Ammonia permeability of the aquaglyceroporins from Plasmodium falciparum, Toxoplasma gondii and Trypansoma brucei

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.

Characterization of SIPs-type aquaporins and their roles in response to environmental cues in rice (Oryza sativa L.)

BACKGROUND

Aquaporins (AQPs) facilitate water diffusion across biological membranes and are involved in all phases of growth and development. Small and basic intrinsic proteins (SIPs) belong to the fourth subfamily of the plant AQPs. Although SIPs are widely present in higher plants, reports on SIPs are limited. Rice is one of the major food crops in the world, and water use is an important factor affecting rice growth and development; therefore, this study aimed to provide information relevant to the function and environmental response of the rice SIP gene family.

RESULTS

The rice (Oryza sativa L. japonica) genome encodes two SIP-like genes, OsSIP1 and OsSIP2, whose products are predominantly located in the endoplasmic reticulum (ER) membrane but transient localization to the plasma membrane is not excluded. Heterologous expression in a yeast aquaglyceroporin-mutant fps1Δ showed that both OsSIP1 and OsSIP2 made the cell more sensitive to KCl, sorbitol and H2O2, indicating facilitated permeation of water and hydrogen peroxide. In addition, the yeast cells expressing OsSIP2 were unable to efflux the toxic methylamine taken up by the endogenous MEP permeases, but OsSIP1 showed subtle permeability to methylamine, suggesting that OsSIP1 may have a wider conducting pore than OsSIP2. Expression profiling in different rice tissues or organs revealed that OsSIP1 was expressed in all tissues tested, whereas OsSIP2 was preferentially expressed in anthers and weakly expressed in other tissues. Consistent with this, histochemical staining of tissues expressing the promoter-β-glucuronidase fusion genes revealed their tissue-specific expression profile. In rice seedlings, both OsSIPs were upregulated to varied levels under different stress conditions, including osmotic shock, high salinity, unfavorable temperature, redox challenge and pathogen attack, as well as by hormonal treatments such as GA, ABA, MeJA, SA. However, a reduced expression of both OsSIPs was observed under dehydration treatment.

CONCLUSIONS

Our results suggest that SIP-like aquaporins are not restricted to the ER membrane and are likely to be involved in unique membrane functions in substrate transport, growth and development, and environmental response.

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.

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.

Experimental malaria-associated acute kidney injury is independent of parasite sequestration and resolves upon antimalarial treatment

Malaria remains a important global disease with more than 200 million cases and 600 000 deaths each year. Malaria-associated acute kidney injury (MAKI) may occur in up to 40% of patients with severe malaria and is associated with increased mortality. Histopathological characteristics of AKI in malaria are acute tubular injury, interstitial nephritis, focal segmental glomerulosclerosis, collapsing glomerulopathy and glomerulonephritis. We observed that C57BL/6 mice infected with Plasmodium berghei NK65 (PbNK65) develop MAKI in parallel with malaria-associated acute respiratory distress syndrome (MA-ARDS). MAKI pathology was associated with proteinuria, acute tubular injury and collapse of glomerular capillary tufts, which resolved rapidly after treatment with antimalarial drugs. Importantly, parasite sequestration was not detected in the kidneys in this model. Furthermore, with the use of skeleton binding protein-1 (SBP-1) KO PbNK65 parasites, we found that parasite sequestration in other organs and its subsequent high parasite load are not required for the development of experimental MAKI. Similar proteinuria, histopathological features, and increases in kidney expression of interferon-γ, TNF-α, kidney injury molecule-1 (KIM-1) and heme oxygenase-1 (HO-1) was observed in both infected groups despite a significant difference in parasite load. Taken together, we introduce a model of experimental AKI in malaria with important similarities to AKI in malaria patients. Therefore, this mouse model might be important to further study the pathogenesis of AKI in malaria.

Metabolic adjustments of blood-stage Plasmodium falciparum in response to sublethal pyrazoleamide exposure

Due to the recurring loss of antimalarial drugs to resistance, there is a need for novel targets, drugs, and combination therapies to ensure the availability of current and future countermeasures. Pyrazoleamides belong to a novel class of antimalarial drugs that disrupt sodium ion homeostasis, although the exact consequences of this disruption in Plasmodium falciparum remain under investigation. In vitro experiments demonstrated that parasites carrying mutations in the metabolic enzyme PfATP4 develop resistance to pyrazoleamide compounds. However, the underlying mechanisms that allow mutant parasites to evade pyrazoleamide treatment are unclear. Here, we first performed experiments to identify the sublethal dose of a pyrazoleamide compound (PA21A092) that caused a significant reduction in growth over one intraerythrocytic developmental cycle (IDC). At this drug concentration, we collected transcriptomic and metabolomic data at multiple time points during the IDC to quantify gene- and metabolite-level alterations in the treated parasites. To probe the effects of pyrazoleamide treatment on parasite metabolism, we coupled the time-resolved omics data with a metabolic network model of P. falciparum. We found that the drug-treated parasites adjusted carbohydrate metabolism to enhance synthesis of myoinositol-a precursor for phosphatidylinositol biosynthesis. This metabolic adaptation caused a decrease in metabolite flux through the pentose phosphate pathway, causing a decreased rate of RNA synthesis and an increase in oxidative stress. Our model analyses suggest that downstream consequences of enhanced myoinositol synthesis may underlie adjustments that could lead to resistance emergence in P. falciparum exposed to a sublethal dose of a pyrazoleamide drug.

