Receptor Subunit GluR1 Research Articles

RNA editing (R/G site) and flip–flop splicing of the AMPA receptor subunit GluR2 in nervous tissue of epilepsy patients

Editing and alternative splicing of mRNA are posttranscriptional steps probably involved in pathophysiological aspects of epilepsy. The present study analyses the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit GluR2 with respect to the expression of (i) editing at the R/G site and (ii) flip–flop cassettes. Nervous tissue from patients with temporal lobe epilepsy was analysed by RT-PCR followed by restriction enzyme assays. Human autoptic tissue served as control. R/G editing status: the relative amount of edited RNA was significantly increased in the hippocampal tissue, whereas no changes were found in neocortical tissues. Flip–flop expression: no significant alterations were found in relative abundance of spliced variants containing the flip exon. The increased editing at the R/G site in the hippocampal tissue of epilepsy patients may enhance responses to glutamate, resulting in a synapse operating at an increased gain.

AMPA receptor properties and coexpression with sodium-calcium exchangers in rat hypothalamic neurons.

The histaminergic tuberomamillary (TM) nucleus, a center for the regulation of wakefulness, is excited by glutamatergic, aminergic and peptidergic inputs. AMPA receptor properties in relation to their expression were investigated in acutely isolated TM neurons with the help of whole-cell patch-clamp recordings combined with single-cell RT-PCR. The mRNAs encoding for the AMPA receptor GluR2 (100% of the neurons) and GluR1 (75%) were the most frequently detected, followed by the mRNA for GluR4 (56%), whereas GluR3 cDNA amplification did not yield a PCR product in any neuron. Flip splice variants prevailed over flop, in keeping with a strong glutamate-response potentiation by cyclothiazide. The expression pattern of AMPA subunits in their two splice variants was correlated with the different subtypes of Na+/Ca2+ (NCX) and Na+/Ca2+/K+ (NCKX) exchangers: glutamate receptor subunits GluR1-4 displayed no coordinated pattern with NCX. However, NCKX2 mRNA occurred only in TM cells with a fast desensitizing glutamate response, where it was coexpressed with the GluR4 subunit in the flop splice variant. NCKX3 mRNA was detected in neurons with fast or slow desensitization of glutamate responses. AMPA receptors in TM neurons were Ca2+-impermeable. As reverse Na+/Ca2+ exchange contributes to the immediate rise in intracellular calcium resulting from glutamate receptor activation, we suggest that the coordinated expression of NCKX2 with the fast desensitizing AMPA receptor-type reflects either a receptor-exchanger coupling or separate mechanisms for maintaining calcium homeostasis in neurons with fast or slow glutamate responses.

Increased expression of AMPA receptor subunits in the nucleus of the solitary tract in the spontaneously hypertensive rat

The expression of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunits GluR1–4 in the nucleus of the solitary tract (NTS) of adult Wistar rats was examined by polymerase chain reaction (PCR), and the neuronal localisation of these receptor subunits in the NTS were confirmed by immunohistochemistry using subunit-specific antibodies. Semi-quantitative PCR was used to investigate differences in AMPA receptor subunit expression between spontaneously hypertensive rats (SH) and age-matched normotensive Wistar Kyoto rats (WKY). All four receptor subunits were expressed in both strains, but compared to WKY, total AMPA receptor and the GluR3 mRNA expressions were significantly higher in SH. No differences were detected in cDNA form the cerebral cortex or cerebellum. Immunolabelling for GluRs 1, 2 and 2/3 in the neuropil relative to neuronal somata in the cardioregulatory areas of the NTS appeared to be increased in SH, with an overall increase in the density of GluR2/3 labelling in the medial and commissural NTS of SH. These results indicate a possible role for changes in AMPA receptor subunit expression in NTS neurones, involving an increase in GluR3 associated with development of hypertension in SH.

