Is TEA an inhibitor for human Aquaporin-1?

KCNJ16-depleted kidney organoids recapitulate tubulopathy and lipid recovery upon statins treatment

BACKGROUND

The KCNJ16 gene has been associated with a novel kidney tubulopathy phenotype, viz. disturbed acid-base homeostasis, hypokalemia and altered renal salt transport. KCNJ16 encodes for Kir5.1, which together with Kir4.1 constitutes a potassium channel located at kidney tubular cell basolateral membranes. Preclinical studies provided mechanistic links between Kir5.1 and tubulopathy, however, the disease pathology remains poorly understood. Here, we aimed at generating and characterizing a novel advanced in vitro human kidney model that recapitulates the disease phenotype to investigate further the pathophysiological mechanisms underlying the tubulopathy and potential therapeutic interventions.

METHODS

We used CRISPR/Cas9 to generate KCNJ16 mutant (KCNJ16+/- and KCNJ16-/-) cell lines from healthy human induced pluripotent stem cells (iPSC) KCNJ16 control (KCNJ16WT). The iPSCs were differentiated following an optimized protocol into kidney organoids in an air-liquid interface.

RESULTS

KCNJ16-depleted kidney organoids showed transcriptomic and potential functional impairment of key voltage-dependent electrolyte and water-balance transporters. We observed cysts formation, lipid droplet accumulation and fibrosis upon Kir5.1 function loss. Furthermore, a large scale, glutamine tracer flux metabolomics analysis demonstrated that KCNJ16-/- organoids display TCA cycle and lipid metabolism impairments. Drug screening revealed that treatment with statins, particularly the combination of simvastatin and C75, prevented lipid droplet accumulation and collagen-I deposition in KCNJ16-/- kidney organoids.

CONCLUSIONS

Mature kidney organoids represent a relevant in vitro model for investigating the function of Kir5.1. We discovered novel molecular targets for this genetic tubulopathy and identified statins as a potential therapeutic strategy for KCNJ16 defects in the kidney.

Aquaglyceroporin Modulators as Emergent Pharmacological Molecules for Human Diseases

Aquaglyceroporins, a sub-class of aquaporins that facilitate the diffusion of water, glycerol and other small uncharged solutes across cell membranes, have been recognized for their important role in human physiology and their involvement in multiple disorders, mostly related to disturbed energy homeostasis. Aquaglyceroporins dysfunction in a variety of pathological conditions highlighted their targeting as novel therapeutic strategies, boosting the search for potent and selective modulators with pharmacological properties. The identification of selective inhibitors with potential clinical applications has been challenging, relying on accurate assays to measure membrane glycerol permeability and validate effective functional blockers. Additionally, biologicals such as hormones and natural compounds have been revealed as alternative strategies to modulate aquaglyceroporins via their gene and protein expression. This review summarizes the current knowledge of aquaglyceroporins’ involvement in several pathologies and the experimental approaches used to evaluate glycerol permeability and aquaglyceroporin modulation. In addition, we provide an update on aquaglyceroporins modulators reported to impact disease, unveiling aquaglyceroporin pharmacological targeting as a promising approach for innovative therapeutics.

Inhibitors of Mammalian Aquaporin Water Channels

Aquaporins (AQPs) are water channel proteins that are essential to life, being expressed in all kingdoms. In humans, there are 13 AQPs, at least one of which is found in every organ system. The structural biology of the AQP family is well-established and many functions for AQPs have been reported in health and disease. AQP expression is linked to numerous pathologies including tumor metastasis, fluid dysregulation, and traumatic injury. The targeted modulation of AQPs therefore presents an opportunity to develop novel treatments for diverse conditions. Various techniques such as video microscopy, light scattering and fluorescence quenching have been used to test putative AQP inhibitors in both AQP-expressing mammalian cells and heterologous expression systems. The inherent variability within these methods has caused discrepancy and many molecules that are inhibitory in one experimental system (such as tetraethylammonium, acetazolamide, and anti-epileptic drugs) have no activity in others. Some heavy metal ions (that would not be suitable for therapeutic use) and the compound, TGN-020, have been shown to inhibit some AQPs. Clinical trials for neuromyelitis optica treatments using anti-AQP4 IgG are in progress. However, these antibodies have no effect on water transport. More research to standardize high-throughput assays is required to identify AQP modulators for which there is an urgent and unmet clinical need.

