Intra-cluster recombination and var transcription switches in the antigenic variation of Plasmodium falciparum

Telomeric heterochromatin propagation and histone acetylation control mutually exclusive expression of antigenic variation genes in malaria parasites

Malaria parasites use antigenic variation to avoid immune clearance and increase the duration of infection in the human host. Variation at the surface of P. falciparum-infected erythrocytes is mediated by the differential control of a family of surface antigens encoded by var genes. Switching of var gene expression occurs in situ, mostly from telomere-associated loci, without detectable DNA alterations, suggesting that it is controlled by chromatin structure. We have identified chromatin modifications at telomeres that spread far into telomere-proximal regions, including var gene loci (>50 kb). One type of modification is mediated by a protein homologous to yeast Sir2 called PfSir2, which forms a chromosomal gradient of heterochromatin structure and histone hypoacetylation. Upon activation of a specific telomere-associated var gene, PfSir2 is removed from the promoter region and acetylation of histone occurs. Our data demonstrate that mutually exclusive transcription of var genes is linked to the dynamic remodeling of chromatin.

Antigenic variation in Plasmodium falciparum is associated with movement of var loci between subnuclear locations

Much of the success of Plasmodium falciparum in establishing persistent infections is attributed to immune evasion through antigenic variation. This process involves periodically exchanging variants of the major surface antigen PfEMP1, a protein also responsible for parasite cytoadherence. PfEMP1 is encoded by genes of the 60-member var family, located at subtelomeric and internal chromosome loci. The active or silenced state of var genes is heritable, and its control by nonsequence information remains puzzling. Using FISH analysis, we demonstrate that both internal and subtelomeric var genes are positioned at the nuclear periphery in their repressed state. Upon activation, the same var genes are still found in the periphery, indicating that this zone can be transcriptionally competent, rather than uniformly silenced. However, activation of a var gene is linked with altered positioning at the nuclear periphery, with subtelomeric var loci exiting chromosome end clusters and being relocated to distinct nuclear sites. Serial sectioning of parasite nuclei reveals areas of both condensed and noncondensed chromatin at the nuclear periphery. Our results demonstrate that regulation of antigenic variation is associated with subnuclear position effects and point to the existence of transcriptionally permissive perinuclear zones for var genes.

Characterization of kinetics of DNA strand-exchange and ATP hydrolysis activities of recombinant PfRad51, a Plasmodium falciparum recombinase

Although homologous recombination-mediated DNA rearrangements are quite widespread in Plasmodium falciparum, the molecular mechanisms involved are essentially unknown. Recent identification of PfRad51 in P. falciparum has suggested that it may play central role during homologous recombination and DNA rearrangements. Full-length recombinant PfRad51 was over expressed in Escherichia coli and purified to near homogeneity. Using optimized enzymatic activity conditions recombinant PfRad51 protein was shown to catalyze DNA strand-exchange reaction, a central step during homologous recombination. Unlike bacterial RecA protein, PfRad51 promoted strand-exchange reaction does not require ATP hydrolysis. The PfRad51 protein also catalyzed ssDNA-dependent ATP hydrolysis and the k(cat) values were similar to those reported for human Rad51. The demonstration of strand-exchange activity of PfRad51 protein, first such report in any protozoan parasite, suggests importance of similar recombination mechanism during DNA rearrangements associated with antigenic variation in P. falciparum.

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.

A SICAvar switching event in Plasmodium knowlesi is associated with the DNA rearrangement of conserved 3′ non-coding sequences

