Aquaporins from pathogenic protozoan parasites: structure, function and potential for chemotherapy

Cloning, expression, and functional identification of aquaporin genes from Eimeria tenella

Avian coccidiosis, caused by Eimeria spp., is one of the major parasitic diseases in chicken. Aquaporins (AQP) are essential mediators of water regulation and nutritional intake in parasites, and it may be a suitable molecule for chemotherapeutic target and vaccine candidate. We identified two aquaporin genes in Eimeria tenella (EtAQP1 and EtAQP2) with their full sequence, and the expression profiles were analyzed across different stages of E. tenella life cycle. The expression of EtAQP1 and EtAQP2 in Xenopus oocytes renders them highly permeable for both water and glycerol. Sugar alcohols up to five carbons and urea pass the pore. The immunohistochemical analysis confirms the restriction of antiserum staining to the surface of transfected Xenopus oocytes. Like other AQP family, EtAQPs are transmembrane proteins that are likely important molecules that facilitate solute uptake for parasite intracellular growth and therapeutic targets.

Intracellular Plasmodium aquaporin 2 is important for sporozoite production in the mosquito vector and malaria transmission

Malaria remains a devastating disease and, with current measures failing to control its transmission, there is a need for novel interventions. A family of proteins that have long been pursued as potential intervention targets are aquaporins, which are channels facilitating the movement of water and other solutes across membranes. We identify an aquaporin in malaria parasites and demonstrate that it is important for completion of Plasmodium development in the mosquito vector. Disruption of AQP2 in the human parasite Plasmodium falciparum and the rodent parasite Plasmodium berghei blocks sporozoite production inside oocysts established on mosquito midguts, greatly limiting parasite infection of salivary glands and transmission to a new host. In vivo epitope tagging of AQP2 in P. berghei, combined with immunofluorescence assays, reveals that the protein is localized in vesicle-like organelles found in the cytoplasm of gametocytes, ookinetes, and sporozoites. The number of these organelles varies between individual parasites and lifecycle stages suggesting that they are likely part of a dynamic endomembrane system. Phylogenetic analysis confirms that AQP2 is unique to malaria and closely related parasites and most closely resembles intracellular aquaporins. Structure prediction analyses identify several unusual features, including a large accessory extracellular loop and an arginine-to-phenylalanine substitution in the selectivity filter principally determining pore function, a unique feature among known aquaporins. This in conjunction with the importance of AQP2 for malaria transmission suggests that AQP2 may be a fruitful target of antimalarial interventions.

Design, synthesis and biological evaluations of quinolone amides against African trypanosomiasis with improved solubility

The human African trypanosomiasis is a devastating parasitic infection, which is caused by the protozoan Trypanosoma brucei and transmitted by the bite of the tsetse fly. An untreated infection usually results in death and only few drugs with significant drawbacks are currently available for treatment. Previous investigations revealed the quinolone amide MB007 as a lead compound with an excellent selectivity for T. b. brucei. Here, new quinolone amides were synthesized for deeper insights into the structure-activity relationship. Furthermore, the aqueous solubility of the compounds was analyzed, as the poor solubility of previous quinolone amides impeded in vivo studies for target identification. The biological evaluation led to the new lead structure 9f, which exhibits a promising in vitro activity against T. b. brucei (IC50 = 22 nM) and showed no cytotoxicity against macrophages. Moreover, compounds 10b and 10c were discovered, which possessed an improved solubility combined with a decent selectivity.

Down the membrane hole: Ion channels in protozoan parasites

Parasitic diseases caused by protozoans are highly prevalent around the world, disproportionally affecting developing countries, where coinfection with other microorganisms is common. Control and treatment of parasitic infections are constrained by the lack of specific and effective drugs, plus the rapid emergence of resistance. Ion channels are main drug targets for numerous diseases, but their potential against protozoan parasites is still untapped. Ion channels are membrane proteins expressed in all types of cells, allowing for the flow of ions between compartments, and regulating cellular functions such as membrane potential, excitability, volume, signaling, and death. Channels and transporters reside at the interface between parasites and their hosts, controlling nutrient uptake, viability, replication, and infectivity. To understand how ion channels control protozoan parasites fate and to evaluate their suitability for therapeutics, we must deepen our knowledge of their structure, function, and modulation. However, methodological approaches commonly used in mammalian cells have proven difficult to apply in protozoans. This review focuses on ion channels described in protozoan parasites of clinical relevance, mainly apicomplexans and trypanosomatids, highlighting proteins for which molecular and functional evidence has been correlated with their physiological functions.

Computational insights for predicting the binding and selectivity of peptidomimetic plasmepsin IV inhibitors against cathepsin D

Plasmepsins (Plms) are aspartic proteases involved in the degradation of human hemoglobin by P. falciparum and are essential for the survival and growth of the parasite. Therefore, Plm enzymes are reported as an important antimalarial drug target. Herein, we have applied molecular docking, molecular dynamics (MD) simulations, and binding free energy with the Linear Interaction Energy (LIE) approach to investigate the binding of peptidomimetic PlmIV inhibitors with a particular focus on understanding their selectivity against the human Asp protease cathepsin D (CatD). The residual decomposition analysis results suggest that amino acid differences in the subsite S3 of PlmIV and CatD are responsible for the higher selectivity of the 5a inhibitor. These findings yield excellent agreement with experimental binding data and provide new details regarding van der Waals and electrostatic interactions of subsite residues as well as structural properties of the PlmIV and CatD systems.

