Manganese decreases glutamate uptake in cultured astrocytes

Stem Cell Therapy and Thiamine Deficiency-Induced Brain Damage

Wernicke's encephalopathy (WE) is a major central nervous system disorder resulting from thiamine deficiency (TD) in which a number of brain regions can develop serious damage including the thalamus and inferior colliculus. Despite decades of research into the pathophysiology of TD and potential therapeutic interventions, little progress has been made regarding effective treatment following the development of brain lesions and its associated cognitive issues. Recent developments in our understanding of stem cells suggest they are capable of repairing damage and improving function in different maladys. This article puts forward the case for the potential use of stem cell treatment as a therapeutic strategy in WE by first examining the effects of TD on brain functional integrity and its consequences. The second half of the paper will address the future benefits of treating TD with these cells by focusing on their nature and their potential to effectively treat neurodegenerative diseases that share some overlapping pathophysiological features with TD. At the same time, some of the obstacles these cells will have to overcome in order to become a viable therapeutic strategy for treating this potentially life-threatening illness in humans will be highlighted.

Acute manganese exposure impairs glutamatergic function in a young mouse model of Alzheimer's disease

Manganese (Mn) is an essential metal that serves as a cofactor for metalloenzymes important in moderating oxidative stress and the glutamate/glutamine cycle. Mn is typically obtained through the diet, but toxic overexposure can occur through other environmental or occupational exposure routes such as inhalation. Mn is known to accumulate in the brain following exposure and may contribute to the etiology of neurodegenerative disorders such as Alzheimer's disease (AD) even in the absence of acute neurotoxicity. In the present study, we used in vitro primary cell culture, ex vivo slice electrophysiology and in vivo behavioral approaches to determine if Mn-induced changes in glutamatergic signaling may be altered by genetic risk factors for AD neuropathology. Primary cortical astrocytes incubated with Mn exhibited early rapid clearance of glutamate compared to saline treated astrocytes but decreased clearance over longer time periods, with no effect of the AD genotype. Further, we found that in vivo exposure to a subcutaneous subacute, high dose of Mn as manganese chloride tetrahydrate (3 ×50 mg/kg MnCl2·4(H2O) over 7 days) resulted in increased expression of cortical GLAST protein regardless of genotype, with no changes in GLT-1. Hippocampal long-term potentiation was not altered in APP/PSEN1 mice at this age and neither was it disrupted following Mn exposure. Mn exposure did increase sensitivity to seizure onset following treatment with the excitatory agonist kainic acid, with differing responses between APP/PSEN1 and control mice. These results highlight the sensitivity of the glutamatergic system to Mn exposure. Experiments were performed in young adult APP/PSEN1 mice, prior to cognitive decline or accumulation of hallmark amyloid plaque pathology and following subacute exposure to Mn. The data support a role of Mn in pathophysiology of AD in early stages of the disease and support the need to better understand neurological consequences of Mn exposure in vulnerable populations.

Mechanisms of manganese-induced neurotoxicity and the pursuit of neurotherapeutic strategies

Chronic exposure to elevated levels of manganese via occupational or environmental settings causes a neurological disorder known as manganism, resembling the symptoms of Parkinson's disease, such as motor deficits and cognitive impairment. Numerous studies have been conducted to characterize manganese's neurotoxicity mechanisms in search of effective therapeutics, including natural and synthetic compounds to treat manganese toxicity. Several potential molecular targets of manganese toxicity at the epigenetic and transcriptional levels have been identified recently, which may contribute to develop more precise and effective gene therapies. This review updates findings on manganese-induced neurotoxicity mechanisms on intracellular insults such as oxidative stress, inflammation, excitotoxicity, and mitophagy, as well as transcriptional dysregulations involving Yin Yang 1, RE1-silencing transcription factor, transcription factor EB, and nuclear factor erythroid 2-related factor 2 that could be targets of manganese neurotoxicity therapies. This review also features intracellular proteins such as PTEN-inducible kinase 1, parkin, sirtuins, leucine-rich repeat kinase 2, and α-synuclein, which are associated with manganese-induced dysregulation of autophagy/mitophagy. In addition, newer therapeutic approaches to treat manganese's neurotoxicity including natural and synthetic compounds modulating excitotoxicity, autophagy, and mitophagy, were reviewed. Taken together, in-depth mechanistic knowledge accompanied by advances in gene and drug delivery strategies will make significant progress in the development of reliable therapeutic interventions against manganese-induced neurotoxicity.

Critical Involvement of Glial Cells in Manganese Neurotoxicity

Over the years, most of the research concerning manganese exposure was restricted to the toxicity of neuronal cells. Manganese is an essential trace element that in high doses exerts neurotoxic effects. However, in the last two decades, efforts have shifted toward a more comprehensive approach that takes into account the involvement of glial cells in the development of neurotoxicity as a brain insult. Glial cells provide structural, trophic, and metabolic support to neurons. Nevertheless, these cells play an active role in adult neurogenesis, regulation of synaptogenesis, and synaptic plasticity. Disturbances in glial cell function can lead to neurological disorders, including neurodegenerative diseases. This review highlights the pivotal role that glial cells have in manganese-induced neurotoxicity as well as the most sounding mechanisms involved in the development of this phenomenon.

Interaction of Manganese and Ammonia in the Brain of Hepatic Encephalopathy Rats

Background: Both ammonia and manganese (Mn) play a key role in the pathogenesis of hepatic encephalopathy (HE) and cause similar morphological and functional changes in astrocytes. Objectives: To investigate the interaction between brain Mn and ammonia in HE rats. Methods: Three rat models of minimal HE (MHE), chronic manganism (CHM), and chronic hyperammonemia (CHA) were constructed. A total of 48 Sprague-Dawley rats were divided into one control group (n = 6), MHE groups (n = 18, among which six rats were used to evaluate the MHE model), CHM groups (n = 12), and CHA groups (n = 12). The CHM, CHA, and the rest of MHE rats were randomly divided further into two subgroups, according to the MgSO4 treatment (oral administration of 496 mg/kg/day for seven weeks): MHE-7W and MHE + Mg-7W; CHM-7W and CHM + Mg-7W; and CHA-7W and CHA + Mg-7W, respectively. Rats’ blood ammonia, brain Mn, glutamine synthetase (GS), and glutamine (GLN) levels were measured and compared among groups. Results: Significantly higher brain Mn content in MHE-7W and CHM-7W rats, higher blood ammonia levels, brain GS activity, and GLN content were observed in MHE-7W, CHM-7W, and CHA-7W rats than in control rats. After MgSO4 treatment for seven weeks, significantly lower brain Mn content, blood ammonia levels, and GLN content were observed in MHE, CHM, and CHA rats. Conclusions: Our study showed that brain Mn accumulation could increase brain ammonia levels, while the accumulation of brain ammonia had no effect on the content of brain Mn.

Fluoxetine and Riluzole Mitigates Manganese-Induced Disruption of Glutamate Transporters and Excitotoxicity via Ephrin-A3/GLAST-GLT-1/Glu Signaling Pathway in Striatum of Mice

Manganese (Mn) is an essential element required for many biological processes and systems in the human body. Mn intoxication increases brain glutamate (Glu) levels causing neuronal damage. Recent studies have reported that ephrin-A3 regulates this glutamate transporter. However, none has explored the role of this crucial molecule in Mn-induced excitotoxicity. The present study investigated whether ephrin-A3/GLAST-GLT-1/Glu signaling pathway participates in Mn-induced excitotoxicity using astrocytes and Kunming mice. The mechanisms were explored using fluoxetine (ephrin-A3 inhibitor) and riluzole (a Glu release inhibitor). Firstly, we demonstrated that Mn exposure (500 μM or 50 mg/kg MnCl₂) significantly increased Mn, ephrin-A3, and Glu levels, and inhibited Na⁺-K⁺ ATPase activity, as well as mRNA and protein levels of GLAST and GLT-1. Secondly, we found that astrocytes and mice pretreated with fluoxetine (100 μM or 15 mg/kg) and riluzole (100 μM or 32 μmol/kg) prior to Mn exposure had lower ephrin-A3 and Glu levels, but higher Na⁺-K⁺ ATPase activity, expression levels of GLAST and GLT-1 than those exposed to 500 μM or 50 mg/kg MnCl₂. Moreover, the morphology of cells and the histomorphology of mice striatum were injured. Results showed that pretreatment with fluoxetine and riluzole attenuated the Mn-induced motor dysfunctions. Together, these results suggest that the ephrin-A3/GLAST-GLT-1/Glu signaling pathway participates in Mn-induced excitotoxicity, and fluoxetine and riluzole can mitigate the Mn-induced excitotoxicity in mice brain.

