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

Estradiol and 3β-diol protect female cortical astrocytes by regulating connexin 43 Gap Junctions

While estrogens have been described to protect or preserve neuronal function in the face of insults such as oxidative stress, the prevailing mechanistic model would suggest that these steroids exert direct effects on the neurons. However, there is growing evidence that glial cells, such as astrocytes, are key cellular mediators of protection. Noting that connexin 43 (Cx43), a protein highly expressed in astrocytes, plays a key role in mediating inter-cellular communication, we hypothesized that Cx43 is a target of estradiol (E2), and the estrogenic metabolite of DHT, 3β-diol. Additionally, we sought to determine if either or both of these hormones attenuate oxidative stress-induced cytotoxicity by eliciting a reduction in Cx43 expression or inhibition of Cx43 channel permeability. Using primary cortical astrocytes, we found that E2 and 3β-diol were each protective against the mixed metabolic/oxidative insult, iodoacetic acid (IAA). Moreover, these effects were blocked by estrogen receptor antagonists. However, E2 and 3β-diol did not alter Cx43 mRNA levels in astrocytes but did inhibit IAA-induced Cx43 gap junction opening/permeability. Taken together, these data implicate astrocyte Cx43 gap junction as an understudied mediator of the cytoprotective effects of estrogens in the brain. Given the wide breadth of disease states associated with Cx43 function/dysfunction, further understanding the relationship between gonadal steroids and Cx43 channels may contribute to a better understanding of the biological basis for sex differences in various diseases.

APOΕ4 lowers energy expenditure in females and impairs glucose oxidation by increasing flux through aerobic glycolysis

Background

Cerebral glucose hypometabolism is consistently observed in individuals with Alzheimer’s disease (AD), as well as in young cognitively normal carriers of the Ε4 allele of Apolipoprotein E (APOE), the strongest genetic predictor of late-onset AD. While this clinical feature has been described for over two decades, the mechanism underlying these changes in cerebral glucose metabolism remains a critical knowledge gap in the field.

Methods

Here, we undertook a multi-omic approach by combining single-cell RNA sequencing (scRNAseq) and stable isotope resolved metabolomics (SIRM) to define a metabolic rewiring across astrocytes, brain tissue, mice, and human subjects expressing APOE4.

Results

Single-cell analysis of brain tissue from mice expressing human APOE revealed E4-associated decreases in genes related to oxidative phosphorylation, particularly in astrocytes. This shift was confirmed on a metabolic level with isotopic tracing of ¹³C-glucose in E4 mice and astrocytes, which showed decreased pyruvate entry into the TCA cycle and increased lactate synthesis. Metabolic phenotyping of E4 astrocytes showed elevated glycolytic activity, decreased oxygen consumption, blunted oxidative flexibility, and a lower rate of glucose oxidation in the presence of lactate. Together, these cellular findings suggest an E4-associated increase in aerobic glycolysis (i.e. the Warburg effect). To test whether this phenomenon translated to APOE4 humans, we analyzed the plasma metabolome of young and middle-aged human participants with and without the Ε4 allele, and used indirect calorimetry to measure whole body oxygen consumption and energy expenditure. In line with data from E4-expressing female mice, a subgroup analysis revealed that young female E4 carriers showed a striking decrease in energy expenditure compared to non-carriers. This decrease in energy expenditure was primarily driven by a lower rate of oxygen consumption, and was exaggerated following a dietary glucose challenge. Further, the stunted oxygen consumption was accompanied by markedly increased lactate in the plasma of E4 carriers, and a pathway analysis of the plasma metabolome suggested an increase in aerobic glycolysis.

Conclusions

Together, these results suggest astrocyte, brain and system-level metabolic reprogramming in the presence of APOE4, a ‘Warburg like’ endophenotype that is observable in young females decades prior to clinically manifest AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00483-y.

Reactive astrocytes: The nexus of pathological and clinical hallmarks of Alzheimer's disease

Astrocyte reactivity is a hallmark of neuroinflammation that arises with Alzheimer’s disease (AD) and nearly every other neurodegenerative condition. While astrocytes certainly contribute to classic inflammatory processes (e.g. cytokine release, waste clearance, and tissue repair), newly emerging technologies for measuring and targeting cell specific activities in the brain have uncovered essential roles for astrocytes in synapse function, brain metabolism, neurovascular coupling, and sleep/wake patterns. In this review, we use a holistic approach to incorporate, and expand upon, classic neuroinflammatory concepts to consider how astrocyte dysfunction/reactivity modulates multiple pathological and clinical hallmarks of AD. Our ever-evolving understanding of astrocyte signaling in neurodegeneration is not only revealing new drug targets and treatments for dementia but is suggesting we reimagine AD pathophysiological mechanisms.

Maneb alters central carbon metabolism and thiol redox status in a toxicant model of Parkinson's disease

The dithiocarbamate fungicide maneb (MB) has attracted interest due to increasing concern of the negative health effects of pesticides, as well as its association with Parkinson's disease (PD). Our laboratory has previously reported distinct phenotypic changes of neuroblastoma cells exposed to acute, sub-toxic levels of MB, including decreased mitochondrial respiration, altered lactate dynamics, and metabolic stress. In this study, we aimed to further define the specific molecular mechanisms of MB toxicity through the comparison of several thiol-containing compounds and their effects on cellular energy metabolism and thiol redox nodes. Extracellular flux analyses and stable isotope labeled tracer metabolomics were employed to evaluate alterations in energy metabolism of SK-N-AS human neuroblastoma cells after acute exposure of an array of compounds, including dithiocarbamates (maneb, nabam, zineb) and other thiol-containing small molecules (glutathione, N-acetylcysteine). These studies revealed MB and its methylated form (MeDTC) as unique toxicants with significant alterations to mitochondrial respiration, proliferation, and glycolysis. We observed MB to significantly impact cellular thiol redox status by oxidizing cellular glutathione and altering the thiol redox status of peroxiredoxin 3 (Prx3, mitochondrial) after acute exposure. Redox Western blotting revealed a MB-specific modification of cellular Prx3, strengthening the argument that MB can preferentially target mitochondrial enzymes containing reactive cysteine thiols. Further, stable isotope tracer metabolomics confirmed our energetics assessments, and demonstrated that MB exposure results in acute derangement of central carbon metabolism. Specifically, we observed shunting of cellular glucose into the pentose-phosphate pathway and reduction of TCA intermediates derived from glucose and glutamine. Also, we report novel lactate utilization for TCA enrichment and glutathione synthesis after MB exposure. In summary, our results further confirm that MB exerts its toxic effects via thiol modification, and significantly transforms central carbon metabolism.

