Host cell remodeling by pathogens: the exomembrane system in Plasmodium-infected erythrocytes

Physiotypic variations lead to variations within the normal range for red blood cells and haemoglobin levels in a healthy human population: An evaluation using generalised additive modelling and hierarchical structure analysis

OBJECTIVES

Haematological parameters have been used for a long time for clinical evaluations, however the dynamics of these parameters has not been studied at length, in healthy populations. We aim to understand the dependence of haematological parameters on human physiotypes.

DESIGN AND METHODS

Using an age and gender restricted healthy human (male) population (n = 100), we attempt to analyse the dynamics of haemoglobin and red blood cells, with reference to age, height and weight of individuals. Using advanced generalised additive modelling and classical hierarchical structural analysis we aim to establish relationships between these parameters and human physiotypes.

RESULTS

We demonstrate that definitive relationships can be established for number of red blood cells, haemoglobin levels, RDW-CV, RDW-SD and weight, height and age of individuals.

CONCLUSION

This study provides a proof of principle, that haematological parameters are dependent on physiotypic variation, within the normal ranges in a healthy population. It may also be noted that there is a definitive influence of height, weight and age on normal ranges and stratification by these factors might therefore make reference intervals narrower, in turn, possibly allowing more precise clinical decisions based on the complete blood count (CBC).

Serum proteomic profile of wild stump-tailed macaques (Macaca arctoides) infected with malaria parasites in Thailand

The number of patients infected with simian malaria is gradually increasing in many countries of Southeast Asia and South America. The most important risk factor for a zoonotic spillover event of malarial infection is mostly influenced by the interaction between humans, monkeys, and vectors. In this study, we determine the protein expression profile of a wild stump-tailed macaque (Macaca arctoides) from a total of 32 blood samples collected from Prachuap Kiri Khan Province, Thailand. The malarial parasite was analyzed using nested polymerase chain reaction (PCR) assays by dividing the samples into three groups: non-infected, mono-infected, and multiple-infected. The identification and differential proteomic expression profiles were determined using liquid chromatography with tandem mass spectrometry (LC-MS/MS) and bioinformatics tools. A total of 9,532 proteins (total proteins) were identified with the filter-based selection methods analysis, and a subset of 440 proteins were found to be different between each group. Within these proteins, the GhostKOALA functional enrichment analysis indicated that 142 important proteins were associated with either of the organismal system (28.87%), genetic information processing (23.24%), environmental information processing (16.20%), metabolism (13.38%), cellular processes (11.97%), or causing human disease (6.34%). Additionally, using interaction network analysis, nine potential reporter proteins were identified. Here, we report the first study on the protein profiles differentially expressed in the serum of wild stump-tailed macaques between non, mono, and multiple malarial infected living in a natural transmission environment. Our findings demonstrate that differentially expressed proteins implicated in host defense through lipid metabolism, involved with TGF pathway were suppressed, while those with the apoptosis pathway, such as cytokines and proinflammation signals were increased. Including the parasite's response via induced hemolysis and disruption of myeloid cells. A greater understanding of the fundamental processes involved in a malarial infection and host response can be crucial for developing diagnostic tools, medication development, and therapies to improve the health of those affected by the disease.

Distribution of malaria parasite-derived phosphatidylcholine in the infected erythrocyte

Malaria parasites modify their host erythrocyte in multiple ways, leading to changes in the deformability, adhesiveness, and permeability of the host erythrocyte. Most of these changes are mediated by proteins exported from the parasite to the host erythrocyte, where these proteins interact with the host cell cytoskeleton or form complexes in the plasma membrane of the infected erythrocyte. In addition, malaria parasites induce the formation of membranous compartments-the parasitophorous vacuole, the tubovesicular network (TVN), the Maurer's clefts and small vesicles-within the infected erythrocyte, a cell that is normally devoid of internal membranes. After infection, changes also occur in the composition and asymmetry of the erythrocyte plasma membrane. Although many aspects of the mechanism of export of parasite proteins have become clear, the mechanism by which these membranous compartments are formed and expanded is almost entirely unknown. To determine whether parasite-derived phospholipids play a part in these processes, we applied a metabolic labeling technique that allows phosphatidylcholine to be labeled with a fluorophore. As the host erythrocyte cannot synthesize phospholipids, within infected erythrocytes, only parasite-derived phosphatidylcholine will be labeled with this technique. The results revealed that phosphatidylcholine produced by the parasite is distributed throughout the infected erythrocyte, including the TVN and the erythrocyte plasma membrane, but not Maurer's clefts. Interestingly, labeled phospholipids were also detected in the erythrocyte plasma membrane very soon after invasion of the parasites, indicating that the parasite may add phospholipids to the host erythrocyte during invasion. IMPORTANCE Here, we describe a previously unappreciated way in which the malaria parasite interacts with the host erythrocyte, namely, by the transfer of parasite phospholipids to the erythrocyte plasma membrane. This likely has important consequences for the survival of the parasite in the host cell and the host organism. We show that parasite-derived phospholipids are transferred from the parasite to the host erythrocyte plasma membrane and that other internal membranes that are produced after the parasite has invaded the cell are produced, at least in part, using parasite-derived phospholipids. The one exception to this is the Maurer's cleft, a membranous organelle that is involved in the transport of parasite proteins to the surface of the erythrocyte. This reveals that the Maurer's cleft is produced in a different manner than the other parasite-induced membranes. Overall, these findings provide a platform for the study of a new aspect of the host-parasite interaction.

