Invasion of red blood cells by malaria parasites

CRISPR/Cas9-engineering of Kell null erythrocytes to unveil host targeted irresistible antimalarial

Malaria elimination faces challenges from drug resistance, stemming from mutations within the parasite's genetic makeup. Genetic adaptations in key erythrocyte proteins offer malaria protection in endemic regions. Emulating nature's approach, and implementing methodologies to render indispensable host proteins inactive, holds the potential to reshape antimalarial therapy. This study delves into the functional implication of the single-span membrane protein Kell ectodomain, which shares consensus sequence with the zinc endopeptidase family, possesses extracellular enzyme activity crucial for parasite invasion into host erythrocytes. Through generating Kell-null erythrocytes from an erythroid progenitor, BEL-A, we demonstrate the indispensable nature of Kell activity in P. falciparum invasion. Additionally, thiorphan, a metallo-endopeptidase inhibitor, which specifically inhibits Kell activity, inhibited Plasmodium infection at nanomolar concentrations. Interestingly, individuals in malaria-endemic regions exhibit low Kell expression and activity, indicating a plausible Plasmodium-induced evolutionary pressure. Both thiorphan and its prodrug racecadotril, demonstrated potent antimalarial activity in vivo, highlighting Kell's protease role in invasion and proposing thiorphan as a promising host-oriented antimalarial therapeutic.

The crystal structure of an uncharacterized domain of P113 from Plasmodium falciparum

The surface protein P113 serves as a membrane-anchored protein that tethers the Plasmodium falciparum RH5 complex, including its associated partners CyRPA and RIPR, to the parasite surface. This anchoring mechanism ensures the proper localization and stabilization of RH5, facilitating its critical interaction with the host erythrocyte receptor basigin during erythrocyte invasion. Here, the helical-rich domain of P113 (residues 311-679) from a Plasmodium species was expressed, purified and crystallized to elucidate its structural and functional characteristics. The recombinant protein, with a molecular weight of approximately 44 kDa, was confirmed to be monomeric in solution. Crystallization in 0.5 mM MES pH 6.0, 22% PEG 3350 yielded high-quality crystals, enabling the determination of the structure of the apo form at 1.7 Å resolution. The structure revealed a predominant α-helical composition, with two distinct left-handed orthogonal four-helix bundles formed by helices α1-α4 and α6-α9 connected by a disordered region. Sequence analysis demonstrated high conservation of P113 across all human-infecting Plasmodium species, including P. vivax, P. malariae, P. falciparum and P. ovale, as well as in Plasmodium species infecting primates and rodents. Protein-protein interaction analysis using the STRING tool identified P113 as a hub protein that interacts with ten proteins, including small nuclear ribonucleoprotein, DNA polymerase delta small subunit and RIPR, which is part of the RH5-CyRPA-RIPR complex. AlphaFold predictions further elucidated the interaction patterns, revealing moderate to strong interaction scores (0.39-0.74) with key partners. Notably, the helical-rich domain of P113 was identified as the critical binding region for PF3D7_0308000, with key interaction sites mapped to residues Asp475, Arg381, Lys386, Asn390, Asp392 and Lys533. These findings provide critical insights into the structural and functional roles of P113 and its interaction network, advancing our understanding of its molecular mechanisms in Plasmodium biology.

Stable Plasmodium falciparum merozoite surface protein-1 allelic diversity despite decreasing parasitaemia in children with multiple malaria infections

BACKGROUND

Individuals experiencing recurrent malaria infections encounter a variety of alleles with each new infection. This ongoing allelic diversity influences the development of naturally acquired immunity and it can inform vaccine efficacy. To investigate the diversity and infection variability, Plasmodium falciparum merozoite surface protein 1 (PfMSP1), a crucial protein for parasite invasion and immune response, was assessed in parasites isolated from children in the Junju cohort, Kilifi County, who experienced at least 10 febrile malaria episodes over a span of 5 years.

METHODS

Pfmsp1 C-terminal region (Pfmsp119) was genotyped using PCR followed by capillary sequencing in blood samples collected from the children. Sequenced reads were trimmed and aligned to the P. falciparum 3D7 reference genome. Single nucleotide polymorphisms in the Pfmsp119 region were identified from the alignment and grouped to distinct microhaplotypes whose changing frequency over time were examined across the multiple infection episodes. In addition, the variability of infections in the population was assessed using nucleotide and haplotype diversity indices.

RESULTS

A total of eleven microhaplotypes were observed across all malaria episodes. There were 3 prevalent microhaplotypes, E-KSNG-L, Q-KSNG-L, and Q-KSNG-F in the population. Conversely, microhaplotypes such as Q-KNNG-L, E-KSSR-L, E-KNNG-L, E-KSSG-L, E-TSSR-L (3D7), Q-TSSR-L, E-TSSG-L, and E-KSNG-F were rare and maintained at low frequencies. High allelic replacements were observed, however some individuals experienced consecutive re-infections with the same microhaplotype. Notably, PfMSP119 allelic diversity as measured by haplotype diversity was stable, while nucleotide diversity decreased over time with decreasing parasitemia. Parasite PfMSP119 allelic diversity remained stable over the multiple malaria episodes, despite declining parasitaemia levels. In addition, there are reveal dynamic PfMSP119 allelic replacements across parasite infection episodes.

CONCLUSIONS

Allelic diversity was stable over time in individuals, based on this limited polymorphic region and small sample size, suggesting that there are no significant shifts in allele frequencies or replacements due to alleles being maintained under balancing selection. The dominant alleles in the population are those frequently observed in these children with multiple malaria episodes, further suggesting that early exposure to dominant alleles does not shift their frequency in the population or prevent repeat infection with the same alleles in subsequent infections. However, a blood stage merozoite vaccine is likely to require a multi-allelic formulation.

Chemically Engineered Peptide Efficiently Blocks Malaria Parasite Entry into Red Blood Cells

Chemical peptide engineering, enabled by residue insertion, backbone cyclization, and incorporation of an additional disulfide bond, led to a unique cyclic peptide that efficiently inhibits the invasion of red blood cells by malaria parasites. The engineered peptide exhibits a 20-fold enhanced affinity toward its receptor (PfAMA1) compared to the native peptide ligand (PfRON2), as determined by surface plasmon resonance. In-vitro parasite growth inhibition assay revealed augmented potency of the engineered peptide. The structure of the PfAMA1-cyclic peptide complex, predicted by the deep learning-based structure prediction tool ColabFold-AlphaFold2 with protein-cyclic peptide complex offset, provided valuable insights into the observed activity of the peptide analogs. Rational editing of the peptide backbone and side chain described here proved to be an effective strategy for designing peptide-based inhibitors to interfere with disease-related protein-protein interactions.

Emerging nanotechnology-driven drug delivery solutions for malaria: Addressing drug resistance and improving therapeutic success

Malaria remains the fifth deadliest parasitic infection worldwide, despite significant advancements in technology. A major challenge in combating this disease lies in the growing resistance of malaria parasites to antimalarial drugs and insect vectors to insecticides. The emerging inefficacy of artemisinin-based combination therapies (ACTs) further exacerbates the issue. Additionally, the absence of a highly effective malaria vaccine continues to be a significant obstacle. The complex biology of the malaria parasite and the multifaceted nature of the disease contribute to these challenges. Recent advancements in nanotechnology offer promising solutions in malaria treatment, providing benefits such as improved drug stability, sustained release, and targeted delivery to specific cells. Encapsulation technology, in particular, addresses critical limitations like poor solubility, low bioavailability, and frequent dosing requirements. Thus, this review explores innovative strategies to combat malaria, focusing on nanotechnology-based antimalarial formulations and their evaluation in vitro and in vivo. Moreover, the study highlights the SAR of potent antimalarial compounds, molecular markers linked with drug resistance, ACTs, advocates for eco-friendly approaches, nanotechnology-driven vaccines, and new antimalarial agents with their specific targets.

