Glucose and lactate supply to the synapse

Aerobic production and utilization of lactate satisfy increased energy demands upon neuronal activation in hippocampal slices and provide neuroprotection against oxidative stress

Ever since it was shown for the first time that lactate can support neuronal function in vitro as a sole oxidative energy substrate, investigators in the field of neuroenergetics have been debating the role, if any, of this glycolytic product in cerebral energy metabolism. Our experiments employed the rat hippocampal slice preparation with electrophysiological and biochemical methodologies. The data generated by these experiments (a) support the hypothesis that lactate, not pyruvate, is the end-product of cerebral aerobic glycolysis; (b) indicate that lactate plays a major and crucial role in affording neural tissue to respond adequately to glutamate excitation and to recover unscathed post-excitation; (c) suggest that neural tissue activation is accompanied by aerobic lactate and NADH production, the latter being produced when the former is converted to pyruvate by mitochondrial lactate dehydrogenase (mLDH); (d) imply that NADH can be utilized as an endogenous scavenger of reactive oxygen species (ROS) to provide neuroprotection against ROS-induced neuronal damage.

Copper export from cultured astrocytes

Copper is an essential trace metal that is required as a catalytic co-factor or a structural component of several important enzymes. However, since excess of copper can also harm cells due to its potential to catalyse the generation of toxic reactive oxygen species, transport of copper and the cellular copper content are tightly regulated. Astrocytes are known to efficiently take up copper ions, but it was not known whether these cells are also able to export copper. Treatment of astrocyte-rich primary cultures for 24 h with copper chloride caused a concentration-dependent increase in the specific cellular copper content. During further 24 h incubation in the absence of copper chloride, the copper-loaded astrocytes remained viable and released up to 45% of the accumulated copper. The rate of copper export was proportional to the amount of cellular copper, was almost completely prevented by lowering the incubation temperature to 4 °C and was partly prevented by the endocytosis inhibitor amiloride. Copper export is most likely mediated by the copper ATPase ATP7A, since this transporter is expressed in astrocyte cultures and its cellular location is strongly affected by the absence or the presence of extracellular copper. The potential of cultured astrocytes to export copper suggests that astrocytes provide neighbouring cells in brain with this essential trace element.

Brain energy metabolism: focus on astrocyte-neuron metabolic cooperation

The energy requirements of the brain are very high, and tight regulatory mechanisms operate to ensure adequate spatial and temporal delivery of energy substrates in register with neuronal activity. Astrocytes-a type of glial cell-have emerged as active players in brain energy delivery, production, utilization, and storage. Our understanding of neuroenergetics is rapidly evolving from a "neurocentric" view to a more integrated picture involving an intense cooperativity between astrocytes and neurons. This review focuses on the cellular aspects of brain energy metabolism, with a particular emphasis on the metabolic interactions between neurons and astrocytes.

NBCe1 mediates the acute stimulation of astrocytic glycolysis by extracellular K+

Excitatory synaptic transmission stimulates brain tissue glycolysis. This phenomenon is the signal detected in FDG-PET imaging and, through enhanced lactate production, is also thought to contribute to the fMRI signal. Using a method based on Förster resonance energy transfer in mouse astrocytes, we have recently observed that a small rise in extracellular K(+) can stimulate glycolysis by >300% within seconds. The K(+) response was blocked by ouabain, but intracellular engagement of the Na(+)/K(+) ATPase pump with Na(+) was ineffective, suggesting that the canonical feedback regulatory pathway involving the Na(+) pump and ATP depletion is only permissive and that a second mechanism is involved. Because of their predominant K(+) permeability and high expression of the electrogenic Na(+)/HCO(3)(-) cotransporter NBCe1, astrocytes respond to a rise in extracellular K(+) with plasma membrane depolarization and intracellular alkalinization. In the present article, we show that a fast glycolytic response can be elicited independently of K(+) by plasma membrane depolarization or by intracellular alkalinization. The glycolytic response to K(+) was absent in astrocytes from NBCe1 null mice (Slc4a4) and was blocked by functional or pharmacological inhibition of the NBCe1. Hippocampal neurons acquired K(+)-sensitive glycolysis upon heterologous NBCe1 expression. The phenomenon could also be reconstituted in HEK293 cells by coexpression of the NBCe1 and a constitutively open K(+) channel. We conclude that the NBCe1 is a key element in a feedforward mechanism linking excitatory synaptic transmission to fast modulation of glycolysis in astrocytes.

