Induction of nitric oxide synthase in glial cells

A novel role of lactosylceramide in the regulation of lipopolysaccharide/interferon-gamma-mediated inducible nitric oxide synthase gene expression: implications for neuroinflammatory diseases

In the present study a possible role of glycosphingolipids (GSLs) in inducible nitric oxide synthase (iNOS) gene expression and nitric oxide (NO) production after spinal cord injury (SCI) in rats has been established. In primary rat astrocytes lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) treatment increased the intracellular levels of lactosylceramide (LacCer) and induced iNOS gene expression. d-Threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol.HCI (PDMP), a glucosylceramide synthase and LacCer synthase (galactosyltransferase, GalT-2) inhibitor, inhibited LPS/IFN-gamma induced iNOS expression, which was reversed by exogenously supplied LacCer, but not by other glycosphingolipids. LPS/IFN-gamma caused a rapid increase in the activity of GalT-2 and synthesis of LacCer. Silencing of GalT-2 gene with the use of antisense oligonucleotides resulted in decreased LPS/IFN-gamma-induced iNOS, TNF-alpha, and IL-1beta gene expression. The PDMP-mediated reduction in LacCer production and inhibition of iNOS expression correlated with decreased Ras and ERK1/2 activation along with decreased IkappaB phosphorylation, NF-kappaB DNA binding activity, and NF-kappaB-luciferase reporter activity. LacCer-mediated Ras activation was redox-mediated and was attenuated by antioxidants N-acetyl cysteine (NAC) and pyrrolidine dithiocarbamate (PDTC). In vivo administration of PDMP after SCI resulted in improved functional outcome (Basso, Beattie, Bresnahan score); inhibition of iNOS, TNF-alpha, and IL-1beta expression; decreased neuronal apoptosis; and decreased tissue necrosis and demyelination. The in vivo studies supported the conclusions drawn from cell culture studies and provided evidence for the possible role of GalT-2 and LacCer in SCI-induced inflammation and pathology. To our knowledge this is the first report of a role of LacCer in iNOS expression and the advantage of GSL depletion in attenuating post-SCI inflammation to improve the outcome of SCI.

Differential roles of neuronal and endothelial nitric oxide synthases during carrageenan-induced inflammatory hyperalgesia

The present study investigated the role of neuronal nitric oxide synthase (nNOS) in carrageenan-induced inflammatory pain by combining genomic and pharmacological strategies. Intrathecal injection of the nNOS inhibitor 7-nitroindazole dose-dependently inhibited carrageenan-induced thermal hyperalgesia in both early and late phases in wild-type mice. However in nNOS knockout mice, carrageenan-induced thermal hyperalgesia remained intact in the early phase but was reduced in the late phase. Spinal Ca2+ -dependent nitric oxide synthase (NOS) activity in nNOS knockout mice was significantly lower than that in wild-type mice. Following carrageenan injection, although the spinal Ca2+ -dependent NOS activity in both wild-type and knockout mice increased, the enzyme activity in nNOS knockout mice reached a level similar to that in wild-type mice. On the other hand, no significant difference in spinal Ca2+ -independent NOS activity was noted between wild-type and nNOS knockout mice before and after carrageenan injection. Furthermore, intrathecal administration of the endothelial NOS (eNOS) inhibitor L-N5-(1-iminoethyl)-ornithinein nNOS knockout mice inhibited the thermal hyperalgesia in both early and late phases, though this inhibitor had no effect in wild-type mice. Meanwhile, Western blot showed that eNOS expression in the spinal cord of nNOS knockout mice was up-regulated compared with wild-type mice; immunohistochemical staining showed that the spinal eNOS was mainly distributed in superficial laminae of the dorsal horn. Finally, double staining with confocal analysis showed that the enhanced spinal eNOS was expressed in astrocytes, but not in neurons. Our current results indicate that nNOS plays different roles in the two phases of carrageenan-induced inflammatory pain. In this model, enhanced spinal eNOS appears to compensate for the role of nNOS in nNOS knockout mice.

Role of nitric oxide after brain ischaemia

Ischaemic stroke is the second or third leading cause of death in developed countries. In the last two decades substantial research and efforts have been made to understand the biochemical mechanisms involved in brain damage and to develop new treatments. The evidence suggests that nitric oxide (NO) can exert both protective and deleterious effects depending on factors such as the NOS isoform and the cell type by which NO is produced or the temporal stage after the onset of the ischaemic brain injury. Immediately after brain ischaemia, NO release from eNOS is protective mainly by promoting vasodilation; however, after ischaemia develops, NO produced by overactivation of nNOS and, later, NO release by de novo expression of iNOS contribute to the brain damage. This review article summarizes experimental and clinical data supporting the dual role of NO in brain ischaemia and the mechanisms by which NO is regulated after brain ischaemia. We also review NO-based therapeutic strategies for stroke treatment, not only those directly linked with the NO pathway such as NO donors and NOS inhibitors but also those partially related like statins, aspirin or lubeluzole.

Immunopathogenesis of brain abscess

Brain abscess represents a significant medical problem despite recent advances made in detection and therapy. Due to the emergence of multi-drug resistant strains and the ubiquitous nature of bacteria, the occurrence of brain abscess is likely to persist. Our laboratory has developed a mouse experimental brain abscess model allowing for the identification of key mediators in the CNS anti-bacterial immune response through the use of cytokine and chemokine knockout mice. Studies of primary microglia and astrocytes from neonatal mice have revealed that S. aureus, one of the main etiologic agents of brain abscess in humans, is a potent stimulus for proinflammatory mediator production. Recent evidence from our laboratory indicates that Toll-like receptor 2 plays a pivotal role in the recognition of S. aureus and its cell wall product peptidoglycan by glia, although other receptors also participate in the recognition event. This review will summarize the consequences of S. aureus on CNS glial activation and the resultant neuroinflammatory response in the experimental brain abscess model.

Effects of neuroinflammation on glia-glia gap junctional intercellular communication: a perspective

Gap junctions serve as intercellular conduits that allow for the direct transfer of small molecular weight molecules (up to 1 kDa) including ions involved in cellular excitability, metabolic precursors, and second messengers. The observation of extensive intercellular coupling and large numbers of gap junctions in the central nervous system (CNS) suggests a syncytium-like organization of glial compartments. Inflammation is a hallmark of various CNS diseases such as bacterial and viral infections, multiple sclerosis, Alzheimer's disease, and cerebral ischemia. A general consequence of brain inflammation is reactive gliosis typified by astrocyte hypertrophy and proliferation of astrocytes and microglia. Changes in gap junction intercellular communication as reflected by alterations in dye coupling and connexin expression have been associated with numerous CNS inflammatory diseases, which may have dramatic implications on the survival of neuronal and glial populations in the context of neuroinflammation. A review of the effects of inflammatory products on glia-glia gap junctional communication and glial glutamate release is presented. In addition, the hypothesis of a "syncytial switch" based upon differential regulation of gap junction expression in astrocytes and microglia during normal CNS homeostasis and neuroinflammation is proposed.

Effects of progesterone on the inflammatory response to brain injury in the rat

The effects of progesterone on the cellular inflammatory response to frontal cortex injury were examined on postsurgical days 1, 3, 5, 7 and 9 in male rats treated with progesterone (4 mg/kg) and/or vehicle. Rats with bilateral contusions showed increased levels of edema on days 1, 3 and 5, more reactive astrocytes on days 3, 5, 7 and 9, and more macrophages/activated microglia on days 1, 3, 5 and 9 compared to shams. The number of neurons in the medial dorsal nucleus (MDN) of the thalamus reduced on days 5 and 9 after injury compared to shams. Progesterone reduced edema levels and increased the accumulation of macrophages/activated microglia compared to vehicle controls (p<0.025); however, these changes in the inflammatory response were not related to MDN neuronal survival. Our results confirm the possibility that one way progesterone mediates its neuroprotective effects following injury is through its actions on the inflammatory response.