Implications of Manganese in Chronic Acquired Hepatocerebral Degeneration

Neurological symptoms can be one of the over-riding symptoms in patients with liver cirrhosis. Patients can present with subtle changes in mood or neurological function due to hepatic encephalopathy (HE), to more severe presentations including stupor and coma. While HE, in its severe form, can be clinically easy to diagnose, more subtle forms may be more difficult to recognize. Other neurological diseases may indeed be overlooked in the context of cirrhosis or confuse the physician regarding the diagnosis. Chronic acquired hepatocerebral degeneration (CAHD) is an uncommon problem occurring in patients with cirrhosis characterised by a Parkinsonian-like neurological presentation with damage to the brain secondary to manganese (Mn) deposition. Here we describe a case of a patient with a neurological presentation of liver disease with a review of the current CAHD literature. In conclusion, CAHD is a rare condition occurring in liver cirrhosis that should always be considered in patients with neurological manifestations of chronic liver disease.

Arundic Acid Increases Expression and Function of Astrocytic Glutamate Transporter EAAT1 Via the ERK, Akt, and NF-κB Pathways

Glutamate is the major excitatory neurotransmitter in the brain, but excessive synaptic glutamate must be removed to prevent excitotoxic injury and death. Two astrocytic glutamate transporters, excitatory amino acid transporter (EAAT) 1 and 2, play a major role in eliminating excess glutamate from the synapse. Dysregulation of EAAT1 contributes to the pathogenesis of multiple neurological disorders, such as Alzheimer's disease (AD), ataxia, traumatic brain injuries, and glaucoma. In the present study, we investigated the effect of arundic acid on EAAT1 to determine its efficacy in enhancing the expression and function of EAAT1, and its possible mechanisms of action. The studies were carried out in human astrocyte H4 cells as well as in human primary astrocytes. Our findings show that arundic acid upregulated EAAT1 expression at the transcriptional level by activating nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Arundic acid increased astrocytic EAAT1 promoter activity, messenger RNA (mRNA)/protein levels, and glutamate uptake, while pharmacological inhibition of NF-κB or mutation on NF-κB binding sites in the EAAT1 promoter region abrogated these effects. Arundic acid increased NF-κB reporter activity and induced NF-κB nuclear translocation as well as its bindings to the EAAT1 promoter. Furthermore, arundic acid activated the Akt and ERK signaling pathways to enhance EAAT1 mRNA/protein levels. Finally, arundic acid attenuated manganese-induced decrease in EAAT1 expression by inhibiting expression of the transcription factor Ying Yang 1 (YY1). These results demonstrate that arundic acid increases the expression and function of EAAT1 via the Akt, ERK, and NF-κB signaling pathways, and reverses Mn-induced EAAT1 repression by inhibiting the Mn-induced YY1 activation.

Manganese Control of Glutamate Transporters' Gene Expression

Manganese (Mn) is an essential trace element, serving as a cofactor for several enzymes involved in various cellular and biochemical reactions in human body. However, chronic overexposure to Mn from occupational or environmental sources induces a neurological disorder, characterized by psychiatric, cognitive, and motor abnormalities, referred to as manganism. Mn-induced neurotoxicity is known to target astrocytes since these cells preferentially accumulate Mn. Astrocytes are the most abundant non-neuronal glial cells in the brain, and they play a critical role in maintaining the optimal glutamate levels to prevent excitotoxic death. The fine regulation of glutamate in the brain is accomplished by two major glutamate transporters - glutamate transporter-1 (GLT-1) and glutamate aspartate transporter (GLAST) that are predominantly expressed in astrocytes. Excitotoxic neuronal injury has been demonstrated as a critical mechanism involved in Mn neurotoxicity and implicated in the pathological signs of multiple neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. Recent evidences also establish that Mn directly deregulates the expression and function of both astrocytic glutamate transporters by decreasing mRNA and protein levels of GLT-1 and GLAST. Herein, we will review the mechanisms of Mn-induced gene regulation of glutamate transporters at the transcriptional level and their role in Mn toxicity.

Pathogenesis of hepatic encephalopathy in cirrhosis: the concept of synergism revisited

The concept of synergistic mechanisms as the pathophysiologic basis of hepatic encephalopathy started with the pioneering work of Les Zieve in Minneapolis some 60 years ago where synergistic actions of the liver-derived toxins ammonia, methanethiol, and octanoic acid were described. More recently, synergistic actions of ammonia and manganese, a toxic metal that is normally eliminated via the hepatobiliary route and shown to accumulate in brain in liver failure, on the glutamatergic neurotransmitter system were described. The current upsurge of interest in brain inflammation (neuroinflammation) in relation to the CNS complications of liver failure has added a third dimension to the synergy debate. The combined actions of ammonia, manganese and pro-inflammatory cytokines in brain in liver failure result in oxidative/nitrosative stress resulting from activation of glutamate (NMDA) receptors and consequent nitration of key brain proteins. One such protein, glutamine synthetase, the sole enzyme responsible for brain ammonia removal is nitrated and inactivated in brain in liver failure. Consequently, brain ammonia levels increase disproportionately resulting in alterations of brain excitability, impaired brain energy metabolism, encephalopathy and brain swelling. Experimental therapeutic approaches for which proof-of-principle has been established include the NMDA receptor antagonist memantine, N-acetyl cysteine (recently shown to have antioxidant properties at both hepatic and cerebral levels) and probiotics.

Size-Dependent Toxicity Differences of Intratracheally Instilled Manganese Oxide Nanoparticles: Conclusions of a Subacute Animal Experiment

Incomplete information on toxicological differences of micro- and nanometer-sized particles raised concerns about the effects of the latter on health and environment. Besides chemical composition, size and surface-to-volume ratio of nanoparticles (NPs) can affect toxicity. To investigate size-dependent toxicity differences, we used particles made of dioxide of the neurotoxic heavy metal manganese (Mn), typically found in inhaled metal fumes, in three size ranges (size A, 9.14 ± 1.98 nm; size B, 42.36 ± 8.06 nm; size C, 118.31 ± 25.37 nm). For modeling the most frequent route of exposure to Mn, NPs were given to rats for 6 weeks by intratracheal instillation. Of each NP size, 3 or 6 mg/kg body weight was given while control animals were vehicle treated. Neurotoxicity was assessed by measuring spontaneous locomotor activity in an open field and by recording spontaneous and evoked electrical activity from the somatosensory cortical area. Mn content of brain, lung, and blood, measured by ICP-MS, were correlated to the observed functional alterations to see the relationship between Mn load and toxic effects. Body weight gain and organ weights were measured as general toxicological indices. The toxicity of size A and size B NPs proved to be stronger compared to size C NPs, seen most clearly in decreased body weight gain and altered spontaneous cortical activity, which were also well correlated to the internal Mn dose. Our results showed strong effect of size on NP toxicity, thus, beyond inappropriateness of toxicity data of micrometer-sized particles in evaluation of NP exposure, differentiation within the nano range may be necessary.

