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

Mass Spectrometric and Artificial Intelligence-Based Identification of the Secretome of Plasmodium falciparum Merozoites to Provide Novel Candidates for Vaccine Development Pipeline

PURPOSE

Merozoites are the only extracellular form of blood stage parasites, making it a worthwhile target. Multiple invasins that are stored in the merozoite apical organelles, are secreted just prior to invasion, and mediates its interaction with RBC. A comprehensive identification of all these secreted invasins is lacking and this study addresses that gap.

EXPERIMENTAL DESIGN

Pf3D7 merozoites were enriched and triggered to discharge apical organelle contents by exposure to ionic conditions mimicking that of blood plasma. The secreted proteins were separated from cellular contents and both the fractions were subjected to proteomic analysis. Also, the identified secreted proteins were subjected to GO, PPI network analysis, and AI-based in silico approach to understand their vaccine candidacy.

RESULTS

A total of 63 proteins were identified in the secretory fraction with membrane and apical organellar localization. This includes various MSPs, micronemal EBAs and rhoptry bulb proteins, which play a crucial role in initial and late merozoite attachment, and majority of them qualified as vaccine candidates.

CONCLUSION AND CLINICAL RELEVANCE

We, for the first time, report the secretory repertoire of merozoite and its status for vaccine candidacy. This information can be utilized to develop better invasion blocking multisubunit vaccines, comprising of immunological epitopes from several secreted invasins.

Tryptophan C-mannosylation is critical for Plasmodium falciparum transmission

Tryptophan C-mannosylation stabilizes proteins bearing a thrombospondin repeat (TSR) domain in metazoans. Here we show that Plasmodium falciparum expresses a DPY19 tryptophan C-mannosyltransferase in the endoplasmic reticulum and that DPY19-deficiency abolishes C-glycosylation, destabilizes members of the TRAP adhesin family and inhibits transmission to mosquitoes. Imaging P. falciparum gametogenesis in its entirety in four dimensions using lattice light-sheet microscopy reveals defects in ΔDPY19 gametocyte egress and exflagellation. While egress is diminished, ΔDPY19 microgametes still fertilize macrogametes, forming ookinetes, but these are abrogated for mosquito infection. The gametogenesis defects correspond with destabilization of MTRAP, which we show is C-mannosylated in P. falciparum, and the ookinete defect is concordant with defective CTRP secretion on the ΔDPY19 background. Genetic complementation of DPY19 restores ookinete infectivity, sporozoite production and C-mannosylation activity. Therefore, tryptophan C-mannosylation by DPY19 ensures TSR protein quality control at two lifecycle stages for successful transmission of the human malaria parasite.

Here, Lopaticki et al. show that Plasmodium falciparum expresses a Dpy19 C-mannosyltransferase in the endoplasmic reticulum that glycosylates TSR domains. Functional characterization shows that PfDpy19 plays a critical role in transmission through mosquitoes as PfDpy19-deficiency abolishes C-glycosylation and destabilizes proteins relevant for gametogenesis and oocyst formation.

Thrombospondin Related Anonymous Protein Superfamily in Vector-Borne Apicomplexans: The Parasite’s Toolkit for Cell Invasion

Apicomplexan parasites transmitted by vectors, including Babesia spp. and Plasmodium spp., cause severe disease in both humans and animals. These parasites have a complex life cycle during which they migrate, invade, and replicate in contrasting hosts such as the mammal and the invertebrate vector. The interaction of parasites with the host cell is mediated by adhesive proteins which play a key role in the different cellular processes regarding successful progression of the life cycle. Thrombospondin related anonymous protein (TRAP) is a superfamily of adhesins that are involved in motility, invasion and egress of the parasite. These proteins are stored and released from apical organelles and have either one or two types of adhesive domains, namely thrombospondin type 1 repeat and von Willebrand factor type A, that upon secretion are located in the extracellular portion of the molecule. Proteins from the TRAP superfamily have been intensively studied in Plasmodium species and to a lesser extent in Babesia spp., where they have proven to be functionally relevant throughout the entire parasite’s journey both in the arthropod vector and in the mammalian host. In recent years new findings provided answers to the role of TRAP proteins and in some cases the function of these adhesins during the parasite’s life cycle was redefined. In this review we will discuss the current knowledge of the diverse roles of the TRAP superfamily in vector-borne parasites from Class Aconoidasida. We will focus on the varied approaches that allowed the understanding of protein function and the relevance of TRAP- superfamily throughout the entire parasite’s cell cycle.

