Subunit-specific role for the amino-terminal domain of AMPA receptors in synaptic targeting

Using FASS-LTP in postmortem mice brain tissues to assess pathological synaptic function

BACKGROUND

Study of synaptic integrity using conventional electrophysiology is a gold standard for quantitative assessment of neurodegeneration. Fluorescence assisted single-synapse long-term potentiation (FASS-LTP) provides a high throughput method to assess the synaptic integrity of neurotransmission within and between different brain regions as a measure of pharmacological efficacy in translational models.

NEW METHOD

We adapted the existing method to our purpose by adding a step during the thawing of frozen samples, by an extra step of placing them on a rocker at room temperature for 30 minutes immediately following thawing with constant mixing on a shaker. This allowed for gradual, uniform thawing, effectively separating the synaptosomes. Our study demonstrates FASS-LTP on four brain regions at 6- and 12-month periods in the 3xTg-AD mouse model, treating sibling cohorts with VU0155069 (a small molecule inhibitor) or vehicle (0.9 % saline).

RESULTS

Our findings demonstrate the robust ability of the FASS-LTP technique to characterize the functional synaptic integrity maintained by disease-treatment therapies in multiple brain regions longitudinally using frozen brain tissue.

COMPARISON WITH EXISTING METHODS

By providing a detailed, user-friendly protocol for this well-known analysis and including a recovery step improved the ability to robustly replicate the FASS-LTP between different brain regions. This may be extrapolated to a translational use on human clinical samples to improve understanding of the therapeutic impact on synaptic performance related to glutamate neurotransmission.

CONCLUSIONS

FASS-LTP method offers a robust analysis of synaptosomes isolated from frozen tissue samples, demonstrating greater reproducibility in rodent and human synapses in physiological and pathological states.

Activity-dependent diffusion trapping of AMPA receptors as a key step for expression of early LTP

This review focuses on the activity-dependent diffusion trapping of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) as a crucial mechanism for the expression of early long-term potentiation (LTP), a process central to learning and memory. Despite decades of research, the precise mechanisms by which LTP induction leads to an increase in AMPAR responses at synapses have been elusive. We review the different hypotheses that have been put forward to explain the increased AMPAR responsiveness during LTP. We discuss the dynamic nature of AMPAR complexes, including their constant turnover and activity-dependent modifications that affect their synaptic accumulation. We highlight a hypothesis suggesting that AMPARs are diffusively trapped at synapses through activity-dependent interactions with protein-based binding slots in the post-synaptic density (PSD), offering a potential explanation for the increased synaptic strength during LTP. Furthermore, we outline the challenges still to be addressed before we fully understand the functional roles and molecular mechanisms of AMPAR dynamic nanoscale organization in LTP. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Nanoscale organization is changed in native, surface AMPARs by mouse brain region and tauopathy

Synaptic AMPA receptors (AMPARs) on neuronal plasma membranes are correlated with learning and memory. Using a unique labeling and super-resolution imaging, we have visualized the nanoscale synaptic and extra-synaptic organization of native surface AMPARs for the first time in mouse brain slices as a function of brain region and tauopathy. We find that the fraction of surface AMPARs organized in synaptic clusters is two-times smaller in the hippocampus compared to the motor and somatosensory cortex. In 6 months old PS19 model of tauopathy, synaptic and extrasynaptic distributions are disrupted in the hippocampus but not in the cortex. Thus, this optimized super-resolution imaging tool allows us to observe synaptic deterioration at the onset of tauopathy before apparent neurodegeneration.

Tuning synaptic strength by regulation of AMPA glutamate receptor localization

Long-term potentiation (LTP) of excitatory synapses is a leading model to explain the concept of information storage in the brain. Multiple mechanisms contribute to LTP, but central amongst them is an increased sensitivity of the postsynaptic membrane to neurotransmitter release. This sensitivity is predominantly determined by the abundance and localization of AMPA-type glutamate receptors (AMPARs). A combination of AMPAR structural data, super-resolution imaging of excitatory synapses, and an abundance of electrophysiological studies are providing an ever-clearer picture of how AMPARs are recruited and organized at synaptic junctions. Here, we review the latest insights into this process, and discuss how both cytoplasmic and extracellular receptor elements cooperate to tune the AMPAR response at the hippocampal CA1 synapse.

Allosteric competition and inhibition in AMPA receptors

Excitatory neurotransmission is principally mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-subtype ionotropic glutamate receptors (AMPARs). Negative allosteric modulators are therapeutic candidates that inhibit AMPAR activation and can compete with positive modulators to control AMPAR function through unresolved mechanisms. Here we show that allosteric inhibition pushes AMPARs into a distinct state that prevents both activation and positive allosteric modulation. We used cryo-electron microscopy to capture AMPARs bound to glutamate, while a negative allosteric modulator, GYKI-52466, and positive allosteric modulator, cyclothiazide, compete for control of the AMPARs. GYKI-52466 binds in the ion channel collar and inhibits AMPARs by decoupling the ligand-binding domains from the ion channel. The rearrangement of the ligand-binding domains ruptures the cyclothiazide site, preventing positive modulation. Our data provide a framework for understanding allostery of AMPARs and for rational design of therapeutics targeting AMPARs in neurological diseases.

Synaptic Mechanisms of Delay Eyeblink Classical Conditioning: AMPAR Trafficking and Gene Regulation in an In Vitro Model

An in vitro model of delay eyeblink classical conditioning was developed to investigate synaptic plasticity mechanisms underlying acquisition of associative learning. This was achieved by replacing real stimuli, such as an airpuff and tone, with patterned stimulation of the cranial nerves using an isolated brainstem preparation from turtle. Here, our primary findings regarding cellular and molecular mechanisms for learning acquisition using this unique approach are reviewed. The neural correlate of the in vitro eyeblink response is a replica of the actual behavior, and features of conditioned responses (CRs) resemble those observed in behavioral studies. Importantly, it was shown that acquisition of CRs did not require the intact cerebellum, but the appropriate timing did. Studies of synaptic mechanisms indicate that conditioning involves two stages of AMPA receptor (AMPAR) trafficking. Initially, GluA1-containing AMPARs are targeted to synapses followed later by replacement by GluA4 subunits that support CR expression. This two-stage process is regulated by specific signal transduction cascades involving PKA and PKC and is guided by distinct protein chaperones. The expression of the brain-derived neurotrophic factor (BDNF) protein is central to AMPAR trafficking and conditioning. BDNF gene expression is regulated by coordinated epigenetic mechanisms involving DNA methylation/demethylation and chromatin modifications that control access of promoters to transcription factors. Finally, a hypothesis is proposed that learning genes like BDNF are poised by dual chromatin features that allow rapid activation or repression in response to environmental stimuli. These in vitro studies have advanced our understanding of the cellular and molecular mechanisms that underlie associative learning.

