Roles of stargazin and phosphorylation in the control of AMPA receptor subcellular distribution

Auxiliary subunits provide new insights into regulation of AMPA receptor trafficking

Glutamate is a major excitatory neurotransmitter in the vertebrate brain. Among the ionotropic glutamate receptors, α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) glutamate receptors are the major receptors mediating excitatory fast synaptic transmission. AMPA receptors are also responsible for modifying synaptic strength through the regulation of their numbers at synapses. Their high regulatability, therefore, could contribute to the mechanisms of synaptic plasticity. The mechanisms regulating AMPA receptor trafficking have evoked great interest through the decades. Recent studies show that in the brain, AMPA receptors make complexes with transmembrane AMPA regulatory proteins (TARPs), which serve as auxiliary subunits. TARPs are required for AMPA receptor function and trafficking. After the initial discovery of TARPs, several other AMPA receptor auxiliary subunits were identified: CNIH-2, CNIH-3, CKAMP44, SynDIG1, SOL-1, SOL-2 and GSG-1L. This review discusses progress in identifying the role of auxiliary subunits in AMPA receptor trafficking.

The balance between receptor recycling and trafficking toward lysosomes determines synaptic strength during long-term depression

The strength of excitatory synaptic transmission depends partly on the number of AMPA receptors (AMPARs) at the postsynaptic surface and, thus, can be modulated by membrane trafficking events. These processes are critical for some forms of synaptic plasticity, such as long-term potentiation and long-term depression (LTD). In the case of LTD, AMPARs are internalized and dephosphorylated in response to NMDA receptor activation. However, the fate of the internalized receptors upon LTD induction and its relevance for synaptic function is still a matter of debate. Here we examined the functional contribution of receptor recycling versus degradation for LTD in rat hippocampal slices, and their correlation with receptor dephosphorylation. We observed that GluA1 undergoes sequential dephosphorylation and degradation in lysosomes after LTD induction. However, this degradation does not have functional consequences for the regulation of synaptic strength, and therefore, for the expression of LTD. In contrast, the partition of internalized AMPARs between Rab7-dependent trafficking (toward lysosomes) or Rab11-dependent endosomes (recycling back toward synapses) is the key factor determining the extent of synaptic depression upon LTD induction. This sorting decision is related to the phosphorylation status of GluA1 Ser845, the dephosphorylated receptors being those preferentially targeted for lysosomal degradation. Altogether, these new data contribute to clarify the fate of AMPARs during LTD and emphasize the importance of membrane sorting decisions to determine the outcome of synaptic plasticity.

Disrupted cortical function underlies behavior dysfunction due to social isolation

Stressful events during early childhood can have a profound lifelong influence on emotional and cognitive behaviors. However, the mechanisms by which stress affects neonatal brain circuit formation are poorly understood. Here, we show that neonatal social isolation disrupts molecular, cellular, and circuit developmental processes, leading to behavioral dysfunction. Neonatal isolation prevented long-term potentiation and experience-dependent synaptic trafficking of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors normally occurring during circuit formation in the rodent barrel cortex. This inhibition of AMPA receptor trafficking was mediated by an increase of the stress glucocorticoid hormone and was associated with reduced calcium/calmodulin-dependent protein kinase type II (CaMKII) signaling, resulting in attenuated whisker sensitivity at the cortex. These effects led to defects in whisker-dependent behavior in juvenile animals. These results indicate that neonatal social isolation alters neuronal plasticity mechanisms and perturbs the initial establishment of a normal cortical circuit, which potentially explains the long-lasting behavioral effects of neonatal stress.

