Hemoglobin catabolism and host-parasite heme balance in chloroquine-sensitive and chloroquine-resistant Plasmodium berghei infections

Plasma metabolomics reveals membrane lipids, aspartate/asparagine and nucleotide metabolism pathway differences associated with chloroquine resistance in Plasmodium vivax malaria

Background

Chloroquine (CQ) is the main anti-schizontocidal drug used in the treatment of uncomplicated malaria caused by Plasmodium vivax. Chloroquine resistant P. vivax (PvCR) malaria in the Western Pacific region, Asia and in the Americas indicates a need for biomarkers of resistance to improve therapy and enhance understanding of the mechanisms associated with PvCR. In this study, we compared plasma metabolic profiles of P. vivax malaria patients with PvCR and chloroquine sensitive parasites before treatment to identify potential molecular markers of chloroquine resistance.

Methods

An untargeted high-resolution metabolomics analysis was performed on plasma samples collected in a malaria clinic in Manaus, Brazil. Male and female patients with Plasmodium vivax were included (n = 46); samples were collected before CQ treatment and followed for 28 days to determine PvCR, defined as the recurrence of parasitemia with detectable plasma concentrations of CQ ≥100 ng/dL. Differentially expressed metabolic features between CQ-Resistant (CQ-R) and CQ-Sensitive (CQ-S) patients were identified using partial least squares discriminant analysis and linear regression after adjusting for covariates and multiple testing correction. Pathway enrichment analysis was performed using Mummichog.

Results

Linear regression and PLS-DA methods yielded 69 discriminatory features between CQ-R and CQ-S groups, with 10-fold cross-validation classification accuracy of 89.6% using a SVM classifier. Pathway enrichment analysis showed significant enrichment (p<0.05) of glycerophospholipid metabolism, glycosphingolipid metabolism, aspartate and asparagine metabolism, purine and pyrimidine metabolism, and xenobiotics metabolism. Glycerophosphocholines levels were significantly lower in the CQ-R group as compared to CQ-S patients and also to independent control samples.

Conclusions

The results show differences in lipid, amino acids, and nucleotide metabolism pathways in the plasma of CQ-R versus CQ-S patients prior to antimalarial treatment. Metabolomics phenotyping of P. vivax samples from patients with well-defined clinical CQ-resistance is promising for the development of new tools to understand the biological process and to identify potential biomarkers of PvCR.

Quantitative characterization of hemozoin in Plasmodium berghei and vivax

The incidence and global distribution of chloroquine resistant (CR) Plasmodium vivax infection has increased since emerging in 1989. The mechanism of resistance in CR P. vivax has not been defined. The resistance likely relates to the formation and disposition of hemozoin as chloroquine's primary mechanism of action involves disruption of hemozoin formation. CR P. berghei strains, like CR P. vivax strains, are confined to reticulocyte host cells and reportedly they do not accumulate appreciable intraerythrocytic hemozoin. Reports comparing hemozoin production between P. vivax strains and CR to chloroquine sensitive (CS) P. berghei are absent. Here we compare in vivo patterns of hemozoin formation and distribution in blood, spleen and liver tissue of male Swiss mice infected with CS or CR P. berghei not treated with chloroquine and CR P. berghei also treated with chloroquine. Light microscopy, laser desorption mass spectrometry and a colorimetric hemozoin assay detect trace hemozoin in the blood of CR P. berghei infected mice but significant hemozoin accumulation in liver and spleen tissue. Field emission in lens scanning electron microscopy reveals CR P. berghei hemozoin crystals are morphologically smaller but similar to those formed by CS parasites. CR P. berghei produces approximately five-fold less total hemozoin than CS strain. Lipid analysis of CS and CR P. berghei sucrose gradient purified bloodstage hemozoin indicates a similar lipid environment around the isolated hemozoin, predominately monopalmitic glycerol and monostearic glycerol. In contrast to CR and CS P. berghei, colorimetric hemozoin analysis of P. vivax strains indicates similar amounts of hemozoin are produced despite differing chloroquine sensitivities. These results suggest CR P. berghei forms significant hemozoin which accumulates in liver and spleen tissues and that the P. vivax chloroquine resistance mechanism differs from P. berghei.

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Chloroquine resistant Plasmodium berghei release measurable hemozoin into tissues with blood hemozoin 100 times less per parasite while total in all tissues is only 5 times less than chloroquine sensitive.

