Hyperthermic injury of oligodendrocyte precursor cells: implications for dysmyelination disorders

Implication of gamma-secretase in neuregulin-induced maturation of oligodendrocytes

Increasing evidences suggest that, after neuregulin (NRG) stimulation, ErbB4 undergoes a series of proteolysis, including gamma-secretase cleavage. The released ErbB4 intracellular domain (EICD) is translocated into nucleus and has a transcriptional function. Although NRG-ErbB4 signaling mediates maturation of oligodendrocytes (OLs), the role of EICD and gamma-secretase in this process remains elusive. Here, we showed that NRG-ErbB4 interaction accumulated EICD in the nucleus and promoted the expression of myelin basic protein expression in OLs. Conversely, inhibitor of ErbB4 or gamma-secretase blocked the capacity of NRG. Nuclear accumulation of EICD did not influence maturation of neurons and astrocytes and early development of OLs. We also found that EICD translocation accorded a temporal pattern, consistent with the developmental gradient of hippocampus. Our data suggest that gamma-secretase activation and EICD nuclear translocation are required for OL maturation induced by NRG, and ErbB4 acts as a functional receptor depending on a new signaling cascade.

GVS-111 prevents oxidative damage and apoptosis in normal and Down's syndrome human cortical neurons

The neuroprotective activity of a novel N-acylprolyl-containing dipeptide analog of the nootropic 2-oxo-1-pyrrolidine acetamide (Piracetam) designated as GVS-111 (DVD-111/Noopept) was tested in two in vitro models of neuronal degeneration mediated by oxidative stress: normal human cortical neurons treated with H(2)O(2), and Down's syndrome (DS) cortical neurons. Incubation of normal cortical neurons with 50 microM H(2)O(2) for 1h resulted in morphological and structural changes consistent with neuronal apoptosis and in the degeneration of more than 60% of the neurons present in the culture. GVS-111 significantly increased neuronal survival after H(2)O(2)-treatment displaying a dose-dependent neuroprotective activity from 10nM to 100 microM, and an IC(50) value of 1.21+/-0.07 microM. GVS-111 inhibited the accumulation of intracellular free radicals and lipid peroxidation damage in neurons treated with H(2)O(2) or FeSO(4), suggesting an antioxidant mechanism of action. GVS-111 exhibited significantly higher neuroprotection compared to the standard cognition enhancer Piracetam, or to the antioxidants Vitamin E, propyl gallate and N-tert-butyl-2-sulpho-phenylnitrone (s-PBN). In DS cortical cultures, chronic treatment with GVS-111 significantly reduced the appearance of degenerative changes and enhanced neuronal survival. The results suggest that the neuroprotective effect of GVS-111 against oxidative damage and its potential nootropic activity may present a valuable therapeutic combination for the treatment of mental retardation and chronic neurodegenerative disorders.

Glial cells as targets for cytotoxic immune mediators

Oligodendrocytes and Schwann cells are the glia principally responsible for the synthesis and maintenance of myelin. Damage may occur to these cells in a number of conditions, but perhaps the most studied are the idiopathic inflammatory demyelinating diseases, multiple sclerosis in the CNS, and Guillain-Barré syndrome and its variants in the peripheral nervous system (PNS). This article explores the effects on these cells of cytotoxic immunological and inflammatory mediators: similarities are revealed, of which perhaps the most important is the sensitivity of both Schwann cells and oligodendrocytes to many such agents. This area of research is, however, characterised and complicated by numerous and often very substantial inter-observer discrepancies. Marked variability in cell culture techniques, and in assays of cell damage and death, provide artifactual explanations for some of this variability; true inter-species differences also contribute. Not the least important conclusion centres on the limited capacity of in vitro studies to reveal disease mechanisms: cell culture findings merely illustrate possibilities which must then be tested ex vivo using human tissue samples affected by the relevant disease.

Mechanisms of damage to myelin and oligodendrocytes and their relevance to disease

Oligodendrocytes synthesize and maintain myelin in the central nervous system (CNS). Damage may occur to these cells in a number of conditions, including infections, exposure to toxins, injury, degeneration, or autoimmune disease, arising both in the course of human disease and in experimental animal models of demyelination and dysmyelination; multiple sclerosis is the commonest human demyelinating disorder. Conventional classical accounts of the pathology of this and other myelin diseases have given great insights into their core features, but there remain considerable uncertainties concerning the timing, means and cause(s) of oligodendrocyte and myelin damage. At present, therapeutic efforts largely concentrate on immune manipulation and damage limitation, an approach that has produced only modest effects in multiple sclerosis. One reason for this must be the limited understanding of the mechanisms underlying cell damage - clearly, successful therapeutic strategies for preserving the oligodendrocyte-myelin unit must depend on knowledge of how oligodendrocyte damage and death occurs. In this review, mechanisms of oligodendrocyte and myelin damage are considered, and attempts made to relate them to disease processes, clinical and experimental. The hallmarks of different cell death processes are described, and oligodendrocyte-myelin injury by cellular and soluble mediators is discussed, both in vitro and invivo. Recent developments concerning the pathological involvement of oligodendrocytes in neurodegenerative disease are summarized. Finally, these neuropathological and applied neurobiological observations are drawn together in the context of multiple sclerosis.

The multiple sclerosis lesion: initiated by a localized hypoperfusion in a central nervous system where mechanisms allowing leukocyte infiltration are readily upregulated?

