Redesigned TetR-Aptamer System To Control Gene Expression in Plasmodium falciparum

The dynamin-related protein PfDyn2 is essential for both apicoplast and mitochondrial fission in Plasmodium falciparum

Dynamins, or dynamin-related proteins (DRPs), are large mechano-sensitive GTPases that mediate membrane dynamics or organellar fission/fusion events. Plasmodium falciparum encodes three dynamin-like proteins whose functions are poorly understood. Here, we demonstrate that one of these dynamin-related proteins, PfDyn2, is required to divide both the apicoplast and the mitochondrion, a striking divergence from the biology of related parasites. Using super-resolution and ultrastructure expansion microscopy (U-ExM), we show that PfDyn2 is expressed in dividing schizonts, and that it localizes to both the apicoplast and the mitochondrion. Our use of long-term, live-cell microscopy allows for the visualization of apicoplast and mitochondrial division in live parasites at super resolution for the first time, and demonstrates that in PfDyn2-deficient parasites, while the apicoplast and mitochondrion increase in size and complexity, they do not undergo fission. We also show that these organellar fission defects prevent successful individualization of the schizont mass and the formation of new daughter cells, or merozoites because the basal complex, the cytokinetic ring of Plasmodium, cannot fully contract in PfDyn2-deficient parasites, a phenotype secondary to physical blockage by undivided organelles occluding the ring. PfDyn2's singular role in mediating both apicoplast and mitochondrial fission has not been observed in other organisms possessing two endosymbiotic organelles, including other Apicomplexans, thus reflecting a unique, potentially exploitable method of organellar division in P. falciparum.IMPORTANCEPlasmodium falciparum remains a significant global pathogen, causing over 200 million infections and over 600,000 deaths per year. One significant obstacle to the control of malaria is increasing resistance to first-line artemisinin-based antimalarials. Another is a lack of basic knowledge about the cell biology of the parasite. Along with the mitochondrion, Plasmodium contains a second organelle descended from an endosymbiotic event, the apicoplast. Both organelles are common targets for antimalarials, but because many proteins involved in organellar fission are not conserved in Plasmodium, until now, the mechanisms underlying apicoplast and mitochondrial division have been unknown. In this study, we demonstrate that PfDyn2, a dynamin-related protein (DRP), is required for the division of both organelles. We also show that defects in organellar division hinder segmentation of the schizont and formation of invasive merozoites by preventing full contraction of the basal complex. By demonstrating its necessity for the proper division of both the apicoplast and the mitochondria, this study highlights PfDyn2 as a potential target for new antimalarials.

tRNA modifying enzymes MnmE and MnmG are essential for Plasmodium falciparum apicoplast maintenance

The circular genome of the Plasmodium falciparum apicoplast contains a complete minimal set of tRNAs, positioning the apicoplast as an ideal model for studying the fundamental factors required for protein translation. Modifications at tRNA wobble base positions, such as xm5s2U, are critical for accurate protein translation. These modifications are ubiquitously found in tRNAs decoding two-family box codons ending in A or G in prokaryotes and in eukaryotic organelles. Here, we investigated the xm5s2U biosynthetic pathway in the apicoplast organelle of P. falciparum. Through comparative genomics, we identified orthologs of enzymes involved in this process: SufS, MnmA, MnmE, and MnmG. While SufS and MnmA were previously shown to catalyze s2U modifications, we now show that MnmE and MnmG are apicoplast-localized and contain features required for xm5s2U biosynthetic activity. Notably, we found that P. falciparum lacks orthologs of MnmC, MnmL, and MnmM, suggesting that the parasites contain a minimal xm5s2U biosynthetic pathway similar to that found in bacteria with reduced genomes. Deletion of either MnmE or MnmG resulted in apicoplast disruption and parasite death, mimicking the phenotype observed in ΔmnmA and ΔsufS parasites. Our data strongly support the presence and essentiality of xm5s2U modifications in apicoplast tRNAs. This study advances our understanding of the minimal requirements for protein translation in the apicoplast organelle.

PfMORC protein regulates chromatin accessibility and transcriptional repression in the human malaria parasite, Plasmodium falciparum

The environmental challenges the human malaria parasite, Plasmodium falciparum, faces during its progression into its various lifecycle stages warrant the use of effective and highly regulated access to chromatin for transcriptional regulation. Microrchidia (MORC) proteins have been implicated in DNA compaction and gene silencing across plant and animal kingdoms. Accumulating evidence has shed light on the role MORC protein plays as a transcriptional switch in apicomplexan parasites. In this study, using the CRISPR/Cas9 genome editing tool along with complementary molecular and genomics approaches, we demonstrate that PfMORC not only modulates chromatin structure and heterochromatin formation throughout the parasite erythrocytic cycle, but is also essential to the parasite survival. Chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) experiments suggests that PfMORC binds to not only sub-telomeric regions and genes involved in antigenic variation but may also play a role in modulating stage transition. Protein knockdown experiments followed by chromatin conformation capture (Hi-C) studies indicate that downregulation of PfMORC impairs key histone marks and induces the collapse of the parasite heterochromatin structure leading to its death. All together these findings confirm that PfMORC plays a crucial role in chromatin structure and gene regulation, validating this factor as a strong candidate for novel antimalarial strategies.

