Cell-cell interactions during the migration of myelin-forming cells transplanted in the demyelinated spinal cord

Reciprocal Interactions between Oligodendrocyte Precursor Cells and the Neurovascular Unit in Health and Disease

Oligodendrocyte precursor cells (OPCs) are mostly known for their capability to differentiate into oligodendrocytes and myelinate axons. However, they have been observed to frequently interact with cells of the neurovascular unit during development, homeostasis, and under pathological conditions. The functional consequences of these interactions are largely unclear, but are increasingly studied. Although OPCs appear to be a rather homogenous cell population in the central nervous system (CNS), they present with an enormous potential to adapt to their microenvironment. In this review, it is summarized what is known about the various roles of OPC-vascular interactions, and the circumstances under which they have been observed.

Four Seasons for Schwann Cell Biology, Revisiting Key Periods: Development, Homeostasis, Repair, and Aging

Like the seasons of the year, all natural things happen in stages, going through adaptations when challenged, and Schwann cells are a great example of that. During maturation, these cells regulate several steps in peripheral nervous system development. The Spring of the cell means the rise and bloom through organized stages defined by time-dependent regulation of factors and microenvironmental influences. Once matured, the Summer of the cell begins: a high energy stage focused on maintaining adult homeostasis. The Schwann cell provides many neuron-glia communications resulting in the maintenance of synapses. In the peripheral nervous system, Schwann cells are pivotal after injuries, balancing degeneration and regeneration, similarly to when Autumn comes. Their ability to acquire a repair phenotype brings the potential to reconnect axons to targets and regain function. Finally, Schwann cells age, not only by growing old, but also by imposed environmental cues, like loss of function induced by pathologies. The Winter of the cell presents as reduced activity, especially regarding their role in repair; this reflects on the regenerative potential of older/less healthy individuals. This review gathers essential information about Schwann cells in different stages, summarizing important participation of this intriguing cell in many functions throughout its lifetime.

The Impact of Fixation on the Detection of Oligodendrocyte Precursor Cell Morphology and Vascular Associations

Oligodendrocyte precursor cells (OPCs) display numerous protrusions that extend into the surrounding parenchyma in the brain. Depending on the preparation of the tissue analyzed, these protrusions are more or less visible. We applied six different fixation methods and compared the effect of prolonged and stronger fixation on fluorescence intensity of platelet-derived growth factor receptor alpha, a surface marker of OPCs. Importantly, the fluorescence signal is mostly lost on protrusions as compared to the cell body, which has to be considered for specific analyses. Additionally, we show numerous contacts established between OPCs and the brain vasculature, which will contribute to the understanding of the interactions between these two elements.

Schwann cells: Rescuers of central demyelination

The presence of peripheral myelinating cells in the central nervous system (CNS) has gained the neurobiologist attention over the years. Despite the confirmed presence of Schwann cells in the CNS in pathological conditions, and the long list of their beneficial effects on central remyelination, the cues that impede or allow Schwann cells to successfully conquer and remyelinate central axons remain partially undiscovered. A better knowledge of these factors stands out as crucial to foresee a rational therapeutic approach for the use of Schwann cells in CNS repair. Here, we review the diverse origins of Schwann cells into the CNS, both peripheral and central, as well as the CNS components that inhibit Schwann survival and migration into the central parenchyma. Namely, we analyze the astrocyte- and the myelin-derived components that restrict Schwann cells into the CNS. Finally, we highlight the unveiled mode of invasion of these peripheral cells through the central environment, using blood vessels as scaffolds to pave their ways toward demyelinated lesions. In short, this review presents the so far uncovered knowledge of this complex CNS-peripheral nervous system (PNS) relationship.

Blood vessels guide Schwann cell migration in the adult demyelinated CNS through Eph/ephrin signaling

Schwann cells (SC) enter the central nervous system (CNS) in pathophysiological conditions. However, how SC invade the CNS to remyelinate central axons remains undetermined. We studied SC migratory behavior ex vivo and in vivo after exogenous transplantation in the demyelinated spinal cord. The data highlight for the first time that SC migrate preferentially along blood vessels in perivascular extracellular matrix (ECM), avoiding CNS myelin. We demonstrate in vitro and in vivo that this migration route occurs by virtue of a dual mode of action of Eph/ephrin signaling. Indeed, EphrinB3, enriched in myelin, interacts with SC Eph receptors, to drive SC away from CNS myelin, and triggers their preferential adhesion to ECM components, such as fibronectin via integrinβ1 interactions. This complex interplay enhances SC migration along the blood vessel network and together with lesion-induced vascular remodeling facilitates their timely invasion of the lesion site. These novel findings elucidate the mechanism by which SC invade and contribute to spinal cord repair.

Myelin-Associated Glycoprotein Inhibits Schwann Cell Migration and Induces Their Death

Remyelination of CNS axons by Schwann cells (SCs) is not efficient, in part due to the poor migration of SCs into the adult CNS. Although it is known that migrating SCs avoid white matter tracts, the molecular mechanisms underlying this exclusion have never been elucidated. We now demonstrate that myelin-associated glycoprotein (MAG), a well known inhibitor of neurite outgrowth, inhibits rat SC migration and induces their death via γ-secretase-dependent regulated intramembrane proteolysis of the p75 neurotrophin receptor (also known as p75 cleavage). Blocking p75 cleavage using inhibitor X (Inh X), a compound that inhibits γ-secretase activity before exposing to MAG or CNS myelin improves SC migration and survival in vitro Furthermore, mouse SCs pretreated with Inh X migrate extensively in the demyelinated mouse spinal cord and remyelinate axons. These results suggest a novel role for MAG/myelin in poor SC-myelin interaction and identify p75 cleavage as a mechanism that can be therapeutically targeted to enhance SC-mediated axon remyelination in the adult CNS.SIGNIFICANCE STATEMENT Numerous studies have used Schwann cells, the myelin-making cells of the peripheral nervous system to remyelinate adult CNS axons. Indeed, these transplanted cells successfully remyelinate axons, but unfortunately they do not migrate far and so remyelinate only a few axons in the vicinity of the transplant site. It is believed that if Schwann cells could be induced to migrate further and survive better, they may represent a valid therapy for remyelination. We show that myelin-associated glycoprotein or CNS myelin, in general, inhibit rodent Schwann cell migration and induce their death via cleavage of the neurotrophin receptor p75. Blockade of p75 cleavage using a specific inhibitor significantly improves migration and survival of the transplanted Schwann cells in vivo.

