Journal of Neuroscience Research 00:00–00 (2014)

Review Remyelination in Multiple Sclerosis: Cellular Mechanisms and Novel Therapeutic Approaches John A. Olsen1 and Eitan M. Akirav1,2* 1

Research Institute, Islet Biology, Winthrop-University Hospital, Mineola, New York Stony Brook University School of Medicine, Stony Brook, New York

2

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. VC 2014 Wiley Periodicals, Inc. Key words: myelination; demyelination; remyelination; multiple sclerosis; oligodendrocytes

The central nervous system (CNS) relies on a network of neuronal cells to transmit electrical impulses known as action potentials (du Bois-Reymond, 1843; Finkelstein, 2003) Many of these neurons possess extensions known as axons that can carry action potentials across a considerable distance from one nerve cell to the next. The axons of many neurons are insulated by a lipidrich membrane that allows rapid conduction of impulses and ensures the delivery of the action potential to the target cell (Hartline and Colman, 2007). The German pathologist Rudolf Virchow designated the term myelin for this membrane that coats axons (Virchow et al., C 2014 Wiley Periodicals, Inc. V

1854). Myelin is produced in the CNS by glial cells known as oligodendrocytes (ODs), first discovered by Pıo del Rıo Hortega in 1921 (English translation by IglesiasRozas and Garrosa, 2012). Extensions of OD cell membranes branch out and ensheath axons in several layers of myelin (Bunge et al., 1962; Lee et al., 2005; Fig. 1A). Axons can lose their myelin sheath through a process of demyelination, which can be caused by disease or physical injury. Once an axon has been demyelinated, its ability to transmit action potentials is impaired (Waxman, 1977; Smith et al., 1981; Felts et al., 1997), and the exposed nerve fiber is susceptible to degeneration (Ferguson et al., 1997). Loss of myelin can result in a wide range of neurological disorders, including reduced motor function, impaired cognitive abilities, and vision problems. Remyelination of these exposed axons can occur naturally within the CNS of several organisms, including humans. Among the various demyelinating diseases, multiple sclerosis (MS) has been the most extensively researched, and a considerable effort has been directed to remyelination research with the intent of creating new therapies for the treatment of this disease. MS patients suffer from lesions in both white and gray matter of the CNS (Kidd et al., 1999; Chang et al., 2012; Damasceno et al., 2014). These lesions consist of demyelinated axons, with a lack of myelin-producing ODs (Chang et al., 2002). Inflammation in these lesions is caused by an immune response to myelin (Kerlero de Rosbo et al., 1993), which can cause continuous damage to ODs and neurons and inhibit successful remyelination. This Review provides a summary of known processes of remyelination in response to demyelination in MS as well as several therapies designed to aid in the restoration of demyelinated axons. *Correspondence to: Eitan M. Akirav, 222 Station Plaza North, Mineola, NY 11501. E-mail: [email protected] Received 3 June 2014; Revised 5 September 2014; Accepted 11 September 2014 Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jnr.23493

2

Olsen and Akirav

Fig. 1. Dynamic changes in axonal myelination during multiple sclerosis progression. A: Neurons in the CNS with axons myelinated by oligodendrocytes. B: Immunopathogenic destruction of myelin sheath and phagocytosis of resulting myelin debris by macrophages and activated microglia. C: Oligodendrocyte progenitor cells migrating to a site of demyelination. D: Mature oligodendrocytes remyelinate exposed axons.

DEMYELINATION IN MS Both mouse and human studies suggest that MS is mediated primarily by T cells (Traugott et al., 1983; Hauser et al., 1986; Fletcher et al., 2010), although some reports also show a role for B cells in the pathogenicity of MS (Oliver et al., 2003; Akirav et al., 2011; Disanto et al., 2012). These cells cross into the CNS (Larochelle et al., 2011) as a result of increased permeabilization of the blood–brain barrier, observable by dynamic contrastenhanced magnetic resonance imaging (MRI; Cramer et al., 2013), and produce an autoimmune response to various myelin and nonmyelin antigens (Kerlero de Rosbo et al., 1993; Silber et al., 2002; Mathey et al., 2007; Lee et al., 2011). The resulting demyelination creates the white and gray matter lesions characteristic of MS, identifiable by MRI (Weiner, 2009; Damasceno et al., 2014). T cells, activated microglia, and macrophages are present in active sites of demyelination, releasing proinflammatory cytokines, such as tumor necrosis factor-a and the free radical nitric oxide, both of which

play a central role in in vitro demyelination and OD death (Smith et al., 1999; Dasgupta et al., 2002; Smith and Lassmann, 2002; di Penta et al., 2013; Fig. 1B). Macrophages clear the myelin debris formed from these autoimmune attacks, creating myelin-free lesions with impaired signal transduction among axons (Boven et al., 2006). Lesions that are actively expanding consist of several distinct regions (Henderson et al., 2009). The inner region is identifiable by excessive demyelination and macrophages containing myelin debris. The intermediate region surrounding the inner region also contains myelincontaining macrophages, with less demyelination observed. The outer peripheral region of active lesions displays activated microglia and preserved myelin. There are several subtypes of MS that can be categorized based on disease progression (Bitsch and Bruck, 2002). Relapsing–remitting MS is characterized by periodic relapses of disease symptoms followed by a temporary remission. Secondary progressive MS typically begins with several rounds of relapse and remission, eventually Journal of Neuroscience Research

