Parasite aquaporins: Current developments in drug facilitation and resistance

Background

Although being situated in a niche, research on parasite aquaporins is a lively field that has provided new insight into basic aquaporin structure–function relationships and physiological roles of water and solute transport. Moreover, it bears the potential to find novel approaches to antiparasitic chemotherapy.

Scope of review

Here, we summarize the current knowledge about the structure and substrate selectivity of aquaporins from protozoan and helminth parasites, review the current views on their physiological roles, and discuss their potency for chemotherapy.

Major conclusions

Parasite aquaporins fulfill highly diverse tasks in the physiology of the various organisms, yet their general protein structure is well conserved. Aquaporins are directly (antimonials) and indirectly (melarsoprol, pentamidine) linked to the uptake of antiparasitic drugs. Unfortunately, drug-like aquaporin inhibitors are still missing.

General significance

Aquaporins expression levels determine the degree of parasite resistance against certain drugs. Further studies on parasite aquaporins may provide data about overcoming drug resistance mechanisms or even spark novel treatments. This article is part of a Special Issue entitled Aquaporins.     

Aquaporins in salivary glands and pancreas

Background

Salivary glands and pancreas are involved in saliva secretion, pancreatic fluid secretion and insulin secretion. These functions are essential for proper oral, pancreatic and glucose homeostasis. Aquaporins are water-permeable transmembrane protein involved in the physiology of these secretory gland functions.

Scope of review

This review gives an overview of the morphology of salivary glands and pancreas, the expression and localization of aquaporins, the secretion roles and mechanisms, the physiological roles of aquaporins, and the role of aquaporins in pathophysiological conditions.

Major conclusions

Several aquaporins are expressed in salivary glands and pancreas, and some play important physiological roles. Modulation of aquaporin expression and/or trafficking may contribute to the pathogenesis of diseases affecting salivary glands and pancreas glands such as xerostomic conditions, pancreatic insufficiencies and diabetes.

General significance

Aquaporins are involved in physiological and pathophysiological processes in salivary glands and pancreas. They could represent therapeutic targets for the treatment of diseases affecting the salivary glands and pancreas. This article is part of a Special Issue entitled Aquaporins.     

Aquaporins in the eye: Expression,function, and roles in ocular disease

Background

All thirteen known mammalian aquaporins have been detected in the eye. Moreover, aquaporins have been identified as playing essential roles in ocular functions ranging from maintenance of lens and corneal transparency to production of aqueous humor to maintenance of cellular homeostasis and regulation of signal transduction in the retina.

Scope of review

This review summarizes the expression and known functions of ocular aquaporins and discusses their known and potential roles in ocular diseases.

Major conclusions

Aquaporins play essential roles in all ocular tissues. Remarkably, not all aquaporin function as a water permeable channel and the functions of many aquaporins in ocular tissues remain unknown. Given their vital roles in maintaining ocular function and their roles in disease, aquaporins represent potential targets for future therapeutic development.

General significance

Since aquaporins play key roles in ocular physiology, an understanding of these functions is important to improving ocular health and treating diseases of the eye. It is likely that future therapies for ocular diseases will rely on modulation of aquaporin expression and/or function. This article is part of a Special Issue entitled Aquaporins.     

Aquaporins and membrane diffusion of CO2 in living organisms

Background

Determination of CO₂ diffusion rates in living cells revealed inconsistencies with existing models about the mechanisms of membrane gas transport. Mainly, these discrepancies exist in the determined CO₂ diffusion rates of bio-membranes, which were orders of magnitudes below those for pure lipid bilayers or theoretical considerations as well as in the observation that membrane insertion of specific aquaporins was rescuing high CO₂ transport rates. This effect was confirmed by functional aquaporin protein analysis in heterologous expression systems as well as in bacteria, plants and partly in mammals.

Scope of Review

This review summarizes the arguments in favor of and against aquaporin facilitated membrane diffusion of CO₂ and reports about its importance for the physiology of living organisms.

Major Conclusions

Most likely, the aquaporin tetramer forming an additional fifth pore is required for CO₂ diffusion facilitation. Aquaporin tetramer formation, membrane integration and disintegration could provide a mechanism for regulation of cellular CO₂ exchange. The physiological importance of aquaporin mediated CO₂ membrane diffusion could be shown for plants and cyanobacteria and partly for mammals.

