Tunicamycin-induced inhibition of functional expression of glutamate receptors in Xenopus oocytes

N-glycosylation of the AMPA-type glutamate receptor regulates cell surface expression and tetramer formation affecting channel function

The AMPA-type glutamate receptor (AMPA-R) plays a primary role in principal excitatory synaptic transmission and many neuronal functions including synaptic plasticity that underlie learning and memory. N-glycosylation is one of the major post-translational modifications of membrane proteins, but its specific roles in neurons remain largely unknown. AMPA-R subunits are N-glycosylated at their extracellular domains during their biosynthesis in the lumen of the endoplasmic reticulum and Golgi system. Six N-glycosylation sites are presumed to exist in the extracellular domain of GluA1, which is a member of the AMPA-R subunits. We observed that the intracellular trafficking and cell surface expression were strongly suppressed in the GluA1 mutants lacking N-glycans at N63/N363 in HEK293T cells. Multimer analysis using Blue Native-PAGE displayed the impaired tetramer formation in the glycosylation mutants (N63S and N363S), indicating that the mis-transport was caused by impaired tetramer formation. N63S and N363S mutants were primarily degraded via the lysosomal pathway. Flag-tagged N363S GluA1, but not N63S GluA1, expressed in primary cortical neuron cultures prepared from GluA1 knockout mice was observed to localize at the cell surface. Co-expression of GluA2 partially rescued tetramer formation and the cell surface expression of N363S GluA1 but not N63S GluA1, in HEK293T cells. Electrophysiological analysis also demonstrated functional heteromers of N363S GluA1 with GluA2. These data suggest that site-specific N-glycans on GluA1 subunit regulates tetramer formation, intracellular trafficking, and cell surface expression of AMPA-R. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Tunicamycin inhibits NMDA and AMPA receptor responses independently of N-glycosylation

In a whole-cell patch-clamp configuration, currents through N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor channels were monitored in cultured rat hippocampal neurons, and those currents were depressed to 25 and 28% of basal levels, respectively, by 3-min treatment with tunicamycin (10 microM), an inhibitor of protein N-glycosylation. Tunicamycin (10 microM) reduced amplitude of population spikes elicited in the dentate gyrus of rat hippocampal slices, reaching 78% of basal levels 60 min after the beginning of treatment, and long-term potentiation (LTP) of the perforant path was never induced in the presence of tunicamycin. Tunicamycin, thus, appears to serve as a modulator for NMDA and AMPA receptors, regardless of N-glycosylation, thereby inhibiting neurotransmission and LTP in the dentate gyrus.

N-glycosylation sites on the nicotinic ACh receptor subunits regulate receptor channel desensitization and conductance

The present study investigated the effects of N-glycosylation sites on Torpedo acetylcholine (ACh) receptors expressed in Xenopus oocytes by monitoring whole-cell membrane currents and single-channel currents from excised patches. Receptors with the mutant subunit at the asparagine residue on the conserved N-glycosylation site (mbetaN141D, mgammaN141D, or mdeltaN143D) or the serine/threonine residue (mbetaT143A, mgammaS143A, or mdeltaS145A) delayed the rate of current decay as compared with wild-type receptors, and the most striking effect was found with receptors with mbetaT143A or mgammaS143A. For wild-type receptors, the lectin concanavalin A, that binds to glycosylated membrane proteins with high affinity, mimicked this effect. Receptors with mbetaN141D or mdeltaN143D exhibited lower single-channel conductance, but those with mbetaT143A, mgammaS143A, or mdeltaS145A otherwise revealed higher conductance than wild-type receptors. Mean opening time of single-channel currents was little affected by the mutation. N-glycosylation sites, thus, appear to play a role in the regulation of ACh receptor desensitization and ion permeability.

Characterization of the functional role of the N-glycans in the AMPA receptor ligand-binding domain

The ligand-binding domains of AMPA receptor subunits carry two conserved N-glycosylation sites. In order to gain insight into the functional role of the corresponding N-glycans, we examined how the elimination of glycosylation at these sites (N407 and N414) affects the ligand-binding characteristics, structural stability, cell-surface expression, and channel properties of homomeric GluR-D (GluR4) receptor and its soluble ligand-binding domain (S1S2). GluR-D S1S2 protein expressed as a secreted protein in insect cells was found to be glycosylated at N407 and N414. No major differences in the ligand-binding properties were observed between the 'wild-type' S1S2 and non-glycosylated N407D/N414Q double mutant, or between S1S2 proteins expressed in the presence or absence of tunicamycin, an inhibitor of N-glycosylation. Purified glycosylated and non-glycosylated S1S2 proteins also showed similar thermostabilities as determined by CD spectroscopy. Full-length homomeric GluR-D receptor with N407D/N414Q mutation was expressed on the surface of HEK293 cells like the wild-type GluR-D. In outside-out patches, GluR-D and the N407D/N414Q mutant produced similar rapidly desensitizing current responses to glutamate and AMPA. We therefore report that the two conserved ligand-binding domain glycans do not play any major role in receptor-ligand interactions, do not impart a stabilizing effect on the ligand-binding domain, and are not critical for the formation and surface localization of homomeric GluR-D AMPA receptors in HEK293 cells.

