A synthetic peptide ligand of neural cell adhesion molecule (NCAM) IgI domain prevents NCAM internalization and disrupts passive avoidance learning

The neural cell adhesion molecule (NCAM) mediates cell adhesion and signal transduction through trans-homophilic- and/or cis-heterophilic-binding mechanisms. Intraventricular infusions of anti-NCAM have revealed a functional requirement of NCAM for the consolidation of memory in rats and chicks in a specific interval 6-8 h after training. We have now extended these studies to a synthetic peptide ligand of NCAM (C3) with an affinity for the IgI domain and the capability of inhibiting NCAM-mediated neurite outgrowth in vitro. Intraventricular administration of a single 5 microg bolus of C3 strongly inhibited recall of a passive avoidance response in adult rats, when given during training or in the 6-8-h posttraining period. The effect of C3 on memory consolidation was similar to that obtained with anti-NCAM as the amnesia was not observed until the 48-h recall time. The unique amnesic action of C3 during training could be related to disrupted NCAM internalization following training. In the 3-4-h posttraining period NCAM 180, the synapse-associated isoform, was down-regulated in the hippocampal dentate gyrus. This effect was mediated by ubiquitination and was prevented by C3 administration during training. These findings indicate NCAM to be involved in both the acquisition and consolidation of a passive avoidance response in the rat. Moreover, the study provides the first in vivo evidence for NCAM internalization in learning and identifies a synthetic NCAM ligand capable of modulating memory processes in vivo.     

Identification of a peptide sequence involved in homophilic binding in the neural cell adhesion molecule NCAM

The neural cell adhesion molecule NCAM is capable of mediating cell-cell adhesion via homophilic interactions. In this study, three strategies have been combined to identify regions of NCAM that participate directly in NCAM-NCAM binding: analysis of domain deletion mutations, mapping of epitopes of monoclonal antibodies, and use of synthetic peptides to inhibit NCAM activity. Studies on L cells transfected with NCAM mutant cDNAs using cell aggregation and NCAM-covasphere binding assays indicate that the third immunoglobulin-like domain is involved in homophilic binding. The epitopes of four monoclonal antibodies that have been previously shown to affect cell-cell adhesion mediated by NCAM were also mapped to domain 3. Overlapping hexapeptides were synthesized on plastic pins and assayed for binding with these monoclonal antibodies. One of them (PP) reacted specifically with the sequence KYSFNY. Synthetic oligopeptides containing the PP epitope were potent and specific inhibitors of NCAM binding activity. A substratum containing immobilized peptide conjugates also exhibited adhesiveness for neural retinal cells. Cell attachment was specifically inhibited by peptides that contained the PP-epitope and by anti-NCAM univalent antibodies. The shortest active peptide has the sequence KYSFNYDGSE, suggesting that this site is directly involved in NCAM homophilic interaction.     

Neuroplastin-65 and a mimetic peptide derived from its homophilic binding site modulate neuritogenesis and neuronal plasticity

Neuroplastin-65 (Np65) is a brain-specific cell adhesion molecule belonging to the immunoglobulin superfamily. Homophilic trans-interaction of Np65 mediates adhesion between cells and modulates synaptic plasticity. This interaction solely occurs through the first immunoglobulin (Ig) module of Np65, but the exact binding mechanism has not yet been elucidated. In this study, we identify the homophilic binding motif of Np65 and show that a synthetic peptide modeled after this motif, termed enplastin, binds to Np65. We demonstrate that both Np65- and enplastin-induced intracellular signaling depends on fibroblast growth factor receptor, p38 mitogen-activated protein kinase, Ca(2+) /calmodulin-dependent protein kinase, and cytoplasmic Ca(2+) concentration. In addition, we show that interference with Np65 homophilic binding by enplastin has an inhibitory effect on Np65-mediated neurite outgrowth in vitro and on the initial phase of spatial learning in rats.     

Attractor neural networks and spatial maps in hippocampus

Attractor neural network theory has been proposed as a theory for long-term memory. Recent studies of hippocampal place cells, including a study by Leutgeb et al. in this issue of Neuron, address the potential role of attractor dynamics in the formation of hippocampal representations of spatial maps.     

