Loss of polysialic residues accelerates CNS neural precursor differentiation in pathological conditions

Slit1 Protein Regulates SVZ-Derived Precursor Mobilization in the Adult Demyelinated CNS

Slit1 is a secreted axon guidance molecule, also involved in adult neurogenesis. In physiological conditions, Slit1 loss promotes ectopic dispersal of SVZ-derived neural precursors (SVZ-NPCs) into periventricular structures such as the corpus callosum. Demyelination of the corpus callosum triggers SVZ-NPC migration to ectopic locations and their recruitment by the lesion, suggesting a possible role for Slit1 in SVZ-NPCs ectopic dispersal regulation in pathological conditions. Here, we have investigated the function of Slit1 protein in the recruitment of SVZ-NPCs after CNS demyelination. We find that the dynamics of oligodendrogenesis and temporal profile of developmental myelination in Slit1^–/– mice are similar to Slit1^+/− controls. SVZ micro-dissection and RT-PCR from wild-type mice, show that Slits and Robos are physiologically regulated at the transcriptional level in response to corpus callosum demyelination suggesting their role in the process of SVZ-NPC ectopic migration in demyelinating conditions. Moreover, we find that the number of SVZ-NPCs recruited by the lesion increases in Sli1^–/– mice compared to Slit1^+/− mice, leading to higher numbers of Olig2⁺ cells within the lesion. Time-lapse video-microscopy of immuno-purified NPCs shows that Slit1-deficient cells migrate faster and make more frequent directional changes than control NPCs, supporting a cell-autonomous mechanism of action of Slit1 in NPC migration. In conclusion, while Slit1 does not affect the normal developmental process of oligodendrogenesis and myelination, it regulates adult SVZ-NPC ectopic migration in response to demyelination, and consequently oligodendrocyte renewal within the lesion.

EndoN treatment allows neuroblasts to leave the rostral migratory stream and migrate towards a lesion within the prefrontal cortex of rats

The binding properties of neural cell adhesion molecule are modulated by a polysialic acid moiety. This plays an important role in the migration of adult born neuroblasts from their area of origin, the subventricular zone, towards the olfactory bulb. Polysialisation increases the migration speed of the cells and helps to prevent the neuroblasts from leaving their migration route, the rostral migratory stream. Here, we evaluated the potential of intraventricular application of endoneuraminidase-N, an enzyme that specifically cleaves polysialic acid from neural cell adhesion molecule, in a rat model for structural prefrontal cortex damage. As expected, endoneuraminidase-N caused the rostral migratory stream to become wider, with a less uniform cellular orientation. Furthermore, endoneuraminidase-N treatment caused the neuroblasts to leave the rostral migratory stream and migrate towards the lesioned tissue. Despite the neuroblasts not being differentiated into neurons after a survival time of three weeks, this technique provides a solid animal model for future work on the migration and differentiation of relocated neuroblasts and might provide a basis for a future endogenous stem cell-based therapy for structural brain damage. The experiments were approved by the local animal care committee (522-27-11/02-00, 115; Senatorin für Wissenschaft, Gesundheit und Verbraucherschutz, Bremen, Germany) on February 10, 2016.

Transgenic overexpression of polysialyltransferase ST8SiaIV under the control of a neuron-specific promoter does not affect brain development but impairs exploratory behavior

A large body of the literature has demonstrated that the polysialic acid (polySia) modification of the neural cell adhesion molecule (NCAM) is a key regulator of cellular interactions during brain development, maintenance and plasticity. To properly fulfill these functions, polySia concentration has to be carefully controlled. This is done by the regulation of the expression of the two polySia-synthesizing enzymes ST8SiaII and ST8SiaIV. From this point of view we and others have demonstrated that downregulation of ST8SiaIV during oligodendrocyte differentiation is a prerequisite for efficient myelin formation and maintenance. Here, we addressed the question whether the prevention of polySia downregulation in neurons affects brain and particularly myelin development and functioning. For this purpose, we developed transgenic (tg) mouse lines overexpressing the polysialyltransferase ST8SiaIV in neurons. tg expression of ST8SiaIV prevented the postnatal downregulation of polySia, and most of the polySias in the forebrain and brain stem of adult tg mice were associated with NCAM-140 and NCAM-180 isoforms. Structural examination of the brain revealed no overt abnormalities of axons and myelin. In addition, ultrastructural and western blot analyses indicated normal myelin development. However, behavioral studies revealed reduced rearing activity, a measure for exploratory behavior, while parameters of motor activity were not affected in tg mice. Taken together, these results suggest that a persisting presence of polySia in neurons has no major effect on brain structure, myelination and myelin maintenance, but causes mild behavioral changes.

