Transgenic Lewis rats overexpressing the proteolipid protein gene: myelin degeneration and its effect on T cell-mediated experimental autoimmune encephalomyelitis

Cerebral White Matter Alterations Associated With Oligodendrocyte Vulnerability in Organic Acidurias: Insights in Glutaric Aciduria Type I

The white matter is an important constituent of the central nervous system, containing axons, oligodendrocytes, and its progenitor cells, astrocytes, and microglial cells. Oligodendrocytes are central for myelin synthesis, the insulating envelope that protects axons and allows normal neural conduction. Both, oligodendrocytes and myelin, are highly vulnerable to toxic factors in many neurodevelopmental and neurodegenerative disorders associated with disturbances of myelination. Here we review the main alterations in oligodendrocytes and myelin observed in some organic acidurias/acidemias, which correspond to inherited neurometabolic disorders biochemically characterized by accumulation of potentially neurotoxic organic acids and their derivatives. The yet incompletely understood mechanisms underlying the high vulnerability of OLs and/or myelin in glutaric acidemia type I, the most prototypical cerebral organic aciduria, are particularly discussed.

Mutation of Proteolipid Protein 1 Gene: From Severe Hypomyelinating Leukodystrophy to Inherited Spastic Paraplegia

Pelizaeus-Merzbacher Disease (PMD) is an inherited leukodystrophy affecting the central nervous system (CNS)-a rare disorder that especially concerns males. Its estimated prevalence is 1.45-1.9 per 100,000 individuals in the general population. Patients affected by PMD exhibit a drastic reduction or absence of myelin sheaths in the white matter areas of the CNS. The Proteolipid Protein 1 (PLP1) gene encodes a transmembrane proteolipid protein. PLP1 is the major protein of myelin, and it plays a key role in the compaction, stabilization, and maintenance of myelin sheaths. Its function is predominant in oligodendrocyte development and axonal survival. Mutations in the PLP1 gene cause the development of a wide continuum spectrum of leukopathies from the most severe form of PMD for whom patients exhibit severe CNS hypomyelination to the relatively mild late-onset type 2 spastic paraplegia, leading to the concept of PLP1-related disorders. The genetic diversity and the biochemical complexity, along with other aspects of PMD, are discussed to reveal the obstacles that hinder the development of treatments. This review aims to provide a clinical and mechanistic overview of this spectrum of rare diseases.

Acute brain injuries trigger microglia as an additional source of the proteoglycan NG2

NG2 is a type I transmembrane glycoprotein known as chondroitin sulfate proteoglycan 4 (CSPG4). In the healthy central nervous system, NG2 is exclusively expressed by oligodendrocyte progenitor cells and by vasculature pericytes. A large body of immunohistochemical studies showed that under pathological conditions such as acute brain injuries and experimental autoimmune encephalomyelitis (EAE), a number of activated microglia were NG2 immuno-positive, suggesting NG2 expression in these cells. Alternative explanations for the microglial NG2 labeling consider the biochemical properties of NG2 or the phagocytic activity of activated microglia. Reportedly, the transmembrane NG2 proteoglycan can be cleaved by a variety of proteases to deposit the NG2 ectodomain into the extracellular matrix. The ectodomain, however, could also stick to the microglial surface. Since microglia are phagocytic cells engulfing debris of dying cells, it is difficult to identify a genuine expression of NG2. Recent studies showing (1) pericytes giving rise to microglial after stroke, and (2) immune cells of NG2-EYFP knock-in mice lacking NG2 expression in an EAE model generated doubts for the de novo expression of NG2 in microglia after acute brain injuries. In the current study, we took advantage of three knock-in mouse lines (NG2-CreERT2, CX₃CR₁-EGFP and NG2-EYFP) to study NG2 expression indicated by transgenic fluorescent proteins in microglia after tMCAO (transient middle cerebral artery occlusion) or cortical stab wound injury (SWI). We provide strong evidence that NG2-expressing cells, including OPCs and pericytes, did not differentiate into microglia after acute brain injuries, whereas activated microglia did express NG2 in a disease-dependent manner. A subset of microglia continuously activated the NG2 gene at least within the first week after tMCAO, whereas within 3 days after SWI a limited number of microglia at the lesion site transiently expressed NG2. Immunohistochemical studies demonstrated that these microglia with NG2 gene activity also synthesized the NG2 protein, suggesting activated microglia as an additional source of the NG2 proteoglycan after acute brain injuries.

