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

Fluorescence lifetime imaging of AMPA receptor endocytosis in living neurons: effects of Aβ and PP1

The relative amount of AMPA receptors expressed at the surface of neurons can be measured using superecliptic pHluorin (SEP) labeling at their N-terminus. However, the high signal variability resulting from protein overexpression in neurons and the low signal observed in intracellular vesicles make quantitative characterization of receptor trafficking difficult. Here, we establish a real-time live-cell assay of AMPAR trafficking based on fluorescence lifetime imaging (FLIM), which allows for simultaneous visualization of both surface and intracellular receptors. Using this assay, we found that elevating amyloid-beta (Aβ) levels leads to a strong increase in intracellular GluA1 and GluA2-containing receptors, indicating that Aβ triggers the endocytosis of these AMPARs. In APP/PS1 Alzheimer's disease model mouse neurons, FLIM revealed strikingly different AMPAR trafficking properties for GluA1- and GluA3-containing receptors, suggesting that chronic Aβ exposure triggered the loss of both surface and intracellular GluA3-containing receptors. Interestingly, overexpression of protein phosphatase 1 (PP1) also resulted in GluA1 endocytosis as well as depressed synaptic transmission, confirming the important role of phosphorylation in regulating AMPAR trafficking. This new approach allows for the quantitative measurement of extracellular pH, small changes in receptor trafficking, as well as simultaneous measurement of surface and internalized AMPARs in living neurons, and could therefore be applied to several different studies in the future.

Novel crosstalk mechanisms between GluA3 and Epac2 in synaptic plasticity and memory in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease which accounts for the most cases of dementia worldwide. Impaired memory, including acquisition, consolidation, and retrieval, is one of the hallmarks in AD. At the cellular level, dysregulated synaptic plasticity partly due to reduced long-term potentiation (LTP) and enhanced long-term depression (LTD) underlies the memory deficits in AD. GluA3 containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are one of key receptors involved in rapid neurotransmission and synaptic plasticity. Recent studies revealed a novel form of GluA3 involved in neuronal plasticity that is dependent on cyclic adenosine monophosphate (cAMP), rather than N-methyl-d-aspartate (NMDA). However, this cAMP-dependent GluA3 pathway is specifically and significantly impaired by amyloid beta (Aβ), a pathological marker of AD. cAMP is a key second messenger that plays an important role in modulating memory and synaptic plasticity. We previously reported that exchange protein directly activated by cAMP 2 (Epac2), acting as a main cAMP effector, plays a specific and time-limited role in memory retrieval. From electrophysiological perspective, Epac2 facilities the maintenance of LTP, a cellular event closely associated with memory retrieval. Additionally, Epac2 was found to be involved in the GluA3-mediated plasticity. In this review, we comprehensively summarize current knowledge regarding the specific roles of GluA3 and Epac2 in synaptic plasticity and memory, and their potential association with AD.

Oxidative stress and inflammation cause auditory system damage via glial cell activation and dysregulated expression of gap junction proteins in an experimental model of styrene-induced oto/neurotoxicity

BACKGROUND

Redox imbalance and inflammation have been proposed as the principal mechanisms of damage in the auditory system, resulting in functional alterations and hearing loss. Microglia and astrocytes play a crucial role in mediating oxidative/inflammatory injury in the central nervous system; however, the role of glial cells in the auditory damage is still elusive.

OBJECTIVES

Here we investigated glial-mediated responses to toxic injury in peripheral and central structures of the auditory pathway, i.e., the cochlea and the auditory cortex (ACx), in rats exposed to styrene, a volatile compound with well-known oto/neurotoxic properties.

METHODS

Male adult Wistar rats were treated with styrene (400 mg/kg daily for 3 weeks, 5/days a week). Electrophysiological, morphological, immunofluorescence and molecular analyses were performed in both the cochlea and the ACx to evaluate the mechanisms underlying styrene-induced oto/neurotoxicity in the auditory system.

RESULTS

We showed that the oto/neurotoxic insult induced by styrene increases oxidative stress in both cochlea and ACx. This was associated with macrophages and glial cell activation, increased expression of inflammatory markers (i.e., pro-inflammatory cytokines and chemokine receptors) and alterations in connexin (Cxs) and pannexin (Panx) expression, likely responsible for dysregulation of the microglia/astrocyte network. Specifically, we found downregulation of Cx26 and Cx30 in the cochlea, and high level of Cx43 and Panx1 in the ACx.

CONCLUSIONS

Collectively, our results provide novel evidence on the role of immune and glial cell activation in the oxidative/inflammatory damage induced by styrene in the auditory system at both peripheral and central levels, also involving alterations of gap junction networks. Our data suggest that targeting glial cells and connexin/pannexin expression might be useful to attenuate oxidative/inflammatory damage in the auditory system.

Molecular Mechanisms of AMPA Receptor Trafficking in the Nervous System

Synaptic plasticity enhances or reduces connections between neurons, affecting learning and memory. Postsynaptic AMPARs mediate greater than 90% of the rapid excitatory synaptic transmission in glutamatergic neurons. The number and subunit composition of AMPARs are fundamental to synaptic plasticity and the formation of entire neural networks. Accordingly, the insertion and functionalization of AMPARs at the postsynaptic membrane have become a core issue related to neural circuit formation and information processing in the central nervous system. In this review, we summarize current knowledge regarding the related mechanisms of AMPAR expression and trafficking. The proteins related to AMPAR trafficking are discussed in detail, including vesicle-related proteins, cytoskeletal proteins, synaptic proteins, and protein kinases. Furthermore, significant emphasis was placed on the pivotal role of the actin cytoskeleton, which spans throughout the entire transport process in AMPAR transport, indicating that the actin cytoskeleton may serve as a fundamental basis for AMPAR trafficking. Additionally, we summarize the proteases involved in AMPAR post-translational modifications. Moreover, we provide an overview of AMPAR transport and localization to the postsynaptic membrane. Understanding the assembly, trafficking, and dynamic synaptic expression mechanisms of AMPAR may provide valuable insights into the cognitive decline associated with neurodegenerative diseases.

Early life stress lastingly alters the function and AMPA-receptor composition of glutamatergic synapses in the hippocampus of male mice

Early postnatal life is a sensitive period of development that shapes brain structure and function later in life. Exposure to stress during this critical time window can alter brain development and may enhance the susceptibility to psychopathology and neurodegenerative disorders later in life. The developmental effects of early life stress (ELS) on synaptic function are not fully understood, but could provide mechanistic insights into how ELS modifies later brain function and disease risk. We here assessed the effects of ELS on synaptic function and composition in the hippocampus of male mice. Mice were subjected to ELS by housing dams and pups with limited bedding and nesting material from postnatal days (P) 2-9. Synaptic strength was measured in terms of miniature excitatory postsynaptic currents (mEPSCs) in the hippocampal dentate gyrus at three different developmental stages: the early postnatal phase (P9), preadolescence (P21, at weaning) and adulthood at 3 months of age (3MO). Hippocampal synaptosome fractions were isolated from P9 and 3MO tissue and analyzed for protein content to assess postsynaptic composition. Finally, dendritic spine density was assessed in the DG at 3MO. At P9, ELS increased mEPSC frequency and amplitude. In parallel, synaptic composition was altered as PSD-95, GluA3 and GluN2B content were significantly decreased. The increased mEPSC frequency was sustained up to 3MO, at which age, GluA3 content was significantly increased. No differences were found in dendritic spine density. These findings highlight how ELS affects the development of hippocampal synapses, which could provide valuable insight into mechanisms how ELS alters brain function later in life.

