Accessibility of substituted cysteines in TM2 and TM10 transmembrane segments in the Plasmodium falciparum equilibrative nucleoside transporter PfENT1.

Infection with Plasmodium species parasites causes malaria. Plasmodium parasites are purine auxotrophic. They import purines via an equilibrative nucleoside transporter (ENT). In P. falciparum, the most virulent species, the equilibrative nucleoside transporter 1 (PfENT1) represents the primary purine uptake pathway. This transporter is a potential target for the development of antimalarial drugs. In the absence of a high-resolution structure for either PfENT1 or a homologous ENT, we used the substituted cysteine accessibility method (SCAM) to investigate the membrane-spanning domain structure of PfENT1 to identify potential inhibitor-binding sites. We previously used SCAM to identify water-accessible residues that line the permeation pathway in transmembrane segment 11 (TM11). TM2 and TM10 lie adjacent to TM11 in an ab initio model of a homologous Leishmania donovani nucleoside transporter. To identify TM2 and TM10 residues in PfENT1 that are at least transiently on the water-accessible transporter surface, we assayed the reactivity of single cysteine-substitution mutants with three methanethiosulfonate (MTS) derivatives. Cysteines substituted for 12 of 14 TM2 segment residues reacted with MTS-ethyl-ammonium-biotin (MTSEA-biotin). At eight positions, MTSEA-biotin inhibited transport, and at four positions substrate transport was potentiated. On an α helical wheel projection of TM2, the four positions where potentiation occurred were located in a cluster on one side of the helix. In contrast, although MTSEA-biotin inhibited 9 of 10 TM10 cysteine-substituted mutants, the reactive residues did not form a pattern consistent with either an α helix or ß sheet. These results may help identify the binding site(s) of PfENT1 inhibitors.

Plasmodium falciparum var gene expression is modified by host immunity.

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a potentially important family of immune targets, which play a central role in the host-parasite interaction by binding to various host molecules. They are encoded by a diverse family of genes called var, of which there are approximately 60 copies in each parasite genome. In sub-Saharan Africa, although P. falciparum infection occurs throughout life, severe malarial disease tends to occur only in childhood. This could potentially be explained if (i) PfEMP1 variants differ in their capacity to support pathogenesis of severe malaria and (ii) this capacity is linked to the likelihood of each molecule being recognized and cleared by naturally acquired antibodies. Here, in a study of 217 Kenyan children with malaria, we show that expression of a group of var genes "cys2," containing a distinct pattern of cysteine residues, is associated with low host immunity. Expression of cys2 genes was associated with parasites from young children, those with severe malaria, and those with a poorly developed antibody response to parasite-infected erythrocyte surface antigens. Cys-2 var genes form a minor component of all genomic var repertoires analyzed to date. Therefore, the results are compatible with the hypothesis that the genomic var gene repertoire is organized such that PfEMP1 molecules that confer the most virulence to the parasite tend also to be those that are most susceptible to the development of host immunity. This may help the parasite to adapt effectively to the development of host antibodies through modification of the host-parasite relationship.

A mosaic of RNA binding and protein interaction motifs in a bifunctional mitochondrial tRNA import factor from Leishmania tropica.

Proteins that participate in the import of cytosolic tRNAs into mitochondria have been identified in several eukaryotic species, but the details of their interactions with tRNA and other proteins are unknown. In the kinetoplastid protozoon Leishmania tropica, multiple proteins are organized into a functional import complex. RIC8A, a tRNA-binding subunit of this complex, has a C-terminal domain that functions as subunit 6b of ubiquinol cytochrome c reductase (complex III). We show that the N-terminal domain, unique to kinetoplastid protozoa, is structurally similar to the appended S15/NS1 RNA-binding domain of aminoacyl tRNA synthetases, with a helix-turn-helix motif. Structure-guided mutagenesis coupled with in vitro assays showed that helix alpha1 contacts tRNA whereas helix alpha2 targets the protein for assembly into the import complex. Inducible expression of a helix 1-deleted variant in L. tropica resulted in formation of an inactive import complex, while the helix 2-deleted variant was unable to assemble in vivo. Moreover, a protein-interaction assay showed that the C-terminal domain makes allosteric contacts with import receptor RIC1 complexed with tRNA. These results help explain the origin of the bifunctionality of RIC8A, and the allosteric changes accompanying docking and release of tRNA during import.

