Erythrocyte invasion by Plasmodium falciparum: multiple ligand-receptor interactions and phenotypic switching

Identification of Novel, Potent, and Selective Compounds against Malaria Using Glideosomal-Associated Protein 50 as a Drug Target

Phylum apicomplexan consists of parasites, such as Plasmodium and Toxoplasma. These obligate intracellular parasites enter host cells via an energy-dependent process using specialized machinery, called the glideosome. In the present study, we used Plasmodium falciparum GAP50, a glideosome-associated protein, as a target to screen 951 different compounds from diverse chemical libraries. Using different screening methods, eight compounds (Hayatinine, Curine, MMV689758 (Bedaquiline), MMV1634402 (Brilacidin), and MMV688271, MMV782353, MMV642550, and USINB4-124-8) were identified, which showed promising binding affinity (KD < 75 μM), along with submicromolar range antiparasitic efficacy and selectivity index > 100 fold for malaria parasite. These eight compounds were effective against Chloroquine-resistant PfINDO and Artemisinin-resistant PfCam3.1R359T strains. Studies on the effect of these compounds at asexual blood stages showed that these eight compounds act differently at different developmental stages, indicating the binding of these compounds to other Plasmodium proteins, in addition to PfGAP50. We further studied the effects of compounds (Bedaquiline and USINB4-124-8) in an in vivoPlasmodium berghei mouse model of malaria. Importantly, the oral delivery of Bedaquiline (50 mg/kg b. wt.) showed substantial suppression of parasitemia, and three out of seven mice were cured of the infection. Thus, our study provides new scaffolds for the development of antimalarials that can act at multiple Plasmodium lifecycle stages.

Calcium in the Backstage of Malaria Parasite Biology

The calcium ion (Ca2+) is a ubiquitous second messenger involved in key biological processes in prokaryotes and eukaryotes. In Plasmodium species, Ca2+ signaling plays a central role in the parasite life cycle. It has been associated with parasite development, fertilization, locomotion, and host cell infection. Despite the lack of a canonical inositol-1,4,5-triphosphate receptor gene in the Plasmodium genome, pharmacological evidence indicates that inositol-1,4,5-triphosphate triggers Ca2+ mobilization from the endoplasmic reticulum. Other structures such as acidocalcisomes, food vacuole and mitochondria are proposed to act as supplementary intracellular Ca2+ reservoirs. Several Ca2+-binding proteins (CaBPs) trigger downstream signaling. Other proteins with no EF-hand motifs, but apparently involved with CaBPs, are depicted as playing an important role in the erythrocyte invasion and egress. It is also proposed that a cross-talk among kinases, which are not members of the family of Ca2+-dependent protein kinases, such as protein kinases G, A and B, play additional roles mediated indirectly by Ca2+ regulation. This statement may be extended for proteins directly related to invasion or egress, such as SUB1, ERC, IMC1I, IMC1g, GAP45 and EBA175. In this review, we update our understanding of aspects of Ca2+-mediated signaling correlated to the developmental stages of the malaria parasite life cycle.

Diversify and Conquer: The Vaccine Escapism of Plasmodium falciparum

Over the last century, a great deal of effort and resources have been poured into the development of vaccines to protect against malaria, particularly targeting the most widely spread and deadly species of the human-infecting parasites: Plasmodium falciparum. Many of the known proteins the parasite uses to invade human cells have been tested as vaccine candidates. However, precisely because of the importance and immune visibility of these proteins, they tend to be very diverse, and in many cases redundant, which limits their efficacy in vaccine development. With the advent of genomics and constantly improving sequencing technologies, an increasingly clear picture is emerging of the vast genomic diversity of parasites from different geographic areas. This diversity is distributed throughout the genome and includes most of the vaccine candidates tested so far, playing an important role in the low efficacy achieved. Genomics is a powerful tool to search for genes that comply with the most desirable attributes of vaccine targets, allowing us to evaluate function, immunogenicity and also diversity in the worldwide parasite populations. Even predicting how this diversity might evolve and spread in the future becomes possible, and can inform novel vaccine efforts.

PfCDPK1 mediated signaling in erythrocytic stages of Plasmodium falciparum

Calcium Dependent Protein Kinases are key effectors of calcium signaling in malaria parasite. PfCDPK1 is critical for asexual development of Plasmodium falciparum, but its precise function and substrates remain largely unknown. Using a conditional knockdown strategy, we here establish that this kinase is critical for the invasion of host erythrocytes. Furthermore, using a multidisciplinary approach involving comparative phosphoproteomics we gain insights into the underlying molecular mechanisms. We identify substrates of PfCDPK1, which includes proteins of Inner Membrane Complex and glideosome-actomyosin motor assembly. Interestingly, PfCDPK1 phosphorylates PfPKA regulatory subunit (PfPKA-R) and regulates PfPKA activity in the parasite, which may be relevant for the process of invasion. This study delineates the signaling network of PfCDPK1 and sheds light on mechanisms via which it regulates invasion.Calcium dependent protein kinase 1 (CDPK1) plays an important role in asexual development of Plasmodium falciparum. Using phosphoproteomics and conditional knockdown of CDPK1, the authors here identify CDPK1 substrates and a cross-talk between CDPK1 and PKA, and show the role of CDPK1 in parasite invasion.

