Invasion of red blood cells by malaria parasites

Association of the GNAS locus with severe malaria

Functional studies have demonstrated an interaction between the stimulatory G protein alpha subunit (G-alpha-s) and the malaria parasite at a cellular level. Obstruction of signal transduction via the erythrocyte G-alpha-s subunit reduced invasion by Plasmodium falciparum parasites. We sought to determine whether this signal pathway had an impact at the disease level by testing polymorphisms in the gene encoding G-alpha-s (GNAS) for association with severe malaria in a large multi-centre study encompassing family and case-control studies from The Gambia, Kenya and Malawi, and a case-control study from Ghana. We gained power to detect association using meta-analysis across the seven studies, with an overall sample size approximating 4,000 cases and 4,000 controls. Out of 12 SNPs investigated in the 19 kb GNAS region, four presented signals of association (P < 0.05) with severe malaria. The strongest single-locus association demonstrated an odds ratio of 1.13 (1.05-1.21), P = 0.001. Three of the loci presenting significant associations were clustered at the 5-prime end of the GNAS gene. Accordingly, haplotypes constructed from these loci demonstrated significant associations with severe malaria [OR = 0.88 (0.81-0.96), P = 0.005 and OR = 1.12 (1.03-1.20), P = 0.005]. The evidence presented here indicates that the influence of G-alpha-s on erythrocyte invasion efficacy may, indeed, alter individual susceptibility to disease.

Reticulocyte-binding protein homologue 5 - an essential adhesin involved in invasion of human erythrocytes by Plasmodium falciparum

Invasion of erythrocytes is a prerequisite in the life history of the malaria parasite. Members of the reticulocyte-binding homologue family (PfRh) have been implicated in the invasion process and in some cases have been shown to act as adhesins, binding to specific receptors on the erythrocyte surface. We have identified a further, putatively essential, PfRh family member in the most virulent human malaria Plasmodium falciparum, called PfRh5, which binds to an unknown class of glycosylated receptors on the erythrocyte surface. This protein is an atypical PfRh family member, being much smaller than others and lacking a transmembrane and cytosolic region at the C-terminus. This suggests it may be part of a functional protein complex. PfRh5 localises to the rhoptries in merozoites and follows the tight junction during the process of erythrocyte invasion. Furthermore, rabbit immune serum raised against a portion of the ecto-domain, inhibits parasite invasion in vitro. We hypothesise an essential role for the PfRh5 adhesin in erythrocyte selection and commitment to invasion. Given its small size, we believe PfRh5 may prove to be a valuable candidate for inclusion in a multi-component anti-malarial vaccine.

Irreversible effect of cysteine protease inhibitors on the release of malaria parasites from infected erythrocytes

By studying the inactivation of malaria parasite culture by cysteine protease inhibition using confocal microscopy of living cells and electron microscopy of high-pressure frozen and freeze-substituted cells, we report the precise step in the release of malaria parasites from erythrocytes that is likely regulated by cysteine proteases: the opening of the erythrocyte membrane, liberating parasites for the next round of infection. Inhibition of cysteine proteases within the last few minutes of cycle does not affect rupture of the parasitophorus vacuole but irreversibly blocks the subsequent rupture of the host cell membrane, locking in resident parasites, which die within a few hours of captivity. This irreversible inactivation of mature parasites inside host cells makes plasmodial cysteine proteases attractive targets for antimalarials, as parasite-specific cysteine protease inhibitors may significantly augment multi-target drug cocktails.

Interactions of the malaria parasite and its mammalian host

A hallmark of Plasmodium development inside its mammalian victim is the remarkable restriction to the host species. Adaptation to an intracellular life style in specific target cells is determined by multiple parasite-host interactions. The first line of crosstalk occurs during intradermal sporozoite injection by an Anopheles mosquito. The following expansion in the liver is highly efficient and leads to successful establishment of the parasite population. During the periodic waves of fevers and chills the parasite destroys and re-infects red blood cells. Recent advances in experimental genetics and imaging techniques begin to expose the complex interactions at the changing parasite-host interfaces. Understanding the cellular and molecular mechanisms of target cell recognition, nutrient acquisition, and hijacking of cellular and immune functions may ultimately explain the elaborate biology of a medically important single cell eukaryote.

Interaction of an atypical Plasmodium falciparum ETRAMP with human apolipoproteins

BACKGROUND

In order to establish a successful infection in the human host, the malaria parasite Plasmodium falciparum must establish interactions with a variety of human proteins on the surface of different cell types, as well as with proteins inside the host cells. To better understand this aspect of malaria pathogenesis, a study was conducted with the goal of identifying interactions between proteins of the parasite and those of its human host.

METHODS

A modified yeast two-hybrid methodology that preferentially selects protein fragments that can be expressed in yeast was used to conduct high-throughput screens with P. falciparum protein fragments against human liver and cerebellum libraries. The resulting dataset was analyzed to exclude interactions that are not likely to occur in the human host during infection.

RESULTS

An initial set of 2,200 interactions was curated to remove proteins that are unlikely to play a role in pathogenesis based on their annotation or localization, and proteins that behave promiscuously in the two-hybrid assay, resulting in a final dataset of 456 interactions. A cluster that implicates binding between P. falciparum PFE1590w/ETRAMP5, a putative parasitophorous vacuole membrane protein, and human apolipoproteins ApoA, ApoB and ApoE was selected for further analysis. Different isoforms of ApoE, which are associated with different outcomes of malaria infection, were shown to display differential interactions with PFE1590w.

CONCLUSION

A dataset of interactions between proteins of P. falciparum and those of its human host was generated. The preferential interaction of the P. falciparum PFE1590w protein with the human ApoE epsilon3 and ApoE epsilon4 isoforms, but not the ApoE epsilon2 isoform, supports the hypothesis that ApoE genotype affects risk of malaria infection. The dataset contains other interactions of potential relevance to disease that may identify possible vaccine candidates and drug targets.