Antibodies against a short region of PfRipr inhibit Plasmodium falciparum merozoite invasion and PfRipr interaction with Rh5 and SEMA7A

Plasmodium falciparum merozoite invasion into erythrocytes is an essential step of the blood-stage cycle, survival of parasites, and malaria pathogenesis. P. falciparum merozoite Rh5 interacting protein (PfRipr) forms a complex with Rh5 and CyRPA in sequential molecular events leading to erythrocyte invasion. Recently we described PfRipr as a conserved protein that induces strain-transcending growth inhibitory antibodies in in vitro assays. However, being a large and complex protein of 1086 amino acids (aa) with 87 cysteine residues, PfRipr is difficult to express in conventional expression systems towards vaccine development. In this study we sought to identify the most potent region of PfRipr that could be developed to overcome difficulties related to protein expression, as well as to elucidate the invasion inhibitory mechanism of anti-PfRipr antibodies. Using the wheat germ cell-free system, Ecto- PfRipr and truncates of approximately 200 aa were expressed as soluble proteins. We demonstrate that antibodies against PfRipr truncate 5 (PfRipr_5: C₇₂₀-D₉₃₄), a region within the PfRipr C-terminal EGF-like domains, potently inhibit merozoite invasion. Furthermore, the antibodies strongly block PfRipr/Rh5 interaction, as well as that between PfRipr and its erythrocyte-surface receptor, SEMA7A. Taken together, PfRipr_5 is a potential candidate for further development as a blood-stage malaria vaccine.

Three F-actin assembly centers regulate organelle inheritance, cell-cell communication and motility in Toxoplasma gondii

Toxoplasma gondii possesses a limited set of actin-regulatory proteins and relies on only three formins (FRMs) to nucleate and polymerize actin. We combined filamentous actin (F-actin) chromobodies with gene disruption to assign specific populations of actin filaments to individual formins. FRM2 localizes to the apical juxtanuclear region and participates in apicoplast inheritance. Restricted to the residual body, FRM3 maintains the intravacuolar cell-cell communication. Conoidal FRM1 initiates a flux of F-actin crucial for motility, invasion and egress. This flux depends on myosins A and H and is controlled by phosphorylation via PKG (protein kinase G) and CDPK1 (calcium-dependent protein kinase 1) and by methylation via AKMT (apical lysine methyltransferase). This flux is independent of microneme secretion and persists in the absence of the glideosome-associated connector (GAC). This study offers a coherent model of the key players controlling actin polymerization, stressing the importance of well-timed post-translational modifications to power parasite motility.

Cellular dissection of malaria parasite invasion of human erythrocytes using viable Plasmodium knowlesi merozoites

