What roles do growth factors play in CNS remyelination?

Newly Identified Deficiencies in the Multiple Sclerosis Central Nervous System and Their Impact on the Remyelination Failure

The pathogenesis of multiple sclerosis (MS) remains enigmatic and controversial. Myelin sheaths in the central nervous system (CNS) insulate axons and allow saltatory nerve conduction. MS brings about the destruction of myelin sheaths and the myelin-producing oligodendrocytes (ODCs). The conundrum of remyelination failure is, therefore, crucial in MS. In this review, the roles of epidermal growth factor (EGF), normal prions, and cobalamin in CNS myelinogenesis are briefly summarized. Thereafter, some findings of other authors and ourselves on MS and MS-like models are recapitulated, because they have shown that: (a) EGF is significantly decreased in the CNS of living or deceased MS patients; (b) its repeated administration to mice in various MS-models prevents demyelination and inflammatory reaction; (c) as was the case for EGF, normal prion levels are decreased in the MS CNS, with a strong correspondence between liquid and tissue levels; and (d) MS cobalamin levels are increased in the cerebrospinal fluid, but decreased in the spinal cord. In fact, no remyelination can occur in MS if these molecules (essential for any form of CNS myelination) are lacking. Lastly, other non-immunological MS abnormalities are reviewed. Together, these results have led to a critical reassessment of MS pathogenesis, partly because EGF has little or no role in immunology.

Epidermal Growth Factor in the CNS: A Beguiling Journey from Integrated Cell Biology to Multiple Sclerosis. An Extensive Translational Overview

This article reviews the wealth of papers dealing with the different effects of epidermal growth factor (EGF) on oligodendrocytes, astrocytes, neurons, and neural stem cells (NSCs). EGF induces the in vitro and in vivo proliferation of NSCs, their migration, and their differentiation towards the neuroglial cell line. It interacts with extracellular matrix components. NSCs are distributed in different CNS areas, serve as a reservoir of multipotent cells, and may be increased during CNS demyelinating diseases. EGF has pleiotropic differentiative and proliferative effects on the main CNS cell types, particularly oligodendrocytes and their precursors, and astrocytes. EGF mediates the in vivo myelinotrophic effect of cobalamin on the CNS, and modulates the synthesis and levels of CNS normal prions (PrP^Cs), both of which are indispensable for myelinogenesis and myelin maintenance. EGF levels are significantly lower in the cerebrospinal fluid and spinal cord of patients with multiple sclerosis (MS), which probably explains remyelination failure, also because of the EGF marginal role in immunology. When repeatedly administered, EGF protects mouse spinal cord from demyelination in various experimental models of autoimmune encephalomyelitis. It would be worth further investigating the role of EGF in the pathogenesis of MS because of its multifarious effects.

Antibody-mediated neutralization of myelin-associated EphrinB3 accelerates CNS remyelination

Remyelination in multiple sclerosis (MS) lesions often remains incomplete despite the presence of oligodendrocyte progenitor cells (OPCs). Amongst other factors, successful remyelination depends on the phagocytic clearance of myelin debris. However, the proteins in myelin debris that act as potent and selective inhibitors on OPC differentiation and inhibit CNS remyelination remain unknown. Here, we identify the transmembrane signalling protein EphrinB3 as important mediator of this inhibition, using a protein analytical approach in combination with a primary rodent OPC assay. In the presence of EphrinB3, OPCs fail to differentiate. In a rat model of remyelination, infusion of EphrinB3 inhibits remyelination. In contrast, masking EphrinB3 epitopes using antibodies promotes remyelination. Finally, we identify EphrinB3 in MS lesions and demonstrate that MS lesion extracts inhibit OPC differentiation while antibody-mediated masking of EphrinB3 epitopes promotes it. Our findings suggest that EphrinB3 could be a target for therapies aiming at promoting remyelination in demyelinating disease.

