A single tryptophan on M2 of glutamate receptor channels confers high permeability to divalent cations

Coupling of a viral K+-channel with a glutamate-binding-domain highlights the modular design of ionotropic glutamate-receptors

Ionotropic glutamate receptors (iGluRs) mediate excitatory neuronal signaling in the mammalian CNS. These receptors are critically involved in diverse physiological processes; including learning and memory formation, as well as neuronal damage associated with neurological diseases. Based on partial sequence and structural similarities, these complex cation-permeable iGluRs are thought to descend from simple bacterial proteins emerging from a fusion of a substrate binding protein (SBP) and an inverted potassium (K+)-channel. Here, we fuse the pore module of the viral K+-channel KcvATCV-1 to the isolated glutamate-binding domain of the mammalian iGluR subunit GluA1 which is structural homolog to SBPs. The resulting chimera (GluATCV*) is functional and displays the ligand recognition characteristics of GluA1 and the K+-selectivity of KcvATCV-1. These results are consistent with a conserved activation mechanism between a glutamate-binding domain and the pore-module of a K+-channel and support the expected phylogenetic link between the two protein families.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology 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, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Na+/Cl- dipole couples agonist binding to kainate receptor activation

Kainate-selective ionotropic glutamate receptors (GluRs) require external Na+ and Cl- as well as the neurotransmitter L-glutamate for activation. Although, external anions and cations apparently coactivate kainate receptors (KARs) in an identical manner, it has yet to be established how ions of opposite charge achieve this. An additional complication is that KARs are subject to other forms of cation modulation via extracellular acidification (i.e., protons) and divalent ions. Consequently, other cation species may compete with Na+ to regulate the time KARs remain in the open state. Here we designed experiments to unravel how external ions regulate GluR6 KARs. We show that GluR6 kinetics are unaffected by alterations in physiological pH but that divalent and alkali metal ions compete to determine the time course of KAR channel activity. Additionally, Na+ and Cl- ions coactivate GluR6 receptors by establishing a dipole, accounting for their common effect on KARs. Using charged amino acids as tethered ions, we further demonstrate that the docking order is fixed with cations binding first, followed by anions. Together, our findings identify the dipole as a novel gating feature that couples neurotransmitter binding to KAR activation.

The AMPA receptor subunit GluR1 regulates dendritic architecture of motor neurons

The morphology of the mature motor neuron dendritic arbor is determined by activity-dependent processes occurring during a critical period in early postnatal life. The abundance of the AMPA receptor subunit GluR1 in motor neurons is very high during this period and subsequently falls to a negligible level. To test the role of GluR1 in dendrite morphogenesis, we reintroduced GluR1 into rat motor neurons at the end of the critical period and quantitatively studied the effects on dendrite architecture. Two versions of GluR1 were studied that differed by the amino acid in the "Q/R" editing site. The amino acid occupying this site determines single-channel conductance, ionic permeability, and other essential electrophysiologic properties of the resulting receptor channels. We found large-scale remodeling of dendritic architectures in a manner depending on the amino acid occupying the Q/R editing site. Alterations in the distribution of dendritic arbor were not prevented by blocking NMDA receptors. These observations suggest that the expression of GluR1 in motor neurons modulates a component of the molecular substrate of activity-dependent dendrite morphogenesis. The control of these events relies on subunit-specific properties of AMPA receptors.

The AMPA receptor subunit GluR1 regulates dendritic architecture of motor neurons

The morphology of the mature motor neuron dendritic arbor is determined by activity-dependent processes occurring during a critical period in early postnatal life. The abundance of the AMPA receptor subunit GluR1 in motor neurons is very high during this period and subsequently falls to a negligible level. To test the role of GluR1 in dendrite morphogenesis, we reintroduced GluR1 into rat motor neurons at the end of the critical period and quantitatively studied the effects on dendrite architecture. Two versions of GluR1 were studied that differed by the amino acid in the "Q/R" editing site. The amino acid occupying this site determines single-channel conductance, ionic permeability, and other essential electrophysiologic properties of the resulting receptor channels. We found large-scale remodeling of dendritic architectures in a manner depending on the amino acid occupying the Q/R editing site. Alterations in the distribution of dendritic arbor were not prevented by blocking NMDA receptors. These observations suggest that the expression of GluR1 in motor neurons modulates a component of the molecular substrate of activity-dependent dendrite morphogenesis. The control of these events relies on subunit-specific properties of AMPA receptors.

