Dual role of CaMKII-dependent SAP97 phosphorylation in mediating trafficking and insertion of NMDA receptor subunit NR2A

DLG1 functions upstream of SDCCAG3 and IFT20 to control ciliary targeting of polycystin-2

Polarized vesicular trafficking directs specific receptors and ion channels to cilia, but the underlying mechanisms are poorly understood. Here we describe a role for DLG1, a core component of the Scribble polarity complex, in regulating ciliary protein trafficking in kidney epithelial cells. Conditional knockout of Dlg1 in mouse kidney causes ciliary elongation and cystogenesis, and cell-based proximity labeling proteomics and fluorescence microscopy show alterations in the ciliary proteome upon loss of DLG1. Specifically, the retromer-associated protein SDCCAG3, IFT20, and polycystin-2 (PC2) are reduced in the cilia of DLG1-deficient cells compared to control cells. This phenotype is recapitulated in vivo and rescuable by re-expression of wild-type DLG1, but not a Congenital Anomalies of the Kidney and Urinary Tract (CAKUT)-associated DLG1 variant, p.T489R. Finally, biochemical approaches and Alpha Fold modelling suggest that SDCCAG3 and IFT20 form a complex that associates, at least indirectly, with DLG1. Our work identifies a key role for DLG1 in regulating ciliary protein composition and suggests that ciliary dysfunction of the p.T489R DLG1 variant may contribute to CAKUT.

PP1β opposes classic PP1 function, inhibiting spine maturation and promoting LTP

Protein phosphatase 1 (PP1) regulates synaptic plasticity and has been described as a molecular constraint on learning and memory. There are three neuronal isoforms, PP1α, PP1β, and PP1γ, but little is known about their individual functions. PP1α and PP1γ are assumed to mediate the effects of PP1 on learning and memory based on their enrichment at dendritic spines and their preferential binding to neurabin and spinophilin, major PP1 synaptic scaffolding proteins. However, it was recently discovered that human de novo PP1β mutations cause intellectual disability, suggesting an important but ill-defined role for PP1β. In this study, we investigated the functions of each PP1 isoform in hippocampal synaptic physiology using conditional CA1-specific knockout mice. In stark contrast to classic PP1 function, we found that PP1β promotes synaptic plasticity as well as spatial memory. These changes in synaptic plasticity and memory are accompanied by changes in GluA1 phosphorylation, GluN2A levels, and dendritic spine density and morphology, including silent synapse number. These functions of PP1β reveal a previously unidentified signaling pathway regulating spine maturation and plasticity, broadening our understanding of the complex role of PP1 in synaptic physiology.

Targeting N-Methyl-d-Aspartate Receptors in Neurodegenerative Diseases

N-methyl-d-aspartate receptors (NMDARs) are the main class of ionotropic receptors for the excitatory neurotransmitter glutamate. They play a crucial role in the permeability of Ca2+ ions and excitatory neurotransmission in the brain. Being heteromeric receptors, they are composed of several subunits, including two obligatory GluN1 subunits (eight splice variants) and regulatory GluN2 (GluN2A~D) or GluN3 (GluN3A~B) subunits. Widely distributed in the brain, they regulate other neurotransmission systems and are therefore involved in essential functions such as synaptic transmission, learning and memory, plasticity, and excitotoxicity. The present review will detail the structure, composition, and localization of NMDARs, their role and regulation at the glutamatergic synapse, and their impact on cognitive processes and in neurodegenerative diseases (Alzheimer's, Huntington's, and Parkinson's disease). The pharmacology of different NMDAR antagonists and their therapeutic potentialities will be presented. In particular, a focus will be given on fluoroethylnormemantine (FENM), an investigational drug with very promising development as a neuroprotective agent in Alzheimer's disease, in complement to its reported efficacy as a tomography radiotracer for NMDARs and an anxiolytic drug in post-traumatic stress disorder.

DLG1 functions upstream of SDCCAG3 and IFT20 to control ciliary targeting of polycystin-2

Polarized vesicular trafficking directs specific receptors and ion channels to cilia, but the underlying mechanisms are poorly understood. Here we describe a role for DLG1, a core component of the Scribble polarity complex, in regulating ciliary protein trafficking in kidney epithelial cells. Conditional knockout of Dlg1 in mouse kidney caused ciliary elongation and cystogenesis, and cell-based proximity labelling proteomics and fluorescence microscopy showed alterations in the ciliary proteome upon loss of DLG1. Specifically, the retromer-associated protein SDCCAG3, IFT20 and polycystin-2 (PC2) were reduced in cilia of DLG1 deficient cells compared to control cells. This phenotype was recapitulated in vivo and rescuable by re-expression of wildtype DLG1, but not a Congenital Anomalies of the Kidney and Urinary Tract (CAKUT)-associated DLG1 variant, p.T489R. Finally, biochemical approaches and Alpha Fold modelling suggested that SDCCAG3 and IFT20 form a complex that associates, at least indirectly, with DLG1. Our work identifies a key role for DLG1 in regulating ciliary protein composition and suggests that ciliary dysfunction of the p.T489R DLG1 variant may contribute to CAKUT.

Trafficking of NMDA receptors is essential for hippocampal synaptic plasticity and memory consolidation

NMDA receptor (NMDAR) plays a vital role in brain development and normal physiological functions. Surface trafficking of NMDAR contributes to the modulation of synaptic functions and information processing. However, it remains unclear whether NMDAR trafficking is independent of long-term potentiation (LTP) and whether it regulates behavior. Here, we report that LTP of AMPAR and NMDAR can occur concurrently and that NMDAR trafficking can regulate AMPAR trafficking and AMPAR-mediated LTP. By contrast, AMPAR trafficking does not impact NMDAR-mediated LTP. Using SAP97-interfering peptide and SAP97 knockin (KI) rat, we show that the effect is mediated by GluN2A-subunit-containing NMDARs. At the behavior level, impaired NMDAR trafficking results in deficits in consolidation, but not acquisition, of fear memory. Collectively, our results suggest the essential role of NMDAR trafficking in LTP and memory consolidation.

Chronic caffeine consumption improves the acute REM sleep deprivation-induced spatial memory impairment while altering NMDA receptor subunit expression in male rats

Caffeine is a psychostimulant substance that is mostly used to prevent fatigue, increase alertness, and ameliorate sleep loss situations. In this study, we aimed to investigate the effect of chronic caffeine consumption on learning and memory functions and related genes in REM (rapid-eye-movement) sleep-deprived rats. During the neonatal period [postnatal day (PND) 28] Wistar albino male rats (n=32) were randomly assigned into four groups: control (C), caffeine application (Cf), acute REM sleep-deprivation (RD), and caffeine application+acute RD (Cf+RD). The 48 hours of RD was executed when caffeine administration was completed. The learning and memory performance was evaluated by the Morris Water Maze Test (MWMT). Following this, the rats were decapitated to isolate hippocampus tissues. In MWMT, time spent in the targeted quadrant decreased significantly in the RD group compared to the C and Cf+RD group. NR2A expression level increased in the RD group compared to C, Cf, and Cf+RD groups (p<0.05). NR2B expression level increased in RD and Cf +RD groups compared to C and Cf groups (p<0.05). BDNF and c-Fos expression levels did not differ significantly between the groups. RD impaired hippocampal spatial memory performance in the MWMT test. Our results indicated that chronic caffeine consumption has a therapeutic effect on spatial memory deterioration impairment caused by RD. Furthermore, it seems that the effect of caffeine RD on the hippocampus may be mediated by NR2A.