Malian field isolates provide insight into Plasmodium malariae intra-erythrocytic development and invasion

Plasmodium malariae is the third most prevalent human malaria parasite species and contributes significantly to morbidity. Nevertheless, our comprehension of this parasite's biology remains limited, primarily due to its frequent co-infections with other species and the lack of a continuous in vitro culture system. To effectively combat and eliminate this overlooked parasite, it is imperative to acquire a better understanding of this species. In this study, we embarked on an investigation of P. malariae, including exploring its clinical disease characteristics, molecular aspects of red blood cell (RBC) invasion, and host-cell preferences. We conducted our research using parasites collected from infected individuals in Mali. Our findings revealed anaemia in most of P. malariae infected participants presented, in both symptomatic and asymptomatic cases. Regarding RBC invasion, quantified by an adapted flow cytometry based method, our study indicated that none of the seven antibodies tested, against receptors known for their role in P. falciparum invasion, had any impact on the ability of P. malariae to penetrate the host cells. However, when RBCs were pre-treated with various enzymes (neuraminidase, trypsin, and chymotrypsin), we observed a significant reduction in P. malariae invasion, albeit not a complete blockade. Furthermore, in a subset of P. malariae samples, we observed the parasite's capability to invade reticulocytes. These results suggest that P. malariae employs alternative pathways to enter RBCs of different maturities, which may differ from those used by P. falciparum.

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.

TKL family kinases in human apicomplexan pathogens

Apicomplexan parasites are the primary causative agents of many human diseases, including malaria, toxoplasmosis, and cryptosporidiosis. These opportunistic pathogens undergo complex life cycles with multiple developmental stages, wherein many key steps are regulated by phosphorylation mechanisms. The genomes of apicomplexan pathogens contain protein kinases from different groups including tyrosine kinase-like (TKL) family proteins. Although information on the role of TKL kinases in apicomplexans is quite limited, recent studies have revealed the important role of this family of proteins in apicomplexan biology. TKL kinases in these protozoan pathogens show unique organization with many novel domains thus making them attractive candidates for drug development. In this mini review, we summarize the current understanding of the role of TKL kinases in human apicomplexan pathogens' (Toxoplasma gondii, Plasmodium falciparum and Cryptosporidium parvum) biology and pathogenesis.

Application of optical tweezer technology reveals that PfEBA and PfRH ligands, not PfMSP1, play a central role in Plasmodium falciparum merozoite-erythrocyte attachment

Malaria pathogenesis and parasite multiplication depend on the ability of Plasmodium merozoites to invade human erythrocytes. Invasion is a complex multi-step process involving multiple parasite proteins which can differ between species and has been most extensively studied in P. falciparum. However, dissecting the precise role of individual proteins has to date been limited by the availability of quantifiable phenotypic assays. In this study, we apply a new approach to assigning function to invasion proteins by using optical tweezers to directly manipulate recently egressed P. falciparum merozoites and erythrocytes and quantify the strength of attachment between them, as well as the frequency with which such attachments occur. Using a range of inhibitors, antibodies, and genetically modified strains including some generated specifically for this work, we quantitated the contribution of individual P. falciparum proteins to these merozoite-erythrocyte attachment interactions. Conditional deletion of the major P. falciparum merozoite surface protein PfMSP1, long thought to play a central role in initial attachment, had no impact on the force needed to pull merozoites and erythrocytes apart, whereas interventions that disrupted the function of several members of the EBA-175 like Antigen (PfEBA) family and Reticulocyte Binding Protein Homologue (PfRH) invasion ligand families did have a significant negative impact on attachment. Deletion of individual PfEBA and PfRH ligands reinforced the known redundancy within these families, with the deletion of some ligands impacting detachment force while others did not. By comparing over 4000 individual merozoite-erythrocyte interactions in a range of conditions and strains, we establish that the PfEBA/PfRH families play a central role in P. falciparum merozoite attachment, not the major merozoite surface protein PfMSP1.

NDV targets the invasion pathway in malaria parasite through cell surface sialic acid interaction

Merozoites utilize sialic acids on the red blood cell (RBC) cell surface to rapidly adhere to and invade the RBCs. Newcastle disease virus (NDV) displays a strong affinity toward membrane-bound sialic acids. Incubation of NDV with the malaria parasites dose-dependently reduces its cellular viability. The antiplasmodial activity of NDV is specific, as incubation with Japanese encephalitis virus, duck enteritis virus, infectious bronchitis virus, and influenza virus did not affect the parasite propagation. Interestingly, NDV is reducing more than 80% invasion when RBCs are pretreated with the virus. Removal of the RBC surface proteins or the NDV coat proteins results in disruption of the virus binding to RBC. It suggests the involvement of specific protein: ligand interaction in virus binding. We established that the virus engages with the parasitized RBCs (PRBCs) through its hemagglutinin neuraminidase (HN) protein by recognizing sialic acid-containing glycoproteins on the cell surface. Blocking of the HN protein with free sialic acid or anti-HN antibodies abolished the virus binding as well as its ability to reduce parasite growth. Interestingly, the purified HN from the virus alone could inhibit the parasite's growth in a dose-dependent manner. NDV binds strongly to knobless murine parasite strain Plasmodium yoelii and restricted the parasite growth in mice. Furthermore, the virus was found to preferentially target the PRBCs compared to normal erythrocytes. Immunolocalization studies reveal that NDV is localized on the plasma membrane as well as weakly inside the PRBC. NDV causes neither any infection nor aggregation of the human RBCs. Our findings suggest that NDV is a potential candidate for developing targeted drug delivery platforms for the Plasmodium-infected RBCs.

Plasmodium falciparum proteases as new drug targets with special focus on metalloproteases

Malaria poses a significant global health threat particularly due to the prevalence of Plasmodium falciparum infection. With the emergence of parasite resistance to existing drugs including the recently discovered artemisinin, ongoing research seeks novel therapeutic avenues within the malaria parasite. Proteases are promising drug targets due to their essential roles in parasite biology, including hemoglobin digestion, merozoite invasion, and egress. While exploring the genomic landscape of Plasmodium falciparum, it has been revealed that there are 92 predicted proteases, with only approximately 14 of them having been characterized. These proteases are further distributed among 26 families grouped into five clans: aspartic proteases, cysteine proteases, metalloproteases, serine proteases, and threonine proteases. Focus on metalloprotease class shows further role in organelle processing for mitochondria and apicoplasts suggesting the potential of metalloproteases as viable drug targets. Holistic understanding of the parasite intricate life cycle and identification of potential drug targets are essential for developing effective therapeutic strategies against malaria and mitigating its devastating global impact.

Protein lipidation in health and disease: molecular basis, physiological function and pathological implication

Posttranslational modifications increase the complexity and functional diversity of proteins in response to complex external stimuli and internal changes. Among these, protein lipidations which refer to lipid attachment to proteins are prominent, which primarily encompassing five types including S-palmitoylation, N-myristoylation, S-prenylation, glycosylphosphatidylinositol (GPI) anchor and cholesterylation. Lipid attachment to proteins plays an essential role in the regulation of protein trafficking, localisation, stability, conformation, interactions and signal transduction by enhancing hydrophobicity. Accumulating evidence from genetic, structural, and biomedical studies has consistently shown that protein lipidation is pivotal in the regulation of broad physiological functions and is inextricably linked to a variety of diseases. Decades of dedicated research have driven the development of a wide range of drugs targeting protein lipidation, and several agents have been developed and tested in preclinical and clinical studies, some of which, such as asciminib and lonafarnib are FDA-approved for therapeutic use, indicating that targeting protein lipidations represents a promising therapeutic strategy. Here, we comprehensively review the known regulatory enzymes and catalytic mechanisms of various protein lipidation types, outline the impact of protein lipidations on physiology and disease, and highlight potential therapeutic targets and clinical research progress, aiming to provide a comprehensive reference for future protein lipidation research.

Genetic complexity alters drug susceptibility of asexual and gametocyte stages of Plasmodium falciparum to antimalarial candidates

Malaria elimination requires interventions able to target both the asexual blood stage (ABS) parasites and transmissible gametocyte stages of Plasmodium falciparum. Lead antimalarial candidates are evaluated against clinical isolates to address key concerns regarding efficacy and to confirm that the current, circulating parasites from endemic regions lack resistance against these candidates. While this has largely been performed on ABS parasites, limited data are available on the transmission-blocking efficacy of compounds with multistage activity. Here, we evaluated the efficacy of lead antimalarial candidates against both ABS parasites and late-stage gametocytes side-by-side, against clinical P. falciparum isolates from southern Africa. We additionally correlated drug efficacy to the genetic diversity of the clinical isolates as determined with a panel of well-characterized, genome-spanning microsatellite markers. Our data indicate varying sensitivities of the isolates to key antimalarial candidates, both for ABS parasites and gametocyte stages. While ABS parasites were efficiently killed, irrespective of genetic complexity, antimalarial candidates lost some gametocytocidal efficacy when the gametocytes originated from genetically complex, multiple-clone infections. This suggests a fitness benefit to multiclone isolates to sustain transmission and reduce drug susceptibility. In conclusion, this is the first study to investigate the efficacy of antimalarial candidates on both ABS parasites and gametocytes from P. falciparum clinical isolates where the influence of parasite genetic complexity is highlighted, ultimately aiding the malaria elimination agenda.

