Erythrocyte entry by malarial parasites. A moving junction between erythrocyte and parasite

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.

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.

Opposing roles of murine duffy antigen receptor for chemokine and murine CXC chemokine receptor-2 receptors in murine melanoma tumor growth

The Duffy antigen receptor for chemokines (DARC) has been classified as a "silent" receptor, as it can bind CXC and CC chemokines to undergo ligand-induced receptor internalization, but is not coupled to trimeric G proteins required for the classic G protein-coupled receptor-mediated signaling. CXC chemokine receptor-2 (CXCR2) has been shown to play a major role in tumor angiogenesis. To test the hypothesis that these two chemokine receptors might play opposing roles in the growth of melanoma tumors, we developed a transgenic mouse model, where the preproendothelin promoter/enhancer (PPEP) is used to drive expression of either murine DARC (mDARC) or murine CXCR2 (mCXCR2) in endothelial cells. We show herein that the growth of melanoma tumor xenografts, established from s.c. injection of immortalized murine melanocytes overexpressing macrophage inflammatory protein-2, was inhibited or enhanced in the PPEP-mDARC and PPEP-mCXCR2 transgenic mice, respectively, compared with control mice. The early tumors formed in mDARC transgenic mice exhibited a significantly higher number of infiltrating leukocytes compared with either the control or mCXCR2 transgenic mice, suggesting a potential role for DARC expressed on endothelial cells in leukocyte migration. In addition, the tumor-associated angiogenesis in mDARC transgenic mice was reduced when compared with the control. Conversely, tumor angiogenesis was significantly increased in mCXCR2 transgenic mice. Results indicate that endothelial cell overexpression of mDARC increased leukocyte trafficking to the tumor, reduced the growth of blood vessels into the tumor, and reduced the growth rate of the tumor, whereas endothelial cell overexpression of mCXCR2 had the reverse effect on tumor angiogenesis and growth.

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.

Plasmodium yoelii: Experimental evidences for the conserved epitopes between mouse and human malaria parasite, Plasmodium falciparum

Bioinformatic analyses of gene homologues have revealed functionally conserved epitopes between human and rodent malaria parasites. Here, we present experimental evidence for the presence of functionally and antigenically conserved domains between Plasmodium falciparum and Plasmodium yoelii asexual blood-stages. Merozoite released soluble proteins (MRSPs) from both P. falciparum and P. yoelii bound to heterologous mouse or human red blood cells, respectively. The presence of conserved antigenic epitopes between the two species of parasites was evident by the inhibitory effect of antibodies, developed against P. yoelii in convalescent mice, on P. falciparum growth and merozoite reinvasion in vitro. Furthermore, mice immunized with P. falciparum MRSPs were protected from infection by a P. yoelii challenge. These data indicate that different species of Plasmodium contain antigenically conserved interspecies domains, which are immunogenic and, thus constitute a potential novel antigen source for vaccine development and testing using a mouse model.

Erythrocyte invasion: vocabulary and grammar of the Plasmodium rhoptry

Malaria is a dangerous infectious disease caused by obligate intracellular protozoan Plasmodium parasites. In the vertebrate host, erythrocyte recognition and establishment of a nascent parasitophorous vacuole are essential processes, and are largely achieved using molecules located in the microorganelles of the invasive-stage parasites. Recent proteome analyses of the phylogenetically related Toxoplasma parasite have provided protein catalogs for these microorganelles, which can now be used to identify orthologous proteins in the Plasmodium genome. Of importance is the formation of a complex between the proteins secreted from the rhoptry neck portion (RONs) and micronemes (AMA1), which localize at the moving junction during parasite invagination into the host cell. In this article I review the largely unexplored paradigm of the malaria merozoite rhoptry, focusing on the high molecular weight rhoptry protein complex (the RhopH complex), and speculate on its grammar during invasion.

Single-chain antibody fragment specific for Plasmodium vivax Duffy binding protein

Phage display of single-chain variable fragment (scFv) antibodies is a powerful tool for selecting important, useful, and specific human antibodies. We constructed a library from three patients infected with Plasmodium vivax. Panning on recombinant PvRII enriched a population of scFvs that recognized region II of the P. vivax Duffy binding protein (DBP). Three clones of scFvs that reacted with PvRII were selected, and their biological functions were analyzed. These scFvs inhibited erythrocyte binding to DBP. Clone SFDBII92 had the greatest affinity (dissociation constant = 3.62 x 10(-8) M) and the greatest inhibition activity (50% inhibitory concentration approximately 2.9 microg/ml) to DBP. Thus, we demonstrated that human neutralizing antibody could be made from malaria patients using phage display and that these neutralizing scFvs should prove valuable for developing both passive and active immunization strategies based on DBP.

Progress in the molecular biology of malaria and the immunology of nematode infections

Japan is one of a small number of countries to have successfully controlled or eliminated major parasitic diseases, including malaria, filariasis, schistosomiasis and enteric parasitoses. Of particular importance in this success was a close collaboration between primary research and public health efforts. Many Japanese researchers continue to study malaria, particularly the areas of genetics and immunology, and this should contribute to global parasite eradication strategies. Furthermore, studies of immunoregulation of nematode infection using the interleukin-18 pathway, most of which have been conducted in Japan, are helping to improve researchers' understanding of human immune mechanisms and host-parasite interactions.