Autocrine signals increase ovine mesenchymal stem cells migration through Aquaporin-1 and CXCR4 overexpression

Sheep is a relevant large animal model that is frequently used to test innovative tissue engineering (TE) approaches especially for bone reconstruction. Mesenchymal stem cells (MSCs) are used in TE applications because they represent key component of adult tissue repair. Importantly, MSCs from different species show similar characteristics, which facilitated their application in translational studies using animal models. Nowadays, many researches are focusing on the use of ovine mesenchymal stem cells (oMSCs) in orthopedic preclinical settings for regenerative medicine purposes. Therefore, there is a need to amplify our knowledge on the mechanisms underlying the behaviour of these cells. Recently, several studies have shown that MSC function is largely dependent on factors that MSCs release in the environment, as well as, in conditioned medium (CM). It has been demonstrated that MSCs through autocrine and paracrine signals are able to stimulate proliferation, migration, and differentiation of different type of cells including themselves. In this study, we investigated the effects of the CM produced by oMSCs on oMSCs themselves and we explored the signal pathways involved. We observed that CM caused an enhancement of oMSC migration. Furthermore, we found that CM increased levels of two membrane proteins involved in cell migration, Aquaporin 1 (AQP1), and C-X-C chemokine receptor type 4 (CXCR4), and activated Akt and Erk intracellular signal pathways. In conclusion, taken together our results suggest the high potential of autologous CM as a promising tool to modulate behaviour of MSCs thus improving their use in therapeutically approaches.

Aquaporins: Novel Targets for Age-Related Ocular Disorders

Aquaporins (AQPs), a large family of membrane protein channels that facilitate transport of water and other small solutes, play important roles in physiological functions and human diseases. Up till now, 13 types of AQPs, numbered 0 through 12, have been identified in various mammalian tissues. Homologous genes for AQPs in amphibians, insects, and bacteria highlight the evolutionary conservation and, thus, the importance of these membrane channels. Many members of the AQP family are expressed in the eye. AQP1, which is a water-selective channel, is expressed in the anterior chamber (cornea, ciliary body, trabecular meshwork) and posterior chamber (retina and microvessels in choroid), controlling the fluid homeostasis in the eye. Mice knockout studies have indicated that AQP1 plays an important function in the eye by suggesting its role in aqueous humor dynamics and retina angiogenesis. This review will focus on the role of AQP1 as a novel target for ocular disorders such as glaucoma and age-related macular degeneration, and it will discuss challenges and advances in identifying modulators of AQP1 function that could be useful in clinical applications.

Potential of Mean Force Calculations of Solute Permeation Across UT-B and AQP1: A Comparison between Molecular Dynamics and 3D-RISM

Membrane channels facilitate the efficient and selective flux of various solutes across biological membranes. A common approach to investigate the selectivity of a channel has been the calculation of potentials of mean force (PMFs) for solute permeation across the pore. PMFs have been frequently computed from molecular dynamics (MD) simulations, yet the three-dimensional reference interaction site model (3D-RISM) has been suggested as a computationally efficient alternative to MD. Whether the two methods yield comparable PMFs for solute permeation has remained unclear. In this study, we calculated potentials of mean force for water, ammonia, urea, molecular oxygen, and methanol across the urea transporter B (UT-B) and aquaporin-1 (AQP1), using 3D-RISM, as well as using MD simulations and umbrella sampling. To allow direct comparison between the PMFs from 3D-RISM and MD, we ensure that all PMFs refer to a well-defined reference area in the bulk or, equivalently, to a well-defined density of channels in the membrane. For PMFs of water permeation, we found reasonable agreement between the two methods, with differences of ≲3 kJ mol-1. In contrast, we found stark discrepancies for the PMFs for all other solutes. Additional calculations confirm that discrepancies between MD and 3D-RISM are not explained by the choice for the closure relation, the definition the reaction coordinate (center of mass-based versus atomic site-based), details of the molecule force field, or fluctuations of the protein. Comparison of the PMFs suggests that 3D-RISM may underestimate effects from hydrophobic solute-channel interactions, thereby, for instance, missing the urea binding sites in UT-B. Furthermore, we speculate that the orientational averages inherent to 3D-RISM might lead to discrepancies in the narrow channel lumen. These findings suggest that current 3D-RISM solvers provide reasonable estimates for the PMF for water permeation, but that they are not suitable to study the selectivity of membrane channels with respect to uncharged nonwater solutes.

