Sequence coding for a sexual stage specific protein of Plasmodium falciparum

Biology of Plasmodium falciparum gametocyte sex ratio and implications in malaria parasite transmission

While significant advances have been made in understanding Plasmodium falciparum gametocyte biology and its relationship with malaria parasite transmission, the gametocyte sex ratio contribution to this process still remains a relevant research question. The present review discusses the biology of sex determination in P. falciparum, the underlying host and parasite factors, the sex specific susceptibility to drugs, the effect of sex ratio dynamics on malaria parasite transmission and the development of gametocyte sex specific diagnosis tools. Despite the inherent differences across several studies and approaches, the emerging picture highlights a potentially relevant contribution of the P. falciparum gametocyte sex ratio in the modulation of malaria parasite transmission. The increasing availability of molecular methods to measure gametocyte sex ratio will enable evaluation of important parameters, such as the impact of drug treatment on gametocyte sex ratio in vitro and in vivo as well as the changes of gametocyte sex ratios in natural infections, key steps towards elucidating how these parameters affect parasite infectiousness to the mosquito vectors.

Molecular make-up of the Plasmodium parasitophorous vacuolar membrane

Plasmodium, the causative agent of malaria, is an obligate, intracellular, eukaryotic cell that invades, replicates, and differentiates within hepatocytes and erythrocytes. Inside a host cell, a second membrane delineates the developing pathogen in addition to the parasite plasma membrane, resulting in a distinct cellular compartment, termed parasitophorous vacuole (PV). The PV membrane (PVM) constitutes the parasite-host cell interface and is likely central to nutrient acquisition, host cell remodeling, waste disposal, environmental sensing, and protection from innate defense. Over the past two decades, a number of parasite-encoded PVM proteins have been identified. They include multigene families and protein complexes, such as early-transcribed membrane proteins (ETRAMPs) and the Plasmodium translocon for exported proteins (PTEX). Nearly all Plasmodium PVM proteins are restricted to this genus and display transient and stage-specific expression. Here, we provide an overview of the PVM proteins of Plasmodium blood and liver stages. Biochemical and experimental genetics data suggest that some PVM proteins are ideal targets for novel anti-malarial intervention strategies.

Plasmodium falciparum: mRNA co-expression and protein co-localisation of two gene products upregulated in early gametocytes

Genes encoding Plasmodium falciparum proteins Pfs16 and Pfpeg3/mdv1, specifically appearing in the parasitophorous vacuole of the early gametocytes, are upregulated at the onset of sexual differentiation. Analysis of asexual development in gametocyte producing and non-producing clones of P. falciparum indicated that these genes are also transcribed at a low level in asexual parasites, although their protein products are not detectable in these stages by immunofluorescence. Immunoelectron microscopic analysis of stage II gametocytes indicated that Pfs16 and Pfpeg3/mdv1 proteins co-localise in the parasitophorous vacuole membrane and in all derived membranous structures (such as the multi-laminate membrane whorls of the circular clefts in the infected erythrocyte cytoplasm and the membranes of the gametocyte food vacuoles). In this analysis both proteins were also observed for the first time in the membrane and in the lumen of distinct cleft-like structures in the erythrocyte cytoplasm.

Quantification of Plasmodium falciparum gametocytes in differential stages of development by quantitative nucleic acid sequence-based amplification

Two quantitative nucleic acid sequence-based amplification assays (QT-NASBA) based on Pfs16 and Pfs25, have been developed to quantify sexual stage commitment and mature gametocytes of Plasmodium falciparum. Pfs16 mRNA is expressed in all sexual forms including sexually committed ring stages while expression of Pfs25 mRNA is restricted to late stage gametocytes. Both assays showed a sensitivity of one sexual stage parasite/microl of blood. Blood samples from experimentally infected non-immune human volunteers were tested for Plasmodium falciparum by standard microscopy, a previously developed asexual 18S rRNA QT-NASBA, Pfs16 and Pfs25 mRNA QT-NASBA. Pfs16 QT-NASBA was positive in 9 out of 10 volunteers within 48 h after first detection of 18S rRNA, mostly before or at the day of positive microscopy. In contrast, the Pfs25 mRNA QT-NASBA was negative during the 28 days of follow-up, but consistently positive in gametocyte samples from naturally infected Kenyan patients. These data suggest that sexual stage commitment can occur early in the blood-stage infection without successful maturation into infectious gametocytes. In conclusion, Pfs16 and Pfs25 QT-NASBA assays in combination with a previously developed asexual stage QT-NASBA allow for the separate quantification of all developmental stages present in the circulation. The application of sexual stage QT-NASBA assays may contribute to a better understanding of the biology and epidemiology of malaria transmission.

