Plasmodium, human and Anopheles genomics and malaria

Propulsive cell entry diverts pathogens from immune degradation by remodeling the phagocytic synapse

Phagocytosis is a critical immune function for infection control and tissue homeostasis. This process is typically described as non-moving pathogens being internalized and degraded in phagolysosomes. For pathogens that evade immune degradation, the prevailing view is that virulence factors that biochemically disrupt the biogenesis of phagoslysosomes are required. In contrast, here we report that physical forces exerted by pathogens during cell entry divert them away from the canonical phagolysosomal degradation pathway, and this altered intracellular fate is determined at the time of phagocytic synapse formation. We used the eukaryotic parasite Toxoplasma gondii as a model because live Toxoplasma uses gliding motility to actively invade into host cells. To differentiate the effect of physical forces from that of virulence factors in phagocytosis, we developed a strategy that used magnetic forces to induce propulsive entry of inactivated Toxoplasma into macrophage cells. Experiments and computer simulations collectively reveal that large propulsive forces suppress productive activation of receptors by hindering their spatial segregation from phosphatases at the phagocytic synapse. Consequently, the inactivated parasites, instead of being degraded in phagolysosomes, are engulfed into vacuoles that fail to mature into degradative units, following an intracellular pathway strikingly similar to that of the live motile parasite. Using opsonized beads, we further confirmed that this mechanism is general, not specific to the parasite used. These results reveal previously unknown aspects of immune evasion by demonstrating how physical forces exerted during active cell entry, independent of virulence factors, can help pathogens circumvent phagolysosomal degradation.

A review of recent research on Theileria parva: Implications for the infection and treatment vaccination method for control of East Coast fever

The infection and treatment (ITM) live vaccination method for control of Theileria parva infection in cattle is increasingly being adopted, particularly in Maasai pastoralist systems. Several studies indicate positive impacts on human livelihoods. Importantly, the first detailed protocol for live vaccine production at scale has recently been published. However, quality control and delivery issues constrain vaccination sustainability and deployment. There is evidence that the distribution of T. parva is spreading from endemic areas in East Africa, North into Southern Sudan and West into Cameroon, probably as a result of anthropogenic movement of cattle. It has also recently been demonstrated that in Kenya, T. parva derived from cape buffalo can 'breakthrough' the immunity induced by ITM. However, in Tanzania, breakthrough has not been reported in areas where cattle co-graze with buffalo. It has been confirmed that buffalo in northern Uganda national parks are not infected with T. parva and R. appendiculatus appears to be absent, raising issues regarding vector distribution. Recently, there have been multiple field population genetic studies using variable number tandem repeat (VNTR) sequences and sequencing of antigen genes encoding targets of CD8+ T-cell responses. The VNTR markers generally reveal high levels of diversity. The antigen gene sequences present within the trivalent Muguga cocktail are relatively conserved among cattle transmissible T. parva populations. By contrast, greater genetic diversity is present in antigen genes from T. parva of buffalo origin. There is also evidence from several studies for transmission of components of stocks present within the Muguga cocktail, into field ticks and cattle following induction of a carrier state by immunization. In the short term, this may increase live vaccine effectiveness, through a more homogeneous challenge, but the long-term consequences are unknown.

Genetic approach towards a vaccine against malaria

Malaria is a major concern for international health authorities. Millions of people contract it every year in the world due to a parasite of the Plasmodium genus. Due to the complexity of the parasite biology and genetics, there is currently no vaccine against the disease. However, due to the great resistance both to the medicines and to the insecticides used to combat the disease, it has become essential to obtain a vaccine as the necessary tool to prevent transmission and eliminate the disease. The bibliometric data indicate that interest in vaccines has been growing steadily since the 1980s. But nowadays, a powerful tool is used: the Plasmodium genome. This allows us to improve the fight against the disease. Knowing the sequences of the genes that favor the appearance of drug resistance, or those that encode for proteins with greater antigenic response, is a tool that can become fundamental. This article reviews the state of the art on vaccines and genetics, in the fight against malaria, and analyzes the fixed photo that the worldwide research on the disease poses.

Identification of microRNAs expressed in the midgut of Aedes albopictus during dengue infection

BACKGROUND

The midgut is the first barrier to dengue virus (DENV) infections of mosquitoes and therefore is a major bottleneck for the subsequent development of vector competence. However, the molecular mechanisms responsible for this barrier are unknown.

RESULTS

We constructed three small RNA libraries from the midguts of adult Aedes albopictus females that had been fed on either sugar solution, an uninfected blood meal, or a blood meal infected with DENV-2, and112 conserved microRNAs represented by 173 miRNA sequences were identified, with 34 novel microRNAs predicted by Mireap, RNAfold and Sfold software. In addition, the expression of aal-miR-1174, aal-miR-2951 and aal-miR-956 was confirmed via stem-loop quantitative real-time PCR (qRT-PCR). Compared with microRNA expression profiles of mosquitoes that had ingested a regular blood meal, 43 microRNAs were upregulated and 4were downregulated in mosquitoes that had ingested a DENV-2-infected blood meal. Among the differentially expressed microRNAs, miR-1767, miR-276-3p, miR-4448 and miR-4728-5p were verified via stem-loop qRT-PCR.

CONCLUSIONS

Analyses indicated that the changing patterns in miRNA expression during DENV-2 infection were significant and varied at different time points post infection. Most miRNA were upregulated at 24 h but were downregulated at 48 h post DENV-2 intake. The aal-miR-4728-5p was chosen for an in vitro transient transfection assay, and the results show that this miRNA enhances DENV replication in C6/36 cells. This study provides the first information on microRNAs expressed in the midgut of Ae. albopictus and describes species-specific changes in their expression levels following infection by DENV-2.

Recent developments in drug discovery against the protozoal parasites Cryptosporidium and Toxoplasma

Apicomplexan parasites cause some of the most devastating human diseases, including malaria, toxoplasmosis, and cryptosporidiosis. New drug discovery is imperative in light of increased resistance. In this digest article, we briefly explore some of the recent and promising developments in new drug discovery against two apicomplexan parasites, Cryptosporidium and Toxoplasma.

