Artemisinin and a series of novel endoperoxide antimalarials exert early effects on digestive vacuole morphology

The use and abuse of heme in apicomplexan parasites

SIGNIFICANCE

Heme is an essential prosthetic group for most life on Earth. It functions in numerous cellular redox reactions, including in antioxidant defenses and at several stages of the electron transport chain in prokaryotes and eukaryotic mitochondria. Heme also functions as a sensor and transport molecule for gases such as oxygen. Heme is a complex organic molecule and can only be synthesized through a multienzyme pathway from simpler precursors. Most free-living organisms synthesize their own heme by a broadly conserved metabolic pathway. Parasites are adept at scavenging molecules from their hosts, and heme is no exception.

RECENT ADVANCES

In this review we examine recent advances in understanding heme usage and acquisition in Apicomplexa, a group of parasites that include the causative agents of malaria, toxoplasmosis, and several major parasites of livestock.

CRITICAL ISSUES

Heme is critical to the survival of Apicomplexa, although the functions of heme in these organisms remain poorly understood. Some Apicomplexa likely scavenge heme from their host organisms, while others retain the ability to synthesize heme. Surprisingly, some Apicomplexa may be able to both synthesize and scavenge heme. Several Apicomplexa live in intracellular environments that contain high levels of heme. Since heme is toxic at high concentrations, parasites must carefully regulate intracellular heme levels and develop mechanisms to detoxify excess heme. Indeed, drugs interfering with heme detoxification serve as major antimalarials.

FUTURE DIRECTIONS

Understanding heme requirements and regulation in apicomplexan parasites promises to reveal multiple targets for much-needed therapeutic intervention against these parasites.

Mechanisms of in vitro resistance to dihydroartemisinin in Plasmodium falciparum

The recent reports of artemisinin (ART) resistance in the Thai-Cambodian border area raise a serious concern on the long-term efficacy of ARTs. To elucidate the resistance mechanisms, we performed in vitro selection with dihydroartemisinin (DHA) and obtained two parasite clones from Dd2 with more than 25-fold decrease in susceptibility to DHA. The DHA-resistant clones were more tolerant of stressful growth conditions and more resistant to several commonly used antimalarial drugs than Dd2. The result is worrisome as many of the drugs are currently used as ART partners in malaria control. This study showed that the DHA resistance is not limited to ring stage, but also occurred in trophozoites and schizonts. Microarray and biochemical analyses revealed pfmdr1 amplification, elevation of the antioxidant defence network, and increased expression of many chaperones in the DHA-resistant parasites. Without drug pressure, the DHA-resistant parasites reverted to sensitivity in approximately 8 weeks, accompanied by de-amplification of pfmdr1 and reduced antioxidant activities. The parallel decrease and increase in pfmdr1 copy number and antioxidant activity and the up and down of DHA sensitivity strongly suggest that pfmdr1 and antioxidant defence play a role in in vitro resistance to DHA, providing potential molecular markers for ART resistance.

In vitro sensitivity of Plasmodium falciparum from China-Myanmar border area to major ACT drugs and polymorphisms in potential target genes

Drug resistance has always been one of the most important impediments to global malaria control. Artemisinin resistance has recently been confirmed in the Greater Mekong Subregion (GMS) and efforts for surveillance and containment are intensified. To determine potential mechanisms of artemisinin resistance and monitor the emergence and spread of resistance in other regions of the GMS, we investigated the in vitro sensitivity of 51 culture-adapted parasite isolates from the China-Myanmar border area to four drugs. The 50% inhibitory concentrations (IC₅₀s) of dihydroartemisinin, mefloquine and lumefantrine were clustered in a relatively narrow, 3- to 6-fold range, whereas the IC₅₀ range of artesunate was 12-fold. We assessed the polymorphisms of candidate resistance genes pfcrt, pfmdr1, pfATP6, pfmdr6 and pfMT (a putative metabolite/drug transporter). The K76T mutation in pfcrt reached fixation in the study parasite population, whereas point mutations in pfmdr1 and pfATP6 had low levels of prevalence. In addition, pfmdr1 gene amplification was not detected. None of the mutations in pfmdr1 and pfATP6 was associated significantly with in vitro sensitivity to artemisinin derivatives. The ABC transporter gene pfmdr6 harbored two point mutations, two indels, and number variations in three simple repeats. Only the length variation in a microsatellite repeat appeared associated with altered sensitivity to dihydroartemisinin. The PfMT gene had two point mutations and one codon deletion; the I30N and N496- both reached high levels of prevalence. However, none of the SNPs or haplotypes in PfMT were correlated significantly with resistance to the four tested drugs. Compared with other parasite populations from the GMS, our studies revealed drastically different genotype and drug sensitivity profiles in parasites from the China-Myanmar border area, where artemisinins have been deployed extensively for over 30 years.

