Energy metabolism in astrocytes: high rate of oxidative metabolism and spatiotemporal dependence on glycolysis/glycogenolysis

Effect of Gastrodin on Early Brain Injury and Neurological Outcome After Subarachnoid Hemorrhage in Rats

Gastrodin is a phenolic glycoside that has been demonstrated to provide neuroprotection in preclinical models of central nervous system disease, but its effect in subarachnoid hemorrhage (SAH) remains unclear. In this study, we showed that intraperitoneal administration of gastrodin (100 mg/kg per day) significantly attenuated the SAH-induced neurological deficit, brain edema, and increased blood-brain barrier permeability in rats. Meanwhile, gastrodin treatment significantly reduced the SAH-induced elevation of glutamate concentration in the cerebrospinal fluid and the intracellular Ca²⁺ overload. Moreover, gastrodin suppressed the SAH-induced microglial activation, astrocyte activation, and neuronal apoptosis. Mechanistically, gastrodin significantly reduced the oxidative stress and inflammatory response, up-regulated the expression of nuclear factor erythroid 2-related factor 2, heme oxygenase-1, phospho-Akt and B-cell lymphoma 2, and down-regulated the expression of BCL2-associated X protein and cleaved caspase-3. Our results suggested that the administration of gastrodin provides neuroprotection against early brain injury after experimental SAH.

Does shuttling of glycogen-derived lactate from astrocytes to neurons take place during neurotransmission and memory consolidation?

Glycogen levels in resting brain and its utilization rates during brain activation are high, but the functions fulfilled by glycogenolysis in living brain are poorly understood. Studies in cultured astrocytes have identified glycogen as the preferred fuel to provide ATP for Na⁺ ,K⁺ -ATPase for the uptake of extracellular K⁺ and for Ca²⁺ -ATPase to pump Ca²⁺ into the endoplasmic reticulum. Studies in astrocyte-neuron co-cultures led to the suggestion that glycogen-derived lactate is shuttled to neurons as oxidative fuel to support glutamatergic neurotransmission. Furthermore, both knockout of brain glycogen synthase and inhibition of glycogenolysis prior to a memory-evoking event impair memory consolidation, and shuttling of glycogen-derived lactate as neuronal fuel was postulated to be required for memory. However, lactate shuttling has not been measured in any of these studies, and procedures to inhibit glycogenolysis and neuronal lactate uptake are not specific. Testable alternative mechanisms to explain the observed findings are proposed: (i) disruption of K⁺ and Ca²⁺ homeostasis, (ii) release of gliotransmitters, (iii) imposition of an energy crisis on astrocytes and neurons by inhibition of mitochondrial pyruvate transport by compounds used to block neuronal monocarboxylic acid transporters, and (iv) inhibition of astrocytic filopodial movements that secondarily interfere with glutamate and K⁺ uptake from the synaptic cleft. Evidence that most pyruvate/lactate derived from glycogen is not oxidized and does not accumulate suggests predominant glycolytic metabolism of glycogen to support astrocytic energy demands. Sparing of blood-borne glucose for use by neurons is a reasonable explanation for the requirement for glycogenolysis in neurotransmission and memory processing.

Parkinsonian Neurotoxins Impair the Pro-inflammatory Response of Glial Cells

In the case of Parkinson's disease (PD), epidemiological studies have reported that pesticide exposure is a risk factor for its pathology. It has been suggested that some chemical agents, such as rotenone and paraquat, that inhibit the mitochondrial respiratory chain (in the same way as the PD mimetic toxin 1-methyl-4-phenylpyridinium, MPP+) are involved in the development of PD. However, although the neurotoxic effect of such compounds has been widely reported using in vivo and in vitro experimental approaches, their direct effect on the glial cells remains poorly characterized. In addition, the extent to which these toxins interfere with the immune response of the glial cells, is also underexplored. We used mouse primary mixed glial and microglial cultures to study the effect of MPP+ and rotenone on glial activation, in the absence and the presence of a pro-inflammatory stimulus (lipopolysaccharide plus interferon-γ, LPS+IFN-γ). We determined the mRNA expression of the effector molecules that participate in the inflammatory response (pro-inflammatory cytokines and enzymes), as well as the nitric oxide (NO) and cytokine production. We also studied the phagocytic activity of the microglial cells. In addition, we evaluated the metabolic changes associated with the observed effects, through the measurement of adenosine triphosphate (ATP) production and the expression of genes involved in the control of metabolic pathways. We observed that exposure of the glial cultures to the neurotoxins, especially rotenone, impaired the pro-inflammatory response induced by LPS/IFN-γ. MPP+ and rotenone also impaired the phagocytic activity of the microglial cells, and this effect was potentiated in the presence of LPS/IFN-γ. The deficit in ATP production that was detected, mainly in MPP+ and rotenone-treated mixed glial cultures, may be responsible for the effects observed. These results show that the response of glial cells to a pro-inflammatory challenge is altered in the presence of toxins inhibiting mitochondrial respiratory chain activity, suggesting that the glial immune response is impaired by such agents. This may have relevant consequences for brain function and the central nervous system's (CNS's) response to insults.

Extracellular ATP and glutamate drive pyruvate production and energy demand to regulate mitochondrial respiration in astrocytes

Astrocytes respond to energetic demands by upregulating glycolysis, lactate production, and respiration. This study addresses the role of respiration and calcium regulation of respiration as part of the astrocyte response to the workloads caused by extracellular ATP and glutamate. Extracellular ATP (100 μM to 1 mM) causes a Ca²⁺ -dependent workload and fall of the cytosolic ATP/ADP ratio which acutely increases astrocytes respiration. Part of this increase is related to a Ca²⁺ -dependent upregulation of cytosolic pyruvate production. Conversely, glutamate (200 μM) causes a Na⁺ , but not Ca²⁺ , dependent workload even though glutamate-induced Ca²⁺ signals readily reach mitochondria. The glutamate workload triggers a rapid fall in the cytosolic ATP/ADP ratio and stimulation of respiration. These effects are mimicked by D-aspartate a nonmetabolized agonist of the glutamate transporter, but not by a metabotropic glutamate receptor agonist, indicating a major role of Na⁺ -dependent workload in stimulated respiration. Glutamate-induced increase in respiration is linked to a rapid increase in glycolytic pyruvate production, suggesting that both glutamate and extracellular ATP cause an increase in astrocyte respiration fueled by workload-induced increase in pyruvate production. However, glutamate-induced pyruvate production is partly resistant to glycolysis blockers (iodoacetate), indicating that oxidative consumption of glutamate also contributes to stimulated respiration. As stimulation of respiration by ATP and glutamate are similar and pyruvate production smaller in the first case, the results suggest that the response to extracellular ATP is a Ca²⁺ -dependent upregulation of respiration added to glycolysis upregulation. The global contribution of astrocyte respiratory responses to brain oxygen consumption is an open question.

State-Dependent Changes in Brain Glycogen Metabolism

A fundamental understanding of glycogen structure, concentration, polydispersity and turnover is critical to qualify the role of glycogen in the brain. These molecular and metabolic features are under the control of neuronal activity through the interdependent action of neuromodulatory tone, ionic homeostasis and availability of metabolic substrates, all variables that concur to define the state of the system. In this chapter, we briefly describe how glycogen responds to selected behavioral, nutritional, environmental, hormonal, developmental and pathological conditions. We argue that interpreting glycogen metabolism through the lens of brain state is an effective approach to establish the relevance of energetics in connecting molecular and cellular neurophysiology to behavior.