Lactate Shuttles in Neuroenergetics-Homeostasis, Allostasis and Beyond

Understanding brain energy metabolism—neuroenergetics—is becoming increasingly important as it can be identified repeatedly as the source of neurological perturbations. Within the scientific community we are seeing a shift in paradigms from the traditional neurocentric view to that of a more dynamic, integrated one where astrocytes are no longer considered as being just supportive, and activated microglia have a profound influence. Lactate is emerging as the “good guy,” contrasting its classical “bad guy” position in the now superseded medical literature. This review begins with the evolution of the concept of “lactate shuttles”; goes on to the recent shift in ideas regarding normal neuroenergetics (homeostasis)—specifically, the astrocyte–neuron lactate shuttle; and progresses to covering the metabolic implications whereby homeostasis is lost—a state of allostasis, and the function of microglia. The role of lactate, as a substrate and shuttle, is reviewed in light of allostatic stress, and beyond—in an acute state of allostatic stress in terms of physical brain trauma, and reflected upon with respect to persistent stress as allostatic overload—neurodegenerative diseases. Finally, the recently proposed astrocyte–microglia lactate shuttle is discussed in terms of chronic neuroinflammatory infectious diseases, using tuberculous meningitis as an example. The novelty extended by this review is that the directionality of lactate, as shuttles in the brain, in neuropathophysiological states is emerging as crucial in neuroenergetics.

Effects of Systemic Metabolic Fuels on Glucose and Lactate Levels in the Brain Extracellular Compartment of the Mouse

Classic neuroenergetic research has emphasized the role of glucose, its transport and its metabolism in sustaining normal neural function leading to the textbook statement that it is the necessary and sole metabolic fuel of the mammalian brain. New evidence, including the Astrocyte-to-Neuron Lactate Shuttle hypothesis, suggests that the brain can use other metabolic substrates. To further study that possibility, we examined the effect of intraperitoneally administered metabolic fuels (glucose, fructose, lactate, pyruvate, ß-hydroxybutyrate, and galactose), and insulin, on blood, and extracellular brain levels of glucose and lactate in the adult male CD1 mouse. Primary motor cortex extracellular levels of glucose and lactate were monitored in freely moving mice with the use of electrochemical electrodes. Blood concentration of these same metabolites were obtained by tail vein sampling and measured with glucose and lactate meters. Blood and extracellular fluctuations of glucose and lactate were monitored for a 2-h period. We found that the systemic injections of glucose, fructose, lactate, pyruvate, and ß-hydroxybutyrate increased blood lactate levels. Apart for a small transitory rise in brain extracellular lactate levels, the main effect of the systemic injection of glucose, fructose, lactate, pyruvate, and ß-hydroxybutyrate was an increase in brain extracellular glucose levels. Systemic galactose injections produced a small rise in blood glucose and lactate but almost no change in brain extracellular lactate and glucose. Systemic insulin injections led to a decrease in blood glucose and a small rise in blood lactate; however brain extracellular glucose and lactate monotonically decreased at the same rate. Our results support the concept that the brain is able to use alternative fuels and the current experiments suggest some of the mechanisms involved.

Metabolic Changes Following Perinatal Asphyxia: Role of Astrocytes and Their Interaction with Neurons

Perinatal Asphyxia (PA) represents an important cause of severe neurological deficits including delayed mental and motor development, epilepsy, major cognitive deficits and blindness. The interaction between neurons, astrocytes and endothelial cells plays a central role coupling energy supply with changes in neuronal activity. Traditionally, experimental research focused on neurons, whereas astrocytes have been more related to the damage mechanisms of PA. Astrocytes carry out a number of functions that are critical to normal nervous system function, including uptake of neurotransmitters, regulation of pH and ion concentrations, and metabolic support for neurons. In this work, we aim to review metabolic neuron-astrocyte interactions with the purpose of encourage further research in this area in the context of PA, which is highly complex and its mechanisms and pathways have not been fully elucidated to this day.

Connexin-based channels contribute to metabolic pathways in the oligodendroglial lineage

Oligodendrocyte precursor cells (OPCs) undergo a series of energy-consuming developmental events; however, the uptake and trafficking pathways for their energy metabolites remain unknown. In the present study, we found that 2-NBDG, a fluorescent glucose analog, can be delivered between astrocytes and oligodendrocytes through connexin-based gap junction channels but cannot be transferred between astrocytes and OPCs. Instead, connexin hemichannel-mediated glucose uptake supports OPC proliferation, and ethidium bromide uptake or increase of 2-NBDG uptake rate is correlated with intracellular Ca(2+) elevation in OPCs, indicating a Ca(2+)-dependent activation of connexin hemichannels. Interestingly, deletion of connexin 43 (Cx43, also known as GJA1) in astrocytes inhibits OPC proliferation by decreasing matrix glucose levels without impacting on OPC hemichannel properties, a process that also occurs in corpus callosum from acute brain slices. Thus, dual functions of connexin-based channels contribute to glucose supply in oligodendroglial lineage, which might pave a new way for energy-metabolism-directed oligodendroglial-targeted therapies.

Computational Analysis of AMPK-Mediated Neuroprotection Suggests Acute Excitotoxic Bioenergetics and Glucose Dynamics Are Regulated by a Minimal Set of Critical Reactions

Loss of ionic homeostasis during excitotoxic stress depletes ATP levels and activates the AMP-activated protein kinase (AMPK), re-establishing energy production by increased expression of glucose transporters on the plasma membrane. Here, we develop a computational model to test whether this AMPK-mediated glucose import can rapidly restore ATP levels following a transient excitotoxic insult. We demonstrate that a highly compact model, comprising a minimal set of critical reactions, can closely resemble the rapid dynamics and cell-to-cell heterogeneity of ATP levels and AMPK activity, as confirmed by single-cell fluorescence microscopy in rat primary cerebellar neurons exposed to glutamate excitotoxicity. The model further correctly predicted an excitotoxicity-induced elevation of intracellular glucose, and well resembled the delayed recovery and cell-to-cell heterogeneity of experimentally measured glucose dynamics. The model also predicted necrotic bioenergetic collapse and altered calcium dynamics following more severe excitotoxic insults. In conclusion, our data suggest that a minimal set of critical reactions may determine the acute bioenergetic response to transient excitotoxicity and that an AMPK-mediated increase in intracellular glucose may be sufficient to rapidly recover ATP levels following an excitotoxic insult.

