Plasmodium falciparum phospholipase C hydrolyzing sphingomyelin and lysocholinephospholipids is a possible target for malaria chemotherapy

Global analysis of putative phospholipases in Plasmodium falciparum reveals an essential role of the phosphoinositide-specific phospholipase C in parasite maturation

For its replication within red blood cells, the malaria parasite depends on a highly active and regulated lipid metabolism. Enzymes involved in lipid metabolic processes such as phospholipases are, therefore, potential drug targets. Here, using reverse genetics approaches, we show that only 1 out of the 19 putative phospholipases expressed in asexual blood stages of Plasmodium falciparum is essential for proliferation in vitro, pointing toward a high level of redundancy among members of this enzyme family. Using conditional mislocalization and gene disruption techniques, we show that this essential phosphoinositide-specific phospholipase C (PI-PLC, PF3D7_1013500) has a previously unrecognized essential role during intracellular parasite maturation, long before its previously perceived role in parasite egress and invasion. Subsequent lipidomic analysis suggests that PI-PLC mediates cleavage of phosphatidylinositol bisphosphate (PIP2) in schizont-stage parasites, underlining its critical role in regulating phosphoinositide levels in the parasite. IMPORTANCE The clinical symptoms of malaria arise due to repeated rounds of replication of Plasmodium parasites within red blood cells (RBCs). Central to this is an intense period of membrane biogenesis. Generation of membranes not only requires de novo synthesis and acquisition but also the degradation of phospholipids, a function that is performed by phospholipases. In this study, we investigate the essentiality of the 19 putative phospholipase enzymes that the human malaria parasite Plasmodium falciparum expresses during its replication within RBCs. We not only show that a high level of functional redundancy exists among these enzymes but, at the same time, also identify an essential role for the phosphoinositide-specific phospholipase C in parasite development and cleavage of the phospholipid phosphatidylinositol bisphosphate.

A Plasmodium falciparum lysophospholipase regulates host fatty acid flux via parasite lipid storage to enable controlled asexual schizogony

Phospholipid metabolism is crucial for membrane biogenesis and homeostasis of Plasmodium falciparum. To generate such phospholipids, the parasite extensively scavenges, recycles, and reassembles host lipids. P. falciparum possesses an unusually large number of lysophospholipases, whose roles and importance remain to be elucidated. Here, we functionally characterize one P. falciparum lysophospholipase, PfLPL3, to reveal its key role in parasite propagation during asexual blood stages. PfLPL3 displays a dynamic localization throughout asexual stages, mainly localizing in the host-parasite interface. Inducible knockdown of PfLPL3 disrupts parasite development from trophozoites to schizont, inducing a drastic reduction in merozoite progenies. Detailed lipidomic analyses show that PfLPL3 generates fatty acids from scavenged host lipids to generate neutral lipids. These are then timely mobilized to allow schizogony and merozoite formation. We then identify inhibitors of PfLPL3 from Medicine for Malaria Venture (MMV) with potent antimalarial activity, which could also serve as pertinent chemical tools to study parasite lipid synthesis.

Host-Erythrocytic Sphingosine-1-Phosphate Regulates Plasmodium Histone Deacetylase Activity and Exhibits Epigenetic Control over Cell Death and Differentiation

The evolution of resistance to practically all antimalarial drugs poses a challenge to the current malaria elimination and eradication efforts. Given that the epigenome of Plasmodium falciparum governs several crucial parasite functions, pharmaceutical interventions with transmission-blocking potential that target epigenetic molecular markers and regulatory mechanisms are likely to encounter drug resistance. In the malaria parasite, histone deacetylases (HDACs) are essential epigenetic modulators that regulate cellular transcriptional rearrangements, notably the molecular mechanisms underlying parasite proliferation and differentiation. We establish "lipid sequestration" as a mechanism by which sphingolipids, specifically Sphingosine-1-Phosphate (S1P) (a metabolic product of Sphingosine Kinase 1 [SphK-1]), regulate epigenetic reprogramming in the parasite by interacting with, and modulating, the histone-deacetylation activity of PfHDAC-1, thereby regulating Plasmodium pathogenesis. Furthermore, we demonstrate that altering host S1P levels with PF-543, a potent and selective Sphk-1 inhibitor, dysregulates PfHDAC-1 activity, resulting in a significant increase in the global histone acetylation signals and, consequently, transcriptional modulation of genes associated with gametocytogenesis, virulence, and proliferation. Our findings point to a hitherto unrecognized functional role for host S1P-mediated sphingolipid signaling in modulating PfHDAC-1's enzymatic activity and, as a result, the parasite's dynamic genome-wide transcriptional patterns. The epigenetic regulation of parasite proliferation and sexual differentiation offers a novel approach for developing host-targeted therapeutics to combat malaria resistance to conventional regimens. IMPORTANCE Sphingolipid is an 18-carbon amino-alcohol-containing lipid with a sphingosine backbone, which when phosphorylated by sphingosine kinase 1 (SphK-1), generates sphingosine-1-phosphate (S1P), an essential lipid signaling molecule. Dysregulation of S1P function has been observed in a variety of pathologies, including severe malaria. The malaria parasite Plasmodium acquires a host S1P pool for its growth and survival. Here, we describe the molecular attuning of histone deacetylase-1 (PfHDAC-1), a crucial epigenetic modulator that contributes to the establishment of epigenetic chromatin states and parasite survival, in response to S1P binding. Our findings highlight the host lipid-mediated epigenetic regulation of malaria parasite key genes.

Analysis of sex-specific lipid metabolism of P. falciparum points to importance of sphingomyelin for gametocytogenesis

Male and female Plasmodium falciparum gametocytes are the parasite lifecycle stage responsible for transmission of malaria from the human host to mosquito vector. Not only are gametocytes able to survive in radically different host environments, but they are also precursors for male and female gametes that reproduce sexually soon after ingestion by the mosquito. Here we investigate the sex-specific lipid metabolism of gametocytes within their host red blood cell. Comparison of the male and female lipidome identifies cholesteryl esters and dihydrosphingomyelin enrichment in female gametocytes. Chemical inhibition of each of these lipid types in mature gametocytes suggests dihydrosphingomyelin synthesis but not cholesteryl ester synthesis is important for gametocyte viability. Genetic disruption of each of the two sphingomyelin synthase gene points towards sphingomyelin synthesis contributing to gametocytogenesis. This study shows that gametocytes are distinct from asexual stages, and that the lipid composition is also vastly different between male and female gametocytes, reflecting the different cellular roles these stages play. Together our results highlight the sex-specific nature of gametocyte lipid metabolism that has the potential to be targeted to block malaria transmission.