Myelin Restoration in MS

leading to an uninterrupted progression in disability. Primary progressive MS is characterized by a continuous progression of illness with an absence of relapse or remission. Extensive research has been conducted on axon damage with the goal of understanding the nature of the different subtypes of MS (Bitsch et al., 2000; Kornek et al., 2000). Because of the complexity of MS, study of the characteristics of remyelination as well as inflammation within each subtype is required. Understanding the mechanisms behind each subtype will provide the means for developing more specialized forms of treatment. REMYELINATION IN THE CNS Remyelination is a process that involves multiple steps and is triggered in response to demyelination. Neural progenitor cells must proliferate, migrate to areas of demyelination, differentiate into mature ODs, and successfully myelinate exposed axons. Precise timing and coordination among a range of different cell types are essential for the mechanism of remyelination to succeed (Fig. 1). Migration and Differentiation In humans, neurogenesis by neural stem cells (NSCs) takes place in the subventricular zone of the brain, by a thin layer of dividing cells (Wang et al., 2011). These cells are induced by extracellular signaling to form oligodendrocyte progenitor cells (OPCs; Raff et al., 1983) that are capable of proliferating and migrating to sites of demyelination in the CNS (Maki et al., 2013; Fig. 1C). Growth factor molecules, such as hepatocyte growth factor, basic fibroblast growth factor, and vascular endothelial growth factor A, have been shown to influence both cell migration and differentiation of precursor cells in vitro (de Castro and Bribian, 2005; Hayakawa et al., 2011). These precursors undergo several stages of differentiation before forming mature ODs capable of myelin production. In addition to the contribution of NSCs to remyelination, populations of local OPCs distributed throughout the CNS have been identified (Chang et al., 2000) and can also contribute to remyelination (Kuhlmann et al., 2008). Each stage of OD maturation is marked by unique combinations of biomarkers, which can be detected with complementary antibodies. Although markers used to characterize each stage of differentiation are well established and accepted, caution must be observed when referring to the cell of a particular stage by name. Terms such as OPCs and oligodendrocyte precursor cells (OPRs; Wolswijk, 1998) are used interchangeably in different articles when describing cells in the early stages of differentiation. Early-stage cells are identifiable by the A2B5 ganglioside (Berg and Schachner, 1982; Dietrich et al., 2002; Eisenbarth et al., 1979) and the NG2 proteoglycan (Stallcup, 1981; Chang et al., 2000). The maturation of these cells to a preoligodendrocyte stage is marked by the expression of the O4 sulfatide (Sommer and Schachner, 1981; Armstrong et al., 1992). Differentiation continues to the immature oligodendrocyte, which is Journal of Neuroscience Research

3

positive for galactosylceramides (Radin and Akahori, 1961; Monge et al., 1986) and O4 and negative for A2B5 and NG2. Mature oligodendrocytes represent the final stage of differentiation, capable of producing myelin and exhibiting biomarkers such as proteolipid protein (PLP; Eng et al., 1968; Carnegie, 1971), myelin basic protein (MBP; Folch and Lees, 1951), myelin associated glycoprotein (MAG; Quarles et al., 1973; McIntyre et al., 1979), and myelin oligodendrocyte glycoprotein (MOG; Lebar et al., 1986; Slavin et al., 1997). A single OD is capable of forming myelin segments on multiple axons (Friedman et al., 1989). However, ODs that are unsuccessful in making contact and sheathing axons undergo programmed cell death, thereby highlighting the importance of targeted OD migration (Barres et al., 1993; Barres and Raff, 1999). Myelin Production Fully matured ODs produce the myelin sheath that insulates the axons of neurons. Myelin is a lipid-rich membrane, with 30% of its composition consisting of proteins (Jackman et al., 2009). Membrane proteins such as MAG play a role in maintaining contact between myelin and axons (Poltorak et al., 1987), whereas PLP and MBP are among the proteins believed to aid in stabilizing the structure of the myelin sheath (Schwob et al., 1985). After an OD has matured, processes guided by the extracellular matrix extend out from its cell body and initiate the remyelination of exposed axons (Oh et al., 1999; Fig. 1D). During remyelination, synthesis of the myelin membrane and its associated proteins occurs as several layers of myelin are wrapped around the axon until remyelination is complete. To sustain this process efficiently, it appears that the synthesis of many of these membrane proteins takes place at the site of remyelination rather than within the OD cell body (Simons and Trajkovic, 2006). A network of microtubules spanning the distance from the OD cell body to the myelin sheath directs ribosomes and mRNA to the site of myelin synthesis (Colman et al., 1982; Sherman and Brophy, 2005; Kosturko et al., 2005). The polymerization of this microtubule network might also support the extension of the OD processes that branch out from the cell body (Song et al., 2001). Limitations of Remyelination Although remyelination is possible, it has several limitations. Lesions that are produced in the CNS of MS patients exhibit remyelinated axons whose myelin sheaths are both thinner and shorter in length than the myelin sheaths on normal axons (Hanafy and Sloane, 2011). A decrease in length of the myelin sheath can have a detrimental effect on conduction speed across the axon (Huxley and Stampfli, 1949; Wu et al., 2012), and myelin sheath thickness is equally important to conduction (Rushton, 1951). The g-ratio is a measurement of axon diameter in relation to myelin thickness (Rushton, 1951) and can be utilized for detecting axons that have been remyelinated (Fig. 2). To calculate the g-ratio of a

4

Olsen and Akirav

Fig. 2. Variation in g-ratios of myelin thickness in relation to axon diameter between native and remyelinated axons.

myelinated axon, the diameter of the axon is divided by the total diameter of both the axon and the myelin sheath. Normally, myelinated axons have a g-ratio between 0.6 and 0.8, whereas an axon that has been remyelinated typically has a g-ratio between 0.8 and 1 (Arnett et al., 2003; Michailov et al., 2004). Although these smaller myelin segments are typically able to restore normal function to demyelinated axons (Coman et al., 2006), it is clear that factors that direct myelination during normal development are inhibited, limited, or absent in the process of remyelination. Whether this occurrence is the result of unfavorable conditions that exist within the demyelinated lesions or a natural mechanism for late-born ODs to form shorter segments along axons remains to be ascertained (Young et al., 2013). This illustrates the requirement for continued studies directed to identifying the factors involved in myelination of the CNS during development (Fancy et al., 2011). Such studies could provide useful insights for enhancing the restorative effects of therapybased remyelination. The process of remyelination is limited in the progressive subtypes of MS (Bramow et al., 2010). Several explanations for this limitation include a depletion of available OPCs resulting from extensive rounds of remyelination, axonal damage resulting in an inability to associate with myelin (Chang et al., 2002), and formation of glial scars (Fawcett and Asher, 1999) that might impede migration of OPCs to demyelinated axons (Fok-Seang et al., 1995). Because of the continued worsening of symptoms in patients with progressive MS, additional studies are required to identify new therapeutic options for therapy to stop or reverse the disease. Availability of myelin components might also play a role in the process of remyelination. Cholesterol is a major constituent, accounting for roughly 27% of the dry weight of myelin (Saher et al., 2011). A disruption in cholesterol availability has been shown to impose a limitation on the myelination process. Knockout mice with a

