The N'-terminal domain of glyceraldehyde-3-phosphate dehydrogenase of the apicomplexan Plasmodium falciparum mediates GTPase Rab2-dependent recruitment to membranes

Analysis of diagnostic biomarkers for malaria: Prospects on rapid diagnostic test (RDT) development

Accurate malaria diagnosis remains a formidable challenge in remote regions of malaria-endemic areas globally. Existing diagnostic methods predominantly rely on microscopy and rapid diagnostic tests (RDTs). While RDTs offer advantages such as rapid results and reduced dependence on highly skilled technicians compared to microscopy, persistent challenges emphasize the critical need to identify novel diagnostic biomarkers to further enhance RDT based malaria diagnosis. This comprehensive review presents a range of promising diagnostic targets. These targets could be useful in developing more robust, accurate, and effective diagnostic tools. Such tools are crucial for the detection of the Plasmodium falciparum (P.falcipaum) malaria parasite. The potential biomarkers discussed here significantly address the challenges posed by HRP2 gene deletion in P.falciparum. Researchers, RDT manufacturers, industrial and other stakeholders involved in malaria diagnosis can harness the crucial information described in this article, to drive the development of advanced RDTs as viable alternatives. By diversifying the available tools for diagnosis, we can attempt to enhance our ability to knock out malaria effectively and contribute to better health outcomes for people residing in malaria-endemic regions. This review serves as a valuable resource for advancing research and development in the field of malaria diagnostics, ultimately aiding to the global fight against this devastating ancient disease.

The three Plasmodium falciparum Aurora-related kinases display distinct temporal and spatial associations with mitotic structures in asexual blood stage parasites and gametocytes

Aurora kinases are crucial regulators of mitotic cell cycle progression in eukaryotes. The protozoan malaria parasite Plasmodium falciparum replicates via schizogony, a specialized mode of cell division characterized by consecutive asynchronous rounds of nuclear division by closed mitosis followed by a single cytokinesis event producing dozens of daughter cells. P. falciparum encodes three Aurora-related kinases (PfARKs) that have been reported essential for parasite proliferation, but their roles in regulating schizogony have not yet been explored in great detail. Here, we engineered transgenic parasite lines expressing GFP-tagged PfARK1-3 to provide a systematic analysis of their expression timing and subcellular localization throughout schizogony as well as in the non-dividing gametocyte stages, which are essential for malaria transmission. We demonstrate that all three PfARKs display distinct and highly specific and exclusive spatiotemporal associations with the mitotic machinery. In gametocytes, PfARK3 is undetectable, and PfARK1 and PfARK2 show male-specific expression in late-stage gametocytes, consistent with their requirement for endomitosis during male gametogenesis in the mosquito vector. Our combined data suggest that PfARK1 and PfARK2 have non-overlapping roles in centriolar plaque maturation, assembly of the mitotic spindle, kinetochore-spindle attachment and chromosome segregation, while PfARK3 seems to be exquisitely involved in daughter cell cytoskeleton assembly and cytokinesis. These important new insights provide a reliable foundation for future research aiming at the functional investigation of these divergent and possibly drug-targetable Aurora-related kinases in mitotic cell division of P. falciparum and related apicomplexan parasites.IMPORTANCEMalaria parasites replicate via non-conventional modes of mitotic cell division, such as schizogony, employed by the disease-causing stages in the human blood or endomitosis during male gametogenesis in the mosquito vector. Understanding the molecular mechanisms regulating cell division in these divergent unicellular eukaryotes is not only of scientific interest but also relevant to identify potential new antimalarial drug targets. Here, we carefully examined the subcellular localization of all three Plasmodium falciparum Aurora-related kinases (ARKs), distantly related homologs of Aurora kinases that coordinate mitosis in model eukaryotes. Detailed fluorescence microscopy-based analyses revealed distinct, specific, and exclusive spatial associations for each parasite ARK with different components of the mitotic machinery and at different phases of the cell cycle during schizogony and gametocytogenesis. This comprehensive set of results closes important gaps in our fragmentary knowledge on this important group of kinases and offers a valuable source of information for future functional studies.

Next Generation Chemiluminescent Probes for Antimalarial Drug Discovery

Malaria is caused by parasites of the Plasmodium genus and remains one of the most pressing human health problems. The spread of parasites resistant to or partially resistant to single or multiple drugs, including frontline antimalarial artemisinin and its derivatives, poses a serious threat to current and future malaria control efforts. In vitro drug assays are important for identifying new antimalarial compounds and monitoring drug resistance. Due to its robustness and ease of use, the [3H]-hypoxanthine incorporation assay is still considered a gold standard and is widely applied, despite limited sensitivity and the dependence on radioactive material. Here, we present a first-of-its-kind chemiluminescence-based antimalarial drug screening assay. The effect of compounds on P. falciparum is monitored by using a dioxetane-based substrate (AquaSpark β-D-galactoside) that emits high-intensity luminescence upon removal of a protective group (β-D-galactoside) by a transgenic β-galactosidase reporter enzyme. This biosensor enables highly sensitive, robust, and cost-effective detection of asexual, intraerythrocytic P. falciparum parasites without the need for parasite enrichment, washing, or purification steps. We are convinced that the ultralow detection limit of less than 100 parasites of the presented biosensor system will become instrumental in malaria research, including but not limited to drug screening.

Comparative proteomic analysis across the developmental stages of the Eimeria tenella

Eimeria tenella is the main pathogen responsible for coccidiosis in chickens. The life cycle of E. tenella is, arguably, the least complex of all Coccidia, with only one host. However, it presents different developmental stages, either in the environment or in the host and either intracellular or extracellular. Its signaling and metabolic pathways change with its different developmental stages. Until now, little is known about the developmental regulation and transformation mechanisms of its life cycle. In this study, protein profiles from the five developmental stages, including unsporulated oocysts (USO), partially sporulated (7 h) oocysts (SO7h), sporulated oocysts (SO), sporozoites (S) and second-generation merozoites (M2), were harvested using the label-free quantitative proteomics approach. Then the differentially expressed proteins (DEPs) for these stages were identified. A total of 314, 432, 689, and 665 DEPs were identified from the comparison of SO7h vs USO, SO vs SO7h, S vs SO, and M2 vs S, respectively. By conducting weighted gene coexpression network analysis (WGCNA), six modules were dissected. Proteins in blue and brown modules were calculated to be significantly positively correlated with the E. tenella developmental stages of sporozoites (S) and second-generation merozoites (M2), respectively. In addition, hub proteins with high intra-module degree were identified. Gene Ontology (GO) and Kyoto Encyclopedia of Gene and Genomes (KEGG) pathway enrichment analyses revealed that hub proteins in blue modules were involved in electron transport chain and oxidative phosphorylation. Hub proteins in the brown module were involved in RNA splicing. These findings provide new clues and ideas to enhance our fundamental understanding of the molecular mechanisms underlying parasite development.

Genetic validation of PfFKBP35 as an antimalarial drug target

Plasmodium falciparum accounts for the majority of over 600,000 malaria-associated deaths annually. Parasites resistant to nearly all antimalarials have emerged and the need for drugs with alternative modes of action is thus undoubted. The FK506-binding protein PfFKBP35 has gained attention as a promising drug target due to its high affinity to the macrolide compound FK506 (tacrolimus). Whilst there is considerable interest in targeting PfFKBP35 with small molecules, a genetic validation of this factor as a drug target is missing and its function in parasite biology remains elusive. Here, we show that limiting PfFKBP35 levels are lethal to P. falciparum and result in a delayed death-like phenotype that is characterized by defective ribosome homeostasis and stalled protein synthesis. Our data furthermore suggest that FK506, unlike the action of this drug in model organisms, exerts its antiproliferative activity in a PfFKBP35-independent manner and, using cellular thermal shift assays, we identify putative FK506-targets beyond PfFKBP35. In addition to revealing first insights into the function of PfFKBP35, our results show that FKBP-binding drugs can adopt non-canonical modes of action - with major implications for the development of FK506-derived molecules active against Plasmodium parasites and other eukaryotic pathogens.