Environmental enrichment promotes neural plasticity and cognitive ability in fish

Different kinds of experience during early life can play a significant role in the development of an animal''s behavioural phenotype. In natural contexts, this influences behaviours from anti-predator responses to navigation abilities. By contrast, for animals reared in captive environments, the homogeneous nature of their experience tends to reduce behavioural flexibility. Studies with cage-reared rodents indicate that captivity often compromises neural development and neural plasticity. Such neural and behavioural deficits can be problematic if captive-bred animals are being reared with the intention of releasing them as part of a conservation strategy. Over the last decade, there has been growing interest in the use of environmental enrichment to promote behavioural flexibility in animals that are bred for release. Here, we describe the positive effects of environmental enrichment on neural plasticity and cognition in juvenile Atlantic salmon (Salmo salar). Exposing fish to enriched conditions upregulated the forebrain expression of NeuroD1 mRNA and improved learning ability assessed in a spatial task. The addition of enrichment to the captive environment thus promotes neural and behavioural changes that are likely to promote behavioural flexibility and improve post-release survival.     

Cellular correlates to spatial learning in the rat hippocampus

Learning through exploration gives increased synaptic field potentials in the perforant path/dentate synapses, largely due to an activity-dependent brain temperature increase. After temperature compensation, spatial learning was associated with small, but significant, STP-like changes of the field potential lasting 20–30 min. A group of spatially trained adult rats showed faster spatial learning and about 10% higher basal dendritic spine density (LY-filled) compared to two control groups. With unchanged dendritic length and branching pattern, the results suggest the formation of new synapses.     

Dual synaptic plasticity in the hippocampus: Hebbian and spatiotemporal learning dynamics

We assume that Hebbian learning dynamics (HLD) and spatiotemporal learning dynamics (SLD) are involved in the mechanism of synaptic plasticity in the hippocampal neurons. While HLD is driven by pre- and postsynaptic spike timings through the backpropagating action potential, SLD is evoked by presynaptic spike timings alone. Since the backpropagation attenuates as it nears the distal dendrites, we assume an extreme case as a neuron model where HLD exists only at proximal dendrites and SLD exists only at the distal dendrites. We examined how the synaptic weights change in response to three types of synaptic inputs in computer simulations. First, in response to a Poisson train having a constant mean frequency, the synaptic weights in HLD and SLD are qualitatively similar. Second, SLD responds more rapidly than HLD to synchronous input patterns, while each responds to them. Third, HLD responds more rapidly to more frequent inputs, while SLD shows fluctuating synaptic weights. These results suggest an encoding hypothesis in that a transient synchronous structure in spatiotemporal input patterns will be encoded into distal dendrites through SLD and that persistent synchrony or firing rate information will be encoded into proximal dendrites through HLD.     

A small peptide mimetic of brain‐derived neurotrophic factor promotes peripheral myelination

The expression of the neurotrophins and their receptors is essential for peripheral nervous system development and myelination. We have previously demonstrated that brain‐derived neurotrophic factor (BDNF) exerts contrasting influences upon Schwann cell myelination in vitro – promoting myelination via neuronally expressed p75NTR, but inhibiting myelination via neuronally expressed TrkB. We have generated a small peptide called cyclo‐d PAKKR that structurally mimics the region of BDNF that binds p75NTR. Here, we have investigated whether utilizing cyclo‐d PAKKR to selectively target p75NTR is an approach that could exert a unified promyelinating response. Like BDNF, cyclo‐d PAKKR promoted myelination of nerve growth factor‐dependent neurons in vitro, an effect dependent on the neuronal expression of p75NTR. Importantly, cyclo‐d PAKKR also significantly promoted the myelination of tropomyosin‐related kinase receptor B‐expressing neurons in vitro, whereas BDNF exerted a significant inhibitory effect. This indicated that while BDNF exerted a contrasting influence upon the myelination of distinct subsets of dorsal root ganglion (DRG) neurons in vitro, cyclo‐d PAKKR uniformly promoted their myelination. Local injection of cyclo‐d PAKKR adjacent to the developing sciatic nerve in vivo significantly enhanced myelin protein expression and significantly increased the number of myelinated axons. These results demonstrate that cyclo‐d PAKKR promotes peripheral myelination in vitro and in vivo, suggesting it is a strategy worthy of further investigation for the treatment of peripheral demyelinating diseases.     