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Chloroquine resistant P. bergheihemozoin crystals are morphologically smaller but similar to those formed by chloroquine sensitive parasites.

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Chloroquine resistance in P. vivax is distinct from P. berghei even though both infect reticulocytes.

Evidence for the contribution of the hemozoin synthesis pathway of the murine Plasmodium yoelii to the resistance to artemisinin-related drugs

Plasmodium falciparum malaria is a major global health problem, causing approximately 780,000 deaths each year. In response to the spreading of P. falciparum drug resistance, WHO recommended in 2001 to use artemisinin derivatives in combination with a partner drug (called ACT) as first-line treatment for uncomplicated falciparum malaria, and most malaria-endemic countries have since changed their treatment policies accordingly. Currently, ACT are often the last treatments that can effectively and rapidly cure P. falciparum infections permitting to significantly decrease the mortality and the morbidity due to malaria. However, alarming signs of emerging resistance to artemisinin derivatives along the Thai-Cambodian border are of major concern. Through long-term in vivo pressures, we have been able to select a murine malaria model resistant to artemisinins. We demonstrated that the resistance of Plasmodium to artemisinin-based compounds depends on alterations of heme metabolism and on a loss of hemozoin formation linked to the down-expression of the recently identified Heme Detoxification Protein (HDP). These artemisinins resistant strains could be able to detoxify the free heme by an alternative catabolism pathway involving glutathione (GSH)-mediation. Finally, we confirmed that artemisinins act also like quinolines against Plasmodium via hemozoin production inhibition. The work proposed here described the mechanism of action of this class of molecules and the resistance to artemisinins of this model. These results should help both to reinforce the artemisinins activity and avoid emergence and spread of endoperoxides resistance by focusing in adequate drug partners design. Such considerations appear crucial in the current context of early artemisinin resistance in Asia.

Revealing parasite influence in metabolic pathways in Apicomplexa infected patients

Background

As an obligate intracellular parasite, Apicomplexa interacts with the host in the special living environment, competing for energy and nutrients from the host cells by manipulating the host metabolism. Previous studies of host-parasite interaction mainly focused on using cellular and biochemical methods to investigate molecular functions in metabolic pathways of parasite infected hosts. Computational approaches taking advantage of high-throughput biological data and topology of metabolic pathways have a great potential in revealing the details and mechanism of parasites-to-host interactions. A new analytical method was designed in this work to study host-parasite interactions in human cells infected with Plasmodium falciparum and Cryptosporidium parvum.

Results

We introduced a new method that analyzes the host metabolic pathways in divided parts: host specific subpathways and host-parasite common subpathways. Upon analysis on gene expression data from cells infected by Plasmodium falciparum or Cryptosporidium parvum, we found: (i) six host-parasite common subpathways and four host specific subpathways were significantly altered in plasmodium infected human cells; (ii) plasmodium utilized fatty acid biosynthesis and elongation, and Pantothenate and CoA biosynthesis to obtain nutrients from host environment; (iii) in Cryptosporidium parvum infected cells, most of the host-parasite common enzymes were down-regulated, whereas the host specific enzymes up-regulated; (iv) the down-regulation of common subpathways in host cells might be caused by competition for the substrates and up-regulation of host specific subpathways may be stimulated by parasite infection.

Conclusion

Results demonstrated a significantly coordinated expression pattern between the two groups of subpathways. The method helped expose the impact of parasite infection on host cell metabolism, which was previously concealed in the pathway enrichment analysis. Our approach revealed detailed subpathways and metabolic information are important to the symbiosis in two kinds of the apicomplex parasites, and highlighted its significance in research and understanding of parasite-host interactions.

SELDI-TOF-MS analysis of chloroquine resistant and sensitivePlasmodium falciparum strains

The resistance of the malarial parasite Plasmodium falciparum to chloroquine represents an emerging problem since neither mode of drug action nor mechanisms of resistance are fully elucidated. We describe a protein expression profiling approach by SELDI-TOF-MS as a useful tool for studying the proteome of malarial parasites. Reproducible and complex protein profiles of the P. falciparum strains K1, Dd2, HB3 and 3D7 were measured on four array types. Hierarchical clustering led to a clear separation of the two major subgroups "resistant" and "sensitive" as well as of the four parasite strains. Our study delivers sets of regulated proteins derived from extensive comparative analyses of 64 P. falciparum protein profiles. A group of 12 peaks reflecting proteome changes under chloroquine treatment and a set of 10 potential chloroquine resistance markers were defined. Three of these regulated peaks were preparatively enriched, purified and identified. They were shown to represent the plasmodial EXP-1 protein, also called circumsporozoite-related antigen, as well as the alpha- and beta- (delta-) chains of human hemoglobin.