Noise-induced auditory damage affects hippocampus causing memory deficits in a model of early age-related hearing loss

Several studies identified noise-induced hearing loss (NIHL) as a risk factor for sensory aging and cognitive decline processes, including neurodegenerative diseases, such as dementia and age-related hearing loss (ARHL). Although the association between noise- and age-induced hearing impairment has been widely documented by epidemiological and experimental studies, the molecular mechanisms underlying this association are not fully understood as it is not known how these risk factors (aging and noise) can interact, affecting memory processes. We recently found that early noise exposure in an established animal model of ARHL (C57BL/6 mice) accelerates the onset of age-related cochlear dysfunctions. Here, we extended our previous data by investigating what happens in central brain structures (auditory cortex and hippocampus), to assess the relationship between hearing and memory impairment and the possible combined effect of noise and sensory aging on the cognitive domain. To this aim, we exposed juvenile C57BL/6 mice of 2 months of age to repeated noise sessions (60 min/day, pure tone of 100 dB SPL, 10 kHz, 10 consecutive days) and we monitored auditory threshold by measuring auditory brainstem responses (ABR), spatial working memory, by using the Y-maze test, and basal synaptic transmission by using ex vivo electrophysiological recordings, at different time points (1, 4 and 7 months after the onset of noise exposure, corresponding to 3, 6 and 9 months of age). We found that hearing loss, along with accelerated presbycusis onset, can induce persistent synaptic alterations in the auditory cortex. This was associated with decreased memory performance and oxidative-inflammatory injury in the hippocampus, the extra-auditory structure involved in memory processes. Collectively, our data confirm the critical relationship between auditory and memory circuits, suggesting that the combined detrimental effect of noise and sensory aging on hearing function can be considered a high-risk factor for both sensory and cognitive degenerative processes, given that early noise exposure accelerates presbycusis phenotype and induces hippocampal-dependent memory dysfunctions.

Protein quality control of N-methyl-D-aspartate receptors

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated cation channels that mediate excitatory neurotransmission and are critical for synaptic development and plasticity in the mammalian central nervous system (CNS). Functional NMDARs typically form via the heterotetrameric assembly of GluN1 and GluN2 subunits. Variants within GRIN genes are implicated in various neurodevelopmental and neuropsychiatric disorders. Due to the significance of NMDAR subunit composition for regional and developmental signaling at synapses, properly folded receptors must reach the plasma membrane for their function. This review focuses on the protein quality control of NMDARs. Specifically, we review the quality control mechanisms that ensure receptors are correctly folded and assembled within the endoplasmic reticulum (ER) and trafficked to the plasma membrane. Further, we discuss disease-associated variants that have shown disrupted NMDAR surface expression and function. Finally, we discuss potential targeted pharmacological and therapeutic approaches to ameliorate disease phenotypes by enhancing the expression and surface trafficking of subunits harboring disease-associated variants, thereby increasing their incorporation into functional receptors.

The Shaping of AMPA Receptor Surface Distribution by Neuronal Activity

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

Auditory sensory deprivation induced by noise exposure exacerbates cognitive decline in a mouse model of Alzheimer's disease

Although association between hearing impairment and dementia has been widely documented by epidemiological studies, the role of auditory sensory deprivation in cognitive decline remains to be fully understood. To address this issue we investigated the impact of hearing loss on the onset and time-course of cognitive decline in an animal model of Alzheimer's disease (AD), that is the 3×Tg-AD mice and the underlying mechanisms. We found that hearing loss induced by noise exposure in the 3×Tg-AD mice before the phenotype is manifested caused persistent synaptic and morphological alterations in the auditory cortex. This was associated with earlier hippocampal dysfunction, increased tau phosphorylation, neuroinflammation, and redox imbalance, along with anticipated memory deficits compared to the expected time-course of the neurodegenerative phenotype. Our data suggest that a mouse model of AD is more vulnerable to central damage induced by hearing loss and shows reduced ability to counteract noise-induced detrimental effects, which accelerates the neurodegenerative disease onset.

Twenty Years of SynGAP Research: From Synapses to Cognition

SynGAP is a potent regulator of biochemical signaling in neurons and plays critical roles in neuronal function. It was first identified in 1998, and has since been extensively characterized as a mediator of synaptic plasticity. Because of its involvement in synaptic plasticity, SynGAP has emerged as a critical protein for normal cognitive function. In recent years, mutations in the SYNGAP1 gene have been shown to cause intellectual disability in humans and have been linked to other neurodevelopmental disorders, such as autism spectrum disorders and schizophrenia. While the structure and biochemical function of SynGAP have been well characterized, a unified understanding of the various roles of SynGAP at the synapse and its contributions to neuronal function remains to be achieved. In this review, we summarize and discuss the current understanding of the multifactorial role of SynGAP in regulating neuronal function gathered over the last two decades.

Optogenetic Control of Spine-Head JNK Reveals a Role in Dendritic Spine Regression

In this study, we use an optogenetic inhibitor of c-Jun NH₂-terminal kinase (JNK) in dendritic spine sub-compartments of rat hippocampal neurons. We show that JNK inhibition exerts rapid (within seconds) reorganization of actin in the spine-head. Using real-time Förster resonance energy transfer (FRET) to measure JNK activity, we find that either excitotoxic insult (NMDA) or endocrine stress (corticosterone), activate spine-head JNK causing internalization of AMPARs and spine retraction. Both events are prevented upon optogenetic inhibition of JNK, and rescued by JNK inhibition even 2 h after insult. Moreover, we identify that the fast-acting anti-depressant ketamine reduces JNK activity in hippocampal neurons suggesting that JNK inhibition may be a downstream mediator of its anti-depressant effect. In conclusion, we show that JNK activation plays a role in triggering spine elimination by NMDA or corticosterone stress, whereas inhibition of JNK facilitates regrowth of spines even in the continued presence of glucocorticoid. This identifies that JNK acts locally in the spine-head to promote AMPAR internalization and spine shrinkage following stress, and reveals a protective function for JNK inhibition in preventing spine regression.

Role of cyclic nucleotides and their downstream signaling cascades in memory function: Being at the right time at the right spot

A plethora of studies indicate the important role of cAMP and cGMP cascades in neuronal plasticity and memory function. As a result, altered cyclic nucleotide signaling has been implicated in the pathophysiology of mnemonic dysfunction encountered in several diseases. In the present review we provide a wide overview of studies regarding the involvement of cyclic nucleotides, as well as their upstream and downstream molecules, in physiological and pathological mnemonic processes. Next, we discuss the regulation of the intracellular concentration of cyclic nucleotides via phosphodiesterases, the enzymes that degrade cAMP and/or cGMP, and via A-kinase-anchoring proteins that refine signal compartmentalization of cAMP signaling. We also provide an overview of the available data pointing to the existence of specific time windows in cyclic nucleotide signaling during neuroplasticity and memory formation and the significance to target these specific time phases for improving memory formation. Finally, we highlight the importance of emerging imaging tools like Förster resonance energy transfer imaging and optogenetics in detecting, measuring and manipulating the action of cyclic nucleotide signaling cascades.

