The complex world of oligodendroglial differentiation inhibitors

Blocked, delayed, or obstructed: What causes poor white matter development in intrauterine growth restricted infants?

Poor white matter development in intrauterine growth restricted (IUGR) babies remains a major, untreated problem in neonatology. New therapies, guided by an understanding of the mechanisms that underlie normal and abnormal oligodendrocyte development and myelin formation, are required. Much of our knowledge of the mechanisms that underlie impaired myelination come from studies in adult demyelinating disease, preterm brain injury, or experimental models of hypoxia-ischemia. However, relatively less is known for IUGR which is surprising because IUGR is a leading cause of perinatal mortality and morbidity, second only to premature birth. IUGR is also a significant risk factor for the later development of cerebral palsy, and is a greater risk compared to some of the more traditionally researched antecedents - asphyxia and inflammation. Recent evidence suggests that the white matter injury and reduced myelination in the brains of some preterm babies is due to impaired maturation of oligodendrocytes thereby resulting in the reduced capacity to synthesize myelin. Therefore, it is not surprising that the hypomyelination observable in the central nervous system of IUGR infants has similarly lead to investigations identifying a delay or blockade in the progress of maturation of oligodendrocytes in these infants. This review will discuss current ideas thought to account for the poor myelination often present in the neonate's brain following IUGR, and discuss novel interventions that are promising as treatments that promote oligodendrocyte maturation, and thereby repair the myelination deficits that otherwise persist into infancy and childhood and lead to neurodevelopmental abnormalities.

NCAM1 is the Target of miRNA-572: Validation in the Human Oligodendroglial Cell Line

The neural cell adhesion molecule 1 (NCAM1) is a fundamental protein in cell-cell interaction and in cellular developmental processes, and its dysregulation is involved in a number of diseases including multiple sclerosis. Studies in rats suggest that the modulation of NCAM1 expression is regulated by miRNA-572, but no data are available confirming such interaction in the human system. We analyzed whether this is the case using a human oligodendroglial cell line (MO3.13). MO3.13 cells were transfected with miRNA-572 mimic and inhibitor separately; NCAM1 mRNA and protein expression levels were analyzed at different time points after transfection. Results indicated that NCAM1 expression is increased after transfection with miRNA-572 inhibitor, whereas it is decreased after transfection with the mimic (p < 0.005). The interaction between NCAM1 and miRNA-572 was subsequently confirmed in a Vero cell line that does not express NCAM1, by luciferase assay after transfection with NCAM1. These results confirm that miRNA-572 regulates NCAM1 and for the first time demonstrate that this interaction regulates NCAM1 expression in human cells. Data herein also support the hypothesis that miRNA-572 is involved in diseases associated with NCAM1 deregulation, suggesting its possible use as a biomarker in these diseases.

MiR-146a promotes remyelination in a cuprizone model of demyelinating injury

The death of mature oligodendrocytes (OLs) which are the sole myelinating cells of the central nervous system (CNS), leads to demyelination and functional deficits. Currently, there is lack of effective remyelination therapies for patients with demyelinating diseases. MicroRNAs (miRNAs) mediate OL function. We hypothesized that miR-146a, by inactivating interleukin-1 receptor-associated kinase 1 (IRAK1), promotes differentiation of oligodendrocyte progenitor cells (OPCs) and thereby enhances remyelination. To test this hypothesis, a demyelination model induced by a cuprizone (CPZ) diet was employed, in which C57BL/6J mice were fed with a CPZ diet for 5weeks. After termination of CPZ diet, the mice were randomly treated with continuous infusion of miR-146a mimics or mimic controls into the corpus callosum for 7days. Compared to the mimic control, infusion of miR-146a mimics facilitated remyelination assessed by increased myelin basic proteins in the corpus callosum, which was associated with augmentation of newly generated mature OLs. Infusion of miR-146a mimics also substantially elevated miR-146a levels in the corpus callosum and fluorescently tagged miR-146a mimics were mainly detected in OPCs. Western blot and double immmunofluorescent staining analysis showed that the miR-146a treatment considerably reduced IRAK1 protein levels and the number of IRAK1-positive cells, respectively. Collectively, these data indicate that exogenous miR-146a enhances remyelination, possibly by promoting OPCs to differentiate into myelinated OLs via targeting IRAK1.

Heterogeneous populations of neural stem cells contribute to myelin repair

As ingenious as nature's invention of myelin sheaths within the mammalian nervous system is, as fatal can be damage to this specialized lipid structure. Long-term loss of electrical insulation and of further supportive functions myelin provides to axons, as seen in demyelinating diseases such as multiple sclerosis (MS), leads to neurodegeneration and results in progressive disabilities. Multiple lines of evidence have demonstrated the increasing inability of oligodendrocyte precursor cells (OPCs) to replace lost oligodendrocytes (OLs) in order to restore lost myelin. Much research has been dedicated to reveal potential reasons for this regeneration deficit but despite promising approaches no remyelination-promoting drugs have successfully been developed yet. In addition to OPCs neural stem cells of the adult central nervous system also hold a high potential to generate myelinating OLs. There are at least two neural stem cell niches in the brain, the subventricular zone lining the lateral ventricles and the subgranular zone of the dentate gyrus, and an additional source of neural stem cells has been located in the central canal of the spinal cord. While a substantial body of literature has described their neurogenic capacity, still little is known about the oligodendrogenic potential of these cells, even if some animal studies have provided proof of their contribution to remyelination. In this review, we summarize and discuss these studies, taking into account the different niches, the heterogeneity within and between stem cell niches and present current strategies of how to promote stem cell-mediated myelin repair.

Taking Advantage of Nature's Gift: Can Endogenous Neural Stem Cells Improve Myelin Regeneration?

Irreversible functional deficits in multiple sclerosis (MS) are directly correlated to axonal damage and loss. Neurodegeneration results from immune-mediated destruction of myelin sheaths and subsequent axonal demyelination. Importantly, oligodendrocytes, the myelinating glial cells of the central nervous system, can be replaced to some extent to generate new myelin sheaths. This endogenous regeneration capacity has so far mainly been attributed to the activation and recruitment of resident oligodendroglial precursor cells. As this self-repair process is limited and increasingly fails while MS progresses, much interest has evolved regarding the development of remyelination-promoting strategies and the presence of alternative cell types, which can also contribute to the restoration of myelin sheaths. The adult brain comprises at least two neurogenic niches harboring life-long adult neural stem cells (NSCs). An increasing number of investigations are beginning to shed light on these cells under pathological conditions and revealed a significant potential of NSCs to contribute to myelin repair activities. In this review, these emerging investigations are discussed with respect to the importance of stimulating endogenous repair mechanisms from germinal sources. Moreover, we present key findings of NSC-derived oligodendroglial progeny, including a comprehensive overview of factors and mechanisms involved in this process.

