Myelination and remyelination of aggregate rat brain cell cultures enriched with macrophages

Ozanimod for the treatment of relapsing forms of multiple sclerosis

Multiple sclerosis (MS) is an inflammatory disease that causes chronic neurological disability in young adults. Modulation of sphingosine 1-phosphate (S1P) receptors, a group of receptors that, among other things, regulate egression of lymphocytes from lymph nodes, has proven to be effective in treating relapsing MS. Fingolimod, the first oral S1P receptor modulator, has demonstrated potent efficacy and tolerability, but can cause undesirable side effects due to its interaction with a wide range of S1P receptor subtypes. This review will focus on ozanimod, a more selective S1P receptor modulator, which has recently received approval for relapsing MS. We summarize ozanimod's mechanism of action, and efficacy and safety from clinical trials that demonstrate its utility as another treatment option for relapsing MS.

Neuropathology of vitamin B12 deficiency in the Cd320-/- mouse

In humans, vitamin B12 deficiency causes peripheral and CNS manifestations. Loss of myelin in the peripheral nerves and the spinal cord (SC) contributes to peripheral neuropathy and motor deficits. The metabolic basis for the demyelination and brain disorder is unknown. The transcobalamin receptor-knockout mouse ( Cd320-/-) develops cobalamin (Cbl) deficiency in the nervous system, with mild anemia. A decreased S-adenosylmethionine: S-adenosylhomocysteine ratio and increased methionine were seen in the brain with no significant changes in neurotransmitter metabolites. The structural pathology in the SC presented as loss of myelin in the axonal tracts with inflammation. The sciatic nerve (SN) showed increased nonuniform, internodal segments suggesting demyelination, and remyelination in progress. Consistent with these changes, the Cd320-/- mouse showed an increased latency to thermal nociception. Further, lower amplitude of compound action potential in the SN suggested that the functional capacity of the heavily myelinated axons were preferentially compromised, leading to loss of peripheral sensation. Although the metabolic basis for the demyelination and the structural and functional alterations of the nervous system in Cbl deficiency remain unresolved, the Cd320-/- mouse provides a unique model to investigate the pathologic consequences of vitamin B12 deficiency. -Arora, K., Sequeira, J. M., Alarcon, J. M., Wasek, B., Arning, E., Bottiglieri, T., Quadros, E. V. Neuropathology of vitamin B12 deficiency in the Cd320-/- mouse.

A silver lining of neuroinflammation: Beneficial effects on myelination

Myelin accelerates action potential conduction velocity and provides essential energy support for axons. Unfortunately, myelin and myelinating cells are often vulnerable to injury or disease, resulting in myelin damage, which in turn can lead to axon dysfunction, overt pathology and neurological impairment. Inflammation is a common component of trauma and disease in both the CNS and PNS and therefore an active inflammatory response is often considered deleterious to myelin health. While inflammation can certainly damage myelin, inflammatory processes also can positively affect oligodendrocyte lineage progression, myelin debris clearance, oligodendrocyte metabolism and myelin repair. In the periphery, inflammatory cascades can also augment myelin repair, including processes initiated by infiltrating immune cells as well as by local Schwann cells. In this review, various aspects of inflammation beneficial to myelin repair are discussed and should be considered when designing or implementing anti-inflammatory therapies for CNS and PNS injury involving myelinating cells.

Liver X receptors alpha and beta promote myelination and remyelination in the cerebellum

The identification of new pathways governing myelination provides innovative avenues for remyelination. Liver X receptors (LXRs) α and β are nuclear receptors activated by oxysterols that originated from the oxidation of cholesterol. They are crucial for cholesterol homeostasis, a major lipid constituent of myelin sheaths that are formed by oligodendrocytes. However, the role of LXRs in myelin generation and maintenance is poorly understood. Here, we show that LXRs are involved in myelination and remyelination processes. LXRs and their ligands are present in oligodendrocytes. We found that mice invalidated for LXRs exhibit altered motor coordination and spatial learning, thinner myelin sheaths, and reduced myelin gene expression. Conversely, activation of LXRs by either 25-hydroxycholesterol or synthetic TO901317 stimulates myelin gene expression at the promoter, mRNA, and protein levels, directly implicating LXRα/β in the transcriptional control of myelin gene expression. Interestingly, activation of LXRs also promotes oligodendroglial cell maturation and remyelination after lysolecithin-induced demyelination of organotypic cerebellar slice cultures. Together, our findings represent a conceptual advance in the transcriptional control of myelin gene expression and strongly support a new role of LXRs as positive modulators in central (re)myelination processes.

From precursors to myelinating oligodendrocytes: contribution of intrinsic and extrinsic factors to white matter plasticity in the adult brain

Oligodendrocyte precursor cells (OPC) are glial cells that metamorphose into myelinating oligodendrocytes during embryogenesis and early stages of post-natal life. OPCs continue to divide throughout adulthood and some eventually differentiate into oligodendrocytes in response to demyelinating lesions. There is growing evidence that OPCs are also involved in activity-driven de novo myelination of previously unmyelinated axons and myelin remodeling in adulthood. In this review, we summarize the interwoven factors and cascades that promote the activation, recruitment and differentiation of OPCs into myelinating oligodendrocytes in the adult brain based mostly on results found in the study of demyelinating diseases. The goal of the review was to draw a complete picture of the transformation of OPCs into mature oligodendrocytes to facilitate the study of this transformation in both the normal and diseased adult brain.

