AMPARs and synaptic plasticity: the last 25 years

Biochemical Computation for Spine Structural Plasticity

The structural plasticity of dendritic spines is considered to be essential for various forms of synaptic plasticity, learning, and memory. The process is mediated by a complex signaling network consisting of numerous species of molecules. Furthermore, the spatiotemporal dynamics of the biochemical signaling are regulated in a complicated manner because of geometrical restrictions from the unique morphology of the dendritic branches and spines. Recent advances in optical techniques have enabled the exploration of the spatiotemporal aspects of the signal regulations in spines and dendrites and have provided many insights into the principle of the biochemical computation that underlies spine structural plasticity.

USP46: a new piece of the memory puzzle?

Long-term potentiation (LTP) and long-term depression (LTD) are crucial for synaptic plasticity, and are driven by AMPA receptor (AMPAR) trafficking. Recent findings indicate that the ubiquitin-proteasome system, the main protein degradation machinery of the cell, plays a significant role in memory formation by regulating the induction and maintenance of LTP. Although previously suggested as a possibility, deubiquitination of mammalian AMPARs had not been demonstrated, and the search for an enzyme that mediates the processes continued. This Editorial Highlight discusses the relevance of a study published in the current issue of Journal of Neurochemistry, in which the authors Huo and collaborators now identified ubiquitin-specific peptidase 46 (USP46) as a specific AMPAR deubiquitinase.

Synaptic Function of Rab11Fip5: Selective Requirement for Hippocampal Long-Term Depression

Postsynaptic AMPA-type glutamate receptors (AMPARs) are among the major determinants of synaptic strength and can be trafficked into and out of synapses. Neuronal activity regulates AMPAR trafficking during synaptic plasticity to induce long-term changes in synaptic strength, including long-term potentiation (LTP) and long-term depression (LTD). Rab family GTPases regulate most membrane trafficking in eukaryotic cells; particularly, Rab11 and its effectors are implicated in mediating postsynaptic AMPAR insertion during LTP. To explore the synaptic function of Rab11Fip5, a neuronal Rab11 effector and a candidate autism-spectrum disorder gene, we performed shRNA-mediated knock-down and genetic knock-out (KO) studies. Surprisingly, we observed robust shRNA-induced synaptic phenotypes that were rescued by a Rab11Fip5 cDNA but that were nevertheless not observed in conditional KO neurons. Both in cultured neurons and acute slices, KO of Rab11Fip5 had no significant effect on basic parameters of synaptic transmission, indicating that Rab11Fip5 is not required for fundamental synaptic operations, such as neurotransmitter release or postsynaptic AMPAR insertion. KO of Rab11Fip5 did, however, abolish hippocampal LTD as measured both in acute slices or using a chemical LTD protocol in cultured neurons but did not affect hippocampal LTP. The Rab11Fip5 KO mice performed normally in several behavioral tasks, including fear conditioning, but showed enhanced contextual fear extinction. These are the first findings to suggest a requirement for Rab11Fip5, and presumably Rab11, during LTD.

GRIP1 is required for homeostatic regulation of AMPAR trafficking

Homeostatic plasticity is a negative feedback mechanism that stabilizes neurons during periods of perturbed activity. The best-studied form of homeostatic plasticity in the central nervous system is the scaling of excitatory synapses. Postsynaptic AMPA-type glutamate receptors (AMPARs) can be inserted into synapses to compensate for neuronal inactivity or removed to compensate for hyperactivity. However, the molecular mechanisms underlying the homeostatic regulation of AMPARs remain elusive. Here, we show that the expression of GRIP1, a multi-PDZ (postsynaptic density 95/discs large/zona occludens) domain AMPAR-binding protein, is bidirectionally altered by neuronal activity. Furthermore, we observe a subcellular redistribution of GRIP1 and a change in the binding of GRIP1 to GluA2 during synaptic scaling. Using a combination of biochemical, genetic, and electrophysiological methods, we find that loss of GRIP1 blocks the accumulation of surface AMPARs and the scaling up of synaptic strength that occur in response to chronic activity blockade. Collectively, our data point to an essential role of GRIP1-mediated AMPAR trafficking during inactivity-induced synaptic scaling.

The pharmacology of nociceptor priming

Nociceptors and neurons in the central nervous system (CNS) that receive nociceptive input show remarkable plasticity in response to injury. This plasticity is thought to underlie the development of chronic pain states. Hence, further understanding of the molecular mechanisms driving and maintaining this plasticity has the potential to lead to novel therapeutic approaches for the treatment of chronic pain states. An important concept in pain plasticity is the presence and persistence of "hyperalgesic priming." This priming arises from an initial injury and results in a remarkable susceptibility to normally subthreshold noxious inputs causing a prolonged pain state in primed animals. Here we describe our current understanding of how this priming is manifested through changes in signaling in the primary nociceptor as well as through memory like alterations at CNS synapses. Moreover, we discuss how commonly utilized analgesics, such as opioids, enhance priming therefore potentially contributing to the development of persistent pain states. Finally we highlight where these priming models draw parallels to common human chronic pain conditions. Collectively, these advances in our understanding of pain plasticity reveal a variety of targets for therapeutic intervention with the potential to reverse rather than palliate chronic pain states.

Canalization of genetic and pharmacological perturbations in developing primary neuronal activity patterns

The function of the nervous system depends on the integrity of synapses and the patterning of electrical activity in brain circuits. The rapid advances in genome sequencing reveal a large number of mutations disrupting synaptic proteins, which potentially result in diseases known as synaptopathies. However, it is also evident that every normal individual carries hundreds of potentially damaging mutations. Although genetic studies in several organisms show that mutations can be masked during development by a process known as canalization, it is unknown if this occurs in the development of the electrical activity in the brain. Using longitudinal recordings of primary cultured neurons on multi-electrode arrays from mice carrying knockout mutations we report evidence of canalization in development of spontaneous activity patterns. Phenotypes in the activity patterns in young cultures from mice lacking the Gria1 subunit of the AMPA receptor were ameliorated as cultures matured. Similarly, the effects of chronic pharmacological NMDA receptor blockade diminished as cultures matured. Moreover, disturbances in activity patterns by simultaneous disruption of Gria1 and NMDA receptors were also canalized by three weeks in culture. Additional mutations and genetic variations also appeared to be canalized to varying degrees. These findings indicate that neuronal network canalization is a form of nervous system plasticity that provides resilience to developmental disruption.

This article is part of the Special Issue entitled ‘Synaptopathy – from Biology to Therapy’.

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Development of network activity in cultures with synaptic mutations was recorded.

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Synchronous burst firing with theta periodicity was observed as cultures matured.

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Gria1 deletion and chronic NMDA-R blockade disrupted network activity patterns.

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Dlg2 knockout disrupted network activity, other synaptic genes had minimal effects.

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Network activity phenotypes early in development were canalized as cultures matured.

Neurolastin, a Dynamin Family GTPase, Regulates Excitatory Synapses and Spine Density

Membrane trafficking and spinogenesis contribute significantly to changes in synaptic strength during development and in various paradigms of synaptic plasticity. GTPases of the dynamin family are key players regulating membrane trafficking. Here, we identify a brain-specific dynamin family GTPase, neurolastin (RNF112/Znf179), with closest homology to atlastin. We demonstrate that neurolastin has functional GTPase and RING domains, making it a unique protein identified with this multi-enzymatic domain organization. We also show that neurolastin is a peripheral membrane protein that localizes to endosomes and affects endosomal membrane dynamics via its RING domain. In addition, neurolastin knockout mice have fewer dendritic spines, and rescue of the wild-type phenotype requires both the GTPase and RING domains. Furthermore, we find fewer functional synapses and reduced paired pulse facilitation in neurolastin knockout mice. Thus, we identify neurolastin as a dynamin family GTPase that affects endosome size and spine density.