Cryogenic enrichment of Plasmodium falciparum gametocytes from spiked whole blood

Each individual cell type typically requires a unique set of conditions for optimal cryopreservation outcome, which relates to its specific response to cryoprotective agent (CPA) toxicity, osmotic behavior and sensitivity to ice crystallization. Cryopreservation of heterogenous cell populations is therefore exceedingly difficult as it requires separate and often conflicting conditions for each cell type. Conversely, these contrasting conditions could be utilized to favor cryogenic preference of a single cell population within a heterogenous sample, leading to its enrichment by elimination of remaining cells. To establish proof-of-concept for this overall approach, a protocol was developed for the cryogenic enrichment of Plasmodium falciparum gametocytes from whole blood. To accomplish this goal, we evaluated the effects of CPAs and cooling conditions during cryopreservation of whole blood samples spiked with P. falciparum gametocytes. We identified that cooling to -80 °C at a rate of -1 °C/min in the presence of 11 % glycerol selectively favors recovery of gametocytes. This protocol eliminates 95.3 ± 1.7 % of total blood cells and recovers 43.2 ± 6.5 % of parasites, leading to a 19-fold enrichment as assessed by microscopic examination of blood smears. This protocol is tunable, where gametocyte enrichment 900-fold may be feasible, however there is an apparent tradeoff in overall parasite recovery. Although translation of this protocol for point-of-care testing for malaria presents many challenges, the overall approach of cryogenic purification may prove useful for alternative diagnostic applications.

Characterization of merozoite-specific thrombospondin-related anonymous protein (MTRAP) in Plasmodium vivax and P. knowlesi parasites

Plasmodium vivax, the most widespread human malaria parasite, and P. knowlesi, an emerging Plasmodium that infects humans, are the phylogenetically closest malarial species that infect humans, which may induce cross-species reactivity across most co-endemic areas in Southeast Asia. The thrombospondin-related anonymous protein (TRAP) family is indispensable for motility and host cell invasion in the growth and development of Plasmodium parasites. The merozoite-specific TRAP (MTRAP), expressed in blood-stage merozoites, is supposed to be essential for human erythrocyte invasion. We aimed to characterize MTRAPs in blood-stage P. vivax and P. knowlesi parasites and ascertain their cross-species immunoreactivity. Recombinant P. vivax and P. knowlesi MTRAPs of full-length ectodomains were expressed in a mammalian expression system. The MTRAP-specific immunoglobulin G, obtained from immune animals, was used in an immunofluorescence assay for subcellular localization and invasion inhibitory activity in blood-stage parasites was determined. The cross-species humoral immune responses were analyzed in the sera of patients with P. vivax or P. knowlesi infections. The MTRAPs of P. vivax (PvMTRAP) and P. knowlesi (PkMTRAP) were localized on the rhoptry body of merozoites in blood-stage parasites. Both anti-PvMTRAP and anti-PkMTRAP antibodies inhibited erythrocyte invasion of blood-stage P. knowlesi parasites. The humoral immune response to PvMTRAP showed high immunogenicity, longevity, and cross-species immunoreactivity with P. knowlesi. MTRAPs are promising candidates for development of vaccines and therapeutics against vivax and knowlesi malaria.

The Need for Novel Asexual Blood-Stage Malaria Vaccine Candidates for Plasmodium falciparum

Extensive control efforts have significantly reduced malaria cases and deaths over the past two decades, but in recent years, coupled with the COVID-19 pandemic, success has stalled. The WHO has urged the implementation of a number of interventions, including vaccines. The modestly effective RTS,S/AS01 pre-erythrocytic vaccine has been recommended by the WHO for use in sub-Saharan Africa against Plasmodium falciparum in children residing in moderate to high malaria transmission regions. A second pre-erythrocytic vaccine, R21/Matrix-M, was also recommended by the WHO on 3 October 2023. However, the paucity and limitations of pre-erythrocytic vaccines highlight the need for asexual blood-stage malaria vaccines that prevent disease caused by blood-stage parasites. Few asexual blood-stage vaccine candidates have reached phase 2 clinical development, and the challenges in terms of their efficacy include antigen polymorphisms and low immunogenicity in humans. This review summarizes the history and progress of asexual blood-stage malaria vaccine development, highlighting the need for novel candidate vaccine antigens/molecules.

Plasmodium chabaudi Merozoites Obtained through a Simpler Method Do Not Survive in Classically Activated Macrophages

Malaria is caused by apicomplexan parasites of the Plasmodium genus. Plasmodium chabaudi is an excellent animal model for the study of human malaria caused by P. falciparum. Merozoites invade erythrocytes but are also found in other host cells including macrophages from the spleen and liver. Methodologies for obtaining merozoites usually involve treatment with protease inhibitors. However, merozoites obtained in this way may have their enzymatic profile altered and, therefore, are not ideal for cell-interaction assays. We report the obtainment of P. chabaudi merozoites naturally egressed from a synchronous erythrocyte population infected with schizonts forms. Merozoites had their infectivity and ultrastructure analyzed. Interaction assays were performed with mice erythrocytes and classically activated mice peritoneal macrophages, a very well-established classic model. Obtained merozoites were able to kill mice and efficiently infect erythrocytes. Interestingly, a lower merozoite:erythrocyte ratio resulted in a higher percentage of infected erythrocytes. We describe a simpler method for obtaining viable and infective merozoites. Classically activated macrophages killed merozoites, suggesting that these host cells may not serve as reservoirs for these parasites. These findings have implications for our understanding of P. chabaudi merozoite biology and may improve the comprehension of their host-parasite relationship.

In silico modeling revealed phytomolecules derived from Cymbopogon citratus (DC.) leaf extract as promising candidates for malaria therapy

The emergence of varying levels of resistance to currently available antimalarial drugs significantly threatens global health. This factor heightens the urgency to explore bioactive compounds from natural products with a view to discovering and developing newer antimalarial drugs with novel mode of actions. Therefore, we evaluated the inhibitory effects of sixteen phytocompounds from Cymbopogon citratus leaf extract against Plasmodium falciparum drug targets such as P. falciparum circumsporozoite protein (PfCSP), P. falciparum merozoite surface protein 1 (PfMSP1) and P. falciparum erythrocyte membrane protein 1 (PfEMP1). In silico approaches including molecular docking, pharmacophore modeling and 3D-QSAR were adopted to analyze the inhibitory activity of the compounds under consideration. The molecular docking results indicated that a compound swertiajaponin from C. citratus exhibited a higher binding affinity (-7.8 kcal/mol) to PfMSP1 as against the standard artesunate-amodiaquine (-6.6 kcal/mol). Swertiajaponin also formed strong hydrogen bond interactions with LYS29, CYS30, TYR34, ASN52, GLY55 and CYS28 amino acid residues. In addition, quercetin another compound from C. citratus exhibited significant binding energies -6.8 and -8.3 kcal/mol with PfCSP and PfEMP1, respectively but slightly lower than the standard artemether-lumefantrine with binding energies of -7.4 kcal/mol against PfCSP and -8.7 kcal/mol against PfEMP1. Overall, the present study provides evidence that swertiajaponin and other phytomolecules from C. citratus have modulatory properties toward P. falciparum drug targets and thus may warrant further exploration in early drug discovery efforts against malaria. Furthermore, these findings lend credence to the folkloric use of C. citratus for malaria treatment.Communicated by Ramaswamy H. Sarma.

