Surface expression and metabolic half-life of AMPA receptors in cultured rat cerebellar granule cells

Genetically encoded intrabody probes for labeling and manipulating AMPA-type glutamate receptors

Tools for visualizing and manipulating protein dynamics in living cells are critical for understanding cellular function. Here we leverage recently available monoclonal antibody sequences to generate a set of affinity tags for labeling and manipulating AMPA-type glutamate receptors (AMPARs), which mediate nearly all excitatory neurotransmission in the central nervous system. These antibodies can be produced from heterologous cells for exogenous labeling applications or directly expressed in living neurons as intrabodies, where they bind their epitopes in the endoplasmic reticulum and co-traffic to the cell surface for visualization with cell impermeant fluorescent dyes. We show these reagents do not perturb AMPAR trafficking, function, mobility, or synaptic recruitment during plasticity and therefore can be used as probes for monitoring endogenous receptors in living neurons. We also adapt these reagents to deplete AMPARs from the cell surface by trapping them in the endoplasmic reticulum, providing a simple approach for loss of excitatory neurotransmission. The strategies outlined here serve as a template for generating similar reagents targeting diverse proteins as more antibody sequences become available.

The Shaping of AMPA Receptor Surface Distribution by Neuronal Activity

Neurotransmission is critically dependent on the number, position, and composition of receptor proteins on the postsynaptic neuron. Of these, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) are responsible for the majority of postsynaptic depolarization at excitatory mammalian synapses following glutamate release. AMPARs are continually trafficked to and from the cell surface, and once at the surface, AMPARs laterally diffuse in and out of synaptic domains. Moreover, the subcellular distribution of AMPARs is shaped by patterns of activity, as classically demonstrated by the synaptic insertion or removal of AMPARs following the induction of long-term potentiation (LTP) and long-term depression (LTD), respectively. Crucially, there are many subtleties in the regulation of AMPARs, and exactly how local and global synaptic activity drives the trafficking and retention of synaptic AMPARs of different subtypes continues to attract attention. Here we will review how activity can have differential effects on AMPAR distribution and trafficking along with its subunit composition and phosphorylation state, and we highlight some of the controversies and remaining questions. As the AMPAR field is extensive, to say the least, this review will focus primarily on cellular and molecular studies in the hippocampus. We apologise to authors whose work could not be cited directly owing to space limitations.

FORTIS: a live-cell assay to monitor AMPA receptors using pH-sensitive fluorescence tags

The real-time live fluorescent monitoring of surface AMPA receptors (AMPARs) could open new opportunities for drug discovery and phenotypic screening concerning neuropsychiatric disorders. We have developed FORTIS, a tool based on pH sensitivity capable of detecting subtle changes in surface AMPARs at a neuronal population level. The expression of SEP-GluA1 or pHuji-GluA1 recombinant AMPAR subunits in mammalian neurons cultured in 96-well plates enables surface AMPARs to be monitored with a microplate reader. Thus, FORTIS can register rapid changes in surface AMPARs induced by drugs or genetic modifications without having to rely on conventional electrophysiology or imaging. By combining FORTIS with pharmacological manipulations, basal surface AMPARs, and plasticity-like changes can be monitored. We expect that employing FORTIS to screen for changes in surface AMPARs will accelerate both neuroscience research and drug discovery.

Ligand-directed two-step labeling to quantify neuronal glutamate receptor trafficking

The regulation of glutamate receptor localization is critical for development and synaptic plasticity in the central nervous system. Conventional biochemical and molecular biological approaches have been widely used to analyze glutamate receptor trafficking, especially for α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate-type glutamate receptors (AMPARs). However, conflicting findings have been reported because of a lack of useful tools for analyzing endogenous AMPARs. Here, we develop a method for the rapid and selective labeling of AMPARs with chemical probes, by combining affinity-based protein labeling and bioorthogonal click chemistry under physiological temperature in culture medium. This method allows us to quantify AMPAR distribution and trafficking, which reveals some unique features of AMPARs, such as a long lifetime and a rapid recycling in neurons. This method is also successfully expanded to selectively label N-methyl-D-aspartate-type glutamate receptors. Thus, bioorthogonal two-step labeling may be a versatile tool for investigating the physiological and pathophysiological roles of glutamate receptors in neurons.

The analysis of AMPA-type glutamate receptor (AMPAR) trafficking is essential for understanding molecular mechanisms of learning and memory, but the analytical tools are currently limited. Here, the authors report a method that combines affinity-based receptor labeling and bioorthogonal click chemistry to quantify AMPAR distribution and trafficking under physiological conditions.

D-Aspartic Acid in Vertebrate Reproduction: Animal Models and Experimental Designs‡

This article reviews the animal models and experimental designs that have been used during the past twenty years to demonstrate the prominent role played by d-aspartate (d-Asp) in the reproduction of vertebrates, from amphibians to humans. We have tabulated the findings of in vivo and in vitro experiments that demonstrate the effects of d-Asp uptake on hormone production and gametogenesis in vertebrate animal models. The contribution of each animal model to the existing knowledge on the role of d-Asp in reproductive processes has been discussed. A critical analysis of experimental designs has also been carried out. Experiments performed on wild animal species suggest a role of d-Asp in the mechanisms that regulate the reproductive cycle. Several in vivo and in vitro studies carried out on mouse and rat models have facilitated an understanding of the molecular pathways activated by D-Asp in both steroidogenesis and spermatogenesis, with particular emphasis on testosterone biosynthesis. Some attempts using d-Asp for the improvement of reproductive activity in animals of commercial interest have yielded mixed results. The increased transcriptome activity of enzymes and receptors involved in the reproductive activity in d-Asp-treated broiler roosters revealed further details on the mechanism of action of d-Asp on the reproductive processes. The close relationship between d-Asp and reproductive activity has emerged, particularly in relation to its effects exerted on semen quality, proposing therapeutic applications of this amino acid in andrology and in medically-assisted procreation techniques.

The Reactive Plasticity of Hippocampal Ionotropic Glutamate Receptors in Animal Epilepsies

Ionotropic glutamate receptors (iGluRs) mediate the synaptic and metabolic actions of glutamate. These iGluRs are classified within the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type, kainate-type, and N-methyl-d-aspartate (NMDA)-type functional receptor families. The iGluR assemblies are regulated by transcription, alternative splicing, and cytoplasmic post-translational modifications. The iGluR subunit proteins are transported from the endoplasmic reticulum, inserted into the synaptic membranes, and anchored at their action site by different scaffolding and interacting proteins. The functional properties of iGluRs depend on their subunit composition, the amino acid sequence of the protein domains, and the scaffolding proteins in the synaptic membranes. The iGluRs are removed from the membranes by enzymatic action and endocytosis. Hippocampal iGluRs are rearranged through the upregulation and downregulation of the subunits following deafferentation and epileptic seizures. The rearrangement of iGluRs and the alteration of their subunit composition transform neurons into “pathological” cells, determining the further plasticity or pathology of the hippocampal formation. In the present review, we summarize the expression of AMPA, kainate, and NMDA receptor subunits following deafferentation, repeated mild seizures, and status epilepticus. We compare our results to literature descriptions, and draw conclusions as to the reactive plasticity of iGluRs in the hippocampus.