Novel Biomarkers of Alzheimer's Disease: Based Upon N-methyl-D-aspartate Receptor Hypoactivation and Oxidative Stress

Early detection and prevention of Alzheimer’s disease (AD) is important. The current treatment for early AD is acetylcholine esterase inhibitors (AChEIs); however, the efficacy is poor. Besides, AChEI did not show efficacy in mild cognitive impairment (MCI). Beta-amyloid (Aβ) deposits have been regarded to be highly related to the pathogenesis of AD. However, many clinical trials aiming at the clearance of Aβ deposits failed to improve the cognitive decline of AD, even at its early phase. There should be other important mechanisms unproven in the course of AD and MCI. Feasible biomarkers for the diagnosis and treatment response of AD are lacking to date. The N-methyl-D-aspartate receptor (NMDAR) activation plays an important role in learning and memory. On the other hand, oxidative stress has been regarded to contribute to aging with the assumption that free radicals damage cell constituents and connective tissues. Our recent study found that an NMDAR enhancer, sodium benzoate (the pivotal inhibitor of D-amino acid oxidase [DAAO]), improved the cognitive and global function of patients with early-phase AD. Further, we found that peripheral DAAO levels were higher in patients with MCI and AD than healthy controls. We also found that sodium benzoate was able to change the activity of antioxidant. These pieces of evidence suggest that the NMDAR function is associated with anti-oxidation, and have potential to be biomarkers for the diagnosis and treatment response of AD.

Protein-protein interactions at the NMDA receptor complex: From synaptic retention to synaptonuclear protein messengers

N-methyl-d-aspartate receptors (NMDARs) are glutamate-gated ion channels that support essential functions throughout the brain. NMDARs are tetramers composed of the GluN1 subunit in complex with GluN2- and GluN3-type regulatory subunits, resulting in the formation of various receptor subtypes throughout the central nervous system (CNS), characterised by different kinetics, biophysical and pharmacological properties, and the abilities to interact with specific partners at dendritic spines. NMDARs are expressed at high levels, are widely distributed throughout the brain, and are involved in several physiological and pathological conditions. Here, we will focus on the GluN2A- and GluN2B-containing NMDARs found at excitatory synapses and their interactions with plasticity-relevant proteins, such as the postsynaptic density family of membrane-associated guanylate kinases (PSD-MAGUKs), Ca²⁺/calmodulin-dependent kinase II (CaMKII) and synaptonuclear protein messengers. The dynamic interactions between NMDAR subunits and various proteins regulating synaptic receptor retention and synaptonuclear signalling mediated by protein messengers suggest that the NMDAR serves as a key molecular player that coordinates synaptic activity and cell-wide events that require gene transcription. Importantly, protein-protein interactions at the NMDAR complex can also contribute to synaptic dysfunction in several brain disorders. Therefore, the modulation of the molecular composition of the NMDAR complex might represent a novel pharmacological approach for the treatment of certain disease states.

Regulation of NMDA glutamate receptor functions by the GluN2 subunits

The N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate the flux of calcium (Ca²⁺ ) into the post-synaptic compartment. Ca²⁺ influx subsequently triggers the activation of various intracellular signalling cascades that underpin multiple forms of synaptic plasticity. Functional NMDARs are assembled as heterotetramers composed of two obligatory GluN1 subunits and two GluN2 or GluN3 subunits. Four different GluN2 subunits (GluN2A-D) are present throughout the central nervous system; however, they are differentially expressed, both developmentally and spatially, in a cell- and synapse-specific manner. Each GluN2 subunit confers NMDARs with distinct ion channel properties and intracellular trafficking pathways. Regulated membrane trafficking of NMDARs is a dynamic process that ultimately determines the number of NMDARs at synapses, and is controlled by subunit-specific interactions with various intracellular regulatory proteins. Here we review recent progress made towards understanding the molecular mechanisms that regulate the trafficking of GluN2-containing NMDARs, focusing on the roles of several key synaptic proteins that interact with NMDARs via their carboxyl termini.

Profile of cortical N-methyl-D-aspartate receptor subunit expression associates with inherent motor impulsivity in rats

Impulsivity is a multifaceted behavioral manifestation with implications in several neuropsychiatric disorders. Glutamate neurotransmission through the N-methyl-D-aspartate receptors (NMDARs) in the medial prefrontal cortex (mPFC), an important brain region in decision-making and goal-directed behaviors, plays a key role in motor impulsivity. We discovered that inherent motor impulsivity predicted responsiveness to D-cycloserine (DCS), a partial NMDAR agonist, which prompted the hypothesis that inherent motor impulsivity is associated with the pattern of expression of cortical NMDAR subunits (GluN1, GluN2A, GluN2B), specifically the protein levels and synaptosomal trafficking of the NMDAR subunits. Outbred male Sprague-Dawley rats were identified as high (HI) or low (LI) impulsive using the one-choice serial reaction time task. Following phenotypic identification, mPFC synaptosomal protein was extracted from HI and LI rats to assess the expression pattern of the NMDAR subunits. Synaptosomal trafficking and stabilization for the GluN2 subunits were investigated by co-immunoprecipitation for postsynaptic density 95 (PSD95) and synapse associated protein 102 (SAP102). HI rats had lower mPFC GluN1 and GluN2A, but higher GluN2B and pGluN2B synaptosomal protein expression versus LI rats. Further, higher GluN2B:PSD95 and GluN2B:SAP102 protein:protein interactions were detected in HI versus LI rats. Thus, the mPFC NMDAR subunit expression pattern and/or synaptosomal trafficking associates with high inherent motor impulsivity. Increased understanding of the complex regulation of NMDAR balance within the mPFC as it relates to inherent motor impulsivity may lead to a better understanding of risk factors for impulse-control disorders.

Subunit contribution to NMDA receptor hypofunction and redox sensitivity of hippocampal synaptic transmission during aging

We examined the contribution of N-methyl-D-aspartate receptor (NMDAR) subunits in the redox-mediated decline in NMDAR function during aging. GluN2A and GluN2B selective antagonists decreased peak NMDAR currents to a similar extent in young and aged animals, indicating that a shift in diheteromeric GluN2 subunits does not underlie the age-related decrease in the NMDAR synaptic function. Application of dithiothreitol (DTT) in aged animals, increased peak NMDAR synaptic currents, prolonged the decay time, and increased the sensitivity of the synaptic response to the GluN2B antagonist, ifenprodil, indicating that DTT increased the contribution of GluN2B subunits to the synaptic response. The DTT-mediated increase in NMDAR function was inhibited by partial blockade of NMDARs, and this inhibition was rescued by increasing Ca2+ concentration in the recording medium. The results indicate that DTT-mediated potentiation requires Ca2+ influx through NMDAR activity. Finally, redox regulation of NMDAR function depends on the activity of Ca2+/calmodulin-dependent protein kinase II (CaMKII). The results indicate that activity-dependent NMDAR synaptic plasticity is suppressed by redox-mediated inhibition of CaMKII activation during aging. The redox regulation of NMDARs represents a suppression of a metaplasticity mechanism, which can disrupt synaptic plasticity and cognition associated with neurological or psychiatric diseases, and aging.