The potential of anti-malarial compounds derived from African medicinal plants, part III: an in silico evaluation of drug metabolism and pharmacokinetics profiling

Background

Malaria is an endemic disease affecting many countries in Tropical regions. In the search for compound hits for the design and/or development of new drugs against the disease, many research teams have resorted to African medicinal plants in order to identify lead compounds. Three-dimensional molecular models were generated for anti-malarial compounds of African origin (from 'weakly' active to 'highly' active), which were identified from literature sources. Selected computed molecular descriptors related to absorption, distribution, metabolism, excretion and toxicity (ADMET) of the phytochemicals have been analysed and compared with those of known drugs in order to access the 'drug-likeness' of these compounds.

Results

In the present study, more than 500 anti-malarial compounds identified from 131 distinct medicinal plant species belonging to 44 plant families from the African flora have been considered. On the basis of Lipinski's 'Rule of Five', about 70% of the compounds were predicted to be orally bioavailable, while on the basis of Jorgensen's 'Rule of Three', a corresponding >80% were compliant. An overall drug-likeness parameter indicated that approximately 55% of the compounds could be potential leads for the development of drugs.

Conclusions

From the above analyses, it could be estimated that >50% of the compounds exhibiting anti-plasmodial/anti-malarial activities, derived from the African flora, could be starting points for drug discovery against malaria. The 3D models of the compounds have been included as an accompanying file and could be employed in virtual screening.

Electronic supplementary material

The online version of this article (doi:10.1186/s13588-014-0006-x) contains supplementary material, which is available to authorized users.

Medical entomology--back to the future?

Some of problems and challenges facing Medical/Veterinary Entomology are presented from my perspective, focusing on the current millennium. Topics include anthropogenic environmental changes created by population growth, administrative problems hindering science's response to these changes, and some of the scientific discoveries potentially providing solutions. As the title implies, many recent research discoveries have yet to be translated into major changes in control approaches for the major vectorborne public health problems, thereby providing an interesting mix of modern surveillance technology used to track problems and direct historical intervention solutions.

Malaria's many mates: past, present, and future of the systematics of the order Haemosporida

Malaria has been one of the most important diseases of humans throughout history and continues to be a major public health concern. The 5 species of Plasmodium that cause the disease in humans are part of the order Haemosporida, a diverse group of parasites that all have heteroxenous life cycles, alternating between a vertebrate host and a free-flying, blood-feeding dipteran vector. Traditionally, the identification and taxonomy of these parasites relied heavily on life-history characteristics, basic morphological features, and the host species infected. However, molecular approaches to resolving the phylogeny of the group have sometimes challenged many of these traditional hypotheses. One of the greatest debates has concerned the origin of the most virulent of the human-infecting parasites, Plasmodium falciparum, with early results suggesting a close relationship with an avian parasite. Subsequent phylogenetic studies placed it firmly within the mammalian clade instead, but the avian origin hypothesis has been revived with recent genome-based analyses. The rooting of the tree of Haemosporida has also been inconsistent, and the various topologies that result certainly affect our interpretation of the history of the group. There is clearly a pressing need to obtain a much more complete degree of taxon sampling of haemosporidians, as well as a greater number of characters before confidence can be placed in any hypothesis regarding the evolutionary history of the order. There are numerous challenges moving forward, particularly for generating complete genome sequences of avian and saurian parasites.

Assessing the genetic diversity of the vir genes in Indian Plasmodium vivax population

Variant surface antigens (VSAs) present on the surface of parasitized erythrocytes facilitate many Plasmodium spp. to escape the host immune system during infection. Multigene families coding for VSAs exist in several Plasmodium spp. and are located on telomeric and subtelomeric regions of the chromosomes. P. vivax genome also contains a multigene superfamily vir (variant interspersed repeats), present in the subtelomeric region with a possible role in immune evasion like the var gene in P. falciparum. Blood samples from 148 symptomatic malaria cases were collected from five different regions of India, viz. Mangalore, Rourkela, Goa, Delhi and Jabalpur. P. vivax isolates (74 single infections) were sequenced for four vir genes (viz. vir 27, vir 4, vir 12 and vir 21) and analyzed for the genetic variability existing in different populations of India. The results indicate that vir genes in different P. vivax populations in India are highly divergent both within and between the isolates. High levels of single nucleotide polymorphisms (SNPs) were observed attributing to the existing polymorphism for all the four vir genes studied across the population. Detailed knowledge of the genetic variation among the vir genes will help in understanding the evolutionary aspects of vir genes and may also provide basis for understanding the disease chronicity.

An in vivo transfection approach elucidates a role for Aedes aegypti thioester-containing proteins in flaviviral infection

Mosquitoes transmit pathogens that cause infectious diseases of global importance. Techniques to easily introduce genes into mosquitoes, however, limit investigations of the interaction between microbes and their arthropod vectors. We now show that a cationic liposome significantly enhances delivery and expression of plasmid DNA in Aedes aegypti and Anopheles gambiae mosquitoes. We then introduced the genes for Ae. aegypti thioester-containing proteins (AeTEPs), which are involved in the control of flaviviral infection, into mosquitoes using this technique. In vivo transfection of AeTEP-1 into Ae. aegypti significantly reduced dengue virus infection, suggesting that the approach can further our understanding of pathogen-mosquito interactions.

Evaluation of the genetic diversity of domain II of Plasmodium vivax Apical Membrane Antigen 1 (PvAMA-1) and the ensuing strain-specific immune responses in patients from Sri Lanka

