Cloned human aquaporin-1 is a cyclic GMP-gated ion channel

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.

Differential Inhibition of Water and Ion Channel Activities of Mammalian Aquaporin-1 by Two Structurally Related Bacopaside Compounds Derived from the Medicinal Plant Bacopa monnieri

Aquaporin-1 (AQP1) is a major intrinsic protein that facilitates flux of water and other small solutes across cell membranes. In addition to its function as a water channel in maintaining fluid homeostasis, AQP1 also acts as a nonselective cation channel gated by cGMP, a property shown previously to facilitate rapid cell migration in a AQP1-expressing colon cancer cell line. Here we report two new modulators of AQP1 channels, bacopaside I and bacopaside II, isolated from the medicinal plant Bacopa monnieri Screening was conducted in the Xenopus oocyte expression system, using quantitative swelling and two-electrode voltage clamp techniques. Results showed bacopaside I blocked both the water (IC50 117 μM) and ion channel activities of AQP1 but did not alter AQP4 activity, whereas bacopaside II selectively blocked the AQP1 water channel (IC50 18 μM) without impairing the ionic conductance. These results fit with predictions from in silico molecular modeling. Both bacopasides were tested in migration assays using HT29 and SW480 colon cancer cell lines, with high and low levels of AQP1 expression, respectively. Bacopaside I (IC50 48 μM) and bacopaside II (IC50 14 μM) impaired migration of HT29 cells but had minimal effect on SW480 cell migration. Our results are the first to identify differential AQP1 modulators isolated from a medicinal plant. Bacopasides could serve as novel lead compounds for pharmaceutic development of selective aquaporin modulators.

Astrocyte Aquaporin Dynamics in Health and Disease

The family of aquaporins (AQPs), membrane water channels, consists of diverse types of proteins that are mainly permeable to water; some are also permeable to small solutes, such as glycerol and urea. They have been identified in a wide range of organisms, from microbes to vertebrates and plants, and are expressed in various tissues. Here, we focus on AQP types and their isoforms in astrocytes, a major glial cell type in the central nervous system (CNS). Astrocytes have anatomical contact with the microvasculature, pia, and neurons. Of the many roles that astrocytes have in the CNS, they are key in maintaining water homeostasis. The processes involved in this regulation have been investigated intensively, in particular regulation of the permeability and expression patterns of different AQP types in astrocytes. Three aquaporin types have been described in astrocytes: aquaporins AQP1 and AQP4 and aquaglyceroporin AQP9. The aim here is to review their isoforms, subcellular localization, permeability regulation, and expression patterns in the CNS. In the human CNS, AQP4 is expressed in normal physiological and pathological conditions, but astrocytic expression of AQP1 and AQP9 is mainly associated with a pathological state.

Aquaporins: Multiple Roles in the Central Nervous System

Aquaporins (AQPs) represent a diverse family of membrane proteins found in prokaryotes and eukaryotes. The primary aquaporins expressed in the mammalian brain are AQP1, which is densely packed in choroid plexus cells lining the ventricles, and AQP4, which is abundant in astrocytes and concentrated especially in the end-feet structures that surround capillaries throughout the brain and are present in glia limitans structures, notably in osmosensory areas such the supraoptic nucleus. Water movement in brain tissues is carefully regulated from the micro- to macroscopic levels, with aquaporins serving key roles as multifunctional elements of complex signaling assemblies. Intriguing possibilities suggest links for AQP1 in Alzheimer's disease, AQP4 as a target for therapy in brain edema, and a possible contribution of AQP9 in Parkinson's disease. For all the aquaporins, new contributions to physiological functions are likely to continue to be discovered with ongoing work in this rapidly expanding field of research. NEUROSCIENTIST 13(5):470—485, 2007.

Pharmacological blockade of aquaporin-1 water channel by AqB013 restricts migration and invasiveness of colon cancer cells and prevents endothelial tube formation in vitro

Background

Aquaporins (AQP) are water channel proteins that enable fluid fluxes across cell membranes, important for homeostasis of the tissue environment and for cell migration. AQP1 knockout mouse models of human cancers showed marked inhibition of tumor-induced angiogenesis, and in pre-clinical studies of colon adenocarcinomas, forced over-expression of AQP1 was shown to increase angiogenesis, invasion and metastasis. We have synthesized small molecule antagonists of AQP1. Our hypothesis is that inhibition of AQP1 will reduce migration and invasiveness of colon cancer cells, and the migration and tube-forming capacity of endothelial cells in vitro.

Methods

Expression of AQP1 in cell lines was assessed by quantitative (q) PCR, western blot and immunofluorescence, while expression of AQP1 in human colon tumour tissue was assessed by immunohistochemistry. The effect of varying concentrations of the AQP1 inhibitor AqB013 was tested on human colon cancer cell lines expressing high versus low levels of AQP1, using wound closure (migration) assays, matrigel invasion assays, and proliferation assays. The effect of AqB013 on angiogenesis was tested using an endothelial cell tube-formation assay.

Results

HT29 colon cancer cells with high AQP1 levels showed significant inhibition of migration compared to vehicle control of 27.9 % ± 2.6 % (p < 0.0001) and 41.2 % ± 2.7 (p <0.0001) treated with 160 or 320 μM AqB013 respectively, whereas there was no effect on migration of HCT-116 cells with low AQP1 expression. In an invasion assay, HT29 cells treated with 160 μM of AqB013, showed a 60.3 % ± 8.5 % decrease in invasion at 144 hours (p < 0.0001) and significantly decreased rate of invasion compared with the vehicle control (F-test, p = 0.001). Almost complete inhibition of endothelial tube formation (angiogenesis assay) was achieved at 80 μM AqB013 compared to vehicle control (p < 0.0001).

Conclusion

These data provide good evidence for further testing of the inhibitor as a therapeutic agent in colon cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-016-0310-6) contains supplementary material, which is available to authorized users.

