Veterinary trypanocidal benzoxaboroles are peptidase-activated prodrugs.

Livestock diseases caused by Trypanosoma congolense, T. vivax and T. brucei, collectively known as nagana, are responsible for billions of dollars in lost food production annually. There is an urgent need for novel therapeutics. Encouragingly, promising antitrypanosomal benzoxaboroles are under veterinary development. Here, we show that the most efficacious subclass of these compounds are prodrugs activated by trypanosome serine carboxypeptidases (CBPs). Drug-resistance to a development candidate, AN11736, emerged readily in T. brucei, due to partial deletion within the locus containing three tandem copies of the CBP genes. T. congolense parasites, which possess a larger array of related CBPs, also developed resistance to AN11736 through deletion within the locus. A genome-scale screen in T. brucei confirmed CBP loss-of-function as the primary mechanism of resistance and CRISPR-Cas9 editing proved that partial deletion within the locus was sufficient to confer resistance. CBP re-expression in either T. brucei or T. congolense AN11736-resistant lines restored drug-susceptibility. CBPs act by cleaving the benzoxaborole AN11736 to a carboxylic acid derivative, revealing a prodrug activation mechanism. Loss of CBP activity results in massive reduction in net uptake of AN11736, indicating that entry is facilitated by the concentration gradient created by prodrug metabolism.

Diverged composition and regulation of the Trypanosoma brucei origin recognition complex that mediates DNA replication initiation.

Initiation of DNA replication depends upon recognition of genomic sites, termed origins, by AAA+ ATPases. In prokaryotes a single factor binds each origin, whereas in eukaryotes this role is played by a six-protein origin recognition complex (ORC). Why eukaryotes evolved a multisubunit initiator, and the roles of each component, remains unclear. In Trypanosoma brucei, an ancient unicellular eukaryote, only one ORC-related initiator, TbORC1/CDC6, has been identified by sequence homology. Here we show that three TbORC1/CDC6-interacting factors also act in T. brucei nuclear DNA replication and demonstrate that TbORC1/CDC6 interacts in a high molecular complex in which a diverged Orc4 homologue and one replicative helicase subunit can also be found. Analysing the subcellular localization of four TbORC1/CDC6-interacting factors during the cell cycle reveals that one factor, TbORC1B, is not a static constituent of ORC but displays S-phase restricted nuclear localization and expression, suggesting it positively regulates replication. This work shows that ORC architecture and regulation are diverged features of DNA replication initiation in T. brucei, providing new insight into this key stage of eukaryotic genome copying.

The Leishmania major†BBSome subunit BBS1 is essential for parasite virulence in the mammalian host.

Bardet-Biedl syndrome (BBS) is a human genetic disorder with a spectrum of symptoms caused by primary cilium dysfunction. The disease is caused by mutations in one of at least 17 identified genes, of which seven encode subunits of the BBSome, a protein complex required for specific trafficking events to and from the primary cilium. The molecular mechanisms associated with BBSome function remain to be fully elucidated. Here, we generated null and complemented mutants of the BBSome subunit BBS1 in the protozoan parasite, Leishmania. In the absence of BBS1, extracellular parasites have no apparent defects in growth, flagellum assembly, motility or differentiation in vitro but there is accumulation of vacuole-like structures close to the flagellar pocket. Infectivity of these parasites for macrophages in vitro is reduced compared with wild-type controls but the null parasites retain the ability to differentiate to the intracellular amastigote stage. However, infectivity of BBS1 null parasites is severely compromised in a BALB/c mouse footpad model. We hypothesize that the absence of BBS1 in Leishmania leads to defects in specific trafficking events that affect parasite persistence in the host. This is the first report of an association between the BBSome complex and pathogen infectivity.

Alternatively activated macrophages elicited by helminth infection can be reprogrammed to enable microbial killing.

