Identification and characterization of heme-interacting proteins in the malaria parasite,Plasmodium falciparum

The degradation of hemoglobin by the malaria parasite, Plasmodium falciparum, produces free ferriprotoporphyrin IX (FP) as a toxic by-product. In the presence of FP-binding drugs such as chloroquine, FP detoxification is inhibited, and the build-up of free FP is thought to be a key mechanism in parasite killing. In an effort to identify parasite proteins that might interact preferentially with FP, we have used a mass spectrometry approach. Proteins that bind to FP immobilized on agarose include P. falciparum glyceraldehyde-3-phosphate dehydrogenase (PfGAPDH), P. falciparum glutathione reductase (PfGR), and P. falciparum protein disulfide isomerase. To examine the potential consequences of FP binding, we have examined the ability of FP to inhibit the activities of GAPDH and GR from P. falciparum and other sources. FP inhibits the enzymic activity of PfGAPDH with a Ki value of 0.2 microm, whereas red blood cell GAPDH is much less sensitive. By contrast, PfGR is more resistant to FP inhibition (Ki > 25 microm) than its human counterpart. We also examined the ability of FP to inhibit the activities of the additional antioxidant enzymes, P. falciparum thioredoxin reductase, which exhibits a Ki value of 1 microm, and P. falciparum glutaredoxin, which shows more moderate sensitivity to FP. The exquisite sensitivity of PfGAPDH to FP may indicate that the glycolytic pathway of the parasite is particularly susceptible to modulation by FP stress. Inhibition of this pathway may drive flux through the pentose phosphate pathway ensuring sufficient production of reducing equivalents to counteract the oxidative stress induced by FP build-up.     

Identification and initial characterization of three novel cyclin-related proteins of the human malaria parasite Plasmodium falciparum

The molecular mechanisms regulating cell proliferation and development during the life cycle of malaria parasites remain to be elucidated. The peculiarities of the cell cycle organization during Plasmodium falciparum schizogony suggest that the modalities of cell cycle control in this organism may differ from those in other eukaryotes. Indeed, existing data concerning Plasmodium cell cycle regulators such as cyclin-dependent kinases reveal structural and functional properties that are divergent from those of their homologues in other systems. The work presented here lies in the context of the exploitation of the recently available P. falciparum genome sequence toward the characterization of putative cell cycle regulators. We describe the in silico identification of three open reading frames encoding proteins with maximal homology to various members of the cyclin family and demonstrate that the corresponding polypeptides are expressed in the erythrocytic stages of the infection. We present evidence that these proteins possess cyclin activity by demonstrating either their association with histone H1 kinase activity in parasite extracts or their ability to activate PfPK5, a P. falciparum cyclin-dependent kinase homologue, in vitro. Furthermore, we show that RINGO, a protein with no sequence homology to cyclins but that is nevertheless a strong activator of mammalian CDK1/2, is also a strong activator of PfPK5 in vitro. This raises the possibility that "cryptic" cell cycle regulators may be found among the 50% of the open reading frames in the P. falciparum genome that display no homology to any known proteins.     

Genetic analysis of polymorphic proteins of the human malaria parasite Plasmodium falciparum

Five polymorphic proteins, detected by two-dimensional electrophoresis, were analysed in the parents and progeny of a cross between two clones of the malaria parasite Plasmodium falciparum. The information obtained showed that different forms of each protein were determined by allelic variants of each respective gene. One protein was identified as the parasite enzyme adenosine deaminase. Recombinant parasites were produced at a higher than expected frequency.     

Proteomics of the human malaria parasite Plasmodium falciparum

The lethal species of malaria parasite, Plasmodium falciparum, continues to exact a huge toll of mortality and morbidity, particularly in sub-Saharan Africa. Completion of the genome sequence of this organism and advances in proteomics and mass spectrometry have opened up unprecedented opportunities for understanding the complex biology of this parasite and how it responds to drug challenge and other interventions. This review describes recent progress that has been made in applying proteomics technology to this important pathogen and provides a look forward to likely future developments.     

