Astrocytes promote process outgrowth by adult human oligodendrocytes in vitro through interaction between bFGF and astrocyte extracellular matrix

Oligodendrocytes from forebrain are highly vulnerable to AMPA/kainate receptor-mediated excitotoxicity

Little is known of the molecular mechanisms that trigger oligodendrocyte death and demyelination in many acute central nervous system insults. Since oligodendrocytes express functional alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate-type glutamate receptors, we examined the possibility that oligodendrocyte death can be mediated by glutamate receptor overactivation. Oligodendrocytes in primary cultures from mouse forebrain were selectively killed by low concentrations of AMPA, kainate or glutamate, or by deprivation of oxygen and glucose. This toxicity could be blocked by the AMPA/kainate receptor antagonist 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX). In vivo, differentiated oligodendrocytes in subcortical white matter expressed AMPA receptors and were selectively injured by microstereotaxic injection of AMPA but not NMDA. These data suggest that oligodendrocytes share with neurons a high vulnerability to AMPA/kainate receptor-mediated death, a mechanism that may contribute to white matter injury in CNS disease.

Macrophages in CNS remyelination: friend or foe?

Hematogenous macrophages and resident brain microglia are agents of demyelination in multiple sclerosis (MS) and paradoxically may also participate in remyelination. In vitro studies have shown that macrophage enrichment of aggregate brain cultures promotes myelination per se and enhances the capacity to remyelinate following a demyelinating episode. It has been hypothesized that remyelination in MS is implemented by surviving dedifferentiated oligodendrocytes or by newly recruited progenitors that migrate, proliferate and synthesize myelin in response to signalling molecules in the local environment. We postulate that macrophage-derived cytokines or growth factors may directly or indirectly promote oligodendroglial proliferation and differentiation, contributing to myelin repair in inflammatory demyelinating disease.

Oligodendrocytes utilize a matrix metalloproteinase, MMP-9, to extend processes along an astrocyte extracellular matrix

Matrix metalloproteinases (MMPs), the key effectors of extracellular matrix remodeling, have been demonstrated to regulate the extension of neurites from neuronal cell bodies. In this report we have addressed the hypothesis that oligodendrocytes (OLs) may utilize a similar mechanism in extending their processes during the initial phase of myelination. Furthermore, given our previous findings linking protein kinase C (PKC) to the OL process outgrowth, we tested the postulate that this signal transduction pathway may regulate MMPs and thus the process outgrowth phenotype. We demonstrate that in response to pharmacologic activators of PKC, cultured human OLs augment their process extension with a concomitant increase in the activity of an MMP, MMP-9, as measured by gelatin zymography. Similarly, the phorbol ester-enhanced process extension and increased MMP-9 activity were both inhibited by calphostin C, a selective PKC inhibitor. Also, MMP inhibitors such as 1,10-phenanthroline and synthetic dipeptides that inactivate the MMP catalytic site negated the 4beta-phorbol-12,13-dibutyrate (PDB)-mediated process extension, further supporting the key role of MMPs in process extension in vitro. Finally, the elevation of MMP-9 protein expression in the mouse corpus callosum, a tissue rich in OL and myelin, coincided with the previously documented temporal increase in myelination that occurs postnatally. Taken together, these data suggest that MMP-9 constitutes an important mediator of OL process outgrowth, and that this protease in turn can be regulated by PKC. The results are relevant not only to the initial steps of myelination during development, but also to the attempted remyelination that has been shown to occur in pathologic conditions such as MS.