Origin, composition, organization and function of the inner membrane complex of Plasmodium falciparum gametocytes

The most virulent of the human malaria parasites, Plasmodium falciparum, undergoes a remarkable morphological transformation as it prepares itself for sexual reproduction and transmission via mosquitoes. Indeed P. falciparum is named for the unique falciform or crescent shape of the mature sexual stages. Once the metamorphosis is completed the mature gametocyte releases from sequestration sites and enters the circulation making it accessible to feeding mosquitoes. Early ultrastructural studies showed that gametocyte elongation is driven by the assembly of a system of flattened cisternal membrane compartments underneath the parasite plasma membrane and a supporting network of microtubules. Here we describe the molecular composition and origin of the sub-pellicular membrane complex, and show that it is analogous to the inner membrane complex, an organelle with structural and motor functions that is well conserved across the apicomplexa. We identify novel cross-linking elements that may help stabilize the inner membrane complex during gametocyte development. We show that changes in gametocyte morphology are associated with an increase in cellular deformability and postulate that this enables the gametocytes to circulate in the blood stream without being detected and removed by the mechanical filtering mechanisms in the host's spleen.

Antitumor activity of artemisinin and its derivatives: from a well-known antimalarial agent to a potential anticancer drug

Improvement of quality of life and survival of cancer patients will be greatly enhanced by the development of highly effective drugs to selectively kill malignant cells. Artemisinin and its analogs are naturally occurring antimalarials which have shown potent anticancer activity. In primary cancer cultures and cell lines, their antitumor actions were by inhibiting cancer proliferation, metastasis, and angiogenesis. In xenograft models, exposure to artemisinins substantially reduces tumor volume and progression. However, the rationale for the use of artemisinins in anticancer therapy must be addressed by a greater understanding of the underlying mechanisms involved in their cytotoxic effects. The primary targets for artemisinin and the chemical base for its preferential effects on heterologous tumor cells need yet to be elucidated. The aim of this paper is to provide an overview of the recent advances and new development of this class of drugs as potential anticancer agents.

Investigating the antimalarial action of 1,2,4-trioxolanes with fluorescent chemical probes

The 1,2,4-trioxolanes are a new class of synthetic peroxidic antimalarials currently in human clinical trials. The well-known reactivity of the 1,2,4-trioxolane ring toward inorganic ferrous iron and ferrous iron heme is proposed to play a role in the antimalarial action of this class of compounds. We have designed structurally relevant fluorescent chemical probes to study the subcellular localization of 1,2,4-trioxolanes in cultured Plasmodium falciparum parasites. Microscopy experiments revealed that a probe fluorescently labeled on the adamantane ring accumulated specifically in digestive vacuole-associated neutral lipid bodies within the parasite while an isosteric, but nonperoxidic, congener did not. Probes fluorescently labeled on the cyclohexane ring showed no distinct localization pattern. In their subcellular localization and peroxidative effects, 1,2,4-trioxolane probes behave much like artemisinin-based probes studied previously. Our results are consistent with a role for adamantane-derived carbon-centered radicals in the antimalarial action of 1,2,4-trioxolanes, as hypothesized previously on the basis of chemical reactivity studies.

Semisynthetic Artemisinin and Synthetic Peroxide Antimalarials

Since the discovery of the endoperoxide sesquiterpene lactone artemisinin, numerous second-generation semisynthetic artemisinins and synthetic peroxides have been prepared and tested for their antimalarial properties. Using a case-study approach, we describe the discovery of the investigational semisynthetic artemisinins artelinic acid (8) and artemisone (9), and the structurally diverse synthetic peroxides arteflene (10), fenozan B07 (11), arterolane (12), PA1103/SAR116242 (13), and RKA182 (14).

The Plasmodium falciparum Ca(2+)-ATPase PfATP6: insensitive to artemisinin, but a potential drug target

The disease malaria, caused by the parasite Plasmodium falciparum, remains one of the most important causes of morbidity and mortality in sub-Saharan Africa. In the absence of an efficient vaccine, the medical treatment of malaria is dependent on the use of drugs. Since artemisinin is a powerful anti-malarial drug which has been proposed to target a particular Ca2+-ATPase (PfATP6) in the parasite, it has been important to characterize the molecular properties of this enzyme. PfATP6 is a 139 kDa protein composed of 1228 amino acids with a 39% overall identity with rabbit SERCA1a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 1a). PfATP6 conserves all sequences and motifs that are important for the function and/or structure of a SERCA, such as two high-affinity Ca2+-binding sites, a nucleotide-binding site and a phosphorylation site. We have been successful in isolating PfATP6 after heterologous expression in yeast and affinity chromatography in a pure, active and stable detergent-solubilized form. With this preparation, we have characterized and compared with the eukaryotic SERCA1a isoform the substrate (Ca2+ and ATP) -dependency for PfATP6 activity as well as the specific inhibition/interaction of the protein with drugs. Our data fully confirm that PfATP6 is a SERCA, but with a distinct pharmacological profile: compared with SERCA1a, it has a lower affinity for thapsigargin and much higher affinity for cyclopiazonic acid. On the other hand, we were not able to demonstrate any inhibition by artemisinin and were also not able to monitor any binding of the drug to the isolated enzyme. Thus it is unlikely that PfATP6 plays an important role as a target for artemisinin in the parasite P. falciparum.

