Rounding precedes rupture and breakdown of vacuolar membranes minutes before malaria parasite egress from erythrocytes

Bottoms up! Malaria parasite invasion the right way around

A critical part of the malaria parasite's life cycle is invasion of red blood cells (RBCs) by merozoites. Inside RBCs, the parasite forms a schizont, which undergoes segmentation to produce daughter merozoites. These cells are released, establishing cycles of invasion. Traditionally, merozoites are represented as nonmotile, egg-shaped cells that invade RBCs 'narrower end' first and pack within schizonts with this narrower end facing outwards. Here, we discuss recent evidence and re-evaluate previous data which suggest that merozoites are capable of motility and have spherical or elongated-teardrop shapes. Furthermore, merozoites invade RBCs 'wider end' first and pack within schizonts with this wider end facing outwards. We encourage the field to review this revised model and consider its implications for future studies.

Activation of the Plasmodium Egress Effector Subtilisin-Like Protease 1 Is Mediated by Plasmepsin X Destruction of the Prodomain

Following each round of replication, daughter merozoites of the malaria parasite Plasmodium falciparum escape (egress) from the infected host red blood cell (RBC) by rupturing the parasitophorous vacuole membrane (PVM) and the RBC membrane (RBCM). A proteolytic cascade orchestrated by a parasite serine protease, subtilisin-like protease 1 (SUB1), regulates the membrane breakdown. SUB1 activation involves primary autoprocessing of the 82-kDa zymogen to a 54-kDa (p54) intermediate that remains bound to its inhibitory propiece (p31) postcleavage. A second processing step converts p54 to the terminal 47-kDa (p47) form of SUB1. Although the aspartic protease plasmepsin X (PM X) has been implicated in the activation of SUB1, the mechanism remains unknown. Here, we show that upon knockdown of PM X, the inhibitory p31-p54 complex of SUB1 accumulates in the parasites. Using recombinant PM X and SUB1, we show that PM X can directly cleave both p31 and p54. We have mapped the cleavage sites on recombinant p31. Furthermore, we demonstrate that the conversion of p54 to p47 can be effected by cleavage at either SUB1 or PM X cleavage sites that are adjacent to one another. Importantly, once the p31 is removed, p54 is fully functional inside the parasites, suggesting that the conversion to p47 is dispensable for SUB1 activity. Relief of propiece inhibition via a heterologous protease is a novel mechanism for subtilisin activation. IMPORTANCE Malaria parasites replicate inside a parasitophorous vacuole within the host red blood cells. The exit of mature progeny from the infected host cells is essential for further dissemination. Parasite exit is a highly regulated, explosive process that involves membrane breakdown. To do this, the parasite utilizes a serine protease called SUB1 that proteolytically activates various effector proteins. SUB1 activity is dependent on an upstream protease called PM X, although the mechanism was unknown. Here, we describe the molecular basis for PM X-mediated SUB1 activation. PM X proteolytically degrades the inhibitory segment of SUB1, thereby activating it. The involvement of a heterologous protease is a novel mechanism for subtilisin activation.

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.

Babesia divergens egress from host cells is orchestrated by essential and druggable kinases and proteases

Apicomplexan egress from host cells is fundamental to the spread of infection and is poorly characterized in Babesia spp., parasites of veterinary importance and emerging zoonoses. Through the use of video microscopy, transcriptomics and chemical genetics, we have implicated signaling, proteases and gliding motility as key drivers of egress by Babesia divergens. We developed reverse genetics to perform a knockdown screen of putative mediators of egress, identifying kinases and proteases involved in distinct steps of egress (ASP3, PKG and CDPK4) and invasion (ASP2, ASP3 and PKG). Inhibition of egress leads to continued intracellular replication, indicating exit from the replication cycle is uncoupled from egress. Chemical genetics validated PKG, ASP2 and ASP3 as druggable targets in Babesia spp. All taken together, egress in B. divergens more closely resembles T. gondii than the more evolutionarily-related Plasmodium spp. We have established a molecular framework for biological and translational studies of B. divergens egress.

