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      The Amino Acid Transporters of the Glutamate/GABA-Glutamine Cycle and Their Impact on Insulin and Glucagon Secretion

      1 , 2 , * , 3 , 4

      Frontiers in Endocrinology

      Frontiers Media S.A.

      GABA, glutamate, glutamine, insulin, SAT2, Slc38a2, Slc38a3, SN1

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          Abstract

          Intercellular communication is pivotal in optimizing and synchronizing cellular responses to keep homeostasis and to respond adequately to external stimuli. In the central nervous system (CNS), glutamatergic and GABAergic signals are postulated to be dependent on the glutamate/GABA-glutamine cycle for vesicular loading of neurotransmitters, for inactivating the signal and for the replenishment of the neurotransmitters. Islets of Langerhans release the hormones insulin and glucagon, but share similarities with CNS cells in for example transcriptional control of development and differentiation, and chromatin methylation. Interestingly, CNS proteins involved in secretion of the neurotransmitters and emitting their responses as well as the regulation of these processes, are also found in islet cells. Moreover, high levels of glutamate, GABA, and glutamine and their respective vesicular and plasma membrane transporters have been shown in the islet cells and there is emerging support for these amino acids and their transporters playing important roles in the maturation and secretion of insulin and glucagon. In this review, we will discuss the feasibility of recent data in the field in relation to the biophysical properties of the transporters (Slc1, Slc17, Slc32, and Slc38) and physiology of hormone secretion in islets of Langerhans.

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          Most cited references 90

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          The expression of vesicular glutamate transporters defines two classes of excitatory synapse.

          The quantal release of glutamate depends on its transport into synaptic vesicles. Recent work has shown that a protein previously implicated in the uptake of inorganic phosphate across the plasma membrane catalyzes glutamate uptake by synaptic vesicles. However, only a subset of glutamate neurons expresses this vesicular glutamate transporter (VGLUT1). We now report that excitatory neurons lacking VGLUT1 express a closely related protein that has also been implicated in phosphate transport. Like VGLUT1, this protein localizes to synaptic vesicles and functions as a vesicular glutamate transporter (VGLUT2). The complementary expression of VGLUT1 and 2 defines two distinct classes of excitatory synapse.
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            Identification and characterization of the vesicular GABA transporter.

            Synaptic transmission involves the regulated exocytosis of vesicles filled with neurotransmitter. Classical transmitters are synthesized in the cytoplasm, and so must be transported into synaptic vesicles. Although the vesicular transporters for monoamines and acetylcholine have been identified, the proteins responsible for packaging the primary inhibitory and excitatory transmitters, gamma-aminobutyric acid (GABA) and glutamate remain unknown. Studies in the nematode Caenorhabditis elegans have implicated the gene unc-47 in the release of GABA. Here we show that the sequence of unc-47 predicts a protein with ten transmembrane domains, that the gene is expressed by GABA neurons, and that the protein colocalizes with synaptic vesicles. Further, a rat homologue of unc-47 is expressed by central GABA neurons and confers vesicular GABA transport in transfected cells with kinetics and substrate specificity similar to those previously reported for synaptic vesicles from the brain. Comparison of this vesicular GABA transporter (VGAT) with a vesicular transporter for monoamines shows that there are differences in the bioenergetic dependence of transport, and these presumably account for the differences in structure. Thus VGAT is the first of a new family of neurotransmitter transporters.
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              Glutamine synthetase: glial localization in brain.

              Light microscopy immunohistochemical techniques were used to examine the distribution of glutamine synthetase in rat brain. Glutamine synthetase was found to be localized in the glial cells. Neuronal cell bodies, endothelial cells, and choroid epithelium contained no enzyme. The findings indicate that glia have a crucial role in glutamic acid, gamma-aminobutyric acid, and ammonia metabolism in brain.
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                Author and article information

                Contributors
                Journal
                Front Endocrinol (Lausanne)
                Front Endocrinol (Lausanne)
                Front. Endocrinol.
                Frontiers in Endocrinology
                Frontiers Media S.A.
                1664-2392
                01 November 2013
                31 December 2013
                2013
                : 4
                Affiliations
                1Institute for Medical Informatics, Oslo University Hospital , Oslo, Norway
                2Centre for Cancer Biomedicine, University of Oslo , Oslo, Norway
                3Institute of Basic Medical Sciences, University of Oslo , Oslo, Norway
                4The Biotechnology Centre of Oslo, University of Oslo , Oslo, Norway
                Author notes

                Edited by: Leif Hertz, China Medical University, China

                Reviewed by: Leif Hertz, China Medical University, China; Michael Byrne Robinson, University of Pennsylvania, USA

                *Correspondence: Monica Jenstad, Institute for Medical Informatics, Oslo University Hospital, Radiumhospitalet, PO Box 4953 Nydalen, Oslo NO-0424, Norway e-mail: mjenstad@ 123456gmail.com

                This article was submitted to Cellular Endocrinology, a section of the journal Frontiers in Endocrinology.

                10.3389/fendo.2013.00199
                3876026
                Copyright © 2013 Jenstad and Chaudhry.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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                Figures: 2, Tables: 0, Equations: 0, References: 90, Pages: 8, Words: 6925
                Categories
                Endocrinology
                Mini Review

                Endocrinology & Diabetes

                sat2, glutamine, glutamate, sn1, slc38a3, slc38a2, insulin, gaba

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