Donepezil promotes differentiation of neural stem cells into mature oligodendrocytes at the expense of astrogenesis

Oligodendrocytes are the myelin-forming cells of the central nervous system. Oligodendrocyte loss and failure of myelin development result in serious human disorders, including multiple sclerosis. Previously, using oligodendrocyte progenitor cells, we have shown that donepezil, which is an acetylcholinesterase inhibitor developed for the treatment of Alzheimer's disease, stimulates myelin gene expression and oligodendrocyte differentiation. Here, we aimed to analyze the effects of donepezil on primary mouse embryonic neural stem cells (NSCs). Donepezil treatment led to impaired self-renewal ability and increased apoptosis. These effects appeared to be mediated through the Akt/Bad signaling pathway. Using neurosphere differentiation analysis, we observed that donepezil leads to reduced numbers of astrocytes and increased numbers of oligodendrocytes and neurons. Consistent with this finding, mRNA and protein levels for the oligodendrocyte markers myelin-associated glycoprotein, 2', 3'-cyclic-nucleotide 3'-phosphodiesterase (CNPase), and myelin basic protein, as well as the neuronal marker β-tubulin type III (Tuj1) were up-regulated. In contrast, the expression of the astrocyte marker glial fibrillary acidic protein (GFAP) was down-regulated by donepezil in a dose- and time-dependent manner. Moreover, donepezil increased oligodendrocyte differentiation, resulting in a reduction in the differentiation of NSCs into astrocytes, by suppressing the activation of signal transducer and activator of transcription 3 (STAT3), SMAD1/5/9, and the downstream target gene GFAP, even under astrocyte-inducing conditions. These results suggest that efficient differentiation of NSCs into oligodendrocytes by donepezil may indicate a novel therapeutic role for this drug in promoting repair in demyelinated lesions in addition to its role in preventing astrogenesis.

Inflammation severely alters thyroid hormone signaling in the central nervous system during experimental allergic encephalomyelitis in rat: Direct impact on OPCs differentiation failure

Differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes is severely impaired by inflammatory cytokines and this could lead to remyelination failure in inflammatory/demyelinating diseases. Due to the role of thyroid hormone in the maturation of OPCs and developmental myelination, in this study we investigated (i) the possible occurrence of dysregulation of thyroid hormone signaling in the CNS tissue during experimental neuroinflammation; (ii) the possible impact of inflammatory cytokines on thyroid hormone signaling and OPCs differentiation in vitro. The disease model is the experimental allergic encephalomyelitis in female Dark-Agouti rats, whereas in vitro experiments were carried out in OPCs derived from neural stem cells. The main results are the following: (i) a strong upregulation of cytokine mRNA expression level was found in the spinal cord during experimental allergic encephalomyelitis; (ii) thyroid hormone signaling in the spinal cord (thyroid hormone receptors; deiodinase; thyroid hormone membrane transporter) is substantially downregulated, due to the upregulation of the thyroid hormone inactivating enzyme deiodinase 3 and the downregulation of thyroid hormone receptors, as investigated at mRNA expression level; (iii) when exposed to inflammatory cytokines, deiodinase 3 is upregulated in OPCs as well, and OPCs differentiation is blocked; (iv) deiodinase 3 inhibition by iopanoic acid recovers OPCs differentiation in the presence on inflammatory cytokines. These data suggest that cellular hypothyroidism occurs during experimental allergic encephalomyelitis, possibly impacting on thyroid hormone-dependent cellular processes, including maturation of OPCs into myelinating oligodendrocytes. GLIA 2016;64:1573-1589.

LINGO-1-Fc-Transduced Neural Stem Cells Are Effective Therapy for Chronic Stage Experimental Autoimmune Encephalomyelitis

The chronic stage multiple sclerosis (MS), an inflammatory demyelinating disease of the central nervous system (CNS), remains refractory to current treatments. This refractory nature may be due to the fact that current treatments are primarily immunomodulatory, which prevent further demyelination but lack the capacity to promote remyelination. Several approaches, including transplantation of neural stem cells (NSCs) or antagonists to LINGO-1, a key part of the receptor complex for neuroregeneration inhibitors, have been effective in suppressing the acute stage of experimental autoimmune encephalomyelitis (EAE), an animal model of MS. However, their effect on the chronic stage EAE is not known. Here, we show that transplantation of NSCs had only a slight therapeutic effect when treatment started at the chronic stage of EAE (e.g., injected at day 40 postimmunization). However, NSCs engineered to produce LINGO-1-Fc, a soluble LINGO-1 antagonist, significantly promoted neurological recovery as demonstrated by amelioration of clinical signs, improvement in axonal integrity, and enhancement of oligodendrocyte maturation and neuron repopulation. Significantly enhanced NAD production and Sirt2 expression were also found in the CNS of mice treated with LINGO-1-Fc-producing NSC. Moreover, differentiation of LINGO-1-Fc-producing NSCs into oligodendrocytes in vitro was largely diminished by an NAMPT inhibitor, indicating that LINGO-1-Fc enhances the NAMPT/NAD/Sirt2 pathway. Together, our study establishes a CNS-targeted, novel LINGO-1-Fc delivery system using NSCs, which represents a novel and effective NSC-based gene therapy approach for the chronic stage of MS.

Persistent 7-tesla phase rim predicts poor outcome in new multiple sclerosis patient lesions

BACKGROUND

In some active multiple sclerosis (MS) lesions, a strong immune reaction at the lesion edge may contain growth and thereby isolate the lesion from the surrounding parenchyma. Our previous studies suggest that this process involves opening of the blood-brain barrier in capillaries at the lesion edge, seen on MRI as centripetal contrast enhancement and a colocalized phase rim. We hypothesized that using these features to characterize early lesion evolution will allow in vivo tracking of tissue degeneration and/or repair, thus improving the evaluation of potential therapies for chronic active lesions.

METHODS

Centripetally and centrifugally enhancing lesions were studied in 17 patients with MS using 7-tesla MRI. High-resolution, susceptibility-weighted, T1-weighted (before/after gadolinium), and dynamic contrast-enhanced scans were acquired at baseline and months 1, 3, 6, and 12. For each lesion, time evolution of the phase rim, lesion volume, and T1 hypointensity were assessed. In autopsies of 3 progressive MS cases, the histopathology of the phase rim was determined.

RESULTS

In centripetal lesions, a phase rim colocalized with initial contrast enhancement. In 12 of 22, this phase rim persisted after enhancement resolved. Compared with centripetal lesions with transient rim, those with persistent rim had less volume shrinkage and became more T1 hypointense between months 3 and 12. No centrifugal lesions developed phase rims at any time point. Pathologically, persistent rims corresponded to an iron-laden inflammatory myeloid cell population at the edge of chronic demyelinated lesions.

CONCLUSION

In early lesion evolution, a persistent phase rim in lesions that shrink least and become more T1 hypointense over time suggests that the rim might mark failure of early lesion repair and/or irreversible tissue damage. In later stages of MS, phase rim lesions continue to smolder, exerting detrimental effects on affected brain tissue.

TRIAL REGISTRATION

NCT00001248.

FUNDING

The Intramural Research Program of NINDS supported this study.

