Glucose deprivation neuronal injury in vitro is modified by withdrawal of extracellular glutamine

Glucose Metabolism and AMPK Signaling Regulate Dopaminergic Cell Death Induced by Gene (α-Synuclein)-Environment (Paraquat) Interactions

While environmental exposures are not the single cause of Parkinson's disease (PD), their interaction with genetic alterations is thought to contribute to neuronal dopaminergic degeneration. However, the mechanisms involved in dopaminergic cell death induced by gene-environment interactions remain unclear. In this work, we have revealed for the first time the role of central carbon metabolism and metabolic dysfunction in dopaminergic cell death induced by the paraquat (PQ)-α-synuclein interaction. The toxicity of PQ in dopaminergic N27 cells was significantly reduced by glucose deprivation, inhibition of hexokinase with 2-deoxy-D-glucose (2-DG), or equimolar substitution of glucose with galactose, which evidenced the contribution of glucose metabolism to PQ-induced cell death. PQ also stimulated an increase in glucose uptake, and in the levels of glucose transporter type 4 (GLUT4) and Na⁺-glucose transporters isoform 1 (SGLT1) proteins, but only inhibition of GLUT-like transport with STF-31 or ascorbic acid reduced PQ-induced cell death. Importantly, while autophagy protein 5 (ATG5)/unc-51 like autophagy activating kinase 1 (ULK1)-dependent autophagy protected against PQ toxicity, the inhibitory effect of glucose deprivation on cell death progression was largely independent of autophagy or mammalian target of rapamycin (mTOR) signaling. PQ selectively induced metabolomic alterations and adenosine monophosphate-activated protein kinase (AMPK) activation in the midbrain and striatum of mice chronically treated with PQ. Inhibition of AMPK signaling led to metabolic dysfunction and an enhanced sensitivity of dopaminergic cells to PQ. In addition, activation of AMPK by PQ was prevented by inhibition of the inducible nitric oxide syntase (iNOS) with 1400W, but PQ had no effect on iNOS levels. Overexpression of wild type or A53T mutant α-synuclein stimulated glucose accumulation and PQ toxicity, and this toxic synergism was reduced by inhibition of glucose metabolism/transport and the pentose phosphate pathway (6-aminonicotinamide). These results demonstrate that glucose metabolism and AMPK regulate dopaminergic cell death induced by gene (α-synuclein)-environment (PQ) interactions.

Glutamine effect on cultured granule neuron death induced by glucose deprivation and chemical hypoxia

Using a specific fluorescent probe of mitochondrial membrane potential (tetramethylrhodamine ethyl ester), we have shown that glucose deprivation (GD) of cultured cerebellar granule neurons (CGN) for 3 h lowers mitochondrial membrane potential in these cells. Longer glucose starvation (24 h) causes CGN death that is not prevented by blockers of ionotropic glutamate receptors (MK-801 (10 µM) and NBQX (10 µM)). Glutamine or pyruvate (2 mM) maintain membrane potential of mitochondria and decrease CGN death under GD conditions. In the presence of glucose the mitochondrial respiratory chain blocker rotenone induces neuron death potentiated by glutamine. The potentiation effect is completely prevented by blockers of ionotropic glutamate receptors. These results show that glutamine under conditions of GD can be utilized by mitochondria as substrate, but at the same time, in the case of mitochondrial function deterioration, metabolism of this amino acid results in glutamate accumulation to toxic level.

Cellular mechanisms of brain hypoglycemia

Data on intracellular processes induced by a low glucose level in nerve tissue are presented. The involvement of glutamate and adenosine receptors, mitochondria, reactive oxygen species (ROS), and calcium ions in the development of hypoglycemia-induced damage of neurons is considered. Hypoglycemia-induced calcium overload of neuronal mitochondria is suggested to be responsible for the increased ROS production by mitochondria.

Glutamate-induced deregulation of calcium homeostasis and mitochondrial dysfunction in mammalian central neurones

Delayed neuronal death following prolonged (10-15 min) stimulation of Glu receptors is known to depend on sustained elevation of cytosolic Ca(2+) concentration ([Ca(2+)](i)) which may persist far beyond the termination of Glu exposure. Mitochondrial depolarization (MD) plays a central role in this Ca(2+) deregulation: it inhibits the uniporter-mediated Ca(2+) uptake and reverses ATP synthetase which enhances greatly ATP consumption during Glu exposure. MD-induced inhibition of Ca(2+) uptake in the face of continued Ca(2+) influx through Glu-activated channels leads to a secondary increase of [Ca(2+)](i) which, in its turn, enhances MD and thus [Ca(2+)](i). Antioxidants fail to suppress this pathological regenerative process which indicates that reactive oxygen species are not involved in its development. In mature nerve cells (>11 DIV), the post-glutamate [Ca(2+)](i) plateau associated with profound MD usually appears after 10-15 min Glu (100 microM) exposure. In contrast, in young cells (<9 DIV) delayed Ca(2+) deregulation (DCD) occurs only after 30-60 min Glu exposure. This difference is apparently determined by a dramatic increase in the susceptibility of mitochondia to Ca(2+) overload during nerve cells maturation. The exact mechanisms of Glu-induced profound MD and its coupling with the impairment of Ca(2+) extrusion following toxic Glu challenge is not clarified yet. Their elucidation demands a study of dynamic changes in local concentrations of ATP, Ca(2+), H(+), Na(+) and protein kinase C using novel methodological approaches.

