Subcellular localization of Pfs16, a Plasmodium falciparum gametocyte antigen

Skeleton binding protein 1 localizes to the Maurer's cleft and interacts with PfHSP70-1 and PfHSP70-x in Plasmodium falciparum gametocyte-infected erythrocytes

Plasmodium falciparum accounts for the majority of malaria deaths, due to pathology provoked by the ability of infected erythrocytes to adhere to vascular endothelium within deep tissues. The parasite recognizes endothelium by trafficking and displaying protein ligands on the surface of asexual stage infected erythrocytes, such as members of the large family of pathogenic proteins, P. falciparum erythrocyte membrane protein 1 (PfEMP1). Parasite-encoded skeleton binding protein 1 (SBP1) plays an important role in the transport of these binding-related surface proteins, via cleft-like membranous structures termed Maurer's clefts, which are present within the cytoplasm of infected erythrocytes. Erythrocytes infected with gametocyte stages accumulate in the extravascular compartment of bone marrow; and it was suggested that their surface-expressed adhesion molecule profile and protein trafficking mechanisms might differ from those in asexual stage parasites. Protein trafficking mechanisms via Maurer's clefts have been well investigated in asexual stage parasite-infected erythrocytes; but little is known regarding the gametocyte stages. In this study, we characterized SBP1 during gametocyte maturation and demonstrated that SBP1 is expressed and localizes to dot-like Maurer's cleft structures in the cytoplasm of gametocyte-infected erythrocytes. Co-immunoprecipitation and mass spectrometry assays indicated that SBP1 interacts with the molecular chaperones PfHSP70-1 and PfHSP70-x. Localization analysis suggested that some PfHSP70-1 and/or PfHSP70-x localize in a dot-like pattern within the cytoplasm of immature gametocyte-infected erythrocytes. These findings suggest that SBP1 may interact with HSP70 chaperones in the infected erythrocyte cytoplasm during the immature gametocyte stages.

Expression of the MSPDBL2 antigen in a discrete subset of Plasmodium falciparum schizonts is regulated by GDV1 but may not be linked to sexual commitment

The Plasmodium falciparum merozoite surface protein MSPDBL2 is a polymorphic antigen targeted by acquired immune responses, and normally expressed in only a minority of mature schizonts. The potential relationship of MSPDBL2 to sexual commitment is examined, as variable mspdbl2 transcript levels and proportions of MSPDBL2-positive mature schizonts in clinical isolates have previously correlated with levels of many sexual stage parasite gene transcripts, although not with the master regulator ap2-g. It is demonstrated that conditional overexpression of the gametocyte development protein GDV1, which promotes sexual commitment, also substantially increases the proportion of MSPDBL2-positive schizonts in culture. Conversely, truncation of the gdv1 gene is shown to prevent any expression of MSPDBL2. However, across diverse P. falciparum cultured lines, the variable proportions of MSPDBL2 positivity in schizonts do not correlate significantly with variable gametocyte conversion rates, indicating it is not involved in sexual commitment. Confirming this, examining a line with endogenous hemagglutinin-tagged AP2-G showed that the individual schizonts expressing MSPDBL2 are mostly different from those expressing AP2-G. Using a selection-linked integration system, modified P. falciparum lines were engineered to express an intact or disrupted version of MSPDBL2, showing the protein is not required for sexual commitment or early gametocyte development. Asexual parasite multiplication rates were also not affected by expression of either intact or disrupted MSPDBL2 in a majority of schizonts. Occurring alongside sexual commitment, the role of the discrete MSPDBL2-positive schizont subpopulation requires further investigation in natural infections where it is under immune selection.

IMPORTANCE

Malaria parasites in the blood are remarkably variable, able to switch antigenic targets so they may survive within humans who have already developed specific immune responses. This is one of the challenges in developing vaccines against malaria. MSPDBL2 is a target of naturally acquired immunity expressed in minority proportions of schizonts, the end stages of each 2-day replication cycle in red blood cells which contain merozoites prepared to invade new red blood cells. Results show that the proportion of schizonts expressing MSPDBL2 is positively controlled by the expression of the regulatory gametocyte development protein GDV1. It was previously known that expression of GDV1 leads to increased expression of AP2-G which causes parasites to switch to sexual development, so a surprising finding here is that MSPDBL2-positive parasites are mostly distinct from those that express AP2-G. This discrete antigenic subpopulation of mostly asexual parasites is regulated alongside sexually committed parasites, potentially enabling survival under stress conditions.

Plasmodium parasitophorous vacuole membrane protein Pfs16 promotes malaria transmission by silencing mosquito immunity

With rising cases for the first time in years, malaria remains a significant public health burden. The sexual stage of the malaria parasite infects mosquitoes to transmit malaria from host to host. Hence, an infected mosquito plays an essential role in malaria transmission. Plasmodium falciparum is the most dominant and dangerous malaria pathogen. Previous studies identified a sexual stage-specific protein 16 (Pfs16) localized to the parasitophorous vacuole membrane (PVM). Here we elucidate the function of Pfs16 during malaria transmission. Our structural analysis identified Pfs16 as an alpha-helical integral membrane protein with one transmembrane domain connecting to two regions across PVM. ELISA assays showed that insect cell-expressed recombinant Pfs16 (rPfs16) interacted with An. gambiae midguts, and microscopy found that rPfs16 bound to midgut epithelial cells. Transmission-blocking assays demonstrated that polyclonal antibodies against Pfs16 significantly reduced the number of oocysts in mosquito midguts. However, on the contrary, feeding rPfs16 increased the number of oocysts. Further analysis revealed that Pfs16 reduced the activity of mosquito midgut caspase 3/7, a key enzyme in the mosquito Jun-N-terminal kinase (JNK) immune pathway. We conclude that Pfs16 facilitates parasites to invade mosquito midguts by actively silencing the mosquito's innate immunity through its interaction with the midgut epithelial cells. Therefore, Pfs16 is a potential target to control malaria transmission.