In vitro modeling of central nervous system myelination and remyelination

This review aims to summarize the current techniques to study myelination and remyelination in culture systems. We attempt to put these into historical context, and to identify the strengths and weaknesses of each approach, which vary depending on the experimental question to be tested. We discuss the difficulty and importance of quantification of myelination and in particular remyelination. Finally, we provide our predictions of how these techniques will and should develop in the future.

Inflammation in neurological disorders: a help or a hindrance?

Inflammation of the central nervous system (CNS) (neuroinflammation) is now recognized to be a feature of all neurological disorders. In multiple sclerosis, there is prominent infiltration of various leukocyte subsets into the CNS. Even when there is no significant inflammatory infiltrates, such as in Parkinson or Alzheimer disease, there is intense activation of microglia with resultant elevation of many inflammatory mediators within the CNS. An extensive dataset describes neuroinflammation to have detrimental consequences, but results emerging largely over the past decade have indicated that aspects of the inflammatory response are beneficial for CNS outcomes. Benefits of neuroinflammation now include neuroprotection, the mobilization of neural precursors for repair, remyelination, and even axonal regeneration. The findings that neuroinflammation can be beneficial should not be surprising as a properly directed inflammatory response in other tissues is a natural healing process after an insult. In this article, we review the data that highlight the dual aspects of neuroinflammation in being a hindrance on the one hand but also a significant help for recovery of the CNS on the other. We consider how the inflammatory response may be beneficial or injurious, and we describe strategies to harness the beneficial aspects of neuroinflammation.

Fingolimod (FTY720) enhances remyelination following demyelination of organotypic cerebellar slices

Remyelination, which occurs subsequent to demyelination, contributes to functional recovery and is mediated by oligodendrocyte progenitor cells (OPCs) that have differentiated into myelinating cells. Therapeutics that impact remyelination in the CNS could be critical determinants of long-term functional outcome in multiple sclerosis (MS). Fingolimod is a S1P receptor modulator in MS clinical trials due to systemic anti-inflammatory properties, yet may impact cells within the CNS by crossing the blood-brain barrier. Previous studies using isolated dissociated cultures indicate that neural cells express S1P receptors and respond to receptor engagement. Our objective was to assess the effects of fingolimod on myelin-related processes within a multicellular environment that maintains physiological cell-cell interactions, using organotypic cerebellar slice cultures. Fingolimod treatment had no impact on myelin under basal conditions. Fingolimod treatment subsequent to lysolecithin-induced demyelination enhanced remyelination and process extension by OPCs and mature oligodendrocytes, while increasing microglia numbers and immunoreactivity for the astrocytic marker glial fibrillary acidic protein. The number of phagocytosing microglia was not increased by fingolimod. Using S1P receptor specific agonists and antagonists, we determined that fingolimod-induced effects on remyelination and astrogliosis were mediated primarily through S1P3 and S1P5, whereas enhanced microgliosis was mediated through S1P1 and S1P5. Taken together, these data demonstrate that fingolimod modulates multiple neuroglial cell responses, resulting in enhanced remyelination in organotypic slice cultures that maintain the complex cellular interactions of the mammalian brain.

Paving the axonal highway: from stem cells to myelin repair

Multiple sclerosis (MS), a demyelinating disorder of the central nervous system (CNS), remains among the most prominent and devastating diseases in contemporary neurology. Despite remarkable advances in anti-inflammatory therapies, the inefficiency or failure of myelin-forming oligodendrocytes to remyelinate axons and preserve axonal integrity remains a major impediment for the repair of MS lesions. To this end, the enhancement of remyelination through endogenous and exogenous repair mechanisms and the prevention of axonal degeneration are critical objectives for myelin repair therapies. Thus, recent advances in uncovering myelinating cell sources and the intrinsic and extrinsic factors that govern neural progenitor differentiation and myelination may pave a way to novel strategies for myelin regeneration. The scope of this review is to discuss the potential sources of stem/progenitor cells for CNS remyelination and the molecular mechanisms underlying oligodendrocyte myelination.

In vitro and in vivo pharmacological models to assess demyelination and remyelination

In making a selection of cellular tools and animal models for generating screening assays in the search for new drugs, one needs to take into consideration the practicality of their use in the drug discovery process. Conducting high-throughput primary screens using libraries of small molecules, close to 1 million members in size, requires the generation of large numbers of cells which are easily acquired, reliably enriched, and reproducibly responsive to standard positive controls. These cells need to be similar in form and function to their counterparts in human disease. In vitro assays that can be mechanized by using robots can therefore save time and costs. In selecting in vivo models, consideration must be given to the species and strain of animal chosen, the appropriateness of the model to human disease, the extent of animal husbandry required during the in-life pharmacological assessment, the technical aspects of generating the model and harvesting the tissues for analyses, the cost of research tools in terms of time and money (demyelinating and remyelinating agents, amount of compound to be generated), and the length of time required for drug testing in the model. A consideration of the translational aspects of the in vivo model compared to those used in the clinic is also important. These themes will be developed with examples for drug discovery in the field of CNS demyelination and repair, specifically as it pertains to multiple sclerosis.