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 Effects of Sindbis Viral Vectors on Neuronal Function

Viral vectors are attractive tools to express genes in neurons. Transduction of neurons with a recombinant, replication-deficient Sindbis viral vector is a method of choice for studying the effects of short-term protein overexpression on neuronal function. However, to which extent Sindbis by itself may affect neurons is not fully understood. We assessed effects of neuronal transduction with a Sindbis viral vector on the transcriptome and proteome in organotypic hippocampal slice cultures, and analyzed the electrophysiological properties of individual CA1 neurons, at 24 h and 72 h after viral vector injection. Whereas Sindbis caused substantial gene expression alterations, changes at the protein level were less pronounced. Alterations in transcriptome and proteome were predominantly limited to proteins involved in mediating anti-viral innate immune responses. Sindbis transduction did not affect the intrinsic electrophysiological properties of individual neurons: the membrane potential and neuronal excitability were similar between transduced and non-transduced CA1 neurons up to 72 h after Sindbis injection. Synaptic currents also remained unchanged upon Sindbis transduction, unless slices were massively infected for 72 h. We conclude that Sindbis viral vectors at low transduction rates are suitable for studying short-term effects of a protein of interest on electrophysiological properties of neurons, but not for studies on the regulation of gene expression.

AMPA receptors and their minions: auxiliary proteins in AMPA receptor trafficking

To correctly transfer information, neuronal networks need to continuously adjust their synaptic strength to extrinsic stimuli. This ability, termed synaptic plasticity, is at the heart of their function and is, thus, tightly regulated. In glutamatergic neurons, synaptic strength is controlled by the number and function of AMPA receptors at the postsynapse, which mediate most of the fast excitatory transmission in the central nervous system. Their trafficking to, at, and from the synapse, is, therefore, a key mechanism underlying synaptic plasticity. Intensive research over the last 20 years has revealed the increasing importance of interacting proteins, which accompany AMPA receptors throughout their lifetime and help to refine the temporal and spatial modulation of their trafficking and function. In this review, we discuss the current knowledge about the roles of key partners in regulating AMPA receptor trafficking and focus especially on the movement between the intracellular, extrasynaptic, and synaptic pools. We examine their involvement not only in basal synaptic function, but also in Hebbian and homeostatic plasticity. Included in our review are well-established AMPA receptor interactants such as GRIP1 and PICK1, the classical auxiliary subunits TARP and CNIH, and the newest additions to AMPA receptor native complexes.

Palmitoylation-mediated synaptic regulation of AMPA receptor trafficking and function

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) is a major glutamate-gated ion channel in the brain and is important for synaptic transmission, synaptic plasticity, and learning. Palmitoylation, a post-translational modification, is a critical process regulating AMPAR trafficking, synaptic function and plasticity, and learning and memory in health and diseases. In this review, we discuss current knowledge on the palmitoylation-dependent regulation of AMPAR trafficking and functions. We focus on the palmitoylation of AMPARs and other synaptic proteins that directly or indirectly interact with AMPARs, including postsynaptic density 95, glutamate receptor-interacting protein/AMPAR-binding protein, A-kinase anchoring protein 79/150, and protein interacting with C kinase 1. Finally, we discuss what future studies should address in the field of palmitoylation-dependent AMPAR trafficking and function with regard to physiology and neurodegenerative diseases.

Preferential generation of Ca2+-permeable AMPA receptors by AKAP79-anchored protein kinase C proceeds via GluA1 subunit phosphorylation at Ser-831

AMPA-type glutamate receptors (AMPARs) mediate fast excitatory neurotransmission in the mammalian central nervous system. Preferential AMPAR subunit assembly favors heteromeric GluA1/GluA2 complexes. The presence of the GluA2 subunit generates Ca²⁺-impermeable (CI) AMPARs that have linear current-voltage (I-V) relationships. However, diverse forms of synaptic plasticity and pathophysiological conditions are associated with shifts from CI to inwardly rectifying, GluA2-lacking, Ca²⁺-permeable (CP) AMPARs on time scales ranging from minutes to days. These shifts have been linked to GluA1 phosphorylation at Ser-845, a protein kinase A (PKA)-targeted site within its intracellular C-terminal tail, often in conjunction with protein kinase A anchoring protein 79 (AKAP79; AKAP150 in rodents), which targets PKA to GluA1. However, AKAP79 may impact GluA1 phosphorylation at other sites by interacting with other signaling enzymes. Here, we evaluated the ability of AKAP79, its signaling components, and GluA1 phosphorylation sites to induce CP-AMPARs under conditions in which CI-AMPARs normally predominate. We found that GluA1 phosphorylation at Ser-831 is sufficient for the appearance of CP-AMPARs and that AKAP79-anchored protein kinase C (PKC) primarily drives the appearance of these receptors via this site. In contrast, other AKAP79-signaling components and C-terminal tail GluA1 phosphorylation sites exhibited a permissive role, limiting the extent to which AKAP79 promotes CP-AMPARs. This may reflect the need for these sites to undergo active phosphorylation/dephosphorylation cycles that control their residency within distinct subcellular compartments. These findings suggest that AKAP79, by orchestrating phosphorylation, represents a key to a GluA1 phosphorylation passcode, which allows the GluA1 subunit to escape GluA2 dominance and promote the appearance of CP-AMPARs.

Phosphorylation Induces Conformational Rigidity at the C-Terminal Domain of AMPA Receptors

The intracellular C-terminal domain (CTD) of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor undergoes phosphorylation at specific locations during long-term potentiation. This modification enhances conductance through the AMPA receptor ion channel and thus potentially plays a crucial role in modulating receptor trafficking and signaling. However, because the CTD structure is largely unresolved, it is difficult to establish if phosphorylation induces conformational changes that might play a role in enhancing channel conductance. Herein, we utilize single-molecule Förster resonance energy transfer (smFRET) spectroscopy to probe the conformational changes of a section of the AMPA receptor CTD, under the conditions of point-mutated phosphomimicry. Multiple analysis algorithms fail to identify stable conformational states within the smFRET distributions, consistent with a lack of well-defined secondary structure. Instead, our results show that phosphomimicry induces conformational rigidity to the CTD, and such rigidity is electrostatically tunable.

Polyamine-mediated channel block of ionotropic glutamate receptors and its regulation by auxiliary proteins

Most excitatory neurotransmission in the mammalian brain is mediated by a family of plasma membrane-bound signaling proteins called ionotropic glutamate receptors (iGluRs). iGluRs assemble at central synapses as tetramers, forming a central ion-channel pore whose primary function is to rapidly transport Na⁺ and Ca²⁺ in response to binding the neurotransmitter l-glutamic acid. The pore of iGluRs is also accessible to bulkier cytoplasmic cations, such as the polyamines spermine, spermidine, and putrescine, which are drawn into the permeation pathway, but get stuck and block the movement of other ions. The degree of this polyamine-mediated channel block is highly regulated by processes that control the free cytoplasmic polyamine concentration, the membrane potential, or the iGluR subunit composition. Recently, an additional regulation by auxiliary proteins, most notably transmembrane AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor regulatory proteins (TARPs), cornichons, and neuropilin and tolloid-like proteins (NETOs), has been identified. Here, I review what we have learned of polyamine block of iGluRs and its regulation by auxiliary subunits. TARPs, cornichons, and NETOs attenuate the channel block by enabling polyamines to exit the pore. As a result, polyamine permeation occurs at more negative and physiologically relevant membrane potentials. The structural basis for enhanced polyamine transport remains unresolved, although alterations in both channel architecture and charge-screening mechanisms have been proposed. That auxiliary subunits can attenuate the polyamine block reveals an unappreciated impact of polyamine permeation in shaping the signaling properties of neuronal AMPA- and kainate-type iGluRs. Moreover, enhanced polyamine transport through iGluRs may have a role in regulating cellular polyamine levels.

