A rhoptry-protein-associated mechanism of clonal phenotypic variation in rodent malaria

Plasmodium rhoptries: how things went pear-shaped

Plasmodium parasites have three sets of specialised secretory organelles at the apical end of their invasive forms--rhoptries, micronemes and dense granules. The contents of these organelles are responsible for or contribute to host cell invasion and modification, and at least four apical proteins are leading vaccine candidates. Given the unusual nature of Plasmodium invasion, it is not surprising that unique proteins are involved in this process. Nowhere is this more evident than in rhoptries. We have collated data from several recent studies to compile a rhoptry proteome. Discussion is focussed here on rhoptry content and function.

Variant proteins of Plasmodium vivax are not clonally expressed in natural infections

Plasmodium vivax is the most widely distributed human malaria parasite and responsible for 70-80 million clinical cases each year and a large socio-economical burden. The sequence of a chromosome end from P. vivax revealed the existence of a multigene superfamily, termed vir (P. vivax variant antigens), that can be subdivided into different subfamilies based on sequence similarity analysis and which represents close to 10-20% of the coding sequences of the parasite. Here we show that there is a vast repertoire of vir genes abundantly expressed in isolates obtained from human patients, that different vir gene subfamilies are transcribed in mature asexual blood stages by individual parasites, that VIR proteins are not clonally expressed and that there is no significant difference in the recognition of VIR-tags by immune sera of first-infected patients compared with sera of multiple-infected patients. These data provide to our knowledge the first comprehensive study of vir genes and their encoding variant proteins in natural infections and thus constitute a baseline for future studies of this multigene superfamily. Moreover, whereas our data are consistent with a major role of vir genes in natural infections, they are inconsistent with a predominant role in the strict sense of antigenic variation.

Invasion by P. falciparum merozoites suggests a hierarchy of molecular interactions

Central to the pathology of malaria disease are the repeated cycles of parasite invasion and destruction of human erythrocytes. In Plasmodium falciparum, the most virulent species causing malaria, erythrocyte invasion involves several specific receptor-ligand interactions that direct the pathway used to invade the host cell, with parasites varying in their dependency on these different pathways. Gene disruption of a key invasion ligand in the 3D7 parasite strain, the P. falciparum reticulocyte binding-like homolog 2b (PfRh2b), resulted in the parasite invading via a novel pathway. Here, we show results that suggest the molecular basis for this novel pathway is not due to a molecular switch but is instead mediated by the redeployment of machinery already present in the parent parasite but masked by the dominant role of PfRh2b. This would suggest that interactions directing invasion are organized hierarchically, where silencing of dominant invasion ligands reveal underlying alternative pathways. This provides wild parasites with the ability to adapt to immune-mediated selection or polymorphism in erythrocyte receptors and has implications for the use of invasion-related molecules in candidate vaccines.

A mathematical model for a new mechanism of phenotypic variation in malaria

The Py235 merozoite rhoptry protein of the rodent malaria agent Plasmodium (yoelii) yoeli is encoded by the Py235 multigene family whose members are transcribed during the parasite's asexual life-cycle in a fashion where single schizonts subsequently give rise to sets of merozoites containing distinct Py235 transcripts. Homologues of Py235 are found in other malaria species, and antibodies to both Py235 and P. falciparum homologues inhibit merozoite invasion, suggesting a unique survival strategy involving immune evasion and host adaptation. Using a mathematical approach to model this free-living stage of Plasmodium in interaction with specific antibodies and a heterogeneous red blood cell population, we investigate if, and under what conditions, this mechanism of clonal phenotypic variation can play a role in immune evasion and adaptation to a dynamic erythropoietic environment.

Apical expression of three RhopH1/Clag proteins as components of the Plasmodium falciparum RhopH complex

The Plasmodium falciparum high molecular mass rhoptry protein ('PfRhopH') complex is important for parasite growth and comprises three distinct gene products: RhopH1, RhopH2 and RhopH3. We have previously shown that P. falciparum RhopH1 is encoded by either PFC0110w (clag3.2) or PFC0120w (clag3.1), members of the previously-named clag (cytoadherence-linked asexual gene) multigene family. In this report, we have further characterized rhoph1/clag members in terms of gene structure, transcription and protein expression. The cDNA sequences for all five rhoph1/clag members were determined, confirming previous in silico predictions of intron-exon boundaries. All member genes were transcribed in HB3 and 3D7 parasite lines, but clag3.2 was not transcribed in Dd2 parasites. The peak abundance of transcripts for all genes was observed during the late schizont stage. Antisera specific to Clag2 and Clag3.1 localized these proteins to the apical end of merozoites in segmented schizonts, and both proteins are found to be components of the PfRhopH complex. PfRhopH complex that was immunoprecipitated with anti-Clag9 antibody contained neither Clag2 nor Clag3.1, thereby suggesting that PfRhopH complexes contain only individual rhoph1/clag gene products. Since the PfRhopH complex binds the erythrocyte surface, and RhopH2 and RhopH3 are encoded by single copy genes, the RhopH1/Clag proteins may serve to confer some degree of specificity to the roles of the individual complexes.

Molecular mechanism for switching of P. falciparum invasion pathways into human erythrocytes

The malaria parasite, Plasmodium falciparum, exploits multiple ligand-receptor interactions, called invasion pathways, to invade the host erythrocyte. Strains of P. falciparum vary in their dependency on sialated red cell receptors for invasion. We show that switching from sialic acid-dependent to -independent invasion is reversible and depends on parasite ligand use. Expression of P. falciparum reticulocyte-binding like homolog 4 (PfRh4) correlates with sialic acid-independent invasion, and PfRh4 is essential for switching invasion pathways. Differential activation of PfRh4 represents a previously unknown mechanism to switch invasion pathways and provides P. falciparum with exquisite adaptability in the face of erythrocyte receptor polymorphisms and host immune responses.