The deubiquitinating enzyme USP46 regulates AMPA receptor ubiquitination and trafficking

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPARs) are the primary mediators for inter-neuronal communication and play a crucial role in higher brain functions including learning and memory. Our previous work demonstrated that AMPARs are subject to ubiquitination by the E3 ligase Nedd4, resulting in EPS15-mediated receptor internalization and Ubiquitin (Ub)-proteasome pathway (UPP)-dependent degradation. Protein ubiquitination is a highly dynamic and reversible process, achieved via the balance between ubiquitination and deubiquitination. However, deubiquitination of mammalian AMPARs and the responsible deubiquitinating enzymes remain elusive. In this study, we identify USP46 as the deubiquitinating enzyme for AMPARs. We find that AMPARs are subject to K63 type ubiquitination, and USP46 is able to deubiquitinate AMPARs in vivo and in vitro. In heterologous cells and neurons, expression of USP46 results in a significant reduction in AMPAR ubiquitination, accompanied by a reduced rate in AMPAR degradation and an increase in surface AMPAR accumulation. By contrast, knockdown of USP46 by RNAi leads to elevated AMPAR ubiquitination and a reduction in surface AMPARs at synapses in neurons. Consistently, miniature excitatory postsynaptic currents recordings show reduced synaptic strength in neurons expressing USP46-selective RNAi. These results demonstrate USP46-mediated regulation of AMPAR ubiquitination and turnover, which may play an important role in synaptic plasticity and brain function. Protein ubiquitination is a highly dynamic and reversible process, achieved via the balance between ubiquitination and deubiquitination. The glutamatergic AMPARs, which mediate most of the excitatory synaptic transmission in the brain, are known to be subjected to Nedd4-mediated ubiquitination; however, the deubiquitination process and the responsible deubiquitinating enzymes (DUBs) for mammalian AMPARs remain elusive. We find that AMPARs are subject to K63-type ubiquitination, and identify USP46 as the DUB for AMPARs. USP46 deubiquitinates AMPARs in vitro and in vivo. Up- or down-regulation of USP46 leads to changes in AMPAR ubiquitination, surface expression, and trafficking, as well as the strength of synaptic transmission. USP46-mediated regulation of AMPAR ubiquitination and turnover may play an important role in synaptic plasticity and brain function.

Short-term use of antiepileptic drugs is neurotoxic to the immature brain

Previous studies have shown that the long-term use of antiepileptic drugs can cause nervous system damage. However, short-term antiepileptic drug treatment is frequently given to infants, especially neonates, to control seizure. Whether the short-term use of antiepileptic drugs is neurotoxic remains unclear. In the present study, immature rats, 3–21 days of age, were intraperitoneally injected with phenobarbital and/or topiramate for 3 consecutive days. Hematoxylin-eosin and immunohistochemical staining revealed that phenobarbital and topiramate, individually or in combination, were cytotoxic to hippocampal CA1 neurons and inhibited the expression of GluR1 and NR2B, excitatory glutamate receptor subunits. Furthermore, the combination of the two drugs caused greater damage than either drug alone. The results demonstrate that the short-term use of antiepileptic drugs damages neurons in the immature brain and that the combined use of antiepileptic drugs exacerbates damage. Our findings suggest that clinicians should consider the potential neurotoxic risk associated with the combined use of antiepileptic drugs in the treatment of seizure.

Ionotropic GABA and Glutamate Receptor Mutations and Human Neurologic Diseases

The advent of whole exome/genome sequencing and the technology-driven reduction in the cost of next-generation sequencing as well as the introduction of diagnostic-targeted sequencing chips have resulted in an unprecedented volume of data directly linking patient genomic variability to disorders of the brain. This information has the potential to transform our understanding of neurologic disorders by improving diagnoses, illuminating the molecular heterogeneity underlying diseases, and identifying new targets for therapeutic treatment. There is a strong history of mutations in GABA receptor genes being involved in neurologic diseases, particularly the epilepsies. In addition, a substantial number of variants and mutations have been found in GABA receptor genes in patients with autism, schizophrenia, and addiction, suggesting potential links between the GABA receptors and these conditions. A new and unexpected outcome from sequencing efforts has been the surprising number of mutations found in glutamate receptor subunits, with the GRIN2A gene encoding the GluN2A N-methyl-d-aspartate receptor subunit being most often affected. These mutations are associated with multiple neurologic conditions, for which seizure disorders comprise the largest group. The GluN2A subunit appears to be a locus for epilepsy, which holds important therapeutic implications. Virtually all α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor mutations, most of which occur within GRIA3, are from patients with intellectual disabilities, suggesting a link to this condition. Similarly, the most common phenotype for kainate receptor variants is intellectual disability. Herein, we summarize the current understanding of disease-associated mutations in ionotropic GABA and glutamate receptor families, and discuss implications regarding the identification of human mutations and treatment of neurologic diseases.

MicroRNA-137 Controls AMPA-Receptor-Mediated Transmission and mGluR-Dependent LTD

Mutations affecting the levels of microRNA miR-137 are associated with intellectual disability and schizophrenia. However, the pathophysiological role of miR-137 remains poorly understood. Here, we describe a highly conserved miR-137-binding site within the mRNA encoding the GluA1 subunit of AMPA-type glutamate receptors (AMPARs) and confirm that GluA1 is a direct target of miR-137. Postsynaptic downregulation of miR-137 at the CA3-CA1 hippocampal synapse selectively enhances AMPAR-mediated synaptic transmission and converts silent synapses to active synapses. Conversely, miR-137 overexpression selectively reduces AMPAR-mediated synaptic transmission and silences active synapses. In addition, we find that miR-137 is transiently upregulated in response to metabotropic glutamate receptor 5 (mGluR5), but not mGluR1 activation. Consequently, acute interference with miR-137 function impedes mGluR-LTD expression. Our findings suggest that miR-137 is a key factor in the control of synaptic efficacy and mGluR-dependent synaptic plasticity, supporting the notion that glutamatergic dysfunction contributes to the pathogenesis of miR-137-linked cognitive impairments.

Structure, Distribution, and Function of Neuronal/Synaptic Spinules and Related Invaginating Projections

Neurons and especially their synapses often project long thin processes that can invaginate neighboring neuronal or glial cells. These "invaginating projections" can occur in almost any combination of postsynaptic, presynaptic, and glial processes. Invaginating projections provide a precise mechanism for one neuron to communicate or exchange material exclusively at a highly localized site on another neuron, e.g., to regulate synaptic plasticity. The best-known types are postsynaptic projections called "spinules" that invaginate into presynaptic terminals. Spinules seem to be most prevalent at large very active synapses. Here, we present a comprehensive review of all kinds of invaginating projections associated with both neurons in general and more specifically with synapses; we describe them in all animals including simple, basal metazoans. These structures may have evolved into more elaborate structures in some higher animal groups exhibiting greater synaptic plasticity. In addition to classic spinules and filopodial invaginations, we describe a variety of lesser-known structures such as amphid microvilli, spinules in giant mossy terminals and en marron/brush synapses, the highly specialized fish retinal spinules, the trophospongium, capitate projections, and fly gnarls, as well as examples in which the entire presynaptic or postsynaptic process is invaginated. These various invaginating projections have evolved to modify the function of a particular synapse, or to channel an effect to one specific synapse or neuron, without affecting those nearby. We discuss how they function in membrane recycling, nourishment, and cell signaling and explore how they might change in aging and disease.

