Erythrocyte-binding activity of Plasmodium yoelii apical membrane antigen-1 expressed on the surface of transfected COS-7 cells

Invasion-inhibitory peptides chosen by natural selection analysis as an antimalarial strategy

Plasmodium vivax's biological complexity has restricted in vitro culture development for characterising antigens involved in erythrocyte invasion and their immunological relevance. The murine model is proposed as a suitable alternative in the search for therapeutic candidates since Plasmodium yoelii uses homologous proteins for its invasion. The AMA-1 protein is essential for parasite invasion of erythrocytes as it is considered an important target for infection control. This study has focused on functional PyAMA-1 peptides involved in host-pathogen interaction; the protein is located in regions under negative selection as determined by bioinformatics analysis. It was found that pyama1 has two highly conserved regions amongst species (>70%) under negative selection. Fourteen synthetic peptides spanning both conserved regions were evaluated; 5 PyAMA-1 peptides having high specific binding (HABP) to murine erythrocytes were identified. The parasite's invasion inhibition capability was analysed through in vitro assays, suggesting that peptides 42681 (43-ENTERSIKLINPWDKYMEKY-62), 42903 (206-RYSSNDANNENQPFSFTPEK-225) and 42904 (221-FTPEKIENYKDLSYLTKNLR-240) had greater than 50% inhibition profile and restricted P. yoelii intra-erythrocyte development. This work proposes that the screening of conserved HABPs under negative selective pressure might be good candidates for developing a synthetic anti-malarial vaccine since they share functionally-relevant characteristics, such as interspecies conservation, specific RBC binding profile, invasion and parasite development inhibition capability, and the predicted B-epitopes within were recognised by sera obtained from experimentally-infected mice.

Babesia Bovis Ligand-Receptor Interaction: AMA-1 Contains Small Regions Governing Bovine Erythrocyte Binding

Apical membrane antigen 1 is a microneme protein which plays an indispensable role during Apicomplexa parasite invasion. The detailed mechanism of AMA-1 molecular interaction with its receptor on bovine erythrocytes has not been completely defined in Babesia bovis. This study was focused on identifying the minimum B. bovis AMA-1-derived regions governing specific and high-affinity binding to its target cells. Different approaches were used for detecting ama-1 locus genetic variability and natural selection signatures. The binding properties of twelve highly conserved 20-residue-long peptides were evaluated using a sensitive and specific binding assay based on radio-iodination. B. bovis AMA-1 ectodomain structure was modelled and refined using molecular modelling software. NetMHCIIpan software was used for calculating B- and T-cell epitopes. The B. bovis ama-1 gene had regions under functional constraint, having the highest negative selective pressure intensity in the Domain I encoding region. Interestingly, B. bovis AMA-1-DI (¹⁰⁰YMQKFDIPRNHGSGIYVDLG¹¹⁹ and ¹²⁰GYESVGSKSYRMPVGKCPVV¹³⁹) and DII (³⁰²CPMHPVRDAIFGKWSGGSCV³²¹)-derived peptides had high specificity interaction with erythrocytes and bound to a chymotrypsin and neuraminidase-treatment sensitive receptor. DI-derived peptides appear to be exposed on the protein’s surface and contain predicted B- and T-cell epitopes. These findings provide data (for the first-time) concerning B. bovis AMA-1 functional subunits which are important for establishing receptor-ligand interactions which could be used in synthetic vaccine development.

On the Evolution and Function of Plasmodium vivax Reticulocyte Binding Surface Antigen (pvrbsa)

The RBSA protein is encoded by a gene described in Plasmodium species having tropism for reticulocytes. Since this protein is antigenic in natural infections and can bind to target cells, it has been proposed as a potential candidate for an anti-Plasmodium vivax vaccine. However, genetic diversity (a challenge which must be overcome for ensuring fully effective vaccine design) has not been described at this locus. Likewise, the minimum regions mediating specific parasite-host interaction have not been determined. This is why the rbsa gene’s evolutionary history is being here described, as well as the P. vivax rbsa (pvrbsa) genetic diversity and the specific regions mediating parasite adhesion to reticulocytes. Unlike what has previously been reported, rbsa was also present in several parasite species belonging to the monkey-malaria clade; paralogs were also found in Plasmodium parasites invading reticulocytes. The pvrbsa locus had less diversity than other merozoite surface proteins where natural selection and recombination were the main evolutionary forces involved in causing the observed polymorphism. The N-terminal end (PvRBSA-A) was conserved and under functional constraint; consequently, it was expressed as recombinant protein for binding assays. This protein fragment bound to reticulocytes whilst the C-terminus, included in recombinant PvRBSA-B (which was not under functional constraint), did not. Interestingly, two PvRBSA-A-derived peptides were able to inhibit protein binding to reticulocytes. Specific conserved and functionally important peptides within PvRBSA-A could thus be considered when designing a fully-effective vaccine against P. vivax.

Plasmodium vivax ligand-receptor interaction: PvAMA-1 domain I contains the minimal regions for specific interaction with CD71+ reticulocytes

The malarial parasite’s invasion is complex, active and coordinated, involving many low and high affinity interactions with receptors on target cell membrane. Proteomics analysis has described around 40 proteins in P. vivax which could be involved in reticulocyte invasion; few have been studied with the aim of elucidating how many of them establish specific interactions with their respective host cells. Given the importance of knowing which of the parasite’s protein regions are functionally important for invasion, minimum regions mediating specific interaction between Plasmodium vivax apical membrane antigen 1 (PvAMA-1) and its host cell were here elucidated. The region covering PvAMA-1 domains I and II (PvAMA-DI-II) specifically bound to the CD71⁺ red blood cell subpopulation. A 20 residue-long region (⁸¹EVENAKYRIPAGRCPVFGKG¹⁰⁰) located in domain I was capable of inhibiting PvAMA-DI-II recombinant protein binding to young reticulocytes (CD71⁺CD45⁻) and rosette formation. This conserved peptide specifically interacted with high affinity with reticulocytes (CD71⁺) through a neuraminidase- and chymotrypsin-treatment sensitive receptor. Such results showed that, despite AMA-1 having universal functions during late Plasmodium invasion stages, PvAMA-1 had reticulocyte-preferring binding regions, suggesting that P. vivax target cell selection is not just restricted to initial interactions but maintained throughout the erythrocyte invasion cycle, having important implications for designing a specific anti-P. vivax vaccine.

