Joanne M. Morrisey

Specific role of mitochondrial electron transport in blood-stage Plasmodium falciparum

The origin of all mitochondria can be traced to the symbiotic arrangement that resulted in the emergence of eukaryotes in a world that was exclusively populated by prokaryotes. This arrangement, however, has been in continuous genetic flux: the varying degrees of gene loss and transfer from the mitochondrial genome in different eukaryotic lineages seem to signify an ongoing ‘conflict’ between the host and the symbiont. Eukaryotic parasites belonging to the phylum Apicomplexa provide an excellent example to support this view. These organisms contain the smallest mitochondrial genomes known, with an organization that differs among various genera; one genus, Cryptosporidium, seems to have lost the entire mitochondrial genome. Here we show that erythrocytic stages of the human malaria parasite Plasmodium falciparum seem to maintain an active mitochondrial electron transport chain to serve just one metabolic function: regeneration of ubiquinone required as the electron acceptor for dihydroorotate dehydrogenase, an essential enzyme for pyrimidine biosynthesis. Transgenic P. falciparum parasites expressing Saccharomyces cerevisiae dihydroorotate dehydrogenase, which does not require ubiquinone as an electron acceptor, were completely resistant to inhibitors of mitochondrial electron transport. Maintenance of mitochondrial membrane potential, however, was essential in these parasites, as indicated by their hypersensitivity to proguanil, a drug that collapsed the membrane potential in the presence of electron transport inhibitors. Thus, acquisition of just one enzyme can render mitochondrial electron transport nonessential in erythrocytic stages of P. falciparum.

Resistance mutations reveal the atovaquone‐binding domain of cytochrome b in malaria parasites

Atovaquone represents a class of antimicrobial agents with a broad‐spectrum activity against various parasitic infections, including malaria, toxoplasmosis and Pneumocystis pneumonia. In malaria parasites, atovaquone inhibits mitochondrial electron transport at the level of the cytochrome bc1 complex and collapses mitochondrial membrane potential. In addition, this drug is unique in being selectively toxic to parasite mitochondria without affecting the host mitochondrial functions. A better understanding of the structural basis for the selective toxicity of atovaquone could help in designing drugs against infections caused by mitochondria‐containing parasites. To that end, we derived nine independent atovaquone‐resistant malaria parasite lines by suboptimal treatment of mice infected with Plasmodium yoelii; these mutants exhibited resistance to atovaquone‐mediated collapse of mitochondrial membrane potential as well as inhibition of electron transport. The mutants were also resistant to the synergistic effects of atovaquone/ proguanil combination. Sequencing of the mitochondrially encoded cytochrome b gene placed these mutants into four categories, three with single amino acid changes and one with two adjacent amino acid changes. Of the 12 nucleotide changes seen in the nine independently derived mutants 11 replaced A:T basepairs with G:C basepairs, possibly because of reactive oxygen species resulting from atovaquone treatment. Visualization of the resistance‐conferring amino acid positions on the recently solved crystal structure of the vertebrate cytochrome bc1 complex revealed a discrete cavity in which subtle variations in hydrophobicity and volume of the amino acid side‐chains may determine atovaquone‐binding affinity, and thereby selective toxicity. These structural insights may prove useful in designing agents that selectively affect cytochrome bc1 functions in a wide range of eukaryotic pathogens.