Bumetanide Derivatives AqB007 and AqB011 Selectively Block the Aquaporin-1 Ion Channel Conductance and Slow Cancer Cell Migration

Aquaporins (AQPs) in the major intrinsic family of proteins mediate fluxes of water and other small solutes across cell membranes. AQP1 is a water channel, and under permissive conditions, a nonselective cation channel gated by cGMP. In addition to mediating fluid transport, AQP1 expression facilitates rapid cell migration in cell types including colon cancers and glioblastoma. Work here defines new pharmacological derivatives of bumetanide that selectively inhibit the ion channel, but not the water channel, activity of AQP1. Human AQP1 was analyzed in the Xenopus laevis oocyte expression system by two-electrode voltage clamp and optical osmotic swelling assays. The aquaporin ligand bumetanide derivative AqB011 was the most potent blocker of the AQP1 ion conductance (IC50 of 14 μM), with no effect on water channel activity (at up to 200 μM). The order of potency for inhibition of the ionic conductance was AqB011 > AqB007 >> AqB006 ≥ AqB001. Migration of human colon cancer (HT29) cells was assessed with a wound-closure assay in the presence of a mitotic inhibitor. AqB011 and AqB007 significantly reduced migration rates of AQP1-positive HT29 cells without affecting viability. The order of potency for AQP1 ion channel block matched the order for inhibition of cell migration, as well as in silico modeling of the predicted order of energetically favored binding. Docking models suggest that AqB011 and AqB007 interact with the intracellular loop D domain, a region involved in AQP channel gating. Inhibition of AQP1 ionic conductance could be a useful adjunct therapeutic approach for reducing metastasis in cancers that upregulate AQP1 expression.

Beyond water homeostasis: Diverse functional roles of mammalian aquaporins

BACKGROUND

Aquaporin (AQP) water channels are best known as passive transporters of water that are vital for water homeostasis.

SCOPE OF REVIEW

AQP knockout studies in whole animals and cultured cells, along with naturally occurring human mutations suggest that the transport of neutral solutes through AQPs has important physiological roles. Emerging biophysical evidence suggests that AQPs may also facilitate gas (CO2) and cation transport. AQPs may be involved in cell signalling for volume regulation and controlling the subcellular localization of other proteins by forming macromolecular complexes. This review examines the evidence for these diverse functions of AQPs as well their physiological relevance.

MAJOR CONCLUSIONS

As well as being crucial for water homeostasis, AQPs are involved in physiologically important transport of molecules other than water, regulation of surface expression of other membrane proteins, cell adhesion, and signalling in cell volume regulation.

GENERAL SIGNIFICANCE

Elucidating the full range of functional roles of AQPs beyond the passive conduction of water will improve our understanding of mammalian physiology in health and disease. The functional variety of AQPs makes them an exciting drug target and could provide routes to a range of novel therapies.

Aquaporin water channels in the mammary gland: from physiology to pathophysiology and neoplasia

Aquaporins are membrane proteins that play fundamental roles in water and small solute transport across epithelial and endothelial barriers. Recent studies suggest that several aquaporin proteins are present in the mammary gland. Immunohistochemical techniques have confirmed the presence of aquaporin 1 (AQP1) and AQP3 water channels in rat, mouse, bovine and human mammary glands. Studies suggest that in addition to AQP1 and AQP3 AQP4, AQP5 and AQP7 proteins are expressed in different locations in the mammary gland. Aquaporins play key roles in tumor biology and are involved in cell growth, migration and formation of ascites via increased water permeability of micro-vessels. Emerging evidence suggests that expression of these proteins is altered in mammary tumors and in breast cancer cell lines although it is not yet clear whether this is a cause or a consequence of neoplastic development. This review analyzes the expression and potential functional roles of aquaporin water channels in the mammary gland. The physiological mechanisms involved in the transport of water and small solutes across mammary endothelial and epithelial barriers are discussed in the context of milk production and lactation. This paper also reviews papers from the recent cancer literature that implicate aquaporins in mammary neoplasia.

Molecular and functional characterization of multiple aquaporin water channel proteins from the western tarnished plant bug, Lygus hesperus

Aquaporins (AQPs) are integral membrane channel proteins that facilitate the bidirectional transfer of water or other small solutes across biological membranes involved in numerous essential physiological processes. In arthropods, AQPs belong to several subfamilies, which contribute to osmoregulation, respiration, cryoprotection, anhydrobiosis, and excretion. We cloned and characterized five novel AQPs from the western tarnished plant bug, Lygus hesperus, a polyphagous insect pest of food and fiber crops throughout western North America. The L. hesperus AQPs (LhAQP1-5) belong to different phylogenetic subfamilies, have unique transcription profiles and cellular localizations, and all transport water (but not glycerol) when heterologously expressed in Xenopus laevis oocytes. Our results demonstrate that multiple AQPs with possible compensatory functions are produced in L. hesperus that likely play important roles in maintaining water homeostasis in this important insect pest.

Physiological roles of aquaporin-4 in brain

Aquaporin-4 (AQP4) is one of the most abundant molecules in the brain and is particularly prevalent in astrocytic membranes at the blood-brain and brain-liquor interfaces. While AQP4 has been implicated in a number of pathophysiological processes, its role in brain physiology has remained elusive. Only recently has evidence accumulated to suggest that AQP4 is involved in such diverse functions as regulation of extracellular space volume, potassium buffering, cerebrospinal fluid circulation, interstitial fluid resorption, waste clearance, neuroinflammation, osmosensation, cell migration, and Ca(2+) signaling. AQP4 is also required for normal function of the retina, inner ear, and olfactory system. A review will be provided of the physiological roles of AQP4 in brain and of the growing list of data that emphasize the polarized nature of astrocytes.

Optimization of 3D Poisson-Nernst-Planck model for fast evaluation of diverse protein channels

We show the accuracy and applicability of our fast algorithmic implementation of a three-dimensional Poisson-Nernst-Planck (3D-PNP) flow model for characterizing different protein channels. Due to its high computational efficiency, our model can predict the full current-voltage characteristics of a channel within minutes, based on the experimental 3D structure of the channel or its computational model structure. Compared with other methods, such as Brownian dynamics, which currently needs a few weeks of the computational time, or even much more demanding molecular dynamics modeling, 3D-PNP is the only available method for a function-based evaluation of very numerous tentative structural channel models. Flow model tests of our algorithm and its optimal parametrization are provided for five native channels whose experimental structures are available in the protein data bank (PDB) in an open conductive state, and whose experimental current-voltage characteristics have been published. The channels represent very different geometric and structural properties, which makes it the widest test to date of the accuracy of 3D-PNP on real channels. We test whether the channel conductance, rectification, and charge selectivity obtained from the flow model, could be sufficiently sensitive to single-point mutations, related to unsignificant changes in the channel structure. Our results show that the classical 3D-PNP model, under proper parametrization, is able to achieve a qualitative agreement with experimental data for a majority of the tested characteristics and channels, including channels with narrow and irregular conductivity pores. We propose that although the standard PNP model cannot provide insight into complex physical phenomena due to its intrinsic limitations, its semiquantitative agreement is achievable for rectification and selectivity at a level sufficient for the bioinformatical purpose of selecting the best structural models with a great advantage of a very short computational time.

