Gene inactivation of Pf11-1 of Plasmodium falciparum by chromosome breakage and healing: identification of a gametocyte-specific protein with a potential role in gametogenesis

Target-agnostic identification of human antibodies to Plasmodium falciparum sexual forms reveals cross-stage recognition of glutamate-rich repeats

Circulating sexual stages of Plasmodium falciparum (Pf) can be transmitted from humans to mosquitoes, thereby furthering the spread of malaria in the population. It is well established that antibodies can efficiently block parasite transmission. In search for naturally acquired antibodies targets on sexual stages, we established an efficient method for target-agnostic single B cell activation followed by high-throughput selection of human monoclonal antibodies (mAbs) reactive to sexual stages of Pf in the form of gametes and gametocyte extracts. We isolated mAbs reactive against a range of Pf proteins including well-established targets Pfs48/45 and Pfs230. One mAb, B1E11K, was cross-reactive to various proteins containing glutamate-rich repetitive elements expressed at different stages of the parasite life cycle. A crystal structure of two B1E11K Fab domains in complex with its main antigen, RESA, expressed on asexual blood stages, showed binding of B1E11K to a repeating epitope motif in a head-to-head conformation engaging in affinity-matured homotypic interactions. Thus, this mode of recognition of Pf proteins, previously described only for Pf circumsporozoite protein (PfCSP), extends to other repeats expressed across various stages. The findings augment our understanding of immune-pathogen interactions to repeating elements of the Plasmodium parasite proteome and underscore the potential of the novel mAb identification method used to provide new insights into the natural humoral immune response against Pf.

Computational approach for decoding malaria drug targets from single-cell transcriptomics and finding potential drug molecule

Malaria is a deadly disease caused by Plasmodium parasites. While potent drugs are available in the market for malaria treatment, over the years, Plasmodium parasites have successfully developed resistance against many, if not all, front-line drugs. This poses a serious threat to global malaria eradication efforts, and the continued discovery of new drugs is necessary to tackle this debilitating disease. With recent unprecedented progress in machine learning techniques, single-cell transcriptomic in Plasmodium offers a powerful tool for identifying crucial proteins as a drug target and subsequent computational prediction of potential drugs. In this study, We have implemented a mutual-information-based feature reduction algorithm with a classification algorithm to select important proteins from transcriptomic datasets (sexual and asexual stages) for Plasmodium falciparum and then constructed the protein-protein interaction (PPI) networks of the proteins. The analysis of this PPI network revealed key proteins vital for the survival of Plasmodium falciparum. Based on the function and identification of a few strong binding sites on a couple of these key proteins, we computationally predicted a set of potential drug molecules using a deep learning-based technique. Lead drug molecules that satisfy ADMET and drug-likeliness properties are finally reported out of the generated drugs. The study offers a general computational pipeline to identify crucial proteins using scRNA-seq data sets and further development of potential new drugs.

Target-agnostic identification of human antibodies to Plasmodium falciparum sexual forms reveals cross stage recognition of glutamate-rich repeats

Circulating sexual stages of Plasmodium falciparum (Pf) can be transmitted from humans to mosquitoes, thereby furthering the spread of malaria in the population. It is well established that antibodies (Abs) can efficiently block parasite transmission. In search for naturally acquired Ab targets on sexual stages, we established an efficient method for target-agnostic single B cell activation followed by high-throughput selection of human monoclonal antibodies (mAbs) reactive to sexual stages of Pf in the form of gamete and gametocyte extract. We isolated mAbs reactive against a range of Pf proteins including well-established targets Pfs48/45 and Pfs230. One mAb, B1E11K, was cross-reactive to various proteins containing glutamate-rich repetitive elements expressed at different stages of the parasite life cycle. A crystal structure of two B1E11K Fab domains in complex with its main antigen, RESA, expressed on asexual blood stages, showed binding of B1E11K to a repeating epitope motif in a head-to-head conformation engaging in affinity-matured homotypic interactions. Thus, this mode of recognition of Pf proteins, previously described only for PfCSP, extends to other repeats expressed across various stages. The findings augment our understanding of immune-pathogen interactions to repeating elements of the Plasmodium parasite proteome and underscore the potential of the novel mAb identification method used to provide new insights into the natural humoral immune response against Pf .

Impact Statement

A naturally acquired human monoclonal antibody recognizes proteins expressed at different stages of the Plasmodium falciparum lifecycle through affinity-matured homotypic interactions with glutamate-rich repeats.

Genomics reveals heterogeneous Plasmodium falciparum transmission and selection signals in Zambia

BACKGROUND

Genomic surveillance is crucial for monitoring malaria transmission and understanding parasite adaptation to interventions. Zambia lacks prior nationwide efforts in malaria genomic surveillance among African countries.

METHODS

We conducted genomic surveillance of Plasmodium falciparum parasites from the 2018 Malaria Indicator Survey in Zambia, a nationally representative household survey of children under five years of age. We whole-genome sequenced and analyzed 241 P. falciparum genomes from regions with varying levels of malaria transmission across Zambia and estimated genetic metrics that are informative about transmission intensity, genetic relatedness between parasites, and selection.

RESULTS

We provide genomic evidence of widespread within-host polygenomic infections, regardless of epidemiological characteristics, underscoring the extensive and ongoing endemic malaria transmission in Zambia. Our analysis reveals country-level clustering of parasites from Zambia and neighboring regions, with distinct separation in West Africa. Within Zambia, identity by descent (IBD) relatedness analysis uncovers local spatial clustering and rare cases of long-distance sharing of closely related parasite pairs. Genomic regions with large shared IBD segments and strong positive selection signatures implicate genes involved in sulfadoxine-pyrimethamine and artemisinin combination therapies drug resistance, but no signature related to chloroquine resistance. Furthermore, differences in selection signatures, including drug resistance loci, are observed between eastern and western Zambian parasite populations, suggesting variable transmission intensity and ongoing drug pressure.

CONCLUSIONS

Our findings enhance our understanding of nationwide P. falciparum transmission in Zambia, establishing a baseline for analyzing parasite genetic metrics as they vary over time and space. These insights highlight the urgency of strengthening malaria control programs and surveillance of antimalarial drug resistance.

Genomics reveals heterogeneous Plasmodium falciparum transmission and population differentiation in Zambia and bordering countries

Genomic surveillance plays a critical role in monitoring malaria transmission and understanding how the parasite adapts in response to interventions. We conducted genomic surveillance of malaria by sequencing 241 Plasmodium falciparum genomes from regions with varying levels of malaria transmission across Zambia. We found genomic evidence of high levels of within-host polygenomic infections, regardless of epidemiological characteristics, underscoring the extensive and ongoing endemic malaria transmission in the country. We identified country-level clustering of parasites from Zambia and neighboring countries, and distinct clustering of parasites from West Africa. Within Zambia, our identity by descent (IBD) relatedness analysis uncovered spatial clustering of closely related parasite pairs at the local level and rare cases of long-distance sharing. Genomic regions with large shared IBD segments and strong positive selection signatures identified genes involved in sulfadoxine-pyrimethamine and artemisinin combination therapies drug resistance, but no signature related to chloroquine resistance. Together, our findings enhance our understanding of P. falciparum transmission nationwide in Zambia and highlight the urgency of strengthening malaria control programs and surveillance of antimalarial drug resistance.