High cholesterol triggers white matter alterations and cognitive deficits in a mouse model of cerebrovascular disease: benefits of simvastatin

Transgenic mice overexpressing transforming growth factor-β1 (TGF mice) display impaired cerebrovascular reactivity, cerebral hypoperfusion and neurovascular uncoupling, but no overt cognitive deficits until old age. Cardiovascular diseases are a major risk factor for vascular cognitive impairment and dementia (VCID). We investigated the impact of a high cholesterol diet (HCD) on cerebrovascular and cognitive function in adult (6 months) and aged (12 months) TGF mice, together with the potential benefit of simvastatin (SV), an anti-cholesterol drug with pleiotropic effects, in adult mice. HCD increased blood, but not brain, cholesterol levels in treated mice, which SV did not reduce. In WT mice, HCD induced small, albeit significant, impairment in endothelium-dependent dilatory function. In TGF mice, HCD worsened the established brain vessel dilatory dysfunction in an age-dependent manner and increased the number of string vessels in the white matter (WM), alterations respectively normalized and significantly countered by SV. HCD triggered cognitive decline only in TGF mice at both ages, a deficit prevented by SV. Concurrently, HCD upregulated galectin−3 immunoreactivity in WM microglial cells, a response significantly reduced in SV-treated TGF mice. Grey matter astrogliosis and microgliosis were not affected by HCD or SV. In the subventricular zone of adult HCD-treated TGF mice, SV promoted oligogenesis and migration of oligodendrocyte progenitor cells. The results demonstrate that an underlying cerebrovascular pathology increases vulnerability to cognitive failure when combined to another risk factor for dementia, and that WM alterations are associated with this loss of function. The results further indicate that myelin repair mechanisms, as triggered by SV, may bear promise in preventing or delaying cognitive decline related to VCID.

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.

Donepezil promotes differentiation of neural stem cells into mature oligodendrocytes at the expense of astrogenesis

Oligodendrocytes are the myelin-forming cells of the central nervous system. Oligodendrocyte loss and failure of myelin development result in serious human disorders, including multiple sclerosis. Previously, using oligodendrocyte progenitor cells, we have shown that donepezil, which is an acetylcholinesterase inhibitor developed for the treatment of Alzheimer's disease, stimulates myelin gene expression and oligodendrocyte differentiation. Here, we aimed to analyze the effects of donepezil on primary mouse embryonic neural stem cells (NSCs). Donepezil treatment led to impaired self-renewal ability and increased apoptosis. These effects appeared to be mediated through the Akt/Bad signaling pathway. Using neurosphere differentiation analysis, we observed that donepezil leads to reduced numbers of astrocytes and increased numbers of oligodendrocytes and neurons. Consistent with this finding, mRNA and protein levels for the oligodendrocyte markers myelin-associated glycoprotein, 2', 3'-cyclic-nucleotide 3'-phosphodiesterase (CNPase), and myelin basic protein, as well as the neuronal marker β-tubulin type III (Tuj1) were up-regulated. In contrast, the expression of the astrocyte marker glial fibrillary acidic protein (GFAP) was down-regulated by donepezil in a dose- and time-dependent manner. Moreover, donepezil increased oligodendrocyte differentiation, resulting in a reduction in the differentiation of NSCs into astrocytes, by suppressing the activation of signal transducer and activator of transcription 3 (STAT3), SMAD1/5/9, and the downstream target gene GFAP, even under astrocyte-inducing conditions. These results suggest that efficient differentiation of NSCs into oligodendrocytes by donepezil may indicate a novel therapeutic role for this drug in promoting repair in demyelinated lesions in addition to its role in preventing astrogenesis.

