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Journal articleBeaghton A, Hammond A, Nolan T, et al., 2017,
Requirements for Driving Antipathogen Effector Genes into Populations of Disease Vectors by Homing
, Genetics, Vol: 205, Pages: 1587-1596 -
Journal articleSimoes ML, Dong Y, Hammond A, et al., 2017,
The Anopheles FBN9 immune factor mediates Plasmodium species-specific defense through transgenic fat body expression
, Developmental and Comparative Immunology, Vol: 67, Pages: 257-265 -
Journal articleOkell LC, Churcher TS, 2016,
Generating the evidence base for malaria elimination: the situation in Haiti
, Lancet Global Health, Vol: 5, Pages: e16-e17, ISSN: 2214-109X -
Journal articleParr JB, Verity R, Doctor SM, et al., 2016,
Pfhrp2-deleted Plasmodium falciparum parasites in the Democratic Republic of the Congo: a national cross-sectional survey
, Journal of Infectious Diseases, Vol: 216, Pages: 36-44, ISSN: 0022-1899Background.Rapid diagnostic tests (RDTs) account for more than two-thirds of malaria diagnoses in Africa. Deletions of the Plasmodium falciparum hrp2 (pfhrp2) gene cause false-negative RDT results and have never been investigated on a national level. Spread of pfhrp2-deleted P. falciparum mutants, resistant to detection by HRP2-based RDTs, would represent a serious threat to malaria elimination efforts.Methods.Using a nationally representative cross-sectional study of 7,137 children under five years of age from the Democratic Republic of Congo (DRC), we tested 783 subjects with RDT-/PCR+ results using PCR assays to detect and confirm deletions of the pfhrp2 gene. Spatial and population genetic analyses were employed to examine the distribution and evolution of these parasites.Results.We identified 149 pfhrp2-deleted parasites, representing 6.4% of all P. falciparum infections country-wide (95% confidence interval 5.1–8.0%). Bayesian spatial analyses identified statistically significant clustering of pfhrp2 deletions near Kinshasa and Kivu. Population genetic analysis revealed significant genetic differentiation between wild-type and pfhrp2-deleted parasite populations (GST = .046, p ≤ .00001).Conclusions.Pfhrp2-deleted P. falciparum is a common cause of RDT-/PCR+ malaria among asymptomatic children in the DRC and appears to be clustered within select communities. Surveillance for these deletions is needed, and alternatives to HRP2-specific RDTs may be necessary.
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Journal articleBargieri DY, Thiberge S, Tay CL, et al., 2016,
Plasmodium merozoite TRAP family protein Is essential for vacuole membrane disruption and gamete egress from erythrocytes
, Cell Host & Microbe, Vol: 20, Pages: 618-630, ISSN: 1934-6069Surface-associated TRAP (thrombospondin-related anonymous protein) family proteins are conserved across the phylum of apicomplexan parasites. TRAP proteins are thought to play an integral role in parasite motility and cell invasion by linking the extracellular environment with the parasite submembrane actomyosin motor. Blood stage forms of the malaria parasite Plasmodium express a TRAP family protein called merozoite-TRAP (MTRAP) that has been implicated in erythrocyte invasion. Using MTRAP-deficient mutants of the rodent-infecting P. berghei and human-infecting P. falciparum parasites, we show that MTRAP is dispensable for erythrocyte invasion. Instead, MTRAP is essential for gamete egress from erythrocytes, where it is necessary for the disruption of the gamete-containing parasitophorous vacuole membrane, and thus for parasite transmission to mosquitoes. This indicates that motor-binding TRAP family members function not just in parasite motility and cell invasion but also in membrane disruption and cell egress.
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Journal articleJohnson S, Rahmani R, Drew DR, et al., 2016,
Truncated latrunculins as actin inhibitors targeting plasmodium falciparum motility and host-cell invasion
, Journal of Medicinal Chemistry, Vol: 59, Pages: 10994-11005, ISSN: 0022-2623Polymerization of the cytosolic protein actin is critical to cell movement and host-cell invasion by the malaria parasite, Plasmodium falciparum. Any disruption to actin polymerization dynamics will render the parasite incapable of invading a host cell and thereby unable to cause infection. Here, we explore the potential of using truncated latrunculins as potential chemotherapeutics for the treatment of malaria. Exploration of the binding interactions of the natural actin inhibitor latrunculins, with actin revealed how a truncated core of the inhibitor could retain its key interaction features with actin. This truncated core was synthesised and subjected to preliminary structure-activity relationship studies to generate a focused set of analogues. Biochemical analyses of these analogues demonstrate their 6-fold increased activity compared with latrunculin B against Plasmodium falciparum and a 16-fold improved selectivity ex vivo. These data establish the latrunculin core as a potential focus for future structure-based drug design of chemotherapeutics against malaria.
