Tackling relapsing Plasmodium vivax and zoonotic Plasmodium knowlesi infections is critical to reducing malaria incidence and mortality worldwide. Understanding the biology of these important and related parasites was previously constrained by the lack of robust molecular and genetic approaches. Here, we establish CRISPR-Cas9 genome editing in a culture-adapted P. knowlesi strain and define parameters for optimal homology-driven repair. We establish a scalable protocol for the production of repair templates by PCR and demonstrate the flexibility of the system by tagging proteins with distinct cellular localisations. Using iterative rounds of genome-editing we generate a transgenic line expressing P. vivax Duffy binding protein (PvDBP), a lead vaccine candidate. We demonstrate that PvDBP plays no role in reticulocyte restriction but can alter the macaque/human host cell tropism of P. knowlesi. Critically, antibodies raised against the P. vivax antigen potently inhibit proliferation of this strain, providing an invaluable tool to support vaccine development.

The efficacy of current antimalarial drugs is threatened by reduced susceptibility of Plasmodium falciparum to artemisinin, associated with mutations in pfkelch13. Another gene with variants known to modulate the response to artemisinin encodes the μ subunit of the AP-2 adaptin trafficking complex. To elucidate the cellular role of AP-2μ in P. falciparum, we performed a conditional gene knockout, which severely disrupted schizont organization and maturation, leading to mislocalization of key merozoite proteins. AP-2μ is thus essential for blood-stage replication. We generated transgenic P. falciparum parasites expressing hemagglutinin-tagged AP-2μ and examined cellular localization by fluorescence and electron microscopy. Together with mass spectrometry analysis of coimmunoprecipitating proteins, these studies identified AP-2μ-interacting partners, including other AP-2 subunits, the K10 kelch-domain protein, and PfEHD, an effector of endocytosis and lipid mobilization, but no evidence was found of interaction with clathrin, the expected coat protein for AP-2 vesicles. In reverse immunoprecipitation experiments with a clathrin nanobody, other heterotetrameric AP-complexes were shown to interact with clathrin, but AP-2 complex subunits were absent. IMPORTANCE We examine in detail the AP-2 adaptin complex from the malaria parasite Plasmodium falciparum. In most studied organisms, AP-2 is involved in bringing material into the cell from outside, a process called endocytosis. Previous work shows that changes to the μ subunit of AP-2 can contribute to drug resistance. Our experiments show that AP-2 is essential for parasite development in blood but does not have any role in clathrin-mediated endocytosis. This suggests that a specialized function for AP-2 has developed in malaria parasites, and this may be important for understanding its impact on drug resistance.

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.

The symptoms of malaria occur during the blood stage of infection, when parasites invade and replicate within human erythrocytes. The five-component PfPCRCR complex, containing PfRH5, PfCyRPA, PfRIPR, PfCSS and PfPTRAMP, is essential for erythrocyte invasion by the deadliest human malaria parasite, Plasmodium falciparum. Invasion can be prevented by antibodies or nanobodies against each of these five conserved proteins, making them the leading blood stage malaria vaccine candidates. However, little is known about the molecular mechanism by which PfPCRCR functions during invasion. Here we present the structure of the PfRCR complex, containing PfRH5, PfCyRPA and PfRIPR, determined by cryogenic-electron microscopy. This reveals that PfRIPR consists of an ordered multi-domain core flexibly linked to an elongated tail. We test the hypothesis that PfRH5 opens to insert into the membrane, but instead show that a rigid, disulphide-locked PfRH5 can mediate efficient erythrocyte invasion. Finally, we show that the elongated tail of PfRIPR, which is the target of growth-neutralising antibodies, binds to the PfCSS-PfPTRAMP complex on the parasite membrane. Therefore, a modular PfRIPR is linked to the merozoite membrane through an elongated tail, while its structured core presents PfCyRPA and PfRH5 to interact with erythrocyte receptors. This provides novel insight into the mechanism of erythrocyte invasion and opens the way to new approaches in rational vaccine design.

