Microarray-based survey of a subset of putative olfactory genes in the mosquito Anopheles gambiae

Gene drives: an alternative approach to malaria control?

Genetic modification for the control of mosquitoes is frequently touted as a solution for a variety of vector-borne diseases. There has been some success using non-insecticidal methods like sterile or incompatible insect techniques to control arbovirus diseases. However, control by genetic modifications to reduce mosquito populations or create mosquitoes that are refractory to infection with pathogens are less developed. The advent of CRISPR-Cas9-mediated gene drives may advance this mechanism of control. In this review, use and progress of gene drives for vector control, particularly for malaria, is discussed. A brief history of population suppression and replacement gene drives in mosquitoes, rapid advancement of the field over the last decade and how genetic modification fits into the current scope of vector control are described. Mechanisms of alternative vector control by genetic modification to modulate mosquitoes' immune responses and anti-parasite effector molecules as part of a combinational strategy to combat malaria are considered. Finally, the limitations and ethics of using gene drives for mosquito control are discussed.

Expression Pattern and Ligand Binding Characteristics Analysis of Chemosensory Protein SnitCSP2 from Sirex nitobei

Sirex nitobei is an important wood-boring wasp to conifers native to Asia, causing considerable economic and ecological damage. However, the current control means cannot achieve better efficiency, and it is expected to clarify the molecular mechanism of protein-ligand binding for effective pest control. This study analyzed the expression pattern of CSP2 in S. nitobei (SnitCSP2) and its features of binding to the screened ligands using molecular docking and dynamic simulations. The results showed that SnitCSP2 was significantly expressed in female antennae. Molecular docking and dynamic simulations revealed that SnitCSP2 bound better to the host plant volatile (+)-α-pinene and symbiotic fungal volatiles terpene and (-)-globulol than other target ligands. By the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) method, the free binding energies of the three complexes were calculated as -44.813 ± 0.189 kJ/mol, -50.446 ± 0.396 kJ/mol, and -56.418 ± 0.368 kJ/mol, and the van der Waals energy was found to contribute significantly to the stability of the complexes. Some key amino acid residues were also identified: VAL13, GLY14, LYS61, MET65, and LYS68 were important for the stable binding of (+)-α-pinene by SnitCSP2, while for terpenes, ILE16, ALA25, TYR26, CYS29, GLU39, THR37, and GLY40 were vital for a stable binding system. We identified three potential ligands and analyzed the interaction patterns of the proteins with them to provide a favorable molecular basis for regulating insect behavioral interactions and developing new pest control strategies.

The structure of AgamOBP5 in complex with the natural insect repellents Carvacrol and Thymol: Crystallographic, fluorescence and thermodynamic binding studies

Among several proteins participating in the olfactory perception process of insects, Odorant Binding Proteins (OBPs) are today considered valid targets for the discovery of compounds that interfere with their host-detection behavior. The 3D structures of Anopheles gambiae mosquito AgamOBP1 in complex with the known synthetic repellents DEET and Icaridin have provided valuable information on the structural characteristics that govern their selective binding. However, no structure of a plant-derived repellent bound to an OBP has been available until now. Herein, we present the novel three-dimensional crystal structures of AgamOBP5 in complex with two natural phenolic monoterpenoid repellents, Carvacrol and Thymol, and the MPD molecule. Structural analysis revealed that both monoterpenoids occupy a binding site (Site-1) by adopting two alternative conformations. An additional Carvacrol was also bound to a secondary site (Site-2) near the central cavity entrance. A protein-ligand hydrogen-bond network supplemented by van der Waals interactions spans the entire binding cavity, bridging α4, α6, and α3 helices and stabilizing the overall structure. Fluorescence competition and Differential Scanning Calorimetry experiments verified the presence of two binding sites and the stabilization effect on AgamOBP5. While Carvacrol and Thymol bind to Site-1 with equal affinity in the submicromolar range, they exhibit a significantly lower and distinct binding capacity for Site-2 with Kd's of ~7 μΜ and ~18 μΜ, respectively. Finally, a comparison of AgamOBP5 complexes with the AgamOBP4-Indole structure revealed that variations of ligand-interacting aminoacids such as A109T, I72M, A112L, and A105T cause two structurally similar and homologous proteins to display different binding specificities.

A Comparative Evaluation of the Structural and Dynamic Properties of Insect Odorant Binding Proteins

Insects devote a major part of their metabolic resources to the production of odorant binding proteins (OBPs). Although initially, these proteins were implicated in the solubilisation, binding and transport of semiochemicals to olfactory receptors, it is now recognised that they may play diverse, as yet uncharacterised, roles in insect physiology. The structures of these OBPs, the majority of which are known as “classical” OBPs, have shed some light on their potential functional roles. However, the dynamic properties of these proteins have received little attention despite their functional importance. Structural dynamics are encoded in the native protein fold and enable the adaptation of proteins to substrate binding. This paper provides a comparative review of the structural and dynamic properties of OBPs, making use of sequence/structure analysis, statistical and theoretical physics-based methods. It provides a new layer of information and additional methodological tools useful in unravelling the relationship between structure, dynamics and function of insect OBPs. The dynamic properties of OBPs, studied by means of elastic network models, reflect the similarities/dissimilarities observed in their respective structures and provides insights regarding protein motions that may have important implications for ligand recognition and binding. Furthermore, it was shown that the OBPs studied in this paper share conserved structural ‘core’ that may be of evolutionary and functional importance.

Screening of olfactory genes related to blood-feeding behaviors in Culex pipiens quinquefasciatus and Culex pipiens molestus by transcriptome analysis

Background

Culex pipiens quinquefasciatus Say (Cx. quinquefasciatus) and Culex pipiens form molestus Forskal (Cx. molestus) in the Culex pipiens complex group show considerable differences in host seeking, blood feeding, mating behavior and in vector competence. Blood-feeding mosquito behaviors are closely related to their olfactory gene expression and olfactory gene repertoire composition. Comparing olfactory genes between these two subspecies with significantly different blood-feeding behaviors can support further research on the molecular mechanism of the Culex pipiens complex olfactory sensory system, providing a new approach for determining candidate attractant or repellent compounds.

Methods

Non-blood-feeding (NBF) and post-blood-feeding (PBF) olfactory system transcriptomes of the two subspecies were sequenced, and the biological functions of their differentially expressed genes were described by bioinformatics analysis. A quantitative polymerase chain reaction (qPCR) was applied to validate the RNA-seq data. The roles of particular olfactory receptors in Cx. quinquefasciatus blood-feeding behaviors were evaluated by RNAi.

Results

Five, 7, 24, and 3 Cx. quinquefasciatus-specific OBPs, Cx. molestus-specific OBPs, Cx. quinquefasciatus-specific ORs and Cx. molestus-specific ORs were identified, respectively. The majority of selected ORs were consistent with the predicted transcriptome sequencing results after qRT-PCR validation. OR5 was expressed only in Cx. quinquefasciatus, and OR65 was the only gene upregulated after blood feeding in Cx. molestus. The blood-feeding rates of the OR5 and OR78 dsRNA groups were significantly lower (4.3%±3.1% and 13.3%±11.5%) than those of the enhanced green fluorescence protein (EGFP) group (64.5%±8.7%).

Conclusion

Most OBPs and ORs were expressed in both subspecies but showed divergence in expression level. OR5 and OR65 might be species-specific expressed genes that regulate the olfactory behaviors of Cx. quinquefasciatus and Cx. molestus, respectively. The RNA interference of OR5 and OR78 could inhibit the blood-feeding behavior of Cx. quinquefasciatus, providing new targets for screening effective repellent compounds to control mosquito-borne diseases effectively and efficiently.

The transcriptomic gene expression of the olfactory tissues of Cx. quinquefasciatusthe and Cx. molestus differ significantly. The majority of ORs and OBPs are expressed in both subspecies but are clearly differentiated in expression level. OR5 and OR65 may be species-specific olfactory genes expressed in Cx. quinquefasciatus and Cx. molestus, respectively. After the microinjection of OR5-dsRNA and OR78-dsRNA into female Cx. quinquefasciatus adults, the blood-feeding rate was significantly lower than that of the control group, suggesting that OR5 and OR78 are associated with the blood-feeding behavior of Cx. quinquefasciatus.

Transcriptome Profiling of Starvation in the Peripheral Chemosensory Organs of the Crop Pest Spodoptera littoralis Caterpillars

Simple Summary

Starvation increases olfactory sensitivity in a manner that enhances the search for food in animals, including insects. However, the molecular mechanisms via which starvation modulates olfactory receptor neuron function are poorly understood. In this study, we sequenced and compared the whole transcriptomes of the main olfactory organs (antennae and palps) of fed and starved caterpillars from the species Spodoptera littoralis. We revealed that transcripts involved in several biological processes are regulated upon starvation. These processes include glucose metabolism, immune defense, foraging activity, and olfaction. In this last process, we evidenced regulation of chemosensory proteins and odorant-degrading enzymes, known to play a role in the dynamics and the sensitivity of the olfactory receptor neuron response. Our results identify new elements in the cascade of olfactory neuron modulation, in addition to insulin, GABA, and short neuropeptide F signaling.