Mitosis in the human malaria parasite Plasmodium falciparum

Malaria is caused by intraerythrocytic protozoan parasites belonging to Plasmodium spp. (phylum Apicomplexa) that produce significant morbidity and mortality, mostly in developing countries. Plasmodium parasites have a complex life cycle that includes multiple stages in anopheline mosquito vectors and vertebrate hosts. During the life cycle, the parasites undergo several cycles of extreme population growth within a brief span, and this is critical for their continued transmission and a contributing factor for their pathogenesis in the host. As with other eukaryotes, successful mitosis is an essential requirement for Plasmodium reproduction; however, some aspects of Plasmodium mitosis are quite distinct and not fully understood. In this review, we will discuss the current understanding of the architecture and key events of mitosis in Plasmodium falciparum and related parasites and compare them with the traditional mitotic events described for other eukaryotes.     

The membrane potential of the intraerythrocytic malaria parasite Plasmodium falciparum

The membrane potential (Deltapsi) of the mature asexual form of the human malaria parasite, Plasmodium falciparum, isolated from its host erythrocyte using a saponin permeabilization technique, was investigated using both the radiolabeled Deltapsi indicator tetraphenylphosphonium ([(3)H]TPP(+)) and the fluorescent Deltapsi indicator DiBAC(4)(3) (bis-oxonol). For isolated parasites suspended in a high Na(+), low K(+) solution, Deltapsi was estimated from the measured distribution of [(3)H]TPP(+) to be -95 +/- 2 mV. Deltapsi was reduced by the specific V-type H(+) pump inhibitor bafilomycin A(1), by the H(+) ionophore CCCP, and by glucose deprivation. Acidification of the parasite cytosol (induced by the addition of lactate) resulted in a transient hyperpolarization, whereas a cytosolic alkalinization (induced by the addition of NH(4)(+)) resulted in a transient depolarization. A decrease in the extracellular pH resulted in a membrane depolarization, whereas an increase in the extracellular pH resulted in a membrane hyperpolarization. The parasite plasma membrane depolarized in response to an increase in the extracellular K(+) concentration and hyperpolarized in response to a decrease in the extracellular K(+) concentration and to the addition of the K(+) channel blockers Ba(2+) or Cs(+) to the suspending medium. The data are consistent with Deltapsi of the intraerythrocytic P. falciparum trophozoite being due to the electrogenic extrusion of H(+) via the V-type H(+) pump at the parasite surface. The current associated with the efflux of H(+) is countered, in part, by the influx of K(+) via Ba(2+)- and Cs(+)-sensitive K(+) channels in the parasite plasma membrane.     

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.     

Centrins, cell cycle regulation proteins in human malaria parasite Plasmodium falciparum

Molecules and cellular mechanisms that regulate the process of cell division in malaria parasites remain poorly understood. In this study we isolate and characterize the four Plasmodium falciparum centrins (PfCENs) and, by growth complementation studies, provide evidence for their involvement in cell division. Centrins are cytoskeleton proteins with key roles in cell division, including centrosome duplication, and possess four Ca(2+)-binding EF hand domains. By means of phylogenetic analysis, we were able to decipher the evolutionary history of centrins in eukaryotes with particular emphasis on the situation in apicomplexans and other alveolates. Plasmodium possesses orthologs of four distinct centrin paralogs traceable to the ancestral alveolate, including two that are unique to alveolates. By real time PCR and/or immunofluorescence, we determined the expression of PfCEN mRNA or protein in sporozoites, asexual blood forms, gametocytes, and in the oocysts developing inside mosquito mid-gut. Immunoelectron microscopy studies showed that centrin is expressed in close proximity with the nucleus of sporozoites and asexual schizonts. Furthermore, confocal and widefield microscopy using the double staining with alpha-tubulin and centrin antibodies strongly suggested that centrin is associated with the parasite centrosome. Following the episomal expression of the four PfCENs in a centrin knock-out Leishmania donovani parasite line that exhibited a severe growth defect, one of the PfCENs was able to partially restore Leishmania growth rate and overcome the defect in cytokinesis in such mutant cell line. To our knowledge, this study is the first characterization of a Plasmodium molecule that is involved in the process of cell division. These results provide the opportunity to further explore the role of centrins in cell division in malaria parasites and suggest novel targets to construct genetically modified, live attenuated malaria vaccines.     

