A conserved molecular motor drives cell invasion and gliding motility across malaria life cycle stages and other apicomplexan parasites

Rational engineering of an antimalarial peptide with enhanced proteolytic stability and preserved parasite invasion inhibitory activity

We describe rational chemical engineering to enhance the proteolytic stability of a chimeric peptide using a combination of unique strategies that involve the incorporation of a series of d-amino acids into the parent l-peptide sequence and restricting the conformational freedom of the peptide by covalent stitching. We hypothesize that replacing a stretch of sequence of an unstructured peptide motif with d-amino acids would increase its proteolytic stability without significantly affecting its affinity to the target protein. Also, considering the Cβ-Cβ distances, replacing an appropriate pair of residues with cysteine to form an additional disulfide bond in the molecule would provide additional stability to the engineered peptide. To verify this hypothesis, we have implemented these strategies to a previously reported peptidic inhibitor RR, against P. falciparum invasion into red blood cells (RBCs) and designed two novel heterochiral chimeric peptides, RR-I and RR-II. We have demonstrated that these peptides exhibit remarkable inhibitory activity with dramatically enhanced proteolytic stability. Finally, we have designed a cyclic analog, RR-III, to enhance the stability of the peptide against endopeptidases. The RR-III peptide exhibits the same inhibitory activity as RR-II while demonstrating impressive resistance to enzymatic degradation and prolonged stability in human plasma. These developments hold promise for a new generation of peptide-based therapeutics, showcasing the potential of residue selection for tailored modifications, as demonstrated in this work.

The Plasmodium circumsporozoite protein

The circumsporozoite protein (CSP) is one of the most studied proteins of the malaria parasite. It is the target of the only licensed malaria vaccines and is essential for sporozoite formation and infectivity. Yet, the mechanisms by which CSP functions and its interactions with other proteins are only beginning to be understood. Here we review the current state of knowledge of CSP structure and function, as sporozoites develop in the mosquito and establish infection in the mammalian host, and outline outstanding questions that need to be addressed.

APEX-based proximity labeling in Plasmodium identifies a membrane protein with dual functions during mosquito infection

Transmission of the malaria parasite Plasmodium to mosquitoes necessitates gamete egress from red blood cells to allow zygote formation and ookinete motility to enable penetration of the midgut epithelium. Both processes are dependent on the secretion of proteins from distinct sets of specialized vesicles. Inhibiting some of these proteins has shown potential for blocking parasite transmission to the mosquito. To identify new transmission blocking vaccine candidates, we aimed to define the microneme content from ookinetes of the rodent model organism Plasmodium berghei using APEX2-mediated rapid proximity-dependent biotinylation. Besides known proteins of ookinete micronemes, this identified over 50 novel candidates and sharpened the list of a previous survey based on subcellular fractionation. Functional analysis of a first candidate uncovered a dual role for this membrane protein in male gametogenesis and ookinete midgut traversal. Mutation of a putative trafficking motif in the C-terminus affected ookinete to oocyst transition but not gamete formation. This suggests the existence of distinct functional and transport requirements for Plasmodium proteins in different parasite stages.

Application of optical tweezer technology reveals that PfEBA and PfRH ligands, not PfMSP1, play a central role in Plasmodium falciparum merozoite-erythrocyte attachment

Malaria pathogenesis and parasite multiplication depend on the ability of Plasmodium merozoites to invade human erythrocytes. Invasion is a complex multi-step process involving multiple parasite proteins which can differ between species and has been most extensively studied in P. falciparum. However, dissecting the precise role of individual proteins has to date been limited by the availability of quantifiable phenotypic assays. In this study, we apply a new approach to assigning function to invasion proteins by using optical tweezers to directly manipulate recently egressed P. falciparum merozoites and erythrocytes and quantify the strength of attachment between them, as well as the frequency with which such attachments occur. Using a range of inhibitors, antibodies, and genetically modified strains including some generated specifically for this work, we quantitated the contribution of individual P. falciparum proteins to these merozoite-erythrocyte attachment interactions. Conditional deletion of the major P. falciparum merozoite surface protein PfMSP1, long thought to play a central role in initial attachment, had no impact on the force needed to pull merozoites and erythrocytes apart, whereas interventions that disrupted the function of several members of the EBA-175 like Antigen (PfEBA) family and Reticulocyte Binding Protein Homologue (PfRH) invasion ligand families did have a significant negative impact on attachment. Deletion of individual PfEBA and PfRH ligands reinforced the known redundancy within these families, with the deletion of some ligands impacting detachment force while others did not. By comparing over 4000 individual merozoite-erythrocyte interactions in a range of conditions and strains, we establish that the PfEBA/PfRH families play a central role in P. falciparum merozoite attachment, not the major merozoite surface protein PfMSP1.

Apicoplast-derived isoprenoids are essential for biosynthesis of GPI protein anchors, and consequently for egress and invasion in Plasmodium falciparum

Glycophosphatidylinositol (GPI) anchors are the predominant glycoconjugate in Plasmodium parasites, enabling modified proteins to associate with biological membranes. GPI biosynthesis commences with donation of a mannose residue held by dolichol-phosphate at the endoplasmic reticulum membrane. In Plasmodium dolichols are derived from isoprenoid precursors synthesised in the Plasmodium apicoplast, a relict plastid organelle of prokaryotic origin. We found that treatment of Plasmodium parasites with apicoplast inhibitors decreases the synthesis of isoprenoid and GPI intermediates resulting in GPI-anchored proteins becoming untethered from their normal membrane association. Even when other isoprenoids were chemically rescued, GPI depletion led to an arrest in schizont stage parasites, which had defects in segmentation and egress. In those daughter parasites (merozoites) that did form, proteins that would normally be GPI-anchored were mislocalised, and when these merozoites were artificially released they were able to attach to but not invade new red blood cells. Our data provides further evidence for the importance of GPI biosynthesis during the asexual cycle of P. falciparum, and indicates that GPI biosynthesis, and by extension egress and invasion, is dependent on isoprenoids synthesised in the apicoplast.

Disruption of Plasmodium falciparum kinetochore proteins destabilises the nexus between the centrosome equivalent and the mitotic apparatus

Plasmodium falciparum is the causative agent of malaria and remains a pathogen of global importance. Asexual blood stage replication, via a process called schizogony, is an important target for the development of new antimalarials. Here we use ultrastructure-expansion microscopy to probe the organisation of the chromosome-capturing kinetochores in relation to the mitotic spindle, the centriolar plaque, the centromeres and the apical organelles during schizont development. Conditional disruption of the kinetochore components, PfNDC80 and PfNuf2, is associated with aberrant mitotic spindle organisation, disruption of the centromere marker, CENH3 and impaired karyokinesis. Surprisingly, kinetochore disruption also leads to disengagement of the centrosome equivalent from the nuclear envelope. Severing the connection between the nucleus and the apical complex leads to the formation of merozoites lacking nuclei. Here, we show that correct assembly of the kinetochore/spindle complex plays a previously unrecognised role in positioning the nascent apical complex in developing P. falciparum merozoites.

The novel Plasmodium berghei protein S14 is essential for sporozoite gliding motility and infectivity

Plasmodium sporozoites are the infective forms of the malaria parasite in the mosquito and vertebrate host. Gliding motility allows sporozoites to migrate and invade mosquito salivary glands and mammalian hosts. Motility and invasion are powered by an actin-myosin motor complex linked to the glideosome, which contains glideosome-associated proteins (GAPs), MyoA and the myosin A tail-interacting protein (MTIP). However, the role of several proteins involved in gliding motility remains unknown. We identified that the S14 gene is upregulated in sporozoite from transcriptome data of Plasmodium yoelii and further confirmed its transcription in P. berghei sporozoites using real-time PCR. C-terminal 3×HA-mCherry tagging revealed that S14 is expressed and localized on the inner membrane complex of the sporozoites. We disrupted S14 in P. berghei and demonstrated that it is essential for sporozoite gliding motility, and salivary gland and hepatocyte invasion. The gliding and invasion-deficient S14 knockout sporozoites showed normal expression and organization of inner membrane complex and surface proteins. Taken together, our data show that S14 plays a role in the function of the glideosome and is essential for malaria transmission.

