Oligodendrocyte survival and function in the long-lived strain of the myelin deficient rat

Activin receptors regulate the oligodendrocyte lineage in health and disease

The most prevalent neurological disorders of myelin include perinatal brain injury leading to cerebral palsy in infants and multiple sclerosis in adults. Although these disorders have distinct etiologies, they share a common neuropathological feature of failed progenitor differentiation into myelin-producing oligodendrocytes and lack of myelin, for which there is an unmet clinical need. Here, we reveal that a molecular pathology common to both disorders is dysregulation of activin receptors and that activin receptor signaling is required for the majority of myelin generation in development and following injury. Using a constitutive conditional knockout of all activin receptor signaling in oligodendrocyte lineage cells, we discovered this signaling to be required for myelination via regulation of oligodendrocyte differentiation and myelin compaction. These processes were found to be dependent on the activin receptor subtype Acvr2a, which is expressed during oligodendrocyte differentiation and axonal ensheathment in development and following myelin injury. During efficient myelin regeneration, Acvr2a upregulation was seen to coincide with downregulation of Acvr2b, a receptor subtype with relatively higher ligand affinity; Acvr2b was shown to be dispensable for activin receptor-driven oligodendrocyte differentiation and its overexpression was sufficient to impair the abovementioned ligand-driven responses. In actively myelinating or remyelinating areas of human perinatal brain injury and multiple sclerosis tissue, respectively, oligodendrocyte lineage cells expressing Acvr2a outnumbered those expressing Acvr2b, whereas in non-repairing lesions Acvr2b+ cells were increased. Thus, we propose that following human white matter injury, this increase in Acvr2b expression would sequester ligand and consequently impair Acvr2a-driven oligodendrocyte differentiation and myelin formation. Our results demonstrate dysregulated activin receptor signaling in common myelin disorders and reveal Acvr2a as a novel therapeutic target for myelin generation following injury across the lifespan.

Transplanted human glial-restricted progenitors can rescue the survival of dysmyelinated mice independent of the production of mature, compact myelin

The therapeutic effect of glial progenitor transplantation in diseases of dysmyelination is currently attributed to the formation of new myelin. Using magnetic resonance imaging (MRI), we show that the therapeutic outcome in dysmyelinated shiverer mice is dependent on the extent of cell migration but not the presence of mature and compact myelin. Human or mouse glial restricted progenitors (GRPs) were transplanted into rag2^-/- shiverer mouse neonates and followed for over one year. Mouse GRPs produced mature myelin as detected with multi-parametric MRI, but showed limited migration without extended animal lifespan. In sharp contrast, human GRPs migrated extensively and significantly increased animal survival, but production of mature myelin did not occur until 46weeks post-grafting. We conclude that human GRPs can extend the survival of transplanted shiverer mice prior to production of mature myelin, while mouse GRPs fail to extend animal survival despite the early presence of mature myelin. This paradox suggests that transplanted GRPs provide therapeutic benefits through biological processes other than the formation of mature myelin capable to foster rapid nerve conduction, challenging the current dogma of the primary role of myelination in regaining function of the central nervous system.

Is involvement of inflammation underestimated in Pelizaeus-Merzbacher disease?

Pelizaeus-Merzbacher disease (PMD) is a severe hypomyelinating leukodystrophy resulting from proteolipid protein 1 gene (PLP1) mutations leading to oligodendrocyte loss. While neuroinflammation has recently become a common feature and actor in neurodegenerative diseases, the involvement of inflammation in PMD physiopathology is still highly debated despite evidence for strong astrogliosis and microglial cell activation. Activation of the innate immune system, and more particularly, of microglia and astrocytes, is mostly associated with the deleterious role of neuroinflammation. However, in diseases such as multiple sclerosis, microglia appear beneficial for repair based on their role in myelin debris removal or recruitment and differentiation of oligodendrocyte progenitor cells. In this review, we will discuss recent published data in terms of their relevance to the role of microglia in PMD evolution, and of their impact on the improvement of therapeutic approaches combining immunomodulation and cell therapy to promote optimal recovery. © 2016 Wiley Periodicals, Inc.

