Malaria, sexual development and transmission: retrospect and prospect

Plasmodium falciparum Development from Gametocyte to Oocyst: Insight from Functional Studies

Malaria elimination may never succeed without the implementation of transmission-blocking strategies. The transmission of Plasmodium spp. parasites from the human host to the mosquito vector depends on circulating gametocytes in the peripheral blood of the vertebrate host. Once ingested by the mosquito during blood meals, these sexual forms undergo a series of radical morphological and metabolic changes to survive and progress from the gut to the salivary glands, where they will be waiting to be injected into the vertebrate host. The design of effective transmission-blocking strategies requires a thorough understanding of all the mechanisms that drive the development of gametocytes, gametes, sexual reproduction, and subsequent differentiation within the mosquito. The drastic changes in Plasmodium falciparum shape and function throughout its life cycle rely on the tight regulation of stage-specific gene expression. This review outlines the mechanisms involved in Plasmodium falciparum sexual stage development in both the human and mosquito vector, and zygote to oocyst differentiation. Functional studies unravel mechanisms employed by P. falciparum to orchestrate the expression of stage-specific functional products required to succeed in its complex life cycle, thus providing us with potential targets for developing new therapeutics. These mechanisms are based on studies conducted with various Plasmodium species, including predominantly P. falciparum and the rodent malaria parasites P. berghei. However, the great potential of epigenetics, genomics, transcriptomics, proteomics, and functional genetic studies to improve the understanding of malaria as a disease remains partly untapped because of limitations in studies using human malaria parasites and field isolates.

Sex in protists: A new perspective on the reproduction mechanisms of trypanosomatids

The Protist kingdom individuals are the most ancestral representatives of eukaryotes. They have inhabited Earth since ancient times and are currently found in the most diverse environments presenting a great heterogeneity of life forms. The unicellular and multicellular algae, photosynthetic and heterotrophic organisms, as well as free-living and pathogenic protozoa represents the protist group. The evolution of sex is directly associated with the origin of eukaryotes being protists the earliest protagonists of sexual reproduction on earth. In eukaryotes, the recombination through genetic exchange is a ubiquitous mechanism that can be stimulated by DNA damage. Scientific evidences support the hypothesis that reactive oxygen species (ROS) induced DNA damage can promote sexual recombination in eukaryotes which might have been a decisive factor for the origin of sex. The fact that some recombination enzymes also participate in meiotic sex in modern eukaryotes reinforces the idea that sexual reproduction emerged as consequence of specific mechanisms to cope with mutations and alterations in genetic material. In this review we will discuss about origin of sex and different strategies of evolve sexual reproduction in some protists such that cause human diseases like malaria, toxoplasmosis, sleeping sickness, Chagas disease, and leishmaniasis.

ApiAP2 Gene-Network Regulates Gametocytogenesis in Plasmodium Parasites

Malaria is a mosquito-borne infectious disease, caused by unicellular Apicomplexan protozoa of the genus Plasmodium. The sexual stage of Plasmodium is one of the most fascinating aspects of the Plasmodium life cycle, yet relatively less explored until now. The production of sexually fit gametocytes through gametocytogenesis is essential to the transmission of the Plasmodium parasite into an anopheline mosquito vector. Understanding how gametocytogenesis is regulated promotes the identification of novel drug targets and also the development of transmission-blocking vaccines that would help reduce the disease burden in endemic areas. Transcriptional regulation in Plasmodium parasites is primarily controlled by a family of twenty-seven Apicomplexan Apetela 2 (ApiAP2) genes which act in a cascade to enable the parasite to progress through its asexual replication as well as gametocytogenesis. Here, we review the latest progress made on members of the ApiAP2 family characterized as key players of the transcriptional machinery of gametocytes. Further, we will highlight the transcriptional regulation network of ApiAP2 genes at each stage of gametocytogenesis.

Gametogenesis in Plasmodium: Delving Deeper to Connect the Dots

In the coming decades, eliminating malaria is the foremost goal of many tropical countries. Transmission control, along with an accurate and timely diagnosis of malaria, effective treatment and prevention are the different aspects that need to be met synchronously to accomplish the goal. The current review is focused on one of these aspects i.e., transmission control, by looking deeper into the event called gametogenesis. In the Plasmodium life cycle, gametocytes are the first life forms of the sexual phase. The transmission of the parasite and the disease is critically dependent on the number, viability and sex ratio of mature gametocytes and their further development inside mosquito vectors. Gametogenesis, the process of conversion of gametocytes into viable gametes, takes place inside the mosquito midgut, and is a tightly regulated event with fast and multiple rounds of DNA replication and diverse cellular changes going on within a short period. Interrupting the gametocyte-gamete transition is ought to restrict the successful transmission and progression of the disease and hence an area worth exploring for designing transmission-blocking strategies. This review summarizes an in-depth and up-to-date understanding of the biochemical and physiological mechanism of gametogenesis in Plasmodium, which could be targeted to control parasite and malaria transmission. This review also raises certain key questions regarding gametogenesis biology in Plasmodium and brings out gaps that still accompany in understanding the spectacular process of gametogenesis.