Neuronal Cell Death Induced by Mechanical Percussion Trauma in Cultured Neurons is not Preceded by Alterations in Glucose, Lactate and Glutamine Metabolism

Traumatic brain injury (TBI) is a devastating neurological disorder that usually presents in acute and chronic forms. Brain edema and associated increased intracranial pressure in the early phase following TBI are major consequences of acute trauma. On the other hand, neuronal injury, leading to neurobehavioral and cognitive impairments, that usually develop months to years after single or repetitive episodes of head trauma, are major consequences of chronic TBI. The molecular mechanisms responsible for TBI-induced injury, however, are unclear. Recent studies have suggested that early mitochondrial dysfunction and subsequent energy failure play a role in the pathogenesis of TBI. We therefore examined whether oxidative metabolism of (13)C-labeled glucose, lactate or glutamine is altered early following in vitro mechanical percussion-induced trauma (5 atm) to neurons (4-24 h), and whether such events contribute to the development of neuronal injury. Cell viability was assayed using the release of the cytoplasmic enzyme lactate dehydrogenase (LDH), together with fluorescence-based cell staining (calcein and ethidium homodimer-1 for live and dead cells, respectively). Trauma had no effect on the LDH release in neurons from 1 to 18 h. However, a significant increase in LDH release was detected at 24 h after trauma. Similar findings were identified when traumatized neurons were stained with fluorescent markers. Additionally (13)C-labeling of glutamate showed a small, but statistically significant decrease at 14 h after trauma. However, trauma had no effect on the cycling ratio of the TCA cycle at any time-period examined. These findings indicate that trauma does not cause a disturbance in oxidative metabolism of any of the substrates used for neurons. Accordingly, such metabolic disturbance does not appear to contribute to the neuronal death in the early stages following trauma.

Protective effect of edaravone against manganese-induced toxicity in cultured rat astrocytes

Manganese (Mn), a trace metal, is essential for maintaining the normal regulation of many biochemical and cellular processes. However, accumulation of Mn due to excessive environmental exposure leads to neurological impairment, referred to as manganism. Edaravone (EDA) is a potent free radical scavenger that has been clinically shown to reduce the neuronal injury after cerebral ischemia. In the present study, we aimed to examine the protective effects of EDA against Mn toxicity in astrocyte cultures. Astrocyte cultures were prepared from cerebral cortices of newborn Sprague-Dawley rats. The experiments were performed between 16 and 18 days of cultures. Astrocytes were treated in DMEM medium containing Mn (1-1000μM) for 24h to test Mn toxicity. In order to assess the effect of EDA, cells were pre-treated with different doses of EDA (10, 100 and 1000μM) 6h before Mn treatment. Cell viability (MTT), apoptotic cell death (Hoechst test) and lipid peroxide levels were evaluated in cultures. Our results showed that Mn significantly and dose-dependently reduced cell viability in astrocyte cultures. The apoptotic cell death and lipid peroxides were significantly higher in Mn treated cultures. Treatment of astrocytes with EDA successfully suppressed oxidative stress and cell death due to Mn exposure. The findings of the present study suggest that Mn cytotoxicity is mainly associated with ROS generation and apoptotic cell death. Besides, EDA may have beneficial effects against Mn toxicity. However, further studies are needed to elucidate the molecular mechanisms underlying protective effect of EDA.

Differential interaction of hGDH1 and hGDH2 with manganese: Implications for metabolism and toxicity

Manganese (Mn) is an essential trace element that serves as co-factor for many important mammalian enzymes. In humans, the importance of this cation is highlighted by the fact that low levels of Mn cause developmental and metabolic abnormalities and, on the other hand, chronic exposure to excessive amounts of Mn is characterized by neurotoxicity, possibly mediated by perturbation of astrocytic mitochondrial energy metabolism. Here we sought to study the effect of Mn on the two human glutamate dehydrogenases (hGDH1 and hGDH2, respectively), key mitochondrial enzymes involved in numerous cellular processes, including mitochondrial metabolism, glutamate homeostasis and neurotransmission, and cell signaling. Our studies showed that, compared to magnesium (Mg) and calcium (Ca), Mn exerted a significant inhibitory effect on both human isoenzymes with hGDH2 being more sensitive than hGDH1, especially under conditions of low ADP levels. Specifically, in the presence of 0.25 mM ADP, the Mn IC50 was 1.14 ± 0.02 mM and 1.54 ± 0.08 mM for hGDH2 and for hGDH1, respectively (p = 0.0001). Increasing Mn levels potentiated this differential effect, with 3 mM Mn inhibiting hGDH2 by 96.5% and hGDH1 by 70.2%. At 1 mM ADP, the Mn IC50 was 1.84 ± 0.02 mM and 2.04 ± 0.07 mM (p = 0.01) for hGDH2 and hGDH1, respectively, with 3 mM Mn inhibiting hGDH2 by 93.6% and hGDH1 by 70.9%. These results were due to the sigmoidal inhibitory curve of Mn that was more pronounced for hGDH2 than for hGDH1. Indeed, at 0.25 mM, the Hill coefficient value was higher for hGDH2 (3.42 ± 0.20) than for hGDH1 (1.94 ± 0.25; p = 0.0002) indicating that interaction of Mn with hGDH2 was substantially more co-operative than for hGDH1. These findings, showing an enhanced sensitivity of the hGDH2 isoenzyme to Mn, especially at low ADP levels, might be of pathophysiological relevance under conditions of Mn neurotoxicity.

Role of transcription factor yin yang 1 in manganese-induced reduction of astrocytic glutamate transporters: Putative mechanism for manganese-induced neurotoxicity

Astrocytes are the most abundant non-neuronal glial cells in the brain. Once relegated to a mere supportive role for neurons, contemporary dogmas ascribe multiple active roles for these cells in central nervous system (CNS) function, including maintenance of optimal glutamate levels in synapses. Regulation of glutamate levels in the synaptic cleft is crucial for preventing excitotoxic neuronal injury. Glutamate levels are regulated predominantly by two astrocytic glutamate transporters, glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST). Indeed, the dysregulation of these transporters has been linked to several neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and Parkinson's disease (PD), as well as manganism, which is caused by overexposure to the trace metal, manganese (Mn). Although Mn is an essential trace element, its excessive accumulation in the brain as a result of chronic occupational or environmental exposures induces a neurological disorder referred to as manganism, which shares common pathological features with Parkinsonism. Mn decreases the expression and function of both GLAST and GLT-1. Astrocytes are commonly targeted by Mn, and thus reduction in astrocytic glutamate transporter function represents a critical mechanism of Mn-induced neurotoxicity. In this review, we will discuss the role of astrocytic glutamate transporters in neurodegenerative diseases and Mn-induced neurotoxicity.

Genetic dys-regulation of astrocytic glutamate transporter EAAT2 and its implications in neurological disorders and manganese toxicity

Astrocytic glutamate transporters, the excitatory amino acid transporter (EAAT) 2 and EAAT1 (glutamate transporter 1 and glutamate aspartate transporter in rodents, respectively), are the main transporters for maintaining optimal glutamate levels in the synaptic clefts by taking up more than 90% of glutamate from extracellular space thus preventing excitotoxic neuronal death. Reduced expression and function of these transporters, especially EAAT2, has been reported in numerous neurological disorders, including amyotrophic lateral sclerosis, Alzheimer's disease, Parkinson's disease, schizophrenia and epilepsy. The mechanism of down-regulation of EAAT2 in these diseases has yet to be fully established. Genetic as well as transcriptional dys-regulation of these transporters by various modes, such as single nucleotide polymorphisms and epigenetics, resulting in impairment of their functions, might play an important role in the etiology of neurological diseases. Consequently, there has been an extensive effort to identify molecular targets for enhancement of EAAT2 expression as a potential therapeutic approach. Several pharmacological agents increase expression of EAAT2 via nuclear factor κB and cAMP response element binding protein at the transcriptional level. However, the negative regulatory mechanisms of EAAT2 have yet to be identified. Recent studies, including those from our laboratory, suggest that the transcriptional factor yin yang 1 plays a critical role in the repressive effects of various neurotoxins, such as manganese (Mn), on EAAT2 expression. In this review, we will focus on transcriptional epigenetics and translational regulation of EAAT2.