Molecular dynamics simulations of the conformational changes of the glutamate receptor ligand-binding core in the presence of glutamate and kainate

Excitatory synaptic transmission is mediated by ionotropic glutamate receptors (iGluRs) through the induced transient opening of transmembrane ion channels. The three-dimensional structure of the extracellular ligand-binding core of iGluRs shares the overall features of bacterial periplasmic binding proteins (PBPs). In both families of proteins, the ligand-binding site is arranged in two domains separated by a cleft and connected by two peptide stretches. PBPs undergo a typical hinge motion of the two domains associated with ligand binding that leads to a conformational change from an open to a closed form. The common architecture suggests a similar closing mechanism in the ligand-binding core of iGluRs induced by the binding of specific agonists. Starting from the experimentally determined kainate-bound closed form of the S1S2 GluR2 construct, we have studied by means of molecular dynamics simulations the opening motion of the ligand-binding core in the presence and in the absence of both glutamate and kainate. Our results suggest that the opening/closing interdomain hinge motions are coupled to conformational changes in the insertion region of the transmembrane segments. These changes are triggered by the interaction of the agonists with the essential Glu 209 residue. A plausible mechanism for the coupling of agonist binding to channel gating is discussed.

NMDA channel gating is influenced by a tryptophan residue in the M2 domain but calcium permeation is not altered

N-Methyl-D-aspartate (NMDA) receptors are susceptible to open-channel block by dizolcipine (MK-801), ketamine and Mg(2+) and are permeable to Ca(2+). It is thought that a tryptophan residue in the second membrane-associated domain (M2) may form part of the binding site for open-channel blockers and contribute to Ca(2+) permeability. We tested this hypothesis using recombinant wild-type and mutant NMDA receptors expressed in HEK-293 cells. The tryptophan was mutated to a leucine (W-5L) in both the NMDAR1 and NMDAR2A subunits. MK-801 and ketamine progressively inhibited currents evoked by glutamate, and the rate of inhibition was increased by the W-5L mutation. An increase in open channel probability accounted for the acceleration. Fluctuation analysis of the glutamate-evoked current revealed that the NMDAR1 W-5L mutation increased channel mean open time, providing further evidence for an alteration in gating. However, the equilibrium affinities of Mg(2+) and ketamine were largely unaffected by the W-5L mutation, and Ca(2+) permeability was not decreased. Therefore, the M2 tryptophan residue of the NMDA channel is not involved in Ca(2+) permeation or the binding of open-channel blockers, but plays an important role in channel gating.

Identification of an aspartic residue in the P-loop of the vanilloid receptor that modulates pore properties

Vanilloid receptor subunit 1 (VR1) is a nonselective cation channel that integrates multiple pain-producing stimuli. VR1 channels are blocked with high efficacy by the well established noncompetitive antagonist ruthenium red and exhibit high permeability to divalent cations. The molecular determinants that define these functional properties remain elusive. We have addressed this question and evaluated by site-specific neutralization the contribution on pore properties of acidic residues located in the putative VR1 pore region. Mutant receptors expressed in Xenopus oocytes exhibited capsaicin-operated ionic currents akin to those of wild type channels. Incorporation of glutamine residues at Glu(648) and Glu(651) rendered minor effects on VR1 pore attributes, while Glu(636) slightly modulated pore blockade. In contrast, replacement of Asp(646) by asparagine decreased 10-fold ruthenium red blockade efficacy and reduced 4-fold the relative permeability of the divalent cation Mg(2+) with respect to Na(+) without changing the selectivity of monovalent cations. At variance with wild type channels and E636Q, E648Q, and E651Q mutant receptors, ruthenium red blockade of D646N mutants was weakly sensitive to extracellular pH acidification. Collectively, our results suggest that Asp(646) is a molecular determinant of VR1 pore properties and imply that this residue may form a ring of negative charges that structures a high affinity binding site for cationic molecules at the extracellular entryway.

Intersegment hydrogen bonds as possible structural determinants of the N/Q/R site in glutamate receptors

Specific electrophysiological and pharmacological properties of ionic channels in NMDA, AMPA, and kainate subtypes of ionotropic glutamate receptors (GluRs) are determined by the Asn (N), Gln (Q), and Arg (R) residues located at homologous positions of the pore-lining M2 segments (the N/Q/R site). Presumably, the N/Q/R site is located at the apex of the reentrant membrane loop and forms the narrowest constriction of the pore. Although the shorter Asn residues are expected to protrude in the pore to a lesser extent than the longer Gln residues, the effective dimension of the NMDA channel (corresponding to the size of the largest permeant organic cation) is, surprisingly, smaller than that of the AMPA channel. To explain this paradox, we propose that the N/Q/R residues form macrocyclic structures (rings) stabilized by H-bonds between a NH(2) group in the side chain of a given M2 segment and a C==O group of the main chain in the adjacent M2 segment. Using Monte Carlo minimization, we have explored conformational properties of the rings. In the Asn, but not in the Gln ring, the side-chain oxygens protruding into the pore may facilitate ion permeation and accept H-bonds from the blocking drugs. In this way, the model explains different electrophysiological and pharmacological properties of NMDA and non-NMDA GluR channels. The ring of H-bonded polar residues at the pore narrowing resembles the ring of four Thr(75) residues observed in the crystallographic structure of the KcsA K(+) channel.