The Osmopressor Response Is Linked to Upregulation of Aquaporin-1 Tyrosine Phosphorylation on Red Blood Cell Membranes

Studies in patients with an impaired efferent baroreflex led us to discover that ingesting water induces a robust increase in blood pressure and vascular resistance. This response was also present in healthy subjects with intact baroreflexes, described as osmopressor response. This study was to discover the physiology of the osmopressor response by determining functional activation of the aquaporin-1 water channel receptor on red blood cell membranes in young healthy subjects. In a randomized, controlled, crossover fashion, 22 young healthy subjects (age, 19–27 years) ingested either 500 or 50 mL of water. Heart rate, blood pressure, cardiac index, and total peripheral vascular resistance were measured using a Finometer hemodynamic monitor. Blood sampling was performed at 5 minutes before and at 25 and 50 minutes after either the water ingestion or control session. Immunoblotting for aquaporin-1 tyrosine phosphorylation was performed before and after subjects ingested either 500 or 50 mL of water. At 25 minutes after the ingestion of 500 mL of water, total peripheral resistance increased significantly, and plasma osmolality decreased. Functional expression of aquaporin-1 tyrosine phosphorylation on red blood cell membranes increased significantly at 25 and 50 minutes after subjects ingested 500 mL of water compared with that before water ingestion. This study concludes that water ingestion produces upregulation of aquaporin-1 tyrosine phosphorylation on red blood cell, which presents as a novel biological marker that occurs simultaneously with the osmopressor response.

Differential expression and seasonal variation on aquaporins 1 and 9 in the male genital system of big fruit-eating bat Artibeus lituratus

Efferent ductules and epididymis are involved in water and solute transport, which is indispensable for storage and maintenance of the sperm viability. The reabsorption process involves proteins such as aquaporins (AQP), which has been described in the male genital system of limited species, including primate, rodents, cats and dogs. To contribute with information about AQPs in the male system, here we investigated the distribution of AQP1 and AQP9 in the tropical bat Artibeus lituratus, along the annual reproductive cycle. A. lituratus is a seasonal breeder with natural variation in components of the androgen and estrogen responsive system, thus being a good model for exploring the AQPs modulation. AQP1 was found restricted to differentiating spermatids, efferent ductules epithelium and venular endothelia along the male tract. AQP9 was detected throughout the epididymis being more abundant in the cauda and ductus deferens, but was not found in testis, rete testis and efferent ductules. Contrasting with AQP1 which appear to be constitutively expressed, there was seasonal variation in AQP9 expression, which was reduced in regressed epididymis. The AQP9 does not appear to be modulated by estradiol or androgens, but possibly by other factor related to luminal sperm. The establishment of specific function for aquaporins in the male tract remains undetermined; however, the cellular distribution presently found are compatible with the main function of AQP1, as a selective water channel, and AQP9, which is a conduct for water and a plethora of neutral solutes present in the epididymis milieu such as glycerol and urea.

Stimulation of aquaporin-mediated fluid transport by cyclic GMP in human retinal pigment epithelium in vitro

PURPOSE

The retinal pigment epithelium (RPE) expresses aquaporin-1 (AQP1) and components of the natriuretic peptide signaling pathway. We hypothesized that stimulation of the natriuretic signaling pathway in RPE with atrial natriuretic peptide (ANP) and with membrane-permeable analogs of cGMP would induce a net apical-to-basal transport of fluid.

METHODS

The hypothesis was tested using human RPE cultures that retain properties seen in vivo. Confluent monolayers were treated with ANP or membrane-permeable cGMP analogs in the presence of anantin, H-8, and an AQP1 inhibitor, AqB013. Fluid movement from the apical to basal chambers was measured by weight and used to calculate net fluid transport.

RESULTS

Our results demonstrated a 40% increase in net apical-to-basal fluid transport by ANP (5 μM) that was inhibited completely by the ANP receptor antagonist anantin and a 60% increase in net apical-to-basal fluid transport in response to the extracellularly applied membrane-permeable cGMP analog pCPT-cGMP (50 μM), which was not affected by the protein kinase G inhibitor H-8. The aquaporin antagonist AqB013 (20 μM) inhibited the cGMP-stimulated RPE fluid flux.

CONCLUSIONS

The effect of cGMP is consistent with an enhancement of the net fluid flux in RPE mediated by AQP1 channels. Pharmacologic activation of cGMP signaling and concomitant stimulation of fluid uptake from the subretinal space could offer insights into a new approach to treating or reducing the risk of retinal detachment.

Structure, function and translational relevance of aquaporin dual water and ion channels

Aquaporins have been assumed to be selective for water alone, and aquaglyceroporins are accepted as carrying water and small uncharged solutes including glycerol. This review presents an expanded view of aquaporins as channels with more complex mechanisms of regulation and diverse repertoires of substrate permeabilities than were originally appreciated in the early establishment of the field. The role of aquaporins as dual water and gated ion channels is likely to have physiological and potentially translational relevance, and can be evaluated with newly developed molecular and pharmacological tools. Ion channel activity has been shown for Aquaporins -0, -1, and -6, Drosphila Big Brain, and plant Nodulin-26. Although the concept of ion channel function in aquaporins remains controversial, research advances are beginning to define not only the ion channel function but also the detailed molecular mechanisms that govern and mediate the multifunctional capabilities. With regard to physiological relevance, the adaptive benefit of expression of ion channel activity in aquaporins, implied by amino acid sequence conservation of the ion channel gating domains, suggests they provide more than water or glycerol and solute transport. Dual ion and water channels are of interest for understanding the modulation of transmembrane fluid gradients, volume regulation, and possible signal transduction in tissues expressing classes of aquaporins that have the dual function capability. Other aquaporin classes might be found in future work to have ion channel activities, pending identification of the possible signaling pathways that could govern activation.

The activity of human aquaporin 1 as a cGMP-gated cation channel is regulated by tyrosine phosphorylation in the carboxyl-terminal domain

In addition to a constitutive water channel activity, several studies suggest Aquaporin-1 (AQP1) functions as a nonselective monovalent cation channel activated by intracellular cGMP, although variability in responsiveness between preparations has led to controversy in the field. Data here support the hypothesis that responsiveness of the AQP1 ionic conductance to cGMP is governed by tyrosine phosphorylation. Wild-type and mutant human AQP1 channels expressed in Xenopus laevis oocytes were characterized by two-electrode voltage clamp and optical osmotic swelling analyses. Quadruple mutation by site-directed mutagenesis of barrier hydrophobic residues (Val50, Leu54, Leu170, Leu174) to alanines in the central pore induced inward rectification of the ionic current and shifted reversal potential by approximately +10 mV, indicating increased permeability of tetraethylammonium ion. Introduction of cysteine at lysine 51 in the central pore (K51C) in a cysteine-less template created new sensitivity to block of the conductance by mercuric ion. Mutations of candidate consensus sites and pharmacological manipulation of serine and threonine phosphorylation did not alter cGMP-dependent responses; however, mutation of tyrosine Y253C or pharmacological dephosphorylation prevented ion channel activation. Modification of Y253C by covalent addition of a negatively charged group [2-sulfonatoethyl methanethiosulfonate sodium salt (MTSES)] rescued the cGMP-activated conductance response, an effect reversed by dithiothreitol. Results support the proposal that phosphorylation of tyrosine Tyr253 in the carboxyl terminal domain, confirmed by Western blot, acts as a master switch regulating responsiveness of AQP1 ion channels to cGMP, and the tetrameric central pore is the ion permeation pathway. These findings advance resolution of a standing controversy and expand our understanding of AQP1 as a multifunctional regulated channel.

