Structure and Pharmacologic Modulation of Inhibitory Glycine Receptors

Abstract

Glycine receptors (GlyR) are inhibitory Cys-loop ion channels that contribute to the control of excitability along the central nervous system (CNS). GlyR are found in the spinal cord and brain stem, and more recently they were reported in higher regions of the CNS such as the hippocampus and nucleus accumbens. GlyR are involved in motor coordination, respiratory rhythms, pain transmission, and sensory processing, and they are targets for relevant physiologic and pharmacologic modulators. Several studies with protein crystallography and cryoelectron microscopy have shed light on the residues and mechanisms associated with the activation, blockade, and regulation of pentameric Cys-loop ion channels at the atomic level. Initial studies conducted on the extracellular domain of acetylcholine receptors, ion channels from prokaryote homologs-Erwinia chrysanthemi ligand-gated ion channel (ELIC), Gloeobacter violaceus ligand-gated ion channel (GLIC)-and crystallized eukaryotic receptors made it possible to define the overall structure and topology of the Cys-loop receptors. For example, the determination of pentameric GlyR structures bound to glycine and strychnine have contributed to visualizing the structural changes implicated in the transition between the open and closed states of the Cys-loop receptors. In this review, we summarize how the new information obtained in functional, mutagenesis, and structural studies have contributed to a better understanding of the function and regulation of GlyR.

Graphical abstract

Fig. 1.

Topology and schematic structure of GlyR. (A) Representation of a monomer of α1 GlyR where the different regions of the receptor are presented: extracellular domain (ECD, gray), transmembrane domains (TM, yellow) highlighting the TM2 which is part of the channel pore (magenta), the intracellular loop domain (IL, light blue), and C-terminal region (cyan). (B) Pentameric arrangement of subunits to form functional GlyR (C) Representation of dimer α1–α1 GlyR where the glycine binding site (GBS) is located. The loops (A–C) and β-strands (D–F) that comprise the GBS are also labeled. The amino acids from the principal (F44, F63, R65, L117, L127, S129) and complementary subunit (F158, Y202, T204, F207) are colored red and orange, respectively. The IL is based on the α1 GlyR full model described previously by Burgos et al. (2015a). All images were created using PyMOL.

Fig. 2.

Putative binding sites for selected important modulators in the α1 GlyR structure. (A) Representation of a homopentameric α1 GlyR (PDB:3JAE) where the binding sites for low (magenta) and high (blue) Zn²⁺concentrations are shown. Amino acids forming the shared cavity for ethanol (red) and general anesthetics (orange) are also shown. (B) Detailed view of amino acids involved in Zn²⁺ potentiation by low concentrations (<10 μM), and (C) an internal pore view that show amino acids related to the inhibitory effects of Zn²⁺. (D) Ethanol and general anesthetics binding site; principal amino acids are colored red while the residues that complement the cavity are colored orange. All images were generated using PyMOL.