Distribution of free D-serine in vertebrate brains

Biosynthesis and Degradation of Free D-Amino Acids and Their Physiological Roles in the Periphery and Endocrine Glands

It was long believed that D-amino acids were either unnatural isomers or laboratory artifacts, and that the important functions of amino acids were exerted only by L-amino acids. However, recent investigations have revealed a variety of D-amino acids in mammals that play important roles in physiological functions, including free D-serine and D-aspartate that are crucial in the central nervous system. The functions of several D-amino acids in the periphery and endocrine glands are also receiving increasing attention. Here, we present an overview of recent advances in elucidating the physiological roles of D-amino acids, especially in the periphery and endocrine glands.

Astrocytic D -amino acid oxidase degrades D -serine in the hindbrain

_D -serine modulates excitatory neurotransmission by binding to N-methyl-_D -aspartate glutamate receptors. _D- amino acid oxidase (DAO) degrades _D -amino acids, such as _D -serine, in the central nervous system, and is associated with neurological and psychiatric disorders. However, cell types that express brain DAO remain controversial, and whether brain DAO influences systemic _D -amino acids in addition to brain _D -serine remains unclear. Here, we created astrocyte-specific DAO-conditional knockout mice. Knockout in glial fibrillary acidic protein (GFAP)-positive cells eliminated DAO expression in the hindbrain and increased _D -serine levels significantly in the cerebellum. Brain DAO did not influence levels of _D -amino acids in the forebrain or periphery. These results show that astrocytic DAO regulates _D -serine specifically in the hindbrain.

Astrocytic contribution to glutamate-related central respiratory chemoreception in vertebrates

Central respiratory chemoreceptors play a key role in the respiratory homeostasis by sensing CO₂ and H⁺ in brain and activating the respiratory neural network. This ability of specific brain regions to respond to acidosis and hypercapnia is based on neuronal and glial mechanisms. Several decades ago, glutamatergic transmission was proposed to be involved as a main mechanism in central chemoreception. However, a complete identification of mechanism has been elusive. At the rostral medulla, chemosensitive neurons of the retrotrapezoid nucleus (RTN) are glutamatergic and they are stimulated by ATP released by RTN astrocytes in response to hypercapnia. In addition, recent findings show that caudal medullary astrocytes in brainstem can also contribute as CO₂ and H⁺ sensors that release D-serine and glutamate, both gliotransmitters able to activate the respiratory neural network. In this review, we describe the mammalian astrocytic glutamatergic contribution to the central respiratory chemoreception trying to trace in vertebrates the emergence of several components involved in this process.

Glycine agonism in ionotropic glutamate receptors

Ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate the majority of excitatory neurotransmission in the vertebrate CNS. Classified as AMPA, kainate, delta and NMDA receptors, iGluRs are central drivers of synaptic plasticity widely considered as a major cellular substrate of learning and memory. Surprisingly however, five out of the eighteen vertebrate iGluR subunits do not bind glutamate but glycine, a neurotransmitter known to mediate inhibitory neurotransmission through its action on pentameric glycine receptors (GlyRs). This is the case of GluN1, GluN3A, GluN3B, GluD1 and GluD2 subunits, all also binding the D amino acid d-serine endogenously present in many brain regions. Glycine and d-serine action and affinities broadly differ between glycinergic iGluR subtypes. On 'conventional' GluN1/GluN2 NMDA receptors, glycine (or d-serine) acts in concert with glutamate as a mandatory co-agonist to set the level of receptor activity. It also regulates the receptor's trafficking and expression independently of glutamate. On 'unconventional' GluN1/GluN3 NMDARs, glycine acts as the sole agonist directly triggering opening of excitatory glycinergic channels recently shown to be physiologically relevant. On GluD receptors, d-serine on its own mediates non-ionotropic signaling involved in excitatory and inhibitory synaptogenesis, further reinforcing the concept of glutamate-insensitive iGluRs. Here we present an overview of our current knowledge on glycine and d-serine agonism in iGluRs emphasizing aspects related to molecular mechanisms, cellular function and pharmacological profile. The growing appreciation of the critical influence of glycine and d-serine on iGluR biology reshapes our understanding of iGluR signaling diversity and complexity, with important implications in neuropharmacology.