The ferredoxin redox system - an essential electron distributing hub in the apicoplast of Apicomplexa

The apicoplast, a relict plastid found in most species of the phylum Apicomplexa, harbors the ferredoxin redox system which supplies electrons to enzymes of various metabolic pathways in this organelle. Recent reports in Toxoplasma gondii and Plasmodium falciparum have shown that the iron-sulfur cluster (FeS)-containing ferredoxin is essential in tachyzoite and blood-stage parasites, respectively. Here we review ferredoxin's crucial contribution to isoprenoid and lipoate biosynthesis as well as tRNA modification in the apicoplast, highlighting similarities and differences between the two species. We also discuss ferredoxin's potential role in the initial reductive steps required for FeS synthesis as well as recent evidence that offers an explanation for how NADPH required by the redox system might be generated in Plasmodium spp.

GAPDH mediates drug resistance and metabolism in Plasmodium falciparum malaria parasites

Efforts to control the global malaria health crisis are undermined by antimalarial resistance. Identifying mechanisms of resistance will uncover the underlying biology of the Plasmodium falciparum malaria parasites that allow evasion of our most promising therapeutics and may reveal new drug targets. We utilized fosmidomycin (FSM) as a chemical inhibitor of plastidial isoprenoid biosynthesis through the methylerythritol phosphate (MEP) pathway. We have thus identified an unusual metabolic regulation scheme in the malaria parasite through the essential glycolytic enzyme, glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Two parallel genetic screens converged on independent but functionally analogous resistance alleles in GAPDH. Metabolic profiling of FSM-resistant gapdh mutant parasites indicates that neither of these mutations disrupt overall glycolytic output. While FSM-resistant GAPDH variant proteins are catalytically active, they have reduced assembly into the homotetrameric state favored by wild-type GAPDH. Disrupted oligomerization of FSM-resistant GAPDH variant proteins is accompanied by altered enzymatic cooperativity and reduced susceptibility to inhibition by free heme. Together, our data identifies a new genetic biomarker of FSM-resistance and reveals the central role of GAPDH in MEP pathway control and antimalarial sensitivity.

Malaria is a life-threatening mosquito-borne infection that remains an enormous public health threat worldwide, with over 600,000 deaths reported in 2020 alone. The parasites that cause malaria invade and replicate within human red blood cells. This unique environment provides the malaria parasite with almost unlimited supply of sugar in the form of glucose, which the parasite uses for energy and as building blocks to grow and divide. Parasites break down glucose, and must use these breakdown products to make new molecules, including a very important class of compounds called isoprenoids. Malaria parasites normally die when they are treated with a drug, called fosmidomycin, that inhibits this process. To understand how parasites regulate this critical function, in this study we identified parasites that were resistant to fosmidomycin. These fosmidomycin-resistant cells had mutations in an enzyme that is critical for sugar breakdown, called glyceraldehyde phosphate dehydrogenase (GAPDH). We find that parasites with mutant GAPDH enzymes still break down sugar normally, but are not inhibited by other changes in the cell that happen upon fosmidomycin treatment. These results reveal a new and important role for the enzyme GAPDH as a control-point for downstream metabolism in malaria parasites.

In vitro models for human malaria: targeting the liver stage

The Plasmodium liver stage represents a vulnerable therapeutic target to prevent disease progression as the parasite resides in the liver before clinical representation caused by intraerythrocytic development. However, most antimalarial drugs target the blood stage of the parasite's life cycle, and the few drugs that target the liver stage are lethal to patients with a glucose-6-phosphate dehydrogenase deficiency. Furthermore, implementation of in vitro liver models to study and develop novel therapeutics against the liver stage of human Plasmodium species remains challenging. In this review, we focus on the progression of in vitro liver models developed for human Plasmodium spp. parasites, provide a brief review on important assay requirements, and lastly present recommendations to improve models to enhance the discovery process of novel preclinical therapeutics.

The 3-phosphoinositide-dependent protein kinase 1 is an essential upstream activator of protein kinase A in malaria parasites

Cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) signalling is essential for the proliferation of Plasmodium falciparum malaria blood stage parasites. The mechanisms regulating the activity of the catalytic subunit PfPKAc, however, are only partially understood, and PfPKAc function has not been investigated in gametocytes, the sexual blood stage forms that are essential for malaria transmission. By studying a conditional PfPKAc knockdown (cKD) mutant, we confirm the essential role for PfPKAc in erythrocyte invasion by merozoites and show that PfPKAc is involved in regulating gametocyte deformability. We furthermore demonstrate that overexpression of PfPKAc is lethal and kills parasites at the early phase of schizogony. Strikingly, whole genome sequencing (WGS) of parasite mutants selected to tolerate increased PfPKAc expression levels identified missense mutations exclusively in the gene encoding the parasite orthologue of 3-phosphoinositide-dependent protein kinase-1 (PfPDK1). Using targeted mutagenesis, we demonstrate that PfPDK1 is required to activate PfPKAc and that T189 in the PfPKAc activation loop is the crucial target residue in this process. In summary, our results corroborate the importance of tight regulation of PfPKA signalling for parasite survival and imply that PfPDK1 acts as a crucial upstream regulator in this pathway and potential new drug target.

Phosphoproteomic Comparison of Four Eimeria tenella Life Cycle Stages

Protein phosphorylation is an important post-translational modification (PTM) involved in diverse cellular functions. It is the most prevalent PTM in both Toxoplasma gondii and Plasmodium falciparum, but its status in Eimeria tenella has not been reported. Herein, we performed a comprehensive, quantitative phosphoproteomic profile analysis of four stages of the E. tenella life cycle: unsporulated oocysts (USO), partially sporulated (7 h) oocysts (SO7h), sporulated oocysts (SO), and sporozoites (S). A total of 15,247 phosphorylation sites on 9514 phosphopeptides corresponding to 2897 phosphoproteins were identified across the four stages. In addition, 456, 479, and 198 differentially expressed phosphoproteins (DEPPs) were identified in the comparisons SO7h vs. USO, SO vs. SO7h, and S vs. SO, respectively. Gene Ontology (GO) term and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of DEPPs suggested that they were involved in diverse functions. For SO7h vs. USO, DEPPs were mainly involved in cell division, actin cytoskeleton organization, positive regulation of transport, and pyruvate metabolism. For SO vs. SO7h, they were related to the peptide metabolic process, translation, and RNA transport. DEPPs in the S vs. SO comparison were associated with the tricarboxylic acid metabolic process, positive regulation of ATPase activity, and calcium ion binding. Time course sequencing data analysis (TCseq) identified six clusters with similar expression change characteristics related to carbohydrate metabolism, cytoskeleton organization, and calcium ion transport, demonstrating different regulatory profiles across the life cycle of E. tenella. The results revealed significant changes in the abundance of phosphoproteins during E. tenella development. The findings shed light on the key roles of protein phosphorylation and dephosphorylation in the E. tenella life cycle.

