An analysis of the early events when oligodendrocyte precursor cells are triggered to differentiate by thyroid hormone, retinoic acid, or PDGF withdrawal

Cholesterol biosynthesis defines oligodendrocyte precursor heterogeneity between brain and spinal cord

Brain and spinal cord oligodendroglia have distinct functional characteristics, and cell-autonomous loss of individual genes can result in different regional phenotypes. However, a molecular basis for these distinctions is unknown. Using single-cell analysis of oligodendroglia during developmental myelination, we demonstrate that brain and spinal cord precursors are transcriptionally distinct, defined predominantly by cholesterol biosynthesis. We further identify the mechanistic target of rapamycin (mTOR) as a major regulator promoting cholesterol biosynthesis in oligodendroglia. Oligodendroglia-specific loss of mTOR decreases cholesterol biosynthesis in both the brain and the spinal cord, but mTOR loss in spinal cord oligodendroglia has a greater impact on cholesterol biosynthesis, consistent with more pronounced deficits in developmental myelination. In the brain, mTOR loss results in a later adult myelin deficit, including oligodendrocyte death, spontaneous demyelination, and impaired axonal function, demonstrating that mTOR is required for myelin maintenance in the adult brain.

Using single-cell RNA sequencing, Khandker et al. reveal that oligodendroglia in the brain and spinal cord are distinct. These differences arise from mechanisms regulating cholesterol acquisition, necessary for maintenance of the lipid-rich myelin sheath, and involve mTOR in the regulation of cholesterol biosynthesis in oligodendroglia.

mTOR Signaling Regulates Metabolic Function in Oligodendrocyte Precursor Cells and Promotes Efficient Brain Remyelination in the Cuprizone Model

In demyelinating diseases, such as multiple sclerosis, primary loss of myelin and subsequent neuronal degeneration throughout the CNS impair patient functionality. While the importance of mechanistic target of rapamycin (mTOR) signaling during developmental myelination is known, no studies have yet directly examined the function of mTOR signaling specifically in the oligodendrocyte (OL) lineage during remyelination. Here, we conditionally deleted Mtor from adult oligodendrocyte precursor cells (OPCs) using Ng2-CreERT in male adult mice to test its function in new OLs responsible for remyelination. During early remyelination after cuprizone-induced demyelination, mice lacking mTOR in adult OPCs had unchanged OL numbers but thinner myelin. Myelin thickness recovered by late-stage repair, suggesting a delay in myelin production when Mtor is deleted from adult OPCs. Surprisingly, loss of mTOR in OPCs had no effect on efficiency of remyelination after lysophosphatidylcholine lesions in either the spinal cord or corpus callosum, suggesting that mTOR signaling functions specifically in a pathway dysregulated by cuprizone to promote remyelination efficiency. We further determined that cuprizone and inhibition of mTOR cooperatively compromise metabolic function in primary rat OLs undergoing differentiation. Together, our results support the conclusion that mTOR signaling in OPCs is required to overcome the metabolic dysfunction in the cuprizone-demyelinated adult brain.SIGNIFICANCE STATEMENT Impaired remyelination by oligodendrocytes contributes to the progressive pathology in multiple sclerosis, so it is critical to identify mechanisms of improving remyelination. The goal of this study was to examine mechanistic target of rapamycin (mTOR) signaling in remyelination. Here, we provide evidence that mTOR signaling promotes efficient remyelination of the brain after cuprizone-mediated demyelination but has no effect on remyelination after lysophosphatidylcholine demyelination in the spinal cord or brain. We also present novel data revealing that mTOR inhibition and cuprizone treatment additively affect the metabolic profile of differentiating oligodendrocytes, supporting a mechanism for the observed remyelination delay. These data suggest that altered metabolic function may underlie failure of remyelination in multiple sclerosis lesions and that mTOR signaling may be of therapeutic potential for promoting remyelination.

The mechanistic target of rapamycin pathway downregulates bone morphogenetic protein signaling to promote oligodendrocyte differentiation

Oligodendrocyte precursor cells (OPCs) differentiate and mature into oligodendrocytes, which produce myelin in the central nervous system. Prior studies have shown that the mechanistic target of rapamycin (mTOR) is necessary for proper myelination of the mouse spinal cord and that bone morphogenetic protein (BMP) signaling inhibits oligodendrocyte differentiation, in part by promoting expression of inhibitor of DNA binding 2 (Id2). Here we provide evidence that mTOR functions specifically in the transition from early stage OPC to immature oligodendrocyte by downregulating BMP signaling during postnatal spinal cord development. When mTOR is deleted from the oligodendrocyte lineage, expression of the FK506 binding protein 1A (FKBP12), a suppressor of BMP receptor activity, is reduced, downstream Smad activity is increased and Id2 expression is elevated. Additionally, mTOR inhibition with rapamycin in differentiating OPCs alters the transcriptional complex present at the Id2 promoter. Deletion of mTOR in oligodendroglia in vivo resulted in fewer late stage OPCs and fewer newly formed oligodendrocytes in the spinal cord with no effect on OPC proliferation or cell cycle exit. Finally, we demonstrate that inhibiting BMP signaling rescues the rapamycin-induced deficit in myelin protein expression. We conclude that mTOR promotes early oligodendrocyte differentiation by suppressing BMP signaling in OPCs.

An autoregulatory cell cycle timer integrates growth and specification in chick wing digit development

A fundamental question is how proliferation and growth are timed during embryogenesis. Although it has been suggested that the cell cycle could be a timer, the underlying mechanisms remain elusive. Here we describe a cell cycle timer that operates in Sonic hedgehog (Shh)-expressing polarising region cells of the chick wing bud. Our data are consistent with Shh signalling stimulating polarising region cell proliferation via Cyclin D2, and then inhibiting proliferation via a Bmp2-p27^kip1 pathway. When Shh signalling is blocked, polarising region cells over-proliferate and form an additional digit, which can be prevented by applying Bmp2 or by inhibiting D cyclin activity. In addition, Bmp2 also restores posterior digit identity in the absence of Shh signalling, thus indicating that it specifies antero-posterior (thumb to little finger) positional values. Our results reveal how an autoregulatory cell cycle timer integrates growth and specification and are widely applicable to many tissues.