A novel class of sulphonamides potently block malaria transmission by targeting a Plasmodium vacuole membrane protein

Phenotypic cell-based screens are critical tools for discovering candidate drugs for development, yet identification of the cellular target and mode of action of a candidate drug is often lacking. Using an imaging-based screen, we recently discovered an N-[(4-hydroxychroman-4-yl)methyl]-sulphonamide (N-4HCS) compound, DDD01035881, that blocks male gamete formation in the malaria parasite life cycle and subsequent transmission of the parasite to the mosquito with nanomolar activity. To identify the target(s) of DDD01035881, and of the N-4HCS class of compounds more broadly, we synthesised a photoactivatable derivative, probe 2. Photoaffinity labelling of probe 2 coupled with mass spectrometry identified the 16 kDa Plasmodium falciparum parasitophorous vacuole membrane protein Pfs16 as a potential parasite target. Complementary methods including cellular thermal shift assays confirmed that the parent molecule DDD01035881 stabilised Pfs16 in lysates from activated mature gametocytes. Combined with high-resolution, fluorescence and electron microscopy data, which demonstrated that parasites inhibited with N-4HCS compounds phenocopy the targeted deletion of Pfs16 in gametocytes, these data implicate Pfs16 as a likely target of DDD01035881. This finding establishes N-4HCS compounds as being flexible and effective starting candidates from which transmission-blocking antimalarials can be developed in the future.

Plasmodium falciparum Sexual Commitment Rate Variation among Clinical Isolates and Diverse Laboratory-Adapted Lines

Asexual blood-stage malaria parasites must produce sexual progeny to infect mosquitoes. It is important to understand the scope and causes of intraspecific variation in sexual commitment rates, particularly for the major human parasite P. falciparum. First, two alternative assay methods of measuring sexual commitment were compared to test a genetically modified P. falciparum line with elevated commitment rates inducible by overexpression of GDV1. The methods yielded correlated measurements with higher sensitivity and precision being achieved by one employing detection of the early gametocyte differentiation marker Pfs16. Thus, this was used to survey a diverse range of parasite lines and test each in multiple biological replicate assays in a serum-free medium supplemented with Albumax. There were differences among six recent clinical isolates from Ghana in their mean rates of sexual commitment per cycle, ranging from 3.3% to 12.2%. Among 13 diverse long-term laboratory-adapted lines, mean sexual commitment rates for most ranged from 4.7% to 13.4%, a few had lower rates with means from 0.3 to 1.6%, and one with a nonfunctional ap2-g gene always showed zero commitment. Among a subset of lines tested for the effects of exogenous choline to suppress commitment, there were significant differences. As expected, there was no effect in a line that had lost the gdv1 gene and that had generally low commitment, whereas the others showed quantitatively variable but significant responses to choline, suggesting potential trait variation. The results indicated the value of performing multiple replicate assays for understanding the variation of this key reproductive trait that likely affects transmission. IMPORTANCE Only sexual-stage malaria parasites are transmitted from human blood to mosquitoes. Thus, it is vital to understand variations in sexual commitment rates because these may be modifiable or susceptible to blocking. Two different methods of commitment rate measurement were first compared, demonstrating higher sensitivity and precision by the detection of an early differentiation marker, which was subsequently used to survey diverse lines. Clinical isolates from Ghana showed significant variation in mean per-cycle commitment rates and variation among biological replicates. Laboratory-adapted lines of diverse origins had a wider range with most being within the range observed for the clinical isolates, while a minority consistently had lower or zero rates. There was quantitative variation in the effects when adding choline to suppress commitment, indicating differing responsiveness of parasites to this environmental modification. Performing multiple assay replicates and comparisons of diverse isolates was important to understand this trait and its potential effects on transmission.

Studies of the Parasite-Midgut Interaction Reveal Plasmodium Proteins Important for Malaria Transmission to Mosquitoes

Malaria transmission relies on parasite-mosquito midgut interaction. The interactive proteins are hypothesized to be ideal targets to block malaria transmission to mosquitoes. We chose 76 genes that contain signal peptide-coding regions and are upregulated and highly abundant at sexual stages. Forty-six of these candidate genes (60%) were cloned and expressed using the baculovirus expression system in insect cells. Six of them, e.g., PF3D7_0303900, PF3D7_0406200 (Pfs16), PF3D7_1204400 (Pfs37), PF3D7_1214800, PF3D7_1239400, and PF3D7_1472800 were discovered to interact with blood-fed mosquito midgut lysate. Previous works showed that among these interactive proteins, knockout the orthologs of Pfs37 or Pfs16 in P. berghei reduced oocysts in mosquitoes. Here we further found that anti-Pfs16 polyclonal antibody significantly inhibited P. falciparum transmission to Anopheles gambiae. Investigating these candidate proteins will improve our understanding of malaria transmission and discover new targets to break malaria transmission.

