Peripheral viral challenge increases c-fos level in cerebral neurons

Peripheral viral infection can substantially alter brain function. We have previously shown that intraperitoneal (i.p.) injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC), engenders hyperexcitability of cerebral neurons. Because neuronal activity is invariably associated with their expression of the Cfos gene, the present study was undertaken to determine whether PIC challenge also increases neuronal c-fos protein level. Female C57BL/6 mice were i.p. injected with PIC, and neuronal c-fos was analyzed in the motor cortex by immunohistochemistry. PIC challenge instigated a robust increase in the number of c-fos-positive neurons. This increase reached approximately tenfold over control at 24 h. Also, the c-fos staining intensity of individual neurons increased. AMG-487, a specific inhibitor of the chemokine receptor CXCR3, profoundly attenuated the accumulation of neuronal c-fos, indicating the activation of CXCL10/CXCR3 axis as the trigger of the process. Together, these results show that the accumulation of c-fos is a viable readout to assess the response of cerebral neurons to peripheral PIC challenge, and to elucidate the underlying molecular mechanisms.

Neuronal CXCL10/CXCR3 Axis Mediates the Induction of Cerebral Hyperexcitability by Peripheral Viral Challenge

Peripheral infections can potently exacerbate neuropathological conditions, though the underlying mechanisms are poorly understood. We have previously demonstrated that intraperitoneal (i.p.) injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC) induces a robust generation of CXCL10 chemokine in the hippocampus. The hippocampus also features hyperexcitability of neuronal circuits following PIC challenge. The present study was undertaken to determine the role of CXCL10 in mediating the development of hyperexcitability in response to PIC challenge. Briefly, young female C57BL/6 mice were i.p. injected with PIC, and after 24 h, the brains were analyzed by confocal microscopy. CXCL10 staining of neuronal perikarya and a less intense staining of the neuropil was observed in the hippocampus and cortex. CXCL10 staining was also evident in a subpopulation of astrocytes, whereas microglia were CXCL10 negative. CXCR3, the cognate receptor of CXCL10 was present exclusively on neurons, indicating that the CXCL10/CXCR3 axis operates through an autocrine/paracrine neuronal signaling. Blocking cerebral CXCR3 through intracerebroventricular injection of a specific inhibitor, AMG487, abrogated PIC challenge-induced increase in basal synaptic transmission and long-term potentiation (LTP), as well as the reduction of paired-pulse facilitation (PPF), in the hippocampus. The PIC-mediated abolishment of hippocampal long-term depression (LTD) was also restored after administration of AMG487. Moreover, CXCR3 inhibition attenuated seizure hypersensitivity induced by PIC challenge. The efficacy of AMG487 strongly strengthens the notion that CXCL10/CXCR3 axis mediates the induction of cerebral hyperexcitability by PIC challenge.

Peripheral viral challenge exacerbates experimental autoimmune encephalomyelitis

Peripheral viral infections are potent triggers of exacerbation in multiple sclerosis (MS). Here, we used a preclinical model of MS, the experimental autoimmune encephalomyelitis (EAE) to corroborate this comorbidity in an experimental setting. EAE was induced by immunization of mice with MOG peptide, and paralysis was scored using a 5-point scale. At the onset of the chronic phase of the disease (Days 42-58 after MOG injection) the animals were divided into low responders (LR) and high responders (HR) with the mean score of 1.5 and 2.5, respectively. The acute phase response (APR) was induced by intraperitoneal injections of a viral mimetic, polyinosinic-polycytidylic acid (PIC). Two daily injections were performed on Days 42 and 44 (PIC_42,44 challenge) and on Days 54, 55 and 56 (PIC_54,55,56 challenge). PIC_42,44 challenge had no effect of EAE disease, whereas PIC_54,55,56 challenge rapidly increased paralysis but only in HR group. This exacerbation ultimately led to animal death by Day 58. These results demonstrate that antiviral APR is a potent exacerbator of EAE, and that this activity directly correlates with the severity of the disease. This in turn, indicates that antiviral APR might play a pivot role in linking peripheral viral infections with MS exacerbations.

Peripheral viral challenge elevates extracellular glutamate in the hippocampus leading to seizure hypersusceptibility

Peripheral viral infections increase seizure propensity and intensity in susceptible individuals. We have modeled this comorbidity by demonstrating that the acute phase response instigated by an intraperitoneal (i.p.) injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC), induces protracted hypersusceptibility to kainic acid-induced seizures. We have further demonstrated that PIC challenge robustly increases the level of tonic extracellular glutamate and neuronal excitability in the hippocampus. This study was undertaken to determine a relationship between tonic glutamate and seizure susceptibility following PIC challenge. Briefly, glutamate-sensing microelectrodes were permanently implanted into the CA1 of 8-week-old female C57BL/6 mice. Following a 3-day recovery, acute phase response was induced by i.p. injection of 12 mg/kg of PIC, while saline-injected mice served as controls. Tonic glutamate was measured at 1, 2, 3 and 4 days after PIC challenge. PIC challenge induced an approximately fourfold increase in tonic glutamate levels measured after 24 h. The levels gradually declined to the baseline values within 4 days. Twenty-four hours after PIC challenge, the mice featured an approximately threefold increase in cumulative seizure scores and twofold increase in the duration of status epilepticus induced by subcutaneous injection of 12 mg/kg of kainic acid. Seizure scores positively correlated with pre-seizure tonic glutamate. Moreover, seizures resulted in a profound (76%) elevation of extracellular glutamate in the CA1 of PIC-challenged but not saline-injected mice. Our results implicate the increase in extracellular glutamate as a mediator of seizure hypersusceptibility induced by peripheral viral challenge.

Peripherally restricted viral challenge elevates extracellular glutamate and enhances synaptic transmission in the hippocampus

Peripheral infections increase the propensity and severity of seizures in susceptible populations. We have previously shown that intraperitoneal injection of a viral mimic, polyinosinic-polycytidylic acid (PIC), elicits hypersusceptibility of mice to kainic acid (KA)-induced seizures. This study was undertaken to determine whether this seizure hypersusceptibility entails alterations in glutamate signaling. Female C57BL/6 mice were intraperitoneally injected with PIC, and after 24 h, glutamate homeostasis in the hippocampus was monitored using the enzyme-based microelectrode arrays. PIC challenge robustly increased the level of resting extracellular glutamate. While pre-synaptic potassium-evoked glutamate release was not affected, glutamate uptake was profoundly impaired and non-vesicular glutamate release was augmented, indicating functional alterations of astrocytes. Electrophysiological examination of hippocampal slices from PIC-challenged mice revealed a several fold increase in the basal synaptic transmission as compared to control slices. PIC challenge also increased the probability of pre-synaptic glutamate release as seen from a reduction of paired-pulse facilitation and synaptic plasticity as seen from an enhancement of long-term potentiation. Altogether, our results implicate a dysregulation of astrocytic glutamate metabolism and an alteration of excitatory synaptic transmission as the underlying mechanism for the development of hippocampal hyperexcitability, and consequently seizure hypersusceptibility following peripheral PIC challenge. Peripheral infections/inflammations enhance seizure susceptibility. Here, we explored the effect of peritoneal inflammation induced by a viral mimic on glutamate homeostasis and glutamatergic neurotransmission in the mouse hippocampus. We found that peritoneal inflammation elevated extracellular glutamate concentration and enhanced the probability of pre-synaptic glutamate release resulting in hyperexcitability of neuronal networks. These mechanisms are likely to underlie the enhanced seizure propensity.

