Mechanisms of CaMKII action in long-term potentiation

Sleep and protein synthesis-dependent synaptic plasticity: impacts of sleep loss and stress

Sleep has been ascribed a critical role in cognitive functioning. Several lines of evidence implicate sleep in the consolidation of synaptic plasticity and long-term memory. Stress disrupts sleep while impairing synaptic plasticity and cognitive performance. Here, we discuss evidence linking sleep to mechanisms of protein synthesis-dependent synaptic plasticity and synaptic scaling. We then consider how disruption of sleep by acute and chronic stress may impair these mechanisms and degrade sleep function.

Analysis of synaptic gene expression in the neocortex of primates reveals evolutionary changes in glutamatergic neurotransmission

Increased relative brain size characterizes the evolution of primates, suggesting that enhanced cognition plays an important part in the behavioral adaptations of this mammalian order. In addition to changes in brain anatomy, cognition can also be regulated by molecular changes that alter synaptic function, but little is known about modifications of synapses in primate brain evolution. The aim of the current study was to investigate the expression patterns and evolution of 20 synaptic genes from the prefrontal cortex of 12 primate species. The genes investigated included glutamate receptors, scaffolding proteins, synaptic vesicle components, as well as factors involved in synaptic vesicle release and structural components of the nervous system. Our analyses revealed that there have been significant changes during primate brain evolution in the components of the glutamatergic signaling pathway in terms of gene expression, protein expression, and promoter sequence changes. These results could entail functional modifications in the regulation of specific genes related to processes underlying learning and memory.

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.

Environment and Brain Plasticity: Towards an Endogenous Pharmacotherapy

Brain plasticity refers to the remarkable property of cerebral neurons to change their structure and function in response to experience, a fundamental theoretical theme in the field of basic research and a major focus for neural rehabilitation following brain disease. While much of the early work on this topic was based on deprivation approaches relying on sensory experience reduction procedures, major advances have been recently obtained using the conceptually opposite paradigm of environmental enrichment, whereby an enhanced stimulation is provided at multiple cognitive, sensory, social, and motor levels. In this survey, we aim to review past and recent work concerning the influence exerted by the environment on brain plasticity processes, with special emphasis on the underlying cellular and molecular mechanisms and starting from experimental work on animal models to move to highly relevant work performed in humans. We will initiate introducing the concept of brain plasticity and describing classic paradigmatic examples to illustrate how changes at the level of neuronal properties can ultimately affect and direct key perceptual and behavioral outputs. Then, we describe the remarkable effects elicited by early stressful conditions, maternal care, and preweaning enrichment on central nervous system development, with a separate section focusing on neurodevelopmental disorders. A specific section is dedicated to the striking ability of environmental enrichment and physical exercise to empower adult brain plasticity. Finally, we analyze in the last section the ever-increasing available knowledge on the effects elicited by enriched living conditions on physiological and pathological aging brain processes.

AMPARs and synaptic plasticity: the last 25 years

The study of synaptic plasticity and specifically LTP and LTD is one of the most active areas of research in neuroscience. In the last 25 years we have come a long way in our understanding of the mechanisms underlying synaptic plasticity. In 1988, AMPA and NMDA receptors were not even molecularly identified and we only had a simple model of the minimal requirements for the induction of plasticity. It is now clear that the modulation of the AMPA receptor function and membrane trafficking is critical for many forms of synaptic plasticity and a large number of proteins have been identified that regulate this complex process. Here we review the progress over the last two and a half decades and discuss the future challenges in the field.

Human immunodeficiency virus-1 Tat protein increases the number of inhibitory synapses between hippocampal neurons in culture

Synaptodendritic damage correlates with cognitive decline in many neurodegenerative diseases, including human immunodeficiency virus-1 (HIV-1)-associated neurocognitive disorders (HAND). Because HIV-1 does not infect neurons, viral-mediated toxicity is indirect, resulting from released neurotoxins such as the HIV-1 protein transactivator of transcription (Tat). We compared the effects of Tat on inhibitory and excitatory synaptic connections between rat hippocampal neurons using an imaging-based assay that quantified clusters of the scaffolding proteins gephyrin or PSD95 fused to GFP. Tat (24 h) increased the number of GFP-gephyrin puncta and decreased the number of PSD95-GFP puncta. The effects of Tat on inhibitory and excitatory synapse number were mediated via the low-density lipoprotein receptor-related protein and subsequent Ca(2+) influx through GluN2A-containing NMDA receptors (NMDARs). The effects of Tat on synapse number required cell-autonomous activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Ca(2+) buffering experiments suggested that loss of excitatory synapses required activation of CaMKII in close apposition to the NMDAR, whereas the increase in inhibitory synapses required Ca(2+) diffusion to a more distal site. The increase in inhibitory synapses was prevented by inhibiting the insertion of GABAA receptors into the membrane. Synaptic changes induced by Tat (16 h) were reversed by blocking either GluN2B-containing NMDARs or neuronal nitric oxide synthase, indicating changing roles for pathways activated by NMDAR subtypes during the neurotoxic process. Compensatory changes in the number of inhibitory and excitatory synapses may serve as a novel mechanism to reduce network excitability in the presence of HIV-1 neurotoxins; these changes may inform the development of treatments for HAND.

Are AMPA receptor positive allosteric modulators potential pharmacotherapeutics for addiction?

Positive allosteric modulators (PAMs) of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are a diverse class of compounds that increase fast excitatory transmission in the brain. AMPA PAMs have been shown to facilitate long-term potentiation, strengthen communication between various cortical and subcortical regions, and some of these compounds increase the production and release of brain-derived neurotrophic factor (BDNF) in an activity-dependent manner. Through these mechanisms, AMPA PAMs have shown promise as broad spectrum pharmacotherapeutics in preclinical and clinical studies for various neurodegenerative and psychiatric disorders. In recent years, a small collection of preclinical animal studies has also shown that AMPA PAMs may have potential as pharmacotherapeutic adjuncts to extinction-based or cue-exposure therapies for the treatment of drug addiction. The present paper will review this preclinical literature, discuss novel data collected in our laboratory, and recommend future research directions for the possible development of AMPA PAMs as anti-addiction medications.