Live-cell fluorescence imaging of microgametogenesis in the human malaria parasite Plasmodium falciparum

Formation of gametes in the malaria parasite occurs in the midgut of the mosquito and is critical to onward parasite transmission. Transformation of the male gametocyte into microgametes, called microgametogenesis, is an explosive cellular event and one of the fastest eukaryotic DNA replication events known. The transformation of one microgametocyte into eight flagellated microgametes requires reorganisation of the parasite cytoskeleton, replication of the 22.9 Mb genome, axoneme formation and host erythrocyte egress, all of which occur simultaneously in <20 minutes. Whilst high-resolution imaging has been a powerful tool for defining stages of microgametogenesis, it has largely been limited to fixed parasite samples, given the speed of the process and parasite photosensitivity. Here, we have developed a live-cell fluorescence imaging workflow that captures the entirety of microgametogenesis. Using the most virulent human malaria parasite, Plasmodium falciparum, our live-cell approach captured early microgametogenesis with three-dimensional imaging through time (4D imaging) and microgamete release with two-dimensional (2D) fluorescence microscopy. To minimise the phototoxic impact to parasites, acquisition was alternated between 4D fluorescence, brightfield and 2D fluorescence microscopy. Combining live-cell dyes specific for DNA, tubulin and the host erythrocyte membrane, 4D and 2D imaging together enables definition of the positioning of newly replicated and segregated DNA. This combined approach also shows the microtubular cytoskeleton, location of newly formed basal bodies, elongation of axonemes and morphological changes to the erythrocyte membrane, the latter including potential echinocytosis of the erythrocyte membrane prior to microgamete egress. Extending the utility of this approach, the phenotypic effects of known transmission-blocking inhibitors on microgametogenesis were confirmed. Additionally, the effects of bortezomib, an untested proteasomal inhibitor, revealed a clear block of DNA replication, full axoneme nucleation and elongation. Thus, as well as defining a framework for broadly investigating microgametogenesis, these data demonstrate the utility of using live imaging to validate potential targets for transmission-blocking antimalarial drug development.

Direct injection of Amblyomma americanum ticks with Cytauxzoon felis

Cytauxzoon felis is a tick-borne hemoprotozoan parasite that causes life-threatening disease in domestic cats in the United States. Currently, the platforms for C. felis research are limited to natural or experimental infection of domestic cats. This study aims to develop an alternative model by infecting Amblyomma americanum ticks with C. felis via direct injection. Amblyomma americanum adults were injected with C. felis-infected feline erythrocytes through two routes: directly into the digestive tract through the anal pore (IA injection), or percutaneously into the tick hemocoel (IH injection). RNAscope® in situ hybridization (ISH) was used to visualize the parasites within the ticks at different time points after injection. Four months after injection, ticks were divided into 3 infestation groups based on injection methods and inoculum type and fed on 3 naïve cats to assess the ticks' ability to transmit C. felis. Prior to the transmission challenge, selected ticks from each infestation group were tested for C. felis RNA via reverse transcription-PCR (RT-PCR). In both IA- and IH-injected ticks, ISH signals were observed in ticks up to 3 weeks after injection. The number of hybridization signals notably decreased over time, and no signals were detected by 4 months after injection. Prior to the transmission challenge, 37-57% of the sampled ticks were positive for C. felis RNA via RT-PCR. While the majority of injected ticks successfully attached and fed to repletion on all 3 cats during the transmission challenge, none of the cats became infected with C. felis. These results suggest that injected C. felis remained alive in ticks but was unable to progress to infective sporozoites after injection. It is unclear why this infection technique had been successful for other closely related tick-borne hemoprotozoa and not for C. felis. This outcome may be associated with uncharacterized differences in the C. felis life cycle, the lack of the feeding or molting in our model or absence of gametocytes in the inoculum. Nonetheless, our study demonstrated the potential of using ticks as an alternative model to study C. felis. Future improvement of a tick model for C. felis should consider other tick species for the injection model or utilize infection methods that more closely emulate the natural infection process.

The sexual side of parasitic protists

Much of the vast evolutionary landscape occupied by Eukaryotes is dominated by protists. Though parasitism has arisen in many lineages, there are three main groups of parasitic protists of relevance to human and livestock health: the Apicomplexa, including the malaria parasite Plasmodium and coccidian pathogens of livestock such as Eimeria; the excavate flagellates, encompassing a diverse range of protist pathogens including trypanosomes, Leishmania, Giardia and Trichomonas; and the Amoebozoa, including pathogenic amoebae such as Entamoeba. These three groups represent separate, deep branches of the eukaryote tree, underlining their divergent evolutionary histories. Here, I explore what is known about sex in these three main groups of parasitic protists.

Gametocyte Sex Ratio: The Key to Understanding Plasmodium falciparum Transmission?

