Requirement of glycolytic and mitochondrial energy supply for loading of Ca(2+) stores and InsP(3)-mediated Ca(2+) signaling in rat hippocampus astrocytes

Evidence that hindbrain astrocytes in the rat detect low glucose with a glucose transporter 2-phospholipase C-calcium release mechanism

Astrocytes generate robust cytoplasmic calcium signals in response to reductions in extracellular glucose. This calcium signal, in turn, drives purinergic gliotransmission, which controls the activity of catecholaminergic (CA) neurons in the hindbrain. These CA neurons are critical to triggering glucose counter-regulatory responses (CRRs) that, ultimately, restore glucose homeostasis via endocrine and behavioral means. Although the astrocyte low-glucose sensor involvement in CRR has been accepted, it is not clear how astrocytes produce an increase in intracellular calcium in response to a decrease in glucose. Our ex vivo calcium imaging studies of hindbrain astrocytes show that the glucose type 2 transporter (GLUT2) is an essential feature of the astrocyte glucosensor mechanism. Coimmunoprecipitation assays reveal that the recombinant GLUT2 binds directly with the recombinant Gq protein subunit that activates phospholipase C (PLC). Additional calcium imaging studies suggest that GLUT2 may be connected to a PLC-endoplasmic reticular-calcium release mechanism, which is amplified by calcium-induced calcium release (CICR). Collectively, these data help outline a potential mechanism used by astrocytes to convert information regarding low-glucose levels into intracellular changes that ultimately regulate the CRR.

Hindbrain astrocytes and glucose counter-regulation

Hindbrain astrocytes are emerging as critical components in the regulation of homeostatic functions by either modulating synaptic activity or serving as primary detectors of physiological parameters. Recent studies have suggested that the glucose counter-regulation response (CRR), a critical defense against hypoglycemic emergencies, is dependent on glucoprivation-sensitive astrocytes in the hindbrain. This subpopulation of astrocytes produces a robust calcium signal in response to glucopenic stimuli. Both ex vivo and in vivo evidence suggest that low-glucose sensitive astrocytes utilize purinergic gliotransmission to activate catecholamine neurons in the hindbrain that are critical to the generation of the integrated CRR. Lastly, reports in the clinical literature suggest that an uncontrolled activation of CRR may as part of the pathology of severe traumatic injury. Work in our laboratory also suggests that this pathological hyperglycemia resulting from traumatic injury may be caused by the action of thrombin (generated by tissue trauma or bleeding) on hindbrain astrocytes. Similar to their glucopenia-sensitive neighbors, these hindbrain astrocytes may trigger hyperglycemic responses by their interactions with catecholaminergic neurons.

Response of catecholaminergic neurons in the mouse hindbrain to glucoprivic stimuli is astrocyte dependent

Hindbrain catecholaminergic (CA) neurons are required for critical autonomic, endocrine, and behavioral counterregulatory responses (CRRs) to hypoglycemia. Recent studies suggest that CRR initiation depends on hindbrain astrocyte glucose sensors (McDougal DH, Hermann GE, Rogers RC. Front Neurosci 7: 249, 2013; Rogers RC, Ritter S, Hermann GE. Am J Physiol Regul Integr Comp Physiol 310: R1102-R1108, 2016). To test the proposition that hindbrain CA responses to glucoprivation are astrocyte dependent, we utilized transgenic mice in which the calcium reporter construct (GCaMP5) was expressed selectively in tyrosine hydroxylase neurons (TH-GCaMP5). We conducted live cell calcium-imaging studies on tissue slices containing the nucleus of the solitary tract (NST) or the ventrolateral medulla, critical CRR initiation sites. Results show that TH-GCaMP5 neurons are robustly activated by a glucoprivic challenge and that this response is dependent on functional astrocytes. Pretreatment of hindbrain slices with fluorocitrate (an astrocytic metabolic suppressor) abolished TH-GCaMP5 neuronal responses to glucoprivation, but not to glutamate. Pharmacologic results suggest that the astrocytic connection with hindbrain CA neurons is purinergic via P2 receptors. Parallel imaging studies on hindbrain slices of NST from wild-type C57BL/6J mice, in which astrocytes and neurons were prelabeled with a calcium reporter dye and an astrocytic vital dye, show that both cell types are activated by glucoprivation but astrocytes responded significantly sooner than neurons. Pretreatment of these hindbrain slices with P2 antagonists abolished neuronal responses to glucoprivation without interruption of astrocyte responses; pretreatment with fluorocitrate eliminated both astrocytic and neuronal responses. These results support earlier work suggesting that the primary detection of glucoprivic signals by the hindbrain is mediated by astrocytes.

Haloacetic Acid Water Disinfection Byproducts Affect Pyruvate Dehydrogenase Activity and Disrupt Cellular Metabolism

The disinfection of drinking water has been a major public health achievement. However, haloacetic acids (HAAs), generated as byproducts of water disinfection, are cytotoxic, genotoxic, mutagenic, carcinogenic, and teratogenic. Previous studies of monoHAA-induced genotoxicity and cell stress demonstrated that the toxicity was due to inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), leading to disruption of cellular metabolism and energy homeostasis. DiHAAs and triHAAs are also produced during water disinfection, and whether they share mechanisms of action with monoHAAs is unknown. In this study, we evaluated the effects of mono-, di-, and tri-HAAs on cellular GAPDH enzyme kinetics, cellular ATP levels, and pyruvate dehydrogenase complex (PDC) activity. Here, treatments conducted in Chinese hamster ovary (CHO) cells revealed differences among mono-, di-, and triHAAs in their molecular targets. The monoHAAs, iodoacetic acid and bromoacetic acid, were the strongest inhibitors of GAPDH and greatly reduced cellular ATP levels. Chloroacetic acid, diHAAs, and triHAAs were weaker inhibitors of GAPDH and some increased the levels of cellular ATP. HAAs also affected PDC activity, with most HAAs activating PDC. The primary finding of this work is that mono- versus multi-HAAs address different molecular targets, and the results are generally consistent with a model in which monoHAAs activate the PDC through GAPDH inhibition-mediated disruption in cellular metabolites, including altering ATP-to-ADP and NADH-to-NAD ratios. The monoHAA-mediated reduction in cellular metabolites results in accelerated PDC activity by way of metabolite-ratio-dependent PDC regulation. DiHAAs and triHAAs are weaker inhibitors of GAPDH, but many also increase cellular ATP levels, and we suggest that they increase PDC activity by inhibiting pyruvate dehydrogenase kinase.