Myelinated, synapsing cultures of murine spinal cord--validation as an in vitro model of the central nervous system

Research in central nervous system (CNS) biology and pathology requires in vitro models, which, to recapitulate the CNS in vivo, must have extensive myelin and synapse formation under serum-free (defined) conditions. However, finding such a model has proven difficult. The technique described here produces dense cultures of myelinated axons, with abundant synapses and nodes of Ranvier, that are suitable for both morphological and biochemical analysis. Cellular and molecular events were easily visualised using conventional microscopy. Ultrastructurally, myelin sheaths were of the appropriate thickness relative to axonal diameter (G-ratio). Production of myelinated axons in these cultures was consistent and repeatable, as shown by statistical analysis of multiple experimental repeats. Myelinated axons were so abundant that from one litter of embryonic mice, myelin was produced in amounts sufficient for bulk biochemical analysis. This culture method was assessed for its ability to generate an in vitro model of the CNS that could be used for both neurobiological and neuropathological research. Myelin protein kinetics were investigated using a myelin fraction isolated from the cultures. This fraction was found to be superior, quantitatively and qualitatively, to the fraction recovered from standard cultures of dissociated oligodendrocytes, or from brain slices. The model was also used to investigate the roles of specific molecules in the pathogenesis of inflammatory CNS diseases. Using the defined conditions offered by this culture system, dose-specific, inhibitory effects of inflammatory cytokines on myelin formation were demonstrated, unequivocally. The method is technically quick, easy and reliable, and should have wide application to CNS research.

Hypothesized role of galactocerebroside and NKT cells in the etiology of multiple sclerosis

According to the molecular mimicry theory, multiple sclerosis (MS) develops when the immune system mistakenly attacks a component of the myelin sheath that is structurally similar to a foreign epitope. The glycolipid galactocerebroside (GalC) is a major component of myelin. As lipids comprise between 70% and 85% of myelin, glycolipids should be investigated as candidate autoantigens in MS. GalC displays broad structural similarities to the Borrelia burgdorferi glycolipid antigen BbGL-2 and to the Sphingomonas antigen GalAGSL. In principle, therefore, these bacteria may induce an autoimmune attack on the myelin sheath. GalC is also structurally similar to natural killer T (NKT) cell ligand alpha-galactosylceramide (alpha-GalCer). Further studies must be performed to clarify the role of GalC in the activation of NKT cells and the development of MS.

Remyelination in multiple sclerosis

Remyelination is the phenomenon by which new myelin sheaths are generated around axons in the adult central nervous system (CNS). This follows the pathological loss of myelin in diseases like multiple sclerosis (MS). Remyelination can restore conduction properties to axons (thereby restoring neurological function) and is increasingly believed to exert a neuroprotective role on axons. Remyelination occurs in many MS lesions but becomes increasingly incomplete/inadequate and eventually fails in the majority of lesions and patients. Efforts to understand the causes for this failure of regeneration have fueled research into the biology of remyelination and the complex, interdependent cellular and molecular factors that regulate this process. Examination of the mechanisms of repair of experimental lesions has demonstrated that remyelination occurs in two major phases. The first consists of colonization of lesions by oligodendrocyte progenitor cells (OPCs), the second the differentiation of OPCs into myelinating oligodendrocytes that contact demyelinated axons to generate functional myelin sheaths. Several intracellular and extracellular molecules have been identified that mediate these two phases of repair. Theoretically, the repair of demyelinating lesions can be promoted by enhancing the intrinsic repair process (by providing one or more remyelination-enhancing factors or via immunoglobulin therapy). Alternatively, endogenous repair can be bypassed by introducing myelinogenic cells into demyelinated areas; several cellular candidates have been identified that can mediate repair of experimental demyelinating lesions. Future challenges confronting therapeutic strategies to enhance remyelination will involve the translation of findings from basic science to clinical demyelinating disease.