GlmS mediated knock-down of a phospholipase expedite alternate pathway to generate phosphocholine required for phosphatidylcholine synthesis in Plasmodium falciparum

Phospholipid synthesis is crucial for membrane proliferation in malaria parasites during the entire cycle in the host cell. The major phospholipid of parasite membranes, phosphatidylcholine (PC), is mainly synthesized through the Kennedy pathway. The phosphocholine required for this synthetic pathway is generated by phosphorylation of choline derived from catabolism of the lyso-phosphatidylcholine (LPC) scavenged from the host milieu. Here we have characterized a Plasmodium falciparum lysophospholipase (PfLPL20) which showed enzymatic activity on LPC substrate to generate choline. Using GFP- targeting approach, PfLPL20 was localized in vesicular structures associated with the neutral lipid storage bodies present juxtaposed to the food-vacuole. The C-terminal tagged glmS mediated inducible knock-down of PfLPL20 caused transient hindrance in the parasite development, however, the parasites were able to multiply efficiently, suggesting that PfLPL20 is not essential for the parasite. However, in PfLPL20 depleted parasites, transcript levels of enzyme of SDPM pathway (Serine Decarboxylase-Phosphoethanolamine Methyltransferase) were altered along with upregulation of phosphocholine and SAM levels; these results show upregulation of alternate pathway to generate the phosphocholine required for PC synthesis through the Kennedy pathway. Our study highlights presence of alternate pathways for lipid homeostasis/membrane-biogenesis in the parasite; these data could be useful to design future therapeutic approaches targeting phospholipid metabolism in the parasite.

Enzymatic Transition States and Drug Design

Transition state theory teaches that chemically stable mimics of enzymatic transition states will bind tightly to their cognate enzymes. Kinetic isotope effects combined with computational quantum chemistry provides enzymatic transition state information with sufficient fidelity to design transition state analogues. Examples are selected from various stages of drug development to demonstrate the application of transition state theory, inhibitor design, physicochemical characterization of transition state analogues, and their progress in drug development.

Substrate specificity of the neutral sphingomyelinase from Trypanosoma brucei

The kinetoplastid parasite Trypanosoma brucei causes African trypanosomiasis in both humans and animals. Infections place a significant health and economic burden on developing nations in sub-Saharan Africa, but few effective anti-parasitic treatments are currently available. Hence, there is an urgent need to identify new leads for drug development. The T. brucei neutral sphingomyelinase (TbnSMase) was previously established as essential to parasite survival, consequently being identified as a potential drug target. This enzyme may catalyse the single route to sphingolipid catabolism outside the T. brucei lysosome. To obtain new insight into parasite sphingolipid catabolism, the substrate specificity of TbnSMase was investigated using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Recombinant TbnSMase was shown to degrade sphingomyelin, inositol-phosphoceramide and ethanolamine-phosphoceramide sphingolipid substrates, consistent with the sphingolipid complement of the parasites. TbnSMase also catabolized ceramide-1-phosphate, but was inactive towards sphingosine-1-phosphate. The broad-range specificity of this enzyme towards sphingolipid species is a unique feature of TbnSMase. Additionally, ESI-MS/MS analysis revealed previously uncharacterized activity towards lyso-phosphatidylcholine despite the enzyme's inability to degrade phosphatidylcholine. Collectively, these data underline the enzyme's importance in choline homoeostasis and the turnover of sphingolipids in T. brucei.

Role of phospholipid synthesis in the development and differentiation of malaria parasites in the blood

The life cycle of malaria parasites in both their mammalian host and mosquito vector consists of multiple developmental stages that ensure proper replication and progeny survival. The transition between these stages is fueled by nutrients scavenged from the host and fed into specialized metabolic pathways of the parasite. One such pathway is used by Plasmodium falciparum, which causes the most severe form of human malaria, to synthesize its major phospholipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. Much is known about the enzymes involved in the synthesis of these phospholipids, and recent advances in genetic engineering, single-cell RNA-Seq analyses, and drug screening have provided new perspectives on the importance of some of these enzymes in parasite development and sexual differentiation and have identified targets for the development of new antimalarial drugs. This Minireview focuses on two phospholipid biosynthesis enzymes of P. falciparum that catalyze phosphoethanolamine transmethylation (PfPMT) and phosphatidylserine decarboxylation (PfPSD) during the blood stages of the parasite. We also discuss our current understanding of the biochemical, structural, and biological functions of these enzymes and highlight efforts to use them as antimalarial drug targets.

Effect of antiprotozoal molecules on hypnospores of Perkinsus spp. parasite

Perkinsus protozoan parasites have been associated with high mortality of bivalves worldwide, including Brazil. The use of antiproliferative drugs to treat the Perkinsosis is an unusual prophylactic strategy. However, because of their environment impact it could be used to control parasite proliferation in closed system, such as hatchery. This study evaluated the anti-Perkinsus activity potential of synthesized and commercial compounds. Viability of hypnospores of Perkinsus spp. was assessed in vitro. Cells were incubated with three 2-amino-thiophene (6AMD, 6CN, 5CN) and one acylhydrazone derivatives (AMZ-DCL), at the concentrations of 31.25; 62.5; 125; 250 and 500 μM and one commercial chlorinated phenoxy phenol derivative, triclosan (2, 5, 10 and 20 μM), for 24-48 h. Two synthetic molecules (6CN and AMZ-DCL) caused a significant decline (38 and 39%, respectively) in hypnospores viability, at the highest concentration (500 μM), after 48 h. Triclosan was the most cytotoxic compound, causing 100% of mortality at 20 μM after 24 h and at 10 μM after 48 h. Cytotoxic effects of the compounds 6CN, AMZ-DCL, and triclosan were investigated by measuring parasite's zoosporulation, morphological changes and metabolic activities (esterase activity, production of reactive oxygen species and lipid content). Results showed that zoosporulation occurred in few cell. Triclosan caused changes in the morphology of hypnospores. The 6CN and AMZ-DCL did not alter the metabolic activities studied whilst Triclosan significantly increased the production of reactive oxygen species and changed the amount and distribution of lipids in the hypnospores. These results suggest that three compounds had potential to be used as antiprotozoal drugs, although further investigation of their mechanism of action must be enlightened.

