Regulation of Sexual Commitment and Gametocytogenesis in Malaria Parasites

Host factors influencing sexual differentiation and transmission of Plasmodium: A comprehensive review

Malaria, a severe parasitic disease caused by Plasmodium infections, remains a major global health challenge. Efforts to eradicate malaria are complicated by the parasite's intricate life cycle, which alternates between vertebrate hosts and mosquito vectors. Host-derived factors and parasite-sourced components exert crucial roles in regulating this biological process. This review explores the critical role of host-derived factors in shaping Plasmodium sexual differentiation and transmission. We examine how vertebrate and mosquito host-specific factors either promote or restrict parasite development, influencing the transition from vertebrates to mosquitoes. Understanding these host-mediated mechanisms is crucial for developing novel transmission-blocking strategies to reduce malaria prevalence. By highlighting key interactions between hosts and parasites, this review provides insights into potential interventions that could disrupt Plasmodium transmission and contribute to malaria control efforts.

PfGCN5 is essential for Plasmodium falciparum survival and transmission and regulates Pf H2B.Z acetylation and chromatin structure

Plasmodium falciparum causes most malaria deaths. Its developmental transitions and environmental adaptation are partially regulated by epigenetic mechanisms. Plasmodium falciparum GCN5 (PfGCN5) is an epigenetic regulator that acetylates lysines and can also bind to acetylated lysine residues on histones via its bromodomain (BRD). Here, we showed that PfGCN5 was essential for parasite transmission and survival in human blood and mosquitoes. PfGCN5 regulated genes important for metabolism and development and its BRD was required at euchromatic gene promoters for their proper expression and for acetylation of the variant histone Pf H2B.Z. However, PfGCN5 was most abundant in heterochromatin and loss of the PfGCN5 BRD de-repressed heterochromatic genes and increased levels of acetylated Pf H2B.Z in heterochromatin. The PfGCN5 BRD-binding compound L-45 phenocopied deletion of the PfGCN5 BRD, identifying PfGCN5 as a promising drug target for BRD inhibitors. Thus, PfGCN5 appears to directly contribute to activating euchromatic promoters, but PfGCN5 is also critical for maintaining repressive heterochromatin structure.

Ectopic overexpression of Plasmodium falciparum DNA-/RNA-binding Alba proteins misregulates virulence gene homeostasis during asexual blood development

Alba domain-containing proteins are ubiquitously found in archaea and eukaryotes. By binding to either DNA, RNA, or DNA:RNA hybrids, these proteins function in genome stabilization, chromatin organization, gene regulation, and/or translational modulation. In the malaria parasite Plasmodium falciparum, six Alba domain proteins PfAlba1-6 have been described, of which PfAlba1 has emerged as a "master regulator" of translation during parasite intra-erythrocytic development (IED). Given that a tight control of gene expression is especially important during IED, when malaria pathogenesis manifests, in this study, we focus on three other P. falciparum Albas, PfAlba2-4. Because genetic manipulation of the genomic loci of PfAlba2-4 was unsuccessful, we overexpressed each of these proteins from an episome under a strong constitutive promoter. We observed that PfAlba2 or PfAlba3 overexpression strongly reduced parasite growth and impacted IED stage transitions. In contrast, elevated levels of PfAlba4 were well-tolerated by the parasite. In keeping with this, differential gene expression analysis using RNA-seq of PfAlba2 or PfAlba3 overexpressing strains revealed a significant misregulation of mRNAs encoding virulence factors, such as those related to erythrocyte invasion; a general repression of var gene expression was also apparent. PfAlba4 overexpression, on the other hand, did not significantly perturb the steady-state transcriptome of IED stages and appeared to enhance var mRNA levels. Moreover, distinct sets of genes were targeted by each PfAlba for regulation. Taken together, this study highlights the nonredundant roles of PfAlba proteins in the P. falciparum IED, emphasizing their importance in subtelomeric chromatin biology and RNA regulation.IMPORTANCEThe malaria parasite Plasmodium falciparum tightly controls the expression of its genes at the epigenetic, transcriptional, post-transcriptional, and translational levels to synthesize essential proteins, including virulence factors, in a timely and spatially coordinated manner. A family of six proteins implicated in this process is called PfAlba, characterized by the presence of the DNA-, RNA- or DNA:RNA hybrid-binding Alba domain. To better understand the cellular pathways regulated by this protein family, we overexpressed three PfAlbas during P. falciparum intra-erythrocytic growth and found that high levels of PfAlba2 and PfAlba3 were detrimental to parasite development. This was accompanied by significant changes in the parasite's transcriptome, either with regards to mRNA steady-state levels or expression timing. PfAlba4 overexpression, on the other hand, was well-tolerated by the parasite. Overall, our results delineate specific pathways targeted by individual PfAlbas for regulation and link PfAlba2/PfAlba3 to mutually exclusive expression of the virulence-promoting surface antigen PfEMP1.

Targeting Bottlenecks in Malaria Transmission: Antibody-Epitope Descriptions Guide the Design of Next-Generation Biomedical Interventions

Malaria continues to pose a significant burden to global health. Thus, a strong need exists for the development of a diverse panel of intervention strategies and modalities to combat malaria and achieve elimination and eradication goals. Deploying interventions that target bottlenecks in the transmission life cycle of the causative agent of malaria, Plasmodium parasites, is an attractive strategy. The development of highly potent antibody-based biologics, including vaccines, can be greatly facilitated by an in-depth molecular understanding of antibody-epitope interactions. Here, we provide an overview of structurally characterized antibodies targeting lead vaccine candidates expressed during the bottlenecks of the Plasmodium life cycle which include the pre-erythrocytic and sexual stages. The repeat region of the circumsporozoite protein (CSP), domain 1 of Pfs230 and domains 1 and 3 of Pfs48/45 are critical Plasmodium regions targeted by the most potent antibodies at the two bottlenecks of transmission, with other promising targets emerging and requiring further characterization.

Genome-wide gene expression profiles throughout human malaria parasite liver stage development in humanized mice

Gene expression of Plasmodium falciparum (Pf) liver-stage (LS) parasites has remained poorly characterized, although they are major vaccine and drug targets. Using a human liver-chimaeric mouse model and a fluorescent parasite line (PfNF54CSPGFP), we isolated PfLS and performed transcriptomics on key LS developmental phases. We linked clustered gene expression to ApiAP2, a major family of transcription factors that regulate the parasite life cycle. This provided insights into transcriptional regulation of LS infection and expression of essential LS metabolic and biosynthetic pathways. We observed expression of antigenically variant PfEMP1 proteins and the major Pf protein export machine PTEX and identified protein candidates that might be exported by LS parasites. Comparing Pf and P. vivax LS transcriptomes, we uncovered differences in their expression of sexual commitment factors. This data will aid LS research and vaccine and drug target identification for prevention of malaria infection.

The Plasmodium falciparum histone methyltransferase PfSET10 is dispensable for the regulation of antigenic variation and gene expression in blood-stage parasites

The malaria parasite Plasmodium falciparum employs antigenic variation of the virulence factor P. falciparum erythrocyte membrane protein 1 (PfEMP1) to escape adaptive immune responses during blood infection. Antigenic variation of PfEMP1 occurs through epigenetic switches in the mutually exclusive expression of individual members of the multi-copy var gene family. var genes are located in perinuclear clusters of transcriptionally inactive heterochromatin. Singular var gene activation is linked to locus repositioning into a dedicated zone at the nuclear periphery and deposition of histone 3 lysine 4 di-/trimethylation (H3K4me2/3) and H3K9 acetylation marks in the promoter region. While previous work identified the putative H3K4-specific methyltransferase PfSET10 as an essential enzyme and positive regulator of var gene expression, a recent study reported conflicting data. Here, we used iterative genome editing to engineer a conditional PfSET10 knockout line tailored to study the function of PfSET10 in var gene regulation. We demonstrate that PfSET10 is not required for mutually exclusive var gene expression and switching. We also show that PfSET10 is dispensable not only for asexual parasite proliferation but also for sexual conversion and gametocyte differentiation. Furthermore, comparative RNA-seq experiments revealed that PfSET10 plays no obvious role in regulating gene expression during asexual parasite development and gametocytogenesis. Interestingly, however, PfSET10 shows different subnuclear localization patterns in asexual and sexual stage parasites and female-specific expression in mature gametocytes. In summary, our work confirms in detail that PfSET10 is not involved in regulating var gene expression and is not required for blood-stage parasite viability, indicating PfSET10 may be important for life cycle progression in the mosquito vector or during liver stage development.IMPORTANCEThe malaria parasite Plasmodium falciparum infects hundreds of millions of people every year. To survive and proliferate in the human bloodstream, the parasites need to escape recognition by the host's immune system. To achieve this, P. falciparum can change the expression of surface antigens via a process called antigenic variation. This fascinating survival strategy is based on infrequent switches in the expression of single members of the var multigene family. Previous research reported conflicting results on the role of the epigenetic regulator PfSET10 in controlling mutually exclusive var gene expression and switching. Here, we unequivocally demonstrate that PfSET10 is neither required for antigenic variation nor the expression of any other proteins during blood-stage infection. This information is critical in directing our attention toward exploring alternative molecular mechanisms underlying the control of antigenic variation and investigating the function of PfSET10 in other life cycle stages.

