Functional conservation of the malaria vaccine antigen MSP-119across distantly related Plasmodium species

The C-terminal region of Plasmodium falciparum merozoite surface protein 1 (MSP-119) is at present a leading malaria vaccine candidate. Antibodies against the epidermal growth factor-like domains of MSP-1 19are associated with immunity to P. falciparum and active immunization with recombinant forms of the molecule protect against malaria challenge in various experimental systems. These findings, with the knowledge that epidermal growth factor-like domains in other molecules have essential binding functions, indicate the importance of this protein in merozoite invasion of red blood cells. Despite extensive molecular epidemiological investigations, only limited sequence polymorphism has been identified in P. falciparum MSP-119 (refs. 9-11). This indicates its sequence is functionally constrained, and is used in support of the use of MSP-119 as a vaccine. Here, we have successfully complemented the function of most of P. falciparum MSP-119 with the corresponding but highly divergent sequence from the rodent parasite P. chabaudi. The results indicate that the role of MSP-119 in red blood cell invasion is conserved across distantly related Plasmodium species and show that the sequence of P. falciparum MSP-119 is not constrained by function.

Analysis of mefloquine resistance and amplification of pfmdr1 in multidrug-resistant Plasmodium falciparum isolates from Thailand

Resistance to quinoline-containing compound has been associated with the Plasmodium falciparum multidrug resistance 1 (pfmdr1) gene. We analyzed wild P. falciparum isolates with high levels of chloroquine and mefloquine resistance for their macrorestriction maps of chromosome 5 and sequence of pfmdr1. Two types of chromosome 5 amplification were found. Eleven of 62 resistant isolates displayed Bgl 1 fragments larger than 100 kb. Twenty-nine isolates possessed multiple copies of the fragments. We failed to detect any amplification of this region on chromosome 5 in 22 mefloquine-resistant isolates, suggesting that other mechanisms can mediate the mefloquine-resistant phenotype. There was no direct association between pfmdr1 mutations and chloroquine sensitivity. Resistant lines could have Asn-86 and Tyr-184 or Phe-184, the predicted sequence of those chloroquine-sensitive isolates. No mutation at Asn-1042 and Asp-1246 was detected among these chloroquine-resistant isolates. Therefore, a few base substitutions in the pfmdr1 gene may not be sufficient to account for all chloroquine-resistant phenotypes.

Transfection of the human malaria parasite Plasmodium falciparum

In the past few years, methods have been developed which allow the introduction of exogenous DNA into the human malaria parasite Plasmodium falciparum. This important technical advance known as parasite transfection, provides powerful new tools to study the function of Plasmodium proteins and their roles in biology and disease. Already it has allowed the analysis of promoter function and has been successfully applied to establish the role of particular molecules and/or mutations in the biology of this parasite. This review summarises the current state of the technology and how it has been applied to dissect the function of the P. falciparum genome.

The adhesion of Plasmodium falciparum-infected erythrocytes to chondroitin sulfate A is mediated by P. falciparum erythrocyte membrane protein 1

Chondroitin sulfate A (CSA) is an important receptor for the sequestration of Plasmodium falciparum in the placenta, but the parasite ligand involved in adhesion has not previously been identified. Here we report the identification of a var gene transcribed in association with binding to CSA and present evidence that the P. falciparum erythrocyte membrane protein 1 product of the gene is the parasite ligand mediating CSA binding. Description of this gene and the implication of P. falciparum erythrocyte membrane protein 1 as the parasite ligand paves the way to a more detailed understanding of the pathogenesis of placental infection and potential therapeutic strategies targeting the interaction.

Nuclear-encoded proteins target to the plastid in Toxoplasma gondii and Plasmodium falciparum