Inherited and acquired disorders of myelin: The underlying myelin pathology

Remyelination is a major therapeutic goal in human myelin disorders, serving to restore function to demyelinated axons and providing neuroprotection. The target disorders that might be amenable to the promotion of this repair process are diverse and increasing in number. They range primarily from those of genetic, inflammatory to toxic origin. In order to apply remyelinating strategies to these disorders, it is essential to know whether the myelin damage results from a primary attack on myelin or the oligodendrocyte or both, and whether indeed these lead to myelin breakdown and demyelination. In some disorders, myelin sheath abnormalities are prominent but demyelination does not occur. This review explores the range of human and animal disorders where myelin pathology exists and focusses on defining the myelin changes in each and their cause, to help define whether they are targets for myelin repair therapy.

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We reviewed myelin disorders of the CNS in humans and animals.

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Myelin damage results from primary attack on the oligodendrocyte or myelin sheath.

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All major categories of disease can affect CNS myelin.

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Myelin vacuolation is common, yet does not always result in demyelination.

Autophagy promotes oligodendrocyte survival and function following dysmyelination in a long-lived myelin mutant

The Long-Evans shaker (les) rat has a mutation in myelin basic protein that results in severe CNS dysmyelination and subsequent demyelination during development. During this time, les oligodendrocytes accumulate cytoplasmic vesicles, including lysosomes and membrane-bound organelles. However, the mechanism and functional relevance behind these oligodendrocyte abnormalities in les have not been investigated. Using high-magnification electron microscopy, we identified the accumulations in les oligodendrocytes as early and late autophagosomes. Additionally, immunohistochemistry and Western blots showed an increase in autophagy markers in les. However, autophagy did not precede the death of les oligodendrocytes. Instead, upregulating autophagy promoted membrane extensions in les oligodendrocytes in vitro. Furthermore, upregulating autophagy in les rats via intermittent fasting increased the proportion of myelinated axons as well as myelin sheath thickness in les and control rats. Overall, this study provides insight into the abnormalities described in les as well as identifying a novel mechanism that promotes the survival and function of oligodendrocytes.

Myelin loss does not lead to axonal degeneration in a long-lived model of chronic demyelination

Current dogma suggests that chronically demyelinated axons are at risk for degeneration, with axonal loss resulting in permanent disability in myelin disease. However, the trophic role of the myelin sheath in long-term axonal survival is incompletely understood. Previous observations of the effect of dysmyelination or demyelination on axonal survival in the myelin mutants has been limited because of their short life span. In this study, we used the Long-Evans shaker (les) rat, which can live up to 9 months, to study axonal health and survival after chronic demyelination. At 2 weeks, ∼29% of medium and ∼47% of large fiber axons are myelinated in les spinal cord. However, by 3 months, no medium and ∼<1% of large-diameter axons retain myelin. After demyelination, axons have a reduced-caliber, abnormal neurofilament distribution and an increase in mitochondrial number. However, there are no signs of axonal degeneration in les rats up to 9 months. Instead, there is a profound increase in oligodendrocytes, which were found to express BDNF, NT-3, and IGF-1. Importantly, this study provides in vivo evidence that mature glial cells produce various neurotrophic factors that may aid in the survival of axons after chronic demyelination.

The myelin mutants as models to study myelin repair in the leukodystrophies

The leukodystrophies are rare and serious genetic disorders of the central nervous system that primarily affect children who frequently die early in life or have significantly delayed motor and mental milestones that result in long-term disability. Although with some of these disorders, early intervention with bone marrow or cord blood transplantation has been proven useful, it has not yet been determined that such therapies promote myelin repair of the central nervous system. Research on experimental therapies aimed at myelin repair is aided by the ability to test cell replacement strategies in genetic models in which the mutations and neuropathology match the human disorder. Thus, models exist of Pelizaeus-Merzbacher disease and the lysosomal storage disorder, Krabbe disease, which reflect the clinical and pathological course of the human disorders. Collectively, animals with mutations in myelin genes are called the myelin mutants, and they include rodent models such as the shiverer mouse that have been extensively used to study myelination by exogenous cell transplantation. These studies have encompassed many permutations of the age of the recipient, type of transplanted cell, site of engraftment, and so forth, and they offer hope that the scaling up of myelin produced by transplanted cells will have clinical significance in treating patients. Here we review these models and discuss their relative importance and use in such translational approaches. We discuss how grafts are identified and functional outcomes are measured. Finally, we briefly discuss the cells that have been successfully transplanted, which may be used in future clinical trials.