Mutational Widening of Constrictions in a Formate-Nitrite/H+ Transporter Enables Aquaporin-Like Water Permeability and Proton Conductance

The unrelated protein families of the microbial formate–nitrite transporters (FNTs) and aquaporins (AQP) likely adapted the same protein fold through convergent evolution. FNTs facilitate weak acid anion/H⁺ cotransport, whereas AQP water channels strictly exclude charged substrates including protons. The FNT channel–like transduction pathway bears two lipophilic constriction sites that sandwich a highly conserved histidine residue. Because of lacking experiments, the function of these constrictions is unclear, and the protonation status of the central histidine during substrate transport remains a matter of debate. Here, we introduced constriction-widening mutations into the prototypical FNT from Escherichia coli, FocA, and assayed formate/H⁺ transport properties, water/solute permeability, and proton conductance. We found that enlargement of these constrictions concomitantly decreased formate/formic acid transport. In contrast to wildtype FocA, the mutants were unable to make use of a transmembrane proton gradient as a driving force. A construct in which both constrictions were eliminated exhibited water permeability, similar to AQPs, although accompanied by a proton conductance. Our data indicate that the lipophilic constrictions mainly act as barriers to isolate the central histidine from the aqueous bulk preventing protonation via proton wires. These results are supportive of an FNT transport model in which the central histidine is uncharged, and weak acid substrate anion protonation occurs in the vestibule regions of the transporter before passing the constrictions.

Longitudinal [18F]FDG and [13N]NH3 PET/CT imaging of brain and spinal cord in a canine hemisection spinal cord injury model

To further understand the neurological changes induced by spinal cord injury (SCI) in its acute and subacute stages, we evaluated longitudinal changes in glucose and glutamate metabolism in the spinal cord and brain regions of a canine hemisection SCI model. [18F]FDG and [13N]NH3 positron-emission tomography (PET) with computed tomography (CT) was performed before SCI and at 1, 3, 7, 14, and 21 days after SCI. Spinal cord [18F]FDG uptake increased and peaked at 3 days post SCI. Similar changes were observed in the brain regions but were not statistically significant. Compared to the acute phase of SCI, [13N]NH3 uptake increased in the subacute stage and peaked at 7 days post SCI in all analyzed brain regions. But in spinal cord, no [13N]NH3 uptake was detected before SCI when the blood-spinal cord barrier (BSCB) was intact, then gradually increased when the BSCB was damaged after SCI. [13N]NH3 uptake was significantly correlated with plasma levels of the BSCB disruption marker, monocyte chemoattractant protein-1 (MCP-1). Overall, we showed that SCI induced in vivo changes in glucose uptake in both the spinal cord and the examined brain regions, and changes in glutamine synthetase activity in the latter. Moreover, our results suggest that [13N]NH3 PET may serve as a potential method for assessing BSCB permeability in vivo.

The Ionophores CCCP and Gramicidin but Not Nigericin Inhibit Trypanosoma brucei Aquaglyceroporins at Neutral pH

Human African trypanosomiasis (HAT) is caused by Trypanosoma brucei parasites. The T. brucei aquaglyceroporin isoform 2, TbAQP2, has been linked to the uptake of pentamidine. Negative membrane potentials and transmembrane pH gradients were suggested to promote transport of the dicationic antitrypanosomal drug. Application of ionophores to trypanosomes further hinted at direct inhibition of TbAQP2 by carbonyl cyanide m-chlorophenyl hydrazone (CCCP). Here, we tested for direct effects of three classical ionophores (CCCP, nigericin, gramicidin) on the functionality of TbAQP2 and the related TbAQP3 at conditions that are independent from the membrane potential or a proton gradient. We expressed TbAQP2 and TbAQP3 in yeast, and determined permeability of uncharged glycerol at neutral pH using stopped-flow light scattering. The mobile proton carrier CCCP directly inhibited TbAQP2 glycerol permeability at an IC₅₀ of 2 µM, and TbAQP3 to a much lesser extent (IC₅₀ around 1 mM) likely due to different selectivity filter layouts. Nigericin, another mobile carrier, left both isoforms unaffected. The membrane-integral pore-forming gramicidin evenly inhibited TbAQP2 and TbAQP2 in the double-digit micromolar range. Our data exemplify the need for suitable controls to detect unwanted ionophore side effects even when used at concentrations that are typically recommended to disturb the transmembrane ion distribution.

Application of the Brown dynamics fluctuation-dissipation theorem to the study of Plasmodium berghei transporter protein PbAQP

In this article, the Brownian dynamics fluctuation-dissipation theorem (BD-FDT) is applied to the study of transport of neutral solutes across the cellular membrane of Plasmodium berghei (Pb), a disease-causing parasite. Pb infects rodents and causes symptoms in laboratory mice that are comparable to human malaria caused by Plasmodium falciparum (Pf). Due to the relative ease of its genetic engineering, P. berghei has been exploited as a model organism for the study of human malaria. P. berghei expresses one type of aquaporin (AQP), PbAQP, and, in parallel, P. falciparum expresses PfAQP. Either PbAQP or PfAQP is a multifunctional channel protein in the plasma membrane of the rodent/human malarial parasite for homeostasis of water, uptake of glycerol, and excretion of some metabolic wastes across the cell membrane. This FDT-study of the channel protein PbAQP is to elucidate how and how strongly it interacts with water, glycerol, and erythritol. It is found that erythritol, which binds deep inside the conducting pore of PbAQP/PfAQP, inhibits the channel protein's functions of conducting water, glycerol etc. This points to the possibility that erythritol, a sugar substitute, may inhibit the malarial parasites in rodents and in humans.