Immune evasion of Plasmodium falciparum by RIFIN via inhibitory receptors

Malaria is among the most serious infectious diseases affecting humans, accounting for approximately half a million deaths annually¹. Plasmodium falciparum is the causative agent of most life-threatening malaria cases. Acquired immunity to malaria is inefficient, even after repeated exposures to P. falciparum²; immune regulatory mechanisms employed by P. falciparum remain largely unclear. Here, we show that P. falciparum uses immune inhibitory receptors for immune evasion. RIFINs, products of a polymorphic multigene family comprising approximately 150–200 genes per parasite genome³, are expressed on the surface of infected erythrocytes. We found that a subset of RIFINs binds to either leucocyte immunoglobulin-like receptor B1 (LILRB1) or leucocyte-associated immunoglobulin-like receptor 1 (LAIR1). LILRB1-binding RIFINs inhibited activation of LILRB1-expressing B cells and NK cells. Furthermore, interactions between LILRB1 and P. falciparum-infected erythrocytes isolated from malaria patients were associated with severe malaria, although an extended study with larger sample sizes is required to confirm the findings. These results suggest that P. falciparum has acquired multiple RIFINs to evade the host immune system by targeting immune inhibitory receptors.

Phospholipases during membrane dynamics in malaria parasites

Plasmodium parasites, the causative agents of malaria, display a well-regulated lipid metabolism required to ensure their survival in the human host as well as in the mosquito vector. The fine-tuning of lipid metabolic pathways is particularly important for the parasites during the rapid erythrocytic infection cycles, and thus enzymes involved in lipid metabolic processes represent prime targets for malaria chemotherapeutics. While plasmodial enzymes involved in lipid synthesis and acquisition have been studied in the past, to date not much is known about the roles of phospholipases for proliferation and transmission of the malaria parasite. These phospholipid-hydrolyzing esterases are crucial for membrane dynamics during host cell infection and egress by the parasite as well as for replication and cell signaling, and thus they are considered important virulence factors. In this review, we provide a comprehensive bioinformatic analysis of plasmodial phospholipases identified to date. We further summarize previous findings on the lipid metabolism of Plasmodium, highlight the roles of phospholipases during parasite life-cycle progression, and discuss the plasmodial phospholipases as potential targets for malaria therapy.

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.

Novel Plasmodium falciparum metabolic network reconstruction identifies shifts associated with clinical antimalarial resistance

Background

Malaria remains a major public health burden and resistance has emerged to every antimalarial on the market, including the frontline drug, artemisinin. Our limited understanding of Plasmodium biology hinders the elucidation of resistance mechanisms. In this regard, systems biology approaches can facilitate the integration of existing experimental knowledge and further understanding of these mechanisms.

Results

Here, we developed a novel genome-scale metabolic network reconstruction, iPfal17, of the asexual blood-stage P. falciparum parasite to expand our understanding of metabolic changes that support resistance. We identified 11 metabolic tasks to evaluate iPfal17 performance. Flux balance analysis and simulation of gene knockouts and enzyme inhibition predict candidate drug targets unique to resistant parasites. Moreover, integration of clinical parasite transcriptomes into the iPfal17 reconstruction reveals patterns associated with antimalarial resistance. These results predict that artemisinin sensitive and resistant parasites differentially utilize scavenging and biosynthetic pathways for multiple essential metabolites, including folate and polyamines. Our findings are consistent with experimental literature, while generating novel hypotheses about artemisinin resistance and parasite biology. We detect evidence that resistant parasites maintain greater metabolic flexibility, perhaps representing an incomplete transition to the metabolic state most appropriate for nutrient-rich blood.

Conclusion

Using this systems biology approach, we identify metabolic shifts that arise with or in support of the resistant phenotype. This perspective allows us to more productively analyze and interpret clinical expression data for the identification of candidate drug targets for the treatment of resistant parasites.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3905-1) contains supplementary material, which is available to authorized users.

Inhibition of acid sphingomyelinase by tricyclic antidepressants and analogons

Amitriptyline, a tricyclic antidepressant, has been used in the clinic to treat a number of disorders, in particular major depression and neuropathic pain. In the 1970s the ability of tricyclic antidepressants to inhibit acid sphingomyelinase (ASM) was discovered. The enzyme ASM catalyzes the hydrolysis of sphingomyelin to ceramide. ASM and ceramide were shown to play a crucial role in a wide range of diseases, including cancer, cystic fibrosis, diabetes, Alzheimer's disease, and major depression, as well as viral (e.g., measles virus) and bacterial (e.g., Staphylococcus aureus, Pseudomonas aeruginosa) infections. Ceramide molecules may act in these diseases by the alteration of membrane biophysics, the self-association of ceramide molecules within the cell membrane and the ultimate formation of larger ceramide-enriched membrane domains/platforms. These domains were shown to serve the clustering of certain receptors such as CD95 and may also act in the above named diseases. The potential to block the generation of ceramide by inhibiting the ASM has opened up new therapeutic approaches for the treatment of these conditions. Since amitriptyline is one of the longest used clinical drugs and side effects are well studied, it could potentially become a cheap and easily accessible medication for patients suffering from these diseases. In this review, we aim to provide an overview of current in vitro and in vivo studies and clinical trials utilizing amitriptyline to inhibit ASM and contemplate possible future applications of the drug.

Identification of Sphingomyelinase on the Surface of Chlamydia pneumoniae: Possible Role in the Entry into Its Host Cells

We have recently suggested a novel mechanism, autoendocytosis, for the entry of certain microbes into their hosts, with a key role played by the sphingomyelinase-catalyzed topical conversion of sphingomyelin to ceramide, the differences in the biophysical properties of these two lipids providing the driving force. The only requirement for such microbes to utilize this mechanism is that they should have a catalytically active SMase on their outer surface while the target cells should expose sphingomyelin in the external leaflet of their plasma membrane. In pursuit of possible microbial candidates, which could utilize this putative mechanism, we conducted a sequence similarity search for SMase. Because of the intriguing cellular and biochemical characteristics of the poorly understood entry of Chlamydia into its host cells these microbes were of particular interest. SMase activity was measured in vitro from isolated C. pneumoniae elementary bodies (EB) and in the lysate from E. coli cells transfected with a plasmid expressing CPn0300 protein having sequence similarity to SMase. Finally, pretreatment of host cells with exogenous SMase resulting in loss plasma membrane sphingomyelin attenuated attachment of EB.

