Sexual development in Plasmodium parasites: knowing when it's time to commit

H3K36 methylation reprograms gene expression to drive early gametocyte development in Plasmodium falciparum

Background

The Plasmodium sexual gametocyte stages are the only transmissible form of the malaria parasite and are thus responsible for the continued transmission of the disease. Gametocytes undergo extensive functional and morphological changes from commitment to maturity, directed by an equally extensive control program. However, the processes that drive the differentiation and development of the gametocyte post-commitment, remain largely unexplored. A previous study reported enrichment of H3K36 di- and tri-methylated (H3K36me2&3) histones in early-stage gametocytes. Using chromatin immunoprecipitation followed by high-throughput sequencing, we identify a stage-specific association between these repressive histone modifications and transcriptional reprogramming that define a stage II gametocyte transition point.

Results

Here, we show that H3K36me2 and H3K36me3 from stage II gametocytes are associated with repression of genes involved in asexual proliferation and sexual commitment, indicating that H3K36me2&3-mediated repression of such genes is essential to the transition from early gametocyte differentiation to intermediate development. Importantly, we show that the gene encoding the transcription factor AP2-G as commitment master regulator is enriched with H3K36me2&3 and actively repressed in stage II gametocytes, providing the first evidence of ap2-g gene repression in post-commitment gametocytes. Lastly, we associate the enhanced potency of the pan-selective Jumonji inhibitor JIB-04 in gametocytes with the inhibition of histone demethylation including H3K36me2&3 and a disruption of normal transcriptional programs.

Conclusions

Taken together, our results provide the first description of an association between global gene expression reprogramming and histone post-translational modifications during P. falciparum early sexual development. The stage II gametocyte-specific abundance of H3K36me2&3 manifests predominantly as an independent regulatory mechanism targeted towards genes that are repressed post-commitment. H3K36me2&3-associated repression of genes is therefore involved in key transcriptional shifts that accompany the transition from early gametocyte differentiation to intermediate development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-021-00393-9.

Targeting Gametocytes of the Malaria Parasite Plasmodium falciparum in a Functional Genomics Era: Next Steps

Mosquito transmission of the deadly malaria parasite Plasmodium falciparum is mediated by mature sexual forms (gametocytes). Circulating in the vertebrate host, relatively few intraerythrocytic gametocytes are picked up during a bloodmeal to continue sexual development in the mosquito vector. Human-to-vector transmission thus represents an infection bottleneck in the parasite's life cycle for therapeutic interventions to prevent malaria. Even though recent progress has been made in the identification of genetic factors linked to gametocytogenesis, a plethora of genes essential for sexual-stage development are yet to be unraveled. In this review, we revisit P. falciparum transmission biology by discussing targetable features of gametocytes and provide a perspective on a forward-genetic approach for identification of novel transmission-blocking candidates in the future.

Transgenic pyrimethamine-resistant plasmodium falciparum reveals transmission-blocking potency of P218, a novel antifolate candidate drug

Antimalarial drugs capable of targeting multiple parasite stages, particularly the transmissible stages, can be valuable tools for advancing the malaria elimination agenda. Current antifolate drugs such as pyrimethamine can inhibit replicative parasite stages in both humans and mosquitoes, but antifolate resistance remains a challenge. The lack of reliable gametocyte-producing, antifolate-resistant Plasmodium falciparum laboratory strain hinders the study of new antifolate compounds that can overcome antifolate resistance including development stages in the mosquito. We used clustered regularly interspaced short palindromic repeats-Cas9 genome editing to develop a transgenic gametocyte-producing strain of P. falciparum with quadruple mutations (N51I, C59R, S108N, I164L) in the dihydrofolate reductase (dhfr) gene, using NF54 as a parental strain. The transgenic parasites exhibited pyrimethamine resistance while maintaining their gametocyte-producing activity. We then demonstrated that pyrimethamine could no longer inhibit male gametocyte exflagellation in the transgenic parasite. In contrast, P218, the novel antifolate, designed to overcome antifolate resistance, potently inhibited exflagellation. The exflagellation IC₅₀ of P218 was five times lower than the asexual stage half maximal inhibitory concentration (IC₅₀), suggesting a strong barrier for transmission of P218-resistant parasites. The transgenic gametocyte-producing, pyrimethamine-resistant parasite is a robust system for evaluating novel antifolate compounds against non-asexual stage development.

PfAP2-G2 Is Associated to Production and Maturation of Gametocytes in Plasmodium falciparum via Regulating the Expression of PfMDV-1

Gametocyte is the sole form of the Plasmodium falciparum which is transmissible to the mosquito vector. Here, we report that an Apicomplexan Apetala2 (ApiAP2) family transcription factor, PfAP2-G2 (Pf3D7_1408200), plays a role in the development of gametocytes in P. falciparum by regulating the expression of PfMDV-1 (Pf3D7_1216500). Reverse transcriptase-quantitative PCR (RT-qPCR) analysis showed that PfAP2-G2 was highly expressed in the ring stage. Indirect immunofluorescence assay showed nuclear localization of PfAP2-G2 in asexual stages. The knockout of PfAP2-G2 led to a ~95% decrease in the number of mature gametocytes with a more substantial influence on the production and maturation of the male gametocytes, resulting in a higher female/male gametocyte ratio. To test the mechanism of this phenotype, RNA-seq and RT-qPCR showed that disruption of PfAP2-G2 led to the down-regulation of male development gene-1 (PfMDV-1) in asexual stages. We further found that PfAP2-G2 was enriched at the transcriptional start site (TSS) of PfMDV-1 by chromatin immunoprecipitation and qPCR assay in both ring stage and schizont stage, which demonstrated that PfMDV-1 is one of the targets of PfAP2-G2. In addition, RT-qPCR also showed that PfAP2-G (Pf3D7_1222600), the master regulator for sexual commitment, was also down-regulated in the PfAP2-G2 knockout parasites in the schizont stage, but no change in the ring stage. This phenomenon suggested that PfAP2-G2 played a role at the asexual stage for the development of parasite gametocytes and warrants further investigations in regulatory pathways of PfAP2-G2.

Three-dimensional chromatin in infectious disease-A role for gene regulation and pathogenicity?

The recent Coronavirus Disease 2019 pandemic has once again reminded us the importance of understanding infectious diseases. One important but understudied area in infectious disease research is the role of nuclear architecture or the physical arrangement of the genome in the nucleus in controlling gene regulation and pathogenicity. Recent advances in research methods, such as Genome-wide chromosome conformation capture using high-throughput sequencing (Hi-C), have allowed for easier analysis of nuclear architecture and chromosomal reorganization in both the infectious disease agents themselves as well as in their host cells. This review will discuss broadly on what is known about nuclear architecture in infectious disease, with an emphasis on chromosomal reorganization, and briefly discuss what steps are required next in the field.

Comparative proteomic analysis of kinesin-8B deficient Plasmodium berghei during gametogenesis