Protein Prenylation and Hsp40 in Thermotolerance of Plasmodium falciparum Malaria Parasites

During its complex life cycle, the malaria parasite survives dramatic environmental stresses, including large temperature shifts. Protein prenylation is required during asexual replication of Plasmodium falciparum, and the canonical heat shock protein 40 protein (HSP40; PF3D7_1437900) is posttranslationally modified with a 15-carbon farnesyl isoprenyl group. In other organisms, farnesylation of Hsp40 orthologs controls their localization and function in resisting environmental stress. In this work, we find that plastidial isopentenyl pyrophosphate (IPP) synthesis and protein farnesylation are required for malaria parasite survival after cold and heat shock. Furthermore, loss of HSP40 farnesylation alters its membrane attachment and interaction with proteins in essential pathways in the parasite. Together, this work reveals that farnesylation is essential for parasite survival during temperature stress. Farnesylation of HSP40 may promote thermotolerance by guiding distinct chaperone-client protein interactions.

The Road Less Traveled? Unconventional Protein Secretion at Parasite-Host Interfaces

Protein secretion in eukaryotic cells is a well-studied process, which has been known for decades and is dealt with by any standard cell biology textbook. However, over the past 20 years, several studies led to the realization that protein secretion as a process might not be as uniform among different cargos as once thought. While in classic canonical secretion proteins carry a signal sequence, the secretory or surface proteome of several organisms demonstrated a lack of such signals in several secreted proteins. Other proteins were found to indeed carry a leader sequence, but simply circumvent the Golgi apparatus, which in canonical secretion is generally responsible for the modification and sorting of secretory proteins after their passage through the endoplasmic reticulum (ER). These alternative mechanisms of protein translocation to, or across, the plasma membrane were collectively termed “unconventional protein secretion” (UPS). To date, many research groups have studied UPS in their respective model organism of choice, with surprising reports on the proportion of unconventionally secreted proteins and their crucial roles for the cell and survival of the organism. Involved in processes such as immune responses and cell proliferation, and including far more different cargo proteins in different organisms than anyone had expected, unconventional secretion does not seem so unconventional after all. Alongside mammalian cells, much work on this topic has been done on protist parasites, including genera Leishmania, Trypanosoma, Plasmodium, Trichomonas, Giardia, and Entamoeba. Studies on protein secretion have mainly focused on parasite-derived virulence factors as a main source of pathogenicity for hosts. Given their need to secrete a variety of substrates, which may not be compatible with canonical secretion pathways, the study of mechanisms for alternative secretion pathways is particularly interesting in protist parasites. In this review, we provide an overview on the current status of knowledge on UPS in parasitic protists preceded by a brief overview of UPS in the mammalian cell model with a focus on IL-1β and FGF-2 as paradigmatic UPS substrates.

The catalytic subunit of Plasmodium falciparum casein kinase 2 is essential for gametocytogenesis

Casein kinase 2 (CK2) is a pleiotropic kinase phosphorylating substrates in different cellular compartments in eukaryotes. In the malaria parasite Plasmodium falciparum, PfCK2 is vital for asexual proliferation of blood-stage parasites. Here, we applied CRISPR/Cas9-based gene editing to investigate the function of the PfCK2α catalytic subunit in gametocytes, the sexual forms of the parasite that are essential for malaria transmission. We show that PfCK2α localizes to the nucleus and cytoplasm in asexual and sexual parasites alike. Conditional knockdown of PfCK2α expression prevented the transition of stage IV into transmission-competent stage V gametocytes, whereas the conditional knockout of pfck2a completely blocked gametocyte maturation already at an earlier stage of sexual differentiation. In summary, our results demonstrate that PfCK2α is not only essential for asexual but also sexual development of P. falciparum blood-stage parasites and encourage studies exploring PfCK2α as a potential target for dual-active antimalarial drugs.

Biogenesis and maintenance of the apicoplast in model apicomplexan parasites

The apicoplast is a non-photosynthetic relict plastid of Apicomplexa that evolved from a secondary symbiotic system. During its evolution, most of the genes derived from its alga ancestor were lost. Only genes involved in several valuable metabolic pathways, such as the synthesis of isoprenoid precursors, heme, and fatty acids, have been transferred to the host genome and retained to help these parasites adapt to a complex life cycle and various living environments. The biological function of an apicoplast is essential for most apicomplexan parasites. Considering their potential as drug targets, the metabolic functions of this symbiotic organelle have been intensively investigated through computational and biological means. Moreover, we know that not only organellar metabolic functions are linked with other organelles, but also their biogenesis processes have developed and evolved to tailor their biological functions and proper inheritance. Several distinct features have been found in the biogenesis process of apicoplasts. For example, the apicoplast borrows a dynamin-related protein (DrpA) from its host to implement organelle division. The autophagy system has also been repurposed for linking the apicoplast and centrosome during replication and the division process. However, many vital questions remain to be answered about how these parasites maintain and properly inherit this symbiotic organelle. Here we review our current knowledge about its biogenesis process and discuss several critical questions remaining to be answered in this field.

The NTP generating activity of pyruvate kinase II is critical for apicoplast maintenance in Plasmodium falciparum

The apicoplast of Plasmodium falciparum parasites is believed to rely on the import of three-carbon phosphate compounds for use in organelle anabolic pathways, in addition to the generation of energy and reducing power within the organelle. We generated a series of genetic deletions in an apicoplast metabolic bypass line to determine which genes involved in apicoplast carbon metabolism are required for blood-stage parasite survival and organelle maintenance. We found that pyruvate kinase II (PyrKII) is essential for organelle maintenance, but that production of pyruvate by PyrKII is not responsible for this phenomenon. Enzymatic characterization of PyrKII revealed activity against all NDPs and dNDPs tested, suggesting that it may be capable of generating a broad range of nucleotide triphosphates. Conditional mislocalization of PyrKII resulted in decreased transcript levels within the apicoplast that preceded organelle disruption, suggesting that PyrKII is required for organelle maintenance due to its role in nucleotide triphosphate generation.

PfMAP-2 is essential for male gametogenesis in the malaria parasite Plasmodium falciparum

In malaria parasites, male gametogenesis is a proliferative stage essential for parasite transmission to the mosquito vector. It is a rapid process involving three rounds of genome replication alternating with closed endomitoses, and assembly of axonemes to produce eight flagellated motile microgametes. Studies in Plasmodium berghei have highlighted tight regulation of gametogenesis by a network of kinases. The P. berghei MAPK homologue PbMAP-2 is dispensable for asexual development but important at the induction of axoneme motility. However, in P. falciparum, causing the most severe form of human malaria, PfMAP-2 was suggested to be essential for asexual proliferation indicating distinct functions for MAP-2 in these two Plasmodium species. We here show that PfMAP-2 is dispensable for asexual growth but important for male gametogenesis in vitro. Similar to PbMAP-2, PfMAP-2 is required for initiating axonemal beating but not for prior DNA replication or axoneme formation. In addition, single and double null mutants of PfMAP-2 and the second P. falciparum MAPK homologue PfMAP-1 show no defect in asexual proliferation, sexual commitment or gametocytogenesis. Our results suggest that MAPK activity plays no major role in the biology of both asexual and sexual blood stage parasites up until the point of male gametogenesis.