Relationship Between Synaptic AMPAR and Spine Dynamics: Impairments in the FXS Mouse

Structural dynamics of dendritic spines are important for memory and learning and are impaired in neurodevelopmental disorders such as fragile X syndrome. Spine dynamics are regulated by activity-dependent mechanisms that involve modulation of AMPA receptors (AMPAR); however, the relationship between AMPAR and spine dynamics in vivo and how these are altered in FXS mouse model is not known. Here, we tracked AMPAR and spines over multiple days in vivo in the cortex and found that dendritic spines in the fmr1 KO mouse were denser, smaller, had higher turnover rates and contained less sGluA2 compared to littermate controls. Although, KO spines maintained the relationship between AMPAR and spine stability, AMPAR levels in the KO were more dynamic with larger proportion of spines showing multiple dynamic events of AMPAR. Directional changes in sGluA2 were also observed in newly formed and eliminated spines, with KO spines displaying greater loss of AMPAR before elimination. Thus, we demonstrate that AMPAR levels within spines not are only continuously dynamic, but are also predictive of spine behavior, with impairments observed in the fmr1 KO mice.

Self-crowding of AMPA receptors in the excitatory postsynaptic density can effectuate anomalous receptor sub-diffusion

AMPA receptors (AMPARs) and their associations with auxiliary transmembrane proteins are bulky structures with large steric-exclusion volumes. Hence, self-crowding of AMPARs, depending on the local density, may affect their lateral diffusion in the postsynaptic membrane as well as in the highly crowded postsynaptic density (PSD) at excitatory synapses. Earlier theoretical studies considered only the roles of transmembrane obstacles and the AMPAR-binding submembranous scaffold proteins in shaping receptor diffusion within PSD. Using lattice model of diffusion, the present study investigates the additional impacts of self-crowding on the anomalousity and effective diffusion coefficient (D_eff) of AMPAR diffusion. A recursive algorithm for avoiding false self-blocking during diffusion simulation is also proposed. The findings suggest that high density of AMPARs in the obstacle-free membrane itself engenders strongly anomalous diffusion and severe decline in D_eff. Adding transmembrane obstacles to the membrane accentuates the anomalousity arising from self-crowding due to the reduced free diffusion space. Contrarily, enhanced AMPAR-scaffold binding, either through increase in binding strength or scaffold density or both, ameliorates the anomalousity resulting from self-crowding. However, binding has differential impacts on D_eff depending on the receptor density. Increase in binding causes consistent decrease in D_eff for low and moderate receptor density. For high density, binding increases D_eff as long as it reduces anomalousity associated with intense self-crowding. Given a sufficiently strong binding condition when diffusion acquires normal behavior, further increase in binding causes decrease in D_eff. Supporting earlier experimental observations are mentioned and implications of present findings to the experimental observations on AMPAR diffusion are also drawn.

The transmembrane AMPA receptors (AMPARs) prominently exhibit lateral diffusion in the postsynaptic membrane at excitatory synapses. Steric obstructions to AMPAR diffusion due to the crowd of other relatively static transmembrane proteins and binding of AMPARs to the submembranous scaffold proteins in the specialized region of postsynaptic density (PSD) are well known to retard receptor diffusion, which causes receptor trapping and accumulation within PSD. However, AMPARs are significantly bulky structures and may also obstruct their own diffusion paths in the presence of their high density. It is shown here that intense self-crowding of AMPARs may lead to highly obstructed and confined receptor diffusion even in the obstacle-free medium, and the presence of other obstacles further aggravates this effect. AMPAR-scaffold binding reduces confined diffusion arising from self-crowding and strong binding engenders normal diffusion even at high receptor density. However, it overall causes reduction in the effective diffusion coefficient of the receptor diffusion.

R/G editing in GluA2Rflop modulates the functional difference between GluA1 flip and flop variants in GluA1/2R heteromeric channels

In α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors, RNA editing and alternative splicing generate sequence variants, and those variants, as in GluA2-4 AMPA receptor subunits, generally show different properties. Yet, earlier studies have shown that the alternatively spliced, flip and flop variants of GluA1 AMPA receptor subunit exhibit no functional difference in homomeric channel form. Using a laser-pulse photolysis technique, combined with whole-cell recording, we measured the rate of channel opening, among other kinetic properties, for a series of AMPA channels with different arginine/glycine (R/G) editing and flip/flop status. We find that R/G editing in the GluA2 subunit modulates the channel properties in both homomeric (GluA2Q) and complex (GluA2Q/2R and GluA1/2R) channel forms. However, R/G editing is only effective in flop channels. Specifically, editing at the R/G site on the GluA2R flop isoform accelerates the rate of channel opening and desensitization for GluA1/2R channels more pronouncedly with the GluA1 being in the flop form than in the flip form; yet R/G editing has no effect on either channel-closing rate or EC₅₀. Our results suggest R/G editing via GluA2R serve as a regulatory mechanism to modulate the function of GluA2R-containing, native receptors involved in fast excitatory synaptic transmission.

Synaptic plasticity through activation of GluA3-containing AMPA-receptors

Excitatory synaptic transmission is mediated by AMPA-type glutamate receptors (AMPARs). In CA1 pyramidal neurons of the hippocampus two types of AMPARs predominate: those that contain subunits GluA1 and GluA2 (GluA1/2), and those that contain GluA2 and GluA3 (GluA2/3). Whereas subunits GluA1 and GluA2 have been extensively studied, the contribution of GluA3 to synapse physiology has remained unclear. Here we show in mice that GluA2/3s are in a low-conductance state under basal conditions, and although present at synapses they contribute little to synaptic currents. When intracellular cyclic AMP (cAMP) levels rise, GluA2/3 channels shift to a high-conductance state, leading to synaptic potentiation. This cAMP-driven synaptic potentiation requires the activation of both protein kinase A (PKA) and the GTPase Ras, and is induced upon the activation of β-adrenergic receptors. Together, these experiments reveal a novel type of plasticity at CA1 hippocampal synapses that is expressed by the activation of GluA3-containing AMPARs.

GluA1 subunit ubiquitination mediates amyloid-β-induced loss of surface α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors

The accumulation of soluble amyloid-β (Aβ) peptides produces profound neuronal changes in the brain during the pathogenesis of Alzheimer's disease. Excessive levels of Aβ disrupt excitatory synaptic transmission by promoting the removal of synaptic AMPA receptors (AMPARs), dendritic spine loss, and synaptic depression. Recently, activity-dependent ubiquitination of the GluA1 subunit has been shown to regulate the intracellular sorting of AMPARs toward late endosomes for degradation. However, whether this ubiquitin signaling pathway mediates Aβ-induced loss of surface AMPARs is unknown. In this study, we demonstrate that acute exposure of cultured neurons to soluble Aβ oligomers induces AMPAR ubiquitination concomitant with the removal of AMPARs from the plasma membrane. Importantly, expression of the GluA1 ubiquitin-deficient mutants inhibited the adverse effects of Aβ on the surface expression of AMPARs in neurons. Furthermore, we revealed the cross-talk between GluA1 ubiquitination and phosphorylation, in particular phosphorylation at Ser-845, which is crucial for AMPAR recycling and is known to be dephosphorylated in the presence of Aβ. Our data showed that the GluA1 ubiquitin-deficient mutant enhances GluA1 phosphorylation on Ser-845. Conversely, the GluA1 S845D phosphomimetic mutant reduced binding with Nedd4-1 and hence the ubiquitination of AMPARs. Importantly, the GluA1 S845D mutant also prevented Aβ-induced removal of surface AMPARs. Taken together, these findings provide the first demonstration of the dynamic cross-modulation of GluA1 ubiquitination and phosphorylation, a process that is perturbed by Aβ, in regulating the membrane sorting decision that ultimately determines the number of AMPARs on the cell surface.