The pathobiology of myelin mutants reveal novel biological functions of the MBP and PLP genes

Substantial biological data indicate that the myelin basic protein (MBP) and myelin proteolipid protein (PLP/DM20) genes produce products with functions beyond that of serving as myelin structural proteins. Much of this evidence comes from studies on naturally-occurring and man-made mutations of these genes in mice and other species. This review focuses upon recent evidence showing the existence of other products of these genes that may account for some of these other functions, and recent studies providing evidence for alternative biological functions of PLP/DM20. The MBP and PLP/DM20 genes each encode the classic MBP and PLP isoforms, as well as a second family of proteins that are not involved in myelin structure. The biological roles of these other products of the genes are becoming clarified. The non-classic MBP gene products appear to be components of transcriptional complexes in the nucleus, and they also may be involved in signaling pathways in T-cells and in neural cells. The non-classic PLP/DM20 gene products appear to be components of intracellular transport vesicles in oligodendrocytes. There is evidence for other functions of the classic PLP/DM20 proteins, including a role in neural cell death mechanisms, autocrine and paracrine regulation of oligodendrocytes and neurons, intracellular transport and oligodendrocyte migration.

Genetic background determines phenotypic severity of the Plp rumpshaker mutation

The rumpshaker mutation of the proteolipid protein (Plp) gene causes dysmyelination in man and mouse. We show that the phenotype in the mouse depends critically on the genetic background in which the mutation is expressed. On the C3H background there is normal longevity whereas changing to a C57BL/6 strain results in seizures and death at around postnatal day 30. The more severe phenotype is associated with less myelin and reduced levels of major myelin proteins. There are also more apoptotic cells, including oligodendrocytes, increased numbers of proliferating cells, increased numbers of NG2+ oligodendrocyte progenitors and increased microglia compared to the milder phenotype. The number of mature oligodendrocytes is similar to wild-type in both strains of mutant, however, suggesting that increased oligodendrocyte death is matched by increased generation from progenitors. The dichotomy of phenotype probably reflects the influence of modifying loci. The localization of these putative modifying genes and their mode of action remain to be determined.

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.

Pluripotent stem cells engrafted into the normal or lesioned adult rat spinal cord are restricted to a glial lineage

Proliferating populations of undifferentiated neural stem cells were isolated from the embryonic day 14 rat cerebral cortex or the adult rat subventricular zone. These cells were pluripotent through multiple passages, retaining the ability to differentiate in vitro into neurons, astrocytes, and oligodendrocytes. Two weeks to 2 months after engraftment of undifferentiated, BrdU-labeled stem cells into the normal adult spinal cord, large numbers of surviving cells were seen. The majority of the cells differentiated with astrocytic phenotype, although some oligodendrocytes and undifferentiated, nestin-positive cells were detected; NeuN-positive neurons were not seen. Labeled cells were also engrafted into the contused adult rat spinal cord (moderate NYU Impactor injury), either into the lesion cavity or into the white or gray matter both rostral and caudal to the injury epicenter. Up to 2 months postgrafting, the majority of cells either differentiated into GFAP-positive astrocytes or remained nestin positive. No BrdU-positive neurons or oligodendrocytes were observed. These results show robust survival of engrafted stem cells, but a differentiated phenotype restricted to glial lineages. We suggest that in vitro induction prior to transplantation will be necessary for these cells to differentiate into neurons or large numbers of oligodendrocytes.

Proteolipid protein gene: Pelizaeus-Merzbacher disease in humans and neurodegeneration in mice

The dosage of the myelin gene and mutant forms of the protein can affect the CNS and PNS. Pelizaeus-Merzbacher disease (PMD) is a myelin disorder of the CNS that arises from both mutational mechanisms. Investigating the molecular basis of PMD in patients and animal models is furthering our understanding of the disease, dosage sensitivity and proteolipid protein function during myelinogenesis.

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.

