Mechanisms Underlying the Process of Demyelination in Multiple Sclerosis

The author generalizes and analyzes the published data and her own findings related to the cellular and molecular mechanisms underlying a demyelinating disease, multiple sclerosis. The mechanisms of the immunopathogenic process in multiple sclerosis, the involvement of microglia and astrocytes in destruction of the myelin sheaths, and injury of oligodendrocytes are discussed. Experimental models used for examination of the processes of demyelination of the nerve tissue in vitro (tissue cultures) and in vivo (experimental allergic encephalomyelitis) are also described.     

Astrogliosis during acute and chronic cuprizone demyelination and implications for remyelination

In multiple sclerosis, microglia/macrophage activation and astrocyte reactivity are important components of the lesion environment that can impact remyelination. The current study characterizes these glial populations relative to expression of candidate regulatory molecules in cuprizone demyelinated corpus callosum. Importantly, periods of recovery after acute or chronic cuprizone demyelination are examined to compare conditions of efficient versus limited remyelination, respectively. Microglial activation attenuates after early demyelination. In contrast, astrocyte reactivity persists throughout demyelination and a 6-week recovery period following either acute or chronic demyelination. This astrocyte reaction is characterized by (a) early proliferation, (b) increased expression of GFAP (glial fibrillary acidic protein), Vim (vimentin), Fn1 (fibronectin) and CSPGs (chondroitin sulphate proteoglycans) and (c) elaboration of a dense network of processes. Glial processes elongated in the axonal plane persist throughout lesion areas during both the robust remyelination that follows acute demyelination and the partial remyelination that follows chronic demyelination. However, prolonged astrocyte reactivity with chronic cuprizone treatment does not progress to barrier formation, i.e. dense compaction of astrocyte processes to wall off the lesion area. Multiple candidate growth factors and inflammatory signals in the lesion environment show strong correlations with GFAP across the acute cuprizone demyelination and recovery time course, yet there is more divergence across the progression of chronic cuprizone demyelination and recovery. However, differential glial scar formation does not appear to be responsible for differential remyelination during recovery in the cuprizone model. The astrocyte phenotype and lesion characteristics in this demyelination model inform studies to identify triggers of non-remyelinating sclerosis in chronic multiple sclerosis lesions.     

The membrane attack complex of complement causes severe demyelination associated with acute axonal injury.

Complement is implicated in pathology in the human demyelinating disease multiple sclerosis and in animal models that mimic the demyelination seen in multiple sclerosis. However, the components of the complement system responsible for demyelination in vivo remain unidentified. In this study, we show that C6-deficient (C6-) PVG/c rats, unable to form the membrane attack complex (MAC), exhibit no demyelination and significantly reduced clinical score in the Ab-mediated experimental autoimmune encephalomyelitis model when compared with matched C6-sufficient (C6+) rats. In C6+ rats, perivenous demyelination appeared, accompanied by abundant mononuclear cell infiltration and axonal injury. Neither demyelination nor axonal damage was seen in C6- rats, whereas levels of mononuclear cell infiltration were equivalent to those seen in C6+ rats. Reconstitution of C6 to C6- rats yielded pathology and clinical disease indistinguishable from that in C6+ rats. We conclude that demyelination and axonal damage occur in the presence of Ab and require activation of the entire complement cascade, including MAC deposition. In the absence of MAC deposition, complement activation leading to opsonization and generation of the anaphylatoxins C5a and C3a is insufficient to initiate demyelination.     

Bystander CD8 T cell-mediated demyelination after viral infection of the central nervous system

Multiple sclerosis, a chronic inflammatory disease of the CNS, is characterized by immune-mediated demyelination. Many patients have a remitting-relapsing course of disease with exacerbations often following unrelated microbial illnesses. The relationship between the two events remains obscure. One possibility is that T cells specific for the inciting microbial pathogen are able to effect demyelination at a site of ongoing inflammation within the CNS. This possibility was examined in mice infected with mouse hepatitis virus, a well-described model of virus-induced demyelination. Using transgenic TCR/recombination activation gene 2(-/-) mice with only non-mouse hepatitis virus-specific T cells, we show that CD8 T cells are able to cause demyelination in the absence of cognate Ag in the CNS, but only if specifically activated. These findings demonstrate a novel mechanism for immune-mediated neuropathology and show that activated CD8 T cells may serve as important mediators of bystander demyelination during times of infection, including in patients with multiple sclerosis.     

