Single-cell imaging tools for brain energy metabolism: a review

Glial metabolism versatility regulates mushroom body-driven behavioral output in Drosophila

Providing metabolic support to neurons is now recognized as a major function of glial cells that is conserved from invertebrates to vertebrates. However, research in this field has focused for more than two decades on the relevance of lactate and glial glycolysis for neuronal energy metabolism, while overlooking many other facets of glial metabolism and their impact on neuronal physiology, circuit activity, and behavior. Here, we review recent work that has unveiled new features of glial metabolism, especially in Drosophila, in the modulation of behavioral traits involving the mushroom bodies (MBs). These recent findings reveal that spatially and biochemically distinct modes of glucose-derived neuronal fueling are implemented within the MB in a memory type-specific manner. In addition, cortex glia are endowed with several antioxidant functions, whereas astrocytes can serve as pro-oxidant agents that are beneficial to redox signaling underlying long-term memory. Finally, glial fatty acid oxidation seems to play a dual fail-safe role: first, as a mode of energy production upon glucose shortage, and, second, as a factor underlying the clearance of excessive oxidative load during sleep. Altogether, these integrated studies performed in Drosophila indicate that glial metabolism has a deterministic role on behavior.

Genetically encoded fluorescent sensor to monitor intracellular arginine methylation

Arginine methylation (ArgMet), as a post-translational modification, plays crucial roles in RNA processing, transcriptional regulation, signal transduction, DNA repair, apoptosis and liquid-liquid phase separation (LLPS). Since arginine methylation is associated with cancer pathogenesis and progression, protein arginine methyltransferases have gained interest as targets for anti-cancer therapy. Despite considerable process made to elucidate (patho)physiological mechanisms regulated by arginine methylation, there remains a lack of tools to visualize arginine methylation with high spatiotemporal resolution in live cells. To address this unmet need, we generated an ArgMet-sensitive genetically encoded, Förster resonance energy transfer-(FRET) based biosensor, called GEMS, capable of quantitative real-time monitoring of ArgMet dynamics. We optimized these biosensors by using different ArgMet-binding domains, arginine-glycine-rich regions and adjusting the linkers within the biosensors to improve their performance. Using a set of mammalian cell lines and modulators, we demonstrated the applicability of GEMS for monitoring changes in arginine methylation with single-cell and temporal resolution. The GEMS can facilitate the in vitro screening to find potential protein arginine methyltransferase inhibitors and will contribute to a better understanding of the regulation of ArgMet related to differentiation, development and disease.

Metabolic FRET sensors in intact organs: Applying spectral unmixing to acquire reliable signals

In multicellular organisms, metabolic coordination across multiple tissues and cell types is essential to satisfy regionalized energetic requirements and respond coherently to changing environmental conditions. However, most metabolic assays require the destruction of the biological sample, with a concomitant loss of spatial information. Fluorescent metabolic sensors and probes are among the most user-friendly techniques for collecting metabolic information with spatial resolution. In a previous work, we have adapted to an animal system, Drosophila melanogaster, genetically encoded metabolic FRET-based sensors that had been previously developed in single-cell systems. These sensors provide semi-quantitative data on the stationary concentrations of key metabolites of the bioenergetic metabolism: lactate, pyruvate, and 2-oxoglutarate. The use of these sensors in intact organs required the development of an image processing method that minimizes the contribution of spatially complex autofluorescence patterns, that would obscure the FRET signals. In this article, we show step by step how to design FRET-based sensor experiments and how to process the fluorescence signal to obtain reliable FRET values.

Gray and white matter astrocytes differ in basal metabolism but respond similarly to neuronal activity

Astrocytes are a heterogeneous population of glial cells in the brain, which adapt their properties to the requirements of the local environment. Two major groups of astrocytes are protoplasmic astrocytes residing in gray matter as well as fibrous astrocytes of white matter. Here, we compared the energy metabolism of astrocytes in the cortex and corpus callosum as representative gray matter and white matter regions, in acute brain slices taking advantage of genetically encoded fluorescent nanosensors for the NADH/NAD⁺ redox ratio and for ATP. Astrocytes of the corpus callosum presented a more reduced basal NADH/NAD⁺ redox ratio, and a lower cytosolic concentration of ATP compared to cortical astrocytes. In cortical astrocytes, the neurotransmitter glutamate and increased extracellular concentrations of K⁺ , typical correlates of neuronal activity, induced a more reduced NADH/NAD⁺ redox ratio. While application of glutamate decreased [ATP], K⁺ as well as the combination of glutamate and K⁺ resulted in an increase of ATP levels. Strikingly, a very similar regulation of metabolism by K⁺ and glutamate was observed in astrocytes in the corpus callosum. Finally, strong intrinsic neuronal activity provoked by application of bicuculline and withdrawal of Mg²⁺ caused a shift of the NADH/NAD⁺ redox ratio to a more reduced state as well as a slight reduction of [ATP] in gray and white matter astrocytes. In summary, the metabolism of astrocytes in cortex and corpus callosum shows distinct basal properties, but qualitatively similar responses to neuronal activity, probably reflecting the different environment and requirements of these brain regions.

Mass spectrometry-based strategies for single-cell metabolomics

Single cell analysis has drawn increasing interest from the research community due to its capability to interrogate cellular heterogeneity, allowing refined tissue classification and facilitating novel biomarker discovery. With the advancement of relevant instruments and techniques, it is now possible to perform multiple omics including genomics, transcriptomics, metabolomics or even proteomics at single cell level. In comparison with other omics studies, single-cell metabolomics (SCM) represents a significant challenge since it involves many types of dynamically changing compounds with a wide range of concentrations. In addition, metabolites cannot be amplified. Although difficult, considerable progress has been made over the past decade in mass spectrometry (MS)-based SCM in terms of processing technologies and biochemical applications. In this review, we will summarize recent progress in the development of promising MS platforms, sample preparation methods and SCM analysis of various cell types (including plant cell, cancer cell, neuron, embryo cell, and yeast cell). Current limitations and future research directions in the field of SCM will also be discussed.

