Nickel and extracellular acidification inhibit the water permeability of human aquaporin-3 in lung epithelial cells

Silver ions disrupt K⁺ homeostasis and cellular integrity in intact barley (Hordeum vulgare L.) roots

The heavy metals silver, gold, and mercury can strongly inhibit aquaporin-mediated water flow across plant cell membranes, but critical examinations of their side effects are rare. Here, the short-lived radiotracer (42)K is used to demonstrate that these metals, especially silver, profoundly change potassium homeostasis in roots of intact barley (Hordeum vulgare L.) plants, by altering unidirectional K(+) fluxes. Doses as low as 5 μM AgNO(3) rapidly reduced K(+) influx to 5% that of controls, and brought about pronounced and immediate increases in K(+) efflux, while higher doses of Au(3+) and Hg(2+) were required to produce similar responses. Reduced influx and enhanced efflux of K(+) resulted in a net loss of >40% of root tissue K(+) during a 15 min application of 500 μM AgNO(3), comprising the entire cytosolic potassium pool and about a third of the vacuolar pool. Silver also brought about major losses of UV-absorbing compounds, total electrolytes, and NH(4)(+). Co-application, with silver, of the channel blockers Cs(+), TEA(+), or Ca(2+), did not affect the enhanced efflux, ruling out the involvement of outwardly rectifying ion channels. Taken together with an examination of propidium iodide staining under confocal microscopy, the results indicate that silver ions affect K(+) homeostasis by directly inhibiting K(+) influx at lower concentrations, and indirectly inhibiting K(+) influx and enhancing K(+) efflux, via membrane destruction, at higher concentrations. Ni(2+), Cd(2+), and Pb(2+), three heavy metals not generally known to affect aquaporins, did not enhance K(+) efflux or cause propidium iodide incorporation. The study reveals strong and previously unknown effects of major aquaporin inhibitors and recommends caution in their application.

Gating of aquaporins by heavy metals in Allium cepa L. epidermal cells

Changes in the water permeability, aquaporin (AQP) activity, of leaf cells were investigated in response to different heavy metals (Zn(2+), Pb(2+), Cd(2+), Hg(2+)). The cell pressure probe experiments were performed on onion epidermal cells as a model system. Heavy metal solutions at different concentrations (0.05 μM-2 mM) were used in our experiments. We showed that the investigated metal ions can be arranged in order of decreasing toxicity (expressed as a decrease in water permeability) as follows: Hg>Cd>Pb>Zn. Our results showed that β-mercaptoethanol treatment (10 mM solution) partially reverses the effect of AQP gating. The magnitude of this reverse differed depending on the metal and its concentration. The time course studies of the process showed that the gating of AQPs occurred within the first 10 min after the application of a metal. We also showed that after 20-40 min from the onset of metal treatment, the water flow through AQPs stabilized and remained constant. We observed that irrespective of the metal applied, the effect of AQP gating can be recorded within the first 10 min after the administration of metal ions. More generally, our results indicate that the toxic effects of investigated metal ions on the cellular level may involve AQP gating.

Design and characterization of genetically engineered zebrafish aquaporin-3 mutants highly permeable to the cryoprotectant ethylene glycol

Background

Increasing cell membrane permeability to water and cryoprotectants is critical for the successful cryopreservation of cells with large volumes. Artificial expression of water-selective aquaporins or aquaglyceroporins (GLPs), such as mammalian aquaporin-3 (AQP3), enhances cell permeability to water and cryoprotectants, but it is known that AQP3-mediated water and solute permeation is limited and pH dependent. To exploit further the possibilities of using aquaporins in cryobiology, we investigated the functional properties of zebrafish (Danio rerio) GLPs.

Results

Water, glycerol, propylene glycol and ethylene glycol permeability of zebrafish Aqp3a, -3b, -7, -9a, -9b, -10a and -10b, and human AQP3, was examined. Expression in Xenopus laevis oocytes indicated that the permeability of DrAqp3a and -3b to ethylene glycol was higher than for glycerol or propylene glycol under isotonic conditions, unlike other zebrafish GLPs and human AQP3, which were more permeable to glycerol. In addition, dose-response experiments and radiolabeled ethylene glycol uptake assays suggested that oocytes expressing DrAqp3b were permeated by this cryoprotectant more efficiently than those expressing AQP3. Water and ethylene glycol transport through DrAqp3a and -3b were, however, highest at pH 8.5 and completely abolished at pH 6.0. Point mutations in the DrAqp3b amino acid sequence rendered two constructs, DrAqp3b-T85A showing higher water and ethylene glycol permeability at neutral and alkaline pH, and DrAqp3b-H53A/G54H/T85A, no longer inhibited at acidic pH but less permeable than the wild type. Finally, calculation of permeability coefficients for ethylene glycol under concentration gradients confirmed that the two DrAqp3b mutants were more permeable than wild-type DrAqp3b and/or AQP3 at neutral pH, resulting in a 2.6- to 4-fold increase in the oocyte intracellular concentration of ethylene glycol.

Conclusion

By single or triple point mutations in the DrAqp3b amino acid sequence, we constructed one mutant with enhanced ethylene glycol permeability and another with reduced pH sensitivity. The DrAqp3b and the two mutant constructs may be useful for application in cryobiology.