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Journal articleBaum J,
Plasmodium MTRAP is essential for vacuole membrane disruption and host cell egress by malaria gametes
, Cell Host & Microbe, ISSN: 1934-6069Surface-associated TRAP (thrombospondin-related anonymous protein) family proteins are conservedacross the phylum of apicomplexan parasites. TRAP proteins are thought to play an integral role inparasite motility and cell invasion by linking the extracellular environment with the parasitesubmembrane actomyosin motor. Blood-stage forms of the malaria parasite Plasmodium express aTRAP family protein called merozoite-TRAP (MTRAP) that has been implicated in erythrocyteinvasion. Using MTRAP deficient mutants of the rodent-infecting P. berghei and human-infecting P.falciparum parasites, we show that MTRAP is dispensable for erythrocyte invasion. Instead, MTRAPis essential for gamete egress from erythrocytes, where it is necessary for the disruption of the gametecontainingparasitophorous vacuole membrane, and thus for parasite transmission to mosquitoes. Thisindicates that motor-binding TRAP family members function not just in parasite motility and cellinvasion but also in membrane disruption and cell egress.
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Journal articleSlater HC, Okell LC, Ghani AC, 2016,
Mathematical Modelling to Guide Drug Development for Malaria Elimination
, Trends in Parasitology, Vol: 33, Pages: 175-184, ISSN: 1471-5007Mathematical models of the dynamics of a drug within the host are now frequently used to guide drug development. These generally focus on assessing the efficacy and duration of response to guide patient therapy. Increasingly, antimalarial drugs are used at the population level, to clear infections, provide chemoprevention, and to reduce onward transmission of infection. However, there is less clarity on the extent to which different drug properties are important for these different uses. In addition, the emergence of drug resistance poses new threats to longer-term use and highlights the need for rational drug development. Here, we argue that integrating within-host pharmacokinetic and pharmacodynamic (PK/PD) models with mathematical models for the population-level transmission of malaria is key to guiding optimal drug design to aid malaria elimination.
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Journal articleTardieux I, Baum J, 2016,
Reassessing the mechanics of parasite motility and host-cell invasion
, Journal of Cell Biology, Vol: 214, Pages: 507-515, ISSN: 1540-8140The capacity to migrate is fundamental to multicellular and single-celled life. Apicomplexan parasites, an ancient protozoan clade that includes malaria parasites (Plasmodium) and Toxoplasma, achieve remarkable speeds of directional cell movement. This rapidity is achieved via a divergent actomyosin motor system, housed within a narrow compartment that lies underneath the length of the parasite plasma membrane. How this motor functions at a mechanistic level during motility and host cell invasion is a matter of debate. Here, we integrate old and new insights toward refining the current model for the function of this motor with the aim of revitalizing interest in the mechanics of how these deadly pathogens move.
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Journal articleLamas Oliveira Marques S, 2016,
Routine in vitro culture of P. falciparum gametocytes to evaluate novel transmission-blocking interventions.
, Nature Protocols, Vol: 11, Pages: 1668-1680, ISSN: 1754-2189The prevention of parasite transmission from the human host to the mosquito has been recognized as a vital tool for malaria eradication campaigns. However, transmission-blocking antimalarial drug and/or vaccine discovery and development is currently hampered by the expense and difficulty of producing mature Plasmodium falciparum gametocytes in vitro-the parasite stage responsible for mosquito infection. Current protocols for P. falciparum gametocyte culture usually require complex parasite synchronization and addition of stimulating and/or inhibitory factors, and they may not have demonstrated the essential property of mosquito infectivity. This protocol details all the steps required for reliable P. falciparum gametocyte production and highlights common factors that influence culture success. The protocol can be completed in 15 d, and particular emphasis is placed upon operating a gametocyte culture facility on a continuous cycle. In addition, we show how functionally viable gametocytes can be used to evaluate transmission-blocking drugs both in a field setting and at high throughput (HTP) for drug discovery.
