Distribution and characterisation of the 235 kDa rhoptry multigene family within the genomes of virulent and avirulent lines of Plasmodium yoelii

Identification of a reticulocyte-specific binding domain of Plasmodium vivax reticulocyte-binding protein 1 that is homologous to the PfRh4 erythrocyte-binding domain

The Plasmodium vivax reticulocyte-binding protein (RBP) family was identified based on the annotation of adhesive ligands in the P. vivax genome. Reticulocyte-specific interactions with the PvRBPs (PvRBP1 and PvRBP2) were previously reported. Plasmodium falciparum reticulocyte-binding protein homologue 4 (PfRh4, a homologue of PvRBP1) was observed to possess erythrocyte-binding activity via complement receptor 1 on the erythrocyte surface. However, the reticulocyte-binding mechanisms of P. vivax are unclear because of the large molecular mass of PvRBP1 (>326 kDa) and the difficulty associated with in vitro cultivation. In the present study, 34 kDa of PvRBP1a (PlasmoDB ID: PVX_098585) and 32 kDa of PvRBP1b (PVX_098582) were selected from a 30 kDa fragment of PfRh4 for reticulocyte-specific binding activity analysis. Both PvRBP1a and PvRBP1b were found to be localized at the microneme in the mature schizont-stage parasites. Naturally acquired immune responses against PvRBP1a-34 and PvRBP1b-32 were observed lower than PvDBP-RII. The reticulocyte-specific binding activities of PvRBP1a-34 and PvRBP1b-32 were significantly higher than normocyte binding activity and were significantly reduced by chymotrypsin treatment. PvRBP1a and 1b, bind to reticulocytes and that this suggests that these ligands may have an important role in P. vivax merozoite invasion.

Linkage maps from multiple genetic crosses and loci linked to growth-related virulent phenotype in Plasmodium yoelii

Plasmodium yoelii is an excellent model for studying malaria pathogenesis that is often intractable to investigate using human parasites; however, genetic studies of the parasite have been hindered by lack of genome-wide linkage resources. Here, we performed 14 genetic crosses between three pairs of P. yoelii clones/subspecies, isolated 75 independent recombinant progeny from the crosses, and constructed a high-resolution linkage map for this parasite. Microsatellite genotypes from the progeny formed 14 linkage groups belonging to the 14 parasite chromosomes, allowing assignment of sequence contigs to chromosomes. Growth-related virulent phenotypes from 25 progeny of one of the crosses were significantly associated with a major locus on chromosome 13 and with two secondary loci on chromosomes 7 and 10. The chromosome 10 and 13 loci are both linked to day 5 parasitemia, and their effects on parasite growth rate are independent but additive. The locus on chromosome 7 is associated with day 10 parasitemia. The chromosome 13 locus spans ~220 kb of DNA containing 51 predicted genes, including the P. yoelii erythrocyte binding ligand, in which a C741Y substitution in the R6 domain is implicated in the change of growth rate. Similarly, the chromosome 10 locus spans ~234 kb with 71 candidate genes, containing a member of the 235-kDa rhoptry proteins (Py235) that can bind to the erythrocyte surface membrane. Atypical virulent phenotypes among the progeny were also observed. This study provides critical tools and information for genetic investigations of virulence and biology of P. yoelii.

Changes in parasite virulence induced by the disruption of a single member of the 235 kDa rhoptry protein multigene family of Plasmodium yoelii