Emerging Themes in PDZ Domain Signaling: Structure, Function, and Inhibition

Post-synaptic density-95, disks-large and zonula occludens-1 (PDZ) domains are small globular protein-protein interaction domains widely conserved from yeast to humans. They are composed of ∼90 amino acids and form a classical two α-helical/six β-strand structure. The prototypical ligand is the C-terminus of partner proteins; however, they also bind internal peptide sequences. Recent findings indicate that PDZ domains also bind phosphatidylinositides and cholesterol. Through their ligand interactions, PDZ domain proteins are critical for cellular trafficking and the surface retention of various ion channels. In addition, PDZ proteins are essential for neuronal signaling, memory, and learning. PDZ proteins also contribute to cytoskeletal dynamics by mediating interactions critical for maintaining cell-cell junctions, cell polarity, and cell migration. Given their important biological roles, it is not surprising that their dysfunction can lead to multiple disease states. As such, PDZ domain-containing proteins have emerged as potential targets for the development of small molecular inhibitors as therapeutic agents. Recent data suggest that the critical binding function of PDZ domains in cell signaling is more than just glue, and their binding function can be regulated by phosphorylation or allosterically by other binding partners. These studies also provide a wealth of structural and biophysical data that are beginning to reveal the physical features that endow this small modular domain with a central role in cell signaling.

Roles of Gasotransmitters in Synaptic Plasticity and Neuropsychiatric Conditions

Synaptic plasticity is important for maintaining normal neuronal activity and proper neuronal functioning in the nervous system. It is crucial for regulating synaptic transmission or electrical signal transduction to neuronal networks, for sharing essential information among neurons, and for maintaining homeostasis in the body. Moreover, changes in synaptic or neural plasticity are associated with many neuropsychiatric conditions, such as schizophrenia (SCZ), bipolar disorder (BP), major depressive disorder (MDD), and Alzheimer's disease (AD). The improper maintenance of neural plasticity causes incorrect neurotransmitter transmission, which can also cause neuropsychiatric conditions. Gas neurotransmitters (gasotransmitters), such as hydrogen sulfide (H2S), nitric oxide (NO), and carbon monoxide (CO), play roles in maintaining synaptic plasticity and in helping to restore such plasticity in the neuronal architecture in the central nervous system (CNS). Indeed, the upregulation or downregulation of these gasotransmitters may cause neuropsychiatric conditions, and their amelioration may restore synaptic plasticity and proper neuronal functioning and thereby improve such conditions. Understanding the specific molecular mechanisms underpinning these effects can help identify ways to treat these neuropsychiatric conditions.

Cryptic protein-protein interaction motifs in the cytoplasmic domain of MHCI proteins

BACKGROUND

Major histocompatibility complex class I (MHCI) proteins present antigenic peptides for immune surveillance and play critical roles in nervous system development and plasticity. Most MHCI are transmembrane proteins. The extracellular domain of MHCI interacts with immunoreceptors, peptides, and co-receptors to mediate immune signaling. While the cytoplasmic domain also plays important roles in endocytic trafficking, cross-presentation of extracellularly derived antigens, and CTL priming, the molecular mediators of cytoplasmic signaling by MHCI remain largely unknown.

RESULTS

Here we show that the cytoplasmic domain of MHCI contains putative protein-protein interaction domains known as PDZ (PSD95/disc large/zonula occludens-1) ligands. PDZ ligands are motifs that bind to PDZ domains to organize and mediate signaling at cell-cell contacts. PDZ ligands are short, degenerate motifs, and are therefore difficult to identify via sequence homology alone, but several lines of evidence suggest that putative PDZ ligand motifs in MHCI are under positive selective pressure. Putative PDZ ligands are found in all of the 99 MHCI proteins examined from diverse species, and are enriched in the cytoplasmic domain, where PDZ interactions occur. Both the position of the PDZ ligand and the class of ligand motif are conserved across species, as well as among genes within a species. Non-synonymous substitutions, when they occur, frequently preserve the motif. Of the many specific possible PDZ ligand motifs, a handful are strikingly and selectively overrepresented in MHCI's cytoplasmic domain, but not elsewhere in the same proteins. Putative PDZ ligands in MHCI encompass conserved serine and tyrosine residues that are targets of phosphorylation, a post-translational modification that can regulate PDZ interactions. Finally, proof-of-principle in vitro interaction assays demonstrate that the cytoplasmic domains of particular MHCI proteins can bind directly and specifically to PDZ1 and PDZ4&5 of MAGI-1, and identify a conserved PDZ ligand motif in the classical MHCI H2-K that is required for this interaction.

CONCLUSIONS

These results identify cryptic protein interaction motifs in the cytoplasmic domain of MHCI. In so doing, they suggest that the cytoplasmic domain of MHCI could participate in previously unsuspected PDZ mediated protein-protein interactions at neuronal as well as immunological synapses.

The Regulation of GluN2A by Endogenous and Exogenous Regulators in the Central Nervous System

The NMDA receptor is the most widely studied ionotropic glutamate receptor, and it is central to many physiological and pathophysiological processes in the central nervous system. GluN2A is one of the two main types of GluN2 NMDA receptor subunits in the forebrain. The proper activity of GluN2A is important to brain function, as the abnormal regulation of GluN2A may induce some neuropsychiatric disorders. This review will examine the regulation of GluN2A by endogenous and exogenous regulators in the central nervous system.

Central role for NMDA receptors in redox mediated impairment of synaptic function during aging and Alzheimer's disease

Increased human longevity has magnified the negative impact that aging can have on cognitive integrity of older individuals experiencing some decline in cognitive function. Approximately 30% of the elderly will have cognitive problems that influence their independence. Impaired executive function and memory performance are observed in normal aging and yet can be an early sign of a progressive cognitive impairment of Alzheimer's disease (AD), the most common form of dementia. Brain regions that are vulnerable to aging exhibit the earliest pathology of AD. Senescent synaptic function is observed as a shift in Ca²⁺-dependent synaptic plasticity and similar mechanisms are thought to contribute to the early cognitive deficits associated with AD. In the case of aging, intracellular redox state mediates a shift in Ca²⁺ regulation including N-methyl-d-aspartate (NMDA) receptor hypofunction and increased Ca²⁺ release from intracellular stores to alter synaptic plasticity. AD can interact with these aging processes such that molecules linked to AD, β-amyloid (Aβ) and mutated presenilin 1 (PS1), can also degrade NMDA receptor function, promote Ca²⁺ release from intracellular stores, and may increase oxidative stress. Thus, age is one of the most important predictors of AD and brain aging likely contributes to the onset of AD. The focus of this review article is to provide an update on mechanisms that contribute to the senescent synapse and possible interactions with AD-related molecules, with special emphasis on regulation of NMDA receptors.

Synaptic signalling and its interface with neuropathologies: snapshots from the past, present and future

This 'Past to Future' Review as part of the 60th anniversary year of the Journal of Neurochemistry focuses on synaptic transmission and associated signalling, and seeks to identify seminal progress in neurochemistry over the last 10 years which has advanced our understanding of neuronal communication in brain. The approach adopted analyses neurotransmitters on a case by case basis (i.e. amino acids, monoamines, acetylcholine, neuropeptides, ATP/purines and gasotransmitters) to highlight novel findings that have changed the way we view each type of transmitter, to explore commonalities and interactions, and to note how new insights have changed the way we view the biology of degenerative, psychiatric and behavioural conditions. Across all transmitter systems there was remarkable growth in the identification of targets likely to provide therapeutic benefit and which undoubtedly was driven by the elucidation of circuit function and new vistas of synaptic signalling. There has been an increasing trend to relate signalling to disease, notably for Alzheimer's and Parkinson's disease and related conditions, and which has occurred for each transmitter family. Forebrain circuitry and tonic excitatory control have been the centre of great attention yielding novel findings that will impact upon cognitive, emotional and addictive behaviours. Other impressive insights focus on gasotransmitters integrating activity as volume transmitters. Exciting developments in how serotonin, cholinergic, l-glutamate, galanin and adenosine receptors and their associated signalling can be beneficially targeted should underpin the development of new therapies. Clearly integrated, multifaceted neurochemistry has changed the way we view synaptic signalling and its relevance to pathobiology. Highlighted are important advances in synaptic signalling over the last decade in the Journal of Neurochemistry. Across all transmitter systems elucidation of circuit function, and notably molecular insights, have underpinned remarkable growth in the identification of targets likely to provide therapeutic benefit in neuropathologies. Another commonality was wide interest in forebrain circuitry and its tonic excitatory control. Increasingly observations relate to signalling in disease and behavioural conditions. This article is part of the 60th Anniversary special issue.