The conjugation of an AQP1-directed immunotoxin in the study of site-directed therapy within the CNS

PURPOSE

The water channel, aquaporin (AQP)1, is highly specific to the choroid plexus (CP) epithelium within the brain. It is therefore a potential target through which therapeutic agents could be selectively directed to the CP. Here we describe the conjugation of a monoclonal antibody (mAb), raised against an extra-cellular domain of AQP1, to the A chain of ricin (RCA). This reagent should allow study of a highly specific chemical lesion of the CP.

METHODS

A published method was used to couple the anti-AQP1 mAb to the A chain of ricin using an SPDP cross-linker. The conjugate was purified by Superdex S-200 chromatography. Column fractions were analysed by SDS-PAGE and Western blotting. Fractions containing conjugate were assayed for cytotoxicity on rat CP cells in culture, using a WST-1 viability assay to assess cell death.

RESULTS

SDS-PAGE and Western blotting demonstrated separation of conjugated anti-AQP1 mAb from unconjugated ricin by the Superdex column. Cultured rat CP cells were killed with an IC(50) of 1.81 μg/ml when exposed to the anti-AQP1 mAb-RCA conjugation reaction products. Selectivity of this reagent was demonstrated by the higher IC(50) exhibited by non-AQP1-expressing primary fibroblast cultures and cultures exposed to the reaction products from the non-specific IgG-RCA conjugation attempt.

CONCLUSION

We have been able to demonstrate selective cytotoxicity in vitro of an anti-AQP1 mAb-RCA conjugate. We hope that this work will generate further interest in the use of this site-specific conjugate to target the CP in conditions such as hydrocephalus, tumours that arise from here, and other AQP1-expressing tumours.

Identification and characterization of functional aquaporin water channel protein from alimentary tract of whitefly, Bemisia tabaci

Some hemipteran xylem and phloem-feeding insects have evolved specialized alimentary structures or filter chambers that rapidly transport water for excretion or osmoregulation. In the whitefly, Bemisia tabaci, mass movement of water through opposing alimentary tract tissues within the filter chamber is likely facilitated by an aquaporin protein. B. tabaci aquaporin-1 (BtAQP1) possesses characteristic aquaporin topology and conserved pore-forming residues found in water-specific aquaporins. As predicted for an integral transmembrane protein, recombinant BtAQP1 expressed in cultured insect cells localized within the plasma membrane. BtAQP1 is primarily expressed in early instar nymphs and adults, where in adults it is localized in the filter chamber and hindgut. Xenopus oocytes expressing BtAQP1 were water permeable and mercury-sensitive, both characteristics of classical water-specific aquaporins. These data support the hypothesis that BtAQP1 is a water transport protein within the specialized filter chamber of the alimentary tract and functions to translocate water across tissues for maintenance of osmotic pressure and/or excretion of excess dietary fluid.

Major intrinsic proteins in biomimetic membranes

Biological membranes define the structural and functional boundaries in living cells and their organelles. The integrity of the cell depends on its ability to separate inside from outside and yet at the same time allow massive transport of matter in and out the cell. Nature has elegantly met this challenge by developing membranes in the form of lipid bilayers in which specialized transport proteins are incorporated. This raises the question: is it possible to mimic biological membranes and create a membrane based sensor and/or separation device? In the development of a biomimetic sensor/separation technology, a unique class of membrane transport proteins is especially interesting-the major intrinsic proteins (MIPs). Generally, MIPs conduct water molecules and selected solutes in and out of the cell while preventing the passage of other solutes, a property critical for the conservation of the cells internal pH and salt concentration. Also known as water channels or aquaporins they are highly efficient membrane pore proteins some of which are capable of transporting water at very high rates up to 10(9) molecules per second. Some MIPs transport other small, uncharged solutes, such as glycerol and other permeants such as carbon dioxide, nitric oxide, ammonia, hydrogen peroxide and the metalloids antimonite, arsenite, silicic and boric acid depending on the effective restriction mechanism of the protein. The flux properties of MIPs thus lead to the question ifMIPs can be used in separation devices or as sensor devices based on, e.g., the selective permeation of metalloids. In principle a MIP based membrane sensor/separation device requires the supporting biomimetic matrix to be virtually impermeable to anything but water or the solute in question. In practice, however, a biomimetic support matrix will generally have finite permeabilities to both electrolytes and non-electrolytes. The feasibility of a biomimetic MIP device thus depends on the relative transport contribution from both protein and biomimetic support matrix. Also the biomimetic matrix must be encapsulated in order to protect it and make it sufficiently stable in a final application. Here, I specifically discuss the feasibility of developing osmotic biomimetic MIP membranes, but the technical issues are of general concern in the design ofbiomimetic membranes capable of supporting selective transmembrane fluxes.

Aquaporin biology and nervous system

Our understanding of the movement of water through cell membranes has been greatly advanced by the discovery of a family of water-specific, membrane-channel proteins: the Aquaporins (AQPs). These proteins are present in organisms at all levels of life, and their unique permeability characteristics and distribution in numerous tissues indicate diverse roles in the regulation of water homeostasis.Phenotype analysis of AQP knock-out mice has confirmed the predicted role of AQPs in osmotically driven transepithelial fluid transport, as occurs in the urinary concentrating mechanism and glandular fluid secretion. Regarding their expression in nervous system, there are evidences suggesting that AQPs are differentially expressed in the peripheral versus central nervous system and that channel-mediated water transport mechanisms may be involved in cerebrospinal fluid formation, neuronal signal transduction and information processing.Moreover, a number of recent studies have revealed the importance of mammalian AQPs in both physiological and pathophysiological mechanisms and have suggested that pharmacological modulation of AQP expression and activity may provide new tools for the treatment of variety of human disorders in which water and small solute transport may be involved. For all the AQPs, new contributions to physiological functions are likely to be discovered with ongoing work in this rapidly expanding field of research.