Immunity against sexual stage Plasmodium falciparum and Plasmodium vivax parasites

The efficient spread of malaria from infected humans to mosquitoes is a major challenge for malaria elimination initiatives. Gametocytes are the only Plasmodium life stage infectious to mosquitoes. Here, we summarize evidence for naturally acquired anti-gametocyte immunity and the current state of transmission blocking vaccines (TBV). Although gametocytes are intra-erythrocytic when present in infected humans, developing Plasmodium falciparum gametocytes may express proteins on the surface of red blood cells that elicit immune responses in naturally exposed individuals. This immune response may reduce the burden of circulating gametocytes. For both P. falciparum and Plasmodium vivax, there is a solid evidence that antibodies against antigens present on the gametocyte surface, when co-ingested with gametocytes, can influence transmission to mosquitoes. Transmission reducing immunity, reducing the burden of infection in mosquitoes, is a well-acknowledged but poorly quantified phenomenon that forms the basis for the development of TBV. Transmission enhancing immunity, increasing the likelihood or intensity of transmission to mosquitoes, is more speculative in nature but is convincingly demonstrated for P. vivax. With the increased interest in malaria elimination, TBV and monoclonal antibodies have moved to the center stage of malaria vaccine development. Methodologies to prioritize and evaluate products are urgently needed.

Antibodies to PfsEGXP, an Early Gametocyte-Enriched Phosphoprotein, Predict Decreased Plasmodium falciparum Gametocyte Density in Humans

Background

Antigametocyte-specific immune responses may regulate Plasmodium falciparum gametocyte density, providing the rationale for pursuing transmission-blocking vaccines (TBVs) that target gametocytes in the human host.

Methods

To identify novel antigametocyte TBV antigens, we interrogated the gametocyte proteome with our whole proteome differential screening method using plasma from a treatment-reinfection study conducted in western Kenya. At the start of the high-transmission season, 144 males (12-35 years) were enrolled and treated with quinine and doxycycline, peripheral venous blood samples were obtained, volunteers were observed, and weekly blood films were obtained for 18 weeks to quantify gametocytemia. Using plasma pooled from individuals with low versus high gametocyte carriage, we differentially screened a P falciparum gametocyte stage complementary deoxyribonucleic acid expression library.

Results

We identified 8 parasite genes uniquely recognized by gametocyte-resistant but not by gametocyte-susceptible individuals. Antibodies to one of these antigens, PfsEGXP, predicted lower gametocytemia measured over the 18-week transmission season (P = .021). When analyzed dichotomously, anti-PfsEGXP responders had 31% lower gametocyte density over 18 weeks of follow-up, compared with nonresponders (P = .04).

Conclusions

PfsEGXP is one of the first reported gametocyte-specific target of antibodies that predict decreased gametocyte density in humans and supports our novel TBV antigen discovery platform.

A seven-helix protein constitutes stress granules crucial for regulating translation during human-to-mosquito transmission of Plasmodium falciparum

The complex life-cycle of the human malaria parasite Plasmodium falciparum requires a high degree of tight coordination allowing the parasite to adapt to changing environments. One of the major challenges for the parasite is the human-to-mosquito transmission, which starts with the differentiation of blood stage parasites into the transmissible gametocytes, followed by the rapid conversion of the gametocytes into gametes, once they are taken up by the blood-feeding Anopheles vector. In order to pre-adapt to this change of host, the gametocytes store transcripts in stress granules that encode proteins needed for parasite development in the mosquito. Here we report on a novel stress granule component, the seven-helix protein 7-Helix-1. The protein, a homolog of the human stress response regulator LanC-like 2, accumulates in stress granules of female gametocytes and interacts with ribonucleoproteins, such as CITH, DOZI, and PABP1. Malaria parasites lacking 7-Helix-1 are significantly impaired in female gametogenesis and thus transmission to the mosquito. Lack of 7-Helix-1 further leads to a deregulation of components required for protein synthesis. Consistently, inhibitors of translation could mimic the 7-Helix-1 loss-of-function phenotype. 7-Helix-1 forms a complex with the RNA-binding protein Puf2, a translational regulator of the female-specific antigen Pfs25, as well as with pfs25-coding mRNA. In accord, gametocytes deficient of 7-Helix-1 exhibit impaired Pfs25 synthesis. Our data demonstrate that 7-Helix-1 constitutes stress granules crucial for regulating the synthesis of proteins needed for life-cycle progression of Plasmodium in the mosquito vector.

Sequential Membrane Rupture and Vesiculation during Plasmodium berghei Gametocyte Egress from the Red Blood Cell

Malaria parasites alternate between intracellular and extracellular stages and successful egress from the host cell is crucial for continuation of the life cycle. We investigated egress of Plasmodium berghei gametocytes, an essential process taking place within a few minutes after uptake of a blood meal by the mosquito. Egress entails the rupture of two membranes surrounding the parasite: the parasitophorous vacuole membrane (PVM), and the red blood cell membrane (RBCM). High-speed video microscopy of 56 events revealed that egress in both genders comprises four well-defined phases, although each event is slightly different. The first phase is swelling of the host cell, followed by rupture and immediate vesiculation of the PVM. These vesicles are extruded through a single stabilized pore of the RBCM, and the latter is subsequently vesiculated releasing the free gametes. The time from PVM vesiculation to completion of egress varies between events. These observations were supported by immunofluorescence microscopy using antibodies against proteins of the RBCM and PVM. The combined results reveal dynamic re-organization of the membranes and the cortical cytoskeleton of the erythrocyte during egress.

