The malaria parasite progressively dismantles the host erythrocyte cytoskeleton for efficient egress

Critical review of single-cell mechanotyping approaches for biomedical applications

Accurate mechanical measurements of cells has the potential to improve diagnostics, therapeutics and advance understanding of disease mechanisms, where high-resolution mechanical information can be measured by deforming individual cells. Here we evaluate recently developed techniques for measuring cell-scale stiffness properties; while many such techniques have been developed, much of the work examining single-cell stiffness is impacted by difficulties in standardization and comparability, giving rise to large variations in reported mechanical moduli. We highlight the role of underlying mechanical theories driving this variability, and note opportunities to develop novel mechanotyping devices and theoretical models that facilitate convenient and accurate mechanical characterisation. Moreover, many high-throughput approaches are confounded by factors including cell size, surface friction, natural population heterogeneity and convolution of elastic and viscous contributions to cell deformability. We nevertheless identify key approaches based on deformability cytometry as a promising direction for further development, where both high-throughput and accurate single-cell resolutions can be realized.

Expansion microscopy of apicomplexan parasites

Apicomplexan parasites comprise significant pathogens of humans, livestock and wildlife, but also represent a diverse group of eukaryotes with interesting and unique cell biology. The study of cell biology in apicomplexan parasites is complicated by their small size, and historically this has required the application of cutting-edge microscopy techniques to investigate fundamental processes like mitosis or cell division in these organisms. Recently, a technique called expansion microscopy has been developed, which rather than increasing instrument resolution like most imaging modalities, physically expands a biological sample. In only a few years since its development, a derivative of expansion microscopy known as ultrastructure-expansion microscopy (U-ExM) has been widely adopted and proven extremely useful for studying cell biology of Apicomplexa. Here, we review the insights into apicomplexan cell biology that have been enabled through the use of U-ExM, with a specific focus on Plasmodium, Toxoplasma and Cryptosporidium. Further, we summarize emerging expansion microscopy modifications and modalities and forecast how these may influence the field of parasite cell biology in future.

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.

Role of Host Small GTPases in Apicomplexan Parasite Infection

The Apicomplexa are obligate intracellular parasites responsible for several important human diseases. These protozoan organisms have evolved several strategies to modify the host cell environment to create a favorable niche for their survival. The host cytoskeleton is widely manipulated during all phases of apicomplexan intracellular infection. Moreover, the localization and organization of host organelles are altered in order to scavenge nutrients from the host. Small GTPases are a class of proteins widely involved in intracellular pathways governing different processes, from cytoskeletal and organelle organization to gene transcription and intracellular trafficking. These proteins are already known to be involved in infection by several intracellular pathogens, including viruses, bacteria and protozoan parasites. In this review, we recapitulate the mechanisms by which apicomplexan parasites manipulate the host cell during infection, focusing on the role of host small GTPases. We also discuss the possibility of considering small GTPases as potential targets for the development of novel host-targeted therapies against apicomplexan infections.

Plasmodium parasitophorous vacuole membrane-resident protein UIS4 manipulates host cell actin to avoid parasite elimination

Parasite-derived PVM-resident proteins are critical for complete parasite development inside hepatocytes, although the function of most of these proteins remains unknown. Here, we show that the upregulated in infectious sporozoites 4 (UIS4) protein, resident at the PVM, interacts with the host cell actin. By suppressing filamentous actin formation, UIS4 avoids parasite elimination. Host cell actin dynamics increases around UIS4-deficient parasites, which is associated with subsequent parasite elimination. Notably, parasite elimination is impaired significantly by the inhibition of host myosin-II, possibly through relieving the compression generated by actomyosin complexes at the host-parasite interface. Together, these data reveal that UIS4 has a critical role in the evasion of host defensive mechanisms, enabling hence EEF survival and development.

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Plasmodium PVM-resident protein UIS4 interacts with host cell actin

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Host actin dynamics is altered around exoerythocytic forms (EEFs) lacking UIS4

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Actin activity around EEFs lacking UIS4 is associated with parasite elimination

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Parasite elimination depends on actomyosin complexes formed around the PVM

RBC membrane biomechanics and Plasmodium falciparum invasion: probing beyond ligand-receptor interactions

A critical step in malaria blood-stage infections is the invasion of red blood cells (RBCs) by merozoite forms of the Plasmodium parasite. Much progress has been made in defining the parasite ligands and host receptors that mediate this critical step. However, less well understood are the RBC biophysical determinants that influence parasite invasion. In this review we explore how Plasmodium falciparum merozoites interact with the RBC membrane during invasion to modulate RBC deformability and facilitate invasion. We further highlight RBC biomechanics-related polymorphisms that might have been selected for in human populations due to their ability to reduce parasite invasion. Such an understanding will reveal the translational potential of targeting host pathways affecting RBC biomechanical properties for the treatment of malaria.

Lessons Learned for Pathogenesis, Immunology, and Disease of Erythrocytic Parasites: Plasmodium and Babesia

Malaria caused by Plasmodium species and transmitted by Anopheles mosquitoes affects large human populations, while Ixodes ticks transmit Babesia species and cause babesiosis. Babesiosis in animals has been known as an economic drain, and human disease has also emerged as a serious healthcare problem in the last 20–30 years. There is limited literature available regarding pathogenesis, immunity, and disease caused by Babesia spp. with their genomes sequenced only in the last decade. Therefore, using previous studies on Plasmodium as the foundation, we have compared similarities and differences in the pathogenesis of Babesia and host immune responses. Sexual life cycles of these two hemoparasites in their respective vectors are quite similar. An adult Anopheles female can take blood meal several times in its life such that it can both acquire and transmit Plasmodia to hosts. Since each tick stage takes blood meal only once, transstadial horizontal transmission from larva to nymph or nymph to adult is essential for the release of Babesia into the host. The initiation of the asexual cycle of these parasites is different because Plasmodium sporozoites need to infect hepatocytes before egressed merozoites can infect erythrocytes, while Babesia sporozoites are known to enter the erythrocytic cycle directly. Plasmodium metabolism, as determined by its two- to threefold larger genome than different Babesia, is more complex. Plasmodium replication occurs in parasitophorous vacuole (PV) within the host cells, and a relatively large number of merozoites are released from each infected RBC after schizogony. The Babesia erythrocytic cycle lacks both PV and schizogony. Cytoadherence that allows the sequestration of Plasmodia, primarily P. falciparum in different organs facilitated by prominent adhesins, has not been documented for Babesia yet. Inflammatory immune responses contribute to the severity of malaria and babesiosis. Antibodies appear to play only a minor role in the resolution of these diseases; however, cellular and innate immunity are critical for the clearance of both pathogens. Inflammatory immune responses affect the severity of both diseases. Macrophages facilitate the resolution of both infections and also offer cross-protection against related protozoa. Although the immunosuppression of adaptive immune responses by these parasites does not seem to affect their own clearance, it significantly exacerbates diseases caused by coinfecting bacteria during coinfections.