Glycosylation of proteins during a critical time window is necessary for the maintenance of long-term potentiation in the hippocampal CA1 region

This paper focuses on the role of glycoproteins in activity-dependent synaptic plasticity. The effect of the different inhibitors of protein glycosylation, Tunicamycin, Brefeldin A and Swainsonine, on long-term potentiation was studied in the CA1 region of rat hippocampal slices. Bath application of the inhibitors 60 min before and during tetanization did not interfere with the induction of long-term potentiation of the field excitatory postsynaptic potential. However, the potentiation in inhibitor-treated slices decreased to baseline levels during 90-180 min. Significant differences in the potentiation in non-treated slices were detectable 80 min (Tunicamycin), 60 min (Brefeldin A) and 75 min (Swainsonine) after tetanization, thus indicating the prevention of long-term potentiation maintenance. The application of Swainsonine 120 and 240 min after tetanization did not influence the potentiated field excitatory postsynaptic potential. These data demonstrate the need for undisturbed glycoprotein processing in a time window around long-term potentiation induction to maintain later phases of long-term potentiation and essential functional implications of protein glycosylation in mechanisms underlying synaptic plasticity.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

A recessive deletion in the GlcNAc-1-phosphotransferase gene results in peri-implantation embryonic lethality

Formation of the dolichol oligosaccharide precursor is essential for the production of asparagine- (N-) linked oligosaccharides (N-glycans) in eukaryotic cells. The first step in precursor biosynthesis requires the enzyme UDP-GlcNAc: dolichol phosphate N-acetylglucosamine-1-phosphate transferase (GPT). Without GPT activity, subsequent steps necessary in constructing the oligosaccharide precursor cannot occur. Inhibition of this biosynthetic step using tunicamycin, a GlcNAc analog, produces a deficiency in N-glycosylation in cell lines and embryonic lethality during preimplantation development in vitro, suggesting that N-glycan formation is essential in early embryogenesis. In exploring structure-function relationships among N-glycans, and since tunicamycin has various reported biochemical activities; we have generated a germline deletion in the mouse GPT gene. GPT mutant embryos were analyzed and the phenotypes obtained were compared with previous studies using tunicamycin. We find that embryos homozygous for a deletion in the GPT gene complete preimplantation development and also implant in the uterine epithelium, but die shortly thereafter between days 4-5 postfertilization with cell degeneration apparent among both embryonic and extraembryonic cell types. Of cells derived from these early embryos, neither trophoblast nor embryonic endodermal lineages are able to survive in culture in vitro. These results indicate that GPT function is essential in early embryogenesis and suggest that N-glycosylation is needed for the viability of cells comprising the peri-implantation stage embryo.

Downregulation and subcellular redistribution of the gamma-aminobutyric acidA receptor induced by tunicamycin in cultured brain neurons

The significance of N-linked glycosylation and oligosaccharide processing was examined for the expression of gamma-aminobutyric acidA receptor (GABA(A)R) in cultured neurons derived from chick embryo brains. Incubation of cultures with 5 microg/ml of tunicamycin for 24 h blocked the binding of 3H-flunitrazepam and 3H-muscimol, probes for the benzodiazepine and GABA sites on the receptor, by about 20% and 28%, respectively. The loss of ligand binding was due to a reduction in the number of binding sites with no significant changes in receptor affinity. Light microscopic immunocytochemistry also revealed that the treatment reduced approximately 13% of the intensity of GABA(A)R immunoreactivity in the neuronal somata. Furthermore, the fraction of intracellular receptors was decreased to 24% from 34% of control in the presence of the agent, as revealed by trypsinization of cells in situ followed by 3H-flunitrazepam binding. The molecular weight of the receptor subunit protein was lowered around 0.5 kDa after tunicamycin treatment, in accordance with that following N-glycosidase F digestion, indicating the blockade of N-linked glycosylation of GABA(A)R by tunicamycin. Moreover, intense inhibitions of 91% and 44%, respectively, were detected to the general galactosylation and mannosylation in the tunicamycin-treated cells, whereas the protein synthesis was hindered by 13%, through assaying the incorporation of 3H-sugars and 3H-leucine. Nevertheless, treatment with castanospermine or swainsonine (10 microg/ml, 24 h), inhibitors to maturation of oligosaccharides, failed to produce significant changes in the ligand binding. In addition, in situ hybridization analysis showed that these three inhibitors did not perturb the mRNA of GABA(A)Ralpha1-subunit. The data suggest that tunicamycin causes the downregulation and subcellular redistribution of GABA(A)R by producing irregularly glycosylated receptors and modifying their localization. Both galactosylation and mannosylation during the process of N-linked glycosylation may be important for the functional expression and intracellular transport of GABA(A)R.