Abnormal aquaporin-3 protein expression in hyperproliferative skin disorders

Non-melanoma skin cancers (NMSCs) and psoriasis represent common hyperproliferative skin disorders, with approximately one million new NMSC diagnoses each year in the United States alone and a psoriasis prevalence of about 2% worldwide. We recently demonstrated that the glycerol channel, aquaporin-3 (AQP3) and the enzyme phospholipase D2 (PLD2) interact functionally in epidermal keratinocytes of the skin to inhibit their proliferation. However, others have suggested that AQP3 is pro-proliferative in keratinocytes and is upregulated in the NMSC, squamous cell carcinoma (SCC). To evaluate the AQP3/PLD2 signaling module in skin diseases, we determined their levels in SCC, basal cell carcinoma (BCC) and psoriasis as compared to normal epidermis. Skin biopsies with the appropriate diagnoses (10 normal, 5 SCC, 13 BCC and 10 plaque psoriasis samples) were obtained from the pathology archives and examined by immunohistochemistry using antibodies recognizing AQP3 and PLD2. In normal epidermis AQP3, an integral membrane protein, was localized mainly to the plasma membrane and PLD2 to the cell periphery, particularly in suprabasal layers. In BCC, AQP3 and PLD2 levels were reduced as compared to the normal-appearing overlying epidermis. In SCC, AQP3 staining was "patchy," with areas of reduced AQP3 immunoreactivity exhibiting positivity for Ki67, a marker of proliferation. PLD2 staining was unchanged in SCC. In psoriasis, AQP3 staining was usually observed in the cytoplasm rather than in the membrane. Also, in the majority of psoriatic samples, PLD2 showed weak immunoreactivity or aberrant localization. These results suggest that abnormalities in the AQP3/PLD2 signaling module correlate with hyperproliferation in psoriasis and the NMSCs.

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.

Effect of maturation on the expression of aquaporin 3 in mouse oocyte

This study aimed to investigate whether aquaporin 3 (Aqp3) mRNAs are expressed in immature oocytes and altered during in vitro maturation process. Five- to 6-week-old female ICR mice were primed by gonadotropin for 24 and 48 h. Immature oocytes obtained 48 h after priming were also matured in vitro for 17 to 18 h. In vivo matured oocytes were obtained after 48 h priming followed by hCG injection. Total RNAs were extracted from 80 to 150 oocytes in each experimental group, and the levels of Aqp3 mRNA were quantified by real-time reverse transcriptase polymerase chain reaction. The experiments were repeated twice using different oocytes. The Aqp3 mRNA was expressed in immature oocytes, as well as in in vitro and in vivo matured oocytes. The expression level was higher in immature oocytes obtained 48 h after priming (17.2 ± 8.6, mean ± SD) than those with no priming (5.7 ± 0.8) or obtained 24 h after priming (2.5 ± 0.8). The expression of Aqp3 mRNA decreased after in vitro maturation (1.2 ± 0.5), which was similar to in vivo matured oocytes (1.0 ± 0.0). Our work demonstrated that Aqp3 mRNA expression increased during the development of immature oocyte but decreased after completion of in vitro maturation. The results indicate that AQP3 is certainly needed for the acquisition of immature oocytes’ full growing potential within antral follicles.

Differential water permeability and regulation of three aquaporin 4 isoforms

Aquaporin 4 (AQP4) is expressed in the perivascular glial endfeet and is an important pathway for water during formation and resolution of brain edema. In this study, we examined the functional properties and relative unit water permeability of three functional isoforms of AQP4 expressed in the brain (M1, M23, Mz). The M23 isoform gave rise to square arrays when expressed in Xenopus laevis oocytes. The relative unit water permeability differed significantly between the isoforms in the order of M1 > Mz > M23. None of the three isoforms were permeable to small osmolytes nor were they affected by changes in external K(+) concentration. Upon protein kinase C (PKC) activation, oocytes expressing the three isoforms demonstrated rapid reduction of water permeability, which correlated with AQP4 internalization. The M23 isoform was more sensitive to PKC regulation than the longer isoforms and was internalized significantly faster. Our results suggest a specific role for square array formation.

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.

Expression and functional characterization of four aquaporin water channels from the European eel (Anguilla anguilla)

The European eel is a euryhaline teleost which has been shown to differentially up- and downregulate aquaporin (AQP) water channels in response to changes in environmental salinity. We have characterized the transport properties of four aquaporins localized to osmoregulatory organs - gill, esophagus, intestine and kidney. By sequence comparison these four AQP orthologs resemble human AQP1 (eel AQP1), AQP3 (eel AQP3) and AQP10 (AQPe). The fourth member is a duplicate form of AQP1 (AQP1dup) thought to arise from a duplication of the teleost genome. Using heterologous expression in Xenopus oocytes we demonstrate that all four eel orthologs transport water and are mercury inhibitable. Eel AQP3 and AQPe also transport urea and glycerol, making them aquaglyceroporins. Eel AQP3 is dramatically inhibited by extracellular acidity (91% and 69% inhibition of water and glycerol transport respectively at pH 6.5) consistent with channel gating by protons. Maximal water flux of eel AQP3 occurred around pH 8.2 - close to the physiological pH of plasma in the eel. Exposure of AQP-expressing oocytes to heavy metals revealed that eel AQP3 is highly sensitive to extracellular nickel and zinc (88.3% and 86.3% inhibition, respectively) but less sensitive to copper (56.4% inhibition). Surprisingly, copper had a stimulatory effect on eel AQP1 (153.7% activity of control). Copper, nickel and zinc did not affect AQP1dup or AQPe. We establish that all four eel AQP orthologs have similar transport profiles to their human counterparts, with eel AQP3 exhibiting some differences in its sensitivity to metals. This is the first investigation of the transport properties and inhibitor sensitivity of salinity-regulated aquaporins from a euryhaline species. Our results indicate a need to further investigate the deleterious effects of metal pollutants on AQP-containing epithelial cells of the gill and gastrointestinal tract at environmentally appropriate concentrations.