Functional expression of constitutive nitric oxide synthases regulated by voltage-gated Na+ and Ca2+ channels in cultured human astrocytes

We report the functional characterization of constitutive nitric oxide synthase(s) (NOS) such as neuronal and endothelial NOS in cultured human astrocytes. Exposure of cultured human astrocytes to 1 microM veratridine or 50 mM KCl produced a pronounced increase in a calmodulin-dependent NOS activity estimated from cGMP formation. The functional expression of voltage-gated Na(+) channel, which is estimated by the response to veratridine, appeared to be earlier (at second day in culture) than that of voltage-gated Ca(2+) channels, which are estimated by the response to the KCl stimulation (at fourth day in culture). The KCl-evoked NO synthesis was totally reversed by L-type Ca(2+) channel blockers such as nifedipine and verapamil, but not by omega-conotoxin GVIA, an N-type Ca(2+) channel blocker, or omega-agatoxin IVA, a P/Q-type Ca(2+) channel blocker. In addition, verapamil abolished the KCl-induced increase in the intracellular free Ca(2+) concentration. RT-PCR analysis revealed that mRNA for neuronal and endothelial NOS was expressed in human astrocytes. In addition, Western blot analysis and double labeling of NOS and glial fibrillary acidic protein (GFAP) showed that cultured human astrocytes expressed neuronal NOS and endothelial NOS as well as the alpha(1) subunit of Ca(2+) channel. These results suggest that human astrocytes express constitutive NOS that are regulated by voltage-gated L-type Ca(2+) channel as well as Na(+) channel.

Presence and role of nitric oxide in the central nervous system of the freshwater snail Planorbarius corneus: possible implication in neuron–microglia communication

The aim of the present study was to investigate the involvement of nitric oxide (NO) as a messenger molecule in neuron-microglia communication in the central nervous system (CNS) of the freshwater snail Planorbarius corneus. The presence of both neuronal (nNOS) and inducible nitric oxide synthase (iNOS) was studied using NADPH-diaphorase (NADPH-d) histochemistry and NOS immunocytochemistry. The experiments were performed on whole ganglia and cultured microglial cells after different activation modalities, such as treatment with lipopolysaccharide and adenosine triphosphate and/or maintaining ganglia in culture medium till 7 days. In sections, nNOS immunoreactivity was found only in neurons and nNOS-positive elements were less numerous than NADPH-d-positive ones, with which they partially overlapped. The iNOS immunoreactivity was observed only after activation, in both nerve and microglial cells. We also found that the number of iNOS-immunoreactive neurons and microglia varied, depending on the activation modalities. In microglial cell cultures, iNOS was expressed in the first generation of cells only after activation, whereas a second generation, proliferated after ganglia activation, expressed iNOS even in the unstimulated condition.

Inhibition of inducible nitric oxide synthase expression by baicalein in endotoxin/cytokine-stimulated microglia

Excessive production of nitric oxide (NO) in the central nervous system (CNS) mediated by activation of microglia has been implicated in neurotoxicity after stresses such as ischemia. Baicalein, a polyphenolic flavonoid antioxidant, is known to have anti-inflammatory, anticarcinogenic, and neuroprotective effects. In the present study, we report the inhibitory effect of baicalein on endotoxin/cytokine-induced NO production and inducible nitric oxide synthase (iNOS) gene expression in microglia. Baicalein abolished the endotoxin/cytokine-induced expression of iNOS protein, iNOS mRNA, and iNOS promoter activity in a parallel concentration-dependent manner. The suppression of iNOS expression was not mediated through the down-regulation of tumor necrosis factor-alpha (TNF-alpha) by baicalein because TNF-alpha failed to enhance endotoxin/cytokine-induced NO production in microglia. From the electrophoretic mobility shift assay (EMSA), we found that baicalein exerted a distinct inhibitory effect on the DNA binding activity of transcription factors, and this was significantly greater in nuclear factor IL-6 (NF-IL6) than in nuclear factor kappa B (NF-kappaB) and activated protein 1 (AP-1). Although extracellular signal-regulated kinase (ERK) is critical to iNOS expression, endotoxin/cytokine-stimulated phosphorylation of ERK1/2 was not significantly inhibited by baicalein. These results indicate that NF-IL6 inactivation could be the major determinant for the suppression of NO production by baicalein in microglia. Furthermore, it suggests that the inhibitory effect of baicalein on microglia activation and neurotoxic factor production is responsible for its neuroprotective action.

Toll-like receptor 2 (TLR2) mediates astrocyte activation in response to the Gram-positive bacterium Staphylococcus aureus

Astrocytes play an important role in initiating and regulating CNS immune responses through the release of proinflammatory cytokines and chemokines. Here we demonstrate that primary astrocytes are capable of recognizing the Gram-positive bacterium Staphylococcus aureus and its cell wall product peptidoglycan (PGN) and respond by producing numerous proinflammatory mediators including interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), macrophage inflammatory protein-1beta (MIP-1beta), MIP-2, and monocyte chemoattractant protein (MCP-1). Astrocytes have recently been shown to express Toll-like receptor 2 (TLR2), a pattern recognition receptor important for recognizing structural components of various Gram-positive bacteria, fungi, and protozoa. However, the functional significance of TLR2 in mediating astrocyte activation remains unknown. Primary astrocytes from TLR2 knockout mice were used to evaluate the role of TLR2 in astrocyte responses to S. aureus and PGN. The results demonstrate that TLR2 is essential for maximal proinflammatory cytokine and chemokine production, but not phagocytosis, in primary astrocytes following S. aureus and PGN exposure. In addition, both stimuli led to a significant increase in TLR2 mRNA expression in wild-type astrocytes as assessed by real-time quantitative RT-PCR. These findings suggest that astrocytes may play a key role in the initial antibacterial immune response in the CNS through engagement of TLR2.

High nitric oxide production in autistic disorder: a possible role for interferon-gamma

BACKGROUND

Neuroimmune regulation abnormalities have been implicated in the pathophysiology of autistic disorder. Nitric oxide (NO) is involved in immune reactivity and is known to affect brain neurodevelopmental processes. Recent evidence indicates that NO, and cytokines involved in NO production, may be high in children with autism. The purpose of this study was to verify that plasma NO is high in children with autism and determine whether this elevation is related to plasma levels of cytokines involved in NO production.

METHODS

The metabolites of NO, nitrite, and nitrate (NOx), along with the cytokines interferon-gamma (IFN-gamma), tumor necrosis factor-alpha, and interleukin-1beta, were measured in plasma of 29 children with autism (mean age +/- SD = 6.1 +/- 2.8 years) and 27 age- and gender-matched healthy comparison subjects using commercially available assay kits.

RESULTS

Plasma levels of NOx were significantly higher in the autistic subjects (p =.006); plasma levels of the cytokines did not differ between groups. NOx and IFN-gamma levels were positively correlated in the autistic subjects (r =.51; p =.005).

CONCLUSIONS

These results confirm that plasma NO is high in some children with autism and suggest that this elevation may be related to IFN-gamma activity.

The neurobiology of glia in the context of water and ion homeostasis

Astrocytes are highly complex cells that respond to a variety of external stimulations. One of the chief functions of astrocytes is to optimize the interstitial space for synaptic transmission by tight control of water and ionic homeostasis. Several lines of work have, over the past decade, expanded the role of astrocytes and it is now clear that astrocytes are active participants in the tri-partite synapse and modulate synaptic activity in hippocampus, cortex, and hypothalamus. Thus, the emerging concept of astrocytes includes both supportive functions as well as active modulation of neuronal output. Glutamate plays a central role in astrocytic-neuronal interactions. This excitatory amino acid is cleared from the neuronal synapses by astrocytes via glutamate transporters, and is converted into glutamine, which is released and in turn taken up by neurons. Furthermore, metabotropic glutamate receptor activation on astrocytes triggers via increases in cytosolic Ca(2+) a variety of responses. For example, calcium-dependent glutamate release from the astrocytes modulates the activity of both excitatory and inhibitory synapses. In vivo studies have identified the astrocytic end-foot processes enveloping the vessel walls as the center for astrocytic Ca(2+) signaling and it is possible that Ca(2+) signaling events in the cellular component of the blood-brain barrier are instrumental in modulation of local blood flow as well as substrate transport. The hormonal regulation of water and ionic homeostasis is achieved by the opposing effects of vasopressin and atrial natriuretic peptide on astroglial water and chloride uptake. In conjuncture, the brain appears to have a distinct astrocytic perivascular system, involving several potassium channels as well as aquaporin 4, a membrane water channel, which has been localized to astrocytic endfeet and mediate water fluxes within the brain. The multitask functions of astrocytes are essential for higher brain function. One of the major challenges for future studies is to link receptor-mediated signaling events in astrocytes to their roles in metabolism, ion, and water homeostasis.

