Nitrosative stress in primary glial cultures after induction of the inducible isoform of nitric oxide synthase (i-NOS)

Redox control of microglial function: molecular mechanisms and functional significance

Neurodegenerative diseases are characterized by chronic microglial over-activation and oxidative stress. It is now beginning to be recognized that reactive oxygen species (ROS) produced by either microglia or the surrounding environment not only impact neurons but also modulate microglial activity. In this review, we first analyze the hallmarks of pro-inflammatory and anti-inflammatory phenotypes of microglia and their regulation by ROS. Then, we consider the production of reactive oxygen and nitrogen species by NADPH oxidases and nitric oxide synthases and the new findings that also indicate an essential role of glutathione (γ-glutamyl-l-cysteinylglycine) in redox homeostasis of microglia. The effect of oxidant modification of macromolecules on signaling is analyzed at the level of oxidized lipid by-products and sulfhydryl modification of microglial proteins. Redox signaling has a profound impact on two transcription factors that modulate microglial fate, nuclear factor kappa-light-chain-enhancer of activated B cells, and nuclear factor (erythroid-derived 2)-like 2, master regulators of the pro-inflammatory and antioxidant responses of microglia, respectively. The relevance of these proteins in the modulation of microglial activity and the interplay between them will be evaluated. Finally, the relevance of ROS in altering blood brain barrier permeability is discussed. Recent examples of the importance of these findings in the onset or progression of neurodegenerative diseases are also discussed. This review should provide a profound insight into the role of redox homeostasis in microglial activity and help in the identification of new promising targets to control neuroinflammation through redox control of the brain.

Anti-inflammatory and anti-oxidant activity of anionic dendrimer-N-acetyl cysteine conjugates in activated microglial cells

Dendrimers are emerging as potential intracellular drug delivery vehicles. Understanding and improving the cellular efficacy of dendrimer-drug conjugates, can lead to significant in vivo benefits. This study explores efficacy of anionic polyamidoamine (PAMAM-COOH) dendrimer-N-acetyl cysteine (NAC) conjugates for applications in neuroinflammation. The anti-oxidative and anti-inflammatory effects of PAMAM-(COOH)(46)-(NAC)(18) conjugate is evaluated on microglial cells in vitro. Cell entry and localization of PAMAM-(COOH)(62)-(FITC)(2) conjugate in BV-2 microglial cells were assessed using flow cytometry and confocal microscopy. ELISA assays were used to evaluate markers of oxidative stress (ROS, NO) and inflammation (TNF-alpha) after stimulation of microglial cells with lipopolysaccharides (LPS), following treatment with increasing doses of free N-acetyl-L-cysteine (NAC) or PAMAM-(COOH)(46)-(NAC)(18) conjugate containing an equivalent molar concentration of NAC. Flow cytometry and confocal microscopy demonstrated the PAMAM-(COOH)(62)-(FITC)(2) conjugate entered BV-2 cells rapidly with significant increase in fluorescence within 15 min and localized mostly in the cytoplasm. PAMAM-(COOH)(46)-(NAC)(18) conjugate was non-toxic, and significantly reduced ROS, NO and TNF-alpha release by activated microglial cells after 24 h and 72 h stimulation of LPS following 3h pre-treatment when compared to the same concentration of free NAC (P<0.05 or P<0.01). Anionic PAMAM dendrimer-NAC conjugate was synthesized with a glutathione sensitive linker for intracellular release. The non-toxic conjugate is a more effective anti-oxidant and anti-inflammatory agent when compared to free NAC in vitro. The conjugate showed significant efficacy even at the lowest dose (0.5mM NAC), where the activity was comparable or better than that of free drug at 8mM (16x higher dosage). The improved efficacy of the conjugate, when combined with the intrinsic neuroinflammation-targeting ability of the PAMAM dendrimers, may provide new opportunities for in vivo applications.

Microglial cells in astroglial cultures: a cautionary note

Primary rodent astroglial-enriched cultures are the most popular model to study astroglial biology in vitro. From the original methods described in the 1970's a great number of minor modifications have been incorporated into these protocols by different laboratories. These protocols result in cultures in which the astrocyte is the predominant cell type, but astrocytes are never 100% of cells in these preparations. The aim of this review is to bring attention to the presence of microglia in astroglial cultures because, in my opinion, the proportion of and the role that microglial cells play in astroglial cultures are often underestimated. The main problem with ignoring microglia in these cultures is that relatively minor amounts of microglia can be responsible for effects observed on cultures in which the astrocyte is the most abundant cell type. If the relative contributions of astrocytes and microglia are not properly assessed an observed effect can be erroneously attributed to the astrocytes. In order to illustrate this point the case of NO production in activated astroglial-enriched cultures is examined. Lipopolysaccharide (LPS) induces nitric oxide (NO) production in astroglial-enriched cultures and this effect is very often attributed to astrocytes. However, a careful review of the published data suggests that LPS-induced NO production in rodent astroglial-enriched cultures is likely to be mainly microglial in origin. This review considers cell culture protocol factors that can affect the proportion of microglial cells in astroglial cultures, strategies to minimize the proportion of microglia in these cultures, and specific markers that allow the determination of such microglial proportions.

