Vesicular uptake and exocytosis of L-aspartate is independent of sialin

FASEB J. 2013 Mar;27(3):1264-74. doi: 10.1096/fj.12-206300. Epub 2012 Dec 6.

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenosine Triphosphate / metabolism
  • Animals
  • Aspartic Acid / metabolism*
  • Brain / metabolism*
  • Exocytosis / physiology*
  • Male
  • Mice
  • Mice, Knockout
  • Nerve Tissue Proteins / metabolism*
  • Neurotransmitter Agents / metabolism*
  • Organic Anion Transporters / metabolism*
  • PC12 Cells
  • Rats
  • Rats, Wistar
  • Symporters / metabolism*
  • Synaptic Vesicles / metabolism*

Substances

  • Nerve Tissue Proteins
  • Neurotransmitter Agents
  • Organic Anion Transporters
  • Symporters
  • sialic acid transport proteins
  • Aspartic Acid
  • Adenosine Triphosphate