Effect and localization of trifluralin in Plasmodium falciparum gametocytes: an electron microscopic study

Repurposing the mitotic machinery to drive cellular elongation and chromatin reorganisation in Plasmodium falciparum gametocytes

The sexual stage gametocytes of the malaria parasite, Plasmodium falciparum, adopt a falciform (crescent) shape driven by the assembly of a network of microtubules anchored to a cisternal inner membrane complex (IMC). Using 3D electron microscopy, we show that a non-mitotic microtubule organizing center (MTOC), embedded in the parasite's nuclear membrane, orients the endoplasmic reticulum and the nascent IMC and seeds cytoplasmic microtubules. A bundle of microtubules extends into the nuclear lumen, elongating the nuclear envelope and capturing the chromatin. Classical mitotic machinery components, including centriolar plaque proteins, Pfcentrin-1 and -4, microtubule-associated protein, End-binding protein-1, kinetochore protein, PfNDC80 and centromere-associated protein, PfCENH3, are involved in the nuclear microtubule assembly/disassembly process. Depolymerisation of the microtubules using trifluralin prevents elongation and disrupts the chromatin, centromere and kinetochore organisation. We show that the unusual non-mitotic hemispindle plays a central role in chromatin organisation, IMC positioning and subpellicular microtubule formation in gametocytes.

Inhibition of chloroplast translation as a new target for herbicides

The rise in herbicide resistance over recent decades threatens global agriculture and food security and so discovery of new modes of action is increasingly important. Here we reveal linezolid, an oxazolidinone antibiotic that inhibits microbial translation, is also herbicidal. To validate the herbicidal mode of action of linezolid we confirmed its micromolar inhibition is specific to chloroplast translation and did not affect photosynthesis directly. To assess the herbicide potential of linezolid, testing against a range of weed and crop species found it effective pre- and post-emergence. Using structure-activity analysis we identified the critical elements for herbicidal activity, but importantly also show, using antimicrobial susceptibility assays, that separation of antibacterial and herbicidal activities was possible. Overall these results validate chloroplast translation as a viable herbicidal target.

Improved herbicide discovery using physico-chemical rules refined by antimalarial library screening

Herbicides have physico-chemical properties not unlike orally-delivered human drugs, but are known to diverge in their limits for proton donors, partition coefficients and molecular weight. To further refine rules specific for herbicides, we exploited the close evolutionary relationship between Plasmodium falciparum and plants by screening the entire Malaria Box, a chemical library of novel chemical scaffolds with activity against the blood stage of P. falciparum. Initial screening against Arabidopsis thaliana on agar media and subsequently on soil demonstrated the crucial nature of log P and formal charge are to active molecules. Using this information, a weighted scoring system was applied to a large chemical library of liver-stage effective antimalarial leads, and of the six top-scoring compounds, one had potency comparable to that of commercial herbicides. This novel compound, MMV1206386, has no close structural analogues among commercial herbicides. Physiological profiling suggested that MMV1206386 has a new mode of action and overall demonstrates how weighted rules can help during herbicide discovery programs.

Reversible host cell remodeling underpins deformability changes in malaria parasite sexual blood stages

The sexual blood stage of the human malaria parasite Plasmodium falciparum undergoes remarkable biophysical changes as it prepares for transmission to mosquitoes. During maturation, midstage gametocytes show low deformability and sequester in the bone marrow and spleen cords, thus avoiding clearance during passage through splenic sinuses. Mature gametocytes exhibit increased deformability and reappear in the peripheral circulation, allowing uptake by mosquitoes. Here we define the reversible changes in erythrocyte membrane organization that underpin this biomechanical transformation. Atomic force microscopy reveals that the length of the spectrin cross-members and the size of the skeletal meshwork increase in developing gametocytes, then decrease in mature-stage gametocytes. These changes are accompanied by relocation of actin from the erythrocyte membrane to the Maurer's clefts. Fluorescence recovery after photobleaching reveals reversible changes in the level of coupling between the membrane skeleton and the plasma membrane. Treatment of midstage gametocytes with cytochalasin D decreases the vertical coupling and increases their filterability. A computationally efficient coarse-grained model of the erythrocyte membrane reveals that restructuring and constraining the spectrin meshwork can fully account for the observed changes in deformability.