MULTIPLE, REDUNDANT CARBOXYLIC ACID TRANSPORTERS SUPPORT MITOCHONDRIAL METABOLISM IN PLASMODIUM FALCIPARUM

The mitochondrion of the deadliest human malaria parasite, Plasmodium falciparum, is an essential source of cellular acetyl-CoA during the asexual blood-stage of the parasite life cycle. Blocking mitochondrial acetyl-CoA synthesis leads to a hypoacetylated proteome and parasite death. We previously determined that mitochondrial acetyl-CoA is primarily synthesized from glucose-derived pyruvate by α-ketoacid dehydrogenases. Here, we asked if inhibiting the import of glycolytic pyruvate across the mitochondrial inner membrane would affect acetyl-CoA production and, thus, could be a potential target for antimalarial drug development. We selected the two predicted mitochondrial pyruvate carrier proteins ( Pf MPC1 and Pf MPC2) for genetic knockout and isotopic metabolite tracing via HPLC-MS metabolomic analysis. Surprisingly, we observed that asexual blood-stage parasites could survive the loss of either or both Pf MPCs with only minor growth defects, despite a substantial reduction in the amount of glucose-derived isotopic labelling into acetyl-CoA. Furthermore, genetic deletion of two additional mitochondrial carboxylic acid transporters - DTC (di/tricarboxylic acid carrier) and YHM2 (a putative citrate/α-ketoglutarate carrier protein) - only mildly affected asexual blood-stage replication, even in the context of Pf MPC deficiency. Although we observed no added impact on the incorporation of glucose carbon into acetyl-CoA in these quadruple knockout mutants, we noted a large decrease in glutamine-derived label in tricarboxylic acid cycle metabolites, suggesting that DTC and YHM2 both import glutamine derivatives into the mitochondrion. Altogether, our results expose redundant routes used to fuel the blood-stage malaria parasite mitochondrion with imported carbon from two major sources - glucose and glutamine.

SIGNIFICANCE

The mitochondrion of malaria parasites generates key molecules, such as acetyl-CoA, that are required for numerous cellular processes. To support mitochondrial biosynthetic pathways, the parasites must transport carbon sources into this organelle. By studying how the mitochondrion obtains pyruvate, a molecule derived from glucose, we have uncovered redundant carbon transport systems that ensure parasite survival in red blood cells. This metabolic redundancy poses a challenge for drug development, as it enables the parasite to adapt and survive by relying on alternative pathways when one is disrupted.

Mapping Plasmodium transitions and interactions in the Anopheles female

The human malaria parasite, Plasmodium falciparum , relies on Anopheles mosquitoes for transmission. Once ingested during blood feeding, most parasites die in the mosquito midgut lumen or during epithelium traversal. How surviving ookinetes interact with midgut cells and form oocysts is unknown, yet these steps are essential to initiate a remarkable, similarly uncharacterized growth process culminating in the production of thousands of infectious sporozoites. Here, using single-cell RNA sequencing of both parasites and mosquito cells across four time points and two metabolic conditions, we unveil key processes shaping developmental transitions and mosquito-parasite interactions occurring in the midgut. In depth functional analyses reveal processes regulating oocyst growth and identify the transcription factor Pf SIP2 as essential for sporozoite infection of human hepatocytes. By combining the analysis of shared mosquito-parasite barcodes with confocal microscopy, we discover that parasites preferentially interact with midgut progenitor cells during epithelial crossing, potentially using their basal location as an exit landmark. Additionally, we unveil tight connections between extracellular late oocysts and surrounding muscle cells that may ensure parasites adhere to the midgut without damaging it. Ultimately, our study provides fundamental insight into the molecular events characterizing previously inaccessible biological transitions and mosquito-parasite interactions, and identifies candidates for transmission-blocking strategies.

Identification of a divalent metal transporter required for cellular iron metabolism in malaria parasites

Plasmodium falciparum malaria parasites invade and multiply inside red blood cells (RBCs), the most iron-rich compartment in humans. Like all cells, P. falciparum requires nutritional iron to support essential metabolic pathways, but the critical mechanisms of iron acquisition and trafficking during RBC infection have remained obscure. Parasites internalize and liberate massive amounts of heme during large-scale digestion of RBC hemoglobin within an acidic food vacuole (FV) but lack a heme oxygenase to release porphyrin-bound iron. Although most FV heme is sequestered into inert hemozoin crystals, prior studies indicate that trace heme escapes biomineralization and is susceptible to nonenzymatic degradation within the oxidizing FV environment to release labile iron. Parasites retain a homolog of divalent metal transporter 1 (DMT1), a known mammalian iron transporter, but its role in P. falciparum iron acquisition has not been tested. Our phylogenetic studies indicate that P. falciparum DMT1 (PfDMT1) retains conserved molecular features critical for metal transport. We localized this protein to the FV membrane and defined its orientation in an export-competent topology. Conditional knockdown of PfDMT1 expression is lethal to parasites, which display broad cellular defects in iron-dependent functions, including impaired apicoplast biogenesis and mitochondrial polarization. Parasites are selectively rescued from partial PfDMT1 knockdown by supplementation with exogenous iron, but not other metals. These results support a cellular paradigm whereby PfDMT1 is the molecular gatekeeper to essential iron acquisition by blood-stage malaria parasites and suggest that therapeutic targeting of PfDMT1 may be a potent antimalarial strategy.

A kalihinol analog disrupts apicoplast function and vesicular trafficking in P. falciparum malaria

We report the discovery of MED6-189, an analog of the kalihinol family of isocyanoterpene natural products that is effective against drug-sensitive and drug-resistant Plasmodium falciparum strains, blocking both asexual replication and sexual differentiation. In vivo studies using a humanized mouse model of malaria confirm strong efficacy of the compound in animals with no apparent hemolytic activity or toxicity. Complementary chemical, molecular, and genomics analyses revealed that MED6-189 targets the parasite apicoplast and acts by inhibiting lipid biogenesis and cellular trafficking. Genetic analyses revealed that a mutation in PfSec13, which encodes a component of the parasite secretory machinery, reduced susceptibility to the drug. Its high potency, excellent therapeutic profile, and distinctive mode of action make MED6-189 an excellent addition to the antimalarial drug pipeline.