Energy Metabolism of the Brain, Including the Cooperation between Astrocytes and Neurons, Especially in the Context of Glycogen Metabolism

Glycogen metabolism has important implications for the functioning of the brain, especially the cooperation between astrocytes and neurons. According to various research data, in a glycogen deficiency (for example during hypoglycemia) glycogen supplies are used to generate lactate, which is then transported to neighboring neurons. Likewise, during periods of intense activity of the nervous system, when the energy demand exceeds supply, astrocyte glycogen is immediately converted to lactate, some of which is transported to the neurons. Thus, glycogen from astrocytes functions as a kind of protection against hypoglycemia, ensuring preservation of neuronal function. The neuroprotective effect of lactate during hypoglycemia or cerebral ischemia has been reported in literature. This review goes on to emphasize that while neurons and astrocytes differ in metabolic profile, they interact to form a common metabolic cooperation.

[Advanced MRI techniques of the fetal brain]

CLINICAL/METHODICAL ISSUE

Evaluation of the normal and pathological fetal brain.

STANDARD RADIOLOGICAL METHODS

Magnetic resonance imaging (MRI).

METHODICAL INNOVATIONS

Advanced MRI of the fetal brain.

PERFORMANCE

Diffusion tensor imaging (DTI) is used in clinical practice, all other methods are used at a research level.

ACHIEVEMENTS

Serving as standard methods in the future.

PRACTICAL RECOMMENDATIONS

Combined structural and functional data for all gestational ages will allow more specific insight into the developmental processes of the fetal brain. This gain of information will help provide a common understanding of complex spatial and temporal procedures of early morphological features and their impact on cognitive and sensory abilities.

Exhausting exercise and tissue-specific expression of monocarboxylate transporters in rainbow trout

Transmembrane lactate movements are mediated by monocarboxylate transporters (MCTs), but these proteins have never been characterized in rainbow trout. Our goals were to clone potential trout MCTs, determine tissue distribution, and quantify the effects of exhausting exercise on MCT expression. Such information could prove important to understand the mechanisms underlying the classic "lactate retention" seen in trout white muscle after intense exercise. Four isoforms were identified and partially characterized in rainbow trout: MCT1a, MCT1b, MCT2, and MCT4. MCT1b was the most abundant in heart and red muscle but poorly expressed in the gill and brain where MCT1a and MCT2 were prevalent. MCT expression was strongly stimulated by exhausting exercise in brain (MCT2: +260%) and heart (MCT1a: +90% and MCT1b: +50%), possibly to increase capacity for lactate uptake in these highly oxidative tissues. By contrast, the MCTs of gill, liver, and muscle remained unaffected by exercise. This study provides a possible functional explanation for postexercise "lactate retention" in trout white muscle. Rainbow trout may be unable to release large lactate loads rapidly during recovery because: 1) they only poorly express MCT4, the main lactate exporter found in mammalian glycolytic muscles; 2) the combined expression of all trout MCTs is much lower in white muscle than in any other tissue; and 3) exhausting exercise fails to upregulate white muscle MCT expression. In this tissue, carbohydrates act as an "energy spring" that alternates between explosive power release during intense swimming (glycogen to lactate) and recoil during protracted recovery (slow glycogen resynthesis from local lactate).

MR spectroscopy of the fetal brain: is it possible without sedation?

BACKGROUND AND PURPOSE

The quality of spectroscopic studies may be limited because of unrestricted fetal movement. Sedation is recommended to avoid motion artefacts. However, sedation involves side effects. The aim of this study was to assess the feasibility and quality of brain (1)H-MR spectroscopy in unsedated fetuses and to evaluate whether quality is dependent on the type of spectra, fetal presentation, GA, and/or fetal pathology.

MATERIALS AND METHODS

Seventy-five single-voxel spectroscopic studies of the fetal brain, performed at gestational weeks 19-38 at 1.5T, were evaluated retrospectively. A PRESS (TE = 144 or 35 ms) was used. Fetal presentation, GA, and kind of pathology were recorded. The quality of the spectra was assessed by reviewing the spectral appearance (line width, signal-to-noise) of the creatine resonance obtained relative to concentrations (ratios-to-creatine) of choline, myo-inositol, and NAA.

RESULTS

Of 75 studies, 50 (66.6%) were rated as readable: short TE = 17/50 (34%), long TE = 33/50 (66%), cephalic presentation in 36/50 (72%) studies, breech in 10/50 (20%) studies, and "other" presentation in 4/50 (8%) studies (mean GA, 31.0 weeks). Twenty-eight of 50 fetuses (56%) showed normal development (short TE = 12/28, long TE = 16/28), and 22/50 (44%) showed pathology. Of the 75 studies, 25 (33.3%) were not readable: short TE = 14/25 (56%), long TE = 11/25 (44%), cephalic presentation in 20/25 (80%) studies, breech in 4/25 (16%) studies, and other presentation in 1 study (4%) (mean GA, 30.1 week). Thirteen of 25 fetuses (52%) showed normal development; 12/25 (48%) showed pathology. Statistical analysis revealed no impact of the different parameters on the quality of spectra.

CONCLUSIONS

Single-voxel spectroscopy can be performed in approximately two-thirds of unsedated fetuses, regardless of the type of spectra, fetal presentation, GA, and pathology.

Brain metabolism in fetal intrauterine growth restriction: a proton magnetic resonance spectroscopy study

OBJECTIVE

The purpose of this study was to investigate alterations in brain metabolism in fetuses with intrauterine growth restriction (IUGR) and evidence of cerebral redistribution of blood flow.

