Résumé
Functional GABAB receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on GABAB receptors [11, 72]) are formed from the heterodimerization of two similar 7TM subunits termed GABAB1 and GABAB2 [11, 71, 28, 72, 85]. GABAB receptors are widespread in the CNS and regulate both pre- and postsynaptic activity. The GABAB1 subunit, when expressed alone, binds both antagonists and agonists, but the affinity of the latter is generally 10-100-fold less than for the native receptor. Co-expression of GABAB1 and GABAB2 subunits allows transport of GABAB1 to the cell surface and generates a functional receptor that can couple to signal transduction pathways such as high-voltage-activated Ca2+ channels (Cav2.1, Cav2.2), or inwardly rectifying potassium channels (Kir3) [12, 11, 5]. The GABAB1 subunit harbours the GABA (orthosteric)-binding site within an extracellular domain (ECD) venus flytrap module (VTM), whereas the GABAB2 subunit mediates G protein-coupled signalling [11, 71, 40, 39]. The two subunits interact by direct allosteric coupling [63], such that GABAB2 increases the affinity of GABAB1 for agonists and reciprocally GABAB1 facilitates the coupling of GABAB2 to G proteins [71, 54, 39]. GABAB1 and GABAB2 subunits assemble in a 1:1 stoichiometry by means of a coiled-coil interaction between α-helices within their carboxy-termini that masks an endoplasmic reticulum retention motif (RXRR) within the GABAB1 subunit but other domains of the proteins also contribute to their heteromerization [5, 71, 15]. Recent evidence indicates that higher order assemblies of GABAB receptor comprising dimers of heterodimers occur in recombinant expression systems and in vivo and that such complexes exhibit negative functional cooperativity between heterodimers [70, 22]. Adding further complexity, KCTD (potassium channel tetramerization proteins) 8, 12, 12b and 16 associate as tetramers with the carboxy terminus of the GABAB2 subunit to impart altered signalling kinetics and agonist potency to the receptor complex [84, 3, 79] and are reviewed by [73]. The molecular complexity of GABAB receptors is further increased through association with trafficking and effector proteins [Schwenk et al., 2016, Nature Neuroscience 19(2): 233-42] and reviewed by [69]. Four isoforms of the human GABAB1 subunit have been cloned. The predominant GABAB1a and GABAB1b isoforms, which are most prevalent in neonatal and adult brain tissue respectively, differ in their ECD sequences as a result of the use of alternative transcription initiation sites. GABAB1a-containing heterodimers localise to distal axons and mediate inhibition of glutamate release in the CA3-CA1 terminals, and GABA release onto the layer 5 pyramidal neurons, whereas GABAB1b-containing receptors occur within dendritic spines and mediate slow postsynaptic inhibition [75, 89]. Only the 1a and 1b variants are identified as components of native receptors [11]. Additional GABAB1 subunit isoforms have been described in rodents and humans [55] and reviewed by [5].