A Critical Role for the GluA1 Accessory Protein, SAP97, in Cocaine Seeking

A growing body of evidence indicates that the transport of GluA1 subunit-containing calcium-permeable AMPA receptors (CP-AMPARs) to synapses in subregions of the nucleus accumbens promotes cocaine seeking. Consistent with these findings, the present results show that administration of the CP-AMPAR antagonist, Naspm, into the caudal lateral core or caudal medial shell of the nucleus accumbens attenuated cocaine priming-induced reinstatement of drug seeking. Moreover, viral-mediated overexpression of 'pore dead' GluA1 subunits (via herpes simplex virus (HSV) GluA1-Q582E) in the lateral core or medial shell attenuated the reinstatement of cocaine seeking. The overexpression of wild-type GluA1 subunits (via HSV GluA1-WT) in the medial shell, but not the lateral core, enhanced the reinstatement of cocaine seeking. These results indicate that activation of GluA1-containing AMPARs in subregions of the nucleus accumbens reinstates cocaine seeking. SAP97 and 4.1N are proteins involved in GluA1 trafficking to and stabilization in synapses; SAP97-GluA1 interactions also influence dendritic growth. We next examined potential roles of SAP97 and 4.1N in cocaine seeking. Viral-mediated expression of a microRNA that reduces SAP97 protein expression (HSV miSAP97) in the medial accumbens shell attenuated cocaine seeking. In contrast, a virus that overexpressed a dominant-negative form of a 4.1N C-terminal domain (HSV 4.1N-CTD), which prevents endogenous 4.1N binding to GluA1 subunits, had no effect on cocaine seeking. These results indicate that the GluA1 subunit accessory protein SAP97 may represent a novel target for pharmacotherapeutic intervention in the treatment of cocaine craving.

Activity-induced synaptic delivery of the GluN2A-containing NMDA receptor is dependent on endoplasmic reticulum chaperone Bip and involved in fear memory

The N-methyl-D-aspartate receptor (NMDAR) in adult forebrain is a heterotetramer mainly composed of two GluN1 subunits and two GluN2A and/or GluN2B subunits. The synaptic expression and relative numbers of GluN2A- and GluN2B-containing NMDARs play critical roles in controlling Ca(2+)-dependent signaling and synaptic plasticity. Previous studies have suggested that the synaptic trafficking of NMDAR subtypes is differentially regulated, but the precise molecular mechanism is not yet clear. In this study, we demonstrated that Bip, an endoplasmic reticulum (ER) chaperone, selectively interacted with GluN2A and mediated the neuronal activity-induced assembly and synaptic incorporation of the GluN2A-containing NMDAR from dendritic ER. Furthermore, the GluN2A-specific synaptic trafficking was effectively disrupted by peptides interrupting the interaction between Bip and GluN2A. Interestingly, fear conditioning in mice was disrupted by intraperitoneal injection of the interfering peptide before training. In summary, we have uncovered a novel mechanism for the activity-dependent supply of synaptic GluN2A-containing NMDARs, and demonstrated its relevance to memory formation.

Dynamic regulation of a cell adhesion protein complex including CADM1 by combinatorial analysis of FRAP with exponential curve-fitting

Protein components of cell adhesion machinery show continuous renewal even in the static state of epithelial cells and participate in the formation and maintenance of normal epithelial architecture and tumor suppression. CADM1 is a tumor suppressor belonging to the immunoglobulin superfamily of cell adhesion molecule and forms a cell adhesion complex with an actin-binding protein, 4.1B, and a scaffold protein, MPP3, in the cytoplasm. Here, we investigate dynamic regulation of the CADM1-4.1B-MPP3 complex in mature cell adhesion by fluorescence recovery after photobleaching (FRAP) analysis. Traditional FRAP analysis were performed for relatively short period of around 10 min. Here, thanks to recent advances in the sensitive laser detector systems, we examine FRAP of CADM1 complex for longer period of 60 min and analyze the recovery with exponential curve-fitting to distinguish the fractions with different diffusion constants. This approach reveals that the fluorescence recovery of CADM1 is fitted to a single exponential function with a time constant (τ) of approximately 16 min, whereas 4.1B and MPP3 are fitted to a double exponential function with two τs of approximately 40-60 sec and 16 min. The longer τ is similar to that of CADM1, suggesting that 4.1B and MPP3 have two distinct fractions, one forming a complex with CADM1 and the other present as a free pool. Fluorescence loss in photobleaching analysis supports the presence of a free pool of these proteins near the plasma membrane. Furthermore, double exponential fitting makes it possible to estimate the ratio of 4.1B and MPP3 present as a free pool and as a complex with CADM1 as approximately 3:2 and 3:1, respectively. Our analyses reveal a central role of CADM1 in stabilizing the complex with 4.1B and MPP3 and provide insight in the dynamics of adhesion complex formation.

Transcriptome profiling and pathway analysis of genes expressed differentially in participants with or without a positive response to topiramate treatment for methamphetamine addiction

Background

Developing efficacious medications to treat methamphetamine dependence is a global challenge in public health. Topiramate (TPM) is undergoing evaluation for this indication. The molecular mechanisms underlying its effects are largely unknown. Examining the effects of TPM on genome-wide gene expression in methamphetamine addicts is a clinically and scientifically important component of understanding its therapeutic profile.

Methods

In this double-blind, placebo-controlled clinical trial, 140 individuals who met the DSM-IV criteria for methamphetamine dependence were randomized to receive either TPM or placebo, of whom 99 consented to participate in our genome-wide expression study. The RNA samples were collected from whole blood for 50 TPM- and 49 placebo-treated participants at three time points: baseline and the ends of weeks 8 and 12. Genome-wide expression profiles and pathways of the two groups were compared for the responders and non-responders at Weeks 8 and 12. To minimize individual variations, expression of all examined genes at Weeks 8 and 12 were normalized to the values at baseline prior to identification of differentially expressed genes and pathways.

Results

At the single-gene level, we identified 1054, 502, 204, and 404 genes at nominal P values < 0.01 in the responders vs. non-responders at Weeks 8 and 12 for the TPM and placebo groups, respectively. Among them, expression of 159, 38, 2, and 21 genes was still significantly different after Bonferroni corrections for multiple testing. Many of these genes, such as GRINA, PRKACA, PRKCI, SNAP23, and TRAK2, which are involved in glutamate receptor and GABA receptor signaling, are direct targets for TPM. In contrast, no TPM drug targets were identified in the 38 significant genes for the Week 8 placebo group. Pathway analyses based on nominally significant genes revealed 27 enriched pathways shared by the Weeks 8 and 12 TPM groups. These pathways are involved in relevant physiological functions such as neuronal function/synaptic plasticity, signal transduction, cardiovascular function, and inflammation/immune function.

Conclusion

Topiramate treatment of methamphetamine addicts significantly modulates the expression of genes involved in multiple biological processes underlying addiction behavior and other physiological functions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12920-014-0065-x) contains supplementary material, which is available to authorized users.