Regulation of water permeability through aquaporin-4

Aquaporin-4 (AQP4) is a predominant water channel protein in mammalian brains that is distributed with the highest density in the perivascular and subpial astrocyte end-feet. AQP4 is a critical component of an integrated water and potassium homeostasis. Expression and regulation of AQP4 have been studied to understand the roles of AQP4 in physiology and several pathological conditions. Indeed, AQP4 has been implicated in several neurological conditions, such as brain edema and seizure. AQP4 is dynamically regulated at different levels: channel gating, subcellular distribution, phosphorylation, protein-protein interactions and orthogonal array formation. In this review, we focus on the short-term regulation of AQP4. Phosphorylation of AQP4 is important; AQP4 is inhibited when Ser180 is phosphorylated and activated when Ser111 is phosphorylated. AQP4 is also regulated by several metal ions. These metal ions inhibit AQP4 by interacting with the Cys178 residue located in the cytoplasmic loop D, suggesting that AQP4 is regulated by intracellular signaling pathways in response to extracellular stimuli. Recently, it was demonstrated that AQP4 may be inhibited by arylsulfonamides, antiepileptic drugs and other related chemical compounds. Structural analysis of AQP4 may guide a drug design for AQP4.

Molecular recognition in biomolecules studied by statistical-mechanical integral-equation theory of liquids

Recent progress in the theory of molecular recognition in biomolecules is reviewed, which has been made based on the statistical mechanics of liquids or the RISM/3D-RISM theory during the last five years in the authors' group. The method requires just the structure of protein and the potential energy parameters for the biomolecules and solutions as inputs. The calculation is carried out in two steps. The first step is to obtain the pair correlation functions for solutions consisting of water and ligands based on the RISM theory. Then, given the pair correlation functions prepared in the first step, we calculate the 3D-distribution functions of water and ligands around and inside protein based on the 3D-RISM theory. The molecular recognition of a ligand by the protein is realized by the 3D-distribution functions: if one finds some conspicuous peaks in the distribution of a ligand inside protein, then the ligand is regarded as "recognized" by the protein. Some biochemical processes are investigated, which are intimately related to the molecular recognition of small ligands including water, noble gases, and ions by a protein.

Design, synthesis and assaying of potential aquaporin inhibitors

The aquaporin protein family performs fundamental tasks in the physiology of several organs in the human body. Their roles in several disorders known to involve water movement make them attractive targets for the development of novel drug therapies.This chapter describes assays commonly used to study the water permeability across AQPs. It also describes the effect of some known inhibitors of aquaporins on water permeability, such as mercury, gold, silver, copper, phloretin, tetraethyl ammonium salts and acetazolamide compounds.

Discovery of the aquaporins and development of the field

The study of water transport began long before the molecular identification of water channels with studies of water-permeable tissues. The discovery of the first aquaporin, AQP1, occurred during experiments focused on the identity of the Rh blood group antigens. Since then the field has expanded dramatically to study aquaporins in all types of organisms. In mammals, some of the aquaporins transport only water. However, there are some family members that collectively transport a diverse set of solutes. The aquaporins can be regulated by factors that affect channel permeability or subcellular localization. An extensive set of studies examines the physiological role of many of the mammalian aquaporins. However, much is still to be discovered about the physiological role of this membrane protein family.

Structure-function analysis of plant aquaporin AtPIP2;1 gating by divalent cations and protons

Water channel proteins, AQPs (aquaporins), of the PIP (plasma membrane intrinsic protein) subfamily, provide a means for fine and quick adjustments of the plant water status. A molecular model for gating of PIPs by cytosolic protons (H(+)) and divalent cations was derived from the atomic structure of spinach SoPIP2;1 (Spinacia oleracea PIP2;1) in an open- and a closed-pore conformation. In the present study, we produced the Arabidopsis AtPIP2;1 (Arabidopsis thaliana PIP2;1) homologue in Pichia pastoris, either WT (wild-type) or mutations at residues supposedly involved in gating. Stopped-flow spectrophotometric measurements showed that, upon reconstitution in proteoliposomes, all forms function as water channels. The first functional evidence for a direct gating of PIPs by divalent (bivalent) cations was obtained. In particular, cadmium and manganese were identified, in addition to calcium (Ca(2+)) and H(+) as potent inhibitors of WT AtPIP2;1. Our results further show that His(199), the previously identified site for H(+) sensing, but also N-terminal located Glu(31), and to a lesser extent Asp(28), are involved in both divalent-cation- and H(+)-mediated gating. In contrast, mutation of Arg(124) rendered AtPIP2;1 largely insensitive to Ca(2+) while remaining fully sensitive to H(+). The role of these residues in binding divalent cations and/or stabilizing the open or closed pore conformations is discussed.

Contribution of aquaporins to cellular water transport observed by a microfluidic cell volume sensor

Here we demonstrate that an impedance-based microfluidic cell volume sensor can be used to study the roles of aquaporin (AQP) in cellular water permeability and screen AQP-specific drugs. Human embryonic kidney (HEK-293) cells were transiently transfected with AQP3- or AQP4-encoding genes to express AQPs in plasma membranes. The swelling of cells in response to hypotonic stimulation was traced in real time using the sensor. Two time constants were obtained by fitting the swelling curves with a two-exponential function, a fast time constant associated with osmotic water permeability of AQP-expressing cells and a slow phase time constant associated mainly with water diffusion through lipid bilayers in the nontransfected cells. The AQP-expressing cells showed at least 10× faster osmotic water transport than control cells. Using the volume sensor, we examined the effects of Hg²⁺ and Ni²⁺ on the water transport via AQPs. Hg²⁺ inhibited the water flux in AQP3-expressing cells irreversibly, while Ni²⁺ blocked the AQP3 channels reversibly. Neither of the two ions blocked the AQP4 channels. The microfluidic volume sensor can sense changes in cell volume in real time, which enables perfusion of various reagents sequentially. It provides a convenient tool for studying the effect of reagents on the function and regulation mechanism of AQPs.

