Arm-specific dynamics of chromosome evolution in malaria mosquitoes

Background  

The malaria mosquito species of subgenus Cellia have rich inversion polymorphisms that correlate with environmental variables. Polymorphic inversions tend to cluster on the chromosomal arms 2R and 2L but not on X, 3R and 3L in Anopheles gambiae and homologous arms in other species. However, it is unknown whether polymorphic inversions on homologous chromosomal arms of distantly related species from subgenus Cellia nonrandomly share similar sets of genes. It is also unclear if the evolutionary breakage of inversion-poor chromosomal arms is under constraints.     

Mosquito immune responses and compatibility between Plasmodium parasites and anopheline mosquitoes

Background  

Functional screens based on dsRNA-mediated gene silencing identified several Anopheles gambiae genes that limit Plasmodium berghei infection. However, some of the genes identified in these screens have no effect on the human malaria parasite Plasmodium falciparum; raising the question of whether different mosquito effector genes mediate anti-parasitic responses to different Plasmodium species.     

The molecular evolution of four anti-malarial immune genes in the <Emphasis Type="Italic">Anopheles gambiae</Emphasis> species complex

Background  

If the insect innate immune system is to be used as a potential blocking step in transmission of malaria, then it will require targeting one or a few genes with highest relevance and ease of manipulation. The problem is to identify and manipulate those of most importance to malaria infection without the risk of decreasing the mosquito's ability to stave off infections by microbes in general. Molecular evolution methodologies and concepts can help identify such genes. Within the setting of a comparative molecular population genetic and phylogenetic framework, involving six species of the Anopheles gambiae complex, we investigated whether a set of four pre-selected immunity genes (gambicin, NOS, Rel2 and FBN9) might have evolved under selection pressure imposed by the malaria parasite.     

Construction of a Plasmodium falciparum Rab-interactome identifies CK1 and PKA as Rab-effector kinases in malaria parasites

Background information

The pathology causing stages of the human malaria parasite Plasmodium falciparum reside within red blood cells that are devoid of any regulated transport system. The parasite, therefore, is entirely responsible for mediating vesicular transport within itself and in the infected erythrocyte cytoplasm, and it does so in part via its family of 11 Rab GTPases. Putative functions have been ascribed to Plasmodium Rabs due to their homology with Rabs of yeast, particularly with Saccharomyces that has an equivalent number of rab/ypt genes and where analyses of Ypt function is well characterized.

Results

Rabs are important regulators of vesicular traffic due to their capacity to recruit specific effectors. In order to identify P. falciparum Rab (PfRab) effectors, we first built a Ypt‐interactome by exploiting genetic and physical binding data available at the Saccharomyces genome database (SGD). We then constructed a PfRab‐interactome using putative parasite Rab‐effectors identified by homology to Ypt‐effectors. We demonstrate its potential by wet‐bench testing three predictions; that casein kinase‐1 (PfCK1) is a specific Rab5B interacting protein and that the catalytic subunit of cAMP‐dependent protein kinase A (PfPKA‐C) is a PfRab5A and PfRab7 effector.

Conclusions

The establishment of a shared set of physical Ypt/PfRab‐effector proteins sheds light on a core set Plasmodium Rab‐interactants shared with yeast. The PfRab‐interactome should benefit vesicular trafficking studies in malaria parasites. The recruitment of PfCK1 to PfRab5B+ and PfPKA‐C to PfRab5A+ and PfRab7+ vesicles, respectively, suggests that PfRab‐recruited kinases potentially play a role in early and late endosome function in malaria parasites.     

Mitochondrial genes support a common origin of rodent malaria parasites and Plasmodium falciparum's relatives infecting great apes

Background  

Plasmodium falciparum is responsible for the most acute form of human malaria. Most recent studies demonstrate that it belongs to a monophyletic lineage specialized in the infection of great ape hosts. Several other Plasmodium species cause human malaria. They all belong to another distinct lineage of parasites which infect a wider range of primate species. All known mammalian malaria parasites appear to be monophyletic. Their clade includes the two previous distinct lineages of parasites of primates and great apes, one lineage of rodent parasites, and presumably Hepatocystis species. Plasmodium falciparum and great ape parasites are commonly thought to be the sister-group of all other mammal-infecting malaria parasites. However, some studies supported contradictory origins and found parasites of great apes to be closer to those of rodents, or to those of other primates.     

