Identification, expression and characterisation of a Babesia bovis hexose transporter

Identification and characterisation of two high-affinity glucose transporters from the spoilage yeast Brettanomyces bruxellensis

The yeast Brettanomyces bruxellensis (syn. Dekkera bruxellensis) is an emerging and undesirable contaminant in industrial low-sugar ethanol fermentations that employ the yeast Saccharomyces cerevisiae. High-affinity glucose import in B. bruxellensis has been proposed to be the mechanism by which this yeast can outcompete S. cerevisiae. The present study describes the characterization of two B. bruxellensis genes (BHT1 and BHT3) believed to encode putative high-affinity glucose transporters. In vitro-generated transcripts of both genes as well as the S. cerevisiae HXT7 high-affinity glucose transporter were injected into Xenopus laevis oocytes and subsequent glucose uptake rates were assayed using 14C-labelled glucose. At 0.1 mM glucose, Bht1p was shown to transport glucose five times faster than Hxt7p. pH affected the rate of glucose transport by Bht1p and Bht3p, indicating an active glucose transport mechanism that involves proton symport. These results suggest a possible role for BHT1 and BHT3 in the competitive ability of B. bruxellensis.

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).

Evolution of the Apicomplexan Sugar Transporter Gene Family Repertoire

Apicomplexan protist parasites utilize host sugars transported into the parasite by sugar transporter proteins for use as an energy source. We performed a phylum-wide phylogenetic analysis of the apicomplexan sugar transporter repertoire. Phylogenetic analyses revealed six major subfamilies of apicomplexan sugar transporters. Transporters in one subfamily have undergone expansions in Piroplasma species and Gregarina niphandrodes, while other subfamilies are highly divergent and contain genes found in only one or two species. Analyses of the divergent apicomplexan subfamilies revealed their presence in ciliates, indicating their alveolate ancestry and subsequent loss in chromerids and many apicomplexans.

Drug target identification in protozoan parasites

INTRODUCTION

Despite the fact that diseases caused by protozoan parasites represent serious challenges for public health, animal production and welfare, only a limited panel of drugs has been marketed for clinical applications.

AREAS COVERED

Herein, the authors investigate two strategies, namely whole organism screening and target-based drug design. The present pharmacopoeia has resulted from whole organism screening, and the mode of action and targets of selected drugs are discussed. However, the more recent extensive genome sequencing efforts and the development of dry and wet lab genomics and proteomics that allow high-throughput screening of interactions between micromolecules and recombinant proteins has resulted in target-based drug design as the predominant focus in anti-parasitic drug development. Selected examples of target-based drug design studies are presented, and calcium-dependent protein kinases, important drug targets in apicomplexan parasites, are discussed in more detail.

EXPERT OPINION

Despite the enormous efforts in target-based drug development, this approach has not yet generated market-ready antiprotozoal drugs. However, whole-organism screening approaches, comprising of both in vitro and in vivo investigations, should not be disregarded. The repurposing of already approved and marketed drugs could be a suitable strategy to avoid fastidious approval procedures, especially in the case of neglected or veterinary parasitoses.

Correlation between glucose uptake and membrane potential in Leishmania parasites isolated from DCL patients with therapeutic failure: a proof of concept

Besides infection with drug-resistant parasites, therapeutic failure in leishmaniasis may be caused by altered drug pharmacokinetics, re-infection, and host immunologic compromise. Our aim has been to evaluate if relapses that occur in patients suffering from diffuse cutaneous leishmaniasis (DCL) associate with changes in the fitness of infecting organisms. Therefore, in isolates from patients suffering DCL, we correlated glucose uptake and plasma membrane potential and compared the results with those obtained from reference strains. The data demonstrate that Leishmania parasites causing DCL incorporate glucose at an efficient rate, albeit without significant changes in the plasma membrane potential as their corresponding reference strains. The isolate that did not change its accumulation rate of glucose compared to its reference strain expressed a less polarized membrane potential that was insensitive to mitochondrial inhibitors, suggesting a metabolic dysfunction that may result in glycolysis being the main source of ATP. The results constitute a proof of concept that indicates that parasites causing DCL adapted well to drug pressure and expressed an increased fitness. That is, that in Leishmania mexicana and Leishmania amazonensis, parasites isolated from DCL patients, a strong modification of the parasite physiology might occur. As consequences, the parasites adapted well to drug pressure, increased their fitness, and they had an efficient glucose uptake rate albeit not significant changes in membrane potential as their corresponding reference strains. Further validation of the concepts herein established and whether or not the third isolate corresponds with a drug-resistant phenotype need to be demonstrated at the genetic level.