Activity-dependent regulation of NMDA receptors in substantia nigra dopaminergic neurones

N-Methyl-d-aspartate receptors (NMDARs) are Ca(2+)-permeable glutamate receptors that play a critical role in synaptic plasticity and promoting cell survival. However, overactive NMDARs can trigger cell death signalling pathways and have been implicated in substantia nigra pars compacta (SNc) pathology in Parkinson's disease. Calcium ion influx through NMDARs recruits Ca(2+)-dependent proteins that can regulate NMDAR activity. The surface density of NMDARs can also be regulated dynamically in response to receptor activity via Ca(2+)-independent mechanisms. We have investigated the activity-dependent regulation of NMDARs in SNc dopaminergic neurones. Repeated whole-cell agonist applications resulted in a decline in the amplitude of NMDAR currents (current run-down) that was use dependent and not readily reversible. Run-down was reduced by increasing intracellular Ca(2+) buffering or by reducing Ca(2+) influx but did not appear to be mediated by the same regulatory proteins that cause Ca(2+)-dependent run-down in hippocampal neurones. The NMDAR current run-down may be mediated in part by a Ca(2+)-independent mechanism, because intracellular dialysis with a dynamin-inhibitory peptide reduced run-down, suggesting a role for clathrin-mediated endocytosis in the regulation of the surface density of receptors. Synaptic NMDARs were also subject to current run-down during repeated low-frequency synaptic stimulation in a Ca(2+)-dependent but dynamin-independent manner. Thus, we report, for the first time, regulation of NMDARs in SNc dopaminergic neurones by changes in intracellular Ca(2+) at both synaptic and extrasynaptic sites and provide evidence for activity-dependent changes in receptor trafficking. These mechanisms may contribute to intracellular Ca(2+) homeostasis in dopaminergic neurones by limiting Ca(2+) influx through the NMDAR.

Subunit-specific trafficking mechanisms regulating the synaptic expression of Ca(2+)-permeable AMPA receptors

AMPA receptors are the main excitatory neurotransmitter receptor in the brain, and hence regulating the number or properties of synaptic AMPA receptors brings about critical changes in synaptic transmission. Synaptic plasticity is thought to underlie learning and memory, and can be brought about by decreasing or increasing the number of AMPA receptors localised to synaptic sites by precisely regulating AMPA receptor trafficking. AMPA receptors are tetrameric assemblies of subunits GluA1-4, and the vast majority are GluA1/2 and GluA2/3 heteromers. The inclusion of GluA2 subunit is critical because it renders the AMPA receptor channel impermeable to Ca(2+) ions. The vast majority of synaptic AMPA receptors in the brain contain GluA2, but relatively recent discoveries indicate that an increasing number of specific forms of synaptic plasticity involve not only an alteration of the number of synaptic AMPA receptors, but also changes to their GluA2 content. The resulting change in AMPA receptor Ca(2+) permeability clearly has profound consequences for synaptic transmission and intracellular signalling events. The subunit-specific trafficking mechanisms that cause such changes represent an emerging field of research with implications for an increasing number of physiological or pathological situations, and are the topic of this review.

Untangling the two-way signalling route from synapses to the nucleus, and from the nucleus back to the synapses

During learning and memory, it has been suggested that the coordinated electrical activity of hippocampal neurons translates information about the external environment into internal neuronal representations, which then are stored initially within the hippocampus and subsequently into other areas of the brain. A widely held hypothesis posits that synaptic plasticity is a key feature that critically modulates the triggering and the maintenance of such representations, some of which are thought to persist over time as traces or tags. However, the molecular and cell biological basis for these traces and tags has remained elusive. Here, we review recent findings that help clarify some of the molecular and cellular mechanisms critical for these events, by untangling a two-way signalling crosstalk route between the synapses and the neuronal soma. In particular, a detailed interrogation of the soma-to-synapse delivery of immediate early gene product Arc/Arg3.1, whose induction is triggered by heightened synaptic activity in many brain areas, teases apart an unsuspected 'inverse' synaptic tagging mechanism that likely contributes to maintaining the contrast of synaptic weight between strengthened and weak synapses within an active ensemble.

The synaptic plasticity and memory hypothesis: encoding, storage and persistence

The synaptic plasticity and memory hypothesis asserts that activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the encoding and trace storage of the type of memory mediated by the brain area in which it is observed. Criteria for establishing the necessity and sufficiency of such plasticity in mediating trace storage have been identified and are here reviewed in relation to new work using some of the diverse techniques of contemporary neuroscience. Evidence derived using optical imaging, molecular-genetic and optogenetic techniques in conjunction with appropriate behavioural analyses continues to offer support for the idea that changing the strength of connections between neurons is one of the major mechanisms by which engrams are stored in the brain.

Expression mechanisms underlying long-term potentiation: a postsynaptic view, 10 years on

This review focuses on the research that has occurred over the past decade which has solidified a postsynaptic expression mechanism for long-term potentiation (LTP). However, experiments that have suggested a presynaptic component are also summarized. It is argued that the pairing of glutamate uncaging onto single spines with postsynaptic depolarization provides the final and most elegant demonstration of a postsynaptic expression mechanism for NMDA receptor-dependent LTP. The fact that the magnitude of this LTP is similar to that evoked by pairing synaptic stimulation and depolarization leaves little room for a substantial presynaptic component. Finally, recent data also require a revision in our thinking about the way AMPA receptors (AMPARs) are recruited to the postsynaptic density during LTP. This recruitment is independent of subunit type, but does require an adequate reserve pool of extrasynaptic receptors.

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.