Genetic mapping in the human malaria parasite Plasmodium falciparum

The Plasmodium falciparum genome sequence has boosted hopes for a new era of malaria research and for the application of comprehensive molecular knowledge to disease control, but formidable obstacles remain: approximately 60% of the predicted P. falciparum proteins have no known functions or homologues, and most life cycle stages of this haploid eukaryotic parasite are relatively intractable to cultivation and biochemical manipulation. Genetic mapping based on high-resolution maps saturated with single-nucleotide polymorphisms or microsatellites is now providing effective strategies for discovering candidate genes determining important parasite phenotypes. Here we review classical linkage studies using laboratory crosses and population associations that are now amenable to genome-wide approaches and are revealing multiple candidate genes involved in complex drug responses. Moreover, mapping by linkage disequilibrium is practicable in cases where chromosomal segments flanking drug-selected genes have been preserved in populations during relatively recent P. falciparum evolution. We discuss the advantages and limitations of these various genetic mapping strategies, results from which offer complementary insights to those emerging from gene knockout experiments and/or high-throughput genomic technologies.     

The Clp chaperones and proteases of the human malaria parasite Plasmodium falciparum

The Clp chaperones and proteases play an important role in protein homeostasis in the cell. They are highly conserved across prokaryotes and found also in the mitochondria of eukaryotes and the chloroplasts of plants. They function mainly in the disaggregation, unfolding and degradation of native as well as misfolded proteins. Here, we provide a comprehensive analysis of the Clp chaperones and proteases in the human malaria parasite Plasmodium falciparum. The parasite contains four Clp ATPases, which we term PfClpB1, PfClpB2, PfClpC and PfClpM. One PfClpP, the proteolytic subunit, and one PfClpR, which is an inactive version of the protease, were also identified. Expression of all Clp chaperones and proteases was confirmed in blood-stage parasites. The proteins were localized to the apicoplast, a non-photosynthetic organelle that accommodates several important metabolic pathways in P. falciparum, with the exception of PfClpB2 (also known as Hsp101), which was found in the parasitophorous vacuole. Both PfClpP and PfClpR form mostly homoheptameric rings as observed by size-exclusion chromatography, analytical ultracentrifugation and electron microscopy. The X-ray structure of PfClpP showed the protein as a compacted tetradecamer similar to that observed for Streptococcus pneumoniae and Mycobacterium tuberculosis ClpPs. Our data suggest the presence of a ClpCRP complex in the apicoplast of P. falciparum.     

Within-host competition and drug resistance in the human malaria parasite Plasmodium falciparum

Infections with the malaria parasite Plasmodium falciparum typically comprise multiple strains, especially in high-transmission areas where infectious mosquito bites occur frequently. However, little is known about the dynamics of mixed-strain infections, particularly whether strains sharing a host compete or grow independently. Competition between drug-sensitive and drug-resistant strains, if it occurs, could be a crucial determinant of the spread of resistance. We analysed 1341 P. falciparum infections in children from Angola, Ghana and Tanzania and found compelling evidence for competition in mixed-strain infections: overall parasite density did not increase with additional strains, and densities of individual chloroquine-sensitive (CQS) and chloroquine-resistant (CQR) strains were reduced in the presence of competitors. We also found that CQR strains exhibited low densities compared with CQS strains (in the absence of chloroquine), which may underlie observed declines of chloroquine resistance in many countries following retirement of chloroquine as a first-line therapy. Our observations support a key role for within-host competition in the evolution of drug-resistant malaria. Malaria control and resistance-management efforts in high-transmission regions may be significantly aided or hindered by the effects of competition in mixed-strain infections. Consideration of within-host dynamics may spur development of novel strategies to minimize resistance while maximizing the benefits of control measures.     