ST8SIA2 promotes oligodendrocyte differentiation and the integrity of myelin and axons

ST8SIA2 is a polysialyltransferase that attaches polysialic acid to the glycoproteins NCAM1 and CADM1. Polysialylation is involved in brain development and plasticity. ST8SIA2 is a schizophrenia candidate gene, and St8sia2^−/− mice exhibit schizophrenia‐like behavior. We sought to identify new pathological consequences of ST8SIA2 deficiency. Our proteomic analysis suggested myelin impairment in St8sia2^−/− mice. Histological and immune staining together with Western blot revealed that the onset of myelination was not delayed in St8sia2^−/− mice, but the content of myelin was lower. Ultrastructure analysis of the corpus callosum showed thinner myelin sheaths, smaller and irregularly shaped axons, and white matter lesions in adult St8sia2^−/− mice. Then we evaluated oligodendrocyte differentiation in vivo and in vitro. Fewer OLIG2+ cells in the cortex and corpus callosum, together with the higher percentage of undifferentiated oligodenroglia in St8sia2^−/− mice suggested an impairment in oligodendrocyte generation. Experiment on primary cultures of oligodendrocyte precursor cells (OPCs) confirmed a cell‐autonomous effect of ST8SIA2 in oligodendroglia, and demonstrated that OPC to oligodendrocyte transition is inhibited in St8sia2^−/− mice. Concluding, ST8SIA2‐mediated polysialylation influences on oligodendrocyte differentiation, and oligodendrocyte deficits in St8sia2 mice are a possible cause of the demyelination and degeneration of axons, resembling nerve fiber alterations in schizophrenia. GLIA 2016;65:34–49

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.

Neuronal adhesion and synapse organization in recovery after brain injury

Few specific therapeutic targets exist to manage brain injury, despite the prevalence of stroke or traumatic brain injury. With traumatic brain injury, characteristic neuronal changes include axonal swelling and degeneration, and the loss of synapses, the sites of communication between neurons. This is followed by axonal sprouting and alterations in synaptic markers in recovery. The resulting changes in neuronal connectivity are likely to contribute to the effects of traumatic brain injury on cognitive functions and the underlying mechanisms may represent points of therapeutic intervention. In agreement, animal studies implicate adhesion and signaling molecules that organize synapses as molecular players in neuronal recovery. In this article, the authors focus on the role of cell surface interactions in the recovery after brain injury in humans and animals. The authors review cellular and synaptic alterations that occur with injury and how changes in cell adhesion, protein expression and modification may be involved in recovery. The changes in neuronal surface interactions as potential targets and their possible value for the development of therapeutics are also discussed.

Oxygen levels and the regulation of cell adhesion in the nervous system: a control point for morphogenesis in development, disease and evolution?

In this article, I discuss the hallmarks of hypoxia in vitro and in vivo and review work showing that many types of stem cell proliferate more robustly in lowered oxygen. I then discuss recent studies showing that alterations in the levels and the types of cell and substrate adhesion molecules are a notable response to reduced O(2) levels in both cultured primary neural stem cells and brain tissues in response to hypoxia in vivo. The ability of O(2) levels to regulate adhesion molecule expression is linked to the Wnt signaling pathway, which can control and be controlled by adhesion events. The ability of O(2) levels to influence cell adhesion also has far-reaching implications for development, ischemic trauma and neural regeneration, as well as for cancer and other diseases. Finally I discuss the possibility that the fluctuations in O(2) levels known to have occurred over evolutionary time could, by influencing adhesion systems, have contributed to early symbiotic events in unicellular organisms and to the emergence of multicellularity. It is not my intention to be exhaustive in these domains, which are far from my own field of study. Rather this article is meant to provoke and stimulate thinking about molecular evolution involving O(2) sensing and signaling during eras of geologic and atmospheric change that might inform modern studies on development and disease.

