Quaternary ammonium compounds as water channel blockers. Specificity, potency, and site of action

Disruption of ion homeostasis in the neurogliovascular unit underlies the pathogenesis of ischemic cerebral edema

Cerebral edema is a major cause of morbidity and mortality following ischemic stroke, but its underlying molecular pathophysiology is incompletely understood. Recent data have revealed the importance of ion flux via channels and transporters expressed in the neurogliovascular unit in the development of ischemia-triggered cytotoxic edema, vasogenic edema, and hemorrhagic conversion. Disruption of homeostatic mechanisms governing cell volume regulation and epithelial/endothelial ion transport due to ischemia-associated energy failure results in the thermodynamically driven re-equilibration of solutes and water across the CSF-blood and blood-brain barriers that ultimately increases the brain's extravascular volume. Additionally, hypoxia, inflammation, and other stress-triggered increases in the functional expression of ion channels and transporters normally expressed at low levels in the neurogliovascular unit cause disruptions in ion homeostasis that contribute to ischemic cerebral edema. Here, we review the pathophysiological significance of several molecular mediators of ion transport expressed in the neurogliovascular unit, including targets of existing FDA-approved drugs, which might be potential nodes for therapeutic intervention.

Chondrocyte channel transcriptomics: do microarray data fit with expression and functional data?

To date, a range of ion channels have been identified in chondrocytes using a number of different techniques, predominantly electrophysiological and/or biomolecular; each of these has its advantages and disadvantages. Here we aim to compare and contrast the data available from biophysical and microarray experiments. This letter analyses recent transcriptomics datasets from chondrocytes, accessible from the European Bioinformatics Institute (EBI). We discuss whether such bioinformatic analysis of microarray datasets can potentially accelerate identification and discovery of ion channels in chondrocytes. The ion channels which appear most frequently across these microarray datasets are discussed, along with their possible functions. We discuss whether functional or protein data exist which support the microarray data. A microarray experiment comparing gene expression in osteoarthritis and healthy cartilage is also discussed and we verify the differential expression of 2 of these genes, namely the genes encoding large calcium-activated potassium (BK) and aquaporin channels.

Aquaporin inhibition by gold(III) compounds: new insights

Aquaporins (AQPs) are membrane water/glycerol channels with essential roles in biological systems, as well as being promising targets for therapy and imaging. Using a stopped-flow method, a series of gold(III), platinum(II) and copper(II) complexes bearing nitrogen donor ligands, such as 1,10-phenatroline, 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, 4,4'-diamino-2,2'-bipyridine and 2,2';6',2"-terpyridine, were evaluated in human red blood cells expressing AQP1 and AQP3, responsible for water and glycerol movement, respectively. The results showed that the gold(III) complexes selectively modulate AQP3 over AQP1. Molecular modeling and density functional theory (DFT) calculations were subsequently performed to rationalize the observations and to investigate the possible molecular mechanism through which these gold compounds act on their putative target (AQP3). In the absence of any crystallographic data, a previously reported homology model was used for this purpose. Combined, the findings of this study show that potent and selective modulation of these solute channels is possible, however further investigation is required into the selectivity of this class of agents against all AQP isoforms and their potential therapeutic uses.

Discovery of novel human aquaporin-1 blockers

Human aquaporin-1 (hAQP1) is a water channel found in many tissues and potentially involved in several human pathologies. Selective inhibitors of hAQP1 are discussed as novel treatment opportunities for glaucoma, brain edema, inflammatory pain, and certain types of cancer. However, only very few potent and chemically attractive blockers have been reported to date. In this study we present three novel hAQP1 blockers that have been identified by virtual screening and inhibit water flux through hAQP1 in Xenopus laevis oocyte swelling assays at low micromolar concentrations. The newly discovered compounds display no chemical similarity to hitherto known hAQP1 blockers and bind at the extracellular entrance of the channel, close to the ar/R selectivity filter. Furthermore, mutagenesis studies showed that Lys36, which is not conserved among the hAQP family, is crucially involved in binding and renders the discovered compounds suitable as leads for the development of selective hAQP1 inhibitors.

Venus flytrap biomechanics: forces in the Dionaea muscipula trap

Biomechanics of morphing structures in the Venus flytrap has attracted the attention of scientists during the last 140 years. The trap closes in a tenth of a second if a prey touches a trigger hair twice. The driving force of the closing process is most likely due to the elastic curvature energy stored and locked in the leaves, which is caused by a pressure differential between the upper and lower layers of the leaf. The trap strikes, holds and compresses the prey. We have developed new methods for measuring all these forces involved in the hunting cycle. We made precise calibration of the piezoelectric sensor and performed direct measurements of the average impact force of the trap closing using a high speed video camera for the determination of time constants. The new equation for the average impact force was derived. The impact average force between rims of two lobes in the Venus flytrap was found equal to 149 mN and the corresponding pressure between the rims was about 41 kPa. Direct measurements of the constriction force in the trap of Dionaea muscipula was performed during gelatin digestion. This force increases in the process of digestion from zero to 450 mN with maximal constriction pressure created by the lobes reaching to 9 kPa. The insects and different small prey have little chance to escape after the snap of the trap. The prey would need to overpower the "escaping" force which is very strong and can reach up to 4N.