Oligodendrocyte precursors migrate along vasculature in the developing nervous system

Oligodendrocytes myelinate axons in the central nervous system and develop from oligodendrocyte precursor cells (OPCs) that must first migrate extensively during brain and spinal cord development. We show that OPCs require the vasculature as a physical substrate for migration. We observed that OPCs of the embryonic mouse brain and spinal cord, as well as the human cortex, emerge from progenitor domains and associate with the abluminal endothelial surface of nearby blood vessels. Migrating OPCs crawl along and jump between vessels. OPC migration in vivo was disrupted in mice with defective vascular architecture but was normal in mice lacking pericytes. Thus, physical interactions with the vascular endothelium are required for OPC migration. We identify Wnt-Cxcr4 (chemokine receptor 4) signaling in regulation of OPC-endothelial interactions and propose that this signaling coordinates OPC migration with differentiation.

Youth and environmental enrichment generate serum exosomes containing miR-219 that promote CNS myelination

Although commonly considered a disease of white matter, gray matter demyelination is increasingly recognized as an important component of multiple sclerosis (MS) pathogenesis, particularly in the secondary progressive disease phase. Extent of damage to gray matter is strongly correlated to decline in memory and cognitive dysfunction in MS patients. Aging likewise occurs with cognitive decline from myelin loss, and age-associated failure to remyelinate significantly contributes to MS progression. However, recent evidence demonstrates that parabiotic exposure of aged animals to a youthful systemic milieu can promote oligodendrocyte precursor cell (OPC) differentiation and improve remyelination. In the current study, we focus on this potential for stimulating remyelination, and show it involves serum exosomes that increase OPCs and their differentiation into mature myelin-producing cells-both under control conditions and after acute demyelination. Environmental enrichment (EE) of aging animals produced exosomes that mimicked this promyelinating effect. Additionally, stimulating OPC differentiation via exosomes derived from environmentally enriched animals is unlikely to deplete progenitors, as EE itself promotes proliferation of neural stem cells. We found that both young and EE serum-derived exosomes were enriched in miR-219, which is necessary and sufficient for production of myelinating oligodendrocytes by reducing the expression of inhibitory regulators of differentiation. Accordingly, protein transcript levels of these miR-219 target mRNAs decreased following exosome application to slice cultures. Finally, nasal administration of exosomes to aging rats also enhanced myelination. Thus, peripheral circulating cells in young or environmentally enriched animals produce exosomes that may be a useful therapy for remyelination.

Schwann cell-free adult canine olfactory ensheathing cell preparations from olfactory bulb and mucosa display differential migratory and neurite growth-promoting properties in vitro

Background

Transplantation of olfactory ensheathing cells (OEC) and Schwann cells (SC) is a promising therapeutic strategy to promote axonal growth and remyelination after spinal cord injury. Previous studies mainly focused on the rat model though results from primate and porcine models differed from those in the rat model. Interestingly, canine OECs show primate-like in vitro characteristics, such as absence of early senescence and abundance of stable p75^NTR expression indicating that this species represents a valuable translational species for further studies. So far, few investigations have tested different glial cell types within the same study under identical conditions. This makes it very difficult to evaluate contradictory or confirmatory findings reported in various studies. Moreover, potential contamination of OEC preparations with Schwann cells was difficult to exclude. Thus, it remains rather controversial whether the different glial types display distinct cellular properties.

Results

Here, we established cultures of Schwann cell-free OECs from olfactory bulb (OB-OECs) and mucosa (OM-OECs) and compared them in assays to Schwann cells. These glial cultures were obtained from a canine large animal model and used for monitoring migration, phagocytosis and the effects on in vitro neurite growth. OB-OECs and Schwann cells migrated faster than OM-OECs in a scratch wound assay. Glial cell migration was not modulated by cGMP and cAMP signaling, but activating protein kinase C enhanced motility. All three glial cell types displayed phagocytic activity in a microbead assay. In co-cultures with of human model (NT2) neurons neurite growth was maximal on OB-OECs.

Conclusions

These data provide evidence that OB- and OM-OECs display distinct migratory behavior and interaction with neurites. OB-OECs migrate faster and enhance neurite growth of human model neurons better than Schwann cells, suggesting distinct and inherent properties of these closely-related cell types. Future studies will have to address whether, and how, these cellular properties correlate with the in vivo behavior after transplantation.

CNS-resident glial progenitor/stem cells produce Schwann cells as well as oligodendrocytes during repair of CNS demyelination

After central nervous system (CNS) demyelination-such as occurs during multiple sclerosis-there is often spontaneous regeneration of myelin sheaths, mainly by oligodendrocytes but also by Schwann cells. The origins of the remyelinating cells have not previously been established. We have used Cre-lox fate mapping in transgenic mice to show that PDGFRA/NG2-expressing glia, a distributed population of stem/progenitor cells in the adult CNS, produce the remyelinating oligodendrocytes and almost all of the Schwann cells in chemically induced demyelinated lesions. In contrast, the great majority of reactive astrocytes in the vicinity of the lesions are derived from preexisting FGFR3-expressing cells, likely to be astrocytes. These data resolve a long-running debate about the origins of the main players in CNS remyelination and reveal a surprising capacity of CNS precursors to generate Schwann cells, which normally develop from the embryonic neural crest and are restricted to the peripheral nervous system.