Antigenic polymorphism displayed by malaria parasites is a skewed schema to escape the host immune system. The prevailing genetic diversity at domain II of the Plasmodium vivax Apical Membrane Antigen-1 (Pvama-1DII) was characterized in 64 single clone P. vivax isolates from Sri Lanka, where unstable malaria prevails with low intensity. In Sri Lanka, the Pvama-1DII gene showed meager meiotic recombination with the enclosure of single nucleotide polymorphisms (SNPs). Eleven amino acid (a.a.) variant positions defined 21 a.a. haplotypes with 9 unique to the island, where the predominant haplotype, H1, was identical to the reference Salvador I strain. A further 376 globally dispersed isolates defined 38 a.a. haplotypes (H22-H59), with 4 and 26 haplotypes exclusive to India and Thailand, respectively. The phylogenetic tree revealed no clustering, where most isolates had a very recent common origin. The polymorphism detected in PvAMA-1DII B and T cell epitopes evidenced an immune evasion mechanism exploited by the parasite. Majority of Sri Lankan patients developed antibody responses to both conformational and linear B cell epitopes. The ensuing strain-specific immunity due to extensive antigenic polymorphism was evaluated by aligning a.a. sequences of PvAMA-1DII with the homologous total (IgM+IgG) antibody responses assayed by in-house established indirect ELISAs against 7 PvAMA-1DII overlapping synthetic peptides, P01-P07. While the antibody responses to P01-P03, P06, P07 harbouring P. vivax clinical isolates with polymorphic a.a. haplotype to Sal I was clearly strain-transcending (cross-reactive), individuals with isolates identical to the Sal I strain observed varying antibody prevalence against the seven PvAMA-1DII Sal-I synthetic peptides, with the highest prevalence detected against P04. Synthetic peptide P04, spanning a.a. positions 302-324 of the PvAMA-1DII of the Sal I strain that included the epitope recognized by the invasion inhibitory 4G2 monoclonal antibody of PfAMA-1, was highly conserved in all 440 local and global P. vivax isolates examined. A functional role for this region is reinforced by the highly immunogenic nature of P04, and could point towards a presumably "protective" anti-P04 antibody response that elicited an isotype switch from IgM to IgG, with increasing exposure to malaria exclusively in endemic residents. Thus the conserved and seemingly "protective" nature of the domain II loop of PvAMA-1 makes it a putative contender to be included in a cocktail vaccine against P. vivax asexual erythrocytic stages in Sri Lanka.

Falcipain-2 inhibitors

Malaria, particularly that one caused by Plasmodium falciparum, remains a serious health problem in Africa, South America, and many parts of Asia where it afflicts about 500 million people and is responsible for the death of more than one million children each year. The main reasons for the persistence of malaria are the emergence of resistance to common antimalarial drugs, inadequate control of mosquito vectors, and the lack of effective vaccines. Therefore, the identification and characterization of new targets for antimalarial chemotherapy are of urgent priority. This review is focused on inhibitors of falcipain-2, a cysteine protease from P. falciparum, which represents one of the most promising targets for antimalarial drug design. Falcipain-2 is a key enzyme in the life cycle of P. falciparum since it degrades hemoglobin, at the early trophozoite stage, and cleaves ankyrin and protein 4.1, the cytoskeletal elements vital to the stability of red cell membrane, at the schizont stage. The main classes of falcipain-2 inhibitors are peptides or peptidomimetics bearing the most popular pharmacophores of cysteine protease inhibitors, such as vinyl sulfones, halomethyl ketones, and aldehydes. Furthermore, many other chemotypes have been identified as inhibitors of falcipain-2, such as isoquinolines, thiosemicarbazones, and chalcones. These inhibitors represent all classes, which, to the best of our knowledge, have been disclosed in journal articles to date.

Current developments in genomics and personalized health care: impact on public health

The knowledge gained from the characterization of genomes, especially the human genome, holds considerable potential for the development of new health care innovations for prevention, diagnosis, and management of many diseases in the coming decade. However, owing to the presence of highly complex scientific, economic, social, and ethical issues associated with this field, societies will need to be better prepared for the era of postgenomics and its consequences. It is important to ensure that the benefits of genomics are distributed fairly among all the countries of the world and that the well-tried and more conventional approaches to medical research and practice are not neglected while the medical potential of genomics is being explored. In this report, the author focuses mainly on human genomics, its applications, development of related technologies and issues related to the dissemination of knowledge derived from genome information, and finally, their impact on global health care.

Antimalarial peroxides

The problem of endemic malaria continues unabated globally. Malaria affects 40 % of the global population, causing an estimated annual mortality of 1.5-2.7 million people. The World Health Organization (WHO) estimates that 90 % of these deaths occur in sub-Saharan Africa among infants under the age of five. While a vaccine against malaria continues to be elusive, chemotherapy remains the most viable alternative towards treatment of the disease. During last years, the situation has become urgent in many ways, but mainly because of the development of chloroquine-resistant (CQR) strains of Plasmodium falciparum (Pf). The discovery that artemisinin (ART, 1), an active principle of Artemisia annua L., expresses a significant antimalarial activity, especially against CQR strains, opened new approaches for combating malaria. Since the early 1980s, hundreds of semi-synthetic and synthetic peroxides have been developed and tested for their antimalarial activity, the results of which were extensively reviewed. In addition, in therapeutic practice, there is no reported case of drug resistance to these antimalarial peroxides. This review summarizes recent achievements in the area of peroxide drug development for malaria chemotherapy.

Genomewide expression profiling of cryptolepine-induced toxicity in Saccharomyces cerevisiae

We have used the budding yeast Saccharomyces cerevisiae to identify genes that may confer sensitivity in vivo to the antimalarial and cytotoxic agent cryptolepine. Five S. cerevisiae strains, with different genetic backgrounds in cell permeability and DNA damage repair mechanisms, were exposed to several concentrations of cryptolepine. Cryptolepine showed a relatively mild toxicity for wild-type strains, which was augmented by either increasing cell permeability (Deltaerg6 or ISE2 strains) or disrupting DNA damage repair (Deltarad52 strains). These results are compatible with the ability of cryptolepine to intercalate into DNA and thus promote DNA lesions. The effects of low concentrations of cryptolepine (20% and 40% inhibitory concentrations [IC(20) and IC(40)]) were analyzed by comparing the gene expression profiles of treated and untreated Deltaerg6 yeast cells. Significant changes in expression levels were observed for 349 genes (117 upregulated and 232 downregulated). General stress-related genes constituted the only recognizable functional cluster whose expression was increased upon cryptolepine treatment, making up about 20% of upregulated genes. In contrast, analysis of the characteristics of downregulated genes revealed a specific effect of cryptolepine on genes related to iron transport or acid phosphatases, as well as a significant proportion of genes related to cell wall components. The effects of cryptolepine on the transcription of iron transport-related genes were consistent with a loss of function of the iron sensor Aft1p, indicating a possible disruption of iron metabolism in S. cerevisiae. Since the interference of cryptolepine with iron metabolism is considered one of its putative antimalarial targets, this finding supports the utility of S. cerevisiae in drug-developing schemes.

