Central nervous system demyelination and remyelination in multiple sclerosis and viral models of disease

The mechanisms of myelin injury and repair were studied in acute multiple sclerosis lesions and in a murine model of demyelination induced by a virus. Injury to oligodendrocytes resulting in degeneration of inner glial loops and inner myelin lamellae (dying-back oligodendrogliopathy) was observed by electron microscopy in brain biopsies of acute demyelinating lesions. Attempts at central nervous system remyelination as manifested by thinly myelinated axons and proliferation of oligodendrocytes were observed at the edge of many acute plaques. To develop therapeutic strategies to inhibit demyelination or promote remyelination, mice infected intracranially with Theiler's virus (a picornavirus) were studied. Experimental manipulation of Theiler's virus-infected mice by treatment during chronic demyelinating disease with immunoglobulins directed at normal spinal cord antigens or with monoclonal antibodies which deplete CD4 or CD8-positive T cells reslted in augmentation of new myelin synthesis. These observations suggest that disturbances in the myelinating function of oligodendrocytes, events not accompanied by death of these cells, may be among the earliest pathological events in multiple sclerosis. Experiments using the Theiler's virus model of demyelination indicate that manipulation of the immune response has the potential to promote central nervous system remyelination and functional recovery in multiple sclerosis.     

Myelin repair strategies: a cellular view

PURPOSE OF REVIEW: The development of successful myelin repair strategies depends on the detailed knowledge of the cellular and molecular processes underlying demyelination and remyelination in the central nervous system of animal models and in patients with multiple sclerosis (MS). Based on the complexity of the demyelination and remyelination processes, it should be expected that effective therapeutic approaches will require a combination of strategies for immunomodulation, neuroprotection, and myelin replacement. This brief review highlights recent cellular and molecular findings and indicates that future therapeutic strategies to enhance remyelination may also require combinatorial treatment to accomplish. RECENT FINDINGS: The relapsing-remitting course of some forms of multiple sclerosis has typically fueled hope for effective repair of multiple sclerosis lesions, if demyelinating activity could be attenuated. Recent findings support the potential of endogenous neural stem cells and progenitor cells to generate remyelinating oligodendrocytes. Importantly, interactions with viable axons and supportive astrocytic responses are required for endogenous immature cells to fulfill their potential remyelinating capacity. SUMMARY: The research described here will help in identifying the major obstacles to effective remyelination and potential therapeutic targets to guide development of comprehensive approaches for testing in animal models and eventual treatment of patients with multiple sclerosis.     

Remyelination in multiple sclerosis: Cellular mechanisms and novel therapeutic approaches

The myelin sheath that coats axons allows rapid propagation of electrical impulses across the nervous system. Oligodendrocytes (ODs) are myelin‐producing cells of the central nervous system (CNS) responsible for wrapping the axons of neurons. Multiple sclerosis (MS) is a demyelinating disease of the CNS identifiable by white and gray matter lesions. These lesions consist of axons that have lost their myelin through an autoimmune response to myelin and ODs. Current treatments for MS target the autoimmune aspect of the disease. However, these immunomodulators do not directly enhance the process of remyelination. The ability to remyelinate lesions can be enhanced by neural progenitor cells that can differentiate into ODs and replace lost myelin, although successful remyelination is complex and dependent on multiple factors. The restoration of lost myelin might protect the axon from degeneration and restore optimal conduction of impulses in MS patients, requiring further research on proremyelinating therapies. The combination of immunomodulators and remyelinating enhancers might be the best course of treatment for many MS patients. This Review discusses demyelination in MS, the mechanisms of remyelination, and current therapies designed to promote remyelination in MS patients. © 2014 Wiley Periodicals, Inc.     

