Regulation of lactate metabolism by albumin in rat neurons and astrocytes from primary culture

Lactate Metabolism, Signaling, and Function in Brain Development, Synaptic Plasticity, Angiogenesis, and Neurodegenerative Diseases

Neural tissue requires a great metabolic demand despite negligible intrinsic energy stores. As a result, the central nervous system (CNS) depends upon a continuous influx of metabolic substrates from the blood. Disruption of this process can lead to impairment of neurological functions, loss of consciousness, and coma within minutes. Intricate neurovascular networks permit both spatially and temporally appropriate metabolic substrate delivery. Lactate is the end product of anaerobic or aerobic glycolysis, converted from pyruvate by lactate dehydrogenase-5 (LDH-5). Although abundant in the brain, it was traditionally considered a byproduct or waste of glycolysis. However, recent evidence indicates lactate may be an important energy source as well as a metabolic signaling molecule for the brain and astrocytes-the most abundant glial cell-playing a crucial role in energy delivery, storage, production, and utilization. The astrocyte-neuron lactate-shuttle hypothesis states that lactate, once released into the extracellular space by astrocytes, can be up-taken and metabolized by neurons. This review focuses on this hypothesis, highlighting lactate's emerging role in the brain, with particular emphasis on its role during development, synaptic plasticity, angiogenesis, and disease.

Astrocyte strategies in the energy-efficient brain

Astrocytes generate ATP through glycolysis and mitochondrion respiration, using glucose, lactate, fatty acids, amino acids, and ketone bodies as metabolic fuels. Astrocytic mitochondria also participate in neuronal redox homeostasis and neurotransmitter recycling. In this essay, we aim to integrate the multifaceted evidence about astrocyte bioenergetics at the cellular and systems levels, with a focus on mitochondrial oxidation. At the cellular level, the use of fatty acid β-oxidation and the existence of molecular switches for the selection of metabolic mode and fuels are examined. At the systems level, we discuss energy audits of astrocytes and how astrocytic Ca2+ signaling might contribute to the higher performance and lower energy consumption of the brain as compared to engineered circuits. We finish by examining the neural-circuit dysregulation and behavior impairment associated with alterations of astrocytic mitochondria. We conclude that astrocytes may contribute to brain energy efficiency by coupling energy, redox, and computational homeostasis in neural circuits.

Human serum albumin in neurodegeneration

Serum albumin (SA) exists in relatively high concentrations, in close contact with most cells. However, in the adult brain, except for cerebrospinal fluid (CSF), SA concentration is relatively low. It is mainly produced in the liver to serve as the main protein of the blood plasma. In the plasma, it functions as a carrier, chaperon, antioxidant, source of amino acids, osmoregulator, etc. As a carrier, it facilitates the stable presence and transport of the hydrophobic and hydrophilic molecules, including free fatty acids, steroid hormones, medicines, and metal ions. As a chaperon, SA binds to and protects other proteins. As an antioxidant, thanks to a free sulfhydryl group (-SH), albumin is responsible for most antioxidant properties of plasma. These functions qualify SA as a major player in, and a mirror of, overall health status, aging, and neurodegeneration. The low concentration of SA is associated with cognitive deterioration in the elderly and negative prognosis in multiple sclerosis (MS) and amyotrophic lateral sclerosis (ALS). SA has been shown to be structurally modified in neurological conditions such as Alzheimer's disease (AD). During blood-brain barrier damage albumin enters the brain tissue and could trigger epilepsy and neurodegeneration. SA is able to bind to the precursor agent of the AD, amyloid-beta (Aβ), preventing its toxic effects in the periphery, and is being tested for treating this disease. SA therapy may also be effective in brain rejuvenation. In the current review, we will bring forward the prominent properties and roles of SA in neurodegeneration.

Ambient PM2.5-induced brain injury is associated with the activation of PI3K/AKT/FoxO1 pathway

PM_2.5-related neurological and mental diseases, such as cognitive impairment and stroke, tend to cause disability. Six-week-old male C57BL/6 mice were divided into 6 groups and exposed to concentrated PM_2.5 or filtered air for 2, 4, and 6 months, respectively. The neurobehavioral changes of mice were tested. The weight of the whole brain and olfactory bulbs were recorded at the end of exposure, and the brain structure was observed by hematoxylin and eosin (HE) staining. Serum indicators, mRNA, and protein expressions were detected. The spatial learning memory ability was impaired, and the mice were more anxious after PM_2.5 exposure. Relative brain weight decreased with age, and PM_2.5 exposure exceeded the decrease of relative brain weight. Interestingly, superoxide dismutase (SOD) and albumin decreased in the PM_2.5-exposed groups although neuronal morphology and other serum indicators did not show significant difference between PM and FA groups. Moreover, PM_2.5 induced the increase of plasminogen at 2 months but recovered at 4 months and then increased at 6 months again. The results from protein expression and transcriptomic test demonstrated that PI3K/AKT/FoxO1 pathway might be activated after 6-month PM_2.5 exposure in mice. Indicators albumin, the percentage of albumin over IgG (A/G value), and plasminogen were the main serous changes in mice after early-stage (2 months) and long-term (6 months) PM_2.5 exposure. In addition, early-stage injury induced by PM_2.5 might recover at later time point and display significant injury again with the exposure time. PM_2.5 exposure-induced brain injury might be associated with the activation of PI3K/AKT/FoxO1 pathway.