A novel GDP-dependent pyruvate kinase isozyme from Toxoplasma gondii localizes to both the apicoplast and the mitochondrion

We previously reported a cytosolic pyruvate kinase (EC 2.7.1.40) from Toxoplasma gondii (TgPyKI) that differs from most eukaryotic pyruvate kinases in being regulated by glucose 6-phosphate rather than fructose 1,6-diphosphate. Another putative pyruvate kinase (TgPyKII) was identified from parasite genome, which exhibits 32% amino acid sequence identity to TgPyKI and retains pyruvate kinase signature motifs and amino acids essential for substrate binding and catalysis. Whereas TgPyKI is most closely related to plant/algal enzymes, phylogenetic analysis suggests a proteobacterial origin for TgPyKII. Enzymatic characterization of recombinant TgPyKII shows a high pH optimum at 8.5, and a preference for GDP as a phosphate recipient. Catalytic activity is independent of K+, and no allosteric or regulatory effects were observed in the presence of fructose 1,6-diphosphate, fructose 2,6-diphosphate, glucose 6-phosphate, ribose 5-phosphate, AMP, or ATP. Unlike TgPyKI, native TgPyKII activity was exclusively associated with the membranous fraction of a T. gondii tachyzoite lysate. TgPyKII possesses a long N-terminal extension containing five putative start codons before the conserved region and localizes to both apicoplast and mitochondrion by immunofluorescence assay using native antibody and fluorescent protein fusion to the N-terminal extension. Further deletional and site-directed mutagenesis suggests that a translation product from 1st Met is responsible for the localization to the apicoplast, whereas one from 3rd Met is for the mitochondrion. This is the first study of a potential mitochondrial pyruvate kinase in any system.

Plasmepsins as potential targets for new antimalarial therapy

Malaria is one of the major diseases in the world. Due to the rapid spread of parasite resistance to available antimalarial drugs there is an urgent need for new antimalarials with novel mechanisms of action. Several promising targets for drug intervention have been revealed in recent years. This review addresses the parasitic aspartic proteases termed plasmepsins (Plms) that are involved in the hemoglobin catabolism that occurs during the erythrocytic stage of the malarial parasite life cycle. Four Plasmodium species are responsible for human malaria; P. vivax, P. ovale, P. malariae, and P. falciparum. This review focuses on inhibitors of the haemoglobin-degrading plasmepsins of the most lethal species, P. falciparum; Plm I, Plm II, Plm IV, and histo-aspartic protease (HAP). Previously, Plm II has attracted the most attention. With the identification and characterization of new plasmepsins and the results from recent plasmepsin knockout studies, it now seems clear that in order to achieve high-antiparasitic activities in P. falciparum-infected erythrocytes it is necessary to inhibit several of the haemoglobin-degrading plasmepsins. Herein we summarize the structure-activity relationships of the Plm I, II, IV, and HAP inhibitors. These inhibitors represent all classes which, to the best of our knowledge, have been disclosed in journal articles to date. The 3D structures of inhibitor/plasmepsin II complexes available in the protein data bank are briefly discussed and compared.

Nonmevalonate terpene biosynthesis enzymes as antiinfective drug targets: substrate synthesis and high-throughput screening methods

The nonmevalonate isoprenoid pathway is an established target for antiinfective drug development. This paper describes high-throughput methods for the screening of 2C-methyl-D-erythritol synthase (IspC protein), 4-diphosphocytidyl-2C-methyl-D-erythritol synthase (IspD protein), 4-diphosphocytidyl-2C-methyl-D-erythritol kinase (IspE protein), and 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (IspF protein) against large compound libraries. The assays use up to three auxiliary enzymes. They are all monitored photometrically at 340 nm and are robust as documented by Z-factors of >or=0.86. 13C NMR assays designed for hit verification via direct detection of the primary reaction product are also described. Enzyme-assisted methods for the preparation, on a multigram scale, of isoprenoid biosynthesis intermediates required as substrates for these assays are reported. Notably, these methods enable the introduction of single or multiple 13C labels as required for NMR-monitored assays. The preparation of 4-diphosphosphocytidyl-2C-methyl-D-erythritol 2-phosphate in multigram quantities is described for the first time.

Genetic linkage and association analyses for trait mapping in Plasmodium falciparum

Genetic studies of Plasmodium falciparum laboratory crosses and field isolates have produced valuable insights into determinants of drug responses, antigenic variation, disease virulence, cellular development and population structures of these virulent human malaria parasites. Full-genome sequences and high-resolution haplotype maps of SNPs and microsatellites are now available for all 14 parasite chromosomes. Rapidly increasing genetic and genomic information on Plasmodium parasites, mosquitoes and humans will combine as a rich resource for new advances in our understanding of malaria, its transmission and its manifestations of disease.

Amodiaquine failure associated with erythrocytic glutathione in Plasmodium falciparum malaria

OBJECTIVE

To establish the relationship between production of glutathione and the therapeutic response to amodiaquine (AQ) monotherapy in Plasmodium falciparum non-complicated malaria patients.

METHODOLOGY

Therapeutic response to AQ was evaluated in 32 patients with falciparum malaria in two townships of Antioquia, Colombia, and followed-up for 28 days. For every patient, total glutathione and enzymatic activity (glutathione reductase, GR, and gamma-glutamylcysteine synthetase, gamma-GCS) were determined in parasitized erythrocytes, non-infected erythrocytes and free parasites, on the starting day (day zero, before ingestion of AQ) and on the day of failure (in case of occurrence).

RESULTS

There was found an AQ failure of 31.25%. Independent of the therapeutic response, on the starting day and on the day of failure, lower total glutathione concentration and higher GR activities in parasitized erythrocytes were found, compared with non-infected erythrocytes (p < 0.003). In addition, only on the day of failure, gamma-GCS activity of parasitized erythrocytes was higher, compared with that of healthy erythrocytes (p = 0.01). Parasitized and non-parasitized erythrocytes in therapeutic failure patients (TF) had higher total glutathione on the starting day compared with those of adequate clinical response (ACR) (p < 0.02). Parasitized erythrocytes of TF patients showed lower total glutathione on the failure day, compared with starting day (p = 0.017). No differences was seen in the GR and gamma-GCS activities by compartment, neither between the two therapeutic response groups nor between the two treatment days.

