Different polysialic acid-neural cell adhesion molecule expression patterns in distinct types of mossy fiber boutons in the adult hippocampus

Vascular adventitia generates neuronal progenitors in the monkey hippocampus after ischemia

In the adult hippocampus, neurogenesis proceeds in the subgranular zone (SGZ) of the dentate gyrus (DG), but not in the cornu Ammonis (CA). Recently, we demonstrated in monkeys that transient brain ischemia induces an increase of the neuronal progenitor cells in the SGZ, but not in CA1, in the second week after the insult. To identify the origin of primary neuronal progenitors in vivo, we compared the postischemic monkey DG and CA1, using light and electron microscopy, focusing on specific phenotype markers, as well as the expression of neurotrophic factors. Laser confocal microscopy showed that 1-3% of 5-bromo-2'-deoxyuridine (BrdU)-positive cells in the SGZ after 2-96 h labeling were also positive for neuronal markers such as TUC4, betaIII tubulin, and NeuN on days 9 and 15. In contrast, despite the presence of numerous BrdU-positive cells, CA1 showed no neurogenesis at any time points, and all the progenitors were positive for glial markers: Iba1 or S-100beta on days 4, 9, and 15. Highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive cells were abundant in the SGZ, but were absent in CA1. On day 9, most of the immature neurons positive for betaIII-tubulin in SGZ showed an increase in PSA-NCAM immunoreactivity. The immunoreactivity of brain-derived neurotrophic factor (BDNF) was abundant at the vascular adventitia of the SGZ, but was absent at the adventitia of CA1. BrdU-positive progenitor cells were frequently seen in the vicinity of proliferating blood vessels. Ultrastructural analysis indicated that most of the neuronal progenitor cells and microglia originated from the pericytes of capillaries and/or adventitial cells of arterioles (called vascular adventitia). The detaching adventitial cells showed mitotic figures in the perivascular space, and the resultant neuronal progenitor cells made contact with dendritic spines associated with synaptic vesicles or boutons. These data implicate the vascular adventitia as a novel potential source of neuronal progenitor cells in the postischemic primate SGZ.

Cell types, lineage, and architecture of the germinal zone in the adult dentate gyrus

New neurons continue to be born in the subgranular zone (SGZ) of the dentate gyrus in the hippocampus of adult mammals, including humans. Previous work has shown that astrocytes function as the progenitors of these new neurons through immature intermediate D cells. In the first part of the present study, we determined the structure of each of these progenitors and how they are organized in three dimensions. Serial-section reconstructions of the SGZ, using confocal and electron microscopy demonstrate that SGZ astrocytes form baskets that hold clusters of D cells, largely insulating them from the hilus. Two types of glial fibrillary acidic protein-expressing astrocytes (radial and horizontal) and three classes of doublecortin and PSA-NCAM-positive D cells (D1, D2, D3) were observed. Radial astrocytes appear to interact closely with clusters of D cells forming radial proliferative units. In the second part of this study, we show that retrovirally labeled radial astrocytes give rise to granule neurons. We also used bromodeoxyuridine and [3H]thymidine labeling to study the sequence of appearance of the different D cells after a 7-day treatment with anti-mitotics. This analysis, together with retroviral labeling data, suggest that radial astrocytes divide to generate D1 cells, which in turn divide once to form postmitotic D2 cells. D2 cells mature through a D3 stage to form new granule neurons. These observations provide a model of how the germinal zone of the adult hippocampus is organized and suggest a sequence of cellular stages in the generation of new granule neurons.