D-Aspartate consumption selectively promotes intermediate-term spatial memory and the expression of hippocampal NMDA receptor subunits

d-Aspartate (d-Asp) and d-serine (d-Ser) have been proposed to promote early-phase LTP in vitro and to enhance spatial memory in vivo. Here, we investigated the behavioural effects of chronic consumption of d-Asp and d-Ser on spatial learning of mice together with the expression of NMDA receptors. We also studied the alterations of neurogenesis by morphometric analysis of bromo-deoxyuridine incorporating and doublecortin expressing cells in the hippocampus. Our results specify a time period (3–4 h post-training), within which the animals exposed to d-Asp (but not d-Ser) show a more stable memory during retrieval. The cognitive improvement is due to elimination of transient bouts of destabilization and reconsolidation of memory, rather than to enhanced acquisition. d-Asp also protracted reversal learning probably due to reduced plasticity. Expression of GluN1 and GluN2A subunits was elevated in the hippocampus of d-Asp (but not d-Ser) treated mice. d-Asp or d-Ser did not alter the proliferation of neuronal progenitor cells in the hippocampus. The observed learning-related changes evoked by d-Asp are unlikely to be due to enhanced proliferation and recruitment of new neurones. Rather, they are likely associated with an upregulation of NMDA receptors, as well as a reorganization of receptor subunit assemblies in existing hippocampal/dentate neurons.

DStabilize: A Web Resource to Generate Mirror Images of Biomolecules

Peptides comprising D-amino acids have been shown to be resistant to proteolysis. This makes them potential candidates as probes of cellular interactions, notably protein-biomolecule interactions. However, the empirical conversion of the amino acids that constitute a peptide from L-forms to D-forms will result in abrogation of the normal interactions made by the L-amino acids due to side-chain orientation changes that are associated with the changes in chirality. These interactions can be preserved by reversing the sequence of the D-peptide. We present a web server (http://dstabilize.bii.a-star.edu.sg/) that allows users to convert between L-proteins and D-proteins and for sequence reversal of D-peptides, along with the capability of performing other empirical geometric transforms. This resource allows the user to generate structures of interest easily for subsequent in silico processing.

Advances in D-Amino Acids in Neurological Research

D-amino acids have been known to exist in the human brain for nearly 40 years, and they continue to be a field of active study to today. This review article aims to give a concise overview of the recent advances in D-amino acid research as they relate to the brain and neurological disorders. This work has largely been focused on modulation of the N-methyl-D-aspartate (NMDA) receptor and its relationship to Alzheimer’s disease and Schizophrenia, but there has been a wealth of novel research which has elucidated a novel role for several D-amino acids in altering brain chemistry in a neuroprotective manner. D-amino acids which have no currently known activity in the brain but which have active derivatives will also be reviewed.

l-Serine links metabolism with neurotransmission

Brain energy metabolism is often considered as a succession of biochemical steps that metabolize the fuel (glucose and oxygen) for the unique purpose of providing sufficient ATP to maintain the huge information processing power of the brain. However, a significant fraction (10-15 %) of glucose is shunted away from the ATP-producing pathway (oxidative phosphorylation) and may be used to support other functions. Recent studies have pointed to the marked compartmentation of energy metabolic pathways between neurons and glial cells. Here, we focused our attention on the biosynthesis of l-serine, a non-essential amino acid that is formed exclusively in glial cells (mostly astrocytes) by re-routing the metabolic fate of the glycolytic intermediate, 3-phosphoglycerate (3PG). This metabolic pathway is called the phosphorylated pathway and transforms 3PG into l-serine via three enzymatic reactions. We first compiled the available data on the mechanisms that regulate the flux through this metabolic pathway. We then reviewed the current evidence that is beginning to unravel the roles of l-serine both in the healthy and diseased brain, leading to the notion that this specific metabolic pathway connects glial metabolism with synaptic activity and plasticity. We finally suggest that restoring astrocyte-mediated l-serine homeostasis may provide new therapeutic strategies for brain disorders.

Nutritional Characteristics and Functions of D-Amino Acids in the Chicken

D-Amino acids occur in modest amounts in bacterial proteins and the bacterial cell wall, as well as in peptide antibiotics. Therefore, D-amino acids present in terrestrial vertebrates were believed to be derived from bacteria present in the gastrointestinal tract or fermented food. However, both exogenous and endogenous origins of D-amino acids have been confirmed. Terrestrial vertebrates possess an enzyme for converting certain L-isomers to D-isomers. D-Amino acids have nutritional aspects and functions, some are similar to, and others are different from those of L-isomers. Here, we describe the nutritional characteristics and functions of D-amino acids and also discuss the future perspectives of D-amino acid nutrition in the chicken.

Distinctive Roles of D-Amino Acids in the Homochiral World: Chirality of Amino Acids Modulates Mammalian Physiology and Pathology

Living organisms enantioselectively employ L-amino acids as the molecular architecture of protein synthesized in the ribosome. Although L-amino acids are dominantly utilized in most biological processes, accumulating evidence points to the distinctive roles of D-amino acids in non-ribosomal physiology. Among the three domains of life, bacteria have the greatest capacity to produce a wide variety of D-amino acids. In contrast, archaea and eukaryotes are thought generally to synthesize only two kinds of D-amino acids: D-serine and D-aspartate. In mammals, D-serine is critical for neurotransmission as an endogenous coagonist of N-methyl D-aspartate receptors. Additionally, D-aspartate is associated with neurogenesis and endocrine systems. Furthermore, recognition of D-amino acids originating in bacteria is linked to systemic and mucosal innate immunity. Among the roles played by D-amino acids in human pathology, the dysfunction of neurotransmission mediated by D-serine is implicated in psychiatric and neurological disorders. Non-enzymatic conversion of L-aspartate or L-serine residues to their D-configurations is involved in age-associated protein degeneration. Moreover, the measurement of plasma or urinary D-/L-serine or D-/L-aspartate levels may have diagnostic or prognostic value in the treatment of kidney diseases. This review aims to summarize current understanding of D-amino-acid-associated biology with a major focus on mammalian physiology and pathology.