Pushing Forward: Remyelination as the New Frontier in CNS Diseases

The evolutionary acquisition of myelin sheaths around large caliber axons in the central nervous system (CNS) represented a milestone in the development of vertebrate higher brain function. Myelin ensheathment of axons enabled saltatory conduction and thus accelerated information processing. However, a number of CNS diseases harm or destroy myelin and oligodendrocytes (myelin-producing cells), ultimately resulting in demyelination. In the adult CNS, new oligodendrocytes can be generated from a quiescent pool of precursor cells, which - upon differentiation - can replace lost myelin sheaths. The efficiency of this spontaneous regeneration is limited, which leads to incomplete remyelination and residual clinical symptoms. Here, we discuss CNS pathologies characterized by white matter degeneration and regeneration and highlight drugs that could potentially serve as remyelination therapies.

CXCR7 Is Involved in Human Oligodendroglial Precursor Cell Maturation

Differentiation of oligodendroglial precursor cells (OPCs), a crucial prerequisite for central nervous system (CNS) remyelination in diseases such as Multiple Sclerosis (MS), is modulated by a multitude of extrinsic and intrinsic factors. In a previous study we revealed that the chemokine CXCL12 stimulates rodent OPC differentiation via activation of its receptor CXCR7. We could now demonstrate that CXCR7 is also expressed on NogoA- and Nkx2.2-positive oligodendroglial cells in human MS brains and that stimulation of cultured primary fetal human OPCs with CXCL12 promotes their differentiation as measured by surface marker expression and morphologic complexity. Pharmacological inhibition of CXCR7 effectively blocks these CXCL12-dependent effects. Our findings therefore suggest that a specific activation of CXCR7 could provide a means to promote oligodendroglial differentiation facilitating endogenous remyelination activities.

Impaired oligodendrocyte maturation in preterm infants: Potential therapeutic targets

Preterm birth is an evolving challenge in neonatal health care. Despite declining mortality rates among extremely premature neonates, morbidity rates remain very high. Currently, perinatal diffuse white matter injury (WMI) is the most commonly observed type of brain injury in preterm infants and has become an important research area. Diffuse WMI is associated with impaired cognitive, sensory and psychological functioning and is increasingly being recognized as a risk factor for autism-spectrum disorders, ADHD, and other psychological disturbances. No treatment options are currently available for diffuse WMI and the underlying pathophysiological mechanisms are far from being completely understood. Preterm birth is associated with maternal inflammation, perinatal infections and disrupted oxygen supply which can affect the cerebral microenvironment by causing activation of microglia, astrogliosis, excitotoxicity, and oxidative stress. This intricate interplay of events negatively influences oligodendrocyte development, causing arrested oligodendrocyte maturation or oligodendrocyte cell death, which ultimately results in myelination failure in the developing white matter. This review discusses the current state in perinatal WMI research, ranging from a clinical perspective to basic molecular pathophysiology. The complex regulation of oligodendrocyte development in healthy and pathological conditions is described, with a specific focus on signaling cascades that may play a role in WMI. Furthermore, emerging concepts in the field of WMI and issues regarding currently available animal models are put forward. Novel insights into the molecular mechanisms underlying impeded oligodendrocyte maturation in diffuse WMI may aid the development of novel treatment options which are desperately needed to improve the quality-of-life of preterm neonates.

The Orphan G Protein-coupled Receptor GPR17 Negatively Regulates Oligodendrocyte Differentiation via Gαi/o and Its Downstream Effector Molecules

Recent studies have recognized G protein-coupled receptors as important regulators of oligodendrocyte development. GPR17, in particular, is an orphan G protein-coupled receptor that has been identified as oligodendroglial maturation inhibitor because its stimulation arrests primary mouse oligodendrocytes at a less differentiated stage. However, the intracellular signaling effectors transducing its activation remain poorly understood. Here, we use Oli-neu cells, an immortalized cell line derived from primary murine oligodendrocytes, and primary rat oligodendrocyte cultures as model systems to identify molecular targets that link cell surface GPR17 to oligodendrocyte maturation blockade. We demonstrate that stimulation of GPR17 by the small molecule agonist MDL29,951 (2-carboxy-4,6-dichloro-1H-indole-3-propionic acid) decreases myelin basic protein expression levels mainly by triggering the Gαi/o signaling pathway, which in turn leads to reduced activity of the downstream cascade adenylyl cyclase-cAMP-PKA-cAMP response element-binding protein (CREB). In addition, we show that GPR17 activation also diminishes myelin basic protein abundance by lessening stimulation of the exchange protein directly activated by cAMP (EPAC), thus uncovering a previously unrecognized role for EPAC to regulate oligodendrocyte differentiation. Together, our data establish PKA and EPAC as key downstream effectors of GPR17 that inhibit oligodendrocyte maturation. We envisage that treatments augmenting PKA and/or EPAC activity represent a beneficial approach for therapeutic enhancement of remyelination in those demyelinating diseases where GPR17 is highly expressed, such as multiple sclerosis.

Non-steroidal anti-inflammatory drug indometacin enhances endogenous remyelination

Multiple sclerosis is the most frequent demyelinating disease in the CNS that is characterized by inflammatory demyelinating lesions and axonal loss, the morphological correlate of permanent clinical disability. Remyelination does occur, but is limited especially in chronic disease stages. Despite effective immunomodulatory therapies that reduce the number of relapses the progressive disease phase cannot be prevented. Therefore, promotion of neuroprotective and repair mechanisms, such as remyelination, represents an attractive additional treatment strategy. A number of pathways have been identified that may contribute to impaired remyelination in MS lesions, among them the Wnt/β-catenin pathway. Here, we demonstrate that indometacin, a non-steroidal anti-inflammatory drug (NSAID) that has been also shown to modulate the Wnt/β-catenin pathway in colorectal cancer cells promotes differentiation of primary human and murine oligodendrocytes, myelination of cerebellar slice cultures and remyelination in cuprizone-induced demyelination. Our in vitro experiments using GSK3β inhibitors, luciferase reporter assays and oligodendrocytes expressing a mutant, dominant stable β-catenin indicate that the mechanism of action of indometacin depends on GSK3β activity and β-catenin phosphorylation. Indometacin might represent a promising treatment option to enhance endogenous remyelination in MS patients.

Intracellular Protein Shuttling: A Mechanism Relevant for Myelin Repair in Multiple Sclerosis?

A prominent feature of demyelinating diseases such as multiple sclerosis (MS) is the degeneration and loss of previously established functional myelin sheaths, which results in impaired signal propagation and axonal damage. However, at least in early disease stages, partial replacement of lost oligodendrocytes and thus remyelination occur as a result of resident oligodendroglial precursor cell (OPC) activation. These cells represent a widespread cell population within the adult central nervous system (CNS) that can differentiate into functional myelinating glial cells to restore axonal functions. Nevertheless, the spontaneous remyelination capacity in the adult CNS is inefficient because OPCs often fail to generate new oligodendrocytes due to the lack of stimulatory cues and the presence of inhibitory factors. Recent studies have provided evidence that regulated intracellular protein shuttling is functionally involved in oligodendroglial differentiation and remyelination activities. In this review we shed light on the role of the subcellular localization of differentiation-associated factors within oligodendroglial cells and show that regulation of intracellular localization of regulatory factors represents a crucial process to modulate oligodendroglial maturation and myelin repair in the CNS.

