Biochemical and ultrastructural study of glycogen in cultured astrocytes submitted to the convulsant methionine sulfoximine

Ultrastructural characterization of rat neurons in primary culture

Few studies have addressed the ultrastructure and morphology of neurons in primary pure culture. We therefore use immunohistochemistry and electron microscopy to investigate the ultrastructure of cultured neurons during extended incubation in vitro. Rat cerebral cortex neurons were cultured in Neurobasal™ medium. Adherent cells developed as networks of single neurons or clusters depending on the plating density. Almost all surviving cells were neurons as demonstrated by neurofilament immunolabeling. The number of cultured neurons increased substantially to 14-21 days in vitro (DIV) and then plateaued and subsequently declined. From DIV 1-10 neurons extended large neurites, followed by the development of fine and dense neurites, and neurones survived until DIV 30-50. Notably, numerous mitochondria were observed along fibrous elements within neurites, suggestive of active intracellular trafficking. Electron microscopy also revealed that multiple types of synapses were formed between neurons. These ultrastructural results confirm previous reports of electrophysiological activity in cultured neurons. However many neurons contained distorted mitochondria and abnormal organelles including multilamellar vesicles and multivesicular myeloid bodies. The proportion of neurons containing abnormal organelles increased significantly in culture medium supplemented with antibiotics. On long-term culture neuronal death and apoptotic nuclei were observed. Despite the presence of abnormal organelles, the ultrastructure of cultured neurons was very similar to that of in vivo neurons; in vitro culture therefore provides a useful tool for studies on neuronal development, aging, and neurotransmission.

Glycogen as a Putative Target for Diagnosis and Therapy in Brain Pathologies

Brain glycogen, a glucose polymer, is now considered as a functional energy store to the brain. Indeed, when neurons outpace their own possibilities to provide themselves with energy, astrocytic metabolism is in charge of feeding neurons, since brain glycogen synthesis is mainly due to astrocyte. Therefore, malfunctions or perturbations of astrocytic glycogen content, synthesis, or mobilization may be involved in processes of brain pathologies. This is the case, for example, in epilepsies and gliomas, two different situations in which, brain needs high level of energy during acute or chronic conditions. The purpose of the present paper is to demonstrate how brain glycogen might be relevant in these two pathologies and to pinpoint the possibilities of considering glycogen as a tool for diagnostic and therapeutic approaches in brain pathologies.

Epilepsy, regulation of brain energy metabolism and neurotransmission

Seizures are the result of a sudden and temporary synchronization of neuronal activity, the reason for which is not clearly understood. Astrocytes participate in the control of neurotransmitter storage and neurotransmission efficacy. They provide fuel to neurons, which need a high level of energy to sustain normal and pathological neuronal activities, such as during epilepsy. Various genetic or induced animal models have been developed and used to study epileptogenic mechanisms. Methionine sulfoximine induces both seizures and the accumulation of brain glycogen, which might be considered as a putative energy store to neurons in various animals. Animals subjected to methionine sulfoximine develop seizures similar to the most striking form of human epilepsy, with a long pre-convulsive period of several hours, a long convulsive period during up to 48 hours and a post convulsive period during which they recover normal behavior. The accumulation of brain glycogen has been demonstrated in both the cortex and cerebellum as early as the pre-convulsive period, indicating that this accumulation is not a consequence of seizures. The accumulation results from an activation of gluconeogenesis specifically localized to astrocytes, both in vivo and in vitro. Both seizures and brain glycogen accumulation vary when using different inbred strains of mice. C57BL/6J is the most "resistant" strain to methionine sulfoximine, while CBA/J is the most "sensitive" one. The present review describes the data obtained on methionine sulfoximine dependent seizures and brain glycogen in the light of neurotransmission, highlighting the relevance of brain glycogen content in epilepsies.

Unlocking the Energy Dynamics of Executive Functioning: Linking Executive Functioning to Brain Glycogen

Past work suggests that executive functioning relies on glucose as a depletable energy, such that executive functioning uses a relatively large amount of glucose and is impaired when glucose is low. Glucose from the bloodstream is one energy source for the brain, and glucose stored in the brain as glycogen is another. A review of the literature on glycogen suggests that executive functioning uses it in much the same way as glucose, such that executive functioning uses glycogen and is impaired when glycogen is low. Findings on stress, physical persistence, glucose tolerance, diabetes, sleep, heat, and other topics provide general support for this view.