Allosteric Competition and Inhibition in AMPA Receptors

Excitatory neurotransmission is principally mediated by AMPA-subtype ionotropic glutamate receptors (AMPARs). Dysregulation of AMPARs is the cause of many neurological disorders and how therapeutic candidates such as negative allosteric modulators inhibit AMPARs is unclear. Here, we show that non-competitive inhibition desensitizes AMPARs to activation and prevents positive allosteric modulation. We dissected the noncompetitive inhibition mechanism of action by capturing AMPARs bound to glutamate and the prototypical negative allosteric modulator, GYKI-52466, with cryo-electron microscopy. Noncompetitive inhibition by GYKI-52466, which binds in the transmembrane collar region surrounding the ion channel, negatively modulates AMPARs by decoupling glutamate binding in the ligand binding domain from the ion channel. Furthermore, during allosteric competition between negative and positive modulators, negative allosteric modulation by GKYI-52466 outcompetes positive allosteric modulators to control AMPAR function. Our data provide a new framework for understanding allostery of AMPARs and foundations for rational design of therapeutics targeting AMPARs in neurological diseases.

Synaptic memory and CaMKII

Ca2+/calmodulin-dependent protein kinase II (CaMKII) and long-term potentiation (LTP) were discovered within a decade of each other and have been inextricably intertwined ever since. However, like many marriages, it has had its up and downs. Based on the unique biochemical properties of CaMKII, it was proposed as a memory molecule before any physiological linkage was made to LTP. However, as reviewed here, the convincing linkage of CaMKII to synaptic physiology and behavior took many decades. New technologies were critical in this journey, including in vitro brain slices, mouse genetics, single-cell molecular genetics, pharmacological reagents, protein structure, and two-photon microscopy, as were new investigators attracted by the exciting challenge. This review tracks this journey and assesses the state of this marriage 40 years on. The collective literature impels us to propose a relatively simple model for synaptic memory involving the following steps that drive the process: 1) Ca2+ entry through N-methyl-d-aspartate (NMDA) receptors activates CaMKII. 2) CaMKII undergoes autophosphorylation resulting in constitutive, Ca2+-independent activity and exposure of a binding site for the NMDA receptor subunit GluN2B. 3) Active CaMKII translocates to the postsynaptic density (PSD) and binds to the cytoplasmic C-tail of GluN2B. 4) The CaMKII-GluN2B complex initiates a structural rearrangement of the PSD that may involve liquid-liquid phase separation. 5) This rearrangement involves the PSD-95 scaffolding protein, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and their transmembrane AMPAR-regulatory protein (TARP) auxiliary subunits, resulting in an accumulation of AMPARs in the PSD that underlies synaptic potentiation. 6) The stability of the modified PSD is maintained by the stability of the CaMKII-GluN2B complex. 7) By a process of subunit exchange or interholoenzyme phosphorylation CaMKII maintains synaptic potentiation in the face of CaMKII protein turnover. There are many other important proteins that participate in enlargement of the synaptic spine or modulation of the steps that drive and maintain the potentiation. In this review we critically discuss the data underlying each of the steps. As will become clear, some of these steps are more firmly grounded than others, and we provide suggestions as to how the evidence supporting these steps can be strengthened or, based on the new data, be replaced. Although the journey has been a long one, the prospect of having a detailed cellular and molecular understanding of learning and memory is at hand.

Structural mobility tunes signalling of the GluA1 AMPA glutamate receptor

AMPA glutamate receptors (AMPARs), the primary mediators of excitatory neurotransmission in the brain, are either GluA2 subunit-containing and thus Ca2+-impermeable, or GluA2-lacking and Ca2+-permeable1. Despite their prominent expression throughout interneurons and glia, their role in long-term potentiation and their involvement in a range of neuropathologies2, structural information for GluA2-lacking receptors is currently absent. Here we determine and characterize cryo-electron microscopy structures of the GluA1 homotetramer, fully occupied with TARPγ3 auxiliary subunits (GluA1/γ3). The gating core of both resting and open-state GluA1/γ3 closely resembles GluA2-containing receptors. However, the sequence-diverse N-terminal domains (NTDs) give rise to a highly mobile assembly, enabling domain swapping and subunit re-alignments in the ligand-binding domain tier that are pronounced in desensitized states. These transitions underlie the unique kinetic properties of GluA1. A GluA2 mutant (F231A) increasing NTD dynamics phenocopies this behaviour, and exhibits reduced synaptic responses, reflecting the anchoring function of the AMPAR NTD at the synapse. Together, this work underscores how the subunit-diverse NTDs determine subunit arrangement, gating properties and ultimately synaptic signalling efficiency among AMPAR subtypes.

Contrastsing synaptic roles of MDGA1 and MDGA2

Neurodevelopmental disorders are frequently linked to mutations in synaptic organizing molecules. MAM domain containing glycosylphosphatidylinositol anchor 1 and 2 (MDGA1 and MDGA2) are a family of synaptic organizers suggested to play an unusual role as synaptic repressors, but studies offer conflicting evidence for their localization. Using epitope-tagged MDGA1 and MDGA2 knock-in mice, we found that native MDGAs are expressed throughout the brain, peaking early in postnatal development. Surprisingly, endogenous MDGA1 was enriched at excitatory, but not inhibitory, synapses. Both shRNA knockdown and CRISPR/Cas9 knockout of MDGA1 resulted in cell-autonomous, specific impairment of AMPA receptor-mediated synaptic transmission, without affecting GABAergic transmission. Conversely, MDGA2 knockdown/knockout selectively depressed NMDA receptor-mediated transmission but enhanced inhibitory transmission. Our results establish that MDGA2 acts as a synaptic repressor, but only at inhibitory synapses, whereas both MDGAs are required for excitatory transmission. This nonoverlapping division of labor between two highly conserved synaptic proteins is unprecedented.