The vicious circle of hypometabolism in neurodegenerative diseases: Ways and mechanisms of metabolic correction

Hypometabolism, characterized by decreased brain glucose consumption, is a common feature of many neurodegenerative diseases. Initial hypometabolic brain state, created by characteristic risk factors, may predispose the brain to acquired epilepsy and sporadic Alzheimer's and Parkinson's diseases, which are the focus of this review. Analysis of available data suggests that deficient glucose metabolism is likely a primary initiating factor for these diseases, and that resulting neuronal dysfunction further promotes the metabolic imbalance, establishing an effective positive feedback loop and a downward spiral of disease progression. Therefore, metabolic correction leading to the normalization of abnormalities in glucose metabolism may be an efficient tool to treat the neurological disorders by counteracting their primary pathological mechanisms. Published and preliminary experimental results on this approach for treating Alzheimer's disease and epilepsy models support the efficacy of metabolic correction, confirming the highly promising nature of the strategy. © 2017 Wiley Periodicals, Inc.

Hindbrain glucoprivation effects on gastric vagal reflex circuits and gastric motility in the rat are suppressed by the astrocyte inhibitor fluorocitrate

Fasting and hypoglycemia elicit powerful gastrointestinal contractions. Whereas the relationship between utilizable nutrient and gastric motility is well recognized, the explanation of this phenomenon has remained incomplete. A relatively recent controversial report suggested that astrocytes in the dorsal hindbrain may be the principal detectors of glucoprivic stimuli. Our own studies also show that a subset of astrocytes in the solitary nucleus (NST) is activated by low glucose. It is very likely that information about glucopenia may directly impact gastric control because the hindbrain is also the location of the vago-vagal reflex circuitry regulating gastric motility. Our in vivo single unit neurophysiological recordings in intact rats show fourth ventricular application of 2-deoxyglucose (2-DG) inhibits NST neurons and activates dorsal motor nucleus (DMN) neurons involved in the gastric accommodation reflex. Additionally, as shown in earlier studies, either systemic insulin or central 2-DG causes an increase in gastric motility. These effects on motility were blocked by fourth ventricle pretreatment with the astrocyte inactivator fluorocitrate. Fluorocitrate administered alone has no effect on gastric-NST or -DMN neuron responsiveness, or on gastric motility. These results suggest that glucoprivation-induced increases in gastric motility are dependent on intact hindbrain astrocytes.

Astrocytes in the nucleus of the solitary tract are activated by low glucose or glucoprivation: evidence for glial involvement in glucose homeostasis

Glucose homeostasis is maintained through interplay between central and peripheral control mechanisms which are aimed at storing excess glucose following meals and mobilizing these same stores during periods of fasting. The nucleus of the solitary tract (NST) in the dorsal medulla has long been associated with the central detection of glucose availability and the control of glucose homeostasis. Recent evidence has emerged which supports the involvement of astrocytes in glucose homeostasis. The aim of the present study was to investigate whether NST-astrocytes respond to physiologically relevant decreases in glucose availability, in vitro, as well as to the presence of the glucoprivic compound 2-deoxy-D-Glucose. This report demonstrates that some NST-astrocytes are capable of responding to low glucose or glucoprivation by increasing cytoplasmic calcium; a change that reverses with restoration of normal glucose availability. While some NST-neurons also demonstrate an increase in calcium signaling during low glucose availability, this effect is smaller and somewhat delayed compared to those observed in adjacent astrocytes. TTX did not abolish these hypoglycemia mediated responses of astrocytes, suggesting that NST-astrocytes may be directly sensing low glucose levels as opposed to responding to neuronal detection of hypoglycemia. Thus, chemodetection of low glucose by NST-astrocytes may play an important role in the autonomic regulation of glucose homeostasis.

Pyruvate remediation of cell stress and genotoxicity induced by haloacetic acid drinking water disinfection by-products

Monohaloacetic acids (monoHAAs) are a major class of drinking water disinfection by-products (DBPs) and are cytotoxic, genotoxic, mutagenic, and teratogenic. We propose a model of toxic action based on monoHAA-mediated inhibition of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as a target cytosolic enzyme. This model predicts that GAPDH inhibition by the monoHAAs will lead to a severe reduction of cellular ATP levels and repress the generation of pyruvate. A loss of pyruvate will lead to mitochondrial stress and genomic DNA damage. We found a concentration-dependent reduction of ATP in Chinese hamster ovary cells after monoHAA treatment. ATP reduction per pmol monoHAA followed the pattern of iodoacetic acid (IAA) > bromoacetic acid (BAA) >> chloroacetic acid (CAA), which is the pattern of potency observed with many toxicological endpoints. Exogenous supplementation with pyruvate enhanced ATP levels and attenuated monoHAA-induced genomic DNA damage as measured with single cell gel electrophoresis. These data were highly correlated with the SN 2 alkylating potentials of the monoHAAs and with the induction of toxicity. The results from this study strongly support the hypothesis that GAPDH inhibition and the possible subsequent generation of reactive oxygen species is linked with the cytotoxicity, genotoxicity, teratogenicity, and neurotoxicity of these DBPs.

Astrocytes in the hindbrain detect glucoprivation and regulate gastric motility

Glucoprivation is a strong signal for the initiation of gastrointestinal contractions. While this relationship between utilizable nutrient levels and gastric motility has been recognized for more than 100 years, the explanation of this phenomenon has remained incomplete. Using widely differing approaches, recent work has suggested that the hindbrain is responsible for this chemoreflex effect. Surprisingly, astrocytes may be the main glucodetector elements under hypoglycemic conditions. Our own work using in vitro live cell calcium imaging shows that astrocytes in the NST increase cytoplasmic calcium in a concentration dependent manner in reaction to reductions in glucose. This effect is reversed on restoration of normal glucose concentrations. In vivo single unit neurophysiological recordings show that brainstem neurons driving gastric motility are activated by glucoprivic stimuli. Studies in intact animals verify that both dorsal medullary and systemic glucoprivation significantly increases gastric motility. Astrocyte inactivation with fluorocitrate blocks the pro-motility effects of glucoprivation. Thus, it appears that intact astrocyte signaling may be essential to glucoregulatory control over gastric motility.