A Place at the Table: LTD as a Mediator of Memory Genesis

Resolving how our brains encode information requires an understanding of the cellular processes taking place during memory formation. Since the 1970s, considerable effort has focused on determining the properties and mechanisms underlying long-term potentiation (LTP) at glutamatergic synapses and how these processes influence initiation of new memories. However, accumulating evidence suggests that long-term depression (LTD) of synaptic strength, particularly at glutamatergic synapses, is a bona fide learning and memory mechanism in the mammalian brain. The known range of mechanisms capable of inducing LTD has been extended to those including NMDAR-independent forms, neuromodulator-dependent LTD, synaptic depression following stress, and non-synaptically induced forms. The examples of LTD observed at the hippocampal CA1 synapse to date demonstrate features consistent with LTP, including homo- and heterosynaptic expression, extended duration beyond induction (several hours to weeks), and association with encoding of distinct types of memories. Canonical mechanisms through which synapses undergo LTD include activation of phosphatases, initiation of protein synthesis, and dynamic regulation of presynaptic glutamate release and/or postsynaptic glutamate receptor endocytosis. Here, we will discuss the pre- and postsynaptic changes underlying LTD, recent advances in the identification and characterization of novel mechanisms underlying LTD, and how engagement of these processes constitutes a cellular analog for the genesis of specific types of memories.

Functions of kinesin superfamily proteins in neuroreceptor trafficking

Synaptic plasticity is widely regarded as the cellular basis of learning and memory. Understanding the molecular mechanism of synaptic plasticity has been one of center pieces of neuroscience research for more than three decades. It has been well known that the trafficking of α-amino-3-hydroxy-5-methylisoxazoloe-4-propionic acid- (AMPA-) type, N-methyl-D-aspartate- (NMDA-) type glutamate receptors to and from synapses is a key molecular event underlying many forms of synaptic plasticity. Kainate receptors are another type of glutamate receptors playing important roles in synaptic transmission. In addition, GABA receptors also play important roles in modulating the synaptic plasticity. Kinesin superfamily proteins (also known as KIFs) transport various cargos in both anterograde and retrograde directions through the interaction with different adaptor proteins. Recent studies indicate that KIFs regulate the trafficking of NMDA receptors, AMPA receptors, kainate receptors, and GABA receptors and thus play important roles in neuronal activity. Here we review the essential functions of KIFs in the trafficking of neuroreceptor and synaptic plasticity.

The discovery and characterization of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor potentiator N-{(3S,4S)-4-[4-(5-cyano-2-thienyl)phenoxy]tetrahydrofuran-3-yl}propane-2-sulfonamide (PF-04958242)

A unique tetrahydrofuran ether class of highly potent α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor potentiators has been identified using rational and structure-based drug design. An acyclic lead compound, containing an ether-linked isopropylsulfonamide and biphenyl group, was pharmacologically augmented by converting it to a conformationally constrained tetrahydrofuran to improve key interactions with the human GluA2 ligand-binding domain. Subsequent replacement of the distal phenyl motif with 2-cyanothiophene to enhance its potency, selectivity, and metabolic stability afforded N-{(3S,4S)-4-[4-(5-cyano-2-thienyl)phenoxy]tetrahydrofuran-3-yl}propane-2-sulfonamide (PF-04958242, 3), whose preclinical characterization suggests an adequate therapeutic index, aided by low projected human oral pharmacokinetic variability, for clinical studies exploring its ability to attenuate cognitive deficits in patients with schizophrenia.

FUS regulates AMPA receptor function and FTLD/ALS-associated behaviour via GluA1 mRNA stabilization

FUS is an RNA/DNA-binding protein involved in multiple steps of gene expression and is associated with amyotrophic lateral sclerosis (ALS) and fronto-temporal lobar degeneration (FTLD). However, the specific disease-causing and/or modifying mechanism mediated by FUS is largely unknown. Here we evaluate intrinsic roles of FUS on synaptic functions and animal behaviours. We find that FUS depletion downregulates GluA1, a subunit of AMPA receptor. FUS binds GluA1 mRNA in the vicinity of the 3′ terminus and controls poly (A) tail maintenance, thus regulating stability. GluA1 reduction upon FUS knockdown reduces miniature EPSC amplitude both in cultured neurons and in vivo. FUS knockdown in hippocampus attenuates dendritic spine maturation and causes behavioural aberrations including hyperactivity, disinhibition and social interaction defects, which are partly ameliorated by GluA1 reintroduction. These results highlight the pivotal role of FUS in regulating GluA1 mRNA stability, post-synaptic function and FTLD-like animal behaviours.

FUS is an RNA/DNA-binding protein involved in gene expression regulation and associated with amyotrophic lateral sclerosis and frontotemporal dementia (FTLD) but the disease-causing mechanisms are unclear. Here the authors show that FUS regulates the stability of GluA1 mRNA and dendritic maturation and plays a role in FTLD-associated behaviours.

Regulation of AMPA receptor phosphorylation by the neuropeptide PACAP38

Dynamic changes in synaptic strength are thought to be critical for higher brain function such as learning and memory. Alterations in synaptic strength can result from modulation of AMPA receptor (AMPAR) function and trafficking to synaptic sites. The phosphorylation state of AMPAR subunits is one mechanism by which cells regulate receptor function and trafficking. Receptor phosphorylation is in turn regulated by extracellular signals; these include neuronal activity, neuropeptides, and neuromodulators such as dopamine and norepinephrine (NE). Although numerous studies have reported that the neuropeptide pituitary adenylate cyclase activating polypeptide 38 (PACAP38) alters hippocampal CA1 synaptic strength and GluA1 synaptic localization, its effect on AMPAR phosphorylation state has not been explored. We determined that PACAP38 stimulation of hippocampal cultures increased phosphorylation of S845, and decreased phosphorylation of T840 on the GluA1 AMPAR subunit. Increases in GluA1 S845 phosphorylation primarily occurred via PAC1 and VPAC2 receptor activation, whereas a reduction in GluA1 T840 phosphorylation was largely driven by PAC1 receptor activation and to a lesser extent by VPAC1 and VPAC2 receptor activation. GluA1 S845 phosphorylation could be blocked by a PKA inhibitor, and GluA1 T840 dephosphorylation could be blocked by a protein phosphatase 1/2A (PP1/PP2A) inhibitor and was partly blocked by a NMDA receptor (NMDAR) antagonist. These results demonstrate that the neuropeptide PACAP38 inversely regulates the phosphorylation of two distinct sites on GluA1 and may play an important role modulating AMPAR function and synaptic plasticity in the brain.