Plasmodium knowlesi variant antigens are expressed at the surface of infected erythrocytes and are encoded by the Schizont Infected Cell Agglutination variant antigen (SICAvar) multigene family. The 3' region of the SICAvar gene locus encoding the 205 kDa variant antigen expressed in the Pk1(B+)1+ parasites was found to be altered compared to the Pk1(A+) parental clone. Here we report that this alteration is the result of a DNA rearrangement and that the original and altered 205 SICAvar alleles appear to encode bona fide variant antigens. Importantly, 205A and 205B SICAvar RNA sequences are detectable in similar apparent quantities as determined by quantitative real-time reverse-transcription polymerase chain reaction (qRT-PCR) amplification experiments. However, expression of the 205 kDa SICA protein at the surface of the infected erythrocyte is not characteristic of the Pk1(A+) parasites and the 205 SICAvar transcript has not been detected in Pk1(A+) parasites by northern blot analysis. Furthermore, we report that many distinct SICAvar transcripts were detected in P. knowlesi Pk1(B+)1+ cDNA library hybridization screens. Of special interest, in light of these data, distinctive differences at the 3' end of the 205A and 205B alleles are observed, which may be of functional importance. An analysis of the 3' untranslated region (UTR) of SICAvar genes in more than 100 sequences revealed a surprising common sequence pattern characterized by blocks of imperfect, GT-rich, heptad repeated motifs (Block I), followed by A and T rich homopolymers (Block II) and in a large number of genes, GC-rich segments (Block III). We show that this region undergoes extensive recombination and that the preferential stability of the 205 SICAvar transcript in Pk1(B+)1+ parasites may be associated with the presence of its specific Block III sequences. We speculate that the conserved yet polymorphic SICAvar 3'UTR sequences, and comparable regions in P. falciparum var genes, function in the stage-specific and developmentally regulated post-transcriptional gene silencing (PTGS) of variant antigen transcripts.

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.

Transcription of a subtelomerically located var gene variant in Plasmodium falciparum appears to require the truncation of an adjacent var gene

The Plasmodium falciparum R29 clone preferentially transcribes the R29var gene variant on rosette selection, unlike other isogenic clones from the same parasite lineage. Characterisation of the R29var gene locus revealed that this gene lies internal to, and is in a tail-to-tail orientation with, a second var gene variant (A4var) at one end of chromosome 13. In the R29 clone, a spontaneous deletion event between these two var variants deletes all of the A4var gene and the subtelomeric repetitive sequence arrays. We have previously shown that a simple disruption of the A4var gene is not sufficient to preferentially activate the R29var gene in rosette-selected parasites. We therefore hypothesised that the truncation of the chromosome end may be a key factor in predisposing the R29var variant to transcription under rosette selection conditions. Here, we have generated a panel of isogenic parasite clones with both intact and truncated A4var-R29var loci, and show that R29var transcription is only detected in rosette-selected clones with a truncated locus. Furthermore, we present provisional data describing the relative frequency with which this spontaneous deletion event occurs. These data have implications in our understanding of how spontaneous deletion events within subtelomeric var loci may affect transcription of these var gene variants.

Evidence for the importance of genetic structuring to the structural and functional specialization of the Plasmodium falciparum var gene family

The var gene family encodes Plasmodium falciparum erythrocyte membrane 1 (PfEMP1) proteins that act as virulence factors responsible for both antigenic variation and cytoadherence of infected erythrocytes. These proteins orchestrate infected erythrocyte sequestration from blood circulation and contribute to adhesion-based complications of P. falciparum malaria infections. For this study, we analysed the genetic organization and strain structure of var genes and present evidence for three separately evolving groups that have, in part, functionally diverged and differ between subtelomeric and central chromosomal locations. Our analyses suggest that a recombination hierarchy limits reassortment between groups and may explain why some var genes are unusually conserved between parasite strains. This recombination hierarchy, coupled with binding and immune selection, shapes the variant antigen repertoire and has structural, functional and evolutionary consequences for the PfEMP1 protein family that are directly relevant to malaria pathogenesis.

Identification and molecular characterisation of DNA damaging agent induced expression of Plasmodium falciparum recombination protein PfRad51

Rad51 protein, the eukaryotic homologue of Escherichia coli RecA protein plays a pivotal role in recombinational repair mechanism. We have identified a new homologue of Rad51 from the apicomplexan parasite Plasmodium falciparum, designated PfRad51. The PfRad51 gene codes for a 351 amino acid polypeptide with a predicted molecular mass of 38720, and shares 66-75% sequence identity within the catalytic region with Rad51 from human, yeast and other protozoan parasites such as Trypanosoma and Leishmania. The expression of PfRad51 transcript as well as protein in the intra-erythrocytic in vitro culture of P. flalciparum was found to be up-regulated in response to the DNA damaging agent methyl methanesulfonate, suggesting its functional involvement in recombinational repair process. PfRad51 is the first apicomplexan gene identified that codes for a recombination protein, and it offers an excellent model for studying DNA damage inducible gene expression in such parasites.