Compartmentation of GAPDH

The concept of the cytosol as a space that contains discrete zones of metabolites is discussed relative to the contribution of GAPDH. GAPDH is directed to very specific cell compartments. This chapter describes the utilization of GAPDH's enzymatic function for focal demands (i.e. ATP/ADP and NAD(+)/NADH), and offers a speculative role for GAPDH as perhaps moderating local concentrations of inorganic phosphate and hydrogen ions (i.e. co-substrate and co-product of the glycolytic reaction, respectively). Where known, the structural features of the binding between GAPDH and the compartment components are discussed. The nuances, which are associated with the intracellular distribution of GAPDH, appear to be specific to the cell-type, particularly with regards to the various plasma membrane proteins to which GAPDH binds. The chapter includes discussion on the curious observation of GAPDH being localized to the external surface of the plasma membrane in a human cell type. The default perspective has been that GAPDH localization is synonymous with compartmentation of glycolytic energy. The chapter discusses GAPDH translocation to the nucleus and to non-nuclear cellular structures, emphasizing its glycolytic function. Nevertheless, it is becoming clear that alternate functions of GAPDH play a role in compartmentation, particularly in the translocation to the nucleus.

Hypoxia enhances high-voltage-activated calcium currents in rat primary cortical neurons via calcineurin

Hypoxia regulates neuronal ion channels, sometimes resulting in seizures. We evaluated the effects of brief sustained hypoxia (1% O(2), 4h) on voltage-gated calcium channels (VGCCs) in cultured rat primary cortical neurons. High-voltage activated (HVA) Ca(2+) currents were acquired immediately after hypoxic exposure or after 48h recovery in 95% air/5% CO(2). Maximal Ca(2+) current density increased 1.5-fold immediately after hypoxia, but reverted to baseline after 48h normoxia. This enhancement was primarily due to an increase in L-type VGCC activity, since nimodipine-insensitive residual Ca(2+) currents were unchanged. The half-maximal potentials of activation and steady-state inactivation were unchanged. The calcineurin inhibitors FK-506 (in the recording pipette) or cyclosporine A (during hypoxia) prevented the post-hypoxic increase in HVA Ca(2+) currents, while rapamycin and okadaic acid did not. L-type VGCCs were the source of Ca(2+) for calcineurin activation, as nimodipine during hypoxia prevented post-hypoxic enhancement. Hypoxia transiently potentiated L-type VGCC currents via calcineurin, suggesting a positive feedback loop to amplify neuronal calcium signaling that may contribute to seizure generation.

Copper accelerates glycolytic flux in cultured astrocytes

Astrocyte-rich primary cultures were used to investigate the consequences of a copper exposure on the glucose metabolism of astrocytes. After application of CuCl(2) (30 μM) the specific cellular copper content increased from initial 1.5 ± 0.2 nmol/mg to a steady state level of 7.9 ± 0.9 nmol/mg within about 12 h. The copper accumulation was accompanied by a significant increase in the extracellular lactate concentration. The stimulating effect of copper on the lactate production remained after removal of extracellular copper. Copper treatment accelerated the rates of both glucose consumption and lactate production by about 60%. The copper induced acceleration of glycolytic flux was prevented by inhibition of protein synthesis, and additive to the stimulation of glycolysis observed for inhibitors of respiration or prolyl hydroxylases. A copper induced stimulation of glycolytic flux in astrocytes could have severe consequences for the glucose metabolism of the brain in conditions of copper overload.

Oxidatively modified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Alzheimer's disease: many pathways to neurodegeneration

Recently, the oxidoreductase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), has become a subject of interest as more and more studies reveal a surfeit of diverse GAPDH functions, extending beyond traditional aerobic metabolism of glucose. As a result of multiple isoforms and cellular locales, GAPDH is able to come in contact with a variety of small molecules, proteins, membranes, etc., that play important roles in normal and pathologic cell function. Specifically, GAPDH has been shown to interact with neurodegenerative disease-associated proteins, including the amyloid-beta protein precursor (AbetaPP). Studies from our laboratory have shown significant inhibition of GAPDH dehydrogenase activity in Alzheimer's disease (AD) brain due to oxidative modification. Although oxidative stress and damage is a common phenomenon in the AD brain, it would seem that inhibition of glycolytic enzyme activity is merely one avenue in which AD pathology affects neuronal cell development and survival, as oxidative modification can also impart a toxic gain-of-function to many proteins, including GAPDH. In this review, we examine the many functions of GAPDH with respect to AD brain; in particular, the apparent role(s) of GAPDH in AD-related apoptotic cell death is emphasized.

Multifunctional roles of enolase in Alzheimer's disease brain: beyond altered glucose metabolism

Enolase enzymes are abundantly expressed, cytosolic carbon-oxygen lyases known for their role in glucose metabolism. Recently, enolase has been shown to possess a variety of different regulatory functions, beyond glycolysis and gluconeogenesis, associated with hypoxia, ischemia, and Alzheimer's disease (AD). AD is an age-associated neurodegenerative disorder characterized pathologically by elevated oxidative stress and subsequent damage to proteins, lipids, and nucleic acids, appearance of neurofibrillary tangles and senile plaques, and loss of synapse and neuronal cells. It is unclear if development of a hypometabolic environment is a consequence of or contributes to AD pathology, as there is not only a significant decline in brain glucose levels in AD, but also there is an increase in proteomics identified oxidatively modified glycolytic enzymes that are rendered inactive, including enolase. Previously, our laboratory identified alpha-enolase as one the most frequently up-regulated and oxidatively modified proteins in amnestic mild cognitive impairment (MCI), early-onset AD, and AD. However, the glycolytic conversion of 2-phosphoglycerate to phosphoenolpyruvate catalyzed by enolase does not directly produce ATP or NADH; therefore it is surprising that, among all glycolytic enzymes, alpha-enolase was one of only two glycolytic enzymes consistently up-regulated from MCI to AD. These findings suggest enolase is involved with more than glucose metabolism in AD brain, but may possess other functions, normally necessary to preserve brain function. This review examines potential altered function(s) of brain enolase in MCI, early-onset AD, and AD, alterations that may contribute to the biochemical, pathological, clinical characteristics, and progression of this dementing disorder.

