Synaptic anchorage of AMPA receptors by cadherins through neural plakophilin-related arm protein AMPA receptor-binding protein complexes

Palmitoylation of synaptic proteins: roles in functional regulation and pathogenesis of neurodegenerative diseases

Palmitoylation is a type of lipid modification that plays an important role in various aspects of neuronal function. Over the past few decades, several studies have shown that the palmitoylation of synaptic proteins is involved in neurotransmission and synaptic functions. Palmitoyl acyltransferases (PATs), which belong to the DHHC family, are major players in the regulation of palmitoylation. Dysregulated palmitoylation of synaptic proteins and mutated/dysregulated DHHC proteins are associated with several neurodegenerative diseases, such as Alzheimer's disease (AD), Huntington's disease (HD), and Parkinson's disease (PD). In this review, we summarize the recent discoveries on the subcellular distribution of DHHC proteins and analyze their expression patterns in different brain cells. In particular, this review discusses how palmitoylation of synaptic proteins regulates synaptic vesicle exocytotic fusion and the localization, clustering, and transport of several postsynaptic receptors, as well as the role of palmitoylation of other proteins in regulating synaptic proteins. Additionally, some of the specific known associations of these factors with neurodegenerative disorders are explored, with a few suggestions for the development of therapeutic strategies. Finally, this review provides possible directions for future research to reveal detailed and specific mechanisms underlying the roles of synaptic protein palmitoylation.

Prenatal exposure to valproic acid reduces synaptic δ-catenin levels and disrupts ultrasonic vocalization in neonates

Valproic acid (VPA) is an effective and commonly prescribed drug for epilepsy and bipolar disorder. However, children born from mothers treated with VPA during pregnancy exhibit an increased incidence of autism spectrum disorder (ASD). Although VPA may impair brain development at the cellular level, the mechanism of VPA-induced ASD has not been completely addressed. A previous study has found that VPA treatment strongly reduces δ-catenin mRNA levels in cultured human neurons. δ-catenin is important for the control of glutamatergic synapses and is strongly associated with ASD. VPA inhibits dendritic morphogenesis in developing neurons, an effect that is also found in neurons lacking δ-catenin expression. We thus hypothesize that prenatal exposure to VPA significantly reduces δ-catenin levels in the brain, which impairs glutamatergic synapses to cause ASD. Here, we found that prenatal exposure to VPA markedly reduced δ-catenin levels in the brain of mouse pups. VPA treatment also impaired dendritic branching in developing mouse cortical neurons, which was partially reversed by elevating δ-catenin expression. Prenatal VPA exposure significantly reduced synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor levels and postsynaptic density 95 (PSD95) in the brain of mouse pups, indicating dysfunctions in glutamatergic synaptic transmission. VPA exposure also significantly altered ultrasonic vocalization (USV) in newly born pups when they were isolated from their nest. Moreover, VPA-exposed pups show impaired hypothalamic response to isolation, which is required to produce animals' USVs following isolation from the nest. Therefore, these results suggest that VPA-induced ASD pathology can be mediated by the loss of δ-catenin functions.

Prenatal exposure to valproic acid reduces synaptic δ-catenin levels and disrupts ultrasonic vocalization in neonates

Valproic acid (VPA) is an effective and commonly prescribed drug for epilepsy and bipolar disorder. However, children born from mothers treated with VPA during pregnancy exhibit an increased incidence of autism spectrum disorder (ASD). Although VPA may impair brain development at the cellular level, the mechanism of VPA-induced ASD has not been completely addressed. A previous study has found that VPA treatment strongly reduces δ-catenin mRNA levels in cultured human neurons. δ-catenin is important for the control of glutamatergic synapses and is strongly associated with ASD. VPA inhibits dendritic morphogenesis in developing neurons, an effect that is also found in neurons lacking δ-catenin expression. We thus hypothesize that prenatal exposure to VPA significantly reduces δ-catenin levels in the brain, which impairs glutamatergic synapses to cause ASD. Here, we found that prenatal exposure to VPA markedly reduced δ-catenin levels in the brain of mouse pups. VPA treatment also impaired dendritic branching in developing mouse cortical neurons, which was reversed by elevating δ-catenin expression. Prenatal VPA exposure significantly reduced synaptic AMPA receptor levels and postsynaptic density 95 (PSD95) in the brain of mouse pups, indicating dysfunctions in glutamatergic synaptic transmission. VPA exposure also significantly altered ultrasonic vocalization (USV) in newly born pups when they were isolated from their nest. Moreover, VPA-exposed pups show impaired hypothalamic response to isolation, which is required to produce animals' USVs following isolation from the nest. Therefore, these results suggest that VPA-induced ASD pathology can be mediated by the loss of δ-catenin functions.

Highlights

Prenatal exposure of valproic acid (VPA) in mice significantly reduces synaptic δ-catenin protein and AMPA receptor levels in the pups' brains.VPA treatment significantly impairs dendritic branching in cultured cortical neurons, which is reversed by increased δ-catenin expression.VPA exposed pups exhibit impaired communication such as ultrasonic vocalization.Neuronal activation linked to ultrasonic vocalization is absent in VPA-exposed pups.The loss of δ-catenin functions underlies VPA-induced autism spectrum disorder (ASD) in early childhood.

δ-Catenin controls astrocyte morphogenesis via layer-specific astrocyte-neuron cadherin interactions

Astrocytes control the formation of specific synaptic circuits via cell adhesion and secreted molecules. Astrocyte synaptogenic functions are dependent on the establishment of their complex morphology. However, it is unknown if distinct neuronal cues differentially regulate astrocyte morphogenesis. δ-Catenin was previously thought to be a neuron-specific protein that regulates dendrite morphology. We found δ-catenin is also highly expressed by astrocytes and required both in astrocytes and neurons for astrocyte morphogenesis. δ-Catenin is hypothesized to mediate transcellular interactions through the cadherin family of cell adhesion proteins. We used structural modeling and biochemical analyses to reveal that δ-catenin interacts with the N-cadherin juxtamembrane domain to promote N-cadherin surface expression. An autism-linked δ-catenin point mutation impaired N-cadherin cell surface expression and reduced astrocyte complexity. In the developing mouse cortex, only lower-layer cortical neurons express N-cadherin. Remarkably, when we silenced astrocytic N-cadherin throughout the cortex, only lower-layer astrocyte morphology was disrupted. These findings show that δ-catenin controls astrocyte-neuron cadherin interactions that regulate layer-specific astrocyte morphogenesis.

The autism-associated loss of δ-catenin functions disrupts social behavior

δ-catenin is expressed in excitatory synapses and functions as an anchor for the glutamatergic AMPA receptor (AMPAR) GluA2 subunit in the postsynaptic density. The glycine 34 to serine (G34S) mutation in the δ-catenin gene has been found in autism spectrum disorder (ASD) patients and results in loss of δ-catenin functions at excitatory synapses, which is presumed to underlie ASD pathogenesis in humans. However, how the G34S mutation causes loss of δ-catenin functions to induce ASD remains unclear. Here, using neuroblastoma cells, we identify that the G34S mutation increases glycogen synthase kinase 3β (GSK3β)-dependent δ-catenin degradation to reduce δ-catenin levels, which likely contributes to the loss of δ-catenin functions. Synaptic δ-catenin and GluA2 levels in the cortex are significantly decreased in mice harboring the δ-catenin G34S mutation. The G34S mutation increases glutamatergic activity in cortical excitatory neurons while it is decreased in inhibitory interneurons, indicating changes in cellular excitation and inhibition. δ-catenin G34S mutant mice also exhibit social dysfunction, a common feature of ASD. Most importantly, pharmacological inhibition of GSK3β activity reverses the G34S-induced loss of δ-catenin function effects in cells and mice. Finally, using δ-catenin knockout mice, we confirm that δ-catenin is required for GSK3β inhibition-induced restoration of normal social behavior in δ-catenin G34S mutant animals. Taken together, we reveal that the loss of δ-catenin functions arising from the ASD-associated G34S mutation induces social dysfunction via alterations in glutamatergic activity and that GSK3β inhibition can reverse δ-catenin G34S-induced synaptic and behavioral deficits.