Myelin in the Central Nervous System: Structure, Function, and Pathology

Oligodendrocytes generate multiple layers of myelin membrane around axons of the central nervous system to enable fast and efficient nerve conduction. Until recently, saltatory nerve conduction was considered the only purpose of myelin, but it is now clear that myelin has more functions. In fact, myelinating oligodendrocytes are embedded in a vast network of interconnected glial and neuronal cells, and increasing evidence supports an active role of oligodendrocytes within this assembly, for example, by providing metabolic support to neurons, by regulating ion and water homeostasis, and by adapting to activity-dependent neuronal signals. The molecular complexity governing these interactions requires an in-depth molecular understanding of how oligodendrocytes and axons interact and how they generate, maintain, and remodel their myelin sheaths. This review deals with the biology of myelin, the expanded relationship of myelin with its underlying axons and the neighboring cells, and its disturbances in various diseases such as multiple sclerosis, acute disseminated encephalomyelitis, and neuromyelitis optica spectrum disorders. Furthermore, we will highlight how specific interactions between astrocytes, oligodendrocytes, and microglia contribute to demyelination in hereditary white matter pathologies.

Chemical Screening Identifies Enhancers of Mutant Oligodendrocyte Survival and Unmasks a Distinct Pathological Phase in Pelizaeus-Merzbacher Disease

Pelizaeus-Merzbacher disease (PMD) is a fatal X-linked disorder caused by loss of myelinating oligodendrocytes and consequent hypomyelination. The underlying cellular and molecular dysfunctions are not fully defined, but therapeutic enhancement of oligodendrocyte survival could restore functional myelination in patients. Here we generated pure, scalable quantities of induced pluripotent stem cell-derived oligodendrocyte progenitor cells (OPCs) from a severe mouse model of PMD, Plp1jimpy. Temporal phenotypic and transcriptomic studies defined an early pathological window characterized by endoplasmic reticulum (ER) stress and cell death as OPCs exit their progenitor state. High-throughput phenotypic screening identified a compound, Ro 25-6981, which modulates the ER stress response and rescues mutant oligodendrocyte survival in jimpy, in vitro and in vivo, and in human PMD oligocortical spheroids. Surprisingly, increasing oligodendrocyte survival did not restore subsequent myelination, revealing a second pathological phase. Collectively, our work shows that PMD oligodendrocyte loss can be rescued pharmacologically and defines a need for multifactorial intervention to restore myelination.

Differences in T cell cytotoxicity and cell death mechanisms between progressive multifocal leukoencephalopathy, herpes simplex virus encephalitis and cytomegalovirus encephalitis

During the appearance of human immunodeficiency virus infection in the 1980 and the 1990s, progressive multifocal leukoencephalopathy (PML), a viral encephalitis induced by the JC virus, was the leading opportunistic brain infection. As a result of the use of modern immunomodulatory compounds such as Natalizumab and Rituximab, the number of patients with PML is once again increasing. Despite the presence of PML over decades, little is known regarding the mechanisms leading to death of infected cells and the role the immune system plays in this process. Here we compared the presence of inflammatory T cells and the targeting of infected cells by cytotoxic T cells in PML, herpes simplex virus encephalitis (HSVE) and cytomegalovirus encephalitis (CMVE). In addition, we analyzed cell death mechanisms in infected cells in these encephalitides. Our results show that large numbers of inflammatory cytotoxic T cells are present in PML lesions. Whereas in HSVE and CMVE, single or multiple appositions of CD8⁺ or granzyme-B⁺ T cells to infected cells are found, in PML such appositions are significantly less apparent. Analysis of apoptotic pathways by markers such as activated caspase-3, caspase-6, poly(ADP-ribose) polymerase-1 (PARP-1) and apoptosis-inducing factor (AIF) showed upregulation of caspase-3 and loss of caspase-6 from mitochondria in CMVE and HSVE infected cells. Infected oligodendrocytes in PML did not upregulate activated caspase-3 but instead showed translocation of PARP-1 from nucleus to cytoplasm and AIF from mitochondria to nucleus. These findings suggest that in HSVE and CMVE, cells die by caspase-mediated apoptosis induced by cytotoxic T cells. In PML, on the other hand, infected cells are not eliminated by the immune system but seem to die by virus-induced PARP and AIF translocation in a type of cell death defined as parthanatos.

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.

Modulation of the Innate Immune Response by Human Neural Precursors Prevails over Oligodendrocyte Progenitor Remyelination to Rescue a Severe Model of Pelizaeus-Merzbacher Disease

Pelizaeus-Merzbacher disease (PMD) results from an X-linked misexpression of proteolipid protein 1 (PLP1). This leukodystrophy causes severe hypomyelination with progressive inflammation, leading to neurological dysfunctions and shortened life expectancy. While no cure exists for PMD, experimental cell-based therapy in the dysmyelinated shiverer model suggested that human oligodendrocyte progenitor cells (hOPCs) or human neural precursor cells (hNPCs) are promising candidates to treat myelinopathies. However, the fate and restorative advantages of human NPCs/OPCs in a relevant model of PMD has not yet been addressed. Using a model of Plp1 overexpression, resulting in demyelination with progressive inflammation, we compared side-by-side the therapeutic benefits of intracerebrally grafted hNPCs and hOPCs. Our findings reveal equal integration of the donor cells within presumptive white matter tracks. While the onset of exogenous remyelination was earlier in hOPCs-grafted mice than in hNPC-grafted mice, extended lifespan occurred only in hNPCs-grafted animals. This improved survival was correlated with reduced neuroinflammation (microglial and astrocytosis loads) and microglia polarization toward M2-like phenotype followed by remyelination. Thus modulation of neuroinflammation combined with myelin restoration is crucial to prevent PMD pathology progression and ensure successful rescue of PMD mice. These findings should help to design novel therapeutic strategies combining immunomodulation and stem/progenitor cell-based therapy for disorders associating hypomyelination with inflammation as observed in PMD.

