NCAM180 and glutamate receptor subtypes in potentiated spine synapses: an immunogold electron microscopic study

Single nuclei RNA-seq reveals a medium spiny neuron glutamate excitotoxicity signature prior to the onset of neuronal death in an ovine Huntington's disease model

Huntington's disease (HD) is a neurodegenerative genetic disorder caused by an expansion in the CAG repeat tract of the huntingtin (HTT) gene resulting in behavioural, cognitive, and motor defects. Current knowledge of disease pathogenesis remains incomplete, and no disease course-modifying interventions are in clinical use. We have previously reported the development and characterisation of the OVT73 transgenic sheep model of HD. The 73 polyglutamine repeat is somatically stable and therefore likely captures a prodromal phase of the disease with an absence of motor symptomatology even at 5-years of age and no detectable striatal cell loss. To better understand the disease-initiating events we have undertaken a single nuclei transcriptome study of the striatum of an extensively studied cohort of 5-year-old OVT73 HD sheep and age matched wild-type controls. We have identified transcriptional upregulation of genes encoding N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors in medium spiny neurons, the cell type preferentially lost early in HD. Further, we observed an upregulation of astrocytic glutamate uptake transporters and medium spiny neuron GABAA receptors, which may maintain glutamate homeostasis. Taken together, these observations support the glutamate excitotoxicity hypothesis as an early neurodegeneration cascade-initiating process but the threshold of toxicity may be regulated by several protective mechanisms. Addressing this biochemical defect early may prevent neuronal loss and avoid the more complex secondary consequences precipitated by cell death.

Sex-specific role of hippocampal NMDA-Erk-mTOR signaling in fear extinction of adolescent mice

Adolescence is a key phase of development for perturbations in fear extinction, with inability to adequately manage fear a potent factor for developing psychiatric disorders in adulthood. However, while behavioral correlates of adolescent fear regulation are established to a degree, molecular mediators of extinction learning in adolescence remain largely unknown. In this study, we observed fear acquisition and fear extinction (across 4 and 7 days) of adolescent and adult mice of both sexes and investigated how hippocampal levels of different plasticity markers relate to extinction learning. While fear was acquired evenly in males and females of both ages, fear extinction was found to be impaired in adolescent males. We also observed lower levels of GluA1, GLUN2A and GLUN2B subunits in male adolescents following fear acquisition, with an increase in their expression, as well as the activity of Erk-mTOR pathway over subsequent extinction sessions, which was paralleled with improved extinction learning. On the other hand, we detected no changes in plasticity-related proteins after fear acquisition in females, with alterations in GluA1, GluA4 and GLUN2B levels across fear extinction sessions. Additionally, we did not discern any pattern regarding the Erk-mTOR activity in female mice associated with their extinction performance. Overall, our research identifies sex-specific synaptic properties in the hippocampus that underlie developmentally regulated differences in fear extinction learning. We also point out hippocampal NMDA-Erk-mTOR signaling as the driving force behind successful fear extinction in male adolescents, highlighting this pathway as a potential therapeutic target for fear-related disorders in the adolescent population.

Sex and chronic stress alter the distribution of glutamate receptors within rat hippocampal CA3 pyramidal cells following oxycodone conditioned place preference

Glutamate receptors have a key role in the neurobiology of opioid addiction. Using electron microscopic immunocytochemical methods, this project elucidates how sex and chronic immobilization stress (CIS) impact the redistribution of GluN1 and GluA1 within rat hippocampal CA3 pyramidal cells following oxycodone (Oxy) conditioned place preference (CPP). Four groups of female and male Sprague-Dawley rats subjected to CPP were used: Saline- (Sal) and Oxy-injected (3 mg/kg, I.P.) naïve rats; and Sal- and Oxy-injected CIS rats. GluN1: In both naive and CIS rats, Sal-females compared to Sal-males had elevated cytoplasmic and total dendritic GluN1. Following Oxy CPP, near plasmalemmal, cytoplasmic, and total GluN1 decreased in CA3 dendrites of unstressed females suggesting reduced pools of GluN1 available for ligand binding. Following CIS, Oxy-males (which did not acquire CPP) had increased GluN1 in all compartments of dendrites and spines of CA3 neurons. GluA1: There were no differences in the distribution GluA1 in any cellular compartments of CA3 dendrites in naïve females and males following either Sal or Oxy CPP. CIS alone increased the percent of GluA1 in CA3 dendritic spines in males compared to females. CIS Oxy-males compared to CIS Sal-males had an increase in cytoplasmic and total dendritic GluA1. Thus, in CIS Oxy-males increased pools of GluN1 and GluA1 are available for ligand binding in CA3 neurons. Together with our prior experiments, these changes in GluN1 and GluA1 following CIS in males may contribute to an increased sensitivity of CA3 neurons to glutamate excitation and a reduced capacity to acquire Oxy CPP.

Exploring the involvement of Tac2 in the mouse hippocampal stress response through gene networking

Tachykinin 2 (Tac2) is expressed in a number of areas throughout the brain, including the hippocampus. However, knowledge about its function has been only well explored in the hypothalamus in the context of reproductive health. In this study, we identified and validated increased hippocampal Tac2 mRNA expression in response to chronic mild stress in mice. Expression quantitative trait locus (eQTL) analysis showed Tac2 is cis-regulated in the hippocampus. Using a systems genetics approach, we constructed a Tac2 co-expression network to better understand the relationship between Tac2 and the hippocampal stress response. Our network identified 69 total genes associated with Tac2, several of which encode major neuropeptides involved in hippocampal stress signaling as well as critical genes for producing neural plasticity, indicating that Tac2 is involved in these processes. Pathway analysis for the member of Tac2 gene network revealed a strong connection between Tac2 and neuroactive ligand-receptor interaction, calcium signaling pathway, as well as cardiac muscle contraction. In addition, we also identified 46 stress-related phenotypes, specifically fear conditioning response, that were significantly correlated with Tac2 expression. Our results provide evidence for Tac2 as a strong candidate gene who likely plays a role in hippocampal stress processing and neural plasticity.