CONCLUSION

This study is a first approach to explaining P. falciparum therapeutic failure in humans through differences in glutathione metabolism in TF and ACR patients. These results suggest a role for glutathione in the therapeutic failure to antimalarials.

The co-evolution of people, plants, and parasites: biological and cultural adaptations to malaria

The urgency generated by drug-resistant strains of malaria has accelerated anti-malarial drug research over the last two decades. While synthetic pharmaceutical agents continue to dominate research, attention increasingly has been directed to natural products. The present paper explores the larger context in which plant use occurs and considers how the selection of medicinal plants has evolved over millennia as part of the larger human effort to mediate illness. First attention is directed to indigenous medicinal plants whose anti-malarial activity is based on an oxidant mode of action, by which intracellular constituents lose electrons (become more electropositive). Next, parallels are drawn between these plant substances and a suite of malaria-protective genetic traits: glucose-6-phosphate dehydrogenase deficiency; haemoglobins S, C and E; α- and β-thalassemias. These erythrocyte anomalies are classic examples of Darwinian evolution, occurring in high frequency in populations who have experienced considerable selective pressure from malaria. Characterized by discrete loci and pathophysiologies, they are united through the phenomenon of increased erythrocyte oxidation. In this model, then, oxidant anti-malarial plants are culturally constructed analogues, and molecular mimics, of these genetic adaptations. To further reinforce the scheme, it is noted that the anti-malarial action of pharmaceutical agents such as chloroquine and mefloquine duplicates both the genetic anomalies and the folk therapeutic models based in oxidant plants. This discussion coheres around a theoretical foundation that relates plant secondary metabolites (oxidants) to plasmodial biochemistry and human biological and cultural adaptations to malaria. Co-evolution provides a theoretical link that illuminates how medical cultures manage the relationships among humans, plants, herbivores and their respective pathogens.

Assembling a global database of malaria parasite prevalence for the Malaria Atlas Project

BACKGROUND

Open access to databases of information generated by the research community can synergize individual efforts and are epitomized by the genome mapping projects. Open source models for outputs of scientific research funded by tax-payers and charities are becoming the norm. This has yet to be extended to malaria epidemiology and control.

METHODS

The exhaustive searches and assembly process for a global database of malaria parasite prevalence as part of the Malaria Atlas Project (MAP) are described. The different data sources visited and how productive these were in terms of availability of parasite rate (PR) data are presented, followed by a description of the methods used to assemble a relational database and an associated geographic information system. The challenges facing spatial data assembly from varied sources are described in an effort to help inform similar future applications.

RESULTS

At the time of writing, the MAP database held 3,351 spatially independent PR estimates from community surveys conducted since 1985. These include 3,036 Plasmodium falciparum and 1,347 Plasmodium vivax estimates in 74 countries derived from 671 primary sources. More than half of these data represent malaria prevalence after the year 2000.

CONCLUSION

This database will help refine maps of the global spatial limits of malaria and be the foundation for the development of global malaria endemicity models as part of MAP. A widespread application of these maps is envisaged. The data compiled and the products generated by MAP are planned to be released in June 2009 to facilitate a more informed approach to global malaria control.

Structure-based design of peptides that self-assemble into regular polyhedral nanoparticles

Artificial particulate systems such as polymeric beads and liposomes are being applied in drug delivery, drug targeting, antigen display, vaccination, and other technologies. Here we used computer modeling to design a novel type of nanoparticles composed of peptides as building blocks. We verified the computer models via solid-phase peptide synthesis and biophysical analyses. We describe the structure-based design of a novel type of nanoparticles with regular polyhedral symmetry and a diameter of about 16 nm, which self-assembles from single polypeptide chains. Each peptide chain is composed of two coiled coil oligomerization domains with different oligomerization states joined by a short linker segment. In aqueous solution the peptides form nanoparticles of about 16 nm diameter. Such peptide nanoparticles are ideally suited for medical applications such as drug targeting and drug delivery systems, such as imaging devices, or they may be used for repetitive antigen display.

InVitro and inVivo Antimalarial Activity of Derivatives of 1,10-Phenanthroline Framework

A series of trisubstituted 1,10-phenanthrolines was prepared. These compounds exhibited mild to high biological activities in vitro both toward chloroquino-resistant FcB1-Columbia and FcM29-Cameron strains and Nigerian chloroquino-sensitive strain of Plasmodium falciparum. Cytotoxicity of the most active compounds was estimated showing that one compound (10) exhibited a selective activity against malaria parasite (selectivity indexes of 52 and 144). Antiplasmodial activity of this derivative was optimized by N-10 alkylation and the phenanthrolinium salt (15) submitted to an in vivo study using mice infected by P. vinckei petteri showing an ED50 of 7.86 mg/kg/day.

Light deprivation affects larval development and arrestin gene expression in Anopheles stephensi

The role of light exposure on the final stages of development of Anopheles stephensi larvae to pupae and adult mosquitoes was explored. We demonstrated a significant reduction in the development of adult mosquitoes when larvae were bred in the absence of light compared with the control group bred in alternating 12 h of light and 12 h of dark. To correlate these findings at the molecular level, RNA levels of the visual arrestin gene were examined. Arrestins are an important gene family that play a role in the vectorial capacity of Anopheles and mediate neurotransmission as well as olfactory and visual sensory reception in insects. Semiquantitative polymerase chain reaction showed a reduction in the expression of the visual arrestin gene in pupae that developed from larvae in the absence of light compared with larvae bred under normal conditions.

Isoprenoid biosynthetic pathways as anti-infective drug targets

IPP (isopentenyl diphosphate) and DMAPP (dimethylallyl diphosphate) serve as the universal precursors for the biosynthesis of isoprenoids. Besides the well-known mevalonate pathway, the existence of a second biosynthetic pathway conducive to IPP and DMAPP formation through 1-deoxy-D-xylulose 5-phosphate and 2C-methyl-D-erythritol 4-phosphate was discovered approx. 10 years ago in plants and certain eubacteria. It is now known that this pathway is widely distributed in the bacterial kingdom including major human pathogens, such as Mycobacterium tuberculosis and Helicobacter pylori. The pathway is also essential in the malaria vector Plasmodium falciparum. During the last few years, the genes, enzymes, intermediates and mechanisms of the biosynthetic route have been elucidated by a combination of comparative genomics, enzymology, advanced NMR technology and crystallography. The results provide the basis for the development of novel anti-infective drugs.

