The mosquito genome--a breakthrough for public health

A generic schema and data collection forms applicable to diverse entomological studies of mosquitoes

BACKGROUND

Standardized schemas, databases, and public data repositories are needed for the studies of malaria vectors that encompass a remarkably diverse array of designs and rapidly generate large data volumes, often in resource-limited tropical settings lacking specialized software or informatics support.

RESULTS

Data from the majority of mosquito studies conformed to a generic schema, with data collection forms recording the experimental design, sorting of collections, details of sample pooling or subdivision, and additional observations. Generically applicable forms with standardized attribute definitions enabled rigorous, consistent data and sample management with generic software and minimal expertise. Forms use now includes 20 experiments, 8 projects, and 15 users at 3 research and control institutes in 3 African countries, resulting in 11 peer-reviewed publications.

CONCLUSION

We have designed generic data schema that can be used to develop paper or electronic based data collection forms depending on the availability of resources. We have developed paper-based data collection forms that can be used to collect data from majority of entomological studies across multiple study areas using standardized data formats. Data recorded on these forms with standardized formats can be entered and linked with any relational database software. These informatics tools are recommended because they ensure that medical entomologists save time, improve data quality, and data collected and shared across multiple studies is in standardized formats hence increasing research outputs.

Maternal germline-specific genes in the Asian malaria mosquito Anopheles stephensi: characterization and application for disease control

Anopheles stephensi is a principal vector of urban malaria on the Indian subcontinent and an emerging model for molecular and genetic studies of mosquito biology. To enhance our understanding of female mosquito reproduction, and to develop new tools for basic research and for genetic strategies to control mosquito-borne infectious diseases, we identified 79 genes that displayed previtellogenic germline-specific expression based on RNA-Seq data generated from 11 life stage-specific and sex-specific samples. Analysis of this gene set provided insights into the biology and evolution of female reproduction. Promoters from two of these candidates, vitellogenin receptor and nanos, were used in independent transgenic cassettes for the expression of artificial microRNAs against suspected mosquito maternal-effect genes, discontinuous actin hexagon and myd88. We show these promoters have early germline-specific expression and demonstrate 73% and 42% knockdown of myd88 and discontinuous actin hexagon mRNA in ovaries 48 hr after blood meal, respectively. Additionally, we demonstrate maternal-specific delivery of mRNA and protein to progeny embryos. We discuss the application of this system of maternal delivery of mRNA/miRNA/protein in research on mosquito reproduction and embryonic development, and for the development of a gene drive system based on maternal-effect dominant embryonic arrest.

Hepatitis C virus infection may lead to slower emergence of P. falciparum in blood

Background

Areas endemic for Plasmodium falciparum, hepatitis B virus (HBV) and hepatitis C virus (HCV) overlap in many parts of sub-Saharan Africa. HBV and HCV infections develop in the liver, where takes place the first development stage of P. falciparum before its further spread in blood. The complex mechanisms involved in the development of hepatitis may potentially influence the development of the liver stage of malaria parasites. Understanding the molecular mechanisms of these interactions could provide new pathophysiological insights for treatment strategies in Malaria.

Methodology

We studied a cohort of 319 individuals living in a village where the three infections are prevalent. The patients were initially given a curative antimalarial treatment and were then monitored for the emergence of asexual P. falciparum forms in blood, fortnightly for one year, by microscopy and polymerase chain reaction.

Principal Findings

At inclusion, 65 (20.4%) subjects had detectable malaria parasites in blood, 36 (11.3%) were HBV chronic carriers, and 61 (18.9%) were HCV chronic carriers. During follow-up, asexual P. falciparum forms were detected in the blood of 203 patients. The median time to P. falciparum emergence in blood was respectively 140 and 120 days in HBV- and HBV+ individuals, and 135 and 224 days in HCV- and HCV+ individuals. HCV carriage was associated with delayed emergence of asexual P. falciparum forms in blood relative to patients without HCV infection.

Conclusions

This pilot study represents first tentative evidence of a potential epidemiological interaction between HBV, HCV and P. falciparum infections. Age is an important confounding factor in this setting however multivariate analysis points to an interaction between P. falciparum and HCV at the hepatic level with a slower emergence of P. falciparum in HCV chronic carriers. More in depth analysis are necessary to unravel the basis of hepatic interactions between these two pathogens, which could help in identifying new therapeutic approaches against malaria.