Malaria parasite pre-erythrocytic stage infection: gliding and hiding

In malaria, the red blood cell-infectious form of the Plasmodium parasite causes illness and the possible death of infected hosts. The initial infection in the liver caused by the mosquito-borne sporozoite parasite stage, however, causes little pathology and no symptoms. Nevertheless, pre-erythrocytic parasite stages are attracting passionate research efforts not least because they are the most promising targets for malaria vaccine development. Here, we review how the infectious sporozoite makes its way to the liver and subsequently develops within hepatocytes. We discuss the factors, both parasite and host, involved in the interactions that occur during this "silent" phase of infection.

Morphology and kinetics of the three distinct phases of red blood cell invasion by Plasmodium falciparum merozoites

The invasion of red blood cells (RBCs) is an essential event in the life cycle of all malaria-causing Plasmodium parasites; however, there are major gaps in our knowledge of this process. Here, we use video microscopy to address the kinetics of RBC invasion in the human malaria parasite Plasmodium falciparum. Under in vitro conditions merozoites generally recognise new target RBCs within 1 min of their release from their host RBC. Parasite entry ensues and is complete on average 27.6s after primary contact. This period can be divided into two distinct phases. The first is an approximately 11s 'pre-invasion' phase that involves an often dramatic RBC deformation and recovery process. The second is the classical 'invasion' phase where the merozoite becomes internalised within the RBC in a approximately 17s period. After invasion, a third 'echinocytosis' phase commences when about 36 s after every successful invasion a dramatic dehydration-type morphology was adopted by the infected RBC. During this phase, the echinocytotic effect reached a peak over the next 23.4s, after which the infected RBC recovered over a 5-11 min period. By then the merozoite had assumed an amoeboid-like state and was apparently free in the cytoplasm. A comparison of our data with that of an earlier study of the distantly related primate parasite Plasmodium knowlesi indicated remarkable similarities, suggesting that the kinetics of invasion are conserved across the Plasmodium genus. This study provides a morphological and kinetic framework onto which the invasion-associated physiological and molecular events can be overlaid.

Host-cell invasion by malaria parasites: insights from Plasmodium and Toxoplasma

Recent years have seen tremendous progress in our understanding of malaria parasite molecular biology. To a large extent, this progress follows significant developments in genetic, molecular and chemical tools available to study the malaria parasites and related Apicomplexa, in particular Toxoplasma gondii. One area of major advancement has been in understanding parasite host-cell invasion, a process that utilizes several essential molecular mechanisms that are conserved across the different lifecycle stages. Here, we summarize some of the most recent experimental data that shed light on the events underlying preparation and execution of malaria parasite invasion and how these insights might relate to the development of new antimalarial drugs.

PfRH5: a novel reticulocyte-binding family homolog of plasmodium falciparum that binds to the erythrocyte, and an investigation of its receptor

Multiple interactions between parasite ligands and their receptors on the human erythrocyte are a condition of successful Plasmodium falciparum invasion. The identification and characterization of these receptors presents a major challenge in the effort to understand the mechanism of invasion and to develop the means to prevent it. We describe here a novel member of the reticulocyte-binding family homolog (RH) of P. falciparum, PfRH5, and show that it binds to a previously unrecognized receptor on the RBC. PfRH5 is expressed as a 63 kDa protein and localized at the apical end of the invasive merozoite. We have expressed a fragment of PfRH5 which contains the RBC-binding domain and exhibits the same pattern of interactions with the RBC as the parent protein. Attachment is inhibited if the target cells are exposed to high concentrations of trypsin, but not to lower concentrations or to chymotrypsin or neuraminidase. We have determined the affinity, copy number and apparent molecular mass of the receptor protein. Thus, we have shown that PfRH5 is a novel erythrocyte-binding ligand and the identification and partial characterization of the new RBC receptor may indicate the existence of an unrecognized P. falciparum invasion pathway.

Characterisation of the Plasmodium vivax Pv38 antigen

This study describes the identification and characterisation of Pv38, based on the available genomic sequence of Plasmodium vivax and previous studies done with its Plasmodium falciparum homologue: Pf38. Pv38 is a 355 amino acid long peptide encoded by a single exon gene, for which orthologous genes have been identified in other Plasmodium species by bioinformatic approaches. As for Pf38, Pv38 was found to contain a s48/45 domain which is usually found in proteins displayed on gametocytes surface. The association of Pv38 with detergent-resistant membranes (DRMs), its expression in mature blood stages of the parasite (mainly schizonts) and the detection of its recombinant protein by sera from Aotus monkeys previously exposed to the parasite, were here assessed to further characterise this new antigen.

Switching Plasmodium falciparum genes on and off for erythrocyte invasion

Culture-adapted lines of the malaria parasite Plasmodium falciparum use alternative pathways for the invasion of erythrocytes. The expression of parasite ligands that are involved in the different pathways varies among parasite lines. Recently, several studies have attempted to characterize the use of different invasion pathways and the expression of specific invasion ligands in field isolates, opening the way to understand how invasion occurs in natural infections. In this review, these findings are discussed in the context of the most recent data on invasion by culture-adapted parasites to describe the current understanding of how wild parasites invade, how the variant expression of invasion ligands relates to switching between alternative invasion pathways and why so many different pathways are needed.

Genetic analysis of the cytoplasmic domain of the PfRh2b merozoite invasion protein of Plasmodium falciparum

Apicomplexan parasites employ multiple adhesive ligands for recognition and entry into host cells. The Duffy binding-like (DBL) and the reticulocyte binding protein-like (RBL) families are central to the invasion of erythrocytes by the malaria parasite. These type-1 transmembrane proteins are composed of large ectodomains and small conserved cytoplasmic tail domains. The cytoplasmic tail domain of the micronemal DBL protein EBA-175 is required for a functional ligand-receptor interaction, but not for correct trafficking and localisation. Here we focus on the cytoplasmic tail domain of the rhoptry-localised Plasmodium falciparum RBL PfRh2b. We have identified a conserved sequence of six amino acids, enriched in acidic residues, in the cytoplasmic tail domains of RBL proteins from Plasmodium spp. Genetic analyses reveal that the entire cytoplasmic tail and the conserved motif within the cytoplasmic tail are indispensable for invasion P. falciparum. Site-directed mutagenesis of the conserved moiety reveals that changes in the order of the amino acids of the conserved moiety, but not the charge of the sequence, can be tolerated. Shuffling of the motif has no effect on either invasion phenotype or PfRh2b expression and trafficking. Although the PfRh2b gene can be readily disrupted, our results suggest that modification of the PfRh2b cytoplasmic tail results in strong dominant negative activity, highlighting important differences between the PfRh2b and EBA-175 invasion ligands.