Neurodegeneration and plasticity

Neurofibrillary degeneration, associated with the formation of paired helical filaments (PHF), is one of the critical neuropathological hallmarks of Alzheimer's disease (AD). Although the microtubule-associated protein tau in a hyperphosphorylated form has been established as primary PHF constituent, the process of tau phosphorylation and its potential link to degeneration is not very well understood, mostly because of the lack of a physiological in vivo model of PHF-like tau phosphorylation. PHF formation in AD follows a hierarchical pattern of development throughout different cortical areas, which closely matches the pattern of neuronal plasticity in the adult brain. Those brain areas are most early and most severely affected which are involved in the regulation of memory, learning, perception, self-awareness, consciousness, and higher brain functions that require a life-long re-fitting of connectivity, a process based on a particularly high degree of plasticity. Failures of synaptic plasticity are, thus, assumed to represent early events in the course of AD that eventually lead to alteration of tau phosphorylation. Recently, we have used the hibernation cycle, a physiological model of adaptation associated with an extraordinary high degree of structural neuronal plasticity, to analyze the potential link between synaptic plasticity, synaptic detachment and the regulation of tau phosphorylation. During torpor, a natural state of hypothermia, synaptic contacts between mossy fibers and hippocampal pyramidal neurons undergo dramatic regressive changes that are fully reversible very rapidly during euthermy. This rapid, reversible, and repeated regression of synaptic and dendritic components on CA3 neurons is associated with a reversible PHF-like phosphorylation of tau at a similar time course. The repeated formation and degradation of PHF-tau might, thus, represent a physiological mechanism not necessarily associated with pathological effects. These findings implicate an essential link between neuronal plasticity and PHF-like phosphorylation of tau, potentially involved in neurofibrillary degeneration.

Neurobiological and endocrine correlates of individual differences in spatial learning ability

The polysialylated neural cell adhesion molecule (PSA-NCAM) has been implicated in activity-dependent synaptic remodeling and memory formation. Here, we questioned whether training-induced modulation of PSA-NCAM expression might be related to individual differences in spatial learning abilities. At 12 h posttraining, immunohistochemical analyses revealed a learning-induced up-regulation of PSA-NCAM in the hippocampal dentate gyrus that was related to the spatial learning abilities displayed by rats during training. Specifically, a positive correlation was found between latency to find the platform and subsequent activated PSA levels, indicating that greater induction of polysialylation was observed in rats with the slower acquisition curve. At posttraining times when no learning-associated activation of PSA was observed, no such correlation was found. Further experiments revealed that performance in the massed water maze training is related to a pattern of spatial learning and memory abilities, and to learning-related glucocorticoid responsiveness. Taken together, our findings suggest that the learning-related neural circuits of fast learners are better suited to solving the water maze task than those of slow learners, the latter relying more on structural reorganization to form memory, rather than the relatively economic mechanism of altering synaptic efficacy that is likely used by the former.

Neural cell adhesion molecule (NCAM) null mice do not show a deficit in odour discrimination learning

Polysialylated neural cell adhesion molecule (PSA-NCAM) is predominantly expressed during development where it regulates biological functions including axon targeting and neuronal precursor cell migration. Although dramatically down regulated after birth in most regions of the nervous system, PSA-NCAM remains highly expressed into adulthood in areas that have ongoing regeneration and plasticity such as in the olfactory bulb and hippocampus. Consequently, lack of PSA-NCAM in NCAM null mice results in distinct morphological changes to these areas. The functional correlates of these changes are not well defined although there have been reports that learning is impaired in NCAM null mice. In the present study, we assessed the ability of old and young NCAM null mice to learn an odour discrimination task. We tested male and female experimental and control animals of two different ages: 30-60 days and 12-15 months. During 4 days of training, NCAM null and C57BL/6J received trials where one odour (CS+) was paired with sugar while another odour (CS-) was not. In a subsequent preference test, conducted in the absence of sugar, all animals, regardless of strain or age, spent significantly more time digging in the CS+ odour than in the CS- odour. In addition, there was no significant difference in digging behaviour in the CS+ between the NCAM null and the control animals. These data indicate that deletion of the NCAM gene may change the morphology of the olfactory bulb but does not interfere with the ability to learn an odour discrimination task.