Identifying Plasmodium falciparum cytoadherence-linked asexual protein 3 (CLAG 3) sequences that specifically bind to C32 cells and erythrocytes

Adhesion of mature asexual stage Plasmodium falciparum parasite-infected erythrocytes (iRBC) to the vascular endothelium is a critical event in the pathology of Plasmodium falciparum malaria. It has been suggested that the clag gene family is essential in cytoadherence to endothelial receptors. Primers used in PCR and RT-PCR assays allowed us to determine that the gene encoding CLAG 3 (GenBank accession no. NP_473155) is transcribed in the Plasmodium falciparum FCB2 strain. Western blot showed that antisera produced against polymerized synthetic peptides from this protein recognized a 142-kDa band in P. falciparum schizont lysate. Seventy-one 20-amino-acid-long nonoverlapping peptides, spanning the CLAG 3 (cytoadherence-linked asexual protein on chromosome 3) sequence were tested in C32 cell and erythrocyte binding assays. Twelve CLAG peptides specifically bound to C32 cells (which mainly express CD36) with high affinity, hereafter referred to as high-affinity binding peptides (HABPs). Five of them also bound to erythrocytes. HABP binding to C32 cells and erythrocytes was independent of peptide charge or peptide structure. Affinity constants were between 100 nM and 800 nM. Cross-linking and SDS-PAGE analysis allowed two erythrocyte binding proteins of around 26 kDa and 59 kDa to be identified, while proteins of around 53 kDa were identified as possible receptor sites for C-32 cells. The HABPs' role in Plasmodium falciparum invasion inhibition was determined. Such an approach analyzing various CLAG 3 regions may elucidate their functions and may help in the search for new antigens important for developing antimalarial vaccines.

Cachexia in malaria and heart failure: therapeutic considerations in clinical practice

Cachexia is an independent prognostic marker of survival in many chronic diseases including heart failure and malaria. Morbidity and mortality from malaria is high in most of the third world where it presents a very challenging public health problem. Malaria may present in the UK as fever in the returning traveller or as fever in overseas visitors. How and why cachexia develops in malaria in a manner similar to the cachexia of chronic heart failure and the treatment strategies that would alter outcomes in both diseases are discussed in this review.

Cross-talk between nitric oxide and transforming growth factor-beta1 in malaria

Malaria has re-emerged as a global health problem, leading to an increased focus on the cellular and molecular biology of the mosquito Anopheles and the parasite Plasmodium with the goal of identifying novel points of intervention in the parasite life cycle. Anti-parasite defenses mounted by both mammalian hosts and Anopheles can suppress the growth of Plasmodium. Nonetheless, the parasite is able to escape complete elimination in vivo, perhaps by thwarting or co-opting these mechanisms for its own survival, as do numerous other pathogens. Among the defense systems used by the mammalian host against Plasmodium is the synthesis of nitric oxide (NO), catalyzed by an inducible NO synthase (iNOS). Nitric oxide produced by the action of an inducible Anopheles stephensi NO synthase (AsNOS) may be central to the anti-parasitic arsenal of this mosquito. In mammals, iNOS can be modulated by members of the transforming growth factor-beta (TGF-beta) cytokine superfamily. Transforming growth factor-beta is produced as an inactive precursor that is activated following dissociation of certain inhibitory proteins, a process that can be promoted by reaction products of NO as well as by hemin. Ingestion by Anopheles of blood containing Plasmodium initiates parasite development, blood digestion which results in the accumulation of hematin (hemin) in the insect midgut, and induction of both AsNOS and TGF-beta-like (As60A) gene expression in the midgut epithelium. Active mammalian TGF-beta1 can be detected in the A. stephensi midgut up to 48h post-ingestion and latent TGF-beta1 can be activated by midgut components in vitro, a process that is potentiated by NO and that may involve hematin. Further, mammalian TGF-beta1 is perceived as a cytokine by A. stephensi cells in vitro and can alter Plasmodium development in vivo. Bloodfeeding by Anopheles, therefore, results in a juxtaposition of evolutionarily conserved mosquito and mammalian TGF-beta superfamily homologs that may influence transmission dynamics of Plasmodium in endemic regions.

The unusually large Plasmodium telomerase reverse-transcriptase localizes in a discrete compartment associated with the nucleolus

Telomerase replicates chromosome ends, a function necessary for maintaining genome integrity. We have identified the gene that encodes the catalytic reverse transcriptase (RT) component of this enzyme in the malaria parasite Plasmodium falciparum (PfTERT) as well as the orthologous genes from two rodent and one simian malaria species. PfTERT is predicted to encode a basic protein that contains the major sequence motifs previously identified in known telomerase RTs (TERTs). At ∼2500 amino acids, PfTERT is three times larger than other characterized TERTs. We observed remarkable sequence diversity between TERT proteins of different Plasmodial species, with conserved domains alternating with hypervariable regions. Immunofluorescence analysis revealed that PfTERT is expressed in asexual blood stage parasites that have begun DNA synthesis. Surprisingly, rather than at telomere clusters, PfTERT typically localizes into a discrete nuclear compartment. We further demonstrate that this compartment is associated with the nucleolus, hereby defined for the first time in P.falciparum.

Detection of four Plasmodium species in blood from humans by 18S rRNA gene subunit-based and species-specific real-time PCR assays

There have been reports of increasing numbers of cases of malaria among migrants and travelers. Although microscopic examination of blood smears remains the "gold standard" in diagnosis, this method suffers from insufficient sensitivity and requires considerable expertise. To improve diagnosis, a multiplex real-time PCR was developed. One set of generic primers targeting a highly conserved region of the 18S rRNA gene of the genus Plasmodium was designed; the primer set was polymorphic enough internally to design four species-specific probes for P. falciparum, P. vivax, P. malarie, and P. ovale. Real-time PCR with species-specific probes detected one plasmid copy of P. falciparum, P. vivax, P. malariae, and P. ovale specifically. The same sensitivity was achieved for all species with real-time PCR with the 18S screening probe. Ninety-seven blood samples were investigated. For 66 of them (60 patients), microscopy and real-time PCR results were compared and had a crude agreement of 86% for the detection of plasmodia. Discordant results were reevaluated with clinical, molecular, and sequencing data to resolve them. All nine discordances between 18S screening PCR and microscopy were resolved in favor of the molecular method, as were eight of nine discordances at the species level for the species-specific PCR among the 31 samples positive by both methods. The other 31 blood samples were tested to monitor the antimalaria treatment in seven patients. The number of parasites measured by real-time PCR fell rapidly for six out of seven patients in parallel to parasitemia determined microscopically. This suggests a role of quantitative PCR for the monitoring of patients receiving antimalaria therapy.

