To what extent is oligodendrocyte progenitor migration a limiting factor in the remyelination of multiple sclerosis lesions?

Dynamic changes of oligodendrogenesis in neonatal rats with hypoxic-ischemic white matter injury

BACKGROUND

White matter injury (WMI) is an important type of preterm brain injury, which may result in severe neurological sequelae and lack of effective treatments. It is ascertained that selective vulnerability of oligodendrocytes is closely related to the WMI in preterm infants. But the alteration of the endogenous oligodendrogenesis over long time after hypoxic-ischemic WMI is still not clearly elucidated.

METHODS

We adopted an animal model of hypoxic-ischemic WMI in 3-day-old neonatal Sprague-Dawley rats. Immunofluorescence staining and western blotting were used to detect dynamic changes of oligodendrogenesis in the white matter region on postoperative day (POD) 1, 3, 7, 14, 28, 56 and 84.

RESULTS

In the sham group, the oligodendrocyte lineage in the white matter reached a developmental peak from POD 3 to 14. The proliferation and development of oligodendrocyte precursor cells (OPCs) occurred primarily within POD 14. The number of mature oligodendrocytes showed an upward trend and a dynamic change in proliferation over time. While in the WMI group, the oligodendrocyte lineage was upregulated on POD1 and 3 but downregulated on POD 7 and 14. The proliferation of OPCs increased on POD 1 and decreased on POD 3 and 7, with the total number of OPCs significantly reduced from POD 3 to 14. The number of mature oligodendrocytes decreased from POD 3 to 28, and return to the level of the sham group on POD 56 and 84, whereas the MBP expression was still significantly downregulated on POD 56 and 84.

CONCLUSIONS

Hypoxia-ischemia can have a long-term dynamic effect on the endogenous oligodendrogenesis of neonatal rat brain white matter. The proliferation of OPCs was promoted on POD 1 but inhibited from POD 3 to 14, which may be an early intervention target to improve oligodendrogenesis. The number of mature oligodendrocytes recover to the normal on POD 56 and 84 but the myelination is still blocked, which suggests it is essential to promote the maturation of oligodendrocyte and its function recovery at the same time within POD 28. Such efforts will provide the opportunity to test new interventions in pre-clinical studies for their promising clinical application.

7T MRI Differentiates Remyelinated from Demyelinated Multiple Sclerosis Lesions

Objective

To noninvasively assess myelin status in chronic white matter lesions of multiple sclerosis (MS), we developed and evaluated a simple classification scheme based on T1 relaxation time maps derived from 7‐tesla postmortem and in vivo MRI.

Methods

Using the MP2RAGE MRI sequence, we classified 36 lesions from 4 postmortem MS brains as “long‐T1,” “short‐T1,” and “mixed‐T1” by visual comparison to neocortex. Within these groups, we compared T1 times to histologically derived measures of myelin and axons. We performed similar analysis of 235 chronic lesions with known date of onset in 25 MS cases in vivo and in a validation cohort of 222 lesions from 66 MS cases, investigating associations with clinical and radiological outcomes.

Results

Postmortem, lesions classified qualitatively as long‐T1, short‐T1, and mixed‐T1 corresponded to fully demyelinated, fully remyelinated, and mixed demyelinated/remyelinated lesions, respectively (p ≤ 0.001). Demyelination (rather than axon loss) dominantly contributed to initial T1 prolongation. We observed lesions with similar characteristics in vivo, allowing manual classification with substantial interrater and excellent intrarater reliability. Short‐T1 lesions were most common in the deep white matter, whereas long‐T1 and mixed‐T1 lesions were prevalent in the juxtacortical and periventricular white matter (p = 0.02) and were much more likely to have paramagnetic rims suggesting chronic inflammation (p < 0.001). Older age at the time of lesion formation portended less remyelination (p = 0.007).

Interpretation

7‐tesla T1 mapping with MP2RAGE, a clinically available MRI method, allows qualitative and quantitative classification of chronic MS lesions according to myelin content, rendering straightforward the tracking of lesional myelination changes over time. ANN NEUROL 2021;90:612–626

Oligodendrocyte progenitors as environmental biosensors

The past decade has seen an important revision of the traditional concept of the role and function of glial cells. From "passive support" for neurons, oligodendrocyte lineage cells are now recognized as metabolic exchangers with neurons, a cellular interface with blood vessels and responders to gut-derived metabolites or changes in the social environment. In the developing brain, the differentiation of neonatal oligodendrocyte progenitors (nOPCs) is required for normal brain function. In adulthood, the differentiation of adult OPCs (aOPCs) serves an important role in learning, behavioral adaptation and response to myelin injury. Here, we propose the concept of OPCs as environmental biosensors, which "sense" chemical and physical stimuli over time and adjust to the new challenges by modifying their epigenome and consequent transcriptome. Because epigenetics defines the ability of the cell to "adapt" gene expression to changes in the environment, we propose a model of OPC differentiation resulting from time-dependent changes of the epigenomic landscape in response to declining mitogens, raising hormone levels, neuronal activity, changes in space constraints or stiffness of the extracellular matrix. We propose that the intrinsically different functional properties of aOPCs compared to nOPCs result from the accrual of "epigenetic memories" of distinct events, which are "recorded" in the nuclei of OPCs as histone and DNA marks, defining a "unique epigenomic landscape" over time.