Minireview: The value of looking backward: the essential role of the hindbrain in counterregulatory responses to glucose deficit

This review focuses on evidence indicating a key role for the hindbrain in mobilizing behavioral, autonomic and endocrine counterregulatory responses to acute and profound glucose deficit, and identifies hindbrain norepinephrine (NE) and epinephrine (E) neurons as essential mediators of some of these responses. It has become clear that hindbrain NE/E neurons are functionally diverse. However, considerable progress has been made in identifying the particular NE/E neurons important for particular glucoregulatory responses. Although it is not yet known whether NE/E neurons are directly activated by glucose deficit, compelling evidence indicates that if they are not, the primary glucoreceptor cells must be located in the immediate vicinity these neurons. Hindbrain studies identifying cellular markers associated with glucose-sensing functions in other brain regions are discussed, as are studies examining the relationship of these markers to counterregulatory responses of NE/E neurons. Further investigations to identify glucose-sensing cells (neurons, ependymocytes, or glia) controlling counterregulatory responses are crucial, as are studies to determine the specific functions of glucose-sensing cells throughout the brain. Likewise, examination of the roles (if any) of hindbrain counterregulatory systems in managing glucose homeostasis under basal, nonglucoprivic conditions will also be important for a full understanding of energy homeostasis. Nevertheless, the accumulated evidence demonstrates that hindbrain glucose sensors and NE/E neurons are essential players in triggering counterregulatory responses to emergencies of glucose deficit.

In vivo evidence for lactate as a neuronal energy source

Cerebral energy metabolism is a highly compartmentalized and complex process in which transcellular trafficking of metabolites plays a pivotal role. Over the past decade, a role for lactate in fueling the energetic requirements of neurons has emerged. Furthermore, a neuroprotective effect of lactate during hypoglycemia or cerebral ischemia has been reported. The majority of the current evidence concerning lactate metabolism at the cellular level is based on in vitro data; only a few recent in vivo results have demonstrated that the brain preferentially utilizes lactate over glucose. Using voltage-sensitive dye (VSD) imaging, beta-probe measurements of radiotracer kinetics, and brain activation by sensory stimulation in the anesthetized rat, we investigated several aspects of cerebral lactate metabolism. The present study is the first in vivo demonstration of the maintenance of neuronal activity in the presence of lactate as the primary energy source. The loss of the voltage-sensitive dye signal found during severe insulin-induced hypoglycemia is completely prevented by lactate infusion. Thus, lactate has a direct neuroprotective effect. Furthermore, we demonstrate that the brain readily oxidizes lactate in an activity-dependent manner. The washout of 1-[(11)C]L-lactate, reflecting cerebral lactate oxidation, was observed to increase during brain activation from 0.077 ± 0.009 to 0.105 ± 0.007 min(-1). Finally, our data confirm that the brain prefers lactate over glucose as an energy substrate when both substrates are available. Using [(18)F]fluorodeoxyglucose (FDG) to measure the local cerebral metabolic rate of glucose, we demonstrated a lactate concentration-dependent reduction of cerebral glucose utilization during experimentally increased plasma lactate levels.

Fast and reversible stimulation of astrocytic glycolysis by K+ and a delayed and persistent effect of glutamate

Synaptic activity is followed within seconds by a local surge in lactate concentration, a phenomenon that underlies functional magnetic resonance imaging and whose causal mechanisms are unclear, partly because of the limited spatiotemporal resolution of standard measurement techniques. Using a novel Förster resonance energy transfer-based method that allows real-time measurement of the glycolytic rate in single cells, we have studied mouse astrocytes in search for the mechanisms responsible for the lactate surge. Consistent with previous measurements with isotopic 2-deoxyglucose, glutamate was observed to stimulate glycolysis in cultured astrocytes, but the response appeared only after a lag period of several minutes. Na(+) overloads elicited by engagement of the Na(+)-glutamate cotransporter with d-aspartate or application of the Na(+) ionophore gramicidin also failed to stimulate glycolysis in the short term. In marked contrast, K(+) stimulated astrocytic glycolysis by fourfold within seconds, an effect that was observed at low millimolar concentrations and was also present in organotypic hippocampal slices. After removal of the agonists, the stimulation by K(+) ended immediately but the stimulation by glutamate persisted unabated for >20 min. Both stimulations required an active Na(+)/K(+) ATPase pump. By showing that small rises in extracellular K(+) mediate short-term, reversible modulation of astrocytic glycolysis and that glutamate plays a long-term effect and leaves a metabolic trace, these results support the view that astrocytes contribute to the lactate surge that accompanies synaptic activity and underscore the role of these cells in neurometabolic and neurovascular coupling.

