The Astrocyte: Metabolic Hub of the Brain

Astrocytic metabolism has taken center stage. Interposed between the neuron and the vasculature, astrocytes exert control over the fluxes of energy and building blocks required for neuronal activity and plasticity. They are also key to local detoxification and waste recycling. Whereas neurons are metabolically rigid, astrocytes can switch between different metabolic profiles according to local demand and the nutritional state of the organism. Their metabolic state even seems to be instructive for peripheral nutrient mobilization and has been implicated in information processing and behavior. Here, we summarize recent progress in our understanding of astrocytic metabolism and its effects on metabolic homeostasis and cognition.

Oligodendrocyte-axon metabolic coupling is mediated by extracellular K+ and maintains axonal health

The integrity of myelinated axons relies on homeostatic support from oligodendrocytes (OLs). To determine how OLs detect axonal spiking and how rapid axon-OL metabolic coupling is regulated in the white matter, we studied activity-dependent calcium (Ca2+) and metabolite fluxes in the mouse optic nerve. We show that fast axonal spiking triggers Ca2+ signaling and glycolysis in OLs. OLs detect axonal activity through increases in extracellular potassium (K+) concentrations and activation of Kir4.1 channels, thereby regulating metabolite supply to axons. Both pharmacological inhibition and OL-specific inactivation of Kir4.1 reduce the activity-induced axonal lactate surge. Mice lacking oligodendroglial Kir4.1 exhibit lower resting lactate levels and altered glucose metabolism in axons. These early deficits in axonal energy metabolism are associated with late-onset axonopathy. Our findings reveal that OLs detect fast axonal spiking through K+ signaling, making acute metabolic coupling possible and adjusting the axon-OL metabolic unit to promote axonal health.

Enlightening brain energy metabolism

Brain tissue metabolism is distributed across several cell types and subcellular compartments, which activate at different times and with different temporal patterns. The introduction of genetically-encoded fluorescent indicators that are imaged using time-lapse microscopy has opened the possibility of studying brain metabolism at cellular and sub-cellular levels. There are indicators for sugars, monocarboxylates, Krebs cycle intermediates, amino acids, cofactors, and energy nucleotides, which inform about relative levels, concentrations and fluxes. This review offers a brief survey of the metabolic indicators that have been validated in brain cells, with some illustrative examples from the literature. Whereas only a small fraction of the metabolome is currently accessible to fluorescent probes, there are grounds to be optimistic about coming developments and the application of these tools to the study of brain disease.

Metabolic switch in the aging astrocyte supported via integrative approach comprising network and transcriptome analyses

Dysregulated central-energy metabolism is a hallmark of brain aging. Supplying enough energy for neurotransmission relies on the neuron-astrocyte metabolic network. To identify genes contributing to age-associated brain functional decline, we formulated an approach to analyze the metabolic network by integrating flux, network structure and transcriptomic databases of neurotransmission and aging. Our findings support that during brain aging: (1) The astrocyte undergoes a metabolic switch from aerobic glycolysis to oxidative phosphorylation, decreasing lactate supply to the neuron, while the neuron suffers intrinsic energetic deficit by downregulation of Krebs cycle genes, including mdh1 and mdh2 (Malate-Aspartate Shuttle); (2) Branched-chain amino acid degradation genes were downregulated, identifying dld as a central regulator; (3) Ketone body synthesis increases in the neuron, while the astrocyte increases their utilization, in line with neuronal energy deficit in favor of astrocytes. We identified candidates for preclinical studies targeting energy metabolism to prevent age-associated cognitive decline.

Deficits in mitochondrial TCA cycle and OXPHOS precede rod photoreceptor degeneration during chronic HIF activation

BACKGROUND

Major retinal degenerative diseases, including age-related macular degeneration, diabetic retinopathy and retinal detachment, are associated with a local decrease in oxygen availability causing the formation of hypoxic areas affecting the photoreceptor (PR) cells. Here, we addressed the underlying pathological mechanisms of PR degeneration by focusing on energy metabolism during chronic activation of hypoxia-inducible factors (HIFs) in rod PR.

METHODS

We used two-photon laser scanning microscopy (TPLSM) of genetically encoded biosensors delivered by adeno-associated viruses (AAV) to determine lactate and glucose dynamics in PR and inner retinal cells. Retinal layer-specific proteomics, in situ enzymatic assays and immunofluorescence studies were used to analyse mitochondrial metabolism in rod PRs during chronic HIF activation.

RESULTS

PRs exhibited remarkably higher glycolytic flux through the hexokinases than neurons of the inner retina. Chronic HIF activation in rods did not cause overt change in glucose dynamics but an increase in lactate production nonetheless. Furthermore, dysregulation of the oxidative phosphorylation pathway (OXPHOS) and tricarboxylic acid (TCA) cycle in rods with an activated hypoxic response decelerated cellular anabolism causing shortening of rod photoreceptor outer segments (OS) before onset of cell degeneration. Interestingly, rods with deficient OXPHOS but an intact TCA cycle did not exhibit these early signs of anabolic dysregulation and showed a slower course of degeneration.

CONCLUSION

Together, these data indicate an exceeding high glycolytic flux in rods and highlight the importance of mitochondrial metabolism and especially of the TCA cycle for PR survival in conditions of increased HIF activity.