A vestigial, nonphotosynthetic plastid has been identified recently in protozoan parasites of the phylum Apicomplexa. The apicomplexan plastid, or "apicoplast," is indispensable, but the complete sequence of both the Plasmodium falciparum and Toxoplasma gondii apicoplast genomes has offered no clue as to what essential metabolic function(s) this organelle might perform in parasites. To investigate possible functions of the apicoplast, we sought to identify nuclear-encoded genes whose products are targeted to the apicoplast in Plasmodium and Toxoplasma. We describe here nuclear genes encoding ribosomal proteins S9 and L28 and the fatty acid biosynthetic enzymes acyl carrier protein (ACP), beta-ketoacyl-ACP synthase III (FabH), and beta-hydroxyacyl-ACP dehydratase (FabZ). These genes show high similarity to plastid homologues, and immunolocalization of S9 and ACP verifies that the proteins accumulate in the plastid. All the putatively apicoplast-targeted proteins bear N-terminal presequences consistent with plastid targeting, and the ACP presequence is shown to be sufficient to target a recombinant green fluorescent protein reporter to the apicoplast in transgenic T. gondii. Localization of ACP, and very probably FabH and FabZ, in the apicoplast implicates fatty acid biosynthesis as a likely function of the apicoplast. Moreover, inhibition of P. falciparum growth by thiolactomycin, an inhibitor of FabH, indicates a vital role for apicoplast fatty acid biosynthesis. Because the fatty acid biosynthesis genes identified here are of a plastid/bacterial type, and distinct from those of the equivalent pathway in animals, fatty acid biosynthesis is potentially an excellent target for therapeutics directed against malaria, toxoplasmosis, and other apicomplexan-mediated diseases.

Allelic exchange at the endogenous genomic locus in Plasmodium falciparum proves the role of dihydropteroate synthase in sulfadoxine-resistant malaria

We have exploited the recently developed ability to trans- fect the malaria parasite Plasmodium falciparum to investigate the role of polymorphisms in the enzyme dihydropteroate synthase (DHPS), identified in sulfadoxine-resistant field isolates. By using a truncated form of the dhps gene, specific mutations were introduced into the endogenous gene by allelic replacement such that they were under the control of the endogenous promoter. Using this approach a series of mutant dhps alleles that mirror P.falciparum variants found in field isolates were found to confer different levels of sulfadoxine resistance. This analysis shows that alteration of Ala437 to Gly (A437G) confers on the parasite a 5-fold increase in sulfadoxine resistance and addition of further mutations increases the level of resistance to 24-fold above that seen for the transfectant expressing the wild-type dhps allele. This indicates that resistance to high levels of sulfadoxine in P.falciparum has arisen by an accumulation of mutations and that Gly437 is a key residue, consistent with its occurrence in most dhps alleles from resistant isolates. These studies provide proof that the mechanism of resistance to sulfadoxine in P.falciparum involves mutations in the dhps gene and determines the relative contribution of these mutations to this phenotype.

Effect of altered serum lipid concentrations on the IC50 of halofantrine against Plasmodium falciparum

Halofantrine (Hf) is a highly lipophilic antimalarial which significantly associates with triglyceride (TG) rich plasma lipoproteins, and this is likely manifest as a decrease in the free fraction of drug. This study assessed the effect of using growth media containing 10% serum containing different concentrations of TG (i.e. TG-rich plasma lipoproteins) on the IC50 of Hf determined using continuous in vitro culture of Plasmodium falciparum. Serum was collected from a human subject in either a fasted state or at various times after ingestion of a fatty meal. There was a linear and statistically significant 2.5-fold increase in the IC50 of Hf across a 6-fold range of increasing TG concentrations, with the increased IC50 values being ascribed to a decreased free fraction of Hf in the growth media due to sequestration by TG-rich lipoproteins. Chloroquine diphosphate, which is hydrophilic and not significantly bound by TG-rich lipoproteins, was used as a control and its IC50 values were independent of TG concentrations. These data indicate that consideration should be given to the adoption of standard conditions for the collection of serum with respect to pre- or postprandial states, and that subject- and disease-related factors which alter plasma lipoprotein profiles should be considered when interpreting the IC50 profile of Hf (and possibly other lipophilic antimalarials). Furthermore, although food is known to affect the pharmacokinetics of Hf, these data suggest that altered plasma lipoprotein profiles could also influence its pharmacodynamic profile.

Mutations in dihydropteroate synthase are responsible for sulfone and sulfonamide resistance in Plasmodium falciparum

Plasmodium falciparum causes the most severe form of malaria in humans. An important class of drugs in malaria treatment is the sulfone/sulfonamide group, of which sulfadoxine is the most commonly used. The target of sulfadoxine is the enzyme dihydropteroate synthase (DHPS), and sequencing of the DHPS gene has identified amino acid differences that may be involved in the mechanism of resistance to this drug. In this study we have sequenced the DHPS gene in 10 isolates from Thailand and identified a new allele of DHPS that has a previously unidentified amino acid difference. We have expressed eight alleles of P. falciparum PPPK-DHPS in Escherichia coli and purified the functional enzymes to homogeneity. Strikingly, the Ki for sulfadoxine varies by almost three orders of magnitude from 0.14 microM for the DHPS allele from sensitive isolates to 112 microM for an enzyme expressed in a highly resistant isolate. Comparison of the Ki of different sulfonamides and the sulfone dapsone has suggested that the amino acid differences in DHPS would confer cross-resistance to these compounds. These results show that the amino acid differences in the DHPS enzyme of sulfadoxine-resistant isolates of P. falciparum are central to the mechanism of resistance to sulfones and sulfonamides.