Reticulocyte Binding Protein Homologue 5 is a target of balancing selection in the Plasmodium falciparum population of Papua New Guinea

Plasmodium falciparum Reticulocyte Binding Protein Homologue (RH5), a leading malaria vaccine candidate, is essential for erythrocyte invasion by the parasite, interacting with the human host receptor, basigin. RH5 has a small number of polymorphisms relative to other blood-stage antigens, and in vitro studies have shown that vaccine-induced antibodies raised against RH5 are strain-transcending, however most studies investigating RH5 diversity have been done in Africa. Understanding the genetic diversity and evolution of malaria antigens in other regions is important for their validation as vaccine candidates. In this study the rh5 gene was sequenced in 677 samples from a longitudinal cohort of Papua New Guinean (PNG) children aged 1-3 years. Of 677 samples successfully sequenced, 566 were identified as independent infections (i.e. one of each pair of identical sequences within hosts were removed). A total of 14 non-synonymous polymorphisms were identified, eight that are 'common' in the population (minor allele frequency > 1%), with 44 haplotypes ranging in frequency from 1% to 21%. Modeling of common SNPs to the cryo-EM structure of the RH5/CyRPA/RIPR complex mapped them to the Basigin binding site and near the contact point of CyRPA. Tajima's D analyses of the corresponding nucleotide sequences produced positive values indicating potential hotspots of balancing selection. We attempted to confirm whether these signals were due to immune selection by measuring the rate of polymorphism between independent infections within the same host, and the association with clinical symptoms, however, no such associations were identified. Together these results suggest that while there is evidence of balancing selection driving RH5 diversity in the PNG P. falciparum population, immune escape was not observed within the cohort of young children. Limited immunity and therefore low selective pressure may explain this result, alternatively other evolutionary forces may contribute to balancing selection at the RH5-BSG binding interface in PNG.

Ultrastructural alterations due to sodium metavanadate treatment in the blood stages of Plasmodium yoelii yoelii

Malaria is a potentially mortal disease caused by parasites of the genus Plasmodium spp. It has a wide distribution in the world and unfortunately there are several factors that make its control difficult; among which the development of pharmacological resistance to the different drugs used to treat this disease stands out, which makes it necessary to design new compounds that have an antimalarial effect. Previous studies have shown that vanadium has a broad antiparasitic spectrum and is also safe for the host, so the objective of this research was to evaluate the antimalarial potential of sodium metavanadate (SM) and to analyze the ultrastructural changes in parasites exposed. The method consisted of inoculating CD-1 male mice with Plasmodium yoelii yoelii and administering a 10 mg/kg/day dose of SM orally for 4 days. On the fifth day, whole blood samples were obtained, processed for ultrastructural analysis, and the changes in the different parasite stages were compared against the control. Our results showed that SM decreased parasitemia compared to the group that did not receive treatment and modified the ultrastructure in all parasitic stages because it damaged the membranes, causing alterations mainly in the nucleus and in the mitochondria as well as the loss of cellular organization, which could affect the integrity of these parasites and decrease its viability.

Effective cell membrane tension protects red blood cells against malaria invasion

A critical step in how malaria parasites invade red blood cells (RBCs) is the wrapping of the membrane around the egg-shaped merozoites. Recent experiments have revealed that RBCs can be protected from malaria invasion by high membrane tension. While cellular and biochemical aspects of parasite actomyosin motor forces during the malaria invasion have been well studied, the important role of the biophysical forces induced by the RBC membrane-cytoskeleton composite has not yet been fully understood. In this study, we use a theoretical model for lipid bilayer mechanics, cytoskeleton deformation, and membrane-merozoite interactions to systematically investigate the influence of effective RBC membrane tension, which includes contributions from the lipid bilayer tension, spontaneous tension, interfacial tension, and the resistance of cytoskeleton against shear deformation on the progression of membrane wrapping during the process of malaria invasion. Our model reveals that this effective membrane tension creates a wrapping energy barrier for a complete merozoite entry. We calculate the tension threshold required to impede the malaria invasion. We find that the tension threshold is a nonmonotonic function of spontaneous tension and undergoes a sharp transition from large to small values as the magnitude of interfacial tension increases. We also predict that the physical properties of the RBC cytoskeleton layer-particularly the resting length of the cytoskeleton-play key roles in specifying the degree of the membrane wrapping. We also found that the shear energy of cytoskeleton deformation diverges at the full wrapping state, suggesting the local disassembly of the cytoskeleton is required to complete the merozoite entry. Additionally, using our theoretical framework, we predict the landscape of myosin-mediated forces and the physical properties of the RBC membrane in regulating successful malaria invasion. Our findings on the crucial role of RBC membrane tension in inhibiting malaria invasion can have implications for developing novel antimalarial therapeutic or vaccine-based strategies.

Pathogenicity and virulence of malaria: Sticky problems and tricky solutions

Infections with Plasmodium falciparum and Plasmodium vivax cause over 600,000 deaths each year, concentrated in Africa and in young children, but much of the world's population remain at risk of infection. In this article, we review the latest developments in the immunogenicity and pathogenesis of malaria, with a particular focus on P. falciparum, the leading malaria killer. Pathogenic factors include parasite-derived toxins and variant surface antigens on infected erythrocytes that mediate sequestration in the deep vasculature. Host response to parasite toxins and to variant antigens is an important determinant of disease severity. Understanding how parasites sequester, and how antibody to variant antigens could prevent sequestration, may lead to new approaches to treat and prevent disease. Difficulties in malaria diagnosis, drug resistance, and specific challenges of treating P. vivax pose challenges to malaria elimination, but vaccines and other preventive strategies may offer improved disease control.

Pre-gelation staining expansion microscopy for visualisation of the Plasmodium liver stage

Fluorescence and light microscopy are important tools in the history of natural science. However, the resolution of microscopes is limited by the diffraction of light. One possible method to circumvent this physical restriction is the recently developed expansion microscopy (ExM). However, the original ultrastructure ExM (U-ExM) protocol is very time-consuming, and some epitopes are lost during the process. In this study, we developed a shortened pre-gelation staining ExM (PS-ExM) protocol and tested it to investigate the Plasmodium liver stage. The protocol presented in this study allows expanding of pre-stained samples, which results in shorter incubation times, better preservation of some epitopes and the advantage that non-expanded controls can be performed alongside using the same staining protocol. The protocol applicability was accessed throughout the Plasmodium liver stage, showing isotropic five-fold expansion. Furthermore, we used PS-ExM to visualise parasite mitochondria as well as the association of lysosomes to the parasitophorous vacuole membrane (PVM) as an example of visualising host-pathogen interaction. We are convinced that this new tool will be helpful for a deeper understanding of the biology of the Plasmodium liver stage.

Protein kinase PfPK2 mediated signalling is critical for host erythrocyte invasion by malaria parasite

Signalling pathways in malaria parasite remain poorly defined and major reason for this is the lack of understanding of the function of majority of parasite protein kinases and phosphatases in parasite signalling and its biology. In the present study, we have elucidated the function of Protein Kinase 2 (PfPK2), which is known to be indispensable for the survival of human malaria parasite Plasmodium falciparum. We demonstrate that it is involved in the invasion of host erythrocytes, which is critical for establishing infection. In addition, PfPK2 may also be involved in the maturation of the parasite post-invasion. PfPK2 regulates the release of microneme proteins like Apical Membrane Antigen 1 (AMA1), which facilitates the formation of Tight Junction between the merozoite and host erythrocyte- a key step in the process of invasion. Comparative phosphoproteomics studies revealed that PfPK2 may be involved in regulation of several key proteins involved in invasion and signalling. Furthermore, PfPK2 regulates the generation of cGMP and the release of calcium in the parasite, which are key second messengers for the process of invasion. These and other studies have shed light on a novel signalling pathway in which PfPK2 acts as an upstream regulator of important cGMP-calcium signalling, which plays an important role in parasite invasion.

The structure of a Plasmodium vivax Tryptophan Rich Antigen domain suggests a lipid binding function for a pan-Plasmodium multi-gene family

Tryptophan Rich Antigens (TRAgs) are encoded by a multi-gene family found in all Plasmodium species, but are significantly expanded in P. vivax and closely related parasites. We show that multiple P. vivax TRAgs are expressed on the merozoite surface and that one, PVP01_0000100 binds red blood cells with a strong preference for reticulocytes. Using X-ray crystallography, we solved the structure of the PVP01_0000100 C-terminal tryptophan rich domain, which defines the TRAg family, revealing a three-helical bundle that is conserved across Plasmodium and has structural homology with lipid-binding BAR domains involved in membrane remodelling. Biochemical assays confirm that the PVP01_0000100 C-terminal domain has lipid binding activity with preference for sulfatide, a glycosphingolipid present in the outer leaflet of plasma membranes. Deletion of the putative orthologue in P. knowlesi, PKNH_1300500, impacts invasion in reticulocytes, suggesting a role during this essential process. Together, this work defines an emerging molecular function for the Plasmodium TRAg family.