Optical diffraction tomography and image reconstruction to measure host cell alterations caused by divergent Plasmodium species

Malaria is a life-threatening infectious disease caused by parasites of the genus Plasmodium. Understanding the biological features of various parasite forms is important for the optical diagnosis and defining pathological states, which are often constrained by the lack of ambient visualization approaches. Here, we employ a label-free tomographic technique to visualize the host red blood cell (RBC) remodeling process and quantify changes in biochemical properties arising from parasitization. Through this, we provide a quantitative body of information pertaining to the influence of host cell environment on growth, survival, and replication of P. falciparum and P. vivax in their respective host cells: mature erythrocytes and young reticulocytes. These exquisite three-dimensional measurements of infected red cells demonstrats the potential of evolving 3D imaging to advance our understanding of Plasmodium biology and host-parasite interactions.

2,3-Diphosphoglycerate and the Protective Effect of Pyruvate Kinase Deficiency against Malaria Infection-Exploring the Role of the Red Blood Cell Membrane

Malaria remains a major world public health problem, contributing to poverty and inequality. It is urgent to find new efficacious tools with few adverse effects. Malaria has selected red blood cell (RBC) alterations linked to resistance against infection, and understanding the protective mechanisms involved may be useful for developing host-directed tools to control Plasmodium infection. Pyruvate kinase deficiency has been associated with resistance to malaria. Pyruvate kinase-deficient RBCs display an increased concentration of 2,3-diphosphoglycerate (2,3-DPG). We recently showed that 2,3-DPG impacts in vitro intraerythrocytic parasite growth, induces a shift of the metabolic profile of infected cells (iRBCs), making it closer to that of noninfected ones (niRBCs), and decreases the number of parasite progenies that invade new RBCs. As an increase of 2,3-DPG content may also have an adverse effect on RBC membrane and, consequently, on the parasite invasion, in this study, we explored modifications of the RBC morphology, biomechanical properties, and RBC membrane on Plasmodium falciparum in vitro cultures treated with 2,3-DPG, using atomic force microscopy (AFM)-based force spectroscopy and other experimental approaches. The presence of infection by P. falciparum significantly increased the rigidity of parasitized cells and influenced the morphology of RBCs, as parasitized cells showed a decrease of the area-to-volume ratio. The extracellular addition of 2,3-DPG also slightly affected the stiffness of niRBCs, making it more similar to that of infected cells. It also changed the niRBC height, making the cells appear more elongated. Moreover, 2,3-DPG treatment influenced the cell surface charge, becoming more negative in treated RBCs than in untreated ones. The results indicate that treatment with 2,3-DPG has only a mild effect on RBCs in comparison with the effect of the presence of the parasite on the host cell. 2,3-DPG is an endogenous host metabolite, which may, in the future, originate a new antimalarial tool with few adverse effects on noninfected cells.

Activity-Based Protein Profiling of Human and Plasmodium Serine Hydrolases and Interrogation of Potential Antimalarial Targets

Malaria remains a global health issue requiring the identification of novel therapeutic targets to combat drug resistance. Metabolic serine hydrolases are druggable enzymes playing essential roles in lipid metabolism. However, very few have been investigated in malaria-causing parasites. Here, we used fluorophosphonate broad-spectrum activity-based probes and quantitative chemical proteomics to annotate and profile the activity of more than half of predicted serine hydrolases in P. falciparum across the erythrocytic cycle. Using conditional genetics, we demonstrate that the activities of four serine hydrolases, previously annotated as essential (or important) in genetic screens, are actually dispensable for parasite replication. Of importance, we also identified eight human serine hydrolases that are specifically activated at different developmental stages. Chemical inhibition of two of them blocks parasite replication. This strongly suggests that parasites co-opt the activity of host enzymes and that this opens a new drug development strategy against which the parasites are less likely to develop resistance.

•

P. falciparum has 48 predicted metabolic SHs. Many react with the ABP, FP-N₃

•

The activity of 25 PfSHs and 8 HsSHs was profiled throughout the asexual life cycle

•

Catalytic mutants of 4 PfSHs (formerly held essential) had no parasite growth effect

•

Selective inhibitors for 2 HsSHs (APEH and LPLA2) affected parasite growth

Machine Learning and Its Applications for Protozoal Pathogens and Protozoal Infectious Diseases

In recent years, massive attention has been attracted to the development and application of machine learning (ML) in the field of infectious diseases, not only serving as a catalyst for academic studies but also as a key means of detecting pathogenic microorganisms, implementing public health surveillance, exploring host-pathogen interactions, discovering drug and vaccine candidates, and so forth. These applications also include the management of infectious diseases caused by protozoal pathogens, such as Plasmodium, Trypanosoma, Toxoplasma, Cryptosporidium, and Giardia, a class of fatal or life-threatening causative agents capable of infecting humans and a wide range of animals. With the reduction of computational cost, availability of effective ML algorithms, popularization of ML tools, and accumulation of high-throughput data, it is possible to implement the integration of ML applications into increasing scientific research related to protozoal infection. Here, we will present a brief overview of important concepts in ML serving as background knowledge, with a focus on basic workflows, popular algorithms (e.g., support vector machine, random forest, and neural networks), feature extraction and selection, and model evaluation metrics. We will then review current ML applications and major advances concerning protozoal pathogens and protozoal infectious diseases through combination with correlative biology expertise and provide forward-looking insights for perspectives and opportunities in future advances in ML techniques in this field.