Phosphorylation of Ca 2+/calmodulin-dependent protein kinase type ii and the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (ampa) receptor in response to a threonine-devoid diet

The anterior piriform cortex (APC) functions as a chemosensor for indispensable amino acid deficiency and responds to this deficiency with increased activity, as indicated by observations including averaged evoked-potentials and c- fos expression in the APC. Little is known of the intracellular signaling mechanisms that mediate this deficiency-related increase in neuronal excitability, but previous studies have shown effects on intracellular Ca 2+ in deficient APC slices in vitro. In the present study we hypothesized that indispensable amino acid deficiency increases intraneuronal Ca 2+, resulting in autophosphorylation of calcium/calmodulin-dependent protein kinase type II (CaMKII) in vivo. Results demonstrated that phosphorylation levels of CaMKII (pCaMKII) in APC neurons increased at 20 and 40 min after a single meal of threonine-devoid diet. Phosphorylation of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunit (GluR1) at the serine 831 (S831) site was modestly increased in the APC in response to a threonine-devoid meal. The GluR1 subunit also showed increased phosphorylation at the 845 (S845) site, suggesting additional signaling mechanisms. Although phosphorylation of CaMKII was sustained, phosphorylation of the GluR1 subunit returned to control levels by 40 min. These effects of amino acid deficiency did not occur throughout the brain as neither CaMKII nor GluR1 showed increased phosphorylation in the neocortex. These findings support the notion that calcium and glutamate signaling in the APC, but not throughout the brain, are triggered during early responses to amino acid deficiency. They also suggest that longer-term changes in APC neurons in response to such a deficiency may be mediated at least in part by CaMKII.

Ischemia-induced changes of AMPA-type glutamate receptor subunit expression pattern in the rat retina: a real-time quantitative PCR study.

To investigate whether the previously observed decrease in immunoreactivity of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor subunits GluR1, -2, -3, and -4 after ischemia-reperfusion in the rat retina is associated with changes at the mRNA expression level. Furthermore, to study possible changes in the ratios of alternative splice variants of GluR2 and -4 and possible changes in the subunit composition of the receptor complex after ischemia-reperfusion. The ischemia-induced changes were related to expression levels of immediate early genes, c-fos and c-jun, and to expression levels of different cell-type-specific transcripts. A 60-minute ischemic event was induced unilaterally in the rat eye by cannulating the anterior chamber and raising the intraocular pressure. Reperfusion was allowed to occur for 2 hours up to 28 days. Total RNA was isolated from the retinas and transcript levels were assessed by real-time quantitative PCR (qPCR). A differential decrease was observed in the expression levels of all AMPA-type GluR subunits 2 hours after ischemia-reperfusion, with a significant downregulation of GluR2 and -3 transcript levels. At the long-term (72 hours-4 weeks), expression levels for all four subunits were decreased by approximately 64%. No changes were observed, either in the expression ratio of GluR2 and -4 splice variants, or in the relative expression of the different subunits. Immediate early genes c-fos and c-jun were transiently upregulated. Expression levels of the ganglion-cell-specific transcripts Thy-1 and neurofilament and of the AII-amacrine-specific transcript parvalbumin decreased after ischemia-reperfusion, whereas the ON bipolar cell transcripts mGluR6 and PKCalpha did not show ischemia-induced changes. Shortly after ischemia-reperfusion immunolabeling of GluR1, -2/3, and -4 is strongly decreased, whereas the corresponding mRNA levels are not affected, indicating degradation at the protein level. In contrast, the GluR2 mRNA level is reduced, whereas immunostaining is not yet affected, suggesting that the GluR2 protein is relatively stable under postischemia conditions. The long-term decrease in mRNA levels of all AMPA-type GluR subunits suggests that ischemia affects a main component of the excitatory retinal neurotransmission. It remains to be investigated whether these changes contribute to the subsequent neurodegeneration.

Ischemia-induced Alterations of AMPA-type glutamate receptor subunit. Expression patterns in the rat retina—an immunocytochemical study