NMDA-induced stimulation of glycolysis in developing hippocampal cell cultures

Developmental changes in energy metabolism of primary hippocampal cell cultures from newborn rats were investigated during the first 3 weeks. These changes were measured by intensity of and number of cells exhibiting NAD(P)H fluorescence in response to NMDA-induced activation of neuronal activity. We observed gradual changes of stimulation-evoked NAD(P)H signaling over the first 3 weeks, such that at day 7 and 16, this stimulation is minimal, while at 5 and 12 days, it is maximal. These results describe a biphasic pattern that was similar to earlier findings from experiments investigating developmental changes in population spike amplitudes or glutamate release in young rats. Inhibition of mitochondrial respiration by KCN revealed that the NMDA-evoked stimulation of energy metabolism is mainly due to increased glycolytic activity.

Astrocytic calcium signaling: mechanism and implications for functional brain imaging

Astrocytes are electrically non-excitable cells that, on a slow time scale of seconds, integrate synaptic transmission by dynamic increases in cytosolic Ca2+. A number of groups have recently shown that astrocytic Ca2+ signaling regulates vascular tones and that astrocytes play a central role in functional hyperemia by Ca2+ -dependent release of Prostaglandin E2 (PGE2). Astrocytes are, however, not simple detectors of excitatory transmission, since a number of neuromodulator and hormones trigger elevations in astrocytic Ca2+ independently of synaptic transmission. Furthermore, astrocytes exhibit ex vivo intrinsic Ca2+ excitability, or spontaneous increases in Ca2+ that are not triggered by receptor activation. The notion that astrocytes can regulate vascular tone independently of synaptic transmission challenges the notion that changes in the blood oxygenation level dependent (BOLD) signal is directly proportional to neuronal activity and may thus require a reevaluation of the large body of data accumulated using functional magnetic resonance imaging (fMRI).

D-beta-hydroxybutyrate prevents glutamate-mediated lipoperoxidation and neuronal damage elicited during glycolysis inhibition in vivo

Excitotoxic neuronal death mediated by over-activation of glutamate receptors has been implicated in ischemia, hypoglycemia and some neurodegenerative diseases. It involves oxidative stress and is highly facilitated during impairment of energy metabolism. We have shown previously that in vivo systemic glycolysis inhibition with iodoacetate (IOA), exacerbates glutamate excitotoxicity. We have now investigated whether this effect involves oxidative damage to membrane lipids, as evaluated by the presence of thiobarbituric acid-reactive substances. We have also tested whether the ketone body, D-beta-hydroxybutyrate (D-BHB), prevents lipoperoxidation and tissue damage. Results show that glutamate intrastriatal injection in control rats transiently enhances lipoperoxidation, while in IOA-treated animals increased lipoperoxidation is sustained. Treatment with D-BHB significantly reduces striatal lesions and lipoperoxidation. Vitamin E also reduced neuronal damage and lipoperoxidation. Results suggest that glycolysis impairment favors a pro-oxidant condition and situates oxidative damage as an important mediator of in vivo induced excitotoxicity. Results provide evidence for the neuroprotective effect of D-BHB against glutamate toxicity.

Interaction of the brain-specific protein p42IP4/centaurin-alpha1 with the peptidase nardilysin is regulated by the cognate ligands of p42IP4, PtdIns(3,4,5)P3 and Ins(1,3,4,5)P4, with stereospecificity

The brain-specific protein p42IP4, also called centaurin-alpha1, specifically binds phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. Here, we investigate the interaction of p42IP4/centaurin-alpha1 with nardilysin (NRDc), a member of the M16 family of zinc metalloendopeptidases. Members of this peptidase family exhibit enzymatic activity and also act as receptors for other proteins. We found that p42IP4/centaurin-alpha1 binds specifically to NRDc from rat brain. We further detected that centaurin-alpha2, a protein that is highly homologous to p42IP4/centaurin-alpha1 and expressed ubiquitously, also binds to NRDc. In vivo interaction was demonstrated by co-immunoprecipitation of p42IP4/centaurin-alpha1 with NRDc from rat brain. The acidic domain of NRDc (NRDc-AD), which does not participate in catalysis, is sufficient for the protein interaction with p42IP4. Interestingly, preincubation of p42IP4 with its cognate ligands D-Ins(1,3,4,5)P4 and the lipid diC8PtdIns(3,4,5)P3 negatively modulates the interaction between the two proteins. D-Ins(1,3,4,5)P4 and diC8PtdIns(3,4,5)P3 suppress the interaction with virtually identical concentration dependencies. This inhibition is highly ligand specific. The enantiomer L-Ins(1,3,4,5)P4 is not effective. Similarly, the phosphoinositides diC8PtdIns(3,4)P2, diC8PtdIns(3,5)P2 and diC8PtdIns(4,5)P2 all have no influence on the interaction. Further experiments revealed that endogenous p42IP4 from rat brain binds to glutathione-S-transferase (GST)-NRDc-AD. The proteins dissociate from each other when incubated with D-Ins(1,3,4,5)P4, but not with inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In summary, we demonstrate that p42IP4 binds to NRDc via the NRDc-AD, and that this interaction is controlled by the cognate cellular ligands of p42IP4/centaurin-alpha1. Thus, specific ligands of p42IP4 can modulate the recruitment of proteins, which are docked to p42IP4, to specific cellular compartments.