Leonurine suppresses neuroinflammation through promoting oligodendrocyte maturation

Focal inflammation and remyelination failure are major hallmarks of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). In this study, we found that leonurine, a bioactive alkaloid, alleviated EAE disease severity along with reduced central nervous system inflammation and myelin damage. During the pathogenesis of EAE, leonurine dramatically suppressed the recruitment of encephalitogenic T cells into the central nervous system, whereas did not impair periphery immune responses and microglia activation. Mechanistically, leonurine protected mice against demyelination along with enhanced remyelination through promoting the maturation of oligodendrocytes in both EAE and cuprizone‐induced demyelination mouse models. Moreover, we identified that the expression of demethylase jumonji domain‐containing protein D3 was significantly enhanced upon treatment of leonurine, which suppressed the trimethylation of histone H3 lysine‐27 and enhanced oligodendrocyte maturation accordingly. Collectively, our study identified the therapeutic effect of leonurine on EAE model, which potentially represents a promising therapeutic strategy for multiple sclerosis, even other demyelination disorders.     

Signaling cascades activated upon antibody cross-linking of myelin oligodendrocyte glycoprotein: potential implications for multiple sclerosis

Antibody-induced demyelination is an important component of pathology in multiple sclerosis. In particular, antibodies to myelin oligodendrocyte glycoprotein (MOG) are elevated in multiple sclerosis patients, and they have been implicated as mediators of demyelination. We have shown previously that antibody cross-linking of MOG in oligodendrocytes results in the repartitioning of MOG into glycosphingolipid-cholesterol membrane microdomains ("lipid rafts"), followed by changes in the phosphorylation of specific proteins, including dephosphorylation of beta-tubulin and the beta subunit of the trimeric G protein and culminating in rapid and dramatic morphological alterations. In order to further elucidate the mechanism of anti-MOG-mediated demyelination, we have carried out a proteomic analysis to identify the set of proteins for which the phosphorylation states or expression levels are altered upon anti-MOG treatment. We demonstrate that treatment of oligodendrocytes with anti-MOG alone leads to an increase in calcium influx and activation of the MAPK/Akt pathways that is independent of MOG repartitioning. However, further cross-linking of anti-MOG.MOG complexes with a secondary anti-IgG results in the lipid raft-dependent phosphorylation of specific proteins related to cellular stress response and cytoskeletal stability. Oligodendrocyte survival is not compromised by these treatments. We discuss the possible significance of the anti-MOG-induced signaling cascade in relation to the initial steps of MOG-mediated demyelination.     

IL-2 suppression of IL-12p70 by a recombinant HSV-1 expressing IL-2 induces T-cell auto-reactivity and CNS demyelination

To evaluate the role of cellular infiltrates in CNS demyelination in immunocompetent mice, we have used a model of multiple sclerosis (MS) in which different strains of mice are infected with a recombinant HSV-1 expressing IL-2. Histologic examination of the mice infected with HSV-IL-2 demonstrates that natural killer cells, dendritic cells, B cells, and CD25 (IL-2rα) do not play any role in the HSV-IL-2-induced demyelination. T cell depletion, T cell knockout and T cell adoptive transfer experiments suggest that both CD8(+) and CD4(+) T cells contribute to HSV-IL-2-induced CNS demyelination with CD8(+) T cells being the primary inducers. In the adoptive transfer studies, all of the transferred T cells irrespective of their CD25 status at the time of transfer were positive for expression of FoxP3 and depletion of FoxP3 blocked CNS demyelination by HSV-IL-2. The expression levels of IL-12p35 relative to IL-12p40 differed in BM-derived macrophages infected with HSV-IL-2 from those infected with wild-type HSV-1. HSV-IL-2-induced demyelination was blocked by injecting HSV-IL-2-infected mice with IL-12p70 DNA. This study demonstrates that suppression of the IL-12p70 function of macrophages by IL-2 causes T cells to become auto-aggressive. Interruption of this immunoregulatory axis results in demyelination of the optic nerve, the spinal cord and the brain by autoreactive T cells in the HSV-IL-2 mouse model of MS.     