Enhanced expression of a recombinant malaria candidate vaccine in Escherichia coli by codon optimization

This study was conducted to compare the expression of three constructs of a multistage candidate vaccine (FALVAC-1) against Plasmodium falciparum in an Escherichia coli system: a synthetic gene with P. falciparum codons, a synthetic gene with optimized E. coli codons, and a synthetic gene with P. falciparum codons co-transformed with a RIG plasmid, which encodes three tRNAs (AG(A/G), ATA, GGA) that recognize rare E. coli codons. The expression of the protein increased at least threefold with codon optimization. The presence of the RIG plasmid in the co-transforming cells did not significantly increase the expression level of the gene with P. falciparum codons. The growth of cells transformed by the construct with P. falciparum codons was significantly slower than that of cells transformed by the construct with optimized E. coli codons after induction of protein expression with IPTG. The cells containing the non-codon optimized gene co-expressed with RIG plasmid had the slowest growth at all time points in culture. Thus, codon optimization significantly increases the yield of P. falciparum candidate vaccines in the E. coli expression system.

Perspectives in anti-infective drug design. The late steps in the biosynthesis of the universal terpenoid precursors, isopentenyl diphosphate and dimethylallyl diphosphate

A mevalonate-independent pathway for the biosynthesis of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) that has been elucidated during the last decade is essential in plants, many eubacteria and apicomplexan parasites, but is absent in Archaea and animals. The enzymes of the pathway are potential targets for the development of novel antibiotic, antimalarial and herbicidal agents. This review is focused on the late steps of this pathway. The intermediate 2C-methyl-D-erythritol 2,4-cyclodiphosphate is converted into IPP and DMAPP via 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate by the consecutive action of the iron-sulfur proteins IspG and IspH. IPP and DMAPP can be interconverted by IPP isomerase which is essential in microorganisms using the mevalonate pathway, whereas its presence is optional in microorganisms using the non-mevalonate pathway. A hitherto unknown family of IPP isomerases using FMN as coenzyme has been discovered recently in Archaea and certain eubacteria.

High-throughput generation of P. falciparum functional molecules by recombinational cloning

Large-scale functional genomics studies for malaria vaccine and drug development will depend on the generation of molecular tools to study protein expression. We examined the feasibility of a high-throughput cloning approach using the Gateway system to create a large set of expression clones encoding Plasmodium falciparum single-exon genes. Master clones and their ORFs were transferred en masse to multiple expression vectors. Target genes (n = 303) were selected using specific sets of criteria, including stage expression and secondary structure. Upon screening four colonies per capture reaction, we achieved 84% cloning efficiency. The genes were subcloned in parallel into three expression vectors: a DNA vaccine vector and two protein expression vectors. These transfers yielded a 100% success rate without any observed recombination based on single colony screening. The functional expression of 95 genes was evaluated in mice with DNA vaccine constructs to generate antibody against various stages of the parasite. From these, 19 induced antibody titers against the erythrocytic stages and three against sporozoite stages. We have overcome the potential limitation of producing large P. falciparum clone sets in multiple expression vectors. This approach represents a powerful technique for the production of molecular reagents for genome-wide functional analysis of the P. falciparum genome and will provide for a resource for the malaria resource community distributed through public repositories.

Transforming growth factor-βs and related gene products in mosquito vectors of human malaria parasites: signaling architecture for immunological crosstalk

The participation of a divergent mosquito transforming growth factor-beta (TGF-beta) and mammalian TGF-beta1 in the Anopheles stephensi response to malaria parasite development [Infect. Genet. Evol. 1 (2001) 131-141; Infect. Immun. 71 (2003) 3000-3009] suggests that a network of Anopheles TGF-beta ligands and signaling pathways figure prominently in immune defense of this important vector group. To provide a basis for identifying the roles of these proteins in Anopheles innate immunity, we identified six predicted TGF-beta ligand-encoding genes in the Anopheles gambiae genome, including two expressed, diverged copies of 60A, the first evidence of ligand gene duplication outside of chordates. In addition to five predicted type I and II receptors, we identified three Smad genes in the A. gambiae genome that would be predicted to support both TGF-beta/Activin- and bone morphogenetic protein (BMP)-like signaling. All three Smad genes are expressed in an immunocompetent A. stephensi cell line and in the A. stephensi midgut epithelium, confirming that a conserved signaling architecture is in place to support signaling by divergent exogenous and endogenous TGF-beta superfamily proteins.

Stereoselectivity in the pharmacodynamics and pharmacokinetics of the chiral antimalarial drugs

Several of the antimalarial drugs are chiral and administered as the racemate. These drugs include chloroquine, hydroxychloroquine, quinacrine, primaquine, mefloquine, halofantrine, lumefantrine and tafenoquine. Quinine and quinidine are also stereoisomers, although they are given separately rather than in combination. From the perspective of antimalarial activity, most of these agents demonstrate little stereoselectivity in their effects in vitro. Mefloquine, on the other hand, displays in vitro stereoselectivity against some strains of P. falciparum, with a eudismic ratio of almost 2 : 1 in favour of the (+)-enantiomer. Additionally, for some of these agents (e.g. halofantrine, primaquine, chloroquine), stereoselectivity has been noted in the ability of the enantiomers to cause certain adverse effects. In recent years, stereospecific analytical methods capable of measuring the individual enantiomers after the administration of racemic drugs have been reported for a number of chiral antimalarial drugs. These assays have revealed that almost all the studied antimalarial drugs display stereoselectivity in their pharmacokinetics, leading to enantioselectivity in their plasma concentrations. Whereas the oral absorption of these agents appears to be non-stereoselective, stereoselectivity is often seen in their volume of distribution and/or clearance. With regard to distribution, plasma protein binding of some chiral antimalarial drugs exhibits a significant degree of stereoselectivity, leading to stereoselective distribution to blood cells and other tissues. Because of their low hepatic extraction ratios, stereoselective plasma protein binding also contributes to the stereoselectivity in the metabolism of these drugs. Chiral metabolites are formed from some parent antimalarial drugs, although stereoselective aspects of the pharmacokinetics of the metabolites are not well understood. It is concluded that knowledge of the stereoselective aspects of these agents may be helpful in better understanding their mechanisms of action and possibly optimising their clinical safety and/or effectiveness.

