Genetic analysis of the cytoplasmic domain of the PfRh2b merozoite invasion protein of Plasmodium falciparum

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.

Localization and function of a Plasmodium falciparum protein (PF3D7_1459400) during erythrocyte invasion

IMPACT STATEMENT

Plasmodium falciparum malaria is a global health problem. Erythrocyte invasion by P. falciparum merozoites appears to be a promising target to curb malaria. We have identified and characterized a novel protein that is involved in erythrocyte invasion. Our data on protein subcellular localization, stage-specific protein expression pattern, and merozoite invasion inhibition by α-peptide antibodies suggest a role for PF3D7_1459400 protein during P. falciparum erythrocyte invasion. Even more, the human immunoepidemiology data present PF3D7_1459400 protein as an immunogenic antigen which could be further exploited for the development of new anti-infective therapy against malaria.

Functional Analysis Reveals Geographical Variation in Inhibitory Immune Responses Against a Polymorphic Malaria Antigen

Background

Plasmodium falciparum reticulocyte-binding protein homologue 2b (PfRh2b) is an invasion ligand that is a potential blood-stage vaccine candidate antigen; however, a naturally occurring deletion within an immunogenic domain is present at high frequencies in Africa and has been associated with alternative invasion pathway usage. Standardized tools that provide antigenic specificity in in vitro assays are needed to functionally assess the neutralizing potential of humoral responses against malaria vaccine candidate antigens.

Methods

Transgenic parasite lines were generated to express the PfRh2b deletion. Total immunoglobulin G (IgG) from individuals residing in malaria-endemic regions in Tanzania, Senegal, and Mali were used in growth inhibition assays with transgenic parasite lines.

Results

While the PfRh2b deletion transgenic line showed no change in invasion pathway utilization compared to the wild-type in the absence of specific antibodies, it outgrew wild-type controls in competitive growth experiments. Inhibition differences with total IgG were observed in the different endemic sites, ranging from allele-specific inhibition to allele-independent inhibitory immune responses.

Conclusions

The PfRh2b deletion may allow the parasite to escape neutralizing antibody responses in some regions. This difference in geographical inhibition was revealed using transgenic methodologies, which provide valuable tools for functionally assessing neutralizing antibodies against vaccine-candidate antigens in regions with varying malaria endemicity.

Specific phosphorylation of the PfRh2b invasion ligand of Plasmodium falciparum

Red blood cell invasion by the malaria parasite Plasmodium falciparum relies on a complex protein network that uses low and high affinity receptor–ligand interactions. Signal transduction through the action of specific kinases is a control mechanism for the orchestration of this process. In the present study we report on the phosphorylation of the CPD (cytoplasmic domain) of P. falciparum Rh2b (reticulocyte homologue protein 2b). First, we identified Ser³²³³ as the sole phospho-acceptor site in the CPD for in vitro phosphorylation by parasite extract. We provide several lines of evidence that this phosphorylation is mediated by PfCK2 (P. falciparum casein kinase 2): phosphorylation is cAMP independent, utilizes ATP as well as GTP as phosphate donors, is inhibited by heparin and tetrabromocinnamic acid, and is mediated by purified PfCK2. We raised a phospho-specific antibody and showed that Ser³²³³ phosphorylation occurs in the parasite prior to host cell egress. We analysed the spatiotemporal aspects of this phosphorylation using immunoprecipitated endogenous Rh2b and minigenes expressing the CPD either at the plasma or rhoptry membrane. Phosphorylation of Rh2b is not spatially restricted to either the plasma or rhoptry membrane and most probably occurs before Rh2b is translocated from the rhoptry neck to the plasma membrane.

An EGF-like protein forms a complex with PfRh5 and is required for invasion of human erythrocytes by Plasmodium falciparum

Invasion of erythrocytes by Plasmodium falciparum involves a complex cascade of protein-protein interactions between parasite ligands and host receptors. The reticulocyte binding-like homologue (PfRh) protein family is involved in binding to and initiating entry of the invasive merozoite into erythrocytes. An important member of this family is PfRh5. Using ion-exchange chromatography, immunoprecipitation and mass spectroscopy, we have identified a novel cysteine-rich protein we have called P. falciparumRh5 interacting protein (PfRipr) (PFC1045c), which forms a complex with PfRh5 in merozoites. Mature PfRipr has a molecular weight of 123 kDa with 10 epidermal growth factor-like domains and 87 cysteine residues distributed along the protein. In mature schizont stages this protein is processed into two polypeptides that associate and form a complex with PfRh5. The PfRipr protein localises to the apical end of the merozoites in micronemes whilst PfRh5 is contained within rhoptries and both are released during invasion when they form a complex that is shed into the culture supernatant. Antibodies to PfRipr1 potently inhibit merozoite attachment and invasion into human red blood cells consistent with this complex playing an essential role in this process.

The malaria parasite invades red blood cells by binding to proteins on the surface of this host cell. A family of proteins called P. falciparum reticulocyte binding-like homologue (PfRh) proteins are important for recognition of the red blood cell and activation of the invasion process. An important member of the PfRh family is PfRh5. We have identified a novel cysteine-rich protein we have called P. falciparumRh5 interacting protein (PfRipr), which forms a complex with PfRh5 in merozoites. PfRipr has 10 epidermal growth factor-like domains and is expressed in mature schizont stages where it is processed into two polypeptides that associate and form a complex with PfRh5. The PfRipr protein localises to the apical end of the merozoites in micronemes whilst PfRh5 is contained within rhoptries and both are released during invasion when they form a complex that is released into the culture supernatant. Antibodies to PfRipr1 can potently inhibit merozoite attachment and invasion into human red blood cells consistent with this complex playing an essential role in this process.

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.

Reticulocyte and erythrocyte binding-like proteins function cooperatively in invasion of human erythrocytes by malaria parasites

Plasmodium falciparum causes the most severe form of malaria in humans and invades erythrocytes using multiple ligand-receptor interactions. Two important protein families involved in erythrocyte binding are the erythrocyte binding-like (EBL) and the reticulocyte binding-like (RBL or P. falciparum Rh [PfRh]) proteins. We constructed P. falciparum lines lacking expression of EBL proteins by creating single and double knockouts of the corresponding genes for eba-175, eba-181, and eba-140 and show that the EBL and PfRh proteins function cooperatively, consistent with them playing a similar role in merozoite invasion. We provide evidence that PfRh and EBL proteins functionally interact, as loss of function of EBA-181 ablates the ability of PfRh2a/b protein antibodies to inhibit merozoite invasion. Additionally, loss of function of some ebl genes results in selection for increased transcription of the PfRh family. This provides a rational basis for considering PfRh and EBL proteins for use as a combination vaccine against P. falciparum. We immunized rabbits with combinations of PfRh and EBL proteins to test the ability of antibodies to block merozoite invasion in growth inhibition assays. A combination of EBA-175, PfRh2a/b, and PfRh4 recombinant proteins induced antibodies that potently blocked merozoite invasion. This validates the use of a combination of these ligands as a potential vaccine that would have broad activity against P. falciparum.

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.