Three-dimensional Architecture of Cyrilia lignieresi Gametocyte-stage Development Inside Red Blood Cells

The Haemogregarinidae family (Apicomplexa: Adeleina) comprises hemoprotozoa that infect mammals, birds, amphibians, fish and reptiles. Some morphological characteristics of the Cyrilia lignieresi have been described previously, but the parasite-erythrocyte relationship is still poorly understood. In order to understand the structural architecture of Cyrilia lignieresi-infected red blood cells, electron microscopy-based three-dimensional reconstruction was carried out using TEM as well as FIB-SEM tomography. Results showed that development of the macrogametocyte-stage inside the red blood cell is related to an increase in cleft-like structures in the host cell cytoplasm. Furthermore, other aspects related to parasite intraerythrocytic development were explored by 3D visualization techniques. We observed the invagination of a large extension of the Inner Membrane Complex on the parasite body, which results from or induces a folding of the posterior end of the parasite. Small tubular structures were seen associated with areas related to Inner Membrane Complex folding. Taken together, results provide new information on the remodeling of erythrocytes induced by the protozoan C. lignieresi.

Lipid transport proteins in malaria, from Plasmodium parasites to their hosts

Eukaryotic unicellular pathogens from the genus Plasmodium are the etiological agents of malaria, a disease that persists over a wide range of vertebrate species, including humans. During its dynamic lifecycle, survival in the different hosts depends on the parasite's ability to establish a suitable environmental milieu. To achieve this, specific host processes are exploited to support optimal growth, including extensive modifications to the infected host cell. These modifications include the formation of novel membranous structures, which are induced by the parasite. Consequently, to maintain a finely tuned and dynamic lipid environment, the organisation and distribution of lipids to different cell sites likely requires specialised lipid transfer proteins (LTPs). Indeed, several parasite and host-derived LTPs have been identified and shown to be essential at specific stages. Here we describe the roles of LTPs in parasite development and adaptation to its host including how the latest studies are profiting from the improved genetic, lipidomic and imaging toolkits available to study Plasmodium parasites. Lastly, a list of predicted Plasmodium LTPs is provided to encourage research in this field.

Native structure of the RhopH complex, a key determinant of malaria parasite nutrient acquisition

Malaria parasites invade and replicate within human red blood cells, which lack nuclei and have minimal metabolic activity. To survive, the parasites create new pathways that alter the permeability of the red blood cell membrane, allowing them to import nutrients and export waste. Here, we present the native structure of the three-membered RhopH protein complex, which plays a key role in this process. We determined the structure of this essential complex from a heterogeneous mixture of proteins enriched directly from parasite cell lysate, using a cryo-electron microscopy–enabled endogenous structural proteomics approach. The native structure of the RhopH complex in a soluble, trafficking state helps elucidate the long-standing question of how parasite transmembrane proteins are trafficked to the erythrocyte membrane.

The RhopH complex is implicated in malaria parasites’ ability to invade and create new permeability pathways in host erythrocytes, but its mechanisms remain poorly understood. Here, we enrich the endogenous RhopH complex in a native soluble form, comprising RhopH2, CLAG3.1, and RhopH3, directly from parasite cell lysates and determine its atomic structure using cryo–electron microscopy (cryo-EM), mass spectrometry, and the cryoID program. CLAG3.1 is positioned between RhopH2 and RhopH3, which both share substantial binding interfaces with CLAG3.1 but make minimal contacts with each other. The forces stabilizing individual subunits include 13 intramolecular disulfide bonds. Notably, CLAG3.1 residues 1210 to 1223, previously predicted to constitute a transmembrane helix, are embedded within a helical bundle formed by residues 979 to 1289 near the C terminus of CLAG3.1. Buried in the core of the RhopH complex and largely shielded from solvent, insertion of this putative transmembrane helix into the erythrocyte membrane would likely require a large conformational rearrangement. Given the unusually high disulfide content of the complex, it is possible that such a rearrangement could be initiated by the breakage of allosteric disulfide bonds, potentially triggered by interactions at the erythrocyte membrane. This first direct observation of an exported Plasmodium falciparum transmembrane protein—in a soluble, trafficking state and with atomic details of buried putative membrane-insertion helices—offers insights into the assembly and trafficking of RhopH and other parasite-derived complexes to the erythrocyte membrane. Our study demonstrates the potential the endogenous structural proteomics approach holds for elucidating the molecular mechanisms of hard-to-isolate complexes in their native, functional forms.