Proteolipid protein modulates preservation of peripheral axons and premature death when myelin protein zero is lacking

Protein zero (P0) is the major structural component of peripheral myelin. Lack of this adhesion protein from Schwann cells causes a severe dysmyelinating neuropathy with secondary axonal degeneration in humans with the neuropathy Dejerine-Sottas syndrome (DSS) and in the corresponding mouse model (P0(null)-mice). In the mammalian CNS, the tetraspan-membrane protein PLP is the major structural myelin constituent and required for the long-term preservation of myelinated axons, which fails in hereditary spastic paraplegia (SPG type-2) and the relevant mouse model (Plp(null)-mice). The Plp-gene is also expressed in Schwann cells but PLP is of very low abundance in normal peripheral myelin; its function has thus remained enigmatic. Here we show that the abundance of PLP but not of other tetraspan myelin proteins is strongly increased in compact peripheral myelin of P0(null)-mice. To determine the functional relevance of PLP expression in the absence of P0, we generated P0(null)*Plp(null)-double-mutant mice. Compared with either single-mutant, P0(null)*Plp(null)-mice display impaired nerve conduction, reduced motor functions, and premature death. At the morphological level, axonal segments were frequently non-myelinated but in a one-to-one relationship with a hypertrophic Schwann cell. Importantly, axonal numbers were reduced in the vital phrenic nerve of P0(null)*Plp(null)-mice. In the absence of P0, thus, PLP also contributes to myelination by Schwann cells and to the preservation of peripheral axons. These data provide a link between the Schwann cell-dependent support of peripheral axons and the oligodendrocyte-dependent support of central axons.

Progesterone antagonist therapy in a Pelizaeus-Merzbacher mouse model

Pelizaeus-Merzbacher disease (PMD) is a severe hypomyelinating disease, characterized by ataxia, intellectual disability, epilepsy, and premature death. In the majority of cases, PMD is caused by duplication of PLP1 that is expressed in myelinating oligodendrocytes. Despite detailed knowledge of PLP1, there is presently no curative therapy for PMD. We used a Plp1 transgenic PMD mouse model to test the therapeutic effect of Lonaprisan, an antagonist of the nuclear progesterone receptor, in lowering Plp1 mRNA overexpression. We applied placebo-controlled Lonaprisan therapy to PMD mice for 10 weeks and performed the grid slip analysis to assess the clinical phenotype. Additionally, mRNA expression and protein accumulation as well as histological analysis of the central nervous system were performed. Although Plp1 mRNA levels are increased 1.8-fold in PMD mice compared to wild-type controls, daily Lonaprisan treatment reduced overexpression at the RNA level to about 1.5-fold, which was sufficient to significantly improve the poor motor phenotype. Electron microscopy confirmed a 25% increase in the number of myelinated axons in the corticospinal tract when compared to untreated PMD mice. Microarray analysis revealed the upregulation of proapoptotic genes in PMD mice that could be partially rescued by Lonaprisan treatment, which also reduced microgliosis, astrogliosis, and lymphocyte infiltration.

Thymic stromal lymphopoietin is expressed in the intact central nervous system and upregulated in the myelin-degenerative central nervous system

Thymic stromal lymphopoietin (TSLP) is an epithelial cytokine expressed at barrier surfaces of the skin, gut, nose, lung, and the maternal/fetal interphase. At these sites, it is important for the generation and maintenance of non-inflammatory, tissue-resident dendritic cell responses. We show here that TSLP is also expressed in the central nervous system (CNS) where it is produced by choroid plexus epithelial cells and astrocytes in the spinal cord. Under conditions of low-grade myelin degeneration, the numbers of TSLP-expressing astrocytes increase, and microglia express transcripts for the functional TSLP receptor dimer indicating that these cells are targets for TSLP in the myelin-degenerative CNS.

Gait abnormalities and progressive myelin degeneration in a new murine model of Pelizaeus-Merzbacher disease with tandem genomic duplication

Pelizaeus-Merzbacher disease (PMD) is a hypomyelinating leukodystrophy caused by mutations of the proteolipid protein 1 gene (PLP1), which is located on the X chromosome and encodes the most abundant protein of myelin in the central nervous sytem. Approximately 60% of PMD cases result from genomic duplications of a region of the X chromosome that includes the entire PLP1 gene. The duplications are typically in a head-to-tail arrangement, and they vary in size and gene content. Although rodent models with extra copies of Plp1 have been developed, none contains an actual genomic rearrangement that resembles those found in PMD patients. We used mutagenic insertion chromosome engineering resources to generate the Plp1dup mouse model by introducing an X chromosome duplication in the mouse genome that contains Plp1 and five neighboring genes that are also commonly duplicated in PMD patients. The Plp1dup mice display progressive gait abnormalities compared with wild-type littermates. The single duplication leads to increased transcript levels of Plp1 and four of the five other duplicated genes over wild-type levels in the brain beginning the second postnatal week. The Plp1dup mice also display altered transcript levels of other important myelin proteins leading to a progressive degeneration of myelin. Our results show that a single duplication of the Plp1 gene leads to a phenotype similar to the pattern seen in human PMD patients with duplications.

