Identification of basal complex protein that is essential for maturation of transmission-stage malaria parasites

Malaria remains a global driver of morbidity and mortality. To generate new antimalarials, one must elucidate the fundamental cell biology of Plasmodium falciparum, the parasite responsible for the deadliest cases of malaria. A membranous and proteinaceous scaffold called the inner membrane complex (IMC) supports the parasite during morphological changes, including segmentation of daughter cells during asexual replication and formation of transmission-stage gametocytes. The basal complex lines the edge of the IMC during segmentation and likely facilitates IMC expansion. It is unknown, however, what drives IMC expansion during gametocytogenesis. We describe the discovery of a basal complex protein, PfBLEB, which we find to be essential for gametocytogenesis. Parasites lacking PfBLEB harbor defects in IMC expansion and are unable to form mature gametocytes. This article demonstrates a role for a basal complex protein outside of asexual division, and, importantly, highlights a potential molecular target for the ablation of malaria transmission.

A Malaria Parasite Cross Reveals Genetic Determinants of Plasmodium falciparum Growth in Different Culture Media

What genes determine in vitro growth and nutrient utilization in asexual blood-stage malaria parasites? Competition experiments between NF54, clone 3D7, a lab-adapted African parasite, and a recently isolated Asian parasite (NHP4026) reveal contrasting outcomes in different media: 3D7 outcompetes NHP4026 in media containing human serum, while NHP4026 outcompetes 3D7 in media containing AlbuMAX, a commercial lipid-rich bovine serum formulation. To determine the basis for this polymorphism, we conducted parasite genetic crosses using humanized mice and compared genome-wide allele frequency changes in three independent progeny populations cultured in media containing human serum or AlbuMAX. This bulk segregant analysis detected three quantitative trait loci (QTL) regions [on chromosome (chr) 2 containing aspartate transaminase AST; chr 13 containing EBA-140; and chr 14 containing cysteine protease ATG4] linked with differential growth in serum or AlbuMAX in each of the three independent progeny pools. Selection driving differential growth was strong (s = 0.10 – 0.23 per 48-hour lifecycle). We conducted validation experiments for the strongest QTL on chr 13: competition experiments between ΔEBA-140 and 3D7 wildtype parasites showed fitness reversals in the two medium types as seen in the parental parasites, validating this locus as the causative gene. These results (i) demonstrate the effectiveness of bulk segregant analysis for dissecting fitness traits in P. falciparum genetic crosses, and (ii) reveal intimate links between red blood cell invasion and nutrient composition of growth media. Use of parasite crosses combined with bulk segregant analysis will allow systematic dissection of key nutrient acquisition/metabolism and red blood cell invasion pathways in P. falciparum.

Six-year trend analysis of malaria prevalence at University of Gondar Specialized Referral Hospital, Northwest Ethiopia, from 2014 to 2019

Globally, malaria is the major public health disease caused by plasmodium species and transmitted by the bite of the female anopheles mosquito. Assessment of the trend of malaria prevalence is important in the control and prevention of the disease. Therefore, the objective of this study was to assess the six year trend of malaria prevalence at the University of Gondar Comprehensive Specialized Hospital, northwest Ethiopia, from 2014 to 2019. A retrospective laboratory registration logbook review study was conducted on the malaria blood film examination results at the University of Gondar Comprehensive Specialized Hospital. The data was collected by using a data extraction tool and entered into SPSS version 20 for analysis. Descriptive statistics were used to summarize the socio-demographic characteristics of study participants and presented by graphs, tables and texts. The binary logistic regression was also used to test the association the trend of malaria prevalence and different factors like sex, age, year, and season. From a total of 17,500 malaria blood film examinations, 1341 (7.7%) were confirmed for malaria parasites. Of the confirmed malaria cases, 47.2%, 45.6% and 7.2% were P. vivax, P. falciparum and mixed infection, respectively. The proportion of P. vivax was the predominant species in the first three study years (2014-2016) and P. falciparum became the predominant species in the last three study years (2017-2019). The odds of malaria prevalence was lower by 68%, 60% and 69% in the year 2017, 2018 and 2019 compared to 2014, respectively. It was also 1.41 times higher in males than in females. Moreover, the odds of malaria prevalence were 1.60, 1.64, 2.45 and 1.82 times higher in the age group of < 5, 5-14, 15-24 and 25-54 years old compared to the older age groups (> 54 years old), respectively. Even there was a significant declining in prevalence trend; malaria is still a major public health problem. The study showed that there was high seasonal fluctuation from year to year. Moreover, males and the younger age groups were more affected than females and old age groups, respectively. Therefore, malaria prevention and control activities should be strengthened and require extra efforts by considering these variability.

A Prioritized and Validated Resource of Mitochondrial Proteins in Plasmodium Identifies Unique Biology

Plasmodium species have a single mitochondrion that is essential for their survival and has been successfully targeted by antimalarial drugs. Most mitochondrial proteins are imported into this organelle, and our picture of the Plasmodium mitochondrial proteome remains incomplete. Many data sources contain information about mitochondrial localization, including proteome and gene expression profiles, orthology to mitochondrial proteins from other species, coevolutionary relationships, and amino acid sequences, each with different coverage and reliability. To obtain a comprehensive, prioritized list of Plasmodium falciparum mitochondrial proteins, we rigorously analyzed and integrated eight data sets using Bayesian statistics into a predictive score per protein for mitochondrial localization. At a corrected false discovery rate of 25%, we identified 445 proteins with a sensitivity of 87% and a specificity of 97%. They include proteins that have not been identified as mitochondrial in other eukaryotes but have characterized homologs in bacteria that are involved in metabolism or translation. Mitochondrial localization of seven Plasmodium berghei orthologs was confirmed by epitope labeling and colocalization with a mitochondrial marker protein. One of these belongs to a newly identified apicomplexan mitochondrial protein family that in P. falciparum has four members. With the experimentally validated mitochondrial proteins and the complete ranked P. falciparum proteome, which we have named PlasmoMitoCarta, we present a resource to study unique proteins of Plasmodium mitochondria. IMPORTANCE The unique biology and medical relevance of the mitochondrion of the malaria parasite Plasmodium falciparum have made it the subject of many studies. However, we actually do not have a comprehensive assessment of which proteins reside in this organelle. Many omics data are available that are predictive of mitochondrial localization, such as proteomics data and expression data. Individual data sets are, however, rarely complete and can provide conflicting evidence. We integrated a wide variety of available omics data in a manner that exploits the relative strengths of the data sets. Our analysis gave a predictive score for the mitochondrial localization to each nuclear encoded P. falciparum protein and identified 445 likely mitochondrial proteins. We experimentally validated the mitochondrial localization of seven of the new mitochondrial proteins, confirming the quality of the complete list. These include proteins that have not been observed mitochondria before, adding unique mitochondrial functions to P. falciparum.

Common host variation drives malaria parasite fitness in healthy human red cells

The replication of Plasmodium falciparum parasites within red blood cells (RBCs) causes severe disease in humans, especially in Africa. Deleterious alleles like hemoglobin S are well-known to confer strong resistance to malaria, but the effects of common RBC variation are largely undetermined. Here, we collected fresh blood samples from 121 healthy donors, most with African ancestry, and performed exome sequencing, detailed RBC phenotyping, and parasite fitness assays. Over one-third of healthy donors unknowingly carried alleles for G6PD deficiency or hemoglobinopathies, which were associated with characteristic RBC phenotypes. Among non-carriers alone, variation in RBC hydration, membrane deformability, and volume was strongly associated with P. falciparum growth rate. Common genetic variants in PIEZO1, SPTA1/SPTB, and several P. falciparum invasion receptors were also associated with parasite growth rate. Interestingly, we observed little or negative evidence for divergent selection on non-pathogenic RBC variation between Africans and Europeans. These findings suggest a model in which globally widespread variation in a moderate number of genes and phenotypes modulates P. falciparum fitness in RBCs.

Single-Cell RNA Sequencing Reveals Cellular Heterogeneity and Stage Transition under Temperature Stress in Synchronized Plasmodium falciparum Cells

The malaria parasite has a complex life cycle exhibiting phenotypic and morphogenic variations in two different hosts by existing in heterogeneous developmental states. To investigate this cellular heterogeneity of the parasite within the human host, we performed single-cell RNA sequencing of synchronized Plasmodium cells under control and temperature treatment conditions. Using the Malaria Cell Atlas (https://www.sanger.ac.uk/science/tools/mca) as a guide, we identified 9 subtypes of the parasite distributed across known intraerythrocytic stages. Interestingly, temperature treatment results in the upregulation of the AP2-G gene, the master regulator of sexual development in a small subpopulation of the parasites. Moreover, we identified a heterogeneous stress-responsive subpopulation (clusters 5, 6, and 7 [∼10% of the total population]) that exhibits upregulation of stress response pathways under normal growth conditions. We also developed an online exploratory tool that will provide new insights into gene function under normal and temperature stress conditions. Thus, our study reveals important insights into cell-to-cell heterogeneity in the parasite population under temperature treatment that will be instrumental toward a mechanistic understanding of cellular adaptation and population dynamics in Plasmodium falciparum. IMPORTANCE The malaria parasite has a complex life cycle exhibiting phenotypic variations in two different hosts accompanied by cell-to-cell variability that is important for stress tolerance, immune evasion, and drug resistance. To investigate cellular heterogeneity determined by gene expression, we performed single-cell RNA sequencing (scRNA-seq) of about 12,000 synchronized Plasmodium cells under physiologically relevant normal (37°C) and temperature stress (40°C) conditions phenocopying the cyclic bouts of fever experienced during malarial infection. In this study, we found that parasites exhibit transcriptional heterogeneity in an otherwise morphologically synchronized culture. Also, a subset of parasites is continually committed to gametocytogenesis and stress-responsive pathways. These observations have important implications for understanding the mechanisms of drug resistance generation and vaccine development against the malaria parasite.

