Quantitative imaging of glutathione in hippocampal neurons and glia in culture using monochlorobimane

Loss of glutathione homeostasis associated with neuronal senescence facilitates TRPM2 channel activation in cultured hippocampal pyramidal neurons

Background

Glutathione (GSH) plays an important role in neuronal oxidant defence. Depletion of cellular GSH is observed in neurodegenerative diseases and thereby contributes to the associated oxidative stress and Ca²⁺ dysregulation. Whether depletion of cellular GSH, associated with neuronal senescence, directly influences Ca²⁺ permeation pathways is not known. Transient receptor potential melastatin type 2 (TRPM2) is a Ca²⁺ permeable non-selective cation channel expressed in several cell types including hippocampal pyramidal neurons. Moreover, activation of TRPM2 during oxidative stress has been linked to cell death. Importantly, GSH has been reported to inhibit TRPM2 channels, suggesting they may directly contribute to Ca²⁺ dysregulation associated with neuronal senescence. Herein, we explore the relation between cellular GSH and TRPM2 channel activity in long-term cultures of hippocampal neurons.

Results

In whole-cell voltage-clamp recordings, we observe that TRPM2 current density increases in cultured pyramidal neurons over time in vitro. The observed increase in current density was prevented by treatment with NAC, a precursor to GSH synthesis. Conversely, treatment of cultures maintained for 2 weeks in vitro with L-BSO, which depletes GSH by inhibiting its synthesis, augments TRPM2 currents. Additionally, we demonstrate that GSH inhibits TRPM2 currents through a thiol-independent mechanism, and produces a 3.5-fold shift in the dose-response curve generated by ADPR, the intracellular agonist for TRPM2.

Conclusion

These results indicate that GSH plays a physiologically relevant role in the regulation of TRPM2 currents in hippocampal pyramidal neurons. This interaction may play an important role in aging and neurological diseases associated with depletion of GSH.

Oxidative stress induced by glutathione depletion reproduces pathological modifications of TDP-43 linked to TDP-43 proteinopathies

TAR DNA-binding protein 43 (TDP-43) is a major component of ubiquitin-positive inclusion of TDP-43 proteinopathies including amyotrophic lateral sclerosis and frontotemporal lobar degeneration with ubiquitinated inclusions, which is now referred to as FTLD-TDP. TDP-43 in the aberrant inclusion is known to be hyperphosphorylated at C-terminal sites, to be truncated at the N-terminal region, and to re-distribute from nucleus to cytoplasm or neurite. The pathogenic role of these modifications, however, has not been clarified. Furthermore, there is no evidence about the initial cause of these modifications. Herein we show that ethacrynic acid (EA), which is able to increase cellular oxidative stress through glutathione depletion, induces TDP-43 C-terminal phosphorylation at serine 403/404 and 409/410, insolubilization, C-terminal fragmentation, and cytoplasmic distribution in NSC34 cells and primary cortical neurons. In the investigation using a nonphosphorylable mutant of TDP-43, there was no evidence that C-terminal phosphorylation of TDP-43 contributes to its solubility or distribution under EA induction. Our findings suggest that oxidative stress induced by glutathione depletion is associated with the process of the pathological TDP-43 modifications and provide new insight for TDP-43 proteinopathies.

Antioxidants, redox signaling, and pathophysiology in schizophrenia: an integrative view

Schizophrenia (SZ) is a brain disorder that has been intensively studied for over a century; yet, its etiology and multifactorial pathophysiology remain a puzzle. However, significant advances have been made in identifying numerous abnormalities in key biochemical systems. One among these is the antioxidant defense system (AODS) and redox signaling. This review summarizes the findings to date in human studies. The evidence can be broadly clustered into three major themes: perturbations in AODS, relationships between AODS alterations and other systems (i.e., membrane structure, immune function, and neurotransmission), and clinical implications. These domains of AODS have been examined in samples from both the central nervous system and peripheral tissues. Findings in patients with SZ include decreased nonenzymatic antioxidants, increased lipid peroxides and nitric oxides, and homeostatic imbalance of purine catabolism. Reductions of plasma antioxidant capacity are seen in patients with chronic illness as well as early in the course of SZ. Notably, these data indicate that many AODS alterations are independent of treatment effects. Moreover, there is burgeoning evidence indicating a link among oxidative stress, membrane defects, immune dysfunction, and multineurotransmitter pathologies in SZ. Finally, the body of evidence reviewed herein provides a theoretical rationale for the development of novel treatment approaches.

Insulin-like growth factor 1 and transforming growth factor-β stimulate cystine/glutamate exchange activity in dental pulp cells

INTRODUCTION

The growth factors insulin-like growth factor (IGF-1) and transforming growth factor-β (TGF-β) are protective to dental pulp cells in culture against the toxicity of the composite materials Durafill VS and Flow Line (Henry Schein Inc, New York, NY). Because the toxicity of these materials is mediated by oxidative stress, it seemed possible that the protective effects of IGF-1 and TGF-β were through the enhancement of an endogenous antioxidant mechanism.

METHODS

We used cultured dental pulp cells to determine the mechanism of the protective effects of IGF-1 and TGF-β, focusing on the glutathione system and the role of cystine/glutamate exchange (system xc-).

RESULTS

We found that the toxicity of Durafill VS and Flow Line was attenuated by the addition of glutathione monoethylester, suggesting a specific role for the cellular antioxidant glutathione. Supporting this hypothesis, we found that IGF-1 and TGF-β were protective against the toxicity of the glutathione synthesis inhibitor buthionine sulfoximine. Because levels of cellular cystine are the limiting factor in the production of glutathione, we tested the effects of IGF-1 and TGF-β on cystine uptake. Both growth factors stimulated system xc-mediated cystine uptake. Furthermore, they attenuated the glutathione depletion induced by Durafill VS and Flow Line.

CONCLUSIONS

The results suggest that IGF-1 and TGF-β are protective through the stimulation of system xc-mediated cystine uptake, leading to maintenance of cellular glutathione. This novel action of growth factors on dental pulp cells has implications not only for preventing toxicity of dental materials but also for the general function of these cells.

Iron handling in hippocampal neurons: activity-dependent iron entry and mitochondria-mediated neurotoxicity

The characterization of iron handling in neurons is still lacking, with contradictory and incomplete results. In particular, the relevance of non-transferrin-bound iron (NTBI), under physiologic conditions, during aging and in neurodegenerative disorders, is undetermined. This study investigates the mechanisms underlying NTBI entry into primary hippocampal neurons and evaluates the consequence of iron elevation on neuronal viability. Fluorescence-based single cell analysis revealed that an increase in extracellular free Fe(2+) (the main component of NTBI pool) is sufficient to promote Fe(2+) entry and that activation of either N-methyl-d-aspartate receptors (NMDARs) or voltage operated calcium channels (VOCCs) significantly potentiates this pathway, independently of changes in intracellular Ca(2+) concentration ([Ca(2+) ](i) ). The enhancement of Fe(2+) influx was accompanied by a corresponding elevation of reactive oxygen species (ROS) production and higher susceptibility of neurons to death. Interestingly, iron vulnerability increased in aged cultures. Scavenging of mitochondrial ROS was the most powerful protective treatment against iron overload, being able to preserve the mitochondrial membrane potential and to safeguard the morphologic integrity of these organelles. Overall, we demonstrate for the first time that Fe(2+) and Ca(2+) compete for common routes (i.e. NMDARs and different types of VOCCs) to enter primary neurons. These iron entry pathways are not controlled by the intracellular iron level and can be harmful for neurons during aging and in conditions of elevated NTBI levels. Finally, our data draw the attention to mitochondria as a potential target for the treatment of the neurodegenerative processes induced by iron dysmetabolism.