Molecular remodeling mechanisms of the neural somatodendritic compartment

Neuronal cells use the process of vesicle trafficking to manipulate the populations of neurotransmitter receptors and other membrane proteins. Long term potentiation (LTP) is a long-lived increase in synaptic strength between neurons and increases postsynaptic dendritic spine size and the concentration of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptor (AMPAR) located in the postsynaptic density. AMPAR is removed from the cell surface via clathrin-mediated endocytosis. While the adaptor protein 2 (AP2) complex of endocytosis seems to have the components needed to allow temporal and spatial regulations of internalization, many accessory proteins are involved, such as epidermal growth factor receptor phosphorylation substrate 15 (Eps15). A sequence of repeats in the Eps15 protein is known as the Eps15 homology (EH) domain. It has affinity for asparagine-proline-phenylalanine (NPF) sequences that are contained within vesicle trafficking proteins such as epsin, Rab11 family interacting protein 2 (Rab11-FIP2), and Numb. After endocytosis, a pool of AMPAR is stored in the endosomal recycling compartment that can be transported to the dendritic spine surface upon stimulation during LTP for lateral diffusion into the postsynaptic density. Rab11 and the Eps15 homologue EHD1 are involved in receptor recycling. EHD family members are also involved in transcytosis of the neuronal cell adhesion molecule neuron-glia cell adhesion molecule (NgCAM) from the somatodendritic compartment to the axon. Neurons have a unique morphology comprising many projections of membrane that is constructed in part by the effects of the Eps15 homologue, intersectin. Morphogenesis in the somatodendritic compartment is becoming better understood, but there is still much exciting territory to explore, especially regarding the roles of various EH domain-NPF interactions in endocytic and recycling processes.

Differences in AMPA and kainate receptor interactomes facilitate identification of AMPA receptor auxiliary subunit GSG1L

AMPA receptor (AMPA-R) complexes consist of channel-forming subunits, GluA1-4, and auxiliary proteins, including TARPs, CNIHs, synDIG1, and CKAMP44, which can modulate AMPA-R function in specific ways. The combinatorial effects of four GluA subunits binding to various auxiliary subunits amplify the functional diversity of AMPA-Rs. The significance and magnitude of molecular diversity, however, remain elusive. To gain insight into the molecular complexity of AMPA and kainate receptors, we compared the proteins that copurify with each receptor type in the rat brain. This interactome study identified the majority of known interacting proteins and, more importantly, provides candidates for additional studies. We validate the claudin homolog GSG1L as a newly identified binding protein and unique modulator of AMPA-R gating, as determined by detailed molecular, cellular, electrophysiological, and biochemical experiments. GSG1L extends the functional variety of AMPA-R complexes, and further investigation of other candidates may reveal additional complexity of ionotropic glutamate receptor function.

The role of metaplasticity mechanisms in regulating memory destabilization and reconsolidation

Memory allows organisms to predict future events based on prior experiences. This requires encoded information to persist once important predictors are extracted, while also being modifiable in response to changes within the environment. Memory reconsolidation may allow stored information to be modified in response to related experience. However, there are many boundary conditions beyond which reconsolidation may not occur. One interpretation of these findings is that the event triggering memory retrieval must contain new information about a familiar stimulus in order to induce reconsolidation. Presently, the mechanisms that affect the likelihood of reconsolidation occurring under these conditions are not well understood. Here we speculate on a number of systems that may play a role in protecting memory from being destabilized during retrieval. We conclude that few memories may enter a state in which they cannot be modified. Rather, metaplasticity mechanisms may serve to alter the specific reactivation cues necessary to destabilize a memory. This might imply that destabilization mechanisms can differ depending on learning conditions.

Compartmentalized versus global synaptic plasticity on dendrites controlled by experience

Synapses in the brain are continuously modified by experience, but the mechanisms are poorly understood. In vitro and theoretical studies suggest threshold-lowering interactions between nearby synapses that favor clustering of synaptic plasticity within a dendritic branch. Here, a fluorescently tagged AMPA receptor-based optical approach was developed permitting detection of single-synapse plasticity in mouse cortex. Sensory experience preferentially produced synaptic potentiation onto nearby dendritic synapses. Such clustering was significantly reduced by expression of a phospho-mutant AMPA receptor that is insensitive to threshold-lowering modulation for plasticity-driven synaptic incorporation. In contrast to experience, sensory deprivation caused homeostatic synaptic enhancement globally on dendrites. Clustered synaptic potentiation produced by experience could bind behaviorally relevant information onto dendritic subcompartments; global synaptic upscaling by deprivation could equally sensitize all dendritic regions for future synaptic input.