In Vitro Evaluation of Biocompatibility of Uncoated Thermally Reduced Graphene and Carbon Nanotube-Loaded PVDF Membranes with Adult Neural Stem Cell-Derived Neurons and Glia

Graphene, graphene-based nanomaterials (GBNs), and carbon nanotubes (CNTs) are being investigated as potential substrates for the growth of neural cells. However, in most in vitro studies, the cells were seeded on these materials coated with various proteins implying that the observed effects on the cells could not solely be attributed to the GBN and CNT properties. Here, we studied the biocompatibility of uncoated thermally reduced graphene (TRG) and poly(vinylidene fluoride) (PVDF) membranes loaded with multi-walled CNTs (MWCNTs) using neural stem cells isolated from the adult mouse olfactory bulb (termed aOBSCs). When aOBSCs were induced to differentiate on coverslips treated with TRG or control materials (polyethyleneimine-PEI and polyornithine plus fibronectin-PLO/F) in a serum-free medium, neurons, astrocytes, and oligodendrocytes were generated in all conditions, indicating that TRG permits the multi-lineage differentiation of aOBSCs. However, the total number of cells was reduced on both PEI and TRG. In a serum-containing medium, aOBSC-derived neurons and oligodendrocytes grown on TRG were more numerous than in controls; the neurons developed synaptic boutons and oligodendrocytes were more branched. In contrast, neurons growing on PVDF membranes had reduced neurite branching, and on MWCNTs-loaded membranes oligodendrocytes were lower in numbers than in controls. Overall, these findings indicate that uncoated TRG may be biocompatible with the generation, differentiation, and maturation of aOBSC-derived neurons and glial cells, implying a potential use for TRG to study functional neuronal networks.

Metabolic reprogramming by the pyruvate dehydrogenase kinase-lactic acid axis: Linking metabolism and diverse neuropathophysiologies

Emerging evidence indicates that there is a complex interplay between metabolism and chronic disorders in the nervous system. In particular, the pyruvate dehydrogenase (PDH) kinase (PDK)-lactic acid axis is a critical link that connects metabolic reprogramming and the pathophysiology of neurological disorders. PDKs, via regulation of PDH complex activity, orchestrate the conversion of pyruvate either aerobically to acetyl-CoA, or anaerobically to lactate. The kinases are also involved in neurometabolic dysregulation under pathological conditions. Lactate, an energy substrate for neurons, is also a recently acknowledged signaling molecule involved in neuronal plasticity, neuron-glia interactions, neuroimmune communication, and nociception. More recently, the PDK-lactic acid axis has been recognized to modulate neuronal and glial phenotypes and activities, contributing to the pathophysiologies of diverse neurological disorders. This review covers the recent advances that implicate the PDK-lactic acid axis as a novel linker of metabolism and diverse neuropathophysiologies. We finally explore the possibilities of employing the PDK-lactic acid axis and its downstream mediators as putative future therapeutic strategies aimed at prevention or treatment of neurological disorders.

Pyruvate to Lactate Metabolic Changes during Neurodevelopment Measured Dynamically Using Hyperpolarized 13C Imaging in Juvenile Murine Brain

Hyperpolarized 13C magnetic resonance imaging has recently been used to dynamically image metabolism in vivo. This technique provides the capability to investigate metabolic changes in mouse brain development over multiple time points. In this study, we used 13C magnetic resonance spectroscopic imaging and hyperpolarized 13C-1-labeled pyruvate to analyze its conversion into lactate. We also applied T2-weighted anatomical imaging to examine brain volume changes starting from postnatal day 18 (P18). We combined these results with body weight measurements for a comprehensive interpretation of mouse brain maturation. Both the produced lactate level and pyruvate to lactate conversion rate decreased with increasing age in a linear manner. Total brain volume remained the same after P18, even though body weight continued to grow exponentially. Our results have shown that the rate of metabolism of 13C-1 pyruvate to lactate in brain is high in the young mouse and decreases with age. The brain at P18 is still relatively immature and continues to develop even as the total brain volume remains the same.

HIF-1 and c-Src mediate increased glucose uptake induced by endothelin-1 and connexin43 in astrocytes

In previous work we showed that endothelin-1 (ET-1) increases the rate of glucose uptake in astrocytes, an important aspect of brain function since glucose taken up by astrocytes is used to supply the neurons with metabolic substrates. In the present work we sought to identify the signalling pathway responsible for this process in primary culture of rat astrocytes. Our results show that ET-1 promoted an increase in the transcription factor hypoxia-inducible factor-1α (HIF-1α) in astrocytes, as shown in other cell types. Furthermore, HIF-1α-siRNA experiments revealed that HIF-1α participates in the effects of ET-1 on glucose uptake and on the expression of GLUT-1, GLUT-3, type I and type II hexokinase. We previously reported that these effects of ET-1 are mediated by connexin43 (Cx43), the major gap junction protein in astrocytes. Indeed, our results show that silencing Cx43 increased HIF-1α and reduced the effect of ET-1 on HIF-1α, indicating that the effect of ET-1 on HIF-1α is mediated by Cx43. The activity of oncogenes such as c-Src can up-regulate HIF-1α. Since Cx43 interacts with c-Src, we investigated the participation of c-Src in this pathway. Interestingly, both the treatment with ET-1 and with Cx43-siRNA increased c-Src activity. In addition, when c-Src activity was inhibited neither ET-1 nor silencing Cx43 were able to up-regulate HIF-1α. In conclusion, our results suggest that ET-1 by down-regulating Cx43 activates c-Src, which in turn increases HIF-1α leading to the up-regulation of the machinery required to take up glucose in astrocytes. Cx43 expression can be reduced in response not only to ET-1 but also to various physiological and pathological stimuli. This study contributes to the identification of the signalling pathway evoked after Cx43 down-regulation that results in increased glucose uptake in astrocytes. Interestingly, this is the first evidence linking Cx43 to HIF-1, which is a master regulator of glucose metabolism.

