Nerve-dependent expression of high polysialic acid neural cell adhesion molecule in neurohypophysial astrocytes of adult rats

Complete spatial characterisation of N-glycosylation upon striatal neuroinflammation in the rodent brain

BACKGROUND

Neuroinflammation is an underlying pathology of all neurological conditions, the understanding of which is still being comprehended. A specific molecular pathway that has been overlooked in neuroinflammation is glycosylation (i.e., post-translational addition of glycans to the protein structure). N-glycosylation is a specific type of glycosylation with a cardinal role in the central nervous system (CNS), which is highlighted by congenital glycosylation diseases that result in neuropathological symptoms such as epilepsy and mental retardation. Changes in N-glycosylation can ultimately affect glycoproteins' functions, which will have an impact on cell machinery. Therefore, characterisation of N-glycosylation alterations in a neuroinflammatory scenario can provide a potential target for future therapies.

METHODS

With that aim, the unilateral intrastriatal injection of lipopolysaccharide (LPS) in the adult rat brain was used as a model of neuroinflammation. In vivo and post-mortem, quantitative and spatial characterisation of both neuroinflammation and N-glycome was performed at 1-week post-injection of LPS. These aspects were investigated through a multifaceted approach based on positron emission tomography (PET), quantitative histology, reverse transcription-quantitative polymerase chain reaction (RT-qPCR), liquid chromatography and matrix-assisted laser desorption ionisation mass spectrometry imaging (MALDI-MSI).

RESULTS

In the brain region showing LPS-induced neuroinflammation, a significant decrease in the abundance of sialylated and core fucosylated structures was seen (approximately 7.5% and 8.5%, respectively), whereas oligomannose N-glycans were significantly increased (13.5%). This was confirmed by MALDI-MSI, which provided a high-resolution spatial distribution of N-glycans, allowing precise comparison between normal and diseased brain hemispheres.

CONCLUSIONS

Together, our data show for the first time the complete profiling of N-glycomic changes in a well-characterised animal model of neuroinflammation. These data represent a pioneering step to identify critical targets that may modulate neuroinflammation in neurodegenerative diseases.

Region-Specific Characterization of N-Glycans in the Striatum and Substantia Nigra of an Adult Rodent Brain

N-glycan alterations in the nervous system can result in different neuropathological symptoms such as mental retardation, seizures, and epilepsy. Studies have reported the characterization of N-glycans in rodent brains, but there is a lack of spatial resolution as either the tissue samples were homogenized or specific proteins were selected for analysis of glycosylation. We hypothesize that region-specific resolution of N-glycans isolated from the striatum and substantia nigra (SN) can give an insight into the establishment and pathophysiological degeneration of neural circuitry in Parkinson's disease. Specific objectives of the study include isolation of N-glycans from the rat striatum and SN; reproducibility, resolution, and relative quantitation of N-glycome using ultra-performance liquid chromatography (UPLC), weak anion exchange-UPLC, and lectin histochemistry. The total N-glycomes from the striatum and SN were characterized using database mining (GlycoStore), exoglycosidase digestions, and liquid chromatography-mass spectrometry. It revealed significant differences in complex and oligomannose type N-glycans, sialylation (mono-, di-, and tetra-), fucosylation (tri-, core, and outer arm), and galactosylation (di-, tri-, and tetra-) between striatum and SN N-glycans with the detection of phosphorylated N-glycans in SN which were not detected in the striatum. This study presents the most comprehensive comparative analysis of relative abundances of N-glycans in the striatum and SN of rodent brains, serving as a foundation for identifying "brain-type" glycans as biomarkers or therapeutic targets and their modulation in neurodegenerative disorders.

Sialic acids in the brain: gangliosides and polysialic acid in nervous system development, stability, disease, and regeneration

Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

Sequences prior to conserved catalytic motifs of polysialyltransferase ST8Sia IV are required for substrate recognition

Polysialic acid on the neural cell adhesion molecule (NCAM) modulates cell-cell adhesion and signaling, is required for proper brain development, and plays roles in neuronal regeneration and the growth and invasiveness of tumor cells. Evidence indicates that NCAM polysialylation is highly protein-specific, requiring an initial polysialyltransferase-NCAM protein-protein interaction. Previous work suggested that a polybasic region located prior to the conserved polysialyltransferase catalytic motifs may be involved in NCAM recognition, but not overall enzyme activity (Foley, D. A., Swartzentruber, K. G., and Colley, K. J. (2009) J. Biol. Chem. 284, 15505-15516). Here, we employ a competition assay to evaluate the role of this region in substrate recognition. We find that truncated, catalytically inactive ST8SiaIV/PST proteins that include the polybasic region, but not those that lack this region, compete with endogenous ST8SiaIV/PST and reduce NCAM polysialylation in SW2 small cell lung carcinoma cells. Replacing two polybasic region residues, Arg(82) and Arg(93), eliminates the ability of a full-length, catalytically inactive enzyme (PST H331K) to compete with SW2 cell ST8SiaIV/PST and block NCAM polysialylation. Replacing these residues singly or together in ST8SiaIV/PST substantially reduces or eliminates NCAM polysialylation, respectively. In contrast, replacing Arg(82), but not Arg(93), substantially reduces the ability of ST8SiaIV/PST to polysialylate neuropilin-2 and SynCAM 1, suggesting that Arg(82) plays a general role in substrate recognition, whereas Arg(93) specifically functions in NCAM recognition. Taken together, our results indicate that the ST8SiaIV/PST polybasic region plays a critical role in substrate recognition and suggest that different combinations of basic residues may mediate the recognition of distinct substrates.

Polysialic acid and activity-dependent synapse remodeling

Polysialic acid (PSA) is a large carbohydrate added post-translationally to the extracellular domain of the Neural Cell Adhesion Molecule (NCAM) that influences its adhesive and other functional properties. PSA-NCAM is widely distributed in the developing nervous system where it promotes dynamic cell interactions, like those responsible for axonal growth, terminal sprouting and target innervation. Its expression becomes restricted in the adult nervous system where it is thought to contribute to various forms of neuronal and glial plasticity. We here review evidence, obtained mainly from hypothalamic neuroendocrine centers and the olfactory system, that it intervenes in structural synaptic plasticity and accompanying neuronal-glial transformations, making possible the formation and elimination of synapses that occur under particular physiological conditions. While the mechanism of action of this complex sugar is unknown, it is now clear that it is a necessary molecular component of various cell transformations, including those responsible for activity-dependent synaptic remodeling.

Activity-Dependent Structural and Functional Plasticity of Astrocyte-Neuron Interactions

Observations from different brain areas have established that the adult nervous system can undergo significant experience-related structural changes throughout life. Less familiar is the notion that morphological plasticity affects not only neurons but glial cells as well. Yet there is abundant evidence showing that astrocytes, the most numerous cells in the mammalian brain, are highly mobile. Under physiological conditions as different as reproduction, sensory stimulation, and learning, they display a remarkable structural plasticity, particularly conspicuous at the level of their lamellate distal processes that normally ensheath all portions of neurons. Distal astrocytic processes can undergo morphological changes in a matter of minutes, a remodeling that modifies the geometry and diffusion properties of the extracellular space and relationships with adjacent neuronal elements, especially synapses. Astrocytes respond to neuronal activity via ion channels, neurotransmitter receptors, and transporters on their processes; they transmit information via release of neuroactive substances. Where astrocytic processes are mobile then, astrocytic-neuronal interactions become highly dynamic, a plasticity that has important functional consequences since it modifies extracellular ionic homeostasis, neurotransmission, gliotransmission, and ultimately neuronal function at the cellular and system levels. Although a complete picture of intervening cellular mechanisms is lacking, some have been identified, notably certain permissive molecular factors common to systems capable of remodeling (cell surface and extracellular matrix adhesion molecules, cytoskeletal proteins) and molecules that appear specific to each system (neuropeptides, neurotransmitters, steroids, growth factors) that trigger or reverse the morphological changes.