Molecular Mechanisms of Persistence in Protozoan Parasites

Protozoan parasites are known for their remarkable capacity to persist within the bodies of vertebrate hosts, which frequently results in prolonged infections and the recurrence of diseases. Understanding the molecular mechanisms that underlie the event of persistence is of paramount significance to develop innovative therapeutic approaches, given that these pathways still need to be thoroughly elucidated. The present article provides a comprehensive overview of the latest developments in the investigation of protozoan persistence in vertebrate hosts. The focus is primarily on the function of persisters, their formation within the host, and the specific molecular interactions between host and parasite while they persist. Additionally, we examine the metabolomic, transcriptional, and translational changes that protozoan parasites undergo during persistence within vertebrate hosts, focusing on major parasites such as Plasmodium spp., Trypanosoma spp., Leishmania spp., and Toxoplasma spp. Key findings of our study suggest that protozoan parasites deploy several molecular and physiological strategies to evade the host immune surveillance and sustain their persistence. Furthermore, some parasites undergo stage differentiation, enabling them to acclimate to varying host environments and immune challenges. More often, stressors such as drug exposure were demonstrated to impact the formation of protozoan persisters significantly. Understanding the molecular mechanisms regulating the persistence of protozoan parasites in vertebrate hosts can reinvigorate our current insights into host-parasite interactions and facilitate the development of more efficacious disease therapeutics.

Recent increase in low complexity polygenomic infections and sialic acid-independent invasion pathways in Plasmodium falciparum from Western Gambia

BACKGROUND

The malaria parasite Plasmodium falciparum utilizes multiple alternative receptor-ligand interactions for the invasion of human erythrocytes. While some P. falciparum clones make use of sialic acid (SA) residues on the surface of the human glycophorin receptors to invade the erythrocyte, others use alternative receptors independent of sialic acid residues. We hypothesized that over the years, intensified malaria control interventions and declining prevalence in The Gambia have resulted in a selection of parasites with a dominant invasion pathways and ligand expression profiles.

METHODS

Blood samples were collected from 65 malaria-infected participants with uncomplicated malaria across 3 years (2015, 2016, and 2021). Genetic diversity was determined by genotyping the merozoite surface protein 2 (msp2) polymorphic gene of P. falciparum. Erythrocyte invasion phenotypes were determined using neuraminidase, trypsin, and chymotrypsin enzymes, known to cleave different receptors from the surface of the erythrocyte. Schizont-stage transcript levels were obtained for a panel of 6 P. falciparum invasion ligand genes (eba175, eba181, Rh2b, Rh4, Rh5, and clag2) using 48 successfully cultured isolates.

RESULTS

Though the allelic heterozygosity of msp2 repeat region decreased as expected with reduced transmission, there was an increase in infections with more than a single msp2 allelotype from 2015 to 2021. The invasion phenotypes of these isolates were mostly SA independent with a continuous increase from 2015 to 2021. Isolates from 2021 were highly inhibited by chymotrypsin treatment compared to isolates from 2015 and 2016. Higher invasion inhibition for 2021 isolates was further obtained following erythrocyte treatment with a combination of chymotrypsin and trypsin. The transcript levels of invasion ligand genes varied across years. However, levels of clag2, a rhoptry-associated protein, were higher in 2015 and 2016 isolates than in 2021 isolates, while Rh5 levels were higher in 2021 compared to other years.

CONCLUSIONS

Overall, these findings suggest increasing mixed infections with an increase in the use of sialic-acid independent invasion pathways by P. falciparum clinical isolates in the Western part of Gambia.

A heparin-binding protein of Plasmodium berghei is associated with merozoite invasion of erythrocytes

BACKGROUND

Malaria caused by Plasmodium species is a prominent public health concern worldwide, and the infection of a malarial parasite is transmitted to humans through the saliva of female Anopheles mosquitoes. Plasmodium invasion is a rapid and complex process. A critical step in the blood-stage infection of malarial parasites is the adhesion of merozoites to red blood cells (RBCs), which involves interactions between parasite ligands and receptors. The present study aimed to investigate a previously uncharacterized protein, PbMAP1 (encoded by PBANKA_1425900), which facilitates Plasmodium berghei ANKA (PbANKA) merozoite attachment and invasion via the heparan sulfate receptor.

METHODS

PbMAP1 protein expression was investigated at the asexual blood stage, and its specific binding activity to both heparan sulfate and RBCs was analyzed using western blotting, immunofluorescence, and flow cytometry. Furthermore, a PbMAP1-knockout parasitic strain was established using the double-crossover method to investigate its pathogenicity in mice.

RESULTS

The PbMAP1 protein, primarily localized to the P. berghei membrane at the merozoite stage, is involved in binding to heparan sulfate-like receptor on RBC surface of during merozoite invasion. Furthermore, mice immunized with the PbMAP1 protein or passively immunized with sera from PbMAP1-immunized mice exhibited increased immunity against lethal challenge. The PbMAP1-knockout parasite exhibited reduced pathogenicity.

CONCLUSIONS

PbMAP1 is involved in the binding of P. berghei to heparan sulfate-like receptors on RBC surface during merozoite invasion.

Cultivation of Asexual Intraerythrocytic Stages of Plasmodium falciparum

Successfully developed in 1976, the continuous in vitro culture of Plasmodium falciparum has many applications in the field of malaria research. It has become an important experimental model that directly uses a human pathogen responsible for a high prevalence of morbidity and mortality in many parts of the world and is a major source of biological material for immunological, biochemical, molecular, and pharmacological studies. Until present, the basic techniques described by Trager and Jensen and Haynes et al. remain unchanged in many malaria research laboratories. Nonetheless, different factors, including culture media, buffers, serum substitutes and supplements, sources of erythrocytes, and conditions of incubation (especially oxygen concentration), have been modified by different investigators to adapt the original technique in their laboratories or enhance the in vitro growth of the parasites. The possible effects and benefits of these modifications for the continuous cultivation of asexual intraerythrocytic stages of P. falciparum, as well as future challenges in developing a serum-free cultivation system and axenic cultures, are discussed.

Course of Plasmodium infection studied using 2D-COS on human erythrocytes

BACKGROUND

The threat of malaria is still present in the world. Recognizing the type of parasite is important in determining a treatment plan. The golden routine involves microscopic diagnostics of Giemsa-stained thin blood smears, however, alternative methods are also constantly being sought, in order to gain an additional insight into the course of the disease. Spectroscopic methods, e.g., Raman spectroscopy, are becoming increasingly popular, due to the non-destructive nature of these techniques.

METHODS

The study included patients hospitalized for malaria caused by Plasmodium falciparum or Plasmodium vivax, in the Department of Infectious Diseases at the University Hospital in Krakow, Poland, as well as healthy volunteers. The aim of this study was to assess the possibility of using Raman spectroscopy and 2D correlation (2D-COS) spectroscopy in understanding the structural changes in erythrocytes depending on the type of attacking parasite. EPR spectroscopy and two-trace two-dimensional (2T2D) correlation was also used to examine the specificity of paramagnetic centres found in the infected human blood.

RESULTS

Two-dimensional (2D) correlation spectroscopy facilitates the identification of the hidden relationship, allowing for the discrimination of Raman spectra obtained during the course of disease in human red blood cells, infected by P. falciparum or P. vivax. Synchronous cross-peaks indicate the processes taking place inside the erythrocyte during the export of the parasite protein towards the cell membrane. In contrast, moieties that generate asynchronous 2D cross-peaks are characteristic of the respective ligand-receptor domains. These changes observed during the course of the infection, have different dynamics for P. falciparum and P. vivax, as indicated by the asynchronous correlation cross-peaks. Two-trace two-dimensional (2T2D) spectroscopy, applied to EPR spectra of blood at the beginning of the infection, showed differences between P. falciparum and P. vivax.

CONCLUSIONS

A unique feature of 2D-COS is the ability to discriminate the collected Raman and EPR spectra. The changes observed during the course of a malaria infection have different dynamics for P. falciparum and P. vivax, indicated by the reverse sequence of events. For each type of parasite, a specific recycling process for iron was observed in the infected blood.