Antioxidant Protection of NADPH-Depleted Oligodendrocyte Precursor Cells Is Dependent on Supply of Reduced Glutathione

The pentose phosphate pathway is the main source of NADPH, which by reducing oxidized glutathione, contributes to antioxidant defenses. Although oxidative stress plays a major role in white matter injury, significance of NADPH for oligodendrocyte survival has not been yet investigated. It is reported here that the NADPH antimetabolite 6-amino-NADP (6AN) was cytotoxic to cultured adult rat spinal cord oligodendrocyte precursor cells (OPCs) as well as OPC-derived oligodendrocytes. The 6AN-induced necrosis was preceded by increased production of superoxide, NADPH depletion, and lower supply of reduced glutathione. Moreover, survival of NADPH-depleted OPCs was improved by the antioxidant drug trolox. Such cells were also protected by physiological concentrations of the neurosteroid dehydroepiandrosterone (10⁻⁸ M). The protection by dehydroepiandrosterone was associated with restoration of reduced glutathione, but not NADPH, and was sensitive to inhibition of glutathione synthesis. A similar protective mechanism was engaged by the cAMP activator forskolin or the G protein-coupled estrogen receptor (GPER/GPR30) ligand G1. Finally, treatment with the glutathione precursor N-acetyl cysteine reduced cytotoxicity of 6AN. Taken together, NADPH is critical for survival of OPCs by supporting their antioxidant defenses. Consequently, injury-associated inhibition of the pentose phosphate pathway may be detrimental for the myelination or remyelination potential of the white matter. Conversely, steroid hormones and cAMP activators may promote survival of NADPH-deprived OPCs by increasing a NADPH-independent supply of reduced glutathione. Therefore, maintenance of glutathione homeostasis appears as a critical effector mechanism for OPC protection against NADPH depletion and preservation of the regenerative potential of the injured white matter.

Thyroid Hormone Signaling in Oligodendrocytes: from Extracellular Transport to Intracellular Signal

Thyroid hormone plays an important role in central nervous system (CNS) development, including the myelination of variable axonal calibers. It is well-established that thyroid hormone is required for the terminal differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes by inducing rapid cell-cycle arrest and constant transcription of pro-differentiation genes. This is well supported by the hypomyelinating phenotypes exhibited by patients with congenital hypothyroidism, cretinism. During development, myelinating oligodendrocytes only appear after the formation of neural circuits, indicating that the timing of oligodendrocyte differentiation is important. Since fetal and post-natal serum thyroid hormone levels peak at the stage of active myelination, it is suspected that the timing of oligodendrocyte development is finely controlled by thyroid hormone. The essential machinery for thyroid hormone signaling such as deiodinase activity (utilized by cells to auto-regulate the level of thyroid hormone), and nuclear thyroid hormone receptors (for gene transcription) are expressed on oligodendrocytes. In this review, we discuss the known and potential thyroid hormone signaling pathways that may regulate oligodendrocyte development and CNS myelination. Moreover, we evaluate the potential of targeting thyroid hormone signaling for white matter injury or disease.

Mechanisms of cell-cell interaction in oligodendrogenesis and remyelination after stroke

White matter damage is a clinically important aspect of several central nervous system diseases, including stroke. Cerebral white matter primarily consists of axonal bundles ensheathed with myelin secreted by mature oligodendrocytes, which play an important role in neurotransmission between different areas of gray matter. During the acute phase of stroke, damage to oligodendrocytes leads to white matter dysfunction through the loss of myelin. On the contrary, during the chronic phase, white matter components promote an environment, which is favorable for neural repair, vascular remodeling, and remyelination. For effective remyelination to take place, oligodendrocyte precursor cells (OPCs) play critical roles by proliferating and differentiating into mature oligodendrocytes, which help to decrease the burden of axonal injury. Notably, other types of cells contribute to these OPC responses under the ischemic conditions. This mini-review summarizes the non-cell autonomous mechanisms in oligodendrogenesis and remyelination after white matter damage, focusing on how OPCs receive support from their neighboring cells. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

The effect of methimazole-induced postnatal hypothyroidism on the retinal maturation and on the Sirtuin 2 level

PURPOSE

To investigate the effect of methimazole-induced postnatal hypothyroidism on the retinal maturation and to study Sirtuin 2 (SIRT2) level in the hypothyroidic rat retina.

METHODS

Twenty newborn Wistar albino rat pups were used in this prospective, randomized study. Wistar albino rats, weight 250-300 g, were impregnated (without addition of any drug) and were fed normally. Rat pups were randomly divided into two groups and were fed with breast milk. After weaning till they were 90 days of age, rat pups received the same water as their lactating mothers drank. Group 1 (methimazole (MMI)-induced hypothyroidy group), rats were given MMI-water, whereas, in Group 2, normal tap water. When the pups were 90 days of age, 20 rat pups were decapitated and the eyes were isolated. Eyes were investigated using histological, histomorphometric and immunohistochemistrical techniques.

RESULTS

No histological difference was seen between the groups stained with hematoxylin and eosin. In both groups the retinal layer structures and cells were observed as normal. The examples in the groups had a normal distribution for retinal thickness (pixel) measure. The mean value (mean ± std. deviation) was 554.7 ± 228.4 in the control group and 494.7 ± 129.4 in the hypothyroidy group. There was no significance between the groups in terms of retinal thickness (p = 0.231). However, immunohistochemistry revealed that SIRT2 was weaker stained in the ganglion cell layer and visual cell layer in the hypothyroidy group compared to the control group.

CONCLUSION

Postnatal hypothyroidism altered the retinal cytoarchitecture and layering which are regulated by thyroid hormones (THs) during retinal maturation in the postnatal period. THs may act by the induction of the SIRT family proteins or through their receptors. Postnatal screenings for THs levels are very important to provide normal retinal development.

Interaction of mTOR and Erk1/2 signaling to regulate oligodendrocyte differentiation

A multitude of factors regulate oligodendrocyte differentiation and remyelination, and to elucidate the mechanisms underlying this process, we analyzed the interactions of known signaling pathways involved in these processes. Previous work from our lab and others shows that Akt, mTOR, and Erk 1/2 are major signaling pathways regulating oligodendrocyte differentiation and myelination in vitro and in vivo. However, the relative contribution of the different pathways has been difficult to establish because the impact of inhibiting one pathway in in vitro cell culture models or in vivo may alter signaling through the other pathway. These studies were undertaken to clarify the interactions between these major pathways and understand more specifically the crosstalk between them. Oligodendrocyte differentiation in vitro required Akt, mTOR, and Erk 1/2 signaling, as inhibition of Akt, mTOR, or Erk 1/2 resulted in a significant decrease of myelin basic protein mRNA and protein expression. Interestingly, while inhibition of the Erk1/2 pathway had little impact on Akt/mTOR signaling, inhibition of the Akt/mTOR pathways significantly increased Erk1/2 signaling, although not enough to overcome the loss of Akt/mTOR signaling in the regulation of oligodendrocyte differentiation. Furthermore, such crosstalk was also noted in an in vivo context, after mTOR inhibition by rapamycin treatment of perinatal pups. GLIA 2014;62:2096-2109.