Cell volume regulation in chondrocytes

Chondrocytes are the cells within cartilage which produce and maintain the extracellular matrix. Volume regulation in these cells is vital to their function and occurs in several different physiological and pathological contexts. Firstly, chondrocytes exist within an environment of changing osmolarity and compressive loads. Secondly, in osteoarthritic joint failure, cartilage water content changes and there is a notable increase in chondrocyte apoptosis. Thirdly, endochondral ossification requires chondrocyte swelling in association with hypertrophy. Regulatory volume decrease (RVD) and regulatory volume increase (RVI) have both been observed in articular chondrocytes and this review focuses on the mechanisms identified to account for these. There has been evidence so far to suggest TRPV4 is central to RVD; however other elements of the pathway have not yet been identified. Unlike RVD, RVI appears less robust in articular chondrocytes and there have been fewer mechanistic studies; the primary focus being on the Na(+)-K(+)-2Cl(-) co-transporter. The clinical significance of chondrocyte volume regulation remains unproven. Importantly however, transcript abundances of several ion channels implicated in volume control are changed in chondrocytes from osteoarthritic cartilage. A critical question is whether disturbances of volume regulation mechanisms lead to, result from or are simply coincidental to cartilage damage.

Aquaporin-9 protein is the primary route of hepatocyte glycerol uptake for glycerol gluconeogenesis in mice

It has been hypothesized that aquaporin-9 (AQP9) is part of the unknown route of hepatocyte glycerol uptake. In a previous study, leptin receptor-deficient wild-type mice became diabetic and suffered from fasting hyperglycemia whereas isogenic AQP9(-/-) knock-out mice remained normoglycemic. The reason for this improvement in AQP9(-/-) mice was not established before. Here, we show increased glucose output (by 123% ± 36% S.E.) in primary hepatocyte culture when 0.5 mM extracellular glycerol was added. This increase depended on AQP9 because it was absent in AQP9(-/-) cells. Likewise, the increase was abolished by 25 μM HTS13286 (IC(50) ~ 2 μM), a novel AQP9 inhibitor, which we identified in a small molecule library screen. Similarly, AQP9 deletion or chemical inhibition eliminated glycerol-enhanced glucose output in perfused liver preparations. The following control experiments suggested inhibitor specificity to AQP9: (i) HTS13286 affected solute permeability in cell lines expressing AQP9, but not in cell lines expressing AQPs 3, 7, or 8. (ii) HTS13286 did not influence lactate- and pyruvate-dependent hepatocyte glucose output. (iii) HTS13286 did not affect glycerol kinase activity. Our experiments establish AQP9 as the primary route of hepatocyte glycerol uptake for gluconeogenesis and thereby explain the previously observed, alleviated diabetes in leptin receptor-deficient AQP9(-/-) mice.

Functional characterization of water transport and cellular localization of three aquaporin paralogs in the salmonid intestine

Intestinal water absorption is greatly enhanced in salmonids upon acclimation from freshwater (FW) to seawater (SW); however, the molecular mechanism for water transport is unknown. We conducted a pharmacological characterization of water absorption in the rainbow trout intestine along with an investigation of the distribution and cellular localization of three aquaporins (Aqp1aa, -1ab, and -8ab) in pyloric caeca, middle (M), and posterior (P) intestine of the Atlantic salmon. In vitro iso-osmotic water absorption (J(v)) was higher in SW than FW-trout and was inhibited by (mmol L(-1)): 0.1 KCN (41%), 0.1 ouabain (72%), and 0.1 bumetanide (82%) suggesting that active transport, Na(+), K(+)-ATPase and Na(+), K(+), 2Cl(-)-co-transport are involved in establishing the driving gradient for water transport. J(v) was also inhibited by 1 mmol L(-1) HgCl(2), serosally (23% in M and 44% in P), mucosally (27% in M), or both (61% in M and 58% in P), suggesting involvement of both apical and basolateral aquaporins in water transport. The inhibition was antagonized by 5 mmol L(-1) mercaptoethanol. By comparison, 10 mmol L(-1) mucosal tetraethylammonium, an inhibitor of certain aquaporins, inhibited J(v) by 20%. In the presence of glucose, mucosal addition of phloridzin inhibited water transport by 20%, suggesting that water transport is partially linked to the Na(+)-glucose co-transporter. Using polyclonal antibodies against salmon Aqp1aa, -1ab, and -8ab, we detected Aqp1aa, and -1ab immunoreactivity in the brush border and sub-apical region of enterocytes in all intestinal segments. The Aqp8ab antibody showed a particularly strong immunoreaction in the brush border and sub-apical region of enterocytes throughout the intestine and also stained lateral membranes and peri-nuclear regions though at lower intensity. The present localization of three aquaporins in both apical and lateral membranes of salmonid enterocytes facilitates a model for transcellular water transport in the intestine of SW-acclimated salmonids.

Mercury inhibits the L170C mutant of aquaporin Z by making waters clog the water channel

We conduct in silico experiments of the L170C mutant of the Escherichia coli aquaporin Z (AQPZ) with and without mercury bonded to residue Cys 170. We find that bonding mercury to Cys 170 does not induce consequential structural changes to the protein. We further find that mercury does not stick in the middle of the water channel to simply occlude water permeation, but resides on the wall of the water pore. However, we observe that the water permeation coefficient of L170C-Hg(+) (with one mercury ion bonded to Cys 170) is approximately half of that of the mercury-free L170C. We examine the interactions between the mercury ion and the waters in its vicinity and find that five to six waters are strongly attracted by the mercury ion, occluding the space of the water channel. Therefore we conclude that mercury, at low concentration, inhibits AQPZ-L170C mutant by making water molecules clog the water channel.