Comparable attenuation of Abeta(25-35)-induced neurotoxicity by quercitrin and 17beta-estradiol in cultured rat hippocampal neurons

In the present work, potential protective effects of quercitrin (a phytoestrogen) on Abeta-induced neurotoxicity in cultured rat hippocampal neurons were investigated in comparison with 17beta-estradiol. Cell viability, oxidative status, and antioxidative potentials were used as comparative parameters. Co-exposure of cultured neurons to Abeta(25-35) with either quercitrin or 17beta-estradiol (50-100 microM) for 72 h attenuated Abeta(25-35)-induced neurotoxicity and lipid peroxidation, but not Abeta(25-35)-induced ROS accumulation. However, only 17beta-estradiol counteracted a reduction in glutathione content and only quercitrin counteracted a reduction in glutathione peroxidase activity. Both compounds displayed no effects on superoxide dismutase activity. A specific estrogen receptor antagonist, ICI 182780, did not abolish neuroprotective effects of quercitrin and 17beta-estradiol. These findings suggested that quercitrin and 17beta-estradiol attenuated Abeta(25-35)-induced neurotoxicity in a comparable manner. Underlying neuroprotective mechanisms of both compounds were probably not related to estrogen receptor-mediated genomic mechanisms but might involve with their antioxidant and free radical scavenging properties.

Maternal diet-induced obesity alters mitochondrial activity and redox status in mouse oocytes and zygotes

The negative impact of obesity on reproductive success is well documented but the stages at which development of the conceptus is compromised and the mechanisms responsible for the developmental failure still remain unclear. Recent findings suggest that mitochondria may be a contributing factor. However to date no studies have directly addressed the consequences of maternal obesity on mitochondria in early embryogenesis.

Using an established murine model of maternal diet induced obesity and a live cell dynamic fluorescence imaging techniques coupled with molecular biology we have investigated the underlying mechanisms of obesity-induced reduced fertility. Our study is the first to show that maternal obesity prior to conception is associated with altered mitochondria in mouse oocytes and zygotes. Specifically, maternal diet-induced obesity in mice led to an increase in mitochondrial potential, mitochondrial DNA content and biogenesis. Generation of reactive oxygen species (ROS) was raised while glutathione was depleted and the redox state became more oxidised, suggestive of oxidative stress. These altered mitochondrial properties were associated with significant developmental impairment as shown by the increased number of obese mothers who failed to support blastocyst formation compared to lean dams. We propose that compromised oocyte and early embryo mitochondrial metabolism, resulting from excessive nutrient exposure prior to and during conception, may underlie poor reproductive outcomes frequently reported in obese women.

Mechanism of neurodegeneration of neurons with mitochondrial DNA mutations

Mutations of mitochondrial DNA are associated with a wide spectrum of disorders, primarily affecting the central nervous system and muscle function. The specific consequences of mitochondrial DNA mutations for neuronal pathophysiology are not understood. In order to explore the impact of mitochondrial mutations on neuronal biochemistry and physiology, we have used fluorescence imaging techniques to examine changes in mitochondrial function in neurons differentiated from mouse embryonic stem-cell cybrids containing mitochondrial DNA polymorphic variants or mutations. Surprisingly, in neurons carrying a severe mutation in respiratory complex I (<10% residual complex I activity) the mitochondrial membrane potential was significantly increased, but collapsed in response to oligomycin, suggesting that the mitochondrial membrane potential was maintained by the F₁F_o ATPase operating in ‘reverse’ mode. In cells with a mutation in complex IV causing ∼40% residual complex IV activity, the mitochondrial membrane potential was not significantly different from controls. The rate of generation of mitochondrial reactive oxygen species, measured using hydroethidium and signals from the mitochondrially targeted hydroethidine, was increased in neurons with both the complex I and complex IV mutations. Glutathione was depleted, suggesting significant oxidative stress in neurons with a complex I deficiency, but not in those with a complex IV defect. In the neurons with complex I deficiency but not the complex IV defect, neuronal death was increased and was attenuated by reactive oxygen species scavengers. Thus, in neurons with a severe mutation of complex I, the maintenance of a high potential by F₁F_o ATPase activity combined with an impaired respiratory chain causes oxidative stress which promotes cell death.

Multiphoton imaging reveals differences in mitochondrial function between nephron segments

Mitochondrial dysfunction may play a role in the pathogenesis of several renal diseases. Although functional roles and metabolic demands differ among tubule segments, relatively little is known about the properties of mitochondria in different parts of the nephron. Clinically, the proximal tubule seems particularly vulnerable to mitochondrial toxicity. In this study, we used multiphoton imaging of live rat kidney slices to investigate differences in mitochondrial function along the nephron. The mitochondrial membrane potential was markedly higher in distal than proximal tubules. Inhibition of respiration rapidly collapsed the membrane potential in proximal tubules, but potential was better maintained in distal tubules. Inhibition of the F1F(o)-ATPase abolished this difference, suggesting that maintenance of potential via ATPase activity is more effective in distal than proximal tubules. Immunostaining revealed that the ratio of the expression of ATPase to IF1, an endogenous inhibitor of the mitochondrial ATPase, was lower in proximal tubules than in distal tubules. Production of reactive oxygen species was higher in proximal than distal cells, but inhibition of NADPH oxidase eliminated this difference. Glutathione levels were higher in proximal tubules. Overall, mitochondria in the proximal tubules were in a more oxidized state than those in the distal tubules. In summary, there are axial differences in mitochondrial function along the nephron, which may contribute to the pattern and pathophysiology of some forms of renal injury.

Fertilization differently affects the levels of cyclin B1 and M-phase promoting factor activity in maturing and metaphase II mouse oocytes

Fertilization affects levels of cyclin B1 and M-phase promoting factor (MPF) activity in maturing and metaphase II mouse oocytes in two distinct ways. In metaphase II oocytes, it leads to a Ca2+-dependent, continuous degradation of cyclin B1 and inactivation of cyclin dependent kinase (CDC2A)–cyclin B1 complex (MPF). In this paper, we show that neither mono- nor polyspermic fertilization of prometaphase I and metaphase I oocytes triggered degradation of cyclin B1. However, polyspermic fertilization of prometaphase I oocytes led to a transient decrease in MPF activity that lasted for 2 h. The inactivation of MPF in polyspermic prometaphase I oocytes did not depend on the fertilization-induced increase in the cytoplasmic concentration of free Ca2+ions, but was caused, at least in part, by dephosphorylation of CDC2A at threonine 161 (Thr161). We found that polyspermic fertilization did not affect glutathione levels in prometaphase I oocytes, and concluded that the decrease in MPF activity and dephosphorylation of CDC2A at Thr161 in polyspermic prometaphase I oocytes were not caused by a change in the redox status of the cell induced by an introduction of excessive amount of sperm protamines. Instead, we propose that inactivation of MPF activity in polyspermic maturing oocytes is caused by a change in nucleo-cytoplasmic ratio that leads to a ‘titration’ of kinases and phosphatases responsible for keeping MPF in an active state. This idea is supported by the finding that oocytes fused with thymocytes rather than spermatozoa also showed a transient decrease in MPF activity.