Activation of the Plasmodium egress effector subtilisin-like protease 1 is achieved by plasmepsin X destruction of the propiece

Following each round of replication, daughter merozoites of the malaria parasite Plasmodium falciparum escape (egress) from the infected host red blood cell (RBC) by rupturing the parasitophorous vacuole membrane (PVM) and the RBC membrane (RBCM). A proteolytic cascade orchestrated by the parasite’s serine protease, subtilisin-like protease 1 (SUB1) regulates the membrane breakdown. SUB1 activation involves primary auto-processing of the 82 kDa zymogen to a 54 kDa (p54) intermediate that remains bound to its inhibitory propiece (p31) post cleavage. A second processing step converts p54 to the terminal 47 kDa (p47) form of SUB1. Although the aspartic protease plasmepsin X (PM X) has been implicated in the activation of SUB1, the mechanism remains unknown. Here, we show that upon knockdown of PM X the inhibitory p31/p54 complex of SUB1 accumulates in the parasites. Using recombinant PM X and SUB1, we show that PM X can directly cleave both p31 and p54. We have mapped the cleavage sites on recombinant p31. Furthermore, we demonstrate that the conversion of p54 to p47 can be effected by cleavage at either a SUB1 or PM X cleavage site that are adjacent to one another. Importantly once the p31 is removed, p54 is fully functional inside the parasites suggesting that the conversion to p47 is dispensable for SUB1 activity. Relief of propiece inhibition via a heterologous protease is a novel mechanism for subtilisin activation.

Significance Statement

Malaria parasites replicate inside a parasitophorous vacuole within the host red blood cells. Exit of mature progeny from the infected host cells is essential for further dissemination. Parasite exit is a highly regulated, explosive process that involves membrane breakdown. To do this, the parasite utilizes a serine protease, called the subtilisin-like protease 1 or SUB1 that proteolytically activates various effector proteins. SUB1 activity is dependent on an upstream protease, called plasmepsin X (PM X), although the mechanism was unknown. Here we describe the molecular basis for PM X mediated SUB1 activation. PM X proteolytically degrades the inhibitory segment of SUB1, thereby activating it. Involvement of a heterologous protease is a novel mechanism for subtilisin activation.

Efficient generation of mNeonGreen Plasmodium falciparum reporter lines enables quantitative fitness analysis

CRISPR editing has enabled the rapid creation of fluorescent Plasmodium transgenic lines, facilitating a deeper understanding of parasite biology. The impact of genetic perturbations such as gene disruption or the introduction of drug resistance alleles on parasite fitness is typically quantified in competitive growth assays between the query line and a wild type reference. Although fluorescent reporter lines offer a facile and frequently used method to measure relative growth, this approach is limited by the strain background of the existing reporter, which may not match the growth characteristics of the query strains, particularly if these are slower-growing field isolates. Here, we demonstrate an efficient CRISPR-based approach to generate fluorescently labelled parasite lines using mNeonGreen derived from the LanYFP protein in Branchiostoma lanceolatum, which is one of the brightest monomeric green fluorescent proteins identified. Using a positive-selection approach by insertion of an in-frame blasticidin S deaminase marker, we generated a Dd2 reporter line expressing mNeonGreen under the control of the pfpare (P. falciparum Prodrug Activation and Resistance Esterase) locus. We selected the pfpare locus as an integration site because it is highly conserved across P. falciparum strains, expressed throughout the intraerythrocytic cycle, not essential, and offers the potential for negative selection to further enrich for integrants. The mNeonGreen@pare line demonstrates strong fluorescence with a negligible fitness defect. In addition, the construct developed can serve as a tool to fluorescently tag other P. falciparum strains for in vitro experimentation.

