expressing untagged AQP3 (GLIP) and each of the four tagged AQP3 constructs; [3H]glycerol uptake was not increased in oocytes expressing AQP1, AQP2, AQP4, AQP5, or MIP. In oocytes microinjected with 5 ng of cRNA, average Pf values (in cm/s x 10(-3)) were 0.67 +/- 0.06 (control), 19 +/- 2 (AQP1), 10 +/- 1 (AQP2), 8 +/- 2 (AQP3), 29 +/- 1 (AQP4), 10 +/- 1 (AQP5), and 1.3 +/- 0.2 (MIP), and they were relatively insensitive to the presence, identity, or location of the epitope tag. Pf values were not affected by protein kinase A or C activation. After normalization for plasma membrane expression by immunoprecipitation of microdissected plasma membranes, single channel water permeabilities (pf, referenced to the AQP1 pf of 6 x 10(-14) cm3/s) were (in cm3/s x 10(-14)) 3.3 +/- 0.2 (AQP2), 2.1 +/- 0.3 (AQP3), 24 +/- 0.6 (AQP4), 5.0 +/- 0.4 (AQP5), and 0.25 +/- 0.05 (MIP); pf values were insensitive to epitope identity and location. These results indicate very different intrinsic water permeabilities for the mammalian aquaporin homologs, with the pf value for AQP4 remarkably higher than those for the others. The pf values establish limits on aquaporin tissue densities required for physiological function and suggest significant structural and functional differences among the aquaporins.

Purified vesicles of tobacco cell vacuolar and plasma membranes exhibit dramatically different water permeability and water channel activity

The vacuolar membrane or tonoplast (TP) and the plasma membrane (PM) of tobacco suspension cells were purified by free-flow electrophoresis (FFE) and aqueous two-phase partitioning, with enrichment factors from a crude microsomal fraction of >/=4- to 5-fold and reduced contamination by other cellular membranes. For each purified fraction, the mean apparent diameter of membrane vesicles was determined by freeze-fracture electron microscopy, and the osmotic shrinking kinetics of the vesicles were characterized by stopped-flow light scattering. Osmotic water permeability coefficients (Pf) of 6.1 +/- 0.2 and 7.6 +/- 0.9 microm . s(-1) were deduced for PM-enriched vesicles purified by FFE and phase partitioning, respectively. The associated activation energies (Ea; 13.7 +/- 1.0 and 13.4 +/- 1.4 kcal . mol(-1), respectively) suggest that water transport in the purified PM occurs mostly by diffusion across the lipid matrix. In contrast, water transport in TP vesicles purified by FFE was characterized by (i) a 100-fold higher Pf of 690 +/- 35 microm . s(-1), (ii) a reduced Ea of 2.5 +/- 1.3 kcal . mol(-1), and (iii) a reversible inhibition by mercuric chloride, up to 83% at 1 mM. These results provide functional evidence for channel-mediated water transport in the TP, and more generally in a higher plant membrane. A high TP Pf suggests a role for the vacuole in buffering osmotic fluctuations occurring in the cytoplasm. Thus, the differential water permeabilities and water channel activities observed in the tobacco TP and PM point to an original osmoregulatory function for water channels in relation to the typical compartmentation of plant cells.

Expression of an Arabidopsis plasma membrane aquaporin in Dictyostelium results in hypoosmotic sensitivity and developmental abnormalities

The rd28 gene of Arabidopsis thaliana encodes a water channel protein, or aquaporin, of the plasma membrane. A construct in which transcription of the rd28 cDNA is controlled by the Dictyostelium actin15 promoter was transformed into Dictyostelium discoideum cells. Transformants contained RD28 protein in their plasma membranes. When shifted to a low-osmotic-strength buffer, cells expressing rd28 swelled rapidly and burst, indicating that the plant aquaporin allowed rapid water entry in the amoebae. The rate of osmotic lysis was a function of the osmotic pressure of the buffer. We also selected transformants in which the expression of the rd28 cDNA is driven by the promoter of the prespore cotB gene. These transformants accumulated rd28 mRNA uniquely in prespore cells. In low-osmotic-strength buffer, the cotB::rd28 cells aggregated and formed normally proportioned slugs but failed to form normal fruiting bodies. The number of spores was reduced 20-fold, and the stalks of the fruiting bodies were abnormally short. The consequences of expressing RD28 in prespore cells could be partially overcome by increasing the osmolarity of the medium. Under these conditions, the cotB::rd28 cells formed fruiting bodies of more normal appearance, and the number of viable spores increased slightly. Because prespore cells have to shrink and dehydrate to form spores, it was not unexpected that expression of an aquaporin would disrupt this process, but it was surprising to find that stalk differentiation was also affected by expression of rd28 in prespore cells. It appears that osmotic stress on prespore cells alters their ability to signal terminal differentiation in prestalk cells. The results provide independent confirmation that plant aquaporins can function in the cells of other organisms, and that D. discoideum can be used to study the properties of these water channels.