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Journal articleGalizi R, Hammond A, Kyrou K, et al., 2016,
A CRISPR-Cas9 sex-ratio distortion system for genetic control.
, Scientific Reports, Vol: 6, ISSN: 2045-2322Genetic control aims to reduce the ability of insect pest populations to cause harm via the release of modified insects. One strategy is to bias the reproductive sex ratio towards males so that a population decreases in size or is eliminated altogether due to a lack of females. We have shown previously that sex ratio distortion can be generated synthetically in the main human malaria vector Anopheles gambiae, by selectively destroying the X-chromosome during spermatogenesis, through the activity of a naturally-occurring endonuclease that targets a repetitive rDNA sequence highly-conserved in a wide range of organisms. Here we describe a CRISPR-Cas9 sex distortion system that targets ribosomal sequences restricted to the member species of the Anopheles gambiae complex. Expression of Cas9 during spermatogenesis resulted in RNA-guided shredding of the X-chromosome during male meiosis and produced extreme male bias among progeny in the absence of any significant reduction in fertility. The flexibility of CRISPR-Cas9 combined with the availability of genomic data for a range of insects renders this strategy broadly applicable for the species-specific control of any pest or vector species with an XY sex-determination system by targeting sequences exclusive to the female sex chromosome.
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Journal articleBane KS, Lepper S, Kehrer J, et al., 2016,
The Actin Filament-Binding Protein Coronin Regulates Motility in Plasmodium Sporozoites
, PLOS Pathogens, Vol: 12, ISSN: 1553-7366Parasites causing malaria need to migrate in order to penetrate tissue barriers and enter host cells. Here we show that the actin filament-binding protein coronin regulates gliding motility in Plasmodium berghei sporozoites, the highly motile forms of a rodent malaria-causing parasite transmitted by mosquitoes. Parasites lacking coronin show motility defects that impair colonization of the mosquito salivary glands but not migration in the skin, yet result in decreased transmission efficiency. In non-motile sporozoites low calcium concentrations mediate actin-independent coronin localization to the periphery. Engagement of extracellular ligands triggers an intracellular calcium release followed by the actin-dependent relocalization of coronin to the rear and initiation of motility. Mutational analysis and imaging suggest that coronin organizes actin filaments for productive motility. Using coronin-mCherry as a marker for the presence of actin filaments we found that protein kinase A contributes to actin filament disassembly. We finally speculate that calcium and cAMP-mediated signaling regulate a switch from rapid parasite motility to host cell invasion by differentially influencing actin dynamics.
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Journal articleWalker PG, Griffin JT, Ferguson NM, et al., 2016,
Estimating the most efficient allocation of interventions to achieve reductions in Plasmodium falciparum malaria burden and transmission in Africa: a modelling study
, Lancet Global Health, Vol: 4, Pages: e474-e484, ISSN: 2214-109XBACKGROUND: Reducing the burden of malaria is a global priority, but financial constraints mean that available resources must be allocated rationally to maximise their effect. We aimed to develop a model to estimate the most efficient (ie, minimum cost) ordering of interventions to reduce malaria burden and transmission. We also aimed to estimate the efficiency of different spatial scales of implementation. METHODS: We combined a dynamic model capturing heterogeneity in malaria transmission across Africa with financial unit cost data for key malaria interventions. We combined estimates of patterns of malaria endemicity, seasonality in rainfall, and mosquito composition to map optimum packages of these interventions across Africa. Using non-linear optimisation methods, we examined how these optimum packages vary when control measures are deployed and assessed at national, subnational first administrative (provincial), or fine-scale (5 km(2) pixel) spatial scales. FINDINGS: The most efficient package in a given setting varies depending on whether disease reduction or elimination is the target. Long-lasting insecticide-treated nets are generally the most cost-effective first intervention to achieve either goal, with seasonal malaria chemoprevention or indoor residual spraying added second depending on seasonality and vector species. These interventions are estimated to reduce malaria transmission to less than one case per 1000 people per year in 43·4% (95% CI 40·0-49·0) of the population at risk in Africa. Adding three rounds of mass drug administration per year is estimated to increase this proportion to 90·9% (95% CI 86·9-94·6). Further optimisation can be achieved by targeting policies at the provincial level, achieving an estimated 32·1% (95% CI 29·6-34·5) cost saving relative to adopting country-wide policies. Nevertheless, we predict that only 26 (95% CI 22-29) of 41 countries could reduce transmissio
