Phosphoethanolamine methyltransferases in phosphocholine biosynthesis: functions and potential for antiparasite therapy

An Introduction to Spiral Steroids

In addition to classical steroids, which have cholesterol as a precursor, there are steroids with 7-dehydrocholesterol as a precursor. This review describes the identification of these steroids, their biosynthesis, and some aspects of their function. There are three classes of these compounds, distinguished by the number of their carbon atoms, 23, 24, and 25. Each class has a spiral steroid and is a phosphodiester. Up until these investigations, no spiral steroids or steroid phosphodiesters were known. There are at least 13 compounds, of which six have been purified to near homogeneity; each one has been characterized by its mass and proposed composition, and they function by regulating the NaK-ATPase. Based on the tissues in which they have been detected, each class of compound seems to regulate a different isoform of the NaK-ATPase. This is an important site of endocrine regulation.

Adapt or Die: Targeting Unique Transmission-Stage Biology for Malaria Elimination

Plasmodium parasites have a complex life cycle that includes development in the human host as well as the Anopheles vector. Successful transmission of the parasite between its host and vector therefore requires the parasite to balance its investments in asexual replication and sexual reproduction, varying the frequency of sexual commitment to persist within the human host and generate future opportunities for transmission. The transmission window is extended further by the ability of stage V gametocytes to circulate in peripheral blood for weeks, whereas immature stage I to IV gametocytes sequester in the bone marrow and spleen until final maturation. Due to the low gametocyte numbers in blood circulation and with the ease of targeting such life cycle bottlenecks, transmission represents an efficient target for therapeutic intervention. The biological process of Plasmodium transmission is a multistage, multifaceted process and the past decade has seen a much deeper understanding of the molecular mechanisms and regulators involved. Clearly, specific and divergent processes are used during transmission compared to asexual proliferation, which both poses challenges but also opportunities for discovery of transmission-blocking antimalarials. This review therefore presents an update of our molecular understanding of gametocyte and gamete biology as well as the status of transmission-blocking activities of current antimalarials and lead development compounds. By defining the biological components associated with transmission, considerations for the development of new transmission-blocking drugs to target such untapped but unique biology is suggested as an important, main driver for transmission-blocking drug discovery.

A Dual, Systematic Approach to Malaria Diagnostic Biomarker Discovery

Background

The emergence and spread of Plasmodium falciparum parasites that lack HRP2/3 proteins and the resulting decreased utility of HRP2-based malaria rapid diagnostic tests (RDTs) prompted the World Health Organization and other global health stakeholders to prioritize the discovery of novel diagnostic biomarkers for malaria.

Methods

To address this pressing need, we adopted a dual, systematic approach by conducting a systematic review of the literature for publications on diagnostic biomarkers for uncomplicated malaria and a systematic in silico analysis of P. falciparum proteomics data for Plasmodium proteins with favorable diagnostic features.

Results

Our complementary analyses led us to 2 novel malaria diagnostic biomarkers compatible for use in an RDT format: glyceraldehyde 3-phosphate dehydrogenase and dihydrofolate reductase-thymidylate synthase.

Conclusions

Overall, our results pave the way for the development of next-generation malaria RDTs based on new antigens by identifying 2 lead candidates with favorable diagnostic features and partially de-risked product development prospects.

Erythrocyte sphingosine kinase regulates intraerythrocytic development of Plasmodium falciparum

The sphingolipid pool is key regulator of vital cellular functions in Plasmodium falciparum a causative agent for deadly malaria. Erythrocytes, the host for asexual stage of Plasmodium, are major reservoir for Sphingosine-1-phosphate (S1P). Erythrocyte possesses Sphingosine kinase (SphK) that catalyzed its biosynthesis from sphingosine (Sph). Since, Plasmodium lacks SphK homologous protein it can be envisaged that it co-opts sphingolipids from both intraerythrocytic as well as extracellular pools for its growth and development. Herein, by sphingosine-NBD probing, we report that infected erythrocytes imports Sph from extracellular pool, which is converted to S1P and thereby taken by P. falciparum. Next, by targeting of the SphK through specific inhibitor N,N-Dimethylsphingosine DMS, we show a reduction in erythrocyte endogenous S1P pool and SphK-phosphorylation that led to inhibition in growth and development of ring stage P. falciparum. Owing to the role of S1P in erythrocyte glycolysis we analyzed uptake of NBD-Glucose and production of lactate in DMS treated and untreated plasmodium. DMS treatment led to decreased glycolysis in Plasmodium. Interestingly the host free Plasmodium did not show any effect on glycolysis with DMS treatment indicating its host-mediated effect. Further to understand the in-vivo anti-plasmodial effects of exogenous and endogenous erythrocyte S1P level, Sphingosine-1-phosphate lyase (S1PL) inhibitor (THI), S1P and SphK-1 inhibitor (DMS), were used in Plasmodium berghei ANKA (PbA) mice model. DMS treatment led to reduction of endogenous S1P conferred significant decrease in parasite load, whereas the plasma level S1P modulated by (THI) and exogenous S1P have no effect on growth of Plasmodium. This suggested erythrocyte endogenous S1P pool is important for Plasmodium growth whereas the plasma level S1P has no effect. Altogether, this study provides insight on cellular processes regulated by S1P in P. falciparum and highlights the novel mechanistically distinct molecular target i.e. SphK-1.