Invasion of the erythrocyte by the merozoites of the malaria parasite is a complex process involving a range of receptor-ligand interactions. Two protein families termed Erythrocyte Binding Like (EBL) proteins and Reticulocyte Binding Protein Homologues (RH) play an important role in host cell recognition by the merozoite. In the rodent malaria parasite, Plasmodium yoelii, the 235 kDa rhoptry proteins (Py235) are coded for by a multigene family and are members of the RH. In P. yoelii Py235 as well as a single member of EBL have been shown to be key mediators of virulence enabling the parasite to invade a wider range of host erythrocytes. One member of Py235, PY01365 is most abundantly transcribed in parasite populations and the protein specifically binds to erythrocytes and is recognized by the protective monoclonal antibody 25.77, suggesting a key role of this particular member in virulence. Recent studies have indicated that overall levels of Py235 expression are essential for parasite virulence. Here we show that disruption of PY01365 in the virulent YM line directly impacts parasite virulence. Furthermore the disruption of PY01365 leads to a reduction in the number of schizonts that express members of Py235 that react specifically with the mcAb 25.77. Erythrocyte binding assays show reduced binding of Py235 to red blood cells in the PY01365 knockout parasite as compared to YM. While our results identify PY01365 as a mediator of parasite virulence, they also confirm that other members of Py235 are able to substitute for PY01365.

Targeted disruption of py235ebp-1: invasion of erythrocytes by Plasmodium yoelii using an alternative Py235 erythrocyte binding protein

Plasmodium yoelii YM asexual blood stage parasites express multiple members of the py235 gene family, part of the super-family of genes including those coding for Plasmodium vivax reticulocyte binding proteins and Plasmodium falciparum RH proteins. We previously identified a Py235 erythrocyte binding protein (Py235EBP-1, encoded by the PY01365 gene) that is recognized by protective mAb 25.77. Proteins recognized by a second protective mAb 25.37 have been identified by mass spectrometry and are encoded by two genes, PY01185 and PY05995/PY03534. We deleted the PY01365 gene and examined the phenotype. The expression of the members of the py235 family in both the WT and gene deletion parasites was measured by quantitative RT-PCR and RNA-Seq. py235ebp-1 expression was undetectable in the knockout parasite, but transcription of other members of the family was essentially unaffected. The knockout parasites continued to react with mAb 25.77; and the 25.77-binding proteins in these parasites were the PY01185 and PY05995/PY03534 products. The PY01185 product was also identified as erythrocyte binding. There was no clear change in erythrocyte invasion profile suggesting that the PY01185 gene product (designated PY235EBP-2) is able to fulfill the role of EBP-1 by serving as an invasion ligand although the molecular details of its interaction with erythrocytes have not been examined. The PY01365, PY01185, and PY05995/PY03534 genes are part of a distinct subset of the py235 family. In P. falciparum, the RH protein genes are under epigenetic control and expression correlates with binding to distinct erythrocyte receptors and specific invasion pathways, whereas in P. yoelii YM all the genes are expressed and deletion of one does not result in upregulation of another. We propose that simultaneous expression of multiple Py235 ligands enables invasion of a wide range of host erythrocytes even in the presence of antibodies to one or more of the proteins and that this functional redundancy at the protein level gives the parasite phenotypic plasticity in the absence of differences in gene expression.

Gene encoding erythrocyte binding ligand linked to blood stage multiplication rate phenotype in Plasmodium yoelii yoelii

Variation in the multiplication rate of blood stage malaria parasites is often positively correlated with the severity of the disease they cause. The rodent malaria parasite Plasmodium yoelii yoelii has strains with marked differences in multiplication rate and pathogenicity in the blood. We have used genetic analysis by linkage group selection (LGS) to identify genes that determine differences in multiplication rate. Genetic crosses were generated between genetically unrelated, fast- (17XYM) and slowly multiplying (33XC) clones of P. y. yoelii. The uncloned progenies of these crosses were placed under multiplication rate selection in blood infections in mice. The selected progenies were screened for reduction in intensity of quantitative genetic markers of the slowly multiplying parent. A small number of strongly selected markers formed a linkage group on P. y. yoelii chromosome 13. Of these, that most strongly selected marked the gene encoding the P. yoelii erythrocyte binding ligand (pyebl), which has been independently identified by Otsuki and colleagues [Otsuki H, et al. (2009) Proc Natl Acad Sci USA 106:10.1073/pnas.0811313106] as a major determinant of virulence in these parasites. In an analysis of a previous genetic cross in P. y. yoelii, pyebl alleles of fast- and slowly multiplying parents segregated with the fast and slow multiplication rate phenotype in the cloned recombinant progeny, implying the involvement of the pyebl locus in determining the multiplication rate. Our genome-wide LGS analysis also indicated effects of at least 1 other locus on multiplication rate, as did the findings of Otsuki and colleagues on virulence in P. y. yoelii.