Expression, Purification, and Biological Characterization of Babesia microti Apical Membrane Antigen 1

The intraerythrocytic apicomplexan Babesia microti, the primary causative agent of human babesiosis, is a major public health concern in the United States and elsewhere. Apicomplexans utilize a multiprotein complex that includes a type I membrane protein called apical membrane antigen 1 (AMA1) to invade host cells. We have isolated the full-length B. microti AMA1 (BmAMA1) gene and determined its nucleotide sequence, as well as the amino acid sequence of the AMA1 protein. This protein contains an N-terminal signal sequence, an extracellular region, a transmembrane region, and a short conserved cytoplasmic tail. It shows the same domain organization as the AMA1 orthologs from piroplasm, coccidian, and haemosporidian apicomplexans but differs from all other currently known piroplasmida, including other Babesia and Theileria species, in lacking two conserved cysteines in highly variable domain III of the extracellular region. Minimal polymorphism was detected in BmAMA1 gene sequences of parasite isolates from six babesiosis patients from Nantucket. Immunofluorescence microscopy studies showed that BmAMA1 is localized on the cell surface and cytoplasm near the apical end of the parasite. Native BmAMA1 from parasite lysate and refolded recombinant BmAMA1 (rBmAMA1) expressed in Escherichia coli reacted with a mouse anti-BmAMA1 antibody using Western blotting. In vitro binding studies showed that both native BmAMA1 and rBmAMA1 bind to human red blood cells (RBCs). This binding is trypsin and chymotrypsin treatment sensitive but neuraminidase independent. Incubation of B. microti parasites in human RBCs with a mouse anti-BmAMA1 antibody inhibited parasite growth by 80% in a 24-h assay. Based on its antigenically conserved nature and potential role in RBC invasion, BmAMA1 should be evaluated as a vaccine candidate.

An Extended Surface Loop on Toxoplasma gondii Apical Membrane Antigen 1 (AMA1) Governs Ligand Binding Selectivity

Apicomplexan parasites are the causative agents of globally prevalent diseases including malaria and toxoplasmosis. These obligate intracellular pathogens have evolved a sophisticated host cell invasion strategy that relies on a parasite-host cell junction anchored by interactions between apical membrane antigens (AMAs) on the parasite surface and rhoptry neck 2 (RON2) proteins discharged from the parasite and embedded in the host cell membrane. Key to formation of the AMA1-RON2 complex is displacement of an extended surface loop on AMA1 called the DII loop. While conformational flexibility of the DII loop is required to expose the mature RON2 binding groove, a definitive role of this substructure has not been elucidated. To establish a role of the DII loop in Toxoplasma gondii AMA1, we engineered a form of the protein where the mobile portion of the loop was replaced with a short Gly-Ser linker (TgAMA1ΔDIIloop). Isothermal titration calorimetry measurements with a panel of RON2 peptides revealed an influential role for the DII loop in governing selectivity. Most notably, an Eimeria tenella RON2 (EtRON2) peptide that showed only weak binding to TgAMA1 bound with high affinity to TgAMA1ΔDIIloop. To define the molecular basis for the differential binding, we determined the crystal structure of TgAMA1ΔDIIloop in complex with the EtRON2 peptide. When analyzed in the context of existing AMA1-RON2 structures, spatially distinct anchor points in the AMA1 groove were identified that, when engaged, appear to provide the necessary traction to outcompete the DII loop. Collectively, these data support a model where the AMA1 DII loop serves as a structural gatekeeper to selectively filter out ligands otherwise capable of binding with high affinity in the AMA1 apical groove. These data also highlight the importance of considering the functional implications of the DII loop in the ongoing development of therapeutic intervention strategies targeting the AMA1-RON2 invasion complex.

Babesia divergens and Neospora caninum apical membrane antigen 1 structures reveal selectivity and plasticity in apicomplexan parasite host cell invasion

Host cell invasion by the obligate intracellular apicomplexan parasites, including Plasmodium (malaria) and Toxoplasma (toxoplasmosis), requires a step-wise mechanism unique among known host-pathogen interactions. A key step is the formation of the moving junction (MJ) complex, a circumferential constriction between the apical tip of the parasite and the host cell membrane that traverses in a posterior direction to enclose the parasite in a protective vacuole essential for intracellular survival. The leading model of MJ assembly proposes that Rhoptry Neck Protein 2 (RON2) is secreted into the host cell and integrated into the membrane where it serves as the receptor for apical membrane antigen 1 (AMA1) on the parasite surface. We have previously demonstrated that the AMA1-RON2 interaction is an effective target for inhibiting apicomplexan invasion. To better understand the AMA1-dependant molecular recognition events that promote invasion, including the significant AMA1-RON2 interaction, we present the structural characterization of AMA1 from the apicomplexan parasites Babesia divergens (BdAMA1) and Neospora caninum (NcAMA1) by X-ray crystallography. These studies offer intriguing structural insight into the RON2-binding surface groove in the AMA1 apical domain, which shows clear evidence for receptor-ligand co-evolution, and the hyper variability of the membrane proximal domain, which in Plasmodium is responsible for direct binding to erythrocytes. By incorporating the structural analysis of BdAMA1 and NcAMA1 with existing AMA1 structures and complexes we were able to define conserved pockets in the AMA1 apical groove that could be targeted for the design of broadly reactive therapeutics.

Immunogenicity of single versus mixed allele vaccines of Plasmodium vivax Duffy binding protein region II

The Duffy binding protein (DBP) of Plasmodium vivax is vital for host erythrocyte invasion. DBP region II (DBPII) contains critical residues for receptor recognition and anti-DBPII antibodies have been shown to inhibit erythrocyte binding and invasion, thereby making the molecule an attractive vaccine candidate against P. vivax blood stages. Similar to other blood-stage antigens, allelic variation within the DBPII and associated strain-specific immunity is a major challenge for development of a broadly effective vaccine against P. vivax malaria. We hypothesized that immunization with a vaccine composed of multiple DBP alleles or a modified epitope DBP (DEKnull) will be more effective in producing a broadly reactive and inhibitory antibody response to diverse DBPII alleles than a single allele vaccine. In this study, we compared single, naturally occurring DBPII allele immunizations (Sal1, 7.18, P) and DEKnull with a combination of (Sal1, 7.18, P) alleles. Quantitative analysis by ELISA demonstrated that the multiple allele vaccine tend to be more immunogenic than any of the single allele vaccines when tested for reactivity against a panel of DBPII allelic variants whereas DEKnull was less immunogenic than the mixed-allele vaccine but similar in reactivity to the single allele vaccines. Further analysis for functional efficacy by in vitro erythrocyte-binding inhibition assays demonstrated that the multiple allele immunization produced a stronger strain-neutralizing response than the other vaccination strategies even though inhibition remained biased toward some alleles. Overall, there was no correlation between antibody titer and functional inhibition. These data suggest that a multiple allele vaccine may enhance immunogenicity of a DBPII vaccine but further investigation is required to optimize this vaccine strategy to achieve broader coverage against global P. vivax strains.

Independent roles of apical membrane antigen 1 and rhoptry neck proteins during host cell invasion by apicomplexa

During invasion, apicomplexan parasites form an intimate circumferential contact with the host cell, the tight junction (TJ), through which they actively glide. The TJ, which links the parasite motor to the host cell cytoskeleton, is thought to be composed of interacting apical membrane antigen 1 (AMA1) and rhoptry neck (RON) proteins. Here we find that, in Plasmodium berghei, while both AMA1 and RON4 are important for merozoite invasion of erythrocytes, only RON4 is required for sporozoite invasion of hepatocytes, indicating that RON4 acts independently of AMA1 in the sporozoite. Further, in the Toxoplasma gondii tachyzoite, AMA1 is dispensable for normal RON4 ring and functional TJ assembly but enhances tachyzoite apposition to the cell and internalization frequency. We propose that while the RON proteins act at the TJ, AMA1 mainly functions on the zoite surface to permit correct attachment to the cell, which may facilitate invasion depending on the zoite-cell combination.

Host cell invasion by apicomplexans: what do we know?