Plasmodium SEY1 is a novel druggable target that contributes to imidazolopiperazine mechanism of action

The precise mode of action of ganaplacide (KAF156), a phase III antimalarial candidate, remains elusive. Here we employ omics-based methods with the closely related chemical analog, GNF179, to search for potential Plasmodium targets. Ranking potential targets derived from chemical genetics and proteomic affinity chromatography methodologies identifies SEY1, or Synthetic Enhancement of YOP1, which is predicted to encode an essential dynamin-like GTPase implicated in homotypic fusion of endoplasmic reticulum (ER) membranes. We demonstrate that GNF179 decreases Plasmodium SEY1 melting temperature. We further show that GNF179 binds to recombinant Plasmodium SEY1 and subsequently inhibits its GTPase activity, which is required for maintaining ER architecture. Using ultrastructure expansion microscopy, we find GNF179 treatment changes parasite ER and Golgi morphology. We also confirm that SEY1 is an essential gene in P. falciparum. These data suggest that SEY1 may contribute to the mechanism of action of imidazolopiperazines and is a new and attractive druggable target.

tRNA lysidinylation is essential for the minimal translation system found in the apicoplast of Plasmodium falciparum

For decades, researchers have sought to define minimal genomes to elucidate the fundamental principles of life and advance biotechnology. tRNAs, essential components of this machinery, decode mRNA codons into amino acids. The apicoplast of malaria parasites encodes 25 tRNA isotypes in its organellar genome - the lowest number found in known translation systems. Efficient translation in such minimal systems depends heavily on post-transcriptional tRNA modifications, especially at the wobble anticodon position. Lysidine modification at the wobble position (C34) of tRNACAU distinguishes between methionine (AUG) and isoleucine (AUA) codons, altering the amino acid delivered by this tRNA and ensuring accurate protein synthesis. Lysidine is formed by the enzyme tRNA isoleucine lysidine synthetase (TilS) and is nearly ubiquitous in bacteria and essential for cellular viability. We identified a TilS ortholog (PfTilS) located in the apicoplast of Plasmodium falciparum parasites. By complementing PfTilS with a bacterial ortholog, we demonstrated that the lysidinylation activity of PfTilS is critical for parasite survival and apicoplast maintenance, likely due to its impact on apicoplast protein translation. Our findings represent the first characterization of TilS in an endosymbiotic organelle, advancing eukaryotic organelle research and our understanding of minimal translational machinery. Due to the absence of lysidine modifications in humans, this research also exposes a potential vulnerability in malaria parasites that could be targeted by antimalarial strategies.

Plasmodium RON11 triggers biogenesis of the merozoite rhoptry pair and is essential for erythrocyte invasion

Malaria is a global and deadly human disease caused by the apicomplexan parasites of the genus Plasmodium. Parasite proliferation within human red blood cells (RBCs) is associated with the clinical manifestations of the disease. This asexual expansion within human RBCs begins with the invasion of RBCs by P. falciparum, which is mediated by the secretion of effectors from 2 specialized club-shaped secretory organelles in merozoite-stage parasites known as rhoptries. We investigated the function of the Rhoptry Neck Protein 11 (RON11), which contains 7 transmembrane domains and calcium-binding EF-hand domains. We generated conditional mutants of the P. falciparum RON11. Knockdown of RON11 inhibits parasite growth by preventing merozoite invasion. The loss of RON11 did not lead to any defects in processing of rhoptry proteins but instead led to a decrease in the amount of rhoptry proteins. We utilized ultrastructure expansion microscopy (U-ExM) to determine the effect of RON11 knockdown on rhoptry biogenesis. Surprisingly, in the absence of RON11, fully developed merozoites had only 1 rhoptry each. The single rhoptry in RON11-deficient merozoites were morphologically typical with a bulb and a neck oriented into the apical polar ring. Moreover, rhoptry proteins are trafficked accurately to the single rhoptry in RON11-deficient parasites. These data show that in the absence of RON11, the first rhoptry is generated during schizogony but upon the start of cytokinesis, the second rhoptry never forms. Interestingly, these single-rhoptry merozoites were able to attach to host RBCs but are unable to invade RBCs. Instead, RON11-deficient merozoites continue to engage with RBC for prolonged periods eventually resulting in echinocytosis, a result of secreting the contents from the single rhoptry into the RBC. Together, our data show that RON11 triggers the de novo biogenesis of the second rhoptry and functions in RBC invasion.

Functionality of the V-type ATPase during asexual growth and development of Plasmodium falciparum

Vacuolar type ATPases (V-type ATPases) are highly conserved hetero-multisubunit proton pumping machineries found in all eukaryotes. They utilize ATP hydrolysis to pump protons, acidifying intracellular or extracellular compartments, and are thus crucial for various biological processes. Despite their evolutionary conservation in malaria parasites, this proton pump remains understudied. To understand the localization and biological functions of Plasmodium falciparum V-type ATPase, we employed CRISPR/Cas9 to endogenously tag the subunit A of the V1 domain. V1A (PF3D7_1311900) was tagged with a triple hemagglutinin epitope and the TetR-DOZI-aptamer system for conditional expression under the regulation of anhydrotetracycline. Via immunofluorescence assays, we identified that V-type ATPase is expressed throughout the intraerythrocytic developmental cycle and is mainly localized to the digestive vacuole and parasite plasma membrane. Immuno-electron microscopy further revealed that V-type ATPase is also localized on secretory organelles in merozoites. Knockdown of V1A led to cytosolic pH imbalance and blockage of hemoglobin digestion in the digestive vacuole, resulting in an arrest of parasite development in the trophozoite-stage and, ultimately, parasite demise. Using bafilomycin A1, a specific inhibitor of V-type ATPases, we found that the P. falciparum V-type ATPase is likely involved in parasite invasion but is not critical for ring-stage development. Further, we detected a large molecular weight complex in blue native-PAGE (∼1.0 MDa), corresponding to the total molecular weights of V1 and Vo domains. Together, we show that V-type ATPase is localized to multiple subcellular compartments in P. falciparum, and its functionality throughout the asexual cycle varies depending on the parasite developmental stages.

PfMORC protein regulates chromatin accessibility and transcriptional repression in the human malaria parasite, Plasmodium falciparum

The environmental challenges the human malaria parasite, Plasmodium falciparum, faces during its progression into its various lifecycle stages warrant the use of effective and highly regulated access to chromatin for transcriptional regulation. Microrchidia (MORC) proteins have been implicated in DNA compaction and gene silencing across plant and animal kingdoms. Accumulating evidence has shed light into the role MORC protein plays as a transcriptional switch in apicomplexan parasites. In this study, using CRISPR/Cas9 genome editing tool along with complementary molecular and genomics approaches, we demonstrate that PfMORC not only modulates chromatin structure and heterochromatin formation throughout the parasite erythrocytic cycle, but is also essential to the parasite survival. Chromatin immunoprecipitation followed by deep sequencing (ChIP-seq) experiments suggest that PfMORC binds to not only sub-telomeric regions and genes involved in antigenic variation but may also play a role in modulating stage transition. Protein knockdown experiments followed by chromatin conformation capture (Hi-C) studies indicate that downregulation of PfMORC impairs key histone marks and induces the collapse of the parasite heterochromatin structure leading to its death. All together these findings confirm that PfMORC plays a crucial role in chromatin structure and gene regulation, validating this factor as a strong candidate for novel antimalarial strategies.