Relapsing fever spirochetes contain chromosomal genes with unique direct tandemly repeated sequences

Genome sequencing of the relapsing fever spirochetes Borrelia hermsii and Borrelia turicatae identified three open reading frames (ORFs) on the chromosomes that contained internal, tandemly repeated amino acid sequences that were absent in the Lyme disease spirochete Borrelia burgdorferi. The predicted amino acid sequences of these genes (BH0209, BH0512, and BH0553) have hydrophobic N termini, indicating that these proteins may be secreted. B. hermsii transcribed the three ORFs in vitro, and the BH0512- and BH0553-encoded proteins (PBH-512 and PBH-553) were produced in vitro and in experimentally infected mice. PBH-512 and PBH-553 were on the spirochete's outer surface, and antiserum to these proteins reduced the adherence of B. hermsii to red blood cells. PCR analyses of 28 isolates of B. hermsii and 8 isolates of B. turicatae demonstrated polymorphism in each gene correlated with the number of repeats. Serum samples from relapsing fever patients reacted with recombinant PBH-512 and PBH-553, suggesting that these proteins are produced during human infection. These polymorphic proteins may be involved in the pathogenicity of these relapsing fever spirochetes and provide a mechanism for antigenic heterogeneity within their populations.

Genes coding for tryptophan-rich proteins are transcribed throughout the asexual cycle of Plasmodium falciparum

Multigene families are a common feature in Plasmodia spp. and constitute a substantial content of the parasite genome. Here, we analyse the structural organisation and sequence diversity of two further members of the Trp-rich multigene family of P. falciparum. The complete DNA sequence of both genes was determined from a series of laboratory adapted and field isolates. Based on the amino acid sequences, we have termed them tryptophan-rich antigen-3 (TrpA-3) and lysine-tryptophan-rich antigen (LysTrpA). Analysis of the genes using reverse transcriptase-polymerase chain reaction (RT-PCR), showed that both genes are transcribed and that introns are spliced out at predicted positions. Gene expression profiles obtained from microarray analysis indicate that both genes are expressed in the mid-stages of the asexual cycle. In-frame stop codons were detected which interrupted the reading frame of LysTrpA. Whereas the number of the Trp-rich proteins is rather low in P. falciparum, P. chabaudi, P. berghei and P. yoelii, this family seems to have 15 or more members in P. knowlesi and P. vivax.

Using expression information to discover new drug and vaccine targets in the malaria parasitePlasmodium falciparum

The recent completion of the malaria parasite Plasmodium falciparum genome has opened the door for applying a variety of genomic-based systems biology approaches that complement existing gene-by-gene methods of investigation. Transcriptomic analyses of P. falciparum using DNA microarrays has allowed for the rapid elucidation of gene function, parasite drug response, and in vivo expression profiles, as well as general mechanisms guiding the parasite life cycle that are vital to disease pathogenesis. The results of these studies have identified promising novel gene targets for the development of new drug and vaccine therapies.

Conservation and developmental control of alternative splicing in maebl among malaria parasites

Genes of malaria parasites and other unicellular organisms have larger exons with fewer and smaller introns than metaozoans. Such differences in gene structure are perceived to extend to simpler mechanisms for transcriptional control and mRNA processing. Instead, we discovered a surprisingly complex level of post-transcriptional mRNA processing in analysis of maebl transcripts in several Plasmodium species. Mechanisms for internal alternative cis-splicing and exon skipping were active in multiple life cycle stages to change exon structure in the deduced coding sequence (CDS). The major alternatively spliced transcript utilized a less favorable acceptor splice site, which shifted codon triplet usage to a different CDS with a hydrophilic C terminus, changing the canonical type I membrane MAEBL product to a predicted soluble isoform. We found that developmental control of the alternative splicing pattern was distinct from the canonical splicing pattern. Western blot analysis indicated that MAEBL expression was better correlated with the appearance of the canonical ORF1 transcript. Together these data reveal that RNA metabolism in unicellular eukaryotes like Plasmodium is more sophisticated than believed and may have a significant role regulating gene expression in Plasmodium.

Variant surface antigens, virulence genes and the pathogenesis of malaria

The first Molecular Approaches to Malaria meeting was held 2-5 February 2000 in Lorne, Australia. Following the meeting, Brian Cooke, Mats Wahlgren and Ross Coppel predicted that research into the molecular details of the mechanisms behind sequestration of parasitized erythrocytes would "become increasingly more complicated, with further interactions, receptors, ligands and functional domains". Furthermore, they cautioned that "the challenge will be not to lose ourselves in the molecular detail, but remain focused on the role of [the var genes and other multigene families] in pathogenesis of malaria". We contemplate on these statements, following the recent second Molecular Approaches to Malaria meeting, which was held at the same venue on 2-5 February 2004.

Parasite ligand–host receptor interactions during invasion of erythrocytes by Plasmodium merozoites

Malaria parasites must recognise and invade different cells during their life cycle. The efficiency with which Plasmodium falciparum invades erythrocytes of all ages is an important virulence factor, since the ability of the parasite to reach high levels of parasitemia is often associated with severe pathology and morbidity. The merozoite invasion of erythrocytes is a highly complex, multi-step process that is dependent on a cascade of specific molecular interactions. Although many proteins are known to play an important role in invasion, their functional characteristics remain unclear. Therefore, a complete understanding of the molecular interactions that are the basis of the invasion process is absolutely crucial, not only in improving our knowledge about the basic biology of the malarial parasite, but also for the development of intervention strategies to counter the disease. Here we review the current state of knowledge about the receptor-ligand interactions that mediate merozoite invasion of erythrocytes.

Virulence in malaria: an evolutionary viewpoint

Malaria parasites cause much morbidity and mortality to their human hosts. From our evolutionary perspective, this is because virulence is positively associated with parasite transmission rate. Natural selection therefore drives virulence upwards, but only to the point where the cost to transmission caused by host death begins to outweigh the transmission benefits. In this review, we summarize data from the laboratory rodent malaria model, Plasmodium chabaudi, and field data on the human malaria parasite, P. falciparum, in relation to this virulence trade-off hypothesis. The data from both species show strong positive correlations between asexual multiplication, transmission rate, infection length, morbidity and mortality, and therefore support the underlying assumptions of the hypothesis. Moreover, the P. falciparum data show that expected total lifetime transmission of the parasite is maximized in young children in whom the fitness cost of host mortality balances the fitness benefits of higher transmission rates and slower clearance rates, thus exhibiting the hypothesized virulence trade-off. This evolutionary explanation of virulence appears to accord well with the clinical and molecular explanations of pathogenesis that involve cytoadherence, red cell invasion and immune evasion, although direct evidence of the fitness advantages of these mechanisms is scarce. One implication of this evolutionary view of virulence is that parasite populations are expected to evolve new levels of virulence in response to medical interventions such as vaccines and drugs.