The inhibitory effect of cromolyn sodium and ketotifen on Toxoplasma gondii entrance into host cells in vitro and in vivo

Toxoplasma gondii is a protozoan with worldwide distribution and in spite of increasing information about its biology, treatment of toxoplasmosis is restricted to a few drugs and unfortunately using of each of drugs is associated with significant side effects in patients. This study was designed to evaluate the efficacy of cromolyn sodium and ketotifen as alternative drugs for the treatment of toxoplasmosis. In vitro; in case group, concentrations of 1, 5, 10 and 15 µg/ml of ketotifen and cromolyn sodium were added to RPMI medium containing peritoneal macrophages. After 1 h incubation and adding tachyzoites to medium, efficacy rate of these drugs in entrance inhibition of Toxoplasma tachyzoites into macrophages were evaluated after 30 and 60 min. In vivo; case groups received ketotifen and cromolyn sodium with different concentrations at various times. Control groups received none of drugs. We found that in vitro; after 60 min the best efficacy of these drugs in inhibition of cell entrance of Toxoplasma was observed at 15 µg/ml (78.9 ± 1.70 and 91.97 ± 0.37 %, respectively) (P < 0.05). In vivo; after 60 min ketotifen at 2 mg/kg in 3 h before tachyzoite injection (69.83 ± 2.25 %), and cromolyn sodium, at 10 mg/kg in 6 h before tachyzoite injection (80.47 ± 2/49 %) had the best effect on inhibition of Toxoplasma entry into the cells (P < 0.05). Our findings show that ketotifen and cromolyn sodium are suitable drugs for entrance inhibition of tachyzoites into nucleated cells in vitro and in vivo.

Characterization of trifluralin binding with recombinant tubulin from Trypanosoma brucei

The binding kinetics of five novel trifluralin analogs with recombinant alpha- and beta-tubulin proteins from Trypanosoma brucei rhodesiense was determined. Native tubulin from rats was used to determine the extent of binding of each analog to mammalian tubulin. The results of this study clearly demonstrate two important characteristics of the binding of these trifluralins to tubulin. Firstly, they have specific affinity for trypanosomal tubulin compared with mammalian tubulin irrespective of the chemical composition of the trifluralin analog tested. Secondly, they have a stronger affinity for trypanosomal alpha-tubulin compared with trypanosomal beta-tubulin. In addition, compounds 1007, 1008, 1016, and 1017 have strong binding affinities for alpha-tubulin, with limited binding affinity for mammalian tubulin, which indicates that these compounds selectively bind to trypanosomal tubulin.

Mutations in alpha-tubulin confer dinitroaniline resistance at a cost to microtubule function

Protozoan microtubules are sensitive to disruption by dinitroanilines, compounds that kill intracellular Toxoplasma gondii parasites without affecting microtubules in vertebrate host cells. We previously isolated a number of resistant Toxoplasma lines that harbor mutations to the alpha1-tubulin gene. Some of the mutations are localized in or near the M and N loops, domains that coordinate lateral interactions between protofilaments. Other resistance mutations map to a computationally identified binding site beneath the N loop. Allelic replacement of wild-type alpha1-tubulin with the individual mutations is sufficient to confer dinitroaniline resistance. Some mutations seem to increase microtubule length, suggesting that they increase subunit affinity. All mutations are associated with replication defects that decrease parasite viability. When parasites bearing the N loop mutation Phe52Tyr are grown without dinitroaniline selection, they spontaneously acquired secondary mutations in the M loop (Ala273Val) or in an alpha-tubulin-specific insert that stabilizes the M loop (Asp367Val). Parasites with the double mutations have both reduced resistance and diminished incidence of replication defects, suggesting that the secondary mutations decrease protofilament affinity to increase parasite fitness.