Sequence and functional divergence of gametocyte-specific parasitophorous vacuole membrane proteins in Plasmodium parasites

Plasmodium parasites develop within red blood cells in a Parasitophorous Vacuole enclosed by a Membrane, the PVM. The protein family ETRAMP (Early Transcribed Membrane Protein) comprises small proteins inserted in the PVM via a single transmembrane domain. Among those, Pfs16 is specifically found in P. falciparum gametocyte PVM. The P. berghei gene PBANKA_1003900 is syntenic with pfs16. The encoded proteins have a similar domain structure but the overall protein similarity is low. A transcript of the P. berghei gene is only found in gametocytes and ookinetes and a C-terminal mCherry fusion of the protein revealed its presence only in gametocytes. A knock-out mutant of the PBANKA_1003900 gene was not affected in sexual development and ookinete formation was similar to WT. The mutation had no adverse effect on transmission through the mosquito although there was a reduction of the number of oocysts formed by the mutant parasites.

Specific expression and export of the Plasmodium falciparum Gametocyte EXported Protein-5 marks the gametocyte ring stage

BACKGROUND

Plasmodium falciparum sexual development plays a fundamental role in the transmission and spread of malaria. The ability to generate gametocytes can be lost during culture in vitro, often associated with the loss of a subtelomeric region of chromosome 9. Gametocytogenesis starts with erythrocyte invasion by a sexually committed merozoite, but the first available specific marker of sexual differentiation appears only from 24 h post invasion.

METHODS

Specific antibodies and gene fusions were produced to study the timing of expression and the sub-cellular localization of the P. falciparum Gametocyte EXported Protein-5 (PfGEXP5), encoded in the subtelomeric region of chromosome 9. Expression patterns were examined in wild-type parasites and in parasite lines mutated in the Apetala2-G (AP2-G) transcription factor, governing a cascade of early sexual stage specific genes.

RESULTS

PfGEXP5 is highly expressed in early sexual stages and it is actively exported to the infected erythrocyte cytoplasm from as early as 14 h post-invasion in haemozoin-free, ring stage-like parasites. The pattern of PfGEXP5 expression and export is similar in wild-type parasites and in independent AP2-G defective parasite lines unable to produce gametocytes.

CONCLUSIONS

PfGEXP5 represents the earliest post-invasion sexual stage marker described to date. This provides a tool that can be used to identify sexually committed ring stage parasites in natural infections. This early gametocyte marker would enable the identification and mapping of malaria transmission reservoirs in human populations and the study of gametocyte sequestration dynamics in infected individuals. The fact that regulation of PfGEXP5 does not depend on the AP2-G master regulator of parasite sexual development suggests that, after sexual commitment, differentiation progresses through multiple checkpoints in the early phase of gametocytogenesis.

Changes in the transcriptome of the malaria parasite Plasmodium falciparum during the initial phase of transmission from the human to the mosquito

BACKGROUND

The transmission of the malaria parasite Plasmodium falciparum from the human to the mosquito is mediated by dormant sexual precursor cells, the gametocytes, which become activated in the mosquito midgut. Because gametocytes are the only parasite stages able to establish an infection in the mosquito, they play a crucial role in spreading the tropical disease. The human-to-mosquito transmission triggers important molecular changes in the gametocytes, which initiate gametogenesis and prepare the parasite for life-cycle progression in the insect vector.

RESULTS

To better understand gene regulations during the initial phase of malaria parasite transmission, we focused on the transcriptome changes that occur within the first half hour of parasite development in the mosquito. Comparison of mRNA levels of P. falciparum gametocytes before and 30 min following activation using suppression subtractive hybridization (SSH) identified 126 genes, which changed in expression during gametogenesis. Among these, 17.5% had putative functions in signaling, 14.3% were assigned to cell cycle and gene expression, 8.7% were linked to the cytoskeleton or inner membrane complex, 7.9% were involved in proteostasis and 6.4% in metabolism, 12.7% were cell surface-associated proteins, 11.9% were assigned to other functions, and 20.6% represented genes of unknown function. For 40% of the identified genes there has as yet not been any protein evidence.For a subset of 27 genes, transcript changes during gametogenesis were studied in detail by real-time RT-PCR. Of these, 22 genes were expressed in gametocytes, and for 15 genes transcript expression in gametocytes was increased compared to asexual blood stage parasites. Transcript levels of seven genes were particularly high in activated gametocytes, pointing at functions downstream of gametocyte transmission to the mosquito. For selected genes, a regulated expression during gametogenesis was confirmed on the protein level, using quantitative confocal microscopy.

CONCLUSIONS

The obtained transcriptome data demonstrate the regulations of gene expression immediately following malaria parasite transmission to the mosquito. Our findings support the identification of proteins important for sexual reproduction and further development of the mosquito midgut stages and provide insights into the genetic basis of the rapid adaption of Plasmodium to the insect vector.