Effectiveness of seasonal malaria chemoprevention (SMC) treatments when SMC is implemented at scale: Case-control studies in 5 countries

BACKGROUND

Seasonal malaria chemoprevention (SMC) has shown high protective efficacy against clinical malaria and severe malaria in a series of clinical trials. We evaluated the effectiveness of SMC treatments against clinical malaria when delivered at scale through national malaria control programmes in 2015 and 2016.

METHODS AND FINDINGS

Case-control studies were carried out in Mali and The Gambia in 2015, and in Burkina Faso, Chad, Mali, Nigeria, and The Gambia in 2016. Children aged 3-59 months presenting at selected health facilities with microscopically confirmed clinical malaria were recruited as cases. Two controls per case were recruited concurrently (on or shortly after the day the case was detected) from the neighbourhood in which the case lived. The primary exposure was the time since the most recent course of SMC treatment, determined from SMC recipient cards, caregiver recall, and administrative records. Conditional logistic regression was used to estimate the odds ratio (OR) associated with receipt of SMC within the previous 28 days, and SMC 29 to 42 days ago, compared with no SMC in the past 42 days. These ORs, which are equivalent to incidence rate ratios, were used to calculate the percentage reduction in clinical malaria incidence in the corresponding time periods. Results from individual countries were pooled in a random-effects meta-analysis. In total, 2,126 cases and 4,252 controls were included in the analysis. Across the 7 studies, the mean age ranged from 1.7 to 2.4 years and from 2.1 to 2.8 years among controls and cases, respectively; 42.2%-50.9% and 38.9%-46.9% of controls and cases, respectively, were male. In all 7 individual case-control studies, a high degree of personal protection from SMC against clinical malaria was observed, ranging from 73% in Mali in 2016 to 98% in Mali in 2015. The overall OR for SMC within 28 days was 0.12 (95% CI: 0.06, 0.21; p < 0.001), indicating a protective effectiveness of 88% (95% CI: 79%, 94%). Effectiveness against clinical malaria for SMC 29-42 days ago was 61% (95% CI: 47%, 72%). Similar results were obtained when the analysis was restricted to cases with parasite density in excess of 5,000 parasites per microlitre: Protective effectiveness 90% (95% CI: 79%, 96%; P<0.001), and 59% (95% CI: 34%, 74%; P<0.001) for SMC 0-28 days and 29-42 days ago, respectively. Potential limitations include the possibility of residual confounding due to an association between exposure to malaria and access to SMC, or differences in access to SMC between patients attending a clinic and community controls; however, neighbourhood matching of cases and controls, and covariate adjustment, attempted to control for these aspects, and the observed decline in protection over time, consistent with expected trends, argues against a major bias from these sources.

CONCLUSIONS

SMC administered as part of routine national malaria control activities provided a very high level of personal protection against clinical malaria over 28 days post-treatment, similar to the efficacy observed in clinical trials. The case-control design used in this study can be used at intervals to ensure SMC treatments remain effective.

Identification of the asymptomatic Plasmodium falciparum and Plasmodium vivax gametocyte reservoir under different transmission intensities

Background

Understanding epidemiological variables affecting gametocyte carriage and density is essential to design interventions that most effectively reduce malaria human-to-mosquito transmission.

Methodology/Principal findings

Plasmodium falciparum and P. vivax parasites and gametocytes were quantified by qPCR and RT-qPCR assays using the same methodologies in 5 cross-sectional surveys involving 16,493 individuals in Brazil, Thailand, Papua New Guinea, and Solomon Islands. The proportion of infections with detectable gametocytes per survey ranged from 44–94% for P. falciparum and from 23–72% for P. vivax. Blood-stage parasite density was the most important predictor of the probability to detect gametocytes. In moderate transmission settings (prevalence by qPCR>5%), parasite density decreased with age and the majority of gametocyte carriers were children. In low transmission settings (prevalence<5%), >65% of gametocyte carriers were adults. Per survey, 37–100% of all individuals positive for gametocytes by RT-qPCR were positive by light microscopy for asexual stages or gametocytes (overall: P. falciparum 178/348, P. vivax 235/398).

Conclusions/Significance

Interventions to reduce human-to-mosquito malaria transmission in moderate-high endemicity settings will have the greatest impact when children are targeted. In contrast, all age groups need to be included in control activities in low endemicity settings to achieve elimination. Detection of infections by light microscopy is a valuable tool to identify asymptomatic blood stage infections that likely contribute most to ongoing transmission at the time of sampling.

Plasmodium vivax and Plasmodium falciparum cause the vast majority of all human malaria cases. Across all transmission settings, a large proportion of infections of the two species remain asymptomatic. These infections are not diagnosed and treated by control programs focusing on clinical cases. They can carry gametocytes, the sexual stage of the parasite that establishes infections in mosquitos, thus asymptomatic infections contribute to transmission. In order to determine who is likely to contribute to transmission, gametocyte densities were measured by sensitive molecular methods in afebrile individuals in four countries. The proportion of infections with gametocytes varied greatly among surveys, and was higher in regions that had experienced low transmission for extended periods of time. In moderate-high transmission settings, gametocyte densities were particularly high in children below six years, highlighting the importance that interventions to reduce transmission include this age group. The majority of gametocyte carriers was positive by light microscopy. The comprehensive data on gametocyte carriage presented here lays the foundation for the development of more effective screen and treat activities to reduce malaria transmission.

Malaria parasites both repress host CXCL10 and use it as a cue for growth acceleration

Pathogens are thought to use host molecular cues to control when to initiate life-cycle transitions, but these signals are mostly unknown, particularly for the parasitic disease malaria caused by Plasmodium falciparum. The chemokine CXCL10 is present at high levels in fatal cases of cerebral malaria patients, but is reduced in patients who survive and do not have complications. Here we show a Pf 'decision-sensing-system' controlled by CXCL10 concentration. High CXCL10 expression prompts P. falciparum to initiate a survival strategy via growth acceleration. Remarkably, P. falciparum inhibits CXCL10 synthesis in monocytes by disrupting the association of host ribosomes with CXCL10 transcripts. The underlying inhibition cascade involves RNA cargo delivery into monocytes that triggers RIG-I, which leads to HUR1 binding to an AU-rich domain of the CXCL10 3'UTR. These data indicate that when the parasite can no longer keep CXCL10 at low levels, it can exploit the chemokine as a cue to shift tactics and escape.

Plasmodium falciparum DDX3X is a nucleocytoplasmic protein and requires N-terminal for DNA helicase activity

Malaria is a haemato-protozoan disease which causes thousands of deaths every year. Due to the alarming increase of drug resistant strains of P. falciparum, malaria is now becoming more deadly. Helicases are the most important components of the cellular machinery without which cells are unable to survive. The importance of helicases has been proven in variety of organisms. In this study we have reported detailed biochemical characterization of human homologue of DDX3X from Plasmodium falciparum (PfDDX3X). Our study revealed that PfDDX3X is ATP- dependent DNA helicase whereas in human host it is ATP-dependent RNA helicase. We show that N-terminal is essential for its activity and it is present in nucleus and cytoplasm in intraerythrocytic developmental stages of P. falciparum 3D7 strain. Also, it is highly expressed in the schizont stage of P. falciparum 3D7strain. The present study suggests that a protein can perform different functions in different systems. The present study will help to understand the basic biology of malaria parasite P. falciparum.

The private life of malaria parasites: Strategies for sexual reproduction

Malaria parasites exhibit a complex lifecycle, requiring extensive asexual replication in the liver and blood of the vertebrate host, and in the haemocoel of the insect vector. Yet, they must also undergo a single round of sexual reproduction, which occurs in the vector's midgut upon uptake of a blood meal. Sexual reproduction is obligate for infection of the vector and thus, is essential for onwards transmission to new hosts. Sex in malaria parasites involves several bottlenecks in parasite number, making the stages involved attractive targets for blocking disease transmission. Malaria parasites have evolved a suite of adaptations ("strategies") to maximise the success of sexual reproduction and transmission, which could undermine transmission-blocking interventions. Yet, understanding parasite strategies may also reveal novel opportunities for such interventions. Here, we outline how evolutionary and ecological theories, developed to explain reproductive strategies in multicellular taxa, can be applied to explain two reproductive strategies (conversion rate and sex ratio) expressed by malaria parasites within the vertebrate host.