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

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

Transporters MCT8 and OATP1C1 maintain murine brain thyroid hormone homeostasis

Allan-Herndon-Dudley syndrome (AHDS), a severe form of psychomotor retardation with abnormal thyroid hormone (TH) parameters, is linked to mutations in the TH-specific monocarboxylate transporter MCT8. In mice, deletion of Mct8 (Mct8 KO) faithfully replicates AHDS-associated endocrine abnormalities; however, unlike patients, these animals do not exhibit neurological impairments. While transport of the active form of TH (T3) across the blood-brain barrier is strongly diminished in Mct8 KO animals, prohormone (T4) can still enter the brain, possibly due to the presence of T4-selective organic anion transporting polypeptide (OATP1C1). Here, we characterized mice deficient for both TH transporters, MCT8 and OATP1C1 (Mct8/Oatp1c1 DKO). Mct8/Oatp1c1 DKO mice exhibited alterations in peripheral TH homeostasis that were similar to those in Mct8 KO mice; however, uptake of both T3 and T4 into the brains of Mct8/Oatp1c1 DKO mice was strongly reduced. Evidence of TH deprivation in the CNS of Mct8/Oatp1c1 DKO mice included highly decreased brain TH content as well as altered deiodinase activities and TH target gene expression. Consistent with delayed cerebellar development and reduced myelination, Mct8/Oatp1c1 DKO mice displayed pronounced locomotor abnormalities. Intriguingly, differentiation of GABAergic interneurons in the cerebral cortex was highly compromised. Our findings underscore the importance of TH transporters for proper brain development and provide a basis to study the pathogenic mechanisms underlying AHDS.

E2F1 coregulates cell cycle genes and chromatin components during the transition of oligodendrocyte progenitors from proliferation to differentiation

Cell cycle exit is an obligatory step for the differentiation of oligodendrocyte progenitor cells (OPCs) into myelinating cells. A key regulator of the transition from proliferation to quiescence is the E2F/Rb pathway, whose activity is highly regulated in physiological conditions and deregulated in tumors. In this paper we report a lineage-specific decline of nuclear E2F1 during differentiation of rodent OPC into oligodendrocytes (OLs) in developing white matter tracts and in cultured cells. Using chromatin immunoprecipitation (ChIP) and deep-sequencing in mouse and rat OPCs, we identified cell cycle genes (i.e., Cdc2) and chromatin components (i.e., Hmgn1, Hmgn2), including those modulating DNA methylation (i.e., Uhrf1), as E2F1 targets. Binding of E2F1 to chromatin on the gene targets was validated and their expression assessed in developing white matter tracts and cultured OPCs. Increased expression of E2F1 gene targets was also detected in mouse gliomas (that were induced by retroviral transformation of OPCs) compared with normal brain. Together, these data identify E2F1 as a key transcription factor modulating the expression of chromatin components in OPC during the transition from proliferation to differentiation.

In vivo analysis of kallikrein-related peptidase 6 (KLK6) function in oligodendrocyte development and the expression of myelin proteins

Oligodendrocytes are important for not only nerve conduction but also central nervous system (CNS) development and neuronal survival in a variety of conditions. Kallikrein-related peptidase 6 (KLK6) is expressed in oligodendrocytes in the CNS and its expression is changed in several physiological and pathological conditions, especially following spinal cord injury (SCI) and experimental autoimmune encephalomyelitis. In this study, we investigated the functions of KLK6 in oligodendrocyte lineage cell development and the production of myelin proteins using KLK6-deficient (KLK6(-/-)) mice. KLK6(-/-) mice were born without apparent defects and lived as long as wild-type (WT) mice. There was no significant difference in the numbers of oligodendrocyte precursor cells and mature oligodendrocytes in the adult naive spinal cord between WT and KLK6(-/-) mice. However, there were fewer mature oligodendrocytes in the KLK6(-/-) spinal cord than in the WT spinal cord at postnatal day 7 (P7). Expression of myelin basic protein (MBP) and oligodendrocyte-specific protein/claudin-11, major myelin proteins, was also decreased in the KLK6(-/-) spinal cord compared with the WT spinal cord at P7-21. Moreover, after SCI, the amount of MBP in the damaged spinal cords of KLK6(-/-) mice was significantly less than that in the damaged spinal cords of WT mice. These results indicate that KLK6 plays a functional role in oligodendrocyte development and the expression of myelin proteins.

Oligodendrogenesis from neural stem cells: perspectives for remyelinating strategies

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

Cell-type dependent response of melanoma cells to aloe emodin

Intrinsic characteristics of melanoma cells such as expression of inducible nitric oxide synthase (iNOS), redox status, and activity of signaling pathways involved in proliferation, differentiation and cell death define the response of the cells to the diverse treatments. In this context we compared the effectiveness of herbal antaquinone aloe emodin (AE) against mouse B16 melanoma and human A375, different in initial activity of ERK1/2, constitutive iNOS expression and basal level of reactive oxygen species (ROS). Both cell lines are sensitive to AE treatment. However, while the agent induces differentiation of B16 cells toward melanocytes, in A375 cells promoted massive apoptosis. Differentiation of B16 cells, characterized by enhanced melanin production and tyrosinase activity, was mediated by H(2)O(2) production synchronized with rapid p53 accumulation and enhanced expression of cyclins D1 and D3. Caspase mediated apoptosis triggered in A375 cells was accompanied with Bcl-2 but not iNOS down-regulation. In addition, opposite regulation of Akt-ERK1/2 axis in AE treated B16 and A375 cells correlated with different outcome of the treatment. However, AE in a dose-dependent manner rescued both B16 and A375 cells from doxorubicin- or paclitaxel-induced killing. These data indicate that caution is warranted when AE is administrated to the patients with conventional chemotherapy.

