Heterogeneous expression of GABA receptor-like subunits LCCH3 and GRD reveals functional diversity of Apis mellifera GABA receptors.

BACKGROUND AND PURPOSE: Despite a growing awareness, the annual losses of honeybee colonies worldwide continue to reach threatening levels for food safety and global biodiversity. Among the biotic and abiotic stresses that may be responsible for these losses, pesticides, including those targeting GABA receptors, are one of the major drivers. Most insect genome include a GABA receptor subunit gene, Rdl, and two GABA-like receptor subunit genes, Lcch3 and Grd. Most studies have focused on Rdl which forms homomeric GABA-gated chloride channels, and a complete analysis of all the putative molecular combinations of GABA receptors is still lacking. EXPERIMENTAL APPROACH: In this study, we have cloned the Rdl, Grd and Lcch3 genes of Apis mellifera and undertaken a systematic characterization of the resulting GABA receptors in the Xenopus oocyte system. KEY RESULTS: We have shown that these subunits are able to interact with each other thus forming GABA-gated heteromeric channels with peculiar properties. Strikingly, these heteromers are always more sensitive than AmRDL homomer to all the pharmacological agents tested. In particular, when expressed together, Grd and Lcch3 form a non-selective cationic channel that opens at low concentrations of GABA and with sensitivity to insecticides similar to that of homomeric Rdl channels. CONCLUSION AND IMPLICATIONS: For off-target species like the honeybee, the chronic sublethal exposure to insecticides constitutes a major threat. At these concentration ranges, our work points out that homomeric RDL receptors may not be the most pertinent target to study and others GABA receptor subtypes should be considered in order to understand the molecular mechanisms of sublethal toxicity to insecticides.