Impact of Field Isolate Identified Nonsynonymous Single Nucleotide Polymorphisms on Plasmodium falciparum Equilibrative Nucleoside Transporter 1 Inhibitor Efficacy.

Plasmodium falciparum causes the most severe form of malaria and causes approximately 500 000 deaths per year. P. falciparum parasites resistant to current antimalarial treatments are spreading. Therefore, it is imperative to develop new antimalarial drugs. Malaria parasites are purine auxotrophic. They rely on purine import from the host erythrocyte via Equilibrative Nucleoside Transporters (ENTs). Recently, inhibitors of the P. falciparum ENT1 (PfENT1) that inhibit proliferation of malaria parasites in culture have been identified as promising starting points for antimalarial drug development. Genome sequencing of P. falciparum field isolates has identified nonsynonymous single nucleotide polymorphisms (SNPs) in the gene encoding PfENT1. Here we evaluate the impact of these PfENT1 SNPs on purine substrate affinity and inhibitor efficacy. We expressed each PfENT1-SNP in Saccharomyces cerevisiae. Using PfENT1-SNP-expressing yeast, we characterized the PfENT1 purine substrate affinity using radiolabeled substrate uptake inhibition experiments. Four of the 13 SNPs altered affinity for one or more purines by up to 7-fold. Three of the SNPs reduced the potency of a subset of the inhibitors by up to 7-fold. One SNP, Q284E, reduced the potency of all six inhibitor chemotypes. We tested drug efficacy in available parasite strains containing PfENT1 SNPs. While PfENT1-SNP-expressing yeast had decreased sensitivity to PfENT1 inhibitors, parasite strains containing SNPs showed similar or more potent inhibition of proliferation with all PfENT1 inhibitors. Thus, parasite strains bearing PfENT1 SNPs are not resistant to these PfENT1 inhibitors. This supports PfENT1 as a promising target for further development of novel antimalarial drugs.

Identification via a Parallel Hit Progression Strategy of Improved Small Molecule Inhibitors of the Malaria Purine Uptake Transporter that Inhibit Plasmodium falciparum Parasite Proliferation.

Emerging resistance to current antimalarial medicines underscores the importance of identifying new drug targets and novel compounds. Malaria parasites are purine auxotrophic and import purines via the Plasmodium falciparum equilibrative nucleoside transporter type 1 (PfENT1). We previously showed that PfENT1 inhibitors block parasite proliferation in culture. Our goal was to identify additional, possibly more optimal chemical starting points for a drug discovery campaign. We performed a high throughput screen (HTS) of GlaxoSmithKline's 1.8 million compound library with a yeast-based assay to identify PfENT1 inhibitors. We used a parallel progression strategy for hit validation and expansion, with an emphasis on chemical properties in addition to potency. In one arm, the most active hits were tested for human cell toxicity; 201 had minimal toxicity. The second arm, hit expansion, used a scaffold-based substructure search with the HTS hits as templates to identify over 2000 compounds; 123 compounds had activity. Of these 324 compounds, 175 compounds inhibited proliferation of P. falciparum parasite strain 3D7 with IC50 values between 0.8 and ∼180 µM. One hundred forty-two compounds inhibited PfENT1 knockout (pfent1Δ) parasite growth, indicating they also hit secondary targets. Thirty-two hits inhibited growth of 3D7 but not pfent1Δ parasites. Thus, PfENT1 inhibition was sufficient to block parasite proliferation. Therefore, PfENT1 may be a viable target for antimalarial drug development. Six compounds with novel chemical scaffolds were extensively characterized in yeast-, parasite-, and human-erythrocyte-based assays. The inhibitors showed similar potencies against drug sensitive and resistant P. falciparum strains. They represent attractive starting points for development of novel antimalarial drugs.