Interaction between microglia and oligodendrocyte cell progenitors involves Golli proteins

Multiple sclerosis (MS) is an autoimmune and chronic inflammatory disease characterized by plaques, areas of destroyed myelin sheaths in the CNS, which results in multiple disabilities for patients. In addition to demyelinated plaques, pathophysiological studies have shown "shadow plaques" that represent areas of partial remyelination. New myelin can be made by oligodendrocytes (OLs) generated from oligodendrocyte progenitor cells (OPCs) that pre-exist in the demyelinated area or recruited from surrounding areas. To successfully repopulate the demyelinated area, OPCs have to proliferate, migrate, and differentiate into mature OLs capable of forming myelin. Identifying factors that influence remyelination is a current topic in developmental neurobiology. Previously, we showed that Golli proteins, which have a broad distribution in the nervous and immune systems, are present both in OPCs and activated microglia around MS lesions. We hypothesized that in response to inflammation, Golli proteins may promote proliferation of OPCs through microglial cells. To test this, we established neonatal mouse brain slice and cell cultures and used lipopolysaccharide (LPS) to induce inflammation. In LPS-treated brain slices, Golli proteins displayed increased expression in the cortical subventricular zone. Furthermore, Golli proteins were demonstrated only in the conditioned medium from LPS-treated microglial cell cultures (LPS-MCM), and were absent in either conditioned medium from LPS-treated astrocytes or control media. Finally, proliferation of purified OPCs was promoted with LPS-MCM or Golli proteins, but not with LPS alone. In summary, these results demonstrate that activated microglia are beneficial for proliferation of OPCs and suggest possible involvement of Golli proteins as one of mediators in this process.

Insulin-like growth factor I and II expression and modulation in amoeboid microglial cells by lipopolysaccharide and retinoic acid

Insulin-like growth factors I and II are known to regulate the development of the CNS. We examined the developmental changes in insulin-like growth factor I and insulin-like growth factor II expression in the postnatal rat corpus callosum. Insulin-like growth factor I and insulin-like growth factor II mRNA expression increased at 3 days as compared with 1 day whereas the protein expression increased up to 7 days. Insulin-like growth factor I and insulin-like growth factor II immunoexpression was specifically localized in round cells confirmed by double immunofluorescence with OX-42 to be the amoeboid microglial cells. Insulin-like growth factor I expression was observed up to 7 days in amoeboid microglial cells while insulin-like growth factor II expression was detected in 1-3 day old rats. Exposure of primary rat microglial cell cultures to lipopolysaccharide increased insulin-like growth factor I and insulin-like growth factor II mRNA and protein expression significantly along with their immunoexpression in microglial cells. The lipopolysaccharide-induced increase in insulin-like growth factor I and insulin-like growth factor II mRNA and protein expression was significantly decreased with all-trans-retinoic acid. We conclude that insulin-like growth factor I and insulin-like growth factor II expression in amoeboid microglial cells in the developing brain is related to their activation. Once the activation is inhibited, either by transformation of the amoeboid microglial cells into ramified microglia regarded as resting cells or as shown by the effect of all-trans-retinoic acid administration, insulin-like growth factor I and insulin-like growth factor II mRNA and protein expression is downregulated.

The role of complement in immunological demyelination of the mammalian spinal cord

STUDY DESIGN

Specificity of serum complement component to elicit immunological demyelination.

OBJECTIVES

To assess the role of complement components and pathways in experimental immunological demyelination of the adult rat spinal cord.

SETTING

ICORD, University of British Columbia, Vancouver, Canada.

SUBJECTS

We used 32 adult male Sprague-Dawley rats, of approximately 220 g weight.

METHODS

Rats received intraspinal infusions of demyelinating reagents, delivered by osmotic minipump, for a 7-day infusion at 0.5 microl/h. Reagents consisted of a polyclonal antibody to galactocerebroside and human serum complement. Complement sera deficient for a single component were used to assess the role of the alternative pathway, the classical pathway, and the membrane attack complex. Demyelination was assessed, at 7 days, ultrastructurally.

RESULTS

Removal of C3 protein, common to classical and alternative complement pathways, or C4 protein, a classical pathway protein, resulted in no demyelination. However, complement deficient in Factor B, an alternative pathway protein, produced effective demyelination. Upon removal of C5 or C6, membrane attack complex proteins, demyelination was also observed.

CONCLUSION

This suggests that the classical pathway is sufficient for the protocol to demyelinate the adult rat spinal cord, and that the membrane attack complex is also not required.

Macrophage-depletion induced impairment of experimental CNS remyelination is associated with a reduced oligodendrocyte progenitor cell response and altered growth factor expression

Although macrophages are mediators of CNS demyelination, they are also implicated in remyelination. To examine the role of macrophages in CNS remyelination, adult rats were depleted of monocytes using clodronate liposomes and demyelination induced in the spinal cord white matter using lysolecithin. In situ hybridization for scavenger receptor-B and myelin basic protein (MBP) revealed a transiently impaired macrophage response associated with delayed remyelination in liposome-treated animals. Macrophage reduction corresponded with delayed recruitment of PDGFRalpha+ oligodendrocyte progenitor cells (OPCs), which preceded changes in myelin phagocytosis, indicating a macrophage effect on OPCs independent of myelin debris clearance. Macrophage-depletion induced changes in the mRNA expression of insulin-like growth factor-1 and transforming growth factor beta1, but not platelet-derived growth factor-A and fibroblast growth factor-2. These data suggest that the macrophage response to toxin-induced demyelination influences the growth factor environment, thereby affecting the behavior of OPCs and hence the efficiency of remyelination.

