The molecular mechanisms driving Plasmodium cell division

Malaria, a vector borne disease, is a major global health and socioeconomic problem caused by the apicomplexan protozoan parasite Plasmodium. The parasite alternates between mosquito vector and vertebrate host, with meiosis in the mosquito and proliferative mitotic cell division in both hosts. In the canonical eukaryotic model, cell division is either by open or closed mitosis and karyokinesis is followed by cytokinesis; whereas in Plasmodium closed mitosis is not directly accompanied by concomitant cell division. Key molecular players and regulatory mechanisms of this process have been identified, but the pivotal role of certain protein complexes and the post-translational modifications that modulate their actions are still to be deciphered. Here, we discuss recent evidence for the function of known proteins in Plasmodium cell division and processes that are potential novel targets for therapeutic intervention. We also identify key questions to open new and exciting research to understand divergent Plasmodium cell division.

DNA-binding protein PfAP2-P regulates parasite pathogenesis during malaria parasite blood stages

Malaria-associated pathogenesis such as parasite invasion, egress, host cell remodelling and antigenic variation requires concerted action by many proteins, but the molecular regulation is poorly understood. Here we have characterized an essential Plasmodium-specific Apicomplexan AP2 transcription factor in Plasmodium falciparum (PfAP2-P; pathogenesis) during the blood-stage development with two peaks of expression. An inducible knockout of gene function showed that PfAP2-P is essential for trophozoite development, and critical for var gene regulation, merozoite development and parasite egress. Chromatin immunoprecipitation sequencing data collected at timepoints matching the two peaks of pfap2-p expression demonstrate PfAP2-P binding to promoters of genes controlling trophozoite development, host cell remodelling, antigenic variation and pathogenicity. Single-cell RNA sequencing and fluorescence-activated cell sorting revealed de-repression of most var genes in Δpfap2-p parasites. Δpfap2-p parasites also overexpress early gametocyte marker genes, indicating a regulatory role in sexual stage conversion. We conclude that PfAP2-P is an essential upstream transcriptional regulator at two distinct stages of the intra-erythrocytic development cycle.

Meiosis in Plasmodium: how does it work?

Meiosis is sexual cell division, a process in eukaryotes whereby haploid gametes are produced. Compared to canonical model eukaryotes, meiosis in apicomplexan parasites appears to diverge from the process with respect to the molecular mechanisms involved; the biology of Plasmodium meiosis, and its regulation by means of post-translational modification, are largely unexplored. Here, we discuss the impact of technological advances in cell biology, evolutionary bioinformatics, and genome-wide functional studies on our understanding of meiosis in the Apicomplexa. These parasites, including Plasmodium falciparum, Toxoplasma gondii, and Eimeria spp., have significant socioeconomic impact on human and animal health. Understanding this key stage during the parasite's life cycle may well reveal attractive targets for therapeutic intervention.

Plasmodium ARK2 and EB1 drive unconventional spindle dynamics, during chromosome segregation in sexual transmission stages

The Aurora family of kinases orchestrates chromosome segregation and cytokinesis during cell division, with precise spatiotemporal regulation of its catalytic activities by distinct protein scaffolds. Plasmodium spp., the causative agents of malaria, are unicellular eukaryotes with three unique and highly divergent aurora-related kinases (ARK1-3) that are essential for asexual cellular proliferation but lack most canonical scaffolds/activators. Here we investigate the role of ARK2 during sexual proliferation of the rodent malaria Plasmodium berghei, using a combination of super-resolution microscopy, mass spectrometry, and live-cell fluorescence imaging. We find that ARK2 is primarily located at spindle microtubules in the vicinity of kinetochores during both mitosis and meiosis. Interactomic and co-localisation studies reveal several putative ARK2-associated interactors including the microtubule-interacting protein EB1, together with MISFIT and Myosin-K, but no conserved eukaryotic scaffold proteins. Gene function studies indicate that ARK2 and EB1 are complementary in driving endomitotic division and thereby parasite transmission through the mosquito. This discovery underlines the flexibility of molecular networks to rewire and drive unconventional mechanisms of chromosome segregation in the malaria parasite.