Metabolic Recruitment in Brain Tissue

Information processing imposes urgent metabolic demands on neurons, which have negligible energy stores and restricted access to fuel. Here, we discuss metabolic recruitment, the tissue-level phenomenon whereby active neurons harvest resources from their surroundings. The primary event is the neuronal release of K⁺ that mirrors workload. Astrocytes sense K⁺ in exquisite fashion thanks to their unique coexpression of NBCe1 and α2β2 Na⁺/K⁺ ATPase, and within seconds switch to Crabtree metabolism, involving GLUT1, aerobic glycolysis, transient suppression of mitochondrial respiration, and lactate export. The lactate surge serves as a secondary recruiter by inhibiting glucose consumption in distant cells. Additional recruiters are glutamate, nitric oxide, and ammonium, which signal over different spatiotemporal domains. The net outcome of these events is that more glucose, lactate, and oxygen are made available. Metabolic recruitment works alongside neurovascular coupling and various averaging strategies to support the inordinate dynamic range of individual neurons.

Dietary Fiber Modulates the Release of Gut Bacterial Products Preventing Cognitive Decline in an Alzheimer's Mouse Model

Fiber intake is associated with a lower risk for Alzheimer´s disease (AD) in older adults. Intake of plant-based diets rich in soluble fiber promotes the production of short-chain fatty acids (SCFAs: butyrate, acetate, propionate) by gut bacteria. Butyrate administration has antiinflammatory actions, but propionate promotes neuroinflammation. In AD patients, gut microbiota dysbiosis is a common feature even in the prodromal stages of the disease. It is unclear whether the neuroprotective effects of fiber intake rely on gut microbiota modifications and specific actions of SCFAs in brain cells. Here, we show that restoration of the gut microbiota dysbiosis through the intake of soluble fiber resulted in lower propionate and higher butyrate production, reduced astrocyte activation and improved cognitive function in 6-month-old male APP/PS1 mice. The neuroprotective effects were lost in antibiotic-treated mice. Moreover, propionate promoted higher glycolysis and mitochondrial respiration in astrocytes, while butyrate induced a more quiescent metabolism. Therefore, fiber intake neuroprotective action depends on the modulation of butyrate/propionate production by gut bacteria. Our data further support and provide a mechanism to explain the beneficial effects of dietary interventions rich in soluble fiber to prevent dementia and AD. Fiber intake restored the concentration of propionate and butyrate by modulating the composition of gut microbiota in male transgenic (Tg) mice with Alzheimer´s disease. Gut dysbiosis was associated with intestinal damage and high propionate levels in control diet fed-Tg mice. Fiber-rich diet restored intestinal integrity and promoted the abundance of butyrate-producing bacteria. Butyrate concentration was associated with better cognitive performance in fiber-fed Tg mice. A fiber-rich diet may prevent the development of a dysbiotic microbiome and the related cognitive dysfunction in people at risk of developing Alzheimer´s disease.

How expensive is the astrocyte?

The energy cost of information processing is thought to be chiefly neuronal, with a minor fraction attributed to glial cells. However, there is compelling evidence that astrocytes capture synaptic K⁺ using their Na⁺/K⁺ ATPase, and not solely through Kir4.1 channels as was once thought. When this active buffering is taken into account, the cost of astrocytes rises by >200%. Gram-per-gram, astrocytes turn out to be as expensive as neurons. This conclusion is supported by 3D reconstruction of the neuropil showing similar mitochondrial densities in neurons and astrocytes, by cell-specific transcriptomics and proteomics, and by the rates of the tricarboxylic acid cycle. Possible consequences for reactive astrogliosis and brain disease are discussed.

CO2 signaling mediates neurovascular coupling in the cerebral cortex

Neurovascular coupling is a fundamental brain mechanism that regulates local cerebral blood flow (CBF) in response to changes in neuronal activity. Functional imaging techniques are commonly used to record these changes in CBF as a proxy of neuronal activity to study the human brain. However, the mechanisms of neurovascular coupling remain incompletely understood. Here we show in experimental animal models (laboratory rats and mice) that the neuronal activity-dependent increases in local CBF in the somatosensory cortex are prevented by saturation of the CO2-sensitive vasodilatory brain mechanism with surplus of exogenous CO2 or disruption of brain CO2/HCO3- transport by genetic knockdown of electrogenic sodium-bicarbonate cotransporter 1 (NBCe1) expression in astrocytes. A systematic review of the literature data shows that CO2 and increased neuronal activity recruit the same vasodilatory signaling pathways. These results and analysis suggest that CO2 mediates signaling between neurons and the cerebral vasculature to regulate brain blood flow in accord with changes in the neuronal activity.

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.