A novel synaptic junction preparation for the identification and characterization of cleft proteins

Identification of synaptic cleft components has been hampered by the lack of a suitable preparation enriched in synaptic junctions devoid of adjoining peripheral membranes. Prior strategies for the isolation of synaptic junctions, relying on detergents for the removal of peripheral membranes, resulted in substantial loss of membranes lining the cleft. Here, a novel, detergent-free method is described for the preparation of a synaptic junction (SJ) fraction, using phospholipase A2. Limited digestion of synaptic plasma membrane (SPM) fraction with phospholipase A2 followed by centrifugation over a sucrose cushion results in selective removal of membranes peripheral to the cleft while junctional membranes remain relatively intact as observed by electron microscopy. Enrichment in synaptic junctional structures and loss of membranes peripheral to the junctional area are further verified by demonstrating enrichment in PSD-95 and loss in mGluR5, respectively. The SJ fraction is enriched in neuroligins and neurexins, in agreement with immuno-electron microscopy data showing their selective localization to the junctional area. Among additional cell adhesion molecules tested, N-cadherin and specific isoforms of the SynCAM and SALM families also show marked enrichment in the SJ fraction, suggesting preferential localization at the synaptic cleft while others show little enrichment or decrease, suggesting that they are not restricted to or concentrated at the synaptic cleft. Treatment of the SJ fraction with glycosidases results in electrophoretic mobility shifts of all cell adhesion molecules tested, indicating glycosylation at the synaptic cleft. Biochemical and ultrastructural data presented indicate that the novel synaptic junction preparation can be used as a predictive tool for the identification and characterization of the components of the synaptic cleft.

Neural Cell Adhesion Molecules of the Immunoglobulin Superfamily Regulate Synapse Formation, Maintenance, and Function

Immunoglobulin superfamily adhesion molecules are among the most abundant proteins in vertebrate and invertebrate nervous systems. Prominent family members are the neural cell adhesion molecules NCAM and L1, which were the first to be shown to be essential not only in development but also in synaptic function and as key regulators of synapse formation, synaptic activity, plasticity, and synaptic vesicle recycling at distinct developmental and activity stages. In addition to interacting with each other, adhesion molecules interact with ion channels and cytokine and neurotransmitter receptors. Mutations in their genes are linked to neurological disorders associated with abnormal development and synaptic functioning. This review presents an overview of recent studies on these molecules and their crucial impact on neurological disorders.

Regulation of extrasynaptic signaling by polysialylated NCAM: Impact for synaptic plasticity and cognitive functions

The activation of synaptic N-methyl-d-aspartate-receptors (NMDARs) is crucial for induction of synaptic plasticity and supports cell survival, whereas activation of extrasynaptic NMDARs inhibits long-term potentiation and triggers neurodegeneration. A soluble polysialylated form of the neural cell adhesion molecule (polySia-NCAM) suppresses signaling through peri-/extrasynaptic GluN2B-containing NMDARs. Genetic or enzymatic manipulations blocking this mechanism result in impaired synaptic plasticity and learning, which could be repaired by reintroduction of polySia, or inhibition of either GluN1/GluN2B receptors or downstream signaling through RasGRF1 and p38 MAP kinase. Ectodomain shedding of NCAM, and hence generation of soluble NCAM, is controlled by metalloproteases of a disintegrin and metalloprotease (ADAM) family. As polySia-NCAM is predominantly associated with GABAergic interneurons in the prefrontal cortex, it is noteworthy that EphrinA5/EphA3-induced ADAM10 activity promotes polySia-NCAM shedding in these neurons. Thus, in addition to the well-known regulation of synaptic NMDARs by the secreted molecule Reelin, shed polySia-NCAM may restrain activation of extrasynaptic NMDARs. These data support a concept that GABAergic interneuron-derived extracellular proteins control the balance in synaptic/extrasynaptic NMDAR-mediated signaling in principal cells. Strikingly, dysregulation of Reelin or polySia expression is linked to schizophrenia. Thus, targeting of the GABAergic interneuron-principle cell communication and restoring the balance in synaptic/extrasynaptic NMDARs represent promising strategies for treatment of psychiatric diseases.

Insulin treatment restores glutamate (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor function in the hippocampus of diabetic rats

Type 1 diabetes is associated with cognitive dysfunction. Cognitive processing, particularly memory acquisition, depends on the regulated enhancement of expression and function of glutamate receptor subtypes in the hippocampus. Impairment of memory was been detected in rodent models of type 1 diabetes induced by streptozotocin (STZ). This study examines the functional properties of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and the expression of synaptic molecules that regulate glutamatergic synaptic transmission in the hippocampus of STZ-diabetic rats. The AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) and single-channel properties of synaptosomal AMPA receptors were examined after 4 weeks of diabetes induction. Results show that amplitude and frequency of mEPSCs recorded from CA1 pyramidal neurons were decreased in diabetic rats. In addition, the single-channel properties of synaptic AMPA receptors from diabetic rat hippocampi were different from those of controls. These impairments in synaptic currents gated by AMPA receptors were accompanied by decreased protein levels of AMPA receptor subunit GluR1, the presynaptic protein synaptophysin, and the postsynaptic anchor protein postsynaptic density protein 95 in the hippocampus of diabetic rats. Neural cell adhesion molecule (NCAM), an extracellular matrix molecule abundantly expressed in the brain, and the polysialic acid (PSA) attached to NCAM were also downregulated in the hippocampus of diabetic rats. Insulin treatment, when initiated at the onset of diabetes induction, reduced these effects. These findings suggest that STZ-induced diabetes may result in functional deteriorations in glutamatergic synapses in the hippocampus of rats and that these effects may be reduced by insulin treatment.