Plasmodium yoelii: combinatorial expression of variants of the 235 kDa rhoptry antigen during infection

The 235 kDa rhoptry protein Py235 of Plasmodium yoelii, has been implicated in erythrocyte invasion by the merozoite forms of the parasite. Py235 is encoded by a large, highly polymorphic gene family, members of which appear to be differentially transcribed. However, it is not clear how many variants are expressed at the protein level during an infection cycle and whether or not these variants are expressed selectively or combinatorially. Certain monoclonal antibodies to Py235 have been shown to attenuate parasite virulence upon passive transfer into mice, suggesting that this antigen or its derivatives may be useful vaccine candidates. To provide a basis for this, we sought to identify those variants that are recognised by the host immune system, and to establish the pattern of expression of the antigen in mice during infection. Using Py235 monoclonal antibodies as probes, we isolated distinct antigenic variants from an expression library, suggesting that the antigen repertoire is potentially large and that different Py235 variants may be produced during infection. The implications of these observations are discussed with respect to the ability of a cloned parasite line to express distinct antigenic variants in vivo.

Two Plasmodium rhomboid proteases preferentially cleave different adhesins implicated in all invasive stages of malaria

Invasion of host cells by the malaria pathogen Plasmodium relies on parasite transmembrane adhesins that engage host-cell receptors. Adhesins must be released by cleavage before the parasite can enter the cell, but the processing enzymes have remained elusive. Recent work indicates that the Toxoplasma rhomboid intramembrane protease TgROM5 catalyzes this essential cleavage. However, Plasmodium does not encode a direct TgROM5 homolog. We examined processing of the 14 Plasmodium falciparum adhesins currently thought to be involved in invasion by both model and Plasmodium rhomboid proteases in a heterologous assay. While most adhesins contain aromatic transmembrane residues and could not be cleaved by nonparasite rhomboid proteins, including Drosophila Rhomboid-1, Plasmodium falciparum rhomboid protein (PfROM)4 (PFE0340c) was able to process these adhesins efficiently and displayed novel substrate specificity. Conversely, PfROM1 (PF11_0150) shared specificity with rhomboid proteases from other organisms and was the only PfROM able to cleave apical membrane antigen 1 (AMA1). PfROM 1 and/or 4 was thus able to cleave diverse adhesins including TRAP, CTRP, MTRAP, PFF0800c, EBA-175, BAEBL, JESEBL, MAEBL, AMA1, Rh1, Rh2a, Rh2b, and Rh4, but not PTRAMP, and cleavage relied on the adhesin transmembrane domains. Swapping transmembrane regions between BAEBL and AMA1 switched the relative preferences of PfROMs 1 and 4 for these two substrates. Our analysis indicates that PfROMs 1 and 4 function with different substrate specificities that together constitute the specificity of TgROM5 to cleave diverse adhesins. This is the first enzymatic analysis of Plasmodium rhomboid proteases and suggests an involvement of PfROMs in all invasive stages of the malaria lifecycle, in both the vertebrate host and the mosquito vector.

Malaria is a devastating global disease that afflicts over 10% of the world's population, claiming between 1 and 3 million lives annually. Invasion of host cells by the malaria parasite Plasmodium ultimately requires enzymes to release close contacts made between the parasite and host cell, but these enzymes have not been identified. Rhomboid enzymes were previously found to be involved in this process in the related pathogen Toxoplasma. The present work examined the activity of Plasmodium rhomboid enzymes, and revealed that two Plasmodium rhomboid enzymes can cleave most, if not all, of the proteins currently known to mediate contacts between the parasite and host-cell membranes during invasion. The two rhomboid enzymes had different specificities for the different target proteins, but together could process all of the proteins that the similar Toxoplasma rhomboid enzyme could process alone. This analysis suggests that rhomboid enzymes may be essential for the ability of the parasite to invade host cells through different pathways both in the human and mosquito hosts, and therefore offers a possible new therapeutic target to explore for treating or controlling the devastating effects of malaria.

Rhomboid proteins: conserved membrane proteases with divergent biological functions

The rhomboid gene was discovered in Drosophila, where it encodes a seven transmembrane protein that is the signal-generating component of epidermal growth factor (EGF) receptor signaling during development. Although metazoan developmental regulators are rarely conserved outside the animal kingdom, rhomboid proteins are conserved in all kingdoms of life, but the significance of this remains unclear. Recent biochemical reconstitution and high-resolution crystal structures have provided proof that rhomboid proteins function as novel intramembrane proteases, with a serine protease-like catalytic apparatus embedded within the membrane bilayer, buried in a hydrophilic cavity formed by a protein ring. A thorough consideration of all known examples of rhomboid function suggests that, despite biochemical similarity in mechanism and specificity, rhomboid proteins function in diverse processes including quorum sensing in bacteria, mitochondrial membrane fusion, apoptosis, and stem cell differentiation in eukaryotes; rhomboid proteins are also now starting to be linked to human disease, including early-onset blindness, diabetes, and parasitic diseases. Regulating cell signaling is at the heart of rhomboid protein function in many, but not all, of these processes. Further study of these novel enzymes promises to reveal the evolutionary path of rhomboid protein function, which could provide insights into the forces that drive the molecular evolution of regulatory mechanisms.