Invasion of host cells by malaria parasites: a tale of two protein families

Malaria parasites are obligate intracellular parasites whose invasive stages select and invade the unique host cell in which they can develop with exquisite specificity and efficacy. Most studies aimed at elucidating the molecules and the mechanisms implicated in the selection and invasion processes have been conducted on the merozoite, the stage that invades erythrocytes to perpetuate the pathological cycles of parasite multiplication in the blood. Bioinformatic analysis has helped identify the members of two parasite protein families, the reticulocyte-binding protein homologues (RBL) and erythrocyte binding like (EBL), in recently sequenced genomes of different Plasmodium species. In this article we review data from classical studies and gene disruption experiments that are helping to illuminate the role of these proteins in the selection-invasion processes. The manner in which subsets of proteins from each of the families act in concert suggests a model to explain the ability of the parasites to use alternate pathways of invasion. Future perspectives and implications are discussed.

Variable expression of the 235 kDa rhoptry protein ofPlasmodium yoeliimediate host cell adaptation and immune evasion

The severity of infections caused by the malaria parasite Plasmodium is in part due to the rapid multiplication cycles in the blood of an infected individual. A fundamental step in this phenomenon is the invasion of selected erythrocytes of the host by the parasite. The py235 rhoptry protein multigene family of the rodent malaria parasite Plasmodium yoelii has been implicated in mediating host cell selection during erythrocyte invasion and virulence. Here we show using quantitative real-time polymerase chain reaction and Western blot analysis that variations in the amounts of py235 may be a mechanism that the parasite uses to define its host cell repertoire. High levels of py235 expression leads to a wider range of erythrocytes invaded and therefore increased virulence. In contrast, to evade PY235-specific immunity, the parasite downregulates py235 thereby decreasing the host cell repertoire and virulence. These results demonstrate a new mechanism where variations in the amounts of parasite ligand define the parasite host cell repertoire and enable it to evade host immunity.

Delineation of epitopes on the Py235 rhoptry antigen of Plasmodium yoelii YM

The 235-kDa antigenic rhoptry protein Py235 of Plasmodium yoelii is encoded by a large, highly polymorphic gene family. Monoclonal antibodies to some of these antigens have been shown to attenuate the virulence of the lethal YM strain of the parasite, converting a potentially fatal YM infection to a fulminating one typical of the nonlethal 17X strain, by inducing a switch in target cell preference from mature red blood cells to reticulocytes. The reason for this is not known but would suggest that antigenic determinants of Py235 may be useful in or as subunit vaccines. To identify such determinants, we constructed an epitope expression library of one Py235 variant and screened the library with the antibodies. Thus, we mapped 5- and 12-amino acid epitopes to the C-terminus of the antigen. Both epitopes were more reactive with protective than with nonprotective monoclonal antibodies. This may explain the differential protection conferred by these antibodies upon their passive transfer into mice.

Molecular analysis of erythrocyte invasion in Plasmodium falciparum isolates from Senegal

The human malaria parasite, Plasmodium falciparum, utilizes multiple ligand-receptor interactions for the invasion of human erythrocytes. Members of the reticulocyte binding protein homolog (PfRh) family have been shown to be critical for directing parasites to alternative erythrocyte receptors that define invasion pathways. Recent studies have identified gene amplification, sequence polymorphism, and variant expression of PfRh paralogs as mechanisms underlying discrimination between pathways for invasion. In this study, we find considerable heterogeneity in the invasion profiles of clonal, uncultured P. falciparum parasite isolates from a low-transmission area in Senegal. Molecular analyses revealed minimal variation in protein expression levels of the PfRh ligands, PfRh1, PfRh2a, and PfRh2b, and an absence of gene amplification in these isolates. However, significant sequence polymorphism was found within repeat regions of PfRh1, PfRh2a, and PfRh2b. Furthermore, we identified a large sequence deletion ( approximately 0.58 kb) in the C-terminal region of the PfRh2b gene at a high prevalence in this population. In contrast to findings of earlier studies, we found no associations between specific sequence variants and distinct invasion pathways. Overall these data highlight the importance of region-specific elaborations in PfRh sequence and expression polymorphisms, which has important implications in our understanding of how the malaria parasite responds to polymorphisms in erythrocyte receptors and/or evades the immune system.

