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

• Published in: The FASEB journal Vol. 27; no. 3; p. 1264
• Main Authors: Morland, Cecilie, Nordengen, Kaja, Larsson, Max, Prolo, Laura M, Farzampour, Zoya, Reimer, Richard J, Gundersen, Vidar
• Format: Journal Article
• Language: English
• Published: United States 01.03.2013
• Subjects: 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
• ISSN: 1530-6860
• DOI: 10.1096/fj.12-206300

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.