ALS genetic modifiers that increase survival of SOD1 mice and are suitable for therapeutic development

Amyotrophic lateral sclerosis (ALS) is a frequently fatal motor neuron disease without any cure. To find molecular therapeutic targets, several studies crossed transgenic ALS murine models with animals transgenic for some ALS target genes. We aimed to revise the new discoveries and new works in this field. We selected the 10 most promising genes, according to their capability when down-regulated or up-regulated in ALS animal models, for increasing life span and mitigating disease progression: XBP-1, NogoA and NogoB, dynein, heavy and medium neurofilament, NOX1 and NOX2, MLC-mIGF-1, NSE-VEGF, and MMP-9. Interestingly, some crucial modifier genes have been described as being involved in common pathways, the most significant of which are inflammation and cytoskeletal activities. The endoplasmic reticulum also seems to play an important role in ALS pathogenesis, as it is involved in different selected gene pathways. In addition, these genes have evident links to each other, introducing the hypothesis of a single unknown, common pathway involving all of these identified genes and others to be discovered.

Tissue transglutaminase activity is involved in the differentiation of oligodendrocyte precursor cells into myelin-forming oligodendrocytes during CNS remyelination

During normal brain development, axons are myelinated by mature oligodendrocytes (OLGs). Under pathological, demyelinating conditions within the central nervous system (CNS), axonal remyelination is only partially successful because oligodendrocyte precursor cells (OPCs) largely remain in an undifferentiated state resulting in a failure to generate myelinating OLGs. Tissue Transglutaminase (TG2) is a multifunctional enzyme, which amongst other functions, is involved in cell differentiation. Therefore, we hypothesized that TG2 contributes to differentiation of OPCs into OLGs and thereby stimulates remyelination. In vivo studies, using the cuprizone model for de- and remyelination in TG2(-/-) and wild-type mice, showed that during remyelination expression of proteolipid protein mRNA, as a marker for remyelination, in the corpus callosum lags behind in TG2(-/-) mice resulting in less myelin formation and, moreover, impaired recovery of motor behavior. Subsequent in vitro studies showed that rat OPCs express TG2 protein and activity which reduces when the cells have matured into OLGs. Furthermore, when TG2 activity is pharmacologically inhibited, the differentiation of OPCs into myelin-forming OLGs is dramatically reduced. We conclude that TG2 plays a prominent role in remyelination of the CNS, probably through stimulating OPC differentiation into myelin-forming OLGs. Therefore, manipulating TG2 activity may represent an interesting new target for remyelination in demyelinating diseases.

Remyelination-promoting human IgMs: developing a therapeutic reagent for demyelinating disease

Promoting remyelination following injury to the central nervous system (CNS) promises to be an effective neuroprotective strategy to limit the loss of surviving axons and prevent disability. Studies confirm that multiple sclerosis (MS) and spinal cord injury lesions contain myelinating cells and their progenitors. Recruiting these endogenous cells to remyelinate may be of therapeutic value. This review addresses the use of antibodies reactive to CNS antigens to promote remyelination. Antibody-induced remyelination in a virus-mediated model of chronic spinal cord injury was initially observed in response to treatment with CNS reactive antisera. Monoclonal mouse and human IgMs, which bind to the surface of oligodendrocytes and myelin, were later identified that were functionally equivalent to antisera. A recombinant form of a human remyelination-promoting IgM (rHIgM22) targets areas of CNS injury and promotes maximal remyelination within 5 weeks after a single low dose (25 microg/kg). The IgM isoform of this reparative antibody is required for in vivo function. We hypothesize that the IgM clusters membrane domains and associated signaling molecules on the surface of target cells. Current therapies for MS are designed to modulate inflammation. In contrast, remyelination promoting IgMs are the first potential therapeutic molecules designed to induce tissue repair by acting within the CNS at sites of damage on the cells responsible for myelin synthesis.