[New therapeutic strategies for remyelination in multiple sclerosis]

Multiple sclerosis (MS) is characterized by oligodendrocyte death and myelin sheath destruction of the central nervous system (CNS) in response to autoinflammatory processes. Besides demyelination axonal degeneration constitutes the second histopathological hallmark of this disease. A large number of immunomodulatory and targeted immunosuppression treatments have been approved for relapsing remitting (RR) MS where they effectively reduce relapse rates; however, currently no treatment options exist to repair injured axonal tracts or myelin damage that accumulates over time particularly in progressive MS. In light of the growing available therapeutic repertoire of highly potent immunomodulatory medications there is an increasing interest in the development of therapies aimed at neutralizing neurodegenerative damage. Endogenous remyelination processes occur mainly as a result of oligodendrocyte precursor cell (OPC) activation, recruitment and maturation; however, this repair activity appears to be limited and increasingly fails during disease progression. Based on these observations OPCs are considered as promising targets for the regenerative treatment of all stages of MS. This article presents an overview of approved medications with a suggested role in regeneration, regenerative treatments that are currently being tested in clinical trials, as well as promising future therapeutic approaches derived from basic glial cell research aiming at the promotion of the endogenous repair activity of the brain.

Inflammatory response of endothelial cells to a human endogenous retrovirus associated with multiple sclerosis is mediated by TLR4

The MSRV (multiple sclerosis-associated retrovirus) belongs to the human endogenous retrovirus HERV-W family. The envelope protein originating from the MSRV has been found in most patients with multiple sclerosis (MS). This protein (Env-ms) has pro-inflammatory properties for several types of immune cells and could therefore play a role in MS pathogenesis by promoting the leukocyte diapedesis observed in the central nervous system of patients. Our study aims to analyze the effects of Env-ms on the blood-brain barrier (BBB) at a molecular and functional level. We demonstrate that the recombinant MSRV envelope is able to stimulate several inflammatory parameters in a human BBB in vitro model, the HCMEC/D3 brain endothelial cell line. Indeed, Env-ms induces over-expression of ICAM-1, a major mediator of leukocyte adhesion to endothelial cells, in a dose-dependent manner as well as a strong dose-dependent production of the pro-inflammatory cytokines IL-6 and IL-8. Furthermore, using a silencing approach with siRNAs, we show that Env-ms is recognized via the Toll-like receptor 4 receptor, a pattern recognition receptor of innate immunity present on endothelial cells. We also show, using functional assays, that treatment of brain endothelial cells with Env-ms significantly stimulated the adhesion and the transmigration of activated immune cells through a monolayer of endothelial cells. These findings support the hypothesis that MSRV could be involved in the pathogenesis of MS disease or at least in maintenance of inflammatory conditions, thus fueling the auto-immune disorder. MSRV could also play a role in other chronic inflammatory diseases.

Oligodendroglial maturation is dependent on intracellular protein shuttling

Multiple sclerosis is an autoimmune disease of the CNS resulting in degeneration of myelin sheaths and loss of oligodendrocytes, which means that protection and electrical insulation of axons and rapid signal propagation are impaired, leading to axonal damage and permanent disabilities. Partial replacement of lost oligodendrocytes and remyelination can occur as a result of activation and recruitment of resident oligodendroglial precursor cells. However, the overall remyelination capacity remains inefficient because precursor cells often fail to generate new oligodendrocytes. Increasing evidence points to the existence of several molecular inhibitors that act on these cells and interfere with their cellular maturation. The p57kip2 gene encodes one such potent inhibitor of oligodendroglial differentiation and this study sheds light on the underlying mode of action. We found that subcellular distribution of the p57kip2 protein changed during differentiation of rat, mouse, and human oligodendroglial cells both in vivo and in vitro. Nuclear export of p57kip2 was correlated with promoted myelin expression, higher morphological phenotypes, and enhanced myelination in vitro. In contrast, nuclear accumulation of p57kip2 resulted in blocked oligodendroglial differentiation. Experimental evidence suggests that the inhibitory role of p57kip2 depends on specific interactions with binding proteins such as LIMK-1, CDK2, Mash1, and Hes5 either by controlling their site of action or their activity. Because functional restoration in demyelinating diseases critically depends on the successful generation of oligodendroglial cells, a therapeutic need that is currently unmet, the regulatory mechanism described here might be of particular interest for identifying suitable drug targets and devising novel therapeutic approaches.

Promoting remyelination in multiple sclerosis: current drugs and future prospects

Myelin destruction due to inflammatory oligodendrocyte cell damage or death in conjunction with axonal degeneration are among the major histopathological hallmarks of multiple sclerosis (MS). The majority of available immunomodulatory medications for MS are approved for relapsing-remitting (RR) MS, for which they reduce relapse rate, MRI measures of inflammation, and the accumulation of disability. These medications are, however, of little benefit during progressive MS where axonal degeneration following demyelination outweighs inflammation. This has sparked great interest in the development of new remyelination therapies aimed at reversing the neurodegenerative damage observed in this disease. Remyelination as a result of oligodendrocyte production from oligodendrocyte precursor cells (OPCs) is considered a promising potential target for the treatment of all stages of MS. In this review we present an overview of a) approved medications (some of them FDA-and EMA-approved for other diseases) with a proposed role in regeneration, b) regenerative treatments under investigation in clinical trials, and c) promising future therapeutic approaches aiming specifically at facilitating endogenous repair.

Transcript profiling of different types of multiple sclerosis lesions yields FGF1 as a promoter of remyelination

Chronic demyelination is a pathological hallmark of multiple sclerosis (MS). Only a minority of MS lesions remyelinates completely. Enhancing remyelination is, therefore, a major aim of future MS therapies. Here we took a novel approach to identify factors that may inhibit or support endogenous remyelination in MS. We dissected remyelinated, demyelinated active, and demyelinated inactive white matter MS lesions, and compared transcript levels of myelination and inflammation-related genes using quantitative PCR on customized TaqMan Low Density Arrays. In remyelinated lesions, fibroblast growth factor (FGF) 1 was the most abundant of all analyzed myelination-regulating factors, showed a trend towards higher expression as compared to demyelinated lesions and was significantly higher than in control white matter. Two MS tissue blocks comprised lesions with adjacent de- and remyelinated areas and FGF1 expression was higher in the remyelinated rim compared to the demyelinated lesion core. In functional experiments, FGF1 accelerated developmental myelination in dissociated mixed cultures and promoted remyelination in slice cultures, whereas it decelerated differentiation of purified primary oligodendrocytes, suggesting that promotion of remyelination by FGF1 is based on an indirect mechanism. The analysis of human astrocyte responses to FGF1 by genome wide expression profiling showed that FGF1 induced the expression of the chemokine CXCL8 and leukemia inhibitory factor, two factors implicated in recruitment of oligodendrocytes and promotion of remyelination. Together, this study presents a transcript profiling of remyelinated MS lesions and identified FGF1 as a promoter of remyelination. Modulation of FGF family members might improve myelin repair in MS.