Injury to retinal ganglion cell axons increases polysialylated neural cell adhesion molecule (PSA-NCAM) in the adult rodent superior colliculus

The adult mammalian central nervous system (CNS) exhibits a limited regenerative response to injury. It is well established that polysialylated neural cell adhesion molecule (PSA-NCAM) contributes to nervous system plasticity. In the visual system, PSA-NCAM participates in retinal ganglion cell (RGC) axon growth during development and specifically influences RGC innervation of its principle target tissue, the superior colliculus (SC). The goals of this study were to determine whether PSA-NCAM is expressed in the normal adult mouse SC and to evaluate PSA-NCAM expression following RGC injury. In the normal rostral, but not caudal, SC we find that PSA-NCAM is present in the retinorecipient layers; however, PSA-NCAM and RGC axons do not co-localize. In the deeper collicular layers, PSA-NCAM is observed as several distinct patches that occur at the same depth along the medial-lateral axis throughout the colliculus. RGC axotomy denervates predominantly the contralateral colliculus, where increased PSA-NCAM levels are seen at 7 and 10 days after the injury. Further evaluation of the retinorecipient layers of the partially denervated SC reveals that some intact CTB-traced RGC axons (less than 5%) labeled from the ipsilateral eye do co-localize with PSA-NCAM. This study is the first characterization of PSA-NCAM expression in the normal and partially denervated adult SC and may indicate that PSA-NCAM is involved in attempted visual system remodeling after injury.

Use of polysialic acid in repair of the central nervous system

Polysialic acid (PSA), a large cell-surface carbohydrate that regulates cell interactions, is used during vertebrate development to promote precursor cell migration and axon path-finding. The induction of PSA expression in damaged adult CNS tissues could help them to rebuild by creating conditions permissive for architectural remodeling. This possibility has been explored in two contexts, the regeneration of axons and the recruitment of endogenous neural precursors to a lesion. Glial scars that form at CNS injury sites block axon regeneration. It has been found that transfection of scar astrocytes by a viral vector encoding polysialyltransferase leads to sustained expression of high levels of PSA. With this treatment, a substantial portion of severed corticospinal tract axon processes were able to grow through a spinal injury site. In the studies of precursor cell migration to a cortical lesion, it was found that induced PSA expression in a path extending from the subventricular zone to a lesion near the cortical surface increased recruitment of BrdU/nestin-positive cells along the path and into the injury site. These displaced precursors were able to differentiate in a regionally appropriate manner. These findings suggest that induced PSA expression can be used as a strategy for promoting tissue repair involving both replacement of cells and rebuilding of neural connections.

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.

Reorganization of the rat cerebellar cortex during postnatal development following cisplatin treatment

We examined the effects of the antitumor agent cisplatin on the development and plasticity of cerebellar cytoarchitecture. Since knowledge of the parallel and climbing fiber-Purkinje cell system is important in order to determine the architectural basis of cerebellar function, we used immunofluorescence for vesicular glutamate transporters (VGluT1 and VGluT2) to evaluate the trend of synaptogenesis of parallel and climbing fibers on Purkinje cells in the cerebellum vermis after a single injection of cisplatin to 10-day-old rats, i.e., during a crucial period of cerebellar development. The temporal and spatial patterns of VGluT1 and VGluT2 immunoreactivity after the early cisplatin injury provided evidence that remodeling of excitatory afferents and Purkinje cell dendrites occurs. After an early slow down of Purkinje cell dendrite growth, 7 days following the treatment, the extension of the molecular layer was reduced, as was parallel fiber innervation, but VGluT1 immunoreactive fibers contacted Purkinje cell dendrite branches extending within the external granular layer. VGluT2 immunopositive climbing fiber varicosities were still largely present on the soma and stem dendrites of Purkinje cells. Twenty days after the cisplatin injection, the thickness of the VGluT1 immunopositive molecular layer was reduced. VGluT2 climbing fiber varicosities were found on the remodeled Purkinje cell dendrites, as in controls, although at a lower density. Alterations in the immunoreactivity for polysialic acid neural cell adhesion molecule (PSA-NCAM) during the recovery phase suggest that this molecule plays a fundamental role not only during development, but also in the reorganization of neuroarchitecture. The changes were restricted to the neocerebellar vermis and were likely dependent on the different timing of lobule formation. The results of these investigations reveal the existence of vulnerability windows of the cerebellum to exposure to experimental or environmental cytotoxic agents during a critical period in development.

Remodeling of astrocytes, a prerequisite for synapse turnover in the adult brain? Insights from the oxytocin system of the hypothalamus

Neurons, including their synapses, are generally ensheathed by fine processes of astrocytes, but this glial coverage can be altered under different physiological conditions that modify neuronal activity. Changes in synaptic connectivity accompany astrocytic transformations so that an increased number of synapses are associated with reduced astrocytic coverage of postsynaptic elements, whereas synaptic numbers are reduced on reestablishment of glial coverage. A system that exemplifies activity-dependent structural synaptic plasticity in the adult brain is the hypothalamo-neurohypophysial system, and in particular, its oxytocin component. Under strong, prolonged activation (parturition, lactation, chronic dehydration), extensive portions of somatic and dendritic surfaces of magnocellular oxytocin neurons are freed of intervening astrocytic processes and become directly juxtaposed. Concurrently, they are contacted by an increased number of inhibitory and excitatory synapses. Once stimulation is over, astrocytic processes again cover oxytocinergic surfaces and synaptic numbers return to baseline levels. Such observations indicate that glial ensheathment of neurons is of consequence to neuronal function, not only directly, for example by modifying synaptic transmission, but indirectly as well, by preparing neuronal surfaces for synapse turnover.

A study of the role of neuro-glial remodeling in the oxytocin system at lactation

Under conditions of strong secretion of neurohypophysial hormone, such as during parturition, lactation and dehydration, the hypothalamic oxytocin-system displays a remarkable morphological plasticity such that astrocytic coverage of its neurones diminishes, their surfaces become directly juxtaposed and contacted by an increased number of synapses. A growing body of evidence indicates that these anatomical changes have an impact on glutamatergic neurotransmission in the supraoptic nucleus, and may be therefore of physiological consequence. We here evaluated the consequences of the inhibition of such plasticity on the overall activity of the oxytocin system during lactation. Remodeling was prevented by performing hypothalamic microinjections in gestating rats of endoneuraminidase, an enzyme that removes polysialic acid from the neural cell adhesion molecule. Our earlier studies established that the presence of polysialic acid is a prerequisite for remodeling of the oxytocin system in the supraoptic and paraventricular nuclei. In dams in which polysialic acid was absent in all magnocellular nuclei after bilateral endoneuraminidase injections, parturition was normal and neither the frequency nor the amplitude of suckling-induced reflex milk ejections was different from vehicle-treated dams. The weight gain of pups was also normal as was water intake by the dams. We then assessed the electrical activity of antidromically identified magnocellular neurones in the polysialic acid-free supraoptic nucleus of isoflurane-anesthetized lactating rats. Basal and bursting activity characteristic of oxytocin neurones before each reflex milk ejection was not significantly different from that recorded in the supraoptic nucleus of rats with normal levels of polysialic acid. Our results indicate that neuro-glial remodeling, despite its role on fine modulation of oxytocin neuronal activity, is not essential to parturition and lactation.

