Stimulation of the myelin basic protein gene expression by 9-cis-retinoic acid and thyroid hormone: activation in the context of its native promoter

RXR agonist, 9-cis-13,14-dihydroretinoic acid (9CDHRA), reduces damage and protects from demyelination in transsynaptic degeneration model

Neurodegenerative and demyelinating disease, such as multiple sclerosis (MS) are at the forefront of medical research and the discovery of new drugs and therapeutics. One phenomenon of degeneration seen in these diseases is transsynaptic degeneration (TSD), where damage from one axon spreads to the other axons that are connected to it synaptically. It has previously been found that demyelination occurs prior to neuronal loss in an experimental form of induced TSD. Retinoid-x receptor (RXR) agonists have been shown to promote remyelination. Therefore, this study aimed to reveal the effects of a novel endogenous RXR-γ agonist, 9-cis-13,14-dihydroretinoic acid (9CDHRA), on preventing or restoring the effects of TSD. 9CDHRA was administered to mice following optic nerve crush (ONC) procedures, and electrophysiology (visual evoked potential, VEP) and histological (immunofluorescent) assessments were performed. It was found that 9CDHRA treatment effectively delayed glial activation and reduced the presence of apoptosis at the site of injury and further anterogradely in the visual system, including the lateral geniculate nucleus (LGN) and primary visual cortex (V1). Most notably, 9CDHRA was able to maintain myelin levels following ONC, and effectively protected from demyelination. This was corroborated by VEP recordings with improved P1 latency. The promising findings regarding the injury attenuating and myelin protecting properties of 9CDHRA necessitates further investigations into the potential therapeutic uses of this compound.

The role of myelin in neurodegeneration: implications for drug targets and neuroprotection strategies

Myelination of axons in the central nervous system offers numerous advantages, including decreased energy expenditure for signal transmission and enhanced signal speed. The myelin sheaths surrounding an axon consist of a multi-layered membrane that is formed by oligodendrocytes, while specific glycoproteins and lipids play various roles in this formation process. As beneficial as myelin can be, its dysregulation and degeneration can prove detrimental. Inflammation, oxidative stress, and changes in cellular metabolism and the extracellular matrix can lead to demyelination of these axons. These factors are hallmark characteristics of certain demyelinating diseases including multiple sclerosis. The effects of demyelination are also implicated in primary degeneration in diseases such as glaucoma and Alzheimer's disease, as well as in processes of secondary degeneration. This reveals a relationship between myelin and secondary processes of neurodegeneration, including resultant degeneration following traumatic injury and transsynaptic degeneration. The role of myelin in primary and secondary degeneration is also of interest in the exploration of strategies and targets for remyelination, including the use of anti-inflammatory molecules or nanoparticles to deliver drugs. Although the use of these methods in animal models of diseases have shown to be effective in promoting remyelination, very few clinical trials in patients have met primary end points. This may be due to shortcomings or considerations that are not met while designing a clinical trial that targets remyelination. Potential solutions include diversifying disease targets and requiring concomitant interventions to promote rehabilitation.

Thyroid hormone-dependent oligodendroglial cell lineage genomic and non-genomic signaling through integrin receptors

Multiple sclerosis (MS) is a heterogeneous autoimmune disease whereby the pathological sequelae evolve from oligodendrocytes (OLs) within the central nervous system and are targeted by the immune system, which causes widespread white matter pathology and results in neuronal dysfunction and neurological impairment. The progression of this disease is facilitated by a failure in remyelination following chronic demyelination. One mediator of remyelination is thyroid hormone (TH), whose reliance on monocarboxylate transporter 8 (MCT8) was recently defined. MCT8 facilitates the entry of THs into oligodendrocyte progenitor cell (OPC) and pre-myelinating oligodendrocytes (pre-OLs). Patients with MS may exhibit downregulated MCT8 near inflammatory lesions, which emphasizes an inhibition of TH signaling and subsequent downstream targeted pathways such as phosphoinositide 3-kinase (PI3K)-Akt. However, the role of the closely related mammalian target of rapamycin (mTOR) in pre-OLs during neuroinflammation may also be central to the remyelination process and is governed by various growth promoting signals. Recent research indicates that this may be reliant on TH-dependent signaling through β1-integrins. This review identifies genomic and non-genomic signaling that is regulated through mTOR in TH-responsive pre-OLs and mature OLs in mouse models of MS. This review critiques data that implicates non-genomic Akt and mTOR signaling in response to TH-dependent integrin receptor activation in pre-OLs. We have also examined whether this can drive remyelination in the context of neuroinflammation and associated sequelae. Importantly, we outline how novel therapeutic small molecules are being designed to target integrin receptors on oligodendroglial lineage cells and whether these are viable therapeutic options for future use in clinical trials for MS.

Oxaprozin Analogues as Selective RXR Agonists with Superior Properties and Pharmacokinetics

The retinoid X receptors (RXR) are ligand-activated transcription factors involved in multiple regulatory networks as universal heterodimer partners for nuclear receptors. Despite their high therapeutic potential in many pathologies, targeting of RXR has only been exploited in cancer treatment as the currently available RXR agonists suffer from exceptional lipophilicity, poor pharmacokinetics (PK), and adverse effects. Aiming to overcome the limitations and to provide improved RXR ligands, we developed a new potent RXR ligand chemotype based on the nonsteroidal anti-inflammatory drug oxaprozin. Systematic structure-activity relationship analysis enabled structural optimization toward low nanomolar potency similar to the well-established rexinoids. Cocrystal structures of the most active derivatives demonstrated orthosteric binding, and in vivo profiling revealed superior PK properties compared to current RXR agonists. The optimized compounds were highly selective for RXR activation and induced RXR-regulated gene expression in native cellular and in vivo settings suggesting them as excellent chemical tools to further explore the therapeutic potential of RXR.

Hormonal Regulation of Oligodendrogenesis II: Implications for Myelin Repair

Alterations in myelin, the protective and insulating sheath surrounding axons, affect brain function, as is evident in demyelinating diseases where the loss of myelin leads to cognitive and motor dysfunction. Recent evidence suggests that changes in myelination, including both hyper- and hypo-myelination, may also play a role in numerous neurological and psychiatric diseases. Protecting myelin and promoting remyelination is thus crucial for a wide range of disorders. Oligodendrocytes (OLs) are the cells that generate myelin, and oligodendrogenesis, the creation of new OLs, continues throughout life and is necessary for myelin plasticity and remyelination. Understanding the regulation of oligodendrogenesis and myelin plasticity within disease contexts is, therefore, critical for the development of novel therapeutic targets. In our companion manuscript, we review literature demonstrating that multiple hormone classes are involved in the regulation of oligodendrogenesis under physiological conditions. The majority of hormones enhance oligodendrogenesis, increasing oligodendrocyte precursor cell differentiation and inducing maturation and myelin production in OLs. Thus, hormonal treatments present a promising route to promote remyelination. Here, we review the literature on hormonal regulation of oligodendrogenesis within the context of disorders. We focus on steroid hormones, including glucocorticoids and sex hormones, peptide hormones such as insulin-like growth factor 1, and thyroid hormones. For each hormone, we describe whether they aid in OL survival, differentiation, or remyelination, and we discuss their mechanisms of action, if known. Several of these hormones have yielded promising results in both animal models and in human conditions; however, a better understanding of hormonal effects, interactions, and their mechanisms will ultimately lead to more targeted therapeutics for myelin repair.