Characterization of merozoite-specific thrombospondin-related anonymous protein (MTRAP) in Plasmodium vivax and P. knowlesi parasites

Plasmodium vivax, the most widespread human malaria parasite, and P. knowlesi, an emerging Plasmodium that infects humans, are the phylogenetically closest malarial species that infect humans, which may induce cross-species reactivity across most co-endemic areas in Southeast Asia. The thrombospondin-related anonymous protein (TRAP) family is indispensable for motility and host cell invasion in the growth and development of Plasmodium parasites. The merozoite-specific TRAP (MTRAP), expressed in blood-stage merozoites, is supposed to be essential for human erythrocyte invasion. We aimed to characterize MTRAPs in blood-stage P. vivax and P. knowlesi parasites and ascertain their cross-species immunoreactivity. Recombinant P. vivax and P. knowlesi MTRAPs of full-length ectodomains were expressed in a mammalian expression system. The MTRAP-specific immunoglobulin G, obtained from immune animals, was used in an immunofluorescence assay for subcellular localization and invasion inhibitory activity in blood-stage parasites was determined. The cross-species humoral immune responses were analyzed in the sera of patients with P. vivax or P. knowlesi infections. The MTRAPs of P. vivax (PvMTRAP) and P. knowlesi (PkMTRAP) were localized on the rhoptry body of merozoites in blood-stage parasites. Both anti-PvMTRAP and anti-PkMTRAP antibodies inhibited erythrocyte invasion of blood-stage P. knowlesi parasites. The humoral immune response to PvMTRAP showed high immunogenicity, longevity, and cross-species immunoreactivity with P. knowlesi. MTRAPs are promising candidates for development of vaccines and therapeutics against vivax and knowlesi malaria.

Novel quinolinepiperazinyl-aryltetrazoles targeting the blood stage of Plasmodium falciparum

The emergence of drug resistance against the frontline antimalarials is a major challenge in the treatment of malaria. In view of emerging reports on drug-resistant strains of Plasmodium against artemisinin combination therapy, a dire need is felt for the discovery of novel compounds acting against novel targets in the parasite. In this study, we identified a novel series of quinolinepiperazinyl-aryltetrazoles (QPTs) targeting the blood stage of Plasmodium. In vitro anti-plasmodial activity screening revealed that most of the compounds showed IC50 < 10 μM against chloroquine-resistant PfINDO strain, with the most promising lead compounds 66 and 75 showing IC50 values of 2.25 and 1.79 μM, respectively. Further, compounds 64-66, 68, 75-77 and 84 were found to be selective (selectivity index >50) in their action against Pf over a mammalian cell line, with compounds 66 and 75 offering the highest selectivity indexes of 178 and 223, respectively. Explorations into the action of lead compounds 66 and 75 revealed their selective cidal activity towards trophozoites and schizonts. In a ring-stage survival assay, 75 showed cidal activity against the early rings of artemisinin-resistant PfCam3.1R539T. Further, 66 and 75 in combination with artemisinin and pyrimethamine showed additive to weak synergistic interactions. Of these two in vitro lead molecules, only 66 restricted rise in the percentage of parasitemia to about 10% in P. berghei-infected mice with a median survival time of 28 days as compared to the untreated control, which showed the percentage of parasitemia >30%, and a median survival of 20 days. Promising antimalarial activity, high selectivity, and additive interaction with artemisinin and pyrimethamine indicate the potential of these compounds to be further optimized chemically as future drug candidates against malaria.

The Multiple Roles of LCCL Domain-Containing Proteins for Malaria Parasite Transmission

Multi-protein complexes are crucial for various essential biological processes of the malaria parasite Plasmodium, such as protein synthesis, host cell invasion and adhesion. Especially during the sexual phase of the parasite, which takes place in the midgut of the mosquito vector, protein complexes are required for fertilization, sporulation and ultimately for the successful transmission of the parasite. Among the most noticeable protein complexes of the transmission stages are the ones formed by the LCCL domain-containing protein family that play critical roles in the generation of infective sporozoites. The six members of this protein family are characterized by numerous adhesive modules and domains typically found in secreted proteins. This review summarizes the findings of expression and functional studies on the LCCL domain-containing proteins of the human pathogenic P. falciparum and the rodent-infecting P. berghei and discusses the common features and differences of the homologous proteins.

Characterization of 2,4-Dianilinopyrimidines Against Five P. falciparum Kinases PfARK1, PfARK3, PfNEK3, PfPK9, and PfPKB

Plasmodium kinases are increasingly recognized as potential novel antiplasmodial targets for the treatment of malaria, but only a small subset of these kinases have had structure-activity relationship (SAR) campaigns reported. Herein we report the discovery of CZC-54252 (1) as an inhibitor of five P. falciparum kinases PfARK1, PfARK3, PfNEK3, PfPK9, and PfPKB. 39 analogues were evaluated against all five kinases to establish SAR at three regions of the kinase active site. Nanomolar inhibitors of each kinase were discovered. We identified common and divergent SAR trends across all five kinases, highlighting substituents in each region that improve potency and selectivity for each kinase. Potent analogues were evaluated against the P. falciparum blood stage. Eight submicromolar inhibitors were discovered, of which 37 demonstrated potent antiplasmodial activity (EC50 = 0.16 μM). Our results provide an understanding of features needed to inhibit each individual kinase and lay groundwork for future optimization efforts toward novel antimalarials.

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.

Plasmodium vivax merozoite-specific thrombospondin-related anonymous protein (PvMTRAP) interacts with human CD36, suggesting a novel ligand-receptor interaction for reticulocyte invasion

BACKGROUND

The Plasmodium vivax merozoite restrictively invades immature erythrocytes, suggesting that its ligand(s) might interact with corresponding receptor(s) that are selectively abundant on reticulocytes to complete the invasion. Finding the ligand‒receptor interaction involved in P. vivax invasion is critical to vivax malaria management; nevertheless, it remains to be unraveled.

METHODS

A library of reticulocyte receptors and P. vivax ligands were expressed by a HEK293E mammalian cell expression system and were then used to screen the interaction using enzyme-linked immunosorbent assay (ELISA). A flow cytometry-based erythrocyte binding assay and bio-layer interferometry experiment were further utilized to cellularly and quantitatively identify the ligand‒receptor interaction, respectively.

RESULTS

Plasmodium vivax merozoite-specific thrombospondin-related anonymous protein (PvMTRAP) was found to interact with human CD36 using systematic screening. This interaction was specific at a molecular level from in vitro analysis and comparable to that of P. vivax Duffy binding protein (PvDBP) and Duffy antigen receptor for chemokines (DARC) (KD: 37.0 ± 1.4 nM and 7.7 ± 0.5 nM, respectively). Flow cytometry indicated that PvMTRAP preferentially binds to reticulocytes, on which CD36 is selectively present.

CONCLUSIONS

Human CD36 is selectively abundant on reticulocytes and is able to interact specifically with PvMTRAP, suggesting that it may function as a ligand and receptor during the invasion of reticulocytes by P. vivax.