Profiling MHC II immunopeptidome of blood-stage malaria reveals that cDC1 control the functionality of parasite-specific CD4 T cells

In malaria, CD4 Th1 and T follicular helper (T_FH) cells are important for controlling parasite growth, but Th1 cells also contribute to immunopathology. Moreover, various regulatory CD4 T‐cell subsets are critical to hamper pathology. Yet the antigen‐presenting cells controlling Th functionality, as well as the antigens recognized by CD4 T cells, are largely unknown. Here, we characterize the MHC II immunopeptidome presented by DC during blood‐stage malaria in mice. We establish the immunodominance hierarchy of 14 MHC II ligands derived from conserved parasite proteins. Immunodominance is shaped differently whether blood stage is preceded or not by liver stage, but the same ETRAMP‐specific dominant response develops in both contexts. In naïve mice and at the onset of cerebral malaria, CD8α⁺ dendritic cells (cDC1) are superior to other DC subsets for MHC II presentation of the ETRAMP epitope. Using in vivo depletion of cDC1, we show that cDC1 promote parasite‐specific Th1 cells and inhibit the development of IL‐10⁺CD4 T cells. This work profiles the P. berghei blood‐stage MHC II immunopeptidome, highlights the potency of cDC1 to present malaria antigens on MHC II, and reveals a major role for cDC1 in regulating malaria‐specific CD4 T‐cell responses.

GDV1 induces sexual commitment of malaria parasites by antagonizing HP1-dependent gene silencing

Malaria is caused by Plasmodium parasites that proliferate in the bloodstream. During each replication cycle, some parasites differentiate into gametocytes, the only forms able to infect the mosquito vector and transmit malaria. Sexual commitment is triggered by activation of AP2-G, the master transcriptional regulator of gametocytogenesis. Heterochromatin protein 1 (HP1)-dependent silencing of ap2-g prevents sexual conversion in proliferating parasites. In this study, we identified Plasmodium falciparum gametocyte development 1 (GDV1) as an upstream activator of sexual commitment. We found that GDV1 targeted heterochromatin and triggered HP1 eviction, thus derepressing ap2-g Expression of GDV1 was responsive to environmental triggers of sexual conversion and controlled via a gdv1 antisense RNA. Hence, GDV1 appears to act as an effector protein that induces sexual differentiation by antagonizing HP1-dependent gene silencing.

Plasmodium falciparum Maf1 Confers Survival upon Amino Acid Starvation

The target of rapamycin complex 1 (TORC1) pathway is a highly conserved signaling pathway across eukaryotes that integrates nutrient and stress signals to regulate the cellular growth rate and the transition into and maintenance of dormancy. The majority of the pathway's components, including the central TOR kinase, have been lost in the apicomplexan lineage, and it is unknown how these organisms detect and respond to nutrient starvation in its absence. Plasmodium falciparum encodes a putative ortholog of the RNA polymerase (Pol) III repressor Maf1, which has been demonstrated to modulate Pol III transcription in a TOR-dependent manner in a number of organisms. Here, we investigate the role of P. falciparum Maf1 (PfMaf1) in regulating RNA Pol III expression under conditions of nutrient starvation and other stresses. Using a transposon insertion mutant with an altered Maf1 expression profile, we demonstrated that proper Maf1 expression is necessary for survival of the dormancy-like state induced by prolonged amino acid starvation and is needed for full recovery from other stresses that slow or stall the parasite cell cycle. This Maf1 mutant is defective in the downregulation of pre-tRNA synthesis under nutrient-limiting conditions, indicating that the function of Maf1 as a stress-responsive regulator of structural RNA transcription is conserved in P. falciparum Recent work has demonstrated that parasites carrying artemisinin-resistant K13 alleles display an enhanced ability to recover from drug-induced growth retardation. We show that one such artemisinin-resistant line displays greater regulation of pre-tRNA expression and higher survival upon prolonged amino acid starvation, suggesting that overlapping, PfMaf1-associated pathways may regulate growth recovery from both artemisinin treatment and amino acid starvation.IMPORTANCE Eukaryote organisms sense changes in their environment and integrate this information through signaling pathways to activate response programs to ensure survival. The TOR pathway is a well-studied signaling pathway found throughout eukaryotes that is known to integrate a variety of signals to regulate organismal growth in response to starvation and other stresses. The human malaria parasite Plasmodium falciparum appears to have lost the TOR pathway over the course of evolution, and it is unclear how the parasite modulates its growth in response to starvation and drug treatment. Here, we show that Maf1, a protein regulated by TOR in other eukaryotes, plays an important role in maintaining the parasite's viability in the face of starvation and other forms of stress. This suggests that PfMaf1 is a component of a yet-to-be-described nutrient and stress response pathway.

Membrane skeletal association and post-translational allosteric regulation of Toxoplasma gondii GAPDH1

When Toxoplasma gondii egresses from the host cell, glyceraldehyde-3-phosphate dehydrogenase 1 (GAPDH1), which is primary a glycolysis enzyme but actually a quintessential multifunctional protein, translocates to the unique cortical membrane skeleton. Here, we report the 2.25 Å resolution crystal structure of the GAPDH1 holoenzyme in a quaternary complex providing the basis for the molecular dissection of GAPDH1 structure-function relationships Knockdown of GAPDH1 expression and catalytic site disruption validate the essentiality of GAPDH1 in intracellular replication but we confirmed that glycolysis is not strictly essential. We identify, for the first time, S-loop phosphorylation as a novel, critical regulator of enzymatic activity that is consistent with the notion that the S-loop is critical for cofactor binding, allosteric activation and oligomerization. We show that neither enzymatic activity nor phosphorylation state correlate with the ability to translocate to the cortex. However, we demonstrate that association of GAPDH1 with the cortex is mediated by the N-terminus, likely palmitoylation. Overall, glycolysis and cortical translocation are functionally decoupled by post-translational modifications.

D-Glyceraldehyde-3-Phosphate Dehydrogenase Structure and Function

Aside from its well-established role in glycolysis, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has been shown to possess many key functions in cells. These functions are regulated by protein oligomerization , biomolecular interactions, post-translational modifications , and variations in subcellular localization . Several GAPDH functions and regulatory mechanisms overlap with one another and converge around its role in intermediary metabolism. Several structural determinants of the protein dictate its function and regulation. GAPDH is ubiquitously expressed and is found in all domains of life. GAPDH has been implicated in many diseases, including those of pathogenic, cardiovascular, degenerative, diabetic, and tumorigenic origins. Understanding the mechanisms by which GAPDH can switch between its functions and how these functions are regulated can provide insights into ways the protein can be modulated for therapeutic outcomes.

Identification of GAPDH on the surface of Plasmodium sporozoites as a new candidate for targeting malaria liver invasion

Cha et al. show that Plasmodium GAPDH on the sporozoite surface acts as a ligand for binding Kupffer cell CD68, an interaction that is critical for parasite liver invasion. Thus, Plasmodium GAPDH is a candidate antigen for a prehepatic malaria vaccine.

Malaria transmission begins when an infected mosquito delivers Plasmodium sporozoites into the skin. The sporozoite subsequently enters the circulation and infects the liver by preferentially traversing Kupffer cells, a macrophage-like component of the liver sinusoidal lining. By screening a phage display library, we previously identified a peptide designated P39 that binds to CD68 on the surface of Kupffer cells and blocks sporozoite traversal. In this study, we show that the P39 peptide is a structural mimic of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) on the sporozoite surface and that GAPDH directly interacts with CD68 on the Kupffer cell surface. Importantly, an anti-P39 antibody significantly inhibits sporozoite liver invasion without cross-reacting with mammalian GAPDH. Therefore, Plasmodium-specific GAPDH epitopes may provide novel antigens for the development of a prehepatic vaccine.

Generation of Plasmodium falciparum parasite-inhibitory antibodies by immunization with recombinantly-expressed CyRPA

BACKGROUND

The pathogenesis of malaria is primarily associated with blood-stage infection and there is strong evidence that antibodies specific for parasite blood-stage antigens can control parasitaemia. This provides a strong rationale for incorporation of asexual blood-stage antigen components into an effective multivalent malaria subunit vaccine. On the basis of available genome-wide transcriptomic and proteomic data, previously uncharacterized Plasmodium falciparum open reading frames were screened for new blood stage vaccine candidates. This has led to the identification of the cysteine-rich protective antigen (PfCyRPA), which forms together with PfRH5 and PfRipr a multiprotein complex that is crucial for erythrocyte invasion.