Hemozoin accumulation in Garnham bodies of Plasmodium falciparum gametocytes

Garnham bodies are curious objects exclusive in erythrocytes containing sexual forms (gametocytes) of Plasmodium falciparum. Although the name is familiar, only a few photographs of Garnham bodies (G-bodies) have been published. Considering that other objects in malaria-infected erythrocytes, such as Schuffner's dots of Plasmodium vivax and Maurer's clefts of P. falciparum, have been found to have some functions, it has become necessary to pay closer attention to G-bodies. The present study presents previously unknown features of G-bodies and suggests a protective role for them. Wild isolates of P. falciparum were encouraged to grow in vitro under conditions that promote gametocytogenesis. Thin and thick smears of the cells were stained with Giemsa stain and examined under a light microscope. Production of G-bodies was detected in two isolates both in immature and mature gametocytes. Sometimes, the objects are found both at the top and below the parasite, contrary to previous suggestion of it being only on one side. They are highly diverse in morphology, including those that are shaped like m or S. Hemozoin accumulation was detected in some of the bodies, indicating direct opening into the cystoplasm of the parasite. It is possible that hemozoin was first produced in the parasite's food vacuole before being transported to G-bodies. Alternatively, hemoglobin transport vesicles could first accumulate in G-bodies where metabolically released ferriprotoporphyrin IX (FP) could be polymerized; but this would need acidic environment comparable to that in food vacuole. Electron microscopy has revealed that G-bodies consist of membranous whorls and it has been demonstrated experimentally that both infected and uninfected membranes promote β-hematin formation. Whatever the mechanism, storing hemozoin in G-bodies outside the cytoplasm of the parasite could provide intraerythrocytic sexual forms of P. falciparum additional protection against FP toxicity.

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.

Epidemiology and infectivity of Plasmodium falciparum and Plasmodium vivax gametocytes in relation to malaria control and elimination

Malaria remains a major cause of morbidity and mortality in the tropics, with Plasmodium falciparum responsible for the majority of the disease burden and P. vivax being the geographically most widely distributed cause of malaria. Gametocytes are the sexual-stage parasites that infect Anopheles mosquitoes and mediate the onward transmission of the disease. Gametocytes are poorly studied despite this crucial role, but with a recent resurgence of interest in malaria elimination, the study of gametocytes is in vogue. This review highlights the current state of knowledge with regard to the development and longevity of P. falciparum and P. vivax gametocytes in the human host and the factors influencing their distribution within endemic populations. The evidence for immune responses, antimalarial drugs, and drug resistance influencing infectiousness to mosquitoes is reviewed. We discuss how the application of molecular techniques has led to the identification of submicroscopic gametocyte carriage and to a reassessment of the human infectious reservoir. These components are drawn together to show how control measures that aim to reduce malaria transmission, such as mass drug administration and a transmission-blocking vaccine, might better be deployed.

Blocking Plasmodium falciparum Malaria Transmission with Drugs: The Gametocytocidal and Sporontocidal Properties of Current and Prospective Antimalarials

Drugs that kill or inhibit the sexual stages of Plasmodium could potentially amplify or synergize the impact of other interventions by blocking transmission to mosquitoes. Primaquine and other 8-aminoquinolines have long offered such potential, but safety and other concerns have limited their use. Although transmission-blocking properties are not often a priority of drug discovery efforts, a number of interesting gametocytocidal and/or sporontocidal drug candidates have emerged in recent years. Some still bear significant technical and safety concerns, while others have passed clinical trials and are on the verge of entering the antimalarial armamentarium. Recent advances in our knowledge of gametocyte differentiation, gametogenesis and sporogony have also led to the identification of a large array of potential new targets for drugs that might interfere with malaria transmission. This review examines the properties of existing and prospective drugs, mechanisms of action, counter-indications and their potential role in regional malaria elimination efforts.

Malaria parasites form filamentous cell-to-cell connections during reproduction in the mosquito midgut

Physical contact is important for the interaction between animal cells, but it can represent a major challenge for protists like malaria parasites. Recently, novel filamentous cell-cell contacts have been identified in different types of eukaryotic cells and termed nanotubes due to their morphological appearance. Nanotubes represent small dynamic membranous extensions that consist of F-actin and are considered an ancient feature evolved by eukaryotic cells to establish contact for communication. We here describe similar tubular structures in the malaria pathogen Plasmodium falciparum, which emerge from the surfaces of the forming gametes upon gametocyte activation in the mosquito midgut. The filaments can exhibit a length of > 100 μm and contain the F-actin isoform actin 2. They actively form within a few minutes after gametocyte activation and persist until the zygote transforms into the ookinete. The filaments originate from the parasite plasma membrane, are close ended and express adhesion proteins on their surfaces that are typically found in gametes, like Pfs230, Pfs48/45 or Pfs25, but not the zygote surface protein Pfs28. We show that these tubular structures represent long-distance cell-to-cell connections between sexual stage parasites and demonstrate that they meet the characteristics of nanotubes. We propose that malaria parasites utilize these adhesive "nanotubes" in order to facilitate intercellular contact between gametes during reproduction in the mosquito midgut.

Malaria gametocytogenesis

Male and female gametocytes are the components of the malaria parasite life cycle which are taken up from an infected host bloodstream by mosquitoes and thus mediate disease transmission. These gamete precursors are morphologically and functionally quite distinct from their asexual blood stage counterparts and this is reflected in their distinct patterns of gene expression, cellular development and metabolism. Recent transcriptome, proteome and reverse genetic studies have added valuable information to that obtained from traditional studies. However, we still have no answer to the fundamental question regarding sexual development: 'what triggers gametocytogenesis'? In the current climate of eradication/elimination, tackling transmission by killing gametocytes has an important place on the agenda because most antimalarial drugs, whilst killing asexual blood stage parasites, have no effect on the transmissible stages.