PfAP2-MRP DNA-binding protein is a master regulator of parasite pathogenesis during malaria parasite blood stages

Malaria pathogenicity results from the parasite's ability to invade, multiply within and then egress from the host red blood cell (RBC). Infected RBCs are remodeled, expressing antigenic variant proteins (such as PfEMP1, coded by the var gene family) for immune evasion and survival. These processes require the concerted actions of many proteins, but the molecular regulation is poorly understood. We have characterized an essential Plasmodium specific Apicomplexan AP2 (ApiAP2) transcription factor in Plasmodium falciparum (PfAP2-MRP; Master Regulator of Pathogenesis) during the intraerythrocytic developmental cycle (IDC). An inducible gene knockout approach showed that PfAP2-MRP is essential for development during the trophozoite stage, and critical for var gene regulation, merozoite development and parasite egress. ChIP-seq experiments performed at 16 hour post invasion (h.p.i.) and 40 h.p.i. matching the two peaks of PfAP2-MRP expression, demonstrate binding of PfAP2-MRP to the promoters of genes controlling trophozoite development and host cell remodeling at 16 h.p.i. and antigenic variation and pathogenicity at 40 h.p.i. Using single-cell RNA-seq and fluorescence-activated cell sorting, we show de-repression of most var genes in Δpfap2-mrp parasites that express multiple PfEMP1 proteins on the surface of infected RBCs. In addition, the Δpfap2-mrp parasites overexpress several early gametocyte marker genes at both 16 and 40 h.p.i., indicating a regulatory role in the sexual stage conversion. Using the Chromosomes Conformation Capture experiment (Hi-C), we demonstrate that deletion of PfAP2-MRP results in significant reduction of both intra-chromosomal and inter-chromosomal interactions in heterochromatin clusters. We conclude that PfAP2-MRP is a vital upstream transcriptional regulator controlling essential processes in two distinct developmental stages during the IDC that include parasite growth, chromatin structure and var gene expression.

Plasmodium ARK2-EB1 axis drives the unconventional spindle dynamics, scaffold formation and chromosome segregation of sexual transmission stages

Mechanisms of cell division are remarkably diverse, suggesting the underlying molecular networks among eukaryotes differ extensively. The Aurora family of kinases orchestrates the process of chromosome segregation and cytokinesis during cell division through precise spatiotemporal regulation of their catalytic activities by distinct scaffolds. Plasmodium spp., the causative agents of malaria, are unicellular eukaryotes that have three divergent aurora-related kinases (ARKs) and lack most canonical scaffolds/activators. The parasite uses unconventional modes of chromosome segregation during endomitosis and meiosis in sexual transmission stages within mosquito host. This includes a rapid threefold genome replication from 1N to 8N with successive cycles of closed mitosis, spindle formation and chromosome segregation within eight minutes (termed male gametogony). Kinome studies had previously suggested likely essential functions for all three Plasmodium ARKs during asexual mitotic cycles; however, little is known about their location, function, or their scaffolding molecules during unconventional sexual proliferative stages. Using a combination of super-resolution microscopy, mass spectrometry, omics and live-cell fluorescence imaging, we set out to investigate the contribution of the atypical Aurora paralog ARK2 to proliferative sexual stages using rodent malaria model Plasmodium berghei. We find that ARK2 primarily localises to the spindle apparatus associated with kinetochores during both mitosis and meiosis. Interactomics and co-localisation studies reveal a unique ARK2 scaffold at the spindle including the microtubule plus end-binding protein EB1 and lacking some other conserved molecules. Gene function studies indicate complementary functions of ARK2 and EB1 in driving endomitotic divisions and thereby parasite transmission. Our discovery of a novel Aurora spindle scaffold underlines the emerging flexibility of molecular networks to rewire and drive unconventional mechanisms of chromosome segregation in the malaria parasite Plasmodium.

Plasmodium ARK2-EB1 axis drives the unconventional spindle dynamics, scaffold formation and chromosome segregation of sexual transmission stages