Current concepts of PLP and its role in the nervous system

Proteolipid protein (PLP) and its smaller isoform DM20 constitute the major myelin proteins of the CNS. Mutations of the X-linked Plp gene cause the heterogeneous syndromes of Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia (SPG) in man and similar dysmyelinating disorders in a range of animal species. A variety of mutations including missense mutations, deletions, and duplications are responsible. Missense mutations cause a predicted alteration in primary structure of the encoded protein(s) and are generally associated with early onset of signs and generalised dysmyelination. The severity of the phenotype varies according to the particular codon involved and the influence of uncharacterised modifying genes. There is some evidence that the dysmyelination results from the altered protein acquiring a novel function deleterious to the oligodendrocyte's function. Transgenic mice carrying extra copies of the Plp gene provide a valid model of PMD/SPG due to gene duplication. Depending on the gene dosage, the phenotype can vary from early onset of severe and lethal dysmyelination through to a very late onset of a tract-specific demyelination and axonal degeneration. Mice with a null mutation of the Plp gene assemble and maintain normal amounts of myelin but develop a progressive axonopathy, again demonstrating tract specificity. The results indicate that the functions of PLP are far from clear. There is good evidence that it is involved in the formation of the intraperiod line of myelin, and the results from the knockout and transgenic mice suggest a role in the interaction of oligodendrocyte and axon.

The immune status of the myelin deficient rat and its immune responses to transplanted allogeneic glial cells

This study examined the immunological responsiveness of the myelin deficient (md) rat, and its immune response to transplanted allogeneic glial cells, with and without immunosuppression therapy. Skin grafts from an ACI strain of rat were found to be acutely rejected by Wistar md rats. Anti-donor cytotoxic antibody was produced and alloreactive T helper cells were expanded in these mutants after skin sensitization. Equivalent high frequencies of precursor cytotoxic T lymphocytes (pCTLs) specific to the ACi MHC antigens were observed in both normal controls and mutants. An immune response was noted when allogeneic glial cells were transplanted into the spinal cords of md rats, which was effectively suppressed for 2 weeks post transplantation by treatment with either cyclosporin A (CsA) or a monoclonal antibody to the interleukin-2 receptor (IL-2R). These results demonstrate that md rats are immunocompetent, but that CNS allograft rejection can be prevented by immunosuppressive agents.

Myelination following transplantation of EGF-responsive neural stem cells into a myelin-deficient environment

Epidermal growth factor (EGF)-responsive stem cells have been identified in the murine central nervous system. These cells can be isolated from the brain and maintained in an undifferentiated state in vitro in the presence of EGF. After removing EGF, the cells cease mitosis and can be induced to differentiate into neurons, astrocytes, and oligodendrocytes. We demonstrate that when the undifferentiated stem cells (nestin-positive) are injected into the myelin-deficient rat spinal cord, they respond to cues within the mutant CNS and differentiate into myelinating oligodendrocytes, in contrast to their behavior in vitro, where they mainly form astrocytes. The cells provide a valuable model system for the study of the development of early oligodendrocytes from multipotent neural stem cells. Because these cells are influenced to divide using growth factors, rather than oncogenes, and because they appear to make appropriate lineage decisions when transplanted into a mutant environment, they may provide an excellent source of cells for a variety of future therapies using cellular transplantation.

Proteolipid protein regulates the survival and differentiation of oligodendrocytes

Proteolipid protein (PLP) has been postulated to play a critical role in the early differentiation of oligodendrocytes (OLs) in addition to its known role as a structural component of myelin. To identify this early function, we blocked the synthesis of PLP in glial cultures with antisense oligodeoxynucleotides that targeted the PLP initiation codon. Primary glial cultures were incubated with phosphorothioate-protected oligodeoxynucleotides (S-ODNs) for up to 11 d. PLP in OLs was reduced >90%. OLs treated with antisense S-ODNs appeared strikingly healthy as judged by (1) immunocytochemical staining for myelin glycolipids and myelin basic protein, (2) their prolonged survival compared with untreated cultures, and (3) their ability to re-establish membrane sheets after removal of the S-ODNs. Our studies show that PLP is required for elaboration and stability of the myelin membrane sheets made by most OLs, but it is not necessary for the network of processes established by OLs. More importantly, the number of OLs in the antisense-treated cultures was nearly sevenfold greater after a 10-11 d incubation with S-ODNs than in control cultures. The number of proliferating OL progenitors was not increased in the antisense-treated cultures, indicating that the increase in the number of OLs was attributable to prolonged OL survival. The tissue culture studies reveal that the absence of PLP/DM20 has the positive effect of promoting OL survival but the negative effect of preventing their full differentiation. This finding clarifies many of the paradoxical findings seen in the PLP mutants, the PLP overexpressers, and the PLP- animals.