Quinolone-3-Diarylethers: A New Class of Antimalarial Drug

ELQ-300, an investigational drug for treating and preventing malaria, shows potent transmission-blocking activity in rodent models of malaria. Taking the Bite Out of Malaria Malaria is spread from person to person by mosquitoes that inject 8 to 10 sporozoite forms of the parasite in a single bite. The sporozoites reproduce in the liver to produce 10,000 to 30,000 merozoites before the liver schizont ruptures and parasites flood into the bloodstream where the absolute parasite burden may increase to a thousand billion (1012) circulating parasites. Some of these parasites develop into gametocytes that may be ingested by another mosquito where they progress through ookinete, oocyst, and sporozoite stages to complete the cycle. Like quinine, most antimalarial drugs in use today target only the symptomatic blood stage. The efficacy of these drugs has been compromised by resistance, and so there is a pressing need for new drugs that target multiple stages of the parasite life cycle for use in malaria treatment and prevention. Clearly, it is advantageous to strike at the liver stage where parasite numbers are low, to diminish the likelihood of selecting for a resistant mutant and before the infection has a chance to weaken the defenses of the human host. In a new study, Nilsen and colleagues describe ELQ-300, a 4(1H)-quinolone-3-diarylether, which targets the liver and blood stages, including the forms that are crucial to disease transmission (gametocytes, zygotes, and ookinetes). In mouse models of malaria, a single oral dose of 0.03 mg/kg prevented sporozoite-induced infections, whereas four daily doses of 1 mg/kg achieved complete cures of patent infections. ELQ-300 is a preclinical candidate that may be coformulated with other antimalarials to prevent and treat malaria, with the potential to aid in eradication of the disease. The goal for developing new antimalarial drugs is to find a molecule that can target multiple stages of the parasite’s life cycle, thus impacting prevention, treatment, and transmission of the disease. The 4(1H)-quinolone-3-diarylethers are selective potent inhibitors of the parasite’s mitochondrial cytochrome bc1 complex. These compounds are highly active against the human malaria parasites Plasmodium falciparum and Plasmodium vivax. They target both the liver and blood stages of the parasite as well as the forms that are crucial for disease transmission, that is, the gametocytes, the zygote, the ookinete, and the oocyst. Selected as a preclinical candidate, ELQ-300 has good oral bioavailability at efficacious doses in mice, is metabolically stable, and is highly active in blocking transmission in rodent models of malaria. Given its predicted low dose in patients and its predicted long half-life, ELQ-300 has potential as a new drug for the treatment, prevention, and, ultimately, eradication of human malaria.

Host-parasite Interactions Revealed by Plasmodium falciparum Metabolomics

Intracellular pathogens have devised mechanisms to exploit their host cells to ensure their survival and replication. The malaria parasite Plasmodium falciparum relies on an exchange of metabolites with the host for proliferation. Here we describe a mass spectrometry-based metabolomic analysis of the parasite throughout its 48 hr intraerythrocytic developmental cycle. Our results reveal a general modulation of metabolite levels by the parasite, with numerous metabolites varying in phase with the developmental cycle. Others differed from uninfected cells irrespective of the developmental stage. Among these was extracellular arginine, which was specifically converted to ornithine by the parasite. To identify the biochemical basis for this effect, we disrupted the plasmodium arginase gene in the rodent malaria model P. berghei. These parasites were viable but did not convert arginine to ornithine. Our results suggest that systemic arginine depletion by the parasite may be a factor in human malarial hypoargininemia associated with cerebral malaria pathogenesis.

Branched tricarboxylic acid metabolism in Plasmodium falciparum

A central hub of carbon metabolism is the tricarboxylic acid cycle, which serves to connect the processes of glycolysis, gluconeogenesis, respiration, amino acid synthesis and other biosynthetic pathways. The protozoan intracellular malaria parasites (Plasmodium spp.), however, have long been suspected of possessing a significantly streamlined carbon metabolic network in which tricarboxylic acid metabolism plays a minor role. Blood-stage Plasmodium parasites rely almost entirely on glucose fermentation for energy and consume minimal amounts of oxygen, yet the parasite genome encodes all of the enzymes necessary for a complete tricarboxylic acid cycle. Here, by tracing 13C-labelled compounds using mass spectrometry we show that tricarboxylic acid metabolism in the human malaria parasite Plasmodium falciparum is largely disconnected from glycolysis and is organized along a fundamentally different architecture from the canonical textbook pathway. We find that this pathway is not cyclic, but rather is a branched structure in which the major carbon sources are the amino acids glutamate and glutamine. As a consequence of this branched architecture, several reactions must run in the reverse of the standard direction, thereby generating two-carbon units in the form of acetyl-coenzyme A. We further show that glutamine-derived acetyl-coenzyme A is used for histone acetylation, whereas glucose-derived acetyl-coenzyme A is used to acetylate amino sugars. Thus, the parasite has evolved two independent production mechanisms for acetyl-coenzyme A with different biological functions. These results significantly clarify our understanding of the Plasmodium metabolic network and highlight the ability of altered variants of central carbon metabolism to arise in response to unique environments.