Reporter parasite lines: valuable tools for the study of Plasmodium biology

The human malaria parasite Plasmodium falciparum causes the most severe form of malaria in endemic regions and is transmitted via mosquito bites. To better understand the biology of this deadly pathogen, a variety of P. falciparum reporter lines have been generated using transgenic approaches to express reporter proteins, such as fluorescent proteins and luciferases. This review discusses the advances in recently generated P. falciparum transgenic reporter lines, which will aid in the investigation of parasite physiology and the discovery of novel antimalarial drugs. Future prospects for the generation of new and superior human malaria parasite reporter lines are also discussed, and unresolved questions in malaria biology are highlighted to help boost support for the development and implementation of malaria treatments.

The three Plasmodium falciparum Aurora-related kinases display distinct temporal and spatial associations with mitotic structures in asexual blood stage parasites and gametocytes

Aurora kinases are crucial regulators of mitotic cell cycle progression in eukaryotes. The protozoan malaria parasite Plasmodium falciparum replicates via schizogony, a specialized mode of cell division characterized by consecutive asynchronous rounds of nuclear division by closed mitosis followed by a single cytokinesis event producing dozens of daughter cells. P. falciparum encodes three Aurora-related kinases (PfARKs) that have been reported essential for parasite proliferation, but their roles in regulating schizogony have not yet been explored in great detail. Here, we engineered transgenic parasite lines expressing GFP-tagged PfARK1-3 to provide a systematic analysis of their expression timing and subcellular localization throughout schizogony as well as in the non-dividing gametocyte stages, which are essential for malaria transmission. We demonstrate that all three PfARKs display distinct and highly specific and exclusive spatiotemporal associations with the mitotic machinery. In gametocytes, PfARK3 is undetectable, and PfARK1 and PfARK2 show male-specific expression in late-stage gametocytes, consistent with their requirement for endomitosis during male gametogenesis in the mosquito vector. Our combined data suggest that PfARK1 and PfARK2 have non-overlapping roles in centriolar plaque maturation, assembly of the mitotic spindle, kinetochore-spindle attachment and chromosome segregation, while PfARK3 seems to be exquisitely involved in daughter cell cytoskeleton assembly and cytokinesis. These important new insights provide a reliable foundation for future research aiming at the functional investigation of these divergent and possibly drug-targetable Aurora-related kinases in mitotic cell division of P. falciparum and related apicomplexan parasites.IMPORTANCEMalaria parasites replicate via non-conventional modes of mitotic cell division, such as schizogony, employed by the disease-causing stages in the human blood or endomitosis during male gametogenesis in the mosquito vector. Understanding the molecular mechanisms regulating cell division in these divergent unicellular eukaryotes is not only of scientific interest but also relevant to identify potential new antimalarial drug targets. Here, we carefully examined the subcellular localization of all three Plasmodium falciparum Aurora-related kinases (ARKs), distantly related homologs of Aurora kinases that coordinate mitosis in model eukaryotes. Detailed fluorescence microscopy-based analyses revealed distinct, specific, and exclusive spatial associations for each parasite ARK with different components of the mitotic machinery and at different phases of the cell cycle during schizogony and gametocytogenesis. This comprehensive set of results closes important gaps in our fragmentary knowledge on this important group of kinases and offers a valuable source of information for future functional studies.

PfHDAC1 is an essential regulator of P. falciparum asexual proliferation and host cell invasion genes with a dynamic genomic occupancy responsive to artemisinin stress

Plasmodium falciparum, the deadly protozoan parasite responsible for malaria, has a tightly regulated gene expression profile closely linked to its intraerythrocytic development cycle. Epigenetic modifiers of the histone acetylation code have been identified as key regulators of the parasite's transcriptome but require further investigation. In this study, we map the genomic distribution of Plasmodium falciparum histone deacetylase 1 (PfHDAC1) across the erythrocytic asexual development cycle and find it has a dynamic occupancy over a wide array of developmentally relevant genes. Overexpression of PfHDAC1 results in a progressive increment in parasite load over consecutive rounds of the asexual infection cycle and is associated with enhanced gene expression of multiple families of host cell invasion factors (merozoite surface proteins, rhoptry proteins, etc.) and with increased merozoite invasion efficiency. With the use of class-specific inhibitors, we demonstrate that PfHDAC1 activity in parasites is crucial for timely intraerythrocytic development. Interestingly, overexpression of PfHDAC1 results in decreased sensitivity to frontline-drug dihydroartemisinin in parasites. Furthermore, we identify that artemisinin exposure can interfere with PfHDAC1 abundance and chromatin occupancy, resulting in enrichment over genes implicated in response/resistance to artemisinin. Finally, we identify that dihydroartemisinin exposure can interrupt the in vitro catalytic deacetylase activity and post-translational phosphorylation of PfHDAC1, aspects that are crucial for its genomic function. Collectively, our results demonstrate PfHDAC1 to be a regulator of critical functions in asexual parasite development and host invasion, which is responsive to artemisinin exposure stress and deterministic of resistance to it.

IMPORTANCE

Malaria is a major public health problem, with the parasite Plasmodium falciparum causing most of the malaria-associated mortality. It is spread by the bite of infected mosquitoes and results in symptoms such as cyclic fever, chills, and headache. However, if left untreated, it can quickly progress to a more severe and life-threatening form. The World Health Organization currently recommends the use of artemisinin combination therapy, and it has worked as a gold standard for many years. Unfortunately, certain countries in southeast Asia and Africa, burdened with a high prevalence of malaria, have reported cases of drug-resistant infections. One of the major problems in controlling malaria is the emergence of artemisinin resistance. Population genomic studies have identified mutations in the Kelch13 gene as a molecular marker for artemisinin resistance. However, several reports thereafter indicated that Kelch13 is not the main mediator but rather hinted at transcriptional deregulation as a major determinant of drug resistance. Earlier, we identified PfGCN5 as a global regulator of stress-responsive genes, which are known to play a central role in artemisinin resistance generation. In this study, we have identified PfHDAC1, a histone deacetylase as a cell cycle regulator, playing an important role in artemisinin resistance generation. Taken together, our study identified key transcriptional regulators that play an important role in artemisinin resistance generation.

Post-Translational Modifications of Proteins of Malaria Parasites during the Life Cycle

Post-translational modifications (PTMs) are essential for regulating protein functions, influencing various fundamental processes in eukaryotes. These include, but are not limited to, cell signaling, protein trafficking, the epigenetic control of gene expression, and control of the cell cycle, as well as cell proliferation, differentiation, and interactions between cells. In this review, we discuss protein PTMs that play a key role in the malaria parasite biology and its pathogenesis. Phosphorylation, acetylation, methylation, lipidation and lipoxidation, glycosylation, ubiquitination and sumoylation, nitrosylation and glutathionylation, all of which occur in malarial parasites, are reviewed. We provide information regarding the biological significance of these modifications along all phases of the complex life cycle of Plasmodium spp. Importantly, not only the parasite, but also the host and vector protein PTMs are often crucial for parasite growth and development. In addition to metabolic regulations, protein PTMs can result in epitopes that are able to elicit both innate and adaptive immune responses of the host or vector. We discuss some existing and prospective results from antimalarial drug discovery trials that target various PTM-related processes in the parasite or host.

Identification of an inhibitory pocket in falcilysin provides a new avenue for malaria drug development

Identification of new druggable protein targets remains the key challenge in the current antimalarial development efforts. Here we used mass-spectrometry-based cellular thermal shift assay (MS-CETSA) to identify potential targets of several antimalarials and drug candidates. We found that falcilysin (FLN) is a common binding partner for several drug candidates such as MK-4815, MMV000848, and MMV665806 but also interacts with quinoline drugs such as chloroquine and mefloquine. Enzymatic assays showed that these compounds can inhibit FLN proteolytic activity. Their interaction with FLN was explored systematically by isothermal titration calorimetry and X-ray crystallography, revealing a shared hydrophobic pocket in the catalytic chamber of the enzyme. Characterization of transgenic cell lines with lowered FLN expression demonstrated statistically significant increases in susceptibility toward MK-4815, MMV000848, and several quinolines. Importantly, the hydrophobic pocket of FLN appears amenable to inhibition and the structures reported here can guide the development of novel drugs against malaria.