Apicomplexan zoites enter host cells by forming and actively moving through a tight junction (TJ) formed between the parasite and host cell surfaces. Although the TJ was first described decades ago, its molecular characterization has proved difficult mainly because of its transient existence during an internalization process that lasts only seconds. In the past 7 years, work has led to a model of the TJ in which the association between AMA1 and RON proteins structures the TJ and bridges the cytoskeletons of the two cells. However, more recent work questions this view. Here, we critically discuss the current model and speculate on alternative models of the AMA1-RON association and of the apicomplexan TJ.

Conserved and variant epitopes of Plasmodium vivax Duffy binding protein as targets of inhibitory monoclonal antibodies

The Duffy binding protein (DBP) is a vital ligand for Plasmodium vivax blood-stage merozoite invasion, making the molecule an attractive vaccine candidate against vivax malaria. Similar to other blood-stage vaccine candidates, DBP allelic variation eliciting a strain-specific immunity may be a major challenge for development of a broadly effective vaccine against vivax malaria. To understand whether conserved epitopes can be the target of neutralizing anti-DBP inhibition, we generated a set of monoclonal antibodies to DBP and functionally analyzed their reactivity to a panel of allelic variants. Quantitative analysis by enzyme-linked immunosorbent assay (ELISA) determined that some monoclonal antibodies reacted strongly with epitopes conserved on all DBP variants tested, while reactivity of others was allele specific. Qualitative analysis characterized by anti-DBP functional inhibition using an in vitro erythrocyte binding inhibition assay indicated that there was no consistent correlation between the endpoint titers and functional inhibition. Some monoclonal antibodies were broadly inhibitory while inhibition of others varied significantly by target allele. These data demonstrate a potential for vaccine-elicited immunization to target conserved epitopes but optimization of DBP epitope target specificity and immunogenicity may be necessary for protection against diverse P. vivax strains.

Identification of a highly antigenic linear B cell epitope within Plasmodium vivax apical membrane antigen 1 (AMA-1)

Apical membrane antigen 1 (AMA-1) is considered to be a major candidate antigen for a malaria vaccine. Previous immunoepidemiological studies of naturally acquired immunity to Plasmodium vivax AMA-1 (PvAMA-1) have shown a higher prevalence of specific antibodies to domain II (DII) of AMA-1. In the present study, we confirmed that specific antibody responses from naturally infected individuals were highly reactive to both full-length AMA-1 and DII. Also, we demonstrated a strong association between AMA-1 and DII IgG and IgG subclass responses. We analyzed the primary sequence of PvAMA-1 for B cell linear epitopes co-occurring with intrinsically unstructured/disordered regions (IURs). The B cell epitope comprising the amino acid sequence 290–307 of PvAMA-1 (SASDQPTQYEEEMTDYQK), with the highest prediction scores, was identified in domain II and further selected for chemical synthesis and immunological testing. The antigenicity of the synthetic peptide was identified by serological analysis using sera from P. vivax-infected individuals who were knowingly reactive to the PvAMA-1 ectodomain only, domain II only, or reactive to both antigens. Although the synthetic peptide was recognized by all serum samples specific to domain II, serum with reactivity only to the full-length protein presented 58.3% positivity. Moreover, IgG reactivity against PvAMA-1 and domain II after depletion of specific synthetic peptide antibodies was reduced by 18% and 33% (P = 0.0001 for both), respectively. These results suggest that the linear epitope SASDQPTQYEEEMTDYQK is highly antigenic during natural human infections and is an important antigenic region of the domain II of PvAMA-1, suggesting its possible future use in pre-clinical studies.

Non-apical membrane antigen 1 (AMA1) IgGs from Malian children interfere with functional activity of AMA1 IgGs as judged by growth inhibition assay

BACKGROUND

Apical membrane antigen 1 (AMA1) is one of the best-studied blood-stage malaria vaccine candidates. When an AMA1 vaccine was tested in a malaria naïve population, it induced functionally active antibodies judged by Growth Inhibition Assay (GIA). However, the same vaccine failed to induce higher growth-inhibitory activity in adults living in a malaria endemic area. Vaccination did induce functionally active antibodies in malaria-exposed children with less than 20% inhibition in GIA at baseline, but not in children with more than that level of baseline inhibition.

METHODS

Total IgGs were purified from plasmas collected from the pediatric trial before and after immunization and pools of total IgGs were made. Another set of total IgGs was purified from U.S. adults immunized with AMA1 (US-total IgG). From these total IgGs, AMA1-specific and non-AMA1 IgGs were affinity purified and the functional activity of these IgGs was evaluated by GIA. Competition ELISA was performed with the U.S.-total IgG and non-AMA1 IgGs from malaria-exposed children.

RESULTS

AMA1-specific IgGs from malaria-exposed children and U.S. vaccinees showed similar growth-inhibitory activity at the same concentrations. When mixed with U.S.-total IgG, non-AMA1 IgGs from children showed an interference effect in GIA. Interestingly, the interference effect was higher with non-AMA1 IgGs from higher titer pools. The non-AMA1 IgGs did not compete with anti-AMA1 antibody in U.S.-total IgG in the competition ELISA.

CONCLUSION

Children living in a malaria endemic area have a fraction of IgGs that interferes with the biological activity of anti-AMA1 antibody as judged by GIA. While the mechanism of interference is not resolved in this study, these results suggest it is not caused by direct competition between non-AMA1 IgG and AMA1 protein. This study indicates that anti-malaria IgGs induced by natural exposure may interfere with the biological effect of antibody induced by an AMA1-based vaccine in the target population.

Focus on the ringleader: the role of AMA1 in apicomplexan invasion and replication

Apicomplexan parasites exhibit an unusual mechanism of host cell penetration. A central player in this process is the protein apical membrane antigen 1 (AMA1). Although essential for invasion, the precise functional roles AMA1 plays have been unclear. Several recent studies have provided important functional insight into its role within the multiprotein complex that comprises the moving junction (MJ). Initially formed at the apical tip of the invading parasite, the MJ represents a ring-like region of contact between the surfaces of the invading parasite and the host cell as the invaginated host plasma membrane is forced inward by the penetrating parasite. This review discusses these and other recent insights into AMA1 with particular emphasis on studies conducted in Plasmodium and Toxoplasma.

Plasmodium berghei circumvents immune responses induced by merozoite surface protein 1- and apical membrane antigen 1-based vaccines

Background

Two current leading malaria blood-stage vaccine candidate antigens for Plasmodium falciparum, the C-terminal region of merozoite surface protein 1 (MSP1₁₉) and apical membrane antigen 1 (AMA1), have been prioritized because of outstanding protective efficacies achieved in a rodent malaria Plasmodium yoelii model. However, P. falciparum vaccines based on these antigens have had disappointing outcomes in clinical trials. Discrepancies in the vaccine efficacies observed between the P. yoelii model and human clinical trials still remain problematic.