Distinctive PSA-NCAM and NCAM hallmarks in glutamate-induced dendritic atrophy and synaptic disassembly

Dendritic and synapse remodeling are forms of structural plasticity that play a critical role in normal hippocampal function. Neural cell adhesion molecule (NCAM) and its polysialylated form (PSA-NCAM) participate in neurite outgrowth and synapse formation and plasticity. However, it remains unclear whether they contribute to dendritic retraction and synaptic disassembly. Cultured hippocampal neurons exposed to glutamate (5 µM) showed a reduced MAP-2 (+) area in the absence of neuronal death 24 h after the insult. Concomitantly, synapse loss, revealed by decreased synaptophysin and post-synaptic density-95 cluster number and area, together with changes in NCAM and PSA-NCAM levels were found. Dendritic atrophy and PSA-NCAM reduction proved NMDA-receptor dependent. Live-imaging experiments evidenced dendritic atrophy 4 h after the insult; this effect was preceded by smaller NCAM clusters (1 h) and decreased surface and total PSA-NCAM levels (3 h). Simultaneously, total NCAM cluster number and area remained unchanged. The subsequent synapse disassembly (6 h) was accompanied by reductions in total NCAM cluster number and area. A PSA mimetic peptide prevented both the dendritic atrophy and the subsequent synaptic changes (6 h) but had no effect on the earliest synaptic remodeling (3 h). Thus, NCAM-synaptic reorganization and PSA-NCAM level decrease precede glutamate-induced dendritic atrophy, whereas the NCAM level reduction is a delayed event related to synapse loss. Consequently, distinctive stages in PSA-NCAM/NCAM balance seem to accompany glutamate-induced dendritic atrophy and synapse loss.

NCAM deficiency in the mouse forebrain impairs innate and learned avoidance behaviours

The neural cell adhesion molecule (NCAM) has been implicated in the development and plasticity of neural circuits and the control of hippocampus- and amygdala-dependent learning and behaviour. Previous studies in constitutive NCAM null mutants identified emotional behaviour deficits related to disturbances of hippocampal and amygdala functions. Here, we studied these behaviours in mice conditionally deficient in NCAM in the postmigratory forebrain neurons. We report deficits in both innate and learned avoidance behaviours, as observed in elevated plus maze and passive avoidance tasks. In contrast, general locomotor activity, trait anxiety or neophobia were unaffected by the mutation. Altered avoidance behaviour of the conditional NCAM mutants was associated with a deficit in serotonergic signalling, as indicated by their reduced responsiveness to (±)-8-hydroxy-2-(dipropylamino)-tetralin-induced hypothermia. Another serotonin-dependent behaviour, namely intermale aggression that is massively increased in constitutively NCAM-deficient mice, was not affected in the forebrain-specific mutants. Our data suggest that genetically or environmentally induced changes of NCAM expression in the late postnatal and mature forebrain determine avoidance behaviour and serotonin (5-HT)1A receptor signalling.

The neural cell adhesion molecule promotes maturation of the presynaptic endocytotic machinery by switching synaptic vesicle recycling from adaptor protein 3 (AP-3)- to AP-2-dependent mechanisms

Newly formed synapses undergo maturation during ontogenetic development via mechanisms that remain poorly understood. We show that maturation of the presynaptic endocytotic machinery in CNS neurons requires substitution of the adaptor protein 3 (AP-3) with AP-2 at the presynaptic plasma membrane. In mature synapses, AP-2 associates with the intracellular domain of the neural cell adhesion molecule (NCAM). NCAM promotes binding of AP-2 over binding of AP-3 to presynaptic membranes, thus favoring the substitution of AP-3 for AP-2 during formation of mature synapses. The presynaptic endocytotic machinery remains immature in adult NCAM-deficient (NCAM-/-) mice accumulating AP-3 instead of AP-2 and its partner protein AP180 in synaptic membranes and vesicles. NCAM deficiency or disruption of the NCAM/AP-2 complex in wild-type (NCAM+/+) neurons by overexpression of AP-2 binding-defective mutant NCAM interferes with efficient retrieval of the synaptic vesicle v-SNARE synaptobrevin 2. Abnormalities in synaptic vesicle endocytosis and recycling may thus contribute to neurological disorders associated with mutations in NCAM.

SynProt: A Database for Proteins of Detergent-Resistant Synaptic Protein Preparations

Chemical synapses are highly specialized cell–cell contacts for communication between neurons in the CNS characterized by complex and dynamic protein networks at both synaptic membranes. The cytomatrix at the active zone (CAZ) organizes the apparatus for the regulated release of transmitters from the presynapse. At the postsynaptic side, the postsynaptic density constitutes the machinery for detection, integration, and transduction of the transmitter signal. Both pre- and postsynaptic protein networks represent the molecular substrates for synaptic plasticity. Their function can be altered both by regulating their composition and by post-translational modification of their components. For a comprehensive understanding of synaptic networks the entire ensemble of synaptic proteins has to be considered. To support this, we established a comprehensive database for synaptic junction proteins (SynProt database) primarily based on proteomics data obtained from biochemical preparations of detergent-resistant synaptic junctions. The database currently contains 2,788 non-redundant entries of rat, mouse, and some human proteins, which mainly have been manually extracted from 12 proteomic studies and annotated for synaptic subcellular localization. Each dataset is completed with manually added information including protein classifiers as well as automatically retrieved and updated information from public databases (UniProt and PubMed). We intend that the database will be used to support modeling of synaptic protein networks and rational experimental design.

