Glucocorticoid-induced upregulation of proteolipid protein and myelin-associated glycoprotein genes in C6 cells

Preliminary Evidence of Increased Hippocampal Myelin Content in Veterans with Posttraumatic Stress Disorder

Recent findings suggest the formation of myelin in the central nervous system by oligodendrocytes is a continuous process that can be modified with experience. For example, a recent study showed that immobilization stress increased oligodendrogensis in the dentate gyrus of adult rat hippocampus. Because changes in myelination represents an adaptive form of brain plasticity that has a greater reach in the adult brain than other forms of plasticity (e.g., neurogenesis), the objective of this “proof of concept” study was to examine whether there are differences in myelination in the hippocampi of humans with and without post-traumatic stress disorder (PTSD). We used the ratio of T1-weighted/T2-weighted magnetic resonance image (MRI) intensity to estimate the degree of hippocampal myelination in 19 male veterans with PTSD and 19 matched trauma-exposed male veterans without PTSD (mean age: 43 ± 12 years). We found that veterans with PTSD had significantly more hippocampal myelin than trauma-exposed controls. There was also found a positive correlation between estimates of hippocampal myelination and PTSD and depressive symptom severity. To our knowledge, this is the first study to examine hippocampal myelination in humans with PTSD. These results provide preliminary evidence for stress-induced hippocampal myelin formation as a potential mechanism underlying the brain abnormalities associated with vulnerability to stress.

Stress and glucocorticoids promote oligodendrogenesis in the adult hippocampus

Stress can exert long-lasting changes on the brain that contribute to vulnerability to mental illness, yet mechanisms underlying this long-term vulnerability are not well understood. We hypothesized that stress may alter the production of oligodendrocytes in the adult brain, providing a cellular and structural basis for stress-related disorders. We found that immobilization stress decreased neurogenesis and increased oligodendrogenesis in the dentate gyrus (DG) of the adult rat hippocampus and that injections of the rat glucocorticoid stress hormone corticosterone (cort) were sufficient to replicate this effect. The DG contains a unique population of multipotent neural stem cells (NSCs) that give rise to adult newborn neurons, but oligodendrogenic potential has not been demonstrated in vivo. We used a nestin-CreER/YFP transgenic mouse line for lineage tracing and found that cort induces oligodendrogenesis from nestin-expressing NSCs in vivo. Using hippocampal NSCs cultured in vitro, we further showed that exposure to cort induced a pro-oligodendrogenic transcriptional program and resulted in an increase in oligodendrogenesis and decrease in neurogenesis, which was prevented by genetic blockade of glucocorticoid receptor (GR). Together, these results suggest a novel model in which stress may alter hippocampal function by promoting oligodendrogenesis, thereby altering the cellular composition and white matter structure.

Multiple sclerosis: neuroprotective alliance of estrogen-progesterone and gender

The potential of 17β-estradiol and progesterone as neuroprotective factors is well-recognized. Persuasive data comes from in vitro and animal models reflecting a wide range of CNS disorders. These studies have endeavored to translate findings into human therapies. Nonetheless, few human studies show promising results. Evidence for neuroprotection was obtained in multiple sclerosis (MS) patients. This chronic inflammatory and demyelinating disease shows a female-to-male gender prevalence and disturbances in sex steroid production. In MS-related animal models, steroids ameliorate symptoms and protect from demyelination and neuronal damage. Both hormones operate in dampening central and brain-intrinsic immune responses and regulating local growth factor supply, oligodendrocyte and astrocyte function. This complex modulation of cell physiology and system stabilization requires the gamut of steroid-dependent signaling pathways. The identification of molecular and cellular targets of sex steroids and the understanding of cell-cell interactions in the pathogenesis will offer promise of novel therapy strategies.