Plasmodium blood stages, responsible for human to vector transmission, termed gametocytes, are the precursor cells that develop into gametes in the mosquito. Male gametogenesis works as a bottleneck for the parasite life cycle, where, during a peculiar and rapid exflagellation, a male gametocyte produces 8 intracellular axonemes that generate by budding 8 motile gametes. Understanding the molecular mechanisms of gametogenesis is key to design strategies for controlling malaria transmission. In the rodent P. berghei, the microtubule-based motor kinesin-8B (PbKIN8B) is essential for flagellum assembly during male gametogenesis and its gene disruption impacts on completion of the parasitic life cycle. In efforts to improve our knowledge about male gametogenesis, we performed an iTRAQ-based quantitative proteomic comparison of P. berghei mutants with disrupted kinesin-8B gene (ΔPbkin8B) and wild type parasites. During the 15 min of gametogenesis, ΔPbkin8B parasites exhibited important motor protein dysregulation that suggests an essential role of PbKIN8B for the correct interaction or integration of axonemal proteins within the growing axoneme. The energy metabolism of ΔPbkin8B mutants was further affected, as well as the response to stress proteins, protein synthesis, as well as chromatin organisation and DNA processes, although endomitoses seemed to occur. SIGNIFICANCE: Malaria continues to be a global scourge, mainly in subtropical and tropical areas. The disease is caused by parasites from the Plasmodium genus. Plasmodium life cycle alternates between female Anopheles mosquitoes and vertebrate hosts through bites. Gametocytes are the parasite blood forms responsible for transmission from vertebrates to vectors. Inside the mosquito midgut, after stimulation, male and female gametocytes transform into gametes resulting in fertilization. During male gametogenesis, one gametocyte generates eight intracytoplasmic axonemes that generate, by budding, flagellated motile gametes involving a process termed exflagellation. Sexual development has a central role in ensuring malaria transmission. However, molecular data on male gametogenesis and particularly on intracytoplasmic axoneme assembly are still lacking. Since rodent malaria parasites permit the combination of in vivo and in vitro experiments and reverse genetic studies, our group investigated the molecular events in rodent P. berghei gametogenesis. The P. berghei motor ATPase kinesin-8B is proposed as an important component for male gametogenesis. We generated Pbkin8B gene-disrupted gametocytes (ΔPbkin8B) that were morphologically similar to the wild- type (WT) parasites. However, in mutants, male gametogenesis is impaired, male gametocytes are disabled in their ability to assemble axonemes and to exflagellate to release gametes, reducing fertilization drastically. Using a comparative quantitative proteomic analysis, we associated the nonfunctional axoneme of the mutants with the abnormal differential expression of proteins essential to axoneme organisation and stability. We also observed a differential dysregulation of proteins involved in protein biosynthesis and degradation, chromatin organisation and DNA processes in ΔPbkin8B parasites, although DNA condensation, mitotic spindle formation and endomitoses seem to occur. This is the first functional proteomic study of a kinesin gene-disrupted Plasmodium parasite providing new insights into Plasmodium male gametogenesis.

Multistage and transmission-blocking targeted antimalarials discovered from the open-source MMV Pandemic Response Box

Chemical matter is needed to target the divergent biology associated with the different life cycle stages of Plasmodium. Here, we report the parallel de novo screening of the Medicines for Malaria Venture (MMV) Pandemic Response Box against Plasmodium asexual and liver stage parasites, stage IV/V gametocytes, gametes, oocysts and as endectocides. Unique chemotypes were identified with both multistage activity or stage-specific activity, including structurally diverse gametocyte-targeted compounds with potent transmission-blocking activity, such as the JmjC inhibitor ML324 and the antitubercular clinical candidate SQ109. Mechanistic investigations prove that ML324 prevents histone demethylation, resulting in aberrant gene expression and death in gametocytes. Moreover, the selection of parasites resistant to SQ109 implicates the druggable V-type H⁺-ATPase for the reduced sensitivity. Our data therefore provides an expansive dataset of compounds that could be redirected for antimalarial development and also point towards proteins that can be targeted in multiple parasite life cycle stages.

Here, Reader et al. screen the Medicines for Malaria Venture Pandemic Response Box in parallel against Plasmodiumasexual and liver stage parasites, stage IV/V gametocytes, gametes, oocysts and as endectocides. They identify two potent transmission-blocking drugs: a histone demethylase inhibitor ML324 and the antitubercular SQ109.

Plasmodium-a brief introduction to the parasites causing human malaria and their basic biology

Malaria is one of the most devastating infectious diseases of humans. It is problematic clinically and economically as it prevails in poorer countries and regions, strongly hindering socioeconomic development. The causative agents of malaria are unicellular protozoan parasites belonging to the genus Plasmodium. These parasites infect not only humans but also other vertebrates, from reptiles and birds to mammals. To date, over 200 species of Plasmodium have been formally described, and each species infects a certain range of hosts. Plasmodium species that naturally infect humans and cause malaria in large areas of the world are limited to five-P. falciparum, P. vivax, P. malariae, P. ovale and P. knowlesi. The first four are specific for humans, while P. knowlesi is naturally maintained in macaque monkeys and causes zoonotic malaria widely in South East Asia. Transmission of Plasmodium species between vertebrate hosts depends on an insect vector, which is usually the mosquito. The vector is not just a carrier but the definitive host, where sexual reproduction of Plasmodium species occurs, and the parasite's development in the insect is essential for transmission to the next vertebrate host. The range of insect species that can support the critical development of Plasmodium depends on the individual parasite species, but all five Plasmodium species causing malaria in humans are transmitted exclusively by anopheline mosquitoes. Plasmodium species have remarkable genetic flexibility which lets them adapt to alterations in the environment, giving them the potential to quickly develop resistance to therapeutics such as antimalarials and to change host specificity. In this article, selected topics involving the Plasmodium species that cause malaria in humans are reviewed.

Plasmodium falciparum gametocyte carriage in symptomatic patients shows significant association with genetically diverse infections, anaemia, and asexual stage density

Background

Multi-genotype malaria infections are frequent in endemic area, and people commonly harbour several genetically distinct Plasmodium falciparum variants. The influence of genetic multiplicity and whether some specific genetic variants are more or less likely to invest into gametocyte production is not clearly understood. This study explored host and parasite-related risk factors for gametocyte carriage, and the extent to which some specific P. falciparum genetic variants are associated with gametocyte carriage.

Methods

Gametocytes and asexual forms were detected by light microscopy on thick smears collected between 2010 and 2012 in Nanoro, Burkina Faso. Merozoite surface protein 1 and 2 were genotyped by nested PCR on clinical samples. Associations between gametocyte carriage and factors, including multiplicity of infection, parasite density, patient age, gender, haemoglobin (Hb) level, and body temperature were assessed. The relationship between the presence of a particular msp1 and msp2 genetic variants and gametocyte carriage was also explored.

Results

Of the 724 samples positive to P. falciparum and successfully genotyped, gametocytes were found in 48 samples (6.63%). There was no effect of patient gender, age and body temperature on gametocyte carriage. However, the probability of gametocyte carriage significantly increased with increasing values of multiplicity of infection (MOI). Furthermore, there was a negative association between parasite density and gametocyte carriage. MOI decreased with parasite density in gametocyte-negative patients, but increased in gametocyte carriers. The probability of gametocyte carriage decreased with Hb level. Finally, the genetic composition of the infection influenced gametocyte carriage. In particular, the presence of RO33 increased the odds of developing gametocytes by 2 while the other allelic families K1, MAD20, FC27, and 3D7 had no significant impact on the occurrence of gametocytes in infected patients.

Conclusion

This study provides insight into potential factors influencing gametocyte production in symptomatic patients. The findings contribute to enhance understanding of risk factors associated with gametocyte carriage in humans.

Trial registration NCT01232530.

Long-lasting infectivity of Plasmodium vivax present in malarial patient blood to Anopheles aquasalis

Experimental studies for understanding the relationship between Plasmodium vivax and its vector hosts are difficult, because of to the lack of a long-term, in vitro continuous culture system unavailability of infected blood samples, seasonality of the disease, and the concentration of most cases in remote areas. This study evaluates the duration of the infectivity of P. vivax to Anopheles aquasalis after collecting blood from malaria-infected patients. Blood was collected from patients and stored at 4 °C and 37 °C. Every day, for 4 days, the blood was fed to An. aquasalis adult females, and a Giemsa-stained thick blood smear was mounted to account for sexual (gametocytes) and asexual (trophozoites and schizonts) stages and calculate parasitemia. Oocysts in the midgut of the mosquitoes were counted on the seventh day after feeding. Kruskal-Wallis test was used to compare the mean number of oocysts (MO) and the parasite density (PD) in each storage condition and post-infection time-points. The Mann-Whitney test was used to compare the number of oocysts for each day between temperatures. The results show that P. vivax stored at 4 °C and at 37 °C has its infectivity to An. aquasalis preserved for 2 days and 3 days, respectively. Infection rate (IR), PD and MO were higher on the day of blood collection and decreased gradually over time. The parasite density (number of parasites/μL) diminished faster at 4 °C than at 37 °C. In this study, a preservation protocol is shown for long-lasting infectivity of P. vivax in a blood sample taken from malaria-infected patients. These results show that infectivity of P. vivax stored at 4 °C and at 37 °C to An. aquasalis persist until 3 days after blood collection, but parasite density, infection rate, and mean of oocysts decreased 24h after blood collection. Since the malaria cases are increasingly far from the urban areas these results indicate that is possible, losing some infectivity, to realize experimental infections several dozen hours after the blood collection. However, it is necessary to improve the procedures for preserving P. vivax gametocytes for mosquito infection in the laboratory.