Molecular aspects of malaria pathogenesis

Plasmodium falciparum being the most lethal plasmodiae is still a major cause of the disease burden and mortality in malaria endemic areas. Due to the wide spread drug resistance in combination with poor socio-economic situation in the vast majority of the endemic countries, malaria is today a great global challenge. The scientific community is, however, progressing. The 23 Mb genome of P. falciparum has been decoded and publicly available. Data of transcriptional profiling at certain developmental stages have already been generated. More than 50% of P. falciparum genes are transcribed constitutively in all the developmental stages of parasite life cycle. Functional disruption of these genes might have implications for parasite growth and development. Available microarray data indicate that P. falciparum preferentially expresses rif and stevor gene families at gametocyte and sporozoite stages while var genes are predominantly expressed at the erythrocytic stage. Gene regulation mechanisms of the variant gene families in P. falciparum are still not understood though some regulatory elements have been proposed. The occurrence of severe malaria is determined by both parasite and human host factors. Sequestration and antigenic variation are two of the evasion mechanisms utilized by P. falciparum in order to escape the human host defences. Understanding the molecular mechanisms underlying these phenomena is of a major importance and interest in malaria research. Here, we summarize and highlight the recent progress in molecular aspects of severe malaria.

Rapid evolution of an erythrocyte invasion gene family: the Plasmodium reichenowi Reticulocyte Binding Like (RBL) genes

Malarial merozoites use an array of ligands, including members of the Reticulocyte Binding Like (RBL) super-family of invasion proteins, to identify and invade erythrocytes. RBL family members are large Type I membrane anchored proteins expressed at the invasive end of merozoites that share homology with the Reticulocyte Binding Proteins 1 and 2 (PvRBP1 and 2) of Plasmodium vivax. Plasmodium species vary widely both in the number and sequence of their RBL genes, with the recently completed Plasmodium falciparum genome containing five RBL genes. Of these, three encode proteins shown to be involved in erythrocyte invasion, a fourth is a pseudogene, and the role of the fifth is as yet unclear. In order to identify sequence similarities and differences that may have functional implications for erythrocyte invasion as well as to gain insights into the recent evolutionary history of the P. falciparum RBL genes, we have sequenced all five corresponding RBL genes from the chimpanzee parasite Plasmodium reichenowi, which is the closest phylogenetic relative of P. falciparum, yet is unable to invade human erythrocytes. Two of the five P. falciparum RBL genes have highly conserved complete open reading frames in both species, while the other three genes show evidence of gene conversion and rapid evolution. The RBL super-family, therefore, appears to be surprisingly dynamic and divergent, implying that it is involved in species-specific aspects of erythrocyte recognition and invasion.

Adhesion of normal and Plasmodium falciparum ring-infected erythrocytes to endothelial cells and the placenta involves the rhoptry-derived ring surface protein-2

Recent findings have challenged the current view of Plasmodium falciparum (P falciparum) blood-stage biology by demonstrating the cytoadhesion of early ring-stage-infected erythrocytes (rIEs) to host endothelial cells and placental syncytiotrophoblasts. The adhesion of rIEs was observed only in parasites that bind to the placenta via chondroitin sulfate A (CSA). In this work, a panel of mouse monoclonal antibodies (mAbs) that specifically inhibit cytoadhesion of rIEs but not of mature IEs was generated The previously described ring surface protein 2 (RSP-2), a 42-kDa protein, was identified as the target of the ring-stage-specific mAbs. Time course surface fluorescence experiments revealed a short overlap (approximately 4 hours) of expression between RSP-2 and P falciparum erythrocyte membrane protein 1 (PfEMP1). Their consecutive expression enables IEs to adhere to endothelial cells during the entire blood-stage cycle. During this study, a new phenotype was detected in parasite cultures, the adhesion of normal erythrocytes (nEs) to endothelial cells. All adherent nEs were coated with RSP-2. Immunolocalization studies show that RSP-2 is a rhoptry-derived protein that is discharged onto the erythrocyte membrane during contact with merozoites. Our results identify RSP-2 as a key molecule in sequestration of young blood-stage forms and nEs to endothelial cells.

Measuring immune selection

Immune responses that kill pathogens or reduce their reproductive rate are generally important in protecting hosts from infection and disease. Pathogens that escape the full impact of such responses will survive, and any heritable genetic basis of this evasion will be selected. Due to the memory component of vertebrate immune responses, pathogens with rare alleles of a target antigen can have an advantage over those with common alleles, leading to the maintenance of a polymorphism. At the genetic level, there ought to be detectable signatures of balancing selection in the genes encoding these antigens. Here, methods for identifying these selective signatures are reviewed. Their practical utility for identifying which antigens are targets of protective immune responses is discussed.

Organization, transcription, and expression of rhoptry associated protein genes in the Babesia bigemina rap-1 locus

The Babesia bigemina rap-1 gene locus contains five tandemly arranged copies of rap-1a genes. However, the size of the locus, as defined by conserved, unrelated orfs at the 5' and 3' ends, suggests that additional genes may be present. In this study, we identified all additional genes in the locus and characterized their pattern of expression in merozoites. The rap-1a genes are separated by 3.38-kbp intergenic (IG) regions, each of which contains an identical copy of a related gene designated rap-1b. One additional copy of rap-1b and one copy of another related gene designated rap-1c is present in the 3' end of the locus. Common sequence features that define the Babesia rap-1 family are present in rap-1b and rap-1c, but otherwise these genes average only 27% identity to rap-1a. Homologues of the rap-1b and rap-1c genes identified in diverse B. bigemina strains have a high degree of predicted amino acid sequence conservation (averaging >90%), with the largest number of changes in the carboxyl end of RAP-1c. We tested whether all rap-1 genes in the locus are co-transcribed in merozoites using RT-PCR, Northern blots, and quantitative real-time PCR. Rap-1a genes produce the most abundant transcripts of the family, while rap-1b transcripts are the least abundant despite the large number of gene copies. Similar patterns of transcription were observed whether merozoites were obtained from in vitro cultures or in vivo infection. Immunoblot analysis of merozoites revealed the expected RAP-1a expression but failed to detect expressed RAP-1b and RAP-1c, indicating that expression of the rap-1 genes is regulated both at the transcriptional and translational levels.