Phosphorylation of serine 256 is required for cAMP-dependent regulatory exocytosis of the aquaporin-2 water channel

The aquaporin-2 (AQP2) vasopressin water channel is translocated to the apical membrane upon vasopressin stimulation. Phosphorylation of serine 256 of AQP2 by cAMP-dependent protein kinase has been shown, but its relation to vasopressin-regulated translocation has not been elucidated. To address this question, wild type (WT) AQP2 and a mutant with alanine in place of serine 256 of AQP2 (S256A) were expressed in LLC-PK1 cells by electroporation. Measurements by a stopped-flow light-scattering method revealed that the osmotic water permeability (Pf) of LLC-PK1 cells transfected with WT was 69.6 +/- 6.5 microm/s (24.8 +/- 2.2 microm/s for mock-transfected), and stimulation by 500 microM 8-(4-chlorophenylthio)-cAMP increased the Pf by 85 +/- 12%. When S256A AQP2 was transfected, the cAMP-dependent increase in the Pf was only 8 +/- 5%. After cAMP stimulation, the increase in surface expression of AQP2 determined by surface biotin labeling was 4 +/- 10%, significantly less than that for WT (88 +/- 5%). In addition, an in vivo [32P]orthophosphate labeling assay demonstrated significant phosphorylation of WT AQP2 and only minimal phosphorylation of S256A AQP2 in LLC-PK1 cells. Our results indicated that serine 256 of AQP2 is necessary for regulatory exocytosis and that cAMP-responsive redistribution of AQP2 may be regulated by phosphorylation of AQP2.

AQUAPORINS AND WATER PERMEABILITY OF PLANT MEMBRANES

The mechanisms of plant membrane water permeability have remained elusive until the recent discovery in both vacuolar and plasma membranes of a class of water channel proteins named aquaporins. Similar to their animal counterparts, plant aquaporins have six membrane-spanning domains and belong to the MIP superfamily of transmembrane channel proteins. Their very high efficiency and selectivity in transporting water molecules have been mostly characterized using heterologous expression in Xenopus oocytes. However, techniques set up to measure the osmotic water permeability of plant membranes such as transcellular osmosis, pressure probe measurements, or stopped-flow spectrophotometry are now being used to analyze the function of plant aquaporins in their native membranes. Multiple mechanisms, at the transcriptional and posttranslational levels, control the expression and activity of the numerous aquaporin isoforms found in plants. These studies suggest a general role for aquaporins in regulating transmembrane water transport during the growth, development, and stress responses of plants. Future research will investigate the integrated function of aquaporins in long-distance water transport and cellular osmoregulation.

The aquaporin family of water channel proteins in clinical medicine

The aquaporins are a family of membrane channel proteins that serve as selective pores through which water crosses the plasma membranes of many human tissues and cell types. The sites where aquaporins are expressed implicate these proteins in renal water reabsorption, cerebrospinal fluid secretion and reabsorption, generation of pulmonary secretions, aqueous humor secretion and reabsorption, lacrimation, and multiple other physiologic processes. Determination of the aquaporin gene sequences and their chromosomal locations has provided insight into the structure and pathophysiologic roles of these proteins, and primary and secondary involvement of aquaporins is becoming apparent in diverse clinical disorders. Aquaporin-1 (AQP1) is expressed in multiple tissues including red blood cells, and the Colton blood group antigens represent a polymorphism on the AQP1 protein. AQP2 is restricted to renal collecting ducts and has been linked to congenital nephrogenic diabetes insipidus in humans and to lithium-induced nephrogenic diabetes insipidus and fluid retention from congestive heart failure in rat models. Congenital cataracts result from mutations in the mouse gene encoding the lens homolog Aqp0 (Mip). The present understanding of aquaporin physiology is still incomplete; identification of additional members of the aquaporin family will affect future studies of multiple disorders of water distribution throughout the body. In some tissues, the aquaporins may participate in the transepithelial movement of fluid without being rate limiting, so aquaporins may be involved in clinical disorders without being causative. As outlined in this review, our challenge is to identify disease states in which aquaporins are involved, to define the aquaporins' roles mechanistically, and to search for ways to exploit this information therapeutically.

