Analysis of female salivary gland proteins of the Anopheles barbirostris complex (Diptera: Culicidae) in Thailand

Anopheles salivary antigens as serological biomarkers of vector exposure and malaria transmission: A systematic review with multilevel modelling

Background

Entomological surveillance for malaria is inherently resource-intensive and produces crude population-level measures of vector exposure which are insensitive in low-transmission settings. Antibodies against Anopheles salivary proteins measured at the individual level may serve as proxy biomarkers for vector exposure and malaria transmission, but their relationship is yet to be quantified.

Methods

A systematic review of studies measuring antibodies against Anopheles salivary antigens (PROSPERO: CRD42020185449). Multilevel modelling (to account for multiple study-specific observations [level 1], nested within study [level 2], and study nested within country [level 3]) estimated associations between seroprevalence with Anopheles human biting rate (HBR) and malaria transmission measures.

Results

From 3981 studies identified in literature searches, 42 studies across 16 countries were included contributing 393 study-specific observations of anti-Anopheles salivary antibodies determined in 42,764 samples. A positive association between HBR (log transformed) and seroprevalence was found; overall a twofold (100% relative) increase in HBR was associated with a 23% increase in odds of seropositivity (OR: 1.23, 95% CI: 1.10-1.37; p<0.001). The association between HBR and Anopheles salivary antibodies was strongest with concordant, rather than discordant, Anopheles species. Seroprevalence was also significantly positively associated with established epidemiological measures of malaria transmission: entomological inoculation rate, Plasmodium spp. prevalence, and malarial endemicity class.

Conclusions

Anopheles salivary antibody biomarkers can serve as a proxy measure for HBR and malaria transmission, and could monitor malaria receptivity of a population to sustain malaria transmission. Validation of Anopheles species-specific biomarkers is important given the global heterogeneity in the distribution of Anopheles species. Salivary biomarkers have the potential to transform surveillance by replacing impractical, inaccurate entomological investigations, especially in areas progressing towards malaria elimination.

Funding

Australian National Health and Medical Research Council, Wellcome Trust.

Protein expression in female salivary glands of pyrethroid-susceptible and resistant strains of Aedes aegypti mosquitoes

BACKGROUND

A group of insecticides called pyrethroids has been used extensively worldwide and development of pyrethroid resistance within mosquito populations, especially in Aedes aegypti, has rapidly spread through populations. In this study, SDS-PAGE, 2-DE coupled with NanoLC-MS, and bioinformatics were used to analyze the female salivary gland proteins of pyrethroid-susceptible (PMD) and pyrethroid-resistant (PMD-R and UPK-R) strains of Ae. aegypti mosquitoes for the first time.

RESULTS

SDS-PAGE analysis revealed that among the three strains at least nine major proteins were detected but one protein band (20 kDa) was found only in the PMD strain. Two-dimensional gel electrophoresis analysis revealed 19 similarly expressed proteins in the salivary glands of the three strains involved in blood-feeding process, stress response, immunogenic response, and metabolic process and five additional major protein spots differentially expressed in the susceptible and resistant strains. Comparative analysis of the expression volume of each protein spot between the PMD and the PMD-R strains showed three downregulated proteins of the PMD-R mosquitoes. For UPK-R strains, six major proteins were downregulated when compared to the PMD strain. Additionally, four downregulated proteins were found in the UPK-R when compared to the PMD-R strain. These results suggest that pyrethroids might induce alteration of salivary gland proteins in resistant mosquitoes. Network analysis by STITCH database 5.0 showed that SRPN23 interacted with sodium and calcium ions, suggesting that SRPN23 might be involved in insecticide resistance.

CONCLUSIONS

Information obtained from this study will be useful for further studies on the roles of differentially expressed salivary gland proteins in resistance to insecticides and viral transmission.

Salivary Gland Proteome during Adult Development and after Blood Feeding of Female Anopheles dissidens Mosquitoes (Diptera: Culicidae)

Understanding changes in mosquito salivary proteins during the time that sporozoite maturation occurs and after blood feeding may give information regarding the roles of salivary proteins during the malarial transmission. Anopheles dissidens (formerly Anopheles barbirostris species A1) is a potential vector of Plasmodium vivax in Thailand. In this study, analyses of the proteomic profiles of female An. dissidens salivary glands during adult development and after blood feeding were carried out using two-dimensional gel electrophoresis coupled with nano-liquid chromatography-mass spectrometry. Results showed at least 17 major salivary gland proteins present from day one to day 21 post emergence at 8 different time points sampled. Although there was variation observed, the patterns of protein expression could be placed into one of four groups. Fifteen protein spots showed significant depletion after blood feeding with the percentages of the amount of depletion ranging from 8.5% to 68.11%. The overall results identified various proteins, including a putative mucin-like protein, an anti-platelet protein, a long form D7 salivary protein, a putative gVAG protein precursor, a D7-related 3.2 protein, gSG7 salivary proteins, and a gSG6 protein. These results allow better understanding of the changes of the salivary proteins during the adult mosquito development. They also provide candidate proteins to investigate any possible link or not between sporozoite maturation, or survival of skin stage sporozoites, and salivary proteins.

Molecular characterization of the malaria vector Anopheles barbirostris van der Wulp in Sri Lanka

Background

Anopheles barbirostris is a vector of malaria in Sri Lanka. The taxon exists as a species complex in the Southeast Asian region. Previous studies using molecular markers suggest that there are more than 4 distinct clades within the An. barbirostris complex in Southeast Asia. The present study characterizes Sri Lankan An. barbirostris using mtDNA cytochrome oxidase subunit I (COI) and ribosomal RNA internal transcribed spacer 2 (ITS2) gene sequences.