METHODS

Glycosylated and non-glycosylated variants of recombinant PfCyRPA were expressed and produced as secreted protein in mammalian cells. Adjuvanted formulations of purified PfCyRPA were tested to assess whether they can effectively elicit parasite inhibitory antibodies, and to investigate whether or not the glycosylation status affects antibody binding. For this purpose, two sets of PfCyRPA-specific mouse monoclonal antibodies (mAbs) have been raised and evaluated for functional activity.

RESULTS

Generated PfCyRPA-specific mAbs, irrespective of the immunogen's glycosylation status, showed substantial parasite in vitro growth-inhibitory activity due to inhibition of erythrocyte invasion by merozoites. Furthermore, passive immunization experiments in P. falciparum infected NOD-scid IL2Rγ (null) mice engrafted with human erythrocytes demonstrated potent in vivo growth-inhibitory activity of generated mAbs.

CONCLUSIONS

Recombinantly expressed PfCyRPA tested as adjuvanted vaccine formulations in mice elicited antibodies that significantly inhibit P. falciparum asexual blood stage parasite growth both in vitro and in vivo. These findings render PfCyRPA a promising blood-stage candidate antigen for inclusion into a multicomponent malaria subunit vaccine.

Critical protein GAPDH and its regulatory mechanisms in cancer cells

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), initially identified as a glycolytic enzyme and considered as a housekeeping gene, is widely used as an internal control in experiments on proteins, mRNA, and DNA. However, emerging evidence indicates that GAPDH is implicated in diverse functions independent of its role in energy metabolism; the expression status of GAPDH is also deregulated in various cancer cells. One of the most common effects of GAPDH is its inconsistent role in the determination of cancer cell fate. Furthermore, studies have described GAPDH as a regulator of cell death; other studies have suggested that GAPDH participates in tumor progression and serves as a new therapeutic target. However, related regulatory mechanisms of its numerous cellular functions and deregulated expression levels remain unclear. GAPDH is tightly regulated at transcriptional and posttranscriptional levels, which are involved in the regulation of diverse GAPDH functions. Several cancer-related factors, such as insulin, hypoxia inducible factor-1 (HIF-1), p53, nitric oxide (NO), and acetylated histone, not only modulate GAPDH gene expression but also affect protein functions via common pathways. Moreover, posttranslational modifications (PTMs) occurring in GAPDH in cancer cells result in new activities unrelated to the original glycolytic function of GAPDH. In this review, recent findings related to GAPDH transcriptional regulation and PTMs are summarized. Mechanisms and pathways involved in GAPDH regulation and its different roles in cancer cells are also described.

Exploiting Unique Structural and Functional Properties of Malarial Glycolytic Enzymes for Antimalarial Drug Development

Metabolic enzymes have been known to carry out a variety of functions besides their normal housekeeping roles known as “moonlighting functions.” These functionalities arise from structural changes induced by posttranslational modifications and/or binding of interacting proteins. Glycolysis is the sole source of energy generation for malaria parasite Plasmodium falciparum, hence a potential pathway for therapeutic intervention. Crystal structures of several P. falciparum glycolytic enzymes have been solved, revealing that they exhibit unique structural differences from the respective host enzymes, which could be exploited for their selective targeting. In addition, these enzymes carry out many parasite-specific functions, which could be of potential interest to control parasite development and transmission. This review focuses on the moonlighting functions of P. falciparum glycolytic enzymes and unique structural differences and functional features of the parasite enzymes, which could be exploited for therapeutic and transmission blocking interventions against malaria.

Evolutionary implications of localization of the signaling scaffold protein parafusin to both cilia and the nucleus

Parafusin (PFUS), a 63 kDa protein first discovered in the eukaryote Paramecium and known for its role in apicomplexan exocytosis, provides a model for the common origin of cellular systems employing scaffold proteins for targeting and signaling. PFUS is closely related to eubacterial rather than archeal phosphoglucomutases (PGM) - as we proved by comparison of their 88 sequences - but has no PGM activity. Immunofluorescence microscopy analysis with a PFUS-specific peptide antibody showed presence of this protein around the base region of primary cilia in a variety of mammalian cell types, including mouse embryonic (MEFs) and human foreskin fibroblasts (hFFs), human carcinoma stem cells (NT-2 cells), and human retinal pigment epithelial (RPE) cells. Further, PFUS localized to the nucleus of fibroblasts, and prominently to nucleoli of MEFs. Localization studies were confirmed by Western blot analysis, showing that the PFUS antibody specifically recognizes a single protein of ca. 63 kDa in both cytoplasmic and nuclear fractions. Finally, immunofluorescence microscopy analysis showed that PFUS localized to nuclei and cilia in Paramecium. These results support the suggestion that PFUS plays a role in signaling between nucleus and cilia, and that the cilium and the nucleus both evolved around the time of eukaryotic emergence. We hypothesize that near the beginnings of eukaryotic cell evolution, scaffold proteins such as PFUS arose as peripheral membrane protein identifiers for cytoplasmic membrane trafficking and were employed similarly during the subsequent evolution of exocytic, nuclear transport, and ciliogenic mechanisms.

Characterization of the small exported Plasmodium falciparum membrane protein SEMP1

Survival and virulence of the human malaria parasite Plasmodium falciparum during the blood stage of infection critically depend on extensive host cell refurbishments mediated through export of numerous parasite proteins into the host cell. The parasite-derived membranous structures called Maurer's clefts (MC) play an important role in protein trafficking from the parasite to the red blood cell membrane. However, their specific function has yet to be determined. We identified and characterized a new MC membrane protein, termed small exported membrane protein 1 (SEMP1). Upon invasion it is exported into the RBC cytosol where it inserts into the MCs before it is partly translocated to the RBC membrane. Using conventional and conditional loss-of-function approaches we showed that SEMP1 is not essential for parasite survival, gametocytogenesis, or PfEMP1 export under culture conditions. Co-IP experiments identified several potential interaction partners, including REX1 and other membrane-associated proteins that were confirmed to co-localize with SEMP1 at MCs. Transcriptome analysis further showed that expression of a number of exported parasite proteins was up-regulated in SEMP1-depleted parasites. By using Co-IP and transcriptome analysis for functional characterization of an exported parasite protein we provide a new starting point for further detailed dissection and characterisation of MC-associated protein complexes.

A var gene upstream element controls protein synthesis at the level of translation initiation in Plasmodium falciparum

Clonally variant protein expression in the malaria parasite Plasmodium falciparum generates phenotypic variability and allows isogenic populations to adapt to environmental changes encountered during blood stage infection. The underlying regulatory mechanisms are best studied for the major virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1 is encoded by the multicopy var gene family and only a single variant is expressed in individual parasites, a concept known as mutual exclusion or singular gene choice. var gene activation occurs in situ and is achieved through the escape of one locus from epigenetic silencing. Singular gene choice is controlled at the level of transcription initiation and var 5' upstream (ups) sequences harbour regulatory information essential for mutually exclusive transcription as well as for the trans-generational inheritance of the var activity profile. An additional level of control has recently been identified for the var2csa gene, where an mRNA element in the 5' untranslated region (5' UTR) is involved in the reversible inhibition of translation of var2csa transcripts. Here, we extend the knowledge on post-transcriptional var gene regulation to the common upsC type. We identified a 5' UTR sequence that inhibits translation of upsC-derived mRNAs. Importantly, this 5' UTR element efficiently inhibits translation even in the context of a heterologous upstream region. Further, we found var 5' UTRs to be significantly enriched in uAUGs which are known to impair the efficiency of protein translation in other eukaryotes. Our findings suggest that regulation at the post-transcriptional level is a common feature in the control of PfEMP1 expression in P. falciparum.