Angiotensin II and hypertonicity modulate proximal tubular aquaporin 1 expression

Aquaporin 1 (AQP1) is the major water channel in the renal proximal tubule (PT) and thin descending limb of Henle, but its regulation remains elusive. Here, we investigated the effect of ANG II, a key mediator of body water homeostasis, on AQP1 expression in immortalized rat proximal tubule cells (IRPTC) and rat kidney. Real-time PCR on IRPTC exposed to ANG II for 12 h revealed a biphasic effect AQP1 mRNA increased dose dependently in response to 10(-12) to 10(-8) M ANG II but decreased by 50% with 10(-7) M ANG II. The twofold increase of AQP1 mRNA in the presence of 10(-8) M ANG II was abolished by the AT(1) receptor blocker losartan. Hypertonicity due to either NaCl or mannitol also upregulated AQP1 mRNA by three- and twofold, respectively. Immunocytochemistry and Western blotting revealed a two- to threefold increase in AQP1 protein expression in IRPTC exposed concomitantly to ANG II (10(-8)M) and hypertonic medium (either NaCl or mannitol), indicating that these stimuli were not additive. Three-dimensional reconstruction of confocal images suggested that AQP1 expression was increased by ANG II in both the apical and basolateral poles of IRPTC. In vivo studies showed that short-term ANG II infusion had a diuretic effect, while this effect was attenuated after several days of ANG II infusion. After 10 days, we observed a twofold increase in AQP1 expression in the PT and thin descending limb of Henle of ANG II-infused rats that was abolished when rats were treated with the selective AT(1)-receptor antagonist olmesartan. Thus ANG II increases AQP1 expression in vitro and in vivo via direct interaction with the AT(1) receptor, providing an important regulatory mechanism to link PT water reabsorption to body fluid homeostasis via the renin-angiotensin system.

Physiological roles of aquaporins in the choroid plexus

The choroid plexus is a specialized tissue that lines subdomains within the four ventricles of the brain where most of the cerebrospinal fluid is produced. Maintenance of an equilibrium in volume and composition of the cerebrospinal fluid (CSF) is vital for a normal brain function, ensuring an optimal environment for the neurons. The necessarily high water permeability of the choroid plexus barrier is made possible by the abundant expression of a water channel, Aquaporin-1 (AQP1), on the apical side of the membrane from early stages of development through adulthood. Data from studies of AQP1 suggest that it also can contribute as a gated ion channel, and suggest that the AQP1-mediated ionic conductance has physiological significance for the regulation of cerebrospinal fluid secretion. The regulation of AQP1 ion channels could be one of several transport mechanisms that contribute to the decreased CSF secretion in response to endogenous signaling molecules such as atrial natriuretic peptide. Numerous classes of ion channels and transporters are targeted specifically to each side of the cellular membrane, and they all work in concert to secrete CSF. Several signaling cascades have a direct effect on transporters and ion channels present in the choroid plexus epithelium, altering their transport activity and therefore modulating the net transcellular movement of solutes and water. Several neurotransmitters, neuropeptides, and growth factors can influence CSF secretion by direct effect on transport mechanisms of the epithelium. The mammalian choroid plexus receives innervation from noradrenergic sympathetic fibers, cholinergic and peptidergic fibers that modulate CSF secretion. Water imbalance in the brain can have life-threatening consequences resulting from altered excitability and neurodegeneration, disruption of the supply of nutrients, loss of signaling molecules, and the accumulation of unwanted toxins and metabolites. Understanding the mechanisms involved in the modulation of CSF secretion is of fundamental importance. An appreciation of AQP1 as an ion channel in addition to its role as a water channel should offer new targets for therapeutic strategies in diseases involving water imbalance in the brain.

Roles for novel pharmacological blockers of aquaporins in the treatment of brain oedema and cancer

1. Aquaporins (AQPs) are targets for drug discovery for basic research and medicine. Human diseases involving fluid imbalances and oedema are of major concern and involve tissues in which AQPs are expressed. The range of functional properties of AQPs is continuing to expand steadily with ongoing research in the field. 2. Gating domains in AQPs are molecular sites for drug actions. Discovery of the arylsulphonamide AqB013 as an antagonist for AQP1 and AQP4 provided the first pharmacological agent with translational promise for the treatment of diseases in which AQPs have been implicated. The putative binding site for AqB013 in the internal vestibule of the AQP water pore involves amino acid residues that are located in the AQP loop D gating domain. 3. Aquaporins have been proposed as novel targets in cancer and oedema and are associated with a surprising array of important processes in the brain and body, such as angiogenesis, cell migration, development and neuropathological diseases. Functions beyond their simple role as water channels are suggested by the subtype-specific regulation of AQP expression. In both cancer and brain oedema, current therapies are limited and new pharmacological approaches focused on AQPs offer exciting potential for clinical advances.

The role of aquaporins in excretion in insects

One of the aspects of insect osmoregulation that has most intrigued researchers is the ability of a simple tubular epithelium, such as the Malpighian tubule, to create both hypo- and hyperosmotic urine. Indeed,Ramsay's initial observation that isolated tubules could secrete a hypoosmotic urine led him to attribute the phenomenon to the active transport of water. In the ensuing decades several models for solute recycling have been proposed,but only in the last 15 years has it become clear that tubule water permeability is due to the presence of aquaporins (AQPs), the ubiquitous water transport proteins. There are 13 known human AQPs, and they are tissue and even membrane specific. It is now clear that the number and type of AQPs within a membrane are the major determinants of its water transport capacity. There are many gene homologs for the AQPs, so proof of function requires expression of the protein in a defined system. Within the insects, only seven AQPs have been functionally expressed and, of these, four directly or indirectly function in excretion. In this paper we review the basic structure and general function of AQPs and then examine the source, localization and functional attributes of those isolated from insects.

Bypassing vasopressin receptor signaling pathways in nephrogenic diabetes insipidus

Water reabsorption in the kidney represents a critical physiological event in the maintenance of body water homeostasis. This highly regulated process relies largely on vasopressin (VP) action and on the VP-sensitive water channel (AQP2) that is expressed in principal cells of the kidney collecting duct. Defects in the VP signaling pathway and/or in AQP2 cell surface expression can lead to an inappropriate reduction in renal water reabsorption and the development of nephrogenic diabetes insipidus, a disease characterized by polyuria and polydipsia. This review focuses on the major regulatory steps that are involved in AQP2 trafficking and function. Specifically, we begin with a discussion on VP-receptor-independent mechanisms of AQP2 trafficking, with special emphasis on the nitric oxide-cyclic guanosine monophosphate signaling pathway, followed by a review of the mechanisms that govern AQP2 endocytosis and exocytosis. We then discuss emerging data illustrating roles played by the actin cytoskeleton on AQP2 trafficking, and lastly we consider elements that affect AQP2 protein expression in cells. Recent advances in each topic are summarized and are presented in the context of their potential to serve as a basis for the development of novel therapies that may ultimately improve life quality of nephrogenic diabetes insipidus patients.

