A pan-apicomplexan phosphoinositide-binding protein acts in malarial microneme exocytosis

Atlas of Plasmodium falciparum intraerythrocytic development using expansion microscopy

Apicomplexan parasites exhibit tremendous diversity in much of their fundamental cell biology, but study of these organisms using light microscopy is often hindered by their small size. Ultrastructural expansion microscopy (U-ExM) is a microscopy preparation method that physically expands the sample by ~4.5×. Here, we apply U-ExM to the human malaria parasite Plasmodium falciparum during the asexual blood stage of its lifecycle to understand how this parasite is organized in three dimensions. Using a combination of dye-conjugated reagents and immunostaining, we have cataloged 13 different P. falciparum structures or organelles across the intraerythrocytic development of this parasite and made multiple observations about fundamental parasite cell biology. We describe that the outer centriolar plaque and its associated proteins anchor the nucleus to the parasite plasma membrane during mitosis. Furthermore, the rhoptries, Golgi, basal complex, and inner membrane complex, which form around this anchoring site while nuclei are still dividing, are concurrently segregated and maintain an association to the outer centriolar plaque until the start of segmentation. We also show that the mitochondrion and apicoplast undergo sequential fission events while maintaining an association with the outer centriolar plaque during cytokinesis. Collectively, this study represents the most detailed ultrastructural analysis of P. falciparum during its intraerythrocytic development to date and sheds light on multiple poorly understood aspects of its organelle biogenesis and fundamental cell biology.

Atlas of Plasmodium falciparum intraerythrocytic development using expansion microscopy

Apicomplexan parasites exhibit tremendous diversity in much of their fundamental cell biology, but study of these organisms using light microscopy is often hindered by their small size. Ultrastructural expansion microscopy (U-ExM) is a microscopy preparation method that physically expands the sample ~4.5x. Here, we apply U-ExM to the human malaria parasite Plasmodium falciparum during the asexual blood stage of its lifecycle to understand how this parasite is organized in three-dimensions. Using a combination of dye-conjugated reagents and immunostaining, we have catalogued 13 different P. falciparum structures or organelles across the intraerythrocytic development of this parasite and made multiple observations about fundamental parasite cell biology. We describe that the outer centriolar plaque and its associated proteins anchor the nucleus to the parasite plasma membrane during mitosis. Furthermore, the rhoptries, Golgi, basal complex, and inner membrane complex, which form around this anchoring site while nuclei are still dividing, are concurrently segregated and maintain an association to the outer centriolar plaque until the start of segmentation. We also show that the mitochondrion and apicoplast undergo sequential fission events while maintaining an association with the outer centriolar plaque during cytokinesis. Collectively, this study represents the most detailed ultrastructural analysis of P. falciparum during its intraerythrocytic development to date, and sheds light on multiple poorly understood aspects of its organelle biogenesis and fundamental cell biology.

A Pyridyl-Furan Series Developed from the Open Global Health Library Block Red Blood Cell Invasion and Protein Trafficking in Plasmodium falciparum through Potential Inhibition of the Parasite's PI4KIIIB Enzyme

With the resistance increasing to current antimalarial medicines, there is an urgent need to discover new drug targets and to develop new medicines against these targets. We therefore screened the Open Global Health Library of Merck KGaA, Darmstadt, Germany, of 250 compounds against the asexual blood stage of the deadliest malarial parasite Plasmodium falciparum, from which eight inhibitors with low micromolar potency were found. Due to its combined potencies against parasite growth and inhibition of red blood cell invasion, the pyridyl-furan compound OGHL250 was prioritized for further optimization. The potency of the series lead compound (WEHI-518) was improved 250-fold to low nanomolar levels against parasite blood-stage growth. Parasites selected for resistance to a related compound, MMV396797, were also resistant to WEHI-518 as well as KDU731, an inhibitor of the phosphatidylinositol kinase PfPI4KIIIB, suggesting that this kinase is the target of the pyridyl-furan series. Inhibition of PfPI4KIIIB blocks multiple stages of the parasite's life cycle and other potent inhibitors are currently under preclinical development. MMV396797-resistant parasites possess an E1316D mutation in PfPKI4IIIB that clusters with known resistance mutations of other inhibitors of the kinase. Building upon earlier studies that showed that PfPI4KIIIB inhibitors block the development of the invasive merozoite parasite stage, we show that members of the pyridyl-furan series also block invasion and/or the conversion of merozoites into ring-stage intracellular parasites through inhibition of protein secretion and export into red blood cells.