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.

Characterization of a PLP-overexpressing transgenic rat, a model for the connatal form of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) most frequently results from duplication of the Plp1 gene with a correlation between disease severity and increasing copy number of the gene. Animal models of PMD, in particular those overexpressing the Plp1 gene, have been sought in attempts to provide systems in which potential therapies can be tested. Here we describe a rat model of the severe connatal form of PMD and provide a detailed characterization of its pathology and molecular biology, prior to testing therapeutic approaches. We determined the exact copy number of Plp1, and the resulting effects on RNA and protein expression. Distinct differences in myelin and disparate distributions of myelin protein markers in comparison to wild-type controls were observed. Altered expression of Plp1 also caused an increase in the apoptotic cell death of oligodendrocytes. These results provide the platform from which to test the effectiveness of in vivo therapies.

Axon-glial interaction in the CNS: what we have learned from mouse models of Pelizaeus-Merzbacher disease

In the central nervous system (CNS) the majority of axons are surrounded by a myelin sheath, which is produced by oligodendrocytes. Myelin is a lipid-rich insulating material that facilitates the rapid conduction of electrical impulses along the myelinated nerve fibre. Proteolipid protein and its isoform DM20 constitute the most abundant protein component of CNS myelin. Mutations in the PLP1 gene encoding these myelin proteins cause Pelizaeus-Merzbacher disease and the related allelic disorder, spastic paraplegia type 2. Animal models of these diseases, particularly models lacking or overexpressing Plp1, have shed light on the interplay between axons and oligodendrocytes, and how one component influences the other.

Increased Plp1 gene expression leads to massive microglial cell activation and inflammation throughout the brain

PMD (Pelizaeus-Merzbacher disease) is a rare neurodegenerative disorder that impairs motor and cognitive functions and is associated with a shortened lifespan. The cause of PMD is mutations of the PLP1 [proteolipid protein 1 gene (human)] gene. Transgenic mice with increased Plp1 [proteolipid protein 1 gene (non-human)] copy number model most aspects of PMD patients with duplications. Hypomyelination and demyelination are believed to cause the neurological abnormalities in mammals with PLP1 duplications. We show, for the first time, intense microglial reactivity throughout the grey and white matter of a transgenic mouse line with increased copy number of the native Plp1 gene. Activated microglia in the white and grey matter of transgenic mice are found as early as postnatal day 7, before myelin commences in normal cerebra. This finding indicates that degeneration of myelin does not cause the microglial response. Microglial numbers are doubled due to in situ proliferation. Compared with the jp (jimpy) mouse, which has much more oligodendrocyte death and hardly any myelin, microglia in the overexpressors show a more dramatic microglial reactivity than jp, especially in the grey matter. Predictably, many classical markers of an inflammatory response, including TNF-α (tumour necrosis factor-α) and IL-6, are significantly up-regulated manyfold. Because inflammation is believed to contribute to axonal degeneration in multiple sclerosis and other neurodegenerative diseases, inflammation in mammals with increased Plp1 gene dosage may also contribute to axonal degeneration described in patients and rodents with PLP1 increased gene dosage.

Oligodendrocytes: biology and pathology

Oligodendrocytes are the myelinating cells of the central nervous system (CNS). They are the end product of a cell lineage which has to undergo a complex and precisely timed program of proliferation, migration, differentiation, and myelination to finally produce the insulating sheath of axons. Due to this complex differentiation program, and due to their unique metabolism/physiology, oligodendrocytes count among the most vulnerable cells of the CNS. In this review, we first describe the different steps eventually culminating in the formation of mature oligodendrocytes and myelin sheaths, as they were revealed by studies in rodents. We will then show differences and similarities of human oligodendrocyte development. Finally, we will lay out the different pathways leading to oligodendrocyte and myelin loss in human CNS diseases, and we will reveal the different principles leading to the restoration of myelin sheaths or to a failure to do so.

