1,2,3-Triazole derivatives: synthesis, docking, cytotoxicity analysis and in vivo antimalarial activity.

Malaria remains one of the most important parasitic diseases in the world. The multidrug-resistant Plasmodium strains make the treatment currently available for malaria less effective. Therefore, the development of new drugs is necessary to overcome therapy resistance. Triazole derivatives exhibit several biological activities and provide a moiety that is promising from the biological perspective. Due to the structural similarity to NADH, it is believed that triazoles can bind to the active site of the Plasmodium lactate dehydrogenase (pLDH) enzyme. The present work evaluates the antimalarial activity of 1,2,3-triazole derivatives by in silico, in vitro, and in vivo studies. Preliminary in silico ADMET studies of the compounds demonstrated good pharmacokinetic properties. In silico docking analysis against LDH of Plasmodium berghei (PbLDH) showed that all compounds presented interactions with the catalytic residue in the active site and affinity similar to that presented by chloroquine; the most common antimalarial drug. Cytotoxicity and hemolysis by these derivatives were evaluated in vitro. The compounds 1, 2, 5, 8, and 9 proved to be non-cytotoxic in the performed tests. In vivo antimalarial activity was evaluated using mice infected with Plasmodium berghei NK65. The five compounds tested exhibited antimalarial activity until nine days post-infection. The compound 5 showed promising activities, with about 70% parasitemia suppression. Considering the in vitro and in vivo studies, we believe the compound 5 to be the most promising molecule for further studies in antimalarial chemotherapy.

Fitness of sulfadoxine-resistant Plasmodium berghei harboring a single mutation in dihydropteroate synthase (DHPS).

Genetic changes conferring drug resistance are generally believed to impose fitness costs to pathogens in the absence of the drug. However, the fitness of resistant parasites against sulfadoxine/pyrimethamine has been inconclusive in Plasmodium falciparum. This is because resistance is conferred by the complex combination of mutations in dihydropteroate synthase (dhps) and dihydrofolate reductase (dhfr), which makes it difficult to separately assess the extent and magnitude of the costs imposed by mutations in dhps and dhfr. To assess the fitness costs imposed by sulfadoxine resistance alone, we generated a transgenic rodent malaria parasite, P. berghei clone harboring an A394G mutation in dhps (PbDHPS-A394G), corresponding to the causative mutation for sulfadoxine resistance in P. falciparum (PfDHPS-A437G). A four-day suppressive test confirmed that the PbDHPS-A394G clone was resistant to sulfadoxine. PbDHPS-A394G and wild-type clones showed similar growth rates and gametocyte production. This observation was confirmed in competitive experiments in which PbDHPS-A394G and wild-type clones were co-infected into mice to directly assess the survival competition between them. In the mosquitoes, there were no significant differences in oocyst production between PbDHPS-A394G and wild-type. These results indicate that the PbDHPS-A394G mutation alters the parasites to sulfadoxine resistance but may not impose fitness disadvantages during the blood stages in mice and oocyst formation in mosquitoes. These results partly explain the persistence of the PfDHPS-A437G mutant in the natural parasite populations.

Mapping the substrate specificity of the Plasmodium M1 and M17 aminopeptidases.

During malarial infection, Plasmodium parasites digest human hemoglobin to obtain free amino acids for protein production and maintenance of osmotic pressure. The Plasmodium M1 and M17 aminopeptidases are both postulated to have an essential role in the terminal stages of the hemoglobin digestion process and are validated drug targets for the design of new dual-target anti-malarial compounds. In this study, we profiled the substrate specificity fingerprints and kinetic behaviors of M1 and M17 aminopeptidases from Plasmodium falciparum and Plasmodium vivax, and the mouse model species, Plasmodium berghei. We found that although the Plasmodium M1 aminopeptidases share a largely similar, broad specificity at the P1 position, the P. falciparum M1 displays the greatest diversity in specificity and P. berghei M1 showing a preference for charged P1 residues. In contrast, the Plasmodium M17 aminopeptidases share a highly conserved preference for hydrophobic residues at the P1 position. The aminopeptidases also demonstrated intra-peptide sequence specificity, particularly the M1 aminopeptidases, which showed a definitive preference for peptides with fewer negatively charged intrapeptide residues. Overall, the P. vivax and P. berghei enzymes had a faster substrate turnover rate than the P. falciparum enzymes, which we postulate is due to subtle differences in structural dynamicity. Together, these results build a kinetic profile that allows us to better understand the catalytic nuances of the M1 and M17 aminopeptidases from different Plasmodium species.