Recombinant probes reveal dynamic localization of CaMKIIα within somata of cortical neurons

In response to NMDA receptor stimulation, CaMKIIα moves rapidly from a diffuse distribution within the shafts of neuronal dendrites to a clustered postsynaptic distribution. However, less is known about CaMKIIα localization and trafficking within neuronal somata. Here we use a novel recombinant probe capable of labeling endogenous CaMKIIα in living rat neurons to examine its localization and trafficking within the somata of cortical neurons. This probe, which was generated using an mRNA display selection, binds to endogenous CaMKIIα at high affinity and specificity following expression in rat cortical neurons in culture. In ∼45% of quiescent cortical neurons, labeled clusters of CaMKIIα 1-4 μm in diameter were present. Upon exposure to glutamate and glycine, CaMKIIα clusters disappeared in a Ca(2+)-dependent manner within seconds. Moreover, minutes after the removal of glutamate and glycine, the clusters returned to their original configuration. The clusters, which also appear in cortical neurons in sections taken from mouse brains, contain actin and disperse upon exposure to cytochalasin D, an actin depolymerizer. In conclusion, within the soma, CaMKII localizes and traffics in a manner that is distinct from its localization and trafficking within the dendrites.

Reaction-diffusion in the NEURON simulator

In order to support research on the role of cell biological principles (genomics, proteomics, signaling cascades and reaction dynamics) on the dynamics of neuronal response in health and disease, NEURON's Reaction-Diffusion (rxd) module in Python provides specification and simulation for these dynamics, coupled with the electrophysiological dynamics of the cell membrane. Arithmetic operations on species and parameters are overloaded, allowing arbitrary reaction formulas to be specified using Python syntax. These expressions are then transparently compiled into bytecode that uses NumPy for fast vectorized calculations. At each time step, rxd combines NEURON's integrators with SciPy's sparse linear algebra library.

Plasticity of dendritic spines: subcompartmentalization of signaling

The ability to induce and study neuronal plasticity in single dendritic spines has greatly advanced our understanding of the signaling mechanisms that mediate long-term potentiation. It is now clear that in addition to compartmentalization by the individual spine, subcompartmentalization of biochemical signals occurs at specialized microdomains within the spine. The spatiotemporal coordination of these complex cascades allows for the concomitant remodeling of the postsynaptic density and actin spinoskeleton and for the regulation of membrane traffic to express functional and structural plasticity. Here, we highlight recent findings in the signaling cascades at spine microdomains as well as the challenges and approaches to studying plasticity at the spine level.

Synaptic NMDA receptor stimulation activates PP1 by inhibiting its phosphorylation by Cdk5

Synaptic stimulation promotes proteasome-dependent degradation of p35, inactivation of Cdk5, and decreased phosphorylation of PP1, allowing PP1 to act in the induction of long-term depression.

The serine/threonine protein phosphatase protein phosphatase 1 (PP1) is known to play an important role in learning and memory by mediating local and downstream aspects of synaptic signaling, but how PP1 activity is controlled in different forms of synaptic plasticity remains unknown. We find that synaptic N-methyl-d-aspartate (NMDA) receptor stimulation in neurons leads to activation of PP1 through a mechanism involving inhibitory phosphorylation at Thr320 by Cdk5. Synaptic stimulation led to proteasome-dependent degradation of the Cdk5 regulator p35, inactivation of Cdk5, and increased auto-dephosphorylation of Thr320 of PP1. We also found that neither inhibitor-1 nor calcineurin were involved in the control of PP1 activity in response to synaptic NMDA receptor stimulation. Rather, the PP1 regulatory protein, inhibitor-2, formed a complex with PP1 that was controlled by synaptic stimulation. Finally, we found that inhibitor-2 was critical for the induction of long-term depression in primary neurons. Our work fills a major gap regarding the regulation of PP1 in synaptic plasticity.

Molecular mechanisms underlying neuronal synaptic plasticity: systems biology meets computational neuroscience in the wilds of synaptic plasticity

Interactions among signaling pathways that are activated by transmembrane receptors produce complex networks and emergent dynamical behaviors that are implicated in synaptic plasticity. Temporal dynamics and spatial aspects are critical determinants of cell responses such as synaptic plasticity, although the mapping between spatiotemporal activity pattern and direction of synaptic plasticity is not completely understood. Computational modeling of neuronal signaling pathways has significantly contributed to understanding signaling pathways underlying synaptic plasticity. Spatial models of signaling pathways in hippocampal neurons have revealed mechanisms underlying the spatial distribution of extracellular signal-related kinase (ERK) activation in hippocampal neurons. Other spatial models have demonstrated that the major role of anchoring proteins in striatal and hippocampal synaptic plasticity is to place molecules near their activators. Simulations of yet other models have revealed that the spatial distribution of synaptic plasticity may differ for potentiation versus depression. In general, the most significant advances have been made by interactive modeling and experiments; thus, an interdisciplinary approach should be applied to investigate critical issues in neuronal signaling pathways. These issues include identifying which transmembrane receptors are key for activating ERK in neurons, and the crucial targets of kinases that produce long-lasting synaptic plasticity. Although the number of computer programs for computationally efficient simulation of large reaction-diffusion networks is increasing, parameter estimation and sensitivity analysis in these spatial models remain more difficult than in single compartment models. Advances in live cell imaging coupled with further software development will continue to accelerate the development of spatial models of synaptic plasticity.

Rapid effects of oestrogen on synaptic plasticity: interactions with actin and its signalling proteins

Oestrogen rapidly enhances fast excitatory postsynaptic potentials, facilitates long-term potentiation (LTP) and increases spine numbers. Each effect likely contributes to the influence of the steroid on cognition and memory. In the present review, we first describe a model for the substrates of LTP that includes an outline of the synaptic events occurring during induction, expression and consolidation. Briefly, critical signalling pathways involving the small GTPases RhoA and Rac/Cdc42 are activated by theta burst-induced calcium influx and initiate actin filament assembly via phosphorylation (inactivation) of cofilin. Reorganisation of the actin cytoskeleton changes spine and synapse morphology, resulting in increased concentrations of AMPA receptors at stimulated contacts. We then use the synaptic model to develop a specific hypothesis about how oestrogen affects both baseline transmission and plasticity. Brief infusions of 17β-oestradiol (E2 ) reversibly stimulate the RhoA, cofilin phosphorylation and actin polymerisation cascade of the LTP machinery; blocking this eliminates the effects of the steroid on transmission. We accordingly propose that E2 induces a weak form of LTP and thereby increases synaptic responses, a hypothesis that also accounts for how it markedly enhances theta burst induced potentiation. Although the effects of E2 on the cytoskeleton could be a result of the direct activation of small GTPases by oestrogen receptors on the synaptic membrane, the hormone also activates tropomyosin-related kinase B receptors for brain-derived neurotrophic factor, a neurotrophin that engages the RhoA-cofilin sequence and promotes LTP. The latter observations raise the possibility that E2 produces its effects on synaptic physiology via transactivation of neighbouring receptors that have prominent roles in the management of spine actin, synaptic physiology and plasticity.