UNC-43/CaMKII-triggered anterograde signals recruit GABAARs to mediate inhibitory synaptic transmission and plasticity at C. elegans NMJs

Disturbed inhibitory synaptic transmission has functional impacts on neurodevelopmental and psychiatric disorders. An essential mechanism for modulating inhibitory synaptic transmission is alteration of the postsynaptic abundance of GABA_ARs, which are stabilized by postsynaptic scaffold proteins and recruited by presynaptic signals. However, how GABAergic neurons trigger signals to transsynaptically recruit GABA_ARs remains elusive. Here, we show that UNC-43/CaMKII functions at GABAergic neurons to recruit GABA_ARs and modulate inhibitory synaptic transmission at C. elegans neuromuscular junctions. We demonstrate that UNC-43 promotes presynaptic MADD-4B/Punctin secretion and NRX-1α/Neurexin surface delivery. Together, MADD-4B and NRX-1α recruit postsynaptic NLG-1/Neuroligin and stabilize GABA_ARs. Further, the excitation of GABAergic neurons potentiates the recruitment of NLG-1-stabilized-GABA_ARs, which depends on UNC-43, MADD-4B, and NRX-1. These data all support that UNC-43 triggers MADD-4B and NRX-1α, which act as anterograde signals to recruit postsynaptic GABA_ARs. Thus, our findings elucidate a mechanism for pre- and postsynaptic communication and inhibitory synaptic transmission and plasticity.

Compulsive methamphetamine self-administration in the presence of adverse consequences is associated with increased hippocampal mRNA expression of cellular adhesion molecules

Methamphetamine (METH) is a popular but harmful psychostimulant. METH use disorder (MUD) is characterized by compulsive and continued use despite adverse life consequences. METH users experience impairments in learning and memory functions that are thought to be secondary to METH-induced abnormalities in the hippocampus. Recent studies have reported that about 50% of METH users develop MUD, suggesting that there may be differential molecular effects of METH between the brains of individuals who met criteria for addiction and those who did not after being exposed to the drug. The present study aimed at identifying potential transcriptional differences between compulsive and non-compulsive METH self-administering male rats by measuring global gene expression changes in the hippocampus using RNA sequencing. Herein, we used a model of METH self-administration (SA) accompanied by contingent foot-shock punishment. This approach led to the separation of animals into shock-resistant rats (compulsive) that continued to take METH and shock-sensitive rats (non-compulsive) that suppressed their METH intake in the presence of punished METH taking. Rats were euthanized 2 h after the last METH SA plus foot-shock session. Their hippocampi were immediately removed, frozen, and used later for RNA sequencing and qRT-PCR analyses. RNA sequencing analyses revealed differential expression of mRNAs encoding cell adhesion molecules (CAMs) between the two rat phenotypes. qRT-PCR analyses showed significant higher levels of Cdh1, Glycam1, and Mpzl2 mRNAs in the compulsive rats in comparison to non-compulsive rats. The present results implicate altered CAM expression in the hippocampus in the behavioral manifestations of continuous compulsive METH taking in the presence of adverse consequences. Our results raise the novel possibility that altered CAM expression might play a role in compulsive METH taking and the cognitive impairments observed in MUD patients.

The autism-associated loss of δ-catenin functions disrupts social behaviors

δ-catenin is expressed in excitatory synapses and functions as an anchor for the glutamatergic AMPA receptor (AMPAR) GluA2 subunit in the postsynaptic density. The glycine 34 to serine (G34S) mutation in the δ-catenin gene is found in autism spectrum disorder (ASD) patients and induces loss of δ-catenin functions at excitatory synapses, which is presumed to underlie ASD pathogenesis in humans. However, how the G34S mutation causes loss of δ-catenin functions to induce ASD remains unclear. Here, using neuroblastoma cells, we discover that the G34S mutation generates an additional phosphorylation site for glycogen synthase kinase 3β (GSK3β). This promotes δ-catenin degradation and causes the reduction of δ-catenin levels, which likely contributes to the loss of δ-catenin functions. Synaptic δ-catenin and GluA2 levels in the cortex are significantly decreased in mice harboring the δ-catenin G34S mutation. The G34S mutation increases glutamatergic activity in cortical excitatory neurons while it is decreased in inhibitory interneurons, indicating changes in cellular excitation and inhibition. δ-catenin G34S mutant mice also exhibit social dysfunction, a common feature of ASD. Most importantly, inhibition of GSK3β activity reverses the G34S-induced loss of δ-catenin function effects in cells and mice. Finally, using δ-catenin knockout mice, we confirm that δ-catenin is required for GSK3β inhibition-induced restoration of normal social behaviors in δ-catenin G34S mutant animals. Taken together, we reveal that the loss of δ-catenin functions arising from the ASD-associated G34S mutation induces social dysfunction via alterations in glutamatergic activity and that GSK3β inhibition can reverse δ-catenin G34S-induced synaptic and behavioral deficits.

Significance Statement

δ-catenin is important for the localization and function of glutamatergic AMPA receptors at synapses in many brain regions. The glycine 34 to serine (G34S) mutation in the δ-catenin gene is found in autism patients and results in the loss of δ-catenin functions. δ-catenin expression is also closely linked to other autism-risk genes involved in synaptic structure and function, further implying that it is important for the autism pathophysiology. Importantly, social dysfunction is a key characteristic of autism. Nonetheless, the links between δ-catenin functions and social behaviors are largely unknown. The significance of the current research is thus predicated on filling this gap by discovering the molecular, cellular, and synaptic underpinnings of the role of δ-catenin in social behaviors.

Effects of cereblon on stress-activated redox proteins and core behavior

The mechanisms underlying the vulnerability and resilience of an individual to stress are only partly understood. Response to stress is determined by behavioral and biochemical changes in the brain. Chronic ultra-mild stress (CUMS) induces an anhedonic-like state in mice that resembles symptoms of human depression. This study reports the role of cereblon (CRBN) in regulating the metabolic and antioxidant status of neuronal tissues in the mouse model of CUMS. Intriguingly, Crbn^-/- (KO) mice showed resilient responsiveness, both at the behavioral and proteomic levels. Several core behaviors were also differentially altered by CUMS in KO mice. Liquid chromatography with tandem mass spectrometry (LC-MS/MS)-based proteome analysis of whole brain lysate (WBL) showed an enriched chaperonic, metabolic, and antioxidant status in the brains of KO subjects, including several members of DNAJ chaperones, creatine kinase, quinone oxidoreductase, superoxide dismutase (SOD1), glutathione S-transferase Mu (GSTM), peroxiredoxin-6 (PRDX6), and thioredoxin. Pathological phosphorylation as characterized by aggregation of tau and α-synuclein (α-syn) was significantly reduced in the neuronal tissues of KO mouse model of CUMS as compared to wild type (WT) mice. Furthermore, significantly increased SOD1 activity and reduced lipid peroxidation were observed in Crbn-KO systems. Integrated signaling pathways were also identified in CRBN-specific sub-networks constructed from protein-protein interaction analysis by STRING. The present study highlights the roles of CRBN in regulating the stress response (SR) and reshaping metabolic status in the brains of mice exposed to CUMS. A better understanding of the molecular mechanisms of depression and neurodegeneration can improve the development of novel treatments.