Hormonal Regulation of Oligodendrogenesis I: Effects across the Lifespan

The brain’s capacity to respond to changing environments via hormonal signaling is critical to fine-tuned function. An emerging body of literature highlights a role for myelin plasticity as a prominent type of experience-dependent plasticity in the adult brain. Myelin plasticity is driven by oligodendrocytes (OLs) and their precursor cells (OPCs). OPC differentiation regulates the trajectory of myelin production throughout development, and importantly, OPCs maintain the ability to proliferate and generate new OLs throughout adulthood. The process of oligodendrogenesis, the creation of new OLs, can be dramatically influenced during early development and in adulthood by internal and environmental conditions such as hormones. Here, we review the current literature describing hormonal regulation of oligodendrogenesis within physiological conditions, focusing on several classes of hormones: steroid, peptide, and thyroid hormones. We discuss hormonal regulation at each stage of oligodendrogenesis and describe mechanisms of action, where known. Overall, the majority of hormones enhance oligodendrogenesis, increasing OPC differentiation and inducing maturation and myelin production in OLs. The mechanisms underlying these processes vary for each hormone but may ultimately converge upon common signaling pathways, mediated by specific receptors expressed across the OL lineage. However, not all of the mechanisms have been fully elucidated, and here, we note the remaining gaps in the literature, including the complex interactions between hormonal systems and with the immune system. In the companion manuscript in this issue, we discuss the implications of hormonal regulation of oligodendrogenesis for neurological and psychiatric disorders characterized by white matter loss. Ultimately, a better understanding of the fundamental mechanisms of hormonal regulation of oligodendrogenesis across the entire lifespan, especially in vivo, will progress both basic and translational research.

Towards a comprehensive etiopathogenetic and pathophysiological theory of multiple sclerosis

Background: Multiple sclerosis (MS) is a neurodegenerative disease caused by dysfunction of the immune system that affects the central nervous system (CNS). It is characterized by demyelination, chronic inflammation, neuronal and oligodendrocyte loss and reactive astrogliosis. It can result in physical disability and acute neurological and cognitive problems. Despite the gains in knowledge of immunology, cell biology, and genetics in the last five decades, the ultimate etiology or specific elements that trigger MS remain unknown. The objective of this review is to propose a theoretical basis for MS etiopathogenesis.Methods: Search was done by accessing PubMed/Medline, EBSCO, and PsycINFO databases. The search string used was "(multiple sclerosis* OR EAE) AND (pathophysiology* OR etiopathogenesis)". The electronic databases were searched for titles or abstracts containing these terms in all published articles between January 1, 1960, and June 30, 2019. The search was filtered down to 362 articles which were included in this review.Results: A framework to better understand the etiopathogenesis and pathophysiology of MS can be derived from four essential factors; mitochondria dysfunction (MtD) & oxidative stress (OS), vitamin D (VD), sex hormones and thyroid hormones. These factors play a direct role in MS etiopathogenesis and have a modulatory effect on many other factors involved in the disease.Conclusions: For better MS prevention and treatment outcomes, efforts should be geared towards treating thyroid problems, sex hormone alterations, VD deficiency, sleep problems and melatonin alterations. MS patients should be encouraged to engage in activities that boost total antioxidant capacity (TAC) including diet and regular exercise and discouraged from activities that promote OS including smoking and alcohol consumption.

Perinatal and postweaning diets high in omega-3 fatty acids have age- and sex-specific effects on the fatty acid composition of the cerebellum and brainstem of C57BL/6 mice

The sex- and age-specific effects of omega (n)-3 polyunsaturated fatty acids (PUFA) enriched diets on brainstem and cerebellar fatty acid composition, and the expression of stearoyl-CoA desaturase (SCD)-1 and myelin basic protein (MBP) were investigated in C57BL/6 mice. Female mice were fed diets (20% fat, w/w) high or low in n-3 PUFA before mating, during pregnancy and lactation; and offspring (both males and females) were weaned onto their mother's designated diet for 16 weeks. A diet high in n-3 PUFA caused an accretion of docosahexaenoic acid in the cerebellum. Monounsaturated fatty acids increased from weaning to 16 weeks in the cerebellum. The changes in the cerebellar fatty acids were more pronounced in females, with a significant effect of diet. A diet high in n-3 PUFA increased cerebellar SCD-1 and MBP mRNA expression. These findings are novel and demonstrate that the effects of n-3 PUFA are brain region, age- and sex-specific.

Nutritional supplementation of vitamin A and health-related outcomes in patients with multiple sclerosis: A protocol for a systematic review and meta-analysis of randomized clinical trials

BACKGROUND

Multiple sclerosis (MS) is a chronic immune mediated disease which affects the central nervous system (CNS), having a substantial financial, functional, and quality of life (QOL) impact on these people. The vitamin A supplementation has been studied as a therapeutic possibility for in MS. Therefore, the objective of this protocol is to build an outline for a future systematic review, which will provide up-to-date available evidence about the clinical impact of nutritional supplementation of vitamin A in the outcomes related to the symptoms in patients with this pathology.

METHODS

The search will be performed in the following databases: PubMed, Embase, Scopus, cinahl, Scielo, Web of Science, the Cochrane Library and Science Direct, randomized clinical trials published until May 2019 that evaluate the relationship of the supplementation of vitamin A and health-related outcomes in patients with MS will be included. Preferred Reporting Items for Systematic review and Meta-Analysis Protocols (PRISMA-P) will be used to outline the protocol, and PRISMA to the systematic review. Undergraduate handbook of quality of evidence and strength of recommendations for decision making in health (GRADE) will be used to assess the quality of evidence and the strength of the recommendation, and the JADAD scale to assess the internal validity of selected studies. For the extraction of all the data found a database in Microsoft Excel will be created. For the summary of the findings the Cochrane Collaboration Handbook recommendations will be used, and for the meta-analysis standard statistical techniques the RevMan software will be used.

RESULTS

In this study, we hope to find a considerable number of articles presenting evidence about the effectiveness of vitamin A supplementation in patients with MS.

