Species-specific inhibition of cerebral malaria in mice coinfected with Plasmodium spp

Recent epidemiological observations suggest that clinical evolution of Plasmodium falciparum infections might be influenced by the concurrent presence of another Plasmodium species, and such mixed-species infections are now known to occur frequently in residents of most areas of endemicity. We used mice infected with P. berghei ANKA (PbA), a model for cerebral malaria (CM), to investigate the influence of experimental mixed-species infections on the expression of this pathology. Remarkably, the development of CM was completely inhibited by the simultaneous presence of P. yoelii yoelii but not that of P. vinckei or another line of P. berghei. In the protected coinfected mice, the accumulation of CD8(+) T cells in the brain vasculature, a pivotal step in CM pathogenesis, was found to be abolished. Protection from CM was further found to be associated with species-specific suppression of PbA multiplication. These observations establish the concept of mixed Plasmodium species infections as potential modulators of pathology and open novel avenues to investigate mechanisms implicated in the pathogenesis of malaria.

Genetically attenuated, P36p-deficient malarial sporozoites induce protective immunity and apoptosis of infected liver cells

Immunization with Plasmodium sporozoites that have been attenuated by gamma-irradiation or specific genetic modification can induce protective immunity against subsequent malaria infection. The mechanism of protection is only known for radiation-attenuated sporozoites, involving cell-mediated and humoral immune responses invoked by infected hepatocytes cells that contain long-lived, partially developed parasites. Here we analyzed sporozoites of Plasmodium berghei that are deficient in P36p (p36p(-)), a member of the P48/45 family of surface proteins. P36p plays no role in the ability of sporozoites to infect and traverse hepatocytes, but p36p(-) sporozoites abort during development within the hepatocyte. Immunization with p36p(-) sporozoites results in a protective immunity against subsequent challenge with infectious wild-type sporozoites, another example of a specifically genetically attenuated sporozoite (GAS) conferring protective immunity. Comparison of biological characteristics of p36p(-) sporozoites with radiation-attenuated sporozoites demonstrates that liver cells infected with p36p(-) sporozoites disappear rapidly as a result of apoptosis of host cells that may potentiate the immune response. Such knowledge of the biological characteristics of GAS and their evoked immune responses are essential for further investigation of the utility of an optimized GAS-based malaria vaccine.

Gene expression in Plasmodium berghei ookinetes and early oocysts in a co-culture system with mosquito cells

Using an in vitro development system for Plasmodium berghei sporogonic stages and microarray technology we examined parasite gene expression during ookinete invasion of Aedes cells and the ensuing oocyst development. A number of genes were found to be differentially expressed. The most prominent class of up-regulated elements corresponded to products involved in protein synthesis and metabolism. Furthermore, several previously studied genes with a known in vivo developmental profile matched published data. A large number of genes with a hitherto unknown function during the life cycle stages studied also show a differential pattern of expression, indicating the involvement of their products in control and execution of active developmental processes.

Murine malaria parasite sequestration: CD36 is the major receptor, but cerebral pathology is unlinked to sequestration

Sequestration of malaria-parasite-infected erythrocytes in the microvasculature of organs is thought to be a significant cause of pathology. Cerebral malaria (CM) is a major complication of Plasmodium falciparum infections, and PfEMP1-mediated sequestration of infected red blood cells has been considered to be the major feature leading to CM-related pathology. We report a system for the real-time in vivo imaging of sequestration using transgenic luciferase-expressing parasites of the rodent malaria parasite Plasmodium berghei. These studies revealed that: (i) as expected, lung tissue is a major site, but, unexpectedly, adipose tissue contributes significantly to sequestration, and (ii) the class II scavenger-receptor CD36 to which PfEMP1 can bind is also the major receptor for P. berghei sequestration, indicating a role for alternative parasite ligands, because orthologues of PfEMP1 are absent from rodent malaria parasites, and, importantly, (iii) cerebral complications still develop in the absence of CD36-mediated sequestration, dissociating parasite sequestration from CM-associated pathology. Real-time in vivo imaging of parasitic processes may be used to evaluate the molecular basis of pathology and develop strategies to prevent pathology.