Brain proteomics supports the role of glutamate metabolism and suggests other metabolic alterations in protein l-isoaspartyl methyltransferase (PIMT)-knockout mice

Protein l-isoaspartyl methyltransferase (PIMT) repairs the isoaspartyl residues (isoAsp) that originate from asparagine deamidation and aspartic acid (Asp) isomerization to Asp residues. Deletion of the gene encoding PIMT in mice (Pcmt1) leads to isoAsp accumulation in all tissues measured, especially in the brain. These PIMT-knockout (PIMT-KO) mice have perturbed glutamate metabolism and die prematurely of epileptic seizures. To elucidate the role of PIMT further, brain proteomes of PIMT-KO mice and controls were analyzed. The isoAsp levels from two of the detected 67 isoAsp sites (residue 98 from calmodulin and 68 from glyceraldehyde-3-phosphate dehydrogenase) were quantified and found to be significantly increased in PIMT-KO mice (p < 0.01). Additionally, the abundance of at least 151 out of the 1017 quantified proteins was found to be altered in PIMT-KO mouse brains. Gene ontology analysis revealed that many down-regulated proteins are involved in cellular amino acid biosynthesis. For example, the serine synthesis pathway was suppressed, possibly leading to reduced serine production in PIMT-KO mice. Additionally, the abundances of enzymes in the glutamate-glutamine cycle were altered toward the accumulation of glutamate. These findings support the involvement of PIMT in glutamate metabolism and suggest that the absence of PIMT also affects other processes involving amino acid synthesis and metabolism.

Food vacuole associated enolase in plasmodium undergoes multiple post-translational modifications: evidence for atypical ubiquitination

Plasmodium enolase localizes to several sub-cellular compartments viz. cytosol, nucleus, cell membrane, food vacuole (FV) and cytoskeleton, without having any organelle targeting signal sequences. This enzyme has been shown to undergo multiple post-translational modifications (PTMs) giving rise to several variants that show organelle specific localization. It is likely that these PTMs may be responsible for its diverse distribution and moonlighting functions. While most variants have a MW of ~50 kDa and are likely to arise due to changes in pI, food vacuole (FV) associated enolase showed three forms with MW~50, 65 and 75 kDa. Evidence from immuno-precipitation and western analysis indicates that the 65 and 75 kDa forms of FV associated enolase are ubiquitinated. Using mass spectrometry (MS), definitive evidence is obtained for the nature of PTMs in FV associated variants of enolase. Results showed several modifications, viz. ubiquitination at K147, phosphorylation at Y148 and acetylation at K142 and K384. MS data also revealed the conjugation of three ubiquitin (Ub) molecules to enolase through K147. Trimeric ubiquitin has a linear peptide linkage between the NH₂-terminal methionine of the first ubiquitin (Ub1) and the C-terminal G76 of the second (Ub2). Ub2 and third ubiquitin (Ub3) were linked through an atypical isopeptide linkage between K6 of Ub2 and G76 of Ub3, respectively. Further, the tri-ubiquitinated form was found to be largely associated with hemozoin while the 50 and 65 kDa forms were present in the NP-40 soluble fraction of FV. Mass spectrometry results also showed phosphorylation of S42 in the cytosolic enolase from P. falciparum and T337 in the cytoskeleton associated enolase from P. yoelii. The composition of food vacuolar proteome and likely interactors of enolase are also being reported.

Functional diversity

There is increasing evidence to support a gene economy model that is fully based on the principles of evolution in which a limited number of proteins does not necessarily reflect a finite number of biochemical processes. The concept of 'gene sharing' proposes that a single protein can have alternate functions that are typically attributed to other proteins. GAPDH appears to play this role quite well in that it exhibits more than one function. GAPDH represents the prototype for this new paradigm of protein multi-functionality. The chapter discusses the diverse functions of GAPDH among three broad categories: cell structure, gene expression and signal transduction. Protein function is curiously re-specified given the cell's unique needs. GAPDH provides the cell with the means of linking metabolic activity to various cellular processes. While interpretations may often lead to GAPDH's role in meeting focal energy demands, this chapter discusses several other very distinct GAPDH functions (i.e. membrane fusogenic properties) that are quite different from its ability to catalyze oxidative phosphorylation of the triose, glyceraldehyde 3-phosphate. It is suggested that a single protein participates in multiple processes in the structural organization of the cell, controls the transmission of genetic information (i.e. GAPDH's involvement may not be finite) and mediates intracellular signaling.

Broad distribution of TPI-GAPDH fusion proteins among eukaryotes: evidence for glycolytic reactions in the mitochondrion?

Glycolysis is a central metabolic pathway in eukaryotic and prokaryotic cells. In eukaryotes, the textbook view is that glycolysis occurs in the cytosol. However, fusion proteins comprised of two glycolytic enzymes, triosephosphate isomerase (TPI) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), were found in members of the stramenopiles (diatoms and oomycetes) and shown to possess amino-terminal mitochondrial targeting signals. Here we show that mitochondrial TPI-GAPDH fusion protein genes are widely spread across the known diversity of stramenopiles, including non-photosynthetic species (Bicosoeca sp. and Blastocystis hominis). We also show that TPI-GAPDH fusion genes exist in three cercozoan taxa (Paulinella chromatophora, Thaumatomastix sp. and Mataza hastifera) and an apusozoan protist, Thecamonas trahens. Interestingly, subcellular localization predictions for other glycolytic enzymes in stramenopiles and a cercozoan show that a significant fraction of the glycolytic enzymes in these species have mitochondrial-targeted isoforms. These results suggest that part of the glycolytic pathway occurs inside mitochondria in these organisms, broadening our knowledge of the diversity of mitochondrial metabolism of protists.

Organellar proteomics reveals hundreds of novel nuclear proteins in the malaria parasite Plasmodium falciparum

BACKGROUND

The post-genomic era of malaria research provided unprecedented insights into the biology of Plasmodium parasites. Due to the large evolutionary distance to model eukaryotes, however, we lack a profound understanding of many processes in Plasmodium biology. One example is the cell nucleus, which controls the parasite genome in a development- and cell cycle-specific manner through mostly unknown mechanisms. To study this important organelle in detail, we conducted an integrative analysis of the P. falciparum nuclear proteome.

RESULTS

We combined high accuracy mass spectrometry and bioinformatic approaches to present for the first time an experimentally determined core nuclear proteome for P. falciparum. Besides a large number of factors implicated in known nuclear processes, one-third of all detected proteins carry no functional annotation, including many phylum- or genus-specific factors. Importantly, extensive experimental validation using 30 transgenic cell lines confirmed the high specificity of this inventory, and revealed distinct nuclear localization patterns of hitherto uncharacterized proteins. Further, our detailed analysis identified novel protein domains potentially implicated in gene transcription pathways, and sheds important new light on nuclear compartments and processes including regulatory complexes, the nucleolus, nuclear pores, and nuclear import pathways.

CONCLUSION

Our study provides comprehensive new insight into the biology of the Plasmodium nucleus and will serve as an important platform for dissecting general and parasite-specific nuclear processes in malaria parasites. Moreover, as the first nuclear proteome characterized in any protist organism, it will provide an important resource for studying evolutionary aspects of nuclear biology.