Incorporation of 3T3-L1 Cells To Mimic Bioaccumulation in a Microscale Cell Culture Analog Device for Toxicity Studies

Deficiencies in the early ADMET (absorption, distribution, metabolism, elimination, and toxicity) information on drug candidates extract a significant economic penalty on pharmaceutical firms. We have developed a microscale cell culture analog (microCCA) device that can potentially provide better, faster, and more efficient prediction of human and animal responses to a wide range of chemicals. The system described in this paper is a simple four-chamber microCCA ("lung"-"liver"-"fat"-"other tissue") designed on the basis of a physiologically based pharmacokinetics (PBPK) model of a rat. Cultures of L2, HepG2/C3A, and differentiated 3T3-L1 adipocytes were selected to mimic the key functions of the lung, liver, and fat compartments, respectively. Here, we have demonstrated the application of the microCCA system to study bioaccumulation, distribution, and toxicity of selected compounds. Results from the bioaccumulation study reveal that hydrophobic compounds such as fluoranthene preferentially accumulated in the fat chamber. Only a small amount of fluoranthene was observed in the liver and lung chambers. In addition, the presence of the differentiated 3T3-L1 adipocytes in the microCCA device significantly reduced naphthalene and naphthoquinone-induced glutathione (GSH) depletion. These findings suggest the potential utilization of the microCCA system to assess ADMET characteristics of the compound of interest prior to animal or human trials.

The proteasome and intracellular redox status: implications for apoptotic regulation in lens epithelial cells

PURPOSE

This study aimed to investigate redox regulation of the proteasome as well as the effect of proteasome inhibition on intracellular oxidative status and apoptosis.

METHODS

Oxidative stress was induced in cultured human lens epithelial cells (HLECs) and intact mouse lenses by 100 microM H2O2. HLECs were also exposed to the reduced and the oxidized forms of glutathione (GSH/GSSG) and the reducing agent dithiotreitol (DTT). The chymotrypsin-like, the trypsin-like, and the peptidylglutamyl peptidase activities of the proteasome were measured using synthetic fluorogenic substrates. Superoxide as well as peroxide production, mitochondrial membrane potential, and the level of GSH was measured in HLECs after proteasome inhibition by MG-132 or lactacystin. Apoptosis was determined by measuring caspase-3 activation and by studying apoptotic nuclei after staining with Hoechst 33342.

RESULTS

All three peptidase activities of the proteasome were inhibited by 100 microM H2O2 and by the oxidized form of glutathione (GSSG), whereas the reduced form (GSH) stimulated chymotrypsin-like and peptidylglutamyl peptidase activities in HLECs lysates. Intact mouse lenses exposed to 100 microM H2O2 exhibited loss of transparency and trends of decreased chymotrypsin-like proteasome activity as well as decreased GSH levels. Inhibition of the proteasome in cultured HLECs caused significant increase in apoptosis and disturbed intracellular redox balance. Simultaneous addition of exogenous GSH completely abolished the increased apoptosis seen after MG-132 treatment.

CONCLUSIONS

This study supports the hypothesis that intracellular proteolytic and oxidative regulatory systems are tightly coupled. The current data also indicate that apoptosis by proteasome inhibition is mediated through oxidative mechanisms.

Bioenergetic and oxidative effects of free 3-nitrotyrosine in culture: selective vulnerability of dopaminergic neurons and increased sensitivity of non-dopaminergic neurons to dopamine oxidation

Protein bound and free 3-nitrotyrosine (3NT) levels are elevated in neurodegenerative diseases and have been used as evidence for peroxynitrite generation. Intrastriatal injection of free 3NT causes dopaminergic neuron injury and represents a new mouse model of Parkinson's disease (PD). We are investigating the nature of free 3NT neurotoxicity. In primary ventral midbrain cultures, free 3NT damaged dopaminergic neurons, while adjacent non-dopaminergic neurons were unaffected. Combined treatment with free 3NT and subtoxic amounts of dopamine caused extensive death of non-dopaminergic forebrain neurons in culture. Free 3NT alone directly inhibited mitochondrial complex I, decreased ATP, sensitized neurons to mitochondrial depolarization, and increased superoxide production. Subtoxic concentrations of rotenone (instead of free 3NT) caused similar results. Additionally, free 3NT and dopamine combined increased extraneuronal hydrogen peroxide and decreased intraneuronal glutathione levels more than dopamine alone. Oxidative and bioenergetic processes have been proposed to contribute to neurodegeneration in PD. As free 3NT is a compound that is increased in PD, damages dopamine neurons in vivo and in vitro and has detrimental effects on neuronal bioenergetics, it is possible that free 3NT is an endogenous contributing factor to neuronal loss, in addition to being a marker of oxidative and nitrative processes.

Methylglyoxal causes strong weakening of detoxifying capacity and apoptotic cell death in rat hippocampal neurons

The hippocampus is known to play a crucial role in learning and memory. Recent data from literature show that cognitive problems, common to aged or diabetic patients, may be related to accumulation of toxic alpha-oxoaldehydes such as methylglyoxal. Thus, it is possible that methylglyoxal could be, at least in part, responsible for the impairment of cognitive functions, and the knowledge of the mechanisms through which this compound elicits neuronal toxicity could be useful for the development of possible therapeutic strategies. We previously reported a high susceptibility of hippocampal neurons to methylglyoxal, through an oxidation-dependent mechanism. In the present study, we extend our investigation on the molecular mechanisms which underlie methylglyoxal toxicity, focusing on possible effects on expression and activity of glyoxalases, its main detoxifying enzymes, and glutathione peroxidase, as well as on the levels of reduced glutathione. We also investigate methylglyoxal-induced modulation of brain derived neurotrophic factor and proinflammatory cytokines. Our results show that methylglyoxal causes a dramatic depletion of reduced glutathione and a significant inhibition of both glyoxalase and glutathione peroxidase activities. Furthermore, methylglyoxal treatment seems to affect the expression of inflammatory cytokines and survival factors. In conclusion, our findings suggest that methylglyoxal-induced neurotoxicity occurs through the impairment of detoxification pathway and depletion of reduced glutathione. This, in turn, triggers widespread apoptotic cell death, occurring through the convergence of both mitochondrial and Fas-receptor pathways.