Abstract

Starvation is frequently encountered by animals under fluctuating food conditions in nature, and response to it is vital for life span. Many studies have investigated the behavioral and physiological responses to starvation. In particular, starvation is known to induce changes in olfactory behaviors and olfactory sensitivity to food odorants, but the underlying mechanisms are not well understood. Here, we investigated the transcriptional changes induced by starvation in the chemosensory tissues of the caterpillar Spodoptera littoralis, using Illumina RNA sequencing. Gene expression profiling revealed 81 regulated transcripts associated with several biological processes, such as glucose metabolism, immune defense, response to stress, foraging activity, and olfaction. Focusing on the olfactory process, we observed changes in transcripts encoding proteins putatively involved in the peri-receptor events, namely, chemosensory proteins and odorant-degrading enzymes. Such modulation of their expression may drive fluctuations in the dynamics and the sensitivity of the olfactory receptor neuron response. In combination with the enhanced presynaptic activity mediated via the short neuropeptide F expressed during fasting periods, this could explain an enhanced olfactory detection process. Our observations suggest that a coordinated transcriptional response of peripheral chemosensory organs participates in the regulation of olfactory signal reception and olfactory-driven behaviors upon starvation.

OBP14 (Odorant-Binding Protein) Sensing in Adelphocoris lineolatus Based on Peptide Nucleic Acid and Graphene Oxide

OBPs play a crucial role in the recognition of ligands and are involved in the initial steps of semiochemical perception. The diverse expression of OBP genes allows them to participate in different physiological functions in insects. In contrast to classic OBPs with typical olfactory roles in A. lineolatus, the physiological functions of Plus-C OBPs remain largely unknown. In addition, detection of the expression of insect OBP genes by conventional methods is difficult in vitro. Here, we focused on AlinOBP14, a Plus-C OBP from A. lineolatus, and we developed a PNA-GO-based mRNA biosensor to detect the expression of AlinOBP14. The results demonstrated that AlinOBP14 plays dual roles in A. lineolatus. The AlinOBP14 is expressed beneath the epidermis of the vertex and gena in heads of A. lineolatus, and it functions as a carrier for three terpenoids, while AlinOBP14 is also expressed in the peripheral antennal lobe and functions as a carrier for endogenous compounds such as precursors for juvenile hormone (JH) and JHⅢ. Our investigation provides a new method to detect the expression of OBP genes in insects, and the technique will facilitate the use of these genes as potential targets for novel insect behavioral regulation strategies against the pest.

Antennal Enriched Odorant Binding Proteins Are Required for Odor Communication in Glossina f. fuscipes

Olfaction is orchestrated at different stages and involves various proteins at each step. For example, odorant-binding proteins (OBPs) are soluble proteins found in sensillum lymph that might encounter odorants before reaching the odorant receptors. In tsetse flies, the function of OBPs in olfaction is less understood. Here, we investigated the role of OBPs in Glossina fuscipes fuscipes olfaction, the main vector of sleeping sickness, using multidisciplinary approaches. Our tissue expression study demonstrated that GffLush was conserved in legs and antenna in both sexes, whereas GffObp44 and GffObp69 were expressed in the legs but absent in the antenna. GffObp99 was absent in the female antenna but expressed in the male antenna. Short odorant exposure induced a fast alteration in the transcription of OBP genes. Furthermore, we successfully silenced a specific OBP expressed in the antenna via dsRNAi feeding to decipher its function. We found that silencing OBPs that interact with 1-octen-3-ol significantly abolished flies' attraction to 1-octen-3-ol, a known attractant for tsetse fly. However, OBPs that demonstrated a weak interaction with 1-octen-3-ol did not affect the behavioral response, even though it was successfully silenced. Thus, OBPs' selective interaction with ligands, their expression in the antenna and their significant impact on behavior when silenced demonstrated their direct involvement in olfaction.

Volatile allosteric antagonists of mosquito odorant receptors inhibit human-host attraction

Odorant-dependent behaviors in insects are triggered by the binding of odorant ligands to the variable subunits of heteromeric olfactory receptors. Previous studies have shown, however, that specific odor binding to ORco, the common subunit of odorant receptor heteromers, may allosterically alter olfactory receptor function and profoundly affect subsequent behavioral responses. Using an insect cell-based screening platform, we identified and characterized several antagonists of the odorant receptor coreceptor of the African malaria vector Anopheles gambiae (AgamORco) in a small collection of natural volatile organic compounds. Because some of the identified antagonists were previously shown to strongly repel Anopheles and Culex mosquitoes, we examined the bioactivities of the identified antagonists against Aedes, the third major genus of the Culicidae family. The tested antagonists inhibited the function of Ae. aegypti ORco ex vivo and repelled adult Asian tiger mosquitoes (Ae. albopictus). Binary mixtures of specific antagonists elicited higher repellency than single antagonists, and binding competition assays suggested that this enhanced repellence is due to antagonist interaction with distinct ORco sites. Our results also suggest that the enhanced mosquito repellency by antagonist mixtures is due to additive rather than synergistic effects of the specific antagonist combinations on ORco function. Taken together, these findings provide novel insights concerning the molecular aspects of odorant receptor function. Moreover, our results demonstrate that a simple screening assay may be used for the identification of allosteric modifiers of olfactory-driven behaviors capable of providing enhanced personal protection against multiple mosquito-borne infectious diseases.

Identification and expression of chemosensory receptor genes in the egg parasitoid Trissolcus basalis

Parasitic wasps largely rely on chemosenses to locate resources. Understanding the evolution of their chemoreceptors can help elucidate the mechanisms underlying host adaptation and speciation. Trissolcus basalis is a biological control agent of the southern green stink bug, a pantropical pest, and is ideal for investigating the evolution of chemoreceptors. We identified 34 gustatory receptors, 170 odorant receptors, one odorant co-receptor, and 23 ionotropic receptors. Comparison with other Hymenoptera revealed species-specific expansions of 21 Grs and 53 Ors. Most of these Or expansions have 9 exons. Gender- and tissue-specific analyses showed that 5 Grs and 54 Ors are expressed only in antennae in both sexes, 66 Ors in female antennae only, and 4 Ors in male antennae alone. The identification and expression profile of chemosensory receptor genes in T. basalis helps in understanding the link between the evolution of chemoreceptors and speciation in parasitic wasps.

RNA-Seq and differential gene expression analysis in Temora stylifera copepod females with contrasting non-feeding nauplii survival rates: an environmental transcriptomics study

Background

Copepods are fundamental components of pelagic food webs, but reports on how molecular responses link to reproductive success in natural populations are still scarce. We present a de novo transcriptome assembly and differential expression (DE) analysis in Temora stylifera females collected in the Gulf of Naples, Mediterranean Sea, where this copepod dominates the zooplankton community. High-Throughput RNA-Sequencing and DE analysis were performed from adult females collected on consecutive weeks (May 23rd and 30th 2017), because opposite naupliar survival rates were observed. We aimed at detecting key genes that may have influenced copepod reproductive potential in natural populations and whose expression was potentially affected by phytoplankton-derived oxylipins, lipoxygenase-derived products strongly impacting copepod naupliar survival.

Results

On the two sampling dates, temperature, salinity, pH and oxygen remained stable, while variations in phytoplankton cell concentration, oxylipin concentration and oxylipin-per-diatom-cell production were observed. T. stylifera naupliar survival was 25% on May 23rd and 93% on May 30th. De novo assembly generated 268,665 transcripts (isoforms) and 120,749 unique ‘Trinity predicted genes’ (unigenes), of which 50% were functionally annotated. Out of the 331 transcript isoforms differentially expressed between the two sampling dates, 119 sequences were functionally annotated (58 up- and 61 down-regulated). Among predicted genes (unigenes), 144 sequences were differentially expressed and 31 (6 up-regulated and 25 down-regulated) were functionally annotated. Most of the significantly down-regulated unigenes and isoforms were A5 Putative Odorant Binding Protein (Obp). Other differentially expressed sequences (isoforms and unigenes) related to developmental metabolic processes, protein ubiquitination, response to stress, oxidation-reduction reactions and hydrolase activities. DE analysis was validated through Real Time-quantitative PCR of 9 unigenes and 3 isoforms.

Conclusions

Differential expression of sequences involved in signal detection and transduction, cell differentiation and development offered a functional interpretation to the maternally-mediated low naupliar survival rates observed in samples collected on May 23rd. Down-regulation of A5 Obp along with higher quantities of oxylipins-per-litre and oxylipins-per-diatom-cell observed on May 23rd could suggest oxylipin-mediated impairment of naupliar survival in natural populations of T. stylifera. Our results may help identify biomarker genes explaining variations in copepod reproductive responses at a molecular level.

Species and sex-specific chemosensory gene expression in Anopheles coluzzii and An. quadriannulatus antennae

BACKGROUND

Olfactory cues drive mosquito behaviors such as host-seeking, locating sugar sources and oviposition. These behaviors can vary between sexes and closely related species. For example, the malaria vector Anopheles coluzzii is highly anthropophilic, whereas An. quadriannulatus is not. These behavioral differences may be reflected in chemosensory gene expression.