Myelin Repair: From Animal Models to Humans

It is widely thought that brain repair does not occur, but myelin regeneration provides clear evidence to the contrary. Spontaneous remyelination may occur after injury or in multiple sclerosis (MS). However, the efficiency of remyelination varies considerably between MS patients and between the lesions of each patient. Myelin repair is essential for optimal functional recovery, so a profound understanding of the cells and mechanisms involved in this process is required for the development of new therapeutic strategies. In this review, we describe how animal models and modern cell tracing and imaging methods have helped to identify the cell types involved in myelin regeneration. In addition to the oligodendrocyte progenitor cells identified in the 1990s as the principal source of remyelinating cells in the central nervous system (CNS), other cell populations, including subventricular zone-derived neural progenitors, Schwann cells, and even spared mature oligodendrocytes, have more recently emerged as potential contributors to CNS remyelination. We will also highlight the conditions known to limit endogenous repair, such as aging, chronic inflammation, and the production of extracellular matrix proteins, and the role of astrocytes and microglia in these processes. Finally, we will present the discrepancies between observations in humans and in rodents, discussing the relationship of findings in experimental models to myelin repair in humans. These considerations are particularly important from a therapeutic standpoint.

Macroglial diversity: white and grey areas and relevance to remyelination

Macroglia, comprising astrocytes and oligodendroglial lineage cells, have long been regarded as uniform cell types of the central nervous system (CNS). Although regional morphological differences between these cell types were initially described after their identification a century ago, these differences were largely ignored. Recently, accumulating evidence suggests that macroglial cells form distinct populations throughout the CNS, based on both functional and morphological features. Moreover, with the use of refined techniques including single-cell and single-nucleus RNA sequencing, additional evidence is emerging for regional macroglial heterogeneity at the transcriptional level. In parallel, several studies revealed the existence of regional differences in remyelination capacity between CNS grey and white matter areas, both in experimental models for successful remyelination as well as in the chronic demyelinating disease multiple sclerosis (MS). In this review, we provide an overview of the diversity in oligodendroglial lineage cells and astrocytes from the grey and white matter, as well as their interplay in health and upon demyelination and successful remyelination. In addition, we discuss the implications of regional macroglial diversity for remyelination in light of its failure in MS. Since the etiology of MS remains unknown and only disease-modifying treatments altering the immune response are available for MS, the elucidation of macroglial diversity in grey and white matter and its putative contribution to the observed difference in remyelination efficiency between these regions may open therapeutic avenues aimed at enhancing endogenous remyelination in either area.

A novel myelin protein zero transgenic zebrafish designed for rapid readout of in vivo myelination

Demyelination occurs following many neurological insults, most notably in multiple sclerosis (MS). Therapeutics that promote remyelination could slow the neurological decline associated with chronic demyelination; however, in vivo testing of candidate small molecule drugs and signaling cascades known to impact myelination is expensive and labor intensive. Here, we describe the development of a novel zebrafish line which uses the putative promoter of Myelin Protein Zero (mpz), a major structural protein in myelin, to drive expression of Enhanced Green Fluorescent Protein (mEGFP) specifically in the processes and nascent internodes of myelinating glia. We observe that changes in fluorescence intensity in Tg(mpz:mEGFP) larvae are a reliable surrogate for changes in myelin membrane production per se in live larvae following bath application of drugs. These changes in fluorescence are strongly predictive of changes in myelin-specific mRNAs [mpz, 36K and myelin basic protein (mbp)] and protein production (Mbp). Finally, we observe that certain drugs alter nascent internode number and length, impacting the overall amount of myelin membrane synthesized and a number of axons myelinated without significantly changing the number of myelinating oligodendrocytes. These studies demonstrate that the Tg(mpz:mEGFP) reporter line responds effectively to positive and negative small molecule regulators of myelination, and could be useful for identifying candidate drugs that specifically target myelin membrane production in vivo. Combined with high throughput cell-based screening of large chemical libraries and automated imaging systems, this transgenic line is useful for rapid large scale whole animal screening to identify novel myelinating small molecule compounds in vivo.