Methodology and Results

In this study, we assessed the protective efficacies of a series of MSP1₁₉- and AMA1-based vaccines using the P. berghei rodent malarial parasite and its transgenic models. Immunization of mice with a baculoviral-based vaccine (BBV) expressing P. falciparum MSP1₁₉ induced high titers of PfMSP1₁₉-specific antibodies that strongly reacted with P. falciparum blood-stage parasites. However, no protection was achieved following lethal challenge with transgenic P. berghei expressing PfMSP1₁₉ in place of native PbMSP1₁₉. Similarly, neither P. berghei MSP1₁₉- nor AMA1-BBV was effective against P. berghei. In contrast, immunization with P. yoelii MSP1₁₉- and AMA1-BBVs provided 100% and 40% protection, respectively, against P. yoelii lethal challenge. Mice that naturally acquired sterile immunity against P. berghei became cross-resistant to P. yoelii, but not vice versa.

Conclusion

This is the first study to address blood-stage vaccine efficacies using both P. berghei and P. yoelii models at the same time. P. berghei completely circumvents immune responses induced by MSP1₁₉- and AMA1-based vaccines, suggesting that P. berghei possesses additional molecules and/or mechanisms that circumvent the host's immune responses to MSP1₁₉ and AMA1, which are lacking in P. yoelii. Although it is not known whether P. falciparum shares these escape mechanisms with P. berghei, P. berghei and its transgenic models may have potential as useful tools for identifying and evaluating new blood-stage vaccine candidate antigens for P. falciparum.

Genetic polymorphism and effect of natural selection at domain I of apical membrane antigen-1 (AMA-1) in Plasmodium vivax isolates from Myanmar

Malaria is endemic or hypoendemic in Myanmar and the country still contributes to the high level of malaria deaths in South-East Asia. Although information on the nature and extent of population diversity within malaria parasites in the country is essential not only for understanding the epidemic situation but also to establish a proper control strategy, very little data is currently available on the extent of genetic polymorphisms of the malaria parasites in Myanmar. In this study, we analyzed the genetic polymorphism and natural selection at domain I of the apical membrane antigen-1 (AMA-1) among Plasmodium vivax Myanmar isolates. A total of 34 distinguishable haplotypes were identified among the 76 isolates sequenced. Comparison with the previously available PvAMA-1 sequences in the GenBank database revealed that 21 of them were new haplotypes that have never been reported till date. The difference between the rate of nonsynonymous (dN) and synonymous (dS) mutations was positive (dN-dS, 0.013+/-0.005), suggesting the domain I is under positive natural selection. The Tajima's D statistics was found to be -0.74652, suggesting that the gene has evolved under population size expansion and/or positive selection. The minimum recombination events were also high, indicating that recombination may occur within the domain I resulting in allelic diversity of PvAMA-1. Our results collectively suggest that PvAMA-1 displays high genetic polymorphism among Myanmar P. vivax isolates with highly diversifying selection at domain I. These results have significant implications in understanding the nature of P. vivax population circulating in Myanmar as well as providing useful information for malaria vaccine development based on this antigen.

Babesia divergens apical membrane antigen 1 and its interaction with the human red blood cell

Multiple parasite ligand-erythrocyte receptor interactions must occur for successful Babesia and Plasmodium invasion of the human red cell. One such parasite ligand is the apical membrane antigen 1 (AMA1) which is a conserved apicomplexan protein present in the micronemes and then secreted onto the surface of the merozoite. Much evidence exists for a vital role for AMA1 in host cell invasion; however, its interaction with the host erythrocyte has remained controversial. In this paper, we present a detailed characterization of a Babesia divergens homolog of AMA1 (BdAMA1), and taking advantage of the relatively high amounts of native BdAMA1 available from the parasite culture system, show that proteolytic products of native BdAMA1 bind to a trypsin- and chymotrypsin-sensitive receptor on the red blood cell. Moreover, immuno-electron microscopic images of the B. divergens merozoite captured during invasion offer additional evidence of the presence of BdAMA1 on the red cell membrane. Given the importance of AMA1 in invasion and the central role invasion plays in pathogenesis, these studies have implications both for novel drug design and for the development of new vaccine approaches aimed at interfering with AMA1 function.

Functional analysis of the leading malaria vaccine candidate AMA-1 reveals an essential role for the cytoplasmic domain in the invasion process

A key process in the lifecycle of the malaria parasite Plasmodium falciparum is the fast invasion of human erythrocytes. Entry into the host cell requires the apical membrane antigen 1 (AMA-1), a type I transmembrane protein located in the micronemes of the merozoite. Although AMA-1 is evolving into the leading blood-stage malaria vaccine candidate, its precise role in invasion is still unclear. We investigate AMA-1 function using live video microscopy in the absence and presence of an AMA-1 inhibitory peptide. This data reveals a crucial function of AMA-1 during the primary contact period upstream of the entry process at around the time of moving junction formation. We generate a Plasmodium falciparum cell line that expresses a functional GFP-tagged AMA-1. This allows the visualization of the dynamics of AMA-1 in live parasites. We functionally validate the ectopically expressed AMA-1 by establishing a complementation assay based on strain-specific inhibition. This method provides the basis for the functional analysis of essential genes that are refractory to any genetic manipulation. Using the complementation assay, we show that the cytoplasmic domain of AMA-1 is not required for correct trafficking and surface translocation but is essential for AMA-1 function. Although this function can be mimicked by the highly conserved cytoplasmic domains of P. vivax and P. berghei, the exchange with the heterologous domain of the microneme protein EBA-175 or the rhoptry protein Rh2b leads to a loss of function. We identify several residues in the cytoplasmic tail that are essential for AMA-1 function. We validate this data using additional transgenic parasite lines expressing AMA-1 mutants with TY1 epitopes. We show that the cytoplasmic domain of AMA-1 is phosphorylated. Mutational analysis suggests an important role for the phosphorylation in the invasion process, which might translate into novel therapeutic strategies.

Malaria is one of the most lethal parasitic diseases worldwide, causing more than 1 million fatalities per annum. Drug resistance is widespread, and a vaccine is not available. One of the leading blood stage vaccine candidates is the apical membrane antigen 1 (AMA-1), which is well-conserved among apicomplexan parasites. Although this protein plays an essential role in the invasion of human red blood cells, little is known about the molecular mechanisms underlying its function. In this study we use live video microscopy in the presence and absence of an invasion-inhibiting molecule to investigate the function of AMA-1 in real time. We establish a complementation assay based on strain-specific AMA-1 inhibition that allows functional characterization of this protein on a molecular level. Using this approach, we provide evidence that the intracellular cytoplasmic domain of AMA-1 is directly involved in the invasion process and that this domain is phosphorylated. We conclude that the phosphorylation of AMA-1 might represent an attractive target for novel therapeutic drug strategies independent of the polymorphic extracellular domain.