Metabotropic glutamate receptor 3 protects neurons from glucose-induced oxidative injury by increasing intracellular glutathione concentration

High glucose concentrations cause oxidative injury and programmed cell death in neurons, and can lead to diabetic neuropathy. Activating the type 3 metabotropic glutamate receptor (mGluR3) prevents glucose-induced oxidative injury in dorsal root ganglion neurons co-cultured with Schwann cells. To determine the mechanisms of protection, studies were performed in rat dorsal root ganglion neuron-Schwann cell co-cultures. The mGluR3 agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate prevented glucose-induced inner mitochondrial membrane depolarization, reactive oxygen species accumulation, and programmed cell death, and increased glutathione (GSH) concentration in co-cultured neurons and Schwann cells, but not in neurons cultured without Schwann cells. Protection was diminished in neurons treated with the GSH synthesis inhibitor l-buthionine-sulfoximine, suggesting that mGluR-mediated protection requires GSH synthesis. GSH precursors and the GSH precursor GSH-ethyl ester also protected neurons from glucose-induced injury, indicating that GSH synthesis in Schwann cells, and transport of reaction precursors to neurons, may underlie mGluR-mediated neuroprotection. These results support the conclusions that activating glial mGluR3 protects neurons from glucose-induced oxidative injury by increasing free radical scavenging and stabilizing mitochondrial function, through increased GSH antioxidant defense.

Three distinct mechanisms generate oxygen free radicals in neurons and contribute to cell death during anoxia and reoxygenation

Ischemia is a major cause of brain damage, and patient management is complicated by the paradoxical injury that results from reoxygenation. We have now explored the generation of reactive oxygen species (ROS) in hippocampal and cortical neurons in culture in response to oxygen and glucose deprivation or metabolic inhibition and reoxygenation. Fluorescence microscopy was used to measure the rate of ROS generation using hydroethidine, dicarboxyfluorescein diacetate, or MitoSOX. ROS generation was correlated with changing mitochondrial potential (rhodamine 123), [Ca2+]c (fluo-4, fura-2, or Indo-1), or ATP consumption, indicated by increased [Mg2+]c. We found that three distinct mechanisms contribute to neuronal injury by generating ROS and oxidative stress, each operating at a different stage of ischemia and reperfusion. In response to hypoxia, mitochondria generate an initial burst of ROS, which is curtailed once mitochondria depolarize or prevented by previous depolarization with uncoupler. A second phase of ROS generation that followed after a delay was blocked by the xanthine oxidase (XO) inhibitor oxypurinol. This phase correlated with a rise in [Mg2+]c, suggesting XO activation by accumulating products of ATP consumption. A third phase of ROS generation appeared at reoxygenation. This was blocked by NADPH oxidase inhibitors and was absent in cells from gp91(phox-/-) knock-out mice. It was Ca2+ dependent, suggesting activation by increased [Ca2+]c during anoxia, itself partly attributable to glutamate release. Inhibition of either the NADPH oxidase or XO was significantly neuroprotective. Thus, oxidative stress contributes to cell death over and above the injury attributable to energy deprivation.

Ketones inhibit mitochondrial production of reactive oxygen species production following glutamate excitotoxicity by increasing NADH oxidation

Dietary protocols that increase serum levels of ketones, such as calorie restriction and the ketogenic diet, offer robust protection against a multitude of acute and chronic neurological diseases. The underlying mechanisms, however, remain unclear. Previous studies have suggested that the ketogenic diet may reduce free radical levels in the brain. Thus, one possibility is that ketones may mediate neuroprotection through antioxidant activity. In the present study, we examined the effects of the ketones beta-hydroxybutyrate and acetoacetate on acutely dissociated rat neocortical neurons subjected to glutamate excitotoxicity using cellular electrophysiological and single-cell fluorescence imaging techniques. Further, we explored the effects of ketones on acutely isolated mitochondria exposed to high levels of calcium. A combination of beta-hydroxybutyrate and acetoacetate (1 mM each) decreased neuronal death and prevented changes in neuronal membrane properties induced by 10 microM glutamate. Ketones also significantly decreased mitochondrial production of reactive oxygen species and the associated excitotoxic changes by increasing NADH oxidation in the mitochondrial respiratory chain, but did not affect levels of the endogenous antioxidant glutathione. In conclusion, we demonstrate that ketones reduce glutamate-induced free radical formation by increasing the NAD+/NADH ratio and enhancing mitochondrial respiration in neocortical neurons. This mechanism may, in part, contribute to the neuroprotective activity of ketones by restoring normal bioenergetic function in the face of oxidative stress.

Bioenergetics of mitochondria in cultured neurons and their role in glutamate excitotoxicity

The pathologic activation of NMDA receptors by glutamate is a major contributor to neuronal cell death after stroke. Receptor activation causes a massive influx of calcium into the neuron that is accumulated by the mitochondria. The favored hypothesis is that the calcium loaded mitochondria generate reactive oxygen species that damage and ultimately killed the neuron. In this review this hypothesis is critically re-examined with an emphasis on the role played by deficits in ATP generation. Novel techniques are developed to monitor the bioenergetic status of in situ mitochondria in cultured neurons. Applying these techniques to a model of glutamate excitotoxicity suggests that enhanced reactive oxygen species are a consequence rather than a cause of failed cytoplasmic calcium homeostasis (delayed calcium deregulation, [DCD]), but that prior oxidative damage facilitates DCD by damaging mitochondrial ATP generation. This impacts on current hypotheses relating to the neuroprotective effects of mild mitochondrial uncoupling.

Alzheimer’s Disease and Cholesterol: The Fat Connection

Since the discovery of the significance of the cholesterol-carrying apolipoprotein E and cholesterolaemia as major risk factors for Alzheimer's Disease (AD) there has been a mounting interest in the role of this lipid as a possible pathogenic agent. In this review we analyse the current evidence linking cholesterol metabolism and regulation in the CNS with the known mechanisms underlying the development of Alzheimer's Disease. Cholesterol is known to affect amyloid-beta generation and toxicity, although it must be considered that the results studies using the statin class of drugs to lower plasma cholesterol may be affected by other effects associated with these drugs. Finally, we report some of our results pointing at the interplay between neurons and astrocytes and NADPH oxidase activation as a new candidate mechanism linking cholesterol and AD pathology.

Regulation of redox metabolism in the mouse oocyte and embryo

Energy homeostasis of the oocyte is a crucial determinant of fertility. Following ovulation, the oocyte is exposed to the unique environment of the Fallopian tube, and this is reflected in a highly specialised biochemistry. The minute amounts of tissue available have made the physiological analysis of oocyte intermediary metabolism almost impossible. We have therefore used confocal imaging of mitochondrial and cytosolic redox state under a range of conditions to explore the oxidative metabolism of intermediary substrates. It has been known for some time that the early mouse embryo metabolises external pyruvate and lactate but not glucose to produce ATP. We now show at the level of single oocytes, that supplied glucose has no effect on the redox potential of the oocyte. Pyruvate is a cytosolic oxidant but a mitochondrial reductant, while lactate is a strong cytosolic reductant via the activity of lactate dehydrogenase. Unexpectedly, lactate-derived pyruvate appears to be diverted from mitochondrial oxidation. Our approach also reveals that the level of reduced glutathione (GSH) in the oocyte is maintained by glutathione reductase, which oxidises intracellular NADPH to reduce oxidised glutathione. Surprisingly, NADPH does not seem to be supplied by the pentose phosphate pathway in the unfertilised oocyte but rather by cytosolic NADP-dependent isocitrate dehydrogenase. Remarkably, we also found that the oxidant action of pyruvate impairs development, demonstrating the fundamental importance of redox state on early development.