Aquaporin Water Channels and Hydrocephalus

The aquaporins (AQPs) are a family of water-transporting proteins that are broadly expressed in mammalian cells. Two AQPs in the central nervous system, AQP1 and AQP4, might play a role in hydrocephalus and are thus potential drug targets. AQP1 is expressed in the ventricular-facing membrane of choroid plexus epithelial cells, where it facilitates the secretion of cerebrospinal fluid (CSF). AQP4 is expressed in astrocyte foot processes and ependymal cells lining ventricles, where it appears to facilitate the transport of excess water out of the brain. Altered expression of these AQPs in experimental animal models of hydrocephalus and limited human specimens suggests their involvement in the pathophysiology of hydrocephalus, as do data in knockout mice demonstrating a protective effect of AQP1 deletion and a deleterious effect of AQP4 deletion in hydrocephalus. Though significant questions remain, including the precise contribution of AQP1 to CSF secretion in humans and the mechanisms by which AQP4 facilitates clearance of excess brain water, AQP1 and AQP4 have been proposed as potential drug targets to reduce ventricular enlargement in hydrocephalus.

Inhibition of AQP1 Hampers Osteosarcoma and Hepatocellular Carcinoma Progression Mediated by Bone Marrow-Derived Mesenchymal Stem Cells

The complex cross-talk between tumor cells and their surrounding stromal environment plays a key role in the pathogenesis of cancer. Among several cell types that constitute the tumor stroma, bone marrow-derived mesenchymal stem cells (BM-MSCs) selectively migrate toward the tumor microenvironment and contribute to the active formation of tumor-associated stroma. Therefore, here we elucidate the involvement of BM-MSCs to promote osteosarcoma (OS) and hepatocellular carcinoma (HCC) cells migration and invasion and deepening the role of specific pathways. We analyzed the function of aquaporin 1 (AQP1), a water channel known to promote metastasis and neoangiogenes. AQP1 protein levels were analyzed in OS (U2OS) and HCC (SNU-398) cells exposed to conditioned medium from BM-MSCs. Tumor cell migration and invasion in response to BM-MSC conditioned medium were evaluated through a wound healing assay and Boyden chamber, respectively. The results showed that the AQP1 level was increased in both tumor cell lines after treatment with BM-MSC conditioned medium. Moreover, BM-MSCs-mediated tumor cell migration and invasion were hampered after treatment with AQP1 inhibitor. These data suggest that the recruitment of human BM-MSCs into the tumor microenvironment might cause OS and HCC cell migration and invasion through involvement of AQP1.

Rapid Identification of Novel Inhibitors of the Human Aquaporin-1 Water Channel

Aquaporins (AQPs) are a family of membrane proteins that function as channels facilitating water transport in response to osmotic gradients. These play critical roles in several normal physiological and pathological states and are targets for drug discovery. Selective inhibition of the AQP1 water channel may provide a new approach for the treatment of several disorders including ocular hypertension/glaucoma, congestive heart failure, brain swelling associated with a stroke, corneal and macular edema, pulmonary edema, and otic disorders such as hearing loss and vertigo. We developed a high-throughput assay to screen a library of compounds as potential AQP1 modulators by monitoring the fluorescence dequenching of entrapped calcein in a confluent layer of AQP1-overexpressing CHO cells that were exposed to a hypotonic shock. Promising candidates were tested in a Xenopus oocyte-swelling assay, which confirmed the identification of two lead classes of compounds belonging to aromatic sulfonamides and dihydrobenzofurans with IC50 s in the low micromolar range. These selected compounds directly inhibited water transport in AQP1-enriched stripped erythrocyte ghosts and in proteoliposomes reconstituted with purified AQP1. Validation of these lead compounds, by the three independent assays, establishes a set of attractive AQP1 blockers for developing novel, small-molecule functional modulators of human AQP1.