Stable transgene expression in Plasmodium falciparum

Plasmid vectors designed to express transgenes and a selectable marker in Plasmodiumfalciparum were constructed. These consist of a selectable gene cassette comprising the Toxoplasma gondii dihydrofolate reductase-thymidylate synthase (DHFR-TS) gene mutated to confer pyrimethamine resistance flanked by either Plasmodium chabaudi DHFR-TS or P. falciparum calmodulin promoter sequences and the P. falciparum histidine rich protein 2 3' region. Also, each vector includes a different expression cassette driven by various Plasmodium transcriptional control sequences. Initially, the chloramphenicol acetyl transferase (CAT) reporter gene was cloned into the expression site of two vectors, pCC6-CAT and pCC13-CAT, which were identical except for the orientation of the expression cassette with respect to the selectable gene cassette. Approximately 8-fold more CAT activity was detected when the direction of transcription of the expression cassettes was in a head to head, rather than a tail to head, orientation. Importantly, it was found that stable transfection could only be achieved when the gene cassettes were in the head to head direction suggesting that this orientation also has an effect on the level of expression of the selectable marker. All other plasmids were designed with the cassettes in a head to head orientation. With the exception of pCC6-CAT and a second vector pHC4-CAT, stable transfectants were obtained with each vector in which the CAT gene had been inserted into the expression cassette. This is the first time vectors for the stable expression in Plasmodium parasites of transgenes other than a selectable marker have been described.

The antimalarial drug, chloroquine, interacts with lactate dehydrogenase from Plasmodium falciparum

We have previously shown that a radioiodinated photoreactive analogue of chloroquine, [125I]N-(4-(4-diethylamino-1-methylbutylamino)quinolin-6-yl) -4-azido-2-hydroxybenzamide ([125I]ASA-Q), specifically labels two proteins in Plasmodium falciparum with apparent molecular weights (Mr) of 42 and 33 kDa (Foley M, Deady LW, Ng K, Cowman AF, Tilley L. J Biol Chem 1994:269:6955-6961). We now report the identification of the 33 kDa protein. The 33 kDa protein was purified from Plasmodium falciparum using photoaffinity labeling with [125I]ASA-Q to monitor the enrichment process. N-terminal sequence analysis of the purified protein revealed exact identity of the first 35 amino acids with P. falciparum lactate dehydrogenase (PfLDH). The plasmodial enzyme was cloned and expressed in E. coli and the recombinant protein used to produce a rabbit antiserum. Immunoprecipitation using affinity-purified anti-PfLDH antibodies confirmed the identity of the 33 kDa CQ-binding protein. The enzyme activity of purified PfLDH was not significantly affected by chloroquine indicating that PfLDH is not a direct target of CQ. PfLDH was, however, shown to be exquisitely sensitive to inhibition by free heme and chloroquine protected against this inhibitory effect.

The chromosomal organization of the Plasmodium falciparum var gene family is conserved

The var gene family of Plasmodium falciparum encodes the protein PfEMP1 which is located on the surface of infected erythrocytes and is the receptor that mediates binding to ligands on endothelial cells. This family of proteins is responsible for antigenic variation and differences in binding phenotype to ligands such as CD36 and ICAM1. We have compared the organization of the var gene family in three in vitro cloned lines of P. falciparum and show that most var genes are located in the subtelomeric region of each chromosome closely linked to the repetitive sequence rep20. While most chromosomes possess var genes in the subtelomeric region, in each in vitro cloned line there are some chromosomes that have deleted subtelomeric repetitive regions which include var genes. Comparison of the location of var genes in a field isolate showed that it does not have any detectable subtelomeric deletions as all chromosomes contain var genes and rep20 sequences. We have detected three chromosomes (4, 7 and 12) that contain var gene loci in more stable central regions and the position of these genes on chromosome 4 in the cloned lines analysed is conserved. The location of most of the var gene family in the subtelomeric region of the genome of P. falciparum has important implications for the generation of antigenic diversity of the PfEMP1 protein.

