The Heptahelical Domain of GABA_B2 Is Activated Directly by CGP7930, a Positive Allosteric Modulator of the GABA_B Receptor*

Abstract

The γ-aminobutyric acid, type B (GABA_B) receptor is well recognized as being composed of two subunits, GABA_B1 and GABA_B2. Both subunits share structural homology with other class-III G-protein-coupled receptors. They are composed of two main domains: a heptahelical domain (HD) typical of all G-protein-coupled receptors and a large extracellular domain (ECD). Although GABA_B1 binds GABA, GABA_B2 is required for GABA_B1 to reach the cell surface. However, it is still not demonstrated whether the association of these two subunits is always required for function in the brain. Indeed, GABA_B2 plays a major role in the coupling of the heteromer to G-proteins, such that it is possible that GABA_B2 can transmit a signal in the absence of GABA_B1. Today only ligands interacting with GABA_B1 ECD have been identified. Thus, the compounds acting exclusively on the GABA_B2 subunit will be helpful in analyzing the specific role of this subunit in the brain. Here, we explored the mechanism of action of CGP7930, a compound described as a positive allosteric regulator of the GABA_B receptor. We showed that it activates the wild type GABA_B receptor but with a low efficacy. The GABA_B2 HD is necessary for this effect, although one cannot exclude that CGP7930 could also bind to GABA_B1. Of interest, CGP7930 could activate GABA_B2 expressed alone and is the first described agonistof GABA_B2. Finally, we show that CGP7930 retains its agonist activity on a GABA_B2 subunit deleted of its ECD. This demonstrates that the HD of GABA_B2 behaves similar to a rhodopsin-like receptor, because it can reach the cell surface alone, can couple to G-protein, and be activated by agonists. These data open new strategies for studying the mechanism of activation of GABA_B receptor and examine any possible role of homomeric GABA_B2 receptors.