Functional Chimeras of the Human α7 Acetylcholine Receptor Provide Insights into Allosteric Modulation

Abstract

The human α7 neuronal acetylcholine receptor (α7nAChR) is a promising drug target for treatment of psychiatric and neurological disorders. A group of positive allosteric modulators (PAMs) specific to α7nAChR are thought to act through the transmembrane domain (TMD), but the structural basis of functional modulation remains unclear. In this study, we constructed multiple chimeras between the TMD of human α7nAChR and the extracellular domain (ECD) of a bacterial homolog, ELIC, for which a high-resolution structure has been determined. Functions of the chimeras were evaluated in Xenopus oocytes by two-electrode voltage clamp electrophysiology. Functional ELIC-α7nAChR chimeras were obtained when their ECD-TMD interfaces were modified to resemble either the ELIC or α7nAChR interface, but only the latter chimera with the entirely native α7nAChR TMD retained the unique pharmacology of α7nAChR evoked by the modulators, including insensitivity to the anesthetic propofol, potentiation by the PAMs ivermectin, PNU-120596, and TQS, as well as activation by 4BP-TQS. None of the ELIC-α7nAChR chimeras exhibited the fast desensitization characteristic of α7nAChR. Functional analysis of a series of chimeras indicated that efficient coupling at loop 7, in addition to efficient coupling at loop 2 or loop 9, was essential for a channel to be functional. The most efficient coupling at the ECD-TMD interface required optimization of all three loops. Altogether, the study suggests the interdependent of the TMD on the ECD-TMD interface for channel gating, desensitization, and allosteric modulation. (Funded in part by grants from the NIH: R01GM069766, R01GM057257, and R37GM049202)