The pharmacogenetics of antimalaria artemisinin combination therapy

INTRODUCTION

Plasmodium falciparum malaria is one of the world's most lethal infectious diseases, commanding millions of drug administrations per year. The pharmacogenetics of these drugs is poorly known, although its application can be pivotal for the optimized management of this disease.

AREAS COVERED

The main components of artemisinin combination therapy (ACT), the worldwide main antimalarial strategy, are metabolized by the polymorphic CYP3A4 (mefloquine, artemether, lumefantrine), CYP2C8 (amodiaquine), CYP2A6 (artesunate) and CYP1A1/2 (amodiaquine/desethylamodiaquine), with dihydroartemisinin being acted by Phase II UDP-glucuronosyltransferases. The worldwide adoption of ACT is leading to a large number of antimalarial treatments. Simultaneously, the feared development of parasite drug resistance might drive dosing increases. In these scenarios of increased drug exposure, pharmacogenetics can be a key tool supporting evidence-based medicine aiming for the longest possible useful lifespan of this important chemotherapy.

EXPERT OPINION

Translation in this moment is not operationally possible at an individual level, but large population studies are achievable for: i) the development of robust pharmacogenetics markers; and ii) the parallel development of a pharmacogenetic cartography of malaria settings. Advances in the understanding of antimalarial pharmacogenetics are urgent in order to protect the exposed populations, enhance the effectiveness of ACT and, consequently, contributing for the long aimed elimination of the disease.

Effects of highly active novel artemisinin-chloroquinoline hybrid compounds on β-hematin formation, parasite morphology and endocytosis in Plasmodium falciparum

4-Aminoquinolines were hybridized with artemisinin and 1,4-naphthoquinone derivatives via the Ugi-four-component condensation reaction, and their biological activities investigated. The artemisinin-containing compounds 6a-c and its salt 6c-citrate were the most active target compounds in the antiplasmodial assays. However, despite the potent in vitro activities, they also displayed cytotoxicity against a mammalian cell-line, and had lower therapeutic indices than chloroquine. Morphological changes in parasites treated with these artemisinin-containing hybrid compounds were similar to those observed after addition of artemisinin. These hybrid compounds appeared to share mechanism(s) of action with both chloroquine and artemisinin: they exhibited potent β-hematin inhibitory activities; they caused an increase in accumulation of hemoglobin within the parasites that was intermediate between the increase observed with artesunate and chloroquine; and they also appeared to inhibit endocytosis as suggested by the decrease in the number of transport vesicles in the parasites. No cross-resistance with chloroquine was observed for these hybrid compounds, despite the fact that they contained the chloroquinoline moiety. The hybridization strategy therefore appeared to be borrowing the best from both classes of antimalarials.

Tracking Glideosome-associated protein 50 reveals the development and organization of the inner membrane complex of Plasmodium falciparum

The most deadly of the human malaria parasites, Plasmodium falciparum, has different stages specialized for invasion of hepatocytes, erythrocytes, and the mosquito gut wall. In each case, host cell invasion is powered by an actin-myosin motor complex that is linked to an inner membrane complex (IMC) via a membrane anchor called the glideosome-associated protein 50 (PfGAP50). We generated P. falciparum transfectants expressing green fluorescent protein (GFP) chimeras of PfGAP50 (PfGAP50-GFP). Using immunoprecipitation and fluorescence photobleaching, we show that C-terminally tagged PfGAP50-GFP can form a complex with endogenous copies of the linker protein PfGAP45 and the myosin A tail domain-interacting protein (MTIP). Full-length PfGAP50-GFP is located in the endoplasmic reticulum in early-stage parasites and then redistributes to apical caps during the formation of daughter merozoites. In the final stage of schizogony, the PfGAP50-GFP profile extends further around the merozoite surface. Three-dimensional (3D) structured illumination microscopy reveals the early-stage IMC as a doubly punctured flat ellipsoid that separates to form claw-shaped apposed structures. A GFP fusion of PfGAP50 lacking the C-terminal membrane anchor is misdirected to the parasitophorous vacuole. Replacement of the acid phosphatase homology domain of PfGAP50 with GFP appears to allow correct trafficking of the chimera but confers a growth disadvantage.

