title: A role for glutamate transporters in the regulation of insulin secretion.
abstract: In the brain, glutamate is an extracellular transmitter that mediates cell-to-cell communication. Prior to synaptic release it is pumped into vesicles by vesicular glutamate transporters (VGLUTs). To inactivate glutamate receptor responses after release, glutamate is taken up into glial cells or neurons by excitatory amino acid transporters (EAATs). In the pancreatic islets of Langerhans, glutamate is proposed to act as an intracellular messenger, regulating insulin secretion from β-cells, but the mechanisms involved are unknown. By immunogold cytochemistry we show that insulin containing secretory granules express VGLUT3. Despite the fact that they have a VGLUT, the levels of glutamate in these granules are low, indicating the presence of a protein that can transport glutamate out of the granules. Surprisingly, in β-cells the glutamate transporter EAAT2 is located, not in the plasma membrane as it is in brain cells, but exclusively in insulin-containing secretory granules, together with VGLUT3. In EAAT2 knock out mice, the content of glutamate in secretory granules is higher than in wild type mice. These data imply a glutamate cycle in which glutamate is carried into the granules by VGLUT3 and carried out by EAAT2. Perturbing this cycle by knocking down EAAT2 expression with a small interfering RNA, or by over-expressing EAAT2 or a VGLUT in insulin granules, significantly reduced the rate of granule exocytosis. Simulations of granule energetics suggest that VGLUT3 and EAAT2 may regulate the pH and membrane potential of the granules and thereby regulate insulin secretion. These data suggest that insulin secretion from β-cells is modulated by the flux of glutamate through the secretory granules.
authors.affiliation: Department of Anatomy and the CMBN, University of Oslo, Oslo, Norway.
authors.forename: Runhild
authors.initials: R
authors.lastname: Gammelsaeter
authors.forename: Thierry
authors.initials: T
authors.lastname: Coppola
authors.forename: Païkan
authors.initials: P
authors.lastname: Marcaggi
authors.forename: Jon
authors.initials: J
authors.lastname: Storm-Mathisen
authors.forename: Farrukh A
authors.initials: FA
authors.lastname: Chaudhry
authors.forename: David
authors.initials: D
authors.lastname: Attwell
authors.forename: Romano
authors.initials: R
authors.lastname: Regazzi
authors.forename: Vidar
authors.initials: V
authors.lastname: Gundersen
mesh_terms: D026922:Amino Acid Transport Systems, Acidic / Q000378:metabolism*; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D027342:Excitatory Amino Acid Transporter 2 / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000378:metabolism / Q000648:ultrastructure; D008564:Membrane Potentials; D051379:Mice; D021381:Protein Transport; D051381:Rats; D017208:Rats, Wistar; D022142:Secretory Vesicles / Q000378:metabolism / Q000648:ultrastructure; D050496:Vesicular Glutamate Transport Proteins / Q000378:metabolism*
chemical_list: D026922:Amino Acid Transport Systems, Acidic; D027342:Excitatory Amino Acid Transporter 2; D018698:Glutamic Acid; D007328:Insulin; D050496:Vesicular Glutamate Transport Proteins; C482103:Slc17a8 protein, rat; C493396:Slc1a2 protein, mouse; C493395:Slc1a2 protein, rat; C504941:Slc17a8 protein, mouse