Integrative Metabolomic and Transcriptomic Analyses Reveal Metabolic Changes and Its Molecular Basis in Rice Mutants of the Strigolactone Pathway

Plants have evolved many metabolites to meet the demands of growth and adaptation. Although strigolactones (SLs) play vital roles in controlling plant architecture, their function in regulating plant metabolism remains elusive. Here we report the integrative metabolomic and transcriptomic analyses of two rice SL mutants, d10 (a biosynthesis mutant) and d14 (a perception mutant). Both mutants displayed a series of metabolic and transcriptional alterations, especially in the lipid, flavonoid, and terpenoid pathways. Levels of several diterpenoid phytoalexins were substantially increased in d10 and d14, together with the induction of terpenoid gene cluster and the corresponding upstream transcription factor WRKY45, an established determinant of plant immunity. The fact that WRKY45 is a target of IPA1, which acted as a downstream transcription factor of SL signaling, suggests that SLs contribute to plant defense through WRKY45 and phytoalexins. Moreover, our data indicated that SLs may modulate rice metabolism through a vast number of clustered or tandemly duplicated genes. Our work revealed a central role of SLs in rice metabolism. Meanwhile, integrative analysis of the metabolome and transcriptome also suggested that SLs may contribute to metabolite-associated growth and defense.

Mosquito metabolomics reveal that dengue virus replication requires phospholipid reconfiguration via the remodeling cycle

Dengue virus (DENV) subdues cell membranes for its cellular cycle by reconfiguring phospholipids in humans and mosquitoes. Here, we determined how and why DENV reconfigures phospholipids in the mosquito vector. By inhibiting and activating the de novo phospholipid biosynthesis, we demonstrated the antiviral impact of de novo-produced phospholipids. In line with the virus hijacking lipids for its benefit, metabolomics analyses indicated that DENV actively inhibited the de novo phospholipid pathway and instead triggered phospholipid remodeling. We demonstrated the early induction of remodeling during infection by using isotope tracing in mosquito cells. We then confirmed in mosquitoes the antiviral impact of de novo phospholipids by supplementing infectious blood meals with a de novo phospholipid precursor. Eventually, we determined that phospholipid reconfiguration was required for viral genome replication but not for the other steps of the virus cellular cycle. Overall, we now propose that DENV reconfigures phospholipids through the remodeling cycle to modify the endomembrane and facilitate formation of the replication complex. Furthermore, our study identified de novo phospholipid precursor as a blood determinant of DENV human-to-mosquito transmission.

Reduction of Sphingosine Kinase 1 Phosphorylation and Activity in Plasmodium-Infected Erythrocytes

Sphingosine-1-phosphate (S1P), a bioactive lipid mediator is involved in an array of biological processes and linked to pathological manifestations. Erythrocyte is known as the major reservoir for S1P as they lack S1P-degrading enzymes (S1P lyase and S1P phosphohydrolase) and harbor sphingosine kinase-1 (SphK-1) essential for sphingosine conversion to S1P. Reduced S1P concentration in serum was correlated with disease severity in patients with Plasmodium falciparum and Plasmodium vivax infections. Herein, we aimed to identify the underlying mechanism and contribution of host erythrocytes toward depleted S1P levels in Plasmodium-infected patients vs. healthy individuals. The level and activity of SphK-1 were measured in vitro in both uninfected and cultured P. falciparum-infected erythrocytes. Infected erythrocytes demonstrated a significant decrease in SphK-1 level in a time-dependent manner. We found that 10–42 h post invasion (hpi), SphK1 level was predominantly reduced to ∼50% in rings, trophozoites, and schizonts compared to uninfected erythrocytes. We next analyzed the phosphorylation status of SphK-1, a modification responsible for its activity and S1P production, in both uninfected control and Plasmodium-infected erythrocytes. Almost ∼50% decrease in phosphorylation of SphK-1 was observed that could be corroborated with significant reduction in the production and release of S1P in infected erythrocytes. Serum S1P levels were studied in parallel in P. falciparum (N = 15), P. vivax (N = 36)-infected patients, and healthy controls (N = 6). The findings revealed that S1P concentration was significantly depleted in uncomplicated malaria cases and was found to be lowest in complicated malaria and thrombocytopenia in both P. falciparum and P. vivax-infected groups (^∗∗p < 0.01). The lower serum S1P level could be correlated with the reduced platelet count defining the role of S1P level in platelet formation. In conclusion, erythrocyte SphK-1 and S1P levels were studied in Plasmodium-infected individuals and erythrocytes that helped in characterizing the complications associated with malaria and thrombocytopenia, providing insights into the contribution of host erythrocyte biology in malaria pathogenesis. Finally, this study proposes the use of S1P and its analog as a novel adjunct therapy for malaria complications.