Zoonotic Transmissions and Host Switches of Malaria Parasites

Malaria is a deadly disease that affects the health of hundreds of millions of people annually. There are five Plasmodium parasite species that can naturally infect humans, including Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale and Plasmodium knowlesi. Some of the parasites can also infect various non-human primates. Parasites mainly infecting monkeys such as Plasmodium cynomolgi (in fact P. knowlesi was considered as a parasite of monkeys for years) can also be transmitted to human hosts. Recently, many new Plasmodium species were discovered in African apes, and it is possible that some of the parasites can be transmitted to humans in the future. Here, we searched PubMed and the internet via Google and selected articles concerning zoonotic transmission and evolution of selected malaria parasite species. We reviewed the current advances in the relevant topics emphasizing on transmissions of malaria parasites between humans and non-human primates. We also briefly discuss the transmissions of some avian malaria parasites between wild birds and domestic fowls. Zoonotic malaria transmissions are widespread, which poses a threat to public health. More studies on parasite species identification in non-human primates, transmission, and evolution are needed to reduce or prevent transmission of malaria parasites from non-human primates to humans.

Prevalence, characteristics and risk factors of imported and local malaria cases in North-Western Province, Zambia: a cross-sectional study

BACKGROUND

Imported malaria is a major challenge for countries that are in malaria elimination stage such as Zambia. Legitimate cross-border activities add to the risk of transmission, necessitating determination of prevalence, characteristics and risk factors of imported and local malaria.

METHODS

This cross-sectional study was conducted in 103 consented child and adult patients with clinical malaria symptoms, from selected health facilities in north-western Zambia. Patient demographic data and blood samples for malaria microscopy and full blood count were obtained. Chi-square and penalized logistic regression were performed to describe the characteristics and assess the risk factors of imported and local malaria in North-Western Province.

RESULTS

Overall, malaria prevalence was 78.6% with 93.8% Plasmodium falciparum and 6.2% other species. The local cases were 72 (88.9%) while the imported were 9 (11.1%) out of the 81 positive participants. About 98.6% of the local cases were P. falciparum compared to 55.6% (χ2 = 52.4; p < 0.01) P. falciparum among the imported cases. Among the imported cases, 44% were species other than P. falciparum (χ2 = 48; p < 0.01) while among the local cases only 1.4% were. Gametocytes were present in 44% of the imported malaria cases and only in 2.8% of the local cases (χ2 = 48; p < 0.01). About 48.6% of local participants had severe anaemia compared to 33.3% of participants from the two neighbouring countries who had (χ2 = 4.9; p = 0.03). In the final model, only country of residence related positively to presence of species other than P. falciparum (OR = 39.0, CI [5.9, 445.9]; p < 0.01) and presence of gametocytes (OR = 23.1, CI [4.2, 161.6]; p < 0.01).

CONCLUSION

Malaria prevalence in North-Western Province is high, with P. falciparum as the predominant species although importation of Plasmodium ovale and Plasmodium malariae is happening as well. Country of residence of patients is a major risk factor for malaria species and gametocyte presence. The need for enhanced malaria control with specific focus on border controls to detect and treat, for specific diagnosis and treatment according to species obtaining, for further research in the role of species and gametocytaemia in imported malaria, cannot be overemphasized.

Host-Malaria Parasite Interactions and Impacts on Mutual Evolution

Malaria is the most deadly parasitic disease, affecting hundreds of millions of people worldwide. Malaria parasites have been associated with their hosts for millions of years. During the long history of host-parasite co-evolution, both parasites and hosts have applied pressure on each other through complex host-parasite molecular interactions. Whereas the hosts activate various immune mechanisms to remove parasites during an infection, the parasites attempt to evade host immunity by diversifying their genome and switching expression of targets of the host immune system. Human intervention to control the disease such as antimalarial drugs and vaccination can greatly alter parasite population dynamics and evolution, particularly the massive applications of antimalarial drugs in recent human history. Vaccination is likely the best method to prevent the disease; however, a partially protective vaccine may have unwanted consequences that require further investigation. Studies of host-parasite interactions and co-evolution will provide important information for designing safe and effective vaccines and for preventing drug resistance. In this essay, we will discuss some interesting molecules involved in host-parasite interactions, including important parasite antigens. We also discuss subjects relevant to drug and vaccine development and some approaches for studying host-parasite interactions.

Mediterranean Diet: Lipids, Inflammation, and Malaria Infection

The Mediterranean diet (MedDiet) consists of consumption of vegetables and healthy oils and have beneficial effects on metabolic and inflammatory diseases. Our goal here is to discuss the role of fatty acid content in MedDiet, mostly omega-3, omega-6, and omega-9 on malaria. Malaria affects millions of people around the globe. The parasite Plasmodium causes the disease. The metabolic and inflammatory alterations in the severe forms have damaging consequences to the host. The lipid content in the MedDiet holds anti-inflammatory and pro-resolutive features in the host and have detrimental effects on the Plasmodium. The lipids from the diet impact the balance of pro- and anti-inflammation, thus, lipids intake from the diet is critical to parasite elimination and host tissue damage caused by an immune response. Herein, we go into the cellular and molecular mechanisms and targets of the MedDiet fatty acids in the host and the parasite, reviewing potential benefits of the MedDiet, on inflammation, malaria infection progression, and clinical outcome.

A protein palmitoylation cascade regulates microtubule cytoskeleton integrity in Plasmodium

Morphogenesis of many protozoans depends on a polarized establishment of cytoskeletal structures. In malaria-causing parasites, this can be observed when a round zygote develops into an elongated motile ookinete within the mosquito stomach. This morphogenesis is mediated by the pellicle cytoskeletal structures, including the inner membrane complex (IMC) and the underlying subpellicular microtubules (SPMs). How the parasite maintains the IMC-SPM connection and establishes a dome-like structure of SPM to support cell elongation is unclear. Here, we show that palmitoylation of N-terminal cysteines of two IMC proteins (ISP1/ISP3) regulates the IMC localization of ISP1/ISP3 and zygote-to-ookinete differentiation. Palmitoylation of ISP1/ISP3 is catalyzed by an IMC-residing palmitoyl-S-acyl-transferase (PAT) DHHC2. Surprisingly, DHHC2 undergoes self-palmitoylation at C-terminal cysteines via its PAT activity, which controls DHHC2 localization in IMC after zygote formation. IMC-anchored ISP1 and ISP3 interact with microtubule component β-tubulin, serving as tethers to maintain the proper structure of SPM during zygote elongation. This study identifies the first PAT-substrate pair in malaria parasites and uncovers a protein palmitoylation cascade regulating microtubule cytoskeleton.

Single-cell transcription analysis of Plasmodium vivax blood-stage parasites identifies stage- and species-specific profiles of expression

Plasmodium vivax and P. falciparum, the parasites responsible for most human malaria worldwide, exhibit striking biological differences, which have important clinical consequences. Unfortunately, P. vivax, unlike P. falciparum, cannot be cultivated continuously in vitro, which limits our understanding of its biology and, consequently, our ability to effectively control vivax malaria. Here, we describe single-cell gene expression profiles of 9,215 P. vivax parasites from bloodstream infections of Aotus and Saimiri monkeys. Our results show that transcription of most P. vivax genes occurs during short periods of the intraerythrocytic cycle and that this pattern of gene expression is conserved in other Plasmodium species. However, we also identify a strikingly high proportion of species-specific transcripts in late schizonts, possibly associated with the specificity of erythrocyte invasion. Our findings provide new and robust markers of blood-stage parasites, including some that are specific to the elusive P. vivax male gametocytes, and will be useful for analyzing gene expression data from laboratory and field samples.