The Multiple Roles of LCCL Domain-Containing Proteins for Malaria Parasite Transmission

Multi-protein complexes are crucial for various essential biological processes of the malaria parasite Plasmodium, such as protein synthesis, host cell invasion and adhesion. Especially during the sexual phase of the parasite, which takes place in the midgut of the mosquito vector, protein complexes are required for fertilization, sporulation and ultimately for the successful transmission of the parasite. Among the most noticeable protein complexes of the transmission stages are the ones formed by the LCCL domain-containing protein family that play critical roles in the generation of infective sporozoites. The six members of this protein family are characterized by numerous adhesive modules and domains typically found in secreted proteins. This review summarizes the findings of expression and functional studies on the LCCL domain-containing proteins of the human pathogenic P. falciparum and the rodent-infecting P. berghei and discusses the common features and differences of the homologous proteins.

In vitro production of cat-restricted Toxoplasma pre-sexual stages

Sexual reproduction of Toxoplasma gondii, confined to the felid gut, remains largely uncharted owing to ethical concerns regarding the use of cats as model organisms. Chromatin modifiers dictate the developmental fate of the parasite during its multistage life cycle, but their targeting to stage-specific cistromes is poorly described1,2. Here we found that the transcription factors AP2XII-1 and AP2XI-2 operate during the tachyzoite stage, a hallmark of acute toxoplasmosis, to silence genes necessary for merozoites, a developmental stage critical for subsequent sexual commitment and transmission to the next host, including humans. Their conditional and simultaneous depletion leads to a marked change in the transcriptional program, promoting a full transition from tachyzoites to merozoites. These in vitro-cultured pre-gametes have unique protein markers and undergo typical asexual endopolygenic division cycles. In tachyzoites, AP2XII-1 and AP2XI-2 bind DNA as heterodimers at merozoite promoters and recruit MORC and HDAC3 (ref. 1), thereby limiting chromatin accessibility and transcription. Consequently, the commitment to merogony stems from a profound epigenetic rewiring orchestrated by AP2XII-1 and AP2XI-2. Successful production of merozoites in vitro paves the way for future studies on Toxoplasma sexual development without the need for cat infections and holds promise for the development of therapies to prevent parasite transmission.

Novel secretory organelles of parasite origin - at the center of host-parasite interaction

Reorganization of cell organelle-deprived host red blood cells by the apicomplexan malaria parasite Plasmodium falciparum enables their cytoadherence to endothelial cells that line the microvasculature. This increases the time red blood cells infected with mature developmental stages remain within selected organs such as the brain to avoid the spleen passage, which can lead to severe complications and cumulate in patient death. The Maurer's clefts are a novel secretory organelle of parasite origin established by the parasite in the cytoplasm of the host red blood cell in order to facilitate the establishment of cytoadherence by conducting the trafficking of immunovariant adhesins to the host cell surface. Another important function of the organelle is the sorting of other proteins the parasite traffics into its host cell. Although the organelle is of high importance for the pathology of malaria, additional putative functions, structure, and genesis remain shrouded in mystery more than a century after its discovery. In this review, we highlight our current knowledge about the Maurer's clefts and other novel secretory organelles established within the host cell cytoplasm by human-pathogenic malaria parasites and other parasites that reside within human red blood cells.

A Candidate Bacterial-Type Amino Acid Decarboxylase Is Essential for Male Gamete Exflagellation and Mosquito Transmission of the Malaria Parasite

A frequent side effect of chemotherapy against malaria parasite blood infections is a dramatic induction of the sexual blood stages, thereby enhancing the risk of future malaria transmissions. The polyamine biosynthesis pathway has been suggested as a candidate target for transmission-blocking anti-malarial drug development. Herein, we describe the role of a bacterial-type amino acid decarboxylase (AAD) in the life cycle of the malaria model parasite Plasmodium yoelii. Hallmarks of AAD include a conserved catalytic lysine residue and high-level homology to arginine/lysine/ornithine decarboxylases of pathogenic bacteria. By targeted gene deletion, we show that AAD plays an essential role in the exflagellation of microgametes, resulting in complete absence of sporozoites in the mosquito vector. These data highlight the central role of the biosysthesis of polyamines in the final steps of male gamete sexual development of the malaria parasite and, hence, onward transmission to mosquitoes.

A versatile Plasmodium falciparum reporter line expressing NanoLuc enables highly sensitive multi-stage drug assays

Transgenic luciferase-expressing Plasmodium falciparum parasites have been widely used for the evaluation of anti-malarial compounds. Here, to screen for anti-malarial drugs effective against multiple stages of the parasite, we generate a P. falciparum reporter parasite that constitutively expresses NanoLuciferase (NanoLuc) throughout its whole life cycle. The NanoLuc-expressing P. falciparum reporter parasite shows a quantitative NanoLuc signal in the asexual blood, gametocyte, mosquito, and liver stages. We also establish assay systems to evaluate the anti-malarial activity of compounds at the asexual blood, gametocyte, and liver stages, and then determine the 50% inhibitory concentration (IC₅₀) value of several anti-malarial compounds. Through the development of this robust high-throughput screening system, we identify an anti-malarial compound that kills the asexual blood stage parasites. Our study highlights the utility of the NanoLuc reporter line, which may advance anti-malarial drug development through the improved screening of compounds targeting the human malarial parasite at multiple stages.

The role of long noncoding RNAs in malaria parasites

The human malaria parasites, including Plasmodium falciparum, persist as a major cause of global morbidity and mortality. The recent stalling of progress toward malaria elimination substantiates a need for novel interventions. Controlled gene expression is central to the parasite's numerous life cycle transformations and adaptation. With few specific transcription factors (TFs) identified, crucial roles for chromatin states and epigenetics in parasite transcription have become evident. Although many chromatin-modifying enzymes are known, less is known about which factors mediate their impacts on transcriptional variation. Like those of higher eukaryotes, long noncoding RNAs (lncRNAs) have recently been shown to have integral roles in parasite gene regulation. This review aims to summarize recent developments and key findings on the role of lncRNAs in P. falciparum.

Genetic Ablation of a Female-Specific Apetala 2 Transcription Factor Blocks Oocyst Shedding in Cryptosporidium parvum

The apicomplexan parasite Cryptosporidium is a leading global cause of diarrheal disease, and the infection poses a particularly grave threat to young children and those with weakened immune function. Infection occurs by ingestion of meiotic spores called oocysts, and transmission relies on fecal shedding of new oocysts. The entire life cycle thus occurs in a single host and features asexual as well as sexual forms of replication. Here, we identify and locus tag two Apetala 2-type (AP2) transcription factors and demonstrate that they are exclusively expressed in male and female gametes, respectively. To enable functional studies of essential genes in Cryptosporidium parvum, we develop and validate a small-molecule-inducible gene excision system, which we apply to the female factor AP2-F to achieve conditional gene knockout. Analyzing this mutant, we find the factor to be dispensable for asexual growth and early female fate determination in vitro but to be required for oocyst shedding in infected animals in vivo. Transcriptional analyses conducted in the presence or absence of AP2-F revealed that the factor controls the transcription of genes encoding crystalloid body proteins, which are exclusively expressed in female gametes. In C. parvum, the organelle is restricted to sporozoites, and its loss in other apicomplexan parasites leads to blocked transmission. Overall, our development of conditional gene ablation in C. parvum provides a robust method for genetic analysis in this parasite that enabled us to identify AP2-F as an essential regulator of transcription required for oocyst shedding and transmission. IMPORTANCE The parasite Cryptosporidium infects millions of people worldwide each year, leading to life-threatening diarrheal disease in young children and immunosuppressed individuals. There is no vaccine and only limited treatment. Transmission occurs via the fecal-oral route by an environmentally resilient spore-like oocyst. Infection takes place in the intestinal epithelium, where parasites initially propagate asexually before transitioning to male and female gametes, with sex leading to the formation of new oocysts. The essential role of sexual development for continuous infection and transmission makes it an attractive target for therapy and prevention. To study essential genes and potential drug targets across the life cycle, we established inducible gene excision for C. parvum. We determined that the female-specific transcription factor AP2-F is not required for asexual growth and early female development in vitro but is necessary for oocyst shedding in vivo. This work enhances the genetic tools available to study Cryptosporidium gene function.