CD36-mediated nonopsonic phagocytosis of erythrocytes infected with stage I and IIA gametocytes of Plasmodium falciparum

Gametocytes, the sexual stages of malaria parasites (Plasmodium spp.) that are transmissible to mosquitoes, have been the focus of much recent research as potential targets for novel drug and vaccine therapies. However, little is known about the host clearance of gametocyte-infected erythrocytes (GEs). Using a number of experimental strategies, we found that the scavenger receptor CD36 mediates the uptake of nonopsonized erythrocytes infected with stage I and IIA gametocytes of Plasmodium falciparum by monocytes and culture-derived macrophages (Mphis). Light microscopy and immunofluorescence assays revealed that stage I and IIA gametocytes were readily internalized by monocytes and Mphis. Pretreating monocytes and Mphis with a monoclonal antibody that blocked CD36 resulted in a significant reduction in phagocytosis, as did treating GEs with low concentrations of trypsin to remove P. falciparum erythrocyte membrane protein 1 (PfEMP-1), a parasite ligand for CD36. Pretreating monocytes and Mphis with peroxisome proliferator-activated receptor gamma-retinoid X receptor agonists, which specifically upregulate CD36, resulted in a significant increase in the phagocytosis of GEs. Murine CD36 on mouse Mphis also mediated the phagocytosis of P. falciparum stage I and IIA gametocytes, as determined by receptor blockade with anti-murine CD36 monoclonal antibodies and the lack of uptake by CD36-null Mphis. These results indicate that phagocytosis of stage I and IIA gametocytes by monocytes and Mphis appears to be mediated to a large extent by the interaction of PfEMP-1 and CD36, suggesting that CD36 may play a role in innate clearance of these early sexual stages.

Plasmodium falciparum: detection of a novel asparagine-rich protein on the surface of sporozoite

We had previously cloned and characterized a gene for a novel asparagine-rich protein from P. falciparum (PfARP), a target of natural human immune response. The antibodies to PfARP were localized to the surface of parasitized red blood cells and reacted with intracellular components in all erythrocytic asexual and sexual stages of the parasite. We here describe reactivity of antibodies against this novel PfARP on the surface of mosquito stage sporozoite of P. falciparum by indirect immunofluorescence assay and immunoelectron microscopy, the latter revealing a highly periodic punctate pattern of distribution of PfARP on the surface of sporozoite. These results suggest a possibility that PfARP might represent yet another sporozoite surface protein.

Identification of an endoplasmic reticulum-resident calcium-binding protein with multiple EF-hand motifs in asexual stages of Plasmodium falciparum

An endoplasmic reticulum-located, calcium-binding protein, with an apparent molecular weight (Mr) of approximately 40,000 (PfERC), has been identified in the asexual stages of the malaria parasite, Plasmodium falciparum. This protein appears to be equivalent to a previously described gametocyte protein, Pfs40, which was reported to be expressed on the gametocyte surface (Rawlings DJ, Kaslow DC. J Biol Chem 1992;267:3976-3982). Sequencing of the 3' end of the gene revealed the omission of a single base in the 3' region of the published sequence. The corrected gene sequence encodes a C-terminal IDEL motif, which indicates residency of the 40 kDa protein within the endoplasmic reticulum. The predicted C-terminal region also appears to contain a sixth EF-hand calcium-binding domain, which suggests that PfERC is related to previously reported ER-localized calcium-binding proteins, namely reticulocalbin and ERC-55 (Ozawa M. J. Biochem. 1995;117:1113-1119; Weis K, Griffiths G, Lamond AI. J. Biol. Chem. 1994;269:19142-19150). The presence of the 40 kDa calcium-binding protein in malaria parasites was confirmed using 45Ca2+-blotting and partial protein sequencing of the corresponding Coomassie blue-stained polypeptide. Confocal immunofluorescence microscopy of asexual stage parasites was used to show that PfERC co-localizes with the known ER-located protein, Pfgrp. Analysis of immunoblots of tightly synchronized parasites showed that expression of PfERC increases with increasing maturity of the parasite. We propose that PfERC is a member of the reticulocalbin family of calcium-binding proteins and may play a role in protein trafficking in the malaria parasite.

