Effect of OBPs on the response of olfactory receptors

Structural basis for odorant recognition of the insect odorant receptor OR-Orco heterocomplex

Insects detect and discriminate a diverse array of chemicals using odorant receptors (ORs), which are ligand-gated ion channels comprising a divergent odorant-sensing OR and a conserved odorant receptor co-receptor (Orco). In this work, we report structures of the ApOR5-Orco heterocomplex from the pea aphid Acyrthosiphon pisum alone and bound to its known activating ligand, geranyl acetate. In these structures, three ApOrco subunits serve as scaffold components that cannot bind the ligand and remain relatively unchanged. Upon ligand binding, the pore-forming helix S7b of ApOR5 shifts outward from the central pore axis, causing an asymmetrical pore opening for ion influx. Our study provides insights into odorant recognition and channel gating of the OR-Orco heterocomplex and offers structural resources to support development of innovative insecticides and repellents for pest control.

A highly expressed odorant receptor from the yellow fever mosquito, AaegOR11, responds to (+)- and (-)-fenchone and a phenolic repellent

The cornerstone of the reverse chemical ecology approach is the identification of odorant receptors (OR) sensitive to compounds in a large panel of odorants. In this approach, we de-orphanize ORs and, subsequently, measure behaviors elicited by these semiochemicals. After that, we evaluate behaviorally active compounds for applications in insect vector management. Intriguingly, multiple ORs encoded by genes highly expressed in mosquito antennae do not respond to any test odorant. One such case is CquiOR125 from the southern house mosquito, Culex quinquefasciatus Say. To better understand CquiOR125's role in Culex mosquito olfaction, we have cloned a CquiOR125 orthologue in the genome of the yellow fever mosquito, Aedes aegypti (L.), AaegOR11. Unlike the unresponsive nature of the orthologue in Cx. quinquefasciatus, oocytes co-expressing AaegOR11 and AaegOrco elicited robust responses when challenged with fenchone, 2,3-dimethylphenol, 3,4-dimethylphenol, 4-methycyclohexanol, and acetophenone. Interestingly, AaegOR11 responded strongly and equally to (+)- and (-)-fenchone, with no chiral discrimination. Contrary to reports in the literature, fenchone did not show any repellency activity against Ae. aegypti or Cx. quinquefasciatus. Laboratory and field tests did not show significant increases in egg captures in cups filled with fenchone solutions compared to control cups. The second most potent ligand, 2,3-dimethylphenol, showed repellency activity stronger than that elicited by DEET at the same dose. We, therefore, concluded that AaegOR11 is a mosquito repellent sensor. It is feasible that CquiOR125 responds to repellents that remain elusive.

Functional Interaction Between Drosophila Olfactory Sensory Neurons and Their Support Cells

Insects detect volatile chemicals using antennae, which house a vast variety of olfactory sensory neurons (OSNs) that innervate hair-like structures called sensilla where odor detection takes place. In addition to OSNs, the antenna also hosts various support cell types. These include the triad of trichogen, tormogen, and thecogen support cells that lie adjacent to their respective OSNs. The arrangement of OSN supporting cells occurs stereotypically for all sensilla and is widely conserved in evolution. While insect chemosensory neurons have received considerable attention, little is known about the functional significance of the cells that support them. For instance, it remains unknown whether support cells play an active role in odor detection, or only passively contribute to homeostasis, e.g., by maintaining sensillum lymph composition. To investigate the functional interaction between OSNs and support cells, we used optical and electrophysiological approaches in Drosophila. First, we characterized the distribution of various supporting cells using genetic markers. By means of an ex vivo antennal preparation and genetically-encoded Ca²⁺ and K⁺ indicators, we then studied the activation of these auxiliary cells during odor presentation in adult flies. We observed acute responses and distinct differences in Ca²⁺ and K⁺ fluxes between support cell types. Finally, we observed alterations in OSN responses upon thecogen cell ablation in mature adults. Upon inducible ablation of thecogen cells, we notice a gain in mechanical responsiveness to mechanical stimulations during single-sensillum recording, but a lack of change to the neuronal resting activity. Taken together, these results demonstrate that support cells play a more active and responsive role during odor processing than previously thought. Our observations thus reveal that support cells functionally interact with OSNs and may be important for the extraordinary ability of insect olfactory systems to dynamically and sensitively discriminate between odors in the turbulent sensory landscape of insect flight.