Spinal cord lesions of multiple sclerosis

The neuropathological findings of the spinal cord lesions of six human multiple sclerosis cases are described. The spinal cord was extensively necrotic and occasionally cystic in five remitting and relapsing cases. The lesions became more severe as the disease course prolonged and relapses increased. The spinal cords of two cases in particular, with a duration of illness of more than 5 years, were severely atrophic. In these cases, peripheral type remyelination was prominent, although central type remyelination was minimal. In contrast, in mouse spinal cords, in which experimental demyelination and remyelination were induced by ethidium bromide, the degree of central type remyelination and peripheral type remyelination was almost the same. Longitudinal sections of the transitional zone between the areas of central type remyelination and peripheral type remyelination contrained Ranvier nodes, in which central type myelin and peripheral type myelin were situated side by side around a central type axon. These transitional zones were similar to those of the normal transitional zone between the central nervous system and peripheral nervous system of the nerve roots. One chronic progressive case, despite the very long duration of illness, showed classical sharply demarcated demyelinated lesions with marked fibrillary gliosis. The spinal cord of this case was not atrophic and axons were well preserved.     

Stem cell therapy for central nervous system demyelinating disease

Recent advances in cell-based therapies for demyelinating central nervous system diseases have demonstrated the ability to restore damaged neuronal architecture and function. Demyelinated axons in patients with multiple sclerosis can spontaneously remyelinate over time; however, the rate and extent at which remyelination occurs is inadequate for complete recovery. Previous attempts aimed at regenerating myelin-forming cells have been successful but limited by the multifocal nature of the lesions and the inability to produce large numbers of myelin-producing cells in culture. Stem cell-based therapy can overcome these limitations to some extent and may prove useful in the future treatment of demyelinating diseases.     

Strategies for myelin regeneration: lessons learned from development

Myelin regeneration is indispensably important for patients suffering from several central nervous system （CNS） disorders such as multiple sclerosis （MS） and spinal cord injury （SCI）, because it is not only essential for restoring neurophysiology, but also protects denuded axons for secondary degeneration. Understanding the cellular and molecular mechanisms underlying remyelination is critical for the development of remyelination-specific therapeutic approaches. As remyelination shares certain common mechanisms with developmental myelination, knowledge from study of developmental myelination contributes greatly to emerging myelin regeneration therapies, best evidenced as the recently developed human anti-Nogo receptor interacting protein-1 （LINGO-1） monoclonal antibodies to treat MS patients in clinical trials.     

The pathogenesis and basis for treatment in multiple sclerosis

The central concept underlying ideas on the pathogenesis of multiple sclerosis is that inflammatory events cause acute injury of axons and myelin. The phases of symptom onset, recovery, persistence and progression in multiple sclerosis can be summarized as functional impairment with intact structure due to direct effects of inflammatory mediators, demyelination and axonal injury with recovery through plasticity and remyelination, and chronic axonal loss due to failure of enduring remyelination from loss of trophic support for axons normally provided by cells of the oligodendrocyte lineage.     

Acutely damaged axons are remyelinated in multiple sclerosis and experimental models of demyelination

Remyelination is in the center of new therapies for the treatment of multiple sclerosis to resolve and improve disease symptoms and protect axons from further damage. Although remyelination is considered beneficial in the long term, it is not known, whether this is also the case early in lesion formation. Additionally, the precise timing of acute axonal damage and remyelination has not been assessed so far. To shed light onto the interrelation between axons and the myelin sheath during de‐ and remyelination, we employed cuprizone‐ and focal lysolecithin‐induced demyelination and performed time course experiments assessing the evolution of early and late stage remyelination and axonal damage. We observed damaged axons with signs of remyelination after cuprizone diet cessation and lysolecithin injection. Similar observations were made in early multiple sclerosis lesions. To assess the correlation of remyelination and axonal damage in multiple sclerosis lesions, we took advantage of a cohort of patients with early and late stage remyelinated lesions and assessed the number of APP‐ and SMI32‐ positive damaged axons and the density of SMI31‐positive and silver impregnated preserved axons. Early de‐ and remyelinating lesions did not differ with respect to axonal density and axonal damage, but we observed a lower axonal density in late stage demyelinated multiple sclerosis lesions than in remyelinated multiple sclerosis lesions. Our findings suggest that remyelination may not only be protective over a long period of time, but may play an important role in the immediate axonal recuperation after a demyelinating insult.     