Peptidic Boronic Acid Plasmodium falciparum SUB1 Inhibitors with Improved Selectivity over Human Proteasome

Plasmodium falciparum subtilisin-like serine protease 1 (PfSUB1) is essential for egress of invasive merozoite forms of the parasite, rendering PfSUB1 an attractive antimalarial target. Here, we report studies aimed to improve drug-like properties of peptidic boronic acid PfSUB1 inhibitors including increased lipophilicity and selectivity over human proteasome (H20S). Structure-activity relationship investigations revealed that lipophilic P3 amino acid side chains as well as N-capping groups were well tolerated in retaining PfSUB1 inhibitory potency. At the P1 position, replacing the methyl group with a carboxyethyl substituent led to boralactone PfSUB1 inhibitors with remarkably improved selectivity over H20S. Combining lipophilic end-capping groups with the boralactone reduced the selectivity over H20S. However, compound 4c still showed >60-fold selectivity versus H20S and low nanomolar PfSUB1 inhibitory potency. Importantly, this compound inhibited the growth of a genetically modified P. falciparum line expressing reduced levels of PfSUB1 13-fold more efficiently compared to a wild-type parasite line.

Identification of a divalent metal transporter required for cellular iron metabolism in malaria parasites

Plasmodium falciparum malaria parasites invade and multiply inside red blood cells (RBCs), the most iron-rich compartment in humans. Like all cells, P. falciparum requires nutritional iron to support essential metabolic pathways, but the critical mechanisms of iron acquisition and trafficking during RBC infection have remained obscure. Parasites internalize and liberate massive amounts of heme during large-scale digestion of RBC hemoglobin within an acidic food vacuole (FV) but lack a heme oxygenase to release porphyrin-bound iron. Although most FV heme is sequestered into inert hemozoin crystals, prior studies indicate that trace heme escapes biomineralization and is susceptible to non-enzymatic degradation within the oxidizing FV environment to release labile iron. Parasites retain a homolog of divalent metal transporter 1 (DMT1), a known mammalian iron transporter, but its role in P. falciparum iron acquisition has not been tested. Our phylogenetic studies indicate that P. falciparum DMT1 (PfDMT1) retains conserved molecular features critical for metal transport. We localized this protein to the FV membrane and defined its orientation in an export-competent topology. Conditional knockdown of PfDMT1 expression is lethal to parasites, which display broad cellular defects in iron-dependent functions, including impaired apicoplast biogenesis and mitochondrial polarization. Parasites are selectively rescued from partial PfDMT1 knockdown by supplementation with exogenous iron, but not other metals. These results support a cellular paradigm whereby PfDMT1 is the molecular gatekeeper to essential iron acquisition by blood-stage malaria parasites and suggest that therapeutic targeting of PfDMT1 may be a potent antimalarial strategy.

Elucidating the spatio-temporal dynamics of the Plasmodium falciparum basal complex

Asexual replication of Plasmodium falciparum occurs via schizogony, wherein 16-36 daughter cells are produced within the parasite during one semi-synchronized cytokinetic event. Schizogony requires a divergent contractile ring structure known as the basal complex. Our lab has previously identified PfMyoJ (PF3D7_1229800) and PfSLACR (PF3D7_0214700) as basal complex proteins recruited midway through segmentation. Using ultrastructure expansion microscopy, we localized both proteins to a novel basal complex subcompartment. While both colocalize with the basal complex protein PfCINCH upon recruitment, they form a separate, more basal subcompartment termed the posterior cup during contraction. We also show that PfSLACR is recruited to the basal complex prior to PfMyoJ, and that both proteins are removed unevenly as segmentation concludes. Using live-cell microscopy, we show that actin dynamics are dispensable for basal complex formation, expansion, and contraction. We then show that EF-hand containing P. falciparum Centrin 2 partially localizes to this posterior cup of the basal complex and that it is essential for growth and replication, with variable defects in basal complex contraction and synchrony. Finally, we demonstrate that free intracellular calcium is necessary but not sufficient for basal complex contraction in P. falciparum. Thus, we demonstrate dynamic spatial compartmentalization of the Plasmodium falciparum basal complex, identify an additional basal complex protein, and begin to elucidate the unique mechanism of contraction utilized by P. falciparum, opening the door for further exploration of Apicomplexan cellular division.

Apicoplast-Resident Processes: Exploiting the Chink in the Armour of Plasmodium falciparum Parasites

The discovery of a relict plastid, also known as an apicoplast (apicomplexan plastid), that houses housekeeping processes and metabolic pathways critical to Plasmodium parasites' survival has prompted increased research on identifying potent inhibitors that can impinge on apicoplast-localised processes. The apicoplast is absent in humans, yet it is proposed to originate from the eukaryote's secondary endosymbiosis of a primary symbiont. This symbiotic relationship provides a favourable microenvironment for metabolic processes such as haem biosynthesis, Fe-S cluster synthesis, isoprenoid biosynthesis, fatty acid synthesis, and housekeeping processes such as DNA replication, transcription, and translation, distinct from analogous mammalian processes. Recent advancements in comprehending the biology of the apicoplast reveal it as a vulnerable organelle for malaria parasites, offering numerous potential targets for effective antimalarial therapies. We provide an overview of the metabolic processes occurring in the apicoplast and discuss the organelle as a viable antimalarial target in light of current advances in drug discovery. We further highlighted the relevance of these metabolic processes to Plasmodium falciparum during the different stages of the lifecycle.