Ca-permeable AMPA receptors in homeostatic synaptic plasticity

Neurons possess diverse mechanisms of homeostatic adaptation to overall changes in neural and synaptic activity, which are critical for proper brain functions. Homeostatic regulation of excitatory synapses has been studied in the context of synaptic scaling, which allows neurons to adjust their excitatory synaptic gain to maintain their activity within a dynamic range. Recent evidence suggests that one of the main mechanisms underlying synaptic scaling is by altering the function of postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), including synaptic expression of Ca²⁺-permeable (CP-) AMPARs. CP-AMPARs endow synapses with unique properties, which may benefit adaptation of neurons to periods of inactivity as would occur when a major input is lost. This review will summarize how synaptic expression of CP-AMPARs is regulated during homeostatic synaptic plasticity in the context of synaptic scaling, and will address the potential functional consequences of altering synaptic CP-AMPAR content.

Regulation of AMPA receptor surface diffusion by PSD-95 slots

Excitatory synaptic transmission is largely mediated by AMPA receptors (AMPARs) present at the postsynaptic density. Recent studies in single molecule tracking of AMPAR has revealed that extrasynaptic AMPARs are highly mobile and thus might serve as a readily available pool for their synaptic recruitment during synaptic plasticity events such as long-term potentiation (LTP). Because this hypothesis relies on the cell's ability to increase the number of diffusional traps or 'slots' at synapses during LTP, we will review a number of protein-protein interactions that might impact AMPARs lateral diffusion and thus potentially serve as slots. Recent studies have identified the interaction between the AMPAR-Stargazin complex and PSD-95 as the minimal components of the diffusional trapping slot. We will overview the molecular basis of this critical interaction, its activity-dependent regulation and its potential contribution to LTP.

Posttranslational regulation of AMPA receptor trafficking and function

In the mammalian central nervous system, the majority of fast excitatory synaptic transmission is mediated by glutamate acting on AMPA-type ionotropic glutamate receptors. The abundance of AMPA receptors at the synapse can be modulated through receptor trafficking, which dynamically regulates many fundamental brain functions, including learning and memory. Reversible posttranslational modifications, including phosphorylation, palmitoylation and ubiquitination of AMPA receptor subunits are important regulatory mechanisms for controlling synaptic AMPA receptor expression and function. In this review, we highlight recent advances in the study of AMPA receptor posttranslational modifications and discuss how these modifications regulate AMPA receptor trafficking and function at synapses.

AMPA receptor regulation during synaptic plasticity in hippocampus and neocortex

Discovery of long-term potentiation (LTP) in the dentate gyrus of the rabbit hippocampus by Bliss and Lømo opened up a whole new field to study activity-dependent long-term synaptic modifications in the brain. Since then hippocampal synapses have been a key model system to study the mechanisms of different forms of synaptic plasticity. At least for the postsynaptic forms of LTP and long-term depression (LTD), regulation of AMPA receptors (AMPARs) has emerged as a key mechanism. While many of the synaptic plasticity mechanisms uncovered in at the hippocampal synapses apply to synapses across diverse brain regions, there are differences in the mechanisms that often reveal the specific functional requirements of the brain area under study. Here we will review AMPAR regulation underlying synaptic plasticity in hippocampus and neocortex. The main focus of this review will be placed on postsynaptic forms of synaptic plasticity that impinge on the regulation of AMPARs using hippocampal CA1 and primary sensory cortices as examples. And through the comparison, we will highlight the key similarities and functional differences between the two synapses.

Reinforcement-related regulation of AMPA glutamate receptor subunits in the ventral tegmental area enhances motivation for cocaine

Chronic cocaine use produces numerous biological changes in brain, but relatively few are functionally associated with cocaine reinforcement. Here we show that daily intravenous cocaine self-administration, but not passive cocaine administration, induces dynamic upregulation of the AMPA glutamate receptor subunits GluR1 and GluR2 in the ventral tegmental area (VTA) of rats. Increases in GluR1 protein and GluR1(S845) phosphorylation are associated with increased GluR1 mRNA in self-administering animals, whereas increased GluR2 protein levels occurred despite substantial decreases in GluR2 mRNA. We investigated the functional significance of GluR1 upregulation in the VTA on cocaine self-administration using localized viral-mediated gene transfer. Overexpression of GluR1(WT) in rat VTA primarily infected dopamine neurons (75%) and increased AMPA receptor-mediated membrane rectification in these neurons with AMPA application. Similar GluR1(WT) overexpression potentiated locomotor responses to intra-VTA AMPA, but not NMDA, infusions. In cocaine self-administering animals, overexpression of GluR1(WT) in the VTA markedly increased the motivation for cocaine injections on a progressive ratio schedule of cocaine reinforcement. In contrast, overexpression of protein kinase A-resistant GluR1(S845A) in the VTA reduced peak rates of cocaine self-administration on a fixed ratio reinforcement schedule. Neither viral vector altered sucrose self-administration, and overexpression of GluR1(WT) or GluR1(S845A) in the adjacent substantia nigra had no effect on cocaine self-administration. Together, these results suggest that dynamic regulation of AMPA receptors in the VTA during cocaine self-administration contributes to cocaine addiction by acting to facilitate subsequent cocaine use.