Aquaporin water channel AgAQP1 in the malaria vector mosquito Anopheles gambiae during blood feeding and humidity adaptation

Altered patterns of malaria endemicity reflect, in part, changes in feeding behavior and climate adaptation of mosquito vectors. Aquaporin (AQP) water channels are found throughout nature and confer high-capacity water flow through cell membranes. The genome of the major malaria vector mosquito Anopheles gambiae contains at least seven putative AQP sequences. Anticipating that transmembrane water movements are important during the life cycle of A. gambiae, we identified and characterized the A. gambiae aquaporin 1 (AgAQP1) protein that is homologous to AQPs known in humans, Drosophila, and sap-sucking insects. When expressed in Xenopus laevis oocytes, AgAQP1 transports water but not glycerol. Similar to mammalian AQPs, water permeation of AgAQP1 is inhibited by HgCl(2) and tetraethylammonium, with Tyr185 conferring tetraethylammonium sensitivity. AgAQP1 is more highly expressed in adult female A. gambiae mosquitoes than in males. Expression is high in gut, ovaries, and Malpighian tubules where immunofluorescence microscopy reveals that AgAQP1 resides in stellate cells but not principal cells. AgAQP1 expression is up-regulated in fat body and ovary by blood feeding but not by sugar feeding, and it is reduced by exposure to a dehydrating environment (42% relative humidity). RNA interference reduces AgAQP1 mRNA and protein levels. In a desiccating environment (<20% relative humidity), mosquitoes with reduced AgAQP1 protein survive significantly longer than controls. These studies support a role for AgAQP1 in water homeostasis during blood feeding and humidity adaptation of A. gambiae, a major mosquito vector of human malaria in sub-Saharan Africa.

A high throughput screen identifies chemical modulators of the laminin-induced clustering of dystroglycan and aquaporin-4 in primary astrocytes

BACKGROUND

Aquaporin-4 (AQP4) constitutes the principal water channel in the brain and is clustered at the perivascular astrocyte endfeet. This specific distribution of AQP4 plays a major role in maintaining water homeostasis in the brain. A growing body of evidence points to a role of the dystroglycan complex and its interaction with perivascular laminin in the clustering of AQP4 at perivascular astrocyte endfeet. Indeed, mice lacking components of this complex or in which laminin-dystroglycan interaction is disrupted show a delayed onset of brain edema due to a redistribution of AQP4 away from astrocyte endfeet. It is therefore important to identify inhibitory drugs of laminin-dependent AQP4 clustering which may prevent or reduce brain edema.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study we used primary rat astrocyte cultures to screen a library of >3,500 chemicals and identified 6 drugs that inhibit the laminin-induced clustering of dystroglycan and AQP4. Detailed analysis of the inhibitory drug, chloranil, revealed that its inhibition of the clustering is due to the metalloproteinase-2-mediated ß-dystroglycan shedding and subsequent loss of laminin interaction with dystroglycan. Furthermore, chemical variants of chloranil induced a similar effect on ß-dystroglycan and this was prevented by the antioxidant N-acetylcysteine.

CONCLUSION/SIGNIFICANCE

These findings reveal the mechanism of action of chloranil in preventing the laminin-induced clustering of dystroglycan and AQP4 and validate the use of high-throughput screening as a tool to identify drugs that modulate AQP4 clustering and that could be tested in models of brain edema.

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.

Aquaporin 1, a potential therapeutic target for migraine with aura

The pathophysiology of migraine remains largely unknown. However, evidence regarding the molecules participating in the pathophysiology of migraine has been accumulating. Water channel proteins, known as aquaporins (AQPs), notably AQP-1 and AQP-4, appears to be involved in the pathophysiology of several neurological diseases. This review outlines newly emerging evidence indicating that AQP-1 plays an important role in pain signal transduction and migraine and could therefore serve as a potential therapeutic target for these diseases.

Exploring transmembrane diffusion pathways with molecular dynamics

Transmembrane exchange of materials is a fundamental process in biology. Molecular dynamics provides a powerful method to investigate in great detail various aspects of the phenomenon, particularly the permeation of small uncharged molecules, which continues to pose a challenge to experimental studies. We will discuss some of the recent simulation studies investigating the role of lipid-mediated and protein-mediated mechanisms in permeation of water and gas molecules across the membrane.

Piscine aquaporins: an overview of recent advances

Aquaporins are a superfamily of integral membrane proteins that facilitate the rapid and yet highly selective flux of water and other small solutes across biological membranes. Since their discovery, they have been documented throughout the living biota, with the majority of research focusing on mammals and plants. Here, we review available data for piscine aquaporins, including Agnatha (jawless fish), Chondrichthyes (chimaeras, sharks, and rays), Dipnoi (lungfishes), and Teleostei (ray-finned bony fishes). Recent evidence suggests that the aquaporin superfamily has specifically expanded in the chordate lineage consequent to serial rounds of whole genome duplication, with teleost genomes harboring the largest number of paralogs. The selective retention and dichotomous clustering of most duplicated paralogs in Teleostei, with differential tissue expression profiles, implies that novel or specialized physiological functions may have evolved in this clade. The recently proposed new nomenclature of the piscine aquaporin superfamily is discussed in relation to the phylogenetic signal and genomic synteny, with the teleost aquaporin-8 paralogs used as a case study to illustrate disparities between the underlying codons, molecular phylogeny, and physical locus. Structural data indicate that piscine aquaporins display similar channel restriction residues found in the tetrapod counterparts, and hence their functional properties seem to be conserved. However, emerging evidence suggests that regulation of aquaporin function in teleosts may have diverged in some cases. Cell localization and experimental studies imply that the physiological roles of piscine aquaporins extend at least to osmoregulation, reproduction, and early development, although in most cases their specific functions remain to be elucidated.

