Mechanisms of inhibition and activation of extrasynaptic αβ GABAA receptors

Abstract

Type A GABA (γ-aminobutyric acid) receptors represent a diverse population in the mammalian brain, forming pentamers from combinations of α-, β-, γ-, δ-, ε-, ρ-, θ- and π-subunits1. αβ, α4βδ, α6βδ and α5βγ receptors favour extrasynaptic localization, and mediate an essential persistent (tonic) inhibitory conductance in many regions of the mammalian brain1,2. Mutations of these receptors in humans are linked to epilepsy and insomnia3,4. Altered extrasynaptic receptor function is implicated in insomnia, stroke and Angelman and Fragile X syndromes1,5, and drugs targeting these receptors are used to treat postpartum depression6. Tonic GABAergic responses are moderated to avoid excessive suppression of neuronal communication, and can exhibit high sensitivity to Zn2+ blockade, in contrast to synapse-preferring α1βγ, α2βγ and α3βγ receptor responses5,7–12. Here, to resolve these distinctive features, we determined structures of the predominantly extrasynaptic αβ GABAA receptor class. An inhibited state bound by both the lethal paralysing agent α-cobratoxin13 and Zn2+ was used in comparisons with GABA–Zn2+ and GABA-bound structures. Zn2+ nullifies the GABA response by non-competitively plugging the extracellular end of the pore to block chloride conductance. In the absence of Zn2+, the GABA signalling response initially follows the canonical route until it reaches the pore. In contrast to synaptic GABAA receptors, expansion of the midway pore activation gate is limited and it remains closed, reflecting the intrinsic low efficacy that characterizes the extrasynaptic receptor. Overall, this study explains distinct traits adopted by αβ receptors that adapt them to a role in tonic signalling.

Lipid nanodisc scaffold and size alters the structure of a pentameric ligand-gated ion channel

“…Nevertheless, it is noteworthy that the diameter of the cryo-EM density of the GABA A R in MSP2N2 nanodiscs was also estimated to be ~9 nm 15 ; thus, the GABA A R in conventional MSP nanodiscs is likely to also interact with the nanodisc scaffold, especially through M4, possibly altering channel structure. This may explain why a recent structure of the α1β3 GABA A R showed a closed activation gate despite having an estimated peak open probability of ~0.6 in HEK293 cells 30 . It may also explain why systematic differences were noted between α1β3γ2 and α1β2γ2 GABA A R structures in MSP2N2 and saposin nanodiscs, respectively 15,36 .…”

“…A a counter-clockwise twisting of the ECD, when viewed from the extracellular space, can be appreciated in spMSP1D1 ELIC , saposin ELIC , and SMA ELIC relative to apo-spMSP1D1 ELIC (Supplementary Fig. 9a), which is a conserved conformational change associated with activation in all pLGICs 18,22,26,28,29,30,31,32 . In contrast, MSP1E3D1 ELIC shows minimal counter-clockwise twisting of the ECD compared to apo-MSP1E3D1 ELIC (Supplementary Fig.…”

Molecular Mingling: Multimodal Predictions of Ligand Promiscuity in Pentameric Ligand-Gated Ion Channels

“…For heteromeric GABA A Rs, the subunit arrangement for α1βγ2 assemblies is known ( Figure 1 ). Binary α1β3 assemblies feature 2α:3β stoichiometry ( Kasaragod et al, 2022 ), containing two GABA sites and a histamine site. Information for arrangements with other subunits is lacking or controversially debated at the time of writing.…”

Recent Insight into Lipid Binding and Lipid Modulation of Pentameric Ligand-Gated Ion Channels

“…The highest resolution structure (2.8 Å) within a set of human α1β3 GABA A R structures solved in bovine brain lipid nanodiscs also exhibit numerous regions of electron density at the periphery of the TMD in both leaflets of the bilayer, although the regions of electron density are too small to provide detailed insight into the modes of lipid binding [ 92 ]. Of note, one region of electron density penetrates an inter-subunit site between the principal α1 and complementary β3 subunits, as observed above in the original α1β2γ2 GABA A R structure.…”