Depletion of reduced glutathione enhances motor neuron degeneration in vitro and in vivo

The mechanism of selective and age-dependent motor neuron degeneration in human amyotrophic lateral sclerosis (ALS) has not been defined and the role of glutathione (GSH) in association with motor neuron death remains largely unknown. A motor neuron-like cell culture system and a transgenic mouse model were used to study the effect of cellular GSH alteration on motor neuron cell death. Exposure of NSC34 motor neuron-like cells to ethacrynic acid (EA) or l-buthionine sulfoximine (BSO) dramatically reduced the cellular GSH levels, and was accompanied by increased production of reactive oxygen species (ROS) measured by the dichlorofluorescin (DCF) fluorescent oxidation assay. In addition, GSH depletion enhanced oxidative stress markers, AP-1 transcriptional activation, c-Jun, c-Fos and heme oxygenase-1 (HO-1) expression in NSC34 cells analyzed by a luciferase reporter, Western blotting and quantitative PCR assays respectively. Furthermore, depletion of GSH decreased mitochondrial function, facilitated apoptosis inducing factor (AIF) translocation, cytochrome c release, and caspase 3 activation, and consequently led to motor neuron-like cell apoptosis. In an ALS-like transgenic mouse model overexpressing mutant G93A-Cu, Zn-superoxide dismutase (SOD1) gene, we showed that the reduction of GSH in the spinal cord and motor neuron cells is correlated with AIF translocation, caspase 3 activation, and motor neuron degeneration during ALS-like disease onset and progression. Taken together, the in vitro and in vivo data presented in the current report demonstrated that decreased GSH promotes multiple apoptotic pathways contributing, at least partially, to motor neuron degeneration in ALS.

Formation and Rapid Export of the Monochlorobimane–Glutathione Conjugate in Cultured Rat Astrocytes

Monochlorobimane (MCB) is often used to visualize glutathione (GSH) levels in cultured cells, since it is quickly converted to a fluorescent GSH conjugate (GS-MCB). To test for consequences of MCB application on the GSH metabolism of astrocytes, we have studied rat astrocyte-rich primary cultures as model system. MCB caused a concentration dependent rapid decrease in the cellular GSH content. Simultaneously, a transient accumulation of GS-MCB in the cells was observed with a maximal content 5 min after MCB application. The cellular accumulation was followed by a rapid release of GS-MCB into the medium with a maximal initial export rate of 27.9 +/- 6.5 nmol h(-1) mg protein(-1). Transporters of the family of multidrug resistance proteins (Mrps) are likely to be involved in this export, since the Mrp inhibitor MK571 lowered the export rate by 60%. These data demonstrate that, due to its rapid export from astrocytes, GS-MCB is only under well-defined conditions a reliable indicator of the cellular GSH concentration and that MK571 can be used to maintain maximal GS-MCB levels in astrocytes.

Different fates of intracellular glutathione determine different modalities of apoptotic nuclear vesiculation

U937 monocytic cells show two main apoptotic nuclear morphologies, budding and cleavage, that are the result of two independent morphological routes, since they never interconvert one into the other, and are differently modulated by stressing or physiological apoptogenic agents [Exp Cell Res 1996; 223:340-347]. With the aim of understanding which biochemical alterations are at the basis of these alternative apoptotic morphologies, we performed an in situ analysis that showed that in U937 cells intracellular glutathione (GSH) is lost in cells undergoing apoptosis by cleavage, whereas it is maintained in apoptotic budding cells. Lymphoma cells BL41 lose GSH in apoptosis, and show the cleavage nuclear morphology; the same cells latently infected with Epstein Barr Virus (E2r line) undergo apoptosis without GSH depletion and show the budding nuclear morphology. GSH depletion is not only concomitant to, but is the determinant of the cleavage route, since the inhibition of apoptotic GSH efflux with cystathionine or methionine shifts the apoptotic morphology from cleavage to budding. Accordingly, cystathionine or methionine antagonizes apoptosis in the all-cleavage BL41, without affecting the all-budding E2r.

Calcium microdomains and oxidative stress

The phenomenon of calcium microdomains is firmly established in the field of subcellular physiology. These regions of localized, transient calcium increase are exemplified by the spontaneous 'sparks' released through the ryanodine receptor in myocytes, but include subplasmalemmal microdomains, focal calcium oscillations and microdomains enclosed within organelles, such as the endoplasmic reticulum, golgi and mitochondria. Increasing evidence suggests that oxidative stress regulates both the formation and disappearance of microdomains. Calcium release channels and transporters are all modulated by redox state, while several mechanisms that generate oxidative or nitrosative stress are regulated by calcium. Here, we discuss the evidence for the regulation of calcium microdomains by redox state, and, by way of example, demonstrate that the frequency of calcium sparks in cardiomyocytes is increased in response to oxidative stress. We consider the evidence for the existence of analogous microdomains of reactive oxygen and nitrogen species and suggest that the refinement of imaging techniques for these species might lead to similar concepts. The interaction between Ca(2+) microdomains and proteins that modulate their formation results in a complex and dynamic, spatial signaling mechanism, which is likely to be broadly applicable to different cell types, adding new dimensions to the calcium signaling 'toolkit'.

Diabetic neuropathy and oxidative stress

This review will focus on the impact of hyperglycemia-induced oxidative stress in the development of diabetes-related neural dysfunction. Oxidative stress occurs when the balance between the production of reactive oxygen species (ROS) and the ability of cells or tissues to detoxify the free radicals produced during metabolic activity is tilted in the favor of the former. Although hyperglycemia plays a key role in inducing oxidative stress in the diabetic nerve, the contribution of other factors, such as endoneurial hypoxia, transition metal imbalances, and hyperlipidemia have been also suggested. The possible sources for the overproduction of ROS in diabetes are widespread and include enzymatic pathways, auto-oxidation of glucose, and mitochondrial superoxide production. Increase in oxidative stress has clearly been shown to contribute to the pathology of neural and vascular dysfunction in diabetes. Potential therapies for preventing increased oxidative stress in diabetic nerve dysfunction will be discussed.