Mixed allele malaria vaccines: host protection and within-host selection

Malaria parasites are frequently polymorphic at the antigenic targets of many candidate vaccines, presumably as a consequence of selection pressure from protective immune responses. Conventional wisdom is therefore that vaccines directed against a single variant could select for non-target variants, rendering the vaccine useless. Many people have argued that a solution is to develop vaccines containing the products of more than one variant of the target. However, we are unaware of any evidence that multi-allele vaccines better protect hosts against parasites or morbidity. Moreover, selection of antigen-variants is not the only evolution that could occur in response to vaccination. Increased virulence could also be favored if more aggressive strains are less well controlled by vaccine-induced immunity. Virulence and antigenic identity have been confounded in all studies so far, and so we do not know formally from any animal or human studies whether vaccine failure has been due to evasion of protective responses by variants at target epitopes, or whether vaccines are just less good at protecting against more aggressive strains. Using the rodent malaria model Plasmodium chabaudi and recombinant apical membrane antigen-1 (AMA-1), we tested whether a bi-allelic vaccine afforded greater protection from parasite infection and morbidity than did vaccination with the component alleles alone. We also tested the effect of mono- and bi-allelic vaccination on within-host selection of mixed P. chabaudi infections, and whether parasite virulence mediates pathogen titres in immunized hosts. We found that vaccination with the bi-allelic AMA-1 formulation did not afford the host greater protection from parasite infection or morbidity than did mono-allelic AMA-1 immunization. Mono-allelic immunization increased the frequency of heterologous clones in mixed clone infections. There was no evidence that any type of immunization regime favored virulence. A single AMA-1 variant is a component of candidate malaria vaccines current in human trials; our results suggest that adding extra AMA-1 alleles to these vaccines would not confer clinical benefits, but that that mono-allelic vaccines could alter AMA-1 allele frequencies in natural populations.

Receptor-based identification of an inhibitory peptide against blood stage malaria

Plasmodium falciparum uses multiple host receptors to attach and invade human erythrocytes. Glycophorins have been implicated as receptors for parasite invasion in human erythrocytes. Here, we screened a phage display cDNA library of P. falciparum (FCR3, a sialic acid-dependent strain) using purified glycophorins and erythrocytes as bait. Several phage clones were identified that bound to immobilized glycophorins and contained the same 74 bp insert encoding the 7-amino acids sequence ETTLKSF. A similar screen using intact human erythrocytes in solution identified additional phage clones containing the same 7-amino acids sequence. Using ELISA and immunofluorescence, direct binding of ETTLKSF peptide to glycophorins and erythrocytes was confirmed. Pull-down and protease treatment assays suggest that ETTLKSF peptide specifically interacts with glycophorin C. The synthetic ETTLKSF peptide partially blocks merozoite invasion in human erythrocytes. Further characterization of ETTLKSF peptide could lead to the development of a novel class of inhibitors against the blood stage malaria.

Erythrocyte binding protein PfRH5 polymorphisms determine species-specific pathways of Plasmodium falciparum invasion

Some human malaria Plasmodium falciparum parasites, but not others, also cause disease in Aotus monkeys. To identify the basis for this variation, we crossed two clones that differ in Aotus nancymaae virulence and mapped inherited traits of infectivity to erythrocyte invasion by linkage analysis. A major pathway of invasion was linked to polymorphisms in a putative erythrocyte binding protein, PfRH5, found in the apical region of merozoites. Polymorphisms of PfRH5 from the A. nancymaae-virulent parent transformed the nonvirulent parent to a virulent parasite. Conversely, replacements that removed these polymorphisms from PfRH5 converted a virulent progeny clone to a nonvirulent parasite. Further, a proteolytic fragment of PfRH5 from the infective parasites bound to A. nancymaae erythrocytes. Our results also suggest that PfRH5 is a parasite ligand for human infection, and that amino acid substitutions can cause its binding domain to recognize different human erythrocyte surface receptors.

Malaria: mechanisms of erythrocytic infection and pathological correlates of severe disease

Malaria is an ancient disease that continues to cause enormous human morbidity and mortality. The life cycle of the causative parasite involves multiple tissues in two distinct host organisms, mosquitoes and humans. However, all the clinical symptoms of malaria are a consequence of infection of human erythrocytes. An understanding of the basic mechanisms that govern parasite invasion, remodeling, growth, and reinvasion of erythrocytes and the complex events leading to tissue pathology may yield new diagnostics and treatments for malaria. This approach is revealing a more complete picture of the most serious syndrome associated with this infection-cerebral malaria. We focus on the most recent understanding of the molecular basis of infection, summarize our finding from an ongoing pediatric cerebral malaria autopsy study in Malawi, and integrate these insights to malarial pathology.

Antibodies targeting the PfRH1 binding domain inhibit invasion of Plasmodium falciparum merozoites

Invasion by the malaria merozoite depends on recognition of specific erythrocyte surface receptors by parasite ligands. Plasmodium falciparum uses multiple ligands, including at least two gene families, reticulocyte binding protein homologues (RBLs) and erythrocyte binding proteins/ligands (EBLs). The combination of different RBLs and EBLs expressed in a merozoite defines the invasion pathway utilized and could also play a role in parasite virulence. The binding regions of EBLs lie in a conserved cysteine-rich domain while the binding domain of RBL is still not well characterized. Here, we identify the erythrocyte binding region of the P. falciparum reticulocyte binding protein homologue 1 (PfRH1) and show that antibodies raised against the functional binding region efficiently inhibit invasion. In addition, we directly demonstrate that changes in the expression of RBLs can constitute an immune evasion mechanism of the malaria merozoite.