Reelin controls progenitor cell migration in the healthy and pathological adult mouse brain

Understanding the signals that control migration of neural progenitor cells in the adult brain may provide new therapeutic opportunities. Reelin is best known for its role in regulating cell migration during brain development, but we now demonstrate a novel function for reelin in the injured adult brain. First, we show that Reelin is upregulated around lesions. Second, experimentally increasing Reelin expression levels in healthy mouse brain leads to a change in the migratory behavior of subventricular zone-derived progenitors, triggering them to leave the rostral migratory stream (RMS) to which they are normally restricted during their migration to the olfactory bulb. Third, we reveal that Reelin increases endogenous progenitor cell dispersal in periventricular structures independently of any chemoattraction but via cell detachment and chemokinetic action, and thereby potentiates spontaneous cell recruitment to demyelination lesions in the corpus callosum. Conversely, animals lacking Reelin signaling exhibit reduced endogenous progenitor recruitment at the lesion site. Altogether, these results demonstrate that beyond its known role during brain development, Reelin is a key player in post-lesional cell migration in the adult brain. Finally our findings provide proof of concept that allowing progenitors to escape from the RMS is a potential therapeutic approach to promote myelin repair.

Removal of polysialic acid triggers dispersion of subventricularly derived neuroblasts into surrounding CNS tissues

Cells generated in the subventricular zone give rise to neuroblasts that migrate to the olfactory bulb (OB) along the rostral migratory stream (RMS). The polysialylated form of neural cell adhesion molecule (PSA-NCAM) is expressed by these cells, and has been shown to both promote their migration and suppress differentiation induced by NCAM. In the present study, enzymatic removal of PSA from these neuroblasts using PSA-specific endoneuraminidase has been found not only to disrupt the tangential migration and cellular organization of the RMS, but also to cause a massive dispersion of BrdU (5-bromo-2'-deoxyuridine)-labeled neuroblasts into surrounding brain regions, including cortex and striatum. These dispersed cells are capable of differentiation, some into mature neurons, and could potentially be of value in the repair of CNS injury. Although the removal of PSA by genetic deletion of NCAM also results in a smaller OB and a swollen RMS, the cells do not escape the RMS in large numbers. These findings suggest that the presence of NCAM without PSA plays a role in the dispersion process, possibly by inducing a new pattern of migration associated with NCAM-dependent differentiation.

Polysialic acid neural cell adhesion molecule (PSA-NCAM) is an adverse prognosis factor in glioblastoma, and regulates olig2 expression in glioma cell lines

BACKGROUND

Glioblastoma multiforme (GBM) is the most aggressive and frequent brain tumor, albeit without cure. Although patient survival is limited to one year on average, significant variability in outcome is observed. The assessment of biomarkers is needed to gain better knowledge of this type of tumor, help prognosis, design and evaluate therapies. The neurodevelopmental polysialic acid neural cell adhesion molecule (PSA-NCAM) protein is overexpressed in various cancers. Here, we studied its expression in GBM and evaluated its prognosis value for overall survival (OS) and disease free survival (DFS).

METHODS

We set up a specific and sensitive enzyme linked immunosorbent assay (ELISA) test for PSA-NCAM quantification, which correlated well with PSA-NCAM semi quantitative analysis by immunohistochemistry, and thus provides an accurate quantitative measurement of PSA-NCAM content for the 56 GBM biopsies analyzed. For statistics, the Spearman correlation coefficient was used to evaluate the consistency between the immunohistochemistry and ELISA data. Patients' survival was estimated by using the Kaplan-Meier method, and curves were compared using the log-rank test. On multivariate analysis, the effect of potential risk factors on the DFS and OS were evaluated using the cox regression proportional hazard models. The threshold for statistical significance was p = 0.05.

RESULTS

We showed that PSA-NCAM was expressed by approximately two thirds of the GBM at variable levels. On univariate analysis, PSA-NCAM content was an adverse prognosis factor for both OS (p = 0.04) and DFS (p = 0.0017). On multivariate analysis, PSA-NCAM expression was an independent negative predictor of OS (p = 0.046) and DFS (p = 0.007). Furthermore, in glioma cell lines, PSA-NCAM level expression was correlated to the one of olig2, a transcription factor required for gliomagenesis.

CONCLUSION

PSA-NCAM represents a valuable biomarker for the prognosis of GBM patients.