mutation in the squalene synthase gene, an enzyme necessary for cholesterol synthesis, develop myelin in the CNS at a delayed rate compared with wild-type mice (Saher et al., 2005). The composition of myelin in these knockout mice is also irregular. Proper cholesterol production and distribution in the CNS are clearly necessary for the mechanism of myelination, which might translate to its importance in remyelination as well. Studies have revealed the presence of undifferentiated OPCs in MS lesions (Chang et al., 2002). This finding highlights another obstacle in the process of remyelination, the inability of OPCs to mature into myelin-producing ODs. There are several factors that might be responsible for this phenomenon (Franklin, 2002). A delay in OPC migration could result in missed differentiation cues at the lesion site. An absence of any of these signaling molecules might inhibit differentiation. This underscores the importance of identifying the different cell types responsible for the production of molecules required for differentiation. In a recent example, the M2 phenotype of macrophages and microglia was studied, focusing on its role in the differentiation of ODs and remyelination (Miron et al., 2013). Unlike the M1 phenotype, which is characterized by proinflammatory activity, the M2 phenotype is typically observed in antiinflammatory reactions. With depletion of M2 cells within lesions in mice, a decrease in mature OD biomarkers was observed. M2 cell secretion of activin-A, a member of the transforming growth factor-b protein superfamily, was identified as a key factor in the process of M2-mediated OD differentiation. Maturation of ODs is a prerequisite for remyelination, so continued research in this field will benefit the design of therapies used in MS. REMYELINATION-BASED THERAPY Currently approved treatments for MS involve the use of immunomodulators to reduce the inflammatory outbreaks Journal of Neuroscience Research

Myelin Restoration in MS

caused by the immune system (Filippini et al., 2013). The purpose of these drugs is to inhibit various immune cells and cytokines. By targeting these cells and molecules, these treatments are designed to suppress inflammatory outbreaks, to prevent new inflammatory outbreaks, and to prevent migration of immune cells across the blood– brain barrier. However, treating the recurring inflammation does not necessarily ensure remyelination in the demyelinated lesions. It is therefore crucial to design therapeutic strategies aimed at promoting remyelination in the CNS following the onset of MS. Several different experimental animal models are used to study disease mechanisms in MS and to evaluate the efficacy of new therapeutic drug candidates. One of the most widely used models is experimental autoimmune encephalomyelitis (EAE; Constantinescu et al., 2011). EAE is commonly induced by injection of myelinderived antigen in combination with an adjuvant capable of eliciting an inflammatory response. Such a response results in CNS lesions similar to those observed in MS patients. A nonimmune approach for induction of OD damage relies on the toxic effects of cuprizone, a copper chelator, which, when added to the animal’s diet, can result in demyelination of the corpus callosum (Gudi et al., 2009). Removal of cuprizone from the diet results in remyelination of the affected areas, allowing for focused studies of remyelination mechanisms. A more recently used model for OD-specific damage employs the use of genetic ablation of ODs by diphtheria toxin (DT). In this animal model, the DT receptor is expressed exclusively on ODs. Exposure to DT results in a selective elimination of ODs in the CNS. (Brockschnieder et al., 2004; Buch et al., 2005; Traka et al., 2010). This model offers researchers the opportunity to target the loss of ODs specifically, eliminating the potential for bystander effects caused by the death of other cell types. Ongoing debate exists whether animal models such as EAE are appropriate models of MS (Sriram and Steiner, 2005), given the fact that many therapies show promising results in animal models but fail to show success in human trials. Nevertheless, animal studies are invaluable in providing a starting point for initial drug development, including those drugs targeting remyelination. Cell-Induced Therapy The ability of stem cells to differentiate and replace cells that have been lost from disease or injury makes multipotent cell transplantation a candidate for treatment of a neurodegenerative disease such as MS. A variety of different studies have used various mouse models of MS with different stem cell types and provided a proof of principle for this approach. Labeled neurospheres derived from the periventricular region of the forebrain ventricles in C57BL/6 mice have been administered both intravenously (IV) and intracerebroventricularly (IC) to EAE C57BL/6 mice (Pluchino et al., 2003). After injection, donor neural precursors were observed in areas of demyelination and Journal of Neuroscience Research

5

areas undergoing remyelination in the CNS. Several donor cells’ membranes were associated with myelin sheaths surrounding axons, indicating that some of the donor cells might have differentiated into mature ODs and initiated remyelination. In addition to differentiating into mature ODs, the donor cells play a role in stimulating migration of endogenous OPCs to areas of demyelination. Mice with EAE injected either IC or IV with neural progenitors also display improved motor function over EAE mice that did not receive neural progenitor injections. Similar results have been observed with the injection of these neural progenitors in the cuprizoneinduced model of MS in C57BL/6 mice (Einstein et al., 2009). Injection of the neural progenitors in this model appeared to provide a trophic effect for endogenous OPCs to proliferate and mature. The growth factors platelet-derived growth factor-AA and fibroblast growth factor-2 are among the factors secreted from the donor neural progenitors that appear to contribute to the trophic effects on endogenous OPC proliferation and differentiation. Mesenchymal stem cells (MSCs) have also been used for transplantation in mouse models of MS. Adipose MSCs were isolated from C57BL/6 mice and administered by IV injection into mice subjected to the cuprizone model of MS (Hedayatpour et al., 2013). IV injections were given upon removal of cuprizone from the diet to observe whether the donor cells would improve the process of remyelination typically observed with this change in diet. Transplanted cells were identified in the corpus callosum of mice injected with the MSCs, providing evidence that these donor cells are able to cross the blood– brain barrier and migrate to sites of demyelination. Compared with mice that did not receive transplants, mice that were given MSC IV injections displayed significant improvement in remyelination of lesions. Although MSCs appear to be promising candidates for MS therapy, their potential to differentiate into multiple cell types presents a certain degree of risk. In an attempt to avoid this occurrence, MSCs can be cultured in neural progenitor medium to produce MSC-derived neural progenitors (M-NPs). M-NPs display reduced potential to differentiate into cell types such as osteoblasts or adipocytes, thereby alleviating some of the risk inherent in MSC transplants (Harris et al., 2012a). Additionally, M-NPs have displayed anti-T-cell properties in vitro (Harris et al., 2012a). In an effort to test the efficacy of M-NPs in vivo, researchers collected MSCs from C57BL/6 mice and used them to culture M-NPs, which they injected intrathecally into the cisterna magna of EAE mice (Harris et al., 2012b). EAE mice that received multiple injections of M-NPs showed improvement in motor function scores over time. Immunohistochemistry revealed that M-NPs had migrated to inflammatory sites within the CNS, with a reduction in T-cell concentration at these sites. Compared with EAE mice that had not received M-NP injections, M-NP-treated mice displayed significantly less demyelination in spinal cord crosssections. These findings have provided the foundation for