Transgenic expression of AQP1 in the fiber cells of AQP0 knockout mouse: effects on lens transparency

Mutations and knockout of aquaporin 0 (AQP0) result in dominant lens cataract. To date, several functions have been proposed for AQP0; however, two functions, water permeability and cell-to-cell adhesion have been supported by several investigators and only water channel function has been readily authenticated by in vitro and ex vivo studies. Lens shifts protein expression from the more efficient AQP1 in the equatorial epithelial cells to the less efficient water channel, AQP0, in the differentiating secondary fiber cells; perhaps, AQP0 performs a distinctive function. If AQP0 has only water permeability function, can the more efficient water channel AQP1 transgenically expressed in the fiber cells compensate and restore lens transparency in the AQP0 knockout (AQP0(-/-)) mouse? To investigate, we generated a transgenic wild-type mouse line expressing AQP1 in the fiber cells using alphaA-crystallin promoter. These transgenic mice (TgAQP1(+/+)) showed increase in fiber cell membrane water permeability without any morphological, anatomical or physiological defects compared to the wild type indicating that the main purpose of the shift in expression from AQP1 to AQP0 may not be to lessen the membrane water permeability. Further, we transgenically expressed AQP1 in the lens fiber cells of AQP0 knockout mouse (TgAQP1(+/+)/AQP0(-/-)) to determine whether AQP1 could restore AQP0 water channel function and regain lens transparency. Fiber cells of these mice showed 2.6 times more water permeability than the wild type. Transgene AQP1 reduced the severity of lens cataract and prevented dramatic acceleration of cataractogenesis. However, lens fiber cells showed deformities and lack of compact cellular architecture. Loss of lens transparency due to the absence of AQP0 was not completely restored indicating an additional function for AQP0. In vitro studies showed that AQP0 is capable of cell-to-cell adhesion while AQP1 is not. To our knowledge, this is the first report which uses an animal model to demonstrate that AQP0 may have an additional function, possibly cell-to-cell adhesion.

42K analysis of sodium-induced potassium efflux in barley: mechanism and relevance to salt tolerance

*Stimulation of potassium (K(+)) efflux by sodium (Na(+)) has been the subject of much recent attention, and its mechanism has been attributed to the activities of specific classes of ion channels. *The short-lived radiotracer (42)K(+) was used to test this attribution, via unidirectional K(+)-flux analysis at the root plasma membrane of intact barley (Hordeum vulgare), in response to NaCl, KCl, NH(4)Cl and mannitol, and to channel inhibitors. *Unidirectional K(+) efflux was strongly stimulated by NaCl, and K(+) influx strongly suppressed. Both effects were ameliorated by elevated calcium (Ca(2+)). As well, K(+) efflux was strongly stimulated by KCl, NH(4)Cl and mannitol , and NaCl also stimulated (13)NH(4)(+) efflux. The Na(+)-stimulated K(+) efflux was insensitive to cesium (Cs(+)) and pH 4.2, weakly sensitive to the K(+)-channel blocker tetraethylammonium (TEA(+)) and quinine, and moderately sensitive to zinc (Zn(2+)) and lanthanum (La(3+)). *We conclude that the stimulated efflux is: specific neither to Na(+) as effector nor K(+) as target; composed of fluxes from both cytosol and vacuole; mediated neither by outwardly-rectifying K(+) channels nor nonselective cation channels; attributable, alternatively, to membrane disintegration brought about by ionic and osmotic components; of limited long-term significance, unlike the suppression of K(+) influx by Na(+), which is a greater threat to K(+) homeostasis under salt stress.

Aquaporin-1: New Developments and Perspectives for Peritoneal Dialysis

Peritoneal dialysis involves diffusive and convective transport and osmosis through the highly vascularized peritoneal membrane. Several lines of evidence have demonstrated that the water channel aquaporin-1 (AQP1) corresponds to the ultrasmall pore predicted by the model of peritoneal transport. Proof-of-principle studies have shown that upregulation of the expression of AQP1 in peritoneal capillaries results in increased water permeability and ultrafiltration, without affecting the osmotic gradient or small solute permeability. Conversely, studies in Aqp1 mice have shown that haplo-insufficiency for AQP1 results in significant attenuation of water transport. Recent studies have demonstrated that AQP1 is involved in the migration of different cell types, including endothelial cells. In parallel, chemical screening has identified lead compounds that could act as antagonists or agonists of AQPs, with description of putative binding sites and potential mechanisms of gating the water channel. By modulating water transport, these pharmacological agents could have clinically relevant effects in targeting specific tissues or disease states.

Metalloid transport by aquaglyceroporins: consequences in the treatment of human diseases

Metalloids can severely harm human physiology in a toxicological sense if taken up from the environment in acute high doses or chronically. However, arsenic or antimony containing drugs are still being used as treatment and are often the sole regime for certain forms of cancer, mainly types of leukemia and diseases caused by parasites, such as sleeping sickness or leishmaniasis. In this chapter, we give an outline of the positive effects of arsenicals and antimonials against such diseases, we summarize data on uptake pathways through human and parasite aquaglyceroporins and we discuss the progress and options in the development of therapeutic aquaporin and aquaglyceroporin inhibitor compounds.