Defined inflammatory states in astrocyte cultures: correlation with susceptibility towards CD95-driven apoptosis

A complete cytokine mix (CCM) or its individual components tumour necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma) were used to switch resting murine astrocytes to reactive states. The transformation process was characterized by differential up-regulation of interleukin-6 (IL-6), cyclooxygenase-2 (COX-2) and inducible nitric oxide synthetase (iNOS) mRNA and protein and a subsequent release of prostaglandin E2, nitric oxide (NO) and IL-6. Both CD95L and anti-CD95 antibodies triggered caspase activation followed by apoptotic death in fully pro-inflammatory astrocytes, whereas resting cells were totally resistant. Two other death-inducing ligands, TNF and TNF-related apoptosis-inducing ligand (TRAIL) did not induce apoptosis in reactive astrocytes. The switch in astrocyte sensitivity was accompanied by up-regulation of caspase-8 and CD95 as well as the capacity to recruit Fas-associated death domain (FADD) to the activated death receptor complex. Neither CD95-mediated death, nor other inflammatory parameters were affected by inhibition of iNOS or COX, respectively. Accordingly, IFN-gamma was absolutely essential for up-regulation of iNOS, but not for the switch in apoptosis sensitivity. In contrast, p38 kinase activity was identified as an important controller of both the inflammatory reaction and apoptosis both in astrocytes stimulated with CCM and in glia exposed to TNF and IL-1 only.

Protection against murine leukemia virus-induced spongiform myeloencephalopathy in mice overexpressing Bcl-2 but not in mice deficient for interleukin-6, inducible nitric oxide synthetase, ICE, Fas, Fas ligand, or TNF-R1 genes

Some murine leukemia viruses (MuLVs), among them Cas-Br-E and ts-1 MuLVs, are neurovirulent, inducing spongiform myeloencephalopathy and hind limb paralysis in susceptible mice. It has been shown that the env gene of these viruses harbors the determinant of neurovirulence. It appears that neuronal loss occurs by an indirect mechanism, since the target motor neurons have not been found to be infected. However, the pathogenesis of the disease remains unclear. Several lymphokines, cytokines, and other cellular effectors have been found to be aberrantly expressed in the brains of infected mice, but whether these are required for the development of the neurodegenerative lesions is not known. In an effort to identify the specific effectors which are indeed required for the initiation and/or development of spongiform myeloencephalopathy, we inoculated gene-deficient (knockout [KO]) mice with ts-1 MuLV. We show here that interleukin-6 (IL-6), inducible nitric oxide synthetase (iNOS), ICE, Fas, Fas ligand (FasL), and TNF-R1 KO mice still develop signs of disease. However, transgenic mice overexpressing Bcl-2 in neurons (NSE/Bcl-2) were largely protected from hind limb paralysis and had less-severe spongiform lesions. These results indicate that motor neuron death occurs in this disease at least in part by a Bcl-2-inhibitable pathway not requiring the ICE, iNOS, Fas/FasL, TNF-R1, and IL-6 gene products.

Development of immune cellular biosensing system for assessing chemicals on inducible nitric oxide synthase signaling activator

An immune cellular biosensing system has been constructed to assess immunomodulating effects of chemicals. Production of nitric oxide in the immune cellular biosensing system was used as readout of an immune cellular response for assessing the immunomodulating effects of chemicals. The macrophage-like cell line RAW264.7, which has signaling pathways of inducible nitric oxide synthase, was employed in the cellular biosensing system. The immune cellular biosensing system consisted of a Pt counter electrode, an Ag/AgCl reference electrode, and a gold electrode onto which a polyion complex layer was coated to allow adherence of the RAW264.7 cells. As the results of evaluating effects of a polyion complex layer on cell viabilities by using WST-8 assay, the polyion complex layer did not affect RAW264.7 cells. The polyion-coated gold electrode could measure NO without the drawback of electrochemical interference that occurs with differential pulse voltammetry. The detection limit of the immune cellular biosensing system was 4.2 nM released NO as measured by double potential step chronoamperometry. The potent immune activating abilities of lipopolysaccharide and interferon-gamma could be assessed by the cellular biosensing system; NO release from cells was detected within 600 ms.

Inflammatory neurodegeneration mediated by nitric oxide, glutamate, and mitochondria

In inflammatory, infectious, ischemic, and neurodegenerative pathologies of the central nervous system (CNS) glia become "activated" by inflammatory mediators, and express new proteins such as the inducible isoform of nitric oxide synthase (iNOS). Although these activated glia have benefi- cial roles, in vitro they potently kill cocultured neurons, and there is increasing evidence that they contribute to pathology in vivo. Nitric oxide (NO) from iNOS appears to be a key mediator of such glial-induced neuronal death. The high sensitivity of neurons to NO is partly due to NO causing inhibition of respiration, rapid glutamate release from both astrocytes and neurons, and subsequent excitotoxic death of the neurons. NO is a potent inhibitor of mitochondrial respiration, due to reversible binding of NO to cytochrome oxidase in competition with oxygen, resulting in inhibition of energy production and sensitization to hypoxia. Activated astrocytes or microglia cause a potent inhibition of respiration in cocultured neurons due to glial NO inhibiting cytochrome oxidase within the neurons, resulting in ATP depletion and glutamate release. In some conditions, glutamate- induced neuronal death can itself be mediated by N-methyl-D-aspartate (NMDA)-receptor activation of the neuronal isoform of NO synthase (nNOS) causing mitochondrial damage. In addition NO can be converted to a number of reactive derivatives such as peroxynitrite, NO2, N2O3, and S-nitrosothiols that can kill cells in part by inhibiting mitochondrial respiration or activation of mitochondrial permeability transition, triggering neuronal apoptosis or necrosis.

CAT2 arginine transporter deficiency significantly reduces iNOS-mediated NO production in astrocytes

We have previously demonstrated that genetic ablation of cationic amino acid transporter 2 (Cat2) significantly inhibits nitric oxide (NO) production by inducible nitric oxide synthase (iNOS) in activated macrophages. Here we report that iNOS activity is impaired by 84% in activated Cat2-deficient astrocytes. Cat2 ablation appears to reduce astrocyte NO synthesis by decreasing the uptake of the sole precursor, arginine, as well as by reducing the expression of iNOS following activation. Excessive or dysregulated NO production by activated astrocytes and other CNS cell types has been implicated in the pathogenesis of neurological disorders. Our results support the idea that manipulation of CAT2 transporter function might be useful for the therapeutic modulation of iNOS activity.

Up-regulation of inducible nitric oxide synthase in the substantia nigra by lipopolysaccharide causes microglial activation and neurodegeneration

The present study was designed to examine whether expression of iNOS was involved in LPS-induced neurodegeneration in rat substantia nigra (SN) and to study the role of NO in the loss of the SN dopaminergic neurons. In Western blot analysis, iNOS was induced in the SN after injection of LPS in a time- and dose-dependent manner. Immunofluorescence and immunohistochemical analyses revealed that the iNOS is located in a fully activated microglia with the characteristic amoeboid morphology. Furthermore, LPS-induced loss of dopaminergic neurons was significantly inhibited by the administration of L-N(G)-nitroarginine, a selective inhibitor of NOS, and the glucocorticoid dexamethasone. These inhibiting agents for iNOS reduced LPS-induced microglial activation, suggesting that NO has a role in inflammatory-mediated microglial activation. These results demonstrate that LPS induces the expression of iNOS in activated microglia in the SN, and that NO and/or its metabolites may play a crucial role in inflammation-mediated degeneration of dopaminergic neurons.