Ectopic expression of polysialylated neural cell adhesion molecule in adult macaque Schwann cells promotes their migration and remyelination potential in the central nervous system

Recent findings suggested that inducing neural cell adhesion molecule polysialylation in rodents is a promising strategy for promoting tissue repair in the injured central nervous system. Since autologous grafting of Schwann cells is one potential strategy to promote central nervous system remyelination, it is essential to show that such a strategy can be translated to adult primate Schwann cells and is of interest for myelin diseases. Adult macaque Schwann cells were transduced with a lentiviral vector encoding sialyltransferase, an enzyme responsible for neural cell adhesion molecule polysialylation. In vitro, we found that ectopic expression of polysialylate promoted adult macaque Schwann cell migration and improved their integration among astrocytes in vitro without modifying their antigenic properties as either non-myelinating or pro-myelinating. In addition, forced expression of polysialylate in adult macaque Schwann cells decreased their adhesion with sister cells. To investigate the ability of adult macaque Schwann cells to integrate and migrate in vivo, focally induced demyelination was targeted to the spinal cord dorsal funiculus of nude mice, and both control and sialyltransferase expressing Schwann cells overexpressing green fluorescein protein were grafted remotely from the lesion site. Analysis of the spatio-temporal distribution of the grafted Schwann cells performed in toto and in situ, showed that in both groups, Schwann cells migrated towards the lesion site. However, migration of sialyltransferase expressing Schwann cells was more efficient than that of control Schwann cells, leading to their accelerated recruitment by the lesion. Moreover, ectopic expression of polysialylated neural cell adhesion molecule promoted adult macaque Schwann cell interaction with reactive astrocytes when exiting the graft, and their ‘chain-like’ migration along the dorsal midline. The accelerated migration of sialyltransferase expressing Schwann cells to the lesion site enhanced their ability to compete for myelin repair with endogenous cells, while control Schwann cells were unable to do so. Finally, remyelination by the exogenous sialyltransferase expressing Schwann cells restored the normal distribution of paranodal and nodal elements on the host axons. These greater performances of sialyltransferase expressing Schwann cell correlated with their sustained expression of polysialylated neural cell adhesion molecule at early times when migrating from the graft to the lesion, and its progressive downregulation at later times during remyelination. These results underline the potential therapeutic benefit to genetically modify Schwann cells to overcome their poor migration capacity and promote their repair potential in demyelinating disorders of the central nervous system.

Transplantation of oligodendrocyte progenitor cells in animal models of leukodystrophies

Leukodystrophies represent a wide variety of hereditary disorders of the white matter in the central nervous system, where the patients, mostly in infancy or childhood, suffer from progressive and often fatal neurological symptoms due to either a delay or lack of myelin development or loss of myelin. As only supportive therapies are available for the majority of the leukodystrophies, replacing genetically defective oligodendrocytes with intact oligodendrocytes by transplantation has a potential as a curative therapy. Animal models of leukodystrophies have been valuable in developing effective strategies of myelin repair in human diseases. This chapter discusses the animal models of leukodystrophies and describes methods for (a) derivation of mouse oligodendrocyte progenitor cells (OPCs) in vitro as a source of donor myelin-forming cells and (b) transplantation of OPCs into the brain and spinal cord of mouse models of leukodystrophies.

Astrocytes in multiple sclerosis: a product of their environment

It has long been thought that astrocytes, like other glial cells, simply provide a support mechanism for neuronal function in the healthy and inflamed central nervous system (CNS). However, recent evidence suggests that astrocytes play an active and dual role in CNS inflammatory diseases such as multiple sclerosis (MS). Astrocytes not only have the ability to enhance immune responses and inhibit myelin repair, but they can also be protective and limit CNS inflammation while supporting oligodendrocyte and axonal regeneration. The particular impact of these cells on the pathogenesis and repair of an inflammatory demyelinating process is dependent upon a number of factors, including the stage of the disease, the type and microenvironment of the lesion, and the interactions with other cell types and factors that influence their activation. In this review, we summarize recent data supporting the idea that astrocytes play a complex role in the regulation of CNS autoimmunity.

Activation, Proliferation and Commitment of Endogenous Stem/Progenitor Cells to the Oligodendrocyte Lineage by TS1 in a Rat Model of Dysmyelination

Wild-type and myelin-deficient rats received a single intraparenchymal injection of TS1, a specific combination of IGF-1 and transferrin (Tf), into their corpus callosum at postnatal day 4. The fate of endogenous stem cells in the brain was examined by the expression of the stem cell marker nestin, together with Tf, neurofilaments and glial fibrillary acidic protein from 2 to 14 days after injection. Treated mutants lacked nestin expression in the ventricular wall and had an increase in nestin-labeled radial cell processes in the subventricular regions, and extended into the parenchyma. The subventricular zone was populated by healthy new oligodendrocytes (OLs). BrdU incorporation showed that these cells originated by proliferation and were identified as OLs based upon Tf expression. Thus, TS1 is an effective treatment to promote endogenous subventricular zone progenitor proliferation, migration and OL lineage specification. This strategy offers for the first time the possibility of myelin restoration to treat myelin disorders.

