Assembly of GABAA receptor subunits determines sorting and localization in polarized cells

The GABAA receptor, the principal inhibitory receptor in the CNS, is distributed on cell bodies, dendrites, and in some cells at axon hillocks and presynaptic terminals. The dendritic distribution is crucial for shunting of excitatory synaptic inputs. Molecular cloning has revealed that the GABAA receptor can be formed by a diverse set of subunits and by separately encoded subunit isoforms, the expression of each of which differs in distinct areas of the central nervous system and during development. Why different genes exist to encode these isoforms is not clear, but may be linked to functional differences. Here we show that assembly of specific isoforms also codes for sorting and localization of the receptor complex. Confocal microscopy and immunoblot analysis of epithelial cells transfected with the complementary DNAs encoding the alpha 1 and beta 1 GABAA receptor subunits and probed with subunit isoform-specific antibodies show that the alpha 1 subunit is targeted to the basolateral surface, and that the beta 1 subunit is sorted to the apical membrane. In cells where alpha 1 and beta 1 isoforms are co-expressed, assembly of the beta 1 with the alpha 1 subunit isoform re-routes the alpha 1 subunit to the apical surface. The ability to assemble complexes of different isoform composition and to target these to specific regions of the cell surface would enable neurons to modulate GABAA receptor distribution and possibly alter the composition of its synapses in response to transcriptional levels of specific subunit isoforms.