Ranking of Molecular Biomarker Interaction with Targeted DNA Nucleobases via Full Atomistic Molecular Dynamics

DNA-based sensors can detect disease biomarkers, including acetone and ethanol for diabetes and H2S for cardiovascular diseases. Before experimenting on thousands of potential DNA segments, we conduct full atomistic steered molecular dynamics (SMD) simulations to screen the interactions between different DNA sequences with targeted molecules to rank the nucleobase sensing performance. We study and rank the strength of interaction between four single DNA nucleotides (Adenine (A), Guanine (G), Cytosine (C), and Thymine (T)) on single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) with acetone, ethanol, H2S and HCl. By sampling forward and reverse interaction paths, we compute the free-energy profiles of eight systems for the four targeted molecules. We find that dsDNA react differently than ssDNA to the targeted molecules, requiring more energy to move the molecule close to DNA as indicated by the potential of mean force (PMF). Comparing the PMF values of different systems, we obtain a relative ranking of DNA base for the detection of each molecule. Via the same procedure, we could generate a library of DNA sequences for the detection of a wide range of chemicals. A DNA sensor array built with selected sequences differentiating many disease biomarkers can be used in disease diagnosis and monitoring.

Challenges and achievements in the therapeutic modulation of aquaporin functionality

Aquaporin (AQP) water and solute channels have basic physiological functions throughout the human body. AQP-facilitated water permeability across cell membranes is required for rapid reabsorption of water from pre-urine in the kidneys and for sustained near isosmolar water fluxes e.g. in the brain, eyes, inner ear, and lungs. Cellular water permeability is further connected to cell motility. AQPs of the aquaglyceroporin subfamily are necessary for lipid degradation in adipocytes and glycerol uptake into the liver, as well as for skin moistening. Modulation of AQP function is desirable in several pathophysiological situations, such as nephrogenic diabetes insipidus, Sjögren's syndrome, Menière's disease, heart failure, or tumors to name a few. Attempts to design or to find effective small molecule AQP inhibitors have yielded only a few hits. Challenges reside in the high copy number of AQP proteins in the cell membranes, and spatial restrictions in the protein structure. This review gives an overview on selected physiological and pathophysiological conditions in which modulation of AQP functions appears beneficial and discusses first achievements in the search of drug-like AQP inhibitors.

Mechanisms underlying turgor regulation in the estuarine alga Vaucheria erythrospora (Xanthophyceae) exposed to hyperosmotic shock

Aquatic organisms are often exposed to dramatic changes in salinity in the environment. Despite decades of research, many questions related to molecular and physiological mechanisms mediating sensing and adaptation to salinity stress remain unanswered. Here, responses of Vaucheria erythrospora, a turgor-regulating xanthophycean alga from an estuarine habitat, have been investigated. The role of ion uptake in turgor regulation was studied using a single cell pressure probe, microelectrode ion flux estimation (MIFE) technique and membrane potential (Em ) measurements. Turgor recovery was inhibited by Gd(3+) , tetraethylammonium chloride (TEA), verapamil and orthovanadate. A NaCl-induced shock rapidly depolarized the plasma membrane while an isotonic sorbitol treatment hyperpolarized it. Turgor recovery was critically dependent on the presence of Na(+) but not K(+) and Cl(-) in the incubation media. Na(+) uptake was strongly decreased by amiloride and changes in net Na(+) and H(+) fluxes were oppositely directed. This suggests active uptake of Na(+) in V. erythrospora mediated by an antiport Na(+) /H(+) system, functioning in the direction opposite to that of the SOS1 exchanger in higher plants. The alga also retains K(+) efficiently when exposed to high NaCl concentrations. Overall, this study provides insights into mechanisms enabling V. erythrospora to regulate turgor via ion movements during hyperosmotic stress.