STUDY DESIGN

Biometry and Doppler assessment of blood flow was assessed with ultrasound in 28 fetuses with IUGR and cerebral redistribution and in 41 appropriately grown control subjects. Proton magnetic resonance spectroscopy of the fetal brain was then performed to determine the presence of choline (Cho), creatine (Cr), N-acetylaspartate (NAA), and lactate and to generate ratios for NAA:Cho, NAA:Cr, and Cho:Cr.

RESULTS

Sixty-five percent of spectra were interpretable: N-acetylaspartate, choline, and creatine peaks were identified in all these spectra; lactate was present in 5 IUGR fetuses and in 3 appropriately grown fetuses. NAA:Cr and NAA:Cho ratios were significantly lower in IUGR fetuses with cerebral redistribution.

CONCLUSION

Cerebral redistribution is associated with altered brain metabolism that is evidenced by a reduction in NAA:Cho and NAA:Cr ratios.

Activated astroglia during chronic inflammation in Alzheimer's disease--do they neglect their neurosupportive roles?

Alzheimer's disease (AD) is a neurodegenerative disorder, characterized histopathologically by the extracellular deposition of beta-amyloid peptide in senile plaques, as well as intracellular neurofibrillary tangles (NFT) of hyperphosphorylated tau protein, extensive neuronal loss and synaptic changes in the hippocampus and cerebral cortex. In addition, the AD brain shows chronic inflammation characterized by an abundance of reactive astrocytes and activated microglia. In the healthy brain, astrocytes provide essential services for brain homeostasis and neuronal function, including metabolic support for neurons in the form of lactate, glutamate uptake and conversion into glutamine, and synthesis of glutathione and its precursors. In AD, a large body of evidence now suggests that by transforming from a basal to a reactive state, astrocytes neglect their neurosupportive functions, thus rendering neurons vulnerable to neurotoxins including pro-inflammatory cytokines and reactive oxygen species. This review will explain the normal functions of astrocytes, and how these cells might be activated to turn into inflammatory cells, actively contributing to neurodegeneration and neglecting their neurosupportive roles ("neuro-neglect hypothesis"). Furthermore, it is proposed that astrocytes might be promising target of therapeutic intervention for Alzheimer's disease, if these compromised functions can be normalized with pharmacological agents that are specifically designed to return astrocytes to a quiescent phenotype or supplement factors which activated astrocytes lack to produce.

ATP-sensitive potassium channel-mediated lactate effect on orexin neurons: implications for brain energetics during arousal

Active neurons have a high demand for energy substrate, which is thought to be mainly supplied as lactate by astrocytes. Heavy lactate dependence of neuronal activity suggests that there may be a mechanism that detects and controls lactate levels and/or gates brain activation accordingly. Here, we demonstrate that orexin neurons can behave as such lactate sensors. Using acute brain slice preparations and patch-clamp techniques, we show that the monocarboxylate transporter blocker alpha-cyano-4-hydroxycinnamate (4-CIN) inhibits the spontaneous activity of orexin neurons despite the presence of extracellular glucose. Furthermore, fluoroacetate, a glial toxin, inhibits orexin neurons in the presence of glucose but not lactate. Thus, orexin neurons specifically use astrocyte-derived lactate. The effect of lactate on firing activity is concentration dependent, an essential characteristic of lactate sensors. Furthermore, lactate disinhibits and sensitizes these neurons for subsequent excitation. 4-CIN has no effect on the activity of some arcuate neurons, indicating that lactate dependency is not universal. Orexin neurons show an indirect concentration-dependent sensitivity to glucose below 1 mm, responding by hyperpolarization, which is mediated by ATP-sensitive potassium channels composed of Kir6.1 and SUR1 subunits. In conclusion, our study suggests that lactate is a critical energy substrate and a regulator of the orexin system. Together with the known effects of orexins in inducing arousal, food intake, and hepatic glucose production, as well as lactate release from astrocytes in response to neuronal activity, our study suggests that orexin neurons play an integral part in balancing brain activity and energy supply.

Human fetal brain chemistry as detected by proton magnetic resonance spectroscopy

Magnetic resonance spectroscopy represents an invaluable tool for the in vivo study of brain development at the chemistry level. Whereas magnetic resonance spectroscopy has received wide attention in pediatric and adult settings, only a few studies were performed on the human fetal brain. They revealed changes occurring throughout gestation in the levels of the main metabolites detected by proton magnetic resonance spectroscopy (N-acetylaspartate, choline, myo-inositol, creatine, and glutamate), providing a reference for the normal metabolic brain development. Throughout the third trimester of gestation, N-acetylaspartate gradually increases, whereas choline undergoes a slow reduction during the process of myelination. Less clear are the modifications in creatine, myo-inositol, and glutamate levels. Under conditions of fetal distress, the meaning of lactate detection is unclear, and further studies are needed. Another field for investigation involves the possibility of early detection of glutamate levels in fetuses at risk for hypoxic-ischemic encephalopathy, because the role of glutamate excitotoxicity in this context is well-established. Because metabolic modifications may precede functional or morphologic changes in the central nervous system, magnetic resonance spectroscopy may likely serve as a powerful, noninvasive tool for the early diagnosis and prognosis of different pathologic conditions.

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

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

Combining metabolic flux analysis tools and 13C NMR to estimate intracellular fluxes of cultured astrocytes

In this work, brain cell metabolism was investigated by (13)C NMR spectroscopy and metabolic flux analysis (MFA). Monotypic cultures of astrocytes were incubated with labeled glucose for 38 h, and the distribution of the label was analyzed by (13)C NMR spectroscopy. The analysis of the spectra reveals two distinct physiological states characterized by different ratios of pyruvate carboxylase to pyruvate dehydrogenase activities (PC/PDH). Intracellular flux distributions for both metabolic states were estimated by MFA using the isotopic information and extracellular rate measurements as constraints. The model was subsequently checked with the consistency index method. From a biological point of view, the occurrence of the two physiological states appears to be correlated with the presence or absence of extracellular glutamate. Concerning the model, it can be stated that the metabolic network and the set of constraints adopted provide a consistent and robust characterization of the astrocytic metabolism, allowing for the calculation of central intracellular fluxes such as pyruvate recycling, the anaplerotic flux mediated by pyruvate carboxylase, and the glutamine formation through glutamine synthetase.