Heterogeneity of D-Serine Distribution in the Human Central Nervous System

D-serine is an endogenous ligand for N-methyl-D-aspartate glutamate receptors. Accumulating evidence including genetic associations of D-serine metabolism with neurological or psychiatric diseases suggest that D-serine is crucial in human neurophysiology. However, distribution and regulation of D-serine in humans are not well understood. Here, we found that D-serine is heterogeneously distributed in the human central nervous system (CNS). The cerebrum contains the highest level of D-serine among the areas in the CNS. There is heterogeneity in its distribution in the cerebrum and even within the cerebral neocortex. The neocortical heterogeneity is associated with Brodmann or functional areas but is unrelated to basic patterns of cortical layer structure or regional expressional variation of metabolic enzymes for D-serine. Such D-serine distribution may reflect functional diversity of glutamatergic neurons in the human CNS, which may serve as a basis for clinical and pharmacological studies on D-serine modulation.

d-Amino Acid Levels in Perfused Mouse Brain Tissue and Blood: A Comparative Study

The l-enantiomer is the predominant type of amino acid in all living systems. However, d-amino acids, once thought to be "unnatural", have been found to be indigenous even in mammalian systems and increasingly appear to be functioning in essential biological and neurological roles. Both d- and l-amino acid levels in the hippocampus, cortex, and blood samples from NIH Swiss mice are reported. Perfused brain tissues were analyzed for the first time, thereby eliminating artifacts due to endogenous blood, and decreased the mouse-to-mouse variability in amino acid levels. Total amino acid levels (l- plus d-enantiomers) in brain tissue are up to 10 times higher than in blood. However, all measured d-amino acid levels in brain tissue are typically ∼10 to 2000 times higher than blood levels. There was a 13% reduction in almost all measured d-amino acid levels in the cortex compared to those in the hippocampus. There is an approximate inverse relationship between the prevalence of an amino acid and the percentage of its d-enantiomeric form. Interestingly, glutamic acid, unlike all other amino acids, had no quantifiable level of its d-antipode. The bioneurological reason for the unique and conspicuous absence/removal of this d-amino acid is yet unknown. However, results suggest that d-glutamate metabolism is likely a unidirectional process and not a cycle, as per the l-glutamate/glutamine cycle. The results suggest that there might be unreported d-amino acid racemases in mammalian brains. The regulation and function of specific other d-amino acids are discussed.

Participation of D-serine in the development and reproduction of the silkworm Bombyx mori

The silkworm Bombyx mori contains high concentrations of free D-serine, an optical isomer of L-serine. To elucidate its function, we first investigated the localization of D-serine in various organs of silkworm larvae, pupae, and adult moths. Using immunohistochemical analysis with an anti-D-serine antibody, we found D-serine in the microvilli of midgut goblet and cylindrical cells and in peripheral matrix components of testicular and ovarian cells. By spectrophotometric analysis, D-serine was also found in the hemolymph and fat body. D-Alanine was not detected in the various organs by immunohistochemistry. Serine racemase, which catalyzes the inter-conversion of L- and D-serine, was found to co-localize with D-serine, and D-serine production from L-serine by intrinsic serine racemase was suggested. O-Phospho-L-serine is an inhibitor of serine racemase, and it was administered to the larvae to reduce the D-serine level. This reagent decreased the midgut caspase-3 level and caused a delay in spermatogenesis and oogenesis. The reagent also decreased mature sperm and egg numbers, suggesting D-serine participation in these processes. D-Serine administration induced an increase in pyruvate levels in testis, midgut, and fat body, indicating conversion of D-serine to pyruvate. On the basis of these results, together with our previous investigation of ATP biosynthesis in testis, we consider the possible involvement of D-serine in ATP synthesis for metamorphosis and reproduction.