Evolution, function and roles in drug sensitivity of trypanosome aquaglyceroporins

Aquaglyceroporins (AQPs) are membrane proteins that function in osmoregulation and the uptake of low molecular weight solutes, in particular glycerol and urea. The AQP family is highly conserved, with two major subfamilies having arisen very early in prokaryote evolution and retained by eukaryotes. A complex evolutionary history indicates multiple lineage-specific expansions, losses and not uncommonly a complete loss. Consequently, the AQP family is highly evolvable and has been associated with significant events in life on Earth. In the African trypanosomes, a role for the AQP2 paralogue, in sensitivity to two chemotherapeutic agents, pentamidine and melarsoprol, is well established, albeit with the mechanisms for cell entry and resistance unclear until very recently. Here, we discuss AQP evolution, structure and mechanisms by which AQPs impact drug sensitivity, suggesting that AQP2 stability is highly sensitive to mutation while serving as the major uptake pathway for pentamidine.

AQPX-cluster aquaporins and aquaglyceroporins are asymmetrically distributed in trypanosomes

Major Intrinsic Proteins (MIPs) are membrane channels that permeate water and other small solutes. Some trypanosomatid MIPs mediate the uptake of antiparasitic compounds, placing them as potential drug targets. However, a thorough study of the diversity of these channels is still missing. Here we place trypanosomatid channels in the sequence-function space of the large MIP superfamily through a sequence similarity network. This analysis exposes that trypanosomatid aquaporins integrate a distant cluster from the currently defined MIP families, here named aquaporin X (AQPX). Our phylogenetic analyses reveal that trypanosomatid MIPs distribute exclusively between aquaglyceroporin (GLP) and AQPX, being the AQPX family expanded in the Metakinetoplastina common ancestor before the origin of the parasitic order Trypanosomatida. Synteny analysis shows how African trypanosomes specifically lost AQPXs, whereas American trypanosomes specifically lost GLPs. AQPXs diverge from already described MIPs on crucial residues. Together, our results expose the diversity of trypanosomatid MIPs and will aid further functional, structural, and physiological research needed to face the potentiality of the AQPXs as gateways for trypanocidal drugs.

The mutation G133D on Leishmania guyanensis AQP1 is highly destabilizing as revealed by molecular modeling and hypo-osmotic shock assay

The Leishmania aquaglyceroporin 1 (AQP1) plays an important role in osmoregulation and antimony (Sb) uptake, being determinant for resistance to antimony. We have previously demonstrated that G133D mutation on L. guyanensis AQP1 (LgAQP1) leads to reduced Sb uptake. Here, we investigated the effects of G133D mutation on LgAQP1 structure, associated with Sb uptake and alterations in osmoregulation capacity. High confidence molecular models of wild-type LgAQP1 as well as the LgAQP1::G133D mutant were constructed and optimized via comparative homology modeling. Computational methods from the mCSM platform were used to evaluate the effects on protein stability and on its ability to bind to glycerol. Functional validation of the disruptive effect of the mutation on LgAQP1 was done by challenging the parasites with hypo-osmotic chock. Glycine 133 is on transmembrane helix 3, buried in the membrane in both open and closed conformation. G133D mutation was predicted to be highly destabilizing, as it alters the helical bundling arrangement in order to accommodate the aspartic acid side chain. The shift in helices also resulted in fewer favorable contacts with glycerol in the channel, which would explain the reduced affinity for similar small molecules as SbO₃. Under hypo-osmotic condition, L. guyanensis AQP1G133D presented a 3-fold increase in cellular volume and pronounced delay to recover osmosis homeostasis when compared to the wild-type, a profile that was enhanced in LgAQP1-/- mutants. In conclusion, G133D is a highly disruptive mutation that will destabilize the monomer, compromise tetramer formation and alter pore conformation, leading to reduced Sb uptake and deficient osmoregulation.

Genomic and Transcriptomic Analysis for Identification of Genes and Interlinked Pathways Mediating Artemisinin Resistance in Leishmania donovani

Current therapy for visceral leishmaniasis (VL), compromised by drug resistance, toxicity, and high cost, demands for more effective, safer, and low-cost drugs. Artemisinin has been found to be an effectual drug alternative in experimental models of leishmaniasis. Comparative genome and transcriptome analysis of in vitro-adapted artesunate-resistant (K133AS-R) and -sensitive wild-type (K133WT) Leishmania donovani parasites was carried out using next-generation sequencing and single-color DNA microarray technology, respectively, to identify genes and interlinked pathways contributing to drug resistance. Whole-genome sequence analysis of K133WT vs. K133AS-R parasites revealed substantial variation among the two and identified 240 single nucleotide polymorphisms (SNPs), 237 insertion deletions (InDels), 616 copy number variations (CNVs) (377 deletions and 239 duplications), and trisomy of chromosome 12 in K133AS-R parasites. Transcriptome analysis revealed differential expression of 208 genes (fold change ≥ 2) in K133AS-R parasites. Functional categorization and analysis of modulated genes of interlinked pathways pointed out plausible adaptations in K133AS-R parasites, such as (i) a dependency on lipid and amino acid metabolism for generating energy, (ii) reduced DNA and protein synthesis leading to parasites in the quiescence state, and (iii) active drug efflux. The upregulated expression of cathepsin-L like protease, amastin-like surface protein, and amino acid transporter and downregulated expression of the gene encoding ABCG2, pteridine receptor, adenylatecyclase-type receptor, phosphoaceylglucosamine mutase, and certain hypothetical proteins are concordant with genomic alterations suggesting their potential role in drug resistance. The study provided an understanding of the molecular basis linked to artemisinin resistance in Leishmania parasites, which may be advantageous for safeguarding this drug for future use.