Circumsporozoite protein gene from Plasmodium reichenowi, a chimpanzee malaria parasite evolutionarily related to the human malaria parasite Plasmodium falciparum

We have cloned and sequenced the gene encoding the circumsporozoite (CS) protein of Plasmodium reichenowi a Plasmodium falciparum-like malaria parasite of chimpanzees. Comparison of the two CS proteins reveals both similarities and differences in these two evolutionarily related parasites that have adapted to different hosts. The P. reichenowi CS protein has a new repeat sequence, NVNP, in addition to the P. falciparum-like NANP and NVDP repeats. In the immunodominant TH2R and TH3R regions of the CS protein, the amino acid sequences are similar in both parasite proteins. The differences in the two proteins exist in domains around the conserved regions, Region I and Region II, which are otherwise conserved in the CS proteins of P. falciparum analyzed to date. Studies of parasite protein genes of evolutionarily related malaria parasites, together with other immunologic and biologic characteristics, will help better understand the evolution and host parasite relationship of malaria parasites and may provide a tool for identifying protein determinants for malaria vaccine development.     

Characterization of adenine phosphoribosyltransferase from the human malaria parasite, Plasmodium falciparum

Because of their inability to synthesize purines de novo, malaria parasites rely on purine phosphoribosyltransferases (PRTases) to convert purine bases salvaged from the host cell (the erythrocyte) into the corresponding purine nucleoside monophosphates. Our studies with late trophozoites of the human malaria parasite, Plasmodium falciparum, showed that virtually all of the purine PRTase activity is accounted for by two distinct enzymes. One enzyme utilizes hypoxanthine, guanine and xanthine (Queen, S.A., Vander Jagt, D. and Reyes, P. (1988) Mol. Biochem. Parasitol. 30, 123-134). The second enzyme utilizes only adenine and is the subject of this paper. This latter enzyme exhibits a biphasic pH-activity profile and is moderately to weakly inhibited by several divalent metal ions. Several of the properties of the P. falciparum enzyme were found to differ significantly from those of human erythrocyte adenine PRTase. (1) The molecular weight (18,000) of the parasite enzyme is smaller than that of the host cell enzyme. (2) The parasite enzyme, unlike the erythrocyte enzyme, is not significantly inhibited by sulfhydryl reagents. (3) 6-Mercaptopurine and 2,6-diaminopurine proved to be competitive inhibitors of the parasite enzyme (Ki 0.70 and 1.0 mM, respectively); on the other hand, the human enzyme is not inhibited by these agents. (4) The Km for adenine (0.80 microM) and 5-phosphoribosyl-1-pyrophosphate (0.70 microM) displayed by the parasite enzyme are significantly smaller than the corresponding Km values shown by the erythrocyte enzyme. These distinctions between the parasite and host enzymes point to the possibility that adenine PRTase of P. falciparum may represent a potential target for chemotherapeutic attack.     