Heme as trigger and target for trioxane-containing antimalarial drugs

Heme is not only just the binding site responsible for oxygen transport by hemoglobin, but it is also the prosthetic group of many different heme-containing enzymes, such as cytochromes P450, peroxidases, catalase, and several proteins involved in electron transfer. Heme plays a key role in the mechanism of action of many different antimalarial drugs. In degrading the host's hemoglobin, the malaria parasite Plasmodium and several other heme-eating parasites are faced with this redox-active metal complex. Heme is able to induce the toxic reductive cascade of molecular oxygen, which leads to the production of destructive hydroxyl radicals. Plasmodium detoxifies heme by converting it into a redox-inactive iron(III) polymer called hemozoin. Artemisinin, a natural drug containing a biologically important 1,2,4-trioxane structure, is now the first-line treatment for multidrug-resistant malaria. The peroxide moiety in artemisinin reacts in the presence of the flat, achiral iron(II)-heme; the mechanism does not reflect the classical "key and lock" paradigm for drugs. Instead, the reductive activation of the peroxide function generates a short-lived alkoxy radical, which quickly rearranges to a C-centered primary radical. This radical alkylates heme via an intramolecular process to produce covalent heme-drug adducts. The accumulation of non-polymerizable redox-active heme derivatives, a consequence of heme alkylation, is thought to be toxic for the parasite. The alkylation of heme by artemisinin has been demonstrated in malaria-infected mice, indicating that heme is acting as the trigger and target of artemisinin. The alkylation of heme by artemisinin is not limited to this natural compound: the mechanism is invoked for a large number of antimalarial semisynthetic derivatives. Synthetic trioxanes or trioxolanes also alkylate heme, and their alkylation ability correlates well with their antimalarial efficacy. In addition, several reports have demonstrated the cytotoxicity of artemisinin derivatives toward several tumor cell lines. Deoxy analogues were just one-fiftieth as active or less, showing the importance of the peroxide bridge. The involvement of heme in anticancer activity has thus also been proposed. The anticancer mechanism of endoperoxide-containing molecules, however, remains a challenging area, but one that offers promising rewards for research success. Although it is not a conventional biological target, heme is the master piece of the mechanism of action of peroxide-containing antimalarial drugs and could well serve as a target for future anticancer drugs.

Cryo transmission X-ray imaging of the malaria parasite, P. falciparum

Cryo transmission X-ray microscopy in the "water window" of photon energies has recently been introduced as a method that exploits the natural contrast of biological samples. We have used cryo tomographic X-ray imaging of the intra-erythrocytic malaria parasite, Plasmodium falciparum, to undertake a survey of the cellular features of this important human pathogen. We examined whole hydrated cells at different stages of growth and defined some of the structures with different X-ray density, including the parasite nucleus, cytoplasm, digestive vacuole and the hemoglobin degradation product, hemozoin. As the parasite develops from an early cup-shaped morphology to a more rounded shape, puncta of hemozoin are formed; these coalesce in the mature trophozoite into a central compartment. In some trophozoite stage parasites we observed invaginations of the parasite surface and, using a selective permeabilization process, showed that these remain connected to the RBC cytoplasm. Some of these invaginations have large openings consistent with phagocytic structures and we observed independent endocytic vesicles in the parasite cytoplasm which appear to play a role in hemoglobin uptake. In schizont stage parasites staggered mitosis was observed and X-ray-dense lipid-rich structures were evident at their apical ends of the developing daughter cells. Treatment of parasites with the antimalarial drug artemisinin appears to affect parasite development and their ability to produce the hemoglobin breakdown product, hemozoin.

Investigations into the role of the Plasmodium falciparum SERCA (PfATP6) L263E mutation in artemisinin action and resistance

Artemisinin-based combination therapies (ACTs) are highly effective for the treatment of Plasmodium falciparum malaria, yet their sustained efficacy is threatened by the potential spread of parasite resistance. Recent studies have provided evidence that artemisinins can inhibit the function of PfATP6, the P. falciparum ortholog of the ER calcium pump SERCA, when expressed in Xenopus laevis oocytes. Inhibition was significantly reduced in an L263E variant, which introduced the mammalian residue into a putative drug-binding pocket. To test the hypothesis that this single mutation could decrease P. falciparum susceptibility to artemisinins, we implemented an allelic-exchange strategy to replace the wild-type pfatp6 allele by a variant allele encoding L263E. Transfected P. falciparum clones were screened by PCR analysis for disruption of the endogenous locus and introduction of the mutant L263E allele under the transcriptional control of a calmodulin promoter. Expression of the mutant allele was demonstrated by reverse transcriptase (RT) PCR and verified by sequence analysis. Parasite clones expressing wild-type or L263E variant PfATP6 showed no significant difference in 50% inhibitory concentrations (IC(50)s) for artemisinin or its derivatives dihydroartemisinin and artesunate. Nonetheless, hierarchical clustering analysis revealed a trend toward reduced susceptibility that neared significance (artemisinin, P approximately = 0.1; dihydroartemisinin, P = 0.053 and P = 0.085; and artesunate, P = 0.082 and P = 0.162 for the D10 and 7G8 lines, respectively). Notable differences in the distribution of normalized IC(50)s provided evidence of decreased responsiveness to artemisinin and dihydroartemisinin (P = 0.02 for the D10 and 7G8 lines), but not to artesunate in parasites expressing mutant PfATP6.