Ultrastructure of synapses in the mammalian brain

The morphology and molecular composition of synapses provide the structural basis for synaptic function. This article reviews the electron microscopy of excitatory synapses on dendritic spines, using data from rodent hippocampus, cerebral cortex, and cerebellar cortex. Excitatory synapses have a prominent postsynaptic density, in contrast with inhibitory synapses, which have less dense presynaptic or postsynaptic specializations and are usually found on the cell body or proximal dendritic shaft. Immunogold labeling shows that the presynaptic active zone provides a scaffold for key molecules involved in the release of neurotransmitter, whereas the postsynaptic density contains ligand-gated ionic channels, other receptors, and a complex network of signaling molecules. Delineating the structure and molecular organization of these axospinous synapses represents a crucial step toward understanding the mechanisms that underlie synaptic transmission and the dynamic modulation of neurotransmission associated with short- and long-term synaptic plasticity.

Restoration of synaptic plasticity and learning in young and aged NCAM-deficient mice by enhancing neurotransmission mediated by GluN2A-containing NMDA receptors

Neural cell adhesion molecule (NCAM) is the predominant carrier of the unusual glycan polysialic acid (PSA). Deficits in PSA and/or NCAM expression cause impairments in hippocampal long-term potentiation and depression (LTP and LTD) and are associated with schizophrenia and aging. In this study, we show that impaired LTP in adult NCAM-deficient (NCAM(-/-)) mice is restored by increasing the activity of the NMDA subtype of glutamate receptor (GluN) through either reducing the extracellular Mg2+ concentration or applying d-cycloserine (DCS), a partial agonist of the GluN glycine binding site. Pharmacological inhibition of the GluN2A subtype reduced LTP to the same level in NCAM(-/-) and wild-type (NCAM(+/+)) littermate mice and abolished the rescue by DCS in NCAM(-/-) mice, suggesting that the effects of DCS are mainly mediated by GluN2A. The insufficient contribution of GluN to LTD in NCAM(-/-) mice was also compensated for by DCS. Furthermore, impaired contextual and cued fear conditioning levels were restored in NCAM(-/-) mice by administration of DCS before conditioning. In 12-month-old NCAM(-/-), but not NCAM(+/+) mice, there was a decline in LTP compared with 3-month-old mice that could be rescued by DCS. In 24-month-old mice of both genotypes, there was a reduction in LTP that could be fully restored by DCS in NCAM(+/+) mice but only partially restored in NCAM(-/-) mice. Thus, several deficiencies of NCAM(-/-) mice can be ameliorated by enhancing GluN2A-mediated neurotransmission with DCS.

The neuronal K-Cl cotransporter KCC2 influences postsynaptic AMPA receptor content and lateral diffusion in dendritic spines

The K-Cl cotransporter KCC2 plays an essential role in neuronal chloride homeostasis, and thereby influences the efficacy and polarity of GABA signaling. Although KCC2 is expressed throughout the somatodendritic membrane, it is remarkably enriched in dendritic spines, which host most glutamatergic synapses in cortical neurons. KCC2 has been shown to influence spine morphogenesis and functional maturation in developing neurons, but its function in mature dendritic spines remains unknown. Here, we report that suppressing KCC2 expression decreases the efficacy of excitatory synapses in mature hippocampal neurons. This effect correlates with a reduced postsynaptic aggregation of GluR1-containing AMPA receptors and is mimicked by a dominant negative mutant of KCC2 interaction with cytoskeleton but not by pharmacological suppression of KCC2 function. Single-particle tracking experiments reveal that suppressing KCC2 increases lateral diffusion of the mobile fraction of AMPA receptor subunit GluR1 in spines but not in adjacent dendritic shafts. Increased diffusion was also observed for transmembrane but not membrane-anchored recombinant neuronal cell adhesion molecules. We suggest that KCC2, likely through interactions with the actin cytoskeleton, hinders transmembrane protein diffusion, and thereby contributes to their confinement within dendritic spines.

Immobilized pool of NCAM180 in the postsynaptic membrane is homeostatically replenished by the flux of NCAM180 from extrasynaptic regions

Homeostatic mechanisms maintaining high levels of adhesion molecules in synapses over prolonged periods of time remain incompletely understood. We used fluorescence recovery after photobleaching experiments to analyze the steady state turnover of the immobile pool of green fluorescent protein-labeled NCAM180, the largest postsynaptically accumulating isoform of the neural cell adhesion molecule (NCAM). We show that there is a continuous flux of NCAM180 to the postsynaptic membrane from nonsynaptic regions of dendrites by diffusion. In the postsynaptic membrane, the newly delivered NCAM180 slowly intermixes with the immobilized pool of NCAM180. Preferential immobilization and accumulation of NCAM180 in the postsynaptic membrane is reduced after disruption of the association of NCAM180 with the spectrin cytoskeleton and in the absence of the homophilic interactions of NCAM180 in synapses. Our observations indicate that the homophilic interactions and binding to the cytoskeleton promote immobilization of NCAM180 and its accumulation in the postsynaptic membrane. Flux of NCAM180 from extrasynaptic regions and its slow intermixture with the immobile pool of NCAM180 in the postsynaptic membrane may be important for the continuous homeostatic replenishment of NCAM180 protein at synaptic contacts without compromising the long term synaptic contact stability.

Building and remodeling synapses

Synaptic junctions are generated by adhesion proteins that bridge the synaptic cleft to firmly anchor pre- and postsynaptic membranes. Several cell adhesion molecule (CAM) families localize to synapses, but it is not yet completely understood how each synaptic CAM family contributes to synapse formation and/or structure, and whether or how smaller groups of CAMs serve as minimal, functionally cooperative adhesive units upon which structure is based. Synapse structure and function evolve over the course of development, and in mature animals, synapses are composed of a greater number of proteins, surrounded by a stabilizing extracellular matrix, and often contacted by astrocytic processes. Thus, in mature networks undergoing plasticity, persistent changes in synapse strength, morphology, or number must be accompanied by selective and regulated remodeling of the neuropil. Recent work indicates that regulated, extracellular proteolysis may be essential for this, and rather than simply acting permissively to enable synapse plasticity, is more likely playing a proactive role in driving coordinated synaptic structural and functional modifications that underlie persistent changes in network activity.