Surface trafficking of NMDA receptors: gathering from a partner to another

Understanding the molecular and cellular pathways by which neurons integrate signals from different neurotransmitter systems has been among the major challenges of modern neuroscience. The ionotropic glutamate NMDA receptor plays a key role in the maturation and plasticity of glutamate synapses, both in physiology and pathology. It recently appeared that the surface distribution of NMDA receptors is dynamically regulated through lateral diffusion, providing for instance a powerful way to rapidly affect the content and composition of synaptic receptors. The ability of various neuromodulators to regulate NMDA receptor signaling revealed that this receptor can also serve as a molecular integrator of the ambient neuronal environment. Although still in its infancy, we here review our current understanding of the cellular regulation of NMDA receptor surface dynamics. We specifically discuss the roles of well-known modulators, such as dopamine, and membrane interactors in these regulatory processes, exemplifying the recent evidence that the direct interaction between NMDAR and dopamine receptors regulates their surface diffusion and distribution. In addition to the well-established modulation of NMDA receptor signaling by intracellular pathways, the surface dynamics of the receptor is now emerging as the first level of regulation, opening new pathophysiological perspectives for innovative therapeutical strategies.

Glutamate drugs and pharmacogenetics of OCD: a pathway-based exploratory approach

INTRODUCTION

Neuropharmacology research in glutamate-modulating drugs supports their development and use in the management of neuropsychiatric disorders, including major depression, Alzheimer's disorder and schizophrenia. Concomitantly, there is a growing use of these agents used in the treatment of obsessive-compulsive disorder (OCD).

AREAS COVERED

This article provides a review of glutamate-modulating drugs used in the treatment of OCD. Specifically, the authors examine riluzole, N-acetylcysteine, d-cycloserine, glycine, ketamine, memantine and acamprosate as treatments. Furthermore, recent genetic epidemiology research findings are presented with a focus on the positional candidate genes SLC1A1 (a glutamate transporter), ADAR3 (an RNA-editing enzyme), RYR3 (a Ca(2+) channel), PBX1 (a homeobox transcription factor) and a GWAS candidate gene, DLGAP1 (a protein interacting with post-synaptic density). These genetic findings are submitted to a curated bioinformatics database to conform a biological network for discerning potential pharmacological targets.

EXPERT OPINION

In the genetically informed network, known genes and identified key connecting components, including DLG4 (a developmental gene), PSD-95 (a synaptic scaffolding protein) and PSEN1 (presenilin, a regulator of secretase), conform a group of potential pharmacological targets. These potential targets can be explored, in the future, to deliver new therapeutic approaches to OCD. There is also the need to develop a better understanding of neuroprotective mechanisms as a foundation for future OCD drug discovery.

The effects of early-life seizures on hippocampal dendrite development and later-life learning and memory

Severe childhood epilepsy is commonly associated with intellectual developmental disabilities. The reasons for these cognitive deficits are likely multifactorial and will vary between epilepsy syndromes and even among children with the same syndrome. However, one factor these children have in common is the recurring seizures they experience - sometimes on a daily basis. Supporting the idea that the seizures themselves can contribute to intellectual disabilities are laboratory results demonstrating spatial learning and memory deficits in normal mice and rats that have experienced recurrent seizures in infancy. Studies reviewed here have shown that seizures in vivo and electrographic seizure activity in vitro both suppress the growth of hippocampal pyramidal cell dendrites. A simplification of dendritic arborization and a resulting decrease in the number and/or properties of the excitatory synapses on them could help explain the observed cognitive disabilities. There are a wide variety of candidate mechanisms that could be involved in seizure-induced growth suppression. The challenge is designing experiments that will help focus research on a limited number of potential molecular events. Thus far, results suggest that growth suppression is NMDA receptor-dependent and associated with a decrease in activation of the transcription factor CREB. The latter result is intriguing since CREB is known to play an important role in dendrite growth. Seizure-induced dendrite growth suppression may not occur as a single process in which pyramidal cells dendrites simply stop growing or grow slower compared to normal neurons. Instead, recent results suggest that after only a few hours of synchronized epileptiform activity in vitro dendrites appear to partially retract. This acute response is also NMDA receptor dependent and appears to be mediated by the Ca(+2)/calmodulin-dependent phosphatase, calcineurin. An understanding of the staging of seizure-induced growth suppression and the underlying molecular mechanisms will likely prove crucial for developing therapeutic strategies aimed at ameliorating the intellectual developmental disabilities associated with intractable childhood epilepsy.

The timing of activity is a regulatory signal during development of neural connections

In PNS and CNS remarkable rearrangements occur soon after the connections are laid down in the course of embryonic life. These processes clearly follow the period of developmental cell death and mostly take place during the very beginning of postnatal life. They consist in changes of the peripheral fields of neurons, marked by elimination of many inputs, while others undergo further maturation and strengthening. Along the efforts to uncover the signals that regulate development, it turned out that while the initial construction of the circuits is heavily based on chemical cues, the subsequent rearrangement is markedly influence by activity. Here we describe experiments testing the influence on developmental plasticity of a particular aspect of activity, the timing of nerve impulses in the competing inputs. Two recent investigations are reviewed, indicating strikingly similar developmental features in quite different systems, neuromuscular and visual. A sharp contrast between the effects of synchrony and asynchrony emerges, indicating that Hebb-related activity rules are important not only for learning but also for development.