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.

Targeting Channels and Transporters in Protozoan Parasite Infections

Infectious diseases caused by pathogenic protozoa are among the most significant causes of death in humans. Therapeutic options are scarce and massively challenged by the emergence of resistant parasite strains. Many of the current anti-parasite drugs target soluble enzymes, generate unspecific oxidative stress, or act by an unresolved mechanism within the parasite. In recent years, collections of drug-like compounds derived from large-scale phenotypic screenings, such as the malaria or pathogen box, have been made available to researchers free of charge boosting the identification of novel promising targets. Remarkably, several of the compound hits have been found to inhibit membrane proteins at the periphery of the parasites, i.e., channels and transporters for ions and metabolites. In this review, we will focus on the progress made on targeting channels and transporters at different levels and the potential for use against infections with apicomplexan parasites mainly Plasmodium spp. (malaria) and Toxoplasma gondii (toxoplasmosis), with kinetoplastids Trypanosoma brucei (sleeping sickness), Trypanosoma cruzi (Chagas disease), and Leishmania ssp. (leishmaniasis), and the amoeba Entamoeba histolytica (amoebiasis).

Aquaglyceroporin PbAQP is required for efficient progression through the liver stage of Plasmodium infection

The discovery of aquaglyceroporins (AQP) has highlighted a new mechanism of membrane solute transport that may hold therapeutic potential for controlling parasitic infections, including malaria. Plasmodium parasites express a single AQP at the plasma membrane that functions as a channel for water, nutrients and waste into and out cells. We previously demonstrated that Plasmodium berghei targeted for PbAQP deletion are deficient in glycerol import and less virulent than wild-type parasites during the blood developmental stage. Here, we have examined the contribution of PbAQP to the infectivity of P. berghei in the liver. PbAQP is expressed in the sporozoite mosquito stage and is detected at low levels in intrahepatic parasites at the onset of hepatocyte infection. As the parasites progress to late hepatic stages, PbAQP transcription increases and PbAQP localizes to the plasma membrane of hepatic merozoites. Compared to wild-type parasites, PbAQP-null sporozoites exhibit a delay in blood stage infection due to slower replication in hepatocytes, resulting in retardation of merosome production. Furthermore, PbAQP disruption results in a significant reduction in erythrocyte infectivity by hepatocyte-derived merozoites. Hepatic merozoites incorporate exogenous glycerol into glycerophospholipids and PbAQP-null merozoites contain less phosphatidylcholine than wild-type merozoites, underlining the contribution of Plasmodium AQP to phospholipid syntheses.

RNA metabolism is the primary target of formamide in vivo

The synthesis, processing and function of coding and non-coding RNA molecules and their interacting proteins has been the focus of a great deal of research that has boosted our understanding of key molecular pathways that underlie higher order events such as cell cycle control, development, innate immune response and the occurrence of genetic diseases. In this study, we have found that formamide preferentially weakens RNA related processes in vivo. Using a non-essential Schizosaccharomyces pombe gene deletion collection, we identify deleted loci that make cells sensitive to formamide. Sensitive deletions are significantly enriched in genes involved in RNA metabolism. Accordingly, we find that previously known temperature-sensitive splicing mutants become lethal in the presence of the drug under permissive temperature. Furthermore, in a wild type background, splicing efficiency is decreased and R-loop formation is increased in the presence of formamide. In addition, we have also isolated 35 formamide-sensitive mutants, many of which display remarkable morphology and cell cycle defects potentially unveiling new players in the regulation of these processes. We conclude that formamide preferentially targets RNA related processes in vivo, probably by relaxing RNA secondary structures and/or RNA-protein interactions, and can be used as an effective tool to characterize these processes.

Transporters of Trypanosoma brucei-phylogeny, physiology, pharmacology

Trypanosoma brucei comprise the causative agents of sleeping sickness, T. b. gambiense and T. b. rhodesiense, as well as the livestock-pathogenic T. b. brucei. The parasites are transmitted by the tsetse fly and occur exclusively in sub-Saharan Africa. T. brucei are not only lethal pathogens but have also become model organisms for molecular parasitology. We focus here on membrane transport proteins of T. brucei, their contribution to homeostasis and metabolism in the context of a parasitic lifestyle, and their pharmacological role as potential drug targets or routes of drug entry. Transporters and channels in the plasma membrane are attractive drug targets as they are accessible from the outside. Alternatively, they can be exploited to selectively deliver harmful substances into the trypanosome's interior. Both approaches require the targeted transporter to be essential: in the first case to kill the trypanosome, in the second case to prevent drug resistance due to loss of the transporter. By combining functional and phylogenetic analyses, we were mining the T. brucei predicted proteome for transporters of pharmacological significance. Here, we review recent progress in the identification of transporters of lipid precursors, amino acid permeases and ion channels in T. brucei.