Neural cell adhesion molecule modulates dopaminergic signaling and behavior by regulating dopamine D2 receptor internalization

The dopaminergic system plays an important role in the etiology of schizophrenia, and most antipsychotic drugs exert their functions by blocking dopamine D(2) receptors (D(2)Rs). Since the signaling strength mediated by D(2)Rs is regulated by internalization and degradation processes, it is crucial to identify molecules that modulate D(2)R localization at the cell surface. Here, we show that the neural cell adhesion molecule (NCAM) promotes D(2)R internalization/desensitization and subsequent degradation via direct interaction with a short peptide in the third intracellular loop of the D(2)R. NCAM deficiency in mice leads to increased numbers of D(2)Rs at the cell surface and augmented D(2)R signaling as a result of impaired D(2)R internalization. Furthermore, NCAM-deficient mice show higher sensitivity to the psychostimulant apomorphine and exaggerated activity of dopamine-related locomotor behavior. These results demonstrate that, in addition to its classical function in cell adhesion, NCAM is involved in regulating the trafficking of the neurotransmitter receptor D(2)R as well as receptor-mediated signaling and behavior, thus implicating NCAM as modulator of the dopaminergic system and a potential pharmacological target for dopamine-related neurological and psychiatric disorders.

Modulation of synaptic transmission and plasticity by cell adhesion and repulsion molecules

Adhesive and repellent molecular cues guide migrating cells and growing neurites during development. They also contribute to synaptic function, learning and memory in adulthood. Here, we review the roles of cell adhesion molecules of the immunoglobulin superfamily (Ig-CAMs) and semaphorins (some of which also contain Ig-like domains) in regulation of synaptic transmission and plasticity. Interestingly, among the seven studied Ig-CAMs, the neuronal cell adhesion molecule proved to be important for all tested forms of hippocampal plasticity, while its associated unusual glycan polysialic acid is necessary and sufficient part for synaptic plasticity only at CA3-CA1 synapses. In contrast, Thy-1 and L1 specifically regulate long-term potentiation (LTP) at synapses formed by entorhinal axons in the dentate gyrus and cornu ammonis, respectively. Contactin-1 is important for long-term depression but not for LTP at CA3-CA1 synapses. Analysis of CHL1-deficient mice illustrates that at intermediate stages of development a deficit in a cell adhesion molecule is compensated but appears as impaired LTP during early and late postnatal development. The emerging mechanisms by which adhesive Ig-CAMs contribute to synaptic plasticity involve regulation of activities of NMDA receptors and L-type Ca2+ channels, signaling via mitogen-activated protein kinase p38, changes in GABAergic inhibition and motility of synaptic elements. Regarding repellent molecules, available data for semaphorins demonstrate their activity-dependent regulation in normal and pathological conditions, synaptic localization of their receptors and their potential to elevate or inhibit synaptic transmission either directly or indirectly.

N-methyl-D-aspartate receptor independent changes in expression of polysialic acid-neural cell adhesion molecule despite blockade of homosynaptic long-term potentiation and heterosynaptic long-term depression in the awake freely behaving rat dentate gyrus

Investigations examining the role of polysialic acid (PSA) on the neural cell adhesion molecule (NCAM) in synaptic plasticity have yielded inconsistent data. Here, we addressed this issue by determining whether homosynaptic long-term potentiation (LTP) and heterosynaptic long-term depression (LTD) induce changes in the distribution of PSA-NCAM in the dentate gyrus (DG) of rats in vivo. In addition, we also examined whether the observed modifications were initiated via the activation of N-methyl-D-aspartate (NMDA) receptors. Immunocytochemical analysis showed an increase in PSA-NCAM positive cells both at 2 and 24 h following high-frequency stimulation of either medial or lateral perforant paths, leading to homosynaptic LTP and heterosynaptic LTD, respectively, in the medial molecular layer of the DG. Analysis of sub-cellular distribution of PSA-NCAM by electron microscopy showed decreased PSA dendritic labelling in LTD rats and a sub-cellular relocation towards the spines in LTP rats. Importantly, these modifications were found to be independent of the activation of NMDA receptors. Our findings suggest that strong activation of the granule cells up-regulates PSA-NCAM synthesis which then incorporates into activated synapses, representing NMDA-independent plastic processes that act synergistically on LTP/LTD mechanisms without participating in their expression.

Developmental regulation of neural cell adhesion molecule in human prefrontal cortex

Neural cell adhesion molecule (NCAM) is a membrane-bound cell recognition molecule that exerts important functions in normal neurodevelopment including cell migration, neurite outgrowth, axon fasciculation, and synaptic plasticity. Alternative splicing of NCAM mRNA generates three main protein isoforms: NCAM-180, -140, and -120. Ectodomain shedding of NCAM isoforms can produce an extracellular 105-115 kilodalton soluble neural cell adhesion molecule fragment (NCAM-EC) and a smaller intracellular cytoplasmic fragment (NCAM-IC). NCAM also undergoes a unique post-translational modification in brain by the addition of polysialic acid (PSA)-NCAM. Interestingly, both PSA-NCAM and NCAM-EC have been implicated in the pathophysiology of schizophrenia. The developmental expression patterns of the main NCAM isoforms and PSA-NCAM have been described in rodent brain, but no studies have examined NCAM expression across human cortical development. Western blotting was used to quantify NCAM in human postmortem prefrontal cortex in 42 individuals ranging in age from mid-gestation to early adulthood. Each NCAM isoform (NCAM-180, -140, and -120), post-translational modification (PSA-NCAM) and cleavage fragment (NCAM-EC and NCAM-IC) demonstrated developmental regulation in frontal cortex. NCAM-180, -140, and -120, as well as PSA-NCAM, and NCAM-IC all showed strong developmental regulation during fetal and early postnatal ages, consistent with their identified roles in axon growth and plasticity. NCAM-EC demonstrated a more gradual increase from the early postnatal period to reach a plateau by early adolescence, potentially implicating involvement in later developmental processes. In summary, this study implicates the major NCAM isoforms, PSA-NCAM and proteolytically cleaved NCAM in pre- and postnatal development of the human prefrontal cortex. These data provide new insights on human cortical development and also provide a basis for how altered NCAM signaling during specific developmental intervals could affect synaptic connectivity and circuit formation, and thereby contribute to neurodevelopmental disorders.