Single-Fluorophore Indicator to Explore Cellular and Sub-cellular Lactate Dynamics

Lactate is an energy substrate and an intercellular signal, which can be monitored in intact cells with the genetically encoded FRET indicator Laconic. However, the structural complexity, need for sophisticated equipment, and relatively small fluorescent change limit the use of FRET indicators for subcellular targeting and development of high-throughput screening methodologies. Using the bacterial periplasmic binding protein TTHA0766 from Thermus thermophilus, we have now developed a single-fluorophore indicator for lactate, CanlonicSF. This indicator exhibits a maximal fluorescence change of 200% and a K_D of ∼300 μM. The fluorescence is not affected by other monocarboxylates. The lactate indicator was not significantly affected by Ca²⁺ at the physiological concentrations prevailing in the cytosol, endoplasmic reticulum, and extracellular space, but was affected by Ca²⁺ in the low micromolar range. Targeting the indicator to the endoplasmic reticulum revealed for the first time sub-cellular lactate dynamics. Its improved lactate-induced fluorescence response permitted the development of a multiwell plate assay to screen for inhibitors of the monocarboxylate transporters MCTs, a pharmaceutical target for cancer and inflammation. The functionality of the indicator in living tissue was demonstrated in the brain of Drosophila melanogaster larvae. CanlonicSF is well suited to explore lactate dynamics with sub-cellular resolution in intact systems.

Review on data analysis methods for mesoscale neural imaging in vivo

Significance: Mesoscale neural imaging in vivo has gained extreme popularity in neuroscience for its capacity of recording large-scale neurons in action. Optical imaging with single-cell resolution and millimeter-level field of view in vivo has been providing an accumulated database of neuron-behavior correspondence. Meanwhile, optical detection of neuron signals is easily contaminated by noises, background, crosstalk, and motion artifacts, while neural-level signal processing and network-level coordinate are extremely complicated, leading to laborious and challenging signal processing demands. The existing data analysis procedure remains unstandardized, which could be daunting to neophytes or neuroscientists without computational background. Aim: We hope to provide a general data analysis pipeline of mesoscale neural imaging shared between imaging modalities and systems. Approach: We divide the pipeline into two main stages. The first stage focuses on extracting high-fidelity neural responses at single-cell level from raw images, including motion registration, image denoising, neuron segmentation, and signal extraction. The second stage focuses on data mining, including neural functional mapping, clustering, and brain-wide network deduction. Results: Here, we introduce the general pipeline of processing the mesoscale neural images. We explain the principles of these procedures and compare different approaches and their application scopes with detailed discussions about the shortcomings and remaining challenges. Conclusions: There are great challenges and opportunities brought by the large-scale mesoscale data, such as the balance between fidelity and efficiency, increasing computational load, and neural network interpretability. We believe that global circuits on single-neuron level will be more extensively explored in the future.

A perspective on astrocyte regulation of neural circuit function and animal behavior

Studies over the past two decades have demonstrated that astrocytes are tightly associated with neurons and play pivotal roles in neural circuit development, operation, and adaptation in health and disease. Nevertheless, precisely how astrocytes integrate diverse neuronal signals, modulate neural circuit structure and function at multiple temporal and spatial scales, and influence animal behavior or disease through aberrant excitation and molecular output remains unclear. This Perspective discusses how new and state-of-the-art approaches, including fluorescence indicators, opto- and chemogenetic actuators, genetic targeting tools, quantitative behavioral assays, and computational methods, might help resolve these longstanding questions. It also addresses complicating factors in interpreting astrocytes’ role in neural circuit regulation and animal behavior, such as their heterogeneity, metabolism, and inter-glial communication. Research on these questions should provide a deeper mechanistic understanding of astrocyte-neuron assemblies’ role in neural circuit function, complex behaviors, and disease.

Visualizing physiological parameters in cells and tissues using genetically encoded indicators for metabolites

The study of metabolism is undergoing a renaissance. Since the year 2002, over 50 genetically-encoded fluorescent indicators (GEFIs) have been introduced, capable of monitoring metabolites with high spatial/temporal resolution using fluorescence microscopy. Indicators are fusion proteins that change their fluorescence upon binding a specific metabolite. There are indicators for sugars, monocarboxylates, Krebs cycle intermediates, amino acids, cofactors, and energy nucleotides. They permit monitoring relative levels, concentrations, and fluxes in living systems. At a minimum they report relative levels and, in some cases, absolute concentrations may be obtained by performing ad hoc calibration protocols. Proper data collection, processing, and interpretation are critical to take full advantage of these new tools. This review offers a survey of the metabolic indicators that have been validated in mammalian systems. Minimally invasive, these indicators have been instrumental for the purposes of confirmation, rebuttal and discovery. We envision that this powerful technology will foster metabolic physiology.

Genetically Encoded Sensors to Study Metabolism in Drosophila

The rather recent development of genetically encoded metabolite sensors has changed the way we can study metabolism in living cells, ex vivo tissues, and in vivo immensely. In recent years, these sensors have also been adapted for use in Drosophila tissues. Here, we describe a standard protocol to image such sensors in ex vivo Drosophila larval brains using the glucose sensor FLII12Pglu-700μδ6. The protocol, however, can be adapted for the use of other sensors, tissues, and can even be used in vivo.

Astrocyte-neuron metabolic cooperation shapes brain activity

The brain has almost no energy reserve, but its activity coordinates organismal function, a burden that requires precise coupling between neurotransmission and energy metabolism. Deciphering how the brain accomplishes this complex task is crucial to understand central facets of human physiology and disease mechanisms. Each type of neural cell displays a peculiar metabolic signature, forcing the intercellular exchange of metabolites that serve as both energy precursors and paracrine signals. The paradigm of this biological feature is the astrocyte-neuron couple, in which the glycolytic metabolism of astrocytes contrasts with the mitochondrial oxidative activity of neurons. Astrocytes generate abundant mitochondrial reactive oxygen species and shuttle to neurons glycolytically derived metabolites, such as L-lactate and L-serine, which sustain energy needs, conserve redox status, and modulate neurotransmitter-receptor activity. Conversely, early disruption of this metabolic cooperation may contribute to the initiation or progression of several neurological diseases, thus requiring innovative therapies to preserve brain energetics.

How Energy Supports Our Brain to Yield Consciousness: Insights From Neuroimaging Based on the Neuroenergetics Hypothesis

Consciousness is considered a result of specific neuronal processes and mechanisms in the brain. Various suggested neuronal mechanisms, including the information integration theory (IIT), global neuronal workspace theory (GNWS), and neuronal construction of time and space as in the context of the temporospatial theory of consciousness (TTC), have been laid forth. However, despite their focus on different neuronal mechanisms, these theories neglect the energetic-metabolic basis of the neuronal mechanisms that are supposed to yield consciousness. Based on the findings of physiology-induced (sleep), pharmacology-induced (general anesthesia), and pathology-induced [vegetative state/unresponsive wakeful syndrome (VS/UWS)] loss of consciousness in both human subjects and animals, we, in this study, suggest that the energetic-metabolic processes focusing on ATP, glucose, and γ-aminobutyrate/glutamate are indispensable for functional connectivity (FC) of normal brain networks that renders consciousness possible. Therefore, we describe the energetic-metabolic predispositions of consciousness (EPC) that complement the current theories focused on the neural correlates of consciousness (NCC).

