Aquaporins: Water selective channels in biological membranes. Molecular structure and tissue distribution

Dual functional characteristic of human aquaporin 10 for solute transport

BACKGROUND/AIMS

Although aquaglyceroporins have been generally believed to operate in a channel mode, which is of nonsaturable nature, for glycerol as well as for water, we recently found that human aquaporin 9 (hAQP9) operates in a carrier-mediated mode, which is of saturable nature, for glycerol. Based on the finding, we assumed that such a characteristic might be shared by the other aquaglyceroporins and examined the functional characteristics of hAQP10, which is an intestine-specific aquaglyceroporin.

METHODS

Transport assays were conducted using Xenopus laevis oocytes expressing hAQP10 derived from the microinjected cRNA.

RESULTS

The transport of glycerol by hAQP10 was found to be highly saturable with a Michaelis constant of 10.4 μM and specifically inhibited by several glycerol analogs such as monoacetin. Furthermore, when glycerol was preloaded in hAQP10-expressing oocytes, its efflux was trans-stimulated by extracellular glycerol. These results indicate the involvement of a carrier-mediated mechanism in glycerol transport by hAQP10. Interestingly, a channel mechanism was also found to be involved in part in hAQP10-mediated glycerol transport.

CONCLUSION

The present study unveiled the uniquely dual functional characteristic of hAQP10 as a carrier/channel for solute transport, providing a novel insight into its operation mechanism, which would help further elucidate its physiological role.

Roles of aquaporin-3 water channels in volume-regulatory water flow in a human epithelial cell line

Membrane water transport is an essential event not only in the osmotic cell volume change but also in the subsequent cell volume regulation. Here we investigated the route of water transport involved in the regulatory volume decrease (RVD) that occurs after osmotic swelling in human epithelial Intestine 407 cells. The diffusion water permeability coefficient (Pd) measured by NMR under isotonic conditions was much smaller than the osmotic water permeability coefficient (Pf) measured under an osmotic gradient. Temperature dependence of Pf showed the Arrhenius activation energy (Ea) of a low value (1.6 kcal/mol). These results indicate an involvement of a facilitated diffusion mechanism in osmotic water transport. A mercurial water channel blocker (HgCl(2)) diminished the Pf value. A non-mercurial sulfhydryl reagent (MMTS) was also effective. These blockers of water channels suppressed the RVD. RT-PCR and immunocytochemistry demonstrated predominant expression of AQP3 water channel in this cell line. Downregulation of AQP3 expression induced by treatment with antisense oligodeoxynucleotides was found to suppress the RVD response. Thus, it is concluded that AQP3 water channels serve as an essential pathway for volume-regulatory water transport in, human epithelial cells.

Aquaporin 2 mutations in nephrogenic diabetes insipidus

Water reabsorption in the renal collecting duct is regulated by the antidiuretic hormone vasopressin (AVP). When the vasopressin V2 receptor, present on the basolateral site of the renal principal cell, becomes activated by AVP, aquaporin-2 (AQP2) water channels will be inserted in the apical membrane, and in this fashion, water can be reabsorbed from the pro-urine into the interstitium. The essential role of the vasopressin V2 receptor and AQP2 in the maintenance of body water homeostasis became clear when it was shown that mutations in their genes cause nephrogenic diabetes insipidus, a disorder in which the kidney is unable to concentrate urine in response to AVP. This review describes the current knowledge on AQP2 mutations in nephrogenic diabetes insipidus.

Protein kinase A-dependent phosphorylation of aquaporin-1

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. Previous results from our laboratory demonstrated that water channel activity of AQP1 was significantly increased by protein kinase A (PKA) activators such as cyclic-AMP (cAMP) and forskolin. The purpose of this study is to determine whether PKA-dependent protein phosphorylation is involved in the regulation of water channel activity of AQP1. Results presented here suggest that catalytic subunit of protein kinase A significantly increased the amount of phosphorylated AQP1 protein. In addition, these results indicated that cAMP-responsive redistribution of AQP1 may be regulated by phosphorylation of AQP1. Moreover, they provide new insights on the molecular mechanisms for regulating water balance in several tissues involving rapid water transport such as ciliary epithelium. In addition, they suggest important potential roles for AQP1 in several clinical disorders involving rapid water transport such as glaucoma.