The dynamin-related protein Dyn2 is essential for both apicoplast and mitochondrial fission in Plasmodium falciparum

Dynamins, or dynamin-related proteins (DRPs), are large mechano-sensitive GTPases mediating membrane dynamics or organellar fission/fusion events. Plasmodium falciparum encodes three dynamin-like proteins whose functions are poorly understood. Here, we demonstrate that PfDyn2 mediates both apicoplast and mitochondrial fission. Using super-resolution and ultrastructure expansion microscopy, we show that PfDyn2 is expressed in the schizont stage and localizes to both the apicoplast and mitochondria. Super-resolution long-term live cell microscopy shows that PfDyn2-deficient parasites cannot complete cytokinesis because the apicoplast and mitochondria do not undergo fission. Further, the basal complex or cytokinetic ring in Plasmodium cannot fully contract upon PfDyn2 depletion, a phenotype secondary to physical blockage of undivided organelles in the middle of the ring. Our data suggest that organellar fission defects result in aberrant schizogony, generating unsuccessful merozoites. The unique biology of PfDyn2, mediating both apicoplast and mitochondrial fission, has not been observed in other organisms possessing two endosymbiotic organelles.

Highlights

PfDyn2 is essential for schizont-stage development.PfDyn2 mediates both apicoplast and mitochondrial fission.Deficiency of PfDyn2 leads to organellar fission failures and blockage of basal complex contraction.Addition of apicoplast-derived metabolite IPP does not rescue the growth defects.

Plasmodium RON11 triggers biogenesis of the merozoite rhoptry pair and is essential for erythrocyte invasion

Malaria is a global and deadly human disease caused by the apicomplexan parasites of the genus Plasmodium. Parasite proliferation within human red blood cells (RBC) is associated with the clinical manifestations of the disease. This asexual expansion within human RBCs, begins with the invasion of RBCs by P. falciparum, which is mediated by the secretion of effectors from two specialized club-shaped secretory organelles in merozoite-stage parasites known as rhoptries. We investigated the function of the Rhoptry Neck Protein 11 (RON11), which contains seven transmembrane domains and calcium-binding EF-hand domains. We generated conditional mutants of the P. falciparum RON11. Knockdown of RON11 inhibits parasite growth by preventing merozoite invasion. The loss of RON11 did not lead to any defects in processing of rhoptry proteins but instead led to a decrease in the amount of rhoptry proteins. We utilized ultrastructure expansion microscopy (U-ExM) to determine the effect of RON11 knockdown on rhoptry biogenesis. Surprisingly, in the absence of RON11, fully developed merozoites had only one rhoptry each. The single rhoptry in RON11 deficient merozoites were morphologically typical with a bulb and a neck oriented into the apical polar ring. Moreover, rhoptry proteins are trafficked accurately to the single rhoptry in RON11 deficient parasites. These data show that in the absence of RON11, the first rhoptry is generated during schizogony but upon the start of cytokinesis, the second rhoptry never forms. Interestingly, these single-rhoptry merozoites were able to attach to host RBCs but are unable to invade RBCs. Instead, RON11 deficient merozoites continue to engage with RBC for prolonged periods eventually resulting in echinocytosis, a result of secreting the contents from the single rhoptry into the RBC. Together, our data show that RON11 triggers the de novo biogenesis of the second rhoptry and functions in RBC invasion.

Plasmodium falciparum utilizes pyrophosphate to fuel an essential proton pump in the ring stage and the transition to trophozoite stage

During asexual growth and replication cycles inside red blood cells, the malaria parasite Plasmodium falciparum primarily relies on glycolysis for energy supply, as its single mitochondrion performs little or no oxidative phosphorylation. Post merozoite invasion of a host red blood cell, the ring stage lasts approximately 20 hours and was traditionally thought to be metabolically quiescent. However, recent studies have shown that the ring stage is active in several energy-costly processes, including gene transcription, protein translation, protein export, and movement inside the host cell. It has remained unclear whether a low glycolytic flux alone can meet the energy demand of the ring stage over a long period post invasion. Here, we demonstrate that the metabolic by-product pyrophosphate (PPi) is a critical energy source for the development of the ring stage and its transition to the trophozoite stage. During early phases of the asexual development, the parasite utilizes Plasmodium falciparum vacuolar pyrophosphatase 1 (PfVP1), an ancient pyrophosphate-driven proton pump, to export protons across the parasite plasma membrane. Conditional deletion of PfVP1 leads to a delayed ring stage that lasts nearly 48 hours and a complete blockage of the ring-to-trophozoite transition before the onset of parasite death. This developmental arrest can be partially rescued by an orthologous vacuolar pyrophosphatase from Arabidopsis thaliana, but not by the soluble pyrophosphatase from Saccharomyces cerevisiae, which lacks proton pumping activities. Since proton-pumping pyrophosphatases have been evolutionarily lost in human hosts, the essentiality of PfVP1 suggests its potential as an antimalarial drug target. A drug target of the ring stage is highly desired, as current antimalarials have limited efficacy against this stage.

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.

Essential function of alveolin PfIMC1g in the Plasmodium falciparum asexual blood stage

IMPORTANCE

Infection by the Plasmodium falciparum parasite is responsible for the most severe form of human malaria. The asexual blood stage of the parasite, which occurs inside human red blood cells, is responsible for the symptoms of malaria and is the target of most antimalarial drugs. Plasmodium spp. rely on their highly divergent cytoskeletal structures to scaffold their cell division, sustain the mechanical stress of invasion, and survive in both the human bloodstream and the mosquito. We investigate the function of a class of divergent intermediate filament-like proteins called alveolins in the clinically important blood stage. The functional role of individual alveolins in Plasmodium remains poorly understood due to pleiotropic effects of gene knockouts and redundancy among alveolins. We evaluate the localization and essentiality of the four asexual-stage alveolins and find that PfIMC1g and PfIMC1c are essential. Furthermore, we demonstrate that PfIMC1g is critical for survival of the parasite post-invasion.