Plasmodium falciparum var genes are regulated by two regions with separate promoters, one upstream of the coding region and a second within the intron

Antigenic variation in Plasmodium falciparum malaria parasites results from switches in expression among members of the multicopy var gene family. This family is subject to allelic exclusion by which particular genes are expressed while the rest of the family remains transcriptionally silent. Evidence from reporter constructs indicates that var gene silencing involves a cooperative interaction between the var intron and an upstream element and requires transition of the parasites through S-phase of the cell cycle. These findings implicate chromatin assembly in the process of regulating var gene expression and antigenic variation. Here we characterize the var intron and the elements within it that are necessary for var transcriptional silencing. Alignments of var introns show a highly conserved structure that consists of three discreet regions with distinct base pair compositions. The middle region is highly AT-rich and is sufficient to silence an associated var promoter. Constructs that include a typical var intron upstream of a reporter gene or drug-selectable marker reveal that the intron also possesses promoter activity, presumably providing an explanation for the origin of the previously described var "sterile" transcripts. Deletions that disable the promoter activity of the intron also eliminate its ability to function as a silencer. These findings suggest that interactions between the regions of these two promoters and the generation of the sterile transcripts play a significant role in regulating var gene expression.

Identification of nuclear proteins that interact differentially with Plasmodium falciparum var gene promoters

The Plasmodium falciparum virulence factor PfEMP1 is responsible for both antigenic variation and cytoadherence of infected erythrocytes in malaria. Approximately 50 var genes per parasite genome code for this highly polymorphic surface protein. We showed recently that chromosome-central and subtelomeric var genes are controlled by different promoters. Here, we report that transcriptional repression of var genes located in different chromosomal regions occurs by different mechanisms. Subtelomeric var gene transcription is repressed 4-8 h before that of chromosome-central var genes. Both repression events coincide with the shifted expression of two distinct nuclear proteins binding specifically to conserved sequence motifs, SPE1 and CPE, present in the respective promoter. Furthermore, a reiterated and highly conserved subtelomeric var promoter element (SPE2) interacts with a nuclear factor exclusively expressed during S-phase. Promoter analysis by transient transfection suggested direct involvement of these interactions in var gene repression and silencing, and identified regions implicated in transcriptional activation of var genes.

Regulation of antigenic variation in Plasmodium falciparum: censoring freedom of expression?

Plasmodium falciparum employs a strategy of clonal antigenic variation to evade the host immune response during the intraerythrocytic stage of its life cycle. The major variant parasite molecule is the P. falciparum erythrocyte membrane protein (PfEMP)1, which is encoded by the var multigene family. The parasite switches between different PfEMP1 molecules through regulation of var transcription. Recent studies have shed considerable light on this process, but much remains unknown. However, striking parallels between transcriptional control of var and genes in other organisms provide direction for future studies.

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.

Effect of var gene disruption on switching in Plasmodium falciparum

The molecular mechanisms underpinning switching of variant antigens on the surface of Plasmodium falciparum-infected erythrocytes are poorly understood. We tested the hypothesis that insertional disruption of the A4var gene, one of two var genes located within the subtelomeric region of one end of chromosome 13, would result in a preferential switch in transcription to the adjacent R29var gene upon rosette selection. In this way, we aimed to mimic the preferential transcription of R29var in rosetting R29 parasites, a parasite line in which the A4var gene is deleted through a chromosome end truncation. Initial analysis of the knock-out parasite lines shows that the insertional disruption of the A4var gene prevents A4 PfEMP1 expression, but that switching transcription to other var gene variants is unaffected. Furthermore, analysis of var transcription in the knock-out parasite line during rosette selection shows that, rather than facilitating a switch to R29var gene transcription, this event was suppressed in the transfectants. These data, and the implications for epigenetic transcriptional control of var genes, are discussed.