Novel phospho-switch function of delta-catenin in dendrite development

In neurons, dendrites form the major sites of information receipt and integration. It is thus vital that, during development, the dendritic arbor is adequately formed to enable proper neural circuit formation and function. While several known processes shape the arbor, little is known of those that govern dendrite branching versus extension. Here, we report a new mechanism instructing dendrites to branch versus extend. In it, glutamate signaling activates mGluR5 receptors to promote Ckd5-mediated phosphorylation of the C-terminal PDZ-binding motif of delta-catenin. The phosphorylation state of this motif determines delta-catenin's ability to bind either Pdlim5 or Magi1. Whereas the delta:Pdlim5 complex enhances dendrite branching at the expense of elongation, the delta:Magi1 complex instead promotes lengthening. Our data suggest that these complexes affect dendrite development by differentially regulating the small-GTPase RhoA and actin-associated protein Cortactin. We thus reveal a "phospho-switch" within delta-catenin, subject to a glutamate-mediated signaling pathway, that assists in balancing the branching versus extension of dendrites during neural development.

Roles of palmitoylation in structural long-term synaptic plasticity

Long-term potentiation (LTP) and long-term depression (LTD) are important cellular mechanisms underlying learning and memory processes. N-Methyl-d-aspartate receptor (NMDAR)-dependent LTP and LTD play especially crucial roles in these functions, and their expression depends on changes in the number and single channel conductance of the major ionotropic glutamate receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) located on the postsynaptic membrane. Structural changes in dendritic spines comprise the morphological platform and support for molecular changes in the execution of synaptic plasticity and memory storage. At the molecular level, spine morphology is directly determined by actin cytoskeleton organization within the spine and indirectly stabilized and consolidated by scaffold proteins at the spine head. Palmitoylation, as a uniquely reversible lipid modification with the ability to regulate protein membrane localization and trafficking, plays significant roles in the structural and functional regulation of LTP and LTD. Altered structural plasticity of dendritic spines is also considered a hallmark of neurodevelopmental disorders, while genetic evidence strongly links abnormal brain function to impaired palmitoylation. Numerous studies have indicated that palmitoylation contributes to morphological spine modifications. In this review, we have gathered data showing that the regulatory proteins that modulate the actin network and scaffold proteins related to AMPAR-mediated neurotransmission also undergo palmitoylation and play roles in modifying spine architecture during structural plasticity.

Targeting of δ-catenin to postsynaptic sites through interaction with the Shank3 N-terminus

BACKGROUND

Neurodevelopmental disorders such as autism spectrum disorder (ASD) may be caused by alterations in genes encoding proteins that are involved in synapse formation and function. This includes scaffold proteins such as Shank3, and synaptic adhesion proteins such as Neurexins or Neuroligins. An important question is whether the products of individual risk genes cooperate functionally (exemplified in the interaction of Neurexin with Neuroligin isoforms). This might suggest a common pathway in pathogenesis. For the SHANK3 gene, heterozygous loss of function, as well as missense mutations have been observed in ASD cases. Several missense mutations affect the N-terminal part of Shank3 which contains the highly conserved Shank/ProSAP N-terminal (SPN) and Ankyrin repeat (Ank) domains. The role of these domains and the relevance of these mutations for synaptic function of Shank3 are widely unknown.

METHODS

We used purification from a synaptic protein fraction, as well as a variety of biochemical and cell biological approaches to identify proteins which associate with the Shank3 N-terminus at postsynaptic sites.

RESULTS

We report here that δ-catenin, which is encoded by CTNND2, an autism candidate gene, directly interacts with the Ank domain of Shank3 at postsynaptic sites through its Armadillo-repeat domain. The interaction is not affected by well-known posttranslational modifications of δ-catenin, i.e. by phosphorylation or palmitoylation. However, an ASD-associated mutation in the SPN domain of Shank3, L68P, significantly increases the interaction of Shank3 with δ-catenin. By analysis of postsynaptic fractions from mice, we show that the lack of SPN-Ank containing, large isoforms of Shank3 results in the loss of postsynaptic δ-catenin. Further, expression of Shank3 variants containing the N-terminal domains in primary cultured neurons significantly increased the presence of coexpressed δ-catenin at postsynaptic sites.

LIMITATIONS

Work in model organisms such as mice, and in primary cultured neurons may not reproduce faithfully the situation in human brain neurons. Work in primary cultured neurons was also hampered by lack of a specific antibody for endogenous δ-catenin.

CONCLUSIONS

Our data show that the interaction between Shank3 N-terminus and δ-catenin is required for the postsynaptic targeting of δ-catenin. Failure of proper targeting of δ-catenin to postsynaptic sites may contribute to the pathogenesis of autism spectrum disorder.

Trafficking and Activity of Glutamate and GABA Receptors: Regulation by Cell Adhesion Molecules

The efficient targeting of ionotropic receptors to postsynaptic sites is essential for the function of chemical excitatory and inhibitory synapses, constituting the majority of synapses in the brain. A growing body of evidence indicates that cell adhesion molecules (CAMs), which accumulate at synapses at the earliest stages of synaptogenesis, are critical for this process. A diverse variety of CAMs assemble into complexes with glutamate and GABA receptors and regulate the targeting of these receptors to the cell surface and synapses. Presynaptically localized CAMs provide an additional level of regulation, sending a trans-synaptic signal that can regulate synaptic strength at the level of receptor trafficking. Apart from controlling the numbers of receptors present at postsynaptic sites, CAMs can also influence synaptic strength by modulating the conductivity of single receptor channels. CAMs thus act to maintain basal synaptic transmission and are essential for many forms of activity dependent synaptic plasticity. These activities of CAMs may underlie the association between CAM gene mutations and synaptic pathology and represent fundamental mechanisms by which synaptic strength is dynamically tuned at both excitatory and inhibitory synapses.

Role of Palmitoylation of Postsynaptic Proteins in Promoting Synaptic Plasticity

Many postsynaptic proteins undergo palmitoylation, the reversible attachment of the fatty acid palmitate to cysteine residues, which influences trafficking, localization, and protein interaction dynamics. Both palmitoylation by palmitoyl acyl transferases (PAT) and depalmitoylation by palmitoyl-protein thioesterases (PPT) is regulated in an activity-dependent, localized fashion. Recently, palmitoylation has received attention for its pivotal contribution to various forms of synaptic plasticity, the dynamic modulation of synaptic strength in response to neuronal activity. For instance, palmitoylation and depalmitoylation of the central postsynaptic scaffold protein postsynaptic density-95 (PSD-95) is important for synaptic plasticity. Here, we provide a comprehensive review of studies linking palmitoylation of postsynaptic proteins to synaptic plasticity.