Electrical coupling between hippocampal neurons: contrasting roles of principal cell gap junctions and interneuron gap junctions

There is considerable experimental evidence, anatomical and physiological, that gap junctions exist in the hippocampus. Electrical coupling through these gap junctions may be divided into three types: between principal neurons, between interneurons and at mixed chemical (glutamatergic)/electrical synapses. An approach, combining in vitro experimental with modeling techniques, sheds some light on the functional consequences of electrical coupling, for network oscillations and for seizures. Additionally, in vivo experiments, using mouse connexin knockouts, suggest that the presence of electrical coupling is important for optimal performance on selected behavioral tasks; however, the interpretation of such data, in cellular terms, has so far proven difficult. Given that invertebrate central pattern generators so often depend on both chemical and electrical synapses, our hypothesis is that hippocampus-mediated and -influenced behaviors will act likewise. Experiments, likely hard ones, will be required to test this intuition.

Central sensitization of the spino-parabrachial-amygdala pathway that outlasts a brief nociceptive stimulus

KEY POINTS

Chronic pain is disabling because sufferers form negative associations between pain and activities, such as work, leading to the sufferer limiting these activities. Pain information arriving in the amygdala is responsible for forming these associations and contributes to us feeling bad when we are in pain. Ongoing injuries enhance the delivery of pain information to the amygdala. If we want to understand why chronic pain can continue without ongoing injury, it is important to know whether this facilitation continues once the injury has healed. In the present study, we show that a 2 min noxious heat stimulus, without ongoing injury, is able to enhance delivery of pain information to the amygdala for 3 days. If the noxious heat stimulus is repeated, this enhancement persists even longer. These changes may prime this information pathway so that subsequent injuries may feel even worse and the associative learning that results in pain-related avoidance may be promoted.

ABSTRACT

Pain is an important defence against dangers in our environment; however, some clinical conditions produce pain that outlasts this useful role and persists even after the injury has healed. The experience of pain consists of somatosensory elements of intensity and location, negative emotional/aversive feelings and subsequent restrictions on lifestyle as a result of a learned association between certain activities and pain. The amygdala contributes negative emotional value to nociceptive sensory information and forms the association between an aversive response and the environment in which it occurs. It is able to form this association because it receives nociceptive information via the spino-parabrachio-amygdaloid pathway and polymodal sensory information via cortical and thalamic inputs. Synaptic plasticity occurs at the parabrachial-amygdala synapse and other brain regions in chronic pain conditions with ongoing injury; however, very little is known about how plasticity occurs in conditions with no ongoing injury. Using immunohistochemistry, electrophysiology and behavioural assays, we show that a brief nociceptive stimulus with no ongoing injury is able to produce long-lasting synaptic plasticity at the rat parabrachial-amygdala synapse. We show that this plasticity is caused by an increase in postsynaptic AMPA receptors with a transient change in the AMPA receptor subunit, similar to long-term potentiation. Furthermore, this synaptic potentiation primes the synapse so that a subsequent noxious stimulus causes prolonged potentiation of the nociceptive information flow into the amygdala. As a result, a second injury could have an increased negative emotional value and promote associative learning that results in pain-related avoidance.

Extracellular Cyclic GMP Modulates Membrane Expression of The GluA1 and GluA2 Subunits of AMPA Receptor in Cerebellum: Molecular Mechanisms Involved

There is increasing evidence that extracellular cGMP modulates glutamatergic neurotransmission and some forms of learning. However, the underlying mechanisms remain unknown. We proposed the hypotheses that extracellular cGMP may regulate membrane expression of AMPA receptors. To do this extracellular cGMP should act on a membrane protein and activate signal transduction pathways modulating phosphorylation of the GluA1 and/or GluA2 subunits. It has been shown that extracellular cGMP modulates glycine receptors. The aims of this work were to assess: 1) whether extracellular cGMP modulates membrane expression of GluA1 and GluA2 subunits of AMPA receptors in cerebellum in vivo; 2) whether this is mediated by glycine receptors; 3) the role of GluA1 and GluA2 phosphorylation and 4) identify steps of the intracellular pathways involved. We show that extracellular cGMP modulates membrane expression of GluA1 and GluA2 in cerebellum in vivo and unveil the mechanisms involved. Extracellular cGMP reduced glycine receptor activation, modulating cAMP, protein kinases and phosphatases, and GluA1 and GluA2 phosphorylation, resulting in increased GluA1 and reduced GluA2 membrane expression. Extracellular cGMP therefore modulates membrane expression of AMPA receptors and glutamatergic neurotransmission. The steps identified may be therapeutic targets to improve neurotransmission and neurological function in pathological situations with abnormal glutamatergic neurotransmission.

Timely regulated sorting from early to late endosomes is required to maintain cerebellar long-term depression

An important feature of long-term synaptic plasticity is the prolonged maintenance of plastic changes in synaptic transmission. The trafficking of AMPA-type glutamate receptors (AMPARs) is involved in the expression of many forms of synaptic plasticity, yet the subsequent events accomplishing the maintenance of plastic changes in synaptic AMPAR numbers are not fully understood. Here, we find that maintenance of cerebellar long-term depression results from a reduction in the number of AMPARs residing within endocytic recycling pathways. We then develop a genetically encoded, photosensitive inhibitor of late endosome sorting and use this to discover that initial maintenance of long-term depression relies on timely regulated late endosome sorting, which exhibits a threshold as well as switch-like behavior. Thus, our results indicate that recycling AMPAR numbers are reduced by a switching machinery of transient late endosome sorting, and that this process enables the transition from basal synaptic transmission to long-term depression maintenance.

Long term depression (LTD) of the cerebellum is known to be mediated by postsynaptic trafficking of glutamate receptor AMPAR. Here, Kim and colleagues show that early- to late-endosomal sorting of AMPAR represents the switch from expression to maintenance phase of cerebellar LTD.

Synaptic AMPA receptor composition in development, plasticity and disease

AMPA receptors (AMPARs) are assemblies of four core subunits, GluA1-4, that mediate most fast excitatory neurotransmission. The component subunits determine the functional properties of AMPARs, and the prevailing view is that the subunit composition also determines AMPAR trafficking, which is dynamically regulated during development, synaptic plasticity and in response to neuronal stress in disease. Recently, the subunit dependence of AMPAR trafficking has been questioned, leading to a reappraisal of this field. In this Review, we discuss what is known, uncertain, conjectured and unknown about the roles of the individual subunits, and how they affect AMPAR assembly, trafficking and function under both normal and pathological conditions.

Postsynaptic Neurotransmitter Receptor Reserve Pools for Synaptic Potentiation

At excitatory and inhibitory synapses, an immediate transfer of additional neurotransmitter receptors from non-synaptic positions to the synapse mediates synaptic long-term potentiation (LTP). Different types of non-synaptic reserve pools permit the rapid supply of transmembrane neurotransmitter receptors. Recycling endosomes (REs) serve as an intracellular reservoir of receptors that is delivered to the plasma membrane on LTP induction. Furthermore, AMPA receptors at the non-synaptic plasma membrane provide an extrasynaptic reserve pool that is also important to potentiate synapse function. Finally, bidirectional synaptic versus extrasynaptic trapping of freely diffusing plasma membrane GABAA receptors (GABAARs) by scaffolding proteins modulates synaptic transmission. Here we discuss novel findings regarding neurotransmitter receptor reservoirs and potential reserve pool mechanisms for synaptic potentiation.