Plasmepsin X activates the PCRCR complex of Plasmodium falciparum by processing PfRh5 for erythrocyte invasion

Plasmodium falciparum causes the most severe form of malaria in humans. The protozoan parasite develops within erythrocytes to mature schizonts, that contain more than 16 merozoites, which egress and invade fresh erythrocytes. The aspartic protease plasmepsin X (PMX), processes proteins and proteases essential for merozoite egress from the schizont and invasion of the host erythrocyte, including the leading vaccine candidate PfRh5. PfRh5 is anchored to the merozoite surface through a 5-membered complex (PCRCR), consisting of Plasmodium thrombospondin-related apical merozoite protein, cysteine-rich small secreted protein, Rh5-interacting protein and cysteine-rich protective antigen. Here, we show that PCRCR is processed by PMX in micronemes to remove the N-terminal prodomain of PhRh5 and this activates the function of the complex unmasking a form that can bind basigin on the erythrocyte membrane and mediate merozoite invasion. The ability to activate PCRCR at a specific time in merozoite invasion most likely masks potential deleterious effects of its function until they are required. These results provide an important understanding of the essential role of PMX and the fine regulation of PCRCR function in P. falciparum biology.

Genomic conservation and putative downstream functionality of the phosphatidylinositol signalling pathway in the cnidarian-dinoflagellate symbiosis

The mutualistic cnidarian-dinoflagellate symbiosis underpins the evolutionary success of stony corals and the persistence of coral reefs. However, a molecular understanding of the signalling events that lead to the successful establishment and maintenance of this symbiosis remains unresolved. For example, the phosphatidylinositol (PI) signalling pathway has been implicated during the establishment of multiple mutualistic and parasitic interactions across the kingdoms of life, yet its role within the cnidarian-dinoflagellate symbiosis remains unexplored. Here, we aimed to confirm the presence and assess the specific enzymatic composition of the PI signalling pathway across cnidaria and dinoflagellates by compiling 21 symbiotic anthozoan (corals and sea anemones) and 28 symbiotic dinoflagellate (Symbiodiniaceae) transcriptomic and genomic datasets and querying genes related to this pathway. Presence or absence of PI-kinase and PI-phosphatase orthologs were also compared between a broad sampling of taxonomically related symbiotic and non-symbiotic species. Across the symbiotic anthozoans analysed, there was a complete and highly conserved PI pathway, analogous to the pathway found in model eukaryotes. The Symbiodiniaceae pathway showed similarities to its sister taxon, the Apicomplexa, with the absence of PI 4-phosphatases. However, conversely to Apicomplexa, there was also an expansion of homologs present in the PI5-phosphatase and PI5-kinase groups, with unique Symbiodiniaceae proteins identified that are unknown from non-symbiotic unicellular organisms. Additionally, we aimed to unravel the putative functionalities of the PI signalling pathway in this symbiosis by analysing phosphoinositide (PIP)-binding proteins. Analysis of phosphoinositide (PIP)-binding proteins showed that, on average, 2.23 and 1.29% of the total assemblies of anthozoan and Symbiodiniaceae, respectively, have the potential to bind to PIPs. Enrichment of Gene Ontology (GO) terms associated with predicted PIP-binding proteins within each taxon revealed a broad range of functions, including compelling links to processes putatively involved in symbiosis regulation. This analysis establishes a baseline for current understanding of the PI pathway across anthozoans and Symbiodiniaceae, and thus a framework to target future research.

How Apicomplexa Parasites Secrete and Build Their Invasion Machinery

Apicomplexa are obligatory intracellular parasites that sense and actively invade host cells. Invasion is a conserved process that relies on the timely and spatially controlled exocytosis of unique specialized secretory organelles termed micronemes and rhoptries. Microneme exocytosis starts first and likely controls the intricate mechanism of rhoptry secretion. To assemble the invasion machinery, micronemal proteins-associated with the surface of the parasite-interact and form complexes with rhoptry proteins, which in turn are targeted into the host cell. This review covers the molecular advances regarding microneme and rhoptry exocytosis and focuses on how the proteins discharged from these two compartments work in synergy to drive a successful invasion event. Particular emphasis is given to the structure and molecular components of the rhoptry secretion apparatus, and to the current conceptual framework of rhoptry exocytosis that may constitute an unconventional eukaryotic secretory machinery closely related to the one described in ciliates. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Toxoplasma gondii’s Basal Complex: The Other Apicomplexan Business End Is Multifunctional