A role for proteolytic regulation of δ-catenin in remodeling a subpopulation of dendritic spines in the rodent brain

Neural wiring and activity are essential for proper brain function and behavioral outputs and rely on mechanisms that guide the formation, elimination, and remodeling of synapses. During development, it is therefore vital that synaptic densities and architecture are tightly regulated to allow for appropriate neural circuit formation and function. δ-Catenin, a component of the cadherin-catenin cell adhesion complex, has been demonstrated to be a critical regulator of synaptic density and function in the developing central neurons. In this study, we identified forms of δ-catenin that include only the N-terminal (DcatNT) or the C-terminal (DcatCT) regions. We found that these δ-catenin forms are differentially expressed in different regions of the male mouse brain. Our results also indicated that in rat primary cortical culture, these forms are generated in an activity-dependent manner by Ca²⁺-dependent and calpain-mediated cleavage of δ-catenin or in an activity-independent but lysosome-dependent manner. Functionally, loss of the domain containing the calpain-cleavage sites allowing for generation of DcatCT and DcatNT perturbed the density of a subpopulation of dendritic protrusions in rat hippocampal neurons. This subpopulation likely included protrusions that are either in transition toward becoming mature mushroom spines or in the process of being eliminated. By influencing this subpopulation of spines, proteolytic processing of δ-catenin can likely regulate the balance between mature and immature dendritic protrusions in coordination with neural activity. We conclude that by undergoing cleavage, δ-catenin differentially regulates the densities of subpopulations of dendritic spines and contributes to proper neural circuit wiring in the developing brain.

Regulation of AMPA receptor trafficking and exit from the endoplasmic reticulum

A fundamental property of the brain is its ability to modify its function in response to its own activity. This ability for self-modification depends to a large extent on synaptic plasticity. It is now appreciated that for excitatory synapses, a significant part of synaptic plasticity depends upon changes in the post synaptic response to glutamate released from nerve terminals. Modification of the post synaptic response depends, in turn, on changes in the abundances of AMPA receptors in the post synaptic membrane. In this review, we consider mechanisms of trafficking of AMPA receptors to and from synapses that take place in the early trafficking stages, starting in the endoplasmic reticulum (ER) and continuing into the secretory pathway. We consider mechanisms of AMPA receptor assembly in the ER, highlighting the role of protein synthesis and the selective properties of specific AMPA receptor subunits, as well as regulation of ER exit, including the roles of chaperones and accessory proteins and the incorporation of AMPA receptors into COPII vesicles. We consider these processes in the context of the mechanism of mGluR LTD and discuss a compelling role for the dendritic ER membrane that is found proximal to synapses. The review illustrates the important, yet little studied, contribution of the early stages of AMPA receptor trafficking to synaptic plasticity.

Structural Mechanisms of Gating in Ionotropic Glutamate Receptors

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate the majority of excitatory neurotransmission in the central nervous system. iGluRs open their ion channels in response to binding of the neurotransmitter glutamate, rapidly depolarize the postsynaptic neuronal membrane, and initiate signal transduction. Recent studies using X-ray crystallography and cryo-electron microscopy have determined full-length iGluR structures that (1) uncover the receptor architecture in an unliganded, resting state, (2) reveal conformational changes produced by ligands in order to activate iGluRs, open their ion channels, and conduct ions, and (3) show how activated, glutamate-bound iGluRs can adopt a nonconducting desensitized state. These new findings, combined with the results of previous structural and functional experiments, kinetic and molecular modeling, mutagenesis, and biochemical analyses, provide new views on the structural mechanisms of iGluR gating.

Functional roles of p120ctn family of proteins in central neurons

The cadherin-catenin complex in central neurons is associated with a variety of cytosolic partners, collectively called catenins. The p120ctn members are a family of catenins that are distinct from the more ubiquitously expressed α- and β-catenins. It is becoming increasingly clear that the functional roles of the p120ctn family of catenins in central neurons extend well beyond their functional roles in non-neuronal cells in partnering with cadherin to regulate adhesion. In this review, we will provide an overview of the p120ctn family in neurons and their varied functional roles in central neurons. Finally, we will examine the emerging roles of this family of proteins in neurodevelopmental disorders.

Cadherins mediate cocaine-induced synaptic plasticity and behavioral conditioning

Drugs of abuse alter synaptic connections in the ‘reward circuit’ of the brain, which leads to long-lasting behavioral changes that underlie addiction. Here we show that cadherin adhesion molecules play a critical role in mediating synaptic plasticity and behavioral changes driven by cocaine. We demonstrate that cadherin is essential for long-term potentiation (LTP) in the ventral tegmental area (VTA), and is recruited to the synaptic membrane of excitatory inputs onto dopaminergic neurons following cocaine-mediated behavioral conditioning. Furthermore, we show that stabilization of cadherin at the membrane of these synapses blocks cocaine-induced synaptic plasticity, leading to a significant reduction in conditioned place preference induced by cocaine. Our findings identify cadherins and associated molecules as targets of interest for understanding pathological plasticity associated with addiction.

Lithium increases synaptic GluA2 in hippocampal neurons by elevating the δ-catenin protein

Lithium (Li⁺) is a drug widely employed for treating bipolar disorder, however the mechanism of action is not known. Here we study the effects of Li⁺ in cultured hippocampal neurons on a synaptic complex consisting of δ-catenin, a protein associated with cadherins whose mutation is linked to autism, and GRIP, an AMPA receptor (AMPAR) scaffolding protein, and the AMPAR subunit, GluA2. We show that Li⁺ elevates the level of δ-catenin in cultured neurons. δ-catenin binds to the ABP and GRIP proteins, which are synaptic scaffolds for GluA2. We show that Li⁺ increases the levels of GRIP and GluA2, consistent with Li⁺-induced elevation of δ-catenin. Using GluA2 mutants, we show that the increase in surface level of GluA2 requires GluA2 interaction with GRIP. The amplitude but not the frequency of mEPSCs was also increased by Li⁺ in cultured hippocampal neurons, confirming a functional effect and consistent with AMPAR stabilization at synapses. Furthermore, animals fed with Li⁺ show elevated synaptic levels of δ-catenin, GRIP, and GluA2 in the hippocampus, also consistent with the findings in cultured neurons. This work supports a model in which Li⁺ stabilizes δ-catenin, thus elevating a complex consisting of δ-catenin, GRIP and AMPARs in synapses of hippocampal neurons. Thus, the work suggests a mechanism by which Li⁺ can alter brain synaptic function that may be relevant to its pharmacologic action in treatment of neurological disease.

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

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

Genetic alterations of δ-catenin/NPRAP/Neurojungin (CTNND2): functional implications in complex human diseases

Some genes involved in complex human diseases are particularly vulnerable to genetic variations such as single nucleotide polymorphism, copy number variations, and mutations. For example, Ras mutations account for over 30 % of all human cancers. Additionally, there are some genes that can display different variations with functional impact in different diseases that are unrelated. One such gene stands out: δ-catenin/NPRAP/Neurojungin with gene designation as CTNND2 on chromosome 5p15.2. Recent advances in genome wide association as well as molecular biology approaches have uncovered striking involvement of δ-catenin gene variations linked to complex human disorders. These disorders include cancer, bipolar disorder, schizophrenia, autism, Cri-du-chat syndrome, myopia, cortical cataract-linked Alzheimer's disease, and infectious diseases. This list has rapidly grown longer in recent years, underscoring the pivotal roles of δ-catenin in critical human diseases. δ-Catenin is an adhesive junction-associated protein in the delta subfamily of the β-catenin superfamily. δ-Catenin functions in Wnt signaling to regulate gene expression and modulate Rho GTPases of the Ras superfamily in cytoskeletal reorganization. δ-Catenin likely lies where Wnt signaling meets Rho GTPases and is a unique and vulnerable common target for mutagenesis in different human diseases.

