Highly sialylated N-CAM is expressed in adult mouse optic nerve and retina

Sialic Acids in Neurology

Sialic acid (Sia) is involved in many biological activities and commonly occurs as a monosialyl residue at the nonreducing terminal end of glycoconjugates. The loss of activity of UDP-GlcNAc2-epimerase/ManNAc kinase, which is a key enzyme in Sia biosynthesis, is lethal to the embryo, which clearly indicates the importance of Sia in embryogenesis. Occasionally, oligo/polymeric Sia structures such as disialic acid (diSia), oligosialic acid (oligoSia), and polysialic acid (polySia) occur in glycoconjugates. In particular, polySia, a well-known epitope that commonly occurs in neuroinvasive bacteria and vertebrate brains, is one of the most well-known and biologically/neurologically important glycotopes in vertebrates. The biological effects of polySia, especially on neural cell-adhesion molecules, have been well studied, and in-depth knowledge regarding polySia has been accumulated. In addition, the importance of diSia and oligoSia epitopes has been reported. In this chapter, the recent advances in the study of diSia, oligoSia, and polySia residues in glycoproteins in neurology, and their history, definition, occurrence, analytical methods, biosynthesis, and biological functions evaluated by phenotypes of gene-targeted mice, biochemical features, and related diseases are described.

Alterations in the polysialylated neural cell adhesion molecule and retinal ganglion cell density in mice with diabetic retinopathy

AIM

To investigate the impact of polysialylated neural cell adhesion molecule (PSA-NCAM) on the survival of retinal ganglion cells (RGCs) in the experimentally induced diabetes in mice.

METHODS

Diabetes was induced in 2.5 months old Swiss Webster mice by intraperitoneal injection of streptozotocin (STZ, 90 mg/kg) once daily for two consecutive days. Examination of the proteins of interest in the retinas from diabetic mice at 2mo after diabetes induction was performed using immunohistochemistry and Western blot analysis. RGCs were counted in the wholemounted retinas, and Brn3a marker was used.

RESULTS

Examination of retinas from diabetic mice at 2mo after diabetes induction revealed a considerable reduction in RGC density. Our experiments also demonstrated a redistribution of PSA-NCAM in the retina of diabetic animals. PSA-NCAM immunoreactivity was diminished in the inner part of the retina where RGCs were located. In contrast, an enhanced PSA-NCAM immunoreactivity was detected in the outer layers of the retina. PSA-NCAM signal was co-localized with glial fibrillary acidic protein immunoreactivity in the Müller cell branches. Previous studies have shown that matrix metalloproteinase-9 (MMP-9) is responsible for the reduction in PSA-NCAM levels in neuronal cells. The reduced levels of PSA-NCAM in inner layers (nerve fiber layer, ganglion cell layer) were accompanied by the increased expression of MMP-9. In contrast, in the outer retinal layers, the expression of MMP-9 was much less pronounced.

CONCLUSION

MMP-9 induces PSA-NCAM shedding in the inner part of the retina and the decreased level of PSA-NCAM in the inner part of the retina might be, at least in part, responsible for the loss of RGCs in diabetic mice.

Polysialic acid blocks mononuclear phagocyte reactivity, inhibits complement activation, and protects from vascular damage in the retina

Age‐related macular degeneration (AMD) is a major cause of blindness in the elderly population. Its pathophysiology is linked to reactive oxygen species (ROS) and activation of the complement system. Sialic acid polymers prevent ROS production of human mononuclear phagocytes via the inhibitory sialic acid‐binding immunoglobulin‐like lectin‐11 (SIGLEC11) receptor. Here, we show that low‐dose intravitreal injection of low molecular weight polysialic acid with average degree of polymerization 20 (polySia avDP20) in humanized transgenic mice expressing SIGLEC11 on mononuclear phagocytes reduced their reactivity and vascular leakage induced by laser coagulation. Furthermore, polySia avDP20 prevented deposition of the membrane attack complex in both SIGLEC11 transgenic and wild‐type animals. In vitro, polySia avDP20 showed two independent, but synergistic effects on the innate immune system. First, polySia avDP20 prevented tumor necrosis factor‐α, vascular endothelial growth factor A, and superoxide production by SIGLEC11‐positive phagocytes. Second, polySia avDP20 directly interfered with complement activation. Our data provide evidence that polySia avDP20 ameliorates laser‐induced damage in the retina and thus is a promising candidate to prevent AMD‐related inflammation and angiogenesis.

Polysialylation at Early Stages of Oligodendrocyte Differentiation Promotes Myelin Repair

Polysialic acid is a glycan modification of the neural cell adhesion molecule (NCAM) produced by the polysialyltransferases ST8SIA2 and ST8SIA4. Polysialic acid has been detected in multiple sclerosis plaques, but its beneficial or adverse role in remyelination is elusive. Here, we show that, despite a developmental delay, myelination at the onset and during cuprizone-induced demyelination was unaffected in male Ncam1^-/- or St8sia2^-/- mice. However, remyelination, restoration of oligodendrocyte densities, and motor recovery after the cessation of cuprizone treatment were compromised. Impaired differentiation of NCAM- or ST8SIA2-negative oligodendrocyte precursors suggested an underlying cell-autonomous mechanism. In contrast, premature differentiation in ST8SIA4-negative cultures explained the accelerated remyelination previously observed in St8sia4^-/- mice. mRNA profiling during differentiation of human stem cell-derived and primary murine oligodendrocytes indicated that the opposing roles of ST8SIA2 and ST8SIA4 arise from sequential expression. We also provide evidence that potentiation of ST8SIA2 by 9-cis-retinoic acid and artificial polysialylation of oligodendrocyte precursors by a bacterial polysialyltransferase are mechanisms to promote oligodendrocytic differentiation. Thus, differential targeting of polysialyltransferases and polysialic acid engineering are promising strategies to advance the treatment of demyelinating diseases.SIGNIFICANCE STATEMENT The beneficial or adverse role of polysialic acid (polySia) in myelin repair is a long-standing question. As a modification of the neural cell adhesion molecule (NCAM), polySia is produced by the polysialyltransferases ST8SIA2 and ST8SIA4. Here we demonstrate that NCAM and ST8SIA2 promote oligodendrocyte differentiation and myelin repair as well as motor recovery after cuprizone-induced demyelination. In contrast, ST8SIA4 delays oligodendrocyte differentiation, explaining its adverse role in remyelination. These opposing roles of the polysialyltransferases are based on different expression profiles. 9-cis-retinoic acid enhances ST8SIA2 expression, providing a mechanism for understanding how it supports oligodendrocyte differentiation and remyelination. Furthermore, artificial polysialylation of the cell surface promotes oligodendrocyte differentiation. Thus, boosting ST8SIA2 and engineering of polySia are promising strategies for improving myelin repair.