The Apicomplexa are famously named for their apical complex, a constellation of organelles at their apical end dedicated to invasion of their host cells. In contrast, at the other end of the cell, the basal complex (BC) has been overshadowed since it is much less prominent and specific functions were not immediately obvious. However, in the past decade a staggering array of functions have been associated with the BC and strides have been made in understanding its structure. Here, these collective insights are supplemented with new data to provide an overview of the understanding of the BC in Toxoplasma gondii. The emerging picture is that the BC is a dynamic and multifunctional complex, with a series of (putative) functions. The BC has multiple roles in cell division: it is the site where building blocks are added to the cytoskeleton scaffold; it exerts a two-step stretch and constriction mechanism as contractile ring; and it is key in organelle division. Furthermore, the BC has numerous putative roles in ‘import’, such as the recycling of mother cell remnants, the acquisition of host-derived vesicles, possibly the uptake of lipids derived from the extracellular medium, and the endocytosis of micronemal proteins. The latter process ties the BC to motility, whereas an additional role in motility is conferred by Myosin C. Furthermore, the BC acts on the assembly and/or function of the intravacuolar network, which may directly or indirectly contribute to the establishment of chronic tissue cysts. Here we provide experimental support for molecules acting in several of these processes and identify several new BC proteins critical to maintaining the cytoplasmic bridge between divided parasites. However, the dispensable nature of many BC components leaves many questions unanswered regarding its function. In conclusion, the BC in T. gondii is a dynamic and multifunctional structure at the posterior end of the parasite.

A Phosphoinositide-Binding Protein Acts in the Trafficking Pathway of Hemoglobin in the Malaria Parasite Plasmodium falciparum

Phosphoinositide lipids play key roles in a variety of processes in eukaryotic cells, but our understanding of their functions in the malaria parasite Plasmodium falciparum is still very much limited. To gain a deeper comprehension of the roles of phosphoinositides in this important pathogen, we attempted gene inactivation for 24 putative effectors of phosphoinositide metabolism. Our results reveal that 79% of the candidates are refractory to genetic deletion and are therefore potentially essential for parasite growth. Inactivation of the gene coding for a Plasmodium-specific putative phosphoinositide-binding protein, which we named PfPX1, results in a severe growth defect. We show that PfPX1 likely binds phosphatidylinositol-3-phosphate and that it localizes to the membrane of the digestive vacuole of the parasite and to vesicles filled with host cell cytosol and labeled with endocytic markers. Critically, we provide evidence that it is important in the trafficking pathway of hemoglobin from the host erythrocyte to the digestive vacuole. Finally, inactivation of PfPX1 renders parasites resistant to artemisinin, the frontline antimalarial drug. Globally, the minimal redundancy in the putative phosphoinositide proteins uncovered in our work supports that targeting this pathway has potential for antimalarial drug development. Moreover, our identification of a phosphoinositide-binding protein critical for the trafficking of hemoglobin provides key insight into this essential process. IMPORTANCE Malaria represents an enormous burden for a significant proportion of humanity, and the lack of vaccines and problems with drug resistance to all antimalarials demonstrate the need to develop new therapeutics. Inhibitors of phosphoinositide metabolism are currently being developed as antimalarials but our understanding of this biological pathway is incomplete. The malaria parasite lives inside human red blood cells where it imports hemoglobin to cover some of its nutritional needs. In this work, we have identified a phosphoinositide-binding protein that is important for the transport of hemoglobin in the parasite. Inactivation of this protein decreases the ability of the parasite to proliferate. Our results have therefore identified a potential new target for antimalarial development.

Cell biological analysis reveals an essential role for Pfcerli2 in erythrocyte invasion by malaria parasites

Merozoite invasion of host red blood cells (RBCs) is essential for survival of the human malaria parasite Plasmodium falciparum. Proteins involved with RBC binding and invasion are secreted from dual-club shaped organelles at the apical tip of the merozoite called the rhoptries. Here we characterise P. falciparum Cytosolically Exposed Rhoptry Leaflet Interacting protein 2 (PfCERLI2), as a rhoptry bulb protein that is essential for merozoite invasion. Phylogenetic analyses show that cerli2 arose through an ancestral gene duplication of cerli1. We show that PfCERLI2 is essential for blood-stage growth and localises to the cytosolic face of the rhoptry bulb. Inducible knockdown of PfCERLI2 led to a proportion of merozoites failing to invade and was associated with elongation of the rhoptry organelle during merozoite development and inhibition of rhoptry antigen processing. These findings identify PfCERLI2 as a protein that has key roles in rhoptry biology during merozoite invasion.

