Association between cell swelling and glycogen content in cultured astrocytes

Chronic corticosterone exposure reduces hippocampal glycogen level and induces depression-like behavior in mice

Long-term exposure to stress or high glucocorticoid levels leads to depression-like behavior in rodents; however, the cause remains unknown. Increasing evidence shows that astrocytes, the most abundant cells in the central nervous system (CNS), are important to the nervous system. Astrocytes nourish and protect the neurons, and serve as glycogen repositories for the brain. The metabolic process of glycogen, which is closely linked to neuronal activity, can supply sufficient energy substrates for neurons. The research team probed into the effects of chronic corticosterone (CORT) exposure on the glycogen level of astrocytes in the hippocampal tissues of male C57BL/6N mice in this study. The results showed that chronic CORT injection reduced hippocampal neurofilament light protein (NF-L) and synaptophysin (SYP) levels, induced depression-like behavior in male mice, reduced hippocampal glycogen level and glycogen synthase activity, and increased glycogen phosphorylase activity. The results suggested that the reduction of the hippocampal glycogen level may be the mechanism by which chronic CORT treatment damages hippocampal neurons and induces depression-like behavior in male mice.

Roles of cell volume in molecular and cellular biology

Extracellular tonicity and volume regulation control a great number of molecular and cellular functions including: cell proliferation, apoptosis, migration, hormone and neuromediator release, gene expression, ion channel and transporter activity and metabolism. The aim of this review is to describe these effects and to determine if they are direct or are secondarily the result of the activity of second messengers.

Glutamine as a mediator of ammonia neurotoxicity: A critical appraisal

Ammonia is a major neurotoxin implicated in hepatic encephalopathy (HE). Here we discuss evidence that many aspects of ammonia toxicity in HE-affected brain are mediated by glutamine (Gln), synthesized in excess from ammonia and glutamate by glutamine synthetase (GS), an astrocytic enzyme. The degree to which Gln is increased in brains of patients with HE was found to positively correlate with the grade of HE. In animals with HE, a GS inhibitor, methionine sulfoximine (MSO), reversed a spectrum of manifestations of ammonia toxicity, including brain edema and increased intracranial pressure, even though MSO itself increased brain ammonia levels. MSO inhibited, while incubation with Gln reproduced the oxidative stress and cell swelling observed in ammonia-exposed cultured astrocytes. Recent studies have shown that astrocytes swell subsequent to Gln transport into mitochondria and its degradation back to ammonia, which then generates reactive oxygen species and the mitochondrial permeability transition. This sequence of events led to the formulation of the "Trojan Horse" hypothesis. Further verification of the role of Gln in the pathogenesis of HE will have to account for: (1) modification of the effects of Gln by interaction of astrocytes with other CNS cells; and (2) direct effects of Gln on these cells. Recent studies have demonstrated a "Trojan Horse"-like effect of Gln in microglia, as well as an interference by Gln with the activation of the NMDA/NO/cGMP pathway by ammonia as measured in whole brain, a process that likely also involves neurons.

Astrocytes are more resistant to focal cerebral ischemia than neurons and die by a delayed necrosis

Several recent reports proposed that astrocyte death might precede neuronal demise after focal ischemia, contrary to the conventional view that astrocytes are more resistant to injury than neurons. Interestingly, there are findings supporting each of these opposing views. To clarify these controversies, we assessed astrocyte viability after 2-h middle cerebral artery occlusion in mice. In contrast to neighboring neurons, astrocytes were alive and contained glycogen across the ischemic area 6 h after reperfusion, and at the expanding outer border of the infarct at later time points. These glycogen-positive astrocytes had intact plasma membranes. Astrocytes lost plasmalemma integrity much later than neurons: 19 +/- 22 (mean +/- standard deviation), 58 +/- 14 and 69 +/- 3% of astrocytes in the perifocal region became permeable to propidium iodide (PI) at 6, 24, 72 h after ischemia, respectively, in contrast to 81 +/- 2, 96 +/- 3, 97 +/- 2% of neurons. Although more astrocytes in the cortical and subcortical core regions were PI-positive, their numbers were considerably less than those of neurons. Lysosomal rupture (monitored by deoxyribonuclease II immunoreactivity) followed a similar time course. Cytochrome-c immunohistochemistry showed that astrocytes maintained mitochondrial integrity longer than neurons. EM confirmed that astrocyte ultrastructure including mitochondria and lysosomes disintegrated much later than that of neurons. We also found that astrocytes died by a delayed necrosis without significantly activating apoptotic mechanisms although they rapidly swelled at the onset of ischemia.

