Bioavailable iron and heme metabolism in Plasmodium falciparum

Stage-specific pharmacodynamic chloroquine and pyronaridine action on artemisinin ring-stage resistant Kelch C580Y mutation Plasmodium falciparum correlates to hemozoin inhibition process

The antimalarial quinolines pyronaridine and chloroquine both inhibit hemozoin crystallization, predominately produced by Plasmodium falciparum intra-erythrocytic trophozoite stage parasites. Pyronaridine extends activity to ring-stage chloroquine-sensitive parasites, in contrast to chloroquine. Here, we investigated chloroquine and pyronaridine hemozoin inhibition type correlated to stage-specific activity on chloroquine-resistant ring-stage artemisinin sensitive and resistant P. falciparum CamWT and CamWT-C580Y parasites. Pyronaridine (2.8 μM) is tenfold more potent at beta-hematin nucleation than chloroquine (40 μM). Both pyronaridine and chloroquine (0.2 and 0.7 μM, respectively) had similar sub-μM inhibition of beta-hematin extension. P. falciparum CamWT-C580Y parasites produce smaller width hemozoin crystals which extend less than isogenic CamWT hemozoin. Stage-specific pulse dose pyronaridine and chloroquine on CamWT-C580Y or CamWT isogenic parasites observed 3- to 4-fold higher pyronaridine IC50s compared to 10- to 15-fold higher chloroquine on most CamWT-C580Y to CamWT stages. These findings collectively show that hemozoin nucleation inhibition widens stage-specific pyronaridine activity on P. falciparum drug-resistant parasites.

Iron transport pathways in the human malaria parasite Plasmodium falciparum revealed by RNA-sequencing

Host iron deficiency is protective against severe malaria as the human malaria parasite Plasmodium falciparum depends on bioavailable iron from its host to proliferate. The essential pathways of iron acquisition, storage, export, and detoxification in the parasite differ from those in humans, as orthologs of the mammalian transferrin receptor, ferritin, or ferroportin, and a functional heme oxygenase are absent in P. falciparum. Thus, the proteins involved in these processes may be excellent targets for therapeutic development, yet remain largely unknown. Here, we show that parasites cultured in erythrocytes from an iron-deficient donor displayed significantly reduced growth rates compared to those grown in red blood cells from healthy controls. Sequencing of parasite RNA revealed diminished expression of genes involved in overall metabolism, hemoglobin digestion, and metabolite transport under low-iron versus control conditions. Supplementation with hepcidin, a specific ferroportin inhibitor, resulted in increased labile iron levels in erythrocytes, enhanced parasite replication, and transcriptional upregulation of genes responsible for merozoite motility and host cell invasion. Through endogenous GFP tagging of differentially expressed putative transporter genes followed by confocal live-cell imaging, proliferation assays with knockout and knockdown lines, and protein structure predictions, we identified six proteins that are likely required for ferrous iron transport in P. falciparum. Of these, we localized PfVIT and PfZIPCO to cytoplasmic vesicles, PfMRS3 to the mitochondrion, and the novel putative iron transporter PfE140 to the plasma membrane for the first time in P. falciparum. PfNRAMP/PfDMT1 and PfCRT were previously reported to efflux Fe2+ from the digestive vacuole. Our data support a new model for parasite iron homeostasis, in which PfE140 is involved in iron uptake across the plasma membrane, PfMRS3 ensures non-redundant Fe2+ supply to the mitochondrion as the main site of iron utilization, PfVIT transports excess iron into cytoplasmic vesicles, and PfZIPCO exports Fe2+ from these organelles in case of iron scarcity. These results provide new insights into the parasite's response to differential iron availability in its environment and into the mechanisms of iron transport in P. falciparum as promising candidate targets for future antimalarial drugs.

Identification of a divalent metal transporter required for cellular iron metabolism in malaria parasites

Plasmodium falciparum malaria parasites invade and multiply inside red blood cells (RBCs), the most iron-rich compartment in humans. Like all cells, P. falciparum requires nutritional iron to support essential metabolic pathways, but the critical mechanisms of iron acquisition and trafficking during RBC infection have remained obscure. Parasites internalize and liberate massive amounts of heme during large-scale digestion of RBC hemoglobin within an acidic food vacuole (FV) but lack a heme oxygenase to release porphyrin-bound iron. Although most FV heme is sequestered into inert hemozoin crystals, prior studies indicate that trace heme escapes biomineralization and is susceptible to non-enzymatic degradation within the oxidizing FV environment to release labile iron. Parasites retain a homolog of divalent metal transporter 1 (DMT1), a known mammalian iron transporter, but its role in P. falciparum iron acquisition has not been tested. Our phylogenetic studies indicate that P. falciparum DMT1 (PfDMT1) retains conserved molecular features critical for metal transport. We localized this protein to the FV membrane and defined its orientation in an export-competent topology. Conditional knockdown of PfDMT1 expression is lethal to parasites, which display broad cellular defects in iron-dependent functions, including impaired apicoplast biogenesis and mitochondrial polarization. Parasites are selectively rescued from partial PfDMT1 knockdown by supplementation with exogenous iron, but not other metals. These results support a cellular paradigm whereby PfDMT1 is the molecular gatekeeper to essential iron acquisition by blood-stage malaria parasites and suggest that therapeutic targeting of PfDMT1 may be a potent antimalarial strategy.