Instability of aquaglyceroporin (AQP) 2 contributes to drug resistance in Trypanosoma brucei

Defining mode of action is vital for both developing new drugs and predicting potential resistance mechanisms. Sensitivity of African trypanosomes to pentamidine and melarsoprol is predominantly mediated by aquaglyceroporin 2 (TbAQP2), a channel associated with water/glycerol transport. TbAQP2 is expressed at the flagellar pocket membrane and chimerisation with TbAQP3 renders parasites resistant to both drugs. Two models for how TbAQP2 mediates pentamidine sensitivity have emerged; that TbAQP2 mediates pentamidine translocation across the plasma membrane or via binding to TbAQP2, with subsequent endocytosis and presumably transport across the endosomal/lysosomal membrane, but as trafficking and regulation of TbAQPs is uncharacterised this remains unresolved. We demonstrate that TbAQP2 is organised as a high order complex, is ubiquitylated and is transported to the lysosome. Unexpectedly, mutation of potential ubiquitin conjugation sites, i.e. cytoplasmic-oriented lysine residues, reduced folding and tetramerization efficiency and triggered ER retention. Moreover, TbAQP2/TbAQP3 chimerisation, as observed in pentamidine-resistant parasites, also leads to impaired oligomerisation, mislocalisation and increased turnover. These data suggest that TbAQP2 stability is highly sensitive to mutation and that instability contributes towards the emergence of drug resistance.

Molecular Modeling of Aquaporins from Leishmania major

Aquaporins are membrane proteins responsible for permeating water, ions, dissolved gases, and other small molecular weight compounds through the protective cell membranes of living organisms. These proteins have been gaining increased importance as targets for treating a variety of parasitic diseases, since they control key physiological processes in the life cycle of parasitic protozoans, such as the uptake of nutrients, release of metabolites, and alleviation of osmotic stress. In this work, we use homology modeling to build three-dimensional structures for the four main aquaporins encoded and expressed by Leishmania major, a protozoan that causes leishmaniasis and affects millions of people worldwide. Physico-chemical properties of the proposed models for LmAQP1, LmAQPα, LmAQPβ, and LmAQPγ are then investigated using molecular dynamics simulations and the reference interaction site model (RISM) molecular theory of solvation. Pore characteristics, water permeation, and potential of mean force across the AQP channels for water, methanol, urea, ammonia, and carbon dioxide are examined and compared with results obtained for a protozoan (Plasmodium falciparum) aquaporin for which a crystal structure is available.

Plant Aquaporins: Diversity, Evolution and Biotechnological Applications

The plasma membrane forms a permeable barrier that separates the cytoplasm from the external environment, defining the physical and chemical limits in each cell in all organisms. The movement of molecules and ions into and out of cells is controlled by the plasma membrane as a critical process for cell stability and survival, maintaining essential differences between the composition of the extracellular fluid and the cytosol. In this process aquaporins (AQPs) figure as important actors, comprising highly conserved membrane proteins that carry water, glycerol and other hydrophilic molecules through biomembranes, including the cell wall and membranes of cytoplasmic organelles. While mammals have 15 types of AQPs described so far (displaying 18 paralogs), a single plant species can present more than 120 isoforms, providing transport of different types of solutes. Such aquaporins may be present in the whole plant or can be associated with different tissues or situations, including biotic and especially abiotic stresses, such as drought, salinity or tolerance to soils rich in heavy metals, for instance. The present review addresses several aspects of plant aquaporins, from their structure, classification, and function, to in silico methodologies for their analysis and identification in transcriptomes and genomes. Aspects of evolution and diversification of AQPs (with a focus on plants) are approached for the first time with the aid of the LCA (Last Common Ancestor) analysis. Finally, the main practical applications involving the use of AQPs are discussed, including patents and future perspectives involving this important protein family.

Aquaporin 1 is located on the intestinal basolateral membrane in Toxocara canis and might play a role in drug uptake

Background

Aquaporins (AQPs) are a family of integral membrane channel proteins that facilitate the transport of water and other small solutes across cell membranes. AQPs appear to play crucial roles in parasite survival and represent possible drug targets for novel intervention strategy. In this work, we investigated the tissue distribution and biological roles of an aquaporin TcAQP1 in the neglected parasitic nematode Toxocara canis.

Methods

Recombinant C-terminal hydrophilic domain of AQP1 of T. canis (rTcAQP1c) and polyclonal antibody against rTcAQP1c were produced to analyse the tissue expression of native TcAQP1 in adult (female and male) worms using an immunohistochemical approach. RNA interference (RNAi), quantitative real-time PCR (qRT-PCR) and nematocidal assays were performed to investigate the functional roles of TcAQP1 in the adult stage of T. canis.

Results

Immunofluorescence analysis showed that TcAQP1 was localised predominantly in the epithelial linings of the reproductive tract and basolateral membrane of the intestine in the adult stage (female and male) of T. canis, indicating important roles in reproduction, nutrient absorption and/or osmoregulation. Treatment with silencing RNA for 24 h resulted in a significant reduction of Tc-aqp-1 mRNA level in adult T. canis, though no phenotypical change was observed. The efficient gene knockdown compromised the nematocidal activity of albendazole in vitro, suggesting the role of TcAQP1 in drug uptake.

Conclusions

The findings of this study provide important information about tissue expression and functional roles of TcAQP1 protein in adult T. canis. Understanding the biological functions of this protein in other developmental stages of T. canis and related parasitic nematodes would contribute to the discovery of novel diagnostic or anthelmintic targets.

Adaptation and Therapeutic Exploitation of the Plasma Membrane of African Trypanosomes

African trypanosomes are highly divergent from their metazoan hosts, and as part of adaptation to a parasitic life style have developed a unique endomembrane system. The key virulence mechanism of many pathogens is successful immune evasion, to enable survival within a host, a feature that requires both genetic events and membrane transport mechanisms in African trypanosomes. Intracellular trafficking not only plays a role in immune evasion, but also in homeostasis of intracellular and extracellular compartments and interactions with the environment. Significantly, historical and recent work has unraveled some of the connections between these processes and highlighted how immune evasion mechanisms that are associated with adaptations to membrane trafficking may have, paradoxically, provided specific sensitivity to drugs. Here, we explore these advances in understanding the membrane composition of the trypanosome plasma membrane and organelles and provide a perspective for how transport could be exploited for therapeutic purposes.