Novel variants in human Aquaporin-4 reduce cellular water permeability

Cerebral edema contributes significantly to morbidity and mortality after brain injury and stroke. Aquaporin-4 (AQP4), a water channel expressed in astrocytes, plays a key role in brain water homeostasis. Genetic variants in other aquaporin family members have been associated with disease phenotypes. However, in human AQP4, only one non-synonymous single-nucleotide polymorphism (nsSNP) has been reported, with no characterization of protein function or disease phenotype. We analyzed DNA from an ethnically diverse cohort of 188 individuals to identify novel AQP4 variants. AQP4 variants were constructed by site-directed mutagenesis and expressed in cells. Water permeability assays in the cells were used to measure protein function. We identified 24 variants in AQP4 including four novel nsSNPs (I128T, D184E, I205L and M224T). We did not observe the previously documented M278T in our sample. The nsSNPs found were rare ( approximately 1-2% allele frequency) and heterozygous. Computational analysis predicted reduced function mutations. Protein expression and membrane localization were similar for reference AQP4 and the five AQP4 mutants. Cellular assays confirmed that four variant AQP4 channels reduced normalized water permeability to between 26 and 48% of the reference (P < 0.001), while the M278T mutation increased normalized water permeability (P < 0.001). We identified multiple novel AQP4 SNPs and showed that four nsSNPs reduced water permeability. The previously reported M278T mutation resulted in gain of function. Our experiments provide insight into the function of the AQP4 protein. These nsSNPs may have clinical implications for patients with cerebral edema and related disorders.

IBMX-elicited inhibition of water permeability in the isolated rabbit conjunctival epithelium

Agents expected to increase intracellular cAMP levels were tested on the diffusional water permeability (P(dw)) of isolated rabbit conjunctival epithelia given recent indications of the apical expression of AQP5, a water channel homologue regulated by cAMP in other cell systems. For these experiments, segments of conjunctivae were mounted between Ussing-type hemichambers under short-circuit conditions. Unidirectional water fluxes (J(dw)) were measured by adding (3)H(2)O to one hemichamber and sampling from the other, while the electrical parameters (I(sc) and R(t)) were recorded simultaneously. J(dw) were determined under control conditions and after the introduction of forskolin, dibutyryl-cAMP, rolipram and IBMX. All agents reduced J(dw), with rolipram and IBMX the most effective inhibitors (~28% reduction), while simultaneously evoking stimulations of the I(sc); suggesting that cAMP regulates ionic transport and P(dw) independently. This observation was consistent with the elimination of the IBMX-elicited I(sc) stimulations by the PKA inhibitor, H89, and the ineffectiveness of the sulfonamide in preventing the J(dw) reductions produced by the xanthine. Data from mannitol fluxes and Arrhenius plots indicated that the IBMX-elicited P(dw) reduction occurred at the level of water-transporting channels, but the specific moiety was not identified. Instead it was observed that lipophiles commonly used in other systems to uncouple cellular communication precluded the effects of IBMX on J(dw), but the mechanism for these results was not directly linked to gap-junction blockade in the conjunctiva, as assessed by the transepithelial electrical parameters. Putatively, agents such as heptanol, by also fluidizing the bilayer, may have changed the conformation of a water channel in a manner preventing down-regulation by IBMX. Nevertheless, this study uncovered an apparently unique response to cAMP elevation exhibited by the conjunctiva, namely that P(dw) declines via an H89-insensitive pathway under conditions whereby PKA-dependent electrolyte transport might be over stimulated due to excessive cAMP levels (e.g., PDE inhibition).

Curcumin attenuates EGF-induced AQP3 up-regulation and cell migration in human ovarian cancer cells

OBJECTIVE

Aquaporin (AQP) water channels are expressed in high-grade tumor cells of different tissue origins. Based on the involvement of AQPs in angiogenesis and cell migration as well as our previous studies which show that AQP3 is involved in human skin fibroblasts cell migration, in this study, we investigated whether AQP3 is expressed in cultured human ovarian cancer cell line CaOV3 cells, and whether AQP3 expression in these cells enhances cell migration and metastatic potential.

METHODS

Cultured CaOV3 cells were treated with EGF and/or various reagents and subjected to cell migration assay by phagokinetic track mobility assay or biochemical analysis for expression or activation of proteins by SDS-PAGE/Western blot analysis.

RESULTS

In this study, we demonstrate that AQP3 is expressed in CaOV3 cells. EGF induces CaOV3 migration and up-regulates AQP3 expression. EGF-induced cell migration is inhibited by specific AQP3 siRNA knockdown or AQP3 water transport inhibitor CuSO4 and NiCl2. We also find that curcumin, a well known anti-ovarian cancer drug, down-regulates AQP3 expression and reduces cell migration in CaOV3, and the effects of curcumin are mediated, at least in part, by its inhibitory effects on EGFR and downstream AKT/ERK activation.

CONCLUSIONS

Collectively, our results provide evidence for AQP3-facilitated ovarian cancer cell migration, suggesting a novel function for AQP3 expression in high-grade tumors. The results that curcumin inhibits EGF-induced up-regulation of AQP3 and cell migration, provide a new explanation for the anticancer potential of curcumin.