ER to synapse trafficking of NMDA receptors

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. There are three distinct subtypes of ionotropic glutamate receptors (GluRs) that have been identified including 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid receptors (AMPARs), N-methyl-D-aspartate receptors (NMDARs) and kainate receptors. The most common GluRs in mature synapses are AMPARs that mediate the fast excitatory neurotransmission and NMDARs that mediate the slow excitatory neurotransmission. There have been large numbers of recent reports studying how a single neuron regulates synaptic numbers and types of AMPARs and NMDARs. Our current research is centered primarily on NMDARs and, therefore, we will focus in this review on recent knowledge of molecular mechanisms occurring (1) early in the biosynthetic pathway of NMDARs, (2) in the transport of NMDARs after their release from the endoplasmic reticulum (ER); and (3) at the plasma membrane including excitatory synapses. Because a growing body of evidence also indicates that abnormalities in NMDAR functioning are associated with a number of human psychiatric and neurological diseases, this review together with other chapters in this issue may help to enhance research and to gain further knowledge of normal synaptic physiology as well as of the etiology of many human brain diseases.

Scribble1/AP2 complex coordinates NMDA receptor endocytic recycling

The appropriate trafficking of glutamate receptors to synapses is crucial for basic synaptic function and synaptic plasticity. It is now accepted that NMDA receptors (NMDARs) internalize and are recycled at the plasma membrane but also exchange between synaptic and extrasynaptic pools; these NMDAR properties are also key to governing synaptic plasticity. Scribble1 is a large PDZ protein required for synaptogenesis and synaptic plasticity. Herein, we show that the level of Scribble1 is regulated in an activity-dependent manner and that Scribble1 controls the number of NMDARs at the plasma membrane. Notably, Scribble1 prevents GluN2A subunits from undergoing lysosomal trafficking and degradation by increasing their recycling to the plasma membrane following NMDAR activation. Finally, we show that a specific YxxR motif on Scribble1 controls these mechanisms through a direct interaction with AP2. Altogether, our findings define a molecular mechanism to control the levels of synaptic NMDARs via Scribble1 complex signaling.

The PDZ domain of the guanine nucleotide exchange factor PDZGEF directs binding to phosphatidic acid during brush border formation

PDZGEF is a guanine nucleotide exchange factor for the small G protein Rap. It was recently found that PDZGEF contributes to establishment of intestinal epithelial polarity downstream of the kinase Lkb1. By binding to phosphatidic acid enriched at the apical membrane, PDZGEF locally activates Rap2a resulting in induction of brush border formation via a pathway that includes the polarity players TNIK, Mst4 and Ezrin. Here we show that the PDZ domain of PDZGEF is essential and sufficient for targeting PDZGEF to the apical membrane of polarized intestinal epithelial cells. Inhibition of PLD and consequently production of phosphatidic acid inhibitis targeting of PDZGEF to the plasma membrane. Furthermore, localization requires specific positively charged residues within the PDZ domain. We conclude that local accumulation of PDZGEF at the apical membrane during establishment of epithelial polarity is mediated by electrostatic interactions between positively charged side chains in the PDZ domain and negatively charged phosphatidic acid.

Emerging role of the scaffolding protein Dlg1 in vesicle trafficking

Discs large 1 (Dlg1) is a modular scaffolding protein implicated in the control of cell polarity through assembly of specific multiprotein complexes, including receptors, ion channels and signaling proteins, at specialized zones of the plasma membrane. Recent data have shown that in addition to these well-known interaction partners, Dlg1 may also recruit components of the vesicle trafficking machinery either to the plasma membrane or to transport vesicles. Here, we discuss Dlg1 function in vesicle formation, targeting, tethering and fusion, in both the exocytotic and endocytotic pathways. These pathways contribute to cell functions as major and diverse as glutamatergic activity in the neurons, membrane homeostasis in Schwann cell myelination, insulin stimulation of glucose transport in adipocytes, or endothelial secretion of the hemostatic protein, von Willebrand factor (VWF).

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.

Crystal structure of the guanylate kinase domain from discs large homolog 1 (DLG1/SAP97)

Discs large homolog 1 (DLG1/SAP97) is involved in the development and regulation of neuronal and immunological synapses. DLG1 is a member of the membrane associated guanylate kinase (MAGUK) family of proteins, which function as molecular scaffolds. The C-terminal guanylate kinase (GK) domain of DLG1 binds peptides with a phosphorylated serine residue. In this study, we solved the crystal structure of the GK domain of human DLG1. The C-terminal tail of DLG1 is bound to the peptide-binding site of an adjacent symmetry-related DLG1 GK molecule. The binding direction of the C-terminal tail to the peptide-binding site is opposite to that of the phosphorylated LGN peptide in complex with the rat DLG1 GK domain. The C-terminal tail forms a 310 helix, which is also different from the conformation of the phosphorylated LGN peptide. Nevertheless, the side chain interactions of the C-terminal tail with the DLG1 GK domain are similar to those of the phosphorylated LGN peptide.

GluN2A versus GluN2B: twins, but quite different

N-Methyl-D-aspartate receptors (NMDARs) play vital roles in the central nervous system, as they are primary mediators of Ca(2+) influx during synaptic activity. The subunits that compose NMDARs share similar topological structures but are distinct in distribution and pharmacological properties, as well as physiological and pathological functions, which make the NMDAR one of the most complex and elusive ionotropic glutamate receptors. In this review, we focus on GluN2A and GluN2B, the primary NMDAR subunits in the cortex and hippocampus, and discuss their differences in developmental expression, brain distribution, trafficking, and functional properties during neuronal activity.

The anchoring protein SAP97 influences the trafficking and localisation of multiple membrane channels

SAP97 is a member of the MAGUK family of proteins that play a major role in the trafficking and targeting of membrane ion channels and cytosolic structural proteins in multiple cell types. Within neurons, SAP97 is localised throughout the secretory trafficking pathway and at the postsynaptic density (PSD). SAP97 differs from other MAGUK family members largely in its long N-terminus and in the sequences between the SH3 and GUK domains, where SAP97 undergoes significant alternative splicing to produce multiple SAP97 isoforms. These splice insertions endow SAP97 with differential cellular localisation patterns and functional roles within neurons. With regard to membrane ion channels, SAP97 forms multi-protein complexes with AMPA and NMDA-type glutamate receptors, and Kv1.4, Kv4.2, and Kir2.2 potassium channels, playing a major role in trafficking and anchoring ion channel surface expression. This highlights SAP97 not only as a regulator of neuronal excitability, synaptic function and plasticity in the brain, but also as a target for the pathophysiology of a number of neurological disorders. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

CaMKII regulation in information processing and storage

The Ca(2+)/Calmodulin(CaM)-dependent protein kinase II (CaMKII) is activated by Ca(2+)/CaM, but becomes partially autonomous (Ca(2+)-independent) upon autophosphorylation at T286. This hallmark feature of CaMKII regulation provides a form of molecular memory and is indeed important in long-term potentiation (LTP) of excitatory synapse strength and memory formation. However, emerging evidence supports a direct role in information processing, while storage of synaptic information may instead be mediated by regulated interaction of CaMKII with the NMDA receptor (NMDAR) complex. These and other CaMKII regulation mechanisms are discussed here in the context of the kinase structure and their impact on postsynaptic functions. Recent findings also implicate CaMKII in long-term depression (LTD), as well as functional roles at inhibitory synapses, lending renewed emphasis on better understanding the spatiotemporal control of CaMKII regulation.