Oxidative and antioxidative potential of brain microglial cells

Microglial cells are the resident immune cells of the central nervous system. These cells defend the central nervous system against invading microorganisms and clear the debris from damaged cells. Upon activation, microglial cells produce a large number of neuroactive substances that include cytokines, proteases, and prostanoids. In addition, activated microglial cells release radicals, such as superoxide and nitric oxide, that are products of the enzymes NADPH oxidase and inducible nitric oxide synthase, respectively. Microglia-derived radicals, as well as their reactive reaction products hydrogen peroxide and peroxynitrite, have the potential to harm cells and have been implicated in contributing to oxidative damage and neuronal cell death in neurological diseases. For self-protection against oxidative damage, microglial cells are equipped with efficient antioxidative defense mechanisms. These cells contain glutathione in high concentrations, substantial activities of the antioxidative enzymes superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase, as well as NADPH-regenerating enzymes. Their good antioxidative potential protects microglial cells against oxidative damage that could impair important functions of these cells in defense and repair of the brain.

Cytosolic and mitochondrial glutathione in microglial cells are differentially affected by oxidative/nitrosative stress

Glutathione (GSH), the major cellular protectant against reactive oxygen and nitrogen species, is compartmentalized in a cytosolic (c) and a mitochondrial (mt) pool. We investigated how c-GSH and mt-GSH are differentially affected by endogenously produced nitric oxide (NO). Microglial cell line (N9) cultures were immunostimulated with lipopolysaccharide/interferon-gamma to elicit the inducible isoform of NO synthase (iNOS). Despite a significant reduction in total GSH, the mt-GSH remained nearly unaffected by iNOS-mediated NO production. To investigate possible consequences of GSH depletion on the mitochondrial membrane potential, we used buthionine sulfoximine (BSO) to reduce separately the c-GSH, whereas ethacrynic acid (EA) was applied to deplete both mt-GSH and c-GSH. The mitochondrial membrane potential was more vulnerable to NO exposure in EA-pretreated cultures than in BSO-pretreated cultures, indicated by a potentiated release of tetramethylrhodamine from mitochondria into the cytosol. To relate the EA-mediated decrease in mitochondrial membrane potential to the oxidant buildup after GSH depletion, we loaded the cells with the oxidant-sensitive fluorochrome 2',7'-dihydrodichlorofluorescein (DCF) diacetate. EA treatment caused an increase in DCF fluorescence over time that was potentiated when the iNOS expression was stimulated. Inhibition of NO production abolished this effect. We conclude that endogenous NO production in microglial cells does not compromise the mt-GSH pool which, in turn, might explain the ability of these cells to combat high-output NO production.

Activation of murine microglial cell lines by lipopolysaccharide and interferon-gamma causes NO-mediated decreases in mitochondrial and cellular function

Activation of murine microglial and macrophage cell lines with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) resulted in the induction of the inducible form of nitric oxide synthase (NOS) and the release of micromolar amounts of NO into the surrounding medium. The synthesis of NO was associated with increased cellular membrane damage as assessed by trypan blue dye exclusion and the leakage of lactate dehydrogenase into the cell culture medium. However, the synthesis and release of cytokines was largely unaffected. NO-mediated cell damage was also accompanied by a marked decrease in the intracellular levels of reduced glutathione and ATP. In addition, significant inhibition of mitochondrial respiratory chain enzyme activities was seen following cellular activation. However, citrate synthase activity (a mitochondrial matrix enzyme) was not detectable in the extracellular supernatants, suggesting preservation of the integrity of the mitochondrial inner membrane following activation. These effects were largely prevented by the addition of the NOS inhibitor, N-guanidino monomethyl L-arginine during the activation period. Our observations demonstrate that induction of NOS activity in microglia results in damage to the plasma membrane leading to a loss of glutathione, complex-specific inhibition of the mitochondrial electron transport chain and depletion of cellular ATP. Our data suggest that pharmacological modulation of NOS activity in activated microglia in vivo may prevent cellular damage to bystander cells such as neurons, astrocytes and oligodendrocytes, as well as to microglia themselves.