RNA oxidation and zinc in hepatic encephalopathy and hyperammonemia

Hepatic encephalopathy is a neuropsychiatric manifestation of acute and chronic liver failure. Ammonia plays a key role in the pathogenesis of hepatic encephalopathy by inducing astrocyte swelling and/or sensitizing astrocytes to swelling by a heterogeneous panel of precipitating factors and conditions. Whereas astrocyte swelling in acute liver failure contributes to a clinically overt brain edema, a low grade glial edema without clinically overt brain edema is observed in hepatic encephalopathy in liver cirrhosis. Astrocyte swelling produces reactive oxygen and nitrogen oxide species (ROS/RNOS), which again increase astrocyte swelling, thereby creating a self-amplifying signaling loop. Astroglial swelling and ROS/RNOS increase protein tyrosine nitration and may account for neurotoxic effects of ammonia and other precipitants of hepatic encephalopathy. Recently, RNA oxidation and an increase of free intracellular zinc ([Zn(2+)](i)) were identified as further consequences of astrocyte swelling and ROS/RNOS production. An elevation of [Zn(2+)](i) mediates mRNA expression of metallothionein and the peripheral benzodiazepine receptor (PBR) induced by hypoosmotic astrocyte swelling. Further, Zn(2+) mediates RNA oxidation in ammonia-treated astrocytes. In the brain of hyperammonemic rats oxidized RNA localizes in part to perivascular astrocyte processes and to postsynaptic dendritic spines. RNA oxidation may impair postsynaptic protein synthesis, which is critically involved in learning and memory consolidation. RNA oxidation offers a novel explanation for multiple disturbances of neurotransmitter systems and gene expression and the cognitive deficits observed in hepatic encephalopathy.

Hypoosmotic swelling and ammonia increase oxidative stress by NADPH oxidase in cultured astrocytes and vital brain slices

The role of NADPH oxidase (NOX) and the regulatory subunit p47(phox) for hypoosmotic ROS generation was studied in cultured rat astrocytes and brain slices of wilde type and p47(phox) knock-out mice. Cultured rat astrocytes express mRNAs encoding for the regulatory subunit p47(phox), NOX1, 2, and 4, and the dual oxidases (DUOX)1 and 2, but not NOX3. Hypoosmotic (205 mosmol/L) swelling of cultured astrocytes induced a rapid generation of ROS that was accompanied by serine phosphorylation of p47(phox) and prevented by the NADPH oxidase inhibitor apocynin. Apocynin also impaired the hypoosmotic tyrosine phosphorylation of Src. Both, hypoosmotic ROS generation and p47(phox) serine phosphorylation were sensitive to the acidic sphingomyelinase inhibitors AY9944 and desipramine, the protein kinase C (PKC)zeta-inhibitory pseudosubstrate peptide, the NMDA receptor antagonist MK-801 and the intracellular Ca(2+) chelator BAPTA-AM. Also hypoosmotic exposure of wilde type mouse cortical brain slices increased ROS generation, which was allocated in part to the astrocytes and which was absent in presence of apocynin and in cortical brain slices from p47(phox) knock-out mice. Also ammonia induced a rapid ROS production in cultured astrocytes and brain slices, which was sensitive to apocynin. The data suggest that astrocyte swelling triggers a p47(phox)-dependent NADPH oxidase-catalyzed ROS production. The findings further support a close interrelation between osmotic and oxidative stress in astrocytes, which may be relevant to different brain pathologies including hepatic encephalopathy.

Pathogenesis of hepatic encephalopathy: the tumour necrosis factor-alpha theory

Hepatic encephalopathy (HE) is a major complication for acute and chronic liver failure. Despite several decades of intensive clinical and basic research, the pathogenesis of HE is still incompletely understood, and the precise mechanisms causing brain dysfunction in liver failure are still not fully established. Several theories concerning the pathogenesis of HE have been previously suggested, including the ammonia theory, which received the most attention. These theories are not mutually exclusive and the validity of none of them has been definitely proved experimentally. In this review article, an attractive theory concerning the pathogenesis of HE, the tumour necrosis factor-alpha (TNF) theory, is presented and comprehensively discussed after accumulation of sufficient data which indicate that the pro-inflammatory cytokine, TNF, is strongly involved in the pathogenesis of HE associated with both acute and chronic liver failure. This theory seems to be superior to all other previous theories in the pathogenesis of HE, and may induce development of other beneficial therapeutical modalities for HE directed towards inhibition of TNF production and/or action, and towards enhancement of its degradation.