Changes in the expression of DNA-binding/differentiation protein inhibitors in neurons and glial cells of the gerbil hippocampus following transient global cerebral ischemia

Inhibitors of DNA-binding/differentiation (ID) proteins bind to basic helix-loop-helix (bHLH) transcription factors, including those that regulate differentiation and cell-cycle progression during development, and regulate gene transcription. However, little is known about the role of ID proteins in the brain under transient cerebral ischemic conditions. In the present study, we examined the effects of ischemia-reperfusion (I-R) injury on the immunoreactivity and protein levels of IDs 1–4 in the gerbil hippocampus proper Cornu Ammonis regions CA1–3 following 5 min of transient cerebral ischemia. Strong ID1 immunoreactivity was detected in the nuclei of pyramidal neurons in the hippocampal CA1–3 regions; immunoreactivity was significantly changed following I-R in the CA1 region, but not in the CA2/3 region. Five days following I-R, ID1 immunoreactivity was not detected in the CA1 pyramidal neurons. ID1 immunoreactivity was detected only in GABAergic interneurons in the ischemic CA1 region. Weak ID4 immunoreactivity was detected in non-pyramidal cells, and immunoreactivity was again only changed in the ischemic CA1 region. Five days following I-R, strong ID4 immunoreactivity was detected in non-pyramidal cells, which were identified as microglia, and not astrocytes, in the ischemic CA1 region. Furthermore, changes in the protein levels of ID1 and ID4 in the ischemic CA1 region studied by western blot were consistent with patterns of immunoreactivity. In summary, these results indicate that immunoreactivity and protein levels of ID1 and ID4 are distinctively altered following transient cerebral ischemia only in the CA1 region, and that the changes in ID1 and ID4 expression may relate to the ischemia-induced delayed neuronal death.

Gene co-expression network and function modules in three types of glioma

The aim of the present study was to identify the disease‑associated genes and their functions involved in the development of three types of glioma (astrocytoma, glioblastoma and oligodendroglioma) with DNA microarray technology, and to analyze their differences and correlations. First, the gene expression profile GSE4290 was downloaded from the Gene Expression Omnibus database, then the probe‑level data were pre‑processed and the differentially expressed genes (DEGs) were identified with limma package in R language. Gene functions of the selected DEGs were further analyzed with the Database for Annotation, Visualization and Integrated Discovery. After the co‑expression network of DEGs was constructed by Cytoscape, the functional modules were mined and enrichment analysis was performed, and then the similarities and differences between any two types of glioma were compared. A total of 1151 genes between normal and astrocytoma tissues, 684 genes between normal and malignant glioma tissues, and 551 genes between normal and oligodendroglioma tissues were filtered as DEGs, respectively. By constructing co‑expression networks of DEGs, a total of 77232, 455 and 987 interactions were involved in the differentially co‑expressed networks of astrocytoma, oligodendroglioma and glioblastoma, respectively. The functions of DEGs were consistent with the modules in astrocytoma, glioblastoma and oligodendroglioma, which were mainly enriched in neuron signal transmission, immune responses and synthesis of organic acids, respectively. Model functions of astrocytoma and glioblastoma were similar (mainly related with immune response), while the model functions of oligodendroglioma differed markedly from that of the other two types. The identification of the associations among these three types of glioma has potential clinical utility for improving the diagnosis of different types of glioma in the future. In addition, these results have marked significance in studying the underlying mechanisms, distinguishing between normal and cancer tissues, and examining novel therapeutic strategies for patients with glioma.

Oligodendrocyte Lineage Cells in Chronic Demyelination of Multiple Sclerosis Optic Nerve

Reports that chronically demyelinated multiple sclerosis brain and spinal cord lesions contained immature oligodendrocyte lineage cells have generated major interest aimed at the potential for promotion of endogenous repair. Despite the prominence of the optic nerve as a lesion site and its importance in clinical disease assessment, no detailed studies of multiple sclerosis‐affected optic nerve exist. This study aims to provide insight into the cellular pathology of chronic demyelination in multiple sclerosis through direct morphological and immunohistochemical analysis of optic nerve in conjunction with observations from an experimental cat optic nerve model of successful remyelination. Myelin staining was followed by immunohistochemistry to differentially label neuroglia. Digitally immortalized sections were then analyzed to generate quantification data and antigenic phenotypes including maturational stages within the oligodendrocyte lineage. It was found that some chronically demyelinated multiple sclerosis optic nerve lesions contained oligodendroglial cells and that heterogeneity existed in the presence of myelin sheaths, oligodendrocyte maturational stages and extent of axonal investment. The findings advance our understanding of oligodendrocyte activity in chronically demyelinated human optic nerve and may have implications for studies aimed at enhancement of endogenous repair in multiple sclerosis.

Effect of type-2 astrocytes on the viability of dorsal root ganglion neurons and length of neuronal processes

The role of type-2 astrocytes in the repair of central nervous system injury remains poorly understood. In this study, using a relatively simple culture condition in vitro, type-2 astrocytes, differentiated from oligodendrocyte precursor cells by induction with bone morphogenetic protein-4, were co-cultured with dorsal root ganglion neurons. We examined the effects of type-2 astrocytes differentiated from oligodendrocyte precursor cells on the survival and growth of dorsal root ganglion neurons. Results demonstrated that the number of dorsal root ganglion neurons was higher following co-culture of oligodendrocyte precursor cells and type-2 astrocytes than when cultured alone, but lower than that of neurons co-cultured with type-1 astrocytes. The length of the longest process and the length of all processes of a single neuron were shortest in neurons cultured alone, followed by neurons co-cultured with type-2 astrocytes, then neurons co-cultured with oligodendrocyte precursor cells, and longest in neurons co-cultured with type-1 astrocytes. These results indicate that co-culture with type-2 astrocytes can increase neuronal survival rate and process length. However, compared with type-1 astrocytes and oligodendrocyte precursor cells, the promotion effects of type-2 astrocytes on the growth of dorsal root ganglion neurons were weaker.

Activation of sodium-dependent glutamate transporters regulates the morphological aspects of oligodendrocyte maturation via signaling through calcium/calmodulin-dependent kinase IIβ's actin-binding/-stabilizing domain

Signaling via the major excitatory amino acid glutamate has been implicated in the regulation of various aspects of the biology of oligodendrocytes, the myelinating cells of the central nervous system (CNS). In this respect, cells of the oligodendrocyte lineage have been described to express a variety of glutamate-responsive transmembrane proteins including sodium-dependent glutamate transporters. The latter have been well characterized to mediate glutamate clearance from the extracellular space. However, there is increasing evidence that they also mediate glutamate-induced intracellular signaling events. Our data presented here show that the activation of oligodendrocyte expressed sodium-dependent glutamate transporters, in particular GLT-1 and GLAST, promotes the morphological aspects of oligodendrocyte maturation. This effect was found to be associated with a transient increase in intracellular calcium levels and a transient phosphorylation event at the serine (S)(371) site of the calcium sensor calcium/calmodulin-dependent kinase type IIβ (CaMKIIβ). The potential regulatory S(371) site is located within CaMKIIβ's previously defined actin-binding/-stabilizing domain, and phosphorylation events within this domain were identified in our studies as a requirement for sodium-dependent glutamate transporter-mediated promotion of oligodendrocyte maturation. Furthermore, our data provide good evidence for a role of these phosphorylation events in mediating detachment of CaMKIIβ from filamentous (F)-actin, and hence allowing a remodeling of the oligodendrocyte's actin cytoskeleton. Taken together with our recent findings, which demonstrated a crucial role of CaMKIIβ in regulating CNS myelination in vivo, our data strongly suggest that a sodium-dependent glutamate transporter-CaMKIIβ-actin cytoskeleton axis plays an important role in the regulation of oligodendrocyte maturation and CNS myelination.