Bidirectional astrocytic GLUT1 activation by elevated extracellular K. Glia 2020;69:1012-1021. [PMID: 33277953 DOI: 10.1002/glia.23944]

The acute rise in interstitial K⁺ that accompanies neural activity couples the energy demand of neurons to the metabolism of astrocytes. The effects of elevated K⁺ on astrocytes include activation of aerobic glycolysis, inhibition of mitochondrial respiration and the release of lactate. Using a genetically encoded FRET glucose sensor and a novel protocol based on 3-O-methylglucose trans-acceleration and numerical simulation of glucose dynamics, we report that extracellular K⁺ is also a potent and reversible modulator of the astrocytic glucose transporter GLUT1. In cultured mouse astrocytes, the stimulatory effect developed within seconds, engaged both the influx and efflux modes of the transporter, and was detected even at 1 mM incremental K⁺ . The modulation of GLUT1 explains how astrocytes are able to maintain their glucose pool in the face of strong glycolysis stimulation. We propose that the stimulation of GLUT1 by K⁺ supports the production of lactate by astrocytes and the timely delivery of glucose to active neurons.

A Dual Nanosensor Approach to Determine the Cytosolic Concentration of ATP in Astrocytes

Adenosine triphosphate (ATP) is the central energy carrier of all cells and knowledge on the dynamics of the concentration of ATP ([ATP]) provides important insights into the energetic state of a cell. Several genetically encoded fluorescent nanosensors for ATP were developed, which allow following the cytosolic [ATP] at high spatial and temporal resolution using fluorescence microscopy. However, to calibrate the fluorescent signal to [ATP] has remained challenging. To estimate basal cytosolic [ATP] ([ATP]₀) in astrocytes, we here took advantage of two ATP nanosensors of the ATeam-family (ATeam1.03; ATeam1.03YEMK) with different affinities for ATP. Altering [ATP] by external stimuli resulted in characteristic pairs of signal changes of both nanosensors, which depend on [ATP]₀. Using this dual nanosensor strategy and epifluorescence microscopy, [ATP]₀ was estimated to be around 1.5 mM in primary cultures of cortical astrocytes from mice. Furthermore, in astrocytes in acutely isolated cortical slices from mice expressing both nanosensors after stereotactic injection of AAV-vectors, 2-photon microscopy revealed [ATP]₀ of 0.7 mM to 1.3 mM. Finally, the change in [ATP] induced in the cytosol of cultured cortical astrocytes by application of azide, glutamate, and an increased extracellular concentration of K⁺ were calculated as −0.50 mM, −0.16 mM, and 0.07 mM, respectively. In summary, the dual nanosensor approach adds another option for determining the concentration of [ATP] to the increasing toolbox of fluorescent nanosensors for metabolites. This approach can also be applied to other metabolites when two sensors with different binding properties are available.

Fluorescent Biosensors for Neuronal Metabolism and the Challenges of Quantitation

Over the past decade, genetically encoded fluorescent biosensors that report metabolic changes have become valuable tools for understanding brain metabolism. These sensors have been targeted to specific brain regions and cell types in different organisms to track multiple metabolic processes at single cell (and subcellular) resolution. Here, we review genetically encoded biosensors used to study metabolism in the brain. We particularly focus on the principles needed to use these sensors quantitatively while avoiding false inferences from variations in sensor fluorescence that arise from differences in expression level or environmental influences such as pH or temperature.

Imaging of the Lactate/Pyruvate Ratio Using a Genetically Encoded Förster Resonance Energy Transfer Indicator

The ratio between the cytosolic concentrations of lactate and pyruvate is a direct readout of the balance between glycolysis and mitochondrial oxidative metabolism. Current approaches do not allow detection of the lactate/pyruvate ratio in a single readout with high spatial/temporal resolution in living systems. Using a Förster resonance energy transfer (FRET)-based screening strategy, we found that the orphan transcriptional factor LutR from Bacillus licheniformis is an endogenous sensor of the lactate/pyruvate ratio, suitable for use as a binding moiety to develop a lactate/pyruvate ratio FRET-based genetically encoded indicator, Lapronic. The sensitivity of the indicator to lactate and pyruvate was characterized through changes in the fluorescence FRET ratio and validated with isothermal titration calorimetry. Lapronic was insensitive to physiological pH and temperature and did not respond to structurally related molecules acetate and β-hydroxybutyrate or cofactors NAD⁺ and NADH. Lapronic was expressed in HEK 293 cells showing a homogeneous cytosolic localization and was also targeted to the mitochondrial matrix. A calibration protocol was designed to quantitatively assess the lactate/pyruvate ratio in intact mammalian cells. Purified protein from Escherichia coli showed robust stability over time and was found suitable for lactate/pyruvate ratio detection in biological samples. We envision that Lapronic will be of practical interest for basic and applied research.

A highly responsive pyruvate sensor reveals pathway-regulatory role of the mitochondrial pyruvate carrier MPC

Mitochondria generate ATP and building blocks for cell growth and regeneration, using pyruvate as the main substrate. Here we introduce PyronicSF, a user-friendly GFP-based sensor of improved dynamic range that enables real-time subcellular quantitation of mitochondrial pyruvate transport, concentration and flux. We report that cultured mouse astrocytes maintain mitochondrial pyruvate in the low micromolar range, below cytosolic pyruvate, which means that the mitochondrial pyruvate carrier MPC is poised to exert ultrasensitive control on the balance between respiration and anaplerosis/gluconeogenesis. The functionality of the sensor in living tissue is demonstrated in the brain of Drosophila melanogaster larvae. Mitochondrial subpopulations are known to coexist within a given cell, which differ in their morphology, mobility, membrane potential, and vicinity to other organelles. The present tool can be used to investigate how mitochondrial diversity relates to metabolism, to study the role of MPC in disease, and to screen for small-molecule MPC modulators.