Astrocytes regulate brain extracellular pH via a neuronal activity-dependent bicarbonate shuttle

Brain cells continuously produce and release protons into the extracellular space, with the rate of acid production corresponding to the levels of neuronal activity and metabolism. Efficient buffering and removal of excess H+ is essential for brain function, not least because all the electrogenic and biochemical machinery of synaptic transmission is highly sensitive to changes in pH. Here, we describe an astroglial mechanism that contributes to the protection of the brain milieu from acidification. In vivo and in vitro experiments conducted in rodent models show that at least one third of all astrocytes release bicarbonate to buffer extracellular H+ loads associated with increases in neuronal activity. The underlying signalling mechanism involves activity-dependent release of ATP triggering bicarbonate secretion by astrocytes via activation of metabotropic P2Y1 receptors, recruitment of phospholipase C, release of Ca2+ from the internal stores, and facilitated outward HCO3- transport by the electrogenic sodium bicarbonate cotransporter 1, NBCe1. These results show that astrocytes maintain local brain extracellular pH homeostasis via a neuronal activity-dependent release of bicarbonate. The data provide evidence of another important metabolic housekeeping function of these glial cells.

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.

Arousal-induced cortical activity triggers lactate release from astrocytes

It has been suggested that, in states of arousal, release of noradrenaline and β-adrenergic signalling affect long-term memory formation by stimulating astrocytic lactate production from glycogen. However, the temporal relationship between cortical activity and cellular lactate fluctuations upon changes in arousal remains to be fully established. Also, the role of β-adrenergic signalling and brain glycogen metabolism on neural lactate dynamics in vivo is still unknown. Here, we show that an arousal-induced increase in cortical activity triggers lactate release into the extracellular space, and this correlates with a fast and prominent lactate dip in astrocytes. The immediate drop in astrocytic lactate concentration and the parallel increase in extracellular lactate levels underline an activity-dependent lactate release from astrocytes. Moreover, when β-adrenergic signalling is blocked or the brain is depleted of glycogen, the arousal-evoked cellular lactate surges are significantly reduced. We provide in vivo evidence that cortical activation upon arousal triggers lactate release from astrocytes, a rise in intracellular lactate levels mediated by β-adrenergic signalling and the mobilization of lactate from glycogen stores.

Aerobic Glycolysis in the Brain: Warburg and Crabtree Contra Pasteur

Information processing is onerous. Curiously, active brain tissue does not fully oxidize glucose and instead generates a local surplus of lactate, a phenomenon termed aerobic glycolysis. Why engage in inefficient ATP production by glycolysis when energy demand is highest and oxygen is plentiful? Aerobic glycolysis is associated to classic biochemical effects known by the names of Pasteur, Warburg and Crabtree. Here we discuss these three interdependent phenomena in brain cells, in light of high-resolution data of neuronal and astrocytic metabolism in culture, tissue slices and in vivo, acquired with genetically-encoded fluorescent sensors. These sensors are synthetic proteins that can be targeted to specific cell types and subcellular compartments, which change their fluorescence in response to variations in metabolite concentration. A major site of acute aerobic glycolysis is the astrocyte. In this cell, a Crabtree effect triggered by K⁺ coincides with a Warburg effect mediated by NO, superimposed on a slower longer-lasting Warburg effect caused by glutamate and possibly by NH₄⁺. The compounded outcome is that more fuel (lactate) and more oxygen are made available to neurons, on demand. Meanwhile neurons consume both glucose and lactate, maintaining a strict balance between glycolysis and respiration, commanded by the Na⁺ pump. We conclude that activity-dependent Warburg and Crabtree effects in brain tissue, and the resulting aerobic glycolysis, do not reflect inefficient energy generation but the marshalling of astrocytes for the purpose of neuronal ATP generation. It remains to be seen whether neurons contribute to aerobic glycolysis under physiological conditions.

Monocarboxylate transporter 4 (MCT4) is a high affinity transporter capable of exporting lactate in high-lactate microenvironments

Monocarboxylate transporter 4 (MCT4) is an H⁺-coupled symporter highly expressed in metastatic tumors and at inflammatory sites undergoing hypoxia or the Warburg effect. At these sites, extracellular lactate contributes to malignancy and immune response evasion. Intriguingly, at 30-40 mm, the reported K_m of MCT4 for lactate is more than 1 order of magnitude higher than physiological or even pathological lactate levels. MCT4 is not thought to transport pyruvate. Here we have characterized cell lactate and pyruvate dynamics using the FRET sensors Laconic and Pyronic. Dominant MCT4 permeability was demonstrated in various cell types by pharmacological means and by CRISPR/Cas9-mediated deletion. Respective K_m values for lactate uptake were 1.7, 1.2, and 0.7 mm in MDA-MB-231 cells, macrophages, and HEK293 cells expressing recombinant MCT4. In MDA-MB-231 cells MCT4 exhibited a K_m for pyruvate of 4.2 mm, as opposed to >150 mm reported previously. Parallel assays with the pH-sensitive dye 2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) indicated that previous K_m estimates based on substrate-induced acidification were severely biased by confounding pH-regulatory mechanisms. Numerical simulation using revised kinetic parameters revealed that MCT4, but not the related transporters MCT1 and MCT2, endows cells with the ability to export lactate in high-lactate microenvironments. In conclusion, MCT4 is a high-affinity lactate transporter with physiologically relevant affinity for pyruvate.