Expression of aquaporins in Xenopus laevis oocytes and glial cells as detected by diffusion-weighted 1H NMR spectroscopy and photometric swelling assay

Expression of aquaporins (AQP) and water permeability were studied in Xenopus laevis oocytes and immobilized glial cells by a pulsed-field gradient spin echo NMR technique and a photometric swelling assay. Oocytes injected with poly(A) RNA from C6-BU-1 cells showed increased swelling behavior under hypoosmotic stress due to expressed water channels as compared to control oocytes. The swelling could be reversibly inhibited by HgCl2. Furthermore, the intracellular relaxation time and the apparent intracellular diffusion coefficient of water in oocytes were determined by diffusion-weighted 1H NMR experiments to be T2=36 ms and Dapp, intra=0.18x10-3 mm2/s. In immobilized C6 and F98 cells the mean exchange time of intracellular water was found to be 51 ms which increased to 75 ms upon chronic treatment (4 days) in hypertonic medium. Additional hybrid depletion experiments with antisense oligonucleotides directed against AQP1 were performed on oocytes and C6 cells. Moreover, different water channel subtypes of glial cells were assessed by a reverse transcriptase polymerase chain reaction assay. With this, the mRNA encoding AQP1 could be detected in primary cultures and glial cell lines, whereas AQP4 mRNA was found in astroglia-rich primary cultures, but not in F98 and C6 cells. Our results show that water permeability in glial cells is mainly mediated by water channels which play an important role in the regulation of water flow in brain under normal and pathological conditions.

Cloning and functional expression of human aquaporin8 cDNA and analysis of its gene

The cDNA and the gene of human aquaporin8 (AQP8) were cloned from human testis cDNA and a genomic library, respectively. The AQP8 cDNA encodes 261 amino acids. The identity of the amino acid sequence to other aquaporins is highest with a plant water channel, gamma-TIP (40.4%), while AQP2 and AQP3 are 28.9 and 29.5% identical to human AQP8, respectively. The human AQP8 is only 74.9% identical to rat AQP8 and 76.0% identical to mouse AQP8. In Northern blot analysis, approximately 1.35-kb human AQP8 mRNA was expressed in pancreas and colon, but not in other tissues. Absence of human AQP8 in testis is noteworthy as rat AQP8 was abundantly expressed in testis. The expression of human AQP8 cRNA in Xenopus oocytes increased osmotic water permeability by 10-fold. AQP8 was not permeable to urea nor to glycerol. The AQP8 gene has five introns, and the locations of exon-intron boundaries were different from those of the other mammalian aquaporins, suggesting its separate phylogenetic origin.

Water signal attenuation in diffusion-weighted 1H NMR experiments during cerebral ischemia: influence of intracellular restrictions, extracellular tortuosity, and exchange

The "concept of restricted intracellular water diffusion at permeable boundaries", which was recently used to model diffusion-weighted 1H NMR experiments on glioma cells, was applied to measurements on the rat brain in vivo. Combined with the "concept of extracellular tortuosity", various physiological states of the brain were simulated. Hereby, a variable intracellular volume fraction, intracellular exchange time, and extracellular tortuosity factor were considered for young, adult, and ischemic rat brains. The model simulated the cytotoxic shift of extracellular water, changes in membrane permeability and tissue morphology, and was able to explain the diffusion time dependence as well as the non-monoexponentiality of the diffusion attenuation curves. Preliminary diffusion time dependent experiments on the healthy rat brain (1H NMR imaging) agreed well with the theoretical concept. Hereby, the intracellular water signal was separated from extracellular signal contributions by large diffusion weighting. It showed the characteristic of restricted diffusion as well as a signal decay due to the exchange of intracellular water across the plasma membrane. A map of the mean intracellular exchange time for water in living animal brain was determined, and the upper limit in rat brain was evaluated to 15 ms. The presented methods can be applied to correlate local differences in a map of exchange times with tissue morphology and to detect pathological deviations of the exchange time, e.g., during ischemia.