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Journal articleMarshall JM, Touré M, Ouédraogo AL, et al., 2016,
Key traveller groups of relevance to spatial malaria transmission: a survey of movement patterns in four sub-Saharan African countries
, Malaria Journal, Vol: 15, ISSN: 1475-2875 -
Journal articleHall AB, Papathanos P-A, Sharma A, et al., 2016,
Radical remodeling of the Y chromosome in a recent radiation of malaria mosquitoes
, Proceedings of the National Academy of Sciences of the United States of America, Vol: 113, Pages: E2114-E2123 -
Journal articleWhite MT, Shirreff G, Karl S, et al., 2016,
Variation in relapse frequency and the transmission potential of Plasmodium vivax malaria
, Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol: 283, ISSN: 0080-4649There is substantial variation in the relapse frequency of Plasmodium vivax malaria, with fast-relapsing strains in tropical areas, and slow-relapsing strains in temperate areas with seasonal transmission. We hypothesize that much of the phenotypic diversity in P. vivax relapses arises from selection of relapse frequency to optimize transmission potential in a given environment, in a process similar to the virulence trade-off hypothesis. We develop mathematical models of P. vivax transmission and calculate the basic reproduction number R0 to investigate how transmission potential varies with relapse frequency and seasonality. In tropical zones with year-round transmission, transmission potential is optimized at intermediate relapse frequencies of two to three months: slower-relapsing strains increase the opportunity for onward transmission to mosquitoes, but also increase the risk of being outcompeted by faster-relapsing strains. Seasonality is an important driver of relapse frequency for temperate strains, with the time to first relapse predicted to be six to nine months, coinciding with the duration between seasonal transmission peaks. We predict that there is a threshold degree of seasonality, below which fast-relapsing tropical strains are selected for, and above which slow-relapsing temperate strains dominate, providing an explanation for the observed global distribution of relapse phenotypes.
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Journal articleChen PB, Ding S, Zanghì G, et al., 2016,
Plasmodium falciparum PfSET7: enzymatic characterization and cellular localization of a novel protein methyltransferase in sporozoite, liver and erythrocytic stage parasites.
, Scientific Reports, Vol: 6, ISSN: 2045-2322Epigenetic control via reversible histone methylation regulates transcriptional activation throughout the malaria parasite genome, controls the repression of multi-copy virulence gene families and determines sexual stage commitment. Plasmodium falciparum encodes ten predicted SET domain-containing protein methyltransferases, six of which have been shown to be refractory to knock-out in blood stage parasites. We have expressed and purified the first recombinant malaria methyltransferase in sufficient quantities to perform a full enzymatic characterization and reveal the ill-defined PfSET7 is an AdoMet-dependent histone H3 lysine methyltransferase with highest activity towards lysines 4 and 9. Steady-state kinetics of the PfSET7 enzyme are similar to previously characterized histone methyltransferase enzymes from other organisms, however, PfSET7 displays specific protein substrate preference towards nucleosomes with pre-existing histone H3 lysine 14 acetylation. Interestingly, PfSET7 localizes to distinct cytoplasmic foci adjacent to the nucleus in erythrocytic and liver stage parasites, and throughout the cytoplasm in salivary gland sporozoites. Characterized recombinant PfSET7 now allows for target based inhibitor discovery. Specific PfSET7 inhibitors can aid in further investigating the biological role of this specific methyltransferase in transmission, hepatic and blood stage parasites, and may ultimately lead to the development of suitable antimalarial drug candidates against this novel class of essential parasite enzymes.