The GlpF residue Trp219 is part of an amino-acid cluster crucial for aquaglyceroporin oligomerization and function

The vestibule loop regions of aquaglyceroporins are involved in accumulation of glycerol inside the channel pore. Even though most loop regions do not show high sequence similarity among aquaglyceroporins, loop E is highly conserved in aquaglyceroporins, but not in members of the homologous aquaporins. Specifically, a tryptophan residue is extremely conserved within this loop. We have investigated the role of this residue (Trp219) that deeply protrudes into the protein and potentially interacts with adjacent loops, using the E. coli aqualgyeroporin GlpF as a model. Replacement of Trp219 affects the activity of GlpF and impairs the stability of the tetrameric protein. Furthermore, we have identified an amino acid cluster involving Trp219 that stabilizes the GlpF tetramer. Based on our results we propose that Trp219 is key for formation of a defined vestibule structure, which is crucial for glycerol accumulation as well as for the stability of the active GlpF tetramer.

Aquaglyceroporins Are the Entry Pathway of Boric Acid in Trypanosoma brucei

The boron element possesses a range of different effects on living beings. It is essential to beneficial at low concentrations, but toxic at excessive concentrations. Recently, some boron-based compounds have been identified as promising molecules against Trypanosoma brucei, the causative agent of sleeping sickness. However, until now, the boron metabolism and its access route into the parasite remained elusive. The present study addressed the permeability of T. brucei aquaglyceroporins (TbAQPs) for boric acid, the main natural boron species. To this end, the three TbAQPs were expressed in Saccharomyces cerevisiae and Xenopus laevis oocytes. Our findings in both expression systems showed that all three TbAQPs are permeable for boric acid. Especially TbAQP2 is highly permeable for this compound, displaying one of the highest conductances reported for a solute in these channels. Additionally, T. brucei aquaglyceroporin activities were sensitive to pH. Taken together, these results establish that TbAQPs are channels for boric acid and are highly efficient entry pathways for boron into the parasite. Our findings stress the importance of studying the physiological functions of boron and their derivatives in T. brucei, as well as the pharmacological implications of their uptake by trypanosome aquaglyceroporins.

A proteomics approach reveals molecular manipulators of distinct cellular processes in the salivary glands of Glossina m. morsitans in response to Trypanosoma b. brucei infections

Background

Glossina m. morsitans is the primary vector of the Trypanosoma brucei group, one of the causative agents of African trypanosomoses. The parasites undergo metacyclogenesis, i.e. transformation into the mammalian-infective metacyclic trypomastigote (MT) parasites, in the salivary glands (SGs) of the tsetse vector. Since the MT-parasites are largely uncultivable in vitro, information on the molecular processes that facilitate metacyclogenesis is scanty.

Methods

To bridge this knowledge gap, we employed tandem mass spectrometry to investigate protein expression modulations in parasitized (T. b. brucei-infected) and unparasitized SGs of G. m. morsitans. We annotated the identified proteins into gene ontologies and mapped the up- and downregulated proteins within protein-protein interaction (PPI) networks.

Results

We identified 361 host proteins, of which 76.6 % (n = 276) and 22.3 % (n = 81) were up- and downregulated, respectively, in parasitized SGs compared to unparasitized SGs. Whilst 32 proteins were significantly upregulated (> 10-fold), only salivary secreted adenosine was significantly downregulated. Amongst the significantly upregulated proteins, there were proteins associated with blood feeding, immunity, cellular proliferation, homeostasis, cytoskeletal traffic and regulation of protein turnover. The significantly upregulated proteins formed major hubs in the PPI network including key regulators of the Ras/MAPK and Ca²⁺/cAMP signaling pathways, ubiquitin-proteasome system and mitochondrial respiratory chain. Moreover, we identified 158 trypanosome-specific proteins, notable of which were proteins in the families of the GPI-anchored surface glycoproteins, kinetoplastid calpains, peroxiredoxins, retrotransposon host spot multigene and molecular chaperones. Whilst immune-related trypanosome proteins were over-represented, membrane transporters and proteins involved in translation repression (e.g. ribosomal proteins) were under-represented, potentially reminiscent of the growth-arrested MT-parasites.

Conclusions

Our data implicate the significantly upregulated proteins as manipulators of diverse cellular processes in response to T. b. brucei infection, potentially to prepare the MT-parasites for invasion and evasion of the mammalian host immune defences. We discuss potential strategies to exploit our findings in enhancement of trypanosome refractoriness or reduce the vector competence of the tsetse vector.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1714-z) contains supplementary material, which is available to authorized users.

Phylogenomic and functional analyses of salmon lice aquaporins uncover the molecular diversity of the superfamily in Arthropoda

BACKGROUND

An emerging field in biomedical research is focusing on the roles of aquaporin water channels in parasites that cause debilitating or lethal diseases to their vertebrate hosts. The primary vectorial agents are hematophagous arthropods, including mosquitoes, flies, ticks and lice, however very little is known concerning the functional diversity of aquaporins in non-insect members of the Arthropoda. Here we conducted phylogenomic and functional analyses of aquaporins in the salmon louse, a marine ectoparasitic copepod that feeds on the skin and body fluids of salmonids, and used the primary structures of the isolated channels to uncover the genomic repertoires in Arthropoda.