A mosquito needs to ingest at least one male and one female gametocyte to become infected with malaria. The sex of Plasmodium falciparum gametocytes can be determined microscopically but recent transcriptomics studies paved the way for the development of molecular methods that allow sex-ratio assessments at much lower gametocyte densities. These sex-specific gametocyte diagnostics were recently used to examine gametocyte dynamics in controlled and natural infections as well as the impact of different antimalarial drugs. It is currently unclear to what extent sex-specific gametocyte diagnostics obviate the need for mosquito feeding assays to formally assess transmission potential. Here, we review recent and historic assessments of gametocyte sex ratio in relation to host and parasite characteristics, treatment, and transmission potential.

Recent RNA sequencing studies have uncovered a number of P. falciparum gametocyte sex-specific targets and provided new insights in gametocyte biology.

After decades when gametocyte sex-ratio research was restricted to nonhuman malarias or in vitro experiments, molecular tools for assessing gametocyte sex ratio are now increasingly available for use in natural P. falciparum infections.

Evidence that gametocyte sex ratio is influenced by total gametocyte density and antimalarial treatment, and improves predictions of transmission potential, highlight the relevance of understanding the gametocyte sex ratio during natural infections.

The finding that the most widely used P. falciparum gametocyte marker Pfs25 is expressed predominantly by female gametocytes and has non-negligible levels of background expression in asexual parasites necessitates a re-evaluation of existing gametocyte data.

Regulation of Sexual Commitment and Gametocytogenesis in Malaria Parasites

Sexual differentiation of malaria parasites from the asexual blood stage into gametocytes is an essential part of the life cycle, as gametocytes are the form that is taken up by the mosquito host. Because of the essentiality of this process for transmission to the mosquito, gametocytogenesis is an extremely attractive target for therapeutic interventions. The subject of this review is the considerable progress that has been made in recent years in elucidating the molecular mechanisms governing this important differentiation process. In particular, a number of critical transcription factors and epigenetic regulators have emerged as crucial elements in the regulation of commitment. The identification of these factors has allowed us to understand better than ever before the events occurring prior to and during commitment to sexual development and offers potential for new therapeutic interventions.

A whole parasite transmission-blocking vaccine for malaria: an ignored strategy

Malaria vaccine approaches can be divided into ‘subunit’ and ‘whole parasite’, and these can be directed at the sporozoite, liver stage, asexual or sexual stages. All combinations of approach and stage are under development with the exception of a whole parasite sexual stage (gametocyte) vaccine. A gametocyte vaccine would aim primarily to block transmission of malaria from the human host to the mosquito vector and as such is referred to as a ‘transmission-blocking vaccine’. An immunological feature of whole parasite vaccines for the sporozoite/liver stage and for the asexual blood stage is the reliance on cellular immunity involving T-cells to control parasite growth. T-cells can also respond vigorously to gametocytes and kill them in the vertebrate host and/or arrest their development. To date, cellular immunity has not been exploited in transmission-blocking vaccine development. Here, the data supporting a gametocyte whole parasite vaccine are reviewed and a strategy for vaccine development and testing is outlined.

Simulating within-vector generation of the malaria parasite diversity

Plasmodium falciparum, the most virulent human malaria parasite, undergoes asexual reproduction within the human host, but reproduces sexually within its vector host, the Anopheles mosquito. Consequently, the mosquito stage of the parasite life cycle provides an opportunity to create genetically novel parasites in multiply-infected mosquitoes, potentially increasing parasite population diversity. Despite the important implications for disease transmission and malaria control, a quantitative mapping of how parasite diversity entering a mosquito relates to diversity of the parasite exiting, has not been undertaken. To examine the role that vector biology plays in modulating parasite diversity, we develop a two-part model framework that estimates the diversity as a consequence of different bottlenecks and expansion events occurring during the vector-stage of the parasite life cycle. For the underlying framework, we develop the first stochastic model of within-vector P. falciparum parasite dynamics and go on to simulate the dynamics of two parasite subpopulations, emulating multiply infected mosquitoes. We show that incorporating stochasticity is essential to capture the extensive variation in parasite dynamics, particularly in the presence of multiple parasites. In particular, unlike deterministic models, which always predict the most fit parasites to produce the most sporozoites, we find that occasionally only parasites with lower fitness survive to the sporozoite stage. This has important implications for onward transmission. The second part of our framework includes a model of sequence diversity generation resulting from recombination and reassortment between parasites within a mosquito. Our two-part model framework shows that bottlenecks entering the oocyst stage decrease parasite diversity from what is present in the initial gametocyte population in a mosquito’s blood meal. However, diversity increases with the possibility for recombination and proliferation in the formation of sporozoites. Furthermore, when we begin with two parasite subpopulations in the initial gametocyte population, the probability of transmitting more than two unique parasites from mosquito to human is over 50% for a wide range of initial gametocyte densities.

Novel lead structures with both Plasmodium falciparum gametocytocidal and asexual blood stage activity identified from high throughput compound screening

BACKGROUND

Blocking malaria transmission is an important step in eradicating malaria. In the field, transmission requires the production of sexual stage Plasmodium parasites, called gametocytes, which are not effectively killed by the commonly used anti-malarials allowing individuals to remain infectious after clearance of asexual parasites.

