Characterization of PfRhop148, a novel rhoptry protein of Plasmodium falciparum

Functional screening reveals Toxoplasma prenylated proteins required for endocytic trafficking and rhoptry protein sorting

In the apicomplexans, endocytosed cargos (e.g., hemoglobin) are trafficked to a specialized organelle for digestion. This follows a unique endocytotic process at the micropore/cytostome in these parasites. However, the mechanism underlying endocytic trafficking remains elusive, due to the repurposing of classical endocytic proteins for the biogenesis of apical organelles. To resolve this issue, we have exploited the genetic tractability of the model apicomplexan Toxoplasma gondii, which ingests host cytosolic materials (e.g., green fluorescent protein[GFP]). We determined an association between protein prenylation and endocytic trafficking, and using an alkyne-labeled click chemistry approach, the prenylated proteome was characterized. Genome editing, using clustered regularly interspaced short palindromic repaet/CRISPR-associated nuclease 9 (CRISPR/Cas9), was efficiently utilized to generate genetically modified lines for the functional screening of 23 prenylated candidates. This identified four of these proteins that regulate the trafficking of endocytosed GFP vesicles. Among these proteins, Rab1B and YKT6.1 are highly conserved but are non-classical endocytic proteins in eukaryotes. Confocal imaging analysis showed that Rab1B and Ras are substantially localized to both the trans-Golgi network and the endosome-like compartments in the parasite. Conditional knockdown of Rab1B caused a rapid defect in secretory trafficking to the rhoptry bulb, suggesting a trafficking intersection role for the key regulator Rab1B. Further experiments confirmed a critical role for protein prenylation in regulating the stability/activity of these proteins (i.e., Rab1B and YKT6.1) in the parasite. Our findings define the molecular basis of endocytic trafficking and reveal a potential intersection function of Rab1B on membrane trafficking in T. gondii. This might extend to other related protists, including the malarial parasites. IMPORTANCE The protozoan Toxoplasma gondii establishes a permissive niche, in host cells, that allows parasites to acquire large molecules such as proteins. Numerous studies have demonstrated that the parasite repurposes the classical endocytic components for secretory sorting to the apical organelles, leaving the question of endocytic transport to the lysosome-like compartment unclear. Recent studies indicated that endocytic trafficking is likely to associate with protein prenylation in malarial parasites. This information promoted us to examine this association in the model apicomplexan T. gondii and to identify the key components of the prenylated proteome that are involved. By exploiting the genetic tractability of T. gondii and a host GFP acquisition assay, we reveal four non-classical endocytic proteins that regulate the transport of endocytosed cargos (e.g., GFP) in T. gondii. Thus, we extend the principle that protein prenylation regulates endocytic trafficking and elucidate the process of non-classical endocytosis in T. gondii and potentially in other related protists.

Intersection of endocytic and exocytic systems in Toxoplasma gondii

Host cytosolic proteins are endocytosed by Toxoplasma gondii and degraded in its lysosome-like compartment, the vacuolar compartment (VAC), but the dynamics and route of endocytic trafficking remain undefined. Conserved endocytic components and plant-like features suggest T. gondii endocytic trafficking involves transit through early and late endosome-like compartments (ELCs) and potentially the trans-Golgi network (TGN) as in plants. However, exocytic trafficking to regulated secretory organelles, micronemes and rhoptries, also proceeds through ELCs and requires classical endocytic components, including a dynamin-related protein, DrpB. Here, we show that host cytosolic proteins are endocytosed within 7 minutes post-invasion, trafficked through ELCs en route to the VAC, and degraded within 30 minutes. We could not definitively interpret if ingested protein is trafficked through the TGN. We also found that parasites ingest material from the host cytosol throughout the parasite cell cycle. Ingested host proteins colocalize with immature microneme proteins, proM2AP and proMIC5, in transit to the micronemes, but not with the immature rhoptry protein proRON4, indicating that endocytic trafficking of ingested protein intersects with exocytic trafficking of microneme proteins. Finally, we show that conditional expression of a DrpB dominant negative mutant increases T. gondii ingestion of host-derived proteins, suggesting that DrpB is not required for parasite endocytosis.