Parasite-Mediated Remodeling of the Host Microfilament Cytoskeleton Enables Rapid Egress of Trypanosoma cruzi following Membrane Rupture

Chagas’ disease arises as a direct consequence of the lytic cycle of Trypanosoma cruzi in the mammalian host. While invasion is well studied for this pathogen, study of egress has been largely neglected. Here, we provide the first description of T. cruzi egress documenting a coordinated mechanism by which T. cruzi engineers its escape from host cells in which it has proliferated and which is essential for maintenance of infection and pathogenesis. Our results indicate that this parasite egress is a sudden event involving coordinated remodeling of host cell cytoskeleton and subsequent rupture of host cell plasma membrane. We document that host cells maintain plasma membrane integrity until immediately prior to parasite release and report the sequential transformation of the host cell’s actin cytoskeleton from normal meshwork in noninfected cells to spheroidal cages—a process initiated shortly after amastigogenesis. Quantification revealed gradual reduction in F-actin over the course of infection, and using cytoskeletal preparations and electron microscopy, we were able to observe disruption of the F-actin proximal to intracellular trypomastigotes. Finally, Western blotting experiments suggest actin degradation driven by parasite proteases, suggesting that degradation of cytoskeleton is a principal component controlling the initiation of egress. Our results provide the first description of the cellular mechanism that regulates the lytic component of the T. cruzi lytic cycle. We show graphically how it is possible to preserve the envelope of host cell plasma membrane during intracellular proliferation of the parasite and how, in cells packed with amastigotes, differentiation into trypomastigotes may trigger sudden egress.

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.

20S proteasomes secreted by the malaria parasite promote its growth

Mature red blood cells (RBCs) lack internal organelles and canonical defense mechanisms, making them both a fascinating host cell, in general, and an intriguing choice for the deadly malaria parasite Plasmodium falciparum (Pf), in particular. Pf, while growing inside its natural host, the human RBC, secretes multipurpose extracellular vesicles (EVs), yet their influence on this essential host cell remains unknown. Here we demonstrate that Pf parasites, cultured in fresh human donor blood, secrete within such EVs assembled and functional 20S proteasome complexes (EV-20S). The EV-20S proteasomes modulate the mechanical properties of naïve human RBCs by remodeling their cytoskeletal network. Furthermore, we identify four degradation targets of the secreted 20S proteasome, the phosphorylated cytoskeletal proteins β-adducin, ankyrin-1, dematin and Epb4.1. Overall, our findings reveal a previously unknown 20S proteasome secretion mechanism employed by the human malaria parasite, which primes RBCs for parasite invasion by altering membrane stiffness, to facilitate malaria parasite growth.

Plasmodium falciparum secretes extracellular vesicles (EVs) while growing inside red blood cells (RBCs). Here the authors show that these EVs contain assembled and functional 20S proteasome complexes that remodel the cytoskeleton of naïve human RBCs, priming the RBCs for parasite invasion.

Characterization of the erythrocyte GTPase Rac1 in relation to Plasmodium falciparum invasion

Malaria is still a devastating disease with 228 million cases globally and 405,000 lethal outcomes in 2018, mainly in children under five years of age. The threat of emerging malaria strains resistant to currently available drugs has made the search for novel drug targets compelling. The process by which Plasmodium falciparum parasites invade the host cell has been widely studied, but only a few erythrocyte proteins involved in this process have been identified so far. The erythrocyte protein Rac1 is a GTPase that plays an important role in host cell invasion by many intracellular pathogens. Here we show that Rac1 is recruited in proximity to the site of parasite entry during P. falciparum invasion process and that subsequently localizes to the parasitophorous vacuole membrane. We also suggest that this GTPase may be involved in erythrocyte invasion by P. falciparum, by testing the effect of specific Rac1 inhibitory compounds. Finally, we suggest a secondary role of the erythrocyte GTPase also in parasite intracellular development. We here characterize a new erythrocyte protein potentially involved in P. falciparum invasion of the host cell and propose the human GTPase Rac1 as a novel and promising antimalarial drug target.

Peptide Probes for Plasmodium falciparum MyoA Tail Interacting Protein (MTIP): Exploring the Druggability of the Malaria Parasite Motor Complex

Malaria remains an endemic tropical disease, and the emergence of Plasmodium falciparum parasites resistant to current front-line medicines means that new therapeutic targets are required. The Plasmodium glideosome is a multiprotein complex thought to be essential for efficient host red blood cell invasion. At its core is a myosin motor, Myosin A (MyoA), which provides most of the force required for parasite invasion. Here, we report the design and development of improved peptide-based probes for the anchor point of MyoA, the P. falciparum MyoA tail interacting protein (PfMTIP). These probes combine low nanomolar binding affinity with significantly enhanced cell penetration and demonstrable competitive target engagement with native PfMTIP through a combination of Western blot and chemical proteomics. These results provide new insights into the potential druggability of the MTIP/MyoA interaction and a basis for the future design of inhibitors.

Vibrational Spectroscopic Based Approach for Diagnosing Babesia bovis Infection

Babesia bovis parasites present a serious and significant health concern for the beef and dairy industries in many parts of the world. Difficulties associated with the current diagnostic techniques include the following: they are prone to human error (microscopy) or expensive and time-consuming (polymerase chain reaction) to perform. Little is known about the biochemical changes in blood that are associated with Babesia infections. The discovery of new biomarkers will lead to improved diagnostic outcomes for the cattle industry. Vibrational spectroscopic technologies can record a chemical snapshot of the entire organism and the surrounding cell thereby providing a phenotype of the organism and the host infected cell. Here, we demonstrate the applicability of vibrational spectroscopic imaging techniques including Atomic Force Microscopy Infrared (AFM-IR) and confocal Raman microscopy to discover new biomarkers for B. bovis infections. Furthermore, we applied Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) to detect B. bovis in red blood cells (RBCs). Based on changes in the IR spectral bands, with ATR-FTIR in combination with Partial Least Squares-Discriminant Analysis we were able to discriminate infected samples from controls with a sensitivity and specificity of 92.0% and 91.7%, respectively, in less than 2 min, excluding sample extraction and preparation. The proposed method utilized a lysis approach to remove hemoglobin from the suspension of infected and uninfected cells, which significantly increased the sensitivity and specificity compared to measurements performed on intact infected red blood cells (intact infected RBC, 77.3% and 79.2%). This work represents a holistic spectroscopic study from the level of the single infected RBC using AFM-IR and confocal Raman to the detection of the parasite in a cell population using ATR-FTIR for a babesiosis diagnostic.