Transmission stage investment of malaria parasites in response to in-host competition

Conspecific competition occurs in a multitude of organisms, particularly in parasites, where several clones are commonly sharing limited resources inside their host. In theory, increased or decreased transmission investment might maximize parasite fitness in the face of competition, but, to our knowledge, this has not been tested experimentally. We developed and used a clone-specific, stage-specific, quantitative PCR protocol to quantify Plasmodium chabaudi replication and transmission stage densities in mixed-clone infections. We co-infected mice from two strains with an avirulent and virulent parasite clone and found competitive suppression of in-host (blood-stage) parasite densities and generally corresponding reductions in transmission stage production, with the virulent clone obtaining overall competitive superiority. In response to competitive suppression, there was little evidence of any alteration in transmission stage investment, apart from a small reduction by one of the two clones in one of the two host strains. This alteration did not result in a competitive advantage, although it might have reduced the disadvantage. This study supports much of the current literature, which predicts that conspecific in-host competition will result in a competitive advantage and positive selection for virulent clones and thus the evolution of higher virulence.

PfCCp proteins of Plasmodium falciparum: gametocyte-specific expression and role in complement-mediated inhibition of exflagellation

The sexual phase of the malaria parasite Plasmodium falciparum is essential for transmission of the disease and is accompanied by the co-ordinated expression of sexual stage proteins. Six of these proteins belong to a highly conserved apicomplexan family of multi-domain adhesion proteins, termed PfCCps. PfCCp1, PfCCp2 and PfCCp3 are co-dependently expressed in the parasitophorous vacuole associated with the gametocyte plasma membrane. PfCCp2 and PfCCp3 also play an essential role for parasite development in the mosquito. We show that the six PfCCp proteins are expressed in stages II-V of gametocytogenesis as well as during early gamete formation. The proteins are expressed in association with the surface of both male and female gametocytes and macrogametes, but are not present in exflagellating microgametes. Further, the newly described protein PfCCp4 co-localizes with the transmission blocking candidate Pfs230, with which it forms a protein complex. In contrast to the phenotypes that are observed following targeted gene disruption of PfCCp2, PfCCp3 or Pfs230, the lack of PfCCp4 expression does not inhibit parasite development in the mosquito vector. This indicates a non-essential role for this protein during parasite transmission. Exflagellation assays revealed that antibodies directed against distinct domains of PfCCp1 through PfCCp4 and PfFNPA support a complement-mediated decrease in gametocyte emergence. We conclude that the six PfCCp proteins are specifically expressed during gametocytogenesis and gamete formation, and that select members may represent prospective candidates for transmission blocking vaccines.

Plasmodium falciparum gametocytes: still many secrets of a hidden life

Sexual differentiation and parasite transmission are intimately linked in the life cycle of malaria parasites. The specialized cells providing this crucial link are the Plasmodium gametocytes. These are formed in the vertebrate host and are programmed to mature into gametes emerging from the erythrocytes in the midgut of a blood-feeding mosquito. The ensuing fusion into a zygote establishes parasite infection in the insect vector. Although key mechanisms of gametogenesis and fertilization are becoming progressively clear, the fundamental biology of gametocyte formation still presents open questions, some of which are specific to the human malaria parasite Plasmodium falciparum. Developmental commitment to sexual differentiation, regulation of stage-specific gene expression, the profound molecular and cellular changes accompanying gametocyte specialization, the requirement for tissue-specific sequestration in P. falciparum gametocytogenesis are proposed here as areas for future investigation. The epidemiological relevance of parasite transmission from humans to mosquito in the spread of malaria and of Plasmodium drug resistance genes indicates that understanding molecular mechanisms of gametocyte formation is highly relevant to design strategies able to interfere with the transmission of this disease.

Proteins of the malaria parasite sexual stages: expression, function and potential for transmission blocking strategies

The sexual phase of the malaria pathogen,Plasmodium falciparum, culminates in fertilization within the midgut of the mosquito and represents a crucial step in the completion of the parasite's life-cycle and transmission of the disease. Two decades ago, the first sexual stage-specific surface proteins were identified, among themPfs230,Pfs48/45, andPfs25, which were of scientific interest as candidates for the development of transmission blocking vaccines. A decade later, gene information gained from the sequencing of theP. falciparumgenome led to the identification of numerous additional sexual-stage proteins with antigenic properties and novel enzymes that putatively possess regulatory functions during sexual-stage development. This review aims to summarize the sexual-stage proteins identified to date, to compare their stage specificities and expression patterns and to highlight novel regulative mechanisms of sexual differentiation. The prospective candidacy of select sexual-stage proteins as targets for transmission blocking strategies will be discussed.

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.

Genome-wide identification of genes upregulated at the onset of gametocytogenesis in Plasmodium falciparum

A genome-wide expression analysis was undertaken to identify novel genes specifically activated from early stages of gametocytogenesis in Plasmodium falciparum. A comparative analysis was conducted on sexually induced cultures of reference parasite clone 3D7 and its gametocyteless derivative clone F12. Competitive hybridisations on long-oligomer microarrays representing 4488 P. falciparum genes identified a remarkably small number of transcripts differentially produced in the two clones. Upregulation of the mRNAs for the early gametocyte markers Pfs16 and Pfg27 was however readily detected in 3D7, and such genes were used as reference transcripts in a comparative time course analysis of 3D7 and F12 parasites between 30 and 40 h post-invasion in cultures induced to enter gametocytogenesis. One hundred and seventeen genes had expression profiles which correlated to those of pfs16 and pfg27, and Northern blot analysis and published proteomic data identified those whose expression was gametocyte-specific. Immunofluorescence analysis with antibodies against two of these gene products identified two novel parasite membrane associated, sexual stage-specific proteins. One was produced from stage I gametocytes and the second showed peak production in stage II gametocytes. The two proteins were named Pfpeg-3 and Pfpeg-4, for P. falciparum proteins of early gametocytes.

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.

