Induction of oligodendrocyte differentiation by activators of adenylate cyclase

Unique allosteric regulation of 5-hydroxytryptamine receptor-mediated signal transduction by oleamide

The effects of oleamide, an amidated lipid isolated from the cerebrospinal fluid of sleep-deprived cats, on serotonin receptor-mediated responses were investigated in cultured mammalian cells. In rat P11 cells, which endogenously express the 5-hydroxytryptamine2A (5HT2A) receptor, oleamide significantly potentiated 5HT-induced phosphoinositide hydrolysis. In HeLa cells expressing the 5HT7 receptor subtype, oleamide caused a concentration-dependent increase in cAMP accumulation but with lower efficacy than that observed by 5HT. This effect was not observed in untransfected HeLa cells. Clozapine did not prevent the increase in cAMP elicited by oleamide, and ketanserin caused an approximately 65% decrease. In the presence of 5HT, oleamide had the opposite effect on cAMP, causing insurmountable antagonism of the concentration-effect curve to 5HT, but had no effect on cAMP levels elicited by isoproterenol or forskolin. These results indicate that oleamide can modulate 5HT-mediated signal transduction at different subtypes of mammalian 5HT receptors. Additionally, our data indicate that oleamide acts at an apparent allosteric site on the 5HT7 receptor and elicits functional responses via activation of this site. This represents a unique mechanism of activation for 5HT G protein-coupled receptors and suggests that G protein-coupled neurotransmitter receptors may act like their iontropic counterparts (i.e., gamma-aminobutyric acid type A receptors) in that there may be several binding sites on the receptor that regulate functional activity with varying efficacies.

1,25-Dihydroxyvitamin D3 induces programmed cell death in a rat glioma cell line

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3), a seco-steroid hormone with potential antitumoral activities, has been recently reported to exert cytotoxic effects on C6 glioma cells. However, the molecular mechanisms which trigger this cell death remain unknown. We show here that this 1,25(OH)2D3-induced cell death is dependent upon protein synthesis and is accompanied by the expression of c-myc, p53, and gadd45 genes. Two other genes, coding for interleukin-6 and vaso-endothelial growth factor, are also upregulated after addition of 1,25(OH)2D3. This programmed cell death can be suppressed when cells are treated with forskolin, a drug which increases intracellular cAMP concentration, or with genistein, an inhibitor of tyrosine protein kinases. However, in spite of the demonstration of fragmented DNA in 1,25(OH)2D3-treated cells, the C6.9 cells used in this study do not show the classical morphological features of apoptosis. These results provide the first evidence for the existence of a programmed cell death triggered by 1,25(OH)2D3 in glioma cells and may provide a basis for the development of new therapeutic strategies. In addition, these data also suggest that the treatment of C6.9 cells with 1,25(OH)2D3 may be a useful model to study the molecular mechanisms involved in the programmed cell death of a cell of glial origin.

Nerve growth factor and neurotrophin-3 differentially regulate the proliferation and survival of developing rat brain oligodendrocytes

There is increasing evidence that the neurotrophins, particularly nerve growth factor (NGF) and neurotrophin-3 (NT-3), play a role in the regulation of glial development in the CNS. Recent studies have shown that the proliferation of optic nerve-derived O2A progenitors (OLPs) is potentiated by NT-3 in combination with platelet-derived growth factor, whereas NT-3 alone supports the survival of their differentiated progeny (Barres et al., 1994). In this study, we have examined the expression of the high-affinity neurotrophin receptors (trks) and the low-affinity nerve growth factor receptor p75 in developing oligodendrocytes (OLs). In addition, we have examined the effects of NGF and NT-3 on proliferation and survival of OLPs and OLs, respectively. TrkC, the high-affinity NT-3 receptor, and trkA, the high-affinity NGF receptor, are both expressed from the early OLP through the mature OL stage. The truncated form of trkB, lacking the tyrosine kinase domain, and the low-affinity neurotrophin receptor p75 are expressed at low levels in OLPs and are upregulated in mature OLs. NGF and NT-3 both induced the phosphorylation of mitogen-activated protein kinase (MAPK) in OLPs and in OLs. In both OLPs and OLs, NT-3 sustained the activation of MAPK more than NGF. NT-3 enhanced the proliferation of OLPs and supported the survival of OLs. By contrast, unless coadministered with FGF-2, NGF did not exhibit mitogenic effects on OLPs but did enhance the survival of differentiated OLs. Our data demonstrate the presence of functional trkA and trkC in developing OLs and indicate that both NGF and NT-3 have a broad spectrum of developmental actions on cells of the OL lineage.