Non-Canonical Control of Neuronal Energy Status by the Na+ Pump

Neurons have limited intracellular energy stores but experience acute and unpredictable increases in energy demand. To better understand how these cells repeatedly transit from a resting to active state without undergoing metabolic stress, we monitored their early metabolic response to neurotransmission using ion-sensitive probes and FRET sensors in vitro and in vivo. A short theta burst triggered immediate Na⁺ entry, followed by a delayed stimulation of the Na⁺/K⁺ ATPase pump. Unexpectedly, cytosolic ATP and ADP levels were unperturbed across a wide range of physiological workloads, revealing strict flux coupling between the Na⁺ pump and mitochondria. Metabolic flux measurements revealed a "priming" phase of mitochondrial energization by pyruvate, whereas glucose consumption coincided with delayed Na⁺ pump stimulation. Experiments revealed that the Na⁺ pump plays a permissive role for mitochondrial ATP production and glycolysis. We conclude that neuronal energy homeostasis is not mediated by adenine nucleotides or by Ca²⁺, but by a mechanism commanded by the Na⁺ pump.

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.

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.

Glia in brain energy metabolism: A perspective

Early views of glia as relatively inert, housekeeping cells have evolved, and glia are now recognized as dynamic cells that not only respond to neuronal activity but also sense metabolic changes and regulate neuronal metabolism. This evolution has been aided in part by technical advances permitting progressively better spatial and temporal resolution. Recent advances in cell-type specific genetic manipulation and sub-cellular metabolic probes promise to further this evolution by enabling study of metabolic interactions between intertwined fine neuronal and glial processes in vivo. Views of glia in disease processes have also evolved. Long considered purely reactive, glia and particularly microglia are now seen to play active roles in both promoting and limiting brain injury. At the same time, established concepts of glial energetics are now being linked to areas such as learning and neural network function, topics previously considered far removed from glial biology.

Chaski, a novel Drosophila lactate/pyruvate transporter required in glia cells for survival under nutritional stress

The intercellular transport of lactate is crucial for the astrocyte-to-neuron lactate shuttle (ANLS), a model of brain energetics according to which neurons are fueled by astrocytic lactate. In this study we show that the Drosophila chaski gene encodes a monocarboxylate transporter protein (MCT/SLC16A) which functions as a lactate/pyruvate transporter, as demonstrated by heterologous expression in mammalian cell culture using a genetically encoded FRET nanosensor. chaski expression is prominent in the Drosophila central nervous system and it is particularly enriched in glia over neurons. chaski mutants exhibit defects in a high energy demanding process such as synaptic transmission, as well as in locomotion and survival under nutritional stress. Remarkably, locomotion and survival under nutritional stress defects are restored by chaski expression in glia cells. Our findings are consistent with a major role for intercellular lactate shuttling in the brain metabolism of Drosophila.

SUMCOR: Cascade summing correction for volumetric sources applying MCNP6

The main features of code SUMCOR developed for cascade summing correction for volumetric sources are described. MCNP6 is used to track histories starting from individual points inside the volumetric source, for each set of cascade transitions from the radionuclide. Total and FEP efficiencies are calculated for all gamma-rays and X-rays involved in the cascade. Cascade summing correction is based on the matrix formalism developed by Semkow et al. (1990). Results are presented applying the experimental data sent to the participants of two intercomparisons organized by the ICRM-GSWG and coordinated by Dr. Marie-Cristine Lépy from the Laboratoire National Henri Becquerel (LNE-LNHB), CEA, in 2008 and 2010, respectively and compared to the other participants in the intercomparisons.

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.

Near-critical GLUT1 and Neurodegeneration

Recent articles have drawn renewed attention to the housekeeping glucose transporter GLUT1 and its possible involvement in neurodegenerative diseases. Here we provide an updated analysis of brain glucose transport and the cellular mechanisms involved in its acute modulation during synaptic activity. We discuss how the architecture of the blood-brain barrier and the low concentration of glucose within neurons combine to make endothelial/glial GLUT1 the master controller of neuronal glucose utilization, while the regulatory role of the neuronal glucose transporter GLUT3 emerges as secondary. The near-critical condition of glucose dynamics in the brain suggests that subtle deficits in GLUT1 function or its activity-dependent control by neurons may contribute to neurodegeneration. © 2017 Wiley Periodicals, Inc.

Activation of Wnt Signaling in Cortical Neurons Enhances Glucose Utilization through Glycolysis

The Wnt signaling pathway is critical for a number of functions in the central nervous system, including regulation of the synaptic cleft structure and neuroprotection against injury. Deregulation of Wnt signaling has been associated with several brain pathologies, including Alzheimer's disease. In recent years, it has been suggested that the Wnt pathway might act as a central integrator of metabolic signals from peripheral organs to the brain, which would represent a new role for Wnt signaling in cell metabolism. Energy metabolism is critical for normal neuronal function, which mainly depends on glucose utilization. Brain energy metabolism is important in almost all neurological disorders, to which a decrease in the capacity of the brain to utilize glucose has been linked. However, little is known about the relationship between Wnt signaling and neuronal glucose metabolism in the cellular context. In the present study, we found that acute treatment with the Wnt3a ligand induced a large increase in glucose uptake, without changes in the expression or localization of glucose transporter type 3. In addition, we observed that Wnt3a treatment increased the activation of the metabolic sensor Akt. Moreover, we observed an increase in the activity of hexokinase and in the glycolytic rate, and both processes were dependent on activation of the Akt pathway. Furthermore, we did not observe changes in the activity of glucose-6-phosphate dehydrogenase or in the pentose phosphate pathway. The effect of Wnt3a was independent of both the transcription of Wnt target genes and synaptic effects of Wnt3a. Together, our results suggest that Wnt signaling stimulates glucose utilization in cortical neurons through glycolysis to satisfy the high energy demand of these cells.