Proteome analysis of separated male and female gametocytes reveals novel sex-specific Plasmodium biology

Gametocytes, the precursor cells of malaria-parasite gametes, circulate in the blood and are responsible for transmission from host to mosquito vector. The individual proteomes of male and female gametocytes were analyzed using mass spectrometry, following separation by flow sorting of transgenic parasites expressing green fluorescent protein, in a sex-specific manner. Promoter tagging in transgenic parasites confirmed the designation of stage and sex specificity of the proteins. The male proteome contained 36% (236 of 650) male-specific and the female proteome 19% (101 of 541) female-specific proteins, but they share only 69 proteins, emphasizing the diverged features of the sexes. Of all the malaria life-cycle stages analyzed, the male gametocyte has the most distinct proteome, containing many proteins involved in flagellar-based motility and rapid genome replication. By identification of gender-specific protein kinases and phosphatases and using targeted gene disruption of two kinases, new sex-specific regulatory pathways were defined.

The Phylogeny of malaria: a useful study

Phylogenetic analyses allow an inference of the origins and evolutionary relationships between organisms. Andy Waters, Des Higgins and Tom McCutchan here discuss the benefits of an accurate understanding of these origins and relationships for malaria parasites to the modern fields of vaccine development, pathology and epidemiology.

Plasmodium berghei: the application of cultivation and purification techniques to molecular studies of malaria parasites

Species of malaria parasites that infect rodents provide models for the study of the biology of malaria parasites that infect humans. In this article, Chris Janse and Andy Waters describe some of the recent advances in the cultivation and purification methodology of one of these species, Plasmodium berghei. The improvement of these techniques, and the increasing knowledge about the molecular biology of P. berghei enhance the value of this particular rodent model for the investigation of many aspects of the biology of Plasmodium.

The cytoplasmic ribosomal RNAs of Plasmodium spp

Plasmodium spp maintain several structurally distinct sets of ribosomal RNA genes whose expression is developmentally regulated. This feature sets them apart from all other eukaryotes studied to date. In this review, Thomas McCutchan, Jun Li, Glenn McConkey, John Rogers and Andy Waters give an account of the progress in our understanding of this unusual phenomenon as it relates to the biology of the parasite. They also outline an interesting turnabout in scientific direction involving the use of the parasite as an important new model for the study of the eukaryotic ribosome.

A Plasmodium berghei reference line that constitutively expresses GFP at a high level throughout the complete life cycle

Green fluorescent protein (GFP) is a well-established reporter protein for the examination of biological processes. This report describes a recombinant Plasmodium berghei, PbGFPCON, that constitutively expresses GFP in a growth responsive manner in its cytoplasm from a transgene that is integrated into the genome and controlled by the strong promoter from a P. berghei elongation factor-1alpha gene. All life cycle forms of PbGFPCON except for male gametes can be easily visualized by fluorescent microscopy. PbGFPCON showed similar growth characteristics to wild type P. berghei parasites throughout the whole life cycle and can therefore be used as a reference line for future investigations of parasite-host cell interactions. The principle of automated fluorescence-based counting and sorting of live parasites from host cell backgrounds and different parasite forms from complex mixtures such as asynchronous blood stages is established. PbGFPCON allows the visualization and investigation of live parasite stages that are difficult and labor-intensive to observe, such as the liver and mosquito stages. PbGFPCON can be employed to establish the phenotype of independent mutant parasites. With the recent development of a second, independent selectable marker in P. berghei, PbGFPCON is a useful tool to investigate the effect of further genetic modifications on host-parasite interactions.

Malaria parasite transmission stages: an update

The Molecular Approaches to Malaria 2004 meeting provided an opportunity to see the impressive progress in all research fields and in the four years since the previous Molecular Approaches to Malaria meeting, when much of the Plasmodium falciparum genome sequence was already available. Study of the part of the Plasmodium life cycle associated with transmission through the vector, which begins with the commitment of blood-stage forms to sexual development, has been especially fruitful. This success is a result of several reasons including: (i) the availability of the genome sequence; (ii) the availability of good animal models that allow parasite culture and facile in vivo studies of many of the life cycle stages involved in transmission; (iii) the availability of genetic manipulation technologies for the animal models of malaria, as well as P. falciparum; and (iv) the ability to study lethal gene knockouts at this stage of the life cycle.

A comprehensive survey of the Plasmodium life cycle by genomic, transcriptomic, and proteomic analyses

Plasmodium berghei and Plasmodium chabaudi are widely used model malaria species. Comparison of their genomes, integrated with proteomic and microarray data, with the genomes of Plasmodium falciparum and Plasmodium yoelii revealed a conserved core of 4500 Plasmodium genes in the central regions of the 14 chromosomes and highlighted genes evolving rapidly because of stage-specific selective pressures. Four strategies for gene expression are apparent during the parasites' life cycle: (i) housekeeping; (ii) host-related; (iii) strategy-specific related to invasion, asexual replication, and sexual development; and (iv) stage-specific. We observed posttranscriptional gene silencing through translational repression of messenger RNA during sexual development, and a 47-base 3' untranslated region motif is implicated in this process.