The expanding social network of ionotropic glutamate receptors: TARPs and other transmembrane auxiliary subunits

Ionotropic glutamate receptors (iGluRs) underlie rapid, excitatory synaptic signaling throughout the CNS. After years of intense research, our picture of iGluRs has evolved from them being companionless in the postsynaptic membrane to them being the hub of dynamic supramolecular signaling complexes, interacting with an ever-expanding litany of other proteins that regulate their trafficking, scaffolding, stability, signaling, and turnover. In particular, the discovery that transmembrane AMPA receptor regulatory proteins (TARPs) are AMPA receptor auxiliary subunits that are critical determinants of their trafficking, gating, and pharmacology has changed the way we think about iGluR function. Recently, a number of novel transmembrane proteins have been uncovered that may also serve as iGluR auxiliary proteins. Here we review pivotal developments in our understanding of the role of TARPs in AMPA receptor trafficking and gating, and provide an overview of how newly discovered transmembrane proteins expand our view of iGluR function in the CNS.

The role of glutamate receptors in traumatic brain injury: implications for postsynaptic density in pathophysiology

Traumatic brain injury (TBI) is the major cause of death and disability, and the incidence of TBI continues to increase rapidly. In recent years, increasing attention has been paid to an important structure at the postsynaptic membrane: the postsynaptic density (PSD). Glutamate receptors, as major components of the PSD, are highly responsive to alterations in the glutamate concentration at excitatory synapses and activate intracellular signal transduction via calcium and other second messengers following TBI. PSD scaffold proteins (PSD-95, Homer, and Shank), which anchor glutamate receptors and form a network structure, also have potential effects on these downstream signaling pathways. The changes in the function and structure of these major PSD proteins are also induced by TBI, indicating that there is a more complicated mechanism associated with PSD proteins in the pathophysiological process of TBI.

Enhanced synaptic plasticity in mice with phosphomimetic mutation of the GluA1 AMPA receptor

Phosphorylation of the GluA1 subunit of AMPA receptors has been proposed to regulate receptor trafficking and synaptic transmission and plasticity. However, it remains unclear whether GluA1 phosphorylation is permissive or sufficient for enacting these functional changes. Here we investigate the role of GluA1 phosphorylation at S831 and S845 residues in the hippocampus through the analyses of GluA1 S831D/S845D phosphomimetic knock-in mice. S831D/S845D mice showed normal total and surface expression and subcellular localization of GluA1 as well as intact basal synaptic transmission. In addition, theta-burst stimulation, a protocol that was sufficient to induce robust long-term potentiation (LTP) in WT mice, resulted in LTP of similar magnitude in S831D/S845D mice. However, S831D/S845D mice showed LTP induced with 10-Hz stimulation, a protocol that is weaker than theta-burst stimulation and was not sufficient to induce LTP in WT mice. Moreover, S831D/S845D mice exhibited LTP induced with spike-timing-dependent plasticity (STDP) protocol at a long pre-post interval that was subthreshold for WT mice, although a suprathreshold STDP protocol at a short pre-post interval resulted in similarly robust LTP for WT and S831D/S845D mice. These results indicate that phosphorylation of GluA1 at S831 and S845 is sufficient to lower the threshold for LTP induction, increasing the probability of synaptic plasticity.

Routes, destinations and delays: recent advances in AMPA receptor trafficking

Postsynaptic AMPA-type glutamate receptors (AMPARs) mediate most fast excitatory synaptic transmission and are crucial for many aspects of brain function, including learning, memory and cognition. The number, synaptic localization and subunit composition of synaptic AMPARs are tightly regulated by network activity and by the history of activity at individual synapses. Furthermore, aberrant AMPAR trafficking is implicated in neurodegenerative diseases. AMPARs therefore represent a prime target for drug development and the mechanisms that control their synaptic delivery, retention and removal are the subject of extensive research. Here, we review recent findings that have provided new insights into AMPAR trafficking and that might lead to the development of novel therapeutic strategies.