Automated cell-based assay for screening of aquaporin inhibitors

Aquaporins form water channels that play major roles in a variety of physiological processes so that altered expression or function may underlie pathological conditions. In order to identify compounds that modulate aquaporin function, we have implemented a functional assay based on rapid measurement of osmotically induced cell volume changes to screen several libraries of diverse drugs. The time course of fluorescence changes in calcein-loaded cells was analyzed during an osmotic challenge using a 96-multiwell fluorescence plate reader. This system was validated using astrocyte primary cultures and fibroblasts that strongly express endogenous AQP4 and AQP1 proteins, respectively, as well as AQP4-transfected cells. We screened 3575 compounds, including 418 FDA-approved and commercially available drugs, for their effect on AQP-mediated water transport. Primary screening yielded 10 compounds that affected water transport activity in both astrocytes and AQP4-transfected cells and 42 compounds that altered cell volume regulation in astrocytes. Selected drugs were then analyzed on AQP1-expressing erythrocytes and AQP4-expressing membrane vesicles by stopped-flow light scattering. Four molecules of the National Cancer Institute's chemical library (NSC164914, NSC670229, NSC168597, NSC301460) were identified that differentially affected both AQP4 and AQP1 mediated water transport, with EC50 values between 20 and 50 microM. This fluorescence microplate reader-based assay may, thus, provide a platform for high-throughput screening which, when coupled to a secondary evaluation to confirm target specificity, should allow discovery of AQP-specific compounds for novel therapeutic strategies in the treatment of water balance disorders.

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.

Novel therapeutic targets in Plasmodium falciparum: aquaglyceroporins

BACKGROUND

Malaria is caused by the intracellular parasite Plasmodium falciparum. The constant need for novel malaria therapies is due to the development of resistance against existing drugs.

OBJECTIVE

To summarise attempts to investigate parasitic aquaporins as drug targets in malaria.

METHODS

Starting with a summary of the history of malaria we present aquaporin structure and function relationships. Potential interactions of inhibitors with plasmodial AQP (PfAQP) are discussed. PfAQP blockage is examined in the light of recent work on knock-out parasites. Since PfAQP is able to transport other small solutes the parasites are sensitive to other compounds which are harmless to the human host.

RESULTS/CONCLUSIONS

Total blockage of PfAQP may not lead to the death of the parasite but application of PfAQP as a vehicle for toxic substances may be a further pathway for research.

Syntaxin specificity of aquaporins in the inner medullary collecting duct

Proper targeting of the aquaporin-2 (AQP2) water channel to the collecting duct apical plasma membrane is critical for the urine concentrating mechanism and body water homeostasis. However, the trafficking mechanisms that recruit AQP2 to the plasma membrane are still unclear. Snapin is emerging as an important mediator in the initial interaction of trafficked proteins with target soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (t-SNARE) proteins, and this interaction is functionally important for AQP2 regulation. We show that in AQP2-Madin-Darby canine kidney cells subjected to adenoviral-mediated expression of both snapin and syntaxins, the association of AQP2 with both syntaxin-3 and syntaxin-4 is highly enhanced by the presence of snapin. In pull-down studies, snapin detected AQP2, syntaxin-3, syntaxin-4, and SNAP23 from the inner medullary collecting duct. AQP2 transport activity, as probed by AQP2's urea permeability, was greatly enhanced in oocytes that were coinjected with cRNAs of SNARE components (snapin+syntaxin-3+SNAP23) over those injected with AQP2 cRNA alone. It was not enhanced when syntaxin-3 was replaced by syntaxin-4 (snapin+syntaxin-4+SNAP23). On the other hand, the latter combination significantly enhanced the transport activity of the related AQP3 water channel while the presence of syntaxin-3 did not. This AQP-syntaxin interaction agrees with the polarity of these proteins' expression in the inner medullary collecting duct epithelium. Thus our findings suggest a selectivity of interactions between different aquaporin and syntaxin isoforms, and thus in the regulation of AQP2 and AQP3 activities in the plasma membrane. Snapin plays an important role as a linker between the water channel and the t-SNARE complex, leading to the fusion event, and the pairing with specific t-SNAREs is essential for the specificity of membrane recognition and fusion.

Inhibition of aquaporin-1 and aquaporin-4 water permeability by a derivative of the loop diuretic bumetanide acting at an internal pore-occluding binding site

Aquaporin (AQP) water channels, essential for fluid homeostasis, are expressed in perivascular brain end-feet regions of astroglia (AQP4) and in choroid plexus (AQP1). At a high concentration, the loop diuretic bumetanide has been shown to reduce rat brain edema after ischemic stroke by blocking Na(+)-K(+)-2Cl(-) cotransport. We hypothesized that an additional inhibition of AQP contributes to the protection. We show that osmotic water flux in AQP4-expressing Xenopus laevis oocytes is reduced by extracellular bumetanide (> or =100 microM). The efficacy of block by bumetanide is increased by injection intracellularly. Forty-five synthesized bumetanide derivatives were tested on oocytes expressing human AQP1 and rat AQP4. Of these, one of the most effective was the 4-aminopyridine carboxamide analog, AqB013, which inhibits AQP1 and AQP4 (IC(50) approximately 20 microM, applied extracellularly). The efficacy of block was enhanced by mutagenesis of intracellular AQP4 valine-189 to alanine (V189A, IC(50) approximately 8 microM), confirming the aquaporin as the molecular target of block. In silico docking of AqB013 supported an intracellular candidate binding site in rat AQP4 and suggested that the block involves occlusion of the AQP water pore at the cytoplasmic side. AqB013 at 2 microM had no effect, and 20 microM caused 20% block of human Na(+)-K(+)-2Cl(-) cotransporter activity, in contrast to >90% block of the transporter by bumetanide. AqB013 did not affect X. laevis oocyte Cl(-) currents and did not alter rhythmic electrical conduction in an ex vivo gastric muscle preparation. The identification of AQP-selective pharmacological agents opens opportunities for breakthrough strategies in the treatment of edema and other fluid imbalance disorders.