Lipid synthesis in protozoan parasites: a comparison between kinetoplastids and apicomplexans

Lipid metabolism is of crucial importance for pathogens. Lipids serve as cellular building blocks, signalling molecules, energy stores, posttranslational modifiers, and pathogenesis factors. Parasites rely on a complex system of uptake and synthesis mechanisms to satisfy their lipid needs. The parameters of this system change dramatically as the parasite transits through the various stages of its life cycle. Here we discuss the tremendous recent advances that have been made in the understanding of the synthesis and uptake pathways for fatty acids and phospholipids in apicomplexan and kinetoplastid parasites, including Plasmodium, Toxoplasma, Cryptosporidium, Trypanosoma and Leishmania. Lipid synthesis differs in significant ways between parasites from both phyla and the human host. Parasites have acquired novel pathways through endosymbiosis, as in the case of the apicoplast, have dramatically reshaped substrate and product profiles, and have evolved specialized lipids to interact with or manipulate the host. These differences potentially provide opportunities for drug development. We outline the lipid pathways for key species in detail as they progress through the developmental cycle and highlight those that are of particular importance to the biology of the pathogens and/or are the most promising targets for parasite-specific treatment.

Host-lipidome as a potential target of protozoan parasites

Host-lipidome caters parasite interaction by acting as first line of recognition, attachment on the cell surface, intracellular trafficking, and survival of the parasite inside the host cell. Here, we summarize how protozoan parasites exploit host-lipidome by suppressing, augmenting, engulfing, remodeling and metabolizing lipids to achieve successful parasitism inside the host.

Characterization of a glycerophosphodiesterase with an unusual tripartite distribution and an important role in the asexual blood stages of Plasmodium falciparum

Catabolism of glycerophospholipids during the rapid growth of the asexual intraerythrocytic malaria parasite may contribute to membrane recycling and the acquisition of lipid biosynthetic precursors from the host. To better understand the scope of lipid catabolism in Plasmodium falciparum, we have characterized a malarial homolog of bacterial glycerophosphodiesterases. These enzymes catalyze the hydrolysis of glycerophosphodiesterases that are generated by phospholipase-catalyzed removal of the two acyl groups from glycerophospholipids. The P. falciparum glycerophosphodiesterase (PfGDPD) exhibits an unusual tripartite distribution during the asexual blood stage with pools of enzyme in the parasitophorous vacuole, food vacuole and cytosol. Efforts to disrupt the chromosomal PfGDPD coding sequence were unsuccessful, which implies that the enzyme is important for efficient parasite growth. Tagging of the endogenous pool of PfGDPD with a conditional aggregation domain partially perturbed the distribution of the enzyme in the parasitophorous vacuole but had no discernable effect on growth in culture. Kinetic characterization of the hydrolysis of glycerophosphocholine by recombinant PfGDPD, an Mg(2+)-dependent enzyme, yielded steady-state parameters that were comparable to those of a homologous bacterial glycerophosphodiesterase. Together, these results suggest a physiological role for PfGDPD in glycerophospholipid catabolism in multiple subcellular compartments. Possibilities for what this role might be are discussed.

Acyclic immucillin phosphonates: second-generation inhibitors of Plasmodium falciparum hypoxanthine-guanine-xanthine phosphoribosyltransferase

Plasmodium falciparum, the primary cause of deaths from malaria, is a purine auxotroph and relies on hypoxanthine salvage from the host purine pool. Purine starvation as an antimalarial target has been validated by inhibition of purine nucleoside phosphorylase. Hypoxanthine depletion kills Plasmodium falciparum in cell culture and in Aotus monkey infections. Hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) from P. falciparum is required for hypoxanthine salvage by forming inosine 5'-monophosphate, a branchpoint for all purine nucleotide synthesis in the parasite. Here, we present a class of HGXPRT inhibitors, the acyclic immucillin phosphonates (AIPs), and cell permeable AIP prodrugs. The AIPs are simple, potent, selective, and biologically stable inhibitors. The AIP prodrugs block proliferation of cultured parasites by inhibiting the incorporation of hypoxanthine into the parasite nucleotide pool and validates HGXPRT as a target in malaria.

Limonoid compounds inhibit sphingomyelin biosynthesis by preventing CERT protein-dependent extraction of ceramides from the endoplasmic reticulum

To identify novel inhibitors of sphingomyelin (SM) metabolism, a new and selective high throughput microscopy-based screening based on the toxicity of the SM-specific toxin, lysenin, was developed. Out of a library of 2011 natural compounds, the limonoid, 3-chloro-8β-hydroxycarapin-3,8-hemiacetal (CHC), rendered cells resistant to lysenin by decreasing cell surface SM. CHC treatment selectively inhibited the de novo biosynthesis of SM without affecting glycolipid and glycerophospholipid biosynthesis. Pretreatment with brefeldin A abolished the limonoid-induced inhibition of SM synthesis suggesting that the transport of ceramide (Cer) from the endoplasmic reticulum to the Golgi apparatus is affected. Unlike the Cer transporter (CERT) inhibitor HPA-12, CHC did not change the transport of a fluorescent short chain Cer analog to the Golgi apparatus or the formation of fluorescent and short chain SM from the corresponding Cer. Nevertheless, CHC inhibited the conversion of de novo synthesized Cer to SM. We show that CHC specifically inhibited the CERT-mediated extraction of Cer from the endoplasmic reticulum membranes in vitro. Subsequent biochemical screening of 21 limonoids revealed that some of them, such as 8β-hydroxycarapin-3,8-hemiacetal and gedunin, which exhibits anti-cancer activity, inhibited SM biosynthesis and CERT-mediated extraction of Cer from membranes. Model membrane studies suggest that 8β-hydroxycarapin-3,8-hemiacetal reduced the miscibility of Cer with membrane lipids and thus induced the formation of Cer-rich membrane domains. Our study shows that certain limonoids are novel inhibitors of SM biosynthesis and suggests that some biological activities of these limonoids are related to their effect on the ceramide metabolism.

PG12, a phospholipid analog with potent antimalarial activity, inhibits Plasmodium falciparum CTP:phosphocholine cytidylyltransferase activity

In the human malaria parasite Plasmodium falciparum, the synthesis of the major and essential membrane phospholipid, phosphatidylcholine, occurs via the CDP-choline and the serine decarboxylase phosphoethanolamine methylation (SDPM) pathways, which are fueled by host choline, serine, and fatty acids. Both pathways share the final two steps catalyzed by two essential enzymes, P. falciparum CTP:phosphocholine cytidylyltransferase (PfCCT) and choline-phosphate transferase (PfCEPT). We identified a novel class of phospholipid mimetics, which inhibit the growth of P. falciparum as well as Leishmania and Trypanosoma species. Metabolic analyses showed that one of these compounds, PG12, specifically blocks phosphatidylcholine biosynthesis from both the CDP-choline and SDPM pathways via inhibition of PfCCT. In vitro studies using recombinant PfCCT showed a dose-dependent inhibition of the enzyme by PG12. The potent antimalarial of this compound, its low cytotoxicity profile, and its established mode of action make it an excellent lead to advance for further drug development and efficacy in vivo.