Plasmodium Egress Across the Parasite Life Cycle

Human malaria, caused by infection with Plasmodium parasites, remains one of the most important global public health problems, with the World Health Organization reporting more than 240 million cases and 600,000 deaths annually as of 2020 (World malaria report 2021). Our understanding of the biology of these parasites is critical for development of effective therapeutics and prophylactics, including both antimalarials and vaccines. Plasmodium is a protozoan organism that is intracellular for most of its life cycle. However, to complete its complex life cycle and to allow for both amplification and transmission, the parasite must egress out of the host cell in a highly regulated manner. This review discusses the major pathways and proteins involved in the egress events during the Plasmodium life cycle-merozoite and gametocyte egress out of red blood cells, sporozoite egress out of the oocyst, and merozoite egress out of the hepatocyte. The similarities, as well as the differences, between the various egress pathways of the parasite highlight both novel cell biology and potential therapeutic targets to arrest its life cycle.

The Type III Secretion Effector CteG Mediates Host Cell Lytic Exit of Chlamydia trachomatis

Chlamydia trachomatis is an obligate intracellular bacterium causing ocular and urogenital infections in humans that are a significant burden worldwide. The completion of its characteristic infectious cycle relies on the manipulation of several host cell processes by numerous chlamydial type III secretion effector proteins. We previously identified the C. trachomatis CteG effector and showed it localizes at the host cell plasma membrane at late stages of infection. Here, we showed that, from 48 h post-infection, mammalian cells infected by wild-type C. trachomatis contained more infectious chlamydiae in the culture supernatant than cells infected by a CteG-deficient strain. This phenotype was CteG-dependent as it could be complemented in cells infected by the CteG-deficient strain carrying a plasmid encoding CteG. Furthermore, we detected a CteG-dependent defect on host cell cytotoxicity, indicating that CteG mediates chlamydial lytic exit. Previous studies showed that Pgp4, a global regulator of transcription encoded in the C. trachomatis virulence plasmid, also mediates chlamydial lytic exit. However, by using C. trachomatis strains encoding or lacking Pgp4, we showed that production and localization of CteG are not regulated by Pgp4. A C. trachomatis strain lacking both CteG and Pgp4 was as defective in promoting host cell cytotoxicity as mutant strains lacking only CteG or Pgp4. Furthermore, CteG overproduction in a plasmid suppressed the host cell cytotoxic defect of CteG- and Pgp4-deficient chlamydiae. Overall, we revealed the first chlamydial type III secretion effector involved in host cell lytic exit. Our data indicates that CteG and Pgp4 participate in a single cascade of events, but involving multiple layers of regulation, leading to lysis of host cells and release of the infectious chlamydiae.

Dramatic Consequences of Reducing Erythrocyte Membrane Cholesterol on Plasmodium falciparum

Cholesterol is the most abundant lipid in the erythrocyte. During its blood-stage development, the malaria parasite establishes an active cholesterol gradient across the various membrane systems within the infected erythrocyte. Interestingly, some antimalarial compounds have recently been shown to disrupt cholesterol homeostasis in the intraerythrocytic stages of Plasmodium falciparum. These studies point to the importance of cholesterol for parasite growth. Previously, reduction of cholesterol from the erythrocyte membrane by treatment with methyl-β-cyclodextrin (MβCD) was shown to inhibit parasite invasion and growth. In addition, MβCD treatment of trophozoite-stage P. falciparum was shown to result in parasite expulsion from the host cell. We have revisited these phenomena by using live video microscopy, ultrastructural analysis, and response to antimalarial compounds. By using time-lapse video microscopy of fluorescently tagged parasites, we show that MβCD treatment for just 30 min causes dramatic expulsion of the trophozoite-stage parasites. This forceful expulsion occurs within 10 s. Remarkably, the plasma membrane of the host cell from which the parasite has been expelled does not appear to be compromised. The parasitophorous vacuolar membrane (PVM) continued to surround the extruded parasite, but the PVM appeared damaged. Treatment with antimalarial compounds targeting PfATP4 or PfNCR1 prevented MβCD-mediated extrusion of the parasites, pointing to a potential role of cholesterol dynamics underlying the expulsion phenomena. We also confirmed the essential role of erythrocyte plasma membrane cholesterol for invasion and growth of P. falciparum. This defect can be partially complemented by cholesterol and desmosterol but not with epicholesterol, revealing stereospecificity underlying cholesterol function. Overall, our studies advance previous observations and reveal unusual cell biological features underlying cholesterol depletion of the infected erythrocyte plasma membrane. IMPORTANCE Malaria remains a major challenge in much of the world. Symptoms of malaria are caused by the growth of parasites belonging to Plasmodium spp. inside the red blood cells (RBCs), leading to their destruction. The parasite depends upon its host for much of its nutritional needs. Cholesterol is a major lipid in the RBC plasma membrane, which is the only source of this lipid for malaria parasites. We have previously shown that certain new antimalarial compounds disrupt cholesterol homeostasis in P. falciparum. Here, we use live time-lapse video microscopy to show dramatic expulsion of the parasite from the host RBC when the cholesterol content of the RBC is reduced. Remarkably, this expulsion is inhibited by the antimalarials that disrupt lipid homeostasis. We also show stereospecificity of cholesterol in supporting parasite growth inside RBC. Overall, these results point to a critical role of cholesterol in the physiology of malaria parasites.