General Significance

Taking the mentioned results into account, consequences for our current picture of cell membrane transport emerge. It appears that in some or many instances, membranes might not be as permeable as it was suggested by current bio-membrane models, opening an additional way of controlling the cellular influx or efflux of volatile substances like CO₂. This article is part of a Special Issue entitled Aquaporins.     

Aquaglyceroporins: Generalized metalloid channels

Background

Aquaporins (AQPs), members of a superfamily of transmembrane channel proteins, are ubiquitous in all domains of life. They fall into a number of branches that can be functionally categorized into two major sub-groups: i) orthodox aquaporins, which are water-specific channels, and ii) aquaglyceroporins, which allow the transport of water, non-polar solutes, such as urea or glycerol, the reactive oxygen species hydrogen peroxide, and gases such as ammonia, carbon dioxide and nitric oxide and, as described in this review, metalloids.

Scope of review

This review summarizes the key findings that AQP channels conduct bidirectional movement of metalloids into and out of cells.

Major conclusions

As(OH)₃ and Sb(OH)₃ behave as inorganic molecular mimics of glycerol, a property that allows their passage through AQP channels. Plant AQPs also allow the passage of boron and silicon as their hydroxyacids, boric acid (B(OH)₃) and orthosilicic acid (Si(OH)₄), respectively. Genetic analysis suggests that germanic acid (GeO₂) is also a substrate. While As(III), Sb(III) and Ge(IV) are toxic metalloids, borate (B(III)) and silicate (Si(IV)) are essential elements in higher plants.

General significance

The uptake of environmental metalloids by aquaporins provides an understanding of (i) how toxic elements such as arsenic enter the food chain; (ii) the delivery of arsenic and antimony containing drugs in the treatment of certain forms of leukemia and chemotherapy of diseases caused by pathogenic protozoa; and (iii) the possibility that food plants such as rice could be made safer by genetically modifying them to exclude arsenic while still accumulating boron and silicon. This article is part of a Special Issue entitled Aquaporins.     

Aquaporin-facilitated transmembrane diffusion of hydrogen peroxide

Background

Hydrogen peroxide (H₂O₂) is an important signaling compound that has recently been identified as a new substrate for several members of the aquaporin superfamily in various organisms. Evidence is emerging about the physiological significance of aquaporin-facilitated H₂O₂ diffusion.

Scope of review

This review summarizes current knowledge about aquaporin-facilitated H₂O₂ diffusion across cellular membranes. It focuses on physicochemical and experimental evidence demonstrating the involvement of aquaporins in the transport of this redox signaling compound and discusses the regulation and structural prerequisites of these channels to transmit this signal. It also provides perspectives about the potential importance of aquaporin-facilitated H₂O₂ diffusion processes and places this knowledge in the context of the current understanding of transmembrane redox signaling processes.

Major conclusions

Specific aquaporin isoforms facilitate the passive diffusion of H₂O₂ across biological membranes and control H₂O₂ membrane permeability and signaling in living organisms.

General significance

Redox signaling is a very important process regulating the physiology of cells and organisms in a similar way to the well-characterized hormonal and calcium signaling pathways. Efficient transmembrane diffusion of H₂O₂, a key molecule in the redox signaling network, requires aquaporins and makes these channels important players in this signaling process. Channel-mediated membrane transport allows the fine adjustment of H₂O₂ levels in the cytoplasm, intracellular organelles, the apoplast, and the extracellular space, which are essential for it to function as a signal molecule. This article is part of a Special Issue entitled Aquaporins.     

Arbuscular mycorrhizal symbiosis increases relative apoplastic water flow in roots of the host plant under both well-watered and drought stress conditions

Background and Aims

The movement of water through mycorrhizal fungal tissues and between the fungus and roots is little understood. It has been demonstrated that arbuscular mycorrhizal (AM) symbiosis regulates root hydraulic properties, including root hydraulic conductivity. However, it is not clear whether this effect is due to a regulation of root aquaporins (cell-to-cell pathway) or to enhanced apoplastic water flow. Here we measured the relative contributions of the apoplastic versus the cell-to-cell pathway for water movement in roots of AM and non-AM plants.