Adaptation of a South American malaria vector to laboratory colonization suggests faster-male evolution for mating ability

Background  

Anopheles (Nyssorhynchus) albitarsis (Diptera: Culicidae) is one of the very few South American mosquito vectors of malaria successfully colonized in the laboratory. These vectors are very hard to breed because they rarely mate in artificial conditions. A few years ago a free-mating laboratory colony of An. albitarsis sensu stricto was established after about 30 generations of artificial-mating. To begin to understand the process of adaptation of these malaria vectors to the laboratory we have compared the insemination rates of colony mosquitoes to those from the original population in both artificial and free-mating crosses. We also carried out crossing experiments between the two types of mosquitoes for a preliminary analysis of the genetic basis of such adaptation.     

Molecular evolution of the three short PGRPs of the malaria vectors <Emphasis Type="Italic">An</Emphasis>opheles <Emphasis Type="Italic">gambiae</Emphasis> and <Emphasis Type="Italic">Anopheles arabiensis</Emphasis>in East Africa

Background  

Immune responses to parasites, which start with pathogen recognition, play a decisive role in the control of the infection in mosquitoes. Peptidoglycan recognition proteins (PGRPs) are an important family of pattern recognition receptors that are involved in the activation of these immune reactions. Pathogen pressure can exert adaptive changes in host genes that are crucial components of the vector's defence. The aim of this study was to determine the molecular evolution of the three short PGRPs (PGRP-S1, PGRP-S2 and PGRP-S3) in the two main African malaria vectors - Anopheles gambiae and Anopheles arabiensis.     

The unexpected importance of mosquito oviposition behaviour for malaria: non-productive larval habitats can be sources for malaria transmission

Background

Mosquitoes commute between blood-meal hosts and water. Thus, heterogeneity in human biting reflects underlying spatial heterogeneity in the distribution and suitability of larval habitat as well as inherent differences in the attractiveness, suitability and distribution of blood-meal hosts. One of the possible strategies of malaria control is to identify local vector species and then attack water bodies that contain their larvae.

Methods

Biting and host seeking, not oviposition, have been the focus of most previous studies of mosquitoes and malaria transmission. This study presents a mathematical model that incorporates mosquito oviposition behaviour.

Results

The model demonstrates that oviposition is one potential factor explaining heterogeneous biting and vector distribution in a landscape with a heterogeneous distribution of larval habitat. Adult female mosquitoes tend to aggregate around places where they oviposit, thereby increasing the risk of malaria, regardless of the suitability of the habitat for larval development. Thus, a water body may be unsuitable for adult mosquito emergence, but simultaneously, be a source for human malaria.

Conclusion

Larval density may be a misleading indicator of a habitat's importance for malaria control. Even if mosquitoes could be lured to oviposit in sprayed larval habitats, this would not necessarily mitigate – and might aggravate – the risk of malaria transmission. Forcing mosquitoes to fly away from humans in search of larval habitat may be a more efficient way to reduce the risk of malaria than killing larvae. Thus, draining, fouling, or filling standing water where mosquitoes oviposit can be more effective than applying larvicide.     

The vasa regulatory region mediates germline expression and maternal transmission of proteins in the malaria mosquito Anopheles gambiae: a versatile tool for genetic control strategies

Background  

Germline specific promoters are an essential component of potential vector control strategies which function by genetic drive, however suitable promoters are not currently available for the main human malaria vector Anopheles gambiae.     