Phenotypic variation of Plasmodium falciparum merozoite proteins directs receptor targeting for invasion of human erythrocytes

The members of the phylum Apicomplexa parasitize a wide range of eukaryotic host cells. Plasmodium falciparum, responsible for the most virulent form of malaria, invades human erythrocytes using several specific and high affinity ligand-receptor interactions that define invasion pathways. We find that members of the P. falciparum reticulocyte-binding homolog protein family, PfRh2a and PfRh2b, are expressed variantly in different lines. Targeted gene disruption shows that PfRh2b mediates a novel invasion pathway and that it functions independently of other related proteins. Phenotypic variation of the PfRh protein family allows P. falciparum to exploit different patterns of receptors on the erythrocyte surface and thereby respond to polymorphisms in erythrocyte receptors and to evade the host immune system.

Small variant STEVOR antigen is uniquely located within Maurer's clefts in Plasmodium falciparum-infected red blood cells

Malaria parasite antigens encoded by multigene families are important factors in virulence and in disease pathology. In Plasmodium falciparum, the virulence factor PfEMP-1 is encoded by the var multigene family and is exposed at the infected erythrocyte surface. PfEMP-1 is clonally variant, allowing the parasite to evade host immunity. The recently identified P. falciparum stevor multigene family and its products also have the potential to be involved in similar important aspects of host-parasite interactions. Here, we show tightly regulated stage-specific transcription of stevor occurring over just a few hours of the asexual parasite life cycle. Only a subset of stevor genes are transcribed in parasite populations maintained in cultures and in single micromanipulated parasites. Antibodies against STEVOR recognize proteins of the expected size (approximately 37 kDa) and localize STEVOR in Maurer's clefts, unique membranous structures located in the cytoplasm of infected erythrocytes. The fact that the timing of stevor expression and the location of STEVOR are clearly distinct from those of other parasite variant antigens suggests that this gene family may have a novel role in P. falciparum biology.

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.

Three multigene families in Plasmodium parasites: facts and questions

Multigene families optimise fitness by providing a set of related genes with possibly different temporal and/or topological expression patterns. We analyse here the structural organisation and sequence diversity of the rDNA, sera and var C Plasmodium falciparum families, and discuss their consequences for parasite biology. The low rDNA copy number, which reduces reshuffling, is probably the corollary of the need for functionally distinct rRNAs in the insect and in the vertebrate host. The unusual intra-genome and population rDNA sequence diversity results in cells equipped with mosaic ribosome sets. The functional constraints are such that ribosome compatibility could influence parasite fitness and contribute to population structuring. Unlike the dispersed rDNA units, the sera family is arranged as a tandem gene cluster, with seven contiguous similar genes, and one more distantly related paralog. We address the question of the inclusion criteria in family definition. We discuss the results concerning the SERA proteins expression and function in the context of the long overlooked multigene family. The var C module is shared by var genes, 'orphan' var C and var C pseudogenes. Analysis of 125 var C deduced protein sequences highlights a well-conserved framework, including putative phosphorylation sites, consistent with the proposed function of mediating interaction with cytoskeletal proteins. The 5' and 3' flanking sequences of the var C pseudogenes are heterogeneous. In contrast, the flanking sequences of the uninterrupted var C modules show remarkable conservation. This is interesting in view of the silencing activity of the var intronic sequence on var expression. The 5' flanking sequence dichotomy reported for internal and sub-telomeric var genes extends to the 3' flanking sequences. This has profound implications for transcription regulation and generation of diversity. The var C family suggests a role for pseudogenes as a diversity reservoir and in genome dynamics by promoting ectopic recombination.

Cross-species regulation of malaria parasitaemia in the human host

Longitudinal genetic analysis of the composition of malaria parasites infecting humans has demonstrated that individuals living in endemic areas are chronically infected with multiple genotypes and species of Plasmodium. The accumulation of infections is a consequence of superinfection from the bites of many infected anopheline mosquitoes. The clinical outcome of infection is determined by the host's ability to regulate the density of malaria parasites in the blood. Interestingly, most infections do not cause symptoms of malarial disease after a degree of immunity is acquired. Here, we review data from the first genetic study of the longitudinal dynamics of multiple Plasmodium species and genotypes in humans. The data show that the total parasite density of Plasmodium species oscillates around a threshold and that peaks of infection with each species do not coincide. We propose that malaria parasitaemia is controlled in a density-dependent manner in these semi-immune children. This implies that a cross-species mechanism of parasite regulation exists. A model of how multiple immune responses could act in concert to explain these within host dynamics is discussed in relation to known regulatory mechanisms.