Plasmodium yoelii iron transporter PyDMT1 interacts with host ferritin and is required in full activity for malarial pathogenesis

BACKGROUND

The rapid reproduction of malaria parasites requires proper iron uptake. However, the process of iron absorption by parasites is rarely studied. Divalent metal transporter (DMT1) is a critical iron transporter responsible for uptaking iron. A homolog of human DMT1 exists in the malaria parasite genome, which in Plasmodium yoelii is hereafter named PyDMT1.

RESULTS

PyDMT1 knockout appears to be lethal. Surprisingly, despite dwelling in an iron-rich environment, the parasite cannot afford to lose even partial expression of PyDMT1; PyDMT1 hypomorphs were associated with severe growth defects and quick loss of pathogenicity. Iron supplementation could completely suppress the defect of the PyDMT1 hypomorph during in vitro culturing. Genetic manipulation through host ferritin (Fth1) knockout to increase intracellular iron levels enforced significant growth inhibition in vivo on the normal parasites but not the mutant. In vitro culturing with isolated ferritin knockout mouse erythrocytes completely rescued PyDMT1-hypomorph parasites.

CONCLUSION

A critical iron requirement of malaria parasites at the blood stage as mediated by this newly identified iron importer PyDMT1, and the iron homeostasis in malarial parasites is finely tuned. Tipping the iron balance between the parasite and host will efficiently kill the pathogenicity of the parasite. Lastly, PyDMT1 hypomorph parasites were less sensitive to the action of artemisinin.

Identifying a Deferiprone-Resveratrol Hybrid as an Effective Lipophilic Anti-Plasmodial Agent

Malaria i a serious health problem caused by Plasmodium spp. that can be treated by an anti-folate pyrimethamine (PYR) drug. Deferiprone (DFP) is an oral iron chelator used for the treatment of iron overload and has been recognized for its potential anti-malarial activity. Deferiprone-resveratrol hybrids (DFP-RVT) have been synthesized to present therapeutic efficacy at a level which is superior to DFP. We have focused on determining the lipophilicity, toxicity and inhibitory effects on P. falciparum growth and the iron-chelating activity of labile iron pools (LIPs) by DFP-RVT. According to our findings, DFP-RVT was more lipophilic than DFP (p < 0.05) and nontoxic to blood mononuclear cells. Potency for the inhibition of P. falciparum was PYR > DFP-RVT > DFP in the 3D7 strain (IC₅₀ = 0.05, 16.82 and 47.67 µM, respectively) and DFP-RVT > DFP > PYR in the K1 strain (IC₅₀ = 13.38, 42.02 and 105.61 µM, respectively). The combined treatment of DFP-RVT with PYR additionally enhanced the PYR activity in both strains. DFP-RVT dose-dependently lowered LIP levels in PRBCs and was observed to be more effective than DFP at equal concentrations. Thus, the DFP-RVT hybrid should be considered a candidate as an adjuvant anti-malarial drug through the deprivation of cellular iron.

Characterization of the substrate binding site of an iron detoxifying membrane transporter from Plasmodium falciparum

Background

Plasmodium species are entirely dependent upon their host as a source of essential iron. Although it is an indispensable micronutrient, oxidation of excess ferrous iron to the ferric state in the cell cytoplasm can produce reactive oxygen species that are cytotoxic. The malaria parasite must therefore carefully regulate the processes involved in iron acquisition and storage. A 273 amino acid membrane transporter that is a member of the vacuolar iron transporter (VIT) family and an orthologue of the yeast Ca²⁺-sensitive cross complementer (CCC1) protein plays a major role in cytosolic iron detoxification of Plasmodium species and functions in transport of ferrous iron ions into the endoplasmic reticulum for storage. While this transporter, termed PfVIT, is not critical for viability of the parasite evidence from studies of mice infected with VIT-deficient Plasmodium suggests it could still provide an efficient target for chemoprophylactic treatment of malaria. Individual amino acid residues that constitute the Fe²⁺ binding site of the protein were identified to better understand the structural basis of substrate recognition and binding by PfVIT.

Methods

Using the crystal structure of a recently published plant VIT as a template, a high-quality homology model of PfVIT was constructed to identify the amino acid composition of the transporter’s substrate binding site and to act as a guide for subsequent mutagenesis studies. To test the effect of mutation of the substrate binding-site residues on PfVIT function a yeast complementation assay assessed the ability of overexpressed, recombinant wild type and mutant PfVIT to rescue an iron-sensitive deletion strain (ccc1∆) of Saccharomyces cerevisiae yeast from the toxic effects of a high concentration of extracellular iron.