Lactate Effectively Covers Energy Demands during Neuronal Network Activity in Neonatal Hippocampal Slices

Although numerous experimental data indicate that lactate is efficiently used for energy by the mature brain, the direct measurements of energy metabolism parameters during neuronal network activity in early postnatal development have not been performed. Therefore, the role of lactate in the energy metabolism of neurons at this age remains unclear. In this study, we monitored field potentials and contents of oxygen and NAD(P)H in correlation with oxidative metabolism during intense network activity in the CA1 hippocampal region of neonatal brain slices. We show that in the presence of glucose, lactate is effectively utilized as an energy substrate, causing an augmentation of oxidative metabolism. Moreover, in the absence of glucose lactate is fully capable of maintaining synaptic function. Therefore, during network activity in neonatal slices, lactate can be an efficient energy substrate capable of sustaining and enhancing aerobic energy metabolism.

The NAD+ /NADH redox state in astrocytes: independent control of the NAD+ and NADH content

The intracellular redox state is established by several redox pairs, such as NAD(+) /NADH and NADP(+) /NADPH and glutathione. This redox state is a crucial determinant of cellular metabolism and function. Astrocytes are an important cell population contributing to brain metabolism and brain energy supply, so a careful control of these redox pairs is essential for proper brain function. Despite this, little is known about control of the NAD(+) and NADH content within the brain or in astrocytes. Therefore, we here analyzed the NAD(+) and NADH content of mouse tissue and cultured cortical astrocytes. The NAD(+) /NADH ratio increased in most tissues during development from newborn to adult mice. The basal redox ratio of cultured astrocytes was about 3.8 and similar to the redox ratio of the cortex of newborn mice. Although the NADH content of these cells was highly sensitive to the concentration of energy substrates and to modulation of energy metabolism, the NAD(+) content was surprisingly constant under these conditions. In contrast, application of nicotine amide or nicotinamide mononucleotide, which are precursors for NAD(+) biosynthesis, slowly increased NAD(+) content while leaving NADH levels unaffected. Finally, inhibiting the NAD(+) -degrading enzyme poly-(ADP-ribose)-polymerase increased NAD(+) content slightly without affecting NADH levels, whereas inhibition of sirtuins had no effect. These results indicate that, in addition to converting NAD(+) to NADH and vice versa during redox reactions, the content of both partners of this redox pair is additionally controlled by other mechanisms.

Why does the brain (not) have glycogen?

In the present paper we formulate the hypothesis that brain glycogen is a critical determinant in the modulation of carbohydrate supply at the cellular level. Specifically, we propose that mobilization of astrocytic glycogen after an increase in AMP levels during enhanced neuronal activity controls the concentration of glucose phosphates in astrocytes. This would result in modulation of glucose phosphorylation by hexokinase and upstream cell glucose uptake. This mechanism would favor glucose channeling to activated neurons, supplementing the already rich neuron-astrocyte metabolic and functional partnership with important implications for the energy compounds used to sustain neuronal activity. The hypothesis is based on recent modeling evidence suggesting that rapid glycogen breakdown can profoundly alter the short-term kinetics of glucose delivery to neurons and astrocytes. It is also based on review of the literature relevant to glycogen metabolism during physiological brain activity, with an emphasis on the metabolic pathways identifying both the origin and the fate of this glucose reserve.

Copper accelerates glycolytic flux in cultured astrocytes

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

High resolution measurement of the glycolytic rate

The glycolytic rate is sensitive to physiological activity, hormones, stress, aging, and malignant transformation. Standard techniques to measure the glycolytic rate are based on radioactive isotopes, are not able to resolve single cells and have poor temporal resolution, limitations that hamper the study of energy metabolism in the brain and other organs. A new method is described in this article, which makes use of a recently developed FRET glucose nanosensor to measure the rate of glycolysis in single cells with high temporal resolution. Used in cultured astrocytes, the method showed for the first time that glycolysis can be activated within seconds by a combination of glutamate and K⁺, supporting a role for astrocytes in neurometabolic and neurovascular coupling in the brain. It was also possible to make a direct comparison of metabolism in neurons and astrocytes lying in close proximity, paving the way to a high-resolution characterization of brain energy metabolism. Single-cell glycolytic rates were also measured in fibroblasts, adipocytes, myoblasts, and tumor cells, showing higher rates for undifferentiated cells and significant metabolic heterogeneity within cell types. This method should facilitate the investigation of tissue metabolism at the single-cell level and is readily adaptable for high-throughput analysis.