The intrinsic disorder alphabet. III. Dual personality of serine

Proteins are natural polypeptides consisting of 20 major amino acid residues, content and order of which in a given amino acid sequence defines the ability of a related protein to fold into unique functional state or to stay intrinsically disordered. Amino acid sequences code for both foldable (ordered) proteins/domains and for intrinsically disordered proteins (IDPs) and IDP regions (IDPRs), but these sequence codes are dramatically different. This difference starts with a very general property of the corresponding amino acid sequences, namely, their compositions. IDPs/IDPRs are enriched in specific disorder-promoting residues, whereas amino acid sequences of ordered proteins/domains typically contain more order-promoting residues. Therefore, the relative abundances of various amino acids in ordered and disordered proteins can be used to scale amino acids according to their disorder promoting potentials. This review continues a series of publications on the roles of different amino acids in defining the phenomenon of protein intrinsic disorder and represents serine, which is the third most disorder-promoting residue. Similar to previous publications, this review represents some physico-chemical properties of serine and the roles of this residue in structures and functions of ordered proteins, describes major posttranslational modifications tailored to serine, and finally gives an overview of roles of serine in structure and functions of intrinsically disordered proteins.

Effect of D-serine on spermatogenesis and extracellular signal-regulated protein kinase (ERK) phosphorylation in the testis of the silkworm, Bombyx mori

Although the pupae and larvae of Bombyx mori possess especially large amounts of free d-serine, the physiological role of the amino acid in the silkworm is unknown. We investigated the effect of d-serine on spermatogenesis. A lowered d-serine level throughout larval development caused a delay in spermatogenesis and resulted in reduced numbers of eupyrene sperm. Administration of d-serine transiently increased the activation of extracellular signal-regulated protein kinase1/2 (ERK1/2; hereafter, ERK) by approximately 25% in the testis of day 3 fifth instar larvae. l-Serine had no effect on ERK activation, and other organs did not respond to d-serine. The effect of d-serine on ERK activation was confirmed by administering d-serine dehydratase, an enzyme that specifically degrades d-serine, and the enzyme's inhibitor, hydroxylamine. ERK phosphorylation in the testis was significantly inhibited by Go6983 and U0126, inhibitors of protein kinase C (PKC) and mitogen-associated protein kinase kinase 1/2 (MEK), respectively, but not by H-89, a protein kinase A (PKA) inhibitor, indicating that ERK was activated in the testis via PKC and MEK but not via PKA. The inhibition of ERK phosphorylation by Go6983 or U0126 was reduced by 20-30% by d-serine. Roughly 30% of c-Raf phosphorylation at an inhibitory site (Ser259) was decreased by the addition of d-serine. These results suggest that d-serine activates ERK in the testis of silkworms through a pathway including c-Raf but not PKC or MEK. Immunohistochemistry confirmed d-serine-induced ERK phosphorylation in the testis and revealed the presence of phospho-ERK in the nuclei of spermatocytes and spermatids.

D-Serine in Neuropsychiatric Disorders: New Advances

D-Serine (DSR) is an endogenous amino acid involved in glia-synapse interactions that has unique neurotransmitter characteristics. DSR acts as obligatory coagonist at the glycine site associated with the N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) and has a cardinal modulatory role in major NMDAR-dependent processes including NMDAR-mediated neurotransmission, neurotoxicity, synaptic plasticity, and cell migration. Since either over- or underfunction of NMDARs may be involved in the pathophysiology of neuropsychiatric disorders; the pharmacological manipulation of DSR signaling represents a major drug development target. A first generation of proof-of-concept animal and clinical studies suggest beneficial DSR effects in treatment-refractory schizophrenia, movement, depression, and anxiety disorders and for the improvement of cognitive performance. A related developing pharmacological strategy is the indirect modification of DSR synaptic levels by use of compounds that alter the function of main enzymes responsible for DSR production and degradation. Accumulating data indicate that, during the next decade, we will witness important advances in the understanding of DSR role that will further contribute to elucidating the causes of neuropsychiatric disorders and will be instrumental in the development of innovative treatments.

Activity of D-amino acid oxidase is widespread in the human central nervous system

It has been proposed that D-amino acid oxidase (DAO) plays an essential role in degrading D-serine, an endogenous coagonist of N-methyl-D-aspartate (NMDA) glutamate receptors. DAO shows genetic association with amyotrophic lateral sclerosis (ALS) and schizophrenia, in whose pathophysiology aberrant metabolism of D-serine is implicated. Although the pathology of both essentially involves the forebrain, in rodents, enzymatic activity of DAO is hindbrain-shifted and absent in the region. Here, we show activity-based distribution of DAO in the central nervous system (CNS) of humans compared with that of mice. DAO activity in humans was generally higher than that in mice. In the human forebrain, DAO activity was distributed in the subcortical white matter and the posterior limb of internal capsule, while it was almost undetectable in those areas in mice. In the lower brain centers, DAO activity was detected in the gray and white matters in a coordinated fashion in both humans and mice. In humans, DAO activity was prominent along the corticospinal tract, rubrospinal tract, nigrostriatal system, ponto-/olivo-cerebellar fibers, and in the anterolateral system. In contrast, in mice, the reticulospinal tract and ponto-/olivo-cerebellar fibers were the major pathways showing strong DAO activity. In the human corticospinal tract, activity-based staining of DAO did not merge with a motoneuronal marker, but colocalized mostly with excitatory amino acid transporter 2 and in part with GFAP, suggesting that DAO activity-positive cells are astrocytes seen mainly in the motor pathway. These findings establish the distribution of DAO activity in cerebral white matter and the motor system in humans, providing evidence to support the involvement of DAO in schizophrenia and ALS. Our results raise further questions about the regulation of D-serine in DAO-rich regions as well as the physiological/pathological roles of DAO in white matter astrocytes.