Nicotinic binding sites in cerebral cortex and hippocampus in alzheimer's dementia

Postmortem cerebral neocortical and hippocampal samples were taken from patients who died with dementia of the Alzheimer type (DAT) and individuals without diagnoses of neurological or psychiatric disease (control). Nicotinic binding was assayed with 20 nM [³H]acetylcholine ([³H]ACh) in the presence of atropine, or with 4 nM (-)-[³H]Nic). Binding of both ligands was lower in the following regions from DAT vs. control brains (P0.05): superior, middle and inferior temproal gyri, orbital frontal gyrus, middle frontal gyrus, pre- and postcentral gyri, inferior parietal lobule, and hippocampal endplate. Values of the correlation coefficient (r's) for binding of the nicotinic cholinergic ligands in these regions ranged from 0.70 to 0.93 (P's<0.05), suggesting that [³H]ACh and (-)-[³H]Nic labeled the same sites in human brain. There was no difference in nicotinic binding in the presubiculum, comparing DAT and control samples (P>0.05). Here too, correlations between binding of the two ligands were statistically significant in control and DAT groups (r's=0.92,P's<0.05). Nicotinic binding measured with [³H]ACh, but not (-)-[³H]Nic, was significantly lower in the H₂ (field of Rose) and H₁-subiculum areas of DAT samples compared to control. Correlations between binding of the two ligands in these regions ranged from 0.21 to 0.34 for the two groups (P's>0.05). The findings support a loss of neocortical and hippocampal nicotininc cholinergic binding sites in DAT. Further study is necessary to better characterize the regional losses of nicotinic binding in DAT and to resolve the differences in binding measured by [³H]ACh and (-)-[³H]Nic in the H₁-subiculum and H₂ (field of Rose) regions.     

Distributed neural plasticity for shape learning in the human visual cortex

Expertise in recognizing objects in cluttered scenes is a critical skill for our interactions in complex environments and is thought to develop with learning. However, the neural implementation of object learning across stages of visual analysis in the human brain remains largely unknown. Using combined psychophysics and functional magnetic resonance imaging (fMRI), we show a link between shape-specific learning in cluttered scenes and distributed neuronal plasticity in the human visual cortex. We report stronger fMRI responses for trained than untrained shapes across early and higher visual areas when observers learned to detect low-salience shapes in noisy backgrounds. However, training with high-salience pop-out targets resulted in lower fMRI responses for trained than untrained shapes in higher occipitotemporal areas. These findings suggest that learning of camouflaged shapes is mediated by increasing neural sensitivity across visual areas to bolster target segmentation and feature integration. In contrast, learning of prominent pop-out shapes is mediated by associations at higher occipitotemporal areas that support sparser coding of the critical features for target recognition. We propose that the human brain learns novel objects in complex scenes by reorganizing shape processing across visual areas, while taking advantage of natural image correlations that determine the distinctiveness of target shapes.     

Developmental plasticity in neural circuits controlling birdsong: Sexual differentiation and the neural basis of learning

In many species of passerine songbirds, males learn their song during defined periods of life. Female song in often reduced or absent, as are the brain regions controlling song. Sexual differences in the brain arise because of the action of sex steroids, which trigger the formation of some neural pathways (especially the pathway from the higher vocal center to the robust nucleus) and prevent the atrophy of others in males. These neural changes occur during periods of developmental song learning and can recur during periods of learning in adult birds. The process of learning is correlated with major increases or decreases in the number of neurons in specific neuronal populations, suggesting that the formation or loss of specific neural pathways regulates the ability to learn. Species differences in sexual differentiation and learning allow informative cross-species comparisons of neural structure and behavior. © 1992 John Wiley & Sons, Inc.     

Anti-androgenic effects of bisphenol-A on spatial memory and synaptic plasticity of the hippocampus in mice

Bisphenol-A (BPA) is a common environmental endocrine disruptor. Our recent studies found that exposure to BPA in both adolescent and adulthood sex-specifically impaired spatial memory in male mice. In this study, 11-week-old gonadectomied (GDX) male mice daily received subcutaneous injections of testosterone propionate (TP, 0.5 mg/kg), TP and BPA (0.4 and 4 mg/kg), or vehicle for 45 days. The results of Morris water maze task showed that exposure to BPA did not affect the spatial memory of GDX mice but impaired that of sham (4 mg/kg/day) and TP-treated GDX mice (0.4 mg/kg/day). In addition, BPA reduced the level of testosterone (T) in the serum and brain of sham and TP-treated GDX mice. Exposure to BPA decreased the synaptic density and had an adverse effect on the synaptic interface of the hippocampus in sham and TP-treated GDX mice. The results of western blot analysis further showed that BPA (4 mg/kg) reduced the levels of synaptic proteins (synapsin I and PSD-95) and NMDA receptor subunit NR2B in sham and TP-treated GDX mice. BPA decreased the phosphorylation of ERK1/2 but increased the phosphorylation of p38 in sham and TP-treated GDX mice. These results suggest that impairment of spatial memory and adverse effects on synaptic remodeling of hippocampal neurons in males after long-term BPA exposure is related to the anti-androgen effect of BPA. These effects of BPA may be associated with downregulated synaptic proteins and NMDA receptor through inhibiting ERKs and promoting the p38 pathways.     