18Plasmodium malaria parasites use a unique form of locomotion termed gliding 19 motility to move through host tissues and invade cells. The process is substrate-20 dependent and powered by an actomyosin motor that drives the posterior 21 translocation of extracellular adhesins, which in turn propel the parasite forward. 22Gliding motility is essential for tissue translocation in the sporozoite and ookinete 23 stages, however, the short-lived erythrocyte-invading merozoite stage has never 24 been observed to undergo gliding movement. Here for the first time we reveal that 25 blood stage Plasmodium merozoites use gliding motility for translocation in addition 33 Keywords 34Malaria, Merozoite, Erythrocyte invasion, Gliding motility 35 36 (Russell et al., 1981; Dobrowolski et al., 1996). The system instead relies on the 41 apical presentation of parasite transmembrane adhesins which bind to host 42 substrates and then are drawn towards the parasite posterior by a conserved 43 actomyosin motor running under the surface of the plasma membrane, resulting in 44 the forward propulsion of the parasite (Tardieux et al., 2016; Frenal et al., 2017). 45Motility of invasive forms of malarial parasites (termed "zoites") was first described 46 for the ookinete stage in avian blood (Danilewsky et al., 1889), and then for the 47 sporozoite stage in the mosquito (Grassi et al., 1900). Unlike ookinetes and 48 sporozoites, which must traverse through tissues, no gliding motility has been 49 described for the merozoite, which invades erythrocytes in the bloodstream. Instead, 50 only limited reorientation movement and cellular deformation has been observed 51 across several malarial parasite species, including Plasmodium knowlesi, P. 52 falciparum, and P. yoelii (Dvorak et al., 1975; Gilson et al., 2009;Yahata et al., 2012). 53Due to the short-lived nature and diminished size of merozoites (1-2 µm) relative to 54 other zoites, it was presumed that merozoites do not require motility to encounter 55 erythrocytes in the bloodstream, leading to the consensus that the molecular motor 56 is principally required for penetration of the erythrocyte during invasion (Tardieux et 57 al., 2016). 58Here we show that both P. falciparum and P. knowlesi are capable of gliding 59 motility across both erythrocyte surfaces and polymer coverslips, with distinctive 60 dynamics between the two species. We have additionally developed a scalable 61 65 Results 66 Gliding motility of Plasmodium merozoites 67Here we sought to address the long-standing question of whether malarial 68 merozoites undergo conventional gliding motility. Whilst motility of sporozoites is 69 normally observed on bovine serum albumin-coated glass slides, merozoites do not 70 glide on this substrate. However, when using polymer coverslips with a hydrophilic 71 coating (ibiTreat), we observed motile merozoites. When imaged immediately after 72 erythrocyte egress, merozoites show directional movement on the coverslip surface 73 which displaces them from the hemozoin containing residual b...

Plasmodium knowlesi is a zoonotic malaria parasite in Southeast Asia that can cause severe and fatal malaria in humans. The main hosts are Macaques, but modern diagnostic tools reveal increasing numbers of human infections. After P. falciparum, P. knowlesi is the only other malaria parasite capable of being maintained in long term in vitro culture with human red blood cells (RBCs). Its closer ancestry to other non-falciparum human malaria parasites, more balanced AT-content, larger merozoites and higher transfection efficiencies, gives P. knowlesi some key advantages over P. falciparum for the study of malaria parasite cell/molecular biology. Here, we describe the generation of marker-free CRISPR gene-edited P. knowlesi parasites, the fast and scalable production of transfection constructs and analysis of transfection efficiencies. Our protocol allows rapid, reliable and unlimited rounds of genome editing in P. knowlesi requiring only a single recyclable selection marker.

The efficacy of current antimalarial drugs is threatened by reduced susceptibility of Plasmodium falciparum to artemisinin. In the Mekong region this is associated with mutations in the kelch propeller-encoding domain of pfkelch13, but variants of other parasite proteins are also thought to modulate the response to drug. Evidence from human and rodent studies suggests that the -subunit of the AP-2 adaptin trafficking complex is one such protein of interest. We generated transgenic Plasmodium falciparum parasites encoding the I592T variant of pfap2, orthologous to the I568T mutation associated with in vivo artemisinin resistance in P. chabaudi. When exposed to a four-hour pulse of dihydroartemisin in the ring-stage survival assay, two P. falciparum clones expressing AP-2I592T displayed significant and reproducible survival of 8.0% and 10.3%, respectively, compared to <2% for the 3D7 parental line (P = 0.0011 for each clone). In immunoprecipitation and localisation studies of HA-tagged AP-2 we identified interacting partners including AP-2 AP-1/2 AP-2 and a kelch-domain protein encoded on chromosome 10 of P. falciparum, K10. Conditional knockout indicates that the AP-2 trafficking complex in P. falciparum is essential for the fidelity of merozoite biogenesis and membrane organisation in the mature schizont. We also show that while other heterotetrameric AP-complexes and secretory factors interact with clathrin, AP-2 complex subunits do not. Thus, the AP-2 complex may be diverted from a clathrin-dependent endocytic role seen in most eukaryotes into a Plasmodium-specific function. These findings represent striking divergences from eukaryotic dogma and support a role for intracellular traffic in determining artemisinin sensitivity in vitro, confirming the existence of multiple functional routes to reduced ring-stage artemisinin susceptibility. Therefore, the utility of pfkelch13 variants as resistance markers is unlikely to be universal, and phenotypic surveillance of parasite susceptibility in vivo may be needed to identify threats to our current combination therapies.

Invasion of red blood cells (RBCs) by Plasmodium merozoites is critical to their continued survival within the host. Two major protein families, the Duffy binding-like proteins (DBPs/EBAs) and the reticulocyte binding like proteins (RBLs/RHs) have been studied extensively in P. falciparum and are hypothesized to have overlapping, but critical roles just prior to host cell entry. The zoonotic malaria parasite, P. knowlesi, has larger invasive merozoites and contains a smaller, less redundant, DBP and RBL repertoire than P. falciparum. One DBP (DBPα) and one RBL, normocyte binding protein Xa (NBPXa) are essential for invasion of human RBCs. Taking advantage of the unique biological features of P. knowlesi and iterative CRISPR-Cas9 genome editing, we determine the precise order of key invasion milestones and demonstrate distinct roles for each family. These distinct roles support a mechanism for phased commitment to invasion and can be targeted synergistically with invasion inhibitory antibodies.