Lactate, not pyruvate, is neuronal aerobic glycolysis end product: An in vitro electrophysiological study

For over 60 years, a distinction has been made between aerobic and anaerobic glycolysis based on their respective end products: pyruvate of the former, lactate of the latter. Recently we hypothesized that, in the brain, both aerobic and anaerobic glycolysis terminate with the formation of lactate from pyruvate by the enzyme lactate dehydrogenase (LDH). If this hypothesis is correct, lactate must be the mitochondrial substrate for oxidative energy metabolism via its oxidation to pyruvate, plausibly by a mitochondrial LDH. Here we employed electrophysiology of the rat hippocampal slice preparation to test and monitor the effects of malonate and oxamate, two different LDH inhibitors, and glutamate, a neuronal activator, in experiments, the results of which support the hypothesis that lactate, at least in this in vitro setting, is indeed the principal end product of neuronal aerobic glycolysis.

N-Acetylaspartate in the CNS: from neurodiagnostics to neurobiology

The brain is unique among organs in many respects, including its mechanisms of lipid synthesis and energy production. The nervous system-specific metabolite N-acetylaspartate (NAA), which is synthesized from aspartate and acetyl-coenzyme A in neurons, appears to be a key link in these distinct biochemical features of CNS metabolism. During early postnatal central nervous system (CNS) development, the expression of lipogenic enzymes in oligodendrocytes, including the NAA-degrading enzyme aspartoacylase (ASPA), is increased along with increased NAA production in neurons. NAA is transported from neurons to the cytoplasm of oligodendrocytes, where ASPA cleaves the acetate moiety for use in fatty acid and steroid synthesis. The fatty acids and steroids produced then go on to be used as building blocks for myelin lipid synthesis. Mutations in the gene for ASPA result in the fatal leukodystrophy Canavan disease, for which there is currently no effective treatment. Once postnatal myelination is completed, NAA may continue to be involved in myelin lipid turnover in adults, but it also appears to adopt other roles, including a bioenergetic role in neuronal mitochondria. NAA and ATP metabolism appear to be linked indirectly, whereby acetylation of aspartate may facilitate its removal from neuronal mitochondria, thus favoring conversion of glutamate to alpha ketoglutarate which can enter the tricarboxylic acid cycle for energy production. In its role as a mechanism for enhancing mitochondrial energy production from glutamate, NAA is in a key position to act as a magnetic resonance spectroscopy marker for neuronal health, viability and number. Evidence suggests that NAA is a direct precursor for the enzymatic synthesis of the neuron specific dipeptide N-acetylaspartylglutamate, the most concentrated neuropeptide in the human brain. Other proposed roles for NAA include neuronal osmoregulation and axon-glial signaling. We propose that NAA may also be involved in brain nitrogen balance. Further research will be required to more fully understand the biochemical functions served by NAA in CNS development and activity, and additional functions are likely to be discovered.

Elevated lactate suppresses neuronal firing in vivo and inhibits glucose metabolism in hippocampal slice cultures

Glucose is well accepted as the major fuel for neuronal activity, while it remains controversial whether lactate also supports neural activity. In hippocampal slice cultures, synaptic transmission supported by glucose was reversibly suppressed by lactate. To test whether lactate had a similar inhibitory effect in vivo, lactate was perfused into the hippocampi of unanesthetized rats while recording the firing of nearby pyramidal cells. Lactate perfusion suppressed pyramidal cell firing by 87.5+/-8.3% (n=6). Firing suppression was slow in onset and fully reversible and was associated with increased lactate concentration at the site of the recording electrode. In vivo suppression of neural activity by lactate occurred in the presence of glucose; therefore we tested whether suppression of neural firing was due to lactate interference with glucose metabolism. Competition between glucose and lactate was measured in hippocampal slice cultures. Lactate had no effect on glucose uptake. Lactate suppressed glucose oxidation when applied at an elevated, pathological concentration (10 mM), but not at its physiological concentration (1 mM). Pyruvate (10 mM) also inhibited glucose oxidation but was significantly less effective than lactate. The greater suppressive effect of lactate as compared to pyruvate suggests that alteration of the NAD(+)/NADH ratio underlies the suppression of glucose oxidation by lactate. ATP in slice culture was unchanged in glucose (1 mM), but significantly reduced in lactate (1 mM). ATP in slice culture was significantly increased by combination of glucose (1 mM) and lactate (1 mM). These data suggest that alteration of redox ratio underlies the suppression of neural discharge and glucose metabolism by lactate.

Competition between glucose and lactate as oxidative energy substrates in both neurons and astrocytes: a comparative NMR study

Competition between glucose and lactate as oxidative energy substrates was investigated in both primary cultures of astrocytes and neurons using physiological concentrations (1.1 mm for each). Glucose metabolism was distinguished from lactate metabolism by using alternatively labelled substrates in the medium ([1-13C]glucose + lactate or glucose + [3-13C]lactate). After 4 h of incubation, 1H and 13C-NMR spectra were realized on perchloric acid extracts of both cells and culture media. For astrocytic cultures, spectra showed that amino acids (glutamine and alanine) were more labelled in the glucose-labelled condition, indicating that glucose is a better substrate to support oxidative metabolism in these cells. The opposite was observed on spectra from neuronal cultures, glutamate being much more labelled in the lactate-labelled condition, confirming that neurons consume lactate preferentially as an oxidative energy substrate. Analysis of glutamine and glutamate peaks (singlets or multiplets) also suggests that astrocytes have a less active oxidative metabolism than neurons. In contrast, they exhibit a stronger glycolytic metabolism than neurons as indicated by their high lactate production yield. Using a mathematical model, we have estimated the relative contribution of exogenous glucose and lactate to neuronal oxidative metabolism. Under the aforementioned conditions, it represents 25% for glucose and 75% for lactate. Altogether, these results obtained on separate astrocytic and neuronal cultures support the idea that lactate, predominantly produced by astrocytes, is used as a supplementary fuel by neurons in vivo already under resting physiological conditions.