Glyoxalase pathway is required for normal liver-stage proliferation of Plasmodium berghei.

The glyoxalase system is a ubiquitous detoxification pathway of methylglyoxal, a cytotoxic byproduct of glycolysis. Actively proliferating cells, such as cancer cells, depend on their energy metabolism for glycolysis. Therefore, the glyoxalase system has been evaluated as a target of anticancer drugs. The malaria sporozoite, which is the infective stage of the malaria parasite, actively proliferates and produces thousands of merozoites within 2-3 days in hepatocytes. This is the first step of infection in mammalian hosts. The glyoxalase system appears to play an important role in this active proliferation stage of the malaria parasite in hepatocytes. In this study, we aimed to dissect the role of the glyoxalase system in malaria parasite proliferation in hepatocytes to examine its potential as a target of malaria prevention using a reverse genetics approach. The malaria parasite possesses a glyoxalase system, comprised of glyoxalases and GloI-like protein, in the cytosol and apicoplast. We generated cytosolic glyoxalase II (cgloII) knockout, apicoplast targeted glyoxalase gloII (tgloII) knockout, and cgloII and tgloII double-knockout parasites and performed their phenotypic analysis. We did not observe any defects in the cgloII or tgloII knockout parasites. In contrast, we observed approximately 90% inhibition of the liver-stage proliferation of cgloII and tgloII double-knockout parasites in vivo. These findings suggest that although the glyoxalase system is dispensable, it plays an important role in parasite proliferation in hepatocytes. Additionally, the results indicate a complementary relationship between the cytosolic and apicoplast glyoxalase pathways. We expect that the parasite utilizes a system similar to that observed in cancer cells to enable its rapid proliferation in hepatocytes; this process could be targeted in the development of novel strategies to prevent malaria.

Mitochondrial apurinic/apyrimidinic endonuclease Apn1 is not critical for the completion of the Plasmodium berghei life cycle.

Mitochondrion is an essential organelle in malaria parasite and its DNA must be maintained for optimal function during its complex life cycle. Base excision repair is one of the major pathways by which this is achieved. Apurinic/apyrimidinic (AP) endonucleases are important components of this pathway as they create a nick at the 5'-phosphodiester bond in the AP site and generate free 5'-phosphate and 3'-hydroxyl groups. Two class II AP endonucleases (Apn1 and Ape1) have been annotated in the Plasmodium berghei genome. Using reverse genetic approaches, we provide direct evidence that Apn1 is exclusively localized to the mitochondria of P. berghei. Surprisingly, our gene deletion study revealed a completely dispensable role of Apn1 for the entirety of the P. berghei life cycle. Apn1- parasites were found to successfully grow in the blood. They were transmitted normally to the mosquito midguts and salivary glands. Sporozoites obtained from the salivary glands were infective and achieved similar patency as WT. Our results help emphasize the non-availability of this enzyme as a plausible drug target. We also emphasize the importance of genetic validation of antimalarial drug targets before furthering them down the drug discovery pipeline.

A Plasmodium cysteine protease required for efficient transition from the liver infection stage.

The transitions between developmental stages are critical points in the Plasmodium life cycle. The development of Plasmodium in the livers of their mammalian hosts bridges malaria transmission and the onset of clinical symptoms elicited by red blood cell infection. The egress of Plasmodium parasites from the liver must be a carefully orchestrated process to ensure a successful switch to the blood stage of infection. Cysteine protease activity is known to be required for liver-stage Plasmodium egress, but the crucial cysteine protease(s) remained unidentified. Here, we characterize a member of the papain-like cysteine protease family, Plasmodium berghei serine repeat antigen 4 (PbSERA4), that is required for efficient initiation of blood-stage infection. Through the generation PbSERA4-specific antisera and the creation of transgenic parasites expressing fluorescently tagged protein, we show that PbSERA4 is expressed and proteolytically processed in the liver and blood stages of infection. Targeted disruption of PbSERA4 results in viable and virulent blood-stage parasites. However, upon transmission from mosquitoes to mice, Pbsera4(-) parasites displayed a reduced capacity to initiate a new round of asexual blood-stage replication. Our results from cultured cells indicate that this defect results from an inability of the PbSERA4-deficient parasites to egress efficiently from infected cells at the culmination of liver-stage development. Protection against infection with wildtype P. berghei could be generated in animals in which Pbsera4(-) parasites failed to establish infection. Our findings confirm that liver-stage merozoite release is an active process and demonstrate that this parasite-encoded cysteine protease contributes to parasite escape from the liver.