Pathogenetic interplay between osmotic and oxidative stress: the hepatic encephalopathy paradigm

Hepatic encephalopathy (HE) defines a primary gliopathy associated with acute and chronic liver disease. Astrocyte swelling triggered by ammonia in synergism with different precipitating factors, including hyponatremia, tumor necrosis factor (TNF)-alpha, glutamate and ligands of the peripheral benzodiazepine receptor (PBR), is an early pathogenetic event in HE. On the other hand, reactive nitrogen and oxygen species (RNOS) including nitric oxide are considered to play a major role in HE. There is growing evidence that osmotic and oxidative stresses are closely interrelated. Astrocyte swelling produces RNOS and vice versa. Based on recent investigations, this review proposes a working model that integrates the pathogenetic action of osmotic and oxidative stresses in HE. Under participation of the N-methyl-D-aspartate (NMDA) receptor, Ca(2+), the PBR and organic osmolyte depletion, astrocyte swelling and RNOS production may constitute an autoamplificatory signaling loop that integrates at least some of the signals released by HE-precipitating factors.

Protein tyrosine nitration in hyperammonemia and hepatic encephalopathy

Hepatic encephalopathy is seen as a clinical manifestation of a chronic low grade cerebral edema, which is thought to trigger disturbances of astrocyte function, glioneuronal communication, and finally HE symptoms. In cultured astrocytes, hypoosmotic swelling triggers a rapid oxidative stress response, which involves the action of NADPH oxidase isoenzymes, followed by tyrosine nitration of distinct astrocytic proteins. Oxidative stress and protein tyrosine nitration (PTN) are also observed in response to ammonia, inflammatory cytokines, such as TNF-alpha or interferons, and benzodiazepines with affinity to the peripheral benzodiazepine receptor (PBR). NMDA receptor activation was identified as upstream event in protein tyrosine nitration (PTN). Cerebral PTN is also found in vivo after administration of ammonia, benzodiazepines or lipopolysaccharide and in portocaval shunted rats. PTN predominantly affects astrocytes surrounding cerebral vessels with potential impact on blood-brain-barrier permeability. Among the tyrosine-nitrated proteins, glutamine synthetase, GAPDH, extracellular signal-regulated kinase and the PBR were identified. PTN of glutamine synthetase is associated with inactivation of the enzyme. Thus, factors known to trigger hepatic encephalopathy induce oxidative/nitrosative stress on astrocytes with protein modifications through PTN. The pathobiochemical relevance of astrocytic PTN for the development of HE symptoms remains to be established.

Astrocytes in cerebral ischemic injury: morphological and general considerations

Asrocytic responses constitute one of the earliest and most prominent changes in the CNS following ischemic injury. Astrocytes are known to carry out critical functions such as maintenance of ionic homeostasis, prevention of excitotoxicity, scavenging free radicals, provision of nutrients and growth factors, promotion of neovascularization, and support of synaptogenesis and neurogenesis that potentially may influence the outcome of ischemic injury. This article reviews ischemia-associated alterations in astrocytes and their potential significance. Interactions with neurons, microglia, and endothelial cells are also considered. This article highlights the critical role of astrocytes in the CNS response to ischemic injury.

Hypoosmotic swelling increases protein tyrosine nitration in cultured rat astrocytes