The role of an astrocytic NADPH oxidase in the neurotoxicity of amyloid beta peptides

Amyloid beta peptide (Abeta) accumulates in the CNS in Alzheimer's disease. Both the full peptide (1-42) or the 25-35 fragment are toxic to neurons in culture. We have used fluorescence imaging technology to explore the mechanism of neurotoxicity in mixed asytrocyte/neuronal cultures prepared from rat or mouse cortex or hippocampus, and have found that Abeta acts preferentially on astrocytes but causes neuronal death. Abeta causes sporadic transient increases in [Ca2+]c in astrocytes, associated with a calcium dependent increased generation of reactive oxygen species (ROS) and glutathione depletion. This caused a slow dissipation of mitochondrial potential on which abrupt calcium dependent transient depolarizations were superimposed. The mitochondrial depolarization was reversed by mitochondrial substrates glutamate, pyruvate or methyl succinate, and by NADPH oxidase (NOX) inhibitors, suggesting that it reflects oxidative damage to metabolic pathways upstream of mitochondrial complex I. The Abeta induced increase in ROS and the mitochondrial depolarization were absent in cells cultured from transgenic mice lacking the NOX component, gp91phox. Neuronal death after 24 h of Abeta exposure was dramatically reduced both by NOX inhibitors and in gp91phox knockout mice. Thus, by raising [Ca2+]c in astrocytes, Abeta activates NOX, generating oxidative stress that is transmitted to neurons, causing neuronal death.

Two-photon imaging of glutathione levels in intact brain indicates enhanced redox buffering in developing neurons and cells at the cerebrospinal fluid and blood-brain interface

Glutathione is the major cellular thiol present in mammalian cells and is critical for maintenance of redox homeostasis. However, current assay systems for glutathione lack application to intact animal tissues. To map the levels of glutathione in intact brain with cellular resolution (acute tissue slices and live animals), we have used two-photon imaging of monochlorobimane fluorescence, a selective enzyme-mediated marker for reduced glutathione. Previously, in vitro experiments using purified components and cultured glial cells attributed cellular monochlorobimane fluorescence to a glutathione S-transferase-dependent reaction with GSH. Our results indicate that cells at the cerebrospinal fluid or blood-brain interface, such as lateral ventricle ependymal cells (2.73 +/- 0.56 mm; glutathione), meningeal cells (1.45 +/- 0.09 mm), and astroglia (0.91 +/- 0.08 mm), contain high levels of glutathione. In comparison, layer II cortical neurons contained 20% (0.21 +/- 0.02 mm) the glutathione content of nearby astrocytes. Neuronal glutathione labeling increased 250% by the addition of the cell-permeable glutathione precursor N-acetylcysteine indicating that the monochlorobimane level or glutathione S-transferase activity within neurons was not limiting. Regional mapping showed that glutathione was highest in cells lining the lateral ventricles, specifically ependymal cells and the subventricular zone, suggesting a possible function for glutathione in oxidant homeostasis of developing neuronal progenitors. Consistently, developing neurons in the subgranular zone of dentate gyrus contained 3-fold more glutathione than older neurons found in the neighboring granular layer. In conclusion, mapping of glutathione levels in intact brain demonstrates a unique role for enhanced redox potential in developing neurons and cells at the cerebrospinal fluid and blood-brain interface.

Using nile red-adsorbed gold nanoparticles to locate glutathione within erythrocytes

An aqueous solution of Nile Red (NR)-absorbed 32-nm gold nanoparticles (AuNPs) have been used to sense glutathione (GSH). When the NR product is displaced by GSH on the AuNP surface, the fluorescence of the solution increases and the AuNPs aggregate. To determine the concentration and distribution of GSH within erythrocyte cells, a homemade fluorescence and scattering microscope was constructed. This system allows monitoring, within individual cells, of the uptake and transportation of the NRAuNPs and the displacement of the NR product from the NRAuNP surface by GSH. The fluorescence and scattering images clearly indicate the location of GSH inside the cells; these findings are supported by images recorded using 2,3-naphthalenedicarboxaldehyde, which is a highly selective fluorogenic reagent for GSH. Microscopic fluorescence measurements of the NRAuNPs revealed that the GSH concentration inside erythrocyte cells is 1.30 +/- 0.31 mM. To confirm this result, lysed erythrocyte cells were analyzed by applying capillary electrophoresis in conjunction with laser-induced fluorescence using NRAuNPs; accordingly, the average GSH concentration in a single erythrocyte cell was determined to be 1.32 +/- 0.06 mM.

Acute glutathione depletion restricts mitochondrial ATP export in cerebellar granule neurons

Decreases in GSH pools detected during ischemia sensitize neurons to excitotoxic damage. Thermodynamic analysis predicts that partial GSH depletion will cause an oxidative shift in the thiol redox potential. To investigate the acute bioenergetic consequences, neurons were exposed to monochlorobimane (mBCl), which depletes GSH by forming a fluorescent conjugate. Neurons transfected with redox-sensitive green fluorescent protein showed a positive shift in thiol redox potential synchronous with the formation of the conjugate. Mitochondria within neurons treated with mBCl for 1 h failed to hyperpolarize upon addition of oligomycin to inhibit their ATP synthesis. A decreased ATP turnover was confirmed by monitoring neuronal oxygen consumption in parallel with mitochondrial membrane potential (Deltapsi(m)) and GSH-mBCl formation. mBCl progressively decreased cell respiration, with no effect on mitochondrial proton leak or maximal respiratory capacity, suggesting adequate glycolysis and a functional electron transport chain. This approach to "state 4" could be mimicked by the adenine nucleotide translocator inhibitor bongkrekic acid, which did not further decrease respiration when administered after mBCl. The cellular ATP/ADP ratio was decreased by mBCl, and consistent with mitochondrial ATP export failure, respiration could not respond to an increased cytoplasmic ATP demand by plasma membrane Na(+) cycling; instead, mitochondria depolarized. More prolonged mBCl exposure induced mitochondrial failure, with Deltapsi(m) collapse followed by cytoplasmic Ca(2+) deregulation. The initial bioenergetic consequence of neuronal GSH depletion in this model is thus an inhibition of ATP export, which precedes other forms of mitochondrial dysfunction.

Mitochondria in health and disease: perspectives on a new mitochondrial biology

The integrity of mitochondrial function is fundamental to cell life. It follows that disturbances of mitochondrial function will lead to disruption of cell function, expressed as disease or even death. In this review, I consider recent developments in our knowledge of basic aspects of mitochondrial biology as an essential step in developing our understanding of the contributions of mitochondria to disease. The identification of novel mechanisms that govern mitochondrial biogenesis and replication, and the delicately poised signalling pathways that coordinate the mitochondrial and nuclear genomes are discussed. As fluorescence imaging has made the study of mitochondrial function within cells accessible, the application of that technology to the exploration of mitochondrial bioenergetics is reviewed. Mitochondrial calcium uptake plays a major role in influencing cell signalling and in the regulation of mitochondrial function, while excessive mitochondrial calcium accumulation has been extensively implicated in disease. Mitochondria are major producers of free radical species, possibly also of nitric oxide, and are also major targets of oxidative damage. Mechanisms of mitochondrial radical generation, targets of oxidative injury and the potential role of uncoupling proteins as regulators of radical generation are discussed. The role of mitochondria in apoptotic and necrotic cell death is seminal and is briefly reviewed. This background leads to a discussion of ways in which these processes combine to cause illness in the neurodegenerative diseases and in cardiac reperfusion injury. The demands of mitochondria and their complex integration into cell biology extends far beyond the provision of ATP, prompting a radical change in our perception of mitochondria and placing these organelles centre stage in many aspects of cell biology and medicine.

