1.A.9.3.1
Adult strychnine-sensitive glycine-inhibited chloride (anion selective) heteropentameric channel (GlyR; GLRA1) consisting of α₁- and β-subunits (Cascio, 2004; Sivilotti, 2010). Ivermectin potentiates glycine-induced channel activation (Wang and Lynch, 2012). Molecular sites for the positive allosteric modulation of glycine receptors by endocannabinoids have been identified (Yévenes and Zeilhofer, 2011). Different subunits contribute asymmetrically to channel conductances via residues in the extracellular domain (Moroni et al., 2011; Xiong et al., 2012). Dominant and recessive mutations in GLRA1 are the major causes of hyperekplexia or startle disease (Gimenez et al., 2012).  Open channel 3-d structures are known (Mowrey et al. 2013).  Desensitization is regulated by interactions between the second and third transmembrane segments which affect the ion channel lumen near its intracellular end. The GABAAR and GlyR pore blocker, picrotoxin (TC# 8.C.1), prevents desensitization (Gielen et al. 2015).  The x-ray structure of the α1 GlyR transmembrane domain has been reported (Moraga-Cid et al. 2015), and residue S296 in hGlyR-alpha1 is involved in potentiation by Delta(9)-tetrahydrocannabinol (THC) (Wells et al. 2015).  The structure has also been elucidated by cryo EM (Du et al. 2015) and by x-ray crystalography (Huang et al. 2015). The latter presented a 3.0 A X-ray structure of the human glycine receptor-alpha3 homopentamer in complex with the high affinity, high-specificity antagonist, strychnine. The structure allowed exploration of the molecular recognition of antagonists. Comparisons with previous structures revealed a mechanism for antagonist-induced inactivation of Cys-loop receptors, involving an expansion of the orthosteric binding site in the extracellular domain that is coupled to closure of the ion pore in the transmembrane domain. The GlyR beta8-beta9 loop is an essential regulator of conformational rearrangements during ion channel opening and closing (Schaefer et al. 2017). Association of GlyR with the anchoring protein, gephyrin (Q9NQX3), is due to  a hydrophobic interaction formed by Phe 330 of gephyrin and Phe 398 and Ile 400 of the GlyR beta-loop (Kim et al. 2006). Alcohols and volatile anesthetics enhance the function of inhibitory glycine receptors (GlyRs) by binding to a single anaesthetic binding site (Roberts et al. 2006). Aromatic residues in the GlyR M1, M3 and M4 α-helices are essential for receptor function (Tang and Lummis 2018). The neurological disorder, startle disease, is caused by glycinergic dysfunction, mainly due to missense mutations in genes encoding GlyR subunits (GLRA1 and GLRB). Another neurological disease with a phenotype similar to startle disease is a special form of stiff-person syndrome (SPS), which is most probably due to the development of GlyR autoantibodies (Schaefer et al. 2018). GlyRs can be modulated by positive allosteric modulators (PAMs) that target the extracellular, transmembrane  and intracellular domains (Lara et al. 2019). Mutations in GLRA1 give rise to hyperekplexia (Milenkovic et al. 2018). Neurosteroid binding sites of GABAARs are conserved in the GlyRs (Alvarez and Pecci 2019). The intracellular domain of homomeric glycine receptors modulates agonist efficacy (Ivica et al. 2020). Inhibitory glycinergic transmission in the adult spinal cord is primarily mediated by glycine receptors (GlyRs) containing the alpha1 subunit. Alpha1ins, a longer alpha1 variant with 8 amino acids inserted into the intracellular large loop between TMSs 3 and 4, is expressed in the dorsal horn of the spinal cord, distributed at inhibitory synapses, and it is engaged in negative control over nociceptive signal transduction. Activation of metabotropic glutamate receptor 5 (mGluR5; TC# 9.A.14.7.1) specifically suppressed alpha1ins-mediated glycinergic transmission and evoked pain sensitization. Extracellular signal-regulated kinase (ERK) was critical for mGluR5 to inhibit alpha1ins. By binding to a D-docking site created by the 8-amino-acid insert ERK catalyzed alpha1ins phosphorylation at Ser380, which favored alpha1ins ubiquitination at Lys379 and led to alpha1ins endocytosis. Disruption of the ERK interaction with alpha1ins blocked Ser380 phosphorylation, potentiated glycinergic synaptic currents, and alleviated inflammatory and neuropathic pain (Zhang et al. 2019). The startle disease mutation (αS270T) affects the opening state for activation of presynaptic homomeric