6

Olsen and Akirav

an FDA Phase I approved clinical trial in which MS patients will receive several rounds of autologous M-NPs injected intrathecally into the cerebral spinal fluid. These patients will be monitored at regular intervals to assess the safety and effectiveness of this treatment. Continued research in the area of cell therapy for MS is critical for several reasons. From a previous study (Einstein et al., 2009), it appears that transplanted cells can differentiate into mature ODs and remyelinate axons as well as play a role in inducing the maturation of resident OPCs that in turn induce remyelination. In the latter case, it might not be necessary to introduce exogenous cells directly, given the fact that as yet unidentified cell products might support the maturation of resident OPCs. Additionally, given the immunomodulatory properties of the different donor cell types (Harris et al., 2012a), it is possible that the improvements observed in test subjects are a result of the therapy’s ability to suppress inflammation rather than directly enhance remyelination. Drug-Induced Therapy Several forms of antibody treatment in MS have focused on binding to various antigens of immune cells to prevent their participation in the immune-mediated pathogenicity of the disease (Miller et al., 2003; Hauser et al., 2008). Although this approach might inhibit the autoimmune response in MS, it does not play a direct role in enhancing remyelination. In an effort to utilize antibodies for myelin-restoring treatment, several monoclonal autoantibodies have been identified that effectively bind to the surface of both ODs and myelin and stimulate remyelination (Warrington et al., 2000). The recombinant form of one of these antibodies, rHIgM22, has been used in the treatment of Theiler’s murine encephalomyelitis virus (TMEV; Oleszak et al., 2004) model of MS in mice (Pirko et al., 2004). A single intraperitoneal injection of rHIgM22 was administered 6 months after TMEV induction in SJL/J mice, and a control group was treated with phosphate-buffered saline. Spinal cord MRI examinations were performed on the day before treatment and 5 weeks later. Comparison of MRI examinations showed a decrease in size of 82.8% of lesions in the treated group 5 weeks after treatment, in contrast to a decrease in size of only 18.8% in the control group. The mechanism of this induced remyelination is still being researched; however, it is known that these antibodies induce an influx of Ca21 into OD cells that most likely activates signaling pathways conducive to myelin production (Paz Soldan et al., 2003). The use of rHIgm22 has recently been approved for Phase I clinical trials in MS patients, with the intention of evaluating the safety and efficacy of this treatment. Another antibody treatment currently being evaluated is BIIB033. BIIB033 is an IgG1 monoclonal antibody that binds to the leucine-rich repeat and Igcontaining Nogo receptor-interacting protein-1 (LINGO1; Mi et al., 2013), a transmembrane protein expressed in both ODs and neurons (Mi et al., 2008). LINGO-1 has been shown to regulate OPC differentiation negatively,

making it an attractive target for remyelination therapy (Pepinsky et al., 2014). Several in vivo animal experiments have been performed in an attempt to examine the potential of antibody treatments directed at LINGO-1 (Mi et al., 2007, 2009). In one experiment, anti-LINGO-1 antibody or an isotype control antibody was administered twice per week for 2 weeks to rats with EAE at the time of disease onset. (Mi et al., 2009). Anti-LINGO-1-treated rats developed a milder EAE over the course of the experiment compared with rats treated with the control antibody. Cross-sections of spinal cord stained for myelinated axons revealed significantly higher amounts of remyelinated axons in the anti-LINGO-1-treated mice compared with the control group. Magnetic resonance diffusion tensor imaging likewise revealed greater myelin integrity in spinal cord transverse sections of the anti-LINGO-1 group compared with the control group. Phase I trials for BIIB033, designed to study the safety and pharmacodynamics of the antibody treatment in humans, have concluded and appear to have been well tolerated (Mi et al., 2013). Although the blood–brain barrier would be expected to hinder the crossing of BIIB033 into the CNS, the antibody was detected in the cerebral spinal fluid of MS patients participating in the Phase I trials (Mi et al., 2013). The concentration of BIIB033 that was detected was determined to be effective, and Phase II trials for BIIB033 are currently underway. Another study that shows promise for remyelination therapy recently demonstrated the effects of the FDAapproved drug benztropine on OPC differentiation (Deshmukh et al., 2013). Benztropine is commonly used in the treatment of Parkinson’s disease or to reduce tremors produced as a side effect of other medications. Expression of the mature OD markers MBP and MOG was detected in rat and mouse OPCs cultured with benztropine (Deshmukh et al., 2013). In vivo administration of benztropine in EAE and cuprizone mouse models of MS resulted in a significant improvement in mean EAE clinical scores and substantial remyelination. Mice in the cuprizone group were fed a 0.2% (w/w) diet for 7 weeks and then given a normal diet along with benztropine injections (Deshmukh et al., 2013). At the 2 week time point following the change to the cuprizone-free diet, mice that had received the injections showed significantly higher staining for myelin and mature ODs in the corpus callosum compared with mice that had received no injections. In a different set of experiments, mice with EAE received a combination therapy consisting of benztropine together with either interferon-b or fingolimod (FTY720), two immunosuppressive drugs that have been approved for treatment in MS. Mean EAE clinical scores were significantly lower in mice treated with the combination of benztropine and interferon-b or FTY720 than in mice treated with each drug individually. An added benefit to this combinational approach is the fact that a lower dosage of benztropine was required to achieve these positive effects. This represents an important factor considering the potential dose-dependent side effects associated with benztropine (Modell et al. 1989), a critical aspect for translating these results to human MS. These Journal of Neuroscience Research