Calcium signals induced by amyloid β peptide and their consequences in neurons and astrocytes in culture

In Alzheimer's disease, amyloid beta (Abeta) peptide is deposited in neuritic plaques in the brain. The Abeta peptide 1-42 or the fragment 25-35 are neurotoxic. We here review our recent explorations of the mechanisms of Abeta toxicity in hippocampal cultures. Abeta had no effect on intracellular calcium in neurons but caused striking changes in nearby astrocytes. The [Ca(2+)](c) signals started approximately 5-15 min after Abeta application and consisted of sporadic [Ca(2+)](c) pulses. These were entirely dependent on extracellular Ca(2+), independent of ER Ca(2+) stores and resulted from Ca(2+) influx, probably through Abeta-induced membrane channels. The Ca(2+) signals were closely associated with transient, episodic acidification which may reflect displacement of protons from binding sites or Ca(2+)/2H(+) exchange. Abeta caused an increased rate of generation of reactive oxygen species (ROS), also seen in astrocytes and not in neurons. The increased ROS generation was blocked by inhibitors of the NADPH oxidase, strongly suggesting that this enzyme, normally associated with immune cells, is expressed in astrocytes. ROS generation was also Ca(2+)-dependent, suggesting that Abeta activation of the enzyme may be secondary to the increase in [Ca(2+)](c). Abeta caused delayed neuronal death despite the fact that all responses were seen only in astrocytes. Neurons could not be protected by glutamate receptor antagonists, but were rescued by inhibition of the NADPH oxidase, by antioxidants and by increasing glutathione. These data suggest that Abeta causes Ca(2+)-dependent oxidative stress by activating an astrocytic NADPH oxidase, and that neuronal death follows through a failure of antioxidant support.

Characterization of a range of fura dyes with two-photon excitation

Two-photon excitation (TPE) spectra of Fura-2, -4F, -6F, -FF, and Furaptra were characterized using a tunable (750-850 nM) ultra-short pulse laser. Two-photon fluorescence of these dyes was studied in free solution and in the cytosol of isolated rabbit ventricular cardiomyocytes. The TPE spectra of the Ca(2+)-free and Ca(2+)-bound forms of the dyes were measured in free solution and expressed in terms of the two-photon fluorescence cross section (Goppert-Meyer units). The Fura dyes displayed the same Ca(2+)-free TPE spectrum in the intracellular volume of permeabilized and intact cardiomyocytes. Fluorescence measurements over a range of laser powers confirmed the TPE of both Ca(2+)-free and Ca(2+)-bound forms of the dyes. Single-wavelength excitation at 810 nM was used to determine the effective dissociation constants (K(eff)) and dynamic ranges (R(f)) of Fura-2, -4F, -6F, -FF, and Furaptra dyes (K(eff) = 181 +/- 52 nM, 1.16 +/- 0.016 micro M, 5.18 +/- 0.3 micro M, 19.2 +/- 1 micro M, and 58.5 +/- 2 micro M; and R(f) = 22.4 +/- 3.8, 12.2 +/- 0.34, 6.3 +/- 0.17, 16.1 +/- 2.8, and 25.4 +/- 4, respectively). Single-wavelength excitation of intracellular Fura-4F resolved diastolic and peak [Ca(2+)] in isolated stimulated cardiomyocytes after calibration of the intracellular signal using reversible exposure to low (100 micro M) extracellular [Ca(2+)]. Furthermore, TPE of Fura-4F allowed continuous, long-term (5-10 min) Ca(2+) imaging in ventricular cardiomyocytes using laser-scanning microscopy without significant cellular photodamage or photobleaching of the dye.

Development of a microscale cell culture analog to probe naphthalene toxicity

Prediction of human response to drugs or chemicals is difficult as a result of the complexity of living organisms. We describe an in vitro model that can realistically and inexpensively study the adsorption, distribution, metabolism, elimination, and potential toxicity (ADMET) of chemicals. A microscale cell culture analog (microCCA) is a physical replica of the physiologically based pharmacokinetics (PBPK) model. Such a microfabricated device consists of a fluidic network of channels to mimic the circulatory system and chambers containing cultured mammalian cells representing key functions of animal "organ" systems. This paper describes the application of a two-cell system, four-chamber microCCA ("lung"-"liver"-"other tissue"-"fat") device for proof-of-concept study using naphthalene as a model toxicant. Naphthalene is converted into reactive metabolites (i.e., 1,2-naphthalenediol and 1,2-naphthoquinone) in the "liver" compartment, which then circulate to the "lung" depleting glutathione (GSH) in lung cells. Such microfabricated in vitro devices are potential human surrogates for testing chemicals and pharmaceutics for toxicity and efficacy.

Oxidative stress in the pathogenesis of diabetic neuropathy

Oxidative stress results from a cell or tissue failing to detoxify the free radicals that are produced during metabolic activity. Diabetes is characterized by chronic hyperglycemia that produces dysregulation of cellular metabolism. This review explores the concept that diabetes overloads glucose metabolic pathways, resulting in excess free radical production and oxidative stress. Evidence is presented to support the idea that both chronic and acute hyperglycemia cause oxidative stress in the peripheral nervous system that can promote the development of diabetic neuropathy. Proteins that are damaged by oxidative stress have decreased biological activity leading to loss of energy metabolism, cell signaling, transport, and, ultimately, to cell death. Examination of the data from animal and cell culture models of diabetes, as well as clinical trials of antioxidants, strongly implicates hyperglycemia-induced oxidative stress in diabetic neuropathy. We conclude that striving for superior antioxidative therapies remains essential for the prevention of neuropathy in diabetic patients.

Toxicity of amyloid beta peptide: tales of calcium, mitochondria, and oxidative stress

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-beta (Abeta) peptides. Although the disease undoubtedly reflects the interaction of complex multifactorial processes, Abeta itself is toxic to neurons in vitro and the load of Abeta in vivo correlates well with the degree of cognitive impairment. There has therefore been considerable interest in the mechanism(s) of Abeta neurotoxicity. We here review the basic biology of Abeta processing and consider some of the major areas of focus of this research. It is clear that both AD and Abeta toxicity are characterized by oxidative stress, alterations in the activity of enzymes of intermediary metabolism, and mitochondrial dysfunction, especially impaired activity of cytochrome c oxidase. Studies in vitro also show alterations in cellular calcium signaling. We consider the mechanisms proposed to mediate cell injury and explore evidence to indicate which of these many changes in function are primary and which secondary.