CXCL12/CXCR4/CXCR7 Chemokine Axis in the Central Nervous System: Therapeutic Targets for Remyelination in Demyelinating Diseases

The chemokine CXCL12 plays a vital role in regulating the development of the central nervous system (CNS) by binding to its receptors CXCR4 and CXCR7. Recent studies reported that the CXCL12/CXCR4/CXCR7 axis regulates both embryonic and adult oligodendrocyte precursor cells (OPCs) in their proliferation, migration, and differentiation. The changes in the expression and distribution of CXCL12 and its receptors are tightly associated with the pathological process of demyelination in multiple sclerosis (MS), suggesting that modulating the CXCL12/CXCR4/CXCR7 axis may benefit myelin repair by enhancing OPC recruitment and differentiation. This review aims to integrate the current findings of the CXCL12/CXCR4/CXCR7 signaling pathway in the CNS and to highlight its role in oligodendrocyte development and demyelinating diseases. Furthermore, this review provides potential therapeutic strategies for myelin repair by analyzing the relevance between the pathological changes and the regulatory roles of CXCL12/CXCR4/CXCR7 during MS.

WNK1 is involved in Nogo66 inhibition of OPC differentiation

LINGO-1 is a transmembrane receptor expressed primarily in the central nervous system (CNS) and plays an important role in myelination. Recent studies have indicated that it is also involved in oligodendrocyte precursor cell (OPC) survival and differentiation; however, the downstream signaling pathway underlying OPC development is unknown. In our previous study, we found that LINGO-1 is associated with WNK1 in mediating Nogo-induced neurite extension inhibition by RhoA activation. In an effort to identify the role of LINGO-1-WNK1 in OPCs, we first confirmed that WNK1 is also expressed in OPCs and co-localized with LINGO-1, which suppresses WNK1 expression by RNA interference-attenuated Nogo66-induced inhibition of OPC differentiation. Furthermore, we mapped the WNK1 kinase domain using several fragmented peptides to identify the key region of interaction with LINGO-1. We found that a sequence corresponding to the D6 peptide is necessary for the interaction. Finally, we found that using the TAT-D6 peptide to introduce D6 peptide into primary cultured OPC inhibits the association between LINGO-1 and WNK1 and significantly attenuates Nogo66-induced inhibition of OPC differentiation. Taken together, our results show that WNK1, via a specific region on WNK1 kinase domain, interacts with LINGO-1, thus mediating Nogo66-inhibited OPC differentiation.

Interactions and tradeoffs between cell recruitment, proliferation, and differentiation affect CNS regeneration

Regeneration of central nervous system (CNS) lesions requires movement of progenitor cells and production of their differentiated progeny. Although damage to the CNS clearly promotes these two processes, the interplay between these complex events and how it affects a response remains elusive. Here, we use spatial stochastic modeling to show that tradeoffs arise between production and recruitment during regeneration. Proper spatial control of cell cycle timing can mitigate these tradeoffs, maximizing recruitment, improving infiltration into the lesion, and reducing wasteful production outside of it. Feedback regulation of cell lineage dynamics alone however leads to spatial defects in cell recruitment, suggesting a novel, to our knowledge, hypothesis for the aggregation of cells to the periphery of a lesion in multiple sclerosis. Interestingly, stronger chemotaxis does not correct this aggregation and instead, substantial random cell motions near the site of the lesion are required to improve CNS regeneration.

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.

Oligodendrogenesis from neural stem cells: perspectives for remyelinating strategies

Mobilization of remyelinating cells spontaneously occurs in the adult brain. These cellular resources are specially active after demyelinating episodes in early phases of multiple sclerosis (MS). Indeed, oligodendrocyte precursor cells (OPCs) actively proliferate, migrate to and repopulate the lesioned areas. Ultimately, efficient remyelination is accomplished when new oligodendrocytes reinvest nude neuronal axons, restoring the normal properties of impulse conduction. As the disease progresses this fundamental process fails. Multiple causes seem to contribute to such transient decline, including the failure of OPCs to differentiate and enwrap the vulnerable neuronal axons. Regenerative medicine for MS has been mainly centered on the recruitment of endogenous self-repair mechanisms, or on transplantation approaches. The latter commonly involves grafting of neural precursor cells (NPCs) or neural stem cells (NSCs), with myelinogenic potential, in the injured areas. Both strategies require further understanding of the biology of oligodendrocyte differentiation and remyelination. Indeed, the success of transplantation largely depends on the pre-commitment of transplanted NPCs or NSCs into oligodendroglial cell type, while the endogenous differentiation of OPCs needs to be boosted in chronic stages of the disease. Thus, much effort has been focused on finding molecular targets that drive oligodendrocytes commitment and development. The present review explores several aspects of remyelination that must be considered in the design of a cell-based therapy for MS, and explores more deeply the challenge of fostering oligodendrogenesis. In this regard, we discuss herein a tool developed in our research group useful to search novel oligodendrogenic factors and to study oligodendrocyte differentiation in a time- and cost-saving manner.

