Cerebral glucose metabolism and the glutamine cycle as detected by in vivo and in vitro 13C NMR spectroscopy

Melanoma Brain Metastasis: Biology and Therapeutic Advances

Metastasis to the brain is a frequent complication of advanced melanoma. Historically, patients with melanoma brain metastasis (MBM) have had dismal outcomes, but outcomes have improved with the development of more effective treatments, including stereotactic radiosurgery and effective immune and targeted therapies. Despite these advances, MBM remains a leading cause of death from this disease, and many therapies show decreased efficacy against these tumors compared with extracranial metastases. This differential efficacy may be because of recently revealed unique molecular and immune features of MBMs-which may also provide rational new therapeutic strategies.

New insights into the correlations between circulating tumor cells and target organ metastasis

Organ-specific metastasis is the primary cause of cancer patient death. The distant metastasis of tumor cells to specific organs depends on both the intrinsic characteristics of the tumor cells and extrinsic factors in their microenvironment. During an intermediate stage of metastasis, circulating tumor cells (CTCs) are released into the bloodstream from primary and metastatic tumors. CTCs harboring aggressive or metastatic features can extravasate to remote sites for continuous colonizing growth, leading to further lesions. In the past decade, numerous studies demonstrated that CTCs exhibited huge clinical value including predicting distant metastasis, assessing prognosis and monitoring treatment response et al. Furthermore, increasingly numerous experiments are dedicated to identifying the key molecules on or inside CTCs and exploring how they mediate CTC-related organ-specific metastasis. Based on the above molecules, more and more inhibitors are being developed to target CTCs and being utilized to completely clean CTCs, which should provide promising prospects to administer advanced tumor. Recently, the application of various nanomaterials and microfluidic technologies in CTCs enrichment technology has assisted to improve our deep insights into the phenotypic characteristics and biological functions of CTCs as a potential therapy target, which may pave the way for us to make practical clinical strategies. In the present review, we mainly focus on the role of CTCs being involved in targeted organ metastasis, especially the latest molecular mechanism research and clinical intervention strategies related to CTCs.

7 Tesla and Beyond: Advanced Methods and Clinical Applications in Magnetic Resonance Imaging

ABSTRACT

Ultrahigh magnetic fields offer significantly higher signal-to-noise ratio, and several magnetic resonance applications additionally benefit from a higher contrast-to-noise ratio, with static magnetic field strengths of B0 ≥ 7 T currently being referred to as ultrahigh fields (UHFs). The advantages of UHF can be used to resolve structures more precisely or to visualize physiological/pathophysiological effects that would be difficult or even impossible to detect at lower field strengths. However, with these advantages also come challenges, such as inhomogeneities applying standard radiofrequency excitation techniques, higher energy deposition in the human body, and enhanced B0 field inhomogeneities. The advantages but also the challenges of UHF as well as promising advanced methodological developments and clinical applications that particularly benefit from UHF are discussed in this review article.

Inhibition of Glutamate Release, but Not of Glutamine Recycling to Glutamate, Is Involved in Delaying the Onset of Initial Lithium-Pilocarpine-Induced Seizures in Young Rats by a Non-Convulsive MSO Dose

Initial seizures observed in young rats during the 60 min after administration of pilocarpine (Pilo) were delayed and attenuated by pretreatment with a non-convulsive dose of methionine sulfoximine (MSO). We hypothesized that the effect of MSO results from a) glutamine synthetase block-mediated inhibition of conversion of Glu/Gln precursors to neurotransmitter Glu, and/or from b) altered synaptic Glu release. Pilo was administered 60 min prior to sacrifice, MSO at 75 mg/kg, i.p., 2.5 h earlier. [1,2-¹³C]acetate and [U-¹³C]glucose were i.p.-injected either together with Pilo (short period) or 15 min before sacrifice (long period). Their conversion to Glu and Gln in the hippocampus and entorhinal cortex was followed using [¹³C] gas chromatography-mass spectrometry. Release of in vitro loaded Glu surrogate, [³H]d-Asp from ex vivo brain slices was monitored in continuously collected superfusates. [³H]d-Asp uptake was tested in freshly isolated brain slices. At no time point nor brain region did MSO modify incorporation of [¹³C] to Glu or Gln in Pilo-treated rats. MSO pretreatment decreased by ~37% high potassium-induced [³H]d-Asp release, but did not affect [³H]d-Asp uptake. The results indicate that MSO at a non-convulsive dose delays the initial Pilo-induced seizures by interfering with synaptic Glu-release but not with neurotransmitter Glu recycling.

Melanoma brain metastases: Biological basis and novel therapeutic strategies

The development of brain metastases is the deadliest complication of advanced melanoma and has long been associated with a dismal prognosis. The recent years have seen incredible progress in the development of therapies for melanoma brain metastases (MBM), with both targeted therapies (the BRAF-MEK inhibitor combination) and immune checkpoint inhibitors (the anti-CTLA-4, anti-PD-1 combination) showing impressive levels of activity. Despite this, durations of response for these therapies remain lower at intracranial sites of metastasis compared to extracranial metastases and it has been suggested that there are unique features of the brain microenvironment that contribute to therapeutic escape. In this review, we outline the latest research into the biology and pathophysiology of melanoma brain metastasis development and progression. We then discuss the current status of clinical trial that are open to patients with MBM and end by describing the ongoing challenges for the field.

The bioactivity of neuronal-derived nitric oxide in aging and neurodegeneration: Switching signaling to degeneration

The small and diffusible free radical nitric oxide (^•NO) has fascinated biological and medical scientists since it was promoted from atmospheric air pollutant to biological ubiquitous signaling molecule. Its unique physical chemical properties expand beyond its radical nature to include fast diffusion in aqueous and lipid environments and selective reactivity in a biological setting determined by bioavailability and reaction rate constants with biomolecules. In the brain, ^•NO is recognized as a key player in numerous physiological processes ranging from neurotransmission/neuromodulation to neurovascular coupling and immune response. Furthermore, changes in its bioactivity are central to the molecular pathways associated with brain aging and neurodegeneration. The understanding of ^•NO bioactivity in the brain, however, requires the knowledge of its concentration dynamics with high spatial and temporal resolution upon stimulation of its synthesis. Here we revise our current understanding of the role of neuronal-derived ^•NO in brain physiology, aging and degeneration, focused on changes in the extracellular concentration dynamics of this free radical and the regulation of bioenergetic metabolism and neurovascular coupling.

[13C6,D8]2-deoxyglucose phosphorylation by hexokinase shows selectivity for the β-anomer

A non-radioactive 2-deoxyglucose (2DG) analog has been developed here for hyperpolarized magnetic resonance investigations. The analog, [13C6,D8]2DG, showed 13% polarization in solution (27,000-fold signal enhancement at the C1 site), following a dissolution-DNP hyperpolarization process. The phosphorylation of this analog by yeast hexokinase (yHK) was monitored in real-time with a temporal resolution of 1 s. We show that yHK selectively utilizes the β anomer of the 2DG analog, thus revealing a surprising anomeric specificity of this reaction. Such anomeric selectivity was not observed for the reaction of yHK or bacterial glucokinase with a hyperpolarized glucose analog. yHK is highly similar to the human HK-2, which is overexpressed in malignancy. Thus, the current finding may shed a new light on a fundamental enzyme activity which is utilized in the most widespread molecular imaging technology for cancer detection - positron-emission tomography with 18F-2DG.

