GABAAR isoform and subunit structural motifs determine synaptic and extrasynaptic receptor localisation. (1st June 2020)

• Record Type: Journal Article
• Authors: Hannan, Saad; Minere, Marielle; Harris, Joseph; Izquierdo, Pablo; Thomas, Philip; Tench, Becky; Smart, Trevor G.
• Is Part Of: Neuropharmacology. Volume 169(2020)
• Publication Date: 2020-06-01
• Subjects: Neuropsychopharmacology -- Periodicals; Autonomic Agents -- Periodicals; Neuropsychopharmacologie -- Périodiques; Neuropsychopharmacology; Periodicals; Electronic journals; 615.78
• DOI: 10.1016/j.neuropharm.2019.02.022
• Languages: English
• ISSNs: 0028-3908

Abstract

Abstract: GABAA receptors (GABAA Rs) are the principal inhibitory neurotransmitter receptors in the central nervous system. They control neuronal excitability by synaptic and tonic forms of inhibition mostly mediated by different receptor subtypes located in specific cell membrane subdomains. A consensus suggests that α1-3βγ comprise synaptic GABAA Rs, whilst extrasynaptic α4βδ, α5βγ and αβ isoforms largely underlie tonic inhibition. Although some structural features that enable the spatial segregation of receptors are known, the mobility of key synaptic and extrasynaptic GABAA Rs are less understood, and yet this is a key determinant of the efficacy of GABA inhibition. To address this aspect, we have incorporated functionally silent α-bungarotoxin binding sites (BBS) into prominent hippocampal GABAA R subunits which mediate synaptic and tonic inhibition. Using single particle tracking with quantum dots we demonstrate that GABAA Rs that are traditionally considered to mediate synaptic or tonic inhibition are all able to access inhibitory synapses. These isoforms have variable diffusion rates and are differentially retained upon entering the synaptic membrane subdomain. Interestingly, α2 and α4 subunits reside longer at synapses compared to α5 and δ subunits. Furthermore, a high proportion of extrasynaptic δ-containing receptors exhibited slower diffusion compared to δ subunits at synapses. A chimera formed from δ-subunits, with the intracellular domain of γ2L, reversed this behaviour. In addition, we observed that receptor activation affected the diffusion of extrasynaptic, but not of synaptic GABAA Rs. Overall, we conclude that the differential mobility profiles of key synaptic and extrasynaptic GABAA Rs are determined by receptor subunit composition and intracellular structural motifs. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'. Highlights: GABAA Rs mediating synaptic or tonic inhibition all access inhibitory synapses. Diffusion and retention of GABAA Rs at synapses depends on the subunit composition. Dwell times for α2 and α4 are longer than for α5 and δ at inhibitory synapses. A large proportion of extrasynaptic δ-GABAA Rs exhibit restricted diffusion. The large intracellular loops of δ and γ2L regulate mobility and synaptic trapping.