Time-resolved proximity biotinylation implicates a porin protein in export of transmembrane malaria parasite effectors

The malaria-causing parasite, Plasmodium falciparum completely remodels its host red blood cell (RBC) through the export of several hundred parasite proteins, including transmembrane proteins, across multiple membranes to the RBC. However, the process by which these exported membrane proteins are extracted from the parasite plasma membrane for export remains unknown. To address this question, we fused the exported membrane protein, skeleton binding protein 1 (SBP1), with TurboID, a rapid, efficient and promiscuous biotin ligase (SBP1TbID). Using time-resolved proximity biotinylation and label-free quantitative proteomics, we identified two groups of SBP1TbID interactors - early interactors (pre-export) and late interactors (post-export). Notably, two promising membrane-associated proteins were identified as pre-export interactors, one of which possesses a predicted translocon domain, that could facilitate the export of membrane proteins. Further investigation using conditional mutants of these candidate proteins showed that these proteins were essential for asexual growth and localize to the host-parasite interface during early stages of the intraerythrocytic cycle. These data suggest that they might play a role in ushering membrane proteins from the parasite plasma membrane for export to the host RBC.

Knock-sideways by inducible ER retrieval enables a unique approach for studying Plasmodium-secreted proteins

Malaria parasites uniquely depend on protein secretion for their obligate intracellular lifestyle but approaches for dissecting Plasmodium-secreted protein functions are limited. We report knockER, a unique DiCre-mediated knock-sideways approach to sequester secreted proteins in the ER by inducible fusion with a KDEL ER-retrieval sequence. We show conditional ER sequestration of diverse proteins is not generally toxic, enabling loss-of-function studies. We employed knockER in multiple Plasmodium species to interrogate the trafficking, topology, and function of an assortment of proteins that traverse the secretory pathway to diverse compartments including the apicoplast (ClpB1), rhoptries (RON6), dense granules, and parasitophorous vacuole (EXP2, PTEX150, HSP101). Taking advantage of the unique ability to redistribute secreted proteins from their terminal destination to the ER, we reveal that vacuolar levels of the PTEX translocon component HSP101 but not PTEX150 are maintained in excess of what is required to sustain effector protein export into the erythrocyte. Intriguingly, vacuole depletion of HSP101 hypersensitized parasites to a destabilization tag that inhibits HSP101-PTEX complex formation but not to translational knockdown of the entire HSP101 pool, illustrating how redistribution of a target protein by knockER can be used to query function in a compartment-specific manner. Collectively, our results establish knockER as a unique tool for dissecting secreted protein function with subcompartmental resolution that should be widely amenable to genetically tractable eukaryotes.

Characterization of GEXP15 as a Potential Regulator of Protein Phosphatase 1 in Plasmodium falciparum

The Protein Phosphatase type 1 catalytic subunit (PP1c) (PF3D7_1414400) operates in combination with various regulatory proteins to specifically direct and control its phosphatase activity. However, there is little information about this phosphatase and its regulators in the human malaria parasite, Plasmodium falciparum. To address this knowledge gap, we conducted a comprehensive investigation into the structural and functional characteristics of a conserved Plasmodium-specific regulator called Gametocyte EXported Protein 15, GEXP15 (PF3D7_1031600). Through in silico analysis, we identified three significant regions of interest in GEXP15: an N-terminal region housing a PP1-interacting RVxF motif, a conserved domain whose function is unknown, and a GYF-like domain that potentially facilitates specific protein-protein interactions. To further elucidate the role of GEXP15, we conducted in vitro interaction studies that demonstrated a direct interaction between GEXP15 and PP1 via the RVxF-binding motif. This interaction was found to enhance the phosphatase activity of PP1. Additionally, utilizing a transgenic GEXP15-tagged line and live microscopy, we observed high expression of GEXP15 in late asexual stages of the parasite, with localization predominantly in the nucleus. Immunoprecipitation assays followed by mass spectrometry analyses revealed the interaction of GEXP15 with ribosomal- and RNA-binding proteins. Furthermore, through pull-down analyses of recombinant functional domains of His-tagged GEXP15, we confirmed its binding to the ribosomal complex via the GYF domain. Collectively, our study sheds light on the PfGEXP15-PP1-ribosome interaction, which plays a crucial role in protein translation. These findings suggest that PfGEXP15 could serve as a potential target for the development of malaria drugs.

The Plasmodium falciparum apicoplast cysteine desulfurase provides sulfur for both iron-sulfur cluster assembly and tRNA modification

Iron-sulfur clusters (FeS) are ancient and ubiquitous protein cofactors that play fundamental roles in many aspects of cell biology. These cofactors cannot be scavenged or trafficked within a cell and thus must be synthesized in any subcellular compartment where they are required. We examined the FeS synthesis proteins found in the relict plastid organelle, called the apicoplast, of the human malaria parasite Plasmodium falciparum. Using a chemical bypass method, we deleted four of the FeS pathway proteins involved in sulfur acquisition and cluster assembly and demonstrated that they are all essential for parasite survival. However, the effect that these deletions had on the apicoplast organelle differed. Deletion of the cysteine desulfurase SufS led to disruption of the apicoplast organelle and loss of the organellar genome, whereas the other deletions did not affect organelle maintenance. Ultimately, we discovered that the requirement of SufS for organelle maintenance is not driven by its role in FeS biosynthesis, but rather, by its function in generating sulfur for use by MnmA, a tRNA modifying enzyme that we localized to the apicoplast. Complementation of MnmA and SufS activity with a bacterial MnmA and its cognate cysteine desulfurase strongly suggests that the parasite SufS provides sulfur for both FeS biosynthesis and tRNA modification in the apicoplast. The dual role of parasite SufS is likely to be found in other plastid-containing organisms and highlights the central role of this enzyme in plastid biology.