Pros and cons of ultra-high-field MRI/MRS for human application

Magnetic resonance imaging and spectroscopic techniques are widely used in humans both for clinical diagnostic applications and in basic research areas such as cognitive neuroimaging. In recent years, new human MR systems have become available operating at static magnetic fields of 7 T or higher (≥300 MHz proton frequency). Imaging human-sized objects at such high frequencies presents several challenges including non-uniform radiofrequency fields, enhanced susceptibility artifacts, and higher radiofrequency energy deposition in the tissue. On the other side of the scale are gains in signal-to-noise or contrast-to-noise ratio that allow finer structures to be visualized and smaller physiological effects to be detected. This review presents an overview of some of the latest methodological developments in human ultra-high field MRI/MRS as well as associated clinical and scientific applications. Emphasis is given to techniques that particularly benefit from the changing physical characteristics at high magnetic fields, including susceptibility-weighted imaging and phase-contrast techniques, imaging with X-nuclei, MR spectroscopy, CEST imaging, as well as functional MRI. In addition, more general methodological developments such as parallel transmission and motion correction will be discussed that are required to leverage the full potential of higher magnetic fields, and an overview of relevant physiological considerations of human high magnetic field exposure is provided.

Spatially Resolved Bioenergetic and Genetic Reprogramming Through the Brain of Rats Bearing Implanted C6 Gliomas As Detected by Multinuclear High-Resolution Magic Angle Spinning and Genomic Analysis

We used ¹H, ¹³C HRMAS and genomic analysis to investigate regionally the transition from oxidative to glycolytic phenotype and its relationship with altered gene expression in adjacent biopsies through the brain of rats bearing C6 gliomas. Tumor-bearing animals were anesthetized and infused with a solution of [1-¹³C]-glucose, and small adjacent biopsies were obtained spanning transversally from the contralateral hemisphere (regions I and II), the right and left peritumoral areas (regions III and V, respectively), and the tumor core (region IV). These biopsies were analyzed by ¹H, ¹³C HRMAS and by quantitative gene expression techniques. Glycolytic metabolism, as reflected by the [3-¹³C]-lactate content, increased clearly from regions I to IV, recovering partially to physiological levels in region V. In contrast, oxidative metabolism, as reflected by the [4-¹³C]-glutamate labeling, decreased in regions I-IV, recovering partially in region V. This metabolic shift from normal to malignant metabolic phenotype paralleled changes in the expression of HIF1α, HIF2α, HIF3α genes, downstream transporters, and regulatory glycolytic, oxidative, and anaplerotic genes in the same regions. Together, our results indicate that genetic and metabolic alterations occurring in the brain of rats bearing C6 gliomas colocalize in situ and the profile of genetic alterations in every region can be inferred from the metabolomic profiles observed in situ by multinuclear HRMAS.

The biology and therapeutic management of melanoma brain metastases

The recent years have seen significant progress in the development of systemic therapies to treat patients with advanced melanoma. Use of these new treatment modalities, which include immune checkpoint inhibitors and small molecule BRAF inhibitors, lead to increased overall survival and better outcomes. Although revolutionary, these therapies are often less effective against melanoma brain metastases, and frequently the CNS is the major site of treatment failure. The development of brain metastases remains a serious complication of advanced melanoma that is associated with significant morbidity and mortality. New approaches to both prevent the development of brain metastases and treat established disease are urgently needed. In this review we will outline the mechanisms underlying the development of melanoma brain metastases and will discuss how new insights into metastasis biology are driving the development of new therapeutic strategies. Finally, we will describe the latest data from the ongoing clinical trials for patients with melanoma brain metastases.

Neurovascular-neuroenergetic coupling axis in the brain: master regulation by nitric oxide and consequences in aging and neurodegeneration

The strict energetic demands of the brain require that nutrient supply and usage be fine-tuned in accordance with the specific temporal and spatial patterns of ever-changing levels of neuronal activity. This is achieved by adjusting local cerebral blood flow (CBF) as a function of activity level - neurovascular coupling - and by changing how energy substrates are metabolized and shuttled amongst astrocytes and neurons - neuroenergetic coupling. Both activity-dependent increase of CBF and O₂ and glucose utilization by active neural cells are inextricably linked, establishing a functional metabolic axis in the brain, the neurovascular-neuroenergetic coupling axis. This axis incorporates and links previously independent processes that need to be coordinated in the normal brain. We here review evidence supporting the role of neuronal-derived nitric oxide (^•NO) as the master regulator of this axis. Nitric oxide is produced in tight association with glutamatergic activation and, diffusing several cell diameters, may interact with different molecular targets within each cell type. Hemeproteins such as soluble guanylate cyclase, cytochrome c oxidase and hemoglobin, with which ^•NO reacts at relatively fast rates, are but a few of the key in determinants of the regulatory role of ^•NO in the neurovascular-neuroenergetic coupling axis. Accordingly, critical literature supporting this concept is discussed. Moreover, in view of the controversy regarding the regulation of catabolism of different neural cells, we further discuss key aspects of the pathways through which ^•NO specifically up-regulates glycolysis in astrocytes, supporting lactate shuttling to neurons for oxidative breakdown. From a biomedical viewpoint, derailment of neurovascular-neuroenergetic axis is precociously linked to aberrant brain aging, cognitive impairment and neurodegeneration. Thus, we summarize current knowledge of how both neurovascular and neuroenergetic coupling are compromised in aging, traumatic brain injury, epilepsy and age-associated neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease, suggesting that a shift in cellular redox balance may contribute to divert ^•NO bioactivity from regulation to dysfunction.

Hyperpolarized MRS: New tool to study real-time brain function and metabolism

The advent of dissolution dynamic nuclear polarization (DNP) led to the emergence of a new kind of magnetic resonance (MR) measurements providing the opportunity to probe metabolism in vivo in real time. It has been shown that, following the injection of hyperpolarized substrates prepared using dissolution DNP, specific metabolic bioprobes that can be used to differentiate between healthy and pathological tissue in preclinical and clinical studies can be readily detected by MR thanks to the tremendous signal enhancement. The present article aims at reviewing the studies of cerebral function and metabolism based on the use of hyperpolarized MR. The constraints and future opportunities that this technology could offer are discussed.

Brain glucose metabolism during hypoglycemia in type 1 diabetes: insights from functional and metabolic neuroimaging studies

Hypoglycemia is the most frequent complication of insulin therapy in patients with type 1 diabetes. Since the brain is reliant on circulating glucose as its main source of energy, hypoglycemia poses a threat for normal brain function. Paradoxically, although hypoglycemia commonly induces immediate decline in cognitive function, long-lasting changes in brain structure and cognitive function are uncommon in patients with type 1 diabetes. In fact, recurrent hypoglycemia initiates a process of habituation that suppresses hormonal responses to and impairs awareness of subsequent hypoglycemia, which has been attributed to adaptations in the brain. These observations sparked great scientific interest into the brain's handling of glucose during (recurrent) hypoglycemia. Various neuroimaging techniques have been employed to study brain (glucose) metabolism, including PET, fMRI, MRS and ASL. This review discusses what is currently known about cerebral metabolism during hypoglycemia, and how findings obtained by functional and metabolic neuroimaging techniques contributed to this knowledge.