MSP1(19) miniproteins can serve as targets for invasion inhibitory antibodies in Plasmodium falciparum provided they contain the correct domains for cell surface trafficking

Antibodies from malaria-exposed individuals can agglutinate merozoites released from Plasmodium schizonts, thereby preventing them from invading new erythrocytes. Merozoite coat proteins attached to the plasma membrane are major targets for host antibodies and are therefore considered important malaria vaccine candidates. Prominent among these is the abundant glycosylphosphatidylinositol (GPI)-anchored merozoite surface protein 1 (MSP1) and particularly its C-terminal fragment (MSP1(19)) comprised of two epidermal growth factor (EGF)-like modules. In this paper, we revisit the role of agglutination and immunity using transgenic fluorescent marker proteins. We describe expression of heterologous MSP1(19)'miniproteins' on the surface of Plasmodium falciparum merozoites. To correctly express these proteins, we determined that GPI-anchoring and the presence of a signal sequence do not allow default export of proteins from the endoplasmic reticulum to merozoite surface and that extra sequence elements are required. The EGFs are insufficient for correct trafficking unless they are fused to additional residues that normally reside upstream of this fragment. Antibodies specifically targeting the surface-expressed miniprotein can inhibit erythrocyte invasion in vitro despite the presence of endogenous MSP1. Using a line expressing a green fluorescent protein-MSP1 fusion protein, we demonstrate that one mode of inhibition by antibodies targeting the MSP1(19) domain is the rapid agglutinating of merozoites prior to erythrocyte attachment.

Transcriptional responses of Plasmodium falciparum to alpha-difluoromethylornithine-induced polyamine depletion

Abstract Polyamines are essential polycationic molecules involved in multiple cellular events, including cell differentiation, division and death. Inhibition of polyamine biosynthesis has been considered in diverse therapeutic strategies ranging from tumour suppressors to anti-parasitic agents. In the human malaria parasite, Plasmodium falciparum, inhibition of ornithine decarboxylase (ODC) results in the arrest of schizogony due to polyamine depletion. However, the exact physiological role of the polyamines in the parasite is unknown. Here, we present results of the depletion of polyamines in the malaria parasite by alpha-difluoromethylornithine inhibition of ODC, as observed with differential transcriptome profiling. Upon depletion of their endogenous polyamines, the up- and downregulated parasite transcripts were selected with suppression subtractive hybridisation and differences were detected using blots or DNA microarrays. A direct linkage between polyamine depletion and the differential expression of two distinct transcripts was observed, indicating the existence of a transcriptional feedback response in the P. falciparum transcriptome upon drug challenge. The data presented provide input into the role of the polyamines in the cellular biology of P. falciparum and contribute towards the validation of polyamine biosynthesis as an antimalarial target.

Variation in use of erythrocyte invasion pathways by Plasmodium falciparum mediates evasion of human inhibitory antibodies

Antibodies that inhibit Plasmodium falciparum invasion of erythrocytes are believed to be an important component of immunity against malaria. During blood-stage infection, P. falciparum can use different pathways for erythrocyte invasion by varying the expression and/or utilization of members of 2 invasion ligand families: the erythrocyte-binding antigens (EBAs) and reticulocyte-binding homologs (PfRhs). Invasion pathways can be broadly classified into 2 groups based on the use of sialic acid (SA) on the erythrocyte surface by parasite ligands. We found that inhibitory antibodies are acquired by malaria-exposed Kenyan children and adults against ligands of SA-dependent and SA-independent invasion pathways, and the ability of antibodies to inhibit erythrocyte invasion depended on the pathway used by P. falciparum isolates. Differential inhibition of P. falciparum lines that varied in their use of specific EBA and PfRh proteins pointed to these ligand families as major targets of inhibitory antibodies. Antibodies against recombinant EBA and PfRh proteins were acquired in an age-associated manner, and inhibitory antibodies against EBA175 appeared prominent among some individuals. These findings suggest that variation in invasion phenotype might have evolved as a mechanism that facilitates immune evasion by P. falciparum and that a broad inhibitory response against multiple ligands may be required for effective immunity.

Identification and characterization of a novel Plasmodium falciparum merozoite apical protein involved in erythrocyte binding and invasion

Proteins that coat Plasmodium falciparum merozoite surface and those secreted from its apical secretory organelles are considered promising candidates for the vaccine against malaria. In the present study, we have identified an asparagine rich parasite protein (PfAARP; Gene ID PFD1105w), that harbors a predicted signal sequence, a C-terminal transmembrane region and whose transcription and translation patterns are similar to some well characterized merozoite surface/apical proteins. PfAARP was localized to the apical end of the merozoites by GFP-targeting approach using an inducible, schizont-stage expression system, by immunofluorescence assays using anti-PfAARP antibodies. Immuno-electron microsopic studies showed that PfAARP is localized in the apical ends of the rhoptries in the merozoites. RBC binding assays with PfAARP expressed on COS cells surface showed that it binds to RBCs through its N-terminal region with a receptor on the RBC surface that is sensitive to trypsin and neuraminidase treatments. Sequencing of PfAARP from different P. falciparum strains as well as field isolates showed that the N-terminal region is highly conserved. Recombinant protein corresponding to the N-terminal region of PfAARP (PfAARP-N) was produced in its functional form in E. coli. PfAARP-N showed reactivity with immune sera from individuals residing in P. falciparum endemic area. The anti-PfAARP-N rabbit antibodies significantly inhibited parasite invasion in vitro. Our data on localization, functional assays and invasion inhibition, suggest a role of PfAARP in erythrocyte binding and invasion by the merozoite.

