1.A.10 The Glutamate-gated Ion Channel (GIC) Family of Neurotransmitter Receptors

Members of the GIC family are homo or heterotetrameric complexes in which each of the 4 subunits is of 800-1000 amino acyl residues in length (Mayer, 2006). These subunits may span the membrane three times as putative transmembrane (TM) α-helices with the N-termini (the glutamate-binding domains) localized extracellularly and the C-termini localized cytoplasmically (Gouaux, 2004).  The extracellular amino terminal domain, S1, and the loop domain between TMSs 2 and 3, bind the neurotransmitter (Gouaux, 2004). Between TMSs 1 and 2 is a P-loop which participates in channel formation and ion selectivity. Transmembrane AMPA receptor regulatory proteins and cornichons allosterically regulate AMPA receptor antagonists and potentiators (Schober et al., 2011; Coombs et al., 2012; Kato et al., 2010).

The extracellular domains of iGluRs are loosely packed assemblies with two clearly distinct layers, each of which has both local and global 2-fold axes of symmetry (Mayer, 2011). By contrast, the GluR transmembrane segments have 4-fold symmetry and share a conserved pore loop architecture found in tetrameric voltage-gated ion channels. The striking layered architecture of iGluRs revealed by the 3.6 Å resolution structure of an AMPA receptor homotetramer likely arose from gene fusion events that occurred early in evolution. Although this modular design has greatly facilitated biophysical and structural studies on individual GluR domains, and suggested conserved mechanisms for iGluR gating, recent work is beginning to reveal unanticipated diversity in the structure, allosteric regulation, and assembly of iGluR subtypes (Mayer, 2011).

The Mammalian ionotropic glutamate receptors (18 proteins) regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. They play important roles in numerous neurological diseases. They have multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. Traynelis et al. (2010) discussed glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, and the potential therapeutic use of pharmacological agents acting at glutamate receptors.

The subunits of GIC family channel proteins fall into six subfamilies: α, β, γ, δ, ε and ζ. Two regions in the N-terminal domain of glutamate receptor 3 form the subunit oligomerization interface that controls subtype-specific receptor assembly (Ayalon et al., 2005). The canonical conformational states occupied by most ligand-gated ion channels, and many cell-surface receptors, are the resting, activated, and desensitized states. The AMPA-sensitive GluR2 receptor undergoes conformational rearrangements of the agonist binding cores that occur upon desensitization. Desensitization involves the rupture of an extensive interface between domain 1 of 2-fold related glutamate-binding core subunits, compensating for the ca. 21 degrees of domain closure induced by glutamate binding. The rupture of the domain 1 interface allows the ion channel to close and thereby provides a simple explanation to the long-standing question of how agonist binding is decoupled from ion channel gating upon receptor desensitization (Armstrong et al., 2006). Auxiliary subunits have been described (Yan and Tomita, 2012).

The GIC channels are divided into three types: (1) α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-, (2) kainate- and (3) N-methyl-D-aspartate (NMDA)-selective glutamate receptors. Subunits of the AMPA and kainate classes exhibit 35-40% identity with each other while subunits of the NMDA receptors exhibit 22-24% identity with the former subunits. They possess large N-terminal, extracellular glutamate-binding domains that are homologous to the periplasmic glutamine and glutamate receptors (TC #3.A.1.3.2 and TC #3.A.1.3.4, respectively) of ABC-type uptake permeases (TC #3.A.1) of Gram-negative bacteria. All functionally characterized members of the GIC family are from animals. The different channel (receptor) types exhibit distinct ion selectivities and conductance properties. The NMDA-selective large conductance channels are highly permeable to monovalent cations and Ca²⁺. The AMPA- and kainate-selective ion channels are permeable primarily to monovalent cations with only low permeability to Ca²⁺.

A prokaryotic K⁺-selective glutamate receptor that binds glutamate and forms K⁺-selective ion channels has been characterized (Chen et al., 1999). It shows sequence similarity to both glutamate receptors of eukaryotes and to K⁺ channels of the VIC family (TC #1.A.1). It exhibits 397 amino acyl residues, a signal peptide, and three TMSs flanked by two regions of about 140 residues. It showed highest sequence similarity to the rat δ1 GluR followed by a putative GluR from Arabidopsis thaliana. As a result of these observations, it has been proposed that glutamate receptors of eukaryotes arose from a primordial prokaryotic protein (Chen et al., 1999).

High-resolution structures of the ligand binding core of GluR0, a glutamate receptor ion channel from Synechocystis PCC 6803, have been solved by X-ray diffraction (Mayer et al., 2001). The GluR0 structures reveal homology with bacterial periplasmic binding proteins and the rat GluR2 AMPA subtype neurotransmitter receptor. The ligand binding site is formed by a cleft between two globular alpha/beta domains. L-Glutamate binds in an extended conformation, similar to that observed for glutamine binding protein (GlnBP). However, the L-glutamate γ-carboxyl group interacts exclusively with Asn51 in domain 1, different from the interactions of ligand with domain 2 residues observed for GluR2 and GlnBP. To address how neutral amino acids activate GluR0 gating, Mayer et al. (2001) solved the structure of the binding site complex with L-serine. This revealed solvent molecules acting as surrogate ligand atoms, such that the serine OH group makes solvent-mediated hydrogen bonds with Asn51. The structure of a ligand-free, closed-cleft conformation revealed an extensive hydrogen bond network mediated by solvent molecules. Equilibrium centrifugation analysis revealed dimerization of the GluR0 ligand binding core with a dissociation constant of 0.8 microM. In the crystal, a symmetrical dimer involving residues in domain 1 occurs along a crystallographic 2-fold axis and suggests that tetrameric glutamate receptor ion channels are assembled from dimers of dimers. They propose that ligand-induced conformational changes cause the ion channel to open as a result of an increase in domain 2 separation relative to the dimer interface.