Computational and experimental elucidation of Plasmodium falciparum phosphoethanolamine methyltransferase inhibitors: Pivotal drug target

INTRODUCTION

Plasmodium falciparum synthesizes phosphatidylcholine for the membrane development through serine decarboxylase-phosphoethanolamine methyltransferase pathway for growth in human host. Phosphoethanolamine-methyltransferase (PfPMT) is a crucial enzyme for the synthesis of phosphocholine which is a precursor for phosphatidylcholine synthesis and is considered as a pivotal drug target as it is absent in the host. The inhibition of PfPMT may kill malaria parasite and hence is being considered as potential target for rational antimalarial drug designing.

METHODS

In this study, we have used computer aided drug designing (CADD) approaches to establish potential PfPMT inhibitors from Asinex compound library virtually screened for ADMET and the docking affinity. The selected compounds were tested for in-vitro schizonticidal, gametocidal and cytotoxicity activity. Nontoxic compounds were further studied for PfPMT enzyme specificity and antimalarial efficacy for P. berghei in albino mice model.

RESULTS

Our results have identified two nontoxic PfPMT competitive inhibitors ASN.1 and ASN.3 with better schizonticidal and gametocidal activity which were found to inhibit PfPMT at IC50 1.49μM and 2.31μM respectively. The promising reduction in parasitaemia was found both in orally (50 & 10 mg/kg) and intravenous (IV) (5& 1 mg/kg) however, the better growth inhibition was found in intravenous groups.

CONCLUSION

We report that the compounds containing Pyridinyl-Pyrimidine and Phenyl-Furan scaffolds as the potential inhibitors of PfPMT and thus may act as promising antimalarial inhibitor candidates which can be further optimized and used as leads for template based antimalarial drug development.

A Methyltransferase Trio Essential for Phosphatidylcholine Biosynthesis and Growth

Phosphatidylcholine (PC) is a primary class of membrane lipids in most eukaryotes. In plants, the primary PC biosynthetic pathway and its role in plant growth and development remain elusive due to lack of a mutant model with substantially decreased PC content. Recently, a double mutant of Arabidopsis (Arabidopsis thaliana) PHOSPHO-BASE N-METHYLTRANSFERASE 1 (PMT1) and PMT3 was reported with reduced PC content and defective plant growth. However, residual PC content as well as the nonlethal phenotype of the mutant suggests an additional enzyme contributes to PC biosynthesis. In this article, we report on the role of three PMTs in PC biosynthesis and plant development, with a focus on PMT2. PMT2 had the highest expression level among the three PMTs, and it was highly expressed in roots. The pmt1 pmt2 double mutant enhanced the defects in root growth, cell viability, and PC content of pmt1, suggesting that PMT2 functions together with PMT1 in roots. Chemical inhibition of PMT activity in wild-type roots reproduced the short root phenotype observed in pmt1 pmt2, suggesting that PMT1 and PMT2 are the major PMT isoforms in roots. In shoots, pmt1 pmt2 pmt3 enhanced the phenotype of pmt1 pmt3, showing seedling lethality and further reduced PC content without detectable de novo PC biosynthesis. These results suggest that PMTs catalyze an essential reaction step in PC biosynthesis and that the three PMTs have differential tissue-specific functions in PC biosynthesis and plant growth.