Rapid dispersion of SynGAP from synaptic spines triggers AMPA receptor insertion and spine enlargement during LTP

SynGAP is a Ras-GTPase activating protein highly enriched at excitatory synapses in the brain. Previous studies have shown that CaMKII and the RAS-ERK pathway are critical for several forms of synaptic plasticity including LTP. NMDA receptor-dependent calcium influx has been shown to regulate the RAS-ERK pathway and downstream events that result in AMPA receptor synaptic accumulation, spine enlargement, and synaptic strengthening during LTP. However, the cellular mechanisms whereby calcium influx and CaMKII control Ras activity remain elusive. Using live-imaging techniques, we have found that SynGAP is rapidly dispersed from spines upon LTP induction in hippocampal neurons, and this dispersion depends on phosphorylation of SynGAP by CaMKII. Moreover, the degree of acute dispersion predicts the maintenance of spine enlargement. Thus, the synaptic dispersion of SynGAP by CaMKII phosphorylation during LTP represents a key signaling component that transduces CaMKII activity to small G protein-mediated spine enlargement, AMPA receptor synaptic incorporation, and synaptic potentiation.

Drug development in schizophrenia: are glutamatergic targets still worth aiming at?

PURPOSE OF REVIEW

The ketamine model has dominated drug discovery in schizophrenia over the past decade, supported by genetic and postmortem evidence implicating glutamatergic transmission. This review assesses recent successes and disappointments of glutamatergic agents and identifies promising new directions.

RECENT FINDINGS

Strategies focused on enhancing activity of the N-methyl D-aspartate (NMDA) receptor via direct agonists at the glycine site or by inhibition of glycine reuptake have produced modest and often inconsistent evidence of efficacy, as have approaches to reduce excessive glutamate release by lamotrigine or by mGluR2/3 agonists. Strategies targeting α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors have also met with only limited success. Newer approaches include selective allosteric modulation of NMDA receptor subunits and of mGluR5 receptors. In addition, intracellular pathways downstream of NMDA receptors may also provide new treatment targets, as exemplified by phosphodiesterase (PDE) inhibitors.

SUMMARY

Targeting glutamatergic transmission remains one of the most promising strategies in schizophrenia, particularly early in the course of illness, but therapeutic approaches may require greater specificity for receptor subtype type, illness phase, and individual biology in order to enhance efficacy and overcome problems with reproducibility of clinical results.

Neto-mediated intracellular interactions shape postsynaptic composition at the Drosophila neuromuscular junction

The molecular mechanisms controlling the subunit composition of glutamate receptors are crucial for the formation of neural circuits and for the long-term plasticity underlying learning and memory. Here we use the Drosophila neuromuscular junction (NMJ) to examine how specific receptor subtypes are recruited and stabilized at synaptic locations. In flies, clustering of ionotropic glutamate receptors (iGluRs) requires Neto (Neuropillin and Tolloid-like), a highly conserved auxiliary subunit that is essential for NMJ assembly and development. Drosophila neto encodes two isoforms, Neto-α and Neto-β, with common extracellular parts and distinct cytoplasmic domains. Mutations that specifically eliminate Neto-β or its intracellular domain were generated. When Neto-β is missing or is truncated, the larval NMJs show profound changes in the subtype composition of iGluRs due to reduced synaptic accumulation of the GluRIIA subunit. Furthermore, neto-β mutant NMJs fail to accumulate p21-activated kinase (PAK), a critical postsynaptic component implicated in the synaptic stabilization of GluRIIA. Muscle expression of either Neto-α or Neto-β rescued the synaptic transmission at neto null NMJs, indicating that Neto conserved domains mediate iGluRs clustering. However, only Neto-β restored PAK synaptic accumulation at neto null NMJs. Thus, Neto engages in intracellular interactions that regulate the iGluR subtype composition by preferentially recruiting and/or stabilizing selective receptor subtypes.

Inducible and combinatorial gene manipulation in mouse brain

We have deployed recombinant adeno-associated viruses equipped with tetracycline-controlled genetic switches to manipulate gene expression in mouse brain. Here, we show a combinatorial genetic approach for inducible, cell type-specific gene expression and Cre/loxP mediated gene recombination in different brain regions. Our chemical-genetic approach will help to investigate 'when', 'where', and 'how' gene(s) control neuronal circuit dynamics, and organize, for example, sensory signal processing, learning and memory, and behavior.

Tuning hippocampal synapses by stress-hormones: Relevance for emotional memory formation

While stress is often associated with an increased risk to develop (psycho) pathology, the initial response after exposure to stressors is often highly beneficial and allows individuals to optimally cope with challenging situations. Various neurotransmitters and neuromodulators - such as catecholamines and glucocorticoids - are released upon exposure to stressors and regulate behavioural adaptation to stress and enhance the storage of salient information. Studies over the past years have revealed that catecholamines and glucocorticoids regulate synaptic function and synaptic plasticity - which underlie memory formation - in a highly dynamic manner. In this brief review we will summarise how catecholamines and glucocorticoids regulate synaptic function and discuss how these effects may contribute to acquisition and storage of emotional information. This article is part of a Special Issue entitled SI: Brain and Memory.

miR-26a and miR-384-5p are required for LTP maintenance and spine enlargement

Long-term potentiation (LTP) is a form of synaptic plasticity that results in enhanced synaptic strength. It is associated with the formation and enlargement of dendritic spines—tiny protrusions accommodating excitatory synapses. Both LTP and spine remodelling are crucial for brain development, cognition and the pathophysiology of neurological disorders. The role of microRNAs (miRNAs) in the maintenance of LTP, however, is not well understood. Using next-generation sequencing to profile miRNA transcriptomes, we demonstrate that miR-26a and miR-384-5p specifically affect the maintenance, but not induction, of LTP and different stages of spine enlargement by regulating the expression of RSK3. Using bioinformatics, we also examine the global effects of miRNA transcriptome changes during LTP on gene expression and cellular activities. This study reveals a novel miRNA-mediated mechanism for gene-specific regulation of translation in LTP, identifies two miRNAs required for long-lasting synaptic and spine plasticity and presents a catalogue of candidate ‘LTP miRNAs'.

Long-term potentiation (LTP) is a form of synaptic plasticity that results in enhanced synaptic strength. Here, the authors demonstrate that miR-26a and miR-384-5p affect the maintenance, but not induction, of LTP as well as spine enlargement by regulating the expression of RSK3.

TARP γ-8 glycosylation regulates the surface expression of AMPA receptors

TARP [transmembrane AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor regulatory protein] γ-8 is an auxiliary subunit of AMPA receptors that is widely distributed in the hippocampus. It has been shown that TARP γ-8 promotes surface expression of AMPA receptors; however, how TARP γ-8 regulates the expression of AMPA receptors remains unclear. In the present study, we examined the effect of TARP glycosylation on AMPA receptor trafficking. We first showed that TARP γ-8 is an N-glycosylated protein, which contains two glycosylation sites, Asn53 and Asn56, and compared this with the glycosylation of TARP γ-2 and the AMPA receptor auxiliary protein CNIH-2 (cornichon homologue 2). We next examine the effect of TARP glycosylation on TARP trafficking and also on AMPA receptor surface expression. We find that TARP γ-8 glycosylation is critical for surface expression of both TARP γ-8 and GluA1 in heterologous cells and neurons. Specifically, knockdown of TARP γ-8 causes a decrease in both total and surface AMPA receptors. We find that the expression of unglycosylated TARP γ-8 in cultured neurons is unable to restore GluA1 expression fully. Furthermore, when the maturation of TARP γ-8 is impaired, a large pool of immature GluA1 is retained intracellularly. Taken together, our data reveal an important role for the maturation of TARP γ-8 in the trafficking and function of the AMPA receptor complex.