Plasmodium yoelii: combinatorial expression of variants of the 235 kDa rhoptry antigen during infection

The 235 kDa rhoptry protein Py235 of Plasmodium yoelii, has been implicated in erythrocyte invasion by the merozoite forms of the parasite. Py235 is encoded by a large, highly polymorphic gene family, members of which appear to be differentially transcribed. However, it is not clear how many variants are expressed at the protein level during an infection cycle and whether or not these variants are expressed selectively or combinatorially. Certain monoclonal antibodies to Py235 have been shown to attenuate parasite virulence upon passive transfer into mice, suggesting that this antigen or its derivatives may be useful vaccine candidates. To provide a basis for this, we sought to identify those variants that are recognised by the host immune system, and to establish the pattern of expression of the antigen in mice during infection. Using Py235 monoclonal antibodies as probes, we isolated distinct antigenic variants from an expression library, suggesting that the antigen repertoire is potentially large and that different Py235 variants may be produced during infection. The implications of these observations are discussed with respect to the ability of a cloned parasite line to express distinct antigenic variants in vivo.

Passive immunization with a multicomponent vaccine against conserved domains of apical membrane antigen 1 and 235-kilodalton rhoptry proteins protects mice against Plasmodium yoelii blood-stage challenge infection

During malaria parasite invasion of red blood cells, merozoite proteins bind receptors on the surface of the erythrocyte. Two candidate Plasmodium yoelii adhesion proteins are apical membrane antigen 1 (AMA1) and the 235-kDa rhoptry proteins (P235). Previously, we have demonstrated that passive immunization with monoclonal antibodies (MAbs) 45B1 and 25.77 against AMA1 and P235, respectively, protects against a lethal challenge infection with P. yoelii YM. We show that MAb 45B1 recognizes an epitope located on a conserved surface of PyAMA1, as determined by phage display and analysis of the three-dimensional structure of AMA1, in a region similar to that bound by the P. falciparum AMA1-specific inhibitory antibody 4G2. The epitope recognized by 25.77 could not be assigned. We report here that MAbs 45B1 and 25.77 also protect against challenge with the nonlethal parasite line 17X, in which PyAMA1 has a significantly different amino acid sequence from that in YM. When administered together, the two MAbs acted at least additively in providing protection against challenge with the virulent YM parasite. These results support the concept of developing a multicomponent blood-stage vaccine and the inclusion of polymorphic targets such as AMA1, which these results suggest contain conserved domains recognized by inhibitory antibodies.

Differences in the copy number of the py235 gene family in virulent and avirulent lines of Plasmodium yoelii

The 235kDa rhoptry protein (Py235) of Plasmodium yoelii is coded for by a multigene family. Py235 has been implicated in host cell selection and virulence as antibodies against it have been shown to inhibit invasion of mature red blood cells of the normally virulent P. yoelii YM line and at least one member of this family directly binds to erythrocytes. Differences in py235 sequence and copy number have been postulated to be responsible for the differences in invasion phenotype seen in the avirulent P. yoelii YA line and the YM line. The newly available sequence data for P. yoelii 17X NL 1.1 has now made it possible to investigate this further. A number of approaches including real time PCR was used to determine the exact copy number of individual py235. Except for two cases in YA and one in YM there are no differences in py235 copy number between the two lines and 17X NL 1.1. Analysis of progeny of a genetic cross between YM and an avirulent strain AC yield similar limited variations in copy number. This study shows that the copy number of py235 in the analyzed P. yoelii strains is significantly lower than previous estimates and much more in line with the published genome sequence. The lower copy number as well as the limited difference of py235 in the virulent lines makes it highly unlikely that these are the factors contributing to the differences in invasion observed.