GlyRs, as well as postsynaptic heteromeric GlyRs, but the former are affected more. Both respond to glycine less efficiently (Wu et al. 2020). Cannabinoids exert therapeutic effects on several diseases such as chronic pain and startle disease by targeting glycine receptors (GlyRs). They target a serine residue at position 296 in the third TMS of the alpha1/alpha3 GlyR on the outside of the channel at the lipid interface where cholesterol concentrates. GlyRs are associated with cholesterol/caveolin-rich domains. and cholesterol reduction significantly inhibits cannabinoid potentiation of glycine-activated currents (Yao et al. 2020). Residues involved in glucose sensitivity of recombinant human glycine receptors have been identified (Hussein et al. 2020). Lipid-protein interactions are dependent on the receptor state, suggesting that lipids may regulate the receptor's conformational dynamics ((Dämgen and Biggin 2021)). Some protein-lipid interactions occur at a site at the communication interface between the extracellular and transmembrane domain, and in the active state, cholesterol can bind to the binding site of the positive allosteric modulator, ivermectin (Dämgen and Biggin 2021). An intracellular domain determines the agonist specificity (Ivica et al. 2021). The general anesthetic etomidate and fenamate mefenamic acid oppositely affect GABAAR and GlyR. These drugs potentiated GABAARs but blocked GlyRs (Rossokhin 2020). Alpha 1 glycine receptors are strongly inhibited by two flavanoids, quercetin and naringenin (Breitinger et al. 2021). The glycine receptor beta-subunit A455P variant occurs in a family affected by hyperekplexia syndrome (Aboheimed et al. 2022). Evidence for distinct roles of conserved proline residues in GlyR has been presented (Lummis and Dougherty 2022). Cannabinoids in general, and THC in particular, modulate pain perception via GlyR with possible clinical applications (Alvarez and Alves 2022). A set of functionally essential but differentially charged amino-acid residues in the transmembrane domain of the alpha1 and beta subunits explains asymmetric activation. These findings point to a gating mechanism that is distinct from homomeric receptors but more compatible with heteromeric GlyRs, being clustered at synapses through beta subunit-scaffolding protein interactions (Liu and Wang 2023). Such a mechanism provides a foundation for understanding how gating of the Cys-loop receptor members diverge to accommodate a specific physiological environment. Gallagher et al. 2022 reviewed the structural basis for how current compounds cause positive allosteric modulation of glycine receptors and discusses their therapeutic potential as analgesics. Gibbs et al. 2023 demonstrated distinct compositional and conformational properties of α₁βGlyR. A glycine-elicited conformational change precedes pore opening. Low concentrations of glycine, partial agonists or specific mixtures of glycine and strychnine trigger weakly activating the channel (Shi et al. 2023). Molecular dynamic simulations of a partial agonist bound-closed Cryo-EM structure reveal a highly dynamic nature: a marked structural flexibility at both the extracellular-transmembrane interface and the orthosteric site, generating docking properties. A progressive propagating transition towards channel opening highlights structural plasticity within the mechanism of action of allosteric effectors (Shi et al. 2023). The spatiotemporal expression pattern of the GlyR alpha4 subunit has been studied, and the results suggest that glycinergic signaling modulates social, startle, and anxiety-like behaviors in mice (Darwish et al. 2023).  Human alpha1beta GlyR is a major Cys-loop receptor that mediates inhibitory neurotransmission in the central nervous system of adults. Glycine binding induces cooperative and symmetric structural rearrangements in the neurotransmitter-binding extracellular domain but asymmetrical pore dilation in the transmembrane domain. SA symmetric response in the extracellular domain is consistent with electrophysiological data showing cooperative glycine activation and contribution from both alpha1 and beta subunits. A set of functionally essential but differentially charged amino acid residues in the transmembrane domain of the alpha1 and beta subunits explains asymmetric activation (Liu and Wang 2023). Modelling and molecular dynamics predict the structure and interactions of the glycine receptor intracellular domain (Thompson et al. 2023).