Metabolomic analysis of exercise effects in the POLG mitochondrial DNA mutator mouse brain

Mitochondrial DNA (mtDNA) mutator mice express a mutated form of mtDNA polymerase gamma that results an accelerated accumulation of somatic mtDNA mutations in association with a premature aging phenotype. An exploratory metabolomic analysis of cortical metabolites in sedentary and exercised mtDNA mutator mice and wild-type littermate controls at 9-10 months of age was performed. Pathway analysis revealed deficits in the neurotransmitters acetylcholine, glutamate, and aspartate that were ameliorated by exercise. Nicotinamide adenine dinucleotide (NAD) depletion and evidence of increased poly(adenosine diphosphate-ribose) polymerase 1 (PARP1)activity were apparent in sedentary mtDNA mutator mouse cortex, along with deficits in carnitine metabolites and an upregulated antioxidant response that largely normalized with exercise. These data highlight specific pathways that are altered in the brain in association with an accelerated age-related accumulation of somatic mtDNA mutations. These results may have relevance to age-related neurodegenerative diseases associated with mitochondrial dysfunction, such as Alzheimer's disease and Parkinson's disease and provide insights into potential mechanisms of beneficial effects of exercise on brain function.

Age-related metabolic fatigue during low glucose conditions in rat hippocampus

Previous reports have indicated that with aging, intrinsic brain tissue changes in cellular bioenergetics may hamper the brain's ability to cope with metabolic stress. Therefore, we analyzed the effects of age on neuronal sensitivity to glucose deprivation by monitoring changes in field excitatory postsynaptic potentials (fEPSPs), tissue Po2, and NADH fluorescence imaging in the CA1 region of hippocampal slices obtained from F344 rats (1-2, 3-6, 12-20, and >22 months). Forty minutes of moderate low glucose (2.5 mM) led to approximately 80% decrease of fEPSP amplitudes and NADH decline in all 4 ages that reversed after reintroduction of 10 mM glucose. However, tissue slices from 12 to 20 months and >22-month-old rats were more vulnerable to low glucose: fEPSPs decreased by 50% on average 8 minutes faster compared with younger slices. Tissue oxygen utilization increased after onset of 2.5 mM glucose in all ages of tissue slices, which persisted for 40 minutes in younger tissue slices. But, in older tissue slices the increased oxygen utilization slowly faded and tissue Po2 levels increased toward baseline values after approximately 25 minutes of glucose deprivation. In addition, with age the ability to regenerate NADH after oxidation was diminished. The NAD(+)/NADH ratio remained relatively oxidized after low glucose, even during recovery. In young slices, glycogen levels were stable throughout the exposure to low glucose. In contrast, with aging utilization of glycogen stores was increased during low glucose, particularly in hippocampal slices from >22 months old rats, indicating both inefficient metabolism and increased demand for glucose. Lactate addition (20 mM) improved oxidative metabolism by directly supplementing the mitochondrial NADH pool and maintained fEPSPs in young as well as aged tissue slices, indicating that inefficient metabolism in the aging tissue can be improved by directly enhancing NADH regeneration.

(13)C NMR spectroscopy applications to brain energy metabolism

(13)C nuclear magnetic resonance (NMR) spectroscopy is the method of choice for studying brain metabolism. Indeed, the most convincing data obtained to decipher metabolic exchanges between neurons and astrocytes have been obtained using this technique, thus illustrating its power. It may be difficult for non-specialists, however, to grasp thefull implication of data presented in articles written by spectroscopists. The aim of the review is, therefore, to provide a fundamental understanding of this topic to facilitate the non-specialists in their reading of this literature. In the first part of this review, we present the metabolic fate of (13)C-labeled substrates in the brain in a detailed way, including an overview of some general neurochemical principles. We also address and compare the various spectroscopic strategies that can be used to study brain metabolism. Then, we provide an overview of the (13)C NMR experiments performed to analyze both intracellular and intercellular metabolic fluxes. More particularly, the role of lactate as a potential energy substrate for neurons is discussed in the light of (13)C NMR data. Finally, new perspectives and applications offered by (13)C hyperpolarization are described.

Deuterium-induced isotope effects on the 13C chemical shifts of α-D-glucose pentaacetate

1,2,3,4,6-Penta-O-acetyl-α-D-glucopyranose and the corresponding [1-(2)H], [2-(2)H], [3-(2)H], [4-(2)H], [5-(2)H], and [6,6-(2)H(2)]-labeled compounds were prepared for measuring deuterium/hydrogen-induced effects on (13)C chemical shift (n)Δ (DHIECS) values. A conformational analysis of the nondeuterated compound was achieved using density functional theory (DFT) molecular models that allowed calculation of several structural properties as well as Boltzmann-averaged (13)C NMR chemical shifts by using the gauge-including atomic orbital method. It was found that the DFT-calculated C-H bond lengths correlate with (1)Δ DHIECS.

Glutamate and GABA in Appetite Regulation

Appetite is regulated by a coordinated interplay between gut, adipose tissue, and brain. A primary site for the regulation of appetite is the hypothalamus where interaction between orexigenic neurons, expressing Neuropeptide Y/Agouti-related protein, and anorexigenic neurons, expressing Pro-opiomelanocortin cocaine/Amphetamine-related transcript, controls energy homeostasis. Within the hypothalamus, several peripheral signals have been shown to modulate the activity of these neurons, including the orexigenic peptide ghrelin and the anorexigenic hormones insulin and leptin. In addition to the accumulated knowledge on neuropeptide signaling, presence and function of amino acid neurotransmitters in key hypothalamic neurons brought a new light into appetite regulation. Therefore, the principal aim of this review will be to describe the current knowledge of the role of amino acid neurotransmitters in the mechanism of neuronal activation during appetite regulation and the associated neuronal-astrocytic metabolic coupling mechanisms. Glutamate and GABA dominate synaptic transmission in the hypothalamus and administration of their receptors agonists into hypothalamic nuclei stimulates feeding. By using (13)C High-Resolution Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy based analysis, the Cerdán group has shown that increased neuronal firing in mice hypothalamus, as triggered by appetite during the feeding-fasting paradigm, may stimulate the use of lactate as neuronal fuel leading to increased astrocytic glucose consumption and glycolysis. Moreover, fasted mice showed increased hypothalamic [2-(13)C]GABA content, which may be explained by the existence of GABAergic neurons in key appetite regulation hypothalamic nuclei. Interestingly, increased [2-(13)C]GABA concentration in the hypothalamus of fasted animals appears to result mainly from reduction in GABA metabolizing pathways, rather than increased GABA synthesis by augmented activity of the glutamate-glutamine-GABA cycle.