Infectious Disease Genomics

The history and development of infectious disease genomics are discussed in this chapter. HGP must not be restricted to the human genome and should include model organisms including mouse, bacteria, yeast, fruit fly, and worm. The completed or ongoing genome projects will provide enormous opportunities for the discovery of novel vaccines and drug targets against human pathogens as well as the improvement of diagnosis and discovery of infectious agents and the development of new strategies for invertebrate vector control. The polysaccharide capsule is important for meningococci to escape from complement-mediated killing. With the completion of the genome sequence of a virulent MenB strain, a “reverse vaccinology” approach was applied for the development of a universal MenB vaccine by Novartis. The indispensable fatty acid synthase (FAS) pathway in bacteria has been regarded as a promising target for the development of antimicrobial agents. Through a systematic screening of 250,000 natural product extracts, a Merck team identified a potent and broad-spectrum antibiotic, platensimycin, which is derived from Streptomyces platensis. Vector Biology Network was formed to achieve three goals (1) to develop basic tools for the stable transformation of anopheline mosquitoes by the year 2000; (2) to engineer a mosquito incapable of carrying the malaria parasite by 2005; and (3) to run controlled experiments to test how to drive the engineered genotype into wild mosquito populations by 2010. The most immediate impact of a completely sequenced pathogen genome is for infectious disease diagnosis.

The Yin and Yang of linkage disequilibrium: mapping of genes and nucleotides conferring insecticide resistance in insect disease vectors

Genetic technologies developed in the last 20 years have lead to novel and exciting methods to identify genes and specific nucleotides within genes that control phenotypes in field collected organisms. In this review we define and explain two of these methods: linkage disequilibrium (LD) mapping and quantitative trait nucleotide (QTN) mapping. The power to detect valid genotype-phenotype associations with LD or QTN mapping depends critically on the extent to which segregating sites in a genome assort independently. LD mapping depends on markers being in disequilibrium with the genes that condition expression of the phenotype. In contrast, QTN mapping depends critically upon most proximal loci being at equilibrium. We show that both patterns actually exist in the genome of Anapheles gambiae, the most important malaria vector in sub-Saharan Africa while segregating sites appear to be largely in equilibrium throughout the genome of Aedes aegypti, the vector of Dengue and Yellow fever flaviviruses. We discuss additional approaches that will be needed to identify genes and nucleotides that control phenotypes in field collected organisms, focusing specifically on ongoing studies of genes conferring resistance to insecticides.

How can ontologies help vector biology?

The reach of genomics has now extended to vector biology, with three mosquito genomes already sequenced and more arthropod vector genomes in the pipeline. The availability of these genomes has paved the way for high-throughput investigations on genome-wide gene expression and proteomics in vector biology. Such investigations would not have been possible without parallel progress in bioinformatics. It is now necessary to construct specific ontologies that will enable vector biologists to achieve computer-comprehensible annotation of genes and genomes, but also of various experimental, clinical and surveillance data. This will inevitably lead to the enhanced usage of such controlled vocabularies, and to an effort to develop novel ontologies, particularly in the context of disease control.

Thiol-frozen shape evolution of triangular silver nanoplates

The structural and optical stability of nanoparticles directly influences their applications. The shape evolution of silver nanoplates synthesized in the presence of bis(2-ethylhexyl) sulfosuccinate (AOT) could be effectively frozen using thiols in aqueous solution. These thiols (e.g., 1-hexanethiol, 1-octanethiol, 1-dodecanethiol, and 1-hexadecanethiol) exhibit stronger surface affinity on the silver crystalline surfaces. This is evidenced from both the unchanged shape/size of nanoplates and their unshifted plasmon resonances in optical absorption. To quantitatively explain the thiol-frozen shape evolution mechanism of silver nanoplates at molecular scale, molecular dynamics simulation was performed. The results show that these thiols exhibit larger interaction energies than AOT molecules on the silver atomic surfaces and hence freeze the shape evolution of silver nanoparticles. This thiol-frozen strategy would not only be useful for stabilizing nanoparticles but would also allow the introduction of a wide range of surface chemical functionalities to the nanoparticles for potential applications in nanosensors.

A role for lysozyme in melanization of Sephadex beads in Anopheles gambiae

Melanization of foreign targets in the mosquito, Anopheles gambiae, was studied using a model Sephadex bead system. A mosquito factor that was deposited on beads and prevented bead melanization (MPF) was purified. The N-terminal sequence of the factor identified it as lysozyme c-1 (Lys c-1). Gene silencing of Lys c-1 mediated by RNA interference resulted in a significant reduction in the MPF activity compared with controls. The purified Lys c-1 protein reduced dopachrome formation by mosquito hemolymph phenoloxidase in solution assays in vitro. In vivo, Lys c-1 might inhibit melanization of beads by blocking attachment of critical factors to the bead surface or by inhibiting PO directly. This work indicates that insect lysozymes can play unexpected roles in mediating melanization of foreign targets.