Upregulation of aquaporin-2 water channel expression in chronic heart failure rat

Aquaporin-2 (AQP2) mediates vasopressin-regulated collecting duct water permeability. Chronic heart failure (CHF) is characterized by abnormal renal water retention. We hypothetized that upregulation of aquaporin-2 water channel could account for the water retention in CHF. Male rats underwent either a left coronary artery ligation, a model of CHF, or were sham operated. 31-33 d after surgery, mean arterial pressure (MAP) and cardiac output were measured in conscious animals, and the animals were killed 24 h later. Cardiac output (CO) and plasma osmolality were significantly decreased and plasma vasopressin increased in the CHF as compared to the sham-operated rats. Both mRNA and protein AQP2 were significantly increased in the kidneys of the CHF rats. The effect of oral administration of a nonpeptide V2 vasopressin receptor antagonist, OPC 31260, was therefore investigated. OPC 31260 induced a significant increase in diuresis, decrease in urinary osmolality, and rise in plasma osmolality in the OPC 31260-treated CHF rats as compared to untreated CHF rats. The mRNA and protein AQP2 were significantly diminished in both cortex and inner medulla of the treated CHF rats. In conclusion, an early upregulation of AQP2 is present in CHF rats and this upregulation is inhibited by the administration of a V2 receptor antagonist. The results indicate a major role for vasopressin in the upregulation of AQP2 water channels and water retention in experimental CHF in the rat.

The 17 kDa band identified by multiple anti-aquaporin 2 antisera in rat kidney medulla is a histone

The osmotic water permeability of epithelial cells of the inner medullary collecting duct of the kidney is regulated by antidiuretic hormone (ADH). ADH causes the insertion and removal of cytoplasmic vesicles containing the aquaporin (AQP-2) water channel protein which is recognized by multiple rabbit antipeptide antisera raised against amino acid sequences comprising its cytoplasmic carboxyl terminal. Immunoblots of rat kidney membrane fractions as well as human urine have all shown that AQP-2 is expressed exclusively by collecting duct cells and have identified a 29 kDa band (corresponding to the nonglycosylated AQP-2 protein), a broad 35-45 kDa band (corresponding to the mature glycosylated form of AQP-2 protein) and an additional immunoreactive 17 kDa band of unknown origin. We now report that the 17 kDa band identified by these anti-AQP-2 antisera is not an AQP-2 component but rather a denatured histone protein type H2A1. This binding of anti-AQP-2 antisera to denatured H2A1 present in protein samples derived from both kidney inner medulla and human urine is blocked specifically by preincubation of immunoblots with solutions containing the acidic protein gelatin.

CFTR-dependent membrane insertion is linked to stimulation of the CFTR chloride conductance

We examined the relation between Cl current (Icl) stimulation and cell membrane capacitance (Cm) when cystic fibrosis transmembrane conductance regulator (CFTR) was expressed in Xenopus oocytes. ICl and Cm increased in parallel when oocytes expressing CFTR were stimulated by forskolin (10 microM) and 3-isobutyl-1-methylxanthine (1 mM). The adenosine 3',5'-cyclic monophosphate (cAMP)-induced increase in surface area detected by Cm was confirmed by morphometry in the same oocytes used for the electrical recordings. These increases in ICl and Cm were reversible and were absent from control oocytes not injected with CFTR cRNA. The time to reach peak ICl lagged slightly behind the peak in Cm. ICl was varied by altering CFTR expression level or agonist dose or by expressing different CFTR mutants. In all cases, there was a close correlation between ICl and Cm, and the kinetics of ICl and Cm stimulation were more rapid the larger the magnitude of the stimulated current. The Cm-ICl relation for wild-type CFTR saturated, consistent with a limited capacity of cells to increase their surface area. These results indicate that stimulation of the CFTR ICl is linked closely to increases in membrane area. This suggests that CFTR is present in the membrane vesicles whose insertion is stimulated by cAMP. The contents of these vesicles may provide a link between activation of CFTR and its cAMP-dependent regulation of other channels.