A distinct 5' flanking var gene region regulates Plasmodium falciparum variant erythrocyte surface antigen expression in placental malaria

The Plasmodium falciparum multigene var family codes for approximately 50 variant adhesive proteins expressed in a mutually exclusive manner at the surface of infected red blood cells (iRBCs). Switching expression of var genes can lead to fundamental changes in the adhesive and antigenic properties of iRBCs. For example, a specific phenotypic switch in adhesion from CD36 to chondroitin sulphate A (CSA) is associated with malaria pathogenesis in pregnant women. The factors and DNA elements that control the expression of a particular member of the var gene family during gestational malaria remains enigmatic. Here, we report that the subtelomeric FCR3 varCSA is expressed under the control of a unique DNA element of 1.8 kb, whereas the other members of the var multigene family are flanked by common regulatory elements. The 5' varCSA-type element is conserved as a single copy in laboratory strains and clinical isolates from Brazil and West Africa and contains two distinct repetitive elements of 150 bp and 60 bp respectively. The 5' varCSA-type sequence tags a var gene in the 3D7 genome that is homologous to the FCR3 varCSA gene. A recombinant DBL gamma domain of this var gene showed specific binding to CSA. This subtelomeric varCSA gene is transcribed in the opposite sense when compared with the usual orientation of telomere-adjacent var genes. This unique arrangement might explain why the varCSA gene is relatively conserved in genetically distinct parasites despite being located in a highly recombinogenic chromosome compartment. The 5' untranslated region (UTR) of the varCSA-type sequence is also transcribed in placental isolates that bind to CSA, illustrating an important role for the unique 5' varCSA-type sequence in the regulation of var genes involved in malaria pathogenesis in pregnant women. However, this promoter is not always found to be transcribing var genes selected for expression of products that bind to CSA in vitro. Our work identifies a sequence tag for the identification of varCSA genes in placental isolates for the first time.

Mosaic-like transcription of var genes in single Plasmodium falciparum parasites

The var gene family of Plasmodium falciparum encodes the clonally variant adhesin PfEMP1 present on the surface of infected erythrocytes. A poorly understood mechanism of allelic exclusion controls the expression of PfEMP1. Transcription of var genes is developmentally and, most likely, epigenetically regulated. Here we have studied the transcriptional pattern of 28 members of this multigene family in individual parasites, early in the intraerythrocytic cycle. The results show unique patterns (type and number) of var transcripts in each individual PRBC, with 1-15 mRNA species detected per cell at 2-4 h post-invasion. When a panel of ten single PRBC was analyzed, the var gene coding for the expressed PfEMP1 was transcribed in more cells than any other, although transcripts from this gene did not give the strongest hybridization signal within each individual cell. Chromosomal mapping of transcriptionally active var genes indicated that their distribution reflects that of var loci in the genome, including a pronounced clustering in chromosome 4. These findings, taken together with existing data on var transcription at later developmental stages, suggest that the mosaic-like transcription of multiple var genes detected at the ring stage and the steady transcription of the gene encoding the expressed PfEMP1 are distinct although superimposed events, one of them random and the other taking place under some form of imprinting. With its unique features, the expression of P. falciparum var genes may reveal new principles of gene regulation.

Antigenic diversity in Eimeria maxima and the influence of host genetics and immunization schedule on cross-protective immunity

Eimeria spp. are a group of highly successful intracellular protozoan parasites that develop within enterocytes. Eimeria maxima from the chicken is characterized by high immunogenicity (a small priming infection gives complete immunity to subsequent homologous challenge) and naturally occurring antigenically variant populations that do not completely cross-protect. In this study we examined the expression of antigenic diversity in E. maxima, as manifested by cross-strain protection in a series of inbred chicken lines. The IAH line of Light Sussex chickens and all lines of inbred White Leghorns were susceptible to primary infections with either of two strains (H and W) of E. maxima and were protected completely against challenge with the homologous strain of parasite. The extent of cross-protection against the heterologous parasite strain varied from 0 to almost 100% depending on host genetics. Interestingly, in one inbred line of chickens (line 15I) the cross-protective phenotype was directional and intensely influenced by the infection history of the host. The basis for the observed variation in cross-protection is not known, but our results suggest that the major histocompatibility complex is not a major genetic component of the phenotype. These results are discussed in relation to the number of protective antigens presented by complex pathogens and the development of immunoprotective responses in hosts of different genetic backgrounds.

Transcription of multiple var genes by individual, trophozoite-stage Plasmodium falciparum cells expressing a chondroitin sulphate A binding phenotype

In this study, we detected multiple var gene transcripts within single, mature trophozoite-infected red blood cells (iRBCs) bound to chondroitin sulphate A (CSA). Several of the var detected had previously been demonstrated to encode Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP-1) variants with domains that mediated iRBC adhesion to receptors other than CSA. Parasites expressing the CSA-adherent phenotype transcribed far more of one var than of all others, but this gene was different from the two other var previously purported to encode adhesion to CSA. Previous work suggesting that only single var are transcribed by mature trophozoites needs re-examination in the light of these data from single, infected cells.