Role of the MAGUK protein family in synapse formation and function

Synaptic function is crucially dependent on the spatial organization of the presynaptic and postsynaptic apparatuses and the juxtaposition of both membrane compartments. This precise arrangement is achieved by a protein network at the submembrane region of each cell that is built around scaffold proteins. The membrane-associated guanylate kinase (MAGUK) family of proteins is a widely expressed and well-conserved group of proteins that plays an essential role in the formation and regulation of this scaffolding. Here, we review general features of this protein family, focusing on the discs large and calcium/calmodulin-dependent serine protein kinase subfamilies of MAGUKs in the formation, function, and plasticity of synapses.

Abnormal activity of the MAPK- and cAMP-associated signaling pathways in frontal cortical areas in postmortem brain in schizophrenia

Recent evidence suggests that schizophrenia may result from alterations of integration of signaling mediated by multiple neurotransmitter systems. Abnormalities of associated intracellular signaling pathways may contribute to the pathophysiology of schizophrenia. Proteins and phospho-proteins comprising mitogen activated protein kinase (MAPK) and 3'-5'-cyclic adenosine monophosphate (cAMP)-associated signaling pathways may be abnormally expressed in the anterior cingulate (ACC) and dorsolateral prefrontal cortex (DLPFC) in schizophrenia. Using western blot analysis we examined proteins of the MAPK- and cAMP-associated pathways in these two brain regions. Postmortem samples were used from a well-characterized collection of elderly patients with schizophrenia (ACC=36, DLPFC=35) and a comparison (ACC=33, DLPFC=31) group. Near-infrared intensity of IR-dye labeled secondary antisera bound to targeted proteins of the MAPK- and cAMP-associated signaling pathways was measured using LiCor Odyssey imaging system. We found decreased expression of Rap2, JNK1, JNK2, PSD-95, and decreased phosphorylation of JNK1/2 at T183/Y185 and PSD-95 at S295 in the ACC in schizophrenia. In the DLPFC, we found increased expression of Rack1, Fyn, Cdk5, and increased phosphorylation of PSD-95 at S295 and NR2B at Y1336. MAPK- and cAMP-associated molecules constitute ubiquitous intracellular signaling pathways that integrate extracellular stimuli, modify receptor expression and function, and regulate cell survival and neuroplasticity. These data suggest abnormal activity of the MAPK- and cAMP-associated pathways in frontal cortical areas in schizophrenia. These alterations may underlie the hypothesized hypoglutamatergic function in this illness. Together with previous findings, these data suggest that abnormalities of intracellular signaling pathways may contribute to the pathophysiology of schizophrenia.

Glutamate receptor dynamics and protein interaction: lessons from the NMDA receptor

The plasticity of excitatory glutamate synapses emerged over the last decades as a core cellular mechanism for the encoding and processing of various cognitive functions. This property relies in part on the ability to dynamically adjust the content of glutamate receptors in the postsynaptic membrane. Among these receptors, NMDA receptors (NMDAR), which are composed of two obligatory GluN1 and two regulatory GluN2/3 subunits, play a key role in the induction of many forms of plasticity processes. Understanding how NMDAR subtypes are trafficked and regulated in the synapse has thus captured considerable attention. It has emerged that NMDAR synaptic content relies on an equilibrium between intracellular trafficking and rapid lateral diffusion of the receptor within the synaptic area. Here, we review our current understanding of NMDAR trafficking, mostly the ones at the surface membrane, with a specific focus on the role of interacting PDZ-containing proteins during the journey of NMDAR to and around the synaptic area. The cellular and molecular lessons obtained from examining NMDAR dynamics and regulation by interacting proteins appear to apply to other ionotropic neurotransmitter receptors, and thus shed new light on the modulation of excitatory, inhibitory, and modulatory transmission. This article is part of a Special Issue entitled 'Neuronal Function'.

SAP97-mediated local trafficking is altered in Alzheimer disease patients' hippocampus

Synapse-asssociated protein-97 (SAP97) is responsible for the trafficking of both glutamate receptor subunits, GluR1 and NR2A, and α-secretase ADAM10 to the synaptic membrane. Here we evaluate the trafficking capability of SAP97 in Alzheimer disease (AD) patients' brain. We analyzed autoptic hippocampus and superior frontal gyrus, respectively as an affected and a less affected area, from 6 AD patients (Braak 4) and 6 healthy controls. In hippocampus, but not in superior frontal gyrus, of AD patients, ADAM10 and GluR1 synaptic membrane levels are altered while NR2A localization is not affected. Both immunoprecipitation and pull-down assays demonstrated that SAP97 failed to correctly couple to ADAM10 and GluR1, but not to NR2A. These findings not only indicate SAP97 as a point of convergence between amyloid cascade and synaptic failure in AD, but also allow a different interpretation of AD which can be now perceived as synaptic trafficking defect pathology.

Postsynaptic alteration of NR2A subunit and defective autophosphorylation of alphaCaMKII at threonine-286 contribute to abnormal plasticity and morphology of upper motor neurons in presymptomatic SOD1G93A mice, a murine model for amyotrophic lateral sclerosis

Although amyotrophic lateral sclerosis (ALS) has long been considered as a lower motor neuron (MN) disease, degeneration of upper MNs arising from a combination of mechanisms including insufficient growth factor signaling and enhanced extracellular glutamate levels is now well documented. The observation that these mechanisms are altered in presymptomatic superoxide dismutase (SOD1) mice, an ALS mouse model, suggests that defective primary motor cortex (M1) synaptic activity might precede the onset of motor disturbances. To examine this point, we assessed the composition of AMPAR and NMDAR subunits and of the alphaCa²(+)/calmodulin-dependent kinase autophosphorylation at threonine-286 in the triton insoluble fraction from the M1 in postnatal P80-P85 SOD1(G93A) and wild-type mice. We show that presymptomatic SOD1(G93A) exhibit a selective decrease of NR2A subunit expression and of the alphaCa²(+)/calmodulin-dependent kinase autophosphorylation at threonine-286 in the triton insoluble fraction of upper MNs synapses. These molecular alterations are associated with synaptic plasticity defects, and a reduction in upper MN dendritic outgrowth revealing that abnormal neuronal connectivity in the M1 region precedes the onset of motor symptoms. We suggest that the progressive disruption of M1 corticocortical connections resulting from the SOD1(G93A) mutation might extend to adjacent regions and promote development of cognitive/dementia alterations frequently associated with ALS.

ERK5 pathway regulates the phosphorylation of tumour suppressor hDlg during mitosis

Human disc-large (hDlg) is a scaffold protein critical for the maintenance of cell polarity and adhesion. hDlg is thought to be a tumour suppressor that regulates the cell cycle and proliferation. However, the mechanism and pathways involved in hDlg regulation during these processes is still unclear. Here we report that hDlg is phosphorylated during mitosis, and we establish the identity of at least three residues phosphorylated in hDlg; some are previously unreported. Phosphorylation affects hDlg localisation excluding it from the contact point between the two daughter cells. Our results reveal a previously unreported pathway for hDlg phosphorylation in mitosis and show that ERK5 pathway mediates hDlg cell cycle dependent phosphorylation. This is likely to have important implications in the correct timely mitotic entry and mitosis progression.

p38gamma regulates interaction of nuclear PSF and RNA with the tumour-suppressor hDlg in response to osmotic shock

Activation of p38γ modulates the integrity of the complex formed by the human discs large protein (hDlg) with cytoskeletal proteins, which is important for cell adaptation to changes in environmental osmolarity. Here we report that, in response to hyperosmotic stress, p38γ also regulates formation of complexes between hDlg and the nuclear protein polypyrimidine tract-binding protein-associated-splicing factor (PSF). Following osmotic shock, p38γ in the cell nucleus increases its association with nuclear hDlg, thereby causing dissociation of hDlg-PSF complexes. Moreover, hDlg and PSF bind different RNAs; in response to osmotic shock, p38γ causes hDlg-PSF and hDlg-RNA dissociation independently of its kinase activity. These findings identify a novel nuclear complex and suggest a previously unreported function of p38γ, which is independent of its catalytic activity and could affect mRNA processing and/or gene transcription to aid cell adaptation to osmolarity changes in the environment.