RESULTS

Genomic screening identified 7 aquaporin paralogs in the louse in contrast to 42 in its host the Atlantic salmon. Phylogenetic inference of the louse nucleotides and proteins in relation to orthologs identified in Chelicerata, Myriapoda, Crustacea and Hexapoda revealed that the arthropod aquaporin superfamily can be classified into three major grades (1) classical aquaporins including Big brain (Bib) and Prip-like (PripL) channels (2) aquaglyceroporins (Glp) and (3) unorthodox aquaporins (Aqp12-like). In Hexapoda, two additional subfamilies exist as Drip and a recently classified entomoglyceroporin (Eglp) group. Cloning and remapping the louse cDNAs to the genomic DNA revealed that they are encoded by 1-7 exons, with two of the Glps being expressed as N-terminal splice variants (Glp1_v1, -1_v2, -3_v1, -3_v2). Heterologous expression of the cRNAs in amphibian oocytes demonstrated that PripL transports water and urea, while Bib does not. Glp1_v1, -2, -3_v1 and -3_v2 each transport water, glycerol and urea, while Glp1_v2 and the Aqp12-like channels were retained intracellularly. Transcript abundance analyses revealed expression of each louse paralog at all developmental stages, except for glp1_v1, which is specific to preadult and adult males.

CONCLUSIONS

Our data suggest that the aquaporin repertoires of extant arthropods have expanded independently in the different lineages, but can be phylogenetically classified into three major grades as opposed to four present in deuterostome animals. While the aquaporin repertoire of Atlantic salmon represents a 6-fold redundancy compared to the louse, the functional assays reveal that the permeation properties of the different crustacean grades of aquaporin are largely conserved to the vertebrate counterparts.

Number and regulation of protozoan aquaporins reflect environmental complexity

Protozoa are a diverse group of unicellular eukaryotes. Evidence has accumulated that protozoan aquaporin water and solute channels (AQP) contribute to adaptation in changing environments. Intracellular protozoan parasites live a well-sheltered life. Plasmodium spp. express a single AQP, Toxoplasma gondii two, while Trypanosoma cruzi and Leishamnia spp. encode up to five AQPs. Their AQPs are thought to import metabolic precursors and simultaneously to dispose of waste and to help parasites survive osmotic stress during transmission to and from the insect vector or during kidney passages. Trypanosoma brucei is a protozoan parasite that swims freely in the human blood. Expression and intracellular localization of the three T. brucei AQPs depend on the stage of differentiation during the life cycle, suggesting distinct roles in energy generation, metabolism, and cell motility. Free-living amoebae are in direct contact with the environment, encountering severe and sudden changes in the availability of nutrition, and in the osmotic conditions due to rainfall or drought. Amoeba proteus expresses a single AQP that is present in the contractile vacuole complex required for osmoregulation, whereas Dictyostelium discoideum expresses four AQPs, of which two are present in the single-celled amoeboidal stage and two more in the later multicellular stages preceding spore formation. The number and regulation of protozoan aquaporins may reflect environmental complexity. We highlight the gated AqpB from D. discoideum as an example of how life in the wild is challenged by a complex AQP structure-function relationship.

Functional inhibition of aquaporin-3 with a gold-based compound induces blockage of cell proliferation

AQP3 has been correlated with higher transport of glycerol, increment of ATP content, and larger proliferation capacity. Recently, we described the gold(III) complex Auphen as a very selective and potent inhibitor of AQP3's glycerol permeability (Pgly ). Here we evaluated Auphen effect on the proliferation of various mammalian cell lines differing in AQP3 expression level: no expression (PC12), moderate (NIH/3T3) or high (A431) endogenous expression, cells stably expressing AQP3 (PC12-AQP3), and human HEK293T cells transiently transfected (HEK-AQP3) for AQP3 expression. Proliferation was evaluated in the absence or presence of Auphen (5 μM) by counting number of viable cells and analyzing 5-bromo-2'-deoxyuridine (BrdU) incorporation. Auphen reduced ≈50% the proliferation in A431 and PC12-AQP3, ≈15% in HEK-AQP3 and had no effect in PC12-wt and NIH/3T3. Strong arrest in the S-G2/M phases of the cell cycle, supported by analysis of cyclins (A, B1, D1, E) levels, was observed in AQP3-expressing cells treated with Auphen. Flow-cytometry of propidium iodide incorporation and measurements of mitochondrial dehydrogenases activity confirmed absence of cytotoxic effect of the drug. Functional studies evidenced ≈50% inhibition of A431 Pgly by Auphen, showing that the compound's antiproliferative effect correlates with its ability to inhibit AQP3 Pgly . Role of Cys-40 on AQP3 permeability blockage by Auphen was confirmed by analyzing the mutated protein (AQP3-Ser-40). Accordingly, cells transfected with mutated AQP3 gained resistance to the antiproliferative effect of Auphen. These results highlight an Auphen inhibitory effect on proliferation of cells expressing AQP3 and suggest a targeted therapeutic effect on carcinomas with large AQP3 expression.

New subfamilies of major intrinsic proteins in fungi suggest novel transport properties in fungal channels: implications for the host-fungal interactions

Background

Aquaporins (AQPs) and aquaglyceroporins (AQGPs) belong to the superfamily of Major Intrinsic Proteins (MIPs) and are involved in the transport of water and neutral solutes across the membranes. MIP channels play significant role in plant-fungi symbiotic relationship and are believed to be important in host-pathogen interactions in human fungal diseases. In plants, at least five major MIP subfamilies have been identified. Fungal MIP subfamilies include orthodox aquaporins and five subgroups within aquaglyceroporins. XIP subfamily is common to both plants and fungi. In this study, we have investigated the extent of diversity in fungal MIPs and explored further evolutionary relationships with the plant MIP counterparts.