Development of a wound dressing targeting neutrophil function

Earlier studies show that neutrophils are virtually unable to kill Staphylococcus aureus in vitro. However, upon addition of 10 mM N-acetylcysteine (NAC) or reduced glutathione (GSH) the neutrophil bacterial killing ability becomes excellent. We want to exploit this phenomenon to develop a wound dressing material that will improve neutrophil function. To study the mechanisms behind the downregulation of neutrophil elimination of bacteria, we used different markers for neutrophil function on surface-adhering neutrophils in contact with S. aureus with or without addition of the antioxidants NAC or GSH. Analysis by scanning electron microscopy showed cell shrinkage and numerous cytoplasmic processes on surface-adhering neutrophils exposed to S. aureus. In cells exposed to S. aureus and GSH, the cells were of normal size and the cytoplasm was spread as in normal attachment. Staining for intracellular GSH, a hallmark of oxidative stress, showed little difference between the experimental groups, indicating that the cells were not damaged by traditional oxidative stress. The H(2)O(2) production of neutrophils, measured by Amplex red, was correlated to bacterial exposure and was not affected by the addition of scavengers. The intracellular and extracellular production of ROS was measured by luminol-amplified chemiluminescence. The apparent ROS-production was mostly intracellular and decreased in the presence of scavengers. However, extracellular production of ROS was not affected by the addition of NAC. The production of nitric oxide (NO) was measured spectrophotometrically as the production of nitrate apparently decreased in the presence of scavengers, probably as a result of interference with the reagents in the test system. In conclusion, differences between leukocytes that were able to eliminate S. aureus and those that were not were mainly seen in the morphology of the cells and in cell viability. The morphological findings point to a difference in NO signaling in the absence and presence of ROS scavengers.

Beta-amyloid peptides induce mitochondrial dysfunction and oxidative stress in astrocytes and death of neurons through activation of NADPH oxidase

Beta-amyloid (betaA) peptide is strongly implicated in the neurodegeneration underlying Alzheimer's disease, but the mechanisms of neurotoxicity remain controversial. This study establishes a central role for oxidative stress by the activation of NADPH oxidase in astrocytes as the cause of betaA-induced neuronal death. betaA causes a loss of mitochondrial potential in astrocytes but not in neurons. The mitochondrial response consists of Ca2+-dependent transient depolarizations superimposed on a slow collapse of potential. The slow response is both prevented by antioxidants and, remarkably, reversed by provision of glutamate and other mitochondrial substrates to complexes I and II. These findings suggest that the depolarization reflects oxidative damage to metabolic pathways upstream of mitochondrial respiration. Inhibition of NADPH oxidase by diphenylene iodonium or 4-hydroxy-3-methoxy-acetophenone blocks betaA-induced reactive oxygen species generation, prevents the mitochondrial depolarization, prevents betaA-induced glutathione depletion in both neurons and astrocytes, and protects neurons from cell death, placing the astrocyte NADPH oxidase as a primary target of betaA-induced neurodegeneration.

Changes in intracellular calcium and glutathione in astrocytes as the primary mechanism of amyloid neurotoxicity

Although the accumulation of the neurotoxic peptide beta amyloid (betaA) in the CNS is a hallmark of Alzheimer's disease, the mechanism of betaA neurotoxicity remains controversial. In cultures of mixed neurons and astrocytes, we found that both the full-length peptide betaA (1-42) and the neurotoxic fragment (25-35) caused sporadic cytoplasmic calcium [intracellular calcium ([Ca2+]c)] signals in astrocytes that continued for hours, whereas adjacent neurons were completely unaffected. Nevertheless, after 24 hr, although astrocyte cell death was marginally increased, approximately 50% of the neurons had died. The [Ca2+]c signal was entirely dependent on Ca2+ influx and was blocked by zinc and by clioquinol, a heavy-metal chelator that is neuroprotective in models of Alzheimer's disease. Neuronal death was associated with Ca2+-dependent glutathione depletion in both astrocytes and neurons. Thus, astrocytes appear to be the primary target of betaA, whereas the neurotoxicity reflects the neuronal dependence on astrocytes for antioxidant support.

Glutathione pathways in the brain

The antioxidant glutathione (GSH) is essential for the cellular detoxification of reactive oxygen species in brain cells. A compromised GSH system in the brain has been connected with the oxidative stress occuring in neurological diseases. Recent data demonstrate that besides intracellular functions GSH has also important extracellular functions in brain. In this respect astrocytes appear to play a key role in the GSH metabolism of the brain, since astroglial GSH export is essential for providing GSH precursors to neurons. Of the different brain cell types studied in vitro only astrocytes release substantial amounts of GSH. In addition, during oxidative stress astrocytes efficiently export glutathione disulfide (GSSG). The multidrug resistance protein 1 participates in both the export of GSH and GSSG from astrocytes. This review focuses on recent results on the export of GSH and GSSG from brain cells as well as on the functions of extracellular GSH in the brain. In addition, implications of disturbed GSH pathways in brain for neurodegenerative diseases will be discussed.

Analytical methods to investigate glutathione and related compounds in biological and pathological processes

Reduced glutathione (GSH, gamma-L-glutamyl-L-cysteinylglycine) is a fundamental low-molecular mass antioxidant that serves several biological functions. Upon enzymatic and non-enzymatic oxidation, GSH forms glutathione disulfide (GSSG) and, under particular conditions, may generate other oxidative products. The determination of GSH, its precursors, and metabolites in several bio-matrices is a useful tool in studying oxidative stress. Many separative and non-separative methods have been developed and improved for the assay of GSH and related compounds. At present, high-performance liquid chromatography and capillary electrophoresis are the most used separative techniques to determine GSH and congeners. The review will deal with analytical methods developed over the last few years for the determination of GSH and related compounds, and with the procedures performed in sample pre-treatment in order to minimize analytical errors. Since GSH, GSSG, and related compounds lack of strong chromophores or fluorophores, it is advantageous, in many assays, to derivatize the compounds in order to improve the detection limit with UV-Vis and to allow fluorescence, thus the most commonly used labeling agents are also described.

Glutathione release from cultured brain cells: multidrug resistance protein 1 mediates the release of GSH from rat astroglial cells

To investigate the release of glutathione (GSH) from brain cells, cultures enriched for astroglial cells, neurons, oligodendroglial cells, and microglial cells derived from rat brain were studied. During incubation of astroglial cultures, GSH accumulated in the medium with a rate of 3.1 +/- 0.6 nmol x h(-1) x mg protein(-1). In contrast, only marginal amounts of extracellular GSH were detectable in the media of the other brain cell cultures investigated. The mechanism of GSH release from astroglial cells, as yet, has not been reported. Multidrug resistance protein 1 (Mrp1), a transport protein known to mediate cellular export of glutathione disulfide and glutathione conjugates, is expressed in astroglial cultures. Inhibitors of Mrp1 were used to test for a function of this transporter in mediating GSH release from astroglial cells. The presence of the competitive Mrp1 inhibitor MK571 at a concentration of 50 microM inhibited the rate of GSH release by 63%. In contrast, the low concentration of 1 microM of MK571 increased the rate of GSH release by 83%. This bimodal concentration-dependent effect of MK571 is in accord with literature data for the effects of Mrp1 substrates on GSH release from cells. In addition, the presence of cyclosporin A (10 microM) reduced the GSH release rate significantly and completely blocked the stimulating effect of 1 microM MK571 on the release of GSH from astroglial cells. In conclusion, the data presented are a strong indication that Mrp1 participates in the release of GSH from astroglial cells.