Subunit-specific synaptic delivery of AMPA receptors by auxiliary chaperone proteins TARPγ8 and GSG1L in classical conditioning

AMPA receptor (AMPAR) trafficking has emerged as a fundamental concept for understanding mechanisms of learning and memory as well as many neurological disorders. Classical conditioning is a simple and highly conserved form of associative learning. Our studies use an ex vivo brainstem preparation in which to study cellular mechanisms underlying learning during a neural correlate of eyeblink conditioning. Two stages of AMPAR synaptic delivery underlie conditioning utilizing sequential trafficking of GluA1-containing AMPARs early in conditioning followed by replacement with GluA4 subunits later. Subunit-selective trafficking of AMPARs is poorly understood. Here, we focused on identification of auxiliary chaperone proteins that traffic AMPARs. The results show that auxiliary proteins TARPγ8 and GSG1L are colocalized with AMPARs on abducens motor neurons that generate the conditioning. Significantly, TARPγ8 was observed to chaperone GluA1-containing AMPARs during synaptic delivery early in conditioning while GSG1L chaperones GluA4 subunits later in conditioning. Interestingly, TARPγ8 remains at the membrane surface as GluA1 subunits are withdrawn and associates with GluA4 when they are delivered to synapses. These data indicate that GluA1- and GluA4-containing AMPARs are selectively chaperoned by TARPγ8 and GSG1L, respectively. Therefore, sequential subunit-selective trafficking of AMPARs during conditioning is achieved through the timing of their interactions with specific auxiliary proteins.

GSG1L regulates the strength of AMPA receptor-mediated synaptic transmission but not AMPA receptor kinetics in hippocampal dentate granule neurons

GSG1L is an AMPA receptor (AMPAR) auxiliary subunit that regulates AMPAR trafficking and function in hippocampal CA1 pyramidal neurons. However, its physiological roles in other types of neurons remain to be characterized. Here, we investigated the role of GSG1L in hippocampal dentate granule cells and found that GSG1L is important for the regulation of synaptic strength but is not critical for the modulation of AMPAR deactivation and desensitization kinetics. These data demonstrate a neuronal type-specific role of GSG1L and suggest that physiological function of AMPAR auxiliary subunits may vary in different types of neurons.

NEW & NOTEWORTHY

GSG1L is a newly identified AMPA receptor (AMPAR) auxiliary subunit and plays a unique role in the regulation of AMPAR trafficking and function in hippocampal CA1 pyramidal neurons. However, its role in the regulation of AMPARs in hippocampal dentate granule cells remains to be characterized. The current work reveals that GSG1L regulates strength of AMPAR-mediated synaptic transmission but not the receptor kinetic properties in hippocampal dentate granule neurons.

Amyloid-β effects on synapses and memory require AMPA receptor subunit GluA3

Amyloid-β (Aβ) is a prime suspect for causing cognitive deficits during the early phases of Alzheimer's disease (AD). Experiments in AD mouse models have shown that soluble oligomeric clusters of Aβ degrade synapses and impair memory formation. We show that all Aβ-driven effects measured in these mice depend on AMPA receptor (AMPAR) subunit GluA3. Hippocampal neurons that lack GluA3 were resistant against Aβ-mediated synaptic depression and spine loss. In addition, Aβ oligomers blocked long-term synaptic potentiation only in neurons that expressed GluA3. Furthermore, although Aβ-overproducing mice showed significant memory impairment, memories in GluA3-deficient congenics remained unaffected. These experiments indicate that the presence of GluA3-containing AMPARs is critical for Aβ-mediated synaptic and cognitive deficits.

TARP γ-2 and γ-8 Differentially Control AMPAR Density Across Schaffer Collateral/Commissural Synapses in the Hippocampal CA1 Area

The number of AMPA-type glutamate receptors (AMPARs) at synapses is the major determinant of synaptic strength and varies from synapse to synapse. To clarify the underlying molecular mechanisms, the density of AMPARs, PSD-95, and transmembrane AMPAR regulatory proteins (TARPs) were compared at Schaffer collateral/commissural (SCC) synapses in the adult mouse hippocampal CA1 by quantitative immunogold electron microscopy using serial sections. We examined four types of SCC synapses: perforated and nonperforated synapses on pyramidal cells and axodendritic synapses on parvalbumin-positive (PV synapse) and pravalbumin-negative interneurons (non-PV synapse). SCC synapses were categorized into those expressing high-density (perforated and PV synapses) or low-density (nonperforated and non-PV synapses) AMPARs. Although the density of PSD-95 labeling was fairly constant, the density and composition of TARP isoforms was highly variable depending on the synapse type. Of the three TARPs expressed in hippocampal neurons, the disparity in TARP γ-2 labeling was closely related to that of AMPAR labeling. Importantly, AMPAR density was significantly reduced at perforated and PV synapses in TARP γ-2-knock-out (KO) mice, resulting in a virtual loss of AMPAR disparity among SCC synapses. In comparison, TARP γ-8 was the only TARP expressed at nonperforated synapses, where AMPAR labeling further decreased to a background level in TARP γ-8-KO mice. These results show that synaptic inclusion of TARP γ-2 potently increases AMPAR expression and transforms low-density synapses into high-density ones, whereas TARP γ-8 is essential for low-density or basal expression of AMPARs at nonperforated synapses. Therefore, these TARPs are critically involved in AMPAR density control at SCC synapses.

SIGNIFICANCE STATEMENT

Although converging evidence implicates the importance of transmembrane AMPA-type glutamate receptor (AMPAR) regulatory proteins (TARPs) in AMPAR stabilization during basal transmission and synaptic plasticity, how they control large disparities in AMPAR numbers or densities across central synapses remains largely unknown. We compared the density of AMPARs with that of TARPs among four types of Schaffer collateral/commissural (SCC) hippocampal synapses in wild-type and TARP-knock-out mice. We show that the density of AMPARs correlates with that of TARP γ-2 across SCC synapses and its high expression is linked to high-density AMPAR expression at perforated type of pyramidal cell synapses and synapses on parvalbumin-positive interneurons. In comparison, TARP γ-8 is the only TARP expressed at nonperforated type of pyramidal cell synapses, playing an essential role in low-density or basal AMPAR expression.