This study investigates whether retinal ischemia/reperfusion leads to alterations in the expression of AMPA-type glutamate receptor (AMPAR) subunits GluR1–4. In ischemia-vulnerable hippocampal neurons, a subunit-specific downregulation of GluR2 precedes the actual neurodegeneration. Our purpose was to study whether retinal ischemia induces a similar downregulation of GluR2 preceding the loss of ganglion and amacrine cells. A 60-min ischemic period was followed by reperfusion lasting between 2 h and 7 days. Changes in the expression patterns of GluR1–4 were assessed using immunocytochemistry. In the same sections, alterations in cell density, thickness of retinal layers, and density of apoptotic cells were investigated. Two-hour post-ischemia, GluR1 immunoreactivity was nearly absent from the inner plexiform layer (IPL). Thereafter, labeling intensity recovered slowly and was close to control levels at 7 days, albeit in a thinner IPL. The decrease in GluR2/3 labeling intensity was most profound at 4 h. The recovery of GluR2/3 staining intensity was slow, and staining was still decreased at 7 days. GluR2 immunoreactivity was not attenuated after ischemia. GluR4 labeling showed a similar time course as observed for GluR1, but the decrease in immunoreactivity was less profound and the recovery was nearly complete. The immunostaining of PKCα, a rod bipolar cell marker, was unaffected at all reperfusion times. The reduction of GluR staining preceded both the typical thinning of the IPL and the peak of cell loss, but coincided with a significant swelling of the IPL. In conclusion, retinal ischemia/reperfusion leads to differential changes in the expression of the different AMPA-type GluR subunits, which may affect excitatory synaptic transmission in the inner retina. However, no evidence was found for a preferential loss of GluR2 immunoreactivity that could account for selective neurodegeneration of amacrine and ganglion cells after retinal ischemia.

Picosecond dynamics of the glutamate receptor in response to agonist-induced vibrational excitation.

Conformational changes of proteins are dominated by the excitation and relaxation processes of their vibrational states. To elucidate the mechanism of receptor activation, the conformation dynamics of receptors must be analyzed in response to agonist-induced vibrational excitation. In this study, we chose the bending vibrational mode of the guanidinium group of Arg485 of the glutamate receptor subunit GluR2 based on our previous studies, and we investigated picosecond dynamics of the glutamate receptor caused by the vibrational excitation of Arg485 via molecular dynamics simulations. The vibrational excitation energy in Arg485 in the ligand-binding site initially flowed into Lys730, and then into the J-helix at the subunit interface of the ligand-binding domain. Consequently, the atomic displacement in the subunit interface around an intersubunit hydrogen bond was evoked in about 3 ps. This atomic displacement may perturb the subunit packing of the receptor, triggering receptor activation.

AGS-induced expression of Narp is concomitant with expression of AMPA receptor subunits GluR1 and GluR2 in hippocampus but not inferior colliculus of P77PMC rats

To explore mechanisms of epileptogenesis in audiogenic seizures (AGS), we examined the expression of α-amino-3-hydroxy-5-methyl-4-isoxazoleopropionic acid (AMPA) receptor subunits GluR1 and GluR2 and of the GluR-associated protein Narp in the hippocampus and the inferior colliculus (IC) from AGS-susceptible P77PMC rats after a single AGS and audiogenic kindling. Western blotting and immunohistochemistry showed that Narp was rapidly induced in both the hippocampus and the IC by AGS. In the hippocampus, up-regulation of Narp was concomitant with GluR1 and GluR2 under both conditions of a single AGS and AGS kindling. In the IC, however, Narp was up-regulated, GluR2 down-regulated, and GluR1 unchanged after kindling. In comparison with kindling, neither GluR1 nor GluR2 was changed, while Narp significantly increased in the IC following a single AGS. These findings suggest that down-regulation of AMPA receptor GluR2 subunit in the IC may contribute to AGS-mediated epileptogenesis, and up-regulation of Narp in the IC may be involved in audiogenic seizures.

Structurally dissimilar antimanic agents modulate synaptic plasticity by regulating AMPA glutamate receptor subunit GluR1 synaptic expression.

A growing body of data from clinical and preclinical studies suggests that the glutamatergic system may represent a novel therapeutic target for severe recurrent mood disorders. Since synapse-specific glutamate receptor expression/localization is known to play critical roles in synaptic plasticity, we investigated the effects of mood stabilizers on AMPA receptor expression. Rats were treated chronically with lithium or valproate, hippocampal synaptosomes were isolated, and GluR1 levels were determined. Additionally, hippocampal neurons were prepared from E18 rat embryos and treated with lithium or valproate. Surface expression of GluR1 was determined using a biotinylation assay, and double-immunostaining with anti-GluR1 and anti-synaptotagmin antibodies was used to determine synaptic GluR1 levels. The AMPA receptor subunit GluR1 expression in hippocampal synaptosomes was significantly reduced by both chronic lithium and valproate. Overall, these studies show that AMPA receptor subunit GluR1 is a common target for two structurally highly dissimilar, but highly efficacious, mood stabilizers, lithium and valproate. These studies suggest that regulation of glutamatergically mediated synaptic plasticity may play a role in the treatment of mood disorders, and raise the possibility that agents more directly affecting synaptic GluR1 may represent novel therapies for this devastating illness.