A pair of phospho-base methyltransferases important for phosphatidylcholine biosynthesis in Arabidopsis

Phosphatidylcholine (PtdCho) is a predominant membrane lipid class in eukaryotes. Phospho-base N-methyltransferase (PMT) catalyzes a critical step in PtdCho biosynthesis. However, in Arabidopsis thaliana, the discovery of involvement of the specific PMT isoform in PtdCho biosynthesis remains elusive. Here, we show that PMT1 and PMT3 redundantly play an essential role in phosphocholine (PCho) biosynthesis, a prerequisite for PtdCho production. A pmt1 pmt3 double mutant was devoid of PCho, which affected PtdCho biosynthesis in vivo, showing severe growth defects in post-embryonic development. PMT1 and PMT3 were both highly expressed in the vasculature. The pmt1 pmt3 mutants had specifically affected leaf vein development and showed pale-green seedlings that were rescued by exogenous supplementation of PCho. We suggest that PMT1 and PMT3 are the primary enzymes for PCho biosynthesis and are involved in PtdCho biosynthesis and vascular development in Arabidopsis seedlings.

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.

Biochemical and genetic analyses of N metabolism in maize testcross seedlings: 2. Roots

Intracellular factors differentially affected enzyme activities of N assimilation in the roots of maize testcrosses where alanine aminotransferase and glutamate synthase were the main enzymes regulating the levels of glutamate. N is a key macronutrient for plant growth and development. Breeding maize with improved efficiency in N use could help reduce environmental contamination as well as increase profitability for the farmers. Quantitative trait loci (QTL) mapping of traits related to N metabolism in the root tissue was undertaken in a maize testcross mapping population grown in hydroponic cultures. N concentration was negatively correlated with root and total dry mass. Neither the enzyme activities nor metabolites were appreciably correlated between the root and leaf tissues. Repeatability measures for most of the enzymes were lower than for dry mass. Weak negative correlations between most of the enzymes and dry mass resulted likely from dilution and suggested the presence of excess of enzyme activities for maximal biomass production. Glutamate synthase and alanine aminotransferase each explained more variation in glutamate concentration than either aspartate aminotransferase or asparagine synthetase whereas glutamine synthetase was inconsequential. Twenty-six QTL were identified across all traits. QTL models explained 7-43% of the variance with no significant epistasis between the QTL. Thirteen candidate genes were identified underlying QTL within 1-LOD confidence intervals. All the candidate genes were located in trans configuration, unlinked or even on different chromosomes, relative to the known genomic positions of the corresponding structural genes. Our results have implications in improving NUE in maize and other crop plants.

Phosphoethanolamine-N-methyltransferase is a potential biomarker for the diagnosis of P. knowlesi and P. falciparum malaria

BACKGROUND

Plasmodium knowlesi is recognised as the main cause of human malaria in Southeast Asia. The disease is often misdiagnosed as P. falciparum or P. malariae infections by microscopy, and the disease is difficult to eliminate due to its presence in both humans and monkeys. P. knowlesi infections can rapidly cause severe disease and require prompt diagnosis and treatment. No protein biomarker exists for the rapid diagnostic test (RDT) detection of P. knowlesi infections. Plasmodium knowlesi infections can be diagnosed by PCR.

METHODS AND PRINCIPAL FINDINGS

Phosphoethanolamine-N-methyltransferase (PMT) is involved in malaria lipid biosynthesis and is not found in the human host. The P. falciparum, P. vivax and P. knowlesi PMT proteins were recombinantly expressed in BL21(DE3) Escherichia coli host cells, affinity purified and used to raise antibodies in chickens. Antibodies against each recombinant PMT protein all detected all three recombinant proteins and the native 29 kDa P. falciparum PMT protein on western blots and in ELISA. Antibodies against a PMT epitope (PLENNQYTDEGVKC) common to all three PMT orthologues detected all three proteins. Antibodies against unique peptides from each orthologue of PMT, PfCEVEHKYLHENKE, PvVYSIKEYNSLKDC, PkLYPTDEYNSLKDC detected only the parent protein in western blots and P. falciparum infected red blood cell lysates or blood lysates spiked with the respective proteins. Similar concentrations of PfPMT and the control, PfLDH, were detected in the same parasite lysate. The recombinant PfPMT protein was detected by a human anti-malaria antibody pool.

CONCLUSION

PMT, like the pan-specific LDH biomarker used in RDT tests, is both soluble, present at comparable concentrations in the parasite and constitutes a promising antimalarial drug target. PMT is absent from the human proteome. PMT has the potential as a biomarker for human malaria and in particular as the first P. knowlesi specific protein with diagnostic potential for the identification of a P. knowlesi infection.