Identification of proteases that regulate erythrocyte rupture by the malaria parasite Plasmodium falciparum

Newly replicated Plasmodium falciparum parasites escape from host erythrocytes through a tightly regulated process that is mediated by multiple classes of proteolytic enzymes. However, the identification of specific proteases has been challenging. We describe here a forward chemical genetic screen using a highly focused library of more than 1,200 covalent serine and cysteine protease inhibitors to identify compounds that block host cell rupture by P. falciparum. Using hits from the library screen, we identified the subtilisin-family serine protease PfSU B1 and the cysteine protease dipeptidyl peptidase 3 (DPAP3) as primary regulators of this process. Inhibition of both DPAP3 and PfSUB1 caused a block in proteolytic processing of the serine repeat antigen (SERA) protein SERA5 that correlated with the observed block in rupture. Furthermore, DPAP3 inhibition reduced the levels of mature PfSUB1. These results suggest that two mechanistically distinct proteases function to regulate processing of downstream substrates required for efficient release of parasites from host red blood cells.     

Live-cell fluorescence imaging of microgametogenesis in the human malaria parasite Plasmodium falciparum

Formation of gametes in the malaria parasite occurs in the midgut of the mosquito and is critical to onward parasite transmission. Transformation of the male gametocyte into microgametes, called microgametogenesis, is an explosive cellular event and one of the fastest eukaryotic DNA replication events known. The transformation of one microgametocyte into eight flagellated microgametes requires reorganisation of the parasite cytoskeleton, replication of the 22.9 Mb genome, axoneme formation and host erythrocyte egress, all of which occur simultaneously in <20 minutes. Whilst high-resolution imaging has been a powerful tool for defining stages of microgametogenesis, it has largely been limited to fixed parasite samples, given the speed of the process and parasite photosensitivity. Here, we have developed a live-cell fluorescence imaging workflow that captures the entirety of microgametogenesis. Using the most virulent human malaria parasite, Plasmodium falciparum, our live-cell approach captured early microgametogenesis with three-dimensional imaging through time (4D imaging) and microgamete release with two-dimensional (2D) fluorescence microscopy. To minimise the phototoxic impact to parasites, acquisition was alternated between 4D fluorescence, brightfield and 2D fluorescence microscopy. Combining live-cell dyes specific for DNA, tubulin and the host erythrocyte membrane, 4D and 2D imaging together enables definition of the positioning of newly replicated and segregated DNA. This combined approach also shows the microtubular cytoskeleton, location of newly formed basal bodies, elongation of axonemes and morphological changes to the erythrocyte membrane, the latter including potential echinocytosis of the erythrocyte membrane prior to microgamete egress. Extending the utility of this approach, the phenotypic effects of known transmission-blocking inhibitors on microgametogenesis were confirmed. Additionally, the effects of bortezomib, an untested proteasomal inhibitor, revealed a clear block of DNA replication, full axoneme nucleation and elongation. Thus, as well as defining a framework for broadly investigating microgametogenesis, these data demonstrate the utility of using live imaging to validate potential targets for transmission-blocking antimalarial drug development.     

Cytometric quantification of singlet oxygen in the human malaria parasite Plasmodium falciparum

The malaria parasite Plasmodium falciparum proliferates within human erythrocytes and is thereby exposed to a variety of reactive oxygen species (ROS) such as hydrogen peroxide, hydroxyl radical, superoxide anion, and highly reactive singlet oxygen ((1)O(2)). While most ROS are already well studied in the malaria parasite, singlet oxygen has been neglected to date. In this study we visualized the generation of (1)O(2) by live cell fluorescence microscopy using 3-(p-aminophenyl) fluorescein as an indicator dye. While (1) O(2) is found restrictively in the parasite, its amount varies during erythrocytic schizogony. Since the photosensitizer cercosporin generates defined amounts of (1)O(2) we have established a new cytometric method that allows the stage specific quantification of (1)O(2). Therefore, the parasites were first classified into three main stages according to their respective pixel-area of 200-600 pixels for rings, 700-1,200 pixels for trophozoites and 1,400-2,500 pixels for schizonts. Interestingly the highest mean concentration of endogenous (1)O(2) of 0.34 nM is found in the trophozoites stage, followed by 0.20 nM (ring stage) and 0.10 nM (schizont stage) suggesting that (1)O(2) derives predominantly from the digestion of hemoglobin.