Distinct trafficking and localization of STEVOR proteins in three stages of the Plasmodium falciparum life cycle

The genome of Plasmodium falciparum harbors three extensive multigene families, var, rif, and stevor (for subtelomeric variable open reading frame), located mainly in the subtelomeric regions of the parasite's 14 chromosomes. STEVOR variants are known to be expressed in asexual parasites, but no function has as yet been ascribed to this protein family. We have examined the expression of STEVOR proteins in intraerythrocytic sexual stages, gametocytes, and extracellular sporozoites isolated from infected Anopheles mosquitoes. In gametocytes, stevor transcripts appear transiently early in development but STEVOR proteins persist for several days and are transported out of the parasite, travel through the host cell cytoplasm, and localize to the erythrocyte plasma membrane. In contrast to asexual parasites, gametocytes move STEVOR to the periphery via a trafficking pathway independent of Maurer's clefts. In sporozoites, STEVOR appear dispersed throughout the cytoplasm in vesicle-like structures. The pattern of STEVOR localization we have observed in gametocytes and sporozoites differs significantly from that in asexual parasite stages. STEVOR variants are therefore likely to perform different functions in each stage of the parasites life cycle in which they occur.

Gene expression in Plasmodium: from gametocytes to sporozoites

Completion of the complex developmental program of Plasmodium in the mosquito is essential for parasite transmission, yet this part of its life cycle is still poorly understood. In recent years, considerable progress has been made in the identification and characterization of genes expressed during parasite development in the mosquito. This line of investigation was greatly facilitated by the availability of the genome sequence of several Plasmodium, and by the application of approaches such as proteomics, microarrays, gene disruption by homologous recombination (gene knockout) and by use of subtraction libraries. Here, we review what is presently known about genes expressed in gametocytes and during the Plasmodium life cycle in the mosquito.

Functional analysis of Plasmodium falciparum parasitophorous vacuole membrane protein (Pfs16) during gametocytogenesis and gametogenesis by targeted gene disruption

Gametocytogenesis is a tightly regulated process marked by differentiation through distinct morphological forms and coordinated expression of sexual stage gene products. The earliest known gene product expressed at the onset of Plasmodium falciparum gametocytogenesis is Pfs16 localized on the parasitophorous vacuole membrane (PVM). Targeted gene disruption was undertaken to disrupt expression of Pfs16 and examine its potential role during sexual development. Three independent clones were demonstrated to have the coding sequence of Ps16 gene disrupted by the targeting plasmid by homologous recombination. No full-length transcripts and PVM localized 16 kDa protein were detected. Instead, all three "16ko" clones expressed a protein of 14 kDa recognized by Pfs16 specific antibodies that was mislocalized to an unidentified double membrane compartment in the parasites. Disruption of Pfs16 gene resulted in a significant reduction in gametocyte production, although the small number of gametocytes produced appeared to be normal by molecular and phenotypic evidences. Preliminary observation also suggested impaired ability of male gametocytes to exflagellate in vitro. Pfs16 does not appear to be essential for sexual development, instead may be required for optimal production of sexual parasites. Understanding mechanisms involved in the development of sexual stages of P. falciparum may identify novel targets for drugs and vaccines effective in reducing malaria transmission.

Multiple splice variants encode a novel adenylyl cyclase of possible plastid origin expressed in the sexual stage of the malaria parasite Plasmodium falciparum

We report the characterization of an unusual adenylyl cyclase gene from Plasmodium falciparum, here designated PfACalpha. The level of mRNA expression is maximum during development of gametocytes (the sexual blood stage of the parasite life cycle). The gene is highly interrupted by 22 introns, and reverse transcriptase-PCR analysis revealed that there are multiple mRNA splice variants. One intron has three alternative 3'-splice sites that confer the potential to encode distinct forms of the enzyme using alternative start codons. Deduced amino acid sequences predict membrane-spanning regions, the number of which can vary between two and six depending on the splice variant. Expression of a synthetic form of two of these variants in Xenopus oocytes and in Dictyostelium adenylyl cyclase-deficient mutants, confirms that PfACalpha is a functional adenylyl cyclase. These results identify a novel mechanism in P. falciparum for the generation of multiple isoforms of a key, membrane-bound signaling molecule from a single genomic copy. Comparisons of the catalytic domains of PfACalpha and a second putative P. falciparum adenylyl cyclase (PfACbeta) with those from other species reveal an unexpected similarity with adenylyl cyclases from certain prokaryotes including the cyanobacteria (blue green algae). In addition, the presence of an unusual active site substitution in a position that determines substrate specificity, also characteristic of these prokaryotic forms of the enzyme, further suggests a plastid origin for the Plasmodium cyclases.

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.

Functional characterisation of sexual stage specific proteins in Plasmodium falciparum

The various stages of the malaria parasites in the vertebrate host and in the mosquito vector offer numerous candidates for vaccine and drug development. However, the biological complexity of the parasites and the interaction with the immune system of the host continue to frustrate all such efforts thus far. While most of the targets for drug and vaccine design have focused on the asexual stages, the sexual stages of the parasite are critical for transmission and maintenance of parasites among susceptible vertebrate hosts. Sexual stage parasites undergo a series of morphological and biochemical changes during their development, accompanied by a co-ordinated cascade of a distinct expression pattern of sexual stage specific proteins. Mechanisms underlying the developmental switch from asexual parasite to sexual parasite still remain elusive. Methods that can break the malaria transmission cycle thus occupy a central place in the overall malaria control strategies. This paper provides a review of genes expressed in sexually differentiated Plasmodium. In the past few years, a molecular approach based on targeted gene disruption has revealed fascinating biological roles for many of the sexual stage gene products. In addition, we will briefly discuss other functional genomic approaches employed to study not only sexual but also other aspects of host-parasite biology.

