Protection, pathogenesis and phenotypic plasticity in Plasmodium falciparum malaria

Why does Plasmodium falciparum cause severe illness in some but not all infections? How is clinical immunity acquired? These questions have intrigued investigators since the clinical epidemiology of malaria was first described. The search for answers to both questions has highlighted the changes that take place at the surface of infected red blood cells during the last half of the erythrocytic cycle. These changes specify the antigenic and adhesive or cytoadherence phenotypes for the infected cell. Now the antigenic and adhesive phenotypes appear to be linked and together undergo clonal variation. In this article David Roberts, Beverley-Ann Biggs, Graham Brown and Christopher Newbold explain how clonal phenotypic variation and the linkage between adhesive and antigenic types contribute to our understanding of naturally acquired immunity and of pathogenesis of severe malaria.     

Centrins, cell cycle regulation proteins in human malaria parasite Plasmodium falciparum

Molecules and cellular mechanisms that regulate the process of cell division in malaria parasites remain poorly understood. In this study we isolate and characterize the four Plasmodium falciparum centrins (PfCENs) and, by growth complementation studies, provide evidence for their involvement in cell division. Centrins are cytoskeleton proteins with key roles in cell division, including centrosome duplication, and possess four Ca(2+)-binding EF hand domains. By means of phylogenetic analysis, we were able to decipher the evolutionary history of centrins in eukaryotes with particular emphasis on the situation in apicomplexans and other alveolates. Plasmodium possesses orthologs of four distinct centrin paralogs traceable to the ancestral alveolate, including two that are unique to alveolates. By real time PCR and/or immunofluorescence, we determined the expression of PfCEN mRNA or protein in sporozoites, asexual blood forms, gametocytes, and in the oocysts developing inside mosquito mid-gut. Immunoelectron microscopy studies showed that centrin is expressed in close proximity with the nucleus of sporozoites and asexual schizonts. Furthermore, confocal and widefield microscopy using the double staining with alpha-tubulin and centrin antibodies strongly suggested that centrin is associated with the parasite centrosome. Following the episomal expression of the four PfCENs in a centrin knock-out Leishmania donovani parasite line that exhibited a severe growth defect, one of the PfCENs was able to partially restore Leishmania growth rate and overcome the defect in cytokinesis in such mutant cell line. To our knowledge, this study is the first characterization of a Plasmodium molecule that is involved in the process of cell division. These results provide the opportunity to further explore the role of centrins in cell division in malaria parasites and suggest novel targets to construct genetically modified, live attenuated malaria vaccines.     

Distinct patterns of cytokine regulation in discrete clinical forms of Plasmodium falciparum malaria

The pathogenesis of two of the most severe complications of Plasmodium falciparum malaria, cerebral malaria (CM) and severe malarial anaemia (SA) both appear to involve dysregulation of the immune system. We have measured plasma levels of TNF and its two receptors in Ghanaian children with strictly defined cerebral malaria (CM), severe malarial anaemia (SA), or uncomplicated malaria (UM) in two independent studies in an area of seasonal, hyperendemic transmission of P. falciparum. Levels of TNF, soluble TNF receptor 1 (sTNF-R1) and 2 (sTNF-R2) were found to be significantly higher in CM than in the other clinical categories of P. falciparum malaria patients. Levels of both receptors depended on clinical category, whereas only sTNF-R1 levels were significantly dependent on parasitemia. Detailed analysis of the interrelationship between these variables resolved this pattern further, and identified marked differences between the patient categories. While levels of TNF, sTNF-R1 and sTNF-R2 correlated with parasitemia in UM, this was not the case in CM and SA. Rather, there was a tendency towards high levels of TNF and its receptors in CM and low levels in SA without significant correlation to parasitemia in either category. This, and the fact that malaria-induced increases in plasma levels of IL-10 are much lower in SA compared to CM, suggest that distinct forms of dysregulation of the immune response to infection contribute to the pathogenesis of CM and SA.     

Plasmodium falciparum malaria endemicity in Indonesia in 2010

Background

Malaria control programs require a detailed understanding of the contemporary spatial distribution of infection risk to efficiently allocate resources. We used model based geostatistics (MBG) techniques to generate a contemporary map of Plasmodium falciparum malaria risk in Indonesia in 2010.

Methods

Plasmodium falciparum Annual Parasite Incidence (PfAPI) data (2006–2008) were used to map limits of P. falciparum transmission. A total of 2,581 community blood surveys of P. falciparum parasite rate (PfPR) were identified (1985–2009). After quality control, 2,516 were included into a national database of age-standardized 2–10 year old PfPR data (PfPR_2–10) for endemicity mapping. A Bayesian MBG procedure was used to create a predicted surface of PfPR_2–10 endemicity with uncertainty estimates. Population at risk estimates were derived with reference to a 2010 human population count surface.