BMP signaling mediates astrocyte differentiation of oligodendrocyte progenitor cells

Oligodendrocyte precursor cells (OPCs) can differentiate into oligodendrocytes or astrocytes, depending on cellular microenvironments. OPCs, cultured in medium supplemented with 10% (v/v) fetal bovine serum (FBS), give rise to type II astrocytes that express glial fibrillary acidic protein and a cell surface ganglioside that is recognized by A2B5 monoclonal antibody. However, the factors in FBS that direct the astrocyte differentiation are not determined. Moreover, bone morphogenetic proteins (BMPs) have been reported to be involved in astrocyte differentiation of neural progenitor cells. We therefore examined whether BMPs are responsible for the serum-mediated astrocyte differentiation from OPCs. OPCs were isolated from the spinal cords of Wistar rat embryos (at day 14) using the A2B5 antibody. We measured the concentrations of BMP-2 and BMP-4 in FBS and rat and human sera and the expression of mRNAs for three types of BMP receptors (BMPRIa, Ib and II) in OPCs by RT-PCR. The serum samples of the three species contained BMP-2 and BMP-4, as judged by ELISA with each monoclonal antibody, and the BMP receptor mRNAs are expressed in OPCs. When OPCs were cultured in the medium containing 10% FBS, cells (more than 95%) differentiated into type II astrocytes. However, when OPCs were pretreated with noggin, a soluble antagonist of BMP action, the degree of astrocyte differentiation was markedly decreased from 95.39 to 38.36%. Taken together, these results suggest that BMP signaling may be responsible for the serum-mediated astrocyte differentiation of OPCs. Our findings provide new insights into the molecular basis of differentiation of OPCs.

Mathematical and experimental approaches to identify and predict the effects of chemotherapy on neuroglial precursors

The adverse effects of chemotherapy on normal cells of the body create substantial clinical problems for many cancer patients. However, relatively little is known about the effects, other than promotion of cell death, of such agents on the function of normal precursor cells critical in tissue homeostasis and repair. We have combined mathematical and experimental analyses to identify the effects of sublethal doses of chemotherapy on glial precursor cells of the central nervous system. We modeled the temporal development of a population of precursor and terminally differentiated cells exposed to sublethal doses of carmustine (BCNU), a classic alkylating chemotherapeutic agent used in treatment of gliomas and non-Hodgkin's lymphomas, as a multitype age-dependent branching process. We fitted our model to data from in vitro clonal experiments using the method of pseudo-likelihood. This approach identifies several novel drug effects, including modification of the cell cycle length, the time between division and differentiation, and alteration in the probability of undergoing self-renewal division in precursor cells. These changes of precursor cell function in the chemotherapy-exposed brain may have profound clinic implications.

MAJOR FINDINGS

We applied our computational approach to analyze the effects of BCNU on clonal cultures of oligodendrocyte progenitor cells-one of the best-characterized neural progenitor cells in the mammalian brain. Our analysis reveals that transient exposures to BCNU increased the cell cycle length of progenitor cells and decreased their time to differentiation, while also decreasing the likelihood that they will undergo self-renewing divisions. By investigating the behavior of our mathematical model, we demonstrate that precursor cell populations should recover spontaneously from transient modifications of the timing of division and of differentiation, but such recovery will not happen after alteration of cell fate. These studies identify means by which precursor cell function can be critically compromised by transient exposure to chemotherapy with long-term consequences on the progenitor cell pool even in the absence of drug-induced apoptosis. These analyses also provide novel tools that apply broadly to identify effects of chemotherapeutic agents and other physiological stressors.

Theiler's murine encephalomyelitis virus preferentially infects immature stages of the murine oligodendrocyte precursor cell line BO-1 and blocks oligodendrocytic differentiation in vitro

Theiler's murine encephalomyelitis virus (TMEV)-induced demyelination is an important animal model for multiple sclerosis. The presence of oligodendrocyte precursor cells (OPCs) within demyelinated lesions together with the limited extent of remyelination has raised the question of how OPCs are affected by TMEV. It is well established that oligodendrocytes, astrocytes and microglia are targets during the chronic phase of the disease. However, whether TMEV infection interferes with the capacity of OPCs to generate oligodendrocytes has remained unclear. In the present study, a bipotential murine OPC cell line termed BO-1 was used to determine the antigenic phenotype susceptible to TMEV and the impact of TMEV infection upon cell differentiation. We show here that retinoic acid increased oligodendrocytic differentiation and decreased proliferation and TMEV infection rates. TMEV under serum-free conditions infected about 75% and 60% of early OPCs (NG2(+) and A2B5(+)) and immature oligodendrocytes (CNPase(+)), respectively, but only approximately 18% of mature oligodendrocytes (MBP(+)). Infection with TMEV prior to application of retinoic acid significantly reduced the percentage of MBP(+) BO-1 cells. These data demonstrate that TMEV preferentially infects early stages of the oligodendrocytic lineage and blocks oligodendrocyte maturation. The first demonstration of TMEV-mediated effects on OPC differentiation may shed new light on the pathogenesis of TMEV-induced demyelination and offers an explanation for the limited remyelination observed in vivo.

Ethanol alters the expressions of c-Fos and myelin basic protein in differentiating oligodendrocytes

Myelination occurs in the central nervous system of the human fetus, adolescents, and young adults. Ethanol interferes with myelination in part by altering the composition of the myelin sheath. Here we show that ethanol also affected the expression of the transcription factor c-Fos in differentiating oligodendrocytes (OLGs). Central glial-4 OLG progenitors were induced to differentiate in the absence and presence of 100 mM ethanol, and ethanol-caused changes in the levels of c-Fos and myelin basic protein (MBP) were determined by Western blot analysis at selected developmental stages. The relatively high c-Fos level in progenitors did not immediately decrease to a low level at the onset of differentiation but displayed a downregulation at a later developmental stage. Ethanol delayed the developmental c-Fos downregulation maintaining c-Fos at a 45% higher level at 2 days of differentiation (DoD). Ethanol also decreased the rate of the burst of MBP expression that occurred between 1 and 2 DoD, reducing the MBP level by 47% at 2 DoD. The ethanol-caused delays of c-Fos downregulation and MBP upregulation were both blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIM). Likewise, treatment of OLGs with a low 5-nM concentration of the PKC activator by 12-O-tetradecanoylphorbol-13-acetate mimicked the ethanol effects on the expression of both proteins, effects that were also counteracted by BIM. The results indicate that ethanol-caused delays of the stage-specific c-Fos downregulation and the inhibition of MBP expression both occur through a PKC-mediated mechanism. The ethanol-caused delay in c-Fos downregulation may disrupt normal timing for expression of genes involved in OLG differentiation, and the inhibited MBP expression may alter the myelin sheath composition.

