A Role for the 2′ Residue in the Second Transmembrane Helix of the GABAA Receptor γ2S Subunit in Channel Conductance and Gating

Molecular dynamics investigation of Cl(-) transport through the closed and open states of the 2α12β2γ2 GABA(A) receptor

The α1β2γ2 gamma-aminobutyric type A receptor (GABA(A)R) is one of the most widely expressed GABA(A)R subtypes in the mammalian brain. GABA(A)Rsbelonging to the Cys-loop superfamily of ligand-gated ion channels have been identified as key targets for many clinical drugs, and the motions that govern the gating mechanism are still not well understood. In this study, an open-state GABA(A)R was constructed using the structure of the glutamate-gated chloride channel (GluCl), which has a high sequence identity to GABA(A)R. A closed-state model was constructed using the structure of the nicotinic acetylcholine receptor (nAChR). Molecular dynamics simulations of the open-state and closed-state GABA(A)R were performed. We calculated the electrostatic potential of the two conformations, the pore radius of the two ion channels and the root-mean-square fluctuation. We observed the presence of two positively charged girdles around the ion channel and found flexible regions in the GABA(A)R. Then, the free-energy of chloride ion permeations through the closed-state and open-state G GABA(A)R has been estimated using adaptive biasing force (ABF) simulation. For the closed-state G GABA(A)R, we observed two major energy barriers for chloride ion translocation in the transmembrane domain (TMD). For the open-state GABA(A)R, there was only one energy barrier formed by two Thr261 (α1), two Thr255 (β2) and one Thr271 (γ2). By using ABF simulation, the overall free-energy profile is obtained for Cl(-) transporting through GABA(A)R, which gives a complete map of the ion channel of Cl(-) permeation.

Insights into the biophysical properties of GABA(A) ion channels: modulation of ion permeation by drugs and protein interactions

The fundamental properties of ion channels assure their selectivity for a particular ion, its rapid permeation through a central pore and that such electrical activity is modulated by factors that control the opening and closing (gating) of the channel. All cell types possess ion channels and their regulated flux of ions across the membrane play critical roles in all steps of life. An ion channel does not act alone to control cell excitability but rather forms part of larger protein complexes. The identification of protein interaction partners of ion channels and their influence on both the fundamental biophysical properties of the channel and its expression in the membrane are revealing the many ways in which electrical activity may be regulated. Highlighted here is the novel use of the patch clamp method to dissect out the influence of protein interactions on the activity of individual GABA(A) receptors. The studies demonstrate that ion conduction is a dynamic property of a channel and that protein interactions in a cytoplasmic domain underlie the channel's ability to alter ion permeation. A structural model describing a reorganisation of the conserved cytoplasmic gondola domain and the influence of drugs on this process are presented.

Protein interactions involving the gamma2 large cytoplasmic loop of GABA(A) receptors modulate conductance

Native GABA(A) channels display a single-channel conductance ranging between approximately 10 and 90 pS. Diazepam increases the conductance of some of these native channels but never those of recombinant receptors, unless they are coexpressed with GABARAP. This trafficking protein clusters recombinant receptors in the membrane, suggesting that high-conductance channels arise from receptors that are at locally high concentrations. The amphipathic (MA) helix that is present in the large cytoplasmic loop of every subunit of all ligand-gated ion channels mediates protein-protein interactions. Here we report that when applied to inside-out patches, a peptide mimicking the MA helix of the gamma2 subunit (gamma(381-403)) of the GABA(A) receptor abrogates the potentiating effect of diazepam on both endogenous receptors and recombinant GABA(A) receptors coexpressed with GABARAP, by substantially reducing their conductance. The protein interaction disrupted by the peptide did not involve GABARAP, because a shorter peptide (gamma(386-403)) known to compete with the gamma2-GABARAP interaction did not affect the conductance of recombinant alphabetagamma receptors coexpressed with GABARAP. The requirement for receptor clustering and the fact that the gamma2 MA helix is able to self-associate support a mechanism whereby adjacent GABA(A) receptors interact via their gamma2-subunit MA helices, altering ion permeation through each channel. Alteration of ion-channel function arising from dynamic interactions between ion channels of the same family has not been reported previously and highlights a novel way in which inhibitory neurotransmission in the brain may be differentially modulated.