5-Phenoxy Primaquine Analogs and the Tetraoxane Hybrid as Antimalarial Agents

The rapid emergence of drug resistance to the current antimalarial agents has led to the urgent need for the discovery of new and effective compounds. In this work, a series of 5-phenoxy primaquine analogs with 8-aminoquinoline core (7a–7h) was synthesized and investigated for their antimalarial activity against Plasmodium falciparum. Most analogs showed improved blood antimalarial activity compared to the original primaquine. To further explore a drug hybrid strategy, a conjugate compound between tetraoxane and the representative 5-phenoxy-primaquine analog 7a was synthesized. In our work, the hybrid compound 12 exhibited almost a 30-fold increase in the blood antimalarial activity (IC₅₀ = 0.38 ± 0.11 μM) compared to that of primaquine, with relatively low toxicity against mammalian cells (SI = 45.61). Furthermore, we found that these 5-phenoxy primaquine analogs and the hybrid exhibit significant heme polymerization inhibition, an activity similar to that of chloroquine, which could contribute to their improved antimalarial activity. The 5-phenoxy primaquine analogs and the tetraoxane hybrid could serve as promising candidates for the further development of antimalarial agents.

Composition and stage dynamics of mitochondrial complexes in Plasmodium falciparum

Our current understanding of mitochondrial functioning is largely restricted to traditional model organisms, which only represent a fraction of eukaryotic diversity. The unusual mitochondrion of malaria parasites is a validated drug target but remains poorly understood. Here, we apply complexome profiling to map the inventory of protein complexes across the pathogenic asexual blood stages and the transmissible gametocyte stages of Plasmodium falciparum. We identify remarkably divergent composition and clade-specific additions of all respiratory chain complexes. Furthermore, we show that respiratory chain complex components and linked metabolic pathways are up to 40-fold more prevalent in gametocytes, while glycolytic enzymes are substantially reduced. Underlining this functional switch, we find that cristae are exclusively present in gametocytes. Leveraging these divergent properties and stage dynamics for drug development presents an attractive opportunity to discover novel classes of antimalarials and increase our repertoire of gametocytocidal drugs.

Diversity and natural selection of Merozoite surface Protein-1 in three species of human malaria parasites: Contribution from South-East Asian isolates

The present study aimed to examine the genetic diversity of human malaria parasites (i.e., P. falciparum, P. vivax and P. knowlesi) in Malaysia and southern Thailand targeting the 19-kDa C-terminal region of Merozoite Surface Protein-1 (MSP-1₁₉). This region is essential for the recognition and invasion of erythrocytes and it is considered one of the leading candidates for asexual blood stage vaccines. However, the genetic data of MSP-1₁₉ among human malaria parasites in Malaysia is limited and there is also a need to update the current sequence diversity of this gene region among the Thailand isolates. In this study, genomic DNA was extracted from 384 microscopy-positive blood samples collected from patients who attended the hospitals or clinics in Malaysia and malaria clinics in Thailand from the year 2008 to 2016. The MSP-1₁₉ was amplified using PCR followed by bidirectional sequencing. DNA sequences identified in the present study were subjected to Median-joining network analysis with sequences of MSP-1₁₉ obtained from GenBank. DNA sequence analysis revealed that PfMSP-1₁₉ of Malaysian and Thailand isolates was not genetically conserved as high number of haplotypes were detected and positive selection was prevalent in PfMSP-1₁₉, hence questioning its suitability to be used as a vaccine candidate. A novel haplotype (Q/TNG/L) was also detected in Thailand P. falciparum isolate. In contrast, PvMSP-1₁₉ was highly conserved, however for the first time, a non-synonymous substitution (A1657S) was reported among Malaysian isolates. As for PkMSP-1₁₉, the presence of purifying selection and low nucleotide diversity indicated that it might be a potential vaccine target for P. knowlesi.

The PfAP2-G2 transcription factor is a critical regulator of gametocyte maturation

Differentiation from asexual blood stages to mature sexual gametocytes is required for the transmission of malaria parasites. Here, we report that the ApiAP2 transcription factor, PfAP2-G2 (PF3D7_1408200) plays a critical role in the maturation of Plasmodium falciparum gametocytes. PfAP2-G2 binds to the promoters of a wide array of genes that are expressed at many stages of the parasite life cycle. Interestingly, we also find binding of PfAP2-G2 within the gene body of almost 3,000 genes, which strongly correlates with the location of H3K36me3 and several other histone modifications as well as Heterochromatin Protein 1 (HP1), suggesting that occupancy of PfAP2-G2 in gene bodies may serve as an alternative regulatory mechanism. Disruption of pfap2-g2 does not impact asexual development, but the majority of sexual parasites are unable to mature beyond stage III gametocytes. The absence of pfap2-g2 leads to overexpression of 28% of the genes bound by PfAP2-G2 and none of the PfAP2-G2 bound genes are downregulated, suggesting that it is a repressor. We also find that PfAP2-G2 interacts with chromatin remodeling proteins, a microrchidia (MORC) protein, and another ApiAP2 protein (PF3D7_1139300). Overall our data demonstrate that PfAP2-G2 establishes an essential gametocyte maturation program in association with other chromatin-related proteins.

Intermittent preventive treatment comparing two versus three doses of sulphadoxine pyrimethamine (IPTp-SP) in the prevention of anaemia in pregnancy in Ghana: A cross-sectional study

In 2012 the World Health Organisation (WHO) revised the policy on Intermittent Preventive Treatment with Sulphadoxine Pyrimethamine (IPTp-SP) to at least three doses for improved protection against malaria parasitaemia and its associated effects such as anaemia during pregnancy. We assessed the different SP dosage regimen available under the new policy to determine the dose at which women obtained optimal protection against anaemia during pregnancy. A cross-sectional study was conducted among pregnant women who attended antenatal clinic at four different health facilities in Ghana. The register at the facilities served as a sampling frame and simple random sampling was used to select all the study respondents; they were enrolled consecutively as they kept reporting to the facility to receive antenatal care to obtain the required sample size. The haemoglobin level was checked using the Cyanmethemoglobin method. Multivariable logistic regression was performed to generate odds ratios, confidence intervals and p-values. The overall prevalence of anaemia among the pregnant women was 62.6%. Pregnant women who had taken 3 or more doses of IPTp-SP had anaemia prevalence of 54.1% compared to 66.6% of those who had taken one or two doses IPTp-SP. In the multivariable logistic model, primary (aOR 0.61; p = 0.03) and tertiary education (aOR 0.40; p = <0.001) decreased the odds of anaemia in pregnancy. Further, pregnant women who were anaemic at the time of enrollment (aOR 3.32; p = <0.001) to the Antenatal Care clinic and had malaria infection at late gestation (aOR 2.36; p = <0.001) had higher odds of anaemia in pregnancy. Anaemia in pregnancy remains high in the Northern region of Ghana. More than half of the pregnant women were anaemic despite the use of IPTp-SP. Maternal formal education reduced the burden of anaemia in pregnancy. The high prevalence of anaemia in pregnancy amid IPTp-SP use in Northern Ghana needs urgent attention to avert negative maternal and neonatal health outcomes.

Dynamic association of the H3K64 trimethylation mark with genes encoding exported proteins in Plasmodium falciparum

Epigenetic modifications have emerged as critical regulators of virulence genes and stage-specific gene expression in Plasmodium falciparum. However, the specific roles of histone core epigenetic modifications in regulating the stage-specific gene expression are not well understood. In this study, we report an unconventional trimethylation at lysine 64 on histone 3 (H3K64me3) and characterize its functional relevance in P. falciparum. We show that PfSET4 and PfSET5 proteins of P. falciparum methylate H3K64 and that they prefer the nucleosome as a substrate over free histone 3 proteins. Structural analysis of PfSET5 revealed that it interacts with the nucleosome as a dimer. The H3K64me3 mark is dynamic, being enriched in the ring and trophozoite stages and drastically reduced in the schizont stages. Stage-specific global chromatin immunoprecipitation –sequencing analysis of the H3K64me3 mark revealed the selective enrichment of this methyl mark on the genes of exported family proteins in the ring and trophozoite stages and a significant reduction of the same in the schizont stages. Collectively, our data identify a novel epigenetic mark that is associated with the subset of genes encoding for exported proteins, which may regulate their expression in different stages of P. falciparum.