Molecular mechanisms of AMPAR reversible stabilization at synapses

In the central nervous system, glutamatergic synapses play a central role in the regulation of excitatory neuronal transmission. With the membrane-associated guanylate kinase (MAGUK) family of proteins as their structuring scaffold, glutamatergic receptors serve as the powerhouse of glutamatergic synapses. Glutamatergic receptors can be categorized as metabotropic and ionotropic receptors. The latter are then categorized into N-methyl-d-aspartate, kainate receptors, and α-amino-3-hydroxy-5-methyl-isoxazole-propionic acid receptors (AMPARs). Over the past two decades, genetic tagging technology and super-resolution microscopy have been of the utmost importance to unravel how the different receptors are organized at glutamatergic synapses. At the plasma membrane, receptors are highly mobile but show reduced mobility when at synaptic sites. This partial immobilization of receptors at synaptic sites is attributed to the stabilization/anchoring of receptors with the postsynaptic MAGUK proteins and auxiliary proteins, and presynaptic proteins. These partial immobilizations and localization of glutamatergic receptors within the synaptic sites are fundamental for proper basal transmission and synaptic plasticity. Perturbations of the stabilization of glutamatergic receptors are often associated with cognitive deficits. In this review, we describe the proposed mechanisms for synaptic localization and stabilization of AMPARs, the major players of fast excitatory transmission in the central nervous system.

Plasticity of postsynaptic nanostructure

The postsynaptic density (PSD) of excitatory synapses is built from a wide variety of scaffolding proteins, receptors, and signaling molecules that collectively orchestrate synaptic transmission. Seminal work over the past decades has led to the identification and functional characterization of many PSD components. In contrast, we know far less about how these constituents are assembled within synapses, and how this organization contributes to synapse function. Notably, recent evidence from high-resolution microscopy studies and in silico models, highlights the importance of the precise subsynaptic structure of the PSD for controlling the strength of synaptic transmission. Even further, activity-driven changes in the distribution of glutamate receptors are acknowledged to contribute to long-term changes in synaptic efficacy. Thus, defining the mechanisms that drive structural changes within the PSD are important for a molecular understanding of synaptic transmission and plasticity. Here, we review the current literature on how the PSD is organized to mediate basal synaptic transmission and how synaptic activity alters the nanoscale organization of synapses to sustain changes in synaptic strength.

mGluR5 is transiently confined in perisynaptic nanodomains to shape synaptic function

The unique perisynaptic distribution of postsynaptic metabotropic glutamate receptors (mGluRs) at excitatory synapses is predicted to directly shape synaptic function, but mechanistic insight into how this distribution is regulated and impacts synaptic signaling is lacking. We used live-cell and super-resolution imaging approaches, and developed molecular tools to resolve and acutely manipulate the dynamic nanoscale distribution of mGluR5. Here we show that mGluR5 is dynamically organized in perisynaptic nanodomains that localize close to, but not in the synapse. The C-terminal domain of mGluR5 critically controlled perisynaptic confinement and prevented synaptic entry. We developed an inducible interaction system to overcome synaptic exclusion of mGluR5 and investigate the impact on synaptic function. We found that mGluR5 recruitment to the synapse acutely increased synaptic calcium responses. Altogether, we propose that transient confinement of mGluR5 in perisynaptic nanodomains allows flexible modulation of synaptic function.

Long-term potentiation reconstituted with an artificial TARP/PSD-95 complex

The critical role of AMPA receptor (AMPAR) trafficking in long-term potentiation (LTP) of excitatory synaptic transmission is now well established, but the underlying molecular mechanism is still uncertain. Recent research suggests that PSD-95 captures AMPARs via an interaction with the AMPAR auxiliary subunits-transmembrane AMPAR regulatory proteins (TARPs). To determine if such interaction is a core minimal component of the AMPAR trafficking and LTP mechanism, we engineered artificial binding partners, which individually were biochemically and functionally dead but which, when expressed together, rescue binding and both basal synaptic transmission and LTP. These findings establish the TARP/PSD-95 complex as an essential interaction underlying AMPAR trafficking and LTP.

High-resolution imaging and manipulation of endogenous AMPA receptor surface mobility during synaptic plasticity and learning

Regulation of synaptic neurotransmitter receptor content is a fundamental mechanism for tuning synaptic efficacy during experience-dependent plasticity and behavioral adaptation. However, experimental approaches to track and modify receptor movements in integrated experimental systems are limited. Exploiting AMPA-type glutamate receptors (AMPARs) as a model, we generated a knock-in mouse expressing the biotin acceptor peptide (AP) tag on the GluA2 extracellular N-terminal. Cell-specific introduction of biotin ligase allows the use of monovalent or tetravalent avidin variants to respectively monitor or manipulate the surface mobility of endogenous AMPAR containing biotinylated AP-GluA2 in neuronal subsets. AMPAR immobilization precluded the expression of long-term potentiation and formation of contextual fear memory, allowing target-specific control of the expression of synaptic plasticity and animal behavior. The AP tag knock-in model offers unprecedented access to resolve and control the spatiotemporal dynamics of endogenous receptors, and opens new avenues to study the molecular mechanisms of synaptic plasticity and learning.

Nanoscale rules governing the organization of glutamate receptors in spine synapses are subunit specific

Heterotetrameric glutamate receptors are essential for the development, function, and plasticity of spine synapses but how they are organized to achieve this is not known. Here we show that the nanoscale organization of glutamate receptors containing specific subunits define distinct subsynaptic features. Glutamate receptors containing GluA2 or GluN1 subunits establish nanomodular elements precisely positioned relative to Synaptotagmin-1 positive presynaptic release sites that scale with spine size. Glutamate receptors containing GluA1 or GluN2B specify features that exhibit flexibility: GluA1-subunit containing AMPARs are found in larger spines, while GluN2B-subunit containing NMDARs are enriched in the smallest spines with neither following a strict modular organization. Given that the precise positioning of distinct classes of glutamate receptors is linked to diverse events including cell death and synaptic plasticity, this unexpectedly robust synaptic nanoarchitecture provides a resilient system, where nanopositioned glutamate receptor heterotetramers define specific subsynaptic regions of individual spine synapses.