Identification and characterization of guanosine 5'-monophosphate reductase of Trypanosoma congolense as a drug target

Trypanosoma congolense is one of the most prevalent pathogens which causes trypanosomosis in African animals, resulting in a significant economic loss. In its life cycle, T. congolense is incapable of synthesizing purine nucleotides via a de novo pathway, and thus relies on a salvage pathway to survive. In this study, we identified a gene from T. congolense, TcIL3000_5_1940, as a guanosine 5'-monophosphate reductase (GMPR), an enzyme that modulates the concentration of intracellular guanosine in the pathogen. The recombinant protein was expressed in Escherichia coli, and the gene product was enzymatically confirmed as a unique GMPR, designated as rTcGMPR. This enzyme was constitutively expressed in glycosomes at all of the parasite's developmental stages similar to other purine nucleotide metabolic enzymes. Mycophenolic acid (MPA) was found to inhibit rTcGMPR activity. Hence, it is a potential lead compound for the design of trypanocidal agents, specifically GMPR inhibitor.

Aquaglyceroporin-null trypanosomes display glycerol transport defects and respiratory-inhibitor sensitivity

Aquaglyceroporins (AQPs) transport water and glycerol and play important roles in drug-uptake in pathogenic trypanosomatids. For example, AQP2 in the human-infectious African trypanosome, Trypanosoma brucei gambiense, is responsible for melarsoprol and pentamidine-uptake, and melarsoprol treatment-failure has been found to be due to AQP2-defects in these parasites. To further probe the roles of these transporters, we assembled a T. b. brucei strain lacking all three AQP-genes. Triple-null aqp1-2-3 T. b. brucei displayed only a very moderate growth defect in vitro, established infections in mice and recovered effectively from hypotonic-shock. The aqp1-2-3 trypanosomes did, however, display glycerol uptake and efflux defects. They failed to accumulate glycerol or to utilise glycerol as a carbon-source and displayed increased sensitivity to salicylhydroxamic acid (SHAM), octyl gallate or propyl gallate; these inhibitors of trypanosome alternative oxidase (TAO) can increase intracellular glycerol to toxic levels. Notably, disruption of AQP2 alone generated cells with glycerol transport defects. Consistent with these findings, AQP2-defective, melarsoprol-resistant clinical isolates were sensitive to the TAO inhibitors, SHAM, propyl gallate and ascofuranone, relative to melarsoprol-sensitive reference strains. We conclude that African trypanosome AQPs are dispensable for viability and osmoregulation but they make important contributions to drug-uptake, glycerol-transport and respiratory-inhibitor sensitivity. We also discuss how the AQP-dependent inverse sensitivity to melarsoprol and respiratory inhibitors described here might be exploited.

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.

Aquaporin 4 as a NH3 Channel

Ammonia is a biologically potent molecule, and the regulation of ammonia levels in the mammalian body is, therefore, strictly controlled. The molecular paths of ammonia permeation across plasma membranes remain ill-defined, but the structural similarity of water and NH3 has pointed to the aquaporins as putative NH3-permeable pores. Accordingly, a range of aquaporins from mammals, plants, fungi, and protozoans demonstrates ammonia permeability. Aquaporin 4 (AQP4) is highly expressed at perivascular glia end-feet in the mammalian brain and may, with this prominent localization at the blood-brain-interface, participate in the exchange of ammonia, which is required to sustain the glutamate-glutamine cycle. Here we observe that AQP4-expressing Xenopus oocytes display a reflection coefficient <1 for NH4Cl at pH 8.0, at which pH an increased amount of the ammonia occurs in the form of NH3 Taken together with an NH4Cl-mediated intracellular alkalization (or lesser acidification) of AQP4-expressing oocytes, these data suggest that NH3 is able to permeate the pore of AQP4. Exposure to NH4Cl increased the membrane currents to a similar extent in uninjected oocytes and in oocytes expressing AQP4, indicating that the ionic NH4 (+) did not permeate AQP4. Molecular dynamics simulations revealed partial pore permeation events of NH3 but not of NH4 (+) and a reduced energy barrier for NH3 permeation through AQP4 compared with that of a cholesterol-containing lipid bilayer, suggesting AQP4 as a favored transmembrane route for NH3 Our data propose that AQP4 belongs to the growing list of NH3-permeable water channels.

Pentamidine Is Not a Permeant but a Nanomolar Inhibitor of the Trypanosoma brucei Aquaglyceroporin-2

The chemotherapeutic arsenal against human African trypanosomiasis, sleeping sickness, is limited and can cause severe, often fatal, side effects. One of the classic and most widely used drugs is pentamidine, an aromatic diamidine compound introduced in the 1940s. Recently, a genome-wide loss-of-function screen and a subsequently generated trypanosome knockout strain revealed a specific aquaglyceroporin, TbAQP2, to be required for high-affinity uptake of pentamidine. Yet, the underlying mechanism remained unclear. Here, we show that TbAQP2 is not a direct transporter for the di-basic, positively charged pentamidine. Even though one of the two common cation filters of aquaglyceroporins, i.e. the aromatic/arginine selectivity filter, is unconventional in TbAQP2, positively charged compounds are still excluded from passing the channel. We found, instead, that the unique selectivity filter layout renders pentamidine a nanomolar inhibitor of TbAQP2 glycerol permeability. Full, non-covalent inhibition of an aqua(glycero)porin in the nanomolar range has not been achieved before. The remarkable affinity derives from an electrostatic interaction with Asp265 and shielding from water as shown by structure-function evaluation and point mutation of Asp265. Exchange of the preceding Leu264 to arginine abolished pentamidine-binding and parasites expressing this mutant were pentamidine-resistant. Our results indicate that TbAQP2 is a high-affinity receptor for pentamidine. Taken together with localization of TbAQP2 in the flagellar pocket of bloodstream trypanosomes, we propose that pentamidine uptake is by endocytosis.