Overlapping and distinct roles of CDPK family members in the pre-erythrocytic stages of the rodent malaria parasite, Plasmodium berghei.

Invasion of hepatocytes by Plasmodium sporozoites initiates the pre-erythrocytic step of a malaria infection. Subsequent development of the parasite within hepatocytes and exit from them is essential for starting the disease-causing erythrocytic cycle. Identification of signaling pathways that operate in pre-erythrocytic stages provides insight into a critical step of infection and potential targets for chemoprotection from malaria. We demonstrate that P. berghei homologs of Calcium Dependent Protein Kinase 1 (CDPK1), CDPK4 and CDPK5 play overlapping but distinct roles in sporozoite invasion and parasite egress from hepatocytes. All three kinases are expressed in sporozoites. All three are required for optimal motility of sporozoites and consequently their invasion of hepatocytes. Increased cGMP can compensate for the functional loss of CDPK1 and CDPK5 during sporozoite invasion but cannot overcome loss of CDPK4. CDPK1 and CDPK5 expression is downregulated after sporozoite invasion. CDPK5 reappears in a subset of late stage liver stages and is present in all merosomes. Chemical inhibition of CDPK4 and depletion of CDPK5 in liver stages implicate these kinases in the formation and/or release of merosomes from mature liver stages. Furthermore, depletion of CDPK5 in merosomes significantly delays initiation of the erythrocytic cycle without affecting infectivity of hepatic merozoites. These data suggest that CDPK5 may be required for the rupture of merosomes. Our work provides evidence that sporozoite invasion requires CDPK1 and CDPK5, and suggests that CDPK5 participates in the release of hepatic merozoites.

Male-Specific Protein Disulphide Isomerase Function is Essential for Plasmodium Transmission and a Vulnerable Target for Intervention.

Inhibiting transmission of Plasmodium is an essential strategy in malaria eradication, and the biological process of gamete fusion during fertilization is a proven target for this approach. Lack of knowledge of the mechanisms underlying fertilization have been a hindrance in the development of transmission-blocking interventions. Here we describe a protein disulphide isomerase essential for malarial transmission (PDI-Trans/PBANKA_0820300) to the mosquito. We show that PDI-Trans activity is male-specific, surface-expressed, essential for fertilization/transmission, and exhibits disulphide isomerase activity which is up-regulated post-gamete activation. We demonstrate that PDI-Trans is a viable anti-malarial drug and vaccine target blocking malarial transmission with the use of PDI inhibitor bacitracin (98.21%/92.48% reduction in intensity/prevalence), and anti-PDI-Trans antibodies (66.22%/33.16% reduction in intensity/prevalence). To our knowledge, these results provide the first evidence that PDI function is essential for malarial transmission, and emphasize the potential of anti-PDI agents to act as anti-malarials, facilitating the future development of novel transmission-blocking interventions.

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.

Protein O-Fucosyltransferase 2 Is Not Essential for Plasmodium berghei Development.

Thrombospondin type I repeat (TSR) domains are commonly O-fucosylated by protein O-fucosyltransferase 2 (PoFUT2), and this modification is required for optimal folding and secretion of TSR-containing proteins. The human malaria parasite Plasmodium falciparum expresses proteins containing TSR domains, such as the thrombospondin-related anonymous protein (TRAP) and circumsporozoite surface protein (CSP), which are O-fucosylated. TRAP and CSP are present on the surface of sporozoites and play essential roles in mosquito and human host invasion processes during the transmission stages. Here, we have generated PoFUT2 null-mutant P. falciparum and Plasmodium berghei (rodent) malaria parasites and, by phenotyping them throughout their complete life cycle, we show that PoFUT2 disruption does not affect the growth through the mosquito stages for both species. However, contrary to what has been described previously by others, P. berghei PoFUT2 null mutant sporozoites showed no deleterious motility phenotypes and successfully established blood stage infection in mice. This unexpected result indicates that the importance of O-fucosylation of TSR domains may differ between human and RODENT malaria parasites; complicating our understanding of glycosylation modifications in malaria biology.