Results

We have extensively analyzed the available fungal genomes and examined nearly 400 fungal MIPs. Phylogenetic analysis and homology modeling exhibit the existence of a new MIP cluster distinct from any of the known fungal MIP subfamilies. All members of this cluster are found in microsporidia which are unicellular fungal parasites. Members of this family are small in size, charged and have hydrophobic residues in the aromatic/arginine selectivity filter and these features are shared by small and basic intrinsic proteins (SIPs), one of the plant MIP subfamilies. We have also found two new subfamilies (δ and γ2) within the AQGP group. Fungal AQGPs are the most diverse and possess the largest number of subgroups. We have also identified distinguishing features in loops E and D in the newly identified subfamilies indicating their possible role in channel transport and gating.

Conclusions

Fungal SIP-like MIP family is distinct from any of the known fungal MIP families including orthodox aquaporins and aquaglyceroporins. After XIPs, this is the second MIP subfamily from fungi that may have possible evolutionary link with a plant MIP subfamily. AQGPs in fungi are more diverse and possess the largest number of subgroups. The aromatic/arginine selectivity filter of SIP-like fungal MIPs and the δ AQGPs are unique, hydrophobic in nature and are likely to transport novel hydrophobic solutes. They can be attractive targets for developing anti-fungal drugs. The evolutionary pattern shared with their plant counterparts indicates possible involvement of new fungal MIPs in plant-fungi symbiosis and host-pathogen interactions.

Suppression of aquaporin, a mediator of water channel control in the carcinogenic liver fluke, Opisthorchis viverrini

Background

Opisthorchiasis and Opisthorchis viverrini-associated bile duct cancer represent major public health threats in Thailand and Laos. The tegument of this food borne fluke plays pivotal roles in parasite metabolism, homeostasis and osmoregulation. Excretory/secretory products also pass from the fluke to the biliary environment, products that likely underlie pathogenesis of liver fluke infection. Aquaporins (AQPs), belong to the major intrinsic protein superfamily of integral plasma membrane channel proteins that selectively transport water across cell membranes. AQPs play key roles as water and ion transport channels through the tegument of helminth parasites.

Methods

Here, two forms of AQP mRNAs from the adult developmental stage of O. viverrini, termed O. viverrini aquaporin-1 and -2 (Ov-aqp-1 and -2) were investigated. Roles of Ov-aqp-1 and -2 in the movement of water across the tegument of this carcinogenic liver fluke were investigated using RNA interference.

Results

Ov-AQP-1 and Ov-AQP-2 contain unique characteristic asparagine-proline-alanine (NPA) motifs of AQP transmembrane proteins. Phylogenetic analysis indicated that Ov-AQPs belong to an expanding group of aquaglyceroporin-like water channel proteins characterized from helminth and protozoan parasites, which is pivotal to the specialized requirements of water and solute control during parasitism. Elevated transcription of Ov-aqp-1 was evident in the egg, cercaria, metacercaria and adult stages of O. viverrini, whereas Ov-aqp-2 transcripts were detected at higher level in egg, metacercaria, cercaria and adult stage, respectively. RNA interference using electroporated dsRNA suppressed transcript levels of Ov-aqp-1 and Ov-aqp-2 in adult worms by 58-99% over periods of up to 16 days in vitro. Suppression of Ov-aqp-1 and Ov-aqp-2 in vitro disabled water transport in adult flukes.

Conclusion

The apparently pivotal roles of Ov-AQP in solute homeostasis at the fluke surface suggest that deeper investigation will be informative for the pathophysiology of O. viverrini, and may uncover intervention targets, particularly in view of the singularly notable predilection of this pathogen for residence within ducts of the biliary tree.

Transport proteins of parasitic protists and their role in nutrient salvage

The loss of key biosynthetic pathways is a common feature of important parasitic protists, making them heavily dependent on scavenging nutrients from their hosts. This is often mediated by specialized transporter proteins that ensure the nutritional requirements of the parasite are met. Over the past decade, the completion of several parasite genome projects has facilitated the identification of parasite transporter proteins. This has been complemented by functional characterization of individual transporters along with investigations into their importance for parasite survival. In this review, we summarize the current knowledge on transporters from parasitic protists and highlight commonalities and differences in the transporter repertoires of different parasitic species, with particular focus on characterized transporters that act at the host-pathogen interface.

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 inhibition by gold(III) compounds: new insights

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

Drug resistance in African trypanosomiasis: the melarsoprol and pentamidine story

Melarsoprol and pentamidine represent the two main classes of drugs, the arsenicals and diamidines, historically used to treat the diseases caused by African trypanosomes: sleeping sickness in humans and Nagana in livestock. Cross-resistance to these drugs was first observed over 60 years ago and remains the only example of cross-resistance among sleeping sickness therapies. A Trypanosoma brucei adenosine transporter is well known for its role in the uptake of both drugs. More recently, aquaglyceroporin 2 (AQP2) loss of function was linked to melarsoprol-pentamidine cross-resistance. AQP2, a channel that appears to facilitate drug accumulation, may also be linked to clinical cases of resistance. Here, we review these findings and consider some new questions as well as future prospects for tackling the devastating diseases caused by these parasites.

First cloning and characterization of two functional aquaporin genes from an arbuscular mycorrhizal fungus Glomus intraradices

Arbuscular mycorrhizal (AM) symbiosis is known to stimulate plant drought tolerance. However, the molecular basis for the direct involvement of AM fungi (AMF) in plant water relations has not been established. Two full-length aquaporin genes, namely GintAQPF1 and GintAQPF2, were cloned by rapid amplification of cDNA 5'- and 3'-ends from an AMF, Glomus intraradices. Aquaporin localization, activities and water permeability were examined by heterologous expression in yeast. Gene expression during symbiosis was also analyzed by quantitative real-time polymerase chain reaction. GintAQPF1 was localized to the plasma membrane of yeast, whereas GintAQPF2 was localized to both plasma and intracellular membranes. Transformed yeast cells exhibited a significant decrease in cell volume on hyperosmotic shock and faster protoplast bursting on hypo-osmotic shock. Polyethylene glycol (PEG) stimulated, but glycerol inhibited, the aquaporin activities. Furthermore, the expression of the two genes in arbuscule-enriched cortical cells and extraradical mycelia of maize roots was also enhanced significantly under drought stress. GintAQPF1 and GintAQPF2 are the first two functional aquaporin genes from AMF reported to date. Our data strongly support potential water transport via AMF to host plants, which leads to a better understanding of the important role of AMF in plant drought tolerance.