Parasite-specific antibody and cytokine profiles in newborns from Plasmodium falciparum and Entamoeba histolytica/dispar-infected mothers

Passage of parasites and their antigens across the placenta occurs with metazoan as well as protozoan parasites, and this study addressed to which extent exposure to and infection of mothers with Plasmodium spp. and Entamoeba histolytica/dispar has sensitized their offspring for parasite-specific immune responses. While at delivery none of the mothers presented with an acute malaria attack, 42% were seropositive for P. falciparum. In half of the mothers cysts of E. histolytica/dispar were detected in stool specimen, 51% of them were found seropositive for E. histolytica, and E. histolytica-specific immunoglobulin A (IgA) responses were detected in neonates of seropositive mothers as well. Umbilical cord blood cells (UCBC) from neonates, when activated with the mitogen phytohaemagglutinine (PHA) and bacterial streptolysin O (SL-O), released significantly less interferon (IFN)-gamma, interleukin (IL)-10 and tumor necrosis factor (TNF)-alpha into cell culture supernatants than peripheral blood cells (PBMC) of mothers. In response to Plasmodium- and Entamoeba-specific antigens UCBC and PBMC produced equal amounts of IL-1beta, TNF-alpha, IFN-gamma and IL-5, but PBMC from mothers secreted significantly more IL-10. Parasite-specific production of inflammatory and Th(1)- and Th(2)-type cytokines was similar in newborns of Plasmodium and Entamoeba seropositive and seronegative mothers. In summary, repeated exposure and subclinical infection of mothers with E. histolytica or P. falciparum will suffice to prime in utero their children for inflammatory and both Th(1)- and Th(2)-type cytokine responses, and such broad and mixed cytokine spectrum may be of advantage upon secondary parasite challenge in later life.

Trematodes and snails: an intimate association

Trematode parasites share an intimate relationship with their gastropod intermediate hosts, which act as the vehicle for their development and transmission. They represent an enormous economic and medical burden in developing countries, stimulating much study of snail–trematode interactions. Laboratory-maintained snail–trematode systems and in vitro cell cultures are being used to investigate the molecular dialogue between host and parasite. These dynamic and finely balanced antagonistic relationships, in which parasites strongly influence the physiology of the host, are highly specific and may occasionally demonstrate co-speciation. We consider the mechanisms and responses deployed by trematodes and snails that result in compatibility or rejection of the parasite, and the macroevolutionary implications that they may effect. Although for gastropods the fossil record gives some insight into evolutionary history, elucidation of trematode evolution must rely largely upon molecular approaches, and for both, such techniques have provided fresh and often surprising evidence of their origins and dispersal over time. Co-evolution of snails and trematodes is becoming increasingly apparent at both cellular and population levels; the implications of which are only beginning to be understood for disease control. Untangling the complex interactions of trematodes and snails promise fresh opportunities for intervention to relieve the burden of parasitic disease.

Genome Sequencing and Comparative Genomics of Tropical Disease Pathogens

The sequencing of eukaryotic genomes has lagged behind sequencing of organisms in the other domains of life, archae and bacteria, primarily due to their greater size and complexity. With recent advances in high-throughput technologies such as robotics and improved computational resources, the number of eukaryotic genome sequencing projects has increased significantly. Among these are a number of sequencing projects of tropical pathogens of medical and veterinary importance, many of which are responsible for causing widespread morbidity and mortality in peoples of developing countries. Uncovering the complete gene complement of these organisms is proving to be of immense value in the development of novel methods of parasite control, such as antiparasitic drugs and vaccines, as well as the development of new diagnostic tools. Combining pathogen genome sequences with the host and vector genome sequences is promising to be a robust method for the identification of host-pathogen interactions. Finally, comparative sequencing of related species, especially of organisms used as model systems in the study of the disease, is beginning to realize its potential in the identification of genes, and the evolutionary forces that shape the genes, that are involved in evasion of the host immune response.

Analysis of the Plasmodium and Anopheles transcriptional repertoire during ookinete development and midgut invasion

Plasmodium, the causative agent of malaria, has to undergo sexual differentiation and development in anopheline mosquitoes for transmission to occur. To isolate genes specifically induced in both organisms during the early stages of Plasmodium differentiation in the mosquito, two cDNA libraries were constructed, one enriched for sequences expressed in differentiating Plasmodium berghei ookinetes and another enriched for sequences expressed in Anopheles stephensi guts containing invading ookinetes and early oocysts. Sequencing of 457 ookinete library clones and 652 early oocyst clones represented 175 and 346 unique expressed sequence tags, respectively. Nine of 13 Plasmodium and four of the five Anopheles novel expressed sequence tags analyzed on Northern blots were induced during ookinete differentiation and mosquito gut invasion. Ancaspase-7, an Anopheles effector caspase, is proteolytically activated during Plasmodium invasion of the midgut. WARP, a gene encoding a Plasmodium surface protein with a von Willebrand factor A-like adhesive domain, is expressed only in ookinetes and early oocysts. An anti-WARP polyclonal antibody strongly inhibits (70-92%) Plasmodium development in the mosquito, making it a candidate antigen for transmission blocking vaccines. The present results and those of an accompanying report (Srinivasan, P., Abraham, E. G., Ghosh, A. K., Valenzuela, J., Ribeiro, J. M. C., Dimopoulos G., Kafatos, F. C., Adams, J. H., and Jacobs-Lorena, M. (2004) J. Biol. Chem. 279, 5581-5587) provide the foundation for further analysis of Plasmodium differentiation in the mosquito and of mosquito responses to the parasite.