Implications from predicted B-cell and T-cell epitopes of Plasmodium falciparum merozoite proteins EBA175-RII and Rh5

The leading circumsporozoite protein (CSP) based malaria vaccine, RTS,S, though promising, has shown limited efficacy in field studies. There is therefore, still a need to identify other malaria vaccine targets. Merozoite antigens are potential vaccine candidates, since naturally acquired antibodies generated against them inhibit erythrocyte invasion and in some cases result in the clinical protection from disease. We thus used in silico tools (BCPreds, NetMHCcons and NetMHCIIpan 3.0) to predict B-cell epitopes (BCEs) and T-cell epitopes (TCEs) in two merozoite invasion proteins, EBA175-RII and Rh5. Initially, we validated these tools using CSP to determine whether the algorithms could predict the epitopes in the RTS,S vaccine. In EBA175-RII, we prioritised three BCEs 15REKRKGMKWDCKKKNDRSNY34, 420SNRKLVGKINTNSNYVHRNKQ440 and 528WISKKKEEYNKQAKQYQEYQ547, a CD8+ epitope 553KMYSEFKSI561 and a CD4+ epitope 440QNDKLFRDEWWK VIKKD456. Three Rh5 epitopes were prioritised, a BCE 344SCYNNNFCNTNGIRYHYDEY363, a CD8+ epitope 198STYGKCIAV206 and a Rh5 CD4+ epitope 180TFLDYYKHLSYNSIYHKSSTY200. All these epitopes are in the region involved in the proteins' interaction with their erythrocyte receptors, thus enabling erythrocyte invasion. Therefore, upon validation of their immunogenicity, by ELISA using serum from a malaria endemic population, antibodies to these epitopes may inhibit erythrocyte invasion. All the epitopes we predicted in EBA175-RII and Rh5 are novel. We also identified polymorphic epitopes that may escape host immunity, as some variants were not predicted as epitopes, suggesting that they may not be immunogenic regions. We present a set of epitopes that following in vitro validation provide a set of molecules to screen as potential vaccine candidates.

Ancient human sialic acid variant restricts an emerging zoonotic malaria parasite

Plasmodium knowlesi is a zoonotic parasite transmitted from macaques causing malaria in humans in Southeast Asia. Plasmodium parasites bind to red blood cell (RBC) surface receptors, many of which are sialylated. While macaques synthesize the sialic acid variant N-glycolylneuraminic acid (Neu5Gc), humans cannot because of a mutation in the enzyme CMAH that converts N-acetylneuraminic acid (Neu5Ac) to Neu5Gc. Here we reconstitute CMAH in human RBCs for the reintroduction of Neu5Gc, which results in enhancement of P. knowlesi invasion. We show that two P. knowlesi invasion ligands, PkDBPβ and PkDBPγ, bind specifically to Neu5Gc-containing receptors. A human-adapted P. knowlesi line invades human RBCs independently of Neu5Gc, with duplication of the sialic acid-independent invasion ligand, PkDBPα and loss of PkDBPγ. Our results suggest that absence of Neu5Gc on human RBCs limits P. knowlesi invasion, but that parasites may evolve to invade human RBCs through the use of sialic acid-independent pathways.

Host-parasite interactions that guide red blood cell invasion by malaria parasites

PURPOSE OF REVIEW

Malaria is caused by the infection and proliferation of parasites from the genus Plasmodium in red blood cells (RBCs). A free Plasmodium parasite, or merozoite, released from an infected RBC must invade another RBC host cell to sustain a blood-stage infection. Here, we review recent advances on RBC invasion by Plasmodium merozoites, focusing on specific molecular interactions between host and parasite.

RECENT FINDINGS

Recent work highlights the central role of host-parasite interactions at virtually every stage of RBC invasion by merozoites. Biophysical experiments have for the first time measured the strength of merozoite-RBC attachment during invasion. For P. falciparum, there have been many key insights regarding the invasion ligand PfRh5 in particular, including its influence on host species tropism, a co-crystal structure with its RBC receptor basigin, and its suitability as a vaccine target. For P. vivax, researchers identified the origin and emergence of the parasite from Africa, demonstrating a natural link to the Duffy-negative RBC variant in African populations. For the simian parasite P. knowlesi, zoonotic invasion into human cells is linked to RBC age, which has implications for parasitemia during an infection and thus malaria.