Role of oxidative stress on beta-amyloid neurotoxicity elicited during impairment of energy metabolism in the hippocampus: protection by antioxidants

Age-associated oxidative stress has been implicated in neuronal damage linked with Alzheimer's disease (AD). In addition to the role of beta-amyloid peptide (Abeta) in the pathogenesis of AD, reduced glucose oxidative metabolism and decreased mitochondrial activity have been suggested as associated factors. However, the relationship between Abeta toxicity, metabolic impairment, and oxidative stress is far from being understood. In vivo neurotoxicity of Abeta25-35 peptide has been conflicting. However, in previous studies, we have shown that Abeta25-35 consistently induces synaptic toxicity and neuronal death in the hippocampus in vivo, when administered during moderate glycolytic or mitochondrial inhibition. In the present study, we have investigated whether enhancement of Abeta neurotoxicity during these conditions involves oxidative stress. Results show increased lipoperoxidation (LPO) when Abeta is administered in the hippocampus of rats previously treated with the glycolysis inhibitor, iodoacetate. Neuronal damage and LPO are efficiently prevented by vitamin E, while the spin trapper, alpha-phenyl-N-tert-butyl nitrone, shows partial protection. Abeta stimulates LPO in synaptosomes, but toxicity is only observed in the presence of metabolic inhibitors. Damage and LPO are efficiently prevented by vitamin E. The present results suggest an interaction between oxidative stress and metabolic impairment in the Abeta neurotoxic cascade.

A study of the cytotoxicity of branched-chain phytanic acid with mitochondria and rat brain astrocytes

Phytanic acid, a saturated fatty acid of 20-carbon-atoms with isoprenoic structure, is formed from the phytol-side chain of chlorophyll in ruminants. Degradation of phytanic acid is blocked in Refsum disease by several enzymatic defects of peroxisomal degradation of branched-chain fatty acids. Refsum disease is an inherited neurological disorder progressively developing from early childhood to adultness. Clinical signs are attributed to toxicity of phytanic acid, which accumulates to unusually high levels in the tissue and serum of patients suffering from untreated Refsum disease. We report here that hippocampal astrocytes isolated from rat brain, which were exposed to phytanic acid (50 microM) die within a few hours. In situ depolarization of mitochondria and an increase of cytosolic Ca2+ precede cell death. Therefore, we also investigated the influence of phytanic acid on physiology of mitochondria isolated from rat brain. Mitochondria become functionally impaired by phytanic acid, as indicated by uncoupling (resting state), inhibition of the electron transport (state 3), stimulation of ROS-generation, decline of Ca2+ loading and severe release of cytochrome c. Thus, phytanic acid seems to initiate astrocyte cell death by activating the mitochondrial route of apoptosis.

Disruption of endoplasmic reticulum calcium stores is involved in neuronal death induced by glycolysis inhibition in cultured hippocampal neurons

Disturbances in neuronal calcium homeostasis have been implicated in a variety of neuropathological conditions, including cerebral ischemia, hypoglycemia, and epilepsy, and possibly constitute part of the cell death process associated with chronic neurodegenerative disorders. We investigated if endoplasmic reticulum (ER) calcium stores participate in neuronal death triggered by moderate glycolysis inhibition induced by iodoacetate, an inhibitor of glyceraldehyde-3-phosphate dehydrogenase, in cultured hippocampal neurons. Results show that exposure to iodoacetate leads to a slow partial decrease in cell survival, which is significantly prevented in the absence of Ca(2+) or in the presence of the calcium chelator BAPTA-AM. Treatment with caffeine and a low (1 microM) concentration of ryanodine, which activates the ryanodine receptor (RyR), exacerbates neuronal death, whereas dantrolene and 25 microM ryanodine, which antagonizes RyR, prevents damage. Xestospongin C (XeC), an antagonist of the inositol-3-phosphate (IP(3)) receptor (IP(3)R) also prevents neuronal damage. Inhibitors of the ER calcium ATPase (sarcoendoplasmic reticulum Ca(2+) ATPase; SERCA) have no effect. The decrease in ATP levels induced by iodoacetate is potentiated by caffeine and prevented by dantrolene. Although only a slight increase in glutamate extracellular levels is observed 3.5 hr after iodoacetate exposure, the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, MK-801, efficiently prevents neuronal damage. Taken together, the data suggest that neuronal death induced during moderate glycolysis inhibition involves calcium influx through NMDA receptors and calcium release from intracellular ER stores. These results might be relevant to the understanding the mechanisms involved in neuronal damage related to aging and chronic neurodegenerative diseases, which have been associated with decreased glucose metabolism.

Mechanism of toxicity of the branched‐chain fatty acid phytanic acid, a marker of Refsum disease, in astrocytes involves mitochondrial impairment

Phytanic acid is a saturated branched-chain fatty acid, which is formed by bacterial degradation of chlorophyll in the intestinal tract of ruminants. The methyl group in beta-position prevents degradation of phytanic acid by the beta-oxidation pathway. Therefore, degradation of phytanic acid is initiated by alpha-oxidation in peroxisomes. The inherited peroxisomal disorder Refsum disease is characterised by accumulation of phytanic acid. Unusually high concentrations of phytanic acid can be found in the plasma of Refsum disease patients, who suffer from neurodegeneration and muscle dystrophy. Phytanic acid has been suggested to be causally involved in the clinical symptoms. To elucidate the pathogenic mechanism, we investigated the influence of phytanic acid in rat hippocampal astrocytes by monitoring the cytosolic Ca(2+) concentration, the mitochondrial membrane potential (Deltapsi(m)), the generation of reactive oxygen species as well as the cellular ATP level. In response to phytanic acid (100 microM) cytosolic Ca(2+) was quickly increased. The phytanic acid-evoked Ca(2+) response was transient and involved activation of intracellular Ca(2+) stores. In isolated rat brain mitochondria, phytanic acid dissipated Deltapsi(m) in a reversible and dose-dependent manner. Moreover, phytanic acid released cytochrome c from mitochondria. Depending on the mitochondrial activity state, phytanic acid either stimulated or inhibited the electron flux within the respiratory chain. In addition, phytanic acid induced substantial generation of reactive oxygen species in isolated mitochondria as well as in intact cells. Phytanic acid caused cell death of astrocytes within a few hours of exposure. In conclusion, we suggest that phytanic acid initiates astrocyte cell death by activating the mitochondrial route of apoptosis.