Functional Analysis of Caenorhabditis elegans Glutamate Receptor Subunits by Domain Transplantation

Glutamate receptors are not only abundant and important mediators of fast excitatory synaptic transmission in vertebrates, but they also serve a similar function in invertebrates such as Drosophila and the nematode Caenorhabditis elegans. In C. elegans, an animal with only 302 neurons, 10 different glutamate receptor subunits have been identified and cloned. To study the ion channel properties of these receptor subunits, we recorded glutamate-gated currents from Xenopus oocytes that expressed either C. elegans glutamate receptor subunits or chimeric rat/C. elegans glutamate receptor subunits. The chimeras were constructed between the C. elegans glutamate receptor pore domains and either the rat kainate receptor subunit GluR6, the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunit GluR1, or the N-methyl-d-aspartate (NMDA) receptor subunit NMDAR1-1a. Although native subunits were nonfunctional, 9 of 10 ion pores were found to conduct current upon transplantation into rat receptor subunits. A provisional classification of the C. elegans glutamate receptor subunits was attempted based on functionality of the chimeras. C. elegans glutamate receptor ion pores, at a position homologous to a highly conserved site critical for ion permeation properties in vertebrate glutamate receptor pores, contain amino acids not found in vertebrate glutamate receptors. We show that the pore-constricting Q/R site, which in vertebrate receptors determines calcium permeability and rectification properties of the ion channel, in C. elegans can be occupied by other amino acids, including, surprisingly, lysine and proline, without loss of these properties.

Mechanisms by which dopamine receptors may influence synaptic plasticity.

While dopamine (DA) receptors mediate acute effects of amphetamine and cocaine, chronic drug administration produces many glutamate-dependent adaptations, including LTP in reward-related neuronal circuits. An important question presents itself: How do DA receptors influence glutamate-dependent synaptic plasticity? Alterations in AMPA receptor phosphorylation and trafficking are critical for LTP. We hypothesize that D1 DA receptors modulate these processes, that chronic drug-induced adaptations in D1 receptor signaling, therefore, trigger compensatory changes in AMPA receptor function, and that this ultimately contributes to inappropriate plasticity in addiction-related neuronal circuits. Postnatal rat nucleus accumbens (NAc) cultures were used to study D1 receptor regulation of the AMPA receptor subunit GluR1. We found that D1 receptor stimulation enhances phosphorylation of GluR1 at the protein kinase A (PKA) site. Furthermore, D1 receptor stimulation increases GluR1 surface expression by increasing the rate of GluR1 externalization. The latter effect is prevented by the PKA inhibitors KT5720 and RpcAMPS, whereas the PKA activator SpcAMPS increases the rate of GluR1 externalization. These findings indicate that PKA phosphorylation is important in determining AMPA receptor surface expression and suggest a mechanism by which DA-releasing drugs of abuse may directly tap into fundamental mechanisms that enable synaptic plasticity. A limitation of our current model is that there are no intrinsic glutamate neurons in the NAc and thus no glutamate synapses in NAc cultures. To address this problem, we have restored excitatory synaptic inputs to NAc neurons by co-culturing them with prefrontal cortex (PFC) neurons. We are also studying GluR1 trafficking in PFC cultures. In both systems, synaptic AMPA receptors can be defined based on colocalization of GluR1 and the synaptic marker synaptobrevin. Preliminary results suggest that D1 receptor stimulation or PKA activation leads to increased surface GluR1 expression in PFC neurons but not to insertion into synaptic sites.