CONCLUSION

Currently, many lines of evidence have been produced when it comes to the use of food supplements. This systematic review proposal might provide recent, important, and trusted information for better treatment of patients.

RECORD OF SYSTEMATIC REVIEW

This review was recorded in the International Register of Prospective Systematic Reviews (PROSPERO) on the January 30, 2019 (registration: CRD42019121757). Available at: http://www.crd.york.ac.uk/PROSPERO/display_record.php?ID=CRD42019121757.

A fungicide miconazole ameliorates tri-o-cresyl phosphate-induced demyelination through inhibition of ErbB/Akt pathway

Organophosphorus compound (OP)-induced delayed neuropathy (OPIDN) is characterized by distal axonal degeneration and demyelination of the central and peripheral axons, which leads to progressive muscle weakness, ataxia and paralysis in several days after OP intoxication. This study aimed to investigate the possible use of an imidazole fungicide miconazole as a novel therapy for OPIDN. Adult hens, the most commonly used animal models in OPIDN studies, were orally given tri-o-cresyl phosphate (TOCP). We showed that miconazole, which was administered daily to hens beginning on the 7th day after TOCP exposure, drastically ameliorated the neurotoxic symptoms and histopathological damages in spinal cord and sciatic nerves. Mechanistically, miconazole inhibited the TOCP-induced activation of ErbB/Akt signaling, and enhanced the myelin basic protein (MBP) expression. In a glial cell model sNF96.2 cells, miconazole restored the TOCP-inhibited MBP expression, and promoted cell differentiation as well as cell migration by inhibiting the activation of ErbB/Akt signaling pathway. In sum, miconazole, a synthetic imidazole fungicide, could ameliorate the symptoms and histopathological changes of OPIDN, probably by promoting glial cell differentiation and migration to enhance myelination via inhibiting the activation of ErbB/Akt. Thus, miconazole is a promising candidate therapy for the clinical treatment of OPIDN.

1,25-Dihydroxyvitamin D3 suppressed experimental autoimmune encephalomyelitis through both immunomodulation and oligodendrocyte maturation

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) has recently been found to have the anti-inflammatory potential to suppress experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis; however, its direct effect on neural cells is not clear. In the current study we show that 1,25(OH)2D3 treatment effectively suppressed clinical signs of ongoing EAE and reduced inflammation and demyelination scores in the central nervous system (CNS). The treatment significantly decreased production/expression of pro-inflammatory cytokines IFN-γ, GM-CSF and IL-17A, while it increased anti-inflammatory cytokines IL-4 and IL-10. Further, 1,25(OH)2D3 treatment effectively elevated the numbers of neural stem cells, oligodendrocyte precursor cells, as well as oligodendrocytes in disease lesions in the CNS. These results, together with its in vitro effect of inducing oligodendrocyte differentiation as shown in our previous findings, demonstrate that 1,25(OH)2D3 suppressed EAE not only by its immunomodulatory capacity, but also by its effect on oligodendrocyte differentiation and maturation, and thus has potential for remyelination and neural repair.

Altered Translational Control of Fragile X Mental Retardation Protein on Myelin Proteins in Neuropsychiatric Disorders

Myelin is a specialized structure of the nervous system that both enhances electrical conductance and insulates neurons from external risk factors. In the central nervous system, polarized oligodendrocytes form myelin by wrapping processes in a spiral pattern around neuronal axons through myelin-related gene regulation. Since these events occur at a distance from the cell body, post-transcriptional control of gene expression has strategic advantage to fine-tune the overall regulation of protein contents in situ. Therefore, many research interests have been focused to identify RNA binding proteins and their regulatory mechanism in myelinating compartments. Fragile X mental retardation protein (FMRP) is one such RNA binding protein, regulating its target expression by translational control. Although the majority of works on FMRP have been performed in neurons, it is also found in the developing or mature glial cells including oligodendrocytes, where its function is not well understood. Here, we will review evidences suggesting abnormal translational regulation of myelin proteins with accompanying white matter problem and neurological deficits in fragile X syndrome, which can have wider mechanistic and pathological implication in many other neurological and psychiatric disorders.

Pharmacological approaches to intervention in hypomyelinating and demyelinating white matter pathology

White matter disease afflicts both developing and mature central nervous systems. Both cell intrinsic and extrinsic dysregulation result in profound changes in cell survival, axonal metabolism and functional performance. Experimental models of developmental white matter (WM) injury and demyelination have not only delineated mechanisms of signaling and inflammation, but have also paved the way for the discovery of pharmacological approaches to intervention. These reagents have been shown to enhance protection of the mature oligodendrocyte cell, accelerate progenitor cell recruitment and/or differentiation, or attenuate pathological stimuli arising from the inflammatory response to injury. Here we highlight reports of studies in the CNS in which compounds, namely peptides, hormones, and small molecule agonists/antagonists, have been used in experimental animal models of demyelination and neonatal brain injury that affect aspects of excitotoxicity, oligodendrocyte development and survival, and progenitor cell function, and which have been demonstrated to attenuate damage and improve WM protection in experimental models of injury. The molecular targets of these agents include growth factor and neurotransmitter receptors, morphogens and their signaling components, nuclear receptors, as well as the processes of iron transport and actin binding. By surveying the current evidence in non-immune targets of both the immature and mature WM, we aim to better understand pharmacological approaches modulating endogenous oligodendroglia that show potential for success in the contexts of developmental and adult WM pathology. This article is part of the Special Issue entitled 'Oligodendrocytes in Health and Disease'.

Effects of electroacupuncture and the retinoid X receptor (RXR) signalling pathway on oligodendrocyte differentiation in the demyelinated spinal cord of rats

Objectives

In spinal cord demyelination, some oligodendrocyte precursor cells (OPCs) remain in the demyelinated region but have a reduced capacity to differentiate into oligodendrocytes. This study investigated whether ‘Governor Vessel’ (GV) electroacupuncture (EA) would promote the differentiation of endogenous OPCs into oligodendrocytes by activating the retinoid X receptor γ (RXR-γ)-mediated signalling pathway.

Methods

Adult rats were microinjected with ethidium bromide (EB) into the T10 spinal cord to establish a model of spinal cord demyelination. EB-injected rats remained untreated (EB group, n=26) or received EA treatment (EB+EA group, n=26). A control group (n=26) was also included that underwent dural exposure without EB injection. After euthanasia at 7 days (n=5 per group), 15 days (n=8 per group) or 30 days (n=13 per group), protein expression of RXR-γ in the demyelinated spinal cord was evaluated by immunohistochemistry and Western blotting. In addition, OPCs derived from rat embryonic spinal cord were cultured in vitro, and exogenous 9-cis-RA (retinoic acid) and RXR-γ antagonist HX531 were administered to determine whether RA could activate RXR-γ and promote OPC differentiation.