Roles of polysialic acid in migration and differentiation of neural stem cells

Polysialic acid, a homopolymer of alpha2,8-linked sialic acid, is one of the carbohydrates expressed on neural precursors in the embryonic and adult brain. Polysialic acid, synthesized by two polysialyltransferases (ST8SiaII and ST8SiaIV), mainly modulates functions of the neural cell adhesion molecule (NCAM). Polysialic acid-deficient mice demonstrated that polysialylated NCAM plays crucial roles in various steps of neural development, such as cell survival and cell migration of neural precursors, neuronal guidance, and synapse formation. However, the mechanisms of the diverse phenotypes and molecules affected by polysialic acid remain to be defined. To study the roles of polysialic acid on neural stem cells, analyses of neural stem cells from polysialic acid-deficient and NCAM-deficient mice are useful. Here, we describe how to prepare neural precursor cells from mouse brain and how to analyze migration and differentiation of neurosphere cells in vitro.

Intranasal HB-EGF administration favors adult SVZ cell mobilization to demyelinated lesions in mouse corpus callosum

In the adult rodent brain, the subventricular zone (SVZ) represents a special niche for neural stem cells; these cells proliferate and generate neural progenitors. Most of these migrate along the rostral migratory stream to the olfactory bulb, where they differentiate into interneurons. SVZ-derived progenitors can also be recruited spontaneously to damaged brain areas to replace lost cells, including oligodendrocytes in demyelinated lesions. In this study, we searched for factors able to enhance this spontaneous recruitment of endogenous progenitors. Previous studies have suggested that epidermal growth factor (EGF) could stimulate proliferation, migration, and glial differentiation of SVZ progenitors. In the present study we examined EGF influence on endogenous SVZ cell participation to brain repair in the context of demyelinated lesions. We induced a focal demyelinated lesion in the corpus callosum by lysolecithin injection and showed that intranasal heparin-binding epidermal growth factor (HB-EGF) administration induces a significant increase in SVZ cell proliferation together with a stronger SVZ cell mobilization toward the lesions. Besides, HB-EGF causes a shift of SVZ-derived progenitor cell differentiation toward the astrocytic lineage. However, due to the threefold increase in cell recruitment by EGF treatment, the absolute number of SVZ-derived oligodendrocytes in the lesion of treated mice is higher than in controls. These results suggest that enhancing SVZ cell proliferation could be part of future strategies to promote SVZ progenitor cell mobilization toward brain lesions.

Dissecting polysialic acid and NCAM functions in brain development

The unique modification of the neural cell adhesion molecule (NCAM) by polysialic acid (polySia) is tightly associated with nervous system development and plasticity. The prevailing view that this large carbohydrate polymer acts as an anti-adhesive factor seems straightforward at first sight. However, during almost 25 years of polySia research it became increasingly clear that the impact of polySia on cell surface interactions can not be explained by one unifying mechanism. Recent progress in the generation of mouse models, which partially or completely lack polySia due to ablation of one or both of the two polySia synthesizing enzymes, provides novel insights into the function of this unique post-translational modification. The present review is focused on a phenotype comparison between the newly established mouse strains which combine polySia-deficiency with normal NCAM expression and the well-characterized NCAM negative mouse model. Analysis of shared and individual phenotypes allows a clear distinction between NCAM and polySia functions and revealed that polySia plays a vital role as a specific control element of NCAM-mediated interactions.

Polysialic acid-directed migration and differentiation of neural precursors are essential for mouse brain development

Polysialic acid, which is synthesized by two polysialyltransferases, ST8SiaII and ST8SiaIV, plays an essential role in brain development by modifying the neural cell adhesion molecule (NCAM). It is currently unclear how polysialic acid functions in different processes of neural development. Here we generated mice doubly mutant in both ST8SiaII and ST8SiaIV to determine the effects of loss of polysialic acid on brain development. In contrast to NCAM-deficient, ST8SiaII-deficient, or ST8SiaIV-deficient single mutant mice, ST8SiaII and ST8SiaIV double mutants displayed severe defects in anatomical organization of the forebrain associated with apoptotic cell death. Loss of polysialic acid affected both tangential and radial migration of neural precursors during cortical development, resulting in aberrant positioning of neuronal and glial cells. Glial cell differentiation was aberrantly increased in vivo and in vitro in the absence of polysialic acid. Consistent with these findings, polysialic acid-deficient mice exhibited increased expression of the glial cell marker glial fibrillary acidic protein and a decrease in expression of Pax6, a transcription factor regulating neural cell migration. These results indicate that polysialic acid regulates cell migration and differentiation of neural precursors crucial for brain development.