Animal models of neurodegenerative diseases

The prevalence of neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), increases with age, and the number of affected patients is expected to increase worldwide in the next decades. Accurately understanding the etiopathogenic mechanisms of these diseases is a crucial step for developing disease-modifying drugs able to preclude their emergence or at least slow their progression. Animal models contribute to increase the knowledge on the pathophysiology of neurodegenerative diseases. These models reproduce different aspects of a given disease, as well as the histopathological lesions and its main symptoms. The purpose of this review is to present the main animal models for AD, PD, and Huntington's disease.

Regulation of astrocyte glutamine synthetase in epilepsy

Astrocytes play a crucial role in regulating and maintaining the extracellular chemical milieu of the central nervous system under physiological conditions. Moreover, proliferation of phenotypically altered astrocytes (a.k.a. reactive astrogliosis) has been associated with many neurologic and psychiatric disorders, including mesial temporal lobe epilepsy (MTLE). Glutamine synthetase (GS), which is found in astrocytes, is the only enzyme known to date that is capable of converting glutamate and ammonia to glutamine in the mammalian brain. This reaction is important, because a continuous supply of glutamine is necessary for the synthesis of glutamate and GABA in neurons. The known stoichiometry of glutamate transport across the astrocyte plasma membrane also suggests that rapid metabolism of intracellular glutamate via GS is a prerequisite for efficient glutamate clearance from the extracellular space. Several studies have indicated that the activity of GS in astrocytes is diminished in several brain disorders, including MTLE. It has been hypothesized that the loss of GS activity in MTLE leads to increased extracellular glutamate concentrations and epileptic seizures. Understanding the mechanisms by which GS is regulated may lead to novel therapeutic approaches to MTLE, which is frequently refractory to antiepileptic drugs. This review discusses several known mechanisms by which GS expression and function are influenced, from transcriptional control to enzyme modification.

Manganese in health and disease

Manganese is an important metal for human health, being absolutely necessary for development, metabolism, and the antioxidant system. Nevertheless, excessive exposure or intake may lead to a condition known as manganism, a neurodegenerative disorder that causes dopaminergic neuronal death and parkinsonian-like symptoms. Hence, Mn has a paradoxal effect in animals, a Janus-faced metal. Extensive work has been carried out to understand Mn-induced neurotoxicity and to find an effective treatment. This review focuses on the requirement for Mn in human health as well as the diseases associated with excessive exposure to this metal.

Intranasal exposure to manganese disrupts neurotransmitter release from glutamatergic synapses in the central nervous system in vivo

Chronic exposure to aerosolized manganese induces a neurological disorder that includes extrapyramidal motor symptoms and cognitive impairment. Inhaled manganese can bypass the blood-brain barrier and reach the central nervous system by transport down the olfactory nerve to the brain's olfactory bulb. However, the mechanism by which Mn disrupts neural function remains unclear. Here we used optical imaging techniques to visualize exocytosis in olfactory nerve terminals in vivo in the mouse olfactory bulb. Acute Mn exposure via intranasal instillation of 2-200 μg MnCl(2) solution caused a dose-dependent reduction in odorant-evoked neurotransmitter release, with significant effects at as little as 2 μg MnCl(2) and a 90% reduction compared to vehicle controls with a 200 μg exposure. This reduction was also observed in response to direct electrical stimulation of the olfactory nerve layer in the olfactory bulb, demonstrating that Mn's action is occurring centrally, not peripherally. This is the first direct evidence that Mn intoxication can disrupt neurotransmitter release, and is consistent with previous work suggesting that chronic Mn exposure limits amphetamine-induced dopamine increases in the basal ganglia despite normal levels of dopamine synthesis (Guilarte et al., J Neurochem 2008). The commonality of Mn's action between glutamatergic neurons in the olfactory bulb and dopaminergic neurons in the basal ganglia suggests that a disruption of neurotransmitter release may be a general consequence wherever Mn accumulates in the brain and could underlie its pleiotropic effects.

Manganese and ammonia interactions in the brain of cirrhotic rats: effects on brain ammonia metabolism

Hepatic encephalopathy is a major complication of cirrhosis. Ammonia and manganese have been associated with hepatic encephalopathy underlying mechanisms. Motor impairment and brain edema are common signs of hepatic encephalopathy. In the present study a model of liver damage in rats was combined with ammonia and manganese exposure to evaluate the role of these substances separately and their interactions on brain glutamine, water content and motor coordination. Additionally, we explored brain levels of each substance -Mn and ammonia- in the presence or absence of the other. Liver damage was induced by bile duct ligation. Rats were exposed to MnCl2 in drinking water (1 mg Mn/ml) and to ammonia in chow pellets containing 20% ammonium acetate (w/w). As expected, manganese and ammonia levels increased in the brain of cirrhotic rats exposed to these substances; in these animals, glutamine brain levels also increased and positively correlated with tissue water content in cortex. A three way-ANOVA showed that manganese favored ammonia and glutamine accumulation in brain, and possibly their subsequent deleterious effects, as evidenced by the fact that manganese and ammonia accumulation in the brain of cirrhotic rats severely affected motor function. These results suggest that even when controlling ammonia levels in cirrhotic patients, reduction of manganese intake is also a potential strategy to be considered in clinical practice.

Estrogen attenuates manganese-induced glutamate transporter impairment in rat primary astrocytes

The astrocytic glutamate transporters (GLT-1, GLAST) are critical for removing excess glutamate from synaptic sites, thereby maintaining glutamate homeostasis within the brain. 17β-Estradiol (E2) is one of the most active estrogen hormones possessing neuroprotective effects both in in vivo and in vitro models, and it has been shown to enhance astrocytic glutamate transporter function (Liang et al. in J Neurochem 80:807-814, 2002; Pawlak et al. in Brain Res Mol Brain Res 138:1-7, 2005). However, E2 is not clinically optimal for neuroprotection given its peripheral feminizing and proliferative effects; therefore, brain selective estrogen receptor modulators (neuro SERMs) (Zhao et al. in Neuroscience 132:299-311, 2005) that specifically target estrogenic mechanisms, but lack the systemic estrogen side effects offer more promising therapeutic modality for the treatment of conditions associated with excessive synaptic glutamate levels. This review highlights recent studies from our laboratory showing that E2 and SERMs effectively reverse glutamate transport inhibition in a manganese (Mn)-induced model of glutamatergic deregulation. Specifically, we discuss mechanisms by which E2 restores the expression and activity of glutamate uptake. We advance the hypothesis that E2 and related compounds, such as tamoxifen may offer a potential therapeutic modality in neurodegenerative disorders, which are characterized by altered glutamate homeostasis.

The protective effects of riluzole on manganese-induced disruption of glutamate transporters and glutamine synthetase in the cultured astrocytes

Chronic exposure to excessive manganese (Mn) can lead to manganism, a type of neurotoxicity accomplished with extracellular glutamate (Glu) accumulation. To investigate this accumulation, this study focused on the role of astrocyte glutamate transporters (GluTs) and glutamine synthetase (GS), which have roles in Glu transport and metabolism, respectively. And the possible protective effects of riluzole (a glutamatergic modulator) were studied in relation to Mn exposure. At first, the astrocytes were exposed to 0, 125, 250, and 500 μM MnCl(2) for 24 h, and 100 μM riluzole was pretreated to astrocytes for 6 h before 500 μM MnCl(2) exposure. Then, [(3)H]-glutamate uptake was measured by liquid scintillation counting; Na(+)-K(+) ATPase and GS activities were determined by a colorimetric method; glutamate/aspartate transporter (GLAST), glutamate transporter-1 (GLT-1), and GS mRNA expression were determined by RT-PCR and protein levels were measured by western blotting. The results showed that Mn inhibited Glu uptake, Na(+)-K(+) ATPase and GS activities, GLAST, GLT-1, and GS mRNA, and protein in a concentration-dependent manner. And they were significantly higher for astrocytes pretreated with 100 μM riluzole than the group exposed to 500 μM MnCl(2). The results suggested that Mn disrupted Glu transport and metabolism by inhibiting GluTs and GS. Riluzole activated protective effects on enhancing GluTs and GS to reverse Glu accumulation. In conclusion, Mn exposure results in the disruption of GLAST, GLT-1, and GS expression and function. Furthermore, riluzole attenuates this Mn toxicity.