Gap junction blockage limits intercellular spreading of astrocytic apoptosis induced by metabolic depression

Astrocytes are highly coupled by gap junction channels, which allow transfer of intracellular signalling molecules and metabolites between connected cells. Astrocytic gap junctions remain open during ischemic conditions as previously demonstrated in vitro and in situ. In this study, we investigated the effect of gap junction blockage on iodoacetate-induced ATP depression and cell death progression in astrocytes in primary rat hippocampal cultures. We demonstrated that blockage of gap junctions during iodoacetate-induced inhibition of the glycolysis induced an earlier onset of the ATP depression. Moreover, initiation of apoptotic processes, demonstrated by binding of Annexin V, was critically dependent on the ATP levels. The apoptotic event was also shown to spread and involve neighbouring cells, a process that was inhibited by blockage of gap junction communication. Chelating intracellular calcium using BAPTA-AM decelerated the iodoacetate-induced ATP depression. The chelation also decelerated the spreading of apoptotic processes. Inhibition of caspases did not alter the expansion of cell groups being Annexin V positive. However, the proportion of Annexin V positive cells also being propidium iodide positive was increased after caspase inhibition. The results show that inhibition of gap junctions during cellular metabolic depression interferes with the metabolic status and cell death progression in astrocytes.

Na-K-Cl cotransporter-mediated intracellular Na+ accumulation affects Ca2+ signaling in astrocytes in an in vitro ischemic model

Na-K-Cl cotransporter isoform 1 (NKCC1) plays an important role in maintenance of intracellular Na+, K+, and Cl- levels in astrocytes. We propose that NKCC1 may contribute to perturbations of ionic homeostasis in astrocytes under ischemic conditions. After 3-8 hr of oxygen and glucose deprivation (OGD), NKCC1-mediated 86Rb influx was significantly increased in astrocytes from NKCC1 wild-type (NKCC1+/+) and heterozygous mutant (NKCC1+/-) mice. Phosphorylated NKCC1 protein was increased in NKCC1+/+ astrocytes at 2 hr of OGD. Two hours of OGD and 1 hr of reoxygenation (OGD/REOX) triggered an 3.6-fold increase in intracellular Na+ concentration ([Na+]i) in NKCC1+/+ astrocytes. Inhibition of NKCC1 activity by bumetanide or ablation of the NKCC1 gene significantly attenuated the rise in [Na+]i. Moreover, NKCC1+/+ astrocytes swelled by 10-30% during 20-60 min of OGD. Either genetic ablation of NKCC1 or inhibition of NKCC1 by bumetanide-attenuated OGD-mediated swelling. An NKCC1-mediated increase in [Na+]i may subsequently affect Ca2+ signaling through the Na+/Ca2+ exchanger (NCX). A rise in [Ca2+]i was detected after OGD/REOX in the presence of a sarcoplasmic-endoplasmic reticulum (ER) Ca2+-ATPase inhibitor thapsigargin. Moreover, OGD/REOX led to a significant increase in Ca2+ release from ER Ca2+ stores. Furthermore, KB-R7943 (2-[2-[4(4-nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate), an inhibitor of reverse-mode operation of NCX, abolished the OGD/REOX-induced enhancement in filling of ER Ca2+ stores. OGD/REOX-mediated Ca2+ accumulation in ER Ca2+ stores was absent when NKCC1 activity was ablated or pharmacologically inhibited. These findings imply that stimulation of NKCC1 activity leads to Na+ accumulation after OGD/REOX and that subsequent reverse-mode operation of NCX contributes to increased Ca2+ accumulation by intracellular Ca2+ stores.

The Refsum disease marker phytanic acid, a branched chain fatty acid, affects Ca2+ homeostasis and mitochondria, and reduces cell viability in rat hippocampal astrocytes

The saturated branched chain fatty acid, phytanic acid, a degradation product of chlorophyll, accumulates in Refsum disease, an inherited peroxisomal disorder with neurological clinical features. To elucidate the pathogenic mechanism, we investigated the influence of phytanic acid on cellular physiology of rat hippocampal astrocytes. Phytanic acid (100 microM) induced an immediate transient increase in cytosolic Ca2+ concentration, followed by a plateau. The peak of this biphasic Ca2+ response was largely independent of extracellular Ca2+, indicating activation of cellular Ca2+ stores by phytanic acid. Phytanic acid depolarized mitochondria without causing in situ swelling of mitochondria. The slow decrease of mitochondrial potential is not consistent with fast and simultaneous opening of the mitochondrial permeability transition pore. However, phytanic acid induced substantial generation of reactive oxygen species. Phytanic acid caused astroglia cell death after a few hours of exposure. We suggest that the cytotoxic effect of phytanic acid seems to be due to a combined action on Ca2+ regulation, mitochondrial depolarization, and increased ROS generation in brain cells.

Glutamate-mediated influx of extracellular Ca2+ is coupled with reactive oxygen species generation in cultured hippocampal neurons but not in astrocytes

Generation of reactive oxygen species (ROS) in brain tissue leads to neurodegeneration. The major source of ROS is the mitochondrial respiratory chain. We studied regulation of Ca2+ level, mitochondrial potential, and ROS generation in defined mixed hippocampal cell cultures exposed to glutamate (100 microM). Recordings were made from individually identified astrocytes and neurons to compare the physiologic responses in both cell types. Neurons identified by synaptotagmin immunoreactivity were characterized functionally by the fast Ca2+ increase with K+ (50 mM) stimulation, and the astrocytes identified by glial fibrillary acidic protein (GFAP) staining had the functional characteristic of a transient Ca2+ peak in response to ATP (10 microM) stimulation. We found that the glutamate-mediated Ca2+ response in neurons is due largely to influx of extracellular Ca2+. This is consistent with our finding that in cultured hippocampal neurons, stores depending on the activity of the sarcoendoplasmic reticulum Ca2+ ATPase (SERCA) pump had a low Ca2+ content, regardless of whether the neurons were challenged or not with K+ before applying the SERCA inhibitor cyclopiazonic acid (CPA). Astrocytes displayed a large CPA-mediated Ca2 response, indicating a high level of Ca2+ load in the stores in astrocytes. Importantly, the rise in ROS generation due to glutamate application was cell-type specific. In neurons, glutamate induced a marked rise in generation of ROS, but not in astrocytes. In both astrocytes and neurons, the mitochondrial potential was increased in response to glutamate challenge. We conclude that in neurons, Ca2+ influx accounts for the increased ROS generation in response to glutamate. This might explain the high vulnerability of neurons to glutamate challenge compared to the vulnerability of astrocytes. The high resistance of astrocytes is accompanied by an efficient downregulation of cytosolic Ca2+, which is not found in neurons.