Evolution of the Phosphatidylcholine Biosynthesis Pathways in Green Algae: Combinatorial Diversity of Methyltransferases

Phosphatidylcholine (PC) is one of the most common phospholipids in eukaryotes, although some green algae such as Chlamydomonas reinhardtii are known to lack PC. Recently, we detected PC in four species in the genus Chlamydomonas: C. applanata NIES-2202, C. asymmetrica NIES-2207, C. debaryana NIES-2212, and C. sphaeroides NIES-2242. To reveal the PC biosynthesis pathways in green algae and the evolutionary scenario involved in their diversity, we analyzed the PC biosynthesis genes in these four algae using draft genome sequences. Homology searches suggested that PC in these species is synthesized by phosphoethanolamine-N-methyltransferase (PEAMT) and/or phosphatidylethanolamine-N-methyltransferase (PEMT), both of which are absent in C. reinhardtii. Recombinant PEAMTs from these algae showed methyltransferase activity for phosphoethanolamine but not for monomethyl phosphoethanolamine in vitro, in contrast to land plant PEAMT, which catalyzes the three methylations from phosphoethanolamine to phosphocholine. This suggested an involvement of other methyltransferases in PC biosynthesis. Here, we characterized the putative phospholipid-N-methyltransferase (PLMT) genes of these species by genetic and phylogenetic analysis. Complementation assays using a PC biosynthesis-deficient yeast suggested that the PLMTs of these algae can synthesize PC from phosphatidylethanolamine. These results indicated that the PC biosynthesis pathways in green algae differ from those of land plants, although the enzymes involved are homologous. Phylogenetic analysis suggested that the PEAMTs and PLMTs in these algae were inherited from the common ancestor of green algae. The absence of PC biosynthesis in many Chlamydomonas species is likely a result of parallel losses of PEAMT and PLMT in this genus.

Microbial metabolites regulate host lipid metabolism through NR5A-Hedgehog signalling

Microorganisms and their hosts share the same environment, and microbial metabolic molecules (metabolites) exert crucial effects on host physiology. Environmental factors not only shape the composition of the host's resident microorganisms, but also modulate their metabolism. However, the exact molecular relationship among the environment, microbial metabolites and host metabolism remains largely unknown. Here, we discovered that environmental methionine tunes bacterial methyl metabolism to regulate host mitochondrial dynamics and lipid metabolism in Caenorhabditis elegans through an endocrine crosstalk involving NR5A nuclear receptor and Hedgehog signalling. We discovered that methionine deficiency in bacterial medium decreases the production of bacterial metabolites that are essential for phosphatidylcholine synthesis in C. elegans. Reductions of diundecanoyl and dilauroyl phosphatidylcholines attenuate NHR-25, a NR5A nuclear receptor, and release its transcriptional suppression of GRL-21, a Hedgehog-like protein. The induction of GRL-21 consequently inhibits the PTR-24 Patched receptor cell non-autonomously, resulting in mitochondrial fragmentation and lipid accumulation. Together, our work reveals an environment-microorganism-host metabolic axis regulating host mitochondrial dynamics and lipid metabolism, and discovers NR5A-Hedgehog intercellular signalling that controls these metabolic responses with critical consequences for host health and survival.

Diverse pathways of phosphatidylcholine biosynthesis in algae as estimated by labeling studies and genomic sequence analysis

Phosphatidylcholine (PC) is an almost ubiquitous phospholipid in eukaryotic algae and plants but is not found in a few species, for example Chlamydomonas reinhardtii. We recently found that some species of the genus Chlamydomonas possess PC. In the universal pathway, PC is synthesized de novo by methylation of phosphatidylethanolamine (PE) or transfer of phosphocholine from cytidine diphosphate (CDP)-choline to diacylglycerol. Phosphocholine, the direct precursor to CDP-choline, is synthesized either by methylation of phosphoethanolamine or phosphorylation of choline. Here we analyzed the mechanism of PC biosynthesis in two species of Chlamydomonas (asymmetrica and sphaeroides) as well as in a red alga, Cyanidioschyzon merolae. Comparative genomic analysis of enzymes involved in PC biosynthesis indicated that C. merolae possesses only the PE methylation pathway. Radioactive tracer experiments using [(32) P]phosphate showed delayed labeling of PC with respect to PE, which was consistent with the PE methylation pathway. In Chlamydomonas asymmetrica, labeling of PC was detected from the early time of incubation with [(32) P]phosphate, suggesting the operation of phosphoethanolamine methylation pathway. Genomic analysis indeed detected the genes for the phosphoethanolamine methylation pathway. In contrast, the labeling of PC in C. sphaeroides was slow, suggesting that the PE methylation pathway was at work. These results as well as biochemical and computational results uncover an unexpected diversity of the mechanisms for PC biosynthesis in algae. Based on these results, we will discuss plausible mechanisms for the scattered distribution of the ability to biosynthesize PC in the genus Chlamydomonas.