Results

EA was found to increase the numbers of both OPCs and oligodendrocytes expressing RXR-γ and RALDH2, and promote remyelination in the remyelinated spinal cord. Exogenous 9-cis-RA enhanced the differentiation of OPCs into mature oligodendrocytes by activating RXR-γ.

Conclusions

The results suggest that EA may activate RXR signalling to promote the differentiation of OPCs into oligodendrocytes in spinal cord demyelination.

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.

Mechanostimulation Promotes Nuclear and Epigenetic Changes in Oligodendrocytes

Oligodendrocyte progenitors respond to biophysical or mechanical signals, and it has been reported that mechanostimulation modulates cell proliferation, migration, and differentiation. Here we report the effect of three mechanical stimuli on mouse oligodendrocyte progenitor differentiation and identify the molecular components of the linker of nucleoskeleton and cytoskeleton (LINC) complex (i.e., SYNE1) as transducers of mechanical signals to the nucleus, where they modulate the deposition of repressive histone marks and heterochromatin formation. The expression levels of LINC components increased during progenitor differentiation and silencing the Syne1 gene resulted in aberrant histone marks deposition, chromatin reorganization and impaired myelination. We conclude that spatial constraints, via the actin cytoskeleton and LINC complex, mediate nuclear changes in oligodendrocyte progenitors that favor a default pathway of differentiation. Significance statement: It is recognized that oligodendrocyte progenitors are mechanosensitive cells. However, the molecular mechanisms translating mechanical stimuli into oligodendrocyte differentiation remain elusive. This study identifies components of the mechanotransduction pathway in the oligodendrocyte lineage.

Cocaine-induced loss of white matter proteins in the adult mouse nucleus accumbens is attenuated by administration of a β-lactam antibiotic during cocaine withdrawal

We report significantly decreased white matter protein levels in the nucleus accumbens in an adult mouse model of chronic cocaine abuse. Previous studies from human cocaine abuse patients show disruption of white matter and myelin loss, thus supporting our observations. Understanding the neuropathological mechanisms for white matter disruption in cocaine abuse patients is complicated by polydrug use and other comorbid factors, hindering the development of effective therapeutic strategies to ameliorate damage or compliment rehabilitation programs. In this context, our data further demonstrate that cocaine-induced loss of white matter proteins is absent in mice treated with the β-lactam antibiotic, ceftriaxone, during cocaine withdrawal. Other studies report that ceftriaxone, a glutamate transporter subtype-1 activator, is neuroprotective in murine models of multiple sclerosis, thereby demonstrating potential therapeutic properties for diseases with white matter loss. Cocaine-induced white matter abnormalities likely contribute to the cognitive, motor, and psychological deficits commonly afflicting cocaine abusers, yet the underlying mechanisms responsible for these changes remain unknown. Our observations describe an adult animal model for the study of cocaine-induced myelin loss for the first time, and highlight a potential pharmacological intervention to ameliorate cocaine-induced white matter loss.

Retinol levels are associated with magnetic resonance imaging outcomes in multiple sclerosis

BACKGROUND

Vitamin A has immunomodulatory properties and may regulate the transcription of genes involved in remyelination.

OBJECTIVE

To investigate the association between retinol and disease activity in multiple sclerosis (MS).

METHODS

Cohort study of 88 relapsing-remitting MS patients, originally included in a randomised placebo-controlled trial of omega-3 fatty acids in MS (the OFAMS study), followed prospectively for 24 months with repeated assessments of serum-retinol and magnetic resonance imaging (MRI). All patients were initiated on interferon β-1a after month 6.

RESULTS

Each 1 µmol/L increase in serum-retinol reduced the odds (95% confidence interval) for new T1 gadolinium enhanced (Gd(+)) lesions by 49 (8-70)%, new T2 lesions by 42 (2-66)%, and combined unique activity (CUA) by 46 (3-68)% in simultaneous MRI scans, and 63 (25-82)% for new T1Gd(+) lesions, 49 (3-73)% for new T2 lesions and 43 (12-71)% for CUA the subsequent month. Serum-retinol also predicted new T1Gd(+) and T2 lesions six months ahead. The associations were not affected by HLA-DRB1*15, or serum levels of 25-hydroxyvitamin D, eicosapentaenoic acid or docosahexaenoic acid.

CONCLUSION

Serum retinol is inversely associated with simultaneous and subsequent MRI outcomes in RRMS.

Differential modulation of the oligodendrocyte transcriptome by sonic hedgehog and bone morphogenetic protein 4 via opposing effects on histone acetylation

Differentiation of oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes is regulated by the interplay between extrinsic signals and intrinsic epigenetic determinants. In this study, we analyze the effect that the extracellular ligands sonic hedgehog (Shh) and bone morphogenetic protein 4 (BMP4), have on histone acetylation and gene expression in cultured OPCs. Shh treatment favored the progression toward oligodendrocytes by decreasing histone acetylation and inducing peripheral chromatin condensation. BMP4 treatment, in contrast, inhibited the progression toward oligodendrocytes and favored astrogliogenesis by favoring global histone acetylation and retaining euchromatin. Pharmacological treatment or silencing of histone deacetylase 1 (Hdac1) or histone deacetylase 2 (Hdac2) in OPCs did not affect BMP4-dependent astrogliogenesis, while it prevented Shh-induced oligodendrocyte differentiation and favored the expression of astrocytic genes. Transcriptional profiling of treated OPCs, revealed that BMP4-inhibition of oligodendrocyte differentiation was accompanied by increased levels of Wnt (Tbx3) and Notch-target genes (Jag1, Hes1, Hes5, Hey1, and Hey2), decreased recruitment of Hdac and increased histone acetylation at these loci. Similar upregulation of Notch-target genes and increased histone acetylation were observed in the corpus callosum of mice infused with BMP4 during cuprizone-induced demyelination. We conclude that Shh and Bmp4 differentially regulate histone acetylation and chromatin structure in OPCs and that BMP4 acts as a potent inducer of gene expression, including Notch and Wnt target genes, thereby enhancing the crosstalk among signaling pathways that are known to inhibit myelination and repair.

The complex world of oligodendroglial differentiation inhibitors

Myelination is a central nervous system (CNS) process wherein oligodendrocyte-axon interactions lead to the establishment of myelin sheaths that stabilize, protect, and electrically insulate axons. In inflammatory demyelinating diseases such as multiple sclerosis (MS), the degeneration and eventual loss of functional myelin sheaths slows and blocks saltatory conduction in axons, which results in clinical impairment. However, remyelination can occur, and lesions can be partially repaired, resulting in clinical remission. The recruitment and activation of resident oligodendrocyte precursor cells (OPCs) play a critical role in the repair process because these cells have the capacity to differentiate into functional myelinating cells. Mature oligodendrocytes, however, are thought to have lost the capacity to develop new myelin sheaths and frequently undergo programmed cell death in MS. The endogenous capacity to generate new oligodendrocytes in MS is limited, and this is predominantly due to the presence of inhibitory components that block OPC differentiation and maturation. Here, we present an overview of recently identified negative regulators of oligodendroglial differentiation and their potential relevance for CNS repair in MS. Because currently available immunomodulatory drugs for MS mainly target inflammatory cascades outside the brain and fail to repair existing lesions, achieving more efficient lesion repair constitutes an important goal for future MS therapies.