Challenges and achievements in the therapeutic modulation of aquaporin functionality

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

Disease specific modules and hub genes for intervention strategies: A co-expression network based approach for Plasmodium falciparum clinical isolates

Systems biology approaches that are based on gene expression and bioinformatics analysis have been successful in predicting the functions of many genes in Plasmodium falciparum, a protozoan parasite responsible for most of the deaths due to malaria. However, approaches that can provide information about the biological processes that are active in this parasite in vivo during complicated malaria conditions have been scarcely deployed. Here we report the analysis of a weighted gene co-expression based network for P. falciparum, from non-cerebral clinical complications. Gene expression profiles of 20 P. falciparum clinical isolates were utilized to construct the same. A total of 20 highly interacting modules were identified post network creation. In 12 of these modules, at least 10% of the member genes, were found to be differentially regulated in parasites from patient isolates showing complications, when compared with those from patients with uncomplicated disease. Enrichment analysis helped identify biological processes like oxidation-reduction, electron transport chain, protein synthesis, ubiquitin dependent catabolic processes, RNA binding and purine nucleotide metabolic processes as associated with these modules. Additionally, for each module, highly connected hub genes were identified. Detailed functional analysis of many of these, which have known annotated functions underline their importance in parasite development and survival. This suggests, that other hub genes with unknown functions may also be playing crucial roles in parasite biology, and, are potential candidates for intervention strategies.

Evolution and functional diversity of aquaporins

In this review, we provide a brief synopsis of the evolution and functional diversity of the aquaporin gene superfamily in prokaryotic and eukaryotic organisms. Based upon the latest data, we discuss the expanding list of molecules shown to permeate the central pore of aquaporins, and the unexpected diversity of water channel genes in Archaea and Bacteria. We further provide new insight into the origin by horizontal gene transfer of plant glycerol-transporting aquaporins (NIPs), and the functional co-option and gene replacement of insect glycerol transporters. Finally, we discuss the origins of four major grades of aquaporins in Eukaryota, together with the increasing repertoires of aquaporins in vertebrates.

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.

Potential role of Plasmodium falciparum-derived ammonia in the pathogenesis of cerebral malaria

Cerebral malaria (CM) is the most severe complication associated with Plasmodium falciparum infection. The exact pathogenic mechanisms leading to the development of CM remains poorly understood while the mortality rates remain high. Several potential mechanisms including mechanical obstruction of brain microvasculature, inflammation, oxidative stress, cerebral energy defects, and hemostatic dysfunction have been suggested to play a role in CM pathogenesis. However, these proposed mechanisms, even when considered together, do not fully explain the pathogenesis and clinicopathological features of human CM. This necessitates consideration of alternative pathogenic mechanisms. P. falciparum generates substantial amounts of ammonia as a catabolic by-product, but lacks detoxification mechanisms. Whether this parasite-derived ammonia plays a pathogenic role in CM is presently unknown, despite its potential to cause localized brain ammonia elevation and subsequent neurotoxic effects. This article therefore, explores and proposes a potential role of parasite-derived ammonia in the pathogenesis and neuropathology of CM. A consideration of parasite-derived ammonia as a factor in CM pathogenesis provides plausible explanations of the various features observed in CM patients including how a largely intravascular parasite can cause neuronal dysfunction. It also provides a framework for rational development and testing of novel drugs targeting the parasite's ammonia handling.

How do antimalarial drugs reach their intracellular targets?

Drugs represent the primary treatment available for human malaria, as caused by Plasmodium spp. Currently approved drugs and antimalarial drug leads generally work against parasite enzymes or activities within infected erythrocytes. To reach their specific targets, these chemicals must cross at least three membranes beginning with the host cell membrane. Uptake at each membrane may involve partitioning and diffusion through the lipid bilayer or facilitated transport through channels or carriers. Here, we review the features of available antimalarials and examine whether transporters may be required for their uptake. Our computational analysis suggests that most antimalarials have high intrinsic membrane permeability, obviating the need for uptake via transporters; a subset of compounds appear to require facilitated uptake. We also review parasite and host transporters that may contribute to drug uptake. Broad permeability channels at the erythrocyte and parasitophorous vacuolar membranes of infected cells relax permeability constraints on antimalarial drug design; however, this uptake mechanism is prone to acquired resistance as the parasite may alter channel activity to reduce drug uptake. A better understanding of how antimalarial drugs reach their intracellular targets is critical to prioritizing drug leads for antimalarial development and may reveal new targets for therapeutic intervention.