Impaired hippocampal glucoregulation in the cannabinoid CB1 receptor knockout mice as revealed by an optimized in vitro experimental approach

Several techniques exist to study the rate of glucose uptake and metabolism in the brain but most of them are not sufficiently robust to permit extensive pharmacological analysis. Here we optimized an in vitro measurement of the simultaneous accumulation of the metabolizable and non-metabolizable (3)H and (14)C d-glucose analogues; permitting convenient large-scale studies on glucose uptake and metabolism in brain slices. Next, we performed an extensive pharmacological characterization on the putative glucoregulator role of the endocannabinoid system in the hippocampal slices of the rat, and the wild-type and the CB(1) cannabinoid receptor (CB(1)R) knockout mice. We observed that (3)H-3-O-methylglucose is a poor substrate to measure glucose uptake in the hippocampus. (3)H-2-deoxyglucose is a better substrate but its uptake is still lower than that of (14)C-U-d-glucose, from which the slices constantly metabolize and dissipate (14)C atoms. Thus, uptake and the metabolism values are not to be used as standalones but as differences between a control and a treatment. The CB(1)R knockout mice exhibited ∼10% less glucose uptake and glucose carbon atom dissipation in comparison with the wild-type mice. This may represent congenital defects as acute treatments of the rat and mouse slices with cannabinoid agonists, antagonists and inhibitors of endocannabinoid uptake/metabolism failed to induce robust changes in either the uptake or the metabolism of glucose. In summary, we report here an optimized technique ideal to complement other metabolic approaches of high spatiotemporal resolution. This technique allowed us concluding that CB(1)Rs are at least indirectly involved in hippocampal glucoregulation.

Neuroglial metabolic compartmentation underlying leptin deficiency in the obese ob/ob mice as detected by magnetic resonance imaging and spectroscopy methods

Manganese-Enhanced Magnetic Resonance Imaging (MEMRI), (1)H and (13)C High-Resolution-Magic Angle Spinning (HR-MAS) Spectroscopy, and genomic approaches were used to compare cerebral activation and neuronal and glial oxidative metabolism in ad libitum fed C57BL6/J leptin-deficient, genetically obese ob/ob mice. T(1)-weighted Magnetic Resonance Images across the hypothalamic Arcuate and the Ventromedial nuclei were acquired kinetically after manganese infusion. Neuroglial compartmentation was investigated in hypothalamic biopsies after intraperitoneal injections of [1-(13)C]glucose or [2-(13)C]acetate. Total RNA was extracted to determine the effects of leptin deficiency in the expression of representative genes coding for regulatory enzymes of hypothalamic energy pathways and glutamatergic neurotransmission. Manganese-Enhanced Magnetic Resonance Imaging revealed enhanced cerebral activation in the hypothalamic Arcuate and Ventromedial nuclei of the ob/ob mice. (13)C HR-MAS analysis showed increased (13)C accumulation in the hypothalamic glutamate and glutamine carbons of ob/ob mice after the administration of [1-(13)C]glucose, a primarily neuronal substrate. Hypothalamic expression of the genes coding for glucokinase, phosphofructokinase, pyruvate dehydrogenase, and glutamine synthase was not significantly altered while pyruvate kinase expression was slightly upregulated. In conclusion, leptin deficiency associated with obesity led to increased cerebral activation in the hypothalamic Arcuate and Ventromedial nuclei, concomitant with significant increases in neuronal oxidative metabolism and glutamatergic neurotransmission.

In vivo detection of intermediate metabolic products of [1-(13) C]ethanol in the brain using (13) C MRS

In this study, in vivo (13) C MRS was used to investigate the labeling of brain metabolites after intravenous administration of [1-(13) C]ethanol. After [1-(13) C]ethanol had been administered systemically to rats, (13) C labels were detected in glutamate, glutamine and aspartate in the carboxylic and amide carbon spectral region. (13) C-labeled bicarbonate HCO 3- (161.0 ppm) was also detected. Saturating acetaldehyde C1 at 207.0 ppm was found to have no effect on the ethanol C1 (57.7 ppm) signal intensity after extensive signal averaging, providing direct in vivo evidence that direct metabolism of alcohol by brain tissue is minimal. To compare the labeling of brain metabolites by ethanol with labeling by glucose, in vivo time course data were acquired during intravenous co-infusion of [1-(13) C]ethanol and [(13) C(6) ]-D-glucose. In contrast with labeling by [(13) C(6) ]-D-glucose, which produced doublets of carboxylic/amide carbons with a J coupling constant of 51 Hz, the simultaneously detected glutamate and glutamine singlets were labeled by [1-(13) C]ethanol. As (13) C labels originating from ethanol enter the brain after being converted into [1-(13) C]acetate in the liver, and the direct metabolism of ethanol by brain tissue is negligible, it is suggested that orally or intragastrically administered (13) C-labeled ethanol may be used to study brain metabolism and glutamatergic neurotransmission in investigations involving alcohol administration. In vivo (13) C MRS of rat brain following intragastric administration of (13) C-labeled ethanol is demonstrated.

A comprehensive metabolic profile of cultured astrocytes using isotopic transient metabolic flux analysis and C-labeled glucose

Metabolic models have been used to elucidate important aspects of brain metabolism in recent years. This work applies for the first time the concept of isotopic transient 13C metabolic flux analysis (MFA) to estimate intracellular fluxes in primary cultures of astrocytes. This methodology comprehensively explores the information provided by 13C labeling time-courses of intracellular metabolites after administration of a 13C-labeled substrate. Cells were incubated with medium containing [1-13C]glucose for 24 h and samples of cell supernatant and extracts collected at different time points were then analyzed by mass spectrometry and/or high performance liquid chromatography. Metabolic fluxes were estimated by fitting a carbon labeling network model to isotopomer profiles experimentally determined. Both the fast isotopic equilibrium of glycolytic metabolite pools and the slow labeling dynamics of TCA cycle intermediates are described well by the model. The large pools of glutamate and aspartate which are linked to the TCA cycle via reversible aminotransferase reactions are likely to be responsible for the observed delay in equilibration of TCA cycle intermediates. Furthermore, it was estimated that 11% of the glucose taken up by astrocytes was diverted to the pentose phosphate pathway. In addition, considerable fluxes through pyruvate carboxylase [PC; PC/pyruvate dehydrogenase (PDH) ratio = 0.5], malic enzyme (5% of the total pyruvate production), and catabolism of branched-chained amino acids (contributing with ∼40% to total acetyl-CoA produced) confirmed the significance of these pathways to astrocytic metabolism. Consistent with the need of maintaining cytosolic redox potential, the fluxes through the malate–aspartate shuttle and the PDH pathway were comparable. Finally, the estimated glutamate/α-ketoglutarate exchange rate (∼0.7 μmol mg prot−1 h−1) was similar to the TCA cycle flux. In conclusion, this work demonstrates the potential of isotopic transient MFA for a comprehensive analysis of energy metabolism.

In vivo labeling of B16 melanoma tumor xenograft with a thiol-reactive gadolinium based MRI contrast agent

Murine melanoma B16 cells display on the extracellular side of the plasma membrane a large number of reactive protein thiols (exofacial protein thiols, EPTs). These EPTs can be chemically labeled with Gd-DO3A-PDP, a Gd(III)-based MRI contrast agent bearing a 2-pyridinedithio chemical function for the recognition of EPTs. Uptake of gadolinium up to 10(9) Gd atoms per cell can be achieved. The treatment of B16 cells ex vivo with a reducing agent such as tris(2-carboxyethyl)phosphine (TCEP) results in an increase by 850% of available EPTs and an increase by 45% of Gd uptake. Blocking EPTs with N-ethylmaleimide (NEM) caused a decrease by 84% of available EPTs and a decrease by 55% of Gd uptake. The amount of Gd taken up by B16 cells is therefore dependent upon the availability of EPTs, whose actual level in turn changes according to the extracellular redox microenvironment. Then Gd-DO3A-PDP has been assessed for the labeling of tumor cells in vivo on B16.F10 melanoma tumor-bearing mice. Gd-DO3A-PDP (or Gd-DO3A as the control) has been injected directly into the tumor region at a dose level of 0.1 μmol and the signal enhancement in MR images followed over time. The washout kinetics of Gd-DO3A-PDP from tumor is very slow if compared to that of control Gd-DO3A, and 48 h post injection, the gadolinium-enhancement is still clearly visible. Therefore, B16 cells can be labeled ex vivo as well as in vivo according to a common EPTs-dependent route, provided that high levels of the thiol reactive probe can be delivered to the tumor.