Current and emerging approaches to studying invasion in apicomplexan parasites

In this chapter, we outline the tools and techniques available to study the process of host cell invasion by apicomplexan parasites and we provide specific examples of how these methods have been used to further our understanding of apicomplexan invasive mechanisms. Throughout the chapter we focus our discussion on Toxoplasmagondii, because T. gondii is the most experimentally accessible model organism for studying apicomplexan invasion (discussed further in the section, "Toxoplasma as a Model Apicomplexan") and more is known about invasion in T. gondii than in any other apicomplexan.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

Identification and characterization of the Plasmodium yoelii PyP140/RON4 protein, an orthologue of Toxoplasma gondii RON4, whose cysteine-rich domain does not protect against lethal parasite challenge infection

Previously, we identified a Plasmodium yoelii YM 140-kDa merozoite protein, designated PyP140, which formed a complex with apical membrane antigen 1 (AMA1). Furthermore, we produced a nonprotective monoclonal antibody (MAb), 48F8, that immunoprecipitated metabolically labeled PyP140 and localized the protein to the merozoite's apical end and, less frequently, to the merozoite surface, as observed by immunofluorescence assay (IFA). Here, using MAb 48F8, we have identified the pyp140 gene by screening a P. yoelii lambda-Zap cDNA expression library. The pyp140 cDNA covers approximately 90% of the putative open reading frame (ORF) of PY02159 from the P. yoelii NL genome sequencing project. Analysis of the complete gene identified the presence of two introns. The ORF encodes a 102,407-Da protein with an amino-terminal signal sequence, a series of three unique types of repeats, and a cysteine-rich region. The binding site of MAb 48F8 was also identified. A BLAST search with the deduced amino acid sequence shows significant similarity with the Toxoplasma gondii RON4 protein and the Plasmodium falciparum RON4 protein, and the sequence is highly conserved in other Plasmodium species. We produced the cysteine-rich domain of PyP140/RON4 by using the Pichia pastoris expression system and characterized the recombinant protein biochemically and biophysically. BALB/c mice immunized with the protein formulated in oil-in-water adjuvants produced antibodies that recognize parasitized erythrocytes by IFA and native PyP140/RON4 by immunoblotting but failed to protect against a lethal P. yoelii YM infection. Our results show that PyP140/RON4 is located within the rhoptries or micronemes. It may associate in part with AMA1, but the conserved cysteine-rich domain does not appear to elicit inhibitory antibodies, a finding that is supported by the marked sequence conservation in this protein within Plasmodium spp., suggesting that it is not under immune pressure.

Plasmodium vivax apical membrane antigen-1: comparative recognition of different domains by antibodies induced during natural human infection

The Apical Membrane Antigen-1 (AMA-1) of Plasmodium sp. has been suggested as a vaccine candidate against malaria. This protein seems to be involved in merozoite invasion and its extra-cellular portion contains three distinct domains: DI, DII, and DIII. Previously, we described that Plasmodium vivax AMA-1 (PvAMA-1) ectodomain is highly immunogenic in natural human infections. Here, we expressed each domain, separately or in combination (DI-II or DII-III), as bacterial recombinant proteins to map immunodominant epitopes within the PvAMA-1 ectodomain. IgG recognition was assessed by ELISA using sera of P. vivax-infected individuals collected from endemic regions of Brazil or antibodies raised in immunized mice. The frequencies of responders to recombinant proteins containing the DII were higher than the others and similar to the ones observed against the PvAMA-1 ectodomain. Moreover, ELISA inhibition assays using the PvAMA-1 ectodomain as substrate revealed the presence of many common epitopes within DI-II that are recognized by human immune antibodies. Finally, immunization of mice with the PvAMA-1 ectodomain induced high levels of antibodies predominantly to DI-II. Together, our results indicate that DII is particularly immunogenic during natural human infections, thus indicating that this region could be used as part of an experimental sub-unit vaccine to prevent vivax malaria.

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.

Apical membrane antigen 1: a malaria vaccine candidate in review

Apical membrane antigen 1 (AMA1) is a micronemal protein of apicomplexan parasites that appears to be essential during the invasion of host cells. Immune responses to Plasmodium AMA1 can have profound parasite-inhibitory effects, both as measured in vitro and in animal challenge models, suggesting AMA1 as a potential vaccine component. However, AMA1 is polymorphic, probably as a result of immune selection operating on an important target of naturally occurring immunity. The current understanding of AMA1 will be presented, particularly in relation to the vaccine potential of AMA1 and the approaches being taken towards clinical development.

Fine Mapping of an Epitope Recognized by an Invasion-inhibitory Monoclonal Antibody on the Malaria Vaccine Candidate Apical Membrane Antigen 1

Antibodies that inhibit red blood cell invasion by the Plasmodium merozoite block the erythrocytic cycle responsible for clinical malaria. The invasion-inhibitory monoclonal antibody (mAb) 4G2 recognizes a conserved epitope in the ectodomain of the essential Plasmodium falciparum microneme protein and vaccine candidate, apical membrane antigen 1 (PfAMA1). Here we demonstrate that purified Fab fragments of 4G2 inhibit invasion markedly more efficiently than the intact mAb, suggesting that the invasion-inhibitory activity of this mAb is not due solely to steric effects and that the epitope lies within a functionally critical region of the molecule. We have taken advantage of a synthetic gene encoding a modified form of PfAMA1, and existing x-ray crystal structure data, to fully characterize this epitope. We first validate the gene by demonstrating that it fully complements the function of the authentic gene in P. falciparum. We then use it to identify a group of residues within the previously described domain II loop of PfAMA1 that are critical for recognition by mAb 4G2 and demonstrate that the epitope lies exclusively within this loop with no contributions from residues in other domains of the molecule. This is the first complete characterization of a conserved invasion-inhibitory epitope on PfAMA1. Our results will aid in the design of subunit vaccines designed to generate a broadly effective, focused anti-PfAMA1 protective immune response and may help elucidate the function of PfAMA1.

Passive immunization with a multicomponent vaccine against conserved domains of apical membrane antigen 1 and 235-kilodalton rhoptry proteins protects mice against Plasmodium yoelii blood-stage challenge infection

During malaria parasite invasion of red blood cells, merozoite proteins bind receptors on the surface of the erythrocyte. Two candidate Plasmodium yoelii adhesion proteins are apical membrane antigen 1 (AMA1) and the 235-kDa rhoptry proteins (P235). Previously, we have demonstrated that passive immunization with monoclonal antibodies (MAbs) 45B1 and 25.77 against AMA1 and P235, respectively, protects against a lethal challenge infection with P. yoelii YM. We show that MAb 45B1 recognizes an epitope located on a conserved surface of PyAMA1, as determined by phage display and analysis of the three-dimensional structure of AMA1, in a region similar to that bound by the P. falciparum AMA1-specific inhibitory antibody 4G2. The epitope recognized by 25.77 could not be assigned. We report here that MAbs 45B1 and 25.77 also protect against challenge with the nonlethal parasite line 17X, in which PyAMA1 has a significantly different amino acid sequence from that in YM. When administered together, the two MAbs acted at least additively in providing protection against challenge with the virulent YM parasite. These results support the concept of developing a multicomponent blood-stage vaccine and the inclusion of polymorphic targets such as AMA1, which these results suggest contain conserved domains recognized by inhibitory antibodies.