Characterization of long-term mouse brain aggregating cultures: Evidence for maintenance of neural precursor cells

An extensive characterization of fetal mouse brain cell aggregates has been performed using immunohistochemical and stereological methods. Single cell suspensions from mechanically dissociated cortex and hippocampus were cultured in serum-free, B27-supplemented medium under constant gyratory agitation for up to 56 days. Three-dimensional aggregates started to form immediately after seeding and reached a final average size of 500 microm in diameter. Among the cell types identified, neurons were the most abundant cells in the aggregates, followed by astrocytes, microglia, and oligodendrocytes. Western blotting for synaptophysin and immunostaining for neurotransmitter-related molecules indicated the presence of well-defined phenotypic characteristics of the neurons in this culture system, suggesting functionality. Proliferating cells, many with neural precursor cell properties, were seen throughout the culture period and could be isolated from the aggregates even after 2 months in culture. Neural precursor cells were isolated from the aggregates after more than 1 month in culture; these cells were successfully differentiated into neurons, astrocytes, and oligodendrocytes. The aggregate culture system may provide a versatile tool for molecular dissection of processes identified in mouse models, including transgenic animals and manipulation of neural precursor cells.

Osteopontin is upregulated during in vivo demyelination and remyelination and enhances myelin formation in vitro

We have used in vitro oligodendrocyte differentiation and the in vivo remyelination model, the cuprizone model, to identify genes regulating oligodendrocyte function and remyelination. One of the genes we identified, osteopontin (opn), is a secreted glycoprotein with cytokine-like, chemotactic, and anti-apoptotic properties that contains an Arg-Gly-Asp (RGD) cell adhesion motif-mediating interactions with several integrins. Both microglia and astrocytes in demyelinating brain regions of cuprizone-fed mice expressed OPN protein. Recombinant OPN protein produced in a baculovirus expression system induced proliferation of both the rat CG-4 and the mouse Oli-neu oligodendrocyte precursor (OLP)-like cell lines in a dose-dependent manner. In addition, recombinant OPN treatment stimulated both myelin basic protein (MBP) synthesis and myelin sheath formation in mixed cortical cultures from embryonic mouse brain, an in vitro primary culture model of myelination. Interestingly, myelinating mixed cultures prepared from OPN(-/-) mice contained significantly less MBP compared to wild-type cultures after 17 days in culture. We propose that in the central nervous system, OPN may act as a novel regulator of myelination and remyelination.

Role for TGF-beta1, FGF-2 and PDGF-AA in a myelination of CNS aggregate cultures enriched with macrophages

The increase in myelin basic protein (MBP) synthesis observed in brain aggregate cultures supplemented with macrophages is reflected in elevated supernatant protein levels of the key promoters of oligodendrocyte proliferation, fibroblast growth factor-2 (FGF-2) and platelet-derived growth factor-AA (PDGF-AA), during the premyelinating phase. Although supernatant levels of transforming growth factor-beta1 (TGF-beta1), the most abundant growth factor produced at the transcriptional and translational levels by phagocytic macrophages, were reduced at this stage, it was the only growth factor for which mRNA expression was increased significantly in macrophage-enriched cultures. TGF-beta1, which supports oligodendrocyte differentiation, was increased in the supernatant of macrophage-enriched cultures only after the onset of myelinogenesis. Hence, standard cultures treated with TGF-beta1 during the premyelinating period reproduced effects of macrophage supplementation, inducing an increase in MBP synthesis and in PDGF-AA and FGF-2 bioavailability. A similar increase in MBP synthesis in PDGF-AA treated cultures emphasises its central role in oligodendrocyte progenitor proliferation. In contrast, FGF-2 blocked MBP synthesis in the cultures. In cultures treated with anti-TGF-beta1 antibody before or after the first detection of MBP, supernatant levels of TGF-beta1, FGF-2, and PDGF-AA were reduced with resultant inhibition of myelination. Paradoxically, supraphysiological TGF-beta1 treatment after the onset of myelination had the same effect on myelin accumulation. These results indicate an enabling and regulatory role for TGF-beta1 in oligodendrocyte development and, as a source of TGF-beta1, macrophages in the inflammatory multiple sclerosis lesion, may have the potential to promote remyelination by modulating the growth factor repertoire in demyelinating disease.

Steroids and the reversal of age-associated changes in myelination and remyelination

The myelin sheaths that surround all but the smallest diameter axons within the mammalian central nervous system (CNS) must maintain their structural integrity for many years. Like many tissues, however, this function is prone to the effects of ageing, and various structural anomalies become apparent in the aged CNS. Similarly, the regenerative process by which myelin sheaths, lost as a consequence of exposure to a demyelinating insult, are restored (remyelination) is also affected by age. As animals grow older, the efficiency of remyelination progressively declines. In this article, we review both phenomena and describe how both can be partially reversed by steroid hormones and their derivatives.

