Modulation of cell surface GABAB receptors by desensitization, trafficking and regulated degradation

Figure 1 Structural organization of GABA_B receptors. Functional GABA_B receptors are heterodimers composed of the two subunits GABA_B1 and GABA_B2. Both subunits are heptahelical membrane proteins with a large extracellular located N-terminal domain containing a “Venus flytrap” module and a large intracellular C-terminal domain containing a coiled-coil protein-protein interaction module. GABA_B1 and GABA_B2 heterodimerize via their “Venus flytrap” and coiled-coiled domains. An endoplasmic reticulum (ER) retention/retrieval signal is present distal to the coiled-coil domain in GABA_B1 and prevents ER exit of GABA_B1 unless it is masked by heterodimerization with GABA_B2. The “Venus flytrap” module of GABA_B1 constitutes the GABA binding site, whereas that of GABA_B2 is inactive and not involved in ligand binding. Instead, the heptahelical domain of GABA_B2 contains a binding site for allosteric modulators, which affects the affinity of ligands binding to the GABA site. Binding of GABA results in the recruitment and activation of Gαi/o proteins via GABA_B2. The activated Gαi/o subunit inhibits the adenylyl cyclase, resulting in lowered cAMP levels, while the Gβγ dimer activates K⁺ channels and inhibits Ca²⁺ channels, leading in either case to neuronal inhibition. There exist two isoforms of GABA_B1, named GABA_B1a and GABA_B1b, which are generated by alternative promoter usage. They only differ by the additional presence of two so-called “sushi repeats” (protein-protein interaction modules) in the N-terminal domain of GABA_B1a. GABA: γ-Aminobutyric acid; ATP: Adenosine-5'-triphosphate; cAMP: 3'-5'-cyclic adenosine monophosphate.

Figure 2 Mechanisms of GABA_B receptor desensitization. Three distinct mechanisms have been so far implicated in the desensitization of GABA_B receptors. In cerebellar granule cells, G protein receptor kinase (GRK) 4 and 5 associate with GABA_B receptors and induce desensitization of the receptors in a phosphorylation-independent manner. In cortical and hippocampal neurons, desensitization of the receptors involves the interaction of NEM-sensitive fusion protein (NSF) with GABA_B1 and GABA_B2, which is thought to prime the receptor for phosphorylation by protein kinase C (PKC). Association of potassium channel tetramerization domain-containing (KCTD) proteins 12 and 12b with the C-terminus of GABA_B2 appears to render the receptor complex competent for desensitization. GABA: γ-Aminobutyric acid.

Figure 3 Regulation of cell surface GABA_B receptors by trafficking. A: Under normal conditions GABA_B receptors are constitutively internalized and recycled back to the plasma membrane. Only a small fraction of receptors are sorted to lysosomes for degradation; B: Sustained activation of glutamate receptors [primarily 2-amino-3-(5-methyl-3-oxo-1,2- oxazol-4-yl) propanoic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptors] and L-type voltage-gated Ca²⁺ channels raises intracellular Ca²⁺ levels. This induces phosphorylation of GABA_B1 at serine 867 by calmodulin-dependent protein kinaseII (CaMKII) and of GABA_B2 at serine 783 by adenosine monophosphate (AMP) kinase, followed by slow dephosphorylation, by protein phosphatase 2 (PPA2). These events shift the recycling/degradation equilibrium towards degradation so that the majority of GABA_B receptors are no longer recycled, but instead degraded in lysosomes. Since constitutive endocytosis of the receptors remains unaffected, this mechanism results in a rapid down-regulation of GABA_B receptors. AMPK: 5’AMP-dependent protein kinase; GABA: γ-Aminobutyric acid; VSCCs: Voltage-sensitive calcium channels.