N-Glycosylation is not a prerequisite for glutamate receptor function but Is essential for lectin modulation

All ionotropic glutamate receptor (iGluR) subunits analyzed so far are heavily N-glycosylated at multiple sites on their amino-terminal extracellular domains. Although the exact functional significance of this glycosylation remains to be determined, it has been suggested that N-glycosylation may be a precondition for the formation of functional ion channels. In particular, it has been argued that N-glycosylation is required for the formation of functional ligand binding sites. We analyzed heterologously expressed recombinant glutamate receptors (GluRs) of all three pharmacological subclasses of glutamate receptors, N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and kainate receptors. By expressing the GluR subunits in tunicamycin-treated, nonglycosylating Xenopus laevis oocytes, we determined that in neither case is N-glycosylation required for ion channel function, although for NMDA receptors, functional expression in the absence of N-glycosylation is very low. Furthermore, we analyzed and compared the interaction of the desensitization-inhibiting lectin concanavalin A (ConA) with all functional GluR subunits. We show that although ConA has its most pronounced effects on kainate receptors, it potentiates currents at most other receptor subtypes as well, including certain NMDA receptor subunits, although to a much lesser extent. One notable exception is the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor GluR2, which is not affected by ConA. Furthermore, we show that ConA acts directly via binding to the carbohydrate side chains of the receptor protein.

Expression and functional characterization of a melatonin-sensitive receptor in Xenopus oocytes

Melatonin (MEL) plays a central role in the regulation of seasonal cycles and in the control of circadian rhythms in mammals. Functional MEL-sensitive receptors were expressed in Xenopus laevis oocytes following injection of poly (A)+ RNA from rat brain. Administration of 0.1-100 micromol/l MEL to voltage-clamped oocytes (holding potential: -70 mV) elicited oscillatory inward currents (reversal potential: -24 mV) which could be blocked by 9-anthracenecarboxylic acid and caffeine. After preincubation with pertussis toxin (PTX) the MEL response disappeared. The expressed MEL-sensitive receptor probably activates Ca(2+)-dependent chloride currents via a PTX-sensitive G protein and the phosphoinositol pathway.

An investigation into the role of N-glycosylation in the functional expression of a recombinant heteromeric NMDA receptor

The effect of N-glycosylation on the assembly of N-methyl-D-aspartate (NMDA) heteromeric cloned receptors was studied. Thus human embryonic kidney (HEK) 293 cells were cotransfected with N-methyl-D-aspartate R1 (NR1) and N-methyl-D-aspartate R2A (NR2A) clones and the cells grown post-transfection in the presence of tunicamycin (TM). TM treatment resulted in a decrease of the NR1 subunit with M(r) 117 000 with a concomitant increase in a M(r) 97 000 immunoreactive species previously identified as the non-N-glycosylated NR1 subunit. In parallel, TM caused a dose-dependent inhibition of [3H]MK801 binding to the expressed receptor which was a result of an approximate four-fold reduction in the Dissociation Constant (KD) but with no change in the number of binding sites (Bmax). NMDA receptor cell surface expression was unchanged following TM treatment but it did result in a decrease in the percentage cell death post-transfection compared to control samples. The removal of TM from the cell culture media resulted in a return to the control KD value for [3H]MK801 binding and partial reglycosylation of newly synthesized NR1 subunit. These results demonstrate that N-glycosylation is requisite for the efficient expression of functional NR1/NR2A receptors. Furthermore, they suggest that N-glycosylation may be important for the correct formation of the channel domain of the NR1/NR2A receptor.