Peptidic boronic acids are potent cell-permeable inhibitors of the malaria parasite egress serine protease SUB1

Malaria is a devastating infectious disease, which causes over 400,000 deaths per annum and impacts the lives of nearly half the world's population. The causative agent, a protozoan parasite, replicates within red blood cells (RBCs), eventually destroying the cells in a lytic process called egress to release a new generation of parasites. These invade fresh RBCs to repeat the cycle. Egress is regulated by an essential parasite subtilisin-like serine protease called SUB1. Here, we describe the development and optimization of substrate-based peptidic boronic acids that inhibit Plasmodium falciparum SUB1 with low nanomolar potency. Structural optimization generated membrane-permeable, slow off-rate inhibitors that prevent Pfalciparum egress through direct inhibition of SUB1 activity and block parasite replication in vitro at submicromolar concentrations. Our results validate SUB1 as a potential target for a new class of antimalarial drugs designed to prevent parasite replication and disease progression.

Malaria parasite egress at a glance

All intracellular pathogens must escape (egress) from the confines of their host cell to disseminate and proliferate. The malaria parasite only replicates in an intracellular vacuole or in a cyst, and must undergo egress at four distinct phases during its complex life cycle, each time disrupting, in a highly regulated manner, the membranes or cyst wall that entrap the parasites. This Cell Science at a Glance article and accompanying poster summarises our current knowledge of the morphological features of egress across the Plasmodium life cycle, the molecular mechanisms that govern the process, and how researchers are working to exploit this knowledge to develop much-needed new approaches to malaria control.

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.

Use of a highly specific kinase inhibitor for rapid, simple and precise synchronization of Plasmodium falciparum and Plasmodium knowlesi asexual blood-stage parasites

During the course of the asexual erythrocytic stage of development, Plasmodium spp. parasites undergo a series of morphological changes and induce alterations in the host cell. At the end of this stage, the parasites egress from the infected cell, after which the progeny invade a new host cell. These processes are rapid and occur in a time-dependent manner. Of particular importance, egress and invasion of erythrocytes by the parasite are difficult to capture in an unsynchronized culture, or even a culture that has been synchronized within a window of one to several hours. Therefore, precise synchronization of parasite cultures is of paramount importance for the investigation of these processes. Here we describe a method for synchronizing Plasmodium falciparum and Plasmodium knowlesi asexual blood stage parasites with ML10, a highly specific inhibitor of the cGMP-dependent protein kinase (PKG) that arrests parasite growth approximately 15 minutes prior to egress. This inhibitor allows parasite cultures to be synchronized so that all parasites are within a window of development of several minutes, with a simple wash step. Furthermore, we show that parasites remain viable for several hours after becoming arrested by the compound and that ML10 has advantages, owing to its high specificity and low EC50, over the previously used PKG inhibitor Compound 2. Here, we demonstrate that ML10 is an invaluable tool for the study of Plasmodium spp. asexual blood stage biology and for the routine synchronization of P. falciparum and P. knowlesi cultures.