Remyelination therapy for multiple sclerosis

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

Electromagnetic field stimulation potentiates endogenous myelin repair by recruiting subventricular neural stem cells in an experimental model of white matter demyelination

Electromagnetic fields (EMFs) may affect the endogenous neural stem cells within the brain. The aim of this study was to assess the effects of EMFs on the process of toxin-induced demyelination and subsequent remyelination. Demyelination was induced using local injection of lysophosphatidylcholine within the corpus callosum of adult female Sprague-Dawley rats. EMFs (60 Hz; 0.7 mT) were applied for 2 h twice a day for 7, 14, or 28 days postlesion. BrdU labeling and immunostaining against nestin, myelin basic protein (MBP), and BrdU were used for assessing the amount of neural stem cells within the tissue, remyelination patterns, and tracing of proliferating cells, respectively. EMFs significantly reduced the extent of demyelinated area and increased the level of MBP staining within the lesion area on days 14 and 28 postlesion. EMFs also increased the number of BrdU- and nestin-positive cells within the area between SVZ and lesion as observed on days 7 and 14 postlesion. It seems that EMF potentiates proliferation and migration of neural stem cells and enhances the repair of myelin in the context of demyelinating conditions.

Stem cell transplantation in multiple sclerosis: current status and future prospects

This article provides an overview of the current knowledge relating to the potential use of transplanted stem cells in the treatment of patients with multiple sclerosis (MS). Two types of stem cells, CNS-derived neural stem/precursor cells (NPCs) and bone marrow-derived mesenchymal stem cells (MSCs) are considered to provide reproducible and robust therapeutic effects when intravenously or intrathecally injected into both rodents and primates with experimental autoimmune encephalomyelitis. Furthermore, preliminary safety data concerning the use of intrathecally injected autologous MSCs in patients with progressive MS are available. We discuss how the data gathered to date challenge the narrow view that the therapeutic effects of NPCs and MSCs observed in the treatment of MS are accomplished solely by cell replacement. Both types of stem cell, when transplanted systemically, might instead influence disease outcome by releasing a plethora of factors that are immunomodulatory or neuroprotective, thereby directly or indirectly influencing the regenerative properties of intrinsic CNS stem/precursor cells.

Recovery from chronic demyelination by thyroid hormone therapy: myelinogenesis induction and assessment by diffusion tensor magnetic resonance imaging

The failure of the remyelination processes in multiple sclerosis contributes to the formation of chronic demyelinated plaques that lead to severe neurological deficits. Long-term cuprizone treatment of C57BL/6 mice resulted in pronounced white matter pathology characterized by oligodendrocyte depletion, irreversible demyelination and persistent functional deficits after cuprizone withdrawal. The use of a combination of in vivo diffusion tensor magnetic resonance imaging (DT-MRI) and histological analyses allowed for an accurate longitudinal assessment of demyelination. Injection of triiodothyronine (T(3)) hormone over a 3 week interval after cuprizone withdrawal progressively restored the normal DT-MRI phenotype accompanied by an improvement of clinical signs and remyelination. The effects of T(3) were not restricted to the later stages of remyelination but increased the expression of sonic hedgehog and the numbers of Olig2(+) and PSA-NCAM(+) precursors and proliferative cells. Our findings establish a role for T(3) as an inducer of oligodendrocyte progenitor cells in adult mouse brain following chronic demyelination.

Myelin repair strategies: a cellular view

PURPOSE OF REVIEW

The development of successful myelin repair strategies depends on the detailed knowledge of the cellular and molecular processes underlying demyelination and remyelination in the central nervous system of animal models and in patients with multiple sclerosis (MS). Based on the complexity of the demyelination and remyelination processes, it should be expected that effective therapeutic approaches will require a combination of strategies for immunomodulation, neuroprotection, and myelin replacement. This brief review highlights recent cellular and molecular findings and indicates that future therapeutic strategies to enhance remyelination may also require combinatorial treatment to accomplish.

RECENT FINDINGS

The relapsing-remitting course of some forms of multiple sclerosis has typically fueled hope for effective repair of multiple sclerosis lesions, if demyelinating activity could be attenuated. Recent findings support the potential of endogenous neural stem cells and progenitor cells to generate remyelinating oligodendrocytes. Importantly, interactions with viable axons and supportive astrocytic responses are required for endogenous immature cells to fulfill their potential remyelinating capacity.

SUMMARY

The research described here will help in identifying the major obstacles to effective remyelination and potential therapeutic targets to guide development of comprehensive approaches for testing in animal models and eventual treatment of patients with multiple sclerosis.

