New potent inhibitors of aquaporins: silver and gold compounds inhibit aquaporins of plant and human origin

TNF-alpha-induced aquaporin 9 in synoviocytes from patients with OA and RA

OBJECTIVES

To determine whether aquaporins (AQPs) are expressed in the synovial tissues of patients with OA and RA, and to examine the patterns of expression in patients with and without hydrarthrosis.

METHODS

AQPs were detected in synovial tissue samples from patients with OA and RA using RT-PCR and immunohistochemistry. Fibroblast-like synoviocytes (FLSs) from patients with OA and RA were cultured and stimulated with TNF-alpha. The expression of AQPs in FLSs was examined using RT-PCR and western blot analyses and the function of aquaglyceroporins was examined by a glycerol uptake assay.

RESULTS

AQP1, -3 and -9 mRNAs were expressed in synovial tissues from patients with OA and RA. AQP1, -3 and -9 proteins were also detected by immunohistochemistry. AQP9 mRNA was expressed more strongly in the synovial tissues of OA patients with hydrarthrosis than those without. AQP9 mRNA and protein expression were strongly induced with TNF-alpha treatment in FLSs, whereas the expression of AQP1 and -3 mRNAs was not induced with TNF-alpha treatment. AQP9 as an aquaglyceroporin was induced by TNF-alpha.

CONCLUSIONS

AQP9 mRNA was detected in synovial tissues from OA and RA patients with hydrarthrosis. AQP9 expression was strongly induced in FLSs with TNF-alpha. Although the functions of AQP1, -3 and -9 in synovial tissues remain to be elucidated, it suggested that AQP9 might be related to the pathogenesis of hydrarthrosis and inflammatory synovitis.

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.

Functional analysis of putative genes encoding the PIP2 water channel subfamily in Populus trichocarpa

We located fully sequenced putative genes of the plasma membrane intrinsic proteins (PIPs) family in the Populus trichocarpa (Torr. Gray), genome. Of 23 gene candidates, we assigned eight genes to the PIP2 subfamily. All eight putative genes were expressed in vegetative tissues (roots, leaves, bark and wood), and all of them showed water channel activity after being expressed in Xenopus oocytes. Six of eight proteins were affected by mercury ions. No proteins were affected by the presence of nickel or tungsten ions, or by lowering the pH of bathing external solution from 7.4 to 6.5. The presence of copper ions caused seven of eight PIP2 proteins to increase their water transport capacity by as much as 50%. This systematic study of the PIP2 subfamily of proteins in P. trichocarpa provides a basic overview of their activity as water channels and will be a useful reference for future physiological studies of plant water relations that use P. trichocarpa as a model system.

Aquaporins: translating bench research to human disease

There is considerable potential for translating knowledge of aquaporin structure, function and physiology to the clinic. One area is in aquaporin-based diagnostics. The discovery of AQP4 autoantibodies as a marker of the neuromyelitis optica form of multiple sclerosis has allowed precise diagnosis of this disease. Other aquaporin-based diagnostics are possible. Another area is in aquaporin-based genetics. Genetic diseases caused by loss-of-function mutations in aquaporins include nephrogenic diabetes insipidus and cataracts, and functionally significant aquaporin polymorphisms are beginning to be explored. Perhaps of greatest translational potential is aquaporin-based therapeutics. Information largely from aquaporin knockout mice has implicated key roles of aquaporin-facilitated water transport in transepithelial fluid transport (urinary concentrating, gland fluid secretion), water movement into and out of the brain, cell migration (angiogenesis, tumor metastasis, wound healing) and neural function (sensory signaling, seizures). A subset of aquaporins that transport both water and glycerol, the 'aquaglyceroporins', regulate glycerol content in epidermal, fat and other tissues, and are involved in skin hydration, cell proliferation, carcinogenesis and fat metabolism. Aquaporin-based modulator drugs are predicted to be of broad potential utility in the treatment of edematous states, cancer, obesity, wound healing, epilepsy and glaucoma. These exciting possibilities and their associated challenges are reviewed.

Inhibition of the aquaporin 3 water channel increases the sensitivity of prostate cancer cells to cryotherapy

Aquaporins (AQPs) are intrinsic membrane proteins that facilitate selective water and small solute movement across the plasma membrane. In this study, we investigate the role of inhibiting AQPs in sensitising prostate cancer cells to cryotherapy. PC-3 and DU145 prostate cancer cells were cooled to 0, −5 and −10°C. The expression of AQP3 in response to freezing was determined using real-time quantitative polymerase chain reaction (RT–qPCR) and western blot analysis. Aquaporins were inhibited using mercuric chloride (HgCl₂) and small interfering RNA (siRNA) duplex, and cell survival was assessed using a colorimetric assay. There was a significant increase in AQP3 expression in response to freezing. Cells treated with AQP3 siRNA were more sensitive to cryoinjury compared with control cells (P<0.001). Inhibition of the AQPs by HgCl₂ also increased the sensitivity of both cell lines to cryoinjury and there was a complete loss of cell viability at −10°C (P<0.01). In conclusion, we have shown that AQP3 is involved directly in cryoinjury. Inhibition of AQP3 increases the sensitivity of prostate cancer cells to freezing. This strategy may be exploited in the clinic to improve the efficacy of prostate cryotherapy.