The mitochondrion of Plasmodium falciparum is required for cellular acetyl-CoA metabolism and protein acetylation

Coenzyme A (CoA) biosynthesis is an excellent target for antimalarial intervention. While most studies have focused on the use of CoA to produce acetyl-CoA in the apicoplast and the cytosol of malaria parasites, mitochondrial acetyl-CoA production is less well understood. In the current study, we performed metabolite-labeling experiments to measure endogenous metabolites in Plasmodium falciparum lines with genetic deletions affecting mitochondrial dehydrogenase activity. Our results show that the mitochondrion is required for cellular acetyl-CoA biosynthesis and identify a synthetic lethal relationship between the two main ketoacid dehydrogenase enzymes. The activity of these enzymes is dependent on the lipoate attachment enzyme LipL2, which is essential for parasite survival solely based on its role in supporting acetyl-CoA metabolism. We also find that acetyl-CoA produced in the mitochondrion is essential for the acetylation of histones and other proteins outside of the mitochondrion. Taken together, our results demonstrate that the mitochondrion is required for cellular acetyl-CoA metabolism and protein acetylation essential for parasite survival.

New insights into apicoplast metabolism in blood-stage malaria parasites

The apicoplast of Plasmodium falciparum is the only source of essential isoprenoid precursors and Coenzyme A (CoA) in the parasite. Isoprenoid precursor synthesis relies on the iron-sulfur cluster (FeS) cofactors produced within the apicoplast, rendering FeS synthesis an essential function of this organelle. Recent reports provide important insights into the roles of FeS cofactors and the use of isoprenoid precursors and CoA both inside and outside the apicoplast. Here, we review the recent insights into the roles of these metabolites in blood-stage malaria parasites and discuss new questions that have been raised in light of these discoveries.

A Leak-Free Inducible CRISPRi/a System for Gene Functional Studies in Plasmodium falciparum

By fusing catalytically dead Cas9 (dCas9) to active domains of histone deacetylase (Sir2a) or acetyltransferase (GCN5), this CRISPR interference/activation (CRISPRi/a) system allows gene regulation at the transcriptional level without causing permanent changes in the parasite genome. However, the constitutive expression of dCas9 poses a challenge for studying essential genes, which may lead to adaptive changes in the parasite, masking the true phenotypes. Here, we developed a leak-free inducible CRISPRi/a system by integrating the DiCre/loxP regulon to allow the expression of dCas9-GCN5/-Sir2a upon transient induction with rapamycin, which allows convenient transcriptional regulation of a gene of interest by introducing a guide RNA targeting its transcription start region. Using eight genes that are either silent or expressed from low to high levels during asexual erythrocytic development, we evaluated the robustness and versatility of this system in the asexual parasites. For most genes analyzed, this inducible CRISPRi/a system led to 1.5- to 3-fold up-or downregulation of the target genes at the mRNA level. Alteration in the expression of PfK13 and PfMYST resulted in altered sensitivities to artemisinin. For autophagy-related protein 18, an essential gene related to artemisinin resistance, a >2-fold up- or downregulation was obtained by inducible CRISPRi/a, leading to growth retardation. For the master regulator of gametocytogenesis, PfAP2-G, a >10-fold increase of the PfAP2-G transcripts was obtained by CRISPRa, resulting in >4-fold higher gametocytemia in the induced parasites. Additionally, inducible CRISPRi/a could also regulate gene expression in gametocytes. This inducible epigenetic regulation system offers a fast way of studying gene functions in Plasmodium falciparum. IMPORTANCE Understanding the fundamental biology of malaria parasites through functional genetic/genomic studies is critical for identifying novel targets for antimalarial development. Conditional knockout/knockdown systems are required to study essential genes in the haploid blood stages of the parasite. In this study, we developed an inducible CRISPRi/a system via the integration of DiCre/loxP. We evaluated the robustness and versatility of this system by activating or repressing eight selected genes and achieved up- and downregulation of the targeted genes located in both the euchromatin and heterochromatin regions. This system offers the malaria research community another tool for functional genetic studies.

Metabolic changes accompanying the loss of fumarate hydratase and malate-quinone oxidoreductase in the asexual blood stage of Plasmodium falciparum

In the glucose-rich milieu of red blood cells, asexually replicating malarial parasites mainly rely on glycolysis for ATP production, with limited carbon flux through the mitochondrial tricarboxylic acid (TCA) cycle. By contrast, gametocytes and mosquito-stage parasites exhibit an increased dependence on the TCA cycle and oxidative phosphorylation for more economical energy generation. Prior genetic studies supported these stage-specific metabolic preferences by revealing that six of eight TCA cycle enzymes are completely dispensable during the asexual blood stages of Plasmodium falciparum, with only fumarate hydratase (FH) and malate-quinone oxidoreductase (MQO) being refractory to deletion. Several hypotheses have been put forth to explain the possible essentiality of FH and MQO, including their participation in a malate shuttle between the mitochondrial matrix and the cytosol. However, using newer genetic techniques like CRISPR and dimerizable Cre, we were able to generate deletion strains of FH and MQO in P. falciparum. We employed metabolomic analyses to characterize a double knockout mutant of FH and MQO (ΔFM) and identified changes in purine salvage and urea cycle metabolism that may help to limit fumarate accumulation. Correspondingly, we found that the ΔFM mutant was more sensitive to exogenous fumarate, which is known to cause toxicity by modifying and inactivating proteins and metabolites. Overall, our data indicate that P. falciparum is able to adequately compensate for the loss of FH and MQO, rendering them unsuitable targets for drug development.

Critical role for isoprenoids in apicoplast biogenesis by malaria parasites

Isopentenyl pyrophosphate (IPP) is an essential metabolic output of the apicoplast organelle in Plasmodium falciparum malaria parasites and is required for prenylation-dependent vesicular trafficking and other cellular processes. We have elucidated a critical and previously uncharacterized role for IPP in apicoplast biogenesis. Inhibiting IPP synthesis blocks apicoplast elongation and inheritance by daughter merozoites, and apicoplast biogenesis is rescued by exogenous IPP and polyprenols. Knockout of the only known isoprenoid-dependent apicoplast pathway, tRNA prenylation by MiaA, has no effect on blood-stage parasites and thus cannot explain apicoplast reliance on IPP. However, we have localized an annotated polyprenyl synthase (PPS) to the apicoplast. PPS knockdown is lethal to parasites, rescued by IPP and long- (C₅₀) but not short-chain (≤C₂₀) prenyl alcohols, and blocks apicoplast biogenesis, thus explaining apicoplast dependence on isoprenoid synthesis. We hypothesize that PPS synthesizes long-chain polyprenols critical for apicoplast membrane fluidity and biogenesis. This work critically expands the paradigm for isoprenoid utilization in malaria parasites and identifies a novel essential branch of apicoplast metabolism suitable for therapeutic targeting.