METHODS

The expression of chemosensory genes in the antennae of both sexes of An. coluzzii and An. quadriannulatus was compared using RNA-seq. The sex-biased expression of several genes in An. coluzzii was also compared using qPCR.

RESULTS

The chemosensory expression is mostly similar in the male antennae of An. coluzzii and An. quadriannulatus, with only a few modest differences in expression. A handful of chemosensory genes are male-biased in both species; the highly expressed gustatory receptor AgGr33, odorant binding proteins AgObp25, AgObp26 and possibly AgObp10. Although the chemosensory gene repertoire is mostly shared between the sexes, several highly female-biased AgOrs, AgIrs, and one AgObp were identified, including several whose expression is biased towards the anthropophilic An. coluzzii. Additionally, the expression of several chemosensory genes is biased towards An. coluzzii in both sexes.

CONCLUSIONS

Chemosensory gene expression is broadly similar between species and sexes, but several sex- biased/specific genes were identified. These may modulate sex- and species-specific behaviors. Although the male behavior of these species remains poorly studied, the identification of sex- and species-specific chemosensory genes may provide fertile ground for future work.

Expression of odorant-binding proteins in mouthpart palps of the desert locust Schistocerca gregaria

Odorant-binding proteins (OBPs) are essential molecular elements of the insect chemosensory system, which is composed of the antennae and the mouthpart palps (maxillary and labial). In this study, we have analysed the expression and the sensilla specificity of 14 OBP subtypes in the palps of the desert locust Schistocerca gregaria. The locust palps comprise only a low number of sensilla basiconica but a high number of sensilla chaetica. Employing a variety of approaches, we found that only a subset of the antennal OBP repertoire was expressed in both palp types. These OBPs were previously shown to be expressed either in sensilla basiconica or sensilla chaetica of the antennae. Comparing the expression pattern in the two chemosensory organs revealed similarities and differences; most remarkably, two OBP subtypes, OBP6 and OBP8, were found in both sensilla types on palps, whereas on the antennae they were solely expressed in one sensillum type. Together, the data indicate a differential, but partly overlapping, expression of OBPs in the two sensilla types of the palps. The differences in the expression pattern of OBP subtypes between antennae and palps might be indicative for distinct functions of the OBPs in the two chemosensory organs.

Sex and age modulate antennal chemosensory-related genes linked to the onset of host seeking in the yellow-fever mosquito, Aedes aegypti

The mosquito Aedes aegypti is the primary vector for the fastest growing infectious disease in the world, dengue fever. Disease transmission heavily relies on the ability of female mosquitoes to locate their human hosts. Additionally, males may be found in close proximity to humans, where they can find mates. Host seeking behaviour of both sexes is dependent on adult sexual maturation. Identifying the molecular basis for the onset of host seeking may help to determine targets for future vector control. In this study, we investigate modulation of the host seeking behaviour and the transcript abundance of the main chemoreceptor families between sexes and across ages in newly-emerged mosquitoes. Attraction to human odour was assessed using a Y-tube olfactometer, demonstrating that both males and females display age-dependent regulation of host seeking. The largest increase in transcript abundance was identified for select chemosensory genes in the antennae of young adult Ae. aegypti mosquitoes and reflects the increase in attraction to human odour observed between 1 and 3 day(s) post-emergence in both males and females. Future functional characterisation of the identified differentially abundant genes may provide targets for the development of novel control strategies against vector borne diseases.

Identification of novel bioinspired synthetic mosquito repellents by combined ligand-based screening and OBP-structure-based molecular docking

In this work we report a fast and efficient virtual screening protocol for discovery of novel bioinspired synthetic mosquito repellents with lower volatility and, in all likelihood, increased protection time as compared with their plant-derived parental compounds. Our screening protocol comprises two filtering steps. The first filter is based on the shape and chemical similarity to known plant-derived repellents, whereas the second filter is based on the predicted similarity of the ligand's binding mode to the Anopheles gambiae odorant binding protein (AgamOBP1) relative to that of DEET and Icaridin to the same OBP. Using this protocol, a chemical library containing 42,755 synthetic molecules was screened in silico and sixteen selected compounds were tested for their affinity to AgamOBP1 in vitro and repellence against A. gambiae female mosquitoes using a warm-body repellent assay. One of them showed DEET-like repellence (91%) but with significantly lower volatility (2.84 × 10^-6 mmHg) than either DEET (1.35 × 10^-3 mmHg) or its parental cuminic acid (3.08 × 10^-3 mmHg), and four other compounds were found to exhibit repellent indices between 69 and 79%. Overall, a correlation was not evident between repellence and OBP-binding strength. In contrast, a correlation between binding mode and repellence was found.

A Synergistic Transcriptional Regulation of Olfactory Genes Drives Blood-Feeding Associated Complex Behavioral Responses in the Mosquito Anopheles culicifacies

Decoding the molecular basis of host seeking and blood feeding behavioral evolution/adaptation in the adult female mosquitoes may provide an opportunity to design new molecular strategy to disrupt human-mosquito interactions. Although there is a great progress in the field of mosquito olfaction and chemo-detection, little is known about the sex-specific evolution of the specialized olfactory system of adult female mosquitoes that enables them to drive and manage the complex blood-feeding associated behavioral responses. A comprehensive RNA-Seq analysis of prior and post blood meal olfactory system of An. culicifacies mosquito revealed a minor but unique change in the nature and regulation of key olfactory genes that may play a pivotal role in managing diverse behavioral responses. Based on age-dependent transcriptional profiling, we further demonstrated that adult female mosquito's chemosensory system gradually learned and matured to drive the host-seeking and blood feeding behavior at the age of 5-6 days. A time scale expression analysis of Odorant Binding Proteins (OBPs) unravels unique association with a late evening to midnight peak biting time. Blood meal-induced switching of unique sets of OBP genes and Odorant Receptors (Ors) expression coincides with the change in the innate physiological status of the mosquitoes. Blood meal follows up experiments further provide enough evidence that how a synergistic and concurrent action of OBPs-Ors may drive "prior and post blood meal" associated complex behavioral events. A dominant expression of two sensory appendages proteins (SAP-1 & SAP2) in the legs of An. culicifacies suggests that this mosquito species may draw an extra advantage of having more sensitive appendages than An. stephensi, an urban malarial vector in the Indian subcontinents. Finally, our molecular modeling analysis predicts crucial amino acid residues for future functional characterization of the sensory appendages proteins which may play a central role in regulating multiple behaviors of An. culicifacies mosquito. SIGNIFICANCE  Evolution and adaptation of blood feeding behavior not only favored the reproductive success of adult female mosquitoes but also make them important disease-transmitting vectors. An environmental exposure after emergence may favor the broadly tuned olfactory system of mosquitoes to drive complex behavioral responses. But, how these olfactory derived genetic factors manage female specific "pre and post" blood meal associated complex behavioral responses are not well known. Our findings suggest that a synergistic action of olfactory factors may govern an innate to prime learning strategy to facilitate rapid blood meal acquisition and downstream behavioral activities. A species-specific transcriptional profiling and an in-silico analysis predict that "sensory appendages protein" may be a unique target to design disorientation strategy against the mosquito Anopheles culicifacies.

Odorant-binding protein-based identification of natural spatial repellents for the African malaria mosquito Anopheles gambiae

There is increasing interest in the development of effective mosquito repellents of natural origin to reduce transmission of diseases such as malaria and yellow fever. To achieve this we have employed an in vitro competition assay involving odorant-binding proteins (OBPs) of the malaria mosquito, Anopheles gambiae, with a predominantly female expression bias to identify plant essential oils (EOs) containing bioactive compounds that target mosquito olfactory function. EOs and their fractions capable of binding to such OBPs displayed repellence against female mosquitoes in a laboratory repellent assay. Repellent EOs were subjected to gas chromatographic analysis linked to antennogram (EAG) recordings from female A. gambiae to identify the biologically active constituents. Among these compounds cumin alcohol, carvacrol, ethyl cinnamate and butyl cinnamate proved as effective as DEET at an equivalent dose in the repellent assay, and combinations of carvacrol with either butyl cinnamate or cumin alcohol proved to be significantly more effective than DEET in the assay. When tested as spatial repellents in experimental shelters housing sleeping humans in northern Nigeria a binary mixture of carvacrol plus cumin alcohol caused mosquitoes to leave shelters in significantly higher numbers to those induced by DEET in female Anopheles spp. and in numbers equivalent to that of DEET in Culex spp. mosquitoes. These findings indicate an approach for the identification of biologically active molecules of natural origin serving as repellents for mosquitoes.