PDZ domains and their binding partners: structure, specificity, and modification

PDZ domains are abundant protein interaction modules that often recognize short amino acid motifs at the C-termini of target proteins. They regulate multiple biological processes such as transport, ion channel signaling, and other signal transduction systems. This review discusses the structural characterization of PDZ domains and the use of recently emerging technologies such as proteomic arrays and peptide libraries to study the binding properties of PDZ-mediated interactions. Regulatory mechanisms responsible for PDZ-mediated interactions, such as phosphorylation in the PDZ ligands or PDZ domains, are also discussed. A better understanding of PDZ protein-protein interaction networks and regulatory mechanisms will improve our knowledge of many cellular and biological processes.

Ultrastructural modifications of spine and synapse morphology by SAP97

Synaptic scaffolding proteins from membrane-associated guanylate kinases (MAGUK) family are implicated in synapse formation and functioning. To better understand the role of one of the proteins of this family, SAP97, we studied with electron microscopy the effects of its overexpression on spine and synapse morphology in CA1 pyramidal neurons of rat organotypic hippocampal slice cultures. Dramatic spine enlargement induced by SAP97 overexpression was accompanied by marked morphological changes, with spines enwrapping and engulfing presynaptic terminals. The size and complexity of the PSD was also significantly increased. Similar to PSD-95, SAP97 promoted formation of multi-innervated spines (MIS). In addition, both MAGUK proteins induced multiple excitatory contacts on dendritic shafts suggesting a mechanism for shaft synapse formation. Formation of MIS and shaft synapses was blocked by the nitric oxide synthase (NOS) inhibitor L-NAME. Immunochemistry revealed that overexpression of SAP97 was associated with overexpression of PSD-95 and recruitment of nNOS to the synapse. These data provide evidence for both common and distinct structural alterations produced by overexpression of SAP97 and PSD-95 and demonstrate strong interactions between these two proteins to regulate contact formation through nitric oxide signaling.

Synchronous and asynchronous transmitter release at nicotinic synapses are differentially regulated by postsynaptic PSD-95 proteins

The rate and timing of information transfer at neuronal synapses are critical for determining synaptic efficacy and higher network function. Both synchronous and asynchronous neurotransmitter release shape the pattern of synaptic influences on a neuron. The PSD-95 family of postsynaptic scaffolding proteins, in addition to organizing postsynaptic components at glutamate synapses, acts transcellularly to regulate synchronous glutamate release. Here we show that PSD-95 family members at nicotinic synapses on chick ciliary ganglion neurons in culture execute multiple functions to enhance transmission. Together, endogenous PSD-95 and SAP102 in the postsynaptic cell appear to regulate transcellularly the synchronous release of transmitter from presynaptic terminals onto the neuron while stabilizing postsynaptic nicotinic receptor clusters under the release sites. Endogenous SAP97, in contrast, has no effect on receptor clusters but acts transcellularly from the postsynaptic cell through N-cadherin to enhance asynchronous release. These separate and parallel regulatory pathways allow postsynaptic scaffold proteins to dictate the pattern of cholinergic input a neuron receives; they also require balancing of PSD-95 protein levels to avoid disruptive competition that can occur through common binding domains.

An arginine stretch limits ADAM10 exit from the endoplasmic reticulum

A disintegrin and metalloproteinase 10 (ADAM10) is a type I transmembrane glycoprotein responsible for the ectodomain shedding of a number of proteins implicated in the pathogenesis of diseases ranging from cancer to Alzheimer Disease. ADAM10 is synthesized in an inactive form, which is proteolytically activated during its forward transport along the secretory pathway and at the plasma membrane. Therefore, modulation of its trafficking could provide a mechanism to finely tune its shedding activity. Here we report the identification of an endoplasmic reticulum (ER) retention motif within the ADAM10 intracellular C-terminal tail. Sequential deletion/mutagenesis analyses showed that an arginine-rich ((723)RRR) sequence was responsible for the retention of ADAM10 in the ER and its inefficient surface trafficking. Mutating the second arginine to alanine was sufficient to allow ER exit and surface expression in both heterologous cells and hippocampal neurons. As synapse-associated protein 97 (SAP97) binds ADAM10 at its cytoplasmic tail and facilitates forward ADAM10 trafficking in neurons, we tested whether SAP97 could modulate ER export. However, neither expression nor Ser-39 phosphorylation of SAP97 in heterologous cells or hippocampal neurons were sufficient to allow the ER exit of ADAM10, suggesting that other signaling pathways or alternative binding partners are responsible for ADAM10 ER exit. Together, these results identify a novel mechanism regulating the intracellular trafficking and membrane delivery of ADAM10.

Ca2+/calmodulin-dependent protein kinase II binds to and phosphorylates a specific SAP97 splice variant to disrupt association with AKAP79/150 and modulate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor (AMPAR) activity

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) promotes trafficking and activation of the GluR1 subunit of alpha-amino- 3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors (AMPARs) during synaptic plasticity. GluR1 is also modulated in parallel by multiprotein complexes coordinated by synapse-associated protein 97 (SAP97) that contain A-kinase anchoring protein 79/150 (AKAP79/150), protein kinase A, and protein phosphatase 2B. Here we show that SAP97 is present in CaMKII immune complexes isolated from rodent brain as well as from HEK293 cells co-expressing CaMKIIalpha and SAP97. CaMKIIalpha phosphorylated recombinant SAP97 within immune complexes in vitro and in intact cells. Four alternative mRNA splice variants of SAP97 expressing combinations of four inserts (I2, I3, I4, I5) in the U5 region between Src homology 3 (SH3) and guanylyl kinase-like (GK) domains were identified in rat brain at postnatal day 21. CaMKIIalpha preferentially phosphorylated a full-length SAP97 and a glutathione S-transferase (GST) fusion protein containing the I3 and I5 inserts (SAP97-I3I5 and GST-SH3-I3I5-GK, respectively) and also specifically interacted with GST-SH3-I3I5-GK compared with GST proteins containing other naturally occurring insert combinations. AKAP79/150 also directly and specifically bound only to GST-SH3-I3I5-GK, but CaMKII phosphorylation of GST-SH3-I3I5-GK prevented this interaction. AKAP79-dependent down-regulation of GluR1 AMPAR currents was ablated by overexpression of SAP97-I2I5 (which does not bind AKAP79) or by infusion of active CaMKIIalpha. Collectively, the data suggest that CaMKIIalpha targets a specific SAP97 splice variant to disengage AKAP79/150 from regulating GluR1 AMPARs, providing new insight into protein-protein interactions and phosphorylation events that are required for normal regulation of glutamatergic synaptic transmission, learning, and memory.

Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Central sensitization represents an enhancement in the function of neurons and circuits in nociceptive pathways caused by increases in membrane excitability and synaptic efficacy as well as to reduced inhibition and is a manifestation of the remarkable plasticity of the somatosensory nervous system in response to activity, inflammation, and neural injury. The net effect of central sensitization is to recruit previously subthreshold synaptic inputs to nociceptive neurons, generating an increased or augmented action potential output: a state of facilitation, potentiation, augmentation, or amplification. Central sensitization is responsible for many of the temporal, spatial, and threshold changes in pain sensibility in acute and chronic clinical pain settings and exemplifies the fundamental contribution of the central nervous system to the generation of pain hypersensitivity. Because central sensitization results from changes in the properties of neurons in the central nervous system, the pain is no longer coupled, as acute nociceptive pain is, to the presence, intensity, or duration of noxious peripheral stimuli. Instead, central sensitization produces pain hypersensitivity by changing the sensory response elicited by normal inputs, including those that usually evoke innocuous sensations.

PERSPECTIVE

In this article, we review the major triggers that initiate and maintain central sensitization in healthy individuals in response to nociceptor input and in patients with inflammatory and neuropathic pain, emphasizing the fundamental contribution and multiple mechanisms of synaptic plasticity caused by changes in the density, nature, and properties of ionotropic and metabotropic glutamate receptors.

Disruption of the interaction between myosin VI and SAP97 is associated with a reduction in the number of AMPARs at hippocampal synapses

Myosin VI is an actin-based motor protein that is enriched at the postsynaptic density and appears to interact with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptors (AMPARs) via synapse associated protein 97 (SAP97). Here, we find that a Flag epitope-tagged dominant negative construct that inhibits the interaction between SAP97 and myosin VI (Flag-myoVI-DN) causes a dramatic reduction in the number of synapses and the surface expression of AMPARs in cultured hippocampal neurons. Furthermore, we find that Flag-myoVI-DN also prevents the rapid delivery of AMPARs to synapses that can be induced by the transient activation of N-methyl-d-aspartate receptors. The Flag-myoVI-DN induced decrease in surface AMPARs is not because of reduced AMPAR subunit protein synthesis. Using whole-cell recording, we show that Flag-myoVI-DN also prevents the activity-induced increase in miniature excitatory postsynaptic current frequency that is normally associated with recruitment of AMPARs to the cell surface at synaptic sites that lack these receptors (i.e. 'silent' synapses). Together, these results indicate that myosin VI/SAP97 plays an important role in trafficking and activity-dependent recruitment of AMPARs to synapses.

Dizocilpine (MK-801) induces distinct changes of N-methyl-D-aspartic acid receptor subunits in parvalbumin-containing interneurons in young adult rat prefrontal cortex

N-methyl-D-aspartic acid receptor (NMDAR) hypofunction has long been implicated in schizophrenia and NMDARs on gamma-aminobutyric acid (GABA)ergic interneurons are proposed to play an essential role in the pathogenesis. However, controversial results have been reported regarding the regulation of NMDAR expression, and direct evidence of how NMDAR antagonists act on specific subpopulations of prefrontal interneurons is missing. We investigated the effects of the NMDAR antagonist dizocilpine (MK-801) on the expression of NMDAR subtypes in the identified interneurons in young adult rat prefrontal cortex (PFC) by using laser microdissection and real-time polymerase chain reaction, combined with Western blotting and immunofluorescent staining. We found that MK-801 induced distinct changes of NMDAR subunits in the parvalbumin-immunoreactive (PV-ir) interneurons vs. pyramidal neurons in the PFC circuitry. The messenger RNA (mRNA) expression of all NMDAR subtypes, including NR1 and NR2A to 2D, exhibited inverted-U dose-dependent changes in response to MK-801 treatment in the PFC. In contrast, subunit mRNAs of NMDARs in PV-ir interneurons were significantly down-regulated at low doses, unaltered at medium doses, and significantly decreased again at high doses, suggesting a biphasic dose response to MK-801. The differential effects of MK-801 in mRNA expression of NMDAR subunits were consistent with the protein expression of NR2A and NR2B subunits revealed with Western blotting and double immunofluorescent staining. These results suggest that PV-containing interneurons in the PFC exhibit a distinct responsiveness to NMDAR antagonism and that NMDA antagonist can differentially and dose-dependently regulate the functions of pyramidal neurons and GABAergic interneurons in the prefrontal cortical circuitry.

SAP97 and CASK mediate sorting of NMDA receptors through a previously unknown secretory pathway

Synaptic plasticity is dependent upon the differential sorting, delivery and retention of neurotransmitter receptors, yet the mechanisms underlying these processes are poorly understood. In the present study, we have found that differential sorting of glutamate receptor subtypes begins within the endoplasmic reticulum (ER) of rat hippocampal neurons. While AMPARs are trafficked to the plasma membrane via the conventional somatic Golgi network, NMDARs are diverted from the somatic ER into a specialized ER sub-compartment that bypasses somatic Golgi, merging instead with dendritic Golgi outposts. Intriguingly, this ER sub-compartment is composed of highly mobile vesicles containing the NMDAR subunits NR1 and NR2B, the microtubule-dependent motor protein KIF17, and the postsynaptic adaptor proteins CASK and SAP97. Furthermore, our data demonstrate that the retention and trafficking of NMDARs within this ER sub-compartment requires both CASK and SAP97. These data indicate that NMDARs are sorted away from AMPARs via a non-conventional secretory pathway that utilizes dendritic Golgi outposts.

Postsynaptic scaffolds for nicotinic receptors on neurons

Complex postsynaptic scaffolds determine the structure and signaling capabilities of glutamatergic synapses. Recent studies indicate that some of the same scaffold components contribute to the formation and function of nicotinic synapses on neurons. PDZ-containing proteins comprising the PSD-95 family co-localize with nicotinic acetylcholine receptors (nAChRs) and mediate downstream signaling in the neurons. The PDZ-proteins also promote functional nicotinic innervation of the neurons, as does the scaffold protein APC and transmembrane proteins such as neuroligin and the EphB2 receptor. In addition, specific chaperones have been shown to facilitate nAChR assembly and transport to the cell surface. This review summarizes recent results in these areas and raises questions for the future about the mechanism and synaptic role of nAChR trafficking.

Kv4 potassium channels form a tripartite complex with the anchoring protein SAP97 and CaMKII in cardiac myocytes

Membrane-associated guanylate kinase (MAGUK) proteins are major determinants of the organization of ion channels in the plasma membrane in various cell types. Here, we investigated the interaction between the MAGUK protein SAP97 and cardiac Kv4.2/3 channels, which account for a large part of the outward potassium current, I(to), in heart. We found that the Kv4.2 and Kv4.3 channels C termini interacted with SAP97 via a SAL amino acid sequence. SAP97 and Kv4.3 channels were colocalized in the sarcolemma of cardiomyocytes. In CHO cells, SAP97 clustered Kv4.3 channels in the plasma membrane and increased the current independently of the presence of KChIP and dipeptidyl peptidase-like protein-6. Suppression of SAP97 by using short hairpin RNA inhibited I(to) in cardiac myocytes, whereas its overexpression by using an adenovirus increased I(to). Kv4.3 channels without the SAL sequence were no longer regulated by Ca2+/calmodulin kinase (CaMK)II inhibitors. In cardiac myocytes, pull-down and coimmunoprecipitation assays showed that the Kv4 channel C terminus, SAP97, and CaMKII interact together, an interaction suppressed by SAP97 silencing and enhanced by SAP97 overexpression. In HEK293 cells, SAP97 silencing reproduced the effects of CaMKII inhibition on current kinetics and suppressed Kv4/CaMKII interactions. In conclusion, SAP97 is a major partner for surface expression and CaMKII-dependent regulation of cardiac Kv4 channels.