The prime function of classically activated macrophages (activated by Th1-type signals, such as IFN-gamma) is microbial destruction. Alternatively activated macrophages (activated by Th2 cytokines, such as IL-4 and IL-13) play important roles in allergy and responses to helminth infection. We utilize a murine model of filarial infection, in which adult nematodes are surgically implanted into the peritoneal cavity of mice, as an in vivo source of alternatively activated macrophages. At 3 wk postinfection, the peritoneal exudate cell population is dominated by macrophages, termed nematode-elicited macrophages (NeMphi), that display IL-4-dependent features such as the expression of arginase 1, RELM-alpha (resistin-like molecule alpha), and Ym1. Since increasing evidence suggests that macrophages show functional adaptivity, the response of NeMphi to proinflammatory Th1-activating signals was investigated to determine whether a switch between alternative and classical activation could occur in macrophages differentiated in an in vivo infection setting. Despite the long-term exposure to Th2 cytokines and antiinflammatory signals in vivo, we found that NeMphi were not terminally differentiated but could develop a more classically activated phenotype in response to LPS and IFN-gamma. This was reflected by a switch in the enzymatic pathway for arginine metabolism from arginase to inducible NO synthase and the reduced expression of RELM-alpha and Ym1. Furthermore, this enabled NeMphi to become antimicrobial, as LPS/IFN-gamma-treated NeMphi produced NO that mediated killing of Leishmania mexicana. However, the adaptation to antimicrobial function did not extend to key regulatory pathways, such as IL-12 production, which remained unaltered.

Real-time imaging of Leishmania mexicana-infected early phagosomes: a study using primary macrophages generated from green fluorescent protein-Rab5 transgenic mice.

The small GTPase Rab5 is a key regulator of endosome/phagosome maturation and in intravesicular infections marks a phagosome stage at which decisions over pathogen replication or destruction are integrated. It is currently unclear whether Leishmania-infected phagosomes uniformly pass through a Rab5(+) stage on their intracellular path to compartments with late endosomal/early lysosomal characteristics. Differences in routes and final compartments could have consequences for accessibility to antileishmanial drugs. Here, we generated a unique gfp-rab5 transgenic mouse model to visualize Rab5 recruitment to early parasite-containing phagosomes in primary host cells. Using real-time fluorescence imaging of phagosomes carrying Leishmania mexicana, we determined that parasite-infested phagosomes follow a uniform sequence of transient Rab5 recruitment. Residence in Rab5(+) compartments was much shorter compared with phagosomes harboring latex beads. Furthermore, a comparative analysis of parasite life-cycle stages and mutants deficient in lpg1, the gene encoding the enzyme required for synthesis of the dominant surface lipophosphoglycan, indicated that parasite surface ligands and host cell receptors modulate pathogen residence times in Rab5(+) phagosomes, but, as far as tested, had no significant effect on intracellular L. mexicana survival or replication.

Transgenic, fluorescent Leishmania mexicana allow direct analysis of the proteome of intracellular amastigotes.

Investigating the proteome of intracellular pathogens is often hampered by inadequate methodologies to purify the pathogen free of host cell material. This has also precluded direct proteome analysis of the intracellular, amastigote form of Leishmania spp., protozoan parasites that cause a spectrum of diseases that affect some 12 million patients worldwide. Here a method is presented that combines classic, isopycnic density centrifugation with fluorescent particle sorting for purification by exploiting transgenic, fluorescent parasites to allow direct proteome analysis of the purified organisms. By this approach the proteome of intracellular Leishmania mexicana amastigotes was compared with that of extracellular promastigotes that are transmitted by insect vectors. In total, 509 different proteins were identified by mass spectrometry and database search. This number corresponds to approximately 6% of gene products predicted from the reference genome of Leishmania major. Intracellular amastigotes synthesized significantly more proteins with basic pI and showed a greater abundance of enzymes of fatty acid catabolism, which may reflect their living in acidic habitats and metabolic adaptation to nutrient availability, respectively. Bioinformatics analyses of the genes corresponding to the protein data sets produced clear evidence for skewed codon usage and translational bias in these organisms. Moreover analysis of the subset of genes whose products were more abundant in amastigotes revealed characteristic sequence motifs in 3'-untranslated regions that have been linked to translational control elements. This suggests that proteome data sets may be used to identify regulatory elements in mRNAs. Last but not least, at 6% coverage the proteome identified all vaccine antigens tested to date. Thus, the present data set provides a valuable resource for selection of candidate vaccine antigens.

Genetic validation of Leishmania genes essential for amastigote survival in vivo using N-myristoyltransferase as a model.