Glial precursor cell transplantation therapy for neurotrauma and multiple sclerosis

Traumatic injury to the brain or spinal cord and multiple sclerosis (MS) share a common pathophysiology with regard to axonal demyelination. Despite advances in central nervous system (CNS) repair in experimental animal models, adequate functional recovery has yet to be achieved in patients in response to any of the current strategies. Functional recovery is dependent, in large part, upon remyelination of spared or regenerating axons. The mammalian CNS maintains an endogenous reservoir of glial precursor cells (GPCs), capable of generating new oligodendrocytes and astrocytes. These GPCs are upregulated following traumatic or demyelinating lesions, followed by their differentiation into oligodendrocytes. However, this innate response does not adequately promote remyelination. As a result, researchers have been focusing their efforts on harvesting, culturing, characterizing, and transplanting GPCs into injured regions of the adult mammalian CNS in a variety of animal models of CNS trauma or demyelinating disease. The technical and logistic considerations for transplanting GPCs are extensive and crucial for optimizing and maintaining cell survival before and after transplantation, promoting myelination, and tracking the fate of transplanted cells. This is especially true in trials of GPC transplantation in combination with other strategies such as neutralization of inhibitors to axonal regeneration or remyelination. Overall, such studies improve our understanding and approach to developing clinically relevant therapies for axonal remyelination following traumatic brain injury (TBI) or spinal cord injury (SCI) and demyelinating diseases such as MS.

From stem cells to oligodendrocytes: prospects for brain therapy

Multiple sclerosis is an autoimmune disease that destroys myelin-forming oligodendrocytes of the CNS. While the damage can be partially controlled using anti-inflammatory cytokines and steroids, endogenous repair is insufficient to replace lost cells. Until now cell replenishment (transplant therapy) has been viewed as unlikely to succeed due to allograft rejection in this sensitized immune environment. However, advances in stem cell biology give new hope for deriving patient-specific, autologous oligodendrocytes which may tip the balance to favor repair. The challenge will be to engineer these cells to respond to cues that can target their migration into lesions for brain and spinal cord repair.

Temporal-spatial dynamics in oligodendrocyte and glial progenitor cell numbers throughout ventrolateral white matter following contusion spinal cord injury

The aim of this study was to systematically determine the spatial distribution of oligodendrocytes (CC1(+)) and glial progenitor cells (NG2(+)) throughout the ventral lateral funiculi (VLF) of adult rat thoracic spinal cord white matter over the course of spontaneous hindlimb locomotor recovery following moderate contusion injury. We used the optical fractionator technique to establish an unbiased estimate of total CC1(+) and NG2(+) cell numbers throughout equivalent segments of VLF from normal and injured spinal cords at designated time points. The results demonstrated a greater than 60% loss of oligodendrocytes and a 50% increase in NG2(+) cells 2 days postinjury. Subsequently, there was a significant increase in oligodendrocytes 7 days postinjury that continued throughout the time course of our experiments (42 days) when the total numbers recovered to 80% of controls. Conversely, NG2(+) cell numbers progressively declined after 2 days postinjury but remained significantly higher than controls throughout the experiments. The pattern of acute loss and repopulation of oligodendrocytes in the VLF paralleled the initial recovery of hindlimb weight-bearing function. Whether such improvement is directly related to NG2(+) cell differentiation into functional oligodendrocytes is uncertain. However, of critical importance is that significant cellular dynamics occurred primarily distal to the injury, and these changes were mirrored by significant alterations in the expression of mature myelin proteins. This infers that site-specific genetic or cellular interventions designed to enhance locomotor recovery by fostering remyelination of spared and/or newly established relay circuits may need to target not only the injury site but also rostrocaudal regions.

The molecular orchestra of the migration of oligodendrocyte precursors during development

During development of the central nervous system (CNS), postmitotic cells (including neurons and myelin-generating cells, the oligodendrocytes) migrate from the germinal areas of the neural tube where they originate to their final destination sites. The migration of neurons during development has been extensively studied and has been the topic of detailed reviews. The migration of oligodendrocyte precursor cells (OPCs) is also an extremely complex and precise event, with a widespread migration of OPCs across many regions to colonize the entire CNS. Different mechanisms have been shown to direct the migration of OPCs, among them contact-mediated mechanisms (adhesion molecules) and long-range cues (chemotropic molecules). This review provides a detailed overview and discussion of the cellular and molecular basis of OPCs migration during development. Because it has been shown that neuronal and oligodendroglial lineages share some of these mechanisms, we briefly summarize similarities and differences between these two types of neural cells. We also summarize the changes in the normal migration of OPCs during development that would be relevant for different neurological diseases (including demyelinating diseases, such as multiple sclerosis, and glial cancers). We also examine the relevance of these migratory properties of the oligondendrocytic cell lineage for the repair of neural damage.

