"Evolution and function of hair cell cholinergic receptors"

ELGOYHEN AB

Congreso; Sociedad Argentina de Investigaciones Farmacológicas; 2019

Nicotinic acetylcholine receptors are a family of ligand-gated nonselective cationic channels that participate in fundamentalphysiological processes at both the central and the peripheral nervous system. The extent of calcium entrythrough ligand-gated ion channels defines their distinct functions. The 910 nicotinic cholinergic receptor, expressed incochlear hair cells, is a peculiar member of the family as it shows differences in the extent of calcium permeability acrossspecies. In particular, mammalian 910 receptors are among the ligand-gated ion channels which exhibit the highestcalcium selectivity. This acquired differential property provides the unique opportunity of studying how protein functionwas shaped along evolutionary history, by tracking its evolutionary record and experimentally defining the amino acidchanges involved. We have applied a molecular evolution approach of ancestral sequence reconstruction, together withmolecular dynamics simulations and an evolutionary-based mutagenesis strategy, in order to trace the molecular eventsthat yielded a high calciumpermeable nicotinic 910mammalian receptor. Only three specific amino acid substitutionsin the 9 subunit were directly involved. These are located at the extracellular vestibule and at the exit of the channelpore and not at the transmembrane region 2 of the protein as previously thought. Moreover, we show that these threecritical substitutions only increase calcium permeability in the context of the mammalian but not the avian receptor,stressing the relevance of overall protein structure on defining functional properties. These results highlight the importanceof tracking evolutionarily acquired changes in protein sequence underlying fundamental functional properties ofligand-gated ion channels.