Methods

We used a combination of two experiments using the apoplastic tracer dye light green SF yellowish and sodium azide as an inhibitor of aquaporin activity. Plant water and physiological status, root hydraulic conductivity and apoplastic water flow were measured.

Key Results

Roots of AM plants enhanced significantly relative apoplastic water flow as compared with non-AM plants and this increase was evident under both well-watered and drought stress conditions. The presence of the AM fungus in the roots of the host plants was able to modulate the switching between apoplastic and cell-to-cell water transport pathways.

Conclusions

The ability of AM plants to switch between water transport pathways could allow a higher flexibility in the response of these plants to water shortage according to the demand from the shoot.     

Unraveling aquaporin interaction partners

Background

Insight into protein–protein interactions (PPIs) is highly desirable in order to understand the physiology of cellular events. This understanding is one of the challenges in biochemistry and molecular biology today, especially for eukaryotic membrane proteins where hurdles of production, purification and structural determination must be passed.

Scope of review

We have explored the common strategies used to find medically relevant interaction partners of aquaporins (AQPs). The most frequently used methods to detect direct contact, yeast two-hybrid interaction assay and co-precipitation, are described together with interactions specifically found for the selected targets AQP0, AQP2, AQP4 and AQP5.

Major conclusions

The vast majority of interactions involve the aquaporin C-terminus and the characteristics of the interaction partners are strikingly diverse. While the well-established methods for PPIs are robust, a novel approach like bimolecular fluorescence complementation (BiFC) is attractive for screening many conditions as well as transient interactions. The ultimate goal is structural evaluation of protein complexes in order to get mechanistic insight into how proteins communicate at a molecular level.

General significance

What we learn from the human aquaporin field in terms of method development and communication between proteins can be of major use for any integral membrane protein of eukaryotic origin. This article is part of a Special Issue entitled Aquaporins.     

Aquaporins in cancer

Background

The aquaporins (AQPs) are a family of 13 small hydrophobic integral transmembrane water channel proteins involved in transcellular and transepithelial water movement, transport of fluid and cell migration.

Scope of the review

This review article summarizes our knowledge concerning the involvement of AQPs in tumor growth, angiogenesis and metastatic process.

Major conclusions

Tumor cells types express AQPs and a positive correlation exists between histological tumor grade and the AQP expression. Moreover, AQPs are involved also in tumor edema formation and angiogenesis in several solid and hematological tumors.

General significance

AQPs inhibition in endothelial and tumor cells might limit tumor growth and spread, suggesting a potential therapeutic use in the treatment of tumors. This article is part of a Special Issue entitled Aquaporins.     

Nitrogen availability affects hydraulic conductivity of rice roots,possibly through changes in aquaporin gene expression

Background and aims

Nitrogen (N) availability affects water uptake from the roots, which decreases upon N deprivation and increases upon resupply. The aim of this study was to reveal possible mechanisms of regulation of water transport in roots through physiological and morphological adaptations to N availability.

Methods

The effects of continuous N deprivation and following resupply on root morphology, osmotic hydraulic conductivity, and expression of genes for aquaporins (water channels) were examined in rice (Oryza sativa L.) plants. The effect of local N availability was examined by using a split-root system.

Results

N deprivation decreased the expression of root-specific aquaporin genes, whereas N resupply increased their expression. Changes in aquaporin gene expression were correlated with changes in hydraulic conductivity. N deprivation increased dry matter allocation to the roots. In a split-root experiment, the expression of root-specific aquaporin genes was down-regulated in the N-deprived half, whereas it was up-regulated in the N-supplied half.

Conclusion

Our results suggest that expression of genes for root-specific aquaporins underlies the changes in conductivity during continuous N deprivation and resupply. Rice plants seem to adapt to N availability through coordinated adjustment of root proliferation and abundance of aquaporins.     

Yeast reveals unexpected roles and regulatory features of aquaporins and aquaglyceroporins

Background

The yeast Saccharomyces cerevisiae provides unique opportunities to study roles and regulation of aqua/glyceroporins using frontline tools of genetics and genomics as well as molecular cell and systems biology.