Analysis of individual Plasmodium vivax parasites reveals the tight control of the expression of most genes during the intra-erythrocytic cycle and the differentiation of male and female gametocytes, and highlights differences between the development of P. vivax and P. falciparum.

Diversity and Multiplicity of P. falciparum infections among asymptomatic school children in Mbita, Western Kenya

Multiplicity of infection (MOI) and genetic diversity of P. falciparum infections are important surrogate indicators for assessing malaria transmission intensity in different regions of endemicity. Determination of MOI and diversity of P. falciparum among asymptomatic carriers will enhance our understanding of parasite biology and transmission to mosquito vectors. This study examined the MOI and genetic diversity of P. falciparum parasite populations circulating in Mbita, a region characterized as one of the malaria hotspots in Kenya. The genetic diversity and multiplicity of P. falciparum infections in 95 asymptomatic school children (age 5–15 yrs.) residing in Mbita, western Kenya were assessed using 10 polymorphic microsatellite markers. An average of 79.69% (Range: 54.84–95.74%) of the isolates analysed in this study were polyclonal infections as detected in at least one locus. A high mean MOI of 3.39 (Range: 2.24–4.72) and expected heterozygosity (He) of 0.81 (Range: 0.57–0.95) was reported in the study population. The analysed samples were extensively polyclonal infections leading to circulation of highly genetically diverse parasite populations in the study area. These findings correlated with the expectations of high malaria transmission intensity despite scaling up malaria interventions in the area thereby indicating the need for a robust malaria interventions particularly against asymptomatic carriers in order to attain elimination in the region.

Dissecting the role of PfAP2-G in malaria gametocytogenesis

In the malaria parasite Plasmodium falciparum, the switch from asexual multiplication to sexual differentiation into gametocytes is essential for transmission to mosquitos. The transcription factor PfAP2-G is a key determinant of sexual commitment that orchestrates this crucial cell fate decision. Here we identify the direct targets of PfAP2-G and demonstrate that it dynamically binds hundreds of sites across the genome. We find that PfAP2-G is a transcriptional activator of early gametocyte genes, and identify differences in PfAP2-G occupancy between gametocytes derived via next-cycle and same-cycle conversion. Our data implicate PfAP2-G not only as a transcriptional activator of gametocyte genes, but also as a potential regulator of genes important for red blood cell invasion. We also find that regulation by PfAP2-G requires interaction with a second transcription factor, PfAP2-I. These results clarify the functional role of PfAP2-G during sexual commitment and early gametocytogenesis.

The transcription factor PfAP2-G is a key determinant of sexual commitment in Plasmodium falciparum. Here, Josling et al. define the transcriptional regulatory network of PfAP2-G by identifying its DNA binding sites genome-wide, which vary depending on the route of sexual conversion and rely on interactions with the PfAP2-I transcription factor.

Single-nucleotide polymorphism characterization of gametocyte development 1 gene in Plasmodium falciparum isolates from Baringo, Uasin Gishu, and Nandi Counties, Kenya

INTRODUCTION

Plasmodium falciparum relies on gametocytogenesis to transmit from humans to mosquitoes. Gametocyte development 1 (Pfgdv1) is an upstream activator and epigenetic controller of gametocytogenesis. The emergence of drug resistance is a major public health concern and this requires the development of new strategies that target the transmission of malaria. As a putative drug target, Pfgdv1 has not been characterized to identify its polymorphisms and alleles under selection and how such polymorphisms influence protein structure.

METHODS

This study characterized single-nucleotide polymorphisms (SNPs) in primary sequences (n = 30) of Pfgdv1 gene generated from thirty blood samples collected from patients infected with P. falciparum and secondary sequences (n = 216) retrieved from PlasmoDB. ChromasPro, MUSCLE, Tajima's D statistic, SLAC, and STRUM were used in editing raw sequences, performing multiple sequence alignment (MSA), identifying signatures of selection, detecting codon sites under selection pressure, and determining the effect of SNPs, respectively.

RESULTS

MSA of primary and secondary sequences established the existence of five SNPs, consisting of four non-synonymous substitutions (nsSNPs) (p.P217H, p.R398Q, p.H417N, and p.D497E), and a synonymous substitution (p.S514S). The analysis of amino acid changes reveals that p.P217H, p.R398Q, and p.H417N comprise non-conservative changes. Tajima's D statistic showed that these SNPs were under balancing selection, while SLAC analysis identified p.P217H to be under the strongest positive selection. . Further analysis based on thermodynamics indicated that p.P217H has a destabilizing effect, while p.R398Q and p.D497E have stabilizing effects on the protein structure.

CONCLUSIONS

The existence of four nsSNPs implies that Pfgdv1 has a minimal diversity in the encoded protein. Selection analysis demonstrates that these nsSNPs are under balancing selection in both local and global populations. However, p.P217H exhibits positive directional selection consistent with previous reports where it showed differentiatial selection of P. falciparum in low and high transmission regions. Therefore, in-silico prediction and experimental determination of protein structure are necessary to evaluate Pfgdv1 as a target candidate for drug design and development.

Plasmodium asexual growth and sexual development in the haematopoietic niche of the host

Plasmodium spp. parasites are the causative agents of malaria in humans and animals, and they are exceptionally diverse in their morphology and life cycles. They grow and develop in a wide range of host environments, both within blood-feeding mosquitoes, their definitive hosts, and in vertebrates, which are intermediate hosts. This diversity is testament to their exceptional adaptability and poses a major challenge for developing effective strategies to reduce the disease burden and transmission. Following one asexual amplification cycle in the liver, parasites reach high burdens by rounds of asexual replication within red blood cells. A few of these blood-stage parasites make a developmental switch into the sexual stage (or gametocyte), which is essential for transmission. The bone marrow, in particular the haematopoietic niche (in rodents, also the spleen), is a major site of parasite growth and sexual development. This Review focuses on our current understanding of blood-stage parasite development and vascular and tissue sequestration, which is responsible for disease symptoms and complications, and when involving the bone marrow, provides a niche for asexual replication and gametocyte development. Understanding these processes provides an opportunity for novel therapies and interventions.

Human unconventional T cells in Plasmodium falciparum infection

Malaria is an old scourge of humankind and has a large negative impact on the economic development of affected communities. Recent success in malaria control and reduction of mortality seems to have stalled emphasizing that our current intervention tools need to be complemented by malaria vaccines. Different populations of unconventional T cells such as mucosal-associated invariant T (MAIT) cells, invariant natural killer T (iNKT) cells and γδ T cells are gaining attention in the field of malaria immunology. Significant advances in our basic understanding of unconventional T cell biology in rodent malaria models have been made, however, their roles in humans during malaria are less clear. Unconventional T cells are abundant in skin, gut and liver tissues, and long-lasting expansions and functional alterations were observed upon malaria infection in malaria naïve and malaria pre-exposed volunteers. Here, we review the current understanding of involvement of unconventional T cells in anti-Plasmodium falciparum immunity and highlight potential future research avenues.

Mining the Human Host Metabolome Toward an Improved Understanding of Malaria Transmission

The big data movement has led to major advances in our ability to assess vast and complex datasets related to the host and parasite during malaria infection. While host and parasite genomics and transcriptomics are often the focus of many computational efforts in malaria research, metabolomics represents another big data type that has great promise for aiding our understanding of complex host-parasite interactions that lead to the transmission of malaria. Recent analyses of the complement of metabolites present in human blood, skin and breath suggest that host metabolites play a critical role in the transmission cycle of malaria. Volatile compounds released through breath and skin serve as attractants to mosquitoes, with malaria-infected hosts appearing to have unique profiles that further increase host attractiveness. Inside the host, fluctuations in the levels of certain metabolites in blood may trigger increased production of transmission-competent sexual stages (gametocytes), setting the stage for enhanced transmission of malaria from human to mosquito. Together, these recent discoveries suggest that metabolites of human blood, skin and breath play critical roles in malaria transmission. This review discusses recent advances in this area, with a focus on metabolites that have been identified to play a role in malaria transmission and methods that may lead to an improved understanding of malaria transmission.