Extracellular matrix proteins expressed by human adult astrocytes in vivo and in vitro: an astrocyte surface protein containing the CS1 domain contributes to binding of lymphoblasts

Primary cultures of human astrocytes, expressing glial fibrillary acidic protein (GFAP), were obtained from postmortem brain tissue samples. These cultured astrocytes produced an extracellular matrix (ECM), containing laminin (Ln) and fibronectin (Fn), as shown with specific antibodies. The perinuclear staining observed in these cells indicated that these proteins were de novo synthesized. Monoclonal antibody (mAb) 90.45, which recognizes the CS1 sequence found in an alternatively spliced form of Fn, also stained cultured astrocytes. Immunohistochemical analysis of normal human brain tissue showed positive staining for the CS1 domain, both on protoplasmic and fibrous astrocytes located in the gray and white matter. In contrast to cultured astrocytes, no immunoreactivity for Ln or Fn was found on astrocytes in normal human brain tissue. These in situ data indicate that the CS1 domain expressed by astrocytes is not part of a splicing variant of Fn. Western blot analysis confirmed that the CS1 domain expressed by cultured human astrocytes is part of an astrocyte protein which is different from human Fn. The CS1 domain is a known ligand for the adhesion receptor alpha4beta1 (VLA-4). We found that the human lymphoma cell lines Jurkat and Ramos, which express alpha4beta1, bound to cultured human astrocytes, and that this interaction could be partly blocked by mAb 90.45 or a synthetic CS1 peptide. Thus, the novel CS1-containing surface protein expressed by astrocytes in vitro and in vivo, contributes to binding of lymphoblasts, and therefore may be a relevant adhesion molecule for the recruitment of alpha4-integrin expressing leukocytes into the central nervous system (CNS).

FGF-2 converts mature oligodendrocytes to a novel phenotype

Fibroblast growth factor (FGF)-2 differentially regulates oligodendrocyte progenitor proliferation and differentiation in culture, and modulates gene expression of its own receptors, in a developmental and receptor type-specific manner (Bansal et al., 1996a,b). Three FGF receptors (types 1, 2, 3) are expressed in postmitotic, terminally differentiating oligodendrocytes. Exposure of such cells to FGF-2 results in: (a) the down-regulation of myelin-specific gene expression (e.g., ceramide galactosyltransferase, 2',3'-cyclic nucleotide 3'-phosphohydrolase, myelin basic protein, proteolipid protein), (b) dramatic increases in the length of cellular processes in a time- and dose-dependent manner, (c) re-entrance into the cell cycle without accompanying mitosis, and (d) the alteration of the expression of both low- and high-affinity FGF receptors. Compared to oligodendrocyte progenitors, the differentiated oligodendrocytes treated with FGF-2 incorporate BrdU at a slower rates, exhibit different patterns of both FGF high- and low-affinity (syndecans) receptors, and are morphologically very different. In addition, they do not re-express the progenitor markers A2B5, NG2 or PDGFalpha receptor. Therefore, although the FGF-treated cells lose their differentiated OL/myelin markers, they do not revert to progenitors and clearly represent a different, apparently novel, phenotype both morphologically and biochemically, which we have termed NOLs. These data indicate that terminally differentiated oligodendrocytes retain the plasticity to reprogram their differentiation fate under the influence of environmental factors. The possible significance of this response to FGF relative to normal and pathological physiology is discussed. In particular, on the basis of these data we predict the appearance of cells in and around multiple sclerosis plaques with the phenotype O4+, NG2-, A2B5-, O1-, MBP-.

In situ expression of fibroblast growth factor receptors by oligodendrocyte progenitors and oligodendrocytes in adult mouse central nervous system

Basic fibroblast growth factor (bFGF) induces proliferation and alters differentiation of cultured oligodendrocyte lineage cells. In situ, bFGF is present in normal adult central nervous system (CNS) and upregulated during an early stage of various pathological conditions. We examined the expression of receptors for bFGF (FGFRs) by oligodendrocyte progenitors and oligodendrocytes in situ in normal adult mouse CNS to further understand the potential in situ response to bFGF. We found FGFR immunoreactivity in oligodendrocyte progenitors, identified by expression of NG2 or platelet-derived growth factor alpha receptor (PDGFalphaR), and in oligodendrocytes expressing 2',3'-cyclic nucleotide 3' phosphodiesterase. Particularly interesting is the demonstration that oligodendrocyte progenitors simultaneously expressing receptors for both bFGF and PDGF-AA are present in normal adult CNS. Since in vitro bFGF and PDGF-AA in combination induce oligodendrocyte progenitors from normal adult CNS to undergo rapid proliferation and migration, the in situ coexpression of FGFRs and PDGFalphaR supports the hypothesis that oligodendrocyte progenitors can respond to bFGF and PDGF-AA in situ, and that both growth factors may be critical for repopulation of demyelinated lesions during remyelination.