MitoToxy assay: A novel cell-based method for the assessment of metabolic toxicity in a multiwell plate format using a lactate FRET nanosensor, Laconic

Mitochondrial toxicity is a primary source of pre-clinical drug attrition, black box warning and post-market drug withdrawal. Methods that detect mitochondrial toxicity as early as possible during the drug development process are required. Here we introduce a new method for detecting mitochondrial toxicity based on MDA-MB-231 cells stably expressing the genetically encoded FRET lactate indicator, Laconic. The method takes advantage of the high cytosolic lactate accumulation observed during mitochondrial stress, regardless of the specific toxicity mechanism, explained by compensatory glycolytic activation. Using a standard multi-well plate reader, dose-response curve experiments allowed the sensitivity of the methodology to detect metabolic toxicity induced by classical mitochondrial toxicants. Suitability for high-throughput screening applications was evaluated resulting in a Z’-factor > 0.5 and CV% < 20 inter-assay variability. A pilot screening allowed sensitive detection of commercial drugs that were previously withdrawn from the market due to liver/cardiac toxicity issues, such as camptothecin, ciglitazone, troglitazone, rosiglitazone, and terfenadine, in ten minutes. We envisage that the availability of this technology, based on a fluorescent genetically encoded indicator, will allow direct assessment of mitochondrial metabolism, and will make the early detection of mitochondrial toxicity in the drug development process possible, saving time and resources.

Transcriptome Analysis Identifies Multifaceted Regulatory Mechanisms Dictating a Genetic Switch from Neuronal Network Establishment to Maintenance During Postnatal Prefrontal Cortex Development

The prefrontal cortex (PFC) is one of the latest brain regions to mature, which allows the acquisition of complex cognitive abilities through experience. To unravel the underlying gene expression changes during postnatal development, we performed RNA-sequencing (RNA-seq) in the rat medial PFC (mPFC) at five developmental time points from infancy to adulthood, and analyzed the differential expression of protein-coding genes, long intergenic noncoding RNAs (lincRNAs), and alternative exons. We showed that most expression changes occur in infancy, and that the number of differentially expressed genes reduces toward adulthood. We observed 137 differentially expressed lincRNAs and 796 genes showing alternative exon usage during postnatal development. Importantly, we detected a genetic switch from neuronal network establishment in infancy to maintenance of neural networks in adulthood based on gene expression dynamics, involving changes in protein-coding and lincRNA gene expression as well as alternative exon usage. Our gene expression datasets provide insights into the multifaceted transcriptional regulation of the developing PFC. They can be used to study the basic developmental processes of the mPFC and to understand the mechanisms of neurodevelopmental and neuropsychiatric disorders. Our study provides an important contribution to the ongoing efforts to complete the "brain map", and to the understanding of PFC development.

Tight coupling of astrocyte energy metabolism to synaptic activity revealed by genetically encoded FRET nanosensors in hippocampal tissue

The potassium ion, K+, a neuronal signal that is released during excitatory synaptic activity, produces acute activation of glucose consumption in cultured astrocytes, a phenomenon mediated by the sodium bicarbonate cotransporter NBCe1 ( SLC4A4). We have explored here the relevance of this mechanism in brain tissue by imaging the effect of neuronal activity on pH, glucose, pyruvate and lactate dynamics in hippocampal astrocytes using BCECF and FRET nanosensors. Electrical stimulation of Schaffer collaterals produced fast activation of glucose consumption in astrocytes with a parallel increase in intracellular pyruvate and biphasic changes in lactate . These responses were blocked by TTX and were absent in tissue slices prepared from NBCe1-KO mice. Direct depolarization of astrocytes with elevated extracellular K+ or Ba2+ mimicked the metabolic effects of electrical stimulation. We conclude that the glycolytic pathway of astrocytes in situ is acutely sensitive to neuronal activity, and that extracellular K+ and the NBCe1 cotransporter are involved in metabolic crosstalk between neurons and astrocytes. Glycolytic activation of astrocytes in response to neuronal K+ helps to provide an adequate supply of lactate, a metabolite that is released by astrocytes and which acts as neuronal fuel and an intercellular signal.

Evaluating Cell Metabolism Through Autofluorescence Imaging of NAD(P)H and FAD

SIGNIFICANCE

Optical imaging using the endogenous fluorescence of metabolic cofactors has enabled nondestructive examination of dynamic changes in cell and tissue function both in vitro and in vivo. Quantifying NAD(P)H and FAD fluorescence through an optical redox ratio and fluorescence lifetime imaging (FLIM) provides sensitivity to the relative balance between oxidative phosphorylation and glucose catabolism. Since its introduction decades ago, the use of NAD(P)H imaging has expanded to include applications involving almost every major tissue type and a variety of pathologies. Recent Advances: This review focuses on the use of two-photon excited fluorescence and NAD(P)H fluorescence lifetime techniques in cancer, neuroscience, tissue engineering, and other biomedical applications over the last 5 years. In a variety of cancer models, NAD(P)H fluorescence intensity and lifetime measurements demonstrate a sensitivity to the Warburg effect, suggesting potential for early detection or high-throughput drug screening. The sensitivity to the biosynthetic demands of stem cell differentiation and tissue repair processes indicates the range of applications for this imaging technology may be broad.

CRITICAL ISSUES

As the number of applications for these fluorescence imaging techniques expand, identifying and characterizing additional intrinsic fluorophores and chromophores present in vivo will be vital to accurately measure and interpret metabolic outcomes. Understanding the full capabilities and limitations of FLIM will also be key to future advances.

FUTURE DIRECTIONS

Future work is needed to evaluate whether a combination of different biochemical and structural outcomes using these imaging techniques can provide complementary information regarding the utilization of specific metabolic pathways.

Interrogating metabolism as an electron flow system

Metabolism is generally considered as a neatly organised system of modular pathways, shaped by evolution under selection for optimal cellular growth. This view falls short of explaining and predicting a number of key observations about the structure and dynamics of metabolism. We highlight these limitations of a pathway-centric view on metabolism and summarise studies suggesting how these could be overcome by viewing metabolism as a thermodynamically and kinetically constrained, dynamical flow system. Such a systems-level, first-principles based view of metabolism can open up new avenues of metabolic engineering and cures for metabolic diseases and allow better insights to a myriad of physiological processes that are ultimately linked to metabolism. Towards further developing this view, we call for a closer interaction among physical and biological disciplines and an increased use of electrochemical and biophysical approaches to interrogate cellular metabolism together with the microenvironment in which it exists.