Antigenic variation at the infected red cell surface in malaria

Many pathogens that either rely on an insect vector to complete their life cycle (e.g., Trypanosoma spp. and Borrelia spp.) or exist in a unique ecological niche where transmission from host to host is sporadic (e.g., Neisseria spp.) have evolved strategies to maintain infection of their mammalian hosts for long periods of time in order to ensure their survival. Because they have to survive in the face of a fully functional immune system, a common feature of many of these organisms is their development of sophisticated strategies for immune evasion. For the above organisms and for malaria parasites of the genus Plasmodium, a common theme is the ability to undergo clonal antigenic variation. In all cases, surface molecules that are important targets of the humoral immune response are encoded in the genome as multicopy, nonallelic gene families. Antigenic variation is accomplished by the successive expression of members of these gene families that show little or no immunological cross-reactivity. In the case of malaria parasites, however, some of the molecules that undergo antigenic variation are also major virulence factors, adding an additional level of complication to the host-parasite interaction. In this review, we cover the history of antigenic variation in malaria and then summarize the more recent data with particular emphasis on Plasmodium falciparum, the etiological agent of the most severe form of human malaria.

DNA microarrays for malaria

DNA microarrays are a powerful tool for the analysis of RNA and DNA composition on a whole-genome scale. The first applications of this technology in parasitology are in place. This review examines the various approaches to Plasmodium transcript-profiling that are being adopted using DNA microarray analysis and discusses additional strategies for obtaining and collating information relevant to the search for drug and vaccine candidates in malaria.

Molecular technology and antigenic variation among intraerythrocytic hemoparasites: do we see reality?

Antigenic variation is one mechanism of immune evasion utilized by many microorganisms--encompassing such broad evolutionary groups as viruses, bacteria, and protozoa--to survive the onslaught of a specifically activated host immune system. Because of its importance to the survival of many infectious agents there is considerable interest in understanding this phenomenon. With knowledge of the molecular mechanisms by which these microbes deliberately manipulate their genomes, it may be possible to disrupt the molecular machinery of the responsible genetic mechanisms. Among intraerythrocytic parasites, genetic mechanisms that have been observed or postulated to control antigenic variation include segmental gene conversion, epigenetically controlled in situ transcriptional switching, alterations of chromosomal structure associated with transcriptional control, and recombination during sexual reproduction. Likely, more than one type of mechanism is used by all organisms that undergo antigenic variation. In this paper, both the observed mechanisms and some of the molecular technology used to detect these mechanisms are discussed. While often seemingly straightforward from a technical standpoint, sometimes subtle differences in the methods used to study this process may affect what is observed. Some examples of this phenomenon are discussed in the context of a small selection of intraerythrocytic parasites.

Decoding the language of var genes and Plasmodium falciparum sequestration

Sequestration and rosetting are key determinants of Plasmodium falciparum pathogenesis. They are mediated by a large family of variant proteins called P. falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1 proteins are multispecific binding receptors that are transported to parasite-induced, 'knob-like' binding structures at the erythrocyte surface. To evade immunity and extend infections, parasites clonally vary their expressed PfEMP1. Thus, PfEMP1 are functionally selected for binding while immune selection acts to diversify the family. Here, we describe a new way to analyse PfEMP1 sequence that provides insight into domain function and protein architecture with potential implications for malaria disease.

Malaria. Cooperative silencing elements in var genes

Each Plasmodium falciparum malaria parasite carries about 50 var genes from a diverse family that encode variable adhesion proteins on the infected red blood cells of the host, but individual parasites single out just one var gene for expression and silence all the others. Here we show that this silencing is established during the DNA-synthesis phase (S phase) of the cell cycle and that it depends on the cooperative interaction between two elements in separate control regions of each var gene (the 5'-flanking region and the intron). This finding should help to clarify the mechanisms by which parasites coordinate the silencing and activation of var genes that are responsible for antigenic variation in malaria.