Impact of the PSA-NCAM system on pathophysiology in a chronic rodent model of temporal lobe epilepsy

Polysialylation is a posttranslational modification of the neural cell adhesion molecule (NCAM). In the adult brain, polysialylated NCAM (PSA-NCAM) is restricted to regions of neurogenesis and neuroplasticity, where PSA promotes plastic changes. Because a variety of plastic changes including neurogenesis have been suggested to be functionally involved in the pathophysiology of epilepsies, it is of specific interest to define the impact of the PSA-NCAM system on development and progression of this disease and associated comorbidities. Here, we studied the impact of transient enzymatic depolysialylation of NCAM on the pathophysiology in the amygdala kindling model, a chronic rodent model of temporal lobe epilepsy. The investigations focused on seizure-induced neurogenesis, seizure progression, and on the development of kindling-associated changes in behavior and cognition. Loss of PSA decreased the number of hippocampal newborn cells that incorporated BrdU during the kindling process and the number of new neurons that were ectopically located in the hilus. The persistence of basal dendrites has been suggested to be a hallmark of newborn granule cells in the epileptic brain. Loss of PSA increased the number of cells with persistent basal dendrites. The modification of the hippocampal cell proliferation rate and the fate of newborn neurons which occurred as a consequence of PSA removal did not affect the generation of a hyperexcitable kindled network or associated behavioral changes. Kindling progression was comparable in rats with and without removal of PSA. In contrast, loss of PSA increased acute seizure susceptibility as indicated by reduced seizure thresholds before kindling. The data indicate that hippocampal proliferation rates and ectoptic hilar newborn neurons are less critical for epileptic network generation. The PSA-NCAM system was not substantiated as a target for antiepileptogenic strategies. However, its impact on ectopic newborn neurons gives evidence that modulation of PSA-NCAM function may be a strategy to promote neuroregeneration in different central nervous system insults.

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.

Activation of the subventricular zone in multiple sclerosis: evidence for early glial progenitors

In multiple sclerosis (MS), oligodendrocyte and myelin destruction lead to demyelination with subsequent axonal loss. Experimental demyelination in rodents has highlighted the activation of the subventricular zone (SVZ) and the involvement of progenitor cells expressing the polysialylated form of neural cell adhesion molecule (PSA-NCAM) in the repair process. In this article, we studied the distribution of early PSA-NCAM(+) progenitors in the SVZ and MS lesions in human postmortem brains. Compared with controls, MS SVZ showed a 2- to 3-fold increase in cell density and proliferation, which correlated with enhanced numbers of PSA-NCAM(+) and glial fibrillary acidic protein-positive (GFAP(+)) cells. PSA-NCAM(+) progenitors mainly were Sox9(+), and a few expressed Sox10 and Olig2, markers of oligodendroglial specification. PSA-NCAM(+) progenitors expressing Sox10 and Olig2 also were detected in demyelinated MS lesions. In active and chronic active lesions, the number of PSA-NCAM(+) progenitors was 8-fold higher compared with chronic silent lesions, shadow plaques, and normal-appearing white matter. In active and chronic active lesions, PSA-NCAM(+) progenitors were more frequent in periventricular lesions (30-50%) than in lesions remote from the ventricular wall. These data indicate that, as in rodents, activation of gliogenesis in the SVZ occurs in MS and suggest the mobilization of SVZ-derived early glial progenitors to periventricular lesions, where they could give rise to oligodendrocyte precursors. These early glial progenitors could be a potential target for therapeutic strategies designed to promote myelin repair in MS.

Enriched environment promotes adult neural progenitor cell mobilization in mouse demyelination models

Since the discovery of adult neural stem cells, mobilization of endogenous stem cells from the subventricular zone (SVZ) emerges as a promising strategy to promote brain repair. Here, we examined the effect of environment enrichment on SVZ cell mobilization in demyelinating pathologies. We showed that enriched housing conditions reduced functional impairment in experimental autoimmune encephalomyelitis (EAE), a rodent model of multiple sclerosis. Furthermore, both in a focal demyelination model (lysolecithin injection) and in the inflammatory EAE model, SVZ mitotic activity and the number of SVZ-derived cells in demyelinated areas were significantly increased by environment enrichment. Enriched housing conditions also promoted the oligodendrocyte fate of SVZ-recruited cells in the EAE lesions. Altogether our results show that environment enrichment provides beneficial conditions to promote the mobilization of neural progenitors into demyelinating lesions and to favour functional recovery.