Different proteolipid protein mutants exhibit unique metabolic defects

PMD (Pelizaeus–Merzbacher disease), a CNS (central nervous system) disease characterized by shortened lifespan and severe neural dysfunction, is caused by mutations of the PLP1 (X-linked myelin proteolipid protein) gene. The majority of human PLP1 mutations are caused by duplications; almost all others are caused by missense mutations. The cellular events leading to the phenotype are unknown. The same mutations in non-humans make them ideal models to study the mechanisms that cause neurological sequelae. In the present study we show that mice with Plp1 duplications (Plp1tg) have major mitochondrial deficits with a 50% reduction in ATP, a drastically reduced mitochondrial membrane potential and increased numbers of mitochondria. In contrast, the jp (jimpy) mouse with a Plp1 missense mutation exhibits normal mitochondrial function. We show that PLP in the Plp1tg mice and in Plp1-transfected cells is targeted to mitochondria. PLP has motifs permissive for insertion into mitochondria and deletions near its N-terminus prevent its co-localization to mitochondria. These novel data show that Plp1 missense mutations and duplications of the native Plp1 gene initiate uniquely different cellular responses.

The "window of susceptibility" for inflammation in the immature central nervous system is characterized by a leaky blood-brain barrier and the local expression of inflammatory chemokines

Early in postnatal development, the immature central nervous system (CNS) is more susceptible to inflammation than its adult counterpart. We show here that this "window of susceptibility" is characterized by the presence of leaky vessels in the CNS, and by a global chemokine expression profile which is clearly distinct from the one observed in the adult CNS and has three important characteristics. First, it contains chemokines with known roles in the differentiation and maturation of glia and neurons. Secondly, these chemokines have been described before in inflammatory lesions of the CNS, where they are important for the recruitment of monocytes and T cells. Lastly, the chemokine profile is shaped by pathological changes like oligodendrocyte stress and attempts of myelin repair. Changes in the chemokine expression profile along with a leaky blood-brain barrier pave the ground for an accelerated development of CNS inflammation.

DTI abnormalities in anterior corpus callosum of rats with spike-wave epilepsy

OBJECTIVE

Absence epilepsy is a common seizure disorder in children which can produce chronic psychosocial sequelae. Human patients and rat absence models show bilateral spike-wave discharges (SWD) in cortical regions. We employed diffusion tensor imaging (DTI) in rat absence models to detect abnormalities in white matter pathways connecting regions of seizure activity.

METHODS

We studied Wistar albino Glaxo rats of Rijswijk (WAG/Rij), genetic absence epilepsy rats of Strasbourg (GAERS), and corresponding nonepileptic control strains. Ex vivo DTI was performed at 9.4 T with diffusion gradients applied in 16 orientations. We compared fractional anisotropy (FA), perpendicular (lambda(perpendicular)) and parallel (lambda(||)) diffusivity between groups using t-maps and region of interest (ROI) measurements.

RESULTS

Adult epileptic WAG/Rij rats exhibited a localized decrease in FA in the anterior corpus callosum. This area was confirmed by tractography to interconnect somatosensory cortex regions most intensely involved in seizures. This FA decrease was not present in young WAG/Rij rats before onset of SWD. GAERS, which have more severe SWD than WAG/Rij, exhibited even more pronounced callosal FA decreases. Reduced FA in the epileptic animals originated from an increased lambda(perpendicular) with no significant changes in lambda(||).

INTERPRETATION

Reduced FA with increased lambda(perpendicular) suggests that chronic seizures cause reduction in myelin or decreased axon fiber density in white matter pathways connecting regions of seizure activity. These DTI abnormalities may improve the understanding of chronic neurological difficulties in children suffering with absence epilepsy, and may also serve as a noninvasive biomarker for monitoring beneficial effects of treatment.

Generation of cultured oligodendrocyte progenitor cells from rat neonatal brains

The oligodendrocyte progenitor cell (OPC) is one of the most studied progenitor cells of the body. It has been extensively researched in tissue culture and more recently in vivo using a wide range of markers that recognize transcription factors and cell surface markers and identify its earliest development from neural stem cells onward. Isolation of OPCs in large numbers and in purified preparations has been sought after as a source of cells for the repair of human myelin disorders. It has been proposed that such cells could be used as an exogenous source of cells for the treatment of lesions in multiple sclerosis and the less common genetic myelin disorders such as Pelizaeus-Merzbacher disease. Prior to clinical trials, such approaches can be tested in animal models. Here, we describe the isolation of rat OPCs in culture conditions that provide large numbers of purified populations of cells.

Epileptic seizures and hippocampal damage after cuprizone-induced demyelination in C57BL/6 mice

Epileptic seizures are known to occur in different animal models of demyelination and have also been described in demyelinating diseases of the central nervous system (CNS) such as multiple sclerosis. How myelin deficiency might cause seizures is unknown, but may involve axonal pathology and resultant alterations in neuronal excitability. The cause of seizures occurring in rodent demyelination models is unknown. In the present study, we used EEG/video monitoring to record seizures occurring during chronic demyelination of C57BL/6 mice fed for 12 weeks with 0.2% cuprizone. Furthermore, in the search for a morphological correlate of the seizures, the hippocampal formation was examined histologically. Epileptiform spikes resembling interictal spikes known from chronic epilepsy were recorded in all cuprizone-treated mice, but not in controls. Most cuprizone-treated animals exhibited generalized tonic-clonic seizures upon stress-inducing stimuli. In addition to the known demyelination of the corpus callosum, massive demyelination was found in the hippocampal formation. This was associated with neuronal alterations, including a loss of neurons in the hilus of the dentate gyrus. In view of the role of the dentate gyrus in epileptogenesis, demyelination leading to axonal pathology and thus neuronal damage as observed in the hilus may be causally involved in the paroxysmal alterations observed after prolonged treatment with cuprizone. The present data suggest a potential role of the hippocampal formation for seizures occurring as a consequence of neuronal damage secondary to CNS demyelination.