De- and remyelination in the CNS white and grey matter induced by cuprizone: the old, the new, and the unexpected

The copper chelator cuprizone (bis-cyclohexanone oxaldihydrazone) was established as a neurotoxin in rodents in 1966 by Carlton. During the following years the usefulness of cuprizone feeding in mice to induce oligodendrocyte death with secondary demyelination of the superior cerebellar peduncles was described by Blakemore. In 1998 the cuprizone model experienced a renaissance as the group of Matsushima described the effects of cuprizone on the white matter of the cerebrum and focussed on demyelination in the corpus callosum, where the extent of demyelination could be scored more easily and consistently. Since then the toxic cuprizone model has been widely used to study experimental de- and remyelination in the corpus callosum. Recently, we and others have extended these studies and have shown several new aspects characteristic for this model. Many lessons can be learned from these recent findings that have implications for the basic understanding of remyelination and the design of remyelinating and neuroprotective strategies in demyelinating diseases of the CNS. Although the model is often mentioned in the context of multiple sclerosis, it must always be kept in mind that this model has a fundamentally different induction of demyelination. We highlight the important findings delineated from this model and critically discuss both the advantages and the shortcomings of cuprizone induced demyelination.     

The cyclooxygenase-2 pathway via the PGE₂ EP2 receptor contributes to oligodendrocytes apoptosis in cuprizone-induced demyelination

Cyclooxygenases (COX)-1 and -2 are key enzymes required for the conversion of arachidonic acid to eicosanoids, potent mediators of inflammation. In patients with multiple sclerosis, COX-2 derived prostaglandins (PGs) are elevated in the CSF and COX-2 is up-regulated in demyelinating plaques. However, it is not known whether COX-2 activity contributes to oligodendrocyte death. In cuprizone-induced demyelination, oligodendrocyte apoptosis and a concomitant increase in the gene expression of COX-2 and PGE?-EP2 receptor precede histological demyelination. COX-2 and EP2 receptor were expressed by oligodendrocytes, suggesting a causative role for the COX-2/EP2 pathway in the initiation of oligodendrocyte death and demyelination. COX-2 gene deletion, chronic treatment with the COX-2 selective inhibitor celecoxib, or with the EP2 receptor antagonist AH6809 reduced cuprizone-induced oligodendrocyte apoptosis, the degree of demyelination and motor dysfunction. These data indicate that the PGE? EP2 receptor contributes to oligodendrocyte apoptosis and open possible new therapeutic approaches for multiple sclerosis.     

A New Model of Cuprizone-Mediated Demyelination/Remyelination

In the central nervous system, demyelinating diseases, such as multiple sclerosis, result in devastating long-term neurologic damage, in part because of the lack of effective remyelination in the adult human brain. One model used to understand the mechanisms regulating remyelination is cuprizone-induced demyelination, which allows investigation of remyelination mechanisms in adult animals following toxin-induced demyelination. Unfortunately, the degree of demyelination in the cuprizone model can vary, which complicates understanding the process of remyelination. Previous work in our laboratory demonstrated that the Akt/mTOR pathway regulates active myelination. When given to young postnatal mice, the mTOR inhibitor, rapamycin, inhibits active myelination. In the current study, the cuprizone model was modified by the addition of rapamycin during cuprizone exposure. When administered together, cuprizone and rapamycin produced more complete demyelination and provided a longer time frame over which to investigate remyelination than treatment with cuprizone alone. The consistency in demyelination will allow a better understanding of the mechanisms initiating remyelination. Furthermore, the slower rate of remyelination provides a longer window of time in which to investigate the diverse contributing factors that regulate remyelination. This new model of cuprizone-induced demyelination could potentially aid in identification of new therapeutic targets to enhance remyelination in demyelinating diseases.     

Investigation of neuro-inflammatory parameters in a cuprizone induced mouse model of multiple sclerosis