The role of oligodendroglial dysfunction in amyotrophic lateral sclerosis

SUMMARY 

Healthy oligodendrocytes are crucial for neurons and abnormal functioning of these cells is involved in several neurodegenerative diseases. We will focus on oligodendroglial pathology in amyotrophic lateral sclerosis (ALS), an adult-onset progressive neurodegenerative disease characterized by selective motor neuron loss. Recent discoveries shed new light on the crucial role of oligodendrocytes in this fatal disease. We will first give an overview of the importance of good-functioning oligodendrocytes for neuronal health, in particular for motor neurons. Subsequently, we will discuss the recent data on oligodendroglial abnormalities in ALS. We conclude that oligodendrocytes should be considered as important contributors to motor neuron degeneration. As a consequence, oligodendrocytes are a promising new therapeutic target for ALS and other neurodegenerative diseases.

NF-κB signaling regulates myelination in the CNS

Besides myelination of neuronal axons by oligodendrocytes to facilitate propagation of action potentials, oligodendrocytes also support axon survival and function. A key transcription factor involved in these processes is nuclear factor-κB (NF-κB), a hetero or homodimer of the Rel family of proteins, including p65, c-Rel, RelB, p50, and p52. Under unstimulated, NF-κB remains inactive in the cytoplasm through interaction with NF-κB inhibitors (IκBs). Upon activation of NF-κB the cytoplasmic IκBs gets degradated, allowing the translocation of NF-κB into the nucleus where the dimer binds to the κB consensus DNA sequence and regulates gene transcription. In this review we describe how oligodendrocytes are, directly or indirectly via neighboring cells, regulated by NF-κB signaling with consequences for innate and adaptive immunity and for regulation of cell apoptosis and survival.

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

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

In vivo quantification of white matter microstructure for use in aging: a focus on two emerging techniques

Human brain imaging has seen many advances in the quantification of white matter in vivo. For example, these advances have revealed the association between white matter damage and vascular disease as well as their impact on risk for and development of dementia and depression in an aging population. Current neuroimaging methods to quantify white matter damage provide a foundation for understanding such age-related neuropathology; however, these methods are not as adept at determining the underlying microstructural abnormalities signaling at risk tissue or driving white matter damage in the aging brain. This review will begin with a brief overview of the use of diffusion tensor imaging (DTI) in understanding white matter alterations in aging before focusing in more detail on select advances in both diffusion-based methods and multi-component relaxometry techniques for imaging white matter microstructural integrity within myelin sheaths and the axons they encase. Although DTI greatly extended the field of white matter interrogation, these more recent technological advances will add clarity to the underlying microstructural mechanisms that contribute to white matter damage. More specifically, the methods highlighted in this review may prove more sensitive (and specific) for determining the contribution of myelin versus axonal integrity to the aging of white matter in brain.

ERK1/2 signaling is essential for the chemoattraction exerted by human FGF2 and human anosmin-1 on newborn rat and mouse OPCs via FGFR1

Signaling through fibroblast growth factor receptors (FGFRs) is essential for many cellular processes including proliferation and migration, as well as differentiation events such as myelination. Anosmin-1 is an extracellular matrix (ECM) glycoprotein that interacts with the fibroblast growth factor receptor 1 (FGFR1) to exert its biological actions through this receptor, although the intracellular pathways underlying anosmin-1 signaling remain largely unknown. This protein is defective in the X-linked form of Kallmann syndrome (KS) and has a prominent role in the migration of neuronal and oligodendroglial precursors. We have shown that anosmin-1 exerts a chemotactic effect via FGFR1 on neuronal precursors from the subventricular zone (SVZ) and the essential role of the ERK1/2 signaling. We report here the positive chemotactic effect of FGF2 and anosmin-1 on rat and mouse postnatal OPCs via FGFR1. The same effect was observed with the truncated N-terminal region of anosmin-1 (A1Nt). The introduction in anosmin-1 of the missense mutation F517L found in patients suffering from KS annulled the chemotactic activity; however, the mutant form carrying the disease-causing mutation E514K also found in KS patients, behaved as the wild-type protein. The chemoattraction exhibited by FGF2 and anosmin-1 on OPCs was blocked by the mitogen-activated protein kinase (MAPK) inhibitor U0126, suggesting that the activation of the ERK1/2 MAPK signaling pathway following interaction with the FGFR1 is necessary for FGF2 and anosmin-1 to exert their chemotactic effect. In fact, both proteins were able to induce the phosphorylation of the ERK1/2 kinases after the activation of the FGFR1 receptor.

Regulation of oligodendrocyte precursor migration during development, in adulthood and in pathology

Oligodendrocytes are the myelin-forming cells in the central nervous system (CNS). These cells originate from oligodendrocyte precursor cells (OPCs) during development, and they migrate extensively from oligodendrogliogenic niches along the neural tube to colonise the entire CNS. Like many other such events, this migratory process is precisely regulated by a battery of positional and signalling cues that act via their corresponding receptors and that are expressed dynamically by OPCs. Here, we will review the cellular and molecular basis of this important event during embryonic and postnatal development, and we will discuss the relevance of the substantial number of OPCs existing in the adult CNS. Similarly, we will consider the behaviour of OPCs in normal and pathological conditions, especially in animal models of demyelination and of the demyelinating disease, multiple sclerosis. The spontaneous remyelination observed after damage in demyelinating pathologies has a limited effect. Understanding the cellular and molecular mechanisms underlying the biology of OPCs, particularly adult OPCs, should help in the design of neuroregenerative strategies to combat multiple sclerosis and other demyelinating diseases.

Decoding signaling and function of the orphan G protein-coupled receptor GPR17 with a small-molecule agonist

Replacement of the lost myelin sheath is a therapeutic goal for treating demyelinating diseases of the central nervous system (CNS), such as multiple sclerosis (MS). The G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR) GPR17, which is phylogenetically closely related to receptors of the "purinergic cluster," has emerged as a modulator of CNS myelination. However, whether GPR17-mediated signaling positively or negatively regulates this critical process is unresolved. We identified a small-molecule agonist, MDL29,951, that selectively activated GPR17 even in a complex environment of endogenous purinergic receptors in primary oligodendrocytes. MDL29,951-stimulated GPR17 engaged the entire set of intracellular adaptor proteins for GPCRs: G proteins of the Gα(i), Gα(s), and Gα(q) subfamily, as well as β-arrestins. This was visualized as alterations in the concentrations of cyclic adenosine monophosphate and inositol phosphate, increased Ca²⁺ flux, phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), as well as multifeatured cell activation recorded with label-free dynamic mass redistribution and impedance biosensors. MDL29,951 inhibited the maturation of primary oligodendrocytes from heterozygous but not GPR17 knockout mice in culture, as well as in cerebellar slices from 4-day-old wild-type mice. Because GPCRs are attractive targets for therapeutic intervention, inhibiting GPR17 emerges as therapeutic strategy to relieve the oligodendrocyte maturation block and promote myelin repair in MS.