Piracetam impedes hippocampal neuronal loss during withdrawal after chronic alcohol intake

In previous studies we have demonstrated that prolonged ethanol consumption induced hippocampal neuronal loss. In addition, we have shown that withdrawal after chronic alcohol intake augmented such degenerative activity leading to increased neuronal death in all subregions of the hippocampal formation but in the CA3 field. In an attempt to reverse this situation, we tested, during the withdrawal period, the effects of piracetam (2-oxo-1-pyrrolidine acetamide), a cyclic derivative of gamma-aminobutyric acid, as there is previous evidence that it might act as a neuronoprotective agent. The total number of dentate granule, hilar, and CA3 and CA1 pyramidal cells of the hippocampal formation were estimated using unbiased stereological methods. We found out that in animals treated with piracetam the numbers of dentate granule, hilar, and CA1 pyramidal cells were significantly higher than in pure withdrawn animals, and did not differ from those of alcohol-treated rats that did not undergo withdrawal. These data suggest that piracetam treatment impedes, during withdrawal, the pursuing of neuronal degeneration.

Metabolic differences between primary cultures of astrocytes and neurons from cerebellum and cerebral cortex. Effects of fluorocitrate

Astrocytes and neurons cultured from mouse cerebellum and cerebral cortex were analyzed with respect to content and synthesis of amino acids as well as export of metabolites to the culture medium and the response to fluorocitrate, an inhibitor of aconitase. The intracellular levels of amino acids were similar in the two astrocytic populations. The release of citrate, lactate and glutamine, however, was markedly higher from cerebellar than from cortical astrocytes. Neurons contained higher levels of glutamate, aspartate and GABA than astrocytic cultures. Cortical neurons were especially high in GABA and aspartate, and the level of aspartate increased specifically when the extracellular level of glutamine was elevated. Fluorocitrate inhibited the TCA cycle in the astrocytes, but was less effective in cerebellar neurons. Whereas neurons responded to fluorocitrate with an increase in the formation of lactate, reflecting glycolysis, astrocytes decreased the formation of lactate in the presence of fluorocitrate, indicating that astrocytes to a high degree synthesize pyruvate and hence lactate from TCA cycle intermediates.

The effect of acute hypoglycemia on the cerebral NMDA receptor in newborn piglets

The effects of acute insulin-induced hypoglycemia on the cerebral NMDA receptor in the newborn were examined by determining [3H]MK-801 binding as an index of NMDA receptor function in 6 control and 7 hypoglycemic piglets. In hypoglycemic animals, the glucose clamp technique with constant insulin infusion was used to maintain a blood glucose concentration of 1.2 mmol/l for 120 min before obtaining cerebral cortex for further analysis; controls received a saline infusion. Concentrations of glucose, lactate, ATP, and PCr were measured in cortex, and Na+,K(+)-ATPase activity was determined in a brain cell membrane preparation. [3H]MK-801 binding was evaluated by: (1) saturation binding assays over the range of 0.5-50 nM [3H]MK-801 in the presence of 100 microM glutamate and glycine; and (2) binding assays at 10 nM [3H]MK-801 in the presence of glutamate and/or glycine at 0, 10, or 100 microM. Blood and brain glucose concentrations were significantly lower in hypoglycemic animals than controls. There was no change in brain ATP with hypoglycemia, but PCr was decreased 80% compared to control (P < 0.05). Na+,K(+)-ATPase activity was 13% lower in hypoglycemic animals (P < 0.05). Based on saturation binding data, hypoglycemia had no effect on the number of functional receptors (Bmax), but the apparent affinity was significantly increased, as indicated by a decrease in the Kd (dissociation constant) from the control value of 8.1 +/- 1.6 nM to 5.5 +/- 2.1 nM (P < 0.05). Augmentation of [3H]MK-801 binding by glutamate and glycine alone or in combination was also significantly greater in the hypoglycemic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell death in primary cultures of mouse neurons and astrocytes during exposure to and 'recovery' from hypoxia, substrate deprivation and simulated ischemia

Effects of hypoxia, substrate deprivation and simulated ischemia (combined hypoxia and substrate deprivation) on cell survival during the insult itself and during a 24 h 'recovery' period were studied in primary cultures of mouse astrocytes and in cerebral cortical neuronal-astrocytic co-cultures. Cell death was determined by release of the cytosolic high molecular enzyme lactate dehydrogenase (LDH) as well as morphologically (retention of staining with rhodamine 123 and lack of staining with propidium iodide as an indicator of live cells). Glutamate concentrations were measured in the incubation media at the end of the metabolic insults. Astrocytes were very resistant to hypoxia, but less so to simulated ischemia; under both conditions the glutamate concentrations in the media remained low. Cerebral cortical neurons were almost equally susceptible to damage by hypoxia and by simulated ischemia, although hypoxia had a faster deleterious effects on some of the neurons and simulated ischemia during a long-term insult (9 h) killed all neurons, whereas a non-negligible neuronal subpopulation survived 9 h of hypoxia. Neuronal cell death after long-term hypoxia (but not after simulated ischemia) was correlated with high concentrations of glutamate in the incubation media. After certain insults, most notably relatively short lasting simulated ischemia (3 h) in neurons (which caused no increased cell death during the insult), there was a large release of LDH during the 'recovery' period.

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.