Evolutionary relationships of conserved cysteine-rich motifs in adhesive molecules of malaria parasites

Malaria parasites invade erythrocytes in a process mediated by a series of molecular interactions. Invasion of human erythrocytes by Plasmodium vivax is dependent upon the presence of a single receptor, but P. falciparum, as well as some other species, exhibits the ability to utilize multiple alternative invasion pathways. Conserved cysteine-rich domains play important roles at critical times during this invasion process and at other stages in the life cycle of malaria parasites. Duffy-binding-like (DBL) domains, expressed as a part of the erythrocyte-binding proteins (DBL-EBP), are such essential cysteine-rich ligands that recognize specific host cell surface receptors. DBL-EBP, which are products of the erythrocyte-binding-like (ebl) gene family, act as critical determinants of erythrocyte specificity and are the best-defined ligands from invasive stages of malaria parasites. The ebl genes include the P. falciparum erythrocyte-binding antigen-175 (EBA-175) and P. vivax Duffy-binding protein. DBL domains also mediate cytoadherence as a part of the variant erythrocytic membrane protein-1 (PfEMP-1) antigens expressed from var genes on the surface of P. falciparum-infected erythrocytes. A paralogue of the ebl family is the malarial ligand MAEBL, which has a chimeric structure where the DBL domain is functionally replaced with a distinct cysteine-rich erythrocyte-binding domain with similarity to the apical membrane antigen-1 (AMA-1) ligand domain. The Plasmodium AMA-1 ligand domain, which encompasses the extracellular cysteine domains 1 and 2 and is well conserved in a Toxoplasma gondii AMA-1, has erythrocyte-binding activity distinct from that of MAEBL. These important families of Plasmodium molecules (DBL-EBP, PfEMP-1, MAEBL, AMA-1) are interrelated through the MAEBL. Because MAEBL and the other ebl products have the characteristics expected of homologous ligands involved in equivalent alternative invasion pathways to each other, we sought to better understand their roles during invasion by determining their relative origins in the Plasmodium genome. An analysis of their multiple cysteine-rich domains permitted a unique insight into the evolutionary development of PLASMODIUM: Our data indicate that maebl, ama-1, and ebl genes have ancient origins which predate Plasmodium speciation. The maebl evolved as a single locus, including its unique chimeric structure, in each Plasmodium species, in parallel with the ama-1 and the ebl genes families. The ancient character of maebl, along with its different expression characteristics suggests that MAEBL is unique and does not play an alternative role in invasion to ebl products such as EBA-175. The multiple P. falciparum ebl paralogues that express DBL domains, which have occurred by duplication and diversification, potentially do provide multiple functionally equivalent ligands to EBA-175 for alternative invasion pathways.

Plasmodium chabaudi: rosetting in a rodent malaria model

Rosetting is a property of many malaria parasite species that has been linked to virulence in the major species infecting humans, Plasmodium falciparum. Here, the basic properties of rosettes in the rodent malaria laboratory model, P. chabaudi, were studied with a view to future studies on the role of rosetting in malaria parasite virulence and transmission. Rosetting occurred in 14 out of the 15 P. chabaudi clones studied, varied consistently between clones, and ranged between 9 and 37% at full parasite maturity. Rosetting frequency markedly declined after the mouse reached peak parasitemia, possibly due to host immunity. Consistent with P. falciparum and P. vivax, rosettes in P. chabaudi were disrupted by treatment with trypsin and EDTA. However, P. chabaudi rosettes were insensitive to sulfated glycoconjugates (heparin, heparan sulfate and fucoidan). The molecular basis of rosetting in P. chabaudi is unknown at present, but the results suggest that the molecules involved may differ from those in human-infecting species.

Transcripts of developmentally regulated Plasmodium falciparum genes quantified by real-time RT-PCR

Plasmodium falciparum intraerythrocytic development is a complex process. Development proceeds rapidly from the trophozoite phase of nutrient acquisition and growth through to the synthetic and reproductive schizont phase, which ends with production of new invasive merozoites. During this process, the malaria parasite must express a series of different gene products, depending on its metabolic and synthetic needs. We are particularly interested in the development of the merozoite's organelles in the apical complex, which form during the later schizont stages. We have used quantitative real-time RT-PCR fluorogenic 5' nuclease assays (TaqMan) for the first time on malaria parasites for analysis of erythrocytic stage-specific gene expression. We analyzed transcripts of the P.falciparum eba-175 and other erythrocyte binding-like (ebl) family genes in temperature-synchronized parasites and found ebl genes have tightly controlled, stage-specific transcription. As expected, eba-175 transcripts were abundant only at the end of schizont development in a pattern most common among ebl, including baebl, pebl and jesebl. The maebl transcript pattern was unique, peaking at mid-late trophozoite stage, but absent in late-stage schizonts. ebl-1 demonstrated another pattern of expression, which peaked during mid-schizont stage and then significantly diminished in late-stage schizonts. Our analysis demonstrates that using real-time RT-PCR fluorogenic 5' nuclease assays is a sensitive, quantitative method for analysis of Plasmodium transcripts.

Malaria parasites giving rise to recrudescence in vitro

Recrudescences were simulated in vitro with drug treatment to examine how drug-sensitive parasites survive the treatment. Various numbers of cultured parasites were treated with lethal doses of pyrimethamine or mefloquine for various lengths of time. Recrudescences were observed in parasite populations with larger initial numbers of parasites when the treatment duration was prolonged. Equal numbers of parasitized erythrocytes were treated with various concentrations of pyrimethamine or mefloquine. There was no clear linear relationship between the incidence of recrudescence and the drug concentration. Parasites that had recrudesced were continuously allowed to recrudesce in the succeeding recrudescence experiments. Both the duration from the cessation of treatment to the time at which the recrudescent parasitemia level reached 1% and the growth rate of recrudescent parasites were equal among these recrudescences. The recrudescent parasites in these experiments were as sensitive to the drugs as the parasites tested before treatment were. These results suggest that a parasite culture may contain parasites in some phases that are not killed by drug for up to 10 days, which explains the recrudescences that occur even after treatment.

The pathogenic basis of malaria

Malaria is today a disease of poverty and underdeveloped countries. In Africa, mortality remains high because there is limited access to treatment in the villages. We should follow in Pasteur's footsteps by using basic research to develop better tools for the control and cure of malaria. Insight into the complexity of malaria pathogenesis is vital for understanding the disease and will provide a major step towards controlling it. Those of us who work on pathogenesis must widen our approach and think in terms of new tools such as vaccines to reduce disease. The inability of many countries to fund expensive campaigns and antimalarial treatment requires these tools to be highly effective and affordable.