Role of the JAK/STAT signal transduction pathway in the regulation of gene expression in CNS

Over the last 20 years the JAK/STAT signal transduction pathway has been extensively studied. An enormous amount of data on different cell signal transduction pathways is now available. The JAK/STAT signal transduction pathway is one of the intracellular signaling pathways activated by cytokines and growth factors that was first studied in the hematopoietic system, but recent data demonstrate that this signal transduction is also greatly utilized by other systems. The JAK/STAT pathway is a signaling cascade that links the activation of specific cell membrane receptors to nuclear gene expression. This review is focused on the role of JAK/STAT signal transduction pathway activation in the central nervous system (CNS).

CB(1) cannabinoid receptor-G protein association: a possible mechanism for differential signaling

Effects of cannabinoid compounds on neurons are predominantly mediated by the CB(1) cannabinoid receptor. Onset of signaling cascades in response to cannabimimetic drugs is triggered by the interaction of the cannabinoid receptor with G(i/o) proteins. Much work has been done to delineate the cannabinoid agonist-induced downstream signaling events; however, it remains to define the molecular basis of cannabinoid receptor-G protein interactions that stimulate these signaling pathways. In this review, we discuss several signal transduction pathways, focusing on studies that demonstrate the efficacy of CB(1) receptor agonists through G protein mediated pathways.

Characterization of neurotransmitters and dopamine attenuation of inducible nitric oxide synthase in glioma cells

Inducible nitric oxide synthase (iNOS) plays a significant role in the pathology of central nervous system diseases. Inducible NOS expression is regulated by intracellular adenosine 3',5'-cyclic monophosphate (cAMP) signaling, and astrocytes contain both iNOS and adenylate cyclase-coupled neurotransmitter receptors. The data obtained from the present study indicated that acetylcholine, lambda-amino-n-butyric acid, glutamate, quinolinic acid, N-methyl-D-aspartate and aspartate have no effect on NO(2)(-) production in C6 glioma cells stimulated by lipopolysaccharide and interferon-gamma. However, dopamine (DA) caused inhibition of NO(2)(-) production and iNOS transcription. The effects of DA were not due to homovanillic acid/3,4-dihydroxyphenylacetic acid, the autoxidative products superoxide (O(2)(-))/hydrogen peroxide (H(2)O(2)) or direct reactions with NO(2)(-). Forskolin, adenylate cyclase activator, dose-dependently reduced NO(2)(-). Meanwhile, (+/-) SKF-38393 D(1) receptor agonist attenuated iNOS in a similar fashion to DA. In addition, the results indicated that DA attenuation of iNOS was significantly impeded by the adenylate cyclase inhibitor MDL-12,330A, the D(1) antagonist SCH-23390, the beta2 adrenergic receptor antagonist ICI-118,551 and the beta1 adrenergic receptor antagonist atenolol. In conclusion, it appears that DA attenuates iNOS through a D(1), beta1 and beta2 adrenergic receptor-linked adenylate cyclase-mediated cAMP cascade.

Increased vulnerability of dopaminergic neurons in MPTP-lesioned interleukin-6 deficient mice

To test the hypothesis that neuroinflammation contributes to dopaminergic neuron death in the MPTP-lesioned mouse, we compared nigrostriatal degeneration in interleukin (IL)-6 (+/+) with IL-6 (-/-) mice. In the absence of IL-6, a single injection of MPTP (30 mg/kg) resulted in significantly greater striatal dopamine depletion than that measured in IL-6 (+/+) mice. The observed dopamine depletion was MPTP dose dependent. This loss of striatal dopamine and a significantly greater loss of TH+ cells in the substantia nigra pars compacta in IL-6 (-/-) mice as compared with control IL-6 (+/+) mice, suggest that IL-6 is neuroprotective in the MPTP-lesioned nigrostriatal system. Co-localization experiments identified striatal astrocytes as the source of IL-6 in IL-6 (+/+) mice at 1 and 7 days postinjection of MPTP. The increased sensitivity of dopaminergic neurons to neurotoxicant in the absence of IL-6, is compatible with a neuroprotective activity of IL-6 in the injured nigrostriatal system.

Astrocytes enhance lipopolysaccharide-induced nitric oxide production by microglial cells

Several stimuli result in glial activation and induce nitric oxide (NO) production in microglial and astroglial cells. The bacterial endotoxin lipopolysaccharide (LPS) has been widely used to achieve glial activation in vitro, and several studies show that both microglial and, to a lesser extent, astroglial cell cultures produce NO after LPS treatment. However, NO production in endotoxin-treated astrocyte cultures is controversial. We characterized NO production in microglial, astroglial and mixed glial cell cultures treated with lipopolysaccharide, measured as nitrite accumulation in the culture media. We also identified the NO-producing cells by immunocytochemistry, using specific markers for the inducible NO synthase (iNOS) isoform, microglial and astroglial cells. Only microglial cells showed iNOS immunoreactivity. Thus, contaminating microglial cells were responsible for NO production in the secondary astrocyte cultures. We then analysed the effect of astrocytes on NO production by microglial cells using microglial-astroglial cocultures, and we observed that this production was clearly enhanced in the presence of astroglial cells. Soluble factors released by astrocytes did not appear to be directly responsible for such an effect, whereas nonsoluble factors present in the cell membrane of LPS-treated astrocytes could account, at least in part, for this enhancement.

The essential nutrient pyrroloquinoline quinone may act as a neuroprotectant by suppressing peroxynitrite formation

Pyrroloquinoline quinone (PQQ) is a redox active essential nutrient that can generate or scavenge superoxide depending on its microenvironment. PQQ has been shown previously to be neuroprotective in a rodent stroke model. Here we test whether PQQ interacts with reactive nitrogen species, known to be involved in the pathogenesis of stroke. Using rat forebrain neurons in culture, we determined that the toxicity of SIN-1 was mediated by peroxynitrite and that PQQ could block this toxic action. However, PQQ could not block the toxicity of peroxynitrite itself. Both SIN-1 and peroxynitrite caused ATP depletion, but only SIN-1 evoked ATP depletion was blocked by PQQ. In a cell-free system, PQQ blocked nitration of bovine serum albumin produced by SIN-1, but potentiated peroxynitrite-induced nitration. PQQ was unable to block ATP depletion and cell death induced by NO. donors (DEA/NO, DPT/NO and DETA/NO), indicating that it does not directly interact with nitric oxide, and suggesting that it acts as a superoxide scavenger. PQQ significantly potentiated cGMP accumulation evoked by SIN-1, similar to the effect of superoxide dismutase (SOD). However, unlike SOD, which potentiated neurotoxicity induced by SIN-1, PQQ blocked its toxicity, arguing against the possibility that PQQ functions simply as a SOD mimetic. Indeed, substantially less H2O2 was produced by the incubation of SIN-1 with PQQ, when compared to SOD. These results suggest that PQQ scavenges superoxide without forming toxic levels of H2O2. Therefore, the protective effect of PQQ on stroke might be due, at least in part, to the suppression of peroxynitrite formation.

GTP cyclohydrolase I induction in striatal astrocytes following intrastriatal kainic acid lesion

The cause of premature death of dopamine neurons in patients with Parkinson's disease remains unknown. It is speculated that damaging reactive species resulting from the metabolism of dopamine, nitric oxide, and tetrahydrobiopterin (BH(4)) may be involved. GTP cyclohydrolase I (GCH1) is the first and rate-limiting enzyme in the synthesis of BH(4), an essential cofactor for tyrosine hydroxylase and nitric oxide synthase in dopamine and nitric oxide production, respectively. Our studies have explored BH(4) metabolism in the nigrostriatal system following intrastriatal kainic acid lesion. We have demonstrated that 1 week following kainic acid there was an increase in striatal GCH1 mRNA, protein, and activity. There was also an elevation of BH(4) levels in the striatum. Part of the induction of GCH1 was localized in situ to astrocytes. Further, the striatal lesion caused death of both neurons and astrocytes in striatum, as shown by in situ end labeling. These novel observations suggest that the induction of GTP cyclohydrolase and BH(4) in striatal astrocytes may be mediating death of striatal neuronal and non-neuronal cells. This work supports existing and emerging reports that demonstrate the importance of dopamine metabolism in neuronal death of the nigrostriatal system.