Progesterone attenuates demyelination and microglial reaction in the lysolecithin-injured spinal cord

Progesterone treatment of mice with experimental autoimmune encephalomyelitis has shown beneficial effects in the spinal cord according to enhanced clinical, myelin and neuronal-related parameters. In the present work, we report progesterone effects in a model of primary demyelination induced by the intraspinal injection of lysophospatidylcholine (LPC). C57Bl6 adult male mice remained steroid-untreated or received a single 100 mg progesterone implant, which increased circulating steroid levels to those of mouse pregnancy. Seven days afterwards mice received a single injection of 1% LPC into the dorsal funiculus of the spinal cord. A week after, anesthetized mice were perfused and paraffin embedded sections of the spinal cord stained for total myelin using Luxol Fast Blue (LFB) histochemistry, for myelin basic protein (MBP) immunohistochemistry and for determination of OX-42+ microglia/macrophages. Cryostat sections were also prepared and stained for oligodendrocyte precursors (NG2+ cells) and mature oligodendrocytes (CC1+ cells). A third batch of spinal cords was prepared for analysis of the microglial marker CD11b mRNA using qPCR. Results showed that progesterone pretreatment of LPC-injected mice decreased by 50% the area of demyelination, evaluated by either LFB staining or MBP immunostaining, increased the density of NG2+ cells and of mature, CC1+ oligodendrocytes and decreased the number of OX-42+ cells, respect of steroid-untreated LPC mice. CD11b mRNA was hyperexpressed in LPC-treated mice, but significantly reduced in LPC-mice receiving progesterone. These results indicated that progesterone antagonized LPC injury, an effect involving (a) increased myelination; (b) stimulation of oligodendrocyte precursors and mature oligodendrocytes, and (c) attenuation of the microglial/macrophage response. Thus, use of a focal demyelination model suggests that progesterone exerts promyelinating and anti-inflammatory effects at the spinal cord level.

Effects of progesterone on oligodendrocyte progenitors, oligodendrocyte transcription factors, and myelin proteins following spinal cord injury

Progesterone is emerging as a myelinizing factor for central nervous system injury. Successful remyelination requires proliferation and differentiation of oligodendrocyte precursor cells (OPC) into myelinating oligodendrocytes, but this process is incomplete following injury. To study progesterone actions on remyelination, we administered progesterone (16 mg/kg/day) to rats with complete spinal cord injury. Rats were euthanized 3 or 21 days after steroid treatment. Short progesterone treatment (a) increased the number of OPC without effect on the injury-induced reduction of mature oligodendrocytes, (b) increased mRNA and protein expression for the myelin basic protein (MBP) without effects on proteolipid protein (PLP) or myelin oligodendrocyte glycoprotein (MOG), and (c) increased the mRNA for Olig2 and Nkx2.2 transcription factors involved in specification and differentiation of the oligodendrocyte lineage. Furthermore, long progesterone treatment (a) reduced OPC with a concomitant increase of oligodendrocytes; (b) promoted differentiation of cells that incorporated bromodeoxyuridine, early after injury, into mature oligodendrocytes; (c) increased mRNA and protein expression of PLP without effects on MBP or MOG; and (d) increased mRNA for the Olig1 transcription factor involved in myelin repair. These results suggest that early progesterone treatment enhanced the density of OPC and induced their differentiation into mature oligodendrocytes by increasing the expression of Olig2 and Nkx2.2. Twenty-one days after injury, progesterone favors remyelination by increasing Olig1 (involved in repair of demyelinated lesions), PLP expression, and enhancing oligodendrocytes maturation. Thus, progesterone effects on oligodendrogenesis and myelin proteins may constitute fundamental steps for repairing traumatic injury inflicted to the spinal cord.

Progesterone neuroprotection in traumatic CNS injury and motoneuron degeneration

Studies on the neuroprotective and promyelinating effects of progesterone in the nervous system are of great interest due to their potential clinical connotations. In peripheral neuropathies, progesterone and reduced derivatives promote remyelination, axonal regeneration and the recovery of function. In traumatic brain injury (TBI), progesterone has the ability to reduce edema and inflammatory cytokines, prevent neuronal loss and improve functional outcomes. Clinical trials have shown that short-and long-term progesterone treatment induces a significant improvement in the level of disability among patients with brain injury. In experimental spinal cord injury (SCI), molecular markers of functional motoneurons become impaired, including brain-derived neurotrophic factor (BDNF) mRNA, Na,K-ATPase mRNA, microtubule-associated protein 2 and choline acetyltransferase (ChAT). SCI also produces motoneuron chromatolysis. Progesterone treatment restores the expression of these molecules while chromatolysis subsided. SCI also causes oligodendrocyte loss and demyelination. In this case, a short progesterone treatment enhances proliferation and differentiation of oligodendrocyte progenitors into mature myelin-producing cells, whereas prolonged treatment increases a transcription factor (Olig1) needed to repair injury-induced demyelination. Progesterone neuroprotection has also been shown in motoneuron neurodegeneration. In Wobbler mice spinal cord, progesterone reverses the impaired expression of BDNF, ChAT and Na,K-ATPase, prevents vacuolar motoneuron degeneration and the development of mitochondrial abnormalities, while functionally increases muscle strength and the survival of Wobbler mice. Multiple mechanisms contribute to these progesterone effects, and the role played by classical nuclear receptors, extra nuclear receptors, membrane receptors, and the reduced metabolites of progesterone in neuroprotection and myelin formation remain an exciting field worth of exploration.