Synaptic AMPA receptor composition in development, plasticity and disease

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

GSG1L suppresses AMPA receptor-mediated synaptic transmission and uniquely modulates AMPA receptor kinetics in hippocampal neurons

Regulation of AMPA receptor (AMPAR)-mediated synaptic transmission is a key mechanism for synaptic plasticity. In the brain, AMPARs assemble with a number of auxiliary subunits, including TARPs, CNIHs and CKAMP44, which are important for AMPAR forward trafficking to synapses. Here we report that the membrane protein GSG1L negatively regulates AMPAR-mediated synaptic transmission. Overexpression of GSG1L strongly suppresses, and GSG1L knockout (KO) enhances, AMPAR-mediated synaptic transmission. GSG1L-dependent regulation of AMPAR synaptic transmission relies on the first extracellular loop domain and its carboxyl-terminus. GSG1L also speeds up AMPAR deactivation and desensitization in hippocampal CA1 neurons, in contrast to the effects of TARPs and CNIHs. Furthermore, GSG1L association with AMPARs inhibits CNIH2-induced slowing of the receptors in heterologous cells. Finally, GSG1L KO rats have deficits in LTP and show behavioural abnormalities in object recognition tests. These data demonstrate that GSG1L represents a new class of auxiliary subunit with distinct functional properties for AMPARs.

The molecular mechanism controlling the trafficking and function of AMPARs at synapses are not fully understood. Here the authors show that GSG1L, a membrane protein, negatively regulates AMPAR-mediated synaptic transmission and represents a new class of AMPAR auxiliary subunit.

Ubiquitin-dependent trafficking and turnover of ionotropic glutamate receptors

Changes in synaptic strength underlie the basis of learning and memory and are controlled, in part, by the insertion or removal of AMPA-type glutamate receptors at the postsynaptic membrane of excitatory synapses. Once internalized, these receptors may be recycled back to the plasma membrane by subunit-specific interactions with other proteins or by post-translational modifications such as phosphorylation. Alternatively, these receptors may be targeted for destruction by multiple degradation pathways in the cell. Ubiquitination, another post-translational modification, has recently emerged as a key signal that regulates the recycling and trafficking of glutamate receptors. In this review, we will discuss recent findings on the role of ubiquitination in the trafficking and turnover of ionotropic glutamate receptors and plasticity of excitatory synapses.

Glutamatergic synapse protein composition of wild-type mice is sensitive to in utero MTHFR genotype and the timing of neonatal vigabatrin exposure

The enzyme methylenetetrahydrofolate-reductase (MTHFR) is part of the homocysteine and folate metabolic pathways. In utero, Mthfr-deficient environment has been reported as a risk factor for neurodevelopmental disorders such as autism and neural tube defects. Neonatal disruption of the GABAergic system is also associated with behavioral outcomes. The interaction between Mthfr deficiency and neonatal exposure to the GABA-potentiating drug vigabatrin (GVG) in mice alters anxiety, memory, and social behavior in a gender-dependent manner. In addition, a gender-dependent enhancement of proteins implicated in excitatory synapse plasticity in the cerebral cortex was shown. Here we show that in utero MTHFR deficiency is sufficient to alter the levels of glutamate receptor subunits GluR1, GluR2, and NR2B in the cerebral cortex and hippocampus of adult offspring with a WT genotype. In addition, FMRP1, CAMKII α and γ, and NLG1 levels in WT offspring were vulnerable to the in utero genotype. These effects depend on brain region and the cellular compartment tested. The effect of in utero MTHFR deficiency varies with the age of neonatal GVG exposure to modify GluR1, NR2A, reelin, CAMKII α, and NLG1 levels. These changes in molecular composition of the glutamatergic synapse were associated with increased anxiety-like behavior. Complex, multifactorial disorders of the nervous system show significant association with several genetic and environmental factors. Our data exemplify the contribution of an in utero MTHFR-deficient environment and early exposure to an antiepileptic drug to the basal composition of the glutamatergic synapses. The robust effect is expected to alter synapse function and plasticity and the cortico-hippocampal circuitry.

miR-501-3p mediates the activity-dependent regulation of the expression of AMPA receptor subunit GluA1

The number of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) in synapses determines synaptic strength. AMPAR expression can be regulated locally in dendrites by synaptic activity. The mechanisms of activity-dependent local regulation of AMPAR expression, however, remain unclear. Here, we tested whether microRNAs (miRNAs) are involved in N-methyl-D-aspartate (NMDA) receptor (NMDAR)-dependent AMPAR expression. We used the 3' untranslated region of Gria1, which encodes the AMPA receptor subunit GluA1, to pull down miRNAs binding to it and analyzed these miRNAs using next-generation deep sequencing. Among the identified miRNAs, miR-501-3p is also a computationally predicted Gria1-targeting miRNA. We confirmed that miR-501-3p targets Gria1 and regulates its expression under physiological conditions. The expression of miR-501-3p and GluA1, moreover, is inversely correlated during postnatal brain development. miR-501-3p expression is up-regulated locally in dendrites through the NMDAR subunit GluN2A, and this regulation is required for NMDA-induced suppression of GluA1 expression and long-lasting remodeling of dendritic spines. These findings elucidate a miRNA-mediated mechanism for activity-dependent, local regulation of AMPAR expression in dendrites.

Down-regulation of Stargazin inhibits the enhanced surface delivery of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor GluR1 subunit in rat dorsal horn and ameliorates postoperative pain

BACKGROUND

Stargazin is the first transmembrane protein known to regulate synaptic targeting of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors. However, it is unclear whether regulation of the surface delivery of spinal AMPA receptor subunits by stargazin contributes to postoperative pain development.

METHODS

Western blot analysis was used to examine changes in the surface delivery of AMPA receptor subunits, GluR1 and GluR2, in rat dorsal horn. The interaction between stargazin and GluR1 and GluR2 was examined by coimmunoprecipitation. Expression of stargazin was suppressed by intrathecal administration of small interfering RNA311.

RESULTS

Membrane-bound GluR1, but not GluR2, in ipsilateral dorsal horn was increased at 3 h (1.49 ± 0.15-fold of β-tubulin, mean ± SEM) and 1 day (1.03 ± 0.25) after incision, as compared with that in control rats (naive, 0.63 ± 0.23, P < 0.05, n = 6 per group). The amount of GluR1 coimmunoprecipitated with stargazin was greater at 3 h after incision (1.48 ± 0.31-fold of input) than that in control animals (0.45 ± 0.24, P < 0.05, n = 6 per group). Importantly, the increase in membrane GluR1 at 3 h after incision was normalized to near control level (0.72 ± 0.20-fold of β-tubulin) by pretreatment with intrathecal stargazin small interfering RNA311 (0.87 ± 0.09), but not scrambled small interfering RNA (1.48 ± 0.24) or vehicle (1.25 ± 0.13, P < 0.05, n = 6 per group). Stargazin small interfering RNA311 pretreatment prevented the increase in stargazin-GluR1 interaction and decreased postoperative pain after incision.

CONCLUSIONS

This study suggests a critical role of stargazin-mediated surface delivery of GluR1 subunit in the development of postoperative pain. A better therapeutic strategy for postoperative pain may involve selectively down-regulating spinal stargazin to inhibit synaptic targeting of GluR1 subunit.

