Molecular Determinants Underlying Delta Selective Compound 2 Activity at δ-Containing GABAA Receptors

Delta selective compound 2 (DS2; 4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl]benzamide) is one of the most widely used tools to study selective actions mediated by δ-subunit–containing GABA_A receptors. DS2 was discovered over 10 years ago, but despite great efforts, the precise molecular site of action has remained elusive. Using a combination of computational modeling, site-directed mutagenesis, and cell-based pharmacological assays, we probed three potential binding sites for DS2 and analogs at α₄β₁δ receptors: an α₄⁽⁺⁾δ⁽⁻⁾ interface site in the extracellular domain (ECD), equivalent to the diazepam binding site in αβγ₂ receptors, and two sites in the transmembrane domain (TMD) - one in the α₄⁽⁺⁾β₁⁽⁻⁾ and one in the α₄⁽⁻⁾β₁⁽⁺⁾ interface, with the α₄⁽⁻⁾β₁⁽⁺⁾ site corresponding to the binding site for etomidate and a recently disclosed low-affinity binding site for diazepam. We show that mutations in the ECD site did not abrogate DS2 modulation. However, mutations in the TMD α₄⁽⁺⁾β₁⁽⁻⁾ interface, either α₄(S303L) of the α₄⁽⁺⁾ side or β₁(I289Q) of the β₁⁽⁻⁾ side, convincingly disrupted the positive allosteric modulation by DS2. This was consistently demonstrated both in an assay measuring membrane potential changes and by whole-cell patch-clamp electrophysiology and rationalized by docking studies. Importantly, general sensitivity to modulators was not compromised in the mutated receptors. This study sheds important light on the long-sought molecular recognition site for DS2, refutes the misconception that the selectivity of DS2 for δ-containing receptors is caused by a direct interaction with the δ-subunit, and instead points toward a functional selectivity of DS2 and its analogs via a surprisingly well conserved binding pocket in the TMD.

SIGNIFICANCE STATEMENT

δ-Containing GABA_A receptors represent potential drug targets for the treatment of several neurological conditions with aberrant tonic inhibition, yet no drugs are currently in clinical use. With the identification of the molecular determinants responsible for positive modulation by the known compound delta selective compound 2, the ground is laid for design of ligands that selectively target δ-containing GABA_A receptor subtypes, for better understanding of tonic inhibition, and ultimately, for rational development of novel drugs.