Cell therapy in demyelinating diseases

Multiple sclerosis presents particular and serious problems to those attempting to develop cell-based therapies: the occurrence of innumerable lesions scattered throughout the CNS, axon loss, astrocytosis, and a continuing inflammatory process, to name but a few. Nevertheless, the limited and relatively focused nature of damage to oligodendrocytes and myelin, at least in early disease, the large body of available knowledge concerning the biology of oligodendrocytes, and the success of experimental myelin repair, have allowed cautious optimism that therapies may be possible. Here, we review the clinical and biological problems presented by multiple sclerosis in the context of cell therapies, and the neuroscientific background to the development of strategies for myelin repair. We attempt to highlight those areas where difficulties have yet to be resolved and draw on a variety of more recent experimental findings to speculate on how remyelinating therapies are likely to develop in the foreseeable future.

N-cadherin differentially determines Schwann cell and olfactory ensheathing cell adhesion and migration responses upon contact with astrocytes

Olfactory ensheathing cells (OECs) and Schwann cells provide a cellular environment that promotes axonal outgrowth in several models of CNS injury. However, they exhibit different properties when in contact with astrocytes. Schwann cells, but not OECs, induce characteristics that typify hypertrophy in astrocytes and exhibit a poor capacity to migrate within astrocyte-rich areas, making them less favourable for transplant-mediated repair. N-cadherin has been implicated in the adhesion of Schwann cells to astrocytes. Despite indistinguishable expression of N-cadherin, Schwann cells adhered more strongly to an astrocyte monolayer and migrated more slowly on astrocytes when compared to OECs. We have examined the role of N-cadherin in mediating these cellular interactions using RNA interference and found differing effects. In Schwann cells, suppression of N-cadherin reduced heterotypic and homotypic adhesion and they gained adhesion properties more akin to OECs. In contrast, suppression of N-cadherin in OECs had no effect. These findings imply that N-cadherin is differentially regulated in OECs and Schwann cells.

Remyelination by transplanted olfactory ensheathing cells

The olfactory ensheathing cells (OECs) of the peripheral olfactory system associate with the axons of the first cranial nerve. These axons are not myelinated by OECs because of their very small diameter. However, when OECs are transplanted into areas where they encounter larger-diameter axons, such as in a model of primary demyelination, these cells assume a myelinating phenotype. Myelinating OECs very closely resemble myelinating Schwann cells by all criteria currently examined, including morphology, ultrastructure, biochemistry, and transcriptional regulation. Indeed, it is currently impossible to reliably distinguish myelinating OECs and myelinating Schwann cells that have been transplanted into experimental models of CNS demyelination. This article describes recent studies on the myelinating properties of transplanted OECs, focusing on their intrinsic myelinating potential and how this can be augmented by the presence of meningeal cells. The relative merits of OECs compared with Schwann cells when transplanted into astrocyte-containing lesions in the CNS are discussed together with their potential role in transplanted-mediated repair of demyelinating disease such as multiple sclerosis.

Cell transplantation, myelin repair, and multiple sclerosis

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

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Depletion of endogenous oligodendrocyte progenitors rather than increased availability of survival factors is a likely explanation for enhanced survival of transplanted oligodendrocyte progenitors in X-irradiated compared to normal CNS

Oligodendrocyte progenitors (OPs) survive and migrate following transplantation into adult rat central nervous system (CNS) exposed to high levels of X-irradiation but fail to do so if they are transplanted into normal adult rat CNS. In the context of developing OP transplantation as a potential therapy for repairing demyelinating diseases it is clearly of some importance to understand what changes have occurred in X-irradiated CNS that permit OP survival. This study addressed two alternative hypotheses. Firstly, X-irradiation causes an increase in the availability of OP survival factors, allowing the CNS to support a greater number of progenitors. Secondly, X-irradiation depletes the endogenous OP population thereby providing vacant niches that can be occupied by transplanted OPs. In situ hybridization was used to examine whether X-irradiation causes an increase in mRNA expression of five known OP survival factors, CNTF, IGF-I, PDGF-A, NT-3 and GGF-2. The levels of expression of these factors at 4 and 10 days following exposure of the adult rat spinal cord to X-irradiation remain the same as the expression levels in normal tissue. Using intravenous injection of horseradish peroxidase, no evidence was found of X-irradiation-induced change in blood-brain barrier permeability that might have exposed X-irradiated tissue to serum-derived survival factors. However, in support of the second hypothesis, a profound X-irradiation-induced decrease in the number of OPs was noted. These data suggest that the increased survival of transplanted OPs in X-irradiated CNS is not a result of the increases in the availability of the OP survival factors examined in this study but rather the depletion of endogenous OPs creating 'space' for transplanted OPs to integrate into the host tissue.

Olfactory ensheathing cells and Schwann cells differ in their in vitro interactions with astrocytes

Transplanted olfactory ensheathing cells (OECs) are able to remyelinate demyelinated axons and support regrowth of transected axons after transplantation into the adult CNS. Transplanted Schwann cells (SCs) share these repair properties but have limitations imposed on their behavior by the presence of astrocytes (ACs). Because OECs exist alongside astrocytes in the olfactory bulb, we have hypothesized that they have advantages over SCs in transplant-mediated CNS repair due to an increased ability to integrate and migrate within an astrocytic environment. In this study, we have tested this hypothesis by comparing the interactions between astrocytes and either SCs or OECs, using a range of in vitro assays. We have shown that (1) astrocytes and SCs segregate into defined non-overlapping domains in co-culture, whereas astrocytes and OECs freely intermingle; (2) both SCs and OECs will migrate across astrocyte monolayers, but only OECs will migrate into an area containing astrocytes; (3) SCs spend less time in contact with astrocytes than do OECs; and (4) astrocytes undergo hypertrophy when in contact with SCs, but not with OECs. Expression of N-cadherin has been implicated as a key mediator of the failure of SCs to integrate with astrocytes. However, we found no differences in the intensity of N-cadherin immunoreactivity between SCs and OECs, suggesting that it is not the adhesion molecule that accounts for the observed differences. In addition, the number of astrocytes expressing chondroitin sulfate proteoglycans (CSPG) is increased when astrocytes are co-cultured with Schwann cells compared with the number when astrocytes are grown alone or with OECs. Taken together, these data support the hypothesis that OECs will integrate more extensively than Schwann cells in astrocytic environments and are therefore better candidates for transplant-mediated repair of the damaged CNS.