Role of Brain Glycogen During Ischemia, Aging and Cell-to-Cell Interactions

The astrocyte-neuron lactate transfer shuttle (ANLS) is one of the important metabolic systems that provides a physiological infrastructure for glia-neuronal interactions where specialized architectural organization supports the function. Perivascular astrocyte end-feet take up glucose via glucose transporter 1 to actively regulate glycogen stores, such that high ambient glucose upregulates glycogen and low levels of glucose deplete glycogen stores. A rapid breakdown of glycogen into lactate during increased neuronal activity or low glucose conditions becomes essential for maintaining axon function. However, it fails to benefit axon function during an ischemic episode in white matter (WM). Aging causes a remarkable change in astrocyte architecture characterized by thicker, larger processes oriented parallel to axons, as opposed to vertically-transposing processes. Subsequently, aging axons become more vulnerable to depleted glycogen, although aging axons can use lactate as efficiently as young axons. Lactate equally supports function during aglycemia in corpus callosum (CC), which consists of a mixture of myelinated and unmyelinated axons. Moreover, axon function in CC shows greater resilience to a lack of glucose compared to optic nerve, although both WM tracts show identical recovery after aglycemic injury. Interestingly, emerging evidence implies that a lactate transport system is not exclusive to astrocytes, as oligodendrocytes support the axons they myelinate, suggesting another metabolic coupling pathway in WM. Future studies are expected to unravel the details of oligodendrocyte-axon lactate metabolic coupling to establish that all WM components metabolically cooperate and that lactate may be the universal metabolite to sustain central nervous system function.

Antioxidants prevent inflammation and preserve the optic projection and visual function in experimental neurotrauma

We investigated the role of oxidative stress and the inflammasome in trauma-induced axon degeneration and vision loss using a mouse model. The left eyes of male mice were exposed to over-pressure air waves. Wild-type C57Bl/6 mice were fed normal, high-vitamin-E (VitE), ketogenic or ketogenic-control diets. Mice lacking the ability to produce vitamin C (VitC) were maintained on a low-VitC diet. Visual evoked potentials (VEPs) and retinal superoxide levels were measured in vivo. Tissue was collected for biochemical and histological analysis. Injury increased retinal superoxide, decreased SOD2, and increased cleaved caspase-1, IL-1α, IL-1β, and IL-18 levels. Low-VitC exacerbated the changes and the high-VitE diet mitigated them, suggesting that oxidative stress led to the increase in IL-1α and activation of the inflammasome. The injury caused loss of nearly 50% of optic nerve axons at 2 weeks and astrocyte hypertrophy in mice on normal diet, both of which were prevented by the high-VitE diet. The VEP amplitude was decreased after injury in both control-diet and low-VitC mice, but not in the high-VitE-diet mice. The ketogenic diet also prevented the increase in superoxide levels and IL-1α, but had no effect on IL-1β. Despite this, the ketogenic diet preserved optic nerve axons, prevented astrocyte hypertrophy, and preserved the VEP amplitude. These data suggest that oxidative stress induces priming and activation of the inflammasome pathway after neurotrauma of the visual system. Further, blocking the activation of the inflammasome pathway may be an effective post-injury intervention.

Glycolysis Paradigm Shift Dictates a Reevaluation of Glucose and Oxygen Metabolic Rates of Activated Neural Tissue

In 1988 two seminal studies were published, both instigating controversy. One concluded that “the energy needs of activated neural tissue are minimal, being fulfilled via the glycolytic pathway alone,” a conclusion based on the observation that neural activation increased glucose consumption, which was not accompanied by a corresponding increase in oxygen consumption (Fox et al., 1988). The second demonstrated that neural tissue function can be supported exclusively by lactate as the energy substrate (Schurr et al., 1988). While both studies continue to have their supporters and detractors, the present review attempts to clarify the issues responsible for the persistence of the controversies they have provoked and offer a possible rationalization. The concept that lactate rather than pyruvate, is the glycolytic end-product, both aerobically and anaerobically, and thus the real mitochondrial oxidative substrate, has gained a greater acceptance over the years. The idea of glycolysis as the sole ATP supplier for neural activation (glucose → lactate + 2ATP) continues to be controversial. Lactate oxidative utilization by activated neural tissue could explain the mismatch between glucose and oxygen consumption and resolve the existing disagreements among users of imaging methods to measure the metabolic rates of the two energy metabolic substrates. The postulate that the energy necessary for active neural tissue is supplied by glycolysis alone stems from the original aerobic glycolysis paradigm. Accordingly, glucose consumption is accompanied by oxygen consumption at 1–6 ratio. Since Fox et al. (1988) observed only a minimal if non-existent oxygen consumption compared to glucose consumption, their conclusion make sense. Nevertheless, considering (a) the shift in the paradigm of glycolysis (glucose → lactate; lactate + O₂ + mitochondria → pyruvate → TCA cycle → CO₂ + H₂O + 17ATP); (b) that one mole of lactate oxidation requires only 50% of the amount of oxygen necessary for the oxidation of one mole of glucose; and (c) that lactate, as a mitochondrial substrate, is over eight times more efficient at ATP production than glucose as a glycolytic substrate, suggest that future studies of cerebral metabolic rates of activated neural tissue should include along with the measurements of CMR_O2 and CMR_glucose the measurement of CMR_lactate.

NBCe1 mediates the regulation of the NADH/NAD+ redox state in cortical astrocytes by neuronal signals

Astrocytes are a glial cell type, which is indispensable for brain energy metabolism. Within cells, the NADH/NAD⁺ redox state is a crucial node in metabolism connecting catabolic pathways to oxidative phosphorylation and ATP production in mitochondria. To characterize the dynamics of the intracellular NADH/NAD⁺ redox state in cortical astrocytes Peredox, a genetically encoded sensor for the NADH/NAD⁺ redox state, was expressed in cultured cortical astrocytes as well as in cortical astrocytes in acutely isolated brain slices. Calibration of the sensor in cultured astrocytes revealed a mean basal cytosolic NADH/NAD⁺ redox ratio of about 0.01; however, with a broad distribution and heterogeneity in the cell population, which was mirrored by a heterogeneous basal cellular concentration of lactate. Inhibition of glucose uptake decreased the NADH/NAD⁺ redox state while inhibition of lactate dehydrogenase or of lactate release resulted in an increase in the NADH/NAD⁺ redox ratio. Furthermore, the NADH/NAD⁺ redox state was regulated by the extracellular concentration of K⁺ , and application of the neurotransmitters ATP or glutamate increased the NADH/NAD⁺ redox state dependent on purinergic receptors and glutamate uptake, respectively. This regulation by K⁺ , ATP, and glutamate involved NBCe1 mediated sodium-bicarbonate transport. These results demonstrate that the NADH/NAD⁺ redox state in astrocytes is a metabolic node regulated by neuronal signals reflecting physiological activity, most likely contributing to adjust astrocytic metabolism to energy demand of the brain.

Prokineticin-2 promotes chemotaxis and alternative A2 reactivity of astrocytes

Astrocyte reactivity is disease- and stimulus-dependent, adopting either a proinflammatory A1 phenotype or a protective, anti-inflammatory A2 phenotype. Recently, we demonstrated, using cell culture, animal models and human brain samples, that dopaminergic neurons produce and secrete higher levels of the chemokine-like signaling protein Prokineticin-2 (PK2) as a compensatory protective response against neurotoxic stress. As astrocytes express a high level of PK2 receptors, herein, we systematically characterize the role of PK2 in astrocyte structural and functional properties. PK2 treatment greatly induced astrocyte migration, which was accompanied by a shift in mitochondrial energy metabolism, a reduction in proinflammatory factors, and an increase in the antioxidant genes Arginase-1 and Nrf2. Overexpression of PK2 in primary astrocytes or in the in vivo mouse brain induced the A2 astrocytic phenotype with upregulation of key protective genes and A2 reactivity markers including Arginase-1 and Nrf2, PTX3, SPHK1, and TM4SF1. A small-molecule PK2 agonist, IS20, not only mimicked the protective effect of PK2 in primary cultures, but also increased glutamate uptake by upregulating GLAST. Notably, IS20 blocked not only MPTP-induced reductions in the A2 phenotypic markers SPHK1 and SCL10a6 but also elevation of the of A1 marker GBP2. Collectively, our results reveal that PK2 regulates a novel neuron-astrocyte signaling mechanism by promoting an alternative A2 protective phenotype in astrocytes, which could be exploited for development of novel therapeutic strategies for PD and other related chronic neurodegenerative diseases. PK2 signals through its receptors on astrocytes and promotes directed chemotaxis. PK2-induced astrocyte reactivity leads to an increase in antioxidant and anti-inflammatory proteins while increasing glutamate uptake, along with decreased inflammatory factors. © 2018 Wiley Periodicals, Inc.