Transport proteins determine drug sensitivity and resistance in a protozoan parasite, Trypanosoma brucei

Drug resistance in pathogenic protozoa is very often caused by changes to the ‘transportome’ of the parasites. In Trypanosoma brucei, several transporters have been implicated in uptake of the main classes of drugs, diamidines and melaminophenyl arsenicals. The resistance mechanism had been thought to be due to loss of a transporter known to carry both types of agents: the aminopurine transporter P2, encoded by the gene TbAT1. However, although loss of P2 activity is well-documented as the cause of resistance to the veterinary diamidine diminazene aceturate (DA; Berenil^®), cross-resistance between the human-use arsenical melarsoprol and the diamidine pentamidine (melarsoprol/pentamidine cross resistance, MPXR) is the result of loss of a separate high affinity pentamidine transporter (HAPT1). A genome-wide RNAi library screen for resistance to pentamidine, published in 2012, gave the key to the genetic identity of HAPT1 by linking the phenomenon to a locus that contains the closely related T. brucei aquaglyceroporin genes TbAQP2 and TbAQP3. Further analysis determined that knockdown of only one pore, TbAQP2, produced the MPXR phenotype. TbAQP2 is an unconventional aquaglyceroporin with unique residues in the “selectivity region” of the pore, and it was found that in several MPXR lab strains the WT gene was either absent or replaced by a chimeric protein, recombined with parts of TbAQP3. Importantly, wild-type AQP2 was also absent in field isolates of T. b. gambiense, correlating with the outcome of melarsoprol treatment. Expression of a wild-type copy of TbAQP2 in even the most resistant strain completely reversed MPXR and re-introduced HAPT1 function and transport kinetics. Expression of TbAQP2 in Leishmania mexicana introduced a pentamidine transport activity indistinguishable from HAPT1. Although TbAQP2 has been shown to function as a classical aquaglyceroporin it is now clear that it is also a high affinity drug transporter, HAPT1. We discuss here a possible structural rationale for this remarkable ability.

Identity of a Plasmodium lactate/H(+) symporter structurally unrelated to human transporters

Maintenance of a high glycolytic flow rate is critical for the rapid growth and virulence of malarial parasites. The parasites release two moles of lactic acid per mole of glucose as the anaerobic end product. However, the molecular identity of the Plasmodium lactate transporter is unknown. Here we show that a member of the microbial formate-nitrite transporter family, PfFNT, acts as a lactate/proton symporter in Plasmodium falciparum. Besides L-lactate, PfFNT transports physiologically relevant D-lactate, as well as pyruvate, acetate and formate, and is inhibited by the antiplasmodial compounds phloretin, furosemide and cinnamate derivatives, but not by p-chloromercuribenzene sulfonate (pCMBS). Our data on PfFNT monocarboxylate transport are consistent with those obtained with living parasites. Moreover, PfFNT is the only transporter of the plasmodial glycolytic pathway for which structure information is available from crystals of homologous proteins, rendering it amenable to further evaluation as a novel antimalarial drug target.

Erythritol predicted to inhibit permeation of water and solutes through the conducting pore of P. falciparum aquaporin

Plasmodium falciparum aquaporin (PfAQP) is a multifunctional channel protein in the plasma membrane of the malarial parasite that causes the most severe form of malaria infecting more than a million people a year. This channel protein facilitates transport of water and several solutes across the cell membrane. In order to better elucidate the fundamental interactions between PfAQP and its permeants and among the permeants, I conducted over three microseconds in silico experiments of atomistic models of the PfAQP-membrane system to obtain the free-energy profiles of five permeants (erythritol, water, glycerol, urea, and ammonia) throughout the amphipathic conducting pore of PfAQP. The profiles are analyzed in light of and shown to be consistent with the existent in vitro data. The binding affinities are computed using the free-energy profiles and the permeant fluctuations inside the channel. On this basis, it is predicted that erythritol, a permeant of PfAQP itself having a deep ditch in its permeation passageway, inhibits PfAQP's functions of transporting water and other solutes with an IC50 in the range of high nanomolars. This leads to the possibility that erythritol, a sweetener generally considered safe, may inhibit or kill the malarial parasite in vivo without causing undesired side effects. Experimental studies are hereby called for to directly test this theoretical prediction of erythritol strongly inhibiting PfAQP in vitro and possibly inhibiting P. falciparum in vivo.

Aquaporins with anion/monocarboxylate permeability: mechanisms, relevance for pathogen-host interactions

Classically, aquaporins are divided based on pore selectivity into water specific, orthodox aquaporins and solute-facilitating aquaglyceroporins, which conduct, e.g., glycerol and urea. However, more aquaporin-passing substrates have been identified over the years, such as the gasses ammonia and carbon dioxide or the water-related hydrogen peroxide. It became apparent that not all aquaporins clearly fit into one of only two subfamilies. Furthermore, certain aquaporins from both major subfamilies have been reported to conduct inorganic anions, such as chloride, or monoacids/monocarboxylates, such as lactic acid/lactate. Here, we summarize the findings on aquaporin anion transport, analyze the pore layout of such aquaporins in comparison to prototypical non-selective anion channels, monocarboxylate transporters, and formate–nitrite transporters. Finally, we discuss in which scenarios anion conducting aquaporins may be of physiological relevance.