METHODS

To identify new gametocytocidal compounds, a library of 45,056 compounds with diverse structures was screened using a high throughput gametocyte viability assay. The characteristics of active hits were further evaluated against asexual stage parasites in a growth inhibition assay. Their cytotoxicity were tested against mammalian cells in a cytotoxicity assay. The chemical scaffold similarity of active hits were studied using scaffold cluster analysis.

RESULTS

A set of 23 compounds were identified and further confirmed for their activity against gametocytes. All the 23 confirmed compounds possess dual-activities against both gametocytes responsible for human to mosquito transmission and asexual parasites that cause the clinical symptoms. Three of these compounds were fourfold more active against gametocytes than asexual parasites. Further cheminformatic analysis revealed three sets of novel scaffolds, including highly selective 4-1H-pyrazol-5-yl piperidine analogs.

CONCLUSIONS

This study revealed important new structural scaffolds that can be used as starting points for dual activity anti-malarial drug development.

Mechanisms of mortality in Culicoides biting midges due to Haemoproteus infection

We examined the effects of Haemoproteus infection on the survival and pathology caused in the biting midges. Forty-six females of Culicoides impunctatus were exposed experimentally by allowing them to feed on a naturally infected red-backed shrike infected with Haemoproteus lanii (lineage hRB1, gametocytaemia 5·2%). Seventeen females were fed on an uninfected bird (controls). Dead insects were collected, counted and used for dissection, histological examination and polymerase chain reaction-based testing. Parasites were present in all experimentally infected biting midges, but absent from control insects. Survivorship differed significantly between the control and infected groups. Twelve hours post-exposure (PE), 45 (98%) experimentally infected midges were dead, but all control midges remained alive, and many of them survived until 7 day PE. The migrating ookinetes of H. lanii overfilled midgut, markedly damaged the midgut wall, entered the haemocoel and overfilled the abdomen and thorax of exposed biting midges. Massive infection by migrating ookinetes led to damage of abdomen and thorax of biting midges. The parasites often present in large clumps in the haemocoel in abdomen and thorax, leading to the interruption of the haemolymph circulation. These are the main reasons for rapid death of biting midges after feeding on high-intensity infections of Haemoproteus parasites.

Translational repression of the cpw-wpc gene family in the malaria parasite Plasmodium

The technical challenges of working with the sexual stages of the malaria parasite Plasmodium have hindered the characterization of sexual stage antigens in the quest for a successful malaria transmission-blocking vaccine. One such predicted and largely uncharacterized group of sexual stage candidate antigens is the CPW-WPC family of proteins. CPW-WPC proteins are named for a characteristic domain that contains two conserved motifs, CPxxW and WPC. Conserved across Apicomplexa, this family is also present earlier in the Alveolata in the free-living, non-parasitophorous, photosynthetic chromerids, Chromera and Vitrella. In Plasmodium falciparum and Plasmodium berghei blood stage parasites, the transcripts of all nine cpw-wpc genes have been detected in gametocytes. RNA immunoprecipitation followed by reverse transcriptase-PCR reveals all P. berghei cpw-wpc transcripts to be bound by the translational repressors DOZI and CITH, and thus are likely under translational control prior to transmission from the rodent host to the mosquito vector in P. berghei. The GFP tagging of two endogenous P. berghei genes confirmed translational silencing in the gametocyte and translation in ookinetes. By establishing a luciferase transgene assay, we show that the 3' untranslated region of PF3D7_1331400 controls protein expression of this reporter in P. falciparum gametocytes. Our analyses suggest that cpw-wpc genes are translationally silenced in gametocytes across Plasmodium spp. and activated during ookinete formation and thus may have a role in transmission to the mosquito.

High-Throughput Assay and Discovery of Small Molecules that Interrupt Malaria Transmission

Preventing transmission is an important element of malaria control. However, most of the current available methods to assay for malaria transmission blocking are relatively low throughput and cannot be applied to large chemical libraries. We have developed a high-throughput and cost-effective assay, the Saponin-lysis Sexual Stage Assay (SaLSSA), for identifying small molecules with transmission-blocking capacity. SaLSSA analysis of 13,983 unique compounds uncovered that >90% of well-characterized antimalarials, including endoperoxides and 4-aminoquinolines, as well as compounds active against asexual blood stages, lost most of their killing activity when parasites developed into metabolically quiescent stage V gametocytes. On the other hand, we identified compounds with consistent low nanomolar transmission-blocking activity, some of which showed cross-reactivity against asexual blood and liver stages. The data clearly emphasize substantial physiological differences between sexual and asexual parasites and provide a tool and starting points for the discovery and development of transmission-blocking drugs.