Auxiliary subunits: shepherding AMPA receptors to the plasma membrane

Ionotropic glutamate receptors (iGluRs) are tetrameric ligand-gated cation channels that mediate excitatory signal transmission in the central nervous system (CNS) of vertebrates. The members of the iGluR subfamily of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) mediate most of the fast excitatory signal transmission, and their abundance in the postsynaptic membrane is a major determinant of the strength of excitatory synapses. Therefore, regulation of AMPAR trafficking to the postsynaptic membrane is an important constituent of mechanisms involved in learning and memory formation, such as long-term potentiation (LTP) and long-term depression (LTD). Auxiliary subunits play a critical role in the facilitation and regulation of AMPAR trafficking and function. The currently identified auxiliary subunits of AMPARs are transmembrane AMPA receptor regulatory proteins (TARPs), suppressor of lurcher (SOL), cornichon homologues (CNIHs), synapse differentiation-induced gene I (SynDIG I), cysteine-knot AMPAR modulating proteins 44 (CKAMP44), and germ cell-specific gene 1-like (GSG1L) protein. In this review we summarize our current knowledge of the modulatory influence exerted by these important but still underappreciated proteins.

Inhibitory interneuron classes express complementary AMPA-receptor patterns in macaque primary visual cortex

Glutamate receptors mediate excitatory neurotransmission. A very prevalent type of glutamate receptor in the neocortex is the AMPA receptor (AMPAR). AMPARs mediate fast synaptic transmission and their functionality depends on the subunit composition. In primary visual cortex (area V1), the density and subunit composition of AMPARs differ among cortical layers and among cell types. The AMPARs expressed by the different types of inhibitory interneurons, which are crucial for network function, have not yet been characterized systematically. We investigated the distribution of AMPAR subunits in macaque V1 for three distinct subpopulations of inhibitory interneurons: parvalbumin-immunoreactive (PV-IR) interneurons, calbindin-immunoreactive (CB-IR) interneurons, and calretinin-immunoreactive (CR-IR) interneurons. We found that PV-IR cells, which have previously been identified as fast spiking, show high expression of the GluA2 and GluA3 subunits. In contrast, CB-IR and CR-IR cells, which tend to be intermediate spiking, show high expression of the GluA1 and GluA4 subunits. Thus, our data demonstrate that the expression of AMPARs divides inhibitory interneurons in macaque V1 into two categories that are compatible with existing classification methods based on calcium-binding proteins and firing behavior. Moreover, our findings suggest new approaches to target the different inhibitory interneuron classes pharmacologically in vivo.

ABA renewal involves enhancements in both GluA2-lacking AMPA receptor activity and GluA1 phosphorylation in the lateral amygdala

Fear renewal, the context-specific relapse of fear following fear extinction, is a leading animal model of post-traumatic stress disorders (PTSD) and fear-related disorders. Although fear extinction can diminish fear responses, this effect is restricted to the context where the extinction is carried out, and the extinguished fear strongly relapses when assessed in the original acquisition context (ABA renewal) or in a context distinct from the conditioning and extinction contexts (ABC renewal). We have previously identified Ser831 phosphorylation of GluA1 subunit in the lateral amygdala (LA) as a key molecular mechanism for ABC renewal. However, molecular mechanisms underlying ABA renewal remain to be elucidated. Here, we found that both the excitatory synaptic efficacy and GluA2-lacking AMPAR activity at thalamic input synapses onto the LA (T-LA synapses) were enhanced upon ABA renewal. GluA2-lacking AMPAR activity was also increased during low-threshold potentiation, a potential cellular substrate of renewal, at T-LA synapses. The microinjection of 1-naphtylacetyl-spermine (NASPM), a selective blocker of GluA2-lacking AMPARs, into the LA attenuated ABA renewal, suggesting a critical role of GluA2-lacking AMPARs in ABA renewal. We also found that Ser831 phosphorylation of GluA1 in the LA was increased upon ABA renewal. We developed a short peptide mimicking the Ser831-containing C-tail region of GluA1, which can be phosphorylated upon renewal (GluA1S); thus, the phosphorylated GluA1S may compete with Ser831-phosphorylated GluA1. This GluA1S peptide blocked the low-threshold potentiation when dialyzed into a recorded neuron. The microinjection of a cell-permeable form of GluA1S peptide into the LA attenuated ABA renewal. In support of the GluA1S experiments, a GluA1D peptide (in which the serine at 831 is replaced with a phosphomimetic amino acid, aspartate) attenuated ABA renewal when microinjected into the LA. These findings suggest that enhancements in both the GluA2-lacking AMPAR activity and GluA1 phosphorylation at Ser831 are required for ABA renewal.

The NMDA receptor complex: a multifunctional machine at the glutamatergic synapse

The N-methyl-D-aspartate receptors (NMDARs) are part of a large multiprotein complex at the glutamatergic synapse. The assembly of NMDARs with synaptic proteins offers a means to regulate NMDAR channel properties and receptor trafficking, and couples NMDAR activation to distinct intracellular signaling pathways, thus contributing to the versatility of NMDAR functions. Receptor-protein interactions at the synapse provide a dynamic and powerful mechanism for regulating synaptic efficacy, but can also contribute to NMDAR overactivation-induced excitotoxicity and cellular damage under pathological conditions. An emerging concept is that by understanding the mechanisms and functions of disease-specific protein-protein interactions in the NMDAR complex, we may be able to develop novel therapies based on protein-NMDAR interactions for the treatment of brain diseases in which NMDAR dysfunction is at the root of their pathogenesis.

The relationship between PSD-95 clustering and spine stability in vivo

The appearance and disappearance of dendritic spines, accompanied by synapse formation and elimination may underlie the experience-dependent reorganization of cortical circuits. The exact temporal relationship between spine and synapse formation in vivo remains unclear, as does the extent to which synapse formation enhances the stability of newly formed spines and whether transient spines produce synapses. We used in utero electroporation of DsRedExpress- and eGFP-tagged postsynaptic density protein 95 (PSD-95) to investigate the relationship between spine and PSD stability in mouse neocortical L2/3 pyramidal cells in vivo. Similar to previous studies, spines and synapses appeared and disappeared, even in naive animals. Cytosolic spine volumes and PSD-95-eGFP levels in spines covaried over time, suggesting that the strength of many individual synapses continuously changes in the adult neocortex. The minority of newly formed spines acquired PSD-95-eGFP puncta. Spines that failed to acquire a PSD rarely survived for more than a day. Although PSD-95-eGFP accumulation was associated with increased spine lifetimes, most new spines with a PSD did not convert into persistent spines. This indicates that transient spines may serve to produce short-lived synaptic contacts. Persistent spines that were destined to disappear showed, on average, reduced PSD-95-eGFP levels well before the actual pruning event. Altogether, our data indicate that the PSD size relates to spine stability in vivo.

Sequential delivery of synaptic GluA1- and GluA4-containing AMPA receptors (AMPARs) by SAP97 anchored protein complexes in classical conditioning

Multiple signaling pathways are involved in AMPAR trafficking to synapses during synaptic plasticity and learning. The mechanisms for how these pathways are coordinated in parallel but maintain their functional specificity involves subcellular compartmentalization of kinase function by scaffolding proteins, but how this is accomplished is not well understood. Here, we focused on characterizing the molecular machinery that functions in the sequential synaptic delivery of GluA1- and GluA4-containing AMPARs using an in vitro model of eyeblink classical conditioning. We show that conditioning induces the interaction of selective protein complexes with the key structural protein SAP97, which tightly regulates the synaptic delivery of GluA1 and GluA4 AMPAR subunits. The results demonstrate that in the early stages of conditioning the initial activation of PKA stimulates the formation of a SAP97-AKAP/PKA-GluA1 protein complex leading to synaptic delivery of GluA1-containing AMPARs through a SAP97-PSD95 interaction. This is followed shortly thereafter by generation of a SAP97-KSR1/PKC-GluA4 complex for GluA4 AMPAR subunit delivery again through a SAP97-PSD95 interaction. These data suggest that SAP97 forms the molecular backbone of a protein scaffold critical for delivery of AMPARs to the PSD during conditioning. Together, the findings reveal a cooperative interaction of multiple scaffolding proteins for appropriately timed delivery of subunit-specific AMPARs to synapses and support a sequential two-stage model of AMPAR synaptic delivery during classical conditioning.