Polysialylated Neural Cell Adhesion Molecule Protects Against Light-Induced Retinal Degeneration

Purpose

We previously demonstrated that neural cell adhesion molecule (NCAM) plays an important role in supporting the survival of injured retinal ganglion cells. In the current study, we used light-induced retinal degeneration (LIRD) as a model to investigate whether NCAM plays a functional role in neuroprotection and whether NCAM influences p75^NTR signaling in modulating retinal cell survival.

Methods

Retinas from wild-type (WT) and NCAM deficient (−/−) mice were tested by electroretinogram before and after LIRD, and changes in the protein expressions of NCAM, polysialic acid (PSA)-NCAM, p75^NTR, and active caspase 3 were measured by immunoblot from 0 to 4 days after light induction. The effects of NCAM and PSA-NCAM on p75^NTR were examined by intraocular injections of the p75^NTR function-blocking antibody and/or the removal of PSA with endoneuraminidase-N prior to LIRD.

Results

In WT mice, low levels of active caspase 3 activation were detected on the first day, followed by increases up to 4 days after LIRD. Conversely, in NCAM^−/− mice, higher cleaved caspase 3 levels along with rapid reductions in electroretinogram amplitudes were found earlier at day 1, followed by reduced levels by day 4. The removal of PSA prior to LIRD induced earlier onset of retinal cell death, an effect delayed by the coadministration of endoneuraminidase-N and the p75^NTR function-blocking antibody antiserum.

Conclusions

These results indicate that NCAM protects WT retinas from LIRD; furthermore, the protective effect of NCAM is, at least in part, attributed to its effects on p75^NTR.

Extracellular cues influencing oligodendrocyte differentiation and (re)myelination

There is an increasing number of neurologic disorders found to be associated with loss and/or dysfunction of the CNS myelin sheath, ranging from the classic demyelinating disease, multiple sclerosis, through CNS injury, to neuropsychiatric diseases. The disabling burden of these diseases has sparked a growing interest in gaining a better understanding of the molecular mechanisms regulating the differentiation of the myelinating cells of the CNS, oligodendrocytes (OLGs), and the process of (re)myelination. In this context, the importance of the extracellular milieu is becoming increasingly recognized. Under pathological conditions, changes in inhibitory as well as permissive/promotional cues are thought to lead to an overall extracellular environment that is obstructive for the regeneration of the myelin sheath. Given the general view that remyelination is, even though limited in human, a natural response to demyelination, targeting pathologically 'dysregulated' extracellular cues and their downstream pathways is regarded as a promising approach toward the enhancement of remyelination by endogenous (or if necessary transplanted) OLG progenitor cells. In this review, we will introduce the extracellular cues that have been implicated in the modulation of (re)myelination. These cues can be soluble, part of the extracellular matrix (ECM) or mediators of cell-cell interactions. Their inhibitory and permissive/promotional roles with regard to remyelination as well as their potential for therapeutic intervention will be discussed.

Glaucoma related Proteomic Alterations in Human Retina Samples

Glaucoma related proteomic changes have been documented in cell and animal models. However, proteomic studies investigating on human retina samples are still rare. In the present work, retina samples of glaucoma and non-glaucoma control donors have been examined by a state-of-the-art mass spectrometry (MS) workflow to uncover glaucoma related proteomic changes. More than 600 proteins could be identified with high confidence (FDR < 1%) in human retina samples. Distinct proteomic changes have been observed in 10% of proteins encircling mitochondrial and nucleus species. Numerous proteins showed a significant glaucoma related level change (p < 0.05) or distinct tendency of alteration (p < 0.1). Candidates were documented to be involved in cellular development, stress and cell death. Increase of stress related proteins and decrease of new glaucoma related candidates, ADP/ATP translocase 3 (ANT3), PC4 and SRFS1-interacting protein 1 (DFS70) and methyl-CpG-binding protein 2 (MeCp2) could be documented by MS. Moreover, candidates could be validated by Accurate Inclusion Mass Screening (AIMS) and immunostaining and supported for the retinal ganglion cell layer (GCL) by laser capture microdissection (LCM) in porcine and human eye cryosections. The workflow allowed a detailed view into the human retina proteome highlighting new molecular players ANT3, DFS70 and MeCp2 associated to glaucoma.

Regulation of the neural niche by the soluble molecule Akhirin

Though the adult central nervous system has been considered a comparatively static tissue with little turnover, it is well established today that new neural cells are generated throughout life. Neural stem/progenitor cells (NS/PCs) can self-renew and generate all types of neural cells. The proliferation of NS/PCs, and differentiation and fate determination of PCs are regulated by extrinsic factors such as growth factors, neurotrophins, and morphogens. Although several extrinsic factors that influence neurogenesis have already been reported, little is known about the role of soluble molecules in neural niche regulation. In this review, we will introduce the soluble molecule Akhirin and discuss its role in the eye and spinal cord during development.

Effect of NCAM on aged-related deterioration in vision

The neural cell adhesion molecule (NCAM) is involved in developmental processes and age-associated cognitive decline; however, little is known concerning the effects of NCAM in the visual system during aging. Using anatomical, electrophysiological, and behavioral assays, we analyzed age-related changes in visual function of NCAM deficient (-/-) and wild-type mice. Anatomical analyses indicated that aging NCAM -/- mice had fewer retinal ganglion cells, thinner retinas, and fewer photoreceptor cell layers than age-matched controls. Electroretinogram testing of retinal function in young adult NCAM -/- mice showed a 2-fold increase in a- and b-wave amplitude compared with wild-type mice, but the retinal activity dropped dramatically to control levels when the animals reached 10 months. In behavioral tasks, NCAM -/- mice had no visual pattern discrimination ability and showed premature loss of vision as they aged. Together, these findings demonstrate that NCAM plays significant roles in the adult visual system in establishing normal retinal anatomy, physiology and function, and in maintaining vision during aging.