Identification of Novel, Potent, and Selective Compounds against Malaria Using Glideosomal-Associated Protein 50 as a Drug Target

Phylum apicomplexan consists of parasites, such as Plasmodium and Toxoplasma. These obligate intracellular parasites enter host cells via an energy-dependent process using specialized machinery, called the glideosome. In the present study, we used Plasmodium falciparum GAP50, a glideosome-associated protein, as a target to screen 951 different compounds from diverse chemical libraries. Using different screening methods, eight compounds (Hayatinine, Curine, MMV689758 (Bedaquiline), MMV1634402 (Brilacidin), and MMV688271, MMV782353, MMV642550, and USINB4-124-8) were identified, which showed promising binding affinity (KD < 75 μM), along with submicromolar range antiparasitic efficacy and selectivity index > 100 fold for malaria parasite. These eight compounds were effective against Chloroquine-resistant PfINDO and Artemisinin-resistant PfCam3.1R359T strains. Studies on the effect of these compounds at asexual blood stages showed that these eight compounds act differently at different developmental stages, indicating the binding of these compounds to other Plasmodium proteins, in addition to PfGAP50. We further studied the effects of compounds (Bedaquiline and USINB4-124-8) in an in vivoPlasmodium berghei mouse model of malaria. Importantly, the oral delivery of Bedaquiline (50 mg/kg b. wt.) showed substantial suppression of parasitemia, and three out of seven mice were cured of the infection. Thus, our study provides new scaffolds for the development of antimalarials that can act at multiple Plasmodium lifecycle stages.

Fostering innovation to solve the biomechanics of microbe-host interactions: Focus on the adhesive forces underlying Apicomplexa parasite biology

The protozoa, Toxoplasma gondii and Plasmodium spp., are preeminent members of the Apicomplexa parasitic phylum in large part due to their public health and economic impact. Hence, they serve as model unicellular eukaryotes with which to explore the repertoire of molecular and cellular strategies that specific developmental morphotypes deploy to timely adjust to their host(s) in order to perpetuate. In particular, host tissue- and cell-invasive morphotypes termed zoites alternate extracellular and intracellular lifestyles, thereby sensing and reacting to a wealth of host-derived biomechanical cues over their partnership. In the recent years, biophysical tools especially related to real time force measurement have been introduced, teaching us how creative are these microbes to shape a unique motility system that powers fast gliding through a variety of extracellular matrices, across cellular barriers, in vascular systems or into host cells. Equally performant was this toolkit to start illuminating how parasites manipulate their hosting cell adhesive and rheological properties to their advantage. In this review, besides highlighting major discoveries along the way, we discuss the most promising development, synergy, and multimodal integration in active noninvasive force microscopy methods. These should in the near future unlock current limitations and allow capturing, from molecules to tissues, the many biomechanical and biophysical interplays over the dynamic host and microbe partnership.

Plasmodium falciparum formins are essential for invasion and sexual stage development

The malaria parasite uses actin-based mechanisms throughout its lifecycle to control a range of biological processes including intracellular trafficking, gene regulation, parasite motility and invasion. In this work we assign functions to the Plasmodium falciparum formins 1 and 2 (FRM1 and FRM2) proteins in asexual and sexual blood stage development. We show that FRM1 is essential for merozoite invasion and FRM2 is required for efficient cell division. We also observed divergent functions for FRM1 and FRM2 in gametocyte development. Conditional deletion of FRM1 leads to a delay in gametocyte stage progression. We show that FRM2 controls the actin and microtubule cytoskeletons in developing gametocytes, with premature removal of the protein resulting in a loss of transmissible stage V gametocytes. Lastly, we show that targeting formin proteins with the small molecule inhibitor of formin homology domain 2 (SMIFH2) leads to a multistage block in asexual and sexual stage parasite development.

A PPP-type pseudophosphatase is required for the maintenance of basal complex integrity in Plasmodium falciparum

During its asexual blood stage, P. falciparum replicates via schizogony, wherein dozens of daughter cells are formed within a single parent. The basal complex, a contractile ring that separates daughter cells, is critical for schizogony. In this study, we identify a Plasmodium basal complex protein essential for basal complex maintenance. Using multiple microscopy techniques, we demonstrate that PfPPP8 is required for uniform basal complex expansion and maintenance of its integrity. We characterize PfPPP8 as the founding member of a novel family of pseudophosphatases with homologs in other Apicomplexan parasites. By co-immunoprecipitation, we identify two additional new basal complex proteins. We characterize the unique temporal localizations of these new basal complex proteins (late-arriving) and of PfPPP8 (early-departing). In this work, we identify a novel basal complex protein, determine its specific role in segmentation, identify a new pseudophosphatase family, and establish that the P. falciparum basal complex is a dynamic structure.

Memory CD8+ T cell-mediated protection against liver-stage malaria

Nearly half of the world's population is at risk of malaria, a disease caused by the protozoan parasite Plasmodium, which is estimated to cause more than 240,000,000 infections and kill more than 600,000 people annually. The emergence of Plasmodia resistant to chemoprophylactic treatment highlights the urgency to develop more effective vaccines. In this regard, whole sporozoite vaccination approaches in murine models and human challenge studies have provided substantial insight into the immune correlates of protection from malaria. From these studies, CD8+ T cells have come to the forefront, being identified as critical for vaccine-mediated liver-stage immunity that can prevent the establishment of the symptomatic blood stages and subsequent transmission of infection. However, the unique biological characteristics required for CD8+ T cell protection from liver-stage malaria dictate that more work must be done to design effective vaccines. In this review, we will highlight a subset of studies that reveal basic aspects of memory CD8+ T cell-mediated protection from liver-stage malaria infection.

Plasmodium falciparum GAP40 Plays an Essential Role in Merozoite Invasion and Gametocytogenesis

Cyclic invasion of red blood cells (RBCs) by Plasmodium merozoites is associated with the symptoms and pathology of malaria. Merozoite invasion is powered actively and rapidly by a parasite actomyosin motor called the glideosome. The ability of the glideosome to generate force to support merozoite entry into the host RBCs is thought to rely on its stable anchoring within the inner membrane complex (IMC) through membrane-resident proteins, such as GAP50 and GAP40. Using a conditional knockdown (KD) approach, we determined that PfGAP40 was required for asexual blood-stage replication. PfGAP40 is not needed for merozoite egress from host RBCs or for the attachment of merozoites to new RBCs. PfGAP40 coprecipitates with PfGAP45 and PfGAP50. During merozoite invasion, PfGAP40 is associated strongly with stabilizing the expression levels of PfGAP45 and PfGAP50 in the schizont stage. Although PfGAP40 KD did not influence IMC integrity, it impaired the maturation of gametocytes. In addition, PfGAP40 is phosphorylated, and mutations that block phosphorylation of PfGAP40 at the C-terminal serine residues S370, S372, S376, S405, S409, S420, and S445 reduced merozoite invasion efficiency. Overall, our findings implicate PfGAP40 as an important regulator for the gliding activity of merozoites and suggest that phosphorylation is required for PfGAP40 function. IMPORTANCE Red blood cell invasion is central to the pathogenesis of the malaria parasite, and the parasite proteins involved in this process are potential therapeutic targets. Gliding motility powers merozoite invasion and is driven by a unique molecular motor termed the glideosome. The glideosome is stably anchored to the parasite inner membrane complex (IMC) through membrane-resident proteins. In the present study, we demonstrate the importance of an IMC-resident glideosome component, PfGAP40, that plays a critical role in stabilizing the expression levels of glideosome components in the schizont stage. We determined that phosphorylation of PfGAP40 at C-terminal residues is required for efficient merozoite invasion.