Astrocyte mitochondrial mechanisms of ischemic brain injury and neuroprotection

Research on ischemic brain injury has established a central role of mitochondria in neuron death. Astrocytes are also damaged by ischemia, although the participation of mitochondria in their injury is ill defined. As astrocytes are responsible for neuronal metabolic and trophic support, astrocyte dysfunction will compromise postischemic neuronal survival. Ischemic alterations to astrocyte energy metabolism and the uptake and metabolism of the excitatory amino acid transmitter glutamate may be particularly important. Despite the significance of ischemic astrocyte injury, little is known of the mechanisms responsible for astrocyte death and dysfunction. This review focuses on differences between astrocyte and neuronal metabolism and mitochondrial function, and on neuronal-glial interactions. The potential for astrocyte mitochondria to serve as targets of neuroprotective interventions is also discussed.

Involvement of highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive granule cells in the amygdaloid-kindling-induced sprouting of a hippocampal mossy fiber trajectory

The mossy fiber system in the hippocampus of amygdaloid-kindled rats was examined by using highly polysialylated neural cell adhesion molecule (PSA-NCAM) as a marker for immunohistochemical detection of immature dentate granule cells and mossy fibers in combination with bromodeoxyuridine (BrdU) labeling of newly generated granule cells. Statistically significant increases in BrdU-labeled cells and PSA-NCAM-positive cells occurred in the dentate gyrus following kindling. The increase in PSA-NCAM-immunoreactive neurites was confined to the entire stratum lucidum of CA3. Immunoelectron-microscopic examination also revealed that PSA-NCAM-positive immature synaptic terminals of the sprouting mossy fibers increased in the stratum lucidum of CA3 in the kindled rats. The increase in the numbers of PSA-NCAM-positive granule cells correlated well with the increase in the immunopositive neurites and synaptic terminals on the mossy fiber trajectory. The increase in these PSA-NCAM-immunopositive structures is thought to reflect the enhancement of sprouting and synaptogenesis of mossy fibers by a subset of granule cells newly generated during amygdaloid-kindling and suggests that the reorganization of the mossy fiber system on the normal trajectory at least in part contributes to the acquisition and maintenance of an epileptogenic state.

Reversible paired helical filament-like phosphorylation of tau is an adaptive process associated with neuronal plasticity in hibernating animals

Neurofibrillary pathology [paired helical filaments (PHFs)] formed by the microtubule-associated protein tau in a hyperphosphorylated form is a major hallmark of Alzheimer's disease and related disorders. The process of tau phosphorylation, thought to be of critical importance for PHF formation, and its potential link to neurodegeneration, however, is not understood very well, mostly because of the lack of a physiological in vivo model of PHF-like tau phosphorylation. Here we describe the formation of highly phosphorylated tau, containing a number of PHF-like epitopes in torpor during hibernation. PHF-like phosphorylation of tau was not associated with fibril formation and was fully reversible after arousal. Distribution of PHF-like tau followed a consistent pattern, being most intense in the entorhinal cortex, hippocampus, and isocortical areas. Within the hippocampus, a particularly high labeling was seen in CA3 pyramidal cells. Somewhat lesser reactivity was present in CA1 neurons while dentate gyrus granule cells were not reactive. Formation of PHF-like tau in CA3 neurons was paralleled by the regression of synaptic contacts of the mossy fiber system terminating on CA3 apical dendrites. Mossy fiber afferentation was re-established during arousal, concomitantly with the decrease of PHF-like tau in CA3 neurons. These findings implicate an essential link between neuronal plasticity and PHF-like phosphorylation of tau. The repeated formation and degradation of PHF-like tau might, thus, represent a physiological mechanism not necessarily associated with pathological effects. Hibernation will, therefore, be a valuable model to study the regulation of PHF-like tau-phosphorylation and its cell biological sequelae under physiological in vivo conditions.