Phosphocholine - an agonist of metabotropic but not of ionotropic functions of α9-containing nicotinic acetylcholine receptors

We demonstrated previously that phosphocholine and phosphocholine-modified macromolecules efficiently inhibit ATP-dependent release of interleukin-1β from human and murine monocytes by a mechanism involving nicotinic acetylcholine receptors (nAChR). Interleukin-1β is a potent pro-inflammatory cytokine of innate immunity that plays pivotal roles in host defence. Control of interleukin-1β release is vital as excessively high systemic levels cause life threatening inflammatory diseases. In spite of its structural similarity to acetylcholine, there are no other reports on interactions of phosphocholine with nAChR. In this study, we demonstrate that phosphocholine inhibits ion-channel function of ATP receptor P2X7 in monocytic cells via nAChR containing α9 and α10 subunits. In stark contrast to choline, phosphocholine does not evoke ion current responses in Xenopus laevis oocytes, which heterologously express functional homomeric nAChR composed of α9 subunits or heteromeric receptors containing α9 and α10 subunits. Preincubation of these oocytes with phosphocholine, however, attenuated choline-induced ion current changes, suggesting that phosphocholine may act as a silent agonist. We conclude that phophocholine activates immuno-modulatory nAChR expressed by monocytes but does not stimulate canonical ionotropic receptor functions.

Characterization of Plasmodium phosphatidylserine decarboxylase expressed in yeast and application for inhibitor screening

Phospholipid biosynthesis is critical for the development, differentiation and pathogenesis of several eukaryotic pathogens. Genetic studies have validated the pathway for phosphatidylethanolamine synthesis from phosphatidylserine catalyzed by phosphatidylserine decarboxylase enzymes (PSD) as a suitable target for development of antimicrobials; however no inhibitors of this class of enzymes have been discovered. We show that the Plasmodium falciparum PSD can restore the essential function of the yeast gene in strains requiring PSD for growth. Genetic, biochemical and metabolic analyses demonstrate that amino acids between positions 40 and 70 of the parasite enzyme are critical for proenzyme processing and decarboxylase activity. We used the essential role of Plasmodium PSD in yeast as a tool for screening a library of anti-malarials. One of these compounds is 7-chloro-N-(4-ethoxyphenyl)-4-quinolinamine, an inhibitor with potent activity against P. falciparum, and low toxicity toward mammalian cells. We synthesized an analog of this compound and showed that it inhibits PfPSD activity and eliminates Plasmodium yoelii infection in mice. These results highlight the importance of 4-quinolinamines as a novel class of drugs targeting membrane biogenesis via inhibition of PSD activity.

Structure, function and inhibition of the phosphoethanolamine methyltransferases of the human malaria parasites Plasmodium vivax and Plasmodium knowlesi

Phosphoethanolamine methyltransferases (PMTs) catalyze the three-step methylation of phosphoethanolamine to form phosphocholine, a critical step in the synthesis of phosphatidylcholine in a select number of eukaryotes including human malaria parasites, nematodes and plants. Genetic studies in the malaria parasite Plasmodium falciparum have shown that the methyltransferase PfPMT plays a critical function in parasite development and differentiation. The presence of PMT orthologs in other malaria parasites that infect humans and their absence in mammals make them ideal targets for the development of selective antimalarials with broad specificity against different Plasmodium species. Here we describe the X-ray structures and biochemical properties of PMT orthologs from Plasmodium vivax and Plasmodium knowlesi and show that both enzymes are inhibited by amodiaquine and NSC158011, two drugs with potent antimalarial activity. Metabolic studies in a yeast mutant that relies on PkPMT or PvPMT for survival demonstrated that these compounds inhibit phosphatidylcholine biosynthesis from ethanolamine. Our structural and functional data provide insights into the mechanism of catalysis and inhibition of PMT enzymes and set the stage for a better design of more specific and selective antimalarial drugs.