Does Plasmodium falciparum have an Achilles' heel?

BACKGROUND

Plasmodium falciparum is the parasite that causes the most severe form of malaria responsible for nearly a million deaths a year. Currently, science has been established about its cellular structures, its metabolic processes, and even the molecular structures of its intrinsic membrane proteins responsible for transporting water, nutrient, and waste molecules across the parasite plasma membrane (PPM).

PRESENTATION OF THE HYPOTHESIS

I hypothesize that Plasmodium falciparum has an Achilles' heel that can be attacked with erythritol, the well-known sweetener that is classified as generally safe. This hypothesis is based on the molecular structure of the parasite's membrane and the quantitative mechanics of how erythritol interacts with the multi-functional channel protein expressed in the PPM. Most organisms have in their cell membrane two types of water-channel proteins: aquaporins to maintain hydro-homeostasis across the membrane and aquaglyceroporins to uptake glycerols etc. In contrast, P. falciparum has only one type of such proteins---the multi-functional aquaglyceroporin (PfAQP) expressed in the PPM---to do both jobs. Moreover, the parasite also uses PfAQP to excrete its metabolic wastes (ammonia included) produced at a very high rate in the blood stage. This extremely high efficiency of the bug using one protein for multiple essential tasks makes the parasite fatally vulnerable. Erythritol in the blood stream can kill the parasite by clogging up its PfAQP channel that needs to be open for maintaining hydro-homeostasis and for excreting toxic wastes across the bug's PPM.

TESTING THE HYPOTHESIS

In vitro tests are to measure the growth/death rate of P. falciparum in blood with various erythritol concentrations. In vivo experiments are to administer groups of infected mice with various doses of erythritol and monitor the parasite growth levels from blood samples drawn from each group. Clinic trials can be performed to observe the added effects of administering to patients erythritol along with the known drugs because erythritol was classified as a safe food ingredient.

IMPLICATIONS OF THE HYPOTHESIS

If proven true, erythritol will cure the most severe form of malaria without significant side effects.

Membrane transport in the malaria parasite and its host erythrocyte

As it grows and replicates within the erythrocytes of its host the malaria parasite takes up nutrients from the extracellular medium, exports metabolites and maintains a tight control over its internal ionic composition. These functions are achieved via membrane transport proteins, integral membrane proteins that mediate the passage of solutes across the various membranes that separate the biochemical machinery of the parasite from the extracellular environment. Proteins of this type play a key role in antimalarial drug resistance, as well as being candidate drug targets in their own right. This review provides an overview of recent work on the membrane transport biology of the malaria parasite-infected erythrocyte, encompassing both the parasite-induced changes in the membrane transport properties of the host erythrocyte and the cell physiology of the intracellular parasite itself.

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.

Structural determinants of the hydrogen peroxide permeability of aquaporins

Aquaporins (AQP) conduct small, uncharged molecules, such as water (orthodox AQPs), ammonia (aquaammoniaporins) or glycerol (aquaglyceroporins). The physiological functions of AQPs are involved in osmotic volume regulation or the transport of biochemical precursors and metabolic waste products. The recent identification of hydrogen peroxide (H₂O₂) as a permeant of certain AQPs suggests additional roles in mitigating oxidative stress or enabling paracrine H₂O₂ signalling. Yet, an analysis of the structural requirements of the H₂O₂ permeability of AQPs is missing. We subjected a representative set of wild-type and mutant AQPs to a newly established quantitative phenotypic assay. We confirmed high H₂O₂ permeability of the human aquaammoniaporin AQP8 and found intermediate H₂O₂ permeability of the prototypical orthodox water channel AQP1 from the rat. Differences from an earlier report showing an absence of H₂O₂ permeability of human AQP1 can be explained by expression levels. By generating point mutations in the selectivity filter of rat orthodox aquaporin AQP1, we established a correlation of H₂O₂ permeability primarily with water permeability and secondarily with the pore diameter. Even the narrowest pore of the test set (i.e. rat orthodox aquaporin AQP1 H180F with a pore diameter smaller than that of natural orthodox AQPs) conducted water and H₂O₂. We further found that H₂O₂ permeability of the aquaglyceroporin from the malaria parasite Plasmodium falciparum was lower despite its wider pore diameter. The data suggest that all water-permeable AQPs are H₂O₂ channels, yet H₂O₂ permeability varies with the isoform. Thus, generally, AQPs must be considered as putative players in situations of oxidative stress (e.g. in Plasmodium-infected red blood cells, immune cells, the cardiovascular system or cells expressing AQP8 in their mitochondria).

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.

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.

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.