Plasmodium berghei serine/threonine protein phosphatase PP5 plays a critical role in male gamete fertility.

Sexual development in malaria parasites involves multiple signal transduction pathways mediated by reversible protein phosphorylation. Here, we functionally characterised a protein phosphatase, Ser/Thr protein phosphatase 5 (PbPP5), during sexual development of the rodent malaria parasite Plasmodium berghei. The recombinant protein phosphatase domain displayed obvious protein phosphatase activity and was sensitive to PP1/PP2A inhibitors including cantharidic acid (IC50 = 122.2 nM), cantharidin (IC50 = 74.3 nM), endothall (IC50 = 365.5 nM) and okadaic acid (IC50 = 1.3 nM). PbPP5 was expressed in both blood stages and ookinetes with more prominent expression during sexual development. PbPP5 was localised in the cytoplasm of the parasite and highly concentrated beneath the parasite plasma membrane in free merozoites and ookinetes. Targeted deletion of the pbpp5 gene had no influence on asexual blood-stage parasite multiplication or the survival curve of the infected hosts. However, male gamete formation and fertility were severely affected, resulting in almost complete blockade of ookinete conversion and oocyst development in the Δpbpp5 lines. This sexual development defect was rescued by crossing Δpbpp5 with the female defective Δpbs47 parasite line, but not with the male defective Δpbs48/45 line, thus confirming the essential function of the pbpp5 gene in male gamete fertility. Furthermore, the aforementioned PP1/PP2A inhibitors all had inhibitory effects on exflagellation of male gametocytes and ookinete conversion. In particular, endothall, a selective inhibitor of PP2A, completely blocked exflagellation and ookinete conversion at ∼548.3 nM. This study elucidated an essential function of PbPP5 during male gamete development and fertility.

Functional genetic evaluation of DNA house-cleaning enzymes in the malaria parasite: dUTPase and Ap4AH are essential in Plasmodium berghei but ITPase and NDH are dispensable.

BACKGROUND: Cellular metabolism generates reactive oxygen species. The oxidation and deamination of the deoxynucleoside triphosphate (dNTP) pool results in the formation of non-canonical, toxic dNTPs that can cause mutations, genome instability, and cell death. House-cleaning or sanitation enzymes that break down and detoxify non-canonical nucleotides play major protective roles in nucleotide metabolism and constitute key drug targets for cancer and various pathogens. We hypothesized that owing to their protective roles in nucleotide metabolism, these house-cleaning enzymes are key drug targets in the malaria parasite. METHODS: Using the rodent malaria parasite Plasmodium berghei we evaluate here, by gene targeting, a group of conserved proteins with a putative function in the detoxification of non-canonical nucleotides as potential antimalarial drug targets: they are inosine triphosphate pyrophosphatase (ITPase), deoxyuridine triphosphate pyrophosphatase (dUTPase) and two NuDiX hydroxylases, the diadenosine tetraphosphate (Ap4A) hydrolase and the nucleoside triphosphate hydrolase (NDH). RESULTS: While all four proteins are expressed constitutively across the intraerythrocytic developmental cycle, neither ITPase nor NDH are required for parasite viability. dutpase and ap4ah null mutants, on the other hand, are not viable suggesting an essential function for these proteins for the malaria parasite. CONCLUSIONS: Plasmodium dUTPase and Ap4A could be drug targets in the malaria parasite.

Deciphering the targets of retroviral protease inhibitors in Plasmodium berghei.