A proteomic analysis of salivary glands of female Anopheles gambiae mosquito

Understanding the development of the malaria parasite within the mosquito vector at the molecular level should provide novel targets for interrupting parasitic life cycle and subsequent transmission. Availability of the complete genomic sequence of the major African malaria vector, Anopheles gambiae, allows discovery of such targets through experimental as well as computational methods. In the female mosquito, the salivary gland tissue plays an important role in the maturation of the infective form of the malaria parasite. Therefore, we carried out a proteomic analysis of salivary glands from female An. gambiae mosquitoes. Salivary gland extracts were digested with trypsin using two complementary approaches and analyzed by LC-MS/MS. This led to identification of 69 unique proteins, 57 of which were novel. We carried out a functional annotation of all proteins identified in this study through a detailed bioinformatics analysis. Even though a number of cDNA and Edman degradation-based approaches to catalog transcripts and proteins from salivary glands of mosquitoes have been published previously, this is the first report describing the application of MS for characterization of the salivary gland proteome. Our approach should prove valuable for characterizing proteomes of parasites and vectors with sequenced genomes as well as those whose genomes are yet to be fully sequenced.

Characterization of the c-type lysozyme gene family in Anopheles gambiae

Seven new c-type lysozyme genes were found using the Anopheles gambiae genome sequence, increasing to eight the total number of genes in this family identified in this species. The eight lysozymes in An. gambiae have considerable variation in gene structure and expression patterns. Lys c-6 has the most unusual primary amino acid structure as the predicted protein consists of five lysozyme-like domains. Transcript abundance of each c-type lysozyme was determined by semiquantitative RT-PCR. Lys c-1, c-6 and c-7 are expressed constitutively in all developmental stages from egg to adult. Lys c-2 and c-4 also are found in all stages, but with relatively much higher levels in adults. Conversely, Lys c-3 and c-8 transcripts are highest in larvae. Lys c-1, c-6 and c-7 transcripts are found in nearly all the adult tissue samples examined while Lys c-2 and Lys c-4 are more restricted in their expression. Lys c-1 and c-2 transcripts are clearly immune responsive and are increased significantly 6-12 h post challenge with bacteria. The functional adaptive changes that may have evolved during the expansion of this gene family are briefly discussed in terms of the expression patterns, gene and protein structures.

An Anopheles gambiae salivary gland promoter analysis in Drosophila melanogaster and Anopheles stephensi

Regulatory regions driving gene expression in specific target organs of the African malaria vector Anopheles gambiae are of critical relevance for studies on Plasmodium-Anopheles interactions as well as to devise strategies for blocking malaria parasite development in the mosquito. In order to identify an appropriate salivary gland promoter we analysed the transactivation properties of genomic fragments located just upstream of the An. gambiae female salivary gland-specific genes AgApy and D7r4. An 800 bp fragment from the AgApy gene directed specific expression of the LacZ reporter gene in the salivary glands of transgenic Anopheles stephensi. However, expression levels were lower than expected and the transgene was expressed in the proximal-rather than in the distal-lateral lobes of female glands. Surprisingly, a promoter fragment from the D7r4 gene conferred strong tissue-specific expression in Drosophila melanogaster but only low transcription levels in transgenic An. stephensi. These results imply a certain conservation of gland-specific control elements between the fruit fly and the mosquito suggesting that an increased degree of complexity, probably connected to the evolution of haematophagy, underlies the regulation of tissue-specific expression in mosquito female salivary glands.

Neglected diseases: under-funded research and inadequate health interventions. Can we change this reality?

Can we change this reality?

MalariaSphere: a greenhouse-enclosed simulation of a natural Anopheles gambiae (Diptera: Culicidae) ecosystem in western Kenya

BACKGROUND

The development and implementation of innovative vector control strategies for malaria control in Africa requires in-depth ecological studies in contained semi-field environments. This particularly applies to the development and release of genetically-engineered vectors that are refractory to Plasmodium infection. Here we describe a modified greenhouse, designed to simulate a natural Anopheles gambiae Giles ecosystem, and the first successful trials to complete the life-cycle of this mosquito vector therein.

METHODS

We constructed a local house, planted crops and created breeding sites to simulate the natural ecosystem of this vector in a screen-walled greenhouse, exposed to ambient climate conditions, in western Kenya. Using three different starting points for release (blood-fed females, virgin females and males, or eggs), we allowed subsequent stages of the life-cycle to proceed under close observation until one cycle was completed.

RESULTS

Completion of the life-cycle was observed in all three trials, indicating that the major life-history behaviours (mating, sugar feeding, oviposition and host seeking) occurred successfully.

CONCLUSION

The system described can be used to study the behavioural ecology of laboratory-reared and wild mosquitoes, and lends itself to contained studies on the stability of transgenes, fitness effects and phenotypic characteristics of genetically-engineered disease vectors. The extension of this approach, to enable continuous maintenance of successive and overlapping insect generations, should be prioritized. Semi-field systems represent a promising means to significantly enhance our understanding of the behavioural and evolutionary ecology of African malaria vectors and our ability to develop and evaluate innovative control strategies. With regard to genetically-modified mosquitoes, development of such systems is an essential prerequisite to full field releases.

Malaria control with genetically manipulated insect vectors

At a recent workshop, experts discussed the benefits, risks, and research priorities associated with using genetically manipulated insects in the control of vector-borne diseases.