Targeted gene disruption shows that knobs enable malaria-infected red cells to cytoadhere under physiological shear stress

Knobs at the surface of erythrocytes infected with Plasmodium falciparum have been proposed to be important in adherence of these cells to the vascular endothelium. This structure contains the knob-associated histidine-rich protein (KAHRP) and the adhesion receptor P. falciparum erythrocyte membrane protein 1. We have disrupted the gene encoding KAHRP and show that it is essential for knob formation. Knob-transfectants adhere to CD36 in static assays; when tested under flow conditions that mimic those of postcapillary venules, however, the binding to CD36 was dramatically reduced. These data suggest that knobs on P. falciparum-infected erythrocytes exert an important influence on adherence of parasitized-erythrocytes to microvascular endothelium, an important process in the pathogenesis of P. falciparum infections.

Photoaffinity labeling of mefloquine-binding proteins in human serum, uninfected erythrocytes and Plasmodium falciparum-infected erythrocytes

A photoreactive quinolinemethanol analog, N-[4-[1-hydroxy-2-(dibutylamino)ethyl]quinolin-8yl]-4- azido-2-salicylamide (ASA-MQ) has been synthesized which closely mimics the action of mefloquine. ASA-MQ possesses potent antimalarial activity against a mefloquine-sensitive strain of Plasmodium falciparum and shows decreased activity against a mefloquine-resistant parasite strain. Radioiodinated ASA-MQ has been used in photoaffinity labeling studies to identify mefloquine-interacting proteins in serum, uninfected erythrocytes and Plasmodium falciparum-infected erythrocytes. We have shown that mefloquine interacts specifically with apo-A1, the major protein of serum high density lipoproteins. In addition, mefloquine was shown to interact specifically with the erythrocyte membrane protein, band 7.2b (stomatin). A further two high affinity mefloquine-binding proteins with apparent molecular masses of 22 and 36 kDa were identified in three different strains of Plasmodium falciparum. We suggest that these two mefloquine-binding parasite proteins may be involved in the uptake of mefloquine or may represent macromolecular targets of mefloquine action in malaria parasites.

Modulation of the function of human MDR1 P-glycoprotein by the antimalarial drug mefloquine

MDR1 P-glycoprotein in membranes of human tumor cells of the CEM/VBL100 line was selectively labelled using photoreactive analogs of verapamil, N-(p-azido-3-[125I]salicyl)amino-verapamil ([125I]ASA-V) and prazosin, 2-[4-(4-azido-3-[125I]iodobenzoyl)piperazin-1-yl]4 -amino-6,7-dimethoxyyquinazoline ([125I]ASA-P). Mefloquine, a quinolinemethanol antimalarial drug, was shown to inhibit the labelling of P-glycoprotein with an efficiency similar to that for verapamil, a known chemosensitizer. By contrast, chloroquine competed poorly for the binding site on P-glycoprotein. Mefloquine also inhibited the functional activity of P-glycoprotein. It decreased the rates of extrusion of [3H]vinblastine and the fluorescent dyes, fluo-3 acetomethoxy ester and rhodamine 123, from drug-resistant cells and decreased the level of resistance of these cells to vinblastine. The ability of mefloquine to inhibit P-glycoprotein function may be involved in the neurotoxic side-effects occasionally associated with the use of mefloquine as an antimalarial drug.

Cloning and sequence analysis of a novel member of the ATP-binding cassette (ABC) protein gene family from Plasmodium falciparum

We have employed oligonucleotide primers directed against the Walker A and B ATP-binding consensus motifs in a PCR-approach to clone a novel member of the eukaryotic ABC protein family of genes from Plasmodium falciparum. The novel gene is predicted to encode a 95.5-kDa protein with two ATP-binding folds each containing a Walker A and B consensus motif and an ABC protein signature sequence. The predicted protein is highly hydrophilic and contains numerous phosphorylation consensus sites but does not contain any potential membrane spanning domains. The gene is present on chromosome 11 and is expressed as a 3.3-kb transcript. The closest homologue with known function to the plasmodial gene is the yeast GCN20 gene which is part of the translation initiation pathway in amino acid starved yeast cells. We have therefore tentatively named the gene Plasmodium falciparum GCN20 homologue (pfgcn20). The pfgcn20 encoded Pfgcn20 protein is also highly homologous to a number of ATP-binding subunits of prokaryotic ABC transporters. We speculate that Pfgcn20 may be an example of a eukaryotic ATP-binding cytosolic subunit of a multipeptide ABC transporter.

