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. © FASEB.