New approaches for the identification of drug targets in protozoan parasites

Antiparasitic chemotherapy is an important issue for drug development. Traditionally, novel compounds with antiprotozoan activities have been identified by screening of compound libraries in high-throughput systems. More recently developed approaches employ target-based drug design supported by genomics and proteomics of protozoan parasites. In this chapter, the drug targets in protozoan parasites are reviewed. The gene-expression machinery has been among the first targets for antiparasitic drugs and is still under investigation as a target for novel compounds. Other targets include cytoskeletal proteins, proteins involved in intracellular signaling, membranes, and enzymes participating in intermediary metabolism. In apicomplexan parasites, the apicoplast is a suitable target for established and novel drugs. Some drugs act on multiple subcellular targets. Drugs with nitro groups generate free radicals under anaerobic growth conditions, and drugs with peroxide groups generate radicals under aerobic growth conditions, both affecting multiple cellular pathways. Mefloquine and thiazolides are presented as examples for antiprotozoan compounds with multiple (side) effects. The classic approach of drug discovery employing high-throughput physiological screenings followed by identification of drug targets has yielded the mainstream of current antiprotozoal drugs. Target-based drug design supported by genomics and proteomics of protozoan parasites has not produced any antiparasitic drug so far. The reason for this is discussed and a synthesis of both methods is proposed.

Plasmodial sugar transporters as anti-malarial drug targets and comparisons with other protozoa

Glucose is the primary source of energy and a key substrate for most cells. Inhibition of cellular glucose uptake (the first step in its utilization) has, therefore, received attention as a potential therapeutic strategy to treat various unrelated diseases including malaria and cancers. For malaria, blood forms of parasites rely almost entirely on glycolysis for energy production and, without energy stores, they are dependent on the constant uptake of glucose. Plasmodium falciparum is the most dangerous human malarial parasite and its hexose transporter has been identified as being the major glucose transporter. In this review, recent progress regarding the validation and development of the P. falciparum hexose transporter as a drug target is described, highlighting the importance of robust target validation through both chemical and genetic methods. Therapeutic targeting potential of hexose transporters of other protozoan pathogens is also reviewed and discussed.

Comparison of effects of green tea catechins on apicomplexan hexose transporters and mammalian orthologues

Here we have investigated the inhibitory properties of green tea catechins on the Plasmodium falciparum hexose transporter (PfHT), the Babesia bovis hexose transporter 1 (BboHT1) and the mammalian facilitative glucose transporters, GLUT1 and GLUT5, expressed in Xenopus laevis oocytes. (-)-Epicatechin-gallate (ECG) and (-)-epigallocatechin-gallate (EGCG) inhibited D-glucose transport by GLUT1 and PfHT, and D-fructose transport by GLUT5, with apparent K(i) values between 45 and 117 microM. BboHT1 was more potently inhibited by the ungallated catechins (-)-epicatechin (EC) and (-)-epigallocatechin (EGC), with apparent K(i) values of 108 and 168 microM, respectively. Site-directed mutagenesis experiments provided little further support for previously reported models of catechin binding to hexose transporters. Furthermore, P. falciparum growth inhibition by catechins was not affected by the external D-glucose concentration. Our results provide new data on the inhibitory action of catechins against sugar transporters but were unable to elucidate the antimalarial mechanism of action of these agents.

Membrane transport proteins of the malaria parasite

The malaria parasite-infected erythrocyte is a multi-compartment structure, incorporating numerous different membrane systems. The movement of nutrients, metabolites and inorganic ions into and out of the intraerythrocytic parasite, as well as between subcellular compartments within the parasite, is mediated by transporters and channels - integral membrane proteins that facilitate the movement of solutes across the membrane bilayer. Proteins of this type also play a key role in antimalarial drug resistance. Genes encoding transporters and channels account for at least 2.5% of the parasite genome. However, ascribing functions and physiological roles to these proteins, and defining their roles in drug resistance, is not straightforward. For any given membrane transport protein, a full understanding of its role(s) in the parasitized erythrocyte requires a knowledge of its subcellular localization and substrate specificity, as well as some knowledge of the effects on the parasite of modifying the sequence and/or level of expression of the gene involved. Here we consider recent work in this area, describe a number of newly identified transport proteins, and summarize the likely subcellular localization and putative substrate specificity of all of the candidate membrane transport proteins identified to date.