Hierarchical transcriptional control regulates Plasmodium falciparum sexual differentiation

BACKGROUND

Malaria pathogenesis relies on sexual gametocyte forms of the malaria parasite to be transmitted between the infected human and the mosquito host but the molecular mechanisms controlling gametocytogenesis remains poorly understood. Here we provide a high-resolution transcriptome of Plasmodium falciparum as it commits to and develops through gametocytogenesis.

RESULTS

The gametocyte-associated transcriptome is significantly different from that of the asexual parasites, with dynamic gene expression shifts characterizing early, intermediate and late-stage gametocyte development and results in differential timing for sex-specific transcripts. The transcriptional dynamics suggest strict transcriptional control during gametocytogenesis in P. falciparum, which we propose is mediated by putative regulators including epigenetic mechanisms (driving active repression of proliferation-associated processes) and a cascade-like expression of ApiAP2 transcription factors.

CONCLUSIONS

The gametocyte transcriptome serves as the blueprint for sexual differentiation and will be a rich resource for future functional studies on this critical stage of Plasmodium development, as the intraerythrocytic transcriptome has been for our understanding of the asexual cycle.

Modeling the dynamics of Plasmodium falciparum gametocytes in humans during malaria infection

Renewed efforts to eliminate malaria have highlighted the potential to interrupt human-to-mosquito transmission — a process mediated by gametocyte kinetics in human hosts. Here we study the in vivo dynamics of Plasmodium falciparum gametocytes by establishing a framework which incorporates improved measurements of parasitemia, a novel gametocyte dynamics model and model fitting using Bayesian hierarchical inference. We found that the model provides an excellent fit to the clinical data from 17 volunteers infected with P. falciparum (3D7 strain) and reliably predicts observed gametocytemia. We estimated the sexual commitment rate and gametocyte sequestration time to be 0.54% (95% credible interval: 0.30–1.00%) per asexual replication cycle and 8.39 (6.54–10.59) days respectively. We used the data-calibrated model to investigate human-to-mosquito transmissibility, providing a method to link within-human host infection kinetics to epidemiological-scale infection and transmission patterns.

The G9a Histone Methyltransferase Inhibitor BIX-01294 Modulates Gene Expression during Plasmodium falciparum Gametocyte Development and Transmission

Transmission of the malaria parasite Plasmodium falciparum from the human to the mosquito is initiated by specialized sexual cells, the gametocytes. In the human, gametocytes are formed in response to stress signals and following uptake by a blood-feeding Anopheles mosquito initiate sexual reproduction. Gametocytes need to fine-tune their gene expression in order to develop inside the mosquito to continue life-cycle progression. Previously, we showed that post-translational histone acetylation controls gene expression during gametocyte development and transmission. However, the role of histone methylation remains poorly understood. We here use the histone G9a methyltransferase inhibitor BIX-01294 to investigate the role of histone methylation in regulating gene expression in gametocytes. In vitro assays demonstrated that BIX-01294 inhibits intraerythrocytic replication with a half maximal inhibitory concentration (IC₅₀) of 13.0 nM. Furthermore, BIX-01294 significantly impairs gametocyte maturation and reduces the formation of gametes and zygotes. Comparative transcriptomics between BIX-01294-treated and untreated immature, mature and activated gametocytes demonstrated greater than 1.5-fold deregulation of approximately 359 genes. The majority of these genes are transcriptionally downregulated in the activated gametocytes and could be assigned to transcription, translation, and signaling, indicating a contribution of histone methylations in mediating gametogenesis. Our combined data show that inhibitors of histone methylation may serve as a multi-stage antimalarial.

Transcriptome profiling reveals functional variation in Plasmodium falciparum parasites from controlled human malaria infection studies

BACKGROUND

The transcriptome of Plasmodium falciparum clinical isolates varies according to strain, mosquito bites, disease severity and clinical history. Therefore, it remains a challenge to directly interpret the parasite's transcriptomic information into a more general biological signature in a natural human malaria infection. These confounding variations can be potentially overcome with parasites derived from controlled-human malaria infection (CHMI) studies.

METHODS

We performed CHMI studies in healthy and immunologically naïve volunteers receiving the same P. falciparum strain ((Sanaria® PfSPZ Challenge (NF54)), but with different sporozoite dosage and route of infection. Parasites isolated from these volunteers at the day of patency were subjected to in vitro culture for several generations and synchronized ring-stage parasites were subjected to transcriptome profiling.

FINDINGS

We observed clear deviations between CHMI-derived parasites from volunteer groups receiving different PfSPZ dose and route. CHMI-derived parasites and the pre-mosquito strain used for PfSPZ generation showed significant transcriptional variability for gene clusters associated with malaria pathogenesis, immune evasion and transmission. These transcriptional variation signature clusters were also observed in the transcriptome of P. falciparum isolates from acute clinical infections.

INTERPRETATION

Our work identifies a previously unrecognized transcriptional pattern in malaria infections in a non-immune background. Significant transcriptome heterogeneity exits between parasites derived from human infections and the pre-mosquito strain, implying that the malaria parasites undergo a change in functional state to adapt to its host environment. Our work also highlights the potential use of transcriptomics data from CHMI study advance our understanding of malaria parasite adaptation and transmission in humans. FUND: This work is supported by German Israeli Foundation, German ministry for education and research, MOE Tier 1 from the Singapore Ministry of Education Academic Research Fund, Singapore Ministry of Health's National Medical Research Council, National Institute of Allergy and Infectious Diseases, National Institutes of Health, USA and the German Centre for Infection Research (Deutsches Zentrum für Infektionsforschung-DZIF).

Life cycle progression and sexual development of the apicomplexan parasite Cryptosporidium parvum

The apicomplexan parasite Cryptosporidium is a leading global cause of severe diarrhoeal disease and an important contributor to early childhood mortality. Currently, there are no fully effective treatments or vaccines available. Parasite transmission occurs through ingestion of oocysts, through either direct contact or consumption of contaminated water or food. Oocysts are meiotic spores and the product of parasite sex. Cryptosporidium has a single-host life cycle in which both asexual and sexual processes occur in the intestine of infected hosts. Here, we genetically engineered strains of Cryptosporidium to make life cycle progression and parasite sex tractable. We derive reporter strains to follow parasite development in culture and in infected mice and define the genes that orchestrate sex and oocyst formation through mRNA sequencing of sorted cells. After 2 d, parasites in cell culture show pronounced sexualization, but productive fertilization does not occur and infection falters. By contrast, in infected mice, male gametes successfully fertilize female parasites, which leads to meiotic division and sporulation. To rigorously test for fertilization, we devised a two-component genetic-crossing assay using a reporter that is activated by Cre recombinase. Our findings suggest obligate developmental progression towards sex in Cryptosporidium, which has important implications for the treatment and prevention of the infection.

Infection with Cryptosporidium parvum is a leading cause of severe diarrhoeal disease and childhood mortality worldwide. Using tools they recently developed to genetically engineer Cryptosporidium, the authors define life cycle stage-specific markers and generate reporter parasites, making life cycle progression and parasite sex tractable.

Plasticity and genetic variation in traits underpinning asexual replication of the rodent malaria parasite, Plasmodium chabaudi

BACKGROUND

The ability of malaria (Plasmodium) parasites to adjust investment into sexual transmission stages versus asexually replicating stages is well known, but plasticity in other traits underpinning the replication rate of asexual stages in the blood has received less attention. Such traits include burst size (the number of merozoites produced per schizont), the duration of the asexual cycle, and invasion preference for different ages of red blood cell (RBC).

METHODS

Here, plasticity [environment (E) effects] and genetic variation [genotype (G) effects] in traits relating to asexual replication rate are examined for 4 genotypes of the rodent malaria parasite Plasmodium chabaudi. An experiment tested whether asexual dynamics differ between parasites infecting control versus anaemic hosts, and whether variation in replication rate can be explained by differences in burst size, asexual cycle, and invasion rates.