Peripheral challenge with a viral mimic upregulates expression of the complement genes in the hippocampus

Peripheral challenge with a viral mimetic, polyinosinic-polycytidylic acid (PIC) induces hippocampal hyperexcitability in mice. Here, we characterized this hippocampal response through a whole genome transcriptome analysis. Intraperitoneal injection of PIC resulted in temporal dysregulation of 625 genes in the hippocampus, indicating an extensive genetic reprogramming. The bioinformatics analysis of these genes revealed the complement pathway to be the most significantly activated. The gene encoding complement factor B (CfB) exhibited the highest response, and its upregulation was commensurate with the development of hyperexcitability. Collectively, these results suggest that the induction of hippocampal hyperexcitability may be mediated by the alternative complement cascades.

Peripherally restricted acute phase response to a viral mimic alters hippocampal gene expression

We have previously shown that peripherally restricted acute phase response (APR) elicited by intraperitoneal (i.p.) injection of a viral mimic, polyinosinic-polycytidylic acid (PIC), renders the brain hypersusceptible to excitotoxic insult as seen from profoundly exacerbated kainic acid (KA)-induced seizures. In the present study, we found that this hypersusceptibility was protracted for up to 72 h. RT-PCR profiling of hippocampal gene expression revealed rapid upregulation of 23 genes encoding cytokines, chemokines and chemokine receptors generally within 6 h after PIC challenge. The expression of most of these genes decreased by 24 h. However, two chemokine genes, i.e., Ccl19 and Cxcl13 genes, as well as two chemokine receptor genes, Ccr1 and Ccr7, remained upregulated for 72 h suggesting their possible involvement in the induction and sustenance of seizure hypersusceptibility. Also, 12 genes encoding proteins related to glutamatergic and GABAergic neurotransmission featured initial upregulation or downregulation followed by gradual normalization. The upregulation of the Gabrr3 gene remained upregulated at 72 h, congruent with its plausible role in the hypersusceptible phenotype. Moreover, the expression of ten microRNAs (miRs) was rapidly affected by PIC challenge, but their levels generally exhibited oscillating profiles over the time course of seizure hypersusceptibility. These results indicate that protracted seizure susceptibility following peripheral APR is associated with a robust polygenic response in the hippocampus.

TLR3 ligation protects human astrocytes against oxidative stress

Astrocytic Toll-like receptor 3 (TLR3) plays an important role not only in antiviral response but also in regeneration/healing of the CNS. The present study was undertaken to determine whether the neuroprotective effects of TLR3 signaling also include antioxidative protection. TLR3 ligation in human astrocytes induced protracted resistance of the cells to H(2)O(2) toxicity. Similar resistance was induced by conditioned medium from TLR3-ligated astrocytes indicating the involvement of paracrine signaling mechanisms. Out of 13 major antioxidative genes only the gene encoding superoxide dismutase 2 (SOD2) was postligationally upregulated suggesting that SOD2 is the major enzyme responsible for this protection.

Peripheral immune challenge with viral mimic during early postnatal period robustly enhances anxiety-like behavior in young adult rats

Inflammatory factors associated with immune challenge during early brain development are now firmly implicated in the etiologies of schizophrenia, autism and mood disorders later in life. In rodent models, maternal injections of inflammagens have been used to induce behavioral, anatomical and biochemical changes in offspring that are congruent with those found in human diseases. Here, we studied whether inflammatory challenge during the early postnatal period can also elicit behavioral alterations in adults. At postnatal day 14, rats were intraperitoneally injected with a viral mimic, polyinosinic:polycytidylic acid (PIC). Two months later, these rats displayed remarkably robust and consistent anxiety-like behaviors as evaluated by the open field/defensive-withdrawal test. These results demonstrate that the window of vulnerability to inflammatory challenge in rodents extends into the postnatal period and offers a means to study the early sequelae of events surrounding immune challenge to the developing brain.

Abstract 3255: Tetrahydropyridine derivatives: Novel and potent histone demethylase LSD1 inhibitors

Flavin-dependent histone demethylases play an important role in epigenetic gene regulation. The most studied enzyme, LSD1 (KDM1), catalyzes the oxidative demethylation of methylated Lys4 of histone H3. Known LSD1 inhibitors also inhibit other flavin-dependent enzymes, in particular monoamine oxidase (MAO). Therefore, selective inhibition of LSD1 is a significant drug discovery target. We designed, synthesized and evaluated in vitro a series of eleven N-substituted tetrahydropyridines as inhibitors of flavin-dependent enzymes. Complete chemical structures of the compounds used will be disclosed at the time of presentation. The inhibition of LSD1 was measured in rat glioma C6 cells. Briefly, cells grown in serum-free medium were treated with the inhibitors for 24 h, and the level of Lys4 dimethylated form of histone H3 was measured by immunoblot. MAO A activity was determined using human recombinant enzyme with kynuramine as substrate. The most selective among the inhibitors showed low nanomolar potency toward LSD1 and weak inhibition of MAO A. The observed high potency and substrate selectivity justifies further evaluation of the drug discovery potential of the tetrahydropyridine scaffold.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3255. doi:10.1158/1538-7445.AM2011-3255

A broad upregulation of cerebral chemokine genes by peripherally-generated inflammatory mediators

Previously, we have shown that peripheral challenge of mice with double stranded RNA (dsRNA), a viral mimic, evokes global upregulation of cerebral inflammatory genes and, particularly, genes encoding chemokines. Because chemokine networks are potent modulators of brain function, the present study was undertaken to comprehensively characterize the cerebral response of chemokine ligand and receptor genes to peripheral immune system stimulation. Briefly, C57BL/6 mice were intraperitoneally injected with 12 mg/kg of polyinosinic-polycytidylic acid (PIC) and the expression of 39 mouse chemokine ligand and 20 receptor genes was monitored in the cerebellum by real time quantitative RT-PCR within 24 h. Almost half of the ligand genes featured either transient or sustained upregulation from several- to several thousand-fold. Five CXC type genes, i.e., Cxcl9, Cxcl11, Cxcl10, Cxcl2 and Cxcl1, were the most robustly upregulated, and were followed by six CC type genes, i.e., Ccl2, Ccl7, Ccl5, Ccl12, Ccl4 and Ccl11. Seven genes showed moderate upregulation, whereas the remaining genes were unresponsive. Six receptor genes, i.e., Cxcr2, Ccr7, Cxcr5, Ccr6, Ccr1 and Ccr5, featured a several-fold upregulation. Similar chemokine gene response was observed in the forebrain and brainstem. This upregulation of chemokine genes could be induced in naïve mice by transfer of blood plasma from PIC-challenged mice. Employing oligodeoxynucleotide-labeled PIC we further showed that intraperitoneally injected PIC was not transferred to the blood. In conclusion, peripheral PIC challenge elicits a broad upregulation of cerebral chemokine genes, and this upregulation is mediated by blood-borne agents.

Peripheral immune challenge with dsRNA enhances kainic acid-induced status epilepticus

Clinical evidence implicates peripheral inflammatory diseases as comorbid factors in epilepsy. The present study was designed to determine the effect of the acute phase of antiviral response on seizure susceptibility. Young adult mice were intraperitoneally injected with 12 mg/kg of a viral mimic, polyinosinic:polycytidylic acid (PIC). After 48 h, seizures were induced by subcutaneous injection of kainic acid (KA). PIC-pretreatment profoundly enhances vulnerability to excitotoxic insult as evidenced by increased seizure intensity and extended duration of status epilepticus. These results support the notion that peripheral viral infections may alter brain function resulting in enhanced predilection to seizures.