Evidence that the Malaria Parasite Plasmodium falciparum Putative Rhoptry Protein 2 Localizes to the Golgi Apparatus throughout the Erythrocytic Cycle

Invasion of a red blood cell by Plasmodium falciparum merozoites is an essential step in the malaria lifecycle. Several of the proteins involved in this process are stored in the apical complex of the merozoite, a structure containing secretory organelles that are released at specific times during invasion. The molecular players involved in erythrocyte invasion thus represent potential key targets for both therapeutic and vaccine-based strategies to block parasite development. In our quest to identify and characterize new effectors of invasion, we investigated the P. falciparum homologue of a P. berghei protein putatively localized to the rhoptries, the Putative rhoptry protein 2 (PbPRP2). We show that in P. falciparum, the protein colocalizes extensively with the Golgi apparatus across the asexual erythrocytic cycle. Furthermore, imaging of merozoites caught at different times during invasion show that PfPRP2 is not secreted during the process instead staying associated with the Golgi apparatus. Our evidence therefore suggests that PfPRP2 is a Golgi protein and that it is likely not a direct effector in the process of merozoite invasion.

A malaria vaccine candidate based on an epitope of the Plasmodium falciparum RH5 protein

Background

The Plasmodium falciparum protein RH5 is an adhesin molecule essential for parasite invasion of erythrocytes. Recent studies show that anti-PfRH5 sera have potent invasion-inhibiting activities, supporting the idea that the PfRH5 antigen could form the basis of a vaccine. Therefore, epitopes recognized by neutralizing anti-PfRH5 antibodies could themselves be effective vaccine immunogens if presented in a sufficiently immunogenic fashion. However, the exact regions within PfRH5 that are targets of this invasion-inhibitory activity have yet to be identified.

Methods

A battery of anti-RH5 monoclonal antibodies (mAbs) were produced and screened for their potency by inhibition of invasion assays in vitro. Using an anti-RH5 mAb that completely inhibited invasion as the selecting mAb, affinity-selection using random sequence peptide libraries displayed on virus-like particles of bacteriophage MS2 (MS2 VLPs) was performed. VLPs were sequenced to identify the specific peptide epitopes they encoded and used to raise specific antisera that was in turn tested for inhibition of invasion.

Results

Three anti-RH5 monoclonals (0.1 mg/mL) were able to inhibit invasion in vitro by >95%. Affinity-selection with one of these mAbs yielded a VLP which yielded a peptide whose sequence is identical to a portion of PfRH5 itself. The VLP displaying the peptide binds strongly to the antibody, and in immunized animals elicits an anti-PfRH5 antibody response. The resulting antisera against the specific VLP inhibit parasite invasion of erythrocytes more than 90% in vitro.

Conclusions

Here, data is presented from an anti-PfRH5 mAb that completely inhibits erythrocyte invasion by parasites in vitro, one of the few anti-malarial monoclonal antibodies reported to date that completely inhibits invasion with such potency, adding to other studies that highlight the potential of PfRH5 as a vaccine antigen. The specific neutralization sensitive epitope within RH5 has been identified, and antibodies against this epitope also elicit high anti-invasion activity, suggesting this epitope could form the basis of an effective vaccine against malaria.

Subversion of host cellular functions by the apicomplexan parasites

Rhoptries are club-shaped secretory organelles located at the anterior pole of species belonging to the phylum of Apicomplexa. Parasites of this phylum are responsible for a huge burden of disease in humans and animals and a loss of economic productivity. Members of this elite group of obligate intracellular parasites include Plasmodium spp. that cause malaria and Cryptosporidium spp. that cause diarrhoeal disease. Although rhoptries are almost ubiquitous throughout the phylum, the relevance and role of the proteins contained within the rhoptries varies. Rhoptry contents separate into two intra-organellar compartments, the neck and the bulb. A number of rhoptry neck proteins are conserved between species and are involved in functions such as host cell invasion. The bulb proteins are less well-conserved and probably evolved for a particular lifestyle. In the majority of species studied to date, rhoptry content is involved in formation and maintenance of the parasitophorous vacuole; however some species live free within the host cytoplasm. In this review, we will summarise the knowledge available regarding rhoptry proteins. Specifically, we will discuss the role of the rhoptry kinases that are used by Toxoplasma gondii and other coccidian parasites to subvert the host cellular functions and prevent parasite death.