The shape and size of hemozoin crystals distinguishes diverse Plasmodium species

All Plasmodium species produce a brown birefringent crystal known as malarial pigment or hemozoin. This work compares the morphology of hemozoin from P. falciparum, P. vivax, P. ovale, P. malariae, P. knowlesi, P. brasilianum, P. yoelii and P. gallinaceum. The human, primate and mouse hemozoins have a regular, flat-faced cuboidal morphology with modest size differences in contrast to the larger, regularly irregular barrel shape with a waffle surface of the avian, P. gallinaceum, pigment. Hydrogen peroxide (H2O2), as a biochemical test reagent, can distinguish the hemozoins by different concentrations to degrade half of the crystals. A surface area to volume ratio explains both the appearance and susceptibility to H2O2 degradation. The hemozoin from each species is able to be a template for hemozoin extension inhibitable by the quinolines. P. gallinaceum hemozoin more closely resembles the hemozoin from another avian apicomplexan, Haemoproteus, rather than the hemozoin from the mammalian malaria species. These distinct morphological characteristics between mammalian and avian crystals suggest different biochemical environments that affect morphology.

Clotrimazole binds to heme and enhances heme-dependent hemolysis: proposed antimalarial mechanism of clotrimazole

Two recent studies have demonstrated that clotrimazole, a potent antifungal agent, inhibits the growth of chloroquine-resistant strains of the malaria parasite, Plasmodium falciparum, in vitro. We explored the mechanism of antimalarial activity of clotrimazole in relation to hemoglobin catabolism in the malaria parasite. Because free heme produced from hemoglobin catabolism is highly toxic to the malaria parasite, the parasite protects itself by polymerizing heme into insoluble nontoxic hemozoin or by decomposing heme coupled to reduced glutathione. We have shown that clotrimazole has a high binding affinity for heme in aqueous 40% dimethyl sulfoxide solution (association equilibrium constant: K(a) = 6.54 x 10(8) m(-2)). Even in water, clotrimazole formed a stable and soluble complex with heme and suppressed its aggregation. The results of optical absorption spectroscopy and electron spin resonance spectroscopy revealed that the heme-clotrimazole complex assumes a ferric low spin state (S = 1/2), having two nitrogenous ligands derived from the imidazole moieties of two clotrimazole molecules. Furthermore, we found that the formation of heme-clotrimazole complexes protects heme from degradation by reduced glutathione, and the complex damages the cell membrane more than free heme. The results described herein indicate that the antimalarial activity of clotrimazole might be due to a disturbance of hemoglobin catabolism in the malaria parasite.

Chloroquine - some open questions on its antimalarial mode of action and resistance

During the digestion of its host cell hemoglobin, large amounts of toxic ferriprotoporphyrin IX (FPIX) are generated in the intraerythrocytic malaria parasite. FPIX is detoxified either by being polymerized into hemozoin inside the food vacuole, or through its degradation by glutathione in the cytosol. Chloroquine is able to complex with FPIX, thus inhibiting both processes and thereby generating receptors for its own uptake. These leads to the accumulation of FPIX in the membrane fraction of infected cells that results in membrane permeabilization and disruption of cation homeostasis and concluded in parasite death. Several unresolved questions, such as the site of FPIX:chloroquine complex formation, the role of pH gradient in drug accumulation and resistance, the role of Pgh-1 in resistance, the mode of action of reversers and the involvement of proteins and their mutants in resistance, are discussed. Copyright 1999 Harcourt Publishers Ltd.