Scope of review

S. cerevisiae has two similar orthodox aquaporins. Based on phenotypes mediated by gene deletion or overexpression as well as on their expression pattern, the yeast aquaporins play important roles in key aspects of yeast biology: establishment of freeze tolerance, during spore formation as well as determination of cell surface properties for substrate adhesion and colony formation. Exactly how the aquaporins perform those roles and the mechanisms that regulate their function under such conditions remain to be elucidated. S. cerevisiae also has two different aquaglyceroporins. While the role of one of them, Yfl054c, remains to be determined, Fps1 plays critical roles in osmoregulation by controlling the accumulation of the osmolyte glycerol. Fps1 communicates with two osmo-sensing MAPK signalling pathways to perform its functions but the details of Fps1 regulation remain to be determined.

Major conclusions

Several phenotypes associated with aqua/glyceroporin function in yeasts have been established. However, how water and glycerol transport contribute to the observed effects is not understood in detail. Also many of the basic principles of regulation of yeast aqua/glyceroporins remain to be elucidated.

General significance

Studying the yeast aquaporins and aquaglyceroporins offers rich insight into the life style, evolution and adaptive responses of yeast and rewards us with discoveries of unexpected roles and regulatory mechanisms of members of this ancient protein family. This article is part of a Special Issue entitled Aquaporins.     

OsRPK1, a novel leucine-rich repeat receptor-like kinase,negatively regulates polar auxin transport and root development in rice

Background

Leucine-rich-repeat receptor-like kinases (LRR-RLKs) represent the largest subfamily of putative RLKs in plants. Although several members in this subfamily have been identified, the studies about the relationships between LRR-RLKs and root development are still few. We previously identified a novel LRR-RLK in rice roots, and named it OsRPK1.

Methods

In this study, we first detected OsRPK1 kinase activity in vitro, and assessed its expression profile. We then investigated its biological function using transgenic rice plants over- and under-expressing OsRPK1.

Results

The OsRPK1 gene, which encodes a Ca^2 +-independent Ser/Thr kinase, was predominantly expressed in root tips, leaf blades, and undifferentiated suspension cells, and was markedly induced by treatment with auxin or ABA. Knockdown of OsRPK1 promoted the growth of transgenic rice plants, and increased plant height and tiller numbers. In contrast, over-expressing plants showed undeveloped adventitious roots, lateral roots, and a reduced root apical meristem. OsRPK1 over-expression also inhibited the expression of most auxin efflux carrier OsPIN genes, which was accompanied by changes in PAT and endogenous free IAA distribution in the leaves and roots.

Conclusions

The data indicated that OsRPK1, a novel leucine-rich-repeat receptor-like kinase, affects the root system architecture by negatively regulating polar auxin transport in rice.

General significance

This study demonstrated a common regulatory pathway of root system development in higher plants, which might be initiated by external stimuli via upstream receptor-like kinases and downstream carriers for polar auxin transport.     

Aquaporin-Mediated Reduction in Maize Root Hydraulic Conductivity Impacts Cell Turgor and Leaf Elongation Even without Changing Transpiration