Neuronal mechanism of nociceptin-induced modulation of learning and memory: involvement of N-methyl-D-aspartate receptors

Nociceptin (also called orphanin FQ) is an endogenous heptadecapeptide that activates the opioid receptor-like 1 (ORL1) receptor. Nociceptin system not only affects the nociception and locomotor activity, but also regulates learning and memory in rodents. We have previously reported that long-term potentiation and memory of ORL1 receptor knockout mice are enhanced compared with those in wild-type mice. Here, we show the neuronal mechanism of nociceptin-induced modulation of learning and memory. Retention of fear-conditioned contextual memory was significantly enhanced in the ORL1 receptor knockout mice without any changes in cued conditioned freezing. Inversely, in the wild-type mice retention of contextual, but not cued, conditioning freezing behavior was suppressed by exogenous nociceptin when it was administered into the cerebroventricle immediately after the training. ORL1 receptor knockout mice exhibited a hyperfunction of N-methyl-D-aspartate (NMDA) receptor, as evidenced by an increase in [3H]MK-801 binding, NMDA-evoked 45Ca2+ uptake and activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity and its phosphorylation as compared with those in wild-type mice. The NMDA-induced CaMKII activation in the hippocampal slices of wild-type mice was significantly inhibited by exogenous nociceptin via a pertussis toxin-sensitive pathway. However, the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor GluR1 subunit at Ser831 and Ser845, and NMDA receptor subunit NR2B at Thr286 were phosphorylated similarly after NMDA receptor stimulation in both type of mice. The expressions of GluR1 and GluR2 also did not change, but the levels of polysialylated form of neuronal cell adhesion molecule (N-CAM) were reduced in the ORL1 receptor knockout as compared with wild-type mice. These results suggest that nociceptin system negatively modulates learning and memory through the regulation of NMDA receptor function and the expression of N-CAM.

Polysialic acid in human milk. CD36 is a new member of mammalian polysialic acid-containing glycoprotein

The neural cell adhesion molecule and the voltage-sensitive sodium channel alpha-subunit are the only two molecules in mammals known to be modified by alpha-2,8-linked polysialic acid (polySia). We found a new polySia-containing glycoprotein in human milk and identified it as CD36, a member of the B class of the scavenger receptor superfamily. The polySia-containing glycan chain(s) were removed by alkaline treatment but not by peptide:N-glycanase F digestion, indicating that milk CD36 contained polySia on O-linked glycan chain(s). Polysialylation of CD36 occurs not only in human milk but also in mouse milk. However, CD36 in human platelets is not polysialylated. PolySia CD36 is secreted in milk at any lactation stage and reaches peak level at 1 month after parturition. Thus, it is suggested that polySia of milk CD36 is significant for neonatal development in terms of protection and nutrition.

Calretinin/PSA-NCAM immunoreactive granule cells after hippocampal damage produced by kainic acid and DEDTC treatment in mouse

There is a dramatic increase in the number of lightly immunoreactive calretinin cells in the granular layer of the dentate gyrus of the mouse hippocampus 1 day after excitotoxic injury using kainic acid combined with the zinc chelator diethyldithiocarbamate. At 7 days after treatment, these cells are strongly immunoreactive for calretinin and for the polysialated form of the glycoprotein neural cell adhesion molecule (PSA-NCAM). The reexpression of calretinin and PSA-NCAM after treatment corresponds well with the loss of input from the damaged hilar mossy cells. These cells could be considered immature granule cells since they are immunoreactive to markers for immature cells such as PSA-NCAM, and are not immunoreactive to calbindin D28k and neuronal nuclear specific protein NeuN (present in mature granule cells), or GABA (present in interneurons). Ultrastructural analysis of these cells indicates that they are immature. Labelling of cell proliferation with 5-bromo-2'-deoxyuridine (BrdU) shows that by day 1 no calretinin immunoreactive cell of the dentate gyrus corresponds to newly generated cells. By day 7 only 6% of the calretinin immunoreactive cells in the dentate gyrus are marked for BrdU. Our data indicate that the CR/PSA-NCAM immunoreactive cells of the dentate gyrus, in spite of their immature characteristics, are not the products of reactive neurogenesis. These cells could represent a reservoir of pre-existing not completely differentiated granule cells that react to damage.