Oxytocin neuron activation in NCAM-deficient mice: anatomical and functional consequences

During stimulated neurosecretion in the rat, oxytocin neurons display a reduced glial coverage and receive an increased number of synapses, changes that are reversed on arrest of stimulation. We identified polysialic acid on the neural cell adhesion molecule (NCAM) as an important mediator of such plasticity. To investigate further the role of this cell surface glycoprotein, we examined the oxytocin system in mice genetically deficient in NCAM. First, ultrastructural analyses revealed that in wild-type mice, the supraoptic nucleus (SON) underwent the same remodelling as in the rat because oxytocin neurons had a diminished astrocytic coverage and increased synaptic input during lactation or chronic salt loading. Surprisingly, the SON displayed this morphology in NCAM-deficient mice as well, whether they were nongestating and hydrated, lactating or dehydrated. The oxytocin system in NCAM-deficient mice was abnormally hyperactive, as illustrated by enhanced plasma and intranuclear concentrations of oxytocin and reduced anxiety-related behaviour. Plasma oxytocin concentrations were also high in lactating NCAM-deficient dams but certain parameters of lactation and maternal behaviour were impaired. NCAM-deficient mice survived ingestion of 2% saline for 7 days and had increased plasma oxytocin but they did not cope with more severe osmotic challenges. Our observations highlight further the remarkable capacity of the adult oxytocin system to undergo neuronal and glial remodelling whenever it is activated. That lack of NCAM did not prevent remodelling indicates that NCAM can be substituted by other molecular mechanisms. Finally, while NCAM deficiency greatly enhanced oxytocin release, it led to impaired oxytocin-dependent physiological and behavioural responses.

Neuronal, glial and synaptic remodeling in the adult hypothalamus: functional consequences and role of cell surface and extracellular matrix adhesion molecules

The adult hypothalamo-neurohypophysial system (HNS) undergoes activity-dependent morphological plasticity which modifies astrocytic coverage of its oxytocinergic neurons and their synaptic inputs. Thus, during physiological conditions that enhance central and peripheral release of oxytocin (OT), adjacent somata and dendrites of OT neurons become extensively juxtaposed, without intervening astrocytic processes and receive an increased number of synapses. The morphological changes occur within a few hours and are reversible with termination of stimulation. The reduced astrocytic coverage has direct functional consequences since it modifies extracellular ionic homeostasis, synaptic transmission, and the size and geometry of the extracellular space. It also contributes indirectly to neuronal function by permitting formation of synapses on neuronal surfaces freed of astrocytic processes. Overall, such remodeling is expected to potentiate activated neuronal firing, especially in clusters of tightly packed neurons, an anatomical arrangement characterizing OT neurons. This plasticity connotes dynamic cell interactions that must bring into play cell surface and extracellular matrix adhesive proteins like those intervening in developing neuronal systems undergoing neuronal-glial and synaptogenic transformations. It is worth noting, therefore, that adult HNS neurons and glia continue to express such molecules, including polysialic acid (PSA)-enriched neural cell adhesion molecule (PSA-NCAM) and the glycoprotein, tenascin-C. PSA is a large, complex sugar on the extracellular domain of NCAM considered a negative regulator of adhesion; it occurs in large amounts on the surfaces of HNS neurons and astrocytes. Tenascin-C, on the other hand, possesses adhesive and repulsive properties; it is secreted by HNS astrocytes and occurs in extracellular spaces and on cell surfaces after interaction with appropriate ligands. These molecules have been considered permissive factors for morphological plasticity. However, because of their localization and inherent properties, they may also serve to modulate the extracellular environment and in consequence, synaptic and volume transmission in a system in which the extracellular compartment is constantly being modified.

Removal of PSA from NCAM affects the survival of magnocellular vasopressin- and oxytocin-producing neurons in organotypic cultures of the paraventricular nucleus

The expression of the polysialic acid neural cell adhesion molecule (PSA-NCAM) in the hypothalamo-neurohypophyseal system has been correlated with morphofunctional plasticity. In this study, we investigated the role of PSA-NCAM in the survival of oxytocin (OT)- and vasopressin (VP)-producing magnocellular cells of this system. We used a recently developed organotypic slice culture model of the rat hypothalamic paraventricular nucleus (PVN) in which ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) are potent survival factors for magnocellular neurons. We demonstrate by means of confocal microscopy that cultured magnocellular VP and OT neurons express strong immunoreactivity for PSA-NCAM. Removal of PSA from NCAM by the enzyme Endo N leads to a significant loss of both VP and OT neurons in the presence of low concentrations of CNTF. Endo N treatment did not change cell survival in the presence of LIF. These results suggest that, in addition to its role in neuro-glial plasticity, PSA-NCAM might also influence the trophic factor responsiveness of hypothalamic VP and OT neurosecretory cells.

Chondroitin sulfate proteoglycan phosphacan/RPTPbeta in the hypothalamic magnocellular nuclei

The hypothalamo-neurohypophysial system synthesizes and releases arginine vasopressin (AVP) and oxytocin (OXT) with physiological stimulation. In the present study, we investigated localization of a chondroitin sulfate proteoglycan (CSPG), phosphacan/RPTPbeta, in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of adult rats at both the light and electron microscopic levels. Immunohistochemical analyses demonstrated stronger phosphacan/RPTPbeta immunoreactivity within the SON and PVN compared with adjacent hypothalamic areas. Double labeling experiments showed phosphacan/RPTPbeta immunoreactivity constituting punctate networks to surround the somata and dendrites of AVP- and OXT-secreting magnocellular neurons. Electron microscopic examination further revealed strong phosphacan/RPTPbeta immunoreactivity at extracellular membrane surface of some axons, somata, and dendrites of the SON, but not of synaptic junctions. Interestingly, phosphacan/RPTPbeta immunoreactivity was also observed at extracellular surface membrane between astrocytic processes and neurons rather than between magnocellular neurons. The present results indicate the high expression of the CSPG, phosphacan/RPTPbeta at the extracellular space in the hypothalamic AVP- and OXT-secreting magnocellular neurons.

Activity-related, dynamic neuron-glial interactions in the hypothalamo-neurohypophysial system

Magnocellular neurons located in the supraoptic nucleus send their principal axons to terminate in the neurohypophysis, where they release vasopressin and oxytocin into the blood circulation. This magnocellular hypothalamo-neurohypophysial system is known to undergo dramatic activity-dependent structural plasticity during chronic physiological stimulation, such as dehydration and lactation. This structural plasticity is accompanied not only by synaptic remodeling, increased direct neuronal membrane apposition, and dendritic bundling in the supraoptic nucleus, but also organization of neurovascular contacts in the neurohypophysis. The adjacent glial cells actively participate in these plastic changes in addition to magnocellular neurons themselves. Many molecules that are possibly concerned with dynamic structural remodeling are highly expressed in the hypothalamo-neurohypophysial system, although they are generally at low expression levels in other regions of adult brains. Interestingly, some of them are highly expressed only in embryonic brains. On the basis of function, these molecules are classified mainly into two categories. Cytoskeletal proteins, such as tubulin, microtubule-associated proteins, and intermediate filament proteins, are responsible for changing both glial and neuronal morphology and location. Cell adhesion molecules, belonging to immunoglobulin superfamily proteins and extracellular matrix glycoproteins, also participate in neuronal-glial, neuronal-neuronal, and glial-glial recognition and guidance. Thus, the hypothalamo-neurohypophysial system is an interesting model for elucidating physiological significance and molecular mechanisms of activity-dependent structural plasticity in adult brains.