Limitation of Cell Elongation in Barley (Hordeum vulgare L.) Leaves Through Mechanical and Tissue-Hydraulic Properties

The aim of the present study was to assess the mechanical and hydraulic limitation of growth in leaf epidermal cells of barley (Hordeum vulgare L.) in response to agents which affect cellular water (mercuric chloride, HgCl(2)) and potassium (cesium chloride, CsCl; tetraethylammonium, TEA) transport, pump activity of plasma membrane H(+)-ATPase and wall acidification (fusicoccin, FC). Cell turgor (P) was measured with the cell pressure probe, and cell osmotic pressure (π) was analyzed through picoliter osmometry of single-cell extracts. A wall extensibility coefficient (M) and tissue hydraulic conductance coefficient (L) were derived using the Lockhart equation. There was a significant positive linear relationship between relative elemental growth rate and P, which fit all treatments, with an overall apparent yield threshold of 0.368 MPa. Differences in growth between treatments could be explained through differences in P. A comparison of L and M showed that growth in all except the FC treatment was co-limited through hydraulic and mechanical properties, though to various extents. This was accompanied by significant (0.17-0.24 MPa) differences in water potential (ΔΨ) between xylem and epidermal cells in the leaf elongation zone. In contrast, FC-treated leaves showed ΔΨ close to zero and a 10-fold increase in L.

Molecular dynamics simulations of the transport of reactive oxygen species by mammalian and plant aquaporins

BACKGROUND

Aquaporins are responsible for water transport across lipid membranes. They are also able to transport reactive oxygen species, playing an important role in redox signaling. Certain plant aquaporins have even the ability to be regulated by oxidative stress. However, the underlying mechanisms are still not fully understood.

METHODS

Here, molecular dynamics simulations were employed to determine the activation free energies related to the transport of reactive oxygen species through both mammalian and plant aquaporin models.

RESULTS AND CONCLUSIONS

Both aquaporins may transport hydrogen peroxide (H2O2) and the protonated form of superoxide radicals (HO2). The solution-to-pore transfer free energies were low for small oxy-radicals, suggesting that even highly reactive hydroxyl radicals (HO) might have access to the pore interior and oxidize amino acids responsible for channel selectivity. In the plant aquaporin, no significant change in water permeability was observed upon oxidation of the solvent-exposed disulfide bonds at the extracellular region. During the simulated time scale, the existence of a direct oxidative gating mechanism involving these disulfide bonds could not be demonstrated.

GENERAL SIGNIFICANCE

Simulation results may improve the understanding of redox signaling mechanisms and help in the interpretation of protein oxidative labeling experiments.

Aquaporins: important but elusive drug targets

The aquaporins (AQPs) are a family of small, integral membrane proteins that facilitate water transport across the plasma membranes of cells in response to osmotic gradients. Data from knockout mice support the involvement of AQPs in epithelial fluid secretion, cell migration, brain oedema and adipocyte metabolism, which suggests that modulation of AQP function or expression could have therapeutic potential in oedema, cancer, obesity, brain injury, glaucoma and several other conditions. Moreover, loss-of-function mutations in human AQPs cause congenital cataracts (AQP0) and nephrogenic diabetes insipidus (AQP2), and autoantibodies against AQP4 cause the autoimmune demyelinating disease neuromyelitis optica. Although some potential AQP modulators have been identified, challenges associated with the development of better modulators include the druggability of the target and the suitability of the assay methods used to identify modulators.

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.

Reinvestigation of drugs and chemicals as aquaporin-1 inhibitors using pressure-induced hemolysis in human erythrocytes

Recently, we have found that pressure-induced hemolysis is enhanced by inhibiting water transport via aquaporin-1 (AQP1), as seen in p-chloromercuribenzoate (pCMB)-treated erythrocytes. So, using this method we reinvestigated the functions as AQP1 inhibitors of drugs and chemicals such as acetazolamide, sodium nitroprusside, tetraethylammonium ions (TEA(+)), and dimethylsulfoxide (DMSO). The values of hemolysis at 200 MPa were almost unaffected by acetazolamide or sodium nitroprusside, decreased by TEA(+), and increased significantly by DMSO. Furthermore, the erythrocytes were exposed to pCMB in the presence of TEA(+) or DMSO. The enhancement effect of pCMB on pressure-induced hemolysis was unaffected by TEA(+) but attenuated by DMSO. Taken together, these results suggest that, of drugs and chemicals examined here, DMSO only is an AQP1 inhibitor, but the effect of DMSO is small compared with pCMB.

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.

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-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.

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.