Abnormal organelles in cultured astrocytes are largely enhanced by streptomycin and intensively by gentamicin

The effects of two aminoglycoside antibiotics on cultured astrocyte organelles were investigated in rat, sheep, and human cultured astrocytes using transmission electron microscopy. Marked changes in mitochondrial shapes were observed in cultured or subcultured astrocytes obtained from three species, including humans. As well, new types of organelles were observed: (i) numerous concentric membranes forming vesicles, which were termed multilamellar vesicles; and (ii) many vesicles gathering into membranous structures, which were termed multivesicular myeloid bodies. The number of abnormalities increased proportionally with increasing concentrations of the two aminoglycosides (streptomycin and gentamicin). The incorporation of peroxidase or albumin-gold complex in the abnormal vesicles showed that the endolysosomal system was involved in the formation of these vesicles. Our results show that: abnormal organelles are present in cultured astrocytes; these abnormalities are enhanced by streptomycin and gentamicin; and gentamicin induces more abnormalities than streptomycin. The binding of aminoglycosides to membrane phospholipids may explain the formation of the observed abnormalities in rat, sheep, and human cultured astrocytes.

In vivo and in vitro glycogenic effects of methionine sulfoximine are different in two inbred strains of mice

We investigated the relationship between brain glycogen anabolism and methionine sulfoximine (MSO)-induced seizures in two inbred mouse strains that presented differential susceptibility to the convulsant. CBA/J was considered a MSO-high-reactive strain and C57BL/6J a MSO-low-reactive strain. Accordingly, the dose of MSO needed to induce seizures in CBA/J mice is lower than that in C57BL/6J mice, and CBA/J mice which had seizures, died during the first convulsion. In addition, the time--course of the MSO effect is faster in CBA/J mice than that in C57BL/6J mice. Analyses were performed in C57BL/6J and CBA/J mice after administration of 75 (subconvulsive dose) and 40 mg/kg of MSO (subconvulsive dose, not lethal dose), respectively. In the preconvulsive period, MSO induced an increase in the brain glycogen content of C57BL/6J mice only. Twenty-four hours after MSO administration, the brain glycogen content increased in both strains. The activity and expression of fructose-1,6-bisphosphatase, the last key enzyme of the gluconeogenic pathway, were increased in MSO-treated C57BL/6J mice as compared to control mice, at all experimental time points, whereas they were increased in CBA/J mice only 24 h after MSO administration. These latter results correspond to CBA/J mice that did not have seizures. Interestingly, the differences observed in vivo were consistent with results in primary cultured astrocytes from the two strains. This data suggests that the metabolism impairment, which was not a consequence of seizures, could be related to the difference in seizure susceptibility between the two strains, depending on their genetic background.

Chronic inhibition of glutamine synthetase is not associated with impairment of learning and memory in mice

The convulsant methionine sulfoximine (MSO) is a byproduct of the agenized flour commonly used for feeding domestic animals decades ago. MSO is a powerful glycogenic and epileptogenic agent, and it is an irreversible inhibitor of glutamine synthetase. This latter effect was hypothesized to be responsible for the increase in the incidence of some neuropathologies in humans, such as Alzheimer's disease or Parkinson's disease. In order to test this hypothesis, we chronically administered MSO to two inbred strains of mice, C57BL/6J and BALB/cJ, and analyzed possible alterations in learning and memory features of these mice. Mice were given 20 mg/kg of MSO three times a week for 10 weeks. Spatial learning capabilities assessed with a radial maze were not affected by the long-term MSO treatment, although activity was significantly decreased in BALB/cJ mice. Thus, our data suggest that long-term administration of non-convulsive and non-glycogenic doses of MSO do not alter the spatial memory of mice. Our results do not support the hypothesis that chronic treatment with MSO influences hippocampus-dependent learning abilities in mice.