Comparative genomics of chemosensory protein genes (CSPs) in twenty-two mosquito species (Diptera: Culicidae): Identification, characterization, and evolution

Chemosensory proteins (CSP) are soluble carrier proteins that may function in odorant reception in insects. CSPs have not been thoroughly studied at whole-genome level, despite the availability of insect genomes. Here, we identified/reidentified 283 CSP genes in the genomes of 22 mosquitoes. All 283 CSP genes possess a highly conserved OS-D domain. We comprehensively analyzed these CSP genes and determined their conserved domains, structure, genomic distribution, phylogeny, and evolutionary patterns. We found an average of seven CSP genes in each of 19 Anopheles genomes, 27 CSP genes in Cx. quinquefasciatus, 43 in Ae. aegypti, and 83 in Ae. albopictus. The Anopheles CSP genes had a simple genomic organization with a relatively consistent gene distribution, while most of the Culicinae CSP genes were distributed in clusters on the scaffolds. Our phylogenetic analysis clustered the CSPs into two major groups: CSP1-8 and CSE1-3. The CSP1-8 groups were all monophyletic with good bootstrap support. The CSE1-3 groups were an expansion of the CSP family of genes specific to the three Culicinae species. The Ka/Ks ratios indicated that the CSP genes had been subject to purifying selection with relatively slow evolution. Our results provide a comprehensive framework for the study of the CSP gene family in these 22 mosquito species, laying a foundation for future work on CSP function in the detection of chemical cues in the surrounding environment.

Chemosensory gene expression in olfactory organs of the anthropophilic Anopheles coluzzii and zoophilic Anopheles quadriannulatus

BACKGROUND

Anopheles (An.) coluzzii, one of Africa's primary malaria vectors, is highly anthropophilic. This human host preference contributes greatly to its ability to transmit malaria. In contrast, the closely related An. quadriannulatus prefers to feed on bovids and is not thought to contribute to malaria transmission. The diverged preference for host odor profiles between these sibling species is likely reflected in chemosensory gene expression levels in the olfactory organs. Therefore, we compared the transcriptomes of the antennae and maxillary palps between An. coluzzii and An. quadriannulatus, focusing on the major chemosensory gene families.

RESULTS

While chemosensory gene expression is strongly correlated between the two species, various chemosensory genes show significantly enhanced expression in one of the species. In the antennae of An. coluzzii the expression of six olfactory receptors (Ors) and seven ionotropic receptors (Irs) is considerably enhanced, whereas 11 Ors and 3 Irs are upregulated in An. quadriannulatus. In the maxillary palps, leaving aside Irs with very low level of expression, one Ir is strongly enhanced in each species. In addition, we find divergence in odorant binding protein (Obp) gene expression, with several highly expressed Obps being enhanced in the antennae and palps of An. coluzzii. Finally, the expression of several gustatory receptors (Grs) in the palps appears to be species-specific, including a homolog of a sugar-sensing Drosophila Gr.

CONCLUSIONS

A considerable number of Ors and Irs are differentially expressed between these two closely related species with diverging host preference. These chemosensory genes could play a role in the human host preference of the malaria vector An. coluzzii. Additionally, divergence in Obp expression between the two species suggests a possible role of these odor carrier proteins in determining host preference. Finally, divergence in chemosensory expression in the palps may point towards a possible role for the maxillary palps in host differentiation.

Sex specific molecular responses of quick-to-court protein in Indian malarial vector Anopheles culicifacies: conflict of mating versus blood feeding behaviour

Understanding the molecular basis of mosquito behavioural complexity plays a central role in designing novel molecular tools to fight against their vector-borne diseases. Although the olfactory system plays an important role in guiding and managing many behavioural responses including feeding and mating, but the sex-specific regulation of olfactory responses remain poorly investigated. From our ongoing transcriptomic data annotation of olfactory tissue of blood fed adult female An. culicifacies mosquitoes; we have identified a 383 bp long unique transcript encoding a Drosophila homolog of the quick-to-court protein. Previously this was shown to regulate courtship behaviour in adult male Drosophila. A comprehensive in silico analysis of the quick-to-court (qtc) gene of An. culicifacies (Ac-qtc) predicts a 1536 bp single copy gene encoding 511 amino acid protein, having a high degree of conservation with other insect homologs. The age-dependent increased expression of putative Ac-qtc correlated with the maturation of the olfactory system, necessary to meet the sex-specific conflicting demand of mating (mate finding) versus host-seeking behavioural responses. Sixteen to eighteen hours of starvation did not alter Ac-qtc expression in both sexes, however, blood feeding significantly modulated its response in the adult female mosquitoes, confirming that it may not be involved in sugar feeding associated behavioural regulation. Finally, a dual behavioural and molecular assay indicated that natural dysregulation of Ac-qtc in the late evening might promote the mating events for successful insemination. We hypothesize that Ac-qtc may play a unique role to regulate the sex-specific conflicting demand of mosquito courtship behaviour versus blood feeding behaviour in the adult female mosquitoes. Further elucidation of this molecular mechanism may provide further information to evaluate Ac-qtc as a key molecular target for mosquito-borne disease management.

In Silico Site-Directed Mutagenesis of the Anopheles gambiae Odorant Binding Protein 20

The Anopheles gambiae is a highly anthropophilic mosquito which is the leading vector for malaria. This disease has affected more than 500 million people worldwide. The Anopheles gambiae targets its hosts through the odors of the human skin and sweat where odorant molecules radiate. These odors elicit specific responses from the insect through the odorant – binding proteins (OBP). Recently, a specific type of OBP has been characterized which is known as the Anopheles gambiae odorant – binding protein 20 (AgamOBP20). This OBP is highly expressed in the female mosquito antennae during the peak of its host – seeking behavior and thus may play a role in olfactory perception. The binding site of the AgamOBP20 is composed primarily of hydrophobic residues wherein the importance of each residue is herein analysed to further understand the properties of AgamOBP20. This was carried out through computer – aided site – directed mutagenesis coupled with homology modelling and docking simulations wherein each residue in the binding site was changed to alanine and serine. Probable key amino acid residues were identified as LEU106, LEU107, and MET53 which are hypothesized to play a significant role in the protein – ligand interaction. These residues had the greatest impact in the binding free energy when mutated with alanine and serine. The presented results suggest that steric hindrance and hydrophobic interaction are crucial factors to consider on the manner in which the ligand binds with AgamOBP20. The molecular features and parameters obtained may be utilized for the development of new pesticides and repellents that are able to block the function of AgamOBP20 and may result to the disarray of the host – seeking behavior of the Anopheles gambiae.

A look inside odorant-binding proteins in insect chemoreception

Detection of chemical signals from the environment through olfaction is an indispensable mechanism for maintaining an insect's life, evoking critical behavioral responses. Among several proteins involved in the olfactory perception process, the odorant binding protein (OBP) has been shown to be essential for a normally functioning olfactory system. This paper discusses the role of OBPs in insect chemoreception. Here, structural aspects, mechanisms of action and binding affinity of such proteins are reviewed, as well as their promising application as molecular targets for the development of new strategies for insect population management and other technological purposes.

The Genetic Basis of Host Preference and Resting Behavior in the Major African Malaria Vector, Anopheles arabiensis

Malaria transmission is dependent on the propensity of Anopheles mosquitoes to bite humans (anthropophily) instead of other dead end hosts. Recent increases in the usage of Long Lasting Insecticide Treated Nets (LLINs) in Africa have been associated with reductions in highly anthropophilic and endophilic vectors such as Anopheles gambiae s.s., leaving species with a broader host range, such as Anopheles arabiensis, as the most prominent remaining source of transmission in many settings. An. arabiensis appears to be more of a generalist in terms of its host choice and resting behavior, which may be due to phenotypic plasticity and/or segregating allelic variation. To investigate the genetic basis of host choice and resting behavior in An. arabiensis we sequenced the genomes of 23 human-fed and 25 cattle-fed mosquitoes collected both in-doors and out-doors in the Kilombero Valley, Tanzania. We identified a total of 4,820,851 SNPs, which were used to conduct the first genome-wide estimates of "SNP heritability" for host choice and resting behavior in this species. A genetic component was detected for host choice (human vs cow fed; permuted P = 0.002), but there was no evidence of a genetic component for resting behavior (indoors versus outside; permuted P = 0.465). A principal component analysis (PCA) segregated individuals based on genomic variation into three groups which were characterized by differences at the 2Rb and/or 3Ra paracentromeric chromosome inversions. There was a non-random distribution of cattle-fed mosquitoes between the PCA clusters, suggesting that alleles linked to the 2Rb and/or 3Ra inversions may influence host choice. Using a novel inversion genotyping assay, we detected a significant enrichment of the standard arrangement (non-inverted) of 3Ra among cattle-fed mosquitoes (N = 129) versus all non-cattle-fed individuals (N = 234; χ2, p = 0.007). Thus, tracking the frequency of the 3Ra in An. arabiensis populations may be of use to infer selection on host choice behavior within these vector populations; possibly in response to vector control. Controlled host-choice assays are needed to discern whether the observed genetic component has a direct relationship with innate host preference. A better understanding of the genetic basis for host feeding behavior in An. arabiensis may also open avenues for novel vector control strategies based on driving genes for zoophily into wild mosquito populations.