Dietary cholesterol impairs memory and memory increases brain cholesterol and sulfatide levels

Cholesterol and sulfatides play many important roles in learning and memory. To date, our observations about the effects of cholesterol on learning have been assessed during response acquisition; that is, the learning of a new memory. Here, we report for the first time to our knowledge, on the effect of a cholesterol diet on a previously formed memory. Rabbits were given trace conditioning of the nictitating membrane response for 10 days, then fed a 2% cholesterol diet for 8 weeks, and then assessed for memory recall of the initially learned task. We show that dietary cholesterol had an adverse effect on memory recall. Second, we investigated whether dietary cholesterol caused an increase in brain cholesterol and sulfatide levels in four major brain structures (hippocampus, frontal lobe, brainstem, and cerebellum) using a technique for analyzing myelin and myelin-free fractions separately. Although our data confirm previous findings that dietary cholesterol does not directly affect cholesterol and establish that it does not affect sulfatide levels in the brain, these levels did increase rather significantly in the hippocampus and frontal lobe as a function of learning and memory.

Phosphodiesterase 4B2 gene is an effector of Toll-like receptor signaling in astrocytes

Cyclic AMP is part of an endogenous mechanism that downregulates inflammatory response, and its intracellular concentration is regulated chiefly by cyclic nucleotide phosphodiesterases type 4. The goal of the present study was to determine whether phosphodiesterases 4 are involved in the inflammatory response of astrocytes mediated by Toll-like receptors. Astrocyte cultures established from newborn rat brain were challenged with lipoteichoic acid, a ligand of Toll-like receptor 2, polyinosinic-polycytidylic acid, a ligand of Toll-like receptor 3, or lipopolysaccharide, a ligand of Toll-like receptor 4. After 24 h the expression of genes encoding phosphodiesterase 4A, phosphodiesterase 4B and phosphodiesterase 4D was determined by real time reverse transcription polymerase chain reaction. The challenge of astrocytes with the ligands profoundly up-regulated expression of the phosphodiesterase 4B mRNA, while the phosphodiesterase 4A and 4D mRNA was either unaffected or downregulated. Moreover, Toll-like receptor ligation specifically up-regulated expression of the phosphodiesterase 4B2 transcriptional variant. Thus, polyinosinic-polycytidylic acid, lipopolysaccharide and lipoteichoic acid induced approximately 7-, 5- and 4-fold up-regulation of the message, respectively. Toll-like receptor ligation also led to an over 2-fold increase in the protein level of phosphodiesterase 4B2 as revealed by immunoblot analysis. The inactivation of Rho proteins by pretreatment with toxin B form C. difficile enhanced ligation-induced up-regulation of the phosphodiesterase 4B2 message by 4-9-fold. However, in spite of this increase in the message abundance, there was no increase in the protein level compared to cells challenged with the ligands alone. These results demonstrate that the phosphodiesterase 4B2 gene is an effector of Toll-like receptor signaling in astrocytes, and that its up-regulation at the protein level is controlled by complex mechanisms.

Rho proteins are negative regulators of TLR2, TLR3, and TLR4 signaling in astrocytes

The family of Toll-like receptors (TLRs) expressed by innate immune cells recognizes a spectrum of microbial components as well as molecules released from injured tissues. TLR ligation activates intracellular signaling cascades that culminate in the up-regulation of proinflammatory genes. We have recently demonstrated that the up-regulation of inflammatory cytokines mediated by TLR4 in astrocytes is negatively controlled by the monomeric GTPases of Rho subfamily. The present study was undertaken to examine further the involvement of Rho proteins in the inflammatory response of astrocytes elicited by the ligation of three TLRs that use divergent signaling pathways. Astrocyte cultures established from newborn rat brains were challenged with ligands of TLR2, TLR3, and TLR4. The expression of genes encoding interleukin (IL)-1beta, IL-6, tumor necrosis factor-alpha (TNFalpha), interferon-beta (IFNbeta), and inducible nitric oxide synthase (NOS2) was up-regulated 24 hr after the challenge as determined by real-time RT-PCR. Pretreatment of the cells with toxin B, which specifically inactivates Rho proteins, enhanced the up-regulation of gene expression. The extent of this enhancement was both receptor and gene dependent. The enhancing effect of Rho protein inactivation was also evident at the protein level of IL-6 and NOS2 as revealed by ELISA and immunoblot analyses, respectively. These results suggest that Rho proteins control TLR-mediated up-regulation of inflammatory genes in astrocytes by interfering with multiple events along the signaling pathways.

Insulin enhances proliferation and viability of human umbilical vein endothelial cells

This investigation is a follow-up to our previous in vivo studies revealing that rapid stretch increases tissue insulin in murine skin flaps, coincident with the up-regulation of key angiogenic effectors and enhanced vascularization. In the present study, we used human umbilical vein endothelial cells (HUVECs) as an in vitro model system to determine the role of insulin in the chemical signals regulating the processes of proliferation and viability (survival). MTT-based colorimetric methods demonstrated that insulin enhances proliferation and survival of HUVECs. Western blot analysis revealed that protein kinase B (pAkt [Thr(308)]) and vascular endothelial growth factor (VEGF) were the insulin-responsive intermediates in proliferating endothelial cells (ECs). In insulin-enhanced survival, both pAkt (Thr(308)) and pAkt (Ser(473)) were activated in HUVECs. However, no change in VEGF expression accompanied pAkt activation. The beneficial effects of insulin were abrogated by insulin receptor (IR)/insulin-like growth factor receptor (IGFR) or phosphoinositide-3 kinase (PI3-K) blockade, suggesting that insulin-induced EC proliferation and viability are mediated through pIR/pIGFR and PI3-K effectors. These data provide new insights into the beneficial effects of insulin on vascularization and tissue viability, providing a mechanistic link to the enhancement of healing in acutely stretched skin.

Dehydroalanine analog of glutathione: an electrophilic busulfan metabolite that binds to human glutathione S-transferase A1-1

Elimination of hydrogen sulfide from glutathione (GSH) converts a well known cellular nucleophile to an electrophilic species, gamma-glutamyldehydroalanylglycine (EdAG). We have found that a sulfonium metabolite formed from GSH and busulfan undergoes a facile beta-elimination reaction to give EdAG, which is an alpha,beta-unsaturated dehydroalanyl analog of GSH. EdAG was identified as a metabolite of busulfan in a human liver cytosol fraction. EdAG condenses with GSH in a Michael addition reaction to produce a lanthionine thioether [(2-amino-5-[[3-[2-[[4-amino-5-hydroxy-5-oxopentanoyl]amino]-3-(carboxymethylamino)-3-oxopropyl]sulfanyl-1-(carboxymethylamino)-1-oxopropan-2-yl]amino]-5-oxopentanoic acid); GSG], which is a nonreducible analog of glutathione disulfide. EdAG was less cytotoxic than busulfan to C6 rat glioma cells. GSH and EdAG were equally effective in displacing a glutathione S-transferase (GST) isozyme (human GSTA1-1) from a GSH-agarose column. The finding of an electrophilic metabolite of GSH suggests that alteration of cellular GSH concentrations, irreversible nonreducible glutathionylation of proteins, and interference with GST function may contribute to the toxicity of busulfan.