General and electrophysiological toxic effects of manganese in rats following subacute administration in dissolved and nanoparticle form

In an attempt to model occupational and environmental Mn exposures and their possible interaction, young male Wistar rats were exposed to Mn by oral administration in dissolved form (MnCl(2)·4H(2)O, 14.84 and 59.36 mg/kg b.w.) and by intratracheal application of MnO(2) nanoparticles (2.63 mg/kg b.w.). After 3 and 6 weeks oral, or 3 weeks oral plus 3 weeks intratracheal, exposure, general toxicological, and electrophysiological tests were done. Body weight gain was significantly reduced after 6 and 3 plus 3 weeks exposure, but the effect of the latter on the pace of weight gain was stronger. Organ weights signalized systemic stress and effect on lungs. Changes in evoked electrophysiological responses (cortical sensory evoked potential and nerve action potential) indicated that the 3 plus 3 weeks combined exposure caused equal or higher changes in the latency of these responses than 6 weeks of exposure, although the calculated summed Mn dose in the former case was lower. The results showed the importance of the physicochemical form of Mn in determining the toxic outcome, and suggested that neurofunctional markers of Mn action may indicate the human health effect better than conventional blood Mn measurement.

Effect of glutamate and riluzole on manganese-induced apoptotic cell signaling in neuronally differentiated mouse P19 Cells

Excess exposure to Mn causes a neurological disorder known as manganism which is similar to dystonic movements associated with Parkinson's disease. Manganism is largely restricted to occupations in which high atmospheric levels are prevalent which include Mn miners, welders and those employed in the ferroalloy processing or related industrial settings. T1 weighted MRI images reveal that Mn is deposited to the greatest extent in the globus pallidus, an area of the brain that is presumed to be responsible for the major CNS associated symptoms. Neurons within the globus pallidus receive glutamatergic input from the subthalamic nuclei which has been suggested to be involved in the toxic actions of Mn. The neurotoxic actions of Mn and glutamate are similar in that they both affect calcium accumulation in the mitochondria leading to apoptotic cell death. In this paper, we demonstrate that the combination of Mn and glutamate potentiates toxicity of neuronally differentiated P19 cells over that observed with either agent alone. Apoptotic signals ROS, caspase 3 and JNK were increased in an additive fashion when the two neurotoxins were combined. The anti-glutamatergic drug, riluzole, was shown to attenuate these apoptotic signals and prevent P19 cell death. Results of this study confirm, for the first time, that Mn toxicity is potentiated in the presence of glutamate and that riluzole is an effective antioxidant which protects against both Mn and glutamate toxicity.

Electrophysiological and biochemical response in rats on intratracheal instillation of manganese

Chronic exposure to excess manganese via inhalation of metal fumes causes central nervous system damage. For modelling Mn aerosol inhalation, male Wistar rats were intratracheally instilled with MnCl2 solution (0.5 mg/kg b.w. MnCl2; n=12) 5 days a week for 5 weeks. At the end of the treatment, somatosensory cortical evoked potentials, elicited by double-pulse stimulation, were recorded from the animals in urethane anaesthesia. Body weight gain, organ weights, and Mn level in brain, lung and blood samples were also measured. In brain samples, gene expression level of MnSOD (Mn superoxide dismutase) was determined. The effect of Mn was mainly seen on the evoked potential amplitudes, and on the second:first ratio of these. Tissue Mn concentration was elevated in brain and lungs, but changed hardly in the blood. Relative weight of heart, thymus, lungs and brain was significantly altered. The level of MnSOD transcript in brain tissue decreased. The observed effects showed that Mn had access to the brain and that somatosensory cortical responses evoked by double-pulse stimulation might be suitable biomarkers of Mn intoxication.

Role of glial cells in manganese neurotoxicity

The objectives of this focused review are to (i) provide a systematic overview of recent advances pertaining to the role of glia, namely microglia and astrocytes, in the neuropathology associated with excessive exposure to manganese (Mn), (ii) highlight possible mechanisms and factors involved in Mn-modulated, glia-derived neuroinflammation, and (iii) discuss the implications of excessive neuroinflammation on neuronal injury within the context of Mn overexposure. As this is not meant to be a comprehensive review on the topic of Mn neurotoxicity, the reader may wish to refer to several broader and more comprehensive reviews. After a brief introduction to Mn neurotoxicity, we first discuss the role of glial cells in neurodegeneration. Next, we review existing in vitro and in vivo studies that implicate Mn as a modulator of glial activation and ensuing neuroinflammation. This is followed by an examination of recognized and potential mechanisms that are involved in the modulation of glial inflammatory output by Mn; here the common pathways activated by Mn in glial and neuronal cells, including outcomes of such activation, are also addressed. We finish with a discussion of the implications of Mn-modulated glial activation for neuronal survival and with a list of data gaps in the topic that need to be filled in the future.

Regional cerebral metabolism in mouse under chronic manganese exposure: implications for manganism

Chronic manganese (Mn) exposure in rodents, non-human primates and humans has been linked to Parkinson's disease like condition known as Manganism. Mn being a cofactor for many enzymes in brain has been known to be accumulated in various regions differentially and thus exert toxic effect upon chronic overexposure. In present study, neuropathology of Manganism was investigated by evaluating regional neuronal and astroglial metabolism in mice under chronic Mn exposure. Male C57BL6 mice were treated with MnCl(2) (25 mg/kg, i.p.) for 21 days. Cerebral metabolism was studied by co-infusing [U-(13)C(6)]glucose and [2-(13)C]acetate, and monitoring (13)C labeling of amino acids in brain tissue extract using (1)H-[(13)C] and (13)C-[(1)H]-NMR spectroscopy. Glutamate, choline, N-acetyl aspartate and myo-inositol were found to be reduced in thalamus and hypothalamus indicating a loss in neuronal and astroglial cells due to Mn neurotoxicity. Reduced labeling of Glu(C4) from [U-(13)C(6)]glucose and [2-(13)C]acetate indicates an impairment of glucose oxidation by glutamatergic neurons and glutamate-glutamine neurotransmitter cycle in cortex, striatum, thalamus-hypothalamus and olfactory bulb with chronic Mn exposure. Additionally, reduced labeling of Gln(C4) from [2-(13)C]acetate indicates a decrease in acetate oxidation by astroglia in the same regions. However, GABAergic function was alleviated only in thalamus-hypothalamus. Our findings indicate that chronic Mn impairs excitatory (glutamatergic) function in the majority of regions of brain while inhibitory (GABAergic) activity is perturbed only in basal ganglia.