Astrocytic energy metabolism and glutamate formation--relevance for 13C-NMR spectroscopy and importance of cytosolic/mitochondrial trafficking

Glutamate plays a double role in (13)C-nuclear magnetic resonance (NMR) spectroscopic determination of glucose metabolism in the brain. Bidirectional exchange between initially unlabeled glutamate and labeled α-ketoglutarate, formed from pyruvate via pyruvate dehydrogenase (PDH), indicates the rate of energy metabolism in the tricarboxylic acid (V(TCA)) cycle in neurons (V(PDH, n)) and, with additional computation, also in astrocytes (V(PDH, g)), as confirmed using the astrocyte-specific substrate [(13)C]acetate. Formation of new molecules of glutamate during increased glutamatergic activity occurs only in astrocytes by combined pyruvate carboxylase (V(PC)) and astrocytic PDH activity. V(PDH, g) accounts for ~15% of total pyruvate metabolism in the brain cortex, and V(PC) accounts for another ~10%. Since both PDH-generated and PC-generated pyruvates are needed for glutamate synthesis, ~20/25 (80%) of astrocytic pyruvate metabolism proceed via glutamate formation. Net transmitter glutamate [γ-aminobutyric acid (GABA)] formation requires transfer of newly synthesized α-ketoglutarate to the astrocytic cytosol, α-ketoglutarate transamination to glutamate, amidation to glutamine, glutamine transfer to neurons, its hydrolysis to glutamate and glutamate release (or GABA formation). Glutamate-glutamine cycling, measured as glutamine synthesis rate (V(cycle)), also transfers previously released glutamate/GABA to neurons after an initial astrocytic accumulation and measures predominantly glutamate signaling. An empirically established ~1/1 ratio between glucose metabolism and V(cycle) may reflect glucose utilization associated with oxidation/reduction processes during glutamate production, which together with associated transamination processes are balanced by subsequent glutamate oxidation after cessation of increased signaling activity. Astrocytic glutamate formation and subsequent oxidative metabolism provide large amounts of adenosine triphosphate used for accumulation from extracellular clefts of neuronally released K(+) and glutamate and for cytosolic Ca(2+) homeostasis.

Proton magnetic resonance spectroscopy detection of neurotransmitters in dorsomedial medulla correlate with spontaneous baroreceptor reflex function

Control of heart rate variability via modulation of sympathovagal balance is a key function of nucleus tractus solitarii and the dorsal motor nucleus of the vagus localized in the dorsomedial medulla oblongata. Normal blood pressure regulation involves precise balance of glutamate (Glu)-glutamine-gamma-aminobutyric acid transmitter systems, and angiotensin II modulates these transmitters to produce tonic suppression of reflex function. It is not known, however, whether other brain transmitters/metabolites are indicators of baroreflex function. This study establishes the concept that comprehensive baseline transmitter/metabolite profiles obtained using in vivo (1)H magnetic resonance spectroscopy in rats with well-characterized differences in resting blood pressure and baroreflex function can be used as indices of autonomic balance or baroreflex sensitivity. Transgenic rats with over-expression of renin [m(Ren2)27] or under-expression of glial-angiotensinogen (ASrAogen) were compared with Sprague-Dawley rats. Glu concentration in the dorsal medulla is significantly higher in ASrAogen rats compared with either Sprague-Dawley or (mRen2)27 rats. Glu levels and the ratio of Glu:glutamine correlated positively with indices of higher vagal tone consistent with the importance of these neurotransmitters in baroreflex function. Interestingly, the levels of choline-containing metabolites showed a significant positive correlation with spontaneous baroreflex sensitivity and a negative correlation with sympathetic tone. Thus, we demonstrate the concept that noninvasive assessment of neurochemical biomarkers may be used as an index of baroreflex sensitivity.

Optimized [1-(13)C]glucose infusion protocol for 13C magnetic resonance spectroscopy at 3T of human brain glucose metabolism under euglycemic and hypoglycemic conditions

The effect of insulin-induced hypoglycemia on cerebral glucose metabolism is largely unknown. (13)C MRS is a unique tool to study cerebral glucose metabolism, but the concurrent requirement for [1-(13)C]glucose administration limits its use under hypoglycemic conditions. To facilitate (13)C MRS data analysis we designed separate [1-(13)C]glucose infusion protocols for hyperinsulinemic euglycemic and hypoglycemic clamps in such a way that plasma isotopic enrichment of glucose was stable and comparable under both glycemic conditions. (13)C MR spectra were acquired with optimized (13)C MRS measurement techniques to obtain high quality (13)C MR spectra with these protocols.

A high performance liquid chromatography method with electrochemical detection of gamma-aminobutyric acid, glutamate and glutamine in rat brain homogenates

Determination of gamma-aminobutyric acid (GABA), glutamate (Glu) and glutamine (Gln) in animal models has been important to understand the normal function and clinical aspects of some neurological diseases. Quantification of these amino acid transmitters has conventionally been performed by using a high performance liquid chromatography (HPLC) system. This paper describes an improved HPLC method with electrochemical detection for glutamate, glutamine and GABA determination in brain homogenates. The protocol is based on a precolumn derivatization of amino acids with o-phthalaldehyde and sodium sulfite, a separation through a C18, 5 microm particle size column and an isocratic elution. Several modifications of previous works on methanol percentage, pH, temperature, flow rate and derivatization solution concentration were done to obtain a suitable protocol for amino acid quantification in brain homogenate samples. Total elution time is 35 min approximately. Technical requirements and laboratory expenses of this new protocol are minimal. This technique showed high linearity, repeatability and accuracy.

Hormonal modulation of amino acid neurotransmitter metabolism in the arcuate nucleus of the adult female rat: a novel action of estradiol

Morphological plasticity in response to estradiol is a hallmark of astrocytes in the arcuate nucleus. The functional consequences of these morphological changes have remained relatively unexplored. Here we report that in the arcuate nucleus estradiol significantly increased the protein levels of the two enzymes in the glutamate-glutamine cycle, glutamine synthetase and glutaminase. We further demonstrate that these estradiol-mediated changes in the enzyme protein levels may underlie functional changes in neurotransmitter availability as: 1) total glutamate concentration in the arcuate nucleus was significantly increased and 2) microdialysis revealed a significant increase in extracellular glutamate levels after a synaptic challenge in the presence of estradiol. These data implicate the glutamate-glutamine cycle in the generation and/or maintenance of glutamate and suggest that the difference in extracellular glutamate between estradiol- and oil-treated animals may be related to an increased efficiency of the cycle enzymes. In vivo enzyme activity assays revealed that the estradiol mediated increase in glutamate-glutamine cycle enzymes in the arcuate nucleus led to an increase in gamma-aminobutyric acid and is likely not related to the increase in extracellular glutamate. Thus, we have observed two-independent effects of estradiol on amino acid neurotransmission in the arcuate nucleus. These data suggest a possible functional consequence of the well-established changes in glial morphology that occur in the arcuate nucleus in the presence of estradiol and suggest the importance of neuronal-glial cooperation in the regulation of hypothalamic functions such as food intake and body weight.