Reconstruction and flux-balance analysis of the Plasmodium falciparum metabolic network

Genome-scale metabolic reconstructions can serve as important tools for hypothesis generation and high-throughput data integration. Here, we present a metabolic network reconstruction and flux-balance analysis (FBA) of Plasmodium falciparum, the primary agent of malaria. The compartmentalized metabolic network accounts for 1001 reactions and 616 metabolites. Enzyme-gene associations were established for 366 genes and 75% of all enzymatic reactions. Compared with other microbes, the P. falciparum metabolic network contains a relatively high number of essential genes, suggesting little redundancy of the parasite metabolism. The model was able to reproduce phenotypes of experimental gene knockout and drug inhibition assays with up to 90% accuracy. Moreover, using constraints based on gene-expression data, the model was able to predict the direction of concentration changes for external metabolites with 70% accuracy. Using FBA of the reconstructed network, we identified 40 enzymatic drug targets (i.e. in silico essential genes), with no or very low sequence identity to human proteins. To demonstrate that the model can be used to make clinically relevant predictions, we experimentally tested one of the identified drug targets, nicotinate mononucleotide adenylyltransferase, using a recently discovered small-molecule inhibitor.

The essential neutral sphingomyelinase is involved in the trafficking of the variant surface glycoprotein in the bloodstream form of Trypanosoma brucei

Sphingomyelin is the main sphingolipid in Trypanosoma brucei, the causative agent of African sleeping sickness. In vitro and in vivo characterization of the T. brucei neutral sphingomyelinase demonstrates that it is directly involved in sphingomyelin catabolism. Gene knockout studies in the bloodstream form of the parasite indicate that the neutral sphingomyelinase is essential for growth and survival, thus highlighting that the de novo biosynthesis of ceramide is unable to compensate for the loss of sphingomyelin catabolism. The phenotype of the conditional knockout has given new insights into the highly active endocytic and exocytic pathways in the bloodstream form of T. brucei. Hence, the formation of ceramide in the endoplasmic reticulum affects post-Golgi sorting and rate of deposition of newly synthesized GPI-anchored variant surface glycoprotein on the cell surface. This directly influences the corresponding rate of endocytosis, via the recycling endosomes, of pre-existing cell surface variant surface glycoprotein. The trypanosomes use this coupled endocytic and exocytic mechanism to maintain the cell density of its crucial variant surface glycoprotein protective coat. TbnSMase is therefore genetically validated as a drug target against African trypanosomes, and suggests that interfering with the endocytic transport of variant surface glycoprotein is a highly desirable strategy for drug development against African trypanosomasis.

Estimating novel potential drug targets of Plasmodium falciparum by analysing the metabolic network of knock-out strains in silico

Malaria is one of the world's most common and serious diseases causing death of about 3 million people each year. Its most severe occurrence is caused by the protozoan Plasmodium falciparum. Biomedical research could enable treating the disease by effectively and specifically targeting essential enzymes of this parasite. However, the parasite has developed resistance to existing drugs making it indispensable to discover new drugs. We have established a simple computational tool which analyses the topology of the metabolic network of P. falciparum to identify essential enzymes as possible drug targets. We investigated the essentiality of a reaction in the metabolic network by deleting (knocking-out) such a reaction in silico. The algorithm selected neighbouring compounds of the investigated reaction that had to be produced by alternative biochemical pathways. Using breadth first searches, we tested qualitatively if these products could be generated by reactions that serve as potential deviations of the metabolic flux. With this we identified 70 essential reactions. Our results were compared with a comprehensive list of 38 targets of approved malaria drugs. When combining our approach with an in silico analysis performed recently [Yeh, I., Hanekamp, T., Tsoka, S., Karp, P.D., Altman, R.B., 2004. Computational analysis of Plasmodium falciparum metabolism: organizing genomic information to facilitate drug discovery. Genome Res. 14, 917-924] we could improve the precision of the prediction results. Finally we present a refined list of 22 new potential candidate targets for P. falciparum, half of which have reasonable evidence to be valid targets against micro-organisms and cancer.

Ceramide-enriched membrane domains in infectious biology and development

Ceramide has been shown to be critically involved in multiple biological processes, for instance induction of apoptosis after ligation of death receptors or application of gamma-irradiation or UV-A light, respectively, regulation of cell differentiation, control of tumor cell growth, infection of mammalian cells with pathogenic bacteria and viruses or the control of embryo and organ development to name a few examples. Ceramide molecules form distinct large domains in the cell membrane, which may serve to re-organize cellular receptors and signalling molecules. Thus, in many conditions, ceramide may be involved in the spatial and temporal organisation of specific signalling pathways explaining the pleiotrophic effects of this lipid. Here, we focus on the role of ceramide and ceramide-enriched membrane domains, respectively, in bacterial infections, in particular of the lung, and sepsis. We describe the role of ceramide for infections with Neisseriae gonorhoeae, Staphylococcus aureus and Pseudomonas aeruginosa. Finally, we discuss newly emerging aspects of the cellular function of ceramide, i.e. its role in germ line and embryo development.

Antimalarial drugs - host targets (re)visited

Every year, forty percent of the world population is at risk of contracting malaria. Hopes for the erradication of this disease during the 20th century were dashed by the ability of Plasmodium falciparum, its most deadly causative agent, to develop resistance to available drugs. Efforts to produce an effective vaccine have so far been unsuccessful, enhancing the need to develop novel antimalarial drugs. In this review, we summarize our knowledge concerning existing antimalarials, mechanisms of drug-resistance development, the use of drug combination strategies and the quest for novel anti-plasmodial compounds. We emphasize the potential role of host genes and molecules as novel targets for newly developed drugs. Recent results from our laboratory have shown Hepatocyte Growth Factor/MET signaling to be essential for the establishment of infection in hepatocytes. We discuss the potential use of this pathway in the prophylaxis of malaria infection.