Norepinephrine stimulates glycogenolysis in astrocytes to fuel neurons with lactate

The mechanism of rapid energy supply to the brain, especially to accommodate the heightened metabolic activity of excited states, is not well-understood. We explored the role of glycogen as a fuel source for neuromodulation using the noradrenergic stimulation of glia in a computational model of the neural-glial-vasculature ensemble (NGV). The detection of norepinephrine (NE) by the astrocyte and the coupled cAMP signal are rapid and largely insensitive to the distance of the locus coeruleus projection release sites from the glia, implying a diminished impact for volume transmission in high affinity receptor transduction systems. Glucosyl-conjugated units liberated from glial glycogen by NE-elicited cAMP second messenger transduction winds sequentially through the glycolytic cascade, generating robust increases in NADH and ATP before pyruvate is finally transformed into lactate. This astrocytic lactate is rapidly exported by monocarboxylate transporters to the associated neuron, demonstrating that the astrocyte-to-neuron lactate shuttle activated by glycogenolysis is a likely fuel source for neuromodulation and enhanced neural activity. Altogether, the energy supply for both astrocytes and neurons can be supplied rapidly by glycogenolysis upon neuromodulatory stimulus.

Although efficient compared to computers, the human brain utilizes energy at 10-fold the rate of other organs by mass. How the brain is supplied with sufficient on-demand energy to support its activity in the absence of neuronal storage capacity remains unknown. Neurons are not capable of meeting their own energy requirements, instead energy supply in the brain is managed by an oligocellular cartel composed of neurons, glia and the local vasculature (NGV), wherein glia can provide the ergogenic metabolite lactate to the neuron in a process called the astrocyte-to-neuron shuttle (ANLS). The only means of energy storage in the brain is glycogen, a polymerized form of glucose that is localized largely to astrocytes, but its exact role and conditions of use are not clear. In this computational model we show that neuromodulatory stimulation by norepinephrine induces astrocytes to recover glucosyl subunits from glycogen for use in a glycolytic process that favors the production of lactate. The ATP and NADH produced support metabolism in the astrocyte while the lactate is exported to feed the neuron. Thus, rapid energy demands by both neurons and glia in a stimulated brain can be met by glycogen mobilization.

Fueling thought: Management of glycolysis and oxidative phosphorylation in neuronal metabolism

Yellen reviews how cellular metabolism responds acutely to the intense energy requirements of neurons when they are stimulated.

The brain’s energy demands are remarkable both in their intensity and in their moment-to-moment dynamic range. This perspective considers the evidence for Warburg-like aerobic glycolysis during the transient metabolic response of the brain to acute activation, and it particularly addresses the cellular mechanisms that underlie this metabolic response. The temporary uncoupling between glycolysis and oxidative phosphorylation led to the proposal of an astrocyte-to-neuron lactate shuttle whereby during stimulation, lactate produced by increased glycolysis in astrocytes is taken up by neurons as their primary energy source. However, direct evidence for this idea is lacking, and evidence rather supports that neurons have the capacity to increase their own glycolysis in response to stimulation; furthermore, neurons may export rather than import lactate in response to stimulation. The possible cellular mechanisms for invoking metabolic resupply of energy in neurons are also discussed, in particular the roles of feedback signaling via adenosine diphosphate and feedforward signaling by calcium ions.

Circadian control of p75 neurotrophin receptor leads to alternate activation of Nrf2 and c-Rel to reset energy metabolism in astrocytes via brain-derived neurotrophic factor

Circadian clock genes regulate energy metabolism partly through neurotrophins in the body. The low affinity neurotrophin receptor p75^NTR is a clock component directly regulated by the transcriptional factor Clock:Bmal1 complex. Brain-derived neurotrophic factor (BDNF) is expressed in the brain and plays a key role in coordinating metabolic interactions between neurons and astrocytes. BDNF transduces signals through TrkB and p75^NTR receptors. This review highlights a novel molecular mechanism by which BDNF via circadian control of p75^NTR leads to daily resetting of glucose and glycogen metabolism in brain astrocytes to accommodate their functional interaction with neurons. Astrocytes store glycogen as an energy reservoir to provide active neurons with the glycolytic metabolite lactate. Astrocytes predominantly express the truncated receptor TrkB.T1 which lacks an intracellular receptor tyrosine kinase domain. TrkB.T1 retains the capacity to regulate cell morphology through regulation of Rho GTPases. In contrast, p75^NTR mediates generation of the bioactive lipid ceramide upon stimulation with BDNF and inhibits PKA activation. As ceramide directly activates PKCζ, we discuss the importance of the TrkB.T1-p75^NTR-ceramide-PKCζ signaling axis in the stimulation of glycogen and lipid synthesis and activation of RhoA. Ceramide-PKCζ-casein kinase 2 signaling activates Nrf2 to support oxidative phosphorylation via upregulation of antioxidant enzymes. In the absence of p75^NTR, TrkB.T1 functionally interacts with adenosine A_2AR and dopamine D1R receptors to enhance cAMP-PKA signaling and activate Rac1 and NF-κB c-Rel, favoring glycogen hydrolysis, gluconeogenesis and aerobic glycolysis. Thus, diurnal changes in p75^NTR levels in astrocytes resets energy metabolism via BDNF to accommodate their metabolic interaction with neurons.

Essential Roles of Lactate in Müller Cell Survival and Function

Müller cells are pivotal in sustaining retinal ganglion cells, and an intact energy metabolism is essential for upholding Müller cell functions. The present study aimed to investigate the impact of lactate on Müller cell survival and function. Primary mice Müller cells and human Müller cell lines (MIO-M1) were treated with or without lactate (10 or 20 mM) for 2 and 24 hours. Simultaneously, Müller cells were incubated with or without 6 mM of glucose. L-lactate exposure increased Müller cell survival independently of the presence of glucose. This effect was abolished by the addition of the monocarboxylate inhibitor 4-cinnamic acid to the treatment media, whereas survival continued to increase in response to addition of D-lactate during glucose restriction. ATP levels decreased over time in MIO-M1 cells and remained stable over time in primary Müller cells. Lactate was preferably metabolized in MIO-M1 cells compared to glucose, and 10 mM of L-Lactate exposure prevented complete glycogen depletion in MIO-M1 cells. Glutamate uptake increased after 2 hours and decreased after 24 hours in glucose-restricted Müller cells compared to cells with glucose supplement. The addition of 10 mM of lactate to the treatment media increased glutamate uptake in glucose supplemented and restricted cells. In conclusion, lactate is a key component in maintaining Müller cell survival and function. Hence, lactate administration may be of great future interest, ultimately leading to novel therapies to rescue retinal ganglion cells.

Astrocytic glycogen metabolism in the healthy and diseased brain

The brain contains a fairly low amount of glycogen, mostly located in astrocytes, a fact that has prompted the suggestion that glycogen does not have a significant physiological role in the brain. However, glycogen metabolism in astrocytes is essential for several key physiological processes and is adversely affected in disease. For instance, diminished ability to break down glycogen impinges on learning, and epilepsy, Alzheimer's disease, and type 2 diabetes are all associated with abnormal astrocyte glycogen metabolism. Glycogen metabolism supports astrocytic K⁺ and neurotransmitter glutamate uptake and subsequent glutamine synthesis-three fundamental steps in excitatory signaling at most brain synapses. Thus, there is abundant evidence for a key role of glycogen in brain function. Here, we summarize the physiological brain functions that depend on glycogen, discuss glycogen metabolism in disease, and investigate how glycogen breakdown is regulated at the cellular and molecular levels.

A computational model integrating brain electrophysiology and metabolism highlights the key role of extracellular potassium and oxygen

The human brain is a small organ which uses a disproportionate amount of the total metabolic energy production in the body. While it is well understood that the most significant energy sink is the maintenance of the neuronal membrane potential during the brain signaling activity, the role of astrocytes in the energy balance continues to be the topic of a lot of research. A key function of astrocytes, besides clearing glutamate from the synaptic clefts, is the potassium clearing after neuronal activation. Extracellular potassium plays a significant role in triggering neuronal firing, and elevated concentration of potassium may lead to abnormal firing patterns, e.g., seizures, thus emphasizing the importance of the glial K⁺ buffering role. The predictive mathematical model proposed in this paper elucidates the role of glial potassium clearing in brain energy metabolism, integrating a detailed model of the ion dynamics which regulates neuronal firing with a four compartment metabolic model. Because of the very different characteristic time scales of electrophysiology and metabolism, care must be taken when coupling the two models to ensure that the predictions, e.g., neuronal firing frequencies and the oxygen-glucose index (OGI) of the brain during activation and rest, are in agreement with empirical observations. The temporal multi-scale nature of the problem requires the design of new computational tools to ensure a stable and accurate numerical treatment. The model predictions for different protocols, including combinations of elevated activation and ischemic episodes, are in good agreement with experimental observations reported in the literature.