On-chip surface acoustic wave and micropipette aspiration techniques to assess cell elastic properties

The cytoskeletal mechanics and cell mechanical properties play an important role in cellular behaviors. In this study, in order to provide comprehensive insights into the relationship between different cytoskeletal components and cellular elastic moduli, we built a phase-modulated surface acoustic wave microfluidic device to measure cellular compressibility and a microfluidic micropipette-aspiration device to measure cellular Young's modulus. The microfluidic devices were validated based on experimental data and computational simulations. The contributions of structural cytoskeletal actin filament and microtubule to cellular compressibility and Young's modulus were examined in MCF-7 cells. The compressibility of MCF-7 cells was increased after microtubule disruption, whereas actin disruption had no effect. In contrast, Young's modulus of MCF-7 cells was reduced after actin disruption but unaffected by microtubule disruption. The actin filaments and microtubules were stained to confirm the structural alteration in cytoskeleton. Our findings suggest the dissimilarity in the structural roles of actin filaments and microtubules in terms of cellular compressibility and Young's modulus. Based on the differences in location and structure, actin filaments mainly contribute to tensile Young's modulus and microtubules mainly contribute to compressibility. In addition, different responses to cytoskeletal alterations between acoustophoresis and micropipette aspiration demonstrated that micropipette aspiration was better at detecting the change from actin cortex, while the response to acoustophoresis was governed by microtubule networks.

The Actomyosin Systems in Apicomplexa

The phylum of Apicomplexa groups obligate intracellular parasites that exhibit unique classes of unconventional myosin motors. These parasites also encode a limited repertoire of actins, actin-like proteins, actin-binding proteins and nucleators of filamentous actin (F-actin) that display atypical properties. In the last decade, significant progress has been made to visualize F-actin and to unravel the functional contribution of actomyosin systems in the biology of Toxoplasma and Plasmodium, the most genetically-tractable members of the phylum. In addition to assigning specific roles to each myosin, recent biochemical and structural studies have begun to uncover mechanistic insights into myosin function at the atomic level. In several instances, the myosin light chains associated with the myosin heavy chains have been identified, helping to understand the composition of the motor complexes and their mode of regulation. Moreover, the considerable advance in proteomic methodologies and especially in assignment of posttranslational modifications is offering a new dimension to our understanding of the regulation of actin dynamics and myosin function. Remarkably, the actomyosin system contributes to three major processes in Toxoplasma gondii: (i) organelle trafficking, positioning and inheritance, (ii) basal pole constriction and intravacuolar cell-cell communication and (iii) motility, invasion, and egress from infected cells. In this chapter, we summarize how the actomyosin system harnesses these key events to ensure successful completion of the parasite life cycle.

Host Cytoskeleton Remodeling throughout the Blood Stages of Plasmodium falciparum

The asexual intraerythrocytic development of Plasmodium falciparum, causing the most severe form of human malaria, is marked by extensive host cell remodeling. Throughout the processes of invasion, intracellular development, and egress, the erythrocyte membrane skeleton is remodeled by the parasite as required for each specific developmental stage. The remodeling is facilitated by a plethora of exported parasite proteins, and the erythrocyte membrane skeleton is the interface of most of the observed interactions between the parasite and host cell proteins. Host cell remodeling has been extensively described and there is a vast body of information on protein export or the description of parasite-induced structures such as Maurer's clefts or knobs on the host cell surface. Here we specifically review the molecular level of each host cell-remodeling step at each stage of the intraerythrocytic development of P. falciparum We describe key events, such as invasion, knob formation, and egress, and identify the interactions between exported parasite proteins and the host cell cytoskeleton. We discuss each remodeling step with respect to time and specific requirement of the developing parasite to explain host cell remodeling in a stage-specific manner. Thus, we highlight the interaction with the host membrane skeleton as a key event in parasite survival.

Comparative and functional genomics of the protozoan parasite Babesia divergens highlighting the invasion and egress processes

Babesiosis is considered an emerging disease because its incidence has significantly increased in the last 30 years, providing evidence of the expanding range of this rare but potentially life-threatening zoonotic disease. Babesia divergens is a causative agent of babesiosis in humans and cattle in Europe. The recently sequenced genome of B. divergens revealed over 3,741 protein coding-genes and the 10.7-Mb high-quality draft become the first reference tool to study the genome structure of B. divergens. Now, by exploiting this sequence data and using new computational tools and assembly strategies, we have significantly improved the quality of the B. divergens genome. The new assembly shows better continuity and has a higher correspondence to B. bovis chromosomes. Moreover, we present a differential expression analysis using RNA sequencing of the two different stages of the asexual lifecycle of B. divergens: the free merozoite capable of invading erythrocytes and the intraerythrocytic parasite stage that remains within the erythrocyte until egress. Comparison of mRNA levels of both stages identified 1,441 differentially expressed genes. From these, around half were upregulated and the other half downregulated in the intraerythrocytic stage. Orthogonal validation by real-time quantitative reverse transcription PCR confirmed the differential expression. A moderately increased expression level of genes, putatively involved in the invasion and egress processes, were revealed in the intraerythrocytic stage compared with the free merozoite. On the basis of these results and in the absence of molecular models of invasion and egress for B. divergens, we have proposed the identified genes as putative molecular players in the invasion and egress processes. Our results contribute to an understanding of key parasitic strategies and pathogenesis and could be a valuable genomic resource to exploit for the design of diagnostic methods, drugs and vaccines to improve the control of babesiosis.

Babesiosis has long been recognized as an economically important disease of cattle, but only in the last 40 years has Babesia been recognized as an important pathogen in humans. Babesiosis in humans is caused by one of several species (B. microti, B. divergens, B. duncani and B. venatorum). The complete Babesia lifecycle requires two hosts, the ixodid ticks and a vertebrate host. It is the parasite's ability to first recognize and then invade host erythrocytes that is central to the pathogenesis of babesiosis. Once inside the cell, the parasite begins a cycle of maturation and growth, resulting in merozoites that egress from the red blood cells (RBCs) and seek new, uninfected RBCs to invade, perpetuating the infection. To better understand this asexual lifecycle, the authors focused on the parasite genome and transcriptome of the asexual erythrocytic forms of B. divergens. Through this functional and comparative genomic approach, the authors have identified genes putatively involved in invasion, gliding motility, moving junction formation and egress, providing new insights into the molecular mechanisms of these processes necessary for B. divergens to survive and propagate during its life cycle.