PfATP4 inhibitors in the Medicines for Malaria Venture Malaria Box and Pathogen Box block the schizont-to-ring transition by inhibiting egress rather than invasion

The cation efflux pump Plasmodium falciparum ATPase 4 (PfATP4) maintains Na⁺ homeostasis in malaria parasites and has been implicated in the mechanism of action of many structurally diverse antimalarial agents, including >7% of the antimalarial compounds in the Medicines for Malaria Venture's 'Malaria Box' and 'Pathogen Box'. Recent screens of the 'Malaria Box' and 'Pathogen Box' revealed that many PfATP4 inhibitors prevent parasites from exiting their host red blood cell (egress) or entering new host cells (invasion), suggesting that these compounds may have additional molecular targets involved in egress or invasion. Here, we demonstrate that five PfATP4 inhibitors reduce egress but not invasion. These compounds appear to inhibit egress by blocking the activation of protein kinase G, an enzyme that, once stimulated, rapidly activates parasite egress. We establish a direct link between egress and PfATP4 function by showing that the inhibition of egress is attenuated in a Na⁺-depleted environment and in parasites with a mutation in pfatp4. Finally, we show that PfATP4 inhibitors induce host cell lysis when administered prior to the completion of parasite replication. Since host cell lysis mimics egress but is not followed by invasion, this phenomenon likely explains why several PfATP4 inhibitors were previously classified as invasion inhibitors. Collectively, our results confirm that PfATP4-mediated Na⁺ efflux is critical to the regulation of parasite egress.

Analysis of Plasmodium falciparum myosin B ATPase activity and structure in complex with the calmodulin-like domain of its light chain MLC-B

Myosin B (MyoB) is a class 14 myosin expressed in all invasive stages of the malaria parasite, Plasmodium falciparum. It is not associated with the glideosome complex that drives motility and invasion of host cells. During red blood cell invasion, MyoB remains at the apical tip of the merozoite but is no longer observed once invasion is completed. MyoB is not essential for parasite survival, but when it is knocked out, merozoites are delayed in the initial stages of red blood cell invasion, giving rise to a growth defect that correlates with reduced invasion success. Therefore, further characterization is needed to understand how MyoB contributes to parasite invasion. Here, we have expressed and purified functional MyoB with the help of parasite-specific chaperones Hsp90 and Unc45, characterized its binding to actin and its known light chain MLC-B using biochemical and biophysical methods and determined its low-resolution structure in solution using small angle X-ray scattering. In addition to MLC-B, we found that four other putative regulatory light chains bind to the MyoB IQ2 motif in vitro. The purified recombinant MyoB adopted the overall shape of a myosin, exhibited actin-activated ATPase activity, and moved actin filaments in vitro. Additionally, we determined that the ADP release rate was faster than the ATP turnover number, and thus, does not appear to be rate limiting. This, together with the observed high affinity to actin and the specific localization of MyoB, may point toward a role in tethering and/or force sensing during early stages of invasion.

Complementary crosstalk between palmitoylation and phosphorylation events in MTIP regulates its role during Plasmodium falciparum invasion

Post-translational modifications (PTMs) including phosphorylation and palmitoylation have emerged as crucial biomolecular events that govern many cellular processes including functioning of motility- and invasion-associated proteins during Plasmodium falciparum invasion. However, no study has ever focused on understanding the possibility of a crosstalk between these two molecular events and its direct impact on preinvasion- and invasion-associated protein–protein interaction (PPI) network-based molecular machinery. Here, we used an integrated in silico analysis to enrich two different catalogues of proteins: (i) the first group defines the cumulative pool of phosphorylated and palmitoylated proteins, and (ii) the second group represents a common set of proteins predicted to have both phosphorylation and palmitoylation. Subsequent PPI analysis identified an important protein cluster comprising myosin A tail interacting protein (MTIP) as one of the hub proteins of the glideosome motor complex in P. falciparum, predicted to have dual modification with the possibility of a crosstalk between the same. Our findings suggested that blocking palmitoylation led to reduced phosphorylation and blocking phosphorylation led to abrogated palmitoylation of MTIP. As a result of the crosstalk between these biomolecular events, MTIP’s interaction with myosin A was found to be abrogated. Next, the crosstalk between phosphorylation and palmitoylation was confirmed at a global proteome level by click chemistry and the phenotypic effect of this crosstalk was observed via synergistic inhibition in P. falciparum invasion using checkerboard assay and isobologram method. Overall, our findings revealed, for the first time, an interdependence between two PTM types, their possible crosstalk, and its direct impact on MTIP-mediated invasion via glideosome assembly protein myosin A in P. falciparum. These insights can be exploited for futuristic drug discovery platforms targeting parasite molecular machinery for developing novel antimalarial therapeutics.

Tryptophan C-mannosylation is critical for Plasmodium falciparum transmission

Tryptophan C-mannosylation stabilizes proteins bearing a thrombospondin repeat (TSR) domain in metazoans. Here we show that Plasmodium falciparum expresses a DPY19 tryptophan C-mannosyltransferase in the endoplasmic reticulum and that DPY19-deficiency abolishes C-glycosylation, destabilizes members of the TRAP adhesin family and inhibits transmission to mosquitoes. Imaging P. falciparum gametogenesis in its entirety in four dimensions using lattice light-sheet microscopy reveals defects in ΔDPY19 gametocyte egress and exflagellation. While egress is diminished, ΔDPY19 microgametes still fertilize macrogametes, forming ookinetes, but these are abrogated for mosquito infection. The gametogenesis defects correspond with destabilization of MTRAP, which we show is C-mannosylated in P. falciparum, and the ookinete defect is concordant with defective CTRP secretion on the ΔDPY19 background. Genetic complementation of DPY19 restores ookinete infectivity, sporozoite production and C-mannosylation activity. Therefore, tryptophan C-mannosylation by DPY19 ensures TSR protein quality control at two lifecycle stages for successful transmission of the human malaria parasite.

Here, Lopaticki et al. show that Plasmodium falciparum expresses a Dpy19 C-mannosyltransferase in the endoplasmic reticulum that glycosylates TSR domains. Functional characterization shows that PfDpy19 plays a critical role in transmission through mosquitoes as PfDpy19-deficiency abolishes C-glycosylation and destabilizes proteins relevant for gametogenesis and oocyst formation.

Functional Characterization of the Thrombospondin-Related Paralogous Proteins Rhoptry Discharge Factors 1 and 2 Unveils Phenotypic Plasticity in Toxoplasma gondii Rhoptry Exocytosis

To gain access to the intracellular cytoplasmic niche essential for their growth and replication, apicomplexan parasites such as Toxoplasma gondii rely on the timely secretion of two types of apical organelles named micronemes and rhoptries. Rhoptry proteins are key to host cell invasion and remodeling, however, the molecular mechanisms underlying the tight control of rhoptry discharge are poorly understood. Here, we report the identification and functional characterization of two novel T. gondii thrombospondin-related proteins implicated in rhoptry exocytosis. The two proteins, already annotated as MIC15 and MIC14, were renamed rhoptry discharge factor 1 (RDF1) and rhoptry discharge factor 2 (RDF2) and found to be exclusive of the Coccidia class of apicomplexan parasites. Furthermore, they were shown to have a paralogous relationship and share a C-terminal transmembrane domain followed by a short cytoplasmic tail. Immunofluorescence analysis of T. gondii tachyzoites revealed that RDF1 presents a diffuse punctate localization not reminiscent of any know subcellular compartment, whereas RDF2 was not detected. Using a conditional knockdown approach, we demonstrated that RDF1 loss caused a marked growth defect. The lack of the protein did not affect parasite gliding motility, host cell attachment, replication and egress, whereas invasion was dramatically reduced. Notably, while RDF1 depletion did not result in altered microneme exocytosis, rhoptry discharge was found to be heavily impaired. Interestingly, rhoptry secretion was reversed by spontaneous upregulation of the RDF2 gene in knockdown parasites grown under constant RDF1 repression. Collectively, our results identify RDF1 and RDF2 as additional key players in the pathway controlling rhoptry discharge. Furthermore, this study unveils a new example of compensatory mechanism contributing to phenotypic plasticity in T. gondii.