Advances in Imaging Brain Metabolism

Metabolism is central to neuroimaging because it can reveal pathways by which neuronal and glial cells use nutrients to fuel their growth and function. We focus on advanced magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) methods used in brain metabolic studies. ¹⁷O-MRS and ³¹P-MRS, respectively, provide rates of oxygen use and ATP synthesis inside mitochondria, whereas ¹⁹F-MRS enables measurement of cytosolic glucose metabolism. Calibrated functional MRI (fMRI), an advanced form of fMRI that uses contrast generated by deoxyhemoglobin, provides maps of oxygen use that track neuronal firing across brain regions. ¹³C-MRS is the only noninvasive method of measuring both glutamatergic neurotransmission and cell-specific energetics with signaling and nonsignaling purposes. Novel MRI contrasts, arising from endogenous diamagnetic agents and exogenous paramagnetic agents, permit pH imaging of glioma. Overall, these magnetic resonance methods for imaging brain metabolism demonstrate translational potential to better understand brain disorders and guide diagnosis and treatment.

An auditory brainstem nucleus as a model system for neuronal metabolic demands

The correlation between neuronal activity and metabolism is essential for coding, plasticity, neurological disorders and the interpretation of functional neuroimaging data. Most likely, metabolic requirements depend upon neuron type, and macroscopic energy demands vary with brain region. However, specific needs of individual neuron types are enigmatic. Therefore, we monitored metabolic activity in the lateral superior olive (LSO), an auditory brainstem nucleus containing only one neuron type. LSO neurons exhibit extreme but well-described biophysics with firing rates of several hundred hertz and low input resistances of a few megaohms. We recorded changes in NADH and flavin adenine dinucleotide (FAD) autofluorescence and O₂ concentration in acute brainstem slices of Mongolian gerbils (Meriones unguiculatus) following electrical stimulation. The LSO shows the typical biphasic NADH/FAD response up to a physiologically relevant frequency of 400 Hz. In the same animal, we compared the LSO with the hippocampal CA1 region and the cerebral cortex. The rate of NADH/FADH₂ consumption and regeneration was slowest in LSO. However, frequency dependence was only similar during the consumption phase but varied during regeneration within the three brain regions. Changes in NADH, FAD and O₂ levels and blocking metabolic reactions indicate a pronounced contribution of mitochondrial oxidative phosphorylation in the LSO which is known for the other brain regions as well. Lactate transport and interconversion are involved in LSO metabolism as we found in immunohistochemical and pharmacological experiments. Our findings show that the LSO represents an apt, biophysically distinct model for brain metabolism and that neuronal properties determine metabolic needs.

Mitochondrial DNA depletion by ethidium bromide decreases neuronal mitochondrial creatine kinase: Implications for striatal energy metabolism

Mitochondrial DNA (mtDNA), the discrete genome which encodes subunits of the mitochondrial respiratory chain, is present at highly variable copy numbers across cell types. Though severe mtDNA depletion dramatically reduces mitochondrial function, the impact of tissue-specific mtDNA reduction remains debated. Previously, our lab identified reduced mtDNA quantity in the putamen of Parkinson's Disease (PD) patients who had developed L-DOPA Induced Dyskinesia (LID), compared to PD patients who had not developed LID and healthy subjects. Here, we present the consequences of mtDNA depletion by ethidium bromide (EtBr) treatment on the bioenergetic function of primary cultured neurons, astrocytes and neuron-enriched cocultures from rat striatum. We report that EtBr inhibition of mtDNA replication and transcription consistently reduces mitochondrial oxygen consumption, and that neurons are significantly more sensitive to EtBr than astrocytes. EtBr also increases glycolytic activity in astrocytes, whereas in neurons it reduces the expression of mitochondrial creatine kinase mRNA and levels of phosphocreatine. Further, we show that mitochondrial creatine kinase mRNA is similarly downregulated in dyskinetic PD patients, compared to both non-dyskinetic PD patients and healthy subjects. Our data support a hypothesis that reduced striatal mtDNA contributes to energetic dysregulation in the dyskinetic striatum by destabilizing the energy buffering system of the phosphocreatine/creatine shuttle.

Cellular Origin of [18F]FDG-PET Imaging Signals During Ceftriaxone-Stimulated Glutamate Uptake: Astrocytes and Neurons

Ceftriaxone stimulates astrocytic uptake of the excitatory neurotransmitter glutamate, and it is used to treat glutamatergic excitotoxicity that becomes manifest during many brain diseases. Ceftriaxone-stimulated glutamate transport was reported to drive signals underlying [¹⁸F]fluorodeoxyglucose-positron emission tomographic ([¹⁸F]FDG-PET) metabolic images of brain glucose utilization and interpreted as supportive of the notion of lactate shuttling from astrocytes to neurons. This study draws attention to critical roles of astrocytes in the energetics and imaging of brain activity, but the results are provocative because (1) the method does not have cellular resolution or provide information about downstream pathways of glucose metabolism, (2) neuronal and astrocytic [¹⁸F]FDG uptake were not separately measured, and (3) strong evidence against lactate shuttling was not discussed. Evaluation of potential metabolic responses to ceftriaxone suggests lack of astrocytic specificity and significant contributions by pre- and postsynaptic neuronal compartments. Indeed, astrocytic glycolysis may not make a strong contribution to the [¹⁸F]FDG-PET signal because partial or complete oxidation of one glutamate molecule on its uptake generates enough ATP to fuel uptake of 3 to 10 more glutamate molecules, diminishing reliance on glycolysis. The influence of ceftriaxone on energetics of glutamate-glutamine cycling must be determined in astrocytes and neurons to elucidate its roles in excitotoxicity treatment.

The Glia-Neuron Lactate Shuttle and Elevated ROS Promote Lipid Synthesis in Neurons and Lipid Droplet Accumulation in Glia via APOE/D

Elevated reactive oxygen species (ROS) induce the formation of lipids in neurons that are transferred to glia, where they form lipid droplets (LDs). We show that glial and neuronal monocarboxylate transporters (MCTs), fatty acid transport proteins (FATPs), and apolipoproteins are critical for glial LD formation. MCTs enable glia to secrete and neurons to absorb lactate, which is converted to pyruvate and acetyl-CoA in neurons. Lactate metabolites provide a substrate for synthesis of fatty acids, which are processed and transferred to glia by FATP and apolipoproteins. In the presence of high ROS, inhibiting lactate transfer or lowering FATP or apolipoprotein levels decreases glial LD accumulation in flies and in primary mouse glial-neuronal cultures. We show that human APOE can substitute for a fly glial apolipoprotein and that APOE4, an Alzheimer's disease susceptibility allele, is impaired in lipid transport and promotes neurodegeneration, providing insights into disease mechanisms.

Regulation of mitochondrial dynamics in astrocytes: Mechanisms, consequences, and unknowns

Astrocytes are the major glial cell in the central nervous system. These polarized cells possess numerous processes that ensheath the vasculature and contact synapses. Astrocytes play important roles in synaptic signaling, neurotransmitter synthesis and recycling, control of nutrient uptake, and control of local blood flow. Many of these processes depend on local metabolism and/or energy utilization. While astrocytes respond to increases in neuronal activity and metabolic demand by upregulating glycolysis and glycogenolysis, astrocytes also possess significant capacity for oxidative (mitochondrial) metabolism. Mitochondria mediate energy supply and metabolism, cellular survival, ionic homeostasis, and proliferation. These organelles are dynamic structures undergoing extensive fission and fusion, directed movement along cytoskeletal tracts, and degradation. While many of the mechanisms underlying the dynamics of these organelles and their physiologic roles have been characterized in neurons and other cells, the roles that mitochondrial dynamics play in glial physiology is less well understood. Recent work from several laboratories has demonstrated that mitochondria are present within the fine processes of astrocytes, that their movement is regulated, and that they contribute to local Ca²⁺ signaling within the astrocyte. They likely play a role in local ATP production and metabolism, particularly that of glutamate. Here we will review these and other findings describing the mechanism by which mitochondrial dynamics are regulated in astrocytes, how mitochondrial dynamics might influence astrocyte and brain metabolism, and draw parallels to mitochondrial dynamics in neurons. Additionally, we present new analyses of the size, distribution, and dynamics of mitochondria in astrocytes performed using in vivo using 2-photon microscopy.