Transition from enhanced T cell infiltration to inflammation in the myelin-degenerative central nervous system

Myelin degeneration in the central nervous system (CNS) is often associated with elevated numbers of T cells in brain and spinal cord (SC). In some degenerative diseases, this T cell immigration has no clinical relevance, in others, it may precede severe inflammation and tissue damage. We studied T cells in the myelin-degenerative SC of transgenic (tg) Lewis rats overexpressing the proteolipid protein (PLP). These lymphocytes are T(H)1/T(C)1 cells and represent different T cell clones unique to individual animals. The SC-infiltrating CD8(+) T cell pool is more restricted than its CD4(+) counterpart, possibly due to constrictions in the peripheral CD8(+) T cell repertoire. Some SC-infiltrating T cells are highly motile and cover large distances within their target tissue, others are tethered to MHC class II(+) microglia cells. The activation of the tethered cells may trigger the formation of inflammatory foci and could pave the way for inflammation in degenerative CNS disease.

PLP overexpression perturbs myelin protein composition and myelination in a mouse model of Pelizaeus-Merzbacher disease

Duplication of PLP1, an X-linked gene encoding the major myelin membrane protein of the human CNS, is the most frequent cause of Pelizaeus-Merzbacher disease (PMD). Transgenic mice with extra copies of the wild type Plp1 gene, a valid model of PMD, also develop a dysmyelinating phenotype dependant on gene dosage. In this study we have examined the effect of increasing Plp1 gene dosage on levels of PLP/DM20 and on other representative myelin proteins. In cultured oligodendrocytes and early myelinating oligodendrocytes in vivo, increased gene dosage leads to elevated levels of PLP/DM20 in the cell body. During myelination, small increases in Plp1 gene dosage (mice hemizygous for the transgene) elevate the level of PLP/DM20 in oligodendrocyte soma but cause only minimal and transient effects on the protein composition and structure of myelin suggesting that cells can regulate the incorporation of proteins into myelin. However, larger increases in dosage (mice homozygous for the transgene) are not well tolerated, leading to hypomyelination and alteration in the cellular distribution of PLP/DM20. A disproportionate amount of PLP/DM20 is retained in the cell soma, probably in autophagic vacuoles and lysosomes whereas the level in myelin is reduced. Increased Plp1 gene dosage affects other myelin proteins, particularly MBP, which is transitorily reduced in hemizygous mice but consistently and markedly lower in homozygotes in both myelin and naïve or early myelinating oligodendrocytes. Whether the reduced MBP is implicated in the pathogenesis of dysmyelination is yet to be established.

Transient axonal injury in the absence of demyelination: a correlate of clinical disease in acute experimental autoimmune encephalomyelitis

Axonal degeneration contributes to the transient and permanent neurological deficits seen in multiple sclerosis, an inflammatory disease of the central nervous system. To study the immunological mechanisms causing axonal degeneration, we induced experimental autoimmune encephalomyelitis (EAE) in wildtype Lewis rats and Lewis rats with a slowly progressive myelin degeneration due to proteolipid protein (PLP) overexpression. EAE was triggered either by the transfer of encephalitogenic T-cells alone or by the co-transfer of T-cells with demyelinating antibodies. Inducible nitric oxide synthase (iNOS) expression in perivascular macrophages was associated with a transient functional disturbance of axons, reflected by the focal and reversible accumulation of amyloid precursor protein. Clinical disease correlated with the numbers of APP positive axon spheroids. Demyelination was associated with a further increase of iNOS expression in macrophages and with a higher degree of axonal injury. Our studies suggest that nitric oxide and its metabolites contribute to axonal pathology and possibly also to subsequent neurological dysfunction in EAE.

Transgenic modifications of the rat genome

The laboratory rat (R. norvegicus) is a very important experimental animal in several fields of biomedical research. This review describes the various techniques that have been used to generate transgenic rats: classical DNA microinjection and more recently described techniques such as lentiviral vector-mediated DNA transfer into early embryos, sperm-mediated transgenesis, embryo cloning by nuclear transfer and germline mutagenesis. It will also cover techniques associated to transgenesis such as sperm cryopreservation, embryo freezing and determination of zygosity. The availability of several technologies allowing genetic manipulation in the rat coupled to genomic data will allow biomedical research to fully benefit from the rat as an experimental animal.