Protein phosphatase beta, a putative type-2A protein phosphatase from the human malaria parasite Plasmodium falciparum

Protein phosphatases play a critical role in the regulation of the eukaryotic cell cycle and signal transduction. A putative protein serine/threonine phosphatase gene has been isolated from the human malaria parasite Plasmodium falciparum. The gene has an unusual intron that contains four repeats of 32 nucleotides and displays a high degree of size polymorphism among different strains of P. falciparum. The open reading frame reconstituted by removal of the intron encodes a protein of 466 amino acids with a predicted molecular mass of approximately 53.7 kDa. The encoded protein, termed protein phosphatase beta (PP-beta), is composed of two distinct domains. The C-terminal domain comprises 315 amino acids and exhibits a striking similarity to the catalytic subunits of the type-2A protein phosphatases. Database searches revealed that the catalytic domain has the highest similarity to Schizosaccharomyces pombe Ppa1 (58% identity and 73% similarity). However, it contains a hydrophilic insert consisting of five amino acids. The N-terminal domain comprises 151 amino acid residues and exhibits several striking features, including high levels of charged amino acids and asparagine, and multiple consensus phosphorylation sites for a number of protein kinases. An overall structural comparison of PP-beta with other members of the protein phosphatase 2A group revealed that PP-beta is more closely related to Saccharomyces cerevisiae PPH22. Southern blots of genomic DNA digests and chromosomal separations showed that PP-beta is a single-copy gene and is located on chromosome 9. A 2800-nucleotide transcript of this gene is expressed specifically in the sexual erythrocytic stage (gametocytes). The results indicate that PP-beta may be involved in sexual stage development.

Pfmrk, a MO15-related protein kinase from Plasmodium falciparum. Gene cloning, sequence, stage-specific expression and chromosome localization

Cyclin-dependent kinases (Cdks) play a central role in the regulation of the eukaryotic cell cycle. A novel gene encoding a Cdk-like protein, Pfmrk, has been isolated from the human malaria parasite Plasmodium falciparum. The gene has no introns and comprises an open reading frame encoding a protein of 324 amino acids with a predicted molecular mass of 38 kDa. Database searches revealed a striking similarity to the Cdk subfamily with the highest similarity to human MO15 (Cdk7). The overall sequence of Pfmrk shares 62% similarity and 46% identity with human MO15, in comparison to the 49-58% similarity and 34-43% identity with other human Cdks. Pfmrk contains two unique inserts: one consisting of 5 amino acids just before the cyclin-binding motif and the other composed of 13 amino acids within the T-loop equivalent region. Southern blots of genomic DNA digests and chromosomal separations showed that Pfmrk is a single-copy gene conserved between several parasite strains and is located on chromosome 10. A 2500-nucleotide transcript of this gene is expressed predominantly in the sexual blood stages (gametocytes), suggesting that Pfmrk may be involved in sexual stage development.

COS cell expression cloning of Pfg377, a Plasmodium falciparum gametocyte antigen associated with osmiophilic bodies

We report the deduced protein sequence and preliminary characterization of Pfg377, a novel sexual stage antigen of Plasmodium falciparum. An initial cDNA clone (Pfg377-1) encoding the N-terminal 755 amino acids of Pfg377 was isolated by transfecting a 3D7 gametocyte cDNA library into COS7 cells and selecting using a pool of anti-Pfs230 monoclonal antibodies. The protein encoded by Pfg377-1 included an N-terminal hydrophobic signal sequence, but no apparent transmembrane anchor. Instead, the particular cDNA clone selected was fused in-frame at its 3' end with the coding sequence for the human decay acceleration factor membrane anchor, which had been deliberately placed downstream of the vector polylinker in order to attach potential fusion proteins onto the COS cell surface. Northern blots probed with the Pfg377-1 cDNA demonstrated cross-hybridization to a single approximately 9.5-kb transcript, which was present only in sexual stages, and not in a sexual stages. DNA hybridization was used to obtain a series of overlapping genomic clones which collectively yielded the complete DNA sequence for Pfg377. There are no introns within the gene, which contains a 9360-bp open reading frame and encodes a 377-kDa protein. The Pfg377 protein is highly hydrophilic, and has an essentially non-repetitive structure, with only four very limited regions of tandem repeats. The Pfg377 gene resides on chromosome 12, and immunoelectron microscopy with two different anti-Pfg377 polyclonal antisera raised against two separate recombinant sub-fragments of the protein both indicated that the antigen is located in electron-dense organelles of the gametocytes--the osmiophilic bodies--which are proposed to play a role in parasite emergence from the erythrocyte during gametocyte maturation in the Anopheles mosquito midgut. Although it was selected with anti-Pfs230 antibodies, comparison of the sub-cellular locations and protein sequences of Pfg377 and Pfs2 show them to be completely distinct antigens. We hypothesize that Pfg377-1 was initially isolated because it expresses an epitope which is recognized by (i.e., cross-reacts with) one of the anti-Pfs230 monoclonal antibodies used to select the original transfected COS cells.