Integration between Glycolysis and Glutamate-Glutamine Cycle Flux May Explain Preferential Glycolytic Increase during Brain Activation, Requiring Glutamate

The 1988 observation by Fox et al. (1988) that brief intense brain activation increases glycolysis (pyruvate formation from glucose) much more than oxidative metabolism has been abundantly confirmed. Specifically glycolytic increase was unexpected because the amount of ATP it generates is much smaller than that formed by subsequent oxidative metabolism of pyruvate. The present article shows that preferential glycolysis can be explained by metabolic processes associated with activation of the glutamate-glutamine cycle. The flux in this cycle, which is essential for production of transmitter glutamate and GABA, equals 75% of brain glucose utilization and each turn is associated with utilization of ~1 glucose molecule. About one half of the association between cycle flux and glucose metabolism occurs during neuronal conversion of glutamine to glutamate in a process similar to the malate-aspartate shuttle (MAS) except that glutamate is supplied from glutamine, not formed from α-ketoglutarate (αKG) as during operation of conventional MAS. Regular MAS function is triggered by one oxidative process in the cytosol during glycolysis causing NAD⁺ reduction to NADH. Since NADH cannot cross the mitochondrial membrane (MEM) for oxidation NAD⁺ is re-generated by conversion of cytosolic oxaloacetate (OAA) to malate, which enters the mitochondria for oxidation and in a cyclic process regenerates cytosolic OAA. Therefore MAS as well as the "pseudo-MAS" necessary for neuronal glutamate formation can only operate together with cytosolic reduction of NAD⁺ to NADH. The major process causing NAD⁺ reduction is glycolysis which therefore also must occur during neuronal conversion of glutamine to glutamate and may energize vesicular glutamate uptake which preferentially uses glycolytically derived energy. Another major contributor to the association between glutamate-glutamine cycle and glucose utilization is the need for astrocytic pyruvate to generate glutamate. Although some oxidative metabolism occurs during glutamate formation it is only one half of that during normal tricarboxylic acid (TCA) cycle function. Glutamate's receptor stimulation leads to potassium ion (K⁺) release and astrocytic uptake, preferentially fueled by glycolysis and followed by release and neuronal re-accumulation. The activation-induced preferential glycolysis diminishes with continued activation and is followed by an increased ratio between oxidative metabolism and glycolysis, reflecting oxidation of generated glutamate and accumulated lactate.

Arginine vasopressin altered the expression of monocarboxylate transporters in cultured astrocytes isolated from stroke-prone spontaneously hypertensive rats and congenic SHRpch1_18 rats

Background

Astrocytes support a range of brain functions as well as neuronal survival, but their detailed relationship with stroke-related edema is not well understood. We previously demonstrated that the release of lactate from astrocytes isolated from stroke-prone spontaneously hypertensive rats (SHRSP/Izm) was attenuated under stroke conditions. The supply of lactate to neurons is regulated by astrocytic monocarboxylate transporters (MCTs). The purpose of this study was to examine the contributions of arginine vasopressin (AVP) and/or hypoxia and reoxygenation (H/R) to the regulation of MCTs and neurotrophic factor in astrocytes obtained from SHRSP/Izm and congenic SHRpch1_18 rats.

Methods

We compared AVP-induced lactate levels, MCTs, and brain-derived neurotrophic factor (BDNF) in astrocytes isolated from SHRSP/Izm, SHRpch1_18, and Wistar Kyoto rats (WKY/Izm). The expression levels of genes and proteins were determined by PCR and Western blotting (WB).

Results

The production of lactate induced by AVP was increased in astrocytes from all three strains. However, the levels of lactate were lower in SHRSP/Izm and SHRpch1_18 animals compared with the WKY/Izm strain. Gene expression levels of Slc16a1, Slc16a4, and Bdnf were lowered by AVP in SHRSP/Izm and SHRpch1_18 rats compared with WKY/Izm. The increase of MCT4 that was induced by AVP was blocked by the addition of a specific nitric oxide (NO) chelator, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (CPTIO). Furthermore, AVP increased the expression of iNOS and eNOS proteins in WKY/Izm and SHRSP/Izm rat astrocytes. However, the iNOS expression levels in SHRSP astrocytes differed from those of WKY/Izm astrocytes. The increase of MCT4 protein expression during AVP treatment was blocked by the addition of a specific NF-kB inhibitor, pyrrolidine dithiocarbamate (PDTC). The induction of MCT4 by AVP may be regulated by NO through NF-kB.

Conclusions

These results suggest that the expression of MCTs mediated by AVP may be regulated by NO. The data suggest that AVP attenuated the expression of MCTs in SHRSP/Izm and SHRpch1_18 astrocytes. Reduced expression of MCTs may be associated with decreased lactate production in SHRSP.

Integration between Glycolysis and Glutamate-Glutamine Cycle Flux May Explain Preferential Glycolytic Increase during Brain Activation, Requiring Glutamate

The 1988 observation by Fox et al. (1988) that brief intense brain activation increases glycolysis (pyruvate formation from glucose) much more than oxidative metabolism has been abundantly confirmed. Specifically glycolytic increase was unexpected because the amount of ATP it generates is much smaller than that formed by subsequent oxidative metabolism of pyruvate. The present article shows that preferential glycolysis can be explained by metabolic processes associated with activation of the glutamate-glutamine cycle. The flux in this cycle, which is essential for production of transmitter glutamate and GABA, equals 75% of brain glucose utilization and each turn is associated with utilization of ~1 glucose molecule. About one half of the association between cycle flux and glucose metabolism occurs during neuronal conversion of glutamine to glutamate in a process similar to the malate-aspartate shuttle (MAS) except that glutamate is supplied from glutamine, not formed from α-ketoglutarate (αKG) as during operation of conventional MAS. Regular MAS function is triggered by one oxidative process in the cytosol during glycolysis causing NAD+ reduction to NADH. Since NADH cannot cross the mitochondrial membrane (MEM) for oxidation NAD+ is re-generated by conversion of cytosolic oxaloacetate (OAA) to malate, which enters the mitochondria for oxidation and in a cyclic process regenerates cytosolic OAA. Therefore MAS as well as the "pseudo-MAS" necessary for neuronal glutamate formation can only operate together with cytosolic reduction of NAD+ to NADH. The major process causing NAD+ reduction is glycolysis which therefore also must occur during neuronal conversion of glutamine to glutamate and may energize vesicular glutamate uptake which preferentially uses glycolytically derived energy. Another major contributor to the association between glutamate-glutamine cycle and glucose utilization is the need for astrocytic pyruvate to generate glutamate. Although some oxidative metabolism occurs during glutamate formation it is only one half of that during normal tricarboxylic acid (TCA) cycle function. Glutamate's receptor stimulation leads to potassium ion (K+) release and astrocytic uptake, preferentially fueled by glycolysis and followed by release and neuronal re-accumulation. The activation-induced preferential glycolysis diminishes with continued activation and is followed by an increased ratio between oxidative metabolism and glycolysis, reflecting oxidation of generated glutamate and accumulated lactate.