Structural features of Plasmodium cytochrome b that may underlie susceptibility to 8-aminoquinolines and hydroxynaphthoquinones

Appropriate functioning of mitochondria is critical for survival and growth of erythrocytic stages of malarial parasites, making it an attractive target for antimalarial drugs which may take advantage of unique features of parasite mitochondrial metabolism. We have sequenced the presumptive mitochondrial DNA, the 6-kb element, of Plasmodium falciparum, permitting an analysis of the predicted structure of parasite electron transport proteins. Although the overall structures of the 3 polypeptides, cytochrome c oxidase subunit 1, cytochrome c oxidase subunit 3, and cytochrome b (cyt b), were similar to those from other species, some striking differences were observed, especially for the cyt b. Analysis of the cyt b structure showed that the critical quinone binding sites of the protein are quite divergent from those of other species. Comparative analysis suggests that these changes are the likely cause for the resistance of parasite cytochrome bc1 complex to antimycin and related inhibitors. We suggest that the same features are responsible for increased affinity of the parasite cyt b for antimalarial compounds of class 8-aminoquinolines and hydroxynaphthoquinones, explaining the therapeutic value of these drugs.

The Heme Biosynthesis Pathway is Essential for Plasmodium falciparum Development in Mosquito Stage but not in Blood Stages

Background: Malaria parasites require heme for growth. Results: Genetic disruption of the P. falciparum heme biosynthesis pathway ablated growth in mosquitoes but had no effect on blood-stage growth. Conclusion: The heme biosynthesis pathway is only essential for exoerythrocytic parasite growth and transmission to mosquitoes. Significance: Pathway inhibition is unlikely to be an effective antimalarial drug strategy. Heme salvage mechanisms likely exist in blood stages. Heme is an essential cofactor for aerobic organisms. Its redox chemistry is central to a variety of biological functions mediated by hemoproteins. In blood stages, malaria parasites consume most of the hemoglobin inside the infected erythrocytes, forming nontoxic hemozoin crystals from large quantities of heme released during digestion. At the same time, the parasites possess a heme de novo biosynthetic pathway. This pathway in the human malaria parasite Plasmodium falciparum has been considered essential and is proposed as a potential drug target. However, we successfully disrupted the first and last genes of the pathway, individually and in combination. These knock-out parasite lines, lacking 5-aminolevulinic acid synthase and/or ferrochelatase (FC), grew normally in blood-stage culture and exhibited no changes in sensitivity to heme-related antimalarial drugs. We developed a sensitive LC-MS/MS assay to monitor stable isotope incorporation into heme from its precursor 5-[13C4]aminolevulinic acid, and this assay confirmed that de novo heme synthesis was ablated in FC knock-out parasites. Disrupting the FC gene also caused no defects in gametocyte generation or maturation but resulted in a greater than 70% reduction in male gamete formation and completely prevented oocyst formation in female Anopheles stephensi mosquitoes. Our data demonstrate that the heme biosynthesis pathway is not essential for asexual blood-stage growth of P. falciparum parasites but is required for mosquito transmission. Drug inhibition of pathway activity is therefore unlikely to provide successful antimalarial therapy. These data also suggest the existence of a parasite mechanism for scavenging host heme to meet metabolic needs.

Pyrazoleamide compounds are potent antimalarials that target Na+ homeostasis in intraerythrocytic Plasmodium falciparum

The quest for new antimalarial drugs, especially those with novel modes of action, is essential in the face of emerging drug-resistant parasites. Here we describe a new chemical class of molecules, pyrazoleamides, with potent activity against human malaria parasites and showing remarkably rapid parasite clearance in an in vivo model. Investigations involving pyrazoleamide-resistant parasites, whole-genome sequencing and gene transfers reveal that mutations in two proteins, a calcium-dependent protein kinase (PfCDPK5) and a P-type cation-ATPase (PfATP4), are necessary to impart full resistance to these compounds. A pyrazoleamide compound causes a rapid disruption of Na+ regulation in blood-stage Plasmodium falciparum parasites. Similar effect on Na+ homeostasis was recently reported for spiroindolones, which are antimalarials of a chemical class quite distinct from pyrazoleamides. Our results reveal that disruption of Na+ homeostasis in malaria parasites is a promising mode of antimalarial action mediated by at least two distinct chemical classes.