Mechanisms of cell division are remarkably diverse, suggesting the underlying molecular networks among eukaryotes differ extensively. The Aurora family of kinases orchestrates the process of chromosome segregation and cytokinesis during cell division through precise spatiotemporal regulation of their catalytic activities by distinct scaffolds. Plasmodium spp., the causative agents of malaria, are unicellular eukaryotes that have three divergent aurora-related kinases (ARKs) and lack most canonical scaffolds/activators. The parasite uses unconventional modes of chromosome segregation during endomitosis and meiosis in sexual transmission stages within mosquito host. This includes a rapid threefold genome replication from 1N to 8N with successive cycles of closed mitosis, spindle formation and chromosome segregation within eight minutes (termed male gametogony). Kinome studies had previously suggested likely essential functions for all three Plasmodium ARKs during asexual mitotic cycles; however, little is known about their location, function, or their scaffolding molecules during unconventional sexual proliferative stages. Using a combination of super-resolution microscopy, mass spectrometry, and live-cell fluorescence imaging, we set out to investigate the role of the atypical Aurora paralog ARK2 to proliferative sexual stages using rodent malaria model Plasmodium berghei . We find that ARK2 primarily localises to the spindle apparatus in the vicinity of kinetochores during both mitosis and meiosis. Interactomics and co-localisation studies reveal a unique ARK2 scaffold at the spindle including the microtubule plus end-binding protein EB1, lacking conserved Aurora scaffold proteins. Gene function studies indicate complementary functions of ARK2 and EB1 in driving endomitotic divisions and thereby parasite transmission. Our discovery of a novel Aurora kinase spindle scaffold underlines the emerging flexibility of molecular networks to rewire and drive unconventional mechanisms of chromosome segregation in the malaria parasite Plasmodium .

Apicoplast ribosomal protein S10-V127M enhances artemisinin resistance of a Kelch13 transgenic Plasmodium falciparum

Background

The resistance of Plasmodium falciparum to artemisinin-based (ART) drugs, the front-line drug family used in artemisinin-based combination therapy (ACT) for treatment of malaria, is of great concern. Mutations in the kelch13 (k13) gene (for example, those resulting in the Cys580Tyr [C580Y] variant) were identified as genetic markers for ART-resistant parasites, which suggests they are associated with resistance mechanisms. However, not all resistant parasites contain a k13 mutation, and clearly greater understanding of resistance mechanisms is required. A genome-wide association study (GWAS) found single nucleotide polymorphisms associated with ART-resistance in fd (ferredoxin), arps10 (apicoplast ribosomal protein S10), mdr2 (multidrug resistance protein 2), and crt (chloroquine resistance transporter), in addition to k13 gene mutations, suggesting that these alleles contribute to the resistance phenotype. The importance of the FD and ARPS10 variants in ART resistance was then studied since both proteins likely function in the apicoplast, which is a location distinct from that of K13.

Methods

The reported mutations were introduced, together with a mutation to produce the k13-C580Y variant into the ART-sensitive 3D7 parasite line and the effect on ART-susceptibility using the 0−3 h ring survival assay (RSA_0−3 h) was investigated.

Results and conclusion

Introducing both fd-D193Y and arps10-V127M into a k13-C580Y-containing parasite, but not a wild-type k13 parasite, increased survival of the parasite in the RSA_0−3 h. The results suggest epistasis of arps10 and k13, with arps10-V127M a modifier of ART susceptibility in different k13 allele backgrounds.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-022-04330-3.

Division and Transmission: Malaria Parasite Development in the Mosquito

The malaria parasite life cycle alternates between two hosts: a vertebrate and the female Anopheles mosquito vector. Cell division, proliferation, and invasion are essential for parasite development, transmission, and survival. Most research has focused on Plasmodium development in the vertebrate, which causes disease; however, knowledge of malaria parasite development in the mosquito (the sexual and transmission stages) is now rapidly accumulating, gathered largely through investigation of the rodent malaria model, with Plasmodium berghei. In this review, we discuss the seminal genome-wide screens that have uncovered key regulators of cell proliferation, invasion, and transmission during Plasmodium sexual development. Our focus is on the roles of transcription factors, reversible protein phosphorylation, and molecular motors. We also emphasize the still-unanswered important questions around key pathways in cell division during the vector transmission stages and how they may be targeted in future studies.

Plasmodium SAS4: basal body component of male cell which is dispensable for parasite transmission

The centriole/basal body (CBB) is an evolutionarily conserved organelle acting as a microtubule organising centre (MTOC) to nucleate cilia, flagella, and the centrosome. SAS4/CPAP is a conserved component associated with BB biogenesis in many model flagellated cells. Plasmodium, a divergent unicellular eukaryote and causative agent of malaria, displays an atypical, closed mitosis with an MTOC (or centriolar plaque), reminiscent of an acentriolar MTOC, embedded in the nuclear membrane. Mitosis during male gamete formation is accompanied by flagella formation. There are two MTOCs in male gametocytes: the acentriolar nuclear envelope MTOC for the mitotic spindle and an outer centriolar MTOC (the basal body) that organises flagella assembly in the cytoplasm. We show the coordinated location, association and assembly of SAS4 with the BB component, kinesin-8B, but no association with the kinetochore protein, NDC80, indicating that SAS4 is part of the BB and outer centriolar MTOC in the cytoplasm. Deletion of the SAS4 gene produced no phenotype, indicating that it is not essential for either male gamete formation or parasite transmission.