Results

The combined in silico and mutagenesis approach identified a methionine residue located within the cytoplasmic metal binding domain of the transporter as essential for PfVIT function and provided insight into the structural basis for the Fe²⁺-selectivity of the protein.

Conclusion

The structural model of the metal binding site of PfVIT opens the door for rational design of therapeutics to interfere with iron homeostasis within the malaria parasite.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03827-7.

Doxycycline has distinct apicoplast-specific mechanisms of antimalarial activity

Doxycycline (DOX) is a key antimalarial drug thought to kill Plasmodium parasites by blocking protein translation in the essential apicoplast organelle. Clinical use is primarily limited to prophylaxis due to delayed second-cycle parasite death at 1–3 µM serum concentrations. DOX concentrations > 5 µM kill parasites with first-cycle activity but are thought to involve off-target mechanisms outside the apicoplast. We report that 10 µM DOX blocks apicoplast biogenesis in the first cycle and is rescued by isopentenyl pyrophosphate, an essential apicoplast product, confirming an apicoplast-specific mechanism. Exogenous iron rescues parasites and apicoplast biogenesis from first- but not second-cycle effects of 10 µM DOX, revealing that first-cycle activity involves a metal-dependent mechanism distinct from the delayed-death mechanism. These results critically expand the paradigm for understanding the fundamental antiparasitic mechanisms of DOX and suggest repurposing DOX as a faster acting antimalarial at higher dosing whose multiple mechanisms would be expected to limit parasite resistance.

Ozonide Antimalarials Alkylate Heme in the Malaria Parasite Plasmodium falciparum

The mechanism of action of ozonide antimalarials involves activation by intraparasitic iron and the formation of highly reactive carbon-centered radicals that alkylate malaria parasite proteins. Given free intraparasitic heme is generally thought to be the iron source responsible for ozonide activation and its likely close proximity to the activated drug, we investigated heme as a possible molecular target of the ozonides. Using an extraction method optimized for solubilization of free heme, untargeted LC-MS analysis of ozonide-treated parasites identified several regioisomers of ozonide-alkylated heme, which resulted from covalent modification of the heme porphyrin ring by an ozonide-derived carbon-centered radical. In addition to the intact alkylated heme adduct, putative ozonide-alkylated heme degradation products were also detected. This study directly demonstrates ozonide modification of heme within the malaria parasite Plasmodium falciparum, revealing that this process may be important for the biological activity of ozonide antimalarials.

Inhibitory Mechanisms of DHA/CQ on pH and Iron Homeostasis of Erythrocytic Stage Growth of Plasmodium Falciparum

Malaria is an infectious disease caused by Plasmodium group. The mechanisms of antimalarial drugs DHA/CQ are still unclear today. The inhibitory effects (IC₅₀) of single treatments with DHA/CQ or V-ATPase inhibitor Baf-A1 or combination treatments by DHA/CQ combined with Baf-A1 on the growth of Plasmodium falciparum strain 3D7 was investigated. Intracellular cytoplasmic pH and labile iron pool (LIP) were labeled by pH probe BCECF, AM and iron probe calcein, AM, the fluorescence of the probes was measured by FCM. The effects of low doses of DHA (0.2 nM, 0.4 nM, 0.8 nM) on gene expression of V-ATPases (vapE, vapA, vapG) located in the membrane of DV were tested by RT-qPCR. DHA combined with Baf-A1 showed a synergism effect (CI = 0.524) on the parasite growth in the concentration of IC₅₀. Intracellular pH and irons were effected significantly by different doses of DHA/Baf-A1. Intracellular pH was decreased by CQ combined with Baf-A1 in the concentration of IC₅₀. Intracellular LIP was increased by DHA combined with Baf-A1 in the concentration of 20 IC₅₀. The expression of gene vapA was down-regulated by all low doses of DHA (0.2/0.4/0.8 nM) significantly (p < 0.001) and the expression of vapG/vapE were up-regulated by 0.8 nM DHA significantly (p < 0.001). Interacting with ferrous irons, affecting the DV membrane proton pumping and acidic pH or cytoplasmic irons homeostasis may be the antimalarial mechanism of DHA while CQ showed an effect on cytoplasmic pH of parasite in vitro. Lastly, this article provides us preliminary results and a new idea for antimalarial drugs combination and new potential antimalarial combination therapies.

Erythrocytic ferroportin reduces intracellular iron accumulation, hemolysis, and malaria risk

Malaria parasites invade red blood cells (RBCs), consume copious amounts of hemoglobin, and severely disrupt iron regulation in humans. Anemia often accompanies malaria disease; however, iron supplementation therapy inexplicably exacerbates malarial infections. Here we found that the iron exporter ferroportin (FPN) was highly abundant in RBCs, and iron supplementation suppressed its activity. Conditional deletion of the Fpn gene in erythroid cells resulted in accumulation of excess intracellular iron, cellular damage, hemolysis, and increased fatality in malaria-infected mice. In humans, a prevalent FPN mutation, Q248H (glutamine to histidine at position 248), prevented hepcidin-induced degradation of FPN and protected against severe malaria disease. FPN Q248H appears to have been positively selected in African populations in response to the impact of malaria disease. Thus, FPN protects RBCs against oxidative stress and malaria infection.