Retinoid X receptor gamma signaling accelerates CNS remyelination

The molecular basis of CNS myelin regeneration (remyelination) is poorly understood. We generated a comprehensive transcriptional profile of the separate stages of spontaneous remyelination that follow focal demyelination in the rat CNS and found that transcripts that encode the retinoid acid receptor RXR-γ were differentially expressed during remyelination. Cells of the oligodendrocyte lineage expressed RXR-γ in rat tissues that were undergoing remyelination and in active and remyelinated multiple sclerosis lesions. Knockdown of RXR-γ by RNA interference or RXR-specific antagonists severely inhibited oligodendrocyte differentiation in culture. In mice that lacked RXR-γ, adult oligodendrocyte precursor cells efficiently repopulated lesions after demyelination, but showed delayed differentiation into mature oligodendrocytes. Administration of the RXR agonist 9-cis-retinoic acid to demyelinated cerebellar slice cultures and to aged rats after demyelination caused an increase in remyelinated axons. Our results indicate that RXR-γ is a positive regulator of endogenous oligodendrocyte precursor cell differentiation and remyelination and might be a pharmacological target for regenerative therapy in the CNS.

Drug reprofiling using zebrafish identifies novel compounds with potential pro-myelination effects

Treatment of the autoimmune demyelinating disease multiple sclerosis (MS) requires therapies that both limit and repair damage. While several immunomodulatory treatments exist to limit damage there are currently no treatments that promote the regenerative process of remyelination. A rapid way of screening potential pro-remyelination compounds is therefore required. The use of larval zebrafish in a drug reprofiling screen allows rapid in vivo screening and has been used successfully in the past as an efficient way of identifying new indications for existing drugs. A novel screening platform for potential pro-myelination compounds was developed using zebrafish larvae. Two percent of compounds screened from reprofiling libraries altered oligodendrocyte lineage cell recruitment and/or proliferation, as measured by the numbers of dorsally migrated spinal cord olig2(+) cells. Selective screening identified three compounds that altered levels of myelination, as measured by whole larvae myelin basic protein (mbp) transcript levels; the src family kinase inhibitor PP2, a biogenic amine and a thioxanthene. As well as many previously unrecognised compounds, identified compounds included those with previously known effects on myelin and/or the oligodendrocyte lineage, such as a PPAR agonist, steroid hormones and src family kinase inhibitors. As well as providing methods for further assessment of potentially beneficial compounds, this screen has highlighted 25 targets that are able to alter oligodendrocyte lineage cell recruitment or proliferation and/or mbp transcript levels in vivo and are worthy of further investigation for their potential effects on remyelination.

Epigenetic regulation of oligodendrocyte identity

The interplay of transcription factors and epigenetic modifiers, including histone modifications, DNA methylation and microRNAs during development is essential for the acquisition of specific cell fates. Here, we review the epigenetic "programming" of stem cells into oligodendrocytes, by analyzing three sequential stages of lineage progression. The first transition from pluripotent stem cells to neural precursors is characterized by repression of pluripotency genes and restriction of the lineage potential to the neural fate. The second transition from multipotential precursors to oligodendrocyte progenitors is associated with the progressive loss of plasticity and the repression of neuronal and astrocytic genes. The last step of differentiation of oligodendrocyte progenitors into myelin-forming cells is defined by a model of derepression of myelin genes.

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.

Two-tier transcriptional control of oligodendrocyte differentiation

Oligodendrocytes (OLs) are the myelin-forming cells of the central nervous system (CNS). They differentiate from proliferative OL precursor cells that migrate from the embryonic neuroepithelium throughout the developing CNS before associating with axons and elaborating myelin. Recent research into the regulation of OL differentiation has uncovered a two-stage mechanism of transcriptional control that combines epigenetic repression of transcriptional inhibitors with direct transcriptional activation of myelin genes. This 'two-pronged' approach creates a fail-safe system of genetic control to ensure orderly and unambiguous expression of the myelination program during development and during repair of demyelinated lesions.

Effects of hexabromocyclododecane and polybrominated diphenyl ethers on mRNA expression in chicken (Gallus domesticus) hepatocytes

Hexabromocyclododecane (HBCD) and polybrominated diphenyl ethers (PBDEs) are additive flame retardants used in a wide range of consumer products. Both compounds have been detected in free-living avian species, but toxicological and molecular end points of exposure are limited. An in vitro approach was used to compare concentration-dependent effects of HBCD and the commercial penta-brominated diphenyl ether mixture DE-71 on cytotoxicity and mRNA expression in cultured hepatocytes derived from embryonic chickens. Neither HBCD-alpha, HBCD-technical mixture (TM), nor DE-71 effected hepatocyte viability at the highest concentrations assessed (30-100 microM). Real-time RT-PCR assays were developed to quantify changes in mRNA abundance of genes associated with chicken xenobiotic-sensing orphan nuclear receptor activation, the thyroid hormone (TH) pathway, and lipid regulation. Exposure to >or= 1 microM HBCD-alpha and HBCD-TM resulted in significant upregulation of cytochrome P450 (CYP) 2H1 (fourfold to sevenfold) and CYP3A37 (5- to 30-fold) at 24 and 36 h. In contrast, 30 microM DE-71 caused a twofold increase of CYP2H1 only. UGT1A9 expression was only upregulated by HBCD-alpha to a maximum of fourfold at >or= 1 microM. Transthyretin, thyroid hormone-responsive spot 14-alpha, and liver fatty acid-binding protein were all significantly downregulated (up to sevenfold) for cells exposed to >or= 1 microM HBCD-alpha and HBCD-TM. DE-71 also downregulated these three target genes twofold to fivefold at concentrations >or= 3 microM. Taken together, our results indicate that xenobiotic-metabolizing enzymes and genes associated with the TH pathway and lipid regulation are vulnerable to HBCD and DE-71 administration in cultured avian hepatocytes and might be useful molecular markers of exposure.