Breadth and magnitude of antibody responses to multiple Plasmodium falciparum merozoite antigens are associated with protection from clinical malaria

Individuals living in areas where malaria is endemic are repeatedly exposed to many different malaria parasite antigens. Studies on naturally acquired antibody-mediated immunity to clinical malaria have largely focused on the presence of responses to individual antigens and their associations with decreased morbidity. We hypothesized that the breadth (number of important targets to which antibodies were made) and magnitude (antibody level measured in a random serum sample) of the antibody response were important predictors of protection from clinical malaria. We analyzed naturally acquired antibodies to five leading Plasmodium falciparum merozoite-stage vaccine candidate antigens, and schizont extract, in Kenyan children monitored for uncomplicated malaria for 6 months (n = 119). Serum antibody levels to apical membrane antigen 1 (AMA1) and merozoite surface protein antigens (MSP-1 block 2, MSP-2, and MSP-3) were inversely related to the probability of developing malaria, but levels to MSP-1(19) and erythrocyte binding antigen (EBA-175) were not. The risk of malaria was also inversely associated with increasing breadth of antibody specificities, with none of the children who simultaneously had high antibody levels to five or more antigens experiencing a clinical episode (17/119; 15%; P = 0.0006). Particular combinations of antibodies (AMA1, MSP-2, and MSP-3) were more strongly predictive of protection than others. The results were validated in a larger, separate case-control study whose end point was malaria severe enough to warrant hospital admission (n = 387). These findings suggest that under natural exposure, immunity to malaria may result from high titers antibodies to multiple antigenic targets and support the idea of testing combination blood-stage vaccines optimized to induce similar antibody profiles.

Plasmodium food vacuole plasmepsins are activated by falcipains

Intraerythrocytic malaria parasites use host hemoglobin as a major nutrient source. Aspartic proteases (plasmepsins) and cysteine proteases (falcipains) function in the early steps of the hemoglobin degradation pathway. There is extensive functional redundancy within and between these protease families. Plasmepsins are synthesized as integral membrane proenzymes that are activated by cleavage from the membrane. This cleavage is mediated by a maturase activity whose identity has been elusive. We have used a combination of cell biology, chemical biology, and enzymology approaches to analyze this processing event. These studies reveal that plasmepsin processing occurs primarily via the falcipains; however, if falcipain activity is blocked, autoprocessing can take place, serving as an alternate activation system. These results establish a further level of redundancy between the protease families involved in Plasmodium hemoglobin degradation.

Apicomplexa in mammalian cells: trafficking to the parasitophorous vacuole

Most Apicomplexa reside and multiply in the cytoplasm of their host cell, within a parasitophorous vacuole (PV) originating from both parasite and host cell components. Trafficking of parasite-encoded proteins destined to membrane compartments beyond the confine of the parasite plasma membrane is a process that offers a rich territory to explore novel mechanisms of protein-membrane interactions. Here, we focus on the PVs formed by the asexual stages of two pathogens of medical importance, Plasmodium and Toxoplasma. We compare the PVs of both parasites, with a particular emphasis on their evolutionary divergent compartmentalization within the host cell. We also discuss the existence of peculiar export mechanisms and/or sorting determinants that are potentially involved in the post-secretory targeting of parasite proteins to the PV subcompartments.

The exoneme helps malaria parasites to break out of blood cells

Malaria parasites must invade the erythrocytes of its host, to be able to grow and multiply. Having depleted the host cell of its nutrients, the parasites break out to invade new erythrocytes. In this issue of Cell, Yeoh et al. (2007) discover a new organelle, the exoneme, that contains a protease SUB1, which helps the parasite to escape from old erythrocytes and invade new ones.

Evidence-based annotation of the malaria parasite's genome using comparative expression profiling

A fundamental problem in systems biology and whole genome sequence analysis is how to infer functions for the many uncharacterized proteins that are identified, whether they are conserved across organisms of different phyla or are phylum-specific. This problem is especially acute in pathogens, such as malaria parasites, where genetic and biochemical investigations are likely to be more difficult. Here we perform comparative expression analysis on Plasmodium parasite life cycle data derived from P. falciparum blood, sporozoite, zygote and ookinete stages, and P. yoelii mosquito oocyst and salivary gland sporozoites, blood and liver stages and show that type II fatty acid biosynthesis genes are upregulated in liver and insect stages relative to asexual blood stages. We also show that some universally uncharacterized genes with orthologs in Plasmodium species, Saccharomyces cerevisiae and humans show coordinated transcription patterns in large collections of human and yeast expression data and that the function of the uncharacterized genes can sometimes be predicted based on the expression patterns across these diverse organisms. We also use a comprehensive and unbiased literature mining method to predict which uncharacterized parasite-specific genes are likely to have roles in processes such as gliding motility, host-cell interactions, sporozoite stage, or rhoptry function. These analyses, together with protein-protein interaction data, provide probabilistic models that predict the function of 926 uncharacterized malaria genes and also suggest that malaria parasites may provide a simple model system for the study of some human processes. These data also provide a foundation for further studies of transcriptional regulation in malaria parasites.

In silico discovery of transcription regulatory elements in Plasmodium falciparum

BACKGROUND

With the sequence of the Plasmodium falciparum genome and several global mRNA and protein life cycle expression profiling projects now completed, elucidating the underlying networks of transcriptional control important for the progression of the parasite life cycle is highly pertinent to the development of new anti-malarials. To date, relatively little is known regarding the specific mechanisms the parasite employs to regulate gene expression at the mRNA level, with studies of the P. falciparum genome sequence having revealed few cis-regulatory elements and associated transcription factors. Although it is possible the parasite may evoke mechanisms of transcriptional control drastically different from those used by other eukaryotic organisms, the extreme AT-rich nature of P. falciparum intergenic regions (approximately 90% AT) presents significant challenges to in silico cis-regulatory element discovery.