Yeast Dihydroorotate Dehydrogenase as a New Selectable Marker for Plasmodium falciparum Transfection

Genetic manipulation of Plasmodium falciparum in culture through transfection has provided numerous insights into the molecular and cell biology of this parasite. The procedure is rather cumbersome, and is limited by the number of drug-resistant markers that can be used for selecting transfected parasites. Here we report a new selectable marker that could allow multiple transfections. We have taken advantage of our finding that a critical function of the mitochondrial electron transport chain (mtETC) in the erythrocytic stages of P. falciparum is the regeneration of ubiquinone as co-substrate of dihydroorotate dehydrogenase (DHODH), and that transgenic P. falciparum expressing ubiquinone-independent DHODH from yeast (yDHODH) are resistant to all mtETC inhibitors. We assessed the possibility of using yDHODH as a positive selectable marker for transfections of P. falciparum, including its use in gene disruption strategies. We constructed a transfection vector designed for gene disruption, termed pUF-1, containing the yDHODH gene as the positive selection marker in combination with a previously described fused yeast cytosine deaminase-uracil phosphoribosyl transferase gene as a negative selection marker. Transfection of the D10 strain followed by selection with atovaquone yielded positively selected parasites containing the plasmid, demonstrating that yDHODH can be used as a selective marker. Atovaquone, however, could not be used for such selection with the Dd2 strain of P. falciparum. On the other hand, we demonstrated that yDHODH transgenic parasites could be selected in both strains by Plasmodium DHODH-specific triazolopyrimidine-based inhibitors. Thus, selection with DHODH inhibitors was superior in that it successfully selected transgenic Dd2 parasites, as well as yielded transgenic parasites after a shorter period of selection. As a proof of concept, we have successfully disrupted the type II vacuolar proton-pumping pyrophosphatase gene (PfVP2) in P. falciparum by double crossover recombination, showing that this gene is not essential for the survival of blood stage parasites.

ATP Synthase Complex of Plasmodium falciparum DIMERIC ASSEMBLY IN MITOCHONDRIAL MEMBRANES AND RESISTANCE TO GENETIC DISRUPTION

Background: The role of ATP synthase in blood stages of malaria parasites has been unclear. Results: Canonical subunits were targeted to the mitochondrion, could not be deleted by gene disruption, and were present in large complexes. Conclusion: Plasmodium ATP synthase is likely essential and forms a dimeric complex. Significance Composition, properties, structure, and drugability of the complex should be fully investigated. The rotary nanomotor ATP synthase is a central player in the bioenergetics of most organisms. Yet the role of ATP synthase in malaria parasites has remained unclear, as blood stages of Plasmodium falciparum appear to derive ATP largely through glycolysis. Also, genes for essential subunits of the FO sector of the complex could not be detected in the parasite genomes. Here, we have used molecular genetic and immunological tools to investigate the localization, complex formation, and functional significance of predicted ATP synthase subunits in P. falciparum. We generated transgenic P. falciparum lines expressing seven epitope-tagged canonical ATP synthase subunits, revealing localization of all but one of the subunits to the mitochondrion. Blue native gel electrophoresis of P. falciparum mitochondrial membranes suggested the molecular mass of the ATP synthase complex to be greater than 1 million daltons. This size is consistent with the complex being assembled as a dimer in a manner similar to the complexes observed in other eukaryotic organisms. This observation also suggests the presence of previously unknown subunits in addition to the canonical subunits in P. falciparum ATP synthase complex. Our attempts to disrupt genes encoding β and γ subunits were unsuccessful, suggesting an essential role played by the ATP synthase complex in blood stages of P. falciparum. These studies suggest that, despite some unconventional features and its minimal contribution to ATP synthesis, P. falciparum ATP synthase is localized to the parasite mitochondrion, assembled as a large dimeric complex, and is likely essential for parasite survival.