Synaptic plasticity, memory and the hippocampus: a neural network approach to causality

Two facts about the hippocampus have been common currency among neuroscientists for several decades. First, lesions of the hippocampus in humans prevent the acquisition of new episodic memories; second, activity-dependent synaptic plasticity is a prominent feature of hippocampal synapses. Given this background, the hypothesis that hippocampus-dependent memory is mediated, at least in part, by hippocampal synaptic plasticity has seemed as cogent in theory as it has been difficult to prove in practice. Here we argue that the recent development of transgenic molecular devices will encourage a shift from mechanistic investigations of synaptic plasticity in single neurons towards an analysis of how networks of neurons encode and represent memory, and we suggest ways in which this might be achieved. In the process, the hypothesis that synaptic plasticity is necessary and sufficient for information storage in the brain may finally be validated.     

PeptiSite: A structural database of peptide binding sites in 4D

We developed PeptiSite, a comprehensive and reliable database of biologically and structurally characterized peptide-binding sites, in which each site is represented by an ensemble of its complexes with protein, peptide and small molecule partners. The unique features of the database include: (1) the ensemble site representation that provides a fourth dimension to the otherwise three dimensional data, (2) comprehensive characterization of the binding site architecture that may consist of a multimeric protein assembly with cofactors and metal ions and (3) analysis of consensus interaction motifs within the ensembles and identification of conserved determinants of these interactions. Currently the database contains 585 proteins with 650 peptide-binding sites. http://peptisite.ucsd.edu/ link allows searching for the sites of interest and interactive visualization of the ensembles using the ActiveICM web-browser plugin. This structural database for protein–peptide interactions enables understanding of structural principles of these interactions and may assist the development of an efficient peptide docking benchmark.     

Developmental plasticity in neural circuits controlling birdsong: sexual differentiation and the neural basis of learning.

In many species of passerine songbirds, males learn their song during defined periods of life. Female song is often reduced or absent, as are the brain regions controlling song. Sexual differences in the brain arise because of the action of sex steroids, which trigger the formation of some neural pathways (especially the pathway from the higher vocal center to the robust nucleus) and prevent the atrophy of others in males. These neural changes occur during periods of developmental song learning and can recur during periods of learning in adult birds. The process of learning is correlated with major increases or decreases in the numbers of neurons in specific neuronal populations, suggesting that the formation or loss of specific neural pathways regulates the ability to learn. Species differences in sexual differentiation and learning allow informative cross-species comparisons of neural structure and behavior.     

Polysialytation as a regulator of neural plasticity in rodent learning and aging

Although generally accepted to play an important role in development, the precise functional significance of NCAM remains to be elucidated. Correlative and interventive studies suggest a role for polysialylated NCAM in neurite elaboration. In the adult NCAM polysialylation continues to be expressed in regions of the central nervous system which retain neuroplastic potential. During memory formation modulation of polysialylation on the synapse-enriched isoform of NCAM occurs in the hippocampus. The polysialylated neurons of this structure have been located at the border of the granule cell layer and hilar region of the dentate and their number increases dramatically during memory consolidation. The converse is also true for a profound decline in the basal number of polysialylated neurons occurs with ageing when neural plasticity becomes attenuated. In conclusion, it is suggested that NCAM polysialylation regulates ultrastructural plasticity associated with synaptic elaboration.Abbreviations PSA polysialic acid - NCAM neural cell adhesion molecule - SGL sub-granular cell layer - MF mossy fibers Special issue dedicated to Dr. Robert Balazs.     

A developmentally regulated switch in neuronal responsiveness to NCAM and N-cadherin in the rat hippocampus.

Monolayers of control 3T3 fibroblasts and 3T3 cells expressing transfected NCAM or N-cadherin have been used as a culture substratum for rat hippocampal neurons. Both NCAM and N-cadherin are expressed in the hippocampus through embryonic day 17 (E17) to postnatal day 4 (PND4); however, whereas E17 neurons responded to transfected NCAM by extending considerably longer neurites, PND4 neurons responded very poorly. The converse was true for responsiveness to N-cadherin. These data demonstrate a switch in neuronal responsiveness to NCAM and N-cadherin in the developing hippocampus. NCAM-dependent neurite outgrowth from E17 neurons was largely dependent on the presence of alpha 2-8-linked polysialic acid (PSA) on neuronal NCAM. NCAM-dependent neurite outgrowth could be fully inhibited by pertussis toxin or a combination of L- and N-type calcium channel antagonists thus providing direct evidence concerning the nature of the second messenger pathway activated in primary neurons by cell adhesion molecules (CAMs).     