Activation of the mammalian target of rapamycin (mTOR) is essential for oligodendrocyte differentiation

Although both extrinsic and intrinsic factors have been identified that orchestrate the differentiation and maturation of oligodendrocytes, less is known about the intracellular signaling pathways that control the overall commitment to differentiate. Here, we provide evidence that activation of the mammalian target of rapamycin (mTOR) is essential for oligodendrocyte differentiation. Specifically, mTOR regulates oligodendrocyte differentiation at the late progenitor to immature oligodendrocyte transition as assessed by the expression of stage specific antigens and myelin proteins including MBP and PLP. Furthermore, phosphorylation of mTOR on Ser 2448 correlates with myelination in the subcortical white matter of the developing brain. We demonstrate that mTOR exerts its effects on oligodendrocyte differentiation through two distinct signaling complexes, mTORC1 and mTORC2, defined by the presence of the adaptor proteins raptor and rictor, respectively. Disrupting mTOR complex formation via siRNA mediated knockdown of raptor or rictor significantly reduced myelin protein expression in vitro. However, mTORC2 alone controlled myelin gene expression at the mRNA level, whereas mTORC1 influenced MBP expression via an alternative mechanism. In addition, investigation of mTORC1 and mTORC2 targets revealed differential phosphorylation during oligodendrocyte differentiation. In OPC-DRG cocultures, inhibiting mTOR potently abrogated oligodendrocyte differentiation and reduced numbers of myelin segments. These data support the hypothesis that mTOR regulates commitment to oligodendrocyte differentiation before myelination.

Initiation of oligodendrocyte progenitor cell migration by a PDGF-A activated extracellular regulated kinase (ERK) signaling pathway

During CNS development, oligodendrocyte progenitor (OP) cells migrate from germinal zones to presumptive white matter tracts to generate myelinating oligodendrocytes. In vitro and in vivo studies indicate that platelet-derived growth factor-A (PDGF-A) is a potent chemoattractant for OP cells and important for normal distribution throughout the developing CNS. However, PDGF-A does not localize in concentration gradients corresponding to OP migratory pathways, as would be expected for a chemoattractant to direct migration. Therefore, the mechanism by which PDGF-A regulates OP distribution remains to be clarified. Here we show that PDGF-A induces OP migration and continuous exposure to PDGF-A is not required to maintain migration. Using pharmacological inhibitors, we show that a self-sustaining extracellular-regulated-kinase signaling pathway drives OP migration for up to 72 hours after the initial PDGF stimulus. These findings indicate PDGF-A may act to mobilize OP cells that then respond to distinct directional signals to distribute appropriately within the CNS.

The Yin and Yang of cell cycle progression and differentiation in the oligodendroglial lineage

In white matter disorders such as leukodystrophies (LD), periventricular leucomalacia (PVL), or multiple sclerosis (MS), the hypomyelination or the remyelination failure by oligodendrocyte progenitor cells involves errors in the sequence of events that normally occur during development when progenitors proliferate, migrate through the white matter, contact the axon, and differentiate into myelin-forming oligodendrocytes. Multiple mechanisms underlie the eventual progressive deterioration that typifies the natural history of developmental demyelination in LD and PVL and of adult-onset demyelination in MS. Over the past few years, pathophysiological studies have mostly focused on seeking abnormalities that impede oligodendroglial maturation at the level of migration, myelination, and survival. In contrast, there has been a strikingly lower interest for early proliferative and differentiation events that are likely to be equally critical for white matter development and myelin repair. This review highlights the Yin and Yang principles of interactions between intrinsic factors that coordinately regulate progenitor cell division and the onset of differentiation, i.e. the initial steps of oligodendrocyte lineage progression that are obviously crucial in health and diseases.

Identification of Sox17 as a transcription factor that regulates oligodendrocyte development

Microarray analysis of oligodendrocyte lineage cells purified by fluorescence-activated cell sorting (FACS) from 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP)-enhanced green fluorescent protein (EGFP) transgenic mice revealed Sox17 (SRY-box containing gene 17) gene expression to be coordinately regulated with that of four myelin genes during postnatal development. In CNP-EGFP-positive (CNP-EGFP+) cells, Sox17 mRNA and protein levels transiently increased between postnatal days 2 and 15, with white matter O4+ preoligodendrocytes expressing greater Sox17 levels than Nkx2.2+ (NK2 transcription factor related, locus 2) NG2+, or GalC+ (galactocerebroside) cells. In spinal cord, Sox17 protein expression was undetectable in the primary motor neuron domain between embryonic days 12.5 and 15.5 but was evident in Nkx2.2+ and CC1+ cells. In cultured oligodendrocyte progenitor cells (OPCs), Sox17 levels were maximal in O4+ cells and peaked during the phenotypic conversion from bipolar to multipolar. Parallel increases in Sox17 and p27 occurred before MBP protein expression, and Sox17 upregulation was prevented by conditions inhibiting differentiation. Sox17 downregulation with small interfering RNAs increased OPC proliferation and decreased lineage progression after mitogen withdrawal, whereas Sox17 overexpression in the presence of mitogen had opposite effects. Sox17 overexpression enhanced myelin gene expression in OPCs and directly stimulated MBP gene promoter activity. These findings support important roles for Sox17 in controlling both oligodendrocyte progenitor cell cycle exit and differentiation.

Suppressor of cytokine signaling 3 limits protection of leukemia inhibitory factor receptor signaling against central demyelination

Enhancement of oligodendrocyte survival through activation of leukemia inhibitory factor receptor (LIFR) signaling is a candidate therapeutic strategy for demyelinating disease. However, in other cell types, LIFR signaling is under tight negative regulation by the intracellular protein suppressor of cytokine signaling 3 (SOCS3). We, therefore, postulated that deletion of the SOCS3 gene in oligodendrocytes would promote the beneficial effects of LIFR signaling in limiting demyelination. By studying wild-type and LIF-knockout mice, we established that SOCS3 expression by oligodendrocytes was induced by the demyelinative insult, that this induction depended on LIF, and that endogenously produced LIF was likely to be a key determinant of the CNS response to oligodendrocyte loss. Compared with wild-type controls, oligodendrocyte-specific SOCS3 conditional-knockout mice displayed enhanced c-fos activation and exogenous LIF-induced phosphorylation of signal transducer and activator of transcription 3. Moreover, these SOCS3-deficient mice were protected against cuprizone-induced oligodendrocyte loss relative to wild-type animals. These results indicate that modulation of SOCS3 expression could facilitate the endogenous response to CNS injury.

pH is an intracellular effector controlling differentiation of oligodendrocyte precursors in culture via activation of the ERK1/2 pathway

We reported previously that onset of oligodendrocyte precursor cell (OPC) differentiation is accompanied by an increase in intracellular pH (pH(i)). We show that OPC differentiation is dependent primarily on a permissive pH(i) value. The highest differentiation levels were observed for pH(i) values around 7.15 and inhibition of differentiation was observed at slightly more acidic or alkaline values. Clamping the pH(i) of OPCs at 7.15 caused a transient activation of ERK1/2 that was not observed at more acidic or alkaline values. Furthermore, inhibition of ERK activation with the UO126 compound totally prevented OPC differentiation in response to pH(i) shift. These results indicate that pH(i), acting through the ERK1/2 pathway, is a key determinant for oligodendrocyte differentiation. We also show that this pH(i) pathway is involved in the process of retinoic acid-induced OPC differentiation.