Non-canonical metabolic pathways in the malaria parasite detected by isotope-tracing metabolomics

The malaria parasite, Plasmodium falciparum, proliferates rapidly in human erythrocytes by actively scavenging multiple carbon sources and essential nutrients from its host cell. However, a global overview of the metabolic capacity of intraerythrocytic stages is missing. Using multiplex 13 C-labelling coupled with untargeted mass spectrometry and unsupervised isotopologue grouping, we have generated a draft metabolome of P. falciparum and its host erythrocyte consisting of 911 and 577 metabolites, respectively, corresponding to 41% of metabolites and over 70% of the metabolic reaction predicted from the parasite genome. An additional 89 metabolites and 92 reactions were identified that were not predicted from genomic reconstructions, with the largest group being associated with metabolite damage-repair systems. Validation of the draft metabolome revealed four previously uncharacterised enzymes which impact isoprenoid biosynthesis, lipid homeostasis and mitochondrial metabolism and are necessary for parasite development and proliferation. This study defines the metabolic fate of multiple carbon sources in P. falciparum, and highlights the activity of metabolite repair pathways in these rapidly growing parasite stages, opening new avenues for drug discovery.

Antiplasmodial Compounds from Deep-Water Marine Invertebrates

Novel drug leads for malaria therapy are urgently needed because of the widespread emergence of resistance to all available drugs. Screening of the Harbor Branch enriched fraction library against the Plasmodium falciparum chloroquine-resistant strain (Dd2) followed by bioassay-guided fractionation led to the identification of two potent antiplasmodials; a novel diterpene designated as bebrycin A (1) and the known C21 degraded terpene nitenin (2). A SYBR Green I assay was used to establish a Dd2 EC₅₀ of 1.08 ± 0.21 and 0.29 ± 0.02 µM for bebrycin A and nitenin, respectively. Further analysis was then performed to assess the stage specificity of the inhibitors antiplasmodial effects on the Dd2 intraerythrocytic life cycle. Exposure to bebrycin A was found to block parasite maturation at the schizont stage if added any time prior to late schizogony at 42 hours post invasion, (HPI). In contrast, early life cycle exposure to nitenin (prior to 18 HPI) was identified as crucial to parasite inhibition, suggesting nitenin may target the maturation of the parasite during the transition from ring to early trophozoite (6–18 HPI), a novel property among known antimalarials.

Six New Phenylpropanoid Derivatives from Chemically Converted Extract of Alpinia galanga (L.) and Their Antiparasitic Activities

Chemical conversion of the extract of natural resources is a very attractive way to expand the chemical space to discover bioactive compounds. In order to search for new medicines to treat parasitic diseases that cause high morbidity and mortality in affected countries in the world, the ethyl acetate extract from the rhizome of Alpinia galanga (L.) has been chemically converted by epoxidation using dioxirane generated in situ. The biological activity of chemically converted extract (CCE) of A. galanga (L.) significantly increased the activity against Leishmania major up to 82.6 ± 6.2 % at 25 μg/mL (whereas 2.7 ± 0.8% for the original extract). By bioassay-guided fractionation, new phenylpropanoids (1-6) and four known compounds, hydroquinone (7), 4-hydroxy(4-hydroxyphenyl)methoxy)benzaldehyde (8), isocoumarin cis 4-hydroxymelein (9), and (2S,3S,6R,7R,9S,10S)-humulene triepoxide (10) were isolated from CCE. The structures of isolated compounds were determined by spectroscopic analyses of 1D and 2D NMR, IR, and MS spectra. The most active compound was hydroquinone (7) with IC50 = 0.37 ± 1.37 μg/mL as a substantial active principle of CCE. In addition, the new phenylpropanoid 2 (IC50 = 27.8 ± 0.34 μg/mL) also showed significant activity against L. major compared to the positive control miltefosine (IC50 = 7.47 ± 0.3 μg/mL). The activities of the isolated compounds were also evaluated against Plasmodium falciparum, Trypanosoma brucei gambisense and Trypanosoma brucei rhodeisense. Interestingly, compound 2 was selectively active against trypanosomes with potent activity. To the best of our knowledge, this is the first report on the bioactive "unnatural" natural products from the crude extract of A. galanga (L.) by chemical conversion and on its activities against causal pathogens of leishmaniasis, trypanosomiasis, and malaria.

In vitro growth competition experiments that suggest consequences of the substandard artemisinin epidemic that may be accelerating drug resistance in P. falciparum malaria

Over the past decade, artemisinin (ART)-combination therapies (ACTs) have shown declining efficacy within Southeast Asia (SEA). These resistance-like phenomena manifest as a delayed clearance phenotype (DCP) in some patients treated with ACTs. ACTs are currently the recommended treatment for P. falciparum infections by the World Health Organization (WHO), and they are our last line of defense to effectively treat all strains of malaria. Acceleration of antimicrobial resistance (AMR) is often theorized to be exacerbated by the use of subtherapeutic dosages of drugs ("substandard" drug), which for ACTs has been well documented over the last decade. Troublingly, in 2017, the WHO estimated that nearly 1 in 10 medical products tested in low- and middle-income countries failed to meet quality standards. We have developed a tissue culture-based approach for testing possible connections between substandard treatment and the spread of ACT resistant blood stage forms of P. falciparum. Via sequencing of pfk13, a molecular marker that is predictive for ART resistance (ARTR), we monitor competition of sensitive vs resistant strains over time and under various conditions and define conditions that favor emergence of ARTR parasites. Our findings help to define the conditions under which substandard drug treatments might favor the proliferation of mutant PfK13-mediated drug resistant strains over drug sensitive.

Bioassay-guided isolation of antiplasmodial and antimicrobial constituents from the roots of Terminalia albida

ETHNOPHARMACOLOGICAL RELEVANCE

Terminalia albida (Combretaceae), widely used in Guinean traditional medicine, showed promising activity against Plasmodium falciparum and Candida albicans in previous studies. Bioassay-guided fractionation was carried out in order to isolate the compounds responsible for these activities.

MATERIALS AND METHODS

Fractionation and isolation were performed by flash chromatography, followed by semi-preparative HPLC-DAD-MS. The structural elucidation of the isolated compounds was carried out by 1D and 2D NMR as well as HR-ESI-MS. Isolated compounds were evaluated against Plasmodium falciparum, Candida albicans, Staphylococcus aureus and Escherichia coli, and their cytotoxicity against MRC-5 cells was determined.

RESULTS

Bioassay-guided fractionation of Terminalia albida root resulted in the isolation of 14 compounds (1-14), and their antimicrobial properties were evaluated. Pantolactone (1) (IC₅₀ 0.60 ± 0.03 μM) demonstrated significant activity against P. falciparum. Other compounds, including 3,4,3'-tri-O-methyl-ellagic acid (3), the triterpenes arjunolic acid (5), arjungenin (6), arjunic acid (7) and arjunglucoside II (10), and the phenol glycoside calophymembranside-B (14), were less active and showed IC₅₀ values in the range 5-15 μM. None of the tested compound showed antibacterial or antifungal activity.

CONCLUSION

These results may explain at least in part the activity of the root extract of T. albida against P. falciparum.

Redox interactome in malaria parasite Plasmodium falciparum

The malaria-causing parasite Plasmodium falciparum is a severe threat to human health across the globe. This parasite alone causes the highest morbidity and mortality than any other species of Plasmodium. The parasites dynamically multiply in the erythrocytes of the vertebrate hosts, a large number of reactive oxygen species that damage biological macromolecules are produced in the cell during parasite growth. To relieve this intense oxidative stress, the parasite employs an NADPH-dependent thioredoxin and glutathione system that acts as an antioxidant and maintains redox status in the parasite. The mutual interaction of both redox proteins is involved in various biological functions and the survival of the erythrocytic stage of the parasite. Since the Plasmodium species is deficient in catalase and classical glutathione peroxidase, so their redox balance relies on a complex set of five peroxiredoxins, differentially positioned in the cytosol, mitochondria, apicoplast, and nucleus with partly overlapping substrate preferences. Moreover, Plasmodium falciparum possesses a set of members belonging to the thioredoxin superfamily, such as three thioredoxins, two thioredoxin-like proteins, one dithiol, three monocysteine glutaredoxins, and one redox-active plasmoredoxin with largely redundant functions. This review paper aims to discuss and encapsulate the biological function and current knowledge of the functional redox network of Plasmodium falciparum.

Melatonin action in Plasmodium infection: Searching for molecules that modulate the asexual cycle as a strategy to impair the parasite cycle

Half of the world's population lives in countries at risk of malaria infection, which results in approximately 450,000 deaths annually. Malaria parasites infect erythrocytes in a coordinated manner, with cycle durations in multiples of 24 hours, which reflects a behavior consistent with the host's circadian cycle. Interference in cycle coordination can help the immune system to naturally fight infection. Consequently, there is a search for new drugs that interfere with the cycle duration for combined treatment with conventional antimalarials. Melatonin appears to be a key host hormone responsible for regulating circadian behavior in the parasite cycle. In addition to host factors, there are still unknown factors intrinsic to the parasite that control the cycle duration. In this review, we present a series of reports of indole compounds and melatonin derivatives with antimalarial activity that were tested on several species of Plasmodium to evaluate the cytotoxicity to parasites and human cells, in addition to the ability to interfere with the development of the erythrocytic cycle. Most of the reported compounds had an IC50 value in the low micromolar range, without any toxicity to human cells. Triptosil, an indole derivative of melatonin, was able to inhibit the effect of melatonin in vitro without causing changes to the parasitemia. The wide variety of tested compounds indicates that it is possible to develop a compound capable of safely eliminating parasites from the host and interfering with the life cycle, which is promising for the development of new combined therapies against malaria.