In brain mitochondria the branched-chain fatty acid phytanic acid impairs energy transduction and sensitizes for permeability transition

Phytanic acid (3,7,11,15-tetramethylhexadecanoic acid) accumulates at high levels throughout the body in the adult form of Refsum disease, a peroxisomal genetic disorder. However, it is still unclear why increased levels of phytanic acid have cytotoxic effects. In the present study, we examined the influence of non-esterified phytanic acid on energy-related functions of mitochondria from adult rat brain. Phytanic acid at low concentrations (5-20 microM, i.e. 5-20 nmol/mg of mitochondrial protein) de-energized mitochondria, as indicated by depolarization, stimulation of non-phosphorylating oxygen uptake and inhibition of the reduction of the tetrazolium dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide. The unbranched homologue palmitic acid exerted much smaller effects. In addition, phytanic acid reduced state 3 respiration, which was partly due to inhibition of the ADP/ATP carrier. Phytanic acid decreased the rate of adenine nucleotide exchange and increased the degree of control, which the ADP/ATP carrier has on state 3 respiration. Important for functional consequences is the finding that mitochondria, which are preloaded with small amounts of Ca2+ (100 nmol/mg of protein), became highly sensitized to rapid permeability transition even when only low concentrations of phytanic acid (below 5 microM) were applied. In conclusion, the incorporation of phytanic acid into the inner mitochondrial membrane increases the membrane H+ conductance and disturbs the protein-linked functions in energy coupling. This is most probably essential for the short-term toxicity of phytanic acid. Thus in neural tissue, which becomes enriched with phytanic acid, the reduction in mitochondrial ATP supply and the facilitation of the opening of the permeability transition pore are two major mechanisms by which the branched-chain fatty acid phytanic acid induces the onset of degenerative processes.

Glial perspectives of metabolic states during cerebral hypoxia—calcium regulation and metabolic energy

Cooperation between astrocytes and neurons is a unique interaction between two highly specialized cell types of the brain. Therefore, lack of nutrient supply during ischemia requires tight coordination of metabolism between astrocytes and neurons to keep the brain functions intact. To understand the impact of energy limitation on astrocytes, the functions of astrocytes have to be considered: (i) supplementation of neuronal cells, (ii) modulation of the extracellular milieu, mainly of the glutamate level, and (iii) elimination of reactive oxygen species (ROS). In cultured astrocytes and neurons inhibition of oxidative phosphorylation, using rotenone, was tested. Interestingly, this had only a negligible effect on Ca2+ homeostasis in astrocytes, even in combination with a severe glutamate stress. In contrast, in neurons glutamate in the presence of rotenone induced Ca2+ deregulation. Ca2+ homeostasis is very critical for cell survival. A massive and prolonged Ca2+ rise will lead to deregulation of many processes in such a way that the cells affected can hardly survive. Ca2+ homeostasis depends on the energy-consuming processes, which maintain the steep gradient between intracellular and extracellular Ca2+ concentration. Deprivation of oxygen and glucose during ischemia leads to a depletion of ATP in the brain, due to inhibited glycolytic and mitochondrial activity, whereas energy-consuming processes like ion pumps drain the ATP pools. On the other hand, specific mechanisms can protect brain structures against the massive insult of ischemia. Glycogen, stored in astrocytes, can maintain both neurons and astrocytes alive during short limitation of oxygen and glucose. Moreover, astrocytes can fuel ATP generation by providing lactate for neurons.

Striatal neurones show sustained recovery from severe hypoglycaemic insult

Glucose deprivation provides a reliable model to investigate cellular responses to metabolic dysfunction, and is reportedly associated with permanent cell death in many paradigms. Consistent with previous studies, primary cultures of rat striatal neurones exposed to 24-h hypoglycaemia showed dramatically decreased sodium 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) metabolism (used as a marker of cell viability) and increased TUNEL staining, suggesting widespread DNA damage typical of apoptotic cell death. Remarkably, restoration of normal glucose levels initiated a sustained recovery in XTT staining, along with a concomitant decrease in TUNEL staining, even after 24 h of hypoglycaemia, suggesting recovery of damaged neurones and repair of nicked DNA. No alterations in the levels of four DNA repair proteins could be detected during hypoglycaemia or recovery. A reduction in intracellular calcium concentration was seen in recovered cells. These data suggest that striatal cells do not die after extended periods of glucose deprivation, but survive in a form of suspended animation, with sufficient energy to maintain membrane potential.

Inhibition of the alpha-ketoglutarate dehydrogenase complex alters mitochondrial function and cellular calcium regulation

Mitochondrial dysfunction occurs in many neurodegenerative diseases. The alpha-ketoglutarate dehydrogenase complex (KGDHC) catalyzes a key and arguably rate-limiting step of the tricarboxylic acid cycle (TCA). A reduction in the activity of the KGDHC occurs in brains and cells of patients with many of these disorders and may underlie the abnormal mitochondrial function. Abnormalities in calcium homeostasis also occur in fibroblasts from Alzheimer's disease (AD) patients and in cells bearing mutations that lead to AD. Thus, the present studies test whether the reduction of KGDHC activity can lead to the alterations in mitochondrial function and calcium homeostasis. alpha-Keto-beta-methyl-n-valeric acid (KMV) inhibits KGDHC activity in living N2a cells in a dose- and time-dependent manner. Surprisingly, concentration of KMV that inhibit in situ KGDHC by 80% does not alter the mitochondrial membrane potential (MMP). However, similar concentrations of KMV induce the release of cytochrome c from mitochondria into the cytosol, reduce basal [Ca(2+)](i) by 23% (P<0.005), and diminish the bradykinin (BK)-induced calcium release from the endoplasmic reticulum (ER) by 46% (P<0.005). This result suggests that diminished KGDHC activities do not lead to the Ca(2+) abnormalities in fibroblasts from AD patients or cells bearing PS-1 mutations. The increased release of cytochrome c with diminished KGDHC activities will be expected to activate other pathways including cell death cascades. Reductions in this key mitochondrial enzyme will likely make the cells more vulnerable to metabolic insults that promote cell death.