Differential drug responses on native GABA(A) receptors revealing heterogeneity in extrasynaptic populations in cultured hippocampal neurons

Hippocampal pyramidal neurons potentially express multiple subtypes of GABA(A) receptors at extrasynaptic locations that could therefore respond to different drugs. We activated extrasynaptic GABA(A) receptors in cultured rat hippocampal pyramidal neurons and measured single-channel currents in order to compare the actions of two drugs that potentially target different GABA(A) receptor subtypes. Despite the possible difference in receptor targets of etomidate and diazepam, the two drugs were similar in their actions on native extrasynaptic GABA(A) receptors. Each drug produced three distinct responses that differed significantly in current magnitude, implying heterogeneous GABA(A) receptor populations. In the majority of patches, drug application increased both the single-channel conductance (>40 pS) and the open probability of the channels. By contrast, in the minority of patches, drug application caused an increase in open probability only. In the third group high-conductance channels were observed upon GABA activation and drug application increased their open probability only. The currents potentiated by etomidate or diazepam were substantially larger in patches displaying high-conductance GABA channels compared to those displaying only low-conductance channels. Factors contributing to the large magnitude of these currents were the long mean open time of high-conductance channels and the presence of multiple channels in these patches. In conclusion, we suggest that the local density of extrasynaptic GABA(A) receptors may influence their single-channel properties and may be an additional regulating factor for tonic inhibition and, importantly, differential drug modulation.

A single amino acid in the second transmembrane domain of GABA rho receptors regulates channel conductance

GABAC receptors, expressed predominately in vertebrate retina, are thought to be formed mainly by GABA rho subunits, each of which exhibits distinct physiological and pharmacological properties. In this study, the receptors formed by perch GABA rho subunits were expressed in HEK cells, and their single channel conductances were determined using noise analysis techniques. The receptors formed by the perch rho1A subunit gate a channel with a conductance of 0.2 pS, whereas the receptors formed by GABA rho2 subunits exhibit much higher channel conductances, i.e., 3.2 and 3.5 pS for perch rho2A and rho2B receptors, respectively. A comparison of the amino acid sequences of the channel-forming TMII regions of the various subunits suggested that a single amino acid at position 2' was a potential site for the large differential in conductance. We found that switching the serine residue at that site in the GABA rho2 subunit to the proline residue present in the rho1 subunit reduced the channel conductance to a level similar to that of the wild type rho1 receptor. Conversely, mutating proline to serine in the amino acid sequence of the rho1 receptor significantly increased its unitary conductance. These results indicate that a single amino acid in the TMII region plays an important role in determining the single channel conductance of the GABAC receptors.

The cellular, molecular and ionic basis of GABA(A) receptor signalling

GABA(A) receptors mediate fast synaptic inhibition in the CNS. Whilst this is undoubtedly true, it is a gross oversimplification of their actions. The receptors themselves are diverse, being formed from a variety of subunits, each with a different temporal and spatial pattern of expression. This diversity is reflected in differences in subcellular targetting and in the subtleties of their response to GABA. While activation of the receptors leads to an inevitable increase in membrane conductance, the voltage response is dictated by the distribution of the permeant Cl(-) and HCO(3)(-) ions, which is established by anion transporters. Similar to GABA(A) receptors, the expression of these transporters is not only developmentally regulated but shows cell-specific and subcellular variation. Untangling all these complexities allows us to appreciate the variety of GABA-mediated signalling, a diverse set of phenomena encompassing both synaptic and non-synaptic functions that can be overtly excitatory as well as inhibitory.

GABA increases both the conductance and mean open time of recombinant GABAA channels co-expressed with GABARAP

The single channel properties of recombinant gamma-aminobutyric acid type A (GABA(A))alphabetagamma receptors co-expressed with the trafficking protein GABARAP were investigated using membrane patches in the outside-out patch clamp configuration from transiently transfected L929 cells. In control cells expressing alphabetagamma receptors alone, GABA activated single channels whose main conductance was 30 picosiemens (pS) with a subconductance state of 20 pS, and increasing the GABA concentration did not alter their conductance. In contrast, when GABA(A) receptors were co-expressed with GABARAP, the GABA-activated single channels displayed multiple, high conductances (> or =40 pS), and GABA (> or =10 microM) was able to increase their conductance, up to a maximum of 60 pS. The mean open time of GABA-activated channels in control cells expressing alphabetagamma receptors alone was 2.3 +/- 0.1 ms for the main 30-pS channel and shorter for the subconductance state (20 pS, 0.8 +/- 0.1 ms). Similar values were measured for the 30- and 20-pS channels active in patches from cells co-expressing GABARAP. However higher conductance channels (> or =40 pS) remained open longer, irrespective of whether GABA or GABA plus diazepam activated them. Plotting mean open times against mean conductances revealed a linear relationship between these two parameters. Since high GABA concentrations increase both the maximum single channel conductance and mean open time of GABA(A) channels co-expressed with GABARAP, trafficking processes must influence ion channel properties. This suggests that the organization of extrasynaptic GABA(A) receptors may provide a range of distinct inhibitory currents in the brain and, further, provide differential drug responses.