Identification of Potential Drug Targets in Erythrocyte Invasion Pathway of Plasmodium falciparum

The erythrocyte invasion phase plays a critical role in multiplication, sexual determination, and drug resistance in Plasmodium falciparum. In order to identify the critical genes and pathways in the erythrocyte invasion phase, the gene set (GSE129949) and the RNA-Seq count data for the W2mef strain were used for further analysis. An integrative bioinformatics study was performed to scrutinize genes as potential drug targets. 487 differentially expressed genes (DEGs) with an adjusted P value < 0.001 enriched 47 Gene Ontology (GO) terms that were over-represented based on hyper-geometric analysis P value < 0.01. Protein-Protein interaction network analysis was done using DEGs with higher confidence interactions (PPI score threshold = 0.7). MCODE and cytoHubba apps were utilized to define the hub proteins and rank them based on multiple topological analyses and MCODE scores. Furthermore, Gene Set Enrichment Analysis (GSEA) was carried out by using 322 gene sets from the MPMP database. The genes involved in multiple significant gene sets were determined by leading-edge analysis. Our study identified six genes encoding proteins that could be potential drug targets involved in the erythrocyte invasion phase related to merozoites motility, cell-cycle regulation, G-dependent protein kinase phosphorylation in schizonts, control of microtubule assembly, and sexual commitment. The druggability of those proteins was calculated based on the DCI (Drug Confidence Index) and predicted binding pockets' values. The protein that showed the best binding pocket value was subjected to deep learning-based virtual screening. The study identified the best small molecule inhibitors in terms of drug-binding score against the proteins for inhibitor identification.

Patterns of Heterochromatin Transitions Linked to Changes in the Expression of Plasmodium falciparum Clonally Variant Genes

The survival of malaria parasites in the changing human blood environment largely depends on their ability to alter gene expression by epigenetic mechanisms. The active state of Plasmodium falciparum clonally variant genes (CVGs) is associated with euchromatin characterized by the histone mark H3K9ac, whereas the silenced state is characterized by H3K9me3-based heterochromatin. Expression switches are linked to euchromatin-heterochromatin transitions, but these transitions have not been characterized for the majority of CVGs. To define the heterochromatin distribution patterns associated with the alternative transcriptional states of CVGs, we compared H3K9me3 occupancy at a genome-wide level among several parasite subclones of the same genetic background that differed in the transcriptional state of many CVGs. We found that de novo heterochromatin formation or the complete disruption of a heterochromatin domain is a relatively rare event, and for the majority of CVGs, expression switches can be explained by the expansion or retraction of heterochromatin domains. We identified different modalities of heterochromatin changes linked to transcriptional differences, but despite this complexity, heterochromatin distribution patterns generally enable the prediction of the transcriptional state of specific CVGs. We also found that in some subclones, several var genes were simultaneously in an active state. Furthermore, the heterochromatin levels in the putative regulatory region of the gdv1 antisense noncoding RNA, a regulator of sexual commitment, varied between parasite lines with different sexual conversion rates. IMPORTANCE The malaria parasite P. falciparum is responsible for more than half a million deaths every year. P. falciparum clonally variant genes (CVGs) mediate fundamental host-parasite interactions and play a key role in parasite adaptation to fluctuations in the conditions of the human host. The expression of CVGs is regulated at the epigenetic level by changes in the distribution of a type of chromatin called heterochromatin. Here, we describe at a genome-wide level the changes in the heterochromatin distribution associated with the different transcriptional states of CVGs. Our results also reveal a likely role for heterochromatin at a particular locus in determining the parasite investment in transmission to mosquitoes. Additionally, this data set will enable the prediction of the transcriptional state of CVGs from epigenomic data, which is important for the study of parasite adaptation to the conditions of the host in natural malaria infections.

In vitro production of cat-restricted Toxoplasma pre-sexual stages by epigenetic reprogramming

Sexual reproduction of Toxoplasma gondii , which is restricted to the small intestine of felids, is sparsely documented, due to ethical concerns surrounding the use of cats as model organisms. Chromatin modifiers dictate the developmental fate of the parasite during its multistage life cycle, but their targeting to stage-specific cistromes is poorly described 1 . In this study, we found that transcription factors AP2XII-1 and AP2XI-2, expressed in tachyzoite stage that causes acute toxoplasmosis, can silence genes necessary for merozoites, a developmental stage critical for sexual commitment and transmission to the next host, including humans. Their conditional and simultaneous depletion leads to a drastic change in the transcriptional program, promoting a complete transition from tachyzoites to merozoites. Pre-gametes produced in vitro under these conditions are characterized by specific protein markers and undergo typical asexual endopolygenic division cycles. In tachyzoites, AP2XII-1 and AP2XI-2 bind DNA as heterodimers at merozoite promoters and recruit the epigenitors MORC and HDAC3 1 , which in turn restrict the accessibility of chromatin to the transcriptional machinery. Thus, the commitment to merogony stems from a profound epigenetic rewiring orchestrated by AP2XII-1 and AP2XI-2. This effective in vitro culture of merozoites paves the way to explore Toxoplasma sexual reproduction without the need to infect kittens and has potential for the development of therapeutics to block parasite transmission.

High-throughput analysis of the transcriptional patterns of sexual genes in malaria

BACKGROUND

Plasmodium falciparum (Pf) is the leading protozoan causing malaria, the most devastating parasitic disease. To ensure transmission, a small subset of Pf parasites differentiate into the sexual forms (gametocytes). Since the abundance of these essential parasitic forms is extremely low within the human host, little is currently known about the molecular regulation of their sexual differentiation, highlighting the need to develop tools to investigate Pf gene expression during this fundamental mechanism.

METHODS

We developed a high-throughput quantitative Reverse-Transcription PCR (RT-qPCR) platform to robustly monitor Pf transcriptional patterns, in particular, systematically profiling the transcriptional pattern of a large panel of gametocyte-related genes (GRG). Initially, we evaluated the technical performance of the systematic RT-qPCR platform to ensure it complies with the accepted quality standards for: (i) RNA extraction, (ii) cDNA synthesis and (iii) evaluation of gene expression through RT-qPCR. We then used this approach to monitor alterations in gene expression of a panel of GRG upon treatment with gametocytogenesis regulators.

RESULTS

We thoroughly elucidated GRG expression profiles under treatment with the antimalarial drug dihydroartemisinin (DHA) or the metabolite choline over the course of a Pf blood cycle (48 h). We demonstrate that both significantly alter the expression pattern of PfAP2-G, the gametocytogenesis master regulator. However, they also markedly modify the developmental rate of the parasites and thus might bias the mRNA expression. Additionally, we screened the effect of the metabolites lactate and kynurenic acid, abundant in severe malaria, as potential regulators of gametocytogenesis.

CONCLUSIONS

Our data demonstrate that the high-throughput RT-qPCR method enables studying the immediate transcriptional response initiating gametocytogenesis of the parasites from a very low volume of malaria-infected RBC samples. The obtained data expand the current knowledge of the initial alterations in mRNA profiles of GRG upon treatment with reported regulators. In addition, using this method emphasizes that asexual parasite stage composition is a crucial element that must be considered when interpreting changes in GRG expression by RT-qPCR, specifically when screening for novel compounds that could regulate Pf sexual differentiation.

Protocols for Plasmodium gametocyte production in vitro: an integrative review and analysis

BACKGROUND

The production of Plasmodium gametocytes in vitro is a real challenge. Many protocols have been described, but few have resulted in the production of viable and infectious gametocytes in sufficient quantities to conduct research on-but not limited to-transmission-blocking drug and vaccine development. The aim of this review was to identify and discuss gametocyte production protocols that have been developed over the last two decades.

METHODS

We analyzed the original gametocyte production protocols published from 2000 onwards based on a literature search and a thorough review. A systematic review was performed of relevant articles identified in the PubMed, Web of Sciences and ScienceDirect databases.

RESULTS

A total 23 studies on the production of Plasmodium gametocytes were identified, 19 involving in vitro Plasmodium falciparum, one involving Plasmodium knowlesi and three involving ex vivo Plasmodium vivax. Of the in vitro studies, 90% used environmental stressors to trigger gametocytogenesis. Mature gametocytemia of up to 4% was reported.

CONCLUSIONS

Several biological parameters contribute to an optimal production in vitro of viable and infectious mature gametocytes. The knowledge gained from this systematic review on the molecular mechanisms involved in gametocytogenesis enables reproducible gametocyte protocols with transgenic parasite lines to be set up. This review highlights the need for additional gametocyte production protocols for Plasmodium species other than P. falciparum.