Co-option of Plasmodium falciparum PP1 for egress from host erythrocytes

Asexual proliferation of the Plasmodium parasites that cause malaria follows a developmental program that alternates non-canonical intraerythrocytic replication with dissemination to new host cells. We carried out a functional analysis of the Plasmodium falciparum homolog of Protein Phosphatase 1 (PfPP1), a universally conserved cell cycle factor in eukaryotes, to investigate regulation of parasite proliferation. PfPP1 is indeed required for efficient replication, but is absolutely essential for egress of parasites from host red blood cells. By phosphoproteomic and chemical-genetic analysis, we isolate two functional targets of PfPP1 for egress: a HECT E3 protein-ubiquitin ligase; and GCα, a fusion protein composed of a guanylyl cyclase and a phospholipid transporter domain. We hypothesize that PfPP1 regulates lipid sensing by GCα and find that phosphatidylcholine stimulates PfPP1-dependent egress. PfPP1 acts as a key regulator that integrates multiple cell-intrinsic pathways with external signals to direct parasite egress from host cells.

Malaria parasite plasmepsins: More than just plain old degradative pepsins

Plasmepsins are a group of diverse aspartic proteases in the malaria parasite Plasmodium Their functions are strikingly multifaceted, ranging from hemoglobin degradation to secretory organelle protein processing for egress, invasion, and effector export. Some, particularly the digestive vacuole plasmepsins, have been extensively characterized, whereas others, such as the transmission-stage plasmepsins, are minimally understood. Some (e.g. plasmepsin V) have exquisite cleavage sequence specificity; others are fairly promiscuous. Some have canonical pepsin-like aspartic protease features, whereas others have unusual attributes, including the nepenthesin loop of plasmepsin V and a histidine in place of a catalytic aspartate in plasmepsin III. We have learned much about the functioning of these enzymes, but more remains to be discovered about their cellular roles and even their mechanisms of action. Their importance in many key aspects of parasite biology makes them intriguing targets for antimalarial chemotherapy. Further consideration of their characteristics suggests that some are more viable drug targets than others. Indeed, inhibitors of invasion and egress offer hope for a desperately needed new drug to combat this nefarious organism.

Simultaneous multiple allelic replacement in the malaria parasite enables dissection of PKG function

Over recent years, a plethora of new genetic tools has transformed conditional engineering of the malaria parasite genome, allowing functional dissection of essential genes in the asexual and sexual blood stages that cause pathology or are required for disease transmission, respectively. Important challenges remain, including the desirability to complement conditional mutants with a correctly regulated second gene copy to confirm that observed phenotypes are due solely to loss of gene function and to analyse structure-function relationships. To meet this challenge, here we combine the dimerisable Cre (DiCre) system with the use of multiple lox sites to simultaneously generate multiple recombination events of the same gene. We focused on the Plasmodium falciparum cGMP-dependent protein kinase (PKG), creating in parallel conditional disruption of the gene plus up to two allelic replacements. We use the approach to demonstrate that PKG has no scaffolding or adaptor role in intraerythrocytic development, acting solely at merozoite egress. We also show that a phosphorylation-deficient PKG is functionally incompetent. Our method provides valuable new tools for analysis of gene function in the malaria parasite.

An Endoplasmic Reticulum CREC Family Protein Regulates the Egress Proteolytic Cascade in Malaria Parasites