Dual labeling with a far red probe permits analysis of growth and oxidative stress in P. falciparum-infected erythrocytes

The malaria parasite, Plasmodium falciparum, develops within human erythrocytes, consuming host hemoglobin to support its own growth. Reactive oxygen species (superoxide and hydrogen peroxide) are by-products of hemoglobin digestion and are believed to exert significant oxidative stress on the parasite. We have characterized a cell permeant, far red fluorescent nucleic acid-binding dye, SYTO 61, that can be used to distinguish between uninfected and infected erythrocytes in a flow cytometric format. The spectral properties of SYTO 61 make it suitable for use in combination with the fluorescent reactive oxygen species reporter 5-(and-6)-chloromethyl-2',7'-dichlorodihydro-fluorescein diacetate acetyl ester. We have used this probe combination to measure oxidative stress in different stages of live P. falciparum. Low levels of the oxidized, fluorescent form of the reporter (2',7'-dichlorofluorescein, DCF) are detected in ring stage parasites; the DCF signal increases as the intraerythrocytic parasite matures into the trophozoite stage where active hemoglobin digestion occurs. Treatment of infected erythrocytes with the cysteine protease inhibitor, E-64, which inhibits hemoglobin digestion, decreases the DCF signal. We show that E-64 prevents schizont rupture but also causes delayed lethal effects when ring stage cultures are exposed to the drug. We also examined cultures of parasites in erythrocytes harboring 98% catalase inactivation and found no effect on growth and only a modest increase in DCF oxidation.

The molecular mechanism of action of artemisinin--the debate continues

Despite international efforts to 'roll back malaria' the 2008 World Malaria Report revealed the disease still affects approximately 3 billion people in 109 countries; 45 within the WHO African region. The latest report however does provide some 'cautious optimism'; more than one third of malarious countries have documented greater than 50% reductions in malaria cases in 2008 compared to 2000. The goal of the Member States at the World Health Assembly and 'Roll Back Malaria' (RBM) partnership is to reduce the numbers of malaria cases and deaths recorded in 2000 by 50% or more by the end of 2010. Although malaria is preventable it is most prevalent in poorer countries where prevention is difficult and prophylaxis is generally not an option. The burden of disease has increased by the emergence of multi drug resistant (MDR) parasites which threatens the use of established and cost effective antimalarial agents. After a major change in treatment policies, artemisinins are now the frontline treatment to aid rapid clearance of parasitaemia and quick resolution of symptoms. Since artemisinin and its derivatives are eliminated rapidly, artemisinin combination therapies (ACT's) are now recommended to delay resistance mechanisms. In spite of these precautionary measures reduced susceptibility of parasites to the artemisinin-based component of ACT's has developed at the Thai-Cambodian border, a historical 'hot spot' for MDR parasite evolution and emergence. This development raises serious concerns for the future of the artemsinins and this is not helped by controversy related to the mode of action. Although a number of potential targets have been proposed the actual mechanism of action remains ambiguous. Interestingly, artemisinins have also shown potent and broad anticancer properties in cell lines and animal models and are becoming established as anti-schistosomal agents. In this review we will discuss the recent evidence explaining bioactivation and potential molecular targets in the chemotherapy of malaria and cancer.

Biological actions of artemisinin: insights from medicinal chemistry studies

Artemisinins have become essential antimalarial drugs for increasingly widespread drug-resistant malaria strains. Although tremendous efforts have been devoted to decipher how this class of molecules works, their exact antimalarial mechanism is still an enigma. Several hypotheses have been proposed to explain their actions, including alkylation of heme by carbon-centered free radicals, interference with proteins such as the sarcoplasmic/endoplasmic calcium ATPase (SERCA), as well as damaging of normal mitochondrial functions. Besides artemisinins, other endoperoxides with various backbones have also been synthesized, some of which showed comparable or even higher antimalarial effects. It is noteworthy that among these artemisinin derivatives, some enantiomers displayed similar in vitro malaria killing efficacy. In this article, the proposed mechanisms of action of artemisinins are reviewed in light of medicinal chemistry findings characterized by efficacy-structure studies, with the hope of gaining more insight into how these potent drugs work.