Complementary contribution of CD4 and CD8 T lymphocytes to T-cell infiltration of the intact and the degenerative spinal cord

The central role of T cells in inflammatory reactions of the central nervous system (CNS) is well documented. However, there is little information about the few T cells found within the noninflamed CNS. In particular, the contribution of CD4+ and CD8+ T cells to the lymphocyte pool infiltrating the intact CNS, the location of these cells in CNS white and gray matter, and changes in the cellular composition of T-cell infiltrates coinciding with degeneration are primarily undefined. To address these points, we studied T cells in the intact and degenerative rat spinal cord. In the intact spinal cord, T cells were preferentially located within the gray matter. CD8+ T cells were more numerous than CD4+ lymphocytes. In cases of neuroaxonal degeneration or myelin degeneration/oligodendrocyte death, T cells were predominantly seen in areas of degeneration and were present in increased numbers. These effects were more pronounced for the CD4+ than for the CD8+ T-cell subset. Collectively, these data provide evidence for a clear cellular and compartmental bias in T-cell infiltration of the intact and degenerative spinal cord. This could indicate that CD4+ and CD8+ T cells might fulfill complementary roles in the intact and the diseased organ.

His36Pro point-mutated proteolipid protein retained in the endoplasmic reticulum of oligodendrocytes in theShaking pup

The shaking pup (shp) is a canine mutation that affects the myelin protein proteolipid protein (PLP) and its smaller and less abundant isoform, DM20, with proline replacing histidine(36), resulting in a severe myelin deficiency in the central nervous system. We present evidence that the mutation leads to disrupted trafficking of the shp PLP/DM20 within oligodendrocytes. Immunohistochemical studies revealed significantly reduced levels of PLP/DM20 and other major myelin components such as myelin basic protein (MBP), myelin associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in shp myelin. The distribution of shp PLP/DM20 proteins were altered and mostly retained in perinuclear cytoplasm and proximal processes, which co-localized with distended rough endoplasmic reticulum (RER) within oligodendrocytes. No abnormal accumulation of MAG, MBP, or CNP in the cell body was found. These results suggest that mutated PLP/DM20 in the shp could be selectively retained in RER, causing disruption of their translocation to the periphery to myelinate axons.

Selective and Antigen-Dependent Effects of Myelin Degeneration on Central Nervous System Inflammation

Damage to myelin sheath or oligodendrocytes may precede or even provoke inflammation of the central nervous system (CNS), but the extent to which these degenerative changes affect inflammation remains largely undefined. To study these processes in more detail, we used CNS antigen-specific T cells in the presence or absence of anti-myelin antibodies to induce experimental autoimmune encephalomyelitis (EAE) in transgenic Lewis rats with low-grade subclinical myelin degeneration and associated microglia cell activation, and in wild-type Lewis rats with an intact CNS. We found that myelin degeneration affects the localization of inflammatory lesions, the numbers of T cells recruited to these lesions, and the severity of the resulting clinical disease. In addition, myelin degeneration and associated microglia cell activation jointly enhance the susceptibility of the CNS to the action of anti-myelin antibodies. Our data show that even subtle alterations of myelin and oligodendrocytes may massively amplify the extent of demyelination and tissue damage, involving different immune effector mechanisms. A similar causal relationship might also operate in human patients with multiple sclerosis, where T cell-mediated inflammation and antibody-mediated demyelination have been documented, and where genetic factors might determine the susceptibility of the target tissue for immune-mediated injury.

Molecular pathogenesis of neuroinflammation

The past few years have seen significant progress towards understanding the mechanisms of immune surveillance and inflammation in the nervous system. In this review, the milestones of scientific discovery in this field are discussed, and the strengths and limitations of the different ways of examining the molecular pathogenesis of neuro-inflammation examined. The review is limited to the inflammatory reactions of the central nervous system that occur in multiple sclerosis and experimental autoimmune encephalomyelitis.

Trafficking of PLP/DM20 and cAMP signaling in immortalized jimpy oligodendrocytes