Aquaglyceroporin 2 controls susceptibility to melarsoprol and pentamidine in African trypanosomes

African trypanosomes cause sleeping sickness in humans, a disease that is typically fatal without chemotherapy. Unfortunately, drug resistance is common and melarsoprol-resistant trypanosomes often display cross-resistance to pentamidine. Although melarsoprol/pentamidine cross-resistance (MPXR) has been an area of intense interest for several decades, our understanding of the underlying mechanisms remains incomplete. Recently, a locus encoding two closely related aquaglyceroporins, AQP2 and AQP3, was linked to MPXR in a high-throughput loss-of-function screen. Here, we show that AQP2 has an unconventional "selectivity filter." AQP2-specific gene knockout generated MPXR trypanosomes but did not affect resistance to a lipophilic arsenical, whereas recombinant AQP2 reversed MPXR in cells lacking native AQP2 and AQP3. AQP2 was also shown to be disrupted in a laboratory-selected MPXR strain. Both AQP2 and AQP3 gained access to the surface plasma membrane in insect life-cycle-stage trypanosomes but, remarkably, AQP2 was specifically restricted to the flagellar pocket in the bloodstream stage. We conclude that the unconventional aquaglyceroporin, AQP2, renders cells sensitive to both melarsoprol and pentamidine and that loss of AQP2 function could explain cases of innate and acquired MPXR.

Molecular dynamics simulations of PfAQP from the malarial parasite Plasmodium falciparum

Aquaporins (AQPs) are widely distributed in all kingdoms of life and act as facilitators in the transport of water and other small solutes through cell membranes. Since the plasmodial and human AQPs are different in their primary and secondary structure, an intervention targeting plasmodial AQP without affecting human AQPs is discussed to identify an attractive novel target against malaria. Therefore, it is crucial to understand the action mechanisms of these plasmodial AQPs. To explore the progression of the plasmodial real AQPs in vivo at work, a molecular dynamic simulation system was successfully developed for a PfAQP tetramer in silico. The results showed that the transporting work was not synchronous in the four channels at the same time, and that it was different at different times in the same channel. The hole sizes varied in different channels with time. The structure analysis showed that both hydrophobic and hydrophilic residues composed the inner surface of the channels, and the asparagines Asn-193 and Asn-70 assembled into two motifs of NLA and NPS in the center of the channel in place of the signature motifs of NPA in other AQPs. In brief, we successfully developed an equilibrated PfAQP-lipid system by molecular dynamics simulations, and investigated the structure of the PfAQP channel, which should aid our understanding of the AQP structure and its functional implications.

Functional characterization of a novel aquaporin from Dictyostelium discoideum amoebae implies a unique gating mechanism

The social amoeba Dictyostelium discoideum is a widely used model organism for studying basic functions of protozoan and metazoan cells, such as osmoregulation and cell motility. There is evidence from other species that cellular water channels, aquaporins (AQP), are central to both processes. Yet, data on D. discoideum AQPs is almost absent. Despite cloning of two putative D. discoideum AQPs, WacA, and AqpA, water permeability has not been shown. Further, WacA and AqpA are expressed at the late multicellular stage and in spores but not in amoebae. We cloned a novel AQP, AqpB, from amoeboidal D. discoideum cells. Wild-type AqpB was impermeable to water, glycerol, and urea when expressed in Xenopus laevis oocytes. Neither stepwise truncation of the N terminus nor selected point mutations activated the water channel. However, mutational truncation by 12 amino acids of an extraordinary long intracellular loop induced water permeability of AqpB, hinting at a novel gating mechanism. This AqpB mutant was inhibited by mercuric chloride, confirming the presence of a cysteine residue in the selectivity filter as predicted by our structure model. We detected AqpB by Western blot analysis in a glycosylated and a non-glycosylated form throughout all developmental stages. When expressed in D. discoideum amoebae, AqpB-GFP fusion constructs localized to vacuolar structures, to the plasma membrane, and to lamellipodia-like membrane protrusions. We conclude that the localization pattern in conjunction with channel gating may be indicative of AqpB functions in osmoregulation as well as cell motility of D. discoideum.

Characterization of Leishmania donovani aquaporins shows presence of subcellular aquaporins similar to tonoplast intrinsic proteins of plants

Leishmania donovani, a protozoan parasite, resides in the macrophages of the mammalian host. The aquaporin family of proteins form important components of the parasite-host interface. The parasite-host interface could be a potential target for chemotherapy. Analysis of L. major and L. infantum genomes showed the presence of five aquaporins (AQPs) annotated as AQP9 (230aa), AQP putative (294aa), AQP-like protein (279aa), AQP1 (314aa) and AQP-like protein (596aa). We report here the structural modeling, localization and functional characterization of the AQPs from L. donovani. LdAQP1, LdAQP9, LdAQP2860 and LdAQP2870 have the canonical NPA-NPA motifs, whereas LdAQP putative has a non-canonical NPM-NPA motif. In the carboxyl terminal to the second NPA box of all AQPs except AQP1, a valine/alanine residue was found instead of the arginine. In that respect these four AQPs are similar to tonoplast intrinsic proteins in plants, which are localized to intracellular organelles. Confocal microscopy of L. donovani expressing GFP-tagged AQPs showed an intracellular localization of LdAQP9 and LdAQP2870. Real-time PCR assays showed expression of all aquaporins except LdAQP2860, whose level was undetectable. Three-dimensional homology modeling of the AQPs showed that LdAQP1 structure bears greater topological similarity to the aquaglyceroporin than to aquaporin of E. coli. The pore of LdAQP1 was very different from the rest in shape and size. The cavity of LdAQP2860 was highly irregular and undefined in geometry. For functional characterization, four AQP proteins were heterologously expressed in yeast. In the fps1Δ yeast cells, which lacked the key aquaglyceroporin, LdAQP1 alone displayed an osmosensitive phenotype indicating glycerol transport activity. However, expression of LdAQP1 and LdAQP putative in a yeast gpd1Δ strain, deleted for glycerol production, conferred osmosensitive phenotype indicating water transport activity or aquaporin function. Our analysis for the first time shows the presence of subcellular aquaporins and provides structural and functional characterization of aquaporins in Leishmania donovani.