New Avenues to Design Toxoplasma Vaccines Based on Oocysts and Cysts

Toxoplasmosis is a worldwide disease affecting all warm-blooded animals, including humans. Vaccination strategies aimed at inducing an efficient immune response while preventing transmission have been attempted in the past. While many different approaches can partially protect immunized animals against subsequent infections, full and lasting protection is rarely attained and only with live-attenuated vaccines. In addition, vaccines based on mutant strains that are deficient in forming the chronic phase of the parasite (such as Toxovax™) cannot be extensively used due to their zoonotic potential and the possibility of reversion to virulent phenotypes. An increasing number of studies using emerging genetic-engineering tools have been conducted to design novel vaccines based on recombinant proteins, DNA or delivery systems such as nanoparticles. However, these are usually less efficient due to their antigenic simplicity. In this perspective article we discuss potential target genes and novel strategies to generate live-attenuated long-lasting vaccines based on tissue cysts and oocysts, which are the environmentally resistant chronic forms of Toxoplasma. By selectively disrupting genes important for parasite dissemination, cyst formation and/or sporozoite invasion, alone or in combination, a vaccine based on a live-attenuated strain that elicits a protective immune response while preventing the transmission of Toxoplasma could be created. Finally, further improvements of protocols to generate Toxoplasma sexual stages in vitro might lead to the production of oocysts from such a strain without the need for using mice or cats.

Apicoplast Dynamics During Plasmodium Cell Cycle

The deadly malaria parasite, Plasmodium falciparum, contains a unique subcellular organelle termed the apicoplast, which is a clinically-proven antimalarial drug target. The apicoplast is a plastid with essential metabolic functions that evolved via secondary endosymbiosis. As an ancient endosymbiont, the apicoplast retained its own genome and it must be inherited by daughter cells during cell division. During the asexual replication of P. falciparum inside human red blood cells, both the parasite, and the apicoplast inside it, undergo massive morphological changes, including DNA replication and division. The apicoplast is an integral part of the cell and thus its development is tightly synchronized with the cell cycle. At the same time, certain aspects of its dynamics are independent of nuclear division, representing a degree of autonomy in organelle biogenesis. Here, we review the different aspects of organelle dynamics during P. falciparum intraerythrocytic replication, summarize our current understanding of these processes, and describe the many open questions in this area of parasite basic cell biology.

Thrombospondin Related Anonymous Protein Superfamily in Vector-Borne Apicomplexans: The Parasite’s Toolkit for Cell Invasion

Apicomplexan parasites transmitted by vectors, including Babesia spp. and Plasmodium spp., cause severe disease in both humans and animals. These parasites have a complex life cycle during which they migrate, invade, and replicate in contrasting hosts such as the mammal and the invertebrate vector. The interaction of parasites with the host cell is mediated by adhesive proteins which play a key role in the different cellular processes regarding successful progression of the life cycle. Thrombospondin related anonymous protein (TRAP) is a superfamily of adhesins that are involved in motility, invasion and egress of the parasite. These proteins are stored and released from apical organelles and have either one or two types of adhesive domains, namely thrombospondin type 1 repeat and von Willebrand factor type A, that upon secretion are located in the extracellular portion of the molecule. Proteins from the TRAP superfamily have been intensively studied in Plasmodium species and to a lesser extent in Babesia spp., where they have proven to be functionally relevant throughout the entire parasite’s journey both in the arthropod vector and in the mammalian host. In recent years new findings provided answers to the role of TRAP proteins and in some cases the function of these adhesins during the parasite’s life cycle was redefined. In this review we will discuss the current knowledge of the diverse roles of the TRAP superfamily in vector-borne parasites from Class Aconoidasida. We will focus on the varied approaches that allowed the understanding of protein function and the relevance of TRAP- superfamily throughout the entire parasite’s cell cycle.

Deciphering the Role of Protein Phosphatases in Apicomplexa: The Future of Innovative Therapeutics?

Parasites belonging to the Apicomplexa phylum still represent a major public health and world-wide socioeconomic burden that is greatly amplified by the spread of resistances against known therapeutic drugs. Therefore, it is essential to provide the scientific and medical communities with innovative strategies specifically targeting these organisms. In this review, we present an overview of the diversity of the phosphatome as well as the variety of functions that phosphatases display throughout the Apicomplexan parasites’ life cycles. We also discuss how this diversity could be used for the design of innovative and specific new drugs/therapeutic strategies.

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

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

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

Molecular mechanisms of hematological and biochemical alterations in malaria: A review

Malaria is a dangerous disease that contributes to millions of hospital visits and hundreds of thousands of deaths, especially in children residing in sub-Saharan Africa. Although several interventions such as vector control, case detection, and treatment are already in place, there is no substantive reduction in the disease burden. Several studies in the past have reported the emergence of resistant strains of malaria parasites (MPs) and mosquitoes, and poor adherence and inaccessibility to effective antimalarial drugs as the major factors for this persistent menace of malaria infections. Moreover, victory against MP infections for many years has been hampered by an incomplete understanding of the complex nature of malaria pathogenesis. Very recent studies have identified different complex interactions and hematological alterations induced by malaria parasites. However, no studies have hybridized these alterations for a better understanding of Malaria pathogenesis. Hence, this review thoroughly discusses the molecular mechanisms of all reported hematological and biochemical alterations induced by MPs infections. Specifically, the mechanisms in which MP-infection induces anemia, thrombocytopenia, leukopenia, dyslipidemia, hypoglycemia, oxidative stress, and liver and kidney malfunctions were presented. The study also discussed how MPs evade the host's immune response and suggested strategies to limit evasion of the host's immune response to combat malaria and its complications.

The Basal Complex Protein PfMORN1 Is Not Required for Asexual Replication of Plasmodium falciparum

Plasmodium falciparum, the Apicomplexan parasite that causes the most severe form of human malaria, divides via schizogony during the asexual blood stage of its life cycle. In this method of cell division, multiple daughter cells are generated from a single schizont by segmentation. During segmentation, the basal complex forms at the basal end of the nascent daughter parasites and likely facilitates cell shape and cytokinesis. The requirement and function for each of the individual protein components within the basal complex remain largely unknown in P. falciparum. In this work, we demonstrate that the P. falciparum membrane occupation and recognition nexus repeat-containing protein 1 (PfMORN1) is not required for asexual replication. Following inducible knockout of PfMORN1, we find no detectable defect in asexual parasite morphology or replicative fitness.

IMPORTANCEPlasmodium falciparum parasites cause the most severe form of human malaria. During the clinically relevant blood stage of its life cycle, the parasites divide via schizogony. In this divergent method of cell division, the components for multiple daughter cells are generated within a common cytoplasm. At the end of schizogony, segmentation partitions the organelles into invasive daughter parasites. The basal complex is a ring-shaped molecular machine that is critical for segmentation. The requirement for individual proteins within the basal complex is incompletely understood. We demonstrate that the PfMORN1 protein is dispensable for blood stage replication of P. falciparum. This result highlights important differences between Plasmodium parasites and Toxoplasma gondii, where the ortholog T. gondii MORN1 (TgMORN1) is required for asexual replication.

Gliding motility of Plasmodium merozoites

Plasmodium malaria parasites use a unique substrate-dependent locomotion, termed gliding motility, to migrate through tissues and invade cells. Previously, it was thought that the small labile invasive stages that invade erythrocytes, merozoites, use this motility solely to penetrate target erythrocytes. Here we reveal that merozoites use gliding motility for translocation across host cells prior to invasion. This forms an important preinvasion step that is powered by a conserved actomyosin motor and is regulated by a complex signaling pathway. This work broadens our understanding of the role of gliding motility and invasion in the blood and will have a significant impact on our understanding of blood stage host–pathogen interactions and parasite biology, with implications for interventions targeting erythrocyte invasion.