Osmotic water permeability of rat intestinal brush border membrane vesicles: involvement of aquaporin-7 and aquaporin-8 and effect of metal ions

Water channels AQP7 and AQP8 may be involved in transcellular water movement in the small intestine. We show that both AQP7 and AQP8 mRNA are expressed in rat small intestine. Immunoblot and immunohistochemistry experiments demonstrate that AQP7 and AQP8 proteins are present in the apical brush border membrane of intestinal epithelial cells. We investigated the effect of several metals and pH on the osmotic water permeability (Pf) of brush border membrane vesicles (BBMVs) and of AQP7 and AQP8 expressed in a cell line. Hg2+, Cu2+, and Zn2+caused a significant decrease in the BBMV Pf, whereas Ni2+and Li+had no effect. AQP8-transfected cells showed a reduction in Pfin the presence of Hg2+and Cu2+, whereas AQP7-transfected cells were insensitive to all tested metals. The Pfof both BBMVs and cells transfected with AQP7 and AQP8 was not affected by pH changes within the physiological range, and the Pfof BBMVs alone was not affected by phlorizin or amiloride. Our results indicate that AQP7 and AQP8 may play a role in water movement via the apical domain of small intestine epithelial cells. AQP8 may contribute to the water-imbalance-related clinical symptoms apparent after ingestion of high doses of Hg2+and Cu2+.

Zinc modulation of water permeability reveals that aquaporin 0 functions as a cooperative tetramer

We previously showed that the water permeability of AQP0, the water channel of the lens, increases with acid pH and that His40 is required (Németh-Cahalan, K.L., and J.E. Hall. 2000. J. Biol. Chem. 275:6777–6782; Németh-Cahalan, K.L., K. Kalman, and J.E. Hall. 2004. J. Gen. Physiol. 123:573–580). We have now investigated the effect of zinc (and other transition metals) on the water permeability of AQP0 expressed in Xenopus oocytes and determined the amino acid residues that facilitate zinc modulation. Zinc (1 mM) increased AQP0 water permeability by a factor of two and prevented any additional increase induced by acid pH. Zinc had no effect on water permeability of AQP1, AQP4 or MIPfun (AQP0 from killifish), or on mutants of AQP1 and MIPfun with added external histidines. Nickel, but not copper, had the same effect on AQP0 water permeability as zinc. A fit of the concentration dependence of the zinc effect to the Hill equation gives a coefficient greater than three, suggesting that binding of more than one zinc ion is necessary to enhance water permeability. His40 and His122 are necessary for zinc modulation of AQP0 water permeability, implying structural constraints for zinc binding and functional modulation. The change in water permeability was highly sensitive to a coinjected zinc-insensitive mutant and a single insensitive monomer completely abolished zinc modulation. Our results suggest a model in which positive cooperativity among subunits of the AQP0 tetramer is required for zinc modulation, implying that the tetramer is the functional unit. The results also offer the possibility of a pharmacological approach to manipulate the water permeability and transparency of the lens.

A molecular mechanism directly linking E-cadherin adhesion to initiation of epithelial cell surface polarity

Mechanisms involved in maintaining plasma membrane domains in fully polarized epithelial cells are known, but when and how directed protein sorting and trafficking occur to initiate cell surface polarity are not. We tested whether establishment of the basolateral membrane domain and E-cadherin–mediated epithelial cell–cell adhesion are mechanistically linked. We show that the basolateral membrane aquaporin (AQP)-3, but not the equivalent apical membrane AQP5, is delivered in post-Golgi structures directly to forming cell–cell contacts where it co-accumulates precisely with E-cadherin. Functional disruption of individual components of a putative lateral targeting patch (e.g., microtubules, the exocyst, and soluble N-ethylmaleimide–sensitive factor attachment protein receptors) did not inhibit cell–cell adhesion or colocalization of the other components with E-cadherin, but each blocked AQP3 delivery to forming cell–cell contacts. Thus, components of the lateral targeting patch localize independently of each other to cell–cell contacts but collectively function as a holocomplex to specify basolateral vesicle delivery to nascent cell–cell contacts and immediately initiate cell surface polarity.

A potential role for the phospholipase D2-aquaporin-3 signaling module in early keratinocyte differentiation: production of a phosphatidylglycerol signaling lipid

In keratinocytes aquaporin-3 (AQP3), an efficient glycerol transporter, is associated with phospholipase D2 (PLD2) in caveolin-rich membrane microdomains. PLD catalyzes both phospholipid hydrolysis to produce phosphatidate and a transphosphatidylation reaction using primary alcohols to generate phosphatidylalcohols. As PLD2 can utilize the physiological alcohol glycerol to form phosphatidylglycerol (PG), we hypothesized that AQP3 provides glycerol to PLD2 for PG synthesis, which then modulates keratinocyte function. Acidic medium inhibits AQP3 transport activity; both glycerol uptake and PG synthesis were inhibited by low versus physiological pH. Co-transfection experiments were performed in which AQP3 or empty vector was introduced into keratinocytes simultaneously with reporter constructs in which differentiation or proliferation promoters directed expression of a luciferase reporter gene. AQP3 coexpression decreased the promoter activity of keratin 5, increased that of keratin 10 and enhanced the effect of a differentiating agent on the promoter activity of involucrin, consistent with promotion of early differentiation. Glycerol inhibited DNA synthesis, whereas equivalent concentrations of xylitol or sorbitol, as osmotic controls, had no effect. Direct provision of PG, but not phosphatidylpropanol, inhibited DNA synthesis in proliferative cells. Thus, our results support the idea that AQP3 supplies PLD2 with glycerol for synthesizing PG, a lipid signal that promotes early keratinocyte differentiation.