Plasmodium, human and Anopheles genomics and malaria

The Plasmodium spp. parasites that cause malaria are transmitted to humans by Anopheles spp. mosquitoes. Scientists have now amassed a great body of knowledge about the parasite, its mosquito vector and human host. Yet this year there will be 300-500 million new malaria infections and 1-3 million deaths caused by the disease. We believe that integrated analyses of genome sequence, DNA polymorphisms, and messenger RNA and protein expression profiles will lead to greater understanding of the molecular basis of vector-human and host-parasite interactions and provide strategies to build upon these insights to develop interventions to mitigate human morbidity and mortality from malaria.

Orthology between the genomes of Plasmodium falciparum and rodent malaria parasites: possible practical applications

The work of the consortium that has been formed to complete the entire sequence of the genome of a selected clone of the human malaria parasite, Plasmodium falciparum, is almost finished. Already huge tracts of the genome are available as fully assembled chromosomes or large contigs and the work of initial annotation is in an advanced state. Post-genomic research is in one sense the process of furthering the process of annotation, creating biological atlases and preliminary attempts to make global descriptions of gene transcription and proteome analysis are underway. Comparison between significant amounts of genome data from both closely, and more distantly related organisms, can facilitate the identification of genes themselves, coordinately regulated gene expression groups, gene function and genome organization. Models of malaria can fulfil these functions and in addition provide an experimental system wherein predictions can be tested and basic experimental investigations performed within numerous aspects of disease, pathology, parasite-host and parasite-vector interactions. Comparative genomics in Plasmodium has already been shown to have informative roles in the completion of annotation and the elucidation of gene function. These roles will be illustrated by example and used as the basis for a discussion of the utility of genome information and malaria models in realizing the desired product of Plasmodium genomics, the development of malaria therapies.

Functional analysis of Plasmodium falciparum merozoite antigens: implications for erythrocyte invasion and vaccine development

Malaria is a major human health problem and is responsible for over 2 million deaths per year. It is caused by a number of species of the genus Plasmodium, and Plasmodium falciparum is the causative agent of the most lethal form. Consequently, the development of a vaccine against this parasite is a priority. There are a number of stages of the parasite life cycle that are being targeted for the development of vaccines. Important candidate antigens include proteins on the surface of the asexual merozoite stage, the form that invades the host erythrocyte. The development of methods to manipulate the genome of Plasmodium species has enabled the construction of gain-of-function and loss-of-function mutants and provided new strategies to analyse the role of parasite proteins. This has provided new information on the role of merozoite antigens in erythrocyte invasion and also allows new approaches to address their potential as vaccine candidates.

Stage-specific transcription of distinct repertoires of a multigene family during Plasmodium life cycle

Members of a multigene family in the rodent malaria parasite Plasmodium yoelii yoelii code for 235-kilodalton proteins (Py235) that are located in the merozoite apical complex, are implicated in virulence, and may determine red blood cell specificity. We show that distinct subsets of py235 genes are expressed in sporozoites and hepatic and erythrocytic stages. Antibodies to Py235 inhibited sporozoite invasion of hepatocytes. The switch in expression profile occurred immediately after transition from one stage to another. The results suggest that this differential expression is driven by strong biological requirements and provide evidence that hepatic and erythrocytic merozoites differ.

Malaria and the red cell

Because of the breakdown of malaria control programs, the constant emergence of drug resistant parasites, and, possibly, climatic changes malaria poses a major problem for the developing countries. In addition, because of the speed of international travel it is being seen with increasing frequency as an imported disease in non-tropical countries. This update explores recent information about the pathophysiology of the disease, its protean hematological manifestations, and how carrier frequencies for the common hemoglobin disorders have been maintained by relative resistance to the malarial parasite. In Section I, Dr. Louis Miller and colleagues consider recent information about the pathophysiology of malarial infection, including new information about interactions between the malarial parasite and vascular endothelium. In Section II, Dr. David Roberts discusses what is known about the complex interactions between red cell production and destruction that characterize the anemia of malaria, one of the commonest causes of anemia in tropical countries. In Section III, Dr. David Weatherall reviews recent studies on how the high gene frequencies of the thalassemias and hemoglobin variants have been maintained by heterozygote advantage against malaria and how malaria has shaped the genetic structure of human populations.

A Plasmodium falciparum homologue of Plasmodium vivax reticulocyte binding protein (PvRBP1) defines a trypsin-resistant erythrocyte invasion pathway

Invasion of erythrocytes by Plasmodium merozoites is an intricate process involving multiple receptor-ligand interactions. The glycophorins and an unknown trypsin sensitive factor are all erythrocyte receptors used during invasion by the major human pathogen Plasmodium falciparum. However, only one erythrocyte receptor, Glycophorin A, has a well-established cognate parasite ligand, the merozoite protein erythrocyte binding antigen-175 (EBA-175). The involvement of several other parasite proteins during invasion have been proposed, but no direct evidence links them with a specific invasion pathway. Here we report the identification and characterization of P. falciparum normocyte binding protein 1 (PfNBP1), an ortholog of Plasmodium vivax reticulocyte binding protein-1. PfNBP1 binds to a sialic acid dependent trypsin-resistant receptor on the erythrocyte surface that appears to be distinct from known invasion receptors. Antibodies against PfNBP1 can inhibit invasion of trypsinized erythrocytes and two P. falciparum strains that express truncated PfNBP1 are unable to invade trypsinized erythrocytes. One of these strain, 7G8, also does not invade Glycophorin B-negative erythrocytes. PfNBP1 therefore defines a novel trypsin-resistant invasion pathway and adds a level of complexity to current models for P. falciparum erythrocyte invasion.

Clonal variation of gene expression as a source of phenotypic diversity in parasitic protozoa

Within a cellular clone, individual cells can express different members of a gene family. If the difference in expression is transmitted to daughter cells, 'phenotypic clones' are formed. Such clonal phenotypic variation has evolved independently in phylogenetically distant parasitic protozoa under similar selective pressure: the need for phenotypic diversity at several steps of their life cycle. Here, I review clonal phenotypic variation processes, outline their role in parasite biology and argue that clonal phenotypic variation is complementary to sexual reproduction as a source of phenotypic diversity.