Crystal structure of human aquaporin 4 at 1.8 A and its mechanism of conductance

Aquaporin (AQP) 4 is the predominant water channel in the mammalian brain, abundantly expressed in the blood-brain and brain-cerebrospinal fluid interfaces of glial cells. Its function in cerebral water balance has implications in neuropathological disorders, including brain edema, stroke, and head injuries. The 1.8-A crystal structure reveals the molecular basis for the water selectivity of the channel. Unlike the case in the structures of water-selective AQPs AqpZ and AQP1, the asparagines of the 2 Asn-Pro-Ala motifs do not hydrogen bond to the same water molecule; instead, they bond to 2 different water molecules in the center of the channel. Molecular dynamics simulations were performed to ask how this observation bears on the proposed mechanisms for how AQPs remain totally insulating to any proton conductance while maintaining a single file of hydrogen bonded water molecules throughout the channel.

Identification, functional characterization and expression patterns of a water-specific aquaporin in the brown dog tick, Rhipicephalus sanguineus

Much is known about the physiology of tick salivation, but nothing is known about the movement of water through the cell membranes of salivary glands, a phenomenon usually associated with water channels or aquaporins (AQPs). An AQP, RsAQP1, was identified in a salivary gland cDNA library of Rhipicephalus sanguineus. In the first functional characterization of an acarine AQP, Xenopus oocytes expressing RsAQP1 became water permeable, whereas RsAQP1 did not transport glycerol or urea. RsAQP1 was inhibited by Hg(2+) but not by triethylammonium. Treatment with a protein kinase A activator (cAMP) had no effect on RsAQP1 transport, whereas treatment with a protein kinase C activator (phorbol 12,13-dibutyrate) reduced water flux by 60%. RsAQP1 transcript was present in unfed larvae, nymphs and adult R. sanguineus, but absent in embryos. Partially fed female R. sanguineus expressed RsAQP1 in gut, Malpighian tubules and was particularly abundant in salivary gland tissue, but absent in ovary and synganglion tissues. Because of the importance of water management in tick biology for both the off-host and on-host phases of the life cycle, our findings on tick AQP1 represent a major advancement in our understanding of tick osmoregulation that could potentially be exploited in tick control.

In vitro analysis and modification of aquaporin pore selectivity

Aquaporins enable the passage of a diverse set of solutes besides water. Many novel aquaporin permeants, such as antimonite and arsenite, silicon, ammonia, and hydrogen peroxide, have been described very recently. By the same token, the number of available aquaporin sequences has rapidly increased. Yet, sequence analyses and structure models cannot reliably predict permeability properties. Even the contribution to pore selectivity of individual residues in the channel layout is not fully understood. Here, we describe and discuss established in vitro assays for water and solute permeability. Measurements of volume change due to flux along osmotic or chemical gradients yield quantitative biophysical data, whereas phenotypic growth assays can hint at the relevance of aquaporins in the physiological setting of a certain cell. We also summarize data on the modification of pore selectivity of the prototypical water-specific mammalian aquaporin-1. We show that replacing residues in the pore constriction region allows ammonia, urea, glycerol, and even protons to pass the aquaporin pore.

Protonophore- and pH-insensitive glucose and sucrose accumulation detected by FRET nanosensors in Arabidopsis root tips

Although soil contains only traces of soluble carbohydrates, plant roots take up glucose and sucrose efficiently when supplied in artificial media. Soluble carbohydrates and other small metabolites found in soil are in part products from exudation from plant roots. The molecular nature of the transporters for uptake and exudation is unknown. Here, fluorescence resonance energy transfer (FRET) glucose and sucrose sensors were used to characterize accumulation and elimination of glucose and sucrose in Arabidopsis roots tips. Using an improved image acquisition set-up, FRET responses to perfusion with carbohydrates were detectable in roots within less than 10 sec and over a wide concentration range. Accumulation was fully reversible within 10-180 sec after glucose or sucrose had been withdrawn; elimination may be caused by metabolism and/or efflux. The rate of elimination was unaffected by pre-incubation with high concentrations of glucose, suggesting that elimination is not due to accumulation in a short-term buffer such as the vacuole. Glucose and sucrose accumulation was insensitive to protonophores, was comparable in media differing in potassium levels, and was similar at pH 5.8, 6.8 and 7.8, suggesting that both influx and efflux may be mediated by proton-independent transport systems. High-resolution expression mapping in root tips showed that only a few proton-dependent transport of the STP (Sugar Transport Protein) and SUT/SUC (Sucrose Transporter/Carrier) families are expressed in the external cell layers of root tips. The root expression maps may help to pinpoint candidate genes for uptake and release of carbohydrates from roots.

Homeostatic maintenance of pathogen-containing vacuoles requires TBK1-dependent regulation of aquaporin-1

Membranes are an integral component of many cellular functions and serve as a barrier to keep pathogenic bacteria from entering the nutrient-rich host cytosol. TANK-binding-kinase-1 (TBK1), a kinase of the IkappaB kinase family, is required for maintaining integrity of pathogen-containing vacuoles (PCV) upon bacterial invasion of host cells. Here we investigate how vacuolar integrity is maintained during bacterial infection, even in the presence of bacterial membrane damaging agents. We found that Aquaporin-1 (AQP1), a water channel that regulates swelling of secretory vesicles, associated with PCV. AQP1 levels were elevated in TBK1-deficient cells, and overexpression of AQP1 in wild-type cells led to PCV destabilization, similar to that observed in tbk1(-/-) cells. Inhibition of physiological levels of AQP1 in multiple cell types also led to increased instability of PCV, demonstrating a need for tightly regulated AQP1 function to maintain vacuole homeostasis during bacterial infection. AQP1-dependent modulation of PCV was triggered by bacterially induced membrane damage and ion flux. These results highlight the contribution of water channels to promoting PCV membrane integrity, and reveal an unexpected role for TBK1 and AQP1 in restricting bacterial pathogens to the vacuolar compartment.