EGFR-mediated expression of aquaporin-3 is involved in human skin fibroblast migration

AQP3 (aquaporin-3), known as an integral membrane channel in epidermal keratinocytes, facilitates water and glycerol movement into and out of the skin. Here, we demonstrate that AQP3 is also expressed in cultured human skin fibroblasts, which under normal wound healing processes migrate from surrounding tissues to close the wound. EGF (epidermal growth factor), which induced fibroblast migration, also induced AQP3 expression in a time- and dose-dependent manner. CuSO4 and NiCl2, previously known as AQP3 water transport inhibitors, as well as two other bivalent heavy metals Mn2+ and Co2+, inhibited EGF-induced cell migration in human skin fibroblasts. AQP3 knockdown by small interfering RNA inhibited EGF-induced AQP3 expression and cell migration. Furthermore, an EGFR (EGF receptor) kinase inhibitor, PD153035, blocked EGF-induced AQP3 expression and cell migration. MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase]/ERK inhibitor U0126 and PI3K (phosphoinositide 3-kinase) inhibitor LY294002 also inhibited EGF-induced AQP3 expression and cell migration. Collectively, our findings show for the first time that AQP3 is expressed in human skin fibroblasts and that EGF induces AQP3 expression via EGFR, PI3K and ERK signal transduction pathways. We have provided evidence for a novel role of AQP3 in human skin fibroblast cell migration, which occurs during normal wound healing.

The role of aquaporin 3 in teleost fish

The aquaporin isoform, AQP3 has now been identified in a number of different teleost fish species, with additional DNA sequence information on AQP3 genes in further fish species available in genome databases. In zebrafish (Danio rerio), the AQP3 gene is present as two duplicate isoforms resulting from a teleostean fish genome-wide duplication. A further splicoform/isoform has also been identified in rainbow trout (Oncorhynchus mykiss). The identification of these AQP3 isoforms in other fish species is consequently explored. The role of AQP3 in physiological/osmoregulatory processes, in various teleost organs is then described. In teleost gill, AQP3 is expressed in 'chloride' cells, and in some species, in other epithelial cell types, where it may have a number of different functions including the prevention of dehydration. In eel esophagus, immunohistochemistry shows that AQP3 is expressed in surface epithelial cells in the anterior esophagus, but in mucus cells within the epithelium of the posterior esophagus. In eel intestine, AQP3 is found in macrophage-like cells and probably plays no part in osmoregulatory processes. In the rectum, as in the posterior esophagus AQP3 is expressed in mucus cells. In eel kidney, AQP3 is expressed in a subset of renal tubules, and localizes to the apical pole of tubule cells. There is no apparent change in the location or protein abundance of renal AQP3 following the acclimation of eels from freshwater to seawater.

Expression of heterologous aquaporins for functional analysis in Saccharomyces cerevisiae

In this study the yeast Saccharomyces cerevisiae, which is a genetically tractable model for analysis of osmoregulation, has been used for analysis of heterologous aquaporins. Aquaporin water channels play important roles in the control of water homeostasis in individual cells and multicellular organisms. We have investigated the effects of functional expression of the mammalian aquaporins AQP1 and AQP5 and the aquaglyceroporins AQP3 and AQP9. Expression of aquaporins caused moderate growth inhibition under hyperosmotic stress, while expression of aquaglyceroporins mediated strong growth inhibition due to glycerol loss. Water transport was monitored in protoplasts, where the kinetics of bursting was influenced by presence of aquaporins but not aquaglyceroporins. We observed glycerol transport through aquaglyceroporins, but not aquaporins, in a yeast strain deficient in glycerol production, whose growth depends on glycerol inflow. In addition, a gene reporter assay allowed to indirectly monitor the effect of AQP9-mediated enhanced glycerol loss on osmoadaptation. Transport activity of certain aqua(glycero)porins was diminished by low pH or CuSO4, suggesting that yeast can potentially be used for screening of putative aquaporin inhibitors. We conclude that yeast is a versatile system for functional studies of aquaporins, and it can be developed to screen for compounds of potential pharmacological use.

Aquaporin gating

An acceleration in the rate at which new aquaporin structures are determined means that structural models are now available for mammalian AQP0, AQP1, AQP2 and AQP4, bacterial GlpF, AqpM and AQPZ, and the plant SoPIP2;1. With an apparent consensus emerging concerning the mechanism of selective water transport and proton extrusion, emphasis has shifted towards the issues of substrate selectivity and the mechanisms of aquaporin regulation. In particular, recently determined structures of plant SoPIP2;1, sheep and bovine AQP0, and Escherichia coli AQPZ provide new insights into the underlying structural mechanisms by which water transport rates are regulated in diverse organisms. From these results, two distinct pictures of 'capping' and 'pinching' have emerged to describe aquaporin gating.

Myocardial water handling and the role of aquaporins

Cardiac surgery is performed in approximately 770,000 adults and 30,000 children in the United States of America annually. In this review we outline the mechanistic links between post-operative myocardial stunning and the development of myocardial edema. These interrelated processes cause a decline in myocardial performance that account for significant morbidity and mortality after cardiac surgery. Factors leading to myocardial edema include hemodilution, ischemia and reperfusion as well as osmotic gradients arising from pathological change. Several members of the aquaporin family of water transport proteins have been described in the myocardium although their role in the pathogenesis and resolution of cardiac edema is not established. This review examines evidence for the involvement of aquaporins in myocardial water handling during normal and pathological conditions.

Function of aquaporins in female and male reproductive systems

The flow of water and some other small molecules across cell membranes is important in many of the processes underlying reproduction. The fluid movement is strongly associated with the presence of aquaporins (AQPs) in the female and male reproductive systems. It has been suggested that AQPs mediate water movement into the antral follicle and play important roles in follicle development. AQPs are known to be involved in the early stage of spermatogenesis, in the secretion of tubule liquid and in the concentration and storage of spermatozoa. Fluid reabsorption in some regions of the male reproductive tract is under steroid hormone control and could be mediated by various AQPs. Also AQPs take part in the processes of fertilization, blastocyst formation (as the pathway for transtrophoectodermal water movement during cavitation) and implantation. Alterations in the expression and function or regulation of AQPs have already been demonstrated in disorders of the male reproductive system, such as abnormal sperm motility, the abnormal epididymis and infertility seen in cystic fibrosis, and varicocele. This article extensively reviews the distribution of AQPs in mammalian reproductive tissues and discusses their possible physiological and pathophysiological roles.