Normal temporal and spatial distribution of oligodendrocyte progenitors in the myelin-deficient (md) rat

A point mutation in exon 3 of the proteolipid protein (PLP) gene of the myelin-deficient (md) rat leads to a failure of oligodendrocyte maturation and early death of oligodendrocytes, resulting in dysmyelination. It has been suggested that an alternative-splice isoform of PLP, known as DM-20, might be expressed in oligodendrocyte progenitors in the embryonic central nervous system (CNS), raising the possibility that early development of the oligodendrocyte lineage might also be affected in the md rat. To test this suggestion, we visualized oligodendrocyte progenitors in the embryonic md rat spinal cord and brain by in situ hybridization with a probe to the platelet-derived growth factor alpha receptor (PDGFR). We could detect no abnormalities in the time of first appearance of oligodendrocyte precursors, nor in their subsequent proliferation and dispersal throughout the CNS. These data strongly suggest that the PLP mutation in the md rat primarily or exclusively affects the later stages of oligodendrocyte lineage.

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.

Myelin mutants: model systems for the study of normal and abnormal myelination

Spontaneous mutations that perturb myelination occur in a range of species including man, and together with engineered mutations have been used to study disease, normal myelination and axon/glial inter-relationships. Only a minority of the currently defined mutations have an apparently simple pathogenesis due to lack of a functional protein. Mutations in the myelin basic protein gene lead to a lack of protein, resulting in changes in the structure of myelin, which can be rescued by transgenic complementation. The pathogenesis of autosomal dominant and X-linked mutations affecting either oligodendrocytes or Schwann cells is more complex. Point mutations may act in a dominant negative manner and gene dosage is clearly linked to phenotypic change. Mutations in regulatory genes, such as those encoding transcription factors, can also disturb myelination by selected cell types. Other less-well studied and unexpected consequences of myelin mutations, such as seizures in mutations affecting genes expressed in Schwann cells and axonal changes associated with dysmyelination, are also considered. With the major developments in gene mapping and cloning it is now relevant to study mutations in a variety of species with the real prospect of defining their molecular basis. Examples are given of unusual, but potentially useful, uncharacterized mutations in dog and bovine.

Glial cell transplantation and remyelination of the central nervous system

Glial cell transplantation has proved to be a powerful tool in the study of glial cell biology. The extent of myelination achieved by transplanting myelin-producing cells into the CNS of myelin mutants, or into focal demyelinating lesions has raised hope that such a strategy may have therapeutic applications. Oligodendrocytes or Schwann cells could be used for repair. It is likely that the immature stages of the oligodendrocyte lineage have the best phenotypic characteristics for remyelination when transplanted, either as primary cells or as immortalized cells or cell lines. Prior culturing and growth factor treatment provides opportunities to expand cell populations before transplantation as dissociated cell preparations. Cell lines are attractive candidates for transplantation, but the risk of transformation must be monitored. The application of this technique to human myelin disorders may require proof that migration, division and stable remyelination of axons by the transplanted cells can occur in the presence of gliosis and inflammation.

Glial cell transplants: experimental therapies of myelin diseases

Transplantation of cells into the CNS of human patients with neurodegenerative disorders offers a radical new approach to the treatment of previously incurable diseases. Considerable success has been achieved in Parkinson's disease following transplantation of human fetal dopaminergic neurons. Disorders of myelination of the brain, of either inherited or acquired origin, might also be treated by glial cell transplantation although there are additional challenges. Cells of the oligodendrocyte lineage have been found to be capable of myelinating axons on transplantation into numerous experimental pathological environments, including the CNS of myelin mutants and focal areas of demyelination in normal animals made by injection of myelinotoxic chemicals. In general, primary cells and progenitors are likely to have the greatest myelinating capacity. Cell lines can also be used, but those driven by oncogenes may produce little myelin, and tumor formation is likely. Schwann cells are also a potential source of cells, possibly as a homograft, and may be primed by treatment ex vivo with glial growth factors. The variable CNS milieu seen in human myelin disease will mean that transplanted cells must be able to migrate appropriately and myelinate axons in an adult, pathological environment, and this awaits experimental confirmation. Physiological analysis of transplants in such situations in adult animals will provide the functional data which may expedite clinical trials.