Experimental strategies to promote central nervous system remyelination in multiple sclerosis: Insights gained from the Theiler's virus model system

The Destruction of central nervous system (CNS) myelin, the lipid-rich insulator surrounding axons in the mammalian brain and spinal cord, is the primary pathological finding in multiple sclerosis. Myelin loss can result in a significant clinical deficit, and was originally thought to be permanent, similar to axonal destruction. However, myelin regeneration is now an established phenomenon in both human disease and animal models of CNS demyelination. In this review, the concept of remyelination in demyelinating deseases such as multiple sclerosis is discussed and the usefulness of animal models of CNS demyelination in developing experimental strategies to promote remyelination is examined Special emphasis is given to the Theiler's murine encephalomyelitis model, which has been the primary animal model used to investigate therapies designed specifically to stimulate myelin repair. © 1995 Wiley-Liss, Inc.     

Spontaneous central nervous system remyelination is not altered in NFH-lacZ transgenic mice after chemical demyelination

Harmonious functioning of the nervous system depends on neuron-glia interactions, particularly between the axons and their myelinating cells, i.e., oligodendrocytes (OL) in the central nervous system (CNS). In human demyelinating diseases such as multiple sclerosis (MS), demyelination may be associated with axonal damage, but alterations of the axonal cytoskeleton, which is composed mainly of neurofilaments (NF) and microtubules, are largely unknown, as are the consequences on remyelination. In a model of demyelination induced by lysophosphatidylcholine (LPC), we have shown that demyelination was correlated with a decrease in NF immunolabelling, and that these axonal abnormalities were reduced by platelet-derived growth factor (PDGF)-enhanced remyelination in adult rats. We have analysed the spontaneous remyelination after LPC stereotaxic injection in the CNS of transgenic NFH-lacZ mice, which present axonal atrophy caused by abnormal distribution of NF, associated with hypermyelination in the PNS, and normal myelin thickness in the CNS. Axonal atrophy in the CNS of NFH-lacZ mice was confirmed, but it was not worsened by demyelination. On the contrary, demyelination induced axonal atrophy in wild-type mice, demonstrating that NF are essential for axonal calibre determination. Moreover, an efficient spontaneous remyelination occurred in NFH-lacZ as well as in wild-type mice, indicating that the NF are not necessary for CNS remyelination. These findings point out that NF modifications observed in MS may not be responsible for the lack of remyelination in this disease.     

The role of B cells in multiple sclerosis: rationale for B-cell-targeted therapies

Multiple sclerosis is an inflammatory demyelinating disease of the central nervous system with no clear etiology. Until recently, most studies have emphasized the role of T cells in the pathogenesis of multiple sclerosis. Data suggesting that B cells play a role in the pathogenesis of multiple sclerosis have been accumulating for the past five decades, demonstrating that the cerebrospinal fluid and central nervous system tissues of multiple sclerosis patients contain B cells, plasma cells, antibodies, and immunoglobulins. Data suggest that B cells are involved in antigen capture and presentation to T cells, cytokine production, antibody secretion, demyelination, tissue damage, and remyelination in multiple sclerosis. These advances in the understanding of B-cell and antibody roles in the pathophysiology of multiple sclerosis provide a strong rationale for B-cell-targeted therapies.     

The contribution of oligodendrocytes and oligodendrocyte progenitor cells to central nervous system repair in multiple sclerosis: perspectives for remyelination therapeutic strategies

Oligodencrocytes(OLs) are the main glial cells of the central nervous system involved in myelination of axons. In multiple sclerosis(MS), there is an imbalance between demyelination and remyelination processes, the last one performed by oligodendrocyte progenitor cells(OPCs) and OLs, resulting into a permanent demyelination, axonal damage and neuronal loss. In MS lesions, astrocytes and microglias play an important part in permeabilization of blood-brain barrier and initiation of OPCs proliferation. Migration and differentiation of OPCs are influenced by various factors and the process is finalized by insufficient acummulation of OLs into the MS lesion. In relation to all these processes, the author will discuss the potential targets for remyelination strategies.     