Multiple blood feeding in mosquitoes shortens the Plasmodium falciparum incubation period and increases malaria transmission potential

Many mosquito species, including the major malaria vector Anopheles gambiae, naturally undergo multiple reproductive cycles of blood feeding, egg development and egg laying in their lifespan. Such complex mosquito behavior is regularly overlooked when mosquitoes are experimentally infected with malaria parasites, limiting our ability to accurately describe potential effects on transmission. Here, we examine how Plasmodium falciparum development and transmission potential is impacted when infected mosquitoes feed an additional time. We measured P. falciparum oocyst size and performed sporozoite time course analyses to determine the parasite's extrinsic incubation period (EIP), i.e. the time required by parasites to reach infectious sporozoite stages, in An. gambiae females blood fed either once or twice. An additional blood feed at 3 days post infection drastically accelerates oocyst growth rates, causing earlier sporozoite accumulation in the salivary glands, thereby shortening the EIP (reduction of 2.3 ± 0.4 days). Moreover, parasite growth is further accelerated in transgenic mosquitoes with reduced reproductive capacity, which mimic genetic modifications currently proposed in population suppression gene drives. We incorporate our shortened EIP values into a measure of transmission potential, the basic reproduction number R0, and find the average R0 is higher (range: 10.1%-12.1% increase) across sub-Saharan Africa than when using traditional EIP measurements. These data suggest that malaria elimination may be substantially more challenging and that younger mosquitoes or those with reduced reproductive ability may provide a larger contribution to infection than currently believed. Our findings have profound implications for current and future mosquito control interventions.

Detection and stage classification of Plasmodium falciparum from images of Giemsa stained thin blood films using random forest classifiers

BACKGROUND

The conventional method for the diagnosis of malaria parasites is the microscopic examination of stained blood films, which is time consuming and requires expertise. We introduce computer-based image segmentation and life stage classification with a random forest classifier. Segmentation and stage classification are performed on a large dataset of malaria parasites with ground truth labels provided by experts.

METHODS

We made use of Giemsa stained images obtained from the blood of 16 patients infected with Plasmodium falciparum. Experts labeled the parasite types from each of the images. We applied a two-step approach: image segmentation followed by life stage classification. In segmentation, we classified each pixel as a parasite or non-parasite pixel using a random forest classifier. Performance was evaluated with classification accuracy, Dice coefficient and free-response receiver operating characteristic (FROC) analysis. In life stage classification, we classified each of the segmented objects into one of 8 classes: 6 parasite life stages, early ring, late ring or early trophozoite, mid trophozoite, early schizont, late schizont or segmented, and two other classes, white blood cell or debris.

RESULTS

Our segmentation method gives an average cross-validated Dice coefficient of 0.82 which is a 13% improvement compared to the Otsu method. The Otsu method achieved a True Positive Fraction (TPF) of 0.925 at the expense of a False Positive Rate (FPR) of 2.45. At the same TPF of 0.925, our method achieved an FPR of 0.92, an improvement by more than a factor two. We find that inclusion of average intensity of the whole image as feature for the random forest considerably improves segmentation performance. We obtain an overall accuracy of 58.8% when classifying all life stages. Stages are mostly confused with their neighboring stages. When we reduce the life stages to ring, trophozoite and schizont only, we obtain an accuracy of 82.7%.

CONCLUSION

Pixel classification gives better segmentation performance than the conventional Otsu method. Effects of staining and background variations can be reduced with the inclusion of average intensity features. The proposed method and data set can be used in the development of automatic tools for the detection and stage classification of malaria parasites. The data set is publicly available as a benchmark for future studies.

Discovery of fast-acting dual-stage antimalarial agents by profiling pyridylvinylquinoline chemical space via copper catalyzed azide-alkyne cycloadditions

To identity fast-acting, multistage antimalarial agents, a series of pyridylvinylquinoline-triazole analogues have been synthesized via CuAAC. Most of the compounds display significant inhibitory effect on the drug-resistant malarial Dd2 strain at low submicromolar concentrations. Among the tested analogues, compound 60 is the most potent molecule with an EC₅₀ value of 0.04 ± 0.01 μM. Our current study indicates that compound 60 is a fast-acting antimalarial compound and it demonstrates stage specific action at the trophozoite phase in the P. falciparum asexual life cycle. In addition, compound 60 is active against both early and late stage P. falciparum gametocytes. From a mechanistic perspective, compound 60 shows good activity as an inhibitor of β-hematin formation. Collectively, our findings suggest that fast-acting agent 60 targets dual life stages of the malarial parasites and warrant further investigation of pyridylvinylquinoline hybrids as new antimalarials.

Opsonized antigen activates Vδ2+ T cells via CD16/FCγRIIIa in individuals with chronic malaria exposure

Vγ9Vδ2 T cells rapidly respond to phosphoantigens produced by Plasmodium falciparum in an innate-like manner, without prior antigen exposure or processing. Vδ2 T cells have been shown to inhibit parasite replication in vitro and are associated with protection from P. falciparum parasitemia in vivo. Although a marked expansion of Vδ2 T cells is seen after acute malaria infection in naïve individuals, repeated malaria causes Vδ2 T cells to decline both in frequency and in malaria-responsiveness, and to exhibit numerous transcriptional and phenotypic changes, including upregulation of the Fc receptor CD16. Here we investigate the functional role of CD16 on Vδ2 T cells in the immune response to malaria. We show that CD16+ Vδ2 T cells possess more cytolytic potential than their CD16- counterparts, and bear many of the hallmarks of mature NK cells, including KIR expression. Furthermore, we demonstrate that Vδ2 T cells from heavily malaria-exposed individuals are able to respond to opsonized P.falciparum-infected red blood cells through CD16, representing a second, distinct pathway by which Vδ2 T cells may contribute to anti-parasite effector functions. This response was independent of TCR engagement, as demonstrated by blockade of the phosphoantigen presenting molecule Butyrophilin 3A1. Together these results indicate that Vδ2 T cells in heavily malaria-exposed individuals retain the capacity for antimalarial effector function, and demonstrate their activation by opsonized parasite antigen. This represents a new role both for Vδ2 T cells and for opsonizing antibodies in parasite clearance, emphasizing cooperation between the cellular and humoral arms of the immune system.

Red blood cell tension protects against severe malaria in the Dantu blood group

Malaria has had a major effect on the human genome, with many protective polymorphisms-such as the sickle-cell trait-having been selected to high frequencies in malaria-endemic regions1,2. The blood group variant Dantu provides 74% protection against all forms of severe malaria in homozygous individuals3-5, a similar degree of protection to that afforded by the sickle-cell trait and considerably greater than that offered by the best malaria vaccine. Until now, however, the protective mechanism has been unknown. Here we demonstrate the effect of Dantu on the ability of the merozoite form of the malaria parasite Plasmodium falciparum to invade red blood cells (RBCs). We find that Dantu is associated with extensive changes to the repertoire of proteins found on the RBC surface, but, unexpectedly, inhibition of invasion does not correlate with specific RBC-parasite receptor-ligand interactions. By following invasion using video microscopy, we find a strong link between RBC tension and merozoite invasion, and identify a tension threshold above which invasion rarely occurs, even in non-Dantu RBCs. Dantu RBCs have higher average tension than non-Dantu RBCs, meaning that a greater proportion resist invasion. These findings provide both an explanation for the protective effect of Dantu, and fresh insight into why the efficiency of P. falciparum invasion might vary across the heterogenous populations of RBCs found both within and between individuals.

Antimalarial effect of cell penetrating peptides derived from the junctional region of Plasmodium falciparum dihydrofolate reductase-thymidylate synthase

Dihydrofolate reductase-thymidylate synthase of Plasmodium falciparum (PfDHFR-TS) is an important target of antifolate antimalarial drugs. However, drug resistant parasites are widespread in malaria endemic regions. The unique bifunctional property of PfDHFR-TS could be exploited for the design of allosteric inhibitors that interfere with the active dimer conformation. In this study, peptides were derived from the junctional region (JR) of PfDHFR-TS amino acid sequence in the α_j1 helix (JR-helix) and the DHFR domain that is necessary for interaction with α_j1 helix (JR21). Five peptides were synthesized and tested for inhibition of PfDHFR-TS enzyme by Bacterial inhibition assay (BIA) based on the growth of an E. coli DHFR and TS knockout complemented with a recombinant plasmid expressing PfDHFR-TS enzyme. Significant inhibition was observed for JR21 and JR21 conjugated to cell-penetrating octa-arginine peptide (rR8-JR21) with 50 % inhibitory concentration (IC₅₀) of 3.87 and 1.53 μM, respectively. The JR-helix and rR8-JR-helix peptides were inactive. JR21 and rR8-JR21 peptides showed similar growth inhibitory effects on P. falciparum NF54 parasites cultured in vitro. Treatment with rR8-JR21 delayed parasite development, in which an accumulation of ring stage parasites was observed after 12 h of culture. Minimal red blood cell (RBC) hemolysis was observed at the highest dose of peptide tested. The most potent peptide rR8-JR21 not only compromised the development of the P. falciparum, but also inhibited the parasite growth and has low hemolytic effect on human RBCs.