Inclusion of gametocyte parameters in anti-malarial drug efficacy studies: filling a neglected gap needed for malaria elimination

Standard anti-malarial drug efficacy and drug resistance assessments neglect the gametocyte parameters in their protocols. With the spread of drug resistance and the absence of clinically proven vaccines, the use of gametocytocidal drugs or drug combinations with transmission-blocking activity is a high priority for malaria control and elimination. However, the limited repertoire of gametocytocidal drugs and induction of gametocytogenesis after treatment with certain anti-malarial drugs necessitate both regular monitoring of gametocytocidal activities of anti-malarial drugs in clinical use and the effectiveness of candidate gametocytocidal agents. Therefore, updating current protocols of anti-malarial drug efficacy is needed to reflect the effects of anti-malarial drugs or drug combinations on gametocyte carriage and gametocyte density along with asexual parasite density. Developing protocols of anti-malarial drug efficacy that include gametocyte parameters related to both microscopic and submicroscopic gametocytaemias is important if drugs or drug combinations are to be strategically used in transmission-blocking interventions in the context of malaria elimination. The present piece of opinion highlights the challenges in gametocyte detection and follow-up and discuss the need for including the gametocyte parameter in anti-malarial efficacy studies.

Commit and Transmit: Molecular Players in Plasmodium Sexual Development and Zygote Differentiation

During each cycle of asexual endomitotic division in erythrocytes, the malaria parasite makes a fundamental and crucial decision: to continue to invade and proliferate or to differentiate into gametocytes ready for continuation of sexual development. The proteins and regulatory pathways involved in Plasmodium sexual development have been of great interest in recent years as targets for blocking malaria transmission. However, the 'Holy Grail', the master switch orchestrating asexual-to-sexual commitment and further differentiation, has remained elusive - until now. Here we highlight the recent studies identifying the epigenetic and transcriptional master regulators of sexual commitment and discuss the key players in reversible phosphorylation pathways involved in sexual and zygote differentiation.

Targeting Human Transmission Biology for Malaria Elimination

Malaria remains one of the leading causes of death worldwide, despite decades of public health efforts. The recent commitment by many endemic countries to eliminate malaria marks a shift away from programs aimed at controlling disease burden towards one that emphasizes reducing transmission of the most virulent human malaria parasite, Plasmodium falciparum. Gametocytes, the only developmental stage of malaria parasites able to infect mosquitoes, have remained understudied, as they occur in low numbers, do not cause disease, and are difficult to detect in vivo by conventional methods. Here, we review the transmission biology of P. falciparum gametocytes, featuring important recent discoveries of genes affecting parasite commitment to gametocyte formation, microvesicles enabling parasites to communicate with each other, and the anatomical site where immature gametocytes develop. We propose potential parasite targets for future intervention and highlight remaining knowledge gaps.

Sex-partitioning of the Plasmodium falciparum stage V gametocyte proteome provides insight into falciparum-specific cell biology

One of the critical gaps in malaria transmission biology and surveillance is our lack of knowledge about Plasmodium falciparum gametocyte biology, especially sexual dimorphic development and how sex ratios that may influence transmission from the human to the mosquito. Dissecting this process has been hampered by the lack of sex-specific protein markers for the circulating, mature stage V gametocytes. The current evidence suggests a high degree of conservation in gametocyte gene complement across Plasmodium, and therefore presumably for sex-specific genes as well. To better our understanding of gametocyte development and subsequent infectiousness to mosquitoes, we undertook a Systematic Subtractive Bioinformatic analysis (filtering) approach to identify sex-specific P. falciparum NF54 protein markers based on a comparison with the Dd2 strain, which is defective in producing males, and with syntenic male and female proteins from the reanalyzed and updated P. berghei (related rodent malaria parasite) gametocyte proteomes. This produced a short list of 174 male- and 258 female-enriched P. falciparum stage V proteins, some of which appear to be under strong diversifying selection, suggesting ongoing adaptation to mosquito vector species. We generated antibodies against three putative female-specific gametocyte stage V proteins in P. falciparum and confirmed either conserved sex-specificity or the lack of cross-species sex-partitioning. Finally, our study provides not only an additional resource for mass spectrometry-derived evidence for gametocyte proteins but also lays down the foundation for rational screening and development of novel sex-partitioned protein biomarkers and transmission-blocking vaccine candidates.

A flow cytometry-based quantitative drug sensitivity assay for all Plasmodium falciparum gametocyte stages

Background

Malaria elimination/eradication campaigns emphasize interruption of parasite transmission as a priority strategy. Screening for new drugs and vaccines against gametocytes is therefore urgently needed. However, current methods for sexual stage drug assays, usually performed by counting or via fluorescent markers are either laborious or restricted to a certain stage. Here we describe the use of a transgenic parasite line for assaying drug sensitivity in all gametocyte stages.

Methods

A transgenic parasite line expressing green fluorescence protein (GFP) under the control of the gametocyte-specific gene α-tubulin II promoter was generated. This parasite line expresses GFP in all gametocyte stages. Using this transgenic line, we developed a flow cytometry-based assay to determine drug sensitivity of all gametocyte stages, and tested the gametocytocidal activities of four antimalarial drugs.

Findings

This assay proved to be suitable for determining drug sensitivity of all sexual stages and can be automated. A Z’ factor of 0.79±0.02 indicated that this assay could be further optimized for high-throughput screening. The daily sensitivity of gametocytes to three antimalarial drugs (chloroquine, dihydroartemisinin and pyronaridine) showed a drastic decrease from stage III on, whereas it remained relatively steady for primaquine.