PSA modification of NCAM supports the survival of injured retinal ganglion cells in adulthood

Neural cell adhesion molecule (NCAM) is known as the cell surface glycoprotein, and it belongs to the immunoglobulin superfamily of adhesion molecules. Polysialic acid (PSA) is a carbohydrate attached to NCAM via either of two specific sialyltransferases: ST8SiaII and ST8SiaIV. Polysialylated neural cell adhesion molecule (PSA-NCAM) mediates cell interactions, plays a role in axon growth, migration, synaptic plasticity during development and cell regeneration. Some evidence has shown that PSA-NCAM supports the survival of neurons. It was demonstrated that PSA-NCAM is present in abundance in the retina during development and in adulthood. The aim of this study was to investigate whether PSA-NCAM promotes retinal ganglion cell (RGC) survival in transgenic mice with deficiencies in sialyltransferases or NCAM or after the administration of endoneuraminidase (Endo-N). RGC injury was induced by intravitreal administration of kainic acid (KA). These studies showed that injection of Endo-N after 14 days enhances the toxicity of KA to RGCs in wild-type (WT) mice by 18%. In contrast, in knockout mice (ST8SiaII-/-, ST8SiaIV-/-, NCAM-/-), survival of RGCs after KA injury did not change. Deficiencies of either ST8SiaII or ST8SiaIV did not influence the level of PSA-NCAM in the adult retina, however, in neonatal animals, decreased levels of PSA-NCAM were observed. In knockout ST8SiaII-/- adults, a reduced number of RGCs was detected, whereas in contrast, increased numbers of RGCs were noted in NCAM-/- mice. In conclusion, these data demonstrate that PSA-NCAM supports the survival of injured RGCs in adulthood. However, the role of PSA-NCAM in the adult retina requires further clarification.

Polysialic acid modification of the synaptic cell adhesion molecule SynCAM 1 in human embryonic stem cell-derived oligodendrocyte precursor cells

Oligodendrocyte precursor cells (OPCs) are the progenitors of myelinating oligodendrocytes in brain development and repair. Successful myelination depends on the control of adhesiveness during OPC migration and axon contact formation. The decoration of cell surface proteins with the glycan polysialic acid (polySia) is a key regulatory element of OPC interactions during development and under pathological conditions. By far the major protein carrier of polySia is the neural cell adhesion molecule NCAM, but recently, polysialylation of the synaptic cell adhesion molecule SynCAM 1 has been detected in the developing mouse brain. In mice, polySia-SynCAM 1 is associated with cells expressing NG2, a marker of a heterogeneous precursor cell population, which is the primary source for oligodendrocytes in development and myelin repair but can also give rise to astrocytes and possibly neurons. It is not yet clear if polySia-SynCAM 1 is expressed by OPCs and its occurrence in humans is elusive. By generating uniform human embryonic stem cell-derived OPC cultures, we demonstrate that polySia is present on human OPCs but down-regulated during differentiation into myelin basic protein-positive oligodendrocytes. PolySia on NCAM resides on the isoforms NCAM-180 and NCAM-140, and SynCAM 1 is identified as a novel polySia acceptor in human OPCs.

Characterization of dsRed2-positive cells in the doublecortin-dsRed2 transgenic adult rat retina

Doublecortin (DCX) is predominantly expressed in neuronal precursor cells and young immature neurons of the developing and adult brain, where it is involved in neuronal differentiation, migration and plasticity. Moreover, its expression pattern reflects neurogenesis, and transgenic DCX promoter-driven reporter models have been previously used to investigate adult neurogenesis. In this study, we characterize dsRed2 reporter protein-expressing cells in the adult retina of the transgenic DCX promoter-dsRed2 rat model, with the aim to identify cells with putative neurogenic activity. Additionally, we confirmed the expression of the dsRed2 protein in DCX-expressing cells in the adult hippocampal dentate gyrus. Adult DCX-dsRed2 rat retinas were analyzed by immunohistochemistry for expression of DCX, NF200, Brn3a, Sox2, NeuN, calbindin, calretinin, PKC-a, Otx2, ChAT, PSA-NCAM and the glial markers GFAP and CRALBP, followed by confocal laser-scanning microscopy. In addition, brain sections of transgenic rats were analyzed for dsRed2 expression and co-localization with DCX, NeuN, GFAP and Sox2 in the cortex and dentate gyrus. Endogenous DCX expression in the adult retina was confined to horizontal cells, and these cells co-expressed the DCX promoter-driven dsRed2 reporter protein. In addition, we encountered dsRed2 expression in various other cell types in the retina: retinal ganglion cells (RGCs), a subpopulation of amacrine cells, a minority of bipolar cells and in perivascular cells. Since also RGCs expressed dsRed2, the DCX-dsRed2 rat model might offer a useful tool to study RGCs in vivo under various conditions. Müller glial cells, which have previously been identified as cells with stem cell features and with neurogenic potential, did express neither endogenous DCX nor the dsRed2 reporter. However, and surprisingly, we identified a perivascular glial cell type expressing the dsRed2 reporter, enmeshed with the glia/stem cell marker GFAP and colocalizing with the neural stem cell marker Sox2. These findings suggest the so far undiscovered existence of perivascular associated cell with neural stem cell-like properties in the adult retina.

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.

Levels of CD56+TIM-3- effector CD8 T cells distinguish HIV natural virus suppressors from patients receiving antiretroviral therapy

Prolonged antiretroviral therapy (ART) with effective HIV suppression and reconstitution of CD4 T cells, fails to restore CD8 T cell lytic effector function that is needed to eradicate the viral reservoir. Better understanding of the phenotype and function of circulating CD8 cells in HIV patients will contribute to new targeted therapies directed at increasing CD8 T cell lytic effector function and destruction of the viral reservoir. We show that CD8 T cells from ART treated patients had sharply reduced expression of CD56 (neural cell adhesion molecule-1), a marker associated with cytolytic function whereas elite patients who control HIV in the absence of ART had CD56+ CD8 T cell levels similar to uninfected controls. The CD56+ CD8 T cells had higher perforin upregulation as well as degranulation following stimulation with HIV gag peptides compared with CD56 negative CD8 T cells. Elite patients had the highest frequencies of perforin producing CD56+ CD8 T cells among all HIV+ groups. In patients receiving ART we noted high levels of the exhaustion marker TIM-3 on CD56+ CD8 T cells, implying that defective effector function was related to immune exhaustion. CD56+ CD8 T cells from elite or treated HIV patients responded to PMA plus ionomycin stimulation, and expressed transcription factors T-bet and EOMES at levels similar to uninfected controls. Consequently, the lytic effector defect in chronic HIV disease is due to immune exhaustion and quantitative loss of CD56+ CD8 T cells and this defect is not repaired in patients where viremia is suppressed and CD4 T cells are recovered after ART.

Reconstituting the cytotoxic CD56+ subset of CD8+ T cells through new interventions might improve the lytic effector capacity and contribute to reducing the viral reservoir. Our initial studies indicate that IL-15 treatment partly reverses the CD56 defect, implying that myeloid cell defects could be targeted for immune therapy during chronic HIV disease.