Metabolic turnover of synaptic proteins: kinetics, interdependencies and implications for synaptic maintenance

Chemical synapses contain multitudes of proteins, which in common with all proteins, have finite lifetimes and therefore need to be continuously replaced. Given the huge numbers of synaptic connections typical neurons form, the demand to maintain the protein contents of these connections might be expected to place considerable metabolic demands on each neuron. Moreover, synaptic proteostasis might differ according to distance from global protein synthesis sites, the availability of distributed protein synthesis facilities, trafficking rates and synaptic protein dynamics. To date, the turnover kinetics of synaptic proteins have not been studied or analyzed systematically, and thus metabolic demands or the aforementioned relationships remain largely unknown. In the current study we used dynamic Stable Isotope Labeling with Amino acids in Cell culture (SILAC), mass spectrometry (MS), Fluorescent Non-Canonical Amino acid Tagging (FUNCAT), quantitative immunohistochemistry and bioinformatics to systematically measure the metabolic half-lives of hundreds of synaptic proteins, examine how these depend on their pre/postsynaptic affiliation or their association with particular molecular complexes, and assess the metabolic load of synaptic proteostasis. We found that nearly all synaptic proteins identified here exhibited half-lifetimes in the range of 2-5 days. Unexpectedly, metabolic turnover rates were not significantly different for presynaptic and postsynaptic proteins, or for proteins for which mRNAs are consistently found in dendrites. Some functionally or structurally related proteins exhibited very similar turnover rates, indicating that their biogenesis and degradation might be coupled, a possibility further supported by bioinformatics-based analyses. The relatively low turnover rates measured here (∼0.7% of synaptic protein content per hour) are in good agreement with imaging-based studies of synaptic protein trafficking, yet indicate that the metabolic load synaptic protein turnover places on individual neurons is very substantial.

Genetic deletion of NP1 prevents hypoxic-ischemic neuronal death via reducing AMPA receptor synaptic localization in hippocampal neurons

Background

Trafficking of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionic acid receptors (AMPARs) to excitatory synapses is critical to their synaptic functions. Previously, we have shown induction of neuronal pentraxin 1 (NP1) and its colocalization with AMPAR subunit GluR1 in hypoxic‐ischemic (HI) brain injury. However, the role of NP1 in mediating GluR1 surface expression, trafficking, and clustering at synapses in HI neuronal death is unclear.

Methods and Results

Primary hippocampal neurons, isolated from wild‐type (WT) and NP1‐knockout (C57BL/6 background) mice at DIV 12 to 14 were exposed to 2 to 8 hours of oxygen glucose deprivation (OGD)—in vitro conditions that mimic human stroke. OGD exposure resulted in time‐dependent induction of NP1 (∼4‐fold), enhanced redistribution of AMAP GluR1 receptors at excitatory synapses, and increased neuronal death. We observed a significant increase in surface and synaptic GluR1 clusters that colocalized with PSD‐95 on dendrites with a simultaneous decrease in internalized GluR1. Surface cross‐linking with BS³ showed enhanced membrane insertions of GluR1, and increased phosphorylation at Ser‐845 further supported enhanced surface availability of GluR1 after OGD. NP1 protein colocalized with GluR1 and PSD‐95, and OGD significantly increased their synaptic coclustering. Most strikingly, the genetic deletion of NP1 resulted in decreases in surface GluR1 cluster density, synaptic localization, phospho‐GluR1 (Ser‐845) levels, and neuronal death after OGD compared with WT neurons. AMPA (50 μmol/L) induced NP1 and significant cell death in WT but not in NP1−/− neurons.

Conclusions

Our results indicate that NP1 plays a key role in synaptic clustering of GluR1, suggesting that targeting NP1 might be a practical approach to preventing ischemic brain damage.

A protein cross-linking assay for measuring cell surface expression of glutamate receptor subunits in the rodent brain after in vivo treatments

Trafficking of neurotransmitter receptors between intracellular and cell surface compartments is important for regulating neurotransmission. We developed a method for determining if an in vivo treatment has altered receptor distribution in a particular region of rodent brain. After the treatment, brain slices are rapidly prepared from the region of interest. Then, cell surface-expressed proteins are covalently cross-linked using the membrane-impermeable, bifunctional cross-linker bis(sulfosuccinimidyl)suberate (BS(3)). This increases the apparent molecular weight of surface receptors, while intracellular receptors are not modified. Thus, surface and intracellular receptor pools can be separated and quantified using SDS-PAGE and immunoblotting. This method is particularly useful for analyzing AMPA receptor subunits, offering advantages in accuracy, efficiency, and cost compared to biotinylation. A disadvantage is that some antibodies no longer recognize their target protein after cross-linking. We have used this method to quantify changes in receptor distribution after acute and chronic exposure to psychomotor stimulants.

Epigenetic mechanisms in memory and synaptic function

Although the term 'epigenetics' was coined nearly seventy years ago, its critical function in memory processing by the adult CNS has only recently been appreciated. The hypothesis that epigenetic mechanisms regulate memory and behavior was motivated by the need for stable molecular processes that evade turnover of the neuronal proteome. In this article, we discuss evidence that supports a role for neural epigenetic modifications in the formation, consolidation and storage of memory. In addition, we will review the evidence that epigenetic mechanisms regulate synaptic plasticity, a cellular correlate of memory. We will also examine how the concerted action of multiple epigenetic mechanisms with varying spatiotemporal profiles influence selective gene expression in response to behavioral experience. Finally, we will suggest key areas for future research that will help elucidate the complex, vital and still mysterious, role of epigenetic mechanisms in neural function and behavior.

Glycine dimerization motif in the N-terminal transmembrane domain of the high density lipoprotein receptor SR-BI required for normal receptor oligomerization and lipid transport

Scavenger receptor class B, type I (SR-BI), a CD36 superfamily member, is an oligomeric high density lipoprotein (HDL) receptor that mediates negatively cooperative HDL binding and selective lipid uptake. We identified in the N-terminal transmembrane (N-TM) domain of SR-BI a conserved glycine dimerization motif, G(15)X(2)G(18)X(3)AX(2)G(25), of which the submotif G(18)X(3)AX(2)G(25) significantly contributes to homodimerization and lipid uptake activity. SR-BI variants were generated by mutations (single or multiple Gly → Leu substitutions) or by replacing the N-TM domain with those from other CD36 superfamily members containing (croquemort) or lacking (lysosomal integral membrane protein (LIMP) II) this glycine motif (chimeras). None of the SR-BI variants exhibited altered surface expression (based on antibody binding) or HDL binding. However, the G15L/G18L/G25L triple mutant exhibited reductions in cell surface homo-oligomerization (>10-fold) and the rate of selective lipid uptake (∼ 2-fold). Gly(18) and Gly(25) were necessary for normal lipid uptake activity of SR-BI and the SR-BI/croquemort chimera. The lipid uptake activity of the glycine motif-deficient SR-BI/LIMP II chimera was low but could be increased by introducing glycines at positions 18 and 25. The rate of lipid uptake mediated by SR-BI/LIMP II chimeras was proportional to the extent of receptor oligomerization. Thus, the glycine dimerization motif G(18)X(3)AX(2)G(25) in the N-TM domain of SR-BI contributes substantially to the homo-oligomerization and lipid transport activity of SR-BI but does not influence the negative cooperativity of HDL binding. Oligomerization-independent binding cooperativity suggests that classic allostery is not involved and that the negative cooperativity is probably the consequence of a "lattice effect" (interligand steric interference accompanying binding to adjacent receptors).