Ras and Rap signaling in synaptic plasticity and mental disorders

The Ras family GTPases (Ras, Rap1, and Rap2) and their downstream mitogen-activated protein kinases (ERK, JNK, and p38MAPK) and PI3K signaling cascades control various physiological processes. In neuronal cells, recent studies have shown that these parallel cascades signal distinct forms of AMPA-sensitive glutamate receptor trafficking during experience-dependent synaptic plasticity and adaptive behavior. Interestingly, both hypo- and hyperactivation of Ras/ Rap signaling impair the capacity of synaptic plasticity, underscoring the importance of a "happy-medium" dynamic regulation of the signaling. Moreover, accumulating reports have linked various genetic defects that either up- or down-regulate Ras/Rap signaling with several mental disorders associated with learning disability (e.g., Alzheimer's disease, Angelman syndrome, autism, cardio-facio-cutaneous syndrome, Coffin-Lowry syndrome, Costello syndrome, Cowden and Bannayan-Riley-Ruvalcaba syndromes, fragile X syndrome, neurofibromatosis type 1, Noonan syndrome, schizophrenia, tuberous sclerosis, and X-linked mental retardation), highlighting the necessity of happy-medium dynamic regulation of Ras/Rap signaling in learning behavior. Thus, the recent advances in understanding of neuronal Ras/Rap signaling provide a useful guide for developing novel treatments for mental diseases.

Regulation of AMPA receptors by transmembrane accessory proteins

Regulating the number and function of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors located at the postsynaptic density is a key mechanism underlying synaptic strength and plasticity. Thus, an active area of investigation is the discovery of accessory proteins that regulate AMPA receptor trafficking and biophysical properties. One decade ago, pioneering studies identified the transmembrane protein stargazin as a critical regulator of synaptic targeting of AMPA receptors in cerebellar granule neurons. Stargazin-related family members called TARPs (transmembrane AMPA receptor regulatory proteins) are now recognized as essential auxiliary subunits for AMPA receptors that control both receptor trafficking and channel gating properties in a wide variety of neuronal cell types. Recent studies have identified a diverse array of additional accessory transmembrane proteins with distinct and overlapping functions compared with TARPs. Coupled with the wide variety of established cytoplasmic AMPA receptor accessory proteins, it is clear that AMPA receptor regulation encompasses a previously unrecognized diversity of molecular mechanisms.

Stargazin and AMPA receptor membrane expression is increased in the somatosensory cortex of Genetic Absence Epilepsy Rats from Strasbourg

Absence-like seizures in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS) model are believed to arise in hyperexcitable somatosensory cortical neurons, however the cellular basis of this increased excitability remains unknown. We have previously shown that expression of the Transmembrane AMPA receptor Regulatory Protein (TARP), stargazin, is elevated in the somatosensory cortex of GAERS. TARPs are critical regulators of the trafficking and function of AMPA receptors. Here we examine the developmental expression of stargazin and the impact this may have on AMPA receptor trafficking in the GAERS model. We show that elevated stargazin in GAERS is associated with an increase in AMPA receptor proteins, GluA1 and GluA2 in the somatosensory cortex plasma membrane of adult epileptic GAERS. Elevated stargazin expression is not seen in the epileptic WAG/Rij rat, which is a genetically distinct but phenotypically similar rat model also manifesting absence seizures, indicating that the changes seen in GAERS are unlikely to be a secondary consequence of the seizures. In juvenile (6 week old) GAERS, at the age when seizures are just starting to be expressed, there is elevated stargazin mRNA, but not protein expression for stargazin or the AMPA receptor subunits. In neonatal (7 day old) pre-epileptic GAERS there was no alteration in stargazin mRNA expression in any brain region examined. These data demonstrate that stargazin and AMPA receptor membrane targeting is altered in GAERS, potentially contributing to hyperexcitability in somatosensory cortex, with a developmental time course that would suggest a pathophysiological role in the epilepsy phenotype.