Targeting the Plasmodium vivax equilibrative nucleoside transporter 1 (PvENT1) for antimalarial drug development.

Infection with Plasmodium falciparum and vivax cause most cases of malaria. Emerging resistance to current antimalarial medications makes new drug development imperative. Ideally a new antimalarial drug should treat both falciparum and vivax malaria. Because malaria parasites are purine auxotrophic, they rely on purines imported from the host erythrocyte via Equilibrative Nucleoside Transporters (ENTs). Thus, the purine import transporters represent a potential target for antimalarial drug development. For falciparum parasites the primary purine transporter is the P. falciparum Equilibrative Nucleoside Transporter Type 1 (PfENT1). Recently we identified potent PfENT1 inhibitors with nanomolar IC50 values using a robust, yeast-based high throughput screening assay. In the current work we characterized the Plasmodium vivax ENT1 (PvENT1) homologue and its sensitivity to the PfENT1 inhibitors. We expressed a yeast codon-optimized PvENT1 gene in Saccharomyces cerevisiae. PvENT1-expressing yeast imported both purines ([(3)H]adenosine) and pyrimidines ([(3)H]uridine), whereas wild type (fui1Δ) yeast did not. Based on radiolabel substrate uptake inhibition experiments, inosine had the lowest IC50 (3.8 µM), compared to guanosine (14.9 µM) and adenosine (142 µM). For pyrimidines, thymidine had an IC50 of 183 µM (vs. cytidine and uridine; mM range). IC50 values were higher for nucleobases compared to the corresponding nucleosides; hypoxanthine had a 25-fold higher IC50 than inosine. The archetypal human ENT1 inhibitor 4-nitrobenzylthioinosine (NBMPR) had no effect on PvENT1, whereas dipyridamole inhibited PvENT1, albeit with a 40 µM IC50, a 1000-fold less sensitive than human ENT1 (hENT1). The PfENT1 inhibitors blocked transport activity of PvENT1 and the five known naturally occurring non-synonymous single nucleotide polymorphisms (SNPs) with similar IC50 values. Thus, the PfENT1 inhibitors also target PvENT1. This implies that development of novel antimalarial drugs that target both falciparum and vivax ENT1 may be feasible.

Yeast-based high-throughput screen identifies Plasmodium falciparum equilibrative nucleoside transporter 1 inhibitors that kill malaria parasites.

Equilibrative transporters are potential drug targets; however, most functional assays involve radioactive substrate uptake that is unsuitable for high-throughput screens (HTS). We developed a robust yeast-based growth assay that is potentially applicable to many equilibrative transporters. As proof of principle, we applied our approach to Equilibrative Nucleoside Transporter 1 of the malarial parasite Plasmodium falciparum (PfENT1). PfENT1 inhibitors might serve as novel antimalarial drugs since PfENT1-mediated purine import is essential for parasite proliferation. To identify PfENT1 inhibitors, we screened 64 560 compounds and identified 171 by their ability to rescue the growth of PfENT1-expressing fui1Δ yeast in the presence of a cytotoxic PfENT1 substrate, 5-fluorouridine (5-FUrd). In secondary assays, nine of the highest activity compounds inhibited PfENT1-dependent growth of a purine auxotrophic yeast strain with adenosine as the sole purine source (IC50 0.2-2 µM). These nine compounds completely blocked [(3)H]adenosine uptake into PfENT1-expressing yeast and erythrocyte-free trophozoite-stage parasites (IC50 5-50 nM), and inhibited chloroquine-sensitive and -resistant parasite proliferation (IC50 5-50 µM). Wild-type (WT) parasite IC50 values were up to 4-fold lower compared to PfENT1-knockout (pfent1Δ) parasites. pfent1Δ parasite killing showed a delayed-death phenotype not observed with WT. We infer that, in parasites, the compounds inhibit both PfENT1 and a secondary target with similar efficacy. The secondary target identity is unknown, but its existence may reduce the likelihood of parasites developing resistance to PfENT1 inhibitors. Our data support the hypothesis that blocking purine transport through PfENT1 may be a novel and compelling approach for antimalarial drug development.