Alternative invasion pathways for Plasmodium berghei sporozoites

Invasion of hepatocytes by Plasmodium sporozoites is a prerequisite for establishment of a natural malaria infection. The molecular mechanisms underlying sporozoite invasion are largely unknown. We have previously reported that infection by Plasmodium falciparum and Plasmodium yoelii sporozoites depends on CD81 and cholesterol-dependent tetraspanin-enriched microdomains (TEMs) on the hepatocyte surface. Here we have analyzed the role of CD81 and TEMs during infection by sporozoites from the rodent parasite Plasmodium berghei. We found that depending on the host cell type, P. berghei sporozoites can use several distinct pathways for invasion. Infection of human HepG2, HuH7 and HeLa cells by P. berghei does not depend on CD81 or host membrane cholesterol, whereas both CD81 and cholesterol are required for infection of mouse hepatoma Hepa1-6 cells. In primary mouse hepatocytes, both CD81-dependent and -independent mechanisms participate in P. berghei infection and the relative contribution of the different pathways varies, depending on mouse genetic background. The existence of distinct invasion pathways may explain why P. berghei sporozoites are capable of infecting a wide range of host cell types in vitro. It could also provide a means for human parasites to escape immune responses and face polymorphisms of host receptors. This may have implications for the development of an anti-malarial vaccine targeting sporozoites.

Plasmodium falciparum malaria and ABO blood group: is there any relationship?

There is increasing evidence that Plasmodium falciparum malaria is influenced by ABO blood group but the extent of association between both is yet to be well defined. Studies that investigated association between P. falciparum malaria and ABO blood group were identified using MEDLINE search and were systematically reviewed. There were apparent discrepancies and contradictions in the studies as some reported significant association between both while others observed no significant association. This outcome may reflect the complex interaction between P. falciparum malaria and the host immune responses. However, findings from all studies reviewed suggested that individuals of blood group O are relatively resistant to severe disease caused by P. falciparum infection. It was established that parasitized erythrocytes form rosettes more readily with red blood cells (RBCs) of A, B, or AB groups than with blood group O and this parasite-triggered RBC rosette formation is associated with the severity of clinical disease and with the development of cerebral malaria. Differences in rosetting ability were based on the P. falciparum strain-specific preference of rosetting with non-O blood groups and not only a phenomenon of laboratory-propagated strains, but also exist in wild clinical isolates from all major malarious areas of the world.

Plasmodium falciparum AMA1 binds a rhoptry neck protein homologous to TgRON4, a component of the moving junction in Toxoplasma gondii

Plasmodium falciparum apical membrane antigen 1 (PfAMA1) coimmunoprecipitates with the Plasmodium homologue of TgRON4, a secreted rhoptry neck protein of Toxoplasma gondii that migrates at the moving junction in association with TgAMA1 during invasion. PfRON4 also originates in the rhoptry necks, suggesting that this unusual collaboration of micronemes and rhoptries is a conserved feature of Apicomplexa.

Accelerated denaturation of hemoglobin and the antimalarial action of chloroquine

To study the antimalarial action of chloroquine, normal mouse erythrocytes were used as surrogates for erythrocytoid bodies. These bodies form in the endosomes of intraerythrocytic malaria parasites as they feed on their host and consist of erythrocyte cytoplasm enclosed in a vestige of the erythrocyte membrane. In suspensions of normal erythrocytes or lysates (equivalent to 5 microl of erythrocytes per ml in each case), hemoglobin underwent denaturation when it was incubated at 38 degrees C in 150 mM sodium acetate (pH 5). It is reasonable to assume that the same phenomenon occurs in acidic endosomes. Addition of 100 microM chloroquine to the incubation mixture caused the rate of hemoglobin denaturation to double to 40 nanomoles per hour per ml of packed erythrocytes. This effect required the presence of erythrocyte stroma and was inhibited by reducing the temperature to 24 degrees C or increasing the pH to 6. We propose that the primary antimalarial action of chloroquine is to bind to ferriprotoporphyrin IX (FP) and remove it from oxidized hemoglobin, thus producing toxic FP-chloroquine complexes and an excess of denatured globin. Furthermore, we suggest that these substances inhibit endosomal maturation and thereby cause hemoglobin accumulation in immature endosomes and masking of the lipids needed for FP dimerization. The term "masking" is used to signify that unsaturated lipids are present in parasitized erythrocytes but are specifically unavailable to promote FP dimerization.

Host receptors in malaria merozoite invasion

The clinical manifestations of Plasmodium falciparum malaria are directly linked to the blood stage of the parasite life cycle. At the blood stage, the circulating merozoites invade erythrocytes via a specific invasion pathway often identified with its dependence or independence on sialic acid residues of the host receptor. The invasion process involves multiple receptor-ligand interactions that mediate a complex series of events in a period of approximately 1 min. Although the mechanism by which merozoites invade erythrocytes is not fully understood, recent advances have put a new perspective on the importance of developing a multivalent blood stage-malaria vaccine. In this review, we highlight the role of currently identified host invasion receptors in blood-stage malaria.

Plasmodium falciparum erythrocyte invasion: A conserved myosin associated complex

Host cell invasion by apicomplexan parasites is powered by an actin/myosin motor complex that has been most thoroughly described in Toxoplasma gondii tachyzoites. In T. gondii, two inner membrane complex (IMC) proteins, the peripheral protein TgGAP45 and the transmembrane protein TgGAP50, form a complex with the myosin, TgMyoA, and its light chain, TgMLC1. This complex, referred to as the glideosome, anchors the invasion motor to the IMC. We have identified and characterized orthologues of TgMLC1, TgGAP45 and TgGAP50 in blood-stages of the major human pathogen Plasmodium falciparum, supporting the idea that the same basic complex drives host cell invasion across the apicomplexan phylum. The P. falciparum glideosome proteins are transcribed, expressed and localized in a manner consistent with a role in erythrocyte invasion. Furthermore, PfMyoA interacts with PfMTIP through broadly conserved mechanisms described in other eukaryotes, and forms a complex with PfGAP45 and PfGAP50 in late schizonts and merozoites. P. falciparum is known to use multiple alternative invasion pathways to enter erythrocytes, hampering vaccine development efforts targeting erythrocyte invasion. Our data suggests that the same invasion motor underpins all alternative invasion pathways, making it an attractive target for the development of novel intervention strategies.