The turnover of the H3 deuterons from (2-13C) glutamate and (2-13C) glutamine reveals subcellular trafficking in the brain of partially deuterated rats

We investigated by 13C NMR the turnover of the H3 deuterons of (2-13C) glutamate and (2-13C) glutamine in the brain of partially deuterated rats. Adult animals (150-200 g) fed ad libitum received 50% 2H2O or tap water 9 days before infusing (1-13C) glucose or (2-13C) acetate for 5, 10, 15, 30, 60, or 90 min. The brains were then funnel-frozen and acid extracts were prepared and analyzed by high-resolution 13C NMR. The deuteration of one or the two H3 hydrogens of (2-13C) glutamate or glutamine resulted in single (-0.07 ppm) or double (-0.14 ppm) isotopic shifts upfield of the corresponding C2 perprotonated resonance, demonstrating two sequential deuteration steps. The faster monodeuteration generated 3R or 3S (2-13C, 3-2H) glutamate or glutamine through the alternate activities of cerebral aconitase or isocitrate dehydrogenase, respectively. The slower process produced bideuterated (2-13C, 3,3'-2H2) glutamate or glutamine through the consecutive activity of both enzymes. The kinetics of deuteration was fitted to a Michaelis-Menten model including the apparent K(m)' and Vmax' values for the observed deuterations. Our results revealed different kinetic constants for the alternate and consecutive deuterations, suggesting that these processes were caused by the different cytosolic or mitochondrial isoforms of aconitase and isocitrate dehydrogenase, respectively. The deuterations of (2-13C) glutamate or glutamine followed also different kinetics from (1-13C) glucose or (2-13C) acetate, revealing distinct deuteration environments in the neuronal or glial compartments.

Determination of the glutamate-glutamine cycling flux using two-compartment dynamic metabolic modeling is sensitive to astroglial dilution

Over the last decade (13)C magnetic resonance spectroscopy ((13)C MRS) combined with the infusion of [1-(13)C]glucose has been used to measure the cerebral rate of the glutamate-glutamine cycle (V(cyc)). However, the effect of the astroglial label dilution pathways on the accuracy and precision of the (13)C MRS measurement of V(cyc) has not been evaluated or realized. In this report, we use the numerical Monte Carlo method to study the effect of astroglial dilution on the reliability of extracting V(cyc) using the neuronal-astroglial two-compartment metabolic model and [1-(13)C]glucose infusion. The results show that omission of the astroglial dilution flux leads to a large loss in the sensitivity of the glutamine turnover curve to V(cyc). When the measured isotopic dilution of cerebral glutamine is accounted for in the analysis, the value of V(cyc) can be precisely and accurately determined.

System A transporter SAT2 mediates replenishment of dendritic glutamate pools controlling retrograde signaling by glutamate

Glutamate mediates several modes of neurotransmission in the central nervous system including recently discovered retrograde signaling from neuronal dendrites. We have previously identified the system N transporter SN1 as being responsible for glutamine efflux from astroglia and proposed a system A transporter (SAT) in subsequent transport of glutamine into neurons for neurotransmitter regeneration. Here, we demonstrate that SAT2 expression is primarily confined to glutamatergic neurons in many brain regions with SAT2 being predominantly targeted to the somatodendritic compartments in these neurons. SAT2 containing dendrites accumulate high levels of glutamine. Upon electrical stimulation in vivo and depolarization in vitro, glutamine is readily converted to glutamate in activated dendritic subsegments, suggesting that glutamine sustains release of the excitatory neurotransmitter via exocytosis from dendrites. The system A inhibitor MeAIB (alpha-methylamino-iso-butyric acid) reduces neuronal uptake of glutamine with concomitant reduction in intracellular glutamate concentrations, indicating that SAT2-mediated glutamine uptake can be a prerequisite for the formation of glutamate. Furthermore, MeAIB inhibited retrograde signaling from pyramidal cells in layer 2/3 of the neocortex by suppressing inhibitory inputs from fast-spiking interneurons. In summary, we demonstrate that SAT2 maintains a key metabolic glutamine/glutamate balance underpinning retrograde signaling by dendritic release of the neurotransmitter glutamate.

On the reliability of (13)C metabolic modeling with two-compartment neuronal-glial models

Metabolic modeling of (13)C NMR spectroscopy ((13)C MRS) data using two-compartment neuronal-glial models enabled non-invasive measurements of the glutamate-glutamine cycle rate (V(NT)) in the brain in vivo. However, the reliability of such two-compartment metabolic modeling has not been examined thoroughly. This study uses Monte-Carlo simulations to investigate the reliability of metabolic modeling of (13)C positional enrichment time courses measured in brain amino acids such as glutamate and glutamine during [1-(13)C]- or [1,6-(13)C(2)]glucose infusion. Results show that the determination of V(NT) is not very precise under experimental conditions typical of in vivo NMR studies, whereas the neuronal TCA cycle rate V(TCA(N)) is determined with a much higher precision. Consistent with these results, simulated (13)C positional enrichment curves for glutamate and glutamine are much more sensitive to the value of V(TCA(N)) than to the value of V(NT). We conclude that the determination of the glutamate-glutamine cycle rate V(NT) using (13)C MRS is relatively unreliable when fitting (13)C positional enrichment curves obtained during [1-(13)C] or [1,6-(13)C(2)]glucose infusion. Further developments are needed to improve the determination of V(NT), for example using additional information from (13)C-(13)C isotopomers and/or using glial specific substrates such as [2-(13)C]acetate.

Kinetic properties of the redox switch/redox coupling mechanism as determined in primary cultures of cortical neurons and astrocytes from rat brain

We investigate the mechanisms underlying the redox switch/redox coupling hypothesis by characterizing the competitive consumption of glucose or lactate and the kinetics of pyruvate production in primary cultures of cortical neurons and astrocytes from rat brain. Glucose consumption was determined in neuronal cultures incubated in Krebs ringer bicarbonate buffer (KRB) containing 0.25-5 mM glucose, in the presence and absence of 5 mM lactate as an alternative substrate. Lactate consumption was measured in neuronal cultures incubated with 1-15 mM lactate, in the presence and absence of 1 mM glucose. In both cases, the alternative substrate increased the K(m) (mM) values for glucose consumption (from 2.2 +/- 0.2 to 3.6 +/- 0.1) or lactate consumption (from 7.8 +/- 0.1 to 8.5 +/- 0.1) without significant changes on the corresponding V(max). This is consistent with a competitive inhibition between the simultaneous consumption of glucose and lactate. When cultures of neurons or astrocytes were incubated with increasing lactate concentrations 1-20 mM, pyruvate production was observed with K(m) (mM) and V(max) (nmol/mg/h) values of 1.0 +/- 0.1 and 109 +/- 4 in neurons, or 0.28 +/- 0.1 and 342 +/- 54 in astrocytes. Thus, astrocytes or neurons are able to return to the incubation medium as pyruvate, a significant part of the lactate consumed. Present results support the reversible exchange of reducing equivalents between neurons and astrocytes in the form of lactate or pyruvate. Monocarboxylate exchange is envisioned to operate under near equilibrium, with the transcellular flux directed thermodynamically toward the more oxidized intracellular redox environment.