Glutamate receptors comprise two obligate subunits and two subunits that confer distinct properties and functions to the specific tetramers, which also localize to distinct synaptic spines. Here, the authors use STimulated Emission Depletion nanoscopy (STED) to provide detailed insights into the spatial organization of glutamate receptor types.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

AMPA receptor trafficking and LTP: Carboxy-termini, amino-termini and TARPs

AMPA receptors (AMPARs) are fundamental elements in excitatory synaptic transmission and synaptic plasticity in the CNS. Long term potentiation (LTP), a form of synaptic plasticity which contributes to learning and memory formation, relies on the accumulation of AMPARs at the postsynapse. This phenomenon requires the coordinated recruitment of different elements in the AMPAR complex. Based on recent research reviewed herein, we propose an updated AMPAR trafficking and LTP model which incorporates both extracellular as well as intracellular mechanisms. This article is part of the special Issue on 'Glutamate Receptors - AMPA receptors'.

AMPA receptor anchoring at CA1 synapses is determined by N-terminal domain and TARP γ8 interactions

AMPA receptor (AMPAR) abundance and positioning at excitatory synapses regulates the strength of transmission. Changes in AMPAR localisation can enact synaptic plasticity, allowing long-term information storage, and is therefore tightly controlled. Multiple mechanisms regulating AMPAR synaptic anchoring have been described, but with limited coherence or comparison between reports, our understanding of this process is unclear. Here, combining synaptic recordings from mouse hippocampal slices and super-resolution imaging in dissociated cultures, we compare the contributions of three AMPAR interaction domains controlling transmission at hippocampal CA1 synapses. We show that the AMPAR C-termini play only a modulatory role, whereas the extracellular N-terminal domain (NTD) and PDZ interactions of the auxiliary subunit TARP γ8 are both crucial, and each is sufficient to maintain transmission. Our data support a model in which γ8 accumulates AMPARs at the postsynaptic density, where the NTD further tunes their positioning. This interplay between cytosolic (TARP γ8) and synaptic cleft (NTD) interactions provides versatility to regulate synaptic transmission and plasticity.

Phosphorylation of AMPA receptor subunit GluA1 regulates clathrin-mediated receptor internalization

Synaptic strength is altered during synaptic plasticity by controlling the number of AMPA receptors (AMPARs) at excitatory synapses. During long-term potentiation and synaptic up-scaling, AMPARs are accumulated at synapses to increase synaptic strength. Neuronal activity leads to phosphorylation of AMPAR subunit GluA1 and subsequent elevation of GluA1 surface expression, either by an increase in receptor forward trafficking to the synaptic membrane or a decrease in receptor internalization. However, the molecular pathways underlying GluA1 phosphorylation-induced elevation of surface AMPAR expression are not completely understood. Here, we employ fluorescence recovery after photobleaching (FRAP) to reveal that phosphorylation of GluA1 Serine 845 (S845) predominantly plays a role in receptor internalization than forward trafficking during synaptic plasticity. Notably, internalization of AMPARs depends upon the clathrin adaptor, AP2, which recruits cargo proteins into endocytic clathrin coated pits. In fact, we further reveal that an increase in GluA1 S845 phosphorylation by two distinct forms of synaptic plasticity diminishes the binding of the AP2 adaptor, reducing internalization, and resulting in elevation of GluA1 surface expression. We thus demonstrate a mechanism of GluA1 phosphorylation-regulated clathrin-mediated internalization of AMPARs.

AMPA receptors in the synapse: Very little space and even less time

Glutamate is by far the most abundant neurotransmitter used by excitatory synapses in the vertebrate central nervous system. Once released into the synaptic cleft, it depolarises the postsynaptic membrane and activates downstream signalling pathways resulting in the propagation of the excitatory signal. Initial depolarisation is primarily mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. These ion channels are the first ones to be activated by released glutamate and their kinetics, dynamics and abundance on the postsynaptic membrane defines the strength of the postsynaptic response. This review focuses on native AMPA receptors and synaptic environment they inhabit and considers structural and functional properties of the receptors obtained in heterologous systems in the light of spatial and temporal constraints of the synapse. This article is part of the special Issue on 'Glutamate Receptors - AMPA receptors'.

Biology of AMPA receptor interacting proteins - From biogenesis to synaptic plasticity

AMPA-type glutamate receptors mediate the majority of excitatory synaptic transmission in the central nervous system. Their signaling properties and abundance at synapses are both crucial determinants of synapse efficacy and plasticity, and are therefore under sophisticated control. Unique to this ionotropic glutamate receptor (iGluR) is the abundance of interacting proteins that contribute to its complex regulation. These include transient interactions with the receptor cytoplasmic tail as well as the N-terminal domain locating to the synaptic cleft, both of which are involved in AMPAR trafficking and receptor stabilization at the synapse. Moreover, an array of transmembrane proteins operate as auxiliary subunits that in addition to receptor trafficking and stabilization also substantially impact AMPAR gating and pharmacology. Here, we provide an overview of the catalogue of AMPAR interacting proteins, and how they contribute to the complex biology of this central glutamate receptor.

AMPA Receptors Exist in Tunable Mobile and Immobile Synaptic Fractions In Vivo

AMPA receptor (AMPAR) mobility within synapses has been extensively studied in vitro. However, whether similar mobility properties apply to AMPARs in vivo has yet to be determined. Here, we use two-photon fluorescence recovery after photobleaching (FRAP) to study AMPAR mobility within individual dendritic spines in live animals using an overexpression vector. We demonstrate the existence of mobile and immobile fractions of AMPARs across multiple cortical regions and layers. Additionally, we find that AMPAR mobility can be altered in vivo in response to administration of corticosterone, a condition that mimics exposure to stress.

Endophilin A1 drives acute structural plasticity of dendritic spines in response to Ca2+/calmodulin

Induction of long-term potentiation (LTP) in excitatory neurons triggers a large transient increase in the volume of dendritic spines followed by decays to sustained size expansion, a process termed structural LTP (sLTP) that contributes to the cellular basis of learning and memory. Although mechanisms regulating the early and sustained phases of sLTP have been studied intensively, how the acute spine enlargement immediately after LTP stimulation is achieved remains elusive. Here, we report that endophilin A1 orchestrates membrane dynamics with actin polymerization to initiate spine enlargement in NMDAR-mediated LTP. Upon LTP induction, Ca²⁺/calmodulin enhances binding of endophilin A1 to both membrane and p140Cap, a cytoskeletal regulator. Consequently, endophilin A1 rapidly localizes to the plasma membrane and recruits p140Cap to promote local actin polymerization, leading to spine head expansion. Moreover, its molecular functions in activity-induced rapid spine growth are required for LTP and long-term memory. Thus, endophilin A1 serves as a calmodulin effector to drive acute structural plasticity necessary for learning and memory.