Histone modification analysis reveals common regulators of gene expression in liver and blood stage merozoites of Plasmodium parasites

Gene expression in malaria parasites is subject to various layers of regulation, including histone post-translational modifications (PTMs). Gene regulatory mechanisms have been extensively studied during the main developmental stages of Plasmodium parasites inside erythrocytes, from the ring stage following invasion to the schizont stage leading up to egress. However, gene regulation in merozoites that mediate the transition from one host cell to the next is an understudied area of parasite biology. Here, we sought to characterize gene expression and the corresponding histone PTM landscape during this stage of the parasite lifecycle through RNA-seq and ChIP-seq on P. falciparum blood stage schizonts, merozoites, and rings, as well as P. berghei liver stage merozoites. In both hepatic and erythrocytic merozoites, we identified a subset of genes with a unique histone PTM profile characterized by a region of H3K4me3 depletion in their promoter. These genes were upregulated in hepatic and erythrocytic merozoites and rings, had roles in protein export, translation, and host cell remodeling, and shared a DNA motif. These results indicate that similar regulatory mechanisms may underlie merozoite formation in the liver and blood stages. We also observed that H3K4me2 was deposited in gene bodies of gene families encoding variant surface antigens in erythrocytic merozoites, which may facilitate switching of gene expression between different members of these families. Finally, H3K18me and H2K27me were uncoupled from gene expression and were enriched around the centromeres in erythrocytic schizonts and merozoites, suggesting potential roles in the maintenance of chromosomal organization during schizogony. Together, our results demonstrate that extensive changes in gene expression and histone landscape occur during the schizont-to-ring transition to facilitate productive erythrocyte infection. The dynamic remodeling of the transcriptional program in hepatic and erythrocytic merozoites makes this stage attractive as a target for novel anti-malarial drugs that may have activity against both the liver and blood stages.

Changes in K+ Concentration as a Signaling Mechanism in the Apicomplexa Parasites Plasmodium and Toxoplasma

During their life cycle, apicomplexan parasites pass through different microenvironments and encounter a range of ion concentrations. The discovery that the GPCR-like SR25 in Plasmodium falciparum is activated by a shift in potassium concentration indicates that the parasite can take advantage of its development by sensing different ionic concentrations in the external milieu. This pathway involves the activation of phospholipase C and an increase in cytosolic calcium. In the present report, we summarize the information available in the literature regarding the role of potassium ions during parasite development. A deeper understanding of the mechanisms that allow the parasite to cope with ionic potassium changes contributes to our knowledge about the cell cycle of Plasmodium spp.

Sulfonylpiperazine compounds prevent Plasmodium falciparum invasion of red blood cells through interference with actin-1/profilin dynamics

With emerging resistance to frontline treatments, it is vital that new antimalarial drugs are identified to target Plasmodium falciparum. We have recently described a compound, MMV020291, as a specific inhibitor of red blood cell (RBC) invasion, and have generated analogues with improved potency. Here, we generated resistance to MMV020291 and performed whole genome sequencing of 3 MMV020291-resistant populations. This revealed 3 nonsynonymous single nucleotide polymorphisms in 2 genes; 2 in profilin (N154Y, K124N) and a third one in actin-1 (M356L). Using CRISPR-Cas9, we engineered these mutations into wild-type parasites, which rendered them resistant to MMV020291. We demonstrate that MMV020291 reduces actin polymerisation that is required by the merozoite stage parasites to invade RBCs. Additionally, the series inhibits the actin-1-dependent process of apicoplast segregation, leading to a delayed death phenotype. In vitro cosedimentation experiments using recombinant P. falciparum proteins indicate that potent MMV020291 analogues disrupt the formation of filamentous actin in the presence of profilin. Altogether, this study identifies the first compound series interfering with the actin-1/profilin interaction in P. falciparum and paves the way for future antimalarial development against the highly dynamic process of actin polymerisation.

An analytical model describing the mechanics of erythrocyte membrane wrapping during active invasion of a plasmodium falciparum merozoite

The invasion of a merozoite into an erythrocyte by membrane wrapping is a hallmark of malaria pathogenesis. The invasion involves biomechanical interactions whereby the merozoite exerts actomyosin-based forces to push itself into and through the erythrocyte membrane while concurrently inducing biochemical damage to the erythrocyte membrane. Whereas the biochemical damage process has been investigated, the detailed mechanistic understanding of the invasion mechanics remains limited. Thus, the current study aimed to develop a mathematical model describing the mechanical factors involved in the merozoite invasion into an erythrocyte and explore the invasion mechanics. A shell theory model was developed comprising constitutive, equilibrium and governing equations of the deformable erythrocyte membrane to predict membrane mechanics during the wrapping of an entire non-deformable ellipsoidal merozoite. Predicted parameters include principal erythrocyte membrane deformations and stresses, wrapping and indentation forces, and indentation work. The numerical investigations considered two limits for the erythrocyte membrane deformation during wrapping (4% and 51% areal strain) and erythrocyte membrane phosphorylation (decrease of membrane elastic modulus from 1 to 0.5 kPa). For an intact erythrocyte, the maximum indentation force was 1 and 8.5 pN, and the indentation work was 1.92 × 10^-18 and 1.40 × 10^-17 J for 4% and 51% areal membrane strain. Phosphorylation damage in the erythrocyte membrane reduced the required indentation work by 50% to 0.97 × 10^-18 and 0.70 × 10^-17 J for 4% and 51% areal strain. The current study demonstrated the developed model's feasibility to provide new knowledge on the physical mechanisms of the merozoite invasion process that contribute to the invasion efficiency towards the discovery of new invasion-blocking anti-malaria drugs.

Autoantibodies against red blood cell antigens are common in a malaria endemic area

Plasmodium falciparum malaria can cause severe anemia. Even after treatment, hematocrit can decrease. The role of autoantibodies against erythrocytes is not clearly elucidated and how common they are, or what they are directed against, is still largely unknown. We have investigated antibodies against erythrocytes in healthy adult men living in a highly malaria endemic area in Uganda. We found antibodies in more than half of the individuals, which is significantly more than in a non-endemic area (Sweden). Some of the Ugandan samples had a broad reactivity where it was not possible to determine the exact target of the autoantibodies, but we also found specific antibodies directed against erythrocyte surface antigens known to be of importance for merozoite invasion such as glycophorin A (anti-En^a, anti-M) and glycophorin B (anti-U, anti-S). In addition, several autoantibodies had partial specificities against glycophorin C and the blood group systems Rh, Diego (located on Band 3), Duffy (located on ACKR1), and Cromer (located on CD55), all of which have been described to be important for malaria and therefore of interest for understanding how autoantibodies could potentially stop parasites from entering the erythrocyte. In conclusion, specific autoantibodies against erythrocytes are common in a malaria endemic area.

A C2 domain containing plasma membrane protein of Plasmodium falciparum merozoites mediates calcium-dependent binding and invasion to host erythrocytes

BACKGROUND

Invasion of red blood cells by Plasmodium falciparum merozoites is governed by multiple receptor-ligand interactions which are critical for bridging the two cells together. The critical function of these ligands for invasion and their direct exposure to the host immune system makes them lucrative vaccine candidates. This necessitates the discovery of new adhesins with less redundancy that mediates the binding of merozoite to the red cell, and furthermore invasion into it. Here we have identified a novel membrane associated antigen (PfC2DMA) that is conserved throughout the Plasmodium species and has a membrane targeting C2 domain at its extreme N-terminal region.

METHODS

Recombinant C2_dom was expressed heterologously in bacteria and purified to homogeneity. Mice antisera against C2_dom was raised and used to check the expression and intraparasitic localization of the protein. RBC and Ca²⁺ ion binding activity of C2_dom was also checked.

RESULTS

C2_dom exhibited specific binding to Ca²⁺ ions and not to Mg²⁺ ions. PfC2DMA localized to the surface of merozoite and recombinant C2_dom bound to the surface of human RBCs. RBC receptor modification by treatment with different enzymes showed that binding of C2_dom to RBC surface is neuraminidase sensitive. Mice antisera raised against C2_dom of Pf C2DMA showed invasion inhibitory effects.

CONCLUSION

Our findings suggest that C2_dom of PfC2DMA binds to surface of red cell in a Ca²⁺-dependent manner, advocating a plausible role in invasion and can serve as a potential novel blood stage vaccine candidate.

Multivalent sialic acid materials for biomedical applications

Sialic acid is a kind of monosaccharide expressed on the non-reducing end of glycoproteins or glycolipids. It acts as a signal molecule combining with its natural receptors such as selectins and siglecs (sialic acid-binding immunoglobulin-like lectins) in intercellular interactions like immunological surveillance and leukocyte infiltration. The last few decades have witnessed the exploration of the roles that sialic acid plays in different physiological and pathological processes and the use of sialic acid-modified materials as therapeutics for related diseases like immune dysregulation and virus infection. In this review, we will briefly introduce the biomedical function of sialic acids in organisms and the utilization of multivalent sialic acid materials for targeted drug delivery as well as therapeutic applications including anti-inflammation and anti-virus.

Erythrocyte membrane with CLIPPKF as biomimetic nanodecoy traps merozoites and attaches to infected red blood cells to prevent Plasmodium infection

BACKGROUND

Malaria remains a serious threat to global public health. With poor efficacies of vaccines and the emergence of drug resistance, novel strategies to control malaria are urgently needed.