Swelling of mitochondria in cultured rat hippocampal astrocytes is induced by high cytosolic Ca(2+) load, but not by mitochondrial depolarization

The influence of cytosolic Ca(2+) load and of mitochondrial membrane potential change on mitochondrial morphology was investigated in cultured rat hippocampal astrocytes. The uncoupler FCCP, applied together with oligomycin, depolarized mitochondria rapidly but did not change their morphology. Depolarization was associated with a moderate cytosolic [Ca(2+)](i) rise of up to 0.3 microM. Only high cytosolic Ca(2+) load (above a threshold of 50 microM), which was evoked by application of the ionophore 4-Br-A23187 in Ca(2+)-containing medium, caused drastic change of mitochondrial morphology. The shape change from the typical rod-like to a spherical shape, indicating mitochondrial swelling, was associated with depolarization. Cyclosporin A sensitivity suggests involvement of permeability transition. Thus, a dramatic cytosolic [Ca(2+)](i) rise is required to induce mitochondrial swelling and depolarization. A large but still moderate [Ca(2+)](i) rise evoked by physiological stimulation, however, has no comparable effect.

Simulations of inositol phosphate metabolism and its interaction with InsP(3)-mediated calcium release

Inositol phosphates function as second messengers for a variety of extracellular signals. Ins(1,4,5)P(3) generated by phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate, triggers numerous cellular processes by regulating calcium release from internal stores. The Ins(1,4,5)P(3) signal is coupled to a complex metabolic cascade involving a series of phosphatases and kinases. These enzymes generate a range of inositol phosphate derivatives, many of which have signaling roles of their own. We have integrated published biochemical data to build a mass action model for InsP(3) metabolism. The model includes most inositol phosphates that are currently known to interact with each other. We have used this model to study the effects of a G-protein coupled receptor stimulus that activates phospholipase C on the inositol phosphates. We have also monitored how the metabolic cascade interacts with Ins(1,4,5)P(3)-mediated calcium release. We find temporal dynamics of most inositol phosphates to be strongly influenced by the elaborate networking. We also show that Ins(1,3,4,5)P(4) plays a key role in InsP(3) dynamics and allows for paired pulse facilitation of calcium release. Calcium oscillations produce oscillatory responses in parts of the metabolic network and are in turn temporally modulated by the metabolism of InsP(3).

Mitochondrial polarization in rat hippocampal astrocytes is resistant to cytosolic Ca(2+) loads

The influence of physiological Ca(2+)-inducing stimuli and agents mimicking ischemic conditions on mitochondrial potential was studied in postnatal (P1) hippocampal astrocytes. Cytosolic Ca(2+) loads with characteristic kinetics of rise and duration, detected by Fura-2, were provoked by extracellular Ca(2+) influx, release from InsP(3)-sensitive intracellular stores, or inhibition of the reloading of endoplasmic reticulum Ca(2+) stores. Inhibitors of mitochondrial respiration caused only moderate release of Ca(2+) from intracellular stores, inducing a rise of less than 60 nM. The maximal Ca(2+) rise was found with InsP(3)-mediated responses (500 nM; via ATP) or with ionophore (4-Br-A23187)-mediated Ca(2+) influx from extracellular medium (770 nM). Remarkably, all these agents causing significant rise of cytosolic Ca(2+), only minimally depolarized the mitochondria. Membrane potential of mitochondria was monitored by Rh123 or TMRE. Depolarization was only found with very high cytosolic Ca(2+) levels (above 60 microM; measured by fura FF). These were achieved with external Ca(2+) influx by ionophore in combination with inhibition of glycolysis. Thus, mitochondria in the astrocytes are obviously not sensitive to moderate cytosolic Ca(2+) loads, irrespective of the source of Ca(2+). Furthermore, isolated rat brain mitochondria display a low sensitivity of respiratory activity to Ca(2+), which is consistent with the data obtained with the astrocytes in vitro. The capacity of isolated mitochondria to build up a potential was gradually reduced at low micromolar Ca(2+) and totally compromised only at Ca(2+) concentrations in the 100 microM range.

Endoplasmic reticulum dysfunction--a common denominator for cell injury in acute and degenerative diseases of the brain?

Various physiological, biochemical and molecular biological disturbances have been put forward as mediators of neuronal cell injury in acute and chronic pathological states of the brain such as ischemia, epileptic seizures and Alzheimer's or Parkinson's disease. These include over-activation of glutamate receptors, a rise in cytoplasmic calcium activity and mitochondrial dysfunction. The possible involvement of the endoplasmic reticulum (ER) dysfunction in this process has been largely neglected until recently, although the ER plays a central role in important cell functions. Not only is the ER involved in the control of cellular calcium homeostasis, it is also the subcellular compartment in which the folding and processing of membrane and secretory proteins takes place. The fact that blocking of these processes is sufficient to cause cell damage indicates that they are crucial for normal cell functioning. This review presents evidence that ER function is disturbed in many acute and chronic diseases of the brain. The complex processes taken place in this subcellular compartment are however, affected in different ways in various disorders; whereas the ER-associated degradation of misfolded proteins is affected in Parkinson's disease, it is the unfolded protein response which is down-regulated in Alzheimer's disease and the ER calcium homeostasis that is disturbed in ischemia. Studying the consequences of the observed deteriorations of ER function and identifying the mechanisms causing ER dysfunction in these pathological states of the brain will help to elucidate whether neurodegeneration is indeed caused by these disturbances, and will help to facilitate the search for drugs capable of blocking the pathological process directly at an early stage.