BACKGROUND: Proving that specific genes are essential for the intracellular viability of Leishmania parasites within macrophages remains a challenge for the identification of suitable targets for drug development. This is especially evident in the absence of a robust inducible expression system or functioning RNAi machinery that works in all Leishmania species. Currently, if a target gene of interest in extracellular parasites can only be deleted from its genomic locus in the presence of ectopic expression from a wild type copy, it is assumed that this gene will also be essential for viability in disease-promoting intracellular parasites. However, functional essentiality must be proven independently in both life-cycle stages for robust validation of the gene of interest as a putative target for chemical intervention. METHODS: Here, we have used plasmid shuffle methods in vivo to provide supportive genetic evidence that N-myristoyltransferase (NMT) is essential for Leishmania viability throughout the parasite life-cycle. Following confirmation of NMT essentiality in vector-transmitted promastigotes, a range of mutant parasites were used to infect mice prior to negative selection pressure to test the hypothesis that NMT is also essential for parasite viability in an established infection. RESULTS: Ectopically-expressed NMT was only dispensable under negative selection in the presence of another copy. Total parasite burdens in animals subjected to negative selection were comparable to control groups only if an additional NMT copy, not affected by the negative selection, was expressed. CONCLUSIONS: NMT is an essential gene in all parasite life-cycle stages, confirming its role as a genetically-validated target for drug development.

Leishmania Encodes a Bacterium-like 2,4-Dienoyl-Coenzyme A Reductase That Is Required for Fatty Acid ß-Oxidation and Intracellular Parasite Survival.

Leishmania spp. are protozoan parasites that cause a spectrum of important diseases in humans. These parasites develop as extracellular promastigotes in the digestive tract of their insect vectors and as obligate intracellular amastigotes that infect macrophages and other phagocytic cells in their vertebrate hosts. Promastigote-to-amastigote differentiation is associated with marked changes in metabolism, including the upregulation of enzymes involved in fatty acid ß-oxidation, which may reflect adaptation to the intracellular niche. Here, we have investigated the function of one of these enzymes, a putative 2,4-dienoyl-coenzyme A (CoA) reductase (DECR), which is specifically required for the ß-oxidation of polyunsaturated fatty acids. The Leishmania DECR shows close homology to bacterial DECR proteins, suggesting that it was acquired by lateral gene transfer. It is present in other trypanosomatids that have obligate intracellular stages (i.e., Trypanosoma cruzi and Angomonas) but is absent from dixenous parasites with an exclusively extracellular lifestyle (i.e., Trypanosoma brucei). A DECR-green fluorescent protein (GFP) fusion protein was localized to the mitochondrion in both promastigote and amastigote stages, and the levels of expression increased in the latter stages. A Leishmania major Δdecr null mutant was unable to catabolize unsaturated fatty acids and accumulated the intermediate 2,4-decadienoyl-CoA, confirming DECR's role in ß-oxidation. Strikingly, the L. major Δdecr mutant was unable to survive in macrophages and was avirulent in BALB/c mice. These findings suggest that ß-oxidation of polyunsaturated fatty acids is essential for intracellular parasite survival and that the bacterial origin of key enzymes in this pathway could be exploited in developing new therapies.IMPORTANCE The Trypanosomatidae are protozoan parasites that infect insects, plants, and animals and have evolved complex monoxenous (single host) and dixenous (two hosts) lifestyles. A number of species of Trypanosomatidae, including Leishmania spp., have evolved the capacity to survive within intracellular niches in vertebrate hosts. The adaptations, metabolic and other, that are associated with development of intracellular lifestyles remain poorly defined. We show that genomes of Leishmania and Trypanosomatidae that can survive intracellularly encode a 2,4-dienoyl-CoA reductase that is involved in catabolism of a subclass of fatty acids. The trypanosomatid enzyme shows closest similarity to the corresponding bacterial enzymes and is located in the mitochondrion and essential for intracellular growth of Leishmania The findings suggest that acquisition of this gene by lateral gene transfer from bacteria by ancestral monoxenous Trypanosomatidae likely contributed to the development of a dixenous lifestyle of these parasites.