Immunohistochemical localization of D-serine dehydratase in chicken tissues

Chicken D-serine dehydratase (DSD) degrades d-serine to pyruvate and ammonia. The enzyme requires both pyridoxal 5'-phosphate and Zn(2+) for its activity. d-Serine is a physiological coagonist that regulates the activity of the N-methyl-d-aspartate receptor (NMDAR) for l-glutamate. We have recently found in chickens that d-serine is degraded only by DSD in the brain, whereas it is also degraded to 3-hydroxypyruvate by d-amino acid oxidase (DAO) in the kidney and liver. In mammalian brains, d-serine is degraded only by DAO. It has not been clarified why chickens selectively use DSD for the control of d-serine concentrations in the brain. In the present study, we measured DSD activity in chicken tissues, and examined the cellular localization of DSD using a specific anti-chicken DSD antibody. The highest activity was found in kidney. Skeletal muscles and heart showed no activity. In chicken brain, cerebellum showed about 6-fold-higher activity (1.1 ± 0.3 U/g protein) than cerebrum (0.19 ± 0.03 U/g protein). At the cellular level DSD was demonstrated in proximal tubule cells of the kidney, in hepatocytes, in Bergmann-glia cells of the cerebellum and in astrocytes. The finding of DSD in glial cells seems to be important because d-serine is involved in NMDAR-dependent brain functions.

Effects of D-amino acid oxidase inhibition on memory performance and long-term potentiation in vivo

N-methyl-d-aspartate receptor (NMDAR) activation can initiate changes in synaptic strength, evident as long-term potentiation (LTP), and is a key molecular correlate of memory formation. Inhibition of d-amino acid oxidase (DAAO) may increase NMDAR activity by regulating d-serine concentrations, but which neuronal and behavioral effects are influenced by DAAO inhibition remain elusive. In anesthetized rats, extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded before and after a theta frequency burst stimulation (TBS) of the Schaffer collateral pathway of the CA1 region in the hippocampus. Memory performance was assessed after training with tests of contextual fear conditioning (FC, mice) and novel object recognition (NOR, rats). Oral administration of 3, 10, and 30 mg/kg 4H-furo[3,2-b]pyrrole-5-carboxylic acid (SUN) produced dose-related and steady increases of cerebellum d-serine in rats and mice, indicative of lasting inhibition of central DAAO. SUN administered 2 h prior to training improved contextual fear conditioning in mice and novel object recognition memory in rats when tested 24 h after training. In anesthetized rats, LTP was established proportional to the number of TBS trains. d-cycloserine (DCS) was used to identify a submaximal level of LTP (5× TBS) that responded to NMDA receptor activation; SUN administered at 10 mg/kg 3-4 h prior to testing similarly increased in vivo LTP levels compared to vehicle control animals. Interestingly, in vivo administration of DCS also increased brain d-serine concentrations. These results indicate that DAAO inhibition increased NMDAR-related synaptic plasticity during phases of post training memory consolidation to improve memory performance in hippocampal-dependent behavioral tests.

D-serine as a gliotransmitter and its roles in brain development and disease

The development of new techniques to study glial cells has revealed that they are active participants in the development of functional neuronal circuits. Calcium imaging studies demonstrate that glial cells actively sense and respond to neuronal activity. Glial cells can produce and release neurotransmitter-like molecules, referred to as gliotransmitters, that can in turn influence the activity of neurons and other glia. One putative gliotransmitter, D-serine is believed to be an endogenous co-agonist for synaptic N-methyl-D-aspartate receptors (NMDARs), modulating synaptic transmission and plasticity mediated by this receptor. The observation that D-serine levels in the mammalian brain increase during early development, suggests a possible role for this gliotransmitter in normal brain development and circuit refinement. In this review we will examine the data that D-serine and its associated enzyme serine racemase are developmentally regulated. We will consider the evidence that D-serine is actively released by glial cells and examine the studies that have implicated D-serine as a critical player involved in regulating NMDAR-mediated synaptic transmission and neuronal migration during development. Furthermore, we will consider how dysregulation of D-serine may play an important role in the etiology of neurological and psychiatric diseases.