Remyelination of cytokine- or antibody-demyelinated CNS aggregate cultures is inhibited by macrophage supplementation

Remyelination in CNS aggregate cultures is determined both by macrophage enrichment and the mode of demyelination. Despite the same degree of myelin loss, accumulation of MBP in anti-MOG antibody-demyelinated aggregates overtakes that of controls, while recovery is significantly delayed following IFN-gamma-induced demyelination. In antibody-treated cultures, remyelination was associated with a significant increase in culture supernatant levels of TGF-beta1, FGF-2, and PDGF-AA as well as an induction of TNF-alpha immediately following removal of the demyelinating insult. The impaired recovery in IFN-gamma-treated cultures, denoted by a significant reduction in TGF-beta1, was reversed by treatment with hrTGF-beta1. Macrophage supplementation of the cultures prior to the addition of either demyelinating agent induced a greater degree of myelin loss followed by incomplete remyelination in both cases. This failure to remyelinate was associated in both groups with a several-fold elevation in TNF-alpha and with modest increases in PDGF-AA and FGF-2 in the antibody-treated cultures. In contrast, macrophage supplementation to mature cultures in the absence of any demyelinating treatment resulted in enhanced accumulation of MBP associated with a promyelinative growth factor and TNF-alpha profile similar to that in aggregates enriched with macrophages at the outset of the culture period. Hence, effector elements of the adaptive immune response appear to override promyelinogenic in favor of proinflammatory macrophage factors in mature CNS aggregates, counteracting the potential for myelin repair.

Why does remyelination fail in multiple sclerosis?

Multiple sclerosis is a common cause of neurological disability in young adults. The disease is complex -- its aetiology is multifactorial and largely unknown; its pathology is heterogeneous; and, clinically, it is difficult to diagnose, manage and treat. However, perhaps its most frustrating aspect is the inadequacy of the healing response of remyelination. This regenerative process generally occurs with great efficiency in experimental models, and sometimes proceeds to completion in multiple sclerosis. But as the disease progresses, the numbers of lesions in which demyelination persists increases, significantly contributing to clinical deterioration. Understanding why remyelination fails is crucial for devising effective methods by which to enhance it.

Polyreactive antibodies to glatiramer acetate promote myelin repair in murine model of demyelinating disease

Using a murine model of demyelinating disease, we demonstrate that remyelination of spinal cord axons is promoted by antibodies to glatiramer acetate (GA, Copolymer-1, Copaxone), a therapeutic agent for multiple sclerosis (MS). Glatiramer acetate is a mixture of randomly synthesized peptides that induces both T cell activation and antibody production in all treated individuals. These observations prompted us to compare the independent effects of adoptively transferred GA-reactive T cells and antibodies in mice with chronic inflammatory demyelination induced by Theiler's virus. Transferred T cells had no effect on lesion load or the extent of remyelination. Purified polyclonal GA antibodies also did not alter lesion load, which suggests that neither GA T cells or antibodies were pathogenic. On the contrary, GA antibodies enhanced the normally low level of remyelination in chronic lesions. The antibodies, which were primarily immunoglobulin (Ig) G1 and IgG2, cross-reacted with oligodendrocytes, perivascular infiltrating cells, astrocytes, and neurons in spinal cord sections. In glial cultures they bound subsets of early lineage oligodendrocytes and microglia. Thus, several mechanisms may have contributed to the promotion of remyelination. These results support the hypothesis that the antibody response in GA-treated patients is beneficial by facilitating repair of demyelinated lesions.

Myelin phagocytosis and remyelination of macrophage-enriched central nervous system aggregate cultures

An increased level of myelin basic protein (MBP) degradation peptide 80-89, representative of myelin breakdown, is detected in myelinating foetal rat brain aggregate cultures supplemented with peritoneal macrophages at a time coinciding with the onset of myelination. During the period of myelination, the proportion of activated macrophages/microglia in the aggregates decreases, accompanied by a reduction in the content of MBP degradation products. During the recovery period following a demyelinating episode, the rate of MBP synthesis in antibody-treated standard aggregates was greater than in their medium controls. However, the rate of MBP accumulation was not as efficient in macrophage-enriched aggregates and was associated with persistently raised MBP peptide levels. Thus, as occurs in multiple sclerosis lesions, attempts at remyelination appear to be counterbalanced by macrophage-mediated demyelination, with the continued presence of degraded myelin rendering a local environment that is not fully conducive to remyelination.

Macrophage depletion impairs oligodendrocyte remyelination following lysolecithin-induced demyelination

An association between macrophages and remyelination efficiency has been observed in a variety of different models of CNS demyelination. In order to test whether this association is causal or coincidental, we have examined the effects of macrophage depletion on the rate of remyelination of lysolecithin-induced demyelination in the spinal cord of young adult female rats. Macrophage depletion was achieved by reducing the monocyte contribution to the macrophages within the lesion using the clodronate-liposome technique. This technique not only resulted in a decrease in Ox-42-positive cells in the spleen of treated animals but also in the levels of macrophage scavenger receptor type B mRNA expression within the demyelinating lesion. In animals treated with clodronate-liposomes throughout the remyelination process, there was a significant decrease in the extent of oligodendrocyte remyelination at 3 weeks after lesion induction, but no effect on Schwann cell remyelination. If macrophage depletion was delayed until the second half of the remyelination phase, then there was no effect on the repair outcome, implying that macrophages are required for the early stages of CNS remyelination. The results of this study indicate that the macrophage response is an important component of successful CNS remyelination and that approaches to the treatment of demyelinating disease based on inhibition of the inflammatory response may also impair regenerative events that follow demyelination.