The 5HT5A serotonin receptor is expressed predominantly by astrocytes in which it inhibits cAMP accumulation: a mechanism for neuronal suppression of reactive astrocytes

The mRNA for the 5-hydroxytryptamine receptor 5-HT5A was detected at embryonic day 18 in the rat central nervous system and peaked by postnatal day 20. At all time points examined, 5-HT5A immunoreactivity observed on astrocyte cell bodies and in the stellate processes not only colocalized with the astrocyte-specific marker glial fibrillary acidic protein (GFAP) but was coordinately regulated with GFAP, increasing during development and during gliosis. Transfection of 5-HT5A into glioma cells prevented the 5-HT-induced increase in cAMP observed in untransfected cells and decreased the relative forskolin response by approximately 20%, suggesting that the 5-HT5A receptor couples negatively to adenylyl cyclase in astrocytes. Together, these results indicate a neuron-to-astrocyte serotonergic signaling pathway mediating cAMP concentrations, which could provide a neuronally driven mechanism for regulating astrocyte physiology with relevance to gliosis.

Oligodendrocytes and their precursors require phosphatidylinositol 3-kinase signaling for survival

Signal transduction in response to several growth factors that regulate oligodendrocyte development and survival involves the activation of phosphatidylinositol 3-kinase, which we detect in oligodendrocytes and their precursors. To investigate the role of this enzyme activity, we analyzed cell survival in cultures of oligodendrocytes treated with wortmannin or LY294002, two potent inhibitors of phosphatidylinositol 3-kinase. Cell survival was inhibited by 60–70% in these cultures within 24 hours, as quantitated by a tetrazolium staining assay for viable cells and by measurement of DNA content. Similar results were obtained with oligodendrocyte precursor cells. Nuclei of the dying cells contained fragmented DNA, as revealed by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling assays, indicating that the cells were dying by apoptosis. Moreover, a significant increase in the number of cells with fragmented nuclear DNA was detected as early as 4 hours, well before any significant differences could be detected in glucose transport or cell viability. Exogenous addition of insulin-like growth factor-I, neurotrophin-3, platelet-derived growth factor, basic fibroblast growth factor, ciliary neurotrophic factor, N-acetyl cysteine, vitamin C, vitamin E, progesterone or serum did not prevent cell death in the presence of wortmannin or LY294002. These findings indicate that survival of oligodendrocytes and their precursors depends on a phosphatidylinositol 3-kinase mediated signaling pathway. Inhibition of this critical enzyme activity induces apoptotic cell death, even in the presence of exogenous growth factors or serum.

Prostaglandin E1, E2, and cholera toxin increase transcription of the brain creatine kinase gene in human U87 glioblastoma cells

The creatine kinase isoenzymes play an important role in maintaining ATP levels in some cell types during times of high energy demand. We have previously shown in primary cell cultures from rat brain that glial cells express much higher levels of brain creatine kinase (CKB) mRNA than neurons. In a separate earlier study we observed that transcription of CKB mRNA in glial cells can be stimulated by a forskolin-mediated increase in cAMP via a pathway involving protein kinase A (PKA). In this report, we show that the level of CKB mRNA in human U87 glioblastoma cells can be increased by either prostaglandin E1 (PGE1), prostaglandin E2 (PGE2), or cholera toxin (an activator of G alpha s proteins). The induction of CKB mRNA occurs rapidly (with maximal induction after 6 h), is at the level of transcription, and is mediated specifically through PKA. In addition, the results indicate that both PGE1 and PGE2 use the same or related signal transduction pathways to increase CKB transcription. These results suggest that in glial cells CKB mRNA can be regulated by extracellular signals acting through G-protein-coupled receptors. This study may contribute to an understanding of the mechanisms underlying the previously-reported, early postnatal increase in CKB enzyme activity in rat brain. The results are also discussed with regard to the potential involvement of the expression of prostaglandins and CKB during hypoxia and ischemia.