Alteration of intrinsic amounts of D-serine in the mice lacking serine racemase and D-amino acid oxidase

For elucidation of the regulation mechanisms of intrinsic amounts of D-serine (D-Ser) which modulates the neuro-transmission of N-methyl-D-aspartate receptors in the brain, mutant animals lacking serine racemase (SRR) and D-amino acid oxidase (DAO) were established, and the amounts of D-Ser in the tissues and physiological fluids were determined. D-Ser amounts in the frontal brain areas were drastically decreased followed by reduced SRR activity. On the other hand, a moderate but significant decrease in D-Ser amounts was observed in the cerebellum and spinal cord of SRR knock-out (SRR(-/-)) mice compared with those of control mice, although the amounts of D-Ser in these tissues were low. The amounts of D-Ser in the brain and serum were not altered with aging. To clarify the uptake of exogenous D-Ser into the brain tissues, we have determined the D-Ser of SRR(-/-) mice after oral administration of D-Ser for the first time, and a drastic increase in D-Ser amounts in all the tested tissues was observed. Because both DAO and SRR are present in some brain areas, we have established the double mutant mice lacking SRR and DAO for the first time, and the contribution of both enzymes to the intrinsic D-Ser amounts was investigated. In the frontal brain, most of the intrinsic D-Ser was biosynthesized by SRR. On the other hand, half of the D-Ser present in the hindbrain was derived from the biosynthesis by SRR. These results indicate that the regulation of intrinsic D-Ser amounts is different depending on the tissues and provide useful information for the development of treatments for neuronal diseases.

Crystal structure of a zinc-dependent D-serine dehydratase from chicken kidney

D-serine is a physiological co-agonist of the N-methyl-D-aspartate receptor. It regulates excitatory neurotransmission, which is important for higher brain functions in vertebrates. In mammalian brains, D-amino acid oxidase degrades D-serine. However, we have found recently that in chicken brains the oxidase is not expressed and instead a D-serine dehydratase degrades D-serine. The primary structure of the enzyme shows significant similarities to those of metal-activated D-threonine aldolases, which are fold-type III pyridoxal 5'-phosphate (PLP)-dependent enzymes, suggesting that it is a novel class of D-serine dehydratase. In the present study, we characterized the chicken enzyme biochemically and also by x-ray crystallography. The enzyme activity on D-serine decreased 20-fold by EDTA treatment and recovered nearly completely by the addition of Zn(2+). None of the reaction products that would be expected from side reactions of the PLP-D-serine Schiff base were detected during the >6000 catalytic cycles of dehydration, indicating high reaction specificity. We have determined the first crystal structure of the D-serine dehydratase at 1.9 Å resolution. In the active site pocket, a zinc ion that coordinates His(347) and Cys(349) is located near the PLP-Lys(45) Schiff base. A theoretical model of the enzyme-D-serine complex suggested that the hydroxyl group of D-serine directly coordinates the zinc ion, and that the ε-NH(2) group of Lys(45) is a short distance from the substrate Cα atom. The α-proton abstraction from D-serine by Lys(45) and the elimination of the hydroxyl group seem to occur with the assistance of the zinc ion, resulting in the strict reaction specificity.

D-Aspartate--an important bioactive substance in mammals: a review from an analytical and biological point of view

It was long believed that D-amino acids were either unnatural isomers or laboratorial artifacts and that the important functions of amino acids were exerted only by l-amino acids. However, recent investigations have shown that a variety of D-amino acids are present in mammals and that they play important roles in physiological functions in the body. Among the free d-amino acids that have been identified in mammals, D-aspartate (D-Asp) has been shown to play a crucial role in the neuroendocrine and endocrine systems as well as in the central nervous system. Here, we present an overview of recent studies of free D-Asp, focusing on the analytical methods in real biological matrices, expression and localization in tissues and cells, biological and physiological activities, biosynthesis, degradation, cellular transport, and possible relevance to disease. In addition to frequently used techniques for the enantiomeric determination of amino acids, including high-performance liquid chromatography and enzymatic methods, the recent development of analytical methods is also described.

The glycerophospho metabolome and its influence on amino acid homeostasis revealed by brain metabolomics of GDE1(-/-) mice

GDE1 is a mammalian glycerophosphodiesterase (GDE) implicated by in vitro studies in the regulation of glycerophophoinositol (GroPIns) and possibly other glycerophospho (GroP) metabolites. Here, we show using untargeted metabolomics that GroPIns is profoundly (>20-fold) elevated in brain tissue from GDE1(-/-) mice. Furthermore, two additional GroP metabolites not previously identified in eukaryotic cells, glycerophosphoserine (GroPSer) and glycerophosphoglycerate (GroPGate), were also highly elevated in GDE1(-/-) brains. Enzyme assays with synthetic GroP metabolites confirmed that GroPSer and GroPGate are direct substrates of GDE1. Interestingly, our metabolomic profiles also revealed that serine (both L-and D-) levels were significantly reduced in brains of GDE1(-/-) mice. These findings designate GroPSer as a previously unappreciated reservoir for free serine in the nervous system and suggest that GDE1, through recycling serine from GroPSer, may impact D-serine-dependent neural signaling processes in vivo.