FRET-based imaging of intracellular ATP in organotypic brain slices

Active neurons require a substantial amount of adenosine triphosphate (ATP) to re-establish ion gradients degraded by ion flux across their plasma membranes. Despite this fact, neurons, in contrast to astrocytes, do not contain any significant stores of energy substrates. Recent work has provided evidence for a neuro-metabolic coupling between both cell types, in which increased glycolysis and lactate production in astrocytes support neuronal metabolism. Here, we established the cell type-specific expression of the Förster resonance energy transfer (FRET) based nanosensor ATeam1.03^YEMK ("Ateam") for dynamic measurement of changes in intracellular ATP levels in organotypic brain tissue slices. To this end, adeno-associated viral vectors coding for Ateam, driven by either the synapsin- or glial fibrillary acidic protein (GFAP) promoter were employed for specific transduction of neurons or astrocytes, respectively. Chemical ischemia, induced by perfusion of tissue slices with metabolic inhibitors of cellular glycolysis and mitochondrial respiration, resulted in a rapid decrease in the cellular Ateam signal to a new, low level, indicating nominal depletion of intracellular ATP. Increasing the extracellular potassium concentration to 8 mM, thereby mimicking the release of potassium from active neurons, did not alter ATP levels in neurons. It, however, caused in an increase in ATP levels in astrocytes, a result which was confirmed in acutely isolated tissue slices. In summary, our results demonstrate that organotypic cultured slices are a reliable tool for FRET-based dynamic imaging of ATP in neurons and astrocytes. They moreover provide evidence for an increased ATP synthesis in astrocytes, but not neurons, during periods of elevated extracellular potassium concentrations.

Intravitreal AAV-Delivery of Genetically Encoded Sensors Enabling Simultaneous Two-Photon Imaging and Electrophysiology of Optic Nerve Axons

Myelination of axons by oligodendrocytes is a key feature of the remarkably fast operating CNS. Oligodendrocytes not only tune axonal conduction speed but are also suggested to maintain long-term axonal integrity by providing metabolic support to the axons they ensheath. However, how myelinating oligodendrocytes impact axonal energy homeostasis remains poorly understood and difficult to investigate. Here, we provide a method of how to study electrically active myelinated axons expressing genetically encoded sensors by combining electrophysiology and two-photon imaging of acutely isolated optic nerves. We show that intravitreal adeno-associated viral (AAV) vector delivery is an efficient tool to achieve functional sensor expression in optic nerve axons, which is demonstrated by measuring axonal ATP dynamics following AAV-mediated sensor expression. This novel approach allows for fast expression of any optical sensor of interest to be studied in optic nerve axons without the need to go through the laborious process of producing new transgenic mouse lines. Viral-mediated biosensor expression in myelinated axons and the subsequent combination of nerve recordings and sensor imaging outlines a powerful method to investigate oligodendroglial support functions and to further interrogate cellular mechanisms governing axonal energy homeostasis under physiological and pathological conditions.

Revisiting the role of metabolism during development

An emerging view emphasizes that metabolism is highly regulated in both time and space. In addition, it is increasingly being recognized that metabolic pathways are tightly connected to specific biological processes such as cell signaling, proliferation and differentiation. As we obtain a better view of this spatiotemporal regulation of metabolism, and of the molecular mechanisms that connect metabolism and signaling, we can now move from largely correlative to more functional studies. It is, therefore, a particularly promising time to revisit how metabolism can affect multiple aspects of animal development. In this Review, we discuss how metabolism is mechanistically linked to cellular and developmental programs through both its bioenergetic and metabolic signaling functions. We highlight how metabolism is regulated across various spatial and temporal scales, and discuss how this regulation can influence cellular processes such as cell signaling, gene expression, and epigenetic and post-translational modifications during embryonic development.

Current technical approaches to brain energy metabolism

Neuroscience is a technology-driven discipline and brain energy metabolism is no exception. Once satisfied with mapping metabolic pathways at organ level, we are now looking to learn what it is exactly that metabolic enzymes and transporters do and when, where do they reside, how are they regulated, and how do they relate to the specific functions of neurons, glial cells, and their subcellular domains and organelles, in different areas of the brain. Moreover, we aim to quantify the fluxes of metabolites within and between cells. Energy metabolism is not just a necessity for proper cell function and viability but plays specific roles in higher brain functions such as memory processing and behavior, whose mechanisms need to be understood at all hierarchical levels, from isolated proteins to whole subjects, in both health and disease. To this aim, the field takes advantage of diverse disciplines including anatomy, histology, physiology, biochemistry, bioenergetics, cellular biology, molecular biology, developmental biology, neurology, and mathematical modeling. This article presents a well-referenced synopsis of the technical side of brain energy metabolism research. Detail and jargon are avoided whenever possible and emphasis is given to comparative strengths, limitations, and weaknesses, information that is often not available in regular articles.

Insights into the HyPer biosensor as molecular tool for monitoring cellular antioxidant capacity

Aerobic metabolism brings inexorably the production of reactive oxygen species (ROS), which are counterbalanced by intrinsic antioxidant defenses avoiding deleterious intracellular effects. Redox balance is the resultant of metabolic functioning under environmental inputs (i.e. diet, pollution) and the activity of intrinsic antioxidant machinery. Monitoring of intracellular hydrogen peroxide has been successfully achieved by redox biosensor advent; however, to track the intrinsic disulfide bond reduction capacity represents a fundamental piece to understand better how redox homeostasis is maintained in living cells.

In the present work, we compared the informative value of steady-state measurements and the kinetics of HyPer, a H₂O₂-sensitive fluorescent biosensor, targeted at the cytosol, mitochondrion and endoplasmic reticulum. From this set of data, biosensor signal recovery from an oxidized state raised as a suitable parameter to discriminate reducing capacity of a close environment. Biosensor recovery was pH-independent, condition demonstrated by experiments on pH-clamped cells, and sensitive to pharmacological perturbations of enzymatic disulfide reduction. Also, ten human cell lines were characterized according their H₂O₂-pulse responses, including their capacity to reduce disulfide bonds evaluated in terms of their migratory capacity.

Finally, cellular migration experiments were conducted to study whether migratory efficiency was associated with the disulfide reduction activity. The migration efficiency of each cell type correlates with the rate of signal recovery measured from the oxidized biosensor. In addition, HyPer-expressing cells treated with N-acetyl-cysteine had accelerated recovery rates and major migratory capacities, both reversible effects upon treatment removal. Our data demonstrate that the HyPer signal recovery offers a novel methodological tool to track the cellular impact of redox active biomolecules.

In vivo two-photon imaging of motoneurons and adjacent glia in the ventral spinal cord

BACKGROUND

Interactions between motoneurons and glial cells are pivotal to regulate and maintain functional states and synaptic connectivity in the spinal cord. In vivo two-photon imaging of the nervous system provided novel and unexpected knowledge about structural and physiological changes in the grey matter of the forebrain and in the dorsal white matter of the spinal cord.

NEW METHOD

Here, we describe a novel experimental strategy to investigate the spinal grey matter, i.e. the ventral horn motoneurons and their adjacent glial cells by employing in vivo two-photon laser-scanning microscopy (2P-LSM) in anesthetized transgenic mice.

RESULTS

After retrograde tracer labelling in transgenic mice with cell-specific expression of fluorescent proteins and surgical exposure of the lumbar intumescence groups of motoneurons could be visualized deeply localized in the ventral horn. In this region, morphological responses of microglial cells to ATP could be recorded for an hour. In addition, using in mice with expression of GCaMP3 in astrocytes, physiological Ca²⁺ signals could be recorded after local noradrenalin application.