Peroxynitrite stimulates L-arginine transport system y(+) in glial cells. A potential mechanism for replenishing neuronal L-arginine

We have reported previously that peroxynitrite stimulates L-arginine release from astrocytes, but the mechanism responsible for such an effect remains elusive. To explore this issue, we studied the regulation of L-[(3)H]arginine transport by either exogenous or endogenous peroxynitrite in glial cells. A 2-fold peroxynitrite-mediated stimulation of l-arginine release in C6 cells was found to be Na(+)-independent, was prevented by 5 mm L-arginine and, although only in the presence of Na(+), was blocked by 5 mm L-alanine or L-leucine. Peroxynitrite-mediated stimulation of L-arginine uptake was trans-stimulated by 10 mm L-arginine and was inhibited in a dose-dependent fashion (k(i) of approximately 40 microm) by the system y(+) inhibitor N-ethylmaleimide in C6 cells. Endogenous production of peroxynitrite in lipopolysaccharide-treated astrocytes triggered an increased L-arginine transport activity without affecting Cat1 l-arginine transporter mRNA levels. However, Western blot analyses of peroxynitrite-treated astrocytes and C6 glial cells revealed a 3-nitrotyrosinated anti-Cat1-immunopositive band, strongly suggesting peroxynitrite-mediated Cat1 nitration. Furthermore, peroxynitrite stimulation of L-arginine release was abolished in fibroblast cells homozygous for a targeted inactivation of the Cat1 gene. Finally, peroxynitrite-triggered L-arginine released from astrocytes was efficiently taken up by neurons in an insert-based co-culture system. These results strongly suggest that peroxynitrite-mediated activation of the Cat1 transporter in glial cells may serve as a mechanism focused to replenish L-arginine in the neighboring neurons.

Nitric oxide accounts for an increased glycolytic rate in activated astrocytes through a glycogenolysis-independent mechanism

The possible interference of nitric oxide (NO) in glucose metabolism was studied in activated astrocytes. Lipopolysaccharide (LPS) treatment triggered a NO-mediated increase in glucose consumption and lactate production, suggesting an enhanced rate of glycolysis. Active glycogen synthesis was also observed after LPS treatment, but NO synthase inhibition was unable to prevent this effect. These results strongly suggest that endogenously-formed NO stimulates glycolysis through a glycogenolysis-independent mechanism in astrocytes.

Mimosine prevents the death of glucose-deprived immunostimulated astrocytes by scavenging peroxynitrite

Immunostimulated astrocytes become highly vulnerable to glucose deprivation (Choi and Kim: J Neurosci Res 54:870-875, 1998a). The increased vulnerability is caused by the enhanced level of peroxynitrite endogenously produced in glucose-deprived immunostimulated astrocytes. In the present study, we report that the plant amino acid mimosine can attenuate the increased death by scavenging peroxynitrite. Treatment with mimosine blocked the increase of nitrotyrosine immunoreactivity, a marker of peroxynitrite, in glucose-deprived immunostimulated astrocytes. Furthermore, mimosine directly inhibited the nitration of tyrosine residues of bovine serum albumin and the oxidation of dihydrorhodamine-123 to rhodamine-123 by peroxynitrite. Mimosine has been used experimentally as a cell cycle G1/S phase transition blocker (Lalande: Exp Cell Res 186:332-339, 1990; Hoffman et al.: Cytometry 12:26-32, 1991). Flow cytometry analysis, however, showed that the cytoprotective effect of mimosine was not attributed to its inhibition of cell cycle progression. Furthermore, under our experimental conditions, mimosine did not alter the levels of cell cycle regulatory proteins, including p21(WAF1/CIP1), cyclins D1 and E, and proliferating cell nuclear antigen. In addition, cyclin-dependent kinase inhibitors olomoucine and roscovitine did not block the increased death. These results indicate that mimosine inhibits the augmented death of glucose-deprived immunostimulated astrocytes by scavenging peroxynitrite rather than suppressing the cell cycle progression.

Astrocytic activation and delayed infarct expansion after permanent focal ischemia in rats. Part II: suppression of astrocytic activation by a novel agent (R)-(-)-2-propyloctanoic acid (ONO-2506) leads to mitigation of delayed infarct expansion and early improvement of neurologic deficits

A novel agent, (R)-(-)-2-propyloctanoic acid (ONO-2506), has a unique property in that it modulates functions of activated cultured astrocytes, including pronounced inhibition of S-100beta synthesis. The present study examined whether administration of this agent would mitigate the delayed expansion of infarct volume and the neurologic deficits after permanent middle cerebral artery occlusion (pMCAO) in rats. Daily intravenous administration of ONO-2506 (10 mg/kg) abolished the delayed infarct expansion between 24 and 168 hours after pMCAO, whereas the acute infarct expansion until 24 hours was unaffected. The agent significantly reduced the expression of S-100beta and glial fibrillary acidic protein in the activated astrocytes and the number of terminal deoxynucleotidyl transferase-mediated 2;-deoxyuridine 5;-triphosphate-biotin nick end labeling-positive cells in the periinfarct area. The neurologic deficits were significantly improved, compared with the vehicle-treated groups, as early as 24 hours after the initial administration of ONO-2506. The agent had a wide therapeutic time window of 0 to 48 hours after pMCAO. These results indicate that because of the pharmacologic modulation of astrocytic activation induced by ONO-2506, symptoms can regress whereas delayed expansion of the lesion is arrested. Pharmacologic modulation of astrocytic activation may confer a novel therapeutic strategy against stroke.

Nitric oxide and the other cyclic nucleotide

Nitric oxide (NO) participates in the regulation of the daily activities of cells as well as in cytotoxic events. Elucidating the mechanism(s) by which NO carries out its diverse functions has been the goal of numerous laboratories. In the cardiovascular system, evidence indicates that NO mediates its effects via an activation of soluble guanylyl cyclase (sGC). In other tissues, it is not clear if sGC is an exclusive target for NO or what the functions of cGMP might be. It is also unlikely that the diversity of NO actions is explained solely by changes in cGMP. This review focuses on the evidence that NO modulates cAMP signalling, with specific attention to the effects of NO on adenylyl cyclase (AC) as the target of NO regulation.

Gö6976 protects mesencephalic neurons from lipopolysaccharide-elicited death by inhibiting p38 MAP kinase phosphorylation

Glial activation is associated with inflammation-related neuron degeneration in the brain. A variety of protein kinases are assumed to contribute to the expression of inflammation-related products, such as nitric oxide (NO) and proinflammatory cytokines, however, the mechanisms of glial activation and glia-mediated neurotoxicity remain unclear. We found that the indolocarbazole, Gö6976, originally known as a selective protein kinase C (PKC) inhibitor, protects neurons from glia-mediated damage and suppresses lipopolysaccharide (LPS)-induced microglial production of inflammatory factors. The purpose of the study we report here was to determine the mechanism underlying the neuroprotective effect of Gö6976 in mesencephalic neuron/glia cultures. Gö6976 suppressed LPS-induced neurotoxicity in mesencephalic neuron/glia cultures and the protective effect of Gö6976 paralleled the suppression of p38 mitogen activated protein kinase (MAPK) activation and inhibition of NO production. Gö6976 did not directly inhibit the activity of p38 MAPK; rather, the inhibitor suppressed the phosphorylation of p38 MAPK, suggesting that the target of Gö6976 is a signaling event upstream of p38 MAPK. Although Gö6976 was originally known to be a selective PKC inhibitor, the neuroprotection was not mediated through its reputed effects on PKC activity. This paper demonstrates that the neuroprotective effect of Gö6976 against LPS-induced damage is mediated through the inhibition of proinflammatory factors, such as NO from microglia, by inhibiting the phosphorylation of p38 MAPK.

p38 MAP kinase is involved in lipopolysaccharide-induced dopaminergic neuronal cell death in rat mesencephalic neuron-glia cultures

Immune stimulants, such as the bacterial endotoxin, lipopolysaccharide (LPS), the human immunodeficiency virus-1 coat protein gp120, or beta-amyloid peptides, lead to glial activation and production of various immune mediators, such as nitric oxide (NO) and proinflammatory cytokines in the brain. These mediators appear to contribute to neuronal cell death in neurodegenerative diseases. However, the signaling pathways, which mediate the neurotoxic effect by the endotoxin, are not understood. The purpose of this study was to determine the role of mitogen-activated protein kinase (MAPK) in LPS-induced neurodegeneration using mesencephalic dopaminergic neuron/glia cultures. We have found that the p38 MAPK is important in LPS-induced death of mesencephalic neurons in rat neuron-glia mixed cultures. Upon treatment with 10 ng/ml LPS, the number of dopaminergic neurons decreased by 80% within 48 h, preceded by a significant production of NO by glia. Neuroprotection by selective inhibition of p38 MAPK activity paralleled a decrease in LPS-induced inducible nitric oxide synthase (iNOS) expression. These events were significantly reduced by the selective p38 MAPK inhibitor, SB202190, but not by the inactive analogue SB202474. Inhibition of iNOS activity and NO production by treatment with GW274150 was also neuroprotective. Although the p38 MAPK inhibitor afforded significant neuroprotection from LPS toxicity in the neuron-glia mixed culture, it failed to protect dopaminergic neurons from 6-hydroxy-dopamine-induced toxicity, which acts directly on dopaminergic neurons by inducing hydroxyl radical formation from the mitochondria. The results suggest that p38 MAPK in glia plays a significant role in the LPS-induced death of mesencephalic neurons through induction of nitric oxide synthase and resulting NO production.