High-resolution x-ray structure of human aquaporin 5

Human aquaporin 5 (HsAQP5) facilitates the transport of water across plasma membranes and has been identified within cells of the stomach, duodenum, pancreas, airways, lungs, salivary glands, sweat glands, eyes, lacrimal glands, and the inner ear. AQP5, like AQP2, is subject to posttranslational regulation by phosphorylation, at which point it is trafficked between intracellular storage compartments and the plasma membrane. Details concerning the molecular mechanism of membrane trafficking are unknown. Here we report the x-ray structure of HsAQP5 to 2.0-A resolution and highlight structural similarities and differences relative to other eukaryotic aquaporins. A lipid occludes the putative central pore, preventing the passage of gas or ions through the center of the tetramer. Multiple consensus phosphorylation sites are observed in the structure and their potential regulatory role is discussed. We postulate that a change in the conformation of the C terminus may arise from the phosphorylation of AQP5 and thereby signal trafficking.

Role of aquaporins and regulation of secretory vesicle volume in cell secretion

In exocrine glands, secretory proteins synthesized in the rough endoplasmic reticulum (RER) exhibit vectorial transport from ER through a succession of membrane-bounded components such as Golgi complex, condensing vacuoles and secretory granules. The secretory granules migrate to particular locations within the cell close to the apical membrane prior to the release of their contents into the acinar lumen. Currently, to release intragranular contents, secretory granules have been demonstrated to transiently dock and fuse at ‘porosome’, a permanent cup-shaped structures at the cell membranes. Then swelling of secretory granules occurs to allow explusion of intragranular contents. In this process, water and ion fluxes in the granule membrane appear to contribute to maintain secretory granule integrity and morphology via osmoregulation in secretory granules. Aquaporins (AQPs) are a family of small, hydrophobic, integral membrane proteins, which function as channels to permeate water and small solutes. The AQPs reside constitutively at the plasma membrane in most cell types. However, recent studies have demonstrated that the AQPs are present in secretory granules in exocrine glands, synaptic vesicles and intracellular vesicles in liver and kidney, implying that AQPs in secretory granules and vesicles are involved in their volume regulation. This paper reviews the possible role of AQPs on secretory granules, especially in exocrine glands, in secretory function.

Ion channel diversity, channel expression and function in the choroid plexuses

Knowledge of the diversity of ion channel form and function has increased enormously over the last 25 years. The initial impetus in channel discovery came with the introduction of the patch clamp method in 1981. Functional data from patch clamp experiments have subsequently been augmented by molecular studies which have determined channel structures. Thus the introduction of patch clamp methods to study ion channel expression in the choroid plexus represents an important step forward in our knowledge understanding of the process of CSF secretion.

Two K⁺ conductances have been identified in the choroid plexus: Kv1 channel subunits mediate outward currents at depolarising potentials; Kir 7.1 carries an inward-rectifying conductance at hyperpolarising potentials. Both K⁺ channels are localised at the apical membrane where they may contribute to maintenance of the membrane potential while allowing the recycling of K⁺ pumped in by Na⁺-K⁺ ATPase. Two anion conductances have been identified in choroid plexus. Both have significant HCO₃^- permeability, and may play a role in CSF secretion. One conductance exhibits inward-rectification and is regulated by cyclic AMP. The other is carried by an outward-rectifying channel, which is activated by increases in cell volume. The molecular identity of the anion channels is not known, nor is it clear whether they are expressed in the apical or basolateral membrane. Recent molecular evidence indicates that choroid plexus also expresses the non-selective cation channels such as transient receptor potential channels (TRPV4 and TRPM3) and purinoceptor type 2 (P2X) receptor operated channels. In conclusion, good progress has been made in identifying the channels expressed in the choroid plexus, but determining the precise roles of these channels in CSF secretion remains a challenge for the future.

Anatomical and functional analysis of aquaporin 1, a water channel in primary afferent neurons

Aquaporin 1 (AQP1) is the archetypal member of a family of water channel proteins that contribute to water homeostasis in kidney, lung, and other tissues. Although there is limited evidence that aquaporins are expressed in the nervous system, AQP4 is expressed in glia and AQP9 is present on some neuronal and glial mitochondria. In the present study, we used immunohistochemistry to show that AQP1 is heavily expressed in a population of small diameter primary sensory neurons of dorsal root, trigeminal, and nodose ganglia. AQP1 immunoreactivity is abundant in DRG cell bodies and in both the peripheral and central branches of primary afferent neurons, and colocalizes with markers of nociceptors, notably substance P and IB4. AQP1 expression in DRG is first detectable at embryonic day 15.5, which corresponds to the developmental stage when the majority of fine cutaneous afferents penetrate the dorsal horn. Electron microscopy revealed dense membrane labeling of unmyelinated axons, a few fine diameter myelinated axons, and synaptic terminals in the superficial dorsal horn. Because this restricted and dense expression suggested that AQP1 contributes to nociceptive processing, we studied behavioral responses of wildtype and AQP1 -/- mice in a comprehensive battery of acute and persistent pain tests. We also used in vivo electrophysiology in wildtype and mutant mice to measure the responses of wide dynamic range neurons in lamina V of the dorsal horn to thermal stimulation before and after noxious stimulus-induced sensitization. To date we have not detected a differential phenotype suggestive of a functional contribution of AQP1 to nociceptive processing.

Molecular mechanisms of conduction and selectivity in aquaporin water channels

Aquaporins (AQP) are a family of membrane channels primarily responsible for conducting water across cellular membranes. The availability of a large body of high resolution structural data along with numerous atomic-scale simulation studies have resulted in an unprecedented level of understanding of the mechanism of function and selectivity in AQP. In this article, after summarizing major highlights of structure-functional studies of AQP, we will report on some of our recent large-scale molecular dynamics simulations investigating the mechanisms of permeation of various substances through pure lipid bilayers and through multiple pathways provided by tetrameric structures of different AQP. Comparison of the results obtained for structurally highly homologous, but functionally distinct, AQP allowed us to identify novel mechanisms of gating and selectivity of these channels and to design mutants with experimentally verified, altered properties. When applicable, special attention will be given to specific aromatic amino acids and their involvement in various functional aspects of AQP.