COMPARISON WITH EXISTING METHODS

Previous in vivo imaging protocols were restricted to the superficial dorsal white matter or upper layers of the dorsal horn. Here, we modified a multi-step procedure originally established for a root-crush injury. We adapted it to simultaneously visualize motoneurons and adjacent glial cells in living animals.

CONCLUSION

A modified surgery approach is presented to visualize fluorescently labelled motoneurons and glial cells at a depth of more than 200 μm in the grey matter ventral horn of the mouse spinal cord.

Guidelines on experimental methods to assess mitochondrial dysfunction in cellular models of neurodegenerative diseases

Neurodegenerative diseases are a spectrum of chronic, debilitating disorders characterised by the progressive degeneration and death of neurons. Mitochondrial dysfunction has been implicated in most neurodegenerative diseases, but in many instances it is unclear whether such dysfunction is a cause or an effect of the underlying pathology, and whether it represents a viable therapeutic target. It is therefore imperative to utilise and optimise cellular models and experimental techniques appropriate to determine the contribution of mitochondrial dysfunction to neurodegenerative disease phenotypes. In this consensus article, we collate details on and discuss pitfalls of existing experimental approaches to assess mitochondrial function in in vitro cellular models of neurodegenerative diseases, including specific protocols for the measurement of oxygen consumption rate in primary neuron cultures, and single-neuron, time-lapse fluorescence imaging of the mitochondrial membrane potential and mitochondrial NAD(P)H. As part of the Cellular Bioenergetics of Neurodegenerative Diseases (CeBioND) consortium (www.cebiond.org), we are performing cross-disease analyses to identify common and distinct molecular mechanisms involved in mitochondrial bioenergetic dysfunction in cellular models of Alzheimer’s, Parkinson’s, and Huntington’s diseases. Here we provide detailed guidelines and protocols as standardised across the five collaborating laboratories of the CeBioND consortium, with additional contributions from other experts in the field.

In Vivo Imaging of CNS Injury and Disease

In vivo optical imaging has emerged as a powerful tool with which to study cellular responses to injury and disease in the mammalian CNS. Important new insights have emerged regarding axonal degeneration and regeneration, glial responses and neuroinflammation, changes in the neurovascular unit, and, more recently, neural transplantations. Accompanying a 2017 SfN Mini-Symposium, here, we discuss selected recent advances in understanding the neuronal, glial, and other cellular responses to CNS injury and disease with in vivo imaging of the rodent brain or spinal cord. We anticipate that in vivo optical imaging will continue to be at the forefront of breakthrough discoveries of fundamental mechanisms and therapies for CNS injury and disease.

Novel genetically encoded fluorescent probes enable real-time detection of potassium in vitro and in vivo

Changes in intra- and extracellular potassium ion (K⁺) concentrations control many important cellular processes and related biological functions. However, our current understanding of the spatiotemporal patterns of physiological and pathological K⁺ changes is severely limited by the lack of practicable detection methods. We developed K⁺-sensitive genetically encoded, Förster resonance energy transfer-(FRET) based probes, called GEPIIs, which enable quantitative real-time imaging of K⁺ dynamics. GEPIIs as purified biosensors are suitable to directly and precisely quantify K⁺ levels in different body fluids and cell growth media. GEPIIs expressed in cells enable time-lapse and real-time recordings of global and local intracellular K⁺ signals. Hitherto unknown Ca²⁺-triggered, organelle-specific K⁺ changes were detected in pancreatic beta cells. Recombinant GEPIIs also enabled visualization of extracellular K⁺ fluctuations in vivo with 2-photon microscopy. Therefore, GEPIIs are relevant for diverse K⁺ assays and open new avenues for live-cell K⁺ imaging.

K⁺ plays an important role in physiology and disease, but the lack of high specificity K⁺ sensors limits our understanding of its spatiotemporal dynamics. Here the authors develop genetically-encoded FRET-based probes able to quantify K⁺ concentration in body fluids, cells and specific organelles.

Neuronal Culture Microenvironments Determine Preferences in Bioenergetic Pathway Use

In the brain, metabolic supply and demand is directly coupled to neuronal activation. Methods for culturing primary rodent brain cells have come of age and are geared toward sophisticated modeling of human brain physiology and pathology. However, the impact of the culture microenvironment on neuronal function is rarely considered. Therefore, we investigated the role of different neuronal culture supplements for neuronal survival and metabolic activity in a model of metabolic deprivation of neurons using oxygen deprivation, glucose deprivation, as well as live cell metabolic flux analysis. We demonstrate the impact of neuronal culture conditions on metabolic function and neuronal survival under conditions of metabolic stress. In particular, we find that the common neuronal cell culture supplement B27 protects neurons from cell death under hypoxic conditions and inhibits glycolysis. Furthermore, we present data that B27 as well as the alternative neuronal culture supplement N2 restrict neuronal glucose metabolism. On the contrary, we find that the more modern supplement GS21 promotes neuronal energy metabolism. Our data support the notion that careful control of the metabolic environment is an essential component in modeling brain function and the cellular and molecular pathophysiology of brain disease in culture.

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.

Upregulated energy metabolism in the Drosophila mushroom body is the trigger for long-term memory

Efficient energy use has constrained the evolution of nervous systems. However, it is unresolved whether energy metabolism may resultantly regulate major brain functions. Our observation that Drosophila flies double their sucrose intake at an early stage of long-term memory formation initiated the investigation of how energy metabolism intervenes in this process. Cellular-resolution imaging of energy metabolism reveals a concurrent elevation of energy consumption in neurons of the mushroom body, the fly's major memory centre. Strikingly, upregulation of mushroom body energy flux is both necessary and sufficient to drive long-term memory formation. This effect is triggered by a specific pair of dopaminergic neurons afferent to the mushroom bodies, via the D5-like DAMB dopamine receptor. Hence, dopamine signalling mediates an energy switch in the mushroom body that controls long-term memory encoding. Our data thus point to an instructional role for energy flux in the execution of demanding higher brain functions.

Energy consumption in the brain is thought to respond to changes in neuronal activity, without informational role. Here the authors show that increased energy flux in the mushroom body, driven by a pair of input dopaminergic neurons, is a command for the formation of long-term memory in Drosophila.