Novel secretory organelles of parasite origin - at the center of host-parasite interaction

Reorganization of cell organelle-deprived host red blood cells by the apicomplexan malaria parasite Plasmodium falciparum enables their cytoadherence to endothelial cells that line the microvasculature. This increases the time red blood cells infected with mature developmental stages remain within selected organs such as the brain to avoid the spleen passage, which can lead to severe complications and cumulate in patient death. The Maurer's clefts are a novel secretory organelle of parasite origin established by the parasite in the cytoplasm of the host red blood cell in order to facilitate the establishment of cytoadherence by conducting the trafficking of immunovariant adhesins to the host cell surface. Another important function of the organelle is the sorting of other proteins the parasite traffics into its host cell. Although the organelle is of high importance for the pathology of malaria, additional putative functions, structure, and genesis remain shrouded in mystery more than a century after its discovery. In this review, we highlight our current knowledge about the Maurer's clefts and other novel secretory organelles established within the host cell cytoplasm by human-pathogenic malaria parasites and other parasites that reside within human red blood cells.

Erythropoiesis and Malaria, a Multifaceted Interplay

One of the major pathophysiologies of malaria is the development of anemia. Although hemolysis and splenic clearance are well described as causes of malarial anemia, abnormal erythropoiesis has been observed in malaria patients and may contribute significantly to anemia. The interaction between inadequate erythropoiesis and Plasmodium parasite infection, which partly occurs in the bone marrow, has been poorly investigated to date. However, recent findings may provide new insights. This review outlines clinical and experimental studies describing different aspects of ineffective erythropoiesis and dyserythropoiesis observed in malaria patients and in animal or in vitro models. We also highlight the various human and parasite factors leading to erythropoiesis disorders and discuss the impact that Plasmodium parasites may have on the suppression of erythropoiesis.

The acquisition of humoral immune responses targeting Plasmodium falciparum sexual stages in controlled human malaria infections

Individuals infected with P. falciparum develop antibody responses to intra-erythrocytic gametocyte proteins and exported gametocyte proteins present on the surface of infected erythrocytes. However, there is currently limited knowledge on the immunogenicity of gametocyte antigens and the specificity of gametocyte-induced antibody responses. In this study, we assessed antibody responses in participants of two controlled human malaria infection (CHMI) studies by ELISA, multiplexed bead-based antibody assays and protein microarray. By comparing antibody responses in participants with and without gametocyte exposure, we aimed to disentangle the antibody response induced by asexual and sexual stage parasites. We showed that after a single malaria infection, a significant anti-sexual stage humoral response is induced in malaria-naïve individuals, even after exposure to relatively low gametocyte densities (up to ~1,600 gametocytes/mL). In contrast to antibody responses to well-characterised asexual blood stage antigens that were detectable by day 21 after infection, responses to sexual stage antigens (including transmission blocking vaccine candidates Pfs48/45 and Pfs230) were only apparent at 51 days after infection. We found antigens previously associated with early gametocyte or anti-gamete immunity were highly represented among responses linked with gametocyte exposure. Our data provide detailed insights on the induction and kinetics of antibody responses to gametocytes and identify novel antigens that elicit antibody responses exclusively in individuals with gametocyte exposure. Our findings provide target identification for serological assays for surveillance of the malaria infectious reservoir, and support vaccine development by describing the antibody response to leading vaccine antigens after primary infection.

Single-cell sequencing of the small and AT-skewed genome of malaria parasites

Single-cell genomics is a rapidly advancing field; however, most techniques are designed for mammalian cells. We present a single-cell sequencing pipeline for an intracellular parasite, Plasmodium falciparum, with a small genome of extreme base content. Through optimization of a quasi-linear amplification method, we target the parasite genome over contaminants and generate coverage levels allowing detection of minor genetic variants. This work, as well as efforts that build on these findings, will enable detection of parasite heterogeneity contributing to P. falciparum adaptation. Furthermore, this study provides a framework for optimizing single-cell amplification and variant analysis in challenging genomes.

Plasmodium falciparum sexual parasites regulate infected erythrocyte permeability

To ensure the transport of nutrients necessary for their survival, Plasmodium falciparum parasites increase erythrocyte permeability to diverse solutes. These new permeation pathways (NPPs) have been extensively characterized in the pathogenic asexual parasite stages, however the existence of NPPs has never been investigated in gametocytes, the sexual stages responsible for transmission to mosquitoes. Here, we show that NPPs are still active in erythrocytes infected with immature gametocytes and that this activity declines along gametocyte maturation. Our results indicate that NPPs are regulated by cyclic AMP (cAMP) signaling cascade, and that the decrease in cAMP levels in mature stages results in a slowdown of NPP activity. We also show that NPPs facilitate the uptake of artemisinin derivatives and that phosphodiesterase (PDE) inhibitors can reactivate NPPs and increase drug uptake in mature gametocytes. These processes are predicted to play a key role in P. falciparum gametocyte biology and susceptibility to antimalarials.

Bouyer et al. report that the new permeation pathways (NPP), responsible of modulating erythrocyte permeability to diverse solutes and considered only to be in pathogenic asexual stages of P. falciparum, are also active in erythrocytes infected with immature gametocytes and this activity declines with gametocyte maturation. NPPs are regulated by the cAMP signalling cascade, and the decrease in cAMP levels in mature stages slows NPP activity.

Plasmodium falciparum sexual parasites develop in human erythroblasts and affect erythropoiesis

Plasmodium falciparum gametocytes, the sexual stage responsible for malaria parasite transmission from humans to mosquitoes, are key targets for malaria elimination. Immature gametocytes develop in the human bone marrow parenchyma, where they accumulate around erythroblastic islands. Notably though, the interactions between gametocytes and this hematopoietic niche have not been investigated. Here, we identify late erythroblasts as a new host cell for P falciparum sexual stages and show that gametocytes can fully develop inside these nucleated cells in vitro and in vivo, leading to infectious mature gametocytes within reticulocytes. Strikingly, we found that infection of erythroblasts by gametocytes and parasite-derived extracellular vesicles delay erythroid differentiation, thereby allowing gametocyte maturation to coincide with the release of their host cell from the bone marrow. Taken together, our findings highlight new mechanisms that are pivotal for the maintenance of immature gametocytes in the bone marrow and provide further insights on how Plasmodium parasites interfere with erythropoiesis and contribute to anemia in malaria patients.