Aquaporin-1 channel function is positively regulated by protein kinase C

Aquaporin-1 (AQP1) channels contribute to osmotically induced water transport in several organs including the kidney and serosal membranes such as the peritoneum and the pleura. In addition, AQP1 channels have been shown to conduct cationic currents upon stimulation by cyclic nucleotides. To date, the short term regulation of AQP1 function by other major intracellular signaling pathways has not been studied. In the present study, we therefore investigated the regulation of AQP1 by protein kinase C. AQP1 wild type channels were expressed in Xenopus oocytes. Water permeability was assessed by hypotonic challenges. Activation of protein kinase C (PKC) by 1-oleoyl-2-acetyl-sn-glycerol (OAG) induced a marked increase of AQP1-dependent water permeability. This regulation was abolished in mutated AQP1 channels lacking both consensus PKC phosphorylation sites Thr(157) and Thr(239) (termed AQP1 DeltaPKC). AQP1 cationic currents measured with double-electrode voltage clamp were markedly increased after pharmacological activation of PKC by either OAG or phorbol 12-myristate 13-acetate. Deletion of either Thr(157) or Thr(239) caused a marked attenuation of PKC-dependent current increases, and deletion of both phosphorylation sites in AQP1 DeltaPKC channels abolished the effect. In vitro phosphorylation studies with synthesized peptides corresponding to amino acids 154-168 and 236-250 revealed that both Thr(157) and Thr(239) are phosphorylated by PKC. Upon stimulation by cyclic nucleotides, AQP1 wild type currents exhibited a strong activation. This regulation was not affected after deletion of PKC phosphorylation sites in AQP1 DeltaPKC channels. In conclusion, this is the first study to show that PKC positively regulates both water permeability and ionic conductance of AQP1 channels. This new pathway of AQP1 regulation is independent of the previously described cyclic nucleotide pathway and may contribute to the PKC stimulation of AQP1-modulated processes such as endothelial permeability, angiogenesis, and urine concentration.

Expression of aquaporin water channels in rat taste buds

In order to gain insight into the molecular mechanisms that allow taste cells to respond to changes in their osmotic environment, we have used primarily immunocytochemical and molecular approaches to look for evidence of the presence of aquaporin-like water channels in taste cells. Labeling of isolated taste buds from the fungiform, foliate, and vallate papillae in rat tongue with antibodies against several of the aquaporins (AQPs) revealed the presence of AQP1, AQP2, and AQP5 in taste cells from these areas. AQP3 antibodies failed to label isolated taste buds from any of the papillae. There was an apparent difference in the regional localization of AQP labeling within the taste bud. Antibodies against AQP1 and AQP2 labeled predominantly the basolateral membrane, whereas the AQP5 label was clearly evident on both the apical and basolateral membranes of cells within the taste bud. Double labeling revealed that AQP1 and AQP2 labeled many, but not all, of the same taste cells. Similar double-labeling experiments with anti-AQP2 and anti-AQP5 clearly showed that AQP5 was expressed on or near the apical membranes whereas AQP2 was absent from this area. The presence of these 3 types of AQPs in taste buds but not in non-taste bud-containing epithelia was confirmed using reverse transcription-polymerase chain reaction. Experiments using patch clamp recording showed that the AQP inhibitor, tetraethylammonium, significantly reduced hypoosmotic-induced currents in rat taste cells. We hypothesize that the AQPs may play roles both in the water movement underlying compensatory mechanisms for changes in extracellular osmolarity and, in the case of AQP5 in particular, in the gustatory response to water.

Dominant-negative suppression of big brain ion channel activity by mutation of a conserved glutamate in the first transmembrane domain

The neurogenic protein Drosophila big brain (BIB), which is involved in the process of neuroblast determination, and the water channel aquaporin-1 (AQP1) are among a subset of the major intrinsic protein (MIP) channels that have been found to show gated monovalent cation channel activity. A glutamate residue in the first transmembrane (M1) domain is conserved throughout the MIP family. Mutation of this residue to asparagine in BIB (E71N) knocks out ion channel activity, and when coexpressed with BIB wild-type as shown here generates a dominant-negative effect on ion channel function, measured in the Xenopus oocyte expression system using two-electrode voltage clamp. cRNAs for wild-type and mutant BIB or AQP1 channels were injected individually or as mixtures. The magnitude of the BIB ionic conductance response was greatly reduced by coexpression of the mutant E71N subunit, suggesting a dominant-negative mechanism of action. The analogous mutation in AQP1 (E17N) did not impair ion channel activation by cGMP, but did knock out water channel function, although not via a dominant-negative effect. This contrast in sensitivity between BIB and AQP1 to mutation of the M1 glutamate suggests the possibility of interesting structural differences in the molecular basis of the ion permeation between these two classes of channels. The dominant-negative construct of BIB could be a tool for testing a role for BIB ion channels during nervous system development in Drosophila. The neurogenic protein Drosophila big brain (BIB), which is involved in the process of neuroblast determination, and the water channel aquaporin-1 (AQP1) are among a subset of the major intrinsic protein (MIP) channels that have been found to show gated monovalent cation channel activity. A glutamate residue in the first transmembrane (M1) domain is conserved throughout the MIP family. Mutation of this residue to asparagine in BIB (E71N) knocks out ion channel activity, and when coexpressed with BIB wild-type as shown here generates a dominant-negative effect on ion channel function, measured in the Xenopus oocyte expression system using two-electrode voltage clamp. cRNAs for wild-type and mutant BIB or AQP1 channels were injected individually or as mixtures. The magnitude of the BIB ionic conductance response was greatly reduced by coexpression of the mutant E71N subunit, suggesting a dominant-negative mechanism of action. The analogous mutation in AQP1 (E17N) did not impair ion channel activation by cGMP, but did knock out water channel function, although not via a dominant-negative effect. This contrast in sensitivity between BIB and AQP1 to mutation of the M1 glutamate suggests the possibility of interesting structural differences in the molecular basis of the ion permeation between these two classes of channels. The dominant-negative construct of BIB could be a tool for testing a role for BIB ion channels during nervous system development in Drosophila.

Mechanism of gating and ion conductivity of a possible tetrameric pore in aquaporin-1

While substrate permeation through monomeric pores of aquaporins is well characterized, little is known about the possible tetrameric pore. AQP1 has been suggested to function as an ion channel upon cGMP activation, although this idea has been controversial. Taking a theoretical and experimental approach, we demonstrate that the current might arise through the tetrameric pore and propose a plausible mechanism for conduction and gating. In response to simulated ion permeation, immediate hydration of the putative central pore was facilitated by moderate conformational changes of pore-lining residues. cGMP is found to interact with an unusually arginine-rich, cytoplasmic loop (loop D) facilitating its outward motion, which is hypothesized to trigger the opening of a cytoplasmic gate. Physiological analyses of wild-type AQP1 and a designed mutant in which two arginines of the gating loop are replaced by alanine provide experimental support for identifying a key component of the proposed mechanism.