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Journal articlePothin E, Ferguson NM, Drakeley CJ, et al., 2016,
Estimating malaria transmission intensity from Plasmodium falciparum serological data using antibody density models
, Malaria Journal, Vol: 15, ISSN: 1475-2875Background: Serological data are increasingly being used to monitor malaria transmission intensity and havebeen demonstrated to be particularly useful in areas of low transmission where traditional measures such as EIR andparasite prevalence are limited. The seroconversion rate (SCR) is usually estimated using catalytic models in whichthe measured antibody levels are used to categorize individuals as seropositive or seronegative. One limitationof this approach is the requirement to impose a fixed cut-off to distinguish seropositive and negative individuals.Furthermore, the continuous variation in antibody levels is ignored thereby potentially reducing the precision of theestimate.Methods: An age-specific density model which mimics antibody acquisition and loss was developed to make fulluse of the information provided by serological measures of antibody levels. This was fitted to blood-stage antibodydensity data from 12 villages at varying transmission intensity in Northern Tanzania to estimate the exposure rate asan alternative measure of transmission intensity.Results: The results show a high correlation between the exposure rate estimates obtained and the estimated SCRobtained from a catalytic model (r = 0.95) and with two derived measures of EIR (r = 0.74 and r = 0.81). Estimates ofexposure rate obtained with the density model were also more precise than those derived from catalytic models.Conclusion: This approach, if validated across different epidemiological settings, could be a useful alternative frameworkfor quantifying transmission intensity, which makes more complete use of serological data.
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Journal articleBaum J, Zuccala E, Satchwell T, et al., 2016,
Quantitative phospho-proteomics reveals the Plasmodium merozoite triggers pre-invasion host kinase modification of the red cell cytoskeleton
, Scientific Reports, Vol: 6, ISSN: 2045-2322The invasive blood-stage malaria parasite – the merozoite – induces rapid morphological changes to the target erythrocyte during entry. However, evidence for active molecular changes in the host cell that accompany merozoite invasion is lacking. Here, we use invasion inhibition assays, erythrocyte resealing and high-definition imaging to explore red cell responses during invasion. We show that although merozoite entry does not involve erythrocyte actin reorganisation, it does require ATP to complete the process. Towards dissecting the ATP requirement, we present an in depth quantitative phospho-proteomic analysis of the erythrocyte during each stage of invasion. Specifically, we demonstrate extensive increased phosphorylation of erythrocyte proteins on merozoite attachment, including modification of the cytoskeletal proteins beta-spectrin and PIEZO1. The association with merozoite contact but not active entry demonstrates that parasite-dependent phosphorylation is mediated by host-cell kinase activity. This provides the first evidence that the erythrocyte is stimulated to respond to early invasion events through molecular changes in its membrane architecture.
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Journal articleNolan T, Crisanti A, 2016,
DRIVING OUT MALARIA
, Scientist, Vol: 31, Pages: 24-31 -
Journal articleGriffi JT, Bhatt S, Sinka ME, et al., 2016,
Potential for reduction of burden and local elimination of malaria by reducing Plasmodium falciparum malaria transmission: a mathematical modelling study
, Lancet Infectious Diseases, Vol: 16, Pages: 465-472, ISSN: 1473-3099 -
Journal articleKoch M, Baum J, 2016,
The Mechanics of Malaria Parasite Invasion of the Human Erythrocyte - Towards a Reassessment of the Host Cell Contribution
, Cellular Microbiology, Vol: 18, Pages: 319-329, ISSN: 1462-5822Despite decades of research, we still know little about the mechanics of Plasmodium host cell invasion. Fundamentally, while the essential or non-essential nature of different parasite proteins is becoming clearer, their actual function and how each comes together to govern invasion are poorly understood. Furthermore, in recent years an emerging world view is shifting focus away from the parasite actin–myosin motor being the sole force responsible for entry to an appreciation of host cell dynamics and forces and their contribution to the process. In this review, we discuss merozoite invasion of the erythrocyte, focusing on the complex set of pre-invasion events and how these might prime the red cell to facilitate invasion. While traditionally parasite interactions at this stage have been viewed simplistically as mediating adhesion only, recent work makes it apparent that by interacting with a number of host receptors and signalling pathways, combined with secretion of parasite-derived lipid material, that the merozoite may initiate cytoskeletal re-arrangements and biophysical changes in the erythrocyte that greatly reduce energy barriers for entry. Seen in this light Plasmodium invasion may well turn out to be a balance between host and parasite forces, much like that of other pathogen infection mechanisms.
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Journal articleSlater HC, Griffin JT, Ghani AC, et al., 2016,
Assessing the potential impact of artemisinin and partner drug resistance in sub-Saharan Africa.