The endoplasmic reticulum (ER) is thought to play an essential role during egress of malaria parasites because the ER is assumed to be required for biogenesis and secretion of egress-related organelles. However, no proteins localized to the parasite ER have been shown to play a role in egress of malaria parasites. In this study, we generated conditional mutants of the Plasmodium falciparum endoplasmic reticulum-resident calcium-binding protein (PfERC), a member of the CREC family. Knockdown of the PfERC gene showed that this gene is essential for asexual growth of P. falciparum Analysis of the intraerythrocytic life cycle revealed that PfERC is essential for parasite egress but is not required for protein trafficking or calcium storage. We found that PfERC knockdown prevents the rupture of the parasitophorous vacuole membrane. This is because PfERC knockdown inhibited the proteolytic maturation of the subtilisin-like serine protease SUB1. Using double mutant parasites, we showed that PfERC is required for the proteolytic maturation of the essential aspartic protease plasmepsin X, which is required for SUB1 cleavage. Further, we showed that processing of substrates downstream of the proteolytic cascade is inhibited by PfERC knockdown. Thus, these data establish that the ER-resident CREC family protein PfERC is a key early regulator of the egress proteolytic cascade of malaria parasites.IMPORTANCE The divergent eukaryotic parasites that cause malaria grow and divide within a vacuole inside a host cell, which they have to break open once they finish cell division. The egress of daughter parasites requires the activation of a proteolytic cascade, and a subtilisin-like protease initiates a proteolytic cascade to break down the membranes blocking egress. It is assumed that the parasite endoplasmic reticulum plays a role in this process, but the proteins in this organelle required for egress remain unknown. We have identified an early ER-resident regulator essential for the maturation of the recently discovered aspartic protease in the egress proteolytic cascade, plasmepsin X, which is required for maturation of the subtilisin-like protease. Conditional loss of PfERC results in the formation of immature and inactive egress proteases that are unable to breakdown the vacuolar membrane barring release of daughter parasites.

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.

Hardly Vacuous: The Parasitophorous Vacuolar Membrane of Malaria Parasites

When a malaria parasite invades a host erythrocyte it pushes itself in and invaginates a portion of the host membrane, thereby sealing itself inside and establishing itself in the resulting vacuole. The parasitophorous vacuolar membrane (PVM) that surrounds the parasite is modified by the parasite, using its secretory organelles. To survive within this enveloping membrane, the organism must take in nutrients, secrete wastes, export proteins into the host cell, and eventually egress. Here, we review current understanding of the unique solutions Plasmodium has evolved to these challenges and discuss the remaining questions.

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.

Plasmodium Niemann-Pick type C1-related protein is a druggable target required for parasite membrane homeostasis

Plasmodium parasites possess a protein with homology to Niemann-Pick Type C1 proteins (Niemann-Pick Type C1-Related protein, NCR1). We isolated parasites with resistance-conferring mutations in Plasmodium falciparum NCR1 (PfNCR1) during selections with three diverse small-molecule antimalarial compounds and show that the mutations are causative for compound resistance. PfNCR1 protein knockdown results in severely attenuated growth and confers hypersensitivity to the compounds. Compound treatment or protein knockdown leads to increased sensitivity of the parasite plasma membrane (PPM) to the amphipathic glycoside saponin and engenders digestive vacuoles (DVs) that are small and malformed. Immuno-electron microscopy and split-GFP experiments localize PfNCR1 to the PPM. Our experiments show that PfNCR1 activity is critically important for the composition of the PPM and is required for DV biogenesis, suggesting PfNCR1 as a novel antimalarial drug target.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Pathways of host cell exit by intracellular pathogens

Host cell exit is a critical step in the life-cycle of intracellular pathogens, intimately linked to barrier penetration, tissue dissemination, inflammation, and pathogen transmission. Like cell invasion and intracellular survival, host cell exit represents a well-regulated program that has evolved during host-pathogen co-evolution and that relies on the dynamic and intricate interplay between multiple host and microbial factors. Three distinct pathways of host cell exit have been identified that are employed by three different taxa of intracellular pathogens, bacteria, fungi and protozoa, namely (i) the initiation of programmed cell death, (ii) the active breaching of host cellderived membranes, and (iii) the induced membrane-dependent exit without host cell lysis. Strikingly, an increasing number of studies show that the majority of intracellular pathogens utilize more than one of these strategies, dependent on life-cycle stage, environmental factors and/or host cell type. This review summarizes the diverse exit strategies of intracellular-living bacterial, fungal and protozoan pathogens and discusses the convergently evolved commonalities as well as system-specific variations thereof. Key microbial molecules involved in host cell exit are highlighted and discussed as potential targets for future interventional approaches.