Plasmodium knowlesi, a zoonotic parasite causing severe-to-lethal malaria disease in humans, has only recently been adapted to continuous culture with human red blood cells (RBCs). In comparison with the most virulent human malaria, Plasmodium falciparum, there are, however, few cellular tools available to study its biology, in particular direct investigation of RBC invasion by blood-stage P. knowlesi merozoites. This leaves our current understanding of biological differences across pathogenic Plasmodium spp. incomplete. Here, we report a robust method for isolating viable and invasive P. knowlesi merozoites to high purity and yield. Using this approach, we present detailed comparative dissection of merozoite invasion (using a variety of microscopy platforms) and direct assessment of kinetic differences between knowlesi and falciparum merozoites. We go on to assess the inhibitory potential of molecules targeting discrete steps of invasion in either species via a quantitative invasion inhibition assay, identifying a class of polysulfonate polymer able to efficiently inhibit invasion in both, providing a foundation for pan-Plasmodium merozoite inhibitor development. Given the close evolutionary relationship between P. knowlesi and P. vivax, the second leading cause of malaria-related morbidity, this study paves the way for inter-specific dissection of invasion by all three major pathogenic malaria species.

Malaria Parasite Invasion: Achieving Superb Resolution

It is only in the last decade that sub-cellular resolution of red cell invasion by the malaria parasite Plasmodium falciparum has been possible. Here we look back on the development of methodologies that led to this possibility and the subsequent advancements made in understanding this key event in malaria disease.

Reassessing the mechanics of parasite motility and host-cell invasion

The capacity to migrate is fundamental to multicellular and single-celled life. Apicomplexan parasites, an ancient protozoan clade that includes malaria parasites (Plasmodium) and Toxoplasma, achieve remarkable speeds of directional cell movement. This rapidity is achieved via a divergent actomyosin motor system, housed within a narrow compartment that lies underneath the length of the parasite plasma membrane. How this motor functions at a mechanistic level during motility and host cell invasion is a matter of debate. Here, we integrate old and new insights toward refining the current model for the function of this motor with the aim of revitalizing interest in the mechanics of how these deadly pathogens move.

Tools for attenuation of gene expression in malaria parasites

An understanding of the biology of Plasmodium parasites, which are the causative agents of the disease malaria, requires study of gene function. Various reverse genetic tools have been described for determining gene function. These tools can be broadly grouped as trans- and cis-acting. Trans-acting tools control gene functions through synthetic nucleic acid probe molecules matching the sequence of the gene of interest. Once delivered to the parasite, the probe engages with the mRNA of the target gene and attenuates its function. Cis-acting tools control gene function through elements introduced into the gene of interest by DNA transfection. The expression of the modified gene can be controlled using external agents, typically small molecule ligands. In this review, we discuss the strengths and weaknesses of these tools to guide researchers in selecting the appropriate tool for studies of gene function, and for guiding future refinements of these tools.

Plasmodium Merozoite TRAP Family Protein Is Essential for Vacuole Membrane Disruption and Gamete Egress from Erythrocytes

Surface-associated TRAP (thrombospondin-related anonymous protein) family proteins are conserved across the phylum of apicomplexan parasites. TRAP proteins are thought to play an integral role in parasite motility and cell invasion by linking the extracellular environment with the parasite submembrane actomyosin motor. Blood stage forms of the malaria parasite Plasmodium express a TRAP family protein called merozoite-TRAP (MTRAP) that has been implicated in erythrocyte invasion. Using MTRAP-deficient mutants of the rodent-infecting P. berghei and human-infecting P. falciparum parasites, we show that MTRAP is dispensable for erythrocyte invasion. Instead, MTRAP is essential for gamete egress from erythrocytes, where it is necessary for the disruption of the gamete-containing parasitophorous vacuole membrane, and thus for parasite transmission to mosquitoes. This indicates that motor-binding TRAP family members function not just in parasite motility and cell invasion but also in membrane disruption and cell egress.

