Revealing the sequence and resulting cellular morphology of receptor-ligand interactions during Plasmodium falciparum invasion of erythrocytes

Quantitative phospho-proteomics reveals the Plasmodium merozoite triggers pre-invasion host kinase modification of the red cell cytoskeleton

The invasive blood-stage malaria parasite - the merozoite - induces rapid morphological changes to the target erythrocyte during entry. However, evidence for active molecular changes in the host cell that accompany merozoite invasion is lacking. Here, we use invasion inhibition assays, erythrocyte resealing and high-definition imaging to explore red cell responses during invasion. We show that although merozoite entry does not involve erythrocyte actin reorganisation, it does require ATP to complete the process. Towards dissecting the ATP requirement, we present an in depth quantitative phospho-proteomic analysis of the erythrocyte during each stage of invasion. Specifically, we demonstrate extensive increased phosphorylation of erythrocyte proteins on merozoite attachment, including modification of the cytoskeletal proteins beta-spectrin and PIEZO1. The association with merozoite contact but not active entry demonstrates that parasite-dependent phosphorylation is mediated by host-cell kinase activity. This provides the first evidence that the erythrocyte is stimulated to respond to early invasion events through molecular changes in its membrane architecture.

Merozoite surface proteins in red blood cell invasion, immunity and vaccines against malaria

Malaria accounts for an enormous burden of disease globally, with Plasmodium falciparum accounting for the majority of malaria, and P. vivax being a second important cause, especially in Asia, the Americas and the Pacific. During infection with Plasmodium spp., the merozoite form of the parasite invades red blood cells and replicates inside them. It is during the blood-stage of infection that malaria disease occurs and, therefore, understanding merozoite invasion, host immune responses to merozoite surface antigens, and targeting merozoite surface proteins and invasion ligands by novel vaccines and therapeutics have been important areas of research. Merozoite invasion involves multiple interactions and events, and substantial processing of merozoite surface proteins occurs before, during and after invasion. The merozoite surface is highly complex, presenting a multitude of antigens to the immune system. This complexity has proved challenging to our efforts to understand merozoite invasion and malaria immunity, and to developing merozoite antigens as malaria vaccines. In recent years, there has been major progress in this field, and several merozoite surface proteins show strong potential as malaria vaccines. Our current knowledge on this topic is reviewed, highlighting recent advances and research priorities.

The authors summarize current knowledge of merozoite surface proteins of malaria parasites; their function in invasion, processing of surface proteins before, during and after invasion, their importance as targets of immunity, and the current status of malaria vaccines that target merozoite surface proteins.

The mechanics of malaria parasite invasion of the human erythrocyte - towards a reassessment of the host cell contribution

Despite decades of research, we still know little about the mechanics of Plasmodium host cell invasion. Fundamentally, while the essential or non‐essential nature of different parasite proteins is becoming clearer, their actual function and how each comes together to govern invasion are poorly understood. Furthermore, in recent years an emerging world view is shifting focus away from the parasite actin–myosin motor being the sole force responsible for entry to an appreciation of host cell dynamics and forces and their contribution to the process. In this review, we discuss merozoite invasion of the erythrocyte, focusing on the complex set of pre‐invasion events and how these might prime the red cell to facilitate invasion. While traditionally parasite interactions at this stage have been viewed simplistically as mediating adhesion only, recent work makes it apparent that by interacting with a number of host receptors and signalling pathways, combined with secretion of parasite‐derived lipid material, that the merozoite may initiate cytoskeletal re‐arrangements and biophysical changes in the erythrocyte that greatly reduce energy barriers for entry. Seen in this light Plasmodium invasion may well turn out to be a balance between host and parasite forces, much like that of other pathogen infection mechanisms.

Plasmodium falciparum Adhesins Play an Essential Role in Signalling and Activation of Invasion into Human Erythrocytes

The most severe form of malaria in humans is caused by the protozoan parasite Plasmodium falciparum. The invasive form of malaria parasites is termed a merozoite and it employs an array of parasite proteins that bind to the host cell to mediate invasion. In Plasmodium falciparum, the erythrocyte binding-like (EBL) and reticulocyte binding-like (Rh) protein families are responsible for binding to specific erythrocyte receptors for invasion and mediating signalling events that initiate active entry of the malaria parasite. Here we have addressed the role of the cytoplasmic tails of these proteins in activating merozoite invasion after receptor engagement. We show that the cytoplasmic domains of these type 1 membrane proteins are phosphorylated in vitro. Depletion of PfCK2, a kinase implicated to phosphorylate these cytoplasmic tails, blocks P. falciparum invasion of red blood cells. We identify the crucial residues within the PfRh4 cytoplasmic domain that are required for successful parasite invasion. Live cell imaging of merozoites from these transgenic mutants show they attach but do not penetrate erythrocytes implying the PfRh4 cytoplasmic tail conveys signals important for the successful completion of the invasion process.