Fuelling cerebral activity in exercising man

The metabolic response to brain activation in exercise might be expressed as the cerebral metabolic ratio (MR; uptake O2/glucose + 1/2 lactate). At rest, brain energy is provided by a balanced oxidation of glucose as MR is close to 6, but activation provokes a 'surplus' uptake of glucose relative to that of O2. Whereas MR remains stable during light exercise, it is reduced by 30% to 40% when exercise becomes demanding. The MR integrates metabolism in brain areas stimulated by sensory input from skeletal muscle, the mental effort to exercise and control of exercising limbs. The MR decreases during prolonged exhaustive exercise where blood lactate remains low, but when vigorous exercise raises blood lactate, the brain takes up lactate in an amount similar to that of glucose. This lactate taken up by the brain is oxidised as it does not accumulate within the brain and such pronounced brain uptake of substrate occurs independently of plasma hormones. The 'surplus' of glucose equivalents taken up by the activated brain may reach approximately 10 mmol, that is, an amount compatible with the global glycogen level. It is suggested that a low MR predicts shortage of energy that ultimately limits motor activation and reflects a biologic background for 'central fatigue'.

Quantitative on-line monitoring of cellular glucose and lactate metabolism in vitro with slow perfusion

An on-line in vitro perfusion technique is described that allows the continuous quantification of cellular glucose metabolism in vitro. Using biosensor technology, we measure glucose and lactate metabolism at a minute-to-minute time resolution for periods up to several days. The application of our perfusion-detection technique for in vitro monitoring is demonstrated in a wide variety of cells, including primary neuronal and astroglia cultures, yeast cells, and human lymphocytes. The method shows that variations in oxygen delivery or exposure to a noncompetitive pseudosubstrate (here 2-deoxyglucose) affects normal glucose metabolism. The innovative advantage of the present system is that, in contrast to other devices including a recently described system, metabolism per cell can be quantified. The potential of in vitro on-line monitoring is discussed for application in studying normal and abnormal metabolism, toxic and nontoxic drug effects, and human tissue biopsies.

Lactate: the ultimate cerebral oxidative energy substrate?

Research over the past two decades has renewed the interest in lactate, no longer as a useless end product of anaerobic glycolysis in brain (and other tissues), but as an oxidative substrate for energy metabolism. While this topic would be considered blasphemy only three decades ago, much recent evidence indicates that lactate does play a major role in aerobic energy metabolism in the brain, the heart, skeletal muscle, and possibly in any other tissue and organ. Nevertheless, this concept has challenged the old dogma and ignited a fierce debate, especially among neuroscientists, pitting the supporters of glucose as the major oxidative energy substrate against those who support lactate as a possible alternative to glucose under certain conditions. Meanwhile, researchers working on energy metabolism in skeletal muscle have taken great strides toward bridging between these two extreme positions, while avoiding the high decibels of an emotional debate. Employing their findings along with the existing old and new data on cerebral energy metabolism, it is postulated here that lactate is the only major product of cerebral (and other tissues) glycolysis, whether aerobic or anaerobic, neuronal or astrocytic, under rest or during activation. Consequently, this postulate entails that lactate is a major, if not the only, substrate for the mitochondrial tricarboxylic acid cycle. If proven true, this hypothesis could provide better understanding of the biochemistry and physiology of (cerebral) energy metabolism, while holding important implications in the field of neuroimaging. Concomitantly, it could satisfy both 'glucoseniks' and 'lactatians' in the ongoing debate.

How astrocytes feed hungry neurons

For years glucose was thought to constitute the sole energy substrate for neurons; it was believed to be directly provided to neurons via the extracellular space by the cerebral circulation. It was recently proposed that in addition to glucose, neurons might rely on lactate to sustain their activity. Therefore, it was demonstrated that lactate is a preferred oxidative substrate for neurons not only in vitro but also in vivo. Moreover, the presence of specific monocarboxylate transporters on neurons as well as on astrocytes is consistent with the hypothesis of a transfer of lactate from astrocytes to neurons. Evidence has been provided for a mechanism whereby astrocytes respond to glutamatergic activity by enhancing their glycolytic activity, resulting in increased lactate release. This is accomplished via the uptake of glutamate by glial glutamate transporters, leading to activation of the Na+/K+ ATPase and a stimulation of astrocytic glycolysis. Several recent observations obtained both in vitro and in vivo with different approaches have reinforced this view of brain energetics. Such an understanding might be critically important, not only because it forms the basis of some classical functional brain imaging techniques but also because several neurodegenerative diseases exhibit diverse alterations in energy metabolism.

Monocarboxylate transporters in the central nervous system: distribution, regulation and function

Monocarboxylate transporters (MCTs) are proton-linked membrane carriers involved in the transport of monocarboxylates such as lactate, pyruvate, as well as ketone bodies. They belong to a larger family of transporters composed of 14 members in mammals based on sequence homologies. MCTs are found in various tissues including the brain where three isoforms, MCT1, MCT2 and MCT4, have been described. Each of these isoforms exhibits a distinct regional and cellular distribution in rodent brain. At the cellular level, MCT1 is expressed by endothelial cells of microvessels, by ependymocytes as well as by astrocytes. MCT4 expression appears to be specific for astrocytes. By contrast, the predominant neuronal monocarboxylate transporter is MCT2. Interestingly, part of MCT2 immunoreactivity is located at postsynaptic sites, suggesting a particular role of monocarboxylates and their transporters in synaptic transmission. In addition to variation in expression during development and upon nutritional modifications, new data indicate that MCT expression is regulated at the translational level by neurotransmitters. Understanding how transport of monocarboxylates is regulated could be of particular importance not only for neuroenergetics but also for areas such as functional brain imaging, regulation of food intake and glucose homeostasis, or for central nervous system disorders such as ischaemia and neurodegenerative diseases.

Elevated blood lactate is not a primary cause of anorexia in tumor-bearing rats

Tumor-bearing (TB) rats exhibit elevated concentrations of lactate in blood contiguous with the development of anorexia. Continuous intravenous infusion of lactate into non-TB rats reduced food intake at plasma concentrations lower than those observed in anorectic TB rats. Levels of neuropeptide Y (NPY) were elevated in the ventromedial (VMH) and dorsomedial hypothalamic regions of lactate-infused rats. The addition of the enhancer of pyruvate dehydrogenase activity, dichloroacetate (DCA), to the drinking water of TB rats (0.1-0.4%) normalized blood lactate concentration but had no significant effect on anorexia. However, the elevated concentration of NPY in the VMH of anorectic TB rats was also normalized by the DCA treatment. No alterations in regional hypothalamic levels of corticotropin-releasing factor were observed within any treatment conditions. These results suggest that, although hyperlactatemia may be involved in maintaining elevated NPY concentrations in anorectic TB rats, it does not appear to be a significant factor in the etiology of experimental cancer anorexia.