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.

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.

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.

Acetazolamide reversibly inhibits water conduction by aquaporin-4

Aquaporin-4 (AQP4) has been implicated in cytotoxic brain edema resulting from water intoxication, brain ischemia or meningitis. AQP4 inhibitors suitable for clinical use would thus be expected to help protect against brain edema. Here, we report the effect of inhibitors on water conduction by AQP4 and AQP1 reconstituted into liposomes. Acetazolamide (AZA), an inhibitor of sulfonamide carbonic anhydrase (CA), reversibly inhibits water permeation through AQP4, but not through AQP1. Methazolamide (MZA), another sulfonamide CA inhibitor similar in chemical structure to AZA, shows no significant effect on water conduction by AQP4 or AQP1. Our results thus demonstrate that AZA acts as a reversible inhibitor for AQP4-mediated water conduction and indicate that AZA is specific, at least to some degree, for AQP4. AZA may thus serve as a lead compound for the development of AQP4-specific inhibitors for clinical applications.

A smoke-derived butenolide alleviates HgCl2 and ZnCl2 inhibition of water uptake during germination and subsequent growth of tomato--possible involvement of aquaporins

Aquaporins, concentrated in zones of cell division and enlargement, play a major role in governing the movement of water between neighboring cells during seed germination. The enhanced germination and growth found with smoke-water and butenolide could be the result of better water uptake, suggesting the involvement of aquaporins. The effects of butenolide, known aquaporin inhibitors (HgCl(2) and ZnCl(2)), along with several chemical agents known to reverse the inhibitory effects of mercuric chloride on the activity of aquaporins were tested. Seedlings raised in the presence of butenolide had higher moisture content (93%) compared to those imbibed in water only (85%). This suggests enhanced activity of aquaporins. The presence of aquaporin inhibitors (HgCl(2) and ZnCl(2)) reduced seedling water content and altered root development. The presence of HgCl(2) (10, 20 or 30 microM) reduced the percentage imbibition of seeds by 11-12%. A corresponding gradual decline, from 17.5% (10 microM) to 22.6% (30 microM) (p0.05), in the root length was recorded. Addition of dithiothreitol (DTT, 500 microM), beta-mercaptoethanol (ME, 250 microM) and butenolide (0.1 microM) along with the HgCl(2) overcame the observed inhibitory effects. The presence of ZnCl(2) (12.5 or 25 microM) affected percentage imbibition as well as root length, which was reversed to some extent following addition of the butenolide. Though zinc chloride-mediated inhibition remained unaffected by the presence of DTT and ME, the butenolide reversed the effect. These results are interesting as they suggest additional avenues of research for uncovering the profound effect butenolide has on germination and growth.

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.

Aquaporins and plant water balance

The impact of aquaporin function on plant water balance is discussed. The significance of these proteins for root water uptake, water conductance in the xylem, including embolism refilling and the role of plant aquaporins in leaf physiology, is described. Emphasis is placed on certain aspects of water stress reactions and the correlation of aquaporins to abscisic acid as well as on the relation of water and CO2 permeability in leaves.

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.

Plant aquaporins: membrane channels with multiple integrated functions

Aquaporins are channel proteins present in the plasma and intracellular membranes of plant cells, where they facilitate the transport of water and/or small neutral solutes (urea, boric acid, silicic acid) or gases (ammonia, carbon dioxide). Recent progress was made in understanding the molecular bases of aquaporin transport selectivity and gating. The present review examines how a wide range of selectivity profiles and regulation properties allows aquaporins to be integrated in numerous functions, throughout plant development, and during adaptations to variable living conditions. Although they play a central role in water relations of roots, leaves, seeds, and flowers, aquaporins have also been linked to plant mineral nutrition and carbon and nitrogen fixation.

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+.

Aquaporins and unloading of phloem-imported water in coats of developing bean seeds

Nutrients are imported into developing legume seeds by mass flow through the phloem, and reach developing embryos following secretion from their symplasmically isolated coats. To sustain homeostasis of seed coat water relations, phloem-delivered nutrients and water must exit seed coats at rates commensurate with those of import through the phloem. In this context, coats of developing French bean seeds were screened for expression of aquaporin genes resulting in cloning PvPIP1;1, PvPIP2;2 and PvPIP2;3. These genes were differentially expressed in all vegetative organs, but exhibited their strongest expression in seed coats. In seed coats, expression was localized to cells of the nutrient-unloading pathway. Transport properties of the PvPIPs were characterized by expression in Xenopus oocytes. Only PvPIP2;3 showed significant water channel activity (Pos = 150-200 microm s(-1)) even when the plasma membrane intrinsic proteins (PIPs) were co-expressed in various combinations. Permeability increases to glycerol, methylamine and urea were not detected in oocytes expressing PvPIPs. Transport active aquaporins in native plasma membranes of seed coats were demonstrated by measuring rates of osmotic shrinkage of membrane vesicles in the presence and absence of mercuric chloride and silver nitrate. The functional significance of aquaporins in nutrient and water transport in developing seeds is discussed.