Multiple binding proteins suggest diverse functions for the N-ethylmaleimide sensitive factor

The hexameric ATPase, N-ethylmaleimide sensitive factor (NSF), is essential to vesicular transport and membrane fusion because it affects the conformations and associations of the soluble NSF attachment protein receptor (SNARE) proteins. NSF binds SNAREs through adaptors called soluble NSF attachment proteins (α- or β-SNAP) and disassembles SNARE complexes to recycle the monomers. NSF contains three domains, two nucleotide-binding domains (NSF-D1 and -D2) and an amino terminal domain (NSF-N) that is required for SNAP–SNARE complex binding. Mutagenesis studies indicate that a cleft between the two sub-domains of NSF-N is critical for binding. The structural conservation of N domains in NSF, p97/VCP, and VAT suggests that a similar type of binding site could mediate substrate recognition by other AAA proteins. In addition to SNAP–SNARE complexes, NSF also binds other proteins and protein complexes such as AMPA receptor subunits (GluR2), β2-adrenergic receptor, β-Arrestin1, GATE-16, LMA1, rabs, and rab-containing complexes. The potential for these interactions indicates a broader role for NSF in the assembly/disassembly cycles of several cellular complexes and suggests that NSF may have specific regulatory effects on the functions of the proteins involved in these complexes. The structural requirements for these interactions and their physiological significance will be discussed.

A 2A antagonist prevents dopamine agonist-induced motor complications in animal models of Parkinson’s disease

Adenosine A 2A receptors, abundantly expressed on striatal medium spiny neurons, appear to activate signaling cascades implicated in the regulation of coexpressed ionotropic glutamatergic receptors. To evaluate the contribution of adenosinergic mechanisms to the pathogenesis of the response alterations induced by dopaminergic treatment, we studied the ability of the selective adenosine A 2A receptor antagonist KW-6002 to prevent as well as palliate these syndromes in rodent and primate models of Parkinson’s disease. In rats, KW-6002 reversed the shortened motor response produced by chronic levodopa treatment while reducing levodopa-induced hyperphosphorylation at S845 residues on AMPA receptor GluR1 subunits. In primates, KW-6002 evidenced modest antiparkinsonian activity when given alone. Once-daily coadministration of KW-6002 with apomorphine prevented the development of dyskinesias, which appeared in control animals 7–10 days after initiating apomorphine treatment. Animals initially given apomorphine plus KW-6002 for 3 weeks did not begin to manifest apomorphine-induced dyskinesias until 10–12 days after discontinuing the A 2A antagonist. These results suggest that KW-6002 can attenuate the induction as well as the expression of motor response alterations to chronic dopaminergic stimulation in parkinsonian animals, possibly by blocking A 2A receptor-stimulated signaling pathways. Our findings strengthen the rationale for developing A 2A antagonists as an early treatment strategy for Parkinson’s disease.

Regulation of AMPA receptor dephosphorylation by glutamate receptor agonists

Phosphorylation of the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 at Ser 845 enhances AMPA channel activity. This study demonstrates that Ser 845 is rapidly dephosphorylated upon AMPA receptor activation in nucleus accumbens slices. AMPA-induced dephosphorylation at Ser 845 was blocked by CNQX, an AMPA receptor antagonist, by nifedipine, an L-type Ca 2+ channel antagonist, or by cyclosporin A, a calcineurin inhibitor. N-methyl- d-aspartate (NMDA) treatment also decreased phosphorylation of Ser 845, an effect that was blocked by MK-801, an NMDA receptor antagonist, but not by nifedipine. Accumbens neurons are enriched for dopamine- and cyclic AMP (cAMP)-regulated phosphoprotein, Mr 32,000 (DARPP-32), a potent inhibitor of protein phosphatase 1 (PP1) when phosphorylated by PKA (at Thr 34). We tested the hypothesis that the AMPA/KA or NMDA-stimulated dephosphorylation of DARPP-32 via calcineurin, leading to increased PP1 activity and dephosphorylation of GluR1. AMPA or NMDA treatment decreased phospho-Thr 34-DARPP-32 levels, effects that were blocked by receptor antagonists, or cyclosporin A. However, dephosphorylation of Ser 845 mediated by AMPA or NMDA receptors was unaffected in DARPP-32/inhibitor-1 knockout mice. These data suggest that AMPA- or NMDA-induced dephosphorylation of GluR1 at Ser 845 occurs by a mechanism that is independent of DARPP-32 and PP1, but involves activation of calcineurin. Thus, Ca 2+-dependent dephosphorylation of GluR1 may serve as a negative feedback mechanism for the regulation of AMPA receptor activity in neurons.

