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      Interaction of glucose sensing and leptin action in the brain

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          Abstract

          Background

          In response to energy abundant or deprived conditions, nutrients and hormones activate hypothalamic pathways to maintain energy and glucose homeostasis. The underlying CNS mechanisms, however, remain elusive in rodents and humans.

          Scope of review

          Here, we first discuss brain glucose sensing mechanisms in the presence of a rise or fall of plasma glucose levels, and highlight defects in hypothalamic glucose sensing disrupt in vivo glucose homeostasis in high-fat fed, obese, and/or diabetic conditions. Second, we discuss brain leptin signalling pathways that impact glucose homeostasis in glucose-deprived and excessed conditions, and propose that leptin enhances hypothalamic glucose sensing and restores glucose homeostasis in short-term high-fat fed and/or uncontrolled diabetic conditions.

          Major conclusions

          In conclusion, we believe basic studies that investigate the interaction of glucose sensing and leptin action in the brain will address the translational impact of hypothalamic glucose sensing in diabetes and obesity.

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          Most cited references97

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          Glutamate uptake into astrocytes stimulates aerobic glycolysis: a mechanism coupling neuronal activity to glucose utilization.

          Glutamate, released at a majority of excitatory synapses in the central nervous system, depolarizes neurons by acting at specific receptors. Its action is terminated by removal from the synaptic cleft mostly via Na(+)-dependent uptake systems located on both neurons and astrocytes. Here we report that glutamate, in addition to its receptor-mediated actions on neuronal excitability, stimulates glycolysis--i.e., glucose utilization and lactate production--in astrocytes. This metabolic action is mediated by activation of a Na(+)-dependent uptake system and not by interaction with receptors. The mechanism involves the Na+/K(+)-ATPase, which is activated by an increase in the intracellular concentration of Na+ cotransported with glutamate by the electrogenic uptake system. Thus, when glutamate is released from active synapses and taken up by astrocytes, the newly identified signaling pathway described here would provide a simple and direct mechanism to tightly couple neuronal activity to glucose utilization. In addition, glutamate-stimulated glycolysis is consistent with data obtained from functional brain imaging studies indicating local nonoxidative glucose utilization during physiological activation.
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            Abnormal splicing of the leptin receptor in diabetic mice.

            Mutations in the mouse diabetes (db) gene result in obesity and diabetes in a syndrome resembling morbid human obesity. Previous data suggest that the db gene encodes the receptor for the obese (ob) gene product, leptin. A leptin receptor was recently cloned from choroid plexus and shown to map to the same 6-cM interval on mouse chromosome 4 as db. This receptor maps to the same 300-kilobase interval as db, and has at least six alternatively spliced forms. One of these splice variants is expressed at a high level in the hypothalamus, and is abnormally spliced in C57BL/Ks db/db mice. The mutant protein is missing the cytoplasmic region, and is likely to be defective in signal transduction. This suggests that the weight-reducing effects of leptin may be mediated by signal transduction through a leptin receptor in the hypothalamus.
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              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.
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                Author and article information

                Contributors
                Journal
                Mol Metab
                Mol Metab
                Molecular Metabolism
                Elsevier
                2212-8778
                13 May 2020
                September 2020
                13 May 2020
                : 39
                : 101011
                Affiliations
                [1 ]Department of Physiology, University of Toronto, Toronto, ON, M5S 1A8, Canada
                [2 ]Toronto General Hospital Research Institute, UHN, Toronto, ON, M5G 1L7, Canada
                [3 ]Department of Medicine, University of Toronto, Toronto, ON, M5S 1A8, Canada
                [4 ]Banting and Best Diabetes Centre, University of Toronto, Toronto, ON, M5G 2C4, Canada
                Author notes
                []Corresponding author. Department of Physiology, University of Toronto, Toronto, ON, M5S 1A8, Canada. tony.lam@ 123456uhnres.utoronto.ca
                Article
                S2212-8778(20)30085-5 101011
                10.1016/j.molmet.2020.101011
                7267726
                32416314
                81992a3a-3afb-4676-a316-458f4dabd7c7
                © 2020 The Author(s)

                This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

                History
                : 26 February 2020
                : 27 April 2020
                : 5 May 2020
                Categories
                Review

                hypothalamus,brain,glucose metabolism,glucose sensing,leptin action,lipid sensing,hepatic glucose production,hypoglycemia

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