Developmental regulation of water uptake in wheat

The discovery of aquaporins has provided a new basis for studying and interpreting water relations in plants. However, slow progress has been made in elucidating the functional facets of the aquaporin-mediated water pathway in whole plant systems. While increasing experimental evidence suggests that these proteins are directly involved in mediating water homeostasis at varying environmental conditions, only a few attempts have been made to understand their contribution to overall water transport at different developmental stages. By using a chemical inhibitor (HgCl(2)) of aquaporins function, here we present in planta evidence for both diurnal and developmental regulation of aquaporin activity in wheat. We demonstrate that the greatest sensitivity of water flux to pharmacological blockage occurs at the stage of ear emergence and does not coincide with the phenological stage at which the greatest plant water uptake occurs (milky ripeness). The relationship transpiration flux (Q) vs. soil-leaf water potential difference (DeltaPsi(soil-leaves)) revealed a gradual decrease of plant resistance to water flux from tillering to milky ripeness, both in HgCl(2)-treated and untreated control plants. However, the mercury-inhibition of water flux began to gradually increase at ear emergence, suggesting that a larger portion of water moves through aquaporins from this developmental stage on. Although the intercept of the DeltaPsi(soil-leaves)/Q regression line, i.e. the DeltaPsi required to initiate the water flux through the soil-plant-air continuum, was generally not affected by mercury treatment, a significant mercury effect on the intercept was observed at the stage of ear formation. These findings may have important implications for predicting which strategy plants utilize to optimize water use during their life cycle.

Cytotoxic edema: mechanisms of pathological cell swelling

✓Cerebral edema is caused by a variety of pathological conditions that affect the brain. It is associated with two separate pathophysiological processes with distinct molecular and physiological antecedents: those related to cytotoxic (cellular) edema of neurons and astrocytes, and those related to transcapillary flux of Na+and other ions, water, and serum macromolecules. In this review, the authors focus exclusively on the first of these two processes. Cytotoxic edema results from unchecked or uncompensated influx of cations, mainly Na+, through cation channels. The authors review the different cation channels that have been implicated in the formation of cytotoxic edema of astrocytes and neurons in different pathological states. A better understanding of these molecular mechanisms holds the promise of improved treatments of cerebral edema and of the secondary injury produced by this pathological process.

Uptake of oxytetracycline and its phytotoxicity to alfalfa (Medicago sativa L.)

A series of experiments were conducted in a hydroponic system to investigate the uptake of oxytetracycline (OTC) and its toxicity to alfalfa (Medicago sativa L.). OTC inhibited alfalfa shoot and root growth by up to 61% and 85%, respectively. The kinetics of OTC uptake could be well described by Michaelis-Menten equation with Vmax of 2.25 micromol g-1 fresh weight h-1, and Km of 0.036 mM. The uptake of OTC by alfalfa was strongly inhibited by the metabolic inhibitor, 2,4-DNP (2,4-dinitrophenol), at pH 3.5 and 6.0, but not by the aquaporin competitors, glycerol and Ag+. OTC uptake, however, was significantly inhibited by Hg2+, suggesting that the inhibition of influx was due to general cellular stress rather than the specific action of Hg2+ on aquaporins. Results from the present study suggested that OTC uptake into alfalfa is an energy-dependent process.

Aquaporins: a promising target for drug development

Aquaporins (AQPs) are a family of small hydrophobic, integral membrane proteins that are expressed in all living organisms and play critical roles in controlling the water flow into and out of cells. So far, 13 different AQPs have been identified in mammals (AQP 0-12). AQPs have recently been implicated in various diseases such as cancer, cataract, brain oedema, gallstone disease and nephrogenic diabetes insipidus, as well as in the development of obesity and polycystic kidney disease. Interfering with the expression of AQPs will undoubtedly have therapeutic applications. Hence, in this review, the authors look at each AQP and its association with various pathological conditions in humans and demonstrate that they form potential targets for the treatment of such diseases.

Structural basis of aquaporin inhibition by mercury

The aquaporin family of channels was defined based on the inhibition of water transport by mercurial compounds. Despite the important role of mercurials, little is known about the structural changes involved upon mercury binding leading to channel inhibition. To elucidate the mechanism we designed a mutant, T183C, of aquaporin Z (AqpZ) patterned after the known mercury-sensitive site of aquaporin 1 (AQP1) and determined the X-ray crystal structures of the unbound and mercury blocked states. Superposition of the two structures shows no conformational rearrangement upon mercury binding. In the blocked structure, there are two mercury sites, one bound to Cys183 and occluding the pore, and a second, also bound to the same cysteine but found buried in an interstitial cavity. To test the mechanism of blockade we designed a different mutant, L170C, to produce a more effective mercury block at the pore site. In a dose-response inhibition study, this mutant was 20 times more sensitive to mercury than wild-type AqpZ and four times more sensitive than T183C. The X-ray structure of L170C shows four mercury atoms at, or near, the pore site defined in the T183C structure and no structural change upon mercury binding. Thus, we elucidate a steric inhibition mechanism for this important class of channels by mercury.