Localization of D-serine and serine racemase in neurons and neuroglias in mouse brain

D-serine is a novel candidate for an intrinsic ligand for the glycine site of N-methyl-D-aspartate (NMDA) receptors in mammalian brain. D-serine and serine racemase, which produces D-serine from L-serine, have long been presumed to be localized in astrocytes. However, we have reported that D-serine immunoreactivity was observed in neurons in rats. In the present study, the distributions of D-serine and serine racemase were investigated in combination with marker proteins for neurons, astrocytes and oligodendrocytes in mice. Immunoreactivities for D-serine and serine racemase were found in neurons and oligodendrocytes. These results suggest that D-serine can be produced in neurons as well as glias and used as a neurotransmitter, which control the synaptic function of NMDA receptors.

d-serine enhances extinction of auditory cued fear conditioning via ERK1/2 phosphorylation in mice

Several lines of evidence suggest that the N-methyl-D-aspartate (NMDA) receptor plays a significant role in fear conditioning and extinction. However, our knowledge of the role of D-serine, an endogenous ligand for the glycine site of the NMDA receptor, in fear extinction is quite limited compared to that of D-cycloserine, an exogenous partial agonist for the same site. In the current study, we examined the effects of D-serine on fear extinction and phosphorylation of extracellular signal-regulated kinase (ERK) in the hippocampus, basolateral amygdala (BLA), and medial prefrontal cortex (mPFC) during the process of fear extinction. Systemic administrations of D-serine (2.7 g/kg, i.p.) with or without the ERK inhibitor SL327 (30 mg/kg, i.p.) to C57BL/6J mice were performed before fear extinction in a cued fear conditioning and extinction paradigm. Cytosolic and nuclear ERK 1/2 phosphorylation in the hippocampus, BLA, and mPFC were measured 1h after extinction (E1h), 24h after extinction (E24h), and 1h after recall (R1h) by Western blotting. We found that D-serine enhanced the extinction of fear memory, and the effects of D-serine were reduced by the ERK phosphorylation inhibitor SL327. The Western blot analyses showed that D-serine significantly increased cytosolic ERK 2 phosphorylation at E1h in the hippocampus and cytosolic ERK 1/2 phosphorylation at R1h in the BLA. The present study suggested that D-serine might enhance fear extinction through NMDA receptor-induced ERK signaling in mice, and that D-serine has potential clinical importance for the treatment of anxiety disorders.

Metabolism of the neuromodulator D-serine

Over the past years, accumulating evidence has indicated that D-serine is the endogenous ligand for the glycine-modulatory binding site on the NR1 subunit of N-methyl-D-aspartate receptors in various brain areas. D-Serine is synthesized in glial cells and neurons by the pyridoxal-5' phosphate-dependent enzyme serine racemase, and it is released upon activation of glutamate receptors. The cellular concentration of this novel messenger is regulated by both serine racemase isomerization and elimination reactions, as well as by its selective degradation catalyzed by the flavin adenine dinucleotide-containing flavoenzyme D-amino acid oxidase. Here, we present an overview of the current knowledge of the metabolism of D-serine in human brain at the molecular and cellular levels, with a specific emphasis on the brain localization and regulatory pathways of D-serine, serine racemase, and D-amino acid oxidase. Furthermore, we discuss how D-serine is involved with specific pathological conditions related to N-methyl-D-aspartate receptors over- or down-regulation.

The neurobiology of D-amino acid oxidase and its involvement in schizophrenia

D-amino acid oxidase (DAO) is a flavoenzyme that metabolizes certain D-amino acids, notably the endogenous N-methyl D-aspartate receptor (NMDAR) co-agonist, D-serine. As such, it has the potential to modulate the function of NMDAR and to contribute to the widely hypothesized involvement of NMDAR signalling in schizophrenia. Three lines of evidence now provide support for this possibility: DAO shows genetic associations with the disorder in several, although not all, studies; the expression and activity of DAO are increased in schizophrenia; and DAO inactivation in rodents produces behavioural and biochemical effects, suggestive of potential therapeutic benefits. However, several key issues remain unclear. These include the regional, cellular and subcellular localization of DAO, the physiological importance of DAO and its substrates other than D-serine, as well as the causes and consequences of elevated DAO in schizophrenia. Herein, we critically review the neurobiology of DAO, its involvement in schizophrenia, and the therapeutic value of DAO inhibition. This review also highlights issues that have a broader relevance beyond DAO itself: how should we weigh up convergent and cumulatively impressive, but individually inconclusive, pieces of evidence regarding the role that a given gene may have in the aetiology, pathophysiology and pharmacotherapy of schizophrenia?