The aquaporin gene family of the ectomycorrhizal fungus Laccaria bicolor: lessons for symbiotic functions

Soil humidity and bulk water transport are essential for nutrient mobilization. Ectomycorrhizal fungi, bridging soil and fine roots of woody plants, are capable of modulating both by being integrated into water movement driven by plant transpiration and the nocturnal hydraulic lift. Aquaporins are integral membrane proteins that function as gradient-driven water and/or solute channels. Seven aquaporins were identified in the genome of the ectomycorrhizal basidiomycete Laccaria bicolor and their role in fungal transfer processes was analyzed. Heterologous expression in Xenopus laevis oocytes revealed relevant water permeabilities for three aquaporins. In fungal mycelia, expression of the corresponding genes was high compared with other members of the gene family, indicating the significance of the respective proteins for plasma membrane water permeability. As growth temperature and ectomycorrhiza formation modified gene expression profiles of these water-conducting aquaporins, specific roles in those aspects of fungal physiology are suggested. Two aquaporins, which were highly expressed in ectomycorrhizas, conferred plasma membrane ammonia permeability in yeast. This indicates that these proteins are an integral part of ectomycorrhizal fungus-based plant nitrogen nutrition in symbiosis.

Assessing aquaglyceroporin gene status and expression profile in antimony-susceptible and -resistant clinical isolates of Leishmania donovani from India

OBJECTIVES

Clinical resistance to pentavalent antimonials results from an interplay between uptake, efflux and sequestration in Leishmania. Aquaglyceroporins (AQPs) have been shown to facilitate uptake of trivalent metalloids. Down-regulation of AQP1 in Leishmania results in resistance to trivalent antimony, whereas overexpression of AQP1 in drug-resistant parasites can reverse the resistance. The present work investigates the role of AQP1 in monitoring antimonial resistance in Indian leishmaniasis.

METHODS AND RESULTS

Susceptibility to trivalent antimony as determined in vitro with intracellular amastigotes from both visceral leishmaniasis (VL) and post-kala-azar dermal leishmaniasis (PKDL) patients correlated well with the clinical response. Higher accumulation of trivalent antimony (SbIII) was observed in all susceptible isolates compared with resistant isolates. Reduced accumulation of SbIII correlated, with a few exceptions, with down-regulation of AQP1 RNA as determined by real-time PCR. Cloning and sequencing of the AQP1 gene from both VL and PKDL isolates showed sequence variation in four of the clinical isolates. None of the isolates had an alteration of Glu152 and Arg230, which have been previously shown to affect metalloid transport. Transfection of the AQP1 gene in a sodium antimony gluconate-resistant field isolate conferred susceptibility to the resistant isolate.

CONCLUSIONS

Our studies indicate genetic variation in VL and PKDL isolates. Down-regulation of AQP1 correlates well with clinical drug resistance in a majority of Indian VL and PKDL isolates. AQP1 gene expression at both the genetic and transcriptional level showed positive correlation with SbIII accumulation, with some exceptions.

The Current Knowledge of Invertebrate Aquaporin Water Channels with Particular Emphasis on Insect AQPs

Aquaporins (AQPs) or water channels are some of the most ubiquitous integral membrane proteins, and are present in all living organisms. Their presence in the lipid bilayer of cell membranes considerably increases their permeability to water and, in some cases, to other small solutes. All AQPs, identified thus far, share the same structure, comprising of six transmembrane segments and two conserved regions forming the pore. Depending on the transported solutes, AQPs can be divided into two classes: ‘classical’ aquaporins (permeable only to water) and aquaglyceroporins (permeable also to glycerol and/or other solutes). Many subtypes of AQPs coexist in a single organism. Localization of particular subtypes of AQPs is tissue-specific. AQPs have been well characterized in almost all vertebrate classes. However, little is known about their counterparts in invertebrates. Most of the water channels characterized in invertebrates are found in insects. Therefore, the knowledge of aquaporins in invertebrates is generally limited to the information concerning water channels in this class of organism. Insects are characterized by an astonishing variety of physiological adaptations, notable in their feeding strategies or survival strategies in hostile environments. An example of such, is feeding on blood, or tolerating extreme cold or drought. It is likely that many of these adaptation patterns emerged due to the expression and regulation of particular aquaporins. Here we review the current state of knowledge of invertebrate AQPs (of insects and nematodes) and compare their structure and function with mammalian water channels

Transporters for drug delivery and as drug targets in parasitic protozoa

Parasitic protozoa cause devastating diseases across large regions of the globe, but a lack of economic incentives has resulted in the limited development of drugs against these "neglected diseases." Transporters expressed in the plasma membranes of these parasites offer potential for the development of new drugs. These permeases could be employed in two distinct strategies for drug development: (i) targeting selective delivery of drugs to the parasite and (ii) developing drugs that inhibit essential parasite permeases.