Ionotropic glutamate receptors and voltage-gated Ca²⁺ channels in long-term potentiation of spinal dorsal horn synapses and pain hypersensitivity

Over the last twenty years of research on cellular mechanisms of pain hypersensitivity, long-term potentiation (LTP) of synaptic transmission in the spinal cord dorsal horn (DH) has emerged as an important contributor to pain pathology. Mechanisms that underlie LTP of spinal DH neurons include changes in the numbers, activity, and properties of ionotropic glutamate receptors (AMPA and NMDA receptors) and of voltage-gated Ca²⁺ channels. Here, we review the roles and mechanisms of these channels in the induction and expression of spinal DH LTP, and we present this within the framework of the anatomical organization and synaptic circuitry of the spinal DH. Moreover, we compare synaptic plasticity in the spinal DH with classical LTP described for hippocampal synapses.

CASK regulates SAP97 conformation and its interactions with AMPA and NMDA receptors

SAP97 interacts with AMPA receptors (AMPARs) and NMDA receptors (NMDARs) during sorting and trafficking to synapses. Here we addressed how SAP97 distinguishes between AMPARs and NMDARs and what role the adaptor/scaffold protein, CASK, plays in the process. Using intramolecular SAP97 Förster resonance energy transfer sensors, we demonstrated that SAP97 is in "extended" or "compact" conformations in vivo. SAP97 conformation was regulated by a direct interaction between SAP97 and CASK through L27 protein-interaction domains on each protein. Unbound SAP97 was mostly in the compact conformation, while CASK binding stabilized it in an extended conformation. In HEK cells and rat hippocampal neurons, SAP97 in the compact conformation preferentially associated and colocalized with GluA1-containing AMPARs, and in the extended conformation colocalized with GluN2B-containing NMDARs. Altogether, our findings suggest a molecular mechanism by which CASK binding regulates SAP97 conformation and its subsequent sorting and synaptic targeting of AMPARs and NMDARs during trafficking to synapses.

Effects of adult exposure to bisphenol a on genes involved in the physiopathology of rat prefrontal cortex

Several neurological and behavioral dysfunctions have been reported in animals exposed to bisphenol A (BPA). However, little is known about the impact of adult exposure to BPA on brain physiopathology. Here, we focused on prefrontal cortex (PFC) of rats, because it is an important area for cognitive control, complex behaviors and is altered in many psychopathologies. Gamma-aminobutyric acid (GABA) and serotonin (5-HT) systems are essential for PFC function. Therefore, we examined the effects of adult exposure to BPA on 5α-Reductase (5α-R) and cytochrome P450 aromatase (P450arom), enzymes that synthesize GABAA receptor modulators, and tryptophan hydroxylase (Tph), the rate-limiting enzyme in 5-HT biosynthesis. To gain better understanding of BPA's action in the adult PFC, 84 genes involved in neurotoxicity were also analysed. Adult male and female rats were subcutaneously injected for 4 days with 50 µg/kg/day, the current reference safe dose for BPA. mRNA and protein levels of 5α-R, P450arom and Tph were quantified by real-time RT-PCR and Western blot. Genes linked to neurotoxicity were analyzed by PCR-Array technology. Adult exposure to BPA increased both P450arom and Tph2 expression in PFC of male and female, but decreased 5α-R1 expression in female. Moreover, we identified 17 genes related to PFC functions such as synaptic plasticity and memory, as potential targets of BPA. Our results provided new insights on the molecular mechanisms underlying BPA action in the physiopathology of PFC, but also raise the question about the safety of short-term exposure to it in the adulthood.

Antihypertensive drug Valsartan promotes dendritic spine density by altering AMPA receptor trafficking

Recent studies demonstrated that the antihypertensive drug Valsartan improved spatial and episodic memory in mouse models of Alzheimer's Disease (AD) and human subjects with hypertension. However, the molecular mechanism by which Valsartan can regulate cognitive function is still unknown. Here, we investigated the effect of Valsartan on dendritic spine formation in primary hippocampal neurons, which is correlated with learning and memory. Interestingly, we found that Valsartan promotes spinogenesis in developing and mature neurons. In addition, we found that Valsartan increases the puncta number of PSD-95 and trends toward an increase in the puncta number of synaptophysin. Moreover, Valsartan increased the cell surface levels of AMPA receptors and selectively altered the levels of spinogenesis-related proteins, including CaMKIIα and phospho-CDK5. These data suggest that Valsartan may promote spinogenesis by enhancing AMPA receptor trafficking and synaptic plasticity signaling.

Berbamine inhibits the growth of liver cancer cells and cancer-initiating cells by targeting Ca²⁺/calmodulin-dependent protein kinase II

Liver cancer is the third leading cause of cancer deaths worldwide but no effective treatment toward liver cancer is available so far. Therefore, there is an unmet medical need to identify novel therapies to efficiently treat liver cancer and improve the prognosis of this disease. Here, we report that berbamine and one of its derivatives, bbd24, potently suppressed liver cancer cell proliferation and induced cancer cell death by targeting Ca(2+)/calmodulin-dependent protein kinase II (CAMKII). Furthermore, berbamine inhibited the in vivo tumorigenicity of liver cancer cells in NOD/SCID mice and downregulated the self-renewal abilities of liver cancer-initiating cells. Chemical inhibition or short hairpin RNA-mediated knockdown of CAMKII recapitulated the effects of berbamine, whereas overexpression of CAMKII promoted cancer cell proliferation and increased the resistance of liver cancer cells to berbamine treatments. Western blot analyses of human liver cancer specimens showed that CAMKII was hyperphosphorylated in liver tumors compared with the paired peritumor tissues, which supports a role of CAMKII in promoting human liver cancer progression and the potential clinical use of berbamine for liver cancer therapies. Our data suggest that berbamine and its derivatives are promising agents to suppress liver cancer growth by targeting CAMKII. Mol Cancer Ther; 12(10); 2067-77. ©2013 AACR.

