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Mobilization of astrocyte glycogen is key for processes such as synaptic plasticity and memory formation but the link between neuronal activity and glycogen breakdown is not fully known. Activation of cytosolic soluble adenylyl cyclase (sAC) in astrocytes has been suggested to link neuronal depolarization and glycogen breakdown partly based on experiments employing pharmacological inhibition of sAC. However, several studies have revealed that sAC located within mitochondria is a central regulator of respiration and oxidative phosphorylation. Thus, pharmacological sAC inhibition is likely to affect both cytosolic and mitochondrial sAC and if bioenergetic readouts are studied, the observed effects are likely to stem from inhibition of mitochondrial rather than cytosolic sAC. Here, we report that a pharmacologically induced inhibition of sAC activity lowers mitochondrial respiration, induces phosphorylation of the metabolic master switch AMP-activated protein kinase (AMPK), and decreases glycogen stores in cultured primary murine astrocytes. From these data and our discussion of the literature, mitochondrial sAC emerges as a key regulator of astrocyte bioenergetics. Lastly, we discuss the challenges of investigating the functional and metabolic roles of cytosolic versus mitochondrial sAC in astrocytes employing the currently available pharmacological tool compounds., (© 2021 Wiley Periodicals LLC.)

Glutamate is the primary excitatory neurotransmitter of the brain. Cellular homeostasis of glutamate is of paramount importance for normal brain function and relies on an intricate metabolic collaboration between neurons and astrocytes. Glutamate is extensively recycled between neurons and astrocytes in a process known as the glutamate-glutamine cycle. The recycling of glutamate is closely linked to brain energy metabolism and is essential to sustain glutamatergic neurotransmission. However, a considerable amount of glutamate is also metabolized and serves as a metabolic hub connecting glucose and amino acid metabolism in both neurons and astrocytes. Disruptions in glutamate clearance, leading to neuronal overstimulation and excitotoxicity, have been implicated in several neurodegenerative diseases. Furthermore, the link between brain energy homeostasis and glutamate metabolism is gaining attention in several neurological conditions. In this review, we provide an overview of the dynamics of synaptic glutamate homeostasis and the underlying metabolic processes with a cellular focus on neurons and astrocytes. In particular, we review the recently discovered role of neuronal glutamate uptake in synaptic glutamate homeostasis and discuss current advances in cellular glutamate metabolism in the context of Alzheimer's disease and Huntington's disease. Understanding the intricate regulation of glutamate-dependent metabolic processes at the synapse will not only increase our insight into the metabolic mechanisms of glutamate homeostasis, but may reveal new metabolic targets to ameliorate neurodegeneration., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The medium-chain fatty acids octanoic acid (C8) and decanoic acid (C10) are gaining attention as beneficial brain fuels in several neurological disorders. The protective effects of C8 and C10 have been proposed to be driven by hepatic production of ketone bodies. However, plasma ketone levels correlates poorly with the cerebral effects of C8 and C10, suggesting that additional mechanism are in place. Here we investigated cellular C8 and C10 metabolism in the brain and explored how the protective effects of C8 and C10 may be linked to cellular metabolism. Using dynamic isotope labeling, with [U- 13 C]C8 and [U- 13 C]C10 as metabolic substrates, we show that both C8 and C10 are oxidatively metabolized in mouse brain slices. The 13 C enrichment from metabolism of [U- 13 C]C8 and [U- 13 C]C10 was particularly prominent in glutamine, suggesting that C8 and C10 metabolism primarily occurs in astrocytes. This finding was corroborated in cultured astrocytes in which C8 increased the respiration linked to ATP production, whereas C10 elevated the mitochondrial proton leak. When C8 and C10 were provided together as metabolic substrates in brain slices, metabolism of C10 was predominant over that of C8. Furthermore, metabolism of both [U- 13 C]C8 and [U- 13 C]C10 was unaffected by etomoxir indicating that it is independent of carnitine palmitoyltransferase I (CPT-1). Finally, we show that inhibition of glutamine synthesis selectively reduced 13 C accumulation in GABA from [U- 13 C]C8 and [U- 13 C]C10 metabolism in brain slices, demonstrating that the glutamine generated from astrocyte C8 and C10 metabolism is utilized for neuronal GABA synthesis. Collectively, the results show that cerebral C8 and C10 metabolism is linked to the metabolic coupling of neurons and astrocytes, which may serve as a protective metabolic mechanism of C8 and C10 supplementation in neurological disorders., (© 2021. The Author(s).)