Humic acid induces the generation of nitric oxide in human umbilical vein endothelial cells: stimulation of nitric oxide synthase during cell injury

Humic acid (HA) has been implicated as an etiological factor in the peripheral vasculopathy of blackfoot disease (BFD). In this study, we examined the effects of HA upon the generation of nitric oxide (NO) during the process of lethal cell injury in cultured human umbilical vein endothelial cells (HUVECs). NO production was measured by the formation of nitrite (NO(2)(-)), the stable end-metabolite of NO. Cell death was assessed by measuring the release of intracellular lactate dehydrogenase (LDH). Treatment HUVECs with HA at a concentration of 50, 100, and 200 microg/ml concentration-dependently increased nitrite levels, reaching a peak at 12 h subsequent to HA treatment, with a maximal response of approximately 400 pmole nitrite (from 1 x 10(4) cells). HA-induced nitrite formation was blocked completely by N(G)-nitro-L-arginine methyl ester (L-NAME) and also by N(G)-methyl-L-arginine (L-NMA), both being specific inhibitors of NO synthase. The LDH released from endothelial cells was evoked at from 24 h after the addition of HA (50, 100, 200 microg/ml) in a concentration- and time-dependent manner. The HA-induced LDH release was also reduced by the presence of both L-NAME and L-NMA. The addition of Ca(2+) chelator (BAPTA) inhibited both nitrite formation and LDH release by HA. Moreover, the antioxidants (superoxide dismutase, vitamin C, vitamin E) and protein kinase inhibitor (H7) effectively suppressed HA-induced nitrite formation. These results suggest that HA treatment of endothelial cells stimulates NO production, which can elicit cell injury via the stimulation of Ca(2+)-dependent NO synthase activity by increasing cytosolic Ca(2+) levels. Because the destruction of endothelial cells has been implicated in triggering the onset of BFD, the induction of excessive levels of NO and consequent endothelial-cell injury may be important to the etiology of HA-induced vascular disorders associated with BFD for humans.

Role of CB1 and CB2 receptors in the inhibitory effects of cannabinoids on lipopolysaccharide-induced nitric oxide release in astrocyte cultures

The purpose of this study was to investigate the role of the central cannabinoid receptor (CB(1)) in mediating the actions of the endogenous cannabinoid agonist anandamide and the synthetic cannabinoid CP-55940. Activation of primary mouse astrocyte cultures by exposure to bacterial lipopolysaccharide (LPS) caused a marked (approximately tenfold) increase in nitric oxide (NO) release. Coincubation with the cannabinoid agonists anandamide or CP-55940 markedly inhibited release of NO (-12% to -55%). This effect was abolished by SR-141716A (1 microM), a CB1 receptor antagonist. SR-141716A alone also significantly increased NO release in response to LPS, suggesting that endogenous cannabinoids modify inflammatory responses. In contrast, coincubation with the CB2 receptor antagonist SR-144528 (1 microM) abolished the inhibitory effects of the endogenous cannabinoid anandamide on LPS-induced NO release, although this may reflect nonspecific effects of this ligand or cannabinoid actions through atypical receptors of anandamide. We also showed that endogenous or synthetic cannabinoids inhibit LPS-induced inducible NO synthase expression (mRNA and protein) in astrocyte cultures. These results indicate that CB1 receptors may promote antiinflammatory responses in astrocytes.

Association of elevated glial expression of interleukin-1beta with improved survival in patients with glioblastomas multiforme

OBJECT

The aim of this study was to investigate the association of interleukin-1beta (IL-1beta) expression with improved survival in patients with glioblastomas multiforme (GBMs). Immune and vascular host-tumor interactions play a pivotal role in the control of tumor development, and inflammatory mechanisms may participate in the host's defense against tumor cells. Expression of proinflammatory cytokines and of inducible nitric oxide synthase (iNOS) has been noted in various types of malignant tumors, raising the possibility that endogenous expression of cytokines and the resulting cytotoxic action of sustained NO production play a role in the control of tumor growth. Indeed, human GBMs express variable amounts of iNOS.

METHODS

In this study, the expression of iNOS and of cytokines known to upregulate IL-1beta, tumor necrosis factor-alpha, interferon-gamma or downregulate iNOS transcription (IL-10, transforming growth factor [TGF]beta1, and TGFbeta2) were measured using reverse transcription-polymerase chain reaction with competitor DNA in 39 samples of human GBM. The iNOS level in GBM was positively correlated with IL-1beta messenger (m)RNA, but not with the other cytokines tested. Immunocytochemical double labeling revealed that both anti-iNOS immunoreactivity and anti-IL-1beta immunoreactivity colocalized with glial fibrillary acidic protein immunoreactivity in GBM. Some macrophage/microglial cells also expressed iNOS, but not IL-1beta. Comparison of biological data with clinical parameters indicated that the survival duration was enhanced when levels of IL-1beta mRNA were elevated or when levels of TGFbeta2 were low, but was independent of the level of iNOS mRNA within the tumor.

CONCLUSIONS

Taken together, these data indicate that the proinflammatory cytokine IL-1beta produced within GBM by glial-derived cells has a negative impact on tumor growth through a mechanism independent of iNOS induction.

Nitric oxide production is enhanced in rat brain before oxygen-induced convulsions

Central nervous system oxygen toxicity (CNS O2 toxicity) is preceded by release of hyperoxic vasoconstriction, which increases regional cerebral blood flow (rCBF). These increases in rCBF precede the onset of O2-induced convulsions. We have tested the hypothesis that hyperbaric oxygen (HBO2) stimulates NO* production in the brain that leads to hyperemia and anticipates electrical signs of neurotoxicity. We measured rCBF and EEG responses in rats exposed at 4 to 6 atmospheres (ATA) of HBO2 and correlated them with brain interstitial NO* metabolites (NO(x)) as an index of NO* production. During exposures to hyperbaric oxygen rCBF decreased at 4 ATA, decreased for the initial 30 min at 5 ATA then gradually increased, and increased within 30 min at 6 ATA. Changes in rCBF correlated positively with NO(x) production; increases in rCBF during HBO2 exposure were associated with large increases in NO(x) at 5 and 6 ATA and always preceded EEG discharges as a sign of CNS O2 toxicity. In rats pretreated with L-NAME, rCBF remained maximally decreased throughout 75 min of HBO2 at 4, 5 and 6 ATA. These data provide the first direct evidence that increased NO* production during prolonged HBO2 exposure is responsible for escape from hyperoxic vasoconstriction. The finding suggests that NO* overproduction initiates CNS O2 toxicity by increasing rCBF, which allows excessive O2 to be delivered to the brain.