Oxytocin-secreting neurons: A physiological model of morphological neuronal and glial plasticity in the adult hypothalamus

Oxytocin-secreting neurons of the hypothalamoneurohypophysial system undergo reversible morphological changes whenever they are strongly stimulated. In the hypothalamus, such structural plasticity is represented by modifications in the size and shape of their somata and dendrites, in the extent to which their surfaces are covered by glia, and in the density of their synapses. In the neurohypophysis, there is a parallel reduction in glial (pituicyte) coverage of their axons together, with retraction of pituicyte processes from the perivascular basal lamina and an increase in the number and size of their terminals. These changes occur rapidly, within a few hours. On the other hand, the system returns to its prestimulated condition on arrest of stimulation at a rate that depends on the length of time it has remained activated. Such neuronal-glial changes have several functional consequences. In the hypothalamic nuclei, reduction in astrocytic coverage of oxytocinergic neurons and their synapses modifies extracellular ionic homeostasis and glutamate clearance and, therefore, their overall excitability. Since it results in extensive dendritic bundling, it may also lead to ephaptic interactions and may facilitate dendritic electrotonic coupling. A most important indirect effect may be to permit synaptic remodeling that occurs concomitantly and that results in significant increases in the number of excitatory and inhibitory synapses driving their activity. In the stimulated neurohypophysis, glial retraction results in increased levels of extracellular K+ which can enhance neurohormone release while an enlarged neurovascular contact zone may facilitate diffusion of neurohormone into the circulation. Ongoing work aims to unravel the cell mechanisms and factors underlying such plasticity and has revealed that neurons and glia of the hypothalamoneurohypophysial system continue to express juvenile molecular features associated with similar neuronglial interactions and synaptic events during development and regeneration. They include strong expression of cell surface adhesion molecules like F3/contactin and polysialylated neural cell adhesion molecule, extracellular matrix glycoproteins like tenascin C, and cytoskeletal proteins like vimentin and microtubule-associated protein 1D. Some of these molecules reach the cell surface constitutively while others follow the activity-dependent regulated pathway. We consider many of these molecular features permissive, allowing oxytocin neurons and their glia to undergo morphological remodeling throughout life, provided the proper stimulus intervenes. In the hypothalamic nuclei, one such stimulus is centrally released oxytocin; in the neurohypophysis, an adrenergic, cAMP-mediated mechanism appears responsible.

The role of neural cell adhesion molecules in plasticity and repair

Repair and functional recovery after brain injury critically depends on structural and functional plasticity of preserved neuronal networks. A striking feature of brain structures where tissue reorganization and plasticity occur is a strong expression of the polysialylated neural cell adhesion molecule (PSA-NCAM). An important role of this molecule in various aspects of neuronal and synaptic plasticity has been revealed by many studies. Recently, a new mechanism has been elucidated whereby PSA-NCAM may contribute to signalling mediated by the neurotrophic factor BDNF, thereby sensitizing neurons to this growth factor. This mechanism was shown to be important for activity-induced synaptic plasticity and for the survival and differentiation of cortical neurons. A cross-talk between these molecules may, thus, reveal a key factor for properties of structural plasticity and in particular could mediate the activity-dependent aspects of synaptic network remodeling. Animal models have been developed to assess the role of these molecules in functional recovery after lesions.

Roles, regulation, and mechanism of polysialic acid function during neural development

The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) appeared during the evolution of vertebrates as a new mechanism for regulation of cell interactions. This large and abundant glycoprotein can exert steric effects at the cell surface that lead to the attenuation of cell-cell bonds mediated not only by NCAM but also a variety of other adhesion receptors. PSA-NCAM expression changes both as a result of developmental programs and physiological inputs. This global modulation of cell-cell attachment has been shown to facilitate cell migration, axon pathfinding and targeting, and plastic changes in the embryonic and adult nervous system.

The polysialylated neural cell adhesion molecule reaches cell surfaces of hypothalamic neurons and astrocytes via the constitutive pathway

Understanding how neurons and glia sort and deliver cell adhesion molecules to their cell surface should provide important clues as to how such molecules participate in dynamic neuronal functions in the developing and adult brain. The present study examines translocation of polysialylated neural cell adhesion molecule (PSA-NCAM), a negative regulator of cell adhesion, in cells of the rat hypothalamo-neurohypophysial system in which it is expressed throughout life and which undergo morphological remodelling in response to stimulation. PSA-NCAM expression in this system does not vary markedly in relation to different conditions of regulated neurosecretion, suggesting that the glycoprotein reaches cell surfaces via the constitutive pathway. To study this more directly, we here used immunofluorescence for PSA on NCAM in live, unpermeabilized cells to monitor PSA-NCAM surface expression in organotypic slice cultures from postnatal rat hypothalami. Subsequent immunolabelling for oxytocin confirmed that the cultures included magnocellular oxytocinergic neurons displaying many properties of adult neurosecretory neurons in situ. In the cultures, immunoreaction for PSA-NCAM was visible on the surface of oxytocinergic and non-oxytocinergic axons. This reaction disappeared after exposure of the cultures to endoneuraminidase, an enzyme which specifically cleaves alpha-2-8-linked PSA from NCAM. PSA-NCAM reappeared on axonal surfaces 4h after enzyme washout. Such reexpression was visibly not affected by neuronal activity inhibition (blockade of Ca(2+) channels with Mn(2+), of Na(+) channels with tetrodotoxin, or of glutamate receptors with 6-cyano-7-nitroquinoxaline-2,3-dione or D-2-amino-5-phosphonopentanoic acid) or facilitation (K(+) depolarization or GABA-A receptor blockade with bicuculline). In contrast, PSA-NCAM surface translocation was inhibited reversibly by cooling the cultures at 20 degrees C, a procedure which blocks constitutive secretion and which resulted in accumulation of PSA-NCAM in the cytoplasm of oxytocinergic and non-oxytocinergic neurons. This treatment also revealed PSA-NCAM in the cytoplasm of underlying astrocytes. Our observations provide direct evidence that PSA-NCAM reaches the cell surface of hypothalamic neurons and astrocytes via the constitutive pathway, independently of Ca(2+) entry and enhanced neuronal activity. Thus, PSA-NCAM in the hypothalamo-neurohypophysial system would be continuously available to permit its cells to undergo remodelling whenever the proper stimulus intervenes.

Expression of the IgLON cell adhesion molecules Kilon and OBCAM in hypothalamic magnocellular neurons

The vasopressin (AVP) and oxytocin (OXT) magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular nuclei (PVN) display reversible structural plasticity of neurons and glial cells under different conditions of neuropeptide secretion. In the present study, we investigated the expression of two immunoglobulin superfamily (IgSF) proteins, Kilon and OBCAM, in the magnocellular neurons by using monoclonal antibodies. Anti-Kilon antibody reacted specifically with the bacterially expressed recombinant Kilon but not with the recombinant OBCAM, and similarly anti-OBCAM antibody specifically recognized the recombinant OBCAM. Western blotting analysis revealed the specific expression of Kilon and OBCAM in the SON homogenates. Although Kilon and OBCAM of the SON homogenates were present as the insoluble form, most Kilon was present in the Triton-insoluble fraction, and OBCAM was localized mainly in the Triton-soluble fraction. Immunocytochemistry revealed Kilon and OBCAM immunoreactivity in the magnocellular neurons of the SON and PVN of the rat hypothalamus compared with outside of the SON and PVN in the hypothalamus. The double-labeling study with confocal microscopy further demonstrated that Kilon immunoreactivity was observed mainly in the dendrites of AVP-secreting neurons and also occasionally OXT-secreting neurons. However, OBCAM immunoreactivity was exclusively seen in the dendrites of AVP-secreting magnocellular neurons. Chronic physiological stimulation by 2% NaCl had no effect on the expression levels of either IgLON protein in the SON. Our study thus demonstrated specific expression of Kilon and OBCAM in the hypothalamic magnocellular neurons, particularly in dendrites, suggesting that they confer on magnocellular neurons the ability to rearrange dendritic connectivity.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Differential expression of two adhesion molecules of the immunoglobulin superfamily, F3 and polysialylated NCAM, in hypothalamic magnocellular neurones capable of plasticity

The adult hypothalamo-neurohypophysial system undergoes activity-dependent, reversible morphological changes which result in reduced astrocytic coverage of its neurones and an increase in their synaptic contacts. Our recent observations show that neurones and glia of the hypothalamo-neurohypophysial system continue to express 'embryonic' molecular features which may underlie their capacity to undergo such plasticity. These include expression of cell surface molecules like the glycosyl phosphatidyl inositol (GPI)-linked glycoprotein F3, which intervenes in axonal outgrowth, and the polysialylated isoform of the neural cell adhesion molecule (PSA-NCAM), which reduces cell adhesion and promotes dynamic cell interactions. F3 is colocalised with vasopressin and oxytocin hormones in neurosecretory granules and follows an activity-dependent, regulated pathway for surface expression on neurohypophysial axons. In contrast, PSA-NCAM appears to follow a constitutive pathway, independent of the activity of the hypothalamo-neurohypophysial system, for expression on axonal and glial surfaces, in the hypothalamic magnocellular nuclei and in the neurohypophysis. The role of F3 remains to be determined but in view of its presumptive functions during development, we propose that it promotes remodelling of neurosecretory terminals. On the other hand, we provide direct evidence that surface expression of PSA on NCAM is essential to morphological plasticity since its specific enzymatic degradation in vivo inhibited the neuronal-glial and synaptic changes normally induced by stimulation of secretion from the hypothalamo-neurohypophysial system.

