Ligand-gated ion channels (LGCs) play important roles in synaptic communication and in regulating behaviours. The LGCs have undergone vast gene expansion in nematodes (with over 100 superfamily members) and includes two of the main targets for anti-helminth drugs, levamisole and ivermectin. Yet the majority of C. elegans LGCs remain uncharacterised, with no known ligand or biological function. We have begun an effort to deorphanise the uncharacterised C. elegans LGCs, in particular a subfamily of 12 channels known as the "diverse" group. Using two-electrode voltage clamp recordings from Xenopus oocytes injected with worm LGCs, we identified ligands for 5 of the 12 channels in the diverse subgroup. All are inhibitory anion-selective channels, yet despite sharing close sequence similarity, they are gated by chemically diverse ligands. Three receptors, GGR-1, GGR-2 and LGC-40 are gated by acetylcholine, and more potently, by choline. Since choline is generated at cholinergic synapses through the action of acetylcholinesterases, this raises the possibility that choline may act as a neuromodulator at these synapses in vivo. In contrast, LGC-39 is gated not only by acetylcholine but also by aminergic ligands, in particular octopamine and tyramine. Thus LGC-39 has the capacity to form a polymodal receptor activated by chemically-disparate neurotransmitters.
lgc-39 is expressed in many neurons involved in the reverse locomotor circuit, including AVD, AVA, DAs, VAs and dorsal body wall muscle; all of these neurons receive cholinergic input, in addition AVA is also a major target of the octopaminergic neuron RIC. Endogenous fluorescent tagging of LGC-39 using CRISPR revealed its synaptic localisation in the nerve ring and ventral cord. Tracking experiments indicate that
lgc-39 mutants display altered reversal behaviours; we aim to identify which ligand mediates these
lgc-39 specific reversal phenotypes. Finally, we find that LGC-41 is not activated by any known neurotransmitter, but is activated by betaine, a metabolite chemically related to choline. Endogenous tagged LGC-41 is localised almost exclusively to sensory cilia, including those of ASI. ASI is involved in sensing various environmental stimuli, including food availability. E. coli contains high concentrations of betaine, which is used as an osmoregulator. We therefore hypothesise that LGC-41 may function as a chemosensory receptor for betaine; calcium imaging experiments to test this are in progress. Taken together our findings highlight the remarkable functional diversification amongst the LGCs in C.elegans.