The biphasic NAD(P)H fluorescence response of astrocytes to dopamine reflects the metabolic actions of oxidative phosphorylation and glycolysis

The NAD(+)/NADH redox pair constitutes an important metabolic node connecting catabolic pathways to energy production. We took advantage of the fluorescence of NADH to monitor changes in NADH levels by 2-photon laser scanning microscopy in cultured cortical astrocytes and acutely isolated brain slices in response to dopamine (DA), a major neurotransmitter involved in modulation of attention, motivation, and learning. DA induced a dose-dependent biphasic response of the NAD(P)H fluorescence signal, consisting of an initial decrease followed by a subsequent increase. This response was mediated by D1-receptors, protein kinase A, and 5'-AMP-activated protein kinase signaling. While the initial decrease could be inhibited by blocking mitochondrial respiratory chain, the increase was inhibited by blocking glycolysis. Finally, activation of DA receptors on astrocytes in acutely isolated mouse cortical brain slices also induced an increase in the NAD(P)H fluorescence signal. We conclude that DA activates two opposing components of astrocytic metabolism with different kinetics. This response of the astroglial metabolism might contribute to fine-tuned participation of astrocytes to neuronal activity and functional states of the brain.

Increase brain lactate in hepatic encephalopathy: cause or consequence?

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome which develops as a result of liver failure or disease. Increased concentrations of brain lactate (microdialysate, cerebrospinal fluid, tissue) are commonly measured in patients with HE induced by either acute or chronic liver failure. Whether an increase in brain lactate is a cause or a consequence of HE remains undetermined. A rise in cerebral lactate may occur due to (1) blood-borne lactate (hyperlactataemia) crossing the blood-brain barrier, (2) increased glycolysis due to energy failure or impairment and (3) increased lactate production/release or decreased lactate utilization/uptake. This review explores the different reasons for lactate accumulation in the brain during liver failure and describes the possible roles of lactate in the pathogenesis of HE.

TNF-α and Microglial Hormetic Involvement in Neurological Health & Migraine

Environmental enrichment, i.e., increased intellectual, social, and physical activity makes brain more resilient to subsequent neurological disease. The mechanisms for this effect remain incompletely defined, but evidence shows tumor necrosis factor-alpha (TNF-α) is involved. TNF-α, at acutely high levels, possesses the intrinsic capacity to enhance injury associated with neurological disease. Conversely, the effect of TNF-α at low-levels is nutritive over time, consistent with physiological conditioning hormesis. Evidence shows that neural activity triggers low-level pro-inflammatory signaling involving TNF-α. This low-level TNF-α signaling alters gene expression, resulting in an enhanced resilience to disease. Brain-immune signaling may become maladaptive when increased activity is chronic without sufficient periods of reduced activity necessary for nutritive adaptation. Such tonically increased activity may explain, for example, the transformation of episodic to chronic migraine with related increased susceptibility to spreading depression, the most likely underlying cause of this malady. Thus, TNF-α, whose function is to alter gene expression, and its principal cellular source, microglia, seem powerfully positioned to orchestrate hormetic immune signaling that establishes the phenotype of neurological health and disease from brain activity.

The cytosolic redox state of astrocytes: Maintenance, regulation and functional implications for metabolite trafficking

Astrocytes have important functions in the metabolism of the brain. These cells provide neurons with metabolic substrates for energy production as well as with precursors for neurotransmitter and glutathione synthesis. Both the metabolism of astrocytes and the subsequent supply of metabolites from astrocytes to neurons are strongly affected by alterations in the cellular redox state. The cytosolic redox state of astrocytes depends predominantly on the ratios of the oxidised and reduced partners of the redox pairs NADH/NAD(+), NADPH/NADP(+) and GSH/GSSG. The NADH/NAD(+) pair is predominately in the oxidised state to accept electrons that are produced during glycolysis. In contrast, the redox pairs NADPH/NADP(+) and GSH/GSSG are biased towards the reduced state under unstressed conditions to provide electrons for reductive biosyntheses and antioxidative processes, respectively. In this review article we describe the metabolic processes that maintain the redox pairs in their desired redox states in the cytosol of astrocytes and discuss the consequences of alterations of the normal redox state for the regulation of cellular processes and for metabolite trafficking from astrocytes to neurons.