Findings

DNA was extracted from morphologically identified An. barbirostris specimens from Sri Lanka, the COI and ITS2 regions amplified and their sequences analysed by comparison with other GenBank entries. Maximum likelihood trees suggested that Sri Lankan An. barbirostris constitute a different molecular type most closely related to clade I.

Conclusions

Considering the uncorrected p distances between the clade I and Sri Lankan specimens it is fair to assume that the specimens collected from widely separated locations in Sri Lanka with morphology characteristic of An. barbirostris s.l. form a new molecular type with close resemblance to An. barbirostris s.s from Indonesia and Thailand.

Members of the salivary gland surface protein (SGS) family are major immunogenic components of mosquito saliva

Mosquitoes transmit Plasmodium and certain arboviruses during blood feeding, when they are injected along with saliva. Mosquito saliva interferes with the host's hemostasis and inflammation response and influences the transmission success of some pathogens. One family of mosquito salivary gland proteins, named SGS, is composed of large bacterial-type proteins that in Aedes aegypti were implicated as receptors for Plasmodium on the basal salivary gland surface. Here, we characterize the biology of two SGSs in the malaria mosquito, Anopheles gambiae, and demonstrate their involvement in blood feeding. Western blots and RT-PCR showed that Sgs4 and Sgs5 are produced exclusively in female salivary glands, that expression increases with age and after blood feeding, and that protein levels fluctuate in a circadian manner. Immunohistochemistry showed that SGSs are present in the acinar cells of the distal lateral lobes and in the salivary ducts of the proximal lobes. SDS-PAGE, Western blots, bite blots, and immunization via mosquito bites showed that SGSs are highly immunogenic and form major components of mosquito saliva. Last, Western and bioinformatic analyses suggest that SGSs are secreted via a non-classical pathway that involves cleavage into a 300-kDa soluble fragment and a smaller membrane-bound fragment. Combined, these data strongly suggest that SGSs play an important role in blood feeding. Together with their role in malaria transmission, we propose that SGSs could be used as markers of human exposure to mosquito bites and in the development of disease control strategies.

Cloning, expression, and characterization of salivary apyrase from Aedes albopictus

Apyrases (ATP diphosphohydrolase) hydrolyze the phosphodiester bonds of nucleoside tri- and diphosphates to orthophosphate and mononucleodides. They can inhibit platelet activation by depletion of adenosine diphosphate. In the current study, the Escherichia coli expression vector pET-19b equipped with an N-terminal histidine tag was applied to express the apyrase of Aedes albopictus. The gene-coding mature apyrase protein was amplified by RT-PCR and cloned into pET-19b. Soluble apyrase protein with high purity was successfully obtained by utilization of the suitable renaturation approach and Ni-NTA purification column. Four monoclonal antibodies to apyrase from A. albopictus were produced in male BALB/c mice immunized with the renatured apyrase. Using immunofluorescence assay and immunoblotting analysis, recombinant apyrase showed fine consistency with native apyrase. From kinetic analysis, it had a K (m) of 11.6 μM and V (max) of 1.02 nM/S/μg protein for adenosine triphosphate. Adenosine diphosphate-induced platelet aggregation was inhibited by approximately 6% when 0.4 μM recombinant apyrase was added and by about 9.5% when the concentration of recombinant apyrase was 0.8 μM. The effect on platelet aggregation was dose dependent. In conclusion, the apyrase of A. albopictus was cloned and expressed highly in the E. coli expression system. Recombinant apyrase protein showed biological activity, and anti-apyrase monoclonal antibody was also prepared.

The dominant Anopheles vectors of human malaria in the Asia-Pacific region: occurrence data, distribution maps and bionomic précis

BACKGROUND

The final article in a series of three publications examining the global distribution of 41 dominant vector species (DVS) of malaria is presented here. The first publication examined the DVS from the Americas, with the second covering those species present in Africa, Europe and the Middle East. Here we discuss the 19 DVS of the Asian-Pacific region. This region experiences a high diversity of vector species, many occurring sympatrically, which, combined with the occurrence of a high number of species complexes and suspected species complexes, and behavioural plasticity of many of these major vectors, adds a level of entomological complexity not comparable elsewhere globally. To try and untangle the intricacy of the vectors of this region and to increase the effectiveness of vector control interventions, an understanding of the contemporary distribution of each species, combined with a synthesis of the current knowledge of their behaviour and ecology is needed.

RESULTS

Expert opinion (EO) range maps, created with the most up-to-date expert knowledge of each DVS distribution, were combined with a contemporary database of occurrence data and a suite of open access, environmental and climatic variables. Using the Boosted Regression Tree (BRT) modelling method, distribution maps of each DVS were produced. The occurrence data were abstracted from the formal, published literature, plus other relevant sources, resulting in the collation of DVS occurrence at 10116 locations across 31 countries, of which 8853 were successfully geo-referenced and 7430 were resolved to spatial areas that could be included in the BRT model. A detailed summary of the information on the bionomics of each species and species complex is also presented.

CONCLUSIONS

This article concludes a project aimed to establish the contemporary global distribution of the DVS of malaria. The three articles produced are intended as a detailed reference for scientists continuing research into the aspects of taxonomy, biology and ecology relevant to species-specific vector control. This research is particularly relevant to help unravel the complicated taxonomic status, ecology and epidemiology of the vectors of the Asia-Pacific region. All the occurrence data, predictive maps and EO-shape files generated during the production of these publications will be made available in the public domain. We hope that this will encourage data sharing to improve future iterations of the distribution maps.