Oxidative metabolism in cultured rat astroglia: effects of reducing the glucose concentration in the culture medium and of D-aspartate or potassium stimulation

The glucose concentration in the culture medium may affect the energy metabolism of cultured cells. The oxidative metabolism of glucose in astrocytes might also be affected because the glucose concentration (25 mmol/L) of many culture media is higher than the physiological levels (approximately 3 mmol/L). In the present study, we assessed the effects of reducing the glucose concentration in the culture medium on the oxidative metabolism of glucose in cultured rat astroglia by measuring the oxidation rates of L-[U-14C]lactate or D-[U-14C]glucose to 14CO2. The effects of D-aspartate and elevated extracellular K+ levels on oxidative and glycolytic metabolism in astroglia were also investigated. The rates of [14C]lactate and [14C]glucose oxidation in astroglia cultured in a medium containing 2 mmol/L of glucose (astroglia2) were approximately twofold of those in astroglia cultured in a medium containing 22 mmol/L of glucose (astroglia22). D-Aspartate (500 micromol/L) significantly increased [14C]lactate oxidation by 156% in astroglia22 and by 83% in astroglia2. D-[U-14C]glucose oxidation in astroglia22 and astroglia2 was also increased by 94% and 76%, respectively. In contrast, an elevated extracellular K+ concentration (7.4 mmol/L) did not affect glucose and lactate oxidation, although it increased 2-deoxy-D-[1-14C]glucose phosphorylation. Astroglia grown in the physiological glucose concentration are more dependent on the oxidative metabolism of glucose than that in high-glucose concentration. Glucose concentration in culture medium has a strong influence on astrocytic oxidative capacity in vitro.

Albumin stimulates monocyte chemotactic protein-1 expression in rat embryonic mixed brain cells

Albumin, a blood protein absent from the adult brain in physiological situations, can be brought into contact with brain cells during development or, in adult, following breakdown of the blood-brain barrier occurring as a result of local inflammation. In the present study, we show that ovalbumin and albumin induce the release of monocyte chemotactic protein 1 (MCP-1/CCL2) from rat embryonic mixed brain cells. A short-term exposure to ovalbumin during the cell dissociation procedure is sufficient to generate MCP-1 mRNA. A comparable effect is observed when the cells are incubated for 4 hr with ovalbumin or rat albumin, while MCP-1 messengers are barely detectable following bovine albumin exposure. The amount of MCP-1 protein measured in 4 hr-supernatants of albumin-treated cells followed the same albumin-inducing pattern as that of MCP-1 mRNA, while all albumins tested induced MCP-1 protein after a 17 hr-incubation period. The albumin-induced MCP-1 production is significantly inhibited in calphostin C-treated cells, suggesting the implication of a protein kinase C-dependent signaling pathway. This MCP-1-inducing activity is maintained after a lipid extraction procedure but abolished by proteinase K or trypsin treatments of albumin. The MCP-1 secretion following albumin contact with nervous cells could thus interfere, by chemotactic gradient formation, with the brain infiltration program of blood-derived cells during development or brain injury.

Lactate utilization by brain cells and its role in CNS development

We studied the role played by lactate as an important substrate for the brain during the perinatal period. Under these circumstances, lactate is the main substrate for brain development and is used as a source of energy and carbon skeletons. In fact, lactate is used actively by brain cells in culture. Neurons, astrocytes, and oligodendrocytes use lactate as a preferential substrate for both energy purposes and as precursor of lipids. Astrocytes use lactate and other metabolic substrates for the synthesis of oleic acid, a new neurotrophic factor. Oligodendrocytes mainly use lactate as precursor of lipids, presumably those used to synthesize myelin. Neurons use lactate as a source of energy and as precursor of lipids. During the perinatal period, neurons may use blood lactate directly to meet the need for the energy and carbon skeletons required for proliferation and differentiation. During adult life, however, the lactate used by neurons may come from astrocytes, in which lactate is the final product of glycogen breakdown. It may be concluded that lactate plays an important role in brain development.

Endothelin-1 stimulates the translocation and upregulation of both glucose transporter and hexokinase in astrocytes: relationship with gap junctional communication

We have previously shown that endothelin-1 increases glucose uptake in astrocytes. In the present work we investigate the mechanism through which endothelin-1 (ET-1) increases glucose uptake. Our results show that ET-1 activates a short-term and a long-term mechanism. Thus, ET-1 induced a rapid change in the localization of both GLUT-1 and type I hexokinase. These changes are probably aimed at rapidly increasing the entry and phosphorylation of glucose. In addition, ET-1 upregulated GLUT-1 and type I hexokinase and induced the expression of isoforms not normally expressed in astrocytes, such as GLUT-3 and type II hexokinase. These changes provide astrocytes with the machinery required to sustain a high rate of glucose uptake for a longer period of time. Our previous work had suggested that the effect of ET-1 on glucose uptake was associated with the inhibition of gap junctions. In this work, we compare the effect of ET-1 with that of carbenoxolone, a classical inhibitor of gap junction communication. Carbenoxolone increased glucose uptake to the same extent as ET-1 following the same mechanisms. Thus, carbenoxolone induced a rapid change in the localization of both GLUT-1 and type I hexokinase, upregulated GLUT-1 and type I hexokinase and induced the expression of GLUT-3 and type II hexokinase. When the inhibition of gap junction was prevented by tolbutamide, neither ET-1 nor carbenoxolone were able to increase the levels of GLUT-1, GLUT-3, type I hexokinase or type II hexokinase, indicating that these events are closely related to gap junctions.