Retroviral protease inhibitors (RPIs) such as lopinavir (LP) and saquinavir (SQ) are active against Plasmodium parasites. However, the exact molecular target(s) for these RPIs in the Plasmodium parasites remains poorly understood. We hypothesised that LP and SQ suppress parasite growth through inhibition of aspartyl proteases. Using reverse genetics approach, we embarked on separately generating knockout (KO) parasite lines lacking Plasmepsin 4 (PM4), PM7, PM8, or DNA damage-inducible protein 1 (Ddi1) in the rodent malaria parasite Plasmodium berghei ANKA. We then tested the suppressive profiles of the LP/Ritonavir (LP/RT) and SQ/RT as well as antimalarials; Amodiaquine (AQ) and Piperaquine (PQ) against the KO parasites in the standard 4-day suppressive test. The Ddi1 gene proved refractory to deletion suggesting that the gene is essential for the growth of the asexual blood stage parasites. Our results revealed that deletion of PM4 significantly reduces normal parasite growth rate phenotype (P = 0.003). Unlike PM4_KO parasites which were less susceptible to LP and SQ (P = 0.036, P = 0.030), the suppressive profiles for PM7_KO and PM8_KO parasites were comparable to those for the WT parasites. This finding suggests a potential role of PM4 in the LP and SQ action. On further analysis, modelling and molecular docking studies revealed that both LP and SQ displayed high binding affinities (-6.3 kcal/mol to -10.3 kcal/mol) towards the Plasmodium aspartyl proteases. We concluded that PM4 plays a vital role in assuring asexual stage parasite fitness and might be mediating LP and SQ action. The essential nature of the Ddi1 gene warrants further studies to evaluate its role in the parasite asexual blood stage growth as well as a possible target for the RPIs.

Nuclear Pore Complex Components in the Malaria Parasite Plasmodium berghei.

The nuclear pore complex (NPC) is a large macromolecular assembly of around 30 different proteins, so-called nucleoporins (Nups). Embedded in the nuclear envelope the NPC mediates bi-directional exchange between the cytoplasm and the nucleus and plays a role in transcriptional regulation that is poorly understood. NPCs display modular arrangements with an overall structure that is generally conserved among many eukaryotic phyla. However, Nups of yeast or human origin show little primary sequence conservation with those from early-branching protozoans leaving those of the malaria parasite unrecognized. Here we have combined bioinformatic and genetic methods to identify and spatially characterize Nup components in the rodent infecting parasite Plasmodium berghei and identified orthologs from the human malaria parasite P. falciparum, as well as the related apicomplexan parasite Toxoplasma gondii. For the first time we show the localization of selected Nups throughout the P. berghei life cycle. Largely restricted to apicomplexans we identify an extended C-terminal poly-proline extension in SEC13 that is essential for parasite survival and provide high-resolution images of Plasmodium NPCs obtained by cryo electron tomography. Our data provide the basis for full characterization of NPCs in malaria parasites, early branching unicellular eukaryotes with significant impact on human health.

Development of a bsd-blasticidin selection system in Plasmodium berghei.

Plasmodium berghei is used as a rodent model for the study of malaria. However, multiple genetic manipulations are restricted by the paucity of selectable markers. The bsd-blasticidin selection system is widely used for eukaryotic cells; however, it could not previously be used for P. berghei due to toxicity to the rodent host. Here, we report the application of this selection system in P. berghei using an in vitro selection method. The desired bsd-integrated mutants are enriched by more than 90% within 2 weeks when using this system. Furthermore, the bsd marker can be used sequentially with established pyrimethamine- and puromycin-resistant markers. This system allows deeper understanding of malaria parasite biology through extensive genetic manipulation of P. berghei.

The Glycosylphosphatidylinositol Transamidase Complex Subunit PbGPI16 of Plasmodium berghei Is Important for Inducing Experimental Cerebral Malaria.

In animal models of experimental cerebral malaria (ECM), the glycosylphosphatidylinositols (GPIs) and GPI anchors are the major factors that induce nuclear factor kappa B (NF-κB) activation and proinflammatory responses, which contribute to malaria pathogenesis. GPIs and GPI anchors are transported to the cell surface via a process called GPI transamidation, which involves the GPI transamidase (GPI-T) complex. In this study, we showed that GPI16, one of the GPI-T subunits, is highly conserved among Plasmodium species. Genetic knockout of pbgpi16 (Δpbgpi16) in the rodent malaria parasite Plasmodium berghei strain ANKA led to a significant reduction of the amounts of GPIs in the membranes of merozoites, as well as surface display of several GPI-anchored merozoite surface proteins. Compared with the wild-type parasites, Δpbgpi16 parasites in C57BL/6 mice caused much less NF-κB activation and elicited a substantially attenuated T helper type 1 response. As a result, Δpbgpi16 mutant-infected mice displayed much less severe brain pathology, and considerably fewer Δpbgpi16 mutant-infected mice died from ECM. This study corroborated the GPI toxin as a significant inducer of ECM and further suggested that vaccines against parasite GPIs may be a promising strategy to limit the severity of malaria.