Cognitive dysfunction induced by chronic administration of common cancer chemotherapeutics in rats

Although cognitive dysfunction manifested by severe memory and attention deficits has been reported in up to 70% of cancer patients undergoing chemotherapy, the mechanisms of this serious side effect have not been defined. In particular, it has not been decisively resolved whether the dysfunction is attributable to the chemotherapy or to the malignancy itself. In the present study we tested whether cognitive dysfunction can be induced in an experimental setting by the administration of commonly used chemotherapeutics to rats. Female 10 month old Sprague-Dawley rats were injected intraperitoneally with a combination of 2.5 mg/kg of adriamycin (ADR) and 25 mg/kg of cytoxan (CTX). A total of four doses were given at weekly intervals. The control group was treated with saline only. No mortality and no apparent morbidity were observed in either group. However, the chemotherapeutic treatment severely impaired memory function of rats as measured by a passive avoidance test. This memory deficiency was fully prevented by the administration of an antioxidant, N-acetyl cysteine (NAC) injected subcutaneously three times a week at 200 mg/kg in the course of chemotherapeutic treatment. These results indicate that chemotherapeutic agents alone, i.e., in the absence of malignancy, damage the brain resulting in memory dysfunction. Moreover, the results strongly indicate that the damaging effect is mediated by oxidative stress, as memory dysfunction is preventable by the co-administration of NAC.

Statins enhance toll-like receptor 4-mediated cytokine gene expression in astrocytes: Implication of Rho proteins in negative feedback regulation

Toll-like receptors (TLRs) are sentinels of innate immunity that recognize pathogenic molecules and trigger inflammatory response. Because inflammatory mediators are detrimental to the host, the TLR response is regulated by feedback inhibition. Statins, the inhibitors of isoprenoid biosynthesis, have been shown to be potent modulators of TLR activity, and this modulation may provide insight regarding mechanisms of the feedback inhibition. In the present study, we examined feedback mechanisms that regulate TLR4 activity in astrocytes using statins to perturb postligational signaling. Astrocytic cultures established from newborn rat brains were exposed to lipopolysaccharide (LPS), the ligand for TLR4. The up-regulation of expression of genes encoding interleukin (IL)-1beta, IL-6, and tumor necrosis factor-alpha (TNFalpha) was determined by real-time RT-PCR. Pretreatment of the cells with either atorvastatin or simvastatin enhanced the LPS-induced up-regulation of cytokine gene expression. The most profound enhancement of approximately 17-fold was observed for the Il-6 gene. The enhancements for the Tnfa and Il-1b genes were approximately 5- and 3.5-fold, respectively. Mevalonate fully reversed the effects of statins, indicating that these drugs act through the inhibition of isoprenoid synthesis. The inhibition of protein geranylgeranylation, but not protein farnesylation, mimicked the effects of statins, strongly indicating that the enhancement is mediated by the Rho proteins. In support of this notion, pretreatment of cells with toxin B, a specific inhibitor of the Rho proteins, also enhanced LPS-triggered up-regulation of the cytokine genes. These results indicate that the Rho proteins are involved in the activation of negative feedback inhibition of TLR4 signaling in astrocytes.

BRCA1 contributes to cell cycle arrest and chemoresistance in response to the anticancer agent irofulven

Tumor suppressor gene BRCA1 is frequently mutated in familial breast and ovarian cancer. BRCA1 plays pivotal roles in maintaining genomic stability by interacting with numerous proteins in cell cycle control and DNA repair. Irofulven (6-hydroxymethylacylfulvene, HMAF, MGI 114, NSC 683863) is one of a new class of anticancer agents that are analogs of mushroom-derived illudin toxins. Preclinical studies and clinical trials have demonstrated that irofulven is effective against several tumor cell types. The exact nature of irofulven-induced DNA damage is not completely understood. We demonstrated previously that irofulven activates ATM and its targets, NBS1, SMC1, CHK2, and p53. In this study, we hypothesize that irofulven induces DNA double-strand breaks and that BRCA1 may affect chemosensitivity by controlling cell cycle checkpoints, DNA repair, and genomic stability in response to irofulven treatment. We observed that irofulven induces the formation of chromosome breaks and radials and the activation and foci formation of gamma-H2AX, BRCA1, and RAD51. We also provided evidence that irofulven induces the generation of DNA double-strand breaks. By using BRCA1-deficient or -proficient cells, we demonstrated that in response to irofulven, BRCA1 contributes to the control of S and G(2)/M cell cycle arrest and is critical for repairing DNA double-strand breaks and for RAD51-dependent homologous recombination. Furthermore, we found that BRCA1 deficiency results in increased chromosome damage and chemosensitivity after irofulven treatment.

The role of Toll-like receptors in CNS response to microbial challenge

The recent discovery of the family of Toll-like receptors has vastly expanded our understanding of the mechanisms by which the innate immune system recognizes and responds to a wide variety of microbial and endogenous pathogens. Toll-like receptors are transmembrane proteins that upon ligation with their cognate ligands trigger the production of cytokines, enzymes and other inflammatory agents. In the CNS Toll-like receptors are expressed predominantly by glial cells. In particular, the vastly abundant astrocytes are likely to be the major contributors to inflammatory responses within the CNS. Studies of the murine brain abscess model revealed that Toll-like receptor 2 plays a pivotal role in the generation of immune responses to Staphylococcus aureus. Although Toll-like receptor signaling is essential in antimicrobial defense, it may also lead to bystander injury of CNS tissue.

Nucleotides induce higher order chromatin degradation

Higher order chromatin degradation (HOCD) is a stepwise dismantling of the genome through the excision of chromatin loops and their oligomers at matrix attachment regions (MARs) during the early stages of programmed cell death. Although HOCD ultimately leads to the inactivation of the genome and cell death, a partial HOCD in cells receiving sublethal signals may result in the loss of genetic stability leading to neoplasia, degeneration, and aging. The present study was undertaken to determine the role of protein poly(ADP-ribosyl)ation in HOCD. Nuclei isolated from rat glioma C6 cells were able to carry poly(ADP-ribosyl)ation as assessed by the incorporation of (32)P-NAD(+) into TCA-insoluble fraction. Under the same experimental conditions, millimolar NAD(+) induced rapid HOCD in nuclei. However, while poly(ADP-ribosyl)ation was totally abrogated by specific inhibitor, benzamide, NAD(+)-induced HOCD was unaffected. Benzamide also failed to inhibit HOCD induced by H(2)O(2) exposure in intact cells. These results indicate that HOCD is not mediated through chromatin poly(ADP-ribosyl)ation, and that NAD(+) activates MAR-associated endonuclease or facilitates the access of the enzyme to DNA by other mechanisms. Furthermore, other nucleotides including NADP(+), ATP, UTP, GTP, and CTP were also found to induce HOCD in isolated nuclei indicating that HOCD is controlled by nucleotide-related ligands.

Kinetics of inflammatory response of astrocytes induced by TLR 3 and TLR4 ligation

Toll-like receptors (TLRs) are sentinels of the innate immune system that recognize an array of exogenous and endogenous pathogenic molecules. The ligation of the receptors triggers inflammatory response necessary for pathogen elimination and for the healing process. In the present study we examined inflammatory response of astrocytes elicited by the ligation of TLR3 and TLR4. Astrocytic cultures established from newborn rat brains were exposed to double stranded RNA (dsRNA) and lipopolysaccharide (LPS), the ligands for TLR3 and TLR4, respectively. The expression of cytokine genes was determined by RNase protection assay, and the generation of nitric oxide (NO) was measured by Griess technique. Both ligands upregulated the expression of several cytokines (i.e., IL-1alpha, IL-1beta, IL-6, TNFalpha, GM-CSF, LTbeta, and TGFbeta3) and downregulated the expression of MIF, but have no effect on the expression of IL-2, IL-3, IL-4, IL-5, IL-10, TGFbeta1, TGFbeta2, TNFbeta, and IFNgamma. Although dsRNA upregulated the expression of IFNbeta, LPS did not indicating that the TRIF-dependent branch of TLR4 signaling is inactive in astrocytes. Proinflammatory response as seen from upregulated cytokine expression and NO generation reached a peak within the first day of exposure, and was subsequently abrogated. The cells also became refractory to subsequent stimulation by the ligands indicating the existence of negative feedback mechanisms that control proinflammatory response in astrocytes.

