Transient and persistent phosphorylation of AMPA-type glutamate receptor subunits in cerebellar Purkinje cells

Impaired modulation of AMPA receptors by calcium-dependent processes in streptozotocin-induced diabetic rats

The mechanisms by which diabetes impairs cognitive function are not well-established. In the present study, we determined the electrophysiological and biochemical nature of disturbances in the mechanism of long-term potentiation (LTP) in diabetic rats. As previously reported, the administration of streptozotocin (STZ) was found to reduce the magnitude of LTP in the CA1 region of the hippocampus, while the same treatment did not interact with the capacity of the hippocampus to generate long-term depression induced by low-frequency stimulation. In addition, STZ treatment did not modify the component of excitatory postsynaptic potentials mediated by activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors, suggesting that NMDA receptor function remained intact in STZ-treated slices. At the biochemical level, the capacity of calcium to increase [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (3H-AMPA) binding to glutamate/AMPA receptors in rat brain tissue sections was markedly affected in most regions of the hippocampus of STZ-treated rats. Moreover, changes in 3H-AMPA binding properties elicited by both exogenous phospholipase A2 and melittin, a potent activator of endogenous phospholipases, were also altered in synaptoneurosomes from diabetic rats. Taken together, the present data suggest that the loss of LTP maintenance in STZ-treated rats is more likely the result of disruption of calcium-dependent processes that are suspected to modulate postsynaptic AMPA receptors during synaptic potentiation. Understanding the biochemical factors participating in the impairment of AMPA receptor modulation might provide important clues revealing the very basis of memory deficits in diabetes.

Regulation of excitatory and inhibitory neurotransmitter-gated ion channels by protein phosphorylation

Phosphorylation of ligand-gated ion channels is recognised as a potentially important mechanism for short- and long-term modulation of ion-channel function. Following the discovery of numerous sites of phosphorylation on ligand-gated ion channel proteins, recent studies have demonstrated that neurotransmitter-induced activation of serine/threonine, tyrosine and other kinases can result in the modulation of glutamate, type A gamma-aminobutyric acid (GABAA) and glycine receptors. These findings may have important consequences for our understanding of synaptic transmission and neuronal excitability.

Persistent phosphorylation parallels long-term desensitization of cerebellar purkinje cell AMPA-type glutamate receptors

This study is aimed at testing the hypothesis that sustained phosphorylation underlies long-term desensitization of AMPA receptors, which is thought to be the mechanism of long-term synaptic depression in cerebellar Purkinje cells (PCs). We induced long-term desensitization of AMPA receptors in rat cerebellar slices by (1) a 4-min bath application of quisqualate (0.1 mM) or (2) a 15-min bath application of a protein kinase C (PKC) activator, phorbol-12,13-diacetate (0.5 microM) or -dibutyrate (0.6 microM), followed by a 4-min AMPA (0.1 mM) application. In slices so treated, labeling with an antibody (12P3) against a peptide corresponding to part of AMPA receptor subunit GluR2 including serine 696 and phosphorylated at this serine site revealed phosphorylation of the AMPA receptors in PC dendrites that was sustained for at least 1 hr. At an early phase, within 20 min after the chemical stimulation, the phosphorylation was resistant to an Ca2+ chelator (BAPTA-AM), a metabotropic glutamate receptor antagonist (MCPG), and a PKC inhibitor (calphostin C), whereas at a late phase, 30 min or more after the chemical stimulation, it was blocked by these reagents similarly to long-term desensitization of AMPA receptors. Taken together with data obtained previously using different protocols of chemical stimulation, the present results strongly support the above-mentioned hypothesis.