Synaptic transmission is closely linked to brain energy and neurotransmitter metabolism. However, the extent of brain metabolism of the inhibitory neurotransmitter γ-aminobutyric acid (GABA), and the relative metabolic contributions of neurons and astrocytes, are yet unknown. The present study was designed to investigate the functional significance of brain GABA metabolism using isolated mouse cerebral cortical slices and slices of neurosurgically resected neocortical human tissue of the temporal lobe. By using dynamic isotope labeling, with [ 15 N]GABA and [U- 13 C]GABA as metabolic substrates, we show that both mouse and human brain slices exhibit a large capacity for GABA metabolism. Both the nitrogen and the carbon backbone of GABA strongly support glutamine synthesis, particularly in the human cerebral cortex, indicative of active astrocytic GABA metabolism. This was further substantiated by pharmacological inhibition of the primary astrocytic GABA transporter subtype 3 (GAT3), by (S)-SNAP-5114 or 1-benzyl-5-chloro-2,3-dihydro-1H-indole-2,3-dione (compound 34), leading to significant reductions in oxidative GABA carbon metabolism. Interestingly, this was not the case when tiagabine was used to specifically inhibit GAT1, which is predominantly found on neurons. Finally, we show that acute GABA exposure does not directly stimulate glycolytic activity nor oxidative metabolism in cultured astrocytes, but can be used as an additional substrate to enhance uncoupled respiration. These results clearly show that GABA is actively metabolized in astrocytes, particularly for the synthesis of glutamine, and challenge the current view that synaptic GABA homeostasis is maintained primarily by presynaptic recycling., (© 2020 Wiley Periodicals, Inc.)

AMP-activated protein kinase (AMPK) is an important energy sensor located in cells throughout the human body. From the periphery, AMPK is known to be a metabolic master switch controlling the use of energy fuels. The energy sensor is activated when the energy status of the cell is low, initiating energy-producing pathways and deactivating energy-consuming pathways. All brain cells are crucially dependent on energy production for survival, and the availability of energy substrates must be closely regulated. Intriguingly, the role of AMPK in the regulation of brain cell metabolism has been sparsely investigated, particularly in astrocytes. By investigating metabolism of 13 C-labeled energy substrates in acutely isolated hippocampal slices and cultured astrocytes, with subsequent mass spectrometry analysis, we here show that activation of AMPK increases glycolysis as well as the capacity of the TCA cycle, that is, anaplerosis, through the activity of pyruvate carboxylase (PC) in astrocytes. In addition, we demonstrate that AMPK activation leads to augmented astrocytic glutamate oxidation via pyruvate recycling (i.e., cataplerosis). This regulatory mechanism induced by AMPK activation is mediated via glutamate dehydrogenase (GDH) shown in a CNS-specific GDH knockout mouse. Collectively, these findings demonstrate that AMPK regulates TCA cycle dynamics in astrocytes via PC and GDH activity. AMPK functionality has been shown to be hampered in Alzheimer's and Parkinson's disease and our findings may therefore add to the toolbox for discovery of new metabolic drug targets., (© 2020 Wiley Periodicals, Inc.)

This review discusses aspects of known and putative compartmentalized 3',5'-cyclic adenosine monophosphate (cAMP) signaling in astrocytes, a cell type that has turned out to be a key player in brain physiology and pathology. cAMP has attracted less attention than Ca 2+ in recent years, but could turn out to rival Ca 2+ in its potential to drive cellular functions and responses to intra- and extracellular cues. Further, Ca 2+ and cAMP are known to engage in extensive crosstalk and cAMP signals often take place within subcellular compartments revolving around multi-protein signaling complexes; however, we know surprisingly little about this in astrocytes. Here, we review aspects of astrocytic cAMP signaling, provide arguments for an increased interest in this subject, suggest possible future research directions within the field, and discuss putative drug targets., (© 2019 Wiley Periodicals, Inc.)

Glycogen is the main storage form of glucose in the brain. In contrast with previous beliefs, brain glycogen has recently been shown to play important roles in several brain functions. A fraction of metabolized glucose molecules are being shunted through glycogen before reentering the glycolytic pathway, a phenomenon known as the glycogen shunt. The significance of glycogen in astrocyte energetics is underlined by high activity of the glycogen shunt and the finding that inhibition of glycogen degradation, under some conditions leads to a disproportional increase in glycolytic activity, so-called glycolytic supercompensation. Glycogen phosphorylase, the key enzyme in glycogen degradation, is expressed in two different isoforms in brain, the muscle and the brain isoform. Recent studies have illustrated how these are differently regulated. In the present study, we investigate the role of the two isoforms in glycolytic supercompensation in cultured astrocytes with the expression of either one of the isoforms silenced by siRNA knockdown. When reintroducing glucose to glucose-starved astrocytes, glycolytic activity increased dramatically. Interestingly, the increase was 30% higher in astrocytes not expressing the muscle isoform of glycogen phosphorylase. Based on these results and previously published data we couple the muscle isoform of glycogen phosphorylase to glycolytic supercompensation and glycogen shunt activity, giving insights to the underlying mechanistic of these phenomena.