Active movements in plants: Mechanism of trap closure by Dionaea muscipula Ellis

The Venus flytrap (Dionaea muscipula Ellis) captures insects with one of the most rapid movements in the plant kingdom. We investigated trap closure by mechanical and electrical stimuli using the novel charge-injection method and high-speed recording. We proposed a new hydroelastic curvature mechanism, which is based on the assumption that the lobes possess curvature elasticity and are composed of outer and inner hydraulic layers with different hydrostatic pressure. The open state of the trap contains high elastic energy accumulated due to the hydrostatic pressure difference between the hydraulic layers of the lobe. Stimuli open pores connecting the two layers, water rushes from one hydraulic layer to another, and the trap relaxes to the equilibrium configuration corresponding to the closed state. In this paper we derived equations describing this system based on elasticity Hamiltonian and found closing kinetics. The novel charge-injection stimulation method gives insight into mechanisms of the different steps of signal transduction and response in the plant kingdom.

Channels for water efflux and influx involved in volume regulation of murine spermatozoa

The nature of the membrane channels mediating water transport in murine spermatozoa adjusting to anisotonic conditions was investigated. The volume of spermatozoa subjected to physiologically relevant hypotonic conditions either simultaneously, or after isotonic pre-incubation, with putative water transport inhibitors was monitored. Experiments in which quinine prevented osmolyte efflux, and thus regulatory volume decrease (RVD), revealed whether water influx or efflux was being inhibited. There was no evidence that sodium-dependent solute transporters or facilitative glucose transporters were involved in water transport during RVD of murine spermatozoa since phloretin, cytochalasin B and phloridzin had no effect on volume regulation. However, there was evidence that Hg(2+)- and Ag(+)-sensitive channels were involved in water transport and the possibility that they include aquaporin 8 is discussed. Toxic effects of these heavy metals were ruled out by evidence that mitochondrial poisons had no such effect on volume regulation.

Hyponatremia revisited: translating physiology to practice

The complexity of hyponatremia as a clinical problem is likely caused by the opposite scenarios that accompany this electrolyte disorder regarding pathophysiology (depletional versus dilutional hyponatremia, high versus low vasopressin levels) and therapy (rapid correction to treat cerebral edema versus slow correction to prevent osmotic demyelination, fluid restriction versus fluid resuscitation). For a balanced differentiation between these opposites, an understanding of the pathophysiology of hyponatremia is required. Therefore, in this review an attempt is made to translate the physiology of water balance regulation to strategies that improve the clinical management of hyponatremia. A physiology-based approach to the patient with hyponatremia is presented, first addressing the possibility of acute hyponatremia, and then asking if and if so why vasopressin is secreted non-osmotically. Additional diagnostic recommendations are not to rely too heavily of the assessment of the extracellular fluid volume, to regard the syndrome of inappropriate antidiuresis as a diagnosis of exclusion, and to rationally investigate the pathophysiology of hyponatremia rather than to rely on isolated laboratory values with arbitrary cutoff values. The features of the major hyponatremic disorders are discussed, including diuretic-induced hyponatremia, adrenal and pituitary insufficiency, the syndrome of inappropriate antidiuresis, cerebral salt wasting, and exercise-associated hyponatremia. The treatment of hyponatremia is reviewed from simple saline solutions to the recently introduced vasopressin receptor antagonists, including their promises and limitations. Given the persistently high rates of hospital-acquired hyponatremia, the importance of improving the management of hyponatremia seems both necessary and achievable.

Kinetics and mechanism of Dionaea muscipula trap closing

The Venus flytrap (Dionaea muscipula) possesses an active trapping mechanism to capture insects with one of the most rapid movements in the plant kingdom, as described by Darwin. This article presents a detailed experimental investigation of trap closure by mechanical and electrical stimuli and the mechanism of this process. Trap closure consists of three distinctive phases: a silent phase with no observable movement; an accelerated movement of the lobes; and the relaxation of the lobes in their closed state, resulting in a new equilibrium. Uncouplers and blockers of membrane channels were used to investigate the mechanisms of different phases of closing. Uncouplers increased trap closure delay and significantly decreased the speed of trap closure. Ion channel blockers and aquaporin inhibitors increased time of closing. Transmission of a single electrical charge between a lobe and the midrib causes closure of the trap and induces an electrical signal propagating between both lobes and midrib. The Venus flytrap can accumulate small subthreshold charges, and when the threshold value is reached, the trap closes. Repeated application of smaller charges demonstrates the summation of stimuli. The cumulative character of electrical stimuli points to the existence of electrical memory in the Venus flytrap. The observed fast movement can be explained by the hydroelastic curvature model without invoking buckling instability. The new hydroelastic curvature mechanism provides an accurate description of the authors' experimental data.

Is TEA an inhibitor for human Aquaporin-1?

Excessive water uptake through aquaporins can be life threatening, and disregulation of water permeability causes many diseases. Therefore, reversible aquaporin inhibitors are highly desired. In this paper, we identified the binding site for tetraethylammonium (TEA) of the membrane water channel aquaporin-1 by a combined molecular docking and molecular dynamics simulation approach. The binding site identified from docking studies was independently confirmed with an unbiased molecular dynamics simulation of an aquaporin tetramer embedded in a lipid membrane, surrounded by a 100-mM tetraethylammonium solution in water. A third independent assessment of the binding site was obtained by umbrella sampling simulations. These simulations, in addition, revealed a binding affinity of more than 17 kJ/mol, corresponding to an IC₅₀ value of << 3 mM. Finally, we observed in our simulations a 50% reduction of the water flux in the presence of TEA, in agreement with water permeability measurements on aquaporin expressed in oocytes. These results confirm TEA as a putative lead for an aquaporin-1 inhibitor.

