Abstract
Insect Ionotropic Receptors (IRs) are a relatively uncharted territory. Some studies have documented IR activation by recording neuronal activity in situ, others by their heterologous expression in Xenopus oocytes or mis-expressing IRs from Drosophila melanogaster or from the related D. sechellia into the D. melanogaster "ionotropic receptor decoder" neuron, which lacks the endogenous tuning receptor subunit but expresses IR-coreceptors. In this study, we first made use of Drosophila olfactory sensory neurons (OSNs) different from the "ionotropic receptor decoder", demonstrating that by replacing or introducing IRs alongside the native D. melanogaster ones, functional heteromeric complexes can be formed. IR41a1 from the lepidopteran Cydia pomonella exhibits binding to polyamines and the IR75d from the dipteran Drosophila suzukii binds hexanoic acid. Secondly, expressing D. suzukii's putative acid sensor IR64a into the "ionotropic receptor decoder" of D. melanogaster inhibits the response to the main activators of neighboring neurons from the same sensillum, despite that IR64a does not respond to acids. In situ hybridization on the antennae of D. suzukii unveils wide expression of IR64a in neurons proximal to the sacculus. Structural modeling analysis does not explain its absence of binding to acids; conversely, this approach identifies key amino acids features explaining the binding of hexanoic acid by IR75d. Finally, we have also explored alternative methods to heterologously express IRs based on Human Embryonic Kidney cells (HEK293). Despite observing correct expression of IRs in transfected cells through immunohistochemistry experiments, this approach did not achieve successful deorphanization of these receptors. Our findings highlight the potential use of Drosophila OSNs as a valuable tool for functional characterization of IRs from different insect species: for the first time, we have provided evidence of IR-functionalities within alternative OSNs from the Drosophila's "ionotropic receptor decoder" neuron to functionally characterize and deorphanize IRs from lineages that are evolutionarily distant from the D. melanogaster subgroup, contributing to the understanding of chemosensory modalities in D. suzukii and C. pomonella, two globally significant agricultural pests. Additionally, the unsuccessful deorphanization in HEK cells highlights the complex requirements for IR functionality, supporting the use of Drosophila OSNs as a more suitable expression system.