Panacea within a Pandora's box: the antiparasitic effects of phospholipases A2 (PLA2s) from snake venoms

Parasitic diseases affect millions of individuals worldwide, mainly in low-income regions. There is no cure for most of these diseases, and the treatment relies on drugs that have side effects and lead to drug resistance, emphasizing the urgency to find new treatments. Snake venom has been gaining prominence as a rich source of molecules with antiparasitic potentials, such as phospholipases A₂ (PLA₂s). Here, we compile the findings involving PLA₂s with antiparasitic activities against helminths, Plasmodium, Toxoplasma, and trypanosomatids. We indicate their molecular features, highlighting the possible antiparasitic mechanisms of action of these proteins. We also demonstrate interactions between PLA₂s and some parasite membrane components, shedding light on potential targets for drug design that may provide better treatment for the illnesses caused by parasites.

Defining the Essential Exportome of the Malaria Parasite

To survive inside red blood cells (RBCs), malaria parasites export many proteins to alter their host cell's physiological properties. Although most proteins of this exportome are involved in immune avoidance or in the trafficking of exported proteins to the host membrane, about 20% are essential for parasite survival in culture but little is known about their biological functions. Here, we have combined information from large-scale genetic screens and targeted gene-disruption studies to tabulate all currently known Plasmodium falciparum exported proteins according to their likelihood of being essential. We also discuss the essential functional pathways that exported proteins might be involved in to help direct research efforts towards a more comprehensive understanding of host-cell remodelling.

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.

Transport mechanisms at the malaria parasite-host cell interface

Obligate intracellular malaria parasites reside within a vacuolar compartment generated during invasion which is the principal interface between pathogen and host. To subvert their host cell and support their metabolism, these parasites coordinate a range of transport activities at this membrane interface that are critically important to parasite survival and virulence, including nutrient import, waste efflux, effector protein export, and uptake of host cell cytosol. Here, we review our current understanding of the transport mechanisms acting at the malaria parasite vacuole during the blood and liver-stages of development with a particular focus on recent advances in our understanding of effector protein translocation into the host cell by the Plasmodium Translocon of EXported proteins (PTEX) and small molecule transport by the PTEX membrane-spanning pore EXP2. Comparison to Toxoplasma gondii and other related apicomplexans is provided to highlight how similar and divergent mechanisms are employed to fulfill analogous transport activities.

Human Aquaporins: Functional Diversity and Potential Roles in Infectious and Non-infectious Diseases

Aquaporins (AQPs) are integral membrane proteins and found in all living organisms from bacteria to human. AQPs mainly involved in the transmembrane diffusion of water as well as various small solutes in a bidirectional manner are widely distributed in various human tissues. Human contains 13 AQPs (AQP0-AQP12) which are divided into three sub-classes namely orthodox aquaporin (AQP0, 1, 2, 4, 5, 6, and 8), aquaglyceroporin (AQP3, 7, 9, and 10) and super or unorthodox aquaporin (AQP11 and 12) based on their pore selectivity. Human AQPs are functionally diverse, which are involved in wide variety of non-infectious diseases including cancer, renal dysfunction, neurological disorder, epilepsy, skin disease, metabolic syndrome, and even cardiac diseases. However, the association of AQPs with infectious diseases has not been fully evaluated. Several studies have unveiled that AQPs can be regulated by microbial and parasitic infections that suggest their involvement in microbial pathogenesis, inflammation-associated responses and AQP-mediated cell water homeostasis. This review mainly aims to shed light on the involvement of AQPs in infectious and non-infectious diseases and potential AQPs-target modulators. Furthermore, AQP structures, tissue-specific distributions and their physiological relevance, functional diversity and regulations have been discussed. Altogether, this review would be useful for further investigation of AQPs as a potential therapeutic target for treatment of infectious as well as non-infectious diseases.

Caveolae as Potential Hijackable Gates in Cell Communication

Caveolae are membrane microdomains described in many cell types involved in endocytocis, transcytosis, cell signaling, mechanotransduction, and aging. They are found at the interface with the extracellular environment and are structured by caveolin and cavin proteins. Caveolae and caveolins mediate transduction of chemical messages via signaling pathways, as well as non-chemical messages, such as stretching or shear stress. Various pathogens or signals can hijack these gates, leading to infectious, oncogenic and even caveolin-related diseases named caveolinopathies. By contrast, preclinical and clinical research have fallen behind in their attempts to hijack caveolae and caveolins for therapeutic purposes. Caveolae involvement in human disease is not yet fully explored or understood and, of all their scaffold proteins, only caveolin-1 is being considered in clinical trials as a possible biomarker of disease. This review briefly summarizes current knowledge about caveolae cell signaling and raises the hypothesis whether these microdomains could serve as hijackable “gatekeepers” or “gateways” in cell communication. Furthermore, because cell signaling is one of the most dynamic domains in translating data from basic to clinical research, we pay special attention to translation of caveolae, caveolin, and cavin research into clinical practice.