Integration between Glycolysis and Glutamate-Glutamine Cycle Flux May Explain Preferential Glycolytic Increase during Brain Activation, Requiring Glutamate

The 1988 observation by Fox et al. (1988) that brief intense brain activation increases glycolysis (pyruvate formation from glucose) much more than oxidative metabolism has been abundantly confirmed. Specifically glycolytic increase was unexpected because the amount of ATP it generates is much smaller than that formed by subsequent oxidative metabolism of pyruvate. The present article shows that preferential glycolysis can be explained by metabolic processes associated with activation of the glutamate-glutamine cycle. The flux in this cycle, which is essential for production of transmitter glutamate and GABA, equals 75% of brain glucose utilization and each turn is associated with utilization of ~1 glucose molecule. About one half of the association between cycle flux and glucose metabolism occurs during neuronal conversion of glutamine to glutamate in a process similar to the malate-aspartate shuttle (MAS) except that glutamate is supplied from glutamine, not formed from α-ketoglutarate (αKG) as during operation of conventional MAS. Regular MAS function is triggered by one oxidative process in the cytosol during glycolysis causing NAD⁺ reduction to NADH. Since NADH cannot cross the mitochondrial membrane (MEM) for oxidation NAD⁺ is re-generated by conversion of cytosolic oxaloacetate (OAA) to malate, which enters the mitochondria for oxidation and in a cyclic process regenerates cytosolic OAA. Therefore MAS as well as the "pseudo-MAS" necessary for neuronal glutamate formation can only operate together with cytosolic reduction of NAD⁺ to NADH. The major process causing NAD⁺ reduction is glycolysis which therefore also must occur during neuronal conversion of glutamine to glutamate and may energize vesicular glutamate uptake which preferentially uses glycolytically derived energy. Another major contributor to the association between glutamate-glutamine cycle and glucose utilization is the need for astrocytic pyruvate to generate glutamate. Although some oxidative metabolism occurs during glutamate formation it is only one half of that during normal tricarboxylic acid (TCA) cycle function. Glutamate's receptor stimulation leads to potassium ion (K⁺) release and astrocytic uptake, preferentially fueled by glycolysis and followed by release and neuronal re-accumulation. The activation-induced preferential glycolysis diminishes with continued activation and is followed by an increased ratio between oxidative metabolism and glycolysis, reflecting oxidation of generated glutamate and accumulated lactate.

Effects of Glutamate Receptor Activation on Local Oxygen Changes

Glutamate is ubiquitous throughout the brain and serves as the primary excitatory neurotransmitter. Neurons require energy to fire, and energetic substrates (i.e., O₂, glucose) are renewed via cerebral blood flow (CBF) to maintain metabolic homeostasis. Magnetic resonance brain functionality studies rely on the assumption that CBF and neuronal activity are coupled consistently throughout the brain; however, the origin of neuronal activity does not always coincide with signals indicative of energy consumption (e.g., O₂ decreases) at high spatial resolutions. Therefore, relationships between excitatory neurotransmission and energy use must be evaluated at higher resolutions. In this study, we showed that both endogenously released and exogenously ejected glutamate decrease local tissue O₂ concentrations, but whether hyperemic O₂ restoration followed depended on the stimulus method. Electrically stimulating the glutamatergic corticostriatal pathway evoked biphasic O₂ responses at striatal terminals: first O₂ decreased, then concentrations increased above baseline. Using iontophoresis to locally eject ionotropic glutamate receptor antagonists revealed that these receptors only influenced the O₂ decrease. We compared electrical stimulation to iontophoretic glutamate stimulation, and measured concurrent single-unit activity and O₂ to limit both stimulation and recordings to <50 μm radius from our sensor. Similarly, iontophoretic glutamate delivery elicited monophasic O₂ decreases without subsequent increases.

Spatiotemporal control of mitochondrial network dynamics in astroglial cells

Mitochondria are increasingly recognized for playing important roles in regulating the evolving metabolic state of mammalian cells. This is particularly true for nerve cells, as dysregulation of mitochondrial dynamics is invariably associated with a number of neuropathies. Accumulating evidence now reveals that changes in mitochondrial dynamics and structure may play equally important roles also in the cell biology of astroglial cells. Astroglial cells display significant heterogeneity in their morphology and specialized functions across different brain regions, however besides fundamental differences they seem to share a surprisingly complex meshwork of mitochondria, which is highly suggestive of tightly regulated mechanisms that contribute to maintain this unique architecture. Here, we summarize recent work performed in astrocytes in situ indicating that this may indeed be the case, with astrocytic mitochondrial networks shown to experience rapid dynamic changes in response to defined external cues. Although the mechanisms underlying this degree of mitochondrial re-shaping are far from being understood, recent data suggest that they may contribute to demarcate astrocyte territories undergoing key signalling and metabolic functions.

Glycogenolysis, an Astrocyte-Specific Reaction, is Essential for Both Astrocytic and Neuronal Activities Involved in Learning

In brain glycogen, formed from glucose, is degraded (glycogenolysis) in astrocytes but not in neurons. Although most of the degradation follows the same pathway as glucose, its breakdown product, l-lactate, is released from astrocytes in larger amounts than glucose when glycogenolysis is activated by noradrenaline. However, this is not the case when glycogenolysis is activated by high potassium ion (K⁺) concentrations - possibly because noradrenaline in contrast to high K⁺ stimulates glycogenolysis by an increase not only in free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) but also in cyclic AMP (c-AMP), which may increase the expression of the monocarboxylate transporter through which it is released. Several transmitters activate glycogenolysis in astrocytes and do so at different time points after training. This stimulation is essential for memory consolidation because glycogenolysis is necessary for uptake of K⁺ and stimulates formation of glutamate from glucose, and therefore is needed both for removal of increased extracellular K⁺ following neuronal excitation (which initially occurs into astrocytes) and for formation of transmitter glutamate and GABA. In addition the released l-lactate has effects on neurons which are essential for learning and for learning-related long-term potentiation (LTP), including induction of the neuronal gene Arc/Arg3.1 and activation of gene cascades mediated by CREB and cofilin. Inhibition of glycogenolysis blocks learning, LTP and all related molecular events, but all changes can be reversed by injection of l-lactate. The effect of extracellular l-lactate is due to both astrocyte-mediated signaling which activates noradrenergic activity on all brain cells and to a minor uptake, possibly into dendritic spines.

How Energy Metabolism Supports Cerebral Function: Insights from 13C Magnetic Resonance Studies In vivo

Cerebral function is associated with exceptionally high metabolic activity, and requires continuous supply of oxygen and nutrients from the blood stream. Since the mid-twentieth century the idea that brain energy metabolism is coupled to neuronal activity has emerged, and a number of studies supported this hypothesis. Moreover, brain energy metabolism was demonstrated to be compartmentalized in neurons and astrocytes, and astrocytic glycolysis was proposed to serve the energetic demands of glutamatergic activity. Shedding light on the role of astrocytes in brain metabolism, the earlier picture of astrocytes being restricted to a scaffold-associated function in the brain is now out of date. With the development and optimization of non-invasive techniques, such as nuclear magnetic resonance spectroscopy (MRS), several groups have worked on assessing cerebral metabolism in vivo. In this context, ¹H MRS has allowed the measurements of energy metabolism-related compounds, whose concentrations can vary under different brain activation states. ¹H-[¹³C] MRS, i.e., indirect detection of signals from ¹³C-coupled ¹H, together with infusion of ¹³C-enriched glucose has provided insights into the coupling between neurotransmission and glucose oxidation. Although these techniques tackle the coupling between neuronal activity and metabolism, they lack chemical specificity and fail in providing information on neuronal and glial metabolic pathways underlying those processes. Currently, the improvement of detection modalities (i.e., direct detection of ¹³C isotopomers), the progress in building adequate mathematical models along with the increase in magnetic field strength now available render possible detailed compartmentalized metabolic flux characterization. In particular, direct ¹³C MRS offers more detailed dataset acquisitions and provides information on metabolic interactions between neurons and astrocytes, and their role in supporting neurotransmission. Here, we review state-of-the-art MR methods to study brain function and metabolism in vivo, and their contribution to the current understanding of how astrocytic energy metabolism supports glutamatergic activity and cerebral function. In this context, recent data suggests that astrocytic metabolism has been underestimated. Namely, the rate of oxidative metabolism in astrocytes is about half of that in neurons, and it can increase as much as the rate of neuronal metabolism in response to sensory stimulation.

Local energy on demand: Are 'spontaneous' astrocytic Ca2+-microdomains the regulatory unit for astrocyte-neuron metabolic cooperation?