Astrocyte swelling is observed in different types of brain injury. We studied a potential contribution of swelling to protein tyrosine nitration (PTN) by using cultured rat astrocytes exposed to hypoosmotic (205 mosmol/L) medium. Hypoosmolarity (2 h) increases total PTN by about 2-fold in 2 h. The hypoosmotic PTN is significantly inhibited by the NMDA receptor antagonist MK-801, the nitric oxide synthase (NOS) inhibitor L-NMMA, the extracellular Ca2+ chelator EGTA and the calmodulin antagonist W13, suggesting the involvement of NMDA receptor activation, influx of extracellular Ca2+ and Ca2+/calmodulin-dependent NO synthesis. Further, superoxide dismutase plus catalase and uric acid strongly inhibit hypoosmotic PTN, suggesting the involvement of the toxic metabolite peroxynitrite (ONOO-) as a nitrating agent. Hypoosmotic astrocyte swelling rapidly stimulates generation of reactive oxygen intermediates; this process is prevented by MK-801 and EGTA. In addition, MK-801 inhibits the hypoosmotic elevation of [Ca2+]i. The findings support the view that astrocyte swelling as induced, for example, by toxins relevant for hepatic encephalopathy is sufficient to produce oxidative stress and PTN and thus contributes to altered astroglial and neuronal function.

Direct, noninvasive measurement of brain glycogen metabolism in humans

The concentration and metabolism of the primary carbohydrate store in the brain, glycogen, is unknown in the conscious human brain. This study reports the first direct detection and measurement of glycogen metabolism in the human brain, which was achieved using localized 13C NMR spectroscopy. To enhance the NMR signal, the isotopic enrichment of the glucosyl moieties was increased by administration of 80 g of 99% enriched [1-13C]glucose in four subjects. 3 h after the start of the label administration, the 13C NMR signal of brain glycogen C1 was detected (0.36+/-0.07 micromol/g, mean+/-S.D., n=4). Based on the rate of 13C label incorporation into glycogen and the isotopic enrichment of plasma glucose, the flux through glycogen synthase was estimated at 0.17+/-0.05 micromol/(gh). This study establishes that brain glycogen can be measured in humans and indicates that its metabolism is very slow in the conscious human. The noninvasive detection of human brain glycogen opens the prospect of understanding the role and function of this important energy reserve under various physiological and pathophysiological conditions.

Immunocytochemical localization of glycogen phosphorylase isozymes in rat nervous tissues by using isozyme-specific antibodies

Isozyme-specific antibodies were raised against peptides from the low-homology regions of the sequences of rat glycogen phosphorylase BB and MM isozymes by immunization of rabbits and guinea pigs. Immunocytochemical double-labelling experiments on frozen sections of rat nervous tissues were performed to investigate the isozyme localization pattern. Astrocytes throughout the brain and spinal cord expressed both isozymes in perfect co-localization. Ependymal cells only expressed the BB isozyme. Most neurones were not immunoreactive. The rare neurones that contained glycogen phosphorylase only expressed the BB isozyme. Nearly all of these neurones formed part of the afferent somatosensory system. These findings stress the general importance of glycogen in neural energy metabolism and indicate a special role for the glycogen phosphorylase BB isozyme in neurones in the somatosensory system.

[Treatment of cerebral oedema]

Progress in brain imaging, monitoring and physiopathology allows the identification of brain oedema from brain swelling, determination of its interstitial or intracellular nature, as well as blood-brain barrier permeability and the evaluation of the impact on cerebral haemodynamic. Common treatment of all types of cerebral oedema is based on prevention of self-sustained disorders due to increased intracranial pressure resulting in ischemic cerebral oedema. The specific treatment of each type of cerebral oedema is reviewed. Optimization of conventional anti-oedematous strategies is based on the precise determination of the nature of the cerebral oedema and of the blood-brain barrier status.

Suppression of ammonia-induced swelling by aspartate but not by ornithine in primary cultures of rat astrocytes

Cerebral edema with a rise in intracranial pressure is the hallmark of fulminant hepatic failure (FHF) and acute hyperammonemic (HA) states and is characterized by a poor survival rate. Astrocytes are the cells in brain which are swollen in these conditions. Several hypotheses have been proposed to explain the mechanism of cerebral edema in FHF and treatment strategies have evolved based on these putative mechanisms. Treatment with a mixture of ornithine and aspartate has been proven to be clinically beneficial as it reduces edema and improves the neurological status. It has been suggested that these two amino acids generate the glutamate required for the synthesis of glutamine and that they also enhance urea synthesis in surviving hepatocytes in FHF and HA. Presently, we report that of these two amino acids, only aspartate is effective in suppressing ammonia-induced swelling in primary cultures of astrocytes, while ornithine is ineffective. These results are discussed in relation to the metabolism of aspartate and ornithine in astrocytes, with an emphasis on glutamine synthesis and the malate-aspartate shuttle (MAS). We propose that the ability of aspartate to generate glutamate in the cytosol for glutamine synthesis and oxaloacetate in mitochondria to support the citric acid cycle play a role in its ability to reduce ammonia-induced swelling in astrocytes.