Genome-wide functional analysis reveals key roles for kinesins in the mammalian and mosquito stages of the malaria parasite life cycle

Kinesins are microtubule (MT)-based motors important in cell division, motility, polarity, and intracellular transport in many eukaryotes. However, they are poorly studied in the divergent eukaryotic pathogens Plasmodium spp., the causative agents of malaria, which manifest atypical aspects of cell division and plasticity of morphology throughout the life cycle in both mammalian and mosquito hosts. Here, we describe a genome-wide screen of Plasmodium kinesins, revealing diverse subcellular locations and functions in spindle assembly, axoneme formation, and cell morphology. Surprisingly, only kinesin-13 is essential for growth in the mammalian host while the other 8 kinesins are required during the proliferative and invasive stages of parasite transmission through the mosquito vector. In-depth analyses of kinesin-13 and kinesin-20 revealed functions in MT dynamics during apical cell polarity formation, spindle assembly, and axoneme biogenesis. These findings help us to understand the importance of MT motors and may be exploited to discover new therapeutic interventions against malaria.

Deletion of Plasmodium falciparum ubc13 increases parasite sensitivity to the mutagen, methyl methanesulfonate and dihydroartemisinin

The inducible Di-Cre system was used to delete the putative ubiquitin-conjugating enzyme 13 gene (ubc13) of Plasmodium falciparum to study its role in ubiquitylation and the functional consequence during the parasite asexual blood stage. Deletion resulted in a significant reduction of parasite growth in vitro, reduced ubiquitylation of the Lys63 residue of ubiquitin attached to protein substrates, and an increased sensitivity of the parasite to both the mutagen, methyl methanesulfonate and the antimalarial drug dihydroartemisinin (DHA), but not chloroquine. The parasite was also sensitive to the UBC13 inhibitor NSC697923. The data suggest that this gene does code for an ubiquitin conjugating enzyme responsible for K63 ubiquitylation, which is important in DNA repair pathways as was previously demonstrated in other organisms. The increased parasite sensitivity to DHA in the absence of ubc13 function indicates that DHA may act primarily through this pathway and that inhibitors of UBC13 may both enhance the efficacy of this antimalarial drug and directly inhibit parasite growth.

Inhibition of protein N-myristoylation blocks Plasmodium falciparum intraerythrocytic development, egress and invasion

We have combined chemical biology and genetic modification approaches to investigate the importance of protein myristoylation in the human malaria parasite, Plasmodium falciparum. Parasite treatment during schizogony in the last 10 to 15 hours of the erythrocytic cycle with IMP-1002, an inhibitor of N-myristoyl transferase (NMT), led to a significant blockade in parasite egress from the infected erythrocyte. Two rhoptry proteins were mislocalized in the cell, suggesting that rhoptry function is disrupted. We identified 16 NMT substrates for which myristoylation was significantly reduced by NMT inhibitor (NMTi) treatment, and, of these, 6 proteins were substantially reduced in abundance. In a viability screen, we showed that for 4 of these proteins replacement of the N-terminal glycine with alanine to prevent myristoylation had a substantial effect on parasite fitness. In detailed studies of one NMT substrate, glideosome-associated protein 45 (GAP45), loss of myristoylation had no impact on protein location or glideosome assembly, in contrast to the disruption caused by GAP45 gene deletion, but GAP45 myristoylation was essential for erythrocyte invasion. Therefore, there are at least 3 mechanisms by which inhibition of NMT can disrupt parasite development and growth: early in parasite development, leading to the inhibition of schizogony and formation of “pseudoschizonts,” which has been described previously; at the end of schizogony, with disruption of rhoptry formation, merozoite development and egress from the infected erythrocyte; and at invasion, when impairment of motor complex function prevents invasion of new erythrocytes. These results underline the importance of P. falciparum NMT as a drug target because of the pleiotropic effect of its inhibition.