Human bone marrow stromal cell treatment improves neurological functional recovery in EAE mice

We investigated the treatment of remitting-relapsing experimental autoimmune encephalomyelitis (EAE) in mice with human bone marrow stromal cells (hBMSCs). hBMSCs were injected intravenously into EAE mice upon onset of paresis. Neurological functional tests were scored daily by grading clinical signs (score 0-5). Immunohistochemistry was performed to measure the transplanted hBMSCs, cell proliferation (bromodeoxyuridine, BrdU), oligodendrocyte progenitor cells (NG2), oligodendrocytes (RIP), and brain-derived neurotrophic factor (BDNF). The maximum clinical score and the average clinical scores were significantly decreased in the hBMSC-transplanted mice compared to the phosphate-buffered-saline-treated EAE controls, indicating a significant improvement in function. Demyelination significantly decreased, and BrdU(+) and BDNF(+) cells significantly increased in the hBMSC-treated mice compared to controls. Some BrdU(+) cells were colocalized with NG2(+) and RIP(+) immunostaining. hBMSCs also significantly reduced the numbers of vessels containing inflammatory cell infiltration. These data indicate that hBMSC treatment improved functional recovery after EAE in mice, possibly, via reducing inflammatory infiltrates and demyelination areas, stimulating oligodendrogenesis, and by elevating BDNF expression.

Stem cell therapy for central nervous system demyelinating disease

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

Is there a future for neural transplantation?

Traditionally neural transplantation has had as its central tenet the replacement of missing neurons that have been lost because of neurodegenerative processes, as exemplified by diseases such as Parkinson disease (PD). However, the effectiveness and widespread application of this approach clinically has been limited, primarily because of the poor donor supply of human fetal neural tissue and the incomplete neurobiological understanding of the circuit reconstruction required to normalize function in these diseases. So, in PD the progress from promising neural transplantation in animal models to proof-of-principle, open-labeled clinical transplants, to randomized, placebo-controlled studies of neural transplantation has not been straightforward. The emergence of previously undescribed adverse effects and lack of significant functional advantage in recent clinical studies has been disappointing and has served to cast a new, and perhaps more realistic, perspective on this treatment approach. In fact, there have been calls by some involved in neural transplantation to return to the drawing board before pressing on with further clinical trials, and the return to basic experimentation. This therefore precipitates the question - is there a future for neural transplantation? It is important to remember that there are a number of possible explanations for the disappointing results from the recent clinical trials in PD, ranging from the mode of transplantation to patient selection. Nevertheless, almost irrespective of these reasons for the current trial results, there have always been significant practical and ethical problems with using human fetal tissue, and so a number of alternative cell sources have been investigated. These alternative sources include stem cells, which are attractive for cell-based therapies because of their potential ease of isolation, propagation and manipulation, and their ability in some cases to migrate to areas of pathology and differentiate into specific and appropriate cell types. Furthermore, the availability of stem cells derived from non-embryonic sources (e.g. adult stem cells derived from the sub-ventricular zone) has removed some of the ethical limitations associated with the use of embryonic human tissue. These potentially beneficial aspects of stem cells means that there is a future for neural transplantation as a means of treating patients with a range of neurological disorders, although whether this will ever translate into a truly effective, widely available therapy remains unknown.

Cell-based remyelinating therapies in multiple sclerosis: evidence from experimental studies

PURPOSE OF REVIEW

Spontaneous remyelination occurs in the central nervous system of patients with multiple sclerosis. However, this process is not robust enough to promote a functional and stable recovery of the myelin architecture. The development of cell-based therapies, aimed at promoting multifocal remyelination, is therefore foreseen.

RECENT FINDINGS

Several experimental cell-based strategies aimed at replacing damaged myelin-forming cells have been developed in the last few years. However, most of these therapeutic approaches - although consistently able to form new myelin sheaths at the transplantation site - are unfeasible owing to the mutifocality of the demyelinating process in multiple sclerosis patients and the inability to grow and produce large numbers of differentiated myelin-forming cells in vitro. Stem cell-based therapies that partially overcome these limitations have been proposed recently.

SUMMARY

Stem cell-based remyelinating therapies can be considered a plausible alternative strategy in immune-mediated demyelinating disorders. However, before any potential applications in patients with multiple sclerosis can be envisaged, it is necessary to confront the following preliminary, and still unsolved, questions: (1) the ideal stem cell source for transplantation; (2) the most appropriate route of stem cell administration; and, last but not least, (3) the best approach for achieving an appropriate, functional and long-lasting integration of transplanted stem cells into the host tissue.

Early stages of oligodendrocyte development in the embryonic murine spinal cord proceed normally in the absence ofHoxa2

Recent discoveries have enhanced our knowledge of the transcriptional control of oligodendrocyte (OG) development. In particular, the transcription factors (TFs) Olig2, Pax6, and Nkx2.2 have been shown to be important in the specification and/or maturation of the OG lineage. Although numerous other TFs are expressed by OGs, little is known regarding their role(s) in oligodendrogenesis. One such TF is the homeobox gene Hoxa2, which was recently shown to be expressed by O4(+) pro-oligodendrocytes. The objectives of this study were to examine the expression of Hoxa2 during the early stages of OG development, as well as to determine whether Hoxa2 is required for specification and/or early maturation of OGs. Immunocytochemical analysis of primary mixed glial cultures demonstrated that Hoxa2 was expressed throughout oligodendrogenesis, diminishing only with the acquisition of a myelinating phenotype. Serial transverse spinal cord sections from embryonic days 12.5, 14.25, 16, and 18 Hoxa2(+/+), Hoxa2(+/-), and Hoxa2(-/-) mice were subjected to single and double immunohistochemical analysis in order to examine Hoxa2, Olig2, Nkx2.2, and Pax6 expression profiles. Results obtained from Hoxa2(+/+) and Hoxa2(+/-) mice suggested that Hoxa2 was expressed by migratory oligodendroglial cells. In addition, comparison of spinal cord sections obtained from Hoxa2(+/+), Hoxa2(+/-), and Hoxa2(-/-) mice suggested that specification and early maturation of OGs proceeded normally in the absence of Hoxa2, since there were no obvious alterations in the expression patterns of Olig2, Nkx2.2, and/or Pax6. Hence, although Hoxa2 is expressed throughout OG development, it does not appear to be critical for early stages of oligodendrogenesis in the murine spinal cord.

