Identification of a vesicular aspartate transporter.
Aspartate is an excitatory amino acid that is costored with glutamate in synaptic vesicles of hippocampal neurons and synaptic-like microvesicles (SLMVs) of pinealocytes and is exocytosed and stimulates neighboring cells by binding to specific cell receptors. Although evidence increasingly supports the occurrence of aspartergic neurotransmission, this process is still debated because the mechanism for the vesicular storage of aspartate is unknown. Here, we show that sialin, a lysosomal H(+)/sialic acid cotransporter, is present in hippocampal synaptic vesicles and pineal SLMVs. RNA interference of sialin expression decreased exocytosis of aspartate and glutamate in pinealocytes. Proteoliposomes containing purified sialin actively accumulated aspartate and glutamate to a similar extent when inside positive membrane potential is imposed as the driving force. Sialin carrying a mutation found in people suffering from Salla disease (R39C) was completely devoid of aspartate and glutamate transport activity, although it retained appreciable H(+)/sialic acid cotransport activity. These results strongly suggest that sialin possesses dual physiological functions and acts as a vesicular aspartate/glutamate transporter. It is possible that people with Salla disease lose aspartergic (and also the associated glutamatergic) neurotransmission, and this could provide an explanation for why Salla disease causes severe neurological defects.
Miyaji
Takaaki
T
Department of Membrane Biochemistry, Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences, Okayama 700-8530, Japan.
Echigo
Noriko
N
Hiasa
Miki
M
Senoh
Shigenori
S
Omote
Hiroshi
H
Moriyama
Yoshinori
Y
D000255:Adenosine Triphosphate / Q000378:metabolism; D026922:Amino Acid Transport Systems, Acidic / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism*; D001692:Biological Transport; D002478:Cells, Cultured; D005089:Exocytosis; D018698:Glutamic Acid / Q000378:metabolism; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D051379:Mice; D019158:N-Acetylneuraminic Acid / Q000378:metabolism; D009474:Neurons / Q000378:metabolism*; D027361:Organic Anion Transporters / Q000235:genetics / Q000378:metabolism*; D010802:Phylogeny; D051381:Rats; D017208:Rats, Wistar; D027981:Symporters / Q000235:genetics / Q000378:metabolism*
D026922:Amino Acid Transport Systems, Acidic; D027361:Organic Anion Transporters; D027981:Symporters; C421224:sialic acid transport proteins; D001224:Aspartic Acid; D018698:Glutamic Acid; D000255:Adenosine Triphosphate; D019158:N-Acetylneuraminic Acid