P120-Catenin Regulates Early Trafficking Stages of the N-Cadherin Precursor Complex

It is well established that binding of p120 catenin to the cytoplasmic domain of surface cadherin prevents cadherin endocytosis and degradation, contributing to cell-cell adhesion. In the present work we show that p120 catenin bound to the N-cadherin precursor, contributes to its anterograde movement from the endoplasmic reticulum (ER) to the Golgi complex. In HeLa cells, depletion of p120 expression, or blocking its binding to N-cadherin, increased the accumulation of the precursor in the ER, while it decreased the localization of mature N-cadherin at intercellular junctions. Reconstitution experiments in p120-deficient SW48 cells with all three major isoforms of p120 (1, 3 and 4) had similar capacity to promote the processing of the N-cadherin precursor to the mature form, and its localization at cell-cell junctions. P120 catenin and protein tyrosine phosphatase PTP1B facilitated the recruitment of the N-ethylmaleimide sensitive factor (NSF), an ATPase involved in vesicular trafficking, to the N-cadherin precursor complex. Dominant negative NSF E329Q impaired N-cadherin trafficking, maturation and localization at cell-cell junctions. Our results uncover a new role for p120 catenin bound to the N-cadherin precursor ensuring its trafficking through the biosynthetic pathway towards the cell surface.

The dynamic AMPA receptor extracellular region: a platform for synaptic protein interactions

AMPA receptors (AMPARs) are glutamate-gated cation channels that mediate fast excitatory neurotransmission and synaptic plasticity. Structures of GluA2 homotetramers in distinct functional states, together with simulations, emphasise the loose architecture of the AMPAR extracellular region (ECR). The ECR encompasses ∼80% of the receptor, and consists of the membrane-distal N-terminal domain (NTD) and ligand-binding domain (LBD), which is fused to the ion channel domain. Minimal contacts within and between layers, together with flexible peptide linkers connecting these three domains give rise to an organisation capable of dynamic rearrangements. This building plan is uniquely suited to engage interaction partners in the crowded environment of synapses, permitting the formation of new binding sites and the loss of existing ones. ECR motions are thereby expected to impact signalling as well as synaptic anchorage and may thereby influence AMPAR clustering during synaptic plasticity.

Changes in synaptic plasticity and expression of glutamate receptor subunits in the CA1 and CA3 areas of the hippocampus after transient global ischemia

Excess glutamate release from the presynaptic membrane has been thought to be the major cause of ischemic neuronal death. Although both CA1 and CA3 pyramidal neurons receive presynaptic glutamate input, transient cerebral ischemia induces CA1 neurons to die while CA3 neurons remain relatively intact. This suggests that changes in the properties of pyramidal cells may be the main cause related to ischemic neuronal death. Our previous studies have shown that the densities of dendritic spines and asymmetric synapses in the CA1 area are increased at 12h and 24h after ischemia. In the present study, we investigated changes in synaptic structures in the CA3 area and compared the expression of glutamate receptors in the CA1 and CA3 hippocampal regions of rats after ischemia. Our results demonstrated that the NR2B/NR2A ratio became larger after ischemia although the expression of both the NR2B subunit (activation of apoptotic pathway) and NR2A subunit (activation of survival pathway) decreased in the CA1 area from 6h to 48h after reperfusion. Furthermore, expression of the GluR2 subunit (calcium impermeable) of the AMPA receptor class significantly decreased while the GluR1 subunit (calcium permeable) remained unchanged at the same examined reperfusion times, which subsequently caused an increase in the GluR1/GluR2 ratio. Despite these notable differences in subunit expression, there were no obvious changes in the density of synapses or expression of NMDAR and AMPAR subunits in the CA3 area after ischemia. These results suggest that delayed CA1 neuronal death may be related to the dramatic fluctuation in the synaptic structure and relative upregulation of NR2B and GluR1 subunits induced by transient global ischemia.

Cadherins and catenins in dendrite and synapse morphogenesis

Neurons are highly polarized specialized cells. Neuronal integrity and functional roles are critically dependent on dendritic architecture and synaptic structure, function and plasticity. The cadherins are glycosylated transmembrane proteins that form cell adhesion complexes in various tissues. They are associated with a group of cytosolic proteins, the catenins. While the functional roles of the complex have been extensively investigates in non-neuronal cells, it is becoming increasingly clear that components of the complex have critical roles in regulating dendritic and synaptic architecture, function and plasticity in neurons. Consistent with these functional roles, aberrations in components of the complex have been implicated in a variety of neurodevelopmental disorders. In this review, we discuss the roles of the classical cadherins and catenins in various aspects of dendrite and synapse architecture and function and their relevance to human neurological disorders. Cadherins are glycosylated transmembrane proteins that were initially identified as Ca(2+)-dependent cell adhesion molecules. They are present on plasma membrane of a variety of cell types from primitive metazoans to humans. In the past several years, it has become clear that in addition to providing mechanical adhesion between cells, cadherins play integral roles in tissue morphogenesis and homeostasis. The cadherin family is composed of more than 100 members and classified into several subfamilies, including classical cadherins and protocadherins. Several of these cadherin family members have been implicated in various aspects of neuronal development and function. (1-3) The classical cadherins are associated with a group of cytosolic proteins, collectively called the catenins. While the functional roles of the cadherin-catenin cell adhesion complex have been extensively investigated in epithelial cells, it is now clear that components of the complex are well expressed in central neurons at different stages during development. (4,5) Recent exciting studies have shed some light on the functional roles of cadherins and catenins in central neurons. In this review, we will provide a brief overview of the cadherin superfamily, describe cadherin family members expressed in central neurons, cadherin-catenin complexes in central neurons and then focus on role of the cadherin-catenin complex in dendrite morphogenesis and synapse morphogenesis, function and plasticity. The final section is dedicated to discussion of the emerging list of neural disorders linked to cadherins and catenins. While the roles of cadherins and catenins have been examined in several different types of neurons, the focus of this review is their role in mammalian central neurons, particularly those of the cortex and hippocampus. Accompanying this review is a series of excellent reviews targeting the roles of cadherins and protocadherins in other aspects of neural development.

Organization of TNIK in dendritic spines

Tumor necrosis factor receptor-associated factor 2 (TRAF2)- and noncatalytic region of tyrosine kinase (NCK)-interacting kinase (TNIK) has been identified as an interactor in the psychiatric risk factor, Disrupted in Schizophrenia 1 (DISC1). As a step toward deciphering its function in the brain, we performed high-resolution light and electron microscopic immunocytochemistry. We demonstrate here that TNIK is expressed in neurons throughout the adult mouse brain. In striatum and cerebral cortex, TNIK concentrates in dendritic spines, especially in the vicinity of the lateral edge of the synapse. Thus, TNIK is highly enriched at a microdomain critical for glutamatergic signaling.