Cuprizone, copper chelator, treatment of mouse is a toxic model of multiple sclerosis (MS) in which oligodendrocyte death, demyelination and remyelination can be observed. Understanding T and B cell subset as well as their cytokines involved in MS pathogenesis still requires further scrutiny to better understand immune component of MS. The study presented here, aimed to evaluate relevant cytokines, lymphocytes, and gene expressions profiles during demyelination and remyelination in the cuprizone mouse model of MS. Eighty male C57BL/6J mice fed with 0.2% cuprizone for eight weeks. Cuprizone has been removed from the diet in the following eight weeks. Cuprizone treated and control mice sacrificed biweekly, and corpus callosum of the brain was investigated by staining. Lymphocyte cells of mice analyzed by flow cytometry with CD3e, CD11b, CD19, CD80, CD86, CD4, CD25 and FOXP3 antibodies. IFN-gamma, IL-1alpha, IL-2, IL-5, IL-6, IL-10, IL-17, TNF-alpha cytokines were analyzed in plasma samples. Neuregulin 1 (Nrg1), ciliary neurotrophic factor (Cntf) and C-X-C chemokine receptor type 4 (Cxcr4) gene expressions in corpus callosum sections of the mice brain were quantified. Histochemistry analysis showed that demyelination began at the fourth week of cuprizone administration and total demyelination occurred at the twelfth week in chronic model. Remyelination occurred at the fourth week of following withdrawal of cuprizone from diet. The level of mature and activated T cells, regulatory T cells, T helper cells and mature B cells increased during demyelination and decreased when cuprizone removed from diet. Further, both type 1 and type 2 cytokines together with the proinflammatory cytokines increased. The level of oligodendrocyte maturation and survival genes showed differential gene expression in parallel to that of demyelination and remyelination. In conclusion, for the first-time, involvement of both cellular immune response and antibody response as well as oligodendrocyte maturation and survival factors having role in demyelination and remyelination of cuprizone mouse model of MS have been shown.     

Mitochondrial changes associated with demyelination: consequences for axonal integrity

The loss of myelin sheath (demyelination) renders axons vulnerable to a variety of insults. Axonal degeneration is well recognised in inflammatory demyelinating disorders of the central nervous system (CNS) such as multiple sclerosis (MS) and also certain neurodegenerative diseases. Energy required for nerve impulse conduction and maintenance of structural integrity of axons is met by mitochondria. Based on the distribution of ion channels and the Na(+)/K(+) ATPase, the energy requirements of demyelinated and dysmyelinated axons are likely to differ from myelinated axons. In this review we discuss the changes in mitochondrial presence within axons in relation to presence or absence of healthy myelin sheaths and propose the increase in mitochondrial presence following demyelination as an adaptive process. An energy deficit within demyelinated axons is likely to be more detrimental compared to myelinated axons, judging by the neuropathological findings in primary mitochondrial disorders due to mitochondrial and nuclear DNA mutations and the mitochondrial changes that follow demyelination. Agents that enhance and protect mitochondria, as potential therapy, need to be considered and investigated in earnest for demyelinating disorders of the CNS such as MS.     

Death receptor 6 negatively regulates oligodendrocyte survival, maturation and myelination

Survival and differentiation of oligodendrocytes are important for the myelination of central nervous system (CNS) axons during development and crucial for myelin repair in CNS demyelinating diseases such as multiple sclerosis. Here we show that death receptor 6 (DR6) is a negative regulator of oligodendrocyte maturation. DR6 is expressed strongly in immature oligodendrocytes and weakly in mature myelin basic protein (MBP)-positive oligodendrocytes. Overexpression of DR6 in oligodendrocytes leads to caspase 3 (casp3) activation and cell death. Attenuation of DR6 function leads to enhanced oligodendrocyte maturation, myelination and downregulation of casp3. Treatment with a DR6 antagonist antibody promotes remyelination in both lysolecithin-induced demyelination and experimental autoimmune encephalomyelitis (EAE) models. Consistent with the DR6 antagoinst antibody studies, DR6-null mice show enhanced remyelination in both demyelination models. These studies reveal a pivotal role for DR6 signaling in immature oligodendrocyte maturation and myelination that may provide new therapeutic avenues for the treatment of demyelination disorders such as multiple sclerosis.     

Minocycline reduces remyelination by suppressing ciliary neurotrophic factor expression after cuprizone‐induced demyelination

Remyelination is disrupted in demyelinating diseases such as multiple sclerosis, but the underlying pathogenetic mechanisms are unclear. In this study, we employed the murine cuprizone model of demyelination, in which remyelination occurs after removal of the toxin from the diet, to examine the cellular and molecular changes during demyelination and remyelination. Microglia accumulated in the corpus callosum during weeks 2–4 of the cuprizone diet, and these cells remained activated 2 weeks after the change to the normal diet. To examine the role of microglia in remyelination, mice were treated with minocycline to inactivate these cells after cuprizone‐induced demyelination. Minocycline treatment reduced the number of CC1‐positive oligodendrocytes, as well as levels of myelin basic protein (MBP) and CNPase in the remyelination phase. The expression of CNTF mRNA in the corpus callosum increased after 4 weeks on the cuprizone diet and remained high 2 weeks after the change to the normal diet. Minocycline suppressed CNTF expression during the remyelination phase on the normal diet. Primary culture experiments showed that CNTF was produced by microglia in addition to astrocytes. In vitro, CNTF directly affected the differentiation of oligodendrocytic cells. These findings suggest that minocycline reduces remyelination by suppressing CNTF expression by microglia after cuprizone‐induced demyelination.     