A progressive and cell non-autonomous increase in striatal neural stem cells in the Huntington's disease R6/2 mouse

Neural stem and progenitor cells are located in the subependyma of the adult forebrain. An increase in adult subependymal cell proliferation is reported after various kinds of brain injury. We demonstrate an expansion of neural precursor cells in the postnatal subependyma in a murine genetic disease model of Huntington's disease (HD), the R6/2 mouse. We used the in vitro neurosphere assay as an index of the number of neural stem cells in vivo and to assess proliferation kinetics in vitro and in vivo bromodeoxyuridine labeling to assess the progenitor cell population and their fates. Disease progression in this model leads to an increase in the numbers of neural stem cells in the adult striatal subependyma. This increase is produced cell non-autonomously by events in the R6/2 brains as the mice become increasingly symptomatic. Once the neural stem cell increase is induced in vivo, it is maintained during in vitro passaging of neural stem cells, but the neural stem cell increase is not reproduced during in vitro passaging of neural stem cells from presymptomatic R6/2 mice. In addition, we show that some of the R6/2 neural progenitor cells show a change from their normal migration destiny toward the olfactory bulb. Instead, some of these cells migrate into the striatum, one of the main affected areas in HD. Our findings demonstrate that HD damage recruits precursor cells in two ways: expansion of neural stem cells and altered migration of progenitor cells.

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.

The corpus callosum as an evolutionary innovation

The corpus callosum (CC) is the major interhemispheric fibre bundle in the eutherian brain and has been described as a true evolutionary innovation. This paper reviews the current literature with regard to functional, developmental and genetic concepts that may help elucidate the evolutionary origin of this structure. It has been suggested that the CC arose in the eutherian brain as a more direct and, therefore, more effective system for the interhemispheric integration of topographically organized sensory cortices than the anterior commissure (AC) and hippocampal commissure (HC) already present in nonplacental mammals. It can also be argued, however, that the ability of the CC to integrate the newly evolving motor cortices of placental mammals may have played a role in the evolutionary fixation of this structure. Investigations into the developmental mechanism involved in the formation of the CC and their underlying patterns of gene expression make it possible to formulate a tentative hypothesis about the evolutionary origin of this commissure. This paper suggests that changes in the developmental patterns of the expression of certain regulatory genes may have allowed a first group of callosal pioneering axons to cross the cortical midline. These pioneering fibres may have used the axons of the HC to find their way across the midline. Additional callosal fibres may then have fasciculated with these pioneers. Once the CC had formed in this way, more complex systems of axonal guidance may have evolved over time, thus enabling a gradual increase in the size and complexity of the CC.

Migrating and myelinating potential of neural precursors engineered to overexpress PSA-NCAM

Polysialic acid (PSA) on NCAM is an important modulator of cell-cell interactions during development and regeneration. Here we investigated whether PSA overexpression influences neural cell migration and myelination. We stably expressed a GFP-tagged polysialytransferase, PSTGFP, in mouse neurospheres and induced prolonged PSA synthesis. Using a chick xenograft assay for migration, we show that PSA can instruct precursor migration along the ventral pathway. PSA persistence did not change neural precursor multipotentiality in vitro but induced a delay in oligodendrocyte differentiation. PSTGFP+ precursors showed widespread engraftment in shiverer brain, closely similar to that observed with control precursors expressing a fluorescent protein. Initially, myelination by oligodendrocytes was delayed but, eventually, down-regulation of PSTGFP occurred, allowing myelination to proceed. Thus down-regulation of polysialyltransferases takes place even in cells where its RNA is under the control of a heterologous promoter and engineering PSA overexpression in neural precursors does not cause irreversible unphysiological effects.

