The heptahelical domain of GABAB2 is activated directly by CGP7930, a positive allosteric modulator of the GABAB receptor

Abstract

The γ-aminobutyric acid, type B (GABAB) receptor is well recognized as being composed of two subunits, GABAB1 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 GABAB1 binds GABA, GABA B2 is required for GABAB1 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, GABAB2 plays a major role in the coupling of the heteromer to G-proteins, such that it is possible that GABAB2 can transmit a signal in the absence of GABAB1. Today only ligands interacting with GABAB1 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 GABAB receptor. We showed that it activates the wild type GABAB receptor but with a low efficacy. The GABAB2 HD is necessary for this effect, although one cannot exclude that CGP7930 could also bind to GABAB1. Of interest, CGP7930 could activate GABAB2 expressed alone and is the first described agonist of GABAB2. Finally, we show that CGP7930 retains its agonist activity on a GABAB2 subunit deleted of its ECD. This demonstrates that the HD of GABAB2 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 GABAB receptor and examine any possible role of homomeric GABAB2 receptors.