Aging-related alterations in the expression and distribution of GluR2 and PICK1 in the rat hippocampus

Deficit in synaptic plasticity in the hippocampus frequently occurs during normal aging. Although the protein level and calcium permeability of AMPARs alter with aging, the alteration of AMPARs and their regulatory proteins during aging are far from understanding. Dynamics of GluR2 subunit are dependent on the function of protein interacting with Cα kinase 1 (PICK1), PKCα and calcineurin (CaN). Here, we firstly show that the expression of PICK1 and CaN B decreased significantly in the hippocampus of old rats compared to that of young and adult rats. The decrease was accompanied by a reduction of GluR2 and PKCα and an increase in CaN A. Next, we found that in young and adult rats, the distribution of PICK1 and GluR2 diffused in the cytoplasm of hippocampal neurons, but closely around perinuclear in the hippocampal neurons of old rats. These results suggest that the expression of GluR2, PICK1, PKCα and CaN B significant decreased in the hippocampus and these alterations may lead to altered distribution of GluR2 and PICK1 during aging.

Assembly and intracellular distribution of kainate receptors is determined by RNA editing and subunit composition

Kainate receptors (KARs) modulate neuronal network activity. The molecular mechanisms that control the assembly and trafficking of KARs are unclear. Here, we examined the role of Q/R editing and subunit composition on KAR subunit assembly and subcellular distribution. The majority of GluK2 subunits undergo editing at the Q/R site in the channel pore loop. Cell surface biotinylation, cross-linking, Endoglycosidase-H analysis and gradient separation of KAR subunit assembly states revealed that Q/R editing reduces oligomerization, endoplasmic reticulum (ER) export, plasma membrane expression and stability of homomeric GluK2-containing KARs. These results indicate that Q/R editing of GluK2 may orchestrate channel subunit composition during KAR assembly in the ER. GluK2/GluK5 heteromers are the most abundant KAR subtype in the brain. While subcellular fractionation of brain tissue confirmed that both GluK2/3 and GluK5 are present in synaptosomes and tightly associated with post-synaptic density fractions, biochemical analysis revealed that endogenous GluK2/3 subunits show less complete assembly and trafficking compared with GluK5. In transgenic mice, the loss of the key assembly partner GluK2 leads to dramatic reduction in GluK5 expression. These results support the idea that the assembly and intracellular distribution of KARs is determined by RNA editing at the Q/R site and subunit composition.

Minocycline increases phosphorylation and membrane insertion of neuronal GluR1 receptors

The tetracycline antibiotic minocycline beneficially affects neuronal functioning and also inhibits the enzyme 5-lipoxygenase (5-LOX). We hypothesized that similar to 5-LOX inhibitors, minocycline may increase phosphorylation and membrane insertion of the glutamate receptor GluR1. The experiments were performed in primary cultures of mouse striatal neurons and in the prefrontal cortex and striatum of minocycline-treated mice. In vitro, low micromolar minocycline concentrations increased GluR1 phosphorylation at Ser845 and Ser831 and increased the surface content of GluR1. Minocycline also increased GluR1 phosphorylation in vivo. Increased GluR1 phosphorylation and minocycline treatment have been associated with antidepressant and memory-enhancing activities. Direct consequences of minocycline-increased GluR1 phosphorylation are yet to be established.

Dopamine receptors regulate NMDA receptor surface expression in prefrontal cortex neurons

Interactions between dopamine (DA) and glutamate systems in the prefrontal cortex (PFC) are important in addiction and other psychiatric disorders. Here, we examined DA receptor regulation of NMDA receptor surface expression in postnatal rat PFC neuronal cultures. Immunocytochemical analysis demonstrated that surface expression (synaptic and non-synaptic) of NR1 and NR2B on PFC pyramidal neurons was increased by the D1 receptor agonist SKF 81297 (1 microM, 5 min). Activation of protein kinase A (PKA) did not alter NR1 distribution, indicating that PKA does not mediate the effect of D1 receptor stimulation. However, the tyrosine kinase inhibitor genistein (50 microM, 30 min) completely blocked the effect of SKF 81297 on NR1 and NR2B surface expression. Protein cross-linking studies confirmed that SKF 81297 (1 microM, 5 min) increased NR1 and NR2B surface expression, and further showed that NR2A surface expression was not affected. Genistein blocked the effect of SKF 81297 on NR1 and NR2B. Surface-expressed immunoreactivity detected with a phospho-specific antibody to tyrosine 1472 of NR2B also increased after D1 agonist treatment. Our results show that tyrosine phosphorylation plays an important role in the trafficking of NR2B-containing NMDA receptors in PFC neurons and the regulation of their trafficking by DA receptors.

Formation of accumbens GluR2-lacking AMPA receptors mediates incubation of cocaine craving

Relapse to cocaine use after prolonged abstinence is an important clinical problem. This relapse is often induced by exposure to cues associated with cocaine use. To account for the persistent propensity for relapse, it has been suggested that cue-induced cocaine craving increases over the first several weeks of abstinence and remains high for extended periods. We and others identified an analogous phenomenon in rats that was termed 'incubation of cocaine craving': time-dependent increases in cue-induced cocaine-seeking over the first months after withdrawal from self-administered cocaine. Cocaine-seeking requires the activation of glutamate projections that excite receptors for alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) in the nucleus accumbens. Here we show that the number of synaptic AMPA receptors in the accumbens is increased after prolonged withdrawal from cocaine self-administration by the addition of new AMPA receptors lacking glutamate receptor 2 (GluR2). Furthermore, we show that these new receptors mediate the incubation of cocaine craving. Our results indicate that GluR2-lacking AMPA receptors could be a new target for drug development for the treatment of cocaine addiction. We propose that after prolonged withdrawal from cocaine, increased numbers of synaptic AMPA receptors combined with the higher conductance of GluR2-lacking AMPA receptors causes increased reactivity of accumbens neurons to cocaine-related cues, leading to an intensification of drug craving and relapse.