Epigenetic and Epitranscriptomic Gene Regulation in Plasmodium falciparum and How We Can Use It against Malaria

Malaria, caused by Plasmodium parasites, is still one of the biggest global health challenges. P. falciparum is the deadliest species to humans. In this review, we discuss how this parasite develops and adapts to the complex and heterogenous environments of its two hosts thanks to varied chromatin-associated and epigenetic mechanisms. First, one small family of transcription factors, the ApiAP2 proteins, functions as master regulators of spatio-temporal patterns of gene expression through the parasite life cycle. In addition, chromatin plasticity determines variable parasite cell phenotypes that link to parasite growth, virulence and transmission, enabling parasite adaptation within host conditions. In recent years, epitranscriptomics is emerging as a new regulatory layer of gene expression. We present evidence of the variety of tRNA and mRNA modifications that are being characterized in Plasmodium spp., and the dynamic changes in their abundance during parasite development and cell fate. We end up outlining that new biological systems, like the mosquito model, to decipher the unknowns about epigenetic mechanisms in vivo; and novel methodologies, to study the function of RNA modifications; are needed to discover the Achilles heel of the parasite. With this new knowledge, future strategies manipulating the epigenetics and epitranscriptomic machinery of the parasite have the potential of providing new weapons against malaria.

Functional inactivation of Plasmodium falciparum glycogen synthase kinase GSK3 modulates erythrocyte invasion and blocks gametocyte maturation

Malaria is responsible for hundreds of thousands of deaths every year. The lack of an effective vaccine and the global spread of multidrug resistant parasites hampers the fight against the disease and underlines the need for new antimalarial drugs. Central to the pathogenesis of malaria is the proliferation of Plasmodium parasites within human erythrocytes. Parasites invade erythrocytes via a coordinated sequence of receptor–ligand interactions between the parasite and the host cell. Posttranslational modifications such as protein phosphorylation are known to be key regulators in this process and are mediated by protein kinases. For several parasite kinases, including the Plasmodium falciparum glycogen synthase kinase 3 (PfGSK3), inhibitors have been shown to block erythrocyte invasion. Here, we provide an assessment of PfGSK3 function by reverse genetics. Using targeted gene disruption, we show the active gene copy, PfGSK3β, is not essential for asexual blood stage proliferation, although it modulates efficient erythrocyte invasion. We found functional inactivation leads to a 69% decreased growth rate and confirmed this growth defect by rescue experiments with wildtype and catalytically inactive mutants. Functional knockout of PfGSK3β does not lead to transcriptional upregulation of the second copy of PfGSK3. We further analyze expression, localization, and function of PfGSK3β during gametocytogenesis using a parasite line allowing conditional induction of sexual commitment. We demonstrate PfGSK3β-deficient gametocytes show a strikingly malformed morphology leading to the death of parasites in later stages of gametocyte development. Taken together, these findings are important for our understanding and the development of PfGSK3 as an antimalarial target.

Distinct Histone Post-translational Modifications during Plasmodium falciparum Gametocyte Development

Histones are the building units of nucleosomes, which constitute chromatin. Histone post-translational modifications (PTMs) play an essential role in epigenetic gene regulation. The Plasmodium falciparum genome encodes canonical and variant histones and a collection of conserved enzymes for histone PTMs and chromatin remodeling. Herein, we profiled the P. falciparum histone PTMs during the development of gametocytes, the obligatory stage for parasite transmission. Mass spectrometric analysis of histones extracted from the early, middle, and late stages of gametocytes identified 457 unique histone peptides with 90 PTMs, of which 50% were novel. The gametocyte histone PTMs display distinct patterns from asexual stages, with many new methylation sites in histones H3 and H3.3 (e.g., K14, K18, and K37). Quantitative analyses revealed a high abundance of acetylation in H3 and H4, mono-methylation of H3/H3.3 K37, and ubiquitination of H3BK112, suggesting that these PTMs play critical roles in gametocytes. Gametocyte histones also showed extensive and unique combinations of PTMs. These data indicate that the parasite harbors distinct transcription regulation mechanisms during gametocyte development and lay the foundation for further characterization of epigenetic regulation in the life cycle of the malaria parasite.

Targeting the Plasmodium falciparum proteome and organelles for potential antimalarial drug candidates

There is an unmet need to unearth alternative treatment options for malaria, wherein this quest is more pressing in recent times due to high morbidity and mortality data arising mostly from the endemic countries coupled with partial diversion of attention from the disease in view of the SARS-Cov-2 pandemic. Available therapeutic options for malaria have been severely threatened with the emergence of resistance to almost all the antimalarial drugs by the Plasmodium falciparum parasite in humans, which is a worrying situation. Artemisinin combination therapies (ACT) that have so far been the mainstay of malaria have encountered resistance by malaria parasite in South East Asia, which is regarded as a notorious ground zero for the emergence of resistance to antimalarial drugs. This review analyzes a few key druggable targets for the parasite and the potential of specific inhibitors to mitigate the emerging antimalarial drug resistance problem by providing a concise assessment of the essential proteins of the malaria parasite that could serve as targets. Moreover, this work provides a summary of the advances made in malaria parasite biology and the potential to leverage these findings for antimalarial drug production.

Declines in prevalence alter the optimal level of sexual investment for the malaria parasite Plasmodium falciparum

Like most human pathogens, the malaria parasite Plasmodium falciparum experiences strong selection pressure from public health interventions such as drug treatment. While most commonly studied in the context of drug targets and related pathways, parasite adaptation to control measures likely extends to phenotypes beyond drug resistance. Here, we use modeling to explore how control measures can reduce levels of within-host competition between P. falciparum genotypes and favor higher rates of sexual investment. We validate these predictions with longitudinally sampled genomic data from French Guiana during a period of malaria decline and find that the most strongly selected genes are enriched for transcription factors involved in commitment to and development of the parasite’s sexual gametocyte form.

Successful infectious disease interventions can result in large reductions in parasite prevalence. Such demographic change has fitness implications for individual parasites and may shift the parasite’s optimal life history strategy. Here, we explore whether declining infection rates can alter Plasmodium falciparum’s investment in sexual versus asexual growth. Using a multiscale mathematical model, we demonstrate how the proportion of polyclonal infections, which decreases as parasite prevalence declines, affects the optimal sexual development strategy: Within-host competition in multiclone infections favors a greater investment in asexual growth whereas single-clone infections benefit from higher conversion to sexual forms. At the same time, drug treatment also imposes selection pressure on sexual development by shortening infection length and reducing within-host competition. We assess these models using 148 P. falciparum parasite genomes sampled in French Guiana over an 18-y period of intensive intervention (1998 to 2015). During this time frame, multiple public health measures, including the introduction of new drugs and expanded rapid diagnostic testing, were implemented, reducing P. falciparum malaria cases by an order of magnitude. Consistent with this prevalence decline, we see an increase in the relatedness among parasites, but no single clonal background grew to dominate the population. Analyzing individual allele frequency trajectories, we identify genes that likely experienced selective sweeps. Supporting our model predictions, genes showing the strongest signatures of selection include transcription factors involved in the development of P. falciparum’s sexual gametocyte form. These results highlight how public health interventions impose wide-ranging selection pressures that affect basic parasite life history traits.

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.

Plasmodium falciparum S-Adenosylmethionine Synthetase Is Essential for Parasite Survival through a Complex Interaction Network with Cytoplasmic and Nuclear Proteins

S-adenosylmethionine synthetase (SAMS) is a key enzyme for the synthesis of the lone methyl donor S-adenosyl methionine (SAM), which is involved in transmethylation reactions and hence required for cellular processes such as DNA, RNA, and histone methylation, but also polyamine biosynthesis and proteostasis. In the human malaria parasite Plasmodium falciparum, PfSAMS is encoded by a single gene and has been suggested to be crucial for malaria pathogenesis and transmission; however, to date, PfSAMS has not been fully characterized. To gain deeper insight into the function of PfSAMS, we generated a conditional gene knockdown (KD) using the glmS ribozyme system. We show that PfSAMS localizes to the cytoplasm and the nucleus of blood-stage parasites. PfSAMS-KD results in reduced histone methylation and leads to impaired intraerythrocytic growth and gametocyte development. To further determine the interaction network of PfSAMS, we performed a proximity-dependent biotin identification analysis. We identified a complex network of 1114 proteins involved in biological processes such as cell cycle control and DNA replication, or transcription, but also in phosphatidylcholine and polyamine biosynthesis and proteasome regulation. Our findings highlight the diverse roles of PfSAMS during intraerythrocytic growth and sexual stage development and emphasize that PfSAMS is a potential drug target.