Lines of murine oligodendroglial precursor cells immortalized by an activated neu tyrosine kinase show distinct degrees of interaction with axons in vitro and in vivo

Replication-defective retroviruses expressing the t-neu oncogene, or a hybrid protein with the neu tyrosine kinase linked to the external region of the human epidermal growth factor receptor (egfr-neu), were used to establish lines of murine oligodendroglial precursor cells. Differentiation of the t-neu lines into myelin-associated glycoprotein (MAG)-positive oligodendrocytes was induced by dibutyryl cAMP, and the egfr-neu line showed limited differentiation in vitro upon withdrawal of epidermal growth factor. Cerebellar granule cell neurons expressed mitogens for the cell lines. Upon transplantation into demyelinated lesions, t-neu line cells engaged with the demyelinated axons whereas the egfr-neu line cells differentiated further and ensheathed the axons. These cell lines thus interact with neurons in vitro and in vivo and can be used as tools to define the molecules involved in different stages of neuron-glia interaction.

Transcription of the brain creatine kinase gene in glial cells is modulated by cyclic AMP-dependent protein kinase

The brain creatine kinase (CKB) gene is expressed in a variety of tissues with highest expression seen in the brain. We have previously shown in primary rat brain cell cultures that CKB mRNA levels are high in oligodendrocytes and astrocytes and low in neurons (Molloy et al.: J Neurochem 59:1925-1932, 1992). In this report we show that treatment of human U87 glioblastoma cells with forskolin and IBMX, to elevate intracellular cAMP, induces expression of CKB mRNA from the transiently transfected rat CKB gene by 14-fold and also increases expression from the endogenous human CKB gene. This induction of CKB mRNA i) is due to increased transcription; ii) occurs rapidly (with maximal induction after 6 hr; iii) requires the activity of protein kinase A (PKA), but iv) does not require de novo protein synthesis and, in fact, is superinduced in the presence of cycloheximide. Given the role of oligodendrocytes in the energy-demanding process of myelination and of astrocytes in ion transport, these results have physiological significance, since they suggest that changes in cellular energy requirements in the brain during events, such as glial cell differentiation and increased neuronal activity, may in part be met by a cAMP-mediated modulation of CKB gene expression. Of particular importance is the possible modulation of CKB gene expression during myelinogenesis, since oligodendrocyte differentiation has been shown previously to be stimulated by increases in cAMP.

Oligodendroglial cyclic AMP response element-binding protein: a member of the CREB family of transcription factors

Several laboratories have shown that cyclic AMP (cAMP) plays an important role in inducing oligodendrocyte differentiation and myelin synthesis. Our previous results have shown that oligodendrocytes contain a nuclear protein that binds to the DNA sequence TGACGTCA or cAMP response element (CRE) known to be involved in the transcriptional regulation of cAMP-responsive genes. In this report the oligodendroglial CRE-binding protein was further identified by using two different antibodies which specifically recognize the CRE-binding protein known as CREB. In DNA-shift assays CREB-1(X-12) antibody interacted with the CRE-protein complexes resulting in further retardation ("super shift") of the mobility of the bands in the gels. Immunoprecipitation of oligodendroglial nuclear extracts with CREB(240) antibody prior to the DNA binding assays resulted in a lack of formation of CRE-protein complexes. In addition immunoreaction with CREB(240) antibody identified the CRE-binding species as a 45 kDa phosphoprotein. Immunocytochemical staining with CREB(240) antibody in oligodendrocytes from 10-, 14-, and 18-day-old and adult rats indicated that this protein is expressed before the appearance of myelin basic protein (MBP) which was used as a marker of myelin synthesis. Collectively, these observations support our previous results and indicate that the oligodendroglial CRE-binding protein species is highly homologous to the CREB protein. The developmental expression of this CREB protein supports the idea of a possible role during the early stages of oligodendrocyte differentiation preceding the peak of myelin synthesis in rat CNS.

Rat oligodendrocytes express muscarinic receptors coupled to phosphoinositide hydrolysis and adenylyl cyclase