Areas of demyelination do not attract significant numbers of schwann cells transplanted into normal white matter

If Schwann cell transplantation is to be used as a therapy for demyelinating disease, it is important to know if the number of transplanted cells and their transplantation site affects the extent of remyelination. Primary Schwann cell cultures were obtained from neonatal rat sciatic nerve, purified, and expanded using bovine pituitary extract and forskolin. Areas of persistent demyelination were created in the dorsal funiculus of the thoracolumbar spinal cord of rats by injecting ethidium bromide into white matter exposed to 40 Gy of X-irradiation, and a high and low number of Schwann cells were transplanted, into either the area of demyelination or the dorsal funiculus cranial to the area of demyelination. Animals were perfused 4 weeks after transplantation. After injection of 4 x 10(4) cells into the area of demyelination, the area of Schwann cell remyelination was 0.88 +/- 0.16 mm(2), while following the injection of 3 x 10(3) cells it was significantly smaller, 0.29 +/- 0.09 mm(2). After implantation of Schwann cells 1-3 mm (mean 2.5 mm) cranial to the area of demyelination, only one of the eight animals (a high-dose animal) showed extensive Schwann cell remyelination. In this animal, the cells were transplanted within 1 mm of the area of demyelination, well within the length of tissue over which cells are passively spread by the injection procedure (1-3 mm). Our results show that significant numbers of transplanted Schwann cells are not attracted through normal tissue to areas of demyelination and when transplanted into areas of demyelination the extent of myelination is related to the number of Schwann cells transplanted.

Schwann cells transplanted into normal and X-irradiated adult white matter do not migrate extensively and show poor long-term survival

Although Schwann cells are able to enter the central nervous system (CNS) when the integrity of the glia limitans is disrupted, their ability to migrate through intact CNS remains unclear. We have addressed this issue by transplanting lacZ-labeled Schwann cells into normal adult spinal cord white matter, and into X-irradiated spinal cord (an environment that, unlike normal spinal cord, permits the migration of transplanted oligodendrocyte progenitors). Schwann cell cultures, obtained from neonatal rat sciatic nerve and expanded using bovine pituitary extract and forskolin, were transfected by repeated exposure to retroviral vectors encoding the Escherichia coli lacZ gene. The normal behavior of the transduced cells was confirmed by transplantation into a nonrepairing area of demyelination in the spinal cord, where they formed myelin sheaths around demyelinated axons. A single microliter containing 4 x 10(4) cells was then transplanted into unlesioned normal and X-irradiated white matter of the spinal cord of adult syngeneic rats. One hour after injection, blue cells were observed as a discrete mass within the dorsal funiculus with a longitudinal distribution of 2-3 mm, indicating the extent of passive spread of the injected cells. At subsequent survival times (1, 2, and 4 weeks posttransplantation) blue cells had a distribution that was no more extensive than that seen 1 h after transplantation. However, the number of Schwann cells declined with time following transplantation such that at 4 weeks there were few surviving Schwann cells in both X-irradiated and nonirradiated spinal cord. These results indicate that transplanted Schwann cells do not migrate extensively and show poor long-term survival when introduced into a normal CNS environment.

Schwann cell remyelination is restricted to astrocyte-deficient areas after transplantation into demyelinated adult rat brain

The ability to generate large numbers of Schwann cells from a peripheral nerve biopsy makes them potential candidates for the clinical application of cell transplantation to enhance remyelination in human demyelinating disease. Transplant-derived Schwann cell remyelination has previously been demonstrated in the spinal cord but not for demyelinated axons in the brain, a more likely site for initial clinical intervention. We have transplanted Schwann cells from male neonatal rat sciatic nerves into ethidium bromide-induced areas of demyelination in the deep cerebellar white matter of adult female rats. The extent of Schwann cell remyelination 28 days after transplantation was significantly increased in lesions that received direct injections of Schwann cells compared with non-transplanted lesions. Using in situ hybridisation to identify the rat Y chromosome, transplanted male cells were found to co-localise with the P0 immunoreactive area of Schwann cell remyelination. Combined immunohistochemistry and in situ hybridisation confirmed that many remyelinating Schwann cells were transplant-derived. P0 immunoreactivity and transplanted male cells were found in GFAP-negative, astrocyte-free areas. Transplanted Schwann cells were not identified outside of transplanted lesions, nor did they did not contribute to remyelination of a lesion at a distance from the site of transplantation. Our findings indicate that demyelinated axons in the adult brain can be remyelinated by transplanted Schwann cells but that migration and remyelination are restricted to areas from which astrocytes are absent.