Effects of adrenergic agonists on glycogenolysis in primary cultures of glycogen body cells and telencephalon astrocytes of the chick

The glycogen body (GB) is at the dorsal area of the lumbosacral spinal cord in birds and is composed of uniform cells that are characterized by high-glycogen storage. Previous morphological and embryological examinations suggest that the GB is derived from the neuroepithelium and contains many blood vessels and a few nerve fibers. However, the function of the GB and role of the glycogen are unknown. Mammalian astrocytes are major sites for glycogen stores in the central nervous system. The metabolic features of astrocytes have been defined by using cultured cells. As a first step toward investigating the function of GB, we established primary culture of chicken GB cells and telencephalon astrocytes. The cultured GB cells maintained high glycogen content and glial fibrillary acidic protein (GFAP) in the cytoplasm. The glycogen content of GB cells significantly increased with the glucose concentration in the medium. The effects of adrenergic agonists on glycogenolysis were different between GB cells and telencephalon astrocytes. The telencephalon astrocytes shared similar characteristics of glycogenolysis with mouse astrocytes, which are mainly affected by beta adrenergic receptor. Although GB cells were affected by noradrenalin (both alpha and beta adrenergic agonist), they were not affected by beta adrenergic agonist. These results showed that cultured GB cells were considered as one lineage of astrocytes because of their reactivity to antibody against GFAP; however, the metabolic features of GB cells were different from those of telencephalon astrocytes.

The xenobiotic methionine sulfoximine modulates carbohydrate anabolism and related genes expression in rodent brain

Methionine sulfoximine is a xenobiotic amino acid derived from methionine. One of its major properties is to display a glycogenic activity in the brain. After studying this property, we investigate here a possible action of this xenobiotic on the expression of genes related to carbohydrate anabolism in the brain. Glycogen was studied by the means of electron microscopy. Astrocytes were cultured and the influence of methionine sulfoximine on carbohydrate anabolism in these cells was investigated. In vivo, methionine sulfoximine induced a large increase in glycogen accumulation. It also enhanced the glycogen accumulation in cultured astrocytes principally, when the medium was enriched in glucose. The gluconeogenic enzyme fructose-1,6-bisphosphatase may account for glycogen accumulation. Plasmids were built using antisens cDNA to permanently block the expression of fructose-1,6-bisphosphatase. An eukaryotic vector was used and the expression of fructose-1,6-bisphosphatase gene was under the control of the promoter of the glial fibrillary acidic protein. In this case, the glycogen content in cultured astrocytes largely decreased. This work shows that methionine sulfoximine enhances energy carbohydrate synthesis in the brain. Since this xenobiotic also enhances the expression of some genes related to one of the key step of glucose synthesis, it is possible that genes may be one target of methionine sulfoximine. Next investigations will study the actual effect of methionine sulfoximine in the cells.

Diabetes reduces glutamate oxidation and glutamine synthesis in the retina. The Penn State Retina Research Group

Retinas of diabetic individuals develop early functional changes measurable by electrophysiological and psychometric testing. Using a rat model of diabetes, we previously identified diabetes-induced alterations in metabolism of the neurotransmitter glutamate which may ultimately lead to accumulation of glutamate in the retina (Diabetes, 47: 815, 1998). We therefore investigated the function of enzymes that mediate the synthesis and breakdown of glutamate in retinas from rats made diabetic by injection of streptozotocin. De novo synthesis of nitrogen-containing amino acids including glutamate, glutamine and aspartate was assessed by measuring the rate of carbon fixation in freshly dissected retinas, and was unchanged by diabetes. In contrast, the oxidation of glutamate was significantly reduced in retinas from diabetic rats (62%, P < 0.05). Furthermore, diabetic retinas were less susceptible to inhibition of glutamate oxidation by the transaminase inhibitor aminoxyacetate (80%, N.S.), compared to the significant decrease seen in control rats (61%, P < 0.001). The activity and content of glutamine synthetase were also significantly reduced in retinas from rats diabetic for 2-6 months [range of 48% (P < 0.005) to 83% (P < 0.05) compared to control]. The activity of glutamine synthetase was normalized by acute injections of insulin, but not by reducing blood sugar levels with injections of phlorizin. These results indicate two enzymatic abnormalities in the glutamate metabolism pathway in the retina during diabetes: transamination to alpha-ketoglutarate and amination to glutamine. The reduced flux through these pathways may be associated with the accumulation of glutamate. These results are also consistent with the possibility that some of the glial changes in the retina during diabetes may be caused by hypoinsulinemia rather than hyperglycemia.