Water channels and zymogen granules in salivary glands

Salivary secretion occurs in response to stimulation by neurotransmitters released from autonomic nerve endings. The molecular mechanisms underlying the secretion of water, a main component of saliva, from salivary glands are not known; the plasma membrane is a major barrier to water transport. A 28-kDa integral membrane protein, distributed in highly water-permeable tissues, was identified as a water channel protein, aquaporin (AQP). Thirteen AQPs (AQP0 - AQP12) have been identified in mammals. AQP5 is localized in lipid rafts under unstimulated conditions and translocates to the apical plasma membrane in rat parotid glands upon stimulation by muscarinic agonists. The importance of increases in intracellular calcium concentration [Ca(2+)](i) and the nitric oxide synthase and protein kinase G signaling pathway in the translocation of AQP5 is reviewed in section I. Signals generated by the activation of Ca(2+) mobilizing receptors simultaneously trigger and regulate exocytosis. Zymogen granule exocytosis occurs under the control of essential process, stimulus-secretion coupling, in salivary glands. Ca(2+) signaling is a principal signal in both protein and water secretion from salivary glands induced by cholinergic stimulation. On the other hand, the cyclic adenosine monophosphate (cAMP)/cAMP-dependent protein kinase system has a major role in zymogen granule exocytosis without significant increases in [Ca(2+)](i). In section II, the mechanisms underlying the control of salivary protein secretion and its dysfunction are reviewed.

Ion channel function of aquaporin-1 natively expressed in choroid plexus

Aquaporins are known as water channels; however, an additional ion channel function has been observed for several including aquaporin-1 (AQP1). Using primary cultures of rat choroid plexus, a brain tissue that secretes CSF and abundantly expresses AQP1, we confirmed the ion channel function of AQP1 and assessed its functional relevance. The cGMP-gated cationic conductance associated with AQP1 is activated by an endogenous receptor guanylate cyclase for atrial natriuretic peptide (ANP). Fluid transport assays with confluent polarized choroid plexus cultures showed that AQP1 current activation by 4.5 mum ANP decreases the normal basal-to-apical fluid transport in the choroid plexus; conversely, AQP1 block with 500 mum Cd2+ restores fluid transport. The cGMP-gated conductance in the choroid plexus is lost with targeted knockdown of AQP1 by small interfering RNA (siRNA), as confirmed by immunocytochemistry and whole-cell patch electrophysiology of transiently transfected cells identified by enhanced green fluorescent protein. The properties of the current (permeability to Na+, K+, TEA+, and Cs+; voltage insensitivity; and dependence on cGMP) matched properties characterized previously in AQP1-expressing oocytes. Background K+ and Cl- currents in the choroid plexus were dissected from AQP1 currents using Cs-methanesulfonate recording salines; the background currents recorded in physiological salines were not affected by AQP1-siRNA treatment. These results confirm that AQP1 can function as both a water channel and a gated ion channel. The conclusion that the AQP1-associated cation current contributes to modulating CSF production resolves a lingering concern as to whether an aquaporin ionic conductance can have a physiologically relevant function.

AQPs and control of vesicle volume in secretory cells

Aquaporins (AQPs) are a family of small, hydrophobic, integral membrane proteins. In mammals, they are expressed in many epithelia and endothelia and function as channels that permit water or small solutes to pass. Although the AQPs reside constitutively at the plasma membrane in most cell types, the presence of AQPs in intracellular organelles such as secretory granules and vesicles has currently been demonstrated. The secretory granules and vesicles contain secretory proteins, migrate to particular locations within the cell close to the plasma membrane and release their contents to the outside. During the process, including exocytosis, regulation of secretory granule or vesicle volume is important. This paper reviews the possible role of AQPs in secretory granules and vesicles.

Enhanced Expression of Multidrug resistance-associated Protein 2 and Reduced Expression of Aquaglyceroporin 3 in an Arsenic-resistant Human Cell Line

Arsenic-resistant cells (R15), derived from a human lung adenocarcinoma cell line (CL3), were 10-fold more resistant to sodium arsenite (As(III)). Because R15 cells accumulated less arsenic than parental CL3 cells, this arsenic resistance may be due to higher efflux and/or lower uptake of As(III). We therefore compared expression of the multidrug resistance-associated proteins MRP1, MRP2, and MRP3 in these two cell lines. MRP2 expression was 5-fold higher in R15 cells than in CL3 cells, whereas MRP1 and MRP3 expression levels were similar. Furthermore, verapamil and cyclosporin A, inhibitors of multidrug resistance transporters, significantly reduced the efflux of arsenic from R15. Thus, increased arsenic extrusion by MRP2 may contribute to arsenic resistance in R15 cells. We also examined the expression of several aquaglyceroporins (AQPs), which mediate As(III) uptake by cells. Little AQP7 or AQP9 mRNA was detected by reverse transcription-PCR in either cell line, whereas AQP3 mRNA expression was 2-fold lower in R15 cells than in CL3 cells. When AQP3 expression in CL3 cells was knocked down by RNA interference, CL3 cells accumulated less arsenic and became more resistant to As(III). Conversely, overexpression of AQP3 in human embryonic kidney 293T cells increased arsenic accumulation, and the cells were more susceptible to As(III) than 293T cells transfected with vector alone. These results suggest that AQP3 is involved in As(III) accumulation. Taken together, our results suggest that enhanced expression of MRP2 and lower expression of AQP3 are responsible for lower arsenic accumulation in arsenic-resistant R15 cells.

Aquaporin-5 water channel in lipid rafts of rat parotid glands

Aquaporin-5 (AQP5), an apical plasma membrane (APM) water channel in salivary glands, lacrimal glands, and airway epithelium, has an important role in fluid secretion. The activation of M3 muscarinic acetylcholine receptors (mAChRs) or alpha1-adrenoceptors on the salivary glands induces salivary fluid secretion. AQP5 localizes in lipid rafts and activation of the M3 mAChRs or alpha1-adrenoceptors induced its translocation together with the lipid rafts to the APM in the interlobular ducts of rat parotid glands. This review focuses on the mechanisms of AQP5 translocation together with lipid rafts to the APM in the interlobular duct cells of parotid glands of normal rats and the impairment of AQP5 translocation in diabetes and senescence.