Glutamate receptors at bipolar synapses in the inner plexiform layer of primate retina: light microscopic analysis.

At least 10 different types of bipolar cells have been distinguished in the primate retina. The axon terminals of these cells stratify in distinct strata in the inner plexiform layer and are involved in parallel pathways to distinct types of ganglion cells. Ionotropic glutamate receptor (GluR) subunits also show a stratified distribution in the inner plexiform layer. Here, we investigated whether different types of bipolar cells are associated with different types of ionotropic glutamate receptors in the inner retina of a New World primate, the common marmoset Callithrix jacchus. Vertical cryostat sections through central retina were double labeled with immunohistochemical markers for bipolar cell types and with antibodies to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits GluR1 to 4, kainate receptor subunits GluR6/7, and the NR1C2' subunit of the N-methyl-D-aspartate (NMDA) receptor. The axon terminals of bipolar cell types were reconstructed from confocal sections, and the colocalized immunoreactive puncta were quantified. For all bipolar cell types, immunoreactive puncta for the AMPA receptor subunits GluR2, 2/3, and 4 were colocalized at highest densities, whereas GluR1-immunoreactive puncta were expressed at very low densities. The kainate receptor subunits GluR6/7 were predominantly associated with diffuse bipolar (DB6) and rod bipolar cells. The NMDA receptor subunit NR1C2' was specifically colocalized with flat midget and DB3 axons. These findings suggest that rod and cone bipolar cell types contribute to multiple but distinct glutamate receptor pathways in primate retina.

AMPA receptors mediate acetylcholine release from starburst amacrine cells in the rabbit retina.

The light response of starburst amacrine cells is initiated by glutamate released from bipolar cells. To identify the receptors that mediate this response, we used a combination of anatomical and physiological techniques. An in vivo, rabbit eyecup was preloaded with [(3)H]-choline, and the [(3)H]-acetylcholine (ACh) released into the superfusate was monitored. A photopic, 3 Hz flashing light increased ACh release, and the selective AMPA receptor antagonist, GYKI 53655, blocked this light-evoked response. Nonselective AMPA/kainate agonists increased the release of ACh, but the specific kainate receptor agonist, SYM 2081, did not increase ACh release. Selective AMPA receptor antagonists, GYKI 53655 or GYKI 52466, also blocked the responses to agonists. We conclude that the predominant excitatory input to starburst amacrine cells is mediated by AMPA receptors. We also labeled lightly fixed rabbit retinas with antisera to choline acetyltransferase (ChAT), AMPA receptor subunits GluR1, GluR2/3, or GluR4, and kainate receptor subunits GluR6/7 and KA2. Labeled puncta were observed in the inner plexiform layer with each of these antisera to glutamate receptors, but only GluR2/3-IR puncta and GluR4-IR puncta were found on the ChAT-IR processes. The same was true of starburst cells injected intracellularly with Neurobiotin, and these AMPA receptor subunits were localized to two populations of puncta. The AMPA receptors are expected to desensitize rapidly, enhancing the sensitivity of starburst amacrine cells to moving or other rapidly changing stimuli.

Agonist-specific vibrational excitation of glutamate receptor

Conformational changes of receptors are dominated by the excitation of their collective motions. The most likely energy source of this excitation is considered to be a collision of an agonist with the binding site of a receptor and a consequential excitation of their vibrational modes. In the present study, as an approach to elucidating the mechanism for receptor activation, we chose both the symmetric stretching vibration of the 1C-carboxyl group of glutamate and the bending vibration of the guanidinium group of Arg485 of the glutamate receptor subunit GluR2 based on our previous study, and we quantum-mechanically calculated the vibrational excitation probability of these two vibrations by glutamate collision. The excitation probability of these two vibrations was found to exceed 0.5 about 260 fs after the onset of collision within the first-order perturbation approximation. Taking into account that the period of these vibrations is about 20 fs, we can expect that the vibrational excitation occurs in Arg485 after tens of vibrations during the collision. This vibrational energy may redistribute to the collective motions of the receptor, resulting in global conformational changes in the receptor. We also confirmed that the charge transfer was small between an agonist and the binding site of GluR2. The glutamate receptor may be oscillatory systems that require the energy injection into the specific vibrational modes of the specific amino acid residues to trigger their activation. Such an injection of energy is provided by agonist collision.