Comparative efficacy of HgCl2 with candidate aquaporin-1 inhibitors DMSO, gold, TEA+ and acetazolamide

Aquaporin-1 (AQP1) inhibitors are predicted to have multiple clinical applications. Hg(++) is a non-specific and toxic AQP1 blocker. We compared compounds with reported AQP1 inhibition activity, including DMSO, Au(+++), Ag(+), tetraethylammonium and acetazolamide. Water permeability was measured by stopped-flow light scattering in erythrocytes and volume marker dilution in epithelial cells. Au(+++) inhibited AQP1 with IC50 approximately 14 microM, similar to 10 microM for Hg(++). DMSO slowed osmotic equilibration; however, the apparent inhibition was due to 'osmotic clamp' rather than AQP1 inhibition. Neither tetraethylammonium nor acetazolamide (to 10 mM) inhibited AQP1. Our data indicate the need to identify new AQP1 inhibitors.

Specific aquaporins facilitate the diffusion of hydrogen peroxide across membranes

The metabolism of aerobic organisms continuously produces reactive oxygen species. Although potentially toxic, these compounds also function in signaling. One important feature of signaling compounds is their ability to move between different compartments, e.g. to cross membranes. Here we present evidence that aquaporins can channel hydrogen peroxide (H2O2). Twenty-four aquaporins from plants and mammals were screened in five yeast strains differing in sensitivity toward oxidative stress. Expression of human AQP8 and plant Arabidopsis TIP1;1 and TIP1;2 in yeast decreased growth and survival in the presence of H2O2. Further evidence for aquaporin-mediated H2O2 diffusion was obtained by a fluorescence assay with intact yeast cells using an intracellular reactive oxygen species-sensitive fluorescent dye. Application of silver ions (Ag+), which block aquaporin-mediated water diffusion in a fast kinetics swelling assay, also reversed both the aquaporin-dependent growth repression and the H2O2-induced fluorescence. Our results present the first molecular genetic evidence for the diffusion of H2O2 through specific members of the aquaporin family.

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.

The structure, function and regulation of the nodulin 26-like intrinsic protein family of plant aquaglyceroporins

The nodulin 26-like intrinsic protein family is a group of highly conserved multifunctional major intrinsic proteins that are unique to plants, and which transport a variety of uncharged solutes ranging from water to ammonia to glycerol. Based on structure-function studies, the NIP family can be subdivided into two subgroups (I and II) based on the identity of the amino acids in the selectivity-determining filter (ar/R region) of the transport pore. Both subgroups appear to contain multifunctional transporters with low to no water permeability and the ability to flux multiple uncharged solutes of varying sizes depending upon the composition of the residues of the ar/R filter. NIPs are subject to posttranslational phosphorylation by calcium-dependent protein kinases. In the case of the family archetype, soybean nodulin 26, phosphorylation has been shown to stimulate its transport activity and to be regulated in response to developmental as well as environmental cues, including osmotic stresses. NIPs tend to be expressed at low levels in the plant compared to other MIPs, and several exhibit cell or tissue specific expression that is subject to spatial and temporal regulation during development.

Control of volume and turgor in stomatal guard cells

Water loss from plants is determined by the aperture of stomatal pores in the leaf epidermis, set by the level of vacuolar accumulation of potassium salt, and hence volume and turgor, of a pair of guard cells. Regulation of ion fluxes across the tonoplast, the key to regulation of stomatal aperture, can only be studied by tracer flux measurements. There are two transport systems in the tonoplast. The first is a Ca(2+)-activated channel, inhibited by phenylarsine oxide (PAO), responsible for the release of vacuolar K(+)(Rb(+)) in response to the "drought" hormone, abscisic acid (ABA). This channel is sensitive to pressure, down-regulated at low turgor and up-regulated at high turgor, providing a system for turgor regulation. ABA induces a transient stimulation of vacuolar ion efflux, during which the flux tracks the ion content (volume, turgor), suggesting ABA reduces the set-point of a control system. The second system, which is PAO-insensitive, is responsible for an ion flux from vacuole to cytoplasm associated with inward water flow following a hypo-osmotic transfer. It is suggested that this involves an aquaporin as sensor, and perhaps also as responder; deformation of the aquaporin may render it ion-permeable, or, alternatively, the deformed aquaporin may signal to an associated ion channel, activating it. Treatment with inhibitors of aquaporins, HgCl(2) or silver sulfadiazine, produces a large transient increase in ion release from the vacuole, also PAO-insensitive. It is suggested that this involves the same aquaporin, either rendered directly ion-permeable, or signalling to activate an associated ion channel.