Acute and chronic dopamine receptor stimulation modulates AMPA receptor trafficking in nucleus accumbens neurons cocultured with prefrontal cortex neurons

Postsynaptic interactions between dopamine (DA) and glutamate receptors in the nucleus accumbens (NAc) are critical for addiction. To determine the effect of acute and repeated DA receptor stimulation on AMPA receptor (AMPAR) synaptic targeting in medium spiny NAc neurons, we developed a model system consisting of rat NAc neurons cocultured with prefrontal cortex neurons from enhanced green fluorescent protein-expressing mice. Cortical neurons restore excitatory input onto NAc neurons but are distinguishable based on fluorescence. First, we showed that brief D1-like agonist exposure increased AMPAR insertion onto extrasynaptic regions of NAc neuronal processes through a mechanism requiring protein kinase A. This facilitated the Ca2+/calmodulin dependent protein kinase II (CaMKII)-dependent synaptic incorporation of AMPARs in response to subsequent NMDA receptor (NMDAR) stimulation. Through this mechanism, DA may promote reward- and drug-related plasticity in the NAc. Then, to model effects of repeated in vivo cocaine exposure, we treated cocultures with DA (1 microm, 30 min) on days 7, 9, and 11 in culture. On day 15, NAc neurons exhibited increased synaptic AMPAR levels. This was associated with CaMKII activation and was blocked by the CaMKII inhibitor KN-93 (N-[2-[N-(4-chlorocinnamyl)-N-methylaminomethyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide phosphate salt). Furthermore, D1-like agonist exposure on day 15 no longer increased AMPAR surface expression. This refractoriness was associated with decreased D1 receptor surface expression. NMDAR surface expression was not altered by acute or repeated DA receptor stimulation. These results suggest that (1) after repeated DA treatment, NAc neurons are more responsive to glutamate inputs but D(1)-like receptor regulation of plasticity is impaired, and (2) NAc/prefrontal cortex cocultures are useful for studying dopamine-induced neuroadaptations.

Modeling the role of lateral membrane diffusion in AMPA receptor trafficking along a spiny dendrite

AMPA receptor trafficking in dendritic spines is emerging as a major postsynaptic mechanism for the expression of plasticity at glutamatergic synapses. AMPA receptors within a spine are in a continuous state of flux, being exchanged with local intracellular pools via exo/endocytosis and with the surrounding dendrite via lateral membrane diffusion. This suggests that one cannot treat a single spine in isolation. Here we present a model of AMPA receptor trafficking between multiple dendritic spines distributed along the surface of a dendrite. Receptors undergo lateral diffusion within the dendritic membrane, with each spine acting as a spatially localized trap where receptors can bind to scaffolding proteins or be internalized through endocytosis. Exocytosis of receptors occurs either at the soma or at sites local to dendritic spines via constitutive recycling from intracellular pools. We derive a reaction-diffusion equation for receptor trafficking that takes into account these various processes. Solutions of this equation allow us to calculate the distribution of synaptic receptor numbers across the population of spines, and hence determine how lateral diffusion contributes to the strength of a synapse. A number of specific results follow from our modeling and analysis. (1) Lateral membrane diffusion alone is insufficient as a mechanism for delivering AMPA receptors from the soma to distal dendrites. (2) A source of surface receptors at the soma tends to generate an exponential-like distribution of receptors along the dendrite, which has implications for synaptic democracy. (3) Diffusion mediates a heterosynaptic interaction between spines so that local changes in the constitutive recycling of AMPA receptors induce nonlocal changes in synaptic strength. On the other hand, structural changes in a spine following long term potentiation or depression have a purely local effect on synaptic strength. (4) A global change in the rates of AMPA receptor exo/endocytosis is unlikely to be the sole mechanism for homeostatic synaptic scaling. (5) The dynamics of AMPA receptor trafficking occurs on multiple timescales and varies according to spatial location along the dendrite. Understanding such dynamics is important when interpreting data from inactivation experiments that are used to infer the rate of relaxation to steady-state.

Spine-type-specific recruitment of newly synthesized AMPA receptors with learning

The stabilization of long-term memories requires de novo protein synthesis. How can proteins, synthesized in the soma, act on specific synapses that participate in a given memory? We studied the dynamics of newly synthesized AMPA-type glutamate receptors (AMPARs) induced with learning using transgenic mice expressing the GluR1 subunit fused to green fluorescent protein (GFP-GluR1) under control of the c-fos promoter. We found learning-associated recruitment of newly synthesized GFP-GluR1 selectively to mushroom-type spines in adult hippocampal CA1 neurons 24 hours after fear conditioning. Our results are consistent with a "synaptic tagging" model to allow activated synapses to subsequently capture newly synthesized receptor and also demonstrate a critical functional distinction in the mushroom spines with learning.

Diffusion-trapping model of receptor trafficking in dendrites

We present a model for the diffusive trafficking of protein receptors along the surface of a neuron's dendrite. Distributed along the dendrite are spatially localized trapping regions that represent submicrometer mushroom-like protrusions known as dendritic spines. Within these trapping regions receptors can be internalized via endocytosis and either reinserted into the surface via exocytosis or degraded. We calculate the steady-state distribution of receptors along the dendrite assuming a constant flux of receptors inserted at one end, adjacent to the soma where receptors are synthesized, and use this to investigate the effectiveness of membrane diffusion as a transport mechanism. We also calculate the mean first passage time of a receptor to travel a certain distance along the cable and use this to derive an effective surface diffusivity.

PICK1 interacts with alpha7 neuronal nicotinic acetylcholine receptors and controls their clustering

Central to synaptic function are protein scaffolds associated with neurotransmitter receptors. Alpha7 neuronal nicotinic acetylcholine receptors (nAChRs) modulate network activity, neuronal survival and cognitive processes in the CNS, but protein scaffolds that interact with these receptors are unknown. Here we show that the PDZ-domain containing protein PICK1 binds to alpha7 nAChRs and plays a role in their clustering. PICK1 interacted with the alpha7 cytoplasmic loop in yeast in a PDZ-dependent way, and the interaction was confirmed in recombinant pull-down experiments and by co-precipitation of native proteins. Some alpha7 and PICK1 clusters were adjacent at the surface of SH-SY5Y cells and GABAergic interneurons in hippocampal cultures. Expression of PICK1 caused decreased alpha7 clustering on the surface of the interneurons in a PDZ-dependent way. These data show that PICK1 negatively regulates surface clustering of alpha7 nAChRs on hippocampal interneurons, which may be important in inhibitory functions of alpha7 in the hippocampus.

Running to stand still: ionotropic receptor dynamics at central and peripheral synapses

For synapses to form and function, neurotransmitter receptors must be recruited to a location on the postsynaptic cell in direct apposition to presynaptic neurotransmitter release. However, once receptors are inserted into the postsynaptic membrane, they are not fixed in place but are continually exchanged between synaptic and extrasynaptic regions, and they cycle between the surface and intracellular compartments. This article highlights and compares the current knowledge about the dynamics of acetylcholine receptors at the vertebrate peripheral neuromuscular junction and AMPA, N-methyl-D-aspartate, and gamma-aminobutyric acid receptors in central synapses.