A regenerative approach to the treatment of multiple sclerosis

Progressive phases of multiple sclerosis are associated with inhibited differentiation of the progenitor cell population that generates the mature oligodendrocytes required for remyelination and disease remission. To identify selective inducers of oligodendrocyte differentiation, we performed an image-based screen for myelin basic protein (MBP) expression using primary rat optic-nerve-derived progenitor cells. Here we show that among the most effective compounds identifed was benztropine, which significantly decreases clinical severity in the experimental autoimmune encephalomyelitis (EAE) model of relapsing-remitting multiple sclerosis when administered alone or in combination with approved immunosuppressive treatments for multiple sclerosis. Evidence from a cuprizone-induced model of demyelination, in vitro and in vivo T-cell assays and EAE adoptive transfer experiments indicated that the observed efficacy of this drug results directly from an enhancement of remyelination rather than immune suppression. Pharmacological studies indicate that benztropine functions by a mechanism that involves direct antagonism of M1 and/or M3 muscarinic receptors. These studies should facilitate the development of effective new therapies for the treatment of multiple sclerosis that complement established immunosuppressive approaches.

Mesenchymal stem cell conditioning promotes rat oligodendroglial cell maturation

Oligodendroglial progenitor/precursor cells (OPCs) represent the main cellular source for the generation of new myelinating oligodendrocytes in the adult central nervous system (CNS). In demyelinating diseases such as multiple sclerosis (MS) myelin repair activities based on recruitment, activation and differentiation of resident OPCs can be observed. However, the overall degree of successful remyelination is limited and the existence of an MS-derived anti-oligodendrogenic milieu prevents OPCs from contributing to myelin repair. It is therefore of considerable interest to understand oligodendroglial homeostasis and maturation processes in order to enable the development of remyelination therapies. Mesenchymal stem cells (MSC) have been shown to exert positive immunomodulatory effects, reduce demyelination, increase neuroprotection and to promote adult neural stem cell differentiation towards the oligodendroglial lineage. We here addressed whether MSC secreted factors can boost the OPC’s oligodendrogenic capacity in a myelin non-permissive environment. To this end, we analyzed cellular morphologies, expression and regulation of key factors involved in oligodendroglial fate and maturation of primary rat cells upon incubation with MSC-conditioned medium. This demonstrated that MSC-derived soluble factors promote and accelerate oligodendroglial differentiation, even under astrocytic endorsing conditions. Accelerated maturation resulted in elevated levels of myelin expression, reduced glial fibrillary acidic protein expression and was accompanied by downregulation of prominent inhibitory differentiation factors such as Id2 and Id4. We thus conclude that apart from their suggested application as potential anti-inflammatory and immunomodulatory MS treatment, these cells might also be exploited to support endogenous myelin repair activities.

Neuron-NG2 cell synapses: novel functions for regulating NG2 cell proliferation and differentiation

NG2 cells are a population of CNS cells that are distinct from neurons, mature oligodendrocytes, astrocytes, and microglia. These cells can be identified by their NG2 proteoglycan expression. NG2 cells have a highly branched morphology, with abundant processes radiating from the cell body, and express a complex set of voltage-gated channels, AMPA/kainate, and GABA receptors. Neurons notably form classical and nonclassical synapses with NG2 cells, which have varied characteristics and functions. Neuron-NG2 cell synapses could fine-tune NG2 cell activities, including the NG2 cell cycle, differentiation, migration, and myelination, and may be a novel potential therapeutic target for NG2 cell-related diseases, such as hypoxia-ischemia injury and periventricular leukomalacia. Furthermore, neuron-NG2 cell synapses may be correlated with the plasticity of CNS in adulthood with the synaptic contacts passing onto their progenies during proliferation, and synaptic contacts decrease rapidly upon NG2 cell differentiation. In this review, we highlight the characteristics of classical and nonclassical neuron-NG2 cell synapses, the potential functions, and the fate of synaptic contacts during proliferation and differentiation, with the emphasis on the regulation of the NG2 cell cycle by neuron-NG2 cell synapses and their potential underlying mechanisms.

Inhibition of astroglial NF-κB enhances oligodendrogenesis following spinal cord injury

BACKGROUND

Astrocytes are taking the center stage in neurotrauma and neurological diseases as they appear to play a dominant role in the inflammatory processes associated with these conditions. Previously, we reported that inhibiting NF-κB activation in astrocytes, using a transgenic mouse model (GFAP-IκBα-dn mice), results in improved functional recovery, increased white matter preservation and axonal sparing following spinal cord injury (SCI). In the present study, we sought to determine whether this improvement, due to inhibiting NF-κB activation in astrocytes, could be the result of enhanced oligodendrogenesis in our transgenic mice.

METHODS

To assess oligodendrogenesis in GFAP-IκBα-dn compared to wild-type (WT) littermate mice following SCI, we used bromodeoxyuridine labeling along with cell-specific immuno-histochemistry, confocal microscopy and quantitative cell counts. To further gain insight into the underlying molecular mechanisms leading to increased white matter, we performed a microarray analysis in naïve and 3 days, 3 and 6 weeks following SCI in GFAP-IκBα-dn and WT littermate mice.

RESULTS

Inhibition of astroglial NF-κB in GFAP-IκBα-dn mice resulted in enhanced oligodendrogenesis 6 weeks following SCI and was associated with increased levels of myelin proteolipid protein compared to spinal cord injured WT mice. The microarray data showed a large number of differentially expressed genes involved in inflammatory and immune response between WT and transgenic mice. We did not find any difference in the number of microglia/leukocytes infiltrating the spinal cord but did find differences in their level of expression of toll-like receptor 4. We also found increased expression of the chemokine receptor CXCR4 on oligodendrocyte progenitor cells and mature oligodendrocytes in the transgenic mice. Finally TNF receptor 2 levels were significantly higher in the transgenic mice compared to WT following injury.

CONCLUSIONS

These studies suggest that one of the beneficial roles of blocking NF-κB in astrocytes is to promote oligodendrogenesis through alteration of the inflammatory environment.