The amino-terminal domain of GluA1 mediates LTP maintenance via interaction with neuroplastin-65

Long-term potentiation (LTP) has long been considered as an important cellular mechanism for learning and memory. LTP expression involves NMDA receptor-dependent synaptic insertion of AMPA receptors (AMPARs). However, how AMPARs are recruited and anchored at the postsynaptic membrane during LTP remains largely unknown. In this study, using CRISPR/Cas9 to delete the endogenous AMPARs and replace them with the mutant forms in single neurons, we have found that the amino-terminal domain (ATD) of GluA1 is required for LTP maintenance. Moreover, we show that GluA1 ATD directly interacts with the cell adhesion molecule neuroplastin-65 (Np65). Neurons lacking Np65 exhibit severely impaired LTP maintenance, and Np65 deletion prevents GluA1 from rescuing LTP in AMPARs-deleted neurons. Thus, our study reveals an essential role for GluA1/Np65 binding in anchoring AMPARs at the postsynaptic membrane during LTP.

Trans-synaptic assemblies link synaptic vesicles and neuroreceptors

Synaptic transmission is characterized by fast, tightly coupled processes and complex signaling pathways that require a precise protein organization, such as the previously reported nanodomain colocalization of pre- and postsynaptic proteins. Here, we used cryo-electron tomography to visualize synaptic complexes together with their native environment comprising interacting proteins and lipids on a 2- to 4-nm scale. Using template-free detection and classification, we showed that tripartite trans-synaptic assemblies (subcolumns) link synaptic vesicles to postsynaptic receptors and established that a particular displacement between directly interacting complexes characterizes subcolumns. Furthermore, we obtained de novo average structures of ionotropic glutamate receptors in their physiological composition, embedded in plasma membrane. These data support the hypothesis that synaptic function is carried by precisely organized trans-synaptic units. It provides a framework for further exploration of synaptic and other large molecular assemblies that link different cells or cellular regions and may require weak or transient interactions to exert their function.

Visualizing synaptic plasticity in vivo by large-scale imaging of endogenous AMPA receptors

Elucidating how synaptic molecules such as AMPA receptors mediate neuronal communication and tracking their dynamic expression during behavior is crucial to understand cognition and disease, but current technological barriers preclude large-scale exploration of molecular dynamics in vivo. We have developed a suite of innovative methodologies that break through these barriers: a new knockin mouse line with fluorescently tagged endogenous AMPA receptors, two-photon imaging of hundreds of thousands of labeled synapses in behaving mice, and computer vision-based automatic synapse detection. Using these tools, we can longitudinally track how the strength of populations of synapses changes during behavior. We used this approach to generate an unprecedentedly detailed spatiotemporal map of synapses undergoing changes in strength following sensory experience. More generally, these tools can be used as an optical probe capable of measuring functional synapse strength across entire brain areas during any behavioral paradigm, describing complex system-wide changes with molecular precision.

Trans-synaptic interactions of ionotropic glutamate receptors

Trans-synaptic interactions organize the multiple steps of synaptic development and are critical to generate fully functional neuronal circuits. While trans-synaptic interactions are primarily mediated by cell adhesion molecules (CAMs), some directly involve ionotropic glutamate receptors (iGluRs). Here, we review the expanding extracellular and trans-synaptic proteome of iGluRs. We discuss the role of these molecular networks in specifying the formation of excitatory and inhibitory circuits and in the input-specific recruitment of iGluRs at synapses in various cell types and brain regions. We also shed light on human-specific mutations affecting iGluR-mediated trans-synaptic interactions that may provide unique features to the human brain and contribute to its susceptibility to neurodevelopmental disorders. Together, these data support a view in which iGluR function goes far beyond fast excitatory synaptic transmission by shaping the wiring and the functional properties of neural circuits.

PORCN Negatively Regulates AMPAR Function Independently of Subunit Composition and the Amino-Terminal and Carboxy-Terminal Domains of AMPARs

Most fast excitatory synaptic transmissions in the mammalian brain are mediated by α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors (AMPARs), which are ligand-gated cation channels. The membrane expression level of AMPARs is largely determined by auxiliary subunits in AMPAR macromolecules, including porcupine O-acyltransferase (PORCN), which negatively regulates AMPAR trafficking to the plasma membrane. However, whether PORCN-mediated regulation depends on AMPAR subunit composition or particular regions of a subunit has not been determined. We systematically examined the effects of PORCN on the ligand-gated current and surface expression level of GluA1, GluA2, and GluA3 AMPAR subunits, alone and in combination, as well as the PORCN-GluA interaction in heterologous HEK293T cells. PORCN inhibited glutamate-induced currents and the surface expression of investigated GluA AMPAR subunits in a subunit-independent manner. These inhibitory effects required neither the amino-terminal domain (ATD) nor the carboxy-terminal domain (CTD) of GluA subunits. In addition, PORCN interacted with AMPARs independently of their ATD or CTD. Thus, the functional inhibition of AMPARs by PORCN in transfected heterologous cells was independent of the ATD, CTD, and subunit composition of AMPARs.

Long-term potentiation is independent of the C-tail of the GluA1 AMPA receptor subunit

We tested the proposal that the C-terminal domain (CTD) of the AMPAR subunit GluA1 is required for LTP. We found that a knock-in mouse lacking the CTD of GluA1 expresses normal LTP and spatial memory, assayed by the Morris water maze. Our results support a model in which LTP generates synaptic slots, which capture passively diffusing AMPARs.

Calcium-permeable AMPA receptors are essential to the synaptic plasticity induced by epileptiform activity in rat hippocampal slices

Abnormal neuronal activity during epileptic seizures alters the properties of synaptic plasticity, and, consequently, leads to cognitive impairment. The molecular mechanism of these alterations in synaptic plasticity is still unclear. In the present study, using a 4-aminopyridine (4-AP) in vitro model, we demonstrated that epileptiform activity in rat hippocampal slices initially causes substantial enhancement of field excitatory postsynaptic potential amplitude. However, the potentiation of CA3-CA1 synapses was temporary and switched to long-term depression (LTD) within an hour. Previous studies showed that transient incorporation of calcium-permeable AMPA receptors (CP-AMPARs) is crucial for the consolidation of long-term potentiation (LTP). We confirmed that, in normal conditions, the blockage of CP-AMPARs prevented the consolidation of LTP induced by theta-burst stimulation (TBS). In contrast, the blockage of CP-AMPARs preserved synaptic potentiation induced by epileptiform activity. One hour after a period of epileptiform activity in the hippocampal slices, synaptic plasticity was substantially altered, and the TBS protocol was unable to produce LTP. Moreover, if CP-AMPARs were blocked, the TBS protocol induced LTD. Our results indicate that CP-AMPARs play an essential role in the molecular mechanism of the disturbances of synaptic plasticity caused by epileptiform activity.