Effects of perinatal hypothyroidism on rat behavior and its relation with apoptosis of hippocampus neurons

Thyroid hormone is an important factor for proper development of the mammalian brain. Perinatal hypothyroidism leads to long-term behavior and neuromotor competence alterations in humans and animals. Our study aimed to investigate the effects of perinatal hypothyroidism on behavior changes of rat pups and its relation with the apoptosis of hippocampus neurons. Behavior tests were taken to evaluate the effects caused by perinatal hypothyroidism. TUNEL staining was used to analyze the apoptosis of neurons on CA3 region of hippocampus. The study suggested that perinatal hypothyroidism affects behavior development, as well as leading to the decrease in spatial learning and memory capability. This condition can be improved with hormone substitute treatment. Furthermore, the changes of learning and memory capability are closely related to the increasing number of apoptotic neurons in the hippocampus.

Transcriptional control of oligodendrogenesis

Oligodendrocytes (OGs) assemble the myelin sheath around axons in the central nervous system. Specification of cells into the OG lineage is largely the result of interplay between bone morphogenetic protein, sonic hedgehog and Notch signaling pathways, which regulate expression of transcription factors (TFs) dictating spatial and temporal aspects of oligodendrogenesis. Many of these TFs and others then direct OG development through to a mature myelinating OG. Here we describe signaling pathways and TFs that are inductive, inhibitory, and/or permissive to OG specification and maturation. We develop a basic transcriptional network and identify similarities and differences between regulation of oligodendrogenesis in the spinal cord and brain.

Differential gene expression during development in two oligodendroglial cell lines overexpressing transferrin: a cDNA array analysis

In the central nervous system, transferrin (Tf) is produced by oligodendroglial cells (OLGcs) and is essential for their development. Recently, using the complete cDNA of the human Tf gene, we obtained clones overexpressing Tf in two OLGc lines, N19 and N20.1, which represent different stages of differentiation. We showed that the overexpression of this glycoprotein promotes the maturation and myelinogenic capacity of both cell lines. In this work, using cDNA array technology, we examined changes induced by Tf in 1,176 genes. We found 41 genes differentially expressed in both cell lines, all of them involved in OLGc development. In the less mature cells (N19) overexpressing Tf, there was a significant increase in key enzymes of neurosteroid metabolism, such as cholesterol side chain cleavage cytochrome P450, 3beta-hydroxysteroid dehydrogenase and 5alpha-reductase type 1. In the more mature cell line (N20.1), Tf overexpression produced an induction of several mRNAs of the GABA(A) receptor subunits, of thyroid hormone receptors and of proteins involved in axon-glia interactions such as F3/contactin. In addition, in both cell lines, Tf overexpression induced an increase in the expression of different isoforms of transforming growth factor beta receptors and in several genes related to mitochondrial function and to complex lipid metabolism, crucial steps in myelin synthesis. Differentiation produced by Tf in both cell lines seems to occur by modulation of different genes depending on the maturational stage of the cells. Our findings provide new insights into the molecular basis of OLGc differentiation and on the role played by Tf in this process.

A screen for mutations in zebrafish that affect myelin gene expression in Schwann cells and oligodendrocytes

Myelin is the multi-layered glial sheath around axons in the vertebrate nervous system. Myelinating glia develop and function in intimate association with neurons and neuron-glial interactions control much of the life history of these cells. However, many of the factors that regulate key aspects of myelin development and maintenance remain unknown. To discover new molecules that are important for glial development and myelination, we undertook a screen of zebrafish mutants with previously characterized neural defects. We screened for myelin basic protein (mbp) mRNA by in situ hybridization and identified four mutants (neckless, motionless, iguana and doc) that lacked mbp expression in parts of the peripheral and central nervous systems (PNS or CNS), despite the presence of axons. In all four mutants electron microscopy revealed that myelin-forming glia were present and had formed loose wraps around axons but did not form compact myelin. We found that addition of exogenous retinoic acid (RA) rescued mbp expression in neckless mutant embryos, which lack endogenous RA synthesis. Timed application of the RA synthesis inhibitor DEAB to wild type embryos showed that RA signalling is required at least 48 h before the onset of myelin protein synthesis in both CNS and PNS.

Thyroid hormones and retinoids: a possible link between genes and environment in schizophrenia

Phenotypic discordance for schizophrenia in monozygotic twins clearly indicates involvement of environmental factors as key determinants in disease development. Positive findings from genome scans, linkage and association studies apply in only a minority of those affected, while post-mortem brain investigations reveal altered expression of genes and proteins involved in numerous neurodevelopmental, metabolic and neurotransmitter pathways. Such altered expressions could result, on the one hand, from mutations in coding regions or polymorphisms in the promoter and regulatory regions in genes within those areas identified by gene searches or, on the other hand, from inadequate amounts of modulators, transporters and synthesizers of transcription factors necessary for regulation of the putative genes. Hormones and vitamins are such modulators. They could serve as bridges between genes and environment in schizophrenia. Multiple evidence supports the suggestion of retinoids and thyroid hormones as plausible actors in these roles. Both are not only essential for normal development of the central nervous system but also regulate the expression of many neurotransmitters, their synthesizing enzymes and receptors, and other genes in broader signaling transduction cascades affecting pathways that are altered in response to treatment. Functional and positional candidate genes include retinoic acid and thyroid hormone receptors, retinaldehyde dehydrogenases and deiodinases, which synthesize the powerful morphogens, retinoic acid and triiodothyronine, and the enzymes involved in their inactivation. This review highlights selective evidence supporting the retinoid and thyroid hormone hypotheses of schizophrenia.

Thyroid hormone regulates oligodendrocyte accumulation in developing rat brain white matter tracts

Thyroid hormone (TH) is necessary for normal axonal myelination. Myelin basic protein (MBP) is a structural protein essential for myelin function. In this study, we demonstrate that perinatal hypothyroidism regulates MBP mRNA levels via indirect mechanisms. We observed decreased MBP mRNA accumulation in the hypothyroid rat brain at postnatal (PN) d 10 and 50. Acute TH replacement did not rescue hypothyroid MBP mRNA levels at PN5, 10, or 50. TH is necessary for normal intrahemispheric commissure development including the anterior commissure (AC) and the corpus callosum (CC). We determined that perinatal hypothyroidism decreases AC area and cellularity in the developing rat brain by PN10 and 50. In the developing CC, hypothyroidism initially increases area and cellularity by PN5, but then ultimately decreases area and cellularity by PN50. MBP-expressing oligodendrocytes are a recognized target of TH and are responsible for myelination within intrahemispheric commissures. We found that hypothyroidism reduces the number of mature oligodendrocytes within both the AC and CC. This reduction is noted at PN5, 10, and 50 in the AC and by PN10 and 50 in the CC. Together, these data suggest that TH regulates MBP mRNA levels through indirect mechanisms. These data demonstrate the complex mechanisms whereby TH regulates myelination in the developing brain.