Delayed changes in growth factor gene expression during slow remyelination in the CNS of aged rats

In this study we have examined whether the slower rate of CNS remyelination that occurs with age is associated with a change in growth factor expression patterns, an association that would provide further support for a causal relationship between growth factors and remyelination. Using quantitative in situ hybridization we have shown that there are differences in IGF-I, TGF-beta 1, and PDGF-A mRNA expression during remyelination of lysolecithin-induced demyelination in the spinal cord of young adult and old adult rats. IGF-I and TGF-beta1 mRNA expression in old rats had a delayed and lower peak expression compared to young rats. The initial increase in PDGF-A mRNA expression was delayed in old rats compared to young rats, but after 5 days both age groups had similar patterns of expression, as was the expression pattern of FGF-2 mRNA at all survival times. In neither age group were increases in CNTF, NT-3, or GGF-2 mRNA expression detected. An analysis of the macrophage response using oligonucleotide probes for scavenger receptor-B mRNA indicated that differences in the macrophage response in young and old animals was the likely cause of the age related change in IGF-I and TGF-beta 1 mRNA expression patterns. On the basis of these data we suggest a model of remyelination in which PDGF is involved in the initial phase of oligodendrocyte progenitor recruitment, while IGF-I and TGF-beta 1 trigger the differentiation of the recruited cells into myelinating oligodendrocytes.

Mature oligodendrocyte apoptosis precedes IGF-1 production and oligodendrocyte progenitor accumulation and differentiation during demyelination/remyelination

We have documented changes in the oligodendrocyte population during demyelinating insult to the adult CNS. Feeding of cuprizone to adult mice led to apoptotic death of mature oligodendrocytes followed by profound demyelination of the corpus callosum. A regenerative response was initiated even during active demyelination. Oligodendrocyte progenitors have begun to proliferate and then accumulate within the lesion. Many of these cells may have migrated from the sub-ventricular zone and fornix before their accumulation in the demyelinating corpus callosum. The accumulation of differentiating oligodendrocyte progenitors was followed closely by the reappearance of mature oligodendrocytes and remyelination. Interestingly, an increase in IGF-1 mRNA was detected at Week 3 through Week 7, suggesting potential involvement in remyelination. Other factors, however, such as PDGF, NT3, FGF, jagged, and notch remained unchanged. These results suggest that the mature oligodendroglial population depleted by apoptosis is replaced by a newly formed oligodendroglial population derived from progenitors; these accumulate and seem to differentiate during remyelination.

Reactions of oligodendrocytes to spinal cord injury: cell survival and myelin repair

The aim of this study was to elucidate whether oligodendrocytes die in fiber tracts that are spared by a spinal cord injury but are in close vicinity of inflammatory cells. Adult rat spinal cords were studied histologically 1 day to 2 weeks after a contusion lesion that left the ventral white matter largely intact. Massive oligodendrocyte death occurred in the lesion center, along with the death of neurons, microglia, and astrocytes. Oligodendrocytes, specifically positive for proteolipid protein (PLP) mRNA, were counted in the ventral white matter where axons at the rostral and caudal edges of the lesion were histologically intact. Although these regions contained many macrophages and neutrophils hypothesized to contribute to secondary tissue loss, there was no significant loss of oligodendrocytes. In the ventral funiculus, 3 and 6 mm rostral and caudal to the lesion, oligodendrocyte numbers were also unchanged, in spite of the presence of many activated microglial cells. From day 7 on, oligodendrocytes in close vicinity to the lesion increased their expression of PLP mRNA. We conclude that, at least within the first 2 weeks after a spinal cord contusion lesion, there is no major devastating influence of inflammatory cells or their mediators on oligodendrocytes. When death occurs, it may be due to mechanical trauma, ischemia, or excitotoxicity within the lesion or it may occur as a result of axonal degeneration.