Artemisinin directly targets malarial mitochondria through its specific mitochondrial activation

The biological mode of action of artemisinin, a potent antimalarial, has long been controversial. Previously we established a yeast model addressing its mechanism of action and found mitochondria the key in executing artemisinin's action. Here we present data showing that artemisinin directly acts on mitochondria and it inhibits malaria in a similar way as yeast. Specifically, artemisinin and its homologues exhibit correlated activities against malaria and yeast, with the peroxide bridge playing a key role for their inhibitory action in both organisms. In addition, we showed that artemisinins are distributed to malarial mitochondria and directly impair their functions when isolated mitochondria were tested. In efforts to explore how the action specificity of artemisinin is achieved, we found strikingly rapid and dramatic reactive oxygen species (ROS) production is induced with artemisinin in isolated yeast and malarial but not mammalian mitochondria, and ROS scavengers can ameliorate the effects of artemisinin. Deoxyartemisinin, which lacks an endoperoxide bridge, has no effect on membrane potential or ROS production in malarial mitochondria. OZ209, a distantly related antimalarial endoperoxide, also causes ROS production and depolarization in isolated malarial mitochondria. Finally, interference of mitochondrial electron transport chain (ETC) can alter the sensitivity of the parasite towards artemisinin. Addition of iron chelator desferrioxamine drastically reduces ETC activity as well as mitigates artemisinin-induced ROS production. Taken together, our results indicate that mitochondrion is an important direct target, if not the sole one, in the antimalarial action of artemisinins. We suggest that fundamental differences among mitochondria from different species delineate the action specificity of this class of drugs, and differing from many other drugs, the action specificity of artemisinins originates from their activation mechanism.

Digestive-vacuole genesis and endocytic processes in the early intraerythrocytic stages of Plasmodium falciparum

The digestive vacuole of the malaria parasite Plasmodium falciparum is the site of haemoglobin digestion and haem detoxification, and is the target of chloroquine and other antimalarials. The mechanisms for genesis of the digestive vacuole and transfer of haemoglobin from the host cytoplasm are still debated. Here, we use live-cell imaging and photobleaching to monitor the uptake of the pH-sensitive fluorescent tracer SNARF-1-dextran from the erythrocyte cytoplasm in ring-stage and trophozoite-stage parasites. We compare these results with electron tomography of serial sections of parasites at different stages of growth. We show that uptake of erythrocyte cytoplasm is initiated in mid-ring-stage parasites. The host cytoplasm is internalised via cytostome-derived invaginations and concentrated into several acidified peripheral structures. Haemoglobin digestion and haemozoin formation take place in these vesicles. The ring-stage parasites can adopt a deeply invaginated cup shape but do not take up haemoglobin via macropinocytosis. As the parasite matures, the haemozoin-containing compartments coalesce to form a single acidic digestive vacuole that is fed by haemoglobin-containing vesicles. There is also evidence for haemoglobin degradation in compartments outside the digestive vacuole. The work has implications for the stage specificity of quinoline and endoperoxide antimalarials.

5-Bromo-spiro-[1,2-dioxane-4,4'-tricyclo-[4.3.1.1]undeca-ne]-3'-ol

The title compound, C₁₄H₂₁BrO₃, comprises a seven- (C₇) and three six-membered (1 × O₂C₄ and 2 × C₆) rings, and each adopts a conformation based on a chair. Stability to the mol­ecular structure is afforded by an intra­molecular O—H⋯Br hydrogen bond. In the crystal structure, mol­ecules are arranged into a helical supra­molecular chain along the b axis, linked by C—H⋯O inter­actions, where the O-atom acceptor is one of the dioxane O atoms. The crystal studied was found to be a racemic twin. The major component was present 94% of the time.

Probing the antimalarial mechanism of artemisinin and OZ277 (arterolane) with nonperoxidic isosteres and nitroxyl radicals

Peroxidic antimalarials such as the semisynthetic artemisinins are critically important in the treatment of drug-resistant malaria. Nevertheless, their peroxide bond-dependent mode of action is still not well understood. Using combination experiments with cultured Plasmodium falciparum cells, we investigated the interactions of the nitroxide radical spin trap, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), and four of its analogs with artemisinin and the ozonide drug development candidate OZ277. The antagonism observed for combinations of artemisinin or OZ277 with the TEMPO analogs supports the hypothesis that the formation of carbon-centered radicals is critical for the activity of these two antimalarial peroxides. The TEMPO analogs showed a trend toward greater antagonism with artemisinin than they did with OZ277, an observation that can be explained by the greater tendency of artemisinin-derived carbon-centered radicals to undergo internal self-quenching reactions, resulting in a lower proportion of radicals available for subsequent chemical reactions such as the alkylation of heme and parasite proteins. In a further mechanistic experiment, we tested both artemisinin and OZ277 in combination with their nonperoxidic analogs. The latter had no effect on the antimalarial activities of the former. These data indicate that the antimalarial properties of peroxides do not derive from reversible interactions with parasite targets.