Disruption of lmo1386, a putative DNA translocase gene, affects biofilm formation of Listeria monocytogenes on abiotic surfaces.

The distribution and survival of Listeria monocytogenes (L. monocytogenes) in food processing environment is linked to its ability to form biofilms, however the genetic mechanisms remain unclear. In our previous study, a Himar1 mariner-based transposon mutagenesis was performed and 42 mutants were confirmed to have reduced biofilm formation. Among the 42 biofilm deficient mutants, two isolates (s25-10C and s55-1D) harbored single insertion in lmo1386, a gene encoding a putative DNA translocase. The lmo1386 mutants had impaired biofilm formation in both static and flow conditions. The mutant strain s55-1D was complemented by cloning the entire lmo1386 gene into pPL2-gtcAP, a derivative of the integration vector pPL2 with the L. monocytogenes gtcA promoter. The genetically complemented mutant restored its biofilm phenotype, demonstrating the role of lmo1386 in the biofilm formation of L. monocytogenes. The lmo1386 mutant had reduced initial adhesion ability, which could at least partially contribute to the impaired biofilm phenotype. Additionally, the lmo1386 mutant formed elongated cell chains when grown in a nutrient TSBYE media, while no obvious cell morphology changes were observed when grown in the minimal MWB media. Overall, our findings suggest that the disruption of lmo1386, a putative DNA translocase gene affects the biofilm formation of L. monocytogenes on abiotic surfaces, which may further advance the understanding of the complicated process of biofilm formation.

Transport of purines and purine salvage pathway inhibitors by the Plasmodium falciparum equilibrative nucleoside transporter PfENT1.

Plasmodium falciparum is a purine auxotroph. The transport of purine nucleosides and nucleobases from the host erythrocyte to the parasite cytoplasm is essential to support parasite growth. P. falciparum equilibrative nucleoside transporter 1 (PfENT1) is a major route for purine transport across the parasite plasma membrane. Malarial parasites are sensitive to inhibitors of purine salvage pathway enzymes. The immucillin class of purine nucleoside phosphorylase inhibitors and the adenosine analog, tubercidin, block growth of P. falciparum under in vitro culture conditions. We sought to determine whether these inhibitors utilize PfENT1 to gain access to the parasite cytosol. There is considerable controversy in the literature regarding the K(m) and/or K(i) for purine transport by PfENT1 in the Xenopus oocyte expression system. We show that oocytes metabolize adenosine but not hypoxanthine. For adenosine, metabolism is the rate limiting step in oocyte uptake assays, making hypoxanthine the preferred substrate for PfENT1 transport studies in oocytes. We demonstrate that the K(i) for PfENT1 transport of hypoxanthine and adenosine is in the 300-700microM range. Effects of substrate metabolism on uptake studies may explain conflicting results in the literature regarding the PfENT1 adenosine transport K(m). PfENT1 transports the tubercidin class of compounds. None of the immucillin compounds tested inhibited PfENT1 transport of [(3)H]hypoxanthine or [(3)H]adenosine. Although nucleobases are transported, modifications of the ribose ring in corresponding nucleoside analogs affect substrate recognition by PfENT1. These results provide new insights into PfENT1 and the mechanism by which purine salvage pathway inhibitors are transported into the parasite cytoplasm.