Understanding the essential factors needed for malaria parasite development could help us find new therapeutic targets. This study reveals that N-myristoylation is a posttranslational modification of proteins essential for the parasites’ growth and their invasion of red blood cells.

Genetic disruption of Plasmodium falciparum Merozoite surface antigen 180 (PfMSA180) suggests an essential role during parasite egress from erythrocytes

Plasmodium falciparum, the parasite responsible for severe malaria, develops within erythrocytes. Merozoite invasion and subsequent egress of intraerythrocytic parasites are essential for this erythrocytic cycle, parasite survival and pathogenesis. In the present study, we report the essential role of a novel protein, P. falciparum Merozoite Surface Antigen 180 (PfMSA180), which is conserved across Plasmodium species and recently shown to be associated with the P. vivax merozoite surface. Here, we studied MSA180 expression, processing, localization and function in P. falciparum blood stages. Initially we examined its role in invasion, a process mediated by multiple ligand-receptor interactions and an attractive step for targeting with inhibitory antibodies through the development of a malaria vaccine. Using antibodies specific for different regions of PfMSA180, together with a parasite containing a conditional pfmsa180-gene knockout generated using CRISPR/Cas9 and DiCre recombinase technology, we demonstrate that this protein is unlikely to play a crucial role in erythrocyte invasion. However, deletion of the pfmsa180 gene resulted in a severe egress defect, preventing schizont rupture and blocking the erythrocytic cycle. Our study highlights an essential role of PfMSA180 in parasite egress, which could be targeted through the development of a novel malaria intervention strategy.

cAMP-Dependent Signaling Pathways as Potential Targets for Inhibition of Plasmodium falciparum Blood Stages

We review the role of signaling pathways in regulation of the key processes of merozoite egress and red blood cell invasion by Plasmodium falciparum and, in particular, the importance of the second messengers, cAMP and Ca²⁺, and cyclic nucleotide dependent kinases. cAMP-dependent protein kinase (PKA) is comprised of cAMP-binding regulatory, and catalytic subunits. The less well conserved cAMP-binding pockets should make cAMP analogs attractive drug leads, but this approach is compromised by the poor membrane permeability of cyclic nucleotides. We discuss how the conserved nature of ATP-binding pockets makes ATP analogs inherently prone to off-target effects and how ATP analogs and genetic manipulation can be useful research tools to examine this. We suggest that targeting PKA interaction partners as well as substrates, or developing inhibitors based on PKA interaction sites or phosphorylation sites in PKA substrates, may provide viable alternative approaches for the development of anti-malarial drugs. Proximity of PKA to a substrate is necessary for substrate phosphorylation, but the P. falciparum genome encodes few recognizable A-kinase anchor proteins (AKAPs), suggesting the importance of PKA-regulatory subunit myristylation and membrane association in determining substrate preference. We also discuss how Pf14-3-3 assembles a phosphorylation-dependent signaling complex that includes PKA and calcium dependent protein kinase 1 (CDPK1) and how this complex may be critical for merozoite invasion, and a target to block parasite growth. We compare altered phosphorylation levels in intracellular and egressed merozoites to identify potential PKA substrates. Finally, as host PKA may have a critical role in supporting intracellular parasite development, we discuss its role at other stages of the life cycle, as well as in other apicomplexan infections. Throughout our review we propose possible new directions for the therapeutic exploitation of cAMP-PKA-signaling in malaria and other diseases caused by apicomplexan parasites.

Plasmodium DEH is ER-localized and crucial for oocyst mitotic division during malaria transmission

Cells use fatty acids (FAs) for membrane biosynthesis, energy storage, and the generation of signaling molecules. 3-hydroxyacyl-CoA dehydratase-DEH-is a key component of very long chain fatty acid synthesis. Here, we further characterized in-depth the location and function of DEH, applying in silico analysis, live cell imaging, reverse genetics, and ultrastructure analysis using the mouse malaria model Plasmodium berghei DEH is evolutionarily conserved across eukaryotes, with a single DEH in Plasmodium spp. and up to three orthologs in the other eukaryotes studied. DEH-GFP live-cell imaging showed strong GFP fluorescence throughout the life-cycle, with areas of localized expression in the cytoplasm and a circular ring pattern around the nucleus that colocalized with ER markers. Δdeh mutants showed a small but significant reduction in oocyst size compared with WT controls from day 10 postinfection onwards, and endomitotic cell division and sporogony were completely ablated, blocking parasite transmission from mosquito to vertebrate host. Ultrastructure analysis confirmed degeneration of Δdeh oocysts, and a complete lack of sporozoite budding. Overall, DEH is evolutionarily conserved, localizes to the ER, and plays a crucial role in sporogony.