Plasmodium rhoptries: how things went pear-shaped

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

Immunogenicity and protective efficacy of recombinant vaccine based on the receptor-binding domain of the Plasmodium vivax Duffy binding protein in Aotus monkeys

Invasion of human erythrocytes by Plasmodium vivax requires interaction between Duffy binding protein (PvDBP) and the Duffy blood group antigen. The receptor-binding domain of PvDBP lies in a conserved N-terminal, cysteine-rich region, region II (PvRII). PvRII is a valuable malaria subunit vaccine candidate for asexual blood stages. We have evaluated in Aotus monkeys the immunogenicity and protective efficacy of recombinant PvRII formulated in Freund's and Montanide ISA720 adjuvants. Specific antibody titers were determined by an enzyme-linked immunosorbent assay after each of three doses of 50 microg of protein administered by the subcutaneous route. Immunization with PvRII formulated in Freund's adjuvant yielded higher antibody titers than immunization with the Montanide ISA720 formulation and offered partial protection. Although the Montanide ISA720 formulation was immunogenic, it did not provide any protection. Given the immunogenicity and partial protection observed, further studies are needed to optimize the PvRII vaccine formulation with adjuvants suitable for human use.

Effect of metalloprotease inhibitors on invasion of red blood cell by Plasmodium falciparum

For successful invasion, the malaria merozoite needs to attach to the red blood cell membrane, undergo reorientation, form a junction of the apical end with the host membrane, and internalize. Malaria proteases have been implicated in the invasion process, but their specific cellular functions remain unclear. To demonstrate the involvement of metalloprotease in the process of Plasmodium falciparum merozoite entry into host red blood cell, schizont-infected red blood cells and parasitophorous vacuolar membrane-enclosed merozoite structures were treated with 1,10-phenanthroline, a metal chelator, resulting in a reduction of invasion with IC50 value of 25 and 29 microM, respectively. Absence of an accumulation of schizont stages after treatment with 1,10-phenanthroline indicated that the inhibitory effect was not due to suppression of merozoite release from red blood cells, but on the invasion step. Although treatment with GM6001, a well-known inhibitor of the mammalian matrix and disintegrin metalloprotease family, was less effective, nevertheless this study points to the importance of metal-requiring protease in the process of invasion of host red blood cell by the malaria parasite.

Cellular and Molecular Mechanics of Gliding Locomotion in Eukaryotes

Gliding is a form of substrate-dependent cell locomotion exploited by a variety of disparate cell types. Cells may glide at rates well in excess of 1 microm/sec and do so without the gross distortion of cellular form typical of amoeboid crawling. In the absence of a discrete locomotory organelle, gliding depends upon an assemblage of molecules that links cytoplasmic motor proteins to the cell membrane and thence to the appropriate substrate. Gliding has been most thoroughly studied in the apicomplexan parasites, including Plasmodium and Toxoplasma, which employ a unique assortment of proteins dubbed the glideosome, at the heart of which is a class XIV myosin motor. Actin and myosin also drive the gliding locomotion of raphid diatoms (Bacillariophyceae) as well as the intriguing form of gliding displayed by the spindle-shaped cells of the primitive colonial protist Labyrinthula. Chlamydomonas and other flagellated protists are also able to abandon their more familiar swimming locomotion for gliding, during which time they recruit a motility apparatus independent of that driving flagellar beating.

The rhoptry neck protein RON4 relocalizes at the moving junction during Toxoplasma gondii invasion

Host cell invasion in the Apicomplexa is unique in its dependency on a parasite actin-driven machinery and in the exclusion of most host cell membrane proteins during parasitophorous vacuole (PV) formation. This exclusion occurs at a junction between host cell and parasite plasma membranes that has been called the moving junction, a circumferential zone which forms at the apical tip of the parasite, moves backward and eventually pinches the PV from the host cell membrane. Despite having been described by electron microscopic studies 30 years ago, the molecular nature of this singular structure is still enigmatic. We have obtained a monoclonal antibody that recognizes the moving junction of invading tachyzoites of Toxoplasma gondii, in a pattern clearly distinct from those described so far for microneme and rhoptry proteins. The protein recognized by this antibody has been affinity purified. Mass spectrometry analysis showed that it is a rhoptry neck protein (RON4), a hypothetical protein with homologues restricted to Apicomplexa. Our findings reveals for the first time the participation of rhoptry neck proteins in moving junction formation and strongly suggest the conservation of this structure at the molecular level among Apicomplexa.