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.

Modulation of Mammary Stromal Cell Lactate Dynamics by Ambient Glucose and Epithelial Factors

Hyperglycemia is a risk factor for a variety of human cancers. Increased access to glucose and that tumor metabolize glucose by a glycolytic process even in the presence of oxygen (Warburg effect), provide a framework to analyze a particular set of metabolic adaptation mechanisms that may explain this phenomenon. In the present work, using a mammary stromal cell line derived from healthy tissue that was subjected to a long-term culture in low (5 mM) or high (25 mM) glucose, we analyzed kinetic parameters of lactate transport using a FRET biosensor. Our results indicate that the glucose pre-culture and soluble epithelial factors constitute a stimulus for lactate stromal production, factors that also modify the kinetic parameters and the monocarboxylate transporters expression in stromal cells. We also observed a vectorial flux of lactate from stroma to epithelial cells in a co-culture setting and found that the uptake of lactate by epithelial cells correlates with the degree of malignancy. Glucose preconditioning of the stromal cell stimulated epithelial motility. Our findings suggest that lactate generated by stromal cells in the high glucose condition stimulate epithelial migration. Overall, our results support the notion that glucose not only provides a substrate for tumor nutrition but also behaves as a signal promoting malignancy. J. Cell. Physiol. 232: 136-144, 2017. © 2016 Wiley Periodicals, Inc.

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.

In Vivo Evidence for a Lactate Gradient from Astrocytes to Neurons

Investigating lactate dynamics in brain tissue is challenging, partly because in vivo data at cellular resolution are not available. We monitored lactate in cortical astrocytes and neurons of mice using the genetically encoded FRET sensor Laconic in combination with two-photon microscopy. An intravenous lactate injection rapidly increased the Laconic signal in both astrocytes and neurons, demonstrating high lactate permeability across tissue. The signal increase was significantly smaller in astrocytes, pointing to higher basal lactate levels in these cells, confirmed by a one-point calibration protocol. Trans-acceleration of the monocarboxylate transporter with pyruvate was able to reduce intracellular lactate in astrocytes but not in neurons. Collectively, these data provide in vivo evidence for a lactate gradient from astrocytes to neurons. This gradient is a prerequisite for a carrier-mediated lactate flux from astrocytes to neurons and thus supports the astrocyte-neuron lactate shuttle model, in which astrocyte-derived lactate acts as an energy substrate for neurons.

Treatment Steps, Surgery, and Hospitalization Rates During the First Year of Follow-up in Patients with Inflammatory Bowel Diseases from the 2011 ECCO-Epicom Inception Cohort

BACKGROUND AND AIMS

The ECCO-EpiCom study investigates the differences in the incidence and therapeutic management of inflammatory bowel diseases [IBD] between Eastern and Western Europe. The aim of this study was to analyse the differences in the disease phenotype, medical therapy, surgery, and hospitalization rates in the ECCO-EpiCom 2011 inception cohort during the first year after diagnosis.

METHODS

Nine Western, five Eastern European centres and one Australian centre with 258 Crohn's disease [CD], 380 ulcerative colitis [UC] and 71 IBD unclassified [IBDU] patients [female/male: 326/383; mean age at diagnosis: 40.9 years, SD: 17.3 years] participated. Patients' data were registered and entered in the web-based ECCO-EpiCom database [www.epicom-ecco.eu].

RESULTS

In CD, 36 [19%] Western Europe/Australian and 6 [9%] Eastern European patients received biological therapy [p = 0.04], but the immunosuppressive [IS] use was equal and high in these regions [Eastern Europe vs Western Europe/Australia: 53% vs 45%; p = 0.27]. Surgery was performed in 17 [24%] CD patients in Eastern Europe and 13 [7%] in Western Europe/Australia [p < 0.001, pLogRank = 0.001]. Of CD patients from Eastern Europe, 24 [34%] were hospitalized, and 39 [21%] from Western Europe/Australia, [p = 0.02, pLogRank = 0.01]. In UC, exposure to biologicals and colectomy rates were low and hospitalization rates did not differ between these regions during the 1-year follow-up period [16% vs 16%; p = 0.93].

CONCLUSIONS

During the first year after diagnosis, surgery and hospitalization rates were significantly higher in CD patients in Eastern Europe compared with Western Europe/Australia, whereas significantly more CD patients were treated with biologicals in the Western Europe/Australian centres.