White matter astrocytes in health and disease

Myelination by oligodendrocytes is a highly specialized process that relies on intimate interactions between the axon and the oligodendrocytes. Astrocytes have an important part in facilitating myelination in the CNS, however, comparatively less is known about how they affect myelination. This review therefore summarizes the literature and explores lingering questions surrounding differences between white matter and gray matter astrocytes, how astrocytes support myelination, how their dysfunction in pathological states contributes to myelin pathologies and how astrocytes may facilitate remyelination. We discuss how astrocytes in the white matter are specialized to promote myelination and myelin maintenance by clearance of extracellular ions and neurotransmitters and by secretion of pro-myelinating factors. Additionally, astrocyte-oligodendrocyte coupling via gap junctions is crucial for both myelin formation and maintenance, due to K(+) buffering and possibly metabolic support for oligodendrocytes via the panglial syncytium. Dysfunctional astrocytes aberrantly affect oligodendrocytes, as exemplified by a number of leukodystrophies in which astrocytic pathology is known as the direct cause of myelin pathology. Conversely, in primary demyelinating diseases, such as multiple sclerosis, astrocytes may facilitate remyelination. We suggest that specific manipulation of astrocytes could help prevent myelin pathologies and successfully restore myelin sheaths after demyelination.

Loss of electrostatic cell-surface repulsion mediates myelin membrane adhesion and compaction in the central nervous system

During the development of the central nervous system (CNS), oligodendrocytes wrap their plasma membrane around axons to form a multilayered stack of tightly attached membranes. Although intracellular myelin compaction and the role of myelin basic protein has been investigated, the forces that mediate the close interaction of myelin membranes at their external surfaces are poorly understood. Such extensive bilayer-bilayer interactions are usually prevented by repulsive forces generated by the glycocalyx, a dense and confluent layer of large and negatively charged oligosaccharides. Here we investigate the molecular mechanisms underlying myelin adhesion and compaction in the CNS. We revisit the role of the proteolipid protein and analyze the contribution of oligosaccharides using cellular assays, biophysical tools, and transgenic mice. We observe that differentiation of oligodendrocytes is accompanied by a striking down-regulation of components of their glycocalyx. Both in vitro and in vivo experiments indicate that the adhesive properties of the proteolipid protein, along with the reduction of sialic acid residues from the cell surface, orchestrate myelin membrane adhesion and compaction in the CNS. We suggest that loss of electrostatic cell-surface repulsion uncovers weak and unspecific attractive forces in the bilayer that bring the extracellular surfaces of a membrane into close contact over long distances.

N-CAM exhibits a regulatory function in pathological angiogenesis in oxygen induced retinopathy

BACKGROUND

Diabetic retinopathy and retinopathy of prematurity are diseases caused by pathological angiogenesis in the retina as a consequence of local hypoxia. The underlying mechanism for epiretinal neovascularization (tuft formation), which contributes to blindness, has yet to be identified. Neural cell adhesion molecule (N-CAM) is expressed by Müller cells and astrocytes, which are in close contact with the retinal vasculature, during normal developmental angiogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

Notably, during oxygen induced retinopathy (OIR) N-CAM accumulated on astrocytes surrounding the epiretinal tufts. Here, we show that N-CAM ablation results in reduced vascular tuft formation due to reduced endothelial cell proliferation despite an elevation in VEGFA mRNA expression, whereas retinal developmental angiogenesis was unaffected.

CONCLUSION/SIGNIFICANCE

We conclude that N-CAM exhibits a regulatory function in pathological angiogenesis in OIR. This is a novel finding that can be of clinical relevance in diseases associated with proliferative vasculopathy.

Remyelination after cuprizone induced demyelination is accelerated in mice deficient in the polysialic acid synthesizing enzyme St8siaIV

Polysialic acid (PSA) is a carbohydrate polymer added post-translationally on the neural cell adhesion molecule (NCAM) affecting its adhesion properties. It has been suggested that the presence of PSA in demyelinated lesions in multiple sclerosis could prevent axon-glia interactions inhibiting spontaneous remyelination. The enzyme St8siaIV is one of the two polysialyltransferases responsible for PSA synthesis, and it is predominantly active during adult life. Here we treated 8-10-weeks old St8siaIV deficient and wild-type mice for 5 weeks with cuprizone, which is a reliable model for de- and remyelination in the corpus callosum and cortex. Developmental myelination of the St8siaIV knock-out mice was not disturbed and adult mice showed normal myelin protein expression. Demyelination did not differ between transgenic and wild-type mice but early myelin protein re-expression and thus remyelination were accelerated in St8siaIV knock-out mice during the first week after withdrawal of the toxin. This was mainly due to enhanced oligodendrocyte precursor cells (OPC) differentiation and to a lesser extent to OPC recruitment. These data are proof of principle that PSA expression interferes at least to some extent with remyelination in vivo.

Genesis of rods in the zebrafish retina occurs in a microenvironment provided by polysialic acid-expressing Müller glia

Polysialic acid (polySia) is a posttranslational modification of the neural cell adhesion molecule NCAM, which in the vertebrate brain is dynamically regulated during development and crucially involved in developmental and adult neurogenesis. In the fish retina, new neurons are persistently generated, but the possible contribution of polySia has not yet been addressed. Here we used immunohistochemistry with NCAM- and polySia-specific antibodies to study spatiotemporal expression patterns of NCAM and polySia in the developing and mature zebrafish retina. As early as 2.3 days postfertilization (dpf), NCAM but not polySia was detected on cell somata and fibers of the developing retina. At 4.3 dpf polySia immunoreactivity first appeared in the ventral retina and was localized to the nascent outer nuclear layer (ONL). In mature zebrafish, polySia immunoreactivity in the ONL extended to the entire retina. Colocalization with rhodopsin-EGFP in transgenic zebrafish or the Müller glia-specific protein cellular retinaldehyde-binding protein (CRALBP) revealed that polySia immunoreactivity was confined to the compartment of radial Müller glia processes crossing the ONL and to a small band of processes positioned proximal to the horizontal cell layer of the mature retina. As shown by 5-bromo-2-deoxyuridine (BrdU) labeling, both newly generated rod precursors within the mature ONL and precursors of the marginal zone were polySia-negative. Thus, polySia-negative rod precursors of the mature zebrafish retina face a polySia-NCAM-positive microenvironment presented by radial Müller glia. In view of the prominent role of polySia in other neurogenic systems, this pattern indicates that polySia provides environmental cues that are relevant for the generation of new rods.