Differential expression of the chemosensory transcriptome in two populations of the stemborer Sesamia nonagrioides

Among the proposed mechanisms of local adaptation to different ecological environments, transcriptional changes may play an important role. In this study, we investigated whether such variability occurred within the chemosensory organs of a herbivorous insect, for which chemosensation guides most of its host preferences. A European and an African population of the noctuid Sesamia nonagrioides that display significant differences in their ecological preferences were collected on Zea mays and Typha domingensis, respectively. RNAseq were used between the two populations for digital expression profiling of chemosensory organs from larval antennae and palps. Preliminary data on adult female antennae and ovipositors were also collected. We found 6,550 differentially expressed transcripts in larval antennae and palps. Gene ontology enrichment analyses suggested that transcriptional activity was overrepresented in the French population and that virus and defense activities were overrepresented in the Kenyan population. In addition, we found differential expression of a variety of cytochrome P450s, which may be linked to the different host-plant diets. Looking at olfactory genes, we observed differential expression of numerous candidate odorant-binding proteins, chemosensory proteins, and one olfactory receptor, suggesting that differences in olfactory sensitivity participate in insect adaptation.

Species-specific chemosensory gene expression in the olfactory organs of the malaria vector Anopheles gambiae

Background

The malaria mosquito Anopheles gambiae has a high preference for human hosts, a characteristic that contributes greatly to its capacity for transmitting human malaria. A sibling species, An. quadriannulatus, has a quite different host preference and feeds mostly on bovids. For this reason it does not contribute to human malaria transmission. Host seeking in mosquitoes is modulated by the olfactory system, which is primarily housed in the antennae and maxillary palps. Therefore, the detection of differing host odors by sibling species may be reflected in the expression level of the olfactory genes involved. Accordingly, we compared the transcriptomes of the antennae and maxillary palps of An. gambiae and An. quadriannulatus.

Results

We identified seven relatively abundant olfactory receptors, nine ionotropic receptors and three odorant binding proteins that are substantially up-regulated in An. gambiae antennae. Interestingly, we find that the maxillary palps of An. gambiae contain a species-specific olfactory receptor, Or52, and five An. gambiae-specific gustatory receptors (AgGr48-52) that are relatively abundant. These five gustatory receptors are also expressed in An. gambiae antennae, although at lower level, indicating a likely role in olfaction, rather than gustation. We also document an approximately three-fold higher overall expression of olfaction genes in the maxillary palps of An. quadriannulatus, indicating an important role of this organ in the olfaction system of this species. Finally, the expression of the CO₂ receptor genes is five to six-fold higher in the zoophilic An. quadriannulatus, implying a much higher sensitivity for detecting CO₂.

Conclusions

These results identify potential human host preference genes in the malaria vector An. gambiae. Interestingly, species-specific expression of several gustatory receptors in the olfactory organs indicate a role in olfaction rather than gustation. Additionally, a more expansive role for maxillary palps in olfaction is implicated than previously thought, albeit more so in the zoophilic An. quadriannulatus.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1089) contains supplementary material, which is available to authorized users.

Transcriptome and tissue-specific expression analysis of Obp and Csp genes in the dark black chafer

The dark black chafer, Holotrichia parallela, is an economically important pest in China and worldwide. Traps based on chemical communication are being developed as an alternative control measure to pesticides for this pest, and studies to reveal chemical communication mechanisms in this pest are highly desirable. To systematically analyze genes potentially involved in chemical communication in this pest, we generated a comprehensive transcriptome with combined samples derived from multiple tissues and developmental stages. A total of 43,967 nonredundant sequences (unigenes) with average length of 806 bp were obtained. These unigenes were annotated into different pathways using gene ontology analysis and cluster analysis of orthologous groups of proteins, and kyoto encyclopedia of genes and genomes. In total, 25 transcripts encoding odorant-binding proteins (OBPs) and 16 transcripts encoding chemosensory proteins (CSPs) were identified based on homology searches. Tissue-specific expression profile indicates that OBP17 and CSP7 are likely responsible for male sex pheromone recognition, whereas OBP1-4, OBP9, OBP13-14, OBP17-18, OBP20, OBP22, OBP25, CSP1-7, CSP11, and CSP12-15 are likely responsible for chemical communication between the beetle and environments. Our data shall provide a foundation for further research on the molecular aspects of chemical communication of this insect, and for comparative genomic studies with other species.

The antenna-specific odorant-binding protein AlinOBP13 of the alfalfa plant bug Adelphocoris lineolatus is expressed specifically in basiconic sensilla and has high binding affinity to terpenoids

Odorant-binding proteins (OBPs) are crucial in the olfactory pathway of insects. In the present study, the antenna-enriched OBP AlinOBP13 was investigated because of its potential contribution to the peripheral olfactory perception in the alfalfa plant bug Adelphocoris lineolatus. The results of quantitative reverse transcriptase-PCR showed that the transcript level of AlinOBP13 was higher in the adult stage than in the nymph stages. The transcript levels of AlinOBP13 in the male and female antennae significantly increased after 4 and 8 h of starvation, respectively. Fine ultrastructures of different types of chemosensilla in both female and male antennae were investigated using transmission electron microscopy and immunocytochemical labelling. The results revealed that the anti-AlinOBP13 antiserum strongly and specifically labelled short basiconic sensilla; this antiserum was restricted to the inner lumen and the cavities below the sensillum base of the sensilla. By contrast, multiporous sensilla trichodea, medium long sensilla basiconica, and aporous sensilla chaetica were not labelled. The present study is the first to report an OBP showing specific expression in the short basiconic sensilla of a member of the Hemipteran species. The results of a fluorescence displacement binding assay indicated that recombinant AlinOBP13 showed a more specific binding preference to terpenoids than to sex pheromones and other classes of chemicals. This binding ability was dramatically affected by pH; higher binding affinities were displayed at pH 10.0 than at pH 7.4 and 5.0. In addition, the results of dose-dependent electroantennogram recordings from the antennae showed that both female and male adult bugs responded to the terpenoids tested, suggesting an apparent physiological relevance of AlinOBP13 in A. lineolatus chemoreception. The results of this study suggest that AlinOBP13 functions as a specific carrier of terpenoids and provide insights into the mechanism of A. lineolatus in response to green volatiles.

Transcriptome sequencing and developmental regulation of gene expression in Anopheles aquasalis

BACKGROUND

Anopheles aquasalis is a major malaria vector in coastal areas of South and Central America where it breeds preferentially in brackish water. This species is very susceptible to Plasmodium vivax and it has been already incriminated as responsible vector in malaria outbreaks. There has been no high-throughput investigation into the sequencing of An. aquasalis genes, transcripts and proteins despite its epidemiological relevance. Here we describe the sequencing, assembly and annotation of the An. aquasalis transcriptome.

METHODOLOGY/PRINCIPAL FINDINGS

A total of 419 thousand cDNA sequence reads, encompassing 164 million nucleotides, were assembled in 7544 contigs of ≥ 2 sequences, and 1999 singletons. The majority of the An. aquasalis transcripts encode proteins with their closest counterparts in another neotropical malaria vector, An. darlingi. Several analyses in different protein databases were used to annotate and predict the putative functions of the deduced An. aquasalis proteins. Larval and adult-specific transcripts were represented by 121 and 424 contig sequences, respectively. Fifty-one transcripts were only detected in blood-fed females. The data also reveal a list of transcripts up- or down-regulated in adult females after a blood meal. Transcripts associated with immunity, signaling networks and blood feeding and digestion are discussed.

CONCLUSIONS/SIGNIFICANCE

This study represents the first large-scale effort to sequence the transcriptome of An. aquasalis. It provides valuable information that will facilitate studies on the biology of this species and may lead to novel strategies to reduce malaria transmission on the South American continent. The An. aquasalis transcriptome is accessible at http://exon.niaid.nih.gov/transcriptome/An_aquasalis/Anaquexcel.xlsx.

Daily rhythms in antennal protein and olfactory sensitivity in the malaria mosquito Anopheles gambiae

We recently characterized 24-hr daily rhythmic patterns of gene expression in Anopheles gambiae mosquitoes. These include numerous odorant binding proteins (OBPs), soluble odorant carrying proteins enriched in olfactory organs. Here we demonstrate that multiple rhythmically expressed genes including OBPs and takeout proteins, involved in regulating blood feeding behavior, have corresponding rhythmic protein levels as measured by quantitative proteomics. This includes AgamOBP1, previously shown as important to An. gambiae odorant sensing. Further, electrophysiological investigations demonstrate time-of-day specific differences in olfactory sensitivity of antennae to major host-derived odorants. The pre-dusk/dusk peaks in OBPs and takeout gene expression correspond with peak protein abundance at night, and in turn coincide with the time of increased olfactory sensitivity to odorants requiring OBPs and times of increased blood-feeding behavior. This suggests an important role for OBPs in modulating temporal changes in odorant sensitivity, enabling the olfactory system to coordinate with the circadian niche of An. gambiae.

The roles of kairomones, synomones and pheromones in the chemically-mediated behaviour of male mosquitoes

Despite decades of intensive study of the chemical ecology of female mosquitoes, relatively little is known about the chemical ecology of males. This short review summarizes the current state of knowledge of the chemicals that mediate male mosquito behaviour. Various trophic interactions including insect-plant, insect-host, and insect-insect responses are emphasized. The relevance of the chemical ecology of male mosquitoes in the context of vector control programmes is discussed.