Regional and duration of illness differences in the alteration of NCAM-180 mRNA expression within the cortex of subjects with schizophrenia

Schizophrenia has been proposed to have a neurodevelopmental aetiology. Neural Cell Adhesion Molecule 1 (NCAM1) is involved in several neurodevelopmental processes and abnormal expression of this gene has been associated in the pathology of schizophrenia and, thus, altered NCAM1 expression may be characteristic of the early stages of the illness. Alternative splicing of the NCAM1 transcript produces 3 major isoforms. Using qPCR we analysed mRNA expression of one of these isoforms; the 180 kDa isoform of NCAM1 (NCAM-180), in Brodmann Area (BA) 46, BA10 and BA17, post-mortem, from 15 subjects with a short duration of illness of schizophrenia (<7 years) and 15 control subjects. NCAM-180 mRNA expression was increased in BA46 from subjects with schizophrenia compared to controls (p=0.013). By contrast, there were no significant differences in the expression of NCAM-180 mRNA in BA10 (p=0.575) or BA17 (p=0.772). We then analysed NCAM-180 mRNA expression in BA46 from 15 subjects with a longer duration of illness of schizophrenia (>22 years) and 15 controls. There was no significant difference in NCAM-180 mRNA expression in this second cohort. This data suggests NCAM-180 mRNA expression is altered in a regionally-specific manner in schizophrenia and these changes are associated with the early period following diagnosis.

Site-directed antibodies to low-voltage-activated calcium channel CaV3.3 (alpha1I) subunit also target neural cell adhesion molecule-180

Synthetic peptides of defined amino acid sequence are commonly used as unique antigens for production of antibodies to more complex target proteins. We previously showed that an affinity-purified, site-directed polyclonal antibody (CW90) raised against a peptide antigen (CNGRMPNIAKDVFTKM) anticipated to be specific to a T-type voltage-dependent Ca(2+) channel subunit identified recombinant rat alpha1I/Ca(V)3.3 and two endogenous mouse proteins distinct in their developmental expression and apparent molecular mass (neonatal form 260 kDa, mature form 190 kDa) [Yunker AM, Sharp AH, Sundarraj S, Ranganathan V, Copeland TD, McEnery MW (2003) Immunological characterization of T-type voltage-dependent calcium channel Ca(V)3.1 (alpha 1G) and Ca(V)3.3 (alpha 1I) isoforms reveal differences in their localization, expression, and neural development. Neuroscience 117:321-335]. In the present study, we further characterize the biochemical properties of the CW90 antigens. We show for the first time that recombinant alpha1I/Ca(V)3.3 is modified by N-glycosylation. Using peptide:N-glycosidase F (PNGase F), an enzyme that removes polysaccharides attached at Asn residues, and endoneuraminidase-N (Endo-N), which specifically removes polysialic acid modifications, we reveal that differential glycosylation fully accounts for the large difference in apparent molecular mass between neonatal and adult CW90 antigens and that the neonatal form is polysialylated. As very few proteins are substrates for Endo-N, we carried out extensive analyses and herein present evidence that CW90 reacts with recombinant alpha1I/Ca(V)3.3 as well as endogenous neural cell adhesion molecule-180 (NCAM-180). We demonstrate the basis for CW90 cross-reactivity is a five amino acid epitope (AKDVF) present in both alpha1I/Ca(V)3.3 and NCAM-180. To extend these findings, we introduce a novel polyclonal anti-peptide antibody (CW678) that uniquely recognizes NCAM-180 and a new antibody (CW109) against alpha1I/Ca(V)3.3. Western blot analyses obtained with CW678, CW109 and CW90 on a variety of samples confirm that the endogenous CW90 signals are fully attributed to the two developmental forms of NCAM-180. Using CW678, we present novel data on differentiation-dependent NCAM-180 expression in human neuroblastoma IMR32 cells. These results strongly suggest the need for careful analyses to validate anti-peptide antibodies when targeting membrane proteins of low abundance.

Selective learning and memory impairments in mice deficient for polysialylated NCAM in adulthood

The neural cell adhesion molecule (NCAM) has been implicated in regulating synaptic plasticity mechanisms as well as memory consolidation processes. Attachment of polysialic acid to NCAM (PSA-NCAM) has been reported to down-regulate its adhesive forces, a process hypothesized to be implicated in synapse selection after learning experiences. PSA-NCAM has been critically implicated in hippocampus-related synaptic plasticity and memory storage, but information about its functional role in other brain areas remains scarce. Here, we studied mice deficient for polysialyltransferase-1 (ST8SialV/PST-1), an enzyme which attaches PSA to NCAM during postnatal development and adulthood, and whose deficiency results in a drastic reduction of PSA-NCAM expression throughout the brain in adulthood. Mice were tested for their performance in the water maze and auditory fear conditioning (AFC). We report that ST8SiaIV knockout mice were impaired in spatial as well as reversal learning in the water maze. On the other hand, AFC was intact and ST8SiaIV mice exhibited no impairments in the acquisition or retention of cued fear memories. Spatial orientation learning and reversal learning require complex integration of spatial information and response selection involving the hippocampus and prefrontal cortex, whereas cued fear conditioning is an associative type of emotional memory that highly depends on amygdala function. Therefore, our results indicate that PSA-NCAM contributes differentially to learning processes that differ in the nature of the neural computations involved, which probably reflects a differential role of this molecule in different brain regions.