RESULTS

We developed erythrocyte membrane-camouflaged nanoparticles loaded with artemether based on the growth characteristics of Plasmodium. The nanoparticles could capture the merozoites to inhibit them from repeatedly infecting normal erythrocytes, owing to the interactions between merozoites and heparin-like molecules on the erythrocyte membrane. Modification with a phosphatidylserine-targeting peptide (CLIPPKF) improved the drug accumulation in infected red blood cells (iRBCs) from the externalized phosphatidylserine induced by Plasmodium infection. In Plasmodium berghei ANKA strain (pbANKA)-infected C57BL/6 mice, the nanoparticles significantly attenuated Plasmodium-induced inflammation, apoptosis, and anemia. We observed reduced weight variation and prolonged survival time in pbANKA-challenged mice, and the nanoparticles showed good biocompatibility and negligible cytotoxicity.

CONCLUSION

Erythrocyte membrane-camouflaged nanoparticles loaded with artemether were shown to provide safe and effective protection against Plasmodium infection.

Genetic polymorphism of the thrombospondin-related apical merozoite protein (TRAMP) of Plasmodium knowlesi in Malaysia

Plasmodium knowlesi is a simian malaria parasite that causes significant zoonotic infections in Southeast Asia, particularly in Malaysia. The Plasmodium thrombospondin-related apical merozoite protein (TRAMP) plays an essential role in the invasion of the parasite into its host erythrocyte. The present study investigated the genetic polymorphism and natural selection of the full length PkTRAMP from P. knowlesi clinical isolates from Malaysia. Blood samples (n = 40) were collected from P. knowlesi malaria patients from Peninsular Malaysia and Malaysian Borneo. The PkTRAMP gene was amplified using PCR, followed by cloning into a plasmid vector and sequenced. Results showed that the nucleotide diversity of PkTRAMP was low (π: 0.009). Z-test results indicated negative (purifying) selection of PkTRAMP. The alignment of the deduced amino acid sequences of PkTRAMP of Peninsular Malaysia and Malaysian Borneo revealed 38 dimorphic sites. A total of 27 haplotypes were identified from the amino acid sequence alignment. Haplotype analysis revealed that there was no clustering of PkTRAMP from Peninsular Malaysia and Malaysian Borneo.

Action mechanisms of metallic compounds on Plasmodium spp

BACKGROUND

Malaria is a parasitic disease with the highest morbidity and mortality worldwide. Unfortunately, during the last decades, the causal agent, Plasmodium spp., has developed resistance to chloroquine and artemisinin. For this reason, metallic compounds have been proposed as an optional treatment since they have shown a potential antimalarial effect with diverse action mechanisms in the parasite and the host.

OBJECTIVE

To show the possible targets of metallic compounds in Plasmodium spp.

CONCLUSION

The metallic compounds are an option attractive to treatment for the malaria, for its low cost and its great activity to reduce parasitemia; however is necessary more studies principally in vivo in order to know the interactions that it can have in an experimental model.

Neutrophils in malaria: A double-edged sword role

Neutrophils are the most abundant leukocytes in human peripheral blood. They form the first line of defense against invading foreign pathogens and might play a crucial role in malaria. According to World Health Organization (WHO), malaria is a globally significant disease caused by protozoan parasites from the Plasmodium genus, and it's responsible for 627,000 deaths in 2020. Neutrophils participate in the defense response against the malaria parasite via phagocytosis and reactive oxygen species (ROS) production. Neutrophils might also be involved in the pathogenesis of malaria by the release of toxic granules and the release of neutrophil extracellular traps (NETs). Intriguingly, malaria parasites inhibit the anti-microbial function of neutrophils, thus making malaria patients more susceptible to secondary opportunistic Salmonella infections. In this review, we will provide a summary of the role of neutrophils during malaria infection, some contradicting mouse model neutrophil data and neutrophil-related mechanisms involved in malaria patients' susceptibility to bacterial infection.

The WD40-Protein PfWLP1 Ensures Stability of the PfCCp-Based Adhesion Protein Complex in Plasmodium falciparum Gametocytes

Members of the WD40-repeat protein family can be found in all eukaryotic proteomes where they usually serve as interaction platforms for the assembly of large protein complexes and are therefore essential for the integrity of these complexes. In the malaria parasite Plasmodium falciparum, the WD40-repeat protein PfWLP1 has been shown to interact with members of distinct adhesion protein complexes in the asexual blood stages and gametocyte stages. In this study, we demonstrate that the presence of PfWLP1 is crucial for both the stability of these gametocyte-specific adhesion complexes as well as for gametocyte maturation and gametogenesis. Using reverse genetics, we generated a PfWLP1-knockdown parasite line for functional characterization of the protein. Knockdown of PfWLP1 resulted in a slight reduction of gametocyte numbers and significantly the impaired ability of the gametocytes to exflagellate. PfWLP1-knockdown further led to reduced protein levels of the Limulus coagulation factor C-like (LCCL)-domain proteins PfCCp1 and PfCCp2, which are key components of the adhesion complexes. These findings suggest that the interaction of PfWLP1 with members of the PfCCp-based adhesion complex ensures complex stability and thereby contributes to gametocyte viability and exflagellation.

Antibody response to a new member of the DBL family (EBP2) after a brief Plasmodium vivax exposure

Plasmodium vivax blood-stage invasion into reticulocyte is critical for parasite development. Thus, validation of novel parasite invasion ligands is essential for malaria vaccine development. Recently, we demonstrated that EBP2, a Duffy binding protein (DBP) paralog, is antigenically distinct from DBP and could not be functionally inhibited by anti-DBP antibodies. Here, we took advantage of a small outbreak of P.vivax malaria, located in a non-malarious area of Brazil, to investigate for the first time IgM/IgG antibodies against EBP2 and DEKnull-2 (an engineering DBPII vaccine) among individuals who had their first and brief exposure to P.vivax (16 cases and 22 non-cases). Our experimental approach included 4 cross sectional surveys at 3-month interval (12-month follow-up). The results demonstrated that while a brief initial P.vivax infection was not efficient to induce IgM/ IgG antibodies to either EBP2 or DEKnull-2, IgG antibodies against DEKnull-2 (but not EBP2) were boosted by recurrent blood-stage infections following treatment. Of interest, in most recurrent P. vivax infections (4 out of 6 patients) DEKnull-2 IgG antibodies were sustained for 6 to 12 months. Polymorphisms in the ebp2 gene does not seem to explain EBP2 low immunogenicity as the ebp2 allele associated with the P.vivax outbreak presented high identity to the original EBP2 isolate used as recombinant protein. Although EBP2 antibodies were barely detectable after a primary episode of P.vivax infection, EBP2 was highly recognized by serum IgG from long-term malaria-exposed Amazonians (range from 35 to 92% according to previous malaria episodes). Taken together, the results showed that individuals with a single and brief exposure to P.vivax infection develop very low anti-EBP2 antibodies, which tend to increase after long-term malaria exposure. Finally, the findings highlighted the potential of DEKnull-2 as a vaccine candidate, as in non-immune individuals anti-DEKnull-2 IgG antibodies were boosted even after a brief exposure to P.vivax blood stages.

Defining ER-mitochondria contact dynamics in Plasmodium falciparum by targeting component of phospholipid synthesis pathway, Phosphatidylserine synthase (PfPSS)

The malaria parasite completes the asexual cycle inside the host erythrocyte, which requires extensive membrane biogenesis for its development and multiplication. Metabolic pathways for the synthesis of membrane phospholipids (PL), including phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylserine (PS), are crucial for parasite survival. Here, we have studied the P. falciparum enzyme responsible for PS synthesis, Phosphatidylserine synthase (PfPSS), GFP targeting approach confirmed it to be localized in the parasite ER as well as in ER-protrusions. Detailed high resolution microscopy, using these transgenic parasites expressing PfPSS-GFP, redefined the dynamics of ER during the intraerythrocytic life cycle and its association with the mitochondria. We report for the first time presence of ER-mitochondria contact (ERMC) in Plasmodium; ERMC is formed by PfPSS containing ER-protrusions, which associate with the mitochondria surface throughout the parasite growth cycle. Further, ERMC is found to be stable and refractory to ER and mitochondrial stresses, suggesting that it is formed through strong tethering complexes. PfPSS was found to interact with other major key enzyme involved in PL synthesis, choline/Etn-phosphotransferase (CEPT), which suggest that ER is the major site for PL biosynthesis. Overall, this study defines the morphological organisation of ERMC which mediates PL synthesis/transport in the Plasmodium.