Identification of aquaporin 4 inhibitors using in vitro and in silico methods

The in vitro inhibitory effects and in silico docking energies of 18 compounds with respect to aquaporin 4 (AQP4) were investigated. More than half of the compounds tested showed inhibitory activity in the in vitro functional assay and included the 5-HT(1B/1D) agonists sumatriptan, and rizatriptan. Moreover, the observed inhibitory activity of the compounds used in this study at 20 microM showed a strong correlation with their in silico docking energies, r(2)=0.64, which was consistent with that found in previous studies. The AQP4 inhibitory IC(50) values of three compounds, 2-(nicotinamido)-1,3,4-thiadiazole, sumatriptan and rizatriptan, were subsequently found to be 3, 11, and 2 microM, respectively.

Alleviation of rapid, futile ammonium cycling at the plasma membrane by potassium reveals K+-sensitive and -insensitive components of NH4+ transport

Futile plasma membrane cycling of ammonium (NH4+) is characteristic of low-affinity NH4+ transport, and has been proposed to be a critical factor in NH4+ toxicity. Using unidirectional flux analysis with the positron-emitting tracer 13N in intact seedlings of barley (Hordeum vulgare L.), it is shown that rapid, futile NH4+ cycling is alleviated by elevated K+ supply, and that low-affinity NH4+ transport is mediated by a K+-sensitive component, and by a second component that is independent of K+. At low external [K+] (0.1 mM), NH4+ influx (at an external [NH4+] of 10 mM) of 92 micromol g(-1) h(-1) was observed, with an efflux:influx ratio of 0.75, indicative of rapid, futile NH4+ cycling. Elevating K+ supply into the low-affinity K+ transport range (1.5-40 mM) reduced both influx and efflux of NH4+ by as much as 75%, and substantially reduced the efflux:influx ratio. The reduction of NH4+ fluxes was achieved rapidly upon exposure to elevated K+, within 1 min for influx and within 5 min for efflux. The channel inhibitor La3+ decreased high-capacity NH4+ influx only at low K+ concentrations, suggesting that the K+-sensitive component of NH4+ influx may be mediated by non-selective cation channels. Using respiratory measurements and current models of ion flux energetics, the energy cost of concomitant NH4+ and K+ transport at the root plasma membrane, and its consequences for plant growth are discussed. The study presents the first demonstration of the parallel operation of K+-sensitive and -insensitive NH4+ flux mechanisms in plants.

Limited genetic diversity of the Plasmodium falciparum aquaglyceroporin gene

In Plasmodium falciparum small solutes like water, ammonium, glycerol and others are transported by a parasite-encoded channel into the parasite. The gene encoding this channel is termed P. falciparum aquaglyceroporin (PfAQP) and is a single-copy gene and highly homologous to other aquaporins from other protozoa. Aquaporins are considered to be attractive targets for drug treatment and more so since the human and parasite aquaporins show considerable sequence differences. To investigate whether PfAQP may be suitable as a conserved target for potential aquaporin blocking agents we determined the DNA sequences of PfAQP from 65 parasite strains, either from in vitro cultured laboratory strains or from parasites obtained in an malaria-endemic region of Gabon. Only two non-synonymous mutations were found and functionally tested by a methylamine efflux assay. The efflux activity of all variants tested was similar. The lack of functionally variability suggests an invariable protein core, which may restrict parasite populations from evading therapeutic pressure if PfAQP inhibitors will be found.

Test of blockers of AQP1 water permeability by a high-resolution method: no effects of tetraethylammonium ions or acetazolamide

The effects of putative water channel blockers were tested on AQP1-expressing Xenopus laevis oocytes by a fast optical method with a time resolution of 1 s and a volume resolution of 20 pl. The oocytes were exposed to external hyposmolarity and the osmotic water permeability (Lp) derived from the initial 10 s of volume change. For longer durations, the effective osmotic gradient across the membrane was reduced significantly because of dilution of the intracellular contents and of ion transport across the membrane. The latter was monitored by voltage clamp of the oocytes. In contrast to previous reports based on slower and less sensitive assays, we found no effects of tetraethylammonium ions (TEA+) and acetazolamide on Lp. We have no single explanation for this, but several factors are considered: (a) If the osmotic gradient is assumed to be constant for periods longer than 10 s, the Lp will be underestimated. (b) Hyposmotic gradients implemented by dilution with water will entail changes in the ionic strength as well; this may enhance loss of salt from the oocyte. (c) By voltage clamping the AQP1-expressing oocytes during hyposmotic challenges, we found that TEA+-treated oocytes were more electrically leaky than untreated ones. This may obscure comparisons between the Lp of treated and untreated oocytes. (d) The nature of the ion transport mechanisms in the plasma membrane depends on how oocytes have been prepared for experiments and on their viability as indicated by the membrane potential. These parameters may vary between laboratories.

The structural basis of water permeation and proton exclusion in aquaporins

Aquaporins (AQPs) represent a ubiquitous class of integral membrane proteins that play critical roles in cellular osmoregulations in microbes, plants and mammals. AQPs primarily function as water-conducting channels, whereas members of a sub-class of AQPs, termed aquaglyceroporins, are permeable to small neutral solutes such as glycerol. While AQPs facilitate transmembrane permeation of water and/or small neutral solutes, they preclude the conduction of protons. Consequently, openings of AQP channels allow rapid water diffusion down an osmotic gradient without dissipating electrochemical potentials. Molecular structures of AQPs portray unique features that define the two central functions of AQP channels: effective water permeation and strict proton exclusion. This review describes AQP structures known to date and discusses the mechanisms underlying water permeation, proton exclusion and water permeability regulation.