Camkii-mediated phosphorylation regulates distributions of Syngap-α1 and -α2 at the postsynaptic density

SynGAP, a protein abundant at the postsynaptic density (PSD) of glutamatergic neurons, is known to modulate synaptic strength by regulating the incorporation of AMPA receptors at the synapse. Two isoforms of SynGAP, α1 and α2, which differ in their C-termini, have opposing effects on synaptic strength. In the present study, antibodies specific for SynGAP-α1 and SynGAP-α2 are used to compare the distribution patterns of the two isoforms at the postsynaptic density (PSD) under basal and excitatory conditions. Western immunoblotting shows enrichment of both isoforms in PSD fractions isolated from adult rat brain. Immunogold electron microscopy of rat hippocampal neuronal cultures shows similar distribution of both isoforms at the PSD, with a high density of immunolabel within the PSD core under basal conditions. Application of NMDA promotes movement of SynGAP-α1 as well as SynGAP-α2 out of the PSD core. In isolated PSDs both isoforms of SynGAP can be phosphorylated upon activation of the endogenous CaMKII. Application of tatCN21, a cell-penetrating inhibitor of CaMKII, to hippocampal neuronal cultures blocks NMDA-induced redistribution of SynGAP-α1 and SynGAP-α2. Thus CaMKII activation promotes the removal of two distinct C-terminal SynGAP variants from the PSD.

Nanoscale scaffolding domains within the postsynaptic density concentrate synaptic AMPA receptors

Scaffolding molecules at the postsynaptic membrane form the foundation of excitatory synaptic transmission by establishing the architecture of the postsynaptic density (PSD), but the small size of the synapse has precluded measurement of PSD organization in live cells. We measured the internal structure of the PSD in live neurons at approximately 25 nm resolution using photoactivated localization microscopy (PALM). We found that four major PSD scaffold proteins were each organized in distinctive ∼80 nm ensembles able to undergo striking changes over time. Bidirectional PALM and single-molecule immunolabeling showed that dense nanodomains of PSD-95 were preferentially enriched in AMPA receptors more than NMDA receptors. Chronic suppression of activity triggered changes in PSD interior architecture that may help amplify synaptic plasticity. The observed clustered architecture of the PSD controlled the amplitude and variance of simulated postsynaptic currents, suggesting several ways in which PSD interior organization may regulate the strength and plasticity of neurotransmission.

Exploring the nicotinic acetylcholine receptor-associated proteome with iTRAQ and transgenic mice

Neuronal nicotinic acetylcholine receptors (nAChRs) containing α4 and β2 subunits are the principal receptors in the mammalian central nervous system that bind nicotine with high affinity. These nAChRs are involved in nicotine dependence, mood disorders, neurodegeneration and neuroprotection. However, our understanding of the interactions between α4β2-containing (α4β2(∗)) nAChRs and other proteins remains limited. In this study, we identified proteins that interact with α4β2(∗) nAChRs in a genedose dependent pattern by immunopurifying β2(∗) nAChRs from mice that differ in α4 and β2 subunit expression and performing proteomic analysis using isobaric tags for relative and absolute quantitation (iTRAQ). Reduced expression of either the α4 or the β2 subunit results in a correlated decline in the expression of a number of putative interacting proteins. We identified 208 proteins co-immunoprecipitated with these nAChRs. Furthermore, stratified linear regression analysis indicated that levels of 17 proteins was correlated significantly with expression of α4β2 nAChRs, including proteins involved in cytoskeletal rearrangement and calcium signaling. These findings represent the first application of quantitative proteomics to produce a β2(∗) nAChR interactome and describe a novel technique used to discover potential targets for pharmacological manipulation of α4β2 nAChRs and their downstream signaling mechanisms.

Proteostasis in complex dendrites

Like all cells, neurons are made of proteins that have characteristic synthesis and degradation profiles. Unlike other cells, however, neurons have a unique multipolar architecture that makes ∼10,000 synaptic contacts with other neurons. Both the stability and modifiability of the neuronal proteome are crucial for its information-processing, storage and plastic properties. The cell biological mechanisms that synthesize, modify, deliver and degrade dendritic and synaptic proteins are not well understood but appear to reflect unique solutions adapted to the particular morphology of neurons.

Recognition memory and synaptic plasticity in the perirhinal and prefrontal cortices

Work is reviewed that relates recognition memory to studies of synaptic plasticity mechanisms in perirhinal and prefrontal cortices. The aim is to consider evidence that perirhinal cortex and medial prefrontal cortex store rather than merely transmit information necessary for recognition memory and, if so, to consider what mechanisms are potentially available within these cortices for producing such storage through synaptic change. Interventions with known actions on plasticity mechanisms are reviewed in relation to their effects on recognition memory processes. These interventions importantly include those involving antagonism of glutamatergic and cholinergic receptors but also inhibition of plasticity consolidation and expression mechanisms. It is concluded that there is strong evidence that perirhinal cortex is involved in information storage necessary for object recognition memory and, moreover, that such storage involves synaptic weakening mechanisms including the removal of AMPA glutamate receptors from synapses. There is good evidence that medial prefrontal cortex is necessary for associative and temporal order recognition memory and that this cortex expresses plasticity mechanisms that potentially allow the storage of information. However, the case for medial prefrontal cortex acting as a store requires further support.

The synaptic maintenance problem: membrane recycling, Ca2+ homeostasis and late onset degeneration

Most neurons are born with the potential to live for the entire lifespan of the organism. In addition, neurons are highly polarized cells with often long axons, extensively branched dendritic trees and many synaptic contacts. Longevity together with morphological complexity results in a formidable challenge to maintain synapses healthy and functional. This challenge is often evoked to explain adult-onset degeneration in numerous neurodegenerative disorders that result from otherwise divergent causes. However, comparably little is known about the basic cell biological mechanisms that keep normal synapses alive and functional in the first place. How the basic maintenance mechanisms are related to slow adult-onset degeneration in different diseasesis largely unclear. In this review we focus on two basic and interconnected cell biological mechanisms that are required for synaptic maintenance: endomembrane recycling and calcium (Ca²⁺) homeostasis. We propose that subtle defects in these homeostatic processes can lead to late onset synaptic degeneration. Moreover, the same basic mechanisms are hijacked, impaired or overstimulated in numerous neurodegenerative disorders. Understanding the pathogenesis of these disorders requires an understanding of both the initial cause of the disease and the on-going changes in basic maintenance mechanisms. Here we discuss the mechanisms that keep synapses functional over long periods of time with the emphasis on their role in slow adult-onset neurodegeneration.