The knob protein KAHRP assembles into a ring-shaped structure that underpins virulence complex assembly

Plasmodium falciparum mediates adhesion of infected red blood cells (RBCs) to blood vessel walls by assembling a multi-protein complex at the RBC surface. This virulence-mediating structure, called the knob, acts as a scaffold for the presentation of the major virulence antigen, P. falciparum Erythrocyte Membrane Protein-1 (PfEMP1). In this work we developed correlative STochastic Optical Reconstruction Microscopy–Scanning Electron Microscopy (STORM-SEM) to spatially and temporally map the delivery of the knob-associated histidine-rich protein (KAHRP) and PfEMP1 to the RBC membrane skeleton. We show that KAHRP is delivered as individual modules that assemble in situ, giving a ring-shaped fluorescence profile around a dimpled disk that can be visualized by SEM. Electron tomography of negatively-stained membranes reveals a previously observed spiral scaffold underpinning the assembled knobs. Truncation of the C-terminal region of KAHRP leads to loss of the ring structures, disruption of the raised disks and aberrant formation of the spiral scaffold, pointing to a critical role for KAHRP in assembling the physical knob structure. We show that host cell actin remodeling plays an important role in assembly of the virulence complex, with cytochalasin D blocking knob assembly. Additionally, PfEMP1 appears to be delivered to the RBC membrane, then inserted laterally into knob structures.

The human malaria parasite Plasmodium falciparum causes severe disease, which is initiated by the adhesion of parasite-infected RBCs to receptors on the walls of the host’s capillaries. Adhesion is mediated by a structure called the knob, which acts as a scaffold for the presentation of the virulence protein, P. falciparum erythrocyte membrane protein-1 (PfEMP1). In this work we investigate the assembly of this complex at different stages of parasite development using a multimodal imaging approach that combines dSTORM localization microscopy and scanning electron microscopy (STORM-SEM). We show that the knob-associated histidine-rich protein (KAHRP) is delivered to the RBC membrane skeleton as individual protein modules that assemble into a ring-shaped complex. We provide evidence that host cell remodeling, driven by association of KAHRP with spectrin and the reorganization of actin, is required for assembly of the ring complex, which in turn supports a spiral scaffold that is required for correct knob morphology. Finally, we provide evidence that PfEMP1 is delivered to the RBC membrane before associating with knob complexes.

Conformational Distortions of the Red Blood Cell Spectrin Matrix Nanostructure in Response to Temperature Changes In Vitro

The spectrin matrix is a structural element of red blood cells (RBCs). As such, it affects RBC morphology, membrane deformability, nanostructure, stiffness, and, ultimately, the rheological properties of blood. However, little is known about how temperature affects the spectrin matrix. In this study, the nanostructure of the spectrin network was recorded by atomic force microscopy. We describe how the nanostructure of the RBC spectrin matrix changes from a regular network to a chaotic pattern following an increase in temperature from 20 to 50°C. At 20-37°С, the spectrin network formed a regular structure with dimensions of typically 150 ± 60 nm. At 42-43°С, 83% of the spectrin network assumed an irregular structure. Finally, at 49-50°С the chaotic pattern was observed, and no quantitative estimates of the spectrin structure's parameters could be made. These results can be useful for biophysical studies on the destruction of the spectrin network under pathological conditions, as well as for investigating cell morphology and blood rheology in different diseases.

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

Because Plasmodium falciparum replicates inside of a parasitophorous vacuole (PV) within a human erythrocyte, parasite egress requires the rupture of two limiting membranes. Parasite Ca²⁺ , kinases, and proteases contribute to efficient egress; their coordination in space and time is not known. Here, the kinetics of parasite egress were linked to specific steps with specific compartment markers, using live-cell microscopy of parasites expressing PV-targeted fluorescent proteins, and specific egress inhibitors. Several minutes before egress, under control of parasite [Ca²⁺ ]_i , the PV began rounding. Then after ~1.5 min, under control of PfPKG and SUB1, there was abrupt rupture of the PV membrane and release of vacuolar contents. Over the next ~6 min, there was progressive vacuolar membrane deterioration simultaneous with erythrocyte membrane distortion, lasting until the final minute of the egress programme when newly formed parasites mobilised and erythrocyte membranes permeabilised and then ruptured-a dramatic finale to the parasite cycle of replication.

Sequential Membrane Rupture and Vesiculation during Plasmodium berghei Gametocyte Egress from the Red Blood Cell

Malaria parasites alternate between intracellular and extracellular stages and successful egress from the host cell is crucial for continuation of the life cycle. We investigated egress of Plasmodium berghei gametocytes, an essential process taking place within a few minutes after uptake of a blood meal by the mosquito. Egress entails the rupture of two membranes surrounding the parasite: the parasitophorous vacuole membrane (PVM), and the red blood cell membrane (RBCM). High-speed video microscopy of 56 events revealed that egress in both genders comprises four well-defined phases, although each event is slightly different. The first phase is swelling of the host cell, followed by rupture and immediate vesiculation of the PVM. These vesicles are extruded through a single stabilized pore of the RBCM, and the latter is subsequently vesiculated releasing the free gametes. The time from PVM vesiculation to completion of egress varies between events. These observations were supported by immunofluorescence microscopy using antibodies against proteins of the RBCM and PVM. The combined results reveal dynamic re-organization of the membranes and the cortical cytoskeleton of the erythrocyte during egress.

A new Python library to analyse skeleton images confirms malaria parasite remodelling of the red blood cell membrane skeleton

We present Skan (Skeleton analysis), a Python library for the analysis of the skeleton structures of objects. It was inspired by the “analyse skeletons” plugin for the Fiji image analysis software, but its extensive Application Programming Interface (API) allows users to examine and manipulate any intermediate data structures produced during the analysis. Further, its use of common Python data structures such as SciPy sparse matrices and pandas data frames opens the results to analysis within the extensive ecosystem of scientific libraries available in Python. We demonstrate the validity of Skan’s measurements by comparing its output to the established Analyze Skeletons Fiji plugin, and, with a new scanning electron microscopy (SEM)-based method, we confirm that the malaria parasite Plasmodium falciparum remodels the host red blood cell cytoskeleton, increasing the average distance between spectrin-actin junctions.