Epoxidation of the methamphetamine pyrolysis product, trans-phenylpropene, to trans-phenylpropylene oxide by CYP enzymes and stereoselective glutathione adduct formation

Pyrolytic products of smoked methamphetamine hydrochloride are well established. Among the various degradation products formed, trans-phenylpropene (trans-beta-methylstyrene) is structurally similar to styrene analogues known to be bioactivated by CYP enzymes. In human liver microsomes, trans-phenylpropene was converted to the epoxide trans-phenylpropylene oxide (trans-2-methyl-3-phenyloxirane) and cinnamyl alcohol. Incubation of trans-phenylpropene with microsomes in the presence of enzyme-specific P450 enzyme inhibitors indicated the involvement of CYP2E1, CYP1A2, and CYP3A4 enzymes. Both (R,R)-phenylpropylene oxide and (S,S)-phenylpropylene oxide were formed in human liver microsomal preparations. Enantiomers of trans-phenylpropylene oxide were stereoselectively and regioselectively conjugated in a Phase II drug metabolism reaction catalyzed by human liver cytosolic enzymes consisting of conjugation with glutathione. The structure of the phenylpropylene oxide-glutathione adduct is consistent with nucleophilic ring-opening by attack at the benzylic carbon. Exposure of cultured C6 glial cells to (S,S)-phenylpropylene oxide produced a cytotoxic response in a concentration-dependent manner based on cell degeneration and death.

Vulnerability of brain tissue to inflammatory oxidant, hypochlorous acid

CNS inflammation is a sequela of a variety of neuropathological conditions resulting in extensive tissue loss. Inflammation is mediated primarily by phagocytic cells, but the mechanisms of CNS tissue destruction are not fully understood. Hypochlorous acid (HOCl) is by far the most abundant agent generated by phagocytic cells and may be the major mediator of inflammatory tissue damage. However, the effects of HOCl on nervous tissue have not been examined. In this study we used an in vitro model system of rat brain slices to determine neurotoxicity of HOCl. The slices were exposed to HOCl at pathologically relevant doses, and the incorporation of [3H]leucine into proteins and lipids was analyzed in total homogenate, and in particulate fractions obtained by density gradient centrifugation. The results demonstrated that a brief HOCl exposure profoundly suppressed protein biosynthesis in the slices. Also, lipid synthesis was suppressed in nonmyelin particulate fraction. However, lipid synthesis in myelin was significantly stimulated in HOCl-exposed slices indicating that oligodendrocyte response to the oxidant differs from that of other CNS cells. The effects of HOCl could be blocked by coadministration of antioxidants, i.e., N-acetylcystein (NAC), uric acid (UA) and ascorbic acid (AA). The protective potency of the antioxidants was NAC>UA>AA. In conclusion, our study demonstrated that HOCl generated by phagocytic cells during inflammatory episodes has a potential to damage surrounding CNS tissue, and that tissue damage can be prevented by HOCl scavenging with clinically applicable antioxidants.

Toxicity of compound A to C6 rat glioma cells

The volatile anesthetic, sevoflurane, undergoes degradation by soda lime to form Compound A (2-fluoromethoxy-1,1,3,3,3-pentafluoro-1-propene). Compound A is toxic in vivo with the kidney being the primary target. However, peripheral neuropathy was recently reported in a group of healthy volunteers who received sevoflurane. The present study was undertaken to evaluate the toxicity of Compound A to neural cells. Rat glioma C6 cells were grown in T25 flasks in 5 mL of DMEM/F12 were exposed to Compound A, and the viability of cells was determined at various time points by trypan blue exclusion. Within 1 h after the addition of 10 microL of Compound A, the fragmentation of cell processes and rounding of cell bodies became apparent. The cellular degeneration progressed over time resulting in the loss of all viable cells from the cultures within 6 h. Even brief exposures to Compound A ranging from 5 to 30 min resulted in massive cell death observed 24 h later, and the toxicity was concentration-dependent. These preliminary experiments indicate that Compound A is a potent toxin to glial cells in vitro. A plausible mechanism for this toxicity entails the depletion of intracellular glutathione resulting in oxidative stress of the cells. However, the relatively high doses of Compound A used to observe its effects do not support the toxicity of Compound A to glial cells under clinical conditions.

H2O2-induced higher order chromatin degradation: a novel mechanism of oxidative genotoxicity

The genotoxicity of reactive oxygen species (ROS) is well established. The underlying mechanism involves oxidation of DNA by ROS. However, we have recently shown that hydrogen peroxide (H2O2), the major mediator of oxidative stress, can also cause genomic damage indirectly. Thus, H2O2 at pathologically relevant concentrations rapidly induces higher order chromatin degradation (HOCD), i.e. enzymatic excision of chromatin loops and their oligomers at matrix-attachment regions. The activation of endonuclease that catalyzes HOCD is a signalling event triggered specifically by H2O2. The activation is not mediated by an influx of calcium ions, but resting concentrations of intracellular calcium ions are required for the maintenance of the endonuclease in an active form. Although H2O2-induced HOCD can efficiently dismantle the genome leading to cell death, under sublethal oxidative stress conditions H2O2-induced HOCD may be the major source of somatic mutations.

Hydrogen peroxide mediates higher order chromatin degradation

Although a large body of evidence supports a causative link between oxidative stress and neurodegeneration, the mechanisms are still elusive. We have recently demonstrated that hydrogen peroxide (H(2)O(2)), the major mediator of oxidative stress triggers higher order chromatin degradation (HOCD), i.e. excision of chromatin loops at the matrix attachment regions (MARs). The present study was designed to determine the specificity of H(2)O(2) in respect to HOCD induction. Rat glioma C6 cells were exposed to H(2)O(2) and other oxidants, and the fragmentation of genomic DNA was assessed by field inversion gel electrophoresis (FIGE). S1 digestion before FIGE was used to detect single strand fragmentation. The exposure of C6 cells to H(2)O(2) induced a rapid and extensive HOCD. Thus, within 30 min, total chromatin was single strandedly digested into 50 kb fragments. Evident HOCD was elicited by H(2)O(2) at concentrations as low as 5 micro M. HOCD was mostly reversible during 4-8h following the removal of H(2)O(2) from the medium indicating an efficient relegation of the chromatin fragments. No HOCD was induced by H(2)O(2) in isolated nuclei indicating that HOCD-endonuclease is activated indirectly by cytoplasmic signal pathways triggered by H(2)O(2). The exposure of cells to a synthetic peroxide, i.e. tert-butyrylhydroperoxide (tBH) also induced HOCD, but to a lesser extent than H(2)O(2). Contrary to the peroxides, the exposure of cells to equitoxic concentration of hypochlorite and spermine NONOate, a nitric oxide generator, failed to induce rapid HOCD. These results indicate that rapid HOCD is not a result of oxidative stress per se, but is rather triggered by signaling cascades initiated specifically by H(2)O(2). Furthermore, the rapid and extensive HOCD was observed in several rat and human cell lines challenged with H(2)O(2), indicating that the process is not restricted to glial cells, but rather represents a general response of cells to H(2)O(2).