Repeated restraint stress suppresses neurogenesis and induces biphasic PSA-NCAM expression in the adult rat dentate gyrus

Chronic restraint stress has been shown to induce structural remodelling throughout the interconnected dentate gyrus-CA3 fields. To find out how this stressor affects the rate of adult hippocampal neurogenesis, we subjected rats to acute or chronic restraint stress and assessed the proliferation, survival and differentiation of newly born cells in the dentate gyrus. We also examined polysialylated neural cell adhesion molecule expression, a molecule normally expressed in immature neurons and important for morphological plasticity. The results show that acute restraint stress did not change either the proliferation of dentate gyrus precursor cells or the expression of polysialylated neural cell adhesion molecule, whereas 3 weeks of chronic restraint stress suppressed proliferation by 24% and increased polysialylated neural cell adhesion molecule expression by 40%. The study was extended for an additional 3 weeks to trace the survival and development of the cells born after the initial 3 weeks of restraint. Rats subjected to 6 weeks of daily restraint stress exhibited suppressed cell proliferation and attenuated survival of the recently born cells after the extended time course, resulting in a 47% reduction of granule cell neurogenesis. Furthermore, 6 weeks of chronic stress significantly reduced the total number of granule cells by 13% and the granule cell layer volume by 5%. Expression of polysialylated neural cell adhesion molecule followed a biphasic time course, displaying a significant up-regulation after 3 weeks of daily restraint stress that was lost after 6 weeks of stress. These studies may help us understand the basis for hippocampal shrinkage and raise questions about the ultimate reversibility of the effects of chronic stress.

CRMP-4 expression in the adult cerebral cortex and other telencephalic areas of the lizard Podarcis hispanica

The control of neuritogenesis is crucial for the development, maturation and regeneration of the nervous system. The collapsin response-mediated protein 4 (CRMP-4) is a member of a family of proteins that are involved in neuronal differentiation and axonal outgrowth. In rodents, this protein is expressed in recently generated neurons such as some granule neurons of the dentate gyrus, as well as in certain differentiated neurons undergoing neurite outgrowth or synaptogenesis during adulthood. Since CRMP-4 protein appears to be highly conserved throughout the evolutionary scale, we have used immunocytochemistry to study its distribution in the lizard cerebral cortex. We have found pronounced CRMP-4 immunolabeling in certain neurons of the medial cortex, the homologous region to the dentate gyrus, but also in the dorsal and lateral cortices. Double labeling with 5'-BrdU indicated that these medial cortex neurons were recently generated. However, it is also possible that many of these cells were not new but undergoing some kind of plasticity implicating neurite outgrowth. Similar CRMP-4-labeled neurons and processes were observed in subcortical regions as the PDVR and the nucleus sphericus. Our results show for the first time the expression of CRMP-4 in a reptile brain, where it appears to be expressed in regions where adult neurogenesis and/or neurite outgrowth occur.

PSA-NCAM immunocytochemistry in the cerebral cortex and other telencephalic areas of the lizard Podarcis hispanica: differential expression during medial cortex neuronal regeneration