Astrogliosis in the neonatal and adult murine brain post-trauma: elevation of inflammatory cytokines and the lack of requirement for endogenous interferon-gamma

The relevance of astrogliosis remains controversial, especially with respect to the beneficial or detrimental influence of reactive astrocytes on CNS recovery. This dichotomy can be resolved if the mediators of astrogliosis are identified. We have measured the levels of transcripts encoding inflammatory cytokines in injury systems in which the presence or absence of astrogliosis could be produced selectively. A stab injury to the adult mouse brain using a piece of nitrocellulose (NC) membrane elicited a prompt and marked increase in levels of transcripts for interleukin (IL)-1alpha, IL-1beta, and tumor necrosis factor (TNF)-alpha, which are considered to be microglia/macrophage cytokines. The elevations preceded, or occurred concomitantly with, the rise in glial fibrillary acidic protein mRNA, an early manifestation of astrogliosis. In neonatal mice, IL-1 and TNF-alpha mRNA were elevated to a greater extent by an NC-implant injury, which produced astrogliosis, than after an NC-stab, with minimal astrogliosis. We determined whether endogenous interferon (IFN)-gamma could be responsible for the observed increases in IL-1 and TNF-alpha, because IFN-gamma is a potent microglia/macrophage activator, and because its exogenous administration to rodents enhanced astrogliosis after adult or neonatal insults. A lack of requirement for endogenous IFN-gamma was demonstrated by three lines of evidence. First, no increase in IFN-gamma transcripts could be found at injury. Second, the administration of a neutralizing antibody to IFN-gamma did not attenuate astrogliosis. Third, in IFN-gamma knockout adult mice, astrogliosis and increases in levels of IL-1alpha and TNF-alpha were induced rapidly by injury. The marked elevation of inflammatory cytokines is discussed in the context of astrogliosis and general CNS recovery.

Interferon beta-1b decreases the migration of T lymphocytes in vitro: effects on matrix metalloproteinase-9

In multiple sclerosis (MS), the influx of activated T lymphocytes into the brain parenchyma leads to the subsequent damage of oligodendrocytes, the cells that produce central nervous system (CNS) myelin. We report here that interferon beta-1b (IFNbeta-1b), a drug shown to be efficacious in the treatment of patients with MS, decreases the in vitro migration of activated T lymphocytes through fibronectin (FN), a major component of the basement membrane that surrounds cerebral endothelium. At 1,000 IU/ml, IFNbeta-1b reduced the migratory rate to that of unactivated T cells. In contrast, IFNgamma at 1,000 IU/ml, which caused a similar decrease (25%) in the proliferation rate of T lymphocytes as IFNbeta-1b, did not affect migration. All T-lymphocyte subsets and natural killer (NK) cells were demonstrated by flow cytometry to be equally affected by IFNbeta-1b treatment. 125I-Western blot analyses revealed that IFNbeta-1b treatment resulted in a marked reduction of the ability of T cells to cleave FN. The substrate-degrading capability of T lymphocytes was shown to be due predominantly to the activity of a 92-kd matrix metalloproteinase, MMP-9, whose levels were decreased by IFNbeta-1b. We suggest that the clinical benefits of IFNbeta-1b treatment in MS patients may be in part a result of the ability of this drug to significantly decrease MMP-9 activity, leading to a reduction of T-lymphocyte infiltration into the CNS.