Identification by microarray analysis of aspartate aminotransferase and glutamine synthetase as glucocorticoid target genes in a mouse Schwann cell line

Schwann cells have been identified as targets for glucocorticoids. Besides genes implicated in the myelination process, the target genes of glucocorticoids have not been identified in these cells. For that purpose, we performed microarray analysis on MSC80 (mouse Schwann cells) treated with a synthetic glucocorticoid, dexamethasone. These cells express a functional glucocorticoid receptor (GR), but none of the other steroid receptors. This allowed us to identify genes specifically regulated by GR in the absence of the mineralocorticoid receptor. Among the 5000 genes analyzed, 12 were at least two-fold upregulated and 91 genes were at least two-fold down-regulated upon treatment with dexamethasone. Because of their potential role in Schwann cell homeostasis, we selected, for further analysis, the upregulated genes encoding glutamine synthetase (GS) and cytosolic aspartate aminotransferase (cAspAT). These genes play a crucial role in the glutamate cycle which was shown to be vital in neuron-astrocyte cross-talk in the central nervous system. Their activation was confirmed by semi-quantitative and real-time PCR. A detailed analysis of cAspAT promoter activity revealed that the mechanism of regulation by GR in Schwann cells differs from that in hepatoma cells, suggesting a cell-specific regulation. The transactivation potency of the two Glucocorticoid Responsive Units (GRU) present in the cAspAT promoter seems to be dependent on the levels of the GR in MSC80 cells. Furthermore, we show that an increase in GR levels under certain circumstances could considerably potentiate the effects of glucocorticoids on the cAspAT promoter via synergistic activation of both GRU, To the opposite, an enhancement in GR levels did not further potentiate Dex-activation of the GS promoter, showing a differential mechanism of action of GR in the context of both promoters.

Involvement of {beta}-catenin and unusual behavior of CBP and p300 in glucocorticosteroid signaling in Schwann cells

In the nervous system, glucocorticosteroid hormones play a major role during development and adult life. Myelin-forming cells are among the targets of glucocorticosteroids, which have been shown to promote myelination both in the central and peripheral nervous system. Glucocorticosteroid-stimulated gene transcription is mediated by the glucocorticosteroid receptor (GR) that recruits coactivators of the p160 family, forming a docking platform for secondary coactivators, such as cAMP-response element binding protein (CREB)-binding protein (CBP) or its close homologue, p300. Here, we investigated the role of CBP and p300 in mouse Schwann cells (MSC80). We show that, although the CBP/p300 binding domain of steroid receptor coactivator-1 is crucial for GR transactivation, neither CBP nor p300 enhanced GR transcriptional activation, as shown by overexpression and small interfering RNA (siRNA) knocking-down experiments. Unexpectedly, overexpression of p300, considered as a coactivator of the GR, resulted in inhibition of GR transcriptional activity. Studies with p300 deletion mutants demonstrated that p300-dependent repression is related to its acetyltransferase activity. Functional and pull-down assays showed that beta-catenin may be the coactivator replacing CBP in the GR transcriptional complex. Our results suggest the formation of a GR-coactivator complex within Schwann cells, indicating that glucocorticosteroids may act by means of unusual partners in the nervous system, and we show a repressive effect of p300 on nuclear receptors.

