Neuron-derived D-serine release provides a novel means to activate N-methyl-D-aspartate receptors.
D-serine is a coagonist of N-methyl-D-aspartate (NMDA) receptors that occurs at high levels in the brain. Biosynthesis of D-serine is carried out by serine racemase, which converts L- to D-serine. D-serine has been demonstrated to occur in glial cells, leading to the proposal that astrocytes are the only source of D-serine. We now report significant amounts of serine racemase and D-serine in primary neuronal cultures and neurons in vivo. Several neuronal culture types expressed serine racemase, and D-serine synthesis was comparable with that in glial cultures. Immunohistochemical staining of brain sections with new antibodies revealed the presence of serine racemase and D-serine in neurons. Cortical neurons expressing serine racemase also expressed the NR2a subunit in situ. Neuron-derived D-serine contributes to NMDA receptor activation in cortical neuronal cultures. Degradation of endogenous D-serine by addition of the recombinant enzyme D-serine deaminase diminished NMDA-elicited excitotoxicity. Release of neuronal D-serine was mediated by ionotropic glutamate receptor agonists such as NMDA, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and kainate. Removal of either external Ca2+ or Na+ blocked D-serine release. Release of D-serine was mostly through a cytosolic route because it was insensitive to bafilomycin A1, a potent inhibitor of vesicular neurotransmitter uptake. D-serine was also not transported into purified synaptic vesicles under conditions optimal for the uptake of known transmitters. Our results suggest that neurons are a major source of D-serine. Glutamate-induced neuronal D-serine release provides a novel mechanism for activating NMDA receptors by an autocrine or paracrine way.
Department of Biochemistry, Bruce Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, Haifa 31096, Israel.
Elena
E
Kartvelishvily
Maria
M
Shleper
Livia
L
Balan
Elena
E
Dumin
Herman
H
Wolosker
D000818:Animals; D001253:Astrocytes / Q000378:metabolism; D001921:Brain / Q000378:metabolism; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D007608:Kainic Acid / Q000494:pharmacology; D009474:Neurons / Q000378:metabolism*; D011422:Propionates / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D012694:Serine / Q000737:chemistry* / Q000378:metabolism; D013569:Synapses / Q000378:metabolism
D011422:Propionates; D016194:Receptors, N-Methyl-D-Aspartate; D012694:Serine; C029658:propionic acid; D007608:Kainic Acid