Plasmodium berghei Kinesin-5 Associates With the Spindle Apparatus During Cell Division and Is Important for Efficient Production of Infectious Sporozoites

Kinesin-5 motors play essential roles in spindle apparatus assembly during cell division, by generating forces to establish and maintain the spindle bipolarity essential for proper chromosome segregation. Kinesin-5 is largely conserved structurally and functionally in model eukaryotes, but its role is unknown in the Plasmodium parasite, an evolutionarily divergent organism with several atypical features of both mitotic and meiotic cell division. We have investigated the function and subcellular location of kinesin-5 during cell division throughout the Plasmodium berghei life cycle. Deletion of kinesin-5 had little visible effect at any proliferative stage except sporozoite production in oocysts, resulting in a significant decrease in the number of motile sporozoites in mosquito salivary glands, which were able to infect a new vertebrate host. Live-cell imaging showed kinesin-5-GFP located on the spindle and at spindle poles during both atypical mitosis and meiosis. Fixed-cell immunofluorescence assays revealed kinesin-5 co-localized with α-tubulin and centrin-2 and a partial overlap with kinetochore marker NDC80 during early blood stage schizogony. Dual-color live-cell imaging showed that kinesin-5 is closely associated with NDC80 during male gametogony, but not with kinesin-8B, a marker of the basal body and axonemes of the forming flagella. Treatment of gametocytes with microtubule-specific inhibitors confirmed kinesin-5 association with nuclear spindles and not cytoplasmic axonemal microtubules. Altogether, our results demonstrate that kinesin-5 is associated with the spindle apparatus, expressed in proliferating parasite stages, and important for efficient production of infectious sporozoites.

Real-time dynamics of Plasmodium NDC80 reveals unusual modes of chromosome segregation during parasite proliferation

Eukaryotic cell proliferation requires chromosome replication and precise segregation to ensure daughter cells have identical genomic copies. Species of the genus Plasmodium, the causative agents of malaria, display remarkable aspects of nuclear division throughout their life cycle to meet some peculiar and unique challenges to DNA replication and chromosome segregation. The parasite undergoes atypical endomitosis and endoreduplication with an intact nuclear membrane and intranuclear mitotic spindle. To understand these diverse modes of Plasmodium cell division, we have studied the behaviour and composition of the outer kinetochore NDC80 complex, a key part of the mitotic apparatus that attaches the centromere of chromosomes to microtubules of the mitotic spindle. Using NDC80-GFP live-cell imaging in Plasmodium berghei, we observe dynamic spatiotemporal changes during proliferation, including highly unusual kinetochore arrangements during sexual stages. We identify a very divergent candidate for the SPC24 subunit of the NDC80 complex, previously thought to be missing in Plasmodium, which completes a canonical, albeit unusual, NDC80 complex structure. Altogether, our studies reveal the kinetochore to be an ideal tool to investigate the non-canonical modes of chromosome segregation and cell division in Plasmodium.

A divergent cyclin/cyclin-dependent kinase complex controls the atypical replication of a malaria parasite during gametogony and transmission

Cell cycle transitions are generally triggered by variation in the activity of cyclin-dependent kinases (CDKs) bound to cyclins. Malaria-causing parasites have a life cycle with unique cell-division cycles, and a repertoire of divergent CDKs and cyclins of poorly understood function and interdependency. We show that Plasmodium berghei CDK-related kinase 5 (CRK5), is a critical regulator of atypical mitosis in the gametogony and is required for mosquito transmission. It phosphorylates canonical CDK motifs of components in the pre-replicative complex and is essential for DNA replication. During a replicative cycle, CRK5 stably interacts with a single Plasmodium-specific cyclin (SOC2), although we obtained no evidence of SOC2 cycling by transcription, translation or degradation. Our results provide evidence that during Plasmodium male gametogony, this divergent cyclin/CDK pair fills the functional space of other eukaryotic cell-cycle kinases controlling DNA replication.