Molecular profiling reveals features of clinical immunity and immunosuppression in asymptomatic P. falciparum malaria

Clinical immunity to P. falciparum malaria is non-sterilizing, with adults often experiencing asymptomatic infection. Historically, asymptomatic malaria has been viewed as beneficial and required to help maintain clinical immunity. Emerging views suggest that these infections are detrimental and constitute a parasite reservoir that perpetuates transmission. To define the impact of asymptomatic malaria, we pursued a systems approach integrating antibody responses, mass cytometry, and transcriptional profiling of individuals experiencing symptomatic and asymptomatic P. falciparum infection. Defined populations of classical and atypical memory B cells and a T_H2 cell bias were associated with reduced risk of clinical malaria. Despite these protective responses, asymptomatic malaria featured an immunosuppressive transcriptional signature with upregulation of pathways involved in the inhibition of T-cell function, and CTLA-4 as a predicted regulator in these processes. As proof of concept, we demonstrated a role for CTLA-4 in the development of asymptomatic parasitemia in infection models. The results suggest that asymptomatic malaria is not innocuous and might not support the induction of immune processes to fully control parasitemia or efficiently respond to malaria vaccines.

RhopH2 and RhopH3 export enables assembly of the RhopH complex on P. falciparum-infected erythrocyte membranes

RhopH complexes consists of Clag3, RhopH2 and RhopH3 and are essential for growth of Plasmodium falciparum inside infected erythrocytes. Proteins are released from rhoptry organelles during merozoite invasion and trafficked to the surface of infected erythrocytes and enable uptake of nutrients. RhopH3, unlike other RhopH proteins, is required for parasite invasion, suggesting some cellular processes RhopH proteins function as single players rather than a complex. We show the RhopH complex has not formed during merozoite invasion. Clag3 is directly released into the host cell cytoplasm, whilst RhopH2 and RhopH3 are released into the nascent parasitophorous vacuole. Export of RhopH2 and RhopH3 from the parasitophorous vacuole into the infected erythrocyte cytoplasm enables assembly of Clag3/RhopH2/RhopH3 complexes and incorporation into the host cell membrane concomitant with activation of nutrient uptake. This suggests compartmentalisation prevents premature channel assembly before intact complex is assembled at the host cell membrane.

Structures of plasmepsin X from Plasmodium falciparum reveal a novel inactivation mechanism of the zymogen and molecular basis for binding of inhibitors in mature enzyme

Plasmodium falciparum plasmepsin X (PfPMX), involved in the invasion and egress of this deadliest malarial parasite, is essential for its survival and hence considered as an important drug target. We report the first crystal structure of PfPMX zymogen containing a novel fold of its prosegment. A unique twisted loop from the prosegment and arginine 244 from the mature enzyme is involved in zymogen inactivation; such mechanism, not previously reported, might be common for apicomplexan proteases similar to PfPMX. The maturation of PfPMX zymogen occurs through cleavage of its prosegment at multiple sites. Our data provide thorough insights into the mode of binding of a substrate and a potent inhibitor 49c to PfPMX. We present molecular details of inactivation, maturation, and inhibition of PfPMX that should aid in the development of potent inhibitors against pepsin-like aspartic proteases from apicomplexan parasites.

Untangling population structure and genetic diversity of reticulocyte binding protein 2b (PvRBP2b) erythrocytic stage vaccine candidate in worldwide Plasmodium vivax isolates

BACKGROUNDS

Plasmodium vivax is the predominant Plasmodium species distributed extensively in the Americas and Asia-Pacific areas. Encoded protein by Plasmodium vivax Reticulocyte Binding Proteins (PvRBPs) family member are of critical prominence to parasite invasion and have been considered the significant targets in development of malaria vaccine for the blood stage. As high genetic polymorphism of parasites may impede the effectiveness of vaccine development, more research to unraveling genetic polymorphism of pvrbp2b from various geographical regions seems indispensable to map the exact pattern of field isolates.

METHODOLOGY/PRINCIPAL FINDINGS

The aim of this study was to determine the sequences of Iranian pvrbp2b (nt: 502-1896) gene and then, to ascertain polymorphism of pvrbp2b gene, recombination, the level of genetic distances, evaluation of natural selection, and the prediction of B-cell epitopes of Iranian and global P. vivax isolates. Pvrbp2b partial gene was amplified and sequenced from 60 Iranian P. vivax isolates. Iranian pvrbp2b sequences as well as 95 published sequences from five countries were used to evaluate the genetic diversity and neutral evolution signature in worldwide scale. A total of 38 SNPs were identified among 60 Iranian pvrbp2b sequences (32 non-synonymous and 6 synonymous mutations), and 32 amino acid substitutions were observed in 29 positions as compared to Sal-1 sequence. Worldwide sequence analysis showed that 44 amino acid changes had occurred in 37 positions of which seven polymorphic sites had trimorphic mutations while the rest was dimorphic. The overall nucleotide diversity for Iranian isolates was 0.00431 ± 0.00091 while the level of nucleotide diversity was ranged from 0.00337 ± 0.00076 (Peru) to 0.00452 ± 0.00092 (Thailand) in global scale.

CONCLUSIONS/SIGNIFICANCE

Of amino acid substitutions, 12 replacements were located in the B-cell epitopes in which nine polymorphic sites were positioned in N-terminal and three polymorphic sites in predicted B-cell epitopes of C-terminal, signifying both variable and conserved epitopes for vaccine designing. Using the achieved outcome of the current investigation interrogate questions to the selection of conserved regions of pvrbp2b and understanding polymorphism and immune system pressure to pave a way for developing a vaccine based on PvRBP2b candidate antigen.

Autophagy Pathways in the Genesis of Plasmodium-Derived Microvesicles: A Double-Edged Sword?

Malaria, caused by Plasmodium species (spp.), is a deadly parasitic disease that results in approximately 400,000 deaths per year globally. Autophagy pathways play a fundamental role in the developmental stages of the parasite within the mammalian host. They are also involved in the production of Plasmodium-derived extracellular vesicles (EVs), which play an important role in the infection process, either by providing nutrients for parasite growth or by contributing to the immunopathophysiology of the disease. For example, during the hepatic stage, Plasmodium-derived EVs contribute to parasite virulence by modulating the host immune response. EVs help in evading the different autophagy mechanisms deployed by the host for parasite clearance. During cerebral malaria, on the other hand, parasite-derived EVs promote an astrocyte-mediated inflammatory response, through the induction of a non-conventional host autophagy pathway. In this review, we will discuss the cross-talk between Plasmodium-derived microvesicles and autophagy, and how it influences the outcome of infection.

A new mass spectral library for high-coverage and reproducible analysis of the Plasmodium falciparum-infected red blood cell proteome

BACKGROUND

Plasmodium falciparum causes the majority of malaria mortality worldwide, and the disease occurs during the asexual red blood cell (RBC) stage of infection. In the absence of an effective and available vaccine, and with increasing drug resistance, asexual RBC stage parasites are an important research focus. In recent years, mass spectrometry-based proteomics using data-dependent acquisition has been extensively used to understand the biochemical processes within the parasite. However, data-dependent acquisition is problematic for the detection of low-abundance proteins and proteome coverage and has poor run-to-run reproducibility.

RESULTS

Here, we present a comprehensive P. falciparum-infected RBC (iRBC) spectral library to measure the abundance of 44,449 peptides from 3,113 P. falciparum and 1,617 RBC proteins using a data-independent acquisition mass spectrometric approach. The spectral library includes proteins expressed in the 3 morphologically distinct RBC stages (ring, trophozoite, schizont), the RBC compartment of trophozoite-iRBCs, and the cytosolic fraction from uninfected RBCs. This spectral library contains 87% of all P. falciparum proteins that have previously been reported with protein-level evidence in blood stages, as well as 692 previously unidentified proteins. The P. falciparum spectral library was successfully applied to generate semi-quantitative proteomics datasets that characterize the 3 distinct asexual parasite stages in RBCs, and compared artemisinin-resistant (Cam3.IIR539T) and artemisinin-sensitive (Cam3.IIrev) parasites.

CONCLUSION

A reproducible, high-coverage proteomics spectral library and analysis method has been generated for investigating sets of proteins expressed in the iRBC stage of P. falciparum malaria. This will provide a foundation for an improved understanding of parasite biology, pathogenesis, drug mechanisms, and vaccine candidate discovery for malaria.