Transactivation activity and nucleocytoplasmic transport of β-catenin are independently regulated by its C-terminal end

The key protein in the canonical Wnt pathway is β-catenin, which is phosphorylated both in absence and presence of Wnt signals by different kinases. Upon activation in the cytoplasm, β-catenin can enter into the nucleus to transactivate target gene expression, many of which are cancer-related genes. The mechanism governing β-catenin's nucleocytoplasmic transport has been recently unvealed, although phosphorylation at its C-terminal end and its functional consequences are not completely understood. Serine 646 of β-catenin is a putative CK2 phosphorylation site and lies in a region which has been proposed to be important for its nucleocytoplasmic transport and transactivation activity. This residue was mutated to aspartic acid mimicking CK2-phosphorylation and its effects on β-catenin activity as well as localization were explored. β-Catenin S6464D did not show significant differences in both transcriptional activity and nuclear localization compared to the wild-type form, but displayed a characteristic granular nuclear pattern. Three-dimensional models of nuclei were constructed which showed differences in number and volume of granules, being those from β-catenin S646D more and smaller than the wild-type form. FRAP microscopy was used to compare nuclear export of both proteins which showed a slightly higher but not significant retention of β-catenin S646D. Altogether, these results show that C-terminal phosphorylation of β-catenin seems to be related with its nucleocytoplasmic transport but not transactivation activity.

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.

Incidence and initial disease course of inflammatory bowel diseases in 2011 in Europe and Australia: results of the 2011 ECCO-EpiCom inception cohort

BACKGROUND AND AIMS

The aim of the present study was to validate the IBD (inflammatory bowel diseases) incidence reported in the 2010 ECCO-EpiCom (European Crohn's and Colitis Organization-Epidemiological Committee) inception cohort by including a second independent inception cohort from participating centers in 2011 and an Australian center to investigate whether there is a difference in the incidence of IBD between Eastern and Western European countries and Australia.

METHODS

Fourteen centers from 5 Eastern and 9 Western European countries and one center from Australia participated in the ECCO-EpiCom 2011 inception cohort. Patients' data regarding disease type, socio-demographic factors, extraintestinal manifestations and therapy were entered into the Web-based EpiCom database, www.ecco-epicom.eu.

RESULTS

A total of 711 adult patients were diagnosed during the inclusion year 2011, 178 (25%) from Eastern, 461 (65%) from Western Europe and 72 (10%) from Australia; 259 (37%) patients were diagnosed with Crohn's disease, 380 (53%) with ulcerative colitis and 72 (10%) with IBD unclassified. The mean annual incidence rate for IBD was 11.3/100,000 in Eastern Europe, 14.0/100,000 in Western Europe and 30.3/100,000 in Australia. Significantly more patients were diagnosed with complicated disease at diagnosis in Eastern Europe compared to Western Europe (43% vs. 27%, p=0.02).

CONCLUSION

Incidence rates, disease phenotype and initial treatment characteristics in the 2011 ECCO-EpiCom cohort were not significantly different from that reported in the 2010 cohort.

Functional interaction between bicarbonate transporters and carbonic anhydrase modulates lactate uptake into mouse cardiomyocytes

Blood-derived lactate is a precious energy substrate for the heart muscle. Lactate is transported into cardiomyocytes via monocarboxylate transporters (MCTs) together with H(+), which couples lactate uptake to cellular pH regulation. In this study, we have investigated how the interplay between different acid/base transporters and carbonic anhydrases (CA), which catalyze the reversible hydration of CO2, modulates the uptake of lactate into isolated mouse cardiomyocytes. Lactate transport was estimated both as lactate-induced acidification and as changes in intracellular lactate levels measured with a newly developed Förster resonance energy transfer (FRET) nanosensor. Recordings of intracellular pH showed an increase in the rate of lactate-induced acidification when CA was inhibited by 6-ethoxy-2-benzothiazolesulfonamide (EZA), while direct measurements of lactate flux demonstrated a decrease in MCT transport activity, when CA was inhibited. The data indicate that catalytic activity of extracellular CA increases lactate uptake and counteracts intracellular lactate-induced acidification. We propose a hypothetical model, in which HCO3 (-), formed from cell-derived CO2 at the outer surface of the cardiomyocyte plasma membrane by membrane-anchored, extracellular CA, is transported into the cell via Na(+)/HCO3 (-) cotransport to counteract intracellular acidification, while the remaining H(+) stabilizes extracellular pH at the surface of the plasma membrane during MCT activity to enhance lactate influx into cardiomyocytes.

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

Neurophotonics comes to light at a time in which advances in microscopy and improved calcium reporters are paving the way toward high-resolution functional mapping of the brain. This review relates to a parallel revolution in metabolism. We argue that metabolism needs to be approached both in vitro and in vivo, and that it does not just exist as a low-level platform but is also a relevant player in information processing. In recent years, genetically encoded fluorescent nanosensors have been introduced to measure glucose, glutamate, ATP, NADH, lactate, and pyruvate in mammalian cells. Reporting relative metabolite levels, absolute concentrations, and metabolic fluxes, these sensors are instrumental for the discovery of new molecular mechanisms. Sensors continue to be developed, which together with a continued improvement in protein expression strategies and new imaging technologies, herald an exciting era of high-resolution characterization of metabolism in the brain and other organs.

East-West gradient in the incidence of inflammatory bowel disease in Europe: the ECCO-EpiCom inception cohort

OBJECTIVE

The incidence of inflammatory bowel disease (IBD) is increasing in Eastern Europe. The reasons for these changes remain unknown. The aim of this study was to investigate whether an East-West gradient in the incidence of IBD in Europe exists.