RESULTS

We have developed an algorithm called Gene Enrichment Motif Searching (GEMS) that uses a hypergeometric-based scoring function and a position-weight matrix optimization routine to identify with high-confidence regulatory elements in the nucleotide-biased and repeat sequence-rich P. falciparum genome. When applied to promoter regions of genes contained within 21 co-expression gene clusters generated from P. falciparum life cycle microarray data using the semi-supervised clustering algorithm Ontology-based Pattern Identification, GEMS identified 34 putative cis-regulatory elements associated with a variety of parasite processes including sexual development, cell invasion, antigenic variation and protein biosynthesis. Among these candidates were novel motifs, as well as many of the elements for which biological experimental evidence already exists in the Plasmodium literature. To provide evidence for the biological relevance of a cell invasion-related element predicted by GEMS, reporter gene and electrophoretic mobility shift assays were conducted.

CONCLUSION

This GEMS analysis demonstrates that in silico regulatory element discovery can be successfully applied to challenging repeat-sequence-rich, base-biased genomes such as that of P. falciparum. The fact that regulatory elements were predicted from a diverse range of functional gene clusters supports the hypothesis that cis-regulatory elements play a role in the transcriptional control of many P. falciparum biological processes. The putative regulatory elements described represent promising candidates for future biological investigation into the underlying transcriptional control mechanisms of gene regulation in malaria parasites.

Release of hepatic Plasmodium yoelii merozoites into the pulmonary microvasculature

Plasmodium undergoes one round of multiplication in the liver prior to invading erythrocytes and initiating the symptomatic blood phase of the malaria infection. Productive hepatocyte infection by sporozoites leads to the generation of thousands of merozoites capable of erythrocyte invasion. Merozoites are released from infected hepatocytes as merosomes, packets of hundreds of parasites surrounded by host cell membrane. Intravital microscopy of green fluorescent protein-expressing P. yoelii parasites showed that the majority of merosomes exit the liver intact, adapt a relatively uniform size of 12-18 microm, and contain 100-200 merozoites. Merosomes survived the subsequent passage through the right heart undamaged and accumulated in the lungs. Merosomes were absent from blood harvested from the left ventricle and from tail vein blood, indicating that the lungs effectively cleared the blood from all large parasite aggregates. Accordingly, merosomes were not detectable in major organs such as brain, kidney, and spleen. The failure of annexin V to label merosomes collected from hepatic effluent indicates that phosphatidylserine is not exposed on the surface of the merosome membrane suggesting the infected hepatocyte did not undergo apoptosis prior to merosome release. Merosomal merozoites continued to express green fluorescent protein and did not incorporate propidium iodide or YO-PRO-1 indicating parasite viability and an intact merosome membrane. Evidence of merosomal merozoite infectivity was provided by hepatic effluent containing merosomes being significantly more infective than blood with an identical low-level parasitemia. Ex vivo analysis showed that merosomes eventually disintegrate inside pulmonary capillaries, thus liberating merozoites into the bloodstream. We conclude that merosome packaging protects hepatic merozoites from phagocytic attack by sinusoidal Kupffer cells, and that release into the lung microvasculature enhances the chance of successful erythrocyte invasion. We believe this previously unknown part of the plasmodial life cycle ensures an effective transition from the liver to the blood phase of the malaria infection.

Characterization of two putative potassium channels in Plasmodium falciparum

Background

Potassium channels are essential for cell survival and participate in the regulation of cell membrane potential and electrochemical gradients. During its lifecycle, Plasmodium falciparum parasites must successfully traverse widely diverse environmental milieus, in which K⁺ channel function is likely to be critical. Dramatically differing conditions will be presented to the parasite in the mosquito mid-gut, red blood cell (RBC) cytosol and the human circulatory system.

Methods

In silico sequence analyses identified two open-reading frames in the P. falciparum genome that are predicted to encode for proteins with high homology to K⁺ channels. To further analyse these putative channels, specific antisera were generated and used in immunoblot and immunofluorescence analyses of P. falciparum-infected RBCs. Recombinant genome methods in cultured P. falciparum were used to create genetic knock outs of each K⁺ channel gene to asses the importance of their expression.

Results

Immunoblot and IFA analyses confirmed the expression of the two putative P. falciparum K⁺ channels (PfK1 and PfK2). PfK1 is expressed in all asexual stage parasites, predominantly in late stages and localizes to the RBC membrane. Conversely, PfK2 is predominantly expressed in late schizont and merozoite stage parasites and remains primarily localized to the parasite. Repeated attempts to knockout PfK1 and PfK2 expression by targeted gene disruption proved unsuccessful despite evidence of recombinant gene integration, indicating that pfk1 and pfk2 are apparently refractory to genetic disruption.

Conclusion

Putative K⁺ channel proteins PfK1 and PfK2 are expressed in cultured P. falciparum parasites with differing spatial and temporal patterns. Eventual functional characterization of these channels may reveal future pharmacological targets.

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

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

Plasmodium in the postgenomic era: new insights into the molecular cell biology of malaria parasites

In this review, we bring together some of the approaches toward understanding the cellular and molecular biology of Plasmodium species and their interaction with their host red blood cells. Considerable impetus has come from the development of new methods of molecular genetics and bioinformatics, and it is important to evaluate the wealth of these novel data in the context of basic cell biology. We describe how these approaches are gaining valuable insights into the parasite-host cell interaction, including (1) the multistep process of red blood cell invasion by the merozoite; (2) the mechanisms by which the intracellular parasite feeds on the red blood cell and exports parasite proteins to modify its cytoadherent properties; (3) the modulation of the cell cycle by sensing the environmental tryptophan-related molecules; (4) the mechanism used to survive in a low Ca(2+) concentration inside red blood cells; (5) the activation of signal transduction machinery and the regulation of intracellular calcium; (6) transfection technology; and (7) transcriptional regulation and genome-wide mRNA studies in Plasmodium falciparum.

Studies of Plasmodium falciparum rhoptry-associated membrane antigen (RAMA) protein peptides specifically binding to human RBC

Plasmodium falciparum rhoptry-associated membrane antigen (RAMA) peptides used in normal red blood cell (RBC) binding assays revealed that peptides 33426 (79NINILSSVHRKGRILYDSF97) and 33460 (777HKKREKSISPHSYQKVSTKVQ797) bound with high activity, presenting nanomolar affinity constants. Such high binding activity peptides (HABPs) displayed helicoid and random coil structures as determined by circular dichroism. HABPs inhibited P. falciparumin vitro invasion of normal RBC by up to 61% (depending on concentration), suggesting that some RAMA protein regions could be involved in P. falciparum invasion of RBC. The nature and localisation of receptors on RBC surface responsible for HABP binding were studied using enzyme-treated erythrocytes and structural analysis.