Conclusions

A drug assay was developed to use a single transgenic parasite line for determining drug susceptibility of all gametocyte stages. This assay may be further automated into a high-throughput platform for screening compound libraries against P. falciparum gametocytes.

Quantitative analysis of Plasmodium ookinete motion in three dimensions suggests a critical role for cell shape in the biomechanics of malaria parasite gliding motility

Motility is a fundamental part of cellular life and survival, including for Plasmodium parasites--single-celled protozoan pathogens responsible for human malaria. The motile life cycle forms achieve motility, called gliding, via the activity of an internal actomyosin motor. Although gliding is based on the well-studied system of actin and myosin, its core biomechanics are not completely understood. Currently accepted models suggest it results from a specifically organized cellular motor that produces a rearward directional force. When linked to surface-bound adhesins, this force is passaged to the cell posterior, propelling the parasite forwards. Gliding motility is observed in all three life cycle stages of Plasmodium: sporozoites, merozoites and ookinetes. However, it is only the ookinetes--formed inside the midgut of infected mosquitoes--that display continuous gliding without the necessity of host cell entry. This makes them ideal candidates for invasion-free biomechanical analysis. Here we apply a plate-based imaging approach to study ookinete motion in three-dimensional (3D) space to understand Plasmodium cell motility and how movement facilitates midgut colonization. Using single-cell tracking and numerical analysis of parasite motion in 3D, our analysis demonstrates that ookinetes move with a conserved left-handed helical trajectory. Investigation of cell morphology suggests this trajectory may be based on the ookinete subpellicular cytoskeleton, with complementary whole and subcellular electron microscopy showing that, like their motion paths, ookinetes share a conserved left-handed corkscrew shape and underlying twisted microtubular architecture. Through comparisons of 3D movement between wild-type ookinetes and a cytoskeleton-knockout mutant we demonstrate that perturbation of cell shape changes motion from helical to broadly linear. Therefore, while the precise linkages between cellular architecture and actomyosin motor organization remain unknown, our analysis suggests that the molecular basis of cell shape may, in addition to motor force, be a key adaptive strategy for malaria parasite dissemination and, as such, transmission.

Molecular evidence for the localization of Plasmodium falciparum immature gametocytes in bone marrow

Plasmodium falciparum immature gametocytes are not observed in peripheral blood. However, gametocyte stages in organs such as bone marrow have never been assessed by molecular techniques, which are more sensitive than optical microscopy. We quantified P falciparum sexual stages in bone marrow (n = 174) and peripheral blood (n = 70) of Mozambican anemic children by quantitative polymerase chain reaction targeting transcripts specific for early (PF14_0748; PHISTa), intermediate (PF13_0247; Pfs48/45), and mature (PF10_0303; Pfs25) gametocytes. Among children positive for the P falciparum housekeeping gene (PF08_0085; ubiquitin-conjugating enzyme gene) in bone marrow (n = 136) and peripheral blood (n = 25), prevalence of immature gametocytes was higher in bone marrow than peripheral blood (early: 95% vs 20%, P < .001; intermediate: 80% vs 16%; P < .001), as were transcript levels (P < .001 for both stages). In contrast, mature gametocytes were more prevalent (100% vs 51%, P < .001) and abundant (P < .001) in peripheral blood than in the bone marrow. Severe anemia (3.57, 95% confidence interval 1.49-8.53) and dyserythropoiesis (6.21, 95% confidence interval 2.24-17.25) were independently associated with a higher prevalence of mature gametocytes in bone marrow. Our results highlight the high prevalence and abundance of early sexual stages in bone marrow, as well as the relationship between hematological disturbances and gametocyte development in this tissue.

No asymptomatic malaria parasitaemia found among 108 young children at one health facility in Dar es Salaam, Tanzania

BACKGROUND

Asymptomatic malaria parasitaemia has been reported in areas with high malaria transmission. It may serve as a reservoir for continued transmission, and furthermore complicates diagnostics, as not all individuals with a positive malaria test are necessarily ill due to malaria, although they may present with malaria-like symptoms. Asymptomatic malaria increases with age as immunity to malaria gradually develops. As mortality and morbidity of malaria is higher among younger children it is important to know the prevalence of asymptomatic malaria parasitaemia in this population in order to interpret laboratory results for malaria correctly.

METHODS

A total of 108 children that had neither been treated for malaria nor had a fever the previous four weeks were recruited consecutively at a maternal and child health clinic (MCHC) in Dar es Salaam, Tanzania. A rapid diagnostic test (RDT) for malaria and dried blood spot (DBS) on filter paper were taken from each child. Social and clinical data were recorded. DNA was extracted from the DBS of study participants by a method using InstaGene™ matrix. PCR targeting the Plasmodium mitochondrial genome was performed on all samples.

RESULTS

Median age was 4.6 months (range 0.5-38). All the RDTs were negative. PCR was negative for all study subjects.