Molecular identification of a malaria merozoite surface sheddase

Proteolytic shedding of surface proteins during invasion by apicomplexan parasites is a widespread phenomenon, thought to represent a mechanism by which the parasites disengage adhesin-receptor complexes in order to gain entry into their host cell. Erythrocyte invasion by merozoites of the malaria parasite Plasmodium falciparum requires the shedding of ectodomain components of two essential surface proteins, called MSP1 and AMA1. Both are released by the same merozoite surface "sheddase," but the molecular identity and mode of action of this protease is unknown. Here we identify it as PfSUB2, an integral membrane subtilisin-like protease (subtilase). We show that PfSUB2 is stored in apical secretory organelles called micronemes. Upon merozoite release it is secreted onto the parasite surface and translocates to its posterior pole in an actin-dependent manner, a trafficking pattern predicted of the sheddase. Subtilase propeptides are usually selective inhibitors of their cognate protease, and the PfSUB2 propeptide is no exception; we show that recombinant PfSUB2 propeptide binds specifically to mature parasite-derived PfSUB2 and is a potent, selective inhibitor of MSP1 and AMA1 shedding, directly establishing PfSUB2 as the sheddase. PfSUB2 is a new potential target for drugs designed to prevent erythrocyte invasion by the malaria parasite.

Identification and characterisation of the Plasmodium vivax rhoptry-associated protein 2

Plasmodium vivax is currently the most widespread of the four parasite species causing malaria in humans around the world. It causes more than 75 million clinical episodes per year, mainly on the Asian and American continents. Identifying new antigens to be further tested as anti-P. vivax vaccine candidates has been greatly hampered by the difficulty of maintaining this parasite cultured in vitro. Taking into account that one of the most promising vaccine candidates against Plasmodium falciparum is the rhoptry-associated protein 2, we have identified the P. falciparum rhoptry-associated protein 2 homologue in P. vivax in the present study. This protein has 400 residues, having an N-terminal 21 amino-acid stretch compatible with a signal peptide and, as occurs with its falciparum homologue, it lacks repeat sequences. The protein is expressed in asexual stage P. vivax parasites and polyclonal sera raised against this protein recognised a 46 kDa band in parasite lysate in a Western blot assay.

Cellular adhesive phenomena in apicomplexan parasites of red blood cells

The apicomplexan parasites Babesia and Plasmodium are related, yet phylogenetically distinct haemoprotozoa that infect red blood cells and cause severe diseases of major human and veterinary importance. A variety of cellular and molecular interactions are pivotal in many aspects of the pathogenicity of these two parasites. Comparison of the cellular and molecular mechanisms that culminate in accumulation of parasitised red blood cells in the microvasculature of cattle infected with Babesia bovis (babesiosis) and humans infected with Plasmodium falciparum (falciparum malaria) is particularly instructive given the striking similarities in the pathophysiology of these two important medical and veterinary diseases. While such adhesive phenomena have been studied extensively in malaria, they have received relatively little attention in babesiosis. In this review, we summarise the findings of more than 25 years of research into cellular adhesive phenomena in malaria and speculate on how this body of work can now be applied to Babesia parasites. Such information is fundamental if we are to learn more about the biology of Babesia parasites, the cellular and molecular mechanisms by which they cause infection and disease and how to develop novel therapeutic strategies or vaccines for both Babesia and malaria infections.

Identification of the moving junction complex of Toxoplasma gondii: a collaboration between distinct secretory organelles

Apicomplexan parasites, including Toxoplasma gondii and Plasmodium sp., are obligate intracellular protozoa. They enter into a host cell by attaching to and then creating an invagination in the host cell plasma membrane. Contact between parasite and host plasma membranes occurs in the form of a ring-shaped moving junction that begins at the anterior end of the parasite and then migrates posteriorly. The resulting invagination of host plasma membrane creates a parasitophorous vacuole that completely envelops the now intracellular parasite. At the start of this process, apical membrane antigen 1 (AMA1) is released onto the parasite surface from specialized secretory organelles called micronemes. The T. gondii version of this protein, TgAMA1, has been shown to be essential for invasion but its exact role has not previously been determined. We identify here a trio of proteins that associate with TgAMA1, at least one of which associates with TgAMA1 at the moving junction. Surprisingly, these new proteins derive not from micronemes, but from the anterior secretory organelles known as rhoptries and specifically, for at least two, from the neck portion of these club-shaped structures. Homologues for these AMA1-associated proteins are found throughout the Apicomplexa strongly suggesting that this moving junction apparatus is a conserved feature of this important class of parasites. Differences between the contributing proteins in different species may, in part, be the result of selective pressure from the different niches occupied by these parasites.

The role of malaria merozoite proteases in red blood cell invasion

Invasion of red blood cells by the malaria merozoite is an essential step in the life cycle of this obligate intracellular pathogen. The molecular details of invasion are only recently becoming understood, largely through studies in related apicomplexan parasites such as Toxoplasma. Protease activity is required for successful invasion to disengage interactions between parasite adhesins and host cell receptors. Shedding of at least two essential surface proteins from the merozoite is thought to occur continuously during invasion as the parasite moves into the nascent parasitophorous vacuole. This shedding is performed by way of juxtamembrane cleavage and is mediated by a sheddase, which probably belongs to the subtilisin-like superfamily. Recent revelations have shown that transmembrane adhesins that are secreted onto the Toxoplasma tachyzoite surface and capped to its posterior pole are shed by way of cleavage within their transmembrane domains. A family of intramembrane serine proteases called rhomboids have now been identified within Apicomplexa, and one Toxoplasma rhomboid has been localized to the posterior end of the parasite. This supports their role in capping proteolysis. Proteases involved in invasion constitute potential targets for the development of new protease inhibitor-based drugs.