Beta1 integrins activate a MAPK signalling pathway in neural stem cells that contributes to their maintenance

The emerging evidence that stem cells develop in specialised niches highlights the potential role of environmental factors in their regulation. Here we examine the role of beta1 integrin/extracellular matrix interactions in neural stem cells. We find high levels of beta1 integrin expression in the stem-cell containing regions of the embryonic CNS, with associated expression of the laminin alpha2 chain. Expression levels of laminin alpha2 are reduced in the postnatal CNS, but a population of cells expressing high levels of beta1 remains. Using neurospheres - aggregate cultures, derived from single stem cells, that have a three-dimensional architecture that results in the localisation of the stem cell population around the edge of the sphere - we show directly that beta1 integrins are expressed at high levels on neural stem cells and can be used for their selection. MAPK, but not PI3K, signalling is required for neural stem cell maintenance, as assessed by neurosphere formation, and inhibition or genetic ablation of beta1 integrin using cre/lox technology reduces the level of MAPK activity. We conclude that integrins are therefore an important part of the signalling mechanisms that control neural stem cell behaviour in specific areas of the CNS.

Regionalization and fate specification in neurospheres: the role of Olig2 and Pax6

Neurosphere cultures are widely used to propagate multipotent CNS precursors, but their differentiation into neurons or oligodendrocytes is rather poor. To elucidate fate determination in this system, we examined the expression and function of candidate transcription factors in neurospheres derived from different CNS regions during development and adulthood. We observed prominent down-regulation of most transcription factors present in telencephalic precursors upon growth factor exposure in neurosphere cultures while Olig1 and Olig2 expression was strongly up-regulated. Interference with Olig2 in neurospheres revealed its role in self-renewal during expansion and for the generation of neurons and oligodendrocytes during differentiation. We further show that neurogenesis becomes fully Pax6-dependent in the neurosphere culture system, independent of the region of origin, and that Pax6 overexpression is sufficient to direct almost all neurosphere-derived cells towards neurogenesis. Thus, a pathway combining transcription factors of dorsal and ventral regions is activated in the neurosphere culture model.

Oligodendrocyte progenitor migration in response to injury of glial monolayers requires the polysialic neural cell-adhesion molecule

Injury to the nervous system results in reactive astrogliosis that is a critical determinant of neuronal regeneration. To analyze glial responses to mechanical injury and the role of the polysialic neural cell adhesion molecule (PSA-NCAM) in this process, we established primary glia cultures from newborn rat cerebral cortex. Scratching a confluent monolayer of primary glial cells resulted in two major events: rapid migration of oligodendrocyte progenitor-like (O-2A) cells into the wounded area and development of polarized morphology of type 1 astrocytes at the wound edge. Migrating O-2A progenitors had a bipolar morphology and exhibited A2B5 and O4 immunolabeling. Once these cells were established inside the wounded area, they lost A2B5 immunoreactivity and differentiated into glial fibrillary acidic protein-positive astrocytes. Migrating O-2A cells expressed PSA-NCAM, but type 1 astrocytes at the wound edge did not. Treatment of wounded cultures with Endo-N, which specifically removes PSA from the surface of cells, resulted in a significant decrease in O-2A cell migration into the wounded area and completely blocked the wound closure. Video time-lapse analysis showed that, in the presence of Endo-N, O-2A cells remained motile and migrated short distances but did not move away from the monolayer. These results demonstrate that O-2A progenitors contribute to reactive astrogliosis in culture and that PSA-NCAM is involved in this process by regulating cell migration.

Growth factor treatment promotes mobilization of young but not aged adult subventricular zone precursors in response to demyelination

Precursor cells of the adult mouse subventricular zone (SVZ) are mobilized and recruited by a lysolecithin (LPC)-induced demyelination of the corpus callosum. Because age decreases the proliferation of the SVZ neural precursors as well as the potential for myelin repair of the adult central nervous system, we have compared the ability of young and aged adult neural precursors to respond to LPC-induced demyelination. With age, the SVZ cells lost their capacity to proliferate and to be recruited by the lesion. Whereas a single injection of fibroblast growth factor-2 or transforming growth factor-alpha stimulated the proliferation of SVZ and rostral migratory stream precursors in both groups of animals after demyelination, they favored recruitment at the lesion in young mice but not in aged ones. In vitro experiments using neurospheres derived from young and aged animals indicated that both populations have the same migratory performances. Our in vivo data thus suggest that aged neural precursors may loose their intrinsic capacities to respond to demyelination-induced signals. Alternatively, their function may be altered by modification of the aged extracellular environment.