The 235 kDa rhoptry protein of Plasmodium (yoelii) yoelii: function at the junction

All malaria parasites are obligate intracellular organisms that must clearly recognise and discriminate between different cells during their life cycle. Invasion into a cell is a multi-step event that is marked by initial attachment proceeding to irreversible junction formation and penetration. A 235 kDa rhoptry protein (Py235) in the rodent malaria, Plasmodium yoelii yoelii has been shown to be involved in red blood cell (rbc) binding and is involved in a new mechanism of clonal phenotypic variation that may be important in adaptation and immune evasion. Immunisation studies using Py235 have also revealed a role for this protein in the virulence phenotype seen with P. y. yoelii in laboratory mice. Interestingly, the genes that encode this protein are present as a multi-gene family. In this paper, we examine Py235 at the level of DNA, transcription and expression, discussing the role of this protein during invasion, in virulence and in immune evasion.

Plasmodium falciparum homologue of the genes for Plasmodium vivax and Plasmodium yoelii adhesive proteins, which is transcribed but not translated

The 235-kDa family of rhoptry proteins in Plasmodium yoelii and the two reticulocyte binding proteins of P. vivax comprise a family of proteins involved in host cell selection and erythrocyte invasion. Here we described a member of the gene family found in P. falciparum (PfRH3) that is transcribed in its entirety, under stage-specific control, with correct splicing of the intron, but appears not to be translated, probably due to two reading frameshifts at the 5' end of the gene.

Antibodies and Plasmodium falciparum merozoites

There is considerable interest in using merozoite proteins in a vaccine against falciparum malaria. Observations that antibodies to merozoite surface proteins block invasion are a basis for optimism. This article draws attention to important and varied aspects of how antibodies to Plasmodium falciparum merozoites affect red blood cell invasion.

JlpA, a novel surface-exposed lipoprotein specific to Campylobacter jejuni, mediates adherence to host epithelial cells

A 1116 bp open reading frame (ORF), designated jlpA, encoding a novel species-specific lipoprotein of Campylobacter jejuni TGH9011, was identified from recombinant plasmid pHIP-O. The jlpA gene encodes a polypeptide (JlpA) of 372 amino acid residues with a molecular mass of 42.3 kDa. JlpA contains a typical signal peptide and lipoprotein processing site at the N-terminus. The presence of a lipid moiety on the JlpA molecule was confirmed by the incorporation of [3H]-palmitic acid. Immunoblotting analysis of cell surface extracts prepared using glycine-acid buffer (pH 2.2) and proteinase K digestion of whole cells indicated that JlpA is a surface-exposed lipoprotein in C. jejuni. JlpA is loosely associated with the cell surface, as it is easily extracted from the C. jejuni outer membrane by detergents, such as sarcosyl and Triton X-100. JlpA is released to the culture medium, and its concentration increases in a time-dependent fashion. The adherence of both insertion and deletion mutants of jlpA to HEp-2 epithelial cells was reduced compared with that of parental C. jejuni TGH9011. Adherence of C. jejuni to HEp-2 cells was inhibited in a dose-dependent manner when the bacterium was preincubated with anti-GST-JlpA antibodies or when HEp-2 cells were preincubated with JlpA protein. A ligand-binding immunoblotting assay showed that JlpA binds to HEp-2 cells, which suggests that JlpA is C. jejuni adhesin.

Molecular insights into receptors used by malaria parasites for erythrocyte invasion

The invasion of erythrocytes by malaria parasites is a multi-step process that requires a series of highly specific molecular interactions. Here, the authors review what has been learned about receptor-ligand interactions that mediate erythrocyte invasion. Parasite proteins involved in these interactions are promising candidates for malaria vaccines. Clear understanding of these interactions is important for the rational design of vaccines that attempt to inhibit invasion and prevent malaria.

Sequence diversity and antigenic polymorphism in the Plasmodium yoelii p235 high molecular mass rhoptry proteins and their genes

A gene family in Plasmodium yoelii YM encodes p235, a group of high molecular mass erythrocyte-binding rhoptry proteins. Sequence analysis of 6 cDNA clones from the 3' end of expressed p235 genes divided them into two groups corresponding to genes on chromosomes 1, and 5 and 6, respectively. Twelve partial p235 protein sequences, derived from cDNA sequences from the region with greatest protein sequence similarity to Plasmodium vivax RBP2, fell into three groups, together with one chimeric sequence. A comparison of these cDNA sequences with genomic DNA sequences from the same region suggested that only a subset of the gene repertoire is expressed. Three genomic DNA clones, derived from the 5' end of p235 genes designated E1, E2, and E5 and located on chromosome 5/6, were also obtained and aligned with sequences from the known E8 and E3 genes. In the region of overlap there was only approximately 27% protein sequence identity, indicating that the sequences in this p235 N-terminal region are more diverse than at the C-terminal end. This sequence variation in the expressed genes did not result in antigenically different rhoptry proteins as detected with a panel of p235-specific mAbs. Only one schizont out of 500 examined with mAb 25.86 appeared to be an antigenic variant, with all of the developing merozoites in this schizont being mAb 25.86 negative. No other antigenic variants were detected with the other antibodies, and therefore it is likely that these antibodies recognise conserved epitopes.

Identification of proteins from Plasmodium falciparum that are homologous to reticulocyte binding proteins in Plasmodium vivax

Plasmodium falciparum infections can be fatal, while P. vivax infections usually are not. A possible factor involved in the greater virulence of P. falciparum is that this parasite grows in red blood cells (RBCs) of all maturities whereas P. vivax is restricted to growth in reticulocytes, which represent only approximately 1% of total RBCs in the periphery. Two proteins, expressed at the apical end of the invasive merozoite stage from P. vivax, have been implicated in the targeting of reticulocytes for invasion by this parasite. A search of the P. falciparum genome databases has identified genes that are homologous to the P. vivax rbp-1 and -2 genes. Two of these genes are virtually identical over a large region of the 5' end but are highly divergent at the 3' end. They encode high-molecular-mass proteins of >300 kDa that are expressed in late schizonts and localized to the apical end of the merozoite. To test a potential role in merozoite invasion of RBCs, we analyzed the ability of these proteins to bind to mature RBCs and reticulocytes. No binding to mature RBCs or cell preparations enriched for reticulocytes was detected. We identified a parasite clone that lacks the gene for one of these proteins, showing that the gene is not required for normal in vitro growth. Antibodies to these proteins can inhibit merozoite invasion of RBCs.