Brain-derived neurotrophic factor regulates the expression and synaptic delivery of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunits in hippocampal neurons

Brain-derived neurotrophic factor (BDNF) plays an important role in synaptic plasticity in the hippocampus, but the mechanisms involved are not fully understood. The neurotrophin couples synaptic activation to changes in gene expression underlying long term potentiation and short term plasticity. Here we show that BDNF acutely up-regulates GluR1, GluR2, and GluR3 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunits in 7-day in vitro cultured hippocampal neurons. The increase in GluR1 and GluR2 protein levels in developing cultures was impaired by K252a, a tropomyosin-related [corrected] kinase (Trk) inhibitor, and by translation (emetine and anisomycin) and transcription (alpha-amanitine and actinomycin D) inhibitors [corrected] The increase in GluR1 and GluR2 protein levels in developing cultures was impaired by K252a, a Trk inhibitor, and by translation (emetine and anisomycin) and transcription (alpha-amanitine and actinomycin D) inhibitors. Accordingly, BDNF increased the mRNA levels for GluR1 and GluR2 subunits. Biotinylation studies showed that stimulation with BDNF for 30 min selectively increased the amount of GluR1 associated with the plasma membrane, and this effect was abrogated by emetine. Under the same conditions, BDNF induced GluR1 phosphorylation on Ser-831 through activation of protein kinase C and Ca(2+)-calmodulin-dependent protein kinase II. Chelation of endogenous extracellular BDNF with TrkB-IgG selectively decreased GluR1 protein levels in 14-day in vitro cultures of hippocampal neurons. Moreover, BDNF promoted synaptic delivery of homomeric GluR1 AMPA receptors in cultured organotypic slices, by a mechanism independent of NMDA receptor activation. Taken together, the results indicate that BDNF up-regulates the protein levels of AMPA receptor subunits in hippocampal neurons and induces the delivery of AMPA receptors to the synapse.

Behavioral sensitization to cocaine is associated with increased AMPA receptor surface expression in the nucleus accumbens

Regulation of AMPA receptor trafficking is important for many forms of neuronal plasticity. In this study, a protein cross-linking assay was used to evaluate the contribution of AMPA receptor trafficking to plasticity associated with behavioral sensitization, an animal model of drug addiction. Cross-linking was used to distinguish between cell surface and intracellular AMPA receptors in nucleus accumbens (NAc) tissue obtained from rats treated repeatedly with saline or cocaine. Surface/intracellular (S/I) ratios for glutamate receptor 1 (GluR1) and GluR2/3 subunits were increased 21 d after the last injection in cocaine-sensitized rats but not rats that failed to sensitize, and the magnitude of the S/I ratio for cocaine-sensitized rats was positively correlated with the magnitude of behavioral sensitization. At the 1 d withdrawal time, cocaine did not alter S/I ratios, and there was no correlation between S/I ratios and behavioral sensitization. The majority of surface-expressed GluR1 detected with this assay was associated with synapses, based on coimmunoprecipitation with postsynaptic density protein of 95 kDa. These findings suggest that behavioral sensitization to cocaine is associated with a slowly developing redistribution of AMPA receptors to the surface of NAc neurons. Motor execution of drug-seeking responses depends on activation of AMPA receptors on NAc neurons by glutamate afferents originating in cortical and limbic regions. We propose that drug-seeking responses are more effectively triggered in cocaine-sensitized rats because of increased cell surface expression of AMPA receptors.

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.

Differential regulation of GABA(A) receptor and gephyrin postsynaptic clustering in immature hippocampal neuronal cultures

Gephyrin is a postsynaptic scaffolding protein involved in clustering of glycine- and GABA(A) receptors at inhibitory synapses. The role of gephyrin in GABAergic synapses, the nature of its interactions with GABA(A) receptors, and the mechanisms of targeting to GABAergic synapses are largely unknown. To gain further insights into these questions, the formation of GABA(A) receptor and gephyrin clusters and their distribution relative to presynaptic terminals were investigated in immature cultures of embryonic hippocampal neurons using triple immunofluorescence staining. GABA(A) receptor clusters, labeled for the alpha2 subunit, formed independently of gephyrin clusters, and were distributed on neurites at constant densities, either extrasynaptically or, to a lesser extent, postsynaptically, apposed to synapsin-I-positive axon terminals. In contrast, gephyrin clusters were always associated with GABA(A) receptors and were preferentially localized postsynaptically. Their density increased linearly with the extent of innervation, which developed rapidly during the first week in vitro. These results suggested that GABA(A) receptor clustering is mediated by cell-autonomous mechanisms independent of synapse formation. Their association with gephyrin is dynamically regulated and may contribute to stabilization at postsynaptic sites. Labeling for vesicular glutamate transporters revealed that most synapses in these immature cultures were presumably glutamatergic, implying that postsynaptic GABA(A) receptor and gephyrin clusters initially were located in "mismatched" synapses. However, clusters appropriately localized in GABAergic synapses were distinctly larger and more intensely stained. Altogether, these results demonstrate that the targeting of GABA(A) receptor and gephyrin clusters to GABAergic synapses occurs secondarily and is regulated by presynaptic factors that are not essential for clustering.

Internalized GABA-receptor subunits are transferred to an intracellular pool associated with the postsynaptic density

Endocytosis represents an important mechanism regulating cell-surface expression of neurotransmitter receptors, including GABAA receptors, in neurons. Little is known, however, about trafficking of internalized receptors. Here, we used antibody tagging in living rat hippocampal neurons in culture to monitor GABAA receptor internalization. We show that cell-surface receptors have a homogeneous distribution reflecting their mobility in the membrane. Unexpectedly, internalized GABAA receptors were detected mainly in a subsynaptic pool associated with gephyrin at postsynaptic sites, whereas AMPA-type glutamate receptors were accumulated in the soma. This process was time-dependent and could be prevented by blocking clathrin-coated vesicle endocytosis. In control experiments, the existence of an intracellular pool of GABAA receptors associated with gephyrin was confirmed independently of internalization of surface receptors, and constitutive endocytosis, unrelated to antibody-tagging, could be demonstrated for both AMPA and GABAA receptors using a biotinylation assay. These results suggest that cycling of GABAA receptors between the cell surface and the subsynaptic pool provides a mechanism for the short-term regulation of GABAergic neurotransmission. Furthermore, the close association of gephyrin with internalized GABAA receptors suggests a role in intracellular receptor trafficking.

Spinal Axonal Injury Induces Brief Downregulation of Ionotropic Glutamate Receptors and No Stripping of Synapses in Cord-Projection Central Neurons

Spinal cord injury often damages the axons of cord-projecting central neurons. To determine whether their excitatory inputs are altered following axonal injury, we used rat rubrospinal neurons as a model and examined their excitatory input following upper cervical axotomy. Anterograde tracing showed that the primary afferents from the cerebellum terminated in a pattern similar to that of control animals. Ultrastructurally, neurons in the injured nucleus were contacted by excitatory synapses of normal appearance, with no sign of glial stripping. Since cerebellar fibers are glutamatergic, we examined the expression of ionotropic receptor subunits GluR1-4 and NR1 for AMPA and NMDA receptors, respectively, in control and injured neurons using immunolabeling methods. In control neurons, GluR2 appeared to be low as compared to GluR1, GluR3, and GluR4, while NR1 labeling was intense. Following unilateral tractotomy, the levels of expression of each subunit in axotomized neurons appeared to be normal, with the exception that they were lower than those of control neurons of the nonlesioned side at 2-6 days postinjury. These findings suggest that axotomized neurons are only temporarily protected from excitotoxicity. This is in sharp contrast to the responses of central neurons that innervate peripheral targets, in which both synaptic stripping and reduction of their ionotropic glutamate receptor subunits persist following axotomy. The absence of an injury-induced trimming of afferents and stripping of synapses and the lack of a persistent downregulation of postsynaptic receptors might enable injured cord-projection neurons to continue to control their supraspinal targets during most of their postinjury survival. Although this may support neurons by providing trophic influences, it nevertheless may subject them to excitotoxicity and ultimately lead to their degenerative fate.