Synthetic peptides from Plasmodium falciparum apical membrane antigen 1 (AMA-1) specifically interacting with human hepatocytes

Plasmodium falciparum apical membrane antigen 1 (AMA-1) is expressed during both the sporozoite and merozoite stage of the parasite's life cycle. The role placed by AMA-1 during sporozoite invasion of hepatocytes has not been made sufficiently clear to date. Identifying the sequences involved in binding to hepatocytes is an important step towards understanding the structural basis for sporozoite-hepatocyte interaction. Binding assays between P. falciparum AMA-1 peptides and HepG2 cell were performed in this study to identify possible AMA-1 functional regions. Four AMA-1 high activity binding peptides (HABPs) bound specifically to hepatocytes: 4310 ((74)QHAYPIDHEGAEPAPQEQNL(93)), 4316 ((194)TLDEMRHFYKDNKYVKNLDE(213)), 4321 ((294)VVDNWEKVCPRKNLQNAKFGY(313)) and 4332 ((514)AEVTSNNEVVVKEEYKDEYA(533)). Their binding to these cells became saturable and resistant to treatment with neuraminidase. Most of these peptides were located in AMA-1 domains I and III, these being target regions for protective antibody responses. These peptides interacted with 36 and 58 kDa proteins on the erythrocyte surface. Some of the peptides were found in exposed regions of the AMA-1 protein, thereby facilitating their interaction with host cells. It is thus probable that AMA-1 regions defined by the four peptides mentioned above are involved in sporozoite-hepatocyte interaction.

Distinct mechanisms govern proteolytic shedding of a key invasion protein in apicomplexan pathogens

Apical membrane antigen-1 (AMA1) is a conserved apicomplexan protein that plays an important but undefined role in host cell invasion. We have studied the fate of Plasmodium falciparum AMA1 (PfAMA1) during erythrocyte invasion by the malaria merozoite, and compared it with that of the Toxoplasma gondii orthologue, TgAMA1. Shedding of the PfAMA1 ectodomain goes essentially to completion during invasion, and occurs predominantly or exclusively via juxtamembrane cleavage at the previously identified sheddase cleavage site, Thr517. Only the resulting juxtamembrane stub of the ectodomain is efficiently carried into the host cell, and this remains distributed around the plasma membrane of the intracellular ring-stage parasite. Inhibition of normal shedding, however, results in proteolysis at an intramembrane, rhomboid-like cleavage site, and PfAMA1 is susceptible to cleavage by Drosophila rhomboid-1, showing that it can be a substrate for intramembrane cleavage but is not normally processed in this manner. In contrast, shedding of TgAMA1 from the surface of extracellular tachyzoites occurs exclusively via cleavage within the luminal half of its transmembrane domain by a rhomboid-like protease. Also unlike PfAMA1, complete TgAMA1 shedding does not accompany Toxoplasma invasion as the intact protein was readily detected on the surface of newly invaded tachyzoites. This work reveals unexpected differences in the manner in which Plasmodium and Toxoplasma shed AMA1 from the surface of invasive zoites, and demonstrates the presence at the malaria merozoite surface of a rhomboid-like protease.

Structural comparison of apical membrane antigen 1 orthologues and paralogues in apicomplexan parasites

Apical membrane antigen 1 (AMA1) is a membrane protein present in Plasmodium species and is probably common to all apicomplexan parasites. The recent crystal structure of the complete ectoplasmic region of AMA1 from Plasmodium vivax has shown that it comprises three structural domains and that the first two domains are based on the PAN folding motif. Here, we discuss the consequences of this analysis for the three-dimensional structure of AMA1 from other Plasmodium species and other apicomplexan parasites, and for the Plasmodium paralogue MAEBL. Many polar and apolar interactions observed in the PvAMA1 crystal structure are made by residues that are invariant or highly conserved throughout all Plasmodium orthologues; a subgroup of these residues is also present in other apicomplexan orthologues and in MAEBL. These interactions presumably play a key role in defining the protein fold. Previous studies have shown that the ectoplasmic region of AMA1 must be cleaved from the parasite surface for host-cell invasion to proceed. The cleavage site in the crystal structure is not readily accessible to proteases and we discuss possible consequences of this observation. The three-dimensional distribution of polymorphic sites in PfAMA1 shows that these are all on the surface and that their positions are significantly biased to one side of the ectoplasmic region. Of particular note, a flexible segment in domain II, comprising about 40 residues and devoid of polymorphism, carries an epitope recognized by an invasion-inhibitory monoclonal antibody and a T-cell epitope implicated in the human immune response to AMA1.

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.

Structure of AMA1 from Plasmodium falciparum reveals a clustering of polymorphisms that surround a conserved hydrophobic pocket

Apical membrane antigen 1 (AMA1) is a leading malaria vaccine candidate that possesses polymorphisms that may pose a problem for a vaccine based on this antigen. Knowledge of the distribution of the polymorphic sites on the surface of AMA1 is necessary to obtain a detailed understanding of their significance for vaccine development. For this reason we have sought to determine the three-dimensional structure of AMA1 using x-ray crystallography. The central two-thirds of AMA1 is relatively conserved among Plasmodium species as well as more distantly related apicomplexan parasites, and contains two clusters of disulfide-bonded cysteines termed domains I and II. The crystal structure of this fragment of AMA1 reported here reveals that domains I+II consists of two intimately associated PAN domains. PAN domain I contains many long loops that extend from the domain core and form a scaffold for numerous polymorphic residues. This extreme adaptation of a PAN domain reveals how malaria parasites have introduced significant flexibility and variation into AMA1 to evade protective human antibody responses. The polymorphisms on the AMA1 surface are exclusively located on one side of the molecule, presumably because this region of AMA1 is most accessible to antibodies reacting with the parasite surface. Moreover, the most highly polymorphic residues surround a conserved hydrophobic trough that is ringed by domain I and domain II loops. Precedents set by viral receptor proteins would suggest that this is likely to be the AMA1 receptor binding pocket.

A new release on life: emerging concepts in proteolysis and parasite invasion

Cell invasion by apicomplexan pathogens such as the malaria parasite and Toxoplasma is accompanied by extensive proteolysis of zoite surface proteins (ZSPs) required for attachment and penetration. Although there is still little known about the proteases involved, a conceptual framework is emerging for the roles of proteolysis in cell invasion. Primary processing of ZSPs, which includes the trimming of terminal peptides or segmentation into multiple fragments, is proposed to activate these adhesive ligands for tight binding to host receptors. Secondary processing, which occurs during penetration, results in the shedding of ZSPs by one of two mechanistically distinct ways, shaving or capping. Resident surface proteins are typically shaved from the surface whereas adhesive ligands mobilized from intracellular secretory vesicles are capped to the posterior end of the parasite before being shed during the final steps of penetration. Intriguingly, recent studies have revealed that ZSPs can be released either by being cleaved adjacent to the membrane anchor or actually within the membrane itself. Mounting evidence suggests that intramembrane cleavage is catalysed by one or more integral membrane serine proteases of the Rhomboid family and we propose that several malaria adhesive ligands may be potential substrates for these enzymes. We also discuss the evidence that the key reason for ZSP shedding during invasion is to break the connection between parasite surface ligands and host receptors. The sequential proteolytic events associated with invasion by pathogenic protozoa may represent vulnerable pathways for the future development of synergistic anti-protozoal therapies.