Myelin Restoration in MS

results reinforce the concept of combining immunosuppressant therapy with proremyelinating inducers to optimize treatment in MS. CONCLUSIONS A great deal of research has yielded valuable insights into the pathology behind MS and the mechanism of remyelination in response to myelin assault. Many signaling molecules involved in OPC migration and differentiation have been identified as well as cellular events that take place in the OD during myelin production. Continued research on both oligodendroglia and the environmental conditions present in MS lesions that influence remyelination is required. Understanding the process of remyelination and its role in MS is critical for the development of successful treatments and therapies. In a disease as heterogeneous as MS, a combinational approach consisting of both immunosuppressive and remyelinating therapies might be required to provide a more complete form of therapy. The various subtypes of MS present an additional challenge that might require specialized therapeutic strategies to be formulated for each one. Promising treatments are currently being tested that might significantly reverse damage incurred during demyelination and prevent further progression of the disease. In addition to research focused on enhancement of remyelination, the development of assays designed for early detection of demyelination prior to lesion formation that rely on biomarkers of active remyelination could improve the diagnosis and prognosis of the disease. Such assays would allow therapy to begin before extensive neurological damage can occur, thereby placing less strain on the remyelination process. This makes the identification of reliable biomarkers for the diagnosis of MS and other demyelinating diseases an area of research that requires extensive effort. ACKNOWLEDGMENTS The authors thank Dr. Nancy H. Ruddle for her help with preparing this article. REFERENCES Akirav EM, Xu Y, Ruddle NH. 2011. Resident B cells regulate thymic expression of myelin oligodendrocyte glycoprotein. J Neuroimmunol 235:33–39. Armstrong RC, Dorn HH, Kufta CV, Friedman E, Dubois-Dalcq ME. 1992. Preoligodendrocytes from adult human CNS. J Neurosci 12: 1538–1547. Arnett HA, Wang Y, Matsushima GK, Suzuki K, Ting JP. 2003. Functional genomic analysis of remyelination reveals importance of inflammation in oligodendrocyte regeneration. J Neurosci 23:9824–9832. Barres BA, Raff MC. 1999. Axonal control of oligodendrocyte development. J Cell Biol 147:1123–1128. Barres BA, Jacobson MD, Schmid R, Sendtner M, Raff MC. 1993. Does oligodendrocyte survival depend on axons? Curr Biol 3:489–497. Berg GJ, Schachner M. 1982. Electron-microscopic localization of A2B5 cell surface antigen in monolayer cultures of murine cerebellum and retina. Cell Tissue Res 224:637–645. Journal of Neuroscience Research

7

Bitsch A, Bruck W. 2002. Differentiation of multiple sclerosis subtypes: implications for treatment. CNS Drugs 16:405–418. Bitsch A, Schuchardt J, Bunkowski S, Kuhlmann T, Bruck W. 2000. Acute axonal injury in multiple sclerosis. Correlation with demyelination and inflammation. Brain 123:1174–1183. Boven LA, Van Meurs M, Van Zwam M, Wierenga-Wolf A, Hintzen RQ, Boot RG, Aerts JM, Amor S, Nieuwenhuis EE, Laman JD. 2006. Myelin-laden macrophages are anti-inflammatory, consistent with foam cells in multiple sclerosis. Brain 129:517–526. Bramow S, Frischer JM, Lassmann H, Koch-Henriksen N, Lucchinetti CF, Sorensen PS, Laursen H. 2010. Demyelination versus remyelination in progressive multiple sclerosis. Brain 133:2983–2998. Brockschnieder D, Lappe-Siefke C, Goebbels S, Boesl MR, Nave KA, Riethmacher D. 2004. Cell depletion due to diphtheria toxin fragment A after Cre-mediated recombination. Mol Cell Biol 24:7636–7642. Buch T, Heppner FL, Tertilt C, Heinen TJ, Kremer M, Wunderlich FT, Jung S, Waisman A. 2005. A Cre-inducible diphtheria toxin receptor mediates cell lineage ablation after toxin administration. Nat Methods 2:419–426. Bunge MB, Bunge RP, Pappas GD. 1962. Electron microscopic demonstration of connections between glia and myelin sheaths in the developing mammalian central nervous system. J Cell Biol 12:448–453. Carnegie PR. 1971. Amino acid sequence of the encephalitogenic basic protein from human myelin. Biochem J 123:57–67. Chang A, Nishiyama A, Peterson J, Prineas J, Trapp BD. 2000. NG2positive oligodendrocyte progenitor cells in adult human brain and multiple sclerosis lesions. J Neurosci 20:6404–6412. Chang A, Tourtellotte WW, Rudick R, Trapp BD. 2002. Premyelinating oligodendrocytes in chronic lesions of multiple sclerosis. N Engl J Med 346:165–173. Chang A, Staugaitis SM, Dutta R, Batt CE, Easley KE, Chomyk AM, Yong VW, Fox RJ, Kidd GJ, Trapp BD. 2012. Cortical remyelination: a new target for repair therapies in multiple sclerosis. Ann Neurol 72: 918–926. Colman DR, Kreibich G, Frey AB, Sabatini DD. 1982. Synthesis and incorporation of myelin polypeptides into CNS myelin. J Cell Biol 95: 598–608. Coman I, Aigrot MS, Seilhean D, Reynolds R, Girault JA, Zalc B, Lubetzki C. 2006. Nodal, paranodal, and juxtaparanodal axonal proteins during demyelination and remyelination in multiple sclerosis. Brain 129:3186–3195. Constantinescu CS, Farooqi N, O’Brien K, Gran B. 2011. Experimental autoimmune encephalomyelitis (EAE) as a model for multiple sclerosis (MS). Br J Pharmacol 164:1079–1106. Cramer SP, Simonsen H, Frederiksen JL, Rostrup E, Larsson HB. 2013. Abnormal blood–brain barrier permeability in normal appearing white matter in multiple sclerosis investigated by MRI. Neuroimage Clin 4: 182–189. Damasceno A, Damasceno BP, Cendes F. 2014. The clinical impact of cerebellar grey matter pathology in multiple sclerosis. PLoS One 9: e96193. Dasgupta S, Jana M, Liu X, Pahan K. 2002. Myelin basic protein-primed T cells induce nitric oxide synthase in microglial cells. Implications for multiple sclerosis. J Biol Chem 277:39327–39333. de Castro F, Bribian A. 2005. The molecular orchestra of the migration of oligodendrocyte precursors during development. Brain Res Brain Res Rev 49:227–241. Deshmukh VA, Tardif V, Lyssiotis CA, Green CC, Kerman B, Kim HJ, Padmanabhan K, Swoboda JG, Ahmad I, Kondo T, Gage FH, Theofilopoulos AN, Lawson BR, Schultz PG, Lairson LL. 2013. A regenerative approach to the treatment of multiple sclerosis. Nature 502:327–332. Dietrich J, Noble M, Mayer-Proschel M. 2002. Characterization of A2B51 glial precursor cells from cryopreserved human fetal brain progenitor cells. Glia 40:65–77.