Sialic acid is an essential nutrient for brain development and cognition

The rapid growth of infant brains places an exceptionally high demand on the supply of nutrients from the diet, particularly for preterm infants. Sialic acid (Sia) is an essential component of brain gangliosides and the polysialic acid (polySia) chains that modify neural cell adhesion molecules (NCAM). Sia levels are high in human breast milk, predominately as N-acetylneuraminic acid (Neu5Ac). In contrast, infant formulas contain a low level of Sia consisting of both Neu5Ac and N-glycolylneuraminic acid (Neu5Gc). Neu5Gc is implicated in some human inflammatory diseases. Brain gangliosides and polysialylated NCAM play crucial roles in cell-to-cell interactions, neuronal outgrowth, modifying synaptic connectivity, and memory formation. In piglets, a diet rich in Sia increases the level of brain Sia and the expression of two learning-related genes and enhances learning and memory. The purpose of this review is to summarize the evidence showing the importance of dietary Sia as an essential nutrient for brain development and cognition.

Level and localization of polysialic acid is critical for early peripheral nerve regeneration

PolySia, the most striking post-translational modification of the neural cell adhesion molecule, is down-regulated during postnatal development. After peripheral nerve lesion, polySia is located on neuronal and glial cells normally not synthesizing polySia. However, structural consequences of reduced polySia content for peripheral nerve regeneration have not yet been clear. Furthermore, the contribution of sialyltransferases ST8SiaII and ST8SiaIV for the up-regulation of polySia has not been studied so far. In order to investigate the impact of polySia on regeneration processes of myelinated axons, we examined mouse mutants retaining only one functional sialyltransferase allele. In the absence of ST8SiaII, quantification of myelinated axons revealed a significant decrease in number and size of regenerated fibers without impairment of remyelination. In contrast, St8SiaIV deficiency resulted in increased fiber outgrowth and axonal maturation. Western blot analysis demonstrated that both ST8SiaII and St8SiaIV direct up-regulation of polySia. Cell-specific induction of polySia in myelinating Schwann cells and on regenerated axons in the presence of ST8SiaIV, but not ST8SiaII, indicates that not only the amount of polySia but also its cellular localization has a high impact on the regeneration progress of peripheral nerves.

Molecular cloning and characterization of the expression pattern of the zebrafish alpha2, 8-sialyltransferases (ST8Sia) in the developing nervous system

Sialyltransferases are Golgi type II transmembrane glycoproteins involved in the biosynthesis of sialylated glycolipids and glycoproteins. These sialylated compounds play fundamental roles in the development of a variety of tissues including the nervous system. In this study, we have molecularly cloned from zebrafish sources, the orthologues of the six human alpha2,8-sialyltransferases (ST8Sia), a family of sialyltransferases implicated in the alpha2-8-mono-, oligo-, and poly-sialylation of glycoproteins and gangliosides and we have analysed their expression pattern in the embryonic zebrafish nervous system, using in situ hybridization. Our results show that all six ST8Sia exhibit distinct and overlapping patterns of expression in the developing zebrafish central nervous system with spatial and temporal regulation of the expression of these genes, which suggests a role for the alpha2-8-sialylated compounds in the developing nervous system.

Down-regulation of the axonal polysialic acid-neural cell adhesion molecule expression coincides with the onset of myelination in the human fetal forebrain

The polysialic acid (PSA) modification of neural cell adhesion molecule, which reduces neural cell adhesion molecule (NCAM) - mediated cell adhesion, is involved in several developmental processes, such as cell migration, axonal growth, path finding, and synaptic plasticity. It has been suggested that PSA-NCAM expression may inhibit myelination. To clarify the relationship between myelination and the expression of PSA-NCAM we systematically investigated its expression in the human forebrain from embryonic stage to midgestation (19-24 gestation weeks, gw). Immunofluorescence on cryosections showed that PSA-NCAM is expressed at the earliest stage studied (5.5 gw) in the primordial plexiform layer of the telencephalon, which mainly consists of neuronal processes. At midgestation, cortical axonal tracts in the emerging white matter were PSA-NCAM+, but they were not yet myelinated, based on the lack of myelin basic protein (MBP) immunoreaction. To follow the progression of myelination we developed organotypic slice cultures that included the subventricular and intermediate zones of the fetal forebrain. In freshly prepared slices, similar to cryosections, axonal tracts were PSA-NCAM+ but did not express MBP. After 5 days in culture there was a dramatic increase in MBP expression around the axons of the intermediate zone, which suggested the onset of myelination. Simultaneously with MBP up-regulation PSA-NCAM expression in axons was completely lost, as demonstrated both with immunofluorescence and Western blot analysis. These results support the idea that in the human fetal forebrain axonal PSA-NCAM expression is inversely related to primary myelination.

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.

Down-regulation of polysialic acid is required for efficient myelin formation

Oligodendrocyte precursor cells modify the neural cell adhesion molecule (NCAM) by the attachment of polysialic acid (PSA). Upon further differentiation into mature myelinating oligodendrocytes, however, oligodendrocyte precursor cells down-regulate PSA synthesis. In order to address the question of whether this down-regulation is a necessary prerequisite for the myelination process, transgenic mice expressing the polysialyltransferase ST8SiaIV under the control of the proteolipid protein promoter were generated. In these mice, postnatal down-regulation of PSA in oligodendrocytes was abolished. Most NCAM-120, the characteristic NCAM isoform in oligodendrocytes, carried PSA in the transgenic mice at all stages of postnatal development. Polysialylated NCAM-120 partially co-localized with myelin basic protein and was present in purified myelin. The permanent expression of PSA-NCAM in oligodendrocytes led to a reduced myelin content in the forebrains of transgenic mice during the period of active myelination and in the adult animal. In situ hybridizations indicated a significant decrease in the number of mature oligodendrocytes in the forebrain. Thus, down-regulation of PSA during oligodendrocyte differentiation is a prerequisite for efficient myelination by mature oligodendrocytes. Furthermore, myelin of transgenic mice exhibited structural abnormalities like redundant myelin and axonal degeneration, indicating that the down-regulation of PSA is also necessary for myelin maintenance.