Plasmodium malaria parasites are obligate intracellular protozoans that use a unique form of locomotion, termed gliding motility, to move through host tissues and invade cells. The process is substrate dependent and powered by an actomyosin motor that drives the posterior translocation of extracellular adhesins which, in turn, propel the parasite forward. Gliding motility is essential for tissue translocation in the sporozoite and ookinete stages; however, the short-lived erythrocyte-invading merozoite stage has never been observed to undergo gliding movement. Here we show Plasmodium merozoites possess the ability to undergo gliding motility in vitro and that this mechanism is likely an important precursor step for successful parasite invasion. We demonstrate that two human infective species, Plasmodium falciparum and Plasmodium knowlesi, have distinct merozoite motility profiles which may reflect distinct invasion strategies. Additionally, we develop and validate a higher throughput assay to evaluate the effects of genetic and pharmacological perturbations on both the molecular motor and the complex signaling cascade that regulates motility in merozoites. The discovery of merozoite motility provides a model to study the glideosome and adds a dimension for work aiming to develop treatments targeting the blood stage invasion pathways.

Plasmodium falciparum PhIL1-associated complex plays an essential role in merozoite reorientation and invasion of host erythrocytes

The human malaria parasite, Plasmodium falciparum possesses unique gliding machinery referred to as the glideosome that powers its entry into the insect and vertebrate hosts. Several parasite proteins including Photosensitized INA-labelled protein 1 (PhIL1) have been shown to associate with glideosome machinery. Here we describe a novel PhIL1 associated protein complex that co-exists with the glideosome motor complex in the inner membrane complex of the merozoite. Using an experimental genetics approach, we characterized the role(s) of three proteins associated with PhIL1: a glideosome associated protein- PfGAPM2, an IMC structural protein- PfALV5, and an uncharacterized protein—referred here as PfPhIP (PhIL1 Interacting Protein). Parasites lacking PfPhIP or PfGAPM2 were unable to invade host RBCs. Additionally, the downregulation of PfPhIP resulted in significant defects in merozoite segmentation. Furthermore, the PfPhIP and PfGAPM2 depleted parasites showed abrogation of reorientation/gliding. However, initial attachment with host RBCs was not affected in these parasites. Together, the data presented here show that proteins of the PhIL1-associated complex play an important role in the orientation of P. falciparum merozoites following initial attachment, which is crucial for the formation of a tight junction and hence invasion of host erythrocytes.

Invasion of Plasmodium merozoites into RBCs is a multistep process that involves initial attachment of merozoites to the RBC surface, their reorientation, and subsequent gliding into RBCs using glideosome machinery. The glideosome machinery lies between the plasma membrane and inner membrane complex (IMC) and consists of MyoA, its interacting protein; MTIP, gliding associated proteins (GAPs), and a Photosensitized INA labeled protein (PhIL1)-associated protein complex. Here, we demonstrate that the deletion of any of two components of the PhIL1-associated complex, PfPhIP or PfGAPM2, aborts merozoite reorientation and blocks their invasion into RBCs. The study thus provides new molecular and mechanistic insights into merozoite invasion of RBCs.

Glycosphingolipid GM3 is localized in both exoplasmic and cytoplasmic leaflets of Plasmodium falciparum malaria parasite plasma membrane

Lipid rafts, sterol-rich and sphingolipid-rich microdomains on the plasma membrane are important in processes like cell signaling, adhesion, and protein and lipid transport. The virulence of many eukaryotic parasites is related to raft microdomains on the cell membrane. In the malaria parasite Plasmodium falciparum, glycosylphosphatidylinositol-anchored proteins, which are important for invasion and are possible targets for vaccine development, are localized in the raft. However, rafts are poorly understood. We used quick-freezing and freeze-fracture immuno-electron microscopy to examine the localization of monosialotetrahexosylganglioside (GM1) and monosialodihexosylganglioside (GM3), putative raft microdomain components in P. falciparum and infected erythrocytes. This method immobilizes molecules in situ, minimizing artifacts. GM3 was localized in the exoplasmic (EF) and cytoplasmic leaflets (PF) of the parasite and the parasitophorous vacuole (PV) membranes, but solely in the EF of the infected erythrocyte membrane, as in the case for uninfected erythrocytes. Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂) was localized solely in the PF of erythrocyte, parasite, and PV membranes. This is the first time that GM3, the major component of raft microdomains, was found in the PF of a biological membrane. The unique localization of raft microdomains may be due to P. falciparum lipid metabolism and its unique biological processes, like protein transport from the parasite to infected erythrocytes.

Identification of novel inner membrane complex and apical annuli proteins of the malaria parasite Plasmodium falciparum

The inner membrane complex (IMC) is a defining feature of apicomplexan parasites, which confers stability and shape to the cell, functions as a scaffolding compartment during the formation of daughter cells and plays an important role in motility and invasion during different life cycle stages of these single-celled organisms. To explore the IMC proteome of the malaria parasite Plasmodium falciparum we applied a proximity-dependent biotin identification (BioID)-based proteomics approach, using the established IMC marker protein Photosensitized INA-Labelled protein 1 (PhIL1) as bait in asexual blood-stage parasites. Subsequent mass spectrometry-based peptide identification revealed enrichment of 12 known IMC proteins and several uncharacterized candidate proteins. We validated nine of these previously uncharacterized proteins by endogenous GFP-tagging. Six of these represent new IMC proteins, while three proteins have a distinct apical localization that most likely represents structures described as apical annuli in Toxoplasma gondii. Additionally, various Kelch13 interacting candidates were identified, suggesting an association of the Kelch13 compartment and the IMC in schizont and merozoite stages. This work extends the number of validated IMC proteins in the malaria parasite and reveals for the first time the existence of apical annuli proteins in P. falciparum. Additionally, it provides evidence for a spatial association between the Kelch13 compartment and the IMC in late blood-stage parasites.

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

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

The Ins and Outs of Plasmodium Rhoptries, Focusing on the Cytosolic Side

Parasites of the genus Plasmodium cause human and animal malaria, leading to significant health and economic impacts. A key aspect of the complex life cycle of Plasmodium parasites is the invasion of the parasite into its host cell, which is mediated by secretory organelles. The largest of these organelles, the rhoptry, undergoes rapid and profound physiological changes when it secretes its contents during merozoite and sporozoite invasion of the host erythrocyte and hepatocyte, respectively. Here we discuss recent advancements in our understanding of the dynamic rhoptry biology during the parasite's invasive stages, with a focus on the roles of cytosolically exposed rhoptry-interacting proteins (C-RIPs). We explore potential similarities between the molecular mechanisms driving merozoite and sporozoite rhoptry function.

The Ringleaders: Understanding the Apicomplexan Basal Complex Through Comparison to Established Contractile Ring Systems

The actomyosin contractile ring is a key feature of eukaryotic cytokinesis, conserved across many eukaryotic kingdoms. Recent research into the cell biology of the divergent eukaryotic clade Apicomplexa has revealed a contractile ring structure required for asexual division in the medically relevant genera Toxoplasma and Plasmodium; however, the structure of the contractile ring, known as the basal complex in these parasites, remains poorly characterized and in the absence of a myosin II homolog, it is unclear how the force required of a cytokinetic contractile ring is generated. Here, we review the literature on the basal complex in Apicomplexans, summarizing what is known about its formation and function, and attempt to provide possible answers to this question and suggest new avenues of study by comparing the Apicomplexan basal complex to well-studied, established cytokinetic contractile rings and their mechanisms in organisms such as S. cerevisiae and D. melanogaster. We also compare the basal complex to structures formed during mitochondrial and plastid division and cytokinetic mechanisms of organisms beyond the Opisthokonts, considering Apicomplexan diversity and divergence.