Selection shapes malaria genomes and drives divergence between pathogens infecting hominids versus rodents

Background  

Malaria kills more people worldwide than all inherited human genetic disorders combined. To characterize how the parasites causing this disease adapt to different host environments, we compared the evolutionary genomics of two distinct groups of malaria pathogens in order to identify critical properties associated with infection of different hosts: those parasites infecting hominids (Plasmodium falciparum and P. reichenowi) versus parasites infecting rodent hosts (P. yoelii yoelii, P. berghei, and P. chabaudi). Adaptation by the parasite to its host is likely highly critical to the evolution of these species.     

Induction of multiple pleiotropic drug resistance genes in yeast engineered to produce an increased level of anti-malarial drug precursor,artemisinic acid

Background  

Due to the global occurrence of multi-drug-resistant malarial parasites (Plasmodium falciparum), the anti-malarial drug most effective against malaria is artemisinin, a natural product (sesquiterpene lactone endoperoxide) extracted from sweet wormwood (Artemisia annua). However, artemisinin is in short supply and unaffordable to most malaria patients. Artemisinin can be semi-synthesized from its precursor artemisinic acid, which can be synthesized from simple sugars using microorganisms genetically engineered with genes from A. annua. In order to develop an industrially competent yeast strain, detailed analyses of microbial physiology and development of gene expression strategies are required.     

Understanding and improving access to prompt and effective malaria treatment and care in rural Tanzania: the ACCESS Programme

Background

Malaria is a significant public health problem in Tanzania. Approximately 16 million malaria cases are reported every year and 100,000 to 125,000 deaths occur. Although most of Tanzania is endemic to malaria, epidemics occur in the highlands, notably in Kagera, a region that was subject to widespread malaria epidemics in 1997 and 1998. This study examined the relationship between climate and malaria incidence in Kagera with the aim of determining whether seasonal forecasts may assist in predicting malaria epidemics.

Methods

A regression analysis was performed on retrospective malaria and climatic data during each of the two annual malaria seasons to determine the climatic factors influencing malaria incidence. The ability of the DEMETER seasonal forecasting system in predicting the climatic anomalies associated with malaria epidemics was then assessed for each malaria season.

Results

It was found that malaria incidence is positively correlated with rainfall during the first season (Oct-Mar) (R-squared = 0.73, p < 0.01). For the second season (Apr-Sep), high malaria incidence was associated with increased rainfall, but also with high maximum temperature during the first rainy season (multiple R-squared = 0.79, p < 0.01). The robustness of these statistical models was tested by excluding the two epidemic years from the regression analysis. DEMETER would have been unable to predict the heavy El Niño rains associated with the 1998 epidemic. Nevertheless, this epidemic could still have been predicted using the temperature forecasts alone. The 1997 epidemic could have been predicted from observed temperatures in the preceding season, but the consideration of the rainfall forecasts would have improved the temperature-only forecasts over the remaining years.

Conclusion

These results demonstrate the potential of a seasonal forecasting system in the development of a malaria early warning system in Kagera region.     

Estimating individual exposure to malaria using local prevalence of malaria infection in the field

Background

Heterogeneity in malaria exposure complicates survival analyses of vaccine efficacy trials and confounds the association between immune correlates of protection and malaria infection in longitudinal studies. Analysis may be facilitated by taking into account the variability in individual exposure levels, but it is unclear how exposure can be estimated at an individual level.

Method and Findings

We studied three cohorts (Chonyi, Junju and Ngerenya) in Kilifi District, Kenya to assess measures of malaria exposure. Prospective data were available on malaria episodes, geospatial coordinates, proximity to infected and uninfected individuals and residence in predefined malaria hotspots for 2,425 individuals. Antibody levels to the malaria antigens AMA1 and MSP1₁₄₂ were available for 291 children from Junju. We calculated distance-weighted local prevalence of malaria infection within 1 km radius as a marker of individual''s malaria exposure. We used multivariable modified Poisson regression model to assess the discriminatory power of these markers for malaria infection (i.e. asymptomatic parasitaemia or clinical malaria). The area under the receiver operating characteristic (ROC) curve was used to assess the discriminatory power of the models. Local malaria prevalence within 1 km radius and AMA1 and MSP1₁₄₂ antibodies levels were independently associated with malaria infection. Weighted local malaria prevalence had an area under ROC curve of 0.72 (95%CI: 0.66–0.73), 0.71 (95%CI: 0.69–0.73) and 0.82 (95%CI: 0.80–0.83) among cohorts in Chonyi, Junju and Ngerenya respectively. In a small subset of children from Junju, a model incorporating weighted local malaria prevalence with AMA1 and MSP1₁₄₂ antibody levels provided an AUC of 0.83 (95%CI: 0.79–0.88).

