Generation of Transmission-Competent Human Malaria Parasites with Chromosomally-Integrated Fluorescent Reporters

Let it glow: genetically encoded fluorescent reporters in Plasmodium

The use of fluorescent proteins (FPs) in Plasmodium parasites has been key to understand the biology of this obligate intracellular protozoon. FPs like the green fluorescent protein (GFP) enabled to explore protein localization, promoter activity as well as dynamic processes like protein export and endocytosis. Furthermore, FP biosensors have provided detailed information on physiological parameters at the subcellular level, and fluorescent reporter lines greatly extended the malariology toolbox. Still, in order to achieve optimal results, it is crucial to know exactly the properties of the FP of choice and the genetic scenario in which it will be used. This review highlights advantages and disadvantages of available landing sites and promoters that have been successfully applied for the ectopic expression of FPs in Plasmodium berghei and Plasmodium falciparum. Furthermore, the properties of newly developed FPs beyond DsRed and EGFP, in the visualization of cells and cellular structures as well as in the sensing of small molecules are discussed.

Selective targeting of Plasmodium falciparum Hsp90 disrupts the 26S proteasome

The molecular chaperone heat shock protein 90 (Hsp90) has an essential but largely undefined role in maintaining proteostasis in Plasmodium falciparum, the most lethal malaria parasite. Herein, we identify BX-2819 and XL888 as potent P. falciparum (Pf)Hsp90 inhibitors. Derivatization of XL888's scaffold led to the development of Tropane 1, as a PfHsp90-selective binder with nanomolar affinity. Hsp90 inhibitors exhibit anti-Plasmodium activity against the liver, asexual blood, and early gametocyte life stages. Thermal proteome profiling was implemented to assess PfHsp90-dependent proteome stability, and the proteasome-the main site of cellular protein recycling-was enriched among proteins with perturbed stability upon PfHsp90 inhibition. Subsequent biochemical and cellular studies suggest that PfHsp90 directly promotes proteasome hydrolysis by chaperoning the active 26S complex. These findings expand our knowledge of the PfHsp90-dependent proteome and protein quality control mechanisms in these pathogenic parasites, as well as further characterize this chaperone as a potential antimalarial drug target.

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.

Sexual differentiation in human malaria parasites is regulated by competition between phospholipid metabolism and histone methylation

For Plasmodium falciparum, the most widespread and virulent malaria parasite that infects humans, persistence depends on continuous asexual replication in red blood cells, while transmission to their mosquito vector requires asexual blood-stage parasites to differentiate into non-replicating gametocytes. This decision is controlled by stochastic derepression of a heterochromatin-silenced locus encoding AP2-G, the master transcription factor of sexual differentiation. The frequency of ap2-g derepression was shown to be responsive to extracellular phospholipid precursors but the mechanism linking these metabolites to epigenetic regulation of ap2-g was unknown. Through a combination of molecular genetics, metabolomics and chromatin profiling, we show that this response is mediated by metabolic competition for the methyl donor S-adenosylmethionine between histone methyltransferases and phosphoethanolamine methyltransferase, a critical enzyme in the parasite's pathway for de novo phosphatidylcholine synthesis. When phosphatidylcholine precursors are scarce, increased consumption of SAM for de novo phosphatidylcholine synthesis impairs maintenance of the histone methylation responsible for silencing ap2-g, increasing the frequency of derepression and sexual differentiation. This provides a key mechanistic link that explains how LysoPC and choline availability can alter the chromatin status of the ap2-g locus controlling sexual differentiation.

Genome-wide profiling of histone modifications in Plasmodium falciparum using CUT&RUN

We recently adapted a CUT&RUN protocol for genome-wide profiling of chromatin modifications in the human malaria parasite Plasmodium Using the step-by-step protocol described below, we were able to generate high-quality profiles of multiple histone modifications using only a small fraction of the cells required for ChIP-seq. Using antibodies against two commonly profiled histone modifications, H3K4me3 and H3K9me3, we show here that CUT&RUN profiling is highly reproducible and closely recapitulates previously published ChIP-seq-based abundance profiles of histone marks. Finally, we show that CUT&RUN requires substantially lower sequencing coverage for accurate profiling compared with ChIP-seq.

