Glycine receptor alpha2 (Glycine_rcpt_A2)
Type: Family

Neurotransmitter ligand-gated ion channels are transmembrane receptor-ion channel complexes that open transiently upon binding of specific ligands, allowing rapid transmission of signals at chemical synapses . Five of these ion channel receptor families have been shown to form a sequence-related superfamily:

Nicotinic acetylcholine receptor (AchR), an excitatory cation channel in vertebrates and invertebrates; in vertebrate motor endplates it is composed of alpha, beta, gamma and delta/epsilon subunits; in neurons it is composed of alpha and non-alpha (or beta) subunits .

Glycine receptor, an inhibitory chloride ion channel composed of alpha and beta subunits .

Gamma-aminobutyric acid (GABA) receptor, an inhibitory chloride ion channel; at least four types of subunits (alpha, beta, gamma and delta) are known .

Serotonin 5HT3 receptor, of which there are seven major types (5HT3-5HT7) .

Glutamate receptor, an excitatory cation channel of which at least three types have been described (kainate, N-methyl-D-aspartate (NMDA) and quisqualate) .

These receptors possess a pentameric structure (made up of varying subunits), surrounding a central pore. All known sequences of subunits from neurotransmitter-gated ion-channels are structurally related. They are composed of a large extracellular glycosylated N-terminal ligand-binding domain, followed by three hydrophobic transmembrane regions which form the ionic channel, followed by an intracellular region of variable length. A fourth hydrophobic region is found at the C-terminal of the sequence .

Glycine is a major inhibitory neurotransmitter (NT) in the adult vertebrate central nervous system (CNS). Glycinergic synapses have a well-established role in the processing of motor and sensory information that controls movement, vision and audition . This action of glycine is mediated through its interaction with the glycine receptor (GlyR): an intrinsic chloride channel is opened in response to agonist binding. The subsequent influx of anions prevents membrane depolarisation and neuronal firing induced by excitatory NTs. Strychnine acts as a competitive antagonist of glycine binding, thereby reducing the activity of inhibitory neurones. Poisoning with strychnine is characterised by over-excitation, muscle spasms and convulsions. Whilst glycine is the principal physiological agonist at GlyRs, taurine and beta-alanine also behave as agonists . Compounds that modulate GlyR activity include zinc, some alcohols and anaesthetics, picrotoxin, cocaine and some anticonvulsants. GlyRs were thought for some time to be localised exclusively in the brain stem and spinal cord, but have since been found to be expressed more widely, including the cochlear nuclei, cerebellar cortex and forebrain .

GlyRs belong to the ligand-gated ion channel family, which also includes the inhibitory gamma-aminobutyric acid type A (GABAA) and excitatory nicotinic acetylcholine (nACh) and serotonin type 3 (5-HT3) receptors . Affinity-purified GlyR was found to contain two glycosylated membrane proteins of 48kDa and 56kDa, corresponding to alpha and beta subunits, respectively. Four genes encoding alpha subunits have been identified (GLRA1 to 4), together with a single beta polypeptide (GLRB). The heterogeneity of alpha subunits is further increased by alternative exon splicing, yielding two isoforms of GLRA1 to 3 . The characteristics of different GlyR subtypes, therefore, can be largely explained by their GLRA content.

GlyRs are generally believed to adopt a pentameric structure in vivo: five subunits assemble to form a ring structure with a central pore. Typically, a stoichiometry of 3:2 (alpha:beta) is observed . GlyR subunits share a high overall level of sequence similarity both with themselves and with the subunits of the GABAA and nACh receptors. Four highly conserved segments have been proposed to correspond to transmembrane (TM) α-helices (TM1-4), the second of which is thought to contribute to the pore wall . A long extracellular N-terminal segment precedes TM1 and a long cytoplasmic loop links TM3 and 4. Short cytoplasmic and extracellular loops join TM1-2 and TM2-3, respectively, and a short C-terminal sequence follows TM4. Studies using radiolabelled strychnine have shown the alpha subunit to be responsible for ligand binding, the critical residues for this interaction lying within the N-terminal domain. The beta subunit plays a structural role, contributing one of its TM domains to the pore wall as well as playing a putative role in postsynaptic clustering of the receptor.

In several mammalian species, defects in glycinergic transmission are associated with complex motor disorders. Mutations in the gene encoding GLRA1 give rise to hyperplexia, or startle disease . This is characterised by muscular spasms in response to unexpected light or noise stimuli, similar to the symptoms of sublethal doses of strychnine. The mutations result in amino acid substitutions within the TM1-2 and TM3-4 loops, suggesting that these regions are involved in the transduction of ligand binding into channel activation.

In humans, the GLRA2 gene is located on chromosome Xp22.2-22.1 . In situ hybridisation studies have shown GLRA2 to be expressed in the hippocampus, cerebral cortex and thalamus. GLRA2 trancripts predominate in the neonatal and embyonic CNS, and are replaced postnatally by those of GLRA1 and, to a lesser extent, GLRA3.