Energy substrates for neurons during neural activity: a critical review of the astrocyte-neuron lactate shuttle hypothesis

Glucose had long been thought to fuel oxidative metabolism in active neurons until the recently proposed astrocyte-neuron lactate shuttle hypothesis (ANLSH) challenged this view. According to the ANLSH, activity-induced uptake of glucose takes place predominantly in astrocytes, which metabolize glucose anaerobically. Lactate produced from anaerobic glycolysis in astrocytes is then released from astrocytes and provides the primary metabolic fuel for neurons. The conventional hypothesis asserts that glucose is the primary substrate for both neurons and astrocytes during neural activity and that lactate produced during activity is removed mainly after neural activity. The conventional hypothesis does not assign any particular fraction of glucose metabolism to the aerobic or anaerobic pathways. In this review, the authors discuss the theoretical background and critically review the experimental evidence regarding these two hypotheses. The authors conclude that the experimental evidence for the ANLSH is weak, and that existing evidence and theoretical considerations support the conventional hypothesis.

β-Amyloid Fragment 25–35 Causes Mitochondrial Dysfunction in Primary Cortical Neurons

Beta-amyloid deposition and compromised energy metabolism both occur in vulnerable brain regions in Alzheimer's disease. It is not known whether beta-amyloid is the cause of impairment of energy metabolism, nor whether impaired energy metabolism is specific to neurons. Our results, using primary neuronal cultures, show that 24-h incubation with A beta(25-35) caused a generalized decrease in the specific activity of mitochondrial enzymes per milligram of cellular protein, induced mitochondrial swelling, and decreased total mitochondrial number. Incubation with A beta(25-35) decreased ATP concentration to 58% of control in neurons and 71% of control in astrocytes. Levels of reduced glutathione were also lowered by A beta(25-35) in both neurons (from 5.1 to 2.9 nmol/mg protein) and astrocytes (from 25.2 to 14.9 nmol/mg protein). We conclude that 24-h treatment with extracellular A beta(25-35) causes mitochondrial dysfunction in both astrocytes and neurons, the latter being more seriously affected. In astrocytes mitochondrial impairment was confined to complex I inhibition, whereas in neurons a generalized loss of mitochondria was seen.

Albumin promotes neuronal survival by increasing the synthesis and release of glutamate

It is well known that the presence of albumin within the brain and the CSF is developmentally regulated. However, the physiological relevance of this phenomenon is not well established. We have previously shown that albumin specifically increases the flux of glucose and lactate through the pyruvate dehydrogenase reaction in astrocytes. Here we show that, in neurones, albumin also increases the oxidation of glucose and lactate through the pyruvate dehydrogenase-catalysed reaction, the final purpose of this being the synthesis of glutamate. Thus, in neurones, the presence of albumin strongly increased the synthesis and release of glutamate to the extracellular medium. Our results also suggest that glutamate release caused by albumin is designed to promote neuronal survival. Thus, under culture conditions in which neurones die by apoptosis, the presence of albumin promoted neuronal survival and maintained the differentiation programme of these cells, as judged by the expression of the axonal protein, GAP-43. The effect of albumin on neuronal survival was counteracted by the presence of DNQX, an antagonist of non-NMDA-glutamate receptors, suggesting that the glutamate synthesized and released due to the presence of albumin is responsible for neuronal survival. In addition, the effect of albumin seemed to depend on the activity of the NGF receptor, TrkA, suggesting that the glutamate synthesized and released due to the presence of albumin promotes neuronal survival through the activity of TrkA.

Astrocyte-synthesized oleic acid behaves as a neurotrophic factor for neurons

Unlike in the adult brain, the newborn brain specifically takes up serum albumin during the postnatal period, coinciding with the stage of maximal brain development. Here we shall summarize our knowledge about the role played by albumin in brain development. The role of this protein in brain development is intimately related to its ability to carry fatty acids. Thus, albumin stimulates oleic acid synthesis by astrocytes from the main metabolic substrates available during brain development. Astrocytes internalize albumin in vesicle-like structures by receptor-mediated endocytosis, which is followed by transcytosis, including passage through the endoplasmic reticulum (ER). The presence of albumin in the ER activates the sterol regulatory element-binding protein-1 (SREBP-1) and increases stearoyl-CoA 9-desaturase (SCD) mRNA, the key enzyme in oleic acid synthesis. Oleic acid released by astrocytes is used by neurons for the synthesis of phospholipids and is specifically incorporated into growth cones. In addition, oleic acid promotes axonal growth, neuronal clustering, and the expression of the axonal growth associated protein, GAP-43. All of these observations indicate neuronal differentiation. The effect of oleic acid on GAP-43 synthesis is brought about by the activation of protein kinase C. The expression of GAP-43 is significantly increased by the presence of albumin in neurons co-cultured with astrocytes, indicating that neuronal differentiation takes place by the presence of oleic acid synthesized and released by astrocytes in situ. In conclusion, during brain development the presence of albumin could play an important role by triggering the synthesis and release of oleic acid by astrocytes, thereby inducing neuronal differentiation.

Oligodendrocytes use lactate as a source of energy and as a precursor of lipids

Lactate is an important metabolic substrate for the brain during the postnatal period and also plays a crucial role in the traffic of metabolites between astrocytes and neurons. However, to date there are no clues with regard to lactate utilization by oligodendrocytes, the myelin-forming cells in the brain. In the present work, lactate utilization by oligodendrocytes in culture was investigated and compared with its utilization by cultured neurons, type 1 and type 2 astrocytes. Our results clearly indicate that oligodendrocytes readily use lactate both as a metabolic fuel and as a precursor to build carbon skeletons. Oligodendrocytes oxidize lactate at a higher rate than that observed for neurons and astrocytes, and their rate of lipid synthesis from lactate was at least 6-fold higher than that found in astrocytes or neurons. The rate of glucose utilization through different pathways was also investigated. The flux of glucose through the pentose phosphate pathway and the rate of lipid synthesis were at least 2-fold higher in oligodendrocytes than in astrocytes or neurons. These findings indicate that oligodendrocyte metabolism is designed specifically for the synthesis of lipids, presumably those of myelin.