Novel Thienopyrimidine Inhibitors of Leishmania N-Myristoyltransferase with On-Target Activity in Intracellular Amastigotes.

The leishmaniases, caused by Leishmania species of protozoan parasites, are neglected tropical diseases with millions of cases worldwide. Current therapeutic approaches are limited by toxicity, resistance, and cost. N-Myristoyltransferase (NMT), an enzyme ubiquitous and essential in all eukaryotes, has been validated via genetic and pharmacological methods as a promising anti-leishmanial target. Here we describe a comprehensive structure-activity relationship (SAR) study of a thienopyrimidine series previously identified in a high-throughput screen against Leishmania NMT, across 68 compounds in enzyme- and cell-based assay formats. Using a chemical tagging target engagement biomarker assay, we identify the first inhibitor in this series with on-target NMT activity in leishmania parasites. Furthermore, crystal structure analyses of 12 derivatives in complex with Leishmania major NMT revealed key factors important for future structure-guided optimization delivering IMP-105 (43), a compound with modest activity against Leishmania donovani intracellular amastigotes and excellent selectivity (>660-fold) for Leishmania NMT over human NMTs.

Cloning and functional complementation of ten Schistosoma mansoni phosphodiesterases expressed in the mammalian host stages.

Only a single drug against schistosomiasis is currently available and new drug development is urgently required but very few drug targets have been validated and characterised. However, regulatory systems including cyclic nucleotide metabolism are emerging as primary candidates for drug discovery. Here, we report the cloning of ten cyclic nucleotide phosphodiesterase (PDE) genes of S. mansoni, out of a total of 11 identified in its genome. We classify these PDEs by homology to human PDEs. Male worms displayed higher expression levels for all PDEs, in mature and juvenile worms, and schistosomula. Several functional complementation approaches were used to characterise these genes. We constructed a Trypanosoma brucei cell line in which expression of a cAMP-degrading PDE complements the deletion of TbrPDEB1/B2. Inhibitor screens of these cells expressing only either SmPDE4A, TbrPDEB1 or TbrPDEB2, identified highly potent inhibitors of the S. mansoni enzyme that elevated the cellular cAMP concentration. We further expressed most of the cloned SmPDEs in two pde1Δ/pde2Δ strains of Saccharomyces cerevisiae and some also in a specialised strain of Schizosacharomyces pombe. Five PDEs, SmPDE1, SmPDE4A, SmPDE8, SmPDE9A and SmPDE11 successfully complemented the S. cerevisiae strains, and SmPDE7var also complemented to a lesser degree, in liquid culture. SmPDE4A, SmPDE8 and SmPDE11 were further assessed in S. pombe for hydrolysis of cAMP and cGMP; SmPDE11 displayed considerable preferrence for cGMP over cAMP. These results and tools enable the pursuit of a rigorous drug discovery program based on inhibitors of S. mansoni PDEs.

Targeting the trypanosome kinetochore with CLK1 protein kinase inhibitors.

The kinetochore is a macromolecular structure that assembles on the centromeres of chromosomes and provides the major attachment point for spindle microtubules during mitosis. In Trypanosoma brucei, the proteins that make up the kinetochore are highly divergent; the inner kinetochore comprises at least 20 distinct and essential proteins (KKT1-20) that include four protein kinases-CLK1 (also known as KKT10), CLK2 (also known as KKT19), KKT2 and KKT3. Here, we report the identification and characterization of the amidobenzimidazoles (AB) protein kinase inhibitors that show nanomolar potency against T. brucei bloodstream forms, Leishmania and Trypanosoma cruzi. We performed target deconvolution analysis using a selection of 29 T. brucei mutants that overexpress known essential protein kinases, and identified CLK1 as a primary target. Biochemical studies and the co-crystal structure of CLK1 in complex with AB1 show that the irreversible competitive inhibition of CLK1 is dependent on a Michael acceptor forming an irreversible bond with Cys 215 in the ATP-binding pocket, a residue that is not present in human CLK1, thereby providing selectivity. Chemical inhibition of CLK1 impairs inner kinetochore recruitment and compromises cell-cycle progression, leading to cell death. This research highlights a unique drug target for trypanosomatid parasitic protozoa and a new chemical tool for investigating the function of their divergent kinetochores.