Loss of δ-catenin function in severe autism

Autism is a multifactorial neurodevelopmental disorder affecting more males than females; consequently, under a multifactorial genetic hypothesis, females are affected only when they cross a higher biological threshold. We hypothesize that deleterious variants at conserved residues are enriched in severely affected patients arising from FEMFs (female-enriched multiplex families) with severe disease, enhancing the detection of key autism genes in modest numbers of cases. We show the utility of this strategy by identifying missense and dosage sequence variants in the gene encoding the adhesive junction-associated delta catenin protein (CTNND2) in FEMFs and demonstrating their loss-of-function effect by functional analyses in zebrafish embryos and cultured hippocampal neurons from wildtype and Ctnnd2 null mouse embryos. Finally, through gene expression and network analyses, we highlight a critical role for CTNND2 in neuronal development and an intimate connection to chromatin biology. Our data contribute to the understanding of the genetic architecture of autism and suggest that genetic analyses of phenotypic extremes, such as FEMFs, are of innate value in multifactorial disorders.

δ-Catenin Regulates Spine Architecture via Cadherin and PDZ-dependent Interactions

The ability of neurons to maintain spine architecture and modulate it in response to synaptic activity is a crucial component of the cellular machinery that underlies information storage in pyramidal neurons of the hippocampus. Here we show a critical role for δ-catenin, a component of the cadherin-catenin cell adhesion complex, in regulating spine head width and length in pyramidal neurons of the hippocampus. The loss of Ctnnd2, the gene encoding δ-catenin, has been associated with the intellectual disability observed in the cri du chat syndrome, suggesting that the functional roles of δ-catenin are vital for neuronal integrity and higher order functions. We demonstrate that loss of δ-catenin in a mouse model or knockdown of δ-catenin in pyramidal neurons compromises spine head width and length, without altering spine dynamics. This is accompanied by a reduction in the levels of synaptic N-cadherin. The ability of δ-catenin to modulate spine architecture is critically dependent on its ability to interact with cadherin and PDZ domain-containing proteins. We propose that loss of δ-catenin during development perturbs synaptic architecture leading to developmental aberrations in neural circuit formation that contribute to the learning disabilities in a mouse model and humans with cri du chat syndrome.

Cadherin-based transsynaptic networks in establishing and modifying neural connectivity

It is tacitly understood that cell adhesion molecules (CAMs) are critically important for the development of cells, circuits, and synapses in the brain. What is less clear is what CAMs continue to contribute to brain structure and function after the early period of development. Here, we focus on the cadherin family of CAMs to first briefly recap their multidimensional roles in neural development and then to highlight emerging data showing that with maturity, cadherins become largely dispensible for maintaining neuronal and synaptic structure, instead displaying new and narrower roles at mature synapses where they critically regulate dynamic aspects of synaptic signaling, structural plasticity, and cognitive function. At mature synapses, cadherins are an integral component of multiprotein networks, modifying synaptic signaling, morphology, and plasticity through collaborative interactions with other CAM family members as well as a variety of neurotransmitter receptors, scaffolding proteins, and other effector molecules. Such recognition of the ever-evolving functions of synaptic cadherins may yield insight into the pathophysiology of brain disorders in which cadherins have been implicated and that manifest at different times of life.

GRIP1 interlinks N-cadherin and AMPA receptors at vesicles to promote combined cargo transport into dendrites

The GluA2 subunit of AMPA-type glutamate receptors (AMPARs) regulates excitatory synaptic transmission in neurons. In addition, the transsynaptic cell adhesion molecule N-cadherin controls excitatory synapse function and stabilizes dendritic spine structures. At postsynaptic membranes, GluA2 physically binds N-cadherin, underlying spine growth and synaptic modulation. We report that N-cadherin binds to PSD-95/SAP90/DLG/ZO-1 (PDZ) domain 2 of the glutamate receptor interacting protein 1 (GRIP1) through its intracellular C terminus. N-cadherin and GluA2-containing AMPARs are presorted to identical transport vesicles for dendrite delivery, and live imaging reveals cotransport of both proteins. The kinesin KIF5 powers GluA2/N-cadherin codelivery by using GRIP1 as a multilink interface. Notably, GluA2 and N-cadherin use different PDZ domains on GRIP1 to simultaneously bind the transport complex, and interference with either binding motif impairs the turnover of both synaptic cargoes. Depolymerization of microtubules, deletion of the KIF5 motor domain, or specific blockade of AMPAR exocytosis affects delivery of GluA2/N-cadherin vesicles. At the functional level, interference with this cotransport reduces the number of spine protrusions and excitatory synapses. Our data suggest the concept that the multi-PDZ-domain adaptor protein GRIP1 can act as a scaffold at trafficking vesicles in the combined delivery of AMPARs and N-cadherin into dendrites.

Palmitoylation of δ-catenin by DHHC5 mediates activity-induced synapse plasticity

Synaptic cadherin adhesion complexes are known to be key regulators of synapse plasticity. However, the molecular mechanisms that coordinate activity-induced modifications in cadherin localization and adhesion and subsequent changes in synapse morphology and efficacy, remain unanswered. We demonstrate that the intracellular cadherin binding protein, δ-catenin, is transiently palmitoylated by DHHC5 following enhanced synaptic activity, and that palmitoylation increases δ-catenin/cadherin interactions at synapses. Both the palmitoylation of δ-catenin and its binding to cadherin are required for activity-induced stabilization of N-cadherin at synapses, the enlargement of postsynaptic spines, as well as insertion of GluA1 and GluA2 subunits into the synaptic membrane and the concomitant increase in mEPSC amplitude. Importantly, context-dependent fear conditioning in mice results in increased δ-catenin palmitoylation as well as increased δ-catenin/cadherin associations at hippocampal synapses. Together, this suggests a role for palmitoylated δ-catenin in coordinating activity-dependent changes in synaptic adhesion molecules, synapse structure, and receptor localization that are involved in memory formation.

N-cadherin relocalizes from the periphery to the center of the synapse after transient synaptic stimulation in hippocampal neurons

N-cadherin is a cell adhesion molecule which is enriched at synapses. Binding of N-cadherin molecules to each other across the synaptic cleft has been postulated to stabilize adhesion between the presynaptic bouton and the postsynaptic terminal. N-cadherin is also required for activity-induced changes at synapses, including hippocampal long term potentiation and activity-induced spine expansion and stabilization. We hypothesized that these activity-dependent changes might involve changes in N-cadherin localization within synapses. To determine whether synaptic activity changes the localization of N-cadherin, we used structured illumination microscopy, a super-resolution approach which overcomes the conventional resolution limits of light microscopy, to visualize the localization of N-cadherin within synapses of hippocampal neurons. We found that synaptic N-cadherin exhibits a spectrum of localization patterns, ranging from puncta at the periphery of the synapse adjacent to the active zone to an even distribution along the synaptic cleft. Furthermore, the N-cadherin localization pattern within synapses changes during KCl depolarization and after transient synaptic stimulation. During KCl depolarization, N-cadherin relocalizes away from the central region of the synaptic cleft to the periphery of the synapse. In contrast, after transient synaptic stimulation with KCl followed by a period of rest in normal media, fewer synapses have N-cadherin present as puncta at the periphery and more synapses have N-cadherin present more centrally and uniformly along the synapse compared to unstimulated cells. This indicates that transient synaptic stimulation modulates N-cadherin localization within the synapse. These results bring new information to the structural organization and activity-induced changes occurring at synapses, and suggest that N-cadherin relocalization may contribute to activity dependent changes at synapses.