Independent functions of hsp90 in neurotransmitter release and in the continuous synaptic cycling of AMPA receptors

The delivery of neurotransmitter receptors into synapses is essential for synaptic function and plasticity. In particular, AMPA-type glutamate receptors (AMPA receptors) reach excitatory synapses according to two distinct routes: a regulated pathway, which operates transiently during synaptic plasticity, and a constitutive pathway, which maintains synaptic function under conditions of basal transmission. However, the specific mechanisms that distinguish these two trafficking pathways are essentially unknown. Here, we evaluate the role of the molecular chaperone hsp90 (heat shock protein 90) in excitatory synaptic transmission in the hippocampus. On one hand, we found that hsp90 is necessary for the efficient neurotransmitter release at the presynaptic terminal. In addition, we identified hsp90 as a critical component of the cellular machinery that delivers AMPA receptors into the postsynaptic membrane. Using the hsp90-specific inhibitors radicicol and geldanamycin, we show that hsp90 is required for the constitutive trafficking of AMPA receptors into synapses during their continuous cycling between synaptic and nonsynaptic sites. In contrast, hsp90 function is not required for either the surface delivery of AMPA receptors into the nonsynaptic plasma membrane or for the acute, regulated delivery of AMPA receptors into synapses during plasticity induction (long-term potentiation). The synaptic cycling of AMPA receptors was also blocked by an hsp90-binding tetratricopeptide repeat (TPR) domain, suggesting that the role of hsp90 in AMPA receptor trafficking is mediated by a TPR domain-containing protein. These results demonstrate new roles for hsp90 in synaptic function by controlling neurotransmitter release and, independently, by mediating the continuous cycling of synaptic AMPA receptors.

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 PKCalpha, 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.

Characterization of the intracellular transport of GluR1 and GluR2 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunits in hippocampal neurons

Little is known about the dynamics of the dendritic transport of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) to synapses. Here, using virally expressed green fluorescent protein (GFP)-GluR1 and GFP-GluR2 and confocal photobleach techniques we show near real-time movement of these subunits in living cultured hippocampal neurons. GFP-GluR1 fluorescence was widely distributed throughout the extranuclear compartment with no evidence for discrete intracellular stores. GFP-GluR1 transport was predominantly proximal to distal at rates of 0.2-0.4 mum.s-1. GFP-GluR2 fluorescence was more punctate and localized at or close to the plasma membrane. Overall, GFP-GluR2 movement was less dynamic with distinct mobile and immobile pools. Neither activation nor inhibition of surface-expressed N-methyl-d-aspartate receptors or AMPARs had any significant effect on the rates of GFP-GluR1 or GFP-GluR2 dendritic transport. These results demonstrate that GluR1 is constitutively and rapidly transported throughout the neuron. GluR2, on the other hand, is less mobile, with a majority retained in relatively immobile membrane-associated clusters, with approximately 40% showing synaptic co-localization. Furthermore, the transport of both subunits is activity-independent, suggesting that the regulated delivery of AMPARs to the vicinity of synapses is not a mechanism that is involved in processes such as synaptic plasticity.

Real-time imaging of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA receptor) movements in neurons

The mechanisms that regulate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) synthesis, transport, targeting and surface expression are of fundamental importance for fast excitatory neurotransmission and synaptic plasticity in the mammalian central nervous system. It has become apparent that these control processes involve complex sets of protein-protein interactions and many of the proteins responsible have been identified. We have been working to visualize AMPAR movement in living neurons in order to investigate the effects of blocking protein interactions. Here we outline the approaches used and the results obtained thus far.

Proteins interactions implicated in AMPA receptor trafficking: a clear destination and an improving route map

The mechanisms that regulate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR), synthesis, transport, targeting and surface expression are of fundamental importance to understand the molecular basis of fast excitatory neurotransmission and synaptic plasticity in the mammalian CNS. An area of intense current interest is how AMPARs are directed to the correct locations in the neuron as and when required. This is a multi-layered problem, which involves complex spatio-temporal coordination of multiple protein interactions. Considerable progress has been achieved in identifying a number of proteins that bind directly to AMPAR subunits and the functional consequences of blocking some of these interactions have been determined. This review highlights recent developments in the field.

GABAergic terminals are required for postsynaptic clustering of dystrophin but not of GABA(A) receptors and gephyrin

In rat hippocampal cultures, we show by multilabeling immunocytochemistry that pyramidal cells, which receive little or no GABAergic input, mistarget alpha2-GABA(A) receptors and gephyrin to glutamatergic terminals. This mismatch does not occur in neurons innervated by numerous GABAergic terminals. A similar phenomenon has been reported for isolated autaptic hippocampal neurons (Rao et al., 2000). GABAergic synapses typically form multiple release sites apposed to GABA(A) receptor and gephyrin clusters. Remarkably, dystrophin, a protein highly abundant in skeletal muscle membranes, is extensively colocalized with alpha2-GABA(A) receptors exclusively opposite GABAergic terminals. In addition, selective apposition of syntrophin and beta-dystroglycan to GABAergic presynaptic terminals suggests that the entire dystrophin-associated protein complex (DPC) clusters at GABAergic synapses. In contrast to gephyrin and GABA(A) receptors, DPC proteins are not mistargeted to glutamatergic synapses, indicating independent clustering mechanisms. This was confirmed in hippocampal neurons cultured from GABA(A) receptor gamma2 subunit-deficient mice. Clustering of GABA(A) receptor and gephyrin in these neurons was strongly impaired, whereas clustering of dystrophin and associated proteins was unaffected by the absence of the gamma2 subunit. Our results indicate that accumulation of dystrophin and DPC proteins at GABAergic synapses occurs independently of postsynaptic GABA(A) receptors and gephyrin. We suggest that selective signaling from GABAergic terminals contributes to postsynaptic clustering of dystrophin.

Intact sorting, targeting, and clustering of gamma-aminobutyric acid A receptor subtypes in hippocampal neurons in vitro

The cellular and subcellular distribution of four GABA(A) receptor subtypes, identified by the presence of the alpha1, alpha2, alpha3, or alpha5 subunit, was investigated immunocytochemically in dissociated cultures of hippocampal neurons. We addressed the questions whether (1) cell-type specific expression, (2) axonal/somatodendritic targeting, and (3) synaptic/extrasynaptic clustering of GABA(A) receptor subtypes was retained in vitro. For comparison, the in vivo distribution pattern was assessed in sections from adult rat brain. The differential expression of GABA(A) receptor subunits allowed to identify five morphologically distinct cell types in culture: the alpha1 subunit was observed in glutamic acid decarboxylase-positive interneurons, the alpha2 and alpha5 subunits marked pyramidal-like cells, and the alpha3 subunit labeled three additional cell types, including presumptive hilar cells. All subunits were found in the somatodendritic compartment. In addition, appropriate axonal targeting was evidenced by the intense alpha2, and sometimes alpha3 subunit labeling of axon-initial segments (AIS) of pyramidal cells and hilar cells, respectively. Accordingly, both receptor subtypes were targeted to AIS in vivo, as well. Synaptic receptors were identified by colocalization with gephyrin, a postsynaptic clustering protein, and apposition to presynaptic terminals labeled with synapsin I. In vitro and in vivo, alpha1- and alpha2-receptor subtypes formed numerous synaptic clusters, alpha3-GABA(A) receptors were located either synaptically or extrasynaptically depending on the cell type, whereas alpha5-GABA(A) receptors were extrasynaptic. We conclude that receptor targeting to broad subcellular locations does not require specific GABAergic innervation patterns, which are disturbed in vitro, but depends on protein-protein interactions in the postsynaptic cell that are both subunit- and neuron-specific.