The synthesis, transport, and insertion of jimpy proteolipid protein and DM20 were studied in normal (158N) and jimpy (158JP) immortalized oligodendrocyte lines. Four different expression vectors encoding fusion proteins composed of native PLP and DM20 or jimpy PLP or DM20 were linked to enhanced green fluorescent protein (EGFP). All four transfected fusion proteins had similar distributions in the cell bodies and processes of the two cell types. Both normal and jimpy PLP-EGFP and DM20-EGFP were detected in both cell lines as far as 200 microM from the cell body, indicating synthesis and transport of mutated PLP and DM20 toward the plasma membrane. Immunocytochemistry of fixed normal and jimpy cells with the O10 antibody, which recognizes a conformationally sensitive PLP/DM20 epitope, confirmed that normal and jimpy PLP and DM20 were transported to the plasma membrane. Live staining of normal and jimpy cells transiently transfected with the native PLP showed positive staining, indicating PLP was correctly inserted into the membrane of both normal and jimpy oligodendrocytes. However, live staining of normal and jimpy cells transiently transfected with jimpy PLP showed no positive staining, indicating the mutated protein is abnormally inserted into the plasma membrane. Electrophysiological recordings of the resting membrane potential measured in the whole cell mode of the patch-clamp technique showed the absence of a developmentally regulated negative shift in the membrane potential in jimpy cells compared to normal native or immortalized oligodendrocytes. Treatment of 158N cells and native oligodendrocytes with dibutyryl cAMP (dbcAMP) caused morphological and biochemical differentiation, but failed to do so in 158JP cells, suggesting an abnormal signaling pathway in jimpy. The defect in cAMP signaling in jimpy oligodendrocytes was associated with the suppression of increase in mRNA level of the inducible cAMP early repressor (ICER). When the jimpy oligodendrocyte line was transfected with normal PLP or DM20 and exposed to dbcAMP, the cells failed to differentiate. This finding suggests that improper insertion of jimpy protein into the plasma membrane alters the membrane in such a way that certain signaling pathways are permanently altered. The abnormal insertion of jimpy PLP/DM20 into the plasma membrane may be the basis for the lack of cell signaling and abnormal resting potential in jimpy oligodendrocytes.

Repression of myelin proteolipid protein gene expression is mediated through both general and cell type-specific negative regulatory elements in nonexpressing cells

The myelin proteolipid protein gene (Plp ) is expressed primarily in oligodendrocytes. Yet how the gene remains repressed in nonexpressing cells has not been defined, and potentially could cause adverse effects in an organism if the mechanism for repression was impaired. Previous studies suggest that the first intron contains element(s), which suppress expression in nonexpressing cells, although the identity of these elements within the 8 kb intron was not characterized. Here we report the localization of multiple negative regulatory elements that repress Plp gene expression in nonexpressing cells (+/+ Li). Two of these elements (regions) correspond to those used by Plp expressing cells (N20.1), whilst another acts in a cell type-specific manner (i.e. operational in +/+ Li liver cells, but not N20.1 cells). By gel-shift and DNase I footprinting analyses, the factor(s) that bind to the cell type-specific negative regulatory region appear to be far more abundant in +/+ Li cells than in N20.1 cells. Thus, Plp gene repression is mediated through the combinatorial action of both "general" and cell type-specific negative regulatory elements. Additionally, repression in +/+ Li cells cannot be overcome via an antisilencer/enhancer element, which previously has been shown to function in N20.1 cells.

Endoplasmic reticulum stress in PLP-overexpressing transgenic rats: gray matter oligodendrocytes are more vulnerable than white matter oligodendrocytes

Studies dealing with transport of proteins from the oligodendrocyte cell body to the myelin sheath reveal the presence of different transport pathways. Proteolipid protein (PLP) is synthesized at the rough endoplasmic reticulum (ER) and then processed through the Golgi apparatus and transported to the myelin membranes. Myelin basic protein (MBP) on the other hand is synthesized locally at the ends of cell processes where its messenger RNA is translated on free ribosomes. Here we show that in rats that overexpress PLP, impairment of PLP transport from the cell body to the processes interferes with the translocation of other membrane proteins such as myelin-associated glycoprotein (MAG) and myelin oligodendrocyte glycoprotein (MOG), but not with peripherally translated MBP. In addition, it also impedes the transport of non-myelin proteins, for example the amyloid precursor protein (APP). At the ultrastructural level, the ER of these metabolically disturbed oligodendrocytes revealed extreme swelling of the cisternae, and immunohistochemistry revealed intense expression of the ER chaperone molecule BiP/GRP78 and ER folding enzyme protein disulfide isomerase (PDI). These features suggest that these oligodendrocytes, which were found exclusively in gray matter areas of the spinal cord, started an unfolded protein response while suffering from ER stress. Some of these disturbed oligodendrocytes were seen to undergo programmed cell death. These results indicate that gray matter oligodendrocyte differ from white matter oligodendrocytes in their capacity to stabilize metabolic disturbances by an unfolded protein response.

Classification of demyelinating diseases at the interface between etiology and pathogenesis

The classical demyelinating diseases include the 'autoimmune' inflammatory demyelinating diseases, the inflammatory demyelinating diseases of infectious aetiology, and the demyelinating or dysmyelinating diseases of genetic/hereditary background. In addition, primary demyelination is present in other conditions, such as brain ischaemia and intoxication. Irrespective of the primary aetiology, selective demyelination can be mediated through various pathogenetic pathways: the immune-mediated inflammatory pathway; the metabolic pathway; and the ischaemic/excitotoxic pathway. These pathways are only partly segregated with distinct aetiologies of demyelinating diseases, but they also reflect the way in which the patient copes with the disease-inciting event in relation to their particular genetic background. For future therapeutic strategies it will be important to interfere with the specific pathogenetic pathways of demyelination, which may be common to various demyelinating diseases, but may differ in subgroups of patients who suffer from a particular clinical demyelinating disease entity.