Phosphorylation-Dependent Regulation of Ca2+-Permeable AMPA Receptors During Hippocampal Synaptic Plasticity

Experience-dependent learning and memory require multiple forms of plasticity at hippocampal and cortical synapses that are regulated by N-methyl-D-aspartate receptors (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptors (NMDAR, AMPAR). These plasticity mechanisms include long-term potentiation (LTP) and depression (LTD), which are Hebbian input-specific mechanisms that rapidly increase or decrease AMPAR synaptic strength at specific inputs, and homeostatic plasticity that globally scales-up or -down AMPAR synaptic strength across many or even all inputs. Frequently, these changes in synaptic strength are also accompanied by a change in the subunit composition of AMPARs at the synapse due to the trafficking to and from the synapse of receptors lacking GluA2 subunits. These GluA2-lacking receptors are most often GluA1 homomeric receptors that exhibit higher single-channel conductance and are Ca2+-permeable (CP-AMPAR). This review article will focus on the role of protein phosphorylation in regulation of GluA1 CP-AMPAR recruitment and removal from hippocampal synapses during synaptic plasticity with an emphasis on the crucial role of local signaling by the cAMP-dependent protein kinase (PKA) and the Ca2+calmodulin-dependent protein phosphatase 2B/calcineurin (CaN) that is coordinated by the postsynaptic scaffold protein A-kinase anchoring protein 79/150 (AKAP79/150).

AMPA receptor structure and auxiliary subunits

Fast excitatory synaptic transmission in the mammalian brain is largely mediated by AMPA-type ionotropic glutamate receptors (AMPARs), which are activated by the neurotransmitter glutamate. In synapses, the function of AMPARs is tuned by their auxiliary subunits, a diverse set of membrane proteins associated with the core pore-forming subunits of the AMPARs. Each auxiliary subunit provides distinct functional modulation of AMPARs, ranging from regulation of trafficking to shaping ion channel gating kinetics. Understanding the molecular mechanism of the function of these complexes is key to decoding synaptic modulation and their global roles in cognitive activities, such as learning and memory. Here, we review the structural and molecular complexity of AMPAR-auxiliary subunit complexes, as well as their functional diversity in different brain regions. We suggest that the recent structural information provides new insights into the molecular mechanisms underlying synaptic functions of AMPAR-auxiliary subunit complexes.

Noelin1 Affects Lateral Mobility of Synaptic AMPA Receptors

Lateral diffusion on the neuronal plasma membrane of the AMPA-type glutamate receptor (AMPAR) serves an important role in synaptic plasticity. We investigated the role of the secreted glycoprotein Noelin1 (Olfactomedin-1 or Pancortin) in AMPAR lateral mobility and its dependence on the extracellular matrix (ECM). We found that Noelin1 interacts with the AMPAR with high affinity, however, without affecting rise- and decay time and desensitization properties. Noelin1 co-localizes with synaptic and extra-synaptic AMPARs and is expressed at synapses in an activity-dependent manner. Single-particle tracking shows that Noelin1 reduces lateral mobility of both synaptic and extra-synaptic GluA1-containing receptors and affects short-term plasticity. While the ECM does not constrain the synaptic pool of AMPARs and acts only extrasynaptically, Noelin1 contributes to synaptic potentiation by limiting AMPAR mobility at synaptic sites. This is the first evidence for the role of a secreted AMPAR-interacting protein on mobility of GluA1-containing receptors and synaptic plasticity.

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Noelin1 interacts with high affinity to AMPA receptors (AMPARs)

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Noelin1 is secreted upon cellular stimulation

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(Extra)synaptic AMPAR mobility, but not channel properties, are affected by Noelin1

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Reducing synaptic AMPAR lateral mobility by Noelin1 limits synaptic plasticity

Neto proteins regulate gating of the kainate-type glutamate receptor GluK2 through two binding sites

The neuropilin and tolloid-like (Neto) proteins Neto1 and Neto2 are auxiliary subunits of kainate-type glutamate receptors (KARs) that regulate KAR trafficking and gating. However, how Netos bind and regulate the biophysical functions of KARs remains unclear. Here, we found that the N-terminal domain (NTD) of glutamate receptor ionotropic kainate 2 (GluK2) binds the first complement C1r/C1s-Uegf-BMP (CUB) domain of Neto proteins (i.e. NTD-CUB1 interaction) and that the core of GluK2 (GluK2ΔNTD) binds Netos through domains other than CUB1s (core-Neto interaction). Using electrophysiological analysis in HEK293T cells, we examined the effects of these interactions on GluK2 gating, including deactivation, desensitization, and recovery from desensitization. We found that NTD deletion does not affect GluK2 fast gating kinetics, the desensitization, and the deactivation. We also observed that Neto1 and Neto2 differentially regulate GluK2 fast gating kinetics, which largely rely on the NTD-CUB1 interactions. NTD removal facilitated GluK2 recovery from desensitization, indicating that the NTD stabilizes the GluK2 desensitization state. Co-expression with Neto1 or Neto2 also accelerated GluK2 recovery from desensitization, which fully relied on the NTD-CUB1 interactions. Moreover, we demonstrate that the NTD-CUB1 interaction involves electric attraction between positively charged residues in the GluK2_NTD and negatively charged ones in the CUB1 domains. Neutralization of these charges eliminated the regulatory effects of the NTD-CUB1 interaction on GluK2 gating. We conclude that KARs bind Netos through at least two sites and that the NTD-CUB1 interaction critically regulates Neto-mediated GluK2 gating.

Amyloid-β oligomers suppress subunit-specific glutamate receptor increase during LTP

Introduction

Amyloid-β oligomers (AβOs) are assumed to impair the ability of learning and memory by suppressing the induction of synaptic plasticity, such as long-term potentiation (LTP) in the early stage of Alzheimer's disease. However, the direct molecular mechanism of how AβOs affect excitatory synaptic plasticity remains to be elucidated.

Methods

In order to study the effects of AβOs on LTP-associated changes of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptor (AMPAR) movement, we performed live-cell imaging of fluorescently labeled AMPAR subunit GluA1 or GluA2 with total internal reflection fluorescence microscopy.