Identification and characterization of a new 34 kDa MORN motif-containing sporozoite surface-exposed protein, Cp-P34, unique to Cryptosporidium

Despite the public health impact of childhood diarrhea caused by Cryptosporidium, effective drugs and vaccines against this parasite are unavailable. Efforts to identify vaccine targets have focused on critical externally exposed virulence factors expressed in the parasite s invasive stages. However, no single surface antigen has yet been found that can elicit a significant protective immune response and it is likely that pooling multiple immune targets will be necessary. Discovery of surface proteins on Cryptosporidium sporozoites is therefore vital to this effort to develop a multi-antigenic vaccine. In this study we applied a novel single-domain camelid antibody (VHH) selection method to identify immunogenic proteins expressed on the surface of Cryptosporidium parvum sporozoites. By this approach, VHHs were identified that recognize two sporozoite surface-exposed antigens, the previously identified gp900 and an unrecognized immunogenic protein, Cp-P34. This Cp-P34 antigen, which contains multiple Membrane Occupation and Recognition Nexus (MORN) repeats, is found in excysted sporozoites as well as in the parasite s intracellular stages. Cp-P34 appears to accumulate inside the parasite and transiently appears on the surface of sporozoites to be shed in trails. Identical or nearly identical orthologs of Cp-P34 are found in the Cryptosporidium hominis and Cryptosporidium tyzzeri genomes. Except for the conserved MORN motifs, the Cp-P34 gene shares no significant homology with genes of other protozoans and thus appears to be unique to Cryptosporidium spp. Cp-P34 elicits immune responses in naturally exposed alpacas and warrants further investigation as a potential vaccine candidate.

Botanical candidates from Saudi Arabian flora as potential therapeutics for Plasmodium infection

Malaria is a lethal parasitic disease affecting over two hundred million people worldwide and kills almost half a million people per year. Until now, there is no curative treatment for this disease that has a substantial morbidity. The available chemotherapeutic agents are unable to completely control the infection with the continuous appearance of drug resistance. Consequently, the search for new therapeutic agents with high safety profiles and low side effects is of paramount importance. Several natural products have been investigated and proven to have antimalarial effects either in vivo or in vitro. A large number of plants have been studied globally for their antimalarial activities. However, studies that have been conducted in this field in Saudi Arabia are not enough. This article presents global and local research on the need for novel natural antimalarial agents with a particular emphasis on studies involving plants from Saudi Arabian flora.

Identification and Molecular Dissection of IMC32, a Conserved Toxoplasma Inner Membrane Complex Protein That Is Essential for Parasite Replication

The inner membrane complex (IMC) is a unique organelle of apicomplexan parasites that plays critical roles in parasite motility, host cell invasion, and replication. Despite the common functions of the organelle, relatively few IMC proteins are conserved across the phylum and the precise roles of many IMC components remain to be characterized. Here, we identify a novel component of the Toxoplasma gondii IMC (IMC32) that localizes to the body portion of the IMC and is recruited to developing daughter buds early during endodyogeny. IMC32 is essential for parasite survival, as its conditional depletion results in a complete collapse of the IMC that is lethal to the parasite. We demonstrate that localization of IMC32 is dependent on both an N-terminal palmitoylation site and a series of C-terminal coiled-coil domains. Using deletion analyses and functional complementation, we show that two conserved regions within the C-terminal coiled-coil domains play critical roles in protein function during replication. Together, this work reveals an essential component of parasite replication that provides a novel target for therapeutic intervention of T. gondii and related apicomplexan parasites.IMPORTANCE The IMC is an important organelle that apicomplexan parasites use to maintain their intracellular lifestyle. While many IMC proteins have been identified, only a few central players that are essential for internal budding have been described and even fewer are conserved across the phylum. Here, we identify IMC32, a novel component of the Toxoplasma gondii IMC that localizes to very early daughter buds, indicating a role in the early stages of parasite replication. We then demonstrate that IMC32 is essential for parasite survival and pinpoint conserved regions within the protein that are important for membrane association and daughter cell formation. As IMC32 is unique to these parasites and not present in their mammalian hosts, it serves as a new target for the development of drugs that exclusively affect these important intracellular pathogens.

The Dynamic Roles of the Inner Membrane Complex in the Multiple Stages of the Malaria Parasite

Apicomplexan parasites, such as human malaria parasites, have complex lifecycles encompassing multiple and diverse environmental niches. Invading, replicating, and escaping from different cell types, along with exploiting each intracellular niche, necessitate large and dynamic changes in parasite morphology and cellular architecture. The inner membrane complex (IMC) is a unique structural element that is intricately involved with these distinct morphological changes. The IMC is a double membrane organelle that forms de novo and is located beneath the plasma membrane of these single-celled organisms. In Plasmodium spp. parasites it has three major purposes: it confers stability and shape to the cell, functions as an important scaffolding compartment during the formation of daughter cells, and plays a major role in motility and invasion. Recent years have revealed greater insights into the architecture, protein composition and function of the IMC. Here, we discuss the multiple roles of the IMC in each parasite lifecycle stage as well as insights into its sub-compartmentalization, biogenesis, disassembly and regulation during stage conversion of P. falciparum.

A Comprehensive Gender-related Secretome of Plasmodium berghei Sexual Stages

Plasmodium, the malaria parasite, undergoes a complex life cycle alternating between a vertebrate host and a mosquito vector of the genus Anopheles In red blood cells of the vertebrate host, Plasmodium multiplies asexually or differentiates into gamete precursors, the male and female gametocytes, responsible for parasite transmission. Sexual stage maturation occurs in the midgut of the mosquito vector, where male and female gametes egress from the host erythrocytes to fuse and form a zygote. Gamete egress entails the successive rupture of two membranes surrounding the parasite, the parasitophorous vacuole membrane and the erythrocyte plasma membrane. In this study, we used the rodent model parasite Plasmodium berghei to design a label-free quantitative proteomic approach aimed at identifying gender-related proteins differentially released/secreted by purified mature gametocytes when activated to form gametes. We compared the abundance of molecules secreted by wild type gametocytes of both genders with that of a transgenic line defective in male gamete maturation and egress. This enabled us to provide a comprehensive data set of egress-related molecules and their gender specificity. Using specific antibodies, we validated eleven candidate molecules, predicted as either gender-specific or common to both male and female gametocytes. All of them localize to punctuate, vesicle-like structures that relocate to cell periphery upon activation, but only three of them localize to the gametocyte-specific secretory vesicles named osmiophilic bodies. Our results confirm that the egress process involves a tightly coordinated secretory apparatus that includes different types of vesicles and may put the basis for functional studies aimed at designing novel transmission-blocking molecules.

Decreased K13 Abundance Reduces Hemoglobin Catabolism and Proteotoxic Stress, Underpinning Artemisinin Resistance

Increased tolerance of Plasmodium falciparum to front-line artemisinin antimalarials (ARTs) is associated with mutations in Kelch13 (K13), although the precise role of K13 remains unclear. Here, we show that K13 mutations result in decreased expression of this protein, while mislocalization of K13 mimics resistance-conferring mutations, pinpointing partial loss of function of K13 as the relevant molecular event. K13-GFP is associated with ∼170 nm diameter doughnut-shaped structures at the parasite periphery, consistent with the location and dimensions of cytostomes. Moreover, the hemoglobin-peptide profile of ring-stage parasites is reduced when K13 is mislocalized. We developed a pulse-SILAC approach to quantify protein turnover and observe less disruption to protein turnover following ART exposure when K13 is mislocalized. Our findings suggest that K13 regulates digestive vacuole biogenesis and the uptake/degradation of hemoglobin and that ART resistance is mediated by a decrease in heme-dependent drug activation, less proteotoxicity, and increased survival of parasite ring stages.

Full-length Plasmodium falciparum myosin A and essential light chain PfELC structures provide new anti-malarial targets

Parasites from the genus Plasmodium are the causative agents of malaria. The mobility, infectivity, and ultimately pathogenesis of Plasmodium falciparum rely on a macromolecular complex, called the glideosome. At the core of the glideosome is an essential and divergent Myosin A motor (PfMyoA), a first order drug target against malaria. Here, we present the full-length structure of PfMyoA in two states of its motor cycle. We report novel interactions that are essential for motor priming and the mode of recognition of its two light chains (PfELC and MTIP) by two degenerate IQ motifs. Kinetic and motility assays using PfMyoA variants, along with molecular dynamics, demonstrate how specific priming and atypical sequence adaptations tune the motor's mechano-chemical properties. Supported by evidence for an essential role of the PfELC in malaria pathogenesis, these structures provide a blueprint for the design of future anti-malarials targeting both the glideosome motor and its regulatory elements.

