A Feedback Mechanism Regulates Odorant Receptor Expression in the Malaria Mosquito, Anopheles gambiae

Abstract

Mosquitoes locate and approach humans ( host-seek) when specific Olfactory Neurons (ORNs) in the olfactory periphery activate a specific combination of glomeruli in the mosquito Antennal Lobe (AL). We hypothesize that dysregulating proper glomerular activation in the presence of human odor will prevent host-seeking behavior. In experiments aimed at ectopically activating most ORNs in the presence of human odor, we made a surprising finding: ectopic expression of an AgOr (AgOr2) in Anopheles gambiae ORNs dampens the activity of the expressing neuron. This contrasts studies in Drosophila melanogaster, the typical insect model of olfaction, in which ectopic expression of non-native ORs in ORNs confers ectopic neuronal responses without interfering with native olfactory physiology. To gain insight into this dysfunction in mosquitoes, RNA-seq analyses were performed comparing wild-type antennae to those ectopically expressing AgOr2 in ORNs. Remarkably, almost all Or transcripts were significantly downregulated (except for AgOr2), and additional experiments suggest that it is AgOR2 protein rather than mRNA that mediates this downregulation. Our study shows that ORNs of Anopheles mosquitoes (in contrast to Drosophila) employ a currently unexplored regulatory mechanism of OR expression, which may be adaptable as a vector-control strategy. SIGNIFICANCE STATEMENTStudies in Drosophila melanogaster suggest that insect Olfactory Receptor Neurons (ORNs) do not contain mechanisms by which Odorant Receptors (ORs) regulate OR expression. This has proved useful in studies where ectopic expression of an OR in Drosophila ORNs confers responses to the odorants that activate the newly expressed OR. In experiments in Anopheles gambiae mosquitoes, we found that ectopic expression of an OR in most Anopheles ORNs dampened the activity of the expressing neurons. RNA-seq analyses demonstrated that ectopic OR expression in Anopheles ORNs leads to downregulation of endogenous Or transcripts. Additional experiments suggest that this downregulation required ectopic expression of a functional OR protein. These findings reveal that Anopheles mosquitoes, in contrast to Drosophila, contain a feedback mechanism to regulate OR expression. Mosquito ORNs might employ regulatory mechanisms of OR expression previously thought to occur only in non-insect olfactory systems.