Differential expression of sonic hedgehog immunoreactivity during lesion evolution in autoimmune encephalomyelitis

The signaling molecule Sonic hedgehog (Shh) is involved in several processes of central nervous system development. Recent reports indicate that Shh expression plays a role also in certain pathologic conditions in the adult brain, including multiple sclerosis and its animal model. However, the role of Shh signaling in immune-mediated demyelinating disease remains still uncertain. The aim of our study was to investigate the distribution pattern of Shh immunoreactivity (Shh-IR) during lesion evolution in myelin-oligodendrocyte-glycoprotein-induced experimental autoimmune encephalomyelitis (MOG-EAE), a model strongly mimicking multiple sclerosis. MOG-EAE was actively induced in DA rats. Histologic evaluation was performed with light and confocal microscopy on paraffin-embedded central nervous system sections from days 20 to 120 after active immunization. Shh-IR was present within the lesions of MOG-EAE during all stages of lesion evolution. The highest staining intensity for Shh was found in remyelinating lesions. In actively demyelinating, inactive demyelinated lesions, and in remyelinating lesions, Shh-IR was detected in macrophages, endothelium, and astrocytes. Shh-IR in axons was exclusively present in remyelinating lesions. Although the exact molecular mechanisms of the Shh-signaling pathway in experimental autoimmune encephalomyelitis are yet to be determined, our findings may imply a role of Shh signaling in facilitating remyelination.     

Multiple sclerosis: Re-expression of a developmental gene in chronic lesions correlates with remyelination

Central nervous system tissue from multiple sclerosis and non-multiple sclerosis subjects was studied for the expression of exon 2 myelin basic protein gene products at the protein and message levels by immunocytochemistry and in situ hybridization, respectively. The exon 2-encoded protein sequence is normally expressed during development (myelination) within the 21 · 5- and 20 · 2-kd isoforms of myelin basic protein and is downregulated in the adult central nervous system where the 18 · 5- and 17 · 2-kd isoforms predominate, the latter devoid of exon 2 owing to alternative splicing. Exon 2 myelin basic protein gene products were readily demonstrable in multiple sclerosis samples, the highest levels correlating with remyelination in chronic lesions while normal adult central nervous system and non-multiple sclerosis material showed very low levels and fetal human central nervous system tissue (a positive control) showed high levels. We conclude that recapitulation of ontogenetic events during myelin repair accounts for the increased expression of the exon 2-encoded protein sequence in the adult central nervous system during multiple sclerosis, an event that might underly the previously observed T-cell activation to this protein sequence during relapses.     

Immunosuppression promotes CNS remyelination in chronic virus-induced demyelinating disease.

Immunosuppression using cyclophosphamide or anti-T cell monoclonal antibodies (mAbs) directed at CD4 or CD8 promoted remyelination of CNS axons in the spinal cords of mice infected chronically with Theiler's virus. Treatment with a mAb directed at class II major histocompatibility gene products did not increase the extent of CNS remyelination. Following immunosuppressive treatment, quantitative morphometry revealed a five- to sevenfold increase in new myelin synthesis. Proliferating nervous system cells were identified at the edges of remyelinated lesions by their incorporation of [3H]thymidine. CNS remyelination occurred in mice depleted of selected subsets of T lymphocytes despite the local persistence of viral antigen. These findings indicate that CNS remyelination occurs as a normal consequence of primary myelin injury, but factors associated with immune T cells somehow impair remyelination. Interference with the function of immune T cells enhances CNS remyelination by oligodendrocytes. Similar depletion of immune T cells may allow for enhanced remyelination in the CNS of patients with chronic multiple sclerosis.     

Inhibitory milieu at the multiple sclerosis lesion site and the challenges for remyelination

Regeneration of myelin, following injury, can occur within the central nervous system to reinstate proper axonal conductance and provide trophic support. Failure to do so renders the axons vulnerable, leading to eventual degeneration, and neuronal loss. Thus, it is essential to understand the mechanisms by which remyelination or failure to remyelinate occur, particularly in the context of demyelinating and neurodegenerative disorders. In multiple sclerosis, oligodendrocyte progenitor cells (OPCs) migrate to lesion sites to repair myelin. However, during disease progression, the ability of OPCs to participate in remyelination diminishes coincident with worsening of the symptoms. Remyelination is affected by a broad range of cues from intrinsic programming of OPCs and extrinsic local factors to the immune system and other systemic elements including diet and exercise. Here we review the literature on these diverse inhibitory factors and the challenges they pose to remyelination. Results spanning several disciplines from fundamental preclinical studies to knowledge gained in the clinic will be discussed.     