Contribution of astrocytes to activity-dependent structural plasticity in the adult brain

A striking example of the capacity of adult astrocytes to undergo reversible morphological changes in response to stimuli which enhance neuronal activity is offered by astrocytes of the adult hypothalamo-neurohypophysial system (HNS). The HNS is composed of magnocellular neurons secreting the neurohormones oxytocin and vasopressin from axon terminals in the neurohypophysis. Upon activation of HNS secretion, glial coverage of oxytocin neurons significantly diminishes and their surfaces become extensively juxtaposed. These glial changes are invariably accompanied by structural synaptic remodelling resulting in increased numbers of GABAergic, glutamatergic, and noradrenergic afferents. In the neurohypophysis, they result in an enhanced neurohemal contact area. HNS glia in the adult continue to display "embryonic" features that may allow such activity-dependent structural plasticity. For example, supraoptic astrocytes display a radial glia-like morphology and continue to express vimentin, together with GFAP. All HNS astrocytes secrete extracellular matrix glycoproteins, like tenascin-C; they also express high levels of polysialylated NCAM or PSA-NCAM and the glycoprotein F3, molecules considered essential for neuronal-glial interactions in the developing and lesioned CNS. HNS expression of most of these proteins does not visibly vary under different conditions of neurohormone secretion. We consider them as permissive factors, therefore, allowing HNS cells to undergo remodeling whenever the proper stimuli intervene. In the hypothalamic nuclei, one such stimulus is oxytocin itself which, in synergy with steroids, can induce neuronal-glial remodelling; adrenaline does so in the neurohypophysis.

Cell surface expression of polysialic acid on NCAM is a prerequisite for activity-dependent morphological neuronal and glial plasticity

Polysialic acid (PSA) on the extracellular domain of the neural cell adhesion molecule (NCAM) reduces cell adhesion and is considered an important regulator of cell surface interactions. The hypothalamo-neurohypophysial system (HNS), whose glia, neurons, and synapses undergo striking, reversible morphological changes in response to physiological stimulation, expresses high levels of PSA-NCAM throughout life. Light and electron microscopic immunocytochemistry in normal rats and rats in which cell transport was blocked with colchicine showed that PSA-NCAM is expressed in both HNS neurons and glia, particularly at the level of astrocytic processes that envelop neuronal profiles and can undergo remodeling. Moreover, we confirmed that the overall levels of PSA-NCAM were not greatly altered by stimulation (lactation and chronic salt ingestion). Nevertheless, PSA is essential to morphological plasticity. Using comparative ultrastructural analysis, we found that, after specific enzymatic removal of PSA from NCAM by microinjection of endoneuraminidase close to the hypothalamic magnocellular nuclei in vivo, there was no apparent withdrawal of astrocytic processes nor any increase in synaptic contacts normally induced by lactation and dehydration. Our observations demonstrate, therefore, that expression of PSA on cell surfaces in the adult HNS is indispensable to its capacity for activity-dependent morphological neuronal-glial and synaptic plasticity. The carbohydrate PSA on NCAM can thus be considered a necessary permissive factor to allow neuronal and glial remodeling to occur whenever the proper inductive stimulus intervenes.

Temporal and spacial relationships between PSA-NCAM-expressing, newly generated granule cells, and radial glia-like cells in the adult dentate gyrus

The granule cell layer of the adult dentate gyrus possesses two characteristics of an immature nervous system. The first is that granule cells continue to be generated in the innermost region of the granule cell layer, and newly generated and developing granule cells in the adult express highly polysialylated neural cell adhesion molecule (PSA-NCAM). PSA-NCAM-expressing apical dendrites have dynamically unstable processes such as irregular shafts and many stick-like or fan-shaped fine processes. The second is that radial glia-like cells expressing glial fibrillary acidic protein (GFAP) remain in a similar region of the granular layer. The numbers of PSA-NCAM-expressing granule cells and GFAP-expressing radial glia-like cells show a parallel age-dependent decrease during aging. Moreover, by using confocal laser scanning microscopy and immunoelectron microscopy, we demonstrated that PSA-NCAM-expressing dendrites and GFAP-expressing radial processes are partly in contact with each other, and occasionally the radial glial processes envelop the PSA-NCAM-positive dendritic processes. The temporal and spatial relationship between the two immature elements suggests that the processes of the radial glia-like cells are closely associated with the dendritic growth of the newly generated granule cells in the adult dentate gyrus and that these two immature features of neurons and glia in the dentate gyrus diminish with age.

L1 and laminin: their expression during rat hypophysis ontogenesis and in adult neurohemal areas

L1 is a murine multidomain glycoprotein implicated in cell aggregation, fasciculation. neurite outgrowth and synaptogenesis. Laminin, a trimeric polypeptide, is implicated in neuronal survival, growth cone guidance, neurite outgrowth and cell differentiation. Laminin can also interact with the cell adhesion molecule L1. Their expressions were investigated from embryonic day 15 (E15) to adult in the rat hypophysis, and in adult neurohemal zones. Detected in the neural lobe from E17, the L1 immunoreactivity increased during prenatal development and persisted in adulthood mainly related to the neuropeptidergic fibers. Pituicytes were only labelled on the plasmalemma apposed to axons. In the intermediate lobe, L1 appeared at birth on folliculo-stellate cells extensions, constituting a network which densified during postnatal development. L1 is also expressed in all neurohemal areas on neuronal profiles. Laminin was clearly detectable in the hypophysis at E15 before the first blood vessels penetrate the Rathke pouch. At E20, all the basal membranes of the blood vessels were stained. In the intermediate lobe, a spotted laminin immunoreactivity was detected at E21. At this stage, we observed the staining of intercellular spaces and the intracellular labelling of melanotrophs, concerning reticulum or vesicles. The staining of melanotrophs seemed to maintain during adulthood. In contrast with blood vessels of the adult cerebral tissue, adult capillaries of the neural lobe and the others neuro-hemal zones were intensely labelled with the anti-laminin antibody. These results suggest that neurite outgrowth and neurite guidance could be promoted by L1 and laminin in the neurointermediate lobe. The "intercellular tunnels" could also be an important guidance cue for migrating cells in the intermediate lobe. These data also demonstrate that melanotrophic cells. secreting the laminin, have a role in the ontogenesis of the gland. Finally, we suggest that L1 and laminin can collaborate to reinforce "neurons-capillaries" interactions in neurohemal zones.