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.

In vivo functional assay of a recombinant aquaporin in Pichia pastoris

The water channel protein PvTIP3;1 (alpha-TIP) is a member of the major intrinsic protein (MIP) membrane channel family. We overexpressed this eukaryotic aquaporin in the methylotrophic yeast Pichia pastoris, and immunogold labeling of cellular cryosections showed that the protein accumulated in the plasma membrane, as well as vacuolar and other intracellular membranes. We then developed an in vivo functional assay for water channel activity that measures the change in optical absorbance of spheroplasts following an osmotic shock. Spheroplasts of wild-type P. pastoris displayed a linear relationship between absorbance and osmotic shock level. However, spheroplasts of P. pastoris expressing PvTIP3;1 showed a break in this linear relationship corresponding to hypo-osmotically induced lysis. It is the difference between control and transformed spheroplasts under conditions of hypo-osmotic shock that forms the basis of our aquaporin activity assay. The aquaporin inhibitor mercury chloride blocked water channel activity but had no effect on wild-type yeast. Osmotically shocked yeast cells were affected only slightly by expression of the Escherichia coli glycerol channel GlpF, which belongs to the MIP family but is a weak water channel. The important role that aquaporins play in human physiology has led to a growing interest in their potential as drug targets for treatment of hypertension and congestive heart failure, as well as other fluid overload states. The simplicity of this assay that is specific for water channel activity should enable rapid screening for compounds that modulate water channel activity.

A role for mitochondrial aquaporins in cellular life-and-death decisions?

Mitochondria dominate the process of life-and-death decisions of the cell. Continuous generation of ATP is essential for cell sustenance, but, on the other hand, mitochondria play a central role in the orchestra of events that lead to apoptotic cell death. Changes of mitochondrial volume contribute to the modulation of physiological mitochondrial function, and several ion permeability pathways located in the inner mitochondrial membrane have been implicated in the mediation of physiological swelling-contraction reactions, such as the K+ cycle. However, the channels and transporters involved in these processes have not yet been identified. Osmotic swelling is also one of the fundamental characteristics exhibited by mitochondria in pathological situations, which activates downstream cascades, culminating in apoptosis. The permeability transition pore has long been postulated to be the primary mediator for water movement in mitochondrial swelling during cell death, but its molecular identity remains obscure. Inevitably, accumulating evidence shows that mitochondrial swelling induced by apoptotic stimuli can also occur independently of permeability transition pore activation. Recently, a novel mechanism for osmotic swelling of mitochondria has been described. Aquaporin-8 and -9 channels have been identified in the inner mitochondrial membrane of various tissues, including the kidney, liver, and brain, where they may mediate water transport associated with physiological volume changes, contribute to the transport of metabolic substrates, and/or participate in osmotic swelling induced by apoptotic stimuli. Hence, the recent discovery that aquaporins are expressed in mitochondria opens up new areas of investigation in health and disease.

Oxidative gating of water channels (aquaporins) in corn roots

An oxidative gating of water channels (aquaporins: AQPs) was observed in roots of corn seedlings as already found for the green alga Chara corallina. In the presence of 35 mM hydrogen peroxide (H2O2)--a precursor of hydroxyl radicals (*OH)--half times of water flow (as measured with the aid of pressure probes) increased at the level of both entire roots and individual cortical cells by factors of three and nine, respectively. This indicated decreases in the hydrostatic hydraulic conductivity of roots (Lp(hr)) and of cells (Lp(h)) by the same factors. Unlike other stresses, the plant hormone abscisic acid (ABA) had no ameliorative effect either on root LP(hr) or on cell Lp(h) when AQPs were inhibited by oxidative stress. Closure of AQPs reduced the permeability of acetone by factors of two in roots and 1.5 in cells. This indicated that AQPs were not ideally selective for water but allowed the passage of the organic solute acetone. In the presence of H2O2, channel closure caused anomalous (negative) osmosis at both the root and the cell level. This was interpreted by the fact that in the case of the rapidly permeating solute acetone, channel closure caused the solute to move faster than the water and the reflection coefficient (sigma s) reversed its sign. When H2O2 was removed from the medium, the effects were reversible, again at both the root and the cell level. The results provide evidence of oxidative gating of AQPs, which leads on to inhibition of water uptake by the roots. Possible mechanisms of the oxidative gating of AQPs induced by H2O2 (*OH) are discussed.