Metabolic interactions between glutamatergic and dopaminergic neurotransmitter systems are mediated through D(1) dopamine receptors

Interactions between the dopaminergic and glutamatergic neurotransmission systems were investigated in the adult brain of wild-type (WT) and transgenic mice lacking the dopamine D(1) or D(2) receptor subtypes. Activity of the glutamine cycle was evaluated by using (13)C NMR spectroscopy, and striatal activity was assessed by c-Fos expression and motor coordination. Brain extracts from (1,2-(13)C(2)) acetate-infused mice were prepared and analyzed by (13)C NMR to determine the incorporation of the label into the C4 and C5 carbons of glutamate and glutamine. D(1)R(-/-) mice showed a significantly higher concentration of cerebral (4,5-(13)C(2)) glutamine, consistent with an increased activity of the glutamate-glutamine cycle and of glutamatergic neurotransmission. Conversely, D(2)R(-/-) mice did not show any significant changes in (4,5-(13)C(2)) glutamate or (4,5-(13)C(2)) glutamine, suggesting that alterations in glutamine metabolism are mediated through D(1) receptors. This was confirmed with D(1)R(-/-) and WT mice treated with reserpine, a dopamine-depleting drug, or with reserpine followed by L-DOPA, a dopamine precursor. Exposure to reserpine increased (4,5-(13)C(2)) glutamine in WT to levels similar to those found in untreated D(1)R(-/-) mice. These values were the same as those reached in the reserpine-treated D(1)R(-/-) mice. Treatment of WT animals with L-DOPA returned (4,5-(13)C(2)) glutamine levels to normal, but this was not verified in D(1)R(-/-) animals. Reserpine impaired motor coordination and decreased c-Fos expression, whereas L-DOPA restored both variables to normal values in WT but not in D(1)R(-/-). Together, our results reveal novel neurometabolic interactions between glutamatergic and dopaminergic systems that are mediated through the D(1), but not the D(2), dopamine receptor subtype.

d-Lactate inhibition of memory in a single trial discrimination avoidance task in the young chick

L-Lactate is a metabolite possibly able to meet some neuronal energy demands. However, a clear role for L-lactate in behaviour remains elusive. Administration of the inactive isomer D-lactate (1.75 mM; ic), immediately post-training, resulted in a persistent retention loss from 40 min post-training when used in conjuction with a single trial discrimination avoidance task designed for the young chick. Furthermore, 1mM noradrenaline (ic) administered 20 min post-training overcame the retention loss induced by D-lactate. Although not directly demonstrated in the current study, it is plausible that D-lactate inhibited memory processing by competing with L-lactate for uptake into neurons. The time of onset of the retention loss induced by D-lactate is in accord with findings where the action of noradrenaline is inhibited. The successful challenge of D-lactate inhibition by a high concentration of noradrenaline may suggest a relationship by some unidentified mechanism.

Adaptation of energy metabolism in breast cancer brain metastases

Brain metastases are among the most feared complications in breast cancer, as no therapy exists that prevents or eliminates breast cancer spreading to the brain. New therapeutic strategies depend on specific knowledge of tumor cell properties that allow breast cancer cell growth within the brain tissue. To provide information in this direction, we established a human breast cancer cell model for brain metastasis based on circulating tumor cells from a breast cancer patient and variants of these cells derived from bone or brain lesions in immunodeficient mice. The brain-derived cells showed an increased potential for brain metastasis in vivo and exhibited a unique protein expression profile identified by large-scale proteomic analysis. This protein profile is consistent with either a selection of predisposed cells or bioenergetic adaptation of the tumor cells to the unique energy metabolism of the brain. Increased expression of enzymes involved in glycolysis, tricarboxylic acid cycle, and oxidative phosphorylation pathways suggests that the brain metastatic cells derive energy from glucose oxidation. The cells further showed enhanced activation of the pentose phosphate pathway and the glutathione system, which can minimize production of reactive oxygen species resulting from an enhanced oxidative metabolism. These changes promoted resistance of brain metastatic cells to drugs that affect the cellular redox balance. Importantly, the metabolic alterations are associated with strongly enhanced tumor cell survival and proliferation in the brain microenvironment. Thus, our data support the hypothesis that predisposition or adaptation of the tumor cell energy metabolism is a key element in breast cancer brain metastasis, and raise the possibility of targeting the functional differentiation in breast cancer brain lesions as a novel therapeutic strategy.

Different metabolism of glutamatergic and GABAergic compartments in superfused hippocampal slices characterized by nuclear magnetic resonance spectroscopy

We investigated intermediary metabolism using (13)C-glucose and (13)C-acetate tracers followed by (13)C-nuclear magnetic resonance (NMR) isotopomer analysis in rat hippocampal slice preparations, the most widely used preparation for electrophysiological studies. Slices displayed a stable metabolic activity over a wide range of superfusion periods in the absence or presence of 50 muM 4-aminopyridine (4AP), which triggers an intermittent burst-like neuronal firing. This caused an increase of tricarboxylic acid (TCA)-related amino acids (glutamate, aspartate and GABA) and shortened the time required to reach metabolic and isotopic steady state (3 h in the presence of 4AP and 7 h in its absence). (13)C-NMR isotopomer analysis revealed an increase in TCA flux in astrocytes and in GABA compartments greater than in putative glutamatergic neurons and the fitting of these data further indicated that the metabolic network in GABAergic and glutamatergic compartments has a different design and reacts differently to the stimulation by the presence of 4AP. These results show that (13)C-isotopomer analysis allows estimating metabolic parameters/fluxes under both steady- and non-steady-state metabolic conditions in hippocampal slices, opening the possibility of combining electrophysiological and metabolic studies in the same preparation.

Investigating brain metabolism at high fields using localized 13C NMR spectroscopy without 1H decoupling

Most in vivo 13C NMR spectroscopy studies in the brain have been performed using 1H decoupling during acquisition. Decoupling imposes significant constraints on the experimental setup (particularly for human studies at high magnetic field) in order to stay within safety limits for power deposition. We show here that incorporation of the 13C label from 13C-labeled glucose into brain amino acids can be monitored accurately using localized 13C NMR spectroscopy without the application of 1H decoupling. Using LCModel quantification with prior knowledge of one-bond and multiple-bond J(CH) coupling constants, the uncertainty on metabolites concentrations was only 35% to 91% higher (depending on the carbon resonance of interest) in undecoupled spectra compared to decoupled spectra in the rat brain at 9.4 Tesla. Although less sensitive, 13C NMR without decoupling dramatically reduces experimental constraints on coil setup and pulse sequence design required to keep power deposition within safety guidelines. This opens the prospect of safely measuring 13C NMR spectra in humans at varied brain locations (not only the occipital lobe) and at very high magnetic fields above 4 Tesla.