Glutamate synapses in human cognitive disorders

Accumulating data, including those from large genetic association studies, indicate that alterations in glutamatergic synapse structure and function represent a common underlying pathology in many symptomatically distinct cognitive disorders. In this review, we discuss evidence from human genetic studies and data from animal models supporting a role for aberrant glutamatergic synapse function in the etiology of intellectual disability (ID), autism spectrum disorder (ASD), and schizophrenia (SCZ), neurodevelopmental disorders that comprise a significant proportion of human cognitive disease and exact a substantial financial and social burden. The varied manifestations of impaired perceptual processing, executive function, social interaction, communication, and/or intellectual ability in ID, ASD, and SCZ appear to emerge from altered neural microstructure, function, and/or wiring rather than gross changes in neuron number or morphology. Here, we review evidence that these disorders may share a common underlying neuropathy: altered excitatory synapse function. We focus on the most promising candidate genes affecting glutamatergic synapse function, highlighting the likely disease-relevant functional consequences of each. We first present a brief overview of glutamatergic synapses and then explore the genetic and phenotypic evidence for altered glutamate signaling in ID, ASD, and SCZ.

Single cell electroporation for longitudinal imaging of synaptic structure and function in the adult mouse neocortex in vivo

Longitudinal imaging studies of neuronal structures in vivo have revealed rich dynamics in dendritic spines and axonal boutons. Spines and boutons are considered to be proxies for synapses. This implies that synapses display similar dynamics. However, spines and boutons do not always bear synapses, some may contain more than one, and dendritic shaft synapses have no clear structural proxies. In addition, synaptic strength is not always accurately revealed by just the size of these structures. Structural and functional dynamics of synapses could be studied more reliably using fluorescent synaptic proteins as markers for size and function. These proteins are often large and possibly interfere with circuit development, which renders them less suitable for conventional transfection or transgenesis methods such as viral vectors, in utero electroporation, and germline transgenesis. Single cell electroporation (SCE) has been shown to be a potential alternative for transfection of recombinant fluorescent proteins in adult cortical neurons. Here we provide proof of principle for the use of SCE to express and subsequently image fluorescently tagged synaptic proteins over days to weeks in vivo.

Retinoic Acid and LTP Recruit Postsynaptic AMPA Receptors Using Distinct SNARE-Dependent Mechanisms

Retinoic acid (RA)-dependent homeostatic plasticity and NMDA receptor-dependent long-term potentiation (LTP), a form of Hebbian plasticity, both enhance synaptic strength by increasing the abundance of postsynaptic AMPA receptors (AMPARs). However, it is unclear whether the molecular mechanisms mediating AMPAR trafficking during homeostatic and Hebbian plasticity differ, and it is unknown how RA signaling impacts Hebbian plasticity. Here, we show that RA increases postsynaptic AMPAR abundance using an activity-dependent mechanism that requires a unique SNARE (soluble NSF-attachment protein receptor)-dependent fusion machinery different from that mediating LTP. Specifically, RA-induced AMPAR trafficking did not involve complexin, which activates SNARE complexes containing syntaxin-1 or -3, but not complexes containing syntaxin-4, whereas LTP required complexin. Moreover, RA-induced AMPAR trafficking utilized the Q-SNARE syntaxin-4, whereas LTP utilized syntaxin-3; both additionally required the Q-SNARE SNAP-47 and the R-SNARE synatobrevin-2. Finally, acute RA treatment blocked subsequent LTP expression, probably by increasing AMPAR trafficking. Thus, RA-induced homeostatic plasticity involves a novel, activity-dependent postsynaptic AMPAR-trafficking pathway mediated by a unique SNARE-dependent fusion machinery.

Rhythms and blues: modulation of oscillatory synchrony and the mechanism of action of antidepressant treatments

Treatments for major depressive disorder (MDD) act at different hierarchical levels of biological complexity, ranging from the individual synapse to the brain as a whole. Theories of antidepressant medication action traditionally have focused on the level of cell-to-cell interaction and synaptic neurotransmission. However, recent evidence suggests that modulation of synchronized electrical activity in neuronal networks is a common effect of antidepressant treatments, including not only medications, but also neuromodulatory treatments such as repetitive transcranial magnetic stimulation. Synchronization of oscillatory network activity in particular frequency bands has been proposed to underlie neurodevelopmental and learning processes, and also may be important in the mechanism of action of antidepressant treatments. Here, we review current research on the relationship between neuroplasticity and oscillatory synchrony, which suggests that oscillatory synchrony may help mediate neuroplastic changes related to neurodevelopment, learning, and memory, as well as medication and neuromodulatory treatment for MDD. We hypothesize that medication and neuromodulation treatments may have related effects on the rate and pattern of neuronal firing, and that these effects underlie antidepressant efficacy. Elucidating the mechanisms through which oscillatory synchrony may be related to neuroplasticity could lead to enhanced treatment strategies for MDD.

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.

Synaptic strength is bidirectionally controlled by opposing activity-dependent regulation of Nedd4-1 and USP8

The trafficking of AMPA receptors (AMPARs) to and from synapses is crucial for synaptic plasticity. Previous work has demonstrated that AMPARs undergo activity-dependent ubiquitination by the E3 ubiquitin ligase Nedd4-1, which promotes their internalization and degradation in lysosomes. Here, we define the molecular mechanisms involved in ubiquitination and deubiquitination of AMPARs. We report that Nedd4-1 is rapidly redistributed to dendritic spines in response to AMPAR activation and not in response to NMDA receptor (NMDAR) activation in cultured rat neurons. In contrast, NMDAR activation directly antagonizes Nedd4-1 function by promoting the deubiquitination of AMPARs. We show that NMDAR activation causes the rapid dephosphorylation and activation of the deubiquitinating enzyme (DUB) USP8. Surface AMPAR levels and synaptic strength are inversely regulated by Nedd4-1 and USP8. Strikingly, we show that homeostatic downscaling of synaptic strength is accompanied by an increase and decrease in Nedd4-1 and USP8 protein levels, respectively. Furthermore, we show that Nedd4-1 is required for homeostatic loss of surface AMPARs and downscaling of synaptic strength. This study provides the first mechanistic evidence for rapid and opposing activity-dependent control of a ubiquitin ligase and DUB at mammalian CNS synapses. We propose that the dynamic regulation of these opposing forces is critical in maintaining synapses and scaling them during homeostatic plasticity.

Signalling scaffolds and local organization of cellular behaviour

Cellular responses to environmental cues involve the mobilization of GTPases, protein kinases and phosphoprotein phosphatases. The spatial organization of these signalling enzymes by scaffold proteins helps to guide the flow of molecular information. Allosteric modulation of scaffolded enzymes can alter their catalytic activity or sensitivity to second messengers in a manner that augments, insulates or terminates local cellular events. This Review examines the features of scaffold proteins and highlights examples of locally organized groups of signalling enzymes that drive essential physiological processes, including hormone action, heart rate, cell division, organelle movement and synaptic transmission.

LTP-triggered cholesterol redistribution activates Cdc42 and drives AMPA receptor synaptic delivery

Cholesterol acts as a sensor of NMDA receptor activation and as a trigger of downstream signaling by engaging small GTPase activation and AMPA receptor synaptic delivery during long-term potentiation.