SUMMARY

New studies of the molecular and cellular mechanisms governing RBC invasion by Plasmodium parasites have shed light on various aspects of parasite biology and host cell tropism, and indicate opportunities for malaria control.

Parasite Calcineurin Regulates Host Cell Recognition and Attachment by Apicomplexans

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

Functional analysis of epigenetic regulation of tandem RhopH1/clag genes reveals a role in Plasmodium falciparum growth

The Plasmodium RhopH complex is a high molecular weight antigenic complex consisting of three subunits - RhopH1/clag, RhopH2 and RhopH3 - located in the rhoptry secretory organelles of the invasive merozoite. In Plasmodium falciparum RhopH1/clag is encoded by one of five clag genes. Two highly similar paralogous genes, clag 3.1 and clag 3.2, are mutually exclusively expressed. Here we show clonal switching from the clag 3.2 to the clag 3.1 paralogue in vitro. Chromatin immunoprecitation studies suggest that silencing of either clag 3 paralogue is associated with the enrichment of specific histone modifications associated with heterochromatin. We were able to disrupt the clag 3.2 gene, with a drug cassette inserted into the clag 3.2 locus being readily silenced in a position-dependent and sequence-independent manner. Activation of this drug cassette by drug selection results in parasites with the clag 3.1 locus silenced and lack full-length clag 3.1 or 3.2 transcripts. These clag 3-null parasites demonstrate a significant growth inhibition compared with wild-type parasites, providing the first genetic evidence for a role for these proteins in efficient parasite proliferation. Epigenetic regulation of these chromosomally proximal members of a multigene family provides a mechanism for both immune evasion and functional diversification.

Conserved regions of the Plasmodium falciparum rhoptry-associated protein 3 mediate specific host-pathogen interactions during invasion of red blood cells

Invasion of red blood cells (RBCs) by the Plasmodium falciparum malaria merozoite is mediated by parasite surface molecules and proteins contained within apical organelles that are capable of recognizing receptors on the membrane of RBCs. The identification and characterization of these P. falciparum invasion-associated proteins is the first step for unveiling potential new drug and vaccine target molecules to eradicate this deadly disease. Among the exclusive set of malarial vaccine candidates, the members of the rhoptry-associated protein (RAP) family have been associated with the parasite's binding to and invasion of RBCs. Remarkably, the third member of this family (named RAP-3) has been recently detected on the surface of non-infected RBCs exposed to free merozoites, therefore suggesting the participation of this protein during RBC infection. In this study, the sequence of RAP-3 was finely mapped using synthetic peptides in order to identify which are the specific binding regions involved in RAP3-RBC interactions. Two high-activity binding peptides (HABPs) established high affinity interactions with RBC surface molecules of about 27-90 kDa, which were differentially affected by different enzymatic treatments. RAP-1 and RAP-2 HABPs inhibited binding of RAP-3 HABPs to different extents, thus suggesting the recognition of similar binding sites on RBC membrane, as well as ability of RAP-3 HABPs to inhibit P. falciparum infection in vitro. Altogether, these functional analyses of RAP-3 HABPs strongly suggest a potential role for this protein in RBC invasion, and highlight its HABPs as potential targets to develop a fully protective minimal subunit-based malarial vaccine.

Functional diversification between two related Plasmodium falciparum merozoite invasion ligands is determined by changes in the cytoplasmic domain

The pathogenesis of Plasmodium falciparum depends on efficient invasion into host erythrocytes. Parasite ligands encoded by multi-gene families interact with erythrocyte receptors. P. falciparum reticulocyte binding protein homologues (PfRhs) are expressed at the apical surface of invasive merozoites and have divergent ectodomains that are postulated to bind different erythrocyte receptors. Variant expression of these paralogues results in the use of alternative invasion pathways. Two PfRh proteins, PfRh2a and PfRh2b, are identical for 2700 N-terminal amino acids and differ only in a C-terminal 500 amino acid region, which includes a unique ectodomain, transmembrane domain and cytoplasmic domain. Despite their similarity, PfRh2b is required for a well-defined invasion pathway while PfRh2a is not required or sufficient for this pathway. Mapping the genomic region encoding these proteins revealed a recombinogenic locus with PfRh2a and PfRh2b in a head-to-head orientation. We have generated viable PfRh2a/2b chimeric parasites to identify the regions required for alternative invasion pathway utilization. We find that the differential ability to use these pathways is conferred by the cytoplasmic domains of PfRh2a and PfRh2b, not the ectodomain or transmembrane regions. Our results highlight the importance of the cytoplasmic domain for functional diversification of a major adhesive ligand family in malaria parasites.