Thyroid hormone induction of the adrenoleukodystrophy-related gene (ABCD2)

X-linked adrenoleukodystrophy (X-ALD) is a demyelinating disorder associated with impaired very-long-chain fatty-acid (VLCFA) beta-oxidation caused by mutations in the ABCD1 (ALD) gene that encodes a peroxisomal membrane ABC transporter. ABCD2 (ALDR) displays partial functional redundancy because when overexpressed, it is able to correct the X-ALD biochemical phenotype. The ABCD2 promoter contains a putative thyroid hormone-response element conserved in rodents and humans. In this report, we demonstrate that the element is capable of binding retinoid X receptor and 3,5,3'-tri-iodothyronine (T3) receptor (TRbeta) as a heterodimer and mediating T3 responsiveness of ABCD2 in its promoter context. After a T3 treatment, an induction of the ABCD2 gene was observed in the liver of normal rats but not that of TRbeta-/- mice. ABCD2 was not induced in the brain of the T3-treated rats. However, we report for the first time that induction of the ABCD2 redundant gene is feasible in myelin-producing cells (differentiated CG4 oligodendrocytes). The induction was specific for this cell type because it did not occur in astrocytes. Furthermore, we observed T3 induction of ABCD2 in human and mouse ABCD1-deficient fibroblasts, which was correlated with normalization of the VLCFA beta-oxidation. Finally, ABCD3 (PMP70), a close homolog of ABCD2, was also induced by T3 in the liver of control rats, but not that of TRbeta-/- mice, and in CG4 oligodendrocytes.

TNFalpha downregulates PPARdelta expression in oligodendrocyte progenitor cells: implications for demyelinating diseases

TNFalpha has been implicated in several demyelinating disorders, including multiple sclerosis (MS) and X-adrenoleukodystrophy (X-ALD). TNFalpha abundance is greatly increased in the areas surrounding damaged regions of the central nervous system of patients with MS and X-ALD, but its role in the observed demyelination remains to be elucidated. A class of nuclear receptors, the peroxisome proliferator-activated receptors (PPARs), has been implicated in several physiological and pathological processes. In particular, PPARdelta has been shown to promote oligodendrocyte (OL) survival and differentiation and PPARgamma has been implicated in inflammation. In the present study, we investigate on the effects of TNFalpha on OLs during differentiation in vitro. The results obtained show that TNFalpha treatment impairs PPARdelta expression with concomitant decrease of lignocerolyl-CoA synthase and very-long-chain fatty acid beta-oxidation as well as plasmalogen biosynthesis. We propose a hypothetical model possibly explaining the perturbation effects of proinflammatory cytokines on myelin synthesis, maturation, and turnover.

Thyroid hormone actions on neural cells

1. In addition to its role in cellular metabolic activity, thyroid hormone (TH) is critically involved in growth, development, and function of the central nervous system. In the brain, as in other structures, TH is described to exert its major action by the binding of L-3,5,3'-triiodothyronine (T3), considered as the bioactive form of the hormone, to nuclear thyroid hormone receptors (TR) that function as ligand-dependent transcription factors. 2. The transcription of numerous brain genes was indeed shown to be positively or negatively regulated by TH, turning these TR-mediated effects one explanation for the physiological effects of TH. In this context, the knowledge from TR-knockout studies provides some surprising results, since neonatal hypothyroidism is associated to more significant abnormalities than is TR deficiency. Some (nonexclusive) hypotheses include a permissive effect of TH, allowing derepression of unliganded-TR effects and non-TR-mediated effects of the hormone, further emphasizing the importance of a controlled accessibility of neural cells to TH. 3. On the other hand, T3 was demonstrated to directly act not only on neuronal but also on glial cells proliferation and differentiation, contributing to the harmonious development of the brain. Interestingly, in addition to these direct actions on neuronal and glial cells, several lines of evidence, notably developped in our laboratory, point out the role of thyroid hormone in neuronal-glial interactions.

Thyroid hormone is a critical determinant for the regulation of the cochlear motor protein prestin

The most impressive property of outer hair cells (OHCs) is their ability to change their length at high acoustic frequencies, thus providing the exquisite sensitivity and frequency-resolving capacity of the mammalian hearing organ. Prestin, a protein related to a sulfate/anion transport protein, recently has been identified and proposed as the OHC motor molecule. Homology searches of 1.5 kb of genomic DNA 5' of the coding region of the prestin gene allowed the identification of a thyroid hormone (TH) response element (TRE) in the first intron upstream of the prestin ATG codon. Prestin(TRE) bound TH receptors as a monomer or presumptive heterodimer and mediated a triiodothyronine-dependent transactivation of a heterologous promotor in response to triiodothyronine receptors alpha and beta. Retinoid X receptor-alpha had an additive effect. Expression of prestin mRNA and prestin protein was reduced strongly in the absence of TH. Although prestin protein typically was redistributed to the lateral membrane before the onset of hearing, an immature pattern of prestin protein distribution across the entire OHC membrane was noted in hypothyroid rats. The data suggest TH as a first transcriptional regulator of the motor protein prestin and as a direct or indirect modulator of subcellular prestin distribution.

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.

Are hypothyroidism and iron deficiency precursor cell diseases?

It is neither known why hormonal and nutritional deficiencies only cause neurological abnormalities at particular periods of development, nor is it known why repair only occurs if normal metabolic status is restored within a particular temporal window. We propose that these maladies are precursor cell diseases, in which the normal balance between self-renewal and differentiation is compromised in dividing precursor cells. According to this hypothesis, the windows of vulnerability in these disorders correspond to the timing of particular transitions in CNS precursor cells, as seen in our studies on the effects of thyroid hormone and iron on the generation of oligodendrocytes and their immediate ancestor, the O-2A progenitor cell.

Thyroid hormone and retinoids affect motoneuron phenotype and reaction after axotomy in the spinal cord of adult rats

Motoneuron phenotype in the spinal cord is regulated by an intrinsic genetic program, extrinsic environmental signals and target-derived molecules. Axonal lesions trigger a phenotype switch to foster repair phenomena and axonal re-growth. We have investigated the influence of the long-term treatment with thyroid hormone and all trans retinol palmitate (RA) on motoneuron phenotype and spinal cord reaction to axotomy in adult male rats. Neurochemical markers, investigated by in situ hybridization and immunocytochemistry, included choline acetyltransferase (ChAT), calcitonin gene-related peptide (CGRP) and neurotrophin low affinity receptor p75. Treatment was administered for 56 days and then mid-thigh sciatic axotomy was performed on a number of animals from each experimental groups; the rats were examined 9 days after surgery. The results indicate that: (1) Number and size of ChAT-immunoreactive neurons in the lumbar tract of the spinal cord was reduced in hypothyroid compared to control rats, whereas steady-state level of ChAT mRNA in labelled motoneurons failed to be modified by hypo and hyperthyroidism, but was increased by RA administration; (2) none of the administered treatments did alter CGRP mRNA level, whereas all of them influenced the axotomy-induced changes of motoneuron phenotype; (3) in hyperthyroid rats ChAT mRNA level of lumbar motoneurons not reduced homolateral to lesion while the number of ChAT-IR profiles was pronouncedly reduced; (4) up-regulation of p75 induced by peripheral nerve lesion was reduced in RA-treated rats. These data indicate that the motoneuron phenotype is regulated by transcription factors, which also play a role in phenotype switch regulation after axotomy.