Positively selected modifications in the pore of TbAQP2 allow pentamidine to enter Trypanosoma brucei.

Mutations in the Trypanosoma brucei aquaporin AQP2 are associated with resistance to pentamidine and melarsoprol. We show that TbAQP2 but not TbAQP3 was positively selected for increased pore size from a common ancestor aquaporin. We demonstrate that TbAQP2's unique architecture permits pentamidine permeation through its central pore and show how specific mutations in highly conserved motifs affect drug permeation. Introduction of key TbAQP2 amino acids into TbAQP3 renders the latter permeable to pentamidine. Molecular dynamics demonstrates that permeation by dicationic pentamidine is energetically favourable in TbAQP2, driven by the membrane potential, although aquaporins are normally strictly impermeable for ionic species. We also identify the structural determinants that make pentamidine a permeant although most other diamidine drugs are excluded. Our results have wide-ranging implications for optimising antitrypanosomal drugs and averting cross-resistance. Moreover, these new insights in aquaporin permeation may allow the pharmacological exploitation of other members of this ubiquitous gene family.

African sleeping sickness is a potentially deadly illness caused by the parasite Trypanosoma brucei. The disease is treatable, but many of the current treatments are old and are becoming increasingly ineffective. For instance, resistance is growing against pentamidine, a drug used in the early stages in the disease, as well as against melarsoprol, which is deployed when the infection has progressed to the brain. Usually, cases resistant to pentamidine are also resistant to melarsoprol, but it is still unclear why, as the drugs are chemically unrelated. Studies have shown that changes in a water channel called aquaglyceroporin 2 (TbAQP2) contribute to drug resistance in African sleeping sickness; this suggests that it plays a role in allowing drugs to kill the parasite. This molecular 'drain pipe' extends through the surface of T. brucei, and should allow only water and a molecule called glycerol in and out of the cell. In particular, the channel should be too narrow to allow pentamidine or melarsoprol to pass through. One possibility is that, in T. brucei, the TbAQP2 channel is abnormally wide compared to other members of its family. Alternatively, pentamidine and melarsoprol may only bind to TbAQP2, and then 'hitch a ride' when the protein is taken into the parasite as part of the natural cycle of surface protein replacement. Alghamdi et al. aimed to tease out these hypotheses. Computer models of the structure of the protein were paired with engineered changes in the key areas of the channel to show that, in T. brucei, TbAQP2 provides a much broader gateway into the cell than observed for similar proteins. In addition, genetic analysis showed that this version of TbAQP2 has been actively selected for during the evolution process of T. brucei. This suggests that the parasite somehow benefits from this wider aquaglyceroporin variant. This is a new resistance mechanism, and it is possible that aquaglyceroporins are also larger than expected in other infectious microbes. The work by Alghamdi et al. therefore provides insight into how other germs may become resistant to drugs.

Modulation of Inflammatory Reactions by Low-Dose Ionizing Radiation: Cytokine Release of Murine Endothelial Cells Is Dependent on Culture Conditions.

BACKGROUND: In many European countries, patients with a variety of chronical inflammatory diseases are treated with low-dose radiotherapy (LD-RT). In contrast to high-dose irradiation given to tumor patients, little is known about radiobiological mechanisms underlying this clinical successful LD-RT application. The objective of this study was to gain a better insight into the modulation of inflammatory reactions after LD-RT on the basis of endothelial cells (EC) as major participants and regulators of inflammation. METHODS: Three murine EC lines were cultivated under 2D and 3D culture conditions and irradiated with doses from 0.01 Gy to 2 Gy. To simulate an inflammatory situation, cells were activated with TNF-α. After LD-RT, a screening of numerous inflammatory markers was determined by multiplex assay, followed by detailed analyses of four cytokines (KC, MCP-1, RANTES, and G-CSF). Additionally, the monocyte binding to EC was analyzed. RESULTS: Cytokine concentrations were dependent on culture condition, IR dose, time point after IR, and EC origin. IR caused nonlinear dose-dependent effects on secretion of the proinflammatory cytokines KC, MCP-1, and RANTES. The monocyte adhesion was significantly enhanced after IR as well as activation. CONCLUSIONS: The study shows that LD-RT, also using very low radiation doses, has a clear immunomodulatory effect on EC as major participants and regulators of inflammation.