Results

We estimate 132.8 million people in Indonesia, lived at risk of P. falciparum transmission in 2010. Of these, 70.3% inhabited areas of unstable transmission and 29.7% in stable transmission. Among those exposed to stable risk, the vast majority were at low risk (93.39%) with the reminder at intermediate (6.6%) and high risk (0.01%). More people in western Indonesia lived in unstable rather than stable transmission zones. In contrast, fewer people in eastern Indonesia lived in unstable versus stable transmission areas.

Conclusion

While further feasibility assessments will be required, the immediate prospects for sustained control are good across much of the archipelago and medium term plans to transition to the pre-elimination phase are not unrealistic for P. falciparum. Endemicity in areas of Papua will clearly present the greatest challenge. This P. falciparum endemicity map allows malaria control agencies and their partners to comprehensively assess the region-specific prospects for reaching pre-elimination, monitor and evaluate the effectiveness of future strategies against this 2010 baseline and ultimately improve their evidence-based malaria control strategies.     

Mitosis in the human malaria parasite Plasmodium falciparum

Malaria is caused by intraerythrocytic protozoan parasites belonging to Plasmodium spp. (phylum Apicomplexa) that produce significant morbidity and mortality, mostly in developing countries. Plasmodium parasites have a complex life cycle that includes multiple stages in anopheline mosquito vectors and vertebrate hosts. During the life cycle, the parasites undergo several cycles of extreme population growth within a brief span, and this is critical for their continued transmission and a contributing factor for their pathogenesis in the host. As with other eukaryotes, successful mitosis is an essential requirement for Plasmodium reproduction; however, some aspects of Plasmodium mitosis are quite distinct and not fully understood. In this review, we will discuss the current understanding of the architecture and key events of mitosis in Plasmodium falciparum and related parasites and compare them with the traditional mitotic events described for other eukaryotes.     

Plasmodium falciparum SERA protein peptide analogues having short helical regions induce protection against malaria

Three Plasmodium falciparum serine repeat antigen (SERA) protein peptides were studied by NMR and structure calculations being done in 70:30 water:trifluoroethanol solution. Peptide 22834 was shown to be immunogenic and protective against malaria in Aotus monkeys, whilst native peptide 6737 and its analogue 14096 did not present protection against the disease in these monkeys. Results showed a relationship between these peptides' secondary structure and their function as immunogen against malaria.     

High affinity antibodies to Plasmodium falciparum merozoite antigens are associated with protection from malaria

Background

Malaria kills almost 1 million people every year, but the mechanisms behind protective immunity against the disease are still largely unknown.

Methodology/Principal Findings

In this study, surface plasmon resonance technology was used to evaluate the affinity (measured as k^d) of naturally acquired antibodies to the Plasmodium falciparum antigens MSP2 and AMA1. Antibodies in serum samples from residents in endemic areas bound with higher affinities to AMA1 than to MSP2, and with higher affinities to the 3D7 allele of MSP2-3D7 than to the FC27 allele. The affinities against AMA1 and MSP2-3D7 increased with age, and were usually within similar range as the affinities for the monoclonal antibodies also examined in this study. The finding of MSP2-3D7 type parasites in the blood was associated with a tendency for higher affinity antibodies to both forms of MSP2 and AMA1, but this was significant only when analyzing antibodies against MSP2-FC27, and individuals infected with both allelic forms of MSP2 at the same time showed the highest affinities. Individuals with the highest antibody affinities for MSP2-3D7 at baseline had a prolonged time to clinical malaria during 40 weeks of follow-up, and among individuals who were parasite positive at baseline higher antibody affinities to all antigens were seen in the individuals that did not experience febrile malaria during follow up.

Conclusions/Significance

This study contributes important information for understanding how immunity against malaria arises. The findings suggest that antibody affinity plays an important role in protection against disease, and differs between antigens. In light of this information, antibody affinity measurements would be a key assessment in future evaluation of malaria vaccine formulations.     

Biotransformation of amiodaquine and prophylaxis of Plasmodium falciparum malaria

The authors have devised a specific HPLC method for amodiaquine assay which demonstrated that the drug disappeared rapidly from the blood of subjects under prophylaxis for malaria (10 mg/kg/week in a single oral dose). The main metabolite was identified as the monodesethyl derivative which is the only active form of the drug. The low erythrocytic levels of the metabolite, observed at day +7, might account for the failure in the prophylaxis of P. falciparum malaria with amodiaquine in a few cases. The in vitro activity of monodesethyl amodiaquine should be evaluated during the chemosensitivity tests and the chemoprophylaxis schedule, re-evaluated.     

Triclosan offers protection against blood stages of malaria by inhibiting enoyl-ACP reductase of Plasmodium falciparum

The antimicrobial biocide triclosan [5-chloro-2-(2,4-dichlorophenoxy)phenol] potently inhibits the growth of Plasmodium falciparum in vitro and, in a mouse model, Plasmodium berghei in vivo. Inhibition of [14C]acetate and [14C]malonyl-CoA incorporation into fatty acids in vivo and in vitro, respectively, by triclosan implicate FabI as its target. Here we demonstrate that the enoyl-ACP reductase purified from P. falciparum is triclosan sensitive. Also, we present the evidence for the existence of FabI gene in P. falciparum. We establish the existence of the de novo fatty acid biosynthetic pathway in this parasite, and identify a key enzyme of this pathway for the development of new antimalarials.     