Neurotransmitter receptor trafficking during synaptic plasticity requires the concerted action of multiple signaling pathways and the protein transport machinery. However, little is known about the contribution of lipid metabolism during these processes. In this paper, we addressed the question of the role of cholesterol in synaptic changes during long-term potentiation (LTP). We found that N-methyl-d-aspartate–type glutamate receptor (NMDAR) activation during LTP induction leads to a rapid and sustained loss or redistribution of intracellular cholesterol in the neuron. A reduction in cholesterol, in turn, leads to the activation of Cdc42 and the mobilization of GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid–type glutamate receptors (AMPARs) from Rab11-recycling endosomes into the synaptic membrane, leading to synaptic potentiation. This process is accompanied by an increase of NMDAR function and an enhancement of LTP. These results imply that cholesterol acts as a sensor of NMDAR activation and as a trigger of downstream signaling to engage small GTPase (guanosine triphosphatase) activation and AMPAR synaptic delivery during LTP.

Dynamic increases in AMPA receptor phosphorylation in the rat hippocampus in response to amphetamine

Increasing evidence supports the critical role of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors in psychostimulant action. These receptors are regulated via a phosphorylation-dependent mechanism in their trafficking, distribution, and function. The hippocampus is a brain structure important for learning and memory and is emerging as a critical site for processing psychostimulant effects. To determine whether the hippocampal pool of AMPA receptors is regulated by stimulants, we investigated and characterized the impact of amphetamine (AMPH) on phosphorylation of AMPA receptors in the adult rat hippocampus in vivo. We found that AMPH markedly increased phosphorylation of AMPA receptor GluA1 subunits at serine 845 (S845) in the hippocampus. The effect of AMPH was dose dependent. A single dose of AMPH induced a rapid and transient increase in S845 phosphorylation. Among different hippocampal subfields, AMPH primarily elevated S845 phosphorylation in the Cornu Ammonis area 1 and dentate gyrus. In contrast to S845, serine 831 phosphorylation of GluA1 and serine 880 phosphorylation of GluA2 were not altered by AMPH. In addition, surface expression of hippocampal GluA1 was up-regulated, while the amount of intracellular GluA1 fraction was concurrently reduced in response to AMPH. GluA2 protein levels in either the surface or intracellular pool were insensitive to AMPH. These data demonstrate that the AMPA receptor in the hippocampus is sensitive to dopamine stimulation. Acute AMPH administration induces dose-, time-, site-, and subunit-dependent phosphorylation of AMPA receptors and facilitates surface trafficking of GluA1 AMPA receptors in hippocampal neurons in vivo. Acute injection of amphetamine increased phosphorylation of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 subunits at a protein kinase A (PKA)-sensitive site (S845) in the rat hippocampus. This increase was dose- and time-dependent and correlated with an increase in surface GluA1 expression. Thus, amphetamine can upregulate GluA1 phosphorylation and surface trafficking of GluA1 in hippocampal neurons in vivo.

Corticosterone suppresses vasotocin-enhanced clasping behavior in male rough-skinned newts by novel mechanisms interfering with V1a receptor availability and receptor-mediated endocytosis

In rough-skinned newts, Taricha granulosa, exposure to an acute stressor results in the rapid release of corticosterone (CORT), which suppresses the ability of vasotocin (VT) to enhance clasping behavior. CORT also suppresses VT-induced spontaneous activity and sensory responsiveness of clasp-controlling neurons in the rostromedial reticular formation (Rf). The cellular mechanisms underlying this interaction remain unclear. We hypothesized that CORT blocks VT-enhanced clasping by interfering with V1a receptor availability and/or VT-induced endocytosis. We administered a physiologically active fluorescent VT conjugated to Oregon Green (VT-OG) to the fourth ventricle 9 min after an intraperitoneal injection of CORT (0, 10, 40 μg/0.1mL amphibian Ringers). The brains were collected 30 min post-VT-OG, fixed, and imaged with confocal microscopy. CORT diminished the number of endocytosed vesicles, percent area containing VT-OG, sum intensity of VT-OG, and the amount of VT-V1a within each vesicle; indicating that CORT was interfering with V1a receptor availability and VT-V1a receptor-mediated endocytosis. CORT actions were brain location-specific and season-dependent in a manner that is consistent with the natural and context-dependent expression of clasping behavior. Furthermore, the sensitivity of the Rf to CORT was much higher in animals during the breeding season, arguing for ethologically appropriate seasonal variation in CORT's ability to prevent VT-induced endocytosis. Our data are consistent with the time course and interaction effects of CORT and VT on clasping behavior and neurophysiology. CORT interference with VT-induced endocytosis may be a common mechanism employed by hormones across taxa for mediating rapid context- and season-specific behavioral responses.

Visualization of NMDA receptor-dependent AMPA receptor synaptic plasticity in vivo

Regulation of AMPA receptor (AMPAR) membrane trafficking is critical for synaptic plasticity, as well as for learning and memory. However, the mechanisms of AMPAR trafficking in vivo remain elusive. Using in vivo two-photon microscopy in the mouse somatosensory barrel cortex, we found that acute whisker stimulation led to a significant increase in the intensity of surface AMPAR GluA1 subunit (sGluA1) in both spines and dendritic shafts and a small increase in spine size relative to prestimulation values. Interestingly, the initial spine properties biased spine changes following whisker stimulation. Changes in spine sGluA1 intensity were positively correlated with changes in spine size and dendritic shaft sGluA1 intensity following whisker stimulation. The increase in spine sGluA1 intensity evoked by whisker stimulation was NMDA receptor dependent and long lasting, similar to major forms of synaptic plasticity in the brain. In this study we were able to observe experience-dependent AMPAR trafficking in real time and characterize, in vivo, a major form of synaptic plasticity in the brain.

Prodomain removal enables neto to stabilize glutamate receptors at the Drosophila neuromuscular junction

Stabilization of neurotransmitter receptors at postsynaptic specializations is a key step in the assembly of functional synapses. Drosophila Neto (Neuropillin and Tolloid-like protein) is an essential auxiliary subunit of ionotropic glutamate receptor (iGluR) complexes required for the iGluRs clustering at the neuromuscular junction (NMJ). Here we show that optimal levels of Neto are crucial for stabilization of iGluRs at synaptic sites and proper NMJ development. Genetic manipulations of Neto levels shifted iGluRs distribution to extrajunctional locations. Perturbations in Neto levels also produced small NMJs with reduced synaptic transmission, but only Neto-depleted NMJs showed diminished postsynaptic components. Drosophila Neto contains an inhibitory prodomain that is processed by Furin1-mediated limited proteolysis. neto null mutants rescued with a Neto variant that cannot be processed have severely impaired NMJs and reduced iGluRs synaptic clusters. Unprocessed Neto retains the ability to engage iGluRs in vivo and to form complexes with normal synaptic transmission. However, Neto prodomain must be removed to enable iGluRs synaptic stabilization and proper postsynaptic differentiation.

Synapse development is initiated by genetic programs, but is coordinated by neuronal activity, by communication between the pre- and postsynaptic compartments, and by cellular signals that integrate the status of the whole organisms and its developmental progression. The molecular mechanisms underlining these processes are poorly understood. In particular, how neurotransmitter receptors are recruited and stabilized at central synapses remain the subject of intense research. The Drosophila NMJ is a glutamatergic synapse similar in composition and physiology with mammalian central excitatory synapses. Like mammals, Drosophila utilizes auxiliary subunit(s) to modulate the formation and function of glutamatergic synapses. We have previously reported that Neto is an auxiliary protein essential for functional glutamate receptors and for organization of postsynaptic specializations. Here we report that synapse assembly and NMJ development are exquisitely sensitive to postsynaptic Neto levels. Furthermore, we show that Neto activity is controlled by Furin-type proteases, which regulate the processing and maturation of many developmentally important proteins, from growth factors and neuropeptides to extracellular matrix components. Such concerted control may serve to coordinate synapse assembly with synapse growth and developmental progression.