Neuronal differentiation is triggered by oleic acid synthesized and released by astrocytes

Unlike in the adult brain, the newborn brain specifically takes up serum albumin during the postnatal period, coinciding with the stage of maximal brain development. Here we report that albumin stimulates oleic acid synthesis by astrocytes from the main metabolic substrates available during brain development. Oleic acid released by astrocytes is used by neurons for the synthesis of phospholipids and is specifically incorporated into growth cones. Oleic acid promotes axonal growth, neuronal clustering, and expression of the axonal growth-associated protein-43, GAP-43; all these observations indicating neuronal differentiation. The effect of oleic acid on GAP-43 synthesis is brought about by the activation of protein kinase C, since it was prevented by inhibitors of this kinase, such as H-7, polymyxin or sphingosine. The expression of GAP-43 was significantly increased in neurons co-cultured with astrocytes by the presence of albumin indicating that neuronal differentiation takes place in the presence of oleic acid synthesized and released by astrocytes in situ. In conclusion, during brain development the presence of albumin could play an important role by triggering the synthesis and release of oleic acid by astrocytes, which induces neuronal differentiation.

The enhancement of glucose uptake caused by the collapse of gap junction communication is due to an increase in astrocyte proliferation

We have previously shown that several gap junction uncouplers increase the uptake of glucose in astrocytes. The aim of the present work was to study whether the increase in glucose uptake was a consequence of the inhibition of gap junction communication and the purpose of this effect. Our results show that alpha-glycyrrhetinic acid and endothelin-1 increase the uptake of glucose in highly, but not in poorly, coupled astrocytes. This effect depended on connexin 43 levels and was abolished when the inhibition of gap junction communication was prevented by tolbutamide or ouabain. The inhibition of gap junctions increased the rate of glucose incorporation into DNA and RNA, which was inhibited by treatment with dehydroepiandrosterone, an inhibitor of glucose-6-phosphate dehydrogenase, the regulatory enzyme of the pentose phosphate pathway. The inhibition of gap junctions significantly increased astrocyte proliferation, which was counteracted by tolbutamide. These effects were not observed in poorly coupled astrocytes expressing low levels of connexin 43. The increase in astrocyte proliferation caused by gap junction inhibition was prevented when either glucose uptake or the pentose phosphate pathway were inhibited. We conclude that the inhibition of gap junction communication induces astrocyte proliferation, resulting in an enhancement of glucose uptake and its utilization through the pentose phosphate pathway to provide ribose-5-phosphate for the synthesis of nucleic acids.

Ethanol intake during lactation. II. Effects On pups' liver and brain metabolism

Lactating rats, with litters adjusted to 8 pups on day 1, were divided into 4 groups: control animals (C), which received water and Nuvilab chow ad libitum, and ethanol animals (E), which received 20% (E20), 10% (E10), or 5% (E5) ethanol diluted in the drinking water and Nuvilab chow ad libitum. On day 12 of life, the pups were weighed and decapitated. The intake of 10% and 20% ethanol solutions by the lactating rats decreased the pups' body weight and liver weight. The pups' liver ATP-citrate lyase activity was decreased in all ethanol groups. The pups' brain weight decreased in E20 only. Glucose metabolism and lactate production were studied in the pups' brain slices, which were incubated at 37 degrees C in Krebs-Henseleit buffer under carbogen in the presence of glucose (5 mM) plus 14C-glucose (0.04 microCi) with or without beta-hydroxybutyrate or insulin. Study of the incubated pups' brain slices showed that the intake of the 20% ethanol solution by the dams increased glucose consumption, oxidation, lactate production, and lipogenesis rate from glucose in all media studied, as compared with findings in the C group. In the pups' brain slices, the lactate production and lipogenesis rate from glucose were higher in E10 than in the C group. The addition of beta-hydroxybutyrate to the incubation medium caused a decrease in glucose oxidation in C, E5, and E20 and an increase in glucose consumption in E10. Ingestion of the 5% ethanol solution by dams decreased the pups' brain lipogenesis rate from glucose in all media studied. We concluded that the effects of maternal alcohol intake on the pups' development and metabolism are dose-dependent. High amounts of ethanol intake (10% or 20%) caused a great impairment in the pups' growth, as well as their liver and brain metabolism. The low dose (5%) did not affect the pups' body weight gain or their brain and liver weight, but it did alter brain glucose metabolism.

Induction of glucose-6-phosphate dehydrogenase by lipopolysaccharide contributes to preventing nitric oxide-mediated glutathione depletion in cultured rat astrocytes

Treatment of cultured rat astrocytes with lipopolysaccharide (LPS; 1 microg/ml) increased mRNA expression of glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting step in the pentose phosphate pathway (PPP), in a time-dependent fashion (0-24 h). This effect was accompanied by an increase in G6PD activity (1.74-fold) and in the rate of glucose oxidation through the PPP (6.32-fold). Inhibition of inducible nitric oxide synthase (iNOS) activity by 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT; 50 microM) did not alter the LPS-mediated enhancement of G6PD mRNA expression or PPP activity. Blockade of nuclear factor-kappaB (NF-kappaB) activation by N-benzyloxycarbonyl-Ile-Glu-(O-tert-butyl)-Ala-leucinal (1 microM) prevented the expression of both iNOS mRNA and G6PD mRNA, suggesting that iNOS and G6PD are co-induced by LPS through a common transcriptional pathway involving NF-kappaB activation. Incubation of cells with LPS for 24 h increased intracellular NADPH concentrations (1.63-fold) as compared with untreated cells, but GSH concentrations were not modified by LPS treatment up to 60 h of incubation. However, inhibition of G6PD activity by dehydroepiandrosterone (DHEA; 100 microM), which prevented LPS-mediated enhancements in PPP activity and NADPH concentrations, caused a 50% decrease in the GSH/GSSG ratio after 24-36 h and in GSH concentrations after 60 h of incubation. Furthermore, the changes in glutathione concentrations caused by DHEA were abolished by AMT, suggesting that nitric oxide and/or its reactive derivatives would be involved in this process. From these results, we conclude that LPS-mediated G6PD expression prevents GSH depletion due to nitric oxide and suggest that this phenomenon may be a contributing factor in the defense mechanisms that protect astrocytes against nitric oxide-mediated cell injury.