The lizard medial cortex, a region homologous to the mammalian dentate gyrus, shows postnatal neurogenesis and the surprising ability to replace its neurons after being lesioned specifically with the neurotoxin 3-acetylpyridine. As the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) is expressed during neuronal migration and differentiation, we have studied its distribution in adult lizards and also during the lesion-regeneration process. In the medial cortex of control animals, many labeled fusiform somata, presumably corresponding to migratory neuroblasts, appeared in the inner plexiform layer. There were also scattered immunoreactive granule neurons in the cell layer. Double immunocytochemistry with 5'-bromodeoxyuridine revealed that some of the PSA-NCAM-expressing cells in the inner plexiform and cell layers were generated recently. PSA-NCAM immunoreactivity was also present in the dorsomedial, dorsal, and lateral cortices, as well as in the dorsal ventricular ridge, the nucleus accumbens, and the nucleus sphericus. Twelve hours after the injection of 3-acetylpyridine, some medial cortex granule neurons appeared degenerated, although some of them still expressed PSA-NCAM. One to 2 days after the injection, most granule neurons appeared degenerated and no PSA-NCAM immunoreactivity was detected in the medial cortex cell layer. Four to 7 days after treatment, abundant labeled fusiform cells populated the inner plexiform layer and some immunoreactive somata were seen in the cell layer. Fifteen to 30 days after the neurotoxin injection, the number of PSA-NCAM expressing granule neurons augmented considerably and the level was still above control levels in lizards that survived 42 days. Our results show for the first time the expression of PSA-NCAM in a reptile brain, where it appears to participate in the migration and differentiation of granule neurons during adult neurogenesis and regeneration.

Hippocampal adult neurogenesis occurs in a microenvironment provided by PSA-NCAM-expressing immature neurons

Neurons continue to be generated in the adult hippocampus. In the present study, the early developmental processes of newly generated neurons in the adult rat hippocampus were examined by confocal laser scanning microscopy using a combination of bromodeoxyuridine (BrdU) labeling and immunohistochemistry for highly polysialylated neural cell adhesion molecule (PSA-NCAM) and NeuroD, which are markers for immature neurons, and glial fibrillary acidic protein (GFAP). Rats were injected with BrdU and 2 hours, 1, 3, and 7 days after the injection, the hippocampus was processed for immunohistochemistry. One day after the injection, BrdU-labeled cells were found frequently in clusters consisting of dividing cells, putative undifferentiated cells, NeuroD-positive differentiated neurons, and GFAP-positive astrocytes. Three days later, BrdU-labeled cells were loosely aggregated and BrdU-positive fragmented nuclei were sometimes observed, suggesting that apoptosis occurred in the clusters. These BrdU-labeled nuclei were frequently associated in various ways with the processes of immature PSA-positive granule cells. They are positioned along PSA-positive apical and basal dendrites or surrounded by these processes. By 7 days after the injection, the number of the clusters was reduced and the BrdU-labeled cells had developed dendrites. These cell-to-cell associations support the hypothesis that the clustering and a microenvironment provided by the PSA-expressing immature neurons contribute to the early developmental events of adult neurogenesis, such as proliferation, differentiation, apoptosis, and neurophilic migration in the adult hippocampus.

Non-granule PSA-NCAM immunoreactive neurons in the rat hippocampus

The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) continues to be expressed in the adult hippocampus, mainly in a subset of neurons located in the innermost portion of the granule cell layer. PSA-NCAM immunoreactive neurons have also been described outside this layer in humans, where they are severely reduced in schizophrenic brains. Given this important clinical implication, we were interested in finding whether similar neurons existed in the adult rat hippocampus and to characterize their distribution, morphology and phenotype. PSA-NCAM immunocytochemistry reveals labeled neurons in the subiculum, fimbria, alveus, hilus, and stratum oriens, lucidum and radiatum of CA3 and CA1. They are mainly distributed in the ventral hippocampus, and have polygonal or fusiform somata with multipolar or bipolar morphology. These neurons show long straight dendrites, which reach several strata and even enter the fimbria and the alveus. These dendrites are often varicose, appear devoid of excrescences and apparently do not show spines. Most of these neurons display GABA immunoreactivity and further analysis has shown that a subpopulation expresses calretinin, but not somatostatin, neuropeptide Y, parvalbumin, calbindin or NADPH diaphorase. Our study demonstrates that there is an important subpopulation of PSA-NCAM immunoreactive neurons, many of which can be considered interneurons, outside the rat granule cell layer, probably homologous to those described in the human hippocampus. The presence of the polysialylated form of NCAM in these neurons could indicate that they are undergoing continuous remodeling during adulthood and may have an important role in hippocampal structural plasticity.