Aquaporin-11: a channel protein lacking apparent transport function expressed in brain

Background

The aquaporins are a family of integral membrane proteins composed of two subfamilies: the orthodox aquaporins, which transport only water, and the aquaglyceroporins, which transport glycerol, urea, or other small solutes. Two recently described aquaporins, numbers 11 and 12, appear to be more distantly related to the other mammalian aquaporins and aquaglyceroporins.

Results

We report on the characterization of Aquaporin-11 (AQP11). AQP11 RNA and protein is found in multiple rat tissues, including kidney, liver, testes and brain. AQP11 has a unique distribution in brain, appearing in Purkinje cell dendrites, hippocampal neurons of CA1 and CA2, and cerebral cortical neurons. Immunofluorescent staining of Purkinje cells indicates that AQP11 is intracellular. Unlike other aquaporins, Xenopus oocytes expressing AQP11 in the plasma membrane failed to transport water, glycerol, urea, or ions.

Conclusion

AQP11 is functionally distinct from other proteins of the aquaporin superfamily and could represent a new aquaporin subfamily. Further studies are necessary to elucidate the role of AQP11 in the brain.

Structural mechanism of plant aquaporin gating

Plants counteract fluctuations in water supply by regulating all aquaporins in the cell plasma membrane. Channel closure results either from the dephosphorylation of two conserved serine residues under conditions of drought stress, or from the protonation of a conserved histidine residue following a drop in cytoplasmic pH due to anoxia during flooding. Here we report the X-ray structure of the spinach plasma membrane aquaporin SoPIP2;1 in its closed conformation at 2.1 A resolution and in its open conformation at 3.9 A resolution, and molecular dynamics simulations of the initial events governing gating. In the closed conformation loop D caps the channel from the cytoplasm and thereby occludes the pore. In the open conformation loop D is displaced up to 16 A and this movement opens a hydrophobic gate blocking the channel entrance from the cytoplasm. These results reveal a molecular gating mechanism which appears conserved throughout all plant plasma membrane aquaporins.

Lead induces increased water permeability in astrocytes expressing aquaporin 4

The water channel aquaporin 4 (AQP4) is abundantly expressed in astrocytes. There is now compelling evidence that AQP4 may contribute to an unfavorable course in brain edema. Acute lead intoxication is a condition that causes brain damage preceded by brain edema. Here we report that lead increases AQP4 water permeability (P(f)) in astrocytes. A rat astrocyte cell line that does not express aquaporin 4 was transiently transfected with aquaporin 4 tagged with green fluorescent protein (GFP). Using confocal laser scanning microscopy we measured water permeability in these cells and in AQP4-negative cells located on the same plate. AQP4-expressing astrocytes had a three-fold higher water permeability than astrocytes not expressing AQP4. Lead exposure induced a significant, 40%, increase in water permeability in astrocytes expressing AQP4, but had no effect on P(f) in astrocytes not expressing AQP4. The increase in water permeability persisted after lead washout, while treatment with a lead chelator, meso-2,3-dimercaptosuccinic acid, abolished the lead-induced increase in P(f). The effect of lead was attenuated in the presence of a calcium (Ca(2+))/calmodulin-dependent protein kinase II (CaMKII) inhibitor, but not in the presence of a protein kinase C inhibitor. In cells expressing AQP4 where the consensus site for CaMKII phosphorylation was mutated, lead failed to increase water permeability. Lead exposure also increased P(f) in rat astroglial cells in primary culture, which express endogenous AQP4. Lead had no effect on P(f) in astrocytes transfected with aquaporin 3. In situ hybridization studies on rat brain after oral lead intake for three days showed no change in distribution of AQP4 mRNA. It is suggested that lead-triggered stimulation of water transport in AQP4-expressing astrocytes may contribute to the pathology of acute lead intoxication.

Opposite effects of Ni2+ on Xenopus and rat ENaCs expressed in Xenopus oocytes

The epithelial Na+ channel (ENaC) is modulated by various extracellular factors, including Na+, organic or inorganic cations, and serine proteases. To identify the effect of the divalent Ni2+ cation on ENaCs, we compared the Na+ permeability and amiloride kinetics of Xenopus ENaCs (xENaCs) and rat ENaCs (rENaCs) heterologously expressed in Xenopus oocytes. We found that the channel cloned from the kidney of the clawed toad Xenopus laevis [wild-type (WT) xENaC] was stimulated by external Ni2+, whereas the divalent cation inhibited the channel cloned from the rat colon (WT rENaC). The kinetics of amiloride binding were determined using noise analysis of blocker-induced fluctuation in current adapted for the transoocyte voltage-clamp method, and Na+ conductance was assessed using the dual electrode voltage-clamp (TEVC) technique. The inhibitory effect of Ni2+ on amiloride binding is not species dependent, because Ni2+ decreased the affinity (mainly reducing the association rate constant) of the blocker in both species in competition with Na+. Importantly, using the TEVC method, we found a prominent difference in channel conductance at hyperpolarizing voltage pulses. In WT xENaCs, the initial ohmic current response was stimulated by Ni2+, whereas the secondary voltage-activated current component remained unaffected. In WT rENaCs, only a voltage-dependent block by Ni2+ was obtained. To further study the origin of the xENaC stimulation by Ni2+, and based on the rationale of the well-known high affinity of Ni2+ for histidine residues, we designed alpha-subunit mutants of xENaCs by substituting histidines that were expressed in oocytes, together with WT beta- and gamma-subunits. Changing His215 to Asp in one putative amiloride-binding domain (WYRFHY) in the extracellular loop between Na+ channel membrane segments M1 and M2 had no influence on the stimulatory effect of Ni2+, and neither did complete deletion of this segment. Next, we mutated His416 flanked by His411 and Cys417, a unique site for possible heavy metal ion chelation, and, with this quality, most proximal (approximately 100 amino acids upstream of the second putative amiloride binding site at the pore entrance), was found localized at M2. Replacing His416 with arginine, aspartate, tyrosine, and alanine clearly affected amiloride binding in all cases, as well as Na+ conductance, as expressed in the xENaC current-voltage relationship, especially with regard to aspartate and tyrosine. However, similarly to those obtained with the WYRFHY stretch, none of these mutations could either abolish the stimulating effect of Ni2+ or reverse it to an inhibitory type.