The twists and turns of Maurer's cleft trafficking in P. falciparum-infected erythrocytes

The malaria parasite, Plasmodium falciparum, invades the red blood cells (RBCs) of its human host and initiates a series of morphological rearrangements within the host cell cytoplasm. The mature RBC has no endogenous trafficking machinery; therefore, the parasite generates novel structures to mediate protein transport. These include compartments called the Maurer's clefts (MC), which play an important role in the trafficking of parasite proteins to the surface of the host cell. Recent electron tomography studies have revealed MC as convoluted flotillas of flattened discs that are tethered to the RBC membrane, prompting speculation that the MC could, in one respect, represent an extracellular equivalent of the Golgi apparatus. Visualization of both resident and cargo proteins has helped decipher the signals and routes for trafficking of parasite proteins to the MC and beyond.

Evidence for a common role for the serine-type Plasmodium falciparum serine repeat antigen proteases: implications for vaccine and drug design

Serine repeat antigens (SERAs) are a family of secreted "cysteine-like" proteases of Plasmodium parasites. Several SERAs possess an atypical active-site serine residue in place of the canonical cysteine. The human malaria parasite Plasmodium falciparum possesses six "serine-type" (SERA1 to SERA5 and SERA9) and three "cysteine-type" (SERA6 to SERA8) SERAs. Here, we investigate the importance of the serine-type SERAs to blood-stage parasite development and examine the extent of functional redundancy among this group. We attempted to knock out the four P. falciparum serine-type SERA genes that have not been disrupted previously. SERA1, SERA4, and SERA9 knockout lines were generated, while only SERA5, the most strongly expressed member of the SERA family, remained refractory to genetic deletion. Interestingly, we discovered that while SERA4-null parasites completed the blood-stage cycle normally, they exhibited a twofold increase in the level of SERA5 mRNA. The inability to disrupt SERA5 and the apparent compensatory increase in SERA5 expression in response to the deletion of SERA4 provides evidence for an important blood-stage function for the serine-type SERAs and supports the notion of functional redundancy among this group. Such redundancy is consistent with our phylogenetic analysis, which reveals a monophyletic grouping of the serine-type SERAs across the genus Plasmodium and a predominance of postspeciation expansion. While SERA5 is to some extent further validated as a target for vaccine and drug development, our data suggest that the expression level of other serine-type SERAs is the only barrier to escape from anti-SERA5-specific interventions.

Targeting the Plasmodium vivax Duffy-binding protein

Plasmodium vivax requires interaction with the Duffy antigen receptor for chemokines (DARC) to enable its invasion of human erythrocytes. Interaction with DARC is mediated by the P. vivax Duffy-binding protein (PvDBP) and is essential for junction formation, which is a key step in the invasion process. The receptor-binding domain of PvDBP maps to a conserved cysteine-rich region, referred to as region II (PvRII). Here, we review data on the interaction of PvRII with DARC and explore the potential of targeting this crucial receptor-ligand interaction to develop new intervention strategies against P. vivax.

A Presenilin-like protease associated with Plasmodium falciparum micronemes is involved in erythrocyte invasion

We describe identification of a Plasmodium falciparum microneme protease involved in RBC invasion. From the yeast two-hybrid screening of a P. falciparum cDNA library, we have identified a 47 kDa membrane protein that interacted with the 5ABC domain of human RBC band 3. This protein shared homology with a Presenilin-type aspartyl protease, the signal peptide peptidase (SPP). An antibody raised against a predicted exposed region of this protein reacted specifically to a single band of approximately 47 kDa in the P. falciparum protein extract. Immunofluorescence microscopy suggested that this protein co-localized with the microneme protein EBA-175 in schizonts, and immunoelectron microscopy established that it is primarily localized to micronemes in merozoites. Functional characterization of Plasmodium falciparum signal peptide peptidase (PfSPP), demonstrates that an antibody to PfSPP blocks RBC invasion by P. falciparumin vitro. Native and recombinant PfSPP bound directly to the 5ABC domain of band 3 in solution and the binding of PfSPP to RBCs was chymotrypsin-sensitive, but trypsin and neuraminidase-resistant. Together, these results suggest that host band 3 interacts with PfSPP during RBC invasion presumably following parasite microneme discharge. PfSPP is the first microneme-associated intramembrane aspartyl protease identified in the apicomplexan parasites that interacts with a major transmembrane receptor on host erythrocytes.

Diversity and evolution of the rhoph1/clag multigene family of Plasmodium falciparum

A complex of high-molecular-mass proteins (PfRhopH) of the human malaria parasite Plasmodium falciparum induces host protective immunity and therefore is a candidate for vaccine development. Understanding the level of polymorphism and the evolutionary processes is important for advancements in both vaccine design and knowledge of the evolution of cell invasion in this parasite. In the present study, we sequenced the entire open reading frames of seven genes encoding the proteins of the PfRhopH complex (rhoph2, rhoph3, and five rhoph1/clag gene paralogs). We found that four rhoph1/clag genes (clag2, 3.1, 3.2, and 8) were highly polymorphic. Amino acid substitutions and indels are predominantly clustered around amino acid positions 1000-1200 of these four rhoph1/clag genes. An excess of nonsynonymous substitutions over synonymous substitutions was detected for clag8 and 9, indicating positive selection. The McDonald-Kreitman test with a Plasmodium reichenowi orthologous sequence also supports positive selection on clag8. Based on the ratio of interspecific genetic distance to intraspecific distance, the time to the most recent common ancestor of the clag2 and 8 polymorphisms was estimated to be 1.89 and 0.87 million years ago, respectively, assuming divergence of P. falciparum and P. reichenowi 6 million years ago. In addition to a copy number polymorphism, gene conversion events were detected for the rhoph1/clag genes on chromosome 3, which likely play a role in increasing the diversity of each locus. Our results indicate that a high diversity of the PfRhopH1/Clag multigene family is maintained by diversifying selection forces over a considerably long period.