Binding and Interaction of Dinitroanilines with Apicomplexan and Kinetoplastid α-Tubulin

Despite years of use as commercial herbicides, it is still unclear how dinitroanilines interact with tubulin, how they cause microtubule disassembly, and why they are selectively active against plant and protozoan tubulin. In this work, through a series of computational studies, a common binding site of oryzalin, trifluralin, and GB-II-5 on apicomplexan and kinetoplastid alpha-tubulin is proposed. Furthermore, to investigate how dinitroanilines affect tubulin dynamics, molecular dynamics simulations of Leishmania alpha-tubulin with and without a bound dinitroaniline are performed. The results obtained provide insight into the molecular mechanism by which these compounds interact with tubulin and function to prevent microtubule assembly. Finally, to aid in the design of effective parasitic microtubule inhibitors, several novel dinitroaniline analogues are evaluated. The location of the binding site and the relative binding affinities of the dinitroanilines all agree well with experimental data.

Cellular and molecular actions of dinitroaniline and phosphorothioamidate herbicides on Plasmodium falciparum: Tubulin as a specific antimalarial target

Microtubules play important roles in cell division, motility and structural integrity of malarial parasites. Some microtubule inhibitors disrupt parasite development at very low concentrations, but most of them also kill mammalian cells. However, the dinitroaniline family of herbicides, which bind specifically to plant tubulin, have inhibitory activity on plant cells but are relatively non-toxic to human cells. Certain dinitroanilines are also inhibitory to various protozoal parasites including Plasmodium. Here we demonstrate that the dinitroanilines trifluralin and oryzalin inhibited progression of erythrocytic Plasmodium falciparum through schizogony, blocked mitotic division, and caused accumulation of abnormal microtubular structures. Moreover, radiolabelled trifluralin interacted with purified, recombinant parasite tubulins but to a much lesser extent with bovine tubulins. The phosphorothioamidate herbicide amiprophos-methyl, which has the same herbicidal mechanism as dinitroanilines, also had antimalarial activity and a similar action on schizogony. These data suggest that P. falciparum tubulin contains a dinitroaniline/phosphorothioamidate-binding site that is not conserved in humans and might be a target for new antimalarial drugs.

The restoring effect of trifluralin and benznidazole on the abnormal fatty-acid pattern induced by Trypanosoma cruzi in the liver microsomes of infected mice

The fatty-acid composition of liver lipids from mice infected with Trypanosoma cruzi (clone H510C8C3) or uninfected mice was investigated. The infected animals were treated orally for 30 days, with trifluralin (TFL) or benznidazole (BNZ), each at 100mg/kg.day, or only with the peanut oil used as the drug vehicle. The uninfected mice were also given the peanut oil. The treatments were stopped 10 days before the animals were killed. The liver microsomal lipids of each mouse were isolated and then analysed by gas-liquid chromatography. In terms of the total lipids, untreated infection evoked a significant increase in saturated fatty acids and the members of the n-9 fatty-acid family, with a concomitant decrease in the polyenoates of the n-3 and n-6 fatty-acid series. Each lipid subclass was affected to a different extent, the phospholipids being affected most. All lipid fractions, apart from the cholesterol esters, showed a significant increase in the proportion of n-9 isomers. Infection also produced a marked increase in the absolute amounts of triacylglycerides, cholesterol and cholesterol esters in liver microsomal membranes. After BNZ or TFL treatment, the fatty-acid pattern of mice that had been infected was indistinguishable from that of the control mice. The possible role of desaturase activity in the alterations observed is discussed.

Plasmodium: assessment of the antimalarial potential of trifluralin and related compounds using a rat model of malaria, Rattus norvegicus

A rodent model of malaria, Plasmodium berghei was used to assess the antimalarial potential of dinitroaniline herbicides. Trifluralin, pendimethalin, oryzalin, and benfluralin were all active against P. berghei in vitro at, or close to, submicromolar concentrations, with a rank order of potency similar to that against other protozoa. The dinitroanilines did not elicit a cytotoxic effect against a mammalian cell line at concentrations 100-fold higher than those for activity against P. berghei. Neither trifluralin nor oryzalin exhibited any antimalarial activity in vivo after oral administration at the maximum dose tolerated by the host. In a pharmacokinetic study, it was found that the lack of in vivo antimalarial activity was due to poor absorption. Other DNs which have better absorption characteristics than either trifluralin or oryzalin may offer more scope for antimalarial activity in vivo.