Spatiotemporal and functional characterisation of the Plasmodium falciparum cGMP-dependent protein kinase

Signalling by 3′–5′-cyclic guanosine monophosphate (cGMP) exists in virtually all eukaryotes. In the apicomplexan parasite Plasmodium, the cGMP-dependent protein kinase (PKG) has previously been reported to play a critical role in four key stages of the life cycle. The Plasmodium falciparum isoform (PfPKG) is essential for the initiation of gametogenesis and for blood stage schizont rupture and work on the orthologue from the rodent malaria parasite P. berghei (PbPKG) has shown additional roles in ookinete differentiation and motility as well as liver stage schizont development. In the present study, PfPKG expression and subcellular location in asexual blood stages was investigated using transgenic epitope-tagged PfPKG-expressing P. falciparum parasites. In Western blotting experiments and immunofluorescence analysis (IFA), maximal PfPKG expression was detected at the late schizont stage. While IFA suggested a cytosolic location, a degree of overlap with markers of the endoplasmic reticulum (ER) was found and subcellular fractionation showed some association with the peripheral membrane fraction. This broad localisation is consistent with the notion that PfPKG, as with the mammalian orthologue, has numerous cellular substrates. This idea is further supported by the global protein phosphorylation pattern of schizonts which was substantially changed following PfPKG inhibition, suggesting a complex role for PfPKG during schizogony.

Binding of aldolase and glyceraldehyde-3-phosphate dehydrogenase to the cytoplasmic tails of Plasmodium falciparum merozoite duffy binding-like and reticulocyte homology ligands

Invasion of erythrocytes by Plasmodium falciparum requires a connection between the cytoplasmic tail of the parasite’s ligands for its erythrocyte receptors and the actin-myosin motor of the parasite. For the thromobospondin-related anonymous protein (TRAP) ligand on Plasmodium sporozoites, aldolase forms this connection and requires tryptophan and negatively charged amino acids in the ligand’s cytoplasmic tail. Because of the importance of the Duffy binding-like (DBL) and the reticulocyte homology (RH) ligand families in erythrocyte binding and merozoite invasion, we characterized the ability of their cytoplasmic tails to bind aldolase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), both of which bind actin. We tested the binding of the cytoplasmic peptides of the two ligand families to aldolase and GAPDH. Only the cytoplasmic peptides of some RH ligands showed strong binding to aldolase, and the binding depended on the presence of an aromatic amino acid (phenylalanine or tyrosine), rather than tryptophan, in the context of negatively charged amino acids. The binding was confirmed by surface plasmon resonance analysis and was found to represent affinity similar to that seen with TRAP. An X-ray crystal structure of aldolase at 2.5 Å in the presence of RH2b peptide suggested that the binding site location was near the TRAP-binding site. GAPDH bound to some of the cytoplasmic tails of certain RH and DBL ligands in an aromatic amino acid-dependent manner. Thus, the connection between Plasmodium merozoite ligands and erythrocyte receptors and the actin motor can be achieved through the activity of either aldolase or GAPDH by mechanisms that do not require tryptophan but, rather, other aromatic amino acids.

The invasion of the Plasmodium merozoite into erythrocytes is a critical element in malaria pathogenesis. It is important to understand the molecular details of this process, as this machinery can be a target for both vaccine and drug development. In Plasmodium sporozoites and Toxoplasma tachyzoites, invasion involves a glycolytic enzyme aldolase, linking the cytoplasmic tail domains of the parasite ligands to the actin-myosin motor that drives invasion. This binding requires a tryptophan that cannot be replaced by other aromatic residues. Here we show that aldolase binds the cytoplasmic tails of some P. falciparum merozoite erythrocyte-binding ligands but that the binding involves aromatic residues other than tryptophan. The biological relevance of aldolase binding to cytoplasmic tails of parasite ligands in invasion is demonstrated by our observation that RH2b but not RH2a binds to aldolase and, as previously shown, that RH2b but not RH2a is required for P. falciparum invasion of erythrocytes.

Exosomes from Plasmodium yoelii-infected reticulocytes protect mice from lethal infections

Exosomes are 30-100-nm membrane vesicles of endocytic origin that are released after the fusion of multivesicular bodies (MVBs) with the plasma membrane. While initial studies suggested that the role of exosomes was limited to the removal of proteins during the maturation of reticulocytes to erythrocytes, recent studies indicate that they are produced by different types of cells and are involved in promoting inter-cellular communication and antigen presentation. Here, we describe the isolation and characterization of exosomes from peripheral blood of BALB/c mice infected with the reticulocyte-prone non-lethal Plasmodium yoelii 17X strain. Importantly, proteomic analysis revealed the presence of parasite proteins in these vesicles. Moreover, immunization of mice with purified exosomes elicited IgG antibodies capable of recognizing P. yoelii-infected red blood cells. Furthermore, lethal challenge of immunized mice with the normocyte-prone lethal P. yoelii 17XL strain caused a significant attenuation in the course of parasitaemia, increased survival time, and altered the cell tropism to reticulocytes. These results were obtained also when the exosomes were isolated from a P. yoelii-infected reticulocyte culture indicating that reticulocyte-derived exosomes carry antigens and are involved in immune modulation. Moreover, inclusion of CpG ODN 1826 in exosome immunizations elicited IgG2a and IgG2b antibodies and promoted survival, clearance of parasites and subsequent sterile protection of 83% of the animals challenged with P. yoelli 17XL. To our knowledge, this is the first report of immune responses elicited by exosomes derived from reticulocytes opening new avenues for the modulation of anti-malaria responses.

On the functional diversity of glyceraldehyde-3-phosphate dehydrogenase: biochemical mechanisms and regulatory control

BACKGROUND

New studies provide evidence that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is not simply a classical glycolytic protein of little interest. Instead, it is a multifunctional protein with significant activity in a number of fundamental cell pathways. GAPDH is a highly conserved gene and protein, with a single mRNA transcribed from a unique gene. Control mechanisms must exist which regulate its functional diversity.

SCOPE OF REVIEW

This review focuses on new, timely studies defining not only its diverse activities but also those which define the regulatory mechanisms through which those functions may be controlled. The reader is referred to the author's prior review for the consideration of past reports which first indicated GAPDH multiple activities (Sirover, Biochim. Biophys. Acta 1432 (1999) 159-184.)

CONCLUSIONS

These investigations demonstrate fundamental roles of GAPDH in vivo, dynamic changes in its subcellular localization, and the importance of posttranslational modifications as well as protein:protein interactions as regulatory control mechanisms.

GENERAL SIGNIFICANCE

GAPDH is the prototype "moonlighting" protein which exhibits activities distinct from their classically identified functions. Their participation in diverse cell pathways is essential. Regulatory mechanisms exist which control those diverse activities as well as changes in their subcellular localization as a consequence of those new functions.

The toxoplasma apicoplast phosphate translocator links cytosolic and apicoplast metabolism and is essential for parasite survival

Apicomplexa are unicellular eukaryotic pathogens that carry a vestigial algal endosymbiont, the apicoplast. The physiological function of the apicoplast and its integration into parasite metabolism remain poorly understood and at times controversial. We establish that the Toxoplasma apicoplast membrane-localized phosphate translocator (TgAPT) is an essential metabolic link between the endosymbiont and the parasite cytoplasm. TgAPT is required for fatty acid synthesis in the apicoplast, but this may not be its most critical function. Further analyses demonstrate that TgAPT also functions to supply the apicoplast with carbon skeletons for additional pathways and, indirectly, with energy and reduction power. Genetic ablation of the transporter results in rapid death of parasites. The dramatic consequences of loss of its activity suggest that targeting TgAPT could be a viable strategy to identify antiparasitic compounds.