A De novo Peptide from a High Throughput Peptide Library Blocks Myosin A -MTIP Complex Formation in Plasmodium falciparum

Apicomplexan parasites, through their motor machinery, produce the required propulsive force critical for host cell-entry. The conserved components of this so-called glideosome machinery are myosin A and myosin A Tail Interacting Protein (MTIP). MTIP tethers myosin A to the inner membrane complex of the parasite through 20 amino acid-long C-terminal end of myosin A that makes direct contacts with MTIP, allowing the invasion of Plasmodium falciparum in erythrocytes. Here, we discovered through screening a peptide library, a de-novo peptide ZA1 that binds the myosin A tail domain. We demonstrated that ZA1 bound strongly to myosin A tail and was able to disrupt the native myosin A tail MTIP complex both in vitro and in vivo. We then showed that a shortened peptide derived from ZA1, named ZA1S, was able to bind myosin A and block parasite invasion. Overall, our study identified a novel anti-malarial peptide that could be used in combination with other antimalarials for blocking the invasion of Plasmodium falciparum.

Alkoxyamines Designed as Potential Drugs against Plasmodium and Schistosoma Parasites

Malaria and schistosomiasis are major infectious causes of morbidity and mortality in the tropical and sub-tropical areas. Due to the widespread drug resistance of the parasites, the availability of new efficient and affordable drugs for these endemic pathologies is now a critical public health issue. In this study, we report the design, the synthesis and the preliminary biological evaluation of a series of alkoxyamine derivatives as potential drugs against Plasmodium and Schistosoma parasites. The compounds (RS/SR)-2F, (RR/SS)-2F, and 8F, having IC₅₀ values in nanomolar range against drug-resistant P. falciparum strains, but also five other alkoxyamines, inducing the death of all adult worms of S. mansoni in only 1 h, can be considered as interesting chemical starting points of the series for improvement of the activity, and further structure activity, relationship studies. Moreover, investigation of the mode of action and the rate constants k_d for C-ON bond homolysis of new alkoxyamines is reported, showing a possible alkyl radical mediated biological activity. A theoretical chemistry study allowed us to design new structures of alkoxyamines in order to improve the selectivity index of these drugs.

Building on Surface-Active Ionic Liquids for the Rescuing of the Antimalarial Drug Chloroquine

Ionic liquids derived from classical antimalarials are emerging as a new approach towards the cost-effective rescuing of those drugs. Herein, we disclose novel surface-active ionic liquids derived from chloroquine and natural fatty acids whose antimalarial activity in vitro was found to be superior to that of the parent drug. The most potent ionic liquid was the laurate salt of chloroquine, which presented IC₅₀ values of 4 and 110 nM against a chloroquine-sensitive and a chloroquine-resistant strain of Plasmodium falciparum, respectively, corresponding to an 11- and 6-fold increase in potency as compared to the reference chloroquine bisphosphate salt against the same strains. This unprecedented report opens new perspectives in both the fields of malaria chemotherapy and of surface-active ionic liquids derived from active pharmaceutical ingredients.

Inhibition of Resistance-Refractory P. falciparum Kinase PKG Delivers Prophylactic, Blood Stage, and Transmission-Blocking Antiplasmodial Activity

The search for antimalarial chemotypes with modes of action unrelated to existing drugs has intensified with the recent failure of first-line therapies across Southeast Asia. Here, we show that the trisubstituted imidazole MMV030084 potently inhibits hepatocyte invasion by Plasmodium sporozoites, merozoite egress from asexual blood stage schizonts, and male gamete exflagellation. Metabolomic, phosphoproteomic, and chemoproteomic studies, validated with conditional knockdown parasites, molecular docking, and recombinant kinase assays, identified cGMP-dependent protein kinase (PKG) as the primary target of MMV030084. PKG is known to play essential roles in Plasmodium invasion of and egress from host cells, matching MMV030084's activity profile. Resistance selections and gene editing identified tyrosine kinase-like protein 3 as a low-level resistance mediator for PKG inhibitors, while PKG itself never mutated under pressure. These studies highlight PKG as a resistance-refractory antimalarial target throughout the Plasmodium life cycle and promote MMV030084 as a promising Plasmodium PKG-targeting chemotype.

Use of a highly specific kinase inhibitor for rapid, simple and precise synchronization of Plasmodium falciparum and Plasmodium knowlesi asexual blood-stage parasites

During the course of the asexual erythrocytic stage of development, Plasmodium spp. parasites undergo a series of morphological changes and induce alterations in the host cell. At the end of this stage, the parasites egress from the infected cell, after which the progeny invade a new host cell. These processes are rapid and occur in a time-dependent manner. Of particular importance, egress and invasion of erythrocytes by the parasite are difficult to capture in an unsynchronized culture, or even a culture that has been synchronized within a window of one to several hours. Therefore, precise synchronization of parasite cultures is of paramount importance for the investigation of these processes. Here we describe a method for synchronizing Plasmodium falciparum and Plasmodium knowlesi asexual blood stage parasites with ML10, a highly specific inhibitor of the cGMP-dependent protein kinase (PKG) that arrests parasite growth approximately 15 minutes prior to egress. This inhibitor allows parasite cultures to be synchronized so that all parasites are within a window of development of several minutes, with a simple wash step. Furthermore, we show that parasites remain viable for several hours after becoming arrested by the compound and that ML10 has advantages, owing to its high specificity and low EC50, over the previously used PKG inhibitor Compound 2. Here, we demonstrate that ML10 is an invaluable tool for the study of Plasmodium spp. asexual blood stage biology and for the routine synchronization of P. falciparum and P. knowlesi cultures.

Parasite density in severe malaria in Colombia

BACKGROUND

Colombia has officially adopted the parasite density levels of severe malaria established by the WHO (>50,000 parasites/μl). These values have been inferred from areas of high transmission in Africa and are not consistent with the dynamics of low and unstable transmission in Colombia. The objective of this study was therefore to determine the parasite density values observed in patients with severe malaria and their distribution in the different ecoepidemiological regions of Colombia.

METHODS

A retrospective and descriptive study of confirmed cases of severe malaria was conducted in endemic areas of malaria in Colombia over the period 2014-2017. Data were collected from secondary sources of the Subnational Programs of Malaria Prevention and Control. Person, place, and time variables were selected. The official definition of severe malaria was adopted, and compliance with these criteria was determined. Univariate and bivariate analyses were conducted with absolute and relative frequency measures, and the relevant statistical tests were applied.

RESULTS

The overall parasite density values in Colombia showed a geometric mean of 5,919 parasites/μl (95% CI: 5,608-6,248). By parasite species, the values were 6,151 (95% CI: 5,631-6,718) for Plasmodium falciparum and 5,815 (95% CI: 5,428-6,230) for Plasmodium vivax. The highest parasite density values were recorded in the Amazon ecoepidemiological region (8,177; 95% CI: 6,015-11,116), and the lowest values were recorded in the Andean region (5,026; 95% CI: 2,409-10,480).

CONCLUSIONS

In endemic areas of low and unstable malaria transmission in the Colombian territory, the parasite density levels observed in populations with severe malaria are lower than the officially established values. The parasite density criterion is not really a relevant criterion for the definition of severe cases in Colombia and it certainly not be used to make a clinical decision about the severity of the disease.

Lignans, Amides, and Saponins from Haplophyllum tuberculatum and Their Antiprotozoal Activity

A screening of Sudanese medicinal plants for antiprotozoal activities revealed that the chloroform and water fractions of the ethanolic root extract of Haplophyllum tuberculatum exhibited appreciable bioactivity against Leishmania donovani. The antileishmanial activity was tracked by HPLC-based activity profiling, and eight compounds were isolated from the chloroform fraction. These included lignans tetrahydrofuroguaiacin B (1), nectandrin B (2), furoguaiaoxidin (7), and 3,3'-dimethoxy-4,4'-dihydroxylignan-9-ol (10), and four cinnamoylphenethyl amides, namely dihydro-feruloyltyramine (5), (E)-N-feruloyltyramine (6), N,N'-diferuloylputrescine (8), and 7'-ethoxy-feruloyltyramine (9). The water fraction yielded steroid saponins 11-13. Compounds 1, 2, and 5-13 are reported for the first time from Haplophyllum species and the family Rutaceae. The antiprotozoal activity of the compounds plus two stereoisomeric tetrahydrofuran lignans-fragransin B2 (3) and fragransin B1 (4)-was determined against Leishmania donovani amastigotes, Plasmodium falciparum, and Trypanosoma brucei rhodesiense bloodstream forms, along with their cytotoxicity to rat myoblast L6 cells. Nectandrin B (2) exhibited the highest activity against L. donovani (IC50 4.5 µM) and the highest selectivity index (25.5).