Biochemical characterization and essentiality of Plasmodium fumarate hydratase.

Plasmodium falciparum (Pf), the causative agent of malaria, has an iron-sulfur cluster-containing class I fumarate hydratase (FH) that catalyzes the interconversion of fumarate to malate, a well-known reaction in the tricarboxylic acid cycle. In humans, the same reaction is catalyzed by class II FH that has no sequence or structural homology with the class I enzyme from Plasmodium Fumarate is generated in large quantities in the parasite as a by-product of AMP synthesis and is converted to malate by FH and then used in the generation of the key metabolites oxaloacetate, aspartate, and pyruvate. Previous studies have identified the FH reaction as being essential to P. falciparum, but biochemical characterization of PfFH that may provide leads for the development of specific inhibitors is lacking. Here, we report on the kinetic characterization of purified recombinant PfFH, functional complementation of fh deficiency in Escherichia coli, and mitochondrial localization in the parasite. We found that the substrate analog mercaptosuccinic acid is a potent PfFH inhibitor, with a Ki value in the nanomolar range. The fh gene could not be knocked out in Plasmodium berghei when transfectants were introduced into BALB/c mice; however, fh knockout was successful when C57BL/6 mice were used as host, suggesting that the essentiality of the fh gene to the parasite was mouse strain-dependent.

The shikimate pathway enzyme that generates chorismate is not required for the development of Plasmodium berghei in the mammalian host nor the mosquito vector.

In Plasmodium, the shikimate pathway is a potential target for malaria chemotherapy owing to its absence in the mammalian host. Chorismate, the end product of this pathway, serves as a precursor for aromatic amino acids, Para-aminobenzoic acid and ubiquinone, and is synthesised by Chorismate synthase (CS). Therefore, it follows that the Cs locus may be refractory to genetic manipulation. By utilising a conditional mutagenesis system of yeast Flp/FRT, we demonstrate an unexpectedly dispensable role of CS in Plasmodium. Our studies reiterate the need to establish an obligate reliance on Plasmodium metabolic enzymes through genetic approaches before their selection as drug targets.

Plant hormone cytokinins control cell cycle progression and plastid replication in apicomplexan parasites.

Cytokinins are plant hormones that are involved in regulation of cell proliferation, cell cycle progression, and cell and plastid development. Here, we show that the apicomplexan parasites Toxoplasma gondii and Plasmodium berghei, an opportunistic human pathogen and a rodent malaria agent, respectively, produce cytokinins via a biosynthetic pathway similar to that in plants. Cytokinins regulate the growth and cell cycle progression of T. gondii by mediating expression of the cyclin gene TgCYC4. A natural form of cytokinin, trans-zeatin (t-zeatin), upregulated expression of this cyclin, while a synthetic cytokinin, thidiazuron, downregulated its expression. Immunofluorescence microscopy and quantitative PCR analysis showed that t-zeatin increased the genome-copy number of apicoplast, which are non-photosynthetic plastid, in the parasite, while thidiazuron led to their disappearance. Thidiazuron inhibited growth of T. gondii and Plasmodium falciparum, a human malaria parasite, suggesting that thidiazuron has therapeutic potential as an inhibitor of apicomplexan parasites.

Structured illumination microscopy reveals actin I localization in discreet foci in Plasmodium berghei gametocytes.

Actin has important roles in Plasmodium parasites but its exact function in different life stages is not yet fully elucidated. Here we report the localization of ubiquitous actin I in gametocytes of the rodent model parasite P. berghei. Using an antibody specifically recognizing F-actin and deconvolution microscopy we detected actin I in a punctate pattern in gametocytes. 3D-Structured Illumination Microscopy which allows sub-diffraction limit imaging resolved the signal into structures of less than 130 nm length. A portion of actin I was soluble, but the protein was also found complexed in a stabilized form which could only be completely solubilized by treatment with SDS. An additional population of actin was pelleted at 100 000 × g, consistent with F-actin. Our results suggest that actin in this non-motile form of the parasite is present in short filaments cross-linked to other structures in a cytoskeleton.