Parasite-Mediated Degradation of Synthetic Ozonide Antimalarials Impacts In Vitro Antimalarial Activity

The peroxide bond of the artemisinins inspired the development of a class of fully synthetic 1,2,4-trioxolane-based antimalarials, collectively known as the ozonides. Similar to the artemisinins, heme-mediated degradation of the ozonides generates highly reactive radical species that are thought to mediate parasite killing by damaging critical parasite biomolecules. We examined the relationship between parasite dependent degradation and antimalarial activity for two ozonides, OZ277 (arterolane) and OZ439 (artefenomel), using a combination of in vitro drug stability and pulsed-exposure activity assays. Our results showed that drug degradation is parasite stage dependent and positively correlates with parasite load. Increasing trophozoite-stage parasitemia leads to substantially higher rates of degradation for both OZ277 and OZ439, and this is associated with a reduction in in vitro antimalarial activity. Under conditions of very high parasitemia (∼90%), OZ277 and OZ439 were rapidly degraded and completely devoid of activity in trophozoite-stage parasite cultures exposed to a 3-h drug pulse. This study highlights the impact of increasing parasite load on ozonide stability and in vitro antimalarial activity and should be considered when investigating the antimalarial mode of action of the ozonide antimalarials under conditions of high parasitemia.

Iron Supplementation Alters Heme and Heme Oxygenase 1 (HO-1) Levels In Pregnant Women in Ghana

BACKGROUND

Iron supplementation is recommended for pregnant women to meet their iron requirement for a healthy pregnancy. The benefits and risks of universal iron supplementation during pregnancy in malaria endemic countries are currently being debated. As part of a broader study that focused on the effect of heme/HO-1 on pregnancy outcomes in malaria in pregnancy, we determined the association between iron supplementation and free heme levels in blood of pregnant women with and without malaria in Ghana. We hypothesized that pregnant women with malaria who took iron supplements will have higher levels of Heme/HO-1 than those who did not take iron supplements.

METHODS

A total of 337 women were recruited for this study. Blood samples were collected for malaria diagnosis and heme/HO-1 measurement. Quantification of heme was done using a heme colorimetric assay kit and HO-1 levels were performed using Enzyme-Linked Immunosorbent Assay (ELISA) on plasma samples.

RESULTS

Malaria positive iron supplemented women, in their third trimester, had significantly higher median levels of heme 59.3(43.1 - 60.4) than non-malaria iron supplemented women 35.7(33.0 - 62.2), p = 0.026. Also, malaria positive iron supplemented women had significant higher median levels of HO-16.2(IQR 4.9 - 8.1) than pregnant women who did not take iron supplements 2.9 (IQR 2.1 - 3.8), p = <0.001.

CONCLUSION

Although iron supplementation may be highly beneficial and improve pregnancy outcomes for iron deficient or anemic mothers, it is also likely that iron supplementation for pregnant women who are not iron deficient may put this group of women at risk for adverse pregnancy outcomes. Findings from this study sheds light on the effect of iron supplementation on malaria derived heme in pregnancy, which may inform how iron supplementation is recommended for pregnant women who are not iron deficient.

Iron and malaria: a dangerous liaison?

Malaria increases the burden of anemia in low-income countries, where, according to 2012 data from the World Health Organization, 40% of children are anemic. Moreover, iron is a cofactor for Plasmodium falciparum development, raising fears that iron supplementation might be harmful in patients with P. falciparum infection. The primary objective of this narrative review is to describe current knowledge on the iron-malaria association, including recent findings and substantive qualitative results. Between 2012 and 2016 the MEDLINE database was searched for literature published about malaria and iron levels. Observational studies reported some protection of iron supplementation against malaria among iron-deficient children, while older clinical trials reported increased susceptibility to malaria among iron-supplemented children. However, iron supplements were not significantly associated with increased malaria risk in recent clinical trials or in a 2016 Cochrane review. Evidence of an iron-malaria association is limited by the following factors: the protective effect of control interventions, the limited follow-up of children, and the lack of homogenous iron indicators. The effects of previous health status and possible thresholds in iron levels should be investigated using a gold-standard combination of iron markers. Moreover, the benefits of iron supplementation require further evaluation.

Inhibitory effect of novel iron chelator, 1-(N-acetyl-6-aminohexyl)-3-hydroxy-2-methylpyridin-4-one (CM1) and green tea extract on growth of Plasmodium falciparum

BACKGROUND

Iron is an essential micronutrient required by all living organisms including malaria parasites (Plasmodium spp.) for many biochemical reactions, especially growth and multiplication processes. Therefore, malaria parasite needs to take up the iron from outside or/and inside the parasitized red blood cells (PRBC). Iron chelators are widely used for the treatment of thalassaemia-related iron overload and also inhibit parasite growth at levels that are non-toxic to mammalian cells.