Manganese accumulation in the CNS and associated pathologies

Manganese (Mn) is an essential metal for life. It is a key constituent of clue enzymes in the central nervous system, contributing to antioxidant defenses, energetic metabolism, ammonia detoxification, among other important functions. Until now, Mn transport mechanisms are partially understood; however, it is known that it shares some mechanisms of transport with iron. CNS is susceptible to Mn toxicity because it possesses mechanisms that allow Mn entry and favor its accumulation. Cases of occupational Mn exposure have been extensively reported in the literature; however, there are other ways of exposure, such as long-term parental nutrition and liver failure. Manganism and hepatic encephalopathy are the most common pathologies associated with the effects of Mn exposure. Both pathologies are associated with motor and psychiatric disturbances, related in turn to mechanisms of damage such as oxidative stress and neurotransmitters alterations, the dopaminergic system being one of the most affected. Although manganism and Parkinson's disease share some characteristics, they differ in many aspects that are discussed here. The mechanisms for Mn transport and its participation in manganism and hepatic encephalopathy are also considered in this review. It is necessary to find an effective therapeutic strategy to decrease Mn levels in exposed individuals and to treat Mn long term effects. In the case of patients with chronic liver failure it would be worthwhile to test a low-Mn diet in order to ameliorate symptoms of hepatic encephalopathy possibly related to Mn accumulation.

Concentration of calcium and magnesium and trace elements (zinc, copper, iron and manganese) in cerebrospinal fluid: a try of a pathophysiological classification

The aim of this study is to analyze the variation of the elements (Ca, Mg, Cu, Fe, Zn and Mn) in normal and pathological CSF and develop a classification basing on the increases in cells and proteins and taking into account these variations. A total of 173 cerebrospinal fluids were analyzed. Of these, 37 fulfilled the criteria of normality and, after clinical exploration, were considered to be healthy (control group). The remaining 136 CSFs (pathological group) belonged to people for whom some neurological pathology had been observed in the clinical exploration and whose CSF analysis presented some abnormality. CSF was extracted by puncture in the lumbar cistern. The analysis of metals was performed by atomic absorption spectrophotometry. The statistical values (mean±standard deviation) obtained for each element analyzed in control group were as follows: Ca (mg/dL): 4.95±0.70; Mg (mg/dL): 2.74±0.10; Cu (μg/dL): 15.70±13.50; Fe (μg/dL): 13.10±3.60; Zn (μg/dL): 17.40±9.50 and Mn (μg/dL): 2.50±0.70. In the pathological CSFs, significant increases were found (p<0.050) in relation to the control group for Ca, Cu, Fe, Zn and Mn in groups with an increase of both cells and proteins. A significant decrease of Mg (p<0.050) was found in the groups with cell and protein increases. Given the results obtained in the different subgroups of the proposed classification, we conclude that it is necessary to further categorize the patients' diagnostics in the different subgroups. This would help to validate the classification.

Role of mitogen-activated protein kinases in the mechanism of oxidant-induced cell swelling in cultured astrocytes

Cytotoxic brain edema, usually a consequence of astrocyte swelling, is an important complication of stroke, traumatic brain injury, hepatic encephalopathy, and other neurological disorders. Although mechanisms underlying astrocyte swelling are not fully understood, oxidative stress (OS) has generally been considered an important factor in its pathogenesis. To better understand the mechanism(s) by which OS causes cell swelling, we examined the potential involvement of mitogen-activated protein kinases (MAPKs) in this process. Cultures exposed to theoxidant H(2)O(2) (10, 25, 50 microM) for different time periods (1-24 hr) significantly increased cell swelling in a triphasic manner. Swelling was initially observed at 10 min (peaking at 30 min), which was followed by cell shrinkage at 1 hr. A subsequent increase in cell volume occurred at approximately 6 hr, and the rise lasted for at least 24 hr. Cultures exposed to H(2)O(2) caused the activation of MAPKs (ERK1/2, JNK and p38-MAPK), whereas inhibition of MAPKs diminished cell swelling induced by 10 and 25 microM H(2)O(2). These findings suggest that activation of MAPKs is an important factor in the mediation of astrocyte swelling following oxidative stress.

Effect of mitochondrial toxins on evoked somatosensory activity in rats

Mitochondrial toxins represent an interesting group of neurotoxicants related both to causation and modelling of central nervous damage. 3-nitropropionic acid (3NP), a neurotoxin of herbal and microfungal origin, inhibits succinate dehydrogenase leading thereby to various biochemical and morphological alterations in the brain. Experimental animals treated by 3NP are used to model Huntington’s disease. Manganese, often present in occupational settings and as environmental pollutant, inhibits complex II and III of the mitochondria and is known to cause Parkinson-like CNS damage. In this work, rats were administered a single acute dose of Mn (50 mg Mn2+/kg body weight) or 3-NP (20 mg/kg b.w.) and the alterations of the somatosensory cortical evoked potential elicited by stimulation of the whisker pad and the tail base were observed, together with the changes of the action potential in the tail nerve. Latency and amplitude of the two cortical responses changed in parallel, while those of the tail nerve response remained more or less unaltered. The two mitochondrial toxins studied seem to exert their action centrally, primarily on synaptic transmission, rather than peripherally. Recording of evoked activity could be used to follow-up the nervous system effects of mitochondrial toxins, but it requires further investigation.

Metal toxicity, liver disease and neurodegeneration

Hepatocerebral disorders are serious neuropsychiatric conditions that result from liver failure. These disorders are characterized neuropathologically by varying degrees of neuronal cell death in basal ganglia, cerebellum, and spinal cord, and include clinical entities such as Wilson's Disease, post-shunt myelopathy, hepatic encephalopathy, and acquired non-Wilsonian hepatocerebral degeneration. Morphologic changes to astrocytes (Alzheimer type II astrocytosis) are a major feature of hepatocerebral disorders. Neurological symptoms include Parkinsonism, cognitive dysfunction, and ataxia. Pathophysiologic mechanisms responsible for cerebral dysfunction and neuronal cell death in hepatocerebral disorders include ammonia toxicity and neurotoxic effects of metals such as copper, manganese, and iron. Molecular mechanisms of neurotoxicity include oxidative/nitrosative stress, glutamate (NMDA)-receptor-mediated excitotoxicity, and neuroinflammatory mechanisms. However, neuronal cell death in hepatocerebral disorders is limited by adaptive mechanisms that may include NMDA-receptor down-regulation, the synthesis of neuroprotective steroids and hypothermia. Management and treatment of hepatocerebral disorders include chelation therapy (Wilson's Disease), the use of ammonia-lowering agents (lactulose, antibiotics, ornithine aspartate) and liver transplantation.

A possible neuroprotective action of a vinylic telluride against Mn-induced neurotoxicity

Manganese (Mn) is a metal required by biological systems. However, environmental or occupational exposure to high levels of Mn can produce a neurological disorder called manganism, which has similarities to Parkinson's disease. Diethyl-2-phenyl-2-tellurophenyl vinylphosphonate (DPTVP) is an organotellurium compound with a high antioxidant activity, especially in the brain. The present study was designed to investigate the effects of long-term low-dose exposure to Mn in drinking water on behavioral and biochemical parameters in rats and to determine the effectiveness of vinylic telluride in attenuating the effects of Mn. After 4 months of treatment with MnCl(2) (13.7 mg/kg), rats exhibited clear signs of neurobehavioral toxicity, including a decrease in the number of rearings in the open field and altered motor performance in rotarod. The administration of DPTVP (0.150 micromol/kg, ip, 2 weeks) improved the motor performance of Mn-treated rats, indicating that the compound could be reverting Mn neurotoxicity. Ex vivo, we observed that Mn concentrations in the Mn-treated group were highest in the striatum, consistent with a statistically significant decrease in mitochondrial viability and [(3)H]glutamate uptake, and increased lipid peroxidation. Mn levels in the hippocampus and cortex were indistinguishable from controls, and no significant differences were noted in the ex vivo assays in these areas. Treatment with DPTVP fully reversed the biochemical parameters altered by Mn. Furthermore, DPTVP treatment was also associated with a reduction in striatal Mn levels. Our results demonstrate that DPTVP has neuroprotective activity against Mn-induced neurotoxicity, which may be attributed to its antioxidant activity and/or its effect on striatal Mn transport.