An alternative mechanism for the methylation of phosphoethanolamine catalyzed by Plasmodium falciparum phosphoethanolamine methyltransferase

The phosphobase methylation pathway catalyzed by the phosphoethanolamine methyltransferase in Plasmodium falciparum (PfPMT), the malaria parasite, offers an attractive target for anti-parasitic drug development. PfPMT methylates phosphoethanolamine (pEA) to phosphocholine for use in membrane biogenesis. Quantum mechanics and molecular mechanics (QM/MM) calculations tested the proposed reaction mechanism for methylation of pEA involving the previously identified Tyr-19-His-132 dyad, which indicated an energetically unfavorable mechanism. Instead, the QM/MM calculations suggested an alternative mechanism involving Asp-128. The reaction coordinate involves the stepwise transfer of a proton to Asp-128 via a bridging water molecule followed by a typical Sn2-type methyl transfer from S-adenosylmethionine to pEA. Functional analysis of the D128A, D128E, D128Q, and D128N PfPMT mutants shows a loss of activity with pEA but not with the final substrate of the methylation pathway. X-ray crystal structures of the PfPMT-D128A mutant in complex with S-adenosylhomocysteine and either pEA or phosphocholine reveal how mutation of Asp-128 disrupts a hydrogen bond network in the active site. The combined QM/MM, biochemical, and structural studies identify a key role for Asp-128 in the initial step of the phosphobase methylation pathway in Plasmodium and provide molecular insight on the evolution of multiple activities in the active site of the PMT.

From crystal to compound: structure-based antimalarial drug discovery

Despite a century of control and eradication campaigns, malaria remains one of the world's most devastating diseases. Our once-powerful therapeutic weapons are losing the war against the Plasmodium parasite, whose ability to rapidly develop and spread drug resistance hamper past and present malaria-control efforts. Finding new and effective treatments for malaria is now a top global health priority, fuelling an increase in funding and promoting open-source collaborations between researchers and pharmaceutical consortia around the world. The result of this is rapid advances in drug discovery approaches and technologies, with three major methods for antimalarial drug development emerging: (i) chemistry-based, (ii) target-based, and (iii) cell-based. Common to all three of these approaches is the unique ability of structural biology to inform and accelerate drug development. Where possible, SBDD (structure-based drug discovery) is a foundation for antimalarial drug development programmes, and has been invaluable to the development of a number of current pre-clinical and clinical candidates. However, as we expand our understanding of the malarial life cycle and mechanisms of resistance development, SBDD as a field must continue to evolve in order to develop compounds that adhere to the ideal characteristics for novel antimalarial therapeutics and to avoid high attrition rates pre- and post-clinic. In the present review, we aim to examine the contribution that SBDD has made to current antimalarial drug development efforts, covering hit discovery to lead optimization and prevention of parasite resistance. Finally, the potential for structural biology, particularly high-throughput structural genomics programmes, to identify future targets for drug discovery are discussed.

Nematode phospholipid metabolism: an example of closing the genome-structure-function circle

Parasitic nematodes that infect humans, animals, and plants cause health problems, livestock and agricultural losses, and economic damage worldwide and are important targets for drug development. The growing availability of nematode genomes supports the discovery of new pathways that differ from host organisms and are a starting point for structural and functional studies of novel antiparasitic targets. As an example of how genome data, structural biology, and biochemistry integrate into a research cycle targeting parasites, we summarize the discovery of the phosphobase methylation pathway for phospholipid synthesis in nematodes and compare the phosphoethanolamine methyltransferases (PMTs) from nematodes, plants, and Plasmodium. Crystallographic and biochemical studies of the PMTs in this pathway provide a foundation that guides the next steps that close the genome-structure-function circle.

PAQR-2 may be a regulator of membrane fluidity during cold adaptation

PAQR-2 is a C. elegans homolog of the mammalian adiponectin receptors. We have recently shown that PAQR-2 is essential for the ability of C. elegans to grow at its lower temperature range, i.e., 15 °C, and that the likely role of PAQR-2 during cold adaptation is to regulate membrane fluidity by promoting fatty acid desaturation. Here we present a summary of this work, with an emphasis on placing our C. elegans findings in the context of mammalian biology.

Plasmodium falciparum phosphoethanolamine methyltransferase is essential for malaria transmission