Membrane rafts in host-pathogen interactions

Central elements in the infection of mammalian cells with viral, bacterial and parasitic pathogens include the adhesion of the pathogen to surface receptors of the cell, recruitment of additional receptor proteins to the infection-site, a re-organization of the membrane and, in particular, the intracellular signalosome. Internalization of the pathogen results in the formation of a phagosome that is supposed to fuse with lysosomes to form phagolysosomes, which serve the degradation of the pathogen, an event actively prevented by some pathogens. In summary, these changes in the infected cell permit pathogens to trigger apoptosis (for instance of macrophages paralysing the initial immune response), to invade the cell and/or to survive in the cell, but they also serve the mammalian cell to defeat the infection, for instance by activation of transcription factors and the release of cytokines. Distinct membrane domains in the plasma membrane and intracellular vesicles that are mainly composed of sphingolipids and cholesterol or enriched with the sphingolipid ceramide, are critically involved in all of these events occurring during the infection. These membrane structures are therefore very attractive targets for novel drugs to interfere with bacterial, viral and parasitic infections.

Physiological and pathophysiological aspects of ceramide

Activation of cells by receptor- and nonreceptor-mediated stimuli not only requires a change in the activity of signaling proteins but also requires a reorganization of the topology of the signalosom in the cell. The cell membrane contains distinct domains, rafts that serve the spatial organization of signaling molecules in the cell. Many receptors or stress stimuli transform rafts by the generation of ceramide. These stimuli activate the acid sphingomyelinase and induce a translocation of this enzyme onto the extracellular leaflet of the cell membrane. Surface acid sphingomyelinase generates ceramide that serves to fuse small rafts and to form large ceramide-enriched membrane platforms. These platforms cluster receptor molecules, recruit intracellular signaling molecules to aggregated receptors, and seem to exclude inhibitory signaling factors. Thus ceramide-enriched membrane platforms do not seem to be part of a specific signaling pathway but may facilitate and amplify the specific signaling elicited by the cognate stimulus. This general function may enable these membrane domains to be critically involved in the induction of apoptosis by death receptors and stress stimuli, bacterial and viral infections of mammalian cells, and the regulation of cardiovascular functions.

Enzymatic production of ceramide from sphingomyelin

Due to its major role in maintaining the water-retaining properties of the epidermis, ceramide is of great commercial potentials in cosmetic and pharmaceutical industries such as in hair and skin care products. Chemical synthesis of ceramide is a costly process, and developments of alternative cost-efficient production methods are of great interest. Present study was the first attempt to perform a systematic study on the production of ceramide through enzymatic hydrolysis of sphingomyelin. Sphingomyelin hydrolysis proved to be more efficient in two-phase (water:organic solvent) system than in one-phase (water-saturated organic solvent) system. Among the screened phospholipase C, the Clostridium perfringens enzyme had the highest sphingomyelin conversion rate, with very small temperature dependence. Addition of ethanol to the system markedly enhanced the rate of ceramide formation, and a mixture of ethylacetate:hexane (50:50) was the best organic solvent tested. Other factors such as (NH(4))(2)SO(4), NaCl and CaCl(2) were also tested but excluded for further consideration. On the basis of the initial experiments, the reaction system was optimized using response surface methodology including five factors (enzyme amount, water amount, ethanol amount, reaction time and the hexane ratio of organic solvent). Water content and enzyme amount was shown to have the most significant influence on the hydrolysis reaction in the fitted quadratic model. The efficiency of sphingomyelin hydrolysis was dramatically improved through system evaluation and optimization, with the optimal conditions at 75 min reaction time, 3 Uml(-1) enzyme amount, 6% water amount, 1.8% ethanol amount and 46% hexane in ethylacetate.

Lipid biology of Apicomplexa: perspectives for new drug targets, particularly for Toxoplasma gondii

Development of effective therapies for intracellular eukaryotic pathogens is a serious challenge, given the protected location of these pathogens and the similarity of their biology to that of the host. Identifying cellular processes that are unique to the parasite is therefore a crucial step towards defining appropriate drug targets. In the case of the apicomplexan parasite Toxoplasma gondii, the need to find alternative treatments is imperative because of the poor tolerability and frequent side-effects associated with existing therapeutic strategies. The discovery that the parasite uses lipid synthetic pathways which are different from, or absent in, the mammalian host is now driving a renewed interest in T. gondii lipid biology. Recent achievements in this field are promising and suggest that the elucidation of lipid pathways will provide new opportunities for designing potent antiparasitic strategies.

Crystal structure of SmcL, a bacterial neutral sphingomyelinase C from Listeria

Sphingomyelinases C are enzymes that catalyze the hydrolysis of sphingomyelin in biological membranes to ceramide and phosphorylcholine. Various pathogenic bacteria produce secreted neutral sphingomyelinases C that act as membrane-damaging virulence factors. Mammalian neutral sphingomyelinases C, which display sequence homology to the bacterial enzymes, are involved in sphingolipid metabolism and signaling. This article describes the first structure to be determined for a member of the neutral sphingomyelinase C family, SmcL, from the intracellular bacterial pathogen Listeria ivanovii. The structure has been refined to 1.9-A resolution with phases derived by single isomorphous replacement with anomalous scattering techniques from a single iridium derivative. SmcL adopts a DNase I-like fold, and is the first member of this protein superfamily to have its structure determined that acts as a phospholipase. The structure reveals several unique features that adapt the protein to its phospholipid substrate. These include large hydrophobic beta-hairpin and hydrophobic loops surrounding the active site that may bind and penetrate the lipid bilayer to position sphingomyelin in a catalytically competent position. The structure also provides insight into the proposed general base/acid catalytic mechanism, in which His-325 and His-185 play key roles.

Investigating serum factors promoting erythrocytic growth of Plasmodium falciparum

The elucidation of factors inducing the growth of Plasmodium falciparum can provide critical information about the developmental mechanisms of this parasite and open the way to search for novel targets for malaria chemotherapy. The ability of components of a growth-promoting factor derived from bovine serum and various related substances to sustain growth of P. falciparum was characterized. A simple total lipid fraction (GFS-C) containing non-esterified fatty acids (NEFAs) as essential factors was noted to promote the parasite's growth. Various proteins from a variety of animals were tested, indicating the importance not only of GFS-C, but also of specific proteins, such as bovine and human albumin, in the parasite growth. Several combinations of the NEFAs tested sustained low parasite growth. Among various phospholipids and lysophospholipids tested, lysophosphatidylcholine containing C-18 unsaturated fatty acids was found to sustain the complete development of the parasite in the presence of bovine albumin. Several other lysophospholipids can partially support growth of P. falciparum.