In vitro models for human malaria: targeting the liver stage

The Plasmodium liver stage represents a vulnerable therapeutic target to prevent disease progression as the parasite resides in the liver before clinical representation caused by intraerythrocytic development. However, most antimalarial drugs target the blood stage of the parasite's life cycle, and the few drugs that target the liver stage are lethal to patients with a glucose-6-phosphate dehydrogenase deficiency. Furthermore, implementation of in vitro liver models to study and develop novel therapeutics against the liver stage of human Plasmodium species remains challenging. In this review, we focus on the progression of in vitro liver models developed for human Plasmodium spp. parasites, provide a brief review on important assay requirements, and lastly present recommendations to improve models to enhance the discovery process of novel preclinical therapeutics.

Streamlined and Robust Stage-Specific Profiling of Gametocytocidal Compounds Against Plasmodium falciparum

Malaria elimination is dependent on the ability to target both the pathogenic and transmissible stages of the human malaria parasite, Plasmodium falciparum. These forms of the parasite are differentiated by unique developmental stages, each with their own biological mechanisms and processes. These individual stages therefore also respond differently to inhibitory compounds, and this complicates the discovery of multistage active antimalarial agents. The search for compounds with transmission-blocking activity has focused on screening for activity on mature gametocytes, with only limited descriptions available for the activity of such compounds on immature stage gametocytes. This therefore poses a gap in the profiling of antimalarial agents for pan-reactive, multistage activity to antimalarial leads. Here, we optimized an effective and robust strategy for the simple and cost-effective description of the stage-specific action of gametocytocidal antimalarial compounds.

Phthalimide analogs for antimalarial drug discovery

Malaria remains one of the world's most life-threatening diseases and, thus, it is a major public health concern all around the world. The disease can become devastating if not treated with proper medication in a timely manner. Currently, the number of viable treatment therapies is in continuous decline due to compromised effectiveness, probably owing to the complex life cycle of Plasmodium falciparum. The factors responsible for the unclear status of malaria eradication programmes include ever-developing parasite resistance to the most effective treatments used on the frontline (i.e., artemisinin derivatives) and the paucity of new effective therapeutics. Due to these circumstances, the development of novel effective drug candidates with unique modes of action is essential for overcoming the listed obstacles. As such, the discovery of novel chemical compounds based on validated pharmacophores remains an unmet need in the field of medicinal chemistry. In this area, functionalized phthalimide (Pht) analogs have been explored as potential candidates against various diseases, including malaria. Pht presents a promising bioactive scaffold that can be easily functionalized and thus utilized as a starting point for the development of new antimalarial candidates suitable for preclinical and clinical studies. In this short review, we highlight a wide range of Pht analogs that have been investigated for their activity against various strains of Plasmodium falciparum.

CRISPRing protozoan parasites to better understand the biology of diseases

Precise gene editing techniques are paramount to gain deeper insights into the biological processes such as host-parasite interactions, drug resistance mechanisms, and gene-function relationships. Discovery of CRISPR-Cas9 system has spearheaded mechanistic understanding of protozoan parasite biology as evident from the number of reports in the last decade. Here, we have described the use of CRISPR-Cas9 in understanding the biology of medically important protozoan parasites such as Plasmodium, Leishmania, Trypanosoma, Babesia and Trichomonas. In spite of intrinsic difficulties in genome editing in these protozoan parasites, CRISPR-Cas9 has acted as a catalyst for faster generation of desired transgenic parasites. Modifications in the CRISPR-Cas9 system for improving the efficiency have been useful in better understanding the molecular mechanisms associated with repair of double strand breaks in the parasites. Moreover, improvement in reagents used for CRISPR mediated gene editing have been instrumental in addressing the issue of non-specificity and toxicity for therapeutic use. These application-based modifications may help in further increasing the efficiency of gene editing in protozoan parasites.

Comparative intravital imaging of human and rodent malaria sporozoites reveals the skin is not a species-specific barrier

Malaria infection starts with the injection of Plasmodium sporozoites into the host’s skin. Sporozoites are motile and move in the skin to find and enter blood vessels to be carried to the liver. Here, we present the first characterization of P. falciparum sporozoites in vivo, analyzing their motility in mouse skin and human skin xenografts and comparing their motility to two rodent malaria species. These data suggest that in contrast to the liver and blood stages, the skin is not a species‐specific barrier for Plasmodium. Indeed, P. falciparum sporozoites enter blood vessels in mouse skin at similar rates to the rodent malaria parasites. Furthermore, we demonstrate that antibodies targeting sporozoites significantly impact the motility of P. falciparum sporozoites in mouse skin. Though the sporozoite stage is a validated vaccine target, vaccine trials have been hampered by the lack of good animal models for human malaria parasites. Pre‐clinical screening of next‐generation vaccines would be significantly aided by the in vivo platform we describe here, expediting down‐selection of candidates prior to human vaccine trials.