Cytoskeleton of apicomplexan parasites

The Apicomplexa are a phylum of diverse obligate intracellular parasites including Plasmodium spp., the cause of malaria; Toxoplasma gondii and Cryptosporidium parvum, opportunistic pathogens of immunocompromised individuals; and Eimeria spp. and Theileria spp., parasites of considerable agricultural importance. These protozoan parasites share distinctive morphological features, cytoskeletal organization, and modes of replication, motility, and invasion. This review summarizes our current understanding of the cytoskeletal elements, the properties of cytoskeletal proteins, and the role of the cytoskeleton in polarity, motility, invasion, and replication. We discuss the unusual properties of actin and myosin in the Apicomplexa, the highly stereotyped microtubule populations in apicomplexans, and a network of recently discovered novel intermediate filament-like elements in these parasites.

Dinitroaniline herbicides against protozoan parasites: the case of Trypanosoma cruzi

The drugs presently in use against Chagas disease are very toxic, inducing a great number of side effects. Alternative treatments are necessary, not only for Chagas disease but also for other diseases caused by protozoan parasites where current drugs pose toxicity problems. The plant microtubule inhibitor trifluralin has previously been tested with success against Leishmania, Trypanosoma brucei and several other protozoan parasites. Trypanosoma cruzi, the causative agent of Chagas disease, is also sensitive to the drug. This sensitivity has been correlated with the deduced amino acid sequences of alpha- and beta-tubulin of T. cruzi as compared with plant, mammal and other parasite sequences.

Motile systems in malaria merozoites: how is the red blood cell invaded?

The ability of the malaria parasite to invade erythrocytes is central to the disease process, but is not thoroughly understood. In particular, little attention has been paid to the motor systems driving invasion. Here, Jennifer Pinder, Ruth Fowler and colleagues review motility in the merozoite. The components of an actomyosin motor are present, including a novel unconventional class XIV myosin, now called Pfmyo-A, which, because of its time of synthesis and location, is likely to generate the force required for invasion. In addition, there is a subpellicular microtubule assemblage in falciparum merozoites, the f-MAST, the integrity of which is necessary for invasion.

Characterization of an alpha-tubulin gene of Cryptosporidium parvum

A gene encoding an alpha-tubulin of Cryptosporidium parvum was isolated and characterized. It had no introns, and encoded a 441-amino acid protein whose predicted ORF represented a typical alpha-tubulin protein with a MW of 50.5 kDa. This tubulin had an amino acid sequence similarity with Apicomplexa Plasmodium falciparum and Toxoplasma gondii higher than 88% and shared a number of conserved motifs.

Microtubule Inhibitors as Potential Antimalarial Agents

The Steering Committee on Drugs for Malaria (CHEMAL) of the UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases (TDR) has identified tubulin as a potential drug target, but one that is not yet ;validated'. Several inhibitors of tubulins, the principal proteins of microtubules, are potent inhibitors of the development and multiplication of malarial parasites in culture and in vivo. However, most of these compounds are also inhibitors of mammalian cell proliferation. Here, Angus Bell reviews the structure and properties of microtubules, their roles in Plasmodium cells, and the effects of various microtubule inhibitors on the parasite. He argues that microtubule inhibitors are not equally toxic to all proliferating cells but, by virtue of differential tubulin binding, show selective toxicity that might allow their use as antimalarial drugs.