A mechanism is proposed that may explain the factors that initiate a multiple sclerosis (MS) lesion. It is based upon the following two hypotheses: (i) there is a lower stimulus threshold for upregulating the mechanisms that result in leukocyte infiltration in individuals predisposed to developing MS; (ii) the MS lesion is initiated as a reduction in blood flow to a localized region of white matter. This reduction in blood flow leads to: (a) degenerative white matter changes affecting oligodendrocytes; (b) upregulation of chemokines in the endothelial cells and/or glial cells; and (c) upregulation of cell adhesion molecules on endothelial cells. Signals from the hypoxemic and hypoglycemic glial cells, likely involving myelin molecules and cytokines, result in an inflammatory immune response that results in rampant demyelination. Evidence supporting the proposed mechanism is presented, as well as suggestions on how to test the validity of the proposal.

Peroxide-scavenging deficit underlies oligodendrocyte susceptibility to oxidative stress

Previous work showed that the susceptibility of oligodendroglial progenitors to oxidative stress is related to their low reduced-glutathione (GSH) and high iron contents. This suggests that these cells have a poor ability to scavenge peroxides. All peroxides are scavenged by glutathione peroxidase. Glutathione peroxidase activity requires GSH as an electron donor resulting in the formation of oxidized-glutathione. Cellular GSH content is dependent upon synthesis as well as reduction of oxidized-glutathione. The objectives of the present study were to compare several parameters important in the ability to scavenge peroxides between astrocytes and oligodendroglia. Three stages of oligodendroglial differentiation were examined: the proliferative oligodendrocyte progenitor, the proliferative oligodendroblast, and the post-mitotic oligodendrocyte. We demonstrate that oligodendroglia at all stages of differentiation have less than one-half the content of GSH compared to astrocytes. This low level of GSH is due in part to a lower rate of GSH synthesis in oligodendroglia compared to astrocytes and in part to their having only one-half of the glutathione reductase activity of astrocytes. Glutathione peroxidase activity of oligodendroglia is less than 15% of that found in astrocytes. The low GSH and concomitant low glutathione peroxidase activity would tend to maintain peroxides at levels that are dangerously high if iron is released from iron stores. Oligodendroglia have high iron stores, and thus these findings emphasize how vulnerable the oligodendroglial lineage is to perturbations that result in oxidative stress.

Basic FGF attenuates amyloid beta-peptide-induced oxidative stress, mitochondrial dysfunction, and impairment of Na+/K+-ATPase activity in hippocampal neurons

Basic fibroblast growth factor (bFGF) exhibits trophic activity for many populations of neurons in the brain, and can protect those neurons against excitotoxic, metabolic and oxidative insults. In Alzheimer's disease (AD), amyloid beta-peptide (A beta) fibrils accumulate in plaques which are associated with degenerating neurons. A beta can be neurotoxic by a mechanism that appears to involve induction of oxidative stress and disruption of calcium homeostasis. Plaques in AD brain contain high levels of bFGF suggesting a possible modulatory role for bFGF in the neurodegenerative process. We now report that bFGF can protect cultured hippocampal neurons against A beta25-35 toxicity by a mechanism that involves suppression of reactive oxygen species (ROS) accumulation and maintenance of Na+/K+-ATPase activity. A beta25-35 induced lipid peroxidation, accumulation of H2O2, mitochondrial ROS accumulation, and a decrease in mitochondrial transmembrane potential; each of these effects of A beta25-35 was abrogated in cultures pre-treated with bFGF. Na+/K+-ATPase activity was significantly reduced following exposure to A beta25-35 in control cultures, but not in cultures pre-treated with bFGF. bFGF did not protect neurons from death induced by ouabain (a specific inhibitor of the Na+/K+-ATPase) or 4-hydroxynonenal (an aldehydic product of lipid peroxidation) consistent with a site of action of bFGF prior to induction of oxidative stress and impairment of ion-motive ATPases. By suppressing accumulation of oxyradicals, bFGF may slow A beta-induced neurodegenerative cascades.

Oligodendroglial precursor cell susceptibility to hypoxia is related to poor ability to cope with reactive oxygen species

Oligodendrocyte precursors and astrocytes in 2-week-old rat primary glial cultures survived 24 h of anoxia, suggesting both cell types could survive using glycolysis for ATP synthesis; however, when the hypoxia developed gradually, the majority of oligodendrocyte precursor cells died within 24 h of the beginning of the experiment but astrocytes survived. Similarly when cultures were exposed to an atmosphere of 1% oxygen, but not 2% or greater, oligodendrocyte precursors died within 24 h. Much more lipid peroxidation was seen under conditions of hypoxia than under conditions of anoxia suggesting that oligodendrocyte precursors died under the former condition because of free radical-induced damage. Using 5-(and -6)-carboxy-2',7'-dichlorodihydrofluorescein (DCFH) as an intracellular probe of oxidative stress, we have demonstrated directly on living cells that oligodendrocyte precursors have a poorer ability to scavenge free radicals than astrocytes. Furthermore, when free radicals were induced to form in the cells either by cysteine auto-oxidation or menadione redox cycling, oligodendrocyte precursors were more readily damaged than astrocytes. We conclude that oligodendroglial precursor cells are exquisitively sensitive to reactive oxygen species.