Conditional expression of Toxoplasma gondii apical membrane antigen-1 (TgAMA1) demonstrates that TgAMA1 plays a critical role in host cell invasion

Toxoplasma gondii is an obligate intracellular parasite and an important human pathogen. Relatively little is known about the proteins that orchestrate host cell invasion by T. gondii or related apicomplexan parasites (including Plasmodium spp., which cause malaria), due to the difficulty of studying essential genes in these organisms. We have used a recently developed regulatable promoter to create a conditional knockout of T. gondii apical membrane antigen-1 (TgAMA1). TgAMA1 is a transmembrane protein that localizes to the parasite's micronemes, secretory organelles that discharge during invasion. AMA1 proteins are conserved among apicomplexan parasites and are of intense interest as malaria vaccine candidates. We show here that T. gondii tachyzoites depleted of TgAMA1 are severely compromised in their ability to invade host cells, providing direct genetic evidence that AMA1 functions during invasion. The TgAMA1 deficiency has no effect on microneme secretion or initial attachment of the parasite to the host cell, but it does inhibit secretion of the rhoptries, organelles whose discharge is coupled to active host cell penetration. The data suggest a model in which attachment of the parasite to the host cell occurs in two distinct stages, the second of which requires TgAMA1 and is involved in regulating rhoptry secretion.

Mode of action of invasion-inhibitory antibodies directed against apical membrane antigen 1 of Plasmodium falciparum

Antibodies against apical membrane antigen 1 (AMA-1) of Plasmodium falciparum inhibit merozoite invasion into erythrocytes. Invasion-inhibitory polyclonal AMA-1 antibodies inhibit secondary proteolytic processing and surface redistribution of AMA-1 on merozoites. We present evidence supporting inhibition of processing and redistribution as probable causes of inhibition of invasion by polyclonal antibodies. Polyclonal anti-AMA-1 was much more inhibitory than monoclonal antibody (MAb) 4G2dc1 in an invasion assay. Although both polyclonal and monoclonal immunoglobulin G (IgG) inhibited secondary processing of the 66-kDa form of AMA-1, only polyclonal IgG caused its anomalous processing, inhibited its redistribution, and cross-linked soluble forms of AMA-1 on merozoites. Moreover, Fab fragments of polyclonal IgG that fail to cross-link did not show the enhancement of inhibitory effect over intact IgG, as observed in the case of Fab fragments of MAb 4G2dc1. We propose that although blocking of biologically important sites is a common direct mode of action of anti-AMA-1 antibodies, blocking of AMA-1 secondary processing and redistribution are additional indirect inhibitory mechanisms by which polyclonal IgG inhibits invasion. We also report a processing inhibition assay that uses a C-terminal AMA-1-specific MAb, 28G2dc1, to detect merozoite-bound remnants of processing (approximately 20 kDa from normal processing to 48 and 44 kDa and approximately 10 kDa from anomalous processing to a 52-kDa soluble form of AMA-1). The ratio of intensity of 10-kDa bands to the sum of 10- and 20-kDa bands was positively correlated with inhibition of invasion by polyclonal antibodies. This assay may serve as an important immunochemical correlate for inhibition of invasion.

Domain III of Plasmodium falciparum apical membrane antigen 1 binds to the erythrocyte membrane protein Kx

Plasmodium falciparum apical membrane antigen 1 (AMA1) is located in the merozoite micronemes, an organelle that contains receptors for invasion, suggesting that AMA1 may play a role in this process. However, direct evidence that P. falciparum AMA1 binds to human erythrocytes is lacking. In this study, we determined that domain III of AMA1 binds to the erythrocyte membrane protein, Kx, and that the rate of invasion of Kx(null) erythrocytes is reduced, indicating a significant but not unique role of AMA1 and Kx in parasite invasion of erythrocytes. Domains I/II/III, domains I/II and domain III of AMA1 were expressed on the surface of CHO-K1 cells, and their ability to bind erythrocytes was determined. We observed that each of these domains failed to bind untreated human erythrocytes. In contrast, domain III, but not the other domains of AMA1, bound to trypsin-treated human erythrocytes. We tested the binding of AMA1 to trypsin-treated genetically mutant human erythrocytes, missing various erythrocyte membrane proteins. AMA1 failed to bind trypsin-treated Kx(null) (McLeod) erythrocytes, which lack the Kx protein. Furthermore, treatment of human erythrocytes with trypsin, followed by alpha-chymotrypsin, cleaved Kx and destroyed the binding of AMA1 to human erythrocytes. Lastly, the rate of invasion of Kx null erythrocytes by P. falciparum was significantly lower than Kx-expressing erythrocytes. Taken together, our data suggest that AMA1 plays an important, but not exclusive, role in invasion of human erythrocytes through a process that involves exposure or modification of the erythrocyte surface protein, Kx, by a trypsin-like enzyme.

Crystal structure of the malaria vaccine candidate apical membrane antigen 1

Apical membrane antigen 1 from Plasmodium is a leading malaria vaccine candidate. The protein is essential for host-cell invasion, but its molecular function is unknown. The crystal structure of the three domains comprising the ectoplasmic region of the antigen from P. vivax, solved at 1.8 angstrom resolution, shows that domains I and II belong to the PAN motif, which defines a superfamily of protein folds implicated in receptor binding. We also mapped the epitope of an invasion-inhibitory monoclonal antibody specific for the P. falciparum ortholog and modeled this to the structure. The location of the epitope and current knowledge on structure-function correlations for PAN domains together suggest a receptor-binding role during invasion in which domain II plays a critical part. These results are likely to aid vaccine and drug design.

Proteases in host cell invasion by the malaria parasite

The life cycle of the malaria parasite contains three distinct invasive forms, or zoites. For at least two of these--the sporozoite and the blood-stage merozoite--invasion into their respective host cell requires the activity of parasite proteases. This review summarizes the evidence for this, discusses selected well-described proteolytic modifications linked to invasion, and describes recent progress towards identifying the proteases involved.

Antibody response of naturally infected individuals to recombinant Plasmodium vivax apical membrane antigen-1

In the present study, we evaluate the naturally acquired antibody response to the Plasmodium vivax apical membrane antigen 1 (PvAMA-1), a leading vaccine candidate against malaria. The gene encoding the PvAMA-1 ectodomain region (amino acids 43-487) was cloned by PCR using genomic DNA from a Brazilian individual with patent P. vivax infection. The predicted amino acid sequence displayed a high degree of identity (97.3%) with a previously published sequence from the P. vivax Salvador strain. A recombinant protein representing the PvAMA-1 ectodomain was expressed in Escherichia coli and refolded. By ELISA, this recombinant protein reacted with 85 and 48.5% of the IgG or IgM antibodies, respectively, from Brazilian individuals with patent P. vivax malaria. IgG1 was the predominant subclass of IgG. The frequency of response increased according to the number of malaria episodes, reaching 100% in individuals in their fourth malaria episode. The high degree of recognition of PvAMA-1 by human antibodies was confirmed using a second recombinant protein expressed in Pichia pastoris (PV66/AMA-1). The observation that recognition of the bacterial recombinant PvAMA-1 was only slightly lower than that of the highly immunogenic 19kDa C-terminal domain of the P. vivax Merozoite Surface Protein-1 was also important. DNA sequencing of the PvAMA-1 variable domain from 20 Brazilian isolates confirmed the limited polymorphism of PvAMA-1 suggested by serological analysis. In conclusion, we provide evidence that PvAMA-1 is highly immunogenic during natural infection in humans and displays limited polymorphism in Brazil. Based on these observations, we conclude that PvAMA-1 merits further immunological studies as a vaccine candidate against P. vivax malaria.

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.