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.

alpha-Tocopherol affects neuronal plasticity in adult rat dentate gyrus: the possible role of PKCdelta

Hippocampus dentate gyrus (DG) is characterized by neuronal plasticity processes in adulthood, and polysialylation of NCAM promotes neuronal plasticity. In previous investigations we found that alpha-tocopherol increased the PSA-NCAM-positive granule cell number in adult rat DG, suggesting that alpha-tocopherol may enhance neuronal plasticity. To verify this hypothesis, in the present study, structural remodeling in adult rat DG was investigated under alpha-tocopherol supplementation conditions. PSA-NCAM expression was evaluated by Western blotting, evaluation of PSA-NCAM-positive granule cell density, and morphometric analysis of PSA-NCAM-positive processes. In addition, the optical density of synaptophysin immunoreactivity and the synaptic profile density, examined by electron microscopy, were evaluated. Moreover, considering that PSA-NCAM expression has been found to be related to PKCdelta activity and alpha-tocopherol has been shown to inhibit PKC activity in vitro, Western blotting and immunohistochemistry followed by densitometry were used to analyze PKC. Our results demonstrated that an increase in PSA-NCAM expression and optical density of DG molecular layer synaptophysin immunoreactivity occurred in alpha-tocopherol-treated rats. Electron microscopy analysis showed that the increase in synaptophysin expression was related to an increase in synaptic profile density. In addition, Western blotting revealed a decrease in phospho-PKC Pan and phospho-PKCdelta, demonstrating that alpha-tocopherol is also able to inhibit PKC activity in vivo. Likewise, immunoreactivity for the active form of PKCdelta was lower in alpha-tocopherol-treated rats than in controls, while no changes were found in PKCdelta expression. These results demonstrate that alpha-tocopherol is an exogenous factor affecting neuronal plasticity in adult rat DG, possibly through PKCdelta inhibition.

GDNF family ligands trigger indirect neuroprotective signaling in retinal glial cells

Apoptotic cell death of photoreceptors is the final event leading to blindness in the heterogeneous group of inherited retinal degenerations. GDNF (glial cell-line-derived neurotrophic factor) was found to rescue photoreceptor function and survival very effectively in an animal model of retinal degeneration (M. Frasson, S. Picaud, T. Leveillard, M. Simonutti, S. Mohand-Said, H. Dreyfus, D. Hicks, and J. Sahel, Investig. Ophthalmol. Vis. Sci. 40:2724-2734, 1999). However, the cellular mechanism of GDNF action remained unresolved. We show here that in porcine retina, GDNF receptors GFRalpha-1 and RET are expressed on retinal Mueller glial cells (RMG) but not on photoreceptors. Additionally, RMG express the receptors for the GDNF family members artemin and neurturin (GFRalpha-2 and GFRalpha-3). We further investigated GDNF-, artemin-, and neurturin-induced signaling in isolated primary RMG and demonstrate three intracellular cascades, which are activated in vitro: MEK/ERK, stress-activated protein kinase (SAPK), and PKB/AKT pathways with different kinetics in dependence on stimulating GFL. We correlate the findings to intact porcine retina, where GDNF induces phosphorylation of ERK in the perinuclear region of RMG located in the inner nuclear layer. GDNF signaling resulted in transcriptional upregulation of FGF-2, which in turn was found to support photoreceptor survival in an in vitro assay. We provide here a detailed model of GDNF-induced signaling in mammalian retina and propose that the GDNF-induced rescue effect on mutated photoreceptors is an indirect effect mediated by retinal Mueller glial cells.

Altered expression of CHL1 by glial cells in response to optic nerve injury and intravitreal application of fibroblast growth factor-2

The close homologue of L1 (CHL1) is a member of the L1 family of cell recognition molecules. The protein is expressed by a variety of nerve cell types and subpopulations of glial cells in vivo and promotes elongation of neurites and survival of nerve cells in vitro. Here we demonstrate that glial cells up-regulate expression of CHL1 in response to an intraorbital crush of the adult mouse optic nerve. We also demonstrate that a single intravitreal application of fibroblast growth factor-2 (FGF-2) increases expression of CHL1 in retinal astrocytes and Müller cells. Elevated expression of CHL1 by glial cells in injured optic nerves and astrocytes and Müller cells in FGF-2-treated retinas suggests a role of the protein in the lesioned central nervous system. Results also suggest that trophic factors might exert part of their biological function by modifying expression of cell recognition molecules.

Immunohistological Localization of Tenascin in the Developing and Lesioned Adult Mouse Optic Nerve

To gain insight into the morphogenetic functions of the recognition molecule tenascin in the central nervous system, we have studied its localization in the developing and lesioned adult mouse optic nerve using light and electron microscopic immunocytochemistry. Since tenascin is a secreted molecule, we have analysed the tenascin-synthesizing cells in tissue sections of retinae and optic nerves by in situ hybridization. A weak and homogeneous tenascin immunoreactivity was detectable in the developing retinal nerve fibre layer and optic nerve of 14-day-old mouse embryos, the earliest developmental age investigated. In the optic nerve of neonatal and 1-week-old animals, a high number of tenascin messenger RNA (mRNA)-containing cells were present, and antibodies to tenascin labelled the surfaces of astrocytes and unmyelinated retinal ganglion cell axons. With increasing age, expression of tenascin in the optic nerve was down-regulated at the mRNA and protein levels. At the fourth postnatal week, blood vessels in the optic nerve and collagen fibrils in the vicinity of meningeal fibroblast-like cells still showed significant immunoreactivity, but the optic nerve tissue proper no longer did so. In adult animals, tenascin was no longer detectable in association with blood vessels located in the myelinated part of the optic nerve, and meninges were only weakly immunoreactive. Also, tenascin mRNA-containing cells were no longer detectable in the myelinated part of the adult mouse optic nerve and few labelled cells were found in the meninges. In the retina, ganglion cells contained no detectable levels of tenascin mRNA at any of the developmental ages analysed. No significant up-regulation of tenascin expression was seen in the nerve tissue proper of transected proximal (i.e. retinal) and distal (i.e. cranial) optic nerve stumps of adult mice during the first 4 weeks after lesioning, the time period studied. However, collagen fibrils associated with meningeal fibroblast-like cells and located near the lesion site became strongly tenascin-immunoreactive 2 days after lesioning. Also, some blood vessels at the lesion site became immunoreactive. We conclude that tenascin in the optic nerve is synthesized by glial cells and not by retinal ganglion cells. The detectability of tenascin at embryonic ages suggests that it may mediate neurite growth in vivo. The absence of a strong, lesion-induced up-regulation of tenascin expression in the regeneration-prohibitive mouse optic nerve contrasts with the lesion-induced pronounced up-regulation in the regeneration-permissive peripheral nervous system, and may indicate a functional involvement of tenascin in regenerative processes. The high tenascin positivity of collagen fibrils at early postnatal ages and after lesioning suggests that tenascin expression may be correlated with mitotic activity of the associated meningeal fibroblast-like cells. Finally, tenascin may be involved in the process of vascularization, since the molecule is associated with blood vessels in developing and adult lesioned, but not intact adult, optic nerves.

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.