Neural Cell Adhesion Molecule-associated Polysialic Acid Inhibits NR2B-containing N-Methyl-d-aspartate Receptors and Prevents Glutamate-induced Cell Death

The neural cell adhesion molecule (NCAM) and its associated glycan polysialic acid play important roles in the development of the nervous system and N-methyl-D-aspartate(NMDA)receptor-dependent synaptic plasticity in the adult. Here, we investigated the influence of polysialic acid on NMDA receptor activity. We found that glutamate-elicited NMDA receptor currents in cultured hippocampal neurons were reduced by approximately 30% with the application of polysialic acid or polysialylated NCAM but not by the sialic acid monomer, chondroitin sulfate, or non-polysialylated NCAM. Polysialic acid inhibited NMDA receptor currents elicited by 3 microm glutamate but not by 30 microm glutamate, suggesting that polysialic acid acts as a competitive antagonist, possibly at the glutamate binding site. The polysialic acid induced effects were mimicked and fully occluded by the NR2B subunit specific antagonist, ifenprodil. Recordings from single synaptosomal NMDA receptors reconstituted in lipid bilayers revealed that polysialic acid reduced open probability but not the conductance of NR2B-containing NMDA receptors in a polysialic acid and glutamate concentration-dependent manner. The activity of single NR2B-lacking synaptosomal NMDA receptors was not affected by polysialic acid. Application of polysialic acid to hippocampal cultures reduced excitotoxic cell death induced by low micromolar concentration of glutamate via activation of NR2B-containing NMDA receptors, whereas enzymatic removal of polysialic acid resulted in increased cell death that occluded glutamate-induced excitotoxicity. These observations indicate that the cell adhesion molecule-associated glycan polysialic acid is able to prevent excitotoxicity via inhibition of NR2B subunit-containing NMDA receptors.

MMP-2, VCAM-1 and NCAM-1 expression in the brain of rats with experimental autoimmune encephalomyelitis as a trigger mechanism for synaptic plasticity and pathology

The neural cell adhesion molecules (NCAMs), and vascular cell adhesion molecules (VCAMs) that regulate cell-to-extracellular matrix adhesion, and matrix metalloproteinases (MMPs), modulating the extracellular matrix (ECM), are considered to play an important role in the pathogenesis of experimental autoimmune encephalomyelitis (EAE). Clinical signs appearance and significant increases of MMP-2 expression in CA1 and CA3 subdomains of the hippocampus and around the central canal of the cervical spinal cord, with the clusters of VCAM-1(+) immunoreactive cells localized in the choroid plexus epithelium and hypothalamo-hypophyses portal vessel system indicate an inflammation in acute EAE. Decreased NCAM-1 expression in CA1 and CA3 fields of the hippocampus, and in a lesser degree in the basal ganglia, limbic structure and cervical spinal cord, support the concept that the demyelinating neuroinflammatory damage in an autoimmune brain affect synaptic organization of the brain, altering the balance between extracellular proteases and cell adhesion molecules which appears to be critical for both the brain plasticity and autoimmune processes.

In vivo synaptic plasticity in the dentate gyrus of mice deficient in the neural cell adhesion molecule NCAM or its polysialic acid

The neural cell adhesion molecule NCAM and its associated polysialic acid (PSA) play important roles in synaptic plasticity in the CA1 and/or CA3 regions of the hippocampus in vitro. Here, we address the question of whether NCAM and PSA are involved in regulation of synaptic transmission and plasticity also in vivo at synapses formed by entorhinal cortex axons in the dentate gyrus of mice anaesthetized with urethane. We show that basal synaptic transmission, measured as the slope of field excitatory postsynaptic potentials, was reduced strongly in mice lacking ST8SiaII/STX, the enzyme involved in polysialylation of NCAM in stem cell-derived immature granule cells, but not in mice deficient either in the NCAM glycoprotein or the enzyme ST8SiaIV/PST involved in polysialylation of NCAM in mature neurons. Strikingly, only mice deficient in NCAM, but not in PST or STX, were impaired in long-term potentiation (LTP) induced by theta-burst stimulation, suggesting that LTP in the dentate gyrus depends on the NCAM glycoprotein alone rather than on its associated PSA. As also patterns of synaptic activity during and immediately after induction of LTP were impaired in NCAM-deficient mice, it is likely that induction of LTP requires NCAM. These data are the first to describe that NCAM is necessary for induction of synaptic plasticity in identified synapses in vivo and suggest that polysialylation of NCAM expressed by immature granule cells in the dentate gyrus supports development of basal excitatory synaptic transmission in this region.

PSA-NCAM in mammalian structural plasticity and neurogenesis

Polysialic acid (PSA) is a linear homopolymer of alpha2-8-N acetylneuraminic acid whose major carrier in vertebrates is the neural cell adhesion molecule (NCAM). PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. PSA on NCAM is developmentally regulated thus playing a prominent role in different forms of neural plasticity spanning from embryonic to adult nervous system, including axonal growth, outgrowth and fasciculation, cell migration, synaptic plasticity, activity-induced plasticity, neuronal-glial plasticity, embryonic and adult neurogenesis. The cellular distribution, developmental changes and possible function(s) of PSA-NCAM in the central nervous system of mammals here are reviewed, along with recent findings and theories about the relationships between NCAM protein and PSA as well as the role of different polysialyltransferases. Particular attention is focused on postnatal/adult neurogenesis, an issue which has been deeply investigated in the last decade as an example of persisting structural plasticity with potential implications for brain repair strategies. Adult neurogenic sites, although harbouring all subsequent steps of cell differentiation, from stem cell division to cell replacement, do not faithfully recapitulate development. After birth, they undergo morphological and molecular modifications allowing structural plasticity to adapt to the non-permissive environment of the mature nervous tissue, that are paralled by changes in the expression of PSA-NCAM. The use of PSA-NCAM as a marker for exploring differences in structural plasticity and neurogenesis among mammalian species is also discussed.