Expression and characterization of the alpha 2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-selective glutamate receptor channel in a baculovirus system

Using a baculovirus expression vector system, the alpha 2 subunit of the mouse alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-selective glutamate receptor (GluR) channel was expressed in Spodoptera frugiperda insect cells. Immunoblotting using the antibody made to the synthetic peptide corresponding to the C-terminus of GluR alpha 2 and [35S]methionine/[35S]cysteine metabolic radiolabeling revealed the major 102-kDa and the minor 98-kDa protein bands. Metabolic radiolabeling with tunicamycin suggested that the two bands correspond to glycosylated and unglycosylated forms, respectively. The recombinant GluR alpha 2 proteins expressed in insect cells were also identified by immunofluorescence staining. The results of [3H]AMPA binding assay using whole cells suggested that, in infected Sf21 cells, binding sites of the GluR alpha 2 proteins were possibly located on the extracellular side. Scatchard analysis of AMPA binding showed the following parameters: Kd = 16 nM, Bmax = 1.9 x 10(5) binding sites per cell or 1 pmol/mg protein in the total particulate fraction. The ligand binding characteristics of the receptors expressed in insect cells were examined. From the effect of various agonists on [3H]AMPA binding of the receptors expressed in insect cells, the rank order potency of agonists was quisqualate > AMPA > L-glutamate > kainate. Thus, the baculovirus-insect cell expression system provides high-efficiency expression of the receptor sufficient to permit structural and functional analyses.

Tunicamycin inhibits prostaglandin F2 alpha receptor-mediated phosphoinositide hydrolysis in cultured rat astrocytes

Effect of tunicamycin, an inhibitor of N-linked glycosylation, on prostaglandin (PG) F2 alpha-stimulated phosphoinositide (PI) hydrolysis was examined in cultured rat astrocytes. Pretreatment of cultured astrocytes with tunicamycin (25-250 ng/ml) inhibited subsequent PGF2 alpha (1 microM)-stimulated PI-hydrolysis in concentration- and time-dependent manners. The inhibition completely recovered after removal of tunicamycin and re-incubation for 12 h. Tunicamycin pretreatment (100 ng/ml for 12 h) significantly blocked [35S]methionine incorporation into cultured astrocytes, but cell viability was not affected under the condition. Inhibitors of processing of N-linked sugar chains such as bromoconduritol, 1-deoxymannojirimycin, and swainsonine had no effect on PI response to PGF2 alpha. These observations suggest that PGF2 alpha receptor is N-linked glycosylated.

N-glycosylation site tagging suggests a three transmembrane domain topology for the glutamate receptor GluR1

We investigated the transmembrane topology of the glutamate receptor GluR1 by introducing N-glycosylation sites as reporter sites for an extracellular location of the respective site. Our data show that the N-terminus is extracellular, whereas the C-terminus is intracellular. Most importantly, we found only three transmembrane domains (designated TMD A, TMD B, and TMD C), which correspond to the previously proposed TMDs I, III, and IV, respectively. Contrary to earlier models, the putative channel-lining hydrophobic domain TMD II does not span the membrane, but either lies in close proximity to the intracellular face of the plasma membrane or loops into the membrane without transversing it. Furthermore, the region between TMDs III and IV, in previous models believed to be intracellular, is an entirely extracellular domain.

Ligand-binding properties and N-glycosylation of alpha 1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate(AMPA)-selective glutamate receptor channel expressed in a baculovirus system

The alpha 1 subunit of the mouse alpha-amino-3-hydroxy-5-methyl-4-isoxazole- propionate(AMPA)-selective glutamate receptor channel has been expressed in insect Spodoptera frugiperda cells using a baculovirus system. The recombinant receptor proteins were identified by immunocytochemical detection, Western-blot analysis, and [35S]methionine/[35S]cysteine metabolic labeling experiments. The effect of tunicamycin on the metabolic labeling and immunoblots suggested that the two products, a major protein species of approximately 104 kDa and a minor species of approximately 100 kDa, correspond to glycosylated and non-N-glycosylated forms, respectively, which was also supported by the enzymic deglycosylation experiments. The lack of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate-binding activity of non-N-glycosylated glutamate receptor expressed in the presence of tunicamycin suggested that N-glycosylation is required, directly or indirectly, for functional expression in insect cells for ligand binding. Scatchard analysis of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate binding showed a single binding site with Kd 30 nM and a Bmax value of 2.6 x 10(5) binding sites/cell or 1.5 pmol/mg protein in the total particulate fraction. Among the compounds tested in the competition studies, beta-(3,5-dioxo-1,2,4-oxadiazolidin-2-yl)-L-alanine (quisqualate) was the most potent inhibitor of the 3H-labeled alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate binding (IC50 = 30 nM), followed in decreasing order by alpha-amino-3-hydroxy-5- methyl-4-isoxazole propionate, L-glutamate, 6,7-dinitroquinoxaline-2,3-dione, 6-cyano-7-nitroquinoxaline-2,3-dione, and 2-carboxy-4-(1-methylethenyl)-3-pyrrolidineacetate (kainate). Thus, in this study we present detailed analysis of alpha-amino-3-hydroxy-5-methyl-4- isoxazole-propionate-binding activity of the homomeric (single subunit) glutamate receptor channel of mouse alpha 1 subunit and discuss possible roles of N-glycosylation of the glutamate receptor channel alpha 1 subunit.