Results

Incubation of cultured hippocampal neurons with AβOs for 1–2 days inhibited the increase in GluA1 number and GluA1 exocytosis frequency in both postsynaptic and extrasynaptic membranes during LTP. In contrast, AβOs did not inhibit the increase in GluA2 number or exocytosis frequency.

Discussion

These results suggest that AβOs primarily inhibit the increase in the number of GluA1 homomers and suppress hippocampal LTP expression.

Linking Nanoscale Dynamics of AMPA Receptor Organization to Plasticity of Excitatory Synapses and Learning

The spatiotemporal organization of neurotransmitter receptors in the postsynaptic membrane is a fundamental determinant of synaptic transmission and thus of information processing by the brain. The ionotropic AMPA subtype of glutamate receptors (AMPARs) mediate fast excitatory synaptic transmission in the CNS. The number of AMPARs located en face presynaptic glutamate release sites sets the efficacy of synaptic transmission. Understanding how this number is set and regulated has been the topic of intense research in the last two decades. We showed that AMPARs are not stable in the synapse as initially thought. They continuously enter and exit the postsynaptic density by lateral diffusion, and they exchange between the neuronal surface and intracellular compartments by endocytosis and exocytosis at extrasynaptic sites. Regulation of these various trafficking pathways has emerged as a key mechanism for activity-dependent plasticity of synaptic transmission, a process important for learning and memory. I here present my view of these findings. In particular, the advent of super-resolution microscopy and single-molecule tracking has helped to uncover the intricacy of AMPARs' dynamic organization at the nanoscale. In addition, AMPAR surface diffusion is highly regulated by a variety of factors, including neuronal activity, stress hormones, and neurodegeneration, suggesting that AMPAR diffusion-trapping may play a central role in synapse function. Using innovative tools to understand further the link between receptor dynamics and synapse plasticity is now unveiling new molecular mechanisms of learning. Modifying AMPAR dynamics may emerge as a new target to correct synapse dysfunction in the diseased brain.

Mechanisms underlying the synaptic trafficking of the glutamate delta receptor GluD1

Ionotropic glutamate delta receptors do not bind glutamate and do not generate ionic current, resulting in difficulty in studying the function and trafficking of these receptors. Here, we utilize chimeric constructs, in which the ligand-binding domain of GluD1 is replaced by that of GluK1, to examine its synaptic trafficking and plasticity. GluD1 trafficked to the synapse, but was incapable of expressing long-term potentiation (LTP). The C-terminal domain (CT) of GluD1 has a classic PDZ-binding motif, which is critical for the synaptic trafficking of other glutamate receptors, but we found that its binding to PSD-95 was very weak, and deleting the PDZ-binding motif failed to alter synaptic trafficking. However, deletion of the entire CT abolished synaptic trafficking, but not surface expression. We found that mutation of threonine (T) T923 to an alanine disrupted synaptic trafficking. Therefore, GluD1 receptors have strikingly different trafficking mechanisms compared with AMPARs. These results highlight the diversity of ionotropic glutamate receptor trafficking rules at a single type of synapse. Since this receptor is genetically associated with schizophrenia, our findings may provide an important clue to understand schizophrenia.

Phase Separation-Mediated TARP/MAGUK Complex Condensation and AMPA Receptor Synaptic Transmission

Transmembrane AMPA receptor (AMPAR) regulatory proteins (TARPs) modulate AMPAR synaptic trafficking and transmission via disc-large (DLG) subfamily of membrane-associated guanylate kinases (MAGUKs). Despite extensive studies, the molecular mechanism governing specific TARP/MAGUK interaction remains elusive. Using stargazin and PSD-95 as the representatives, we discover that the entire tail of stargazin (Stg_CT) is required for binding to PSD-95. The PDZ binding motif (PBM) and an Arg-rich motif upstream of PBM conserved in TARPs bind to multiple sites on PSD-95, thus resulting in a highly specific and multivalent stargazin/PSD-95 complex. Stargazin in complex with PSD-95 or PSD-95-assembled postsynaptic complexes form highly concentrated and dynamic condensates via phase separation, reminiscent of stargazin/PSD-95-mediated AMPAR synaptic clustering and trapping. Importantly, charge neutralization mutations in TARP_CT Arg-rich motif weakened TARP's condensation with PSD-95 and impaired TARP-mediated AMPAR synaptic transmission in mice hippocampal neurons. The TARP_CT/PSD-95 interaction mode may have implications for understanding clustering of other synaptic transmembrane proteins.

The AMPA Receptor Code of Synaptic Plasticity

Changes in the properties and postsynaptic abundance of AMPA-type glutamate receptors (AMPARs) are major mechanisms underlying various forms of synaptic plasticity, including long-term potentiation (LTP), long-term depression (LTD), and homeostatic scaling. The function and the trafficking of AMPARs to and from synapses is modulated by specific AMPAR GluA1-GluA4 subunits, subunit-specific protein interactors, auxiliary subunits, and posttranslational modifications. Layers of regulation are added to AMPAR tetramers through these different interactions and modifications, increasing the computational power of synapses. Here we review the reliance of synaptic plasticity on AMPAR variants and propose "the AMPAR code" as a conceptual framework. The AMPAR code suggests that AMPAR variants will be predictive of the types and extent of synaptic plasticity that can occur and that a hierarchy exists such that certain AMPARs will be disproportionally recruited to synapses during LTP/homeostatic scaling up, or removed during LTD/homeostatic scaling down.

Structural biology of glutamate receptor ion channels: towards an understanding of mechanism

Ionotropic glutamate receptors (iGluRs) are tetrameric ion channels that mediate signal transmission at neuronal synapses, where they contribute centrally to the postsynaptic plasticity that underlies learning and memory. Receptor activation by l-glutamate triggers complex allosteric cascades that are transmitted through the layered and highly flexible receptor assembly culminating in opening a cation-selective pore. This process is shaped by the arrangement of the four core subunits as well as the presence of various auxiliary subunits, and is subject to regulation by an array of small molecule modulators targeting a number of sites throughout the complex. Here, we discuss recent structures of iGluR homomers and heteromers illuminating the organization and subunit arrangement of the core tetramer, co-assembled with auxiliary subunits and in complex with allosteric modulators.

