Neuronal release of d-serine: a physiological pathway controlling extracellular d-serine concentration

d-Serine is thought to be a glia-derived transmitter that activates N-methyl d-aspartate receptors (NMDARs) in the brain. Here, we investigate the pathways for d-serine release using primary cultures, brain slices, and in vivo microdialysis. In contrast with the notion that d-serine is exclusively released from astrocytes, we found that d-serine is released by neuronal depolarization both in vitro and in vivo. Veratridine (50 μM) or depolarization by 40 mM KCl elicits a significant release of endogenous d-serine from primary neuronal cultures. Controls with astrocyte cultures indicate that glial cells are insensitive to veratridine, but release d-serine mainly by the opening of volume-regulated anion channels. In cortical slices perfused with veratridine, endogenous d-serine release is 10-fold higher than glutamate receptor-evoked release. Release of d-serine from slices does not require internal or external Ca2+, suggesting a nonvesicular release mechanism. To confirm the neuronal origin of d-serine, we selectively loaded neurons in cortical slices with d-[3H]serine or applied d-alanine, which specifically releases d-serine from neurons. Depolarization with veratridine promotes d-serine release in vivo monitored by high temporal resolution microdialysis of the striatum. Our data indicate that the neuronal pool of d-serine plays a major role in d-serine dynamics, with implications for the regulation of NMDAR transmission. Rosenberg, D., Kartvelishvily, E., Shleper, M., Klinker, C. M. C., Bowser, M. T., Wolosker, H. Neuronal release of d-serine: a physiological pathway controlling extracellular d-serine concentration

Feedback inactivation of D-serine synthesis by NMDA receptor-elicited translocation of serine racemase to the membrane

D-serine is a physiological coagonist of N-methyl D-aspartate receptors (NMDARs) that plays a major role in several NMDAR-dependent events. In this study we investigate mechanisms regulating D-serine production by the enzyme serine racemase (SR). We now report that NMDAR activation promotes translocation of SR to the plasma membrane, which dramatically reduces the enzyme activity. Membrane-bound SR isolated from rat brain is not extracted from the membrane by high detergent and salt concentration, indicating a strong association. Colocalization studies indicate that most membrane-bound SR is located at the plasma membrane and dendrites, with much less SR observed in other types of membrane. NMDAR activation promotes translocation of the cytosolic SR to the membrane, resulting in reduced D-serine synthesis, and this effect is averted by blockade of NMDARs. In primary neuronal cultures, SR translocation to the membrane is blocked by a palmitoylation inhibitor, indicating that membrane binding is mediated by fatty acid acylation of SR. In agreement, we found that SR is acylated in transfected neuroblastoma cells using [3H]palmitate or [3H]octanoic acid as precursors. In contrast to classical S-palmitoylation of cysteines, acylation of SR occurs through the formation of an oxyester bond with serine or threonine residues. In addition, we show that phosphorylation of Thr-227 is also required for steady-state binding of SR to the membrane under basal, nonstimulated condition. We propose that the inhibition of D-serine synthesis caused by translocation of SR to the membrane provides a fail-safe mechanism to prevent NMDAR overactivation in vicinal cells or synapses