C6 cells express a sodium-calcium exchanger/GM1 complex in the nuclear envelope but have no exchanger in the plasma membrane: comparison to astrocytes

Previous work demonstrated the presence of an isoform of Na(+)/Ca(2+) exchanger in the nuclear envelope of neurons and NG108-15 cells that is tightly associated with GM1 ganglioside and potentiated by the latter. This contrasted with the Na(+)/Ca(2+) exchanger(s) in the plasma membrane, which were suggested to associate more loosely with GM1. To study these aspects of Na(+)/Ca(2+) exchanger expression in nonneuronal neural cells, we have examined nuclear and plasma membrane exchanger patterns in astrocytes and C6 cells, a glia-derived line. We find both cell types contain the tightly associated exchanger/GM1 complex in the nuclear envelope but, surprisingly, only astrocytes possess Na(+)/Ca(2+) exchanger activity in the plasma membrane. This is the first reported example of a cell (C6) with Na(+)/Ca(2+) exchangers in the nuclear envelope but not in the plasma membrane. RT-PCR established the presence of the NCX1 subtype in C6 cells and both NCX1 and NCX2 in astrocytes. Comparison was made with NG108-15 cells, which have Na(+)/Ca(2+) exchangers in both nuclear and plasma membranes, and Jurkat cells, which have no Na(+)/Ca(2+) exchanger in either membrane. Culturing of C6 cells in the presence dibutyryl-cAMP caused upregulation of a high molecular weight isoform of the exchanger together with GM1 in the nuclear envelope, resulting in significant elevation of Na(+)/Ca(2+) exchanger activity in the latter. Application of exogenous GM1 to nuclei from non-treated cells also potentiated exchanger activity, although to a lesser degree. The Na(+)/Ca(2+) exchanger/GM1 complex occurs in the inner membrane of the nuclear envelope, suggesting a functional role in transferring Ca(2+) between nucleoplasm and the envelope lumen.

Glucocorticosteroids stimulate the activity of the promoters of peripheral myelin protein-22 and protein zero genes in Schwann cells

To better understand the mechanism by which glucocorticosteroids (GLUC) could enhance myelination in the PNS, cultured rat Schwann cells were transiently transfected with reporter constructs in which luciferase expression was controlled by the promoter region of either the peripheral myelin protein-22 (PMP22) or the protein zero (P(0)) genes. GLUC stimulated the activity of the P(0) promoter and the PMP22 promoters 1 and 2. The effect of GLUC was specific as estradiol and testosterone did not activate the promoters. The antagonist RU486 did not abolish the effect of GLUC, but instead stimulated promoter activities by itself. In the mammary carcinoma cell line 34i, which expresses GLUC receptors, GLUC did not stimulate the P(0) and PMP22 promoters while the promoter of the mouse mammary tumor virus was strongly activated. Thus, the activation by GLUC of the promoter activities of two peripheral myelin protein genes is Schwann cell-specific.

Structure-function analysis of Qk1: a lethal point mutation in mouse quaking prevents homodimerization

Qk1 is a member of the KH domain family of proteins that includes Sam68, GRP33, GLD-1, SF1, and Who/How. These family members are RNA binding proteins that contain an extended KH domain embedded in a larger domain called the GSG (for GRP33-Sam68-GLD-1) domain. An ethylnitrosourea-induced point mutation in the Qk1 GSG domain alters glutamic acid 48 to a glycine and is known to be embryonically lethal in mice. The function of Qk1 and the GSG domain as well as the reason for the lethality are unknown. Here we demonstrate that the Qk1 GSG domain mediates RNA binding and Qk1 self-association. By using in situ chemical cross-linking studies, we showed that the Qk1 proteins exist as homodimers in vivo. The Qk1 self-association region was mapped to amino acids 18 to 57, a region predicted to form coiled coils. Alteration of glutamic acid 48 to glycine (EG) in the Qk1 GSG domain (producing protein Qk1:EG) abolishes self-association but has no effect on the RNA binding activity. The expression of Qk1 or Qk1:EG in NIH 3T3 cells induces cell death by apoptosis. Approximately 90% of the remaining transfected cells are apoptotic 48 h after transfection. Qk1:EG was consistently more potent at inducing apoptosis than was wild-type Qk1. These results suggest that the mouse quaking lethality (EG) occurs due to the absence of Qk1 self-association mediated by the GSG domain.