Ketamine, magnesium and major depression--from pharmacology to pathophysiology and back

The glutamatergic mechanism of antidepressant treatments is now in the center of research to overcome the limitations of monoamine-based approaches. There are several unresolved issues. For the action of the model compound, ketamine, NMDA-receptor block, AMPA-receptor activation and BDNF release appear to be involved in a mechanism, which leads to synaptic sprouting and strengthened synaptic connections. The link to the pathophysiology of depression is not clear. An overlooked connection is the role of magnesium, which acts as physiological NMDA-receptor antagonist: 1. There is overlap between the actions of ketamine with that of high doses of magnesium in animal models, finally leading to synaptic sprouting. 2. Magnesium and ketamine lead to synaptic strengthening, as measured by an increase in slow wave sleep in humans. 3. Pathophysiological mechanisms, which have been identified as risk factors for depression, lead to a reduction of (intracellular) magnesium. These are neuroendocrine changes (increased cortisol and aldosterone) and diabetes mellitus as well as Mg(2+) deficiency. 4. Patients with therapy refractory depression appear to have lower CNS Mg(2+) levels in comparison to health controls. 5. Experimental Mg(2+) depletion leads to depression- and anxiety like behavior in animal models. 6. Ketamine, directly or indirectly via non-NMDA glutamate receptor activation, acts to increase brain Mg(2+) levels. Similar effects have been observed with other classes of antidepressants. 7. Depressed patients with low Mg(2+) levels tend to be therapy refractory. Accordingly, administration of Mg(2+) either alone or in combination with standard antidepressants acts synergistically on depression like behavior in animal models.

CONCLUSION

On the basis of the potential pathophysiological role of Mg(2+)-regulation, it may be possible to predict the action of ketamine and of related compounds based on Mg(2+) levels. Furthermore, screening for compounds to increase neuronal Mg(2+) concentration could be a promising instrument to identify new classes of antidepressants. Overall, any discussion of the glutamatergic system in affective disorders should consider the role of Mg(2+).

CaMKII-dependent phosphorylation of the GTPase Rem2 is required to restrict dendritic complexity

The morphogenesis of the dendritic arbor is a critical aspect of neuronal development, ensuring that proper neural networks are formed. However, the molecular mechanisms that underlie this dendritic remodeling remain obscure. We previously established the activity-regulated GTPase Rem2 as a negative regulator of dendritic complexity. In this study, we identify a signaling pathway whereby Rem2 regulates dendritic arborization through interactions with Ca(2+)/calmodulin-dependent kinases (CaMKs) in rat hippocampal neurons. Specifically, we demonstrate that Rem2 functions downstream of CaMKII but upstream of CaMKIV in a pathway that restricts dendritic complexity. Furthermore, we show that Rem2 is a novel substrate of CaMKII and that phosphorylation of Rem2 by CaMKII regulates Rem2 function and subcellular localization. Overall, our results describe a unique signal transduction network through which Rem2 and CaMKs function to restrict dendritic complexity.

Adaptations in AMPA receptor transmission in the nucleus accumbens contributing to incubation of cocaine craving

Cue-induced cocaine craving in rodents intensifies or "incubates" during the first months of withdrawal from long access cocaine self-administration. This incubation phenomenon is relevant to human users who achieve abstinence but exhibit persistent vulnerability to cue-induced relapse. It is well established that incubation of cocaine craving involves complex neuronal circuits. Here we will focus on neuroadaptations in the nucleus accumbens (NAc), a region of convergence for pathways that control cocaine seeking. A key adaptation is a delayed (~3-4 weeks) accumulation of Ca(2+)-permeable AMPAR receptors (CP-AMPARs) in synapses on medium spiny neurons (MSN) of the NAc. These CP-AMPARs mediate the expression of incubation after prolonged withdrawal, although different mechanisms must be responsible during the first weeks of withdrawal, prior to CP-AMPAR accumulation. The cascade of events leading to CP-AMPAR accumulation is still unclear. However, several candidate mechanisms have been identified. First, mGluR1 has been shown to negatively regulate CP-AMPAR levels in NAc synapses, and it is possible that a withdrawal-dependent decrease in this effect may help explain CP-AMPAR accumulation during incubation. Second, an increase in phosphorylation of GluA1 subunits (at the protein kinase A site) within extrasynaptic homomeric GluA1 receptors (CP-AMPARs) may promote their synaptic insertion and oppose their removal. Finally, elevation of brain-derived neurotrophic factor (BDNF) levels in the NAc may contribute to maintenance of incubation after months of withdrawal, although incubation-related increases in BDNF accumulation do not account for CP-AMPAR accumulation. Receptors and pathways that negatively regulate incubation, such as mGluR1, are promising targets for the development of therapeutic strategies to help recovering addicts maintain abstinence. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

The Caenorhabditis elegans voltage-gated calcium channel subunits UNC-2 and UNC-36 and the calcium-dependent kinase UNC-43/CaMKII regulate neuromuscular junction morphology

Background

The conserved Caenorhabditis elegans proteins NID-1/nidogen and PTP-3A/LAR-RPTP function to efficiently localize the presynaptic scaffold protein SYD-2/α-liprin at active zones. Loss of function in these molecules results in defects in the size, morphology and spacing of neuromuscular junctions.