Immunity against sexual stage Plasmodium falciparum and Plasmodium vivax parasites

The efficient spread of malaria from infected humans to mosquitoes is a major challenge for malaria elimination initiatives. Gametocytes are the only Plasmodium life stage infectious to mosquitoes. Here, we summarize evidence for naturally acquired anti-gametocyte immunity and the current state of transmission blocking vaccines (TBV). Although gametocytes are intra-erythrocytic when present in infected humans, developing Plasmodium falciparum gametocytes may express proteins on the surface of red blood cells that elicit immune responses in naturally exposed individuals. This immune response may reduce the burden of circulating gametocytes. For both P. falciparum and Plasmodium vivax, there is a solid evidence that antibodies against antigens present on the gametocyte surface, when co-ingested with gametocytes, can influence transmission to mosquitoes. Transmission reducing immunity, reducing the burden of infection in mosquitoes, is a well-acknowledged but poorly quantified phenomenon that forms the basis for the development of TBV. Transmission enhancing immunity, increasing the likelihood or intensity of transmission to mosquitoes, is more speculative in nature but is convincingly demonstrated for P. vivax. With the increased interest in malaria elimination, TBV and monoclonal antibodies have moved to the center stage of malaria vaccine development. Methodologies to prioritize and evaluate products are urgently needed.

Erythrocyte Membrane Makeover by Plasmodium falciparum Gametocytes

Plasmodium falciparum sexual parasites, called gametocytes, are the only parasite stages responsible for transmission from humans to Anopheles mosquitoes. During their maturation, P. falciparum gametocytes remodel the structural and mechanical properties of the membrane of their erythrocyte host. This remodeling is induced by the export of several parasite proteins and a dynamic reorganization of the erythrocyte cytoskeleton. Some of these modifications are specific for sexual stages and play a key role for gametocyte maturation, sequestration in internal organs, subsequent release in the bloodstream and ability to persist in circulation. Here we discuss the mechanisms developed by gametocytes to remodel their host cell and the functional relevance of these modifications.

RecQ helicases in the malaria parasite Plasmodium falciparum affect genome stability, gene expression patterns and DNA replication dynamics

The malaria parasite Plasmodium falciparum has evolved an unusual genome structure. The majority of the genome is relatively stable, with mutation rates similar to most eukaryotic species. However, some regions are very unstable with high recombination rates, driving the generation of new immune evasion-associated var genes. The molecular factors controlling the inconsistent stability of this genome are not known. Here we studied the roles of the two putative RecQ helicases in P. falciparum, PfBLM and PfWRN. When PfWRN was knocked down, recombination rates increased four-fold, generating chromosomal abnormalities, a high rate of chimeric var genes and many microindels, particularly in known 'fragile sites'. This is the first identification of a gene involved in suppressing recombination and maintaining genome stability in Plasmodium. By contrast, no change in mutation rate appeared when the second RecQ helicase, PfBLM, was mutated. At the transcriptional level, however, both helicases evidently modulate the transcription of large cohorts of genes, with several hundred genes-including a large proportion of vars-showing deregulated expression in each RecQ mutant. Aberrant processing of stalled replication forks is a possible mechanism underlying elevated mutation rates and this was assessed by measuring DNA replication dynamics in the RecQ mutant lines. Replication forks moved slowly and stalled at elevated rates in both mutants, confirming that RecQ helicases are required for efficient DNA replication. Overall, this work identifies the Plasmodium RecQ helicases as major players in DNA replication, antigenic diversification and genome stability in the most lethal human malaria parasite, with important implications for genome evolution in this pathogen.

Capturing in vivo RNA transcriptional dynamics from the malaria parasite Plasmodium falciparum

To capture the transcriptional dynamics within proliferating cells, methods to differentiate nascent transcription from preexisting mRNAs are desired. One approach is to label newly synthesized mRNA transcripts in vivo through the incorporation of modified pyrimidines. However, the human malaria parasite, Plasmodium falciparum, is incapable of pyrimidine salvage for mRNA biogenesis. To capture cellular mRNA dynamics during Plasmodium development, we engineered parasites that can salvage pyrimidines through the expression of a single bifunctional yeast fusion gene, cytosine deaminase/uracil phosphoribosyltransferase (FCU). We show that expression of FCU allows for the direct incorporation of thiol-modified pyrimidines into nascent mRNAs. Using developmental stage-specific promoters to express FCU-GFP enables the biosynthetic capture and in-depth analysis of mRNA dynamics from subpopulations of cells undergoing differentiation. We demonstrate the utility of this method by examining the transcriptional dynamics of the sexual gametocyte stage transition, a process that is essential to malaria transmission between hosts. Using the pfs16 gametocyte-specific promoter to express FCU-GFP in 3D7 parasites, we found that sexual stage commitment is governed by transcriptional reprogramming and stabilization of a subset of essential gametocyte transcripts. We also measured mRNA dynamics in F12 gametocyte-deficient parasites and demonstrate that the transcriptional program required for sexual commitment and maturation is initiated but likely aborted due to the absence of the PfAP2-G transcriptional regulator and a lack of gametocyte-specific mRNA stabilization. Biosynthetic labeling of Plasmodium mRNAs is incredibly versatile, can be used to measure transcriptional dynamics at any stage of parasite development, and will allow for future applications to comprehensively measure RNA-protein interactions in the malaria parasite.

Expansion of Lysine-rich Repeats in Plasmodium Proteins Generates Novel Localization Sequences That Target the Periphery of the Host Erythrocyte

Repetitive low complexity sequences, mostly assumed to have no function, are common in proteins that are exported by the malaria parasite into its host erythrocyte. We identify a group of exported proteins containing short lysine-rich tandemly repeated sequences that are sufficient to localize to the erythrocyte periphery, where key virulence-related modifications to the plasma membrane and the underlying cytoskeleton are known to occur. Efficiency of targeting is dependent on repeat number, indicating that novel targeting modules could evolve by expansion of short lysine-rich sequences. Indeed, analysis of fragments of GARP from different species shows that two novel targeting sequences have arisen via the process of repeat expansion in this protein. In the protein Hyp12, the targeting function of a lysine-rich sequence is masked by a neighboring repetitive acidic sequence, further highlighting the importance of repetitive low complexity sequences. We show that sequences capable of targeting the erythrocyte periphery are present in at least nine proteins from Plasmodium falciparum and one from Plasmodium knowlesi. We find these sequences in proteins known to be involved in erythrocyte rigidification and cytoadhesion as well as in previously uncharacterized exported proteins. Together, these data suggest that expansion and contraction of lysine-rich repeats could generate targeting sequences de novo as well as modulate protein targeting efficiency and function in response to selective pressure.