The domain on the mouse Duffy protein for Plasmodium yoelii binding and invasion to mouse erythrocytes

Erythrocyte invasion by malaria parasites is a multi-step process requiring specific molecular interactions between merozoites and erythrocyte surface receptors. Human Duffy blood group protein is the receptor for Plasmodium vivax merozoite invasion to red blood cells. The cognate parasite ligand for Duffy protein is a 135 kDa Duffy binding protein (DBP). Previously, we defined the domain on the N-terminus of human Duffy protein required for DBP binding and showed that a 35-mer N-terminal peptide inhibited DBP binding to Duffy positive red cells in vitro. There is no efficient in vitro culture system or small animal model to study P. vivax ligand binding and invasion to red blood cells. Plasmodium yoelii is frequently used to study the interaction between host receptors and parasite ligands. Similar to human parasite P. vivax, rodent malaria parasite P. yoelii also uses Duffy protein on mouse RBCs for invasion. However, the domain on the mouse Duffy for P. yoelii binding is not known. In this communication, using a mouse model, we show that an antibody against the N-terminus of mouse Duffy protein inhibited P. yoelii invasion in the mouse. In addition, by using small peptides from the N-terminal exocellular domain, we defined the domain on the Duffy protein for P. yoelii binding and invasion to mouse erythrocytes. Our results also indicated that small peptides from the host receptor could act as decoy receptors and may be utilized as potential antimalarial drugs.

Alteration in host cell tropism limits the efficacy of immunization with a surface protein of malaria merozoites

Immunization with Plasmodium yoelii merozoite surface protein-8 (PyMSP-8) has been shown to protect mice against lethal P. yoelii 17XL malaria. Here we demonstrate that PyMSP-8-specific antibodies preferentially suppress P. yoelii 17XL growth in mature erythrocytes compared to growth in reticulocytes and do not suppress the growth of nonlethal P. yoelii 17X, a parasite that primarily replicates in reticulocytes. The protection against normocyte-associated P. yoelii malaria parasites is mediated by antibodies that recognize conformational epitopes of PyMSP-8 that are nonpolymorphic. We examined changes in gene expression in reticulocyte-restricted P. yoelii 17XL parasites that escaped neutralization by PyMSP-8-specific antibodies using P. yoelii DNA microarrays. Of interest, Pymsp-8 gene expression decreased, while the expression of msp-1, msp-7, and several rhoptry protein genes increased. Breakthrough parasites also exhibited increases in the expression of a subset of yir and Pyst-a genes that are predicted to encode polymorphic antigens expressed on the surface of infected erythrocytes. These data suggest that changes in the expression of parasite proteins expressed on the merozoite surface, as well as the surface of infected erythrocytes, may alter host cell tropism and contribute to the ability of malaria parasites to evade merozoite-specific, neutralizing antibodies.

Relapsing fever spirochetes contain chromosomal genes with unique direct tandemly repeated sequences

Genome sequencing of the relapsing fever spirochetes Borrelia hermsii and Borrelia turicatae identified three open reading frames (ORFs) on the chromosomes that contained internal, tandemly repeated amino acid sequences that were absent in the Lyme disease spirochete Borrelia burgdorferi. The predicted amino acid sequences of these genes (BH0209, BH0512, and BH0553) have hydrophobic N termini, indicating that these proteins may be secreted. B. hermsii transcribed the three ORFs in vitro, and the BH0512- and BH0553-encoded proteins (PBH-512 and PBH-553) were produced in vitro and in experimentally infected mice. PBH-512 and PBH-553 were on the spirochete's outer surface, and antiserum to these proteins reduced the adherence of B. hermsii to red blood cells. PCR analyses of 28 isolates of B. hermsii and 8 isolates of B. turicatae demonstrated polymorphism in each gene correlated with the number of repeats. Serum samples from relapsing fever patients reacted with recombinant PBH-512 and PBH-553, suggesting that these proteins are produced during human infection. These polymorphic proteins may be involved in the pathogenicity of these relapsing fever spirochetes and provide a mechanism for antigenic heterogeneity within their populations.