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.

Dual-gene, dual-cell type therapy against an excitotoxic insult by bolstering neuroenergetics

Increasing evidence suggests that glutamate activates the generation of lactate from glucose in astrocytes; this lactate is shuttled to neurons that use it as a preferential energy source. We explore this multicellular "lactate shuttle" with a novel dual-cell, dual-gene therapy approach and determine the neuroprotective potential of enhancing this shuttle. Viral vector-driven overexpression of a glucose transporter in glia enhanced glucose uptake, lactate efflux, and the glial capacity to protect neurons from excitotoxicity. In parallel, overexpression of a lactate transporter in neurons enhanced lactate uptake and neuronal resistance to excitotoxicity. Finally, overexpression of both transgenes in the respective cell types provided more protection than either therapy alone, demonstrating that a dual-cell, dual-gene therapy approach gives greater neuroprotection than the conventional single-cell, single-gene strategy.

Neuroenergetics: calling upon astrocytes to satisfy hungry neurons

Classical neuroenergetics states that glucose is the exclusive energy substrate of brain cells and its full oxidation provides all the necessary energy to support brain function. Recent data have revealed a more intricate picture in which astrocytes play a key role in supplying lactate as an additional energy substrate in register with glutamatergic activity.

Increased lactate/pyruvate ratio augments blood flow in physiologically activated human brain

The factors regulating cerebral blood flow (CBF) changes in physiological activation remain the subject of great interest and debate. Recent experimental studies suggest that an increase in cytosolic NADH mediates increased blood flow in the working brain. Lactate injection should elevate NADH levels by increasing the lactate/pyruvate ratio, which is in near equilibrium with the NADH/NAD(+) ratio. We studied CBF responses to bolus lactate injection at rest and in visual stimulation by using positron-emission tomography in seven healthy volunteers. Bolus lactate injection augmented the CBF response to visual stimulation by 38-53% in regions of the visual cortex but had no effect on the resting CBF or the whole-brain CBF. These lactate-induced CBF increases correlated with elevations in plasma lactate/pyruvate ratios and in plasma lactate levels but not with plasma pyruvate levels. Our observations support the hypothesis that an increase in the NADH/NAD(+) ratio activates signaling pathways to selectively increase CBF in the physiologically stimulated brain regions.

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.

Lactate is a preferential oxidative energy substrate over glucose for neurons in culture

The authors investigated concomitant lactate and glucose metabolism in primary neuronal cultures using 13C- and 1H-NMR spectroscopy. Neurons were incubated in a medium containing either [1-13C]glucose and different unlabeled lactate concentrations, or unlabeled glucose and different [3-13C]lactate concentrations. Overall, 13C-NMR spectra of cellular extracts showed that more 13C was incorporated into glutamate when lactate was the enriched substrate. Glutamate 13C-enrichment was also found to be much higher in lactate-labeled than in glucose-labeled conditions. When glucose and lactate concentrations were identical (5.5 mmol/L), relative contributions of glucose and lactate to neuronal oxidative metabolism amounted to 21% and 79%, respectively. Results clearly indicate that when neurons are in the presence of both glucose and lactate, they preferentially use lactate as their main oxidative substrate.

Lactate as a pivotal element in neuron-glia metabolic cooperation

Lactate has been considered for a long time as a metabolic waste and/or a sign of hypoxia in the central nervous system. Nevertheless, clear evidence that lactate can constitute an adequate energy substrate for brain tissue has been provided as early as in the 1950s with the pioneering work of McIlwain in brain slices. Over the years, several studies using different approaches have confirmed that lactate is efficiently oxidized by brain cells in vitro. Moreover, lactate has been shown under certain circumstances to have a neuroprotective effect and support neuronal activity. Similar confirmation of lactate utilization in vivo as well as putative neuroprotection in various excitotoxic models has been provided. Lactate was even shown to restore cognitive performance upon an hypoglycemic episode in humans. More recently, it was proposed that lactate could be produced by astrocytes and released in the extracellular space to form a pool readily available for neurons in case of high energy demands. Several elements support the concept of a lactate shuttle between astrocytes and neurons in the central nervous system. Among them, the description of specific monocarboxylate transporters found on both astrocytes and neurons is an important observation consistent with this concept. Interestingly, lactate shuttles between different cell types within the same organ have been described outside the central nervous system, notably in muscle and testis. Thus, lactate is emerging as a valuable intercellular exchange molecule in different systems including the brain where it might be an essential element of neuron-glia metabolic interactions.

Lactate: a preferred fuel for human brain metabolism in vivo

Recent in vitro studies suggest that lactate, rather than glucose, may be the preferred fuel for neuronal metabolism. The authors examined the effect of lactate on global brain glucose uptake in euglycemic human subjects using 18 fluoro-deoxyglucose (FDG) positron emission tomography (PET). Eight healthy men, aged 40 to 54 years, underwent a 60-minute FDG-PET scan on two occasions in random order. On one occasion, 6.72% sodium lactate was infused at a rate of 50 micro mol. kg-1. min-1 for 20 minutes and then reduced to 30 micro mol. kg-1. min-1; 1.4% sodium bicarbonate was infused as a control on the other occasion. Plasma glucose levels were not different between the two groups (5.3 +/- 0.23 and 5.3 +/- 0.24 mmol/L, P = 0.55). Plasma lactate was significantly elevated by lactate infusion (4.08 +/- 0.35 vs. 0.63 +/- 0.22 mmol/L, P < 0.0005. The whole-brain rate of glucose uptake was significantly reduced by approximately 17% during lactate infusion (0.195 +/- 0.022 vs. 0.234 +/- 0.020 micro mol. g-1. min-1, P = 0.001). The authors conclude that, in vivo in humans, circulating lactate is used by the brain at euglycemia, with sparing of glucose.