Variant antigens of Plasmodium falciparum encoded by the var multigenic family are multifunctional macromolecules

Cytoadhesion of parasitized red blood cells (PRBCs) to vascular endothelial cells (sequestration) and binding of unparasitized RBCs to PRBCs (rosetting) are virulence factors of Plasmodium falciparum, the species responsible for lethal human malaria. Variant antigens involved in both phenomena have been identified as products of the multicopy var gene family. In this review, progress in the understanding of molecular mechanisms of sequestration is summarized, in particular, concerning the structure of var gene products related to specificity of binding to endothelial receptors, and the origin of var gene diversity.

Molecular mechanisms of Plasmodium falciparum placental adhesion

In natural Plasmodium falciparum infections, parasitized erythrocytes (PEs) circulate in the peripheral blood for a period corresponding roughly to the first part of the erythrocytic life cycle (ring stage). Later, in blood-stage development, parasite-encoded adhesion molecules are inserted into the erythrocyte membrane, preventing the circulation of the PEs. The principal molecule mediating PE adhesion is P. falciparum erythrocyte membrane protein 1 (PfEMP1), encoded by the polymorphic var gene family. The population of parasites is subject to clonal antigenic variation through changes in var expression, and a single PfEMP1 variant is expressed at the PE surface in a mutually exclusive manner. In addition to its role in immune evasion, switches in PfEMP1 expression may be associated with fundamental changes in parasite tissue tropism in malaria patients. A switch from CD36 binding to chondroitin sulphate A (CSA) binding may lead to extensive sequestration of PEs in placenta syncytiotrophoblasts. This is probably a key event in malaria pathogenesis during pregnancy. The CSA-binding phenotype of mature PEs is linked to another distinct adhesive phenotype: the recently described CSA-independent cytoadhesion of ring-stage PEs. Thus, a subpopulation of PEs that sequentially displays these two different phenotypes may bind to an individual endothelial cell or syncytiotrophoblast throughout the asexual blood-stage cycle. This suggests that non-circulating (cryptic) parasite subpopulations are present in malaria patients.

The development of genetic tools for dissecting the biology of malaria parasites

Plasmodium parasites are haploid unicellular organisms that cause malaria. In the last decade, transfection systems have been developed for both human and animal model species of Plasmodium, providing a broad range of genetic tools for the study of malaria parasite biology. Transient transfection has been used to provide insight into the regulation of gene expression by Plasmodium spp. The development of stable transfection technologies has provided the opportunity to express transgenes in Plasmodium spp., as well as elucidate the function of proteins by disrupting, modifying, or replacing the genes encoding them. These genetic tools represent an important breakthrough for malaria research and will significantly contribute to our understanding of the biology of the parasite. However, further developments in this technology are still required, especially because the full genome sequence of the major human malaria parasite Plasmodium falciparum will shortly be available. Ultimately, the biological information obtained through genetic manipulation of Plasmodium spp. will facilitate a more rational approach to vaccine and drug design.

Rapid recombination among transfected plasmids, chimeric episome formation and trans gene expression in Plasmodium falciparum

Although recombination is known to be important to generating diversity in the human malaria parasite P. falciparum, the low efficiencies of transfection and the fact that integration of transfected DNA into chromosomes is observed only after long periods (typically 12 weeks or more) have made it difficult to genetically manipulate the blood stages of this major human pathogen. Here we show that co-transfection of a P. falciparum line with two plasmids, one expressing a green fluorescent protein (gfp) reporter and the other expressing a drug resistance marker (Tgdhfr-ts M23), allowed selection of a population in which about approximately 30% of the parasites produce GFP. In these GFP-producing parasites, the transfected plasmids had recombined into chimeric episomes as large as 20 kb and could be maintained under drug pressure for at least 16 weeks. Our data suggest that chimera formation occurs early (detected by 7--14 days) and that it involves homologous recombination favored by presence of the same P. falciparum 5'hrp3 UTR promoting transcription from each plasmid. This indicates the presence of high levels of homologous recombination activity in blood stage parasites that can be used to drive rapid recombination of newly introduced DNA, study mechanisms of recombination, and introduce genes for trans expression in P. falciparum.