The effects of isofagomine, a potent glycogen phosphorylase inhibitor, on glycogen metabolism in cultured mouse cortical astrocytes

A novel inhibitor of liver glycogen phosphorylase, isofagomine, was investigated as a possible inhibitor of the enzyme in the brain and in cultured astrocytes. Additionally, the effect of the drug on norepinephrine (NE) induced glycogen degradation in astrocytes was studied. Astrocytes were cultured from mouse cerebral cortex and homogenates were prepared from the cells as well as from mouse brain. Isofagomine dose-dependently inhibited glycogen phosphorylase when measured in the direction of glycogen degradation in both preparations with IC50 values (mean +/- SEM) of 1.0 +/- 0.1 microM and 3.3 +/- 0.5 microM in brain and astrocyte homogenates, respectively. Moreover, isofagomine at a concentration of 400 microM completely prevented NE induced depletion of glycogen stores and the concomitant lactate production in intact astrocytes. It is suggested that this novel glycogen phosphorylase inhibitor may be a valuable tool to investigate the functional importance of glycogen in astrocytes and in the brain.

Early, middle, and late stages of neural cells from ovine embryo in primary cultures

The utilization of neural cells in culture has importantly increased the knowledge of the nervous system biology. In most studies, the investigations are performed on biological materials coming from common laboratory animals and the extrapolation of the results to other animals is not easy. For some studies, such as developmental biology of the nervous system, prion disease investigations, or agronomical production, the utilization of ovine neural cell cultures presents many advantages. Unfortunately, there are few data on the conditions of culture of such cells. In the present work, we investigated simple ways to obtain neurons and astrocytes from sheep brain. Viable neuronal cell cultures were obtained from 40 to 50 day old fetuses. Their morphologies were quite similar to that of neurons from rodent or chick brain and they were labeled by antineurofilament antibodies. Stages older than 50 days of pregnancy were unable to give viable culture of neurons. The stages of 40 day old fetus to newborn lamb were able to give viable astrocyte cultures. The common protoplasmic astrocytes were obtained and they were labeled by antiglial fibrillary acidic protein antibodies. The astrocytes contained glycogen, thus looking like the common astrocytes from rodents. Neuronal or astroglial cultures can be derived from 26 day old embryos, but the cultures contained contaminating cells. Among the latter cells, there were undifferentiated cells which were flat and epitheloid and which were grouped as islets. These cells could be maintained in culture for a time duration over 7 months, even after two passages. They differentiated principally in astrocytes with a radial configuration. This work shows how some neural cells can be simply and easily cultured from sheep brain. For the first time, neurons were cultured from the sheep embryonic brain. Moreover, stem cells were cultured for more than 7 months and, finally, glycogen accumulation in sheep astrocytes was shown to be the same as that in rodent astrocytes. The oligodendrocyte culture was already documented. Thus, sheep can easily be used as well as other models for neural cell studies.

Direct cloning of astrocytes from primary culture without previous immortalization

In primary cultures, much evidence shows the existence of different subtypes of astrocytes that are not all identified. One methodology for studying these subtypes can be their cloning. The present investigation shows a method for a direct cloning of astrocytes without previous immortalization. Astrocytes from the cerebral cortex of newborn rats were cultured, purified by shaking, and harvested by trypsinization. One single astrocyte was plated in a small volume of a homemade cloning medium. After getting a colony, successive platings were made using larger and larger vessels, up to 60-mm-diameter petri dishes. Then, subcultures were made. The yield of the cloning was similar to that of common eukaryotic cell clonings. All along the cloning procedure, the cells were positively immunostained with anti-glial fibrillary acidic protein antibodies. Cloned cells from some batches were spindle-shaped, looking like fibroblasts. Nevertheless, they were immunostained with anti-glial fibrillary acidic protein antibodies, unlike true fibroblasts. These spindle-shaped astrocytes were compared to cells from an astrocytoma cell line that had the same shape. The growth pattern of the astrocytoma cells was different from that of the astrocytes cloned from the primary cultures. All the types of studied cells contained glycogen. On the basis of the criteria of morphology, of glial fibrillary acidic protein immunolabeling, and of glycogen synthesis, the cloned cells kept the characteristics of astrocytes. This study shows that it is perfectly possible to get clones of astrocytes from one astrocyte without previous immortalization, giving thus a convenient material for the study of astrocyte biology.