Human plasma plasminogen internalization route in Plasmodium falciparum-infected erythrocytes

Background

The intra-erythrocytic development of the malaria parasite Plasmodium falciparum depends on the uptake of a number of essential nutrients from the host cell and blood plasma. It is widely recognized that the parasite imports low molecular weight solutes from the plasma and the consumption of these nutrients by P. falciparum has been extensively analysed. However, although it was already shown that the parasite also imports functional proteins from the vertebrate host, the internalization route through the different infected erythrocyte membranes has not yet been elucidated. In order to further understand the uptake mechanism, the study examined the trafficking of human plasminogen from the extracellular medium into P. falciparum-infected red blood cells.

Methods

Plasmodium falciparum clone 3D7 was cultured in standard HEPES-buffered RPMI 1640 medium supplemented with 0.5% AlbuMAX. Exogenous human plasminogen was added to the P. falciparum culture and the uptake of this protein by the parasites was analysed by electron microscopy and Western blotting. Immunoprecipitation and mass spectrometry were performed to investigate possible protein interactions that may assist plasminogen import into infected erythrocytes. The effect of pharmacological inhibitors of different cellular physiological processes in plasminogen uptake was also tested.

Results

It was observed that plasminogen was selectively internalized by P. falciparum-infected erythrocytes, with localization in plasma membrane erythrocyte and parasite’s cytosol. The protein was not detected in parasitic food vacuole and haemoglobin-containing vesicles. Furthermore, in erythrocyte cytoplasm, plasminogen was associated with the parasite-derived membranous structures tubovesicular network (TVN) and Maurer’s clefts. Several proteins were identified in immunoprecipitation assay and may be involved in the delivery of plasminogen across the P. falciparum multiple compartments.

Conclusion

The findings here reported reveal new features regarding the acquisition of plasma proteins of the host by P. falciparum-infected erythrocytes, a mechanism that involves the exomembrane system, which is distinct from the haemoglobin uptake, clarifying a route that may be potentially targeted for inhibition studies.

Contacting domains segregate a lipid transporter from a solute transporter in the malarial host-parasite interface

The malaria parasite interfaces with its host erythrocyte (RBC) using a unique organelle, the parasitophorous vacuole (PV). The mechanism(s) are obscure by which its limiting membrane, the parasitophorous vacuolar membrane (PVM), collaborates with the parasite plasma membrane (PPM) to support the transport of proteins, lipids, nutrients, and metabolites between the cytoplasm of the parasite and the cytoplasm of the RBC. Here, we demonstrate that the PV has structure characterized by micrometer-sized regions of especially close apposition between the PVM and the PPM. To determine if these contact sites are involved in any sort of transport, we localize the PVM nutrient-permeable and protein export channel EXP2, as well as the PPM lipid transporter PfNCR1. We find that EXP2 is excluded from, but PfNCR1 is included within these regions of close apposition. We conclude that the host-parasite interface is structured to segregate those transporters of hydrophilic and hydrophobic substrates.

Palmitoylated Proteins in Plasmodium falciparum-Infected Erythrocytes: Investigation with Click Chemistry and Metabolic Labeling

The examination of the complex cell biology of the human malaria parasite Plasmodium falciparum usually relies on the time-consuming generation of transgenic parasites. Here, metabolic labeling and click chemistry are employed as a fast transfection-independent method for the microscopic examination of protein S-palmitoylation, an important post-translational modification during the asexual intraerythrocytic replication of P. falciparum. Applying various microscopy approaches such as confocal, single-molecule switching, and electron microscopy, differences in the extent of labeling within the different asexual developmental stages of P. falciparum and the host erythrocytes over time are observed.

Galtonosides A-E: Antiproliferative and Antiplasmodial Cholestane Glycosides from Galtonia regalis

An extract of Galtonia regalis from the Natural Products Discovery Institute showed moderate antiplasmodial activity, with an IC50 value less than 1.25 μg/mL. The two known cholestane glycosides 1 and 2 and the five new cholestane glycosides galtonosides A-E (3-7) were isolated after bioassay-directed fractionation. The structures of the new compounds were determined by interpretation of their NMR and mass spectra. Among these compounds, galtonoside B (4) displayed the most potent antiplasmodial activity, with an IC50 value of 0.214 μM against the drug-resistant Dd2 strain of Plasmodium falciparum.

EXP1 is required for organisation of EXP2 in the intraerythrocytic malaria parasite vacuole

Intraerythrocytic malaria parasites reside within a parasitophorous vacuole membrane (PVM) that closely overlays the parasite plasma membrane. Although the PVM is the site of several transport activities essential to parasite survival, the basis for organisation of this membrane system is unknown. Here, we performed proximity labeling at the PVM with BioID2, which highlighted a group of single-pass integral membrane proteins that constitute a major component of the PVM proteome but whose function remains unclear. We investigated EXP1, the longest known member of this group, by adapting a CRISPR/Cpf1 genome editing system to install the TetR-DOZI-aptamers system for conditional translational control. Importantly, although EXP1 was required for intraerythrocytic development, a previously reported in vitro glutathione S-transferase activity could not account for this essential EXP1 function in vivo. EXP1 knockdown was accompanied by profound changes in vacuole ultrastructure, including apparent increased separation of the PVM from the parasite plasma membrane and formation of abnormal membrane structures. Furthermore, although activity of the Plasmodium translocon of exported proteins was not impacted by depletion of EXP1, the distribution of the translocon pore-forming protein EXP2 but not the HSP101 unfoldase was substantially altered. Collectively, our results reveal a novel PVM defect that indicates a critical role for EXP1 in maintaining proper organisation of EXP2 within the PVM.

