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D-Serine is a D-amino acid that occurs at high levels in the mammalian brain and is an endogenous ligand of the "glycine site" of N-methyl D-aspartate (NMDA) 1 receptors (1-4). NMDA receptors play key roles in excitatory synaptic transmission, plasticity, and learning and memory (5). Overactivation of the NMDA receptor and the resultant influx of calcium into cells is a major culprit in the cell death that occurs following stroke and neurodegenerative diseases. Blockers of the "glycine site" of the receptor are neuroprotective in animal models of stroke (5). Endogenous D-serine is required for NMDA receptor activation, and its removal markedly decreases NMDA receptor activity (3). In the vertebrate retina, endogenous D-serine may also mediate the light-dependent increase in neuronal activity by activating NMDA receptors (6). More recently, D-serine was suggested to play a role in the long term potentiation of synaptic transmission in the hippocampus, indicating a role of endogenous D-serine in long term synaptic plasticity (7).D-Serine is synthesized by serine racemase, a pyridoxal phosphate (PLP)-dependent enzyme enriched in the mammalian brain (8, 9). Serine racemase has high sequence homology with the fold-type II group of PLP enzymes, such as serine/threonine dehydratase and D-serine dehydratase (10, 11). In addition to converting L-to D-serine, serine racemase catalyzes the ␣,␤-elimination of water from L-serine to form pyruvate and ammonia (12). The initial rates of racemization and ␣,␤-elimination of L-serine by serine racemase are strongly stimulated by magnesium and ATP, indicating that the complex Mg⅐ATP is a physiological ligand of the enzyme (12).In accordance with accepted mechanisms of PLP-catalyzed reactions (13-16), a mechanism for racemization and ␣,␤-elimination catalyzed by serine racemase is depicted in Scheme 1. PLP, bound to the enzyme through an internal aldimine with The termination of signaling by a neurotransmitter in the brain normally requires its re-uptake and metabolism. D-Serine signaling is thought to involve its release from cells to

N-methyl d-aspartate receptors (NMDARs) are key excitatory neurotransmitter receptors in the brain and are involved in many physiological processes, including memory formation, synaptic plasticity and development [1]. The NMDARs are composed of multiple subunits and their activity is regulated by numerous mechanisms, including different ligands and interacting proteins [2]. The NMDARs display high permeability to Ca 2+ , which is known to play a central role in syn-aptic plasticity and many signal transduction mechanisms [1]. NMDAR overstimulation promotes neurotoxicity and is implicated in several pathological conditions, such as stroke and neurodegenerative diseases [3]. The NMDARs are unique in their requirement for more than one agonist to operate. Glutamate, the main NMDAR agonist, does not activate the receptors unless a co-agonist binding site located at the NR1 subunit is occupied [4,5]. d-Serine, an unusual d-amino acid present in mammalian brain, is now recognized as a physiological ligand of the NMDAR co-agonist site, mediating several NMDAR-dependent processes [6-15]. At first, the NMDAR co-agonist site was thought to be occupied by glycine. Hence, the co-agonist site is also generally referred to as the 'glycine site'. In addition, to be essential for NMDAR activity, the co-agonist site exerts neuromodulatory roles. Thus, co-agonist binding increases the receptor's affinity for glutamate [16], decreases its desensitization [17] and promotes NMDAR turnover by internalization [18]. Since its discovery, the role of the co-agonist site in regulating the activity of the NMDAR has been The mammalian brain contains unusually high levels of d-serine, a d-amino acid previously thought to be restricted to some bacteria and insects. In the last few years, studies from several groups have demonstrated that d-serine is a physiological co-agonist of the N-methyl d-aspartate (NMDA) type of glutamate receptor-a key excitatory neurotransmitter receptor in the brain. d-Serine binds with high affinity to a co-agonist site at the NMDA receptors and, along with glutamate, mediates several important physiological and pathological processes, including NMDA receptor transmission, synaptic plasticity and neurotoxicity. In recent years, biosynthetic, degrada-tive and release pathways for d-serine have been identified, indicating that d-serine may function as a transmitter. At first, d-serine was described in astrocytes, a class of glial cells that ensheathes neurons and release several transmitters that modulate neurotransmission. This led to the notion that d-serine is a glia-derived transmitter (or gliotransmitter). However, recent data indicate that serine racemase, the d-serine biosynthetic enzyme, is widely expressed in neurons of the brain, suggesting that d-serine also has a neuronal origin. We now review these findings, focusing on recent questions regarding the roles of glia versus neurons in d-serine signaling. Abbreviations ALS, amyotrophic lateral sclerosis; AMPA, a-amino-3-hydroxy-5-methylisoxazole-4-propionic a...

High levels of D-serine occur in the brain, challenging the notion that D-amino acids would not be present or play a role in mammals. D-serine levels in the brain are even higher than many L-amino acids, such as asparagine, valine, isoleucine, and tryptophan, among others. D-serine is synthesized by a serine racemase (SR) enzyme, which directly converts L-to D-serine. We now report that SR is a bifunctional enzyme, producing both D-serine and pyruvate in cultured cells and in vitro. Transfection of SR into HEK 293 cells elicits synthesis of D-serine and augmented release of pyruvate to culture media. We identified substances present in HEK 293 and astrocyte cell extracts that strongly stimulate D-serine production by SR and elicit production of pyruvate. Experiments with recombinant enzyme reveal that Mg 2؉ and ATP present in the cell extracts are physiological cofactors and increase 5-to 10-fold the rates of racemization and production of pyruvate. As much as three molecules of pyruvate are synthesized for each molecule of D-serine produced by SR. This finding constitutes a previously undescribed mechanism underlying D-amino acid synthesis in mammals, different from classical amino acid racemases present in bacteria. Our data link the production of D-serine to the energy metabolism, with implications for the metabolic and transmitter crosstalk between glia and neurons.glutamate receptors ͉ pyruvate ͉ astrocytes I n recent years, it has been shown that astrocytes and neurons exhibit a dynamic bidirectional signaling that profoundly influences neuronal activity and development (1). Astrocytes modulate synaptic transmission by releasing chemical transmitters and eliciting Ca 2ϩ waves in nearby neurons. The search for new transmitter molecules in the brain uncovered the presence of high levels of D-serine in astrocytes, a D-amino acid not previously thought to occur in mammals (2, 3). D-serine is synthesized by a glial-enriched enzyme serine racemase (SR), which directly converts L-to D-serine (4-7). SR does not bear significant homology with bacterial racemases, and little is known about the mechanism and regulation of D-serine production.D-serine released from astrocytes seems to be an endogenous ligand of the N-methyl-D-aspartate (NMDA) receptor (3,8). Depletion of endogenous D-serine in slices and cultured cells strongly diminishes NMDA receptor responses measured biochemically and electrophysiologically (8). Massive stimulation of NMDA receptors is implicated in neural damage after stroke (9), and inhibitors of SR may be useful to prevent stroke damage. Thus, inhibitors of SR provide a strategy to decrease NMDA receptor coactivation and may be useful in conditions in which overstimulation of NMDA receptors plays a pathological role.To clarify the role of D-serine as a modulator of NMDA receptors, one should identify the factors that regulate SR and D-serine signaling. In this paper, we explored mechanisms regulating the production of D-serine by SR. We demonstrate that SR is a unique bifunctional enzyme, produ...