Results

Here we show that the Ca_v2-like voltage-gated calcium channel (VGCC) proteins, UNC-2 and UNC-36, and the calmodulin kinase II (CaMKII), UNC-43, function to regulate the size and morphology of presynaptic domains in C. elegans. Loss of function in unc-2, unc-36 or unc-43 resulted in slightly larger GABAergic neuromuscular junctions (NMJs), but could suppress the synaptic morphology defects found in nid-1/nidogen or ptp-3/LAR mutants. A gain-of-function mutation in unc-43 caused defects similar to those found in nid-1 mutants. Mutations in egl-19, Ca_v1-like, or cca-1, Ca_v3-like, α1 subunits, or the second α2/δ subunit, tag-180, did not suppress nid-1, suggesting a specific interaction between unc-2 and the synaptic extracellular matrix (ECM) component nidogen. Using a synaptic vesicle marker in time-lapse microscopy studies, we observed GABAergic motor neurons adding NMJ-like structures during late larval development. The synaptic bouton addition appeared to form in at least two ways: (1) de novo formation, where a cluster of vesicles appeared to coalesce, or (2) when a single punctum became enlarged and then divided to form two discrete fluorescent puncta. In comparison to wild type animals, we found unc-2 mutants exhibited reduced NMJ dynamics, with fewer observed divisions during a similar stage of development.

Conclusions

We identified UNC-2/UNC-36 VGCCs and UNC-43/CaMKII as regulators of C. elegans synaptogenesis. UNC-2 has a modest role in synapse formation, but a broader role in regulating dynamic changes in the size and morphology of synapses that occur during organismal development. During the late 4th larval stage (L4), wild type animals exhibit synaptic morphologies that are similar to those found in animals lacking NID-1/PTP-3 adhesion, as well as those with constitutive activation of UNC-43. Genetic evidence indicates that the VGCCs and the NID-1/PTP-3 adhesion complex provide opposing functions in synaptic development, suggesting that modulation of synaptic adhesion may underlie synapse development in C. elegans.

Dopamine and full-field illumination activate D1 and D2-D5-type receptors in adult rat retinal ganglion cells

Dopamine can regulate signal generation and transmission by activating multiple receptors and signaling cascades, especially in striatum, hippocampus, and cerebral cortex. Dopamine modulates an even larger variety of cellular properties in retina, yet has been reported to do so by only D1 receptor-driven cyclic adenosine monophosphate (cAMP) increases or D2 receptor-driven cAMP decreases. Here, we test the possibility that dopamine operates differently on retinal ganglion cells, because the ganglion cell layer binds D1 and D2 receptor ligands, and displays changes in signaling components other than cAMP under illumination that should release dopamine. In adult rat retinal ganglion cells, based on patch-clamp recordings, Ca(2+) imaging, and immunohistochemistry, we find that 1) spike firing is inhibited by dopamine and SKF 83959 (an agonist that does not activate homomeric D1 receptors or alter cAMP levels in other systems); 2) D1 and D2 receptor antagonists (SCH 23390, eticlopride, raclopride) counteract these effects; 3) these antagonists also block light-induced rises in cAMP, light-induced activation of Ca(2+) /calmodulin-dependent protein kinase II, and dopamine-induced Ca(2+) influx; and 4) the Ca(2+) rise is markedly reduced by removing extracellular Ca(2+) and by an IP3 receptor antagonist (2-APB). These results provide the first evidence that dopamine activates a receptor in adult mammalian retinal neurons that is distinct from classical D1 and D2 receptors, and that dopamine can activate mechanisms in addition to cAMP and cAMP-dependent protein kinase to modulate retinal ganglion cell excitability.

Structural studies on the regulation of Ca2+/calmodulin dependent protein kinase II

Ca(2+)/calmodulin dependent protein kinase II (CaMKII) is a broadly distributed metazoan Ser/Thr protein kinase that is important in neuronal and cardiac signaling. CaMKII forms oligomeric assemblies, typically dodecameric, in which the calcium-responsive kinase domains are organized around a central hub. We review the results of crystallographic analyses of CaMKII, including the recently determined structure of a full-length and autoinhibited form of the holoenzyme. These structures, when combined with other data, allow informed speculation about how CaMKII escapes calcium-dependence when calcium spikes exceed threshold frequencies.

Synaptic plasticity in hepatic encephalopathy - a molecular perspective

Hepatic encephalopathy (HE)(1) is a common neuropsychiatric complication of both acute and chronic liver disease. Clinical symptoms may include motor disturbances and cognitive dysfunction. Available animal models of HE mimic the deficits in cognitive performance including the impaired ability to learn and memorize information. This review explores the question how HE might affect cognitive functions at molecular levels. Both acute and chronic models of HE constrain the plasticity of glutamatergic neurotransmission. Thus, long-lasting activity-dependent changes in synaptic efficiency, known as long-term potentiation (LTP) and long-term depression (LTD) are significantly impeded. We discuss molecules and signal transduction pathways of LTP and LTD that are targeted by experimental HE, with a focus on ionotropic glutamate receptors of the AMPA-subtype. Finally, a novel strategy of functional proteomic analysis is presented, which, if applied differentially, may provide molecular insight into disease-related dysfunction of membrane protein complexes, i.e. disturbed ionotropic glutamate receptor signaling in HE.

BDNF-induced local protein synthesis and synaptic plasticity

Brain-derived neurotrophic factor (BDNF) is an important regulator of synaptic transmission and long-term potentiation (LTP) in the hippocampus and in other brain regions, playing a role in the formation of certain forms of memory. The effects of BDNF in LTP are mediated by TrkB (tropomyosin-related kinase B) receptors, which are known to be coupled to the activation of the Ras/ERK, phosphatidylinositol 3-kinase/Akt and phospholipase C-γ (PLC-γ) pathways. The role of BDNF in LTP is best studied in the hippocampus, where the neurotrophin acts at pre- and post-synaptic levels. Recent studies have shown that BDNF regulates the transport of mRNAs along dendrites and their translation at the synapse, by modulating the initiation and elongation phases of protein synthesis, and by acting on specific miRNAs. Furthermore, the effect of BDNF on transcription regulation may further contribute to long-term changes in the synaptic proteome. In this review we discuss the recent progress in understanding the mechanisms contributing to the short- and long-term regulation of the synaptic proteome by BDNF, and the role in synaptic plasticity, which is likely to influence learning and memory formation. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.