PTRAMP; a conserved Plasmodium thrombospondin-related apical merozoite protein

A gene encoding a 352 amino acid protein with a putative signal sequence, transmembrane domain and thrombospondin structural homology repeat was identified in the genome of the human malaria parasite, Plasmodium falciparum and the rodent malaria parasite, Plasmodium berghei. The protein localises in the apical organelles of P. falciparum and P. berghei merozoites within intraerythrocytic schizonts and has, therefore, been termed the Plasmodium thrombospondin-related apical merozoite protein (PTRAMP). PTRAMP co-localises with the Apical Merozoite Antigen-1 (AMA-1) in developing micronemes and subsequently relocates onto the merozoite surface. Although the gene appears to be specific to the Plasmodium genus, orthologues are present in the genomes of all malaria parasite species examined suggesting a conserved function in host-cell invasion. PTRAMP, therefore, has all the features to merit further evaluation as a malaria vaccine candidate.

Real-time, in vivo analysis of malaria ookinete locomotion and mosquito midgut invasion

Invasion of the Anopheles mosquito midgut by the Plasmodium ookinete is a critical step in the malaria transmission cycle. We have generated a fluorescent P. berghei transgenic line that expresses GFP in the ookinete and oocyst stages, and used it to perform the first real-time analysis of midgut invasion in the living mosquito as well as in explanted intact midguts whose basolateral plasma membranes were vitally stained. These studies permitted detailed analysis of parasite motile behaviour in the midgut and cell biological analysis of the invasion process. Throughout its journey, the ookinete displays distinct modes of motility: stationary rotation, translocational spiralling and straight-segment motility. Spiralling is based on rotational motility combined with translocation steps and changes in direction, which are achieved by transient attachments of the ookinete's trailing end. As it moves from the apical to the basal side of the midgut epithelium, the ookinete uses a predominant intracellular route and appears to glide on the membrane in foldings of the basolateral domain. However, it traverses serially the cytoplasm of several midgut cells before entering and migrating through the basolateral intercellular space to access the basal lamina. The invaded cells commit apoptosis, and their expulsion from the epithelium invokes wound repair mechanisms including extensive lamellipodia crawling. A 'hood' of lamellipodial origin, provided by the invaded cell, covers the ookinete during its egress from the epithelium. The flexible ookinete undergoes shape changes and temporary constrictions associated with passage through the plasma membranes. Similar observations were made in both A. gambiae and A. stephensi, demonstrating the conservation of P. berghei interactions with these vectors.

Complement-Like Protein TEP1 Is a Determinant of Vectorial Capacity in the Malaria Vector Anopheles gambiae

Anopheles mosquitoes are major vectors of human malaria in Africa. Large variation exists in the ability of mosquitoes to serve as vectors and to transmit malaria parasites, but the molecular mechanisms that determine vectorial capacity remain poorly understood. We report that the hemocyte-specific complement-like protein TEP1 from the mosquito Anopheles gambiae binds to and mediates killing of midgut stages of the rodent malaria parasite Plasmodium berghei. The dsRNA knockdown of TEP1 in adults completely abolishes melanotic refractoriness in a genetically selected refractory strain. Moreover, in susceptible mosquitoes this knockdown increases the number of developing parasites. Our results suggest that the TEP1-dependent parasite killing is followed by a TEP1-independent clearance of dead parasites by lysis and/or melanization. Further elucidation of the molecular mechanisms of TEP1-mediated parasite killing will be of great importance for our understanding of the principles of vectorial capacity in insects.

Gene targeting demonstrates that thePlasmodium bergheisubtilisin PbSUB2 is essential for red cell invasion and reveals spontaneous genetic recombination events

The Plasmodium merozoite proteases involved in the crucial process of erythrocyte invasion are promising targets for novel malaria control strategies. We report here the characterization of the subtilisin-like protease SUB2 from the rodent parasites Plasmodium berghei and Plasmodium yoelii, leading the way to in vivo functional studies of this enzyme. The kinetics of expression and subcellular localization imply a central role for SUB2 in erythrocyte invasion. Through the use of gene targeting strategies, we assessed the relevance of P. berghei SUB2 for the intraerythrocytic cycle. The selection of recombinant Pbsub2-TrimycDuoXpress-tagged parasites and the proper expression of the modified coding region demonstrate that the Pbsub2 locus is accessible to genetic modifications. However, Pbsub2 knock-out parasites were not recovered, confirming the importance of PbSUB2 for P. berghei merozoite stages, and supporting the fact that its Plasmodium falciparum SUB2 orthologue is an attractive drug target candidate. Finally, we identify revertant parasites that have lost the integrated selection cassette while conserving a Pbsub2-tagged gene. These spontaneous reversion events should overcome the scarcity of selectable markers available for this parasite, giving access to multiple gene tagging strategies, which, together with the validation of a TrimycDuoXpress tag, would represent valuable new tools for studying the biology of P. berghei.

