Low Expression in Xenopus Oocytes and Unusual Functional Properties of α1β2γ2 GABAA Receptors with Non-Conventional Subunit Arrangement

Potentiation of the GABAAR reveals variable energetic contributions by etiocholanolone and propofol

The properties of a potentiator are typically evaluated by measuring its ability to enhance the magnitude of the control response. Analysis of the ability of drugs to potentiate responses from receptor channels takes place in the context of particular models to extract parameters for functional effects. In the often-used coagonist model, the agonist generating control activity and the potentiator enhancing the control activity make additive energetic contributions to stabilize the active state of the receptor. The energetic contributions are fixed and, once known, enable calculation of predicted receptor behavior at any concentration combination of agonist and potentiator. Here, we have examined the applicability of the coagonist model by measuring the relationship between the magnitude of receptor potentiation and the level of background activity. Ternary αβγ GABAA receptors were activated by GABA or the allosteric agonist propofol, or by a gain-of-function mutation, and etiocholanolone- or propofol-mediated potentiation of peak responses was measured. We show that the free energy change contributed by the modulators etiocholanolone or propofol is reduced at higher levels of control activity, thereby being in disagreement with basic principles of the coagonist model. Possible mechanisms underlying this discrepancy are discussed.

Concatenated γ-aminobutyric acid type A receptors revisited: Finding order in chaos

Subunit concatenation is a powerful technique used to control the assembly of structurally diverse heteromeric receptors such as GABA_ARs. Liao et al. find that existing GABA_AR concatemers do not assemble as expected and describe refinements that allow expression of uniform receptor populations.

γ-aminobutyric acid type A receptors (GABA_ARs), the major inhibitory neurotransmitter receptors in the mammalian central nervous system, are arguably the most challenging member of the pentameric Cys-loop receptors to study due to their heteromeric structure. When two or more subunits are expressed together in heterologous systems, receptors of variable subunit type, ratio, and orientation can form, precluding accurate interpretation of data from functional studies. Subunit concatenation is a technique that involves the linking of individual subunits and in theory allows the precise control of the uniformity of expressed receptors. In reality, the resulting concatemers from widely used constructs are flexible in their orientation and may therefore assemble with themselves or free GABA_AR subunits in unexpected ways. In this study, we examine functional responses of receptors from existing concatenated constructs and describe refinements necessary to allow expression of uniform receptor populations. We find that dimers from two commonly used concatenated constructs, β-23-α and α-10-β, assemble readily in both the clockwise and the counterclockwise orientations when coexpressed with free subunits. Furthermore, we show that concatemers formed from new tetrameric α-10-β-α-β and α-10-β-α-γ constructs also assemble in both orientations with free subunits to give canonical αβγ receptors. To restrict linker flexibility, we systematically shorten linker lengths of dimeric and pentameric constructs and find optimized constructs that direct the assembly of GABA_ARs only in one orientation, thus eliminating the ambiguity associated with previously described concatemers. Based on our data, we revisit some noncanonical GABA_AR configurations proposed in recent years and explain how the use of some concatenated constructs may have led to wrong conclusions. Our results help clarify current contradictions in the literature regarding GABA_AR subunit stoichiometry and arrangement. The lessons learned from this study may guide future efforts in understanding other related heteromeric receptors.

SAHA (Vorinostat) Corrects Inhibitory Synaptic Deficits Caused by Missense Epilepsy Mutations to the GABAA Receptor γ2 Subunit

The GABA_A receptor (GABA_AR) α1 subunit A295D epilepsy mutation reduces the surface expression of α1^A295Dβ2γ2 GABA_ARs via ER-associated protein degradation. Suberanilohydroxamic acid (SAHA, also known as Vorinostat) was recently shown to correct the misfolding of α1^A295D subunits and thereby enhance the functional surface expression of α1^A295Dβ2γ2 GABA_ARs. Here we investigated whether SAHA can also restore the surface expression of γ2 GABA_AR subunits that incorporate epilepsy mutations (N40S, R43Q, P44S, R138G) known to reduce surface expression via ER-associated protein degradation. As a control, we also investigated the γ2^K289M epilepsy mutation that impairs gating without reducing surface expression. Effects of mutations were evaluated on inhibitory postsynaptic currents (IPSCs) mediated by the major synaptic α1β2γ2 GABA_AR isoform. Recordings were performed in neuron-HEK293 cell artificial synapses to minimise contamination by GABA_ARs of undefined subunit composition. Transfection with α1β2γ2^N40S, α1β2γ2^R43Q, α1β2γ2^P44S and α1β2γ2^R138G subunits produced IPSCs with decay times slower than those of unmutated α1β2γ2 GABA_ARs due to the low expression of mutant γ2 subunits and the correspondingly high expression of slow-decaying α1β2 GABA_ARs. SAHA pre-treatment significantly accelerated the decay time constants of IPSCs consistent with the upregulation of mutant γ2 subunit expression. This increase in surface expression was confirmed by immunohistochemistry. SAHA had no effect on either the IPSC kinetics or surface expression levels of α1β2γ2^K289M GABA_ARs, confirming its specificity for ER-retained mutant γ2 subunits. We also found that α1β2γ2^K289M GABA_ARs and SAHA-treated α1β2γ2^R43Q, α1β2γ2^P44S and α1β2γ2^R138G GABA_ARs all mediated IPSCs that decayed at significantly faster rates than wild type receptors as temperature was increased from 22 to 40°C. This may help explain why these mutations cause febrile seizures (FS). Given that SAHA is approved by therapeutic regulatory agencies for human use, we propose that it may be worth investigating as a treatment for epilepsies caused by the N40S, R43Q, P44S and R138G mutations. Although SAHA has already been proposed as a therapeutic for patients harbouring the α1^A295D epilepsy mutation, the present study extends its potential utility to a new subunit and four new mutations.

α subunits in GABAA receptors are dispensable for GABA and diazepam action

The major isoform of the GABA_A receptor is α₁β₂γ₂. The binding sites for the agonist GABA are located at the β₂+/α₁− subunit interfaces and the modulatory site for benzodiazepines at α₁+/γ₂−. In the absence of α₁ subunits, a receptor was formed that was gated by GABA and modulated by diazepam similarly. This indicates that alternative subunits can take over the role of the α₁ subunits. Point mutations were introduced in β₂ or γ₂ subunits at positions homologous to α₁− benzodiazepine binding and GABA binding positions, respectively. From this mutation work we conclude that the site for GABA is located at a β₂+/β₂− subunit interface and that the diazepam site is located at the β₂+/γ₂− subunit interface. Computational docking leads to a structural hypothesis attributing this non-canonical interaction to a binding mode nearly identical with the one at the α₁+/γ₂− interface. Thus, the β₂ subunit can take over the role of the α₁ subunit for the formation of both sites, its minus side for the GABA binding site and its plus side for the diazepam binding site.