, Malaria Journal, Vol: 15, ISSN: 1475-2875BACKGROUND: Artemisinin and partner drug resistant malaria parasites have emerged in Southeast Asia. If resistance were to emerge in Africa it could have a devastating impact on malaria-related morbidity and mortality. This study estimates the potential impact of artemisinin and partner drug resistance on disease burden in Africa if it were to emerge. METHODS: Using data from Asia and Africa, five possible artemisinin and partner drug resistance scenarios are characterized. An individual-based malaria transmission model is used to estimate the impact of each resistance scenario on clinical incidence and parasite prevalence across Africa. Artemisinin resistance is characterized by slow parasite clearance and partner drug resistance is associated with late clinical failure or late parasitological failure. RESULTS: Scenarios with high levels of recrudescent infections resulted in far greater increases in clinical incidence compared to scenarios with high levels of slow parasite clearance. Across Africa, it is estimated that artemisinin and partner drug resistance at levels similar to those observed in Oddar Meanchey province in Cambodia could result in an additional 78 million cases over a 5 year period, a 7 % increase in cases compared to a scenario with no resistance. A scenario with high levels of slow clearance but no recrudescence resulted in an additional 10 million additional cases over the same period. CONCLUSION: Artemisinin resistance is potentially a more pressing concern than partner drug resistance due to the lack of viable alternatives. However, it is predicted that a failing partner drug will result in greater increases in malaria cases and morbidity than would be observed from artemisinin resistance only.
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Journal articleRiglar DT, Whitehead L, Cowman AF, et al., 2016,
Localization-based imaging of malarial antigens during red cell entry reaffirms role for AMA1 but not MTRAP in invasion
, Journal of Cell Science, Vol: 129, Pages: 228-242, ISSN: 1477-9137Microscopy-based localisation of proteins during malaria parasite invasion of the erythrocyte is widely used for tentative assignment of protein function. To date, however, imaging has been limited by the rarity of invasion events and poor resolution available, given micron size of the parasite, which leads to a lack of quantitative measures for definitive localisation. Here, using computational image analysis we have attempted to assign relative protein localisation during invasion using wide-field deconvolution microscopy. By incorporating three-dimensional information we present a detailed assessment of known parasite effectors predicted to function during entry but as yet untested or for which data is equivocal. Our method, longitudinal intensity profiling, resolves confusion surrounding localisation of apical membrane antigen (AMA1) at the merozoite-erythrocyte junction and predicts that the merozoite thrombospondin related anonymous protein (MTRAP) is unlikely to play a direct role in the mechanics of entry, an observation supported with additional biochemical evidence. This approach sets a benchmark for imaging of complex micron-scale events and cautions against simplistic interpretations of small numbers of representative images for assignment of protein function or prioritisation of candidates as therapeutic targets.
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Journal articleHammond A, Galizi R, Kyrou K, et al., 2016,
A CRISPR-Cas9 gene drive system-targeting female reproduction in the malaria mosquito vector Anopheles gambiae
, Nature Biotechnology, Vol: 34, Pages: 78-83, ISSN: 1087-0156Gene drive systems that enable super-Mendelian inheritance of a transgene have the potential to modify insect populations over a timeframe of a few years. We describe CRISPR-Cas9 endonuclease constructs that function as gene drive systems in Anopheles gambiae, the main vector for malaria. We identified three genes (AGAP005958, AGAP011377 and AGAP007280) that confer a recessive female-sterility phenotype upon disruption, and inserted into each locus CRISPR-Cas9 gene drive constructs designed to target and edit each gene. For each targeted locus we observed a strong gene drive at the molecular level, with transmission rates to progeny of 91.4 to 99.6%. Population modeling and cage experiments indicate that a CRISPR-Cas9 construct targeting one of these loci, AGAP007280, meets the minimum requirement for a gene drive targeting female reproduction in an insect population. These findings could expedite the development of gene drives to suppress mosquito populations to levels that do not support malaria transmission.