Characterisation of PfRON6, a Plasmodium falciparum rhoptry neck protein with a novel cysteine-rich domain

The pathological consequences of malaria infection are the result of parasite replication within red blood cells (RBCs). Invasion into RBCs is mediated by a large repertoire of parasite proteins that are distributed on the parasite surface and within specialised apical secretory organelles. As invasion is an essential step in the parasite life-cycle, targeting invasion-related molecules provides an avenue for therapeutic intervention. We have used genome and transcriptome data available for Plasmodium falciparum to identify proteins likely to be involved in RBC invasion. Of these candidates, we selected a protein which we have dubbed PfRON6 for detailed characterisation. PfRON6 contains a novel cysteine-rich domain that is conserved in other Apicomplexan parasites. We show that PfRON6 is localised in the rhoptry neck of merozoites and is transferred to the newly formed parasitophorous vacuole during invasion. Transfection experiments indicate that the gene which encodes PfRON6 is refractory to integration that disrupts the coding sequence, suggesting its absence is incompatible with the parasite life-cycle. Further, the cysteine-rich domain appears to be functionally important as it cannot be truncated. Taken together, these data identify PfRON6 as a novel and potentially important component of the Plasmodium invasion machinery.

PfRH5: a novel reticulocyte-binding family homolog of plasmodium falciparum that binds to the erythrocyte, and an investigation of its receptor

Multiple interactions between parasite ligands and their receptors on the human erythrocyte are a condition of successful Plasmodium falciparum invasion. The identification and characterization of these receptors presents a major challenge in the effort to understand the mechanism of invasion and to develop the means to prevent it. We describe here a novel member of the reticulocyte-binding family homolog (RH) of P. falciparum, PfRH5, and show that it binds to a previously unrecognized receptor on the RBC. PfRH5 is expressed as a 63 kDa protein and localized at the apical end of the invasive merozoite. We have expressed a fragment of PfRH5 which contains the RBC-binding domain and exhibits the same pattern of interactions with the RBC as the parent protein. Attachment is inhibited if the target cells are exposed to high concentrations of trypsin, but not to lower concentrations or to chymotrypsin or neuraminidase. We have determined the affinity, copy number and apparent molecular mass of the receptor protein. Thus, we have shown that PfRH5 is a novel erythrocyte-binding ligand and the identification and partial characterization of the new RBC receptor may indicate the existence of an unrecognized P. falciparum invasion pathway.

Erythrocyte invasion: vocabulary and grammar of the Plasmodium rhoptry

Malaria is a dangerous infectious disease caused by obligate intracellular protozoan Plasmodium parasites. In the vertebrate host, erythrocyte recognition and establishment of a nascent parasitophorous vacuole are essential processes, and are largely achieved using molecules located in the microorganelles of the invasive-stage parasites. Recent proteome analyses of the phylogenetically related Toxoplasma parasite have provided protein catalogs for these microorganelles, which can now be used to identify orthologous proteins in the Plasmodium genome. Of importance is the formation of a complex between the proteins secreted from the rhoptry neck portion (RONs) and micronemes (AMA1), which localize at the moving junction during parasite invagination into the host cell. In this article I review the largely unexplored paradigm of the malaria merozoite rhoptry, focusing on the high molecular weight rhoptry protein complex (the RhopH complex), and speculate on its grammar during invasion.

Plasmodium falciparum Pf34, a novel GPI-anchored rhoptry protein found in detergent-resistant microdomains