The biology of CNS remyelination: the key to therapeutic advances

Remyelination, the process by which new myelin sheaths are restored to demyelinated axons, represents one of the most compelling examples of adult multipotent progenitor cells contributing to regeneration of the injured CNS. This process can occur with remarkable efficiency in both clinical disease, such as multiple sclerosis, and in experimental models, revealing an impressive ability of the adult CNS to repair itself. However, the inconsistency of remyelination in multiple sclerosis, and the loss of axonal integrity that results from its failure, makes enhancement of remyelination an important therapeutic objective. Identifying potential targets requires a detailed understanding of the cellular and molecular mechanisms of remyelination. A critical step in achieving effective remyelination is the differentiation of precursor cells into mature oligodendrocytes. In experimental models of demyelinating disease in aged animals, as well as in multiple sclerosis, such differentiation appears to be impaired. This is due, at least in part, to changes in environmental signals governing remyelination. In particular, myelin debris within lesions appears to contain powerful inhibitors of precursor cell differentiation. Efficient removal of myelin debris by macrophages may thus facilitate differentiation and permit successful remyelination of damaged axons. This may represent a promising therapeutic target for promoting remyelination in multiple sclerosis and thus limiting the accumulation of irreversible neurological disability.     

Could non-invasive brain-stimulation prevent neuronal degeneration upon ion channel re-distribution and ion accumulation after demyelination?

Fast and efficient transmission of electrical signals in the nervous system is mediated through myelinated nerve fibers. In neuronal diseases such as multiple sclerosis, the conduction properties of axons are disturbed by the removal of the myelin sheath, leaving nerve cells at a higher risk of degenerating. In some cases, the protective myelin sheath of axons can be rebuilt by remyelination through oligodendroglial cells. In any case, however, changes in the ion channel organization occur and may help to restore impulse conduction after demyelination. On the other hand, changes in ion channel distribution may increase the energy demand of axons, thereby increasing the probability of axonal degeneration. Many attempts have been made or discussed in recent years to increase remyelination of affected axons in demyelinating diseases such as multiple sclerosis. These approaches range from pharmacological treatments that reduce inflammatory processes or block ion channels to the modulation of neuronal activity through electrical cortical stimulation. However, these treatments either affect the entire organism(pharmacological) or exert a very local effect(electrodes). Current results show that neuronal activity is a strong regulator of oligodendroglial development. To bridge the gap between global and very local treatments, non-invasive transcranial magnetic stimulation could be considered. Transcranial magnetic stimulation is externally applied to brain areas and experiments with repetitive transcranial magnetic stimulation show that the neuronal activity can be modulated depending on the stimulation parameters in both humans and animals. In this review, we discuss the possibilities of influencing ion channel distribution and increasing neuronal activity by transcranial magnetic stimulation as well as the effect of this modulation on oligodendroglial cells and their capacity to remyelinate previously demyelinated axons. Although the physiological mechanisms underlying the effects of transcranial magnetic stimulation clearly need further investigations, repetitive transcranial magnetic stimulation may be a promising approach for non-invasive neuronal modulation aiming at enhancing remyelination and thus reducing neurodegeneration.     

Remyelination Therapy for Multiple Sclerosis

Multiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system characterized by infiltration of immune cells and progressive damage to myelin and axons. All therapeutics used to treat MS have been developed to target an overactive immune response, with aims to reduce disease activity. Chronic demyelinated axons are further prone to irreversible damage and death, and it is imperative that new therapies address this critical issue. Remyelination, the generation of new myelin in the adult nervous system, is an endogenous repair mechanism that restores function of denuded axons and delays their deterioration. Although remyelination can be extensive in some patients, the majority of cases limit repair only to the acute phase of disease. A significant current drive in new MS therapeutics is to identify targets that can promote remyelination by boosting endogenous oligodendrocyte precursor cells to form new myelin. Also, a number of inhibitory pathways have been identified in chronic MS lesions that prevent oligodendrocyte precursor cells from being properly recruited to demyelinated lesions or interfere with their differentiation to myelin-forming oligodendrocytes. In this review, we introduce the phenomenon of remyelination from the view of experimental models and studies in MS patients, describe a potential role in remyelination for currently available MS mediations, and discuss many avenues that are being actively studied to promote remyelination. The next frontier in MS therapeutics will supplement immunomodulation with agents that directly foster myelin repair, with aims to delay disease progression and recover lost neurological functions.