Lactic Acid Supplementation Increases Quantity and Quality of Gametocytes in Plasmodium falciparum Culture

Histone code: a common language and multiple dialects to meet the different developmental requirements of apicomplexan parasites

Apicomplexan parasites have complex life cycles, often requiring transmission between two different hosts, facing periods of dormancy within the host or in the environment to maximize chances of transmission. To support survival in these different conditions, tightly regulated and correctly timed gene expression is critical. The modification of histones and nucleosome composition makes a significant contribution to this regulation, and as eukaryotes, the fundamental mechanisms underlying this process in apicomplexans are similar to those in model eukaryotic organisms. However, single-celled intracellular parasites face unique challenges, and regulation of gene expression at the epigenetic level provides tight control for responses that must often be rapid and robust. Here, we discuss the recent advances in understanding the dynamics of histone modifications across Apicomplexan life cycles and the molecular mechanisms that underlie epigenetic regulation of gene expression to promote parasite life cycle progression, dormancy, and transmission.

Revisiting the determinants of malaria transmission

Malaria parasites have coevolved with humans over thousands of years, mirroring their migration out of Africa. They persist to this day, despite continuous elimination efforts worldwide. These parasites can adapt to changing environments during infection of human and mosquito, and when expanding the geographical range by switching vector species. Recent studies in the human malaria parasite, Plasmodium falciparum, identified determinants governing the plasticity of sexual conversion rates, sex ratio, and vector competence. Here we summarize the latest literature revealing environmental, epigenetic, and genetic determinants of malaria transmission.

Plasmodium berghei Purified Hemozoin Associated with DNA Strongly Inhibits P. berghei Liver-Stage Development in BALB/c Mice after Intravenous Inoculation

In the acidic lysosome-like digestive vacuole, Plasmodium parasites crystallize heme from hemoglobin into hemozoin, or malaria pigment. Upon release of progeny merozoites, the residual hemozoin is phagocytized by macrophages principally in the liver and spleen where the heme crystals can persist for months to years, as heme oxygenase does not readily degrade the crystal. Previous studies demonstrated hemozoin modulation of monocytes and macrophages. Hemozoin modulates immune function activity of monocytes/macrophages. Here, we used purified/washed hemozoin (W-Hz) isolated from murine Plasmodium berghei infections and intravenously (i.v.) injected it back into naive mice. We characterized the modulating effect of W-Hz on liver-stage replication. Purified washed hemozoin decreases P. berghei liver levels both at 1 week and 1 month after i.v. injection in a dose and time dependent fashion. The injected hemozoin fully protected in nine out of 10 mice given a 50 sporozoite inoculum, and in 10 out of 10 mice against 2,000 sporozoites when they were infected an hour or a day after hemozoin inoculation. DNase treatment at the hemozoin reversed the observed liver load reduction. The liver load reduction was similar in mature B- and T-cell-deficient RAG-1 knockout (KO) mice suggesting an innate immune protection mechanism. This work indicates a role for residual hemozoin in down modulation of Plasmodium liver stages.

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.

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.

The private life of malaria parasites: Strategies for sexual reproduction

Malaria parasites exhibit a complex lifecycle, requiring extensive asexual replication in the liver and blood of the vertebrate host, and in the haemocoel of the insect vector. Yet, they must also undergo a single round of sexual reproduction, which occurs in the vector's midgut upon uptake of a blood meal. Sexual reproduction is obligate for infection of the vector and thus, is essential for onwards transmission to new hosts. Sex in malaria parasites involves several bottlenecks in parasite number, making the stages involved attractive targets for blocking disease transmission. Malaria parasites have evolved a suite of adaptations ("strategies") to maximise the success of sexual reproduction and transmission, which could undermine transmission-blocking interventions. Yet, understanding parasite strategies may also reveal novel opportunities for such interventions. Here, we outline how evolutionary and ecological theories, developed to explain reproductive strategies in multicellular taxa, can be applied to explain two reproductive strategies (conversion rate and sex ratio) expressed by malaria parasites within the vertebrate host.

Activation of Anopheles stephensi Pantothenate Kinase and Coenzyme A Biosynthesis Reduces Infection with Diverse Plasmodium Species in the Mosquito Host

Malaria parasites require pantothenate from both human and mosquito hosts to synthesize coenzyme A (CoA). Specifically, mosquito-stage parasites cannot synthesize pantothenate de novo or take up preformed CoA from the mosquito host, making it essential for the parasite to obtain pantothenate from mosquito stores. This makes pantothenate utilization an attractive target for controlling sexual stage malaria parasites in the mosquito. CoA is synthesized from pantothenate in a multi-step pathway initiated by the enzyme pantothenate kinase (PanK). In this work, we manipulated A. stephensi PanK activity and assessed the impact of mosquito PanK activity on the development of two malaria parasite species with distinct genetics and life cycles: the human parasite Plasmodium falciparum and the mouse parasite Plasmodium yoelii yoelii 17XNL. We identified two putative A. stephensi PanK isoforms encoded by a single gene and expressed in the mosquito midgut. Using both RNAi and small molecules with reported activity against human PanK, we confirmed that A. stephensi PanK manipulation was associated with corresponding changes in midgut CoA levels. Based on these findings, we used two small molecule modulators of human PanK activity (PZ-2891, compound 7) at reported and ten-fold EC50 doses to examine the effects of manipulating A. stephensi PanK on malaria parasite infection success. Our data showed that oral provisioning of 1.3 nM and 13 nM PZ-2891 increased midgut CoA levels and significantly decreased infection success for both Plasmodium species. In contrast, oral provisioning of 62 nM and 620 nM compound 7 decreased CoA levels and significantly increased infection success for both Plasmodium species. This work establishes the A. stephensi CoA biosynthesis pathway as a potential target for broadly blocking malaria parasite development in anopheline hosts. We envision this strategy, with small molecule PanK modulators delivered to mosquitoes via attractive bait stations, working in concert with deployment of parasite-directed novel pantothenamide drugs to block parasite infection in the human host. In mosquitoes, depletion of pantothenate through manipulation to increase CoA biosynthesis is expected to negatively impact Plasmodium survival by starving the parasite of this essential nutrient. This has the potential to kill both wild type parasites and pantothenamide-resistant parasites that could develop under pantothenamide drug pressure if these compounds are used as future therapeutics for human malaria.

Creative interior design by Plasmodium falciparum: Lipid metabolism and the parasite's secret chamber

Malaria parasites conceal themselves within host erythrocytes and establish a necessary logistics system through the three-membrane layered structures of these cells. To establish this system, lipid metabolism is needed for the de novo synthesis of lipids and the recycling of extracellular lipids and erythrocyte lipid components. Cholesterol supply depends on its uptake from the extracellular environment and erythrocyte cytoplasm, but phospholipids can be synthesized on their own. This differential production of lipid species creates unique modifications in the lipid profile of parasitized erythrocytes, which in turn may influence the biophysical and/or mechanical properties of organelles and vesicles and communication among them. Variations in local membrane properties possibly influence the transportation of various molecules such as parasite-derived proteins, because efficiencies in secretion, vesicle fusion and budding are partly determined by the lipid profiles. Comprehensive understanding of the parasite's lipid metabolism and the biophysics of lipid membranes provides fundamental knowledge about these pathogenic organisms and could lead to new anti-malarials.