Forskolin stimulation of water and cation permeability in aquaporin 1 water channels

Aquaporin 1, a six-transmembrane domain protein, is a water channel present in many fluid-secreting and -absorbing cells. In Xenopus oocytes injected with aquaporin 1 complementary RNA, the application of forskolin or cyclic 8-bromo- adenosine 3',5'-monophosphate increased membrane permeability to water and triggered a cationic conductance. The cationic conductance was also induced by direct injection of protein kinase A (PKA) catalytic subunit, reduced by the kinase inhibitor H7, and blocked by HgCl2, an inhibitor of aquaporin 1. The cationic permeability of the aquaporin 1 channel is activated by a cyclic adenosine monophosphate-dependent mechanism that may involve direct or indirect phosphorylation by PKA.

Changes in aquaporin-2 protein contribute to the urine concentrating defect in rats fed a low-protein diet

Low-protein diets cause a urinary concentrating defect in rats and humans. Previously, we showed that feeding rats a low (8%) protein diet induces a change in urea transport in initial inner medullary collecting ducts (IMCDs) which could contribute to the concentrating defect. Now, we test whether decreased osmotic water permeability (Pf) contributes to the concentrating defect by measuring Pf in perfused initial and terminal IMCDs from rats fed 18 or 8% protein for 2 wk. In terminal IMCDs, arginine vasopressin (AVP)-stimulated osmotic water permeability was significantly reduced in rats fed 8% protein compared to rats fed 18% protein. In initial IMCDs, AVP-stimulated osmotic water permeability was unaffected by dietary protein. Thus, AVP-stimulated osmotic water permeability is significantly reduced in terminal IMCDs but not in initial IMCDs. Next, we determined if the amount of immunoreactive aquaporin-2 (AQP2, the AVP-regulated water channel) or AQP3 protein was altered. Protein was isolated from base or tip regions of rat inner medulla and Western analysis performed using polyclonal antibodies to rat AQP2 or AQP3 (courtesy of Dr. M.A. Knepper, National Institutes of Health, Bethesda, MD). In rats fed 8% protein (compared to rats fed 18% protein): (a) AQP2 decreases significantly in both membrane and vesicle fractions from the tip; (b) AQP2 is unchanged in the base; and (c) AQP3 is unchanged. Together, the results suggest that the decrease in AVP-stimulated osmotic water permeability results, at least in part, in the decrease in AQP2 protein. We conclude that water reabsorption, like urea reabsorption, responds to dietary protein restriction in a manner that would limit urine concentrating capacity.

The aquaporin family of water channels in kidney: an update on physiology and pathophysiology of aquaporin-2

The long-standing problem of membrane water transport has been advanced by the recognition of a new family of water transport proteins, the "aquaporins" [1-3]. Not surprisingly, water transport is a major process in kidney physiology, and the biology of aquaporins is most thoroughly understood in that organ. We reviewed in detail the status of aquaporins in the kidney only one year ago [4], but the subsequent progress has dictated the need for an update. This seems especially appropriate in honor of the 100th birthday of Homer Smith, the pioneer whose foresight initiated this field.

The human Aquaporin-5 gene. Molecular characterization and chromosomal localization

The cDNA for the fifth mammalian aquaporin (AQP5) was isolated from rat, and expression was demonstrated in rat salivary and lacrimal glands, cornea, and lung (Raina, S., Preston, G. M., Guggino, W. B., and Agre, P. (1995) J. Biol. Chem. 270, 1908-1912). Here we report the isolation and characterization of the human AQP5 cDNA and gene. The AQP5 cDNA from a human submaxillary gland library contains a 795-base pair open reading frame encoding a 265-amino acid protein. The deduced amino acid sequences of human and rat AQP5 are 91% identical with 6 substitutions in the 22-amino acid COOH-terminal domain. Expression of human AQP5 in Xenopus oocytes conferred mercurial-sensitive osmotic water permeability (Pf) equivalent to other aquaporins. The human AQP5 structural gene resides within a 7. 4-kilobase SalI-EcoRI fragment with four exons corresponding to amino acids 1-121, 122-176, 177-204, and 205-265 separated by introns of 1.2, 0.5, and 0.9 kilobases. A transcription initiation site was identified 518 base pairs upstream of the initiating methionine. Genomic Southern analysis indicated that AQP5 is a single copy gene which localized to human chromosome 12q13; this coincides with the chromosomal locations of the homologous human genes MIP and AQP2, thus confirming 12q13 as the site of an aquaporin gene cluster. The mouse gene localized to distal chromosome 15. This information may permit molecular characterization of AQP5 expression during normal development and in clinical disorders.