Molecular mechanisms of GABAB receptor activation: new insights from the mechanism of action of CGP7930, a positive allosteric modulator

Abstract

The GABA(B) (gamma-aminobutyric acid-B) receptor is composed of two subunits, GABA(B1) and GABA(B2). Both subunits share structural homology with other class-III G-protein-coupled receptors. They contain two main domains, a heptahelical domain typical of all G-protein-coupled receptors and a large ECD (extracellular domain). It has not been demonstrated whether the association of these two subunits is always required for function. However, GABA(B2) plays a major role in coupling with G-proteins, and GABA(B1) has been shown to bind GABA. To date, only ligands interacting with GABA(B1)-ECD have been identified. In the present study, we explored the mechanism of action of CGP7930, a compound described as a positive allosteric regulator of the GABA(B) receptor. We have shown that it can weakly activate the wild-type GABA(B) receptor, but also the GABA(B2) expressed alone, thus being the first described agonist of GABA(B2). CGP7930 retains its weak agonist activity on a GABA(B2) subunit deleted of its ECD. Thus the heptahelical domain of GABA(B2) behaves similar to a rhodopsin-like receptor. These results open new strategies for studying the mechanism of activation of GABA(B) receptor and examine any possible role of GABA(B2).

CGP7930: A Positive Allosteric Modulator of the GABAB Receptor

“…Compounds that stimulate adenylate cyclase possibly have a larger pool of G proteins (mediated by both G O and G S ) than those that inhibit it (mediated by G i ). Importantly, Onali et al (2003) note that there was no effect of CGP7930 alone; that is, it exhibits no agonistic properties in this model, indicating that the doses used in the study were low enough for CGP7930 not to act as an agonist at the GABA B receptor as previously reported (Binet et al 2004b).…”

“…In work on chimeric GABA B receptor subunits, it was discovered that CGP7930 does not require GABA B1 or GABA B2 extracellular domains at the present. Rather, the binding site seems to lie within the heptahelical domain, and although more likely to be within the GABA B2 subunit, the exact binding location remains unclear (Binet et al 2004b).…”

“…CRH‐stimulated adenylate cyclase activity is increased by both baclofen and GABA, an effect that is potentiated when CGP7930 is present (Onali et al 2003). In transfected HEK 293 cells, the EC 50 for GABA was increased three‐fold by 0.1 nM of CGP7930 and ten‐fold by 1.0 nM as measured via a [ 35 S]GTPγS assay (Binet et al 2004b). In membranes of rat granule cell layer, CGP7930 increased the pEC 50 and maximal effect of baclofen.…”

Diversity of structure and function of GABA_B receptors: a complexity of GABA_B-mediated signaling

“…92) CGP7930 directly acts as a PAM and a partial agonist through R2 subunits, which can facilitate agonist responses at low concentrations, and activate the receptor at higher concentrations. 93–95) A more potent compound has also been identified, GS39783, which enhances GABA B receptor-mediated inhibition of cAMP formation and shows anxiolytic-like effects and attenuates rewarding properties of the substances of abuse. 96–98)…”

The GABAB receptor as a target for antidepressant drug action

“…Whereas the GABAA site is a pentameric, ligand-gated ion channel allosterically modulated by benzodiazepines and other anxiolytic and hypnotic agents, the GABAB receptor, a G protein-coupled heterodimer, is the site of action for baclofen, a muscle relaxant (Bowery, 2010). A class III metabotropic site, the GABAB receptor is composed of two 7-transmembrane spanning proteins (Kubo and Tateyama, 2005;Binet et al, 2006). While there are multiple subunit isoforms in various animal species, GABAB1a, GABAB1b and GABAB2 predominate, with coupling of either of the GABAB1 subtypes with GABAB2 essential for insertion into the plasma membrane and receptor function (Kaupmann et al, 1998;Chronwall et al, 2001;Enna and Bowery, 2010).…”