Targeted Phenotypic Screening in Plasmodium falciparum and Toxoplasma gondii Reveals Novel Modes of Action of Medicines for Malaria Venture Malaria Box Molecules

The Malaria Box collection includes 400 chemically diverse small molecules with documented potency against malaria parasite growth, but the underlying modes of action are largely unknown. Using complementary phenotypic screens against Plasmodium falciparum and Toxoplasma gondii, we report phenotype-specific hits based on inhibition of overall parasite growth, apicoplast segregation, and egress or host invasion, providing hitherto unavailable insights into the possible mechanisms affected. First, the Malaria Box library was screened against tachyzoite stage T. gondii and the half-maximal effective concentrations (EC₅₀s) of molecules showing ≥80% growth inhibition at 10 µM were determined. Comparison of the EC₅₀s for T. gondii and P. falciparum identified a subset of 24 molecules with nanomolar potency against both parasites. Thirty molecules that failed to induce acute growth inhibition in T. gondii tachyzoites in a 2-day assay caused delayed parasite death upon extended exposure, with at least three molecules interfering with apicoplast segregation during daughter cell formation. Using flow cytometry and microscopy-based examinations, we prioritized 26 molecules with the potential to inhibit host cell egress/invasion during asexual developmental stages of P. falciparum. None of the inhibitors affected digestive vacuole integrity, ruling out a mechanism mediated by broadly specific protease inhibitor activity. Interestingly, five of the plasmodial egress inhibitors inhibited ionophore-induced egress of T. gondii tachyzoites. These findings highlight the advantage of comparative and targeted phenotypic screens in related species as a means to identify lead molecules with a conserved mode of action. Further work on target identification and mechanism analysis will facilitate the development of antiparasitic compounds with cross-species efficacy. IMPORTANCE The phylum Apicomplexa includes many human and animal pathogens, such as Plasmodium falciparum (human malaria) and Toxoplasma gondii (human and animal toxoplasmosis). Widespread resistance to current antimalarials and the lack of a commercial vaccine necessitate novel pharmacological interventions with distinct modes of action against malaria. For toxoplasmosis, new drugs to effectively eliminate tissue-dwelling latent cysts of the parasite are needed. The Malaria Box antimalarial collection, managed and distributed by the Medicines for Malaria Venture, includes molecules of novel chemical classes with proven antimalarial efficacy. Using targeted phenotypic assays of P. falciparum and T. gondii, we have identified a subset of the Malaria Box molecules as potent inhibitors of plastid segregation and parasite invasion and egress, thereby providing early insights into their probable mode of action. Five molecules that inhibit the egress of both parasites have been identified for further mechanistic studies. Thus, the approach we have used to identify novel molecules with defined modes of action in multiple parasites can expedite the development of pan-active antiparasitic agents.

Cellular Exit Strategies of Intracellular Bacteria

The coevolution of intracellular bacteria with their eukaryotic hosts has presented these pathogens with numerous challenges for their evolutionary progress and survival. Chief among these is the ability to exit from host cells, an event that is fundamentally linked to pathogen dissemination and transmission. Recent years have witnessed a major expansion of research in this area, and this chapter summarizes our current understanding of the spectrum of exit strategies that are exploited by intracellular pathogens. Clear themes regarding the mechanisms of microbial exit have emerged and are most easily conceptualized as (i) lysis of the host cell, (ii) nonlytic exit of free bacteria, and (iii) release of microorganisms into membrane-encased compartments. The adaptation of particular exit strategies is closely linked with additional themes in microbial pathogenesis, including host cell death, manipulation of host signaling pathways, and coincident activation of proinflammatory responses. This chapter will explore the molecular determinants used by intracellular pathogens to promote host cell escape and the infectious advantages each exit pathway may confer, and it will provide an evolutionary framework for the adaptation of these mechanisms.

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.

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.

Quantitative mass spectrometry of human reticulocytes reveal proteome-wide modifications during maturation

Erythropoiesis is marked by progressive changes in morphological, biochemical and mechanical properties of erythroid precursors to generate red blood cells (RBC). The earliest enucleated forms derived in this process, known as reticulocytes, are multi-lobular and spherical. As reticulocytes mature, they undergo a series of dynamic cytoskeletal re-arrangements and the expulsion of residual organelles, resulting in highly deformable biconcave RBCs (normocytes). To understand the significant, yet neglected proteome-wide changes associated with reticulocyte maturation, we undertook a quantitative proteomics approach. Immature reticulocytes (marked by the presence of surface transferrin receptor, CD71) and mature RBCs (devoid of CD71) were isolated from human cord blood using a magnetic separation procedure. After sub-fractionation into triton-extracted membrane proteins and luminal samples (isobaric tags for relative and absolute quantitation), quantitative mass spectrometry was conducted to identify more than 1800 proteins with good confidence and coverage. While most structural proteins (such as Spectrins, Ankyrin and Band 3) as well as surface glycoproteins were conserved, proteins associated with microtubule structures, such as Talin-1/2 and ß-Tubulin, were detected only in immature reticulocytes. Atomic force microscopy (AFM)-based imaging revealed an extended network of spectrin filaments in reticulocytes (with an average length of 48 nm), which shortened during reticulocyte maturation (average spectrin length of 41 nm in normocytes). The extended nature of cytoskeletal network may partly account for increased deformability and shape changes, as reticulocytes transform to normocytes.

Potential Sabotage of Host Cell Physiology by Apicomplexan Parasites for Their Survival Benefits

Plasmodium, Toxoplasma, Cryptosporidium, Babesia, and Theileria are the major apicomplexan parasites affecting humans or animals worldwide. These pathogens represent an excellent example of host manipulators who can overturn host signaling pathways for their survival. They infect different types of host cells and take charge of the host machinery to gain nutrients and prevent itself from host attack. The mechanisms by which these pathogens modulate the host signaling pathways are well studied for Plasmodium, Toxoplasma, Cryptosporidium, and Theileria, except for limited studies on Babesia. Theileria is a unique pathogen taking into account the way it modulates host cell transformation, resulting in its clonal expansion. These parasites majorly modulate similar host signaling pathways, however, the disease outcome and effect is different among them. In this review, we discuss the approaches of these apicomplexan to manipulate the host–parasite clearance pathways during infection, invasion, survival, and egress.

Molecular Signaling Involved in Entry and Exit of Malaria Parasites from Host Erythrocytes

During the blood stage, Plasmodium spp. merozoites invade host red blood cells (RBCs), multiply, exit, and reinvade uninfected RBCs in a continuing cycle that is responsible for all the clinical symptoms associated with malaria. Entry into (invasion) and exit from (egress) RBCs are highly regulated processes that are mediated by an array of parasite proteins with specific functional roles. Many of these parasite proteins are stored in specialized apical secretory vesicles, and their timely release is critical for successful invasion and egress. For example, the discharge of parasite protein ligands to the apical surface of merozoites is required for interaction with host receptors to mediate invasion, and the timely discharge of proteases and pore-forming proteins helps in permeabilization and dismantling of limiting membranes during egress. This review focuses on our understanding of the signaling mechanisms that regulate apical organelle secretion during host cell invasion and egress by malaria parasites. The review also explores how understanding key signaling mechanisms in the parasite can open opportunities to develop novel strategies to target Plasmodium parasites and eliminate malaria.