Isolation of carbohydrate metabolic clones from cultured astrocytes

Astrocytes are the principal sites of glycogen synthesis in the nervous tissue. Growing evidence shows that there are many types of astrocytes. The aim of the present investigation was to isolate different types of astrocytes that display different carbohydrate anabolism. Astrocytes from newborn rat brain were directly cloned from primary cultures without a previous transformation. Many clones were obtained, and they were termed CP clones. Another series of clones, termed SV clones, were obtained after the transfection of the primary cultures by the SV40 T antigen. The effectiveness of the transfection was verified by the rate of DNA synthesis using flow cytometry and by the presence of plasmid DNA in the genomic DNA of the astrocytes using the Southern blot method. After the transfection, the growth velocity increased greatly. The size and shape of the astrocytes were the same for each cell in a given clone, regardless of the cloning method utilized. However, these sizes and shapes could be different from one clone to another in CP clones, whereas all the astrocytes of all the SV clones looked like each other. All the clones obtained stained positively with anti-glial fibrillary acidic protein antibodies. Glycogen stained in the clones using concanavalin A-horseradish peroxidase. The glycogen content was also measured using biochemical analysis. Concordant results obtained using two methods showed that some clones contained an important quantity of glycogen while other clones contained a small amount, in the CP series as well as in the SV series. This property was the same for the intracellular glucose concentrations. The activity of the gluconeogenic enzyme fructose-1, 6-bisphosphatase was measured in each clone using spectrophotometry. This activity was also significantly different from one clone to another. The clones containing large amounts of glycogen had important fructose-1,6-bisphosphatase activity. The present results show that it is possible to clone astrocytes either directly from primary cultures without immortalization or after their transformation. When analyzing these clones, it appears that carbohydrate anabolism can be significantly different from one astrocyte to another. This difference may also exist in vivo.

Regulation of fructose-1,6-bisphosphatase activity in primary cultured astrocytes

In the gluconeogenic pathway, fructose-1,6-bisphosphatase (EC 3. 1. 3. 11) is the last key-enzyme before the synthesis of glucose-6-phosphate. The extreme diversity of cells present in the whole brain does not facilitate in vivo study of this enzyme and makes it difficult to understand the regulatory mechanisms of the related carbohydrate metabolism. It is for instance difficult to grasp the actual effect of ions like potassium, magnesium and manganese on the metabolic process just as it is difficult to grasp the effect of different pH values and the influence of glycogenic compounds such as methionine sulfoximine. The present investigation attempts to study the expression and regulation of fructose-1,6-bisphosphatase in cultured astrocytes. Cerebral cortex of new-born rats was dissociated into single cells that were then plated. The cultured cells were flat and roughly polygonal and were positively immunostained by anti-glial fibrillary acidic protein antibodies. Cultured astrocytes are able to display the activity of fructose-1,6-bisphosphatase. This activity was much higher than that in brain tissue in vivo. Fructose-1,6-bisphosphatase in cultured astrocytes did not require magnesium ions for its activity. The initial velocity observed when the activity was measured in standard conditions was largely increased when the enzyme was incubated with Mn2+. This increase was however followed by a decrease in absorbance resulting in the induction, by the manganese ions, of a singular kinetics in the enzyme activity. Potassium ions also stimulated fructose-1,6-bisphosphatase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Glial glycogen stores affect neuronal survival during glucose deprivation in vitro

Glia perform several energy-dependent functions that may aid neuronal survival under pathological conditions. Glycogen is the major energy reserve in brain, and it is localized almost exclusively to astrocytes. Using murine cortical cell cultures containing both glia and neurons, we examined the effect of altered glial glycogen stores on neuronal survival following glucose deprivation. As previously reported, cultures exposed for several hours to media lacking glucose developed widespread neuronal degeneration without glial degeneration. If glial astrocyte glycogen content was increased to 2-3 times control levels by a 24-h pretreatment with 1 microM insulin or 0.5 mM methionine sulfoximine (MSO), glucose deprivation-induced neuronal degeneration was attenuated. These protective effects were blocked if glycogen levels were reduced back to control levels by a 30-min exposure to 1 mM dibutyryl cyclic AMP or 20 microM norepinephrine prior to glucose deprivation. Astrocyte glycogen stores may be an important factor influencing neuronal survival under conditions of energy substrate limitation.