Acyrthosiphon pisum AQP2: a multifunctional insect aquaglyceroporin

Annotation of the recently sequenced genome of the pea aphid (Acyrthosiphon pisum) identified a gene ApAQP2 (ACYPI009194, Gene ID: 100168499) with homology to the Major Intrinsic Protein/aquaporin superfamily of membrane channel proteins. Phylogenetic analysis suggests that ApAQP2 is a member of an insect-specific clade of this superfamily. Homology model structures of ApAQP2 showed a novel array of amino acids comprising the substrate selectivity-determining "aromatic/arginine" region of the putative transport pore. Subsequent characterization of the transport properties of ApAQP2 upon expression in Xenopus oocytes supports an unusual substrate selectivity profile. Water permeability analyses show that the ApAQP2 protein exhibits a robust mercury-insensitive aquaporin activity. However unlike the water-specific ApAQP1 protein, ApAQP2 forms a multifunctional transport channel that shows a wide permeability profile to a range of linear polyols, including the potentially biologically relevant substrates glycerol, mannitol and sorbitol. Gene expression analysis indicates that ApAQP2 is highly expressed in the insect bacteriocytes (cells bearing the symbiotic bacteria Buchnera) and the fat body. Overall the results demonstrate that ApAQP2 is a novel insect aquaglyceroporin which may be involved in water and polyol transport in support of the Buchnera symbiosis and aphid osmoregulation.

Plasmodium falciparum glutamate dehydrogenase a is dispensable and not a drug target during erythrocytic development

Background

Plasmodium falciparum contains three genes encoding potential glutamate dehydrogenases. The protein encoded by gdha has previously been biochemically and structurally characterized. It was suggested that it is important for the supply of reducing equivalents during intra-erythrocytic development of Plasmodium and, therefore, a suitable drug target.

Methods

The gene encoding the NADP(H)-dependent GDHa has been disrupted by reverse genetics in P. falciparum and the effect on the antioxidant and metabolic capacities of the resulting mutant parasites was investigated.

Results

No growth defect under low and elevated oxygen tension, no up- or down-regulation of a number of antioxidant and NADP(H)-generating proteins or mRNAs and no increased levels of GSH were detected in the D10^Δgdha parasite lines. Further, the fate of the carbon skeleton of [¹³C] labelled glutamine was assessed by metabolomic studies, revealing no differences in the labelling of α-ketoglutarate and other TCA pathway intermediates between wild type and mutant parasites.

Conclusions

First, the data support the conclusion that D10^Δgdha parasites are not experiencing enhanced oxidative stress and that GDHa function may not be the provision of NADP(H) for reductive reactions. Second, the results imply that the cytosolic, NADP(H)-dependent GDHa protein is not involved in the oxidative deamination of glutamate but that the protein may play a role in ammonia assimilation as has been described for other NADP(H)-dependent GDH from plants and fungi. The lack of an obvious phenotype in the absence of GDHa may point to a regulatory role of the protein providing glutamate (as nitrogen storage molecule) in situations where the parasites experience a limiting supply of carbon sources and, therefore, under in vitro conditions the enzyme is unlikely to be of significant importance. The data imply that the protein is not a suitable target for future drug development against intra-erythrocytic parasite development.

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.

The Fps1p aquaglyceroporin facilitates the use of small aliphatic amides as a nitrogen source by amidase-expressing yeasts

Saccharomyces cerevisiae acquires a resistance to high, toxic levels of acetic acid by destabilizing Fps1p, the plasma membrane aquaglyceroporin through which this acid - in its undissociated state - enters the cell. In this study, Fps1p loss was shown to confer resistances to acetic acid, acrolein and allyl alcohol, not just in S. cerevisiae but also in the osmotolerant spoilage yeast Zygosaccharomyces rouxii. However, in Z. rouxii, the loss of Fps1p severely compromised the use of acetamide and several other small amides as sources of nitrogen, an indication that these amides enter the cells of this yeast by passive diffusion through the Fps1p pore. Saccharomyces cerevisiae cannot grow on acetamide, but was conferred with an ability to use this and other small amides as nitrogen sources by heterologous expression of a Z. rouxii ORF (ZrAMD1) with protein sequence identity to the amdS-encoded amidase of Aspergillus nidulans. This capacity of ZrAMD1-expressing S. cerevisiae to assimilate amide nitrogen was severely compromised by the loss of Fps1p. ZrAMD1 appears to encode the major amidase of Z. rouxii as a Zramd1Delta deletant mutant had, like the Zrfps1Delta deletant, lost the ability to assimilate small amides as sources of nitrogen.

Characterization of a novel organelle in Toxoplasma gondii with similar composition and function to the plant vacuole

Toxoplasma gondii belongs to the phylum Apicomplexa and is an important cause of congenital disease and infection in immunocompromised patients. Like most apicomplexans, T. gondii possesses several plant-like features, such as the chloroplast-like organelle, the apicoplast. We describe and characterize a novel organelle in T. gondii tachyzoites, which is visible by light microscopy and possesses a broad similarity to the plant vacuole. Electron tomography shows the interaction of this vacuole with other organelles. The presence of a plant-like vacuolar proton pyrophosphatase (TgVP1), a vacuolar proton ATPase, a cathepsin L-like protease (TgCPL), an aquaporin (TgAQP1), as well as Ca(2+)/H(+) and Na(+)/H(+) exchange activities, supports similarity to the plant vacuole. Biochemical characterization of TgVP1 in enriched fractions shows a functional similarity to the respective plant enzyme. The organelle is a Ca(2+) store and appears to have protective effects against salt stress potentially linked to its sodium transport activity. In intracellular parasites, the organelle fragments, with some markers colocalizing with the late endosomal marker, Rab7, suggesting its involvement with the endocytic pathway. Studies on the characterization of this novel organelle will be relevant to the identification of novel targets for chemotherapy against T. gondii and other apicomplexan parasites as well.

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.