Benjamin Liffner and Miguel Balbin et al. report that the Plasmodium falciparum protein, PfCERLI2, localises to the cytosolic face of the parasite’s rhoptry bulb and is essential for invasion and growth within human red blood cells.

Distinct effects on the secretion of MTRAP and AMA1 in Plasmodium yoelii following deletion of acylated pleckstrin homology domain-containing protein

Plasmodium, the causative agents of malaria, are obligate intracellular organisms. In humans, pathogenesis is caused by the blood stage parasite, which multiplies within erythrocytes, thus erythrocyte invasion is an essential developmental step. Merozoite form parasites released into the blood stream coordinately secrets a panel of proteins from the microneme secretory organelles for gliding motility, establishment of a tight junction with a target naive erythrocyte, and subsequent internalization. A protein identified in Toxoplasma gondii facilitates microneme fusion with the plasma membrane for exocytosis; namely, acylated pleckstrin homology domain-containing protein (APH). To obtain insight into the differential microneme discharge by malaria parasites, in this study we analyzed the consequences of APH deletion in the rodent malaria model, Plasmodium yoelii, using a DiCre-based inducible knockout method. We found that APH deletion resulted in a reduction in parasite asexual growth and erythrocyte invasion, with some parasites retaining the ability to invade and grow without APH. APH deletion impaired the secretion of microneme proteins, MTRAP and AMA1, and upon contact with erythrocytes the secretion of MTRAP, but not AMA1, was observed. APH-deleted merozoites were able to attach to and deform erythrocytes, consistent with the observed MTRAP secretion. Tight junctions were formed, but echinocytosis after merozoite internalization into erythrocytes was significantly reduced, consistent with the observed absence of AMA1 secretion. Together with our observation that APH largely colocalized with MTRAP, but less with AMA1, we propose that APH is directly involved in MTRAP secretion; whereas any role of APH in AMA1 secretion is indirect in Plasmodium.

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.

Plasmodium sporozoites on the move: Switching from cell traversal to productive invasion of hepatocytes

Parasites of the genus Plasmodium, the etiological agent of malaria, are transmitted through the bite of anopheline mosquitoes, which deposit sporozoites into the host skin. Sporozoites migrate through the dermis, enter the bloodstream, and rapidly traffic to the liver. They cross the liver sinusoidal barrier and traverse several hepatocytes before switching to productive invasion of a final one for replication inside a parasitophorous vacuole. Cell traversal and productive invasion are functionally independent processes that require proteins secreted from specialized secretory organelles known as micronemes. In this review, we summarize the current understanding of how sporozoites traverse through cells and productively invade hepatocytes, and discuss the role of environmental sensing in switching from a migratory to an invasive state. We propose that timely controlled secretion of distinct microneme subsets could play a key role in successful migration and infection of hepatocytes. A better understanding of these essential biological features of the Plasmodium sporozoite may contribute to the development of new strategies to fight against the very first and asymptomatic stage of malaria.

Identification and characterisation of a phospholipid scramblase in the malaria parasite Plasmodium falciparum

Recent studies highlight the emerging role of lipids as important messengers in malaria parasite biology. In an attempt to identify interacting proteins and regulators of these dynamic and versatile molecules, we hypothesised the involvement of phospholipid translocases and their substrates in the infection of the host erythrocyte by the malaria parasite Plasmodium spp. Here, using a data base searching approach of the Plasmodium Genomics Resources (www.plasmodb.org), we have identified a putative phospholipid (PL) scramblase in P. falciparum (PfPLSCR) that is conserved across the genus and in closely related unicellular algae. By reconstituting recombinant PfPLSCR into liposomes, we demonstrate metal ion dependent PL translocase activity and substrate preference, confirming PfPLSCR as a bona fide scramblase. We show that PfPLSCR is expressed during asexual and sexual parasite development, localising to different membranous compartments of the parasite throughout the intra-erythrocytic life cycle. Two different gene knockout approaches, however, suggest that PfPLSCR is not essential for erythrocyte invasion and asexual parasite development, pointing towards a possible role in other stages of the parasite life cycle.