Are there common biochemical and molecular mechanisms controlling manganism and parkisonism

Over the past several decades there has been considerable progress in our basic knowledge as to the mechanisms and factors regulating Mn toxicity. The disorder known as manganism is associated with the preferential accumulation of Mn in the globus pallidus of the basal ganglia which is generally considered to be the major and initial site of injury. Because the area of the CNS comprising the basal ganglia is very complex and dependent on the precise function and balance of several neurotransmitters, it is not surprising that symptoms of manganism often overlap with that of Parkinson's disease. The fact that neurological symptoms and onset of Mn toxicity are quite broad and can vary unpredictably probably reflects specific genetic variance of the physiological and biochemical makeup within the basal ganglia in any individual. Differences in response to Mn overexposure are, thus, likely due to underlying genetic variability which ultimately presents in deviations in both susceptibility as well as the characteristics of the neurological lesions and symptoms expressed. Although chronic exposure to Mn is not the initial causative agent provoking Parkinsonism, there is evidence suggesting that persistent exposure can predispose an individual to acquire dystonic movements associated with Parkinson's disease. As noted in this review, there appears to be common threads between the two disorders, as mutations in the genes, parkin and ATP13A2, associated with early onset of Parkinsonism, may also predispose an individual to develop Mn toxicity. Mutations in both genes appear to effect transport of Mn into the cell. These genetic difference coupled with additional environmental or nutritional factors must also be considered as contributing to the severity and onset of manganism.

Estrogen and tamoxifen reverse manganese-induced glutamate transporter impairment in astrocytes

Chronic exposure to manganese (Mn) can cause manganism, a neurodegenerative disorder similar to Parkinson's disease. The toxicity of Mn includes impairment of astrocytic glutamate transporters. 17beta-Estradiol (E2) has been shown to be neuroprotective in various neurodegenerative diseases including Parkinson's disease and Alzheimer's disease, and some selective estrogen receptor modulators, including tamoxifen (TX), also possess neuroprotective properties. We have tested our hypothesis that E2 and TX reverse Mn-induced glutamate transporter impairment in astrocytes. The results established that E2 and TX increased glutamate transporter function and reversed Mn-induced glutamate uptake inhibition, primarily via the up-regulation of glutamate/aspartate transporter (GLAST). E2 and TX also increased astrocytic GLAST mRNA levels and attenuated the Mn-induced inhibition of GLAST mRNA expression. In addition, E2 and TX effectively increased the expression of transforming growth factor beta1, a potential modulator of the stimulatory effects of E2/TX on glutamate transporter function. This effect was mediated by the activation of MAPK/extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3K)/Akt signaling pathways. These novel findings suggest, for the first time, that E2 and TX enhance astrocytic glutamate transporter expression via increased transforming growth factor beta1 expression. Furthermore, the present study is the first to show that both E2 and TX effectively reverse Mn-induced glutamate transport inhibition by restoring its expression and activity, thus offering a potential therapeutic modality in neurodegenerative disorders characterized by altered glutamate homeostasis.

Manganese neurotoxicity: lessons learned from longitudinal studies in nonhuman primates

BACKGROUND

Exposure to excess levels of the essential trace element manganese produces cognitive, psychiatric, and motor abnormalities. The understanding of Mn neurotoxicology is heavily governed by pathologic and neurochemical observations derived from rodent studies that often employ acute Mn exposures. The comparatively sparse studies incorporating in vivo neuroimaging in nonhuman primates provide invaluable insights on the effects of Mn on brain chemistry.

OBJECTIVES

The purpose of this review is to discuss important aspects of Mn neurotoxicology and to synthesize recent findings from one of the largest cohorts of nonhuman primates used to study the neurologic effects of chronic Mn exposure.

DISCUSSION

We reviewed our recent in vivo and ex vivo studies that have significantly advanced the understanding of Mn-induced neurotoxicity. In those studies, we administered weekly doses of 3.3-5.0 (n=4), 5.0-6.7 (n=5), or 8.3-10.0 mg Mn/kg (n=3) for 7-59 weeks to cynomolgus macaque monkeys. Animals expressed subtle deficits in cognition and motor function and decreases in the N-acetylaspartate-to-creatine ratio in the parietal cortex measured by magnetic resonance spectroscopy reflective of neuronal dysfunction. Impaired striatal dopamine release measured by positron emission tomography was observed in the absence of changes in markers of dopamine neuron degeneration. Neuropathology indicated decreased glutamine synthetase expression in the globus pallidus with otherwise normal markers of glutamatergic and GABAergic neurotransmission. Increased amyloid beta (A4) precursor-like protein 1 gene expression with multiple markers of neurodegeneration and glial cell activation was observed in the frontal cortex.

CONCLUSIONS

These findings provide new information on mechanisms by which Mn affects behavior, neurotransmitter function, and neuropathology in nonhuman primates.

The mitochondrial permeability transition, and oxidative and nitrosative stress in the mechanism of copper toxicity in cultured neurons and astrocytes

Copper is an essential element and an integral component of various enzymes. However, excess copper is neurotoxic and has been implicated in the pathogenesis of Wilson's disease, Alzheimer's disease, prion conditions, and other disorders. Although mechanisms of copper neurotoxicity are not fully understood, copper is known to cause oxidative stress and mitochondrial dysfunction. As oxidative stress is an important factor in the induction of the mitochondrial permeability transition (mPT), we determined whether mPT plays a role in copper-induced neural cell injury. Cultured astrocytes and neurons were treated with 20 microM copper and mPT was measured by changes in the cyclosporin A (CsA)-sensitive inner mitochondrial membrane potential (Delta Psi m), employing the potentiometric dye TMRE. In astrocytes, copper caused a 36% decrease in the Delta Psi m at 12 h, which decreased further to 48% by 24 h and remained at that level for at least 72 h. Cobalt quenching of calcein fluorescence as a measure of mPT similarly displayed a 45% decrease at 24 h. Pretreatment with antioxidants significantly blocked the copper-induced mPT by 48-75%. Copper (24 h) also caused a 30% reduction in ATP in astrocytes, which was completely blocked by CsA. Copper caused death (42%) in astrocytes by 48 h, which was reduced by antioxidants (35-60%) and CsA (41%). In contrast to astrocytes, copper did not induce mPT in neurons. Instead, it caused early and extensive death with a concomitant reduction (63%) in ATP by 14 h. Neuronal death was prevented by antioxidants and nitric oxide synthase inhibitors but not by CsA. Copper increased protein tyrosine nitration in both astrocytes and neurons. These studies indicate that mPT, and oxidative and nitrosative stress represent major factors in copper-induced toxicity in astrocytes, whereas oxidative and nitrosative stress appears to play a major role in neuronal injury.

Aquaporin-4 in hepatic encephalopathy

Brain edema is a critical component of hepatic encephalopathy (HE) associated with acute liver failure and such edema appears to be principally due to astrocyte swelling (cytotoxic edema). Ammonia is believed to represent a major factor responsible for astrocyte swelling, although the mechanisms by which ammonia causes such swelling are not completely understood. Recent studies have implicated potential role of oxidative stress, and the mitochondrial permeability transition (mPT). While it is not known how oxidative stress and the mPT cause astrocyte swelling, it is reasonable to suggest that these events may affect one or more plasma membrane proteins involved in water and ion homeostasis in astrocytes. One such protein strongly implicated in brain edema in other neurological conditions is the water channel protein aquaporin-4 (AQP-4), which is abundantly expressed in astrocytes. This article summarizes the potential role of AQP-4 in brain edema in in vivo models of HE, as well as in ammonia-induced cell swelling in cultured astrocytes. The involvement of AQP-4 in the effects of manganese, another toxin implicated in HE, will also be discussed.