The Binding of Plasmodium falciparum Adhesins and Erythrocyte Invasion Proteins to Aldolase Is Enhanced by Phosphorylation

Aldolase has been implicated as a protein coupling the actomyosin motor and cell surface adhesins involved in motility and host cell invasion in the human malaria parasite Plasmodium falciparum. It binds to the cytoplasmic domain (CTD) of type 1 membrane proteins of the thrombospondin-related anonymous protein (TRAP) family. Other type 1 membrane proteins located in the apical organelles of merozoites, the form of the parasite that invades red blood cells, including apical membrane antigen 1 (AMA1) and members of the erythrocyte binding ligand (EBL) and reticulocyte binding homologue (RH) protein families have been implicated in host cell binding and invasion. Using a direct binding method we confirm that TRAP and merozoite TRAP (MTRAP) bind aldolase and show that the interaction is mediated by more than just the C-terminal six amino acid residues identified previously. Single amino acid substitutions in the MTRAP CTD abolished binding to aldolase. The CTDs of AMA1 and members of the EBL and RH protein families also bound to aldolase. MTRAP competed with AMA1 and RH4 for binding to aldolase, indicating overlapping binding sites. MTRAP CTD was phosphorylated in vitro by both calcium dependent kinase 1 (CDPK1) and protein kinase A, and this modification increased the affinity of binding to aldolase by ten-fold. Phosphorylation of the CTD of members of the EBL and RH protein families also increased their affinity for aldolase in some cases. To examine whether or not MTRAP expressed in asexual blood stage parasites is phosphorylated, it was tagged with GFP, purified and analysed, however no phosphorylation was detected. We propose that CTD binding to aldolase may be dynamically modulated by phosphorylation, and there may be competition for aldolase binding between different CTDs. The use and efficiency of alternate invasion pathways may be determined by the affinity of adhesins and cell invasion proteins for aldolase, in addition to their host ligand specificity.

Evaluating the use of heparin for synchronization of in vitro culture of Plasmodium falciparum

The malaria parasite Plasmodium falciparum infects human erythrocytes and reproduces asexually through an intraerythrocytic developmental cycle. In vitro culture of P. falciparum allows investigation of the parasite's blood-stage development, which spans approximately 48h from the time of invasion to the lysis of mature schizonts to release merozoites. To focus on a specific step in the developmental cycle, synchronization techniques are utilized. d-Sorbitol treatment and the Percoll-sorbitol method have been used; however, these techniques have limitations in terms of the degree of synchronization achieved, the amount of synchronized parasite acquired, convenience, reproducibility, and cost. Here, we evaluated an existing synchronization method involving heparin. Heparin reversibly inhibits erythrocyte invasion by P. falciparum merozoites. We confirm that parasite cultures can be inexpensively, reproducibly, and tightly synchronized by combining a sorbitol step to limit cultures to the ring stages and by adding and removing heparin to manipulate the window during which merozoites can invade erythrocytes.

Proteomic Analysis of the Plasmodium berghei Gametocyte Egressome and Vesicular bioID of Osmiophilic Body Proteins Identifies Merozoite TRAP-like Protein (MTRAP) as an Essential Factor for Parasite Transmission

Malaria transmission from an infected host to the mosquito vector requires the uptake of intraerythrocytic sexual precursor cells into the mosquito midgut. For the release of mature extracellular gametes two membrane barriers-the parasite parasitophorous vacuole membrane and the host red blood cell membrane-need to be dissolved. Membrane lysis occurs after the release of proteins from specialized secretory vesicles including osmiophilic bodies. In this study we conducted proteomic analyses of the P. berghei gametocyte egressome and developed a vesicular bioID approach to identify hitherto unknown proteins with a potential function in gametocyte egress. This first Plasmodium gametocyte egressome includes the proteins released by the parasite during the lysis of the parasitophorous vacuole membrane and red blood cell membrane. BioID of the osmiophilic body protein MDV1/PEG3 revealed a vesicular proteome of these gametocyte-specific secretory vesicles. Fluorescent protein tagging and gene deletion approaches were employed to validate and identify a set of novel factors essential for this lysis and egress process. Our study provides the first in vivo bioID for a rodent malaria parasite and together with the first Plasmodium gametocyte egressome identifies MTRAP as a novel factor essential for mosquito transmission. Our data provide an important resource for proteins potentially involved in a key step of gametogenesis.

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.