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

Time to switch gears: how long noncoding RNAs function as epigenetic regulators in Apicomplexan parasites

Long noncoding RNAs (lncRNA) are emerging as important regulators of gene expression in eukaryotes. In recent years, a large repertoire of lncRNA were discovered in Apicomplexan parasites and were implicated in several mechanisms of gene expression, including marking genes for activation, contributing to the formation of subnuclear compartments and organization, regulating the deposition of epigenetic modifications, influencing chromatin and chromosomal structure and manipulating host gene expression. Here, we aim to update recent knowledge on the role of lncRNAs as regulators in Apicomplexan parasites and highlight the possible molecular mechanisms by which they function. We hope that some of the hypotheses raised here will contribute to further investigation and lead to new mechanistic insight and better understanding of the role of lncRNA in parasite's biology.

The Plasmodium falciparum CCCH zinc finger protein MD3 regulates male gametocytogenesis through its interaction with RNA-binding proteins

The transmission of malaria parasites to mosquitoes is dependent on the formation of gametocytes. Once fully matured, gametocytes are able to transform into gametes in the mosquito's midgut, a process accompanied with their egress from the enveloping erythrocyte. Gametocyte maturation and gametogenesis require a well-coordinated gene expression program that involves a wide spectrum of regulatory proteins, ranging from histone modifiers to transcription factors to RNA-binding proteins. Here, we investigated the role of the CCCH zinc finger protein MD3 in Plasmodium falciparum gametocytogenesis. MD3 was originally identified as an epigenetically regulated protein of immature gametocytes and recently shown to be involved in male development in a barcode-based screen in P. berghei. We report that MD3 is mainly present in the cytoplasm of immature male P. falciparum gametocytes. Parasites deficient of MD3 are impaired in gametocyte maturation and male gametocytogenesis. BioID analysis in combination with co-immunoprecipitation assays unveiled an interaction network of MD3 with RNA-binding proteins like PABP1 and ALBA3, with translational initiators, regulators and repressors like elF4G, PUF1, NOT1 and CITH, and with further regulators of gametocytogenesis, including ZNF4, MD1 and GD1. We conclude that MD3 is part of a regulator complex crucial for post-transcriptional fine-tuning of male gametocytogenesis.

Comparative proteomics of vesicles essential for the egress of Plasmodium falciparum gametocytes from red blood cells

Transmission of malaria parasites to the mosquito is mediated by sexual precursor cells, the gametocytes. Upon entering the mosquito midgut, the gametocytes egress from the enveloping erythrocyte while passing through gametogenesis. Egress follows an inside-out mode during which the membrane of the parasitophorous vacuole (PV) ruptures prior to the erythrocyte membrane. Membrane rupture requires exocytosis of specialized egress vesicles of the parasites; that is, osmiophilic bodies (OBs) involved in rupturing the PV membrane, and vesicles that harbor the perforin-like protein PPLP2 (here termed P-EVs) required for erythrocyte lysis. While some OB proteins have been identified, like G377 and MDV1/Peg3, the majority of egress vesicle-resident proteins is yet unknown. Here, we used high-resolution imaging and BioID methods to study the two egress vesicle types in Plasmodium falciparum gametocytes. We show that OB exocytosis precedes discharge of the P-EVs and that exocytosis of the P-EVs, but not of the OBs, is calcium sensitive. Both vesicle types exhibit distinct proteomes with the majority of proteins located in the OBs. In addition to known egress-related proteins, we identified novel components of OBs and P-EVs, including vesicle-trafficking proteins. Our data provide insight into the immense molecular machinery required for the inside-out egress of P. falciparum gametocytes.

Antimalarial target vulnerability of the putative Plasmodium falciparum methionine synthase

Background

Plasmodium falciparum possesses a cobalamin-dependent methionine synthase (MS). MS is putatively encoded by the PF3D7_1233700 gene, which is orthologous and syntenic in Plasmodium. However, its vulnerability as an antimalarial target has not been assessed.

Methods

We edited the PF3D7_1233700 and PF3D7_0417200 (dihydrofolate reductase-thymidylate synthase, DHFR-TS) genes and obtained transgenic P. falciparum parasites expressing epitope-tagged target proteins under the control of the glmS ribozyme. Conditional loss-of-function mutants were obtained by treating transgenic parasites with glucosamine.

Results

DHFR-TS, but not MS mutants showed a significant proliferation defect over 96 h, suggesting that P. falciparum MS is not a vulnerable antimalarial target.