Water channels (aquaporins) and their role for postnatal adaptation

Birth is a transition from an underwater life in the uterus to a terrestrial life in a milieu where supply of water is limited. Rapid adaptation to the new environment is crucial for survival and health of infants. The discovery of a family of molecules-aquaporin (AQP) water channels-that are responsible for regulated water transport across cell membranes has made it possible to identify the molecular mechanisms behind the postnatal homeostatic adaptation and to better understand water imbalance-related disorders in infancy and childhood. Thirteen mammalian AQP isoforms have been identified, most of them having a unique tissue-specific pattern of expression. Most mammalian AQPs can be dynamically regulated, which makes them potential targets for the development of new drugs for diseases associated with disturbances in water homeostasis. This review deals with AQP in kidney, lung, and brain. Evidence is presented that AQPs are expressed in a specific age-dependent manner and that the timed expression of AQPs may have a crucial role during the early postnatal period.

From structure to disease: the evolving tale of aquaporin biology

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. 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. The recognition of aquaporins has stimulated a reconsideration of membrane water permeability by investigators across a wide range of disciplines.

Copper inhibits the water and glycerol permeability of aquaporin-3

Aquaporin-3 (AQP3) is an aquaglyceroporin expressed in erythrocytes and several other tissues. Erythrocytes are, together with kidney and liver, the main targets for copper toxicity. Here we report that both water and glycerol permeability of human AQP3 is inhibited by copper. Inhibition is fast, dose-dependent, and reversible. If copper is dissolved in carbonic acid-bicarbonate buffer, the natural buffer system in our body, doses in the range of those observed in Wilson disease and in copper poisoning caused significant inhibition. AQP7, another aquaglyceroporin, was insensitive to copper. Three extracellular amino acid residues, Trp128, Ser152, and His241, were identified as responsible for the effect of copper on AQP3. We have previously shown that Ser152 is involved in regulation of AQP3 by pH. The fact that Ser152 mediates regulation of AQP3 by copper may explain the phenomenon of exquisite sensitivity of human erythrocytes to copper at acidic pH. When AQP3 was co-expressed with another AQP, only glycerol but not water permeability was inhibited by copper. Our results provide a better understanding of processes that occur in severe copper metabolism defects such as Wilson disease and in copper poisoning.

Aquaporin 3 colocates with phospholipase d2 in caveolin-rich membrane microdomains and is downregulated upon keratinocyte differentiation

Aquaporin 3 is a channel that transports both water and glycerol. Aquaporin 3-deficient mice exhibit skin defects, including decreased glycerol content and impairment of water holding capacity, barrier recovery, and wound healing. Whether aquaporin 3 and its glycerol transporting capacity are involved in regulating keratinocyte function, we have previously shown that phospholipase D2 can metabolize phospholipids in the presence of glycerol to yield phosphatidylglycerol. We hypothesized that aquaporin 3 is involved in the regulation of keratinocyte function by a mechanism involving the interaction between aquaporin 3 and phospholipase D. Using sucrose gradient centrifugation, immunoprecipitation analysis, and confocal microscopy, we found that aquaporin 3 and phospholipase D2 colocalized in caveolin-rich membrane microdomains. In addition, aquaporin 3 expression was downregulated at the transcriptional level and glycerol uptake was reduced upon primary mouse keratinocytes to differentiation in response to an elevated extracellular calcium concentration or 1,25-dihydroxyvitamin D3. Our results suggest that aquaporin 3 and phospholipase D2 form a signaling module in lipid rafts, where aquaporin 3 transports glycerol to phospholipase D2 for the synthesis of phosphatidylglycerol. Phosphatidylglycerol, as a bioactive lipid, could potentially mediate the effects of the aquaporin 3-phospholipase D2 signaling module, with aquaporin 3 as a modulatory unit, in the regulation of keratinocyte function.

Regulation of brain aquaporins

Emerging evidence suggests that brain aquaporins (AQP) play important roles for the dynamic regulation of brain water homeostasis and for the regulation of cerebrospinal fluid production. This review deals with the short- and long-term regulation of AQP4 and AQP9, both expressed in astrocytes, and of AQP1, expressed in the choroid plexus. AQP1 and 4 have in other cell types been shown to be regulated by phosphorylation. Phosphorylation affects the gating of AQP4 and the trafficking and insertion into membrane of AQP1. Mercury inhibits the water permeability of AQP1 and AQP9, but not AQP4. The permeability of AQP4 is increased by lead. AQP4 is also regulated by protein-protein interaction. The assembly between AQP4 and syntrophin is required for the proper localization of AQP4 in the astrocyte plasma membrane that faces capillaries. There is evidence from studies on peripheral tissues that steroid hormones regulate the expression of AQP1, AQP4 and AQP9. There is also evidence that the expression of AQP1 can be regulated by ubiquitination, and that osmolality can regulate the expression of AQP1, AQP4 and AQP9. Further insight into the mechanisms by which brain AQPs are regulated will be of utmost clinical importance, since perturbed water flow via brain AQPs has been implicated in many neurological diseases and since, in brain edema, water flow via AQP4 may have a harmful effect.