Understanding the role of thyroid hormone in Sertoli cell development: a mechanistic hypothesis

More than a decade of research has shown that Sertoli cell proliferation is regulated by thyroid hormone. Neonatal hypothyroidism lengthens the period of Sertoli cell proliferation, leading to increases in Sertoli cell number, testis weight, and daily sperm production (DSP) when euthyroidism is re-established. In contrast, the neonatal Sertoli cell proliferative period is shortened under hyperthyroid conditions, but the mechanism by which thyroid hormone is able to negatively regulate Sertoli cell proliferation has been unclear. Recent progress in the understanding of the cell cycle has provided the opportunity to dissect the molecular targets responsible for thyroid-hormone-mediated effects on Sertoli cell proliferation. In this review, we discuss recent results indicating a critical role for the cyclin-dependent kinase inhibitors (CDKI) p27(Kip1) and p21(Cip1) in establishing Sertoli cell number, testis weight, and DSP, and the ability of thyroid hormone to modulate these CDKIs. Based on these recent results, we propose a working hypothesis for the way in which thyroid hormone regulates the withdrawal of the cell cycle by controlling CDKI degradation. Finally, although Sertoli cells have been shown to have two biologically active thyroid hormone receptor (TR) isoforms, TRalpha1 and TRbeta1, experiments with transgenic mice lacking TRalpha or TRbeta illustrate that only one TR mediates thyroid hormone effects in neonatal Sertoli cells. Although significant gaps in our knowledge still remain, advances have been made toward appreciation of the molecular sequence of events that occur when thyroid hormone stimulates Sertoli cell maturation.

Differential gene expression in neural stem cells and oligodendrocyte precursor cells: a cDNA microarray analysis

The use of neural stem cells (NSCs) or their progeny oligodendrocyte precursor cells (OPCs) represents a promising repair strategy for many neurological disorders. However, the molecular events and biological features during the transition from NSCs to OPCs remain unclear. In the present study, we isolated NSCs from the embryonic rat forebrain and induced them into OPCs by using B104 conditioned medium (B104CM) in vitro. We then employed cDNA array technology to compare changes in gene expression between the two cell populations. Among 1,176 genes examined, 40 were differentially expressed, and some of them may be involved in OPC differentiation from NSCs. Our findings thus provide new insights into the molecular basis of differentiation of OPCs from NSCs.

Estimating the life-span of oligodendrocytes from clonal data on their development in cell culture

This paper presents a new method to analyze clonal data on oligodendrocyte development in cell culture. The process of oligodendrocyte generation from precursor cells is modelled as a multi-type Bellman-Harris branching process as suggested in an earlier paper [K. Boucher, A. Zorin, A.Y. Yakovlev, M. Mayer-Proschel, M. Noble, An alternative stochastic model of generation of oligodendrocytes in cell culture, J. Math. Biol. 43 (2001) 22]. This model has been extended to allow for death of oligodendrocytes as well as a dissimilar distribution of the first mitotic cycle duration as compared to the subsequent cycles of precursor cells, which lengths are assumed to be independent and identically distributed random variables. Since the time-span of oligodendrocytes is not directly observable in clonal data, plausible parametric assumptions are invoked to make estimation problems tractable. In particular, the time to cell death follows a two-parameter gamma distribution, while the lapse of time between the event of cell death and the event of cell disintegration is assumed to be exponentially distributed. A simulated pseudo maximum likelihood method for estimation of model parameters has been developed using simulation-based approximations of the expected numbers and variance-covariance matrices for different types of cells. Finite sample properties of the estimation procedure are studied by computer simulations. The proposed method is illustrated with an analysis of the clonal development of O-2A progenitor cells isolated from the rat optic nerve and the corpus callosum.

Thyroid hormone regulates the cell cycle inhibitor p27Kip1 in postnatal murine Sertoli cells

Thyroid hormone regulates early postnatal Sertoli cell proliferation. Transient neonatal hypothyroidism allows prolonged postnatal Sertoli cell mitogenesis and doubles adult Sertoli cell numbers, testis weight, and sperm production. The mechanism of this effect is unknown. Cell proliferation is stimulated by cyclins and cyclin-dependent kinases and inhibited by cyclin-dependent kinase inhibitors (CDKIs). T(3) regulates the CDKI p27(Kip1) in other cell types, and mice lacking p27(Kip1) have increased testis size. To test the hypothesis that T(3) regulates Sertoli cell mitogenesis by acting through p27(Kip1), we compared expression of p27(Kip1) in Sertoli cells of testes from euthyroid, hypothyroid, and hyperthyroid mice. At postnatal d 5-25, testes were collected and immunostained for p27(Kip1) expression, or Sertoli cells were isolated enzymatically and used for p27(Kip1) Western blotting. p27(Kip1) immunostaining was low in rapidly proliferating 5-d-old Sertoli cells but had increased strongly in nonproliferating 25-d-old Sertoli cells. p27(Kip1) immunostaining was reduced in Sertoli cells from hypothyroid mice compared with euthyroid controls at 10 and 16 d, consistent with increased Sertoli cell proliferation in these mice. Western blotting corroborated the p27(Kip1) immunostaining, and p27(Kip1) expression was greater in Sertoli cells from control compared with hypothyroid mice at postnatal d 10 and 16, but p27(Kip1) expression was comparable by d 25. Hyperthyroidism increased p27(Kip1) immunostaining relative to controls, and Western analysis indicated that Sertoli cells from 10-d-old hyperthyroid mice expressed more p27(Kip1) than control mice. These results indicate that thyroid hormone status affects p27(Kip1) expression in neonatal Sertoli cells, suggesting that T(3) effects on Sertoli cell proliferation may be mediated through this CDKI.