The regulation of proliferation and differentiation in oligodendrocyte progenitor cells by alphaV integrins

We have previously shown that oligodendrocyte progenitor cells exhibit developmental switching between alphav-associated beta integrin subunits to sequentially express alphavbeta1, alphavbeta3 and alphavbeta5 integrins during differentiation in vitro. To understand the role that alphavveta3 integrin may play in regulating oligodendrocyte progenitor cell behaviour, cells of the rat cell line, CG-4, were genetically engineered to constitutively express alphavbeta3 integrin by transfection with full-length human beta3 integrin subunit cDNA. Time-lapse videomicroscopy showed no effect of beta3 expression on cell migration but revealed enhanced proliferation on vitronectin substrata. Comparison of mitotic indices, as measured by 5-bromo-2′-deoxyuridine incorporation, confirmed that human beta3 integrin-expressing cells exhibited enhanced proliferation, as compared to both vector-only transfected, and wild-type CG-4 cells when switched to differentiation medium from growth medium, but only in cultures grown on vitronectin and not on poly-D-lysine. The effects on proliferation were inhibited by a function-blocking antibody specifically directed against the human beta3 integrin subunit. Human beta3 integrin-expressing cells also exhibited reduced differentiation. This differentiation could be reduced still further by a function-blocking monoclonal antibody against alphavbeta5 integrin, as could differentiation in the wild-type CG-4 cells. Taken together, these results suggest that alphavbeta3 integrin may regulate oligodendroglial cell proliferation and that both downregulation of alphavbeta3 integrin expression and signalling through alphavbeta5 integrin may be critical to continued differentiation in vitro.

The glial scar and central nervous system repair

Damage to the central nervous system (CNS) results in a glial reaction, leading eventually to the formation of a glial scar. In this environment, axon regeneration fails, and remyelination may also be unsuccessful. The glial reaction to injury recruits microglia, oligodendrocyte precursors, meningeal cells, astrocytes and stem cells. Damaged CNS also contains oligodendrocytes and myelin debris. Most of these cell types produce molecules that have been shown to be inhibitory to axon regeneration. Oligodendrocytes produce NI250, myelin-associated glycoprotein (MAG), and tenascin-R, oligodendrocyte precursors produce NG2 DSD-1/phosphacan and versican, astrocytes produce tenascin, brevican, and neurocan, and can be stimulated to produce NG2, meningeal cells produce NG2 and other proteoglycans, and activated microglia produce free radicals, nitric oxide, and arachidonic acid derivatives. Many of these molecules must participate in rendering the damaged CNS inhibitory for axon regeneration. Demyelinated plaques in multiple sclerosis consists mostly of scar-type astrocytes and naked axons. The extent to which the astrocytosis is responsible for blocking remyelination is not established, but astrocytes inhibit the migration of both oligodendrocyte precursors and Schwann cells which must restrict their access to demyelinated axons.

N-Cadherin inhibits Schwann cell migration on astrocytes

Astrocytes exclude Schwann cells (SCs) from the central nervous system (CNS) at peripheral nerve entry zones and restrict their migration after transplantation into the CNS. We have modeled the interactions between SCs, astrocytes, and fibroblasts in vitro. Astrocytes and SCs in vitro form separate territories, with sharp boundaries between them. SCs migrate poorly when placed on astrocyte monolayers, but migrate well on various other surfaces such as laminin (LN) and skin fibroblasts. Interactions between individual SCs and astrocytes result in long-lasting adhesive contacts during which the SC is unable to migrate away from the astrocyte. In contrast, SC interactions with fibroblasts are much shorter with less arrest of migration. SCs adhere strongly to astrocytes and other SCs, but less well to substrates that promote migration, such as LN and fibroblasts. SC-astrocyte and SC-SC adhesion is mediated by the calcium-dependent cell adhesion molecule N-cadherin. Inhibition of N-cadherin function by calcium withdrawal, peptides containing the classical cadherin cell adhesion recognition sequence His-Ala-Val, or antibodies directed against this sequence inhibit SC adhesion and increase SC migration on astrocytes. We suggest that N-cadherin-mediated adhesion to astrocytes inhibits the widespread migration of SCs in CNS tissue.

Transplanting myelin-forming cells into the central nervous system: principles and practice

Although transplantation of myelin-forming cells into the central nervous system (CNS) has recently attracted much attention as a potential therapy for repairing persistent demyelination found in the demyelinating diseases such as multiple sclerosis and the leukodystrophies, it is worth remembering that the technique was originally conceived of as an experimental technique for manipulating in vivo environments to study interactions between different cell types in either repair or development. It is in this capacity that the technique is still predominantly used. Nevertheless, information, both technical and biological, that the continued use of the technique yields also often provides material for assessing the feasibility of glial cell transplantation as a therapeutic procedure. In this article, we describe some of the guiding principles of transplantation of myelinogenic cells into the mammalian CNS, focusing initially on the recipient environment and then considering the donor material. The division of the discussion into recipient and donor is one of convenience since in reality the interactions between the two cannot be considered in isolation.

Tangential migration of ameboid microglia in the developing quail retina: mechanism of migration and migratory behavior

Long distance migration of microglial precursors within the central nervous system is essential for microglial colonization of the nervous parenchyma. We studied morphological features of ameboid microglial cells migrating tangentially in the developing quail retina to shed light on the mechanism of migration and migratory behavior of microglial precursors. Many microglial precursors remained attached on retinal sheets containing the inner limiting membrane covered by a carpet of Müller cell endfeet. This demonstrates that most ameboid microglial cells migrate tangentially on Müller cell endfeet. Many of these cells showed a central-to-peripheral polarized morphology, with extensive lamellipodia spreading through grooves flanked by Müller cell radial processes, to which they were frequently anchored. Low protuberances from the vitreal face of microglial precursors were firmly attached to the subjacent basal lamina, which was accessible through gaps in the carpet of Müller cell endfeet. These results suggest a mechanism of migration involving polarized extension of lamellipodia at the leading edge of the cell, strong cell-to-substrate attachment, translocation of the cell body forward, and retraction of the rear of the cell. Other ameboid cells were multipolar, with lamellipodial projections radiating in all directions from the cell body, suggesting that microglial precursors explore the surrounding environment to orient their movement. Central-to-peripheral migration of microglial precursors in the retina does not follow a straight path; instead, these cells perform forward, backward, and sideways movements, as suggested by the occurrence of (a) V-shaped bipolar ameboid cells with their vertex pointing toward either the center or the periphery of the retina, and (b) threadlike processes projecting from either the periphery-facing edge or the center-facing edge of ameboid microglial cells.