GeneTargeter: Automated In Silico Design for Genome Editing in the Malaria Parasite, Plasmodium falciparum

Functional characterization of the multitude of poorly described proteins in the human malarial pathogen, Plasmodium falciparum, requires tools to enable genome-scale perturbation studies. Here, we present GeneTargeter (genetargeter.mit.edu), a software tool for automating the design of homology-directed repair donor vectors to achieve gene knockouts, conditional knockdowns, and epitope tagging of P. falciparum genes. We demonstrate GeneTargeter-facilitated genome-scale design of six different types of knockout and conditional knockdown constructs for the P. falciparum genome and validate the computational design process experimentally with successful donor vector assembly and transfection. The software's modular nature accommodates arbitrary destination vectors and allows customizable designs that extend the genome manipulation outcomes attainable in Plasmodium and other organisms.

A toolbox for conditional control of gene expression in apicomplexan parasites

Apicomplexan parasites encompass diverse pathogens for humans and animals, including the causative agents of malaria and toxoplasmosis, Plasmodium spp. and Toxoplasma gondii. Genetic manipulation of these parasites has become central to explore parasite biology, unravel gene function and identify new targets for therapeutic strategies. Tremendous progress has been achieved over the past years with the advent of next generation sequencing and powerful genome editing methods. In particular, various methods for conditional gene expression have been developed in both Plasmodium and Toxoplasma to knockout or knockdown essential genes, or for inducible expression of master developmental regulators or mutant versions of proteins. Conditional gene expression can be achieved at three distinct levels. At the DNA level, inducible site‐specific recombinases allow conditional genome editing. At the RNA level, regulation can be achieved during transcription, using stage‐specific or regulatable promoters, or post‐transcriptionally through alteration of mRNA stability or translation. At the protein level, several systems have been developed for inducible degradation or displacement of a protein of interest. In this review, we provide an overview of current systems for conditional control of gene expression in Plasmodium and Toxoplasma parasites, highlighting the advantages and limitations of each approach.

Genomic and Genetic Approaches to Studying Antimalarial Drug Resistance and Plasmodium Biology

Recent progress in genomics and molecular genetics has empowered novel approaches to study gene functions in disease-causing pathogens. In the human malaria parasite Plasmodium falciparum, the application of genome-based analyses, site-directed genome editing, and genetic systems that allow for temporal and quantitative regulation of gene and protein expression have been invaluable in defining the genetic basis of antimalarial resistance and elucidating candidate targets to accelerate drug discovery efforts. Using examples from recent studies, we review applications of some of these approaches in advancing our understanding of Plasmodium biology and illustrate their contributions and limitations in characterizing parasite genomic loci associated with antimalarial drug responses.

Dephospho-CoA kinase, a nuclear-encoded apicoplast protein, remains active and essential after Plasmodium falciparum apicoplast disruption

Malaria parasites contain an essential organelle called the apicoplast that houses metabolic pathways for fatty acid, heme, isoprenoid, and iron–sulfur cluster synthesis. Surprisingly, malaria parasites can survive without the apicoplast as long as the isoprenoid precursor isopentenyl pyrophosphate (IPP) is supplemented in the growth medium, making it appear that isoprenoid synthesis is the only essential function of the organelle in blood‐stage parasites. In the work described here, we localized an enzyme responsible for coenzyme A synthesis, DPCK, to the apicoplast, but we were unable to delete DPCK, even in the presence of IPP. However, once the endogenous DPCK was complemented with the E. coli DPCK (EcDPCK), we were successful in deleting it. We were then able to show that DPCK activity is required for parasite survival through knockdown of the complemented EcDPCK. Additionally, we showed that DPCK enzyme activity remains functional and essential within the vesicles present after apicoplast disruption. These results demonstrate that while the apicoplast of blood‐stage P. falciparum parasites can be disrupted, the resulting vesicles remain biochemically active and are capable of fulfilling essential functions.

Some conditions apply: Systems for studying Plasmodium falciparum protein function

Malaria, caused by infection with Plasmodium parasites, remains a significant global health concern. For decades, genetic intractability and limited tools hindered our ability to study essential proteins and pathways in Plasmodium falciparum, the parasite associated with the most severe malaria cases. However, recent years have seen major leaps forward in the ability to genetically manipulate P. falciparum parasites and conditionally control protein expression/function. The conditional knockdown systems used in P. falciparum target all 3 components of the central dogma, allowing researchers to conditionally control gene expression, translation, and protein function. Here, we review some of the common knockdown systems that have been adapted or developed for use in P. falciparum. Much of the work done using conditional knockdown approaches has been performed in asexual, blood-stage parasites, but we also highlight their uses in other parts of the life cycle and discuss new ways of applying these systems outside of the intraerythrocytic stages. With the use of these tools, the field’s understanding of parasite biology is ever increasing, and promising new pathways for antimalarial drug development are being discovered.

The NTP generating activity of pyruvate kinase II is critical for apicoplast maintenance in Plasmodium falciparum

The apicoplast of Plasmodium falciparum parasites is believed to rely on the import of three-carbon phosphate compounds for use in organelle anabolic pathways, in addition to the generation of energy and reducing power within the organelle. We generated a series of genetic deletions in an apicoplast metabolic bypass line to determine which genes involved in apicoplast carbon metabolism are required for blood-stage parasite survival and organelle maintenance. We found that pyruvate kinase II (PyrKII) is essential for organelle maintenance, but that production of pyruvate by PyrKII is not responsible for this phenomenon. Enzymatic characterization of PyrKII revealed activity against all NDPs and dNDPs tested, suggesting that it may be capable of generating a broad range of nucleotide triphosphates. Conditional mislocalization of PyrKII resulted in decreased transcript levels within the apicoplast that preceded organelle disruption, suggesting that PyrKII is required for organelle maintenance due to its role in nucleotide triphosphate generation.