METHODS

Inhibitory effect of 1-(N-acetyl-6-aminohexyl)-3-hydroxy-2-methylpyridin-4-one (CM1) and green tea extract (GTE) on the growth of malaria parasite Plasmodium falciparum was compared with standard chelators including desferrioxamine (DFO), deferiprone (DFP) and deferasirox (DFX). A flow cytometric technique was used to enumerate PRBC stained with SYBR Green I fluorescent dye. The labile iron pool (LIP) was assayed using the calcein-acetoxymethyl fluorescent method.

RESULTS

The IC50 values of DFO, GTE, CM1, DFX and DFP against P. falciparum were 14.09, 21.11, 35.14, 44.71 and 58.25 µM, respectively. Importantly, CM1 was more effective in reducing LIP levels in the P. falciparum culture than DFP (p < 0.05).

CONCLUSIONS

CM1 and GTE exhibit anti-malarial activity. They could interfere with uptake of exogenous iron or deplete the intracellular labile iron pool in malaria parasites, leading to inhibition of their growth.

Erythrocytic Iron Deficiency Enhances Susceptibility to Plasmodium chabaudi Infection in Mice Carrying a Missense Mutation in Transferrin Receptor 1

The treatment of iron deficiency in areas of high malaria transmission is complicated by evidence which suggests that iron deficiency anemia protects against malaria, while iron supplementation increases malaria risk. Iron deficiency anemia results in an array of pathologies, including reduced systemic iron bioavailability and abnormal erythrocyte physiology; however, the mechanisms by which these pathologies influence malaria infection are not well defined. In the present study, the response to malaria infection was examined in a mutant mouse line, Tfrc(MRI24910), identified during an N-ethyl-N-nitrosourea (ENU) screen. This line carries a missense mutation in the gene for transferrin receptor 1 (TFR1). Heterozygous mice exhibited reduced erythrocyte volume and density, a phenotype consistent with dietary iron deficiency anemia. However, unlike the case in dietary deficiency, the erythrocyte half-life, mean corpuscular hemoglobin concentration, and intraerythrocytic ferritin content were unchanged. Systemic iron bioavailability was also unchanged, indicating that this mutation results in erythrocytic iron deficiency without significantly altering overall iron homeostasis. When infected with the rodent malaria parasite Plasmodium chabaudi adami, mice displayed increased parasitemia and succumbed to infection more quickly than their wild-type littermates. Transfusion of fluorescently labeled erythrocytes into malaria parasite-infected mice demonstrated an erythrocyte-autonomous enhanced survival of parasites within mutant erythrocytes. Together, these results indicate that TFR1 deficiency alters erythrocyte physiology in a way that is similar to dietary iron deficiency anemia, albeit to a lesser degree, and that this promotes intraerythrocytic parasite survival and an increased susceptibility to malaria in mice. These findings may have implications for the management of iron deficiency in the context of malaria.

Influence of host iron status on Plasmodium falciparum infection

Iron deficiency affects one quarter of the world's population and causes significant morbidity, including detrimental effects on immune function and cognitive development. Accordingly, the World Health Organization (WHO) recommends routine iron supplementation in children and adults in areas with a high prevalence of iron deficiency. However, a large body of clinical and epidemiological evidence has accumulated which clearly demonstrates that host iron deficiency is protective against falciparum malaria and that host iron supplementation may increase the risk of malaria. Although many effective antimalarial treatments and preventive measures are available, malaria remains a significant public health problem, in part because the mechanisms of malaria pathogenesis remain obscured by the complexity of the relationships that exist between parasite virulence factors, host susceptibility traits, and the immune responses that modulate disease. Here we review (i) the clinical and epidemiological data that describes the relationship between host iron status and malaria infection and (ii) the current understanding of the biological basis for these clinical and epidemiological observations.

Iron and heme metabolism in Plasmodium falciparum and the mechanism of action of artemisinins

During the asexual blood stage of its lifecycle, the malaria parasite Plasmodium falciparum grows and multiplies in the hemoglobin-rich environment of the human erythrocyte. Although the parasite has evolved unique strategies to survive in this environment, its interaction with iron represents an Achilles' heel that is exploited by many antimalarial drugs. Recent work has shed new light on how the parasite deals with hemoglobin breakdown products and on the role of iron as a mediator of the action of the antimalarial drug, artemisinin.

The iron link between malaria and invasive non-typhoid Salmonella infections

Epidemiological studies have demonstrated an association between malaria and invasive non-typhoid Salmonella (NTS) infections, especially in children. We explore the role of iron as a possible cofactor in this association. Malarial disease, among others, is associated with enhanced erythrophagocytosis and inflammation, which increases the iron content of macrophages and thereby also the survival of Salmonella spp. within macrophages. Whether iron supplementation programs augment the risk of invasive NTS infections in malaria-endemic regions is an important global health issue that still needs to be determined.