Astrocytes are a neural cell type critically involved in maintaining brain energy homeostasis as well as signaling. Like neurons, astrocytes are a heterogeneous cell population. Cortical astrocytes show a complex morphology with a highly branched aborization and numerous fine processes ensheathing the synapses of neighboring neurons, and typically extend one process connecting to blood vessels. Recent studies employing genetically encoded fluorescent calcium (Ca²⁺) indicators have described 'spontaneous' localized Ca²⁺-transients in the astrocyte periphery that occur asynchronously, independently of signals in other parts of the cells, and that do not involve somatic Ca²⁺ transients; however, neither it is known whether these Ca²⁺-microdomains occur at or near neuronal synapses nor have their molecular basis nor downstream effector(s) been identified. In addition to Ca²⁺ microdomains, sodium (Na⁺) transients occur in astrocyte subdomains, too, most likely as a consequence of Na⁺ co-transport with the neurotransmitter glutamate, which also regulates mitochondrial movements locally - as do cytoplasmic Ca²⁺ levels. In this review, we cover various aspects of these local signaling events and discuss how structural and biophysical properties of astrocytes might foster such compartmentation. Astrocytes metabolically interact with neurons by providing energy substrates to active neurons. As a single astrocyte branch covers hundreds to thousands of synapses, it is tempting to speculate that these metabolic interactions could occur localized to specific subdomains of astrocytes, perhaps even at the level of small groups of synapses. We discuss how astrocytic metabolism might be regulated at this scale and which signals might contribute to its regulation. We speculate that the astrocytic structures that light up transiently as Ca²⁺-microdomains might be the functional units of astrocytes linking signaling and metabolic processes to adapt astrocytic function to local energy demands. The understanding of these local regulatory and metabolic interactions will be fundamental to fully appreciate the complexity of brain energy homeostasis as well as its failure in disease and may shed new light on the controversy about neuron-glia bi-directional signaling at the tripartite synapse.

Succinate supplementation improves metabolic performance of mixed glial cell cultures with mitochondrial dysfunction

Mitochondrial dysfunction, the inability to efficiently utilise metabolic fuels and oxygen, contributes to pathological changes following traumatic spinal cord or traumatic brain injury (TBI). In the present study, we tested the hypothesis that succinate supplementation can improve cellular energy state under metabolically stressed conditions in a robust, reductionist in vitro model of mitochondrial dysfunction in which primary mixed glial cultures (astrocytes, microglia and oligodendrocytes) were exposed to the mitochondrial complex I inhibitor rotenone. Cellular response was determined by measuring intracellular ATP, extracellular metabolites (glucose, lactate, pyruvate), and oxygen consumption rate (OCR). Rotenone produced no significant changes in glial ATP levels. However, it induced metabolic deficits as evidenced by lactate/pyruvate ratio (LPR) elevation (a clinically-established biomarker for poor outcome in TBI) and decrease in OCR. Succinate addition partially ameliorated these metabolic deficits. We conclude that succinate can improve glial oxidative metabolism, consistent our previous findings in TBI patients’ brains. The mixed glial cellular model may be useful in developing therapeutic strategies for conditions involving mitochondrial dysfunction, such as TBI.

Endogenous Two-Photon Excited Fluorescence Imaging Characterizes Neuron and Astrocyte Metabolic Responses to Manganese Toxicity

As neurodegenerative conditions are increasingly linked to mitochondrial dysfunction, methods for studying brain cell metabolism at high spatial resolution are needed to elucidate neurodegeneration mechanisms. Two-photon excited fluorescence (TPEF) imaging is a non-destructive, high-resolution technique for studying cell metabolism via endogenous fluorescence of reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) and flavin adenine dinucleotide (FAD). We employed TPEF to study the metabolism of primary rat astrocyte and neuronal cultures under normal growth conditions and in response to manganese (Mn) treatment. Histograms of pixel-wise optical redox ratio, defined as FAD/(FAD + NAD(P)H), revealed three distinct redox distributions and significant differences in their relative weights between astrocytes and neurons. When treated with Mn, both cell types exhibited redox ratio shifts consistent with increased oxidative stress. However, the manner in which the redox distributions was affected was distinct for the two cell types. Furthermore, NAD(P)H fluorescence lifetime imaging revealed an increase in bound NAD(P)H fraction upon Mn treatment for neurons, consistent with enhanced apoptosis. Astrocytes showed a decrease in bound fraction, possibly due to a shift towards glycolytic metabolism in response to impaired respiration. These results exhibit TPEF’s utility for characterizing detailed metabolic changes of different brain cell types in response to neurotoxins.

The nature of early astroglial protection-Fast activation and signaling

Our present review is focusing on the uniqueness of balanced astroglial signaling. The balance of excitatory and inhibitory signaling within the CNS is mainly determined by sharp synaptic transients of excitatory glutamate (Glu) and inhibitory γ-aminobutyrate (GABA) acting on the sub-second timescale. Astroglia is involved in excitatory chemical transmission by taking up i) Glu through neurotransmitter-sodium transporters, ii) K⁺ released due to presynaptic action potential generation, and iii) water keeping osmotic pressure. Glu uptake-coupled Na⁺ influx may either ignite long-range astroglial Ca²⁺ transients or locally counteract over-excitation via astroglial GABA release and increased tonic inhibition. Imbalance of excitatory and inhibitory drives is associated with a number of disease conditions, including prevalent traumatic and ischaemic injuries or the emergence of epilepsy. Therefore, when addressing the potential of early therapeutic intervention, astroglial signaling functions combating progress of Glu excitotoxicity is of critical importance. We suggest, that excitotoxicity is linked primarily to over-excitation induced by the impairment of astroglial Glu uptake and/or GABA release. Within this framework, we discuss the acute alterations of Glu-cycling and metabolism and conjecture the therapeutic promise of regulation. We also confer the role played by key carrier proteins and enzymes as well as their interplay at the molecular, cellular, and organ levels. Moreover, based on our former studies, we offer potential prospect on the emerging theme of astroglial succinate sensing in course of Glu excitotoxicity.

Energy metabolism in the rat cortex under thiopental anaesthesia measured In Vivo by 13 C MRS

Barbiturates, commonly used as general anaesthetics, depress neuronal activity and thus cerebral metabolism. Moreover, they are likely to disrupt the metabolic support of astrocytes to neurons, as well as the uptake of nutrients from circulation. By employing ¹³ C magnetic resonance spectroscopy (MRS) in vivo at high magnetic field, we characterized neuronal and astrocytic pathways of energy metabolism in the rat cortex under thiopental anaesthesia. The neuronal tricarboxylic acid (TCA) cycle rate was 0.46 ± 0.02 µmol/g/min, and the rate of the glutamate-glutamine cycle was 0.09 ± 0.02 µmol/g/min. In astrocytes, the TCA cycle rate was 0.16 ± 0.02 µmol/g/min, accounting for a quarter of whole brain glucose oxidation, pyruvate carboxylase rate was 0.02 ± 0.01 µmol/g/min, and glutamine synthetase was 0.12 ± 0.01 µmol/g/min. Relative to previous experiments under light α-chloralose anaesthesia, thiopental reduced oxidative metabolism in neurons and even more so in astrocytes. Interestingly, total oxidative metabolism in the cortex under thiopental anaesthesia surpassed the rate of pyruvate production by glycolysis, indicating substantial utilisation of substrates other than glucose, likely plasma lactate. © 2017 Wiley Periodicals, Inc.

Transcriptomic profile of leg muscle during early growth in chicken

The early growth pattern, especially the age of peak growth, of broilers affects the time to market and slaughter weight, which in turn affect the profitability of the poultry industry. However, the underlying mechanisms regulating chicken growth and development have rarely been studied. This study aimed to identify candidate genes involved in chicken growth and investigated the potential regulatory mechanisms of early growth in chicken. RNA sequencing was applied to compare the transcriptomes of chicken muscle tissues at three developmental stages during early growth. In total, 978 differentially expressed genes (DEGs) (fold change ≥ 2; false discovery rate < 0.05) were detected by pairwise comparison. Functional analysis showed that the DEGs are mainly involved in the processes of cell growth, muscle development, and cellular activities (such as junction, migration, assembly, differentiation, and proliferation). Many of the DEGs are well known to be related to chicken growth, such as MYOD1, GH, IGF2BP2, IGFBP3, SMYD1, CEBPB, FGF2, and IGFBP5. KEGG pathway analysis identified that the DEGs were significantly enriched in five pathways (P < 0.1) related to growth and development: extracellular matrix–receptor interaction, focal adhesion, tight junction, insulin signaling pathway, and regulation of the actin cytoskeleton. A total of 42 DEGs assigned to these pathways are potential candidate genes inducing the difference in growth among the three developmental stages, such as MYH10, FGF2, FGF16, FN1, CFL2, MAPK9, IRS1, PHKA1, PHKB, and PHKG1. Thus, our study identified a series of genes and several pathways that may participate in the regulation of early growth in chicken. These results should serve as an important resource revealing the molecular basis of chicken growth and development.