Dominant negative mutant of ionotropic glutamate receptor subunit GluR3: implications for the role of a cysteine residue for its channel activity and pharmacological properties

We reported that a 33-amino-acid deletion (from tyrosine-715 to glycine-747) in a putative extracellular loop of GluR3 produced a mutant that exhibited dominant negative effects upon the functional expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors [Sekiguchi et al. (1994) J. Biol. Chem. 269, 14559-14565]. In this study, we searched for a key residue in the dominant negative effects to explore the mechanism and examined the role of the residue in the function of the AMPA receptor. We prepared 20 GluR3 mutants with amino acid substitutions within the 33-amino-acid-region, and dominant negative effects were tested electrophysiologically in Xenopus oocytes co-expressing the mutant and normal subunits. Among the mutants, only a GluR3 mutant in which an original cysteine (Cys)-722 was replaced by alanine exhibited a dominant negative effect comparable with that of the original mutant in which the entire 33-amino-acid segment is deleted. The co-expression of the Cys-722 mutant did not inhibit the translation of normal subunits in oocytes. The Cys-722 mutant formed a functional homomeric receptor with significantly higher affinity for glutamate or kainate than a homomeric GluR3 receptor. The Cys-722 mutation greatly enhanced the sensitivity of GluR3 for aniracetam, which alters kinetic properties of AMPA receptors. The kainate-induced currents in oocytes expressing the Cys-722 mutant alone showed strong inward rectification. These results suggest that the Cys-722 in GluR3 is important for dominant negative effects and plays a crucial role in the determination of pharmacological properties in AMPA receptor function.

Characterization of protein kinase A and protein kinase C phosphorylation of the N-methyl-D-aspartate receptor NR1 subunit using phosphorylation site-specific antibodies

Modulation of N-methyl-D-aspartate receptors in the brain by protein phosphorylation may play a central role in the regulation of synaptic plasticity. To examine the phosphorylation of the NR1 subunit of N-methyl-D-aspartate receptors in situ, we have generated several polyclonal antibodies that recognize the NR1 subunit only when specific serine residues are phosphorylated. Using these antibodies, we demonstrate that protein kinase C (PKC) phosphorylates serine residues 890 and 896 and cAMP-dependent protein kinase (PKA) phosphorylates serine residue 897 of the NR1 subunit. Activation of PKC and PKA together lead to the simultaneous phosphorylation of neighboring serine residues 896 and 897. Phosphorylation of serine 890 by PKC results in the dispersion of surface-associated clusters of the NR1 subunit expressed in fibroblasts, while phosphorylation of serine 896 and 897 has no effect on the subcellular distribution of NR1. The PKC-induced redistribution of the NR1 subunit in cells occurs within minutes of serine 890 phosphorylation and reverses upon dephosphorylation. These results demonstrate that PKA and PKC phosphorylate distinct residues within a small region of the NR1 subunit and differentially affect the subcellular distribution of the NR1 subunit.

An alanine residue in the M3-M4 linker lines the glycine binding pocket of the N-methyl-D-aspartate receptor

While attempting to map a central region in the M3-M4 linker of the N-methyl-D-aspartate receptor NR1 subunit, we found that mutation of a single position, Ala-714, greatly reduced the apparent affinity for glycine. Proximal N-glycosylation localized this region to the extracellular space. Glycine affinities of additional Ala-714 mutations correlated with side chain volume. Substitution of alanine 714 with cysteine did not alter glycine sensitivity, although this mutant was rapidly inhibited by dithionitrobenzoate. Glycine protected the A714C mutant from modification by dithionitrobenzoate, whereas the co-agonist L-glutamate was ineffective. These experiments place Ala-714 in the glycine binding pocket of the N-methyl-D-aspartate receptor, a determination not predicted by previous structural models based on bacterial periplasmic binding protein homology.