Parasite maturation and host serum iron influence the labile iron pool of erythrocyte stage Plasmodium falciparum

Iron is a critical and tightly regulated nutrient for both the malaria parasite and its human host. The importance of the relationship between host iron and the parasite has been underscored recently by studies showing that host iron supplementation may increase the risk of falciparum malaria. It is unclear what host iron sources the parasite is able to access. We developed a flow cytometry-based method for measuring the labile iron pool (LIP) of parasitized erythrocytes using the nucleic acid dye STYO 61 and the iron sensitive dye, calcein acetoxymethyl ester (CA-AM). This new approach enabled us to measure the LIP of P. falciparum through the course of its erythrocytic life cycle and in response to the addition of host serum iron sources. We found that the LIP increases as the malaria parasite develops from early ring to late schizont stage, and that the addition of either transferrin or ferric citrate to culture media increases the LIP of trophozoites. Our method for detecting the LIP within malaria parasitized RBCs provides evidence that the parasite is able to access serum iron sources as part of the host vs. parasite arms race for iron.

Interactions between artemisinins and other antimalarial drugs in relation to the cofactor model--a unifying proposal for drug action

Artemisinins are proposed to act in the malaria parasite cytosol by oxidizing dihydroflavin cofactors of redox-active flavoenzymes, and under aerobic conditions by inducing their autoxidation. Perturbation of redox homeostasis coupled with the generation of reactive oxygen species (ROS) ensues. Ascorbic acid-methylene blue (MB), N-benzyl-1,4-dihydronicotinamide (BNAH)-MB, BNAH-lumiflavine, BNAH-riboflavin (RF), and NADPH-FAD-E. coli flavin reductase (Fre) systems at pH 7.4 generate leucomethylene blue (LMB) and reduced flavins that are rapidly oxidized in situ by artemisinins. These oxidations are inhibited by the 4-aminoquinolines piperaquine (PPQ), chloroquine (CQ), and others. In contrast, the arylmethanols lumefantrine, mefloquine (MFQ), and quinine (QN) have little or no effect. Inhibition correlates with the antagonism exerted by 4-aminoquinolines on the antimalarial activities of MB, RF, and artemisinins. Lack of inhibition correlates with the additivity/synergism between the arylmethanols and artemisinins. We propose association via π complex formation between the 4-aminoquinolines and LMB or the dihydroflavins; this hinders hydride transfer from the reduced conjugates to the artemisinins. The arylmethanols have a decreased tendency to form π complexes, and so exert no effect. The parallel between chemical reactivity and antagonism or additivity/synergism draws attention to the mechanism of action of all drugs described herein. CQ and QN inhibit the formation of hemozoin in the parasite digestive vacuole (DV). The buildup of heme-Fe(III) results in an enhanced efflux from the DV into the cytosol. In addition, the lipophilic heme-Fe(III) complexes of CQ and QN that form in the DV are proposed to diffuse across the DV membrane. At the higher pH of the cytosol, the complexes decompose to liberate heme-Fe(III) . The quinoline or arylmethanol reenters the DV, and so transfers more heme-Fe(III) out of the DV. In this way, the 4-aminoquinolines and arylmethanols exert antimalarial activities by enhancing heme-Fe(III) and thence free Fe(III) concentrations in the cytosol. The iron species enter into redox cycles through reduction of Fe(III) to Fe(II) largely mediated by reduced flavin cofactors and likely also by NAD(P)H-Fre. Generation of ROS through oxidation of Fe(II) by oxygen will also result. The cytotoxicities of artemisinins are thereby reinforced by the iron. Other aspects of drug action are emphasized. In the cytosol or DV, association by π complex formation between pairs of lipophilic drugs must adversely influence the pharmacokinetics of each drug. This explains the antagonism between PPQ and MFQ, for example. The basis for the antimalarial activity of RF mirrors that of MB, wherein it participates in redox cycling that involves flavoenzymes or Fre, resulting in attrition of NAD(P)H. The generation of ROS by artemisinins and ensuing Fenton chemistry accommodate the ability of artemisinins to induce membrane damage and to affect the parasite SERCA PfATP6 Ca(2+) transporter. Thus, the effect exerted by artemisinins is more likely a downstream event involving ROS that will also be modulated by mutations in PfATP6. Such mutations attenuate, but cannot abrogate, antimalarial activities of artemisinins. Overall, parasite resistance to artemisinins arises through enhancement of antioxidant defense mechanisms.

Direct tests of enzymatic heme degradation by the malaria parasite Plasmodium falciparum

Malaria parasites generate vast quantities of heme during blood stage infection via hemoglobin digestion and limited de novo biosynthesis, but it remains unclear if parasites metabolize heme for utilization or disposal. Recent in vitro experiments with a heme oxygenase (HO)-like protein from Plasmodium falciparum suggested that parasites may enzymatically degrade some heme to the canonical HO product, biliverdin (BV), or its downstream metabolite, bilirubin (BR). To directly test for BV and BR production by P. falciparum parasites, we DMSO-extracted equal numbers of infected and uninfected erythrocytes and developed a sensitive LC-MS/MS assay to quantify these tetrapyrroles. We found comparable low levels of BV and BR in both samples, suggesting the absence of HO activity in parasites. We further tested live parasites by targeted expression of a fluorescent BV-binding protein within the parasite cytosol, mitochondrion, and plant-like plastid. This probe could detect exogenously added BV but gave no signal indicative of endogenous BV production within parasites. Finally, we recombinantly expressed and tested the proposed heme degrading activity of the HO-like protein, PfHO. Although PfHO bound heme and protoporphyrin IX with modest affinity, it did not catalyze heme degradation in vivo within bacteria or in vitro in UV absorbance and HPLC assays. These observations are consistent with PfHO's lack of a heme-coordinating His residue and suggest an alternative function within parasites. We conclude that P. falciparum parasites lack a canonical HO pathway for heme degradation and thus rely fully on alternative mechanisms for heme detoxification and iron acquisition during blood stage infection.