Muscarinic receptors expressed by rat oligodendrocyte primary cultures were examined by measuring changes in second messengers following exposure to carbachol, an acetylcholine analog, and by polymerase chain reaction. Inositol phosphate levels were measured in [3H]myo-inositol-labelled young oligodendrocyte cultures following stimulation with carbachol. Atropine, a specific muscarinic antagonist, prevented the carbachol-induced accumulation of inositol phosphates. The formation of inositol trisphosphate was concentration- and time-dependent, with the peak at 100 microM carbachol and 10 min. Carbachol increased intracellular calcium levels, which were dependent both on the mobilization of intracellular stores and influx of extracellular calcium. In initial experiments with more selective antagonists, the mobilization of intracellular calcium was preferentially inhibited by pirenzepine, a selective M1 antagonist, but not methoctramine, a selective M2 antagonist, suggesting M1 muscarinic receptor involvement. A role for protein kinase C in the regulation of carbachol-stimulated inositol phosphate formation and intracellular calcium mobilization was demonstrated, as acute pretreatment with phorbol-12,13-myristate acetate abolished the formation of both second messengers. Pretreatment with 100 microM carbachol abolished the 40% increase in the cyclic AMP accumulation stimulated by isoproterenol, a specific beta-adrenergic agonist. In turn, the inhibition was alleviated by pretreatment with atropine, suggesting muscarinic receptor involvement. Polymerase chain reaction carried out with specific m1 and m2 muscarinic receptor oligonucleotide primers, confirmed that these cells express, at least, the two muscarinic receptor subtypes. Without excluding the expression of other subtypes, these results suggest that developing oligodendrocytes express m1 (M1) and m2 (M2) muscarinic receptors capable of mediating phosphoinositide hydrolysis, mobilization of intracellular calcium and the attenuation of beta-adrenergic stimulation of cyclic AMP formation.

Ca2+/calmodulin system: participation on rat sexual hypothalamic differentiation

Modifications of male rat hypothalamic sexual differentiation after neonatal administration of drugs that participate on the Ca2+/calmodulin system (haloperidol, trifluoperazine, penfluridol, pimozide, and verapamil) were studied. Pups treated 72 h after birth were behaviorally tested on day 120 of extrauterine life. Five tests for homotypical behavior were conducted. Afterwards animals were castrated and tested twice for heterotypical (female) behavior under replacement hormonal therapy. Fifty percent (80% in the case of pimozide) of all treated males showed lordotic behavior compared with none of the controls. Haloperidol (39%, lordosis quotient) and pimozide (40%, lordosis quotient) were more active than the others. Results obtained with verapamil were not statistically different from the controls. Pimozide was the most active agent influencing the appetitive masculine behavior (mount latency, intromission latency, and postejaculatory interval). Verapamil was more efficient than the rest of the drugs on the consummatory behavior (mount latency, intromission frequency, interintromission interval, and ejaculatory latency). Our results support the participation of the Ca2+/calmodulin system in hypothalamic sexual differentiation and in the differential modulation of the masculine and feminine behavioral patterns.

Analysis of the human MBP promoter in primary cultures of oligodendrocytes: positive and negative cis-acting elements in the proximal MBP promoter mediate oligodendrocyte-specific expression of MBP

Since the regulation of myelin basic protein expression depends primarily on the initiation of transcription, we analyzed the 5' flanking region of the human myelin basic protein gene in transient transfection studies in primary cultures of developing oligodendrocytes. We demonstrated that 149 base pairs 5' of the initiation of transcription was sufficient to direct oligodendrocyte-specific expression of myelin basic protein. The capsite of the fusion transcript was identical with that of the endogenous myelin basic protein transcript, and chloramphenicol acetyl transferase reporter gene expression was restricted to oligodendrocytes in these cultures. Within this 149 base pair region, one distal, negative cis-acting segment, containing a consensus nuclear factor I site, and one proximal, positive cis-acting segment were identified. The distal segment behaved more negatively in Cos-7 cells than in oligodendrocytes, reducing expression to background levels. Furthermore, these functionally important cis-acting segments bound oligodendrocyte nuclear proteins in a pattern differing from other cells, including Cos-7 cells. Interestingly, the distal segment increased heterologous SV40 promoter activity in oligodendrocytes but had no effect on the SV40 promoter in Cos-7 cells. We conclude that the functionally negative distal segment may mediate oligodendrocyte-specific expression of MBP by restricting its expression in other cells. These experiments strongly support using primary cultures of oligodendrocytes for analyzing the myelin-specific promoters.

Presence of cyclic AMP response element-binding protein in oligodendrocytes

Several lines of evidence indicate that cyclic AMP (cAMP) induces oligodendrocytes differentiation. However, the mechanism(s) of this stimulation remains unknown. Because in several cell types the transcriptional activity of various cAMP-responsive genes is regulated through a cis-acting DNA sequence known as cAMP response element (CRE), we investigated the possible presence of a CRE binding (CREB) protein in myelinating oligodendrocytes. A double-stranded oligonucleotide containing a tandem repeat of the CRE sequence was labeled with T4 kinase in the presence of [32P]ATP and then incubated with a nuclear protein extract from 14-day-old rat brain oligodendrocytes. The reaction mixture was then electrophoresed on nondenaturing polyacrylamide gels. The results indicated the presence of a protein that specifically binds to the CRE sequence. The results were supported by southwestern blotting assays in which the CRE probe bound to a approximately 45-kDa protein species. In separate experiments, it was shown that the 45-kDa protein can be phosphorylated in vitro by the catalytic subunit of protein kinase A. Developmental analysis of CREB protein expression indicated a peak at 14 days of age, preceding the peak of myelinogenesis.