Adenosine: a neuron-glial transmitter promoting myelination in the CNS in response to action potentials

Neuronal activity influences myelination of the brain, but the molecular mechanisms involved are largely unknown. Here, we report that oligodendrocyte progenitor cells (OPCs) express functional adenosine receptors, which are activated in response to action potential firing. Adenosine acts as a potent neuron-glial transmitter to inhibit OPC proliferation, stimulate differentiation, and promote the formation of myelin. This neuron-glial signal provides a molecular mechanism for promoting oligodendrocyte development and myelination in response to impulse activity and may help resolve controversy on the opposite effects of impulse activity on myelination in the central and peripheral nervous systems.

Cyclin-dependent kinase-2 controls oligodendrocyte progenitor cell cycle progression and is downregulated in adult oligodendrocyte progenitors

Proliferation of oligodendrocyte progenitor (OP) cells is a crucial process controlling myelination in the CNS. Previous studies demonstrated a correlation between OP proliferation rate and cyclin E/cyclin-dependent kinase-2 (cdk2) activity. To establish a causal link between cyclin E/cdk2 activity and OP proliferation, we selectively modulated cdk2 activity in vitro by transfection of cultured OP cells. Dominant-negative (Dn)-cdk2 overexpression inhibited mitogen-induced OP cell proliferation, whereas wild-type (wt)-cdk2 prevented cell cycle arrest caused by anti-mitotic signals. Dn-cdk2- or wt-cdk2-mediated regulation of G(1)/S transition, per se, did not influence initiation of OP differentiation. To study the function of cyclin E/cdk2 in OP cells during development in vivo, we analyzed cdk2 and cyclin E expression in cells acutely isolated from transgenic mice expressing the green fluorescent protein (GFP) under the control of the 2'-3'-cyclic nucleotide 3'-phosphodiesterase gene promoter. Both cyclin E/cdk2 protein levels and activity were decreased in GFP(+) oligodendrocyte lineage cells between postnatal days 4 and 30. Immunostaining of NG2(+)/GFP(+) OP cells in brain tissue sections showed a 90% decrease in overall cell proliferation and cdk2 expression between perinatal and adult cells. However, cdk2 expression within the proliferating (i.e., expressing the proliferating cell nuclear antigen) OP cell population was maintained throughout development. Our data indicate that: (1) cyclin E/cdk2 activity plays a pivotal function in OP cell cycle decisions occurring at G(1)/S checkpoint; (2) initiation of OP differentiation is independent of cyclinE/cdk2 checkpoint, and (3) intrinsic differences in cyclin E/cdk2 expression and activity may underlie the slowly proliferative state that characterizes so-called "quiescent" adult OP cells in vivo.

Persistence of oligodendrocyte precursor cells and altered myelination in optic nerve associated to retina degeneration in mice devoid of all thyroid hormone receptors

Thyroid hormone (3,5,3'-triiodo-l-thyronine or T3) exerts a pleiotropic activity during central nervous system development. Hypothyroidism during the fetal and postnatal life results in an irreversible mental retardation syndrome. At the cellular level, T3 is known to act on neuronal and glial lineages and to control cell proliferation, apoptosis, migration, and differentiation. Oligodendrocyte precursor cells (OPC) found at birth in the optic nerves are self-renewing cells that normally differentiate during the first 3 weeks of rodent postnatal life into postmitotic myelinating oligodendrocytes. In vitro, the addition of T3 to OPC is sufficient to trigger their terminal differentiation. The present analysis of T3 receptor knockout mice reveals that the absence of all T3 receptor results in the persistence of OPC proliferation in adult optic nerves, in a default in myelination, and sometimes in the degeneration of the retinal ganglion neurons. Thus, T3 signaling is necessary in vivo to promote the complete differentiation of OPC.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.

Down-regulation of the retinoblastoma protein (rb) is associated with rat oligodendrocyte differentiation

Terminal differentiation of oligodendrocytes is associated with permanent withdrawal from the cell cycle. We studied the expression of the retinoblastoma protein, expression and activity of G1 cyclins and kinases in oligodendrocyte progenitor cells cultured in vitro. We found that Rb stopped to be expressed concomitantly with the activation of CNPase in oligodendrocytes differentiated with thyroid hormone. In contrast, Rb continued to be expressed at reduced levels in oligodendrocytes that were arrested in G1 by removal of mitogens. Cyclin D1, cdk2, and cdk4 kinase activities were decreased in G1-arrested and differentiated oligodendrocytes. Cyclin E, however, continued to be expressed in G1-arrested oligodendrocytes. Inhibition of differentiation induced by mitogens in oligodendrocytes arrested in G1 by Ad-p27 was accompanied by continued expression of Rb, D1, and E cyclins. After removal of mitogens and addition of thyroid hormone, Rb stopped being expressed and CNPase expression was activated with a temporal course similar to that of oligodendrocytes infected with a control adenovirus. Our results indicate that Rb may play an important function in differentiation of oligodendrocytes in response to external mitogens and differentiation factors.

Two molecularly distinct intracellular pathways to oligodendrocyte differentiation: role of a p53 family protein

Both thyroid hormone (TH) and retinoic acid (RA) induce purified rat oligodendrocyte precursor cells in culture to stop division and differentiate. We show that these responses are blocked by the expression of a dominant-negative form of p53. Moreover, both TH and RA cause a transient, immediate early increase in the same 8 out of 13 mRNAs encoding intracellular cell cycle regulators and gene regulatory proteins, but only if protein synthesis is inhibited. Platelet-derived growth factor (PDGF) withdrawal also induces these cells to differentiate, but we show that the intracellular mechanisms involved are different from those involved in the hormone responses: the changes in cell cycle regulators differ, and the differentiation induced by PDGF withdrawal (or that which occurs spontaneously in the presence of PDGF) is not blocked by the dominant-negative p53. These results suggest that TH and RA activate the same intracellular pathway leading to oligodendrocyte differentiation, and that this pathway depends on a p53 family protein. Differentiation that occurs independently of TH and RA apparently involves a different pathway. It is likely that both pathways operate in vivo.