Simultaneous determination of hydrophilic amino acid enantiomers in mammalian tissues and physiological fluids applying a fully automated micro-two-dimensional high-performance liquid chromatographic concept

A validated two-dimensional HPLC system combining a microbore-monolithic ODS column and a narrowbore-enantioselective column has been established for a sensitive and simultaneous analysis of hydrophilic amino acid enantiomers (His, Asn, Ser, Gln, Arg, Asp, allo-Thr, Glu and Thr) and the non-chiral amino acid, Gly, in biological samples. To accomplish this goal, the amino acids were first tagged with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) to the respective fluorescent NBD derivatives which were separated in the first dimension by a micro-reversed-phase column. The automatically collected fractions of the target peaks were then transferred to the second dimension consisting of a Pirkle type enantioselective column generating separation factors higher than 1.13 for all the enantiomeric target analytes. The system was validated using standard amino acids and a rat plasma sample, and analytically satisfactory calibration and precision results were obtained. The present 2D-HPLC system enables the fully automated determination of hydrophilic amino acid enantiomers in mammalian samples. The d-isomers of all the investigated 9 amino acids were found in rat urine but at various enantiomeric ratios.

In vitro evidence that D-serine disturbs the citric acid cycle through inhibition of citrate synthase activity in rat cerebral cortex

The present work investigated the in vitro effects of D-serine (D-Ser) on important parameters of energy metabolism in cerebral cortex of young rats. The parameters analyzed were CO(2) generation from glucose and acetate, glucose uptake and the activities of the respiratory chain complexes I-IV, of the citric acid cycle enzymes citrate synthase, aconitase, isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, succinate dehydrogenase, fumarase and malate dehydrogenase and of creatine kinase and Na(+),K(+)-ATPase. Our results show that D-Ser significantly reduced CO(2) production from acetate, but not from glucose, reflecting an impairment of the citric acid cycle function. Furthermore, D-Ser did not affect glucose uptake. We also observed that the activity of the mitochondrial enzyme citrate synthase from mitochondrial preparations and purified citrate synthase was significantly inhibited by D-Ser, whereas the other activities of the citric acid cycle as well as the activities of complexes I-III, II-III, II and IV of the respiratory chain, creatine kinase and Na(+),K(+)-ATPase were not affected by this D-amino acid. We also found that L-serine did not affect citrate synthase activity from mitochondrial preparations and purified enzyme. The data indicate that D-Ser impairs the citric acid cycle activity via citrate synthase inhibition, therefore compromising energy metabolism production in cerebral cortex of young rats. Therefore, it is presumed that this mechanism may be involved at least in part in the neurological damage found in patients affected by disorders in which D-Ser metabolism is impaired, with altered cerebral concentrations of this D-amino acid.

Localization and expression of serine racemase in Arabidopsis thaliana

Arabidopsis plants transformed by promoter of A. thaliana serine racemase fused with beta-glucuronidase (GUS) reporter gene showed strong GUS staining in elongating and developing cells such as tip regions of primary and lateral roots, developing leaves, and shoot meristems. RT-PCR and digital northern hybridization showed that expression of the serine racemase gene was not induced by L- and D-serine, light irradiation, biotic and abiotic stresses.

d-Amino acid oxidase and serine racemase in human brain: normal distribution and altered expression in schizophrenia

The N-methyl-D-aspartate receptor co-agonist d-serine is synthesized by serine racemase and degraded by D-amino acid oxidase. Both D-serine and its metabolizing enzymes are implicated in N-methyl-D-aspartate receptor hypofunction thought to occur in schizophrenia. We studied D-amino acid oxidase and serine racemase immunohistochemically in several brain regions and compared their immunoreactivity and their mRNA levels in the cerebellum and dorsolateral prefrontal cortex in schizophrenia. D-Amino acid oxidase immunoreactivity was abundant in glia, especially Bergmann glia, of the cerebellum, whereas in prefrontal cortex, hippocampus and substantia nigra, it was predominantly neuronal. Serine racemase was principally glial in all regions examined and demonstrated prominent white matter staining. In schizophrenia, D-amino acid oxidase mRNA was increased in the cerebellum, and as a trend for protein. Serine racemase was increased in schizophrenia in the dorsolateral prefrontal cortex but not in cerebellum, while serine racemase mRNA was unchanged in both regions. Administration of haloperidol to rats did not significantly affect serine racemase or D-amino acid oxidase levels. These findings establish the major cell types wherein serine racemase and D-amino acid oxidase are expressed in human brain and provide some support for aberrant D-serine metabolism in schizophrenia. However, they raise further questions as to the roles of D-amino acid oxidase and serine racemase in both physiological and pathophysiological processes in the brain.