Activity-dependent ubiquitination of GluA1 mediates a distinct AMPA receptor endocytosis and sorting pathway

The accurate trafficking of AMPA receptors (AMPARs) to and from the synapse is a critical component of learning and memory in the brain, whereas dysfunction of AMPAR trafficking is hypothesized to be an underlying mechanism of Alzheimer's disease. Previous work has shown that ubiquitination of integral membrane proteins is a common posttranslational modification used to mediate endocytosis and endocytic sorting of surface proteins in eukaryotic cells. Here we report that mammalian AMPARs become ubiquitinated in response to their activation. Using a mutant of GluA1 that is unable to be ubiquitinated at lysines on its C-terminus, we demonstrate that ubiquitination is required for internalization of surface AMPARs and their trafficking to the lysosome in response to the AMPAR agonist AMPA but not for internalization of AMPARs in response to the NMDA receptor agonist NMDA. Through overexpression or RNA interference-mediated knockdown, we identify that a specific E3 ligase, Nedd4-1 (neural-precursor cell-expressed developmentally downregulated gene 4-1), is necessary for this process. Finally, we show that ubiquitination of GluA1 by Nedd4-1 becomes more prevalent as neurons mature. Together, these data show that ubiquitination of GluA1-containing AMPARs by Nedd4-1 mediates their endocytosis and trafficking to the lysosome. Furthermore, these results provide insight into how hippocampal neurons regulate AMPAR trafficking and degradation with high specificity in response to differing neuronal signaling cues and suggest that changes to this pathway may occur as neurons mature.

Morphine induces AMPA receptor internalization in primary hippocampal neurons via calcineurin-dependent dephosphorylation of GluR1 subunits

Chronic morphine treatment resulting in the alteration of postsynaptic levels of AMPA receptors, thereby modulating synaptic strength, has been reported. However, the mechanism underlying such drug-induced synaptic modification has not been resolved. By monitoring the GluR1 trafficking in primary hippocampal neurons using the pHluorin-GluR1 imaging and biotinylation studies, we observed that prolonged morphine exposure significantly induced loss of synaptic and extrasynaptic GluR1 by internalization. The morphine-induced GluR1 endocytosis was independent of neural network activities or NMDA receptor activities, as neither blocking the sodium channels with tetrodotoxin nor NMDA receptors with dl-APV altered the effects of morphine. Instead, morphine-induced GluR1 endocytosis is attributed to a change in the phosphorylation state of the GluR1 at Ser(845) as morphine significantly decreased the dephosphorylation of GluR1 at this site. Such changes in Ser(845) phosphorylation required morphine-induced activation of calcineurin, based on the observations that a calcineurin inhibitor, FK506, completely abrogated the dephosphorylation, and morphine treatment led to an increase in calcineurin enzymatic activity, even in the presence of dl-APV. Importantly, pretreatment with FK506 and overexpression of the GluR1 mutants, S845D (phospho-mimic) or S845A (phospho-blocking) attenuated the morphine-induced GluR1 endocytosis. Therefore, the calcineurin-mediated GluR1-S845 dephosphorylation is critical for the morphine-induced changes in the postsynaptic AMPA receptor level. Together, these findings reveal a novel molecular mechanism for opioid-induced neuronal adaptation and/or synaptic impairment.

The prion protein as a receptor for amyloid-beta

Increased levels of brain amyloid-beta, a secreted peptide cleavage product of amyloid precursor protein (APP), is believed to be critical in the aetiology of Alzheimer's disease. Increased amyloid-beta can cause synaptic depression, reduce the number of spine protrusions (that is, sites of synaptic contacts) and block long-term synaptic potentiation (LTP), a form of synaptic plasticity; however, the receptor through which amyloid-beta produces these synaptic perturbations has remained elusive. Laurén et al. suggested that binding between oligomeric amyloid-beta (a form of amyloid-beta thought to be most active) and the cellular prion protein (PrP(C)) is necessary for synaptic perturbations. Here we show that PrP(C) is not required for amyloid-beta-induced synaptic depression, reduction in spine density, or blockade of LTP; our results indicate that amyloid-beta-mediated synaptic defects do not require PrP(c).