DESIGN

A prospective, uniformly diagnosed, population based inception cohort of IBD patients in 31 centres from 14 Western and eight Eastern European countries covering a total background population of approximately 10.1 million people was created. One-third of the centres had previous experience with inception cohorts. Patients were entered into a low cost, web based epidemiological database, making participation possible regardless of socioeconomic status and prior experience.

RESULTS

1515 patients aged 15 years or older were included, of whom 535 (35%) were diagnosed with Crohn's disease (CD), 813 (54%) with ulcerative colitis (UC) and 167 (11%) with IBD unclassified (IBDU). The overall incidence rate ratios in all Western European centres were 1.9 (95% CI 1.5 to 2.4) for CD and 2.1 (95% CI 1.8 to 2.6) for UC compared with Eastern European centres. The median crude annual incidence rates per 100,000 in 2010 for CD were 6.5 (range 0-10.7) in Western European centres and 3.1 (range 0.4-11.5) in Eastern European centres, for UC 10.8 (range 2.9-31.5) and 4.1 (range 2.4-10.3), respectively, and for IBDU 1.9 (range 0-39.4) and 0 (range 0-1.2), respectively. In Western Europe, 92% of CD, 78% of UC and 74% of IBDU patients had a colonoscopy performed as the diagnostic procedure compared with 90%, 100% and 96%, respectively, in Eastern Europe. 8% of CD and 1% of UC patients in both regions underwent surgery within the first 3 months of the onset of disease. 7% of CD patients and 3% of UC patients from Western Europe received biological treatment as rescue therapy. Of all European CD patients, 20% received only 5-aminosalicylates as induction therapy.

CONCLUSIONS

An East-West gradient in IBD incidence exists in Europe. Among this inception cohort--including indolent and aggressive cases--international guidelines for diagnosis and initial treatment are not being followed uniformly by physicians.

East-West gradient in the incidence of inflammatory bowel disease in Europe: the ECCO-EpiCom inception cohort

OBJECTIVE

The incidence of inflammatory bowel disease (IBD) is increasing in Eastern Europe. The reasons for these changes remain unknown. The aim of this study was to investigate whether an East-West gradient in the incidence of IBD in Europe exists.

DESIGN

A prospective, uniformly diagnosed, population based inception cohort of IBD patients in 31 centres from 14 Western and eight Eastern European countries covering a total background population of approximately 10.1 million people was created. One-third of the centres had previous experience with inception cohorts. Patients were entered into a low cost, web based epidemiological database, making participation possible regardless of socioeconomic status and prior experience.

RESULTS

1515 patients aged 15 years or older were included, of whom 535 (35%) were diagnosed with Crohn's disease (CD), 813 (54%) with ulcerative colitis (UC) and 167 (11%) with IBD unclassified (IBDU). The overall incidence rate ratios in all Western European centres were 1.9 (95% CI 1.5 to 2.4) for CD and 2.1 (95% CI 1.8 to 2.6) for UC compared with Eastern European centres. The median crude annual incidence rates per 100,000 in 2010 for CD were 6.5 (range 0-10.7) in Western European centres and 3.1 (range 0.4-11.5) in Eastern European centres, for UC 10.8 (range 2.9-31.5) and 4.1 (range 2.4-10.3), respectively, and for IBDU 1.9 (range 0-39.4) and 0 (range 0-1.2), respectively. In Western Europe, 92% of CD, 78% of UC and 74% of IBDU patients had a colonoscopy performed as the diagnostic procedure compared with 90%, 100% and 96%, respectively, in Eastern Europe. 8% of CD and 1% of UC patients in both regions underwent surgery within the first 3 months of the onset of disease. 7% of CD patients and 3% of UC patients from Western Europe received biological treatment as rescue therapy. Of all European CD patients, 20% received only 5-aminosalicylates as induction therapy.

CONCLUSIONS

An East-West gradient in IBD incidence exists in Europe. Among this inception cohort--including indolent and aggressive cases--international guidelines for diagnosis and initial treatment are not being followed uniformly by physicians.

Imaging mitochondrial flux in single cells with a FRET sensor for pyruvate

Mitochondrial flux is currently accessible at low resolution. Here we introduce a genetically-encoded FRET sensor for pyruvate, and methods for quantitative measurement of pyruvate transport, pyruvate production and mitochondrial pyruvate consumption in intact individual cells at high temporal resolution. In HEK293 cells, neurons and astrocytes, mitochondrial pyruvate uptake was saturated at physiological levels, showing that the metabolic rate is determined by intrinsic properties of the organelle and not by substrate availability. The potential of the sensor was further demonstrated in neurons, where mitochondrial flux was found to rise by 300% within seconds of a calcium transient triggered by a short theta burst, while glucose levels remained unaltered. In contrast, astrocytic mitochondria were insensitive to a similar calcium transient elicited by extracellular ATP. We expect the improved resolution provided by the pyruvate sensor will be of practical interest for basic and applied researchers interested in mitochondrial function.

