Subcellular localization of gamma-aminobutyric acid type A receptors is determined by receptor beta subunits.

Abstract

y-aminobutyric acid type A (GABAA) receptors are the major sites of fast synaptic inhibition in the brain. They are constructed from four subunit classes with multiple members: a(1-6), ,3(1-4), y(1-4), and 6(1). The contribution of subunit diversity in determining receptor subcellular targeting was examined in polarized Madin-Darby canine kidney (MDCK) cells. Significant detection of cell surface homomeric receptor expression by a combination of both immunological and electrophysiological methodologies was only found for the .83 subunit. Expression of a/,8 binary combinations resulted in a nonpolarized distribution for dgl1 complexes, but specific basolateral targeting of both a112 and a1j3 complexes. The polarized distribution of these a/"3 complexes was unaffected by the presence of the y2S subunit. Interestingly, delivery of receptors containing the 133 subunit to the basolateral domain occurs via the apical surface. These results show that 18 subunits can selectively target GABAA receptors to distinct cellular locations. Changes in the spatial and temporal expression of ,8-subunit isoforms may therefore provide a mechanism for relocating GABAA receptor function between distinct neuronal domains. Given the critical role of these receptors in mediating synaptic inhibition, the contribution ofdifferent 18 subunits in GABAA receptor function, may have implications in neuronal development and for receptor localization/clustering. ,y-aminobutyric acid type A (GABAA) receptors are the major sites of fast synaptic inhibition in the brain. Molecular cloning has revealed that these receptors are members of a ligandgated ion-channel superfamily that includes both nicotinic acetylcholine and glycine receptors (1). Members of this ion-channel superfamily are pentameric hetero-oligomers whose subunits share a common proposed transmembrane topology. This topology comprises a large N-terminal extracellular domain and four transmembrane domains with a large intracellular loop between transmembrane regions 3 and 4 (1). GABAA receptors can be constructed from a range of homologous subunit types, a(1-6), 13(1-4), -y(1-4), and 8(1), creating considerable potential for structural diversity (1). In situ hybridization and immunocytochemical studies suggest a large temporal and spatial diversity of GABAA receptor structure in the brain with many neuron types often expressing multiple receptor subunits (1).Heterologous expression has been used to determine the minimal structural requirements for the production of GABAgated chloride channels. Homomeric subunit expression (e.g., a, 13, or y) has produced various results, with the detection of GABA-gated chloride channels by some groups (2-4), but not by others (5-9). Similarly, expression of binary combinations (e.g., ay, fry) have produced functional GABAA receptors in some cases (10, 11), but not in others (5-8). However, there is general agreement that expression of receptor a and 13 subunits produces GABA-gated channels and that the coexpression of the y2 or y3 subunit is ...

Detection and Binding Properties of GABAA Receptor Assembly Intermediates

“…5, E and F). These results are in agreement with previous reports demonstrating that ␤ 3 subunits are able to form homo-oligomeric receptors that are expressed on the cell surface (21,30). The observation that the truncated and the full-length subunit could be detected in the permeabilized cells in the same subcellular compartments (Fig.…”

“…These subunits are able to form pentameric receptors (12) and are expressed on the cell surface (4,21,30). C-terminally truncated ␣ 1 or ␤ 3 subunits, containing only the extracellular Nterminal domain also could assemble with each other or with full-length ␤ 3 or ␣ 1 subunits, respectively, but the assembly products remained in intracellular compartments and could not be detected on the cell surface.…”

Alternate Use of Distinct Intersubunit Contacts Controls GABAAReceptor Assembly and Stoichiometry

“…Although homo-oligomeric receptors composed of ␤ 3 subunits could also have been formed under the conditions used, they would not have contributed to the GABA evoked current because these receptors apparently are not gated by GABA (Connolly et al, 1996b). Using various GABA concentrations, it was demonstrated that GABA exhibited an EC 50 of 10.5 Ϯ 2.1 M (mean Ϯ SEM; n ϭ 8 cells from different plates; total of four transfections) (Fig.…”

The diversity of subunit composition in nAChRs: Evolutionary origins, physiologic and pharmacologic consequences

“…The targeting of GABA A (Connolly et al, 1996) or glutamate receptors depends on the identity of the subunits forming the receptors. In addition, the composition in subunits also controls the local movements of glutamate receptors (Shi et al, 2001).…”