Swelling detection for volume regulation in the primitive eukaryote Giardia intestinalis: a common feature of volume detection in present-day eukaryotes

It is increasingly evident that cell swelling is associated with the triggering of many biological processes, including progression of the cell cycle, hormonal response, and gene expression. However, the mechanism by which cell swelling is initially sensed and converted into intracellular signals is still ill-defined. We report here an early event in the detection of cell swelling and initiation of the volume regulatory response in Giardia intestinalis, an ancient representative of the eukaryotic kingdom. Giardial cell swelling, irrespective of the extent, was sensed at a cell volume of 1.06 x isosmotic volume (the threshold volume), at which the transition of the volume regulatory transport system from the 'resting' to the 'open' state occurred. Irreversible modification by p-chloromercuribenzoate (pCMB) and N-ethylmaleimide (NEM) of reduced thiols affected the threshold volume, but in opposing manners: pCMB increased the threshold volume to 1.14 x and NEM decreased to 0.85 x isosmotic volume. The simple modification of the threshold volume by NEM caused a drastic reduction of giardial cell volume under isosmotic conditions, with a process strikingly similar to the opening of mitochondrial permeability transition pore, a causative event in stress-induced programmed cell death. Substantial evidence supports the hypothesis that modulation of the membrane thiol moieties at the threshold volume, causing the 'all-or-nothing' type of swelling detection, represents the event linking cell swelling to the second messenger systems for volume regulation in present eukaryotes. Pathophysiological implications of alteration of the threshold volume are discussed.

Human intestinal permeability

This review focuses on permeability measurements in humans, briefly discussing different perfusion techniques, the relevance of human Peff values, and various aspects of in vivo transport mechanisms. In addition, human Peff values are compared with corresponding data from three preclinical transport models. The regional human jejunal perfusion technique has been validated in several important ways. One of the most important findings is that there is a good correlation between the measured human effective permeability values and the extent of absorption of drugs in humans determined by pharmacokinetic studies. Estimations of the absorption half-lives from the measured Peff agree very well with the time to maximal amount of the dose absorbed achieved after an oral dose in humans. We have also shown that it is possible to determine the Peff for carrier-mediated transported compounds and to classify them according to the proposed biopharmaceutical classification system (BCS). Furthermore, human in vivo permeabilities can be predicted using preclinical permeability models, such as in situ perfusion of rat jejunum, the Caco-2 model, and excised intestinal segments in the Ussing chamber. The permeability of passively transported compounds can be predicted with a particularly high degree of accuracy. However, special care must be taken for drugs with a carrier-mediated transport mechanism, and a scaling factor has to be used. Finally, the data obtained in vivo in humans emphasize the need for more clinical studies investigating the effect of physiological in vivo factors and molecular mechanisms influencing the transport of drugs across the intestinal and as well as other membrane barriers. It will also be important to study the effect of antitransport mechanisms (multidrug resistance, MDR), such as efflux by P-glycoprotein(s) and gut wall metabolism, for example CYP 3A4, on bioavailability.

Monitoring of cell volume and water exchange time in perfused cells by diffusion-weighted 1H NMR spectroscopy

Diffusion of intracellular water was measured in perfused cells embedded in basement membrane gel threads. F98 glioma cells, primary astrocytes, and epithelial KB cells were used and were exposed to osmotic stress, immunosuppressiva, the water channel blocker p-chloromercuriobenzenesulfonate (pCMBS), and apoptotic conditions. With diffusion-weighted 1H NMR spectroscopy changes in the intracellular signal could be monitored and quantified with single signal (ss), constant diffusion time (ct), and constant gradient strength (cg) experiments. The temporal resolution of the ss monitoring was 3.5 s with a standard deviation of 0.5% of the signal intensity and 32 s (3%) with ct monitoring, respectively. A mean intracellular residence time of water was determined with the cg experiment to about 50 ms. Changes of this exchange time from (51.9 +/- 1.0) to (59.0 +/- 1.1) ms were observed during treatment with pCMBS. The changes in the diffusion attenuated signal could be simulated analytically varying the intracellular volume fraction and exchange time by combination of restricted diffusion (Tanner model) and water exchange (Kärger model). This sensitive and noninvasive NMR method on perfused cells allows to determine changes in the intracellular volume and residence time in a simple and accurate manner. Many further applications as anoxia, volume regulation, ischemia and treatment with various pharmaceuticals are conceiveable to follow up their effect on the cell volume and the exchange time of intracellular water.