Decline in rate of colonization of oligodendrocyte progenitor cell (OPC)-depleted tissue by adult OPCs with age

Rates of remyelination decline with age and this has been attributed to slower recruitment of oligodendrocyte progenitor cells (OPCs) into areas of demyelination and slower differentiation of OPCs into remyelinating oligodendrocytes. When considering causes for reduced recruitment rates, intrinsic causes (alterations in biological properties of OPCs) need to be separated from extrinsic causes (age-related differences in the lesion environment). Using 40 Gy of X-irradiation to deplete tissue of its endogenous OPC-population, we examined the effects of age on the rate at which adult rat OPCs colonize OPC-depleted tissue. We found a significant reduction in the rate of colonization between 2 and 10 months of age (0.6 mm/week versus 0.38 mm/week). To determine if this represented an intrinsic property of OPCs or was due to changes in the environment that the cells were recolonizing, OPCs from 10-month-old animals were transplanted into 2-month-old hosts and OPCs from 2-month-old animals were transplanted into 10-month-old hosts. These experiments showed that the transplanted OPCs retained their age-related rate of colonization, indicating that the decline in colonizing rates of OPCs with age reflects an intrinsic property of OPCs. This age-related decline in the ability of OPCs to repopulate OPC-depleted tissue has implications for understanding remyelination failure in multiple sclerosis (MS) and developing therapies for remyelination failure.

Injection of adult neurospheres induces recovery in a chronic model of multiple sclerosis

Widespread demyelination and axonal loss are the pathological hallmarks of multiple sclerosis. The multifocal nature of this chronic inflammatory disease of the central nervous system complicates cellular therapy and puts emphasis on both the donor cell origin and the route of cell transplantation. We established syngenic adult neural stem cell cultures and injected them into an animal model of multiple sclerosis--experimental autoimmune encephalomyelitis (EAE) in the mouse--either intravenously or intracerebroventricularly. In both cases, significant numbers of donor cells entered into demyelinating areas of the central nervous system and differentiated into mature brain cells. Within these areas, oligodendrocyte progenitors markedly increased, with many of them being of donor origin and actively remyelinating axons. Furthermore, a significant reduction of astrogliosis and a marked decrease in the extent of demyelination and axonal loss were observed in transplanted animals. The functional impairment caused by EAE was almost abolished in transplanted mice, both clinically and neurophysiologically. Thus, adult neural precursor cells promote multifocal remyelination and functional recovery after intravenous or intrathecal injection in a chronic model of multiple sclerosis.

Transplanted multipotential neural precursor cells migrate into the inflamed white matter in response to experimental autoimmune encephalomyelitis

Transplanted neural precursor cells remyelinate efficiently acutely demyelinated focal lesions. However, the clinical value of cell transplantation in a chronic, multifocal disease like multiple sclerosis will depend on the ability of transplanted cells to migrate to the multiple disease foci in the brain. Here, we expanded newborn rat neural precursor cells in spheres and transplanted them intracerebroventricularly or intrathecally in rats. The cells were labeled by the nuclear fluorescent dye Hoechst or by incubation with BrdU to enable their identification at 2 days and 2 weeks after transplantation, respectively. Spheres consisted of PSA-NCAM(+), nestin(+), NG2(-) undifferentiated precursor cells that differentiated in vitro into astrocytes, oligodendrocytes, and neurons. Spheres that were transplanted into intact rats remained mostly in the ventricles or in the spinal subarachnoid space. Following transplantation at peak of experimental autoimmune encephalomyelitis, cells migrated into the brain or spinal cord parenchyma, exclusively into inflamed white matter but not into adjacent gray matter regions. After 2 weeks, many transplanted cells had migrated into distant white matter tracts and acquired specific markers of the astroglial and oligodendroglial lineages. Thus, the inflammatory process may attract targeted migration of transplanted precursor cells into the brain parenchyma.

