Heterogeneous expression of GABA receptor-like subunits LCCH3 and GRD reveals functional diversity of GABA receptors in the honeybeeApis mellifera

Background and Purpose Despite a growing awareness, annual losses of honeybee colonies worldwide continue to reach threatening levels for food safety and global biodiversity. Among the biotic and abiotic stresses probably responsible for these losses, pesticides, including those targeting ionotropic GABA receptors, are one of the major drivers. Most insect genomes include the ionotropic GABA receptor subunit gene,Rdl, and two GABA-like receptor subunit genes,Lcch3andGrd.Most studies have focused onRdlwhich forms homomeric GABA-gated chloride channels, and a complete analysis of all possible molecular combinations of GABA receptors is still lacking. Experimental Approach We cloned theRdl,Grd, andLcch3genes ofApis melliferaand systematically characterized the resulting GABA receptors expressed inXenopusoocytes, using electrophysiological assays, fluorescence microscopy and co-immunoprecipitation techniques. Key Results The cloned subunits interacted with each other, forming GABA-gated heteromeric channels with particular properties. Strikingly, these heteromers were always more sensitive thanAmRDL homomer to all the pharmacological agents tested. In particular, when expressed together,GrdandLcch3form a non-selective cationic channel that opens at low concentrations of GABA and with sensitivity to insecticides similar to that of homomericRdlchannels. Conclusion and Implications For off-target species like the honeybee, chronic sublethal exposure to insecticides constitutes a major threat. At these concentration ranges, homomeric RDL receptors may not be the most pertinent target to study and other ionotropic GABA receptor subtypes should be considered in order to understand more fully the molecular mechanisms of sublethal toxicity to insecticides.