Temporal analysis of growth factor mRNA expression in myelinating rat brain aggregate cultures: increments in CNTF, FGF-2, IGF-I, and PDGF-AA mRNA are induced by antibody-mediated demyelination

Myelinogenesis in rat brain aggregate cultures is associated with a pattern of growth factor mRNA expression comparable to that of the developing brain. The rate of increase in platelet-derived growth factor-AA (PDGF-AA) expression was greatest just before the detection of myelin basic protein (MBP) mRNA in the cultures and remained high thereafter, consistent with in vivo observations. Levels of fibroblast growth factor-2 (FGF-2) and of ciliary neurotrophic factor (CNTF) mRNA increased continuously over the period of MBP accumulation. High rates of transforming growth factor beta1 (TGF-beta1), insulin-like growth factor-I (IGF-I), and neurotrophin-3 (NT-3) expression at early time points during the culture gradually decreased over time, indicative of a key regulatory role during oligodendrocyte development. The addition of demyelinative anti-myelin oligodendrocyte glycoprotein (anti-MOG) antibody resulted in a significant increase in MBP peptide fragments with a C-terminus at phenylalanine 89 indicating proteolytic breakdown of MBP after myelin phagocytosis. Immediately after antibody treatment the expression of CNTF mRNA was significantly increased, compared with controls, while that of FGF-2 and IGF-I, and of PDGF-AA peaked during the early and later stages of recovery respectively. Thus, specific growth factors combine to regulate myelination and remyelination in the aggregates; these data have implications for demyelinating disease in which protective growth factor secretion may be central to regeneration.

Axonal sprouting regulates myelin basic protein gene expression in denervated mouse hippocampus

The regulation of oligodendrocyte gene expression and myelination in vivo in the normal and injured adult CNS is still poorly understood. We have analyzed the effects of axotomy-induced axonal sprouting and microglial activation, on oligodendrocyte myelin basic protein (MBP) gene expression from 2 to 35 days after transection of the entorhino-hippocampal perforant path axonal projection. In situ hybridization analysis showed that anterograde axonal and terminal degeneration lead to upregulated oligodendrocyte MBP mRNA expression starting between day 2 and day 4, in (1) the deep part of stratum radiatum of CA3 and the dentate hilus, which display axonal sprouting but no degenerative changes or microglial activation, and (2) the outer part of the molecular layer of the fascia dentata, and in stratum moleculare of CA3 and stratum lacunosum-moleculare of CA1, areas that display dense anterograde axonal and terminal degeneration, myelin degenerative changes, microglial activation and axotomi-induced axonal sprouting. Oligodendrocyte MBP mRNA expression reached maximum in both these areas at day 7. MBP gene transcription remained constant in stratum radiatum, stratum pyramidale and stratum oriens of CA1, areas that were unaffected by perforant path transection. These results provide strong evidence that oligodendrocyte MBP gene expression can be regulated by axonal sprouting independently of microglial activation in the injured adult CNS.

Macrophages: their myelinotrophic or neurotoxic actions depend upon tissue oxidative stress

There are still questions regarding whether macrophages found in MS lesions are agents of recovery or of destruction. To address this, we examined in aggregate cultures prepared from dissociated embryonic spinal cord tissue, with or without addition of exogenous macrophages, the effect of menadione-induced oxidative stress. Similar to findings of other laboratories, we observed that in the absence of oxidative stress macrophage enrichment promoted myelinogenesis. In macrophage-poor cultures, menadione at 5 microM had very little effect upon the status of the aggregate cultures; however, increasing this to 10 and 20 microM did result in some damage to axons and myelin. By contrast, in macrophage enriched cultures, menadione at a concentration as little as 5 microM caused the complete destruction of the aggregates. We suggest that in neural tissues that have sufficiently high macrophage numbers, oxidative stress results in a positive inflammatory feedback loop that results in massive tissue destruction. We further suggest that what we see in macrophage-enriched aggregates subjected to oxidative stress may represent what happens in the Marburg-type of MS lesion.

Myelin basic protein isoforms in myelinating and remyelinating rat brain aggregate cultures

Recent evidence suggests that myelin basic protein (MBP) exon-2-containing isoforms play a significant role in the onset of myelination because they are more abundant during early development. The pattern of expression of MBP exon-2-containing isoforms was studied in rat brain aggregate cultures during myelination to draw comparisons with the developing brain and at remyelination after demyelinative treatment. The pattern of MBP isoform expression in the aggregate cultures was found to be similar to that of the brain and was recapitulated after demyelination with antimyelin antibodies. Macrophage enrichment, resulting in increased accumulation of total MBP in the cultures, did not alter the isoform distribution. Both control and enriched cultures expressed a 16-kDa protein (26+/-9.8% of total MBP for control samples) that reacted with MBP antisera at the onset of myelination (day in vitro 14) but was barely detectable by day in vitro 21. The expression of this protein, also present in postnatal day 6 rat brain but no longer by day 11, has been predicted by reverse transcription polymerase chain reaction in embryonic mouse brain. The results of the present study reinforce the value of the aggregate culture system as a versatile yet accurate model of myelination and remyelination.

Cytokines in multiple sclerosis: pro-inflammation or pro-remyelination?

The targets of the inflammatory response in multiple sclerosis (MS) are myelin and the myelin producing cells, oligodendrocytes. The infiltration of activated immune cells and recruitment of endogenous glia lead to the destruction of myelin and oligodendrocytes, a process that is compounded by the release of cytokines by infiltrating cells. Recent evidence suggests that key cytokines that are responsible for the destruction of myelin may also mediate the process of remyelination and repair. It appears that both inflammatory and repair processes are governed by the temporal and spatial relation of cytokines to their targets.