PfSR1 controls alternative splicing and steady-state RNA levels in Plasmodium falciparum through preferential recognition of specific RNA motifs

Plasmodium species have evolved complex biology to adapt to different hosts and changing environments throughout their life cycle. Remarkably, these adaptations are achieved by a relatively small genome. One way by which the parasite expands its proteome is through alternative splicing (AS). We recently identified PfSR1 as a bona fide Ser/Arg-rich (SR) protein that shuttles between the nucleus and cytoplasm and regulates AS in Plasmodium falciparum. Here we show that PfSR1 is localized adjacent to the Nuclear Pore Complex (NPC) clusters in the nucleus of early stage parasites. To identify the endogenous RNA targets of PfSR1, we adapted an inducible overexpression system for tagged PfSR1 and performed RNA immunoprecipitation followed by microarray analysis (RIP-chip) to recover and identify the endogenous RNA targets that bind PfSR1. Bioinformatic analysis of these RNAs revealed common sequence motifs potentially recognized by PfSR1. RNA-EMSAs show that PfSR1 preferentially binds RNA molecules containing these motifs. Interestingly, we find that PfSR1 not only regulates AS but also the steady-state levels of mRNAs containing these motifs in vivo.

Erythrocyte remodeling by Plasmodium falciparum gametocytes in the human host interplay

The spread of malaria critically relies on the presence of Plasmodium transmission stages - the gametocytes - circulating in the blood of an infected individual, which are taken up by Anopheles mosquitoes. A striking feature of Plasmodium falciparum gametocytes is their long development inside the erythrocytes while sequestered in the internal organs of the human host. Recent studies of the molecular and cellular remodeling of the host erythrocyte induced by P. falciparum during gametocyte maturation are shedding light on how these may affect the establishment and maintenance of sequestration of the immature transmission stages and the subsequent release and circulation of mature gametocytes in the peripheral bloodstream.

Feeling at home from arrival to departure: protein export and host cell remodelling during Plasmodium liver stage and gametocyte maturation

Obligate intracellular pathogens actively remodel their host cells to boost propagation, survival, and persistence. Plasmodium falciparum, the causative agent of the most severe form of malaria, assembles a complex secretory system in erythrocytes. Export of parasite factors to the erythrocyte membrane is essential for parasite sequestration from the blood circulation and a major factor for clinical complications in falciparum malaria. Historic and recent molecular reports show that host cell remodelling is not exclusive to P. falciparum and that parasite-induced intra-erythrocytic membrane structures and protein export occur in several Plasmodia. Comparative analyses of P. falciparum asexual and sexual blood stages and imaging of liver stages from transgenic murine Plasmodium species show that protein export occurs in all intracellular phases from liver infection to sexual differentiation, indicating that mammalian Plasmodium species evolved efficient strategies to renovate erythrocytes and hepatocytes according to the specific needs of each life cycle phase. While the repertoireof identified exported proteins is remarkably expanded in asexual P. falciparum blood stages, the putative export machinery and known targeting signatures are shared across life cycle stages. A better understanding of the molecular mechanisms underlying Plasmodium protein export could assist in designing novel strategies to interrupt transmission between Anopheles mosquitoes and humans.

The landscape of inherited and de novo copy number variants in a Plasmodium falciparum genetic cross

BACKGROUND

Copy number is a major source of genome variation with important evolutionary implications. Consequently, it is essential to determine copy number variant (CNV) behavior, distributions and frequencies across genomes to understand their origins in both evolutionary and generational time frames. We use comparative genomic hybridization (CGH) microarray and the resolution provided by a segregating population of cloned progeny lines of the malaria parasite, Plasmodium falciparum, to identify and analyze the inheritance of 170 genome-wide CNVs.

RESULTS

We describe CNVs in progeny clones derived from both Mendelian (i.e. inherited) and non-Mendelian mechanisms. Forty-five CNVs were present in the parent lines and segregated in the progeny population. Furthermore, extensive variation that did not conform to strict Mendelian inheritance patterns was observed. 124 CNVs were called in one or more progeny but in neither parent: we observed CNVs in more than one progeny clone that were not identified in either parent, located more frequently in the telomeric-subtelomeric regions of chromosomes and singleton de novo CNVs distributed evenly throughout the genome. Linkage analysis of CNVs revealed dynamic copy number fluctuations and suggested mechanisms that could have generated them. Five of 12 previously identified expression quantitative trait loci (eQTL) hotspots coincide with CNVs, demonstrating the potential for broad influence of CNV on the transcriptional program and phenotypic variation.

CONCLUSIONS

CNVs are a significant source of segregating and de novo genome variation involving hundreds of genes. Examination of progeny genome segments provides a framework to assess the extent and possible origins of CNVs. This segregating genetic system reveals the breadth, distribution and dynamics of CNVs in a surprisingly plastic parasite genome, providing a new perspective on the sources of diversity in parasite populations.

Blocking Plasmodium falciparum Malaria Transmission with Drugs: The Gametocytocidal and Sporontocidal Properties of Current and Prospective Antimalarials

Drugs that kill or inhibit the sexual stages of Plasmodium could potentially amplify or synergize the impact of other interventions by blocking transmission to mosquitoes. Primaquine and other 8-aminoquinolines have long offered such potential, but safety and other concerns have limited their use. Although transmission-blocking properties are not often a priority of drug discovery efforts, a number of interesting gametocytocidal and/or sporontocidal drug candidates have emerged in recent years. Some still bear significant technical and safety concerns, while others have passed clinical trials and are on the verge of entering the antimalarial armamentarium. Recent advances in our knowledge of gametocyte differentiation, gametogenesis and sporogony have also led to the identification of a large array of potential new targets for drugs that might interfere with malaria transmission. This review examines the properties of existing and prospective drugs, mechanisms of action, counter-indications and their potential role in regional malaria elimination efforts.

Comparative transcriptional and genomic analysis of Plasmodium falciparum field isolates

Mechanisms for differential regulation of gene expression may underlie much of the phenotypic variation and adaptability of malaria parasites. Here we describe transcriptional variation among culture-adapted field isolates of Plasmodium falciparum, the species responsible for most malarial disease. It was found that genes coding for parasite protein export into the red cell cytosol and onto its surface, and genes coding for sexual stage proteins involved in parasite transmission are up-regulated in field isolates compared with long-term laboratory isolates. Much of this variability was associated with the loss of small or large chromosomal segments, or other forms of gene copy number variation that are prevalent in the P. falciparum genome (copy number variants, CNVs). Expression levels of genes inside these segments were correlated to that of genes outside and adjacent to the segment boundaries, and this association declined with distance from the CNV boundary. This observation could not be explained by copy number variation in these adjacent genes. This suggests a local-acting regulatory role for CNVs in transcription of neighboring genes and helps explain the chromosomal clustering that we observed here. Transcriptional co-regulation of physical clusters of adaptive genes may provide a way for the parasite to readily adapt to its highly heterogeneous and strongly selective environment.