Adapt or Die: Targeting Unique Transmission-Stage Biology for Malaria Elimination

Plasmodium parasites have a complex life cycle that includes development in the human host as well as the Anopheles vector. Successful transmission of the parasite between its host and vector therefore requires the parasite to balance its investments in asexual replication and sexual reproduction, varying the frequency of sexual commitment to persist within the human host and generate future opportunities for transmission. The transmission window is extended further by the ability of stage V gametocytes to circulate in peripheral blood for weeks, whereas immature stage I to IV gametocytes sequester in the bone marrow and spleen until final maturation. Due to the low gametocyte numbers in blood circulation and with the ease of targeting such life cycle bottlenecks, transmission represents an efficient target for therapeutic intervention. The biological process of Plasmodium transmission is a multistage, multifaceted process and the past decade has seen a much deeper understanding of the molecular mechanisms and regulators involved. Clearly, specific and divergent processes are used during transmission compared to asexual proliferation, which both poses challenges but also opportunities for discovery of transmission-blocking antimalarials. This review therefore presents an update of our molecular understanding of gametocyte and gamete biology as well as the status of transmission-blocking activities of current antimalarials and lead development compounds. By defining the biological components associated with transmission, considerations for the development of new transmission-blocking drugs to target such untapped but unique biology is suggested as an important, main driver for transmission-blocking drug discovery.

Systems biology of malaria explored with nonhuman primates

"The Primate Malarias" book has been a uniquely important resource for multiple generations of scientists, since its debut in 1971, and remains pertinent to the present day. Indeed, nonhuman primates (NHPs) have been instrumental for major breakthroughs in basic and pre-clinical research on malaria for over 50 years. Research involving NHPs have provided critical insights and data that have been essential for malaria research on many parasite species, drugs, vaccines, pathogenesis, and transmission, leading to improved clinical care and advancing research goals for malaria control, elimination, and eradication. Whilst most malaria scientists over the decades have been studying Plasmodium falciparum, with NHP infections, in clinical studies with humans, or using in vitro culture or rodent model systems, others have been dedicated to advancing research on Plasmodium vivax, as well as on phylogenetically related simian species, including Plasmodium cynomolgi, Plasmodium coatneyi, and Plasmodium knowlesi. In-depth study of these four phylogenetically related species over the years has spawned the design of NHP longitudinal infection strategies for gathering information about ongoing infections, which can be related to human infections. These Plasmodium-NHP infection model systems are reviewed here, with emphasis on modern systems biological approaches to studying longitudinal infections, pathogenesis, immunity, and vaccines. Recent discoveries capitalizing on NHP longitudinal infections include an advanced understanding of chronic infections, relapses, anaemia, and immune memory. With quickly emerging new technological advances, more in-depth research and mechanistic discoveries can be anticipated on these and additional critical topics, including hypnozoite biology, antigenic variation, gametocyte transmission, bone marrow dysfunction, and loss of uninfected RBCs. New strategies and insights published by the Malaria Host-Pathogen Interaction Center (MaHPIC) are recapped here along with a vision that stresses the importance of educating future experts well trained in utilizing NHP infection model systems for the pursuit of innovative, effective interventions against malaria.

Live imaging of the Cryptosporidium parvum life cycle reveals direct development of male and female gametes from type I meronts

Cryptosporidium is a leading infectious cause of diarrhea around the world associated with waterborne outbreaks, community spread, or zoonotic transmission. The parasite has significant impact on early childhood mortality, and infection is both a consequence and cause of malnutrition and stunting. There is currently no vaccine, and treatment options are very limited. Cryptosporidium is a member of the Apicomplexa, and, as typical for this, protist phylum relies on asexual and sexual reproduction. In contrast to other Apicomplexa, including the malaria parasite Plasmodium, the entire Cryptosporidium life cycle unfolds in a single host in less than 3 days. Here, we establish a model to image life cycle progression in living cells and observe, track, and compare nuclear division of asexual and sexual stage parasites. We establish the length and sequence of the cell cycles of all stages and map the developmental fate of parasites across multiple rounds of invasion and egress. We propose that the parasite executes an intrinsic program of 3 generations of asexual replication, followed by a single generation of sexual stages that is independent of environmental stimuli. We find no evidence for a morphologically distinct intermediate stage (the tetraploid type II meront) but demonstrate direct development of gametes from 8N type I meronts. The progeny of each meront is collectively committed to either asexual or sexual fate, but, importantly, meronts committed to sexual fate give rise to both males and females. We define a Cryptosporidium life cycle matching Tyzzer’s original description and inconsistent with the coccidian life cycle now shown in many textbooks.

5-methylcytosine modification by Plasmodium NSUN2 stabilizes mRNA and mediates the development of gametocytes

5-methylcytosine (m⁵C) is an important epitranscriptomic modification involved in messenger RNA (mRNA) stability and translation efficiency in various biological processes. However, it remains unclear if m⁵C modification contributes to the dynamic regulation of the transcriptome during the developmental cycles of Plasmodium parasites. Here, we characterize the landscape of m⁵C mRNA modifications at single nucleotide resolution in the asexual replication stages and gametocyte sexual stages of rodent (Plasmodium yoelii) and human (Plasmodium falciparum) malaria parasites. While different representations of m⁵C-modified mRNAs are associated with the different stages, the abundance of the m⁵C marker is strikingly enhanced in the transcriptomes of gametocytes. Our results show that m⁵C modifications confer stability to the Plasmodium transcripts and that a Plasmodium ortholog of NSUN2 is a major mRNA m⁵C methyltransferase in malaria parasites. Upon knockout of P. yoelii nsun2 (pynsun2), marked reductions of m⁵C modification were observed in a panel of gametocytogenesis-associated transcripts. These reductions correlated with impaired gametocyte production in the knockout rodent malaria parasites. Restoration of the nsun2 gene in the knockout parasites rescued the gametocyte production phenotype as well as m⁵C modification of the gametocytogenesis-associated transcripts. Together with the mRNA m⁵C profiles for two species of Plasmodium, our findings demonstrate a major role for NSUN2-mediated m⁵C modifications in mRNA transcript stability and sexual differentiation in malaria parasites.

Current status of experimental models for the study of malaria

Infection by malaria parasites (Plasmodium spp.) remains one of the leading causes of morbidity and mortality, especially in tropical regions of the world. Despite the availability of malaria control tools such as integrated vector management and effective therapeutics, these measures have been continuously undermined by the emergence of vector resistance to insecticides or parasite resistance to frontline antimalarial drugs. Whilst the recent pilot implementation of the RTS,S malaria vaccine is indeed a remarkable feat, highly effective vaccines against malaria remain elusive. The barriers to effective vaccines result from the complexity of both the malaria parasite lifecycle and the parasite as an organism itself with consequent major gaps in our understanding of their biology. Historically and due to the practical and ethical difficulties of working with human malaria infections, research into malaria parasite biology has been extensively facilitated by animal models. Animals have been used to study disease pathogenesis, host immune responses and their (dys)regulation and further disease processes such as transmission. Moreover, animal models remain at the forefront of pre-clinical evaluations of antimalarial drugs (drug efficacy, mode of action, mode of resistance) and vaccines. In this review, we discuss commonly used animal models of malaria, the parasite species used and their advantages and limitations which hinder their extrapolation to actual human disease. We also place into this context the most recent developments such as organoid technologies and humanized mice.

Revisiting the Effect of Pharmaceuticals on Transmission Stage Formation in the Malaria Parasite Plasmodium falciparum

Malaria parasites rely on specialized stages, called gametocytes, to ensure human-to-human transmission. The formation of these sexual precursor cells is initiated by commitment of blood stage parasites to the sexual differentiation pathway. Plasmodium falciparum, the most virulent of six parasite species infecting humans, employs nutrient sensing to control the rate at which sexual commitment is initiated, and the presence of stress-inducing factors, including antimalarial drugs, has been linked to increased gametocyte production in vitro and in vivo. These observations suggest that therapeutic interventions may promote gametocytogenesis and malaria transmission. Here, we engineered a P. falciparum reporter line to quantify sexual commitment rates after exposure to antimalarials and other pharmaceuticals commonly prescribed in malaria-endemic regions. Our data reveal that some of the tested drugs indeed have the capacity to elevate sexual commitment rates in vitro. Importantly, however, these effects are only observed at drug concentrations that inhibit parasite survival and only rarely result in a net increase of gametocyte production. Using a drug-resistant parasite reporter line, we further show that the gametocytogenesis-promoting effect of drugs is linked to general stress responses rather than to compound-specific activities. Altogether, we did not observe evidence for mechanistic links between the regulation of sexual commitment and the activity of commonly used pharmaceuticals in vitro. Our data hence does not support scenarios in which currently applied therapeutic interventions would promote the spread of drug-resistant parasites or malaria transmission in general.