Coincidence detection in a neural correlate of classical conditioning is initiated by bidirectional 3-phosphoinositide-dependent kinase-1 signalling and modulated by adenosine receptors

How the neural substrates for detection of paired stimuli are distinct from unpaired stimuli is poorly understood and a fundamental question for understanding the signalling mechanisms for coincidence detection during associative learning. To address this question, we used a neural correlate of eyeblink classical conditioning in an isolated brainstem from the turtle, in which the cranial nerves are directly stimulated in place of using a tone or airpuff. A bidirectional response is activated in <5 min of training, in which phosphorylated 3-phosphoinositide-dependent kinase-1 (p-PDK1) is increased in response to paired and decreased in response to unpaired nerve stimulation and is mediated by the opposing actions of neurotrophin receptors TrkB and p75(NTR) . Surprisingly, blockade of adenosine 2A (A2A ) receptors inhibits both of these responses. Pairing also induces substantially increased surface expression of TrkB that is inhibited by Src family tyrosine kinase and A2A receptor antagonists. Finally, the acquisition of conditioning is blocked by a PDK1 inhibitor. The unique action of A2A receptors to function directly as G proteins and in receptor transactivation to control distinct TrkB and p75(NTR) signalling pathways allows for convergent activation of PDK1 and protein kinase A during paired stimulation to initiate classical conditioning.

Meningeal afferent signaling and the pathophysiology of migraine

Migraine is the most common neurological disorder. Attacks are complex and consist of multiple phases but are most commonly characterized by intense, unilateral, throbbing headache. The pathophysiology contributing to migraine is poorly understood and the disorder is not well managed with currently available therapeutics, often rendering patients disabled during attacks. The mechanisms most likely to contribute to the pain phase of migraine require activation of trigeminal afferent signaling from the cranial meninges and subsequent relay of nociceptive information into the central nervous system in a region of the dorsal brainstem known as the trigeminal nucleus caudalis. Events leading to activation of meningeal afferents are unclear, but nerve endings within this tissue are mechanosensitive and also express a variety of ion channels including acid-sensing ion channels and transient receptor-potential channels. These properties may provide clues into the pathophysiology of migraine by suggesting that decreased extracellular pH and environmental irritant exposure in the meninges contributes to headache. Neuroplasticity is also likely to play a role in migraine given that attacks are triggered by routine events that are typically nonnoxious in healthy patients and clear evidence of sensitization occurs during an attack. Where and how plasticity develops is also not clear but may include events directly on the afferents and/or within the TNC. Among the mediators potentially contributing to plasticity, calcitonin gene-related peptide has received the most attention within the migraine field but other mechanisms may also contribute. Ultimately, greater understanding of the molecules and mechanisms contributing to migraine will undoubtedly lead to better therapeutics and relief for the large number of patients across the globe who suffer from this highly disabling neurological disorder.

Activity-Dependent Ubiquitination of GluA1 and GluA2 Regulates AMPA Receptor Intracellular Sorting and Degradation

AMPA receptors (AMPARs) have recently been shown to undergo post-translational ubiquitination in mammalian neurons. However, the underlying molecular mechanisms are poorly understood and remain controversial. Here, we report that all four AMPAR subunits (GluA1–4) are rapidly ubiquitinated upon brief application of AMPA or bicuculline in cultured neurons. This process is Ca²⁺ dependent and requires the activity of L-type voltage-gated Ca²⁺ channels and Ca²⁺/calmodulin-dependent kinase II. The ubiquitination of all subunits occurs exclusively on AMPARs located on the plasma membrane post-endocytosis. The sites of ubiquitination were mapped to Lys-868 in GluA1 and Lys-870/Lys-882 in GluA2 C-terminals. Mutation of these lysines did not affect basal surface expression or AMPA-induced internalization of GluA1 and GluA2 subunits. Instead, it reduced the intracellular trafficking of AMPARs to the late endosomes and thus protein degradation. These data indicate that ubiquitination is an important regulatory signal for controlling AMPAR function, which may be crucial for synaptic plasticity.

Modulation of excitatory synaptic transmission in rat hippocampal CA3 neurons by triphenyltin, an environmental pollutant

Triphenyltin (TPT) is an organometallic compound that poses a known environmental hazard to some fish and mollusks, as well as mammals. However, its neurotoxic mechanisms in the mammalian brain are still unclear. Thus, we have investigated mechanisms through which TPT modulates glutamatergic synaptic transmission, including spontaneous, miniature, and evoked excitatory postsynaptic currents (sEPSCs, mEPSCs, and eEPSCs respectively), in a rat hippocampal CA3 'synaptic-bouton' preparation. TPT, at environmentally relevant concentrations (30 nM to 1 μM), significantly increased the frequency of sEPSCs and mEPSCs in a concentration-dependent manner, without affecting the currents' amplitudes. The facilitatory effects of TPT on mEPSC frequency were seen even in a Ca(2+)-free external solution containing tetrodotoxin. These effects were further prolonged by adding caffeine, which releases Ca(2+) from intracellular Ca(2+) storage sites. In glutamatergic eEPSCs evoked by paired-pulse stimuli, TPT at concentrations greater than or equal to 100 nM markedly increased the current amplitude by the first pulse and decreased failure rate and pair-pulse ratio. On the other hand, both voltage-dependent Na(+) and Ca(2+) channels were unaffected by submicromolar concentrations of TPT. Overall, these results suggest that TPT, at environmentally relevant concentrations, affects presynaptic transmitter release machinery by directly modulating Ca(2+) storage. Further, findings of this study imply that excitotoxic mechanisms may underlie TPT-induced neuronal damage.

Commonalities between pain and memory mechanisms and their meaning for understanding chronic pain

Pain sensing neurons in the periphery (called nociceptors) and the central neurons that receive their projections show remarkable plasticity following injury. This plasticity results in amplification of pain signaling that is now understood to be crucial for the recovery and survival of organisms following injury. These same plasticity mechanisms may drive a transition to a nonadaptive chronic pain state if they fail to resolve following the termination of the healing process. Remarkable advances have been achieved in the past two decades in understanding the molecular mechanisms that underlie pain plasticity following injury. The mechanisms bear a striking resemblance to molecular mechanisms involved in learning and memory processes in other brain regions, including the hippocampus and cerebral cortex. Here those mechanisms, their commonalities and subtle differences, will be highlighted and their role in causing chronic pain will be discussed. Arising from these data is the striking argument that chronic pain is a disease of the nervous system, which distinguishes this phenomena from acute pain that is frequently a symptom alerting the organism to injury. This argument has important implications for the development of disease modifying therapeutics.

STIM2 regulates PKA-dependent phosphorylation and trafficking of AMPARs

STIMs (STIM1 and STIM2 in mammals) are transmembrane proteins that reside in the endoplasmic reticulum and regulate store-operated Ca²⁺ entry. STIM2 mediates cAMP/PKA-dependent phosphorylation of the AMPA receptor subunit GluA1 in excitatory neurons. In addition, STIM2 promotes cAMP-dependent surface delivery of GluA1.