Differential modulation of gene expression in the NMDA postsynaptic density of schizophrenic and control smokers

Nicotine is known to induce the release of multiple neurotransmitters, including glutamate and dopamine, through activation of nicotinic receptors. Gene expression in the N-methyl-d-aspartate postsynaptic density (NMDA-PSD), as well as other functional groups, was compared in postmortem hippocampus of schizophrenic and nonmentally ill smokers and nonsmokers utilizing a microarray and quantitative RT-PCR approach. The expression of 277 genes was significantly changed between all smokers and nonsmokers. Specific gene groups, most notably genes expressed in the NMDA-PSD, were prevalent among these transcripts. Analysis of the interaction between smoking and schizophrenia identified several genes in the NMDA-PSD that were differentially affected by smoking in patients. The present findings suggest that smoking may differentially modulate glutamatergic function in schizophrenic patients and control subjects. The biological mechanisms underlying chronic tobacco use are likely to differ substantially between these two groups.

Ontogeny of postsynaptic density proteins at glutamatergic synapses

In glutamatergic synapses, glutamate receptors (GluRs) associate with many other proteins involved in scaffolding and signal transduction. The ontogeny of these postsynaptic density (PSD) proteins involves changes in their composition during development, paralleling changes in GluR type and function. In the CA1 region of the hippocampus, at postnatal day 2 (P2), many synapses already have a distinct PSD. We used immunoblot analysis, subcellular fractionation, and quantitative immunogold electron microscopy to examine the distribution of PSD proteins during development of the hippocampus. Synapses at P2 contained substantial levels of NR1 and NR2B and most GluR-associated proteins, including SAP102, SynGAP, the chain of proteins from GluRs/SAP102 through GKAP/Shank/Homer and metabotropic glutamate receptors, and the adhesion factors, cadherin, catenin, neuroligin, and Nr-CAM. Development was marked by substantial decreases in NR2B and SAP102 and increases in NR2A, PSD-95, AMPA receptors, and CaMKII. Other components showed more moderate changes.

Polysialylated neural cell adhesion molecule promotes remodeling and formation of hippocampal synapses

Expression of the neural cell adhesion molecule (NCAM) has been shown to promote long-term potentiation (LTP) and stabilization of synapses during early synaptogenesis. Here, we searched for the mechanisms of synaptogenic activity of NCAM, focusing on the role of polysialic acid (PSA), an unusual carbohydrate preferentially associated with NCAM. We show that enzymatic removal of PSA with endoneuraminidase-N (endo-N) abolished preferential formation of synapses on NCAM-expressing cells in heterogenotypic cocultures of wild-type and NCAM-deficient hippocampal neurons. Transfection of NCAM-deficient neurons with either of three major NCAM isoforms (different in intracellular domains but identical in extracellular domains and carrying PSA) stimulated preferential synapse formation on NCAM isoform-expressing neurons. Enzymatic removal of heparan sulfates from cultured neurons and a mutation in the heparin-binding domain of NCAM diminished synaptogenic activity of neuronally expressed PSA-NCAM, suggesting that interaction of NCAM with heparan sulfate proteoglycans mediates this activity. PSA-NCAM-driven synaptogenesis was also blocked by antagonists to fibroblast growth factor receptor and NMDA subtype of glutamate receptors but not by blockers of non-NMDA glutamate receptors and voltage-dependent Na+ channels. Enzymatic removal of PSA and heparan sulfates also blocked the increase in the number of perforated spine synapses associated with NMDA receptor-dependent LTP in the CA1 region of organotypic hippocampal cultures. Thus, neuronal PSA-NCAM in complex with heparan sulfate proteoglycans promotes synaptogenesis and activity-dependent remodeling of synapses.

Conditional ablation of the neural cell adhesion molecule reduces precision of spatial learning, long-term potentiation, and depression in the CA1 subfield of mouse hippocampus

NCAM, a neural cell adhesion molecule of the immunoglobulin superfamily, is involved in neuronal migration and differentiation, axon outgrowth and fasciculation, and synaptic plasticity. To dissociate the functional roles of NCAM in the adult brain from developmental abnormalities, we generated a mutant in which the NCAM gene is inactivated by cre-recombinase under the control of the calcium-calmodulin-dependent kinase II promoter, resulting in reduction of NCAM expression predominantly in the hippocampus. This mutant (NCAMff+) did not show the overt morphological and behavioral abnormalities previously observed in constitutive NCAM-deficient (NCAM-/-) mice. However, similar to the NCAM-/- mouse, a reduction in long-term potentiation (LTP) in the CA1 region of the hippocampus was revealed. Long-term depression was also abolished in NCAMff+ mice. The deficit in LTP could be rescued by elevation of extracellular Ca2+ concentrations from 1.5 or 2.0 to 2.5 mm, suggesting an involvement of NCAM in regulation of Ca2+-dependent signaling during LTP. Contrary to the NCAM-/- mouse, LTP in the CA3 region was normal, consistent with normal mossy fiber lamination in NCAMff+ as opposed to abnormal lamination in NCAM-/- mice. NCAMff+ mutants did not show general deficits in short- and long-term memory in global landmark navigation in the water maze but were delayed in the acquisition of precise spatial orientation, a deficit that could be overcome by training. Thus, mice conditionally deficient in hippocampal NCAM expression in the adult share certain abnormalities characteristic of NCAM-/- mice, highlighting the role of NCAM in the regulation of synaptic plasticity in the CA1 region.