Mesenchymal stem cells lack efficacy in the treatment of experimental autoimmune neuritis despite in vitro inhibition of T-cell proliferation

Mesenchymal stem cells have been demonstrated to ameliorate experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis, prompting clinical trials in multiple sclerosis which are currently ongoing. An important question is whether this therapeutic effect generalises to other autoimmune neurological diseases. We performed two trials of efficacy of MSCs in experimental autoimmune neuritis (EAN) in Lewis (LEW/Han (M)Hsd) rats, a model of human autoimmune inflammatory neuropathies. No differences between the groups were found in clinical, histological or electrophysiological outcome measures. This was despite the ability of mesenchymal stem cells to inhibit proliferation of CD4+ T-cells in vitro. Therefore the efficacy of MSCs observed in autoimmune CNS demyelination models do not necessarily generalise to the treatment of other forms of neurological autoimmunity.     

The L-coding region of the DA strain of Theiler's murine encephalomyelitis virus causes dysfunction and death of myelin-synthesizing cells

The DA strain and other members of the TO subgroup of Theiler's murine encephalomyelitis virus (TMEV) induce an early transient subclinical neuronal disease followed by a chronic progressive inflammatory demyelination, with persistence of the virus in the central nervous system (CNS) for the life of the mouse. Although TMEV-induced demyelinating disease (TMEV-IDD) is thought to be immune mediated, there is also evidence that supports a role for the virus in directly inducing demyelination. In order to clarify the function of DA virus genes, we generated a transgenic mouse that had tamoxifen-inducible expression of the DA L-coding region in oligodendrocytes (and Schwann cells), a cell type in which the virus is known to persist. Tamoxifen-treated young transgenic mice usually developed an acute progressive fatal paralysis, with abnormalities of the oligodendrocytes and Schwann cells and demyelination, but without significant lymphocytic infiltration; later treatment led to transient weakness with demyelination and persistent expression of the recombined transgene. These findings demonstrate that a high level of expression of DA L can cause the death of myelin-synthesizing cells and death of the mouse, while a lower level of L expression (which can persist) can lead to cellular dysfunction with survival. The results suggest that expression of DA L plays an important role in the pathogenesis of TMEV-IDD. Virus-induced infection and death of oligodendrocytes may play a part in the demyelination of other diseases in which an immune-mediated mechanism has been stressed, including multiple sclerosis.     

The response of NG2-expressing oligodendrocyte progenitors to demyelination in MOG-EAE and MS

Remyelination of primary demyelinated lesions is a common feature of experimental models of multiple sclerosis (MS) and is also suggested to be the normal response to demyelination during the early stages of MS itself. Many lines of evidence have shown that remyelination is preceded by the division of endogenous oligodendrocyte precursor cells (OPCs) in the lesion and its borders. It is suggested that this rapid response of OPCs to repopulate the lesion site and their subsequent differentiation into new oligodendrocytes is the key to the rapid remyelination. Antibodies to the NG2 chondroitin sulphate proteoglycan have proved exceedingly useful in following and quantitating the response of endogenous OPCs to demyelination. Here we review the literature on the response of NG2-expressing OPCs to demyelination and provide some new evidence on their response to the chronic inflammatory demyelinating environment seen in recombinant myelin oligodendrocyte glycoprotein (MOG) induced experimental allergic encephalomyelitis (EAE) in the DA rat. NG2-expressing OPCs responded to the inflammatory demyelination in this model by becoming reactive and increasing in number in a very focal manner. Evidence of NG2⁺OPCs in lesioned areas beginning to express the oligodendrocyte marker CNP was also seen. The response of OPCs appeared to occur following successive relapses but did not always lead to remyelination, with areas of chronic demyelination observed in the spinal cord. The presence of OPCs in the adult human CNS is clearly of vital importance for repair in multiple sclerosis (MS). As in rat tissue, the antibody labels an evenly distributed cell population present in both white and grey matter, distinct from HLA-DR⁺microglia. NG2⁺cells are sparsely distributed in the centre of chronic MS lesions. These cells apparently survive demyelination and exhibit a multi-processed or bipolar morphology in the very hypocellular environment of the lesion.