IMRAS-A clinical trial of mosquito-bite immunization with live, radiation-attenuated P. falciparum sporozoites: Impact of immunization parameters on protective efficacy and generation of a repository of immunologic reagents

Background

Immunization with radiation-attenuated sporozoites (RAS) by mosquito bite provides >90% sterile protection against Plasmodium falciparum (Pf) malaria in humans. RAS invade hepatocytes but do not replicate. CD8+ T cells recognizing parasite-derived peptides on the surface of infected hepatocytes are likely the primary protective mechanism. We conducted a randomized clinical trial of RAS immunization to assess safety, to achieve 50% vaccine efficacy (VE) against controlled human malaria infection (CHMI), and to generate reagents from protected and non-protected subjects for future identification of protective immune mechanisms and antigens.

Methods

Two cohorts (Cohort 1 and Cohort 2) of healthy, malaria-naïve, non-pregnant adults age 18–50 received five monthly immunizations with infected (true-immunized, n = 21) or non-infected (mock-immunized, n = 5) mosquito bites and underwent homologous CHMI at 3 weeks. Immunization parameters were selected for 50% protection based on prior clinical data. Leukapheresis was done to collect plasma and peripheral blood mononuclear cells.

Results

Adverse event rates were similar in true- and mock-immunized subjects. Two true- and two mock-immunized subjects developed large local reactions likely caused by mosquito salivary gland antigens. In Cohort 1, 11 subjects received 810–1235 infected bites; 6/11 (55%) were protected against CHMI vs. 0/3 mock-immunized and 0/6 infectivity controls (VE 55%). In Cohort 2, 10 subjects received 839–1131 infected bites with a higher first dose and a reduced fifth dose; 9/10 (90%) were protected vs. 0/2 mock-immunized and 0/6 controls (VE 90%). Three/3 (100%) protected subjects administered three booster immunizations were protected against repeat CHMI vs. 0/6 controls (VE 100%). Cohort 2 uniquely showed a significant rise in IFN-γ responses after the third and fifth immunizations and higher antibody responses to CSP.

Conclusions

PfRAS were generally safe and well tolerated. Cohort 2 had a higher first dose, reduced final dose, higher antibody responses to CSP and significant rise of IFN-γ responses after the third and fifth immunizations. Whether any of these factors contributed to increased protection in Cohort 2 requires further investigation. A cryobank of sera and cells from protected and non-protected individuals was generated for future immunological studies and antigen discovery.

Trial registration

ClinicalTrials.gov NCT01994525.

Compound isolation and biological activities of Piptadeniastrum africanum (hook.f.) Brennan roots

ETHNOPHARMACOLOGICAL RELEVANCE

The dicotyledonous plant Piptadeniastrum africanum (hook.f.) Brennan (Fabaceae) is used in traditional medicine to treat various human complaints including bronchitis, coughing, urino-genital ailments, meningitis, abdominal pain, treatment of wounds, malaria and gastrointestinal ailments, and is used as a purgative and worm expeller.

AIM OF THE STUDY

The present study describes the phytochemical investigation and the determination of the antimicrobial, antiplasmodial and antitrypanosomal activities of crude extract, fractions and compounds extracted from Piptadeniastrum africanum roots.

MATERIALS AND METHODS

Isolated compounds were obtained using several chromatographic techniques. The structures of all compounds were determined by comprehensive spectroscopic analyses (1D and 2D NMR) and by comparing their NMR data with those found in literature. In vitro antimicrobial activity of samples was evaluated using the microdilution method on bacterial (Escherichia coli, Proteus mirabilis, Staphylococcus aureus) and fungal (Candida krusei) strains, while in vitro cell-growth inhibition activities were assessed against two parasites (Trypanosoma brucei brucei and Plasmodium falciparum strain 3D7). The cytotoxicity properties of samples were assayed against HeLa human cervical carcinoma.

RESULTS

Five compounds were isolated and identified as: tricosanol 1, 5α-stigmasta-7,22-dien-3-β-ol 2, betulinic acid 3, oleanolic acid 4 and piptadenamide 5. This is the first report of the isolation of these five compounds from the roots of P. africanum. The (Hex:EtOAc 50:50) fraction exhibited moderate antibacterial activity against P. mirabilis (MIC 250 μg/mL), while the other fractions and isolated compounds had weak antimicrobial activities. Only the EtOAc fraction presented a moderate antimalarial activity with an IC₅₀ of 16.5 μg/mL. The MeOH crude extract and three fractions (Hexane, Hexane-EtOAc 25% and EtOAc-MeOH 25%) exhibited significant trypanocidal activity with IC₅₀ values of 3.0, 37.5, 3.8 and 9.5 μg/mL, respectively.

CONCLUSION

These results demonstrated a scientific rational of the traditional uses of P. africanum and indicate that this plant should be further investigated to identify some of the chemical components that exhibited the activities reported in this study and therefore may constitute new lead candidates in parasiticidal drug discovery.

Ethnopharmacology-aided antiplasmodial evaluation of six selected plants used for malaria treatment in Nigeria

ETHNOPHARMACOLOGICAL RELEVANCE

Sub - Saharan Africa has a high malaria burden and Nigeria accounts for majority of malaria cases worldwide. The aim of this study was to evaluate selected plants extracts used against malaria in Nigeria for antiplasmodial activity.

MATERIALS AND METHODS

An ethnomedicinal based - approach by literature survey was used to identify plants used in the study. The parts of the plant used were collected and extracted with 70% v/v ethanol; a portion of each extract was used to prepare successive solvent and residual fractions. Chloroquine-sensitive (3D7) P. falciparum strain and human embryonic kidney cells (HEK293) were used for antiplasmodial and cytotoxicity screening respectively. Hemolysis assay was also carried out on red blood cells (RBCs). Test for in vivo efficacy of an active extract was conducted in a mouse model of established P. berghei ANKA-infection.

RESULTS

A total of six plants; Andropogon schirensis, Celtis durandii, Chasmanthera dependens, Daniellia ogea, Icacina trichantha and Triumfetta cordifolia were selected and screened. Triumfetta cordifolia leaf extract was observed to display moderate in vitro antiplasmodial activity (IC₅₀ = 48.09 μg/ml) and was non-toxic to HEK293 cells and erythrocytes. At a dose of 400 mg/kg, T. cordifolia significantly (p<0.001) suppressed parasitemia, significantly (p<0.001) inhibited RBC depletion and prolonged survival in infected mice.

CONCLUSIONS

T. cordifolia ethanol extract possesses antiplasmodial efficacy and this is the first report of its kind on the plant. It is a potential candidate for further studies to identify its mechanism of action.

Assessment of Biological Role and Insight into Druggability of the Plasmodium falciparum Protease Plasmepsin V

Upon infecting a red blood cell (RBC), the malaria parasite Plasmodium falciparum drastically remodels its host by exporting hundreds of proteins into the RBC cytosol. This protein export program is essential for parasite survival. Hence export-related proteins could be potential drug targets. One essential enzyme in this pathway is plasmepsin V (PMV), an aspartic protease that processes export-destined proteins in the parasite endoplasmic reticulum (ER) at the Plasmodium export element (PEXEL) motif. Despite long-standing interest in this enzyme, functional studies have been hindered by the inability of previous technologies to produce a regulatable lethal depletion of PMV. To overcome this technical barrier, we designed a system for stringent post-transcriptional regulation allowing a tightly controlled, tunable knockdown of PMV. Using this system, we found that PMV must be dramatically depleted to affect parasite growth, suggesting the parasite maintains this enzyme in substantial excess. Surprisingly, depletion of PMV arrested parasite growth immediately after RBC invasion, significantly before the death from exported protein deficit that has previously been described. The data suggest that PMV inhibitors can halt parasite growth at two distinct points in the parasite life cycle. However, overcoming the functional excess of PMV in the parasite may require inhibitor concentrations far beyond the enzyme's IC50.

Inactivation of Plasmodium falciparum in whole blood using the amustaline and glutathione pathogen reduction technology

BACKGROUND

Risk of transfusion-transmitted (TT) malaria is mainly associated with whole blood (WB) or red blood cell (RBC) transfusion. Risk mitigation relies mostly on donor deferral while a limited number of countries perform blood testing, both negatively impacting blood availability. This study investigated the efficacy of the pathogen reduction system using amustaline and glutathione (GSH) to inactivate Plasmodium falciparum in WB.

STUDY DESIGN AND METHODS

WB units were spiked with ring stage P. falciparum infected RBCs. Parasite loads were measured in samples at time of infection, after 24 hours at room temperature (RT), and after a 24-hour incubation at RT post-treatment with 0.2 mM amustaline and 2 mM GSH. Serial 10-fold dilutions of the samples were inoculated to RBC cultures and maintained up to 4 weeks. Parasitemia was quantified by cytometry.