Remyelinating strategies for the treatment of multiple sclerosis

Demyelination is the pathological hallmark of multiple sclerosis (MS) lesions. The concept of remyelination has gained acceptance in recent years, but naturally occurring remyelination is incomplete. To improve repair processes, a number of strategies have been explored experimentally and clinical trials are being carried out. In principle, remyelination can be achieved by either promoting endogenous repair mechanisms or by providing an exogenous source of myelinating cells via transplantation. Both approaches have been successful in animal models of demyelination. Besides, many studies have elucidated principal mechanisms of oligodendrocyte biology and remyelination in the central nervous system (CNS). This progress in knowledge also allowed for more specific interventions. First clinical trials to enhance endogenous remyelination have been performed, unfortunately with disappointingly negative results. This illustrates that experimental data cannot be easily transferred to human disease, and more detailed knowledge on the regulatory mechanisms of remyelination in MS is required. Recently, the first MS patient received a transplant of autologous Schwann cells. Many other cell types are being studied experimentally, including stem cells. Despite the ethical problems associated with an embryonic cell source, new developments in stem cell biology indicate that adult stem cells or bone marrow-derived cells may substitute for embryonic cells in the future. In this review, we describe the current views on oligodendrocyte biology, myelination and remyelination, and focus on recent developments leading to reconstructing, remyelinating strategies in MS.

N-cadherin influences migration of oligodendrocytes on astrocyte monolayers

Oligodendrocyte cell migration is required for the development of the nervous system and the repopulation of demyelinated lesions in the adult central nervous system. We have investigated the role of the calcium-dependent adhesion molecules, the cadherins, in oligodendrocyte-astrocyte interaction and oligodendrocyte progenitor migration. Immunostaining demonstrated the expression of N-cadherin on the surfaces of both oligodendrocytes and astrocytes, and oligodendrocyte-like cells adhered to and spread on N-cadherin substrates. The blocking of cadherin function by antisera or specific peptides reduced adhesion of oligodendroglia to astrocyte monolayers, diminished contact time between oligodendrocyte processes and individual astrocytes, and significantly increased the migration of oligodendrocyte-like cells on astrocyte monolayers. Furthermore, a soluble cadherin molecule without adhesive properties increased oligodendroglial proliferation on various extracellular matrix substrates. These data suggest that cadherins are at least partially responsible for the poor migration-promoting properties of astrocytes and that decreasing cell-cell adhesion might effect repopulation of demyelinated multiple sclerosis lesions by oligodendrocyte progenitors.

Purification and analysis of in vivo-differentiated oligodendrocytes expressing the green fluorescent protein

A complete understanding of the molecular mechanisms involved in the formation and repair of the central nervous system myelin sheath requires an unambiguous identification and isolation of in vivo-differentiated myelin-forming cells. In order to develop a novel tool for the analysis of in vivo-differentiated oligodendrocytes, we generated transgenic mice expressing a red-shifted variant of the green fluorescent protein under the control of the proteolipid protein promoter. We demonstrate here that green fluorescent protein-derived fluorescence in the central nervous system of 9-day- to 7-week-old mice is restricted to mature oligodendrocytes, as determined by its spatiotemporal appearance and by both immunocytochemical and electrophysiological criteria. Green fluorescent protein-positive oligodendrocytes could easily be visualized in live and fixed tissue. Furthermore, we show that this convenient and reliable identification now allows detailed physiological analyses of differentiated oligodendrocytes in situ. In addition, we developed a novel tissue culture system for in vivo-differentiated oligodendrocytes. Initial data using this system indicate that, for oligodendrocytes isolated after differentiation in vivo, as yet unidentified factors secreted by astrocytes are necessary for survival and/or reappearance of a mature phenotype in culture.

Remyelination: cellular and gene therapy

Dysfunctional myelination or oligodendroglial abnormalities play a prominent role in a vast array of pediatric neurological diseases of genetic, inflammatory, immunological, traumatic, ischemic, developmental, metabolic, and infectious causes. Recent advances in glial cell biology have suggested that effective remyelination strategies may, indeed, be feasible. Evidence for myelin repair is accumulating in various experimental models of dysmyelinating and demyelinating disease. Attempts at remyelination have either been directed towards creating myelin de novo from exogenous sources of myelin-elaborating cells or promoting an intrinsic spontaneous remyelinating process. Ultimately, some disorders of myelin may require multiple repair strategies, not only the replacement of dysfunctional cells (oligodendroglia) but also the delivery or supplementation of gene products (i.e., growth factors, immune modulators, metabolic enzymes). Although primary oligodendrocytes or oligodendroglial precursors may be effective for glial cell replacement in certain discrete regions and circumstances and although various genetic vectors may be effective for the delivery of therapeutic molecules, multipotent neural stem cells may be most ideally suited for both gene transfer and cell replacement on transplantation into multiple regions of the central nervous system under a wide range of pathological conditions. We propose that, by virtue of their inherent biological properties, neural stem cells possess the multifaceted therapeutic capabilities that many diseases characterized by myelin dysfunction in the pediatric population may demand.