Ubiquitin activation is essential for schizont maturation in Plasmodium falciparum blood-stage development

Ubiquitylation is a common post translational modification of eukaryotic proteins and in the human malaria parasite, Plasmodium falciparum (Pf) overall ubiquitylation increases in the transition from intracellular schizont to extracellular merozoite stages in the asexual blood stage cycle. Here, we identify specific ubiquitylation sites of protein substrates in three intraerythrocytic parasite stages and extracellular merozoites; a total of 1464 sites in 546 proteins were identified (data available via ProteomeXchange with identifier PXD014998). 469 ubiquitylated proteins were identified in merozoites compared with only 160 in the preceding intracellular schizont stage, suggesting a large increase in protein ubiquitylation associated with merozoite maturation. Following merozoite invasion of erythrocytes, few ubiquitylated proteins were detected in the first intracellular ring stage but as parasites matured through trophozoite to schizont stages the apparent extent of ubiquitylation increased. We identified commonly used ubiquitylation motifs and groups of ubiquitylated proteins in specific areas of cellular function, for example merozoite pellicle proteins involved in erythrocyte invasion, exported proteins, and histones. To investigate the importance of ubiquitylation we screened ubiquitin pathway inhibitors in a parasite growth assay and identified the ubiquitin activating enzyme (UBA1 or E1) inhibitor MLN7243 (TAK-243) to be particularly effective. This small molecule was shown to be a potent inhibitor of recombinant PfUBA1, and a structural homology model of MLN7243 bound to the parasite enzyme highlights avenues for the development of P. falciparum specific inhibitors. We created a genetically modified parasite with a rapamycin-inducible functional deletion of uba1; addition of either MLN7243 or rapamycin to the recombinant parasite line resulted in the same phenotype, with parasite development blocked at the schizont stage. Nuclear division and formation of intracellular structures was interrupted. These results indicate that the intracellular target of MLN7243 is UBA1, and this activity is essential for the final differentiation of schizonts to merozoites.

Depleted circulatory complement-lysis inhibitor (CLI) in childhood cerebral malaria returns to normal with convalescence

BACKGROUND

Cerebral malaria (CM), is a life-threatening childhood malaria syndrome with high mortality. CM is associated with impaired consciousness and neurological damage. It is not fully understood, as yet, why some children develop CM. Presented here is an observation from longitudinal studies on CM in a paediatric cohort of children from a large, densely-populated and malaria holoendemic, sub-Saharan, West African metropolis.

METHODS

Plasma samples were collected from a cohort of children with CM, severe malarial anaemia (SMA), uncomplicated malaria (UM), non-malaria positive healthy community controls (CC), and coma and anemic patients without malaria, as disease controls (DC). Proteomic two-dimensional difference gel electrophoresis (2D-DIGE) and mass spectrometry were used in a discovery cohort to identify plasma proteins that might be discriminatory among these clinical groups. The circulatory levels of identified proteins of interest were quantified by ELISA in a prospective validation cohort.

RESULTS

The proteome analysis revealed differential abundance of circulatory complement-lysis inhibitor (CLI), also known as Clusterin (CLU). CLI circulatory level was low at hospital admission in all children presenting with CM and recovered to normal level during convalescence (p < 0.0001). At acute onset, circulatory level of CLI in the CM group significantly discriminates CM from the UM, SMA, DC and CC groups.

CONCLUSIONS

The CLI circulatory level is low in all patients in the CM group at admission, but recovers through convalescence. The level of CLI at acute onset may be a specific discriminatory marker of CM. This work suggests that CLI may play a role in the pathophysiology of CM and may be useful in the diagnosis and follow-up of children presenting with CM.

Plasmodium Condensin Core Subunits SMC2/SMC4 Mediate Atypical Mitosis and Are Essential for Parasite Proliferation and Transmission

Condensin is a multi-subunit protein complex regulating chromosome condensation and segregation during cell division. In Plasmodium spp., the causative agent of malaria, cell division is atypical and the role of condensin is unclear. Here we examine the role of SMC2 and SMC4, the core subunits of condensin, during endomitosis in schizogony and endoreduplication in male gametogenesis. During early schizogony, SMC2/SMC4 localize to a distinct focus, identified as the centromeres by NDC80 fluorescence and chromatin immunoprecipitation sequencing (ChIP-seq) analyses, but do not form condensin I or II complexes. In mature schizonts and during male gametogenesis, there is a diffuse SMC2/SMC4 distribution on chromosomes and in the nucleus, and both condensin I and condensin II complexes form at these stages. Knockdown of smc2 and smc4 gene expression reveals essential roles in parasite proliferation and transmission. The condensin core subunits (SMC2/SMC4) form different complexes and may have distinct functions at various stages of the parasite life cycle.