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.

Guanylyl cyclase activity associated with putative bifunctional integral membrane proteins in Plasmodium falciparum

We report here that guanylyl cyclase activity is associated with two large integral membrane proteins (PfGCalpha and PfGCbeta) in the human malaria parasite Plasmodium falciparum. Unusually, the proteins appear to be bifunctional; their amino-terminal regions have strong similarity with P-type ATPases, and the sequence and structure of the carboxyl-terminal regions conform to that of G protein-dependent adenylyl cyclases, with two sets of six transmembrane sequences, each followed by a catalytic domain (C1 and C2). However, amino acids that are enzymatically important and present in the C2 domain of mammalian adenylyl cyclases are located in the C1 domain of the P. falciparum proteins and vice versa. In addition, certain key residues in these domains are more characteristic of guanylyl cyclases. Consistent with this, guanylyl cyclase activity was obtained following expression of the catalytic domains of PfGCbeta in Escherichia coli. In P. falciparum, expression of both genes was detectable in the sexual but not the asexual blood stages of the life cycle, and PfGCalpha was localized to the parasite/parasitophorous vacuole membrane region of gametocytes. The profound structural differences identified between mammalian and parasite guanylyl cyclases suggest that aspects of this signaling pathway may be mechanistically distinct.

Isolation and functional characterization of two distinct sexual-stage-specific promoters of the human malaria parasite Plasmodium falciparum

Transmission of malaria depends on the successful development of the sexual stages of the parasite within the midgut of the mosquito vector. The differentiation process leading to the production of the sexual stages is delineated by several developmental switches. Arresting the progression through this sexual differentiation pathway would effectively block the spread of the disease. The successful development of such transmission-blocking agents is hampered by the lack of a detailed understanding of the program of gene expression that governs sexual differentiation of the parasite. Here we describe the isolation and functional characterization of the Plasmodium falciparum pfs16 and pfs25 promoters, whose activation marks the developmental switches executed during the sexual differentiation process. We have studied the differential activation of the pfs16 and pfs25 promoters during intraerythrocytic development by transfection of P. falciparum and during gametogenesis and early sporogonic development by transfection of the related malarial parasite P. gallinaceum. Our data indicate that the promoter of the pfs16 gene is activated at the onset of gametocytogenesis, while the activity of the pfs25 promoter is induced following the transition to the mosquito vector. Both promoters have unusual DNA compositions and are extremely A/T rich. We have identified the regions in the pfs16 and pfs25 promoters that are essential for high transcriptional activity. Furthermore, we have identified a DNA-binding protein, termed PAF-1, which activates pfs25 transcription in the mosquito midgut. The data presented here shed the first light on the details of processes of gene regulation in the important human pathogen P. falciparum.

Developmentally regulated expression of pfs16, a marker for sexual differentiation of the human malaria parasite Plasmodium falciparum

Sexual differentiation is essential for the transmission of Plasmodium to mosquitoes and therefore, for the spread of malaria. The molecular mechanisms underlying sexual differentiation are poorly understood but may be elucidated by a detailed study of the regulation of expression of sexual stage specific genes. In the present work we describe the differential expression of the gene encoding the sexual stage specific protein, Pfs16. We have conducted a comparative analysis of pfs16 promoter activity, RNA levels and the rate of de novo protein synthesis during development of Plasmodium falciparum. Furthermore, we have determined the pattern of expression of pfs16 transcripts at the single cell level by in situ hybridisation. We show that the expression of pfs16 is induced immediately following the invasion of a red blood cell in sexually committed ring stage parasites and continues throughout gametocytogenesis and in macrogametes. The expression of pfs16 is regulated at the level of transcription initiation and modulated by a post-transcriptional process. These results demonstrate that the expression of the pfs16 gene is the earliest event in the sexual differentiation process of P. falciparum described to date.

Induction of Plasmodium falciparum sporozoite-neutralizing antibodies upon vaccination with recombinant Pfs16 vaccinia virus and/or recombinant Pfs16 protein produced in yeast

Pfs16 is a sexual stage/sporozoite-specific antigen of Plasmodium falciparum and is a potential candidate for a sporozoite-neutralizing vaccine. To obtain more information on the function of Pfs16 and to investigate its role during transmission and hepatocyte invasion, immunization experiments were performed with both a Pfs16-specific recombinant vaccinia virus and virus-like particles produced in yeast composed of the hepatitis B surface antigen (HBsAg) and antigen Pfs16 fused to HBsAg. Upon transformation of yeast cells, harbouring a genomic copy of the HBsAg gene, with a plasmid carrying the fusion gene Pfs16-HBsAg (Pfs16-S) virus-like hybrid particles composed of HBsAg and Pfs16-S were formed of a size similar to those present in human sera after infection with the hepatitis B virus. Cells infected with recombinant Pfs16 vaccinia virus synthesized a polypeptide of approx. 16 kDa that reacted with a Pfs16-specific polyclonal antibody. Animals vaccinated with the yeast hybrid particles and/or recombinant vaccinia virus both produced Pfs16-specific antibodies. These antibodies showed no transmission-blocking activity, but they efficiently diminished or abolished in vitro invasion of sporozoites into human hepatoma cells (HepG2-A16) and primary human hepatocytes.

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.