Assembly and Heterogeneity of GABAA Receptors

The vast majority of inhibitory neurotransmission in the brain is mediated by γ-aminobutyric acid (GABA). It has been detected in approximately 30% of all synapses and acts via ionotropic GABAA receptors, which mediate fast inhibitory neurotransmission, and metabotropic GABAB receptors, which mediate slower inhibitory effects. GABAA receptors (GABAARs) are chloride channels belonging to the Cys-loop receptor superfamily of ligand-gated ion channels (LGIC), which also includes nicotinic acetylcholine receptors (nAChR), 5-hydroxytryptamine type 3 receptors (5-HT3), and glycine receptors (GlyR). Like most members of this superfamily, GABAARs are pentamers that are assembled from an array of homologous subunits. All subunits share a common structure: each contains a large, extracellular N-terminal domain, which contains the ligand-binding site and the eponymous Cys-loop; four α-helical transmembrane domains (M1-4); a large intracellular loop between the third and fourth transmembrane helices (M3-M4 loop); and a very short, extracellular C-terminal domain (Figure 1a). Nineteen subunits, grouped by sequence homology into eight subunit families, have been identified for the GABAA receptor: α1-6, β1-3, γ1-3, δ, ε, π, and ρ1-3. Several of these subunit subtypes also undergo alternative splicing and/or RNA editing, further increasing the potential diversity of GABAA receptor isoforms. Each subunit exhibits a characteristic expression pattern in the brain; however, these patterns overlap extensively. Indeed, a single neuron can express many subunits simultaneously. Consequently, many but not all of the mathematically-possible GABAAR isoforms could exist somewhere in the brain. The most common isoforms, however, are thought to comprise two α subunits, two β subunits, and one γ or δ subunit (Figure 1b), though this remains a subject of vigorous debate. The large variety of GABAAR isoforms exhibit a concomitant variety of physiological properties. For instance, most receptors containing a γ subunit are located in the synapse, where they mediate phasic inhibition in response to presynaptically-released GABA. These receptors have a relatively low affinity for GABA, activate quickly, desensitize extensively, and deactivate slowly. Conversely, receptors containing a δ subunit are located outside the synapse, where they mediate tonic inhibition in response to low concentrations of ambient GABA. Unsurprisingly, δ-subunit-containing receptors also differ physiologically; they have a relatively high affinity for GABA, activate slowly, desensitize minimally, and deactivate rapidly. Additionally, GABAARs have been linked to many diseases and disorders, including epilepsy, insomnia, anxiety, depression, schizophrenia, alcoholism, and autism. Predictably, then, GABAARs are targeted by numerous drugs, particularly sedatives, anxiolytics, and anticonvulsants; examples include benzodiazepines, zolpidem, etomidate, and propofol . Both the pathology and the pharmacology of GABAARs depend highly upon receptor subunit composition – for instance, epilepsy-associated mutations have been identified only in the α1, β3, γ2, and δ subunits, and benzodiazepines act only at receptor isoforms containing both a γ subunit and certain α subunits. Given the prevalence of GABAAR expression, the pathology resulting from receptor malfunction, and the pharmacological dependence upon isoform Assembly and Heterogeneity of GABAA Receptors