Glutamate decreases pyruvate carboxylase activity and spares glucose as energy substrate in cultured cerebellar astrocytes

The effects of glutamate on [U-(13)C]glucose metabolism were studied in cerebellar astrocytes using (13)C magnetic resonance spectroscopy. Labeled glutamate, glutamine, aspartate, lactate, and alanine were observed both in the cell extracts and in media, and, additionally, labeled glycogen was detected in the cell extracts. However, only labeled lactate and alanine were quantifiable in the medium in addition to [U-(13)C]glucose. In the presence of unlabeled glutamate, the amount of [U-(13)C]glucose removed from the medium was decreased, indicating that glutamate might spare glucose as an energy substrate and thus decrease the uptake of glucose. Labeled glycogen, [4,5-(13)C]glutamate, [3,4,5-(13)C]glutamate, [3,4-(13)C]aspartate, and [U-(13)C]alanine were increased in the presence of glutamate. However, the increase in the amount of [3,4,5-(13)C]glutamate from the second turn in the tricarboxylic acid (TCA) cycle was less pronounced than that of [4,5-(13)C]glutamate from the first turn in the TCA cycle. This indicates the dilution of label, probably resulting from the synthesis of unlabeled oxaloacetate from glutamate in the TCA cycle. Furthermore, exogenous glutamate had an inhibiting effect on pyruvate carboxylation, presumably by formation of oxaloacetate from 2-oxoglutarate derived from glutamate. It could be shown that glucose is a better substrate for energy production than glutamate; it is, however, less efficient in labeling amino acids than glutamate in cerebellar astrocytes.

Activated astrocytes with glycogen accumulation in ischemic penumbra during the early stage of brain infarction: immunohistochemical and electron microscopic studies

Brain infarction was induced in rats by injection of microspheres through the right internal carotid artery, and structural changes in the astrocytes were observed during the early period following the infarction. Necrotic foci, varying in size and shape, were found in the right hemisphere. After immunohistochemical staining for GFAP, GFAP-positive astrocytes in the perinecrotic area known as the ischemic penumbra had distinctly increased in number and size with elongation of cytoplasmic processes 3 days after infarction. Electron microscopic observation revealed that glycogen granules had markedly accumulated in the cytoplasm of astrocytes located in the ischemic penumbra 3 and 5 days after infarction. Seven days after infarction, however, the glycogen granules disappeared from the astrocytes. Intermediate filaments increasingly appeared in the protoplasmic astrocytes after 3 days and were abundant in the activated and hypertrophied astrocytes after 7 days. As a result of our present study, we conclude that: (1) the function of glucose uptake from blood vessels was not impaired in the astrocytes under hypoxic conditions; (2) the astrocytes actively ingested blood glucose through the endothelial cells and accumulated it as glycogen for activation of their functions, and the cell volume increased under hypoxic conditions; (3) the depression of energy metabolism and the decrease in the uptake of energy sources in the nerve cells promoted glycogen accumulation in the astrocytes under hypoxic conditions; (4) intermediate filaments (GFAP filaments) increased in number, coincident with the activation and enlargement of the astrocytes; and (5) protoplasmic astrocytes were transformed into fibrous astrocytes in the ischemic penumbra of the brain infarction.

A review of the causes of central pontine myelinosis: yet another apoptotic illness?

One of the well recognized stimuli for central pontine myelinosis (CPM) is the rapid correction of chronic hyponatraemia. Conventionally this has been perceived to lead to pontine glial cell swelling through osmosis and eventually to cell death. However, although a purely osmotic argument has been central to any patho-physiological understanding of CPM, there are deficiencies in this approach that do not account for why certain individuals develop CPM with relatively mild osmotic insults. Here we review the varying aetiologies of CPM and propose a novel hypothesis for CPM causation by suggesting that individuals predisposed to CPM have inadequate energy provision as well as other factors that result in a pro-apoptotic drive, which renders them susceptible to brain injury from diverse causes. In CPM, the precipitant of brain injury appears to be osmotic stress. Furthermore, this model suggests a number of therapeutic interventions that may prevent or at least mitigate the consequences of CPM.