Localisation-based imaging of malarial antigens during erythrocyte entry reaffirms a role for AMA1 but not MTRAP in invasion

Microscopy-based localisation of proteins during malaria parasite (Plasmodium) invasion of the erythrocyte is widely used for tentative assignment of protein function. To date, however, imaging has been limited by the rarity of invasion events and the poor resolution available, given the micron size of the parasite, which leads to a lack of quantitative measures for definitive localisation. Here, using computational image analysis we have attempted to assign relative protein localisation during invasion using wide-field deconvolution microscopy. By incorporating three-dimensional information we present a detailed assessment of known parasite effectors predicted to function during entry but as yet untested or for which data are equivocal. Our method, termed longitudinal intensity profiling, resolves confusion surrounding the localisation of apical membrane antigen 1 (AMA1) at the merozoite–erythrocyte junction and predicts that the merozoite thrombospondin-related anonymous protein (MTRAP) is unlikely to play a direct role in the mechanics of entry, an observation supported with additional biochemical evidence. This approach sets a benchmark for imaging of complex micron-scale events and cautions against simplistic interpretations of small numbers of representative images for the assignment of protein function or prioritisation of candidates as therapeutic targets.

Highlighted Article: Here we develop a high-definition imaging approach to dissect and assign function to proteins involved in the rapid process of malaria parasite invasion of the human erythrocyte.

Characterization of Inhibitors and Monoclonal Antibodies That Modulate the Interaction between Plasmodium falciparum Adhesin PfRh4 with Its Erythrocyte Receptor Complement Receptor 1

Plasmodium falciparum parasites must invade red blood cells to survive within humans. Entry into red blood cells is governed by interactions between parasite adhesins and red blood cell receptors. Previously we identified that P. falciparum reticulocyte binding protein-like homologue 4 (PfRh4) binds to complement receptor 1 (CR1) to mediate entry of malaria parasites into human red blood cells. In this report we characterize a collection of anti-PfRh4 monoclonal antibodies and CR1 protein fragments that modulate the interaction between PfRh4 and CR1. We identify an anti-PfRh4 monoclonal that blocks PfRh4-CR1 interaction in vitro, inhibits PfRh4 binding to red blood cells, and as a result abolishes the PfRh4-CR1 invasion pathway in P. falciparum. Epitope mapping of anti-PfRh4 monoclonal antibodies identified distinct functional regions within PfRh4 involved in modulating its interaction with CR1. Furthermore, we designed a set of protein fragments based on extensive mutagenesis analyses of the PfRh4 binding site on CR1 and determined their interaction affinities using surface plasmon resonance. These CR1 protein fragments bind tightly to PfRh4 and also function as soluble inhibitors to block PfRh4 binding to red blood cells and to inhibit the PfRh4-CR1 invasion pathway. Our findings can aid future efforts in designing specific single epitope antibodies to block P. falciparum invasion via complement receptor 1.

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.

Macrolides rapidly inhibit red blood cell invasion by the human malaria parasite, Plasmodium falciparum

BACKGROUND

Malaria invasion of red blood cells involves multiple parasite-specific targets that are easily accessible to inhibitory compounds, making it an attractive target for antimalarial development. However, no current antimalarial agents act against host cell invasion.

RESULTS

Here, we demonstrate that the clinically used macrolide antibiotic azithromycin, which is known to kill human malaria asexual blood-stage parasites by blocking protein synthesis in their apicoplast, is also a rapid inhibitor of red blood cell invasion in human (Plasmodium falciparum) and rodent (P. berghei) malarias. Multiple lines of evidence demonstrate that the action of azithromycin in inhibiting parasite invasion of red blood cells is independent of its inhibition of protein synthesis in the parasite apicoplast, opening up a new strategy to develop a single drug with multiple parasite targets. We identified derivatives of azithromycin and erythromycin that are better invasion inhibitors than parent compounds, offering promise for development of this novel antimalarial strategy.

CONCLUSIONS

Safe and effective macrolide antibiotics with dual modalities could be developed to combat malaria and reduce the parasite's options for resistance.

Parasite Calcineurin Regulates Host Cell Recognition and Attachment by Apicomplexans

Apicomplexans invade a variety of metazoan host cells through mechanisms involving host cell receptor engagement and secretion of parasite factors to facilitate cellular attachment. We find that the parasite homolog of calcineurin, a calcium-regulated phosphatase complex central to signal transduction in eukaryotes, also contributes to host cell invasion by the malaria parasite Plasmodium falciparum and related Toxoplasma gondii. Using reverse-genetic and chemical-genetic approaches, we determine that calcineurin critically regulates and stabilizes attachment of extracellular P. falciparum to host erythrocytes before intracellular entry and has similar functions in host cell engagement by T. gondii. Calcineurin-mediated Plasmodium invasion is strongly associated with host receptors required for host cell recognition, and calcineurin function distinguishes this form of receptor-mediated attachment from a second mode of host-parasite adhesion independent of host receptors. This specific role of calcineurin in coordinating physical interactions with host cells highlights an ancestral mechanism for parasitism used by apicomplexans.

Malaria. A forward genetic screen identifies erythrocyte CD55 as essential for Plasmodium falciparum invasion

Efforts to identify host determinants for malaria have been hindered by the absence of a nucleus in erythrocytes, which precludes genetic manipulation in the cell in which the parasite replicates. We used cultured red blood cells derived from hematopoietic stem cells to carry out a forward genetic screen for Plasmodium falciparum host determinants. We found that CD55 is an essential host factor for P. falciparum invasion. CD55-null erythrocytes were refractory to invasion by all isolates of P. falciparum because parasites failed to attach properly to the erythrocyte surface. Thus, CD55 is an attractive target for the development of malaria therapeutics. Hematopoietic stem cell-based forward genetic screens may be valuable for the identification of additional host determinants of malaria pathogenesis.

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.