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Journal articleBaum J, Olshina M, Baumann H, et al., 2016,
Plasmodium actin is incompletely folded by heterologous protein-folding machinery and likely requires the native Plasmodium chaperonin complex to enter a mature functional state
, The FASEB Journal, Vol: 30, Pages: 405-416, ISSN: 0892-6638Actin filament turnover underpins several processes in the life cycle of the malaria parasite, Plasmodium falciparum. Polymerization and depolymerization are especially important for gliding motility, a substrate-dependent form of cell movement that underpins the protozoan parasite’s ability to disseminate and invade host cells. To date, given difficulties in extraction of native actins directly from parasites, much of our biochemical understanding of malarial actin has instead relied on recombinant protein extracted and purified from heterologous protein expression systems. Here, using in vitro transcription-translation methodologies and quantitative protein-binding assays, we explored the folding state of heterologously expressed P. falciparum actin 1 (PfACTI) with the aim of assessing the reliability of current recombinant-protein-based data. We demonstrate that PfACTI, when expressed in non-native systems, is capable of binding to and release from bacterial, yeast, and mammalian chaperonin complexes but appears to be incompletely folded. Characterization of the native Plasmodium folding machinery in silico, the chaperonin containing t-complex protein-1 complex, highlights key divergences between the different chaperonin systems that likely underpins this incomplete folded state. These results highlight the importance of characterizing actin’s folded state and raise concerns about the interpretation of actin polymerization kinetics based solely on protein derived from heterologous expression systems.—Olshina, M. A., Baumann, H., Willison, K. R., Baum, J. Plasmodium actin is incompletely folded by heterologous protein-folding machinery and likely requires the native Plasmodium chaperonin complex to enter a mature functional state.
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Journal articleWu L, van den Hoogen LL, Slater H, et al., 2015,
Comparison of diagnostics for the detection of asymptomatic Plasmodium falciparum infections to inform control and elimination strategies
, Nature, Vol: 528, Pages: S86-S93, ISSN: 0028-0836The global burden of malaria has been substantially reduced over the past two decades. Future efforts to reduce malaria further will require moving beyond the treatment of clinical infections to targeting malaria transmission more broadly in the community. As such, the accurate identification of asymptomatic human infections, which can sustain a large proportion of transmission, is becoming a vital component of control and elimination programmes. We determined the relationship across common diagnostics used to measure malaria prevalence — polymerase chain reaction (PCR), rapid diagnostic test and microscopy — for the detection of Plasmodium falciparum infections in endemic populations based on a pooled analysis of cross-sectional data. We included data from more than 170,000 individuals comparing the detection by rapid diagnostic test and microscopy, and 30,000 for detection by rapid diagnostic test and PCR. The analysis showed that, on average, rapid diagnostic tests detected 41% (95% confidence interval = 26–66%) of PCR-positive infections. Data for the comparison of rapid diagnostic test to PCR detection at high transmission intensity and in adults were sparse. Prevalence measured by rapid diagnostic test and microscopy was comparable, although rapid diagnostic test detected slightly more infections than microscopy. On average, microscopy captured 87% (95% confidence interval = 74–102%) of rapid diagnostic test-positive infections. The extent to which higher rapid diagnostic test detection reflects increased sensitivity, lack of specificity or both, is unclear. Once the contribution of asymptomatic individuals to the infectious reservoir is better defined, future analyses should ideally establish optimal detection limits of new diagnostics for use in control and elimination strategies.
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Journal articleSlater HC, Ross A, Ouedraogo AL, et al., 2015,
Assessing the impact of next-generation rapid diagnostic tests on Plasmodium falciparum malaria elimination strategies
, Nature, Vol: 528, Pages: S94-S101, ISSN: 0028-0836Mass-screen-and-treat and targeted mass-drug-administration strategies are being considered as a means to interrupt transmission of Plasmodium falciparum malaria. However, the effectiveness of such strategies will depend on the extent to which current and future diagnostics are able to detect those individuals who are infectious to mosquitoes. We estimate the relationship between parasite density and onward infectivity using sensitive quantitative parasite diagnostics and mosquito feeding assays from Burkina Faso. We find that a diagnostic with a lower detection limit of 200 parasites per microlitre would detect 55% of the infectious reservoir (the combined infectivity to mosquitoes of the whole population weighted by how often each individual is bitten) whereas a test with a limit of 20 parasites per