Higher order chromatin degradation: implications for neurodegeneration

Higher order chromatin degradation (HOCD) is a hallmark of programmed cell death. HOCD is mediated by enzymatic digestion of the DNA backbone at matrix attachment regions, and ultimately results in the excision of chromatin loops and their oligomers from chromosomes. We have recently demonstrated that hydrogen peroxide (H2O2), the major mediator of oxidative stress, rapidly induces HOCD. This demonstration allowed us to characterize several kinetic features of HOCD. Moreover, H2O2-induced HOCD provides a mechanistic link between oxidative stress and the pathology of neurodegeneration. Thus, in acute neurodegenerative conditions, which feature severe oxidative stress, H2O2-induced HOCD efficiently dismantles the genome, and thus, irreversibly commits cells to death. In chronic neurodegenerative conditions, which feature sublethal but perennial oxidative stress, cells undergo only a partial fragmentation of the genome via H2O2-induced HOCD. If unrepaired of improperly repaired, such a partial fragmentation leads to the generation and accumulation of somatic mutations that are likely to play the key role in delayed degeneration and death of neural cells.

Hydrogen peroxide induces transient dephosphorylation of tau protein in cultured rat oligodendrocytes

Oxidative stress is a major mediator of neurodegeneration. In this study, we tested the effects of oxidative stress induced by a brief exposure to hydrogen peroxide (H(2)O(2)) on the phosphorylation state of the tau protein in oligodendrocytes (OL). Primary oligodendrocyte cultures prepared from newborn rat brains were exposed to millimolar concentrations of H(2)O(2) for up to 15 min, and then incubated in normal medium for up to 12 h. The treatment caused morphological degeneration of OL characterized by the loss of cellular processes apparent approximately 3 h after H(2)O(2) exposure. The morphological degeneration was preceded by a profound dephosphorylation of tau protein revealed by immunoblot using monoclonal tau-1 antibody that recognizes the dephosphorylated epitope. The dephosphorylated form increased dramatically during H(2)O(2) exposure, peaked after 2 h of post-exposure, and returned to the baseline level within 12 h. Total tau protein levels were not changed in the course of the experiment as judged by immunoblotting with phosphorylation-insensitive tau-5 and 46-1 monoclonal antibodies. Our finding demonstrates that oxidative stress induces a rapid but transient dephosphorylation of tau protein that may underlie morphological degeneration of OL.

Higher order chromatin degradation in glial cells: the role of calcium

Higher order chromatin degradation (HOCD), i.e. the scission of nuclear chromatin loops at the matrix attachment regions (MARs), is a hallmark of programmed cell death. We have previously demonstrated that hydrogen peroxide (H(2)O(2)) induces rapid HOCD in cultured oligodendrocytes generating two subpopulations of DNA fragments of >or=400 and 50-200 kb. In the present study, we examined the involvement of calcium in this process. HOCD was induced in primary rat oligodendrocytes by exposure to 1 mM H(2)O(2) and assessed by field inversion gel electrophoresis with and without S1 endonuclease digestion, to detect single and double stranded fragmentation, respectively. Chelating intracellular calcium with BAPTA/AM prior to H(2)O(2) exposure inhibited HOCD in a dose-dependent manner. Complete inhibition of HOCD was attained with 50 muM BAPTA/AM. The pretreatment of cells with desferroxamine mesylate, which may lower intracellular calcium levels, also resulted in a profound inhibition of HOCD, but the initial chromatin digestion into >or=400 kb single stranded DNA fragments was unaffected. Neither removing extracellular calcium nor blocking calcium release from intracellular stores with TMB-8 affected HOCD. Moreover, increasing intracellular calcium with A23187 calcium ionophore did not induce HOCD. Subsequent study in nuclei purified from C6 glioma cells revealed that the endonuclease responsible for HOCD is calcium-independent, but is magnesium-dependent. Magnesium-induced HOCD was not affected by the removal of calcium from nuclei with EGTA, but was practically abrogated in nuclei prepared from BAPTA/AM-pretreated cells. These results indicate that although H(2)O(2)-induced HOCD is not directly mediated by an increase of intracellular calcium concentration, normal resting levels of intracellular calcium are required for the maintenance of MAR-associated endonuclease in an active form.

Hydrogen peroxide induces rapid digestion of oligodendrocyte chromatin into high molecular weight fragments

High molecular weight (HMW) fragmentation of nuclear chromatin was studied in cultured rat oligodendrocytes (OL) exposed to hydrogen peroxide (H2O2). Intact genomic DNA was isolated by agarose embedding, and analyzed by field inversion gel electrophoresis, with and without S1 endonuclease digestion to detect and discriminate between single and double stranded fragmentations, respectively. The exposure of OL to H2O2 resulted in a very rapid degradation of chromosomal DNA into HMW fragments that reflect native chromatin structure. Hence, within 10 min after the addition of 1 mM H2O2, a discrete pool representing approximately 45% of the nuclear chromatin underwent single strand digestion into >400 kb fragments likely at AT-rich matrix attachment regions. Subsequent accumulation of single stand breaks at these regions led to bifilar scission. Ultimately, chromatin within this susceptible pool was cleaved at remaining matrix attachment regions into 50-200 kb fragments. Chromatin digestion could be elicited with H2O2 concentrations as low as 50 microM. After the removal of H2O2, most >400 kb fragments were religated within 2 h; however, digestion into 50-200 kb fragments was irreversible. The DNA digestion was not accompanied by the degradation of nuclear proteins, i.e., lamins A/C and poly (ADP-ribose) polymerase indicating that chromatin fragmentation is unlikely to be mediated by proteolysis. In conclusion, H2O2 at pathologically relevant concentrations induces a very rapid and extensive digestion of OL chromatin into HMW fragments. Because the chromatin fragmentation is only partly reversible, it may be a decisive factor in committing oxidatively stressed OL to degeneration and/or death.

Differentiation dependent activation of the myelin genes in purified oligodendrocytes is highly resistant to hypoglycemia

We have previously demonstrated that the developmental upregulation of myelin-specific genes in mixed glial cultures is strongly attenuated by hypoglycemia. The present study was designed to evaluate the effect of hypoglycemia on differentiation-dependent upregulation of myelin genes in purified oligodendrocyte cultures. The expression of major myelin protein genes, i.e., proteolipid protein (PLP), basic protein (BP) and myelin associated glycoprotein (MAG) were monitored by Northern blot analysis. In control cultures maintained at 6 mg/ml of glucose, the expression of all the genes upregulated rapidly, and plateaued at approximately day 4. A similar pattern of differentiation-dependent upregulation was observed for the gene encoding a lipogenic enzyme, i.e., malic enzyme (ME). In contrast to mixed glial cultures, however, this developmental gene upregulation was not significantly affected by severe hypoglycemia (approximately 0.02 mg/ml). The results indicate that the effect of glucose deprivation on oligodendrocyte genes observed in mixed glial cultures is mediated by other cells. The upregulation of the genes in differentiating oligodendrocytes was accompanied by the production of myelin-related membrane that was isolated by density gradient fractionation. In contrast to the effect on gene expression, this anabolic activity was highly dependent on glucose, as seen from a profound suppression by severe hypoglycemia.

Antisense oligodeoxynucleotides targeted to MAG mRNA profoundly alter BP and PLP mRNA expression in differentiating oligodendrocytes: a caution

The applicability of antisense technology to suppress the expression of myelin associated glycoprotein (MAG) in cultured oligodendrocytes was evaluated. Differentiating oligodendrocyte precursor cells obtained by the shake-off method were exposed to nine unmodified antisense oligodeoxynucleotides (ODNs) targeted to the first seven exons of MAG mRNA. After four days, steady-state levels of MAG, proteolipid protein (PLP) and basic protein (BP) mRNAs were determined by Northern blot analysis. Only ODN annealing to 599-618 nt of the MAG mRNA (the junction of exon 5 and 6) resulted in a significant, 75% decrease in the MAG mRNA level. Unexpectedly, six other anti-MAG ODNs which had no significant effect on the MAG message, greatly increased the level of BP mRNA. The highest upregulation of approximately 12 fold was observed with ODN annealing to 139-168 nt (junction of exon 3 and 4). On the other hand, the 997-1016 ODN decreased the levels of BP and PLP messages by 70-80%. The 599-618 ODN also decreased the PLP mRNA by 85%. The results demonstrate that antisense ODNs targeted to one gene may profoundly alter the expression of other genes, and hence, complicate functional analysis of the targeted protein.