Regulation of gene expression by corticoid hormones in the brain and spinal cord

Glucocorticoids (GC) and mineralocorticoids (MC) have profound regulatory effects upon the central nervous system (CNS). Hormonal regulation affects several molecules essential to CNS function. First, evidences are presented that mRNA expression of the alpha3 and beta1-subunits of the Na,K-ATPase are increased by GC and physiological doses of MC in a region-dependent manner. Instead, high MC doses reduce the beta1 isoform and enzyme activity in amygdaloid and hypothalamic nuclei, an effect which may be related to MC control of salt appetite. The alpha3-subunit mRNA of the Na,K-ATPase is also stimulated by GC in motoneurons of the injured spinal cord, suggesting a role for the enzyme in GC neuroprotection. Second, we provide evidences for hormonal effects on the expression of mRNA for the neuropeptide arginine vasopressin (AVP). Our data show that GC inhibition of AVP mRNA levels in the paraventricular nucleus is sex-hormone dependent. This sexual dimorphism may explain sex differences in the hypothalamic-pituitary-adrenal axis function between female and male rats. Third, steroid effects on the astrocyte marker glial fibrillary acidic protein (GFAP) points to a complex regulatory mechanism. In an animal model of neurodegeneration (the Wobbler mouse) showing pronounced astrogliosis of the spinal cord, in vivo GC treatment down-regulated GFAP immunoreactivity, whereas the membrane-active steroid antioxidant U-74389F up-regulated this protein. It is likely that variations in GFAP protein expression affect spinal cord neurodegeneration in Wobbler mice. Fourth, an interaction between neurotrophins and GC is shown in the injured rat spinal cord. In this model, intensive GC treatment increases immunoreactive low affinity nerve growth factor (NGF) receptor in motoneuron processes. Because GC also increases immunoreactive NGF, this mechanism would support trophism and regeneration in damaged tissues. In conclusion, evidences show that some molecules regulated by adrenal steroids in neurons and glial cells are not only involved in physiological control, but additionally, may play important roles in neuropathology.

Structural characterization of myelin-associated glycoprotein gene core promoter

Myelin-associated glycoprotein (MAG) is emerging as an important molecule involved in the plasticity and regeneration of the central nervous system. In this study, the structure of MAG gene promoter was characterized in cultured rat oligodendrocyte lineage cells. Heterogeneous transcription initiation with five major and eight minor start sites scattered within 72 bp was shown by primer extension analysis. This TATA-less core promoter contains no prominent initiator (Inr) elements associated with the transcription initiation sites, and hence, appears to utilize novel positioning mechanisms. Genomic footprinting analysis revealed several putative protein-binding regions overlapping the initiation sites and containing a multitude of CG-rich sequences. However, no conspicuous alterations in the protein-binding pattern were evident between O2A progenitors in which the gene is inactive, and mature oligodendrocytes with fully upregulated gene. The core promoter DNA features a differentiation-dependent demethylation as shown by genomic sequencing analysis. Three of eight cytosines are totally demethylated in oligodendrocyte chromosomes, indicating that these unmodified bases may be critical for full activation of the promoter. The core promoter is located within an internucleosomal linker, and the upstream regulatory region appears to be organized into an array of nucleosomes with hypersensitive linkers.

Myelin gene expression in glia treated with oligodendroglial trophic factor

Oligodendroglia synthesize myelin in the CNS. In vitro, oligodendroglia may be identified by the binding of monoclonal antibodies against galactocerebroside, a myelin-specific galactolipid. Oligodendroglial trophic factor is a protein mitogen for cells of the oligodendroglial lineage. When oligodendroglia in cerebral white matter cultures are treated with oligodendroglial trophic factor, galactocerebroside-positive cells undergo mitosis but fail to express the myelin structural proteins, myelin basic protein and proteolipid protein. Oligodendroglia treated with oligodendroglial trophic factor, however, do express 2',3'-cyclic nucleotide 3'-phosphodiesterase and myelin-associated glycoprotein in a manner similar to oligodendroglia treated with platelet-derived growth factor. Oligodendroglial trophic factor, therefore, generates a population of somewhat 'immature' oligodendroglia, which are galactocerebroside, myelin-associated glycoprotein and 2', 3'-cyclic nucleotide 3' phosphodiesterase positive but myelin basic protein and proteolipid protein negative.