The complete genomic sequence of Mycoplasma penetrans, an intracellular bacterial pathogen in humans

The complete genomic sequence of an intracellular bacterial pathogen, Mycoplasma penetrans HF-2 strain, was determined. The HF-2 genome consists of a 1 358 633 bp single circular chromosome containing 1038 predicted coding sequences (CDSs), one set of rRNA genes and 30 tRNA genes. Among the 1038 CDSs, 264 predicted proteins are common to the Mycoplasmataceae sequenced thus far and 463 are M.penetrans specific. The genome contains the two-component system but lacks the essential cellular gene, uridine kinase. The relatively large genome of M.penetrans HF-2 among mycoplasma species may be accounted for by both its rich core proteome and the presence of a number of paralog families corresponding to 25.4% of all CDSs. The largest paralog family is the p35 family, which encodes surface lipoproteins including the major antigen, P35. A total of 44 genes for p35 and p35 homologs were identified and 30 of them form one large cluster in the chromosome. The genetic tree of p35 paralogs suggests the occurrence of dynamic chromosomal rearrangement in paralog formation during evolution. Thus, M.penetrans HF-2 may have acquired diverse repertoires of antigenic variation-related genes to allow its persistent infection in humans.

PSLAP, a protein with multiple adhesive motifs, is expressed in Plasmodium falciparum gametocytes

A gene coding for a protein containing two Scavenger Receptor Cysteine-Rich (SRCR) motifs, four Limulus factor C, Coch-5b2 and Lgl1 (LCCL) motifs; and one Polycystin-1, Lipoxygenase and Alpha Toxin (PLAT) motif was cloned from Plasmodium chabaudi and homologues identified in the P. falciparum and P. yoelii genome data bases. At least one of these sequence motifs (SRCR) has adhesive properties in other proteins, therefore, we propose to name this protein PSLAP for Plasmodium SRCR, LCCL Adhesive-like Protein. Southern blotting and chromosome analysis showed that pslap is a single copy gene on chromosome 14 in P. falciparum 3D7. pslap mRNA is strongly expressed in P. falciparum gametocytes, but was undetectable on Northern blots of RNA from the asexual blood stages. Polyclonal antibodies raised to different parts of PSLAP detected a protein expressed in late gametocytes, but not in the early stages of gametocytogenesis or asexual blood stages of P. falciparum. We suggest that PSLAP functions in the mosquito, for example, in modulation of the invertebrate host immune response or in protection against complement factors in the blood meal.

Stage-specific transcription of distinct repertoires of a multigene family during Plasmodium life cycle

Members of a multigene family in the rodent malaria parasite Plasmodium yoelii yoelii code for 235-kilodalton proteins (Py235) that are located in the merozoite apical complex, are implicated in virulence, and may determine red blood cell specificity. We show that distinct subsets of py235 genes are expressed in sporozoites and hepatic and erythrocytic stages. Antibodies to Py235 inhibited sporozoite invasion of hepatocytes. The switch in expression profile occurred immediately after transition from one stage to another. The results suggest that this differential expression is driven by strong biological requirements and provide evidence that hepatic and erythrocytic merozoites differ.

The 235 kDa rhoptry protein of Plasmodium (yoelii) yoelii: function at the junction

All malaria parasites are obligate intracellular organisms that must clearly recognise and discriminate between different cells during their life cycle. Invasion into a cell is a multi-step event that is marked by initial attachment proceeding to irreversible junction formation and penetration. A 235 kDa rhoptry protein (Py235) in the rodent malaria, Plasmodium yoelii yoelii has been shown to be involved in red blood cell (rbc) binding and is involved in a new mechanism of clonal phenotypic variation that may be important in adaptation and immune evasion. Immunisation studies using Py235 have also revealed a role for this protein in the virulence phenotype seen with P. y. yoelii in laboratory mice. Interestingly, the genes that encode this protein are present as a multi-gene family. In this paper, we examine Py235 at the level of DNA, transcription and expression, discussing the role of this protein during invasion, in virulence and in immune evasion.