Developmental-stage-specific triacylglycerol biosynthesis, degradation and trafficking as lipid bodies in Plasmodium falciparum-infected erythrocytes

Triacylglycerol (TAG) serves as a major energy storage molecule in eukaryotes. In Plasmodium, however, this established function of TAG appears unlikely, despite detecting previously considerable amount of TAG associated with intraerythrocytic parasites, because plasmodial cells have very little capacity to oxidize fatty acids. Thus, it is plausible that TAG and its biosynthesis in Plasmodium have other functions. As a first step in understanding the biological significance of TAG and its biosynthesis to the intraerythrocytic proliferation of Plasmodium falciparum, we performed detailed characterization of TAG metabolism and trafficking in parasitized erythrocyte. Metabolic labeling using radiolabeled-oleic and palmitic acids in association with serum albumin, which have been shown to be among the serum essential factors for intraerythrocytic proliferation of P. falciparum, revealed that accumulation of TAG was strikingly pronounced from trophozoite to schizont, whereas TAG degradation became active from schizont to segmented schizont; the consequent products, free fatty acids, were released into the medium during schizont rupture and/or merozoite release. These results were further supported by visualization of lipid bodies through immunofluorescence and electron microscopy. At the schizont stages, there is some evidence that the lipid bodies are partly localized in the parasitophorous vacuole. Interestingly, the discrete formation and/or trafficking of lipid bodies are inhibited by brefeldin A and trifluoperazine. Inhibition by trifluoperazine hints at least that a de novo TAG biosynthetic pathway via phosphatidic acid contributes to lipid body formation. Indeed, biochemical analysis reveals a higher activity of acyl-CoA:diacylglycerol acyltransferase, the principal enzyme in the sn-glycerol-3-phosphate pathway for TAG synthesis, at trophozoite and schizont stages. Together, these results establish that TAG metabolism and trafficking in P. falciparum-infected erythrocyte occurs in a stage-specific manner during the intraerythrocytic cycle and we propose that these unique and dynamic cellular events participate during schizont rupture and/or merozoite release.

Anti-Plasmodium activity of ceramide analogs

Background

Sphingolipids are key molecules regulating many essential functions in eukaryotic cells and ceramide plays a central role in sphingolipid metabolism. A sphingolipid metabolism occurs in the intraerythrocytic stages of Plasmodium falciparum and is associated with essential biological processes. It constitutes an attractive and potential target for the development of new antimalarial drugs.

Methods

The anti-Plasmodium activity of a series of ceramide analogs containing different linkages (amide, methylene or thiourea linkages) between the fatty acid part of ceramide and the sphingoid core was investigated in culture and compared to the sphingolipid analog PPMP (d,1-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol). This analog is known to inhibit the parasite sphingomyelin synthase activity and block parasite development by preventing the formation of the tubovesicular network that extends from the parasitophorous vacuole to the red cell membrane and delivers essential extracellular nutrients to the parasite.

Results

Analogs containing methylene linkage showed a considerably higher anti-Plasmodium activity (IC₅₀ in the low nanomolar range) than PPMP and their counterparts with a natural amide linkage (IC₅₀ in the micromolar range). The methylene analogs blocked irreversibly P. falciparum development leading to parasite eradication in contrast to PPMP whose effect is cytostatic. A high sensitivity of action towards the parasite was observed when compared to their effect on the human MRC-5 cell growth. The toxicity towards parasites did not correlate with the inhibition by methylene analogs of the parasite sphingomyelin synthase activity and the tubovesicular network formation, indicating that this enzyme is not their primary target.

Conclusions

It has been shown that ceramide analogs were potent inhibitors of P. falciparum growth in culture. Interestingly, the nature of the linkage between the fatty acid part and the sphingoid core considerably influences the antiplasmodial activity and the selectivity of analogs when compared to their cytotoxicity on mammalian cells. By comparison with their inhibitory effect on cancer cell growth, the ceramide analogs might inhibit P. falciparum growth through modulation of the endogenous ceramide level.

Evidence that Plasmodium falciparum diacylglycerol acyltransferase is essential for intraerythrocytic proliferation

In triacylglycerol (TAG)-accumulating organisms, the physiological roles of diacylglycerol acyltransferase (DGAT), a principal enzyme in the major biosynthetic pathway for TAG, appear to be diverse. Apicomplexan parasite, Plasmodium falciparum, shows unique features in TAG metabolism and trafficking during intraerythrocytic development, and unlike most eukaryotes, only one open reading frame (ORF) encoding a candidate DGAT could be found in its genome. However, whether this candidate ORF encodes P. falciparum DGAT and its physiological relevance have not been assessed. Here, we demonstrate that the ORF is transcribed as a approximately 3.6 kb single mRNA throughout intraerythrocytic development, markedly elevated at trophozoite, schizont, and segmented schizont, and indeed encodes a protein exhibiting DGAT activity. Further, we provide evidence that the parasite in which the ORF was disrupted via double crossover recombination cannot be enriched, implying a fundamental role of PfDGAT in intraerythrocytic proliferation.

Involvement of ceramide in hyperosmotic shock-induced death of erythrocytes

Erythrocytes lack nuclei and mitochondria, the organelles important for apoptosis of nucleated cells. However, following increase of cytosolic Ca(2+) activity, erythrocytes undergo cell shrinkage, cell membrane blebbing and breakdown of phosphatidylserine asymmetry, all features typical for apoptosis in nucleated cells. The same events are observed following osmotic shock, an effect mediated in part by activation of Ca(2+)-permeable cation channels. However, erythrocyte death following osmotic shock is blunted but not prevented in the absence of extracellular Ca(2+) pointing to additional mechanisms. As shown in this study, osmotic shock (950 mOsm) triggers sphingomyelin breakdown and formation of ceramide. The stimulation of annexin binding following osmotic shock is mimicked by addition of ceramide or purified sphingomyelinase and significantly blunted by genetic (aSM-deficient mice) or pharmacologic (50 microM 3,4-dichloroisocoumarin) knockout of sphingomyelinase. The effect of ceramide is blunted but not abolished in the absence of Ca(2+). Conversely, osmotic shock-induced annexin binding is potentiated in the presence of sublethal concentrations of ceramide. In conclusion, ceramide and Ca(2+) entry through cation channels concert to trigger erythrocyte death during osmotic shock.