8

Olsen and Akirav

di Penta A, Moreno B, Reix S, Fernandez-Diez B, Villanueva M, Errea O, Escala N, Vandenbroeck K, Comella JX, Villoslada P. 2013. Oxidative stress and proinflammatory cytokines contribute to demyelination and axonal damage in a cerebellar culture model of neuroinflammation. PLoS One 8: e54722. Disanto G, Morahan JM, Barnett MH, Giovannoni G, Ramagopalan SV. 2012. The evidence for a role of B cells in multiple sclerosis. Neurology 78: 823–832. du Bois-Reymond E. 1843. Vorl€aufiger Abriss einer Untersuchung ueber den sogenannten Froschstrom und ueber die elektromotorischen Fische. Ann Phys 134:1–30. Einstein O, Friedman-Levi Y, Grigoriadis N, Ben-Hur T. 2009. Transplanted neural precursors enhance host brain-derived myelin regeneration. J Neurosci 29:15694–15702. Eisenbarth GS, Walsh FS, Nirenberg M. 1979. Monoclonal antibody to a plasma membrane antigen of neurons. Proc Natl Acad Sci U S A 76:4913–4917. Eng LF, Chao FC, Gerstl B, Pratt D, Tavaststjerna MG. 1968. The maturation of human white matter myelin. Fractionation of the myelin membrane proteins. Biochemistry 7:4455–4465. Fancy SP, Chan JR, Baranzini SE, Franklin RJ, Rowitch DH. 2011. Myelin regeneration: a recapitulation of development? Annu Rev Neurosci 34:21–43. Fawcett JW, Asher RA. 1999. The glial scar and central nervous system repair. Brain Res Bull 49:377–391. Felts PA, Baker TA, Smith KJ. 1997. Conduction in segmentally demyelinated mammalian central axons. J Neurosci 17:7267–7277. Ferguson B, Matyszak MK, Esiri MM, Perry VH. 1997. Axonal damage in acute multiple sclerosis lesions. Brain 120:393–399. Filippini G, Del Giovane C, Vacchi L, D’Amico R, Di Pietrantonj C, Beecher D, Salanti G. 2013. Immunomodulators and immunosuppressants for multiple sclerosis: a network meta-analysis. Cochrane Database Syst Rev 6:CD008933. Finkelstein G. 2003. M. du Bois-Reymond goes to Paris. Br J Hist Sci 36: 261–300. Fletcher JM, Lalor SJ, Sweeney CM, Tubridy N, Mills KH. 2010. T cells in multiple sclerosis and experimental autoimmune encephalomyelitis. Clin Exp Immunol 162:1–11. Fok-Seang J, Mathews GA, ffrench-Constant C, Trotter J, Fawcett JW. 1995. Migration of oligodendrocyte precursors on astrocytes and meningeal cells. Dev Biol 171:1–15. Folch J, Lees M. 1951. Proteolipides, a new type of tissue lipoproteins; their isolation from brain. J Biol Chem 191:807–817. Franklin RJ. 2002. Why does remyelination fail in multiple sclerosis? Nat Rev Neurosci 3:705–714. Friedman B, Hockfield S, Black JA, Woodruff KA, Waxman SG. 1989. In situ demonstration of mature oligodendrocytes and their processes: an immunocytochemical study with a new monoclonal antibody, rip. Glia 2:380–390. Gudi V, Moharregh-Khiabani D, Skripuletz T, Koutsoudaki PN, Kotsiari A, Skuljec J, Trebst C, Stangel M. 2009. Regional differences between grey and white matter in cuprizone induced demyelination. Brain Res 1283: 127–138. Hanafy KA, Sloane JA. 2011. Regulation of remyelination in multiple sclerosis. FEBS Lett 585:3821–3828. Harris VK, Faroqui R, Vyshkina T, Sadiq SA. 2012a. Characterization of autologous mesenchymal stem cell-derived neural progenitors as a feasible source of stem cells for central nervous system applications in multiple sclerosis. Stem Cells Transl Med 1:536–547. Harris VK, Yan QJ, Vyshkina T, Sahabi S, Liu X, Sadiq SA. 2012b. Clinical and pathological effects of intrathecal injection of mesenchymal stem cell-derived neural progenitors in an experimental model of multiple sclerosis. J Neurol Sci 313:167–177. Hartline DK, Colman DR. 2007. Rapid conduction and the evolution of giant axons and myelinated fibers. Curr Biol 17:R29–R35.

Hauser SL, Bhan AK, Gilles F, Kemp M, Kerr C, Weiner HL. 1986. Immunohistochemical analysis of the cellular infiltrate in multiple sclerosis lesions. Ann Neurol 19:578–587. Hauser SL, Waubant E, Arnold DL, Vollmer T, Antel J, Fox RJ, Bar-Or A, Panzara M, Sarkar N, Agarwal S, Langer-Gould A, Smith CH. 2008. B-cell depletion with rituximab in relapsing–remitting multiple sclerosis. N Engl J Med 358:676–688. Hayakawa K, Pham LD, Som AT, Lee BJ, Guo S, Lo EH, Arai K. 2011. Vascular endothelial growth factor regulates the migration of oligodendrocyte precursor cells. J Neurosci 31:10666–10670. Hedayatpour A, Ragerdi I, Pasbakhsh P, Kafami L, Atlasi N, Pirhajati Mahabadi V, Ghasemi S, Reza M. 2013. Promotion of remyelination by adipose mesenchymal stem cell transplantation in a cuprizone model of multiple sclerosis. Cell J 15:142–151. Henderson AP, Barnett MH, Parratt JD, Prineas JW. 2009. Multiple sclerosis: distribution of inflammatory cells in newly forming lesions. Ann Neurol 66:739–753. Huxley AF, Stampfli R. 1949. Evidence for saltatory conduction in peripheral myelinated nerve fibres. J Physiol 108:315–339. Iglesias-Rozas JR, Garrosa M. 2012. The discovery of oligodendroglia cells by Rio-Hortega: his original articles. 1921. Clin Neuropathol 31: 437–439. Jackman N, Ishii A, Bansal R. 2009. Oligodendrocyte development and myelin biogenesis: parsing out the roles of glycosphingolipids. Physiology 24:290–297. Kerlero de Rosbo N, Milo R, Lees MB, Burger D, Bernard CC, BenNun A. 1993. Reactivity to myelin antigens in multiple sclerosis. Peripheral blood lymphocytes respond predominantly to myelin oligodendrocyte glycoprotein. J Clin Invest 92:2602–2608. Kidd D, Barkhof F, McConnell R, Algra PR, Allen IV, Revesz T. 1999. Cortical lesions in multiple sclerosis. Brain 122:17–26. Kornek B, Storch MK, Weissert R, Wallstroem E, Stefferl A, Olsson T, Linington C, Schmidbauer M, Lassmann H. 2000. Multiple sclerosis and chronic autoimmune encephalomyelitis: a comparative quantitative study of axonal injury in active, inactive, and remyelinated lesions. Am J Pathol 157:267–276. Kosturko LD, Maggipinto MJ, D’Sa C, Carson JH, Barbarese E. 2005. The microtubule-associated protein tumor overexpressed gene binds to the RNA trafficking protein heterogeneous nuclear ribonucleoprotein A2. Mol Biol Cell 16:1938–1947. Kuhlmann T, Miron V, Cui Q, Wegner C, Antel J, Bruck W. 2008. Differentiation block of oligodendroglial progenitor cells as a cause for remyelination failure in chronic multiple sclerosis. Brain 131:1749– 1758. Larochelle C, Alvarez JI, Prat A. 2011. How do immune cells overcome the blood–brain barrier in multiple sclerosis? FEBS Lett 585:3770– 3780. Lebar R, Lubetzki C, Vincent C, Lombrail P, Boutry JM. 1986. The M2 autoantigen of central nervous system myelin, a glycoprotein present in oligodendrocyte membrane. Clin Exp Immunol 66:423–434. Lee J, Gravel M, Zhang R, Thibault P, Braun PE. 2005. Process outgrowth in oligodendrocytes is mediated by CNP, a novel microtubule assembly myelin protein. J Cell Biol 170:661–673. Lee S, Xu L, Shin Y, Gardner L, Hartzes A, Dohan FC, Raine C, Homayouni R, Levin MC. 2011. A potential link between autoimmunity and neurodegeneration in immune-mediated neurological disease. J Neuroimmunol 235:56–69. Maki T, Liang AC, Miyamoto N, Lo EH, Arai K. 2013. Mechanisms of oligodendrocyte regeneration from ventricular–subventricular zonederived progenitor cells in white matter diseases. Front Cell Neurosci 7:275. Mathey EK, Derfuss T, Storch MK, Williams KR, Hales K, Woolley DR, Al-Hayani A, Davies SN, Rasband MN, Olsson T, Moldenhauer A, Velhin S, Hohlfeld R, Meinl E, Linington C. 2007. Neurofascin as