The blunt trichoid sensillum of female mosquitoes, Anopheles gambiae: odorant binding protein and receptor types

In order to find a blood host and to select appropriate oviposition sites female Anopheles gambiae mosquitoes rely on olfactory cues which are sensed by olfactory sensory neurons (OSNs) located within morphologically different sensilla hairs. While the sharp type trichoid sensilla are most abundant and intensely studied, the striking blunt type trichoid sensilla exist only in small numbers and their specific function is unknown. It has been suggested that they may play a role in the detection of chemical cues indicating oviposition sites. With the aim of identifying molecular elements in blunt type trichoid sensilla, which may be relevant for chemosensory function of this sensillum type, experiments were performed which include whole mount fluorescence in situ hybridization (WM-FISH) and fluorescence immunohistochemistry (WM-FIHC). The studies were concentrated on odorant binding proteins (AgOBPs) and odorant receptors (AgORs). WM-FISH approaches using a probe for the plus-C class AgOBP47 led to the labeling of cells, which resembled in number and antennal distribution pattern the blunt type trichoid sensilla. Moreover, WM-FIHC with an antiserum for AgOBP47 allowed to assign the AgOBP47-expressing cells to blunt type trichoid sensilla and to allocate the protein within the sensillum hair shafts. The result of double WM-FISH-experiments and combined WM-FIHC/FISH approaches indicated that the AgOBP47-expressing cells are co-localized with cells, which express AgOR11, AgOR13 and AgOR55. In addition, it turned out that the two receptor types AgOR13 and AgOR55 are co-expressed in the same cells. Together, the results indicate that the blunt type trichoid sensilla contain a characteristic binding protein, plus-C AgOBP47, in the sensillum lymph and two sensory neurons, one cell which express the odorant receptor AgOR11 and a second cell which express the receptor types AgOR13 and AgOR55. The expression of characteristic chemosensory elements in blunt type trichoid sensilla supports the notion that this sensillum type is involved in sensing distinct odorous compounds.

Cloning and expression profiling of odorant-binding proteins in the tarnished plant bug, Lygus lineolaris

In insects, the perception and discrimination of odorants requires the involvement of odorant-binding proteins (OBPs). To gain a better molecular understanding of olfaction in the agronomic pest Lygus lineolaris (the tarnished plant bug), we used a transcriptomics-based approach to identify potential OBPs. In total, 33 putative OBP transcripts, including the previously reported Lygus antennal protein (LAP), were identified based on the characteristic OBP Cys signature and/or sequence similarity with annotated orthologous sequences. The L. lineolaris OBP (LylinOBP) repertoire consists of 20 'classic' OBPs, defined by the spacing of six conserved Cys residues, and 12 'Plus-C' OBPs, defined by the spacing of eight conserved Cys and one conserved Pro residue. Alternative splicing of OBP genes appears to contribute significantly to the multiplicity of LylinOBP sequences. Microarray-based analysis of chemosensory tissues (antennae, legs and proboscis) revealed enrichment of 21 LylinOBP transcripts in antennae, 12 in legs, and 15 in proboscis, suggesting potential roles in olfaction and gustation respectively. PCR-based determination of transcript abundance for a subset of the LylinOBP genes across multiple adult tissues yielded results consistent with the hybridization data.

A proteomic investigation of soluble olfactory proteins in Anopheles gambiae

Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) are small soluble polypeptides that bind semiochemicals in the lymph of insect chemosensilla. In the genome of Anopheles gambiae, 66 genes encode OBPs and 8 encode CSPs. Here we monitored their expression through classical proteomics (2D gel-MS analysis) and a shotgun approach. The latter method proved much more sensitive and therefore more suitable for tiny biological samples as mosquitoes antennae and eggs. Females express a larger number and higher quantities of OBPs in their antennae than males (24 vs 19). OBP9 is the most abundant in the antennae of both sexes, as well as in larvae, pupae and eggs. Of the 8 CSPs, 4 were detected in antennae, while SAP3 was the only one expressed in larvae. Our proteomic results are in fairly good agreement with data of RNA expression reported in the literature, except for OBP4 and OBP5, that we could not identify in our analysis, nor could we detect in Western Blot experiments. The relatively limited number of soluble olfactory proteins expressed at relatively high levels in mosquitoes makes further studies on the coding of chemical messages at the OBP level more accessible, providing for few specific targets. Identification of such proteins in Anopheles gambiae might facilitate future studies on host finding behavior in this important disease vector.

Effects of blood-feeding on olfactory sensitivity of the malaria mosquito Anopheles gambiae: application of mixed linear models to account for repeated measurements

Olfaction plays an important role in the host-seeking behavior of the malaria mosquito Anopheles gambiae. After a complete blood meal, female mosquitoes will not engage in host-seeking behavior until oviposition has occurred. We investigated if peripheral olfactory sensitivity changed after a blood meal by recording electroantennograms (EAGs) of female mosquitoes at three time points (2h, 48 h and 72 h) to 15 volatile kairomones of either human origin or documented to emanate from oviposition sites. The EAG-sensitivity was compared with that of females of similar age post eclosion. As is common practice in electrophysiological studies, the EAG recordings were obtained by repeated stimulation of the same antennal preparations. We introduce mixed linear modeling as an improved statistical analysis for electrophysiological data. Two hours after blood ingestion, olfactory sensitivity as quantified through EAG-recording increased significantly and selectively, i.e. for seven compounds, compared to unfed females of the same age. Such short-term electrophysiological sensitization in the olfactory system as a result of feeding has not been documented before for insects. Sensitization to six compounds persisted until 48 h or 72 h post-blood meal at one or more concentrations. Desensitization was observed at 48 and 72 h pbm in response to two and three kairomones, respectively. For several compounds, sensitization at the EAG-level corresponded with sensitization found previously in single sensillum studies on olfactory neurons in antennal sensilla trichodea of An. gambiae females. These effects are likely to reflect sensitization to oviposition cues, as eggs have matured 48-72 h pbm. Knowledge of changes in olfactory sensitivity to kairomones can be applied to increase trap catches of malaria mosquitoes that have taken a blood meal and need to locate oviposition sites.

Antennal transcriptome profiles of anopheline mosquitoes reveal human host olfactory specialization in Anopheles gambiae

BACKGROUND

Two sibling members of the Anopheles gambiae species complex display notable differences in female blood meal preferences. An. gambiae s.s. has a well-documented preference for feeding upon human hosts, whereas An. quadriannulatus feeds on vertebrate/mammalian hosts, with only opportunistic feeding upon humans. Because mosquito host-seeking behaviors are largely driven by the sensory modality of olfaction, we hypothesized that hallmarks of these divergent host seeking phenotypes will be in evidence within the transcriptome profiles of the antennae, the mosquito's principal chemosensory appendage.

RESULTS

To test this hypothesis, we have sequenced antennal mRNA of non-bloodfed females from each species and observed a number of distinct quantitative and qualitative differences in their chemosensory gene repertoires. In both species, these gene families show higher rates of sequence polymorphisms than the overall rates in their respective transcriptomes, with potentially important divergences between the two species. Moreover, quantitative differences in odorant receptor transcript abundances have been used to model potential distinctions in volatile odor receptivity between the two sibling species of anophelines.

CONCLUSION

This analysis suggests that the anthropophagic behavior of An. gambiae s.s. reflects the differential distribution of olfactory receptors in the antenna, likely resulting from a co-option and refinement of molecular components common to both species. This study improves our understanding of the molecular evolution of chemoreceptors in closely related anophelines and suggests possible mechanisms that underlie the behavioral distinctions in host seeking that, in part, account for the differential vectorial capacity of these mosquitoes.

Crystal and solution studies of the "Plus-C" odorant-binding protein 48 from Anopheles gambiae: control of binding specificity through three-dimensional domain swapping

Much physiological and behavioral evidence has been provided suggesting that insect odorant-binding proteins (OBPs) are indispensable for odorant recognition and thus are appealing targets for structure-based discovery and design of novel host-seeking disruptors. Despite the fact that more than 60 putative OBP-encoding genes have been identified in the malaria vector Anopheles gambiae, the crystal structures of only six of them are known. It is therefore clear that OBP structure determination constitutes the bottleneck for structure-based approaches to mosquito repellent/attractant discovery. Here, we describe the three-dimensional structure of an A. gambiae "Plus-C" group OBP (AgamOBP48), which exhibits the second highest expression levels in female antennae. This structure represents the first example of a three-dimensional domain-swapped dimer in dipteran species. A combined binding site is formed at the dimer interface by equal contribution of each monomer. Structural comparisons with the monomeric AgamOBP47 revealed that the major structural difference between the two Plus-C proteins localizes in their N- and C-terminal regions, and their concerted conformational change may account for monomer-swapped dimer conversion and furthermore the formation of novel binding pockets. Using a combination of gel filtration chromatography, differential scanning calorimetry, and analytical ultracentrifugation, we demonstrate the AgamOBP48 dimerization in solution. Eventually, molecular modeling calculations were used to predict the binding mode of the most potent synthetic ligand of AgamOBP48 known so far, discovered by ligand- and structure-based virtual screening. The structure-aided identification of multiple OBP binders represents a powerful tool to be employed in the effort to control transmission of the vector-borne diseases.

