title: Vesicular uptake and exocytosis of L-aspartate is independent of sialin.
abstract: The mechanism of release and the role of l-aspartate as a central neurotransmitter are controversial. A vesicular release mechanism for l-aspartate has been difficult to prove, as no vesicular l-aspartate transporter was identified until it was found that sialin could transport l-aspartate and l-glutamate when reconstituted into liposomes. We sought to clarify the release mechanism of l-aspartate and the role of sialin in this process by combining l-aspartate uptake studies in isolated synaptic vesicles with immunocyotchemical investigations of hippocampal slices. We found that radiolabeled l-aspartate was taken up into synaptic vesicles. The vesicular l-aspartate uptake, relative to the l-glutamate uptake, was twice as high in the hippocampus as in the whole brain, the striatum, and the entorhinal and frontal cortices and was not inhibited by l-glutamate. We further show that sialin is not essential for exocytosis of l-aspartate, as there was no difference in ATP-dependent l-aspartate uptake in synaptic vesicles from sialin-knockout and wild-type mice. In addition, expression of sialin in PC12 cells did not result in significant vesicle uptake of l-aspartate, and depolarization-induced depletion of l-aspartate from hippocampal nerve terminals was similar in hippocampal slices from sialin-knockout and wild-type mice. Further, there was no evidence for nonvesicular release of l-aspartate via volume-regulated anion channels or plasma membrane excitatory amino acid transporters. This suggests that l-aspartate is exocytotically released from nerve terminals after vesicular accumulation by a transporter other than sialin.
authors.affiliation: Department of Anatomy, University of Oslo, POB 1105 Blindern, 0317 Oslo, Norway.
authors.forename: Cecilie
authors.initials: C
authors.lastname: Morland
authors.forename: Kaja
authors.initials: K
authors.lastname: Nordengen
authors.forename: Max
authors.initials: M
authors.lastname: Larsson
authors.forename: Laura M
authors.initials: LM
authors.lastname: Prolo
authors.forename: Zoya
authors.initials: Z
authors.lastname: Farzampour
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Reimer
authors.forename: Vidar
authors.initials: V
authors.lastname: Gundersen
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism*; D001921:Brain / Q000378:metabolism*; D005089:Exocytosis / Q000502:physiology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism*; D027361:Organic Anion Transporters / Q000378:metabolism*; D016716:PC12 Cells; D051381:Rats; D017208:Rats, Wistar; D027981:Symporters / Q000378:metabolism*; D013572:Synaptic Vesicles / Q000378:metabolism*
chemical_list: D009419:Nerve Tissue Proteins; D018377:Neurotransmitter Agents; D027361:Organic Anion Transporters; D027981:Symporters; C421224:sialic acid transport proteins; D001224:Aspartic Acid; D000255:Adenosine Triphosphate