Immunoelectron microscopic localization of the neural recognition molecules L1, NCAM, and its isoform NCAM180, the NCAM-associated polysialic acid, beta1 integrin and the extracellular matrix molecule tenascin-R in synapses of the adult rat hippocampus

We have investigated the possibility that morphologically different excitatory glutamatergic synapses of the "trisynaptic circuit" in the adult rodent hippocampus, which display different types of long-term potentiation (LTP), may express the immunoglobulin superfamily recognition molecules L1 and NCAM, the extracellular matrix molecule tenascin-R, and the extracellular matrix receptor constituent beta1 integrin in a differential manner. The neural cell adhesion molecules L1, NCAM (all three major isoforms), NCAM180 (the largest major isoform with the longest cytoplasmic domain), beta1 integrin, polysialic acid (PSA) associated with NCAM, and tenascin-R were localized by pre-embedding immunostaining procedures in the CA3/CA4 region (mossy fiber synapses) and in the dentate gyrus (spine synapses) of the adult rat hippocampus. Synaptic membranes of mossy fiber synapses where LTP is expressed presynaptically did not show detectable levels of immunoreactivity for any of the molecules/epitopes studied. L1, NCAM, and PSA, but not NCAM180 or beta1 integrin, were detectable on axonal membranes of fasciculating mossy fibers. In contrast to mossy fiber synapses, spine synapses in the outer third of the molecular layer of the dentate gyrus, which display postsynaptic expression mechanisms of LTP, were both immunopositive and immunonegative for NCAM, NCAM180, beta1 integrin, and PSA. Those spine synapses postsynaptically immunoreactive for NCAM or PSA also showed immunoreactivity on their presynaptic membranes. NCAM180 was not detectable presynaptically in spine synapses. L1 could not be found in spine synapses either pre- or postsynaptically. Also, the extracellular matrix molecule tenascin-R was not detectable in synaptic clefts of all synapses tested, but was amply present between fasciculating axons, axon-astrocyte contact areas, and astrocytic gap junctions. Differences in expression of the membrane-bound adhesion molecules at both types of synapses may reflect the different mechanisms for induction and/or maintenance of synaptic plasticity.

Revisiting the function of PSA-NCAM in the nervous system

Since its first description the polysialylated form of NCAM (PSA-NCAM) is thought to be a major regulator of cell-cell interactions in the nervous system. Over the past few years many crucial questions have been answered concerning PSA biosynthesis and function. Among these are the identification and cloning of the key enzymes that are responsible for its synthesis and the fact that expression of PSA is not restricted to developmental stages but maintained in the adult nervous system. In the adult, PSA has been shown to be not only a marker of structural plasticity but seems to be a major player in these processes. Originally suggested to be a purely anti-adhesive factor, modulating cell-cell interactions in general and by this allowing plasticity, there is now increasing evidence that this might not be the whole story. Instead, it appears possible that PSA-NCAM interacts with secreted signaling molecules and by this fulfills a more instructive function in brain plasticity.

NMDA receptor and nitric oxide synthase activation regulate polysialylated neural cell adhesion molecule expression in adult brainstem synapses

Here we report that synapses in the adult dorsal vagal complex, a gateway for many primary afferent fibers, express a high level of the polysialylated neural cell adhesion molecule (PSA-NCAM). We show that electrical stimulation of the vagal afferents causes a rapid decrease of PSA-NCAM expression both in vivo and in acute slices. Inhibition of NMDA receptor activity completely prevented the decrease. Blockade of calmodulin activation, neuronal nitric oxide (NO) synthase, or soluble guanylyl cyclase and chelation of extracellular NO mimicked this inhibition. Our data provide a mechanistic framework for understanding how activity-linked stimulation of the NMDA-NO-cGMP pathway induces rapid changes in PSA-NCAM expression, which may be associated with long-term depression.