Probing the water permeability of ROMK1 and amphotericin B channels using Xenopus oocytes

Water permeability of ion channels in the plasma membrane of Xenopus oocytes was studied by simultaneously measuring the membrane conductance under two-electrode voltage-clamp and the cell size by video-imaging technique. The basal level of osmotic water permeability of oocyte plasma membrane was 15.9+/-0.98 microm/s (SE, n = 5). Extracellular application of pore-forming antibiotic amphotericin B at 5 microM developed macroscopic conductance of 995+/-70 microS (n = 5) and increased the osmotic water permeability of cell membrane by 44.9+/-4.1 microm/s. Meanwhile, after expressing ROMK1 channels, originally cloned from kidney, virtually no increase in the water permeability was observed even at the conductance level as high as 1113+/-47 microS (n = 5). This result suggests that even though potassium channels, like any others, are considered to be water-filled pores, K+-selective ion-transporting pathway remains virtually water-impermeable in physiological conditions, such as in kidney epithelia where huge water transport takes place at both apical and basolateral sides.

Contribution to the physiological characterization of glycerol active uptake in Saccharomyces cerevisiae

Evidence is presented here that in Saccharomyces cerevisiae IGC 3507, grown either on glycerol, ethanol or acetate, glycerol is transported by a high affinity uptake system of the electrogenic proton symport type, with Km of 1.7 +/- 0.7 mM, Vmax 441 +/- 19 micromolh(-1) g(-1) dry weight and a stoichiometry of 1:1 proton per molecule of glycerol, at 30 degrees C and pH 5.0. No competitors were found among other polyols and sugars. Glycerol maximum accumulation ratios followed p.m.f. with extracellular pH. CCCP prevented glycerol accumulation, and inhibited uptake. NaCl did not interfere with H+/glycerol kinetics and energetics. This transport system was shown to be under glucose repression and inactivation. Glucose-grown cells presented, instead, a lower affinity permease for glycerol, probably a facilitated diffusion. Growth on glucose in the presence of NaCl did not induce the high affinity carrier. The stringent control of cell physiological condition over induction suggests for glycerol proton symport rather a physiological role connected with growth under gluconeogenic conditions.

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.

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.

Stress-wave-induced membrane permeation of red blood cells is facilitated by aquaporins

Stress waves generated by lasers and extracorporeal lithotripters have been shown to transiently increase the permeability of the plasma membrane, without affecting cell viability. Molecules present in the medium can diffuse into the cytoplasm under the concentration gradient. Molecular uptake under stress waves correlates with the presence of functioning aquaporins in the plasma membrane.

Molecular mechanisms for passive and active transport of water

Water crosses cell membranes by passive transport and by secondary active cotransport along with ions. While the first concept is well established, the second is new. The two modes of transport allow cellular H2O homeostasis to be viewed as a balance between H2O leaks and H2O pumps. Consequently, cells can be hyperosmolar relative to their surroundings during steady states. Under physiological conditions, cells from leaky epithelia may be hyperosmolar by roughly 5 mosm liter-1, under dilute conditions, hyperosmolarities up to 40 mosm liter-1 have been recorded. Most intracellular H2O is free to serve as solvent for small inorganic ions. The mechanism of transport across the membrane depends on how H2O interacts with the proteinaceous or lipoid pathways. Osmotic transport of H2O through specific H2O channels such as CHIP 28 is hydraulic if the pore is impermeable to the solute and diffusive if the pore is permeable. Cotransport of ions and H2O can be a result of conformational changes in proteins, which in addition to ion transport also translocate H2O bound to or occlude in the protein. A cellular model of a leaky epithelium based on H2O leaks and H2O pumps quantitatively predicts a number of so-far unexplained observations of H2O transport.

Mutations in aquaporin-1 in phenotypically normal humans without functional CHIP water channels

The gene aquaporin-1 encodes channel-forming integral protein (CHIP), a member of a large family of water transporters found throughout nature. Three rare individuals were identified who do not express CHIP-associated Colton blood group antigens and whose red cells exhibit low osmotic water permeabilities. Genomic DNA analyses demonstrated that two individuals were homozygous for different nonsense mutations (exon deletion or frameshift), and the third had a missense mutation encoding a nonfunctioning CHIP molecule. Surprisingly, none of the three suffers any apparent clinical consequence, which raises questions about the physiological importance of CHIP and implies that other mechanisms may compensate for its absence.