Mechanism of calcium potentiation of the α7 nicotinic acetylcholine receptor.

The alpha 7 nicotinic acetylcholine receptor (nAChR) is among the most abundant types of nAChR in the brain, yet the ability of nerve-released ACh to activate alpha 7 remains enigmatic. In particular, a major population of alpha 7 resides in extra-synaptic regions where the ACh concentration is reduced, owing to dilution and enzymatic hydrolysis, yet ACh shows low potency in activating alpha 7. Using high-resolution single-channel recording techniques, we show that extracellular calcium is a powerful potentiator of alpha 7 activated by low concentrations of ACh. Potentiation manifests as robust increases in the frequency of channel opening and the average duration of the openings. Molecular dynamics simulations reveal that calcium binds to the periphery of the five ligand binding sites and is framed by a pair of anionic residues from the principal and complementary faces of each site. Mutation of residues identified by simulation prevents calcium from potentiating ACh-elicited channel opening. An anionic residue is conserved at each of the identified positions in all vertebrate species of alpha 7. Thus, calcium associates with a novel structural motif on alpha 7 and is an obligate cofactor in regions of limited ACh concentration.