A Potent Kalihinol Analogue Disrupts Apicoplast Function and Vesicular Trafficking in P. falciparum Malaria

Here we report the discovery of MED6-189, a new analogue of the kalihinol family of isocyanoterpene (ICT) natural products. MED6-189 is effective against drug-sensitive and -resistant P. falciparum strains blocking both intraerythrocytic asexual replication and sexual differentiation. This compound was also effective against P. knowlesi and P. cynomolgi. In vivo efficacy studies using a humanized mouse model of malaria confirms strong efficacy of the compound in animals with no apparent hemolytic activity or apparent toxicity. Complementary chemical biology, molecular biology, genomics and cell biological analyses revealed that MED6-189 primarily targets the parasite apicoplast and acts by inhibiting lipid biogenesis and cellular trafficking. Genetic analyses in P. falciparum revealed that a mutation in PfSec13, which encodes a component of the parasite secretory machinery, reduced susceptibility to the drug. The high potency of MED6-189 in vitro and in vivo, its broad range of efficacy, excellent therapeutic profile, and unique mode of action make it an excellent addition to the antimalarial drug pipeline.

Correlates of Iron, Cobalamin, Folate, and Vitamin A Status among Stunted Children: A Cross-Sectional Study in Uganda

Micronutrient deficiencies and stunting are prevalent. We assessed correlates of iron, cobalamin, folate, and vitamin A biomarkers in a cross-sectional study of stunted children aged 12-59 months in eastern Uganda. The biomarkers measured were serum ferritin (S-FE), soluble transferrin receptor (S-TfR), retinol binding protein (S-RBP), plasma cobalamin (P-Cob), methylmalonic acid (P-MMA), and folate (P-Fol). Using linear regression, we assessed socio-demography, stunting severity, malaria rapid test, and inflammation as correlates of micronutrient biomarkers. Of the 750 children, the mean (SD) age was 32.0 (11.7) months, and 45% were girls. Iron stores were depleted (inflammation-corrected S-FE < 12 µg/L) in 43%, and 62% had tissue iron deficiency (S-TfR > 8.3 mg/L). P-Cob was low (<148 pmol/L) and marginal (148-221 pmol/L) in 3% and 20%, and 16% had high P-MMA (>0.75 µmol/L). Inflammation-corrected S-RBP was low (<0.7 µmol/L) in 21% and P-Fol (<14 nmol/L) in 1%. Age 24-59 months was associated with higher S-FE and P-Fol and lower S-TfR. Breastfeeding beyond infancy was associated with lower iron status and cobalamin status, and malaria was associated with lower cobalamin status and tissue iron deficiency (higher S-TfR) despite iron sequestration in stores (higher S-FE). In conclusion, stunted children have iron, cobalamin, and vitamin A deficiencies. Interventions addressing stunting should target co-existing micronutrient deficiencies.

Phenotypic Screens Identify Genetic Factors Associated with Gametocyte Development in the Human Malaria Parasite Plasmodium falciparum

Transmission of the deadly malaria parasite Plasmodium falciparum from humans to mosquitoes is achieved by specialized intraerythrocytic sexual forms called gametocytes. Though the crucial regulatory mechanisms leading to gametocyte commitment have recently come to light, networks of genes that control sexual development remain to be elucidated. Here, we report a pooled-mutant screen to identify genes associated with gametocyte development in P. falciparum. Our results categorized genes that modulate gametocyte progression as hypoproducers or hyperproducers of gametocytes, and the in-depth analysis of individual clones confirmed phenotypes in sexual commitment rates and putative functions in gametocyte development. We present a new set of genes that have not been implicated in gametocytogenesis before and demonstrate the potential of forward genetic screens in isolating genes impacting parasite sexual biology, an exciting step toward the discovery of new antimalarials for a globally significant pathogen. IMPORTANCE Blocking human-to-vector transmission is an essential step toward malaria elimination. Gametocytes are solely responsible for achieving this transmission and represent an opportunity for therapeutic intervention. While these falciform-shaped parasite stages were first discovered in the 1880s, our understanding of the genetic determinants responsible for their formation and molecular mechanisms that drive their development is limited. In this work, we developed a scalable screening methodology with piggyBac mutants to identify genes that influence the development of gametocytes in the most lethal human malaria parasite, P. falciparum. By doing so, we lay the foundation for large-scale functional genomic studies specifically designed to address remaining questions about sexual commitment, maturation, and mosquito infection in P. falciparum. Such functional genetic screens will serve to expedite the identification of essential pathways and processes for the development of novel transmission-blocking agents.