Widespread expression of rat collapsin response-mediated protein 4 in the telencephalon and other areas of the adult rat central nervous system

The rat collapsin response-mediated protein 4 (rCRMP-4) is a member of a family of proteins that are involved in axonal growth. It is found transiently in postmitotic neurons, such as those that are generated in the adult hippocampus. The authors used immunocytochemistry to investigate whether areas of the rat central nervous system (CNS) that retain postnatal neurogenesis express this protein. They found pronounced rCRMP-4 immunoreactivity in recently generated cells in the dentate granular layer, the subventricular zone, the olfactory bulbs, and the rostral migratory stream, four areas in which the production or migration of neurons occurs in adulthood. However, rCRMP-4 immunoreactivity also is expressed in many other regions of the rat brain in which there is no record of adult neurogenesis or neuronal migration, e.g., in the olfactory glomeruli and in neurons of the cerebral cortex. In the hypothalamus, intensely rCRMP-4-labeled neurons populated the supraoptic, paraventricular, and periventricular nuclei as well as the median eminence and the arcuate nucleus. Immunoreactivity for rCRMP-4 also was present in certain neurons of the interpeduncular nucleus, median raphe, superior colliculus, and scattered granule cerebellar neurons. Many of these regions are known to display axonal outgrowth and/or synaptic rearrangement in adulthood and to coexpress the polysialylated form of the neural cell adhesion molecule. Thus, the results of this study suggest that rCRMP-4 expression in the CNS is associated with cells that are migrating or are undergoing axonal growth. Nevertheless, small, rCRMP-4-immunoreactive cells were seen throughout the brain. These cells did not express neuronal, astroglial, or microglial markers, although some of them also were immunoreactive for rip antibody, suggesting an oligodendroglial lineage.

Mice deficient in the polysialyltransferase ST8SiaIV/PST-1 allow discrimination of the roles of neural cell adhesion molecule protein and polysialic acid in neural development and synaptic plasticity

Functional properties of the neural cell adhesion molecule (NCAM) are strongly influenced by polysialylation. We used gene-targeting to generate mice lacking ST8SiaIV/PST-1, one of the polysialyltransferases responsible for addition of polysialic acid (PSA) to NCAM. Mice homozygous for the null mutation reveal normal development of gross anatomical features. In contrast to NCAM-deficient mice, olfactory precursor cells in the rostral migratory stream express PSA and follow their normal pathway. Furthermore, delamination of mossy fibers in the hippocampal CA3 region, as found in NCAM-deficient mice, does not occur in ST8SiaIV mutants. However, during postnatal development these animals show a decrease of PSA in most brain regions compared to wild-type animals. Loss of PSA in the presence of NCAM protein but in the absence of obvious histological changes allowed us to directly address the role of PSA in synaptic plasticity. Schaffer collateral-CA1 synapses, which express PSA in wild types, showed impaired long-term potentiation (LTP) and long-term depression (LTD) in adult mutants. This impairment was age-dependent, following the time course of developmental disappearance of PSA. Contrary to NCAM mutant mice, LTP in ST8SiaIV mutants was undisturbed at mossy fiber-CA3 synapses, which do not express PSA in wild-type mice. The results demonstrate an essential role for ST8SiaIV in synaptic plasticity in hippocampal CA1 synapses, whereas PSA produced by different polysialyltransferase or polysialyltransferases at early stages of differentiation regulates migration of neural precursor cells and correct lamination of mossy fibers. We suggest that NCAM but not PSA is likely to be important for LTP in the hippocampal CA3 region.