Spatiotemporal Imaging of Cellular Energy Metabolism with Genetically-Encoded Fluorescent Sensors in Brain

Abstract

The brain has very high energy requirements and consumes 20% of the oxygen and 25% of the glucose in the human body. Therefore, the molecular mechanism underlying how the brain metabolizes substances to support neural activity is a fundamental issue for neuroscience studies. A well-known model in the brain, the astrocyte-neuron lactate shuttle, postulates that glucose uptake and glycolytic activity are enhanced in astrocytes upon neuronal activation and that astrocytes transport lactate into neurons to fulfill their energy requirements. Current evidence for this hypothesis has yet to reach a clear consensus, and new concepts beyond the shuttle hypothesis are emerging. The discrepancy is largely attributed to the lack of a critical method for real-time monitoring of metabolic dynamics at cellular resolution. Recent advances in fluorescent protein-based sensors allow the generation of a sensitive, specific, real-time readout of subcellular metabolites and fill the current technological gap. Here, we summarize the development of genetically encoded metabolite sensors and their applications in assessing cell metabolism in living cells and in vivo, and we believe that these tools will help to address the issue of elucidating neural energy metabolism.

Keywords: Astrocyte; Energy metabolism; Genetically encoded fluorescent sensor; Neuron; Real time monitoring.

Fig. 1

The main pathways of neural energy metabolism. Depicted are the main substrates and pathways of cellular energy metabolism in the adult brain. In general, the brain utilizes glucose as an obligatory fuel and makes use of ketone bodies under certain conditions such as fasting or strenuous physical activity. Neurons also metabolize glucose through the pentose phosphate pathway to generate NADPH, which protects neurons from oxidative stress. Astrocytes can synthesize glycogen from glucose and mobilize it to support the energy demands of both astrocytes and neurons. GLUT, glucose transporter; MCT, monocarboxylate transporter; GR, glutathione reductase; Gpx, glutathione peroxidase; MPC, mitochondrial pyruvate carrier; TCA, tricarboxylic acid cycle; G-6-P, glucose 6-phosphate; GA3P, glyceraldehyde-3-phosphate; R-5-P, ribulose 5-phosphate; PPP, pentose phosphate pathway; GSSG, oxidized glutathione; GSH, reduced glutathione; ROOH, hydroperoxides.

Fig. 2

The astrocyte-neuron lactate shuttle model. The astrocyte-neuron lactate shuttle model highlights the important role of lactate secreted by astrocytes as the fuel for neurons. As proposed in this model, the neurotransmitter glutamate released upon neuronal excitation is taken up by astrocytes. On one side, astrocytic glutamate is used to synthesize glutamine and recycled in neurons for facilitating glutamate regeneration; on the other side, glutamate stimulates the glycolytic activity and lactate secretion of astrocytes. Thereafter, the lactate is transported into neurons to supply the intense energy consumption of neuronal activities. The metabolite transporters and enzymes predominantly expressed in neurons and astrocytes are listed. GluR: glutamate receptor; GS: glutamine synthetase; EAAT: excitatory amino acid transporter; GLUT1 and GLUT3: glucose transporter 1 and 3; LDH1 and LDH5: lactate dehydrogenase 1 and 5; MCT1, MCT2, and MCT4: monocarboxylate transporter 1, 2, and 4; SNATs: sodium-coupled amino acid transporters.

Fig. 3

Diverse proposed models of neural energy metabolism. At least four models of neural energy metabolism have been proposed based on the current evidence. The first model (①) is the ANLS hypothesis, which proposes that neuronal activity augments astrocytic glycolysis and lactate secretion and feeds neurons with lactate as an energy substrate. The second model (②) suggests that the lactate shuttle noted above occurs only under resting rather than stimulated conditions. The third model (③) postulates the direct uptake of glucose by neurons from the interstitium. The last model (④) states that lactate is transported from astrocytes to oligodendrocytes through gap junctions and that oligodendrocytes nurture and recharge axons with lactate.