AMPA receptor trafficking and the mechanisms underlying synaptic plasticity and cognitive aging

Even in healthy individuals there is an inexorable agerelated decline in cognitive function. This is due, in large part, to reduced synaptic plasticity caused by changes in the molecular composition of the postsynaptic membrane. AMPA receptors (AMPARs) are glutamate-gated cation channels that mediate the overwhelming majority of fast excitatory transmission in the brain. Changes in AMPAR number and/or function are a core feature of synaptic plasticity and age-related cognitive decline, AMPARs are highly dynamic proteins that are subject to highly controlled trafficking, recycling, and/or degradation and replacement. This active regulation of AMPAR synthesis, targeting, synaptic dwell time, and degradation is fundamentally important for memory formation and storage. Further, aberrant AMPAR trafficking and consequent detrimental changes in synapses are strongly implicated in many brain diseases, which represent a vast social and economic burden. The purpose of this article is to provide an overview of the molecular and cellular AMPA receptor trafficking events that control synaptic responsiveness and plasticity, and highlight what is known currently known about how these processes change with age and disease.

Predicting protein-protein interactions in the post synaptic density

The post synaptic density (PSD) is a specialization of the cytoskeleton at the synaptic junction, composed of hundreds of different proteins. Characterizing the protein components of the PSD and their interactions can help elucidate the mechanism of long-term changes in synaptic plasticity, which underlie learning and memory. Unfortunately, our knowledge of the proteome and interactome of the PSD is still partial and noisy. In this study we describe a computational framework to improve the reconstruction of the PSD network. The approach is based on learning the characteristics of PSD protein interactions from a set of trusted interactions, expanding this set with data collected from large scale repositories, and then predicting novel interaction with proteins that are suspected to reside in the PSD. Using this method we obtained thirty predicted interactions, with more than half of which having supporting evidence in the literature. We discuss in details two of these new interactions, Lrrtm1 with PSD-95 and Src with Capg. The first may take part in a mechanism underlying glutamatergic dysfunction in schizophrenia. The second suggests an alternative mechanism to regulate dendritic spines maturation.

AMPA receptor trafficking and the mechanisms underlying synaptic plasticity and cognitive aging

Even in healthy individuals there is an inexorable agerelated decline in cognitive function. This is due, in large part, to reduced synaptic plasticity caused by changes in the molecular composition of the postsynaptic membrane. AMPA receptors (AMPARs) are glutamate-gated cation channels that mediate the overwhelming majority of fast excitatory transmission in the brain. Changes in AMPAR number and/or function are a core feature of synaptic plasticity and age-related cognitive decline, AMPARs are highly dynamic proteins that are subject to highly controlled trafficking, recycling, and/or degradation and replacement. This active regulation of AMPAR synthesis, targeting, synaptic dwell time, and degradation is fundamentally important for memory formation and storage. Further, aberrant AMPAR trafficking and consequent detrimental changes in synapses are strongly implicated in many brain diseases, which represent a vast social and economic burden. The purpose of this article is to provide an overview of the molecular and cellular AMPA receptor trafficking events that control synaptic responsiveness and plasticity, and highlight what is known currently known about how these processes change with age and disease.

The CaMKII/NMDAR complex as a molecular memory

CaMKII is a major synaptic protein that is activated during the induction of long-term potentiation (LTP) by the Ca²⁺ influx through NMDARs. This activation is required for LTP induction, but the role of the kinase in the maintenance of LTP is less clear. Elucidating the mechanisms of maintenance may provide insights into the molecular processes that underlie the stability of stored memories. In this brief review, we will outline the criteria for evaluating an LTP maintenance mechanism. The specific hypothesis evaluated is that LTP is maintained by the complex of activated CaMKII with the NMDAR. The evidence in support of this hypothesis is substantial, but further experiments are required, notably to determine the time course and persistence of complex after LTP induction. Additional work is also required to elucidate how the CaMKII/NMDAR complex produces the structural growth of the synapse that underlies late LTP. It has been proposed by Frey and Morris that late LTP involves the setting of a molecular tag during LTP induction, which subsequently allows the activated synapse to capture the proteins responsible for late LTP. However, the molecular processes by which this leads to the structural growth that underlies late LTP are completely unclear. Based on known binding reactions, we suggest the first molecularly specific version of tag/capture hypothesis: that the CaMKII/NMDAR complex, once formed, serves as a tag, which then leads to a binding cascade involving densin, delta-catenin, and N-cadherin (some of which are newly synthesized). Delta-catenin binds AMPA-binding protein (ABP), leading to the LTP-induced increase in AMPA channel content. The addition of postsynaptic N-cadherin, and the complementary increase on the presynaptic side, leads to a trans-synaptically coordinated increase in synapse size (and more release sites). It is suggested that synaptic strength is stored stably through the combined actions of the CaMKII/NMDAR complex and N-cadherin dimers. These N-cadherin pairs have redundant storage that could provide informational stability in a manner analogous to the base-pairing in DNA.

Mef2 promotes spine elimination in absence of δ-catenin

δ-Catenin is a component of the cadherin-catenin cell adhesion complex and its loss has been implicated in the mental retardation associated with the Cri du chat syndrome. We have previously demonstrated that loss of δ-catenin in a murine model during development results in excessive spine and synaptic density and function. In order to examine the role of potential molecules that might cooperate with δ-catenin to regulate spine density, we focused on Mef2. Our data demonstrate that while loss of δ-catenin does not alter the expression levels of endogenous Mef2, expression of Mef2 in neurons that are knocked down for δ-catenin promotes spine elimination. These results establish a molecular mechanism by which excessive spines in the absence of δ-catenin may be eliminated and may point toward pharmacological therapy for the Cri du chat syndrome.

14-3-3ɛ/ζ Affects the stability of δ-catenin and regulates δ-catenin-induced dendrogenesis

Accumulated evidence suggests that aberrant regulation of δ-catenin leads to pathological consequences such as mental retardation and cognitive dysfunction. This study revealed that 14-3-3ɛ/ζ stabilizes δ-catenin, with different binding regions involved in the interaction. Furthermore, the specific inhibition of the interaction of 14-3-3 with δ-catenin reduced levels of δ-catenin and significantly impaired the capacity of δ-catenin to induce dendritic branching in both NIH3T3 fibroblasts and primary hippocampal neurons. However, the S1094A δ-catenin mutant, which cannot interact with 14-3-3ζ, still retained the capability of inducing dendrogenesis. Taken together, these results elucidate the underlying events that regulate the stability of δ-catenin and δ-catenin-induced dendrogenesis.

Biophysical mechanisms regulating AMPA receptor accumulation at synapses

Controlling the number of AMPA receptors at synapses is fundamental for fast synaptic transmission as well as for long term adaptations in synaptic strength. In this review, we examine the biophysical mechanisms implicated in regulating AMPAR levels at the cell surface and at synapses. We first describe the structure and function of AMPARs, as well as their interactions with various proteins regulating their traffic and function. Second we review the vesicular trafficking mechanism involving exocytosis and endocytosis, by which AMPARs reach the cell surface and are internalized, respectively. Third, we examine the properties of lateral diffusion of AMPARs and their trapping at post-synaptic densities. Finally, we discuss how these two parallel mechanisms are integrated in time and space to control changes in synaptic AMPAR levels in response to plasticity protocols. This review highlights the important role of the extra-synaptic AMPAR pool, which makes an obligatory link between vesicular trafficking and trapping or release at synapses.