DISTRIBUTION OF CRYPTOSPORIDIUM PARVUM SPOROZOITE APICAL ORGANELLES DURING ATTACHMENT TO AND INTERNALIZATION BY CULTURED BILIARY EPITHELIAL CELLS

Although accumulating evidence supports an active role for host cells during Cryptosporidium parvum invasion of epithelia, our knowledge of the underlying parasite-specific processes triggering such events is limited. In an effort to better understand the invasion strategy of C. parvum, we characterized the presence and distribution of the apical organelles (micronemes, dense granules, and rhoptry) through the stages of attachment to, and internalization by, human biliary epithelia, using serial-section electron microscopy. Novel findings include an apparent organized rearrangement of micronemes upon host cell attachment. The apically segregated micronemes were apposed to a central microtubule-like filamentous structure, and the more distal micronemes localized to the periphery and apical region of the parasite during internalization, coinciding with the formation of the anterior vacuole. The morphological observations presented here extend our understanding of parasite-specific processes that occur during attachment to, and internalization by, host epithelial cells.

Formation of a parasitophorous vacuole in a nonadequate experimental host: electron microscopical and X-ray microanalytical study

An unusual mechanism of formation of a parasitophorous vacuole as a result of interaction between an invasive stage of a parasite (merozoites of a protozoon, Mattesia dispora) and defense response of an insect host, Galleria mellonella is reported. The entire ontogenesis of parasitophorous vacuole can be divided into five morphologically clearly discernible stages. They differed, e.g., in the contents and distribution of elements at subcellular level, as determined by direct in situ elemental analysis of single organelles (electron microprobe X-ray analysis). The method was used in conjunction with electron microscopy to investigate the relationship between the host and the parasite.

Reticulocyte-binding protein homologue 1 is required for sialic acid-dependent invasion into human erythrocytes by Plasmodium falciparum

The Apicomplexan parasite responsible for the most virulent form of malaria, Plasmodium falciparum, invades human erythrocytes through multiple ligand-receptor interactions. Some strains of P. falciparum are sensitive to neuraminidase treatment of the host erythrocyte and these parasites have been termed sialic acid-dependent as they utilize receptors containing sialic acid. In contrast, other strains can efficiently invade neuraminidase-treated erythrocytes and hence are sialic acid-independent. The molecular interactions that allow P. falciparum to differentially utilize receptors for merozoite invasion are not understood. The P. falciparum reticulocyte-binding protein homologue (PfRh or PfRBL) family have been implicated in the invasion process but their exact role is unknown. PfRh1, a member of this protein family, appears to be expressed in all parasite lines analysed but there are marked differences in the level of expression between different strains. We have used targeted gene disruption of the PfRh1 gene in P. falciparum to show that the encoded protein is required for sialic acid-dependent invasion of human erythrocytes. The DeltaPfRh1 parasites are able to invade normally; however, they utilize a pattern of ligand-receptor interactions that are more neuraminidase-resistant. Current data suggest a strategy based on the differential function of specific PfRh proteins has evolved to allow P. falciparum parasites to utilize alternative receptors on the erythrocyte surface for evasion of receptor polymorphisms and the host immune system.

Receptors, rafts, and microvesicles in thrombosis and inflammation

Hemostasis at sites of blood vessel injury and its pathologic counterpart, thrombosis, involve a complex interplay between several blood elements: soluble proteins of the blood coagulation system, blood cells (most prominently platelets) and cell fragments, and elements of the vessel wall (endothelial cells and, at sites of injury, the exposed matrix and deeper cellular components). In this review, we focus on ways in which specialized membrane microdomains known as lipid rafts are involved in various phases of hemostasis and thrombosis.

Red cell deformability and invasion by malaria parasites

Malaria parasites enter red cells in a multi-step process involving attachment, membrane deformation, invagination and encapsulation. The molecular basis of red cell rigidity is examined by Geoff Pasvol and lain Wilson, and they discuss its effect on the efficiency of invasion by various Plasmodium spp.

Targeted deletion ofPlasmodium knowlesiDuffy binding protein confirms its role in junction formation during invasion

Red cell invasion by Plasmodium merozoites involves multiple steps such as attachment, apical reorientation, junction formation and entry into a parasitophorous vacuole. These steps are mediated by specific molecular interactions. P. vivax and the simian parasite P. knowlesi require interaction with the Duffy blood group antigen to invade human erythrocytes. P. vivax and P. knowlesi Duffy binding proteins (PvDBP and PkDBP), which bind the Duffy antigen during invasion, share regions of sequence homology and belong to a family of erythrocyte binding proteins (EBPs). By deletion of the gene that encodes PkDBP, we demonstrate that interaction of PkDBP with the Duffy antigen is absolutely necessary for invasion of human erythrocytes by P. knowlesi. Electron microscopy studies reveal that PkDBP knockout parasites are unable to form a junction with human erythrocytes. The interaction of PkDBP with the Duffy antigen is thus necessary for the critical step of junction formation during invasion. These studies provide support for development of intervention strategies that target EBPs to inhibit junction formation and block erythrocyte invasion by malaria parasites.