Feeding active neurons: (re)emergence of a nursing role for astrocytes

Despite unquestionable evidence that glucose is the major energy substrate for the brain, data collected over several decades with different approaches suggest that lactate may represent a supplementary metabolic substrate for neurons. Starting with the pioneering work of McIlwain in the early 1950s which showed that lactate can sustain the respiratory rate of small brain tissue pieces, this idea receives confirmation with more recent studies using nuclear magnetic resonance spectroscopy undoubtedly demonstrating that lactate is efficiently oxidized by neurons, both in vitro and in vivo. Not only is lactate able to maintain ATP levels and promote neuronal survival but it was also found to support neuronal activity, at least if low levels of glucose are present. Despite the early suggestion for a role of astrocytes in metabolic supply to neurons, it is only recently however that they have been considered as a potential source of lactate for neurons. Moreover, it has been proposed that astrocytes might provide lactate to neurons in response to enhanced synaptic activity by a well-characterized mechanism involving glutamate uptake. The description of specific transporters for lactate on both astrocytes and neurons further suggest that there exist a coordinated mechanism of lactate exchange between the two cell types. Thus it is proposed that astrocytes play a nursing role toward neurons by providing lactate as an additional energy substrate especially during periods of enhanced synaptic activity. The importance of this metabolic cooperation within the central nervous system, although not unique if compared to other organs, still remains to be explored.

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.

Effects of lactate on glucose-sensing neurons in the solitary tract nucleus

For nervous tissue, lactate is a valuable energy substrate that can be extracted from glucose by astrocytes and released for neuronal use. Therefore, we hypothesized that the glucose-sensing neurons that signal the glycemic changes involved in the control of body energy homeostasis may be responsive to extracellular lactate as well. To test this hypothesis, neuronal activity was recorded extracellularly in the solitary tract nucleus of anesthetized rats in order to compare the effects of microelectrophoretic applications of glucose and lactate and of moderate hyperglycemia and to assess the possible effects of lactate on the response to glucose. About 90% of the investigated neurons behaved in a similar manner after local ejections of glucose and lactate. Among them, most neurons activated by glucose were also activated by lactate and all neurons depressed by glucose were also depressed by lactate. This result suggests that the response to these two compounds is mediated by a common mechanism related to their utilization as oxidizible substrates. In half of the tested neurons, the response to glucose was eliminated or significantly reduced after repeated lactate ejections. This inhibitory effect is a likely result of a modification in glucose metabolism induced by a high extracellular lactate level. Most glycemia-sensitive neurons responded similarly to moderate hyperglycemia and to local lactate ejection, suggesting that high brain lactate levels might interfere with the brain mechanisms that mediate glucoprivic eating.

Interactions among glucose, lactate and adenosine regulate energy substrate utilization in hippocampal cultures

Glucose is the major energy source during normal adult brain activity. However, it appears that glial-derived lactate is preferred as an energy substrate by neurons following hypoxia-ischemia. We examined factors influencing this switch in energetic bias from glucose to lactate in cultured hippocampal neurons, focusing on the effects of the physiological changes in lactate, glucose and adenosine concentrations seen during hypoxia-ischemia. We show that with typical basal concentrations of lactate and glucose, lactate had no effect on glucose uptake. However, at the concentrations of these metabolites found after hypoxia-ischemia, lactate inhibited glucose uptake. Reciprocally, glucose had no effect on lactate utilization regardless of glucose and lactate concentrations. Furthermore, we find that under hypoglycemic conditions adenosine had a small, but significant, inhibitory effect on glucose uptake. Additionally, adenosine increased lactate utilization. Thus, the relative concentrations of glucose, lactate and adenosine, which are indicative of the energy status of the hippocampus, influence which energy substrates are used. These results support the idea that after hypoxia-ischemia, neurons are biased in the direction of lactate rather than glucose utilization and this is accomplished through a number of regulatory steps.

Direct administration and utilization of [1-13C]glucose by fetal brain and liver tissues under normal and ischemic conditions: 1H, 31P, and 13C NMR studies

Three distinct, maternal-independent routes (e.g. intraamniotic, intraperitoneal and intracerebral), for [1-13C]glucose utilization by fetal brain and liver tissues, were examined by multinuclear magnetic resonance (NMR) spectroscopy before and after vascular occlusion of the maternal-fetal blood flow. Labeled lactate was the major glycolytic product by all routes, but in addition labeled TCA cycle products were also generated. Fractional 13C enrichment in both glucose and lactate were always higher in the ischemic state compared to controls using either one of the three routes studied. After intraperitoneal injection total glucose in the fetal brain was decreased by 85% after 20 min reperfusion following 20 min ischemia, but was elevated up to 170% after 60 min. [1-13C]glucose increased continuously by up to 370% after 60 min. Total glucose in the fetal liver remained unchanged while [1-13C]glucose increased up to 380%. Total lactate level in brain was 50-80% above the control apart from a transient increase (140%) notable after 40 min reperfusion. The kinetics of [3-13C]lactate followed a similar time course. At the same time when lactate was transiently increased in fetal brain, total lactate as well as 13C-labeled lactate showed a transient decrease in liver after 40 min. While the ways of mobilization of energy substrates for maintaining adequate metabolic activity in the fetal brain remain still unclear, the present 13C NMR studies suggest that both liver glucose and lactate can contribute to brain metabolism particularly under ischemic stress.

Monocarboxylate transporter expression in mouse brain

Although glucose is the major metabolic fuel needed for normal brain function, monocarboxylic acids, i.e., lactate, pyruvate, and ketone bodies, can also be utilized by the brain as alternative energy substrates. In most mammalian cells, these substrates are transported either into or out of the cell by a family of monocarboxylate transporters (MCTs), first cloned and sequenced in the hamster. We have recently cloned two MCT isoforms (MCT1 and MCT2) from a mouse kidney cDNA library. Northern blot analysis revealed that MCT1 mRNA is ubiquitous and can be detected in most tissues at a relatively constant level. MCT2 expression is more limited, with high levels of expression confined to testes, kidney, stomach, and liver and lower levels in lung, brain, and epididymal fat. Both MCT1 mRNA and MCT2 mRNA are detected in mouse brain using antisense riboprobes and in situ hybridization. MCT1 mRNA is found throughout the cortex, with higher levels of hybridization in hippocampus and cerebellum. MCT2 mRNA was detected in the same areas, but the pattern of expression was more specific. In addition, MCT1 mRNA, but not MCT2, is localized to the choroid plexus, ependyma, microvessels, and white matter structures such as the corpus callosum. These results suggest a differential expression of the two MCTs at the cellular level.