RESULTS

The within-host environment affected each trait to different extents but generally had similar impacts across genotypes. The dynamics of asexual densities exhibited a genotype by environment effect (G×E), in which one of the genotypes increased replication rate more than the others in anaemic hosts. Burst size and cycle duration varied between the genotypes (G), while burst size increased and cycle duration became longer in anaemic hosts (E). Variation in invasion rates of differently aged RBCs was not explained by environmental or genetic effects. Plasticity in burst size and genotype are the only traits making significant contributions to the increase in asexual densities observed in anaemic hosts, together explaining 46.4% of the variation in replication rate.

CONCLUSIONS

That host anaemia induces several species of malaria parasites to alter conversion rate is well documented. Here, previously unknown plasticity in other traits underpinning asexual replication is revealed. These findings contribute to mounting evidence that malaria parasites deploy a suite of sophisticated strategies to maximize fitness by coping with, or exploiting the opportunities provided by, the variable within-host conditions experienced during infections. That genetic variation and genotype by environment interactions also shape these traits highlights their evolutionary potential. Asexual replication rate is a major determinant of virulence and so, understanding the evolution of virulence requires knowledge of the ecological (within-host environment) and genetic drivers of variation among parasites.

Malaria: The Past and the Present

Malaria is a severe disease caused by parasites of the genus Plasmodium, which is transmitted to humans by a bite of an infected female mosquito of the species Anopheles. Malaria remains the leading cause of mortality around the world, and early diagnosis and fast-acting treatment prevent unwanted outcomes. It is the most common disease in Africa and some countries of Asia, while in the developed world malaria occurs as imported from endemic areas. The sweet sagewort plant was used as early as the second century BC to treat malaria fever in China. Much later, quinine started being used as an antimalaria drug. A global battle against malaria started in 1955, and Croatia declared 1964 to be the year of eradication of malaria. The World Health Organization carries out a malaria control program on a global scale, focusing on local strengthening of primary health care, early diagnosis of the disease, timely treatment, and disease prevention. Globally, the burden of malaria is lower than ten years ago. However, in the last few years, there has been an increase in the number of malaria cases around the world. It is moving towards targets established by the WHO, but that progress has slowed down.

Modelling pathogen load dynamics to elucidate mechanistic determinants of host-Plasmodium falciparum interactions

During infection, increasing pathogen load stimulates both protective and harmful aspects of the host response. The dynamics of this interaction are hard to quantify in humans, but doing so could improve understanding of mechanisms of disease and protection. We sought to model the contributions of parasite multiplication rate and host response to observed parasite load in individual subjects with Plasmodium falciparum malaria, using only data obtained at the time of clinical presentation, and then to identify their mechanistic correlates. We predicted higher parasite multiplication rates and lower host responsiveness in severe malaria cases, with severe anemia being more insidious than cerebral malaria. We predicted that parasite growth-inhibition was associated with platelet consumption, lower expression of CXCL10 and type-1 interferon-associated genes, but increased cathepsin G and matrix metallopeptidase 9 expression. We found that cathepsin G and matrix metallopeptidase 9 directly inhibit parasite invasion into erythrocytes. Parasite multiplication rate was associated with host iron availability and higher complement factor H levels, lower expression of gametocyte-associated genes but higher expression of translation-associated genes in the parasite. Our findings demonstrate the potential of using explicit modelling of pathogen load dynamics to deepen understanding of host-pathogen interactions and identify mechanistic correlates of protection.

Current status and applications of genome-scale metabolic models

Genome-scale metabolic models (GEMs) computationally describe gene-protein-reaction associations for entire metabolic genes in an organism, and can be simulated to predict metabolic fluxes for various systems-level metabolic studies. Since the first GEM for Haemophilus influenzae was reported in 1999, advances have been made to develop and simulate GEMs for an increasing number of organisms across bacteria, archaea, and eukarya. Here, we review current reconstructed GEMs and discuss their applications, including strain development for chemicals and materials production, drug targeting in pathogens, prediction of enzyme functions, pan-reactome analysis, modeling interactions among multiple cells or organisms, and understanding human diseases.

The Diversity, Multiplicity of Infection and Population Structure of P. falciparum Parasites Circulating in Asymptomatic Carriers Living in High and Low Malaria Transmission Settings of Ghana

Background: Diversity in Plasmodium falciparum poses a major threat to malaria control and elimination interventions. This study utilized 12 polymorphic microsatellite (MS) markers and the Msp2 marker to examine diversity, multiplicity of infection (MOI) as well as the population structure of parasites circulating in two sites separated by about 92 km and with varying malaria transmission intensities within the Greater Accra Region of Ghana. Methods: The diversity and MOI of P. falciparum parasites in 160 non-symptomatic volunteers living in Obom (high malaria transmission intensity) and Asutsuare (low malaria transmission intensity) aged between 8 and 60 years was determined using Msp2 genotyping and microsatellite analysis. Results: The prevalence of asymptomatic P. falciparum carriers as well as the parasite density of infections was significantly higher in Obom than in Asutsuare. Samples from Asutsuare and Obom were 100% and 65% clonal, respectively, by Msp2 genotyping but decreased to 50% and 5%, respectively, when determined by MS analysis. The genetic composition of parasites from Obom and Asutsuare were highly distinct, with parasites from Obom being more diverse than those from Asutsuare. Conclusion: Plasmodium falciparum parasites circulating in Obom are genetically more diverse and distinct from those circulating in Asutsuare. The MOI in samples from both Obom and Asutsuare increased when assessed by MS analysis relative to MSP2 genotyping. The TA40 and TA87 loci are useful markers for estimating MOI in high and low parasite prevalence settings.

Less Lipid, More Commitment

Sexual differentiation of the malaria parasite is a pre-requisite for transmission from humans to the mosquito vector and has emerged as a target for intervention in eradication efforts. In this issue of Cell, a study from Marti, Clardy, and colleagues (Brancucci et al., 2017) describes a host-derived lipid lysophosphatidylcholine (LysoPC) that regulates sexual commitment.

Transcriptome and histone epigenome of Plasmodium vivax salivary-gland sporozoites point to tight regulatory control and mechanisms for liver-stage differentiation in relapsing malaria

Plasmodium vivax is the key obstacle to malaria elimination in Asia and Latin America, largely attributed to its ability to form resilient hypnozoites (sleeper cells) in the host liver that escape treatment and cause relapsing infections. The decision to form hypnozoites is made early in the liver infection and may already be set in sporozoites prior to invasion. To better understand these early stages of infection, we undertook a comprehensive transcriptomic and histone epigenetic characterization of P. vivax sporozoites. Through comparisons with recently published proteomic data for the P. vivax sporozoite, our study found that although highly transcribed, transcripts associated with functions needed for early infection of the vertebrate host are not detectable as proteins and may be regulated through translational repression. We identified differential transcription between the sporozoite and published transcriptomes of asexual blood stages and mixed versus hypnozoite-enriched liver stages. These comparisons point to multiple layers of transcriptional, post-transcriptional and post-translational control that appear active in sporozoites and to a lesser extent hypnozoites, but are largely absent in replicating liver schizonts or mixed blood stages. We also characterised histone epigenetic modifications in the P. vivax sporozoite and explored their role in regulating transcription. Collectively, these data support the hypothesis that the sporozoite is a tightly programmed stage to infect the human host and identify mechanisms for hypnozoite formation that may be further explored in liver stage models.

Plasmodium falciparum sexual differentiation in malaria patients is associated with host factors and GDV1-dependent genes

Plasmodium sexual differentiation is required for malaria transmission, yet much remains unknown about its regulation. Here, we quantify early gametocyte-committed ring (gc-ring) stage, P. falciparum parasites in 260 uncomplicated malaria patient blood samples 10 days before maturation to transmissible stage V gametocytes using a gametocyte conversion assay (GCA). Seventy six percent of the samples have gc-rings, but the ratio of gametocyte to asexual-committed rings (GCR) varies widely (0–78%). GCR correlates positively with parasitemia and is negatively influenced by fever, not hematocrit, age or leukocyte counts. Higher expression levels of GDV1-dependent genes, ap2-g, msrp1 and gexp5, as well as a gdv1 allele encoding H²¹⁷ are associated with high GCR, while high plasma lysophosphatidylcholine levels are associated with low GCR in the second study year. The results provide a view of sexual differentiation in the field and suggest key regulatory roles for clinical factors and gdv1 in gametocytogenesis in vivo.