Glycosphingolipids in Plasmodium falciparum. Presence of an active glucosylceramide synthase

Malaria remains a major health problem especially in tropical and subtropical regions of the world, and therefore developing new antimalarial drugs constitutes an urgent challenge. Lipid metabolism has been attracting a lot of attention as an application for malarial chemotherapeutic purposes in recent years. However, little is known about glycosphingolipid biosynthesis in Plasmodium falciparum. In this report we describe for the first time the presence of an active glucosylceramide synthase in the intraerythrocytic stages of the parasite. Two different experiments, using UDP-[(14)C]glucose as donor with ceramides as acceptors, or UDP-glucose as donor and fluorescent ceramides as acceptors, were performed. In both cases, we found that the parasitic enzyme was able to glycosylate only dihydroceramide. The enzyme activity could be inhibited in vitro with low concentrations of d,l-threo-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP). In addition, de novo biosynthesis of glycosphingolipids was shown by metabolic incorporation of [(14)C]palmitic acid and [(14)C]glucose in the three intraerythrocytic stages of the parasite. The structure of the ceramide, monohexosylceramide, trihexosylceramide and tetrahexosylceramide fractions was analysed by UV-MALDI-TOF mass spectrometry. When PPMP was added to parasite cultures, a correlation between arrest of parasite growth and inhibition of glycosphingolipid biosynthesis was observed. The particular substrate specificity of the malarial glucosylceramide synthase must be added to the already known unique and amazing features of P. falciparum lipid metabolism; therefore this enzyme might represent a new attractive target for malarial chemotherapy.

Computational analysis of Plasmodium falciparum metabolism: organizing genomic information to facilitate drug discovery

Identification of novel targets for the development of more effective antimalarial drugs and vaccines is a primary goal of the Plasmodium genome project. However, deciding which gene products are ideal drug/vaccine targets remains a difficult task. Currently, a systematic disruption of every single gene in Plasmodium is technically challenging. Hence, we have developed a computational approach to prioritize potential targets. A pathway/genome database (PGDB) integrates pathway information with information about the complete genome of an organism. We have constructed PlasmoCyc, a PGDB for Plasmodium falciparum 3D7, using its annotated genomic sequence. In addition to the annotations provided in the genome database, we add 956 additional annotations to proteins annotated as "hypothetical" using the GeneQuiz annotation system. We apply a novel computational algorithm to PlasmoCyc to identify 216 "chokepoint enzymes." All three clinically validated drug targets are chokepoint enzymes. A total of 87.5% of proposed drug targets with biological evidence in the literature are chokepoint reactions. Therefore, identifying chokepoint enzymes represents one systematic way to identify potential metabolic drug targets.

A pathway for phosphatidylcholine biosynthesis in Plasmodium falciparum involving phosphoethanolamine methylation

Plasmodium falciparum is the causative agent of the most severe form of human malaria. The rapid multiplication of the parasite within human erythrocytes requires an active production of new membranes. Phosphatidylcholine is the most abundant phospholipid in Plasmodium membranes, and the pathways leading to its synthesis are attractive targets for chemotherapy. In addition to its synthesis from choline, phosphatidylcholine is synthesized from serine via an unknown pathway. Serine, which is actively transported by Plasmodium from human serum and readily available in the parasite, is subsequently converted into phosphoethanolamine. Here, we describe in P. falciparum a plant-like S-adenosyl-l-methionine-dependent three-step methylation reaction that converts phosphoethanolamine into phosphocholine, a precursor for the synthesis of phosphatidylcholine. We have identified the gene, PfPMT, encoding this activity and shown that its product is an unusual phosphoethanolamine methyltransferase with no human homologs. P. falciparum phosphoethanolamine methyltransferase (Pfpmt) is a monopartite enzyme with a single catalytic domain that is responsible for the three-step methylation reaction. Interestingly, Pfpmt activity is inhibited by its product phosphocholine and by the phosphocholine analog, miltefosine. We show that miltefosine can also inhibit parasite proliferation within human erythrocytes. The importance of this enzyme in P. falciparum membrane biogenesis makes it a potential target for malaria chemotherapy.

The Plasmodium falciparum PfGatp is an Endoplasmic Reticulum Membrane Protein Important for the Initial Step of Malarial Glycerolipid Synthesis

During its 48-h asexual life cycle within human erythrocytes, Plasmodium falciparum grows to many times its own size and divides to produce 16-32 new parasites. This rapid multiplication requires active synthesis of new membranes and is fueled by phospholipid precursors and fatty acids that are scavenged from the human host. Plasmodium membrane biogenesis relies heavily on the expression of parasite enzymes that incorporate these precursors into phospholipids. However, little is known about the genes involved in membrane biogenesis or where this process takes place within the parasite. Here, we describe the analysis in P. falciparum of the first step of phospholipid biosynthesis that controls acylation of glycerol 3-phosphate (GPAT) at the sn-1 position. We show that this activity is of parasite origin and is specific for glycerol 3-phosphate substrate. We have identified the gene, PfGAT, encoding this activity in P. falciparum and reconstituted its codon composition for optimal expression in the yeast Saccharomyces cerevisiae. PfGAT complements the lethality of a yeast double mutant gat1Deltagat2Delta, lacking GPAT activity. Biochemical analysis revealed that PfGatp is a low affinity GPAT enzyme with a high specificity for C16:0 and C16:1 substrates. PfGatp is an integral membrane protein of the endoplasmic reticulum expressed throughout the intraerythrocytic life cycle of the parasite but induced mainly at the trophozoite stage. This study, which describes the first protozoan GPAT gene, reveals an important role for the endoplasmic reticulum in the initial step of Plasmodium membrane biogenesis.

Hydrolysis of sphingosylphosphocholine by neutral sphingomyelinases

Sphingosylphosphocholine (SPC), the N-deacylated form of sphingomyelin (SM), is a naturally occurring lipid mediator. However, little is known about the metabolism of SPC. We here report an in vitro assay system for SPC-phospholipase C (PLC). Using this assay system, we demonstrated that nSMase1 and nSMase2, human neutral sphingomyelinases (SMases), are capable of hydrolyzing SPC efficiently under detergent-free conditions. Bacterial and plasmodial neutral SMases also showed SPC-PLC activity. The substrate specificity of neutral SMases that hydrolyze SM, SPC, and monoradyl glycerophosphocholine, but not diradyl glycerophosphocholine, suggested that a hydrogen-bond donor at the C-2 or sn-2 position in the substrate is required for recognition by the enzymes.