The co-expression pattern of odorant binding proteins and olfactory receptors identify distinct trichoid sensilla on the antenna of the malaria mosquito Anopheles gambiae

The initial steps of odorant recognition in the insect olfactory system involve odorant binding proteins (OBPs) and odorant receptors (ORs). While large families of OBPs have been identified in the malaria vector A. gambiae, little is known about their expression pattern in the numerous sensory hairs of the female antenna. We applied whole mount fluorescence in Situ hybridization (WM-FISH) and fluorescence immunohistochemistry (WM-FIHC) to investigate the sensilla co-expression of eight A. gambiae OBPs (AgOBPs), most notably AgOBP1 and AgOBP4, which all have abundant transcripts in female antenna. WM-FISH analysis of female antennae using AgOBP-specific probes revealed marked differences in the number of cells expressing each various AgOBPs. Testing combinations of AgOBP probes in two-color WM-FISH resulted in distinct cellular labeling patterns, indicating a combinatorial expression of AgOBPs and revealing distinct AgOBP requirements for various functional sensilla types. WM-FIHC with antisera to AgOBP1 and AgOBP4 confirmed expression of the respective proteins by support cells and demonstrated a location of OBPs within sensilla trichodea. Based on the finding that AgOBP1 and AgOBP4 as well as the receptor type AgOR2 are involved in the recognition of indole, experiments were performed to explore if the AgOBP-types and AgOR2 are co-expressed in distinct olfactory sensilla. Applying two-color WM-FISH with AgOBP-specific probes and probes specific for AgOR2 revealed a close association of support cells bearing transcripts for AgOBP1 and AgOBP4 and neurons with a transcript for the receptor AgOR2. Moreover, combined WM-FISH/-FIHC approaches using an AgOR2-specific riboprobe and AgOBP-specific antisera revealed the expression of the “ligand-matched” AgOBP1, AgOBP4 and AgOR2 to single trichoid hairs. This result substantiates the notion that a specific response to indole is mediated by an interplay of the proteins.

Transcriptional basis of the acclimation to high environmental temperature at the olfactory receptor organs of Drosophila melanogaster

Background

Environmental temperature directly affects the concentrations of chemicals in the gas phase. Therefore, if the olfactory system does not physiologically adapt to environmental conditions, it may provide inadequate information about the distance to or direction of odor sources. Previous reports have shown at the behavioral level that temperature induces changes in olfactory sensitivity in Drosophila melanogaster. These changes are initiated in the main olfactory receptor organs, the antennae. In this work, we attempted to identify the particular genes responsible for olfactory adaptation to increasing temperatures in these organs based on current knowledge of the molecular basis of olfactory reception.

Results

Whole-genome transcriptional responses to transitory temperature shifts from 21-30°C were analyzed in the third antennal segments of Drosophila. More than 53% of the genome was expressed in these organs; this percentage increased slightly (55%) after heat treatment. However, the expression levels increased for 26%, decreased for 21% and remained constant for 53% of the expressed genes. Analysis of the changes produced in 389 genes related to heat response and olfactory reception, according to the current functional annotations of the Drosophila gene set, showed significant differences in 95 of these genes, which are involved in the heat response (23), perireceptor events in olfaction (50), olfactory and gustatory receptors (18) and G-proteins and transduction cascades (4).

Conclusions

Gene expression was altered in response to environmental heat in the antennae of Drosophila by increasing or decreasing expression. Different acclimation patterns emerged for reception through the basiconic, trichoid and coeloconic sensilla. Changes in genes with a central role in olfactory reception, such as orco, may account for part of the acclimation reported at the behavioral level.

Extensive circadian and light regulation of the transcriptome in the malaria mosquito Anopheles gambiae

Background

Mosquitoes exhibit 24 hr rhythms in flight activity, feeding, reproduction and development. To better understand the molecular basis for these rhythms in the nocturnal malaria vector Anopheles gambiae, we have utilized microarray analysis on time-of-day specific collections of mosquitoes over 48 hr to explore the coregulation of gene expression rhythms by the circadian clock and light, and compare these with the 24 hr rhythmic gene expression in the diurnal Aedes aegypti dengue vector mosquito.

Results

In time courses from An. gambiae head and body collected under light:dark cycle (LD) and constant dark (DD) conditions, we applied three algorithms that detect sinusoidal patterns and an algorithm that detects spikes in expression. This revealed across four experimental conditions 393 probes newly scored as rhythmic. These genes correspond to functions such as metabolic detoxification, immunity and nutrient sensing. This includes glutathione S-transferase GSTE5, whose expression pattern and chromosomal location are shared with other genes, suggesting shared chromosomal regulation; and pulsatile expression of the gene encoding CYP6M2, a cytochrome P450 that metabolizes pyrethroid insecticides. We explored the interaction of light and the circadian clock and highlight the regulation of odorant binding proteins (OBPs), important components of the olfactory system. We reveal that OBPs have unique expression patterns as mosquitoes make the transition from LD to DD conditions. We compared rhythmic expression between An. gambiae and Ae. aegypti heads collected under LD conditions using a single cosine fitting algorithm, and report distinct similarities and differences in the temporal regulation of genes involved in tRNA priming, the vesicular-type ATPase, olfaction and vision between the two species.

Conclusions

These data build on our previous analyses of time-of-day specific regulation of the An. gambiae transcriptome to reveal additional rhythmic genes, an improved understanding of the co-regulation of rhythms in gene expression by the circadian clock and by light, and an understanding of the time-of-day specific regulation of some of these rhythmic processes in comparison with a different species of mosquito. Improved understanding of biological timing at the molecular level that underlies key physiological aspects of mosquito vectors may prove to be important to successful implementation of established and novel insect control methods.

Fatty acid solubilizer from the oral disk of the blowfly

Background

Blowflies are economic pests of the wool industry and potential vectors for epidemics. The establishment of a pesticide-free, environmentally friendly blowfly control strategy is necessary. Blowflies must feed on meat in order to initiate the cascade of events that are involved in reproduction including juvenile hormone synthesis, vitellogenesis, and mating. During feeding blowflies regurgitate salivary lipase, which may play a role in releasing fatty acids from triglycerides that are found in food. However, long-chain fatty acids show low solubility in aqueous solutions. In order to solubilize and ingest the released hydrophobic fatty acids, the blowflies must use a solubilizer.

Methodology

We applied native PAGE, Edman degradation, cDNA cloning, and RT-PCR to characterize a protein that accumulated in the oral disk of the black blowfly, Phormia regina. In situ hybridization was carried out to localize the expression at the cellular level. A fluorescence competitive binding assay was used to identify potential ligands of this protein.

Conclusion

A protein newly identified from P. regina (PregOBP56a) belonged to the classic odorant-binding protein (OBP) family. This gene was expressed in a cluster of cells that was localized between pseudotracheae on the oral disk, which are not accessory cells of the taste peg chemosensory sensilla that normally synthesize OBPs. At pH 7 and pH 6, PregOBP56a bound palmitic, stearic, oleic, and linoleic acids, that are mainly found in chicken meat. The binding affinity of PregOBP56a decreased at pH 5. We propose that PregOBP56a is a protein that solubilizes fatty acids during feeding and subsequently helps to deliver the fatty acids to the midgut where it may help in the process of reproduction. As such, PregOBP56a is a potential molecular target for controlling the blowfly.

A novel mechanism of ligand binding and release in the odorant binding protein 20 from the malaria mosquito Anopheles gambiae

Anopheles gambiae mosquitoes that transmit malaria are attracted to humans by the odor molecules that emanate from skin and sweat. Odorant binding proteins (OBPs) are the first component of the olfactory apparatus to interact with odorant molecules, and so present potential targets for preventing transmission of malaria by disrupting the normal olfactory responses of the insect. AgamOBP20 is one of a limited subset of OBPs that it is preferentially expressed in female mosquitoes and its expression is regulated by blood feeding and by the day/night light cycles that correlate with blood-feeding behavior. Analysis of AgamOBP20 in solution reveals that the apo-protein exhibits significant conformational heterogeneity but the binding of odorant molecules results in a significant conformational change, which is accompanied by a reduction in the conformational flexibility present in the protein. Crystal structures of the free and bound states reveal a novel pathway for entrance and exit of odorant molecules into the central-binding pocket, and that the conformational changes associated with ligand binding are a result of rigid body domain motions in α-helices 1, 4, and 5, which act as lids to the binding pocket. These structures provide new insights into the specific residues involved in the conformational adaptation to different odorants and have important implications in the selection and development of reagents targeted at disrupting normal OBP function.