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

Excessive water uptake through Aquaporins (AQP) can be life-threatening and reversible AQP inhibitors are needed. Here, we determined the specificity, potency, and binding site of tetraethylammonium (TEA) to block Aquaporin water permeability. Using oocytes, externally applied TEA blocked AQP1/AQP2/AQP4 with IC50 values of 1.4, 6.2, and 9.8 microM, respectively. Related tetraammonium compounds yielded some (propyl) or no (methyl, butyl, or pentyl) inhibition. TEA inhibition was lost upon a Tyr to Phe amino acid switch in the external water pore of AQP1/AQP2/AQP4, whereas the water permeability of AQP3 and AQP5, which lack a corresponding Tyr, was not blocked by TEA. Consistent with experimental data, multi-nanosecond molecular dynamics simulations showed one stable binding site for TEA, but not tetramethyl (TMA), in AQP1, resulting in a nearly 50% water permeability inhibition, which was reduced in AQP1-Y186F due to effects on the TEA inhibitory binding region. Moreover, in the simulation TEA interacted with charged residues in the C (Asp128) and E (Asp185) loop, and the A(Tyr37-Asn42-Thr44) loop of the neighboring monomer, but not directly with Tyr186. The loss of TEA inhibition in oocytes expressing properly folded AQP1-N42A or -T44A is in line with the computationally predicted binding mode. Our data reveal that the molecular interaction of TEA with AQP1 differs and is about 1000-fold more effective on AQPs than on potassium channels. Moreover, the observed experimental and simulated similarities open the way for rational design and virtual screening for AQP-specific inhibitors, with quaternary ammonium compounds in general, and TEA in particular as a lead compound.

Modulating the expression of aquaporin genes in planta: A key to understand their physiological functions?

Aquaporins (AQPs) are believed to act as "cellular plumbers", allowing plants to rapidly alter their membrane water permeability in response to environmental cues. This study of AQP regulation at both the RNA and protein levels has revealed a large number of possible mechanisms. Currently, modulation of AQP expression in planta is considered the strategy of choice for elucidating the role of AQPs in plant physiology. This review highlights the fact that this strategy is complicated by many factors, such as the incomplete characterization of transport selectivity of the targeted AQP, the fact that AQPs might act as multifunctional channels with multiple physiological roles, and the number of post-translational regulation mechanisms. The classification of AQPs as constitutive or stress-responsive isoforms is also proposed.

Acetazolamide inhibits osmotic water permeability by interaction with aquaporin-1

Water channel proteins, known as aquaporins, are transmembrane proteins that mediate osmotic water permeability. In a previous study, we found that acetazolamide could inhibit osmotic water transportation across Xenopus oocytes by blocking the function of aquaporin-1 (AQP1). The purpose of the current study was to confirm the effect of acetazolamide on water osmotic permeability using the human embryonic kidney 293 (HEK293) cells transfected with pEGFP/AQP1 and to investigate the interaction between acetazolamide and AQP1. The fluorescence intensity of HEK293 cells transfected with pEGFP/AQP1, which corresponds to the cell volume when the cells swell in a hyposmotic solution, was recorded under confocal laser fluorescence microscopy. The osmotic water permeability was assessed by the change in the ratio of cell fluorescence to certain cell area. Acetazolamide, at concentrations of 1 and 10muM, inhibited the osmotic water permeability in HEK293 cells transfected with pEGFP/AQP1. The direct binding between acetazolamide and AQP1 was detected by surface plasmon resonance. AQP1 was prepared from rat red blood cells and immobilized on a CM5 chip. The binding assay showed that acetazolamide could directly interact with AQP1. This study demonstrated that acetazolamide inhibited osmotic water permeability through interaction with AQP1.

Plasma membrane of Beta vulgaris storage root shows high water channel activity regulated by cytoplasmic pH and a dual range of calcium concentrations

Plasma membrane vesicles isolated by two-phase partitioning from the storage root of Beta vulgaris show atypically high water permeability that is equivalent only to those reported for active aquaporins in tonoplast or animal red cells (Pf=542 microm s(-1)). The values were determined from the shrinking kinetics measured by stopped-flow light scattering. This high Pf was only partially inhibited by mercury (HgCl2) but showed low activation energy (Ea) consistent with water permeation through water channels. To study short-term regulation of water transport that could be the result of channel gating, the effects of pH, divalent cations, and protection against dephosphorylation were tested. The high Pf observed at pH 8.3 was dramatically reduced by medium acidification. Moreover, intra-vesicular acidification (corresponding to the cytoplasmic face of the membrane) shut down the aquaporins. De-phosphorylation was discounted as a regulatory mechanism in this preparation. On the other hand, among divalent cations, only calcium showed a clear effect on aquaporin activity, with two distinct ranges of sensitivity to free Ca2+ concentration (pCa 8 and pCa 4). Since the normal cytoplasmic free Ca2+ sits between these ranges it allows for the possibility of changes in Ca2+ to finely up- or down-regulate water channel activity. The calcium effect is predominantly on the cytoplasmic face, and inhibition corresponds to an increase in the activation energy for water transport. In conclusion, these findings establish both cytoplasmic pH and Ca2+ as important regulatory factors involved in aquaporin gating.