Nitric oxide mediates glutamate-induced mitochondrial depolarization in rat cortical neurons

Mitochondria have been considered to be a target for glutamate neurotoxicity. The aim of the present work was to investigate the mechanisms leading to glutamate-mediated mitochondrial deenergization, as measured by mitochondrial membrane potential and cell respiration in cultured neurons. Glutamate exposure to cells induced pronounced mitochondrial depolarization associated with an impairment in neuronal respiration, leading to neuronal ATP depletion. These effects were prevented by both the nitric oxide (. NO) synthase inhibitor Nomega-nitro-l-arginine methyl ester and by the N-methyl-d-aspartate glutamate-subtype receptor inhibitor d-(-)-2-amino-5-phosphopentanoate. Our results suggest that glutamate causes ATP depletion by collapsing mitochondrial membrane potential through a.NO-mediated mechanism.

The K-ATP channel regulates the effect of Ca2+ on gap junction permeability in cultured astrocytes

Using the scrape-loading technique we show that tolbutamide and glybenzcyclamide, two inhibitors of the K+ channel sensitive to ATP (K-ATP channel), partially prevent the inhibition of gap junction permeability promoted by Ca2+ in cultured astrocytes. This effect was dose-dependent, reaching a maximum at 400 microM and 1 microM of tolbutamide and glybenzcyclamide, respectively. The presence of the Ca2+ ionophore A-23187 strongly reduced the concentration of Ca2+ required to block gap junction permeability but did not abolish the effect of tolbutamide and glybenzcyclamide. These results suggest that the effect of these two compounds are not brought about by control of the intracellular concentration of Ca2+ but probably by the promotion of plasma membrane depolarization.

Glutamate neurotoxicity is associated with nitric oxide-mediated mitochondrial dysfunction and glutathione depletion

The role of mitochondrial energy metabolism in glutamate mediated neurotoxicity was studied in rat neurones in primary culture. A brief (15 min) exposure of the neurones to glutamate caused a dose-dependent (0.01-1 mM) increase in cyclic GMP levels together with delayed (24 h) neurotoxicity and ATP depletion. These effects were prevented by either the nitric oxide (.NO) synthase (NOS) inhibitor Nomega-nitro-L-arginine methyl ester (NAME; 1 mM) or by the N-methyl-D-aspartate (NMDA) glutamate-subtype receptor antagonist D-(-)-2-amino-5-phosphonopentanoate (APV; 0.1 mM). Glutamate exposure (0.1 mM and 1 mM) followed by 24 h of incubation caused the inhibition of succinate-cytochrome c reductase (20-25%) and cytochrome c oxidase (31%) activities in the surviving neurones, without affecting NADH-coenzyme-Q1 reductase activity. The rate of oxygen consumption was impaired in neurones exposed to 1 mM glutamate, either with glucose (by 26%) or succinate (by 39%) as substrates. These effects on the mitochondrial respiratory chain and neuronal respiration, together with the observed glutathione depletion (20%) by glutamate exposure were completely prevented by NAME or APV. Our results suggest that mitochondrial dysfunction and impairment of antioxidant status may account for glutamate-mediated neurotoxicity via a mechanism involving .NO biosynthesis.

A rapid method for the isolation of metabolically active mitochondria from rat neurons and astrocytes in primary culture

A rapid method (about 1.5 h) for the isolation of intact functional mitochondria from neurons and astrocytes in primary culture is described. Mitochondria isolated by this method are metabolically active and tightly coupled as shown by respiratory control ratio values, which were about 4 with glutamate-malate as substrate. The activities of marker enzymes revealed the occurrence of a low degree of cytosolic (5%) or synaptosomal (5.5%) contamination in the mitochondrial fractions. In addition, the activity of citrate synthase was increased by 4 fold in both neuronal and astrocytic mitochondria with respect to values found in cell homogenates. These results confirm that the method affords mitochondrial preparations from cultured brain cells at suitable levels of purity and enrichment for the study of their mitochondrial function. Since mitochondrial damage has been associated with the pathogenesis of certain neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases (P. Chagnon, C. Betard, Y. Robitaille, A. Cholette, D. Gauvreau, Distribution of brain cytochrome oxidase activity in various neurodegenerative disease, Neuroreport 6 (1995) 711-715 [6]; S.J. Kish, C. Bergeron, A. Rajput, S. Dozic, F. Mastrogiacomo, L. Chang, J.M. Wilson, L.M. DiStefano, J.N. Nobrega, Brain cytochrome oxidase in Alzheimer's disease, J. Neurochem. 59 (1992) 776-779 [10]; A.H.V. Schapira, J.M. Cooper, D. Dexter, J.B. Clark, P. Jenner, C.D. Marsden, Mitochondrial complex I deficiency in Parkinson's disease, J. Neurochem. 54 (1990) 823-827 [15]), the method described here shed light on the possible susceptibility of neuronal or astrocytic mitochondria to deleterious effects of these diseases.