Delayed oligodendrocyte degeneration induced by brief exposure to hydrogen peroxide

An in vitro model system of cultured oligodendrocytes was used to determine the susceptibility of these cells to oxidative stress induced by 15 min exposure to millimolar concentrations of hydrogen peroxide (H2O2). Following the exposure, the cells were incubated in normal growth medium, and analyzed at different time points. Although no cell loss was observed during the exposure period, there was a progressive depletion of adherent cells during the postexposure period as seen from either the number of recoverable nuclei, or from total RNA content of the cultures. Both the rate and the extent of cell deletion was directly dependent on H2O2 concentration. Cell death was preceded by structural alterations in the nuclear envelope resulting in "fragile" nuclei which disintegrated during isolation. Northern blot analysis showed that the expression of myelin-specific genes was rapidly downregulated in H2O2-treated cells. On the other hand, the expression of antiapoptotic gene, bcl-2 featured massive but transient upregulation. Oligodendrocyte degeneration also featured genomic DNA degradation into high molecular weight fragments, which are likely to represent cleaved chromosomal loops. The results demonstrate vulnerability of oligodendrocytes to oxidative stress that induces rapid degeneration and ultimately leads to delayed cell death. This feature is highly relevant to oligodendrocyte damage and depletion following ischemic, traumatic, or inflammatory insults to the central nervous system (CNS).

Generation of high efficiency ssDNA hybridization probes by linear polymerase chain reaction (LPCR)

The polymerase chain reaction (PCR) methodology can be employed to produce DNA hybridization probes. The major advantages of this paradigm over other techniques include superior specific activity of the probes, the versatility of sequence selection, the ability to produce short probes, and the simplicity of the procedure. We have further improved the efficiency of PCR probes by generating single stranded (ssDNA) probes that do not reanneal with themselves in solution, and hence, their availability for the interaction with the complementary sequences of the target is profoundly increased. Protocols for 32P-dCTP labeled and digoxigenin-dUTP labeled probes have been elaborated to maximize the incorporation rate of the label as well as to provide for the production of full-length probes. The ssDNA probes may be particularly suitable for nucleic acid detection in tissues by in situ hybridization.

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.

Selenium is required for normal upregulation of myelin genes in differentiating oligodendrocytes

The purpose of this study was to characterize the selenium requirement for the normal differentiation of oligodendrocyte lineage cells. In primary mixed glial cultures prepared from newborn rat brains, the overall growth of cultures, as seen from the total RNA yield, was not significantly affected by selenium. However, 30 nM selenium was required for the normal upregulation the proteolipid protein, basic protein, and myelin-associated glycoprotein gene expression assessed by Northern blot analysis. Selenium deprivation during initial, rapid phase of the gene upregulation irreversibly suppressed the genes, indicating the existence of a critical period in oligodendrocyte differentiation. In purified oligodendrocyte cultures prepared by mechanical dislodging of progenitor (O-2A) cells from mixed glial cultures, total cell number and total RNA yield were virtually unaffected by selenium deprivation; however, the developmental upregulation of the myelin genes was profoundly attenuated. Immunocytochemical analysis confirmed the suppressive effect of selenium deficiency on the differentiation of oligodendrocyte lineage cells, as seen from a significant decrease in the population of GalC+ and O4+ cells. Because the number of GC+ cells was more reduced than the number of O4+ cells, the results indicate that selenium deficiency may specifically inhibit the progression from immature to mature oligodendrocytes.

Chromatin structure and transcriptional activity of MAG gene

Myelin associated glycoprotein (MAG) is an essential component of the periaxonal architecture of the myelin sheath. Because of its potent neurite growth repressive activity, MAG is also likely to play an important role in axonal guidance during the CNS development, and to be responsible for abortive neuronal regeneration in adult CNS. The MAG gene chromatin from approximately -1.6 to +0.6 kb features MNase hypersensitivity that may delineate the gene control region. The proximal upstream region of the gene is organized into an array of five nucleosomes with hypersensitive linkers. The core promoter is located within the first upstream linker that becomes highly hypersensitive in the course of oligodendrocyte differentiation. The adjacent upstream region contains positive and negative enhancers that are likely to streamline oligodendrocyte specific expression of the gene. The TATA-less core promoter contains novel, as yet uncharacterized initiator elements that direct the assembly of transcriptional complexes. The promoter appears to be controlled by both, the addition of activating trans-factors and removal of inhibitory trans-factors as progenitor cells differentiate into mature oligodendrocyte. The developmental activation of the gene is also concomitant with profound DNA demethylation that may provide auxiliary regulatory mechanisms. Hence, the upregulation of the MAG gene in differentiating oligodendrocytes entails chromatin remodeling as well as changes in the assortment of nuclear trans-factors.

Glioma-stimulated chemoattraction of endothelial cells and fibroblasts in vitro: a model for the study of glioma-induced angiogenesis

Induction of angiogenesis is essential for the continued growth of solid tumors, and one critical component of tumor-induced angiogenesis involves the stimulation of microvascular cells to migrate into the growing mass. We have developed a convenient model system utilizing dual co-culture chambers to study cellular chemotaxis induced by glioma cells in vitro. In this system, rat C6 glioma cells induced migration of fibroblasts and brain capillary endothelial cells. The migratory response was directly related to the number of C6 cells serving as stimulus in the lower chamber. Similar migratory responses were induced by C6 cell conditioned medium in a concentration dependent fashion. Medium conditioned by cultured human anaplastic astrocytoma cells was also found to contain potent chemotactic factor(s). This system may ultimately be employed in the identification of particular glioma cell population(s) and secreted factor(s) responsible for the chemoattraction of microvascular cells.

Differentiation-specific demethylation of myelin associated glycoprotein gene in cultured oligodendrocytes

The methylation status of a 4.4-kb 5' end of the myelin-associated glycoprotein (MAG) gene was assessed in cells with different levels of transcriptional activity of the gene, i.e., liver, brain, O-2A oligodendrocyte precursors cells, mature oligodendrocytes, and glioma C6 cells. Purified DNA was digested with methylation-sensitive restriction enzymes, and the cuts were mapped by the indirect end-labeling technique. The restriction sites within the 4.4-kb fragment revealed a highly heterogenous methylation pattern among cells and tissues, and liver DNA was the most heavily methylated. Most of the restriction sites were partly demethylated in the nervous system cells. Notably, two adjacent Hha1 sites at +94 and +96 were fully methylated in liver, but partially demethylated in the brain, OL, and O2A. Two Hpa2 site located at -1836 and at -39 were progressively demethylated in oligodendrocyte lineage cells, indicating specific hypomethylation associated with the oligodendrocytic differentiation. Most of the restriction sites were weakly methylated in the DNA from neoplastic C6 cells, although the Hha1 sites were fully methylated. No clear-cut correlation between the extent of CpG dinucleotide methylation and the chromatin conformation was found. For example, out of four heavily methylated sites only two comapped with MNase hypersensitive sites. Also, the -1836 Hpa2 site whose demethylation is concomitant with oligodendrocytic differentiation seems to be localized within precisely positioned nucleosomal arrays of the MAG gene chromatin. The results indicate that the MAG gene undergoes progressive demethylation concomitant with the oligodendrocyte differentiation/maturation. However, certain CpG dinucleotides remain heavily methylated even in the fully active gene in mature oligodendrocytes, indicating that they may be essential in maintaining proper chromatin structure.