Plasmodium falciparum heterochromatin protein 1 marks genomic loci linked to phenotypic variation of exported virulence factors

Epigenetic processes are the main conductors of phenotypic variation in eukaryotes. The malaria parasite Plasmodium falciparum employs antigenic variation of the major surface antigen PfEMP1, encoded by 60 var genes, to evade acquired immune responses. Antigenic variation of PfEMP1 occurs through in situ switches in mono-allelic var gene transcription, which is PfSIR2-dependent and associated with the presence of repressive H3K9me3 marks at silenced loci. Here, we show that P. falciparum heterochromatin protein 1 (PfHP1) binds specifically to H3K9me3 but not to other repressive histone methyl marks. Based on nuclear fractionation and detailed immuno-localization assays, PfHP1 constitutes a major component of heterochromatin in perinuclear chromosome end clusters. High-resolution genome-wide chromatin immuno-precipitation demonstrates the striking association of PfHP1 with virulence gene arrays in subtelomeric and chromosome-internal islands and a high correlation with previously mapped H3K9me3 marks. These include not only var genes, but also the majority of P. falciparum lineage-specific gene families coding for exported proteins involved in host-parasite interactions. In addition, we identified a number of PfHP1-bound genes that were not enriched in H3K9me3, many of which code for proteins expressed during invasion or at different life cycle stages. Interestingly, PfHP1 is absent from centromeric regions, implying important differences in centromere biology between P. falciparum and its human host. Over-expression of PfHP1 results in an enhancement of variegated expression and highlights the presence of well-defined heterochromatic boundaries. In summary, we identify PfHP1 as a major effector of virulence gene silencing and phenotypic variation. Our results are instrumental for our understanding of this widely used survival strategy in unicellular pathogens.

The Plasmodium falciparum protein Pfg27 is dispensable for gametocyte and gamete production, but contributes to cell integrity during gametocytogenesis

In the human malaria parasite Plasmodium falciparum, gametocyte maturation is a process remarkably longer than in other malaria species, accompanied by expression of 2-300 sexual stage-specific proteins. Disruption of several of their encoding genes so far showed that only the abundant protein Pfg27, produced at the onset of sexual differentiation, is essential for gametocyte production. In contrast with what has been previously described, here we show that P. falciparum pfg27 disruptant lines are able to undergo all stages of gametocyte maturation, and are able to mature into gametes. A fraction of Pfg27-defective gametocytes show, however, distinct abnormalities in intra- and extra-cellular membranous compartments, such as accumulation of parasitophorous vacuole-derived vesicles in the erythrocyte cytoplasm, large intracellular vacuoles and discontinuities in their trilaminar cell membrane. This work revises current knowledge on the role of Pfg27, indicating that the protein is not required for parasite entry into sexual differentiation, and suggesting that it is instead involved in maintaining cell integrity in the uniquely long gametocytogenesis of P. falciparum.

Plasmodium yoelii: novel rhoptry proteins identified within the body of merozoite rhoptries in rodent Plasmodium malaria

The biogenesis, organization and function of the rhoptries are not well understood. Antisera were prepared to synthetic peptides prepared as multiple antigenic peptides (MAPs) obtained from a Plasmodium yoelii merozoite rhoptry proteome analysis. The antisera were used in immunofluorescence and immunoelectron microscopy of schizont-infected erythrocytes. Twenty-seven novel rhoptry proteins representing proteases, metabolic enzymes, secreted proteins and hypothetical proteins, were identified in the body of the rhoptries by immunoelectron microscopy. The merozoite rhoptries contain a heterogeneous mixture of proteins that may initiate host cell invasion and establish intracellular parasite development.

Plasmodium falciparum gametocytes: still many secrets of a hidden life

Sexual differentiation and parasite transmission are intimately linked in the life cycle of malaria parasites. The specialized cells providing this crucial link are the Plasmodium gametocytes. These are formed in the vertebrate host and are programmed to mature into gametes emerging from the erythrocytes in the midgut of a blood-feeding mosquito. The ensuing fusion into a zygote establishes parasite infection in the insect vector. Although key mechanisms of gametogenesis and fertilization are becoming progressively clear, the fundamental biology of gametocyte formation still presents open questions, some of which are specific to the human malaria parasite Plasmodium falciparum. Developmental commitment to sexual differentiation, regulation of stage-specific gene expression, the profound molecular and cellular changes accompanying gametocyte specialization, the requirement for tissue-specific sequestration in P. falciparum gametocytogenesis are proposed here as areas for future investigation. The epidemiological relevance of parasite transmission from humans to mosquito in the spread of malaria and of Plasmodium drug resistance genes indicates that understanding molecular mechanisms of gametocyte formation is highly relevant to design strategies able to interfere with the transmission of this disease.

Proteins of the malaria parasite sexual stages: expression, function and potential for transmission blocking strategies

The sexual phase of the malaria pathogen,Plasmodium falciparum, culminates in fertilization within the midgut of the mosquito and represents a crucial step in the completion of the parasite's life-cycle and transmission of the disease. Two decades ago, the first sexual stage-specific surface proteins were identified, among themPfs230,Pfs48/45, andPfs25, which were of scientific interest as candidates for the development of transmission blocking vaccines. A decade later, gene information gained from the sequencing of theP. falciparumgenome led to the identification of numerous additional sexual-stage proteins with antigenic properties and novel enzymes that putatively possess regulatory functions during sexual-stage development. This review aims to summarize the sexual-stage proteins identified to date, to compare their stage specificities and expression patterns and to highlight novel regulative mechanisms of sexual differentiation. The prospective candidacy of select sexual-stage proteins as targets for transmission blocking strategies will be discussed.

Protein export from Plasmodium parasites

Many prokaryotic and eukaryotic intracellular pathogens survive by altering the host cell through the export of proteins. In contrast to the well-studied prokaryotic export systems, knowledge of protein export in eukaryotic pathogens is scant. The recent discovery that a short protein sequence targets a protein for export from the malaria parasite Plasmodium falciparum has shed light on the possible mechanism of proteins export and has allowed the preliminary identification of several hundred exported proteins. Among the exported proteins are the members of the paralogous protein families, previously identified exported proteins and many uncharacterized proteins. The interaction of the parasite with the host cell is thus much more complex, and involves more parasite proteins, than previously thought.

The Plasmodium falciparum sexual development transcriptome: a microarray analysis using ontology-based pattern identification

The sexual stages of malarial parasites are essential for the mosquito transmission of the disease and therefore are the focus of transmission-blocking drug and vaccine development. In order to better understand genes important to the sexual development process, the transcriptomes of high-purity stage I-V Plasmodium falciparum gametocytes were comprehensively profiled using a full-genome high-density oligonucleotide microarray. The interpretation of this transcriptional data was aided by applying a novel knowledge-based data-mining algorithm termed ontology-based pattern identification (OPI) using current information regarding known sexual stage genes as a guide. This analysis resulted in the identification of a sexual development cluster containing 246 genes, of which approximately 75% were hypothetical, exhibiting highly-correlated, gametocyte-specific expression patterns. Inspection of the upstream promoter regions of these 246 genes revealed putative cis-regulatory elements for sexual development transcriptional control mechanisms. Furthermore, OPI analysis was extended using current annotations provided by the Gene Ontology Consortium to identify 380 statistically significant clusters containing genes with expression patterns characteristic of various biological processes, cellular components, and molecular functions. Collectively, these results, available as part of a web-accessible OPI database (http://carrier.gnf.org/publications/Gametocyte), shed light on the components of molecular mechanisms underlying parasite sexual development and other areas of malarial parasite biology.