Angiotensin III as well as angiotensin II regulates water flow through aquaporins in a clam worm

Angiotensin III has been reported to exist in various animals and tissues. The physiological role, however, is still unclear except that brain angiotensin III is a central regulator of vasopressin release. In this study, angiotensin III as well as angiotensin II enhanced an increase in body weight of clam worms of Perinereis sp. under a hypo-osmotic condition and suppressed a decrease in body weight under a hyper-osmotic condition. When clam worms were treated with tetrachloroaurate (III) after angiotensin-treatment, these enhancing and suppressive effects of the angiotensins under hypo- and hyper-osmotic conditions were inhibited. In contrast, when clam worms were pretreated with tetrachloroaurate (III) before angiotensin-treatment, these effects of angiotensins were not inhibited. Since tetrachloroaurate (III) is a representative blocker of aquaporins, these results indicate that angiotensin III as well as angiotensin II regulates water flow through aquaporins in clam worms.

Regulation of plant aquaporin activity

Accumulating evidence indicates that aquaporins play a key role in plant water relations. Plant aquaporins are part of a large and highly divergent protein family that can be divided into four subfamilies according to amino acid sequence similarity. As in other organisms, plant aquaporins facilitate the transcellular movement of water, but, in some cases, also the flux of small neutral solutes across a cellular membrane. Plant cell membranes are characterized by a large range of osmotic water permeabilities, and recent data indicate that plant aquaporin activity might be regulated by gating mechanisms. The factors affecting the gating behaviour possibly involve phosphorylation, heteromerization, pH, Ca2+, pressure, solute gradients and temperature. Regulation of aquaporin trafficking may also represent a way to modulate membrane water permeability. The aim of this review is to integrate recent molecular and biophysical data on the mechanisms regulating aquaporin activity in plant membranes and to relate them to putative changes in protein structure.

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.

Functional characterization of a putative aquaporin from Encephalitozoon cuniculi, a microsporidia pathogenic to humans

The microsporidia are a group of obligate intracellular parasitic protists that have been implicated as both human and veterinary pathogens. The infectious process of these organisms is believed to be dependent upon the rapid influx of water into spores, presumably via aquaporins (AQPs), transmembrane channels that facilitate osmosis. An AQP-like sequence of the microsporidium Encephalitozoon cuniculi (EcAQP), when cloned and expressed in oocytes of Xenopus laevis, rendered these oocytes highly permeable to water. No permeability to the solutes glycerol or urea was observed. Pre-treatment of EcAQP-expressing oocytes with HgCl(2) failed to inhibit their osmotic permeability, as predicted from EcAQP's lack of mercury-sensitive cysteine residues near the NPA motifs which line the AQP aqueous pore. EcAQP exhibits sequence identity to AQP A of Dictyostelium discoideum (26%) and human AQP 2 (24%). Further study of AQPs in microsporidia and their potential inhibitors may yield novel therapeutic agents for microsporidian infections.

Establishment of HEK293 cell line expressing green fluorescent protein–aquaporin-1 to determine osmotic water permeability

Aquaporin (AQP) is a kind of channel-forming membrane glycoprotein that mediates osmotic water transport. The present study aimed to establish a cell line stably transfected with AQP1 to measure osmotic water permeability. The recombinant plasmid was constructed by subcloning the full-length rat AQP1 cDNA into pEGFP-C3 vector, named pEGFP/AQP1. Human embryonic kidney 293 cells were transfected with pEGFP/AQP1 and selected by G418 to obtain a cell line stably expressing AQP1 tagged with green fluorescent protein. The expression level of AQP1 in the stably transfected cell was detected by reverse transcription polymerase chain reaction and Western blot. The real-time change of fluorescence density, corresponding to cell swelling induced by hyposmotic solution, was recorded under confocal laser scanning microscope and used to assess osmotic water permeability. The typical AQP1 inhibitor, mercuric chloride, validated this osmotic water permeability assay. These results suggested that this transfected cell model could be conveniently used to determine osmotic water permeability.

Tonoplast intrinsic proteins AtTIP2;1 and AtTIP2;3 facilitate NH3 transport into the vacuole

While membrane transporters mediating ammonium uptake across the plasma membrane have been well described at the molecular level, little is known about compartmentation and cellular export of ammonium. (The term ammonium is used to denote both NH3 and NH4+ and chemical symbols are used when specificity is required.) We therefore developed a yeast (Saccharomyces cerevisiae) complementation approach and isolated two Arabidopsis (Arabidopsis thaliana) genes that conferred tolerance to the toxic ammonium analog methylammonium in yeast. Both genes, AtTIP2;1 and AtTIP2;3, encode aquaporins of the tonoplast intrinsic protein subfamily and transported methylammonium or ammonium in yeast preferentially at high medium pH. AtTIP2;1 expression in Xenopus oocytes increased 14C-methylammonium accumulation with increasing pH. AtTIP2;1- and AtTIP2;3-mediated methylammonium detoxification in yeast depended on a functional vacuole, which was in agreement with the subcellular localization of green fluorescent protein-fusion proteins on the tonoplast in planta. Transcript levels of both AtTIPs were influenced by nitrogen supply but did not follow those of the nitrogen-derepressed ammonium transporter gene AtAMT1;1. Transgenic Arabidopsis plants overexpressing AtTIP2;1 did not show altered ammonium accumulation in roots after ammonium supply, although AtTIP2;1 mRNA levels in wild-type plants were up-regulated under these conditions. This study shows that AtTIP2;1 and AtTIP2;3 can mediate the extracytosolic transport of methyl-NH2 and NH3 across the tonoplast membrane and may thus participate in vacuolar ammonium compartmentation.