Efficient transmission of Plasmodium species between humans and Anopheles mosquitoes is a major contributor to the global burden of malaria. Gametocytogenesis, the process by which parasites switch from asexual replication within human erythrocytes to produce male and female gametocytes, is a critical step in malaria transmission and Plasmodium genetic diversity. Nothing is known about the pathways that regulate gametocytogenesis and only few of the current drugs that inhibit asexual replication are also capable of inhibiting gametocyte development and blocking malaria transmission. Here we provide genetic and pharmacological evidence indicating that the pathway for synthesis of phosphatidylcholine in Plasmodium falciparum membranes from host serine is essential for parasite gametocytogenesis and malaria transmission. Parasites lacking the phosphoethanolamine N-methyltransferase enzyme, which catalyzes the limiting step in this pathway, are severely altered in gametocyte development, are incapable of producing mature-stage gametocytes, and are not transmitted to mosquitoes. Chemical screening identified 11 inhibitors of phosphoethanolamine N-methyltransferase that block parasite intraerythrocytic asexual replication and gametocyte differentiation in the low micromolar range. Kinetic studies in vitro as well as functional complementation assays and lipid metabolic analyses in vivo on the most promising inhibitor NSC-158011 further demonstrated the specificity of inhibition. These studies set the stage for further optimization of NSC-158011 for development of a class of dual activity antimalarials to block both intraerythrocytic asexual replication and gametocytogenesis.

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.

(1)H, (13)C, and (15)N chemical shift assignments for PfPMT, a phosphoethanolamine methyltransferase from Plasmodium falciparum

Phosphoethanolamine methyltransferases (PMTs also known as PEAMTs) catalyze the three-step s-adenosyl-methionione-dependent methylation of phosphoethanolamine to form phosphocholine. These enzymes play an important function in the synthesis of phosphatidylcholine, the major phospholipid in the membranes of lower and higher eukaryotes, as well as in the production of the compatible solute and osmoprotectant glycine betaine in plants. Genetic studies in plants, Caenhorhabditis elegans and Plasmodium falciparum have demonstrated that disruption of PMT activity results in severe defects in important cellular processes such as development, replication, survival and sexual maturation and differentiation. Here we report chemical shift assignments for PfPMT, the PMT from Plasmodium falciparum. X-ray crystal structures have been recently reported for complexes of PfPMT, but the structure of the apoenzyme remains unknown. The solution structure of the apoenzyme will help to elucidate important details of the mechanism of substrate binding by PfPMT, as residues comprising the substrate binding site are inaccessible to solvent in the conformation evident in the available crystal structures. In addition to enabling determination of the solution structure of the apoenzyme, the assignments will facilitate additional investigations into the interaction of PfPMT with its substrates and inhibitors.

Crystal structure of phosphoethanolamine methyltransferase from Plasmodium falciparum in complex with amodiaquine

Phosphoethanolamine N-methyltransferase (PMT) is essential for phospholipid biogenesis in the malarial parasite Plasmodium falciparum. PfPMT catalyzes the triple methylation of phosphoethanolamine to produce phosphocholine, which is then used for phosphatidylcholine synthesis. Here we describe the 2.0Å resolution X-ray crystal structure of PfPMT in complex with amodiaquine. To better characterize inhibition of PfPMT by amodiaquine, we determined the IC(50) values of a series of aminoquinolines using a direct radiochemical assay. Both structural and functional analyses provide a possible approach for the development of new small molecule inhibitors of PfPMT.

Structure and reaction mechanism of phosphoethanolamine methyltransferase from the malaria parasite Plasmodium falciparum: an antiparasitic drug target

In the malarial parasite Plasmodium falciparum, a multifunctional phosphoethanolamine methyltransferase (PfPMT) catalyzes the methylation of phosphoethanolamine (pEA) to phosphocholine for membrane biogenesis. This pathway is also found in plant and nematodes, but PMT from these organisms use multiple methyltransferase domains for the S-adenosylmethionine (AdoMet) reactions. Because PfPMT is essential for normal growth and survival of Plasmodium and is not found in humans, it is an antiparasitic target. Here we describe the 1.55 Å resolution crystal structure of PfPMT in complex with AdoMet by single-wavelength anomalous dispersion phasing. In addition, 1.19-1.52 Å resolution structures of PfPMT with pEA (substrate), phosphocholine (product), sinefungin (inhibitor), and both pEA and S-adenosylhomocysteine bound were determined. These structures suggest that domain rearrangements occur upon ligand binding and provide insight on active site architecture defining the AdoMet and phosphobase binding sites. Functional characterization of 27 site-directed mutants identifies critical active site residues and suggests that Tyr-19 and His-132 form a catalytic dyad. Kinetic analysis, isothermal titration calorimetry, and protein crystallography of the Y19F and H132A mutants suggest a reaction mechanism for the PMT. Not only are Tyr-19 and His-132 required for phosphobase methylation, but they also form a "catalytic" latch that locks ligands in the active site and orders the site for catalysis. This study provides the first insight on this antiparasitic target enzyme essential for survival of the malaria parasite; however, further studies of the multidomain PMT from plants and nematodes are needed to understand the evolutionary division of metabolic function in the phosphobase pathway of these organisms.

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.