Oligodendrocyte differentiation and progenitor cell proliferation are independently regulated by cyclic AMP

Oligodendrocytes, the glial cells specialized to synthesize myelin in the central nervous system, differentiate in primary rat brain cell cultures on a schedule similar to that observed in vivo. The schedule of oligodendrocyte differentiation and the rate of oligodendroglial progenitor cell proliferation in vitro are both modulated by 3',5'-cyclic AMP (cAMP). A 24-hour exposure to 1 mM N6,2'O-dibutyryladenosine 3',5'-cyclic monophosphate (dbcAMP) induced a wave of oligodendrocyte differentiation but inhibited proliferation of oligodendroglial progenitors, and reduced by 30-fold the proliferation of progenitors in response to platelet-derived growth factor (PDGF). When cells were grown in the presence of maximally stimulating concentrations of PDGF, the inhibitory effect of cAMP on progenitor cell proliferation was abolished while the stimulatory effect of cAMP on oligodendrocyte differentiation remained, demonstrating that these two cAMP-regulated events are independent.

Phosphorylation of nuclear proteins in myelinating oligodendrocytes and its control by cyclic AMP

Oligodendroglial nuclei isolated from rat brains at different stages of myelinogenesis (10, 18, and 30 days of age) were incubated with [gamma-32P]ATP and extracted with 0.75 M perchloric acid to yield a fraction of nonacidic chromatin proteins. The protein extracts were then analyzed by polyacrylamide gel electrophoresis. The phosphorylation pattern of these proteins was found to be different for different age groups. In 10-day-old rat oligodendrocytes the most extensive phosphorylation occurred in low molecular mass species (less than 30 kDa), in contrast to fractions obtained from 18- and 30-day-old rat oligodendrocytes which showed a significantly higher labeling of the proteins with molecular masses greater than 30 kDa. The phosphorylation of the latter species was greatly stimulated by the presence of cyclic AMP in the incubation media. The results suggest that the phosphorylation of specific nuclear proteins, which may play a regulatory role at different stages of oligodendroglial maturation and myelinogenesis, may be at least partially modulated by intracellular cyclic AMP.

Infection by coronavirus JHM of rat neurons and oligodendrocyte-type-2 astrocyte lineage cells during distinct developmental stages

Primary telencephalic cultures derived from neonatal Wistar Furth rats were able to support the growth of coronavirus JHM if a viable neuronal population was maintained. This occurred under serum-free defined, but not serum-supplemented, growth conditions. The importance of neurons in establishing infections in mixed cultures was confirmed by immunocytochemical and electron microscopic studies. Glia, although more abundant than neurons in these cultures, were less frequently infected during the initial 48 h postinoculation. The two glial lineages present in mixed telencephalic cultures were separated into type-1 astrocytes and oligodendrocyte-type-2 astrocyte (O-2A) lineage cells and individually assessed for their ability to support virus growth. Infection could not be established in type-1 astrocytes regardless of the culture conditions employed, consistent with our previous study (S. Beushausen and S. Dales, Virology 141:89-101, 1985). In contrast, infections could be initiated in selected O-2A lineage cells grown in serum-free medium. Virus multiplication was however significantly reduced by preconditioning the medium with mixed telencephalic or enriched type-1 astrocyte cultures, suggesting that intercellular interactions mediated by soluble factor(s) can influence the infectious process in O-2A lineage cells. This presumption was supported by eliciting similar effects with basic fibroblast growth factor and platelet-derived growth factor, two central nervous system cytokines known to control O-2A differentiation. The presence of these cytokines, which synergistically block O-2A cells from differentiating into oligodendrocytes was correlated with specific and reversible resistance to JHM virus (JHMV) infection. These data, combined with our finding that accelerated terminal differentiation of the oligodendrocyte phenotype confers resistance to JHMV (Beushausen and Dales, Virology, 1985), suggest that the permissiveness of O-2A cells for JHMV is restricted to a discrete developmental stage.