Artemisinin-based combination therapies: a vital tool in efforts to eliminate malaria

Plasmodium falciparum resistance to chloroquine and sulphadoxine-pyrimethamine has led to the recent adoption of artemisinin-based combination therapies (ACTs) as the first line of treatment against malaria. ACTs comprise semisynthetic artemisinin derivatives paired with distinct chemical classes of longer acting drugs. These artemisinins are exceptionally potent against the pathogenic asexual blood stages of Plasmodium parasites and also act on the transmissible sexual stages. These combinations increase the rates of clinical and parasitological cures and decrease the selection pressure for the emergence of antimalarial resistance. This Review article discusses our current knowledge about the mode of action of ACTs, their pharmacological properties and the proposed mechanisms of drug resistance.

The Plasmodium falciparum protein Pfg27 is dispensable for gametocyte and gamete production, but contributes to cell integrity during gametocytogenesis

In the human malaria parasite Plasmodium falciparum, gametocyte maturation is a process remarkably longer than in other malaria species, accompanied by expression of 2-300 sexual stage-specific proteins. Disruption of several of their encoding genes so far showed that only the abundant protein Pfg27, produced at the onset of sexual differentiation, is essential for gametocyte production. In contrast with what has been previously described, here we show that P. falciparum pfg27 disruptant lines are able to undergo all stages of gametocyte maturation, and are able to mature into gametes. A fraction of Pfg27-defective gametocytes show, however, distinct abnormalities in intra- and extra-cellular membranous compartments, such as accumulation of parasitophorous vacuole-derived vesicles in the erythrocyte cytoplasm, large intracellular vacuoles and discontinuities in their trilaminar cell membrane. This work revises current knowledge on the role of Pfg27, indicating that the protein is not required for parasite entry into sexual differentiation, and suggesting that it is instead involved in maintaining cell integrity in the uniquely long gametocytogenesis of P. falciparum.

A phosphatidylcholine-BODIPY 581/591 conjugate allows mapping of oxidative stress in P. falciparum-infected erythrocytes

The chromophore, BODIPY 581/591, has an extended conjugated system that reacts with oxygen centered-radicals leading to changes in its spectral characteristics. Fatty acid-conjugated BODIPY 581/591 transfers readily between membrane bilayers and can be used as a sensor of oxidative stress in cell populations. We report here the use of a phosphatidylcholine (PC) derivative of BODIPY 581/591, which transfers much less rapidly between membranes. This allows the analysis of oxidative stress in individual cells and in different compartments within cells. Quantitative imaging and flow cytometry were used to measure the ratio of fully conjugated to oxidized probe in model systems and in Plasmodium falciparum-infected erythrocytes. We observed an increase in the oxidation of the parasite-associated BODIPY 581/591-PC as the intraerythrocytic parasite matures. By contrast, BODIPY 581/591-PC associated with the erythrocyte membrane experiences a low level of oxidation even in the later stages of parasite development. Treatment with a pro-oxidant compound caused increased oxidation of the probe in the parasite compartment, but less so in the host cell membrane. Conversely, treatment with ferricyanide increases oxidation of the probe in the erythrocyte cell membrane but does not inhibit parasite growth. Chromatographic analysis of the lipids in infected erythrocytes shows no evidence for loss of alpha-tocopherol or the accumulation of lipid hydroperoxides indicating that, despite the increased oxidative stress, the parasite membranes remain protected from substantial lipid oxidation. We have established BODIPY 581/591-PC as a useful probe of the spatial distribution of oxidative stress in P. falciparum-infected erythrocytes; however, the probe appears to be more sensitive to oxidative damage than endogenous lipids.

Antimalarials and the fight against malaria in Brazil

Malaria, known as the “fevers,” has been treated for over three thousand years in China with extracts of plants of the genus Artemisia (including Artemisia annua, A. opiacea, and A. lancea) from which the active compound is artemisin, a sesquiterpene that is highly effective in the treatment of the disease, especially against young forms of the parasite. South American Indians in the seventeenth century already used an extract of the bark of chinchona tree, commonly named “Jesuits’ powder.” Its active compound was isolated in 1820 and its use spread all over the world being used as a prophylactic drug during the construction of the Madeira–Mamoré railroad in the beginning of the twentieth century. During the 1920s to the 1940s, new antimalarial drugs were synthesized to increase the arsenal against this parasite. However, the parasite has presented systematic resistence to conventional antimalarial drugs, driving researchers to find new strategies to treat the disease. In the present review we discuss how Brazil treats Plasmodium-infected patients.