Safety of iron fortification and supplementation in malaria-endemic areas

This review considers the safety of iron supplementation and fortification for the prevention and correction of iron deficiency in malaria-endemic areas, with a focus on potential means whereby provision of additional iron might heighten the risks of malaria and other infections. Iron deficiency itself may increase the risk of morbidity and mortality from malaria and other infections. The available evidence indicates that iron interventions are safe in settings without endemic malaria, and, with adequate health care, in regions with high transmission of malaria and other infections. Without regular surveillance and treatment of malaria and other infections, iron supplementation of individuals who are iron deficient seems safe, but individuals who are iron replete may have an increased risk of adverse outcomes. The mechanisms responsible for harmful effects with iron supplementation have not been established. These are likely to include the effects of (a) increased amounts of absorbed iron, with the production of plasma non-transferrin-bound iron, (b) increased amounts of iron in the gastrointestinal tract, with effects on gastrointestinal structural integrity and on gut microflora, and (c) the complex immune effects of iron interventions. Iron fortification appears to be generally safe, although more data from malaria-endemic areas are needed.

The distribution of the parasitic fauna dictates the distribution of the haemochromatosis genes

No satisfactory explanation has been offered, to date, to account for the prevalence of the haemochromatosis genes in the European population and yet relative paucity of the gene in the tropics. Traditional wisdom suggests that, in antiquity, the haemochromatosis gene, which promotes iron absorption, would have protected ancient man from iron loss resulting from injury either during hunting or through war. However, such an advantage would be equally desirable for other populations where the incidence of the alleles is negligible. Others have tackled the polemic from the another view, postulating that the paucity of the haemochromatosis alleles in populations outside of Europe may be explained by the fact that iron load predisposes to infection and that iron deficiency anaemia is protective against this by limiting parasitic access to host stores of iron. This explanation alone is equally unsatisfactory as European populations are exposed to pathogens and would benefit from any protection afforded by mild anaemia. Others have mooted genetic drift as another alternative explanation. Yet this would be unexpected for a gene which is deleterious. We propose here that the driving force for the propagation of the haemochromatosis alleles was not infection per se but the nature of the parasitic fauna which sojourned with mankind. The tropics are inhabited with multicellular parasitic and highly pathogenic organisms, which consequently have a high demand for iron. The organisms have developed aggressive means of iron extraction from their hosts. Where there is iron in abundance such organisms would have a licence to multiply in an unbridled fashion at the expense of the host. Such a host, due to their increased iron load, would be able to harbour a high parasitic load which would be harmful to the population as a whole, not just the individual with the haemochromatosis allele. As man migrated from the tropics many of the larger pathogens disappeared and man had only to contend with traditional unicellular adversaries. Iron is a critical micronutrient that the host attempts to withhold for invading pathogens. We also advance the theory that the tropical anaemias including sickle cell trait, thalassaemia, glucose-6-phosphate dehydrogenase deficiency, and pyruvate kinase deficiency are an ingenious evolutionary means by the host of withholding iron from tropical pathogens while simultaneously avoiding the deleterious effects of frank iron deficiency and/or iron deficiency anaemia. The mechanism is essentially an immunological passive aggressive orchestrated by man kind.

Experimental cerebral malaria progresses independently of the Nlrp3 inflammasome

Cerebral malaria is the most severe complication of Plasmodium falciparum infection in humans and the pathogenesis is still unclear. Using the P. berghei ANKA infection model of mice, we investigated a potential involvement of Nlrp3 and the inflammasome in the pathogenesis of cerebral malaria. Nlrp3 mRNA expression was upregulated in brain endothelial cells after exposure to P. berghei ANKA. Although beta-hematin, a synthetic compound of the parasites heme polymer hemozoin, induced the release of IL-1beta in macrophages through Nlrp3, we did not obtain evidence for a role of IL-1beta in vivo. Nlrp3 knock-out mice displayed a delayed onset of cerebral malaria; however, mice deficient in caspase-1, the adaptor protein ASC or the IL-1 receptor succumbed as WT mice. These results indicate that the role of Nlrp3 in experimental cerebral malaria is independent of the inflammasome and the IL-1 receptor pathway.