Transplantation of CG4 oligodendrocyte progenitor cells in the myelin-deficient rat brain results in myelination of axons and enhanced oligodendroglial markers

Transplantation of oligodendrocyte (Ol) progenitor cells into the central nervous system is a promising approach for the treatment of myelin disorders. This approach requires providing adequate numbers of healthy cells with myelinating potential. We recently showed the successful transplantation of Ol progenitors into the myelin-deficient (md) rat brain. In the present work, CG4 cells, a cell line with properties of Ol progenitors, were labeled with fast blue and grafted into P3-P5 pups born to carrier mothers. Examination of host brains 2 weeks posttransplant indicated that CG4 cells display a much more extensive migration capacity than their wild-type counterparts. These cells synthesized myelin components. In addition, ultrastructural analysis showed myelin formation along axons of md hosts in various brain regions, including corpus callosum, cerebellum, and brainstem. Furthermore, in situ hybridization studies performed on sagittal sections revealed extensive expression of transferrin-mRNA within the md host parenchyma. The high survival and functional features displayed by CG4 cells after transplantation, together with their striking wide distribution within the host parenchyma, as assessed by the presence of myelinated fibers in mutant hosts, emphasizes the importance of using highly motile and proliferative Ol progenitor cells. Strategies to improve the condition and life span of md rat pups are currently under investigation.

Repair of myelin disease: strategies and progress in animal models

Myelin disorders form an important group of human neurological diseases that are as yet incurable. Recent studies on experimental remyelination have suggested that it might be feasible to repair the CNS, either by transplanting normal myelinating cells or by enhancing endogenous repair. Progress in animal models, particularly in transplanting cells of the oligodendrocyte lineage, has resulted in significant focal remyelination and physiological evidence of restoration of function. These data suggest that focal lesions in multiple sclerosis could be repaired by the transplantation of myelin-forming cells. Future therapies could involve both transplantation and promotion of endogenous repair, and the two approaches could be combined with ex vivo manipulation of the donor tissue.

Inflammation promotes survival and migration of the CG4 oligodendrocyte progenitors transplanted in the spinal cord of both inflammatory and demyelinated EAE rats

Oligodendrocyte progenitor CG4 cells were labeled with bisbenzimide and transplanted in the lumbar spinal cord of rats 15 to 17 days prior to the induction of experimental autoimmune encephalomyelitis (EAE). EAE was induced by immunization with the encephalitogenic peptide of myelin basic protein (amino acids 68-88; C1) in adjuvant, either alone or in combination with a single injection of an anti-myelin oligodendrocyte glycoprotein (MOG) antibody to enhance central nervous system (CNS) demyelination. In control animals without EAE, the survival and migration capacity of CG4 cells was minimal. In striking contrast, both the survival and migration of this oligodendrocyte progenitor cell line were greatly enhanced in animals with EAE. In both disease models, large number of CG4 cells were still found in the spinal cord 50 days after transplantation, by which time they had migrated up to 6 cm from the transplantation site. Migrating CG4 cells were found in the subpial space, around the ependyma and blood vessels, and as well as in the grey and white matter of the CNS parenchyma. In all these locations, the CG4 cells were often associated with reactive astrocytes. These data strongly support the concept that inflammatory responses within the CNS promote, rather than inhibit, the survival and migration of transplanted oligodendrocyte progenitors in the adult CNS.

Schwann cell transplantation and myelin repair of the CNS

Studies with experimental models of dysmyelination and demyelination have shown that rodent Schwann cells including a Schwann cell line, transplanted in the central nervous system compete with host oligodendrocytes to remyelinate denuded central axons of the spinal cord. The myelin produced by transplanted SC around these central nervous system axons is structurally normal and restores, secure nerve conduction. In the presence of a favorable substrate, transplanted Schwann cells migrate over considerable distances (several mm) and are recruited by a demyelinated lesion which they will partially repair Thus Schwann cells, which can also support axonal growth, may be instrumental in central nervous system repair. In addition, the possibility of obtaining large quantities of human and non-human primate Schwann cells, makes it possible to consider autologous Schwann cell transplantation as a potential therapy for demyelinating or traumatic diseases. The various differences which may exist between rodents and humans, however, require further investigation of this possibility in a non-human primate model of demyelination. These experiments should provide not only insights on the potential of autologous transplantation in primates but also a better understanding of the process of central remyelination.

Inhibition of oligodendrocyte precursor motility by oligodendrocyte processes: implications for transplantation-based approaches to multiple sclerosis

Transplantation of oligodendrocyte precursor cells represents a promising approach to the treatment of the chronic demyelinated lesions of multiple sclerosis. In view of the multi-focal nature of the disease it will be necessary for the transplanted oligodendrocyte precursor cells to migrate through normal white matter between lesions. Work in other systems has shown that differentiated oligodendrocytes within white matter express molecules inhibitory for axon outgrowth. In light of this we have examined the effect of oligodendrocytes on the migration of oligodendrocyte precursors in vitro using time lapse video microscopy. We find that oligodendrocytes induce collapse and loss of motility in oligodendrocyte precursor processes, with this effect being lost as oligodendrocytes undergo programmed cell death. We conclude that the inhibitory factors present on differentiated oligodendrocytes may prevent effective migration between lesion in vivo, and that strategies to overcome this inhibition may be required for successful repair.