Axonal remyelination by cord blood stem cells after spinal cord injury

Human umbilical cord blood stem cells (hUCB) hold great promise for therapeutic repair after spinal cord injury (SCI). Here, we present our preliminary investigations on axonal remyelination of injured spinal cord by transplanted hUCB. Adult male rats were subjected to moderate SCI using NYU Impactor, and hUCB were grafted into the site of injury one week after SCI. Immunohistochemical data provides evidence of differentiation of hUCB into several neural phenotypes including neurons, oligodendrocytes and astrocytes. Ultrastructural analysis of axons reveals that hUCB form morphologically normal appearing myelin sheaths around axons in the injured areas of spinal cord. Colocalization studies prove that oligodendrocytes derived from hUCB secrete neurotrophic hormones neurotrophin-3 (NT3) and brain-derived neurotrophic factor (BDNF). Cord blood stem cells aid in the synthesis of myelin basic protein (MBP) and proteolipid protein (PLP) of myelin in the injured areas, thereby facilitating the process of remyelination. Elevated levels of mRNA expression were observed for NT3, BDNF, MBP and PLP in hUCB-treated rats as revealed by fluorescent in situ hybridization (FISH) analysis. Recovery of hind limb locomotor function was also significantly enhanced in the hUCB-treated rats based on Basso-Beattie-Bresnahan (BBB) scores assessed 14 days after transplantation. These findings demonstrate that hUCB, when transplanted into the spinal cord 7 days after weight-drop injury, survive for at least 2 weeks, differentiate into oligodendrocytes and neurons, and enable improved locomotor function. Therefore, hUCB facilitate functional recovery after moderate SCI and may prove to be a useful therapeutic strategy to repair the injured spinal cord.

A recombinant human IgM promotes myelin repair after a single, very low dose

A recombinant human monoclonal IgM, rHIgM22, promotes the synthesis of new myelin when used to treat several animal models of demyelination. rHIgM22 binds to myelin and the surface of oligodendrocytes and accumulates at central nervous system lesions in vivo. The minimal dose of monoclonal IgM required to promote remyelination has a direct bearing on the proposed mechanism of action. A dose ranging study using rHIgM22 was performed in mice with chronic virus-induced demyelination, a model of chronic progressive multiple sclerosis. The lowest tested dose of rHIgM22 effective at promoting spinal cord remyelination was a single 500-ng intraperitoneal bolus injection. A time course study of spinal cord repair performed in chronically demyelinated mice revealed that remyelination plateaued by 5 weeks following treatment with rHIgM22. Two doses of rHIgM22 spaced 5 weeks apart did not increase the extent of remyelination over a single dose. The half-life of rHIgM22 in the mouse systemic circulation was determined to be 15 hr; the human IgM serum concentration was close to zero by 48 hr following antibody administration. We propose that the specificity of rHIgM22 for myelin on living tissue targets the antibody to demyelinated lesions, initiating a long-term reparative effect on the central nervous system.

Beta1-integrin signaling mediates premyelinating oligodendrocyte survival but is not required for CNS myelination and remyelination

Previous reports, including transplantation experiments using dominant-negative inhibition of beta1-integrin signaling in oligodendrocyte progenitor cells, suggested that beta1-integrin signaling is required for myelination. Here, we test this hypothesis using conditional ablation of the beta1-integrin gene in oligodendroglial cells during the development of the CNS. This approach allowed us to study oligodendroglial beta1-integrin signaling in the physiological environment of the CNS, circumventing the potential drawbacks of a dominant-negative approach. We found that beta1-integrin signaling has a much more limited role than previously expected. Although it was involved in stage-specific oligodendrocyte cell survival, beta1-integrin signaling was not required for axon ensheathment and myelination per se. We also found that, in the spinal cord, remyelination occurred normally in the absence of beta1-integrin. We conclude that, although beta1-integrin may still contribute to other aspects of oligodendrocyte biology, it is not essential for myelination and remyelination in the CNS.