Root hydraulic conductivity in plants (Lp_r) exhibits large variations in response to abiotic stimuli. In this study, we investigated the impact of dynamic, aquaporin-mediated changes of Lp_r on leaf growth, water potential, and water flux throughout the plant. For this, we manipulated Lp_r by subjecting roots to four independent treatments, with aquaporin inhibitors applied either to transpiring maize (Zea mays) plants grown in hydroponics or to detopped root systems for estimation of Lp_r. The treatments were acid load at pH 6.0 and 5.0 and hydrogen peroxide and anoxia applied for 1 to 2 h and subsequently reversed. First, we established that acid load affected cell hydraulic conductivity in maize root cortex. Lp_r was reduced by all treatments by 31% to 63%, with half-times of about 15 min, and partly recovered when treatments were reversed. Cell turgor measured in the elongating zone of leaves decreased synchronously with Lp_r, and leaf elongation rate closely followed these changes across all treatments in a dose-dependent manner. Leaf and xylem water potentials also followed changes in Lp_r. Stomatal conductance and rates of transpiration and water uptake were not affected by Lp_r reduction under low evaporative demand. Increased evaporative demand, when combined with acid load at pH 6.0, induced stomatal closure and amplified all other responses without altering their synchrony. Root pressurization reversed the impact of acid load or anoxia on leaf elongation rate and water potential, further indicating that changes in turgor mediated the response of leaf growth to reductions in Lp_r.Leaf growth is an essential process for crop production and is subject to large temporal fluctuations with environmental conditions. There is accumulating evidence that a large part of the changes observed in leaf growth depends on water transport within the plant (Sperry et al., 1998; Bouchabke et al., 2006). It has also been shown that changes in leaf water potential induced by root pressurization can trigger rapid variations of leaf elongation rate in wheat (Triticum aestivum) and barley (Hordeum vulgare; Passioura and Munns, 2000). This raises the question of whether and to what extent low hydraulic conductivity within the plant can limit leaf growth.After the stomata, the root system represents the largest resistance to water flow in the soil-plant atmosphere continuum (Steudle and Peterson, 1998). Root hydraulic conductivity (Lp_r) is affected by environmental stimuli such as drought, salinity, anoxia, low temperature, and nutrient availability (Zhang and Tyerman, 1991; Azaizeh et al., 1992; Birner and Steudle, 1993; Boursiac et al., 2005; Vandeleur et al., 2009). This ability to respond rapidly to fluctuating conditions suggests that Lp_r may participate in plant adaptation to diverse environments (Steudle, 2000). Aquaporins, a large family of water channel proteins located in plasma and intracellular membranes, are the main determinants of water flow across plant cells and tissues (Javot et al., 2003; Maurel et al., 2008). The dynamic changes in Lp_r in response to chemical or environmental stimuli may result from modifications of aquaporin abundance or activity (Carvajal et al., 1996; Tournaire-Roux et al., 2003; Boursiac et al., 2005). In particular, aquaporin regulation by phosphorylation, protonation, and relocalization in intracellular compartments has been reported in response to extracellular stimuli (Guenther et al., 2003; Tournaire-Roux et al., 2003; Vera-Estrella et al., 2004; Boursiac et al., 2008).The first insights into the involvement of aquaporins in physiological processes such as cell enlargement, tissue differentiation, and organ movement have been obtained at the cell or tissue level (Hukin et al., 2002; Moshelion et al., 2002; Wei et al., 2007). It is still unknown to what extent changes in root aquaporin activity impact integrated physiological processes such as shoot growth of intact plants. The importance of aquaporins in controlling physiological processes in adult, transpiring plants is assumed to be rather limited; this is because the proportion of water transport controlled by aquaporins is believed to be much lower than that in slowly transpiring plants (Steudle and Frensch, 1996; Steudle and Peterson, 1998).The manipulation of aquaporin activity offers the possibility to address this question. A classical approach is to alter the expression of aquaporin genes. Down-regulation of genes encoding aquaporins of the Plasma membrane Intrinsic Protein1 (PIP1) and PIP2 subfamilies in Arabidopsis and NtAQP1 in tobacco (Nicotiana tabacum) reduced the ability of these plants to recover after a water deficit treatment (Martre et al., 2002; Siefritz et al., 2002). However, genetic studies have been hindered by phenotypic compensation due to the functional redundancy of aquaporin isoforms in plants (Hachez et al., 2006b). The use of aquaporin inhibitors is a useful tool to investigate the role of root aquaporins in controlling leaf growth. Mercuric chloride (HgCl₂), which blocks aquaporins by binding of Hg²⁺ ions to Cys residues, has been widely used to evaluate the contribution of aquaporins to root water transport (Maggio and Joly, 1995; Carvajal et al., 1996). For instance, Lu and Neumann (1999) have observed that root exposure to 0.5 mm HgCl₂ immediately inhibited leaf growth in water-stressed rice (Oryza sativa) seedlings, thereby suggesting a role for aquaporins in controlling leaf growth. However, the signaling mechanisms involved in leaf growth inhibition remained unclear. Side effects of HgCl₂ application, such as the reduction of membrane potential in root cortex cells or an impaired cell respiration, restrict its usefulness in physiological studies (Wan and Zwiazek, 1999; Zhang and Tyerman, 1999). Manipulating the root environment is an alternative strategy to efficiently alter Lp_r and to assess the significance of such changes on leaf and/or shoot growth. The effects of varying Lp_r by root chilling or anoxia are correlated to leaf growth responses (Malone, 1993; Else et al., 1995, 2001), but the interpretation of these results remains controversial in the absence of precise hydraulic measurements.The goal of this study was to determine whether alterations of root aquaporin activity can influence leaf growth in intact, adult plants via effects on Lp_r and cell turgor in the leaf elongation zone. In this work, we compared three chemical treatments that target aquaporin inactivation in roots via different mechanisms. Each of them could exert side effects, but provided that all treatments resulted in common responses, the role of aquaporins on leaf growth could be established. We have followed, with a high temporal definition, the consequences of experimentally induced changes in Lp_r on water flux, leaf water potential, and leaf elongation rate under different scenarios (three evaporative demands and pressurized or nonpressurized root systems). In addition, we have measured cell turgor in growing leaves using a pressure probe to investigate whether cell turgor responds to changes in root hydraulic conductivity and whether such changes could account for the control of leaf growth.The first treatment used to alter Lp_r was acid loading of the solution surrounding the roots, which causes cytosolic acidification in root cortex cells. This triggers the closure of aquaporins due to the protonation of a conserved His residue (Tournaire-Roux et al., 2003). The second treatment was hydrogen peroxide (H₂O₂) application to the roots, which results in the inhibition of Lp_r in maize (Zea mays) by oxidative gating of aquaporins and/or their internalization (Ye et al., 2004; Aroca et al., 2005; Ye and Steudle, 2006; Boursiac et al., 2008). The third treatment was anoxia, an environmental stress that induces an inhibition of Lp_r in a large array of species through proton-induced closure of aquaporins (Zhang and Tyerman, 1991; Birner and Steudle, 1993; Else et al., 1995; Tournaire-Roux et al., 2003).     

