γ-Aminobutyric Acid Type A (GABAA) Receptor α Subunits Play a Direct Role in Synaptic Versus Extrasynaptic Targeting

GABA(A) receptors (GABA(A)-Rs) are localized at both synaptic and extrasynaptic sites, mediating phasic and tonic inhibition, respectively. Previous studies suggest an important role of gamma 2 and delta subunits in synaptic versus extrasynaptic targeting of GABA(A)-Rs. Here, we demonstrate differential function of gamma 2 and alpha 6 subunits in guiding the localization of GABA(A)-Rs. To study the targeting of specific subtypes of GABA(A)-Rs, we used a molecularly engineered GABAergic synapse model to precisely control the GABA(A)-R subunit composition. We found that in neuron-HEK cell heterosynapses, GABAergic events mediated by alpha 2 beta 3 gamma 2 receptors were very fast (rise time similar to 2 ms), whereas events mediated by alpha 6 beta 3 delta receptors were very slow (rise time similar to 20 ms). Such an order of magnitude difference in rise time could not be attributed to the minute differences in receptor kinetics. Interestingly, synaptic events mediated by alpha 6 beta 3 or alpha 6 beta 3 gamma 2 receptors were significantly slower than those mediated by alpha 2 beta 3 or alpha 2 beta 3 gamma 2 receptors, suggesting a differential role of delta subunit in receptor targeting. This was confirmed by differential targeting of the same delta-gamma 2 chimeric subunits to synaptic or extrasynaptic sites, depending on whether it was co-assembled with the alpha 2 or alpha 6 subunit. In addition, insertion of a gephyrin-binding site into the intracellular domain of alpha 6 and delta subunits brought alpha 6 beta 3 delta receptors closer to synaptic sites. Therefore, the alpha subunits, together with the gamma 2 and delta subunits, play a critical role in governing synaptic versus extrasynaptic targeting of GABA(A)-Rs, possibly through differential interactions with gephyrin.