A synthetic peptide ligand of neural cell adhesion molecule (NCAM), C3d,promotes neuritogenesis and synaptogenesis and modulates presynaptic function in primary cultures of rat hippocampal neurons

The neural cell adhesion molecule (NCAM) plays a key role in morphogenesis of the nervous system and in remodeling of neuronal connections accompanying regenerative and cognitive processes. Recently, a new synthetic ligand of NCAM, the C3-peptide, which binds to the NCAM IgI module, has been identified by means of combinatorial chemistry (R?nn, L. C. B, Olsen, M., Ostergaard, S., Kiselyov, V., Berezin, V., Mortensen, M. T., Lerche, M. H., Jensen, P. H., Soroka, V., Saffell, J. L., Doherty, P., Poulsen, F. M., Bock, E., Holm, A., and Saffells, J. L. (1999) Nat. Biotechnol. 17, 1000-1005). In vitro, the dendrimeric form of C3, termed C3d, disrupts NCAM-mediated cell adhesion, induces neurite outgrowth, and triggers intracellular signaling cascades similar to those activated by homophilic NCAM binding. The peptide may therefore be expected to regulate regeneration and synaptic plasticity. Here we demonstrate that in primary cultures of hippocampal neurons: 1) C3d induces a sustained neuritogenic response, the neuritogenic activity of the compound being dependent on the dose, starting time, and duration of peptide application; 2) the peptide triggers the neuritogenic response by forming an adhesive substratum necessary for NCAM-mediated neurite formation and elongation; 3) C3d promotes synapse formation; and 4) C3d modulates the presynaptic function, causing a transient increase of the function at low (2 and 5 microm) doses and a reduction when applied at a higher concentration (10 microm). The effect of the peptide is dependent on the activation of the fibroblast growth factor receptor. We suggest that C3d may constitute a useful lead for the development of compounds for treatment of various neurodegenerative disorders.     

Altered expression of NCAM in hippocampus and cortex may underlie memory and learning deficits in rats with streptozotocin-induced diabetes mellitus

Neurological and structural changes are paralleled by cognitive deficits in diabetes mellitus. The present study was designed to evaluate the expression of neural cell adhesion molecules (NCAM) in the hippocampus, cortex and cerebellum and to examine cognitive functions in diabetic rats. Diabetes was induced in male albino rats via intraperitoneal streptozotocin injection. Learning and memory behaviors were investigated using a passive avoidance test and a spatial version of the Morris water maze test. NCAM expression was detected in the hippocampus, cortex and cerebellum by an immunoblotting method. The diabetic rats developed significant impairment in learning and memory behaviours as indicated by deficits in passive avoidance and water maze tests as compared to control rats. Expression of NCAM 180 and 120 kDa were found to be higher in hippocampus and cortex of diabetic rat brains compared to those of control, whereas expression of NCAM 140 kDa decreased in these brain regions. Our findings suggest that streptozotocin-induced diabetes impairs cognitive functions and causes an imbalance in expression of NCAM in those brain regions involved in learning and memory. Altered expression of NCAM in hippocampus may be an important cause of learning and memory deficits that occur in diabetes mellitus.     

Characterization, distribution and plasticity of atrial natriuretic factor binding sites in brain

The [125I]iodotyrosyl derivative of atrial natriuretic factor [( 125I])ANF) apparently binds to a single class of high affinity sites in guinea pig brain membrane preparations. Ligand selectivity pattern reveals that the structural requirements of brain [125I]ANF binding sites are similar to those reported in most peripheral tissues. In vitro receptor autoradiographic studies demonstrate that the brain distribution of [125I]ANF binding sites is species dependent. In rat, high levels of binding are found in olfactory bulb, subfornical organ, area postrema, choroid plexus, and ependyma. In guinea pig, these regions are also enriched with [125I]ANF binding in addition to various thalamic nucleic, amygdala, hippocampus, and cerebellum. In monkey, high densities of sites are seen in the cerebellar cortex. This suggests that brain ANF receptor sites could mediate ANF effects related to the central integration of cardiovascular parameters, as well as other actions not associated with these systems. As in the periphery, it appears that brain [125I]ANF binding sites are associated with guanylate cyclase. Moreover, the density of [125I]ANF receptor binding sites is altered in certain brain regions in spontaneously hypertensive rats and in cardiomyopathic hamsters, demonstrating the plasticity of brain ANF receptors. Thus, ANF and ANF receptors are complementary facets of a new neurotransmitter-neuromodulator system present in mammalian brain.