The parasitophorous vacuole of the blood-stage malaria parasite

The pathology of malaria is caused by infection of red blood cells with unicellular Plasmodium parasites. During blood-stage development, the parasite replicates within a membrane-bound parasitophorous vacuole. A central nexus for host-parasite interactions, this unique parasite shelter functions in nutrient acquisition, subcompartmentalization and the export of virulence factors, making its functional molecules attractive targets for the development of novel intervention strategies to combat the devastating impact of malaria. In this Review, we explore the origin, development, molecular composition and functions of the parasitophorous vacuole of Plasmodium blood stages. We also discuss the relevance of the malaria parasite's intravacuolar lifestyle for successful erythrocyte infection and provide perspectives for future research directions in parasitophorous vacuole biology.

Vitamin and cofactor acquisition in apicomplexans: Synthesis versus salvage

The Apicomplexa phylum comprises diverse parasitic organisms that have evolved from a free-living ancestor. These obligate intracellular parasites exhibit versatile metabolic capabilities reflecting their capacity to survive and grow in different hosts and varying niches. Determined by nutrient availability, they either use their biosynthesis machineries or largely depend on their host for metabolite acquisition. Because vitamins cannot be synthesized by the mammalian host, the enzymes required for their synthesis in apicomplexan parasites represent a large repertoire of potential therapeutic targets. Here, we review recent advances in metabolic reconstruction and functional studies coupled to metabolomics that unravel the interplay between biosynthesis and salvage of vitamins and cofactors in apicomplexans. A particular emphasis is placed on Toxoplasma gondii, during both its acute and latent stages of infection.

Malaria infection promotes a selective expression of kinin receptors in murine liver

BACKGROUND

Malaria represents a worldwide medical emergency affecting mainly poor areas. Plasmodium parasites during blood stages can release kinins to the extracellular space after internalization of host kininogen inside erythrocytes and these released peptides could represent an important mechanism in liver pathophysiology by activation of calcium signaling pathway in endothelial cells of vertebrate host. Receptors (B1 and B2) activated by kinins peptides are important elements for the control of haemodynamics in liver and its physiology. The aim of this study was to identify changes in the liver host responses (i.e. kinin receptors expression and localization) and the effect of ACE inhibition during malaria infection using a murine model.

METHODS

Balb/C mice infected by Plasmodium chabaudi were treated with captopril, an angiotensin I-converting enzyme (ACE) inhibitor, used alone or in association with the anti-malarial chloroquine in order to study the effect of ACE inhibition on mice survival and the activation of liver responses involving B1R and B2R signaling pathways. The kinin receptors (B1R and B2R) expression and localization was analysed in liver by western blotting and immunolocalization in different conditions.

RESULTS

It was verified that captopril treatment caused host death during the peak of malaria infection (parasitaemia about 45%). B1R expression was stimulated in endothelial cells of sinusoids and other blood vessels of mice liver infected by P. chabaudi. At the same time, it was also demonstrated that B1R knockout mice infected presented a significant reduction of survival. However, the infection did not alter the B2R levels and localization in liver blood vessels.

CONCLUSIONS

Thus, it was observed through in vivo studies that the vasodilation induced by plasma ACE inhibition increases mice mortality during P. chabaudi infection. Besides, it was also seen that the anti-malarial chloroquine causes changes in B1R expression in liver, even after days of parasite clearance. The differential expression of B1R and B2R in liver during malaria infection may have an important role in the disease pathophysiology and represents an issue for clinical treatments.

Evidence for vesicle-mediated antigen export by the human pathogen Babesia microti

The human pathogen Babesia microti undergoes unique morphogenesis during its development within human and mouse red blood cells and uses a novel vesicle-based system for export of antigens into the host cell and environment.

The apicomplexan parasite Babesia microti is the primary agent of human babesiosis, a malaria-like illness and potentially fatal tick-borne disease. Unlike its close relatives, the agents of human malaria, B. microti develops within human and mouse red blood cells in the absence of a parasitophorous vacuole, and its secreted antigens lack trafficking motifs found in malarial secreted antigens. Here, we show that after invasion of erythrocytes, B. microti undergoes a major morphogenic change during which it produces an interlacement of vesicles (IOV); the IOV system extends from the plasma membrane of the parasite into the cytoplasm of the host erythrocyte. We developed antibodies against two immunodominant antigens of the parasite and used them in cell fractionation studies and fluorescence and immunoelectron microscopy analyses to monitor the mode of secretion of B. microti antigens. These analyses demonstrate that the IOV system serves as a major export mechanism for important antigens of B. microti and represents a novel mechanism for delivery of parasite effectors into the host by this apicomplexan parasite.