Emerging therapies in multiple sclerosis

Multiple sclerosis (MS) is the most common neurological cause of disability in young people. The disease-modifying treatments, IFN-beta and glatiramer acetate, have been widely available over the last decade and have shown a beneficial effect on relapse rate and magnetic resonance imaging parameters of disease activity; however, their effect on disease progression and disability is modest. Therefore, the search for alternative treatment strategies continues. As understanding of the heterogeneous pathophysiology of MS has increased, emphasis has shifted to more selective therapy that targets components of the inflammatory cascade and the promotion of remyelination and neuroprotection. These agents target the blood-brain barrier, systemic immune dysfunction, local inflammation and neurodegeneration. Combination therapies are being investigated for patients who fail first-line treatments. Many new drugs are being developed and tested that address these issues with the aim of finding a more effective and convenient therapy. These include humanized monoclonal antibodies such as daclizumab (IL-2 antagonist), oral immunomodulators such as sirolimus and statins and neuroprotective agents such as NMDA antagonists and Na+-channel blockers. Many of the treatments discussed in this review are still at early stages of development, but provide exciting potential treatment options; others have proved disappointing in larger extended-phase studies.

Functional recovery in traumatic spinal cord injury after transplantation of multineurotrophin-expressing glial-restricted precursor cells

Demyelination contributes to the physiological and behavioral deficits after contusive spinal cord injury (SCI). Therefore, remyelination may be an important strategy to facilitate repair after SCI. We show here that rat embryonic day 14 spinal cord-derived glial-restricted precursor cells (GRPs), which differentiate into both oligodendrocytes and astrocytes, formed normal-appearing central myelin around axons of cultured DRG neurons and had enhanced proliferation and survival in the presence of neurotrophin 3 (NT3) and brain-derived neurotrophin factor (BDNF). We infected GRPs with retroviruses expressing the multineurotrophin D15A (with both BDNF and NT3 activities) and then transplanted them into the contused adult thoracic spinal cord at 9 d after injury. Expression of D15A in the injured spinal cord is five times higher in animals receiving D15A-GRP grafts than ones receiving enhanced green fluorescent protein (EGFP)-GRP or DMEM grafts. Six weeks after transplantation, the grafted GRPs differentiated into mature oligodendrocytes expressing both myelin basic protein (MBP) and adenomatus polyposis coli (APC). Ultrastructural analysis showed that the grafted GRPs formed morphologically normal-appearing myelin sheaths around the axons in the ventrolateral funiculus (VLF) of spinal cord. Expression of D15A significantly increased the percentage of APC+ oligodendrocytes of grafted GRPs (15-30%). Most importantly, 8 of 12 rats receiving grafts of D15A-GRPs recovered transcranial magnetic motor-evoked potential responses, indicating that conduction through the demyelinated VLF axons was restored. Such electrophysiological recovery was not observed in rats receiving grafts of EGFP-GRPs, D15A-NIH3T3 cells, or an injection of an adenovirus expressing D15A. Recovery of hindlimb locomotor function was also significantly enhanced only in the D15A-GRP-grafted animals at 4 and 5 weeks after transplantation. Therefore, combined treatment with neurotrophins and GRP grafts can facilitate functional recovery after traumatic SCI and may prove to be a useful therapeutic strategy to repair the injured spinal cord.

Neural stem cells in inflammatory CNS diseases: mechanisms and therapy

Autoimmune inflammatory diseases of the central nervous system (CNS) are highly complex in their interaction of different cell populations. The main therapy focus in the last years has been the inhibition of the immune system. Recent progress has shown that endogenous as well as transplanted neural stem cells might positively influence the outcome of such diseases. In this review, we discuss the current concept of the underlying pathogenesis with a specific focus on local CNS cells and potential treatment options.