Evaluation of the efficacy of potential therapeutic agents at protecting against nitric oxide synthase-mediated mitochondrial damage in activated astrocytes

Within the central nervous system, nitric oxide is an important physiological messenger. However, when synthesized excessively in neurones, cell death may occur. An impairment of mitochondrial cytochrome oxidase and subsequent cellular energy depletion seems to be a likely mechanism for this neurotoxicity. Within neurones, nitric oxide is synthesized by the constitutive, Ca(2+)-dependent form of nitric oxide synthase (nNOS). Astrocytes, however, possess both the constitutive and the inducible Ca(2+)-independent NOS (iNOS), which is expressed by endotoxin and/or cytokines. In vitro, activation of nNOS rapidly produces neuronal cell death. In contrast to neurones, following induction of iNOS, astrocytes synthesize large quantities of nitric oxide, but cell death is not apparent despite marked damage to mitochondrial cytochrome oxidase. The resistance of astrocytes to nitric oxide synthase-mediated cell damage may be due to their ability to increase their glycolytic rate when mitochondrial ATP synthesis is compromised. On the basis of this phenomenon, we propose that activated astrocytes represent a suitable system for studying the efficacy of potential therapeutic agents at protecting from nitric oxide synthase-mediated mitochondrial damage.

Isolation and characterization of tightly coupled mitochondria from neurons and astrocytes in primary culture

This work provides a rapid method for isolation of intact functional mitochondria from neurons and astrocytes in primary culture. By using this method, it was found that the respiratory control ratio was 1.5-fold greater in neuronal than in astrocytic mitochondria using both NAD-linked (glutamate/malate) and FAD-linked (succinate) substrates. The difference observed in RCR values was due to the lower rate of respiration in state 4 found in neurons as compared to that found in astrocytes, because both cell types showed the same rate of respiration in state 3. The P/O ratio was also higher in neurons than in astrocytes. Our results suggest that the coupling between the mitochondrial respiratory chain and oxidative phosphorylation is stronger in neurons than in astrocytes. These results may be of relevance for the understanding of the differential susceptibility of brain cells to impairments of energy metabolism observed in certain neurodegenerative diseases.

Lactate spares glucose as a metabolic fuel in neurons and astrocytes from primary culture

The effect of lactate on glucose metabolism in neurons and astrocytes from primary culture has been studied. The rates of glucose metabolism through the pentose-phosphate shunt, the pyruvate dehydrogenase-catalyzed reaction, the tricarboxylic acid cycle, the total lipogenesis and the synthesis of glycerol-borne lipids in astrocytes were 2-3 fold higher than in neurons. However, the rate of glucose incorporation into sterols and esterified fatty acids was similar in both types of cells. Total glucose utilization was inhibited by lactate to the same extend in both neurons and astrocytes. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle, in both neurons (60 and 44%, respectively) and astrocytes (64 and 62%, respectively). Glucose incorporation into sterols and fatty acids was also inhibited by lactate in both neurons and astrocytes (57 and 76%, respectively) while the oxidation of glucose in the pentose-phosphate shunt and the synthesis of glycerol-borne lipids was not significantly affected. These results suggest that in the presence of lactate both neurons and astrocytes can utilize lactate as the major metabolic substrate, sparing glucose for the synthesis of NADPH(H+), ribose-5-phosphate and/or glycerol-borne lipids. An interaction between glucose and lactate metabolism at the level of the pyruvate dehydrogenase-catalyzed reaction is suggested.

Inhibition of astrocyte gap junctional communication by ATP depletion is reversed by calcium sequestration

We have studied the possible role of cellular energy status in the regulation of gap junction permeability in rat astrocytes in primary culture. Incubation with the mitochondrial respiratory chain inhibitor antimycin (5 ng/ml) for 16 h caused a significant decrease in ATP concentrations. This effect was accompanied by a dose-dependent inhibition of gap junction permeability as assessed by the scrape-loading/Lucifer yellow transfer technique. No cell death was observed following this treatment. Restoration of cellular ATP levels by a further 24 h incubation in antimycin-free medium reversed the inhibition of Lucifer yellow transfer caused by antimycin. The inhibition of Lucifer yellow transfer brought about by antimycin treatment was also reversed by a short incubation of the cells with the calcium chelator EGTA plus the calcium ionophore A23187. These results suggest that ATP depiction causes a reversible inhibition of gap junction permeability through a calcium-mediated mechanism.

Fuel utilization by early newborn brain is preserved under congenital hypothyroidism in the rat

Mental retardation associated with hypothyroidism may be caused by impairment of brain ketone body-metabolizing enzymes during the suckling period. However, much evidence suggests that, immediately after delivery, lactate, instead of ketone bodies or glucose, may be the best substrate for the brain. In this work, we have studied the effect of experimentally induced congenital hypothyroidism on the rate of lactate, glucose, and 3-hydroxybutyrate utilization in early neonatal brain slices. Methimazole (MMI) administration to the mothers caused a 5.4- and 1.7-fold decrease in neonatal plasma concentrations of L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3), respectively. Propylthiouracil (PTU) administration to the mothers caused a 7.3- and > 2-fold decrease in plasma T4 and T3 concentrations, respectively. MMI-induced hypothyroidism did not significantly modify the rate of lactate, glucose, or 3-hydroxybutyrate oxidation to CO2 and their incorporation into lipids by the neonatal brain. However, PTU-induced hypothyroidism decreased the rate of lactate and glucose oxidation to CO2 and their incorporation into lipids by 17% (p < 0.05). 3-Hydroxybutyrate utilization was not modified by this treatment. Separation by HPLC of the lipids revealed that PTU-mediated inhibition of lipid synthesis from lactate and glucose may be accounted for by specific inhibition of the rate of sterol synthesis (15%, p < 0.05), whereas the rate of phospholipid synthesis was unaffected. These results suggest that the early newborn may develop mechanisms aimed at avoiding the possible brain damage caused by the inhibition of lipid synthesis brought about by mild neonatal hypothyroidism.