Glial lineages and myelination in the central nervous system

Oligodendrocytes, derived from stem cell precursors which arise in subventricular zones of the developing central nervous system, have as their specialist role the synthesis and maintenance of myelin. Astrocytes contribute to the cellular architecture of the central nervous system and act as a source of growth factors and cytokines; microglia are bone-marrow derived macrophages which function as primary immunocompetent cells in the central nervous system. Myelination depends on the establishment of stable relationships between each differentiated oligodendrocyte and short segments of several neighbouring axons. There is growing evidence, especially from studies of glial cell implantation, that oligodendrocyte precursors persist in the adult nervous system and provide a limited capacity for the restoration of structure and function in myelinated pathways damaged by injury or disease.

Schwann cell invasion of the central nervous system of the myelin mutants

Schwann cells are excluded from the CNS during development by the glial limiting membrane, an area of astrocytic specialisation present at the nerve root transitional zone, and at blood vessels in the neuropil. This barrier, however, can be disrupted and, with the highly migratory nature of Schwann cells, can result in their invasion and myelination of the CNS in many pathological situations. In this paper we demonstrate that this occurs in a number of myelin mutants, including the myelin deficient (md) and taiep rats and the canine shaking (sh) pup. While it is still relatively uncommon in the rodent mutants, the sh pup shows extensive Schwann cell invasion along the neuraxis. This invasion involves the spinal cord, brain stem, and cerebellum and increases in amount and distribution with age. In situ hybridisation studies using a Pzero riboprobe suggest that the likely origin of these cells in the sh pup is the nerve roots, primarily the dorsal roots. Paradoxically, Schwann cell myelination of the CNS increases with time in the sh pup despite a marked, progressive gliosis involving the glia limitans and neuropil. Thus the mechanism by which these cells migrate into the CNS through the gliosed nerve root transitional zone or from vasa nervorum remains unknown. Extensive Schwann cell CNS myelination may have therapeutic significance in human myelin disease.

Oligodendrocyte-type 2 astrocyte progenitors use a metalloendoprotease to spread and migrate on CNS myelin

Oligodendrocyte-type 2 astrocyte (O-2A) progenitors are highly motile cells which migrate in the developing and adult central nervous system (CNS). Adult CNS myelin, however, contains inhibitory proteins, the neurite growth inhibitors NI 35/250, which block neurite outgrowth and spreading of many different cell types, such as astrocytes and fibroblasts. In the present study we investigated the spreading of dissociated cells and migration out of aggregates ('spheres') of primary O-2A cultures and of a glial precursor cell line (CG-4) on purified CNS myelin and on CNS tissue. Primary O-2A progenitors and CG-4 cells quickly attached to and spread on CNS myelin-coated culture dishes, showing no inhibition by the neurite growth inhibitors. CG-4 cells migrated with a velocity of 30 microns/h on a CNS myelin protein extract and at 5.7 microns/h on adult spinal cord tissue. Both cell spreading and migration on a CNS substrate could be specifically blocked by metalloprotease blockers like o-phenanthroline and the tetrapeptide carbobenzoxy-phe-ala-phe-tyr-amide, whereas blockers of the serine, aspartyl and cysteine proteases had no effect. On differentiation to astrocytes, the O-2A progenitors lost their ability to spread on CNS myelin. These results suggest the crucial involvement of a metalloprotease in the mechanism of migration on a CNS substrate. In vivo, migration of oligodendrocyte progenitors may be an important aspect of myelin repair following local traumatic, inflammatory or toxin-induced myelin loss.

Myelination of the canine central nervous system by glial cell transplantation: a model for repair of human myelin disease

There is a lack of effective means of promoting remyelination of the central nervous system (CNS) in humans with chronic demyelinating disease. We have investigated the ability of transplanted glia to myelinate areas of the CNS equivalent to focal demyelinated lesions in multiple sclerosis (MS). In these studies we show that transplantation of oligodendrocytes or their progenitors into the CNS of a neonatal or adult canine myelin mutant results in repair of large areas similar in size to many MS plaques. Progenitor or pre-progenitor cells of the oligodendrocyte lineage have the greatest capacity for myelination following grafting, although cells of neonatal origin may also be used. Such an approach may therefore have therapeutic value in the repair of focal lesions in human myelin disease.

Transplanting oligodendrocyte progenitors into the adult CNS

This review covers a number of aspects of the behaviour of oligodendrocyte progenitors following transplantation into the adult CNS. First, an account is given of the ability of transplanted oligodendrocyte progenitors, grown in tissue culture in the presence of PDGF and bFGF, to extensively remyelinate focal areas of persistent demyelination. Secondly, we describe how transplanted clonal cell lines of oligodendrocyte progenitors will differentiate into astrocytes as well oligodendrocytes following transplantation into pathological environments in which both oligodendrocytes and astrocytes are absent, thereby manifesting the bipotentially demonstrable in vitro but not during development. Finally, a series of studies examining the migratory behaviour of transplanted oligodendrocyte progenitors (modelled using the oliodendrocyte progenitor cell line CG4) are described. These show that CG4 cells do not survive (or migrate) when transplanted into the normal adult CNS. However, if they are transplanted into CNS tissue that has previously been exposed to 40 Gy of x-irradiation then transplanted CG4 cells survive, divide and migrate over large distances. Moreover, within an x-irradiated environment, migrating transplanted CG4 cells are able to enter remotely located foci of demyelination and contribute to the remyelination of the demyelinated axons within. These studies demonstrate that although the normal adult CNS does not appear to support survival and migration of the CG4 cell line, it is possible to manipulate the environment in such a way that these nonpermissive properties of the environment can be overcome.