RESULTS

The P. falciparum viability assay has a limit of detection of a single live parasite per sample. Input parasite titer was >5.7 log10 TCID50 per mL. A 24-hour incubation at RT paused parasite development in controls, but they retained viability and infectivity when tested in culture. In contrast, no infectious parasites were detected in the amustaline/GSH-treated sample after 4 weeks of culture.

CONCLUSION

A robust level of P. falciparum inactivation was achieved in WB using amustaline/GSH treatment. Parasite log reduction was >5.7 log10 TCID50 per mL. Development of such a pathogen reduction system may provide an opportunity to reduce the risk of TT malaria and improve blood availability.

Lapatinib, Nilotinib and Lomitapide Inhibit Haemozoin Formation in Malaria Parasites

With the continued loss of antimalarials to resistance, drug repositioning may have a role in maximising efficiency and accelerating the discovery of new antimalarial drugs. Bayesian statistics was previously used as a tool to virtually screen USFDA approved drugs for predicted β-haematin (synthetic haemozoin) inhibition and in vitro antimalarial activity. Here, we report the experimental evaluation of nine of the highest ranked drugs, confirming the accuracy of the model by showing an overall 93% hit rate. Lapatinib, nilotinib, and lomitapide showed the best activity for inhibition of β-haematin formation and parasite growth and were found to inhibit haemozoin formation in the parasite, providing mechanistic insights into their mode of antimalarial action. We then screened the USFDA approved drugs for binding to the β-haematin crystal, applying a docking method in order to evaluate its performance. The docking method correctly identified imatinib, lapatinib, nilotinib, and lomitapide. Experimental evaluation of 22 of the highest ranked purchasable drugs showed a 24% hit rate. Lapatinib and nilotinib were chosen as templates for shape and electrostatic similarity screening for lead hopping using the in-stock ChemDiv compound catalogue. The actives were novel structures worthy of future investigation. This study presents a comparison of different in silico methods to identify new haemozoin-inhibiting chemotherapeutic alternatives for malaria that proved to be useful in different ways when taking into consideration their strengths and limitations.

Structure-Activity Relationship between Thiol Group-Trapping Ability of Morphinan Compounds with a Michael Acceptor and Anti-Plasmodium falciparum Activities

7-Benzylidenenaltrexone (BNTX) and most of its derivatives showed in vitro antimalarial activities against chloroquine-resistant and -sensitive Plasmodium falciparum strains (K1 and FCR3, respectively). In addition, the time-dependent changes of the addition reactions of the BNTX derivatives with 1-propanethiol were examined by ¹H-NMR experiments to estimate their thiol group-trapping ability. The relative chemical reactivity of the BNTX derivatives to trap the thiol group of 1-propanethiol was correlated highly with the antimalarial activity. Therefore, the measurements of the thiol group-trapping ability of the BNTX derivatives with a Michael acceptor is expected to become an alternative method for in vitro malarial activity and related assays.

Epigenetic inhibitors target multiple stages of Plasmodium falciparum parasites

The epigenome of the malaria parasite, Plasmodium falciparum, is associated with regulation of various essential processes in the parasite including control of proliferation during asexual development as well as control of sexual differentiation. The unusual nature of the epigenome has prompted investigations into the potential to target epigenetic modulators with novel chemotypes. Here, we explored the diversity within a library of 95 compounds, active against various epigenetic modifiers in cancerous cells, for activity against multiple stages of P. falciparum development. We show that P. falciparum is differentially susceptible to epigenetic perturbation during both asexual and sexual development, with early stage gametocytes particularly sensitive to epi-drugs targeting both histone and non-histone epigenetic modifiers. Moreover, 5 compounds targeting histone acetylation and methylation show potent multistage activity against asexual parasites, early and late stage gametocytes, with transmission-blocking potential. Overall, these results warrant further examination of the potential antimalarial properties of these hit compounds.

The chromatin bound proteome of the human malaria parasite

Proteins interacting with DNA are fundamental for mediating processes such as gene expression, DNA replication and maintenance of genome integrity. Accumulating evidence suggests that the chromatin of apicomplexan parasites, such as Plasmodium falciparum, is highly organized, and this structure provides an epigenetic mechanism for transcriptional regulation. To investigate how parasite chromatin structure is being regulated, we undertook comparative genomics analysis using 12 distinct eukaryotic genomes. We identified conserved and parasite-specific chromatin-associated domains (CADs) and proteins (CAPs). We then used the chromatin enrichment for proteomics (ChEP) approach to experimentally capture CAPs in P. falciparum. A topological scoring analysis of the proteomics dataset revealed stage-specific enrichments of CADs and CAPs. Finally, we characterized, two candidate CAPs: a conserved homologue of the structural maintenance of chromosome 3 protein and a homologue of the crowded-like nuclei protein, a plant-like protein functionally analogous to animal nuclear lamina proteins. Collectively, our results provide a comprehensive overview of CAPs in apicomplexans, and contribute to our understanding of the complex molecular components regulating chromatin structure and genome architecture in these deadly parasites.

Using two phases of the CD4 T cell response to blood-stage murine malaria to understand regulation of systemic immunity and placental pathology in Plasmodium falciparum infection

Plasmodium falciparum infection and malaria remain a risk for millions of children and pregnant women. Here, we seek to integrate knowledge of mouse and human T helper cell (Th) responses to blood-stage Plasmodium infection to understand their contribution to protection and pathology. Although there is no complete Th subset differentiation, the adaptive response occurs in two phases in non-lethal rodent Plasmodium infection, coordinated by Th cells. In short, cellular immune responses limit the peak of parasitemia during the first phase; in the second phase, humoral immunity from T cell-dependent germinal centers is critical for complete clearance of rapidly changing parasite. A strong IFN-γ response kills parasite, but an excess of TNF compared with regulatory cytokines (IL-10, TGF-β) can cause immunopathology. This common pathway for pathology is associated with anemia, cerebral malaria, and placental malaria. These two phases can be used to both understand how the host responds to rapidly growing parasite and how it attempts to control immunopathology and variation. This dual nature of T cell immunity to Plasmodium is discussed, with particular reference to the protective nature of the continuous generation of effector T cells, and the unique contribution of effector memory T cells.

Immunity against sexual stage Plasmodium falciparum and Plasmodium vivax parasites

The efficient spread of malaria from infected humans to mosquitoes is a major challenge for malaria elimination initiatives. Gametocytes are the only Plasmodium life stage infectious to mosquitoes. Here, we summarize evidence for naturally acquired anti-gametocyte immunity and the current state of transmission blocking vaccines (TBV). Although gametocytes are intra-erythrocytic when present in infected humans, developing Plasmodium falciparum gametocytes may express proteins on the surface of red blood cells that elicit immune responses in naturally exposed individuals. This immune response may reduce the burden of circulating gametocytes. For both P. falciparum and Plasmodium vivax, there is a solid evidence that antibodies against antigens present on the gametocyte surface, when co-ingested with gametocytes, can influence transmission to mosquitoes. Transmission reducing immunity, reducing the burden of infection in mosquitoes, is a well-acknowledged but poorly quantified phenomenon that forms the basis for the development of TBV. Transmission enhancing immunity, increasing the likelihood or intensity of transmission to mosquitoes, is more speculative in nature but is convincingly demonstrated for P. vivax. With the increased interest in malaria elimination, TBV and monoclonal antibodies have moved to the center stage of malaria vaccine development. Methodologies to prioritize and evaluate products are urgently needed.

Cell trace far-red is a suitable erythrocyte dye for multi-color Plasmodium falciparum invasion phenotyping assays

Plasmodium falciparum erythrocyte invasion phenotyping assays are a very useful tool for assessing parasite diversity and virulence, and for characterizing the formation of ligand–receptor interactions. However, such assays need to be highly sensitive and reproducible, and the selection of labeling dyes for differentiating donor and acceptor erythrocytes is a critical factor. We investigated the suitability of cell trace far-red (CTFR) as a dye for P. falciparum invasion phenotyping assays. Using the dyes carboxyfluorescein diacetate succinimidyl ester (CFDA-SE) and dichloro dimethyl acridin one succinimidyl ester (DDAO-SE) as comparators, we used a dye-dilution approach to assess the limitations and specific staining procedures for the applicability of CTFR in P. falciparum invasion phenotyping assays. Our data show that CTFR effectively labels acceptor erythrocytes and provides a stable fluorescent intensity at relatively low concentrations. CTFR also yielded a higher fluorescence intensity relative to DDAO-SE and with a more stable fluorescence intensity over time. Furthermore, CTFR did not affect merozoites invasion of erythrocytes and was not toxic to the parasite’s intraerythrocytic development. Additionally, CTFR offers flexibility in the choice of combinations with several other DNA dyes, which broaden its usage for P. falciparum erythrocyte invasion assays, considering a wider range of flow cytometers with various laser settings.