Apicomplexan parasites are characterised by the presence of specialised organelles, such as rhoptries, located at the apical end of invasive forms that play an important role in invasion of the host cell and formation of the parasitophorous vacuole. In this study, we have characterised a novel Plasmodium falciparum rhoptry protein, Pf34, encoded by a single exon gene located on chromosome 4 and expressed as a 34kDa protein in mature asexual stage parasites. Pf34 is expressed later in the life cycle than the previously described rhoptry protein, Rhoptry Associated Membrane Antigen (RAMA). Orthologues of Pf34 are present in other Plasmodium species and a potential orthologue has also been identified in Toxoplasma gondii. Indirect immunofluorescence assays show that Pf34 is located at the merozoite apex and localises to the rhoptry neck. Pf34, previously demonstrated to be glycosyl-phosphatidyl-inositol (GPI)-anchored [Gilson, P.R., Nebl, T., Vukcevic, D., Moritz, R.L., Sargeant, T., Speed, T.P., Schofield, L., Crabb, B.S. (2006) Identification and stoichiometry of GPI-anchored membrane proteins of the human malaria parasite Plasmodium falciparum. Mol. Cell. Proteomics 5, 1286-1299.], is associated with parasite-derived detergent-resistant microdomains (DRMs). Pf34 is carried into the newly invaded ring, consistent with a role for Pf34 in the formation of the parasitophorous vacuole. Pf34 is exposed to the human immune system during infection and is recognised by human immune sera collected from residents of malaria endemic areas of Vietnam and Papua New Guinea.

Plasmodium rhoptries: how things went pear-shaped

Plasmodium parasites have three sets of specialised secretory organelles at the apical end of their invasive forms--rhoptries, micronemes and dense granules. The contents of these organelles are responsible for or contribute to host cell invasion and modification, and at least four apical proteins are leading vaccine candidates. Given the unusual nature of Plasmodium invasion, it is not surprising that unique proteins are involved in this process. Nowhere is this more evident than in rhoptries. We have collated data from several recent studies to compile a rhoptry proteome. Discussion is focussed here on rhoptry content and function.

Proteomic analysis of rhoptry organelles reveals many novel constituents for host-parasite interactions in Toxoplasma gondii

Rhoptries are specialized secretory organelles that are uniquely present within protozoan parasites of the phylum Apicomplexa. These obligate intracellular parasites comprise some of the most important parasites of humans and animals, including the causative agents of malaria (Plasmodium spp.) and chicken coccidiosis (Eimeria spp.). The contents of the rhoptries are released into the nascent parasitophorous vacuole during invasion into the host cell, and the resulting proteins often represent the literal interface between host and pathogen. We have developed a method for highly efficient purification of rhoptries from one of the best studied Apicomplexa, Toxoplasma gondii, and we carried out a detailed proteomic analysis using mass spectrometry that has identified 38 novel proteins. To confirm their rhoptry origin, antibodies were raised to synthetic peptides and/or recombinant protein. Eleven of 12 of these yielded antibody that showed strong rhoptry staining by immunofluorescence within the rhoptry necks and/or their bulbous base. Hemagglutinin epitope tagging confirmed one additional novel protein as from the rhoptry bulb. Previously identified rhoptry proteins from Toxoplasma and Plasmodium were unique to one or the other organism, but our elucidation of the Toxoplasma rhoptry proteome revealed homologues that are common to both. This study also identified the first Toxoplasma genes encoding rhoptry neck proteins, which we named RONs, demonstrated that toxofilin and Rab11 are rhoptry proteins, and identified novel kinases, phosphatases, and proteases that are likely to play a key role in the ability of the parasite to invade and co-opt the host cell for its own survival and growth.

Characterization of a Membrane-associated Rhoptry Protein of Plasmodium falciparum

Invasive forms of apicomplexan parasites contain secretory organelles called rhoptries that are essential for entry into host cells. We present a detailed characterization of an unusual rhoptry protein of the human malaria parasite Plasmodium falciparum, the rhoptry-associated membrane antigen (RAMA) that appears to have roles in both rhoptry biogenesis and host cell invasion. RAMA is synthesized as a 170-kDa protein in early trophozoites, several hours before rhoptry formation and is transiently localized within the endoplasmic reticulum and Golgi within lipid-rich microdomains. Regions of the Golgi membrane containing RAMA bud to form vesicles that later mature into rhoptries in a process that is inhibitable by brefeldin A. Other rhoptry proteins such as RhopH3 and RAP1 are found in close apposition with RAMA suggesting direct protein-protein interactions. We suggest that RAMA is involved in trafficking of these proteins into rhoptries. In rhoptries, RAMA is proteolytically processed to give a 60-kDa form that is anchored in the inner face of the rhoptry membrane by means of the glycosylphosphatidylinositol anchor. The p60 RAMA form is discharged from the rhoptries of free merozoites and binds to the red blood cell membrane by its most C-terminal region. In early ring stages RAMA is found in association with the parasitophorous vacuole.