Malaria parasites lacking eef1a have a normal S/M phase yet grow more slowly due to a longer G1 phase

Eukaryotic elongation factor 1A (eEF1A) plays a central role in protein synthesis, cell growth and morphology. Malaria parasites possess two identical genes encoding eEF1A (eef1aa and eef1ab). Using pbeef1a-Plasmodium berghei mutants that lack an eEF1a gene, we demonstrate that the level of eEF1A production affects the proliferation of blood stages and parasite fitness. Pbeef1a- parasites can complete the vertebrate and mosquito phases of the life cycle, but the growth phase of the asexual blood stages is extended by up to 20%. Analysis of the cell cycle by flow cytometry as well as transcriptional analyses revealed that the duration of the S and M phases and the number of daughter cells produced were not detectably affected, but that the G1 phase is elongated. Thus, as in budding yeast, a growth threshold must be achieved by blood-stage Plasmodium parasites to permit transition from G1 into S/M phase. Initial analyses indicate that transcriptional events associated with gametocyte development were not remarkably retarded. Insight into protein synthesis and its influence on cell proliferation might be used to generate slow-growing (attenuated) parasites.

Genome sequence and comparative analysis of the model rodent malaria parasite Plasmodium yoelii yoelii

Species of malaria parasite that infect rodents have long been used as models for malaria disease research. Here we report the whole-genome shotgun sequence of one species, Plasmodium yoelii yoelii, and comparative studies with the genome of the human malaria parasite Plasmodium falciparum clone 3D7. A synteny map of 2,212 P. y. yoelii contiguous DNA sequences (contigs) aligned to 14 P. falciparum chromosomes reveals marked conservation of gene synteny within the body of each chromosome. Of about 5,300 P. falciparum genes, more than 3,300 P. y. yoelii orthologues of predominantly metabolic function were identified. Over 800 copies of a variant antigen gene located in subtelomeric regions were found. This is the first genome sequence of a model eukaryotic parasite, and it provides insight into the use of such systems in the modelling of Plasmodium biology and disease.

Analysis of the Plasmodium falciparum proteome by high-accuracy mass spectrometry

The annotated genomes of organisms define a 'blueprint' of their possible gene products. Post-genome analyses attempt to confirm and modify the annotation and impose a sense of the spatial, temporal and developmental usage of genetic information by the organism. Here we describe a large-scale, high-accuracy (average deviation less than 0.02 Da at 1,000 Da) mass spectrometric proteome analysis of selected stages of the human malaria parasite Plasmodium falciparum. The analysis revealed 1,289 proteins of which 714 proteins were identified in asexual blood stages, 931 in gametocytes and 645 in gametes. The last two groups provide insights into the biology of the sexual stages of the parasite, and include conserved, stage-specific, secreted and membrane-associated proteins. A subset of these proteins contain domains that indicate a role in cell-cell interactions, and therefore can be evaluated as potential components of a malaria vaccine formulation. We also report a set of peptides with significant matches in the parasite genome but not in the protein set predicted by computational methods.

Topology and replication of a nuclear episomal plasmid in the rodent malaria Plasmodium berghei

The rodent malaria Plasmodium berghei is one of a small number of species of Plasmodium that can currently be genetically transformed through experimentally controlled uptake of exogenous DNA by bloodstage parasites. Circular DNA containing a selectable marker replicates and is maintained under selection pressure in a randomly segregating episomal form during the first weeks after transformation. In this study, using pulsed field gel electrophoresis and ionising radiation, we show that in dividing asexual blood stage parasites the episomes are completely converted, within 2 weeks post-infection, into non-rearranged circular concatamers ranging in size between about 9 and 15 copies of the monomer. These occur as slow-moving aggregates held together by radiation-sensitive linkers consisting partly of single-stranded DNA. The process generating these complexes is not clear but 2D gel analysis showed that Cairns-type replication origins were absent and it seems most likely that the initial concatamerisation takes place using a rolling circle mechanism followed by circularisation through internal recombination. We propose a model in which continued rolling circle replication of the large circular concatamers and the recombinational activity of the tails of the rolling circles could lead to the formation of the large aggregates.