Recombinant Plasmodium falciparum reticulocyte homology protein 4 binds to erythrocytes and blocks invasion

Plasmodium falciparum invasion of human erythrocytes involves several parasite and erythrocyte receptors that enable parasite invasion by multiple redundant pathways. A key challenge to the development of effective vaccines that block parasite infection of erythrocytes is identifying the players in these pathways and determining their function. Invasion by the parasite clone, Dd2, requires sialic acid on the erythrocyte surface; Dd2/NM is a variant selected for its ability to invade neuraminidase-treated erythrocytes that lack sialic acid. The P. falciparum protein, reticulocyte homology 4 (PfRH4), is uniquely up-regulated in Dd2/NM compared with Dd2, suggesting that it may be a parasite receptor involved in invasion. The aim of the present study was to determine the role of PfRH4 in invasion of erythrocytes and to determine whether it is a target of antibody-mediated blockade and thus a vaccine candidate. We show that both native PfRH4 and a recombinant 30-kDa protein to a conserved region of PfRH4 (rRH4(30)) bind strongly to neuraminidase-treated erythrocytes. rRH4(30) blocks both the erythrocyte binding of the native PfRH4 and invasion of neuraminidase-treated erythrocytes by Dd2/NM. Taken together, these results indicate that PfRH4 is a parasite receptor involved in sialic acid-independent invasion of erythrocytes. Although antibodies to rRH4(30) block binding of the native protein to erythrocytes, these antibodies failed to block invasion. These findings suggest that, although PfRH4 is required for invasion of neuraminidase-treated erythrocytes by Dd2/NM, it is inaccessible for antibody-mediated inhibition of the invasion process.

Invasion of host cells by malaria parasites: a tale of two protein families

Malaria parasites are obligate intracellular parasites whose invasive stages select and invade the unique host cell in which they can develop with exquisite specificity and efficacy. Most studies aimed at elucidating the molecules and the mechanisms implicated in the selection and invasion processes have been conducted on the merozoite, the stage that invades erythrocytes to perpetuate the pathological cycles of parasite multiplication in the blood. Bioinformatic analysis has helped identify the members of two parasite protein families, the reticulocyte-binding protein homologues (RBL) and erythrocyte binding like (EBL), in recently sequenced genomes of different Plasmodium species. In this article we review data from classical studies and gene disruption experiments that are helping to illuminate the role of these proteins in the selection-invasion processes. The manner in which subsets of proteins from each of the families act in concert suggests a model to explain the ability of the parasites to use alternate pathways of invasion. Future perspectives and implications are discussed.

Epigenetic silencing of Plasmodium falciparum genes linked to erythrocyte invasion

The process of erythrocyte invasion by merozoites of Plasmodium falciparum involves multiple steps, including the formation of a moving junction between parasite and host cell, and it is characterised by the redundancy of many of the receptor–ligand interactions involved. Several parasite proteins that interact with erythrocyte receptors or participate in other steps of invasion are encoded by small subtelomerically located gene families of four to seven members. We report here that members of the eba, rhoph1/clag, acbp, and pfRh multigene families exist in either an active or a silenced state. In the case of two members of the rhoph1/clag family, clag3.1 and clag3.2, expression was mutually exclusive. Silencing was clonally transmitted and occurred in the absence of detectable DNA alterations, suggesting that it is epigenetic. This was demonstrated for eba-140. Our data demonstrate that variant or mutually exclusive expression and epigenetic silencing in Plasmodium are not unique to genes such as var, which encode proteins that are exported to the surface of the erythrocyte, but also occur for genes involved in host cell invasion. Clonal variant expression of invasion-related ligands increases the flexibility of the parasite to adapt to its human host.

Plasmodium falciparum is responsible for the most severe forms of human malaria. Invasion of host erythrocytes is an essential step of the complex life cycle of this parasite. There is redundancy in many of the interactions involved in this process, such that the parasite can use different sets of receptor–ligand interactions to invade. Here, we demonstrate that the parasite can turn off the expression of some of the proteins that mediate invasion of erythrocytes. Expression can be turned off without alterations in the genetic information of the parasite by using a mechanism known as epigenetic silencing. This is far more flexible than genetic changes, and permits fast, reversible adaptation. Turning on or off the expression of these proteins did not affect the capacity of the parasite to invade normal or modified red cells, which suggests that the variant expression of these genes may be used by the parasite to escape immune responses from the host. Parasite proteins that participate in erythrocyte invasion are important vaccine candidates. Determining which proteins can be turned off is important because vaccines based on single antigens of the parasite that can be turned off without affecting its growth would have little chance of inducing protective immunity.

Building the perfect parasite: cell division in apicomplexa

Apicomplexans are pathogens responsible for malaria, toxoplasmosis, and crytposporidiosis in humans, and a wide range of livestock diseases. These unicellular eukaryotes are stealthy invaders, sheltering from the immune response in the cells of their hosts, while at the same time tapping into these cells as source of nutrients. The complexity and beauty of the structures formed during their intracellular development have made apicomplexans the darling of electron microscopists. Dramatic technological progress over the last decade has transformed apicomplexans into respectable genetic model organisms. Extensive genomic resources are now available for many apicomplexan species. At the same time, parasite transfection has enabled researchers to test the function of specific genes through reverse and forward genetic approaches with increasing sophistication. Transfection also introduced the use of fluorescent reporters, opening the field to dynamic real time microscopic observation. Parasite cell biologists have used these tools to take a fresh look at a classic problem: how do apicomplexans build the perfect invasion machine, the zoite, and how is this process fine-tuned to fit the specific niche of each pathogen in this ancient and very diverse group? This work has unearthed a treasure trove of novel structures and mechanisms that are the focus of this review.