An in silico down-scaling approach uncovers novel constituents of the Plasmodium-containing vacuole

During blood stage development the malaria parasite resides in a membrane-bound compartment, termed the parasitophorous vacuole (PV). The reasons for this intravacuolar life style and the molecular functions of this parasite-specific compartment remain poorly defined, which is mainly due to our limited knowledge about the molecular make-up of this unique niche. We used an in silico down-scaling approach to select for Plasmodium-specific candidates that harbour signatures of PV residency. Live co-localisation of five endogenously tagged proteins confirmed expression in the PV of Plasmodium berghei blood and liver stages. ER retention was ruled out by addition of the respective carboxyterminal tetrapeptides to a secreted reporter protein. Although all five PV proteins are highly expressed, four proved to be dispensable for parasite development in the mammalian and mosquito host, as revealed by targeted gene deletion. In good agreement with their redundant roles, the knockout parasites displayed no detectable deficiencies in protein export, sequestration, or PV morphology. Together, our approach improved the catalogue of the Plasmodium PV proteome and provides experimental genetics evidence for functional redundancy of several PV proteins.

Plasmodium falciparum

Plasmodium falciparum is a protozoan parasite that causes the most severe form of human malaria. Five other Plasmodium species can also infect humans - P. vivax, P. malariae, P. ovale curtisi, P. ovale wallikeri and P. knowlesi - but P. falciparum is the most prevalent Plasmodium species in the African region, where 90% of all malaria occurs, and it is this species that causes the great majority of malaria deaths. These were reported by the WHO at 438 000 in 2015 from an estimated 214 million cases; importantly, however, figures for the global burden of malaria tend to have wide margins of error due to poor and inaccurate reporting. In this Perspective, features of P. falciparum that are unique among human malaria parasites are highlighted, and current issues surrounding the control and treatment of this major human pathogen are discussed.

Proteomic analysis of extracellular vesicles from a Plasmodium falciparum Kenyan clinical isolate defines a core parasite secretome

BACKGROUND

Many pathogens secrete effector molecules to subvert host immune responses, to acquire nutrients, and/or to prepare host cells for invasion. One of the ways that effector molecules are secreted is through extracellular vesicles (EVs) such as exosomes. Recently, the malaria parasite P. falciparum has been shown to produce EVs that can mediate transfer of genetic material between parasites and induce sexual commitment. Characterizing the content of these vesicles may improve our understanding of P. falciparum pathogenesis and virulence.

METHODS

Previous studies of P. falciparum EVs have been limited to long-term adapted laboratory isolates. In this study, we isolated EVs from a Kenyan P. falciparum clinical isolate adapted to in vitro culture for a short period and characterized their protein content by mass spectrometry (data are available via ProteomeXchange, with identifier PXD006925).

RESULTS

We show that P. falciparum extracellular vesicles ( PfEVs) are enriched in proteins found within the exomembrane compartments of infected erythrocytes such as Maurer's clefts (MCs), as well as the secretory endomembrane compartments in the apical end of the merozoites, suggesting that these proteins play a role in parasite-host interactions. Comparison of this novel clinically relevant dataset with previously published datasets helps to define a core secretome present in Plasmodium EVs.

CONCLUSIONS

P. falciparum extracellular vesicles contain virulence-associated parasite proteins. Therefore, analysis of PfEVs contents from a range of clinical isolates, and their functional validation may improve our understanding of the virulence mechanisms of the parasite, and potentially identify targets for interventions or diagnostics.

Membrane trafficking and remodeling at the host-parasite interface

Membrane shape is functionally linked with many cellular processes. The limiting membrane of vacuoles containing Toxoplasma gondii and Plasmodium apicomplexan parasites lies at the host-parasite interface. This membrane comprises intra-vacuolar and extra-vacuolar tubulo-vesicular deformations, which influence host-parasite cross-talk. Here, underscoring specificities and similarities between the T. gondii and Plasmodium contexts, we present recent findings about vacuolar membrane remodeling and its potential roles in parasite fitness and immune recognition. We review in particular the implication of tubulo-vesicular structures in trapping and/or transporting host and parasite components. Understanding how membrane remodeling influences host-pathogen interactions is expected to be critical in the battle against many intracellular pathogens beyond parasites.

Proteomic analysis of extracellular vesicles from a Plasmodium falciparum Kenyan clinical isolate defines a core parasite secretome

BACKGROUND

Many pathogens secrete effector molecules to subvert host immune responses, to acquire nutrients, and/or to prepare host cells for invasion. One of the ways that effector molecules are secreted is through extracellular vesicles (EVs) such as exosomes. Recently, the malaria parasite P. falciparum has been shown to produce EVs that can mediate transfer of genetic material between parasites and induce sexual commitment. Characterizing the content of these vesicles may improve our understanding of P. falciparum pathogenesis and virulence.

METHODS

Previous studies of P. falciparum EVs have been limited to long-term adapted laboratory isolates. In this study, we isolated EVs from a Kenyan P. falciparum clinical isolate adapted to in vitro culture for a short period and characterized their protein content by mass spectrometry (data are available via ProteomeXchange, with identifier PXD006925).