A new double-antibody sandwich ELISA targeting Plasmodium falciparum aldolase to evaluate anti-malarial drug sensitivity

Background

The standard in vitro test to assess anti-malarial activity of chemical compounds is the [³H]hypoxanthine incorporation assay. It is a radioactivity-based method to measure DNA replication of Plasmodium in red blood cells. The method is highly reproducible, however, the handling of radioactive material is costly, hazardous and requires the availability of appropriate technology and trained staff. Several other ways to evaluate in vitro anti-malarial activity do exist, all with their own assets and limitations.

Methods

The newly developed double-antibody sandwich ELISA described here is based on the properties of a non-overlapping pair of monoclonal antibodies directed against Plasmodium falciparum aldolase. This glycolytic enzyme possesses some unique nucleotide sequences compared to the human isoenzymes and has been highly conserved through evolution. Out of twenty possibilities, the most sensitive antibody pair was selected and used to quantitatively detect parasite aldolase in infected blood lysates.

Results

A total of 34 compounds with anti-malarial activity were tested side-by-side by ELISA and the [³H]hypoxanthine incorporation assay. The novel ELISA provided IC₅₀s closely paralleling those from the radioactivity-based assay (R = 0.99, p < 0.001). At the investigated assay conditions (72 h incubation time, parasitaemia = 0.3%), the assay was found to be reproducible and easy to perform.

Conclusion

The newly developed ELISA presents several advantages over the comparative method, the [³H]hypoxanthine incorporation assay. The assay is highly reproducible, less hazardous (involves no radioactivity) and requires little and cheap technical equipment. Relatively unskilled personnel can conduct this user-friendly assay. All this makes it attractive to be employed in resource-poor laboratories.

Plasmodium falciparum enolase: stage-specific expression and sub-cellular localization

Background

In an earlier study, it was observed that the vaccination with Plasmodium falciparum enolase can confer partial protection against malaria in mice. Evidence has also build up to indicate that enolases may perform several non-glycolytic functions in pathogens. Investigating the stage-specific expression and sub-cellular localization of a protein may provide insights into its moonlighting functions.

Methods

Sub-cellular localization of P. falciparum enolase was examined using immunofluorescence assay, immuno-gold electron microscopy and western blotting.

Results

Enolase protein was detected at every stage in parasite life cycle examined. In asexual stages, enolase was predominantly (≥85–90%) present in soluble fraction, while in sexual stages it was mostly associated with particulate fraction. Apart from cytosol, enolase was found to be associated with nucleus, food vacuole, cytoskeleton and plasma membrane.

Conclusion

Diverse localization of enolase suggests that apart from catalyzing the conversion of 2-phosphoglycericacid into phosphoenolpyruvate in glycolysis, enolase may be involved in a host of other biological functions. For instance, enolase localized on the merozoite surface may be involved in red blood cell invasion; vacuolar enolase may be involved in food vacuole formation and/or development; nuclear enolase may play a role in transcription.

Food vacuole proteome of the malarial parasite Plasmodium falciparum

The Plasmodium falciparum food vacuole (FV) is a lysosome-like organelle where erythrocyte hemoglobin digestion occurs. It is a favorite target in the development of antimalarials. We have used a tandem mass spectrometry approach to investigate the proteome of an FV-enriched fraction and identified 116 proteins. The electron microscopy analysis and the Western blot data showed that the major component of the fraction was the FV and, as expected, the majority of previously known FV markers were recovered. Of particular interest, several proteins involved in vesicle-mediated trafficking were identified, which are likely to play a key role in FV biogenesis and/or FV protein trafficking. Recovery of parasite surface proteins lends support to the cytostomal pathway of hemoglobin ingestion as a FV trafficking route. We have identified 32 proteins described as hypothetical in the databases. This insight into FV protein content provides new clues towards understanding the biological function of this organelle in P. falciparum.

Characterization of a conserved rhoptry-associated leucine zipper-like protein in the malaria parasite Plasmodium falciparum

One of the key processes in the pathobiology of the malaria parasite is the invasion and subsequent modification of the human erythrocyte. In this complex process, an unknown number of parasite proteins are involved, some of which are leading vaccine candidates. The majority of the proteins that play pivotal roles in invasion are either stored in the apical secretory organelles or located on the surface of the merozoite, the invasive stage of the parasite. Using transcriptional and structural features of these known proteins, we performed a genomewide search that identified 49 hypothetical proteins with a high probability of being located on the surface of the merozoite or in the secretory organelles. Of these candidates, we characterized a novel leucine zipper-like protein in Plasmodium falciparum that is conserved in Plasmodium spp. This protein is expressed in late blood stages and localizes to the rhoptries of the parasite. We demonstrate that this Plasmodium sp.-specific protein has a high degree of conservation within field isolates and that it is refractory to gene knockout attempts and thus might play an important role in invasion.

The M18 aspartyl aminopeptidase of the human malaria parasite Plasmodium falciparum

A member of the M18 family of aspartyl aminopeptidases is expressed by all intra-erythrocytic stages of the human malaria parasite Plasmodium falciparum (PfM18AAP), with highest expression levels in rings. Functionally active recombinant enzyme, rPfM18AAP, and native enzyme in cytosolic extracts of malaria parasites are 560-kDa octomers that exhibit optimal activity at neutral pH and require the presence of metal ions to maintain enzymatic activity and stability. Like the human aspartyl aminopeptidase, the exopeptidase activity of PfM18AAP is exclusive to N-terminal acidic amino acids, glutamate and aspartate, making this enzyme of particular interest and suggesting that it may function alongside the malaria cytosolic neutral aminopeptidases in the release of amino acids from host hemoglobin-derived peptides. Whereas immunocytochemical studies using transgenic P. falciparum parasites show that PfM18AAP is expressed in the cytosol, immunoblotting experiments revealed that the enzyme is also trafficked out of the parasite into the surrounding parasitophorous vacuole. Antisense-mediated knockdown of PfM18AAP results in a lethal phenotype as a result of significant intracellular damage and validates this enzyme as a target at which novel antimalarial drugs could be directed. Novel phosphinic derivatives of aspartate and glutamate showed modest inhibition of rPfM18AAP but did not inhibit malaria growth in culture. However, we were able to draw valuable observations concerning the structure-activity relationship of these inhibitors that can be employed in future inhibitor optimization studies.

Selective permeabilization of the host cell membrane of Plasmodium falciparum-infected red blood cells with streptolysin O and equinatoxin II

Plasmodium falciparum develops within the mature RBCs (red blood cells) of its human host in a PV (parasitophorous vacuole) that separates the host cell cytoplasm from the parasite surface. The pore-forming toxin, SLO (streptolysin O), binds to cholesterol-containing membranes and can be used to selectively permeabilize the host cell membrane while leaving the PV membrane intact. We found that in mixtures of infected and uninfected RBCs, SLO preferentially lyses uninfected RBCs rather than infected RBCs, presumably because of differences in cholesterol content of the limiting membrane. This provides a means of generating pure preparations of viable ring stage infected RBCs. As an alternative permeabilizing agent we have characterized EqtII (equinatoxin II), a eukaryotic pore-forming toxin that binds preferentially to sphingomyelin-containing membranes. EqtII lyses the limiting membrane of infected and uninfected RBCs with similar efficiency but does not disrupt the PV membrane. It generates pores of up to 100 nm, which allow entry of antibodies for immunofluorescence and immunogold labelling. The present study provides novel tools for the analysis of this important human pathogen and highlights differences between Plasmodium-infected and uninfected RBCs.