Molecular and physiologic basis of quinoline drug resistance in Plasmodium falciparum malaria

30 years before the discovery of the pfcrt gene, altered cellular drug accumulation in drug-resistant malarial parasites had been well documented. Heme released from catabolized hemoglobin was thought to be a key target for quinoline drugs, and additional modifications to quinoline drug structure in order to improve activity against chloroquine-resistant malaria were performed in a few laboratories. However, parasite cell culture methods were still in their infancy, assays for drug susceptibility were not well standardized, and the power of malarial genetics was decades away. The last 10 years have witnessed explosive progress in elucidation of the biochemistry of chloroquine resistance. This review briefly summarizes that progress, and discusses where additional work is needed.

A new model for hemoglobin ingestion and transport by the human malaria parasite Plasmodium falciparum

The current model for hemoglobin ingestion and transport by intraerythrocytic Plasmodium falciparum malaria parasites shares similarities with endocytosis. However, the model is largely hypothetical, and the mechanisms responsible for the ingestion and transport of host cell hemoglobin to the lysosome-like food vacuole (FV) of the parasite are poorly understood. Because actin dynamics play key roles in vesicle formation and transport in endocytosis, we used the actin-perturbing agents jasplakinolide and cytochalasin D to investigate the role of parasite actin in hemoglobin ingestion and transport to the FV. In addition, we tested the current hemoglobin trafficking model through extensive analysis of serial thin sections of parasitized erythrocytes (PE) by electron microscopy. We find that actin dynamics play multiple, important roles in the hemoglobin transport pathway, and that hemoglobin delivery to the FV via the cytostomes might be required for parasite survival. Evidence is provided for a new model, in which hemoglobin transport to the FV occurs by a vesicle-independent process.

Relationship between antimalarial activity and heme alkylation for spiro- and dispiro-1,2,4-trioxolane antimalarials

The reaction of spiro- and dispiro-1,2,4-trioxolane antimalarials with heme has been investigated to provide further insight into the mechanism of action for this important class of antimalarials. A series of trioxolanes with various antimalarial potencies was found to be unreactive in the presence of Fe(III) hemin, but all were rapidly degraded by reduced Fe(II) heme. The major reaction product from the heme-mediated degradation of biologically active trioxolanes was an alkylated heme adduct resulting from addition of a radical intermediate. Under standardized reaction conditions, a correlation (R2 = 0.88) was found between the extent of heme alkylation and in vitro antimalarial activity, suggesting that heme alkylation may be related to the mechanism of action for these trioxolanes. Significantly less heme alkylation was observed for the clinically utilized artemisinin derivatives compared to the equipotent trioxolanes included in this study.

Pumping iron: a potential target for novel therapeutics against schistosomes

Parasites, as with the vast majority of organisms, are dependent on iron. Pathogens must compete directly with the host for this essential trace metal, which is sequestered within host proteins and inorganic chelates. Not surprisingly, pathogenic prokaryotes and eukaryotic parasites have diverse adaptations to exploit host iron resources. How pathogenic bacteria scavenge host iron is well characterized and is reasonably well known for a few parasitic protozoa, but is poorly understood for metazoan parasites. Strategies of iron acquisition by schistosomes are examined here, with emphasis on possible mechanisms of iron absorption from host serum iron transporters or from digested haem. Elucidation of these metabolic mechanisms could lead to the development of new interventions for the control of schistosomiasis and other helminth diseases.

Current perspectives on the mechanism of action of artemisinins

Artemisinin derivatives are the most recent single drugs approved and introduced for public antimalarial treatment. Although their recommended use is for treatment of Plasmodium falciparum infection, these drugs also act against other parasites, as well as against tumor cells. The mechanisms of action attributed to artemisinin include interference with parasite transport proteins, disruption of parasite mitochondrial function, modulation of host immune function and inhibition of angiogenesis. Artemisinin combination therapies are currently the preferred treatment for malaria. These combinations may prevent the induction of parasite drug resistance. However, in view of the multiple mechanisms involved, especially when additional drugs are used, the combined therapy should be carefully examined for antagonistic effects. It is now a general theory that the crucial mechanism is interference with plasmodial SERCA. Therefore, future development of resistance may be associated with overproduction or mutations of this transporter. However, a general mechanism, such as alterations in general drug transport pathways, is feasible. In this article, we review the evidence for each mechanism of action suggested.

Re-evaluation of how artemisinins work in light of emerging evidence of in vitro resistance

There are more than half a billion cases of malaria every year. Combinations of an artemisinin with other antimalarial drugs are now recommended treatments for Plasmodium falciparum malaria in most endemic areas. These treatment regimens act rapidly to relieve symptoms and effect cure. There is considerable controversy on how artemisinins work and over emerging indications of resistance to this class of antimalarial drugs. Several individual molecules have been proposed as targets for artemisinins, in addition to the idea that artemisinins might have many targets at the same time. Our suggestion that artemisinins inhibit the parasite-encoded sarco–endoplasmic reticulum Ca²⁺-ATPase (SERCA) PfATP6 has gained support from recent observations that a polymorphism in the gene encoding PfATP6 is associated with in vitro resistance to artemether in field isolates of P. falciparum.