A three-step model for the synaptic recruitment of AMPA receptors

The amount of AMPARs at synapses is not a fixed number but varies according to different factors including synaptic development, activity and disease. Because the number of AMPARs sets the strength of synaptic transmission, their trafficking is subject to fine and tight regulation. In this review, we will describe the different steps taken by AMPARs in order to reach the synapse. We propose a three-step mechanism involving exocytosis at extra/perisynaptic sites, lateral diffusion to synapses and a subsequent rate-limiting diffusional trapping step. We will describe how the different trafficking steps are regulated during synaptic plasticity or altered during neurodegenerative diseases such as Alzheimer's.

AMPA receptors are exocytosed in stimulated spines and adjacent dendrites in a Ras-ERK-dependent manner during long-term potentiation

The exocytosis of AMPA receptors is a key step in long-term potentiation (LTP), yet the timing and location of exocytosis and the signaling pathways involved in exocytosis during synaptic plasticity are not fully understood. Here we combine two-photon uncaging with two-photon imaging of a fluorescent label of surface AMPA receptors to monitor individual AMPA receptor exocytosis events near spines undergoing LTP. AMPA receptors that reached the stimulated spine came from a combination of preexisting surface receptors (70-90%) and newly exocytosed receptors (10-30%). We observed exocytosis in both the dendrite and spine under basal conditions. The rate of AMPA receptor exocytosis increased approximately 5-fold during LTP induction and decayed to the basal level within approximately 1 min, both in the stimulated spine and in the dendrite within approximately 3 microm of the stimulated spine. AMPA receptors inserted in the spine were trapped in the spine in an activity-dependent manner. The activity-dependent exocytosis required the Ras-ERK pathway, but not CaMKII. Thus, diffusive Ras-ERK signaling presumably serves as an important means for signaling from synapses to dendritic shafts to recruit AMPA receptors into synapses during LTP.

TARP phosphorylation regulates synaptic AMPA receptors through lipid bilayers

Neurons use neurotransmitters to communicate across synapses, constructing neural circuits in the brain. AMPA-type glutamate receptors are the predominant excitatory neurotransmitter receptors mediating fast synaptic transmission. AMPA receptors localize at synapses by forming protein complexes with transmembrane AMPA receptor regulatory proteins (TARPs) and PSD-95-like membrane-associated guanylate kinases. Among the three classes of ionotropic glutamate receptors (AMPA, NMDA, and kainate type), AMPA receptor activity is most regulatable by neuronal activity to adjust synaptic strength. Here, we mutated the prototypical TARP, stargazin, and found that TARP phosphorylation regulates synaptic AMPA receptor activity in vivo. We also found that stargazin interacts with negatively charged lipid bilayers in a phosphorylation-dependent manner and that the lipid interaction inhibited stargazin binding to PSD-95. Cationic lipids dissociated stargazin from lipid bilayers and enhanced synaptic AMPA receptor activity in a stargazin phosphorylation-dependent manner. Thus, TARP phosphorylation plays a critical role in regulating AMPA receptor-mediated synaptic transmission via a lipid bilayer interaction.

Regulation of nucleus accumbens activity by the hypothalamic neuropeptide melanin-concentrating hormone

The lateral hypothalamus and the nucleus accumbens shell (AcbSh) are brain regions important for food intake. The AcbSh contains high levels of receptor for melanin-concentrating hormone (MCH), a lateral hypothalamic peptide critical for feeding and metabolism. MCH receptor (MCHR1) activation in the AcbSh increases food intake, while AcbSh MCHR1 blockade reduces feeding. Here biochemical and cellular mechanisms of MCH action in the rodent AcbSh are described. A reduction of phosphorylation of GluR1 at serine 845 (pSer(845)) is shown to occur after both pharmacological and genetic manipulations of MCHR1 activity. These changes depend upon signaling through G(i/o), and result in decreased surface expression of GluR1-containing AMPA receptors (AMPARs). Electrophysiological analysis of medium spiny neurons (MSNs) in the AcbSh revealed decreased amplitude of AMPAR-mediated synaptic events (mEPSCs) with MCH treatment. In addition, MCH suppressed action potential firing MSNs through K(+) channel activation. Finally, in vivo recordings confirmed that MCH reduces neuronal cell firing in the AcbSh in freely moving animals. The ability of MCH to reduce cell firing in the AcbSh is consistent with a general model from other pharmacological and electrophysiological studies whereby reduced AcbSh neuronal firing leads to food intake. The current work integrates the hypothalamus into this model, providing biochemical and cellular mechanisms whereby metabolic and limbic signals converge to regulate food intake.