Endothelin-1 regulates glucose utilization in cultured astrocytes by controlling intercellular communication through gap junctions

The role played by endothelin-1 and intercellular communication mediated by gap junctions in the regulation of glucose disposal by astrocytes has been studied in primary culture. Endothelin-1 increased glucose uptake by astrocytes as did one of its putative messenger arachidonic acid and the non-physiological gap junction uncoupler alpha-glycyrrhetinic acid (AGA). None of these agents increased glucose uptake by C6 glioma cells, a cell line in which gap junction proteins are poorly expressed. In confluent astrocytes, the inhibition of gap junction permeability caused by AGA doubled the activity of the pentose phosphate shunt with minimal changes in the activity of the pyruvate dehydrogenase-catalyzed reaction and that of the tricarboxylic acid cycle. By contrast, these effects were not observed in dissociated astrocytes in which intercellular communication is lacking. The scraped loading dye transfer technique was modified to follow the passage of glucose and its metabolites through astrocyte gap junctions. The diffusion of glucose, the phosphorylated derivative glucose-6-phosphate, the phosphorylisable but not metabolisable derivative ortho-methyl-glucose, and the anaerobic glycolytic product L-lactate was much higher in astrocytes than in C6 glioma cells and was inhibited by the inhibition of gap junction permeability caused by endothelin-1, arachidonic acid, octanol, or AGA. It is concluded that gap junction permeability may regulate brain metabolism by controlling the uptake, utilization, and intercellular distribution of glucose and its metabolites in astrocytes.

Astrocyte differentiation in primary culture followed by flow cytometry

Astrocyte proliferation and differentiation in primary culture were followed by flow cytometry. The time-courses of the percentages of astrocytes in the different cell-cycle phases suggest that astrocytes proliferate during the first 10 days in culture thereafter reaching confluence. Two types of astrocytes are identified immunocytochemically: one growing in the bed monolayer, identified as type-1 astrocytes, and another growing on the top of the monolayer, identified as type-2 astrocytes. In addition, three populations identified as being formed of type-1, type-2 and putative progenitor cells, respectively, were followed by flow cytometry. Progenitor cells were the major type 2 h after plating (89%) but their percentage decreases sharply (to 16%) during the first 5 days in culture, with no ensuing changes. In contrast, the percentage of type-1 cells (11%) rapidly increased after plating reaching a maximum 5 days later (73%). Later, it decreased (to 47%) and was maintained thereafter. The percentage of type-2 cells was undetectable immediately after plating but increased from the 3rd to the 10th day with no further changes. Our results suggest that progenitor cells differentiate into type-1 astrocytes triggered by the culture medium but the differentiation of progenitor cells into type-2 astrocytes is brought about by some type-1-secreted factor. In this work we report a rapid and simple method for following the growth and differentiation of rat brain astrocytes in primary culture.

Nitric oxide-mediated mitochondrial damage: a potential neuroprotective role for glutathione

In this study we have investigated the mechanisms leading to mitochondrial damage in cultured neurons following sustained exposure to nitric oxide. Thus, the effects upon neuronal mitochondrial respiratory chain complex activity and reduced glutathione concentration following exposure to either the nitric oxide donor, S-nitroso-N-acetylpenicillamine, or to nitric oxide releasing astrocytes were assessed. Incubation with S-nitroso-N-acetylpenicillamine (1 mM) for 24 h decreased neuronal glutathione concentration by 57%, and this effect was accompanied by a marked decrease of complex I (43%), complex II-III (63%), and complex IV (41%) activities. Incubation of neurons with the glutathione synthesis inhibitor, L-buthionine-[S,R]-sulfoximine caused a major depletion of neuronal glutathione (93%), an effect that was accompanied by a marked loss of complex II-III (60%) and complex IV (41%) activities, although complex I activity was only mildly decreased (34%). In an attempt to approach a more physiological situation, we studied the effects upon glutathione status and mitochondrial respiratory chain activity of neurons incubated in coculture with nitric oxide releasing astrocytes. Astrocytes were activated by incubation with lipopolysaccharide/interferon-gamma for 18 h, thereby inducing nitric oxide synthase and, hence, a continuous release of nitric oxide. Coincubation for 24 h of activated astrocytes with neurons caused a limited loss of complex IV activity and had no effect on the activities of complexes I or II-III. However, neurons exposed to astrocytes had a 1.7-fold fold increase in glutathione concentration compared to neurons cultured alone. Under these coculture conditions, the neuronal ATP concentration was modestly reduced (14%). This loss of ATP was prevented by the nitric oxide synthase inhibitor, NG-monomethyl-L-arginine. These results suggest that the neuronal mitochondrial respiratory chain is damaged by sustained exposure to nitric oxide and that reduced glutathione may be an important defence against such damage.

Metabolic fuel utilization and pyruvate oxidation during the postnatal period

The transplacental supply of nutrients is interrupted at birth, which diverts maternal metabolism to lactation. After birth, energy homeostasis is rapidly regained through milk nutrients which supply the newborn with the fatty acids and ketone bodies required for neonatal development. However, immediately after birth and before the onset of suckling there is a time lapse in which the newborn undergoes a unique kind of starvation. During this period glucose is scarce and ketone bodies are not available owing to the delay in ketogenesis. Under these circumstances, the newborn is supplied with another metabolic fuel, lactate, which is utilized as a source of energy and carbon skeletons. Neonatal rat lung, heart, liver and brain utilize lactate for energy production and lipogenesis. Lactate is also utilized by the brain of human babies with type I glycogenosis. Both rat neurons and astrocytes in primary culture actively use lactate as an oxidizable substrate and as a precursor of phospholipids and sterols. Lactate oxidation is enhanced by dichloroacetate, an inhibitor of the pyruvate dehydrogenase kinase in neurons but not in astrocytes, suggesting that the pyruvate dehydrogenase is regulated differently in each type of cell. Despite the low activity of this enzyme in newborn brain, pyruvate decarboxylation is the main fate of glucose in both neurons and astrocytes. The occurrence of a yeast-like pyruvate decarboxylase activity in neonatal brain may explain these results.