Changes to cholesterol trafficking in macrophages by Leishmania parasites infection.

Leishmania spp. are protozoan parasites that are transmitted by sandfly vectors during blood sucking to vertebrate hosts and cause a spectrum of diseases called leishmaniases. It has been demonstrated that host cholesterol plays an important role during Leishmania infection. Nevertheless, little is known about the intracellular distribution of this lipid early after internalization of the parasite. Here, pulse-chase experiments with radiolabeled cholesteryl esterified to fatty acids bound to low-density lipoproteins indicated that retention of this source of cholesterol is increased in parasite-containing subcellular fractions, while uptake is unaffected. This is correlated with a reduction or absence of detectable NPC1 (Niemann-Pick disease, type C1), a protein responsible for cholesterol efflux from endocytic compartments, in the Leishmania mexicana habitat and infected cells. Filipin staining revealed a halo around parasites within parasitophorous vacuoles (PV) likely representing free cholesterol accumulation. Labeling of host cell membranous cholesterol by fluorescent cholesterol species before infection revealed that this pool is also trafficked to the PV but becomes incorporated into the parasites' membranes and seems not to contribute to the halo detected by filipin. This cholesterol sequestration happened early after infection and was functionally significant as it correlated with the upregulation of mRNA-encoding proteins required for cholesterol biosynthesis. Thus, sequestration of cholesterol by Leishmania amastigotes early after infection provides a basis to understand perturbation of cholesterol-dependent processes in macrophages that were shown previously by others to be necessary for their proper function in innate and adaptive immune responses.

Global Profiling and Inhibition of Protein Lipidation in Vector and Host Stages of the Sleeping Sickness Parasite Trypanosoma brucei.

The enzyme N-myristoyltransferase (NMT) catalyzes the essential fatty acylation of substrate proteins with myristic acid in eukaryotes and is a validated drug target in the parasite Trypanosoma brucei, the causative agent of African trypanosomiasis (sleeping sickness). N-Myristoylation typically mediates membrane localization of proteins and is essential to the function of many. However, only a handful of proteins are experimentally validated as N-myristoylated in T. brucei. Here, we perform metabolic labeling with an alkyne-tagged myristic acid analogue, enabling the capture of lipidated proteins in insect and host life stages of T. brucei. We further compare this with a longer chain palmitate analogue to explore the chain length-specific incorporation of fatty acids into proteins. Finally, we combine the alkynyl-myristate analogue with NMT inhibitors and quantitative chemical proteomics to globally define N-myristoylated proteins in the clinically relevant bloodstream form parasites. This analysis reveals five ARF family small GTPases, calpain-like proteins, phosphatases, and many uncharacterized proteins as substrates of NMT in the parasite, providing a global view of the scope of this important protein modification and further evidence for the crucial and pleiotropic role of NMT in the cell.

Mapping replication dynamics in Trypanosoma brucei reveals a link with telomere transcription and antigenic variation.

Survival of Trypanosoma brucei depends upon switches in its protective Variant Surface Glycoprotein (VSG) coat by antigenic variation. VSG switching occurs by frequent homologous recombination, which is thought to require locus-specific initiation. Here, we show that a RecQ helicase, RECQ2, acts to repair DNA breaks, including in the telomeric site of VSG expression. Despite this, RECQ2 loss does not impair antigenic variation, but causes increased VSG switching by recombination, arguing against models for VSG switch initiation through direct generation of a DNA double strand break (DSB). Indeed, we show DSBs inefficiently direct recombination in the VSG expression site. By mapping genome replication dynamics, we reveal that the transcribed VSG expression site is the only telomeric site that is early replicating - a differential timing only seen in mammal-infective parasites. Specific association between VSG transcription and replication timing reveals a model for antigenic variation based on replication-derived DNA fragility.

Genome-wide mapping reveals single-origin chromosome replication in Leishmania, a eukaryotic microbe.