Conserved residues in the Plasmodium vivax Duffy-binding protein ligand domain are critical for erythrocyte receptor recognition

Malaria merozoite invasion of human erythrocytes depends on recognition of specific erythrocyte surface receptors by parasite ligands. Plasmodium vivax merozoite invasion is totally dependent on the recognition of the Duffy blood group antigen by the parasite ligand Duffy-binding protein (DBP). Receptor recognition by P. vivax relies on a cysteine-rich domain, the DBL domain or region II, at the N terminus of the extracellular portion of DBP. The minimal region of the DBP implicated for receptor recognition lies between cysteines 4 and 8 of the DBL domain, which is a region that also has the highest rate of allelic polymorphisms among parasite isolates. We previously found that allelic polymorphisms in this region altered the P. vivax DBL domain antigenic character, which contrasts with changes in receptor specificity attributed to polymorphisms in some homologous ligands of Plasmodium falciparum. To further investigate the relative importance of conserved and polymorphic residues within this DBL central region, we identified residues critical for receptor recognition by site-directed mutagenesis. Seventy-seven surface-predicted residues of the Sal-1 DBL domain were substituted with alanine and assayed for erythrocyte binding activity by expression of the mutant proteins on the surface of transiently transfected COS cells. The functional effect of alanine substitution varied from nil to complete loss of DBL erythrocyte-binding activity. Mutations that caused loss of ligand function mostly occurred in discontinuous clusters of conserved residues, whereas nearly all mutations in polymorphic residues did not affect erythrocyte binding. These data delineate DBL domain residues essential for receptor recognition.

Erythrocyte invasion by Babesia bovis merozoites is inhibited by polyclonal antisera directed against peptides derived from a homologue of Plasmodium falciparum apical membrane antigen 1

Apical membrane antigen 1 (AMA-1) is a micronemal protein secreted to the surface of merozoites of Plasmodium species and Toxoplasma gondii tachyzoites in order to fulfill an essential but noncharacterized function in host cell invasion. Here we describe cloning and characterization of a Babesia bovis AMA-1 homologue designated BbAMA-1. The overall level of similarity of BbAMA-1 to P. falciparum AMA-1 was low (18%), but characteristic features like a transmembrane domain near the C terminus, a predicted short cytoplasmic C-terminal sequence with conserved sequence properties, and an extracellular domain containing 14 conserved cysteine residues putatively involved in disulfide bridge formation are typical of AMA-1. Rabbit polyclonal antisera were raised against three synthetic peptides derived from the N-terminal region and domains II and III of the putative extracellular domain and were shown to recognize specifically recombinant BbAMA-1 expressed in Escherichia coli. Immunofluorescence microscopy showed that there was labeling of the apical half of merozoites with these antisera. Preincubation of free merozoites with all three antisera reduced the efficiency of invasion of erythrocytes by a maximum of 65%. Antisera raised against the N-terminal peptide detected a 82-kDa protein on Western blots and a 69-kDa protein in the supernatant that was harvested after in vitro invasion, suggesting that proteolytic processing and secretion take place during or shortly after invasion. A combination of two-dimensional Western blotting and metabolic labeling allowing direct identification of spots reacting with the BbAMA-1 peptide antisera together with the very low silver staining intensity of these spots indicated that very low levels of BbAMA-1 are present in Babesia merozoites.

Allelic polymorphisms in apical membrane antigen-1 are responsible for evasion of antibody-mediated inhibition in Plasmodium falciparum

Apical membrane antigen-1 (AMA-1) is a target of antibodies that inhibit invasion of Plasmodium falciparum into human erythrocytes and is a candidate for inclusion in a malaria vaccine. We have identified a line of P. falciparum (W2mef) less susceptible to anti-AMA1 antibodies raised to the protein from a heterologous parasite line (3D7). We have constructed transgenic P. falciparum expressing heterologous AMA-1 alleles. In vitro invasion assays show that these transgenic parasites differ from parental lines in susceptibility to inhibitory antibodies, providing direct evidence that sequence polymorphisms within AMA-1 are responsible for evasion of immune responses that inhibit parasite invasion. We also generated a parasite line that would express a chimeric AMA-1 protein, in which highly polymorphic residues within domain 1 were exchanged. Inhibition assays suggest that these residues are not sufficient for inhibition by invasion-blocking antibodies. This study is the first to use P. falciparum allelic exchange to examine the relationship between genetic diversity and susceptibility to protective antibodies. The findings have important implications for the development of an AMA-1-based malaria vaccine.

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.

A single malaria merozoite serine protease mediates shedding of multiple surface proteins by juxtamembrane cleavage

Erythrocyte invasion by the malaria merozoite is accompanied by the regulated discharge of apically located secretory organelles called micronemes. Plasmodium falciparum apical membrane antigen-1 (PfAMA-1), which plays an indispensable role in invasion, translocates from micronemes onto the parasite surface and is proteolytically shed in a soluble form during invasion. We have previously proposed, on the basis of incomplete mass spectrometric mapping data, that PfAMA-1 shedding results from cleavage at two alternative positions. We now show conclusively that the PfAMA-1 ectodomain is shed from the merozoite solely as a result of cleavage at a single site, just 29 residues away from the predicted transmembrane-spanning sequence. Remarkably, this cleavage is mediated by the same membrane-bound parasite serine protease as that responsible for shedding of the merozoite surface protein-1 (MSP-1) complex, an abundant, glycosylphosphatidylinositol-anchored multiprotein complex. Processing of MSP-1 is essential for invasion. Our results indicate the presence on the merozoite surface of a multifunctional serine sheddase with a broad substrate specificity. We further demonstrate that translocation and shedding of PfAMA-1 is an actin-independent process.

Variation in the expression of a Plasmodium falciparum protein family implicated in erythrocyte invasion

The PfRH protein family of Plasmodium falciparum is implicated in erythrocyte invasion. Here we report variations in the sequence, transcription, and protein expression of four different members of this family in three parasite lines, 3D7, T996, and FCB1. There are sequence polymorphisms in PfRH1, PfRH2a, PfRH2b, and PfRH3, ranging from variations across repeat regions to a 585-bp deletion in the 3' end of PfRH2b in T996. Not all the genes are transcribed: although all members of the family are transcribed in 3D7 and T996, PfRH2a and PfRH2b are not transcribed in FCB1. The PfRH1, PfRH2a, and PfRH2b proteins are expressed in late schizonts and merozoites and are located in apical organelles and on the apical surface. However, the PfRH1 protein does not appear to be correctly targeted to the apex in 3D7 and T996. In contrast, the PfRH1 protein is present at the apical end of FCB1 merozoites, but the PfRH2a and PfRH2b proteins are undetectable. The apparent redundancy in the PfRH family of proteins at the level of gene number and sequence and the variations in transcription and protein expression may allow the parasite to use alternative invasion pathways.

Independent translocation of two micronemal proteins in developing Plasmodium falciparum merozoites

Apical membrane antigen 1 of Plasmodium falciparum (PfAMA1) contains an N-terminal propeptide that is removed prior to the translocation of the mature protein onto the merozoite surface. We localized unprocessed PfAMA1 to the microneme organelles of the intraerythrocytic schizont. The results have suggested that the processed form of PfAMA1 translocates from the microneme compartment independently of another microneme protein, EBA175, which is also involved in the invasion of human erythrocytes.

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

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