CONCLUSION

The study suggests that asymptomatic malaria may not be present in apparently healthy children up to the age of three years in Dar es Salaam, Tanzania. However, because of the small sample size and low median age of the study population, the findings cannot be generalized. Larger studies, including higher age groups, need to be done to clarify whether asymptomatic malaria parasitaemia is present in the general population in the Dar es Salaam area.

Sex andEimeria: a molecular perspective

Eimeriais a common genus of apicomplexan parasites that infect diverse vertebrates, most notably poultry, causing serious disease and economic loss. Like all apicomplexans, eimerians have a complex life cycle characterized by asexual divisions that amplify the parasite population in preparation for sexual reproduction. This can be divided into three events: gametocytogenesis, producing gametocytes from merozoites; gametogenesis, producing microgametes and macrogametes from gametocytes; and fertilization of macrogametes by microgametes, producing diploid zygotes with ensuing meiosis completing the sexual phase. Sexual development inEimeriadepends on the differential expression of stage-specific genes, rather than presence or absence of sex chromosomes. Thus, it involves the generation of specific structures and, implicitly, storage of proteins and regulation of protein expression in macrogametes, in preparation for fertilization. InEimeria, the formation of a unique, resilient structure, the oocyst wall, is essential for completion of the sexual phase and parasite transmission. In this review, we piece together the molecular events that underpin sexual reproduction inEimeriaand use additional details from analogous events inPlasmodiumto fill current knowledge gaps. The mechanisms governing sexual stage formation and subsequent fertilization may represent targets for counteracting parasite transmission.

Antimalarial drug discovery - the path towards eradication

Malaria is a disease that still affects a significant proportion of the global human population. Whilst advances have been made in lowering the numbers of cases and deaths, it is clear that a strategy based solely on disease control year on year, without reducing transmission and ultimately eradicating the parasite, is unsustainable. This article highlights the current mainstay treatments alongside a selection of emerging new clinical molecules from the portfolio of Medicines for Malaria Venture (MMV) and our partners. In each case, the key highlights from each research phase are described to demonstrate how these new potential medicines were discovered. Given the increased focus of the community on eradicating the disease, the strategy for next generation combination medicines that will provide such potential is explained.

P. falciparum infection durations and infectiousness are shaped by antigenic variation and innate and adaptive host immunity in a mathematical model

Many questions remain about P. falciparum within-host dynamics, immunity, and transmission–issues that may affect public health campaign planning. These gaps in knowledge concern the distribution of durations of malaria infections, determination of peak parasitemia during acute infection, the relationships among gametocytes and immune responses and infectiousness to mosquitoes, and the effect of antigenic structure on reinfection outcomes. The present model of intra-host dynamics of P. falciparum implements detailed representations of parasite and immune dynamics, with structures based on minimal extrapolations from first-principles biology in its foundations. The model is designed to quickly and readily accommodate gains in mechanistic understanding and to evaluate effects of alternative biological hypothesis through in silico experiments. Simulations follow the parasite from the liver-stage through the detailed asexual cycle to clearance while tracking gametocyte populations. The modeled immune system includes innate inflammatory and specific antibody responses to a repertoire of antigens. The mechanistic focus provides clear explanations for the structure of the distribution of infection durations through the interaction of antigenic variation and innate and adaptive immunity. Infectiousness to mosquitoes appears to be determined not only by the density of gametocytes but also by the level of inflammatory cytokines, which harmonizes an extensive series of study results. Finally, pre-existing immunity can either decrease or increase the duration of infections upon reinfection, depending on the degree of overlap in antigenic repertoires and the strength of the pre-existing immunity.

Further observations on in vitro hybridization of hemosporidian parasites: patterns of ookinete development in Haemoproteus spp