Two-dimensional gel analysis of secreted proteins induced by interleukin-1 beta in rat astrocytes

Interleukin-1 beta (IL-1 beta) is a pro-inflammatory cytokine produced in the brain by endogenous microglial cells responding to injury. Levels of IL-1 beta are elevated in several neurodegenerative disorders, including Alzheimer's disease. IL-1 beta, which can act as a mitogen for astrocytes, also elicits the expression and secretion of multiple factors and paracrine 'second messengers' such as other cytokines, nerve growth factor, prostaglandins and nitric oxide that may in turn modulate neuronal and glial responses to injury. Utilizing giant, high-resolution two-dimensional gel electrophoresis, we have sought to more fully define the potential range of protein mediators that are secreted by astrocytes treated with IL-1 beta. In cultured rat astrocytes, we observe dramatic increases in the secretion of eight different protein species after 24 h of treatment with human recombinant IL-1 beta (1 U/ml). Seven of the proteins are also induced by tumor necrosis factor-alpha or basic fibroblast growth factor. Based on immunoprecipitation with specific antisera, we have identified three of these proteins as plasminogen activator inhibitor type-1, ceruloplasmin, and complement component C3. The identities of the other proteins, including the IL-1 beta-specific induction, are currently unknown. Characterization of these downstream modulators of IL-1 beta action complements gene-based approaches and will provide a better understanding of astrocyte responses to injury as well as markers for astrocyte activation in neurodegenerative diseases.

Thrombin and PAR-1 activating peptide increase iNOS expression in cytokine-stimulated C6 glioma cells

Thrombin (THR) plays a key role in the brain under physiological and pathological conditions. Several of the biological activities of thrombin have been shown to be mainly driven through activation of protease-activated receptor-1 (PAR-1)-type thrombin receptor. Here we have studied the effect of THR and PAR-1-activating peptide (PAR1-AP), SFLLRN, on cytokine-induced expression of inducible nitric oxide (iNOS), a prominent marker of astroglial activation using the rat C6 glioma cells. In this cell line, THR (1-10 U/mL) and PAR1-AP (1-100 microM) induced a significant concentration-dependent increase both of IFN-gamma- (250 U/mL) or TNF-alpha- (500 U/mL) induced NO release. The observed increase of NO production was related to an enhancement of iNOS expression as measured in cell lysates prepared from different treatments by using SDS-PAGE followed by western blot analysis. The effect of THR, but not that of PAR1-AP, was significantly inhibited by hirulog(TM) (60 microg/mL), a specific and stochiometric THR inhibitor or by cathepsin-G (40 mU/mL), an inhibitor of PAR-1. In conclusion our data suggest a role for THR through activation of PAR-1 in the induction of astroglial iNOS, and further support the hypothesis that THR may function as an important pathophysiological modulator of the inflammatory response.

Expression of glucose transporter GLUT3 by endotoxin in cultured rat astrocytes: the role of nitric oxide

The induction of nitric oxide (NO) synthase in astrocytes by endotoxin and/or cytokine treatment is associated with increased glucose consumption and glycolysis, but the mechanism whereby this phenomenon occurs remains obscure. In this work, we have addressed this issue and found that incubation of cultured rat astrocytes with lipopolysaccharide (LPS; 1 microg/mL) for 24 h increased the level of constitutively expressed GLUT1 glucose transporter mRNA, and triggered GLUT3 mRNA expression, which was absent in normal astrocytes. The occurrence of GLUT3 protein after LPS treatment was corroborated by western blotting and immunocytochemistry. A 4-h incubation of astrocytes in the absence of glucose, or under an oxygen-poor (3%) atmosphere also resulted in GLUT3 mRNA overexpression. Experiments performed with 2-deoxy-D-[U-14C]glucose (at 0.1 mM of D-glucose) confirmed that LPS (0.1-10 microg/mL) dose-dependently increased the rate of glucose uptake (by a factor of 1.6 at 1 microg/mL of LPS), which was paralleled with the increase in NO synthesis. Furthermore, blockade of NO synthase with 2-amino-5,6-dihydro-6-methyl-(4H)-1,3-thiazine (AMT; 50 microM) partially (by 45%) prevented the LPS-mediated increase in glucose uptake. Finally, incubation of astrocytes with the NO donor 1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA; 100 microM) increased by a factor of 1.4 the rate of glucose uptake. We conclude that the increase in GLUT3-driven glucose uptake in astrocytes would have a neuroprotective role under conditions in which NO formation is combined with hypoglycaemia, such as in brain ischemia.

Glial cells of the spinal cord and subcortical white matter up-regulate neuronal nitric oxide synthase in sporadic amyotrophic lateral sclerosis

Several studies have suggested that excessive generation of nitric oxide (NO) may contribute to the pathogenesis of amyotrophic lateral sclerosis (ALS). Recently, a selective induction of the neuronal isoform of nitric oxide synthase (nNOS) in glial cells has been reported in an animal model of familial ALS. We therefore examined in postmortem tissue the expression of nNOS in patients with sporadic ALS and patients without any history of neurological disease. Using immunohistochemistry, we found an up-regulation of nNOS in glial cells of the spinal cord and subcortical white matter in ALS patients compared to controls. The enhanced glial nNOS expression seen in ALS patients could conceivably contribute to motoneuronal degeneration through NO-mediated cytotoxic effects.

Nitration as a mechanism of Na+, K+-ATPase modification during hypoxia in the cerebral cortex of the guinea pig fetus

Previous studies have shown that hypoxia induces nitric oxide synthase-mediated generation of nitric oxide free radicals leading to peroxynitrite production. The present study tests the hypothesis that hypoxia results in NO-mediated modification of Na+, K+-ATPase in the fetal brain. Studies were conducted in guinea pig fetuses of 58-days gestation. The mothers were exposed to FiO2 of 0.07% for 1 hour. Brain tissue hypoxia in the fetus was confirmed biochemically by decreased ATP and phosphocreatine levels. P2 membrane fractions were prepared from normoxic and hypoxic fetuses and divided into untreated and treated groups. The membranes were treated with 0.5 mM peroxynitrite at pH 7.6. The Na+, K+-ATPase activity was determined at 37 degrees C for five minutes in a medium containing 100 mM NaCl, 20 mM KCl, 6.0 mM MgCl2, 50 mM Tris HCl buffer pH 7.4, 3.0 mM ATP with or without 10 mM ouabain. Ouabain sensitive activity was referred to as Na+, K+-ATPase activity. Following peroxynitrite exposure, the activity of Na+, K+-ATPase in guinea pig brain was reduced by 36% in normoxic membranes and further 29% in hypoxic membranes. Enzyme kinetics was determined at varying concentrations of ATP (0.5 mM-2.0 mM). The results indicate that peroxynitrite treatment alters the affinity of the active site of Na+, K+-ATPase for ATP and decreases the Vmax by 35% in hypoxic membranes. When compared to untreated normoxic membranes Vmax decreases by 35.6% in treated normoxic membranes and further to 52% in treated hypoxic membranes. The data show that peroxynitrite treatment induces modification of Na+, K+-ATPase. The results demonstrate that peroxynitrite decreased activity of Na+, K+-ATPase enzyme by altering the active sites as well as the microenvironment of the enzyme. We propose that nitric oxide synthase-mediated formation of peroxynitrite during hypoxia is a potential mechanism of hypoxia-induced decrease in Na+, K+-ATPase activity.

Peripheral nitric oxide in carrageenan-induced inflammation

Recent studies have suggested that nitric oxide (NO) peripherally produced by different nitric oxide synthase (NOS) isoforms contributes to edema formation and development of hyperalgesia. The present study was designed to examine the effects of NOS isoforms on NO release in carrageenan-induced inflammation at various time points. A microdialysis probe was implanted subcutaneously into the glabrous skin of hindpaws of Sprague-Dawley rats under pentobarbital anesthesia. After sample collection to obtain the basal level of the total amount of nitrite and nitrate (NO2-/NO3-), modified Ringer solution, a non-selective NOS inhibitor, NG monomethyl-L-arginine acetate (L-NMMA), or an iNOS inhibitor, aminoguanidine hemisulfate (AG) was perfused through the microdialysis probe. 2 mg of carrageenan was injected into the plantar surface of the probe-implanted hindpaw. Carrageenan was also injected in rats that had undergone sciatic nerve sectioning. Carrageenan significantly increased the dialysate concentrations of NO2-/NO3- for more than 8 h. L-NMMA suppressed the carrageenan-induced increase in NO2-/NO3- concentration. Although AG did not suppress the increase in NO2-/NO3- for the first 2 h after carrageenan injection, significant suppression of the increase in NO2-/NO3- was observed from 2.5 h after carrageenan injection. In the rats in which the sciatic nerves had been denervated, the increases in concentrations of NO2-/NO3- were completely suppressed up to 3 h and partially suppressed 4.5-8 h after carrageenan injection. The results of the current study show that carrageenan induces peripheral release of NO, the production of which is mediated by nNOS in the early phase and by both nNOS and iNOS in the late phase of carrageenan-induced inflammation.