Polysialic acid at the cell surface: biophysics in service of cell interactions and tissue plasticity

Polysialic acid (PSA) is a long polymer of negatively-charged sialic acid associated with the neural cell adhesion molecule. PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. During development the abundant and regulated expression of this carbohydrate is closely correlated with axon pathfinding and targeting, and with certain aspects of muscle formation. Its level can also be modulated by synaptic activity. PSA expression is more restricted in the neonatal and adult brain, being primarily associated with regions capable of morphological or physiological changes. Studies on the function of PSA studies suggest that its primary role is to promote developmentally-controlled and activity-dependent plasticity in cell interactions and thereby facilitate changes in the structure and function of the nervous system. The presence of PSA on a variety of metastatic tumor lines has also attracted the attention of oncologists, and its late appearance in evolution raises interesting questions about the phylogeny of complex tissue formation.

Regulated expression of the cell adhesion glycoprotein F3 in adult hypothalamic magnocellular neurons

F3, a glycoprotein of the immunoglobulin superfamily implicated in axonal growth, occurs in oxytocin (OT)-secreting and vasopressin (AVP)-secreting neurons of the adult hypothalamo-neurohypophysial system (HNS) whose axons undergo morphological changes in response to stimulation. Immunocytochemistry and immunoblot analysis showed that during basal conditions of HNS secretion, there are higher levels of this glycosylphosphatidyl inositol-anchored protein in the neurohypophysis, where their axons terminate, than in the hypothalamic nuclei containing their somata. Physiological stimulation (lactation, osmotic challenge) reversed this pattern and resulted in upregulation of F3 expression, paralleling that of OT and AVP under these conditions. In situ hybridization revealed that F3 expression in the hypothalamus is restricted to its magnocellular neurons and demonstrated a more than threefold increase in F3 mRNA levels in response to stimulation. Confocal and electron microscopy localized F3 in secretory granules in all neuronal compartments, a localization confirmed by detection of F3 immunoreactivity in granule-enriched fractions obtained by sucrose density gradient fractionation of rat neurohypophyses. F3 was not visible on any cell surface in the magnocellular nuclei. In contrast, in the neurohypophysis, it was present not only in secretory granules but also on the surface of axon terminals and glia and in extracellular spaces. Taken together, our observations reveal that the cell adhesion glycoprotein F3 is colocalized with neurohypophysial peptides in secretory granules. It follows, therefore, the regulated pathway of secretion in HNS neurons to be released by exocytosis at their axon terminals in the neurohypophysis, where it may intervene in activity-dependent structural axonal plasticity.

A role of adhesion molecules in neuroglial plasticity

Glial cells are exquisitely sensitive to changes in neuronal activity, and their capacity for structural plasticity including migration is critical for remodeling an repair of nervous tissue. Our in vitro studies suggest that isoforms of the neural cell adhesion molecule (NCAM) carrying an unconventional carbohydrate polymer, polysialic acid (PSA), are involved in these events. We have demonstrated that neurohypophyseal explants from newborn rats generate cellular outgrowth of immature astrocytes displaying the characteristics of oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells previously identified in the optic nerve. Treatment of O-2A cells with the enzyme Endo N, which specifically removes PSA from the cells surface, produced a complete blockade of the dispersion of the O-2A cell population from the explant. Identical effects of Endo N were observed in migration assays using cortical O-2A cells. Neurohypophyseal O-2A cells express functional NMDA class of glutamate receptors and the pharmacological blockade of these receptors inhibit PSA-NCAM biosynthesis and dramatically diminish O-2A cell migration from neurohypophyseal explants. This suggests a potential mechanism through which neuronal activity via glutamate release may regulate PSA-NCAM expression on immature glial cells, which in turn is critical for their migration.

Factors governing activity-dependent structural plasticity of the hypothalamoneurohypophysial system

1. The adult hypothalamoneurohypophysial system (HNS) undergoes reversible morphological changes in response to physiological stimulation. 2. In the hypothalamus, stimulation of neurohormone secretion results in reduced astrocytic coverage of oxytocinergic somata and dendrites so that their surfaces become directly juxtaposed. Concurrently, there is a significant increase in the number of GABAergic, glutamatergic. and noradrenergic synapses impinging on the neurons. 3. In the neurohypophysis, stimulation induces retraction of pituicyte processes from the perivascular area and enlargement and multiplication of neurosecretory terminals. 4. These neuronal-glial and synaptic changes are reversible with cessation of stimulation, thus rendering the HNS an excellent model to study physiologically linked structural neuronal plasticity in the adult CNS. 5. We still do not know the cellular mechanisms and factors underlying such plasticity. Recent studies indicate, however, that the adult HNS expresses molecular characteristics normally associated with histogenesis and/or tissue reorganization in developing or regenerating neural systems. They include expression of cell adhesion molecules such as the highly sialylated isoform of the neural cell adhesion molecule, PSA-NCAM, and the glycoproteins, F3 and tenascin-C. 6. The expression of PSA-NCAM and tenascin-C does not show striking differences in terms of age, sex or physiological condition but that of F3 varies considerably with neurohypophysial stimulation. 7. We postulate that such molecular features allow magnocellular neurons and their glia to undergo neuronal-glial and synaptic plasticity throughout life, provided the proper stimulus intervenes. 8. Thus, in the hypothalamic nuclei, centrally released oxytocin acting in synergy with steroids can induce such plasticity, while adrenaline, acting through beta-adrenergic mechanisms, does so in the neurohypophysis.

Polysialylated neural cell adhesion is involved in target-induced morphological differentiation of arcuate dopaminergic neurons

We have previously shown that the morphological and biochemical maturation of developing rat hypothalamic dopaminergic neurons is accelerated when they are cocultivated with pituitary intermediate lobe cells, one of their targets. Only two subsets of hypothalamic dopaminergic neurons (arcuate, A12, and periventricular, A14, nuclei) may project to the pars intermedia. In order to determine whether the two populations are equally responsive to coculture conditions, we microdissected the hypothalamus of 17-day-old rat fetuses in two fragments containing cell bodies from the A12 and from the A14 regions, prepared neuronal cultures from both portions and incubated them separately with intermediate lobe cells. The presence of intermediate lobe cells increased tyrosine hydroxylase levels in both dopaminergic neuron subsets, but morphological differentiation was accelerated in dopaminergic neurons originating in the arcuate nucleus only. We then investigated whether physical contact between developing arcuate neurons and their target cells was a prerequisite of the morphological effect by interposing a semipermeable membrane between cultivated neurons and intermediate lobe cells in transwell culture dishes. The morphological effect was no longer observed under transwell coculture conditions, pointing to the involvement of membrane-bound molecules. Accordingly, the stimulating effect of coculture on arcuate dopaminergic neurons was completely abolished by the removal of polysialic acid on neural cell adhesion molecules by endoneuraminidase N treatment. Thus, maturation of A12 and A14 dopaminergic neurons exhibits differential susceptibility to intermediate lobe target cells, and polysialylated-NCAM is required for the contact-dependent effect.

Cell biology of polysialic acid

The unusual carbohydrate polysialic acid (PSA), attached uniquely to neural cell adhesion molecule (NCAM) through a developmentally regulated process, modulates neural cell interactions. Major advances in the past two years have increased our understanding of PSA biosynthesis and regulation. Of particular interest is the cloning of the genes encoding polysialyltransferases (PSTs) and the finding that a single enzyme is able to confer polysialylation to NCAM. The electrical activity of neurons and transmembrane signalling are probably major players in controlling both PSA biosynthesis and its expression at the cell surface. A direct causal relationship between PSA expression and activity-induced synaptic plasticity has been reported.