The developmental stages of synaptic plasticity

The brain is programmed to drive behaviour by precisely wiring the appropriate neuronal circuits. Wiring and rewiring of neuronal circuits largely depends on the orchestrated changes in the strengths of synaptic contacts. Here, we review how the rules of synaptic plasticity change during development of the brain, from birth to independence. We focus on the changes that occur at the postsynaptic side of excitatory glutamatergic synapses in the rodent hippocampus and neocortex. First we summarize the current data on the structure of synapses and the developmental expression patterns of the key molecular players of synaptic plasticity, N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, as well as pivotal kinases (Ca(2+)/calmodulin-dependent protein kinase II, protein kinase A, protein kinase C) and phosphatases (PP1, PP2A, PP2B). In the second part we relate these findings to important characteristics of the emerging network. We argue that the concerted and gradual shifts in the usage of plasticity molecules comply with the changing need for (re)wiring neuronal circuits.

The expression of long-term potentiation: reconciling the preists and the postivists

Long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus has been investigated in great detail over the past 40 years. Where and how LTP is actually expressed, however, remain controversial issues. Considerable evidence has been offered to support both pre- and postsynaptic contributions to LTP expression. Though it is widely held that postsynaptic expression mechanisms are the primary contributors to LTP expression, evidence for that conclusion is amenable to alternative explanations. Here, we briefly review some key contributions to the 'locus' debate and describe data that support a dominant role for presynaptic mechanisms. Recognition of the state-dependency of expression mechanisms, and consideration of the consequences of the spatial relationship between postsynaptic glutamate receptors and presynaptic vesicular release sites, lead to a model that may reconcile views from both sides of the synapse.

AMPAR trafficking in synapse maturation and plasticity

Glutamate ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) mediate most fast excitatory synaptic transmission in the central nervous system. The content and composition of AMPARs in postsynaptic membranes (which determine synaptic strength) are dependent on the regulated trafficking of AMPAR subunits in and out of the membranes. AMPAR trafficking is a key mechanism that drives nascent synapse development, and is the main determinant of both Hebbian and homeostatic plasticity in mature synapses. Hebbian plasticity seems to be the biological substrate of at least some forms of learning and memory; while homeostatic plasticity (also known as synaptic scaling) keeps neuronal circuits stable by maintaining changes within a physiological range. In this review, we examine recent findings that provide further understanding of the role of AMPAR trafficking in synapse maturation, Hebbian plasticity, and homeostatic plasticity.

How to scale down postsynaptic strength

Synaptic scaling is a form of synaptic plasticity that contributes to the homeostatic regulation of neuronal activity both in vitro and in vivo, by bidirectionally and proportionally adjusting postsynaptic AMPA receptor (AMPAR) abundance to compensate for chronic perturbations in activity. This proportional regulation of synaptic strength allows synaptic scaling to normalize activity without disrupting the synapse-specific differences in strength thought to underlie memory storage, but how such proportional scaling of synaptic strength is accomplished at the biophysical level is unknown. Here we addressed this question in cultured rat visual cortical pyramidal neurons. We used photoactivation and fluorescence recovery after photobleaching of fluorescently tagged AMPAR to show that scaling down, but not up, decreases the steady-state accumulation of synaptic AMPAR by increasing the rate at which they unbind from and exit the postsynaptic density (Koff). This increase in Koff was not diffusion limited, was independent of AMPAR endocytosis, and was prevented by a scaffold manipulation that specifically blocks scaling down, suggesting that it is accomplished through enhanced dissociation of AMPAR from synaptic scaffold tethers. Finally, simulations show that increasing Koff decreases synaptic strength multiplicatively, by reducing the fractional occupancy of available scaffold "slots." These data demonstrate that scaling down is accomplished through a regulated increase in Koff, which in turn reduces the fractional occupancy of synaptic scaffolds to proportionally reduce synaptic strength.

Exercise modifies α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor expression in striatopallidal neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned mouse

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic-acid-type glutamate receptor (AMPAR) plays a critical role in modulating experience-dependent neuroplasticity, and alterations in AMPAR expression may underlie synaptic dysfunction and disease pathophysiology. Using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of dopamine (DA) depletion, our previous work showed exercise increases total GluA2 subunit expression and the contribution of GluA2-containing channels in MPTP mice. The purpose of this study was to determine whether exercise-dependent changes in AMPAR expression after MPTP are specific to the striatopallidal (D2 R) or striatonigral (D1 R) medium spiny neuron (MSN) striatal projection pathways. Drd2 -eGFP-BAC transgenic mice were used to delineate differences in AMPAR expression between striatal D2 R-MSNs and D1 R-MSNs. Striatal AMPAR expression was assessed by immunohistochemical (IHC) staining, Western immunoblotting (WB) of preparations enriched for postsynaptic density (PSD), and alterations in the current-voltage relationship of MSNs. We found DA depletion results in the emergence of GluA2-lacking AMPARs selectively in striatopallidal D2 R-MSNs and that exercise reverses this effect in MPTP mice. Exercise-induced changes in AMPAR channels observed after DA depletion were associated with alterations in GluA1 and GluA2 subunit expression in postsynaptic protein, D2 R-MSN cell surface expression, and restoration of corticostriatal plasticity. Mechanisms regulating experience-dependent changes in AMPAR expression may provide innovative therapeutic targets to increase the efficacy of treatments for basal ganglia disorders, including Parkinson's disease.

Regulation of AMPA receptor surface trafficking and synaptic plasticity by a cognitive enhancer and antidepressant molecule

The plasticity of excitatory synapses is an essential brain process involved in cognitive functions, and dysfunctions of such adaptations have been linked to psychiatric disorders such as depression. Although the intracellular cascades that are altered in models of depression and stress-related disorders have been under considerable scrutiny, the molecular interplay between antidepressants and glutamatergic signaling remains elusive. Using a combination of electrophysiological and single nanoparticle tracking approaches, we here report that the cognitive enhancer and antidepressant tianeptine (S 1574, [3-chloro-6-methyl-5,5-dioxo-6,11-dihydro-(c,f)-dibenzo-(1,2-thiazepine)-11-yl) amino]-7 heptanoic acid, sodium salt) favors synaptic plasticity in hippocampal neurons both under basal conditions and after acute stress. Strikingly, tianeptine rapidly reduces the surface diffusion of AMPA receptor (AMPAR) through a Ca(2+)/calmodulin-dependent protein kinase II (CaMKII)-dependent mechanism that enhances the binding of AMPAR auxiliary subunit stargazin with PSD-95. This prevents corticosterone-induced AMPAR surface dispersal and restores long-term potentiation of acutely stressed mice. Collectively, these data provide the first evidence that a therapeutically used drug targets the surface diffusion of AMPAR through a CaMKII-stargazin-PSD-95 pathway, to promote long-term synaptic plasticity.