Nmr spectroscopic analysis of regional brain energy metabolism in manganese neurotoxicity

A central question in manganese neurotoxicity concerns the focal neuronal damage in the globus pallidus. In the present study, we investigated specific pathways of [1-(13)C]glucose as well as of [2-(13)C]acetate in this brain region and the frontal cortex following 4-day manganese treatment by high-resolution NMR spectroscopy. Following administration of 50 mg/kg/day manganese, glutamine concentration in the globus pallidus was decreased to 67% of control values but increased in frontal cortex by 56%. Manganese treatment also caused pronounced changes in glutamine-glutamate-GABA interconversion in which region-selective differences were observed in the isotopomer pattern of GABA compared with that of glutamine when including the astrocyte-specific substrate [2-(13)C]acetate. In particular, decreased (13)C-labeled glutamine, synthesized from [1-(13)C]glucose, paralleled accumulation of (13)C-labeled GABA in globus pallidus but not in frontal cortex. On the other hand, increased synthesis of glutamine from [2-(13)C]acetate showed that GABA accumulation was not due to increased synthesis from astrocytic glutamine. Furthermore, treatment with manganese resulted in a selective decrease in N-acetyl-aspartate in the globus pallidus. These data illustrate the potential importance of alterations in neuronal metabolic function. In particular, neuronal metabolic derangements and regional differences in the ability of astrocytes to fulfill their contribution to the glutamine-glutamate-GABA cycle during the early phase of manganese neurotoxicity may be crucial in determining the severity of cellular injury.

Manganese intoxication decreases the expression of manganoproteins in the rat basal ganglia: an immunohistochemical study

Manganese (Mn) is a cofactor for some metalloprotein enzymes, including Mn-superoxide dismutase (Mn-SOD), a mitochondrial enzyme predominantly localized in neurons, and glutamine synthetase (GS), which is selectively expressed in astroglial cells. The detoxifying effects of GS and Mn-SOD in the brain, involve catabolizing glutamate and scavenging superoxide anions, respectively. Mn intoxication is characterized by impaired function of the basal ganglia. However, it is unclear whether regional central nervous system expression of manganoproteins is also affected. Here, we use immunocytochemistry in the adult rat brain, to examine whether Mn overload selectively affects the expression of GS, Mn-SOD, Cu/Zn-SOD, another component of the SOD family, and glial fibrillary acid protein (GFAP), a specific marker of astrocytes. After chronic Mn overload in drinking water for 13 weeks, we found that the number and immunostaining intensity of GS- and Mn-SOD-positive cells was significantly decreased in the striatum and globus pallidus, but not in the cerebral frontal cortex. In addition, we found that GS enzymatic activity was decreased in the strio-pallidal regions but not in the cerebral cortex of Mn-treated animals. In contrast, Cu/Zn-SOD- and GFAP-immunoreactivity was unchanged in both the cerebral cortex and basal ganglia of Mn-treated rats. Thus, we conclude that in response to chronic Mn overload, a down-regulation of some manganoproteins occurs in neurons and astrocytes of the striatum and globus pallidus, probably reflecting the vulnerability of these regions to Mn toxicity.

Low-level manganese exposure alters glutamate metabolism in GABAergic AF5 cells

Recent studies have suggested that the globus pallidus may be a particularly sensitive target of manganese (Mn), however, in vitro studies of the effects of Mn on GABAergic neurons have been restricted by the lack of a cell model expressing GABAergic properties. Here, we investigated the effects of low-level Mn treatment on cellular GABA and glutamate metabolism using the newly characterized AF5 rat neural-derived cell line, which displays GABAergic properties during culture in vitro. Intracellular GABA and glutamate levels were measured along with measurement of the release of GABA and glutamate into the culture medium, glutamine uptake from the culture medium, and the specific effects of Mn on the enzymes directly responsible for the synthesis and degradation of GABA, glutamate decarboxylase (GAD) and GABA transaminase (GABA-T). Our results demonstrate that Mn had no effect on the activities of GAD or GABA-T. Similarly, low-level Mn treatment of AF5 cultures had only a small effect on intracellular GABA levels (114% of control) and no effect on the release of GABA. In contrast, intracellular and extracellular glutamate levels were enhanced to 170 and 198% of control during Mn treatment, respectively, while extracellular glutamine decreased to 73% of controls. Together, these results suggest that glutamate homeostasis may be preferentially affected over GABA in AF5 cells during low-level Mn treatment, suggesting a novel mechanism by which Mn-induced excitotoxicity might arise.

The brain glutamate system in liver failure

Liver failure results in significant alterations of the brain glutamate system. Ammonia and the astrocyte play major roles in such alterations, which affect several components of the brain glutamate system, namely its synthesis, intercellular transport (uptake and release), and function. In addition to the neurological symptoms of hepatic encephalopathy, modified glutamatergic regulation may contribute to other cerebral complications of liver failure, such as brain edema, intracranial hypertension and changes in cerebral blood flow. A better understanding of the cause and precise nature of the alterations of the brain glutamate system in liver failure could lead to new therapeutic avenues for the cerebral complications of liver disease.

L-glutamate activates RhoA GTPase leading to suppression of astrocyte stellation

The actin cytoskeleton is known to support cellular morphological changes. Rho family small GTPases function as switching molecules to promote the convergence of both extracellular and intracellular signals in regulating cytoskeletal organization. Evidence indicates that L-glutamate suppresses morphological changes of astrocytes over a broad spectrum. To test the possibility that L-glutamate affects cytoskeletal reorganization, we investigated its effect on morphological changes induced by manganese exposure. L-glutamate concentration-dependently prevented and reversed manganese-induced astrocyte stellation and cytoskeletal disruption. The suppressive effect of L-glutamate on manganese-induced stellation was mediated by the activation of the glutamate transporter rather than ionotropic or metabotropic glutamate receptors. Pharmacological and biochemical approaches revealed the involvement of Ras homolog gene family, member A (RhoA) activation in L-glutamate-mediated suppression of manganese-induced stellation. The activation of RhoA by L-glutamate was partly through the up-regulation of guanine nucleotide exchange factor phosphorylation and was abrogated by competitive nonsubstrate inhibitors. Furthermore, the hyperphosphorylation of myosin light chain and cofilin through the activation of RhoA following L-glutamate treatment synergistically stabilized actin stress fibres. These results suggest that manganese-induced stellation is suppressed by a mechanism involving glutamate transporters. Our in vitro findings also strongly indicate that astrocyte morphological plasticity is under the control of RhoA and that manganese and L-glutamate regulate astrocyte morphology by modulating this switching molecule under culture conditions.

Astrocytes protect neurons from ammonia toxicity

Ammonia is a neurotoxin that is implicated in the CNS dysfunction associated with hepatic encephalopathy, urea cycle disorders, Reye's syndrome and other neurological conditions. While in vivo studies suggest that astrocytes are the principal target of ammonia toxicity, recent in vitro investigations suggest that neurons may also be directly affected by ammonia. To further examine the issue of neural cell sensitivity to ammonia, pure rat cortical neuronal cultures, as well as co-cultures of neurons and astrocytes, were exposed to 5 mM NH4Cl for 48 h. Cultures were examined for morphological changes by light microscopy, measures of cell death, free radical production and changes in the mitochondrial inner membrane potential. Ammonia caused extensive degenerative changes in pure cultured neurons, while such neuronal changes were minor in the co-cultures. Similarly, processes of pure cultured neurons displayed a significant loss of the mitochondrial inner membrane potential, as compared to neurons in co-cultures. Cell death (LDH release) in ammonia-treated neuronal cultures was twice as great as untreated controls, while in co-cultures ammonia did not significantly increase cell death. Free radical production at 3 min was increased (69%, P<0.05) in pure neuronal cultures but not in co-cultures. The neuroprotective effects observed in co-cultures may have been mediated by the astrocyte's ability to scavenge free radicals, by their detoxification of ammonia and/or by their neurotrophic actions. The neuroprotective action of astrocytes may explain the failure to detect significant pathological changes in neurons in ammonia toxicity in vivo.