Developmental expression of two rat sialyltransferases that modify the neural cell adhesion molecule, N-CAM

Polysialylation of the neural cell adhesion molecule (N-CAM) reduces the efficacy of N-CAM-mediated homophilic binding and is regulated both during development and in regions undergoing neurogenesis or remodeling in the adult. Hamster PST-1 (PST) and rat STX are two related sialytransferases that catalyze the polysialylation of N-CAM. We have isolated a cDNA clone for the rat homologue of PST and compared its amino acid and nucleotide sequence to that of rat STX. This analysis revealed regions of high sequence similarity corresponding to the enzymatic domains of the two molecules. Other regions of lower similarity were used to generate specific probes for in situ hybridization. The distribution of PST and STX mRNAs, polysialic acid, and N-CAM were analyzed at three developmental stages. PST and STX mRNAs were expressed abundantly throughout the nervous system at embryonic day 15 and postnatal day 4 and were coexpressed in most tissues examined. In the adult brain, STX expression was reduced relative to PST and expression of both mRNAs was restricted to subsets of cells in areas undergoing constant synaptic rearrangement including hippocampus and olfactory system. The results suggest that both PST and STX participate in the polysialylation of N-CAM in vivo and that their expression levels are dynamically controlled during development and regeneration.

Modulation of NCAM polysialylation is associated with morphofunctional modifications in the hypothalamo-neurohypophysial system during lactation

Post-transcriptional modification of the neural cell adhesion molecule (NCAM) by polysialic acid significantly decreases NCAM adhesiveness and more generally modifies cell-cell interactions. Polysialic acid-NCAM (PSA-NCAM) is mainly expressed in the developing nervous system. In the adult, its expression is restricted to regions that retain morphological plasticity, such as the hypothalamo-neurohypophysial system during lactation in rats. Since cell-cell interactions and synaptic contacts in the hypothalamo-neurohypophysial system are greatly increased during lactation, we examined whether PSA-NCAM expression is modified during this period. Immunohistochemistry and immunoblotting showed that, compared with virgin rats, PSA-NCAM dramatically decreased during lactation in both the supraoptic nuclei and the neurohypophysis, and returned to its initial level only after weaning. This decrease was progressive and became significant only at the end of the first week of lactation. By contrast, modifications in the level of NCAM protein or changes in the splicing pattern of NCAM mRNAs could not be detected. The decline in polysialic acid on the NCAM molecule could strengthen membrane appositions, thereby stabilizing the newly established synapses and neurohaemal contacts in the hypothalamo-neurohypophysial system that accompany the increased neuronal activity that occurs during lactation. We also studied the regulation of the phosphorylated microtubule-associated protein-1B (MAP1B-P), whose distribution pattern largely overlaps with that of PSA-NCAM in the adult brain. Expression of MAP1B-P was greatly increased during lactation in the hypothalamic axons projecting into the neurohypophysis. Thus, the expression patterns of both PSA-NCAM and MAP1B-P may reflect the permanent structural plasticity characterizing the hypothalamo-neurohypophysial system in the adult.

Expression of high levels of the extracellular matrix glycoprotein, tenascin-C, in the normal adult hypothalamoneurohypophysial system

Glia and neurons of the hypothalamoneurohypophysial system (HNS) undergo reversible morphological changes, which are concomitant with the remodelling of afferents onto the neurons, under different conditions of neurohormone secretion. Here, we show that the adult rat HNS contains high levels of tenascin-C (TN-C), which is an extracellular matrix glycoprotein whose expression is usually associated with neuronal-glial interactions in the developing and lesioned central nervous system (CNS). By using light and electron microscopic immunocytochemical procedures, we visualized TN-C immunoreactivity in the hypothalamic supraoptic (SON) and paraventricular nuclei, where somata of the neurons are localized; in the median eminence, where their axons transit; and in the neurohypophysis, where they terminate. Hypothalamic areas adjacent to the magnocellular nuclei were devoid of immunoreactivity. Electron microscopy of the neurohypophysis showed immunolabelling of perivascular spaces, glial (pituicyte) and axonal surfaces, a type of labelling that also characterized the median eminence. In the hypothalamic nuclei, there was labelling of extracellular spaces and astrocytic surfaces. In normal animals, we detected no cytoplasmic reaction in glia somata, neurons, or endothelial cells. However, in animals treated with the intracellular transport blocker colchicine, there was intracytoplasmic labelling of all HNS glial cells, indicating a glial source for TN-C. Immunoblot analysis revealed TN-C isoforms of apparent high molecular weight (225, 240, and 260 kD) in the SON and median eminence, whereas lower MW forms (190/200 kD) predominated in the neurohypophysis. By using immunocytochemistry and immunoblot analysis, we found no visible differences in TN-C expression in relation to age, sex, or differing neurohypophysial secretion, which suggests that the expression of TN-C is a permanent feature of the HNS.

Functional N-methyl-D-aspartate receptors in O-2A glial precursor cells: a critical role in regulating polysialic acid-neural cell adhesion molecule expression and cell migration

The capacity for long-distance migration of the oligodendrocyte precursor cell, oligodendrocyte-type 2 astrocyte (O-2A), is essential for myelin formation. To study the molecular mechanisms that control this process, we used an in vitro migration assay that uses neurohypophysial explants. We provide evidence that O-2A cells in these preparations express functional N-methyl-D-aspartate (NMDA) receptors, most likely as homomeric complexes of the NR1 subunit. We show that NMDA evokes an increase in cytosolic Ca2+ that can be blocked by the NMDA receptor antagonist AP-5 and by Mg2+. Blocking the activity of these receptors dramatically diminished O-2A cell migration from explants. We also show that NMDA receptor activity is necessary for the expression by O-2A cells of the highly sialylated polysialic acid-neural cell adhesion molecule (PSA-NCAM) that is required for their migration. Thus, glutamate or glutamate receptor ligands may regulate O-2A cell migration by modulating expression of PSA-NCAM. These studies demonstrate how interactions between ionotropic receptors, intracellular signaling, and cell adhesion molecule expression influence cell surface properties, which in turn are critical determinants of cell migration.

Differential expression of tenascin by astrocytes associated with the supraoptic nucleus (SON) of hydrated and dehydrated adult rats

The present study evaluated the expression of tenascin by astrocytes in the supraoptic nucleus and associated ventral glial limitans (SON-VGL) under conditions that induce reversible changes in neuronal organization (dehydration and rehydration). Immunostaining of astroglia cultured from rat neonatal SON-VGL confirmed that these cells are capable of both expressing and secreting tenascin. Observations of immunostained tissue sections from adult rats revealed tenascin immunoreactivity primarily in the VGL and dendritic zone, subjacent to SON neuronal somata. Comparison of immunostained tissues from hydrated and dehydrated animals showed an apparent decrease in the intensity of immunostaining with dehydration. Subsequent Western blots of similar tissues confirmed the presence of the 210-220-kDa tenascin protein in the SON-VGL. SON-VGL tissues from control, dehydrated, and rehydrated rats were then studied by using SDS-PAGE and quantitative gel densitometry. A consistent decrease in tenascin concentration was observed by 6 days of dehydration that, with rehydration, reversed back toward or beyond control levels. Together, these observations indicate that SON-VGL astrocytes variably express tenascin and that this protein may play a role in adult SON plasticity.

PSA-NCAM is required for activity-induced synaptic plasticity

Hippocampal organotypic slice cultures maintained 10-20 days in vitro express a high level of the polysialylated embryonic form of neural cell adhesion molecule (NCAM) (PSA-NCAM). Treatment of the cultures with endoneuraminidase-N selectively removed polysialic acid (PSA) from NCAM and completely prevented induction of long-term potentiation (LTP) and long-term depression (LTD) without affecting cellular or synaptic parameters. Similarly, slices prepared from transgenic mice lacking the NCAM gene exhibited a decaying LTP. No inhibition of N-methyl-D-aspartic acid receptor-dependent synaptic responses was detected. Washout of the enzyme resulted in reexpression of PSA immunoreactivity which correlated with a complete recovery of LTP and LTD. This reexpression was blocked by TTX and low calcium and enhanced by bicuculline. Taken together, these results indicate that neuronal activity regulates the expression of PSA-NCAM at the synapse and that this expression is required for the induction of synaptic plasticity.