Critical role for PAR1 in kallikrein 6-mediated oligodendrogliopathy

Kallikrein 6 (KLK6) is a secreted serine protease preferentially expressed by oligodendroglia in CNS white matter. Elevated levels of KLK6 occur in actively demyelinating multiple sclerosis (MS) lesions and in cases of spinal cord injury (SCI), stroke, and glioblastoma. Taken with recent evidence establishing KLK6 as a CNS-endogenous activator of protease-activated receptors (PARs), we hypothesized that KLK6 activates a subset of PARs to regulate oligodendrocyte physiology and potentially pathophysiology. Here, primary oligodendrocyte cultures derived from wild type or PAR1-deficient mice and the murine oligodendrocyte cell line, Oli-neu, were used to demonstrate that Klk6 (rodent form) mediates loss of oligodendrocyte processes and impedes morphological differentiation of oligodendrocyte progenitor cells (OPCs) in a PAR1-dependent fashion. Comparable gliopathy was also elicited by the canonical PAR1 agonist, thrombin, as well as PAR1-activating peptides (PAR1-APs). Klk6 also exacerbated ATP-mediated oligodendrogliopathy in vitro, pointing to a potential role in augmenting excitotoxicity. In addition, Klk6 suppressed the expression of proteolipid protein (PLP) RNA in cultured oligodendrocytes by a mechanism involving PAR1-mediated Erk1/2 signaling. Microinjection of PAR1 agonists, including Klk6 or PAR1-APs, into the dorsal column white matter of PAR1(+/+) but not PAR1(-/-) mice promoted vacuolating myelopathy and a loss of immunoreactivity for myelin basic protein (MBP) and CC-1(+) oligodendrocytes. These results demonstrate a functional role for Klk6-PAR1 signaling in oligodendroglial pathophysiology and suggest that antagonists of PAR1 or its protease agonists may represent new modalities to moderate demyelination and to promote myelin regeneration in cases of CNS white matter injury or disease.

Digestion products of the PH20 hyaluronidase inhibit remyelination

OBJECTIVE

Oligodendrocyte progenitor cells (OPCs) recruited to demyelinating lesions often fail to mature into oligodendrocytes (OLs) that remyelinate spared axons. The glycosaminoglycan hyaluronan (HA) accumulates in demyelinating lesions and has been implicated in the failure of OPC maturation and remyelination. We tested the hypothesis that OPCs in demyelinating lesions express a specific hyaluronidase, and that digestion products of this enzyme inhibit OPC maturation.

METHODS

Mouse OPCs grown in vitro were analyzed for hyaluronidase expression and activity. Gain of function studies were used to define the hyaluronidases that blocked OPC maturation. Mouse and human demyelinating lesions were assessed for hyaluronidase expression. Digestion products from different hyaluronidases and a hyaluronidase inhibitor were tested for their effects on OPC maturation and functional remyelination in vivo.

RESULTS

OPCs demonstrated hyaluronidase activity in vitro and expressed multiple hyaluronidases, including HYAL1, HYAL2, and PH20. HA digestion by PH20 but not other hyaluronidases inhibited OPC maturation into OLs. In contrast, inhibiting HA synthesis did not influence OPC maturation. PH20 expression was elevated in OPCs and reactive astrocytes in both rodent and human demyelinating lesions. HA digestion products generated by the PH20 hyaluronidase but not another hyaluronidase inhibited remyelination following lysolecithin-induced demyelination. Inhibition of hyaluronidase activity lead to increased OPC maturation and promoted increased conduction velocities through lesions.

INTERPRETATION

We determined that PH20 is elevated in demyelinating lesions and that increased PH20 expression is sufficient to inhibit OPC maturation and remyelination. Pharmacological inhibition of PH20 may therefore be an effective way to promote remyelination in multiple sclerosis and related conditions.

Oligodendrocyte dysfunction in the pathogenesis of amyotrophic lateral sclerosis

Oligodendrocytes are well known targets for immune-mediated and infectious diseases, and have been suggested to play a role in neurodegeneration. Here, we report the involvement of oligodendrocytes and their progenitor cells in the ventral grey matter of the spinal cord in amyotrophic lateral sclerosis, a neurodegenerative disease of motor neurons. Degenerative changes in oligodendrocytes were abundantly present in human patients with amyotrophic lateral sclerosis and in an amyotrophic lateral sclerosis mouse model. In the mouse model, morphological changes in grey matter oligodendrocytes became apparent before disease onset, increasingly so during disease progression, and oligodendrocytes ultimately died. This loss was compensated by increased proliferation and differentiation of oligodendrocyte precursor cells. However, these newly differentiated oligodendrocytes were dysfunctional as suggested by their reduced myelin basic protein and monocarboxylate transporter 1 expression. Mutant superoxide dismutase 1 was found to directly affect monocarboxylate transporter 1 protein expression. Our data suggest that oligodendroglial dysfunction may be a contributor to motor neuron degeneration in amyotrophic lateral sclerosis.

Oligodendrocyte N-methyl-D-aspartate receptor signaling: insights into its functions

Myelination by oligodendrocytes facilitates rapid nerve conduction. Loss of oligodendrocytes and failure of myelination lead to nerve degeneration and numerous demyelinating white matter diseases. N-methyl-D-aspartate (NMDA) receptors, which are key regulators on neuron survival and functions, have been recently identified to express in oligodendrocytes, especially in the myelin sheath. NMDA receptor signaling in oligodendrocytes plays crucial roles in energy metabolism and myelination. In the present review, we highlight the subcellular location-specific impairment of excessive NMDA receptor signaling on oligodendrocyte energy metabolism in soma and myelin, and the mechanisms including Ca(2+) overload, acidotoxicity, mitochondria dysfunction, and impairment of respiratory chains. Conversely, physiological NMDA receptor signaling regulates differentiation and migration of oligodendrocytes. How can we use above knowledge to treat excitotoxic oligodendrocyte loss, congenital myelination deficiency, or postnatal demyelination? A thorough understanding of NMDA receptor signaling-mediated cellular events in oligodendrocytes at the pathophysiological level will no doubt aid in exploring effective therapeutic strategies for demyelinating white matter diseases.

Myelination: do astrocytes play a role?

Astrocytes are the most abundant cell type in the adult central nervous system (CNS), and their functional diversity in response to injury is now being appreciated. Astrocytes have long been considered the main player in the inhibition of CNS repair via the formation of the gliotic scar, but now it is accepted that astrocyte can play an important role in CNS repair and remyelination. Interest in the relationship between astrocytes and myelination focused initially on attempts to understand how the development of plaques of astroglial scar tissue in multiple sclerosis was related to the failure of these lesions to remyelinate. It is now considered that this is an end stage pathological response to injury, and that normally astrocytes play important roles in supporting the development and maintenance of CNS myelin. This review will focus on how this new understanding may be exploited to develop new strategies to enhance remyelination in multiple sclerosis and other diseases.

Developmental changes and subcellular location in inhibitor of DNA binding 2 (Id2) immunoreactivity in the rat Corpus callosum

The mechanisms underlying oligodendrocyte differentiation and myelination are still unclear, but understanding them will be critical for the development of therapies for multiple sclerosis. Inhibitor of DNA binding 2 (Id2) is a transcription factor thought to inhibit oligodendrocyte differentiation, however, it is not known whether the developmental changes and subcellular localization of Id2 are related to myelination. Therefore, we investigated the developmental changes in and the subcellular localization of Id2 immunoreactivity in the rat Corpus callosum, at post-natal developmental stages P0, P7, P14, P21, P42 and P90, by immunohistochemistry. Id2 expression increased from P0 to a peak at P42, the late stage of myelination in the Corpus callosum. Id2 immunostaining decreased slightly, but still remained high at P90. Subcellular localization of Id2 changed from presence in cytoplasm at P14 to the nuclei at P42. Moreover, Id2 was mainly co-localized with CC-1-immunopositive mature oligodendrocytes at P42. These results may be consistent with Id2 inhibitory function in oligodendrocyte differentiation, at the end of myelination or in compaction of myelin in the Corpus callosum of postnatal rat brain.