A dynamic and combinatorial histone code drives malaria parasite asexual and sexual development

Histone posttranslational modifications (PTMs) frequently co-occur on the same chromatin domains or even in the same molecule. It is now established that these “histone codes” are the result of cross talk between enzymes that catalyze multiple PTMs with univocal readout as compared with these PTMs in isolation. Here, we performed a comprehensive identification and quantification of histone codes of the malaria parasite, Plasmodium falciparum. We used advanced quantitative middle-down proteomics to identify combinations of PTMs in both the proliferative, asexual stages and transmissible, sexual gametocyte stages of P. falciparum. We provide an updated, high-resolution compendium of 77 PTMs on H3 and H3.3, of which 34 are newly identified in P. falciparum. Coexisting PTMs with unique stage distinctions were identified, indicating that many of these combinatorial PTMs are associated with specific stages of the parasite life cycle. We focused on the code H3R17me2K18acK23ac for its unique presence in mature gametocytes; chromatin proteomics identified a gametocyte-specific SAGA-like effector complex including the transcription factor AP2-G2, which we tied to this specific histone code, as involved in regulating gene expression in mature gametocytes. Ultimately, this study unveils previously undiscovered histone PTMs and their functional relationship with coexisting partners. These results highlight that investigating chromatin regulation in the parasite using single histone PTM assays might overlook higher-order gene regulation for distinct proliferation and differentiation processes.

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First middle-down chromatin proteomics compendium of the malaria parasite, Plasmodium falciparum.

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Novel histone PTMs (including arginine methylation) in both asexual parasites and transmissible gametocytes.

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Histone PTM cross talk is dynamic life cycle stage stratified.

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Gametocytes rely on histone PTM connectivity to allow onward transmission.

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AP2-G2 is an important effector of H3K18acK23ac in mature gametocytes.

Antimalarial Natural Products

Natural products have made a crucial and unique contribution to human health, and this is especially true in the case of malaria, where the natural products quinine and artemisinin and their derivatives and analogues, have saved millions of lives. The need for new drugs to treat malaria is still urgent, since the most dangerous malaria parasite, Plasmodium falciparum, has become resistant to quinine and most of its derivatives and is becoming resistant to artemisinin and its derivatives. This volume begins with a short history of malaria and follows this with a summary of its biology. It then traces the fascinating history of the discovery of quinine for malaria treatment and then describes quinine's biosynthesis, its mechanism of action, and its clinical use, concluding with a discussion of synthetic antimalarial agents based on quinine's structure. The volume then covers the discovery of artemisinin and its development as the source of the most effective current antimalarial drug, including summaries of its synthesis and biosynthesis, its mechanism of action, and its clinical use and resistance. A short discussion of other clinically used antimalarial natural products leads to a detailed treatment of other natural products with significant antiplasmodial activity, classified by compound type. Although the search for new antimalarial natural products from Nature's combinatorial library is challenging, it is very likely to yield new antimalarial drugs. The chapter thus ends by identifying over ten natural products with development potential as clinical antimalarial agents.

OUP accepted manuscript

Heterochromatin-associated gene silencing controls multiple physiological processes in malaria parasites, however, little is known concerning the regulatory network and cis-acting sequences involved in the organization of heterochromatin and how they modulate heterochromatic gene expression. Based on systematic profiling of genome-wide occupancy of eighteen Apicomplexan AP2 transcription factors by ChIP-seq analysis, we identify and characterize eight heterochromatin-associated factors (PfAP2-HFs), which exhibit preferential enrichment within heterochromatic regions but with differential coverage profiles. Although these ApiAP2s target euchromatic gene loci via specific DNA motifs, they are likely integral components of heterochromatin independent of DNA motif recognition. Systematic knockout screenings of ApiAP2 factors coupled with RNA-seq transcriptomic profiling revealed three activators and three repressors of heterochromatic gene expression including four PfAP2-HFs. Notably, expression of virulence genes is either completely silenced or significantly reduced upon the depletion of PfAP2-HC. Integrated multi-omics analyses reveal autoregulation and feed-forward loops to be common features of the ApiAP2 regulatory network, in addition to the occurrence of dynamic interplay between local chromatin structure and ApiAP2s in transcriptional control. Collectively, this study provides a valuable resource describing the genome-wide landscape of the ApiAP2 family and insights into functional divergence and cooperation within this family during the blood-stage development of malaria parasites.

Expansion microscopy of Plasmodium gametocytes reveals the molecular architecture of a bipartite microtubule organisation centre coordinating mitosis with axoneme assembly

Transmission of malaria-causing parasites to mosquitoes relies on the production of gametocyte stages and their development into gametes. These stages display various microtubule cytoskeletons and the architecture of the corresponding microtubule organisation centres (MTOC) remains elusive. Combining ultrastructure expansion microscopy (U-ExM) with bulk proteome labelling, we first reconstructed in 3D the subpellicular microtubule network which confers cell rigidity to Plasmodium falciparum gametocytes. Upon activation, as the microgametocyte undergoes three rounds of endomitosis, it also assembles axonemes to form eight flagellated microgametes. U-ExM combined with Pan-ExM further revealed the molecular architecture of the bipartite MTOC coordinating mitosis with axoneme formation. This MTOC spans the nuclear membrane linking cytoplasmic basal bodies to intranuclear bodies by proteinaceous filaments. In P. berghei, the eight basal bodies are concomitantly de novo assembled in a SAS6- and SAS4-dependent manner from a deuterosome-like structure, where centrin, γ-tubulin, SAS4 and SAS6 form distinct subdomains. Basal bodies display a fusion of the proximal and central cores where centrin and SAS6 are surrounded by a SAS4-toroid in the lumen of the microtubule wall. Sequential nucleation of axonemes and mitotic spindles is associated with a dynamic movement of γ-tubulin from the basal bodies to the intranuclear bodies. This dynamic architecture relies on two non-canonical regulators, the calcium-dependent protein kinase 4 and the serine/arginine-protein kinase 1. Altogether, these results provide insights into the molecular organisation of a bipartite MTOC that may reflect a functional transition of a basal body to coordinate axoneme assembly with mitosis.

Plasmodium falciparum and Plasmodium vivax Adjust Investment in Transmission in Response to Change in Transmission Intensity: A Review of the Current State of Research

Malaria parasites can adjust the proportion of parasites that develop into gametocytes, and thus the probability for human-to-vector transmission, through changes in the gametocyte conversion rate. Understanding the factors that impact the commitment of malaria parasites to transmission is required to design better control interventions. Plasmodium spp. persist across countries with vast differences in transmission intensities, and in sites where transmission is highly seasonal. Mounting evidence shows that Plasmodium spp. adjusts the investment in transmission according to seasonality of vector abundance, and transmission intensity. Various techniques to determine the investment in transmission are available, i.e., short-term culture, where the conversion rate can be measured most directly, genome and transcriptome studies, quantification of mature gametocytes, and mosquito feeding assays. In sites with seasonal transmission, the proportion of gametocytes, their densities and infectivity are higher during the wet season, when vectors are plentiful. When countries with pronounced differences in transmission intensity were compared, the investment in transmission was higher when transmission was low, thus maximizing the parasite’s chances to be transmitted to mosquitoes. Increased transmissibility of residual infections after a successful reduction of malaria transmission levels need to be considered when designing intervention measures.

Stressed Out About Plasmodium falciparum Gametocytogenesis

Blocking malaria transmission is critical to malaria control programs but remains a major challenge especially in endemic regions with high levels of asymptomatic infections. New strategies targeting the transmissible sexual stages of the parasite, called gametocytes, are needed. This review focuses on P. falciparum gametocytogenesis in vivo and in vitro. Highlighting advances made elucidating genes required for gametocyte production and identifying key questions that remain unanswered such as the factors and regulatory mechanisms that contribute to gametocyte induction, and the mechanism of sequestration. Tools available to begin to address these issues are also described to facilitate advances in our understanding of this important stage of the life cycle.