Manipulation of the Host Cell Membrane during Plasmodium Liver Stage Egress

A crucial step in the life cycle of Plasmodium parasites is the transition from the liver stage to the blood stage. Hepatocyte-derived merozoites reach the blood vessels of the liver inside host cell-derived vesicles called merosomes. The molecular basis of merosome formation is only partially understood. Here we show that Plasmodium berghei liver stage merozoites, upon rupture of the parasitophorous vacuole membrane, destabilize the host cell membrane (HCM) and induce separation of the host cell actin cytoskeleton from the HCM. At the same time, the phospholipid and protein composition of the HCM appears to be substantially altered. This includes the loss of a phosphatidylinositol 4,5-bisphosphate (PIP₂) reporter and the PIP₂-dependent actin-plasma membrane linker ezrin from the HCM. Furthermore, transmembrane domain-containing proteins and palmitoylated and myristoylated proteins, as well as glycosylphosphatidylinositol-anchored proteins, lose their HCM localization. Collectively, these findings provide an explanation of HCM destabilization during Plasmodium liver stage egress and thereby contribute to our understanding of the molecular mechanisms that lead to merosome formation.

Egress from host cells is an essential process for intracellular pathogens, allowing successful infection of other cells and thereby spreading the infection. Here we describe the molecular details of a novel egress strategy of Plasmodium parasites infecting hepatocytes. We show that toward the end of the liver stage, parasites induce a breakdown of the host cell actin cytoskeleton, leading to destabilization of the host cell plasma membrane. This, in turn, results in the formation of membrane vesicles (merosomes), in which parasites can safely migrate from liver tissue to the bloodstream to infect red blood cells and start the pathogenic phase of malaria.

Parasitophorous vacuole poration precedes its rupture and rapid host erythrocyte cytoskeleton collapse in Plasmodium falciparum egress

In the asexual blood stages of malarial infection, merozoites invade erythrocytes and replicate within a parasitophorous vacuole to form daughter cells that eventually exit (egress) by sequential rupture of the vacuole and erythrocyte membranes. The current model is that PKG, a malarial cGMP-dependent protein kinase, triggers egress, activating malarial proteases and other effectors. Using selective inhibitors of either PKG or cysteine proteases to separately inhibit the sequential steps in membrane perforation, combined with video microscopy, electron tomography, electron energy loss spectroscopy, and soft X-ray tomography of mature intracellular Plasmodium falciparum parasites, we resolve intermediate steps in egress. We show that the parasitophorous vacuole membrane (PVM) is permeabilized 10-30 min before its PKG-triggered breakdown into multilayered vesicles. Just before PVM breakdown, the host red cell undergoes an abrupt, dramatic shape change due to the sudden breakdown of the erythrocyte cytoskeleton, before permeabilization and eventual rupture of the erythrocyte membrane to release the parasites. In contrast to the previous view of PKG-triggered initiation of egress and a gradual dismantling of the host erythrocyte cytoskeleton over the course of schizont development, our findings identify an initial step in egress and show that host cell cytoskeleton breakdown is restricted to a narrow time window within the final stages of egress.

Inhibition of an Erythrocyte Tyrosine Kinase with Imatinib Prevents Plasmodium falciparum Egress and Terminates Parasitemia

With half of the world's population at risk for malaria infection and with drug resistance on the rise, the search for mutation-resistant therapies has intensified. We report here a therapy for Plasmodium falciparum malaria that acts by inhibiting the phosphorylation of erythrocyte membrane band 3 by an erythrocyte tyrosine kinase. Because tyrosine phosphorylation of band 3 causes a destabilization of the erythrocyte membrane required for parasite egress, inhibition of the erythrocyte tyrosine kinase leads to parasite entrapment and termination of the infection. Moreover, because one of the kinase inhibitors to demonstrate antimalarial activity is imatinib, i.e. an FDA-approved drug authorized for use in children, translation of the therapy into the clinic will be facilitated. At a time when drug resistant strains of P. falciparum are emerging, a strategy that targets a host enzyme that cannot be mutated by the parasite should constitute a therapeutic mechanism that will retard evolution of resistance.

Nanomechanical properties of composite protein networks of erythroid membranes at lipid surfaces

Erythrocyte membranes have been particularly useful as a model for studies of membrane structure and mechanics. Native erythroid membranes can be electroformed as giant unilamellar vesicles (eGUVs). In the presence of ATP, the erythroid membrane proteins of eGUVs rearrange into protein networks at the microscale. Here, we present a detailed nanomechanical study of individual protein microfilaments forming the protein networks of eGUVs when spread on supporting surfaces. Using Peak Force tapping Atomic Force Microscopy (PF-AFM) in liquid environment we have obtained the mechanical maps of the composite lipid-protein networks supported on solid surface. In the absence of ATP, the protein pool was characterized by a Young's Modulus E_pool≈5-15MPa whereas the complex filaments were found softer after protein supramolecular rearrangement; E_fil≈0.4MPa. The observed protein softening and reassembling could be relevant for understanding the mechanisms of cytoskeleton reorganization found in pathological erythrocytes or erythrocytes that are affected by biological agents.

Reversible host cell remodeling underpins deformability changes in malaria parasite sexual blood stages

The sexual blood stage of the human malaria parasite Plasmodium falciparum undergoes remarkable biophysical changes as it prepares for transmission to mosquitoes. During maturation, midstage gametocytes show low deformability and sequester in the bone marrow and spleen cords, thus avoiding clearance during passage through splenic sinuses. Mature gametocytes exhibit increased deformability and reappear in the peripheral circulation, allowing uptake by mosquitoes. Here we define the reversible changes in erythrocyte membrane organization that underpin this biomechanical transformation. Atomic force microscopy reveals that the length of the spectrin cross-members and the size of the skeletal meshwork increase in developing gametocytes, then decrease in mature-stage gametocytes. These changes are accompanied by relocation of actin from the erythrocyte membrane to the Maurer's clefts. Fluorescence recovery after photobleaching reveals reversible changes in the level of coupling between the membrane skeleton and the plasma membrane. Treatment of midstage gametocytes with cytochalasin D decreases the vertical coupling and increases their filterability. A computationally efficient coarse-grained model of the erythrocyte membrane reveals that restructuring and constraining the spectrin meshwork can fully account for the observed changes in deformability.

Expanding the antimalarial toolkit: Targeting host-parasite interactions

Recent successes in malaria control are threatened by drug-resistant Plasmodium parasites and insecticide-resistant Anopheles mosquitoes, and first generation vaccines offer only partial protection. New research approaches have highlighted host as well as parasite molecules or pathways that could be targeted for interventions. In this study, we discuss host–parasite interactions at the different stages of the Plasmodium life cycle within the mammalian host and the potential for therapeutics that prevent parasite migration, invasion, intracellular growth, or egress from host cells, as well as parasite-induced pathology.