Targeting Malaria Virulence and Remodeling Proteins to the Host Erythrocyte

To establish infection in the host, malaria parasites export remodeling and virulence proteins into the erythrocyte. These proteins can traverse a series of membranes, including the parasite membrane, the parasitophorous vacuole membrane, and the erythrocyte membrane. We show that a conserved pentameric sequence plays a central role in protein export into the host cell and predict the exported proteome in Plasmodium falciparum. We identified 400 putative erythrocyte-targeted proteins corresponding to approximately 8% of all predicted genes, with 225 virulence proteins and a further 160 proteins likely to be involved in remodeling of the host erythrocyte. The conservation of this signal across Plasmodium species has implications for the development of new antimalarials.

Chromosome healing through terminal deletions generated by de novo telomere additions in Saccharomyces cerevisiae

Broken chromosomes healed by de novo addition of a telomere are a major class of genome rearrangements seen in Saccharomyces cerevisiae and similar to rearrangements seen in human tumors. We have analyzed the sequences of 534 independent de novo telomere additions within a 12-kb region of chromosome V. The distribution of events mirrored that of four-base sequences consisting of the GG, GT, and TG dinucleotides, suggesting that de novo telomere additions occur at short regions of homology to the telomerase guide RNA. These chromosomal sequences restrict potential registrations of the added telomere sequence. The first 11 nucleotides of the addition sequences fell into common families that included 91% of the breakpoints. The observed registrations suggest that the 3' end of the TLC1 guide RNA is involved in annealing but not as a template for synthesis. Some families of added sequences can be accounted for by one cycle of annealing and extension, whereas others require a minimum of two. The same pattern emerges for sequences added onto the most common addition sequence, indicating that de novo telomeres are added and extended by the same process. Together, these data indicate that annealing is central to telomerase registration, which limits telomere heterogeneity and resolves the problem of synthesizing Rap1 binding sites by a nonprocessive telomerase with a low-complexity guide RNA sequence.

Identification of telomerase activity in gametocytes of Plasmodium falciparum

Telomerase, a specialized cellular reverse transcriptase, compensates the chromosome shortening during the replication of most eukaryotic cells and contributes to cellular immortalization in cell culture (in vitro) and cancerous cell (in vivo). In the present study, the telomerase activity in the gametocytes of Plasmodium falciparum was investigated. Here, we report for the first time, the presence of telomerase activity in the gametocytes of P. falciparum using P. falciparum telomere repeat amplification protocol (Pf-TRAP) assay and Southern blot hybridization. Telomerase inhibitors such as 7-deaza-dGTP and AZT-TP, when used with the cytoplasmic extract of gametocytes in the Pf-TRAP assay, efficiently inhibit the product, which confirms the presence of telomerase in the gametocytes. The presence of telomerase activity in the laboratory adapted local (eastern India) isolates of P. falciparum indicates that telomerase might be the major player in chromosomal end protection during replication. The finding suggests that telomerase can be a potent target for the transmission blocking vaccine and drugs for combating malaria caused by P. falciparum.

Genome sequence of the human malaria parasite Plasmodium falciparum

The parasite Plasmodium falciparum is responsible for hundreds of millions of cases of malaria, and kills more than one million African children annually. Here we report an analysis of the genome sequence of P. falciparum clone 3D7. The 23-megabase nuclear genome consists of 14 chromosomes, encodes about 5,300 genes, and is the most (A + T)-rich genome sequenced to date. Genes involved in antigenic variation are concentrated in the subtelomeric regions of the chromosomes. Compared to the genomes of free-living eukaryotic microbes, the genome of this intracellular parasite encodes fewer enzymes and transporters, but a large proportion of genes are devoted to immune evasion and host-parasite interactions. Many nuclear-encoded proteins are targeted to the apicoplast, an organelle involved in fatty-acid and isoprenoid metabolism. The genome sequence provides the foundation for future studies of this organism, and is being exploited in the search for new drugs and vaccines to fight malaria.

Male-specific expression of the paralog of malaria transmission-blocking target antigen Pfs230, PfB0400w

Malaria transmission requires that Plasmodium parasites circulating in the vertebrate host develop into male and female gametocytes, which are then taken up by a mosquito to undergo fertilization and further development into infectious sporozoites. To understand the malaria specific events involved in this process, the gene products involved require identification and characterization. This work demonstrates that antibodies generated against the paralog of malaria transmission-blocking antigen Pfs230, PfB0400w, react only with stage V male gametocytes, not gametes or asexual parasites. In contrast, Pfs230 is expressed on the surface of all gametocytes and remains associated with emerged gametes as one of the primary surface antigens for several hours. Consistent with the localization findings, a high molecular weight band is recognized by anti-PfB0400w antibodies on western blots of extracts of late stage gametocytes, not asexual parasites, early (stage II/III) gametocytes, or gametes. PfB0400w mRNA is also not observed in asexual parasites. The transcript levels peak in stage III/IV gametocytes, then sharply decline in gametes. This work identifies a novel male-specific protein with an expression pattern that is distinctly different than its paralog.

Repetitive sequences upstream of the pfg27/25 gene determine polymorphism in laboratory and natural lines of Plasmodium falciparum

The structure of the genomic region located upstream of the gametocyte-specific gene pfg27/25 of Plasmodium falciparum was analysed in laboratory lines and field isolates of the parasite. The gene is located in a subtelomeric region of chromosome 13 in parasite clones 3D7 and HB3. Analysis of laboratory lines and field isolates of P. falciparum indicated that polymorphism upstream of pfg27/25 is mainly due to the structure of a repetitive DNA region located at about half a kilobase from the pfg27/25 coding sequence. Different types of repetitive sequences are present in this region, whose copy number is variable in different parasite lines. In addition a GC-rich sequence element contained in this region, which is proposed to be the startpoint of pfg27/25 mRNA, presents either a direct or a reverse orientation in different parasite lines. Genomic deletions upstream of the pfg27/25 gene are also described in two laboratory lines of the parasite, which eliminate two newly identified malaria genes. orf P and orf Gap, from the genome of these parasites. One of them, orf Gap, deleted from the reference parasite clone 3D7, is abundantly expressed as mature mRNA in asexual parasites. PCR analysis on 64 field isolates of P. falciparum indicated that orf P and orf Gap sequences are present in all tested samples of naturally propagating parasites.