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

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

Protein transport and trafficking inPlasmodium falciparum-infected erythrocytes

The human malarial parasitePlasmodium falciparumextensively modifies its host erythrocyte, and to this end, is faced with an interesting challenge. It must not only sort proteins to common organelles such as endoplasmic reticulum, Golgi and mitochondria, but also target proteins across the ‘extracellular’ cytosol of its host cell. Furthermore, as a member of the phylum Apicomplexa, the parasite has to sort proteins to novel organelles such as the apicoplast, micronemes and rhoptries. In order to overcome these difficulties, the parasite has created a novel secretory system, which has been characterized in ever-increasing detail in the past decade. Along with the ‘hardware’ for a secretory system, the parasite also needs to ‘program’ proteins to enable high fidelity sorting to their correct subcellular location. The nature of these sorting signals has remained until relatively recently, enigmatic. Experimental work has now begun to dissect the sorting signals responsible for correct subcellular targeting of parasite-encoded proteins. In this review we summarize the current understanding of such signals, and comment on their role in protein sorting in this organism, which may become a model for the study of novel protein trafficking mechanisms.

Insect-malaria parasites interactions: the salivary gland

Mosquito salivary glands are organs specialized in the production of a complex mix of molecules that digest carbohydrates from plant nectars, and facilitate blood feeding by the lubrication of mouthparts and the inhibition of homeostasis. Malaria sporozoites invade salivary glands and are injected with the saliva into vertebrate hosts during blood feeding. Sporozoites utilize molecules on their surface coat and outer pellicle membrane to adhere and invade specific regions of the salivary gland lobes. They transverse the secretory cells and are stored in the salivary duct, where transcription of new genes prepares them for vertebrate host invasion. Although it is probably that specific carbohydrate molecules on the surface of salivary glands function as parasites receptors, these have not been identified, neither other molecules nor mechanisms used by the parasite to invade, survive and mature within these organs. The recent advances in the sequence of the genomes of Anopheles gambiae and Plasmodium falciparum, and new developments in genomics and proteomics may help to elucidate the participating molecules, their regulation and interactions.

MAEBL Plasmodium falciparum protein peptides bind specifically to erythrocytes and inhibit in vitro merozoite invasion

MAEBL is an erythrocyte binding protein located in the rhoptries and on the surface of mature merozoites, being expressed at the beginning of schizogony. The structure of MAEBL originally isolated from rodent malaria parasites suggested a molecule likely to be involved in invasion. We thus became interested in identifying possible MAEBL functional regions. Synthetic peptides spanning the MAEBL sequence were tested in erythrocyte binding assays to identify such possible MAEBL functional regions. Nine high activity binding peptides (HABPs) were identified: two were found in the M1 domain, one was found between the M1 and M2 regions, five in the erythrocyte binding domain (M2), and one in the protein's repeat region. The results showed that peptide binding was saturable; some HABPs inhibited in vitro merozoite invasion and specifically bound to a 33kDa protein on red blood cell membrane. HABPs' possible function in merozoite invasion of erythrocytes is also discussed.

Ferriprotoporphyrin IX, phospholipids, and the antimalarial actions of quinoline drugs

Two subclasses of quinoline antimalarial drugs are used clinically. Both act on the endolysosomal system of malaria parasites, but in different ways. Treatment with 4-aminoquinoline drugs, such as chloroquine, causes morphologic changes and hemoglobin accumulation in endocytic vesicles. Treatment with quinoline-4-methanol drugs, such as quinine and mefloquine, also causes morphologic changes, but does not cause hemoglobin accumulation. In addition, chloroquine causes undimerized ferriprotoporphyrin IX (ferric heme) to accumulate whereas quinine and mefloquine do not. On the contrary, treatment with quinine or mefloquine prevents and reverses chloroquine-induced accumulation of hemoglobin and undimerized ferriprotoporphyrin IX. This difference is of particular interest since there is convincing evidence that undimerized ferriprotoporphyrin IX in malaria parasites would interact with and serve as a target for chloroquine. According to the ferriprotoporphyrin IX interaction hypothesis, chloroquine would bind to undimerized ferriprotoporphyrin IX, delay its detoxification, cause it to accumulate, and allow it to exert its intrinsic biological toxicities. The ferriprotoporphyrin IX interaction hypothesis appears to explain the antimalarial action of chloroquine, but a drug target in addition to ferriprotoporphyrin IX is suggested by the antimalarial actions of quinine and mefloquine. This article summarizes current knowledge of the role of ferriprotoporphyrin IX in the antimalarial actions of quinoline drugs and evaluates the currently available evidence in support of phospholipids as a second target for quinine, mefloquine and, possibly, the chloroquine-ferriprotoporphyrin IX complex.

Polymorphism in the Plasmodium falciparum erythrocyte-binding ligand JESEBL/EBA-181 alters its receptor specificity

The malaria parasite lives within erythrocytes and depends on the binding of parasite ligands to host cell surface receptors for invasion. The most virulent human malaria parasite, Plasmodium falciparum, uses multiple ligands, including EBA-175, BAEBL, and JESEBL of the Duffy-binding-like (DBL) family of erythrocyte-binding proteins, for invasion of human erythrocytes. Region II of these parasite ligands is the erythrocyte-binding domain. Previously, we had shown that polymorphism in region II of BAEBL leads to different erythrocyte-binding specificities. We have now identified and characterized the binding specificity of six JESEBL variants. We sequenced region II of JESEBL from 20 P. falciparum clones collected from various parts of the world where malaria is endemic. We observed eight JESEBL variants that contained amino acid polymorphisms at five positions among all clones. Seven of the eight variants could be connected by a single base change that led to an amino acid change. We investigated the functional significance of these polymorphisms by transiently expressing region II from six of JESEBL variants on the surface of Chinese hamster ovary cells. We observed four erythrocyte-binding patterns to enzyme-treated erythrocytes. Thus, P. falciparum DBL ligands JESEBL and BAEBL can recognize multiple receptors on the erythrocyte surface. In contrast to Plasmodium vivax, which has disappeared from West Africa because of the Duffy-negative blood group, P. falciparum may have been successful in endemic areas because it has mutated the ligands of the DBL family to create multiple pathways of invasion, thus making selection of refractory erythrocytes unlikely.