Ionotropic glutamate receptor (GluR) ion channels share structural similarity, albeit an inverted membrane topology, with P-loop channels. Like P-loop channels, prokaryotic GluR subunits (e.g. GluR0 of Synechocystis (TC# 1.A.10.2.1)) have two transmembrane channel-forming segments. In contrast, eukaryotic GluRs have an additional transmembrane segment (M4), located C-terminal to the ion channel core. Although topologically similar to GluR0, mammalian AMPA receptor (GluA1) subunits lacking the M4 segment do not display surface expression. In the AMPA receptor structure, a face in M4 forms intersubunit contacts with the transmembrane helices of the ion channel core (M1 and M3) from another subunit within the homotetramer. Thus, a highly specific interaction of the M4 segment with an adjacent subunit is required for surface expression of AMPA receptors (Salussolia et al., 2011). 

AMPA receptors are homo or heterooligomers of four subunits, GluRA-D (also called GluR1-4). The GluRB subunit of the AMPA receptor, responsible for fast excitatory signaling in the brain and ion selectivity, has been purified in milligram quantities as a homotetramer. It exhibits the expected pharmacological properties. Based on molecular mass and electron microscopic studies, the channel appears to be a dimer of dimers (Safferling et al., 2001). The molecular dimensions are about 11 x 14 x 17 nm, and solvent accessible regions that may form the channel can be seen.

Ligand (neurotransmitter) binding opens the transmembrane pore, but after activation, desensitization results, in which the ligand remains bound, but the ion channel is closed. Using the GluR2 AMPA-sensitive glutamate receptor, Sun et al. (2002) showed (1) that the ligand-binding cores form the dimer interfaces, (2) that stabilization of the intradimer interface reduces desensitization, (3) that destabilization of the intradimer interface enhances desensitization, and (4) receptor activation involves conformational changes within each subunit that result in an increase in the separation of portions of the receptor that are linked to the channel. These results indicate how ligand binding is coupled to channel activation (gating), suggest modes of dimer-dimer interaction in the formation of the tetramer, and show that desensitization results from rearrangement of the dimer interface which disengages the agonist-induced conformational change in the ligand-binding core from the ion channel gate (Sun et al., 2002).

NMDA receptors are always heterotetrameric cation channels that transport Ca²⁺ with subunits NR1, NR2 and NR3 in an (NR1)₂ (NR2)₂ or (NR1)₂ (NR2)(NR3) arrangement (Furukawa et al., 2005). Glycine binds to NR1, and glutamate binds to NR2 and/or NR3, and simultaneous binding of both agonists as well as relief of Mg²⁺ blockage by membrane depolarization is required for channel opening.

Recently, crystal structures of the ligand binding core of NR2A with glutamate and of the NR1-NR2A heterodimer with glycine and glutamate bound. The details of subunit:subunit interaction and of channel opening were reported (Furukawa et al., 2005). As a result, many features including the mechanism of allosteric modulation of channel activity (Jin et al., 2005) and the mechanism of dual agonist action (Olson and Gouaux, 2005) were revealed.

Glutamate receptor ligand binding domain dimer assembly is modulated allosterically by ions (Chaudhry et al., 2009). The activities of many ligand-gated ion channels and cell surface receptors are modulated by small molecules and ions. For kainate, but not AMPA subtype glutamate receptors, the binding of Na⁺ and Cl^- ions to discrete, electrostatically coupled sites in the extracellular ligand binding domain (LBD), regulates dimer assembly. Dimer assembly then regulates the rate of entry into the desensitized state, which occurs when the dimer interface ruptures and the channel closes. Studies on glutamate receptors have defined the LBD dimer assembly as a key functional unit that controls activation and desensitization. Sodium and chloride ions modulate kainate receptor dimer affinity as much as 50-fold, and removal of either Cl^- or Na⁺ disrupts the dimer (Chaudhry et al., 2009).

Ionotropic glutamate receptors (iGluRs) mediate fast excitatory synaptic transmission in the central nervous system. Upon agonist binding, an iGluR opens to allow the flow of cations and subsequently enters into a desensitized state. Dong and Zhou (2011) reported molecular dynamics simulations of an AMPA-subtype iGluR Channel opening and closing were observed in simulations of the activation and desensitization processes, respectively. The motions of the LBD-TMD linkers along the central axis of the receptor and in the lateral plane contributed cooperatively to channel opening and closing. Glutamatergic mechanisms related to epilepsy have been reviewed by Dingledine (2012).

Cys loop, glutamate, and P2X receptors are ligand-gated ion channels (LGICs) with 5, 4, and 3 protomers, respectively. Agonists and competitive antagonists apparently induce opposite motions of the binding pocket (Du et al., 2012). Agonists, usually small, induce closure of the binding pocket, leading to opening of the channel pore, whereas antagonists, usually large, induce opening of the binding pocket, thereby stabilizing the closed pore.

AMPA receptors (AMPAR) are the main ligand-gated ion channels responsible for the fast excitatory synaptic transmission in the mammalian brain. A number of proteins that interact with AMPAR are known to be involved in the trafficking and localization of the receptor and/or the regulation of receptor channel properties. Additionally, the presence of up to 34 proteins may interact as high-confidence constituents of the AMPAR. The inner core of the receptor complex may consist of the GluA tetramer and four auxiliary proteins comprising transmembrane AMPA receptor regulatory proteins and/or cornichons. The other AMPAR interactors, present in lower amount, may form the outer shell of the AMPAR with a range in size and variability (Li et al. 2013).

The generalized transport reaction catalyzed by GIC family channels is:

Me⁺ (or Me²⁺) (out) ⇌ Me⁺ (or Me²⁺) (in).