microlitre would detect 83% and 2 parasites per microlitre would detect 95% of the infectious reservoir. Using mathematical models, we show that increasing the diagnostic sensitivity from 200 parasites per microlitre (equivalent to microscopy or current rapid diagnostic tests) to 2 parasites per microlitre would increase the number of regions where transmission could be interrupted with a mass-screen-and-treat programme from an entomological inoculation rate below 1 to one of up to 4. The higher sensitivity diagnostic could reduce the number of treatment rounds required to interrupt transmission in areas of lower prevalence. We predict that mass-screen-and-treat with a highly sensitive diagnostic is less effective than mass drug administration owing to the prophylactic protection provided to uninfected individuals by the latter approach. In low-transmission settings such as those in Southeast Asia, we find that a diagnostic tool with a sensitivity of 20 parasites per microlitre may be sufficient for targeted mass drug administration because this diagnostic is predicted to identify a similar village population prevalence compared with that currently detected using polym
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Journal articlePenny MA, Verity RV, Bever C, et al., 2015,
Public health impact and cost-effectiveness of the RTS,S/AS01 malaria vaccine: a systematic comparison of predictions from four mathematical models
, The Lancet, Vol: 387, Pages: 367-375, ISSN: 0140-6736BackgroundThe phase 3 trial of the RTS,S/AS01 malaria vaccine candidate showed modest efficacy of the vaccine against Plasmodium falciparum malaria, but was not powered to assess mortality endpoints. Impact projections and cost-effectiveness estimates for longer timeframes than the trial follow-up and across a range of settings are needed to inform policy recommendations. We aimed to assess the public health impact and cost-effectiveness of routine use of the RTS,S/AS01 vaccine in African settings.MethodsWe compared four malaria transmission models and their predictions to assess vaccine cost-effectiveness and impact. We used trial data for follow-up of 32 months or longer to parameterise vaccine protection in the group aged 5–17 months. Estimates of cases, deaths, and disability-adjusted life-years (DALYs) averted were calculated over a 15 year time horizon for a range of levels of Plasmodium falciparum parasite prevalence in 2–10 year olds (PfPR2–10; range 3–65%). We considered two vaccine schedules: three doses at ages 6, 7·5, and 9 months (three-dose schedule, 90% coverage) and including a fourth dose at age 27 months (four-dose schedule, 72% coverage). We estimated cost-effectiveness in the presence of existing malaria interventions for vaccine prices of US$2–10 per dose.FindingsIn regions with a PfPR2–10 of 10–65%, RTS,S/AS01 is predicted to avert a median of 93 940 (range 20 490–126 540) clinical cases and 394 (127–708) deaths for the three-dose schedule, or 116 480 (31 450–160 410) clinical cases and 484 (189–859) deaths for the four-dose schedule, per 100 000 fully vaccinated children. A positive impact is also predicted at a PfPR2–10 of 5–10%, but there is little impact at a prevalence of lower than 3%. At $5 per dose and a PfPR2–10 of 10–65%, we estimated a median incremental cost-effectiveness ratio compared with current interventions of $30 (range 18–2
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Journal articleWhite MT, Verity R, Churcher TS, et al., 2015,
Vaccine approaches to malaria control and elimination: Insights from mathematical models
, Vaccine, Vol: 33, Pages: 7544-7550, ISSN: 1873-2518A licensed malaria vaccine would provide a valuable new tool for malaria control and elimination efforts.Several candidate vaccines targeting different stages ofthe malaria parasite’s lifecycle are currently underdevelopment, with one candidate, RTS,S/AS01 for the prevention of Plasmodium falciparum infection,having recently completed Phase III trials. Predicting the public health impact of a candidate malariavaccine requires using clinical trial data to estimate the vaccine’s efficacy profile—the initial efficacyfollowing vaccination and the pattern of waning of efficacy over time. With an estimated vaccine efficacyprofile, the effects of vaccination on malaria transmission can be simulated with the aid of mathematicalmodels.Here, we provide an overview of methods for estimating the vaccine efficacy profiles of pre-erythrocyticvaccines and transmission-blocking vaccines from clinicaltrial data. In the case of RTS,S/AS01, model estimatesfrom Phase II clinical trial data indicate a bi-phasic exponential profile of efficacy against infection,with efficacy waning rapidly in the first 6 months after vaccination followed by a slower rate of waningover the next 4 years. Transmission-blocking vaccines have yet to be tested in large-scale Phase II orPhase III clinical trials so we review ongoing work investigating how a clinical trial might be designed toensure that vaccine efficacy can be estimated with sufficient statistical power. Finally, we demonstratehow parameters estimated from clinical trials can be used to predict the impact of vaccination campaignson malaria using a mathematical model of malaria transmission
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