Plasmodium falciparum Gametocyte Culture and Mosquito Infection Through Artificial Membrane Feeding

Malaria remains one of the most important public health problems, causing significant morbidity and mortality. Malaria is a mosquito borne disease transmitted through an infectious bite from the female Anopheles mosquito. Malaria control will eventually rely on a multitude of approaches, which includes ways to block transmission to, through and from mosquitoes. To study mosquito stages of malaria parasites in the laboratory, we have optimized a protocol to culture highly infectious Plasmodium falciparum gametocytes, a parasite stage required for transmission from the human host to the mosquito vector. P. falciparum gametocytes mature through five morphologically distinct steps, which takes approximately 1-2 weeks. Gametocyte culture described in this protocol is completed in 15 days and are infectious to mosquitoes from days 15-18. These protocols were developed to maintain a continuous cycle of infection competent gametocytes and to maintain uninterrupted supply of mosquito stages of the parasite. Here, we describe the methodology of gametocyte culture and how to infect mosquitoes with these parasites using glass membrane feeders.

Activity of Epigenetic Inhibitors against Plasmodium falciparum Asexual and Sexual Blood Stages

Earlier genetic and inhibitor studies showed that epigenetic regulation of gene expression is critical for malaria parasite survival in multiple life stages and a promising target for new antimalarials. We therefore evaluated the activity of 350 diverse epigenetic inhibitors against multiple stages of Plasmodium falciparum We observed ≥90% inhibition at 10 μM for 28% of compounds against asexual blood stages and early gametocytes, of which a third retained ≥90% inhibition at 1 μM.

Generation of Novel Plasmodium falciparum NF135 and NF54 Lines Expressing Fluorescent Reporter Proteins Under the Control of Strong and Constitutive Promoters

Transgenic reporter lines of malaria parasites that express fluorescent or luminescent proteins are valuable tools for drug and vaccine screening assays as well as to interrogate parasite gene function. Different Plasmodium falciparum (Pf ) reporter lines exist, however nearly all have been created in the African NF54/3D7 laboratory strain. Here we describe the generation of novel reporter lines, using CRISPR/Cas9 gene modification, both in the standard Pf NF54 background and in a recently described Cambodian P. falciparum NF135.C10 line. Sporozoites of this line show more effective hepatocyte invasion and enhanced liver merozoite development compared to Pf NF54. We first generated Pf NF54 reporter parasites to analyze two novel promoters for constitutive and high expression of mCherry-luciferase and GFP in blood and mosquito stages. The promoter sequences were selected based on available transcriptome data and are derived from two housekeeping genes, i.e., translation initiation factor SUI1, putative (sui1, PF3D7_1243600) and 40S ribosomal protein S30 (40s, PF3D7_0219200). We then generated and characterized reporter lines in the Pf NF135.C10 line which express GFP driven by the sui1 and 40s promoters as well as by the previously used ef1α promoter (GFP@ef1α, GFP@sui1, GFP@40s). The GFP@40s reporter line showed strongest GFP expression in liver stages as compared to the other two lines. The strength of reporter expression by the 40s promoter throughout the complete life cycle, including liver stages, makes transgenic lines expressing reporters by the 40s promoter valuable novel tools for analyses of P. falciparum.

Disruption of the Plasmodium falciparum Life Cycle through Transcriptional Reprogramming by Inhibitors of Jumonji Demethylases

Little is known about the role of the three Jumonji C (JmjC) enzymes in Plasmodium falciparum (Pf). Here, we show that JIB-04 and other established inhibitors of mammalian JmjC histone demethylases kill asexual blood stage parasites and are even more potent at blocking gametocyte development and gamete formation. In late stage parasites, JIB-04 increased levels of trimethylated lysine residues on histones, suggesting the inhibition of P. falciparum Jumonji demethylase activity. These epigenetic defects coincide with deregulation of invasion, cell motor, and sexual development gene programs, including gene targets coregulated by the PfAP2-I transcription factor and chromatin-binding factor, PfBDP1. Mechanistically, we demonstrate that PfJmj3 converts 2-oxoglutarate to succinate in an iron-dependent manner consistent with mammalian Jumonji enzymes, and this catalytic activity is inhibited by JIB-04 and other Jumonji inhibitors. Our pharmacological studies of Jumonji activity in the malaria parasite provide evidence that inhibition of these enzymatic activities is detrimental to the parasite.