Regulation of ionotropic glutamate receptors by their auxiliary subunits

Glutamate receptors are major excitatory receptors in the brain. Recent findings have established auxiliary subunits of glutamate receptors as critical modulators of synaptic transmission, synaptic plasticity, and neurological disorder. The elucidation of the molecular rules governing glutamate receptors and subunits will improve our understanding of synapses and of neural-circuit regulation in the brain.

Distance-dependent homeostatic synaptic scaling mediated by a-type potassium channels

Many lines of evidence suggest that the efficacy of synapses on CA1 pyramidal neuron dendrites increases as a function of distance from the cell body. The strength of an individual synapse is also dynamically modulated by activity-dependent synaptic plasticity, which raises the question as to how a neuron can reconcile individual synaptic changes with the maintenance of the proximal-to-distal gradient of synaptic strength along the dendrites. As the density of A-type potassium channels exhibits a similar gradient from proximal (low)-to-distal (high) dendrites, the A-current may play a role in coordinating local synaptic changes with the global synaptic strength gradient. Here we describe a form of homeostatic plasticity elicited by conventional activity blockade (with tetrodotoxin) coupled with a block of the A-type potassium channel. Following A-type potassium channel inhibition for 12 h, recordings from CA1 somata revealed a significantly higher miniature excitatory postsynaptic current (mEPSC) frequency, whereas in dendritic recordings, there was no change in mEPSC frequency. Consistent with mEPSC recordings, we observed a significant increase in AMPA receptor density in stratum pyramidale but not stratum radiatum. Based on these data, we propose that the differential distribution of A-type potassium channels along the apical dendrites may create a proximal-to-distal membrane potential gradient. This gradient may regulate AMPA receptor distribution along the same axis. Taken together, our results indicate that A-type potassium channels play an important role in controlling synaptic strength along the dendrites, which may help to maintain the computational capacity of the neuron.

Alzheimer's disease: synaptic dysfunction and Abeta

Synapse loss is an early and invariant feature of Alzheimer's disease (AD) and there is a strong correlation between the extent of synapse loss and the severity of dementia. Accordingly, it has been proposed that synapse loss underlies the memory impairment evident in the early phase of AD and that since plasticity is important for neuronal viability, persistent disruption of plasticity may account for the frank cell loss typical of later phases of the disease. Extensive multi-disciplinary research has implicated the amyloid β-protein (Aβ) in the aetiology of AD and here we review the evidence that non-fibrillar soluble forms of Aβ are mediators of synaptic compromise. We also discuss the possible mechanisms of Aβ synaptotoxicity and potential targets for therapeutic intervention.

Stabilization of Ca2+-permeable AMPA receptors at perisynaptic sites by GluR1-S845 phosphorylation

AMPA receptor (AMPAR) channel properties and function are regulated by its subunit composition and phosphorylation. Certain types of neural activity can recruit Ca(2+)-permeable (CP) AMPARs, such as GluR1 homomers, to synapses likely via lateral diffusion from extrasynaptic sites. Here we show that GluR1-S845 phosphorylation can alter the subunit composition of perisynaptic AMPARs by providing stability to GluR1 homomers. Using mice specifically lacking phosphorylation of the GluR1-S845 site (GluR1-S845A mutants), we demonstrate that this site is necessary for maintaining CP-AMPARs. Specifically, in the GluR1-S845A mutants, CP-AMPARs were absent from perisynaptic locations mainly due to lysosomal degradation. This regulation was mimicked by acute desphosphorylation of the GluR1-S845 site in wild-type mice by NMDA application. Furthermore, long-term depression (LTD) was associated with a reduction in perisynaptic CP-AMPAR levels. Our findings suggest that GluR1-S845 is necessary for maintaining CP-AMPARs on the surface, especially at perisynaptic sites, and suggest that the regulation of these receptors is involved in synaptic plasticity.

Viral vector approaches to modify gene expression in the brain

The use of viral vectors as gene transfer tools for the central nervous system has seen a significant growth in the last decade. Improvements in the safety, efficiency and specificity of vectors for clinical applications have proven to be beneficial also for basic neuroscience research. This review will discuss the viral systems currently available to neuroscientists and some of the recent achievements in the study of synaptic function, memory and drug addiction.