Inducible nitric oxide synthase (iNOS) and nitrotyrosine immunoreactivity in the spinal cords of transgenic mice with a G93A mutant SOD1 gene

We performed a prospective, longitudinal immunohistochemical study of the spinal cords of transgenic mice with a G93A mutant SOD1 gene at 4 fixed points in time, using antibodies to inducible nitric oxide synthase (iNOS) and nitrotyrosine. The purpose of this study was to characterize the temporal and topographic distribution of iNOS and nitrotyrosine immunoreactivity in the spinal cord over a certain period, thus illuminating the possible role of increased oxidative damage to the motor system in the neurodegenerative process in this animal model. Specimens from age-matched non-transgenic wild-type mice served as controls. The control mice showed no positive iNOS or nitrotyrosine immuunoreactivity in the somata of anterior horn neurons or their neuronal processes at any age. On the other hand, the transgenic mice demonstrated a common immunostaining pattern of iNOS and nitrotyrosine in the anterior horn neurons. When the mice reached the age of 24 wk (early presymptomatic stage), the anterior horn neurons and their neuronal processes were occasionally immunostained for iNOS and nitrotyrosine; at 28 wk (late presymptomatic stage), the anterior horn neurons were not uncommonly immunostained; at 32 wk (early symptomatic stage) and 35 wk (end-stage), positive iNOS and nitrotyrosine immunoreactivity was frequently observed in proliferated reactive astrocytes as well as in the somata of the anterior horn cells. The selective localization of positive iNOS and nitrotyrosine immunoreactivity in the anterior horn neurons suggests that oxidative stress may be involved in the pathomechanism of degeneration of motor neurons in this transgenic animal model.

JAK/STAT signaling pathway mediates cytokine-induced iNOS expression in primary astroglial cell cultures

The production of nitric oxide by the calcium-independent inducible nitric oxide synthase (iNOS) in glial cells has been implicated in the neuropathogenesis of various diseases. It is well known that in response to lipopolysaccharide (LPS) and cytokines, such as IFN-gamma, glial cells are induced to synthesize large amount of nitric oxide (NO) (Bolaños et al., 1996; Nicoletti et al., 1998). The signaling transduction pathways for iNOS transcription in astroglial cells have however not yet been established. Because IFN-gamma receptor chains are associated with Janus tyrosine kinases (JAK1 and JAK2) (Darnell et al., 1994), we analyzed the involvement of the JAK/STAT signal transduction pathway in iNOS expression. Our study shows increased JAK2 and STAT1 alpha/beta tyrosine phosphorylation in primary astroglial cell culture after treatment with IFN-gamma and LPS. A temporal correlation was observed between JAK2 and STAT1 alpha/beta tyrosine phosphorylation, the appearance of interferon-regulatory factor-1 (IRF-1) mRNA and the iNOS expression. Inhibition experiments showed that JAK2 and STAT1 alpha/beta tyrosine phosphorylation were necessary for IFN gamma-mediated iNOS induction in astroglial cells. We conclude that JAK2 and STAT1 alpha/beta tyrosine phosphorylation is an early event involved in the expression of iNOS in astroglial cells.

The plant flavonoid wogonin suppresses death of activated C6 rat glial cells by inhibiting nitric oxide production

Flavonoids are a group of low molecular weight polyphenolic compounds derived from plants. 5,7-dihydroxy-8-methoxyflavone (Wogonin), a flavonoid originated from the root of Scutellaria baicalensis Georgi, has been shown to exert various anti-inflammatory effects such as inhibition of nitric oxide (NO) and prostaglandin E2 production in macrophages. Because glial cells have been previously shown to undergo NO-dependent apoptosis upon inflammatory activation and this auto-regulatory process may be negatively affected by exogenous factors possessing anti-inflammatory activities, we examined the effects of wogonin on NO production and activation-induced cell death of C6 rat glial cells. Activation of C6 glial cells with lipopolysaccharide (LPS), interferon-gamma, and tumor necrosis factor-alpha induced NO production followed by cell death. Pretreatment of C6 cells with wogonin before LPS and cytokine treatment dose-dependently inhibited NO production as well as death of activated C6 cells. Wogonin-mediated inhibition of NO production was accompanied by suppression of inducible nitric oxide synthase (iNOS) protein induction and nuclear factor kappa B (NF-kappaB) reporter activity. Wogonin, however, did not affect a NO donor-induced cytotoxicity. Taken together, our results indicate that wogonin inhibits activation-induced death of C6 glial cells by suppressing NO production, and these inhibitory effects of wogonin on NO production are exerted through inhibition of NF-kappaB-mediated iNOS induction.

Selective cannabinoid CB1 receptor-mediated inhibition of inducible nitric oxide synthase protein expression in C6 rat glioma cells

We have studied the effects of two cannabinoid receptor agonists, WIN 55,212-2 and cannabinol, on nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) expression in the C6 glioma cell line. After 24 h of lipopolysaccharide (LPS) (1 microg/mL) and interferon-gamma (IFN-gamma) (300 U/mL) stimulation, a significant increase in NO production, evaluated as nitrite, was observed in the culture medium. WIN 55,212-2 (0.1-10000 nM) and cannabinol (0.3-30000 nM), dose-dependently inhibited nitrite production showing a different potency (WIN 55,212-2 EC(50): 4.2 nM; cannabinol EC(50): 700 nM). WIN 55,212-2 (100 nM), given concomitantly to the stimulus also inhibited iNOS expression but had no effect when added to the cells 2 h after LPS/IFN-gamma, indicating a possible interference at the protein synthesis level or at an earlier step, as gene transcription. The cannabinoid CB1 receptor antagonist, SR141716A (0.1-100 nM), but not the cannabinoid CB2 receptor antagonist, SR144528 (0.1-100 nM), reduced in a dose-related manner WIN 55,212-2-and cannabinol-induced inhibition of nitrite production. SR141161A also reversed the WIN 55,212-2-induced inhibition of iNOS expression. These data suggest that selective cannabinoid CB1 receptor activation, by inhibiting iNOS expression and NO overproduction in glial cells, might be helpful in NO-mediated inflammation leading to neurodegeneration.

Neurodegeneration and glia response in rat hippocampus following nitro-L-arginine methyl ester (L-NAME)

Hippocampal neurodegeneration and glia response was examined following administration of the nitric oxide synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME). Male Long-Evans rats received L-NAME (50 mg/kg, ip) either once or twice a day for 4 days. Both dosing schedules decreased NOS-activity by approximately 90%. At 10 and 30 days following cessation of L-NAME (2x/day), moderate neuronal death was evident in CA1-2 pyramidal cells and dentate granule cells. Neurodegeneration was accompanied by increased astrocyte glial fibrillary acidic protein (GFAP) immunoreactivity yet, minimal astrocyte hypertrophy. Microglia response was limited to an increase in ramified microglia at 10 days, returning to normal by 30 days. As early as 4 days post-dosing (2x/day), GFAP mRNA levels were significantly elevated as were mRNA levels for tumor necrosis factor-alpha (TNFalpha), interleukin-1alpha (IL-1alpha), and interleukin 6 (IL-6). No alterations were seen with L-NAME dosing limited to once a day. The co-administration of a hippocampal neurotoxicant, trimethyltin (TMT), with the last dose of L-NAME (2x/day), produced an additive response pattern of neuronal degeneration including both CA1-2 and CA3-4 pyramidal neurons accompanied by TMT-induced astrocyte hypertrophy and prominent microglia reactivity. This was preceded by elevations in mRNA levels for GFAP, TNFalpha, IL-1alpha, and IL-6 similar to those seen with each substance alone. These data suggest that high levels of L-NAME can produce a pro-inflammatory environment in the brain and that neurodegeneration and neuroglia responses in the hippocampus can be induced by an alteration in the balance and regulation of local nitric oxide levels.