Sexual-stage-specific RNA expression of a new Plasmodium falciparum gene detected by in situ hybridisation

A gene, Pf77, transcribed in the sexual stages of Plasmodium falciparum was isolated from a genomic expression library with a polyclonal antibody raised to gametocyte proteins. The entire coding region was obtained from a series of overlapping genomic and cDNA clones (from gametocyte RNA). A single open reading frame is present with no introns and no tandem repeat sequences. A Pf77 probe hybridised to a single transcript present in RNA prepared from purified gametocytes and could not be detected in RNA prepared from asexual blood stages. In situ hybridisation studies confirmed that the expression of Pf77 mRNA is sexual-stage-specific and in addition, showed that Pf77 mRNA is present only in female gametocytes during the vertebrate stages of the parasite's development.

Subcellular localization of Pfs16, a Plasmodium falciparum gametocyte antigen

We have used immunoelectron microscopy to investigate the subcellular location of Pfs16 in Plasmodium falciparum. It was detected in the outer membrane region of gametocytes and more specifically on the parasitophorous vacuole membrane (pvm), since, during gametogenesis when the pvm disintegrates, the majority of the antigen was detected on the remains of this membrane in multilaminated whorls and not on the gamete plasma membrane. The antigen was also present on other gametocyte cellular structures, including those which we believe to be Garnham bodies, present in the host cell cytoplasm of some gametocytes. The antigen was present too on the membrane surrounding cytosomes and the resulting food vacuoles in the parasite cytoplasm.

Antibody responses to Plasmodium falciparum gametocyte antigens during and after malaria attacks in schoolchildren from Madang, Papua New Guinea

Sera from 49 school children in Madang, Papua New Guinea with malaria and follow-up sera from 40 of these cases were tested by competitive ELISA for antibodies capable of inhibiting binding of eight monoclonal antibodies (MoAbs) to Plasmodium falciparum gametocytes. The proportion of sera inhibiting each MoAb ranged from 31.2% to 85.7%. At follow-up, the proportion of inhibitory sera decreased for 3 MoAbs, did not change significantly for 4 MoAbs and increased for one MoAb. When sera were grouped according to whether the follow-up blood slide was positive or negative, further trends emerged for MoAbs against the gamete surface antigen Pfs 48/45. Antibody levels to the IA3-B8 epitope decreased in follow-up positive cases, but remained unchanged for follow-up negative cases. The converse was observed for the IIC5-B10 epitope with an increase of antibody in follow-up positive cases and no change in the negative cases. Amount of antibody to the 3G12/58 epitope decreased when the follow-up was negative but not when it was positive. Increase in antibody to the 3E12/58 epitope occurred at the follow-up sample irrespective of the blood slide result. Thus four distinct patterns of longitudinal antibody response were observed against four epitopes on the same molecule.

A stage specific gene expressed at the onset of gametocytogenesis in Plasmodium falciparum

The gene encoding the gametocyte specific cytoplasmic protein Pfg27/25 of the human malaria parasite Plasmodium falciparum has been cloned. The gene encodes a highly hydrophilic non-repetitive protein which does not share obvious homologies with other polypeptides. The stage specificity of Pfg27/25 is controlled at the stage of the production of stable mRNA, which is detectable only in the sexual stages of the parasite, and contains long additional sequences outside the Pfg27/25 coding region. As the activation of Pfg27/25 gene expression occurs at an early stage of gametocytogenesis, the study of its regulation might provide information on the molecular events occurring after the parasite commitment to sexual differentiation and at the beginning of gametocyte formation.