Systematic analysis of Plasmodium myosins reveals differential expression, localisation, and function in invasive and proliferative parasite stages

The myosin superfamily comprises of actin-dependent eukaryotic molecular motors important in a variety of cellular functions. Although well studied in many systems, knowledge of their functions in Plasmodium, the causative agent of malaria, is restricted. Previously, six myosins were identified in this genus, including three Class XIV myosins found only in Apicomplexa and some Ciliates. The well characterized MyoA is a Class XIV myosin essential for gliding motility and invasion. Here, we characterize all other Plasmodium myosins throughout the parasite life cycle and show that they have very diverse patterns of expression and cellular location. MyoB and MyoE, the other two Class XIV myosins, are expressed in all invasive stages, with apical and basal locations, respectively. Gene deletion revealed that MyoE is involved in sporozoite traversal, MyoF and MyoK are likely essential in the asexual blood stages, and MyoJ and MyoB are not essential. Both MyoB and its essential light chain (MCL-B) are localised at the apical end of ookinetes but expressed at completely different time points. This work provides a better understanding of the role of actomyosin motors in Apicomplexan parasites, particularly in the motile and invasive stages of Plasmodium during sexual and asexual development within the mosquito.

Plasmodium kinesin-8X associates with mitotic spindles and is essential for oocyst development during parasite proliferation and transmission

Kinesin-8 proteins are microtubule motors that are often involved in regulation of mitotic spindle length and chromosome alignment. They move towards the plus ends of spindle microtubules and regulate the dynamics of these ends due, at least in some species, to their microtubule depolymerization activity. Plasmodium spp. exhibit an atypical endomitotic cell division in which chromosome condensation and spindle dynamics in the different proliferative stages are not well understood. Genome-wide shared orthology analysis of Plasmodium spp. revealed the presence of two kinesin-8 motor proteins, kinesin-8X and kinesin-8B. Here we studied the biochemical properties of kinesin-8X and its role in parasite proliferation. In vitro, kinesin-8X has motility and depolymerization activities like other kinesin-8 motors. To understand the role of Plasmodium kinesin-8X in cell division, we used fluorescence-tagging and live cell imaging to define its location, and gene targeting to analyse its function, during all proliferative stages of the rodent malaria parasite P. berghei life cycle. The results revealed a spatio-temporal involvement of kinesin-8X in spindle dynamics and an association with both mitotic and meiotic spindles and the putative microtubule organising centre (MTOC). Deletion of the kinesin-8X gene revealed a defect in oocyst development, confirmed by ultrastructural studies, suggesting that this protein is required for oocyst development and sporogony. Transcriptome analysis of Δkinesin-8X gametocytes revealed modulated expression of genes involved mainly in microtubule-based processes, chromosome organisation and the regulation of gene expression, supporting a role for kinesin-8X in cell division. Kinesin-8X is thus required for parasite proliferation within the mosquito and for transmission to the vertebrate host.

Kinesin-8B controls basal body function and flagellum formation and is key to malaria transmission

Eukaryotic flagella are conserved microtubule-based organelles that drive cell motility. Plasmodium, the causative agent of malaria, has a single flagellate stage: the male gamete in the mosquito. Three rounds of endomitotic division in male gametocyte together with an unusual mode of flagellum assembly rapidly produce eight motile gametes. These processes are tightly coordinated, but their regulation is poorly understood. To understand this important developmental stage, we studied the function and location of the microtubule-based motor kinesin-8B, using gene-targeting, electron microscopy, and live cell imaging. Deletion of the kinesin-8B gene showed no effect on mitosis but disrupted 9+2 axoneme assembly and flagellum formation during male gamete development and also completely ablated parasite transmission. Live cell imaging showed that kinesin-8B-GFP did not co-localise with kinetochores in the nucleus but instead revealed a dynamic, cytoplasmic localisation with the basal bodies and the assembling axoneme during flagellum formation. We, thus, uncovered an unexpected role for kinesin-8B in parasite flagellum formation that is vital for the parasite life cycle.