Journal of Neuroscience Research

Myelin Restoration in MS a novel target for autoantibody-mediated axonal injury. J Exp Med 204:2363–2372. McIntyre LJ, Quarles RH, Brady RO. 1979. Lectin-binding proteins in central nervous system myelin. Detection of glycoproteins of purified myelin on polyacrylamide gels by [3H]concanavalin A binding. Biochem J 183:205–212. Mi S, Hu B, Hahm K, Luo Y, Kam Hui ES, Yuan Q, Wong WM, Wang L, Su H, Chu TH, Guo J, Zhang W, So KF, Pepinsky B, Shao Z, Graff C, Garber E, Jung V, Wu EX, Wu W. 2007. LINGO-1 antagonist promotes spinal cord remyelination and axonal integrity in MOG-induced experimental autoimmune encephalomyelitis. Nat Med 13:1228–1233. Mi S, Sandrock A, Miller RH. 2008. LINGO-1 and its role in CNS repair. Int J Biochem Cell Biol 40:1971–1978. Mi S, Miller RH, Tang W, Lee X, Hu B, Wu W, Zhang Y, Shields CB, Miklasz S, Shea D, Mason J, Franklin RJ, Ji B, Shao Z, Chedotal A, Bernard F, Roulois A, Xu J, Jung V, Pepinsky B. 2009. Promotion of central nervous system remyelination by induced differentiation of oligodendrocyte precursor cells. Ann Neurol 65:304–315. Mi S, Pepinsky RB, Cadavid D. 2013. Blocking LINGO-1 as a therapy to promote CNS repair: from concept to the clinic. CNS Drugs 27: 493–503. Michailov GV, Sereda MW, Brinkmann BG, Fischer TM, Haug B, Birchmeier C, Role L, Lai C, Schwab MH, Nave KA. 2004. Axonal neuregulin-1 regulates myelin sheath thickness. Science 304:700–703. Miller DH, Khan OA, Sheremata WA, Blumhardt LD, Rice GP, Libonati MA, Willmer-Hulme AJ, Dalton CM, Miszkiel KA, O’Connor PW. 2003. A controlled trial of natalizumab for relapsing multiple sclerosis. N Engl J Med 348:15–23. Miron VE, Boyd A, Zhao JW, Yuen TJ, Ruckh JM, Shadrach JL, van Wijngaarden P, Wagers AJ, Williams A, Franklin RJ, ffrench-Constant C. 2013. M2 microglia and macrophages drive oligodendrocyte differentiation during CNS remyelination. Nat Neurosci 16:1211–1218. Modell JG, Tandon R, Beresford TP. 1989. Dopaminergic activity of the antimuscarinic antiparkinsonian agents. J Clin Psychopharmacol 9:347– 351. Monge M, Kadiiski D, Jacque CM, Zalc B. 1986. Oligodendroglial expression and deposition of four major myelin constituents in the myelin sheath during development. An in vivo study. Dev Neurosci 8: 222–235. Oh LY, Larsen PH, Krekoski CA, Edwards DR, Donovan F, Werb Z, Yong VW. 1999. Matrix metalloproteinase-9/gelatinase B is required for process outgrowth by oligodendrocytes. J Neurosci 19:8464–8475. Oleszak EL, Chang JR, Friedman H, Katsetos CD, Platsoucas CD. 2004. Theiler’s virus infection: a model for multiple sclerosis. Clin Microbiol Rev 17:174–207. Oliver AR, Lyon GM, Ruddle NH. 2003. Rat and human myelin oligodendrocyte glycoproteins induce experimental autoimmune encephalomyelitis by different mechanisms in C57BL/6 mice. J Immunol 171: 462–468. Paz Soldan MM, Warrington AE, Bieber AJ, Ciric B, Van Keulen V, Pease LR, Rodriguez M. 2003. Remyelination-promoting antibodies activate distinct Ca21 influx pathways in astrocytes and oligodendrocytes: relationship to the mechanism of myelin repair. Mol Cell Neurosci 22:14–24. Pepinsky RB, Arndt JW, Quan C, Gao Y, Quintero-Monzon O, Lee X, Mi S. 2014. Structure of the LINGO-1-anti-LINGO-1 Li81 antibody complex provides insights into the biology of LINGO-1 and the mechanism of action of the antibody therapy. J Pharmacol Exp Ther 350:110–123. Pirko I, Ciric B, Gamez J, Bieber AJ, Warrington AE, Johnson AJ, Hanson DP, Pease LR, Macura SI, Rodriguez M. 2004. A human antibody that promotes remyelination enters the CNS and decreases lesion load as detected by T2-weighted spinal cord MRI in a virusinduced murine model of MS. FASEB J 18:1577–1579.