Markers for OLN-93 oligodendroglia differentiation

Oligodendrocytes are target cells in the pathogenesis of multiple sclerosis (MS), a chronic demyelinating disease of the central nervous system (CNS). During the course of the disease, inflammatory mediators may damage oligodendrocytes and their myelin sheaths. Differentiation of oligodendrocyte progenitors is an important step in the process of remyelination. In the present study, OLN-93 differentiation was studied in co-culture with C6 astrocytes as a natural source of growth and differentiation factors as well as after exposure to insulin-like growth factor-I (IGF-I). Morphological evaluation showed an increased degree of differentiation of OLN-93 cells after IGF-I administration, but not after co-culture with astrocytes. During early differentiation, 2', 3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and zonula occludens-1 (ZO-1) tight junction protein expression were significantly increased. However, neither astrocyte co-culture nor exposure to IGF-I further increased the expression of these markers. Although reverse transcriptase-polymerase chain reaction revealed myelin basic protein (MBP) mRNA expression not to be affected during differentiation, we did find increased MBP protein expression by Western blotting. ZO-1 protein and DM20 mRNA levels were increased during the course of differentiation and after IGF-I administration. The present findings suggest that ZO-1 may be used as a marker for OLN-93 oligodendroglia differentiation.

Impaired remyelination and depletion of oligodendrocyte progenitors does not occur following repeated episodes of focal demyelination in the rat central nervous system

It has been hypothesized that the progressive failure of remyelination in chronic multiple sclerosis is, in part, the consequence of repeated episodes of demyelination at the same site, eventually depleting oligodendrocyte progenitor cells (OPCs) and exhausting the remyelinating capacity. We investigated the effect of previous focal, ethidium bromide-induced demyelination of brain stem white matter (with intervening recovery) on the efficiency of the remyelination process during second and third subsequent episodes of demyelination, and the OPC response during a second episode of demyelination. Previous focal demyelinating lesions followed by recovery did not result in any retardation of the remyelination process, nor did they alter the proportion of Schwann cell versus oligodendrocyte remyelination. The OPC response during remyelination was quantified by in situ hybridization using a probe to platelet-derived growth factor-alpha receptor (PDGF alpha R), an OPC-expressed mRNA. Following recovery from focal, toxin-induced CNS demyelination, the OPC density returned to levels equivalent to those in normal white matter. Further more, there was no depletion of OPCs following repeated episodes of focal, toxin-induced CNS demyelination at the same site. These results indicate that repeated CNS demyelination, which has the opportunity to repair in the intervening period, is not characterized by impaired remyelination or depletion of OPCs.

Efficient central nervous system remyelination requires T cells

We demonstrate a role for immune functions in the spontaneous remyelination of central nervous system (CNS) axons after lysolecithin-induced demyelination in the spinal cord. Rag-1-deficient mice lack both B cells and T cells and show significantly reduced spontaneous remyelination compared with control mice of matching genetic background. Mice lacking or depleted of either CD4(+) T cells or CD8(+) T cells also exhibit reduced remyelination. These data indicate that T cells are necessary for efficient CNS remyelination. Thus, general nonspecific immunosuppression as a therapeutic approach for the treatment of CNS injury and demyelinating disease may have undesirable effects on subsequent tissue repair.

Lentiviral vectors for gene delivery to normal and demyelinated white matter

Lentiviral vectors are increasingly used for gene delivery to neurons and in experimental models of neurodegeneration. Their use in gene delivery to white matter and their potential value in preventing or repairing CNS demyelination has received less attention. Here we show using a VSV-G-pseudotyped HIV-derived vector expressing the marker gene LacZ that lentiviral vectors transduce the major macroglial cell types present in normal white matter (astrocytes, oligodendrocytes, and oligodendrocyte progenitors). Injection of lentiviral vectors causes an inflammatory response at the injection site characterized by OX42(+) and ED1(+) macrophages, but only a few CD8(+) and no CD4(+) lymphocytes, and mild demyelination. Injection of lentiviral vectors into areas of toxin-induced demyelination resulted in significant numbers of cells expressing the marker gene and was a more effective means of gene delivery than was a LacZ-expressing murine retroviral vector.