Inhibition of glutathione-dependent degradation of heme by chloroquine and amodiaquine as a possible basis for their antimalarial mode of action

We propose here a new and detailed model for the antimalarial action of chloroquine (CQ), based on the its ability to inhibit degradation of heme by glutathione. Heme, which is toxic to the malaria parasite, is formed when the intraerythrocytic malaria parasite ingests and digests inside its food vacuole its host cell cytosol, which consists mainly of hemoglobin. The parasite protects itself against the toxicity of heme by polymerizing some of it to insoluble hemozoin (HZ). We show here that in Plasmodium falciparum at the trophozoite stage only ca. 30% of the heme is converted into hemozoin. We suggest that nonpolymerized heme exits the food vacuole and is subsequently degraded by glutathione, as has been shown before for uninfected erythrocytes. Marginal amounts of free heme could be detected in the membrane fraction of infected cells but nowhere else. It is well established that CQ and amodiaquine (AQ) accumulate in the parasite's food vacuole and inhibit heme polymerization, thereby increasing its efflux out of the food vacuole. We found that these drugs competitively inhibit the degradation of heme by glutathione, thus allowing heme to accumulate in membranes. Incubation of intact infected cells with CQ and AQ results in a marked increase in membrane-associated heme in a dose- and time-dependent manner, and a relationship exists between membrane heme levels and the extent of parasite killing. Heme has been shown to disrupt the barrier properties of membranes and to upset ion homeostasis in CQ-treated malaria-infected cells. In agreement with the predictions of our model, increasing the cellular levels of glutathione leads to increased resistance to CQ, whereas decreasing them results in enhanced sensitivity to the drug. These results insinuate a novel mechanism of drug resistance.

High-level chloroquine resistance of Plasmodium berghei is associated with multiple drug resistance and loss of reversal by calcium antagonists

The chloroquine resistance of Plasmodium falciparum is reversed in vitro by numerous compounds, including calcium antagonists, which could enhance the accumulation of the drug in the parasite food vacuole. However, this mechanism of resistance could be insufficient when the resistance level increases. Using in vitro drug trials on strains of Plasmodium berghei displaying various chloroquine-resistance levels, we confirmed previous results obtained in vivo in the chloroquine-resistant strains of P. berghei are cross-resistant to related drugs (amodiaquine, quinine and mefloquine), the resistance levels to these drugs being related to their analogy to chloroquine. Furthermore, we showed that high-level resistant lines were associated with a loss of drug potentiation by verapamil and nicardipine in vivo, but that the reversal rates obtained in vitro are of low significance. We conclude that the parasite is able to escape the activity of these reversing agents.

Biosynthesis and maturation of the malaria aspartic hemoglobinases plasmepsins I and II

During the intraerythrocytic stage of infection, the malaria parasite Plasmodium falciparum digests most of the host cell hemoglobin. Hemoglobin degradation occurs in the acidic digestive vacuole and is essential for the survival of the parasite. Two aspartic proteases, plasmepsins I and II, have been isolated from the vacuole and shown to make the initial cleavages in the hemoglobin molecule. We have studied the biosynthesis of these two enzymes. Plasmepsin I is synthesized and processed to the mature form soon after the parasite invades the red blood cell, while plasmepsin II synthesis is delayed until later in development. Otherwise, biosynthesis of the plasmepsins is identical. The proplasmepsins are type II integral membrane proteins that are transported through the secretory pathway before cleavage to the soluble form. They are not glycosylated in vivo, despite the presence of several potential glycosylation sites. Proplasmepsin maturation appears to require acidic conditions and is reversibly inhibited by the tripeptide aldehydes N-acetyl-L-leucyl-L-leucyl-norleucinal and N-acetyl-L-leucyl-L-leucyl-methional. These compounds are known to inhibit cysteine proteases and the chymotryptic activity of proteasomes but not aspartic proteases. However, proplasmepsin processing is not blocked by other cysteine protease inhibitors, nor by the proteasome inhibitor lactacystin. Processing is also not blocked by aspartic protease inhibitors. This inhibitor profile suggests that unlike most other aspartic proteases, proplasmepsin maturation may not be autocatalytic in vivo, but instead could require the action of an unusual processing enzyme. Compounds that block processing are expected to be potent antimalarials.

Hemoglobin catabolism and iron utilization by malaria parasites

Erythrocytic malaria parasites transport large quantities of erythrocyte cytoplasm to an acidic food vacuole, where hemoglobin is degraded. Globin is hydrolysed to free amino acids, which are subsequently incorporated into parasite proteins. Potentially toxic heme moieties are polymerized to hemozoin and also probably provide necessary parasite iron. Our understanding of the precise mechanisms of hemoglobin processing is incomplete. However, it is clear that hemoglobin catabolism and related events in the malarial food vacuole are the likely targets of both important antimalarial drugs and of promising new compounds. Thus, a more precise characterization of the metabolism of hemoglobin and iron by malaria parasites should expedite the development of new modes of antimalarial chemotherapy.