Here, the authors quantify early gametocyte-committed ring (gc-ring) stage Plasmodium falciparum parasites in 260 malaria patients 10 days before maturation to transmissible stage V gametocytes, and show that the ratio of circulating gc-rings is positively correlated with parasitemia and negatively correlated with body temperature.

Upregulation of gametocytogenesis in anti-malarial drug-resistant Plasmodium falciparum

The deadliest form of human malaria is primarily caused by the protozoan parasite Plasmodium falciparum. These parasites establish pathogenicity in the human host with a very low number of sexual forms or gametocytes, which are transmitted to the mosquitoes. Several studies have reported that exposing artemisinin-sensitive P. falciparum rings to a low concentration of dihydroartemisinin (DHA) results in dormancy, and the artemisinin-induced dormant (AID) forms are recovered into normal growth stages after 5-20 days. In this study, artemisinin-resistant P. falciparum parasites were tested for the development of AID forms and their recovery. Interestingly, it was found that exposure of an asynchronous culture of artemisinin-resistant P. falciparum IPC 5202 to DHA, a line carrying a mutation in the PfK13 gene that is linked to artemisinin resistance, also results in dormancy. Both the ring and some late stages of these AID forms recovered after 10-15 days. Furthermore, a high proportion of the recovered dormant forms developed into sexual forms or gametocytes after 3-4 weeks, which is almost a 7-8 times higher rate of conversion of asexual to sexual forms (gametocytes) or the malaria transmissible forms. In contrast, only early ring forms of artemisinin-sensitive parasites recovered slowly, and additional exposure of these parasites to artemisinin resulted in complete clearance within a week. This is in contrast to the resistant parasites exposed to a second dose of artemisinin, which resulted in a very high rate of dormancy and recovery into sexual forms or gametocytes.

Clipped histone H3 is integrated into nucleosomes of DNA replication genes in the human malaria parasite Plasmodium falciparum

Post-translational modifications of histone H3 N-terminal tails are key epigenetic regulators of virulence gene expression and sexual commitment in the human malaria parasite Plasmodium falciparum Here, we identify proteolytic clipping of the N-terminal tail of nucleosome-associated histone H3 at amino acid position 21 as a new chromatin modification. A cathepsin C-like proteolytic clipping activity is observed in nuclear parasite extracts. Notably, an ectopically expressed version of clipped histone H3, PfH3p-HA, is targeted to the nucleus and integrates into mononucleosomes. Furthermore, chromatin immunoprecipitation and next-generation sequencing analysis identified PfH3p-HA as being highly enriched in the upstream region of six genes that play a key role in DNA replication and repair: In these genes, PfH3p-HA demarcates a specific 1.5 kb chromatin island adjacent to the open reading frame. Our results indicate that, in P. falciparum, the process of histone clipping may precede chromatin integration hinting at preferential targeting of pre-assembled PfH3p-containing nucleosomes to specific genomic regions. The discovery of a protease-directed mode of chromatin organization in P. falciparum opens up new avenues to develop new anti-malarials.

See-through observation of malaria parasite behaviors in the mosquito vector

Although it is known that malaria parasites proliferate in the midgut of mosquito vector, their detailed behaviors, from gamete maturation to formation of next generation sporozoite, have not been fully understood at cellular or molecular level. This is mainly attributed to technical difficulties of dissection and whole-mount observation, of delicate and opaque mosquito body contents. In addition, blood pigment surrounding parasites immediately after blood meal also complicates tracing mosquito-stage parasites. Recent revolutionary studies have overcome such negative factors in tissue observation by clearing organisms. CUBIC reagents succeeded to remove both light scattering and blood pigment from various mouse tissues, and to whole-organ image fluorescence-labeled cell structures. In this study, we utilized the advanced version of CUBIC technology and high sensitivity fluorescent markers for see-through observation of mosquito vector after engulfment of rodent malaria parasites to clarify their behaviors during mosquito stage. As a result, we succeeded to visualize oocysts, sporozoites, female gametes and ookinetes in the mosquito bodies without any dissection.

Babesia Life Cycle - When Phylogeny Meets Biology

Although Babesia represents an important worldwide veterinary threat and an emerging risk to humans, this parasite has been poorly studied as compared to Plasmodium, its malaria-causing relative. In fact, Babesia employs highly specific survival strategies during its intraerythrocytic development and its intricate journey through the tick vector. This review introduces a substantially extended molecular phylogeny of the order Piroplasmida, challenging previous taxonomic classifications. The intriguing developmental proficiencies of Babesia are highlighted and compared with those of other haemoparasitic Apicomplexa. Molecular mechanisms associated with distinctive events in the Babesia life cycle are emphasized as potential targets for the development of Babesia-specific treatments.

Malaria vaccines in the eradication era: current status and future perspectives

INTRODUCTION

The challenge to eradicate malaria is an enormous task that will not be achieved by current control measures, thus an efficacious and long-lasting malaria vaccine is required. The licensing of RTS, S/AS01 is a step forward in providing some protection, but a malaria vaccine that protects across multiple transmission seasons is still needed. To achieve this, inducing beneficial immune responses while minimising deleterious non-targeted effects will be essential.

AREAS COVERED

This article discusses the current challenges and advances in malaria vaccine development and reviews recent human clinical trials for each stage of infection. Pubmed and ScienceDirect were searched, focusing on cell mediated immunity and how T cell subsets might be targeted in future vaccines using novel adjuvants and emerging vaccine technologies.

EXPERT COMMENTARY

Despite decades of research there is no highly effective licensed malaria vaccine. However, there is cause for optimism as new adjuvants and vaccine systems emerge, and our understanding of correlates of protection increases, especially regarding cellular immunity. The new field of heterologous (non-specific) effects of vaccines also highlights the broader consequences of immunization. Importantly, the WHO led Malaria Vaccine Technology Roadmap illustrates that there is a political will among the global health community to make it happen.

Revisiting the initial steps of sexual development in the malaria parasite Plasmodium falciparum

Human to vector transmission of malaria requires that some blood-stage parasites abandon asexual growth and convert into non-replicating sexual forms called gametocytes. The initial steps of gametocytogenesis remain largely uncharacterized. Here, we study this part of the malaria life cycle in Plasmodium falciparum using PfAP2-G, the master regulator of sexual conversion, as a marker of commitment. We demonstrate the existence of PfAP2-G-positive sexually committed parasite stages that precede the previously known committed schizont stage. We also found that sexual conversion can occur by two different routes: the previously described route in which PfAP2-G-expressing parasites complete a replicative cycle as committed forms before converting into gametocytes upon re-invasion, or a direct route with conversion within the same cycle as initial PfAP2-G expression. The latter route is linked to early PfAP2-G expression in ring stages. Reanalysis of published single-cell RNA-sequencing (RNA-seq) data confirmed the presence of both routes. Consistent with these results, using plaque assays we observed that, in contrast to the prevailing model, many schizonts produced mixed plaques containing both asexual parasites and gametocytes. Altogether, our results reveal unexpected features of the initial steps of sexual development and extend the current view of this part of the malaria life cycle.

Inducing controlled cell cycle arrest and re-entry during asexual proliferation of Plasmodium falciparum malaria parasites

The life cycle of the malaria parasite Plasmodium falciparum is tightly regulated, oscillating between stages of intense proliferation and quiescence. Cyclic 48-hour asexual replication of Plasmodium is markedly different from cell division in higher eukaryotes, and mechanistically poorly understood. Here, we report tight synchronisation of malaria parasites during the early phases of the cell cycle by exposure to DL-α-difluoromethylornithine (DFMO), which results in the depletion of polyamines. This induces an inescapable cell cycle arrest in G₁ (~15 hours post-invasion) by blocking G₁/S transition. Cell cycle-arrested parasites enter a quiescent G₀-like state but, upon addition of exogenous polyamines, re-initiate their cell cycle. This ability to halt malaria parasites at a specific point in their cell cycle, and to subsequently trigger re-entry into the cell cycle, provides a valuable framework to investigate cell cycle regulation in these parasites. We subsequently used gene expression analyses to show that re-entry into the cell cycle involves expression of Ca²⁺-sensitive (cdpk4 and pk2) and mitotic kinases (nima and ark2), with deregulation of the pre-replicative complex associated with expression of pk2. Changes in gene expression could be driven through transcription factors MYB1 and two ApiAP2 family members. This new approach to parasite synchronisation therefore expands our currently limited toolkit to investigate cell cycle regulation in malaria parasites.