Regional localization and developmental profile of acetylcholinesterase-evoked increases in [(3)H]-5-fluororwillardiine binding to AMPA receptors in rat brain

In addition to its role in hydrolyzing the neurotransmitter acetylcholine, the synaptically enriched enzyme acetylcholinesterase (AChE) has been reported to play an important role in the development and remodelling of neural processes and synapses. We have shown previously that AChE causes an increase in binding of the specific AMPA receptor ligand (S)-[(3)H]-5-fluorowillardiine ([(3)H]-FW) to rat brain membranes. In this study we have used quantitative autoradiography to investigate the regional distribution and age-dependence of AChE-evoked increases in the binding of [(3)H]-FW in rat brain. Pretreatment of rat brain sections with AChE caused a marked enhancement of [(3)H]-FW binding to many, but not all, brain areas. The increased [(3)H]-FW binding was blocked by the specific AChE inhibitor BW 284c51. The maximal potentiation of [(3)H]-FW binding occurred at different developmental age-points in different regions with a profile consistent with the peak periods for synaptogenesis in any given region. In addition to its effects on brain sections, AChE also strongly potentiated [(3)H]-FW binding to detergent solubilized AMPA receptors suggesting a direct action on the receptors themselves rather than an indirect effect on the plasma membrane. These findings suggest that modulation of AMPA receptors could provide one molecular mechanism for the previously reported effects of AChE on synapse formation, synaptic plasticity and neurodegeneration.

GluR2 protein synthesis and metabolism in rat hippocampus following transient ischemia and ischemic tolerance induction

In this study we have determined the metabolic half-life, protein synthesis and expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR2 in the hippocampus of the living rat. Synthesized proteins were pulse labeled in vivo using intracarotid infusion or intrahippocampal injection of L-[(35)S] labeled amino acids, and the GluR2 protein immunoprecipitated in order to measure the tracer incorporation at different survival time-points. A limited time course study suggested a metabolic half-life of 144 and 108 h in the CA1 region in control animals following carotid artery infusion and intrahippocampal injection, respectively. Twenty-four hours following a moderate ischemic insult, GluR2 protein synthesis was decreased significantly in both the CA1 and DG/CA3 region, whereas the total protein synthesis was decreased significantly only in the CA1 region. Twenty-four hours following ischemic tolerance induction, a significant increase in GluR2 expression was found in the CA1 region using quantitative Western blotting, while no change was found in the dentate gyrus (DG)/CA3 or in expression of GluR1 protein. Data from labeling experiments did not reveal the reason for the increased amount of GluR2 in the CA1 region of the tolerant animals. This study shows that following global ischemia the GluR2 synthesis is decreased both in the CA1 and DG/CA3, which, together with the found GluR2 metabolic half-life, contradict a selective loss of GluR2 protein as a triggering mechanism for the delayed CA1 pyramidal cell death. Twenty-four hours following tolerance induction, we found an increased GluR2 expression in the CA1 region, suggesting that GluR2 plays a role in the acquisition of ischemic tolerance. Our study suggests the ability of neurons to regulate the AMPA receptor subunit expression through changes in protein synthesis and stability.

Calpain-mediated truncation of rat brain AMPA receptors increases their Triton X-100 solubility

Previous studies have indicated that calpain activation results in the truncation of the C-terminal domains of AMPA and NMDA receptor subunits. The present study determined the distribution of the truncated species of the subunits between Triton-soluble and -insoluble fractions. Western blots were performed with various antibodies to quantify the amounts of the various species of GluR1, GluR2, GluR3 and NR2B subunits. The results indicate that calpain activation decreased the amount of all the intact subunits in Triton-insoluble fractions. Calpain-generated truncated forms of GluR1 and GluR2, but not NR2B, were absent in these fractions, and were recovered in Triton-soluble fractions. These findings suggest that calpain-mediated truncation of AMPA but not NMDA receptor C-terminal domains results in modifications of the interactions between the receptors and postsynaptic densities, and that this mechanism could be involved in activity-dependent changes in the subcellular distribution of AMPA receptors.

The DnaX-binding subunits delta' and psi are bound to gamma and not tau in the DNA polymerase III holoenzyme

The DnaX complex subassembly of the DNA polymerase III holoenzyme is comprised of the DnaX proteins tau and gamma and the auxiliary subunits delta, delta', chi, and psi, which together load the beta processivity factor onto primed DNA in an ATP-dependent reaction. delta' and psi bind directly to DnaX whereas delta and chi bind to delta' and psi, respectively (Onrust, R., Finkelstein, J., Naktinis, V., Turner, J., Fang, L., and O'Donnell, M. (1995) J. Biol. Chem. 270, 13348-13357). Until now, it has been unclear which DnaX protein, tau or gamma, in holoenzyme binds the auxiliary subunits delta, delta', chi,and psi. Treatment of purified holoenzyme with the homobifunctional cross-linker bis(sulfosuccinimidyl)suberate produces covalently cross-linked gamma-delta' and gamma-psi complexes identified by Western blot analysis. Immunodetection of cross-linked species with anti-delta' and anti-psi antibodies revealed that no tau-delta' or tau-psi cross-links had formed, suggesting that the delta' and psi subunits reside only on gamma within holoenzyme.

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.

Shaping excitation at glutamatergic synapses

Glutamatergic synapses vary, exhibiting EPSCs of widely different magnitudes and timecourses. The main contributors to this variability are: presynaptic factors, including release probability, quantal content and vesicle composition; factors that modulate the concentration and longevity of glutamate in the cleft, including diffusion and the actions of glutamate transporters; and postsynaptic factors, including the types and locations of ionotropic glutamate receptors, their numbers, and the nature and locations of associated intracellular signalling systems.

Characterisation and partial purification of the GABA(B) receptor from the rat cerebellum using the novel antagonist [3H]CGP 62349

The novel GABA(B) receptor antagonist [3H]CGP 62349 binds rat cerebellar synaptosomal membranes with high affinity at a single population of sites (K(d) = 0.9 nM, B(max) = 760 fmol/mg protein). Solubilisation with 1% Triton X-100/0.5 M NaCl/10% glycerol resulted in a marked increase in [3H]CGP 62349 binding (K(d) = 0.5 nM, B(max) = 1285 fmol/mg protein). Competition of [3HCGP 35348 = CGP 36742. The GABA(A) ligand isoguvacine did not displace [3H]CGP 62349 binding. Partial purification of [3H]CGP 62349 binding sites was obtained by sucrose density centrifugation and a predominant protein in the peak binding fraction was recognised by an anti-GABA(B) receptor antibody and had a molecular weight similar to the recombinant expressed GABA(B)R1a. These results demonstrate that [3H]CGP 62349 provides a useful additional tool for further characterisation of the pharmacology and biochemistry of the native GABA(B) receptor.