Molecular interactions between parasite and mosquito during midgut invasion as targets to block malaria transmission

Despite considerable effort, malaria remains a major public health burden. Malaria is caused by five Plasmodium species and is transmitted to humans via the female Anopheles mosquito. The development of malaria vaccines against the liver and blood stages has been challenging. Therefore, malaria elimination strategies advocate integrated measures, including transmission-blocking approaches. Designing an effective transmission-blocking strategy relies on a sophisticated understanding of the molecular mechanisms governing the interactions between the mosquito midgut molecules and the malaria parasite. Here we review recent advances in the biology of malaria transmission, focusing on molecular interactions between Plasmodium and Anopheles mosquito midgut proteins. We provide an overview of parasite and mosquito proteins that are either targets for drugs currently in clinical trials or candidates of promising transmission-blocking vaccines.

Malaria Parasite Stress Tolerance Is Regulated by DNMT2-Mediated tRNA Cytosine Methylation

Malaria parasites need to cope with changing environmental conditions that require strong countermeasures to ensure pathogen survival in the human and mosquito hosts. The molecular mechanisms that protect Plasmodium falciparum homeostasis during the complex life cycle remain unknown. Here, we identify cytosine methylation of tRNA^{Asp (GTC)} as being critical to maintain stable protein synthesis. Using conditional knockout (KO) of a member of the DNA methyltransferase family, called Pf-DNMT2, RNA bisulfite sequencing demonstrated the selective cytosine methylation of this enzyme of tRNA^{Asp (GTC)} at position C38. Although no growth defect on parasite proliferation was observed, Pf-DNMT2KO parasites showed a selective downregulation of proteins with a GAC codon bias. This resulted in a significant shift in parasite metabolism, priming KO parasites for being more sensitive to various types of stress. Importantly, nutritional stress made tRNA^{Asp (GTC)} sensitive to cleavage by an unknown nuclease and increased gametocyte production (>6-fold). Our study uncovers an epitranscriptomic mechanism that safeguards protein translation and homeostasis of sexual commitment in malaria parasites. IMPORTANCE P. falciparum is the most virulent malaria parasite species, accounting for the majority of the disease mortality and morbidity. Understanding how this pathogen is able to adapt to different cellular and environmental stressors during its complex life cycle is crucial in order to develop new strategies to tackle the disease. In this study, we identified the writer of a specific tRNA cytosine methylation site as a new layer of epitranscriptomic regulation in malaria parasites that regulates the translation of a subset of parasite proteins (>400) involved in different metabolic pathways. Our findings give insight into a novel molecular mechanism that regulates P. falciparum response to drug treatment and sexual commitment.

The Plasmodium NOT1-G paralogue is an essential regulator of sexual stage maturation and parasite transmission

Productive transmission of malaria parasites hinges upon the execution of key transcriptional and posttranscriptional regulatory events. While much is now known about how specific transcription factors activate or repress sexual commitment programs, far less is known about the production of a preferred mRNA homeostasis following commitment and through the host-to-vector transmission event. Here, we show that in Plasmodium parasites, the NOT1 scaffold protein of the CAF1/CCR4/Not complex is duplicated, and one paralogue is dedicated for essential transmission functions. Moreover, this NOT1-G paralogue is central to the sex-specific functions previously associated with its interacting partners, as deletion of not1-g in Plasmodium yoelii leads to a comparable or complete arrest phenotype for both male and female parasites. We show that, consistent with its role in other eukaryotes, PyNOT1-G localizes to cytosolic puncta throughout much of the Plasmodium life cycle. PyNOT1-G is essential to both the complete maturation of male gametes and to the continued development of the fertilized zygote originating from female parasites. Comparative transcriptomics of wild-type and pynot1-g⁻ parasites shows that loss of PyNOT1-G leads to transcript dysregulation preceding and during gametocytogenesis and shows that PyNOT1-G acts to preserve mRNAs that are critical to sexual and early mosquito stage development. Finally, we demonstrate that the tristetraprolin (TTP)-binding domain, which acts as the typical organization platform for RNA decay (TTP) and RNA preservation (ELAV/HuR) factors is dispensable for PyNOT1-G’s essential blood stage functions but impacts host-to-vector transmission. Together, we conclude that a NOT1-G paralogue in Plasmodium fulfills the complex transmission requirements of both male and female parasites.

Malaria parasites face two bottlenecks in their life cycle: their two transmission events. This study shows that Plasmodium has taken the unorthodox approach of duplicating the gene for the NOT1 RNA regulatory scaffold protein, allowing it to dedicate one paralog to functions that are essential for transmission from mammalian hosts to the mosquito vector.

Chemogenomic Fingerprints Associated with Stage-Specific Gametocytocidal Compound Action against Human Malaria Parasites

Kinase-focused inhibitors previously revealed compounds with differential activity against different stages of Plasmodium falciparum gametocytes. MMV666810, a 2-aminopyrazine, is more active on late-stage gametocytes, while a pyrazolopyridine, MMV674850, preferentially targets early-stage gametocytes. Here, we probe the biological mechanisms underpinning this differential stage-specific killing using in-depth transcriptome fingerprinting. Compound-specific chemogenomic profiles were observed with MMV674850 treatment associated with biological processes shared between asexual blood stage parasites and early-stage gametocytes but not late-stage gametocytes. MMV666810 has a distinct profile with clustered gene sets associated primarily with late-stage gametocyte development, including Ca²⁺-dependent protein kinases (CDPK1 and 5) and serine/threonine protein kinases (FIKK). Chemogenomic profiling therefore highlights essential processes in late-stage gametocytes, on a transcriptional level. This information is important to prioritize compounds that preferentially compromise late-stage gametocytes for further development as transmission-blocking antimalarials.

Single-Cell RNA Sequencing Reveals Cellular Heterogeneity and Stage Transition under Temperature Stress in Synchronized Plasmodium falciparum Cells

The malaria parasite has a complex life cycle exhibiting phenotypic and morphogenic variations in two different hosts by existing in heterogeneous developmental states. To investigate this cellular heterogeneity of the parasite within the human host, we performed single-cell RNA sequencing of synchronized Plasmodium cells under control and temperature treatment conditions. Using the Malaria Cell Atlas (https://www.sanger.ac.uk/science/tools/mca) as a guide, we identified 9 subtypes of the parasite distributed across known intraerythrocytic stages. Interestingly, temperature treatment results in the upregulation of the AP2-G gene, the master regulator of sexual development in a small subpopulation of the parasites. Moreover, we identified a heterogeneous stress-responsive subpopulation (clusters 5, 6, and 7 [∼10% of the total population]) that exhibits upregulation of stress response pathways under normal growth conditions. We also developed an online exploratory tool that will provide new insights into gene function under normal and temperature stress conditions. Thus, our study reveals important insights into cell-to-cell heterogeneity in the parasite population under temperature treatment that will be instrumental toward a mechanistic understanding of cellular adaptation and population dynamics in Plasmodium falciparum. IMPORTANCE The malaria parasite has a complex life cycle exhibiting phenotypic variations in two different hosts accompanied by cell-to-cell variability that is important for stress tolerance, immune evasion, and drug resistance. To investigate cellular heterogeneity determined by gene expression, we performed single-cell RNA sequencing (scRNA-seq) of about 12,000 synchronized Plasmodium cells under physiologically relevant normal (37°C) and temperature stress (40°C) conditions phenocopying the cyclic bouts of fever experienced during malarial infection. In this study, we found that parasites exhibit transcriptional heterogeneity in an otherwise morphologically synchronized culture. Also, a subset of parasites is continually committed to gametocytogenesis and stress-responsive pathways. These observations have important implications for understanding the mechanisms of drug resistance generation and vaccine development against the malaria parasite.

CRISPR/Cas9-engineered inducible gametocyte producer lines as a valuable tool for Plasmodium falciparum malaria transmission research

The malaria parasite Plasmodium falciparum replicates inside erythrocytes in the blood of infected humans. During each replication cycle, a small proportion of parasites commits to sexual development and differentiates into gametocytes, which are essential for parasite transmission via the mosquito vector. Detailed molecular investigation of gametocyte biology and transmission has been hampered by difficulties in generating large numbers of these highly specialised cells. Here, we engineer P. falciparum NF54 inducible gametocyte producer (iGP) lines for the routine mass production of synchronous gametocytes via conditional overexpression of the sexual commitment factor GDV1. NF54/iGP lines consistently achieve sexual commitment rates of 75% and produce viable gametocytes that are transmissible by mosquitoes. We also demonstrate that further genetic engineering of NF54/iGP parasites is a valuable tool for the targeted exploration of gametocyte biology. In summary, we believe the iGP approach developed here will greatly expedite basic and applied malaria transmission stage research.