Molecular genetics of drug resistance in Plasmodium falciparum malaria

Resistance to dihydro folate reductase inhibitors and resistance to chloroquine have been mapped to single genetic loci in Plasmodium falciparum. Specific point mutations in the dihydro folate reductase gene confer different degrees of resistance to two dihydro folate inhibitors, cycloguanil and pyrimethamine, depending on the positions of the mutations and the residues involved. The chloroquine resistance locus has been mapped to a 400 kilobase (kb) segment of chromosome 7 in a P. falciparum cross. Identification and characterization of genes within this segment should lead to an understanding of the rapid drug efflux mechanism responsible for chloroquine resistance.     

1H-NMR structures of the Plasmodium falciparum 1758 erythrocyte binding peptide analogues and protection against malaria

A conserved high activity erythrocyte binding peptide (HAEBP) derived from the 175-erythrocyte binding antigen (EBA-175), coded 1758, was synthesized and analyzed for antigenic and protective activities in Aotus monkeys, together with several of its analogues. Conformational analysis by 1H Nuclear Magnetic Resonance in TFE-solution was done for some of them, as well as the 1758 parent peptide. We show that the conserved 1758 HAEBP (being neither immunogenic nor protective) has an alpha helical structure, whilst its analogues contain beta-turn structures. The 13790 peptide (highly immunogenic and protective for some monkeys) shows a type I beta-turn structure distorted in psi(i + 1) psi(i + 2) angles, whilst immunogenic and non-protective (as well as the non-immunogenic and non-protective peptides) have type III' beta-turns. An understanding of native peptide's correlation with altered peptide three-dimensional structure and resulting immunogenicity and protective activity may lead to a more rational design of multi-antigenic, multi-stage P. falciparum subunit based malaria vaccines.     

Proteomics of the human malaria parasite Plasmodium falciparum

The lethal species of malaria parasite, Plasmodium falciparum, continues to exact a huge toll of mortality and morbidity, particularly in sub-Saharan Africa. Completion of the genome sequence of this organism and advances in proteomics and mass spectrometry have opened up unprecedented opportunities for understanding the complex biology of this parasite and how it responds to drug challenge and other interventions. This review describes recent progress that has been made in applying proteomics technology to this important pathogen and provides a look forward to likely future developments.     

Genetic mapping in the human malaria parasite Plasmodium falciparum

The Plasmodium falciparum genome sequence has boosted hopes for a new era of malaria research and for the application of comprehensive molecular knowledge to disease control, but formidable obstacles remain: approximately 60% of the predicted P. falciparum proteins have no known functions or homologues, and most life cycle stages of this haploid eukaryotic parasite are relatively intractable to cultivation and biochemical manipulation. Genetic mapping based on high-resolution maps saturated with single-nucleotide polymorphisms or microsatellites is now providing effective strategies for discovering candidate genes determining important parasite phenotypes. Here we review classical linkage studies using laboratory crosses and population associations that are now amenable to genome-wide approaches and are revealing multiple candidate genes involved in complex drug responses. Moreover, mapping by linkage disequilibrium is practicable in cases where chromosomal segments flanking drug-selected genes have been preserved in populations during relatively recent P. falciparum evolution. We discuss the advantages and limitations of these various genetic mapping strategies, results from which offer complementary insights to those emerging from gene knockout experiments and/or high-throughput genomic technologies.     

Helicases involved in splicing from malaria parasite Plasmodium falciparum

An interesting element of eukaryotic genomes is the large quantity of non-coding intervening sequences commonly known as introns, which regularly interrupt functional genes and therefore must be removed prior to the use of genetic information by the cell. After splicing, the mature RNA is exported from the nucleus to the cytoplasm. Any error in the process of recognition and removal of introns, or splicing, would lead to change in genetic message and thus has potentially catastrophic consequences. Thus splicing is a highly complex essential step in eukaryotic gene expression. It takes place in spliceosome, which is a dynamic RNA-protein complex made of snRNPs and non-snRNP proteins. The splicing process consists of following sequential steps: spliceosome formation, the first transesterification and second transesterification reactions, release of the mature mRNA and recycling of the snRNPs. The spliceosomal components produce a complex network of RNA-RNA, RNA-protein and protein-protein interactions and spliceosome experience remodeling during each splicing cycle. Helicases are essentially required at almost each step for resolution of RNA-RNA and/or RNA-protein interactions. RNA helicases share a highly conserved helicase domain which includes the motif DExD/H in the single letter amino acid code. This article will focus on members of the DExD/H-box proteins involved specially in splicing in the malaria parasite Plasmodium falciparum.