New Role for Glutamate Receptor Subunit

Excitatory synapses on hippocampal neurons in the mammalian brain are located on top of dendritic spines. Increased formation of spines, protuberances on the postsynaptic cell, is thought to be part of the mechanism by which long-term changes are stored in neurons and maintained after stimulation. AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors often mediate excitatory signals, and Passafaro et al. explored the role of the glutamate receptor 2 (GluR2) subunit of AMPA receptors on spine formation in cultured hippocampal neurons from embryonic rats. Overexpression of GluR2 increased the number of spines and their size, whereas treatment of cells with small interfering RNA to decrease expression of endogenous GluR2 decreased the size and number of spines. Surprisingly, these effects appeared to require only the extracellular N-terminal domain of the receptor and thus may be independent of the better-understood channel properties of the receptor or its C-terminal portion, which interacts with a number of cytoplasmic proteins. The authors discuss mechanisms by which the GluR2 subunits might act. A favored model is that GluR2 mediates interaction with another protein, which might be present in either soluble form or anchored in the membrane of pre- or postsynaptic cells. Such an interacting protein itself, or another interaction partner, might mediate signaling that promotes growth of spines. M. Passafaro, T. Nakagawa, C. Sala, M. Sheng, Induction of dendritic spines by an extracellular domain of AMPA receptor subunit GluR2. Nature 424, 677-681 (2003). [Online Journal]

New Role for Glutamate Receptor Subunit

Excitatory synapses on hippocampal neurons in the mammalian brain are located on top of dendritic spines. Increased formation of spines, protuberances on the postsynaptic cell, is thought to be part of the mechanism by which long-term changes are stored in neurons and maintained after stimulation. AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors often mediate excitatory signals, and Passafaro et al. explored the role of the glutamate receptor 2 (GluR2) subunit of AMPA receptors on spine formation in cultured hippocampal neurons from embryonic rats. Overexpression of GluR2 increased the number of spines and their size, whereas treatment of cells with small interfering RNA to decrease expression of endogenous GluR2 decreased the size and number of spines. Surprisingly, these effects appeared to require only the extracellular N-terminal domain of the receptor and thus may be independent of the better-understood channel properties of the receptor or its C-terminal portion, which interacts with a number of cytoplasmic proteins. The authors discuss mechanisms by which the GluR2 subunits might act. A favored model is that GluR2 mediates interaction with another protein, which might be present in either soluble form or anchored in the membrane of pre- or postsynaptic cells. Such an interacting protein itself, or another interaction partner, might mediate signaling that promotes growth of spines. M. Passafaro, T. Nakagawa, C. Sala, M. Sheng, Induction of dendritic spines by an extracellular domain of AMPA receptor subunit GluR2. Nature 424 , 677-681 (2003). [Online Journal]

Induction of dendritic spines by an extracellular domain of AMPA receptor subunit GluR2.

Synaptic transmission from excitatory nerve cells in the mammalian brain is largely mediated by AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors located at the surface of dendritic spines. The abundance of postsynaptic AMPA receptors correlates with the size of the synapse and the dimensions of the dendritic spine head. Moreover, long-term potentiation is associated with the formation of dendritic spines as well as synaptic delivery of AMPA receptors. The molecular mechanisms that coordinate AMPA receptor delivery and spine morphogenesis are unknown. Here we show that overexpression of the glutamate receptor 2 (GluR2) subunit of AMPA receptors increases spine size and density in hippocampal neurons, and more remarkably, induces spine formation in GABA-releasing interneurons that normally lack spines. The extracellular N-terminal domain (NTD) of GluR2 is responsible for this effect, and heterologous fusion proteins of the NTD of GluR2 inhibit spine morphogenesis. We propose that the NTD of GluR2 functions at the cell surface as part of a receptor-ligand interaction that is important for spine growth and/or stability.