Rewiring of afferent fibers in the somatosensory thalamus of mice caused by peripheral sensory nerve transection

The remodeling of neural circuitry and changes in synaptic efficacy after peripheral sensory nerve injury are considered the basis for functional reorganization in the brain, including changes in receptive fields. However, when or how the remodeling occurs is largely unknown. Here we show the rapid rewiring of afferent fibers in the mature ventral posteromedial thalamic nucleus of mice after transection of the peripheral whisker sensory nerve, using the whole-cell voltage-clamp technique. Transection induced the recruitment of afferent fibers to a thalamic relay neuron within 5-6 d of injury. The rewiring was pathway specific, but not sensory experience dependent or peripheral nerve activity dependent. The newly recruited fibers mediated small EPSCs, and postsynaptic GluA2-containing AMPA receptors were selectively upregulated at the new synapses. This rapid and pathway-specific remodeling of thalamic circuitry may be an initial step in the massive axonal reorganization at supraspinal levels, which occurs months or years after peripheral sensory nerve injury.

Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.

Linkage of N-cadherin to multiple cytoskeletal elements revealed by a proteomic approach in hippocampal neurons

The CNS synapse is an adhesive junction differentiated for chemical neurotransmission and is equipped with presynaptic vesicles and postsynaptic neurotransmitter receptors. Cell adhesion molecule cadherins not only maintain connections between pre- and postsynaptic membranes but also modulate the efficacy of synaptic transmission. Although the components of the cadherin-mediated adhesive apparatus have been studied extensively in various cell systems, the complete picture of these components, particularly at the synaptic junction, remains elusive. Here, we describe the proteomic assortment of the N-cadherin-mediated synaptic adhesion apparatus in cultured hippocampal neurons. N-cadherin immunoprecipitated from Triton X-100-solubilized neuronal extract contained equal amounts of β- and α-catenins, as well as F-actin-related membrane anchor proteins such as integrins bridged with α-actinin-4, and Na(+)/K(+)-ATPase bridged with spectrins. A close relative of β-catenin, plakoglobin, and its binding partner, desmoplakin, were also found, suggesting that a subset of the N-cadherin-mediated adhesive apparatus also anchors intermediate filaments. Moreover, dynein heavy chain and LEK1/CENPF/mitosin were found. This suggests that internalized pools of N-cadherin in trafficking vesicles are conveyed by dynein motors on microtubules. In addition, ARVCF and NPRAP/neurojungin/δ2-catenin, but not p120ctn/δ1-catenin or plakophilins-1, -2, -3, -4 (p0071), were found, suggesting other possible bridges to microtubules. Finally, synaptic stimulation by membrane depolarization resulted in an increased 93-kDa band, which corresponded to proteolytically truncated β-catenin. The integration of three different classes of cytoskeletal systems found in the synaptic N-cadherin complex may imply a dynamic switching of adhesive scaffolds in response to synaptic activity.

Scaffold proteins at the postsynaptic density

Scaffold proteins are abundant and essential components of the postsynaptic density (PSD). They play a major role in many synaptic functions including the trafficking, anchoring, and clustering of glutamate receptors and adhesion molecules. Moreover, they link postsynaptic receptors with their downstream signaling proteins and regulate the dynamics of cytoskeletal structures. By definition, PSD scaffold proteins do not have intrinsic enzymatic activities but are formed by modular and specific domains deputed to form large protein networks. Here, we will discuss the latest findings regarding the structure and functions of major PSD scaffold proteins. Given that scaffold proteins are central components of PSD architecture, it is not surprising that deletion or mutations in their human genes cause severe neuropsychiatric disorders including autism, mental retardation, and schizophrenia. Thus, their dynamic organization and regulation are directly correlated with the essential structure of the PSD and the normal physiology of neuronal synapses.

Synaptic autoregulation by metalloproteases and γ-secretase

The proteolytic machinery comprising metalloproteases and γ-secretase, an intramembrane aspartyl protease involved in Alzheimer's disease, cleaves several substrates in addition to the extensively studied amyloid precursor protein. Some of these substrates, such as N-cadherin, are synaptic proteins involved in synapse remodeling and maintenance. Here we show, in rats and mice, that metalloproteases and γ-secretase are physiologic regulators of synapses. Both proteases are synaptic, with γ-secretase tethered at the synapse by δ-catenin, a synaptic scaffolding protein that also binds to N-cadherin and, through scaffolds, to AMPA receptor and a metalloprotease. Activity-dependent proteolysis by metalloproteases and γ-secretase takes place at both sides of the synapse, with the metalloprotease cleavage being NMDA receptor-dependent. This proteolysis decreases levels of synaptic proteins and diminishes synaptic transmission. Our results suggest that activity-dependent substrate cleavage by synaptic metalloproteases and γ-secretase modifies synaptic transmission, providing a novel form of synaptic autoregulation.

Identification of novel NPRAP/δ-catenin-interacting proteins and the direct association of NPRAP with dynamin 2

Neural plakophilin-related armadillo protein (NPRAP or δ-catenin) is a neuronal-specific protein that is best known for its interaction with presenilin 1 (PS1). Interestingly, the hemizygous loss of NPRAP is associated with severe mental retardation in cri du chat syndrome (CDCS), and mutations in PS1 cause an aggressive, early-onset form of Alzheimer's disease. Until recently, studies on the function of NPRAP have focused on its ability to modulate dendritic protrusion elaboration through its binding to cell adhesion and scaffolding molecules. However, mounting evidence indicates that NPRAP participates in intracellular signaling and exists in the nucleus, where it modulates gene expression. This apparent bifunctional nature suggests an elaborate neuronal role, but how NPRAP came to participate in such distinct subcellular events remains a mystery. To gain insight into this pathway, we immunoprecipitated NPRAP from human SH SY5Y cells and identified several novel interacting proteins by mass spectrometry. These included neurofilament alpha-internexin, interferon regulatory protein 2 binding factors, and dynamins 1 and 2. We further validated dynamin 2/NPRAP colocalization and direct interaction in vivo, confirming their bona fide partnership. Interestingly, dynamin 2 has established roles in endocytosis and actin assembly, and both of these processes have the potential to interface with the cell adhesion and intracellular signaling processes that involve NPRAP. Our data provide new avenues for approaching NPRAP biology and suggest a broader role for this protein than previously thought.

Modulation of fear memory by retrieval and extinction: a clue for memory deconsolidation

Memories are fragile and easily forgotten at first, but after a consolidation period of hours to weeks, are inscribed in our brains as stable traces, no longer vulnerable to conventional amnesic treatments. Retrieval of a memory renders it labile, akin to the early stages of consolidation. This phenomenon has been explored as memory reactivation, in the sense that the memory is temporarily 'deconsolidated', allowing a short time window for amnesic intervention. This window closes again after reconsolidation, which restores the stability of the memory. In contrast to this 'transient deconsolidation' and the short-spanned amnesic effects of consolidation blockers, some specific treatments can disrupt even consolidated memory, leading to apparent amnesia. We propose the term 'amnesic deconsolidation' to describe such processes that lead to disruption of consolidated memory and/or consolidated memory traces. We review studies of these 'amnesic deconsolidation' treatments that enhance memory extinction, alleviate relapse, and reverse learning-induced plasticity. The transient deconsolidation that memory retrieval induces and the amnesic deconsolidation that these regimes induce both seem to dislodge a component that stabilizes consolidated memory. Characterizing this component, at both molecular and network levels, will provide a key to developing clinical treatments for memory-related disorders and to defining the consolidated memory trace.