Coexpression of thyroid hormone receptor isoforms in mouse oligodendrocytes

Double and triple immunocytochemistry with stage-specific markers and specific antireceptor antibodies was used to study expression of nuclear thyroid hormone receptor (TR) isoforms in cultured mouse oligodendrocytes. To evaluate the coexpression of each TR isoform, antibodies were raised in rabbits and mice against specific regions of alpha1-TR and alpha2-TR common to both alpha isoforms and beta1-TR. Their specificities were assessed by Western blotting and by immunocytochemistry on rat hepatocytes. Oligodendrocyte subpopulations were found to coexpress the alpha- and beta1-TR epitopes at defined developmental stages. Both alpha- and beta1-TR isoforms are colocalized in oligodendrocytes during an early stage identified by the marker OL-1, before 2',3'-cyclic nucleotide 3'-phosphohydrolase is expressed. Expression of beta1-TR varies during maturation, and that of alpha-TR decreases during terminal maturation.

Expression and regulation of golli products of myelin basic protein gene during in vitro development of oligodendrocytes

The myelin basic protein (MBP) gene produces two families of proteins, the classic MBPs, important for myelination of the CNS, and the golli proteins, whose biological role in oligodendrocytes (OLs) is still unknown. The goals of this work were to study the in vitro pattern of expression of the golli products during OL differentiation and to compare it with that of the classic MBP products of the gene. Mouse primary glial cultures were analyzed at the mRNA and protein levels with an array of techniques. We found that OLs express golli mRNA primarily during intermediate stages of differentiation, which was confirmed by immunocytochemical analysis. Golli expression was low in proliferating OL progenitors as well as in terminally mature OLs. Golli proteins were found associated with the OL cell soma and nuclei and, to a lesser extent, with the cellular processes. We also found that golli proteins are not targeted to myelin in vitro and in vivo, in contrast to the classic MBPs. Finally, we found that golli expression is regulated during OL development and can be manipulated by growth factors such as basic fibroblast growth factor, neurotrophin-3, and retinoic acid.

Mitogen‐activated protein kinase signalling in oligodendrocytes: a comparison of primary cultures and CG‐4

Oligodendrocytes play a significant role in the central nervous system, as these cells are responsible for myelinating axons and allowing for the efficient conduction of nerve impulses. Therefore, any understanding we can gain about the functional biology of oligodendrocytes will give us important insights into demyelinating diseases such as multiple sclerosis, where oligodendrocytes and myelin are damaged or destroyed. Currently, much attention has focussed on the role of a family of mitogen-activated protein kinases in OL. This kinase family includes the extracellular signal-regulated protein kinases (ERKs), the stress-activated c-Jun N-terminal kinase (JNK), and the 38 kDa high osmolarity glycerol response kinase (p38). The actions of mitogen-activated protein kinases in oligodendrocytes appear to range from proliferation and cell survival to differentiation and cell death. In the past, studies on oligodendrocytes have been hampered by the difficulties inherent in producing large enough quantities of these cells for experimentation. This problem arises in large part due to the post-mitotic nature of mature oligodendrocytes. Over the years, a cell line known as Central Glia-4 (CG-4) has become a popular oligodendrocyte model due to its potentially unlimited capacity for self-renewal. In this review, we will look at the suitability of the Central Glia-4 cell line as an oligodendrocyte model, specifically in respect to studies on mitogen-activated protein kinase signalling in oligodendrocytes.

Rat oligodendrocytes express the vitamin D(3) receptor and respond to 1,25-dihydroxyvitamin D(3)

The present study investigates the presence of vitamin D receptor (VDR) in cells of the rat oligodendrocyte (OL) lineage. VDR transcripts were detected by in situ hybridization in a fraction of rat OL in secondary cultures. The VDR protein was shown to be co-localized in cells that are also recognized by an anti-myelin basic protein (MBP) antibody. Likewise, in vivo, VDR-positive cells were found in the brain white matter, such as the internal capsule of the striatum or the corpus callosum but also in the spinal cord. At least part of these positive cells in vivo correspond to OL, since they were co-stained by an anti-carbonic anhydrase II antiserum. Northern blot analyses of the CG-4 OL cell line demonstrated that the VDR transcripts are already found in the O-2A precursors. There was a two-fold increase in the relative abundance of these transcripts in differentiated OL or in type-2 astrocytes. 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] increased the pool of transcripts encoding its own receptor, the VDR. The hormone also enhanced the abundance of the mRNA of the nerve growth factor (NGF) and of its low-affinity receptor, the p75(NTR) protein. By contrast, the hormone had no effect on the levels of MBP or proteolipid protein (PLP) mRNA. This finding suggests that unlike retinoic acid (RA) or thyroid hormone, 1,25-(OH)(2)D(3) has no regulatory action on the synthesis of myelin proteins.

Oligodendrocyte development and thyroid hormone

Thyroid hormone plays an important role in brain development and is essential to ensure a normal myelination. The effects of thyroid hormone are mediated by nuclear thyroid hormone receptors, which act as ligand-regulated transcription factors. There are several isoforms encoded by two genes, alpha and beta. Developmental studies have shown that alpha isoforms are widely expressed in the fetal brain, while beta isoforms expression is more restricted with a dramatic increase that begins at birth in the rat. Remarkably, receptor number reaches maximal levels by postnatal day 10, when serum thyroid hormone levels also peak and myelination is the most prominent event in the developing rat brain. Likewise, oligodendrocyte precursor cells express alpha isoforms and expression of the beta isoforms is confined to the differentiated oligodendrocytes, suggesting that these isoforms might mediate different thyroid hormone effects in the oligodendrocyte lineage. Thyroid hormone acts at multiple steps in the development of oligodendrocytes: (a) Early in development, it can function as an instructive signal for the generation of oligodendrocytes and enhance the proliferation of the committed preprecursor oligodendrocyte cells. (b) Thyroid hormone regulates the number of oligodendrocyte generated by directly promoting their differentiation. Since oligodendrocytes are produced in vitro after the same period in culture regardless of whether thyroid hormone was added to the cultures, it has been suggested that thyroid hormone is required for neither the timing nor the generation of oligodendrocytes, but is necessary to achieve adequate oligodendrocyte numbers. (c) Finally, thyroid hormone increases morphological and functional maturation of postmitoitic oligodendrocytes by stimulation of the expression of various myelin genes.