Polysialylated neural cell adhesion molecule expression by neurons and astroglial processes in the rat dentate gyrus declines dramatically with increasing age

The expression of polysialylated neurons in the dentate gyrus of the hippocampal formation of young (postnatal day 40), mature (postnatal day 80) and aged (postnatal day 540) male Wistar rats has been investigated by immunohistochemical techniques employing a monoclonal antibody specific for neural cell adhesion molecule-linked alpha 2,8 polysialic acid. A strong immunoreactivity was found on the cell bodies, dendrites and axons of granule-like neuronal cells at the border between the hilar region and the granule cell layer of the young rat. In the mature animal the number of immunoreactive neurons declined dramatically and were virtually absent in the aged group. Using an alternative fixation procedure, glial fibrillary acidic protein-positive and polysialylated astroglia processes were found in close proximity to the dendrites of the polysialylated granule-like cells. The number of astroglial processes traversing the granule cell layer showed a similar age-dependent decline to that observed with the polysialylated neurons. Glial fibrillary acidic protein-positive and polysialylated stellate astroglia were present throughout the hippocampal formation, but did not show the marked age-dependent decline observed with the astroglial processes in the granule cell layer. The neuronal dendrites and astroglial processes exhibited a strict numerical ratio in the young and mature animal and, in double immunofluorescence studies with anti-polysialic acid and anti-glial fibrillary acidic protein, the astroglial processes exhibited apparent points of cell and/or dendritic contact. These findings suggest that loss of polysialylated astroglial processes precedes the decline in polysialylated dentate neurons.

Photoperiod affects the expression of neural cell adhesion molecule and polysialic acid in the hypothalamus of the Siberian hamster

Immunohistochemical and immunoblot procedures were used to examine the effects of inhibitory short day (SD) photoperiod on the expression of neural cell adhesion molecule (NCAM) and its polysialylated form (PSA-NCAM) in the hypothalamus and preoptic area (POA) of the adult male Siberian hamster. In animals that had undergone SD-induced gonadal regression, immunoblot analyses revealed significant reductions in the content of immunoreactive PSA in anterior hypothalamic (AH) and mediobasal hypothalamic (MBH) regions. These changes were accompanied by increased contents of the 180 kDa NCAM isoform in the POA and AH, and decreased content in the MBH. The 140 kDa NCAM isoform also was elevated in the AH. Light microscopic analysis revealed a marked reduction in the density of NCAM-immunoreactive tanycyte-like processes in the MBH of animals exposed to SD. This effect was not blocked by castration, indicating that this may be a primay (sex steroid-independent) effect of altered photoperiod in the hypothalamus. Also, photoperiod-induced alterations in NCAM expression were not evident in non-responsive hamsters that maintained active testes under SD exposure. Collectively, these results are evidence that seasonal changes in photoperiod affect the expression of NCAM and PSA in the hypothalamus. Such changes could help promote plastic morphological rearrangements related to the regulation of seasonal reproductive and/or metabolic cycles.

Expression of a glycosyl phosphatidylinositol-anchored adhesion molecule, the glycoprotein F3, in the adult rat hypothalamo-neurohypophysial system

The F3 cell surface glycoprotein consists of six immunoglobulin-like domains, four fibronectin type III repeats and a glycosylphosphatidylinositol anchor and is found in membrane-bound and soluble form. Until now, it has been localized mainly on axons of subsets of developing and postnatal neurons and has been implicated in axonal growth and synaptogenesis. We here examined its expression in the adult rat hypothalamo-neurohypophysial system composed of magnocellular neurons whose axons can undergo remodelling in adulthood in response to lesion or physiological stimulation. Immunoblot analyses demonstrated high levels of F3 immunoreactivity in the hypothalamic nuclei containing the somata of the neurons, in the median eminence, through which pass their axons and in the neurohypophysis, where they terminate. The amount of F3 detected in the latter was 2-fold that in the hypothalamus. In addition, soluble forms predominated in the neurohypophysis and GPI-linked forms in the hypothalamus. Immunocytochemistry revealed a strong F3 immunoreactivity throughout the neurohypophysis and internal layer of the median eminence, characterized by a punctate labeling of fibers and dense filling of dilatations. In the hypothalamic nuclei, staining of variable intensity was visible in the cytoplasm of some magnocellular somata. In contrast, in colchicine-treated rats, all magnocellular somata throughout the hypothalamus displayed intense labeling while staining in the neurohypophysis was greatly reduced. Our observations reveal that neurons of the adult hypothalamo-neurohypophysial system express high level of F3, even under normal conditions. In view of its distribution and the differing proportions of membrane-bound and soluble forms, we propose that, after synthesis in the hypothalamus, F3 is targeted to the neurohypophysis where it accumulates in neurosecretory terminals or is released into the extracellular space. It remains to be seen whether its expression is linked to the secretion of the neurohypophysial peptides and in particular, to the ability of these neurons to undergo structural remodelling in adulthood.

Expression of neural cell adhesion molecules, NCAMs, and their polysialylated forms, PSA-NCAMs, in the developing rat pituitary gland

Neural cell adhesion molecules (NCAMs) can undergo post-translational modifications, such as the addition of polysialic acid chains, thus generating PSA-NCAMs, which are expressed mainly during development. Since polysialylation considerably modifies NCAM adhesivity, expression of NCAMs and PSA-NCAMs has been investigated in the developing hypophysis by immunohistochemistry. At embryonic day 13 (E13), an antibody against NCAM outlined all cellular profiles in the entire Rathke's pouch; this labelling persisted until adulthood. NCAM expression increased in all lobes during development and concerned all pituitary cell types. In contrast, at E13, PSA-NCAMs were only detected in the neural lobe, solely constituted of pituicytes at this stage, and the tuberal lobe, the only lobe expressing hormonal mRNA at the same stage. PSA-NCAMs expression increased in the neural lobe at E17 with the arrival of the neurosecretory fibres and persisted into adulthood. In the anterior lobe, PSA-NCAMs appeared at E15 where their distribution was similar to that of the differentiating corticotrophic cells; at subsequent stages, their expression extended to the whole anterior lobe. Only two cell types, corticotrophic and somatotrophic cells, remained labelled in the adult gland. In the intermediate lobe, melanotrophic cells never expressed PSA-NCAMs but these were expressed on folliculo-stellate cells at birth, preceding the onset of innervation. These results suggest that NCAMs and PSA-NCAMs play a role in pituitary histogenesis, cell differentiation and neurointermediate lobe innervation.

Expression of the Neural Cell Adhesion Molecule NCAM by Peptide- and Steroid-Producing Endocrine Cells and Tumors: Alternatively Spliced Forms and Polysialylation

The adhesive properties of neural cell adhesion molecules (NCAMs) can be modified by alternative splicing of the primary transcript or by posttranslational modifications, such as sialylation. In this article, we describe distinct forms of alternative splicing and posttranslational modification of the extracellular domain of NCAM of various endocrine tissues and derived tumor cells of the rat and of steroid- and peptide-hormone producing endocrine cells in humans. NCAM-140 is the major isoform expressed in the rat adrenal gland, adenohypophysis, and in granulosa and granulosa-lutein cells. NCAM-180 is predominant in the neurohypophysis. Polysialylated NCAM is expressed in different endocrine tissues and tumor cells of the rat. Different amounts of NCAM mRNA containing the "extra-exon" VASE at the exon 7/8 splice boundary were detected in endocrine cells of rats. Human granulosa cells in culture undergo luteinization. During this process, the VASE-containing NCAM isoform is supplemented by an alternatively spliced isoform without this insert. Thus, modifications of NCAM may be important for adhesive interactions in normal and neoplastic endocrine cells. In addition, the differential expression and the alternative splicing of NCAM during luteinization of granulosa cells raise the possibility that NCAM could be involved in folliculogenesis and the formation of the corpus luteum in humans.