In vivo 13C NMR spectroscopy and metabolic modeling in the brain: a practical perspective

In vivo 13C NMR spectroscopy has the unique capability to measure metabolic fluxes noninvasively in the brain. Quantitative measurements of metabolic fluxes require analysis of the 13C labeling time courses obtained experimentally with a metabolic model. The present work reviews the ingredients necessary for a dynamic metabolic modeling study, with particular emphasis on practical issues.

The redox switch/redox coupling hypothesis

We provide an integrative interpretation of neuroglial metabolic coupling including the presence of subcellular compartmentation of pyruvate and monocarboxylate recycling through the plasma membrane of both neurons and glial cells. The subcellular compartmentation of pyruvate allows neurons and astrocytes to select between glucose and lactate as alternative substrates, depending on their relative extracellular concentration and the operation of a redox switch. This mechanism is based on the inhibition of glycolysis at the level of glyceraldehyde 3-phosphate dehydrogenase by NAD(+) limitation, under sufficiently reduced cytosolic NAD(+)/NADH redox conditions. Lactate and pyruvate recycling through the plasma membrane allows the return to the extracellular medium of cytosolic monocarboxylates enabling their transcellular, reversible, exchange between neurons and astrocytes. Together, intracellular pyruvate compartmentation and monocarboxylate recycling result in an effective transcellular coupling between the cytosolic NAD(+)/NADH redox states of both neurons and glial cells. Following glutamatergic neurotransmission, increased glutamate uptake by the astrocytes is proposed to augment glycolysis and tricarboxylic acid cycle activity, balancing to a reduced cytosolic NAD(+)/NADH in the glia. Reducing equivalents are transferred then to the neuron resulting in a reduced neuronal NAD(+)/NADH redox state. This may eventually switch off neuronal glycolysis, favoring the oxidation of extracellular lactate in the lactate dehydrogenase (LDH) equilibrium and in the neuronal tricarboxylic acid cycles. Finally, pyruvate derived from neuronal lactate oxidation, may return to the extracellular space and to the astrocyte, restoring the basal redox state and beginning a new loop of the lactate/pyruvate transcellular coupling cycle. Transcellular redox coupling operates through the plasma membrane transporters of monocarboxylates, similarly to the intracellular redox shuttles coupling the cytosolic and mitochondrial redox states through the transporters of the inner mitochondrial membrane. Finally, transcellular redox coupling mechanisms may couple glycolytic and oxidative zones in other heterogeneous tissues including muscle and tumors.

Cerebral glutamate metabolism during hypoglycaemia in healthy and type 1 diabetic humans

BACKGROUND

The mechanisms responsible for the progressive failure of hypoglycaemia counterregulation in long-standing type 1 diabetes are poorly understood. Increased brain glucose uptake during hypoglycaemia or alterations of brain energy metabolism could effect glucose sensing by the brain and thus contribute to hypoglycaemia-associated autonomic failure.

MATERIALS AND METHODS

Type 1 diabetic patients (T1DM) and healthy volunteers (CON) were studied before, during and after a hypoglycaemic (50 mg dL(-1)) hyperinsulinaemic (1.5 mU kg(-1) min(-1)) clamp test. The (1)H magnetic resonance spectroscopy of the occipital lobe of the brain was performed employing the STEAM localization technique. The water signal was suppressed by the modified SWAMP method. All spectra were acquired on a 3 Tesla scanner (80 cm MEDSPEC-DBX, Bruker Medical, Ettlingen, Germany) using a 10-cm diameter surface coil.

RESULTS

During hypoglycaemia, T1DM showed blunted endocrine counterregulation. At baseline the brain tissue glucose : creatine ratio was lower in CON than in T1DM (CON 0.13 +/- 0.05 vs. T1DM 0.19 0.11; P < 0.01). During hypoglycaemia glucose : creatine ratios decreased in both groups (CON 0.07 +/- 0.08, P < 0.05; T1DM 0.03 +/- 0.03, P < 0.001). A significant drop in the glutamate : creatine ratio could only be found in CON during hypoglycaemia (CON 1.36 +/- 0.08 vs. 1.26 +/- 0.11; P < 0.01; T1DM 1.32 +/- 0.13 vs. 1.28 +/- 0.15; P = NS). The ratios of glutamine, N-acetylaspartate, choline and myo-inositol : creatine were not different between both groups and did not change throughout the experiment.

CONCLUSIONS

Only in CON does moderate hypoglycaemia reduce intracerebral glutamate concentrations, possibly owing to a slower substrate flux through the tricarboxylic acid cycle in neurones. The maintenance of normal energy metabolism in T1DM during hypoglycaemia might effect glucose sensing in the brain and contribute to hypoglycaemia-associated autonomic failure.

Amino acid transporters ASCT2 and LAT1 in cancer: partners in crime?

Relative to other neutral amino acid transporters, the expression levels of ASCT2 and LAT1, are coordinately elevated in a wide spectrum of primary human cancers, suggesting that they are frequently co-opted to support the "tumor metabolome". Each has recently been shown to play important roles in the growth and survival of cancer cell lines, making them potential targets for cancer therapy. The properties and putative relationship of these two amino acid exchangers are discussed in the context of their demonstrated utility in cancer biology, including cellular growth and survival signaling and integrated links to the mammalian target-of-rapamycin (mTOR) kinase.

Futile cycling of lactate through the plasma membrane of C6 glioma cells as detected by (13C,2H) NMR

We report a novel ((13)C, (2)H) nuclear magnetic resonance (NMR) procedure to investigate lactate recycling through the monocarboxylate transporter of the plasma membrane of cells in culture. C6 glioma cells were incubated with [3-(13)C]lactate in Krebs-Henseleit Buffer containing 50% (2)H(2)O (vol/vol) for up to 30 hr. (13)C NMR analysis of aliquots progressively taken from the medium, showed: (1) a linearly decreasing singlet at approximately 20.85 parts per million (ppm; -0.119 micromol/mg protein/hr) derived from the methyl carbon of [3-(13)C]lactate; and (2) an exponentially increasing shifted singlet at approximately 20.74 ppm (0.227 micromol/ mg protein/hr) from the methyl carbon of [3-(13)C, 2-(2)H]lactate. The shifted singlet appears because during its transit through the cytosol, [3-(13)C]lactate generates [3-(13)C, 2-(2)H]lactate in the lactate dehydrogenase (LDH) equilibrium, which may return to the incubation medium through the reversible monocarboxylate carrier. The methyl group of [3-(13)C, 2-(2)H]lactate is shifted -0.11 ppm with respect to that of [3-(13)C]lactate, making it possible to distinguish between both molecules by (13)C NMR. During incubations with 2.5 mM [1-(13)C]glucose and 3.98 mM [U-(13)C(3)]lactate or with 2.5 mM [1-(13)C]glucose and 3.93 mM [2-(13)C]pyruvate, C2-deuterated lactate was produced only from [1-(13)C]glucose or [U-(13)C(3)]lactate, revealing that this deuteration process is redox sensitive. When [1-(13)C]glucose and [U-(13)C(3)]lactate were used as substrates, no significant [3-(13)C]lactate production from [1-(13)C]glucose was detected, suggesting that glycolytic lactate production may be stopped under the high lactate concentrations prevailing under mild hypoxic or ischemic episodes or during cerebral activation.