Small is fast: astrocytic glucose and lactate metabolism at cellular resolution

Brain tissue is highly dynamic in terms of electrical activity and energy demand. Relevant energy metabolites have turnover times ranging from milliseconds to seconds and are rapidly exchanged between cells and within cells. Until recently these fast metabolic events were inaccessible, because standard isotopic techniques require use of populations of cells and/or involve integration times of tens of minutes. Thanks to fluorescent probes and recently available genetically-encoded optical nanosensors, this Technology Report shows how it is now possible to monitor the concentration of metabolites in real-time and in single cells. In combination with ad hoc inhibitor-stop protocols, these probes have revealed a key role for K⁺ in the acute stimulation of astrocytic glycolysis by synaptic activity. They have also permitted detection of the Warburg effect in single cancer cells. Genetically-encoded nanosensors currently exist for glucose, lactate, NADH and ATP, and it is envisaged that other metabolite nanosensors will soon be available. These optical tools together with improved expression systems and in vivo imaging, herald an exciting era of single-cell metabolic analysis.

Acute feedback control of astrocytic glycolysis by lactate

Neuronal activity is accompanied by a rapid increase in interstitial lactate, which is hypothesized to serve as a fuel for neurons and a signal for local vasodilation. Using FRET microscopy, we report here that the rate of glycolysis in cultured mice astrocytes can be acutely modulated by physiological changes in extracellular lactate. Glycolytic inhibition by lactate was not accompanied by detectable variations in intracellular pH or intracellular ATP and was not dependent of mitochondrial function. Pyruvate was also inhibitory, suggesting that the effect of lactate is not mediated by the NADH/NAD(+) ratio. We propose that lactate serves as a fast negative feedback signal limiting its own production by astrocytes and therefore the amplitude of the lactate surge. The inhibition of glucose usage by lactate was much stronger in resting astrocytes than in K(+)-stimulated astrocytes, which suggests that lactate may also help diverting glucose from resting to active zones.

High resolution measurement of the glycolytic rate

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

General requirement for harvesting antennae at ca and h channels and transporters

The production and dissipation of energy in cells is intimately linked to the movement of small molecules in and out of enzymes, channels, and transporters. An analytical model of diffusion was described previously, which was used to estimate local effects of these proteins acting as molecular sources. The present article describes a simple but more general model, which can be used to estimate the local impact of proteins acting as molecular sinks. The results show that the enzymes, transporters, and channels, whose substrates are present at relatively high concentrations like ATP, Na⁺, glucose, lactate, and pyruvate, do not operate fast enough to deplete their vicinity to a meaningful extent, supporting the notion that for these molecules the cytosol is a well-mixed compartment. One specific consequence of this analysis is that the well-documented cross-talk existing between the Na⁺/K⁺ ATPase and the glycolytic machinery should not be explained by putative changes in local ATP concentration. In contrast, Ca2⁺ and H⁺ transporters like the Na⁺/Ca2⁺ exchanger NCX and the Na⁺/H⁺ exchanger NHE, show experimental rates of transport that are two to three orders of magnitude faster than the rates at which the aqueous phase may possibly feed their binding sites. This paradoxical result implies that Ca2⁺ and H⁺ transporters do not extract their substrates directly from the bulk cytosol, but from an intermediate “harvesting” compartment located between the aqueous phase and the transport site.

Current use and future perspectives of diagnostic and therapeutic lasers in Oral Medicine

Several diagnostic and therapeutic methods are based on the optical properties of lasers. In therapeutic applications, laser light is absorbed in a specific manner, whereas light is scattered, reflected, or transmitted from different structures. Improvements in laser technology allow new procedures and broaden the scope of applications for both diagnosis and therapy. The focus of laser application in Oral Medicine diagnosis should be early detection of oral squamous cell carcinoma. Novel modalities for the detection of oral malignancy are urgently needed, while others must be continuously improved. Optical coherence tomography and laser-induced fluorescence spectroscopy are currently being studied. In addition to diagnosis of non-malignant lesions, laser therapy has been used based upon the biological reactions and molecular wound healing mechanisms as an alternative for the treatment of a variety of oral soft tissue lesions. The aim of the present article is to review current knowledge and future perspectives of lasers in Oral Medicine.

A genetically encoded ratiometric sensor to measure extracellular pH in microdomains bounded by basolateral membranes of epithelial cells

Extracellular pH, especially in relatively inaccessible microdomains between cells, affects transport membrane protein activity and might have an intercellular signaling role. We have developed a genetically encoded extracellular pH sensor capable of detecting pH changes in basolateral spaces of epithelial cells. It consists of a chimerical membrane protein displaying concatenated enhanced variants of cyan fluorescence protein (ECFP) and yellow fluorescence protein (EYFP) at the external aspect of the cell surface. The construct, termed pHCECSensor01, was targeted to basolateral membranes of Madin-Darby canine kidney (MDCK) cells by means of a sequence derived from the aquaporin AQP4. The fusion of pH-sensitive EYFP with pH-insensitive ECFP allows ratiometric pH measurements. The titration curve of pHCECSensor01 in vivo had a pK (a) value of 6.5 +/- 0.04. Only minor effects of extracellular chloride on pHCECSensor01 were observed around the physiological concentrations of this anion. In MDCK cells, the sensor was able to detect changes in pH secondary to H(+) efflux into the basolateral spaces elicited by an ammonium prepulse or lactate load. This genetically encoded sensor has the potential to serve as a noninvasive tool for monitoring changes in extracellular pH microdomains in epithelial and other tissues in vivo.