Increasingly frequent outbreaks of zoonotic infections call for studies of wildlife parasites to reach a better understanding of the mechanisms of host switch, leading to the evolution of new diseases. However, speciation processes have been insufficiently addressed in experimental parasitology studies, primarily due to difficulties in determining and measuring mate-recognition signals in parasites. We investigated patterns of sexual process and ookinete development in avian Haemoproteus (Parahaemoproteus) spp. (Haemosporida, Haemoproteidae) using in vitro experiments on between-lineage hybridization. Eleven mitochondrial cytochrome b (cyt b) lineages belonging to 9 species of hemoproteid were isolated from naturally infected passerine birds. The parasites were identified to species on the basis of morphology of their gametocytes and polymerase chain reaction amplification of segments of the cyt b gene. Sexual process and ookinete development were initiated in vitro by mixing blood containing mature gametocytes with a 3.7% solution of sodium citrate and exposing the mixture to air. Ookinetes of all lineages except Haemoproteus payevskyi (lineage hRW1) and Haemoproteus nucleocondensus (hGRW1) developed; the 2 latter species did not exflagellate. Between-lineage hybridization was initiated by mixing blood containing mature gametocytes of 2 different parasites; the following experiments were performed: (1) Haemoproteus pallidus (lineage hPFC1) × Haemoproteus minutus (lineage hTURDUS2); (2) H. pallidus (hPFC1) × Haemoproteus tartakovskyi (hSISKIN1); (3) Haemoproteus belopolskyi (hHIICT3) × Haemoproteus lanii (hRB1); (4) Haemoproteus balmorali (hSFC1) × H. pallidus (hPFC1); (5) H. belopolskyi (hHIICT1) × Haemoproteus parabelopolskyi (hSYBOR1); (6) H. tartakovskyi (hHAWF1) × H. tartakovskyi (hSISKIN1); (7) H. pallidus (hPFC1) × H. lanii (hRB1); (8) H. tartakovskyi (hHAWF1) × H. parabelopolskyi (hSYBOR1). We report 4 patterns of between-lineage interactions that seem to be common and might prevent mixing lineages during simultaneous sexual process in wildlife: (1) the blockage of ookinete development of both parasites; (2) the development of ookinetes of 1 parasite and blockage of ookinete development of the other; (3) selective within-lineage mating resulting in ookinete development of both parent species and absence of hybrid organisms; (4) absence of selective within-lineage mating resulting in presence of ookinetes of both parents and also development of hybrid organisms with unclear potential for further sporogony. The present study indicates directions for collection of source material in the investigation of mechanisms of reproductive isolation leading to speciation in these parasites. The next steps in these studies should be the development of nuclear markers for distinguishing hemosporidian hybrid organisms and the experimental observation of further development of hybrid ookinetes in vectors.

Function and composition of male accessory gland secretions in Anopheles gambiae: a comparison with other insect vectors of infectious diseases

Human malaria, a major public health burden in tropical and subtropical countries, is transmitted exclusively by the bite of a female Anopheles mosquito. Malaria control strategies aimed at inducing sexual sterility in natural vector populations are an attractive alternative to the use of insecticides. However, despite their importance as disease vectors, limited information is available on the molecular mechanisms regulating fertility in Anopheles mosquitoes. In the major malaria vector, An. gambiae, the full complement of sperm and seminal fluid required for a female's lifelong egg production is obtained from a single mating event. This single mating has important consequences for the physiology and behavior of An. gambiae females: in particular, they become refractory to further insemination, and they start laying eggs. In other insects including Drosophila, similar post-copulatory changes are induced by seminal proteins secreted by the male accessory glands and transferred to the female during mating. In this review, we analyze the current state of knowledge on the function and characterization of male seminal proteins in An. gambiae, and provide a comparative assessment of the role of these male reproductive factors in other mosquito vectors of human disease in which female post-copulatory behavior has been studied. Knowledge of the factors and mechanisms regulating fertility in An. gambiae and other vectors can help the design of novel control strategies to fight the spread of disease.

Gametocytogenesis in malaria parasite: commitment, development and regulation

Malaria parasites have evolved a complicated life cycle alternating between two hosts. Gametocytes are produced in the vertebrate hosts and are obligatory for natural transmission of the parasites through mosquito vectors. The mechanism of sexual development in Plasmodium has been the focus of extensive studies. In the postgenomic era, the advent of genome-wide analytical tools and genetic manipulation technology has enabled rapid advancement of our knowledge in this area. Patterns of gene expression during sexual development, molecular distinction of the two sexes, and mechanisms underlying subsequent formation of gametes and their fertilization have been progressively elucidated. However, the triggers and mechanism of sexual development remain largely unknown. This article provides an update of our understanding of the molecular and cellular events associated with the decision for commitment to sexual development and regulation of gene expression during gametocytogenesis. Insights into the molecular mechanisms of gametocyte development are essential for designing proper control strategies for interruption of malaria transmission and ultimate elimination.

Challenges in antimalarial drug discovery

Malaria is one of the most devastating diseases in the world, affecting almost 225 million people a year, and causing over 780,000 deaths, most of which are children under the age of 5 years. Following the recent call for the eradication of the disease, supported by the WHO, there has been increasing investment into antimalarial drug-discovery projects. These activities are aimed at generating the next generation of molecules focused on the treatment and transmission-blocking of Plasmodium falciparum and Plasmodium vivax endo- and exo-erythrocytic stages of the parasite. This article summarizes the current top-level thinking regarding the prosecution of such endeavors and the disease-specific considerations in project planning.

Mitosis in the human malaria parasite Plasmodium falciparum

Malaria is caused by intraerythrocytic protozoan parasites belonging to Plasmodium spp. (phylum Apicomplexa) that produce significant morbidity and mortality, mostly in developing countries. Plasmodium parasites have a complex life cycle that includes multiple stages in anopheline mosquito vectors and vertebrate hosts. During the life cycle, the parasites undergo several cycles of extreme population growth within a brief span, and this is critical for their continued transmission and a contributing factor for their pathogenesis in the host. As with other eukaryotes, successful mitosis is an essential requirement for Plasmodium reproduction; however, some aspects of Plasmodium mitosis are quite distinct and not fully understood. In this review, we will discuss the current understanding of the architecture and key events of mitosis in Plasmodium falciparum and related parasites and compare them with the traditional mitotic events described for other eukaryotes.

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.