RESULTS

We show that P. falciparum extracellular vesicles ( PfEVs) are enriched in proteins found within the exomembrane compartments of infected erythrocytes such as Maurer's clefts (MCs), as well as the secretory endomembrane compartments in the apical end of the merozoites, suggesting that these proteins play a role in parasite-host interactions. Comparison of this novel clinically relevant dataset with previously published datasets helps to define a core secretome present in Plasmodium EVs.

CONCLUSIONS

P. falciparum extracellular vesicles contain virulence-associated parasite proteins. Therefore, analysis of PfEVs contents from a range of clinical isolates, and their functional validation may improve our understanding of the virulence mechanisms of the parasite, and potentially identify targets for interventions or diagnostics.

Phospholipases during membrane dynamics in malaria parasites

Plasmodium parasites, the causative agents of malaria, display a well-regulated lipid metabolism required to ensure their survival in the human host as well as in the mosquito vector. The fine-tuning of lipid metabolic pathways is particularly important for the parasites during the rapid erythrocytic infection cycles, and thus enzymes involved in lipid metabolic processes represent prime targets for malaria chemotherapeutics. While plasmodial enzymes involved in lipid synthesis and acquisition have been studied in the past, to date not much is known about the roles of phospholipases for proliferation and transmission of the malaria parasite. These phospholipid-hydrolyzing esterases are crucial for membrane dynamics during host cell infection and egress by the parasite as well as for replication and cell signaling, and thus they are considered important virulence factors. In this review, we provide a comprehensive bioinformatic analysis of plasmodial phospholipases identified to date. We further summarize previous findings on the lipid metabolism of Plasmodium, highlight the roles of phospholipases during parasite life-cycle progression, and discuss the plasmodial phospholipases as potential targets for malaria therapy.

The Plasmodium liver-stage parasitophorous vacuole: A front-line of communication between parasite and host

The intracellular development and differentiation of the Plasmodium parasite in the host liver is a prerequisite for the actual onset of malaria disease pathology. Since liver-stage infection is clinically silent and can be completely eliminated by sterilizing immune responses, it is a promising target for urgently needed innovative antimalarial drugs and/or vaccines. Discovered more than 65 years ago, these stages remain poorly understood regarding their molecular repertoire and interaction with their host cells in comparison to the pathogenic erythrocytic stages. The differentiating and replicative intrahepatic parasite resides in a membranous compartment called the parasitophorous vacuole, separating it from the host-cell cytoplasm. Here we outline seminal work that contributed to our present understanding of the fundamental dynamic cellular processes of the intrahepatic malarial parasite with both specific host-cell factors and compartments.

Shedding of host autophagic proteins from the parasitophorous vacuolar membrane of Plasmodium berghei

The hepatic stage of the malaria parasite Plasmodium is accompanied by an autophagy-mediated host response directly targeting the parasitophorous vacuolar membrane (PVM) harbouring the parasite. Removal of the PVM-associated autophagic proteins such as ubiquitin, p62, and LC3 correlates with parasite survival. Yet, it is unclear how Plasmodium avoids the deleterious effects of selective autophagy. Here we show that parasites trap host autophagic factors in the tubovesicular network (TVN), an expansion of the PVM into the host cytoplasm. In proliferating parasites, PVM-associated LC3 becomes immediately redirected into the TVN, where it accumulates distally from the parasite’s replicative centre. Finally, the host factors are shed as vesicles into the host cytoplasm. This strategy may enable the parasite to balance the benefits of the enhanced host catabolic activity with the risk of being eliminated by the cell’s cytosolic immune defence.

Regulation and Essentiality of the StAR-related Lipid Transfer (START) Domain-containing Phospholipid Transfer Protein PFA0210c in Malaria Parasites

StAR-related lipid transfer (START) domains are phospholipid- or sterol-binding modules that are present in many proteins. START domain-containing proteins (START proteins) play important functions in eukaryotic cells, including the redistribution of phospholipids to subcellular compartments and delivering sterols to the mitochondrion for steroid synthesis. How the activity of the START domain is regulated remains unknown for most of these proteins. The Plasmodium falciparum START protein PFA0210c (PF3D7_0104200) is a broad-spectrum phospholipid transfer protein that is conserved in all sequenced Plasmodium species and is most closely related to the mammalian START proteins STARD2 and STARD7. PFA0210c is unusual in that it contains a signal sequence and a PEXEL export motif that together mediate transfer of the protein from the parasite to the host erythrocyte. The protein also contains a C-terminal extension, which is very uncommon among mammalian START proteins. Whereas the biochemical properties of PFA0210c have been characterized, the function of the protein remains unknown. Here, we provide evidence that the unusual C-terminal extension negatively regulates phospholipid transfer activity. Furthermore, we use the genetically tractable Plasmodium knowlesi model and recently developed genetic technology in P. falciparum to show that the protein is essential for growth of the parasite during the clinically relevant asexual blood stage life cycle. Finally, we show that the regulation of phospholipid transfer by PFA0210c is required in vivo, and we identify a potential second regulatory domain. These findings provide insight into a novel mechanism of regulation of phospholipid transfer in vivo and may have important implications for the interaction of the malaria parasite with its host cell.