Rodent malaria models: insights into human disease and parasite biology

The use of rodents as model organisms to study human disease is based on the genetic and physiological similarities between the species. Successful molecular methods to generate transgenic reporter or humanized rodents has rendered rodents as powerful tools for understanding biological processes and host-pathogen interactions relevant to humans. In malaria research, rodent models have been pivotal for the study of liver stages, syndromes arising from blood stages of infection, and malaria transmission to and from the mammalian host. Importantly, many in vivo findings are comparable to pathology observed in humans only when adequate combinations of rodent strains and Plasmodium parasites are used.

Exploring parasite heterogeneity using single-cell RNA-seq reveals a gene signature among sexual stage Plasmodium falciparum parasites

The malaria parasite has a complex lifecycle, including several events of differentiation and stage progression, while actively evading immunity in both its mosquito and human hosts. Important parasite gene expression and regulation during these events remain hidden in rare populations of cells. Here, we combine a capillary-based platform for cell isolation with single-cell RNA-sequencing to transcriptionally profile 165 single infected red blood cells (iRBCs) during the intra-erythrocytic developmental cycle (IDC). Unbiased analyses of single-cell data grouped the cells into eight transcriptional states during IDC. Interestingly, we uncovered a gene signature from the single iRBC analyses that can successfully discriminate between developing asexual and sexual stage parasites at cellular resolution, and we verify five, previously undefined, gametocyte stage specific genes. Moreover, we show the capacity of detecting expressed genes from the variable gene families in single parasites, despite the sparse nature of data. In total, the single parasite transcriptomics holds promise for molecular dissection of rare parasite phenotypes throughout the malaria lifecycle.

Transcriptome analysis based detection of Plasmodium falciparum development in Anopheles stephensi mosquitoes

The Plasmodium life cycle within the mosquito involves the gamete, zygote, motile ookinete, and the oocyst stage that supports sporogony and sporozoite formation. We mapped the P. falciparum transcriptome as the parasite progresses through the oocyst stage of development on days 2, 4, 6, and 8 post-P. falciparum infectious blood meal. Through these genomic studies, we identified 212 novel transmission stage biomarkers including genes that are developmentally expressed at a single time point and genes that are pan-developmentally expressed at all four time points in P. falciparum oocysts. Validation of a small subset of genes at the transcriptional and translational level resulted in identification of a signature of genes/proteins that can detect parasites within the mosquito as early as day 2 post-infectious blood meal and can be used to distinguish early versus late stage P. falciparum oocyst development in the mosquito. Currently, circumsporozoite protein (CSP), which is detectable only after day 7 post-infection, is the only marker used for detection of P. falciparum infection in mosquitoes. Our results open the prospect to develop a non-CSP based detection assay for assessment of P. falciparum infection in mosquitoes and evaluate the effect of intervention measures on malaria transmission in an endemic setting.

Commit, hide and escape: the story of Plasmodium gametocytes

Malaria is the major cause of mortality and morbidity in tropical countries. The causative agent, Plasmodium sp., has a complex life cycle and is armed with various mechanisms which ensure its continuous transmission. Gametocytes represent the sexual stage of the parasite and are indispensable for the transmission of the parasite from the human host to the mosquito. Despite its vital role in the parasite's success, it is the least understood stage in the parasite's life cycle. The presence of gametocytes in asymptomatic populations and induction of gametocytogenesis by most antimalarial drugs warrants further investigation into its biology. With a renewed focus on malaria elimination and advent of modern technology available to biologists today, the field of gametocyte biology has developed swiftly, providing crucial insights into the molecular mechanisms driving sexual commitment. This review will summarise key current findings in the field of gametocyte biology and address the associated challenges faced in malaria detection, control and elimination.

Genome-wide real-time in vivo transcriptional dynamics during Plasmodium falciparum blood-stage development

Genome-wide analysis of transcription in the malaria parasite Plasmodium falciparum has revealed robust variation in steady-state mRNA abundance throughout the 48-h intraerythrocytic developmental cycle (IDC), suggesting that this process is highly dynamic and tightly regulated. Here, we utilize rapid 4-thiouracil (4-TU) incorporation via pyrimidine salvage to specifically label, capture, and quantify newly-synthesized RNA transcripts at every hour throughout the IDC. This high-resolution global analysis of the transcriptome captures the timing and rate of transcription for each newly synthesized mRNA in vivo, revealing active transcription throughout all IDC stages. Using a statistical model to predict the mRNA dynamics contributing to the total mRNA abundance at each timepoint, we find varying degrees of transcription and stabilization for each mRNA corresponding to developmental transitions. Finally, our results provide new insight into co-regulation of mRNAs throughout the IDC through regulatory DNA sequence motifs, thereby expanding our understanding of P. falciparum mRNA dynamics.

Transcriptomic analysis often doesn’t differentiate between newly synthesized and stabilized mRNAs. Using rapid 4-thiouracil incorporation, Painter et al. here define genome-wide active transcription throughout Plasmodium blood-stage developmental stages and identify associated regulatory DNA sequence motifs.

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.

Infectivity of Chronic Malaria Infections and Its Consequences for Control and Elimination

Assessing the importance of targeting the chronic Plasmodium falciparum malaria reservoir is pivotal as the world moves toward malaria eradication. Through the lens of a mathematical model, we show how, for a given malaria prevalence, the relative infectivity of chronic individuals determines what intervention tools are predicted be the most effective. Crucially, in a large part of the parameter space where elimination is theoretically possible, it can be achieved solely through improved case management. However, there are a significant number of settings where malaria elimination requires not only good vector control but also a mass drug administration campaign. Quantifying the relative infectiousness of chronic malaria across a range of epidemiological settings would provide essential information for the design of effective malaria elimination strategies. Given the difficulties obtaining this information, we also provide a set of epidemiological metrics that can be used to guide policy in the absence of such data.

Integrated pathogen load and dual transcriptome analysis of systemic host-pathogen interactions in severe malaria

The pathogenesis of infectious diseases depends on the interaction of host and pathogen. In Plasmodium falciparum malaria, host and parasite processes can be assessed by dual RNA sequencing of blood from infected patients. We performed dual transcriptome analyses on samples from 46 malaria-infected Gambian children to reveal mechanisms driving the systemic pathophysiology of severe malaria. Integrating these transcriptomic data with estimates of parasite load and detailed clinical information allowed consideration of potentially confounding effects due to differing leukocyte proportions in blood, parasite developmental stage, and whole-body pathogen load. We report hundreds of human and parasite genes differentially expressed between severe and uncomplicated malaria, with distinct profiles associated with coma, hyperlactatemia, and thrombocytopenia. High expression of neutrophil granule-related genes was consistently associated with all severe malaria phenotypes. We observed severity-associated variation in the expression of parasite genes, which determine cytoadhesion to vascular endothelium, rigidity of infected erythrocytes, and parasite growth rate. Up to 99% of human differential gene expression in severe malaria was driven by differences in parasite load, whereas parasite gene expression showed little association with parasite load. Coexpression analyses revealed interactions between human and P. falciparum, with prominent co-regulation of translation genes in severe malaria between host and parasite. Multivariate analyses suggested that increased expression of granulopoiesis and interferon-γ-related genes, together with inadequate suppression of type 1 interferon signaling, best explained severity of infection. These findings provide a framework for understanding the contributions of host and parasite to the pathogenesis of severe malaria and identifying new treatments.