Frequent ectopic recombination of virulence factor genes in telomeric chromosome clusters of P. falciparum

Persistent and recurrent infections by Plasmodium falciparum malaria parasites result from the ability of the parasite to undergo antigenic variation and evade host immune attack. P. falciparum parasites generate high levels of variability in gene families that comprise virulence determinants of cytoadherence and antigenic variation, such as the var genes. These genes encode the major variable parasite protein (PfEMP-1), and are expressed in a mutually exclusive manner at the surface of the erythrocyte infected by P. falciparum. Here we identify a mechanism by which var gene sequences undergo recombination at frequencies much higher than those expected from homologous crossover events alone. These recombination events occur between subtelomeric regions of heterologous chromosomes, which associate in clusters near the nuclear periphery in asexual blood-stage parasites or in bouquet-like configurations near one pole of the elongated nuclei in sexual parasite forms. We propose that the alignment of var genes in heterologous chromosomes facilitates gene conversion and promotes the diversity of antigenic and adhesive phenotypes. The association of virulence factors with a specific nuclear subcompartment may also have implications for variation during mitotic recombination in asexual blood stages.

The conserved genome organisation of non-falciparum malaria species: the need to know more

The current knowledge on genomes of non-falciparum malaria species and the potential of model malaria parasites for functional analyses are reviewed and compared with those of the most pathogenic human parasite, Plasmodium falciparum. There are remarkable similarities in overall genome composition among the different species at the level of chromosome organisation and chromosome number, conserved order of individual genes, and even conserved functions of specific gene domains and regulatory control elements. With the initiative taken to sequence the genome of P. falciparum, a wealth of information is already becoming available to the scientific community. In order to exploit the biological information content of a complete genome sequence, simple storage of the bulk of sequence data will be inadequate. The requirement for functional analyses to determine the biological role of the open reading frames is commonly accepted and knowledge of the genomes of the animal model malaria species will facilitate these analyses. Detailed comparative genome information and sequencing of additional Plasmodium genomes will provide a deeper insight into the evolutionary history of the species, the biology of the parasite, and its interactions with the mammalian host and mosquito vector. Therefore, an extended and integrated approach will enhance our knowledge of malaria and will ultimately lead to a more rational approach that identifies and evaluates new targets for anti-malarial drug and vaccine development.

The ves multigene family of B. bovis encodes components of rapid antigenic variation at the infected erythrocyte surface

B. bovis, an intraerythrocytic protozoal parasite, establishes chronic infections in cattle in part through rapid variation of the polymorphic, heterodimeric VESA1 protein on the infected erythrocyte surface and sequestration of mature parasites. We describe the characterization of the ves1 alpha gene encoding the VESA1a subunit, thus providing a description of a gene whose product is involved in rapid antigenic variation in a babesial parasite. This three-exon gene, a member of a multigene family (ves), encodes a polypeptide with no cleavable signal sequence, a single predicted transmembrane segment, and a cysteine/lysine-rich domain. Variation appears to involve creation and modification or loss of a novel, transcribed copy of the gene.

Adhesive receptors on malaria-parasitized red cells

Antigenic variation, rosetting and cytoadhesion are key determinants in the survival and virulence of the malaria parasite Plasmodium falciparum. These properties reside in a multigene protein family called P. falciparum erythrocyte membrane protein I (PfEMPI), encoded by the large and diverse var gene family. PfEMPI plays a central role in the biology of P. falciparum and its interaction with the human host. The molecular mechanism and the domains involved in cytoadherence, rosetting and antigenic variation are beginning to unfold. Domains mediating rosetting and adhesion to several key host receptors have already been identified. Understanding the role of PfEMPI in the pathogenesis and survival of malaria parasites is the key for the development of anti-adhesion vaccines and therapeutics to reduce the mortality and morbidity of P. falciparum infections.

Genes involved in phenotypic and antigenic variation in African trypanosomes and malaria

Large polymorphic gene families that are involved in clonal phenotypic variation have been identified in both African trypanosomes and malaria parasites. Many of these gene families are necessary for host adaptation, allowing the parasite to infect different species of host or types of host cells. In many cases, switching between these functionally variable proteins also results in antigenic variation.

Antigenic variation in Plasmodium falciparum: mechanisms and consequences

In the past year, the major advances in malaria antigenic variation have been concerned with the transcription and switching of variant antigen genes, and the functional expression of regions of the major variant antigen. Also, new variant gene families have been discovered as a result of the Malaria Genome Project.