Journal of Neuroscience Research

9

Pluchino S, Quattrini A, Brambilla E, Gritti A, Salani G, Dina G, Galli R, Del Carro U, Amadio S, Bergami A, Furlan R, Comi G, Vescovi AL, Martino G. 2003. Injection of adult neurospheres induces recovery in a chronic model of multiple sclerosis. Nature 422:688–694. Poltorak M, Sadoul R, Keilhauer G, Landa C, Fahrig T, Schachner M. 1987. Myelin-associated glycoprotein, a member of the L2/HNK-1 family of neural cell adhesion molecules, is involved in neuron–oligodendrocyte and oligodendrocyte–oligodendrocyte interaction. J Cell Biol 105:1893–1899. Quarles RH, Everly JL, Brady RO. 1973. Evidence for the close association of a glycoprotein with myelin in rat brain. J Neurochem 21:1177– 1191. Radin NS, Akahori Y. 1961. Fatty acids of human brain cerebrosides. J Lipid Res 2:335–341. Raff MC, Miller RH, Noble M. 1983. A glial progenitor cell that develops in vitro into an astrocyte or an oligodendrocyte depending on culture medium. Nature 303:390–396. Rushton WA. 1951. A theory of the effects of fibre size in medullated nerve. J Physiol 115:101–122. Saher G, Brugger B, Lappe-Siefke C, Mobius W, Tozawa R, Wehr MC, Wieland F, Ishibashi S, Nave KA. 2005. High cholesterol level is essential for myelin membrane growth. Nat Neurosci 8:468– 475. Saher G, Quintes S, Nave KA. 2011. Cholesterol: a novel regulatory role in myelin formation. Neuroscientist 17:79–93. Schwob VS, Clark HB, Agrawal D, Agrawal HC. 1985. Electron microscopic immunocytochemical localization of myelin proteolipid protein and myelin basic protein to oligodendrocytes in rat brain during myelination. J Neurochem 45:559–571. Sherman DL, Brophy PJ. 2005. Mechanisms of axon ensheathment and myelin growth. Nat Rev Neurosci 6:683–690. Silber E, Semra YK, Gregson NA, Sharief MK. 2002. Patients with progressive multiple sclerosis have elevated antibodies to neurofilament subunit. Neurology 58:1372–1381. Simons M, Trajkovic K. 2006. Neuron–glia communication in the control of oligodendrocyte function and myelin biogenesis. J Cell Sci 119: 4381–4389. Slavin AJ, Johns TG, Orian JM, Bernard CC. 1997. Regulation of myelin oligodendrocyte glycoprotein in different species throughout development. Dev Neurosci 19:69–78. Smith KJ, Lassmann H. 2002. The role of nitric oxide in multiple sclerosis. Lancet Neurol 1:232–241. Smith KJ, Blakemore WF, McDonald WI. 1981. The restoration of conduction by central remyelination. Brain 104:383–404. Smith KJ, Kapoor R, Felts PA. 1999. Demyelination: the role of reactive oxygen and nitrogen species. Brain Pathol 9:69–92. Sommer I, Schachner M. 1981. Monoclonal antibodies (O1 to O4) to oligodendrocyte cell surfaces: an immunocytological study in the central nervous system. Dev Biol 83:311–327. Song J, Goetz BD, Baas PW, Duncan ID. 2001. Cytoskeletal reorganization during the formation of oligodendrocyte processes and branches. Mol Cell Neurosci 17:624–636. Sriram S, Steiner I. 2005. Experimental allergic encephalomyelitis: a misleading model of multiple sclerosis. Ann Neurol 58:939–945. Stallcup WB. 1981. The NG2 antigen, a putative lineage marker: immunofluorescent localization in primary cultures of rat brain. Dev Biol 83: 154–165. Traka M, Arasi K, Avila RL, Podojil JR, Christakos A, Miller SD, Soliven B, Popko B. 2010. A genetic mouse model of adult-onset, pervasive central nervous system demyelination with robust remyelination. Brain 133:3017–3029. Traugott U, Reinherz EL, Raine CS. 1983. Multiple sclerosis. Distribution of T cells, T-cell subsets, and Ia-positive macrophages in lesions of different ages. J Neuroimmunol 4:201–221.

10

Olsen and Akirav

Virchow, Noad H, V€ olckel C. 1854. Notizen. J Prakt Chem 61:250– 256. Wang C, Liu F, Liu YY, Zhao CH, You Y, Wang L, Zhang J, Wei B, Ma T, Zhang Q, Zhang Y, Chen R, Song H, Yang Z. 2011. Identification and characterization of neuroblasts in the subventricular zone and rostral migratory stream of the adult human brain. Cell Res 21:1534–1550. Warrington AE, Asakura K, Bieber AJ, Ciric B, Van Keulen V, Kaveri SV, Kyle RA, Pease LR, Rodriguez M. 2000. Human monoclonal antibodies reactive to oligodendrocytes promote remyelination in a model of multiple sclerosis. Proc Natl Acad Sci U S A 97:6820–6825. Waxman SG. 1977. Conduction in myelinated, unmyelinated, and demyelinated fibers. Arch Neurol 34:585–589.

Weiner HL. 2009. The challenge of multiple sclerosis: how do we cure a chronic heterogeneous disease? Ann Neurol 65:239–248. Wolswijk G. 1998. Chronic stage multiple sclerosis lesions contain a relatively quiescent population of oligodendrocyte precursor cells. J Neurosci 18:601–609. Wu LM, Williams A, Delaney A, Sherman DL, Brophy PJ. 2012. Increasing internodal distance in myelinated nerves accelerates nerve conduction to a flat maximum. Curr Biol 22:1957–1961. Young KM, Psachoulia K, Tripathi RB, Dunn SJ, Cossell L, Attwell D, Tohyama K, Richardson WD. 2013. Oligodendrocyte dynamics in the healthy adult CNS: evidence for myelin remodeling. Neuron 77:873– 885.

Journal of Neuroscience Research