Actomyosin forces and the energetics of red blood cell invasion by the malaria parasite Plasmodium falciparum

All symptoms of malaria disease are associated with the asexual blood stages of development, involving cycles of red blood cell (RBC) invasion and egress by the Plasmodium spp. merozoite. Merozoite invasion is rapid and is actively powered by a parasite actomyosin motor. The current accepted model for actomyosin force generation envisages arrays of parasite myosins, pushing against short actin filaments connected to the external milieu that drive the merozoite forwards into the RBC. In Plasmodium falciparum, the most virulent human malaria species, Myosin A (PfMyoA) is critical for parasite replication. However, the precise function of PfMyoA in invasion, its regulation, the role of other myosins and overall energetics of invasion remain unclear. Here, we developed a conditional mutagenesis strategy combined with live video microscopy to probe PfMyoA function and that of the auxiliary motor PfMyoB in invasion. By imaging conditional mutants with increasing defects in force production, based on disruption to a key PfMyoA phospho-regulation site, the absence of the PfMyoA essential light chain, or complete motor absence, we define three distinct stages of incomplete RBC invasion. These three defects reveal three energetic barriers to successful entry: RBC deformation (pre-entry), mid-invasion initiation, and completion of internalisation, each requiring an active parasite motor. In defining distinct energetic barriers to invasion, these data illuminate the mechanical challenges faced in this remarkable process of protozoan parasitism, highlighting distinct myosin functions and identifying potential targets for preventing malaria pathogenesis.

Inhibition of Plasmepsin V Activity Blocks Plasmodium falciparum Gametocytogenesis and Transmission to Mosquitoes

Plasmodium falciparum gametocytes infect mosquitoes and are responsible for malaria transmission. New interventions that block transmission could accelerate malaria elimination. Gametocytes develop within erythrocytes and activate protein export pathways that remodel the host cell. Plasmepsin V (PMV) is an aspartyl protease that is required for protein export in asexual parasites, but its function and essentiality in gametocytes has not been definitively proven, nor has PMV been assessed as a transmission-blocking drug target. Here, we show that PMV is expressed and can be inhibited specifically in P. falciparum stage I-II gametocytes. PMV inhibitors block processing and export of gametocyte effector proteins and inhibit development of stage II-V gametocytes. Gametocytogenesis in the presence of sublethal inhibitor concentrations results in stage V gametocytes that fail to infect mosquitoes. Therefore, PMV primes gametocyte effectors for export, which is essential for the development and fitness of gametocytes for transmission to mosquitoes.

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

Malaria elimination efforts have been repeatedly hindered by the evolution and spread of multidrug-resistant strains of Plasmodium falciparum. The absence of a commercially available vaccine emphasizes the need for a better understanding of Plasmodium biology in order to further translational research. This has been partly facilitated by targeted gene deletion strategies for the functional analysis of parasite genes. However, genes that are essential for parasite replication in erythrocytes are refractory to such methods, and require conditional knockdown or knockout approaches to dissect their function. One such approach is the TetR-DOZI system that employs multiple synthetic aptamers in the untranslated regions of target genes to control their expression in a tetracycline-dependent manner. Maintaining modified parasites with intact aptamer copies has been challenging since these repeats can be lost by recombination. By interspacing the aptamers with unique sequences, we created a stable genetic system that remains effective at controlling target gene expression.

One of the most powerful approaches to understanding gene function involves turning genes on and off at will and measuring the impact at the cellular or organismal level. This particularly applies to the cohort of essential genes where traditional gene knockouts are inviable. In Plasmodium falciparum, conditional control of gene expression has been achieved by using multicomponent systems in which individual modules interact with each other to regulate DNA recombination, transcription, or posttranscriptional processes. The recently devised TetR-DOZI aptamer system relies on the ligand-regulatable interaction of a protein module with synthetic RNA aptamers to control the translation of a target gene. This technique has been successfully employed to study essential genes in P. falciparum and involves the insertion of several aptamer copies into the 3′ untranslated regions (UTRs), which provide control over mRNA fate. However, aptamer repeats are prone to recombination and one or more copies can be lost from the system, resulting in a loss of control over target gene expression. We rectified this issue by redesigning the aptamer array to minimize recombination while preserving the control elements. As proof of concept, we compared the original and modified arrays for their ability to knock down the levels of a putative essential apicoplast protein (PF3D7_0815700) and demonstrated that the modified array is highly stable and efficient. This redesign will enhance the utility of a tool that is quickly becoming a favored strategy for genetic studies in P. falciparum.

IMPORTANCE Malaria elimination efforts have been repeatedly hindered by the evolution and spread of multidrug-resistant strains of Plasmodium falciparum. The absence of a commercially available vaccine emphasizes the need for a better understanding of Plasmodium biology in order to further translational research. This has been partly facilitated by targeted gene deletion strategies for the functional analysis of parasite genes. However, genes that are essential for parasite replication in erythrocytes are refractory to such methods, and require conditional knockdown or knockout approaches to dissect their function. One such approach is the TetR-DOZI system that employs multiple synthetic aptamers in the untranslated regions of target genes to control their expression in a tetracycline-dependent manner. Maintaining modified parasites with intact aptamer copies has been challenging since these repeats can be lost by recombination. By interspacing the aptamers with unique sequences, we created a stable genetic system that remains effective at controlling target gene expression.

Rapid activation of distinct members of multigene families in Plasmodium spp

The genomes of Plasmodium spp. encode a number of different multigene families that are thought to play a critical role for survival. However, with the exception of the P. falciparum var genes, very little is known about the biological roles of any of the other multigene families. Using the recently developed Selection Linked Integration method, we have been able to activate the expression of a single member of a multigene family of our choice in Plasmodium spp. from its endogenous promoter. We demonstrate the usefulness of this approach by activating the expression of a unique var, rifin and stevor in P. falciparum as well as yir in P. yoelii. Characterization of the selected parasites reveals differences between the different families in terms of mutual exclusive control, co-regulation, and host adaptation. Our results further support the application of the approach for the study of multigene families in Plasmodium and other organisms.

Omelianczyk, Loh et al. activate the expression of a single member of a multigene family in Plasmodium spp. from its endogenous promoter, identifying differences between the different families. This study supports the application of the Selection Linked Integration method for studying multigene families in Plasmodium.

Molecular Characterization and Immuno-Reactivity Patterns of a Novel Plasmodium falciparum Armadillo-Type Repeat Protein, PfATRP

Nearly half of the genes in the Plasmodium falciparum genome have not yet been functionally investigated. We used homology-based structural modeling to identify multiple copies of Armadillo repeats within one uncharacterized gene expressed during the intraerythrocytic stages, PF3D7_0410600, subsequently referred to as P. falciparum Armadillo-Type Repeat Protein (PfATRP). Soluble recombinant PfATRP was expressed in a bacterial expression system, purified to apparent homogeneity and the identity of the recombinant PfATRP was confirmed by mass spectrometry. Affinity-purified α-PfATRP rabbit antibodies specifically recognized the recombinant protein. Immunofluorescence assays revealed that α-PfATRP rabbit antibodies reacted with P. falciparum schizonts. Anti-PfATRP antibody exhibited peripheral staining patterns around the merozoites. Given the localization of PfATRP in merozoites, we tested for an egress phenotype during schizont arrest assays and demonstrated that native PfATRP is inaccessible on the surface of merozoites in intact schizonts. Dual immunofluorescence assays with markers for the inner membrane complex (IMC) and microtubules suggest partial colocalization in both asexual and sexual stage parasites. Using the soluble recombinant PfATRP in a screen of plasma samples revealed that malaria-infected children have naturally acquired PfATRP-specific antibodies.