Nanomolar nitric oxide concentrations quickly and reversibly modulate astrocytic energy metabolism

Nitric oxide (NO) is an intercellular messenger involved in multiple bodily functions. Prolonged NO exposure irreversibly inhibits respiration by covalent modification of mitochondrial cytochrome oxidase, a phenomenon of pathological relevance. However, the speed and potency of NO's metabolic effects at physiological concentrations are incompletely characterized. To this end, we set out to investigate the metabolic effects of NO in cultured astrocytes from mice by taking advantage of the high spatiotemporal resolution afforded by genetically encoded Förster resonance energy transfer (FRET) nanosensors. NO exposure resulted in immediate and reversible intracellular glucose depletion and lactate accumulation. Consistent with cytochrome oxidase involvement, the glycolytic effect was enhanced at a low oxygen level and became irreversible at a high NO concentration or after prolonged exposure. Measurements of both glycolytic rate and mitochondrial pyruvate consumption revealed significant effects even at nanomolar NO concentrations. We conclude that NO can modulate astrocytic energy metabolism in the short term, reversibly, and at concentrations known to be released by endothelial cells under physiological conditions. These findings suggest that NO modulates the size of the astrocytic lactate reservoir involved in neuronal fueling and signaling.

Genetically Encoded Fluorescent Biosensors to Explore AMPK Signaling and Energy Metabolism

Maintenance of energy homeostasis is a basic requirement for cell survival. Different mechanisms have evolved to cope with spatial and temporal mismatch between energy-providing and -consuming processes. Among these, signaling by AMP-activated protein kinase (AMPK) is one of the key players, regulated by and itself regulating cellular adenylate levels. Further understanding its complex cellular function requires deeper insight into its activation patterns in space and time at a single cell level. This may become possible with an increasing number of genetically encoded fluorescent biosensors, mostly based on fluorescence resonance energy transfer, which have been engineered to monitor metabolic parameters and kinase activities. Here, we review basic principles of biosensor design and function and the advantages and limitations of their use and provide an overview on existing FRET biosensors to monitor AMPK activation, ATP concentration, and ATP/ADP ratios, together with other key metabolites and parameters of energy metabolism.

Imaging and spectroscopic approaches to probe brain energy metabolism dysregulation in neurodegenerative diseases

Changes in energy metabolism are generally considered to play an important role in neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's diseases. Whether these changes are causal or simply a part of self-defense mechanisms is a matter of debate. Furthermore, energy defects have often been discussed solely in the context of their probable neuronal origin without considering the cellular heterogeneity of the brain. Recent data point towards the existence of a tri-cellular compartmentation of brain energy metabolism between neurons, astrocytes, and oligodendrocytes, each cell type having a distinctive metabolic profile. Still, the number of methods to follow energy metabolism in patients is extremely limited and existing clinical techniques are blind to most cellular processes. There is a need to better understand how brain energy metabolism is regulated in health and disease through experiments conducted at different scales in animal models to implement new methods in the clinical setting. The purpose of this review is to offer a brief overview of the broad spectrum of methodological approaches that have emerged in recent years to probe energy metabolism in more detail. We conclude that multi-modal neuroimaging is needed to follow non-cell autonomous energy metabolism dysregulation in neurodegenerative diseases.

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.

Mitochondrial dysfunction induces dendritic loss via eIF2α phosphorylation

Mitochondrial dysfunction is associated with neuropathological events, but how it mediates loss of specific neuronal subtypes is unclear. Tsuyama et al. show that mitochondrial dysfunction triggers selective dendritic loss in class IV arborization neurons in a manner dependent on eIF2α phosphorylation and translation inhibition.

Mitochondria are key contributors to the etiology of diseases associated with neuromuscular defects or neurodegeneration. How changes in cellular metabolism specifically impact neuronal intracellular processes and cause neuropathological events is still unclear. We here dissect the molecular mechanism by which mitochondrial dysfunction induced by Prel aberrant function mediates selective dendritic loss in Drosophila melanogaster class IV dendritic arborization neurons. Using in vivo ATP imaging, we found that neuronal cellular ATP levels during development are not correlated with the progression of dendritic loss. We searched for mitochondrial stress signaling pathways that induce dendritic loss and found that mitochondrial dysfunction is associated with increased eIF2α phosphorylation, which is sufficient to induce dendritic pathology in class IV arborization neurons. We also observed that eIF2α phosphorylation mediates dendritic loss when mitochondrial dysfunction results from other genetic perturbations. Furthermore, mitochondrial dysfunction induces translation repression in class IV neurons in an eIF2α phosphorylation-dependent manner, suggesting that differential translation attenuation among neuron subtypes is a determinant of preferential vulnerability.

Targeting of astrocytic glucose metabolism by beta-hydroxybutyrate

The effectiveness of ketogenic diets and intermittent fasting against neurological disorders has brought interest to the effects of ketone bodies on brain cells. These compounds are known to modify the metabolism of neurons, but little is known about their effect on astrocytes, cells that control the supply of glucose to neurons and also modulate neuronal excitability through the glycolytic production of lactate. Here we have used genetically-encoded Förster Resonance Energy Transfer nanosensors for glucose, pyruvate and ATP to characterize astrocytic energy metabolism at cellular resolution. Our results show that the ketone body beta-hydroxybutyrate strongly inhibited astrocytic glucose consumption in mouse astrocytes in mixed cultures, in organotypic hippocampal slices and in acute hippocampal slices prepared from ketotic mice, while blunting the stimulation of glycolysis by physiological and pathophysiological stimuli. The inhibition of glycolysis was paralleled by an increased ability of astrocytic mitochondria to metabolize pyruvate. These results support the emerging notion that astrocytes contribute to the neuroprotective effect of ketone bodies.

Proton Fall or Bicarbonate Rise: GLYCOLYTIC RATE IN MOUSE ASTROCYTES IS PAVED BY INTRACELLULAR ALKALINIZATION

Glycolysis is the primary step for major energy production in the cell. There is strong evidence suggesting that glucose consumption and rate of glycolysis are highly modulated by cytosolic pH/[H(+)], but those can also be stimulated by an increase in the intracellular [HCO3 (-)]. Because proton and bicarbonate shift concomitantly, it remained unclear whether enhanced glucose consumption and glycolytic rate were mediated by the changes in intracellular [H(+)] or [HCO3 (-)]. We have asked whether glucose metabolism is enhanced by either a fall in intracellular [H(+)] or a rise in intracellular [HCO3 (-)], or by both, in mammalian astrocytes. We have recorded intracellular glucose in mouse astrocytes using a FRET-based nanosensor, while imposing different intracellular [H(+)] and [CO2]/[HCO3 (-)]. Glucose consumption and glycolytic rate were augmented by a fall in intracellular [H(+)], irrespective of a concomitant rise or fall in intracellular [HCO3 (-)]. Transport of HCO3 (-) into and out of astrocytes by the electrogenic sodium bicarbonate cotransporter (NBCe1) played a crucial role in causing changes in intracellular pH and [HCO3 (-)], but was not obligatory for the pH-dependent changes in glucose metabolism. Our results clearly show that it is the cytosolic pH that modulates glucose metabolism in cortical astrocytes, and possibly also in other cell types.