A spiral scaffold underlies cytoadherent knobs in Plasmodium falciparum-infected erythrocytes

Much of the virulence of Plasmodium falciparum malaria is caused by cytoadherence of infected erythrocytes, which promotes parasite survival by preventing clearance in the spleen. Adherence is mediated by membrane protrusions known as knobs, whose formation depends on the parasite-derived, knob-associated histidine-rich protein (KAHRP). Knobs are required for cytoadherence under flow conditions, and they contain both KAHRP and the parasite-derived erythrocyte membrane protein PfEMP1. Using electron tomography, we have examined the 3-dimensional structure of knobs in detergent-insoluble skeletons of P falciparum 3D7 schizonts. We describe a highly organized knob skeleton composed of a spiral structure coated by an electron-dense layer underlying the knob membrane. This knob skeleton is connected by multiple links to the erythrocyte cytoskeleton. We used immuno-electron microscopy (EM) to locate KAHRP in these structures. The arrangement of membrane proteins in the knobs, visualized by high-resolution freeze-fracture scanning EM, is distinct from that in the surrounding erythrocyte membrane, with a structure at the apex that likely represents the adhesion site. Thus, erythrocyte knobs in P falciparum infection contain a highly organized skeleton structure underlying a specialized region of membrane. We propose that the spiral and dense coat organize the cytoadherence structures in the knob, and anchor them into the erythrocyte cytoskeleton. The high density of knobs and their extensive mechanical linkage suggest an explanation for the rigidification of the cytoskeleton in infected cells, and for the transmission to the cytoskeleton of shear forces experienced by adhering cells.

Stretching and relaxation of malaria-infected red blood cells

The invasion of red blood cells (RBCs) by malaria parasites is a complex dynamic process, in which the infected RBCs gradually lose their deformability and their ability to recover their original shape is greatly reduced with the maturation of the parasites. In this work, we developed two types of cell model, one with an included parasite, and the other without an included parasite. The former is a representation of real malaria-infected RBCs, in which the parasite is treated as a rigid body. In the latter, where the parasite is absent, the membrane modulus and viscosity are elevated so as to produce the same features present in the parasite model. In both cases, the cell membrane is modeled as a viscoelastic triangular network connected by wormlike chains. We studied the transient behaviors of stretching deformation and shape relaxation of malaria-infected RBCs based on these two models and found that both models can generate results in agreement with those of previously published studies. With the parasite maturation, the shape deformation becomes smaller and smaller due to increasing cell rigidity, whereas the shape relaxation time becomes longer and longer due to the cell's reduced ability to recover its original shape.

Dynamic morphology and cytoskeletal protein changes during spontaneous inside-out vesiculation of red blood cell membranes

Vesicle preparations from cell plasma membranes, red blood cells in particular, are extensively used in transport and enzymic studies and in the fields of drug delivery and drug-transport interactions. Here we investigated the role of spectrin–actin, the main components of the red cell cortical cytoskeleton, in a particular mechanism of vesicle generation found to be relevant to the egress process of Plasmodium falciparum merozoites from infected red blood cells. Plasma membranes from red blood cells lysed in ice-cold media of low ionic strength and free of divalent cations spontaneously and rapidly vesiculate upon incubation at 37 °C rendering high yields of inside-out vesicles. We tested the working hypothesis that the dynamic shape transformations resulted from changes in spectrin–actin configuration within a disintegrating cytoskeletal mesh. We showed that cytoskeletal-free membranes behave like a two-dimensional fluid lacking shape control, that spectrin–actin remain attached to vesiculating membranes for as long as spontaneous movement persists, that most of the spectrin–actin detachment occurs terminally at the time of vesicle sealing and that naked membrane patches increasingly appear during vesiculation. These results support the proposed role of spectrin–actin in spontaneous vesiculation. The implications of these results to membrane dynamics and to the mechanism of merozoite egress are discussed.

Thioamide-based fluorescent protease sensors

Thioamide quenchers can be paired with compact fluorophores to design “turn-on” fluorescent protease substrates. We have used this method to study a variety of serine-, cysteine-, carboxyl-, and metallo-proteases, including trypsin, chymotrypsin, pepsin, thermolysin, papain, and calpain. Since thioamides quench some fluorophores red-shifted from those naturally occurring in proteins, this technique can be used for real time monitoring of protease activity in crude preparations of virtually any protease. We demonstrate the value of this method in three model applications: (1) characterization of papain enzyme kinetics using rapid-mixing experiments, (2) selective monitoring of cleavage at a single site in a peptide with multiple proteolytic sites, and (3) analysis of the specificity of an inhibitor of calpain in cell lysates.

Malaria-infected erythrocyte-derived microvesicles mediate cellular communication within the parasite population and with the host immune system

Humans and mice infected with different Plasmodium strains are known to produce microvesicles derived from the infected red blood cells (RBCs), denoted RMVs. Studies in mice have shown that RMVs are elevated during infection and have proinflammatory activity. Here we present a detailed characterization of RMV composition and function in the human malaria parasite Plasmodium falciparum. Proteomics profiling revealed the enrichment of multiple host and parasite proteins, in particular of parasite antigens associated with host cell membranes and proteins involved in parasite invasion into RBCs. RMVs are quantitatively released during the asexual parasite cycle prior to parasite egress. RMVs demonstrate potent immunomodulatory properties on human primary macrophages and neutrophils. Additionally, RMVs are internalized by infected red blood cells and stimulate production of transmission stage parasites in a dose-dependent manner. Thus, RMVs mediate cellular communication within the parasite population and with the host innate immune system.

Prison break: pathogens' strategies to egress from host cells

A wide spectrum of pathogenic bacteria and protozoa has adapted to an intracellular life-style, which presents several advantages, including accessibility to host cell metabolites and protection from the host immune system. Intracellular pathogens have developed strategies to enter and exit their host cells while optimizing survival and replication, progression through the life cycle, and transmission. Over the last decades, research has focused primarily on entry, while the exit process has suffered from neglect. However, pathogen exit is of fundamental importance because of its intimate association with dissemination, transmission, and inflammation. Hence, to fully understand virulence mechanisms of intracellular pathogens at cellular and systemic levels, it is essential to consider exit mechanisms to be a key step in infection. Exit from the host cell was initially viewed as a passive process, driven mainly by physical stress as a consequence of the explosive replication of the pathogen. It is now recognized as a complex, strategic process termed "egress," which is just as well orchestrated and temporally defined as entry into the host and relies on a dynamic interplay between host and pathogen factors. This review compares egress strategies of bacteria, pathogenic yeast, and kinetoplastid and apicomplexan parasites. Emphasis is given to recent advances in the biology of egress in mycobacteria and apicomplexans.