Paclitaxel arrests growth of intracellular Toxoplasma gondii

Addition of paclitaxel (Taxol) at a concentration of 1 microM to Toxoplasma gondii-infected human foreskin fibroblasts arrested parasite multiplication. Division of the T. gondii tachyzoite nucleus was inhibited, leading to syncytium-like parasite structures within the fibroblasts by 24 h after infection and treatment of the cultures. By 4 days after infection and treatment of the cultures with paclitaxel, this inhibition was irreversible, since the arrested intracellular form was incapable of leaving the host cell, infecting new cells, and initiating the growth of tachyzoites with normal morphology. Specifically, when paclitaxel was added to infected cells for 4 days and then removed by washing and the infected, paclitaxel-treated cells were cultured for 4 more days, there were no remaining T. gondii organisms with normal morphology. Syncytium-like structures in the cultures that were infected and treated with paclitaxel for 8 days were similar in appearance to those in preparations of infected paclitaxel-treated fibroblasts that had been cultured for 24 to 48 h. Pretreatment of the tachyzoites for 1 h with paclitaxel followed by the removal of the paclitaxel by repeatedly centrifuging and resuspending the parasites in fresh medium without paclitaxel and then adding fresh medium prior to culture of the parasites with fibroblasts did not prevent their invasion of fibroblasts but did affect their subsequent ability to replicate within fibroblasts. Pretreatment of the fibroblasts with paclitaxel also diminished subsequent replication of T. gondii in such host cells after 8 days. Thus, paclitaxel alters the ability of T. gondii to replicate in host cells. Inhibition of parasite microtubules by such compounds at concentrations which do not interfere with the function of host cell microtubules may be useful for development of novel medicines to treat T. gondii infections in the future.

Synthesis and evaluation of dinitroanilines for treatment of cryptosporidiosis

The efficacy of a series of dinitroaniline herbicide derivatives for the treatment of Cryptosporidium parvum infections has been studied. The lead compounds oryzalin (compound 1) and trifluralin (compound 2) have low water solubility (<3 ppm) which was alleged to be a major contributor to their poor pharmacokinetic availability. Derivatives of compounds 1 and 2 were synthesized. In these derivatives the functionality at the C-1 amine position or the C-4 position was substituted with groups with various hydrophilicities to determine if a direct relation existed between water solubility and overall activity. The chlorinated precursors of these derivatives were also examined and were found to be less active in the C. parvum assays, a result in direct contrast to earlier work with Leishmania. Enhanced water solubility alone did not overcome the drug availability problem; however, several candidates with similar activities but with toxicities lower than those of the lead compounds were produced.

Microtubule inhibitors: structure-activity analyses suggest rational models to identify potentially active compounds

Trifluralin, a dinitroaniline microtubule inhibitor currently in use as an herbicide, has been shown to inhibit the proliferation of Plasmodium falciparum, Trypanosoma brucei, and several species of Leishmania, in vitro. As a topical formulation, trifluralin is also effective in vivo (in BALB/c mice) against Leishmania major and Leishmania mexicana. Although trifluralin and other dinitroaniline herbicides show significant activity as antiparasitic compounds, disputed indications of potential carcinogenicity will probably limit advanced development of these substances. However, researchers have suggested that the activity of trifluralin is due to an impurity or contaminant, not to trifluralin itself. We have pursued this lead and identified the structure of the active impurity. This compound, chloralin, is 100 times more active than trifluralin. On the basis of its structure, we developed a rational structure-activity model for chloralin. Using this model, we have successfully predicted and tested active analogs in a Leishmania promastigote assay; thus, we have identified the putative mechanism of action of this class of drugs in Leishmania species. Potentially, this will allow the design of noncarcinogenic, active drugs.

Efficacy of the herbicide trifluralin against four P-glycoprotein-expressing strains of Leishmania

Drug resistance has emerged as a major obstacle to chemotherapy for many infectious diseases. Trifluralin, an antimicrotubule herbicide, is a new experimental drug for treatment of leishmaniasis. Here, we found that it was effective against two strains of Leishmania that express the multidrug-resistant genes ldmdr1 and lmpgpA and two strains that express proteins that are immunologically cross-reactive with mammalian P glycoproteins. These results suggest that trifluralin is not subject to counteractions of these multidrug resistance mechanisms of Leishmania species.