STIMs (STIM1 and STIM2 in mammals) are transmembrane proteins that reside in the endoplasmic reticulum (ER) and regulate store-operated Ca²⁺ entry (SOCE). The function of STIMs in the brain is only beginning to be explored, and the relevance of SOCE in nerve cells is being debated. Here we identify STIM2 as a central organizer of excitatory synapses. STIM2, but not its paralogue STIM1, influences the formation of dendritic spines and shapes basal synaptic transmission in excitatory neurons. We further demonstrate that STIM2 is essential for cAMP/PKA-dependent phosphorylation of the AMPA receptor (AMPAR) subunit GluA1. cAMP triggers rapid migration of STIM2 to ER–plasma membrane (PM) contact sites, enhances recruitment of GluA1 to these ER-PM junctions, and promotes localization of STIM2 in dendritic spines. Both biochemical and imaging data suggest that STIM2 regulates GluA1 phosphorylation by coupling PKA to the AMPAR in a SOCE-independent manner. Consistent with a central role of STIM2 in regulating AMPAR phosphorylation, STIM2 promotes cAMP-dependent surface delivery of GluA1 through combined effects on exocytosis and endocytosis. Collectively our results point to a unique mechanism of synaptic plasticity driven by dynamic assembly of a STIM2 signaling complex at ER-PM contact sites.

The organization of AMPA receptor subunits at the postsynaptic membrane

AMPA receptors are the principal mediators of excitatory synaptic transmission in the mammalian central nervous system. The subunit composition of these tetrameric receptors helps to define their functional properties, and may also influence the synaptic trafficking implicated in long-term synaptic plasticity. However, the organization of AMPAR subunits within the synapse remains unclear. Here, we use postembedding immunogold electron microscopy to study the synaptic organization of AMPAR subunits in stratum radiatum of CA1 hippocampus in the adult rat. We find that GluA1 concentrates away from the center of the synapse, extending at least 25 nm beyond the synaptic specialization; in contrast, GluA3 is uniformly distributed along the synapse, and seldom extends beyond its lateral border. The fraction of extrasynaptic GluA1 is markedly higher in small than in large synapses; no such effect is seen for GluA3. These observations imply that different kinds of AMPARs are differently trafficked to and/or anchored at the synapse.

Signaling pathways relevant to cognition-enhancing drug targets

Aging is generally associated with a certain cognitive decline. However, individual differences exist. While age-related memory deficits can be observed in humans and rodents in the absence of pathological conditions, some individuals maintain intact cognitive functions up to an advanced age. The mechanisms underlying learning and memory processes involve the recruitment of multiple signaling pathways and gene expression, leading to adaptative neuronal plasticity and long-lasting changes in brain circuitry. This chapter summarizes the current understanding of how these signaling cascades could be modulated by cognition-enhancing agents favoring memory formation and successful aging. It focuses on data obtained in rodents, particularly in the rat as it is the most common animal model studied in this field. First, we will discuss the role of the excitatory neurotransmitter glutamate and its receptors, downstream signaling effectors [e.g., calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC), extracellular signal-regulated kinases (ERK), mammalian target of rapamycin (mTOR), cAMP response element-binding protein (CREB)], associated immediate early gene (e.g., Homer 1a, Arc and Zif268), and growth factors [insulin-like growth factors (IGFs) and brain-derived neurotrophic factor (BDNF)] in synaptic plasticity and memory formation. Second, the impact of the cholinergic system and related modulators on memory will be briefly reviewed. Finally, since dynorphin neuropeptides have recently been associated with memory impairments in aging, it is proposed as an attractive target to develop novel cognition-enhancing agents.

Genetic risk for schizophrenia: convergence on synaptic pathways involved in plasticity

Recent large-scale genomic studies have revealed two broad classes of risk alleles for schizophrenia: a polygenic component of risk mediated through multiple common risk variants and rarer more highly penetrant submicroscopic chromosomal deletions and duplications, known as copy number variants. The focus of this review is on the emerging findings from the latter and subsequent exome sequencing data of smaller, deleterious single nucleotide variants and indels. In these studies, schizophrenia patients were found to have enriched de novo mutations in genes belonging to the postsynaptic density at glutamatergic synapses, particularly components of the N-methyl-D-aspartate receptor signaling complex, including the PSD-95 complex, activity-regulated cytoskeleton-associated protein interactors, the fragile X mental retardation protein complex, voltage-gated calcium channels, and genes implicated in actin cytoskeletal dynamics. The convergence of these implicated genes onto a coherent biological pathway at the synapse, with a specific role in plasticity, provides a significant advance in understanding pathogenesis and points to new targets for biological investigation. We consider the implications of these studies in the context of existing genetic data and the potential need to reassess diagnostic boundaries of neuropsychiatric disorders before discussing ways forward for more directed mechanistic studies to develop stratified, novel therapeutic approaches in the future.

Coexistence of two forms of LTP in ACC provides a synaptic mechanism for the interactions between anxiety and chronic pain

Chronic pain can lead to anxiety and anxiety can enhance the sensation of pain. Unfortunately, little is known about the synaptic mechanisms that mediate these re-enforcing interactions. Here we characterized two forms of long-term potentiation (LTP) in the anterior cingulate cortex (ACC); a presynaptic form (pre-LTP) that requires kainate receptors and a postsynaptic form (post-LTP) that requires N-methyl-D-aspartate receptors. Pre-LTP also involves adenylyl cyclase and protein kinase A and is expressed via a mechanism involving hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Interestingly, chronic pain and anxiety both result in selective occlusion of pre-LTP. Significantly, microinjection of the HCN blocker ZD7288 into the ACC in vivo produces both anxiolytic and analgesic effects. Our results provide a mechanism by which two forms of LTP in the ACC may converge to mediate the interaction between anxiety and chronic pain.

The dendritic SNARE fusion machinery involved in AMPARs insertion during long-term potentiation

Sorting endosomes carry α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) from their maturation sites to their final destination at the dendritic plasma membrane through both constitutive and regulated exocytosis. Insertion of functional AMPARs into the postsynaptic membrane is essential for maintaining fast excitatory synaptic transmission and plasticity. Despite this crucial role in neuronal function, the machinery mediating the fusion of AMPAR-containing endosomes in dendrites has been largely understudied in comparison to presynaptic vesicle exocytosis. Increasing evidence suggests that similarly to neurotransmitter release, AMPARs insertion relies on the formation of a SNARE complex (soluble NSF-attachment protein receptor), whose composition in dendrites has just begun to be elucidated. This review analyzes recent findings of the fusion machinery involved in regulated AMPARs insertion and discusses how dendritic exocytosis and AMPARs lateral diffusion may work together to support synaptic plasticity.

Synaptic localization of neurotransmitter receptors: comparing mechanisms for AMPA and GABAA receptors

Ionotropic neurotransmitter receptors mediate fast synaptic transmission by localizing at postsynapses. Changes in receptor number at synapses induce synaptic plasticity. Thus, mechanisms for the synaptic localization of receptors in basal transmission and synaptic plasticity have been investigated extensively. Recent findings reveal that synaptic localization of tetrameric AMPA receptors in basal transmission requires the PDZ binding of TARP auxiliary subunits, which modulate receptor properties and pharmacology. On the other hand, pentameric GABAA receptors require multiple receptor subunits for their synaptic localization in basal transmission. AMPA receptors seem to utilize distinct mechanisms for basal synaptic localization and synaptic insertion during plasticity. Revealing precise mechanisms for receptor synaptic localization may establish new approaches to control synaptic transmission.