Pathophysiology of oligodendroglial excitotoxicity

Oligodendrocyte-like cells (OLD) derived from the rat oligodendroglial precursor line, CG-4, express Ca(2+)-permeable non-methyl-D-aspartate glutamate receptor channels (GluR). Exposure to kainate, an L-glutamate analogue, markedly elevates OLC Ca2+ influx and cytosolic [Ca2+], and results in damage to both OLC plasma membrane and OLC nuclear DNA. Two observations indicate that kainate-induced OLC internucleosomal DNA nicking is not simply a delayed consequence of cell necrosis: 1) there is no temporal lag between onset of plasma membrane injury and of DNA nicking; and 2) aurintricarboxylic acid, an endonuclease inhibitor, blocks kainate-induced damage to the plasma membrane. N-acetyl-L-cysteine also inhibits OLC kainate injury, suggesting that reactive oxygen species participate in OLC excitotoxicity. Kainate-induced OLC Ca2+ influx and excitotoxicity are blocked by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), indicating that these kainate effects are mediated by AMPA-GluR. AMPA and L-glutamate fail to elicit OLC damage unless cyclothiazide, an AMPA-GluR desensitization blocker, is present. OLC express both the "flip" and "flop" forms of GluR2, GluR3, and GluR4 mRNAs, but neither flip nor flop GluR1 mRNA. These data, together with the restriction of the desensitization-blocking activity of cyclothiazide to GluR containing flip-encoded GluR subunits, and the sharply diminished Ca2+ permeability of GluR containing edited GluR2, suggest OLC excitotoxicity is mediated by AMPA-GluR that contain flip GluR3 and/or flip GluR4 protein subunits, but neither flip nor flop GluR2 protein subunits. Rapid desensitization of these GluR is likely to be important in protecting cells of the oligodendroglial lineage from excitotoxicity.

A study of the oligomeric state of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-preferring glutamate receptors in the synaptic junctions of porcine brain

The number of the subunits in an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring L-glutamate receptor in the synaptic junctions of porcine brain was investigated in this study. Upon incubation of the synaptic junctions with three cross-linking regents, dimethyl adipimidate (DMA), dimethyl suberimidate (DMS) and N-succinimidyl-(4-azidophenyl)-1,3'-dithiopropionate (SADP), AMPA receptor subunits in higher-molecular-mass aggregates were detected by immunoblotting. These aggregates migrated as proteins of approx. 200, 300 and 400 kDa. The number and identity of the subunits in a solubilized AMPA receptor were also investigated here. Two samples, W1 and W2, enriched in AMPA receptors were prepared from synaptic junctions by a combination of detergent-solubilization, anion-exchange chromatography and wheatgerm agglutinin affinity chromatography. Hydrodynamic behaviour analyses revealed that the majority of the AMPA receptors in either one of these samples were asymmetrical detergent-surrounded particles with a protein mass around 350 kDa. SDS/PAGE analysis revealed that the majority of AMPA receptors in the W1 sample were comprised of dimers of 106 kDa subunits which were covalently linked by disulphide bonds. Cross-linking these receptors with SADP yielded a new band of approx. 400 kDa. The results obtained here, either from the studies of AMPA receptors embedding in synaptic junctions or from those of detergent-solubilized and partially purified receptors, suggest that AMPA receptors contain a basic core structure comprising of four 106 kDa subunits.

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

Ionotropic glutamate receptors (iGluRs) of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate/kainate subtype display lower permeability to Ca2+ than the N-methyl-D-aspartate (NMDA) subtype. The well-documented N/Q/R site on the M2 transmembrane segment (M2) is an important determinant of the distinct Ca2+ permeability exhibited by members of the non-NMDA receptor subfamily. This site, however, does not completely account for the different permeation properties displayed by non-NMDA and NMDA receptors, suggesting the involvement of other molecular determinants. We have identified additional molecular elements on M2 of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate/kainate receptor GluR1 that specify its permeation properties. Higher permeability to divalent over monovalent cations is conferred on GluR1 by a tryptophan at position 577, whereas blockade by external divalent cations is imparted by an asparagine at position 582. Hence, the permeation properties of ionotropic glutamate receptors appear to be primarily specified by two distinct determinants on M2, the well-known N/Q/R site and the newly identified L/W site. These findings substantiate the notion that M2 is a structural component of the pore lining.

Characterization of the ligand-binding domains of glutamate receptor (GluR)-B and GluR-D subunits expressed in Escherichia coli as periplasmic proteins

