RESUMO
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.
Assuntos
Encéfalo/fisiologia , Glicólise/fisiologia , Animais , Astrócitos/metabolismo , Respiração Celular/fisiologia , Glucose/metabolismo , Humanos , Ácido Láctico/metabolismo , Mitocôndrias/metabolismo , Neurônios/metabolismoRESUMO
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.
Assuntos
Astrócitos , Glicólise , Animais , Glucose , Transportador de Glucose Tipo 1/genética , Ácido Láctico , CamundongosRESUMO
Aerobic glycolysis is a phenomenon that in the long term contributes to synaptic formation and growth, is reduced by normal aging, and correlates with amyloid beta deposition. Aerobic glycolysis starts within seconds of neural activity and it is not obvious why energetic efficiency should be compromised precisely when energy demand is highest. Using genetically encoded FRET nanosensors and real-time oxygen measurements in culture and in hippocampal slices, we show here that astrocytes respond to physiological extracellular K+ with an acute rise in cytosolic ATP and a parallel inhibition of oxygen consumption, explained by glycolytic stimulation via the Na+-bicarbonate cotransporter NBCe1. This control of mitochondrial respiration via glycolysis modulation is reminiscent of a phenomenon previously described in proliferating cells, known as the Crabtree effect. Fast brain aerobic glycolysis may be interpreted as a strategy whereby neurons manipulate neighboring astrocytes to obtain oxygen, thus maximizing information processing.
Assuntos
Astrócitos/fisiologia , Glicólise/fisiologia , Hipocampo/fisiologia , Mitocôndrias/fisiologia , Neurônios/fisiologia , Consumo de Oxigênio , Animais , Astrócitos/citologia , Células Cultivadas , Metabolismo Energético , Hipocampo/citologia , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Endogâmicos CBA , Camundongos Knockout , Neurônios/citologia , Simportadores de Sódio-Bicarbonato/fisiologiaRESUMO
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.
Assuntos
Encéfalo/metabolismo , Transportador de Glucose Tipo 1/metabolismo , Glucose/metabolismo , Doenças Neurodegenerativas/metabolismo , Neurônios/metabolismo , Animais , Encéfalo/patologia , Metabolismo Energético/fisiologia , Transportador de Glucose Tipo 1/deficiência , Humanos , Doenças Neurodegenerativas/patologia , Neurônios/patologiaRESUMO
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.
Assuntos
Ácido 3-Hidroxibutírico/farmacologia , Astrócitos/efeitos dos fármacos , Glucose/metabolismo , Hipocampo/efeitos dos fármacos , Animais , Astrócitos/metabolismo , Técnicas Biossensoriais , Técnicas de Cultura de Células , Metabolismo Energético , Feminino , Transferência Ressonante de Energia de Fluorescência , Hipocampo/metabolismo , Técnicas In Vitro , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Endogâmicos CBA , Microscopia de FluorescênciaRESUMO
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.
Assuntos
Astrócitos/efeitos dos fármacos , Canais Iônicos/fisiologia , Ácido Láctico/metabolismo , Potássio/farmacologia , Animais , Animais Recém-Nascidos , Bário/farmacologia , Cádmio/farmacologia , Células Cultivadas , Córtex Cerebral/citologia , Feminino , Fluoresceínas/metabolismo , Glicogênio/metabolismo , Humanos , Técnicas In Vitro , Canais Iônicos/efeitos dos fármacos , Íons/farmacologia , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Camundongos , Camundongos Endogâmicos C57BL , Neurônios/efeitos dos fármacos , Neurônios/fisiologia , Ácido Pirúvico/farmacologia , Córtex Somatossensorial/citologia , Córtex Somatossensorial/fisiologia , TransfecçãoRESUMO
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.