Lead alters the developmental profile of the galactolipid metabolic enzymes in cultured oligodendrocyte lineage cells

Lead is a neurotoxicant that can cause myelin deficits. Galactolipids are expressed during differentiation of oligodendrocyte lineage cells and accumulate in myelin. To examine the impact of lead on oligodendroglial differentiation, galactolipid metabolism in cultured oligodendrocyte lineage cells exposed to the metal was studied. Oligodendrocyte progenitor cells obtained from newborn rat pups were exposed to 1 microM lead acetate for 24 h prior to maintenance of the cells in medium containing the metal salt for 0, 2, or 6 days of differentiation. Lead caused approximately 50% reduction in levels of the galactolipid biosynthetic transferases, UDP-galactose:ceramide galactosyltransferase and 3'-phosphoadenosine-5'-phosphosulfate:galactocerebroside sulfotransferase, as compared to sodium-treated controls, in cultures of oligodendrocyte lineage cells following 2 days of differentiation. The activities of the galactolipid catabolic hydrolases, galactocerebroside-beta-galactosidase and arylsulfatase A, were reduced by 20%. Following 6 days of differentiation, lead-exposed cells exhibited levels of all the enzymes, except for arylsulfatase A, similar to those of the control cells. These results are consistent with the lead-induced delay of oligodendrocyte differentiation, as evidenced by the emergence of stage-specific immunochemical markers and the observed change in the developmental activity profile of 2',3'-cyclic nucleotide 3'-phosphohydrolase. The activity of arylsulfatase A in lead-treated 6-day oligodendrocytes was significantly less than that found in control cultures. This effect is consistent with the lead-induced reduction of arylsulfatase A in human fibroblasts caused by mis-sorting the newly-synthesized enzyme. The perturbation of galactolipid metabolism by lead during developmental maturation of oligodendrocytes may represent a contributing mechanism for lead-induced neurotoxicity.

Behaviour of oligodendrocytes and Schwann cells in an experimental model of toxic demyelination of the central nervous system

Oligodendrocytes and Schwann cells are engaged in myelin production, maintenance and repairing respectively in the central nervous system (CNS) and the peripheral nervous system (PNS). Whereas oligodendrocytes act only within the CNS, Schwann cells are able to invade the CNS in order to make new myelin sheaths around demyelinated axons. Both cells have some limitations in their activities, i.e. oligodendrocytes are post-mitotic cells and Schwann cells only get into the CNS in the absence of astrocytes. Ethidium bromide (EB) is a gliotoxic chemical that when injected locally within the CNS, induce demyelination. In the EB model of demyelination, glial cells are destroyed early after intoxication and Schwann cells are free to approach the naked central axons. In normal Wistar rats, regeneration of lost myelin sheaths can be achieved as early as thirteen days after intoxication; in Wistar rats immunosuppressed with cyclophosphamide the process is delayed and in rats administered cyclosporine it may be accelerated. Aiming the enlightening of those complex processes, all events concerning the myelinating cells in an experimental model are herein presented and discussed.

A role for an AP-1-like site in the expression of the myelin basic protein gene during differentiation

Differentiation of oligodendrocyte progenitors into mature oligodendrocytes involves the timely, cell-type specific expression of a number of different genes. Among these, the expression of the myelin basic protein (MBP) gene closely parallels the course of oligodendrocyte differentiation. To understand how transcription of the myelin basic protein gene is controlled, binding to the distal end of the 5' flanking sequence of the MBP gene was investigated. Specific protein-DNA complexes were localized to an AP-1-like element located between -1230 and -1240. The protein-DNA complexes formed at this site were shown to change as the cells differentiated. In undifferentiated cells two complexes were formed but, as the cells differentiated, binding was nearly completely lost. One of the two complexes was shown to contain a member of the fos family of transcription factors but no jun family members were involved. Mutation of the AP-1-like site resulted in loss of the complex and a change in expression of a reporter construct driven by the mutated promoter sequence. These results demonstrate a role for the AP-1-like site in repression of MBP gene expression in oligodendrocyte progenitor cells.

Cell cycle and cell fate in the nervous system

Recently, a number of molecules originally thought to have a primary role in cell determination have been shown to affect the cell cycle at specific check points, while other molecules discovered for their roles in the cell cycle progression are known to affect the determination and differentiation of neurons. These discoveries have led to a more detailed investigation of the complex molecular machinery that co-ordinates proliferation and differentiation.

In vivo regulation of precursor cells in the subventricular zone of adult rat brain by thyroid hormone and retinoids

The mature central nervous system contains precursor cells in the subventricular zone of the lateral ventricle. In this study we examined the possibility to affect fate of precursor cells through exogenous manipulations. The results indicate that administration of thyroid hormone and retinoic acid increases the expression of Ki67, a nuclear antigen associated with cell proliferation, and of nestin, a marker protein for precursor cells in the subventricular zone of adult male rats. Moreover, retinoic acid increases polysialated-neural cell adhesion molecules (PSA-NCAM)-immunoreactivity. These data suggest that nuclear receptor ligands are potential candidates for fate determination of precursor cells in the subventricular zone also in the adult brain.

AP-1 activity during the growth, differentiation, and death of O-2A lineage cells

Oligodendrocyte differentiation has been correlated with AP-1 activity, being low in progenitors and high in differentiated cells. In this study we have carried out a detailed temporal analysis of AP-1 activity in oligodendrocyte-type-2 astrocyte (O-2A) lineage cells. We show that low AP-1 activity in progenitor cells depended on the application of growth factors. Treatment of cells with B104-conditioned medium induced high AP-1 activity, increased process length, and improved growth. The role of AP-1 in proliferation and process extension was emphasized when progenitor cells overexpressing a c-Jun dominant-negative mutant had impaired growth and shortened processes. AP-1 DNA-binding activity during O-2A differentiation in vitro showed an initial down-regulation followed by up-regulation after 2 days. Increased AP-1 levels in oligodendrocytes were inhibited by overexpression of bcl-2, indicating that AP-1 in mature oligodendrocytes is involved in the regulation of apoptosis. Prevention of cell death by bcl-2 in oligodendrocytes was accompanied by progressive differentiation and expression of MOG and PLP.

Long-term culture of purified postnatal oligodendrocyte precursor cells. Evidence for an intrinsic maturation program that plays out over months

Oligodendrocytes myelinate axons in the vertebrate central nervous system (CNS). They develop from precursor cells (OPCs), some of which persist in the adult CNS. Adult OPCs differ in many of their properties from OPCs in the developing CNS. In this study we have purified OPCs from postnatal rat optic nerve and cultured them in serum-free medium containing platelet-derived growth factor (PDGF), the main mitogen for OPCs, but in the absence of thyroid hormone in order to inhibit their differentiation into oligodendrocytes. We find that many of the cells continue to proliferate for more than a year and progressively acquire a number of the characteristics of OPCs isolated from adult optic nerve. These findings suggest that OPCs have an intrinsic maturation program that progressively changes the cell's phenotype over many months. When we culture the postnatal OPCs in the same conditions but with the addition of basic fibroblast growth factor (bFGF), the cells acquire these mature characteristics much more slowly, suggesting that the combination of bFGF and PDGF, previously shown to inhibit OPC differentiation, also inhibits OPC maturation. The challenge now is to determine the molecular basis of such a protracted maturation program and how the program is restrained by bFGF.