Transcriptional regulation of myelin associated glycoprotein gene expression by cyclic AMP

The treatment of rat glioma C6 cells with 10 microM isoproterenol (Ipt) for 4 days upregulated the expression of the myelin-associated glycoprotein (MAG) gene by approximately 55-fold over the control value. The constant presence of Ipt in the medium was required for the maximal upregulation, as time-restricted exposures to the drug produced only partial, or no upregulation of the gene. No difference in the MAG mRNA stability could be detected in Ipt-treated vs untreated cells indicating that the drug upregulates the MAG gene at transcriptional level. Serum (FCS) strongly attenuated the response of the MAG gene to Ipt. The stimulatory effect of Ipt was profoundly reduced by spermine and H-89, indicating that protein kinase A-dependent protein phosphorylation is involved in the MAG gene activation. Within 30 min after Ipt administration, the c-fos gene was upregulated by 10-fold, and thereafter, its message level decreased and stabilized at approximately 3-fold over control. In contrast, the c-jun gene was downregulated to approximately 20% of control within 30 min after Ipt administration. Subsequently, its message level rose and fell once again within 12 h to approximately half of control, and returned to control level within 72 h.(ABSTRACT TRUNCATED AT 250 WORDS)

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

The effect of dexamethasone on the expression of proteolipid protein (PLP) and myelin-associated glycoprotein (MAG) genes was investigated in rat C6 glioma cells. The steady state level of the respective mRNAs was quantitated by Northern blot analysis. The treatment of cells with dexamethasone transiently upregulated the expression of both genes with peak mRNA levels of approximately 10-fold over control levels occurring at day 3 for the PLP gene and at day 5 for the MAG gene. The effect was directly related to the drug concentration in the range from 10(-9) to 10(-5) M. Combined exposure of the cells to dexamethasone and retinoic acid featured an additive effect on PLP gene expression, whereas MAG gene expression was depressed below detectability level. The dissimilarity in the response of the genes to dexamethasone and retinoic acid supports the contention that the genes are controlled by different mechanisms. Furthermore, the results indicate that the effects of dexamethasone and retinoic acid on the myelin genes are mediated by different regulatory pathways.

Myelin gene activation: a glucose sensitive critical period in development

Glucose deprivation was employed to model caloric undernutrition in newborn rat mixed glial cultures. Six day-old cultures were placed in serum-free media containing glucose concentrations from 0.55 mg/ml to 10 mg/ml. The expression of the myelin PLP, BP, and MAG genes was determined by Northern blot analysis. The activation of the myelin genes began at approximately 6 days in vitro (DIV), and a period of rapid upregulation followed through 14 DIV. The gene activity was directly related to the glucose concentration. The increase in glucose concentration from 0.55 to 1.5 mg/ml (which spans the physiological range) resulted in 2-3 fold increases in expression of the myelin genes, whereas further increases in glucose (2-10 mg/ml) produced only slight additional elevation in the gene activity. We used high glucose (5-6 mg/ml) as control, or low glucose (0.55 mg/ml) as deprived, to delineate possible critical periods of oligodendrocytic differentiation. Cultures were deprived for 4-day intervals, staggered from 6 to 22 DIV. Deprivation from 6 to 10 DIV produced an 80-90% suppression of the myelin gene upregulation at 22 DIV; deprivation from 10 to 14 DIV produced 60-70% suppression; whereas deprivation from 14 to 18 DIV was fully recoverable by 22 DIV. These results show that mixed glial cultures model the developmental pattern of myelin gene expression, as well as their vulnerability. Furthermore, the period of rapid upregulation of the myelin genes appears to be a critical period in development, during which glucose deprivation irreversibly alters oligodendrocyte differentiation.

Spatiotemporal expression of SCIP and myelin genes in rat brain during early postnatal development

The expression of the SCIP and myelin genes (PLP, BP and MAG) was quantitated by Northern blot analysis during early postnatal rat brain development. The SCIP gene was already profoundly upregulated on day 1 postpartum. The SCIP message level in the forebrain was 4- and 17-fold higher than the message level in the cerebellum and brainstem, respectively. By day 20, the SCIP gene expression declined in the forebrain to approximately 5% of the day-1 level, and became undetectable in the brainstem and cerebellum. The myelin gene expression sharply upregulated at approximately day 7-8, and in general, was highest in the brainstem, and lowest in the forebrain. The results demonstrate that the expression of the SCIP gene in the oligodendrocytic lineage cells is relatively low, whereas other cellular elements feature very active expression of the gene during the early postnatal development. Additionally, the variation in the relative ratios of the myelin messages among the brain regions indicates that the levels of expression of the myelin genes are not tightly synchronized.

Myelin isolation: comparison of sedimentation and flotation techniques

Brains from young (20 day old) and adult rats were used to compare myelin yields obtained by sedimentation and flotation techniques. The flotation method consistently gave approx 70% higher yields of myelin than the sedimentation method. Both myelin preparations have virtually identical protein composition as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Electrophoretic analysis revealed substantial concentrations of myelin proteins in the non-myelin particulate fraction obtained by the sedimentation but not by the flotation method. The study indicates that the paradigm of the sedimentation method results in a significant loss of myelin during isolation, and that this loss can be avoided or minimized by employing the flotation method.

Calcium-activated neutral proteinase (calpain) activity in C6 cell line: compartmentation of mu and m calpain

Calcium-activated neutral proteinase (calpain) activity was determined, including in cytosol and membrane fractions, in rat glioma C6 cell line. The mu and m forms of calpain were separated by DEAE and phenylsepharose column chromatography and with removal of the endogenous inhibitor calpastatin. C6 cells contained more mcalpain than the mu isoform. More than 70% of mcalpain activity was membrane-associated and 20% was cytosolic. Isolated plasma membrane also contained 69% of the mcalpain activity. In contrast, approximately 80% of mucalpain activity was cytosolic and 16% was membranous. Half-maximal activity for mu and mcalpain was obtained at 1 microM and 0.2 mM CaCl2, respectively. Trypsin dissociation of cells reduced activity. Triton X-100 stimulated mcalpain activity of the whole homogenate and the membrane pellet but not of the cytosol. Activity of the myelin marker enzyme adenosine 2'3'-cyclic nucleotide 3'-phosphohydrolase (CNPase), was also found in C6 cells. The identification of calpain and CNPase in C6 cells is in keeping with an interpretation that C6 differentiation resembles, at least in part, that of the myelin-forming oligodendroglial cells.

Down regulation of myelin-specific mRNAs in the mechanism of hypomyelination in the undernourished developing brain

The expression of myelin-specific protein genes, i.e. myelin proteolipid (PLP), basic (BP), and myelin associated glycoproteins (MAG) was studied in normal and severely undernourished 20-day-old rats. The undernutrition paradigm resulted in reductions of approximately 50, 25 and 65% in body weight, brain weight and brain myelin yield, respectively. The amount of total brain RNA was not significantly altered, although the amount of cyclophilin (CYC) mRNA was increased. In contrast, the steady-state levels of myelin specific mRNAs were significantly decreased by approximately 40, 20 and 40% for PLP, BP and MAG, respectively. In addition, polyadenylation of the PLP transcript was altered, producing an abnormal ratio of the 1.6 kb to the 3.2 kb PLP mRNAs. The results indicate that down-regulation of myelin-specific gene expression is involved in the mechanisms of hypomyelination in hunger disease, although the individual genes are differently altered. Furthermore, undernutrition may have additional effects on the posttranscriptional processing of the transcripts as indicated by the abnormal size distribution of PLP messages.