Apical membrane antigen 1, a major malaria vaccine candidate, mediates the close attachment of invasive merozoites to host red blood cells

Apical membrane antigen 1 (AMA-1) of Plasmodium merozoites is established as a candidate molecule for inclusion in a human malaria vaccine and is strongly conserved in the genus. We have investigated its function in merozoite invasion by incubating Plasmodium knowlesi merozoites with red cells in the presence of a previously described rat monoclonal antibody (MAb R31C2) raised against an invasion-inhibitory epitope of P. knowlesi AMA-1 and then fixing the material for ultrastructural analysis. We have found that the random, initial, long-range (12 nm) contact between merozoites and red cells occurs normally in the presence of the antibody, showing that AMA-1 plays no part in this stage of attachment. Instead, inhibited merozoites fail to reorientate, so they do not bring their apices to bear on the red cell surface and do not make close junctional apical contact. We conclude that AMA-1 may be directly responsible for reorientation or that the molecule may initiate the junctional contact, which is then presumably dependent on Duffy binding proteins for its completion.

Toxoplasma gondii: perfecting an intracellular life style

Toxoplasma gondii is a widespread protozoan parasite that infects all nucleated cell types of warm-blooded vertebrates. Parasite motility is regulated by polymerization of new actin filaments that provide a substrate for the small myosin TgMyoA. Interaction between the cytoplasmic tails of parasite adhesins and the actin-binding protein aldolase links these cell surface proteins with the cytoskeleton. Translocation of adhesins coupled to extracellular receptors allows the parasite to glide across the substrate. This conserved system is important for active penetration into host cells and tissue migration by T. gondii. Entry into the host cell is accompanied by dramatic remodeling of the intracellular vacuole that the parasite resides in. This compartment resists fusion with host cell endocytic organelles, yet recruits mitochondria and endoplasmic reticulum in order to gain access to host cell nutrients. The combined abilities to actively penetrate host cells and control the fate of the parasite-containing vacuole contributes to the remarkable success of T. gondii as an intracellular parasite.

Distribution of the two forms of Plasmodium falciparum erythrocyte binding antigen-175 (eba-175) gene in Lao PDR

BACKGROUND

The erythrocyte binding antigen 175 (EBA-175) is a 175 kDa antigen of Plasmodium falciparum and plays a major role in erythrocyte recognition by the parasite. The antigen is also supposed to be partly responsible for the invasion of erythrocytes by merozoites. EBA-175 has been sequenced from the FCR-3 and CAMP strains of P. falciparum. The sequences were identical in most parts of the gene. Differences were apparent in a 423 bp segment in the FCR-3 strain, the F-Fragment, that is not found in the CAMP-strain and a 342 bp segment, the C-Fragment, which is present in the CAMP-strain but not in the FCR-3-strain. The aim of this study was to assess the distribution of the two EBA-175-alleles in the Lao PDR.

MATERIALS & METHODS

Altogether, 240 blood-samples were collected in two areas of the country: Attapeu in the south and Lung Namtha in the north. Subsequently, the material was scanned for the F-and C-fragments.

RESULTS

In the whole study population, 52% carried the F-fragment, and 41% the C-fragment while seven percent of the patients were infected with at least two parasite strains and showed both alleles.

CONCLUSION

Distribution of the alleles showed significant differences between the north and the south province. Reasons for this include possible importation of different parasite strains from neighbouring countries.

The cytoplasmic domain of the Plasmodium falciparum ligand EBA-175 is essential for invasion but not protein trafficking

The invasion of host cells by the malaria parasite Plasmodium falciparum requires specific protein-protein interactions between parasite and host receptors and an intracellular translocation machinery to power the process. The transmembrane erythrocyte binding protein-175 (EBA-175) and thrombospondin-related anonymous protein (TRAP) play central roles in this process. EBA-175 binds to glycophorin A on human erythrocytes during the invasion process, linking the parasite to the surface of the host cell. In this report, we show that the cytoplasmic domain of EBA-175 encodes crucial information for its role in merozoite invasion, and that trafficking of this protein is independent of this domain. Further, we show that the cytoplasmic domain of TRAP, a protein that is not expressed in merozoites but is essential for invasion of liver cells by the sporozoite stage, can substitute for the cytoplasmic domain of EBA-175. These results show that the parasite uses the same components of its cellular machinery for invasion regardless of the host cell type and invasive form.

Pathogens: raft hijackers

Throughout evolution, organisms have developed immune-surveillance networks to protect themselves from potential pathogens. At the cellular level, the signalling events that regulate these defensive responses take place in membrane rafts--dynamic microdomains that are enriched in cholesterol and glycosphingolipids--that facilitate many protein-protein and lipid-protein interactions at the cell surface. Pathogens have evolved many strategies to ensure their own survival and to evade the host immune system, in some cases by hijacking rafts. However, understanding the means by which pathogens exploit rafts might lead to new therapeutic strategies to prevent or alleviate certain infectious diseases, such as those caused by HIV-1 or Ebola virus.