Targeted terminal deletions as a tool for functional genomics studies in Plasmodium

We describe a transfection system that induces terminal deletions at specific chromosome ends in malaria parasites using a linear construct containing telomeric repeats at one end and plasmodial sequences able to drive homologous recombination at the other. A site-specific deletion was generated at one extremity of chromosome 5 of Plasmodium berghei, which was stably maintained in the parasite population selected after transfection. The telomeric repeat array introduced with the construct reached the average length observed in natural telomeres of Plasmodium, indicating that in vivo telomere addition occurred at the newly formed extremity. The expression of a mutant dhfr/ts gene conferring pyrimethamine resistance, used as a selectable marker, was not affected by the proximity to the telomeric sequences, either in the presence or absence of drug pressure. In addition, no transcriptional silencing was observed on insertion of the mutant dhfr/ts gene either in subtelomeric or internal positions that are transcriptionally silent in blood-stage parasites. This suggests that the activity of its promoter is not affected by the chromatin organization of the chromosomal context.

Small, clonally variant antigens expressed on the surface of the Plasmodium falciparum-infected erythrocyte are encoded by the rif gene family and are the target of human immune responses

Disease severity in Plasmodium falciparum infections is a direct consequence of the parasite's efficient evasion of the defense mechanisms of the human host. To date, one parasite-derived molecule, the antigenically variant adhesin P. falciparum erythrocyte membrane protein 1 (PfEMP1), is known to be transported to the infected erythrocyte (pRBC) surface, where it mediates binding to different host receptors. Here we report that multiple additional proteins are expressed by the parasite at the pRBC surface, including a large cluster of clonally variant antigens of 30-45 kD. We have found these antigens to be identical to the rifins, predicted polypeptides encoded by the rif multigene family. These parasite products, formerly called rosettins after their identification in rosetting parasites, are prominently expressed by fresh isolates of P. falciparum. Rifins are immunogenic in natural infections and strain-specifically recognized by human immune sera in immunoprecipitation of surface-labeled pRBC extracts. Furthermore, human immune sera agglutinate pRBCs digested with trypsin at conditions such that radioiodinated PfEMP1 polypeptides are not detected but rifins are detected, suggesting the presence of epitopes in rifins targeted by agglutinating antibodies. When analyzed by two-dimensional electrophoresis, the rifins resolved into several isoforms in the pI range of 5.5-6.5, indicating molecular microheterogeneity, an additional potential novel source of antigenic diversity in P. falciparum. Prominent polypeptides of 20, 22, 76-80, 140, and 170 kD were also detected on the surfaces of pRBCs bearing in vitro-propagated or field-isolated parasites. In this report, we describe the rifins, the second family of clonally variant antigens known to be displayed by P. falciparum on the surface of the infected erythrocyte.

Plasmodium ookinete development in the mosquito midgut: a case of reciprocal manipulation

The ookinete is one of the most important stages of Plasmodium development in the mosquito. It is morphologically and biochemically distinct from the earlier sexual stages--gametocytes and zygote, and from the later stages--oocyst and sporozoites. Development to ookinete allows the parasite to escape from the tightly packed blood bolus, to cross the sturdy peritrophic matrix (PM), to be protected from the digestive environment of the midgut lumen, and to invade the gut epithelium. The success of each of these activities may depend on the degree of the biochemical and physical barriers in the mosquito (such as density of blood bolus, thickness of peritrophic matrix, proteolytic activities in the gut lumen etc.) and the ability of the ookinete to overcome these barriers. Ookinete motility, secretion of chitinase, resistance to the digestive enzymes, and recognition/invasion of the midgut epithelium all may play crucial roles in the transformation to oocyst. The overall sporogonic development of Plasmodium, therefore, depends on the results of the two-way manipulations between the parasite and the vector mosquito. Study of ookinete development and of the cellular and biochemical complexities of the mosquito gut may therefore lead to the design of novel strategies to block the transmission of malaria. This article reviews the intricate interactions between the parasite and the mosquito midgut in the context of development and transmission of Plasmodium parasites.

Characterisation of PfSec61, a Plasmodium falciparum homologue of a component of the translocation machinery at the endoplasmic reticulum membrane of eukaryotic cells

Plasmodium falciparum secretes several proteins that cause changes in the erythrocyte membrane enabling it to survive within red blood cells. Little is known of the mechanisms involved in the secretion and targeting of parasite polypeptides to the various cell compartments. The P. falciparum gene homologous to the mammalian Sec61alpha, gene, which encodes a component of the translocation pore in the endoplasmic reticulum of eukaryotic cells, was characterised to investigate the translocation process in the parasite. PfSec61 is present as a unique copy in the parasite genome and was mapped to chromosome 13. It encodes a 40 kDa polypeptide, as shown by immunoblotting and immunoprecipitation of [35S]methionine metabolically-labelled parasite extracts. The deduced amino acid sequence of PfSec61 is 87% similar to the mammalian polypeptide, and the two proteins give similar hydropathy plots. These results strongly suggest that PfSec61 has the same topological orientation and functional role as Sec61alpha. Anti-PfSec61 antibodies were used to investigate the cellular location and kinetics of expression of the polypeptide in the parasite. Immunofluorescence confocal microscopy showed that PfSec61 was located in the parasite cytoplasm, close to the nucleus, in a position consistent with its being in the endoplasmic reticulum.

Plasmodium falciparum telomerase: de novo telomere addition to telomeric and nontelomeric sequences and role in chromosome healing

Telomerase, a specialized cellular reverse transcriptase, compensates for chromosome shortening during the proliferation of most eucaryotic cells and contributes to cellular immortalization. The mechanism used by the single-celled protozoan malaria parasite Plasmodium falciparum to complete the replication of its linear chromosomes is currently unknown. In this study, telomerase activity has for the first time been identified in cell extracts of P. falciparum. The de novo synthesis of highly variable telomere repeats to the 3' end of DNA oligonucleotide primers by plasmodial telomerase is demonstrated. Permutated telomeric DNA primers are extended by the addition of the next correct base. In addition to elongating preexisting telomere sequences, P. falciparum telomerase can also add telomere repeats onto nontelomeric 3' ends. The sequence GGGTT was the predominant initial DNA sequence added to the nontelomeric 3' ends in vitro. Poly(C) at the 3' end of the oligonucleotide significantly alters the precision of the new telomerase added repeats. The efficiency of nontelomeric primer elongation was dependent on the presence of a G-rich cassette upstream of the 3' terminus. Oligonucleotide primers based on natural P. falciparum chromosome breakpoints are efficiently used as telomerase substrates. These results imply that P. falciparum telomerase contributes to chromosome maintenance and to de novo telomere formation on broken chromosomes. Reverse transcriptase inhibitors such as dideoxy GTP efficiently inhibit P. falciparum telomerase activity in vitro. These data point to malaria telomerase as a new target for the development of drugs that could induce parasite cell senescence.

Biology of Giant Proteins of Plasmodium: Resolution on Polyacrylamide-Agarose Composite Gels

The malaria parasite Plasmodium falciparum that infects humans encodes several extremely large proteins with molecular masses in the hundreds of thousands to megadalton range. Studies on the structure, function and antigenicity of these ;giant proteins' are hindered by the inability to resolve them effectively in conventional polyacrylamide gels. In this report, Jochen Wiesner, Denise Mattei, Artur Scherf and Michael Lanzer describe a convenient gel system, based on a composite polyacrylamide-agarose matrix, which facilitates analysis of giant proteins.