Sodium signaling and astrocyte energy metabolism

The Na(+) gradient across the plasma membrane is constantly exploited by astrocytes as a secondary energy source to regulate the intracellular and extracellular milieu, and discard waste products. One of the most prominent roles of astrocytes in the brain is the Na(+) -dependent clearance of glutamate released by neurons during synaptic transmission. The intracellular Na(+) load collectively generated by these processes converges at the Na,K-ATPase pump, responsible for Na(+) extrusion from the cell, which is achieved at the expense of cellular ATP. These processes represent pivotal mechanisms enabling astrocytes to increase the local availability of metabolic substrates in response to neuronal activity. This review presents basic principles linking the intracellular handling of Na(+) following activity-related transmembrane fluxes in astrocytes and the energy metabolic pathways involved. We propose a role of Na(+) as an energy currency and as a mediator of metabolic signals in the context of neuron-glia interactions. We further discuss the possible impact of the astrocytic syncytium for the distribution and coordination of the metabolic response, and the compartmentation of these processes in cellular microdomains and subcellular organelles. Finally, we illustrate future avenues of investigation into signaling mechanisms aimed at bridging the gap between Na(+) and the metabolic machinery. GLIA 2016;64:1667-1676.

Ultrasensitive detection of proteins and sugars at single-cell level

Each cell produces its own responses even if it appears identical to other cells. To analyze these individual cell characteristics, we need to measure trace amounts of molecules in a single cell. Nucleic acids in a single cell can be easily amplified by polymerase chain reaction, but single-cell measurement of proteins and sugars will require de novo techniques. In the present study, we outline the techniques we have developed toward this end. For proteins, our ultrasensitive enzyme-linked immunosorbent assay (ELISA) coupled with thionicotinamide-adenine dinucleotide cycling can detect proteins at subattomoles per assay. For sugars, fluorescence correlation spectroscopy coupled with glucose oxidase-catalyzed reaction allows us to measure glucose at tens of nM. Our methods thus offer versatile techniques for single-cell-level analyses, and they are hoped to strongly promote single-cell biology as well as to develop noninvasive tests in clinical medicine.

High-resolution in vivo optical imaging of stroke injury and repair

Central nervous system (CNS) function and dysfunction are best understood within a framework of interactions between neuronal, glial and vascular compartments comprising the neurovascular unit (NVU), all of which contribute to stroke-induced CNS injury, plasticity, repair, and recovery. Recent advances in in vivo optical microscopy have enabled us to observe and interrogate cells and their processes with high spatial resolution in real time and in their natural environment deep in the brain tissue. Here, we review some of these state-of-the-art imaging techniques with an emphasis on imaging the interactions among the constituents of the NVU during ischemic injury and repair in small animal models. This article is part of a Special Issue entitled SI: Cell Interactions In Stroke.

NH4(+) triggers the release of astrocytic lactate via mitochondrial pyruvate shunting

Neural activity is accompanied by a transient mismatch between local glucose and oxygen metabolism, a phenomenon of physiological and pathophysiological importance termed aerobic glycolysis. Previous studies have proposed glutamate and K(+) as the neuronal signals that trigger aerobic glycolysis in astrocytes. Here we used a panel of genetically encoded FRET sensors in vitro and in vivo to investigate the participation of NH4(+), a by-product of catabolism that is also released by active neurons. Astrocytes in mixed cortical cultures responded to physiological levels of NH4(+) with an acute rise in cytosolic lactate followed by lactate release into the extracellular space, as detected by a lactate-sniffer. An acute increase in astrocytic lactate was also observed in acute hippocampal slices exposed to NH4(+) and in the somatosensory cortex of anesthetized mice in response to i.v. NH4(+). Unexpectedly, NH4(+) had no effect on astrocytic glucose consumption. Parallel measurements showed simultaneous cytosolic pyruvate accumulation and NADH depletion, suggesting the involvement of mitochondria. An inhibitor-stop technique confirmed a strong inhibition of mitochondrial pyruvate uptake that can be explained by mitochondrial matrix acidification. These results show that physiological NH4(+) diverts the flux of pyruvate from mitochondria to lactate production and release. Considering that NH4(+) is produced stoichiometrically with glutamate during excitatory neurotransmission, we propose that NH4(+) behaves as an intercellular signal and that pyruvate shunting contributes to aerobic lactate production by astrocytes.

Channel-mediated lactate release by K⁺-stimulated astrocytes

Excitatory synaptic transmission is accompanied by a local surge in interstitial lactate that occurs despite adequate oxygen availability, a puzzling phenomenon termed aerobic glycolysis. In addition to its role as an energy substrate, recent studies have shown that lactate modulates neuronal excitability acting through various targets, including NMDA receptors and G-protein-coupled receptors specific for lactate, but little is known about the cellular and molecular mechanisms responsible for the increase in interstitial lactate. Using a panel of genetically encoded fluorescence nanosensors for energy metabolites, we show here that mouse astrocytes in culture, in cortical slices, and in vivo maintain a steady-state reservoir of lactate. The reservoir was released to the extracellular space immediately after exposure of astrocytes to a physiological rise in extracellular K(+) or cell depolarization. Cell-attached patch-clamp analysis of cultured astrocytes revealed a 37 pS lactate-permeable ion channel activated by cell depolarization. The channel was modulated by lactate itself, resulting in a positive feedback loop for lactate release. A rapid fall in intracellular lactate levels was also observed in cortical astrocytes of anesthetized mice in response to local field stimulation. The existence of an astrocytic lactate reservoir and its quick mobilization via an ion channel in response to a neuronal cue provides fresh support to lactate roles in neuronal fueling and in gliotransmission.

The Astrocyte: Powerhouse and Recycling Center

Brain metabolism is characterized by fuel monodependence, high-energy expenditure, autonomy from the rest of body, local recycling, and marked division of labor between cell types. Although neurons spend most of the brain's energy on signaling, astrocytes bear the brunt of the metabolic load, controlling the composition of the interstitial fluid, supplying neurons with energy substrates and precursors for biosynthesis, and recycling neurotransmitters, oxidized scavengers, and other waste products. Outstanding questions in this field are the role of oligodendrocytes, the metabolic behavior of the different subtypes of astrocytes during development and disease, and the emerging notion that metabolism may participate directly in information processing.