Characterization of Anopheles stephensi Odorant Receptor 8, an Abundant Component of the Mouthpart Chemosensory Transcriptome

Abstract

Several mosquito species within the genus Anopheles are vectors for human malaria, and the spread of this disease is driven by the propensity of certain species to feed preferentially on humans. The study of olfaction in mosquitoes is important to understand dynamics of host-seeking and host-selection; however, the majority of these studies focus on Anopheles gambiae or An. coluzzii, both vectors of malaria in Sub-Saharan Africa. Other malaria vectors may recognize different chemical cues from potential hosts; therefore, in this study, we investigated An. stephensi, the south Asian malaria mosquito. We specifically focused on the mouthparts (primarily the maxillary palp and labella) that have been much less investigated compared to the antennae but are also important for host-seeking. To provide a broad view of chemoreceptor expression, RNAseq was used to examine the transcriptomes from the mouthparts of host-seeking females, blood-fed females, and males. Notably, AsOr8 had a high transcript abundance in all transcriptomes and was, therefore, cloned and expressed in the Drosophila empty neuron system. This permitted characterization with a panel of odorants that were selected, in part, for their presence in the human odor profile. The responsiveness of AsOr8 to odorants was highly similar to An. gambiae Or8 (AgOr8), except for sulcatone, which was detected by AsOr8 but not AgOr8. Subtle differences in the receptor sensitivity to specific odorants may provide clues to species- or strain-specific approaches to host-seeking and host selection. Further exploration of the profile of An. stephensi chemosensory proteins may yield a better understanding of how different malaria vectors navigate host-finding and host-choice.

The Aversive Response of the Locust Locusta migratoria to 3-Octanone, an Odorant from Fungal Pathogens, Is Mediated by a Chemosensory Protein

“…Among the 18 compounds, only 3 compounds (2, 5-dimethylpyrazine, 3-heptanone, and 2-heptanone) could induce excitatory reactions; most of the others did not respond after stimulation. This result was in contrast to the performance of mosquito OR8 (AgOR8), which has a similar odorant response profile in both endogenous and exogenous neurons (only slightly narrower in the exogenous system than in the endogenous system; the compounds induced excitatory responses from 12 to 9) [20]. Given that these 18 compounds can stimulate strong inhibitory responses of B neurons in locust, the change in the response spectrum of LmigOR3 might stem from the competitive binding of different ORs to the same compound, or the interaction between different neurons in the same sensillum [19].…”

“…Given that these 18 compounds can stimulate strong inhibitory responses of B neurons in locust, the change in the response spectrum of LmigOR3 might stem from the competitive binding of different ORs to the same compound, or the interaction between different neurons in the same sensillum [19]. Furthermore, the differential sensitivity to these ligands observed between at1 empty neurons expressing LmigOR3 and endogenous neurons expressing LmigOR3 differed; specifically, the intensity of compounds that can trigger excitatory reactions decreased from 30 spikes/s < n (exogenous neuron) to 5 < n < 10 spikes/s (endogenous neuron) [23], and some compounds elicited responses that appeared excitatory in empty neurons but inhibitory in endogenous neurons, which was similar to AgOR8 [20]. This might stem from differences in the lymphatic environment and competitive combinations of other LmigORs [22].…”

“…Nevertheless, previous studies have found that the "empty neuron" system has certain limitations. For example, the ideal effect of the system was observed when mosquito ORs were expressed; however, the ideal effect was not observed in a system expressing moth ORs [20,22].…”

“…The "empty neuron" system in Drosophila can be used in non-model insects to study the chemical reaction spectrum of ORs; it accurately reflects the reaction characteristics of exogenous ORs and can be used to measure the specificity and sensitivity of OR reactions [18]. This has become the main system used for studying the functions of ORs [8,[19][20][21]. Nevertheless, previous studies have found that the "empty neuron" system has certain limitations.…”

VectorBase.org updates: bioinformatic resources for invertebrate vectors of human pathogens and related organisms

“…Genome resources have also been used to develop wet lab techniques, for example, for primer design [ 32 ] or a multilocus amplicon sequencing for simultaneous mosquito species identification and detection of parasite infection status [ 33 ]. BLAST [ 34 , 35 ], gene enrichment [ 36 ], and comparative genomic analyses among the same or different species, have been used for phylogenetic and homolog gene predictions [ 37 •, 38 •, 39 •]. Genome assemblies have been improved, creating physical maps [ 40 ], karyotypes [ 41 ], and genome elements identified [ 42 – 44 ] using VectorBase files and tools.…”