Synthesis and chemistry of 2,3-dioxabicyclo[2.2.2]octane-5,6-diols

1,4-Disubstituted 2,3-dioxabicyclo[2.2.2]oct-5-enes were dihydroxylated with osmium tetroxide to yield diols anti to the peroxide linkage in a highly selective manner. Reduction of the peroxide bond furnished cyclohexane-1,2,3,4-tetraols with toxocarol relative stereochemistry in excellent yield. This new methodology was employed to synthesize the natural product (1S,2R,3S,4R,5R)-2-methyl-5-(propan-2-yl)cyclohexane-1,2,3,4-tetrol (1) in a short sequence from (R)-alpha-phellandrene. Moreover, during the study of the chemistry of 2,3-dioxabicyclo[2.2.2]octane-5,6-diols a hitherto unknown rearrangement was discovered which has wide applicability for the synthesis of 1,4-dicarbonyls, including optically enriched synthons. A broad range of mechanistic investigations applicable to this rearrangement are also reported.

Accumulation of artemisinin trioxane derivatives within neutral lipids of Plasmodium falciparum malaria parasites is endoperoxide-dependent

The antimalarial trioxanes, exemplified by the naturally occurring sesquiterpene lactone artemisinin and its semi-synthetic derivatives, contain an endoperoxide pharmacophore that lends tremendous potency against Plasmodium parasites. Despite decades of research, their mechanism of action remains unresolved. A leading model of anti-plasmodial activity hypothesizes that iron-mediated cleavage of the endoperoxide bridge generates cytotoxic drug metabolites capable of damaging cellular macromolecules. To probe the malarial targets of the endoperoxide drugs, we studied the distribution of fluorescent dansyl trioxane derivatives in living, intraerythrocytic-stage Plasmodium falciparum parasites using microscopic imaging. The fluorescent trioxanes rapidly accumulated in parasitized erythrocytes, localizing within digestive vacuole-associated neutral lipid bodies of trophozoites and schizonts, and surrounding the developing merozoite membranes. Artemisinin pre-treatment significantly reduced fluorescent labeling of neutral lipid bodies, while iron chelation increased non-specific cytoplasmic localization. To further explore the effects of endoperoxides on cellular lipids, we used an oxidation-sensitive BODIPY lipid probe to show the presence of artemisinin-induced peroxyl radicals in parasite membranes. Lipid extracts from artemisinin-exposed parasites contained increased amounts of free fatty acids and a novel cholesteryl ester. The cellular accumulation patterns and effects on lipids were entirely endoperoxide-dependent, as inactive dioxolane analogs lacking the endoperoxide moiety failed to label neutral lipid bodies or induce oxidative membrane damage. In the parasite digestive vacuole, neutral lipids closely associate with heme and promote hemozoin formation. We propose that the trioxane artemisinin and its derivatives are activated by heme-iron within the neutral lipid environment where they initiate oxidation reactions that damage parasite membranes.

Dynamic RNA profiling in Plasmodium falciparum synchronized blood stages exposed to lethal doses of artesunate

Background

Translation of the genome sequence of Plasmodium sp. into biologically relevant information relies on high through-put genomics technology which includes transcriptome analysis. However, few studies to date have used this powerful approach to explore transcriptome alterations of P. falciparum parasites exposed to antimalarial drugs.

Results

The rapid action of artesunate allowed us to study dynamic changes of the parasite transcriptome in synchronous parasite cultures exposed to the drug for 90 minutes and 3 hours. Developmentally regulated genes were filtered out, leaving 398 genes which presented altered transcript levels reflecting drug-exposure. Few genes related to metabolic pathways, most encoded chaperones, transporters, kinases, Zn-finger proteins, transcription activating proteins, proteins involved in proteasome degradation, in oxidative stress and in cell cycle regulation. A positive bias was observed for over-expressed genes presenting a subtelomeric location, allelic polymorphism and encoding proteins with potential export sequences, which often belonged to subtelomeric multi-gene families. This pointed to the mobilization of processes shaping the interface between the parasite and its environment. In parallel, pathways were engaged which could lead to parasite death, such as interference with purine/pyrimidine metabolism, the mitochondrial electron transport chain, proteasome-dependent protein degradation or the integrity of the food vacuole.

Conclusion

The high proportion of over-expressed genes encoding proteins exported from the parasite highlight the importance of extra-parasitic compartments as fields for exploration in drug research which, to date, has mostly focused on the parasite itself rather than on its intra and extra erythrocytic environment. Further work is needed to clarify which transcriptome alterations observed reflect a specific response to overcome artesunate toxicity or more general perturbations on the path to cellular death.