Regulation of GABAARs by Transmembrane Accessory Proteins

Abstract

The vast majority of fast inhibitory transmission in the brain is mediated by GABA acting on GABA_A receptors (GABA_ARs), which provides inhibitory balance to excitatory drive and controls neuronal output. GABA_ARs are also effectively targeted by clinically important drugs for treatment in a number of neurological disorders. It has long been hypothesized that function and pharmacology of GABA_ARs are determined by the channel pore-forming subunits. However, recent studies have provided new dimensions in studying GABA_ARs due to several transmembrane proteins that interact with GABA_ARs and modulate their trafficking and function. In this review, we summarize recent findings on these novel GABA_AR transmembrane regulators and highlight a potential avenue to develop new GABA_AR psychopharmacology by targeting these receptor-associated membrane proteins.

Keywords: Clptm1; GABA(A) receptor; GARLH4; Lhfpl4; Shisa7; auxiliary subunit; benzodiazepines.

Figure 1.. (A) Schematic of the GABA_A Receptor (GABA_AR) Subunit and Its Associated Transmembrane (TM) Proteins, Including LH4 [–21], Cleft Lip and Palate Transmembrane Protein f (Clptm1*) [24], and Shisa7 [25].

The TM helix TM2 is the ion pore-lining region, shown in orange, *indicates putative membrane topology. (B) Schematic of GABA_ARs and associated TM proteins with other key inhibitory synaptic components. Clptm1 suppresses forward trafficking of both synaptic and extrasynaptic GABA_ARs from the endoplasmic reticulum to the plasma membrane, δ subunit prevents γ2 and LH4 incorporation into the GABA_AR complex and receptors containing δ subunit are trafficked to extrasynaptic domains. LH4 forms a tripartite complex with neuroligin2 (NL2) and γ2-containing GABA_ARs and is important for stabilization of receptors at the synapse, Lastly, Shisa7 is critical for controlling GABA_AR synaptic abundance and functional properties, but regulation of extrasynaptic GABA_ARs by Shisa7 is currently unknown, Note that currently, the exact stoichiometry of the GABA_AR–LH4, –Clptm1*, or –Shisa7 complexes, and whether these proteins are mutually exclusive in the complex with the receptor remain unclear.

Figure 2.. A Model of the Role of Shisa7 in the Regulation of GABAergic Transmission.

Shisa7 modulates abundance and kinetics of GABA_A Receptors (GABA_ARs) at the inhibitory synapse. Loss of Shisa7 reduces synaptic GABA_ARs and increases the decay time constant (T_decay). Abbreviations: IPSC, inhibitory postsynaptic current; KO, knockout; WT, wild type.

Figure 3.. GABA_A Receptor (GABA_AR) Schematic Showing Binding Subunits of GABA_AR-Targeting Drugs.

GABA_AR positive allosteric modulators (PAMs), including neurosteroids, general anesthetics, barbiturates, and benzodiazepines, have been widely used within clinical contexts, such as postpartum depression, anxiety-related disorders, epilepsy, anesthesia, alcohol withdrawal, and sleep disorders. Benzodiazepines have been reported to primarily bind at the interface of GABA_AR α1/2/3/5 and γ2 subunits and a secondary site at the interface of β/α [–73], zolpidem primarily binds to the α1 subunit [109]. ^aδ subunit can also occupy this site, and ethanol (< 30 mM) binds to both α and δ subunits [110]. ^bNeurosteroids have two major binding sites: one is within the α subunit and the second is at the α/β-subunit interface, although neurosteroids have a higher affinity for the δ-containing GABA_AR [–114].

Figure I.. Shisa Family Members.

(A) Phylogenetic analysis of mammalian Shisa family proteins, based on their sequences (average distance tree) and functional domains, modified from [48]. (B) Schematic of Shisa6–9 proteins, showing the signal peptide (SP), cysteine (C) residues in the cystine knot domain, transmembrane domain (TM), and PDZ type II motif (EVTV). Abbreviation: CKAMPs, cysteine-knot AMPAR modulating proteins.