The var genes of Plasmodium falciparum are located in the subtelomeric region of most chromosomes

PfEMP1, a Plasmodium falciparum-encoded protein on the surface of infected erythrocytes is a ligand that mediates binding to receptors on endothelial cells. The PfEMP1 protein, which is encoded by the large var gene family, shows antigenic variation and changes in binding phenotype associated with alterations in antigenicity. We have constructed a yeast artificial chromosome contig of chromosome 12 from P. falciparum and show that var genes are arranged in four clusters; two lie amongst repetitive subtelomeric sequences and two occur in the more conserved central region. Analysis of parasite chromosomes by pulsed field gel electrophoresis (PFGE) demonstrates that most contain var genes and two-dimensional PFGE has shown that var genes are located at chromosome ends interspersed amongst repetitive sequences present in the subtelomeric complex. Analysis of a var gene located in the subtelomeric region of chromosome 12 has shown that it has close homologues at the opposite end of the chromosome and in the subtelomeric region of two other chromosomes. This suggests that recombination between heterologous chromosomes has occurred in the subtelomeric regions of these chromosomes. The subtelomeric location of var genes dispersed amongst repetitive sequences has important implications for generation of antigenic variants and novel cytoadherent specificities of this protein.

Plasmodium falciparum: chloroquine selection of a cloned line and DNA rearrangements

The chloroquine-sensitive Plasmodium falciparum clone, HB3, was selected for growth in increasing amounts of chloroquine. Increases in the size of chromosome 3 were observed with increased chloroquine selection, with the less chloroquine-sensitive parasites possessing the larger forms of the chromosome. Removal of chloroquine selection resulted in a decrease in size of the larger form to the original size and reexposure of these parasites to chloroquine selected again for parasites with a larger chromosome 3. These results suggest that the increase in size of chromosome 3 was associated with the decreased drug sensitivity in the chloroquine selected parasites. A macrorestriction map of chromosome 3 was constructed using the parent HB3 and drug selected lines. The chromosomal size increase was found to have resulted from DNA amplification occurring in 100-kb amplicons. Other isolates of P. falciparum were screened for large size polymorphism in chromosome 3 to determine if chromosome amplification events in this area of the genome also occurred in field isolates. No size changes that correlated with the amplified region in the chloroquine selected lines were observed.

Plasmodium falciparum: amplification and overexpression of pfmdr1 is not necessary for increased mefloquine resistance

Amplification and overexpression of the pfmdr1 gene has been associated with mefloquine resistance in Plasmodium falciparum. We have selected for parasites more resistant to mefloquine from P. falciparum FAC8, a clone that has three copies of pfmdr1. The parasite lines derived from this selection were up to threefold more resistant to mefloquine. The mefloquine-selected parasites were also more resistant to quinine and halofantrine, suggesting a multidrug resistance phenotype. In contrast to previous findings, the selection of P. falciparum FAC8 on mefloquine did not alter the copy number or the level of expression of pfmdr1. Sequence analysis of pfmdr1 from the selected lines showed no change in the amino acids. These results show that alterations in pfmdr1 are not involved in mediating the increased mefloquine resistance observed in this clone.

Point mutations in the dihydrofolate reductase and dihydropteroate synthetase genes and in vitro susceptibility to pyrimethamine and cycloguanil of Plasmodium falciparum isolates from Papua New Guinea

Plasmodium falciparum isolates from 24 Papua New Guinean patients with symptomatic malaria were tested for susceptibility to pyrimethamine and cycloguanil. Thirteen isolates were sensitive to both agents and the remainder exhibited varying degrees of resistance. No isolates were found to be resistant to one agent yet sensitive to the other and a positive correlation suggesting cross-resistance was found. Parasite DNA extracted from the patients' stained blood slides was amplified and sequenced to examine point mutations in the dihydrofolate reductase (DHFR) and dihydropteroate synthetase genes (DHPS) associated with antifolate resistance. All resistant isolates possessed mutations in the DHFR gene at codon 108, the majority changing from Ser to Asn, but one isolate from Ser to Thr, a change not previously reported in field isolates. A second mutation of the DHFR gene at Cys-59 to Arg was present in isolates with higher level resistance, but not exclusively so. Sequencing the DHPS gene, as a predictor of sulfadoxine resistance, revealed only one example that was different from DHPS alleles of sensitive isolates.