Human aquaporins: Regulators of transcellular water flow

Background

Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular water flow. Consistent with their expression in most tissues, AQPs are associated with diverse physiological and pathophysiological processes.

Scope of review

AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell volume regulation (CVR) is particularly notable. This review examines the regulatory role of AQPs in transcellular water flow, especially in CVR. We focus on key systems of the human body, encompassing processes as diverse as urine concentration in the kidney to clearance of brain oedema.

Major conclusions

AQPs are crucial for the regulation of water homeostasis, providing selective pores for the rapid movement of water across diverse cell membranes and playing regulatory roles in CVR. Gating mechanisms have been proposed for human AQPs, but have only been reported for plant and microbial AQPs. Consequently, it is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane water permeability and a regulator of transcellular water flow.

General significance

Elucidating the mechanisms that regulate transcellular water flow will improve our understanding of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.     

Water channel structures analysed by electron crystallography

Background

The mechanisms underlying water transport through aquaporin (AQP) have been debated for two decades. The water permeation phenomenon of AQP seems inexplicable because the Grotthuss mechanism does not allow for simultaneous fast water permeability and inhibition of proton transfer through the hydrogen bonds of water molecules.

Scope of review

The AQP1 structure determined by electron crystallography provided the first insights into the proton exclusion mechanism despite fast water permeation. Although several studies have provided clues about the mechanism based on the AQP structure, each proposed mechanism remains incomplete. The present review is focused on AQP function and structure solved by electron crystallography in an attempt to fill the gaps between the findings in the absence and presence of lipids.

Major conclusions

Many AQP structures can be superimposed regardless of the determination method. The AQP fold is preserved even under conditions lacking lipids, but the water arrangement in the channel pore differs. The differences might be explained by dipole moments formed by the two short helices in the lipid bilayer. In addition, structure analyses of double-layered two-dimensional crystals of AQP suggest an array formation and cell adhesive function.

General significance

Electron crystallography findings not only have contributed to resolve some of the water permeation mechanisms, but have also elucidated the multiple functions of AQPs in the membrane. The roles of AQPs in the brain remain obscure, but their multiple activities might be important in the regulation of brain and other biological functions. This article is part of a Special Issue entitled Aquaporins.