A role for the polysialic acid-neural cell adhesion molecule in PDGF-induced chemotaxis of oligodendrocyte precursor cells.

Directed migration of oligodendrocyte precursor cells (OPCs) is important for myelin formation and repair but the mechanisms of directional control are poorly understood. Here we have tested the role of polysialic acid-neural cell adhesion molecule (PSA-NCAM) in the directional migration of OPCs towards platelet-derived growth factor (PDGF). Using a Boyden microchemotaxis chamber and the Dunn direct viewing chamber, we show that in concentration gradients of PDGF, PSA-positive OPCs polarize and efficiently migrate towards the source of PDGF (chemotaxis). The loss or inactivation of the polysialic tail of NCAM leads to an altered pattern of OPC migration in response to PDGF gradients. Cells under these conditions, while being polarized and migrating, show no bias of displacement towards the source of PDGF and make random turns. By contrast, directed migration of OPCs towards basic fibroblast growth factor was not affected by the removal of PSA. Moreover, inactivation of PSA does not interfere with the random migration pattern of cells in uniform concentrations of PDGF (chemokinesis). These results suggest that PSA-NCAM is specifically involved in establishing the directionality of OPC migration in response to the concentration gradient of PDGF, but it is not essential for cell motility per se.

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.

Neuregulin signaling regulates neural precursor growth and the generation of oligodendrocytes in vitro.

Neuregulin 1 (Nrg-1) isoforms have been shown to influence the emergence and growth of oligodendrocytes, the CNS myelin-forming cells. We have investigated how Nrg-1 signaling of ErbB receptors specifically controls the early stages of oligodendrocyte generation from multipotential neural precursors (NPs). We show here that embryonic striatal NPs express multiple Nrg-1 transcripts and proteins as well as their specific receptors, ErbB2 and ErbB4, but not ErbB3. The major isoform synthesized by striatal NPs is a transmembrane type III isoform called cysteine-rich domain Nrg-1. To examine the biological effect of Nrg-1, we added soluble ErbB3 (sErbB3) to growing neurospheres. This inhibitor of Nrg-1 bioactivity decreased mitosis of NPs and increased their apoptosis, resulting in a significant reduction in neurosphere size and number. When NPs were induced to migrate and differentiate by adhesion of neurospheres to the substratum, the level of type III isoforms detected by RT-PCR and Western blot decreased in parallel with a reduction in Nrg-1 fluorescence intensity in differentiating astrocytes, neurons, and oligodendrocytes. Pretreatment of growing neurospheres with sErbB3 induced a threefold increase in the proportion of oligodendrocytes generated from NPs migrating out of the neurosphere. This effect was not observed with an unrelated soluble receptor. Addition of sErbB3 during NP growth and differentiation enhanced oligodendrocyte maturation as shown by expression of galactocerebroside and myelin basic protein. We propose that both type III Nrg-1 signaling and soluble ErbB receptors modulate oligodendrocyte development from NPs.

PSA-NCAM is required for activity-induced synaptic plasticity.

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

mCD24 expression in the developing mouse brain and in zones of secondary neurogenesis in the adult.

Interactions mediated by cell surface glycoproteins are considered to be crucial during the formation of the nervous system. Using a monoclonal antibody directed to mCD24, a glycosylphos-phatidylinositol-anchored membrane glycoprotein, we have mapped its distribution throughout the mouse cerebral cortex during development and in young adult. Before birth, mCD24 immunoreactivity was observed in the intermediate zone, the cortical plate and the marginal zone, whereas the ventricular zones were immunonegative. After birth, mCD24 expression declined rapidly in the cortex, except in the corpus callosum (and other commissures in the brain) where immunoreactivity was still found until P20. Furthermore, mCD24 expression was maintained in young adults (until P60, at least) in zones of secondary neurogenesis, such as the granule cells of the dentate gyrus, the subventricular zone lining the anterior part of the lateral ventricles and a zone of cells extending between the striatum and the corpus callosum to the centre of the olfactory bulb. In this area mCD24 and polysialic acid neural cell adhesion molecule stainings were superimposed, and this corresponded to the pathway of migration of the olfactory immature neurons (subependymal layer). A layer of ciliated ependymal cells, lining all the ventricular walls, was also immunoreactive for mCD24. Thus, except for these epithelial-like cells, mCD24 was essentially found associated with differentiating postmitotic neurons. Its spatiotemporal expression, both during development and in the adult, is compatible with a role for this glycoprotein in cell surface recognition and in signalling events occurring during neuronal migration and axonal growth.

mCD24, a glycoprotein transiently expressed by neurons, is an inhibitor of neurite outgrowth.

In the immune system, mCD24, the mouse homolog of the human glycosyl phosphatidylinositol-anchored glycoprotein CD24, may play a role in cell adhesion. In the nervous system, the function of mCD24 has not been determined, but its transient expression by neurons suggests that it may be involved in axon growth in development. Here we show that retinal ganglion cells (RGCs) and dorsal root ganglion (DRG) neurons express mCD24 in the developing but not adult mouse in vivo and in DRG neurons of the injured adult peripheral nervous system (PNS). In vitro, mCD24 was expressed by immature neurons and by a subpopulation of adult DRG neurons. To analyze the possible function of mCD24 in the nervous system, we prepared rat C6 glioma cells stably transfected or retrovirally infected with mCD24 cDNA. The cells did not exhibit changes in their adhesive properties or cell division rate after transfection or infection. When mCD24-expressing cells were used as monolayer substrates for culturing RGCs and DRG neurons, neurite outgrowth was inhibited, depending on neuronal age and on the relative levels of mCD24 in the monolayer. This inhibition, however, was not dependent on the expression of mCD24 by the neurons themselves, because DRG neurons of a mouse deleted of the mCD24 gene showed the same response. These results show that mCD24 interacts in a heterophilic manner with a developmentally regulated molecule expressed by neurons, and they suggest that in vivo, mCD24 may inhibit the further extension or collateral branching of axons in late embryonic development.