Mapping the Anopheles gambiae odorant binding protein 1 (AgamOBP1) using modeling techniques, site directed mutagenesis, circular dichroism and ligand binding assays

The major malaria vector in Sub-Saharan Africa is the Anopheles gambiae mosquito. This species is a key target of malaria control measures. Mosquitoes find humans primarily through olfaction, yet the molecular mechanisms associated with host-seeking behavior remain largely unknown. To further understand the functionality of A. gambiae odorant binding protein 1 (AgamOBP1), we combined in silico protein structure modeling and site-directed mutagenesis to generate 16 AgamOBP1 protein analogues containing single point mutations of interest. Circular dichroism (CD) and ligand-binding assays provided data necessary to probe the effects of the point mutations on ligand binding and the overall structure of AgamOBP1. Far-UV CD spectra of mutated AgamOBP1 variants displayed both substantial decreases to ordered α-helix structure (up to22%) and increases to disordered α-helix structure(up to 15%) with only minimal changes in random coil (unordered) structure. In mutations Y54A, Y122A and W114Q, aromatic side chain removal from the binding site significantly reduced N-phenyl-1-naphthylamine binding. Several non-aromatic mutations (L15T, L19T, L58T, L58Y, M84Q, M84K, H111A, Y122A and L124T) elicited changes to protein conformation with subsequent effects on ligand binding. This study provides empirical evidence for the in silico predicted functions of specific amino acids in AgamOBP1 folding and ligand binding characteristics.

Expression pattern of a 'Plus-C' class odorant binding protein in the antenna of the malaria vector Anopheles gambiae

In the malaria mosquito Anopheles gambiae (Ag), olfaction plays a crucial role in various behaviours, most strikingly in the seeking of females after a blood meal. The first step of odorant recognition in antennal sensilla involves soluble odorant binding proteins (OBPs), which transfer odorous compounds to olfactory receptors (ORs) in the dendritic membrane of sensory neurons. A particular OBP subtype of the 'Plus-C' class, called AgOBP48, is abundantly transcribed in female antennae and partially down-regulated after a blood meal, suggesting a possible role in host detection. In the present study, we have identified the AgOBP48-expressing cells, explored their antennal topography and determined their position relative to cells that express the 'classic' AgOBP1, the AgOR co-receptor (AgOrco) and the receptor AgOR1. By means of two-colour whole-mount fluorescence in situ hybridization it was found that AgOBP48 was expressed in cells, which are closely associated with AgOrco-expressing sensory neurons. Furthermore, AgOBP48 was not expressed in the same cells as AgOBP1, but subpopulations of AgOBP48- and of AgOBP1-expressing cells were found closely associated and adjacent to sensory neurons expressing AgOR1. Together, the results indicate that cells that express either AgOBP48, AgOBP1 or AgOR1 are housed together in distinct olfactory sensilla and that an interplay of the proteins may contribute to the specific responsiveness of the sensillum to distinct odorants.

Female Anopheles gambiae antennae: increased transcript accumulation of the mosquito-specific odorant-binding-protein OBP2

Background

New interventions are required to optimally and sustainably control the Anopheles sp. mosquitoes that transmit malaria and filariasis. The mosquito olfactory system is important in host seeking (transmission) and mate finding (reproduction). Understanding olfactory function could lead to development of control strategies based on repelling parasite-carrying mosquitoes or attracting them into a fatal trap.

Findings

Our initial focus is on odorant binding proteins with differential transcript accumulation between female and male mosquitoes. We report that the odorant binding protein, OBP2 (AGAP003306), had increased expression in the antennae of female vs. male Anopheles gambiae sensu stricto (G3 strain). The increased expression in antennae of females of this gene by quantitative RT-PCR was 4.2 to 32.3 fold in three independent biological replicates and two technical replicate experiments using A. gambiae from two different laboratories. OBP2 is a member of the vast OBP superfamily of insect odorant binding proteins and belongs to the predominantly dipteran clade that includes the Culex oviposition kairomone-binding OBP1. Phylogenetic analysis indicates that its orthologs are present across culicid mosquitoes and are likely to play a conserved role in recognizing a molecule that might be critical for female behavior.

Conclusions

OBP2 has increased mRNA transcript accumulation in the antennae of female as compared to male A. gambiae. This molecule and related molecules may play an important role in female mosquito feeding and breeding behavior. This finding may be a step toward providing a foundation for understanding mosquito olfactory requirements and developing control strategies based on reducing mosquito feeding and breeding success.

Anopheles gambiae odorant binding protein crystal complex with the synthetic repellent DEET: implications for structure-based design of novel mosquito repellents

Insect odorant binding proteins (OBPs) are the first components of the olfactory system to encounter and bind attractant and repellent odors emanating from various sources for presentation to olfactory receptors, which trigger relevant signal transduction cascades culminating in specific physiological and behavioral responses. For disease vectors, particularly hematophagous mosquitoes, repellents represent important defenses against parasitic diseases because they effect a reduction in the rate of contact between the vectors and humans. OBPs are targets for structure-based rational approaches for the discovery of new repellent or other olfaction inhibitory compounds with desirable features. Thus, a study was conducted to characterize the high resolution crystal structure of an OBP of Anopheles gambiae, the African malaria mosquito vector, in complex with N,N-diethyl-m-toluamide (DEET), one of the most effective repellents that has been in worldwide use for six decades. We found that DEET binds at the edge of a long hydrophobic tunnel by exploiting numerous non-polar interactions and one hydrogen bond, which is perceived to be critical for DEET's recognition. Based on the experimentally determined affinity of AgamOBP1 for DEET (K (d) of 31.3 μΜ) and our structural data, we modeled the interactions for this protein with 29 promising leads reported in the literature to have significant repellent activities, and carried out fluorescence binding studies with four highly ranked ligands. Our experimental results confirmed the modeling predictions indicating that structure-based modeling could facilitate the design of novel repellents with enhanced binding affinity and selectivity.

New insights into the mechanism of odorant detection by the malaria-transmitting mosquito Anopheles gambiae

Anopheles gambiae mosquitoes that transmit Plasmodium falciparum malaria use a series of olfactory cues present in human sweat to locate their hosts for a blood meal. Recognition of these odor cues occurs through the interplay of odorant receptors and odorant-binding proteins (OBPs) that bind to odorant molecules and transport and present them to the receptors. Recent studies have implicated potential heterodimeric interactions between two OBPs, OBP1 and OBP4, as important for perception of indole by the mosquito (Biessmann, H., Andronopoulou, E., Biessmann, M. R., Douris, V., Dimitratos, S. D., Eliopoulos, E., Guerin, P. M., Iatrou, K., Justice, R. W., Kröber, T., Marinotti, O., Tsitoura, P., Woods, D. F., and Walter, M. F. (2010) PLoS ONE 5, e9471; Qiao, H., He, X., Schymura, D., Ban, L., Field, L., Dani, F. R., Michelucci, E., Caputo, B., della Torre, A., Iatrou, K., Zhou, J. J., Krieger, J., and Pelosi, P. (2011) Cell. Mol. Life Sci. 68, 1799-1813). Here we present the 2.0 Å crystal structure of the OBP4-indole complex, which adopts a classical odorant-binding protein fold, with indole bound at one end of a central hydrophobic cavity. Solution-based NMR studies reveal that OBP4 exists in a molten globule state and binding of indole induces a dramatic conformational shift to a well ordered structure, and this leads to the formation of the binding site for OBP1. Analysis of the OBP4-OBP1 interaction reveals a network of contacts between residues in the OBP1 binding site and the core of the protein and suggests how the interaction of the two proteins can alter the binding affinity for ligands. These studies provide evidence that conformational ordering plays a key role in regulating heteromeric interactions between OBPs.

Characterization of olfactory genes in the antennae of the Southern house mosquito, Culex quinquefasciatus

Odorant reception in insects is mediated by different families of olfactory proteins. Here we focus on the characterization of odorant-binding proteins (OBPs), "plus-C" odorant-binding proteins ("plus-C" OBPs), chemosensory proteins (CSPs) and sensory neuron membrane proteins (SNMPs) families from the Southern house mosquito, Culex quinquefasciatus, a vector of pathogens implicated in multiple human diseases. Using bioinformatics and molecular approaches, we have identified a diversity of genes in the genome of Culex quinquefasciatus and examined their expression profiles by RT-PCR and real-time quantitative PCR. Based on their high transcript enrichment in female antennae compared to non-olfactory tissues, we have identified twelve OBPs, two "plus-C" OBPs and two SNMPs that likely play important roles in odorant reception. Transcripts of two genes were clearly enriched in female antennae compared to male antennae, whereas other genes displayed relatively equivalent transcript levels in antennae of both sexes. Additionally, eight genes were found to be transcribed at very high levels in female antennae compared to CquiOR7, suggesting they might encode highly abundant olfactory proteins. Comparative analysis across different mosquito species revealed that olfactory genes of Culex quinquefasciatus are related to putative orthologs in other species, indicating that they might perform similar functions. Understanding how mosquitoes are able to detect ecologically relevant odorant cues might help designing better control strategies. We have identified olfactory genes from different families which are likely important in Culex quinquefasciatus behaviors, thus paving the way towards a better understanding of the diversity of proteins involved in the reception of semiochemicals in this species.