Immunomodulation in multiple sclerosis: from immunosuppression to neuroprotection

Multiple sclerosis (MS) is the most common disabling neurological disease of young adulthood. Following advances in the understanding of the immunological mechanisms that underlie the pathogenesis of MS, a growing arsenal of immunomodulatory agents is available. Two classes of immunomodulators are approved for long-term treatment of MS, the efficacy of several promising new concepts is being tested in clinical trials and classical immunosuppressive agents used in MS treatment have been shown to exert specific, immunomodulatory effects. Furthermore, two recent observations have changed our basic understanding of the pathogenesis of MS. First, immune cells in MS lesions have neuroprotective activity, which indicates a beneficial role of neuroinflammation. Second, there is evidence that axonal loss, rather than demyelination, underlies the progression of MS and, hence, constitutes a therapeutic target.

Remyelination-promoting antibodies activate distinct Ca2+ influx pathways in astrocytes and oligodendrocytes: relationship to the mechanism of myelin repair

Our laboratory has identified mouse and human monoclonal antibodies that promote myelin repair in multiple models of demyelinating disease. We have proposed that these antibodies promote remyelination by directly activating central nervous system glia. Intracellular calcium concentration was monitored using a Fura2 ratiometric assay. Repair-promoting antibodies induced distinct Ca2+ signals in both astrocytes and oligodendrocytes. Astrocyte Ca2+ signaling is mediated by a phospholipase C-dependent pathway while oligodendrocyte Ca2+ signaling is mediated via AMPA-sensitive glutamate receptors. An antibody's ability to induce Ca2+ signals is statistically correlated with promotion of myelin repair. These findings support the hypothesis that remyelination-promoting antibodies are acting directly at the surface of glial cells to induce calcium-dependent physiologic reparative function.

Cell transplantation, myelin repair, and multiple sclerosis

A decade ago, therapeutic strategies to remyelinate the CNS in diseases such as multiple sclerosis had much experimental appeal, but translation of laboratory success into clinical treatments appeared to be a long way off. Within the past 12 months, however, the first patients with multiple sclerosis have received intracerebral implants of autologous myelinating cells. Here we review the clinical and biological problems presented by multiple sclerosis disease processes, and the background to the development of myelin-repair strategies. We attempt to highlight those areas where difficulties have yet to be resolved, and draw on various experimental findings to speculate on how remyelinating therapies are likely to develop in the foreseeable future.

Increasing local levels of IGF-I mRNA expression using adenoviral vectors does not alter oligodendrocyte remyelination in the CNS of aged rats

IGF-I, a growth factor that contributes to developmental myelination, shows increased levels of expression within experimental models of remyelination. The pattern of IGF-I mRNA expression changes with the rate of remyelination, with peak levels of expression occurring earlier during rapid remyelination in young adult rats compared to the slower remyelination in old adult rats. In this study we have attempted to accelerate remyelination in old adult rats by using an IGF-expressing adenoviral vector (IGF-I-Ad) to bring forward the timing of peak level of IGF-I expression. Following injection of IGF-I-Ad into focal areas of lysolecithin-induced demyelination in the spinal white matter of old adult rats we created levels of IGF-I mRNA expression at 10 days that were considerably higher than those normally occurring at this time and more similar to those in young animals. However, despite the elevated levels of IGF-I mRNA expression there was no significant change in the extent of oligodendrocyte remyelination compared to saline controls or animals injected with an adenoviral vector expressing LacZ (NT-LacZ-Ad). There was a small increase in Schwann cell remyelination in IGF-I-Ad- and NT-LacZ-Ad-injected animals compared to saline controls. These results indicate that changing the levels of IGF-I directly within demyelinating lesions undergoing remyelination is not sufficient to alter remyelination and that the proremyelinating effects of systemically delivered IGF-I are unlikely to be due to direct effects on the oligodendrocyte lineage.