Developmental Profile of Neural Cell Adhesion Molecule Glycoforms with a Varying Degree of Polymerization of Polysialic Acid Chains

More precise information on the degree of polymerization (DP) of polysialic acid (polySia) chains expressed on neural cell adhesion molecule (NCAM) and its developmental stage-dependent variation are considered important in understanding the mechanism of regulated polysialylation and fine-tuning of NCAM-mediated cell adhesion by polySia. In this paper, first we performed a kinetic study of acid-catalyzed hydrolysis of polySia and report our findings that (a) in (-->8Neu5Ac alpha 2-->)(n)-->8Neu5Ac alpha 2-->3Gal beta 1-->R, the proximal Neu5Ac residue alpha 2-->3 linked to Gal is cleaved about 2.5-4 times faster than the alpha 2-->8 linkages and (b) in contrary to general belief that alpha 2-->8 linkages in polySia are extremely labile, the kinetic consideration showed that they are not so unstable, and every ketosidic bond is hydrolyzed at the same rate. These findings are the basis of our strategy for DP analysis of polySia on NCAM. Second, using the recently developed method that provides base-line resolution of oligo/polySia from DP 2 to >80 with detection thresholds of 1.4 fmol per resolved peak, we have determined the DP of polySia chains expressed in embryonic chicken brains at different developmental stages. Our results support the presence of numerous NCAM glycoforms differing in DPs of oligo/polySia chains and a delicate change in their distribution during development.

Increased NCAM-180 Immunoreactivity and Maintenance of L1 Immunoreactivity in Injured Optic Fibers of Adult Mice

The injury related expression of two axon-growth promoting cell adhesion molecules (CAMs), NCAM-180 which is developmentally downregulated and L1 which is regionally restricted, were compared in optic fibers in the adult mouse. The neuron-specific isoform of NCAM (NCAM-180) is present at very low levels in unlesioned adult optic axons. At 7 days after nerve crush, immunoreactivity was strongly and uniformly increased in optic axons within the nerve and throughout retina. Reactivity in surviving axons had returned to control levels at 4 weeks. To induce regrowth of adult retinal ganglion cell axons retinal explants were placed in culture. Strong NCAM-180 staining was observed on these regenerating optic axons. The neuronal cell adhesion molecule L1 is restricted to retina and to the unmyelinated segment of the optic nerve near the optic nerve head in unlesioned adult animals. Following nerve crush, L1 immunoreactivity was retained within retina and proximal nerve and novel staining was detected in the more distal segment of the optic nerve up to the lesion site where it persisted for at least eight months. The capacity of optic fibers to show increased NCAM-180 immunoreactivity and maintain L1 expression after a lesion may explain why these fibers exhibit relatively good potential for regeneration.

PSA-NCAM distinguishes reactive astrocytes in 6-OHDA-lesioned substantia nigra from those in the striatal terminal fields

6-hydroxydopamine (6-OHDA) lesion of the substantia nigra (SN) causes the appearance of reactive astrocytes not only in the SN but also in the striatal terminal fields, as measured by increased size of the cells and their processes, as well as enhanced expression of glial fibrillary acidic protein (GFAP) and an epitope recognized by monoclonal antibody 19D1. We now demonstrate that polysialylated neural cell adhesion molecule (PSA-NCAM) is induced on reactive astrocytes, as well as on large neurons, on the ipsilateral side of the 6-OHDA-lesioned SN. Colocalization of GFAP and PSA-NCAM was confirmed for reactive astrocytes using a confocal laser scanning microscope. Negligible amounts of PSA-NCAM reactivity were detected contralaterally, although colocalization was noted on astrocytes with sparse, significantly thinner processes. In contrast to the increase of GFAP in the lesioned striatum, few striatal astrocytes expressed PSA-NCAM. In agreement with these results, PSA-NCAM was detected on cultured reactive astrocytes from SN but not reactive striatal astrocytes. Double immunohistochemistry for proliferating cell nuclear antigen (PCNA), a marker of dividing cells, and GFAP demonstrated that reactive astrocytes in lesioned SN were PCNA-positive whereas those in striatum were not. Although NG2 chondroitin sulfate proteoglycan expression also increased in the lesioned SN, NG2 was not colocalized with PSA-NCAM, was not expressed on astrocytes, and labeled only oligodendrocyte precursor cells. Our results suggest that PSA-NCAM can act as a marker for reactive astrocytes only at the site of the lesion and not in the terminal fields, probably because it is reexpressed only when astrocytes divide.

Negative regulation of central nervous system myelination by polysialylated-neural cell adhesion molecule

Many factors have been shown to promote myelination, but few have been shown to be inhibitory. Here, we show that polysialylated-neural cell adhesion molecule (PSA-NCAM) can negatively regulate myelin formation. During development, PSA-NCAM is first expressed on all growing fibers; then, axonal expression is down-regulated and myelin deposition occurs only on PSA-NCAM-negative axons. Similarly, in cocultures of oligodendrocytes and neurons, PSA-NCAM expression on axons is initially high, but decreases as myelination proceeds. Importantly, if expression of PSA-NCAM is prematurely decreased in cultures, by either antibody-mediated internalization or enzymatic removal of the PSA moieties with endoneuraminidase N (endo-N), myelination increases 4- to 5-fold. In the optic nerve, premature cleavage of PSA moieties by intravitreous injection of endo-N also induces a transient increase in the number of myelinated internodes, but does not interfere with the onset of myelination. Previously, we showed that axonal electrical activity strongly induced myelination, which could be prevented by tetrodotoxin (TTX), an action potential blocker. Interestingly, removal of PSA moieties does not reverse the inhibition of myelination by TTX. Together, this suggests that myelination is tightly controlled by both positive (electrical activity) and negative (PSA-NCAM expression) regulatory signals.

Multiple functions of the myelin-associated glycoprotein MAG (siglec-4a) in formation and maintenance of myelin

The myelin-associated glycoprotein, a minor component of myelin in the central and peripheral nervous system, has been implicated in the formation and maintenance of myelin. Although the analysis of MAG null mutants confirms this view, the phenotype of this mutant is surprisingly subtle. In the CNS of MAG-deficient mice, initiation of myelination, formation of morphologically intact myelin sheaths and to a minor extent, integrity of myelin is affected. In the PNS, in comparison, only maintenance of myelin is impaired. Recently, the large isoform of MAG has been identified as the functionally important isoform in the CNS, whereas the small MAG isoform is sufficient to maintain the integrity of myelinated fibers in the PNS. Remarkably, none of the different defects in the MAG mutant is consistently associated with each myelinated fiber. These observations suggest that other molecules performing similar functions as MAG might compensate, at least partially, for the absence of MAG in the null mutant.