Cd2+-induced cytochromecrelease in apoptotic proximal tubule cells: role of mitochondrial permeability transition pore and Ca2+uniporter

Cd2+induces apoptosis of kidney proximal tubule (PT) cells. Mitochondria play a pivotal role in toxic compound-induced apoptosis by releasing cytochrome c. Our objective was to investigate the mechanisms underlying Cd2+-induced cytochrome c release from mitochondria in rat PT cells. Using Hoechst 33342 or MTT assay, 10 μM Cd2+induced ∼5–10% apoptosis in PT cells at 6 and 24 h, which was associated with cytochrome c and apoptosis-inducing factor release at 24 h only. This correlated with previously described maximal intracellular Cd2+concentrations at 24 h, suggesting that elevated Cd2+may directly induce mitochondrial liberation of proapoptotic factors. Indeed, Cd2+caused swelling of energized isolated kidney cortex mitochondria (EC50∼9 μM) and cytochrome c release, which were independent of permeability transition pore (PTP) opening since PTP inhibitors cyclosporin A or bongkrekic acid had no effect. On the contrary, Cd2+inhibited swelling and cytochrome c release induced by PTP openers (PO43−or H2O2+Ca2+). The mitochondrial Ca2+uniporter (MCU) played a key role in mitochondrial damage: 1) MCU inhibitors (La3+, ruthenium red, Ru360) prevented swelling and cytochrome c release; and 2) ruthenium red attenuated Cd2+inhibition of PO43−-induced swelling. Using the Cd2+-sensitive fluorescent indicator FluoZin-1, Cd2+was also taken up by mitoplasts. The aquaporin inhibitor AgNO3abolished Cd2+-induced swelling of mitoplasts. This could be partially mediated by activation of the mitoplast-enriched water channel aquaporin-8. Thus cytosolic Cd2+concentrations exceeding a certain threshold may directly cause mitochondrial damage and apoptotic development by interacting with MCU and water channels in the inner mitochondrial membrane.

Acidocalcisomes and the contractile vacuole complex are involved in osmoregulation in Trypanosoma cruzi

Trypanosoma cruzi, the etiologic agent of Chagas disease, resists extreme fluctuations in osmolarity during its life cycle. T. cruzi possesses a robust regulatory volume decrease mechanism that completely reverses cell swelling when submitted to hypo-osmotic stress. The efflux of amino acids and K+ release could account for only part for this volume reversal. In this work we demonstrate that swelling of acidocalcisomes mediated by an aquaporin and microtubule- and cyclic AMP-mediated fusion of acidocalcisomes to the contractile vacuole complex with translocation of this aquaporin and the resulting water movement are responsible for the volume reversal not accounted for by efflux of osmolytes. Contractile vacuole bladders were isolated by subcellular fractionation in iodixanol gradients, showed a high concentration of basic amino acids and inorganic phosphate, and were able to transport protons in the presence of ATP or pyrophosphate. Taken together, these results strongly support a role for acidocalcisomes and the contractile vacuole complex in osmoregulation and identify a functional role for aquaporin in protozoal osmoregulation.

A functional aquaporin co-localizes with the vacuolar proton pyrophosphatase to acidocalcisomes and the contractile vacuole complex of Trypanosoma cruzi

We cloned an aquaporin gene from Trypanosoma cruzi (TcAQP) that encodes a protein of 231 amino acids, which is highly hydrophobic. The protein has six putative transmembrane domains and the two signature motifs asparagine-proline-alanine (NPA) which have been shown, in other aquaporins, to be involved in the formation of an aqueous channel spanning the bilayer. TcAQP was sensitive to endo H treatment, suggesting that the protein is N-glycosylated. Oocytes of Xenopus laevis expressing TcAQP swelled under hyposmotic conditions indicating water permeability, which was abolished after preincubating oocytes with very low concentrations of the AQP inhibitors HgCl(2) and AgNO(3). glycerol transport was detected. No Immunofluorescence microscopy of T. cruzi expressing GFP-TcAQP showed co-localization of TcAQP with the vacuolar proton pyrophosphatase (V-H(+)-PPase), a marker of acidocalcisomes. This localization was confirmed by Western blotting and immunofluorescence staining using polyclonal antibodies against a C-terminal peptide of TcAQP. In addition, there was a strong anterior labeling in a vacuole, close to the flagellar pocket, that was distinct from the acidocalcisomes and that was identified by immunogold electron microscopy as the contractile vacuole complex. Taking together, the presence of an aquaporin in acidocalcisomes and the contractile vacuole complex of T. cruzi, provides support for the role of these organelles in osmotic adaptations of these parasites.