We recently reported that a functional ligand-binding site of an alpha-amino-5-methyl-3-hydroxy-4-isoxazole propionate (AMPA)-selective glutamate receptor (GluR)-D subunit can be expressed in insect cells as a soluble, N-glycosylated fusion protein consisting of two segments (S1 and S2) that are related by amino acid sequence to bacterial periplasmic binding proteins (Kuusinen, A., Arvola, M., and Keinänen, K., EMBO J. 14, 6327-6332). In an attempt to further characterize the structural determinants for ligand binding, we have now expressed the ligand-binding sites of GluR-B and GluR-D subunits in Escherichia coli as soluble periplasmic proteins. The bacterially expressed S1-S2 fusion proteins bound [3H]AMPA with a high affinity (Kd of 12 nM for GluR-B, Kd of 60 nM for GluR-D) and with a ligand pharmacology typical of native AMPA receptors, indicating that N-linked glycosylation is not required for the formation or the maintenance of the ligand-binding site. The flip and flop splice variants of the GluR-D S1-S2 fusion protein bound [3H]AMPA with equal affinities, whereas deletion of the C-terminal one-third of the S2 segment including the flip/flop sequence resulted in a loss of binding activity. Our results highlight the potential of bacterial expression for the analysis of the binding site and support a close structural similarity between glutamate receptors and bacterial proteins.

The glycine binding site of the N-methyl-D-aspartate receptor subunit NR1: identification of novel determinants of co-agonist potentiation in the extracellular M3-M4 loop region

The N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors is a heterooligomeric membrane protein composed of homologous subunits. Here, the contribution of the M3-M4 loop of the NR1 subunit to the binding of glutamate and the co-agonist glycine was investigated by site-directed mutagenesis. Substitution of the phenylalanine residues at positions 735 or 736 of the M3-M4 loop produced a 15- to 30-fold reduction in apparent glycine affinity without affecting the binding of glutamate and the competitive glycine antagonist 7-chlorokynurenic acid; mutation of both residues caused a >100-fold decrease in glycine affinity. These residues are found in a C-terminal region of the M3-M4 loop that shows significant sequence similarity to bacterial amino acid-binding proteins. Epitope tagging revealed both the N-terminus and the M3-M4 loop to be exposed extracellularly, whereas a C-terminal epitope was localized intracellularly. These results indicate that the M3-M4 loop is part of the ligand-binding pocket of the NR1 subunit and provide the basis for a refined model of the glycine-binding site of the NMDA receptor.

Characterization of multiple phosphorylation sites on the AMPA receptor GluR1 subunit

We have characterized the phosphorylation of the glutamate receptor subunit GluR1, using biochemical and electrophysiological techniques. GluR1 is phosphorylated on multiple sites that are all located on the C-terminus of the protein. Cyclic AMP-dependent protein kinase specifically phosphorylates SER-845 of GluR1 in transfected HEK cells and in neurons in culture. Phosphorylation of this residue results in a 40% potentiation of the peak current through GluR1 homomeric channels. In addition, protein kinase C specifically phosphorylates Ser-831 of GluR1 in HEK-293 cells and in cultured neurons. These results are consistent with the recently proposed transmembrane topology models of glutamate receptors, in which the C-terminus is intracellular. In addition, the modulation of GluR1 by PKA phosphorylation of Ser-845 suggests that phosphorylation of this residue may underlie the PKA-induced potentiation of AMPA receptors in neurons.

Antibody specific for phosphorylated AMPA-type glutamate receptors at GluR2 Ser-696

Possible phosphorylation sites on the Purkinje cell alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate receptor subunits were identified using in vitro kinase assays of 17 synthetic peptides derived from the transmembrane-3 (TM3) domain to the end of C-terminal of a rat glutamate receptor 2 (GluR2). Only two peptides containing Ser-662 and Ser-696 were found to be efficiently phosphorylated by protein kinase C (PKC). The peptide including Ser-696 was also phosphorylated by protein kinase G (PKG). Another peptide containing Thr-692 of a rat GluRA, clone almost identical to GluR1, was phosphorylated by PKC but not by PKG. Antisera recognizing phosphorylated AMPA receptor subunits at GluR2 Ser-696 or the homologous sites of GluR1/3/4 were produced, and the specificity of one of them, named 12P3, was established by enzyme-linked immunosorbent assay (ELISA), immunoblot and immunoprecipitation analyses. 12P3-immunocytochemistry on cerebellar slices demonstrated an AMPA-induced transient AMPA receptor phosphorylation, which appeared in Purkinje cell dendrites as well as somata immediately after AMPA treatment and disappeared after 20 min. This antibody may be a useful tool to study the role of AMPA receptor phosphorylation in producing synaptic plasticity.