Transmission-blocking vaccines: uses and current status of development

Malaria continues to cause incomprehensible human suffering throughout most of the tropics and subtropics: in sub-Saharan Africa it is estimated that 2 million children die each year as a direct cause of infection with Plasmodium. Vector control and malaria chemotherapy that were previously effective in controlling and treating malaria are now largely ineffective due to insecticide-resistant mosquitoes and drug-resistant parasites. As alternatives to these mainstays of control, an intensive effort to develop subunit vaccines targeted at various stages of the life has been undertaken. One such vaccine, directed against the sexual and sporogonic stages and referred to as a transmission-blocking vaccine, offers the hope of controlling malaria in geographically isolated areas, preventing re-introduction of the parasite in malaria-free zones, blocking the spread of drug-resistant or vaccine escape mutants, and reducing exposure to "virulent" strains of parasites. A series of potential transmission-blocking vaccine candidates have identified and the genes encoding these surface proteins have now been isolated and sequenced. One such vaccine candidate, Pfs25, is now being tested in human Phase I safety and immunogenicity studies. Here the use and status of transmission-blocking vaccines are reviewed.

Plasmodium chabaudi: immunogenicity of a highly antigenic glutamate-rich protein

The immunogenicity of a 93-kDa Plasmodium chabaudi protein that contains glutamate-rich tandem repeats was investigated in this study. Immunoblotting with various monoclonal antibodies indicates that this 93-kDa protein is equivalent to a potential P. chabaudi RESA analogue. However, the sequence of the P. chabaudi protein does not exhibit any significant homology to Pf155/RESA. Antibodies against the 93-kDa protein appear early during P. chabaudi infection and reach high titers. The highest antibody titers are found when the parasitemia is descending, suggesting that this protein may play some role in immunity. Immunization of mice with the recombinant protein also results in high antibody titers, indicating that the protein is quite immunogenic. However, mice immunized with recombinant protein and challenged with P. chabaudi do not exhibit a delayed appearance of parasitemia, a reduced parasitemia, or a shortened duration of parasitemia. Glutamate-rich P. falciparum proteins such as Pf155/RESA, are being considered as vaccine candidates. The studies with P. chabaudi suggest that interpretation of serological data using glutamate-rich proteins should proceed with caution. The glutamate-rich repeats, although highly immunogenic, may not be important in host immunity against malaria. However, antibodies that appear late in the P. chabaudi infection do appear to play a role in anti-malarial immunity.

Compartmentalization of genes coding for immunodominant antigens to fragile chromosome ends leads to dispersed subtelomeric gene families and rapid gene evolution in Plasmodium falciparum

Recent studies on the chromosome structure of Plasmodium falciparum have led to two observations: chromosome breakage occurs frequently in subtelomeric regions and the genes coding for a number of immunodominant parasite proteins are located in these fragile chromosomal segments. Toward understanding the biological significance of these observations, we have been studying the variability of a number of these telomeric genes in parasite lines isolated in different regions of the world. In this report, we present evidence that the telomeric location of the resa and the gbp genes of P. falciparum has allowed their dispersion to other chromosomes and eventual alteration. In the first example it is shown that the resa gene has been dispersed to subtelomeric positions on chromosomes 1, 2, 11 and 14 in clinical isolates from West African patients, giving rise to new parasite genotypes and gene linkage groups. Cloning and molecular analysis of the newly detected resa-related sequences reveal that two of the members of the family have diverged from the ancestral copy on chromosome 1, while the third member on chromosome 14 is very homologous to the ancestral copy indicating that it arose from a recent translocation event. In the second example, we show that the gbp genes form a dispersed gene family that maps to at least three different chromosome extremities. The data suggest that the compartmentalization of P. falciparum antigen genes to the chromosome ends lead to gene families scattered on several chromosome extremities. We propose that the generation of segmental aneuploidy is a specific mechanism of genome adaptation of P. falciparum to its host environment. We present a model to explain the duplicative translocation of chromosome termini.

Chromosome healing: spontaneous and programmed de novo telomere formation by telomerase

Telomeres are protective caps for chromosome ends that are essential for genome stability. Broken chromosomes missing a telomere will not be maintained unless the chromosome is 'healed' with the formation of a new telomere. Chromosome healing can be a programmed event following developmentally regulated chromosome fragmentation, or it may occur spontaneously when a chromosome is accidentally broken. In this article we discuss the consequences of telomere loss and the possible mechanisms that the enzyme telomerase employs to form telomeres de novo on broken chromosome ends.

Expression of a Plasmodium gene introduced into subtelomeric regions of Plasmodium berghei chromosomes

Targeted integration of exogenous DNA into the genome of malaria parasites will allow their phenotype to be modulated by means of gene disruption or the stable expression of foreign and mutated genes. Described here is the site-specific integration through reciprocal exchange, and subsequent expression, of a selectable marker gene into the genome of the pathogenic, bloodstage forms of the rodent malaria parasite Plasmodium berghei. Stable integration of a single copy of the marker gene (retained for more than 70 generations in the absence of drug pressure) into a nontranscribed subtelomeric repeat array of different chromosomes was observed. Expression of the gene within the subtelomeres indicated that the previously recorded absence of transcription in these regions could be due to a corresponding absence of genes rather than active silencing mechanisms.

Detection of anti-Plasmodium falciparum antibodies directed against a repetitive peptide of the gametocyte antigen Pfs2400 in malaria patients in Brazil

Sera collected from 164 individuals who had clinical Plasmodium falciparum malaria and came from several areas of Brazil where malaria is endemic were tested for the presence of anti-gametocyte antibodies. Antibodies directed against P. falciparum gametocytes were detected, by IFAT, in the sera of 67.1% of these patients. The prevalence of these antibodies was significantly higher in patients who had undergone multiple attacks of malaria than in those who were experiencing their first attack at the time of serum collection. Although circulating gametocytes were detected in 22% of the patients at this time, there was no difference in the percentages of IFAT positivity between apparent gametocyte 'carriers' and 'non-carriers'. All sera were also tested by ELISA, using a dimer of the nonamer peptide [PEE(L/V)VEEV(I/V)]2, which represents a tandem consensus repeat of the P. falciparum gametocyte antigen, Pfs2400, a target of transmission-blocking antibodies. ELISA demonstrated that 32.9% of the patients had antibodies that reacted with this peptide. Positive ELISA reactions were significantly more frequent amongst the sera of patients who had had multiple malaria attacks than in those undergoing their first malaria episode; positivity was lower in the gametocyte 'carriers' than in their 'non-carriers'. These results demonstrate that anti-gametocyte antibodies, which have already been shown to have potential transmission-blocking activity, are naturally elicited in Brazilian patients, the highest rates of seropositivity occurring after multiple malaria attacks.