Probing the ligand binding domain of the GluR2 receptor by proteolysis and deletion mutagenesis defines domain boundaries and yields a crystallizable construct

Ionotropic glutamate receptors constitute an important family of ligand-gated ion channels for which there is little biochemical or structural data. Here we probe the domain structure and boundaries of the ligand binding domain of the AMPA-sensitive GluR2 receptor by limited proteolysis and deletion mutagenesis. To identify the proteolytic fragments, Maldi mass spectrometry and N-terminal amino acid sequencing were employed. Trypsin digestion of HS1S2 (Chen GQ, Gouaux E. 1997. Proc Natl Acad Sci USA 94:13431-13436) in the presence and absence of glutamate showed that the ligand stabilized the S1 and S2 fragments against complete digestion. Using limited proteolysis and multiple sequence alignments of glutamate receptors as guides, nine constructs were made, folded, and screened for ligand binding activity. From this screen, the S1S21 construct proved to be trypsin- and chymotrypsin-resistant, stable to storage at 4 degrees C, and amenable to three-dimensional crystal formation. The HS1S21 variant was readily prepared on a large scale, the His tag was easily removed by trypsin, and crystals were produced that diffracted to beyond 1.5 A resolution. These experiments, for the first time, pave the way to economical overproduction of the ligand binding domains of glutamate receptors and more accurately map the boundaries of the ligand binding domain.

Structural determinants of the blocker binding site in glutamate and NMDA receptor channels

Glutamate receptor channels of the NMDA-type (N-methyl-D-aspartate) and non-NMDA-type (GluR) differ in their pore properties. The N-site in the M2 transmembrane segment of NMDA receptors (NMDAR), or the corresponding Q/R-site in GluRs, is a pivotal structural determinant of their permeation and blockade characteristics. Substitutions at a second site in M2, the L-site (L577) in GluR1, drastically alter the receptor selectivity to divalent cations. Here we report that M2 mutants carrying an asparagine or a threonine residue at the Q-site of GluR1, along with a tryptophan residue at the L-site, form homomeric GluR1 channels that are highly sensitive to structurally diverse, uncompetitive NMDA antagonists such as arylcyclohexylamines, dibenzocycloheptenimines, and to morphinian and adamantane derivatives. Analysis of the voltage dependence of channel blockade locates the blocker binding site approximately 0.65 partway into the transmembrane electric field in both GluR1 mutants and NMDAR channels. Our results suggest that the homomeric GluR1 double mutants, L577W/Q582N and L577W/Q582T, fairly approximate the pore properties of the heteromeric NMDA receptor and support the structural kinship of their permeation pathways.

Selected peptides targeted to the NMDA receptor channel protect neurons from excitotoxic death

Excitotoxic neuronal death, associated with neurodegeneration and stroke, is triggered primarily by massive Ca2+ influx arising from overactivation of glutamate receptor channels of the N-methyl-D-aspartate (NMDA) subtype. To search for channel blockers, synthetic combinatorial libraries were assayed for block of agonist-evoked currents by the human NR1-NR2A NMDA receptor subunits expressed in amphibian oocytes. A set of arginine-rich hexapeptides selectively blocked the NMDA receptor channel with IC50 approximately 100 nM, a potency similar to clinically tolerated blockers such as memantine, and only marginally blocked on non-NMDA glutamate receptors. These peptides prevent neuronal cell death elicited by an excitotoxic insult on hippocampal cultures.

Ionization, partitioning, and dynamics of tryptophan octyl ester: implications for membrane-bound tryptophan residues

The presence of tryptophan residues as intrinsic fluorophores in most proteins makes them an obvious choice for fluorescence spectroscopic analyses of such proteins. Membrane proteins have been reported to have a significantly higher tryptophan content than soluble proteins. The role of tryptophan residues in the structure and function of membrane proteins has attracted a lot of attention. Tryptophan residues in membrane proteins and peptides are believed to be distributed asymmetrically toward the interfacial region. Tryptophan octyl ester (TOE) is an important model for membrane-bound tryptophan residues. We have characterized this molecule as a fluorescent membrane probe in terms of its ionization, partitioning, and motional characteristics in unilamellar vesicles of dioleoylphosphatidylcholine. The ionization property of this molecule in model membranes has been studied by utilizing its pH-dependent fluorescence characteristics. Analysis of pH-dependent fluorescence intensity and emission maximum shows that deprotonation of the alpha-amino group of TOE occurs with an apparent pKa of approximately 7.5 in the membrane. The fluorescence lifetime of membrane-bound TOE also shows pH dependence. The fluorescence lifetimes of TOE have been interpreted by using the rotamer model for the fluorescence decay of tryptophan. Membrane/water partition coefficients of TOE were measured in both its protonated and deprotonated forms. No appreciable difference was found in its partitioning behavior with ionization. Analysis of fluorescence polarization of TOE as a function of pH showed that there is a decrease in polarization with increasing pH, implying more rotational freedom on deprotonation. This is further supported by pH-dependent red edge excitation shift and the apparent rotational correlation time of membrane-bound TOE. TOE should prove useful in monitoring the organization and dynamics of tryptophan residues incorporated into membranes.