Astrocytes induce oligodendrocyte processes to align with and adhere to axons

In order to study the signals that control the onset of myelination, we cocultured highly purified postnatal retinal ganglion cells and optic nerve oligodendrocytes under serum-free conditions that promote their survival for at least a month and found that no myelination occurred. Although the addition of optic nerve astrocytes induced the oligodendrocyte processes to align with, and adhere to, axons, myelination still did not occur. The effect of astrocytes was mimicked by removal of polysialic acid from both cell types using neuroaminidase. These findings provide evidence for a novel role for astrocytes in controlling the onset of myelination by promoting adhesion of oligodendrocyte processes to axons. They also suggest that other, as yet unidentified, cell-cell interactions are necessary to induce the myelination process itself.

Multistratified expression of polysialic acid and its relationship to VAChT-containing neurons in the inner plexiform layer of adult rat retina

We investigated the localization of polysialic acid (PSA), neural cell adhesion molecule (NCAM), and vesicular acetylcholine transporter (VAChT) in adult rat retina by using immunofluorescence with a confocal laser scanning microscope. Western blot analysis showed a typical broad smear of PSA and isoforms of NCAM (120, 140, and 180 kD). PSA immunofluorescence revealed multistratification in the inner plexiform layer (IPL). Dual immunostaining for PSA and NCAM exhibited the selective co-expression of PSA and NCAM on Müller cells. Moreover, dual immunolabeling for PSA and VAChT completely separated the five strata in the IPL. Strata 1, 3, and 5 were immunoreactive for PSA and Strata 2 and 4 for VAChT. These results suggest the possibility that PSA molecules on Müller cells are spatially related to ON and OFF retinal channels in the IPL.

Transforming growth factor beta1-regulated cell proliferation and expression of neural cell adhesion molecule in B104 neuroblastoma cells: differential effects of ethanol

The expression and activity of factors influencing early neuronal development are altered by ethanol. Such factors include growth factors, for example, platelet-derived growth factor and basic fibroblast growth factor (for cell proliferation), and cell adhesion molecules (for neuronal migration). One agent, transforming growth factor beta1 (TGFbeta1), may affect both events. We tested the hypothesis that ethanol alters myriad TGFbeta1-mediated activities [i.e., cell proliferation and neural cell adhesion molecule (N-CAM) expression] using B104 neuroblastoma cells. TGFbeta1 inhibited the proliferation of B104 cells as evidenced by decreases in cell number and [3H]thymidine ([3H]dT) incorporation. TGFbeta1 induced sustained activation of extracellular signal-regulated kinases (ERKs), which are part of the family of mitogen-activated protein kinases (MAPKs). Treatment with PD98059 (a MAPK kinase blocker) abolished TGFbeta1-regulated inhibition of [3H]dT incorporation. TGFbeta1-mediated growth inhibition was potentiated by ethanol exposure. Ethanol also produced prolonged activation of ERK, an effect that was partially eliminated by treatment with PD98059. On the other hand, TGFbeta1 up-regulated N-CAM expression, and this up-regulation was not affected by treatment with PD98059. Ethanol inhibited the TGFbeta1-induced up-regulation of N-CAM expression in a concentration-dependent manner. Thus, TGFbeta1 affects ERK-dependent cell proliferation and ERK-independent N-CAM expression in B104 cells. Both activities are sensitive to ethanol and may underlie the ethanol-induced alterations in the proliferation and migration of CNS neurons.

Expression of polysialylated NCAM but not L1 or N-cadherin by regenerating adult mouse optic fibers in vitro

This study asks if there might be irreversible maturational changes in adult neurons that limit their capacity to regenerate. Retina from adult and embryonic mouse were placed in culture on laminin substrates so that regenerating adult optic fibers could be compared to growing embryonic fibers. Several cell adhesion molecules (CAMs) known to mediate the growth of embryonic neurites on astrocytes were assayed by immunocytochemistry: L1, N-cadherin, and NCAM. Thy 1.2, a potential CAM with inhibitory activity, was also examined. As in vivo, embryonic fibers were found to express both L1 and N-cadherin. In contrast, regenerating adult fibers had no detectable amounts of either of these CAMs. N-Cadherin is normally down regulated during development so its absence in adult fibers suggests it can not be reexpressed during regeneration. L1 is normally found in the proximal regions of adult optic fibers so its absence indicates it is not expressed or transported in regenerating fibers. Adult regenerating fibers expressed high levels of Thy 1.2, which was undetectable in embryonic optic fibers. Thy 1.2 is normally found in mature fibers, indicating this phenotypic feature is preserved during regeneration. Both adult and embryonic fibers showed strong reactivity for NCAM, which in vivo is normally found in embryonic and at lower levels in adult fibers. Surprisingly, both embryonic and regenerating adult fibers expressed high levels of polysialic acid, which is normally absent in adult fibers. NCAM may be one of few CAMs available to adult optic fibers for regeneration on astrocytes.

Mutation of a gene encoding a protein with extracellular matrix motifs in Usher syndrome type IIa

Usher syndrome type IIa (OMIM 276901), an autosomal recessive disorder characterized by moderate to severe sensorineural hearing loss and progressive retinitis pigmentosa, maps to the long arm of human chromosome 1q41 between markers AFM268ZD1 and AFM144XF2. Three biologically important mutations in Usher syndrome type IIa patients were identified in a gene (USH2A) isolated from this critical region. The USH2A gene encodes a protein with a predicted size of 171.5 kilodaltons that has laminin epidermal growth factor and fibronectin type III motifs; these motifs are most commonly observed in proteins comprising components of the basal lamina and extracellular matrixes and in cell adhesion molecules.

Decrease in highly polysialylated neuronal cell adhesion molecules and in spatial learning during ageing are not correlated

Age-dependent spatial memory impairments have been related to a decline in hippocampal plasticity. Highly polysialylated neuronal cell adhesion molecules (PSA-NCAM) show a strong expression during adulthood within regions associated with neuroplastic events. Furthermore, NCAM molecules have been proposed to mediate neuronal plasticity during learning and memory. The aim of the present study was to examine the effect of ageing on the expression of PSA-NCAM within the hippocampus. To investigate whether age-dependent changes in expression of PSA-NCAM were accentuated in aged rats with learning impairment, animals were in a first step assessed for their cognitive abilities using a Morris water maze. Seven-month-old and 24-month-old-rats were tested for their performance in the Morris water maze. The animals were sacrificed and brain sections were processed for PSA-NCAM immunohistochemistry. Ageing was accompanied by an overall decrease in PSA-NCAM-immunoreactivity (-IR) within the forebrain, presenting a important decrease of the number of PSA-NCAM-IR perikarya within the hippocampus. These results were confirmed by Western blot analysis. No difference in PSA-NCAM immunoreactivity was observed in aged rats with or without spatial learning impairment. It is concluded that although changes in PSA-NCAM accompanied the decrease in cognitive abilities, our data did not evidence a causal relationship between these two parameters.