Malaria proteomics: insights into the parasite-host interactions in the pathogenic space

Proteomics is improving malaria research by providing global information on relevant protein sets from the parasite and the host in connection with its cellular structures and specific functions. In the last decade, reports have described biologically significant elements in the proteome of Plasmodium, which are selectively targeted and quantified, allowing for sensitive and high-throughput comparisons. The identification of molecules by which the parasite and the host react during the malaria infection is crucial to the understanding of the underlying pathogenic mechanisms. Hence, proteomics is playing a major role by defining the elements within the pathogenic space between both organisms that change across the parasite life cycle in association with the host transformation and response. Proteomics has identified post-translational modifications in the parasite and the host that are discussed in terms of functional interactions in malaria parasitism. Furthermore, the contribution of proteomics to the investigation of immunogens for potential vaccine candidates is summarized. The malaria-specific technological advances in proteomics are particularly suited now for identifying host-parasite interactions that could lead to promising targets for therapy, diagnosis or prevention. In this review, we examine the knowledge gained on the biology, pathogenesis, immunity and diagnosis of Plasmodium infection from recent proteomic studies. This article is part of a Special Issue entitled: Trends in Microbial Proteomics.

Recent insights into apicomplexan parasite egress provide new views to a kill

A hallmark of apicomplexan pathogens such as Plasmodium, Toxoplasma and Cryptosporidium is that they invade, replicate within, and then egress from their host cells. Egress usually results in lysis of the host cell, with deleterious consequences for the host. In the case of malaria, for example, much of the disease pathology is associated with cyclical waves of host erythrocyte destruction. This review highlights recent advances in mapping the signaling pathways that lead to egress and the parasite molecules involved in responding to and transmitting those signals. The review also discusses new findings for effector molecules that mediate disruption of the bounding membranes that enclose the intracellular parasite and the manner in which membrane rupture occurs to finally release invasive forms of the parasite.

Membrane active chelators as novel anti-African trypanosome and anti-malarial drugs

Malaria (Plasmodium spp.) and human African trypanosomiasis (Trypanosoma brucei spp.) are vector borne, deadly parasitic diseases. While chemotherapeutic agents for both diseases are available, difficulty in disease eradication and development of drug resistance require that new therapies targeting unexplored pathways or exploiting novel modes of action be developed. Intracellular Plasmodium and extracellular Trypanosoma brucei may have unique and essential requirements for divalent metal ions, beyond that deemed physiological for the host. Membrane Active Chelators (MACs), biologically active only in a hydrophobic lipid environment, are able to bind metal ions at elevated non-physiological concentrations in the vicinity of cell membranes. A dose-response relationship study using validated viability assays revealed that two MAC drugs, DP-b99 and DP-460, were cytotoxic for these parasites in vitro. The 50% effective concentration (EC50) values for DP-b99 and DP-460 were 87 μM and 39 μM for Trypanosoma brucei brucei and 21 μM and 28 μM for erythrocytic Plasmodium falciparum, respectively. Furthermore, drug potency was maintained for at least 24h in serum containing medium at 37°C. While the exact mechanism of action of MACs against intracellular malaria and extracellular African trypanosome parasites has yet to be determined, their potential as antiparasitic agents warrants further investigation.

The antimalarial natural product symplostatin 4 is a nanomolar inhibitor of the food vacuole falcipains

The marine natural product symplostatin 4 (Sym4) has been recognized as a potent antimalarial agent. However, its mode of action and, in particular, direct targets have to date remained elusive. We report a chemical synthesis of Sym4 and show that Sym4-treatment of P. falciparum-infected red blood cells (RBCs) results in the generation of a swollen food vacuole phenotype and a reduction of parasitemia at nanomolar concentrations. We furthermore demonstrate that Sym4 is a nanomolar inhibitor of the P. falciparum falcipains in infected RBCs, suggesting inhibition of the hemoglobin degradation pathway as Sym4's mode of action. Finally, we reveal a critical influence of the unusual methyl-methoxypyrrolinone (mmp) group of Sym4 for potent inhibition, indicating that Sym4 derivatives with such a mmp moiety might represent viable lead structures for the development of antimalarial falcipain inhibitors.

Probing the cytoadherence of malaria infected red blood cells under flow

Malaria is one of the most widespread and deadly human parasitic diseases caused by the Plasmodium (P.) species with the P.falciparum being the most deadly. The parasites are capable of invading red blood cells (RBCs) during infection. At the late stage of parasites’ development, the parasites export proteins to the infected RBCs (iRBC) membrane and bind to receptors of surface proteins on the endothelial cells that line microvasculature walls. Resulting adhesion of iRBCs to microvasculature is one of the main sources of most complications during malaria infection. Therefore, it is important to develop a versatile and simple experimental method to quantitatively investigate iRBCs cytoadhesion and binding kinetics. Here, we developed an advanced flow based adhesion assay to demonstrate that iRBC’s adhesion to endothelial CD36 receptor protein coated channels is a bistable process possessing a hysteresis loop. This finding confirms a recently developed model of cell adhesion which we used to fit our experimental data. We measured the contact area of iRBC under shear flow at different stages of infection using Total Internal Reflection Fluorescence (TIRF), and also adhesion receptor and ligand binding kinetics using Atomic Force Microscopy (AFM). With these parameters, we reproduced in our model the experimentally observed changes in adhesion properties of iRBCs accompanying parasite maturation and investigated the main mechanisms responsible for these changes, which are the contact area during the shear flow as well as the rupture area size.

The proteomics and interactomics of human erythrocytes

In this minireview, we focus on advances in our knowledge of the human erythrocyte proteome and interactome that have occurred since our seminal review on the topic published in 2007. As will be explained, the number of unique proteins has grown from 751 in 2007 to 2289 as of today. We describe how proteomics and interactomics tools have been used to probe critical protein changes in disorders impacting the blood. The primary example used is the work done on sickle cell disease where biomarkers of severity have been identified, protein changes in the erythrocyte membranes identified, pharmacoproteomic impact of hydroxyurea studied and interactomics used to identify erythrocyte protein changes that are predicted to have the greatest impact on protein interaction networks.

Life cycle-dependent cytoskeletal modifications in Plasmodium falciparum infected erythrocytes

Plasmodium falciparum infection of human erythrocytes is known to result in the modification of the host cell cytoskeleton by parasite-coded proteins. However, such modifications and corresponding implications in malaria pathogenesis have not been fully explored. Here, we probed the gradual modification of infected erythrocyte cytoskeleton with advancing stages of infection using atomic force microscopy (AFM). We reported a novel strategy to derive accurate and quantitative information on the knob structures and their connections with the spectrin network by performing AFM-based imaging analysis of the cytoplasmic surface of infected erythrocytes. Significant changes on the red cell cytoskeleton were observed from the expansion of spectrin network mesh size, extension of spectrin tetramers and the decrease of spectrin abundance with advancing stages of infection. The spectrin network appeared to aggregate around knobs but also appeared sparser at non-knob areas as the parasite matured. This dramatic modification of the erythrocyte skeleton during the advancing stage of malaria infection could contribute to the loss of deformability of the infected erythrocyte.