Conclusion

We have proposed an approach to estimating the intensity of an individual''s malaria exposure in the field. The weighted local malaria prevalence can be used as individual marker of malaria exposure in malaria vaccine trials and longitudinal studies of natural immunity to malaria.     

Proteomic analysis of microparticles isolated from malaria positive blood samples

Background

Malaria continues to be a great public health concern due to the significant mortality and morbidity associated with the disease especially in developing countries. Microparticles (MPs), also called plasma membrane derived extracellular vesicles (PMEVs) are subcellular structures that are generated when they bud off the plasma membrane. They can be found in healthy individuals but the numbers tend to increase in pathological conditions including malaria. Although, various studies have been carried out on the protein content of specific cellular derived MPs, there seems to be paucity of information on the protein content of circulating MPs in malaria and their association with the various signs and symptoms of the disease. The aim of this study was therefore to carry out proteomic analyses of MPs isolated from malaria positive samples and compare them with proteins of MPs from malaria parasite culture supernatant and healthy controls in order to ascertain the role of MPs in malaria infection.

Methods

Plasma samples were obtained from forty-three (43) malaria diagnosed patients (cases) and ten (10) healthy individuals (controls). Malaria parasite culture supernatant was obtained from our laboratory and MPs were isolated from them and confirmed using flow cytometry. 2D LC-MS was done to obtain their protein content. Resultant data were analyzed using SPSS Ver. 21.0 statistical software, Kruskal Wallis test and Spearman’s correlation coefficient r.

Results

In all, 1806 proteins were isolated from the samples. The MPs from malaria positive samples recorded 1729 proteins, those from culture supernatant were 333 while the control samples recorded 234 proteins. The mean number of proteins in MPs of malaria positive samples was significantly higher than that in the control samples. Significantly, higher quantities of haemoglobin subunits were seen in MPs from malaria samples and culture supernatant compared to control samples.

Conclusion

A great number of proteins were observed to be carried in the microparticles (MPs) from malaria samples and culture supernatant compared to controls. The greater loss of haemoglobin from erythrocytes via MPs from malaria patients could serve as the initiation and progression of anaemia in P.falciparum infection. Also while some proteins were upregulated in circulating MPs in malaria samples, others were down regulated.

     

Advantages of larval control for African malaria vectors: Low mobility and behavioural responsiveness of immature mosquito stages allow high effective coverage

Background  

Based on sensitivity analysis of the MacDonald-Ross model, it has long been argued that the best way to reduce malaria transmission is to target adult female mosquitoes with insecticides that can reduce the longevity and human-feeding frequency of vectors. However, these analyses have ignored a fundamental biological difference between mosquito adults and the immature stages that precede them: adults are highly mobile flying insects that can readily detect and avoid many intervention measures whereas mosquito eggs, larvae and pupae are confined within relatively small aquatic habitats and cannot readily escape control measures.     

Characterisation and expression of a PP1 serine/threonine protein phosphatase (PfPP1) from the malaria parasite, <Emphasis Type="Italic">Plasmodium falciparum:</Emphasis> demonstration of its essential role using RNA interference

Background  

Reversible protein phosphorylation is relatively unexplored in the intracellular protozoa of the Apicomplexa family that includes the genus Plasmodium, to which belong the causative agents of malaria. Members of the PP1 family represent the most highly conserved protein phosphatase sequences in phylogeny and play essential regulatory roles in various cellular pathways. Previous evidence suggested a PP1-like activity in Plasmodium falciparum, not yet identified at the molecular level.