BACKGROUND: DNA replication initiates on defined genome sites, termed origins. Origin usage appears to follow common rules in the eukaryotic organisms examined to date: all chromosomes are replicated from multiple origins, which display variations in firing efficiency and are selected from a larger pool of potential origins. To ask if these features of DNA replication are true of all eukaryotes, we describe genome-wide origin mapping in the parasite Leishmania. RESULTS: Origin mapping in Leishmania suggests a striking divergence in origin usage relative to characterized eukaryotes, since each chromosome appears to be replicated from a single origin. By comparing two species of Leishmania, we find evidence that such origin singularity is maintained in the face of chromosome fusion or fission events during evolution. Mapping Leishmania origins suggests that all origins fire with equal efficiency, and that the genomic sites occupied by origins differ from related non-origins sites. Finally, we provide evidence that origin location in Leishmania displays striking conservation with Trypanosoma brucei, despite the latter parasite replicating its chromosomes from multiple, variable strength origins. CONCLUSIONS: The demonstration of chromosome replication for a single origin in Leishmania, a microbial eukaryote, has implications for the evolution of origin multiplicity and associated controls, and may explain the pervasive aneuploidy that characterizes Leishmania chromosome architecture.

Global analysis of protein N-myristoylation and exploration of N-myristoyltransferase as a drug target in the neglected human pathogen Leishmania donovani.

N-Myristoyltransferase (NMT) modulates protein function through the attachment of the lipid myristate to the N terminus of target proteins, and is a promising drug target in eukaryotic parasites such as Leishmania donovani. Only a small number of NMT substrates have been characterized in Leishmania, and a global picture of N-myristoylation is lacking. Here, we use metabolic tagging with an alkyne-functionalized myristic acid mimetic in live parasites followed by downstream click chemistry and analysis to identify lipidated proteins in both the promastigote (extracellular) and amastigote (intracellular) life stages. Quantitative chemical proteomics is used to profile target engagement by NMT inhibitors, and to define the complement of N-myristoylated proteins. Our results provide new insight into the multiple pathways modulated by NMT and the pleiotropic effects of NMT inhibition. This work constitutes the first global experimental analysis of protein lipidation in Leishmania, and reveals the extent of NMT-related biology yet to be explored for this neglected human pathogen.

Discovery of high affinity inhibitors of Leishmania donovani N-myristoyltransferase.

N-Myristoyltransferase (NMT) is a potential drug target in Leishmania parasites. Scaffold-hopping from published inhibitors yielded the serendipitous discovery of a chemotype selective for Leishmania donovani NMT; development led to high affinity inhibitors with excellent ligand efficiency. The binding mode was characterised by crystallography and provides a structural rationale for selectivity.

Using a non-image-based medium-throughput assay for screening compounds targeting N-myristoylation in intracellular Leishmania amastigotes.

We have refined a medium-throughput assay to screen hit compounds for activity against N-myristoylation in intracellular amastigotes of Leishmania donovani. Using clinically-relevant stages of wild type parasites and an Alamar blue-based detection method, parasite survival following drug treatment of infected macrophages is monitored after macrophage lysis and transformation of freed amastigotes into replicative extracellular promastigotes. The latter transformation step is essential to amplify the signal for determination of parasite burden, a factor dependent on equivalent proliferation rate between samples. Validation of the assay has been achieved using the anti-leishmanial gold standard drugs, amphotericin B and miltefosine, with EC50 values correlating well with published values. This assay has been used, in parallel with enzyme activity data and direct assay on isolated extracellular amastigotes, to test lead-like and hit-like inhibitors of Leishmania N-myristoyl transferase (NMT). These were derived both from validated in vivo inhibitors of Trypanosoma brucei NMT and a recent high-throughput screen against L. donovani NMT. Despite being a potent inhibitor of L. donovani NMT, the activity of the lead T. brucei NMT inhibitor (DDD85646) against L. donovani amastigotes is relatively poor. Encouragingly, analogues of DDD85646 show improved translation of enzyme to cellular activity. In testing the high-throughput L. donovani hits, we observed macrophage cytotoxicity with compounds from two of the four NMT-selective series identified, while all four series displayed low enzyme to cellular translation, also seen here with the T. brucei NMT inhibitors. Improvements in potency and physicochemical properties will be required to deliver attractive lead-like Leishmania NMT inhibitors.