Architecture of the heteromeric GluA1/2 AMPA receptor in complex with the auxiliary subunit TARP γ8

AMPA-type glutamate receptors (AMPARs) mediate excitatory neurotransmission and are central regulators of synaptic plasticity, a molecular mechanism underlying learning and memory. Although AMPARs act predominantly as heteromers, structural studies have focused on homomeric assemblies. Here, we present a cryo-electron microscopy structure of the heteromeric GluA1/2 receptor associated with two transmembrane AMPAR regulatory protein (TARP) γ8 auxiliary subunits, the principal AMPAR complex at hippocampal synapses. Within the receptor, the core subunits arrange to give the GluA2 subunit dominant control of gating. This structure reveals the geometry of the Q/R site that controls calcium flux, suggests association of TARP-stabilized lipids, and demonstrates that the extracellular loop of γ8 modulates gating by selectively interacting with the GluA2 ligand-binding domain. Collectively, this structure provides a blueprint for deciphering the signal transduction mechanisms of synaptic AMPARs.

Architecture and subunit arrangement of native AMPA receptors elucidated by cryo-EM

Glutamate-gated AMPA receptors mediate the fast component of excitatory signal transduction at chemical synapses throughout all regions of the mammalian brain. AMPA receptors are tetrameric assemblies composed of four subunits, GluA1-GluA4. Despite decades of study, the subunit composition, subunit arrangement, and molecular structure of native AMPA receptors remain unknown. Here we elucidate the structures of 10 distinct native AMPA receptor complexes by single-particle cryo-electron microscopy (cryo-EM). We find that receptor subunits are arranged nonstochastically, with the GluA2 subunit preferentially occupying the B and D positions of the tetramer and with triheteromeric assemblies comprising a major population of native AMPA receptors. Cryo-EM maps define the structure for S2-M4 linkers between the ligand-binding and transmembrane domains, suggesting how neurotransmitter binding is coupled to ion channel gating.

Mechanisms of postsynaptic localization of AMPA-type glutamate receptors and their regulation during long-term potentiation

l-Glutamate is the main excitatory neurotransmitter in the brain, with postsynaptic responses to its release predominantly mediated by AMPA-type glutamate receptors (AMPARs). A critical component of synaptic plasticity involves changes in the number of responding postsynaptic receptors, which are dynamically recruited to and anchored at postsynaptic sites. Emerging findings continue to shed new light on molecular mechanisms that mediate AMPAR postsynaptic trafficking and localization. Accordingly, unconventional secretory trafficking of AMPARs occurs in dendrites, from the endoplasmic reticulum (ER) through the ER-Golgi intermediary compartment directly to recycling endosomes, independent of the Golgi apparatus. Upon exocytosis, AMPARs diffuse in the plasma membrane to reach the postsynaptic site, where they are trapped to contribute to transmission. This trapping occurs through a combination of both intracellular interactions, such as TARP (transmembrane AMPAR regulatory protein) binding to α-actinin-stabilized PSD-95, and extracellular interactions through the receptor amino-terminal domain. These anchoring mechanisms may facilitate precise receptor positioning with respect to glutamate release sites to enable efficient synaptic transmission.

Mechanisms and Role of Dendritic Membrane Trafficking for Long-Term Potentiation

Long-term potentiation (LTP) of excitatory synapses is a major form of plasticity for learning and memory in the central nervous system. While the molecular mechanisms of LTP have been debated for decades, there is consensus that LTP induction activates membrane trafficking pathways within dendrites that are essential for synapse growth and strengthening. Current models suggest that key molecules for synaptic potentiation are sequestered within intracellular organelles, which are mobilized by synaptic activity to fuse with the plasma membrane following LTP induction. While the identity of the factors mobilized to the plasma membrane during LTP remain obscure, the field has narrowly focused on AMPA-type glutamate receptors. Here, we review recent literature and present new experimental data from our lab investigating whether AMPA receptors trafficked from intracellular organelles directly contribute to synaptic strengthening during LTP. We propose a modified model where membrane trafficking delivers distinct factors that are required to maintain synapse growth and AMPA receptor incorporation following LTP. Finally, we pose several fundamental questions that may guide further inquiry into the role of membrane trafficking for synaptic plasticity.

Postsynaptic localization and regulation of AMPA receptors and Cav1.2 by β2 adrenergic receptor/PKA and Ca2+/CaMKII signaling

The synapse transmits, processes, and stores data within its tiny space. Effective and specific signaling requires precise alignment of the relevant components. This review examines current insights into mechanisms of AMPAR and NMDAR localization by PSD-95 and their spatial distribution at postsynaptic sites to illuminate the structural and functional framework of postsynaptic signaling. It subsequently delineates how β2 adrenergic receptor (β2 AR) signaling via adenylyl cyclase and the cAMP-dependent protein kinase PKA is organized within nanodomains. Here, we discuss targeting of β2 AR, adenylyl cyclase, and PKA to defined signaling complexes at postsynaptic sites, i.e., AMPARs and the L-type Ca2+ channel Cav1.2, and other subcellular surface localizations, the role of A kinase anchor proteins, the physiological relevance of the spatial restriction of corresponding signaling, and their interplay with signal transduction by the Ca2+- and calmodulin-dependent kinase CaMKII How localized and specific signaling by cAMP occurs is a central cellular question. The dendritic spine constitutes an ideal paradigm for elucidating the dimensions of spatially restricted signaling because of their small size and defined protein composition.

AMPA Receptor Trafficking for Postsynaptic Potentiation

Long-term potentiation (LTP) of excitatory synaptic strength, which has long been considered a synaptic correlate for learning and memory, requires a fast recruitment of additional α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors (AMPARs) to the postsynaptic sites. As cell biological concepts have been applied to the field and genetic manipulation and microscopic imaging technologies have been advanced, visualization of the trafficking of AMPARs to synapses for LTP has been investigated intensively over the last decade. Recycling endosomes have been reported as intracellular storage organelles to supply AMPARs for LTP through the endocytic recycling pathway. In addition, exocytic domains in the spine plasma membrane, where AMPARs are inserted from the intracellular compartment, and nanodomains, where diffusing AMPARs are trapped and immobilized inside synapses for LTP, have been described. Furthermore, cell surface lateral diffusion of AMPARs from extrasynaptic to synaptic sites has been reported as a key step for AMPAR location to the synaptic sites for LTP. This review article will discuss recent findings and views on the reservoir(s) of AMPARs and their trafficking for LTP expression by focusing on the exocytosis and lateral diffusion of AMPARs, and provide some future directions that need to be addressed in the field of LTP.