Glutamine-Glutamate Cycle Flux Is Similar in Cultured Astrocytes and Brain and Both Glutamate Production and Oxidation Are Mainly Catalyzed by Aspartate Aminotransferase

The glutamine-glutamate cycle provides neurons with astrocyte-generated glutamate/γ-aminobutyric acid (GABA) and oxidizes glutamate in astrocytes, and it returns released transmitter glutamate/GABA to neurons after astrocytic uptake. This review deals primarily with the glutamate/GABA generation/oxidation, although it also shows similarity between metabolic rates in cultured astrocytes and intact brain. A key point is identification of the enzyme(s) converting astrocytic α-ketoglutarate to glutamate and vice versa. Most experiments in cultured astrocytes, including those by one of us, suggest that glutamate formation is catalyzed by aspartate aminotransferase (AAT) and its degradation by glutamate dehydrogenase (GDH). Strongly supported by results shown in Table 1 we now propose that both reactions are primarily catalyzed by AAT. This is possible because the formation occurs in the cytosol and the degradation in mitochondria and they are temporally separate. High glutamate/glutamine concentrations abolish the need for glutamate production from α-ketoglutarate and due to metabolic coupling between glutamate synthesis and oxidation these high concentrations render AAT-mediated glutamate oxidation impossible. This necessitates the use of GDH under these conditions, shown by insensitivity of the oxidation to the transamination inhibitor aminooxyacetic acid (AOAA). Experiments using lower glutamate/glutamine concentration show inhibition of glutamate oxidation by AOAA, consistent with the coupled transamination reactions described here.

Genome-Scale Reconstruction of the Human Astrocyte Metabolic Network

Astrocytes are the most abundant cells of the central nervous system; they have a predominant role in maintaining brain metabolism. In this sense, abnormal metabolic states have been found in different neuropathological diseases. Determination of metabolic states of astrocytes is difficult to model using current experimental approaches given the high number of reactions and metabolites present. Thus, genome-scale metabolic networks derived from transcriptomic data can be used as a framework to elucidate how astrocytes modulate human brain metabolic states during normal conditions and in neurodegenerative diseases. We performed a Genome-Scale Reconstruction of the Human Astrocyte Metabolic Network with the purpose of elucidating a significant portion of the metabolic map of the astrocyte. This is the first global high-quality, manually curated metabolic reconstruction network of a human astrocyte. It includes 5,007 metabolites and 5,659 reactions distributed among 8 cell compartments, (extracellular, cytoplasm, mitochondria, endoplasmic reticle, Golgi apparatus, lysosome, peroxisome and nucleus). Using the reconstructed network, the metabolic capabilities of human astrocytes were calculated and compared both in normal and ischemic conditions. We identified reactions activated in these two states, which can be useful for understanding the astrocytic pathways that are affected during brain disease. Additionally, we also showed that the obtained flux distributions in the model, are in accordance with literature-based findings. Up to date, this is the most complete representation of the human astrocyte in terms of inclusion of genes, proteins, reactions and metabolic pathways, being a useful guide for in-silico analysis of several metabolic behaviors of the astrocyte during normal and pathologic states.

Activity-dependent modulation of intracellular ATP in cultured cortical astrocytes

Brain function is absolutely dependent on an appropriate supply of energy. A shortfall in supply-as occurs, for instance, following stroke-can lead rapidly to irreversible damage to this vital organ. While the consequences of pathophysiological energy depletion have been well documented, much less is known about the physiological energy dynamics of brain cells, although changes in the intracellular concentration of adenosine triphosphate (ATP), the major energy carrier of cells, have been postulated to contribute to cellular signaling. To address this issue more closely, we have investigated intracellular ATP in cultured primary cortical astrocytes by time-lapse microscopy using a genetically encoded fluorescent sensor for ATP. The cytosolic ATP sensor signal decreased after application of the neurotransmitter glutamate in a manner dependent on both glutamate concentration and glutamate transporter activity, but independent of glutamate receptors. The application of dopamine did not affect ATP levels within astrocytes. These results confirm that intracellular ATP levels in astrocytes do indeed respond to changes in physiological activity and pave the way for further studies addressing factors that affect regulation of ATP. © 2017 Wiley Periodicals, Inc.

Lack of appropriate stoichiometry: Strong evidence against an energetically important astrocyte-neuron lactate shuttle in brain

Glutamate-stimulated aerobic glycolysis in astrocytes coupled with lactate shuttling to neurons where it can be oxidized was proposed as a mechanism to couple excitatory neuronal activity with glucose utilization (CMR_glc ) during brain activation. From the outset, this model was not viable because it did not fulfill critical stoichiometric requirements: (i) Calculated glycolytic rates and measured lactate release rates were discordant in cultured astrocytes. (ii) Lactate oxidation requires oxygen consumption, but the oxygen-glucose index (OGI, calculated as CMR_O2 /CMR_glc ) fell during activation in human brain, and the small rise in CMR_O2 could not fully support oxidation of lactate produced by disproportionate increases in CMR_glc . (iii) Labeled products of glucose metabolism are not retained in activated rat brain, indicating rapid release of a highly labeled, diffusible metabolite identified as lactate, thereby explaining the CMR_glc -CMR_O2 mismatch. Additional independent lines of evidence against lactate shuttling include the following: astrocytic oxidation of glutamate after its uptake can help "pay" for its uptake without stimulating glycolysis; blockade of glutamate receptors during activation in vivo prevents upregulation of metabolism and lactate release without impairing glutamate uptake; blockade of β-adrenergic receptors prevents the fall in OGI in activated human and rat brain while allowing glutamate uptake; and neurons upregulate glucose utilization in vivo and in vitro under many stimulatory conditions. Studies in immature cultured cells are not appropriate models for lactate shuttling in adult brain because of their incomplete development of metabolic capability and astrocyte-neuron interactions. Astrocyte-neuron lactate shuttling does not make large, metabolically significant contributions to energetics of brain activation. © 2017 Wiley Periodicals, Inc.

Transient Opening of the Mitochondrial Permeability Transition Pore Induces Microdomain Calcium Transients in Astrocyte Processes

Astrocytes extend highly branched processes that form functionally isolated microdomains, facilitating local homeostasis by redistributing ions, removing neurotransmitters, and releasing factors to influence blood flow and neuronal activity. Microdomains exhibit spontaneous increases in calcium (Ca²⁺), but the mechanisms and functional significance of this localized signaling are unknown. By developing conditional, membrane-anchored GCaMP3 mice, we found that microdomain activity that occurs in the absence of inositol triphosphate (IP3)-dependent release from endoplasmic reticulum arises through Ca²⁺ efflux from mitochondria during brief openings of the mitochondrial permeability transition pore. These microdomain Ca²⁺ transients were facilitated by the production of reactive oxygen species during oxidative phosphorylation and were enhanced by expression of a mutant form of superoxide dismutase 1 (SOD1 G93A) that causes astrocyte dysfunction and neurodegeneration in amyotrophic lateral sclerosis (ALS). By localizing mitochondria to microdomains, astrocytes ensure local metabolic support for energetically demanding processes and enable coupling between metabolic demand and Ca²⁺ signaling events.

Review: Traumatic brain injury and hyperglycemia, a potentially modifiable risk factor

Hyperglycemia after severe traumatic brain injury (TBI) occurs frequently and is associated with poor clinical outcome and increased mortality. In this review, we highlight the mechanisms that lead to hyperglycemia and discuss how they may contribute to poor outcomes in patients with severe TBI. Moreover, we systematically review the proper management of hyperglycemia after TBI, covering topics such as nutritional support, glucose control, moderated hypothermia, naloxone, and mannitol treatment. However, to date, an optimal and safe glycemic target range has not been determined, and may not be safe to implement among TBI patients. Therefore, there is a mandate to explore a reasonable glycemic target range that can facilitate recovery after severe TBI.