Host cell invasion by malaria parasites

The complex life cycle of the malaria parasite includes three specialized invasive stages, distinct both in terms of their cellular architecture and in their choice of target host cell. Despite the dissimilarities between these forms, there are clear parallels in the manner by which they enter their respective host cells. Advances in the area of erythrocyte invasion by the malaria merozoite, outlined here by Chetan Chitnis and Mike Blackman and discussed at the Molecular Approaches to Malaria conference, Lorne, Australia, 2-5 February 2000, will undoubtedly impact on our understanding of mechanisms of cell entry by the other invasive forms. Similarly, recent progress in dissecting the functional role of surface proteins expressed by sporozoite and ookinete stages has provided fascinating insights into general aspects of invasion by all invasive stages of apicomplexan parasites.

The apical organelles of malaria merozoites: host cell selection, invasion, host immunity and immune evasion

Malaria is caused by protozoan parasites belonging to the phylum Apicomplexa. These obligate intracellular parasites depend on the successful invasion of an appropriate host cell for their survival. This article is a broad overview of the molecular strategies employed by the merozoite, an invasive form of the malaria parasite, to successfully invade a suitable red blood cell.

Two Plasmodium falciparum genes express merozoite proteins that are related to Plasmodium vivax and Plasmodium yoelii adhesive proteins involved in host cell selection and invasion

Two related Plasmodium falciparum genes and their encoded proteins have been identified by comparative analyses with Plasmodium vivax reticulocyte binding protein 2 (PvRBP-2). The P. falciparum genes have a structure which suggests that they may be the result of an evolutionary duplication event, as they share more than 8 kb of closely related nucleotide sequence but then have quite divergent unique 3' ends. Between these shared and unique regions is a complex set of repeats, the nature and number of which differs between the two genes, as well as between different P. falciparum strains. Both genes encode large hydrophilic proteins, which are concentrated at the invasive apical end of the merozoite and are predicted to be more than 350 kDa, with an N-terminal signal sequence and a single transmembrane domain near their C termini. Importantly, they also share gene structure and amino acid homology with the Plasmodium yoelii 235-kDa rhoptry protein family, which is also related to PvRBP-2. Together these Plasmodium proteins define an extended family of proteins that appear to function in erythrocyte selection and invasion. As such, they may prove to be essential components of malaria vaccine preparations.

Molecular aspects of severe malaria

Human infections with Plasmodium falciparum may result in severe forms of malaria. The widespread and rapid development of drug resistance in P. falciparum and the resistance of the disease-transmitting mosquitoes to insecticides make it urgent to understand the molecular background of the pathogenesis of malaria to enable the development of novel approaches to combat the disease. This review focuses on the molecular mechanisms of severe malaria caused by the P. falciparum parasite. The nature of severe malaria and the deleterious effects of parasite-derived toxins and host-induced cytokines are introduced. Sequestration, brought about by cytoadherence and rosetting, is linked to severe malaria and is mediated by multiple receptors on the endothelium and red blood cells. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is the ligand responsible for a majority of binding interactions, and the multiply adhesive features of this sticky molecule are presented. Antigenic variation is also a major feature of PfEMP1 and of the surface of the P. falciparum-infected erythrocyte. Possible mechanisms of P. falciparum antigenic variation in asexual stages are further discussed. We conclude this review with a perspective and suggestions of important aspects for future investigations.

Molecular aspects of severe malaria

Human infections with Plasmodium falciparum may result in severe forms of malaria. The widespread and rapid development of drug resistance in P. falciparum and the resistance of the disease-transmitting mosquitoes to insecticides make it urgent to understand the molecular background of the pathogenesis of malaria to enable the development of novel approaches to combat the disease. This review focuses on the molecular mechanisms of severe malaria caused by the P. falciparum parasite. The nature of severe malaria and the deleterious effects of parasite-derived toxins and host-induced cytokines are introduced. Sequestration, brought about by cytoadherence and rosetting, is linked to severe malaria and is mediated by multiple receptors on the endothelium and red blood cells. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is the ligand responsible for a majority of binding interactions, and the multiply adhesive features of this sticky molecule are presented. Antigenic variation is also a major feature of PfEMP1 and of the surface of the P. falciparum-infected erythrocyte. Possible mechanisms of P. falciparum antigenic variation in asexual stages are further discussed. We conclude this review with a perspective and suggestions of important aspects for future investigations.

Targeted disruption of an erythrocyte binding antigen in Plasmodium falciparum is associated with a switch toward a sialic acid-independent pathway of invasion

Erythrocyte invasion by Plasmodium requires molecules present both on the merozoite surface and within the specialized organelles of the apical complex. The Plasmodium erythrocyte binding protein family includes the Plasmodium falciparum sialic acid-binding protein, EBA-175 (erythrocyte binding antigen-175), which binds sialic acid present on glycophorin A of human erythrocytes. We address the role of the conserved 3'-cysteine rich region, the transmembrane, and cytoplasmic domains through targeted gene disruption. Truncation of EBA-175 had no measurable effect on either the level of EBA-175 protein expression or its subcellular localization. Similarly, there appears to be no impairment in the ability of soluble EBA-175 to be released into the culture supernatant after schizont rupture. Additionally, the 3'-cys rich region, transmembrane, and cytoplasmic domains of EBA-175 are apparently non-essential for merozoite invasion. In contrast, erythrocyte invasion via the EBA-175/glycophorin A route appears to have been disrupted to such a degree that the mutant lines have undergone a stable switch in invasion phenotype. As such, EBA-175 appears to have been functionally inactivated within the truncation mutants. The sialic acid-independent invasion pathway within the mutant parasites accounts for approximately 85% of invasion into normal erythrocytes. These data demonstrate the ability of P. falciparum to utilize alternate pathways for invasion of red blood cells, a property that most likely provides a substantial survival advantage in terms of overcoming host receptor heterogeneity and/or immune pressure.